xenial (1) ns-3-tutorial.1.gz

Provided by: ns3-doc_3.22+dfsg-2build1_all bug

NAME

       ns-3-tutorial - ns-3 Tutorial

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

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

       • Tutorial (this document), Manual, and Model Library for the latest release and development treens-3 wiki

       This  document  is written in reStructuredText for Sphinx and is maintained in the doc/tutorial directory
       of ns-3's source code.

INTRODUCTION

       The ns-3 simulator is a discrete-event network simulator targeted primarily for research and  educational
       use.  The ns-3 project, started in 2006, is an open-source project developing ns-3.

       The  purpose  of  this  tutorial is to introduce new ns-3 users to the system in a structured way.  It is
       sometimes difficult for new users to glean essential information from detailed  manuals  and  to  convert
       this  information into working simulations.  In this tutorial, we will build several example simulations,
       introducing and explaining key concepts and features as we go.

       As the tutorial unfolds, we will introduce the full ns-3 documentation and  provide  pointers  to  source
       code for those interested in delving deeper into the workings of the system.

       A few key points are worth noting at the onset:

       • ns-3  is  open-source,  and  the  project  strives  to  maintain an open environment for researchers to
         contribute and share their software.

       • ns-3 is not a backwards-compatible extension of ns-2; it is a new simulator.  The  two  simulators  are
         both  written in C++ but ns-3 is a new simulator that does not support the ns-2 APIs.  Some models from
         ns-2 have already been ported from ns-2 to ns-3. The project will continue to maintain ns-2 while  ns-3
         is being built, and will study transition and integration mechanisms.

   About ns-3
       ns-3  has  been  developed  to  provide  an  open, extensible network simulation platform, for networking
       research and education.  In brief, ns-3 provides models of how packet data networks work and perform, and
       provides  a  simulation  engine  for users to conduct simulation experiments.  Some of the reasons to use
       ns-3 include to perform studies that are more difficult or not possible to perform with real systems,  to
       study  system behavior in a highly controllled, reproducible environment, and to learn about how networks
       work.  Users will note that the available model set in ns-3 focuses on modeling  how  Internet  protocols
       and  networks  work,  but  ns-3 is not limited to Internet systems; several users are using ns-3 to model
       non-Internet-based systems.

       Many simulation tools exist for network simulation studies.  Below are a few distinguishing  features  of
       ns-3 in contrast to other tools.

       • ns-3  is  designed  as  a  set  of libraries that can be combined together and also with other external
         software libraries.  While some simulation platforms provide users with a single, integrated  graphical
         user  interface  environment  in  which all tasks are carried out, ns-3 is more modular in this regard.
         Several external animators and data analysis and visualization tools can be used with  ns-3.   However,
         users should expect to work at the command line and with C++ and/or Python software development tools.

       • ns-3 is primarily used on Linux systems, although support exists for FreeBSD, Cygwin (for Windows), and
         native Windows Visual Studio support is in the process of being developed.

       • ns-3 is not an officially supported software product of any company.  Support for ns-3  is  done  on  a
         best-effort basis on the ns-3-users mailing list.

   For ns-2 Users
       For  those  familiar  with ns-2 (a popular tool that preceded ns-3), the most visible outward change when
       moving to ns-3 is the choice of scripting language.  Programs in ns-2 are scripted in OTcl and results of
       simulations  can be visualized using the Network Animator nam.  It is not possible to run a simulation in
       ns-2 purely from C++ (i.e., as a main() program without any OTcl).  Moreover, some components of ns-2 are
       written  in  C++  and  others  in OTcl.  In ns-3, the simulator is written entirely in C++, with optional
       Python bindings.  Simulation scripts can therefore be written in C++ or in  Python.   New  animators  and
       visualizers  are  available and under current development.  Since ns-3 generates pcap packet trace files,
       other utilities can be used to analyze traces as well.  In this tutorial, we will  first  concentrate  on
       scripting directly in C++ and interpreting results via trace files.

       But  there are similarities as well (both, for example, are based on C++ objects, and some code from ns-2
       has already been ported to ns-3).  We will try to highlight differences  between  ns-2  and  ns-3  as  we
       proceed in this tutorial.

       A  question  that  we often hear is "Should I still use ns-2 or move to ns-3?"  In this author's opinion,
       unless the user is somehow vested in ns-2 (either based on existing personal comfort with  and  knowledge
       of  ns-2,  or  based  on a specific simulation model that is only available in ns-2), a user will be more
       productive with ns-3 for the following reasons:

       • ns-3 is actively maintained with an active, responsive users mailing list, while ns-2 is  only  lightly
         maintained and has not seen significant development in its main code tree for over a decade.

       • ns-3  provides  features  not  available  in  ns-2, such as a implementation code execution environment
         (allowing users to run real implementation code in the simulator)

       • ns-3 provides a lower base level of abstraction compared with ns-2, allowing it to  align  better  with
         how  real  systems are put together.  Some limitations found in ns-2 (such as supporting multiple types
         of interfaces on nodes correctly) have been remedied in ns-3.

       ns-2 has a more diverse set of contributed modules than does ns-3, owing to its long  history.   However,
       ns-3  has more detailed models in several popular areas of research (including sophisticated LTE and WiFi
       models), and its support of implementation code admits a very  wide  spectrum  of  high-fidelity  models.
       Users  may  be  surprised  to  learn that the whole Linux networking stack can be encapsulated in an ns-3
       node, using the Direct Code Execution (DCE) framework.  ns-2 models can  sometimes  be  ported  to  ns-3,
       particularly if they have been implemented in C++.

       If  in  doubt, a good guideline would be to look at both simulators (as well as other simulators), and in
       particular the models available for your research, but keep in mind that your experience may be better in
       using the tool that is being actively developed and maintained (ns-3).

   Contributing
       ns-3  is  a  research  and educational simulator, by and for the research community.  It will rely on the
       ongoing contributions of the community to develop new models, debug or maintain existing ones, and  share
       results.   There  are  a  few policies that we hope will encourage people to contribute to ns-3 like they
       have for ns-2:

       • Open source licensing based on GNU GPLv2 compatibility

       • wikiContributed Code page, similar to ns-2's popular Contributed Code page

       • Open bug tracker

       We realize that if you are reading this document, contributing back to the project is probably  not  your
       foremost  concern  at  this  point, but we want you to be aware that contributing is in the spirit of the
       project and that even the act of dropping us a note about your early experience  with  ns-3  (e.g.  "this
       tutorial section was not clear..."), reports of stale documentation, etc. are much appreciated.

   Tutorial Organization
       The tutorial assumes that new users might initially follow a path such as the following:

       • Try to download and build a copy;

       • Try to run a few sample programs;

       • Look at simulation output, and try to adjust it.

       As a result, we have tried to organize the tutorial along the above broad sequences of events.

RESOURCES

   The Web
       There are several important resources of which any ns-3 user must be aware.  The main web site is located
       at http://www.nsnam.org and provides access  to  basic  information  about  the  ns-3  system.   Detailed
       documentation  is  available  through  the main web site at http://www.nsnam.org/documentation/.  You can
       also find documents relating to the system architecture from this page.

       There  is  a  Wiki   that   complements   the   main   ns-3   web   site   which   you   will   find   at
       http://www.nsnam.org/wiki/.   You  will  find  user  and developer FAQs there, as well as troubleshooting
       guides, third-party contributed code, papers, etc.

       The source code may be found and browsed at http://code.nsnam.org/.  There  you  will  find  the  current
       development  tree  in  the  repository named ns-3-dev. Past releases and experimental repositories of the
       core developers may also be found there.

   Mercurial
       Complex software systems need some way to manage the organization and changes to the underlying code  and
       documentation.   There  are many ways to perform this feat, and you may have heard of some of the systems
       that are currently used to do this.  The Concurrent Version System (CVS) is probably the most well known.

       The ns-3 project uses Mercurial as its source code management system.  Although you do not need  to  know
       much  about  Mercurial  in order to complete this tutorial, we recommend becoming familiar with Mercurial
       and using it to access the source code.  Mercurial has a web site  at  http://www.selenic.com/mercurial/,
       from  which you can get binary or source releases of this Software Configuration Management (SCM) system.
       Selenic     (the     developer     of     Mercurial)      also      provides      a      tutorial      at
       http://www.selenic.com/mercurial/wiki/index.cgi/Tutorial/,      and     a     QuickStart     guide     at
       http://www.selenic.com/mercurial/wiki/index.cgi/QuickStart/.

       You can also find vital information about using Mercurial and ns-3 on the main ns-3 web site.

   Waf
       Once you have source code downloaded to your local system, you  will  need  to  compile  that  source  to
       produce  usable  programs.  Just as in the case of source code management, there are many tools available
       to perform this function.  Probably the most well known of these tools is make.   Along  with  being  the
       most  well  known,  make  is  probably  the most difficult to use in a very large and highly configurable
       system.  Because of this, many alternatives have  been  developed.   Recently  these  systems  have  been
       developed using the Python language.

       The  build system Waf is used on the ns-3 project.  It is one of the new generation of Python-based build
       systems.  You will not need to understand any Python to build the existing ns-3 system.

       For  those  interested  in  the  gory  details  of  Waf,  the   main   web   site   can   be   found   at
       http://code.google.com/p/waf/.

   Development Environment
       As  mentioned  above,  scripting  in ns-3 is done in C++ or Python.  Most of the ns-3 API is available in
       Python, but the models are written in C++ in either case.  A working knowledge of C++ and object-oriented
       concepts  is  assumed  in  this  document.   We  will  take some time to review some of the more advanced
       concepts or possibly unfamiliar language features, idioms and design patterns as they appear.   We  don't
       want  this  tutorial  to  devolve  into  a  C++  tutorial, though, so we do expect a basic command of the
       language.  There are an almost unimaginable number of sources of information on C++ available on the  web
       or in print.

       If  you  are  new  to  C++,  you may want to find a tutorial- or cookbook-based book or web site and work
       through at least the basic features of the language before proceeding.  For instance, this tutorial.

       The ns-3 system uses several components of the GNU "toolchain" for development.  A software toolchain  is
       the  set  of programming tools available in the given environment. For a quick review of what is included
       in the GNU toolchain see, http://en.wikipedia.org/wiki/GNU_toolchain.  ns-3 uses gcc, GNU  binutils,  and
       gdb.   However,  we  do  not  use the GNU build system tools, neither make nor autotools.  We use Waf for
       these functions.

       Typically an ns-3 author will work in Linux  or  a  Linux-like  environment.   For  those  running  under
       Windows,  there  do exist environments which simulate the Linux environment to various degrees.  The ns-3
       project has in the past (but not presently) supported development in the  Cygwin  environment  for  these
       users.   See  http://www.cygwin.com/  for  details  on  downloading,  and  visit  the  ns-3 wiki for more
       information about Cygwin and ns-3.  MinGW is presently not officially supported.  Another alternative  to
       Cygwin  is  to  install  a  virtual machine environment such as VMware server and install a Linux virtual
       machine.

   Socket Programming
       We will assume a basic facility with the Berkeley Sockets API in the examples used in this tutorial.   If
       you  are new to sockets, we recommend reviewing the API and some common usage cases.  For a good overview
       of programming TCP/IP sockets we recommend TCP/IP Sockets in C, Donahoo and Calvert.

       There is an associated web site that includes source for the examples in the book, which you can find at:
       http://cs.baylor.edu/~donahoo/practical/CSockets/.

       If  you  understand the first four chapters of the book (or for those who do not have access to a copy of
       the book, the echo clients and servers shown in  the  website  above)  you  will  be  in  good  shape  to
       understand  the  tutorial.  There is a similar book on Multicast Sockets, Multicast Sockets, Makofske and
       Almeroth.  that covers material you may need to understand if you look at the multicast examples  in  the
       distribution.

GETTING STARTED

       This  section  is  aimed at getting a user to a working state starting with a machine that may never have
       had ns-3 installed.  It covers supported platforms, prerequisites, ways to obtain  ns-3,  ways  to  build
       ns-3, and ways to verify your build and run simple programs.

   Overview
       ns-3  is  built  as a system of software libraries that work together.  User programs can be written that
       links with (or imports from) these libraries.  User programs are written in  either  the  C++  or  Python
       programming languages.

       ns-3  is  distributed as source code, meaning that the target system needs to have a software development
       environment to build the libraries first, then build the  user  program.   ns-3  could  in  principle  be
       distributed  as  pre-built  libraries  for selected systems, and in the future it may be distributed that
       way, but at present, many users actually do their work by editing ns-3 itself, so having the source  code
       around  to  rebuild  the  libraries  is  useful.   If  someone  would like to undertake the job of making
       pre-built libraries and packages for operating systems, please contact the ns-developers mailing list.

       In the following, we'll look at two ways of downloading and building ns-3.  The first is to download  and
       build an official release from the main web site.  The second is to fetch and build development copies of
       ns-3.  We'll walk through both examples since the tools involved are slightly different.

   Downloading ns-3
       The ns-3 system as a whole is a fairly  complex  system  and  has  a  number  of  dependencies  on  other
       components.   Along  with  the  systems  you  will  most  likely  deal with every day (the GNU toolchain,
       Mercurial, a text editor) you will need to ensure that a number of additional libraries  are  present  on
       your  system before proceeding.  ns-3 provides a wiki page that includes pages with many useful hints and
       tips.  One such page is the "Installation" page, http://www.nsnam.org/wiki/Installation.

       The "Prerequisites" section of this wiki page explains which packages are required to support common ns-3
       options,  and  also  provides  the commands used to install them for common Linux variants.  Cygwin users
       will have to use the Cygwin installer (if you are a Cygwin user, you used it to install Cygwin).

       You may want to take this opportunity to explore the ns-3 wiki a bit since there really is  a  wealth  of
       information there.

       From  this point forward, we are going to assume that the reader is working in Linux or a Linux emulation
       environment (Linux, Cygwin, etc.) and has the  GNU  toolchain  installed  and  verified  along  with  the
       prerequisites mentioned above.  We are also going to assume that you have Mercurial and Waf installed and
       running on the target system.

       The ns-3 code is available in Mercurial repositories on the server http://code.nsnam.org.  You  can  also
       download  a  tarball  release  at  http://www.nsnam.org/release/, or you can work with repositories using
       Mercurial.  We recommend using Mercurial unless there's a good reason  not  to.   See  the  end  of  this
       section for instructions on how to get a tarball release.

       The  simplest  way  to  get started using Mercurial repositories is to use the ns-3-allinone environment.
       This is a set of scripts that manages the downloading and building of various subsystems of ns-3 for you.
       We recommend that you begin your ns-3 work in this environment.

       One  practice  is to create a directory called workspace in one's home directory under which one can keep
       local Mercurial repositories.  Any directory name will do, but we'll assume that workspace is used herein
       (note:  repos may also be used in some documentation as an example directory name).

   Downloading ns-3 Using a Tarball
       A  tarball  is  a particular format of software archive where multiple files are bundled together and the
       archive possibly compressed.  ns-3 software releases  are  provided  via  a  downloadable  tarball.   The
       process  for  downloading  ns-3  via  tarball is simple; you just have to pick a release, download it and
       decompress it.

       Let's assume that you, as a user, wish to build ns-3 in a local directory called workspace.  If you adopt
       the workspace directory approach, you can get a copy of a release by typing the following into your Linux
       shell (substitute the appropriate version numbers, of course):

          $ cd
          $ mkdir workspace
          $ cd workspace
          $ wget http://www.nsnam.org/release/ns-allinone-3.22.tar.bz2
          $ tar xjf ns-allinone-3.22.tar.bz2

       If you change into the directory ns-allinone-3.22 you should see a number of files:

          $ ls
          bake      constants.py   ns-3.22               README
          build.py  netanim-3.105  pybindgen-0.16.0.886  util.py

       You are now ready to build the base ns-3 distribution.

   Downloading ns-3 Using Bake
       Bake is a tool for distributed integration and building, developed for the ns-3  project.   Bake  can  be
       used to fetch development versions of the ns-3 software, and to download and build extensions to the base
       ns-3 distribution, such as the Direct Code Execution environment, Network Simulation Cradle,  ability  to
       create new Python bindings, and others.

       In  recent ns-3 releases, Bake has been included in the release tarball.  The configuration file included
       in the released version will allow one to download any software that was  current  at  the  time  of  the
       release.   That  is, for example, the version of Bake that is distributed with the ns-3.21 release can be
       used to fetch components for that ns-3 release or earlier, but can't be  used  to  fetch  components  for
       later releases (unless the bakeconf.xml file is updated).

       You can also get the most recent copy of bake by typing the following into your Linux shell (assuming you
       have installed Mercurial):

          $ cd
          $ mkdir workspace
          $ cd workspace
          $ hg clone http://code.nsnam.org/bake

       As the hg (Mercurial) command executes, you should see something like the following displayed,

          ...
          destination directory: bake
          requesting all changes
          adding changesets
          adding manifests
          adding file changes
          added 339 changesets with 796 changes to 63 files
          updating to branch default
          45 files updated, 0 files merged, 0 files removed, 0 files unresolved

       After the clone command completes, you should have a directory called bake, the contents of which  should
       look something like the following:

          $ ls
          bake                  bakeconf.xml  doc       generate-binary.py  TODO
          bake.py               examples      test

       Notice  that  you  really  just downloaded some Python scripts and a Python module called bake.  The next
       step will be to use those scripts to download and build the ns-3 distribution of your choice.

       There are a few configuration targets available:

       1. ns-3.22:  the module corresponding to the release; it will download components similar to the  release
          tarball.

       2. ns-3-dev:  a similar module but using the development code tree

       3. ns-allinone-3.22:   the  module  that includes other optional features such as click routing, openflow
          for ns-3, and the Network Simulation Cradle

       4. ns-3-allinone:  similar to the released version of the allinone module, but for development code.

       The current development snapshot (unreleased) of ns-3 may be  found  at  http://code.nsnam.org/ns-3-dev/.
       The  developers  attempt  to  keep  these  repository  in  consistent,  working  states but they are in a
       development area with unreleased code present, so you may want  to  consider  staying  with  an  official
       release if you do not need newly- introduced features.

       You  can  find the latest version  of the code either by inspection of the repository list or by going to
       the "ns-3 Releases" web page and clicking on the latest release link.  We'll  proceed  in  this  tutorial
       example with ns-3.22.

       We  are now going to use the bake tool to pull down the various pieces of ns-3 you will be using.  First,
       we'll say a word about running bake.

       bake works by downloading source packages into a source directory, and installing libraries into a  build
       directory.  bake can be run by referencing the binary, but if one chooses to run bake from outside of the
       directory it was downloaded into, it is advisable to put bake into your path, such as follows (Linux bash
       shell  example).   First,  change  into  the  'bake'  directory,  and  then set the following environment
       variables

          $ export BAKE_HOME=`pwd`
          $ export PATH=$PATH:$BAKE_HOME:$BAKE_HOME/build/bin
          $ export PYTHONPATH=$PYTHONPATH:$BAKE_HOME:$BAKE_HOME/build/lib

       This will put the bake.py program  into  the  shell's  path,  and  will  allow  other  programs  to  find
       executables  and  libraries created by bake.  Although several bake use cases do not require setting PATH
       and PYTHONPATH as above, full builds of ns-3-allinone (with the optional packages) typically do.

       Step into the workspace directory and type the following into your shell:

          $ ./bake.py configure -e ns-3.22

       Next, we'l ask bake to check whether we have enough tools to download various components.  Type:

          $ ./bake.py check

       You should see something like the following,

          > Python - OK
          > GNU C++ compiler - OK
          > Mercurial - OK
          > CVS - OK
          > GIT - OK
          > Bazaar - OK
          > Tar tool - OK
          > Unzip tool - OK
          > Unrar tool - is missing
          > 7z  data compression utility - OK
          > XZ data compression utility - OK
          > Make - OK
          > cMake - OK
          > patch tool - OK
          > autoreconf tool - OK

          > Path searched for tools: /usr/lib64/qt-3.3/bin /usr/lib64/ccache
          /usr/local/bin /bin /usr/bin /usr/local/sbin /usr/sbin /sbin
          /home/tomh/bin bin

       In particular, download tools such as Mercurial, CVS, GIT, and Bazaar are our principal concerns at  this
       point,  since  they allow us to fetch the code.  Please install missing tools at this stage, in the usual
       way for your system (if you are able to), or contact your system administrator as needed to install these
       tools.

       Next, try to download the software:

          $ ./bake.py download

       should yield something like:

          >> Searching for system dependency pygoocanvas - OK
          >> Searching for system dependency python-dev - OK
          >> Searching for system dependency pygraphviz - OK
          >> Downloading pybindgen-0.16.0.886 - OK
          >> Searching for system dependency g++ - OK
          >> Searching for system dependency qt4 - OK
          >> Downloading netanim-3.105 - OK
          >> Downloading ns-3.22 - OK

       The  above suggests that three sources have been downloaded.  Check the source directory now and type ls;
       one should see:

          $ ls
          netanim-3.105  ns-3.22  pybindgen-0.16.0.886

       You are now ready to build the ns-3 distribution.

   Building ns-3
   Building with build.py
       When working from a released tarball, the first time you build the ns-3 project you  can  build  using  a
       convenience program found in the allinone directory.  This program is called build.py.  This program will
       get the project configured for you in the most commonly useful  way.   However,  please  note  that  more
       advanced configuration and work with ns-3 will typically involve using the native ns-3 build system, Waf,
       to be introduced later in this tutorial.

       If you downloaded using a tarball you should have a  directory  called  something  like  ns-allinone-3.22
       under your ~/workspace directory.  Type the following:

          $ ./build.py --enable-examples --enable-tests

       Because  we  are  working  with  examples  and  tests in this tutorial, and because they are not built by
       default in ns-3, the arguments for build.py tells it to build them for us.  The program also defaults  to
       building  all  available modules.  Later, you can build ns-3 without examples and tests, or eliminate the
       modules that are not necessary for your work, if you wish.

       You will see lots of typical compiler output messages displayed as the build script  builds  the  various
       pieces you downloaded.  Eventually you should see the following:

          Waf: Leaving directory `/path/to/workspace/ns-allinone-3.22/ns-3.22/build'
          'build' finished successfully (6m25.032s)

          Modules built:
          antenna                   aodv                      applications
          bridge                    buildings                 config-store
          core                      csma                      csma-layout
          dsdv                      dsr                       energy
          fd-net-device             flow-monitor              internet
          lr-wpan                   lte                       mesh
          mobility                  mpi                       netanim (no Python)
          network                   nix-vector-routing        olsr
          point-to-point            point-to-point-layout     propagation
          sixlowpan                 spectrum                  stats
          tap-bridge                test (no Python)          topology-read
          uan                       virtual-net-device        wave
          wifi                      wimax

          Modules not built (see ns-3 tutorial for explanation):
          brite                     click                     openflow
          visualizer

          Leaving directory `./ns-3.22'

       Regarding the portion about modules not built:

          Modules not built (see ns-3 tutorial for explanation):
          brite                     click                     openflow
          visualizer

       This  just  means  that  some  ns-3 modules that have dependencies on outside libraries may not have been
       built, or that the configuration specifically asked not to  build  them.   It  does  not  mean  that  the
       simulator  did  not  build  successfully  or that it will provide wrong results for the modules listed as
       being built.

