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NAME

       ns-3-manual - ns-3 Manual

       This  is  the ns-3 Manual. Primary documentation for the ns-3 project is available in five
       forms:

       · ns-3 Doxygen: Documentation of the public APIs of the simulator

       · Tutorial, Manual  (this  document),  and  Model  Library  for  the  latest  release  and
         development tree

       · ns-3 wiki

CONTENTS

   Organization
       This  chapter  describes  the  overall  ns-3  software  organization and the corresponding
       organization of this manual.

       ns-3 is a discrete-event network simulator in which the simulation  core  and  models  are
       implemented  in  C++.  ns-3  is  built as a library which may be statically or dynamically
       linked to a C++  main  program  that  defines  the  simulation  topology  and  starts  the
       simulator.  ns-3 also exports nearly all of its API to Python, allowing Python programs to
       import an “ns3” module in much the same way as the ns-3 library is linked  by  executables
       in C++.
         [image] Software organization of ns-3.UNINDENT

         The  source  code for ns-3 is mostly organized in the src directory and can be described
         by the diagram in Software organization of ns-3. We will work our way  from  the  bottom
         up; in general, modules only have dependencies on modules beneath them in the figure.

         We first describe the core of the simulator; those components that are common across all
         protocol, hardware, and environmental models.  The simulation  core  is  implemented  in
         src/core.  Packets are fundamental objects in a network simulator and are implemented in
         src/network. These two simulation modules by  themselves  are  intended  to  comprise  a
         generic  simulation  core  that  can  be  used  by different kinds of networks, not just
         Internet-based networks.  The above modules of ns-3 are independent of specific  network
         and device models, which are covered in subsequent parts of this manual.

         In  addition  to  the  above ns-3 core, we introduce, also in the initial portion of the
         manual, two other modules that supplement the core C++-based  API.   ns-3  programs  may
         access  all  of the API directly or may make use of a so-called helper API that provides
         convenient wrappers or encapsulation of low-level API calls. The fact that ns-3 programs
         can  be  written  to  two APIs (or a combination thereof) is a fundamental aspect of the
         simulator.  We also describe how Python is supported in ns-3 before moving onto specific
         models of relevance to network simulation.

         The  remainder  of  the  manual  is  focused  on  documenting  the models and supporting
         capabilities.  The next part focuses on two fundamental objects in ns-3:  the  Node  and
         NetDevice.  Two  special  NetDevice  types are designed to support network emulation use
         cases,  and  emulation  is  described  next.   The  following  chapter  is  devoted   to
         Internet-related  models,  including  the sockets API used by Internet applications. The
         next chapter covers applications, and the following chapter describes additional support
         for simulation, such as animators and statistics.

         The  project  maintains a separate manual devoted to testing and validation of ns-3 code
         (see the ns-3 Testing and Validation manual).

   Random Variables
       ns-3 contains a built-in pseudo-random  number  generator  (PRNG).  It  is  important  for
       serious  users  of the simulator to understand the functionality, configuration, and usage
       of this PRNG, and to decide whether it is sufficient for his or her research use.

   Quick Overview
       ns-3 random numbers are provided via instances of ns3::RandomVariableStream.

       · by default, ns-3 simulations use a fixed  seed;  if  there  is  any  randomness  in  the
         simulation,  each run of the program will yield identical results unless the seed and/or
         run number is changed.

       · in ns-3.3 and earlier, ns-3 simulations used a random seed  by  default;  this  marks  a
         change in policy starting with ns-3.4.

       · in  ns-3.14  and  earlier,  ns-3  simulations  used  a  different  wrapper  class called
         ns3::RandomVariable.    As   of   ns-3.15,   this   class   has   been    replaced    by
         ns3::RandomVariableStream;   the  underlying  pseudo-random  number  generator  has  not
         changed.

       · to obtain randomness across multiple simulation runs,  you  must  either  set  the  seed
         differently   or   set   the   run   number   differently.    To   set   a   seed,  call
         ns3::RngSeedManager::SetSeed() at the beginning of the program; to set a run number with
         the  same  seed, call ns3::RngSeedManager::SetRun() at the beginning of the program; see
         Creating random variables.

       · each RandomVariableStream used in ns-3 has a virtual random number generator  associated
         with  it; all random variables use either a fixed or random seed based on the use of the
         global seed (previous bullet);

       · if you intend to perform multiple runs of  the  same  scenario,  with  different  random
         numbers,  please be sure to read the section on how to perform independent replications:
         Creating random variables.

       Read further for more explanation about the random number facility for ns-3.

   Background
       Simulations use a lot of random numbers; one study found  that  most  network  simulations
       spend  as  much  as 50% of the CPU generating random numbers.  Simulation users need to be
       concerned with the quality of the (pseudo) random numbers  and  the  independence  between
       different streams of random numbers.

       Users need to be concerned with a few issues, such as:

       · the  seeding  of  the  random  number  generator  and  whether  a  simulation outcome is
         deterministic or not,

       · how to acquire different streams  of  random  numbers  that  are  independent  from  one
         another, and

       · how long it takes for streams to cycle

       We  will  introduce  a  few terms here:  a RNG provides a long sequence of (pseudo) random
       numbers.  The length of this sequence is called the cycle length or  period,  after  which
       the  RNG  will repeat itself.  This sequence can be  partitioned into disjoint streams.  A
       stream of a RNG is a contiguous subset or block of the RNG sequence.  For instance, if the
       RNG  period  is  of  length  N, and two streams are provided from this RNG, then the first
       stream might use the first N/2 values and the second stream might produce the  second  N/2
       values.   An  important property here is that the two streams are uncorrelated.  Likewise,
       each stream can be partitioned disjointedly to a number of uncorrelated  substreams.   The
       underlying  RNG  hopefully  produces  a pseudo-random sequence of numbers with a very long
       cycle length, and partitions this into streams and substreams in an efficient manner.

       ns-3 uses the same  underlying  random  number  generator  as  does  ns-2:   the  MRG32k3a
       generator   from   Pierre   L’Ecuyer.    A   detailed   description   can   be   found  in
       http://www.iro.umontreal.ca/~lecuyer/myftp/papers/streams00.pdf.  The  MRG32k3a  generator
       provides  1.8x10^{19}  independent  streams  of  random numbers, each of which consists of
       2.3x10^{15} substreams. Each substream has a period (i.e., the number  of  random  numbers
       before overlap) of 7.6x10^{22}. The period of the entire generator is 3.1x10^{57}.

       Class  ns3::RandomVariableStream  is the public interface to this underlying random number
       generator.  When users create new random variables  (such  as  ns3::UniformRandomVariable,
       ns3::ExponentialRandomVariable,  etc.),  they  create  an  object  that  uses  one  of the
       distinct, independent streams of the random number generator.  Therefore, each  object  of
       type  ns3::RandomVariableStream  has,  conceptually,  its own “virtual” RNG.  Furthermore,
       each ns3::RandomVariableStream can be configured to use one of the set of substreams drawn
       from the main stream.

       An  alternate  implementation  would  be  to  allow  each  RandomVariable  to have its own
       (differently seeded) RNG.  However, we cannot guarantee as  strongly  that  the  different
       sequences  would  be uncorrelated in such a case; hence, we prefer to use a single RNG and
       streams and substreams from it.

   Creating random variables
       ns-3  supports   a   number   of   random   variable   objects   from   the   base   class
       RandomVariableStream.   These  objects  derive  from  ns3::Object and are handled by smart
       pointers.

       The correct way to create these objects is to use  the  templated  CreateObject<>  method,
       such as:

          Ptr<UniformRandomVariable> x = CreateObject<UniformRandomVariable> ();

       then you can access values by calling methods on the object such as:

          myRandomNo = x->GetInteger ();

       If you try to instead do something like this:

          myRandomNo = UniformRandomVariable().GetInteger ();

       your  program  will  encounter  a segmentation fault, because the implementation relies on
       some attribute construction that occurs only when CreateObject is called.

       Much of the  rest  of  this  chapter  now  discusses  the  properties  of  the  stream  of
       pseudo-random  numbers generated from such objects, and how to control the seeding of such
       objects.

   Seeding and independent replications
       ns-3 simulations can be configured to produce deterministic or  random  results.   If  the
       ns-3 simulation is configured to use a fixed, deterministic seed with the same run number,
       it should give the same output each time it is run.

       By default, ns-3 simulations use a fixed seed and run number.  These values are stored  in
       two ns3::GlobalValue instances: g_rngSeed and g_rngRun.

       A  typical  use  case is to run a simulation as a sequence of independent trials, so as to
       compute statistics on a large number of independent runs.  The user can either change  the
       global seed and rerun the simulation, or can advance the substream state of the RNG, which
       is referred to as incrementing the run number.

       A class ns3::RngSeedManager provides  an  API  to  control  the  seeding  and  run  number
       behavior.   This  seeding  and  substream  state  setting must be called before any random
       variables are created; e.g:

          RngSeedManager::SetSeed (3);  // Changes seed from default of 1 to 3
          RngSeedManager::SetRun (7);   // Changes run number from default of 1 to 7
          // Now, create random variables
          Ptr<UniformRandomVariable> x = CreateObject<UniformRandomVariable> ();
          Ptr<ExponentialRandomVariable> y = CreateObject<ExponentialRandomVarlable> ();
          ...

       Which is better, setting a new seed  or  advancing  the  substream  state?   There  is  no
       guarantee that the streams produced by two random seeds will not overlap.  The only way to
       guarantee that two streams do not overlap is to use the substream capability  provided  by
       the  RNG  implementation.   Therefore,  use  the  substream capability to produce multiple
       independent runs of the same simulation. In other words, the more  statistically  rigorous
       way  to  configure multiple independent replications is to use a fixed seed and to advance
       the run number.  This implementation allows  for  a  maximum  of  2.3x10^{15}  independent
       replications using the substreams.

       For  ease  of  use, it is not necessary to control the seed and run number from within the
       program; the user can set the NS_GLOBAL_VALUE environment variable as follows:

          $ NS_GLOBAL_VALUE="RngRun=3" ./waf --run program-name

       Another way to control this is by passing a command-line argument; since this is  an  ns-3
       GlobalValue instance, it is equivalently done such as follows:

          $ ./waf --command-template="%s --RngRun=3" --run program-name

       or, if you are running programs directly outside of waf:

          $ ./build/optimized/scratch/program-name --RngRun=3

       The  above  command-line  variants make it easy to run lots of different runs from a shell
       script by just passing a different RngRun index.

   Class RandomVariableStream
       All random variables should derive from class RandomVariable. This base class  provides  a
       few  methods for globally configuring the behavior of the random number generator. Derived
       classes provide API for drawing random variates from  the  particular  distribution  being
       supported.

       Each  RandomVariableStream  created  in  the simulation is given a generator that is a new
       RNGStream from the underlying PRNG. Used  in  this  manner,  the  L’Ecuyer  implementation
       allows  for  a  maximum  of  1.8x10^19 random variables.  Each random variable in a single
       replication can produce up to 7.6x10^22 random numbers before overlapping.

   Base class public API
       Below are excerpted a few public methods of class  RandomVariableStream  that  access  the
       next value in the substream.

          /**
           * \brief Returns a random double from the underlying distribution
           * \return A floating point random value
           */
          double GetValue (void) const;

          /**
           * \brief Returns a random integer from the underlying distribution
           * \return  Integer cast of ::GetValue()
           */
          uint32_t GetInteger (void) const;

       We  have  already  described  the  seeding  configuration  above. Different RandomVariable
       subclasses may have additional API.

   Types of RandomVariables
       The following types of random variables are provided,  and  are  documented  in  the  ns-3
       Doxygen  or  by  reading  src/core/model/random-variable-stream.h.   Users can also create
       their own custom random variables by deriving from class RandomVariableStream.

       · class UniformRandomVariable

       · class ConstantRandomVariable

       · class SequentialRandomVariable

       · class ExponentialRandomVariable

       · class ParetoRandomVariable

       · class WeibullRandomVariable

       · class NormalRandomVariable

       · class LogNormalRandomVariable

       · class GammaRandomVariable

       · class ErlangRandomVariable

       · class TriangularRandomVariable

       · class ZipfRandomVariable

       · class ZetaRandomVariable

       · class DeterministicRandomVariable

       · class EmpiricalRandomVariable

   Semantics of RandomVariableStream objects
       RandomVariableStream objects derive from ns3::Object and are handled by smart pointers.

       RandomVariableStream instances can also be used  in  ns-3  attributes,  which  means  that
       values  can  be  set  for  them  through  the ns-3 attribute system.  An example is in the
       propagation models for WifiNetDevice:

          TypeId
          RandomPropagationDelayModel::GetTypeId (void)
          {
            static TypeId tid = TypeId ("ns3::RandomPropagationDelayModel")
              .SetParent<PropagationDelayModel> ()
              .SetGroupName ("Propagation")
              .AddConstructor<RandomPropagationDelayModel> ()
              .AddAttribute ("Variable",
                             "The random variable which generates random delays (s).",
                             StringValue ("ns3::UniformRandomVariable"),
                             MakePointerAccessor (&RandomPropagationDelayModel::m_variable),
                             MakePointerChecker<RandomVariableStream> ())
              ;
            return tid;
          }

       Here, the ns-3 user can change the default random variable for this delay model (which  is
       a UniformRandomVariable ranging from 0 to 1) through the attribute system.

   Using other PRNG
       There  is  presently  no  support  for  substituting  a different underlying random number
       generator (e.g., the GNU Scientific Library or the Akaroa package).  Patches are welcome.

   Setting the stream number
       The underlying MRG32k3a generator provides 2^64 independent streams.  In ns-3,  these  are
       assigned sequentially starting from the first stream as new RandomVariableStream instances
       make their first call to GetValue().

       As a result of how these RandomVariableStream objects are assigned to underlying  streams,
       the  assignment  is  sensitive  to  perturbations  of  the  simulation configuration.  The
       consequence is that if any aspect of the simulation configuration is changed, the  mapping
       of RandomVariables to streams may (or may not) change.

       As  a  concrete  example, a user running a comparative study between routing protocols may
       find that the act of changing one routing  protocol  for  another  will  notice  that  the
       underlying mobility pattern also changed.

       Starting  with  ns-3.15,  some  control  has  been  provided  to  users  to allow users to
       optionally fix the assignment  of  selected  RandomVariableStream  objects  to  underlying
       streams.  This is the Stream attribute, part of the base class RandomVariableStream.

       By partitioning the existing sequence of streams from before:

          <-------------------------------------------------------------------------->
          stream 0                                                   stream (2^64 - 1)

       into two equal-sized sets:

          <-------------------------------------------------------------------------->
          ^                                    ^^                                    ^
          |                                    ||                                    |
          stream 0            stream (2^63 - 1)  stream 2^63         stream (2^64 - 1)
          <- automatically assigned -----------><- assigned by user ----------------->

       The  first  2^63  streams  continue  to be automatically assigned, while the last 2^63 are
       given stream indices starting with zero up to 2^63-1.

       The  assignment  of  streams  to  a  fixed  stream  number  is  optional;   instances   of
       RandomVariableStream  that  do  not have a stream value assigned will be assigned the next
       one from the pool of automatic streams.

       To fix a RandomVariableStream  to  a  particular  underlying  stream,  assign  its  Stream
       attribute  to  a  non-negative integer (the default value of -1 means that a value will be
       automatically allocated).

   Publishing your results
       When you publish simulation results, a key piece of  configuration  information  that  you
       should always state is how you used the random number generator.

       · what seeds you used,

       · what RNG you used if not the default,

       · how were independent runs performed,

       · for large simulations, how did you check that you did not cycle.

       It  is  incumbent  on  the  researcher publishing results to include enough information to
       allow others to reproduce his or her results. It is also incumbent on  the  researcher  to
       convince  oneself  that  the random numbers used were statistically valid, and to state in
       the paper why such confidence is assumed.

   Summary
       Let’s review what things you should do when creating a simulation.

       · Decide whether you are running with a fixed seed or random seed; a  fixed  seed  is  the
         default,

       · Decide how you are going to manage independent replications, if applicable,

       · Convince  yourself that you are not drawing more random values than the cycle length, if
         you are running a very long simulation, and

       · When you publish, follow the guidelines above about documenting your use of  the  random
         number generator.

   Hash Functions
       ns-3  provides  a  generic  interface  to general purpose hash functions.  In the simplest
       usage, the hash function returns the 32-bit or 64-bit hash of a  data  buffer  or  string.
       The  default underlying hash function is murmur3, chosen because it has good hash function
       properties and offers a 64-bit version.  The venerable FNV1a hash is also available.

       There is a straight-forward mechanism to add (or provide at  run  time)  alternative  hash
       function implementations.

   Basic Usage
       The simplest way to get a hash value of a data buffer or string is just:

          #include "ns3/hash.h"

          using namespace ns3;

          char * buffer = ...
          size_t buffer_size = ...

          uint32_t  buffer_hash = Hash32 ( buffer, buffer_size);

          std::string s;
          uint32_t  string_hash = Hash32 (s);

       Equivalent functions are defined for 64-bit hash values.

   Incremental Hashing
       In  some  situations  it’s  useful to compute the hash of multiple buffers, as if they had
       been joined together.  (For example, you might want the hash of a packet stream,  but  not
       want to assemble a single buffer with the combined contents of all the packets.)

       This is almost as straight-forward as the first example:

          #include "ns3/hash.h"

          using namespace ns3;

          char * buffer;
          size_t buffer_size;

          Hasher hasher;  // Use default hash function

          for (<every buffer>)
            {
                buffer = get_next_buffer ();
                hasher (buffer, buffer_size);
            }
          uint32_t combined_hash = hasher.GetHash32 ();

       By  default Hasher preserves internal state to enable incremental hashing.  If you want to
       reuse a Hasher object (for  example  because  it’s  configured  with  a  non-default  hash
       function),  but  don’t  want  to  add to the previously computed hash, you need to clear()
       first:

          hasher.clear ().GetHash32 (buffer, buffer_size);

       This reinitializes the internal state before hashing the buffer.

   Using an Alternative Hash Function
       The default hash function is murmur3.  FNV1a is  also  available.   To  specify  the  hash
       function explicitly, use this contructor:

          Hasher hasher = Hasher ( Create<Hash::Function::Fnv1a> () );

   Adding New Hash Function Implementations
       To add the hash function foo, follow the hash-murmur3.h/.cc pattern:

          · Create   a   class   declaration   (.h)   and   definition   (.cc)   inheriting  from
            Hash::Implementation.

          · include the declaration in hash.h (at the point where hash-murmur3.h is included.

          · In  your  own  code,  instantiate  a  Hasher  object  via  the   constructor   Hasher
            (Ptr<Hash::Function::Foo> ())

       If  your  hash  function is a single function, e.g. hashf, you don’t even need to create a
       new class derived from HashImplementation:

          Hasher hasher =
            Hasher ( Create<Hash::Function::Hash32> (&hashf) );

       For this to compile, your hashf has to match one of the function pointer signatures:

          typedef uint32_t (*Hash32Function_ptr) (const char *, const size_t);
          typedef uint64_t (*Hash64Function_ptr) (const char *, const size_t);

   Sources for Hash Functions
       Sources for other hash function implementations include:

          · Peter Kankowski: http://www.strchr.com

          · Arash Partow:    http://www.partow.net/programming/hashfunctions/index.html

          · SMHasher:        http://code.google.com/p/smhasher/

          · Sanmayce:        http://www.sanmayce.com/Fastest_Hash/index.html

   Events and Simulator
       ns-3 is a discrete-event network simulator.  Conceptually, the simulator keeps track of  a
       number of events that are scheduled to execute at a specified simulation time.  The job of
       the simulator is to execute the events in sequential time order.  Once the  completion  of
       an  event  occurs, the simulator will move to the next event (or will exit if there are no
       more events in the event queue).  If, for example, an event scheduled for simulation  time
       “100  seconds”  is  executed, and the next event is not scheduled until “200 seconds”, the
       simulator will immediately jump from 100 seconds to 200 seconds (of  simulation  time)  to
       execute the next event.  This is what is meant by “discrete-event” simulator.

       To make this all happen, the simulator needs a few things:

       1. a  simulator object that can access an event queue where events are stored and that can
          manage the execution of events

       2. a scheduler responsible for inserting and removing events from the queue

       3. a way to represent simulation time

       4. the events themselves

       This chapter of the manual describes  these  fundamental  objects  (simulator,  scheduler,
       time, event) and how they are used.

   Event
       To be completed

   Simulator
       The  Simulator class is the public entry point to access event scheduling facilities. Once
       a couple of events have been scheduled to start the simulation,  the  user  can  start  to
       execute them by entering the simulator main loop (call Simulator::Run). Once the main loop
       starts running, it will sequentially execute all scheduled events in order from oldest  to
       most  recent  until  there  are  either  no  more  events  left  in  the  event  queue  or
       Simulator::Stop has been called.

       To schedule events for execution by the simulator main loop, the Simulator class  provides
       the Simulator::Schedule* family of functions.

       1. Handling event handlers with different signatures

       These  functions are declared and implemented as C++ templates to handle automatically the
       wide variety of C++ event handler signatures used in the wild. For example, to schedule an
       event  to  execute  10  seconds  in  the  future, and invoke a C++ method or function with
       specific arguments, you might write this:

          void handler (int arg0, int arg1)
          {
            std::cout << "handler called with argument arg0=" << arg0 << " and
               arg1=" << arg1 << std::endl;
          }

          Simulator::Schedule(Seconds(10), &handler, 10, 5);

       Which will output:

          handler called with argument arg0=10 and arg1=5

       Of course, these C++ templates  can  also  handle  transparently  member  methods  on  C++
       objects:

       To be completed:  member method example

       Notes:

       · the  ns-3  Schedule  methods  recognize automatically functions and methods only if they
         take less than 5 arguments. If you need them to support more arguments, please,  file  a
         bug report.

       · Readers   familiar   with   the   term   ‘fully-bound   functors’   will  recognize  the
         Simulator::Schedule methods as a way to automatically construct such objects.

       2. Common scheduling operations

       The Simulator API was designed to make it  really  simple  to  schedule  most  events.  It
       provides three variants to do so (ordered from most commonly used to least commonly used):

       · Schedule  methods  which  allow  you to schedule an event in the future by providing the
         delay between the current simulation time and the expiration date of the target event.

       · ScheduleNow methods which allow you to schedule an  event  for  the  current  simulation
         time: they will execute _after_ the current event is finished executing but _before_ the
         simulation time is changed for the next event.

       · ScheduleDestroy methods which allow you to hook in the shutdown process of the Simulator
         to  cleanup  simulation resources: every ‘destroy’ event is executed when the user calls
         the Simulator::Destroy method.

       3. Maintaining the simulation context

       There are two basic ways to schedule events, with and without  context.   What  does  this
       mean?

          Simulator::Schedule (Time const &time, MEM mem_ptr, OBJ obj);

       vs.

          Simulator::ScheduleWithContext (uint32_t context, Time const &time, MEM mem_ptr, OBJ obj);

       Readers  who  invest time and effort in developing or using a non-trivial simulation model
       will know the value of the ns-3 logging framework to debug simple and complex  simulations
       alike.  One  of  the  important features that is provided by this logging framework is the
       automatic display of the network node id associated with the ‘currently’ running event.

       The node id of the currently executing network node is in fact tracked  by  the  Simulator
       class.  It  can  be  accessed  with  the  Simulator::GetContext  method  which returns the
       ‘context’ (a 32-bit integer) associated and stored in the  currently-executing  event.  In
       some  rare  cases,  when  an  event  is  not  associated with a specific network node, its
       ‘context’ is set to 0xffffffff.

       To associate a context to each event, the Schedule, and ScheduleNow methods  automatically
       reuse  the  context of the currently-executing event as the context of the event scheduled
       for execution later.

       In some cases, most notably when simulating the transmission of a packet from  a  node  to
       another, this behavior is undesirable since the expected context of the reception event is
       that of the receiving node, not the sending node. To avoid  this  problem,  the  Simulator
       class provides a specific schedule method: ScheduleWithContext which allows one to provide
       explicitly the node id of the receiving node associated with the receive event.

       XXX: code example

       In some very rare cases, developers might need to modify or  understand  how  the  context
       (node  id)  of the first event is set to that of its associated node. This is accomplished
       by the  NodeList  class:  whenever  a  new  node  is  created,  the  NodeList  class  uses
       ScheduleWithContext to schedule a ‘initialize’ event for this node. The ‘initialize’ event
       thus executes with a context set to that of the node id and can use the normal variety  of
       Schedule methods. It invokes the Node::Initialize method which propagates the ‘initialize’
       event by calling the DoInitialize method for each object associated  with  the  node.  The
       DoInitialize  method  overridden in some of these objects (most notably in the Application
       base class) will schedule some events (most notably  Application::StartApplication)  which
       will   in   turn  scheduling  traffic  generation  events  which  will  in  turn  schedule
       network-level events.

       Notes:

       · Users need to be careful to propagate DoInitialize methods  across  objects  by  calling
         Initialize explicitly on their member objects

       · The  context  id  associated  with each ScheduleWithContext method has other uses beyond
         logging: it is used by an experimental branch of ns-3 to perform parallel simulation  on
         multicore systems using multithreading.

       The  Simulator::*  functions  do  not know what the context is: they merely make sure that
       whatever context you specify with ScheduleWithContext is available when the  corresponding
       event executes with ::GetContext.

       It  is up to the models implemented on top of Simulator::* to interpret the context value.
       In ns-3, the network models interpret the context  as  the  node  id  of  the  node  which
       generated  an  event.  This  is  why  it  is  important  to  call  ScheduleWithContext  in
       ns3::Channel subclasses because we are generating an event from node i to node  j  and  we
       want to make sure that the event which will run on node j has the right context.

   Time
       To be completed

   Scheduler
       To be completed

   Callbacks
       Some  new  users  to  ns-3  are unfamiliar with an extensively used programming idiom used
       throughout the code: the ns-3 callback. This  chapter  provides  some  motivation  on  the
       callback, guidance on how to use it, and details on its implementation.

   Callbacks Motivation
       Consider  that  you  have  two  simulation  models A and B, and you wish to have them pass
       information between them during the simulation. One way that you can do that is  that  you
       can  make  A  and B each explicitly knowledgeable about the other, so that they can invoke
       methods on each other:

          class A {
          public:
            void ReceiveInput ( // parameters );
            ...
          }

          (in another source file:)

          class B {
          public:
            void DoSomething (void);
            ...

          private:
            A* a_instance; // pointer to an A
          }

          void
          B::DoSomething()
          {
            // Tell a_instance that something happened
            a_instance->ReceiveInput ( // parameters);
            ...
          }

       This certainly works, but it has the drawback that it introduces a dependency on A  and  B
       to  know  about  the  other  at  compile  time  (this  makes it harder to have independent
       compilation units in the simulator) and is not generalized; if in a later usage  scenario,
       B  needs  to  talk  to  a completely different C object, the source code for B needs to be
       changed to add a c_instance and so forth. It is easy to see that this  is  a  brute  force
       mechanism of communication that can lead to programming cruft in the models.

       This  is  not  to  say  that  objects should not know about one another if there is a hard
       dependency between them, but that often the  model  can  be  made  more  flexible  if  its
       interactions are less constrained at compile time.

       This  is not an abstract problem for network simulation research, but rather it has been a
       source of problems in previous simulators, when researchers want to extend or  modify  the
       system  to do different things (as they are apt to do in research). Consider, for example,
       a user who wants to add an IPsec security protocol sublayer between TCP and IP:

          ------------                   -----------
          |   TCP    |                   |  TCP    |
          ------------                   -----------
               |           becomes ->        |
          -----------                    -----------
          |   IP    |                    | IPsec   |
          -----------                    -----------
                                             |
                                         -----------
                                         |   IP    |
                                         -----------

       If the simulator has made assumptions, and hard coded into the code, that IP always  talks
       to  a  transport  protocol  above,  the  user  may be forced to hack the system to get the
       desired interconnections. This  is  clearly  not  an  optimal  way  to  design  a  generic
       simulator.

   Callbacks Background
       NOTE:
          Readers familiar with programming callbacks may skip this tutorial section.

       The  basic  mechanism that allows one to address the problem above is known as a callback.
       The ultimate goal is to allow one piece of code to call a  function  (or  method  in  C++)
       without any specific inter-module dependency.

       This  ultimately  means  you  need some kind of indirection – you treat the address of the
       called function as a variable.  This variable is called  a  pointer-to-function  variable.
       The  relationship  between function and pointer-to-function pointer is really no different
       that that of object and pointer-to-object.

       In    C    the     canonical     example     of     a     pointer-to-function     is     a
       pointer-to-function-returning-integer  (PFI).  For  a  PFI  taking one int parameter, this
       could be declared like,:

          int (*pfi)(int arg) = 0;

       What you get from this is a variable named simply pfi that is initialized to the value  0.
       If  you  want  to  initialize  this  pointer  to  something meaningful, you have to have a
       function with a matching signature. In this case:

          int MyFunction (int arg) {}

       If you have this target, you can initialize the variable to point to your function like:

          pfi = MyFunction;

       You can then call MyFunction indirectly using the more suggestive form of the call:

          int result = (*pfi) (1234);

       This is suggestive since it looks like you are dereferencing  the  function  pointer  just
       like  you  would dereference any pointer. Typically, however, people take advantage of the
       fact that the compiler knows what is going on and will just use a shorter form:

          int result = pfi (1234);

       Notice that the function pointer obeys value semantics, so you can pass it around like any
       other  value.  Typically, when you use an asynchronous interface you will pass some entity
       like this to a function which will perform an action and call back  to  let  you  know  it
       completed. It calls back by following the indirection and executing the provided function.

       In  C++ you have the added complexity of objects. The analogy with the PFI above means you
       have a pointer to a member function returning an int  (PMI)  instead  of  the  pointer  to
       function returning an int (PFI).

       The declaration of the variable providing the indirection looks only slightly different:

          int (MyClass::*pmi) (int arg) = 0;

       This  declares a variable named pmi just as the previous example declared a variable named
       pfi. Since the will be to call a method of an instance of a  particular  class,  one  must
       declare that method in a class:

          class MyClass {
          public:
            int MyMethod (int arg);
          };

       Given this class declaration, one would then initialize that variable like this:

          pmi = &MyClass::MyMethod;

       This  assigns  the address of the code implementing the method to the variable, completing
       the indirection. In order to call a method, the code needs a this pointer. This, in  turn,
       means there must be an object of MyClass to refer to. A simplistic example of this is just
       calling a method indirectly (think virtual function):

          int (MyClass::*pmi) (int arg) = 0;  // Declare a PMI
          pmi = &MyClass::MyMethod;           // Point at the implementation code

          MyClass myClass;                    // Need an instance of the class
          (myClass.*pmi) (1234);              // Call the method with an object ptr

       Just like in the C example, you can use this in an asynchronous  call  to  another  module
       which  will  call back using a method and an object pointer. The straightforward extension
       one might consider is to pass a pointer to the object and the  PMI  variable.  The  module
       would just do:

          (*objectPtr.*pmi) (1234);

       to execute the callback on the desired object.

       One  might  ask  at this time, what’s the point? The called module will have to understand
       the concrete type of the calling object in order to properly make the  callback.  Why  not
       just  accept  this, pass the correctly typed object pointer and do object->Method(1234) in
       the code instead of the callback?  This is precisely the problem described above. What  is
       needed  is  a  way to decouple the calling function from the called class completely. This
       requirement led to the development of the Functor.

       A functor is the outgrowth of something invented in the 1960s called  a  closure.   It  is
       basically just a packaged-up function call, possibly with some state.

       A  functor has two parts, a specific part and a generic part, related through inheritance.
       The calling code (the code that executes the callback) will execute a  generic  overloaded
       operator  () of a generic functor to cause the callback to be called. The called code (the
       code that wants to be called back) will have to provide a  specialized  implementation  of
       the  operator  ()  that  performs  the  class-specific work that caused the close-coupling
       problem above.

       With the specific functor and its overloaded operator () created,  the  called  code  then
       gives  the  specialized  code  to  the  module that will execute the callback (the calling
       code).

       The calling code will take a generic functor as a parameter, so an implicit cast  is  done
       in  the  function  call  to convert the specific functor to a generic functor.  This means
       that the calling module just needs to understand the generic functor type. It is decoupled
       from the calling code completely.

       The  information  one  needs  to  make  a  specific  functor is the object pointer and the
       pointer-to-method address.

       The essence of what needs to happen is that the system declares  a  generic  part  of  the
       functor:

          template <typename T>
          class Functor
          {
          public:
            virtual int operator() (T arg) = 0;
          };

       The  caller  defines a specific part of the functor that really is just there to implement
       the specific operator() method:

          template <typename T, typename ARG>
          class SpecificFunctor : public Functor<ARG>
          {
          public:
            SpecificFunctor(T* p, int (T::*_pmi)(ARG arg))
            {
              m_p = p;
              m_pmi = _pmi;
            }

            virtual int operator() (ARG arg)
            {
              (*m_p.*m_pmi)(arg);
            }
          private:
            int (T::*m_pmi)(ARG arg);
            T* m_p;
          };

       Here is an example of the usage:

          class A
          {
          public:
          A (int a0) : a (a0) {}
          int Hello (int b0)
          {
            std::cout << "Hello from A, a = " << a << " b0 = " << b0 << std::endl;
          }
          int a;
          };

          int main()
          {
            A a(10);
            SpecificFunctor<A, int> sf(&a, &A::Hello);
            sf(5);
          }

       NOTE:
          The previous code is not real ns-3 code.  It is simplistic example code  used  only  to
          illustrate  the  concepts  involved and to help you understand the system more.  Do not
          expect to find this code anywhere in the ns-3 tree.

       Notice that there are two variables defined in the class above.  The m_p variable  is  the
       object  pointer  and  m_pmi  is  the  variable  containing  the address of the function to
       execute.

       Notice that when operator() is called, it in turn  calls  the  method  provided  with  the
       object pointer using the C++ PMI syntax.

       To  use  this,  one  could  then declare some model code that takes a generic functor as a
       parameter:

          void LibraryFunction (Functor functor);

       The code that will talk to the model would  build  a  specific  functor  and  pass  it  to
       LibraryFunction:

          MyClass myClass;
          SpecificFunctor<MyClass, int> functor (&myclass, MyClass::MyMethod);

       When LibraryFunction is done, it executes the callback using the operator() on the generic
       functor it was passed, and in this particular case, provides the integer argument:

          void
          LibraryFunction (Functor functor)
          {
            // Execute the library function
            functor(1234);
          }

       Notice that LibraryFunction is completely decoupled from the specific type of the  client.
       The connection is made through the Functor polymorphism.

       The Callback API in ns-3 implements object-oriented callbacks using the functor mechanism.
       This callback API, being based on C++ templates, is type-safe; that is, it performs static
       type  checks to enforce proper signature compatibility between callers and callees.  It is
       therefore more type-safe to use than traditional function pointers,  but  the  syntax  may
       look  imposing at first.  This section is designed to walk you through the Callback system
       so that you can be comfortable using it in ns-3.

   Using the Callback API
       The Callback API is fairly minimal, providing only two services:

       1. callback type declaration: a way to declare a type of callback with a given  signature,
       and,

       2.  callback  instantiation: a way to instantiate a template-generated forwarding callback
       which can forward any calls to another C++ class member method or C++ function.

       This is best observed via walking through an example, based on samples/main-callback.cc.

   Using the Callback API with static functions
       Consider a function:

          static double
          CbOne (double a, double b)
          {
            std::cout << "invoke cbOne a=" << a << ", b=" << b << std::endl;
            return a;
          }

       Consider also the following main program snippet:

          int main (int argc, char *argv[])
          {
            // return type: double
            // first arg type: double
            // second arg type: double
            Callback<double, double, double> one;
          }

       This is an example of a C-style callback – one which does  not  include  or  need  a  this
       pointer.   The  function  template Callback is essentially the declaration of the variable
       containing the pointer-to-function.  In the example above, we explicitly showed a  pointer
       to  a  function  that  returned  an integer and took a single integer as a parameter,  The
       Callback template function is a generic version of that – it is used to declare  the  type
       of a callback.

       NOTE:
          Readers        unfamiliar        with       C++       templates       may       consult
          http://www.cplusplus.com/doc/tutorial/templates/.

       The Callback template requires one mandatory argument (the return type of the function  to
       be  assigned  to  this callback) and up to five optional arguments, which each specify the
       type of the arguments (if your particular callback function has more than five  arguments,
       then this can be handled by extending the callback implementation).

       So  in  the  above example, we have a declared a callback named “one” that will eventually
       hold a function pointer.  The signature of the function that  it  will  hold  must  return
       double  and  must  support  two  double  arguments.  If one tries to pass a function whose
       signature does not match the declared callback, a compilation error will occur.  Also,  if
       one  tries  to  assign  to  a callback an incompatible one, compilation will succeed but a
       run-time     NS_FATAL_ERROR     will     be     raised.       The      sample      program
       src/core/examples/main-callback.cc  demonstrates  both  of these error cases at the end of
       the main() program.

       Now, we need to tie together  this  callback  instance  and  the  actual  target  function
       (CbOne).   Notice  above that CbOne has the same function signature types as the callback–
       this is important.  We can pass in any such  properly-typed  function  to  this  callback.
       Let’s look at this more closely:

          static   double CbOne (double a, double b) {}
                     ^             ^         ^
                     |             |         |
                     |             |         |
          Callback<double,       double,   double> one;

       You can only bind a function to a callback if they have the matching signature.  The first
       template argument is the return type, and the additional template arguments are the  types
       of the arguments of the function signature.

       Now, let’s bind our callback “one” to the function that matches its signature:

          // build callback instance which points to cbOne function
          one = MakeCallback (&CbOne);

       This  call  to  MakeCallback  is,  in  essence,  creating  one of the specialized functors
       mentioned above.  The variable declared using the Callback template function is  going  to
       be playing the part of the generic functor.  The assignment one = MakeCallback (&CbOne) is
       the cast that converts the specialized functor known to the callee to  a  generic  functor
       known to the caller.

       Then, later in the program, if the callback is needed, it can be used as follows:

          NS_ASSERT (!one.IsNull ());

          // invoke cbOne function through callback instance
          double retOne;
          retOne = one (10.0, 20.0);

       The check for IsNull() ensures that the callback is not null – that there is a function to
       call behind this callback.  Then, one() executes the generic operator()  which  is  really
       overloaded  with a specific implementation of operator() and returns the same result as if
       CbOne() had been called directly.

   Using the Callback API with member functions
       Generally, you will not be calling static functions but instead public member functions of
       an  object.   In  this  case, an extra argument is needed to the MakeCallback function, to
       tell the system on which object the function should be invoked.   Consider  this  example,
       also from main-callback.cc:

          class MyCb {
          public:
            int CbTwo (double a) {
                std::cout << "invoke cbTwo a=" << a << std::endl;
                return -5;
            }
          };

          int main ()
          {
            ...
            // return type: int
            // first arg type: double
            Callback<int, double> two;
            MyCb cb;
            // build callback instance which points to MyCb::cbTwo
            two = MakeCallback (&MyCb::CbTwo, &cb);
            ...
          }

       Here,  we  pass  an additional object pointer to the MakeCallback<> function.  Recall from
       the background section above that Operator() will use the pointer to member syntax when it
       executes on an object:

          virtual int operator() (ARG arg)
          {
            (*m_p.*m_pmi)(arg);
          }

       And  so  we  needed to provide the two variables (m_p and m_pmi) when we made the specific
       functor.  The line:

          two = MakeCallback (&MyCb::CbTwo, &cb);

       does precisely that.  In this case, when two () is invoked:

          int result = two (1.0);

       will result in a call tothe CbTwo member function (method) on the  object  pointed  to  by
       &cb.

   Building Null Callbacks
       It  is possible for callbacks to be null; hence it may be wise to check before using them.
       There is a special construct for a null callback, which is preferable  to  simply  passing
       “0” as an argument; it is the MakeNullCallback<> construct:

          two = MakeNullCallback<int, double> ();
          NS_ASSERT (two.IsNull ());

       Invoking  a  null callback is just like invoking a null function pointer: it will crash at
       runtime.

   Bound Callbacks
       A very useful extension to the functor concept is that of a Bound Callback.  Previously it
       was  mentioned  that  closures  were  originally  function  calls  packaged  up  for later
       execution.  Notice that in all of the Callback descriptions above,  there  is  no  way  to
       package up any parameters for use later – when the Callback is called via operator().  All
       of the parameters are provided by the calling function.

       What if it is desired to allow the client function (the one that provides the callback) to
       provide  some  of  the parameters?  Alexandrescu calls the process of allowing a client to
       specify one of the parameters “binding”.  One of the parameters  of  operator()  has  been
       bound (fixed) by the client.

       Some  of  our pcap tracing code provides a nice example of this.  There is a function that
       needs to be called whenever a packet is received.  This  function  calls  an  object  that
       actually writes the packet to disk in the pcap file format.  The signature of one of these
       functions will be:

          static void DefaultSink (Ptr<PcapFileWrapper> file, Ptr<const Packet> p);

       The static keyword means this is a static function which does not need a this pointer,  so
       it  will be using C-style callbacks.  We don’t want the calling code to have to know about
       anything but the Packet.  What we want in the calling code is just a call that looks like:

          m_promiscSnifferTrace (m_currentPkt);

       What we want to do is to bind  the  Ptr<PcapFileWriter>  file  to  the  specific  callback
       implementation  when  it  is  created  and  arrange  for the operator() of the Callback to
       provide that parameter for free.

       We provide the MakeBoundCallback template function for that purpose.  It  takes  the  same
       parameters  as  the  MakeCallback  template  function  but also takes the parameters to be
       bound.  In the case of the example above:

          MakeBoundCallback (&DefaultSink, file);

       will create a specific callback implementation that  knows  to  add  in  the  extra  bound
       arguments.  Conceptually, it extends the specific functor described above with one or more
       bound arguments:

          template <typename T, typename ARG, typename BOUND_ARG>
          class SpecificFunctor : public Functor
           {
           public:
              SpecificFunctor(T* p, int (T::*_pmi)(ARG arg), BOUND_ARG boundArg)
              {
                m_p = p;
                m_pmi = pmi;
                m_boundArg = boundArg;
              }

              virtual int operator() (ARG arg)
              {
                (*m_p.*m_pmi)(m_boundArg, arg);
              }
          private:
              void (T::*m_pmi)(ARG arg);
              T* m_p;
              BOUND_ARG m_boundArg;
           };

       You can see that when the specific functor is created, the bound argument is saved in  the
       functor  /  callback  object  itself.   When  the  operator()  is  invoked with the single
       parameter, as in:

          m_promiscSnifferTrace (m_currentPkt);

       the implementation of operator() adds the bound parameter into the actual function call:

          (*m_p.*m_pmi)(m_boundArg, arg);

       It’s possible to bind two or three arguments  as  well.   Say  we  have  a  function  with
       signature:

          static void NotifyEvent (Ptr<A> a, Ptr<B> b, MyEventType e);

       One can create bound callback binding first two arguments like:

          MakeBoundCallback (&NotifyEvent, a1, b1);

       assuming  a1  and  b1  are  objects  of  type  A  and B respectively.  Similarly for three
       arguments one would have function with a signature:

          static void NotifyEvent (Ptr<A> a, Ptr<B> b, MyEventType e);

       Binding three arguments in done with:

          MakeBoundCallback (&NotifyEvent, a1, b1, c1);

       again assuming a1, b1 and c1 are objects of type A, B and C respectively.

       This kind of binding can be used for exchanging information between objects in simulation;
       specifically,  bound callbacks can be used as traced callbacks, which will be described in
       the next section.

   Traced Callbacks
       Placeholder subsection

   Callback locations in ns-3
       Where are callbacks frequently used in ns-3?  Here are some of the more  visible  ones  to
       typical users:

       · Socket API

       · Layer-2/Layer-3 API

       · Tracing subsystem

       · API between IP and routing subsystems

   Implementation details
       The  code  snippets  above  are  simplistic  and only designed to illustrate the mechanism
       itself.  The actual Callback code is quite complicated and  very  template-intense  and  a
       deep  understanding of the code is not required.  If interested, expert users may find the
       following useful.

       The   code   was   originally   written   based   on   the   techniques    described    in
       http://www.codeproject.com/cpp/TTLFunction.asp.   It  was subsequently rewritten to follow
       the architecture outlined in Modern C++ Design, Generic Programming  and  Design  Patterns
       Applied, Alexandrescu, chapter 5, Generalized Functors.

       This code uses:

       · default  template parameters to saves users from having to specify empty parameters when
         the number of parameters is smaller than the maximum supported number

       · the pimpl idiom: the Callback class is passed around by value and delegates the crux  of
         the work to its pimpl pointer.