   Building with bake
       If you used bake above to fetch source code from project repositories, you may  continue  to  use  it  to
       build ns-3.  Type

          $ ./bake.py build

       and you should see something like:

          >> Building pybindgen-0.16.0.886 - OK
          >> Building netanim-3.105 - OK
          >> Building ns-3.22 - OK

       Hint:  you can also perform both steps, download and build by calling 'bake.py deploy'.

       If there happens to be a failure, please have a look at what the following command tells you; it may give
       a hint as to a missing dependency:

          $ ./bake.py show

       This will list out the various dependencies of the packages you are trying to build.

   Building with Waf
       Up to this point, we have used either the build.py script, or the bake tool, to get started with building
       ns-3.   These  tools  are  useful for building ns-3 and supporting libraries, and they call into the ns-3
       directory to call the Waf build tool to do the actual building.  Most users quickly transition  to  using
       Waf  directly  to  configure and build ns-3.  So, to proceed, please change your working directory to the
       ns-3 directory that you have initially built.

       It's not strictly required at this point, but it will be valuable to take a slight detour and look at how
       to  make  changes to the configuration of the project.  Probably the most useful configuration change you
       can make will be to build the optimized version of the code.  By default you have configured your project
       to  build  the debug version.  Let's tell the project to make an optimized build.  To explain to Waf that
       it should do optimized builds that include the examples and tests, you will need to execute the following
       commands:

          $ ./waf clean
          $ ./waf --build-profile=optimized --enable-examples --enable-tests configure

       This runs Waf out of the local directory (which is provided as a convenience for you).  The first command
       to clean out the previous build is not typically strictly necessary but is good practice (but  see  Build
       Profiles,  below);  it  will  remove  the  previously built libraries and object files found in directory
       build/.  When the project is reconfigured and the build  system  checks  for  various  dependencies,  you
       should see output that looks similar to the following:

          Setting top to                           : .
          Setting out to                           : build
          Checking for 'gcc' (c compiler)          : /usr/bin/gcc
          Checking for cc version                  : 4.2.1
          Checking for 'g++' (c++ compiler)        : /usr/bin/g++
          Checking boost includes                  : 1_46_1
          Checking boost libs                      : ok
          Checking for boost linkage               : ok
          Checking for click location              : not found
          Checking for program pkg-config          : /sw/bin/pkg-config
          Checking for 'gtk+-2.0' >= 2.12          : yes
          Checking for 'libxml-2.0' >= 2.7         : yes
          Checking for type uint128_t              : not found
          Checking for type __uint128_t            : yes
          Checking high precision implementation   : 128-bit integer (default)
          Checking for header stdint.h             : yes
          Checking for header inttypes.h           : yes
          Checking for header sys/inttypes.h       : not found
          Checking for header sys/types.h          : yes
          Checking for header sys/stat.h           : yes
          Checking for header dirent.h             : yes
          Checking for header stdlib.h             : yes
          Checking for header signal.h             : yes
          Checking for header pthread.h            : yes
          Checking for header stdint.h             : yes
          Checking for header inttypes.h           : yes
          Checking for header sys/inttypes.h       : not found
          Checking for library rt                  : not found
          Checking for header netpacket/packet.h   : not found
          Checking for header sys/ioctl.h          : yes
          Checking for header net/if.h             : not found
          Checking for header net/ethernet.h       : yes
          Checking for header linux/if_tun.h       : not found
          Checking for header netpacket/packet.h   : not found
          Checking for NSC location                : not found
          Checking for 'mpic++'                    : yes
          Checking for 'sqlite3'                   : yes
          Checking for header linux/if_tun.h       : not found
          Checking for program sudo                : /usr/bin/sudo
          Checking for program valgrind            : /sw/bin/valgrind
          Checking for 'gsl'                       : yes
          Checking for compilation flag -Wno-error=deprecated-d... support : ok
          Checking for compilation flag -Wno-error=deprecated-d... support : ok
          Checking for compilation flag -fstrict-aliasing... support       : ok
          Checking for compilation flag -fstrict-aliasing... support       : ok
          Checking for compilation flag -Wstrict-aliasing... support       : ok
          Checking for compilation flag -Wstrict-aliasing... support       : ok
          Checking for program doxygen                                     : /usr/local/bin/doxygen
          ---- Summary of optional NS-3 features:
          Build profile                 : debug
          Build directory               : build
          Python Bindings               : enabled
          BRITE Integration             : not enabled (BRITE not enabled (see option --with-brite))
          NS-3 Click Integration        : not enabled (nsclick not enabled (see option --with-nsclick))
          GtkConfigStore                : enabled
          XmlIo                         : enabled
          Threading Primitives          : enabled
          Real Time Simulator           : enabled (librt is not available)
          Emulated Net Device           : enabled (<netpacket/packet.h> include not detected)
          File descriptor NetDevice     : enabled
          Tap FdNetDevice               : not enabled (needs linux/if_tun.h)
          Emulation FdNetDevice         : not enabled (needs netpacket/packet.h)
          PlanetLab FdNetDevice         : not enabled (PlanetLab operating system not detected (see option --force-planetlab))
          Network Simulation Cradle     : not enabled (NSC not found (see option --with-nsc))
          MPI Support                   : enabled
          NS-3 OpenFlow Integration     : not enabled (Required boost libraries not found, missing: system, signals, filesystem)
          SQlite stats data output      : enabled
          Tap Bridge                    : not enabled (<linux/if_tun.h> include not detected)
          PyViz visualizer              : enabled
          Use sudo to set suid bit      : not enabled (option --enable-sudo not selected)
          Build tests                   : enabled
          Build examples                : enabled
          GNU Scientific Library (GSL)  : enabled
          'configure' finished successfully (1.944s)

       Note  the last part of the above output.  Some ns-3 options are not enabled by default or require support
       from the underlying system to work properly.  For instance, to enable XmlTo, the library libxml-2.0  must
       be  found  on  the  system.   If this library were not found, the corresponding ns-3 feature would not be
       enabled and a message would be displayed.  Note further that there is a feature to use the  program  sudo
       to  set the suid bit of certain programs.  This is not enabled by default and so this feature is reported
       as "not enabled."

       Now go ahead and switch back to the debug build that includes the examples and tests.

          $ ./waf clean
          $ ./waf --build-profile=debug --enable-examples --enable-tests configure

       The build system is now configured and you can build the debug versions of the ns-3  programs  by  simply
       typing

          $ ./waf

       Okay,  sorry,  I  made  you  build  the ns-3 part of the system twice, but now you know how to change the
       configuration and build optimized code.

       The build.py script discussed above supports also the --enable-examples and enable-tests  arguments,  but
       in general, does not directly support other waf options; for example, this will not work:

          $ ./build.py --disable-python

       will result in

          build.py: error: no such option: --disable-python

       However, the special operator -- can be used to pass additional options through to waf, so instead of the
       above, the following will work:

          $ ./build.py -- --disable-python

       as it generates the underlying command ./waf configure --disable-python.

       Here are a few more introductory tips about Waf.

   Configure vs. Build
       Some Waf commands are only meaningful during the configure phase and some commands are valid in the build
       phase.   For  example,  if  you  wanted  to  use the emulation features of ns-3, you might want to enable
       setting the suid bit using sudo as described above.  This turns out to be a  configuration-time  command,
       and so you could reconfigure using the following command that also includes the examples and tests.

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

       If you do this, Waf will have run sudo to change the socket creator programs of the emulation code to run
       as root.

       There are many other configure- and build-time options available in Waf.  To explore these options, type:

          $ ./waf --help

       We'll use some of the testing-related commands in the next section.

   Build Profiles
       We already saw how you can configure Waf for debug or optimized builds:

          $ ./waf --build-profile=debug

       There is also an intermediate build profile, release.  -d is a synonym for --build-profile.

       By default Waf puts the build artifacts in the build directory.   You  can  specify  a  different  output
       directory with the --out option, e.g.

          $ ./waf configure --out=foo

       Combining this with build profiles lets you switch between the different compile options in a clean way:

          $ ./waf configure --build-profile=debug --out=build/debug
          $ ./waf build
          ...
          $ ./waf configure --build-profile=optimized --out=build/optimized
          $ ./waf build
          ...

       This  allows  you  to  work with multiple builds rather than always overwriting the last build.  When you
       switch, Waf will only compile what it has to, instead of recompiling everything.

       When you do switch build profiles like this, you have to  be  careful  to  give  the  same  configuration
       parameters  each  time.   It  may  be  convenient  to define some environment variables to help you avoid
       mistakes:

          $ export NS3CONFIG="--enable-examples --enable-tests"
          $ export NS3DEBUG="--build-profile=debug --out=build/debug"
          $ export NS3OPT=="--build-profile=optimized --out=build/optimized"

          $ ./waf configure $NS3CONFIG $NS3DEBUG
          $ ./waf build
          ...
          $ ./waf configure $NS3CONFIG $NS3OPT
          $ ./waf build

   Compilers
       In the examples above, Waf uses the GCC C++ compiler, g++, for building ns-3. However, it's  possible  to
       change  the  C++  compiler used by Waf by defining the CXX environment variable.  For example, to use the
       Clang C++ compiler, clang++,

          $ CXX="clang++" ./waf configure
          $ ./waf build

       One can also set up Waf to do distributed compilation with distcc in a similar way:

          $ CXX="distcc g++" ./waf configure
          $ ./waf build

       More info on distcc and distributed compilation can be found on it's  project  page  under  Documentation
       section.

   Install
       Waf  may  be  used  to  install  libraries  in  various places on the system.  The default location where
       libraries and executables are built is in the build directory, and because  Waf  knows  the  location  of
       these libraries and executables, it is not necessary to install the libraries elsewhere.

       If  users  choose  to  install  things  outside of the build directory, users may issue the ./waf install
       command.  By default, the prefix for installation is /usr/local, so ./waf install will  install  programs
       into  /usr/local/bin,  libraries  into  /usr/local/lib,  and  headers into /usr/local/include.  Superuser
       privileges are typically needed to install to the default prefix, so the typical command  would  be  sudo
       ./waf  install.   When  running  programs  with Waf, Waf will first prefer to use shared libraries in the
       build directory, then will look for libraries in the library path configured in  the  local  environment.
       So  when installing libraries to the system, it is good practice to check that the intended libraries are
       being used.

       Users may choose to install to a different prefix by passing the --prefix option at configure time,  such
       as:

          ./waf configure --prefix=/opt/local

       If later after the build the user issues the ./waf install command, the prefix /opt/local will be used.

       The  ./waf clean command should be used prior to reconfiguring the project if Waf will be used to install
       things at a different prefix.

       In summary, it is not necessary to call ./waf install to use ns-3.  Most users will not need this command
       since  Waf will pick up the current libraries from the build directory, but some users may find it useful
       if their use case involves working with programs outside of the ns-3 directory.

   One Waf
       There is only one Waf script, at the top level of the ns-3 source  tree.   As  you  work,  you  may  find
       yourself  spending  a  lot of time in scratch/, or deep in src/..., and needing to invoke Waf.  You could
       just remember where you are, and invoke Waf like this:

          $ ../../../waf ...

       but that get's tedious, and error prone, and there are better solutions.

       If you have the full ns-3 repository this little gem is a start:

          $ cd $(hg root) && ./waf ...

       Even better is to define this as a shell function:

          $ function waff { cd $(hg root) && ./waf $* ; }

          $ waff build

       If you only have the tarball, an environment variable can help:

          $ export NS3DIR="$PWD"
          $ function waff { cd $NS3DIR && ./waf $* ; }

          $ cd scratch
          $ waff build

       It might be tempting in a module directory to add a trivial waf script along the lines of exec ../../waf.
       Please  don't.   It's confusing to new-comers, and when done poorly it leads to subtle build errors.  The
       solutions above are the way to go.

   Testing ns-3
       You can run the unit tests of the ns-3 distribution by running the ./test.py -c core script:

          $ ./test.py -c core

       These tests are run in parallel by Waf. You should eventually see a report saying that

          92 of 92 tests passed (92 passed, 0 failed, 0 crashed, 0 valgrind errors)

       This is the important message.

       You will also see the summary output from Waf and  the  test  runner  executing  each  test,  which  will
       actually look something like:

          Waf: Entering directory `/path/to/workspace/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/path/to/workspace/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (1.799s)

          Modules built:
          aodv                      applications              bridge
          click                     config-store              core
          csma                      csma-layout               dsdv
          emu                       energy                    flow-monitor
          internet                  lte                       mesh
          mobility                  mpi                       netanim
          network                   nix-vector-routing        ns3tcp
          ns3wifi                   olsr                      openflow
          point-to-point            point-to-point-layout     propagation
          spectrum                  stats                     tap-bridge
          template                  test                      tools
          topology-read             uan                       virtual-net-device
          visualizer                wifi                      wimax

          PASS: TestSuite ns3-wifi-interference
          PASS: TestSuite histogram

          ...

          PASS: TestSuite object
          PASS: TestSuite random-number-generators
          92 of 92 tests passed (92 passed, 0 failed, 0 crashed, 0 valgrind errors)

       This  command  is typically run by users to quickly verify that an ns-3 distribution has built correctly.
       (Note the order of the PASS: ...  lines can vary, which is okay.  What's important is  that  the  summary
       line at the end report that all tests passed; none failed or crashed.)

   Running a Script
       We  typically  run  scripts  under  the  control of Waf.  This allows the build system to ensure that the
       shared library paths are set correctly and that the libraries are  available  at  run  time.   To  run  a
       program, simply use the --run option in Waf.  Let's run the ns-3 equivalent of the ubiquitous hello world
       program by typing the following:

          $ ./waf --run hello-simulator

       Waf first checks to make sure that the program is built correctly and executes a build if required.   Waf
       then executes the program, which produces the following output.

          Hello Simulator

       Congratulations!  You are now an ns-3 user!

       What do I do if I don't see the output?

       If  you  see Waf messages indicating that the build was completed successfully, but do not see the "Hello
       Simulator" output, chances are that you have switched your build mode to optimized in the  Building  with
       Waf  section,  but  have  missed  the  change back to debug mode.  All of the console output used in this
       tutorial uses a special ns-3 logging component that is useful for printing user messages to the  console.
       Output  from this component is automatically disabled when you compile optimized code -- it is "optimized
       out."  If you don't see the "Hello Simulator" output, type the following:

          $ ./waf configure --build-profile=debug --enable-examples --enable-tests

       to tell Waf to build the debug versions of the ns-3 programs that includes the examples and  tests.   You
       must still build the actual debug version of the code by typing

          $ ./waf

       Now, if you run the hello-simulator program, you should see the expected output.

   Program Arguments
       To feed command line arguments to an ns-3 program use this pattern:

          $ ./waf --run <ns3-program> --command-template="%s <args>"

       Substitute  your  program  name  for <ns3-program>, and the arguments for <args>.  The --command-template
       argument to Waf is basically a recipe for constructing the actual command line Waf should use to  execute
       the  program.   Waf  checks  that  the build is complete, sets the shared library paths, then invokes the
       executable using the provided command line template, inserting the program name for the  %s  placeholder.
       (I admit this is a bit awkward, but that's the way it is.  Patches welcome!)

       Another  particularly  useful  example is to run a test suite by itself.  Let's assume that a mytest test
       suite exists (it doesn't).  Above, we used the ./test.py script to run a whole slew of tests in parallel,
       by  repeatedly  invoking  the  real  testing  program, test-runner.  To invoke test-runner directly for a
       single test:

          $ ./waf --run test-runner --command-template="%s --suite=mytest --verbose"

       This passes the arguments to the test-runner program.  Since mytest does not exist, an error message will
       be generated.  To print the available test-runner options:

          $ ./waf --run test-runner --command-template="%s --help"

   Debugging
       To  run  ns-3  programs  under  the  control  of another utility, such as a debugger (e.g. gdb) or memory
       checker (e.g. valgrind), you use a similar --command-template="..." form.

       For example, to run your ns-3 program hello-simulator with the arguments <args> under the gdb debugger:

          $ ./waf --run=hello-simulator --command-template="gdb %s --args <args>"

       Notice that the ns-3 program name goes with the --run argument, and the control utility (here gdb) is the
       first  token in the --commmand-template argument.  The --args tells gdb that the remainder of the command
       line belongs to the "inferior" program.  (Some gdb's don't understand the --args feature.  In this  case,
       omit the program arguments from the --command-template, and use the gdb command set args.)

       We can combine this recipe and the previous one to run a test under the debugger:

          $ ./waf --run test-runner --command-template="gdb %s --args --suite=mytest --verbose"

   Working Directory
       Waf  needs  to  run  from  it's location at the top of the ns-3 tree.  This becomes the working directory
       where output files will be written.  But what if you want to keep those ouf to the ns-3 source tree?  Use
       the --cwd argument:

          $ ./waf --cwd=...

       It  may be more convenient to start with your working directory where you want the output files, in which
       case a little indirection can help:

          $ function waff {
              CWD="$PWD"
              cd $NS3DIR >/dev/null
              ./waf --cwd="$CWD" $*
              cd - >/dev/null
            }

       This embellishment of the previous  version  saves  the  current  working  directory,  cd's  to  the  Waf
       directory, then instructs Waf to change the working directory back to the saved current working directory
       before running the program.

CONCEPTUAL OVERVIEW

       The first thing we need to do before actually starting to look at or write ns-3 code is to explain a  few
       core concepts and abstractions in the system.  Much of this may appear transparently obvious to some, but
       we recommend taking the time to read through this section just to ensure  you  are  starting  on  a  firm
       foundation.

   Key Abstractions
       In  this  section,  we'll  review  some  terms  that are commonly used in networking, but have a specific
       meaning in ns-3.

   Node
       In Internet jargon, a computing device that connects to a network is called a host or  sometimes  an  end
       system.  Because ns-3 is a network simulator, not specifically an Internet simulator, we intentionally do
       not use the term host since it is closely associated with the Internet and its  protocols.   Instead,  we
       use a more generic term also used by other simulators that originates in Graph Theory --- the node.

       In  ns-3  the  basic computing device abstraction is called the node.  This abstraction is represented in
       C++ by the class Node.  The Node class provides methods for managing  the  representations  of  computing
       devices in simulations.

       You  should  think  of  a  Node  as a computer to which you will add functionality.  One adds things like
       applications, protocol stacks and peripheral cards with their associated drivers to enable  the  computer
       to do useful work.  We use the same basic model in ns-3.

   Application
       Typically,  computer  software  is  divided  into  two  broad classes.  System Software organizes various
       computer resources such as memory, processor cycles, disk, network, etc.,  according  to  some  computing
       model.   System  software  usually does not use those resources to complete tasks that directly benefit a
       user.  A user would typically run an application that acquires and uses the resources controlled  by  the
       system software to accomplish some goal.

       Often,  the  line  of  separation  between system and application software is made at the privilege level
       change that happens in operating system traps.  In ns-3 there is no real concept of operating system  and
       especially  no  concept  of  privilege  levels  or  system  calls.   We  do, however, have the idea of an
       application.  Just as software applications run on computers to perform tasks in the "real  world,"  ns-3
       applications run on ns-3 Nodes to drive simulations in the simulated world.

       In  ns-3  the  basic  abstraction  for a user program that generates some activity to be simulated is the
       application.  This abstraction is represented in C++ by the class  Application.   The  Application  class
       provides  methods  for  managing  the  representations  of  our  version  of  user-level  applications in
       simulations.  Developers are  expected  to  specialize  the  Application  class  in  the  object-oriented
       programming  sense  to  create  new applications.  In this tutorial, we will use specializations of class
       Application called UdpEchoClientApplication and UdpEchoServerApplication.  As  you  might  expect,  these
       applications compose a client/server application set used to generate and echo simulated network packets

   Channel
       In  the  real  world,  one can connect a computer to a network.  Often the media over which data flows in
       these networks are called channels.  When you connect your Ethernet cable to the plug in  the  wall,  you
       are  connecting  your computer to an Ethernet communication channel.  In the simulated world of ns-3, one
       connects a Node to an  object  representing  a  communication  channel.   Here  the  basic  communication
       subnetwork abstraction is called the channel and is represented in C++ by the class Channel.

       The  Channel class provides methods for managing communication subnetwork objects and connecting nodes to
       them.  Channels may also be specialized by developers  in  the  object  oriented  programming  sense.   A
       Channel  specialization may model something as simple as a wire.  The specialized  Channel can also model
       things as complicated as a large Ethernet switch, or three-dimensional space full of obstructions in  the
       case of wireless networks.

       We  will  use specialized versions of the Channel called CsmaChannel, PointToPointChannel and WifiChannel
       in this tutorial.  The CsmaChannel, for example, models a version  of  a  communication  subnetwork  that
       implements   a  carrier  sense  multiple  access  communication  medium.   This  gives  us  Ethernet-like
       functionality.

   Net Device
       It used to be the case that if you wanted to connect a computers to a network, you had to buy a  specific
       kind  of  network cable and a hardware device called (in PC terminology) a peripheral card that needed to
       be installed in your computer.  If the peripheral card implemented some networking  function,  they  were
       called  Network  Interface Cards, or NICs.  Today most computers come with the network interface hardware
       built in and users don't see these building blocks.

       A NIC will not work without a software driver to control the hardware.  In Unix (or Linux),  a  piece  of
       peripheral  hardware is classified as a device.  Devices are controlled using device drivers, and network
       devices (NICs) are controlled using network device drivers collectively known as net  devices.   In  Unix
       and Linux you refer to these net devices by names such as eth0.

       In  ns-3  the  net  device abstraction covers both the software driver and the simulated hardware.  A net
       device is "installed" in a Node in order to enable the Node  to  communicate  with  other  Nodes  in  the
       simulation  via  Channels.   Just as in a real computer, a Node may be connected to more than one Channel
       via multiple NetDevices.

       The net device abstraction is represented in C++ by the class NetDevice.  The  NetDevice  class  provides
       methods for managing connections to Node and Channel objects; and may be specialized by developers in the
       object-oriented programming sense.  We will use the several specialized versions of the NetDevice  called
       CsmaNetDevice,  PointToPointNetDevice,  and  WifiNetDevice  in this tutorial.  Just as an Ethernet NIC is
       designed to work with an Ethernet network, the CsmaNetDevice is designed to work with a CsmaChannel;  the
       PointToPointNetDevice  is  designed to work with a PointToPointChannel and a WifiNetNevice is designed to
       work with a WifiChannel.

   Topology Helpers
       In a real network, you will find host computers with added (or built-in) NICs.  In ns-3 we would say that
       you will find Nodes with attached NetDevices.  In a large simulated network you will need to arrange many
       connections between Nodes, NetDevices and Channels.

       Since connecting NetDevices to Nodes, NetDevices to Channels, assigning IP  addresses,   etc.,  are  such
       common  tasks  in  ns-3,  we provide what we call topology helpers to make this as easy as possible.  For
       example, it may take many distinct ns-3 core operations to create a NetDevice, add a MAC address, install
       that  net  device  on  a  Node,  configure the node's protocol stack, and then connect the NetDevice to a
       Channel.  Even more operations would be required to connect multiple devices onto multipoint channels and
       then to connect individual networks together into internetworks.  We provide topology helper objects that
       combine those many distinct operations into an easy to use model for your convenience.

   A First ns-3 Script
       If you downloaded the system as was suggested above, you will have a  release  of  ns-3  in  a  directory
       called  repos  under  your  home  directory.   Change  into that release directory, and you should find a
       directory structure something like the following:

          AUTHORS       examples       scratch        utils      waf.bat*
          bindings      LICENSE        src            utils.py   waf-tools
          build         ns3            test.py*       utils.pyc  wscript
          CHANGES.html  README         testpy-output  VERSION    wutils.py
          doc           RELEASE_NOTES  testpy.supp    waf*       wutils.pyc

       Change into the examples/tutorial directory.  You should see a file named first.cc located  there.   This
       is  a  script  that  will  create a simple point-to-point link between two nodes and echo a single packet
       between the nodes.  Let's take a look at that script line by line, so go ahead and open first.cc in  your
       favorite editor.

   Boilerplate
       The  first  line  in  the  file is an emacs mode line.  This tells emacs about the formatting conventions
       (coding style) we use in our source code.

          /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */

       This is always a somewhat controversial subject, so we might as well get it out of the  way  immediately.
       The ns-3 project, like most large projects, has adopted a coding style to which all contributed code must
       adhere.  If you want to contribute your code to the project, you will eventually have to conform  to  the
       ns-3 coding standard as described in the file doc/codingstd.txt or shown on the project web page here.

       We  recommend  that  you,  well,  just get used to the look and feel of ns-3 code and adopt this standard
       whenever you are working with our code.  All of the development team and contributors have done  so  with
       various amounts of grumbling.  The emacs mode line above makes it easier to get the formatting correct if
       you use the emacs editor.

       The ns-3 simulator is licensed using the GNU General Public License.  You will see  the  appropriate  GNU
       legalese  at  the head of every file in the ns-3 distribution.  Often you will see a copyright notice for
       one of the institutions involved in the ns-3 project above the GPL text and an author listed below.

          /*
           * This program is free software; you can redistribute it and/or modify
           * it under the terms of the GNU General Public License version 2 as
           * published by the Free Software Foundation;
           *
           * This program is distributed in the hope that it will be useful,
           * but WITHOUT ANY WARRANTY; without even the implied warranty of
           * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
           * GNU General Public License for more details.
           *
           * You should have received a copy of the GNU General Public License
           * along with this program; if not, write to the Free Software
           * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA
           */

   Module Includes
       The code proper starts with a number of include statements.

          #include "ns3/core-module.h"
          #include "ns3/network-module.h"
          #include "ns3/internet-module.h"
          #include "ns3/point-to-point-module.h"
          #include "ns3/applications-module.h"

       To help our high-level script users deal with the large number of include files present in the system, we
       group  includes  according  to  relatively  large  modules.   We  provide a single include file that will
       recursively load all of the include files used in each module.  Rather than having  to  look  up  exactly
       what header you need, and possibly have to get a number of dependencies right, we give you the ability to
       load a group of files at a large granularity.  This is not the most efficient approach but  it  certainly
       makes writing scripts much easier.

       Each of the ns-3 include files is placed in a directory called ns3 (under the build directory) during the
       build process to help avoid include file name collisions.  The ns3/core-module.h file corresponds to  the
       ns-3 module you will find in the directory src/core in your downloaded release distribution.  If you list
       this directory you will find a large number of header files.  When you do a build, Waf will place  public
       header files in an ns3 directory under the appropriate build/debug or build/optimized directory depending
       on your configuration.  Waf will also automatically generate a module include file to  load  all  of  the
       public header files.

       Since you are, of course, following this tutorial religiously, you will already have done a

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

       in order to configure the project to perform debug builds that include examples and tests.  You will also
       have done a

          $ ./waf

       to build the project.  So now if you look in the directory ../../build/debug/ns3 you will find  the  four
       module  include files shown above.  You can take a look at the contents of these files and find that they
       do include all of the public include files in their respective modules.

   Ns3 Namespace
       The next line in the first.cc script is a namespace declaration.

          using namespace ns3;

       The ns-3 project is implemented in a C++ namespace called ns3.  This groups all ns-3-related declarations
       in  a  scope outside the global namespace, which we hope will help with integration with other code.  The
       C++ using statement introduces the ns-3 namespace into the current (global) declarative region.  This  is
       a  fancy  way  of  saying  that  after this declaration, you will not have to type ns3:: scope resolution
       operator before all of the ns-3 code in order to use it.  If you are unfamiliar with  namespaces,  please
       consult  almost  any  C++ tutorial and compare the ns3 namespace and usage here with instances of the std
       namespace and the using namespace std; statements you will often find in discussions of cout and streams.

   Logging
       The next line of the script is the following,

          NS_LOG_COMPONENT_DEFINE ("FirstScriptExample");

       We will use this statement as a convenient place to talk about our Doxygen documentation system.  If  you
       look  at  the  project  web site, ns-3 project, you will find a link to "Documentation" in the navigation
       bar.  If you select this link, you will be taken to our documentation page. There is a  link  to  "Latest
       Release"  that  will  take you to the documentation for the latest stable release of ns-3.  If you select
       the "API Documentation" link, you will be taken to the ns-3 API documentation page.

       Along the left side, you will find a graphical representation of the structure of the  documentation.   A
       good  place  to  start is the NS-3 Modules "book" in the ns-3 navigation tree.  If you expand Modules you
       will see a list of ns-3 module documentation.  The concept of module here ties directly into  the  module
       include files discussed above.  The ns-3 logging subsystem is discussed in the C++ Constructs Used by All
       Modules section, so go ahead and expand that documentation node.  Now, expand the Debugging book and then
       select the Logging page.

       You  should now be looking at the Doxygen documentation for the Logging module.  In the list of #define's
       at the top of the page you will see the entry for NS_LOG_COMPONENT_DEFINE.  Before jumping in,  it  would
       probably  be  good  to  look  for  the "Detailed Description" of the logging module to get a feel for the
       overall operation.  You can either scroll down or select  the  "More..."  link  under  the  collaboration
       diagram to do this.

       Once  you  have  a  general  idea  of  what  is  going  on,  go  ahead  and  take  a look at the specific
       NS_LOG_COMPONENT_DEFINE documentation.  I won't duplicate the documentation here, but to summarize,  this
       line declares a logging component called FirstScriptExample that allows you to enable and disable console
       message logging by reference to the name.

   Main Function
       The next lines of the script you will find are,

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

       This is just the declaration of the main function of your program (script).  Just as in any C++  program,
       you  need to define a main function that will be the first function run.  There is nothing at all special
       here.  Your ns-3 script is just a C++ program.

       The next line sets the time resolution to one nanosecond, which happens to be the default value:

          Time::SetResolution (Time::NS);

       The resolution is the smallest time value that can be represented (as well as the smallest  representable
       difference between two time values).  You can change the resolution exactly once.  The mechanism enabling
       this flexibility is somewhat memory hungry, so once the resolution has been set explicitly we release the
       memory, preventing further updates.   (If you don't set the resolution explicitly, it will default to one
       nanosecond, and the memory will be released when the simulation starts.)

       The next two lines of the script are used to enable two logging components that are built into  the  Echo
       Client and Echo Server applications:

          LogComponentEnable("UdpEchoClientApplication", LOG_LEVEL_INFO);
          LogComponentEnable("UdpEchoServerApplication", LOG_LEVEL_INFO);

       If  you  have read over the Logging component documentation you will have seen that there are a number of
       levels of logging verbosity/detail that you can enable on each component.  These two lines of code enable
       debug  logging  at  the  INFO  level  for  echo clients and servers.  This will result in the application
       printing out messages as packets are sent and received during the simulation.

       Now we will get directly to the business of creating a topology and running a  simulation.   We  use  the
       topology helper objects to make this job as easy as possible.

   Topology Helpers
   NodeContainer
       The  next  two lines of code in our script will actually create the ns-3 Node objects that will represent
       the computers in the simulation.

          NodeContainer nodes;
          nodes.Create (2);

       Let's find the documentation for the NodeContainer class before we continue.  Another way to get into the
       documentation  for  a  given  class  is  via the Classes tab in the Doxygen pages.  If you still have the
       Doxygen handy, just scroll up to the top of the page and select the Classes tab.  You should  see  a  new
       set  of  tabs  appear, one of which is Class List.  Under that tab you will see a list of all of the ns-3
       classes.  Scroll down, looking for ns3::NodeContainer.  When you find the class, go ahead and  select  it
       to go to the documentation for the class.

       You  may recall that one of our key abstractions is the Node.  This represents a computer to which we are
       going to add things like protocol stacks, applications and peripheral cards.  The NodeContainer  topology
       helper provides a convenient way to create, manage and access any Node objects that we create in order to
       run a simulation.  The first line above just declares a NodeContainer which we call  nodes.   The  second
       line  calls  the  Create  method  on  the  nodes  object  and asks the container to create two nodes.  As
       described in the Doxygen, the container calls down into the ns-3 system proper to create two Node objects
       and stores pointers to those objects internally.

       The nodes as they stand in the script do nothing.  The next step in constructing a topology is to connect
       our nodes together into a network.  The simplest form of network we support is  a  single  point-to-point
       link between two nodes.  We'll construct one of those links here.

   PointToPointHelper
       We  are constructing a point to point link, and, in a pattern which will become quite familiar to you, we
       use a topology helper object to do the low-level work required to put the link together.  Recall that two
       of  our  key  abstractions  are the NetDevice and the Channel.  In the real world, these terms correspond
       roughly to peripheral cards and network cables.  Typically these two things are intimately tied  together
       and  one cannot expect to interchange, for example, Ethernet devices and wireless channels.  Our Topology
       Helpers follow this intimate coupling and therefore you will use a single PointToPointHelper to configure
       and connect ns-3 PointToPointNetDevice and PointToPointChannel objects in this script.

       The next three lines in the script are,

          PointToPointHelper pointToPoint;
          pointToPoint.SetDeviceAttribute ("DataRate", StringValue ("5Mbps"));
          pointToPoint.SetChannelAttribute ("Delay", StringValue ("2ms"));

       The first line,

          PointToPointHelper pointToPoint;

       instantiates a PointToPointHelper object on the stack.  From a high-level perspective the next line,

          pointToPoint.SetDeviceAttribute ("DataRate", StringValue ("5Mbps"));

       tells the PointToPointHelper object to use the value "5Mbps" (five megabits per second) as the "DataRate"
       when it creates a PointToPointNetDevice object.

       From a more detailed perspective, the string "DataRate" corresponds to what we call an Attribute  of  the
       PointToPointNetDevice.   If  you  look  at  the Doxygen for class ns3::PointToPointNetDevice and find the
       documentation for the GetTypeId method, you will find a list  of   Attributes  defined  for  the  device.
       Among  these  is  the  "DataRate"  Attribute.   Most  user-visible  ns-3  objects  have  similar lists of
       Attributes.  We use this mechanism to easily configure simulations without recompiling as you will see in
       a following section.

       Similar  to the "DataRate" on the PointToPointNetDevice you will find a "Delay" Attribute associated with
       the PointToPointChannel.  The final line,

          pointToPoint.SetChannelAttribute ("Delay", StringValue ("2ms"));

       tells the PointToPointHelper to use the value "2ms" (two milliseconds) as the value of  the  transmission
       delay of every point to point channel it subsequently creates.

   NetDeviceContainer
       At   this  point  in  the  script,  we  have  a  NodeContainer  that  contains  two  nodes.   We  have  a
       PointToPointHelper that is primed and ready to make PointToPointNetDevices and  wire  PointToPointChannel
       objects  between  them.  Just as we used the NodeContainer topology helper object to create the Nodes for
       our simulation, we will ask the PointToPointHelper to do the work involved in creating,  configuring  and
       installing  our  devices  for  us.   We will need to have a list of all of the NetDevice objects that are
       created, so we use a NetDeviceContainer to hold them just as we used a NodeContainer to hold the nodes we
       created.  The following two lines of code,

          NetDeviceContainer devices;
          devices = pointToPoint.Install (nodes);

       will  finish configuring the devices and channel.  The first line declares the device container mentioned
       above and the second does the heavy lifting.  The  Install  method  of  the  PointToPointHelper  takes  a
       NodeContainer  as  a  parameter.   Internally,  a  NetDeviceContainer  is  created.  For each node in the
       NodeContainer (there must be exactly two for a point-to-point link) a  PointToPointNetDevice  is  created
       and  saved  in the device container.  A PointToPointChannel is created and the two PointToPointNetDevices
       are attached.  When objects are created by the PointToPointHelper, the Attributes previously set  in  the
       helper are used to initialize the corresponding Attributes in the created objects.

       After  executing  the  pointToPoint.Install  (nodes)  call we will have two nodes, each with an installed
       point-to-point net device and a single  point-to-point  channel  between  them.   Both  devices  will  be
       configured  to  transmit  data  at  five megabits per second over the channel which has a two millisecond
       transmission delay.

   InternetStackHelper
       We now have nodes and devices configured, but we don't have any protocol stacks installed on  our  nodes.
       The next two lines of code will take care of that.

          InternetStackHelper stack;
          stack.Install (nodes);

       The InternetStackHelper is a topology helper that is to internet stacks what the PointToPointHelper is to
       point-to-point net devices.  The Install method takes  a  NodeContainer  as  a  parameter.   When  it  is
       executed,  it  will  install  an  Internet  Stack  (TCP,  UDP, IP, etc.) on each of the nodes in the node
       container.

   Ipv4AddressHelper
       Next we need to associate the devices on our nodes with IP addresses.  We provide a  topology  helper  to
       manage  the  allocation  of  IP  addresses.   The only user-visible API is to set the base IP address and
       network mask to use when performing the actual address allocation (which is done at a lower level  inside
       the helper).

       The next two lines of code in our example script, first.cc,

          Ipv4AddressHelper address;
          address.SetBase ("10.1.1.0", "255.255.255.0");

       declare  an  address  helper  object  and  tell  it that it should begin allocating IP addresses from the
       network 10.1.1.0 using the mask 255.255.255.0 to define the allocatable bits.  By default  the  addresses
       allocated  will  start  at  one and increase monotonically, so the first address allocated from this base
       will be 10.1.1.1, followed by 10.1.1.2, etc.  The low level ns-3 system actually remembers all of the  IP
       addresses  allocated  and  will  generate  a fatal error if you accidentally cause the same address to be
       generated twice (which is a very hard to debug error, by the way).

       The next line of code,

          Ipv4InterfaceContainer interfaces = address.Assign (devices);

       performs the actual address assignment.  In ns-3 we make the association between  an  IP  address  and  a
       device  using  an  Ipv4Interface  object.   Just  as we sometimes need a list of net devices created by a
       helper  for  future   reference   we   sometimes   need   a   list   of   Ipv4Interface   objects.    The
       Ipv4InterfaceContainer provides this functionality.

       Now  we  have  a  point-to-point network built, with stacks installed and IP addresses assigned.  What we
       need at this point are applications to generate traffic.

   Applications
       Another one of the core abstractions of the ns-3 system is the Application.  In this script  we  use  two
       specializations    of   the   core   ns-3   class   Application   called   UdpEchoServerApplication   and
       UdpEchoClientApplication.  Just as we have in our previous explanations,  we use helper objects  to  help
       configure  and  manage  the underlying objects.  Here, we use UdpEchoServerHelper and UdpEchoClientHelper
       objects to make our lives easier.

   UdpEchoServerHelper
       The following lines of code in our example script, first.cc, are  used  to  set  up  a  UDP  echo  server
       application on one of the nodes we have previously created.

          UdpEchoServerHelper echoServer (9);

          ApplicationContainer serverApps = echoServer.Install (nodes.Get (1));
          serverApps.Start (Seconds (1.0));
          serverApps.Stop (Seconds (10.0));

       The  first  line of code in the above snippet declares the UdpEchoServerHelper.  As usual, this isn't the
       application itself, it is an object used  to  help  us  create  the  actual  applications.   One  of  our
       conventions is to place required Attributes in the helper constructor.  In this case, the helper can't do
       anything useful unless it is provided with a port number that the client also knows about.   Rather  than
       just  picking  one  and  hoping  it  all  works  out,  we  require  the port number as a parameter to the
       constructor.  The constructor, in turn, simply does a SetAttribute with the passed value.  If  you  want,
       you can set the "Port" Attribute to another value later using SetAttribute.

       Similar  to  many  other helper objects, the UdpEchoServerHelper object has an Install method.  It is the
       execution of this method that actually causes the underlying echo server application to  be  instantiated
       and  attached to a node.  Interestingly, the Install method takes a NodeContainter as a parameter just as
       the other Install methods we have seen.  This is actually what is passed to the  method  even  though  it
       doesn't  look  so in this case.  There is a C++ implicit conversion at work here that takes the result of
       nodes.Get (1) (which returns a smart pointer to  a  node  object  ---  Ptr<Node>)  and  uses  that  in  a
       constructor  for  an  unnamed NodeContainer that is then passed to Install.  If you are ever at a loss to
       find a particular method signature in C++ code that compiles and runs just fine, look for these kinds  of
       implicit conversions.

       We  now  see  that echoServer.Install is going to install a UdpEchoServerApplication on the node found at
       index number one of the NodeContainer we used to manage our nodes.  Install will return a container  that
       holds  pointers  to  all of the applications (one in this case since we passed a NodeContainer containing
       one node) created by the helper.

       Applications require a time to "start" generating traffic and may take an optional time  to  "stop".   We
       provide both.  These times are set using  the ApplicationContainer methods Start and Stop.  These methods
       take Time parameters.  In this case, we use an explicit C++ conversion sequence to take  the  C++  double
       1.0  and  convert it to an ns-3 Time object using a Seconds cast.  Be aware that the conversion rules may
       be controlled by the model author, and C++ has its own rules,  so  you  can't  always  just  assume  that
       parameters will be happily converted for you.  The two lines,

          serverApps.Start (Seconds (1.0));
          serverApps.Stop (Seconds (10.0));

       will  cause the echo server application to Start (enable itself) at one second into the simulation and to
       Stop (disable itself) at ten seconds into the simulation.  By virtue of the fact that we have declared  a
       simulation  event (the application stop event) to be executed at ten seconds, the simulation will last at
       least ten seconds.

   UdpEchoClientHelper
       The echo client application is set up in a method substantially similar to that for the server.  There is
       an underlying UdpEchoClientApplication that is managed by an UdpEchoClientHelper.

          UdpEchoClientHelper echoClient (interfaces.GetAddress (1), 9);
          echoClient.SetAttribute ("MaxPackets", UintegerValue (1));
          echoClient.SetAttribute ("Interval", TimeValue (Seconds (1.0)));
          echoClient.SetAttribute ("PacketSize", UintegerValue (1024));

          ApplicationContainer clientApps = echoClient.Install (nodes.Get (0));
          clientApps.Start (Seconds (2.0));
          clientApps.Stop (Seconds (10.0));

       For the echo client, however, we need to set five different Attributes.  The first two Attributes are set
       during construction of the UdpEchoClientHelper.  We pass parameters that  are  used  (internally  to  the
       helper)  to set the "RemoteAddress" and "RemotePort" Attributes in accordance with our convention to make
       required Attributes parameters in the helper constructors.

       Recall that we used an Ipv4InterfaceContainer to keep track of  the  IP  addresses  we  assigned  to  our
       devices.   The  zeroth  interface in the interfaces container is going to correspond to the IP address of
       the zeroth node in the nodes container.  The first interface in the interfaces container  corresponds  to
       the  IP address of the first node in the nodes container.  So, in the first line of code (from above), we
       are creating the helper and telling it so set the remote address of the client  to  be   the  IP  address
       assigned  to  the  node  on which the server resides.  We also tell it to arrange to send packets to port
       nine.

       The "MaxPackets" Attribute tells the client the maximum number of packets we allow it to send during  the
       simulation.   The  "Interval"  Attribute  tells  the  client  how  long  to wait between packets, and the
       "PacketSize" Attribute tells the client how large its packet payloads should be.   With  this  particular
       combination of Attributes, we are telling the client to send one 1024-byte packet.

       Just  as in the case of the echo server, we tell the echo client to Start and Stop, but here we start the
       client one second after the server is enabled (at two seconds into the simulation).

   Simulator
       What we need to do at this point is to actually run the  simulation.   This  is  done  using  the  global
       function Simulator::Run.

          Simulator::Run ();

       When we previously called the methods,

          serverApps.Start (Seconds (1.0));
          serverApps.Stop (Seconds (10.0));
          ...
          clientApps.Start (Seconds (2.0));
          clientApps.Stop (Seconds (10.0));

       we actually scheduled events in the simulator at 1.0 seconds, 2.0 seconds and two events at 10.0 seconds.
       When Simulator::Run is called, the system will begin looking through the list  of  scheduled  events  and
       executing  them.   First  it  will  run  the  event  at  1.0  seconds,  which will enable the echo server
       application (this event may, in turn, schedule many other events).  Then it will run the event  scheduled
       for t=2.0 seconds which will start the echo client application.  Again, this event may schedule many more
       events.  The start event implementation in the echo client application will begin the data transfer phase
       of the simulation by sending a packet to the server.

       The  act  of  sending  the packet to the server will trigger a chain of events that will be automatically
       scheduled behind the scenes and which will perform the mechanics of the  packet  echo  according  to  the
       various timing parameters that we have set in the script.

       Eventually,  since we only send one packet (recall the MaxPackets Attribute was set to one), the chain of
       events triggered by that single client echo request will taper off and the simulation will go idle.  Once
       this  happens,  the  remaining  events will be the Stop events for the server and the client.  When these
       events are executed, there are no further events to process and Simulator::Run returns.   The  simulation
       is then complete.

       All that remains is to clean up.  This is done by calling the global function Simulator::Destroy.  As the
       helper functions (or low level ns-3 code) executed, they arranged it so that hooks were inserted  in  the
       simulator  to  destroy  all  of  the objects that were created.  You did not have to keep track of any of
       these objects yourself --- all you had to do was to call Simulator::Destroy and exit.   The  ns-3  system
       took  care  of  the  hard  part for you.  The remaining lines of our first ns-3 script, first.cc, do just
       that:

            Simulator::Destroy ();
            return 0;
          }

   When the simulator will stop?
       ns-3 is a Discrete Event (DE) simulator. In such a simulator, each event is associated with its execution
       time,  and  the simulation proceeds by executing events in the temporal order of simulation time.  Events
       may cause future events to be scheduled (for example, a timer may reschedule itself to expire at the next
       interval).