       · two pimpl implementations which derive from CallbackImpl FunctorCallbackImpl can be used
         with any functor-type while MemPtrCallbackImpl can  be  used  with  pointers  to  member
         functions.

       · a reference list implementation to implement the Callback’s value semantics.

       This  code  most  notably departs from the Alexandrescu implementation in that it does not
       use type lists to specify and pass around the types of the callback arguments. Of  course,
       it also does not use copy-destruction semantics and relies on a reference list rather than
       autoPtr to hold the pointer.

   Object model
       ns-3 is fundamentally a C++ object system. Objects can be  declared  and  instantiated  as
       usual,  per  C++  rules.  ns-3  also  adds  some  features  to traditional C++ objects, as
       described below, to provide greater functionality and features.  This  manual  chapter  is
       intended to introduce the reader to the ns-3 object model.

       This  section  describes  the  C++ class design for ns-3 objects. In brief, several design
       patterns in  use  include  classic  object-oriented  design  (polymorphic  interfaces  and
       implementations),  separation  of  interface  and  implementation,  the non-virtual public
       interface design pattern, an object  aggregation  facility,  and  reference  counting  for
       memory  management.  Those  familiar  with  component  models  such  as COM or Bonobo will
       recognize elements of the design in the ns-3 object aggregation model, although  the  ns-3
       design is not strictly in accordance with either.

   Object-oriented behavior
       C++  objects,  in  general,  provide  common  object-oriented  capabilities  (abstraction,
       encapsulation, inheritance, and polymorphism) that are  part  of  classic  object-oriented
       design. ns-3 objects make use of these properties; for instance:

          class Address
          {
          public:
            Address ();
            Address (uint8_t type, const uint8_t *buffer, uint8_t len);
            Address (const Address & address);
            Address &operator = (const Address &address);
            ...
          private:
            uint8_t m_type;
            uint8_t m_len;
            ...
          };

   Object base classes
       There  are  three  special base classes used in ns-3. Classes that inherit from these base
       classes can instantiate objects with special properties.  These base classes are:

       · class Object

       · class ObjectBase

       · class SimpleRefCount

       It is not required that ns-3 objects inherit from these  class,  but  those  that  do  get
       special properties. Classes deriving from class Object get the following properties.

       · the ns-3 type and attribute system (see Attributes)

       · an object aggregation system

       · a smart-pointer reference counting system (class Ptr)

       Classes  that  derive from class ObjectBase get the first two properties above, but do not
       get  smart  pointers.  Classes  that  derive  from  class  SimpleRefCount:  get  only  the
       smart-pointer reference counting system.

       In  practice,  class Object is the variant of the three above that the ns-3 developer will
       most commonly encounter.

   Memory management and class Ptr
       Memory management in a C++ program is a complex process, and is often done incorrectly  or
       inconsistently. We have settled on a reference counting design described as follows.

       All  objects  using  reference  counting maintain an internal reference count to determine
       when an object can safely delete itself. Each time  that  a  pointer  is  obtained  to  an
       interface,  the  object’s  reference  count  is  incremented  by  calling Ref(). It is the
       obligation of the user of the pointer to explicitly Unref() the pointer  when  done.  When
       the reference count falls to zero, the object is deleted.

       · When  the  client code obtains a pointer from the object itself through object creation,
         or via GetObject, it does not have to increment the reference count.

       · When client code obtains a pointer from another source (e.g., copying a pointer) it must
         call Ref() to increment the reference count.

       · All users of the object pointer must call Unref() to release the reference.

       The  burden  for calling Unref() is somewhat relieved by the use of the reference counting
       smart pointer class described below.

       Users using a low-level API who wish to explicitly allocate non-reference-counted  objects
       on the heap, using operator new, are responsible for deleting such objects.

   Reference counting smart pointer (Ptr)
       Calling  Ref()  and  Unref()  all  the  time would be cumbersome, so ns-3 provides a smart
       pointer class Ptr similar to Boost::intrusive_ptr. This smart-pointer class  assumes  that
       the  underlying  type  provides  a  pair  of  Ref  and  Unref methods that are expected to
       increment and decrement the internal refcount of the object instance.

       This implementation allows you to manipulate the smart pointer  as  if  it  was  a  normal
       pointer:  you  can compare it with zero, compare it against other pointers, assign zero to
       it, etc.

       It is possible to extract the raw pointer from this smart pointer  with  the  GetPointer()
       and PeekPointer() methods.

       If  you  want  to  store  a newed object into a smart pointer, we recommend you to use the
       CreateObject template functions to create the object and store it in a  smart  pointer  to
       avoid  memory leaks. These functions are really small convenience functions and their goal
       is just to save you a small bit of typing.

   CreateObject and Create
       Objects in C++ may be statically, dynamically, or automatically created.  This holds  true
       for  ns-3  also, but some objects in the system have some additional frameworks available.
       Specifically, reference counted objects are usually allocated using a templated Create  or
       CreateObject method, as follows.

       For objects deriving from class Object:

          Ptr<WifiNetDevice> device = CreateObject<WifiNetDevice> ();

       Please  do  not  create  such objects using operator new; create them using CreateObject()
       instead.

       For objects deriving from class SimpleRefCount, or other objects that support usage of the
       smart pointer class, a templated helper function is available and recommended to be used:

          Ptr<B> b = Create<B> ();

       This is simply a wrapper around operator new that correctly handles the reference counting
       system.

       In summary, use Create<B> if B is not an object but just  uses  reference  counting  (e.g.
       Packet), and use CreateObject<B> if B derives from ns3::Object.

   Aggregation
       The  ns-3  object  aggregation  system is motivated in strong part by a recognition that a
       common use case for ns-2 has been the  use  of  inheritance  and  polymorphism  to  extend
       protocol  models.  For  instance,  specialized versions of TCP such as RenoTcpAgent derive
       from (and override functions from) class TcpAgent.

       However, two problems that have arisen in the ns-2 model  are  downcasts  and  “weak  base
       class.” Downcasting refers to the procedure of using a base class pointer to an object and
       querying it at run time to find out type information, used to explicitly cast the  pointer
       to  a subclass pointer so that the subclass API can be used. Weak base class refers to the
       problems that arise when a class cannot be effectively reused (derived  from)  because  it
       lacks  necessary functionality, leading the developer to have to modify the base class and
       causing proliferation of base class API calls, some  of  which  may  not  be  semantically
       correct for all subclasses.

       ns-3  is  using  a  version of the query interface design pattern to avoid these problems.
       This design is based on elements of the Component Object Model and GNOME  Bonobo  although
       full  binary-level  compatibility  of  replaceable components is not supported and we have
       tried to simplify the syntax and impact on model developers.

   Examples
   Aggregation example
       Node is a good example of the use of aggregation in ns-3.  Note that there are not derived
       classes  of Nodes in ns-3 such as class InternetNode.  Instead, components (protocols) are
       aggregated to a node. Let’s look at how some Ipv4 protocols are added to a node.:

          static void
          AddIpv4Stack(Ptr<Node> node)
          {
            Ptr<Ipv4L3Protocol> ipv4 = CreateObject<Ipv4L3Protocol> ();
            ipv4->SetNode (node);
            node->AggregateObject (ipv4);
            Ptr<Ipv4Impl> ipv4Impl = CreateObject<Ipv4Impl> ();
            ipv4Impl->SetIpv4 (ipv4);
            node->AggregateObject (ipv4Impl);
          }

       Note that the Ipv4 protocols are created using CreateObject().  Then, they are  aggregated
       to the node. In this manner, the Node base class does not need to be edited to allow users
       with a base class Node pointer to access the Ipv4 interface; users may ask the node for  a
       pointer  to  its Ipv4 interface at runtime. How the user asks the node is described in the
       next subsection.

       Note that it is a programming error to aggregate more than one object of the same type  to
       an  ns3::Object.  So,  for  instance,  aggregation is not an option for storing all of the
       active sockets of a node.

   GetObject example
       GetObject is a type-safe way to achieve a safe downcasting and to allow interfaces  to  be
       found on an object.

       Consider  a node pointer m_node that points to a Node object that has an implementation of
       IPv4 previously aggregated to it. The client code wishes to configure a default route.  To
       do so, it must access an object within the node that has an interface to the IP forwarding
       configuration. It performs the following:

          Ptr<Ipv4> ipv4 = m_node->GetObject<Ipv4> ();

       If the node in fact does not have an Ipv4 object aggregated to it, then  the  method  will
       return null. Therefore, it is good practice to check the return value from such a function
       call. If successful, the user can now use the Ptr to the Ipv4 object that  was  previously
       aggregated to the node.

       Another example of how one might use aggregation is to add optional models to objects. For
       instance, an existing Node object may have an “Energy Model” object aggregated  to  it  at
       run time (without modifying and recompiling the node class).  An existing model (such as a
       wireless net device) can then later “GetObject” for the energy model and act appropriately
       if  the  interface has been either built in to the underlying Node object or aggregated to
       it at run time.  However, other nodes need not know anything about energy models.

       We hope that this mode of programming will require much less need for developers to modify
       the base classes.

   Object factories
       A  common  use  case is to create lots of similarly configured objects. One can repeatedly
       call CreateObject() but there is also a factory design pattern in use in the ns-3  system.
       It is heavily used in the “helper” API.

       Class  ObjectFactory can be used to instantiate objects and to configure the attributes on
       those objects:

          void SetTypeId (TypeId tid);
          void Set (std::string name, const AttributeValue &value);
          Ptr<T> Create (void) const;

       The first method allows one to use the ns-3 TypeId system to specify the type  of  objects
       created.  The  second  allows  one to set attributes on the objects to be created, and the
       third allows one to create the objects themselves.

       For example:

          ObjectFactory factory;
          // Make this factory create objects of type FriisPropagationLossModel
          factory.SetTypeId ("ns3::FriisPropagationLossModel")
          // Make this factory object change a default value of an attribute, for
          // subsequently created objects
          factory.Set ("SystemLoss", DoubleValue (2.0));
          // Create one such object
          Ptr<Object> object = factory.Create ();
          factory.Set ("SystemLoss", DoubleValue (3.0));
          // Create another object with a different SystemLoss
          Ptr<Object> object = factory.Create ();

   Downcasting
       A question that has arisen several times is, “If I have a base class pointer (Ptr)  to  an
       object  and  I want the derived class pointer, should I downcast (via C++ dynamic cast) to
       get the derived pointer, or should I use the object aggregation system to  GetObject<>  ()
       to find a Ptr to the interface to the subclass API?”

       The answer to this is that in many situations, both techniques will work.  ns-3 provides a
       templated function for making  the  syntax  of  Object  dynamic  casting  much  more  user
       friendly:

          template <typename T1, typename T2>
          Ptr<T1>
          DynamicCast (Ptr<T2> const&p)
          {
            return Ptr<T1> (dynamic_cast<T1 *> (PeekPointer (p)));
          }

       DynamicCast  works  when  the  programmer has a base type pointer and is testing against a
       subclass pointer. GetObject works when looking for different objects aggregated, but  also
       works  with  subclasses,  in the same way as DynamicCast. If unsure, the programmer should
       use GetObject, as it works in all cases. If the programmer knows the  class  hierarchy  of
       the object under consideration, it is more direct to just use DynamicCast.

   Configuration and Attributes
       In ns-3 simulations, there are two main aspects to configuration:

       · The simulation topology and how objects are connected.

       · The values used by the models instantiated in the topology.

       This  chapter  focuses  on  the  second item above: how the many values in use in ns-3 are
       organized, documented, and modifiable by ns-3 users. The ns-3 attribute system is also the
       underpinning of how traces and statistics are gathered in the simulator.

       In the course of this chapter we will discuss the various ways to set or modify the values
       used by ns-3 model objects.  In increasing order of specificity, these are:

                 ┌─────────────────────────────────┬──────────────────────────────────┐
                 │Method                           │ Scope                            │
                 ├─────────────────────────────────┼──────────────────────────────────┤
                 │Default  Attribute  values   set │ Affect  all  instances  of   the │
                 │when  Attributes  are defined in │ class.                           │
                 │GetTypeId ().                    │                                  │
                 ├─────────────────────────────────┼──────────────────────────────────┤
                 │CommandLine                      │ Affect all future instances.     │
                 │Config::SetDefault()             │                                  │
                 │ConfigStore                      │                                  │
                 ├─────────────────────────────────┼──────────────────────────────────┤
                 │ObjectFactory                    │ Affects  all  instances  created │
                 │                                 │ with the factory.                │
                 ├─────────────────────────────────┼──────────────────────────────────┤
                 │Helper  methods  with   (string/ │ Affects all instances created by │
                 │AttributeValue) parameter pairs  │ the helper.                      │
                 ├─────────────────────────────────┼──────────────────────────────────┤
                 │MyClass::SetX ()                 │ Alters this particular instance. │
                 │Object::SetAttribute ()          │ Generally  this is the only form │
                 │Config::Set()                    │ which can be scheduled to  alter │
                 │                                 │ an  instance once the simulation │
                 │                                 │ is running.                      │
                 └─────────────────────────────────┴──────────────────────────────────┘

       By “specificity” we mean that methods in later rows in the table override the  values  set
       by, and typically affect fewer instances than, earlier methods.

       Before  delving  into  details  of the attribute value system, it will help to review some
       basic properties of class Object.

   Object Overview
       ns-3 is fundamentally a C++ object-based system. By this we  mean  that  new  C++  classes
       (types) can be declared, defined, and subclassed as usual.

       Many  ns-3 objects inherit from the Object base class.  These objects have some additional
       properties that we exploit for organizing the system and improving the  memory  management
       of our objects:

       · “Metadata”  system  that  links  the  class  name to a lot of meta-information about the
         object, including:

         · The base class of the subclass,

         · The set of accessible constructors in the subclass,

         · The set of “attributes” of the subclass,

         · Whether each attribute can be set, or is read-only,

         · The allowed range of values for each attribute.

       · Reference counting smart pointer implementation, for memory management.

       ns-3 objects that use the attribute system derive from either Object or  ObjectBase.  Most
       ns-3  objects  we  will  discuss  derive from Object, but a few that are outside the smart
       pointer memory management framework derive from ObjectBase.

       Let’s review a couple of properties of these objects.

   Smart Pointers
       As introduced in the ns-3 tutorial,  ns-3  objects  are  memory  managed  by  a  reference
       counting smart pointer implementation, class Ptr.

       Smart  pointers  are  used  extensively  in  the ns-3 APIs, to avoid passing references to
       heap-allocated objects that may cause memory leaks.  For most basic usage (syntax),  treat
       a smart pointer like a regular pointer:

          Ptr<WifiNetDevice> nd = ...;
          nd->CallSomeFunction ();
          // etc.

       So how do you get a smart pointer to an object, as in the first line of this example?

   CreateObject
       As we discussed above in Memory-management-and-class-Ptr, at the lowest-level API, objects
       of type Object are not instantiated using operator new as usual but instead by a templated
       function called CreateObject ().

       A typical way to create such an object is as follows:

          Ptr<WifiNetDevice> nd = CreateObject<WifiNetDevice> ();

       You can think of this as being functionally equivalent to:

          WifiNetDevice* nd = new WifiNetDevice ();

       Objects that derive from Object must be allocated on the heap using CreateObject (). Those
       deriving from ObjectBase, such as ns-3 helper functions and packet headers  and  trailers,
       can be allocated on the stack.

       In  some  scripts,  you  may  not  see a lot of CreateObject () calls in the code; this is
       because there are some helper objects in effect that are doing the CreateObject  ()  calls
       for you.

   TypeId
       ns-3 classes that derive from class Object can include a metadata class called TypeId that
       records meta-information about the class, for use in the object aggregation and  component
       manager systems:

       · A unique string identifying the class.

       · The base class of the subclass, within the metadata system.

       · The set of accessible constructors in the subclass.

       · A list of publicly accessible properties (“attributes”) of the class.

   Object Summary
       Putting all of these concepts together, let’s look at a specific example: class Node.

       The  public  header  file  node.h  has  a  declaration that includes a static GetTypeId ()
       function call:

          class Node : public Object
          {
          public:
            static TypeId GetTypeId (void);
            ...

       This is defined in the node.cc file as follows:

          TypeId
          Node::GetTypeId (void)
          {
            static TypeId tid = TypeId ("ns3::Node")
              .SetParent<Object> ()
              .SetGroupName ("Network")
              .AddConstructor<Node> ()
              .AddAttribute ("DeviceList",
                             "The list of devices associated to this Node.",
                             ObjectVectorValue (),
                             MakeObjectVectorAccessor (&Node::m_devices),
                             MakeObjectVectorChecker<NetDevice> ())
              .AddAttribute ("ApplicationList",
                             "The list of applications associated to this Node.",
                             ObjectVectorValue (),
                             MakeObjectVectorAccessor (&Node::m_applications),
                             MakeObjectVectorChecker<Application> ())
              .AddAttribute ("Id",
                             "The id (unique integer) of this Node.",
                             TypeId::ATTR_GET, // allow only getting it.
                             UintegerValue (0),
                             MakeUintegerAccessor (&Node::m_id),
                             MakeUintegerChecker<uint32_t> ())
              ;
            return tid;
          }

       Consider the TypeId of the ns-3 Object  class  as  an  extended  form  of  run  time  type
       information  (RTTI).  The  C++ language includes a simple kind of RTTI in order to support
       dynamic_cast and typeid operators.

       The SetParent<Object> () call in the definition above is  used  in  conjunction  with  our
       object  aggregation  mechanisms  to  allow  safe up- and down-casting in inheritance trees
       during GetObject ().  It also enables subclasses to inherit the Attributes of their parent
       class.

       The  AddConstructor<Node>  () call is used in conjunction with our abstract object factory
       mechanisms to allow us to construct C++  objects  without  forcing  a  user  to  know  the
       concrete class of the object she is building.

       The three calls to AddAttribute () associate a given string with a strongly typed value in
       the class. Notice that you must provide a help string which may be displayed, for example,
       via  command  line  processors. Each Attribute is associated with mechanisms for accessing
       the underlying member variable in the object (for example, MakeUintegerAccessor  ()  tells
       the  generic  Attribute  code  how  to get to the node ID above). There are also “Checker”
       methods which are used to validate values against range limitations, such as  maximum  and
       minimum allowed values.

       When users want to create Nodes, they will usually call some form of CreateObject (),:

          Ptr<Node> n = CreateObject<Node> ();

       or  more  abstractly,  using  an object factory, you can create a Node object without even
       knowing the concrete C++ type:

          ObjectFactory factory;
          const std::string typeId = "ns3::Node'';
          factory.SetTypeId (typeId);
          Ptr<Object> node = factory.Create <Object> ();

       Both of these methods result in  fully  initialized  attributes  being  available  in  the
       resulting Object instances.

       We  next  discuss  how attributes (values associated with member variables or functions of
       the class) are plumbed into the above TypeId.

   Attributes
       The goal of the attribute system is to organize the access of internal member objects of a
       simulation.  This  goal  arises  because,  typically  in  simulation,  users  will cut and
       paste/modify existing simulation scripts, or will use higher-level simulation  constructs,
       but  often  will  be interested in studying or tracing particular internal variables.  For
       instance, use cases such as:

       · “I want to trace the packets on the wireless interface only on the first access point.”

       · “I want to trace the value of the TCP congestion window (every time  it  changes)  on  a
         particular TCP socket.”

       · “I want a dump of all values that were used in my simulation.”

       Similarly,  users may want fine-grained access to internal variables in the simulation, or
       may want to broadly change the initial value  used  for  a  particular  parameter  in  all
       subsequently  created objects. Finally, users may wish to know what variables are settable
       and retrievable in a simulation configuration. This is  not  just  for  direct  simulation
       interaction  on  the  command line; consider also a (future) graphical user interface that
       would like to be able to provide a feature whereby a user might right-click on an node  on
       the  canvas  and see a hierarchical, organized list of parameters that are settable on the
       node and its constituent member objects,  and  help  text  and  default  values  for  each
       parameter.

   Defining Attributes
       We  provide  a  way for users to access values deep in the system, without having to plumb
       accessors (pointers) through the system and walk pointer chains to get to them. Consider a
       class  DropTailQueue  that has a member variable that is an unsigned integer m_maxPackets;
       this member variable controls the depth of the queue.

       If we look at the declaration of DropTailQueue, we see the following:

          class DropTailQueue : public Queue {
          public:
            static TypeId GetTypeId (void);
            ...

          private:
            std::queue<Ptr<Packet> > m_packets;
            uint32_t m_maxPackets;
          };

       Let’s consider things that a user may want to do with the value of m_maxPackets:

       · Set a default value for the system, such that whenever a new DropTailQueue  is  created,
         this member is initialized to that default.

       · Set or get the value on an already instantiated queue.

       The above things typically require providing Set () and Get () functions, and some type of
       global default value.

       In the ns-3 attribute system, these value definitions and accessor function  registrations
       are moved into the TypeId class; e.g.:

          NS_OBJECT_ENSURE_REGISTERED (DropTailQueue);

          TypeId
          DropTailQueue::GetTypeId (void)
          {
            static TypeId tid = TypeId ("ns3::DropTailQueue")
              .SetParent<Queue> ()
              .SetGroupName ("Network")
              .AddConstructor<DropTailQueue> ()
              .AddAttribute ("MaxPackets",
                             "The maximum number of packets accepted by this DropTailQueue.",
                             UintegerValue (100),
                             MakeUintegerAccessor (&DropTailQueue::m_maxPackets),
                             MakeUintegerChecker<uint32_t> ())
              ;

            return tid;
          }

       The AddAttribute () method is performing a number of things for the m_maxPackets value:

       · Binding   the  (usually  private)  member  variable  m_maxPackets  to  a  public  string
         "MaxPackets".

       · Providing a default value (100 packets).

       · Providing some help text defining the meaning of the value.

       · Providing a “Checker” (not used in this example) that can be used to set bounds  on  the
         allowable range of values.

       The  key point is that now the value of this variable and its default value are accessible
       in the attribute namespace, which is based on strings such as "MaxPackets" and TypeId name
       strings.  In  the next section, we will provide an example script that shows how users may
       manipulate these values.

       Note that initialization of the attribute relies on the macro  NS_OBJECT_ENSURE_REGISTERED
       (DropTailQueue) being called; if you leave this out of your new class implementation, your
       attributes will not be initialized correctly.

       While we have described how to create attributes, we still haven’t described how to access
       and  manage  these values. For instance, there is no globals.h header file where these are
       stored; attributes are stored with their classes.  Questions that naturally arise are  how
       do  users  easily  learn  about all of the attributes of their models, and how does a user
       access these attributes, or  document  their  values  as  part  of  the  record  of  their
       simulation?

       Detailed  documentation  of the actual attributes defined for a type, and a global list of
       all defined attributes, are available in the API documentation.   For  the  rest  of  this
       document  we  are  going  to demonstrate the various ways of getting and setting attribute
       values.

   Setting Default Values
   Config::SetDefault and CommandLine
       Let’s look at how a user script might access a specific attribute value.  We’re  going  to
       use  the src/point-to-point/examples/main-attribute-value.cc script for illustration, with
       some details stripped out.  The main function begins:

          // This is a basic example of how to use the attribute system to
          // set and get a value in the underlying system; namely, an unsigned
          // integer of the maximum number of packets in a queue
          //

          int
          main (int argc, char *argv[])
          {

            // By default, the MaxPackets attribute has a value of 100 packets
            // (this default can be observed in the function DropTailQueue::GetTypeId)
            //
            // Here, we set it to 80 packets.  We could use one of two value types:
            // a string-based value or a Uinteger value
            Config::SetDefault ("ns3::DropTailQueue::MaxPackets", StringValue ("80"));
            // The below function call is redundant
            Config::SetDefault ("ns3::DropTailQueue::MaxPackets", UintegerValue (80));

            // Allow the user to override any of the defaults and the above
            // SetDefaults () at run-time, via command-line arguments
            // For example, via "--ns3::DropTailQueue::MaxPackets=80"
            CommandLine cmd;
            // This provides yet another way to set the value from the command line:
            cmd.AddValue ("maxPackets", "ns3::DropTailQueue::MaxPackets");
            cmd.Parse (argc, argv);

       The main thing to notice in the above are the two equivalent calls  to  Config::SetDefault
       ().    This   is   how  we  set  the  default  value  for  all  subsequently  instantiated
       DropTailQueues.  We illustrate that two types  of  Value  classes,  a  StringValue  and  a
       UintegerValue  class,  can  be  used  to  assign  the  value  to  the  attribute  named by
       “ns3::DropTailQueue::MaxPackets”.

       It’s also possible to manipulate Attributes using the CommandLine; we  saw  some  examples
       early  in  the Tutorial.  In particular, it is straightforward to add a shorthand argument
       name, such as --maxPackets, for an Attribute that is particular relevant for  your  model,
       in  this  case "ns3::DropTailQueue::MaxPackets".  This has the additional feature that the
       help string for the Attribute will be printed as part of the usage message for the script.
       For more information see the CommandLine API documentation.

       Now,  we will create a few objects using the low-level API.  Our newly created queues will
       not   have   m_maxPackets   initialized   to   100   packets,   as    defined    in    the
       DropTailQueue::GetTypeId () function, but to 80 packets, because of what we did above with
       default values.:

          Ptr<Node> n0 = CreateObject<Node> ();

          Ptr<PointToPointNetDevice> net0 = CreateObject<PointToPointNetDevice> ();
          n0->AddDevice (net0);

          Ptr<Queue> q = CreateObject<DropTailQueue> ();
          net0->AddQueue(q);

       At this point, we have created a single  Node  (n0)  and  a  single  PointToPointNetDevice
       (net0), and added a DropTailQueue (q) to net0.

   Constructors, Helpers and ObjectFactory
       Arbitrary  combinations of attributes can be set and fetched from the helper and low-level
       APIs; either from the constructors themselves:

          Ptr<GridPositionAllocator> p =
            CreateObjectWithAttributes<GridPositionAllocator>
              ("MinX", DoubleValue (-100.0),
               "MinY", DoubleValue (-100.0),
               "DeltaX", DoubleValue (5.0),
               "DeltaY", DoubleValue (20.0),
               "GridWidth", UintegerValue (20),
               "LayoutType", StringValue ("RowFirst"));

       or from the higher-level helper APIs, such as:

          mobility.SetPositionAllocator
              ("ns3::GridPositionAllocator",
               "MinX", DoubleValue (-100.0),
               "MinY", DoubleValue (-100.0),
               "DeltaX", DoubleValue (5.0),
               "DeltaY", DoubleValue (20.0),
               "GridWidth", UintegerValue (20),
               "LayoutType", StringValue ("RowFirst"));

       We don’t illustrate it here, but you can also configure an ObjectFactory with  new  values
       for  specific  attributes.   Instances  created  by  the  ObjectFactory  will  have  those
       attributes set during construction.  This is very similar to using one of the helper  APIs
       for the class.

       To  review,  there are several ways to set values for attributes for class instances to be
       created in the future:

       · Config::SetDefault ()

       · CommandLine::AddValue ()

       · CreateObjectWithAttributes<> ()

       · Various helper APIs

       But what if you’ve already created an instance, and you want to change the  value  of  the
       attribute?   In  this example, how can we manipulate the m_maxPackets value of the already
       instantiated DropTailQueue?  Here are various ways to do that.

   Changing Values
   SmartPointer
       Assume that a smart pointer (Ptr) to a relevant network device is in hand; in the  current
       example, it is the net0 pointer.

       One  way to change the value is to access a pointer to the underlying queue and modify its
       attribute.

       First, we observe  that  we  can  get  a  pointer  to  the  (base  class)  Queue  via  the
       PointToPointNetDevice attributes, where it is called "TxQueue":

          PointerValue tmp;
          net0->GetAttribute ("TxQueue", tmp);
          Ptr<Object> txQueue = tmp.GetObject ();

       Using  the GetObject () function, we can perform a safe downcast to a DropTailQueue, where
       "MaxPackets" is an attribute:

          Ptr<DropTailQueue> dtq = txQueue->GetObject <DropTailQueue> ();
          NS_ASSERT (dtq != 0);

       Next, we can get the value of an attribute on this  queue.   We  have  introduced  wrapper
       Value  classes for the underlying data types, similar to Java wrappers around these types,
       since the attribute system stores values serialized to strings, and not  disparate  types.
       Here,  the  attribute  value is assigned to a UintegerValue, and the Get () method on this
       value produces the (unwrapped) uint32_t.:

          UintegerValue limit;
          dtq->GetAttribute ("MaxPackets", limit);
          NS_LOG_INFO ("1.  dtq limit: " << limit.Get () << " packets");

       Note that the above downcast is not really needed; we  could  have  gotten  the  attribute
       value directly from txQueue, which is an Object:

          txQueue->GetAttribute ("MaxPackets", limit);
          NS_LOG_INFO ("2.  txQueue limit: " << limit.Get () << " packets");

       Now, let’s set it to another value (60 packets):

          txQueue->SetAttribute("MaxPackets", UintegerValue (60));
          txQueue->GetAttribute ("MaxPackets", limit);
          NS_LOG_INFO ("3.  txQueue limit changed: " << limit.Get () << " packets");

   Config Namespace Path
       An  alternative  way to get at the attribute is to use the configuration namespace.  Here,
       this attribute resides on a known path in this namespace; this approach is useful  if  one
       doesn’t  have  access  to  the  underlying pointers and would like to configure a specific
       attribute with a single statement.:

          Config::Set ("/NodeList/0/DeviceList/0/TxQueue/MaxPackets",
                       UintegerValue (25));
          txQueue->GetAttribute ("MaxPackets", limit);
          NS_LOG_INFO ("4.  txQueue limit changed through namespace: "
                       << limit.Get () << " packets");

       The    configuration    path     often     has     the     form     of     ".../<container
       name>/<index>/.../<attribute>/<attribute>"  to refer to a specific instance by index of an
       object in the container.  In this case the first container is the list of all  Nodes;  the
       second  container  is  the  list  of  all  NetDevices  on  the  chosen Node.  Finally, the
       configuration path usually ends with a succession of member attributes, in this  case  the
       "MaxPackets" attribute of the "TxQueue" of the chosen NetDevice.

       We  could  have  also  used  wildcards to set this value for all nodes and all net devices
       (which in this simple example has the same effect as the previous Config::Set ()):

          Config::Set ("/NodeList/*/DeviceList/*/TxQueue/MaxPackets",
                       UintegerValue (15));
          txQueue->GetAttribute ("MaxPackets", limit);
          NS_LOG_INFO ("5.  txQueue limit changed through wildcarded namespace: "
                       << limit.Get () << " packets");

   Object Name Service
       Another way to get at the attribute is to use  the  object  name  service  facility.   The
       object  name  service  allows  us  to  add  items to the configuration namespace under the
       "/Names/" path with a user-defined name string.  This approach is useful  if  one  doesn’t
       have  access  to  the  underlying  pointers  and it is difficult to determine the required
       concrete configuration namespace path.

          Names::Add ("server", n0);
          Names::Add ("server/eth0", net0);

          ...

          Config::Set ("/Names/server/eth0/TxQueue/MaxPackets", UintegerValue (25));

       Here we’ve added the path elements "server" and "eth0" under the "/Names/" namespace, then
       used the resulting configuration path to set the attribute.

       See Object-names for a fuller treatment of the ns-3 configuration namespace.

   Implementation Details
   Value Classes
       Readers  will  note  the TypeValue classes which are subclasses of the AttributeValue base
       class. These can be thought of as intermediate classes which are used to convert from  raw
       types  to  the  AttributeValues  that  are used by the attribute system.  Recall that this
       database is holding objects of many types serialized to strings. Conversions to this  type
       can  either  be done using an intermediate class (such as IntegerValue, or DoubleValue for
       floating  point  numbers)  or  via  strings.  Direct  implicit  conversion  of  types   to
       AttributeValue  is  not  really  practical.  So in the above, users have a choice of using
       strings or values:

          p->Set ("cwnd", StringValue ("100")); // string-based setter
          p->Set ("cwnd", IntegerValue (100)); // integer-based setter

       The system provides some macros that help users  declare  and  define  new  AttributeValue
       subclasses for new types that they want to introduce into the attribute system:

       · ATTRIBUTE_HELPER_HEADER

       · ATTRIBUTE_HELPER_CPP

       See the API documentation for these constructs for more information.

   Initialization Order
       Attributes  in  the  system  must  not  depend on the state of any other Attribute in this
       system. This is because an ordering of Attribute  initialization  is  not  specified,  nor
       enforced, by the system. A specific example of this can be seen in automated configuration
       programs such as ConfigStore.  Although a given model may arrange it  so  that  Attributes
       are  initialized  in  a  particular  order,  another  automatic  configurator  may  decide
       independently to change Attributes in, for example, alphabetic order.

       Because of this non-specific ordering, no Attribute in the system may have any  dependence
       on any other Attribute. As a corollary, Attribute setters must never fail due to the state
       of another Attribute. No Attribute setter may change (set) any other Attribute value as  a
       result of changing its value.

       This  is  a  very  strong  restriction  and  there  are  cases  where  Attributes must set
       consistently to allow correct operation. To this end we do allow for consistency  checking
       when the attribute is used (cf. NS_ASSERT_MSG or NS_ABORT_MSG).

       In  general,  the attribute code to assign values to the underlying class member variables
       is executed after an object is constructed. But what  if  you  need  the  values  assigned
       before  the  constructor  body  executes,  because  you  need  them  in  the  logic of the
       constructor? There is a way to do this, used for example in the  class  ConfigStore:  call
       ObjectBase::ConstructSelf () as follows:

          ConfigStore::ConfigStore ()
          {
            ObjectBase::ConstructSelf (AttributeConstructionList ());
            // continue on with constructor.
          }

       Beware that the object and all its derived classes must also implement a GetInstanceTypeId
       () method. Otherwise the ObjectBase::ConstructSelf  ()  will  not  be  able  to  read  the
       attributes.

   Adding Attributes
       The  ns-3  system  will  place a number of internal values under the attribute system, but
       undoubtedly users will want to extend this to pick up ones we have missed, or to add their
       own classes to the system.

       There are three typical use cases:

       · Making an existing class data member accessible as an Attribute, when it isn’t already.

       · Making a new class able to expose some data members as Attributes by giving it a TypeId.

       · Creating  an  AttributeValue  subclass  for a new class so that it can be accessed as an
         Attribute.

   Existing Member Variable
       Consider this variable in TcpSocket:

          uint32_t m_cWnd;   // Congestion window

       Suppose that someone working with TCP wanted to get or set  the  value  of  that  variable
       using the metadata system. If it were not already provided by ns-3, the user could declare
       the following addition in the runtime metadata system (to the GetTypeId()  definition  for
       TcpSocket):

          .AddAttribute ("Congestion window",
                         "Tcp congestion window (bytes)",
                         UintegerValue (1),
                         MakeUintegerAccessor (&TcpSocket::m_cWnd),
                         MakeUintegerChecker<uint16_t> ())

       Now,  the  user  with  a  pointer  to  a TcpSocket instance can perform operations such as
       setting and  getting  the  value,  without  having  to  add  these  functions  explicitly.
       Furthermore, access controls can be applied, such as allowing the parameter to be read and
       not written, or bounds checking on the permissible values can be applied.

   New Class TypeId
       Here, we discuss the impact on a user who  wants  to  add  a  new  class  to  ns-3.   What
       additional things must be done to enable it to hold attributes?

       Let’s  assume  our new class, called ns3::MyMobility, is a type of mobility model.  First,
       the class should inherit from its parent class, ns3::MobilityModel.  In the  my-mobility.h
       header file:

          namespace ns3 {

          class MyMobility : public MobilityModel
          {

       This  requires  we  declare the GetTypeId () function.  This is a one-line public function
       declaration:

          public:
            /**
             *  Register this type.
             *  \return The object TypeId.
             */
            static TypeId GetTypeId (void);

       We’ve already introduced what a TypeId definition will look  like  in  the  my-mobility.cc
       implementation file:

          NS_OBJECT_ENSURE_REGISTERED (MyMobility);

          TypeId
          MyMobility::GetTypeId (void)
          {
            static TypeId tid = TypeId ("ns3::MyMobility")
              .SetParent<MobilityModel> ()
              .SetGroupName ("Mobility")
              .AddConstructor<MyMobility> ()
              .AddAttribute ("Bounds",
                             "Bounds of the area to cruise.",
                             RectangleValue (Rectangle (0.0, 0.0, 100.0, 100.0)),
                             MakeRectangleAccessor (&MyMobility::m_bounds),
                             MakeRectangleChecker ())
              .AddAttribute ("Time",
                             "Change current direction and speed after moving for this delay.",
                             TimeValue (Seconds (1.0)),
                             MakeTimeAccessor (&MyMobility::m_modeTime),
                             MakeTimeChecker ())
              // etc (more parameters).
              ;
            return tid;
          }

       If  we  don’t  want to subclass from an existing class, in the header file we just inherit
       from ns3::Object, and in the object file we set  the  parent  class  to  ns3::Object  with
       .SetParent<Object> ().

       Typical mistakes here involve:

       · Not calling NS_OBJECT_ENSURE_REGISTERED ()

       · Not calling the SetParent () method, or calling it with the wrong type.

       · Not calling the AddConstructor () method, or calling it with the wrong type.

       · Introducing a typographical error in the name of the TypeId in its constructor.

       · Not using the fully-qualified C++ typename of the enclosing C++ class as the name of the
         TypeId.  Note that "ns3::" is required.

       None of these mistakes can be detected by the ns-3 codebase, so users are advised to check
       carefully multiple times that they got these right.

   New AttributeValue Type
       From  the  perspective of the user who writes a new class in the system and wants it to be
       accessible as an attribute, there is mainly the matter of writing the conversions  to/from
       strings  and attribute values.  Most of this can be copy/pasted with macro-ized code.  For
       instance, consider a class declaration for Rectangle in the src/mobility/model directory:

   Header File
          /**
           * \brief a 2d rectangle
           */
          class Rectangle
          {
            ...

            double xMin;
            double xMax;
            double yMin;
            double yMax;
          };

       One macro call and two operators, must be added below the class declaration  in  order  to
       turn a Rectangle into a value usable by the Attribute system:

          std::ostream &operator << (std::ostream &os, const Rectangle &rectangle);
          std::istream &operator >> (std::istream &is, Rectangle &rectangle);

          ATTRIBUTE_HELPER_HEADER (Rectangle);

   Implementation File
       In the class definition (.cc file), the code looks like this:

          ATTRIBUTE_HELPER_CPP (Rectangle);

          std::ostream &
          operator << (std::ostream &os, const Rectangle &rectangle)
          {
            os << rectangle.xMin << "|" << rectangle.xMax << "|" << rectangle.yMin << "|"
               << rectangle.yMax;
            return os;
          }
          std::istream &
          operator >> (std::istream &is, Rectangle &rectangle)
           {
            char c1, c2, c3;
            is >> rectangle.xMin >> c1 >> rectangle.xMax >> c2 >> rectangle.yMin >> c3
               >> rectangle.yMax;
            if (c1 != '|' ||
                c2 != '|' ||
                c3 != '|')
              {
                is.setstate (std::ios_base::failbit);
              }
            return is;
          }

       These  stream  operators  simply  convert  from  a  string representation of the Rectangle
       ("xMin|xMax|yMin|yMax") to the underlying  Rectangle.   The  modeler  must  specify  these
       operators and the string syntactical representation of an instance of the new class.

   ConfigStore
       Values  for  ns-3  attributes can be stored in an ASCII or XML text file and loaded into a
       future simulation run.  This feature is known as the ns-3 ConfigStore.  The ConfigStore is
       a specialized database for attribute values and default values.

       Although  it  is  a  separately  maintained  module in the src/config-store/ directory, we
       document it here because of its sole dependency on ns-3 core module and attributes.

       We     can     explore     this     system     by     using      an      example      from
       src/config-store/examples/config-store-save.cc.

       First, all users of the ConfigStore must include the following statement:

          #include "ns3/config-store-module.h"

       Next, this program adds a sample object ConfigExample to show how the system is extended:

          class ConfigExample : public Object
          {
          public:
            static TypeId GetTypeId (void) {
              static TypeId tid = TypeId ("ns3::A")
                .SetParent<Object> ()
                .AddAttribute ("TestInt16", "help text",
                               IntegerValue (-2),
                               MakeIntegerAccessor (&A::m_int16),
                               MakeIntegerChecker<int16_t> ())
                ;
                return tid;
              }
            int16_t m_int16;
          };

          NS_OBJECT_ENSURE_REGISTERED (ConfigExample);

       Next, we use the Config subsystem to override the defaults in a couple of ways:

          Config::SetDefault ("ns3::ConfigExample::TestInt16", IntegerValue (-5));

          Ptr<ConfigExample> a_obj = CreateObject<ConfigExample> ();
          NS_ABORT_MSG_UNLESS (a_obj->m_int16 == -5,
                               "Cannot set ConfigExample's integer attribute via Config::SetDefault");

          Ptr<ConfigExample> a2_obj = CreateObject<ConfigExample> ();
          a2_obj->SetAttribute ("TestInt16", IntegerValue (-3));
          IntegerValue iv;
          a2_obj->GetAttribute ("TestInt16", iv);
          NS_ABORT_MSG_UNLESS (iv.Get () == -3,
                               "Cannot set ConfigExample's integer attribute via SetAttribute");

       The  next  statement is necessary to make sure that (one of) the objects created is rooted
       in the configuration namespace as an object instance.   This  normally  happens  when  you
       aggregate  objects to a ns3::Node or ns3::Channel instance, but here, since we are working
       at the core level, we need to create a new root namespace object:

          Config::RegisterRootNamespaceObject (a2_obj);

   Writing
       Next, we want to output the configuration store.  The examples show how to do  it  in  two
       formats,  XML and raw text.  In practice, one should perform this step just before calling
       Simulator::Run () to save the final configuration just before running the simulation.

       There  are  three  Attributes  that  govern  the  behavior  of  the  ConfigStore:  "Mode",
       "Filename",  and  "FileFormat".   The Mode (default "None") configures whether ns-3 should
       load configuration from a previously saved file (specify "Mode=Load") or save it to a file
       (specify  "Mode=Save").  The Filename (default "") is where the ConfigStore should read or
       write its data.  The FileFormat (default "RawText") governs whether the ConfigStore format
       is plain text or Xml ("FileFormat=Xml")

       The example shows:

          Config::SetDefault ("ns3::ConfigStore::Filename", StringValue ("output-attributes.xml"));
          Config::SetDefault ("ns3::ConfigStore::FileFormat", StringValue ("Xml"));
          Config::SetDefault ("ns3::ConfigStore::Mode", StringValue ("Save"));
          ConfigStore outputConfig;
          outputConfig.ConfigureDefaults ();
          outputConfig.ConfigureAttributes ();

          // Output config store to txt format
          Config::SetDefault ("ns3::ConfigStore::Filename", StringValue ("output-attributes.txt"));
          Config::SetDefault ("ns3::ConfigStore::FileFormat", StringValue ("RawText"));
          Config::SetDefault ("ns3::ConfigStore::Mode", StringValue ("Save"));
          ConfigStore outputConfig2;
          outputConfig2.ConfigureDefaults ();
          outputConfig2.ConfigureAttributes ();

          Simulator::Run ();

          Simulator::Destroy ();

       Note  the  placement  of  these  statements just prior to the Simulator::Run () statement.
       This output logs all of the values in place just prior to starting  the  simulation  (i.e.
       after all of the configuration has taken place).

       After running, you can open the output-attributes.txt file and see:

          default ns3::RealtimeSimulatorImpl::SynchronizationMode "BestEffort"
          default ns3::RealtimeSimulatorImpl::HardLimit "+100000000.0ns"
          default ns3::PcapFileWrapper::CaptureSize "65535"
          default ns3::PacketSocket::RcvBufSize "131072"
          default ns3::ErrorModel::IsEnabled "true"
          default ns3::RateErrorModel::ErrorUnit "EU_BYTE"
          default ns3::RateErrorModel::ErrorRate "0"
          default ns3::RateErrorModel::RanVar "Uniform:0:1"
          default ns3::DropTailQueue::Mode "Packets"
          default ns3::DropTailQueue::MaxPackets "100"
          default ns3::DropTailQueue::MaxBytes "6553500"
          default ns3::Application::StartTime "+0.0ns"
          default ns3::Application::StopTime "+0.0ns"
          default ns3::ConfigStore::Mode "Save"
          default ns3::ConfigStore::Filename "output-attributes.txt"
          default ns3::ConfigStore::FileFormat "RawText"
          default ns3::ConfigExample::TestInt16 "-5"
          global RngSeed "1"
          global RngRun "1"
          global SimulatorImplementationType "ns3::DefaultSimulatorImpl"
          global SchedulerType "ns3::MapScheduler"
          global ChecksumEnabled "false"
          value /$ns3::ConfigExample/TestInt16 "-3"

       In  the  above,  all  of  the default values for attributes for the core module are shown.
       Then, all the values for the ns-3 global values are recorded.  Finally, the value  of  the
       instance  of  ConfigExample that was rooted in the configuration namespace is shown.  In a
       real ns-3 program, many more models, attributes, and defaults would be shown.