       The  initial events are usually triggered by each object, e.g., IPv6 will schedule Router Advertisements,
       Neighbor Solicitations, etc., an Application schedule the first packet sending event, etc.

       When an event is processed, it may generate zero, one or more events.  As a simulation  executes,  events
       are consumed, but more events may (or may not) be generated.  The simulation will stop automatically when
       no further events are in the event queue, or when a special Stop  event  is  found.  The  Stop  event  is
       created through the Simulator::Stop (stopTime); function.

       There  is a typical case where Simulator::Stop is absolutely necessary to stop the simulation: when there
       is a self-sustaining event.  Self-sustaining (or recurring) events  are  events  that  always  reschedule
       themselves. As a consequence, they always keep the event queue non-empty.

       There are many protocols and modules containing recurring events, e.g.:

       • FlowMonitor - periodic check for lost packets

       • RIPng - periodic broadcast of routing tables update

       • etc.

       In  these  cases, Simulator::Stop is necessary to gracefully stop the simulation.  In addition, when ns-3
       is in emulation mode, the RealtimeSimulator is used to keep the simulation clock aligned with the machine
       clock, and Simulator::Stop is necessary to stop the process.

       Many  of  the simulation programs in the tutorial do not explicitly call Simulator::Stop, since the event
       queue will automatically run out of  events.   However,  these  programs  will  also  accept  a  call  to
       Simulator::Stop.   For  example,  the  following  additional  statement in the first example program will
       schedule an explicit stop at 11 seconds:

          +  Simulator::Stop (Seconds (11.0));
             Simulator::Run ();
             Simulator::Destroy ();
             return 0;
           }

       The above wil not actually change  the  behavior  of  this  program,  since  this  particular  simulation
       naturally  ends after 10 seconds.  But if you were to change the stop time in the above statement from 11
       seconds to 1 second, you would notice that the simulation stops before  any  output  is  printed  to  the
       screen (since the output occurs around time 2 seconds of simulation time).

       It  is  important  to  call  Simulator::Stop before calling Simulator::Run; otherwise, Simulator::Run may
       never return control to the main program to execute the stop!

   Building Your Script
       We have made it trivial to build your simple scripts.  All you have to do is to drop your script into the
       scratch   directory  and  it  will  automatically  be  built  if  you  run  Waf.   Let's  try  it.   Copy
       examples/tutorial/first.cc into the scratch directory after changing back into the top level directory.

          $ cd ../..
          $ cp examples/tutorial/first.cc scratch/myfirst.cc

       Now build your first example script using waf:

          $ ./waf

       You should see messages reporting that your myfirst example was built successfully.

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          [614/708] cxx: scratch/myfirst.cc -> build/debug/scratch/myfirst_3.o
          [706/708] cxx_link: build/debug/scratch/myfirst_3.o -> build/debug/scratch/myfirst
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (2.357s)

       You can now run the example (note that if you build your program in the scratch directory you must run it
       out of the scratch directory):

          $ ./waf --run scratch/myfirst

       You should see some output:

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (0.418s)
          Sent 1024 bytes to 10.1.1.2
          Received 1024 bytes from 10.1.1.1
          Received 1024 bytes from 10.1.1.2

       Here  you  see  that  the build system checks to make sure that the file has been build and then runs it.
       You see the logging component on the echo client indicate that it has sent one 1024 byte  packet  to  the
       Echo  Server on 10.1.1.2.  You also see the logging component on the echo server say that it has received
       the 1024 bytes from 10.1.1.1.  The echo server silently echoes the packet and you see the echo client log
       that it has received its packet back from the server.

   Ns-3 Source Code
       Now  that  you  have used some of the ns-3 helpers you may want to have a look at some of the source code
       that implements that functionality.  The most recent code can  be  browsed  on  our  web  server  at  the
       following  link:  http://code.nsnam.org/ns-3-dev.  There, you will see the Mercurial summary page for our
       ns-3 development tree.

       At the top of the page, you will see a number of links,

          summary | shortlog | changelog | graph | tags | files

       Go ahead and select the files link.  This is what the top-level of most of our repositories will look:

          drwxr-xr-x                               [up]
          drwxr-xr-x                               bindings python  files
          drwxr-xr-x                               doc              files
          drwxr-xr-x                               examples         files
          drwxr-xr-x                               ns3              files
          drwxr-xr-x                               scratch          files
          drwxr-xr-x                               src              files
          drwxr-xr-x                               utils            files
          -rw-r--r-- 2009-07-01 12:47 +0200 560    .hgignore        file | revisions | annotate
          -rw-r--r-- 2009-07-01 12:47 +0200 1886   .hgtags          file | revisions | annotate
          -rw-r--r-- 2009-07-01 12:47 +0200 1276   AUTHORS          file | revisions | annotate
          -rw-r--r-- 2009-07-01 12:47 +0200 30961  CHANGES.html     file | revisions | annotate
          -rw-r--r-- 2009-07-01 12:47 +0200 17987  LICENSE          file | revisions | annotate
          -rw-r--r-- 2009-07-01 12:47 +0200 3742   README           file | revisions | annotate
          -rw-r--r-- 2009-07-01 12:47 +0200 16171  RELEASE_NOTES    file | revisions | annotate
          -rw-r--r-- 2009-07-01 12:47 +0200 6      VERSION          file | revisions | annotate
          -rwxr-xr-x 2009-07-01 12:47 +0200 88110  waf              file | revisions | annotate
          -rwxr-xr-x 2009-07-01 12:47 +0200 28     waf.bat          file | revisions | annotate
          -rw-r--r-- 2009-07-01 12:47 +0200 35395  wscript          file | revisions | annotate
          -rw-r--r-- 2009-07-01 12:47 +0200 7673   wutils.py        file | revisions | annotate

       Our example scripts are in the examples directory.  If you click on examples  you  will  see  a  list  of
       subdirectories.   One  of  the  files in tutorial subdirectory is first.cc.  If you click on first.cc you
       will find the code you just walked through.

       The source code is mainly in the src directory.  You can view source  code  either  by  clicking  on  the
       directory  name or by clicking on the files link to the right of the directory name.  If you click on the
       src directory, you will be taken to the listing of the src subdirectories.  If you  then  click  on  core
       subdirectory,  you  will  find  a  list  of  files.  The first file you will find (as of this writing) is
       abort.h.  If you click on the abort.h link, you will be  sent  to  the  source  file  for  abort.h  which
       contains useful macros for exiting scripts if abnormal conditions are detected.

       The  source code for the helpers we have used in this chapter can be found in the src/applications/helper
       directory.  Feel free to poke around in the directory tree to get a feel for what is there and the  style
       of ns-3 programs.

TWEAKING

   Using the Logging Module
       We  have  already taken a brief look at the ns-3 logging module while going over the first.cc script.  We
       will now take a closer look and see what kind of use-cases the logging subsystem was designed to cover.

   Logging Overview
       Many large systems support some kind of message logging facility, and ns-3 is not an exception.  In  some
       cases, only error messages are logged to the "operator console" (which is typically stderr in Unix- based
       systems).  In other systems, warning messages may be  output  as  well  as  more  detailed  informational
       messages.  In some cases, logging facilities are used to output debug messages which can quickly turn the
       output into a blur.

       ns-3 takes the view that all  of  these  verbosity  levels  are  useful  and  we  provide  a  selectable,
       multi-level   approach   to   message  logging.   Logging  can  be  disabled  completely,  enabled  on  a
       component-by-component basis, or enabled globally; and it provides selectable verbosity levels.  The ns-3
       log  module  provides a straightforward, relatively easy to use way to get useful information out of your
       simulation.

       You should understand that we do provide a general purpose mechanism --- tracing --- to get data  out  of
       your  models  which should be preferred for simulation output (see the tutorial section Using the Tracing
       System for more details on our tracing system).  Logging should be preferred for  debugging  information,
       warnings,  error  messages,  or  any  time  you want to easily get a quick message out of your scripts or
       models.

       There are currently seven levels of log messages of increasing verbosity defined in the system.

       • LOG_ERROR --- Log error messages (associated macro: NS_LOG_ERROR);

       • LOG_WARN --- Log warning messages (associated macro: NS_LOG_WARN);

       • LOG_DEBUG --- Log relatively rare, ad-hoc debugging messages (associated macro: NS_LOG_DEBUG);

       • LOG_INFO --- Log informational messages about program progress (associated macro: NS_LOG_INFO);

       • LOG_FUNCTION --- Log a message describing each function called (two associated macros: NS_LOG_FUNCTION,
         used for member functions, and NS_LOG_FUNCTION_NOARGS, used for static functions);

       • LOG_LOGIC -- Log messages describing logical flow within a function (associated macro: NS_LOG_LOGIC);

       • LOG_ALL --- Log everything mentioned above (no associated macro).

       For  each LOG_TYPE there is also LOG_LEVEL_TYPE that, if used, enables logging of all the levels above it
       in addition to it's level.  (As a consequence of this, LOG_ERROR and LOG_LEVEL_ERROR and also LOG_ALL and
       LOG_LEVEL_ALL  are  functionally  equivalent.)   For example, enabling LOG_INFO will only enable messages
       provided by NS_LOG_INFO macro, while enabling  LOG_LEVEL_INFO  will  also  enable  messages  provided  by
       NS_LOG_DEBUG, NS_LOG_WARN and NS_LOG_ERROR macros.

       We  also  provide an unconditional logging macro that is always displayed, irrespective of logging levels
       or component selection.

       • NS_LOG_UNCOND -- Log the associated message unconditionally (no associated log level).

       Each level can be requested singly or cumulatively; and logging can be set up using a  shell  environment
       variable  (NS_LOG)  or by logging system function call.  As was seen earlier in the tutorial, the logging
       system has Doxygen documentation and now would be a good time to peruse the Logging Module  documentation
       if you have not done so.

       Now  that  you  have read the documentation in great detail, let's use some of that knowledge to get some
       interesting information out of the scratch/myfirst.cc example script you have already built.

   Enabling Logging
       Let's use the NS_LOG environment variable to turn on some more  logging,  but  first,  just  to  get  our
       bearings, go ahead and run the last script just as you did previously,

          $ ./waf --run scratch/myfirst

       You should see the now familiar output of the first ns-3 example program

          $ Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (0.413s)
          Sent 1024 bytes to 10.1.1.2
          Received 1024 bytes from 10.1.1.1
          Received 1024 bytes from 10.1.1.2

       It turns out that the "Sent" and "Received" messages you see above are actually logging messages from the
       UdpEchoClientApplication and UdpEchoServerApplication.  We can ask the client application,  for  example,
       to print more information by setting its logging level via the NS_LOG environment variable.

       I  am  going  to  assume  from  here on that you are using an sh-like shell that uses the"VARIABLE=value"
       syntax.  If you are using a csh-like shell, then you will have to convert  my  examples  to  the  "setenv
       VARIABLE value" syntax required by those shells.

       Right   now,  the  UDP  echo  client  application  is  responding  to  the  following  line  of  code  in
       scratch/myfirst.cc,

          LogComponentEnable("UdpEchoClientApplication", LOG_LEVEL_INFO);

       This line of code enables the LOG_LEVEL_INFO level of logging.  When we pass a logging level flag, we are
       actually  enabling  the  given  level  and  all lower levels.  In this case, we have enabled NS_LOG_INFO,
       NS_LOG_DEBUG, NS_LOG_WARN and NS_LOG_ERROR.  We can increase the logging level and get  more  information
       without changing the script and recompiling by setting the NS_LOG environment variable like this:

          $ export NS_LOG=UdpEchoClientApplication=level_all

       This sets the shell environment variable NS_LOG to the string,

          UdpEchoClientApplication=level_all

       The  left  hand side of the assignment is the name of the logging component we want to set, and the right
       hand side is the flag we want to use.  In this case, we are going to turn on all of the debugging  levels
       for  the  application.  If you run the script with NS_LOG set this way, the ns-3 logging system will pick
       up the change and you should see the following output:

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (0.404s)
          UdpEchoClientApplication:UdpEchoClient()
          UdpEchoClientApplication:SetDataSize(1024)
          UdpEchoClientApplication:StartApplication()
          UdpEchoClientApplication:ScheduleTransmit()
          UdpEchoClientApplication:Send()
          Sent 1024 bytes to 10.1.1.2
          Received 1024 bytes from 10.1.1.1
          UdpEchoClientApplication:HandleRead(0x6241e0, 0x624a20)
          Received 1024 bytes from 10.1.1.2
          UdpEchoClientApplication:StopApplication()
          UdpEchoClientApplication:DoDispose()
          UdpEchoClientApplication:~UdpEchoClient()

       The additional debug information provided by the application is from  the  NS_LOG_FUNCTION  level.   This
       shows  every time a function in the application is called during script execution.  Generally, use of (at
       least) NS_LOG_FUNCTION(this) in member functions  is  preferred.  Use  NS_LOG_FUNCTION_NOARGS()  only  in
       static  functions.   Note,  however,  that  there are no requirements in the ns-3 system that models must
       support any particular logging  functionality.  The decision regarding how much information is logged  is
       left  to the individual model developer.  In the case of the echo applications, a good deal of log output
       is available.

       You can now see a log of the function calls that were made to the application.  If you look  closely  you
       will  notice  a  single  colon  between the string UdpEchoClientApplication and the method name where you
       might have expected a C++ scope operator (::).  This is intentional.

       The name is not actually a class name, it is a logging  component  name.   When  there  is  a  one-to-one
       correspondence  between  a  source file and a class, this will generally be the class name but you should
       understand that it is not actually a class name, and there is a single colon there instead  of  a  double
       colon  to  remind you in a relatively subtle way to conceptually separate the logging component name from
       the class name.

       It turns out that in some cases, it can be hard to  determine  which  method  actually  generates  a  log
       message.   If  you  look  in  the  text  above, you may wonder where the string "Received 1024 bytes from
       10.1.1.2" comes from.  You can resolve this by OR'ing the prefix_func level into the  NS_LOG  environment
       variable.  Try doing the following,

          $ export 'NS_LOG=UdpEchoClientApplication=level_all|prefix_func'

       Note  that  the  quotes  are required since the vertical bar we use to indicate an OR operation is also a
       Unix pipe connector.

       Now, if you run the script you will see that the logging system makes sure that every  message  from  the
       given log component is prefixed with the component name.

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (0.417s)
          UdpEchoClientApplication:UdpEchoClient()
          UdpEchoClientApplication:SetDataSize(1024)
          UdpEchoClientApplication:StartApplication()
          UdpEchoClientApplication:ScheduleTransmit()
          UdpEchoClientApplication:Send()
          UdpEchoClientApplication:Send(): Sent 1024 bytes to 10.1.1.2
          Received 1024 bytes from 10.1.1.1
          UdpEchoClientApplication:HandleRead(0x6241e0, 0x624a20)
          UdpEchoClientApplication:HandleRead(): Received 1024 bytes from 10.1.1.2
          UdpEchoClientApplication:StopApplication()
          UdpEchoClientApplication:DoDispose()
          UdpEchoClientApplication:~UdpEchoClient()

       You  can  now see all of the messages coming from the UDP echo client application are identified as such.
       The message "Received 1024 bytes from 10.1.1.2" is now clearly identified as coming from the echo  client
       application.   The  remaining message must be coming from the UDP echo server application.  We can enable
       that component by entering a colon separated list of components in the NS_LOG environment variable.

          $ export 'NS_LOG=UdpEchoClientApplication=level_all|prefix_func:
                         UdpEchoServerApplication=level_all|prefix_func'

       Warning:  You will need to remove the newline after the : in the example text above which is  only  there
       for document formatting purposes.

       Now,  if  you  run  the  script you will see all of the log messages from both the echo client and server
       applications.  You may see that this can be very useful in debugging problems.

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (0.406s)
          UdpEchoServerApplication:UdpEchoServer()
          UdpEchoClientApplication:UdpEchoClient()
          UdpEchoClientApplication:SetDataSize(1024)
          UdpEchoServerApplication:StartApplication()
          UdpEchoClientApplication:StartApplication()
          UdpEchoClientApplication:ScheduleTransmit()
          UdpEchoClientApplication:Send()
          UdpEchoClientApplication:Send(): Sent 1024 bytes to 10.1.1.2
          UdpEchoServerApplication:HandleRead(): Received 1024 bytes from 10.1.1.1
          UdpEchoServerApplication:HandleRead(): Echoing packet
          UdpEchoClientApplication:HandleRead(0x624920, 0x625160)
          UdpEchoClientApplication:HandleRead(): Received 1024 bytes from 10.1.1.2
          UdpEchoServerApplication:StopApplication()
          UdpEchoClientApplication:StopApplication()
          UdpEchoClientApplication:DoDispose()
          UdpEchoServerApplication:DoDispose()
          UdpEchoClientApplication:~UdpEchoClient()
          UdpEchoServerApplication:~UdpEchoServer()

       It is also sometimes useful to be able to see the simulation time at which a log  message  is  generated.
       You can do this by ORing in the prefix_time bit.

          $ export 'NS_LOG=UdpEchoClientApplication=level_all|prefix_func|prefix_time:
                         UdpEchoServerApplication=level_all|prefix_func|prefix_time'

       Again,  you  will  have  to  remove  the  newline  above.   If you run the script now, you should see the
       following output:

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (0.418s)
          0s UdpEchoServerApplication:UdpEchoServer()
          0s UdpEchoClientApplication:UdpEchoClient()
          0s UdpEchoClientApplication:SetDataSize(1024)
          1s UdpEchoServerApplication:StartApplication()
          2s UdpEchoClientApplication:StartApplication()
          2s UdpEchoClientApplication:ScheduleTransmit()
          2s UdpEchoClientApplication:Send()
          2s UdpEchoClientApplication:Send(): Sent 1024 bytes to 10.1.1.2
          2.00369s UdpEchoServerApplication:HandleRead(): Received 1024 bytes from 10.1.1.1
          2.00369s UdpEchoServerApplication:HandleRead(): Echoing packet
          2.00737s UdpEchoClientApplication:HandleRead(0x624290, 0x624ad0)
          2.00737s UdpEchoClientApplication:HandleRead(): Received 1024 bytes from 10.1.1.2
          10s UdpEchoServerApplication:StopApplication()
          10s UdpEchoClientApplication:StopApplication()
          UdpEchoClientApplication:DoDispose()
          UdpEchoServerApplication:DoDispose()
          UdpEchoClientApplication:~UdpEchoClient()
          UdpEchoServerApplication:~UdpEchoServer()

       You can see that the constructor for the UdpEchoServer was called at a  simulation  time  of  0  seconds.
       This  is actually happening before the simulation starts, but the time is displayed as zero seconds.  The
       same is true for the UdpEchoClient constructor message.

       Recall that the scratch/first.cc script started the echo  server  application  at  one  second  into  the
       simulation.   You  can  now see that the StartApplication method of the server is, in fact, called at one
       second.  You can also see that the echo client application is started at a simulation time of two seconds
       as we requested in the script.

       You can now follow the progress of the simulation from the ScheduleTransmit call in the client that calls
       Send to the HandleRead callback in the echo server application.  Note  that  the  elapsed  time  for  the
       packet to be sent across the point-to-point link is 3.69 milliseconds.  You see the echo server logging a
       message telling you that it has echoed the packet and then, after another channel delay, you see the echo
       client receive the echoed packet in its HandleRead method.

       There  is  a  lot  that is happening under the covers in this simulation that you are not seeing as well.
       You can very easily follow the entire process by turning on all of the logging components in the  system.
       Try setting the NS_LOG variable to the following,

          $ export 'NS_LOG=*=level_all|prefix_func|prefix_time'

       The asterisk above is the logging component wildcard.  This will turn on all of the logging in all of the
       components used in the simulation.  I won't reproduce the output here (as of  this  writing  it  produces
       1265  lines  of  output for the single packet echo) but you can redirect this information into a file and
       look through it with your favorite editor if you like,

          $ ./waf --run scratch/myfirst > log.out 2>&1

       I personally use this extremely verbose version of logging when I am presented with a problem and I  have
       no  idea where things are going wrong.  I can follow the progress of the code quite easily without having
       to set breakpoints and step through code in a debugger.  I can just edit up the  output  in  my  favorite
       editor  and search around for things I expect, and see things happening that I don't expect.  When I have
       a general idea about what is going wrong, I transition into a debugger for a fine-grained examination  of
       the  problem.   This  kind  of output can be especially useful when your script does something completely
       unexpected.  If you are stepping using a debugger  you  may  miss  an  unexpected  excursion  completely.
       Logging the excursion makes it quickly visible.

   Adding Logging to your Code
       You  can  add  new  logging  to your simulations by making calls to the log component via several macros.
       Let's do so in the myfirst.cc script we have in the scratch directory.

       Recall that we have defined a logging component in that script:

          NS_LOG_COMPONENT_DEFINE ("FirstScriptExample");

       You now know that you can enable all of the logging for this component by setting the NS_LOG  environment
       variable  to  the  various levels.  Let's go ahead and add some logging to the script.  The macro used to
       add an informational level log message is NS_LOG_INFO.  Go ahead  and  add  one  (just  before  we  start
       creating  the nodes) that tells you that the script is "Creating Topology."  This is done as in this code
       snippet,

       Open scratch/myfirst.cc in your favorite editor and add the line,

          NS_LOG_INFO ("Creating Topology");

       right before the lines,

          NodeContainer nodes;
          nodes.Create (2);

       Now build the script using waf and clear the NS_LOG variable to  turn  off  the  torrent  of  logging  we
       previously enabled:

          $ ./waf
          $ export NS_LOG=

       Now, if you run the script,

          $ ./waf --run scratch/myfirst

       you  will  not  see  your new message since its associated logging component (FirstScriptExample) has not
       been enabled.  In order to see your message you  will  have  to  enable  the  FirstScriptExample  logging
       component  with  a  level  greater than or equal to NS_LOG_INFO.  If you just want to see this particular
       level of logging, you can enable it by,

          $ export NS_LOG=FirstScriptExample=info

       If you now run the script you will see your new "Creating Topology" log message,

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (0.404s)
          Creating Topology
          Sent 1024 bytes to 10.1.1.2
          Received 1024 bytes from 10.1.1.1
          Received 1024 bytes from 10.1.1.2

   Using Command Line Arguments
   Overriding Default Attributes
       Another way you can change how ns-3 scripts behave without editing  and  building  is  via  command  line
       arguments.  We provide a mechanism to parse command line arguments and automatically set local and global
       variables based on those arguments.

       The first step in using the command line argument system is to declare the command line parser.  This  is
       done quite simply (in your main program) as in the following code,

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

            CommandLine cmd;
            cmd.Parse (argc, argv);

            ...
          }

       This  simple  two  line  snippet is actually very useful by itself.  It opens the door to the ns-3 global
       variable and Attribute systems.  Go ahead and add that two lines of code to the scratch/myfirst.cc script
       at  the  start  of  main.   Go  ahead and build the script and run it, but ask the script for help in the
       following way,

          $ ./waf --run "scratch/myfirst --PrintHelp"

       This will ask Waf to run the scratch/myfirst script and pass the command line argument --PrintHelp to the
       script.   The quotes are required to sort out which program gets which argument.  The command line parser
       will now see the --PrintHelp argument and respond with,

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (0.413s)
          TcpL4Protocol:TcpStateMachine()
          CommandLine:HandleArgument(): Handle arg name=PrintHelp value=
          --PrintHelp: Print this help message.
          --PrintGroups: Print the list of groups.
          --PrintTypeIds: Print all TypeIds.
          --PrintGroup=[group]: Print all TypeIds of group.
          --PrintAttributes=[typeid]: Print all attributes of typeid.
          --PrintGlobals: Print the list of globals.

       Let's focus on the --PrintAttributes option.  We have already hinted at the ns-3 Attribute  system  while
       walking through the first.cc script.  We looked at the following lines of code,

          PointToPointHelper pointToPoint;
          pointToPoint.SetDeviceAttribute ("DataRate", StringValue ("5Mbps"));
          pointToPoint.SetChannelAttribute ("Delay", StringValue ("2ms"));

       and  mentioned  that  DataRate  was  actually  an  Attribute of the PointToPointNetDevice.  Let's use the
       command line argument parser to take a look at the Attributes of  the  PointToPointNetDevice.   The  help
       listing  says  that we should provide a TypeId.  This corresponds to the class name of the class to which
       the Attributes belong.  In this case it will be ns3::PointToPointNetDevice.  Let's go ahead and type in,

          $ ./waf --run "scratch/myfirst --PrintAttributes=ns3::PointToPointNetDevice"

       The system will print out all of the Attributes of this kind of net device.   Among  the  Attributes  you
       will see listed is,

          --ns3::PointToPointNetDevice::DataRate=[32768bps]:
            The default data rate for point to point links

       This  is  the  default value that will be used when a PointToPointNetDevice is created in the system.  We
       overrode this default with the Attribute setting in the PointToPointHelper above.  Let's use the  default
       values  for  the  point-to-point  devices  and  channels  by deleting the SetDeviceAttribute call and the
       SetChannelAttribute call from the myfirst.cc we have in the scratch directory.

       Your script should now just declare the PointToPointHelper and not  do  any  set  operations  as  in  the
       following example,

          ...

          NodeContainer nodes;
          nodes.Create (2);

          PointToPointHelper pointToPoint;

          NetDeviceContainer devices;
          devices = pointToPoint.Install (nodes);

          ...

       Go ahead and build the new script with Waf (./waf) and let's go back and enable some logging from the UDP
       echo server application and turn on the time prefix.

          $ export 'NS_LOG=UdpEchoServerApplication=level_all|prefix_time'

       If you run the script, you should now see the following output,

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (0.405s)
          0s UdpEchoServerApplication:UdpEchoServer()
          1s UdpEchoServerApplication:StartApplication()
          Sent 1024 bytes to 10.1.1.2
          2.25732s Received 1024 bytes from 10.1.1.1
          2.25732s Echoing packet
          Received 1024 bytes from 10.1.1.2
          10s UdpEchoServerApplication:StopApplication()
          UdpEchoServerApplication:DoDispose()
          UdpEchoServerApplication:~UdpEchoServer()

       Recall that the last time we looked at the simulation time at which the packet was received by  the  echo
       server, it was at 2.00369 seconds.

          2.00369s UdpEchoServerApplication:HandleRead(): Received 1024 bytes from 10.1.1.1

       Now  it is receiving the packet at 2.25732 seconds.  This is because we just dropped the data rate of the
       PointToPointNetDevice down to its default of 32768 bits per second from five megabits per second.

       If we were to provide a new DataRate using the command line, we could speed our simulation up again.   We
       do this in the following way, according to the formula implied by the help item:

          $ ./waf --run "scratch/myfirst --ns3::PointToPointNetDevice::DataRate=5Mbps"

       This  will  set  the  default  value of the DataRate Attribute back to five megabits per second.  Are you
       surprised by the result?  It turns out that in order to get the original behavior of the script back,  we
       will  have to set the speed-of-light delay of the channel as well.  We can ask the command line system to
       print out the Attributes of the channel just like we did for the net device:

          $ ./waf --run "scratch/myfirst --PrintAttributes=ns3::PointToPointChannel"

       We discover the Delay Attribute of the channel is set in the following way:

          --ns3::PointToPointChannel::Delay=[0ns]:
            Transmission delay through the channel

       We can then set both of these default values through the command line system,

          $ ./waf --run "scratch/myfirst
            --ns3::PointToPointNetDevice::DataRate=5Mbps
            --ns3::PointToPointChannel::Delay=2ms"

       in which case we recover the timing we had when we explicitly set the DataRate and Delay in the script:

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (0.417s)
          0s UdpEchoServerApplication:UdpEchoServer()
          1s UdpEchoServerApplication:StartApplication()
          Sent 1024 bytes to 10.1.1.2
          2.00369s Received 1024 bytes from 10.1.1.1
          2.00369s Echoing packet
          Received 1024 bytes from 10.1.1.2
          10s UdpEchoServerApplication:StopApplication()
          UdpEchoServerApplication:DoDispose()
          UdpEchoServerApplication:~UdpEchoServer()

       Note that the packet is again received by the server at 2.00369 seconds.  We could actually  set  any  of
       the  Attributes  used  in the script in this way.  In particular we could set the UdpEchoClient Attribute
       MaxPackets to some other value than one.

       How would you go about that?  Give it a try.  Remember you have to comment out the place we override  the
       default Attribute and explicitly set MaxPackets in the script.  Then you have to rebuild the script.  You
       will also have to find the syntax for actually setting the new default attribute value using the  command
       line  help  facility.  Once you have this figured out you should be able to control the number of packets
       echoed from the command line.  Since we're nice folks, we'll tell you that your command line  should  end
       up looking something like,

          $ ./waf --run "scratch/myfirst
            --ns3::PointToPointNetDevice::DataRate=5Mbps
            --ns3::PointToPointChannel::Delay=2ms
            --ns3::UdpEchoClient::MaxPackets=2"

   Hooking Your Own Values
       You  can  also  add  your  own  hooks to the command line system.  This is done quite simply by using the
       AddValue method to the command line parser.

       Let's use this facility to specify the number of packets to echo in a completely  different  way.   Let's
       add  a  local  variable  called  nPackets  to the main function.  We'll initialize it to one to match our
       previous default behavior.  To allow the command line parser to change this value, we need  to  hook  the
       value  into  the  parser.   We  do  this  by  adding  a  call  to  AddValue.   Go  ahead  and  change the
       scratch/myfirst.cc script to start with the following code,

          int
          main (int argc, char *argv[])
          {
            uint32_t nPackets = 1;

            CommandLine cmd;
            cmd.AddValue("nPackets", "Number of packets to echo", nPackets);
            cmd.Parse (argc, argv);

            ...

       Scroll down to the point in the script where we set the MaxPackets Attribute and change it so that it  is
       set to the variable nPackets instead of the constant 1 as is shown below.

          echoClient.SetAttribute ("MaxPackets", UintegerValue (nPackets));

       Now  if  you  run  the script and provide the --PrintHelp argument, you should see your new User Argument
       listed in the help display.

       Try,

          $ ./waf --run "scratch/myfirst --PrintHelp"

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (0.403s)
          --PrintHelp: Print this help message.
          --PrintGroups: Print the list of groups.
          --PrintTypeIds: Print all TypeIds.
          --PrintGroup=[group]: Print all TypeIds of group.
          --PrintAttributes=[typeid]: Print all attributes of typeid.
          --PrintGlobals: Print the list of globals.
          User Arguments:
              --nPackets: Number of packets to echo

       If you want to specify the number of packets to echo, you  can  now  do  so  by  setting  the  --nPackets
       argument in the command line,

          $ ./waf --run "scratch/myfirst --nPackets=2"

       You should now see

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (0.404s)
          0s UdpEchoServerApplication:UdpEchoServer()
          1s UdpEchoServerApplication:StartApplication()
          Sent 1024 bytes to 10.1.1.2
          2.25732s Received 1024 bytes from 10.1.1.1
          2.25732s Echoing packet
          Received 1024 bytes from 10.1.1.2
          Sent 1024 bytes to 10.1.1.2
          3.25732s Received 1024 bytes from 10.1.1.1
          3.25732s Echoing packet
          Received 1024 bytes from 10.1.1.2
          10s UdpEchoServerApplication:StopApplication()
          UdpEchoServerApplication:DoDispose()
          UdpEchoServerApplication:~UdpEchoServer()

       You have now echoed two packets.  Pretty easy, isn't it?

       You  can see that if you are an ns-3 user, you can use the command line argument system to control global
       values and Attributes.  If you are a model author, you can add new Attributes to your  Objects  and  they
       will  automatically be available for setting by your users through the command line system.  If you are a
       script author, you can add new variables to your scripts and hook them into the command line system quite
       painlessly.

   Using the Tracing System
       The  whole  point of simulation is to generate output for further study, and the ns-3 tracing system is a
       primary mechanism for this.  Since ns-3 is a C++ program, standard facilities for generating output  from
       C++ programs could be used:

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

       You  could  even  use  the  logging  module  to  add a little structure to your solution.  There are many
       well-known problems generated by such approaches  and  so  we  have  provided  a  generic  event  tracing
       subsystem to address the issues we thought were important.

       The basic goals of the ns-3 tracing system are:

       • For  basic  tasks,  the  tracing  system should allow the user to generate standard tracing for popular
         tracing sources, and to customize which objects generate the tracing;

       • Intermediate users must be able to extend the tracing system to modify the output format generated,  or
         to insert new tracing sources, without modifying the core of the simulator;

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

       The  ns-3 tracing system is built on the concepts of independent tracing sources and tracing sinks, and 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.

       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 sink.  Trace sinks are consumers of the
       events  and  data  provided  by the trace sources.  For example, one could create a trace sink that would
       (when connected to the trace source of the previous example) print out interesting parts of the  received
       packet.

       The  rationale  for  this  explicit  division  is to allow users to attach new types of sinks to existing
       tracing sources, without requiring editing and recompilation of the core of the simulator.  Thus, in  the
       example  above, a user could define a new tracing sink in her script and attach it to an existing tracing
       source defined in the simulation core by editing only the user script.

       In this tutorial, we will walk through some pre-defined sources and  sinks  and  show  how  they  may  be
       customized  with  little user effort.  See the ns-3 manual or how-to sections for information on advanced
       tracing configuration including extending the tracing namespace and creating new tracing sources.

   ASCII Tracing
       ns-3 provides helper functionality that wraps the low-level tracing system to help you with  the  details
       involved in configuring some easily understood packet traces.  If you enable this functionality, you will
       see output in a ASCII files --- thus the name.  For those familiar with ns-2 output, this type  of  trace
       is analogous to the out.tr generated by many scripts.

       Let's  just  jump  right  in  and  add some ASCII tracing output to our scratch/myfirst.cc script.  Right
       before the call to Simulator::Run (), add the following lines of code:

          AsciiTraceHelper ascii;
          pointToPoint.EnableAsciiAll (ascii.CreateFileStream ("myfirst.tr"));

       Like in many other ns-3 idioms, this code uses a  helper object to help create ASCII traces.  The  second
       line  contains  two  nested method calls.  The "inside" method, CreateFileStream() uses an unnamed object
       idiom to create a file stream object on the stack (without an object  name)  and  pass  it  down  to  the
       called method.  We'll go into this more in the future, but all you have to know at this point is that you
       are creating an object representing a file named "myfirst.tr" and passing it into ns-3.  You are  telling
       ns-3  to  deal  with the lifetime issues of the created object and also to deal with problems caused by a
       little-known (intentional) limitation of C++ ofstream objects relating to copy constructors.

       The outside call, to EnableAsciiAll(), tells the helper that you want to  enable  ASCII  tracing  on  all
       point-to-point  devices  in  your  simulation;  and  you  want  the  (provided)  trace sinks to write out
       information about packet movement in ASCII format.

       For those familiar with ns-2, the traced events are equivalent to the popular trace points that log  "+",
       "-", "d", and "r" events.

       You can now build the script and run it from the command line:

          $ ./waf --run scratch/myfirst

       Just  as  you have seen many times before, you will see some messages from Waf and then "'build' finished
       successfully" with some number of messages from the running program.

       When it ran, the program will have created a file named myfirst.tr.  Because of the way that  Waf  works,
       the  file  is  not  created  in  the  local  directory,  it  is created at the top-level directory of the
       repository by default.  If you want to control where the traces are saved you can use the --cwd option of
       Waf  to  specify  this.   We have not done so, thus we need to change into the top level directory of our
       repo and take a look at the ASCII trace file myfirst.tr in your favorite editor.

   Parsing Ascii Traces
       There's a lot of information there in a pretty dense form, but the first thing to notice  is  that  there
       are  a  number  of distinct lines in this file.  It may be difficult to see this clearly unless you widen
       your window considerably.

       Each line in the file corresponds to a trace event.  In this case we are tracing events on  the  transmit
       queue  present  in  every  point-to-point  net  device  in the simulation.  The transmit queue is a queue
       through which every packet destined for a point-to-point channel must pass.  Note that each line  in  the
       trace  file  begins with a lone character (has a space after it).  This character will have the following
       meaning:

       • +: An enqueue operation occurred on the device queue;

       • -: A dequeue operation occurred on the device queue;

       • d: A packet was dropped, typically because the queue was full;

       • r: A packet was received by the net device.

       Let's take a more detailed view of the first line in the trace file.  I'll break it  down  into  sections
       (indented for clarity) with a reference number on the left side:

          +
          2
          /NodeList/0/DeviceList/0/$ns3::PointToPointNetDevice/TxQueue/Enqueue
          ns3::PppHeader (
            Point-to-Point Protocol: IP (0x0021))
            ns3::Ipv4Header (
              tos 0x0 ttl 64 id 0 protocol 17 offset 0 flags [none]
              length: 1052 10.1.1.1 > 10.1.1.2)
              ns3::UdpHeader (
                length: 1032 49153 > 9)
                Payload (size=1024)

       The  first  section  of  this  expanded  trace  event (reference number 0) is the operation.  We have a +
       character, so this corresponds to an enqueue  operation  on  the  transmit  queue.   The  second  section
       (reference  1)  is  the  simulation  time  expressed  in  seconds.   You  may  recall  that  we asked the
       UdpEchoClientApplication to start sending packets at two seconds.  Here we see confirmation that this is,
       indeed, happening.

       The  next  section  of  the  example trace (reference 2) tell us which trace source originated this event
       (expressed in the tracing namespace).  You can think of the tracing namespace somewhat like you  would  a
       filesystem  namespace.   The  root  of  the  namespace  is the NodeList.  This corresponds to a container
       managed in the ns-3 core code that contains all of the nodes that are created in a  script.   Just  as  a
       filesystem  may  have  directories  under the root, we may have node numbers in the NodeList.  The string
       /NodeList/0 therefore refers to the zeroth node in the NodeList which we typically think of as "node  0".
       In  each  node  there  is  a  list  of  devices  that have been installed.  This list appears next in the
       namespace.  You can see that this trace  event  comes  from  DeviceList/0  which  is  the  zeroth  device
       installed in the node.

       The  next  string, $ns3::PointToPointNetDevice tells you what kind of device is in the zeroth position of
       the device list for node zero.  Recall that the operation + found at reference 00 meant that  an  enqueue
       operation  happened  on the transmit queue of the device.  This is reflected in the final segments of the
       "trace path" which are TxQueue/Enqueue.

       The remaining sections in the trace should be fairly intuitive.  References 3-4 indicate that the  packet
       is  encapsulated  in  the point-to-point protocol.  References 5-7 show that the packet has an IP version
       four header and has originated from IP address 10.1.1.1 and is destined  for  10.1.1.2.   References  8-9
       show  that this packet has a UDP header and, finally, reference 10 shows that the payload is the expected
       1024 bytes.

       The next line in the trace file shows the same packet being dequeued from the transmit queue on the  same
       node.

       The  Third  line in the trace file shows the packet being received by the net device on the node with the
       echo server. I have reproduced that event below.

          r
          2.25732
          /NodeList/1/DeviceList/0/$ns3::PointToPointNetDevice/MacRx
            ns3::Ipv4Header (
              tos 0x0 ttl 64 id 0 protocol 17 offset 0 flags [none]
              length: 1052 10.1.1.1 > 10.1.1.2)
              ns3::UdpHeader (
                length: 1032 49153 > 9)
                Payload (size=1024)

       Notice that the trace operation is now r and the simulation time has increased to  2.25732  seconds.   If
       you  have been following the tutorial steps closely this means that you have left the DataRate of the net
       devices and the channel Delay set to their default values.  This time should be familiar as you have seen
       it before in a previous section.

       The  trace  source  namespace  entry (reference 02) has changed to reflect that this event is coming from
       node 1 (/NodeList/1) and the packet reception trace source (/MacRx).  It should be quite easy for you  to
       follow the progress of the packet through the topology by looking at the rest of the traces in the file.

   PCAP Tracing
       The  ns-3  device  helpers  can also be used to create trace files in the .pcap format.  The acronym pcap
       (usually written in lower case) stands for packet capture, and is  actually  an  API  that  includes  the
       definition  of  a  .pcap  file format.  The most popular program that can read and display this format is
       Wireshark (formerly called Ethereal).  However, there are many traffic  trace  analyzers  that  use  this
       packet  format.   We  encourage  users to exploit the many tools available for analyzing pcap traces.  In
       this tutorial, we concentrate on viewing pcap traces with tcpdump.

       The code used to enable pcap tracing is a one-liner.

          pointToPoint.EnablePcapAll ("myfirst");

       Go ahead and insert this line of code after the ASCII tracing code we just added  to  scratch/myfirst.cc.
       Notice  that  we  only passed the string "myfirst," and not "myfirst.pcap" or something similar.  This is
       because the parameter is a prefix, not a complete file name.  The helper will  actually  create  a  trace
       file  for  every point-to-point device in the simulation.  The file names will be built using the prefix,
       the node number, the device number and a ".pcap" suffix.

       In our example script, we will eventually see files named "myfirst-0-0.pcap" and "myfirst-1-0.pcap" which
       are the pcap traces for node 0-device 0 and node 1-device 0, respectively.

       Once you have added the line of code to enable pcap tracing, you can run the script in the usual way:

          $ ./waf --run scratch/myfirst

       If  you  look  at  the  top  level  directory  of  your distribution, you should now see three log files:
       myfirst.tr is the ASCII trace file we have previously examined.   myfirst-0-0.pcap  and  myfirst-1-0.pcap
       are the new pcap files we just generated.

   Reading output with tcpdump
       The easiest thing to do at this point will be to use tcpdump to look at the pcap files.

          $ tcpdump -nn -tt -r myfirst-0-0.pcap
          reading from file myfirst-0-0.pcap, link-type PPP (PPP)
          2.000000 IP 10.1.1.1.49153 > 10.1.1.2.9: UDP, length 1024
          2.514648 IP 10.1.1.2.9 > 10.1.1.1.49153: UDP, length 1024

          tcpdump -nn -tt -r myfirst-1-0.pcap
          reading from file myfirst-1-0.pcap, link-type PPP (PPP)
          2.257324 IP 10.1.1.1.49153 > 10.1.1.2.9: UDP, length 1024
          2.257324 IP 10.1.1.2.9 > 10.1.1.1.49153: UDP, length 1024

       You can see in the dump of myfirst-0-0.pcap (the client device) that the echo packet is sent at 2 seconds
       into the simulation.  If you look at the second dump (myfirst-1-0.pcap) you can  see  that  packet  being
       received  at  2.257324  seconds.   You see the packet being echoed back at 2.257324 seconds in the second
       dump, and finally, you see the packet being received back at the client in the  first  dump  at  2.514648
       seconds.

   Reading output with Wireshark
       If  you are unfamiliar with Wireshark, there is a web site available from which you can download programs
       and documentation:  http://www.wireshark.org/.

       Wireshark is a graphical user interface which can be used for displaying these trace files.  If you  have
       Wireshark available, you can open each of the trace files and display the contents as if you had captured
       the packets using a packet sniffer.

BUILDING TOPOLOGIES

   Building a Bus Network Topology
       In this section we are going to expand our mastery of ns-3 network  devices  and  channels  to  cover  an
       example  of  a  bus network.  ns-3 provides a net device and channel we call CSMA (Carrier Sense Multiple
       Access).

       The ns-3 CSMA device models a simple network in the spirit of Ethernet.  A  real  Ethernet  uses  CSMA/CD
       (Carrier  Sense Multiple Access with Collision Detection) scheme with exponentially increasing backoff to
       contend for the shared transmission medium.  The ns-3 CSMA device and channel models  only  a  subset  of
       this.

       Just  as we have seen point-to-point topology helper objects when constructing point-to-point topologies,
       we will see equivalent CSMA topology helpers in this section.  The  appearance  and  operation  of  these
       helpers should look quite familiar to you.

       We  provide  an  example  script in our examples/tutorial} directory.  This script builds on the first.cc
       script and adds a CSMA network to the point-to-point simulation we've already considered.  Go  ahead  and
       open examples/tutorial/second.cc in your favorite editor.  You will have already seen enough ns-3 code to
       understand most of what is going on in this example, but we will go over the entire  script  and  examine
       some of the output.

       Just  as  in  the first.cc example (and in all ns-3 examples) the file begins with an emacs mode line and
       some GPL boilerplate.

       The actual code begins by loading module include files just as was done in the first.cc example.

          #include "ns3/core-module.h"
          #include "ns3/network-module.h"
          #include "ns3/csma-module.h"
          #include "ns3/internet-module.h"
          #include "ns3/point-to-point-module.h"
          #include "ns3/applications-module.h"
          #include "ns3/ipv4-global-routing-helper.h"

       One thing that can be surprisingly useful is a small bit of ASCII art that shows a cartoon of the network
       topology constructed in the example.  You will find a similar "drawing" in most of our examples.

       In  this  case,  you can see that we are going to extend our point-to-point example (the link between the
       nodes n0 and n1 below) by hanging a bus network off of the right side.  Notice that this is  the  default
       network topology since you can actually vary the number of nodes created on the LAN.  If you set nCsma to
       one, there will be a total of two nodes on the LAN (CSMA channel) --- one required node and  one  "extra"
       node.  By default there are three "extra" nodes as seen below:

          // Default Network Topology
          //
          //       10.1.1.0
          // n0 -------------- n1   n2   n3   n4
          //    point-to-point  |    |    |    |
          //                    ================
          //                      LAN 10.1.2.0

       Then  the  ns-3  namespace  is  used  and  a logging component is defined.  This is all just as it was in
       first.cc, so there is nothing new yet.

          using namespace ns3;

          NS_LOG_COMPONENT_DEFINE ("SecondScriptExample");

       The main program begins with a slightly different twist.  We use a verbose flag to determine  whether  or
       not  the UdpEchoClientApplication and UdpEchoServerApplication logging components are enabled.  This flag
       defaults to true (the logging components are enabled) but allows us to turn off logging during regression
       testing of this example.