       An XML version also exists in output-attributes.xml:

          <?xml version="1.0" encoding="UTF-8"?>
          <ns3>
           <default name="ns3::RealtimeSimulatorImpl::SynchronizationMode" value="BestEffort"/>
           <default name="ns3::RealtimeSimulatorImpl::HardLimit" value="+100000000.0ns"/>
           <default name="ns3::PcapFileWrapper::CaptureSize" value="65535"/>
           <default name="ns3::PacketSocket::RcvBufSize" value="131072"/>
           <default name="ns3::ErrorModel::IsEnabled" value="true"/>
           <default name="ns3::RateErrorModel::ErrorUnit" value="EU_BYTE"/>
           <default name="ns3::RateErrorModel::ErrorRate" value="0"/>
           <default name="ns3::RateErrorModel::RanVar" value="Uniform:0:1"/>
           <default name="ns3::DropTailQueue::Mode" value="Packets"/>
           <default name="ns3::DropTailQueue::MaxPackets" value="100"/>
           <default name="ns3::DropTailQueue::MaxBytes" value="6553500"/>
           <default name="ns3::Application::StartTime" value="+0.0ns"/>
           <default name="ns3::Application::StopTime" value="+0.0ns"/>
           <default name="ns3::ConfigStore::Mode" value="Save"/>
           <default name="ns3::ConfigStore::Filename" value="output-attributes.xml"/>
           <default name="ns3::ConfigStore::FileFormat" value="Xml"/>
           <default name="ns3::ConfigExample::TestInt16" value="-5"/>
           <global name="RngSeed" value="1"/>
           <global name="RngRun" value="1"/>
           <global name="SimulatorImplementationType" value="ns3::DefaultSimulatorImpl"/>
           <global name="SchedulerType" value="ns3::MapScheduler"/>
           <global name="ChecksumEnabled" value="false"/>
           <value path="/$ns3::ConfigExample/TestInt16" value="-3"/>
          </ns3>

       This file can be archived with your simulation script and output data.

   Reading
       Next, we discuss configuring simulations via a stored input configuration file.  There are
       a  couple  of  key  differences  compared  to  writing the final simulation configuration.
       First, we need to place statements such as these at the beginning of the  program,  before
       simulation configuration statements are written (so the values are registered before being
       used in object construction).

          Config::SetDefault ("ns3::ConfigStore::Filename", StringValue ("input-defaults.xml"));
          Config::SetDefault ("ns3::ConfigStore::Mode", StringValue ("Load"));
          Config::SetDefault ("ns3::ConfigStore::FileFormat", StringValue ("Xml"));
          ConfigStore inputConfig;
          inputConfig.ConfigureDefaults ();

       Next, note that loading of input configuration data is limited to Attribute default  (i.e.
       not  instance)  values,  and  global  values.  Attribute instance values are not supported
       because at this stage of the simulation, before any objects are constructed, there are  no
       such  object  instances around.  (Note, future enhancements to the config store may change
       this behavior).

       Second, while the output of ConfigStore state will list everything in  the  database,  the
       input  file  need  only  contain the specific values to be overridden.  So, one way to use
       this class for input file configuration is to generate an initial configuration using  the
       output  ("Save")  "Mode"  described  above,  extract from that configuration file only the
       elements one wishes to change, and move these minimal elements to a new configuration file
       which can then safely be edited and loaded in a subsequent simulation run.

       When  the  ConfigStore  object  is  instantiated,  its  attributes "Filename", "Mode", and
       "FileFormat" must be set, either via command-line or via program statements.

   Reading/Writing Example
       As a more complicated example, let’s assume that we want to read  in  a  configuration  of
       defaults  from  an  input  file  named  input-defaults.xml,  and  write  out the resulting
       attributes to a separate file called output-attributes.xml.:

          #include "ns3/config-store-module.h"
          ...
          int main (...)
          {

            Config::SetDefault ("ns3::ConfigStore::Filename", StringValue ("input-defaults.xml"));
            Config::SetDefault ("ns3::ConfigStore::Mode", StringValue ("Load"));
            Config::SetDefault ("ns3::ConfigStore::FileFormat", StringValue ("Xml"));
            ConfigStore inputConfig;
            inputConfig.ConfigureDefaults ();

            //
            // Allow the user to override any of the defaults and the above Bind () at
            // run-time, viacommand-line arguments
            //
            CommandLine cmd;
            cmd.Parse (argc, argv);

            // setup topology
            ...

            // Invoke just before entering Simulator::Run ()
            Config::SetDefault ("ns3::ConfigStore::Filename", StringValue ("output-attributes.xml"));
            Config::SetDefault ("ns3::ConfigStore::Mode", StringValue ("Save"));
            ConfigStore outputConfig;
            outputConfig.ConfigureAttributes ();
            Simulator::Run ();
          }

   ConfigStore GUI
       There is a GTK-based front end for the ConfigStore.  This allows users to  use  a  GUI  to
       access  and  change  variables.   Screenshots  of  this feature are available in the |ns3|
       Overview presentation.

       To  use  this  feature,  one  must  install  libgtk  and  libgtk-dev;  an  example  Ubuntu
       installation command is:

          $ sudo apt-get install libgtk2.0-0 libgtk2.0-dev

       To check whether it is configured or not, check the output of the step:

          $ ./waf configure --enable-examples --enable-tests

          ---- Summary of optional NS-3 features:
          Python Bindings               : enabled
          Python API Scanning Support   : enabled
          NS-3 Click Integration        : enabled
          GtkConfigStore                : not enabled (library 'gtk+-2.0 >= 2.12' not found)

       In the above example, it was not enabled, so it cannot be used until a suitable version is
       installed and:

          $ ./waf configure --enable-examples --enable-tests
          $ ./waf

       is rerun.

       Usage is almost the same as the  non-GTK-based  version,  but  there  are  no  ConfigStore
       attributes involved:

          // Invoke just before entering Simulator::Run ()
          GtkConfigStore config;
          config.ConfigureDefaults ();
          config.ConfigureAttributes ();

       Now,  when  you  run  the  script,  a  GUI  should  pop  up, allowing you to open menus of
       attributes on different nodes/objects, and then launch the simulation execution  when  you
       are done.

   Future work
       There are a couple of possible improvements:

       · Save a unique version number with date and time at start of file.

       · Save rng initial seed somewhere.

       · Make each RandomVariable serialize its own initial seed and re-read it later.

   Object names
       Placeholder chapter

   Logging
       The  ns-3  logging  facility  can  be  used to monitor or debug the progress of simulation
       programs.  Logging output can be enabled by program statements in your main()  program  or
       by the use of the NS_LOG environment variable.

       Logging  statements  are  not compiled into optimized builds of ns-3.  To use logging, one
       must build the (default) debug build of ns-3.

       The project makes no guarantee about whether logging output  will  remain  the  same  over
       time.  Users are cautioned against building simulation output frameworks on top of logging
       code, as the output and the way the output is enabled may change over time.

   Overview
       ns-3 logging statements are typically used to log various program execution  events,  such
       as the occurrence of simulation events or the use of a particular function.

       For example, this code snippet is from Ipv4L3Protocol::IsDestinationAddress():

          if (address == iaddr.GetBroadcast ())
            {
              NS_LOG_LOGIC ("For me (interface broadcast address)");
              return true;
             }

       If  logging  has  been  enabled for the Ipv4L3Protocol component at a severity of LOGIC or
       above (see below about log severity), the statement will be  printed  out;  otherwise,  it
       will be suppressed.

   Enabling Output
       There  are  two ways that users typically control log output.  The first is by setting the
       NS_LOG environment variable; e.g.:

          $ NS_LOG="*" ./waf --run first

       will run the first tutorial program with all logging output.  (The specifics of the NS_LOG
       format will be discussed below.)

       This can be made more granular by selecting individual components:

          $ NS_LOG="Ipv4L3Protocol" ./waf --run first

       The output can be further tailored with prefix options.

       The second way to enable logging is to use explicit statements in your program, such as in
       the first tutorial program:

          int
          main (int argc, char *argv[])
          {
            LogComponentEnable ("UdpEchoClientApplication", LOG_LEVEL_INFO);
            LogComponentEnable ("UdpEchoServerApplication", LOG_LEVEL_INFO);
            ...

       (The meaning of LOG_LEVEL_INFO, and other possible values, will be discussed below.)

   NS_LOG Syntax
       The NS_LOG environment variable contains a  list  of  log  components  and  options.   Log
       components are separated by `:’ characters:

          $ NS_LOG="<log-component>:<log-component>..."

       Options for each log component are given as flags after each log component:

          $ NS_LOG="<log-component>=<option>|<option>...:<log-component>..."

       Options  control  the  severity  and  level  for  that  component,  and  whether  optional
       information should be included, such as the simulation  time,  simulation  node,  function
       name, and the symbolic severity.

   Log Components
       Generally  a  log  component  refers to a single source code .cc file, and encompasses the
       entire file.

       Some helpers have special methods to enable the logging of all  components  in  a  module,
       spanning  different  compilation  units,  but logically grouped together, such as the ns-3
       wifi code:

          WifiHelper wifiHelper;
          wifiHelper.EnableLogComponents ();

       The NS_LOG log component wildcard `*’ will enable all components.

       To see what log components are defined, any of these will work:

          $ NS_LOG="print-list" ./waf --run ...

          $ NS_LOG="foo"  # a token not matching any log-component

       The first form will print the name and enabled flags for  all  log  components  which  are
       linked  in;  try  it  with  scratch-simulator.   The second form prints all registered log
       components, then exit with an error.

   Severity and Level Options
       Individual messages belong to a single “severity class,” set by  the  macro  creating  the
       message.   In  the  example  above,  NS_LOG_LOGIC(..) creates the message in the LOG_LOGIC
       severity class.

       The following severity classes are defined as enum constants:

                          ┌───────────────┬──────────────────────────────────┐
                          │Severity Class │ Meaning                          │
                          ├───────────────┼──────────────────────────────────┤
                          │LOG_NONE       │ The default, no logging          │
                          ├───────────────┼──────────────────────────────────┤
                          │LOG_ERROR      │ Serious error messages only      │
                          ├───────────────┼──────────────────────────────────┤
                          │LOG_WARN       │ Warning messages                 │
                          ├───────────────┼──────────────────────────────────┤
                          │LOG_DEBUG      │ For use in debugging             │
                          ├───────────────┼──────────────────────────────────┤
                          │LOG_INFO       │ Informational                    │
                          ├───────────────┼──────────────────────────────────┤
                          │LOG_FUNCTION   │ Function tracing                 │
                          ├───────────────┼──────────────────────────────────┤
                          │LOG_LOGIC      │ Control  flow   tracing   within │
                          │               │ functions                        │
                          └───────────────┴──────────────────────────────────┘

       Typically one wants to see messages at a given severity class and higher.  This is done by
       defining inclusive logging “levels”:

                        ┌───────────────────┬──────────────────────────────────┐
                        │Level              │ Meaning                          │
                        ├───────────────────┼──────────────────────────────────┤
                        │LOG_LEVEL_ERROR    │ Only  LOG_ERROR  severity  class │
                        │                   │ messages.                        │
                        ├───────────────────┼──────────────────────────────────┤
                        │LOG_LEVEL_WARNLOG_WARN and above.              │
                        ├───────────────────┼──────────────────────────────────┤
                        │LOG_LEVEL_DEBUGLOG_DEBUG and above.             │
                        ├───────────────────┼──────────────────────────────────┤
                        │LOG_LEVEL_INFOLOG_INFO and above.              │
                        ├───────────────────┼──────────────────────────────────┤
                        │LOG_LEVEL_FUNCTIONLOG_FUNCTION and above.          │
                        ├───────────────────┼──────────────────────────────────┤
                        │LOG_LEVEL_LOGICLOG_LOGIC and above.             │
                        ├───────────────────┼──────────────────────────────────┤
                        │LOG_LEVEL_ALL      │ All severity classes.            │
                        ├───────────────────┼──────────────────────────────────┤
                        │LOG_ALL            │ Synonym for LOG_LEVEL_ALL        │
                        └───────────────────┴──────────────────────────────────┘

       The  severity  class  and level options can be given in the NS_LOG environment variable by
       these tokens:

                                      ┌─────────┬────────────────┐
                                      │Class    │ Level          │
                                      ├─────────┼────────────────┤
                                      │errorlevel_error    │
                                      ├─────────┼────────────────┤
                                      │warnlevel_warn     │
                                      ├─────────┼────────────────┤
                                      │debuglevel_debug    │
                                      └─────────┴────────────────┘

                                      │infolevel_info     │
                                      ├─────────┼────────────────┤
                                      │functionlevel_function │
                                      ├─────────┼────────────────┤
                                      │logiclevel_logic    │
                                      ├─────────┼────────────────┤
                                      │         │ level_all      │
                                      │         │ all            │
                                      │         │ *              │
                                      └─────────┴────────────────┘

       Using a severity class token enables log messages at that  severity  only.   For  example,
       NS_LOG="*=warn"  won’t  output  messages with severity error.  NS_LOG="*=level_debug" will
       output messages at severity levels debug and above.

       Severity   classes   and   levels   can   be   combined    with    the    `|’    operator:
       NS_LOG="*=level_warn|logic" will output messages at severity levels error, warn and logic.

       The NS_LOG severity level wildcard `*’ and all are synonyms for level_all.

       For log components merely mentioned in NS_LOG

          $ NS_LOG="<log-component>:..."

       the default severity is LOG_LEVEL_ALL.

   Prefix Options
       A  number  of  prefixes can help identify where and when a message originated, and at what
       severity.

       The available prefix options (as enum constants) are

                         ┌─────────────────┬──────────────────────────────────┐
                         │Prefix Symbol    │ Meaning                          │
                         ├─────────────────┼──────────────────────────────────┤
                         │LOG_PREFIX_FUNC  │ Prefix the name of  the  calling │
                         │                 │ function.                        │
                         ├─────────────────┼──────────────────────────────────┤
                         │LOG_PREFIX_TIME  │ Prefix the simulation time.      │
                         ├─────────────────┼──────────────────────────────────┤
                         │LOG_PREFIX_NODE  │ Prefix the node id.              │
                         ├─────────────────┼──────────────────────────────────┤
                         │LOG_PREFIX_LEVEL │ Prefix the severity level.       │
                         ├─────────────────┼──────────────────────────────────┤
                         │LOG_PREFIX_ALL   │ Enable all prefixes.             │
                         └─────────────────┴──────────────────────────────────┘

       The prefix options are described briefly below.

       The options can be given in the NS_LOG environment variable by these tokens:

                                       ┌─────────────┬───────────┐
                                       │Token        │ Alternate │
                                       ├─────────────┼───────────┤
                                       │prefix_funcfunc      │
                                       ├─────────────┼───────────┤
                                       │prefix_timetime      │
                                       ├─────────────┼───────────┤
                                       │prefix_nodenode      │
                                       ├─────────────┼───────────┤
                                       │prefix_levellevel     │
                                       ├─────────────┼───────────┤
                                       │prefix_allall       │
                                       │             │ *         │
                                       └─────────────┴───────────┘

       For log components merely mentioned in NS_LOG

          $ NS_LOG="<log-component>:..."

       the default prefix options are LOG_PREFIX_ALL.

   Severity Prefix
       The  severity  class  of a message can be included with the options prefix_level or level.
       For example, this value of NS_LOG enables logging for all log  components  (`*’)  and  all
       severity classes (=all), and prefixes the message with the severity class (|prefix_level).

          $ NS_LOG="*=all|prefix_level" ./waf --run scratch-simulator
          Scratch Simulator
          [ERROR] error message
          [WARN] warn message
          [DEBUG] debug message
          [INFO] info message
          [FUNCT] function message
          [LOGIC] logic message

   Time Prefix
       The simulation time can be included with the options prefix_time or time.  This prints the
       simulation time in seconds.

   Node Prefix
       The simulation node id can be included with the options prefix_node or node.

   Function Prefix
       The name of the calling function can be included with the options prefix_func or func.

   NS_LOG Wildcards
       The log component wildcard `*’ will enable all components.  To enable all components at  a
       specific severity level use *=<severity>.

       The  severity  level option wildcard `*’ is a synonym for all.  This must occur before any
       `|’ characters separating options.  To enable all severity classes, use <log-component>=*,
       or <log-component>=*|<options>.

       The  option  wildcard  `*’ or token all enables all prefix options, but must occur after a
       `|’ character.  To enable a specific severity  class  or  level,  and  all  prefixes,  use
       <log-component>=<severity>|*.

       The  combined  option  wildcard  **  enables all severities and all prefixes; for example,
       <log-component>=**.

       The uber-wildcard *** enables all severities and all  prefixes  for  all  log  components.
       These are all equivalent:

          $ NS_LOG="***" ...      $ NS_LOG="*=all|*" ...        $ NS_LOG="*=*|all" ...
          $ NS_LOG="*=**" ...     $ NS_LOG="*=level_all|*" ...  $ NS_LOG="*=*|prefix_all" ...
          $ NS_LOG="*=*|*" ...

       Be  advised:   even  the  trivial scratch-simulator produces over 46K lines of output with
       NS_LOG="***"!

   How to add logging to your code
       Adding logging to your code is very simple:

       1. Invoke the NS_LOG_COMPONENT_DEFINE (...); macro inside of namespace ns3.
          Create a unique string identifier (usually based on the name of the file  and/or  class
          defined within the file) and register it with a macro call such as follows:

              namespace ns3 {

              NS_LOG_COMPONENT_DEFINE ("Ipv4L3Protocol");
              ...

          This registers Ipv4L3Protocol as a log component.

          (The  macro  was  carefully  written  to  permit  inclusion either within or outside of
          namespace ns3, and usage will vary across the codebase, but the original intent was  to
          register this outside of namespace ns3 at file global scope.)

       2. Add logging statements (macro calls) to your functions and function bodies.

       In  case  you  want to add logging statements to the methods of your template class (which
       are defined in an header file):

       1. Invoke the  NS_LOG_TEMPLATE_DECLARE;  macro  in  the  private  section  of  your  class
          declaration. For instance:

              template <typename Item>
              class Queue : public QueueBase
              {
              ...
              private:
                std::list<Ptr<Item> > m_packets;          //!< the items in the queue
                NS_LOG_TEMPLATE_DECLARE;                  //!< the log component
              };

          This requires you to perform these steps for all the subclasses of your class.

       2. Invoke  the  NS_LOG_TEMPLATE_DEFINE  (...);  macro  in the constructor of your class by
          providing the name of a log component registered by calling the NS_LOG_COMPONENT_DEFINE
          (...); macro in some module. For instance:

              template <typename Item>
              Queue<Item>::Queue ()
                : NS_LOG_TEMPLATE_DEFINE ("Queue")
              {
              }

       3. Add logging statements (macro calls) to the methods of your class.

       In  case  you want to add logging statements to a static member template (which is defined
       in an header file):

       1. Invoke  the  NS_LOG_STATIC_TEMPLATE_DEFINE  (...);  macro  in  your  static  method  by
          providing the name of a log component registered by calling the NS_LOG_COMPONENT_DEFINE
          (...); macro in some module. For instance:

              template <typename Item>
              void
              NetDeviceQueue::PacketEnqueued (Ptr<Queue<Item> > queue,
                                              Ptr<NetDeviceQueueInterface> ndqi,
                                              uint8_t txq, Ptr<const Item> item)
              {

                NS_LOG_STATIC_TEMPLATE_DEFINE ("NetDeviceQueueInterface");
              ...

       2. Add logging statements (macro calls) to your static method.

   Controlling timestamp precision
       Timestamps are printed out in units of seconds.  When used  with  the  default  ns-3  time
       resolution of nanoseconds, the default timestamp precision is 9 digits, with fixed format,
       to allow for 9 digits to be consistently printed  to  the  right  of  the  decimal  point.
       Example:

          +0.000123456s RandomVariableStream:SetAntithetic(0x805040, 0)

       When  the ns-3 simulation uses higher time resolution such as picoseconds or femtoseconds,
       the precision is expanded accordingly; e.g. for picosecond:

          +0.000123456789s RandomVariableStream:SetAntithetic(0x805040, 0)

       When the ns-3 simulation uses a time resolution lower than microseconds, the  default  C++
       precision is used.

       An  example  program  at  src\core\examples\sample-log-time-format.cc  demonstrates how to
       change the timestamp formatting.

       The maximum useful precision is 20 decimal digits, since Time is signed 64 bits.

   Logging Macros
          The logging macros and associated severity levels are

                                 ┌───────────────┬────────────────────────┐
                                 │Severity Class │ Macro                  │
                                 ├───────────────┼────────────────────────┤
                                 │LOG_NONE       │ (none needed)          │
                                 ├───────────────┼────────────────────────┤
                                 │LOG_ERRORNS_LOG_ERROR (...);    │
                                 ├───────────────┼────────────────────────┤
                                 │LOG_WARNNS_LOG_WARN (...);     │
                                 ├───────────────┼────────────────────────┤
                                 │LOG_DEBUGNS_LOG_DEBUG (...);    │
                                 ├───────────────┼────────────────────────┤
                                 │LOG_INFONS_LOG_INFO (...);     │
                                 ├───────────────┼────────────────────────┤
                                 │LOG_FUNCTIONNS_LOG_FUNCTION (...); │
                                 ├───────────────┼────────────────────────┤
                                 │LOG_LOGICNS_LOG_LOGIC (...);    │
                                 └───────────────┴────────────────────────┘

          The macros function as output streamers, so anything you can send to std::cout,  joined
          by << operators, is allowed:

              void MyClass::Check (int value, char * item)
              {
                NS_LOG_FUNCTION (this << arg << item);
                if (arg > 10)
                  {
                    NS_LOG_ERROR ("encountered bad value " << value <<
                                  " while checking " << name << "!");
                  }
                ...
              }

          Note  that  NS_LOG_FUNCTION automatically inserts a `,’ (comma-space) separator between
          each of its arguments.  This simplifies logging of function arguments; just concatenate
          them with << as in the example above.

   Unconditional Logging
       As  a  convenience,  the NS_LOG_UNCOND (...); macro will always log its arguments, even if
       the associated log-component is not enabled at any severity.  This macro does not use  any
       of  the  prefix  options.   Note  that logging is only enabled in debug builds; this macro
       won’t produce output in optimized builds.

   Guidelines
       · Start every class method with NS_LOG_FUNCTION  (this  <<  args...);  This  enables  easy
         function call tracing.

         · Except:   don’t  log  operators  or explicit copy constructors, since these will cause
           infinite recursion and stack overflow.

         · For methods without arguments use the same form: NS_LOG_FUNCTION (this);

         · For static functions:

           · With arguments use NS_LOG_FUNCTION (...); as normal.

           · Without arguments use NS_LOG_FUNCTION_NOARGS ();

       · Use NS_LOG_ERROR for serious error conditions that probably  invalidate  the  simulation
         execution.

       · Use  NS_LOG_WARN for unusual conditions that may be correctable.  Please give some hints
         as to the nature of the problem and how it might be corrected.

       · NS_LOG_DEBUG is usually used in an ad hoc way to understand the execution of a model.

       · Use NS_LOG_INFO for additional information about the execution, such as the  size  of  a
         data structure when adding/removing from it.

       · Use NS_LOG_LOGIC to trace important logic branches within a function.

       · Test  that  your  logging changes do not break the code.  Run some example programs with
         all log components turned on (e.g.  NS_LOG="***").

       · Use an explicit cast for any variable of type  uint8_t  or  int8_t,  e.g.,  NS_LOG_LOGIC
         ("Variable  i  is  "  <<  static_cast<int>  (i));.   Without  the  cast,  the integer is
         interpreted as a char, and the  result  will  be  most  likely  not  in  line  with  the
         expectations.  This is a well documented C++ ‘feature’.

   Tracing
       The  tracing  subsystem  is one of the most important mechanisms to understand in ns-3. In
       most cases, ns-3 users will have a brilliant idea for some  new  and  improved  networking
       feature.  In  order to verify that this idea works, the researcher will make changes to an
       existing system and then run experiments to see how the new feature behaves  by  gathering
       statistics that capture the behavior of the feature.

       In  other words, the whole point of running a simulation is to generate output for further
       study. In ns-3, the subsystem that  enables  a  researcher  to  do  this  is  the  tracing
       subsystem.

   Tracing Motivation
       There  are  many ways to get information out of a program. The most straightforward way is
       to just directly print the information to the standard output, as in,

          #include <iostream>
          ...
          int main ()
          {
            ...
            std::cout << "The value of x is " << x << std::endl;
            ...
          }

       This is workable in small  environments,  but  as  your  simulations  get  more  and  more
       complicated,  you  end up with more and more prints and the task of parsing and performing
       computations on the output begins to get harder and harder.

       Another thing to consider is that every time a new tidbit is  needed,  the  software  core
       must  be  edited and another print introduced. There is no standardized way to control all
       of this output, so the amount of output tends to  grow  without  bounds.  Eventually,  the
       bandwidth required for simply outputting this information begins to limit the running time
       of the simulation. The output files grow to enormous sizes  and  parsing  them  becomes  a
       problem.

       ns-3  provides  a  simple mechanism for logging and providing some control over output via
       Log Components, but the level of control is not very fine  grained  at  all.  The  logging
       module is a relatively blunt instrument.

       It  is desirable to have a facility that allows one to reach into the core system and only
       get the information required without having to change and recompile the core system.  Even
       better  would  be  a  system that notified the user when an item of interest changed or an
       interesting event happened.

       The ns-3 tracing system is designed to work along those lines and is well-integrated  with
       the Attribute and Config substems allowing for relatively simple use scenarios.

   Overview
       The  tracing  subsystem  relies heavily on the ns-3 Callback and Attribute mechanisms. You
       should read and understand the corresponding sections of the manual before  attempting  to
       understand the tracing system.

       The  ns-3  tracing  system  is  built  on  the concepts of independent tracing sources and
       tracing sinks; along with a uniform mechanism for connecting sources to sinks.

       Trace sources are entities that can signal events that happen in a simulation and  provide
       access  to  interesting underlying data. For example, a trace source could indicate when a
       packet is received by a  net  device  and  provide  access  to  the  packet  contents  for
       interested  trace  sinks.  A  trace  source  might also indicate when an interesting state
       change happens in a model. For example, the congestion window of a TCP model  is  a  prime
       candidate for a trace source.

       Trace sources are not useful by themselves; they must be connected to other pieces of code
       that actually do something useful with  the  information  provided  by  the  source.   The
       entities  that  consume  trace  information  are  called  trace  sinks.  Trace sources are
       generators of events and trace sinks are consumers.

       This explicit division allows for large numbers of trace sources to  be  scattered  around
       the system in places which model authors believe might be useful. Unless a user connects a
       trace sink to one of these sources, nothing is output. This arrangement allows  relatively
       unsophisticated  users  to  attach new types of sinks to existing tracing sources, without
       requiring editing and recompiling the core or models of the simulator.

       There can be zero or more consumers of trace events generated by a trace source.  One  can
       think of a trace source as a kind of point-to-multipoint information link.

       The “transport protocol” for this conceptual point-to-multipoint link is an ns-3 Callback.

       Recall  from  the Callback Section that callback facility is a way to allow two modules in
       the system to communicate via function calls while at the same time decoupling the calling
       function  from the called class completely. This is the same requirement as outlined above
       for the tracing system.

       Basically, a trace source is a callback to which multiple  functions  may  be  registered.
       When  a  trace  sink expresses interest in receiving trace events, it adds a callback to a
       list of callbacks held by the trace source. When an interesting event happens,  the  trace
       source  invokes its operator() providing zero or more parameters. This tells the source to
       go through its list of callbacks invoking each one in turn. In this way, the  parameter(s)
       are communicated to the trace sinks, which are just functions.

   The Simplest Example
       It will be useful to go walk a quick example just to reinforce what we’ve said.:

          #include "ns3/object.h"
          #include "ns3/uinteger.h"
          #include "ns3/traced-value.h""
          #include "ns3/trace-source-accessor.h"

          #include <iostream>

          using namespace ns3;

       The  first thing to do is include the required files. As mentioned above, the trace system
       makes heavy use of the Object and Attribute systems. The first two includes bring  in  the
       declarations   for  those  systems.  The  file,  traced-value.h  brings  in  the  required
       declarations for tracing data that obeys value semantics.

       In general, value semantics just means that  you  can  pass  the  object  around,  not  an
       address.  In  order  to  use  value  semantics  at  all you have to have an object with an
       associated copy constructor and assignment operator available. We extend the  requirements
       to  talk  about  the set of operators that are pre-defined for plain-old-data (POD) types.
       Operator=, operator++, operator–, operator+, operator==, etc.

       What this all means is that you will be able to trace changes  to  an  object  made  using
       those operators.:

          class MyObject : public Object
          {
          public:
            static TypeId GetTypeId (void)
            {
              static TypeId tid = TypeId ("MyObject")
                .SetParent (Object::GetTypeId ())
                .AddConstructor<MyObject> ()
                .AddTraceSource ("MyInteger",
                                 "An integer value to trace.",
                                 MakeTraceSourceAccessor (&MyObject::m_myInt))
                ;
              return tid;
            }

            MyObject () {}
            TracedValue<uint32_t> m_myInt;
          };

       Since  the tracing system is integrated with Attributes, and Attributes work with Objects,
       there must be an ns-3 Object for the trace source to live in. The two important  lines  of
       code are the .AddTraceSource and the TracedValue declaration.

       The  .AddTraceSource  provides  the  “hooks”  used  for connecting the trace source to the
       outside world. The TracedValue declaration provides the infrastructure that overloads  the
       operators mentioned above and drives the callback process.:

          void
          IntTrace (Int oldValue, Int newValue)
          {
            std::cout << "Traced " << oldValue << " to " << newValue << std::endl;
          }

       This  is the definition of the trace sink. It corresponds directly to a callback function.
       This function will be  called  whenever  one  of  the  operators  of  the  TracedValue  is
       executed.:

          int
          main (int argc, char *argv[])
          {
            Ptr<MyObject> myObject = CreateObject<MyObject> ();

            myObject->TraceConnectWithoutContext ("MyInteger", MakeCallback(&IntTrace));

            myObject->m_myInt = 1234;
          }

       In  this  snippet,  the first thing that needs to be done is to create the object in which
       the trace source lives.

       The next step, the TraceConnectWithoutContext, forms  the  connection  between  the  trace
       source  and  the  trace  sink.  Notice the MakeCallback template function. Recall from the
       Callback section that this creates the specialized functor responsible for  providing  the
       overloaded  operator() used to “fire” the callback. The overloaded operators (++, –, etc.)
       will use this operator() to actually invoke the callback. The  TraceConnectWithoutContext,
       takes  a  string  parameter  that provides the name of the Attribute assigned to the trace
       source. Let’s ignore the bit about context for now since it is not important yet.

       Finally, the line,:

          myObject->m_myInt = 1234;

       should be interpreted as an invocation of operator= on the member  variable  m_myInt  with
       the  integer  1234  passed  as a parameter. It turns out that this operator is defined (by
       TracedValue) to execute a callback that returns void  and  takes  two  integer  values  as
       parameters – an old value and a new value for the integer in question. That is exactly the
       function signature for the callback function we provided – IntTrace.

       To summarize, a trace source is, in essence, a variable that holds a list of callbacks.  A
       trace  sink  is a function used as the target of a callback. The Attribute and object type
       information systems are used to provide a way to connect trace sources to trace sinks. The
       act  of  “hitting” a trace source is executing an operator on the trace source which fires
       callbacks. This results in the trace sink callbacks registering  interest  in  the  source
       being called with the parameters provided by the source.

   Using the Config Subsystem to Connect to Trace Sources
       The  TraceConnectWithoutContext  call  shown  above in the simple example is actually very
       rarely used in the system. More typically, the Config subsystem is used to allow selecting
       a trace source in the system using what is called a config path.

       For  example,  one might find something that looks like the following in the system (taken
       from examples/tcp-large-transfer.cc):

          void CwndTracer (uint32_t oldval, uint32_t newval) {}

          ...

          Config::ConnectWithoutContext (
            "/NodeList/0/$ns3::TcpL4Protocol/SocketList/0/CongestionWindow",
            MakeCallback (&CwndTracer));

       This should look very familiar. It is the same thing as the previous example, except  that
       a  static  member  function of class Config is being called instead of a method on Object;
       and instead of an Attribute name, a path is being provided.

       The first thing to do is to read the path backward. The last segment of the path  must  be
       an  Attribute  of  an  Object.  In  fact,  if you had a pointer to the Object that has the
       “CongestionWindow” Attribute handy (call it theObject), you could write this just like the
       previous example:

          void CwndTracer (uint32_t oldval, uint32_t newval) {}

          ...

          theObject->TraceConnectWithoutContext ("CongestionWindow", MakeCallback (&CwndTracer));

       It  turns  out  that  the  code  for Config::ConnectWithoutContext does exactly that. This
       function takes a path that represents a chain of Object pointers and follows them until it
       gets  to  the  end of the path and interprets the last segment as an Attribute on the last
       object. Let’s walk through what happens.

       The leading “/” character in the  path  refers  to  a  so-called  namespace.  One  of  the
       predefined  namespaces  in  the  config system is “NodeList” which is a list of all of the
       nodes in the simulation. Items in the list are referred to by indices into  the  list,  so
       “/NodeList/0”  refers  to  the zeroth node in the list of nodes created by the simulation.
       This node is actually a Ptr<Node> and so is a subclass of an ns3::Object.

       As described in the Object-model section, ns-3 supports an object aggregation  model.  The
       next path segment begins with the “$” character which indicates a GetObject call should be
       made  looking  for  the  type  that  follows.   When  a  node   is   initialized   by   an
       InternetStackHelper a number of interfaces are aggregated to the node. One of these is the
       TCP level four protocol. The runtime type of this protocol object is ns3::TcpL4Protocol''.
       When the ``GetObject is executed, it returns a pointer to the object of this type.

       The  TcpL4Protocol  class  defines  an  Attribute  called  “SocketList” which is a list of
       sockets.  Each socket is actually an ns3::Object with its own Attributes.   The  items  in
       the  list  of  sockets are referred to by index just as in the NodeList, so “SocketList/0”
       refers to the zeroth socket in the list of sockets on the zeroth node in  the  NodeList  –
       the first node constructed in the simulation.

       This  socket, the type of which turns out to be an ns3::TcpSocketImpl defines an attribute
       called     “CongestionWindow”     which     is     a      TracedValue<uint32_t>.       The
       Config::ConnectWithoutContext now does a,:

          object->TraceConnectWithoutContext ("CongestionWindow", MakeCallback (&CwndTracer));

       using  the object pointer from “SocketList/0” which makes the connection between the trace
       source defined in the socket to the callback – CwndTracer.

       Now, whenever a change is made to the TracedValue<uint32_t>  representing  the  congestion
       window  in  the  TCP  socket,  the  registered  callback will be executed and the function
       CwndTracer will be called printing out the old  and  new  values  of  the  TCP  congestion
       window.

   Using the Tracing API
       There are three levels of interaction with the tracing system:

       · Beginning user can easily control which objects are participating in tracing;

       · Intermediate  users  can extend the tracing system to modify the output format generated
         or use existing trace sources in different ways,  without  modifying  the  core  of  the
         simulator;

       · Advanced users can modify the simulator core to add new tracing sources and sinks.

   Using Trace Helpers
       The  ns-3 trace helpers provide a rich environment for configuring and selecting different
       trace events and  writing  them  to  files.  In  previous  sections,  primarily  “Building
       Topologies,”  we  have seen several varieties of the trace helper methods designed for use
       inside other (device) helpers.

       Perhaps you will recall seeing some of these variations:

          pointToPoint.EnablePcapAll ("second");
          pointToPoint.EnablePcap ("second", p2pNodes.Get (0)->GetId (), 0);
          csma.EnablePcap ("third", csmaDevices.Get (0), true);
          pointToPoint.EnableAsciiAll (ascii.CreateFileStream ("myfirst.tr"));

       What may not be obvious, though, is that there is  a  consistent  model  for  all  of  the
       trace-related  methods found in the system. We will now take a little time and take a look
       at the “big picture”.

       There are currently two primary use cases of the tracing helpers in ns-3:  Device  helpers
       and protocol helpers. Device helpers look at the problem of specifying which traces should
       be enabled through a node, device pair.  For example, you may want to  specify  that  pcap
       tracing should be enabled on a particular device on a specific node. This follows from the
       ns-3 device conceptual model, and  also  the  conceptual  models  of  the  various  device
       helpers.     Following    naturally    from    this,    the   files   created   follow   a
       <prefix>-<node>-<device> naming convention.

       Protocol helpers look at the problem of specifying which traces should be enabled  through
       a protocol and interface pair. This follows from the ns-3 protocol stack conceptual model,
       and also the conceptual models of internet  stack  helpers.  Naturally,  the  trace  files
       should follow a <prefix>-<protocol>-<interface> naming convention.

       The  trace  helpers  therefore  fall naturally into a two-dimensional taxonomy.  There are
       subtleties that prevent all four classes from behaving identically, but we  do  strive  to
       make  them  all work as similarly as possible; and whenever possible there are analogs for
       all methods in all classes.

                                     ┌────────────────┬──────┬───────┐
                                     │                │ pcap │ ascii │
                                     ├────────────────┼──────┼───────┤
                                     │Device Helper   │      │       │
                                     ├────────────────┼──────┼───────┤
                                     │Protocol Helper │      │       │
                                     └────────────────┴──────┴───────┘

       We use an approach called a mixin to add tracing functionality to our  helper  classes.  A
       mixin  is  a  class  that  provides  functionality  to  that  is  inherited by a subclass.
       Inheriting from a mixin is not considered a form of specialization but is really a way  to
       collect functionality.

       Let’s take a quick look at all four of these cases and their respective mixins.

   Pcap Tracing Device Helpers
       The goal of these helpers is to make it easy to add a consistent pcap trace facility to an
       ns-3 device. We want all of the various flavors of pcap tracing to work  the  same  across
       all  devices, so the methods of these helpers are inherited by device helpers. Take a look
       at src/network/helper/trace-helper.h if you want to follow the discussion while looking at
       real code.

       The  class  PcapHelperForDevice is a mixin provides the high level functionality for using
       pcap tracing in an ns-3 device. Every  device  must  implement  a  single  virtual  method
       inherited from this class.:

          virtual void EnablePcapInternal (std::string prefix, Ptr<NetDevice> nd, bool promiscuous) = 0;

       The  signature  of  this  method reflects the device-centric view of the situation at this
       level. All of the public methods inherited from class  PcapUserHelperForDevice  reduce  to
       calling  this single device-dependent implementation method. For example, the lowest level
       pcap method,:

          void EnablePcap (std::string prefix, Ptr<NetDevice> nd, bool promiscuous = false, bool explicitFilename = false);

       will call the device implementation of EnablePcapInternal directly. All other public  pcap
       tracing   methods   build   on   this  implementation  to  provide  additional  user-level
       functionality. What this means to the user is that all device helpers in the  system  will
       have  all of the pcap trace methods available; and these methods will all work in the same
       way across devices if the device implements EnablePcapInternal correctly.

   Pcap Tracing Device Helper Methods
          void EnablePcap (std::string prefix, Ptr<NetDevice> nd,
                           bool promiscuous = false, bool explicitFilename = false);
          void EnablePcap (std::string prefix, std::string ndName,
                           bool promiscuous = false, bool explicitFilename = false);
          void EnablePcap (std::string prefix, NetDeviceContainer d,
                           bool promiscuous = false);
          void EnablePcap (std::string prefix, NodeContainer n,
                           bool promiscuous = false);
          void EnablePcap (std::string prefix, uint32_t nodeid, uint32_t deviceid,
                           bool promiscuous = false);
          void EnablePcapAll (std::string prefix, bool promiscuous = false);

       In each of the methods shown above, there is a default parameter called  promiscuous  that
       defaults  to  false.  This  parameter  indicates  that the trace should not be gathered in
       promiscuous mode. If you do want your traces to include all traffic  seen  by  the  device
       (and  if the device supports a promiscuous mode) simply add a true parameter to any of the
       calls above. For example,:

          Ptr<NetDevice> nd;
          ...
          helper.EnablePcap ("prefix", nd, true);

       will enable promiscuous mode captures on the NetDevice specified by nd.

       The first two methods also include a default parameter called explicitFilename  that  will
       be discussed below.

       You are encouraged to peruse the Doxygen for class PcapHelperForDevice to find the details
       of these methods; but to summarize …

       You can  enable  pcap  tracing  on  a  particular  node/net-device  pair  by  providing  a
       Ptr<NetDevice>  to  an  EnablePcap  method. The Ptr<Node> is implicit since the net device
       must belong to exactly one Node. For example,:

          Ptr<NetDevice> nd;
          ...
          helper.EnablePcap ("prefix", nd);

       You can  enable  pcap  tracing  on  a  particular  node/net-device  pair  by  providing  a
       std::string  representing  an  object  name  service  string to an EnablePcap method.  The
       Ptr<NetDevice> is looked up from the name string.  Again, the <Node> is implicit since the
       named net device must belong to exactly one Node.  For example,:

          Names::Add ("server" ...);
          Names::Add ("server/eth0" ...);
          ...
          helper.EnablePcap ("prefix", "server/ath0");

       You  can  enable  pcap  tracing  on  a  collection of node/net-device pairs by providing a
       NetDeviceContainer. For each NetDevice in the container the type  is  checked.   For  each
       device  of  the proper type (the same type as is managed by the device helper), tracing is
       enabled. Again, the <Node> is implicit since the found net device must belong  to  exactly
       one Node. For example,:

          NetDeviceContainer d = ...;
          ...
          helper.EnablePcap ("prefix", d);

       You  can  enable  pcap  tracing  on  a  collection of node/net-device pairs by providing a
       NodeContainer. For each Node in the NodeContainer its attached  NetDevices  are  iterated.
       For  each  NetDevice  attached  to  each node in the container, the type of that device is
       checked.  For each device of the proper type (the same type as is managed  by  the  device
       helper), tracing is enabled.:

          NodeContainer n;
          ...
          helper.EnablePcap ("prefix", n);

       You can enable pcap tracing on the basis of node ID and device ID as well as with explicit
       Ptr. Each Node in the system has an integer node ID and each device connected  to  a  node
       has an integer device ID.:

          helper.EnablePcap ("prefix", 21, 1);

       Finally,  you can enable pcap tracing for all devices in the system, with the same type as
       that managed by the device helper.:

          helper.EnablePcapAll ("prefix");

   Pcap Tracing Device Helper Filename Selection
       Implicit in the method descriptions above is the construction of a  complete  filename  by
       the  implementation  method. By convention, pcap traces in the ns-3 system are of the form
       <prefix>-<node id>-<device id>.pcap

       As previously mentioned, every node in the system will have a system-assigned node id; and
       every  device will have an interface index (also called a device id) relative to its node.
       By default, then, a pcap trace file created as a result of enabling tracing on  the  first
       device of node 21 using the prefix “prefix” would be prefix-21-1.pcap.

       You  can always use the ns-3 object name service to make this more clear.  For example, if
       you use the object name service to assign the name “server” to node 21, the resulting pcap
       trace file name will automatically become, prefix-server-1.pcap and if you also assign the
       name “eth0” to the device, your pcap file name will automatically  pick  this  up  and  be
       called prefix-server-eth0.pcap.

       Finally, two of the methods shown above,:

          void EnablePcap (std::string prefix, Ptr<NetDevice> nd, bool promiscuous = false, bool explicitFilename = false);
          void EnablePcap (std::string prefix, std::string ndName, bool promiscuous = false, bool explicitFilename = false);

       have  a  default  parameter  called  explicitFilename.  When  set  to true, this parameter
       disables the automatic filename completion mechanism and allows you to create an  explicit
       filename.  This  option  is  only  available in the methods which enable pcap tracing on a
       single device.

       For example, in order to arrange for a device helper to create a single  promiscuous  pcap
       capture file of a specific name (my-pcap-file.pcap) on a given device, one could:

          Ptr<NetDevice> nd;
          ...
          helper.EnablePcap ("my-pcap-file.pcap", nd, true, true);

       The  first  true parameter enables promiscuous mode traces and the second tells the helper
       to interpret the prefix parameter as a complete filename.