       You  will  see some familiar code that will allow you to change the number of devices on the CSMA network
       via command line argument.  We did something similar when we allowed the number of  packets  sent  to  be
       changed in the section on command line arguments.  The last line makes sure you have at least one "extra"
       node.

       The code consists of variations of previously covered API so you should be entirely comfortable with  the
       following code at this point in the tutorial.

          bool verbose = true;
          uint32_t nCsma = 3;

          CommandLine cmd;
          cmd.AddValue ("nCsma", "Number of \"extra\" CSMA nodes/devices", nCsma);
          cmd.AddValue ("verbose", "Tell echo applications to log if true", verbose);

          cmd.Parse (argc, argv);

          if (verbose)
            {
              LogComponentEnable("UdpEchoClientApplication", LOG_LEVEL_INFO);
              LogComponentEnable("UdpEchoServerApplication", LOG_LEVEL_INFO);
            }

          nCsma = nCsma == 0 ? 1 : nCsma;

       The next step is to create two nodes that we will connect via the point-to-point link.  The NodeContainer
       is used to do this just as was done in first.cc.

          NodeContainer p2pNodes;
          p2pNodes.Create (2);

       Next, we declare another NodeContainer to hold the nodes that will be part of  the  bus  (CSMA)  network.
       First, we just instantiate the container object itself.

          NodeContainer csmaNodes;
          csmaNodes.Add (p2pNodes.Get (1));
          csmaNodes.Create (nCsma);

       The  next  line  of  code Gets the first node (as in having an index of one) from the point-to-point node
       container and adds it to the container of nodes that will get CSMA devices.   The  node  in  question  is
       going to end up with a point-to-point device and a CSMA device.  We then create a number of "extra" nodes
       that compose the remainder of the CSMA network.  Since we already have one node in the  CSMA  network  --
       the  one  that will have both a point-to-point and CSMA net device, the number of "extra" nodes means the
       number nodes you desire in the CSMA section minus one.

       The next bit of code should be quite familiar by now.  We instantiate a PointToPointHelper  and  set  the
       associated  default Attributes so that we create a five megabit per second transmitter on devices created
       using the helper and a two millisecond delay on channels created by the helper.

          PointToPointHelper pointToPoint;
          pointToPoint.SetDeviceAttribute ("DataRate", StringValue ("5Mbps"));
          pointToPoint.SetChannelAttribute ("Delay", StringValue ("2ms"));

          NetDeviceContainer p2pDevices;
          p2pDevices = pointToPoint.Install (p2pNodes);

       We then instantiate a NetDeviceContainer to keep track of the point-to-point net devices and  we  Install
       devices on the point-to-point nodes.

       We  mentioned above that you were going to see a helper for CSMA devices and channels, and the next lines
       introduce them.  The CsmaHelper works just like a PointToPointHelper, but it creates  and  connects  CSMA
       devices  and  channels.   In  the  case  of  a CSMA device and channel pair, notice that the data rate is
       specified by a channel Attribute instead of a device Attribute.  This is because a real CSMA network does
       not  allow  one to mix, for example, 10Base-T and 100Base-T devices on a given channel.  We first set the
       data rate to 100 megabits per second, and then set the  speed-of-light  delay  of  the  channel  to  6560
       nano-seconds (arbitrarily chosen as 1 nanosecond per foot over a 100 meter segment).  Notice that you can
       set an Attribute using its native data type.

          CsmaHelper csma;
          csma.SetChannelAttribute ("DataRate", StringValue ("100Mbps"));
          csma.SetChannelAttribute ("Delay", TimeValue (NanoSeconds (6560)));

          NetDeviceContainer csmaDevices;
          csmaDevices = csma.Install (csmaNodes);

       Just as we created a NetDeviceContainer to hold the devices created by the PointToPointHelper we create a
       NetDeviceContainer  to  hold  the  devices  created by our CsmaHelper.  We call the Install method of the
       CsmaHelper to install the devices into the nodes of the csmaNodes NodeContainer.

       We now have our nodes, devices and channels created, but we have no protocol stacks present.  Just as  in
       the first.cc script, we will use the InternetStackHelper to install these stacks.

          InternetStackHelper stack;
          stack.Install (p2pNodes.Get (0));
          stack.Install (csmaNodes);

       Recall that we took one of the nodes from the p2pNodes container and added it to the csmaNodes container.
       Thus we only need to install the stacks on the remaining p2pNodes node, and  all  of  the  nodes  in  the
       csmaNodes container to cover all of the nodes in the simulation.

       Just  as in the first.cc example script, we are going to use the Ipv4AddressHelper to assign IP addresses
       to our device interfaces.  First we use the network 10.1.1.0 to create the two addresses needed  for  our
       two point-to-point devices.

          Ipv4AddressHelper address;
          address.SetBase ("10.1.1.0", "255.255.255.0");
          Ipv4InterfaceContainer p2pInterfaces;
          p2pInterfaces = address.Assign (p2pDevices);

       Recall  that  we  save  the  created  interfaces  in  a  container to make it easy to pull out addressing
       information later for use in setting up the applications.

       We now need to assign IP addresses to our CSMA device interfaces.  The operation works just as it did for
       the  point-to-point  case,  except we now are performing the operation on a container that has a variable
       number of CSMA devices --- remember we made the  number  of  CSMA  devices  changeable  by  command  line
       argument.   The  CSMA  devices  will be associated with IP addresses from network number 10.1.2.0 in this
       case, as seen below.

          address.SetBase ("10.1.2.0", "255.255.255.0");
          Ipv4InterfaceContainer csmaInterfaces;
          csmaInterfaces = address.Assign (csmaDevices);

       Now we have a topology built, but we need applications.   This  section  is  going  to  be  fundamentally
       similar  to the applications section of first.cc but we are going to instantiate the server on one of the
       nodes that has a CSMA device and the client on the node having only a point-to-point device.

       First, we set up the echo server.  We create a UdpEchoServerHelper and provide a required Attribute value
       to the constructor which is the server port number.  Recall that this port can be changed later using the
       SetAttribute method if desired, but we require it to be provided to the constructor.

          UdpEchoServerHelper echoServer (9);

          ApplicationContainer serverApps = echoServer.Install (csmaNodes.Get (nCsma));
          serverApps.Start (Seconds (1.0));
          serverApps.Stop (Seconds (10.0));

       Recall that the csmaNodes NodeContainer contains one of the nodes created for the point-to-point  network
       and  nCsma  "extra" nodes.  What we want to get at is the last of the "extra" nodes.  The zeroth entry of
       the csmaNodes container will be the point-to-point node.  The easy way to think of this, then, is  if  we
       create  one "extra" CSMA node, then it will be at index one of the csmaNodes container.  By induction, if
       we create nCsma "extra" nodes the last one will be at index nCsma.  You see this exhibited in the Get  of
       the first line of code.

       The  client  application  is  set up exactly as we did in the first.cc example script.  Again, we provide
       required Attributes to the UdpEchoClientHelper in the constructor (in this case the  remote  address  and
       port).   We  tell  the  client to send packets to the server we just installed on the last of the "extra"
       CSMA nodes.  We install the client on the leftmost point-to-point node seen in the topology illustration.

          UdpEchoClientHelper echoClient (csmaInterfaces.GetAddress (nCsma), 9);
          echoClient.SetAttribute ("MaxPackets", UintegerValue (1));
          echoClient.SetAttribute ("Interval", TimeValue (Seconds (1.0)));
          echoClient.SetAttribute ("PacketSize", UintegerValue (1024));

          ApplicationContainer clientApps = echoClient.Install (p2pNodes.Get (0));
          clientApps.Start (Seconds (2.0));
          clientApps.Stop (Seconds (10.0));

       Since we have actually built an internetwork here, we need  some  form  of  internetwork  routing.   ns-3
       provides  what  we  call global routing to help you out.  Global routing takes advantage of the fact that
       the entire internetwork is accessible in the simulation and runs through the all of the nodes created for
       the  simulation  ---  it  does  the  hard  work of setting up routing for you without having to configure
       routers.

       Basically, what happens is that each node behaves  as  if  it  were  an  OSPF  router  that  communicates
       instantly   and  magically  with  all  other  routers  behind  the  scenes.   Each  node  generates  link
       advertisements and communicates  them  directly  to  a  global  route  manager  which  uses  this  global
       information  to  construct  the  routing  tables  for  each  node.   Setting up this form of routing is a
       one-liner:

          Ipv4GlobalRoutingHelper::PopulateRoutingTables ();

       Next we enable pcap tracing.  The first line of code to enable pcap tracing in the point-to-point  helper
       should  be  familiar to you by now.  The second line enables pcap tracing in the CSMA helper and there is
       an extra parameter you haven't encountered yet.

          pointToPoint.EnablePcapAll ("second");
          csma.EnablePcap ("second", csmaDevices.Get (1), true);

       The CSMA network is a multi-point-to-point network.  This means that there can (and  are  in  this  case)
       multiple  endpoints  on  a  shared  medium.  Each of these endpoints has a net device associated with it.
       There are two basic alternatives to gathering trace information from such  a  network.   One  way  is  to
       create  a  trace file for each net device and store only the packets that are emitted or consumed by that
       net device.  Another way is to pick one of the devices and place it in  promiscuous  mode.   That  single
       device  then  "sniffs"  the  network  for all packets and stores them in a single pcap file.  This is how
       tcpdump, for example, works.  That final parameter tells the CSMA helper whether or  not  to  arrange  to
       capture packets in promiscuous mode.

       In  this  example,  we are going to select one of the devices on the CSMA network and ask it to perform a
       promiscuous sniff of the network, thereby emulating what tcpdump would  do.   If  you  were  on  a  Linux
       machine  you  might  do  something  like  tcpdump -i eth0 to get the trace.  In this case, we specify the
       device using csmaDevices.Get(1), which selects the first device in  the  container.   Setting  the  final
       parameter to true enables promiscuous captures.

       The last section of code just runs and cleans up the simulation just like the first.cc example.

            Simulator::Run ();
            Simulator::Destroy ();
            return 0;
          }

       In order to run this example, copy the second.cc example script into the scratch directory and use waf to
       build just as you did with the first.cc example.  If you are in the top-level directory of the repository
       you just type,

          $ cp examples/tutorial/second.cc scratch/mysecond.cc
          $ ./waf

       Warning:   We use the file second.cc as one of our regression tests to verify that it works exactly as we
       think it should in order to make your tutorial experience a positive one.  This means that an  executable
       named  second already exists in the project.  To avoid any confusion about what you are executing, please
       do the renaming to mysecond.cc suggested above.

       If you are following the tutorial religiously (you are, aren't  you)  you  will  still  have  the  NS_LOG
       variable set, so go ahead and clear that variable and run the program.

          $ export NS_LOG=
          $ ./waf --run scratch/mysecond

       Since  we  have  set up the UDP echo applications to log just as we did in first.cc, you will see similar
       output when you run the script.

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (0.415s)
          Sent 1024 bytes to 10.1.2.4
          Received 1024 bytes from 10.1.1.1
          Received 1024 bytes from 10.1.2.4

       Recall that the first message, "Sent 1024 bytes to 10.1.2.4," is the UDP echo client sending a packet  to
       the  server.   In  this  case,  the  server  is  on  a different network (10.1.2.0).  The second message,
       "Received 1024 bytes from 10.1.1.1," is from the UDP echo server, generated when  it  receives  the  echo
       packet.  The final message, "Received 1024 bytes from 10.1.2.4," is from the echo client, indicating that
       it has received its echo back from the server.

       If you now go and look in the top level directory, you will find three trace files:

          second-0-0.pcap  second-1-0.pcap  second-2-0.pcap

       Let's  take  a  moment  to  look  at  the  naming  of  these  files.   They  all  have  the  same   form,
       <name>-<node>-<device>.pcap.   For example, the first file in the listing is second-0-0.pcap which is the
       pcap trace from node zero, device zero.  This is the point-to-point net device on node  zero.   The  file
       second-1-0.pcap  is the pcap trace for device zero on node one, also a point-to-point net device; and the
       file second-2-0.pcap is the pcap trace for device zero on node two.

       If you refer back to the topology illustration at the start of the section, you will see that  node  zero
       is  the  leftmost node of the point-to-point link and node one is the node that has both a point-to-point
       device and a CSMA device.  You will see that node two is the first "extra" node on the CSMA  network  and
       its device zero was selected as the device to capture the promiscuous-mode trace.

       Now,  let's  follow  the echo packet through the internetwork.  First, do a tcpdump of the trace file for
       the leftmost point-to-point node --- node zero.

          $ tcpdump -nn -tt -r second-0-0.pcap

       You should see the contents of the pcap file displayed:

          reading from file second-0-0.pcap, link-type PPP (PPP)
          2.000000 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
          2.017607 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024

       The first line of the dump indicates that the link type is PPP (point-to-point)  which  we  expect.   You
       then  see the echo packet leaving node zero via the device associated with IP address 10.1.1.1 headed for
       IP address 10.1.2.4 (the rightmost CSMA node).  This packet will move over the point-to-point link and be
       received by the point-to-point net device on node one.  Let's take a look:

          $ tcpdump -nn -tt -r second-1-0.pcap

       You should now see the pcap trace output of the other side of the point-to-point link:

          reading from file second-1-0.pcap, link-type PPP (PPP)
          2.003686 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
          2.013921 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024

       Here  we  see  that  the  link  type  is also PPP as we would expect.  You see the packet from IP address
       10.1.1.1 (that was sent at 2.000000 seconds) headed toward IP address 10.1.2.4 appear on this  interface.
       Now, internally to this node, the packet will be forwarded to the CSMA interface and we should see it pop
       out on that device headed for its ultimate destination.

       Remember that we selected node 2 as the promiscuous sniffer node for the CSMA network so let's then  look
       at second-2-0.pcap and see if its there.

          $ tcpdump -nn -tt -r second-2-0.pcap

       You should now see the promiscuous dump of node two, device zero:

          reading from file second-2-0.pcap, link-type EN10MB (Ethernet)
          2.007698 ARP, Request who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1, length 50
          2.007710 ARP, Reply 10.1.2.4 is-at 00:00:00:00:00:06, length 50
          2.007803 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
          2.013815 ARP, Request who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4, length 50
          2.013828 ARP, Reply 10.1.2.1 is-at 00:00:00:00:00:03, length 50
          2.013921 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024

       As  you  can  see, the link type is now "Ethernet".  Something new has appeared, though.  The bus network
       needs ARP, the Address Resolution Protocol.  Node one knows it needs to send the  packet  to  IP  address
       10.1.2.4,  but  it  doesn't  know  the  MAC address of the corresponding node.  It broadcasts on the CSMA
       network (ff:ff:ff:ff:ff:ff) asking for the device that has  IP  address  10.1.2.4.   In  this  case,  the
       rightmost node replies saying it is at MAC address 00:00:00:00:00:06.  Note that node two is not directly
       involved in this exchange, but is sniffing the network and reporting all of the traffic it sees.

       This exchange is seen in the following lines,

          2.007698 ARP, Request who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1, length 50
          2.007710 ARP, Reply 10.1.2.4 is-at 00:00:00:00:00:06, length 50

       Then node one, device one goes ahead and sends the echo packet to the  UDP  echo  server  at  IP  address
       10.1.2.4.

          2.007803 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024

       The  server  receives  the echo request and turns the packet around trying to send it back to the source.
       The server knows that this address is on another network that it reaches via IP address  10.1.2.1.   This
       is because we initialized global routing and it has figured all of this out for us.  But, the echo server
       node doesn't know the MAC address of the first CSMA node, so it has to ARP for it  just  like  the  first
       CSMA node had to do.

          2.013815 ARP, Request who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4, length 50
          2.013828 ARP, Reply 10.1.2.1 is-at 00:00:00:00:00:03, length 50

       The server then sends the echo back to the forwarding node.

          2.013921 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024

       Looking back at the rightmost node of the point-to-point link,

          $ tcpdump -nn -tt -r second-1-0.pcap

       You  can now see the echoed packet coming back onto the point-to-point link as the last line of the trace
       dump.

          reading from file second-1-0.pcap, link-type PPP (PPP)
          2.003686 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
          2.013921 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024

       Lastly, you can look back at the node that originated the echo

          $ tcpdump -nn -tt -r second-0-0.pcap

       and see that the echoed packet arrives back at the source at 2.007602 seconds,

          reading from file second-0-0.pcap, link-type PPP (PPP)
          2.000000 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
          2.017607 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024

       Finally, recall that we added the ability to control the number of CSMA  devices  in  the  simulation  by
       command  line  argument.   You can change this argument in the same way as when we looked at changing the
       number of packets echoed in the first.cc example.  Try running the program with  the  number  of  "extra"
       devices set to four:

          $ ./waf --run "scratch/mysecond --nCsma=4"

       You should now see,

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (0.405s)
          At time 2s client sent 1024 bytes to 10.1.2.5 port 9
          At time 2.0118s server received 1024 bytes from 10.1.1.1 port 49153
          At time 2.0118s server sent 1024 bytes to 10.1.1.1 port 49153
          At time 2.02461s client received 1024 bytes from 10.1.2.5 port 9

       Notice  that  the  echo  server  has  now been relocated to the last of the CSMA nodes, which is 10.1.2.5
       instead of the default case, 10.1.2.4.

       It is possible that you may not be satisfied with a trace file generated  by  a  bystander  in  the  CSMA
       network.   You  may  really want to get a trace from a single device and you may not be interested in any
       other traffic on the network.  You can do this fairly easily.

       Let's take a look at scratch/mysecond.cc and add that code enabling us to be more specific.  ns-3 helpers
       provide  methods  that  take  a  node  number  and device number as parameters.  Go ahead and replace the
       EnablePcap calls with the calls below.

          pointToPoint.EnablePcap ("second", p2pNodes.Get (0)->GetId (), 0);
          csma.EnablePcap ("second", csmaNodes.Get (nCsma)->GetId (), 0, false);
          csma.EnablePcap ("second", csmaNodes.Get (nCsma-1)->GetId (), 0, false);

       We know that we want to create a pcap file with the base name "second" and we also know that  the  device
       of interest in both cases is going to be zero, so those parameters are not really interesting.

       In  order  to  get  the  node number, you have two choices:  first, nodes are numbered in a monotonically
       increasing fashion starting from zero in the order in which you created them.  One  way  to  get  a  node
       number  is to figure this number out "manually" by contemplating the order of node creation.  If you take
       a look at the network topology illustration at the beginning of the file, we did this for you and you can
       see  that  the  last CSMA node is going to be node number nCsma + 1.  This approach can become annoyingly
       difficult in larger simulations.

       An alternate way, which we use here, is to realize that the NodeContainers contain pointers to ns-3  Node
       Objects.   The  Node Object has a method called GetId which will return that node's ID, which is the node
       number we seek.  Let's go take a look at the Doxygen for the  Node  and  locate  that  method,  which  is
       further  down  in  the ns-3 core code than we've seen so far; but sometimes you have to search diligently
       for useful things.

       Go to the Doxygen documentation for your release (recall that you can find it on the project  web  site).
       You  can  get  to  the  Node documentation by looking through at the "Classes" tab and scrolling down the
       "Class List" until you find ns3::Node.  Select ns3::Node and you will be taken to the  documentation  for
       the  Node  class.   If  you  now  scroll down to the GetId method and select it, you will be taken to the
       detailed documentation for the method.  Using the GetId method can make  determining  node  numbers  much
       easier in complex topologies.

       Let's clear the old trace files out of the top-level directory to avoid confusion about what is going on,

          $ rm *.pcap
          $ rm *.tr

       If you build the new script and run the simulation setting nCsma to 100,

          $ ./waf --run "scratch/mysecond --nCsma=100"

       you will see the following output:

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (0.407s)
          At time 2s client sent 1024 bytes to 10.1.2.101 port 9
          At time 2.0068s server received 1024 bytes from 10.1.1.1 port 49153
          At time 2.0068s server sent 1024 bytes to 10.1.1.1 port 49153
          At time 2.01761s client received 1024 bytes from 10.1.2.101 port 9

       Note that the echo server is now located at 10.1.2.101 which corresponds to having 100 "extra" CSMA nodes
       with the echo server on the last one.  If you list the pcap files in the top  level  directory  you  will
       see,

          second-0-0.pcap  second-100-0.pcap  second-101-0.pcap

       The  trace  file second-0-0.pcap is the "leftmost" point-to-point device which is the echo packet source.
       The file second-101-0.pcap corresponds to the rightmost CSMA  device  which  is  where  the  echo  server
       resides.   You  may  have noticed that the final parameter on the call to enable pcap tracing on the echo
       server node was false.  This means that the trace gathered on that node was in non-promiscuous mode.

       To illustrate the difference  between  promiscuous  and  non-promiscuous  traces,  we  also  requested  a
       non-promiscuous  trace  for  the  next-to-last  node.   Go  ahead  and  take  a  look  at the tcpdump for
       second-100-0.pcap.

          $ tcpdump -nn -tt -r second-100-0.pcap

       You can now see that node 100 is really a bystander in the echo  exchange.   The  only  packets  that  it
       receives are the ARP requests which are broadcast to the entire CSMA network.

          reading from file second-100-0.pcap, link-type EN10MB (Ethernet)
          2.006698 ARP, Request who-has 10.1.2.101 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1, length 50
          2.013815 ARP, Request who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.101, length 50

       Now take a look at the tcpdump for second-101-0.pcap.

          $ tcpdump -nn -tt -r second-101-0.pcap

       You can now see that node 101 is really the participant in the echo exchange.

          reading from file second-101-0.pcap, link-type EN10MB (Ethernet)
          2.006698 ARP, Request who-has 10.1.2.101 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1, length 50
          2.006698 ARP, Reply 10.1.2.101 is-at 00:00:00:00:00:67, length 50
          2.006803 IP 10.1.1.1.49153 > 10.1.2.101.9: UDP, length 1024
          2.013803 ARP, Request who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.101, length 50
          2.013828 ARP, Reply 10.1.2.1 is-at 00:00:00:00:00:03, length 50
          2.013828 IP 10.1.2.101.9 > 10.1.1.1.49153: UDP, length 1024

   Models, Attributes and Reality
       This  is  a convenient place to make a small excursion and make an important point.  It may or may not be
       obvious to you, but whenever one is using a simulation, it is important to  understand  exactly  what  is
       being  modeled  and  what is not.  It is tempting, for example, to think of the CSMA devices and channels
       used in the previous section as if they were real Ethernet devices; and to expect a simulation result  to
       directly reflect what will happen in a real Ethernet.  This is not the case.

       A  model  is,  by  definition,  an  abstraction  of  reality.  It is ultimately the responsibility of the
       simulation script author to determine the so-called "range of accuracy" and "domain of applicability"  of
       the simulation as a whole, and therefore its constituent parts.

       In  some  cases, like Csma, it can be fairly easy to determine what is not modeled.  By reading the model
       description (csma.h) you can find that there is no collision detection in the CSMA model  and  decide  on
       how  applicable  its  use  will  be  in your simulation or what caveats you may want to include with your
       results.  In other cases, it can be quite easy to configure behaviors  that  might  not  agree  with  any
       reality  you  can  go  out and buy.  It will prove worthwhile to spend some time investigating a few such
       instances, and how easily you can swerve outside the bounds of reality in your simulations.

       As you have seen, ns-3 provides Attributes which  a  user  can  easily  set  to  change  model  behavior.
       Consider  two  of  the  Attributes  of  the CsmaNetDevice:  Mtu and EncapsulationMode.  The Mtu attribute
       indicates the Maximum Transmission Unit to the device.  This is the size of  the  largest  Protocol  Data
       Unit (PDU) that the device can send.

       The  MTU  defaults to 1500 bytes in the CsmaNetDevice.  This default corresponds to a number found in RFC
       894, "A Standard for the Transmission of IP Datagrams over Ethernet Networks."  The  number  is  actually
       derived  from  the  maximum  packet size for 10Base5 (full-spec Ethernet) networks -- 1518 bytes.  If you
       subtract the DIX encapsulation overhead for Ethernet packets (18 bytes) you will end up  with  a  maximum
       possible  data  size (MTU) of 1500 bytes.  One can also find that the MTU for IEEE 802.3 networks is 1492
       bytes.  This is because LLC/SNAP encapsulation adds an extra eight bytes of overhead to the  packet.   In
       both cases, the underlying hardware can only send 1518 bytes, but the data size is different.

       In  order to set the encapsulation mode, the CsmaNetDevice provides an Attribute called EncapsulationMode
       which can take on the values Dix or Llc.  These correspond to Ethernet and LLC/SNAP framing respectively.

       If one leaves the Mtu at 1500 bytes and changes the encapsulation mode to  Llc,  the  result  will  be  a
       network  that encapsulates 1500 byte PDUs with LLC/SNAP framing resulting in packets of 1526 bytes, which
       would be illegal in many networks, since they can transmit a maximum of  1518  bytes  per  packet.   This
       would  most  likely  result  in  a simulation that quite subtly does not reflect the reality you might be
       expecting.

       Just to complicate the picture, there exist jumbo frames (1500 < MTU <= 9000 bytes) and super-jumbo  (MTU
       >  9000  bytes)  frames  that  are not officially sanctioned by IEEE but are available in some high-speed
       (Gigabit) networks and NICs.  One could leave the  encapsulation  mode  set  to  Dix,  and  set  the  Mtu
       Attribute  on a CsmaNetDevice to 64000 bytes -- even though an associated CsmaChannel DataRate was set at
       10 megabits per second.  This would essentially model an  Ethernet  switch  made  out  of  vampire-tapped
       1980s-style  10Base5  networks  that support super-jumbo datagrams.  This is certainly not something that
       was ever made, nor is likely to ever be made, but it is quite easy for you to configure.

       In the previous example, you used the command line to create a simulation that had 100 Csma  nodes.   You
       could  have  just  as  easily  created  a  simulation with 500 nodes.  If you were actually modeling that
       10Base5 vampire-tap network, the maximum length of a full-spec Ethernet  cable  is  500  meters,  with  a
       minimum tap spacing of 2.5 meters.  That means there could only be 200 taps on a real network.  You could
       have quite easily built an illegal network in that way as  well.   This  may  or  may  not  result  in  a
       meaningful simulation depending on what you are trying to model.

       Similar  situations  can occur in many places in ns-3 and in any simulator.  For example, you may be able
       to position nodes in such a way that they occupy the same space at the same time, or you may be  able  to
       configure amplifiers or noise levels that violate the basic laws of physics.

       ns-3 generally favors flexibility, and many models will allow freely setting Attributes without trying to
       enforce any arbitrary consistency or particular underlying spec.

       The thing to take home from this is that ns-3 is going to  provide  a  super-flexible  base  for  you  to
       experiment  with.   It  is up to you to understand what you are asking the system to do and to  make sure
       that the simulations you create have some meaning and some connection with a reality defined by you.

   Building a Wireless Network Topology
       In this section we are going to further expand our knowledge of ns-3  network  devices  and  channels  to
       cover  an example of a wireless network.  ns-3 provides a set of 802.11 models that attempt to provide an
       accurate MAC-level implementation of the 802.11 specification and a "not-so-slow" PHY-level model of  the
       802.11a specification.

       Just   as  we  have  seen  both  point-to-point  and  CSMA  topology  helper  objects  when  constructing
       point-to-point topologies, we will see equivalent Wifi topology helpers in this section.  The  appearance
       and operation of these helpers should look quite familiar to you.

       We  provide  an  example  script in our examples/tutorial directory.  This script builds on the second.cc
       script and adds a Wifi network.  Go ahead and open examples/tutorial/third.cc in  your  favorite  editor.
       You  will  have already seen enough ns-3 code to understand most of what is going on in this example, but
       there are a few new things, so we will go over the entire script and examine some of the output.

       Just as in the second.cc example (and in all ns-3 examples) the file begins with an emacs mode  line  and
       some GPL boilerplate.

       Take  a  look  at the ASCII art (reproduced below) that shows the default network topology constructed in
       the example.  You can see that we are going to further extend our example by hanging a  wireless  network
       off  of  the  left  side.  Notice that this is a default network topology since you can actually vary the
       number of nodes created on the wired and wireless networks.  Just as in the second.cc script case, if you
       change  nCsma,  it will give you a number of "extra" CSMA nodes.  Similarly, you can set nWifi to control
       how many STA (station) nodes are created in the simulation.  There will always be one AP  (access  point)
       node  on  the  wireless  network.   By  default there are three "extra" CSMA nodes and three wireless STA
       nodes.

       The code begins by loading module include files just as was done in the second.cc example.  There  are  a
       couple  of  new  includes  corresponding to the Wifi module and the mobility module which we will discuss
       below.

          #include "ns3/core-module.h"
          #include "ns3/point-to-point-module.h"
          #include "ns3/network-module.h"
          #include "ns3/applications-module.h"
          #include "ns3/wifi-module.h"
          #include "ns3/mobility-module.h"
          #include "ns3/csma-module.h"
          #include "ns3/internet-module.h"

       The network topology illustration follows:

          // Default Network Topology
          //
          //   Wifi 10.1.3.0
          //                 AP
          //  *    *    *    *
          //  |    |    |    |    10.1.1.0
          // n5   n6   n7   n0 -------------- n1   n2   n3   n4
          //                   point-to-point  |    |    |    |
          //                                   ================
          //                                     LAN 10.1.2.0

       You can see that we are adding a new network device to the node on the left side  of  the  point-to-point
       link  that becomes the access point for the wireless network.  A number of wireless STA nodes are created
       to fill out the new 10.1.3.0 network as shown on the left side of the illustration.

       After the illustration, the ns-3 namespace is used and a logging component is defined.  This  should  all
       be quite familiar by now.

          using namespace ns3;

          NS_LOG_COMPONENT_DEFINE ("ThirdScriptExample");

       The  main  program  begins  just  like  second.cc  by adding some command line parameters for enabling or
       disabling logging components and for changing the number of devices created.

          bool verbose = true;
          uint32_t nCsma = 3;
          uint32_t nWifi = 3;

          CommandLine cmd;
          cmd.AddValue ("nCsma", "Number of \"extra\" CSMA nodes/devices", nCsma);
          cmd.AddValue ("nWifi", "Number of wifi STA devices", nWifi);
          cmd.AddValue ("verbose", "Tell echo applications to log if true", verbose);

          cmd.Parse (argc,argv);

          if (verbose)
            {
              LogComponentEnable("UdpEchoClientApplication", LOG_LEVEL_INFO);
              LogComponentEnable("UdpEchoServerApplication", LOG_LEVEL_INFO);
            }

       Just as in all of the previous examples, the next step is to create two nodes that we  will  connect  via
       the point-to-point link.

          NodeContainer p2pNodes;
          p2pNodes.Create (2);

       Next,  we  see  an  old  friend.   We  instantiate  a  PointToPointHelper  and set the associated default
       Attributes so that we create a five megabit per second transmitter on devices created  using  the  helper
       and  a  two millisecond delay on channels created by the helper.  We then Intall the devices on the nodes
       and the channel between them.

          PointToPointHelper pointToPoint;
          pointToPoint.SetDeviceAttribute ("DataRate", StringValue ("5Mbps"));
          pointToPoint.SetChannelAttribute ("Delay", StringValue ("2ms"));

          NetDeviceContainer p2pDevices;
          p2pDevices = pointToPoint.Install (p2pNodes);

       Next, we declare another NodeContainer to hold the nodes that will be part of the bus (CSMA) network.

          NodeContainer csmaNodes;
          csmaNodes.Add (p2pNodes.Get (1));
          csmaNodes.Create (nCsma);

       The next line of code Gets the first node (as in having an index of one)  from  the  point-to-point  node
       container  and  adds  it  to  the container of nodes that will get CSMA devices.  The node in question is
       going to end up with a point-to-point device and a CSMA device.  We then create a number of "extra" nodes
       that compose the remainder of the CSMA network.

       We  then  instantiate a CsmaHelper and set its Attributes as we did in the previous example.  We create a
       NetDeviceContainer to keep track of the created CSMA net devices and then we Install CSMA devices on  the
       selected nodes.

          CsmaHelper csma;
          csma.SetChannelAttribute ("DataRate", StringValue ("100Mbps"));
          csma.SetChannelAttribute ("Delay", TimeValue (NanoSeconds (6560)));

          NetDeviceContainer csmaDevices;
          csmaDevices = csma.Install (csmaNodes);

       Next,  we  are going to create the nodes that will be part of the Wifi network.  We are going to create a
       number of "station" nodes as specified by the command  line  argument,  and  we  are  going  to  use  the
       "leftmost" node of the point-to-point link as the node for the access point.

          NodeContainer wifiStaNodes;
          wifiStaNodes.Create (nWifi);
          NodeContainer wifiApNode = p2pNodes.Get (0);

       The  next  bit  of  code  constructs  the wifi devices and the interconnection channel between these wifi
       nodes. First, we configure the PHY and channel helpers:

          YansWifiChannelHelper channel = YansWifiChannelHelper::Default ();
          YansWifiPhyHelper phy = YansWifiPhyHelper::Default ();

       For simplicity, this code uses  the  default  PHY  layer  configuration  and  channel  models  which  are
       documented    in   the   API   doxygen   documentation   for   the   YansWifiChannelHelper::Default   and
       YansWifiPhyHelper::Default methods. Once these objects are  created,  we  create  a  channel  object  and
       associate  it  to our PHY layer object manager to make sure that all the PHY layer objects created by the
       YansWifiPhyHelper share the same underlying channel, that is, they share the same wireless medium and can
       communication and interfere:

          phy.SetChannel (channel.Create ());

       Once  the  PHY  helper  is configured, we can focus on the MAC layer. Here we choose to work with non-Qos
       MACs so we use a NqosWifiMacHelper object to set MAC parameters.

          WifiHelper wifi = WifiHelper::Default ();
          wifi.SetRemoteStationManager ("ns3::AarfWifiManager");

          NqosWifiMacHelper mac = NqosWifiMacHelper::Default ();

       The SetRemoteStationManager method tells the helper the type of rate control algorithm to use.  Here,  it
       is asking the helper to use the AARF algorithm --- details are, of course, available in Doxygen.

       Next, we configure the type of MAC, the SSID of the infrastructure network we want to setup and make sure
       that our stations don't perform active probing:

          Ssid ssid = Ssid ("ns-3-ssid");
          mac.SetType ("ns3::StaWifiMac",
            "Ssid", SsidValue (ssid),
            "ActiveProbing", BooleanValue (false));

       This code first creates an 802.11 service set identifier (SSID) object that will be used to set the value
       of  the  "Ssid" Attribute of the MAC layer implementation.  The particular kind of MAC layer that will be
       created by the helper is specified by Attribute as being of  the  "ns3::StaWifiMac"  type.   The  use  of
       NqosWifiMacHelper will ensure that the "QosSupported" Attribute for created MAC objects is set false. The
       combination of these two configurations means that the MAC instance next created will be a non-QoS non-AP
       station  (STA) in an infrastructure BSS (i.e., a BSS with an AP).  Finally, the "ActiveProbing" Attribute
       is set to false.  This means that probe requests will not be sent by MACs created by this helper.

       Once all the station-specific parameters are fully configured, both at the MAC and  PHY  layers,  we  can
       invoke our now-familiar Install method to create the wifi devices of these stations:

          NetDeviceContainer staDevices;
          staDevices = wifi.Install (phy, mac, wifiStaNodes);

       We  have  configured  Wifi  for  all of our STA nodes, and now we need to configure the AP (access point)
       node.  We begin this process by changing the default Attributes of the NqosWifiMacHelper to  reflect  the
       requirements of the AP.

          mac.SetType ("ns3::ApWifiMac",
                       "Ssid", SsidValue (ssid));

       In  this  case,  the  NqosWifiMacHelper is going to create MAC layers of the "ns3::ApWifiMac", the latter
       specifying that a MAC instance configured as an AP should be created, with the helper type implying  that
       the  "QosSupported" Attribute should be set to false - disabling 802.11e/WMM-style QoS support at created
       APs.

       The next lines create the single AP which shares the same set of PHY-level Attributes  (and  channel)  as
       the stations:

          NetDeviceContainer apDevices;
          apDevices = wifi.Install (phy, mac, wifiApNode);

       Now,  we are going to add mobility models.  We want the STA nodes to be mobile, wandering around inside a
       bounding box, and we want to make the AP node stationary.  We use the MobilityHelper to  make  this  easy
       for  us.  First, we instantiate a MobilityHelper object and set some Attributes controlling the "position
       allocator" functionality.

          MobilityHelper mobility;

          mobility.SetPositionAllocator ("ns3::GridPositionAllocator",
            "MinX", DoubleValue (0.0),
            "MinY", DoubleValue (0.0),
            "DeltaX", DoubleValue (5.0),
            "DeltaY", DoubleValue (10.0),
            "GridWidth", UintegerValue (3),
            "LayoutType", StringValue ("RowFirst"));

       This code tells the mobility helper to use a two-dimensional grid to initially place the STA nodes.  Feel
       free to explore the Doxygen for class ns3::GridPositionAllocator to see exactly what is being done.

       We  have  arranged our nodes on an initial grid, but now we need to tell them how to move.  We choose the
       RandomWalk2dMobilityModel which has the nodes move in a random direction at a random speed around  inside
       a bounding box.

          mobility.SetMobilityModel ("ns3::RandomWalk2dMobilityModel",
            "Bounds", RectangleValue (Rectangle (-50, 50, -50, 50)));

       We now tell the MobilityHelper to install the mobility models on the STA nodes.

          mobility.Install (wifiStaNodes);

       We  want  the  access  point  to remain in a fixed position during the simulation.  We accomplish this by
       setting the mobility model for this node to be the ns3::ConstantPositionMobilityModel:

          mobility.SetMobilityModel ("ns3::ConstantPositionMobilityModel");
          mobility.Install (wifiApNode);

       We now have our nodes, devices and channels created, and mobility models chosen for the Wifi  nodes,  but
       we  have  no  protocol  stacks  present.   Just  as  we  have done previously many times, we will use the
       InternetStackHelper to install these stacks.

          InternetStackHelper stack;
          stack.Install (csmaNodes);
          stack.Install (wifiApNode);
          stack.Install (wifiStaNodes);

       Just as in the second.cc example script, we are going to use the Ipv4AddressHelper to assign IP addresses
       to  our  device interfaces.  First we use the network 10.1.1.0 to create the two addresses needed for our
       two point-to-point devices.  Then we use network 10.1.2.0 to assign addresses to  the  CSMA  network  and
       then  we  assign  addresses  from  network  10.1.3.0  to  both the STA devices and the AP on the wireless
       network.

          Ipv4AddressHelper address;

          address.SetBase ("10.1.1.0", "255.255.255.0");
          Ipv4InterfaceContainer p2pInterfaces;
          p2pInterfaces = address.Assign (p2pDevices);

          address.SetBase ("10.1.2.0", "255.255.255.0");
          Ipv4InterfaceContainer csmaInterfaces;
          csmaInterfaces = address.Assign (csmaDevices);

          address.SetBase ("10.1.3.0", "255.255.255.0");
          address.Assign (staDevices);
          address.Assign (apDevices);

       We put the echo server on the "rightmost" node in the illustration at the start of  the  file.   We  have
       done this before.

          UdpEchoServerHelper echoServer (9);

          ApplicationContainer serverApps = echoServer.Install (csmaNodes.Get (nCsma));
          serverApps.Start (Seconds (1.0));
          serverApps.Stop (Seconds (10.0));

       And  we  put  the  echo  client  on  the  last STA node we created, pointing it to the server on the CSMA
       network.  We have also seen similar operations before.

          UdpEchoClientHelper echoClient (csmaInterfaces.GetAddress (nCsma), 9);
          echoClient.SetAttribute ("MaxPackets", UintegerValue (1));
          echoClient.SetAttribute ("Interval", TimeValue (Seconds (1.0)));
          echoClient.SetAttribute ("PacketSize", UintegerValue (1024));

          ApplicationContainer clientApps =
            echoClient.Install (wifiStaNodes.Get (nWifi - 1));
          clientApps.Start (Seconds (2.0));
          clientApps.Stop (Seconds (10.0));

       Since we have built an internetwork here, we need to enable internetwork routing just as we  did  in  the
       second.cc example script.

          Ipv4GlobalRoutingHelper::PopulateRoutingTables ();

       One  thing  that  can  surprise  some  users  is  the fact that the simulation we just created will never
       "naturally" stop.  This is because we asked the wireless access  point  to  generate  beacons.   It  will
       generate  beacons  forever,  and  this  will  result  in simulator events being scheduled into the future
       indefinitely, so we must tell the simulator to stop even though it  may  have  beacon  generation  events
       scheduled.   The  following  line  of  code tells the simulator to stop so that we don't simulate beacons
       forever and enter what is essentially an endless loop.

          Simulator::Stop (Seconds (10.0));

       We create just enough tracing to cover all three networks:

          pointToPoint.EnablePcapAll ("third");
          phy.EnablePcap ("third", apDevices.Get (0));
          csma.EnablePcap ("third", csmaDevices.Get (0), true);

       These three lines of code will start pcap tracing on both of the point-to-point nodes that serves as  our
       backbone, will start a promiscuous (monitor) mode trace on the Wifi network, and will start a promiscuous
       trace on the CSMA network.  This will let us see all of the traffic with a minimum number of trace files.

       Finally, we actually run the simulation, clean up and then exit the program.

            Simulator::Run ();
            Simulator::Destroy ();
            return 0;
          }

       In order to run this example, you have to copy the third.cc example script into the scratch directory and
       use  Waf  to  build just as you did with the second.cc example.  If you are in the top-level directory of
       the repository you would type,

          $ cp examples/tutorial/third.cc scratch/mythird.cc
          $ ./waf
          $ ./waf --run scratch/mythird

       Again, since we have set up the UDP echo applications just as we did in the second.cc  script,  you  will
       see similar output.

          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
          'build' finished successfully (0.407s)
          At time 2s client sent 1024 bytes to 10.1.2.4 port 9
          At time 2.01796s server received 1024 bytes from 10.1.3.3 port 49153
          At time 2.01796s server sent 1024 bytes to 10.1.3.3 port 49153
          At time 2.03364s client received 1024 bytes from 10.1.2.4 port 9

       Recall  that  the first message, Sent 1024 bytes to 10.1.2.4," is the UDP echo client sending a packet to
       the server.  In this case, the client is  on  the  wireless  network  (10.1.3.0).   The  second  message,
       "Received  1024  bytes  from  10.1.3.3," is from the UDP echo server, generated when it receives the echo
       packet.  The final message, "Received 1024 bytes from 10.1.2.4," is from the echo client, indicating that
       it has received its echo back from the server.

       If  you  now go and look in the top level directory, you will find four trace files from this simulation,
       two from node zero and two from node one:

          third-0-0.pcap  third-0-1.pcap  third-1-0.pcap  third-1-1.pcap

       The file "third-0-0.pcap" corresponds to the point-to-point device on node zero -- the left side  of  the
       "backbone".   The file "third-1-0.pcap" corresponds to the point-to-point device on node one -- the right
       side of the "backbone".  The file "third-0-1.pcap" will be the promiscuous (monitor mode) trace from  the
       Wifi  network and the file "third-1-1.pcap" will be the promiscuous trace from the CSMA network.  Can you
       verify this by inspecting the code?

       Since the echo client is on the Wifi network, let's start there.  Let's take a look  at  the  promiscuous
       (monitor mode) trace we captured on that network.

          $ tcpdump -nn -tt -r third-0-1.pcap

       You should see some wifi-looking contents you haven't seen here before:

          reading from file third-0-1.pcap, link-type IEEE802_11 (802.11)
          0.000025 Beacon (ns-3-ssid) [6.0* 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit] IBSS
          0.000308 Assoc Request (ns-3-ssid) [6.0 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit]
          0.000324 Acknowledgment RA:00:00:00:00:00:08
          0.000402 Assoc Response AID(0) :: Successful
          0.000546 Acknowledgment RA:00:00:00:00:00:0a
          0.000721 Assoc Request (ns-3-ssid) [6.0 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit]
          0.000737 Acknowledgment RA:00:00:00:00:00:07
          0.000824 Assoc Response AID(0) :: Successful
          0.000968 Acknowledgment RA:00:00:00:00:00:0a
          0.001134 Assoc Request (ns-3-ssid) [6.0 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit]
          0.001150 Acknowledgment RA:00:00:00:00:00:09
          0.001273 Assoc Response AID(0) :: Successful
          0.001417 Acknowledgment RA:00:00:00:00:00:0a
          0.102400 Beacon (ns-3-ssid) [6.0* 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit] IBSS
          0.204800 Beacon (ns-3-ssid) [6.0* 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit] IBSS
          0.307200 Beacon (ns-3-ssid) [6.0* 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit] IBSS

       You  can  see  that the link type is now 802.11 as you would expect.  You can probably understand what is
       going on and find the IP echo request and response packets in this trace.  We leave it as an exercise  to
       completely parse the trace dump.

       Now, look at the pcap file of the right side of the point-to-point link,

          $ tcpdump -nn -tt -r third-0-0.pcap

       Again, you should see some familiar looking contents:

          reading from file third-0-0.pcap, link-type PPP (PPP)
          2.008151 IP 10.1.3.3.49153 > 10.1.2.4.9: UDP, length 1024
          2.026758 IP 10.1.2.4.9 > 10.1.3.3.49153: UDP, length 1024

       This  is  the  echo  packet  going  from  left  to  right  (from  Wifi to CSMA) and back again across the
       point-to-point link.