   Ascii Tracing Device Helpers
       The behavior of the ASCII trace helper mixin is substantially similar to the pcap version.
       Take  a  look  at  src/network/helper/trace-helper.h  if you want to follow the discussion
       while looking at real code.

       The class AsciiTraceHelperForDevice adds the high  level  functionality  for  using  ASCII
       tracing  to  a  device  helper  class.  As in the pcap case, every device must implement a
       single virtual method inherited from the ASCII trace mixin.:

          virtual void EnableAsciiInternal (Ptr<OutputStreamWrapper> stream, std::string prefix, Ptr<NetDevice> nd) = 0;

       The signature of this method reflects the device-centric view of  the  situation  at  this
       level;  and also the fact that the helper may be writing to a shared output stream. All of
       the public ASCII-trace-related  methods  inherited  from  class  AsciiTraceHelperForDevice
       reduce  to  calling  this single device- dependent implementation method. For example, the
       lowest level ASCII trace methods,:

          void EnableAscii (std::string prefix, Ptr<NetDevice> nd);
          void EnableAscii (Ptr<OutputStreamWrapper> stream, Ptr<NetDevice> nd);

       will call the device implementation of EnableAsciiInternal directly,  providing  either  a
       valid  prefix  or  stream.   All  other  public  ASCII tracing methods will build on these
       low-level functions to provide additional user-level functionality. What this means to the
       user  is  that  all  device helpers in the system will have all of the ASCII trace methods
       available; and these methods will all work in the same way across devices if  the  devices
       implement EnablAsciiInternal correctly.

   Ascii Tracing Device Helper Methods
          void EnableAscii (std::string prefix, Ptr<NetDevice> nd);
          void EnableAscii (Ptr<OutputStreamWrapper> stream, Ptr<NetDevice> nd);

          void EnableAscii (std::string prefix, std::string ndName);
          void EnableAscii (Ptr<OutputStreamWrapper> stream, std::string ndName);

          void EnableAscii (std::string prefix, NetDeviceContainer d);
          void EnableAscii (Ptr<OutputStreamWrapper> stream, NetDeviceContainer d);

          void EnableAscii (std::string prefix, NodeContainer n);
          void EnableAscii (Ptr<OutputStreamWrapper> stream, NodeContainer n);

          void EnableAscii (std::string prefix, uint32_t nodeid, uint32_t deviceid);
          void EnableAscii (Ptr<OutputStreamWrapper> stream, uint32_t nodeid, uint32_t deviceid);

          void EnableAsciiAll (std::string prefix);
          void EnableAsciiAll (Ptr<OutputStreamWrapper> stream);

       You  are  encouraged  to  peruse  the  Doxygen  for class TraceHelperForDevice to find the
       details of these methods; but to summarize …

       There are twice as many methods available  for  ASCII  tracing  as  there  were  for  pcap
       tracing.  This  is  because,  in  addition  to the pcap-style model where traces from each
       unique node/device pair are written to a unique file, we support a model  in  which  trace
       information  for  many node/device pairs is written to a common file.  This means that the
       <prefix>-<node>-<device> file name generation mechanism is  replaced  by  a  mechanism  to
       refer  to  a  common  file;  and  the  number  of  API  methods  is  doubled  to allow all
       combinations.

       Just as in pcap tracing, you can enable ASCII tracing on a particular node/net-device pair
       by  providing  a  Ptr<NetDevice> to an EnableAscii method. The Ptr<Node> is implicit since
       the net device must belong to exactly one Node. For example,:

          Ptr<NetDevice> nd;
          ...
          helper.EnableAscii ("prefix", nd);

       In this case, no trace contexts are written to the ASCII trace file since  they  would  be
       redundant.  The  system  will  pick  the  file  name to be created using the same rules as
       described in the pcap section, except that the file will have the suffix “.tr” instead  of
       “.pcap”.

       If  you  want to enable ASCII tracing on more than one net device and have all traces sent
       to a single file, you can do that as well by using an object to refer to a single file:

          Ptr<NetDevice> nd1;
          Ptr<NetDevice> nd2;
          ...
          Ptr<OutputStreamWrapper> stream = asciiTraceHelper.CreateFileStream ("trace-file-name.tr");
          ...
          helper.EnableAscii (stream, nd1);
          helper.EnableAscii (stream, nd2);

       In this case, trace contexts are written to the ASCII trace file since they  are  required
       to  disambiguate  traces  from  the  two  devices.  Note that since the user is completely
       specifying the file name, the string should include the “.tr” for consistency.

       You can enable  ASCII  tracing  on  a  particular  node/net-device  pair  by  providing  a
       std::string  representing  an  object  name  service  string to an EnablePcap method.  The
       Ptr<NetDevice> is looked up from the name string.  Again, the <Node> is implicit since the
       named net device must belong to exactly one Node.  For example,:

          Names::Add ("client" ...);
          Names::Add ("client/eth0" ...);
          Names::Add ("server" ...);
          Names::Add ("server/eth0" ...);
          ...
          helper.EnableAscii ("prefix", "client/eth0");
          helper.EnableAscii ("prefix", "server/eth0");

       This  would result in two files named prefix-client-eth0.tr and prefix-server-eth0.tr with
       traces for each device in  the  respective  trace  file.  Since  all  of  the  EnableAscii
       functions are overloaded to take a stream wrapper, you can use that form as well:

          Names::Add ("client" ...);
          Names::Add ("client/eth0" ...);
          Names::Add ("server" ...);
          Names::Add ("server/eth0" ...);
          ...
          Ptr<OutputStreamWrapper> stream = asciiTraceHelper.CreateFileStream ("trace-file-name.tr");
          ...
          helper.EnableAscii (stream, "client/eth0");
          helper.EnableAscii (stream, "server/eth0");

       This  would  result  in a single trace file called trace-file-name.tr that contains all of
       the trace events for both devices. The events would  be  disambiguated  by  trace  context
       strings.

       You  can  enable  ASCII  tracing  on  a collection of node/net-device pairs by providing a
       NetDeviceContainer. For each NetDevice in the container the  type  is  checked.  For  each
       device  of  the proper type (the same type as is managed by the device helper), tracing is
       enabled. Again, the <Node> is implicit since the found net device must belong  to  exactly
       one Node.  For example,:

          NetDeviceContainer d = ...;
          ...
          helper.EnableAscii ("prefix", d);

       This  would  result  in a number of ASCII trace files being created, each of which follows
       the <prefix>-<node id>-<device id>.tr convention. Combining  all  of  the  traces  into  a
       single file is accomplished similarly to the examples above:

          NetDeviceContainer d = ...;
          ...
          Ptr<OutputStreamWrapper> stream = asciiTraceHelper.CreateFileStream ("trace-file-name.tr");
          ...
          helper.EnableAscii (stream, d);

       You  can  enable  ascii  tracing  on  a collection of node/net-device pairs by providing a
       NodeContainer. For each Node in the NodeContainer its attached  NetDevices  are  iterated.
       For  each  NetDevice  attached  to  each node in the container, the type of that device is
       checked.  For each device of the proper type (the same type as is managed  by  the  device
       helper), tracing is enabled.:

          NodeContainer n;
          ...
          helper.EnableAscii ("prefix", n);

       This  would  result  in a number of ASCII trace files being created, each of which follows
       the <prefix>-<node id>-<device id>.tr convention. Combining  all  of  the  traces  into  a
       single file is accomplished similarly to the examples above:

       You can enable pcap tracing on the basis of node ID and device ID as well as with explicit
       Ptr. Each Node in the system has an integer node ID and each device connected  to  a  node
       has an integer device ID.:

          helper.EnableAscii ("prefix", 21, 1);

       Of course, the traces can be combined into a single file as shown above.

       Finally,  you can enable pcap tracing for all devices in the system, with the same type as
       that managed by the device helper.:

          helper.EnableAsciiAll ("prefix");

       This would result in a number of ASCII trace files being created, one for every device  in
       the  system  of  the  type  managed  by  the  helper.  All  of these files will follow the
       <prefix>-<node id>-<device id>.tr convention. Combining all of the traces  into  a  single
       file is accomplished similarly to the examples above.

   Ascii Tracing Device Helper Filename Selection
       Implicit in the prefix-style method descriptions above is the construction of the complete
       filenames by the implementation method. By convention, ASCII traces in the ns-3 system are
       of the form <prefix>-<node id>-<device id>.tr.

       As previously mentioned, every node in the system will have a system-assigned node id; and
       every device will have an interface index (also called a device id) relative to its  node.
       By default, then, an ASCII trace file created as a result of enabling tracing on the first
       device of node 21, using the prefix “prefix”, would be prefix-21-1.tr.

       You can always use the ns-3 object name service to make this more clear.  For example,  if
       you  use  the  object  name  service to assign the name “server” to node 21, the resulting
       ASCII trace file name will automatically become, prefix-server-1.tr and if you also assign
       the  name “eth0” to the device, your ASCII trace file name will automatically pick this up
       and be called prefix-server-eth0.tr.

   Pcap Tracing Protocol Helpers
       The goal of these mixins is to make it easy to add a consistent  pcap  trace  facility  to
       protocols.  We want all of the various flavors of pcap tracing to work the same across all
       protocols, so the methods of these helpers are inherited by stack helpers. Take a look  at
       src/network/helper/trace-helper.h  if  you  want to follow the discussion while looking at
       real code.

       In this section we will be illustrating the methods as applied to the protocol  Ipv4.   To
       specify  traces  in similar protocols, just substitute the appropriate type.  For example,
       use a Ptr<Ipv6> instead of a Ptr<Ipv4> and call EnablePcapIpv6 instead of EnablePcapIpv4.

       The class PcapHelperForIpv4 provides the high level functionality for using  pcap  tracing
       in the Ipv4 protocol.  Each protocol helper enabling these methods must implement a single
       virtual method inherited from this class.  There will be  a  separate  implementation  for
       Ipv6,  for  example,  but  the only difference will be in the method names and signatures.
       Different method names are required to disambiguate class Ipv4 from Ipv6  which  are  both
       derived from class Object, and methods that share the same signature.:

          virtual void EnablePcapIpv4Internal (std::string prefix, Ptr<Ipv4> ipv4, uint32_t interface) = 0;

       The  signature  of  this  method  reflects  the protocol and interface-centric view of the
       situation at this level. All of the public methods inherited from class  PcapHelperForIpv4
       reduce  to  calling  this single device-dependent implementation method.  For example, the
       lowest level pcap method,:

          void EnablePcapIpv4 (std::string prefix, Ptr<Ipv4> ipv4, uint32_t interface);

       will call the device implementation of EnablePcapIpv4Internal directly.  All other  public
       pcap  tracing  methods  build  on  this  implementation  to  provide additional user-level
       functionality. What this means to the user is that all protocol helpers in the system will
       have  all of the pcap trace methods available; and these methods will all work in the same
       way across protocols if the helper implements EnablePcapIpv4Internal correctly.

   Pcap Tracing Protocol Helper Methods
       These methods are  designed  to  be  in  one-to-one  correspondence  with  the  Node-  and
       NetDevice-  centric  versions  of  the device versions. Instead of Node and NetDevice pair
       constraints, we use protocol and interface constraints.

       Note that just like in the device version, there are six methods:

          void EnablePcapIpv4 (std::string prefix, Ptr<Ipv4> ipv4, uint32_t interface);
          void EnablePcapIpv4 (std::string prefix, std::string ipv4Name, uint32_t interface);
          void EnablePcapIpv4 (std::string prefix, Ipv4InterfaceContainer c);
          void EnablePcapIpv4 (std::string prefix, NodeContainer n);
          void EnablePcapIpv4 (std::string prefix, uint32_t nodeid, uint32_t interface);
          void EnablePcapIpv4All (std::string prefix);

       You are encouraged to peruse the Doxygen for class PcapHelperForIpv4 to find  the  details
       of these methods; but to summarize …

       You  can  enable  pcap  tracing  on  a  particular  protocol/interface pair by providing a
       Ptr<Ipv4> and interface to an EnablePcap method.  For example,:

          Ptr<Ipv4> ipv4 = node->GetObject<Ipv4> ();
          ...
          helper.EnablePcapIpv4 ("prefix", ipv4, 0);

       You can  enable  pcap  tracing  on  a  particular  node/net-device  pair  by  providing  a
       std::string  representing  an  object  name  service  string to an EnablePcap method.  The
       Ptr<Ipv4> is looked up from the name string.  For example,:

          Names::Add ("serverIPv4" ...);
          ...
          helper.EnablePcapIpv4 ("prefix", "serverIpv4", 1);

       You can enable pcap tracing on a collection of protocol/interface pairs  by  providing  an
       Ipv4InterfaceContainer.  For each Ipv4 / interface pair in the container the protocol type
       is checked. For each protocol of the proper type (the same  type  as  is  managed  by  the
       device helper), tracing is enabled for the corresponding interface.  For example,:

          NodeContainer nodes;
          ...
          NetDeviceContainer devices = deviceHelper.Install (nodes);
          ...
          Ipv4AddressHelper ipv4;
          ipv4.SetBase ("10.1.1.0", "255.255.255.0");
          Ipv4InterfaceContainer interfaces = ipv4.Assign (devices);
          ...
          helper.EnablePcapIpv4 ("prefix", interfaces);

       You  can  enable  pcap  tracing on a collection of protocol/interface pairs by providing a
       NodeContainer. For each Node in the NodeContainer the appropriate protocol is  found.  For
       each  protocol,  its  interfaces  are  enumerated  and tracing is enabled on the resulting
       pairs. For example,:

          NodeContainer n;
          ...
          helper.EnablePcapIpv4 ("prefix", n);

       You can enable pcap tracing on the basis of node ID and interface as well. In  this  case,
       the  node-id is translated to a Ptr<Node> and the appropriate protocol is looked up in the
       node. The resulting protocol and  interface  are  used  to  specify  the  resulting  trace
       source.:

          helper.EnablePcapIpv4 ("prefix", 21, 1);

       Finally,  you  can  enable  pcap tracing for all interfaces in the system, with associated
       protocol being the same type as that managed by the device helper.:

          helper.EnablePcapIpv4All ("prefix");

   Pcap Tracing Protocol Helper Filename Selection
       Implicit in all of the method descriptions above  is  the  construction  of  the  complete
       filenames  by  the  implementation method. By convention, pcap traces taken for devices in
       the ns-3 system are of the form  <prefix>-<node  id>-<device  id>.pcap.  In  the  case  of
       protocol traces, there is a one-to-one correspondence between protocols and Nodes. This is
       because protocol Objects are aggregated to Node Objects. Since there is no global protocol
       id  in  the system, we use the corresponding node id in file naming.  Therefore there is a
       possibility for file name collisions in automatically chosen trace file  names.  For  this
       reason, the file name convention is changed for protocol traces.

       As  previously  mentioned,  every  node in the system will have a system-assigned node id.
       Since there is a one-to-one correspondence between protocol instances and  node  instances
       we  use  the  node id. Each interface has an interface id relative to its protocol. We use
       the convention  “<prefix>-n<node  id>-i<interface  id>.pcap”  for  trace  file  naming  in
       protocol helpers.

       Therefore,  by  default,  a  pcap  trace  file  created as a result of enabling tracing on
       interface 1 of  the  Ipv4  protocol  of  node  21  using  the  prefix  “prefix”  would  be
       “prefix-n21-i1.pcap”.

       You  can always use the ns-3 object name service to make this more clear.  For example, if
       you use the object name service to assign the name “serverIpv4” to the Ptr<Ipv4>  on  node
       21,    the    resulting    pcap    trace    file    name    will   automatically   become,
       “prefix-nserverIpv4-i1.pcap”.

   Ascii Tracing Protocol Helpers
       The behavior of the ASCII trace helpers is substantially similar to the pcap case.  Take a
       look  at  src/network/helper/trace-helper.h  if  you  want  to follow the discussion while
       looking at real code.

       In this section we will be illustrating the methods as applied to the  protocol  Ipv4.  To
       specify  traces  in  similar protocols, just substitute the appropriate type. For example,
       use  a  Ptr<Ipv6>  instead  of  a  Ptr<Ipv4>   and   call   EnableAsciiIpv6   instead   of
       EnableAsciiIpv4.

       The  class  AsciiTraceHelperForIpv4  adds  the  high  level  functionality for using ASCII
       tracing to a protocol helper. Each protocol that enables these methods  must  implement  a
       single virtual method inherited from this class.:

          virtual void EnableAsciiIpv4Internal (Ptr<OutputStreamWrapper> stream, std::string prefix,
                                                Ptr<Ipv4> ipv4, uint32_t interface) = 0;

       The  signature  of  this  method  reflects the protocol- and interface-centric view of the
       situation at this level; and also the fact that the helper may  be  writing  to  a  shared
       output     stream.     All    of    the    public    methods    inherited    from    class
       PcapAndAsciiTraceHelperForIpv4  reduce  to   calling   this   single   device-   dependent
       implementation method. For example, the lowest level ascii trace methods,:

          void EnableAsciiIpv4 (std::string prefix, Ptr<Ipv4> ipv4, uint32_t interface);
          void EnableAsciiIpv4 (Ptr<OutputStreamWrapper> stream, Ptr<Ipv4> ipv4, uint32_t interface);

       will  call the device implementation of EnableAsciiIpv4Internal directly, providing either
       the prefix or the stream. All other public ascii  tracing  methods  will  build  on  these
       low-level functions to provide additional user-level functionality. What this means to the
       user is that all device helpers in the system will have all of  the  ascii  trace  methods
       available;  and  these  methods  will  all  work  in  the same way across protocols if the
       protocols implement EnablAsciiIpv4Internal correctly.

   Ascii Tracing Device Helper Methods
          void EnableAsciiIpv4 (std::string prefix, Ptr<Ipv4> ipv4, uint32_t interface);
          void EnableAsciiIpv4 (Ptr<OutputStreamWrapper> stream, Ptr<Ipv4> ipv4, uint32_t interface);

          void EnableAsciiIpv4 (std::string prefix, std::string ipv4Name, uint32_t interface);
          void EnableAsciiIpv4 (Ptr<OutputStreamWrapper> stream, std::string ipv4Name, uint32_t interface);

          void EnableAsciiIpv4 (std::string prefix, Ipv4InterfaceContainer c);
          void EnableAsciiIpv4 (Ptr<OutputStreamWrapper> stream, Ipv4InterfaceContainer c);

          void EnableAsciiIpv4 (std::string prefix, NodeContainer n);
          void EnableAsciiIpv4 (Ptr<OutputStreamWrapper> stream, NodeContainer n);

          void EnableAsciiIpv4 (std::string prefix, uint32_t nodeid, uint32_t deviceid);
          void EnableAsciiIpv4 (Ptr<OutputStreamWrapper> stream, uint32_t nodeid, uint32_t interface);

          void EnableAsciiIpv4All (std::string prefix);
          void EnableAsciiIpv4All (Ptr<OutputStreamWrapper> stream);

       You are encouraged to peruse the Doxygen for class PcapAndAsciiHelperForIpv4 to  find  the
       details of these methods; but to summarize …

       There  are  twice  as  many  methods  available  for  ASCII tracing as there were for pcap
       tracing. This is because, in addition to the  pcap-style  model  where  traces  from  each
       unique  protocol/interface  pair are written to a unique file, we support a model in which
       trace information for many protocol/interface pairs is written  to  a  common  file.  This
       means  that the <prefix>-n<node id>-<interface> file name generation mechanism is replaced
       by a mechanism to refer to a common file; and the number of  API  methods  is  doubled  to
       allow all combinations.

       Just  as  in pcap tracing, you can enable ASCII tracing on a particular protocol/interface
       pair by providing a Ptr<Ipv4> and an interface to an EnableAscii method.  For example,:

          Ptr<Ipv4> ipv4;
          ...
          helper.EnableAsciiIpv4 ("prefix", ipv4, 1);

       In this case, no trace contexts are written to the ASCII trace file since  they  would  be
       redundant.  The  system  will  pick  the  file  name to be created using the same rules as
       described in the pcap section, except that the file will have the suffix “.tr” instead  of
       “.pcap”.

       If you want to enable ASCII tracing on more than one interface and have all traces sent to
       a single file, you can do that as well by using an object to refer to a  single  file.  We
       have already something similar to this in the “cwnd” example above:

          Ptr<Ipv4> protocol1 = node1->GetObject<Ipv4> ();
          Ptr<Ipv4> protocol2 = node2->GetObject<Ipv4> ();
          ...
          Ptr<OutputStreamWrapper> stream = asciiTraceHelper.CreateFileStream ("trace-file-name.tr");
          ...
          helper.EnableAsciiIpv4 (stream, protocol1, 1);
          helper.EnableAsciiIpv4 (stream, protocol2, 1);

       In  this  case, trace contexts are written to the ASCII trace file since they are required
       to disambiguate traces from the two interfaces. Note that since  the  user  is  completely
       specifying the file name, the string should include the “.tr” for consistency.

       You  can  enable  ASCII  tracing  on  a  particular  protocol  by  providing a std::string
       representing an object name service string to an  EnablePcap  method.   The  Ptr<Ipv4>  is
       looked  up  from the name string.  The <Node> in the resulting filenames is implicit since
       there is a one-to-one correspondence between protocol instances and nodes, For example,:

          Names::Add ("node1Ipv4" ...);
          Names::Add ("node2Ipv4" ...);
          ...
          helper.EnableAsciiIpv4 ("prefix", "node1Ipv4", 1);
          helper.EnableAsciiIpv4 ("prefix", "node2Ipv4", 1);

       This    would    result    in    two    files    named    “prefix-nnode1Ipv4-i1.tr”    and
       “prefix-nnode2Ipv4-i1.tr”  with  traces  for  each interface in the respective trace file.
       Since all of the EnableAscii functions are overloaded to take a stream  wrapper,  you  can
       use that form as well:

          Names::Add ("node1Ipv4" ...);
          Names::Add ("node2Ipv4" ...);
          ...
          Ptr<OutputStreamWrapper> stream = asciiTraceHelper.CreateFileStream ("trace-file-name.tr");
          ...
          helper.EnableAsciiIpv4 (stream, "node1Ipv4", 1);
          helper.EnableAsciiIpv4 (stream, "node2Ipv4", 1);

       This  would result in a single trace file called “trace-file-name.tr” that contains all of
       the trace events for both interfaces. The events would be disambiguated by  trace  context
       strings.

       You  can  enable ASCII tracing on a collection of protocol/interface pairs by providing an
       Ipv4InterfaceContainer. For each protocol of the proper type (the same type as is  managed
       by  the  device  helper),  tracing is enabled for the corresponding interface.  Again, the
       <Node> is implicit since there is a one-to-one correspondence between  each  protocol  and
       its node. For example,:

          NodeContainer nodes;
          ...
          NetDeviceContainer devices = deviceHelper.Install (nodes);
          ...
          Ipv4AddressHelper ipv4;
          ipv4.SetBase ("10.1.1.0", "255.255.255.0");
          Ipv4InterfaceContainer interfaces = ipv4.Assign (devices);
          ...
          ...
          helper.EnableAsciiIpv4 ("prefix", interfaces);

       This  would  result  in a number of ASCII trace files being created, each of which follows
       the <prefix>-n<node id>-i<interface>.tr convention. Combining all of  the  traces  into  a
       single file is accomplished similarly to the examples above:

          NodeContainer nodes;
          ...
          NetDeviceContainer devices = deviceHelper.Install (nodes);
          ...
          Ipv4AddressHelper ipv4;
          ipv4.SetBase ("10.1.1.0", "255.255.255.0");
          Ipv4InterfaceContainer interfaces = ipv4.Assign (devices);
          ...
          Ptr<OutputStreamWrapper> stream = asciiTraceHelper.CreateFileStream ("trace-file-name.tr");
          ...
          helper.EnableAsciiIpv4 (stream, interfaces);

       You  can  enable  ASCII tracing on a collection of protocol/interface pairs by providing a
       NodeContainer. For each Node in the NodeContainer the appropriate protocol is found.   For
       each  protocol,  its  interfaces  are  enumerated  and tracing is enabled on the resulting
       pairs. For example,:

          NodeContainer n;
          ...
          helper.EnableAsciiIpv4 ("prefix", n);

       This would result in a number of ASCII trace files being created, each  of  which  follows
       the  <prefix>-<node  id>-<device  id>.tr  convention.  Combining  all of the traces into a
       single file is accomplished similarly to the examples above:

       You can enable pcap tracing on the basis of node ID and device ID as well. In  this  case,
       the  node-id is translated to a Ptr<Node> and the appropriate protocol is looked up in the
       node.  The resulting protocol and interface  are  used  to  specify  the  resulting  trace
       source.:

          helper.EnableAsciiIpv4 ("prefix", 21, 1);

       Of course, the traces can be combined into a single file as shown above.

       Finally,  you  can  enable ASCII tracing for all interfaces in the system, with associated
       protocol being the same type as that managed by the device helper.:

          helper.EnableAsciiIpv4All ("prefix");

       This would result in a number of ASCII trace files being created, one for every  interface
       in  the system related to a protocol of the type managed by the helper. All of these files
       will follow the <prefix>-n<node id>-i<interface.tr convention. Combining all of the traces
       into a single file is accomplished similarly to the examples above.

   Ascii Tracing Device Helper Filename Selection
       Implicit in the prefix-style method descriptions above is the construction of the complete
       filenames by the implementation method. By convention, ASCII traces in the ns-3 system are
       of the form “<prefix>-<node id>-<device id>.tr.”

       As  previously  mentioned,  every  node in the system will have a system-assigned node id.
       Since there is a one-to-one correspondence between protocols and nodes we use  to  node-id
       to  identify  the  protocol  identity.  Every  interface  on a given protocol will have an
       interface index (also called simply an interface) relative to its  protocol.  By  default,
       then,  an  ASCII trace file created as a result of enabling tracing on the first device of
       node 21, using the prefix  “prefix”,  would  be  “prefix-n21-i1.tr”.  Use  the  prefix  to
       disambiguate multiple protocols per node.

       You  can always use the ns-3 object name service to make this more clear.  For example, if
       you use the object name service to assign the name “serverIpv4” to the  protocol  on  node
       21, and also specify interface one, the resulting ASCII trace file name will automatically
       become, “prefix-nserverIpv4-1.tr”.

   Tracing implementation details
   Data Collection
       This  chapter  describes  the  ns-3  Data  Collection  Framework  (DCF),  which   provides
       capabilities  to  obtain  data  generated  by  models in the simulator, to perform on-line
       reduction and data processing, and to marshal raw or transformed data into various  output
       formats.

       The  framework  presently  supports  standalone  ns-3 runs that don’t rely on any external
       program execution control.  The objects provided by the DCF may be hooked  to  ns-3  trace
       sources to enable data processing.

       The source code for the classes lives in the directory src/stats.

       This  chapter  is  organized  as  follows.   First,  an  overview  of  the architecture is
       presented.  Next, the helpers for these classes  are  presented;  this  initial  treatment
       should  allow  basic  use  of the data collection framework for many use cases.  Users who
       wish to produce output outside of the scope of the current helpers, or who wish to  create
       their  own  data  collection objects, should read the remainder of the chapter, which goes
       into detail about all of  the  basic  DCF  object  types  and  provides  low-level  coding
       examples.

   Design
       The DCF consists of three basic classes:

       · Probe  is  a  mechanism  to instrument and control the output of simulation data that is
         used to monitor interesting events. It produces output in the form of one or  more  ns-3
         trace  sources.   Probe  objects  are  hooked  up  to  one  or  more trace sinks (called
         Collectors), which process samples on-line and prepare them for output.

       · Collector consumes the data generated  by  one  or  more  Probe  objects.   It  performs
         transformations  on  the  data, such as normalization, reduction, and the computation of
         basic statistics. Collector objects do not produce data that is directly output  by  the
         ns-3  run;  instead,  they  output  data  downstream  to  another type of object, called
         Aggregator, which performs that function.  Typically, Collectors output  their  data  in
         the form of trace sources as well, allowing collectors to be chained in series.

       · Aggregator is the end point of the data collected by a network of Probes and Collectors.
         The main responsibility of the Aggregator is to marshal  data  and  their  corresponding
         metadata,  into different output formats such as plain text files, spreadsheet files, or
         databases.

       All three of these classes provide the capability to dynamically turn themselves on or off
       throughout a simulation.

       Any  standalone  ns-3  simulation run that uses the DCF will typically create at least one
       instance of each of the three classes above.
         [image] Data Collection Framework overview.UNINDENT

         The overall flow of data processing is depicted in Data Collection  Framework  overview.
         On  the  left side, a running ns-3 simulation is depicted.  In the course of running the
         simulation, data is made available by models through trace sources, or via other  means.
         The  diagram depicts that probes can be connected to these trace sources to receive data
         asynchronously, or probes can poll for data.  Data is then passed to a collector  object
         that transforms the data.  Finally, an aggregator can be connected to the outputs of the
         collector, to generate plots, files, or databases.
         [image] Data Collection Framework aggregation.UNINDENT

         A variation on the above figure is provided in Data  Collection  Framework  aggregation.
         This  second figure illustrates that the DCF objects may be chained together in a manner
         that downstream  objects  take  inputs  from  multiple  upstream  objects.   The  figure
         conceptually  shows  that  multiple probes may generate output that is fed into a single
         collector; as an example, a collector  that  outputs  a  ratio  of  two  counters  would
         typically  acquire each counter data from separate probes.  Multiple collectors can also
         feed into a single aggregator, which (as its name implies) may collect a number of  data
         streams for inclusion into a single plot, file, or database.

   Data Collection Helpers
       The  full  flexibility of the data collection framework is provided by the interconnection
       of probes, collectors, and aggregators.  Performing all of these interconnections leads to
       many  configuration statements in user programs.  For ease of use, some of the most common
       operations can be combined and  encapsulated  in  helper  functions.   In  addition,  some
       statements involving ns-3 trace sources do not have Python bindings, due to limitations in
       the bindings.

   Data Collection Helpers Overview
       In this section, we provide an overview of some helper classes that have been  created  to
       ease  the  configuration  of the data collection framework for some common use cases.  The
       helpers allow users to form common operations with only a few statements in their  C++  or
       Python  programs.   But,  this  ease  of  use  comes  at  the  cost  of significantly less
       flexibility than low-level configuration can provide, and  the  need  to  explicitly  code
       support for new Probe types into the helpers (to work around an issue described below).

       The  emphasis  on  the  current  helpers is to marshal data out of ns-3 trace sources into
       gnuplot plots or text files, without a high degree of output customization or  statistical
       processing  (initially).   Also,  the  use  is constrained to the available probe types in
       ns-3.  Later sections of this documentation will go into more detail  about  creating  new
       Probe types, as well as details about hooking together Probes, Collectors, and Aggregators
       in custom arrangements.

       To date, two Data Collection helpers have been implemented:

       · GnuplotHelper

       · FileHelper

   GnuplotHelper
       The GnuplotHelper is a helper class for producing output files used to make gnuplots.  The
       overall  goal is to provide the ability for users to quickly make plots from data exported
       in ns-3 trace sources.  By default, a minimal amount of data transformation is  performed;
       the  objective  is  to  generate  plots  with as few (default) configuration statements as
       possible.

   GnuplotHelper Overview
       The GnuplotHelper will create 3 different files at the end of the simulation:

       · A space separated gnuplot data file

       · A gnuplot control file

       · A shell script to generate the gnuplot

       There are two configuration statements that  are  needed  to  produce  plots.   The  first
       statement configures the plot (filename, title, legends, and output type, where the output
       type defaults to PNG if unspecified):

          void ConfigurePlot (const std::string &outputFileNameWithoutExtension,
                              const std::string &title,
                              const std::string &xLegend,
                              const std::string &yLegend,
                              const std::string &terminalType = ".png");

       The second statement hooks the trace source of interest:

          void PlotProbe (const std::string &typeId,
                          const std::string &path,
                          const std::string &probeTraceSource,
                          const std::string &title);

       The arguments are as follows:

       · typeId:  The ns-3 TypeId of the Probe

       · path:  The path in the ns-3 configuration namespace to one or more trace sources

       · probeTraceSource:  Which output of the probe (itself a trace source) should be plotted

       · title:  The title to associate with the dataset(s) (in the gnuplot legend)

       A variant on the PlotProbe above is to specify a fifth  optional  argument  that  controls
       where in the plot the key (legend) is placed.

       A fully worked example (from seventh.cc) is shown below:

          // Create the gnuplot helper.
          GnuplotHelper plotHelper;

          // Configure the plot.
          // Configure the plot.  The first argument is the file name prefix
          // for the output files generated.  The second, third, and fourth
          // arguments are, respectively, the plot title, x-axis, and y-axis labels
          plotHelper.ConfigurePlot ("seventh-packet-byte-count",
                                    "Packet Byte Count vs. Time",
                                    "Time (Seconds)",
                                    "Packet Byte Count",
                                    "png");

          // Specify the probe type, trace source path (in configuration namespace), and
          // probe output trace source ("OutputBytes") to plot.  The fourth argument
          // specifies the name of the data series label on the plot.  The last
          // argument formats the plot by specifying where the key should be placed.
          plotHelper.PlotProbe (probeType,
                                tracePath,
                                "OutputBytes",
                                "Packet Byte Count",
                                GnuplotAggregator::KEY_BELOW);

       In this example, the probeType and tracePath are as follows (for IPv4):

          probeType = "ns3::Ipv4PacketProbe";
          tracePath = "/NodeList/*/$ns3::Ipv4L3Protocol/Tx";

       The  probeType is a key parameter for this helper to work.  This TypeId must be registered
       in the system, and the signature on the Probe’s trace sink must match that  of  the  trace
       source  it  is  being  hooked  to.  Probe types are pre-defined for a number of data types
       corresponding to ns-3 traced values, and for a few other trace source signatures  such  as
       the ‘Tx’ trace source of ns3::Ipv4L3Protocol class.

       Note  that  the trace source path specified may contain wildcards.  In this case, multiple
       datasets are plotted on one plot; one for each matched path.

       The main output produced will be three files:

          seventh-packet-byte-count.dat
          seventh-packet-byte-count.plt
          seventh-packet-byte-count.sh

       At this point, users can either hand edit the .plt file  for  further  customizations,  or
       just run it through gnuplot.  Running sh seventh-packet-byte-count.sh simply runs the plot
       through gnuplot, as shown below.
         [image] 2-D Gnuplot Created by seventh.cc Example..UNINDENT

         It can be seen that the key elements (legend, title, legend placement,  xlabel,  ylabel,
         and  path for the data) are all placed on the plot.  Since there were two matches to the
         configuration path provided, the two data series are shown:

       · Packet Byte Count-0 corresponds to /NodeList/0/$ns3::Ipv4L3Protocol/Tx

       · Packet Byte Count-1 corresponds to /NodeList/1/$ns3::Ipv4L3Protocol/Tx

   GnuplotHelper ConfigurePlot
       The GnuplotHelper’s ConfigurePlot() function can be used to configure plots.

       It has the following prototype:

          void ConfigurePlot (const std::string &outputFileNameWithoutExtension,
                              const std::string &title,
                              const std::string &xLegend,
                              const std::string &yLegend,
                              const std::string &terminalType = ".png");

       It has the following arguments:

                    ┌───────────────────────────────┬──────────────────────────────────┐
                    │Argument                       │ Description                      │
                    ├───────────────────────────────┼──────────────────────────────────┤
                    │outputFileNameWithoutExtension │ Name of gnuplot related files to │
                    │                               │ write with no extension.         │
                    ├───────────────────────────────┼──────────────────────────────────┤
                    │title                          │ Plot  title  string  to  use for │
                    │                               │ this plot.                       │
                    ├───────────────────────────────┼──────────────────────────────────┤
                    │xLegend                        │ The legend for the x  horizontal │
                    │                               │ axis.                            │
                    ├───────────────────────────────┼──────────────────────────────────┤
                    │yLegend                        │ The  legend  for  the y vertical │
                    │                               │ axis.                            │
                    ├───────────────────────────────┼──────────────────────────────────┤
                    │terminalType                   │ Terminal type setting string for │
                    │                               │ output.   The  default  terminal │
                    │                               │ type is “png”.                   │
                    └───────────────────────────────┴──────────────────────────────────┘

       The GnuplotHelper’s ConfigurePlot() function configures plot related parameters  for  this
       gnuplot  helper  so  that  it  will  create  a  space  separated  gnuplot  data file named
       outputFileNameWithoutExtension    +    “.dat”,    a    gnuplot    control    file    named
       outputFileNameWithoutExtension  + “.plt”, and a shell script to generate the gnuplot named
       outputFileNameWithoutExtension + “.sh”.

       An example of how to use this function can be seen in the seventh.cc code described  above
       where it was used as follows:

          plotHelper.ConfigurePlot ("seventh-packet-byte-count",
                                    "Packet Byte Count vs. Time",
                                    "Time (Seconds)",
                                    "Packet Byte Count",
                                    "png");

   GnuplotHelper PlotProbe
       The GnuplotHelper’s PlotProbe() function can be used to plot values generated by probes.

       It has the following prototype:

          void PlotProbe (const std::string &typeId,
                          const std::string &path,
                          const std::string &probeTraceSource,
                          const std::string &title,
                          enum GnuplotAggregator::KeyLocation keyLocation = GnuplotAggregator::KEY_INSIDE);

       It has the following arguments:

                           ┌─────────────────┬──────────────────────────────────┐
                           │Argument         │ Description                      │
                           ├─────────────────┼──────────────────────────────────┤
                           │typeId           │ The   type   ID  for  the  probe │
                           │                 │ created by this helper.          │
                           ├─────────────────┼──────────────────────────────────┤
                           │path             │ Config path to access the  trace │
                           │                 │ source.                          │
                           ├─────────────────┼──────────────────────────────────┤
                           │probeTraceSource │ The   probe   trace   source  to │
                           │                 │ access.                          │
                           ├─────────────────┼──────────────────────────────────┤
                           │title            │ The title to  be  associated  to │
                           │                 │ this dataset                     │
                           ├─────────────────┼──────────────────────────────────┤
                           │keyLocation      │ The  location  of the key in the │
                           │                 │ plot.  The default  location  is │
                           │                 │ inside.                          │
                           └─────────────────┴──────────────────────────────────┘

       The  GnuplotHelper’s  PlotProbe()  function  plots a dataset generated by hooking the ns-3
       trace source with a probe created by the helper, and then plotting  the  values  from  the
       probeTraceSource.   The  dataset  will  have  the  provided title, and will consist of the
       ‘newValue’ at each timestamp.

       If the config path has more than one match in the system because there is a wildcard, then
       one  dataset for each match will be plotted.  The dataset titles will be suffixed with the
       matched characters for each of the wildcards in the config path, separated by spaces.  For
       example,  if the proposed dataset title is the string “bytes”, and there are two wildcards
       in the path, then dataset titles like “bytes-0 0” or “bytes-12  9”  will  be  possible  as
       labels for the datasets that are plotted.

       An  example of how to use this function can be seen in the seventh.cc code described above
       where it was used (with variable substitution) as follows:

          plotHelper.PlotProbe ("ns3::Ipv4PacketProbe",
                                "/NodeList/*/$ns3::Ipv4L3Protocol/Tx",
                                "OutputBytes",
                                "Packet Byte Count",
                                GnuplotAggregator::KEY_BELOW);

   Other Examples
   Gnuplot Helper Example
       A  slightly  simpler   example   than   the   seventh.cc   example   can   be   found   in
       src/stats/examples/gnuplot-helper-example.cc.  The following 2-D gnuplot was created using
       the example.
         [image] 2-D Gnuplot Created by gnuplot-helper-example.cc Example..UNINDENT

         In this example, there is an Emitter object that increments its counter according  to  a
         Poisson process and then emits the counter’s value as a trace source.

          Ptr<Emitter> emitter = CreateObject<Emitter> ();
          Names::Add ("/Names/Emitter", emitter);

       Note  that  because  there  are no wildcards in the path used below, only 1 datastream was
       drawn in the plot.  This single datastream in the plot is simply labeled “Emitter  Count”,
       with no extra suffixes like one would see if there were wildcards in the path.

          // Create the gnuplot helper.
          GnuplotHelper plotHelper;

          // Configure the plot.
          plotHelper.ConfigurePlot ("gnuplot-helper-example",
                                    "Emitter Counts vs. Time",
                                    "Time (Seconds)",
                                    "Emitter Count",
                                    "png");

          // Plot the values generated by the probe.  The path that we provide
          // helps to disambiguate the source of the trace.
          plotHelper.PlotProbe ("ns3::Uinteger32Probe",
                                "/Names/Emitter/Counter",
                                "Output",
                                "Emitter Count",
                                GnuplotAggregator::KEY_INSIDE);

   FileHelper
       The FileHelper is a helper class used to put data values into a file.  The overall goal is
       to provide the ability for users to quickly make formatted text files from  data  exported
       in  ns-3 trace sources.  By default, a minimal amount of data transformation is performed;
       the objective is to generate files with  as  few  (default)  configuration  statements  as
       possible.

   FileHelper Overview
       The FileHelper will create 1 or more text files at the end of the simulation.

       The FileHelper can create 4 different types of text files:

       · Formatted

       · Space separated (the default)

       · Comma separated

       · Tab separated

       Formatted  files  use  C-style  format  strings  and the sprintf() function to print their
       values in the file being written.

       The   following   text   file   with    2    columns    of    formatted    values    named
       seventh-packet-byte-count-0.txt  was  created  using  more  new code that was added to the
       original ns-3 Tutorial example’s code.  Only the first 10 lines of  this  file  are  shown
       here for brevity.

          Time (Seconds) = 1.000e+00    Packet Byte Count = 40
          Time (Seconds) = 1.004e+00    Packet Byte Count = 40
          Time (Seconds) = 1.004e+00    Packet Byte Count = 576
          Time (Seconds) = 1.009e+00    Packet Byte Count = 576
          Time (Seconds) = 1.009e+00    Packet Byte Count = 576
          Time (Seconds) = 1.015e+00    Packet Byte Count = 512
          Time (Seconds) = 1.017e+00    Packet Byte Count = 576
          Time (Seconds) = 1.017e+00    Packet Byte Count = 544
          Time (Seconds) = 1.025e+00    Packet Byte Count = 576
          Time (Seconds) = 1.025e+00    Packet Byte Count = 544

          ...

       The   following   different   text   file   with  2  columns  of  formatted  values  named
       seventh-packet-byte-count-1.txt was also created using the same new code that was added to
       the original ns-3 Tutorial example’s code.  Only the first 10 lines of this file are shown
       here for brevity.

          Time (Seconds) = 1.002e+00    Packet Byte Count = 40
          Time (Seconds) = 1.007e+00    Packet Byte Count = 40
          Time (Seconds) = 1.013e+00    Packet Byte Count = 40
          Time (Seconds) = 1.020e+00    Packet Byte Count = 40
          Time (Seconds) = 1.028e+00    Packet Byte Count = 40
          Time (Seconds) = 1.036e+00    Packet Byte Count = 40
          Time (Seconds) = 1.045e+00    Packet Byte Count = 40
          Time (Seconds) = 1.053e+00    Packet Byte Count = 40
          Time (Seconds) = 1.061e+00    Packet Byte Count = 40
          Time (Seconds) = 1.069e+00    Packet Byte Count = 40

          ...

       The new code that was added to produce the two text files is below.   More  details  about
       this API will be covered in a later section.

       Note that because there were 2 matches for the wildcard in the path, 2 separate text files
       were created.  The first text  file,  which  is  named  “seventh-packet-byte-count-0.txt”,
       corresponds  to  the wildcard match with the “*” replaced with “0”.  The second text file,
       which is named “seventh-packet-byte-count-1.txt”, corresponds to the wildcard  match  with
       the “*” replaced with “1”.  Also, note that the function call to WriteProbe() will give an
       error message if there are no matches for a path that contains wildcards.

          // Create the file helper.
          FileHelper fileHelper;

          // Configure the file to be written.
          fileHelper.ConfigureFile ("seventh-packet-byte-count",
                                    FileAggregator::FORMATTED);

          // Set the labels for this formatted output file.
          fileHelper.Set2dFormat ("Time (Seconds) = %.3e\tPacket Byte Count = %.0f");

          // Write the values generated by the probe.
          fileHelper.WriteProbe ("ns3::Ipv4PacketProbe",
                                 "/NodeList/*/$ns3::Ipv4L3Protocol/Tx",
                                 "OutputBytes");

   FileHelper ConfigureFile
       The FileHelper’s ConfigureFile() function can be used to configure text files.

       It has the following prototype:

          void ConfigureFile (const std::string &outputFileNameWithoutExtension,
                              enum FileAggregator::FileType fileType = FileAggregator::SPACE_SEPARATED);

       It has the following arguments:

                    ┌───────────────────────────────┬──────────────────────────────────┐
                    │Argument                       │ Description                      │
                    ├───────────────────────────────┼──────────────────────────────────┤
                    │outputFileNameWithoutExtension │ Name of  output  file  to  write │
                    │                               │ with no extension.               │
                    ├───────────────────────────────┼──────────────────────────────────┤
                    │fileType                       │ Type  of  file  to  write.   The │
                    │                               │ default type of  file  is  space │
                    │                               │ separated.                       │
                    └───────────────────────────────┴──────────────────────────────────┘

       The  FileHelper’s ConfigureFile() function configures text file related parameters for the
       file helper so that it  will  create  a  file  named  outputFileNameWithoutExtension  plus
       possible  extra  information  from  wildcard  matches  plus  “.txt” with values printed as
       specified by fileType.  The default file type is space-separated.

       An example of how to use this function can be seen in the seventh.cc code described  above
       where it was used as follows:

          fileHelper.ConfigureFile ("seventh-packet-byte-count",
                                    FileAggregator::FORMATTED);

   FileHelper WriteProbe
       The  FileHelper’s WriteProbe() function can be used to write values generated by probes to
       text files.

       It has the following prototype:

          void WriteProbe (const std::string &typeId,
                           const std::string &path,
                           const std::string &probeTraceSource);

       It has the following arguments:

                           ┌─────────────────┬──────────────────────────────────┐
                           │Argument         │ Description                      │
                           ├─────────────────┼──────────────────────────────────┤
                           │typeId           │ The type ID for the probe to  be │
                           │                 │ created.                         │
                           ├─────────────────┼──────────────────────────────────┤
                           │path             │ Config  path to access the trace │
                           │                 │ source.                          │
                           ├─────────────────┼──────────────────────────────────┤
                           │probeTraceSource │ The  probe   trace   source   to │
                           │                 │ access.                          │
                           └─────────────────┴──────────────────────────────────┘

       The  FileHelper’s WriteProbe() function creates output text files generated by hooking the
       ns-3 trace source with a probe created by the helper, and then writing the values from the
       probeTraceSource.  The  output file names will have the text stored in the member variable
       m_outputFileNameWithoutExtension plus “.txt”, and will consist of the ‘newValue’  at  each
       timestamp.

       If the config path has more than one match in the system because there is a wildcard, then
       one output file for each match will be created.  The output file names  will  contain  the
       text  in  m_outputFileNameWithoutExtension  plus  the  matched  characters for each of the
       wildcards in the config path, separated by dashes, plus “.txt”.  For example, if the value
       in  m_outputFileNameWithoutExtension  is the string “packet-byte-count”, and there are two
       wildcards in  the  path,  then  output  file  names  like  “packet-byte-count-0-0.txt”  or
       “packet-byte-count-12-9.txt” will be possible as names for the files that will be created.

       An  example of how to use this function can be seen in the seventh.cc code described above
       where it was used as follows:

          fileHelper.WriteProbe ("ns3::Ipv4PacketProbe",
                                 "/NodeList/*/$ns3::Ipv4L3Protocol/Tx",
                                 "OutputBytes");

   Other Examples
   File Helper Example
       A  slightly  simpler   example   than   the   seventh.cc   example   can   be   found   in
       src/stats/examples/file-helper-example.cc.  This example only uses the FileHelper.

       The  following  text file with 2 columns of formatted values named file-helper-example.txt
       was created using the example.  Only the first 10 lines of this file are  shown  here  for
       brevity.

          Time (Seconds) = 0.203  Count = 1
          Time (Seconds) = 0.702  Count = 2
          Time (Seconds) = 1.404  Count = 3
          Time (Seconds) = 2.368  Count = 4
          Time (Seconds) = 3.364  Count = 5
          Time (Seconds) = 3.579  Count = 6
          Time (Seconds) = 5.873  Count = 7
          Time (Seconds) = 6.410  Count = 8
          Time (Seconds) = 6.472  Count = 9
          ...

       In  this  example,  there  is an Emitter object that increments its counter according to a
       Poisson process and then emits the counter’s value as a trace source.

          Ptr<Emitter> emitter = CreateObject<Emitter> ();
          Names::Add ("/Names/Emitter", emitter);

       Note that because there are no wildcards in the path used below,  only  1  text  file  was
       created.   This  single text file is simply named “file-helper-example.txt”, with no extra
       suffixes like you would see if there were wildcards in the path.

          // Create the file helper.
          FileHelper fileHelper;

          // Configure the file to be written.
          fileHelper.ConfigureFile ("file-helper-example",
                                    FileAggregator::FORMATTED);

          // Set the labels for this formatted output file.
          fileHelper.Set2dFormat ("Time (Seconds) = %.3e\tCount = %.0f");

          // Write the values generated by the probe.  The path that we
          // provide helps to disambiguate the source of the trace.
          fileHelper.WriteProbe ("ns3::Uinteger32Probe",
                                 "/Names/Emitter/Counter",
                                 "Output");

   Scope and Limitations
       Currently, only these Probes have been implemented and connected to the GnuplotHelper  and
       to the FileHelper:

       · BooleanProbe

       · DoubleProbe

       · Uinteger8Probe

       · Uinteger16Probe

       · Uinteger32Probe

       · TimeProbe

       · PacketProbe

       · ApplicationPacketProbe

       · Ipv4PacketProbe

       These  Probes,  therefore,  are  the  only TypeIds available to be used in PlotProbe() and
       WriteProbe().

       In the next few sections, we cover each of the fundamental object types (Probe, Collector,
       and  Aggregator)  in  more  detail,  and  show  how  they  can be connected together using
       lower-level API.

   Probes
       This section  details  the  functionalities  provided  by  the  Probe  class  to  an  ns-3
       simulation, and gives examples on how to code them in a program. This section is meant for
       users interested in developing  simulations  with  the  ns-3  tools  and  using  the  Data
       Collection  Framework,  of  which  the Probe class is a part, to generate data output with
       their simulation’s results.

   Probe Overview
       A Probe object is supposed to be connected to a variable from the simulation whose  values
       throughout  the  experiment  are  relevant  to  the user.  The Probe will record what were
       values assumed by the variable throughout the simulation and pass  such  data  to  another
       member  of  the  Data  Collection  Framework.   While it is out of this section’s scope to
       discuss what happens after the Probe produces its output, it is sufficient to say that, by
       the  end of the simulation, the user will have detailed information about what values were
       stored inside the variable being probed during the simulation.

       Typically, a Probe is connected to an ns-3 trace source.  In  this  manner,  whenever  the
       trace  source exports a new value, the Probe consumes the value (and exports it downstream
       to another object via its own trace source).

       The Probe can be thought of as kind of a filter on trace sources.  The  main  reasons  for
       possibly hooking to a Probe rather than directly to a trace source are as follows:

       · Probes may be dynamically turned on and off during the simulation with calls to Enable()
         and Disable().  For example, the outputting  of  data  may  be  turned  off  during  the
         simulation warmup phase.

       · Probes  may  perform  operations  on  the  data  to extract values from more complicated
         structures; for instance, outputting the packet size value from a received ns3::Packet.

       · Probes register a name in the ns3::Config namespace (using Names::Add ()) so that  other
         objects may refer to them.

       · Probes  provide  a  static method that allows one to manipulate a Probe by name, such as
         what is done in ns2measure [Cic06]

            Stat::put ("my_metric", ID, sample);

         The ns-3 equivalent of the above ns2measure code is, e.g.

            DoubleProbe::SetValueByPath ("/path/to/probe", sample);

   Creation
       Note that a Probe base class object can not be created because  it  is  an  abstract  base
       class,  i.e.  it  has pure virtual functions that have not been implemented.  An object of
       type DoubleProbe, which is a subclass of the Probe class, will be  created  here  to  show
       what needs to be done.

       One declares a DoubleProbe in dynamic memory by using the smart pointer class (Ptr<T>). To
       create a DoubleProbe in dynamic memory with smart pointers, one just  needs  to  call  the
       ns-3 method CreateObject():

          Ptr<DoubleProbe> myprobe = CreateObject<DoubleProbe> ();

       The  declaration  above  creates DoubleProbes using the default values for its attributes.
       There are four attributes  in  the  DoubleProbe  class;  two  in  the  base  class  object
       DataCollectionObject, and two in the Probe base class:

       · “Name” (DataCollectionObject), a StringValue

       · “Enabled” (DataCollectionObject), a BooleanValue

       · “Start” (Probe), a TimeValue

       · “Stop” (Probe), a TimeValue

       One can set such attributes at object creation by using the following method:

          Ptr<DoubleProbe> myprobe = CreateObjectWithAttributes<DoubleProbe> (
              "Name", StringValue ("myprobe"),
              "Enabled", BooleanValue (false),
              "Start", TimeValue (Seconds (100.0)),
              "Stop", TimeValue (Seconds (1000.0)));

       Start and Stop are Time variables which determine the interval of action of the Probe. The
       Probe will only output data if the current time  of  the  Simulation  is  inside  of  that
       interval.   The special time value of 0 seconds for Stop will disable this attribute (i.e.
       keep the Probe on for the whole simulation).  Enabled is a flag that turns the Probe on or
       off,  and must be set to true for the Probe to export data.  The Name is the object’s name
       in the DCF framework.

   Importing and exporting data
       ns-3 trace sources are strongly typed, so the mechanisms for hooking  Probes  to  a  trace
       source  and  for  exporting  data  belong  to  its  subclasses.  For instance, the default
       distribution of ns-3 provides a class DoubleProbe that is designed  to  hook  to  a  trace
       source  exporting a double value.  We’ll next detail the operation of the DoubleProbe, and
       then discuss how other Probe classes may be defined by the user.

   DoubleProbe Overview
       The DoubleProbe connects to a double-valued  ns-3  trace  source,  and  itself  exports  a
       different double-valued ns-3 trace source.

       The following code, drawn from src/stats/examples/double-probe-example.cc, shows the basic
       operations of plumbing the DoubleProbe into a simulation, where it is  probing  a  Counter
       exported by an emitter object (class Emitter).

          Ptr<Emitter> emitter = CreateObject<Emitter> ();
          Names::Add ("/Names/Emitter", emitter);
          ...

          Ptr<DoubleProbe> probe1 = CreateObject<DoubleProbe> ();

          // Connect the probe to the emitter's Counter
          bool connected = probe1->ConnectByObject ("Counter", emitter);

       The  following code is probing the same Counter exported by the same emitter object.  This
       DoubleProbe, however, is  using  a  path  in  the  configuration  namespace  to  make  the
       connection.   Note that the emitter registered itself in the configuration namespace after
       it was created; otherwise, the ConnectByPath would not work.

          Ptr<DoubleProbe> probe2 = CreateObject<DoubleProbe> ();

          // Note, no return value is checked here
          probe2->ConnectByPath ("/Names/Emitter/Counter");

       The next DoubleProbe shown that is shown below will have its value set using its  path  in
       the configuration namespace.  Note that this time the DoubleProbe registered itself in the
       configuration namespace after it was created.

          Ptr<DoubleProbe> probe3 = CreateObject<DoubleProbe> ();
          probe3->SetName ("StaticallyAccessedProbe");

          // We must add it to the config database
          Names::Add ("/Names/Probes", probe3->GetName (), probe3);

       The emitter’s Count() function is now able to  set  the  value  for  this  DoubleProbe  as
       follows:

          void
          Emitter::Count (void)
          {
            ...
            m_counter += 1.0;
            DoubleProbe::SetValueByPath ("/Names/StaticallyAccessedProbe", m_counter);
            ...
          }

       The  above  example shows how the code calling the Probe does not have to have an explicit
       reference to the Probe, but can direct the value setting  through  the  Config  namespace.
       This  is  similar  in functionality to the Stat::Put method introduced by ns2measure paper
       [Cic06], and allows users to temporarily insert Probe statements  like  printf  statements
       within existing ns-3 models.  Note that in order to be able to use the DoubleProbe in this
       example like this, 2 things were necessary:

       1. the stats module header file was included in the example .cc file

       2. the example was made dependent on the stats module in its wscript file.

       Analogous things need to be done in order to add other Probes in other places in the  ns-3
       code base.

       The values for the DoubleProbe can also be set using the function DoubleProbe::SetValue(),
       while  the   values   for   the   DoubleProbe   can   be   gotten   using   the   function
       DoubleProbe::GetValue().

       The  DoubleProbe  exports  double values in its “Output” trace source; a downstream object
       can hook a trace sink (NotifyViaProbe) to this as follows:

          connected = probe1->TraceConnect ("Output", probe1->GetName (), MakeCallback (&NotifyViaProbe));

   Other probes
       Besides the DoubleProbe, the following Probes are also available:

       · Uinteger8Probe connects to an ns-3 trace source exporting an uint8_t.

       · Uinteger16Probe connects to an ns-3 trace source exporting an uint16_t.

       · Uinteger32Probe connects to an ns-3 trace source exporting an uint32_t.

       · PacketProbe connects to an ns-3 trace source exporting a packet.

       · ApplicationPacketProbe connects to an ns-3 trace source exporting a packet and a  socket
         address.

       · Ipv4PacketProbe connects to an ns-3 trace source exporting a packet, an IPv4 object, and
         an interface.

   Creating new Probe types
       To create a new Probe type, you need to perform the following steps:

       · Be sure that your new Probe class is derived from the Probe base class.

       · Be sure that the pure virtual functions that your new  Probe  class  inherits  from  the
         Probe base class are implemented.

       · Find  an  existing  Probe  class that uses a trace source that is closest in type to the
         type of trace source your Probe will be using.

       · Copy that existing Probe class’s header file (.h) and implementation file (.cc)  to  two
         new files with names matching your new Probe.

       · Replace  the  types,  arguments,  and variables in the copied files with the appropriate
         type for your Probe.

       · Make necessary modifications to make the code compile and to make it behave as you would
         like.

   Examples
       Two examples will be discussed in detail here:

       · Double Probe Example

       · IPv4 Packet Plot Example

   Double Probe Example
       The  double probe example has been discussed previously.  The example program can be found
       in src/stats/examples/double-probe-example.cc.  To summarize what occurs in this  program,
       there is an emitter that exports a counter that increments according to a Poisson process.
       In particular, two ways of emitting data are shown:

       1. through a traced variable hooked to one Probe:

             TracedValue<double> m_counter;  // normally this would be integer type

       2. through a counter whose value is posted to a second Probe, referenced by  its  name  in
          the Config system:

              void
              Emitter::Count (void)
              {
                NS_LOG_FUNCTION (this);
                NS_LOG_DEBUG ("Counting at " << Simulator::Now ().GetSeconds ());
                m_counter += 1.0;
                DoubleProbe::SetValueByPath ("/Names/StaticallyAccessedProbe", m_counter);
                Simulator::Schedule (Seconds (m_var->GetValue ()), &Emitter::Count, this);
              }

       Let’s  look  at  the  Probe more carefully.  Probes can receive their values in a multiple
       ways:

       1. by the Probe accessing the trace source directly and connecting a trace sink to it

       2. by the Probe accessing the trace source through the config namespace and  connecting  a
          trace sink to it

       3. by the calling code explicitly calling the Probe’s SetValue() method

       4. by       the       calling       code       explicitly      calling      SetValueByPath
          (“/path/through/Config/namespace”, …)

       The first two techniques are expected to be the most common.  Also  in  the  example,  the
       hooking  of  a  normal  callback  function  is  shown, as is typically done in ns-3.  This
       callback function is not associated with a Probe object.  We’ll call this case 0) below.

          // This is a function to test hooking a raw function to the trace source
          void
          NotifyViaTraceSource (std::string context, double oldVal, double newVal)
          {
            NS_LOG_DEBUG ("context: " << context << " old " << oldVal << " new " << newVal);
          }

       First, the emitter needs to be setup:

          Ptr<Emitter> emitter = CreateObject<Emitter> ();
          Names::Add ("/Names/Emitter", emitter);

          // The Emitter object is not associated with an ns-3 node, so
          // it won't get started automatically, so we need to do this ourselves
          Simulator::Schedule (Seconds (0.0), &Emitter::Start, emitter);

       The various DoubleProbes interact with the emitter in the example as shown below.

       Case 0):

              // The below shows typical functionality without a probe
              // (connect a sink function to a trace source)
              //
              connected = emitter->TraceConnect ("Counter", "sample context", MakeCallback (&NotifyViaTraceSource));
              NS_ASSERT_MSG (connected, "Trace source not connected");

       case 1):

              //
              // Probe1 will be hooked directly to the Emitter trace source object
              //

              // probe1 will be hooked to the Emitter trace source
              Ptr<DoubleProbe> probe1 = CreateObject<DoubleProbe> ();
              // the probe's name can serve as its context in the tracing
              probe1->SetName ("ObjectProbe");

              // Connect the probe to the emitter's Counter
              connected = probe1->ConnectByObject ("Counter", emitter);
              NS_ASSERT_MSG (connected, "Trace source not connected to probe1");

       case 2):

              //
              // Probe2 will be hooked to the Emitter trace source object by
              // accessing it by path name in the Config database
              //

              // Create another similar probe; this will hook up via a Config path
              Ptr<DoubleProbe> probe2 = CreateObject<DoubleProbe> ();
              probe2->SetName ("PathProbe");

              // Note, no return value is checked here
              probe2->ConnectByPath ("/Names/Emitter/Counter");

       case 4) (case 3 is not shown in this example):

              //
              // Probe3 will be called by the emitter directly through the
              // static method SetValueByPath().
              //
              Ptr<DoubleProbe> probe3 = CreateObject<DoubleProbe> ();
              probe3->SetName ("StaticallyAccessedProbe");
              // We must add it to the config database
              Names::Add ("/Names/Probes", probe3->GetName (), probe3);

       And finally, the example shows how the probes can be hooked to generate output:

              // The probe itself should generate output.  The context that we provide
              // to this probe (in this case, the probe name) will help to disambiguate
              // the source of the trace
              connected = probe3->TraceConnect ("Output",
                                                "/Names/Probes/StaticallyAccessedProbe/Output",
                                                MakeCallback (&NotifyViaProbe));
              NS_ASSERT_MSG (connected, "Trace source not .. connected to probe3 Output");

       The following callback is hooked to the Probe in this example for  illustrative  purposes;
       normally, the Probe would be hooked to a Collector object.

          // This is a function to test hooking it to the probe output
          void
          NotifyViaProbe (std::string context, double oldVal, double newVal)
          {
            NS_LOG_DEBUG ("context: " << context << " old " << oldVal << " new " << newVal);
          }

   IPv4 Packet Plot Example
       The  IPv4 packet plot example is based on the fifth.cc example from the ns-3 Tutorial.  It
       can be found in src/stats/examples/ipv4-packet-plot-example.cc.

                node 0                 node 1
          +----------------+    +----------------+
          |    ns-3 TCP    |    |    ns-3 TCP    |
          +----------------+    +----------------+
          |    10.1.1.1    |    |    10.1.1.2    |
          +----------------+    +----------------+
          | point-to-point |    | point-to-point |
          +----------------+    +----------------+
                  |                     |
                  +---------------------+

       We’ll just look at the Probe, as it illustrates that Probes may also  unpack  values  from
       structures (in this case, packets) and report those values as trace source outputs, rather
       than just passing through the same type of data.

       There are other aspects of this example that will be explained later in the documentation.
       The  two types of data that are exported are the packet itself (Output) and a count of the
       number of bytes in the packet (OutputBytes).

          TypeId
          Ipv4PacketProbe::GetTypeId ()
          {
            static TypeId tid = TypeId ("ns3::Ipv4PacketProbe")
              .SetParent<Probe> ()
              .AddConstructor<Ipv4PacketProbe> ()
              .AddTraceSource ( "Output",
                                "The packet plus its IPv4 object and interface that serve as the output for this probe",
                                MakeTraceSourceAccessor (&Ipv4PacketProbe::m_output))
              .AddTraceSource ( "OutputBytes",
                                "The number of bytes in the packet",
                                MakeTraceSourceAccessor (&Ipv4PacketProbe::m_outputBytes))
            ;
            return tid;
          }

       When the Probe’s trace sink gets a packet, if the Probe is enabled, then  it  will  output
       the  packet on its Output trace source, but it will also output the number of bytes on the
       OutputBytes trace source.

          void
          Ipv4PacketProbe::TraceSink (Ptr<const Packet> packet, Ptr<Ipv4> ipv4, uint32_t interface)
          {
            NS_LOG_FUNCTION (this << packet << ipv4 << interface);
            if (IsEnabled ())
              {
                m_packet    = packet;
                m_ipv4      = ipv4;
                m_interface = interface;
                m_output (packet, ipv4, interface);

                uint32_t packetSizeNew = packet->GetSize ();
                m_outputBytes (m_packetSizeOld, packetSizeNew);
                m_packetSizeOld = packetSizeNew;
              }
          }

   References
       [Cic06]
            Claudio Cicconetti, Enzo  Mingozzi,  Giovanni  Stea,  “An  Integrated  Framework  for
            Enabling  Effective  Data  Collection  and Statistical Analysis with ns2, Workshop on
            ns-2 (WNS2), Pisa, Italy, October 2006.

   Collectors
       This section is a placeholder to detail the  functionalities  provided  by  the  Collector
       class to an ns-3 simulation, and gives examples on how to code them in a program.

       Note:  As  of ns-3.18, Collectors are still under development and not yet provided as part
       of the framework.

   Aggregators
       This section details the functionalities provided by  the  Aggregator  class  to  an  ns-3
       simulation.  This section is meant for users interested in developing simulations with the
       ns-3 tools and using the Data Collection Framework, of which the  Aggregator  class  is  a
       part, to generate data output with their simulation’s results.

   Aggregator Overview
       An  Aggregator  object  is  supposed to be hooked to one or more trace sources in order to
       receive input. Aggregators are the end point of the  data  collected  by  the  network  of
       Probes  and  Collectors  during  the simulation.  It is the Aggregator’s job to take these
       values and transform them into their  final  output  format  such  as  plain  text  files,
       spreadsheet files, plots, or databases.

       Typically, an aggregator is connected to one or more Collectors.  In this manner, whenever
       the Collectors’ trace sources export new values, the Aggregator can process the  value  so
       that it can be used in the final output format where the data values will reside after the
       simulation.

       Note the following about Aggregators:

       · Aggregators may be dynamically turned on and off during the  simulation  with  calls  to
         Enable()  and  Disable().  For example, the aggregating of data may be turned off during
         the simulation warmup phase, which means those values won’t be  included  in  the  final
         output medium.

       · Aggregators  receive  data from Collectors via callbacks. When a Collector is associated
         to an aggregator, a call to TraceConnect is made to  establish  the  Aggregator’s  trace
         sink method as a callback.

       To date, two Aggregators have been implemented:

       · GnuplotAggregator

       · FileAggregator

   GnuplotAggregator
       The GnuplotAggregator produces output files used to make gnuplots.

       The GnuplotAggregator will create 3 different files at the end of the simulation:

       · A space separated gnuplot data file

       · A gnuplot control file

       · A shell script to generate the gnuplot

   Creation
       An object of type GnuplotAggregator will be created here to show what needs to be done.

       One  declares  a  GnuplotAggregator  in  dynamic  memory  by using the smart pointer class
       (Ptr<T>). To create a GnuplotAggregator in dynamic memory with smart  pointers,  one  just
       needs   to   call   the   ns-3   method   CreateObject().    The   following   code   from
       src/stats/examples/gnuplot-aggregator-example.cc shows how to do this:

          string fileNameWithoutExtension = "gnuplot-aggregator";

          // Create an aggregator.
          Ptr<GnuplotAggregator> aggregator =
            CreateObject<GnuplotAggregator> (fileNameWithoutExtension);

       The first argument for the constructor,  fileNameWithoutExtension,  is  the  name  of  the
       gnuplot  related  files  to write with no extension.  This GnuplotAggregator will create a
       space separated gnuplot data file named “gnuplot-aggregator.dat”, a gnuplot  control  file
       named  “gnuplot-aggregator.plt”,  and  a  shell  script  to  generate  the gnuplot named +
       “gnuplot-aggregator.sh”.

       The gnuplot that is created can have its key in 4 different locations:

       · No key

       · Key inside the plot (the default)

       · Key above the plot

       · Key below the plot

       The following gnuplot key location enum values are allowed to specify the key’s position:

          enum KeyLocation {
            NO_KEY,
            KEY_INSIDE,
            KEY_ABOVE,
            KEY_BELOW
          };

       If it was desired to have the key below rather than the default position of  inside,  then
       you could do the following.

          aggregator->SetKeyLocation(GnuplotAggregator::KEY_BELOW);

   Examples
       One example will be discussed in detail here:

       · Gnuplot Aggregator Example

   Gnuplot Aggregator Example
       An    example    that    exercises    the    GnuplotAggregator    can    be    found    in
       src/stats/examples/gnuplot-aggregator-example.cc.

       The following 2-D gnuplot was created using the example.
         [image] 2-D Gnuplot Created by gnuplot-aggregator-example.cc Example..UNINDENT

         This code from the example shows how to construct the GnuplotAggregator as was discussed
         above.

          void Create2dPlot ()
          {
            using namespace std;

            string fileNameWithoutExtension = "gnuplot-aggregator";
            string plotTitle                = "Gnuplot Aggregator Plot";
            string plotXAxisHeading         = "Time (seconds)";
            string plotYAxisHeading         = "Double Values";
            string plotDatasetLabel         = "Data Values";
            string datasetContext           = "Dataset/Context/String";

            // Create an aggregator.
            Ptr<GnuplotAggregator> aggregator =
              CreateObject<GnuplotAggregator> (fileNameWithoutExtension);

       Various  GnuplotAggregator  attributes  are  set  including  the  2-D dataset that will be
       plotted.

          // Set the aggregator's properties.
          aggregator->SetTerminal ("png");
          aggregator->SetTitle (plotTitle);
          aggregator->SetLegend (plotXAxisHeading, plotYAxisHeading);

          // Add a data set to the aggregator.
          aggregator->Add2dDataset (datasetContext, plotDatasetLabel);

          // aggregator must be turned on
          aggregator->Enable ();

       Next, the 2-D values  are  calculated,  and  each  one  is  individually  written  to  the
       GnuplotAggregator using the Write2d() function.

            double time;
            double value;

            // Create the 2-D dataset.
            for (time = -5.0; time <= +5.0; time += 1.0)
              {
                // Calculate the 2-D curve
                //
                //                   2
                //     value  =  time   .
                //
                value = time * time;

                // Add this point to the plot.
                aggregator->Write2d (datasetContext, time, value);
              }

            // Disable logging of data for the aggregator.
            aggregator->Disable ();
          }

   FileAggregator
       The FileAggregator sends the values it receives to a file.

       The FileAggregator can create 4 different types of files:

       · Formatted

       · Space separated (the default)

       · Comma separated

       · Tab separated

       Formatted  files  use  C-style  format  strings  and the sprintf() function to print their
       values in the file being written.

   Creation
       An object of type FileAggregator will be created here to show what needs to be done.

       One declares a FileAggregator in dynamic memory by using the smart pointer class (Ptr<T>).
       To  create  a FileAggregator in dynamic memory with smart pointers, one just needs to call
       the      ns-3      method      CreateObject.       The      following      code       from
       src/stats/examples/file-aggregator-example.cc shows how to do this:

          string fileName       = "file-aggregator-formatted-values.txt";

          // Create an aggregator that will have formatted values.
          Ptr<FileAggregator> aggregator =
            CreateObject<FileAggregator> (fileName, FileAggregator::FORMATTED);

       The  first  argument  for the constructor, filename, is the name of the file to write; the
       second argument, fileType, is type of file to write. This  FileAggregator  will  create  a
       file  named “file-aggregator-formatted-values.txt” with its values printed as specified by
       fileType, i.e., formatted in this case.

       The following file type enum values are allowed:

          enum FileType {
            FORMATTED,
            SPACE_SEPARATED,
            COMMA_SEPARATED,
            TAB_SEPARATED
          };

   Examples
       One example will be discussed in detail here:

       · File Aggregator Example

   File Aggregator Example
       An    example    that    exercises    the    FileAggregator    can     be     found     in
       src/stats/examples/file-aggregator-example.cc.

       The following text file with 2 columns of values separated by commas was created using the
       example.

          -5,25
          -4,16
          -3,9
          -2,4
          -1,1
          0,0
          1,1
          2,4
          3,9
          4,16
          5,25

       This code from the example shows how to construct  the  FileAggregator  as  was  discussed
       above.

          void CreateCommaSeparatedFile ()
          {
            using namespace std;

            string fileName       = "file-aggregator-comma-separated.txt";
            string datasetContext = "Dataset/Context/String";

            // Create an aggregator.
            Ptr<FileAggregator> aggregator =
              CreateObject<FileAggregator> (fileName, FileAggregator::COMMA_SEPARATED);

       FileAggregator attributes are set.

          // aggregator must be turned on
          aggregator->Enable ();

       Next,  the  2-D  values  are  calculated,  and  each  one  is  individually written to the
       FileAggregator using the Write2d() function.

            double time;
            double value;

            // Create the 2-D dataset.
            for (time = -5.0; time <= +5.0; time += 1.0)
              {
                // Calculate the 2-D curve
                //
                //                   2
                //     value  =  time   .
                //
                value = time * time;

                // Add this point to the plot.
                aggregator->Write2d (datasetContext, time, value);
              }

            // Disable logging of data for the aggregator.
            aggregator->Disable ();
          }

       The following text file with 2 columns of formatted values  was  also  created  using  the
       example.

          Time = -5.000e+00     Value = 25
          Time = -4.000e+00     Value = 16
          Time = -3.000e+00     Value = 9
          Time = -2.000e+00     Value = 4
          Time = -1.000e+00     Value = 1
          Time = 0.000e+00      Value = 0
          Time = 1.000e+00      Value = 1
          Time = 2.000e+00      Value = 4
          Time = 3.000e+00      Value = 9
          Time = 4.000e+00      Value = 16
          Time = 5.000e+00      Value = 25

       This  code  from  the  example  shows how to construct the FileAggregator as was discussed
       above.

          void CreateFormattedFile ()
          {
            using namespace std;

            string fileName       = "file-aggregator-formatted-values.txt";
            string datasetContext = "Dataset/Context/String";

            // Create an aggregator that will have formatted values.
            Ptr<FileAggregator> aggregator =
              CreateObject<FileAggregator> (fileName, FileAggregator::FORMATTED);

       FileAggregator attributes are set, including the C-style format string to use.

          // Set the format for the values.
          aggregator->Set2dFormat ("Time = %.3e\tValue = %.0f");

          // aggregator must be turned on
          aggregator->Enable ();

       Next, the 2-D values  are  calculated,  and  each  one  is  individually  written  to  the
       FileAggregator using the Write2d() function.

            double time;
            double value;

            // Create the 2-D dataset.
            for (time = -5.0; time <= +5.0; time += 1.0)
              {
                // Calculate the 2-D curve
                //
                //                   2
                //     value  =  time   .
                //
                value = time * time;

                // Add this point to the plot.
                aggregator->Write2d (datasetContext, time, value);
              }

            // Disable logging of data for the aggregator.
            aggregator->Disable ();
          }

   Adaptors
       This  section  details  the  functionalities  provided  by  the  Adaptor  class to an ns-3
       simulation. This section is meant for users interested in developing simulations with  the
       ns-3  tools and using the Data Collection Framework, of which the Adaptor class is a part,
       to generate data output with their simulation’s results.

       Note:  the term ‘adaptor’ may also be spelled ‘adapter’; we  chose  the  spelling  aligned
       with the C++ standard.

   Adaptor Overview
       An Adaptor is used to make connections between different types of DCF objects.

       To date, one Adaptor has been implemented:

       · TimeSeriesAdaptor

   Time Series Adaptor
       The  TimeSeriesAdaptor  lets  Probes  connect  directly to Aggregators without needing any
       Collector in between.

       Both of the implemented DCF helpers utilize TimeSeriesAdaptors in  order  to  take  probed
       values  of  different types and output the current time plus the value with both converted
       to doubles.

       The role of the TimeSeriesAdaptor class is that of  an  adaptor,  which  takes  raw-valued
       probe  data  of  different types and outputs a tuple of two double values.  The first is a
       timestamp, which may be set to different resolutions (e.g. Seconds, Milliseconds, etc.) in
       the future but which is presently hardcoded to Seconds.  The second is the conversion of a
       non-double value to a double value (possibly with loss of precision).

   Scope/Limitations
       This section discusses the scope and limitations of the Data Collection Framework.

       Currently, only these Probes have been implemented in DCF:

       · BooleanProbe

       · DoubleProbe

       · Uinteger8Probe

       · Uinteger16Probe

       · Uinteger32Probe

       · TimeProbe

       · PacketProbe

       · ApplicationPacketProbe

       · Ipv4PacketProbe

       Currently, no Collectors are available in the DCF, although a BasicStatsCollector is under
       development.

       Currently, only these Aggregators have been implemented in DCF:

       · GnuplotAggregator

       · FileAggregator

       Currently, only this Adaptor has been implemented in DCF:

       Time-Series Adaptor.

   Future Work
       This section discusses the future work to be done on the Data Collection Framework.

       Here are some things that still need to be done:

       · Hook up more trace sources in ns-3 code to get more values out of the simulator.

       · Implement more types of Probes than there currently are.

       · Implement more than just the single current 2-D Collector, BasicStatsCollector.

       · Implement more Aggregators.

       · Implement more than just Adaptors.

   Statistical Framework
       This chapter outlines work on simulation data collection and the statistical framework for
       ns-3.

       The source code for the statistical framework lives in the directory src/stats.

   Goals
       Primary objectives for this effort are the following:

       · Provide functionality to record, calculate, and present data and statistics for analysis
         of network simulations.

       · Boost  simulation  performance  by reducing the need to generate extensive trace logs in
         order to collect data.

       · Enable simulation  control  via  online  statistics,  e.g.  terminating  simulations  or
         repeating trials.

       Derived sub-goals and other target features include the following:

       · Integration with the existing ns-3 tracing system as the basic instrumentation framework
         of the internal simulation engine, e.g. network stacks, net devices, and channels.

       · Enabling users to utilize the statistics framework without requiring use of the  tracing
         system.

       · Helping users create, aggregate, and analyze data over multiple trials.

       · Support  for  user  created  instrumentation,  e.g.  of  application specific events and
         measures.

       · Low memory and CPU overhead when the package is not in use.

       · Leveraging existing analysis and output tools as much as possible.   The  framework  may
         provide  some  basic  statistics,  but  the  focus  is  on collecting data and making it
         accessible for manipulation in established tools.

       · Eventual support for distributing independent replications is important but not included
         in the first round of features.

   Overview
       The statistics framework includes the following features:

       · The  core  framework  and  two basic data collectors: A counter, and a min/max/avg/total
         observer.

       · Extensions of those to easily work with times and packets.

       · Plaintext output formatted for OMNet++.

       · Database output using SQLite, a standalone, lightweight, high performance SQL engine.

       · Mandatory and open ended metadata for describing and working with runs.

       · An example based on the notional  experiment  of  examining  the  properties  of  NS-3’s
         default ad hoc WiFi performance.  It incorporates the following:

         · Constructs  of a two node ad hoc WiFi network, with the nodes a parameterized distance
           apart.

         · UDP traffic  source  and  sink  applications  with  slightly  different  behavior  and
           measurement hooks than the stock classes.

         · Data  collection  from the NS-3 core via existing trace signals, in particular data on
           frames transmitted and received by the WiFi MAC objects.

         · Instrumentation of custom applications by connecting new trace  signals  to  the  stat
           framework, as well as via direct updates.  Information is recorded about total packets
           sent and received, bytes transmitted, and end-to-end delay.

         · An example of using packet tags to track end-to-end delay.

         · A simple control script which runs a number of trials of  the  experiment  at  varying
           distances and queries the resulting database to produce a graph using GNUPlot.

   To-Do
       High priority items include:

       · Inclusion of online statistics code, e.g. for memory efficient confidence intervals.

       · Provisions  in  the  data  collectors  for  terminating  runs,  i.e. when a threshold or
         confidence is met.

       · Data collectors for logging samples over time, and output to the various formats.

       · Demonstrate writing simple cyclic event glue to regularly poll some value.

       Each of those should prove straightforward to incorporate in the current framework.

   Approach
       The framework is based around the following core principles:

       · One experiment trial is conducted by one instance of a simulation  program,  whether  in
         parallel or serially.

       · A control script executes instances of the simulation, varying parameters as necessary.

       · Data  is  collected  and  stored  for  plotting  and analysis using external scripts and
         existing tools.

       · Measures within the ns-3 core are taken by connecting the  stat  framework  to  existing
         trace signals.

       · Trace  signals  or direct manipulation of the framework may be used to instrument custom
         simulation code.

       Those basic components of the  framework  and  their  interactions  are  depicted  in  the
       following figure.  [image]

   Example
       This  section  goes through the process of constructing an experiment in the framework and
       producing data for analysis (graphs) from it, demonstrating the structure  and  API  along
       the way.

   Question
       ‘’What  is  the  (simulated)  performance  of  ns-3’s  WiFi  NetDevices (using the default
       settings)?  How far apart can wireless  nodes  be  in  a  simulation  before  they  cannot
       communicate reliably?’‘

       · Hypothesis:  Based  on  knowledge of real life performance, the nodes should communicate
         reasonably well to  at  least  100m  apart.   Communication  beyond  200m  shouldn’t  be
         feasible.

       Although  not a very common question in simulation contexts, this is an important property
       of which simulation developers should have a basic understanding.  It  is  also  a  common
       study done on live hardware.

   Simulation Program
       The  first  thing  to  do  in  implementing  this  experiment is developing the simulation
       program.  The code for this example can be  found  in  examples/stats/wifi-example-sim.cc.
       It does the following main steps.

       · Declaring parameters and parsing the command line using ns3::CommandLine.

            double distance = 50.0;
            string format ("OMNet++");
            string experiment ("wifi-distance-test");
            string strategy ("wifi-default");
            string runID;

            CommandLine cmd;
            cmd.AddValue("distance",   "Distance apart to place nodes (in meters).", distance);
            cmd.AddValue("format",     "Format to use for data output.",             format);
            cmd.AddValue("experiment", "Identifier for experiment.",                 experiment);
            cmd.AddValue("strategy",   "Identifier for strategy.",                   strategy);
            cmd.AddValue("run",        "Identifier for run.",                        runID);
            cmd.Parse (argc, argv);

       · Creating  nodes  and  network  stacks  using  ns3::NodeContainer,  ns3::WiFiHelper,  and
         ns3::InternetStackHelper.

            NodeContainer nodes;
            nodes.Create(2);

            WifiHelper wifi;
            wifi.SetMac("ns3::AdhocWifiMac");
            wifi.SetPhy("ns3::WifiPhy");
            NetDeviceContainer nodeDevices = wifi.Install(nodes);

            InternetStackHelper internet;
            internet.Install(nodes);
            Ipv4AddressHelper ipAddrs;
            ipAddrs.SetBase("192.168.0.0", "255.255.255.0");
            ipAddrs.Assign(nodeDevices);

       · Positioning the nodes using ns3::MobilityHelper.   By  default  the  nodes  have  static
         mobility  and  won’t  move,  but must be positioned the given distance apart.  There are
         several ways to do this; it is done here using ns3::ListPositionAllocator,  which  draws
         positions from a given list.

            MobilityHelper mobility;
            Ptr<ListPositionAllocator> positionAlloc =
              CreateObject<ListPositionAllocator>();
            positionAlloc->Add(Vector(0.0, 0.0, 0.0));
            positionAlloc->Add(Vector(0.0, distance, 0.0));
            mobility.SetPositionAllocator(positionAlloc);
            mobility.Install(nodes);

       · Installing  a  traffic  generator  and  a traffic sink.  The stock Applications could be
         used, but the example includes custom  objects  in  src/test/test02-apps.(cc|h).   These
         have a simple behavior, generating a given number of packets spaced at a given interval.
         As there is only one of  each  they  are  installed  manually;  for  a  larger  set  the
         ns3::ApplicationHelper  class could be used.  The commented-out Config::Set line changes
         the destination of the packets, set to broadcast by default in this example.  Note  that
         in  general  WiFi may have different performance for broadcast and unicast frames due to
         different rate control and MAC retransmission policies.

            Ptr<Node> appSource = NodeList::GetNode(0);
            Ptr<Sender> sender = CreateObject<Sender>();
            appSource->AddApplication(sender);
            sender->Start(Seconds(1));

            Ptr<Node> appSink = NodeList::GetNode(1);
            Ptr<Receiver> receiver = CreateObject<Receiver>();
            appSink->AddApplication(receiver);
            receiver->Start(Seconds(0));

            //  Config::Set("/NodeList/*/ApplicationList/*/$Sender/Destination",
            //              Ipv4AddressValue("192.168.0.2"));

       · Configuring the data and statistics to be collected.  The  basic  paradigm  is  that  an
         ns3::DataCollector  object  is created to hold information about this particular run, to
         which observers and calculators are attached to actually  generate  data.   Importantly,
         run  information  includes  labels  for the ‘’experiment’‘, ‘’strategy’‘, ‘’input’‘, and
         ‘’run’‘.  These are used to later identify and easily group data from multiple trials.

         · The experiment is the study of which this trial is a  member.   Here  it  is  on  WiFi
           performance and distance.

         · The  strategy is the code or parameters being examined in this trial.  In this example
           it is fixed, but an obvious extension would  be  to  investigate  different  WiFi  bit
           rates, each of which would be a different strategy.

         · The  input  is  the  particular  problem  given  to this trial.  Here it is simply the
           distance between the two nodes.

         · The runID is a unique identifier for this trial with which it’s information is  tagged
           for identification in later analysis.  If no run ID is given the example program makes
           a (weak) run ID using the current time.

         Those four pieces of metadata are required, but more may be desired.  They may be  added
         to the record using the ns3::DataCollector::AddMetadata() method.

            DataCollector data;
            data.DescribeRun(experiment, strategy, input, runID);
            data.AddMetadata("author", "tjkopena");

         Actual  observation  and  calculating  is  done by ns3::DataCalculator objects, of which
         several different types exist.  These are created by the simulation program, attached to
         reporting or sampling code, and then registered with the ns3::DataCollector so they will
         be queried later for their output.  One easy observation mechanism is  to  use  existing
         trace sources, for example to instrument objects in the ns-3 core without changing their
         code.  Here a counter is attached directly to a trace signal in the WiFi  MAC  layer  on
         the target node.

            Ptr<PacketCounterCalculator> totalRx = CreateObject<PacketCounterCalculator>();
            totalRx->SetKey("wifi-rx-frames");
            Config::Connect("/NodeList/1/DeviceList/*/$ns3::WifiNetDevice/Rx",
                            MakeCallback(&PacketCounterCalculator::FrameUpdate, totalRx));
            data.AddDataCalculator(totalRx);

         Calculators may also be manipulated directly.  In this example, a counter is created and
         passed to the traffic sink application to be updated when packets are received.

            Ptr<CounterCalculator<> > appRx = CreateObject<CounterCalculator<> >();
            appRx->SetKey("receiver-rx-packets");
            receiver->SetCounter(appRx);
            data.AddDataCalculator(appRx);

         To increment the count, the  sink’s  packet  processing  code  then  calls  one  of  the
         calculator’s update methods.

            m_calc->Update();

         The  program  includes  several  other  examples  as  well,  using  both  the  primitive
         calculators such as ns3::CounterCalculator and those adapted for observing  packets  and
         times.  In src/test/test02-apps.(cc|h) it also creates a simple custom tag which it uses
         to  track  end-to-end  delay   for   generated   packets,   reporting   results   to   a
         ns3::TimeMinMaxAvgTotalCalculator data calculator.

       · Running the simulation, which is very straightforward once constructed.

            Simulator::Run();

       · Generating either OMNet++ or SQLite output, depending on the command line arguments.  To
         do this a ns3::DataOutputInterface object is created and configured.  The specific  type
         of   this   will   determine   the  output  format.   This  object  is  then  given  the
         ns3::DataCollector object which it interrogates to produce the output.

            Ptr<DataOutputInterface> output;
            if (format == "OMNet++") {
              NS_LOG_INFO("Creating OMNet++ formatted data output.");
              output = CreateObject<OmnetDataOutput>();
            } else {
            #   ifdef STAT_USE_DB
                NS_LOG_INFO("Creating SQLite formatted data output.");
                output = CreateObject<SqliteDataOutput>();
            #   endif
            }

            output->Output(data);

       · Freeing any memory used by the simulation.  This should come at  the  end  of  the  main
         function for the example.

            Simulator::Destroy();

   Logging
       To see what the example program, applications, and stat framework are doing in detail, set
       the NS_LOG variable appropriately.  The following will provide  copious  output  from  all
       three.

          $ export NS_LOG=WiFiDistanceExperiment:WiFiDistanceApps

       Note that this slows down the simulation extraordinarily.

   Sample Output
       Compiling and simply running the test program will append OMNet++ formatted output such as
       the following to data.sca.

          run run-1212239121

          attr experiment "wifi-distance-test"
          attr strategy "wifi-default"
          attr input "50"
          attr description ""

          attr "author" "tjkopena"

          scalar wifi-tx-frames count 30
          scalar wifi-rx-frames count 30
          scalar sender-tx-packets count 30
          scalar receiver-rx-packets count 30
          scalar tx-pkt-size count 30
          scalar tx-pkt-size total 1920
          scalar tx-pkt-size average 64
          scalar tx-pkt-size max 64
          scalar tx-pkt-size min 64
          scalar delay count 30
          scalar delay total 5884980ns
          scalar delay average 196166ns
          scalar delay max 196166ns
          scalar delay min 196166ns

   Control Script
       In order to automate data collection at a variety of inputs  (distances),  a  simple  Bash
       script   is   used   to   execute   a   series   of  simulations.   It  can  be  found  at
       examples/stats/wifi-example-db.sh.  The script is meant to be run from the examples/stats/
       directory.

       The  script  runs  through  a  set  of  distances,  collecting  the results into an SQLite
       database.  At each distance five trials are conducted to give a better picture of expected
       performance.   The  entire  experiment  takes only a few dozen seconds to run on a low end
       machine as there is no output during the simulation and little traffic is generated.

          #!/bin/sh

          DISTANCES="25 50 75 100 125 145 147 150 152 155 157 160 162 165 167 170 172 175 177 180"
          TRIALS="1 2 3 4 5"

          echo WiFi Experiment Example

          if [ -e data.db ]
          then
            echo Kill data.db?
            read ANS
            if [ "$ANS" = "yes" -o "$ANS" = "y" ]
            then
              echo Deleting database
              rm data.db
            fi
          fi

          for trial in $TRIALS
          do
            for distance in $DISTANCES
            do
              echo Trial $trial, distance $distance
              ./bin/test02 --format=db --distance=$distance --run=run-$distance-$trial
            done
          done

   Analysis and Conclusion
       Once all trials have been conducted, the script executes  a  simple  SQL  query  over  the
       database using the SQLite command line program.  The query computes average packet loss in
       each set of trials associated with each distance.  It does not take into account different
       strategies,  but the information is present in the database to make some simple extensions
       and do so.  The collected data is then passed to GNUPlot for graphing.

          CMD="select exp.input,avg(100-((rx.value*100)/tx.value)) \
              from Singletons rx, Singletons tx, Experiments exp \
              where rx.run = tx.run AND \
                    rx.run = exp.run AND \
                    rx.name='receiver-rx-packets' AND \
                    tx.name='sender-tx-packets' \
              group by exp.input \
              order by abs(exp.input) ASC;"

          sqlite3 -noheader data.db "$CMD" > wifi-default.data
          sed -i "s/|/   /" wifi-default.data
          gnuplot wifi-example.gnuplot

       The GNUPlot script found at examples/stats/wifi-example.gnuplot simply defines the  output
       format and some basic formatting for the graph.

          set terminal postscript portrait enhanced lw 2 "Helvetica" 14

          set size 1.0, 0.66

          #-------------------------------------------------------
          set out "wifi-default.eps"
          #set title "Packet Loss Over Distance"
          set xlabel "Distance (m) --- average of 5 trials per point"
          set xrange [0:200]
          set ylabel "% Packet Loss"
          set yrange [0:110]

          plot "wifi-default.data" with lines title "WiFi Defaults"

   End Result
       The  resulting  graph  provides  no  evidence that the default WiFi model’s performance is
       necessarily unreasonable and lends some confidence to an at least  token  faithfulness  to
       reality.  More importantly, this simple investigation has been carried all the way through
       using the statistical framework.  Success!  [image]

   RealTime
       ns-3 has been designed for integration into testbed and virtual machine  environments.  To
       integrate  with  real  network  stacks  and emit/consume packets, a real-time scheduler is
       needed to try to lock the simulation clock with the hardware clock.  We  describe  here  a
       component of this: the RealTime scheduler.

       The  purpose of the realtime scheduler is to cause the progression of the simulation clock
       to occur synchronously with respect to some external time base.  Without the  presence  of
       an  external  time  base  (wall clock), simulation time jumps instantly from one simulated
       time to the next.

   Behavior
       When using a non-realtime scheduler (the default in  ns-3),  the  simulator  advances  the
       simulation  time  to  the next scheduled event. During event execution, simulation time is
       frozen. With the realtime scheduler, the behavior  is  similar  from  the  perspective  of
       simulation  models  (i.e.,  simulation time is frozen during event execution), but between
       events, the simulator will attempt to keep the simulation clock aligned with  the  machine
       clock.

       When  an  event  is  finished  executing,  and  the scheduler moves to the next event, the
       scheduler compares the next event execution time with the  machine  clock.   If  the  next
       event  is scheduled for a future time, the simulator sleeps until that realtime is reached
       and then executes the next event.

       It may happen that, due to the processing inherent in the execution of simulation  events,
       that  the  simulator  cannot  keep up with realtime.  In such a case, it is up to the user
       configuration what to do. There are two ns-3 attributes  that  govern  the  behavior.  The
       first  is  ns3::RealTimeSimulatorImpl::SynchronizationMode.  The  two entries possible for
       this attribute are BestEffort (the  default)  or  HardLimit.  In  “BestEffort”  mode,  the
       simulator  will  just  try  to catch up to realtime by executing events until it reaches a
       point where the next event is in the (realtime) future, or else the  simulation  ends.  In
       BestEffort  mode,  then,  it  is possible for the simulation to consume more time than the
       wall clock time. The other option “HardLimit” will cause the simulation to  abort  if  the
       tolerance  threshold is exceeded.  This attribute is ns3::RealTimeSimulatorImpl::HardLimit
       and the default is 0.1 seconds.

       A different mode of operation is one in which simulated time is not frozen during an event
       execution. This mode of realtime simulation was implemented but removed from the ns-3 tree
       because of questions of whether it would be useful.  If users are interested in a realtime
       simulator  for  which  simulation time does not freeze during event execution (i.e., every
       call to Simulator::Now() returns the current wall clock time, not the time  at  which  the
       event started executing), please contact the ns-developers mailing list.

   Usage
       The  usage  of  the  realtime  simulator is straightforward, from a scripting perspective.
       Users  just  need  to  set  the  attribute  SimulatorImplementationType  to  the  Realtime
       simulator, such as follows:

          GlobalValue::Bind ("SimulatorImplementationType",
            StringValue ("ns3::RealtimeSimulatorImpl"));

       There  is a script in examples/realtime/realtime-udp-echo.cc that has an example of how to
       configure the realtime behavior.  Try:

          $ ./waf --run realtime-udp-echo

       Whether the simulator will work in a best effort or hard limit policy fashion is  governed
       by the attributes explained in the previous section.

   Implementation
       The implementation is contained in the following files:

       · src/core/model/realtime-simulator-impl.{cc,h}

       · src/core/model/wall-clock-synchronizer.{cc,h}

       In  order  to create a realtime scheduler, to a first approximation you just want to cause
       simulation time jumps to consume real time. We propose doing this using a  combination  of
       sleep-  and  busy-  waits.  Sleep-waits  cause  the  calling process (thread) to yield the
       processor for some amount of time. Even though this specified amount of time can be passed
       to  nanosecond  resolution,  it  is  actually  converted to an OS-specific granularity. In
       Linux, the granularity is called a Jiffy. Typically this resolution  is  insufficient  for
       our  needs  (on  the  order  of  a  ten milliseconds), so we round down and sleep for some
       smaller number of Jiffies. The process is then awakened  after  the  specified  number  of
       Jiffies  has  passed.  At  this  time,  we  have  some residual time to wait. This time is
       generally smaller than the minimum sleep time, so we busy-wait for the  remainder  of  the
       time.  This  means  that  the  thread  just  sits in a for loop consuming cycles until the
       desired time arrives. After the combination of sleep- and busy-waits, the elapsed realtime
       (wall)  clock  should  agree with the simulation time of the next event and the simulation
       proceeds.

   Helpers
       The above chapters introduced you to various  ns-3  programming  concepts  such  as  smart
       pointers  for reference-counted memory management, attributes, namespaces, callbacks, etc.
       Users who work at this low-level API can interconnect ns-3 objects with fine  granularity.
       However, a simulation program written entirely using the low-level API would be quite long
       and tedious to code. For this reason, a separate so-called “helper API” has been  overlaid
       on  the  core  ns-3  API. If you have read the ns-3 tutorial, you will already be familiar
       with the helper API, since it is the API that new users are typically introduced to first.
       In  this  chapter, we introduce the design philosophy of the helper API and contrast it to
       the low-level API. If you become a heavy user of ns-3, you will likely move back and forth
       between these APIs even in the same program.

       The helper API has a few goals:

       1. the  rest of src/ has no dependencies on the helper API; anything that can be done with
          the helper API can be coded also at the low-level API

       2. Containers: Often simulations will need to do a number of identical actions  to  groups
          of  objects.  The  helper API makes heavy use of containers of similar objects to which
          similar or identical operations can be performed.

       3. The helper API is not  generic;  it  does  not  strive  to  maximize  code  reuse.  So,
          programming  constructs  such as polymorphism and templates that achieve code reuse are
          not  as  prevalent.  For  instance,  there  are  separate  CsmaNetDevice  helpers   and
          PointToPointNetDevice  helpers  but  they  do  not  derive from a common NetDevice base
          class.

       4. The helper API typically works with stack-allocated (vs. heap-allocated)  objects.  For
          some  programs,  ns-3  users may not need to worry about any low level Object Create or
          Ptr handling; they can make do with containers of objects and  stack-allocated  helpers
          that operate on them.

       The  helper API is really all about making ns-3 programs easier to write and read, without
       taking away the power of the low-level interface. The rest of this chapter  provides  some
       examples of the programming conventions of the helper API.

   Making Plots using the Gnuplot Class
       There   are   2   common   methods   to   make   a   plot   using   ns-3  and  gnuplot  (‐
       http://www.gnuplot.info):

       1. Create a gnuplot control file using ns-3’s Gnuplot class.

       2. Create a gnuplot data file using values generated by ns-3.

       This section is about method 1, i.e. it is about how to make a plot using  ns-3’s  Gnuplot
       class.   If  you are interested in method 2, see the “A Real Example” subsection under the
       “Tracing” section in the ns-3 Tutorial.

   Creating Plots Using the Gnuplot Class
       The following steps must be taken in order to create a plot using ns-3’s Gnuplot class:

       1. Modify your code so that is uses the Gnuplot class and its functions.

       2. Run your code so that it creates a gnuplot control file.

       3. Call gnuplot with the name of the gnuplot control file.

       4. View the graphics file that was produced in your favorite graphics viewer.

       See the code from the example plots that are discussed below for details on step 1.

   An Example Program that Uses the Gnuplot Class
       An example program that uses ns-3’s Gnuplot class can be found here:

          src/stats/examples/gnuplot-example.cc

       In order to run this example, do the following:

          $ ./waf --run src/stats/examples/gnuplot-example

       This should produce the following gnuplot control files:

          plot-2d.plt
          plot-2d-with-error-bars.plt
          plot-3d.plt

       In order to process these gnuplot control files, do the following:

          $ gnuplot plot-2d.plt
          $ gnuplot plot-2d-with-error-bars.plt
          $ gnuplot plot-3d.plt

       This should produce the following graphics files:

          plot-2d.png
          plot-2d-with-error-bars.png
          plot-3d.png

       You can view these graphics files in your favorite graphics  viewer.   If  you  have  gimp
       installed on your machine, for example, you can do this:

          $ gimp plot-2d.png
          $ gimp plot-2d-with-error-bars.png
          $ gimp plot-3d.png

   An Example 2-Dimensional Plot
       The following 2-Dimensional plot
         [image]

       was created using the following code from gnuplot-example.cc:

          using namespace std;

          string fileNameWithNoExtension = "plot-2d";
          string graphicsFileName        = fileNameWithNoExtension + ".png";
          string plotFileName            = fileNameWithNoExtension + ".plt";
          string plotTitle               = "2-D Plot";
          string dataTitle               = "2-D Data";

          // Instantiate the plot and set its title.
          Gnuplot plot (graphicsFileName);
          plot.SetTitle (plotTitle);

          // Make the graphics file, which the plot file will create when it
          // is used with Gnuplot, be a PNG file.
          plot.SetTerminal ("png");

          // Set the labels for each axis.
          plot.SetLegend ("X Values", "Y Values");

          // Set the range for the x axis.
          plot.AppendExtra ("set xrange [-6:+6]");

          // Instantiate the dataset, set its title, and make the points be
          // plotted along with connecting lines.
          Gnuplot2dDataset dataset;
          dataset.SetTitle (dataTitle);
          dataset.SetStyle (Gnuplot2dDataset::LINES_POINTS);

          double x;
          double y;

          // Create the 2-D dataset.
          for (x = -5.0; x <= +5.0; x += 1.0)
            {
              // Calculate the 2-D curve
              //
              //            2
              //     y  =  x   .
              //
              y = x * x;

              // Add this point.
              dataset.Add (x, y);
            }

          // Add the dataset to the plot.
          plot.AddDataset (dataset);

          // Open the plot file.
          ofstream plotFile (plotFileName.c_str());

          // Write the plot file.
          plot.GenerateOutput (plotFile);

          // Close the plot file.
          plotFile.close ();

   An Example 2-Dimensional Plot with Error Bars
       The following 2-Dimensional plot with error bars in the x and y directions
         [image]

       was created using the following code from gnuplot-example.cc:

          using namespace std;

          string fileNameWithNoExtension = "plot-2d-with-error-bars";
          string graphicsFileName        = fileNameWithNoExtension + ".png";
          string plotFileName            = fileNameWithNoExtension + ".plt";
          string plotTitle               = "2-D Plot With Error Bars";
          string dataTitle               = "2-D Data With Error Bars";

          // Instantiate the plot and set its title.
          Gnuplot plot (graphicsFileName);
          plot.SetTitle (plotTitle);

          // Make the graphics file, which the plot file will create when it
          // is used with Gnuplot, be a PNG file.
          plot.SetTerminal ("png");

          // Set the labels for each axis.
          plot.SetLegend ("X Values", "Y Values");

          // Set the range for the x axis.
          plot.AppendExtra ("set xrange [-6:+6]");

          // Instantiate the dataset, set its title, and make the points be
          // plotted with no connecting lines.
          Gnuplot2dDataset dataset;
          dataset.SetTitle (dataTitle);
          dataset.SetStyle (Gnuplot2dDataset::POINTS);

          // Make the dataset have error bars in both the x and y directions.
          dataset.SetErrorBars (Gnuplot2dDataset::XY);

          double x;
          double xErrorDelta;
          double y;
          double yErrorDelta;

          // Create the 2-D dataset.
          for (x = -5.0; x <= +5.0; x += 1.0)
            {
              // Calculate the 2-D curve
              //
              //            2
              //     y  =  x   .
              //
              y = x * x;

              // Make the uncertainty in the x direction be constant and make
              // the uncertainty in the y direction be a constant fraction of
              // y's value.
              xErrorDelta = 0.25;
              yErrorDelta = 0.1 * y;

              // Add this point with uncertainties in both the x and y
              // direction.
              dataset.Add (x, y, xErrorDelta, yErrorDelta);
            }

          // Add the dataset to the plot.
          plot.AddDataset (dataset);

          // Open the plot file.
          ofstream plotFile (plotFileName.c_str());

          // Write the plot file.
          plot.GenerateOutput (plotFile);

          // Close the plot file.
          plotFile.close ();

   An Example 3-Dimensional Plot
       The following 3-Dimensional plot
         [image]

       was created using the following code from gnuplot-example.cc:

          using namespace std;

          string fileNameWithNoExtension = "plot-3d";
          string graphicsFileName        = fileNameWithNoExtension + ".png";
          string plotFileName            = fileNameWithNoExtension + ".plt";
          string plotTitle               = "3-D Plot";
          string dataTitle               = "3-D Data";

          // Instantiate the plot and set its title.
          Gnuplot plot (graphicsFileName);
          plot.SetTitle (plotTitle);

          // Make the graphics file, which the plot file will create when it
          // is used with Gnuplot, be a PNG file.
          plot.SetTerminal ("png");

          // Rotate the plot 30 degrees around the x axis and then rotate the
          // plot 120 degrees around the new z axis.
          plot.AppendExtra ("set view 30, 120, 1.0, 1.0");

          // Make the zero for the z-axis be in the x-axis and y-axis plane.
          plot.AppendExtra ("set ticslevel 0");

          // Set the labels for each axis.
          plot.AppendExtra ("set xlabel 'X Values'");
          plot.AppendExtra ("set ylabel 'Y Values'");
          plot.AppendExtra ("set zlabel 'Z Values'");

          // Set the ranges for the x and y axis.
          plot.AppendExtra ("set xrange [-5:+5]");
          plot.AppendExtra ("set yrange [-5:+5]");

          // Instantiate the dataset, set its title, and make the points be
          // connected by lines.
          Gnuplot3dDataset dataset;
          dataset.SetTitle (dataTitle);
          dataset.SetStyle ("with lines");

          double x;
          double y;
          double z;

          // Create the 3-D dataset.
          for (x = -5.0; x <= +5.0; x += 1.0)
            {
            for (y = -5.0; y <= +5.0; y += 1.0)
                {
                  // Calculate the 3-D surface
                  //
                  //            2      2
                  //     z  =  x   *  y   .
                  //
                  z = x * x * y * y;

                  // Add this point.
                  dataset.Add (x, y, z);
                }

            // The blank line is necessary at the end of each x value's data
            // points for the 3-D surface grid to work.
            dataset.AddEmptyLine ();
            }

          // Add the dataset to the plot.
          plot.AddDataset (dataset);

          // Open the plot file.
          ofstream plotFile (plotFileName.c_str());

          // Write the plot file.
          plot.GenerateOutput (plotFile);

          // Close the plot file.
          plotFile.close ();

   Using Python to Run ns-3
       Python bindings allow the C++ code in ns-3 to be called from Python.

       This  chapter  shows  you  how  to  create  a Python script that can run ns-3 and also the
       process of creating Python bindings for a C++ ns-3 module.

   Introduction
       Python bindings provide support for importing ns-3  model  libraries  as  Python  modules.
       Coverage  of  most  of  the  ns-3  C++  API is provided.  The intent has been to allow the
       programmer to write complete simulation scripts in Python, to allow  integration  of  ns-3
       with  other Python tools and workflows.  The intent is not to provide a different language
       choice to author new ns-3 models implemented in Python.

       Python    bindings     for     ns-3     use     a     tool     called     PyBindGen     (‐
       https://github.com/gjcarneiro/pybindgen)  to  create Python modules from the C++ libraries
       built by Waf.  The Python bindings that  PyBindGen  uses  are  maintained  in  a  bindings
       directory in each module, and must be maintained to match the C++ API of that ns-3 module.
       If the C++ API changes,  the  Python  bindings  file  must  either  be  modified  by  hand
       accordingly, or the bindings must be regenerated by an automated scanning process.

       If a user is not interested in Python, he or she may disable the use of Python bindings at
       Waf configure time.  In this case, changes to the C++ API of a provided  module  will  not
       cause the module to fail to compile.

       The process for automatically generating Python bindings relies on a toolchain involving a
       development  installation  of  the  Clang  compiler,   a   program   called   CastXML   (‐
       https://github.com/CastXML/CastXML),     and     a     program    called    pygccxml    (‐
       https://github.com/gccxml/pygccxml).  The toolchain can be installed using the  ns-3  bake
       build tool.

   An Example Python Script that Runs ns-3
       Here  is  some example code that is written in Python and that runs ns-3, which is written
       in C++.  This Python example can be found in examples/tutorial/first.py:

          import ns.applications
          import ns.core
          import ns.internet
          import ns.network
          import ns.point_to_point

          ns.core.LogComponentEnable("UdpEchoClientApplication", ns.core.LOG_LEVEL_INFO)
          ns.core.LogComponentEnable("UdpEchoServerApplication", ns.core.LOG_LEVEL_INFO)

          nodes = ns.network.NodeContainer()
          nodes.Create(2)

          pointToPoint = ns.point_to_point.PointToPointHelper()
          pointToPoint.SetDeviceAttribute("DataRate", ns.core.StringValue("5Mbps"))
          pointToPoint.SetChannelAttribute("Delay", ns.core.StringValue("2ms"))

          devices = pointToPoint.Install(nodes)

          stack = ns.internet.InternetStackHelper()
          stack.Install(nodes)

          address = ns.internet.Ipv4AddressHelper()
          address.SetBase(ns.network.Ipv4Address("10.1.1.0"), ns.network.Ipv4Mask("255.255.255.0"))

          interfaces = address.Assign (devices);

          echoServer = ns.applications.UdpEchoServerHelper(9)

          serverApps = echoServer.Install(nodes.Get(1))
          serverApps.Start(ns.core.Seconds(1.0))
          serverApps.Stop(ns.core.Seconds(10.0))

          echoClient = ns.applications.UdpEchoClientHelper(interfaces.GetAddress(1), 9)
          echoClient.SetAttribute("MaxPackets", ns.core.UintegerValue(1))
          echoClient.SetAttribute("Interval", ns.core.TimeValue(ns.core.Seconds (1.0)))
          echoClient.SetAttribute("PacketSize", ns.core.UintegerValue(1024))

          clientApps = echoClient.Install(nodes.Get(0))
          clientApps.Start(ns.core.Seconds(2.0))
          clientApps.Stop(ns.core.Seconds(10.0))

          ns.core.Simulator.Run()
          ns.core.Simulator.Destroy()

   Running Python Scripts
       waf contains some options that automatically update  the  python  path  to  find  the  ns3
       module.  To run example programs, there are two ways to use waf to take care of this.  One
       is to run a waf shell; e.g.:

          $ ./waf shell
          $ python examples/wireless/mixed-wireless.py

       and the other is to use the –pyrun option to waf:

          $ ./waf --pyrun examples/wireless/mixed-wireless.py

       To run a python script under the C debugger:

          $ ./waf shell
          $ gdb --args python examples/wireless/mixed-wireless.py

       To  run  your  own  Python  script   that   calls   ns-3   and   that   has   this   path,
       /path/to/your/example/my-script.py, do the following:

          $ ./waf shell
          $ python /path/to/your/example/my-script.py

   Caveats
       Python  bindings  for  ns-3  are  a  work  in  progress, and some limitations are known by
       developers.  Some of these limitations (not all) are listed here.

   Incomplete Coverage
       First of all, keep in mind that not 100% of the API is supported in Python.  Some  of  the
       reasons are:

       1. some  of  the  APIs  involve  pointers,  which require knowledge of what kind of memory
          passing semantics (who owns what memory). Such knowledge is not part  of  the  function
          signatures, and is either documented or sometimes not even documented.  Annotations are
          needed to bind those functions;

       2. Sometimes a unusual fundamental data type or C++ construct is used  which  is  not  yet
          supported by PyBindGen;

       3. GCC-XML  does  not  report  template based classes unless they are instantiated. (Note:
          Need to confirm this limitation still exists with CastXML)

       Most of the missing APIs can be wrapped, given enough time, patience, and  expertise,  and
       will  likely  be  wrapped  if bug reports are submitted.  However, don’t file a bug report
       saying “bindings are incomplete”, because we do not have manpower to complete 100% of  the
       bindings.

   Conversion Constructors
       Conversion  constructors  are not fully supported yet by PyBindGen, and they always act as
       explicit constructors when translating an API into Python.  For example, in C++ you can do
       this:

          Ipv4AddressHelper ipAddrs;
          ipAddrs.SetBase ("192.168.0.0", "255.255.255.0");
          ipAddrs.Assign (backboneDevices);

       In Python, for the time being you have to do:

          ipAddrs = ns.internet.Ipv4AddressHelper()
          ipAddrs.SetBase(ns.network.Ipv4Address("192.168.0.0"), ns.network.Ipv4Mask("255.255.255.0"))
          ipAddrs.Assign(backboneDevices)

   CommandLine
       CommandLine::AddValue()  works differently in Python than it does in ns-3.  In Python, the
       first parameter is a string that represents the command-line option name.  When the option
       is  set,  an  attribute  with the same name as the option name is set on the CommandLine()
       object.  Example:

          NUM_NODES_SIDE_DEFAULT = 3

          cmd = ns3.CommandLine()

          cmd.NumNodesSide = None
          cmd.AddValue("NumNodesSide", "Grid side number of nodes (total number of nodes will be this number squared)")

          cmd.Parse(argv)

          [...]

          if cmd.NumNodesSide is None:
              num_nodes_side = NUM_NODES_SIDE_DEFAULT
          else:
              num_nodes_side = int(cmd.NumNodesSide)

   Tracing
       Callback based tracing is not yet properly supported for Python, as new ns-3 API needs  to
       be provided for this to be supported.

       Pcap file writing is supported via the normal API.

       ASCII  tracing  is  supported  since  ns-3.4  via the normal C++ API translated to Python.
       However, ASCII tracing requires the creation of an ostream object to pass into  the  ASCII
       tracing  methods.   In  Python,  the C++ std::ofstream has been minimally wrapped to allow
       this.  For example:

          ascii = ns3.ofstream("wifi-ap.tr") # create the file
          ns3.YansWifiPhyHelper.EnableAsciiAll(ascii)
          ns3.Simulator.Run()
          ns3.Simulator.Destroy()
          ascii.close() # close the file

       There is one caveat: you must not allow the file object to be garbage collected while ns-3
       is  still  using it.  That means that the ‘ascii’ variable above must not be allowed to go
       out of scope or else the program will crash.

   Working with Python Bindings
       Python bindings are built on a module-by-module basis, and can be found in  each  module’s
       bindings directory.

   Overview
       The python bindings are generated into an ‘ns’ namespace.  Examples:

          from ns.network import Node
          n1 = Node()

       or

          import ns.network
          n1 = ns.network.Node()

       The  best  way to explore the bindings is to look at the various example programs provided
       in ns-3; some C++ examples have a corresponding Python example.  There  is  no  structured
       documentation  for  the  Python  bindings  like  there is Doxygen for the C++ API, but the
       Doxygen can be consulted to understand how the C++ API works.

   Python Bindings Workflow
       The process by which Python bindings are handled is the following:

       1. Periodically a developer uses a CastXML (https://github.com/CastXML/CastXML) based  API
          scanning     script,     which     saves     the     scanned    API    definition    as
          bindings/python/ns3_module_*.py files or as Python  files  in  each  modules’  bindings
          directory.  These files are kept under version control in the main ns-3 repository;

       2. Other developers clone the repository and use the already scanned API definitions;

       3. When  configuring  ns-3,  pybindgen  will  be  automatically  downloaded if not already
          installed.  Released ns-3 tarballs will ship a copy of pybindgen.

       If something goes wrong with compiling Python bindings and you just want  to  ignore  them
       and move on with C++, you can disable Python with:

          $ ./waf configure --disable-python ...

       To  add  support  for  modular  bindings to an existing or new ns-3 module, simply add the
       following line to its wscript build() function:

          bld.ns3_python_bindings()

       One must also provide the bindings files (usually by running the scanning framework).

   Regenerating the Python bindings
       ns-3 will fail to successfully compile the Python bindings if the C++ headers are  changed
       and  no longer align with the stored Python bindings.  In this case, the developer has two
       main choices:  1) disable Python as described above, or 2) update the  bindings  to  align
       with the new C++ API.

   Process Overview
       ns-3  has  an  automated  process to regenerate Python bindings from the C++ header files.
       The automated process is only semi-automated at the moment (ns-3.27) because we are in the
       midst  of  transition to new tools.  The current process is outlined below.  In short, the
       process currently requires the following steps.

       1. Prepare  the  system  for  scanning  by  installing  the  prerequisites,  including   a
          development version of clang, the CastXML package, and pygccxml.

       2. Perform a 64-bit scan of the module or modules of interest

       3. Create the 32-bit bindings file from the 64-bit scan output

   Installing a clang development environment
       Make  sure  you have a development version of the clang compiler installed on your system.
       This can take a long time to  build  from  source.   Linux  distributions  provide  binary
       library  packages  such  as  clang-dev  or  clang-devel.   The  version  should not be too
       important; version 3.8 is known to work.  Note that there is a  problem  with  the  Ubuntu
       package  installation  of  clang-dev; see the Installation wiki page for details on how to
       fix using some symlinks.

   Installing other prerequisites
       cxxfilt is a new requirement, typically installed using pip; e.g.

          sudo pip install cxxfilt

       See also the wiki for installation notes for your system.

   Set up a bake build environment
       Try the following commands:

          $ cd bake
          $ export BAKE_HOME=`pwd`
          $ export PATH=$PATH:$BAKE_HOME/build/bin
          $ export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$BAKE_HOME/build/lib
          $ export PYTHONPATH=$PYTHONPATH:$BAKE_HOME/build/lib
          $ mkdir -p build/lib

   Configure
       Perform a configuration at the bake level:

          $ ./bake.py configure -e ns-3-dev -e pygccxml-1.9.1

       The output of bake show should show something like this:

          $ ./bake.py show

       Should say:

          -- System Dependencies --
           > clang-dev - OK
           > g++ - OK
           > libxml2-dev - OK
           > pygoocanvas - OK
           > pygraphviz - OK
           > python-dev - OK
           > qt - OK
           > setuptools - OK

   Download
       Try the following command:

          $ ./bake.py download
           >> Searching for system dependency python-dev - OK
           >> Searching for system dependency clang-dev - OK
           >> Searching for system dependency g++ - OK
           >> Searching for system dependency setuptools - OK
           >> Searching for system dependency pygoocanvas - OK
           >> Searching for system dependency pygraphviz - OK
           >> Searching for system dependency qt - OK
           >> Downloading castxml - OK
           >> Downloading netanim - OK
           >> Downloading pygccxml-1.9.1 - OK
           >> Downloading pygccxml - OK
           >> Downloading pybindgen - OK
           >> Downloading ns-3-dev - OK

   Build
       Try the following commands:

          $ mkdir -p build/lib
          $ ./bake.py build

       It should fail on the ns-3 bindings complilation.

       The output of ‘./waf configure’ can be inspected to see if Python API scanning support  is
       enabled:

          Python API Scanning Support   : enabled

       It may say something like this, if the support is not active:

          Python API Scanning Support   : not enabled (Missing 'pygccxml' Python module)

       In  this  case,  the  user must take steps to install castxml and pygccxml; castxml binary
       must be in the shell’s path, and pygccxml must be in the Python path.

   Rescan 64-bit
       It is important that you scan the failing module with ‘–no32bit-scan’; e.g.:

          $ cd source/ns-3-dev
          $ ./waf --apiscan=wifi --no32bit-scan

   Generate 32-bit
       Once the 64-bit bindings are fixed, if you are a  maintainer  and  need  to  generate  the
       32-bit equivalent:

       1. copy the newly scanned LP64.py file(s) to the ILP32.py filename

       2. on  the  ILP32.py  file(s),  perform global substitition of unsigned long with unsigned
          long long

       Linux (64-bit, as most modern installations use) and MacOS use different data  models,  as
       explained                                                                            here:
       https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.3.0/com.ibm.zos.v2r3.cbcpx01/datatypesize64.htm

       Linux uses the LP64 model, and MacOS (as well as 32-bit Linux) use the ILP32 model.  Users
       will note that there are two versions of bindings files in each ns-3 module directory; one
       with  an  ILP32.py  suffix  and one with an LP64.py suffix.  Only one is used on any given
       platform.  The main difference is in the representation of the  64  bit  integer  type  as
       either a ‘long’ (LP64) or ‘long long’ (ILP32).

       As  of  ns-3.27,  the  CastXML  framework  can  generate  LP64  bindings  by  passing  the
       --no32bit-scan flag.  However, the framework does not automatically generate  32bit  scans
       at  the  moment.  Instead, users must manually generate the ILP32.py equivalents by taking
       all instances of ‘unsigned long’ in the bindings file and  converting  them  to  ‘unsigned
       long long’, such as:

          -    cls.add_instance_attribute('nMarkedBytes', 'std::map< std::string, unsigned long >', is_const=False)
          +    cls.add_instance_attribute('nMarkedBytes', 'std::map< std::string, unsigned long long >', is_const=False)

       In  summary,  to generate LP64 bindings for Linux 64-bit systems, it is sufficient to call
       (e.g. for the core module):

          $ ./waf --apiscan=core --no32bit-scan

       To generate ILP32 bindings, one first must generate the LP64.py file as  above,  and  then
       copy the file to be named with an ILP32.py suffix, and then hand-edit that file, replacing
       all instances of ‘unsigned long’ with ‘unsigned long long’.  ns-3 maintainers are  working
       to better automate this process for future releases.

   Generating bindings on MacOS
       In  principle, this should work (and should generate the 32-bit bindings).  However, it is
       untested and we are not sure what instructions to offer.  We would welcome suggestions  on
       how to enable scanning for MacOS.

   Organization of the Modular Python Bindings
       The  src/<module>/bindings  directory  may  contain  the  following  files,  some  of them
       optional:

       · callbacks_list.py: this is a scanned file, DO NOT TOUCH.  Contains a list of Callback<…>
         template instances found in the scanned headers;

       · modulegen__gcc_LP64.py:  this  is a scanned file, DO NOT TOUCH.  Scanned API definitions
         for the GCC, LP64 architecture (64-bit)

       · modulegen__gcc_ILP32.py: this is a scanned file, DO NOT TOUCH.  Scanned API  definitions
         for the GCC, ILP32 architecture (32-bit)

       · modulegen_customizations.py:  you may optionally add this file in order to customize the
         pybindgen code generation

       · scan-header.h: you may optionally add this file to customize what header file is scanned
         for  the  module.   Basically  this  file  is  scanned instead of ns3/<module>-module.h.
         Typically, the first statement is  #include  “ns3/<module>-module.h”,  plus  some  other
         stuff to force template instantiations;

       · module_helpers.cc:  you  may  add additional files, such as this, to be linked to python
         extension  module,  but  they  have  to  be  registered  in   the   wscript.   Look   at
         src/core/wscript for an example of how to do so;

       · <module>.py:  if  this  file exists, it becomes the “frontend” python module for the ns3
         module, and the extension module (.so file) becomes _<module>.so instead of <module>.so.
         The  <module>.py  file has to import all symbols from the module _<module> (this is more
         tricky than it sounds, see src/core/bindings/core.py for an example), and then  can  add
         some additional pure-python definitions.

   More Information for Developers
       If you are a developer and need more information on ns-3’s Python bindings, please see the
       Python Bindings wiki page.

   Tests
   Overview
       This chapter is concerned with the testing and validation of ns-3 software.

       This chapter provides

       · background about terminology and software testing

       · a description of the ns-3 testing framework

       · a guide to model developers or new model contributors for how to write tests

   Background
       This chapter may be skipped by readers familiar with the basics of software testing.

       Writing defect-free software is a difficult proposition.  There are many dimensions to the
       problem  and  there  is  much  confusion  regarding  what  is  meant by different terms in
       different contexts.  We have found it worthwhile to spend  a  little  time  reviewing  the
       subject and defining some terms.

       Software  testing  may  be  loosely defined as the process of executing a program with the
       intent of finding errors.  When one enters a discussion  regarding  software  testing,  it
       quickly  becomes  apparent  that  there  are  many  distinct  mind-sets with which one can
       approach the subject.

       For  example,  one  could  break  the  process  into  broad  functional  categories   like
       ‘’correctness  testing,’’  ‘’performance  testing,’’ ‘’robustness testing’’ and ‘’security
       testing.’’  Another way to look at the problem is by life-cycle: ‘’requirements testing,’’
       ‘’design testing,’’ ‘’acceptance testing,’’ and ‘’maintenance testing.’’  Yet another view
       is by the scope of the tested system.  In this case one may  speak  of  ‘’unit  testing,’’
       ‘’component  testing,’’  ‘’integration testing,’’ and ‘’system testing.’’  These terms are
       also not standardized  in  any  way,  and  so  ‘’maintenance  testing’’  and  ‘’regression
       testing’’ may be heard interchangeably.  Additionally, these terms are often misused.

       There  are  also  a number of different philosophical approaches to software testing.  For
       example, some organizations advocate writing test programs  before  actually  implementing
       the  desired  software, yielding ‘’test-driven development.’’  Some organizations advocate
       testing from a customer perspective as soon as possible, following  a  parallel  with  the
       agile  development  process:  ‘’test  early  and  test  often.’’  This is sometimes called
       ‘’agile testing.’’  It seems that there is at least one  approach  to  testing  for  every
       development methodology.

       The  ns-3 project is not in the business of advocating for any one of these processes, but
       the project as a whole has requirements that help inform the test process.

       Like all major software products, ns-3 has a number of qualities that must be present  for
       the  product to succeed.  From a testing perspective, some of these qualities that must be
       addressed  are  that  ns-3  must   be   ‘’correct,’’   ‘’robust,’’    ‘’performant’’   and
       ‘’maintainable.’’   Ideally  there should be metrics for each of these dimensions that are
       checked by the tests to identify when  the  product  fails  to  meet  its  expectations  /
       requirements.

   Correctness
       The  essential  purpose  of  testing  is  to  determine  that  a piece of software behaves
       ‘’correctly.’’  For ns-3 this means that if we simulate something, the  simulation  should
       faithfully  represent  some  physical  entity  or  process  to  a  specified  accuracy and
       precision.

       It turns out that there  are  two  perspectives  from  which  one  can  view  correctness.
       Verifying  that  a  particular  model  is  implemented  according  to its specification is
       generically called verification.  The process of deciding that the model  is  correct  for
       its intended use is generically called validation.

   Validation and Verification
       A  computer  model  is  a  mathematical  or  logical  representation  of something. It can
       represent a vehicle, an elephant (see David Harel’s talk about  modeling  an  elephant  at
       SIMUTools  2009, or a networking card.  Models can also represent processes such as global
       warming, freeway traffic flow or a specification of a networking protocol.  Models can  be
       completely   faithful  representations  of  a  logical  process  specification,  but  they
       necessarily can never completely simulate a physical object or process.  In most cases,  a
       number  of  simplifications  are  made  to  the  model  to make simulation computationally
       tractable.

       Every model has a target system that it is attempting to  simulate.   The  first  step  in
       creating  a simulation model is to identify this target system and the level of detail and
       accuracy that the simulation is desired to reproduce.  In the case of a  logical  process,
       the  target  system  may be identified as ‘’TCP as defined by RFC 793.’’  In this case, it
       will probably be desirable to create a model that completely and faithfully reproduces RFC
       793.   In  the  case of a physical process this will not be possible. If, for example, you
       would like to simulate a wireless networking card, you may determine that you need,   ‘’an
       accurate  MAC-level  implementation  of  the  802.11  specification  and […] a not-so-slow
       PHY-level model of the 802.11a specification.’‘

       Once this is done, one can develop an abstract  model  of  the  target  system.   This  is
       typically  an exercise in managing the tradeoffs between complexity, resource requirements
       and accuracy.  The  process  of  developing  an  abstract  model  has  been  called  model
       qualification in the literature.  In the case of a TCP protocol, this process results in a
       design for a collection of objects, interactions and behaviors that will  fully  implement
       RFC  793  in  ns-3.  In the case of the wireless card, this process results in a number of
       tradeoffs to allow the physical layer to be simulated and the design of a  network  device
       and channel for ns-3, along with the desired objects, interactions and behaviors.

       This  abstract  model  is  then  developed into an ns-3 model that implements the abstract
       model as a computer program.  The process of getting the implementation to agree with  the
       abstract model is called model verification in the literature.

       The process so far is open loop. What remains is to make a determination that a given ns-3
       model has some connection to some reality – that a model is an accurate representation  of
       a real system, whether a logical process or a physical entity.

       If one is going to use a simulation model to try and predict how some real system is going
       to behave, there must be some reason to believe your results – i.e., can one trust that an
       inference  made  from  the model translates into a correct prediction for the real system.
       The process of getting the ns-3 model behavior to agree with  the  desired  target  system
       behavior  as  defined by the model qualification process is called model validation in the
       literature. In the case of a TCP implementation, you may want to compare the  behavior  of
       your  ns-3 TCP model to some reference implementation in order to validate your model.  In
       the case of a wireless physical layer simulation, you may want to compare the behavior  of
       your model to that of real hardware in a controlled setting,

       The  ns-3  testing  environment  provides  tools  to  allow  for both model validation and
       testing, and encourages the publication of validation results.

   Robustness
       Robustness is the quality of being able to withstand stresses, or changes in environments,
       inputs  or  calculations,  etc.  A system or design is ‘’robust’’ if it can deal with such
       changes with minimal loss of functionality.

       This kind of testing is usually done with a particular focus.  For example, the system  as
       a  whole  can  be  run  on many different system configurations to demonstrate that it can
       perform correctly in a large number of environments.

       The system can be also be stressed by operating close to or beyond capacity by  generating
       or  simulating  resource  exhaustion  of  various  kinds.  This genre of testing is called
       ‘’stress testing.’‘

       The system and its components may be exposed to so-called ‘’clean tests’’ that demonstrate
       a  positive  result  –  that  is that the system operates correctly in response to a large
       variation of expected configurations.

       The system and its components may also be exposed to ‘’dirty tests’’ which provide  inputs
       outside  the  expected  range.   For example, if a module expects a zero-terminated string
       representation of an integer, a dirty test might provide an unterminated string of  random
       characters  to verify that the system does not crash as a result of this unexpected input.
       Unfortunately, detecting such ‘’dirty’’ input and taking preventive measures to ensure the
       system  does  not fail catastrophically can require a huge amount of development overhead.
       In order to reduce development time, a decision was taken  early  on  in  the  project  to
       minimize  the amount of parameter validation and error handling in the ns-3 codebase.  For
       this reason, we do not spend much time on dirty  testing  –  it  would  just  uncover  the
       results of the design decision we know we took.

       We  do want to demonstrate that ns-3 software does work across some set of conditions.  We
       borrow a couple of definitions to narrow this down a bit.  The domain of applicability  is
       a  set  of  prescribed  conditions  for  which the model has been tested, compared against
       reality to the extent possible, and judged  suitable for use.  The range of accuracy is an
       agreement between the computerized model and reality within a domain of applicability.

       The  ns-3  testing  environment  provides  tools  to allow for setting up and running test
       environments over multiple systems (buildbot) and  provides  classes  to  encourage  clean
       tests  to verify the operation of the system over the expected ‘’domain of applicability’’
       and ‘’range of accuracy.’‘

   Performant
       Okay, ‘’performant’’ isn’t a real English word.  It is, however, a very concise  neologism
       that  is quite often used to describe what we want ns-3 to be: powerful and fast enough to
       get the job done.

       This is really about the broad subject of software performance testing.  One  of  the  key
       things  that  is  done  is  to  compare  two  systems  to  find  which performs better (cf
       benchmarks).  This is used to demonstrate that, for example, ns-3 can perform a basic kind
       of  simulation  at  least as fast as a competing tool, or can be used to identify parts of
       the system that perform badly.

       In the ns-3 test framework, we provide support for timing various kinds of tests.

   Maintainability
       A software product must be maintainable.  This is, again, a very broad  statement,  but  a
       testing  framework can help with the task.  Once a model has been developed, validated and
       verified, we can repeatedly execute the suite of tests for the  entire  system  to  ensure
       that it remains valid and verified over its lifetime.

       When  a  feature  stops functioning as intended after some kind of change to the system is
       integrated, it is  called  generically  a  regression.   Originally  the  term  regression
       referred  to  a  change  that  caused a previously fixed bug to reappear, but the term has
       evolved to describe any kind of change that breaks existing functionality.  There are many
       kinds of regressions that may occur in practice.

       A  local  regression is one in which a change affects the changed component directly.  For
       example, if a component is modified to allocate and free memory  but  stale  pointers  are
       used, the component itself fails.

       A  remote  regression  is  one  in which a change to one component breaks functionality in
       another component.  This reflects  violation  of  an  implied  but  possibly  unrecognized
       contract between components.

       An  unmasked  regression  is  one that creates a situation where a previously existing bug
       that had no affect is suddenly exposed in the system.  This may be as simple as exercising
       a code path for the first time.

       A  performance regression is one that causes the performance requirements of the system to
       be violated.  For example, doing some work in a low level function that  may  be  repeated
       large numbers of times may suddenly render the system unusable from certain perspectives.

       The  ns-3 testing framework provides tools for automating the process used to validate and
       verify the code in nightly test suites to help quickly identify possible regressions.

   Testing framework
       ns-3 consists of a simulation core engine, a set of models, example programs,  and  tests.
       Over time, new contributors contribute models, tests, and examples.  A Python test program
       test.py serves as the test execution manager; test.py can run test code  and  examples  to
       look  for  regressions, can output the results into a number of forms, and can manage code
       coverage analysis tools.  On top of this, we layer buildslaves that  are  automated  build
       robots  that perform robustness testing by running the test framework on different systems
       and with different configuration options.

   Buildslaves
       At the highest level of ns-3 testing are the  buildslaves  (build  robots).   If  you  are
       unfamiliar  with this system look at https://ns-buildmaster.ee.washington.edu:8010/.  This
       is an open-source automated system that allows ns-3 to be rebuilt and  tested  daily.   By
       running  the buildbots on a number of different systems we can ensure that ns-3 builds and
       executes properly on all of its supported systems.

       Users (and developers) typically will not interact with the buildslave system  other  than
       to  read  its  messages  regarding  test  results.  If a failure is detected in one of the
       automated build and test jobs, the buildbot will send an email to the  ns-commits  mailing
       list.  This email will look something like

       In  the  full  details  URL  shown  in  the email, one can find links to the detailed test
       output.

       The buildslave system will do its job quietly if there are no errors, and the system  will
       undergo build and test cycles every day to verify that all is well.

   Test.py
       The  buildbots  use  a Python program, test.py, that is responsible for running all of the
       tests and collecting the resulting reports into a human- readable form.  This  program  is
       also available for use by users and developers as well.

       test.py  is  very flexible in allowing the user to specify the number and kind of tests to
       run; and also the amount and kind of output to generate.

       Before running test.py, make sure that ns3’s examples and tests have been built  by  doing
       the following

          $ ./waf configure --enable-examples --enable-tests
          $ ./waf build

       By  default, test.py will run all available tests and report status back in a very concise
       form.  Running the command

          $ ./test.py

       will result in a number of PASS, FAIL, CRASH or SKIP indications followed by the  kind  of
       test that was run and its display name.

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone-test/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone-test/ns-3-dev/build'
          'build' finished successfully (0.939s)
          FAIL: TestSuite propagation-loss-model
          PASS: TestSuite object-name-service
          PASS: TestSuite pcap-file-object
          PASS: TestSuite ns3-tcp-cwnd
          ...
          PASS: TestSuite ns3-tcp-interoperability
          PASS: Example csma-broadcast
          PASS: Example csma-multicast

       This  mode  is  intended  to  be  used by users who are interested in determining if their
       distribution is working correctly, and by developers who are interested in determining  if
       changes they have made have caused any regressions.

       There  are  a  number of options available to control the behavior of test.py.  if you run
       test.py --help you should see a command summary like:

          Usage: test.py [options]

          Options:
            -h, --help            show this help message and exit
            -b BUILDPATH, --buildpath=BUILDPATH
                                  specify the path where ns-3 was built (defaults to the
                                  build directory for the current variant)
            -c KIND, --constrain=KIND
                                  constrain the test-runner by kind of test
            -e EXAMPLE, --example=EXAMPLE
                                  specify a single example to run (no relative path is
                                  needed)
            -d, --duration        print the duration of each test suite and example
            -e EXAMPLE, --example=EXAMPLE
                                  specify a single example to run (no relative path is
                                  needed)
            -u, --update-data     If examples use reference data files, get them to re-
                                  generate them
            -f FULLNESS, --fullness=FULLNESS
                                  choose the duration of tests to run: QUICK, EXTENSIVE,
                                  or TAKES_FOREVER, where EXTENSIVE includes QUICK and
                                  TAKES_FOREVER includes QUICK and EXTENSIVE (only QUICK
                                  tests are run by default)
            -g, --grind           run the test suites and examples using valgrind
            -k, --kinds           print the kinds of tests available
            -l, --list            print the list of known tests
            -m, --multiple        report multiple failures from test suites and test
                                  cases
            -n, --nowaf           do not run waf before starting testing
            -p PYEXAMPLE, --pyexample=PYEXAMPLE
                                  specify a single python example to run (with relative
                                  path)
            -r, --retain          retain all temporary files (which are normally
                                  deleted)
            -s TEST-SUITE, --suite=TEST-SUITE
                                  specify a single test suite to run
            -t TEXT-FILE, --text=TEXT-FILE
                                  write detailed test results into TEXT-FILE.txt
            -v, --verbose         print progress and informational messages
            -w HTML-FILE, --web=HTML-FILE, --html=HTML-FILE
                                  write detailed test results into HTML-FILE.html
            -x XML-FILE, --xml=XML-FILE
                                  write detailed test results into XML-FILE.xml

       If one specifies an optional output style, one can generate detailed descriptions  of  the
       tests and status.  Available styles are text and HTML.  The buildbots will select the HTML
       option to generate HTML test reports for the nightly builds using

          $ ./test.py --html=nightly.html

       In this case, an HTML file named ‘’nightly.html’’ would be created with a  pretty  summary
       of the testing done.  A ‘’human readable’’ format is available for users interested in the
       details.

          $ ./test.py --text=results.txt

       In the example above, the test suite checking the ns-3 wireless  device  propagation  loss
       models failed.  By default no further information is provided.

       To  further  explore  the  failure,  test.py  allows  a single test suite to be specified.
       Running the command

          $ ./test.py --suite=propagation-loss-model

       or equivalently

          $ ./test.py -s propagation-loss-model

       results in that single test suite being run.

          FAIL: TestSuite propagation-loss-model

       To find detailed information regarding the failure, one must specify the  kind  of  output
       desired.  For example, most people will probably be interested in a text file:

          $ ./test.py --suite=propagation-loss-model --text=results.txt

       This  will  result in that single test suite being run with the test status written to the
       file ‘’results.txt’‘.

       You should find something similar to the following in that file

          FAIL: Test Suite ''propagation-loss-model'' (real 0.02 user 0.01 system 0.00)
          PASS: Test Case "Check ... Friis ... model ..." (real 0.01 user 0.00 system 0.00)
          FAIL: Test Case "Check ... Log Distance ... model" (real 0.01 user 0.01 system 0.00)
            Details:
              Message:   Got unexpected SNR value
              Condition: [long description of what actually failed]
              Actual:    176.395
              Limit:     176.407 +- 0.0005
              File:      ../src/test/ns3wifi/propagation-loss-models-test-suite.cc
              Line:      360

       Notice that the Test Suite is composed of two Test Cases.  The first test case checked the
       Friis  propagation  loss  model  and passed.  The second test case failed checking the Log
       Distance propagation model.  In this case, an SNR of  176.395  was  found,  and  the  test
       expected  a  value of 176.407 correct to three decimal places.  The file which implemented
       the failing test is listed as well as the line of code which triggered the failure.

       If you desire, you could just as easily have written an HTML file using the --html  option
       as described above.

       Typically  a  user  will run all tests at least once after downloading ns-3 to ensure that
       his or her environment  has  been  built  correctly  and  is  generating  correct  results
       according  to  the  test suites.  Developers will typically run the test suites before and
       after making a change to ensure that they have not  introduced  a  regression  with  their
       changes.   In this case, developers may not want to run all tests, but only a subset.  For
       example, the developer might only want to run the unit  tests  periodically  while  making
       changes  to  a  repository.   In  this case, test.py can be told to constrain the types of
       tests being run to a particular class of tests.  The following command will result in only
       the unit tests being run:

          $ ./test.py --constrain=unit

       Similarly, the following command will result in only the example smoke tests being run:

          $ ./test.py --constrain=unit

       To  see a quick list of the legal kinds of constraints, you can ask for them to be listed.
       The following command

          $ ./test.py --kinds

       will result in the following list being displayed:

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone-test/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone-test/ns-3-dev/build'
          'build' finished successfully (0.939s)Waf: Entering directory `/home/craigdo/repos/ns-3-allinone-test/ns-3-dev/build'
          bvt:         Build Verification Tests (to see if build completed successfully)
          core:        Run all TestSuite-based tests (exclude examples)
          example:     Examples (to see if example programs run successfully)
          performance: Performance Tests (check to see if the system is as fast as expected)
          system:      System Tests (spans modules to check integration of modules)
          unit:        Unit Tests (within modules to check basic functionality)

       Any of these kinds of test can be provided as a constraint using the --constraint option.

       To see a quick list of all of the test suites available,  you  can  ask  for  them  to  be
       listed.  The following command,

          $ ./test.py --list

       will result in a list of the test suite being displayed, similar to

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone-test/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone-test/ns-3-dev/build'
          'build' finished successfully (0.939s)

          Test Type    Test Name
          ---------    ---------
          performance  many-uniform-random-variables-one-get-value-call
          performance  one-uniform-random-variable-many-get-value-calls
          performance  type-id-perf
          system       buildings-pathloss-test
          system       buildings-shadowing-test
          system       devices-mesh-dot11s-regression
          system       devices-mesh-flame-regression
          system       epc-gtpu
          ...
          unit         wimax-phy-layer
          unit         wimax-service-flow
          unit         wimax-ss-mac-layer
          unit         wimax-tlv
          example      adhoc-aloha-ideal-phy
          example      adhoc-aloha-ideal-phy-matrix-propagation-loss-model
          example      adhoc-aloha-ideal-phy-with-microwave-oven
          example      aodv
          ...

       Any of these listed suites can be selected to be run by itself using the --suite option as
       shown above.  Examples are handled differently.

       Similarly to test suites, one can run a single C++ example  program  using  the  --example
       option.  Note that the relative path for the example does not need to be included and that
       the executables built for C++ examples do not have extensions.  Furthermore,  the  example
       must  be registered as an example to the test framework; it is not sufficient to create an
       example and run it through test.py; it must be added to  the  relevant  examples-to-run.py
       file, explained below.  Entering

          $ ./test.py --example=udp-echo

       results in that single example being run.

          PASS: Example examples/udp/udp-echo

       You  can  specify  the  directory  where  ns-3  was  built using the --buildpath option as
       follows.

          $ ./test.py --buildpath=/home/craigdo/repos/ns-3-allinone-test/ns-3-dev/build/debug --example=wifi-simple-adhoc

       One can run a single Python example program using the --pyexample option.  Note  that  the
       relative  path  for  the  example  must be included and that Python examples do need their
       extensions.  Entering

          $ ./test.py --pyexample=examples/tutorial/first.py

       results in that single example being run.

          PASS: Example examples/tutorial/first.py

       Because Python examples are not built, you do not need to specify the directory where ns-3
       was built to run them.

       Normally when example programs are executed, they write a large amount of trace file data.
       This  is  normally  saved   to   the   base   directory   of   the   distribution   (e.g.,
       /home/user/ns-3-dev).   When  test.py runs an example, it really is completely unconcerned
       with the trace files.  It just wants to to determine if the example can be built  and  run
       without   error.    Since   this  is  the  case,  the  trace  files  are  written  into  a
       /tmp/unchecked-traces directory.  If you run the above example, you should be able to find
       the associated udp-echo.tr and udp-echo-n-1.pcap files there.

       The list of available examples is defined by the contents of the ‘’examples’’ directory in
       the distribution.  If you select an example for  execution  using  the  --example  option,
       test.py  will not make any attempt to decide if the example has been configured or not, it
       will just try to run it and report the result of the attempt.

       When test.py runs, by default it will first ensure that the  system  has  been  completely
       built.  This can be defeated by selecting the --nowaf option.

          $ ./test.py --list --nowaf

       will result in a list of the currently built test suites being displayed, similar to:

          propagation-loss-model
          ns3-tcp-cwnd
          ns3-tcp-interoperability
          pcap-file
          object-name-service
          random-variable-stream-generators

       Note the absence of the Waf build messages.

       test.py  also supports running the test suites and examples under valgrind.  Valgrind is a
       flexible program for debugging and profiling Linux executables.  By default, valgrind runs
       a  tool  called  memcheck, which performs a range of memory- checking functions, including
       detecting accesses to uninitialised memory, misuse  of  allocated  memory  (double  frees,
       access  after  free,  etc.) and detecting memory leaks.  This can be selected by using the
       --grind option.

          $ ./test.py --grind

       As it runs, test.py and the programs that it runs indirectly, generate  large  numbers  of
       temporary  files.  Usually, the content of these files is not interesting, however in some
       cases it can be useful (for debugging purposes) to view these files.  test.py  provides  a
       --retain  option  which  will  cause  these  temporary  files  to be kept after the run is
       completed.  The files are saved in a directory named testpy-output  under  a  subdirectory
       named  according  to  the current Coordinated Universal Time (also known as Greenwich Mean
       Time).

          $ ./test.py --retain

       Finally, test.py provides a --verbose option which will print large amounts of information
       about  its progress.  It is not expected that this will be terribly useful unless there is
       an error.  In this case, you can get access to the  standard  output  and  standard  error
       reported by running test suites and examples.  Select verbose in the following way:

          $ ./test.py --verbose

       All  of  these options can be mixed and matched.  For example, to run all of the ns-3 core
       test suites under valgrind, in verbose mode, while generating an  HTML  output  file,  one
       would do:

          $ ./test.py --verbose --grind --constrain=core --html=results.html

   TestTaxonomy
       As  mentioned above, tests are grouped into a number of broadly defined classifications to
       allow users to selectively run tests to address the different kinds of testing  that  need
       to be done.

       · Build Verification Tests

       · Unit Tests

       · System Tests

       · Examples

       · Performance Tests

       Moreover, each test is further classified according to the expected time needed to run it.
       Tests are classified as:

       · QUICK

       · EXTENSIVE

       · TAKES_FOREVER

       Note that specifying EXTENSIVE fullness will also run tests in QUICK category.  Specifying
       TAKES_FOREVER  will  run  tests in EXTENSIVE and QUICK categories.  By default, only QUICK
       tests are ran.

       As a rule of thumb, tests that must be run to ensure ns-3 coherence should be QUICK (i.e.,
       take  a  few  seconds).  Tests that could be skipped, but are nice to do can be EXTENSIVE;
       these are tests that typically need minutes. TAKES_FOREVER is left for tests that  take  a
       really  long time, in the order of several minutes.  The main classification goal is to be
       able to run the buildbots in a  reasonable  time,  and  still  be  able  to  perform  more
       extensive tests when needed.

   BuildVerificationTests
       These  are relatively simple tests that are built along with the distribution and are used
       to make sure that the build is pretty much working.  Our current unit tests  live  in  the
       source  files  of  the  code they test and are built into the ns-3 modules; and so fit the
       description of BVTs.  BVTs live in the same source code that is built into the ns-3  code.
       Our current tests are examples of this kind of test.

   Unit Tests
       Unit  tests  are more involved tests that go into detail to make sure that a piece of code
       works as advertised in isolation.  There is really no reason for this kind of test  to  be
       built  into an ns-3 module.  It turns out, for example, that the unit tests for the object
       name service are about the same size as the object name service code itself.   Unit  tests
       are  tests that check a single bit of functionality that are not built into the ns-3 code,
       but live in the same directory as the code it tests.  It  is  possible  that  these  tests
       check  integration  of  multiple  implementation  files  in  a  module  as well.  The file
       src/core/test/names-test-suite.cc  is  an  example  of  this  kind  of  test.   The   file
       src/network/test/pcap-file-test-suite.cc  is  another  example that uses a known good pcap
       file as a test vector file.  This file is stored locally in the src/network directory.

   System Tests
       System tests are those that involve more than one module in the system.  We have  lots  of
       this  kind  of  test  running  in our current regression framework, but they are typically
       overloaded examples.  We provide a new place for  this  kind  of  test  in  the  directory
       src/test.   The  file src/test/ns3tcp/ns3-interop-test-suite.cc is an example of this kind
       of test.  It uses NSC TCP to test the ns-3 TCP implementation.  Often there will  be  test
       vectors  required  for  this  kind of test, and they are stored in the directory where the
       test lives.  For example, ns3tcp-interop-response-vectors.pcap is a file consisting  of  a
       number  of  TCP headers that are used as the expected responses of the ns-3 TCP under test
       to a stimulus generated by the NSC TCP which is used as a ‘’known good’’ implementation.

   Examples
       The examples are tested by the framework to make sure they built and will run.  Nothing is
       checked,  and currently the pcap files are just written off into /tmp to be discarded.  If
       the examples run (don’t crash) they pass this smoke test.

   Performance Tests
       Performance tests are those which exercise a particular part of the system  and  determine
       if the tests have executed to completion in a reasonable time.

   Running Tests
       Tests  are  typically  run  using  the  high  level  test.py program. To get a list of the
       available command-line options, run test.py --help

       The test program test.py will run both tests and those examples that have  been  added  to
       the  list  to  check.   The  difference  between  tests and examples is as follows.  Tests
       generally check that specific simulation output or events conforms to  expected  behavior.
       In contrast, the output of examples is not checked, and the test program merely checks the
       exit status of the example program to make sure that it runs without error.

       Briefly, to run all tests, first one must configure tests during configuration stage,  and
       also (optionally) examples if examples are to be checked:

          $ ./waf --configure --enable-examples --enable-tests

       Then,  build ns-3, and after it is built, just run test.py.  test.py -h will show a number
       of configuration options that modify the behavior of test.py.

       The program test.py invokes, for C++ tests and examples, a lower-level C++ program  called
       test-runner  to  actually  run  the  tests.  As discussed below, this test-runner can be a
       helpful way to debug tests.

   Debugging Tests
       The debugging of the test programs is best performed  running  the  low-level  test-runner
       program.  The  test-runner  is  the  bridge  from  generic Python code to ns-3 code. It is
       written in C++ and uses the automatic test discovery process in the ns-3 code to find  and
       allow execution of all of the various tests.

       The  main  reason  why test.py is not suitable for debugging is that it is not allowed for
       logging to be turned on using the NS_LOG environmental variable when test.py  runs.   This
       limitation does not apply to the test-runner executable. Hence, if you want to see logging
       output from your tests, you have to run them using the test-runner directly.

       In order to execute the test-runner, you run it like any other  ns-3  executable  –  using
       waf.  To get a list of available options, you can type:

          $ ./waf --run "test-runner --help"

       You should see something like the following

          Usage: /home/craigdo/repos/ns-3-allinone-test/ns-3-dev/build/utils/ns3-dev-test-runner-debug [OPTIONS]

          Options:
          --help                 : print these options
          --print-test-name-list : print the list of names of tests available
          --list                 : an alias for --print-test-name-list
          --print-test-types     : print the type of tests along with their names
          --print-test-type-list : print the list of types of tests available
          --print-temp-dir       : print name of temporary directory before running
                                   the tests
          --test-type=TYPE       : process only tests of type TYPE
          --test-name=NAME       : process only test whose name matches NAME
          --suite=NAME           : an alias (here for compatibility reasons only)
                                   for --test-name=NAME
          --assert-on-failure    : when a test fails, crash immediately (useful
                                   when running under a debugger
          --stop-on-failure      : when a test fails, stop immediately
          --fullness=FULLNESS    : choose the duration of tests to run: QUICK,
                                   EXTENSIVE, or TAKES_FOREVER, where EXTENSIVE
                                   includes QUICK and TAKES_FOREVER includes
                                   QUICK and EXTENSIVE (only QUICK tests are
                                   run by default)
          --verbose              : print details of test execution
          --xml                  : format test run output as xml
          --tempdir=DIR          : set temp dir for tests to store output files
          --datadir=DIR          : set data dir for tests to read reference files
          --out=FILE             : send test result to FILE instead of standard output
          --append=FILE          : append test result to FILE instead of standard output

       There  are  a  number of things available to you which will be familiar to you if you have
       looked at test.py.  This should be expected since the test- runner is  just  an  interface
       between  test.py and ns-3.  You may notice that example-related commands are missing here.
       That is because the examples are really not ns-3 tests.  test.py runs them as if they were
       to present a unified testing environment, but they are really completely different and not
       to be found here.

       The first new option that appears here, but not  in  test.py  is  the  --assert-on-failure
       option.   This  option  is useful when debugging a test case when running under a debugger
       like gdb.  When  selected,  this  option  tells  the  underlying  test  case  to  cause  a
       segmentation  violation if an error is detected.  This has the nice side-effect of causing
       program execution to stop (break into the debugger) when an error is detected.  If you are
       using gdb, you could use this option something like,

          $ ./waf shell
          $ cd build/utils
          $ gdb ns3-dev-test-runner-debug
          $ run --suite=global-value --assert-on-failure

       If  an  error  is then found in the global-value test suite, a segfault would be generated
       and the (source level) debugger would stop at the  NS_TEST_ASSERT_MSG  that  detected  the
       error.

       To  run one of the tests directly from the test-runner using waf, you will need to specify
       the test suite to run.  So you could use the shell and do:

          $ ./waf --run "test-runner --suite=pcap-file"

       ns-3 logging is available when you run it this way, such as:
          $ NS_LOG=”Packet” ./waf –run “test-runner –suite=pcap-file”

   Test output
       Many test suites need to write temporary files (such as pcap  files)  in  the  process  of
       running  the  tests.   The  tests then need a temporary directory to write to.  The Python
       test utility (test.py) will provide a temporary file automatically, but if run stand-alone
       this  temporary  directory  must  be  provided.  It can be annoying to continually have to
       provide a --tempdir, so the test runner will figure one out for you if you  don’t  provide
       one.   It  first  looks  for  environment variables named TMP and TEMP and uses those.  If
       neither TMP nor TEMP are defined it picks /tmp.  The code  then  tacks  on  an  identifier
       indicating  what created the directory (ns-3) then the time (hh.mm.ss) followed by a large
       random number.  The test runner creates a directory  of  that  name  to  be  used  as  the
       temporary  directory.   Temporary  files  then  go  into  a  directory  that will be named
       something like

          /tmp/ns-3.10.25.37.61537845

       The time is provided as a  hint  so  that  you  can  relatively  easily  reconstruct  what
       directory  was  used if you need to go back and look at the files that were placed in that
       directory.

       Another class of output is test output like pcap traces that are generated to  compare  to
       reference  output.  The test program will typically delete these after the test suites all
       run.  To disable the deletion of test output, run test.py with the “retain” option:

          $ ./test.py -r

       and test output can be found in the testpy-output/ directory.

   Reporting of test failures
       When you run a test suite using the test-runner it will run the test and  report  PASS  or
       FAIL.   To  run  more  quietly, you need to specify an output file to which the tests will
       write their status using the --out option.  Try,

          $ ./waf --run "test-runner --suite=pcap-file --out=myfile.txt"

   Debugging test suite failures
       To debug test crashes, such as

          CRASH: TestSuite ns3-wifi-interference

       You can access the underlying test-runner program via gdb as follows, and  then  pass  the
       “–basedir=`pwd`” argument to run (you can also pass other arguments as needed, but basedir
       is the minimum needed):

          $ ./waf --command-template="gdb %s" --run "test-runner"
          Waf: Entering directory `/home/tomh/hg/sep09/ns-3-allinone/ns-3-dev-678/build'
          Waf: Leaving directory `/home/tomh/hg/sep09/ns-3-allinone/ns-3-dev-678/build'
          'build' finished successfully (0.380s)
          GNU gdb 6.8-debian
          Copyright (C) 2008 Free Software Foundation, Inc.
          L cense GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
          This is free software: you are free to change and redistribute it.
          There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
          and "show warranty" for details.
          This GDB was configured as "x86_64-linux-gnu"...
          (gdb) r --suite=
          Starting program: <..>/build/utils/ns3-dev-test-runner-debug --suite=ns3-wifi-interference
          [Thread debugging using libthread_db enabled]
          assert failed. file=../src/core/model/type-id.cc, line=138, cond="uid <= m_information.size () && uid != 0"
          ...

       Here is another example of how to use valgrind to debug a memory problem such as:

          VALGR: TestSuite devices-mesh-dot11s-regression

          $ ./waf --command-template="valgrind %s --suite=devices-mesh-dot11s-regression" --run test-runner

   Class TestRunner
       The executables that run dedicated test programs  use  a  TestRunner  class.   This  class
       provides  for  automatic  test  registration  and listing, as well as a way to execute the
       individual tests.  Individual test suites use C++ global constructors to add themselves to
       a  collection  of test suites managed by the test runner.  The test runner is used to list
       all of the available tests and to select a test to be run.  This is a quite  simple  class
       that  provides  three  static  methods  to  provide or Adding and Getting test suites to a
       collection of tests.  See the doxygen for class ns3::TestRunner for details.

   Test Suite
       All ns-3 tests are classified into Test  Suites  and  Test  Cases.   A  test  suite  is  a
       collection  of  test  cases  that  completely  exercise a given kind of functionality.  As
       described above, test suites can be classified as,

       · Build Verification Tests

       · Unit Tests

       · System Tests

       · Examples

       · Performance Tests

       This classification is exported from the TestSuite class.  This  class  is  quite  simple,
       existing  only  as  a place to export this type and to accumulate test cases.  From a user
       perspective, in order to create a new TestSuite in the system one only has to define a new
       class that inherits from class TestSuite and perform these two duties.

       The  following  code will define a new class that can be run by test.py as a ‘’unit’’ test
       with the display name, my-test-suite-name.

          class MySuite : public TestSuite
          {
          public:
            MyTestSuite ();
          };

          MyTestSuite::MyTestSuite ()
            : TestSuite ("my-test-suite-name", UNIT)
          {
            AddTestCase (new MyTestCase, TestCase::QUICK);
          }

          static MyTestSuite myTestSuite;

       The base class takes care of all of the registration and reporting required to be  a  good
       citizen in the test framework.

       Avoid putting initialization logic into the test suite or test case constructors.  This is
       because an instance of the test suite is created at run time (due to the  static  variable
       above) regardless of whether the test is being run or not.  Instead, the TestCase provides
       a virtual DoSetup method that can be specialized to perform setup before DoRun is called.

   Test Case
       Individual tests are created using a TestCase class.  Common models for the use of a  test
       case  include  “one  test  case per feature”, and “one test case per method.”  Mixtures of
       these models may be used.

       In order to create a new test case in the system, all one has to do is to inherit from the
       TestCase  base  class,  override the constructor to give the test case a name and override
       the DoRun method to run the test.  Optionally, override also the DoSetup method.

          class MyTestCase : public TestCase
          {
            MyTestCase ();
            virtual void DoSetup (void);
            virtual void DoRun (void);
          };

          MyTestCase::MyTestCase ()
            : TestCase ("Check some bit of functionality")
          {
          }

          void
          MyTestCase::DoRun (void)
          {
            NS_TEST_ASSERT_MSG_EQ (true, true, "Some failure message");
          }

   Utilities
       There are a number of utilities of various  kinds  that  are  also  part  of  the  testing
       framework.   Examples  include  a generalized pcap file useful for storing test vectors; a
       generic container useful for transient storage of test vectors during test execution;  and
       tools for generating presentations based on validation and verification testing results.

       These  utilities  are  not  documented here, but for example, please see how the TCP tests
       found in src/test/ns3tcp/ use pcap files and reference output.

   How to write tests
       A primary goal of the  ns-3  project  is  to  help  users  to  improve  the  validity  and
       credibility  of  their  results.   There  are  many elements to obtaining valid models and
       simulations, and testing is a major component.  If you contribute models  or  examples  to
       ns-3,  you  may be asked to contribute test code.  Models that you contribute will be used
       for many years by other people, who probably have no idea upon first  glance  whether  the
       model  is  correct.  The test code that you write for your model will help to avoid future
       regressions in the output and will aid future users in understanding the verification  and
       bounds of applicability of your models.

       There  are  many  ways  to  verify  the  correctness of a model’s implementation.  In this
       section, we hope to cover some common cases that can be used as a  guide  to  writing  new
       tests.

   Sample TestSuite skeleton
       When  starting  from  scratch (i.e. not adding a TestCase to an existing TestSuite), these
       things need to be decided up front:

       · What the test suite will be called

       · What type of test it will be (Build  Verification  Test,  Unit  Test,  System  Test,  or
         Performance Test)

       · Where  the  test  code  will  live  (either  in an existing ns-3 module or separately in
         src/test/ directory).  You will have to edit the  wscript  file  in  that  directory  to
         compile your new code, if it is a new file.

       A  program  called  src/create-module.py  is  a  good starting point.  This program can be
       invoked such as create-module.py router for a hypothetical new module called router.  Once
       you  do this, you will see a router directory, and a test/router-test-suite.cc test suite.
       This file can be a starting point for your initial test.  This is a  working  test  suite,
       although  the  actual  tests  performed  are  trivial.  Copy it over to your module’s test
       directory, and do a global substitution of “Router” in that file for something  pertaining
       to  the  model that you want to test.  You can also edit things such as a more descriptive
       test case name.

       You also need to add a block into your wscript to get this test to compile:

          module_test.source = [
              'test/router-test-suite.cc',
              ]

       Before you actually start making this do useful things, it may help  to  try  to  run  the
       skeleton.  Make sure that ns-3 has been configured with the “–enable-tests” option.  Let’s
       assume that your new test suite is called “router” such as here:

          RouterTestSuite::RouterTestSuite ()
            : TestSuite ("router", UNIT)

       Try this command:

          $ ./test.py -s router

       Output such as below should be produced:

          PASS: TestSuite router
          1 of 1 tests passed (1 passed, 0 skipped, 0 failed, 0 crashed, 0 valgrind errors)

       See src/lte/test/test-lte-antenna.cc for a worked example.

   Test macros
       There are a number of macros available for checking  test  program  output  with  expected
       output.  These macros are defined in src/core/model/test.h.

       The main set of macros that are used include the following:

          NS_TEST_ASSERT_MSG_EQ(actual, limit, msg)
          NS_TEST_ASSERT_MSG_NE(actual, limit, msg)
          NS_TEST_ASSERT_MSG_LT(actual, limit, msg)
          NS_TEST_ASSERT_MSG_GT(actual, limit, msg)
          NS_TEST_ASSERT_MSG_EQ_TOL(actual, limit, tol, msg)

       The  first argument actual is the value under test, the second value limit is the expected
       value (or the value to test against), and the last argument msg is the  error  message  to
       print out if the test fails.

       The  first  four  macros  above test for equality, inequality, less than, or greater than,
       respectively.  The fifth macro above tests for equality, but within a  certain  tolerance.
       This  variant  is useful when testing floating point numbers for equality against a limit,
       where you want to avoid a test failure due to rounding errors.

       Finally, there are variants of the above where the keyword ASSERT is replaced  by  EXPECT.
       These  variants are designed specially for use in methods (especially callbacks) returning
       void.  Reserve their use for callbacks that you use in your test programs; otherwise,  use
       the ASSERT variants.

   How to add an example program to the test suite
       One  can  “smoke  test”  that examples compile and run successfully to completion (without
       memory leaks) using the examples-to-run.py script located in your module’s test directory.
       Briefly,  by  including an instance of this file in your test directory, you can cause the
       test runner to execute the examples listed.  It is usually best  to  make  sure  that  you
       select examples that have reasonably short run times so as to not bog down the tests.  See
       the example in src/lte/test/ directory.

   Testing for boolean outcomes
   Testing outcomes when randomness is involved
   Testing output data against a known distribution
   Providing non-trivial input vectors of data
   Storing and referencing non-trivial output data
   Presenting your output test data
   Support
   Creating a new ns-3 model
       This chapter walks through the design process of an ns-3 model.  In many  research  cases,
       users  will  not  be satisfied to merely adapt existing models, but may want to extend the
       core of the simulator in a novel way. We will use the example of adding an ErrorModel to a
       simple  ns-3  link  as  a  motivating  example  of how one might approach this problem and
       proceed through a design and implementation.

       NOTE:
          Documentation

          Here we focus on the process of creating new models and new modules, and  some  of  the
          design choices involved.  For the sake of clarity, we defer discussion of the mechanics
          of documenting models and source code to the Documentation chapter.

   Design Approach
       Consider how you want it to work; what should it do. Think about these things:

       · functionality:  What functionality should it have?  What attributes or configuration  is
         exposed to the user?

       · reusability:   How much should others be able to reuse my design?  Can I reuse code from
         ns-2 to get started?  How does a user integrate the  model  with  the  rest  of  another
         simulation?

       · dependencies:   How can I reduce the introduction of outside dependencies on my new code
         as much as possible (to make it  more  modular)?   For  instance,  should  I  avoid  any
         dependence  on IPv4 if I want it to also be used by IPv6?  Should I avoid any dependency
         on IP at all?

       Do not be hesitant to contact the ns-3-users or ns-developers list if you have  questions.
       In particular, it is important to think about the public API of your new model and ask for
       feedback. It also helps to let others know of your work in  case  you  are  interested  in
       collaborators.

   Example: ErrorModel
       An  error  model  exists in ns-2. It allows packets to be passed to a stateful object that
       determines, based on a random variable, whether the packet is corrupted.  The  caller  can
       then decide what to do with the packet (drop it, etc.).

       The main API of the error model is a function to pass a packet to, and the return value of
       this function is a boolean that tells the caller whether any  corruption  occurred.   Note
       that  depending  on  the  error model, the packet data buffer may or may not be corrupted.
       Let’s call this function “IsCorrupt()”.

       So far, in our design, we have:

          class ErrorModel
          {
          public:
           /**
            * \returns true if the Packet is to be considered as errored/corrupted
            * \param pkt Packet to apply error model to
            */
            bool IsCorrupt (Ptr<Packet> pkt);
          };

       Note that we do not pass a const pointer, thereby allowing  the  function  to  modify  the
       packet  if IsCorrupt() returns true. Not all error models will actually modify the packet;
       whether or not the packet data buffer is corrupted should be documented.

       We may also want specialized versions of this, such as in ns-2, so although it is not  the
       only  design  choice  for  polymorphism,  we  assume  that  we  will subclass a base class
       ErrorModel for specialized classes, such as RateErrorModel, ListErrorModel, etc,  such  as
       is done in ns-2.

       You  may be thinking at this point, “Why not make IsCorrupt() a virtual method?”.  That is
       one approach; the other is to make the public  non-virtual  function  indirect  through  a
       private  virtual  function (this in C++ is known as the non virtual interface idiom and is
       adopted in the ns-3 ErrorModel class).

       Next, should this device have any dependencies on IP or other protocols?  We do  not  want
       to  create  dependencies  on  Internet  protocols (the error model should be applicable to
       non-Internet protocols too), so we’ll keep that in mind later.

       Another consideration is how objects will include this error model.  We  envision  putting
       an explicit setter in certain NetDevice implementations, for example.:

          /**
           * Attach a receive ErrorModel to the PointToPointNetDevice.
           *
           * The PointToPointNetDevice may optionally include an ErrorModel in
           * the packet receive chain.
           *
           * @see ErrorModel
           * @param em Ptr to the ErrorModel.
           */
          void PointToPointNetDevice::SetReceiveErrorModel(Ptr<ErrorModel> em);

       Again,  this  is  not the only choice we have (error models could be aggregated to lots of
       other objects), but it satisfies our primary use case, which is to allow a user  to  force
       errors on otherwise successful packet transmissions, at the NetDevice level.

       After  some  thinking  and  looking  at existing ns-2 code, here is a sample API of a base
       class and first subclass that could be posted for initial review:

          class ErrorModel
          {
          public:
            ErrorModel ();
            virtual ~ErrorModel ();
            bool IsCorrupt (Ptr<Packet> pkt);
            void Reset (void);
            void Enable (void);
            void Disable (void);
            bool IsEnabled (void) const;
          private:
            virtual bool DoCorrupt (Ptr<Packet> pkt) = 0;
            virtual void DoReset (void) = 0;
          };

          enum ErrorUnit
            {
              EU_BIT,
              EU_BYTE,
              EU_PKT
            };

          // Determine which packets are errored corresponding to an underlying
          // random variable distribution, an error rate, and unit for the rate.
          class RateErrorModel : public ErrorModel
          {
          public:
            RateErrorModel ();
            virtual ~RateErrorModel ();
            enum ErrorUnit GetUnit (void) const;
            void SetUnit (enum ErrorUnit error_unit);
            double GetRate (void) const;
            void SetRate (double rate);
            void SetRandomVariable (const RandomVariable &ranvar);
          private:
            virtual bool DoCorrupt (Ptr<Packet> pkt);
            virtual void DoReset (void);
          };

   Scaffolding
       Let’s say that you are ready to start implementing; you have a  fairly  clear  picture  of
       what you want to build, and you may have solicited some initial review or suggestions from
       the list.  One way to approach the next step (implementation) is to create scaffolding and
       fill in the details as the design matures.

       This section walks through many of the steps you should consider to define scaffolding, or
       a non-functional skeleton of what your model will eventually implement. It is usually good
       practice  to  not  wait to get these details integrated at the end, but instead to plumb a
       skeleton of your model into the system early and then add functions later once the API and
       integration seems about right.

       Note  that  you  will want to modify a few things in the below presentation for your model
       since if you follow the error model verbatim, the code you produce will collide  with  the
       existing  error  model.  The below is just an outline of how ErrorModel was built that you
       can adapt to other models.

   Review the ns-3 Coding Style Document
       At this point, you may want to pause and read the ns-3 coding style  document,  especially
       if  you  are  considering  to  contribute your code back to the project.  The coding style
       document is linked off the main project page: ns-3 coding style.

   Decide Where in the Source Tree the Model Should Reside
       All of the ns-3 model source code is in the directory src/.  You will need to choose which
       subdirectory it resides in. If it is new model code of some sort, it makes sense to put it
       into the src/ directory somewhere, particularly for ease of  integrating  with  the  build
       system.

       In  the case of the error model, it is very related to the packet class, so it makes sense
       to implement this in the src/network/ module where ns-3 packets are implemented.

   waf and wscript
       ns-3 uses the Waf build system.  You will want to integrate your new  ns-3  uses  the  Waf
       build  system.  You  will  want  to integrate your new source files into this system. This
       requires that you add your files to the wscript file found in each directory.

       Let’s  start  with  empty  files  error-model.h  and  error-model.cc,  and  add  this   to
       src/network/wscript.  It is really just a matter of adding the .cc file to the rest of the
       source files, and the .h file to the list of the header files.

       Now, pop up to the top level directory and type “./test.py”.  You  shouldn’t  have  broken
       anything by this operation.

   Include Guards
       Next, let’s add some include guards in our header file.:

          #ifndef ERROR_MODEL_H
          #define ERROR_MODEL_H
          ...
          #endif

   namespace ns3
       ns-3  uses  the  ns-3 namespace to isolate its symbols from other namespaces. Typically, a
       user will next put an ns-3 namespace block in both the cc and h file.:

          namespace ns3 {
          ...
          }

       At this point, we have some skeletal files in which we can start defining our new classes.
       The header file looks like this:

          #ifndef ERROR_MODEL_H
          #define ERROR_MODEL_H

          namespace ns3 {

          } // namespace ns3
          #endif

       while the error-model.cc file simply looks like this:

          #include "error-model.h"

          namespace ns3 {

          } // namespace ns3

       These  files should compile since they don’t really have any contents.  We’re now ready to
       start adding classes.

   Initial Implementation
       At this point, we’re still working on some scaffolding, but we can  begin  to  define  our
       classes, with the functionality to be added later.

   Inherit from the Object Class?
       This is an important design step; whether to use class Object as a base class for your new
       classes.

       As described in the chapter on the ns-3 Object-model,  classes  that  inherit  from  class
       Object get special properties:

       · the ns-3 type and attribute system (see Attributes)

       · an object aggregation system

       · a smart-pointer reference counting system (class Ptr)

       Classes  that derive from class ObjectBase} get the first two properties above, but do not
       get smart pointers. Classes that derive from class RefCountBase get only the smart-pointer
       reference counting system.

       In  practice,  class Object is the variant of the three above that the ns-3 developer will
       most commonly encounter.

       In our case, we want to make use of the attribute system, and we will be passing instances
       of this object across the ns-3 public API, so class Object is appropriate for us.

   Initial Classes
       One way to proceed is to start by defining the bare minimum functions and see if they will
       compile. Let’s review what all is needed to implement when we derive from class Object.:

          #ifndef ERROR_MODEL_H
          #define ERROR_MODEL_H

          #include "ns3/object.h"

          namespace ns3 {

          class ErrorModel : public Object
          {
          public:
            static TypeId GetTypeId (void);

            ErrorModel ();
            virtual ~ErrorModel ();
          };

          class RateErrorModel : public ErrorModel
          {
          public:
            static TypeId GetTypeId (void);

            RateErrorModel ();
            virtual ~RateErrorModel ();
          };
          #endif

       A few things to note here. We need to include object.h. The convention in ns-3 is that  if
       the  header  file is co-located in the same directory, it may be included without any path
       prefix. Therefore, if we were implementing  ErrorModel  in  src/core/model  directory,  we
       could  have  just said “#include "object.h"”.  But we are in src/network/model, so we must
       include it as “#include "ns3/object.h"”. Note also that this goes  outside  the  namespace
       declaration.

       Second, each class must implement a static public member function called GetTypeId (void).

       Third,  it is a good idea to implement constructors and destructors rather than to let the
       compiler generate them, and to make the destructor virtual. In C++, note  also  that  copy
       assignment  operator  and copy constructors are auto-generated if they are not defined, so
       if you do not want those, you should implement those as private members.  This  aspect  of
       C++ is discussed in Scott Meyers’ Effective C++ book. item 45.

       Let’s now look at some corresponding skeletal implementation code in the .cc file.:

          #include "error-model.h"

          namespace ns3 {

          NS_OBJECT_ENSURE_REGISTERED (ErrorModel);

          TypeId ErrorModel::GetTypeId (void)
          {
            static TypeId tid = TypeId ("ns3::ErrorModel")
              .SetParent<Object> ()
              .SetGroupName ("Network")
              ;
            return tid;
          }

          ErrorModel::ErrorModel ()
          {
          }

          ErrorModel::~ErrorModel ()
          {
          }

          NS_OBJECT_ENSURE_REGISTERED (RateErrorModel);

          TypeId RateErrorModel::GetTypeId (void)
          {
            static TypeId tid = TypeId ("ns3::RateErrorModel")
              .SetParent<ErrorModel> ()
              .SetGroupName ("Network")
              .AddConstructor<RateErrorModel> ()
              ;
            return tid;
          }

          RateErrorModel::RateErrorModel ()
          {
          }

          RateErrorModel::~RateErrorModel ()
          {
          }

       What  is  the  GetTypeId (void) function? This function does a few things.  It registers a
       unique string into the TypeId system. It establishes  the  hierarchy  of  objects  in  the
       attribute system (via SetParent). It also declares that certain objects can be created via
       the object creation framework (AddConstructor).

       The macro NS_OBJECT_ENSURE_REGISTERED (classname) is needed also once for every class that
       defines  a new GetTypeId method, and it does the actual registration of the class into the
       system.  The Object-model chapter discusses this in more detail.

   Including External Files
   Logging Support
       Here, write a bit about adding |ns3| logging macros.  Note  that  LOG_COMPONENT_DEFINE  is
       done outside the namespace ns3

   Constructor, Empty Function Prototypes
   Key Variables (Default Values, Attributes)
   Test Program 1
   Object Framework
   Adding a Sample Script
       At  this point, one may want to try to take the basic scaffolding defined above and add it
       into the system. Performing this step now allows one to use a simpler model when  plumbing
       into  the  system  and  may also reveal whether any design or API modifications need to be
       made. Once this is done,  we  will  return  to  building  out  the  functionality  of  the
       ErrorModels themselves.

   Add Basic Support in the Class
          /* point-to-point-net-device.h */
            class ErrorModel;

            /**
             * Error model for receive packet events
             */
            Ptr<ErrorModel> m_receiveErrorModel;

   Add Accessor
          void
          PointToPointNetDevice::SetReceiveErrorModel (Ptr<ErrorModel> em)
          {
            NS_LOG_FUNCTION (this << em);
            m_receiveErrorModel = em;
          }

             .AddAttribute ("ReceiveErrorModel",
                             "The receiver error model used to simulate packet loss",
                             PointerValue (),
                             MakePointerAccessor (&PointToPointNetDevice::m_receiveErrorModel),
                             MakePointerChecker<ErrorModel> ())

   Plumb Into the System
          void PointToPointNetDevice::Receive (Ptr<Packet> packet)
          {
            NS_LOG_FUNCTION (this << packet);
            uint16_t protocol = 0;

            if (m_receiveErrorModel && m_receiveErrorModel->IsCorrupt (packet) )
              {
          //
          // If we have an error model and it indicates that it is time to lose a
          // corrupted packet, don't forward this packet up, let it go.
          //
                m_dropTrace (packet);
              }
            else
              {
          //
          // Hit the receive trace hook, strip off the point-to-point protocol header
          // and forward this packet up the protocol stack.
          //
                m_rxTrace (packet);
                ProcessHeader(packet, protocol);
                m_rxCallback (this, packet, protocol, GetRemote ());
                if (!m_promiscCallback.IsNull ())
                  {           m_promiscCallback (this, packet, protocol, GetRemote (),
                                GetAddress (), NetDevice::PACKET_HOST);
                  }
              }
          }

   Create Null Functional Script
          /* simple-error-model.cc */

            // Error model
            // We want to add an error model to node 3's NetDevice
            // We can obtain a handle to the NetDevice via the channel and node
            // pointers
            Ptr<PointToPointNetDevice> nd3 = PointToPointTopology::GetNetDevice
              (n3, channel2);
            Ptr<ErrorModel> em = Create<ErrorModel> ();
            nd3->SetReceiveErrorModel (em);

          bool
          ErrorModel::DoCorrupt (Packet& p)
          {
            NS_LOG_FUNCTION;
            NS_LOG_UNCOND("Corrupt!");
            return false;
          }

       At this point, we can run the program with our trivial ErrorModel plumbed into the receive
       path of the PointToPointNetDevice. It prints out the string  “Corrupt!”  for  each  packet
       received at node n3. Next, we return to the error model to add in a subclass that performs
       more interesting error modeling.

   Add a Subclass
       The trivial base class ErrorModel does not do anything  interesting,  but  it  provides  a
       useful base class interface (Corrupt () and Reset ()), forwarded to virtual functions that
       can be subclassed. Let’s next consider what we call a BasicErrorModel which  is  based  on
       the ns-2 ErrorModel class (in ns-2/queue/errmodel.{cc,h}).

       What  properties do we want this to have, from a user interface perspective? We would like
       for the user to be able to  trivially  swap  out  the  type  of  ErrorModel  used  in  the
       NetDevice. We would also like the capability to set configurable parameters.

       Here are a few simple requirements we will consider:

       · Ability to set the random variable that governs the losses (default is UniformVariable)

       · Ability  to  set the unit (bit, byte, packet, time) of granularity over which errors are
         applied.

       · Ability to set the rate of errors (e.g.  10^-3)  corresponding  to  the  above  unit  of
         granularity.

       · Ability to enable/disable (default is enabled)

   How to Subclass
       We declare BasicErrorModel to be a subclass of ErrorModel as follows,:

          class BasicErrorModel : public ErrorModel
          {
          public:
            static TypeId GetTypeId (void);
            ...
          private:
            // Implement base class pure virtual functions
            virtual bool DoCorrupt (Ptr<Packet> p);
            virtual bool DoReset (void);
            ...
          }

       and  configure  the  subclass  GetTypeId  function  by  setting a unique TypeId string and
       setting the Parent to ErrorModel:

          TypeId RateErrorModel::GetTypeId (void)
          {
            static TypeId tid = TypeId ("ns3::RateErrorModel")
              .SetParent<ErrorModel> ()
              .SetGroupName ("Network")
              .AddConstructor<RateErrorModel> ()
            ...

   Build Core Functions and Unit Tests
   Assert Macros
   Writing Unit Tests
   Adding a New Module to ns-3
       When you have created a group of  related  classes,  examples,  and  tests,  they  can  be
       combined  together into an ns-3 module so that they can be used with existing ns-3 modules
       and by other researchers.

       This chapter walks you through the steps necessary to add a new module to ns-3.

   Step 0 - Module Layout
       All modules can be found in the src directory.  Each module can be found  in  a  directory
       that has the same name as the module.  For example, the spectrum module can be found here:
       src/spectrum.  We’ll be quoting from the spectrum module for illustration.

       A prototypical module has the following directory structure and required files:

          src/
            module-name/
              bindings/
              doc/
              examples/
                wscript
              helper/
              model/
              test/
                examples-to-run.py
              wscript

       Not all directories will be present in each module.

   Step 1 - Create a Module Skeleton
       A python program is provided in the source directory that will create a skeleton for a new
       module.  For the purposes of this discussion we will assume that your new module is called
       new-module.  From the src directory, do the following to create the new module:

          $ ./create-module.py new-module

       Next, cd into new-module; you will find this directory layout:

          $ cd new-module
          $ ls
          doc examples  helper  model  test  wscript

       In more detail, the create-module.py script will create the directories as well as initial
       skeleton  wscript,  .h, .cc and .rst files.  The complete module with skeleton files looks
       like this:

          src/
            new-module/
              doc/
                new-module.rst
              examples/
                new-module-example.cc
                wscript
              helper/
                new-module-helper.cc
                new-module-helper.h
              model/
                new-module.cc
                new-module.h
              test/
                new-module-test-suite.cc
              wscript

       (If required the bindings/ directory listed in Step-0 will be created automatically during
       the build.)

       We  next  walk  through how to customize this module.  Informing waf about the files which
       make up your module is done by editing the two wscript files.  We will  walk  through  the
       main steps in this chapter.

       All  ns-3 modules depend on the core module and usually on other modules.  This dependency
       is specified in the wscript file (at the top level of the module, not the separate wscript
       file in the examples directory!).  In the skeleton wscript the call that will declare your
       new module to waf will look like this (before editing):

          def build(bld):
              module = bld.create_ns3_module('new-module', ['core'])

       Let’s assume that new-module depends on the internet, mobility, and aodv  modules.   After
       editing it the wscript file should look like:

          def build(bld):
              module = bld.create_ns3_module('new-module', ['internet', 'mobility', 'aodv'])

       Note  that  only first level module dependencies should be listed, which is why we removed
       core; the internet module in turn depends on core.

       Your module will most likely have model  source  files.   Initial  skeletons  (which  will
       compile successfully) are created in model/new-module.cc and model/new-module.h.

       If  your  module  will  have  helper  source  files,  then  they  will go into the helper/
       directory; again, initial skeletons are created in that directory.

       Finally, it is good practice to write tests and examples.  These will almost certainly  be
       required  for  new  modules to be accepted into the official ns-3 source tree.  A skeleton
       test suite and test case is created in the test/ directory.  The skeleton test suite  will
       contain  the  below  constructor,  which declares a new unit test named new-module, with a
       single test case consisting of the class NewModuleTestCase1:

          NewModuleTestSuite::NewModuleTestSuite ()
            : TestSuite ("new-module", UNIT)
          {
            AddTestCase (new NewModuleTestCase1);
          }

   Step 3 - Declare Source Files
       The public header and source code files for your new module should  be  specified  in  the
       wscript file by modifying it with your text editor.

       As an example, after declaring the spectrum module, the src/spectrum/wscript specifies the
       source code files with the following list:

          def build(bld):

            module = bld.create_ns3_module('spectrum', ['internet', 'propagation', 'antenna', 'applications'])

            module.source = [
                'model/spectrum-model.cc',
                'model/spectrum-value.cc',
                      .
                      .
                      .
                'model/microwave-oven-spectrum-value-helper.cc',
                'helper/spectrum-helper.cc',
                'helper/adhoc-aloha-noack-ideal-phy-helper.cc',
                'helper/waveform-generator-helper.cc',
                'helper/spectrum-analyzer-helper.cc',
                ]

       The objects resulting from compiling these sources will be assembled into a link  library,
       which will be linked to any programs relying on this module.

       But how do such programs learn the public API of our new module?  Read on!

   Step 4 - Declare Public Header Files
       The  header  files  defining  the  public  API  of  your  model and helpers also should be
       specified in the wscript file.

       Continuing with the spectrum model illustration, the public  header  files  are  specified
       with  the  following  stanza.   (Note  that  the argument to the bld function tells waf to
       install this module’s headers with the other ns-3 headers):

          headers = bld(features='ns3header')

          headers.module = 'spectrum'

          headers.source = [
              'model/spectrum-model.h',
              'model/spectrum-value.h',
                     .
                     .
                     .
              'model/microwave-oven-spectrum-value-helper.h',
              'helper/spectrum-helper.h',
              'helper/adhoc-aloha-noack-ideal-phy-helper.h',
              'helper/waveform-generator-helper.h',
              'helper/spectrum-analyzer-helper.h',
              ]

       Headers made public in this way will be accessible to users of  your  model  with  include
       statements like

          #include "ns3/spectrum-model.h"

       Headers used strictly internally in your implementation should not be included here.  They
       are still accessible to your implemenation by include statements like

          #include "my-module-implementation.h"

   Step 5 - Declare Tests
       If your new module has tests, then  they  must  be  specified  in  your  wscript  file  by
       modifying it with your text editor.

       The spectrum model tests are specified with the following stanza:

          module_test = bld.create_ns3_module_test_library('spectrum')

          module_test.source = [
              'test/spectrum-interference-test.cc',
              'test/spectrum-value-test.cc',
              ]

       See Tests for more information on how to write test cases.

   Step 6 - Declare Examples
       If  your  new  module  has  examples, then they must be specified in your examples/wscript
       file.  (The skeleton top-level wscript will recursively include examples/wscript  only  if
       the examples were enabled at configure time.)

       The spectrum model defines it’s first example in src/spectrum/examples/wscript with

          def build(bld):
            obj = bld.create_ns3_program('adhoc-aloha-ideal-phy',
                                         ['spectrum', 'mobility'])
            obj.source = 'adhoc-aloha-ideal-phy.cc'

       Note  that the second argument to the function create_ns3_program() is the list of modules
       that the program being created depends on; again, don’t forget to  include  new-module  in
       the  list.   It’s  best  practice to list only the direct module dependencies, and let waf
       deduce the full dependency tree.

       Occasionally, for clarity, you may want to split the implementation for your example among
       several  source  files.   In  this  case,  just include those files as additional explicit
       sources of the example:

          obj = bld.create_ns3_program('new-module-example', [new-module])
          obj.source = ['new-module-example.cc', 'new-module-example-part.cc']

       Python examples are specified using the following function call.   Note  that  the  second
       argument  for  the  function  register_ns3_script() is the list of modules that the Python
       example depends on:

          bld.register_ns3_script('new-module-example.py', ['new-module'])

   Step 7 - Examples Run as Tests
       In addition to running explicit test code, the test framework can also be instrumented  to
       run  full  example programs to try to catch regressions in the examples.  However, not all
       examples are suitable for regression tests.  The file test/examples-to-run.py controls the
       invocation of the examples when the test framework runs.

       The     spectrum     model     examples     run    by    test.py    are    specified    in
       src/spectrum/test/examples-to-run.py using the following  two  lists  of  C++  and  Python
       examples:

          # A list of C++ examples to run in order to ensure that they remain
          # buildable and runnable over time.  Each tuple in the list contains
          #
          #     (example_name, do_run, do_valgrind_run).
          #
          # See test.py for more information.
          cpp_examples = [
              ("adhoc-aloha-ideal-phy", "True", "True"),
              ("adhoc-aloha-ideal-phy-with-microwave-oven", "True", "True"),
              ("adhoc-aloha-ideal-phy-matrix-propagation-loss-model", "True", "True"),
          ]

          # A list of Python examples to run in order to ensure that they remain
          # runnable over time.  Each tuple in the list contains
          #
          #     (example_name, do_run).
          #
          # See test.py for more information.
          python_examples = [
              ("sample-simulator.py", "True"),
          ]

       As  indicated  in  the comment, each entry in the C++ list of examples to run contains the
       tuple (example_name, do_run, do_valgrind_run), where

          · example_name is the executable to be run,

          · do_run is a condition under which to run the example, and

          · do_valgrind_run is a condition under which to run the example under valgrind.   (This
            is  needed  because  NSC causes illegal instruction crashes with some tests when they
            are run under valgrind.)

       Note that the two conditions are Python statements that can depend  on  waf  configuration
       variables.  For example,

          ("tcp-nsc-lfn", "NSC_ENABLED == True", "NSC_ENABLED == False"),

       Each  entry  in  the  Python  list  of  examples  to run contains the tuple (example_name,
       do_run), where, as for the C++ examples,

          · example_name is the Python script to be run, and

          · do_run is a condition under which to run the example.

       Again, the condition is a Python statement that can depend on waf configuration variables.
       For example,

          ("realtime-udp-echo.py", "ENABLE_REAL_TIME == False"),

   Step 8 - Configure and Build
       You  can  now  configure,  build and test your module as normal.  You must reconfigure the
       project as a first step so that waf caches the new information in your wscript  files,  or
       else your new module will not be included in the build.

          $ ./waf configure --enable-examples --enable-tests
          $ ./waf build
          $ ./test.py

       Look  for  your  new  module’s  test  suite  (and example programs, if your module has any
       enabled) in the test output.

   Step 9 - Python Bindings
       Adding Python bindings to your module is optional,  and  the  step  is  commented  out  by
       default in the create-module.py script.

          # bld.ns3_python_bindings()

       If  you  want  to  include  Python  bindings (needed only if you want to write Python ns-3
       programs instead of C++ ns-3 programs), you should uncomment the  above  and  install  the
       Python  API  scanning  system  (covered  elsewhere in this manual) and scan your module to
       generate new bindings.

   Creating Documentation
       ns-3 supplies two kinds of documentation:   expository  “user-guide”-style  chapters,  and
       source code API documentation.

       The  “user-guide” chapters are written by hand in reStructuredText format (.rst), which is
       processed by the Python documentation system Sphinx to generate web pages and  pdf  files.
       The API documentation is generated from the source code itself, using Doxygen, to generate
       cross-linked web pages.  Both of these are important:  the Sphinx chapters explain the why
       and overview of using a model; the API documentation explains the how details.

       This  chapter  gives  a  quick  overview  of  these tools, emphasizing preferred usage and
       customizations for ns-3.

       To build all the standard documentation:

          $ ./waf docs

       For more specialized options, read on.

   Documenting with Sphinx
       We use Sphinx to generate expository chapters describing the  design  and  usage  of  each
       module.  Right now you are reading the Documentation Chapter.  If you are reading the html
       version, the Show Source link in the sidebar will show you the reStructuredText source for
       this chapter.

   Adding New Chapters
       Adding a new chapter takes three steps (described in more detail below):

       1. Choose Where? the documentation file(s) will live.

       2. Link from an existing page to the new documentation.

       3. Add the new file to the Makefile.

   Where?
       Documentation for a specific module, foo, should normally go in src/foo/doc/.  For example
       src/foo/doc/foo.rst   would   be   the   top-level   document   for   the   module.    The
       src/create-module.py script will create this file for you.

       Some  models  require  several  .rst  files,  and  figures;  these  should  all  go in the
       src/foo/doc/ directory.  The docs are actually built by a Sphinx Makefile.  For especially
       involved  documentation,  it  may  be helpful to have a local Makefile in the src/foo/doc/
       directory to simplify building the documentation for this module (Antenna is an  example).
       Setting this up is not particularly hard, but is beyond the scope of this chapter.

       In some cases, documentation spans multiple models; the Network chapter is an example.  In
       these cases adding the .rst files directly to doc/models/source/ might be appropriate.

   Link
       Sphinx has to know where your new chapter should appear.   In  most  cases,  a  new  model
       chapter   should   appear   the   in  Models  book.   To  add  your  chapter  there,  edit
       doc/models/source/index.rst

          .. toctree::
             :maxdepth: 1

            organization
            animation
            antenna
            aodv
            applications
            ...

       Add the name of your document (without the .rst extension) to this list.  Please keep  the
       Model chapters in alphabetical order, to ease visual scanning for specific chapters.

   Makefile
       You  also have to add your document to the appropriate Makefile, so make knows to check it
       for updates.  The Models book Makefile is doc/models/Makefile, the Manual book Makefile is
       doc/manual/Makefile.

          # list all model library .rst files that need to be copied to $SOURCETEMP
          SOURCES = \
                  source/conf.py \
                  source/_static \
                  source/index.rst \
                  source/replace.txt \
                  source/organization.rst \
                  ...
                  $(SRC)/antenna/doc/source/antenna.rst \
                  ...

       You add your .rst files to the SOURCES variable.  To add figures, read the comments in the
       Makefile to see which variable should contain your image files.  Again, please keep  these
       in alphabetical order.

   Building Sphinx Docs
       Building   the   Sphinx   documentation  is  pretty  simple.   To  build  all  the  Sphinx
       documentation:

          $ ./waf sphinx

       To build just the Models documentation:

          $ make -C doc/models html

       To see the generated documentation point your browser at doc/models/build/html.

       As you can see, Sphinx uses Make to guide the process.   The  default  target  builds  all
       enabled  output  forms, which in ns-3 are the multi-page html, single-page singlehtml, and
       pdf (latex).  To build just the multi-page html, you add the html target:

          $ make -C doc/models html

       This can be helpful to reduce the build time (and the size of the build  chatter)  as  you
       are writing your chapter.

       Before  committing  your  documentation  to  the repo, please check that it builds without
       errors or warnings.  The build process generates lots of  output  (mostly  normal  chatter
       from  LaTeX),  which  can  make  it  difficult  to see if there are any Sphinx warnings or
       errors.  To find important warnings and errors build just the html  version,  then  search
       the build log for warning or error.

   ns-3 Specifics
       The  Sphinx  documentation  and  tutorial  are pretty good.  We won’t duplicate the basics
       here, instead focusing on preferred usage for ns-3.

       · Start documents with these two lines:

            .. include:: replace.txt
            .. highlight:: cpp

         The first line enables some simple replacements.  For example, typing |ns3|  renders  as
         ns-3.   The second sets the default source code highlighting language explicitly for the
         file, since the parser guess isn’t always accurate.  (It’s  also  possible  to  set  the
         language explicitly for a single code block, see below.)

       · Sections:

         Sphinx  is pretty liberal about marking section headings.  By convention, we prefer this
         hierarchy:

            .. heading hierarchy:
               ------------- Chapter
               ************* Section (#.#)
               ============= Subsection (#.#.#)
               ############# Sub-subsection

       · Syntax Highlighting:

         To use the default syntax highlighter, simply start a sourcecode block:

            ┌──────────────────────────────────────────────┬─────────────────────────────────┐
            │Sphinx Source                                 │ Rendered Output                 │
            ├──────────────────────────────────────────────┼─────────────────────────────────┤
            │                                              │ The Frobnitz is accessed by:    │
            │            The ``Frobnitz`` is accessed by:: │                                 │
            │                                              │             Foo::Frobnitz frob; │
            │              Foo::Frobnitz frob;             │             frob.Set (...);     │
            │              frob.Set (...);                 │                                 │
            └──────────────────────────────────────────────┴─────────────────────────────────┘

         To use a specific syntax highlighter, for example, bash shell commands:

                          ┌─────────────────────────────────┬──────────────────┐
                          │Sphinx Source                    │ Rendered Output  │
                          ├─────────────────────────────────┼──────────────────┤
                          │                                 │                  │
                          │            .. sourcecode:: bash │             $ ls │
                          │                                 │                  │
                          │               $ ls              │                  │
                          └─────────────────────────────────┴──────────────────┘

       · Shorthand Notations:

         These shorthands are defined:

                               ┌────────────────────────┬─────────────────┐
                               │Sphinx Source           │ Rendered Output │
                               ├────────────────────────┼─────────────────┤
                               │                        │ ns-3            │
                               │            |ns3|       │                 │
                               ├────────────────────────┼─────────────────┤
                               │                        │ ns-2            │
                               │            |ns2|       │                 │
                               ├────────────────────────┼─────────────────┤
                               │                        │                 │
                               │            |check|     │                 │
                               ├────────────────────────┼─────────────────┤
                               │                        │ RFC 6282        │
                               │            :rfc:`6282` │                 │
                               └────────────────────────┴─────────────────┘

   Documenting with Doxygen
       We use Doxygen to generate browsable API documentation.   Doxygen  provides  a  number  of
       useful features:

       · Summary table of all class members.

       · Graphs of inheritance and collaboration for all classes.

       · Links to the source code implementing each function.

       · Links to every place a member is used.

       · Links to every object used in implementing a function.

       · Grouping of related classes, such as all the classes related to a specific protocol.

       In addition, we use the TypeId system to add to the documentation for every class

       · The Config paths by which such objects can be reached.

       · Documentation for any Attributes, including Attributes defined in parent classes.

       · Documentation for any Trace sources defined by the class.

       · The memory footprint for each class.

       Doxygen  operates  by  scaning the source code, looking for specially marked comments.  It
       also creates a cross reference, indicating where each file, class, method, and variable is
       used.

   Preferred Style
       The preferred style for Doxygen comments is the JavaDoc style:

          /**
           *  Brief description of this class or method.
           *  Adjacent lines become a single paragraph.
           *
           *  Longer description, with lots of details.
           *
           *  Blank lines separate paragraphs.
           *
           *  Explain what the class or method does, using what algorithm.
           *  Explain the units of arguments and return values.
           *
           *  \note Note any limitations or gotchas.
           *
           *  (For functions with arguments or return valued:)
           *  \param [in] foo Brief noun phrase describing this argument. Note
           *                  that we indicate if the argument is input,
           *                  output, or both.
           *  \param [in,out] bar Note Sentence case, and terminating period.
           *  \param [in] baz Indicate boolean values with \c true or \c false.
           *  \return Brief noun phrase describing the value.
           *
           *  \internal
           *
           *  You can also discuss internal implementation details.
           *  Understanding this material shouldn't be necessary to using
           *  the class or method.
           */
           void ExampleFunction (const int foo, double & bar, const bool baz);

       In  this style the Doxygen comment block begins with two `*’ characters: /**, and precedes
       the item being documented.

       For items needing only a brief description, either of these short forms is appropriate:

          /** Destructor implementation. */
          void DoDispose ();

          int m_count;  //!< Count of ...

       Note the special form of the end of line comment, //!<, indicating that it refers  to  the
       preceding item.

       Some items to note:

       · Use sentence case, including the initial capital.

       · Use punctuation, especially `.’s at the end of sentences or phrases.

       ·

         The \brief tag is not needed; the first sentence will be
                used as the brief description.

       Every  class,  method, typedef, member variable, function argument and return value should
       be documented in all source code files which form the formal API  and  implementation  for
       ns-3,   such  as  src/<module>/model/*,  src/<module>/helper/*  and  src/<module>/utils/*.
       Documentation for items in src/<module>/test/* and src/<module>/examples/*  is  preferred,
       but not required.

   Useful Features
       · Inherited  members will automatically inherit docs from the parent, (but can be replaced
         by local documentation).

         1. Document the base class.

         2. In the sub class mark inherited functions with an ordinary comment:

              // Inherited methods
              virtual void FooBar (void);
              virtual int BarFoo (double baz);

           Note that the signatures have to match exactly, so include the formal argument (void)

           This doesn’t work for static functions; see GetTypeId, below, for an example.

   Building Doxygen Docs
       Building the Doxygen documentation is pretty simple:

          $ ./waf doxygen

       This builds using the default configuration, which generates  documentation  sections  for
       all  items,  even if they do not have explicit comment documentation blocks.  This has the
       effect of suppressing warnings for undocumented items, but makes sure  everything  appears
       in  the  generated  output,  which is usually what you want for general use.  Note that we
       generate documentation even for modules which are disabled, to make it easier to  see  all
       the features available in ns-3.

       When  writing  documentation,  it’s  often  more  useful to see which items are generating
       warnings, typically about missing documentation.  To see the full warnings list,  use  the
       doc/doxygen.warnings.report.sh script:

          $ doc/doxygen.warnings.report.sh

          doxygen.warnings.report.sh:
          Building and running print-introspected-doxygen...done.
          Rebuilding doxygen (v1.8.10) docs with full errors...done.

          Report of Doxygen warnings
          ----------------------------------------

          (All counts are lower bounds.)

          Warnings by module/directory:

          Count Directory
          ----- ----------------------------------
          3414 src/lte/model
          1532 src/wimax/model
           825 src/lte/test
          ....
             1 src/applications/test
           97 additional undocumented parameters.
          ----------------------------------------
           12460 total warnings
             100 directories with warnings

          Warnings by file (alphabetical)

          Count File
          ----- ----------------------------------
            15 examples/routing/manet-routing-compare.cc
            26 examples/stats/wifi-example-apps.h
            12 examples/tutorial/fifth.cc
          ....
            17 utils/python-unit-tests.py
          ----------------------------------------
             771 files with warnings

          Warnings by file (numerical)

          Count File
          ----- ----------------------------------
           273 src/lte/model/lte-rrc-sap.h
           272 src/core/model/simulator.h
           221 src/netanim/model/animation-interface.h
          ....
             1 src/wimax/model/ul-job.cc
          ----------------------------------------
             771 files with warnings

          Doxygen Warnings Summary
          ----------------------------------------
             100 directories
             771 files
           12460 warnings

       (This snippet has a lot of lines suppressed!)

       The  script  modifies the configuration to show all warnings, and to shorten the run time.
       (It shortens the run time primarily by disabling creation of diagrams, such as call trees,
       and  doesn’t  generate  documentation  for  undocumented  items,  in  order to trigger the
       warnings.)  As you can see, at this writing we have a  lot  of  undocumented  items.   The
       report summarizes warnings by module src/*/*, and by file, in alphabetically and numerical
       order.

       The script has a few options to pare things down and make the output more manageable.  For
       help, use the -h option.  Having run it once to do the Doxygen build and generate the full
       warnings log, you can reprocess the log file with various “filters,” without having to  do
       the  full  Doxygen  build  again  by  using  the -s option.  You can exclude warnings from
       */examples/* files (-e option), and/or */test/* files (-t).  Just to be clear, all of  the
       filter  options  do  the  complete  fast  doxygen  build; they just filter doxygen log and
       warnings output.

       Perhaps the most useful option when writing documentation comments is -m  <module>,  which
       will  limit  the  report to just files matching src/<module>/*, and follow the report with
       the actual warning lines.  Combine with -et and you can focus on  the  warnings  that  are
       most urgent in a single module:

          $ doc/doxygen.warnings.report.sh -m mesh/helper
          ...
          Doxygen Warnings Summary
          ----------------------------------------
            1 directories
            3 files
          149 warnings

          Filtered Warnings
          ========================================
          src/mesh/helper/dot11s/dot11s-installer.h:72: warning: Member m_root (variable) of class ns3::Dot11sStack is not documented.
          src/mesh/helper/dot11s/dot11s-installer.h:35: warning: return type of member ns3::Dot11sStack::GetTypeId is not documented
          src/mesh/helper/dot11s/dot11s-installer.h:56: warning: return type of member ns3::Dot11sStack::InstallStack is not documented
          src/mesh/helper/flame/lfame-installer.h:40: warning: Member GetTypeId() (function) of class ns3::FlameStack is not documented.
          src/mesh/helper/flame/flame-installer.h:60: warning: return type of member ns3::FlameStack::InstallStack is not documented
          src/mesh/helper/mesh-helper.h:213: warning: Member m_nInterfaces (variable) of class ns3::MeshHelper is not documented.
          src/mesh/helper/mesh-helper.h:214: warning: Member m_spreadChannelPolicy (variable) of class ns3::MeshHelper is not documented.
          src/mesh/helper/mesh-helper.h:215: warning: Member m_stack (variable) of class ns3::MeshHelper is not documented.
          src/mesh/helper/mesh-helper.h:216: warning: Member m_stackFactory (variable) of class ns3::MeshHelper is not documented.
          src/mesh/helper/mesh-helper.h:209: warning: parameters of member ns3::MeshHelper::CreateInterface are not (all) documented
          src/mesh/helper/mesh-helper.h:119: warning: parameters of member ns3::MeshHelper::SetStandard are not (all) documented

       Finally,  note  that undocumented items (classes, methods, functions, typedefs, etc. won’t
       produce documentation  when  you  build  with  doxygen.warnings.report.sh,  and  only  the
       outermost  item will produce a warning.  As a result, if you don’t see documentation for a
       class  method  in  the  generated  documentation,  the   class   itself   probably   needs
       documentation.

       Now it’s just a matter of understanding the code, and writing some docs!

   ns-3 Specifics
       As  for  Sphinx,  the  Doxygen docs and reference are pretty good.  We won’t duplicate the
       basics here, instead focusing on preferred usage for ns-3.

       · Use Doxygen Modules to group related items.

         In the main header for a module, create a Doxgyen group:

            /**
             *  \defgroup foo Foo protocol.
             *  Implemenation of the Foo protocol.
             */

         The symbol foo is how other  items  can  add  themselves  to  this  group.   The  string
         following  that  will  be the title for the group.  Any futher text will be the detailed
         description for the group page.

       · Document each file, assigning it to the relevant group.  In a header file:

            /**
             *  \file
             *  \ingroup foo
             *  Class Foo declaration.
             */

         or in the corresponding .cc file:

            /**
             *  \file
             *  \ingroup foo
             *  Class FooBar implementation.
             */

       · Mark each associated class as belonging to the group:

            /**
             *  \ingroup foo
             *
             *  FooBar packet type.
            */
            class FooBar

       · Did you know typedefs can have formal arguments?  This enables documentation of function
         pointer signatures:

            /**
             *  Bar callback function signature.
             *
             *  \param ale The size of a pint of ale, in Imperial ounces.
             */
            typedef void (* BarCallback)(const int ale);

       · Copy  the  Attribute  help  strings  from  the  GetTypeId  method  to  use  as the brief
         descriptions of associated members.

       · \bugid{298} will create a link to bug 298 in our Bugzilla.

       · \p foo in a description will format foo the same as the \param foo parameter, making  it
         clear that you are referring to an actual argument.

       · \RFC{301} will create a link to RFC 301.

       · Document  the direction of function arguments with \param [in], etc.  The allowed values
         of the direction  token  are  [in],  [out],  and  [in,out]  (note  the  explicit  square
         brackets), as discussed in the Doxygen docs for \param.

       · Document template arguments with \tparam, just as you use \param for function arguments.

       · For template arguments, indicate if they will be deduced or must be given explicitly:

            /**
             * A templated function.
             * \tparam T \explicit The return type.
             * \tparam U \deduced The argument type.
             * \param [in] a The argument.
             */
            template <typename T, typename U> T Function (U a);

         · Use  \tparam  U  \deduced  because  the  type  U  can be deduced at the site where the
           template is invoked.  Basically deduction can only be done for function arguments.

         · Use \tparam T \explicit because the  type  T  can’t  be  deduced;  it  must  be  given
           explicitly at the invocation site, as in Create<MyObject> (...)

       · \internal  should be used only to set off a discussion of implementation details, not to
         mark private functions (they are already marked, as private!)

       · Don’t create classes with trivial names, such as class A, even in  test  suites.   These
         cause all instances of the class name literal `A’ to be rendered as links.

       As  noted above, static functions don’t inherit the documentation of the same functions in
       the  parent  class.   ns-3  uses  a  few  static  functions  ubiquitously;  the  suggested
       documentation block for these cases is:

       · Default constructor/destructor:

            MyClass ();   //!< Default constructor
            ~MyClass ();  //!< Destructor

       · Dummy destructor and DoDispose:

            /** Dummy destructor, see DoDispose. */
            ~MyClass ();

            /** Destructor implementation */
            virtual void DoDispose ();

       · GetTypeId:

            /**
             *  Register this type.
             *  \return The object TypeId.
             */
            static TypeId GetTypeId (void);

   Enabling Subsets of ns-3 Modules
       As with most software projects, ns-3 is ever growing larger in terms of number of modules,
       lines of code, and memory footprint.  Users, however, may only use a few of those  modules
       at  a  time.   For this reason, users may want to explicitly enable only the subset of the
       possible ns-3 modules that they actually need for their research.

       This chapter discusses how to enable only the ns-3 modules  that  you  are  interested  in
       using.

   How to enable a subset of ns-3’s modules
       If  shared  libraries  are  being  built,  then  enabling a module will cause at least one
       library to be built:

          libns3-modulename.so

       If the module has a test library and test libraries are being built, then

          libns3-modulename-test.so

       will be built, too.  Other modules that the module depends on  and  their  test  libraries
       will also be built.

       By default, all modules are built in ns-3.  There are two ways to enable a subset of these
       modules:

       1. Using waf’s –enable-modules option

       2. Using the ns-3 configuration file

   Enable modules using waf’s –enable-modules option
       To enable only the core module with example and tests, for example, try these commands:

          $ ./waf clean
          $ ./waf configure --enable-examples --enable-tests --enable-modules=core
          $ ./waf build
          $ cd build/debug/
          $ ls

       and the following libraries should be present:

          bindings  libns3-core.so       ns3      scratch  utils
          examples  libns3-core-test.so  samples  src

       Note the ./waf clean step is done here only to make it more obvious which module libraries
       were built.  You don’t have to do ./waf clean in order to enable subsets of modules.

       Running test.py will cause only those tests that depend on module core to be run:

          24 of 24 tests passed (24 passed, 0 skipped, 0 failed, 0 crashed, 0 valgrind errors)

       Repeat  the  above  steps  for  the “network” module instead of the “core” module, and the
       following will be built, since network depends on core:

          bindings  libns3-core.so       libns3-network.so       ns3      scratch  utils
          examples  libns3-core-test.so  libns3-network-test.so  samples  src

       Running test.py will cause those tests that depend on only the core and network modules to
       be run:

          31 of 31 tests passed (31 passed, 0 skipped, 0 failed, 0 crashed, 0 valgrind errors)

   Enable modules using the ns-3 configuration file
       A  configuration  file,  .ns3rc, has been added to ns-3 that allows users to specify which
       modules are to be included in the build.

       When enabling a subset of ns-3 modules, the precedence rules are as follows:

       1. the –enable-modules configure string overrides any .ns3rc file

       2. the .ns3rc file in the top level ns-3 directory is next consulted, if present

       3. the system searches for ~/.ns3rc if the above two are unspecified

       If none of the above limits the modules to be built, all modules that waf knows about will
       be built.

       The  maintained  version  of the .ns3rc file in the ns-3 source code repository resides in
       the utils directory.  The reason for this is if it were in the top-level directory of  the
       repository,  it  would  be  prone  to accidental checkins from maintainers that enable the
       modules they want to use.  Therefore, users need to manually  copy  the  .ns3rc  from  the
       utils  directory to their preferred place (top level directory or their home directory) to
       enable persistent modular build configuration.

       Assuming that you are in the top level ns-3 directory, you can get a copy  of  the  .ns3rc
       file that is in the utils directory as follows:

          $ cp utils/.ns3rc .

       The  .ns3rc  file  should  now  be  in  your top level ns-3 directory, and it contains the
       following:

          #! /usr/bin/env python

          # A list of the modules that will be enabled when ns-3 is run.
          # Modules that depend on the listed modules will be enabled also.
          #
          # All modules can be enabled by choosing 'all_modules'.
          modules_enabled = ['all_modules']

          # Set this equal to true if you want examples to be run.
          examples_enabled = False

          # Set this equal to true if you want tests to be run.
          tests_enabled = False

       Use your favorite editor to modify the .ns3rc file to only enable  the  core  module  with
       examples and tests like this:

          #! /usr/bin/env python

          # A list of the modules that will be enabled when ns-3 is run.
          # Modules that depend on the listed modules will be enabled also.
          #
          # All modules can be enabled by choosing 'all_modules'.
          modules_enabled = ['core']

          # Set this equal to true if you want examples to be run.
          examples_enabled = True

          # Set this equal to true if you want tests to be run.
          tests_enabled = True

       Only the core module will be enabled now if you try these commands:

          $ ./waf clean
          $ ./waf configure
          $ ./waf build
          $ cd build/debug/
          $ ls

       and the following libraries should be present:

          bindings  libns3-core.so       ns3      scratch  utils
          examples  libns3-core-test.so  samples  src

       Note the ./waf clean step is done here only to make it more obvious which module libraries
       were built.  You don’t have to do ./waf clean in order to enable subsets of modules.

       Running test.py will cause only those tests that depend on module core to be run:

          24 of 24 tests passed (24 passed, 0 skipped, 0 failed, 0 crashed, 0 valgrind errors)

       Repeat the above steps for the “network” module instead of  the  “core”  module,  and  the
       following will be built, since network depends on core:

          bindings  libns3-core.so       libns3-network.so       ns3      scratch  utils
          examples  libns3-core-test.so  libns3-network-test.so  samples  src

       Running test.py will cause those tests that depend on only the core and network modules to
       be run:

          31 of 31 tests passed (31 passed, 0 skipped, 0 failed, 0 crashed, 0 valgrind errors)

   Enabling/disabling ns-3 Tests and Examples
       The ns-3 distribution includes many examples and tests that are used to validate the  ns-3
       system.   Users, however, may not always want these examples and tests to be run for their
       installation of ns-3.

       This chapter discusses how to build ns-3 with or without its examples and tests.

   How to enable/disable examples and tests in ns-3
       There are 3 ways to enable/disable examples and tests in ns-3:

       1. Using build.py when ns-3 is built for the first time

       2. Using waf once ns-3 has been built

       3. Using the ns-3 configuration file once ns-3 has been built

   Enable/disable examples and tests using build.py
       You can use build.py to enable/disable examples and tests when ns-3 is built for the first
       time.

       By default, examples and tests are not built in ns-3.

       From  the ns-3-allinone directory, you can build ns-3 without any examples or tests simply
       by doing:

          $ ./build.py

       Running test.py in the top level ns-3 directory now will cause no examples or tests to  be
       run:

          0 of 0 tests passed (0 passed, 0 skipped, 0 failed, 0 crashed, 0 valgrind errors)

       If  you  would  like  build  ns-3  with examples and tests, then do the following from the
       ns-3-allinone directory:

          $ ./build.py --enable-examples --enable-tests

       Running test.py in the top level ns-3 directory will cause all of the examples  and  tests
       to be run:

          170 of 170 tests passed (170 passed, 0 skipped, 0 failed, 0 crashed, 0 valgrind errors)

   Enable/disable examples and tests using waf
       You can use waf to enable/disable examples and tests once ns-3 has been built.

       By default, examples and tests are not built in ns-3.

       From the top level ns-3 directory, you can build ns-3 without any examples or tests simply
       by doing:

          $ ./waf configure
          $ ./waf build

       Running test.py now will cause no examples or tests to be run:

          0 of 0 tests passed (0 passed, 0 skipped, 0 failed, 0 crashed, 0 valgrind errors)

       If you would like build ns-3 with examples and tests, then do the following from  the  top
       level ns-3 directory:

          $ ./waf configure --enable-examples --enable-tests
          $ ./waf build

       Running test.py will cause all of the examples and tests to be run:

          170 of 170 tests passed (170 passed, 0 skipped, 0 failed, 0 crashed, 0 valgrind errors)

   Enable/disable examples and tests using the ns-3 configuration file
       A  configuration file, .ns3rc, has been added to ns-3 that allows users to specify whether
       examples and tests should be built or not.   You  can  use  this  file  to  enable/disable
       examples and tests once ns-3 has been built.

       When enabling disabling examples and tests, the precedence rules are as follows:

       1. the –enable-examples/–disable-examples configure strings override any .ns3rc file

       2. the –enable-tests/–disable-tests configure strings override any .ns3rc file

       3. the .ns3rc file in the top level ns-3 directory is next consulted, if present

       4. the system searches for ~/.ns3rc if the .ns3rc file was not found in the previous step

       If none of the above exists, then examples and tests will not be built.

       The  maintained  version  of the .ns3rc file in the ns-3 source code repository resides in
       the utils directory.  The reason for this is if it were in the top-level directory of  the
       repository,  it  would  be  prone  to accidental checkins from maintainers that enable the
       modules they want to use.  Therefore, users need to manually  copy  the  .ns3rc  from  the
       utils  directory to their preferred place (top level directory or their home directory) to
       enable persistent enabling of examples and tests.

       Assuming that you are in the top level ns-3 directory, you can get a copy  of  the  .ns3rc
       file that is in the utils directory as follows:

          $ cp utils/.ns3rc .

       The  .ns3rc  file  should  now  be  in  your top level ns-3 directory, and it contains the
       following:

          #! /usr/bin/env python

          # A list of the modules that will be enabled when ns-3 is run.
          # Modules that depend on the listed modules will be enabled also.
          #
          # All modules can be enabled by choosing 'all_modules'.
          modules_enabled = ['all_modules']

          # Set this equal to true if you want examples to be run.
          examples_enabled = False

          # Set this equal to true if you want tests to be run.
          tests_enabled = False

       From the top level ns-3 directory, you can build ns-3 without any examples or tests simply
       by doing:

          $ ./waf configure
          $ ./waf build

       Running test.py now will cause no examples or tests to be run:

          0 of 0 tests passed (0 passed, 0 skipped, 0 failed, 0 crashed, 0 valgrind errors)

       If  you  would like build ns-3 with examples and tests, use your favorite editor to change
       the values in the .ns3rc file for examples_enabled and tests_enabled file to be True:

          #! /usr/bin/env python

          # A list of the modules that will be enabled when ns-3 is run.
          # Modules that depend on the listed modules will be enabled also.
          #
          # All modules can be enabled by choosing 'all_modules'.
          modules_enabled = ['all_modules']

          # Set this equal to true if you want examples to be run.
          examples_enabled = True

          # Set this equal to true if you want tests to be run.
          tests_enabled = True

       From the top level ns-3 directory, you can build ns-3 with examples and  tests  simply  by
       doing:

          $ ./waf configure
          $ ./waf build

       Running test.py will cause all of the examples and tests to be run:

          170 of 170 tests passed (170 passed, 0 skipped, 0 failed, 0 crashed, 0 valgrind errors)

   Troubleshooting
       This  chapter  posts  some information about possibly common errors in building or running
       ns-3 programs.

       Please note that the wiki (http://www.nsnam.org/wiki/Troubleshooting) may have contributed
       items.

   Build errors
   Run-time errors
       Sometimes,  errors  can  occur with a program after a successful build. These are run-time
       errors, and can commonly occur when memory is corrupted or pointer values are unexpectedly
       null.

       Here is an example of what might occur:

          $ ./waf --run tcp-point-to-point
          Entering directory '/home/tomh/ns-3-nsc/build'
          Compilation finished successfully
          Command ['/home/tomh/ns-3-nsc/build/debug/examples/tcp-point-to-point'] exited with code -11

       The  error  message  says  that the program terminated unsuccessfully, but it is not clear
       from this information what might be wrong. To examine more closely, try running  it  under
       the gdb debugger:

          $ ./waf --run tcp-point-to-point --command-template="gdb %s"
          Entering directory '/home/tomh/ns-3-nsc/build'
          Compilation finished successfully
          GNU gdb Red Hat Linux (6.3.0.0-1.134.fc5rh)
          Copyright 2004 Free Software Foundation, Inc.
          GDB is free software, covered by the GNU General Public License, and you are
          welcome to change it and/or distribute copies of it under certain conditions.
          Type "show copying" to see the conditions.
          There is absolutely no warranty for GDB.  Type "show warranty" for details.
          This GDB was configured as "i386-redhat-linux-gnu"...Using host libthread_db
          library "/lib/libthread_db.so.1".

          (gdb) run
          Starting program: /home/tomh/ns-3-nsc/build/debug/examples/tcp-point-to-point
          Reading symbols from shared object read from target memory...done.
          Loaded system supplied DSO at 0xf5c000

          Program received signal SIGSEGV, Segmentation fault.
          0x0804aa12 in main (argc=1, argv=0xbfdfefa4)
              at ../examples/tcp-point-to-point.cc:136
          136       Ptr<Socket> localSocket = socketFactory->CreateSocket ();
          (gdb) p localSocket
          $1 = {m_ptr = 0x3c5d65}
          (gdb) p socketFactory
          $2 = {m_ptr = 0x0}
          (gdb) quit
          The program is running.  Exit anyway? (y or n) y

       Note  first the way the program was invoked– pass the command to run as an argument to the
       command template “gdb %s”.

       This tells us that there was an attempt to dereference a null pointer socketFactory.

       Let’s look around line 136 of tcp-point-to-point, as gdb suggests:

          Ptr<SocketFactory> socketFactory = n2->GetObject<SocketFactory> (Tcp::iid);
          Ptr<Socket> localSocket = socketFactory->CreateSocket ();
          localSocket->Bind ();

       The culprit here is that the return value of GetObject is not being  checked  and  may  be
       null.

       Sometimes  you  may need to use the valgrind memory checker for more subtle errors. Again,
       you invoke the use of valgrind similarly:

          $ ./waf --run tcp-point-to-point --command-template="valgrind %s"

SOURCE

       This document is  written  in  reStructuredText  for  Sphinx  and  is  maintained  in  the
       doc/manual directory of ns-3’s source code.

AUTHOR

       ns-3 project

COPYRIGHT

       2018, ns-3 project