       Now, look at the pcap file of the right side of the point-to-point link,

          $ tcpdump -nn -tt -r third-1-0.pcap

       Again, you should see some familiar looking contents:

          reading from file third-1-0.pcap, link-type PPP (PPP)
          2.011837 IP 10.1.3.3.49153 > 10.1.2.4.9: UDP, length 1024
          2.023072 IP 10.1.2.4.9 > 10.1.3.3.49153: UDP, length 1024

       This is also the echo packet going from left to right (from Wifi to  CSMA)  and  back  again  across  the
       point-to-point link with slightly different timings as you might expect.

       The echo server is on the CSMA network, let's look at the promiscuous trace there:

          $ tcpdump -nn -tt -r third-1-1.pcap

       You should see some familiar looking contents:

          reading from file third-1-1.pcap, link-type EN10MB (Ethernet)
          2.017837 ARP, Request who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1, length 50
          2.017861 ARP, Reply 10.1.2.4 is-at 00:00:00:00:00:06, length 50
          2.017861 IP 10.1.3.3.49153 > 10.1.2.4.9: UDP, length 1024
          2.022966 ARP, Request who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4, length 50
          2.022966 ARP, Reply 10.1.2.1 is-at 00:00:00:00:00:03, length 50
          2.023072 IP 10.1.2.4.9 > 10.1.3.3.49153: UDP, length 1024

       This  should be easily understood.  If you've forgotten, go back and look at the discussion in second.cc.
       This is the same sequence.

       Now, we spent a lot of time setting up mobility models for the wireless network and  so  it  would  be  a
       shame  to  finish  up  without  even  showing  that  the  STA nodes are actually moving around during the
       simulation.  Let's do this by hooking into the MobilityModel course change trace source.  This is just  a
       sneak  peek into the detailed tracing section which is coming up, but this seems a very nice place to get
       an example in.

       As mentioned in the "Tweaking ns-3" section, the ns-3 tracing system is divided into  trace  sources  and
       trace  sinks,  and  we  provide  functions to connect the two.  We will use the mobility model predefined
       course change trace source to originate the trace events.  We will need to write a trace sink to  connect
       to  that  source  that  will  display  some  pretty  information for us.  Despite its reputation as being
       difficult, it's really quite simple.  Just before the  main  program  of  the  scratch/mythird.cc  script
       (i.e., just after the NS_LOG_COMPONENT_DEFINE statement), add the following function:

          void
          CourseChange (std::string context, Ptr<const MobilityModel> model)
          {
            Vector position = model->GetPosition ();
            NS_LOG_UNCOND (context <<
              " x = " << position.x << ", y = " << position.y);
          }

       This  code just pulls the position information from the mobility model and unconditionally logs the x and
       y position of the node.  We are going to arrange for this function to be called every time  the  wireless
       node  with the echo client changes its position.  We do this using the Config::Connect function.  Add the
       following lines of code to the script just before the Simulator::Run call.

          std::ostringstream oss;
          oss <<
            "/NodeList/" << wifiStaNodes.Get (nWifi - 1)->GetId () <<
            "/$ns3::MobilityModel/CourseChange";

          Config::Connect (oss.str (), MakeCallback (&CourseChange));

       What we do here is to create a string containing the tracing namespace path of the event to which we want
       to  connect.   First,  we have to figure out which node it is we want using the GetId method as described
       earlier.  In the case of the default number of CSMA and wireless nodes, this turns out to be  node  seven
       and the tracing namespace path to the mobility model would look like,

          /NodeList/7/$ns3::MobilityModel/CourseChange

       Based on the discussion in the tracing section, you may infer that this trace path references the seventh
       node  in  the  global  NodeList.   It  specifies  what  is  called   an   aggregated   object   of   type
       ns3::MobilityModel.   The  dollar sign prefix implies that the MobilityModel is aggregated to node seven.
       The last component of the path means that we are hooking into the "CourseChange" event of that model.

       We make  a  connection  between  the  trace  source  in  node  seven  with  our  trace  sink  by  calling
       Config::Connect  and  passing  this namespace path.  Once this is done, every course change event on node
       seven will be hooked into our trace sink, which will in turn print out the new position.

       If you now run the simulation, you will see the course changes displayed as they happen.

          'build' finished successfully (5.989s)
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 10, y = 0
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 10.3841, y = 0.923277
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 10.2049, y = 1.90708
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 10.8136, y = 1.11368
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 10.8452, y = 2.11318
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 10.9797, y = 3.10409
          At time 2s client sent 1024 bytes to 10.1.2.4 port 9
          At time 2.01796s server received 1024 bytes from 10.1.3.3 port 49153
          At time 2.01796s server sent 1024 bytes to 10.1.3.3 port 49153
          At time 2.03364s client received 1024 bytes from 10.1.2.4 port 9
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 11.3273, y = 4.04175
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 12.013, y = 4.76955
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 12.4317, y = 5.67771
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 11.4607, y = 5.91681
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 12.0155, y = 6.74878
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 13.0076, y = 6.62336
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 12.6285, y = 5.698
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 13.32, y = 4.97559
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 13.1134, y = 3.99715
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 13.8359, y = 4.68851
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 13.5953, y = 3.71789
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 12.7595, y = 4.26688
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 11.7629, y = 4.34913
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 11.2292, y = 5.19485
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 10.2344, y = 5.09394
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.3601, y = 4.60846
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.40025, y = 4.32795
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.14292, y = 4.99761
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.08299, y = 5.99581
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.26068, y = 5.42677
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.35917, y = 6.42191
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.66805, y = 7.14466
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 6.71414, y = 6.84456
          /NodeList/7/$ns3::MobilityModel/CourseChange x = 6.42489, y = 7.80181

TRACING

   Background
       As mentioned in UsingTracingSystem, the whole point of running an ns-3 simulation is to  generate  output
       for  study.   You  have  two  basic strategies to obtain output from ns-3: using generic pre-defined bulk
       output mechanisms and parsing their content to extract interesting information; or somehow developing  an
       output mechanism that conveys exactly (and perhaps only) the information wanted.

       Using  pre-defined  bulk output mechanisms has the advantage of not requiring any changes to ns-3, but it
       may require writing scripts to parse and filter for data of  interest.   Often,  PCAP  or  NS_LOG  output
       messages are gathered during simulation runs and separately run through scripts that use grep, sed or awk
       to parse the messages and reduce and transform the data to a manageable form.  Programs must  be  written
       to  do  the  transformation, so this does not come for free.  NS_LOG output is not considered part of the
       ns-3 API, and can change without warning between releases.  In addition, NS_LOG output is only  available
       in  debug  builds,  so  relying  on  it  imposes a performance penalty.  Of course, if the information of
       interest does not exist in any of the pre-defined output mechanisms, this approach fails.

       If you need to add some tidbit of information to the pre-defined bulk mechanisms, this can  certainly  be
       done; and if you use one of the ns-3 mechanisms, you may get your code added as a contribution.

       ns-3  provides  another  mechanism, called Tracing, that avoids some of the problems inherent in the bulk
       output mechanisms.  It has several important advantages.  First, you can reduce the amount  of  data  you
       have  to  manage by only tracing the events of interest to you (for large simulations, dumping everything
       to disk for post-processing can create I/O bottlenecks).  Second, if you use this method, you can control
       the  format  of  the  output  directly so you avoid the postprocessing step with sed, awk, perl or python
       scripts.  If you desire, your output can be formatted directly into a form  acceptable  by  gnuplot,  for
       example  (see  also  GnuplotHelper).   You  can add hooks in the core which can then be accessed by other
       users, but which will produce no information unless explicitly asked to do so.   For  these  reasons,  we
       believe  that  the ns-3 tracing system is the best way to get information out of a simulation and is also
       therefore one of the most important mechanisms to understand in ns-3.

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

          #include <iostream>
          ...
          void
          SomeFunction (void)
          {
            uint32_t x = SOME_INTERESTING_VALUE;
            ...
            std::cout << "The value of x is " << x << std::endl;
            ...
          }

       Nobody  is  going to prevent you from going deep into the core of ns-3 and adding print statements.  This
       is insanely easy to do and, after all, you have complete control of your  own  ns-3  branch.   This  will
       probably not turn out to be very satisfactory in the long term, though.

       As  the  number of print statements increases in your programs, the task of dealing with the large number
       of outputs will become more and more complicated.  Eventually, you may feel  the  need  to  control  what
       information  is being printed in some way, perhaps by turning on and off certain categories of prints, or
       increasing or decreasing the amount of information you want.  If you continue  down  this  path  you  may
       discover  that  you have re-implemented the NS_LOG mechanism (see UsingLogging).  In order to avoid that,
       one of the first things you might consider is using NS_LOG itself.

       We mentioned above that one way to get information out of ns-3 is to parse  existing  NS_LOG  output  for
       interesting  information.   If  you  discover  that some tidbit of information you need is not present in
       existing log output, you could edit the core of ns-3 and simply add your interesting information  to  the
       output stream.  Now, this is certainly better than adding your own print statements since it follows ns-3
       coding conventions and could potentially be useful to other people as a patch to the existing core.

       Let's  pick  a  random  example.   If  you  wanted  to  add  more  logging  to  the   ns-3   TCP   socket
       (tcp-socket-base.cc)  you  could  just  add  a  new  message  down in the implementation.  Notice that in
       TcpSocketBase::ReceivedAck() there is no log message for the no ACK case.   You  could  simply  add  one,
       changing the code.  Here is the original:

          /** Process the newly received ACK */
          void
          TcpSocketBase::ReceivedAck (Ptr<Packet> packet, const TcpHeader& tcpHeader)
          {
            NS_LOG_FUNCTION (this << tcpHeader);

            // Received ACK. Compare the ACK number against highest unacked seqno
            if (0 == (tcpHeader.GetFlags () & TcpHeader::ACK))
              { // Ignore if no ACK flag
              }
            ...

       To log the no ACK case, you can add a new NS_LOG_LOGIC in the if statement body:

          /** Process the newly received ACK */
          void
          TcpSocketBase::ReceivedAck (Ptr<Packet> packet, const TcpHeader& tcpHeader)
          {
            NS_LOG_FUNCTION (this << tcpHeader);

            // Received ACK. Compare the ACK number against highest unacked seqno
            if (0 == (tcpHeader.GetFlags () & TcpHeader::ACK))
              { // Ignore if no ACK flag
                NS_LOG_LOGIC ("TcpSocketBase " << this << " no ACK flag");
              }
            ...

       This may seem fairly simple and satisfying at first glance, but something to consider is that you will be
       writing code to add NS_LOG statements and you will also have to write  code  (as  in  grep,  sed  or  awk
       scripts)  to  parse the log output in order to isolate your information.  This is because even though you
       have some control over what is output by the logging system, you  only  have  control  down  to  the  log
       component level, which is typically an entire source code file.

       If you are adding code to an existing module, you will also have to live with the output that every other
       developer has found interesting.  You may find that in order to get the small amount of  information  you
       need,  you  may  have to wade through huge amounts of extraneous messages that are of no interest to you.
       You may be forced to save huge log files to disk and process them down to a few lines whenever  you  want
       to do anything.

       Since  there  are  no guarantees in ns-3 about the stability of NS_LOG output, you may also discover that
       pieces of log output which you depend on disappear or change between releases.   If  you  depend  on  the
       structure of the output, you may find other messages being added or deleted which may affect your parsing
       code.

       Finally, NS_LOG output is only available in debug builds, you can't get log output from optimized builds,
       which run about twice as fast.  Relying on NS_LOG imposes a performance penalty.

       For  these reasons, we consider prints to std::cout and NS_LOG messages to be quick and dirty ways to get
       more information out of ns-3, but not suitable for serious work.

       It is desirable to have a stable facility using stable APIs that allow one to reach into the core  system
       and  only  get  the information required.  It is desirable to be able to do this without having to change
       and recompile the core system.  Even better would be a system that notified user code  when  an  item  of
       interest changed or an interesting event happened so the user doesn't have to actively poke around in the
       system looking for things.

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

   Overview
       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.  Every time the congestion window changes  connected
       trace sinks are notified with the old and new value.

       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 data 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.  Inserting trace sources introduces a very small
       execution overhead.

       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.  Your code looking for trace events from
       a particular piece of core code could happily coexist with other code doing something entirely  different
       from the same information.

       Unless  a  user  connects  a trace sink to one of these sources, nothing is output.  By using the tracing
       system, both you and other people hooked to the same trace source are getting exactly what they want  and
       only  what  they  want  out  of the system.  Neither of you are impacting any other user by changing what
       information is output by the system.  If you  happen  to  add  a  trace  source,  your  work  as  a  good
       open-source  citizen may allow other users to provide new utilities that are perhaps very useful overall,
       without making any changes to the ns-3 core.

   Simple Example
       Let's take a few minutes and walk through a simple tracing example.   We  are  going  to  need  a  little
       background on Callbacks to understand what is happening in the example, so we have to take a small detour
       right away.

   Callbacks
       The goal of the Callback system in ns-3 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 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;

       (But  read  the  C++-FAQ Section 33 before writing code like this!)  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, you could provide a
       function that looks like:

          int MyFunction (int arg) {}

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

          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);

       This looks like you are calling a function named pfi, but the compiler is smart enough to  know  to  call
       through the variable pfi indirectly to the function MyFunction.

       Conceptually,  this  is  almost  exactly  how  the  tracing  system  works.  Basically, a trace sink is a
       callback.  When a trace sink expresses interest in receiving trace events, it adds itself as  a  Callback
       to a list of Callbacks internally held by the trace source.  When an interesting event happens, the trace
       source invokes its operator(...) providing zero or more arguments.  The operator(...) eventually  wanders
       down into the system and does something remarkably like the indirect call you just saw, providing zero or
       more parameters, just as the call to pfi above passed one parameter to the target function MyFunction.

       The important difference that the tracing system adds is that for each trace source there is an  internal
       list  of  Callbacks.   Instead  of  just  making  one  indirect  call, a trace source may invoke multiple
       Callbacks.  When a trace sink expresses interest in notifications from a trace source, it basically  just
       arranges to add its own function to the callback list.

       If  you  are  interested in more details about how this is actually arranged in ns-3, feel free to peruse
       the Callback section of the ns-3 Manual.

   Walkthrough: fourth.cc
       We have provided some code to implement what is really the  simplest  example  of  tracing  that  can  be
       assembled.  You can find this code in the tutorial directory as fourth.cc.  Let's walk through it:

          /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
          /*
           * This program is free software; you can redistribute it and/or modify
           * it under the terms of the GNU General Public License version 2 as
           * published by the Free Software Foundation;
           *
           * This program is distributed in the hope that it will be useful,
           * but WITHOUT ANY WARRANTY; without even the implied warranty of
           * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
           * GNU General Public License for more details.
           *
           * You should have received a copy of the GNU General Public License
           * along with this program; if not, write to the Free Software
           * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
           */

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

          #include <iostream>

          using namespace ns3;

       Most  of this code should be quite familiar to you.  As mentioned above, the trace system makes heavy use
       of the Object and Attribute systems, so you will need to include them.   The  first  two  includes  above
       bring  in  the  declarations  for  those systems explicitly.  You could use the core module header to get
       everything at once, but we do the includes explicitly here to illustrate how simple this all really is.

       The file, traced-value.h brings in the required  declarations  for  tracing  of  data  that  obeys  value
       semantics.   In  general,  value  semantics just means that you can pass the object itself around, rather
       than passing the address of the object.  What this all really means is that you will be able to trace all
       changes made to a TracedValue in a really simple way.

       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 next code snippet  declares  and  defines  a  simple
       Object we can work with.

          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),
                                 "ns3::Traced::Value::Int32Callback")
                ;
              return tid;
            }

            MyObject () {}
            TracedValue<int32_t> m_myInt;
          };

       The  two  important  lines  of  code,  above,  with  respect  to  tracing are the .AddTraceSource and the
       TracedValue declaration of m_myInt.

       The .AddTraceSource provides the "hooks" used for connecting  the  trace  source  to  the  outside  world
       through the Config system.  The first argument is a name for this trace source, which makes it visible in
       the Config system. The second argument is a help string.  Now look at the third argument, in  fact  focus
       on  the  argument of the third argument: &MyObject::m_myInt. This is the TracedValue which is being added
       to the class; it is always a class data member.  (The final argument is the name of  a  typedef  for  the
       TracedValue  type, as a string.  This is used to generate documentation for the correct Callback function
       signature, which is useful especially for more general types of Callbacks.)

       The TracedValue<> declaration provides the infrastructure that drives the callback process.  Any time the
       underlying value is changed the TracedValue mechanism will provide both the old and the new value of that
       variable, in this case an int32_t value.  The trace sink function for  this  TracedValue  will  need  the
       signature

          void (* TracedValueCallback)(const int32_t oldValue, const int32_t newValue);

       All  trace  sinks  hooking  this  trace source must have this signature.  We'll discuss below how you can
       determine the required callback signature in other cases.

       Sure enough, continuing through fourth.cc we see:

          void
          IntTrace (int32_t oldValue, int32_t newValue)
          {
            std::cout << "Traced " << oldValue << " to " << newValue << std::endl;
          }

       This is the definition of a matching trace sink.   It  corresponds  directly  to  the  callback  function
       signature.  Once it is connected, this function will be called whenever the TracedValue changes.

       We  have now seen the trace source and the trace sink.  What remains is code to connect the source to the
       sink, which happens in main:

          int
          main (int argc, char *argv[])
          {
            Ptr<MyObject> myObject = CreateObject<MyObject> ();
            myObject->TraceConnectWithoutContext ("MyInteger", MakeCallback(&IntTrace));

            myObject->m_myInt = 1234;
          }

       Here we first create the MyObject instance in which the trace source lives.

       The next step, the TraceConnectWithoutContext, forms the connection between  the  trace  source  and  the
       trace  sink.   The  first  argument  is  just the trace source name "MyInteger" we saw above.  Notice the
       MakeCallback template function.  This function does the magic required  to  create  the  underlying  ns-3
       Callback  object and associate it with the function IntTrace.  TraceConnect makes the association between
       your provided function and overloaded operator() in the traced variable referred to  by  the  "MyInteger"
       Attribute.  After this association is made, the trace source will "fire" your provided callback function.

       The code to make all of this happen is, of course, non-trivial, but the essence is that you are arranging
       for something that looks just like the pfi() example above  to  be  called  by  the  trace  source.   The
       declaration  of  the  TracedValue<int32_t>  m_myInt;  in  the  Object itself performs the magic needed to
       provide the overloaded assignment operators that will use the operator() to actually invoke the  Callback
       with  the  desired  parameters.   The  .AddTraceSource  performs the magic to connect the Callback to the
       Config system, and TraceConnectWithoutContext performs the magic to connect your function  to  the  trace
       source, which is specified by Attribute name.

       Let's ignore the bit about context for now.

       Finally, the line assigning a value to m_myInt:

          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.

       Since m_myInt is a TracedValue, this operator is defined 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 who registering interest in the source being called with the parameters provided by the source.

       If you now build and run this example,

          $ ./waf --run fourth

       you will see the output from the IntTrace function execute as soon as the trace source is hit:

          Traced 0 to 1234

       When we executed the code, myObject->m_myInt = 1234;, the trace source fired and  automatically  provided
       the  before  and after values to the trace sink.  The function IntTrace then printed this to the standard
       output.

   Connect with Config
       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 select a trace source in the system using what
       is called a Config path.  We saw an example  of  this  in  the  previous  section  where  we  hooked  the
       "CourseChange" event when we were experimenting with third.cc.

       Recall  that  we  defined a trace sink to print course change information from the mobility models of our
       simulation.  It should now be a lot more clear to you what this function is doing:

          void
          CourseChange (std::string context, Ptr<const MobilityModel> model)
          {
            Vector position = model->GetPosition ();
            NS_LOG_UNCOND (context <<
              " x = " << position.x << ", y = " << position.y);
          }

       When we connected the "CourseChange" trace source to the above trace sink,  we  used  a  Config  path  to
       specify  the  source when we arranged a connection between the pre-defined trace source and the new trace
       sink:

          std::ostringstream oss;
          oss << "/NodeList/"
              << wifiStaNodes.Get (nWifi - 1)->GetId ()
              << "/$ns3::MobilityModel/CourseChange";

          Config::Connect (oss.str (), MakeCallback (&CourseChange));

       Let's try and make some sense of what is  sometimes  considered  relatively  mysterious  code.   For  the
       purposes  of  discussion,  assume that the Node number returned by the GetId() is "7".  In this case, the
       path above turns out to be

          "/NodeList/7/$ns3::MobilityModel/CourseChange"

       The last segment of a config path must be an Attribute of an Object.  In fact, if you had  a  pointer  to
       the  Object  that  has  the  "CourseChange" Attribute handy, you could write this just like we did in the
       previous example.  You know by now that we typically store pointers to our Nodes in a NodeContainer.   In
       the third.cc example, the Nodes of interest are stored in the wifiStaNodes NodeContainer.  In fact, while
       putting the path together, we used this container to get a Ptr<Node> which we used to call  GetId().   We
       could have used this Ptr<Node> to call a Connect method directly:

          Ptr<Object> theObject = wifiStaNodes.Get (nWifi - 1);
          theObject->TraceConnectWithoutContext ("CourseChange", MakeCallback (&CourseChange));

       In  the  third.cc  example,  we  actually  wanted  an additional "context" to be delivered along with the
       Callback parameters (which will be explained below) so we could actually  use  the  following  equivalent
       code:

          Ptr<Object> theObject = wifiStaNodes.Get (nWifi - 1);
          theObject->TraceConnect ("CourseChange", MakeCallback (&CourseChange));

       It turns out that the internal code for Config::ConnectWithoutContext and Config::Connect actually find a
       Ptr<Object> and call the appropriate TraceConnect method at the lowest level.

       The Config functions take a path that represents a chain of Object pointers.   Each  segment  of  a  path
       corresponds  to  an  Object Attribute.  The last segment is the Attribute of interest, and prior segments
       must be typed to contain or find Objects.  The Config code parses and "walks" this path until it gets  to
       the final segment of the path.  It then interprets the last segment as an Attribute on the last Object it
       found while  walking  the  path.   The  Config  functions  then  call  the  appropriate  TraceConnect  or
       TraceConnectWithoutContext  method on the final Object.  Let's see what happens in a bit more detail when
       the above path is walked.

       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/7" refers to the eighth Node in the list  of
       nodes  created  during  the  simulation  (recall  indices  start  at  0').   This reference is actually a
       ``Ptr<Node>` and so is a subclass of an ns3::Object.

       As described in the Object  Model  section  of  the  ns-3  Manual,  we  make  widespread  use  of  object
       aggregation.   This  allows  us  to  form  an  association  between  different Objects without building a
       complicated inheritance tree or predeciding what objects will be part of  a  Node.   Each  Object  in  an
       Aggregation can be reached from the other Objects.

       In  our  example the next path segment being walked begins with the "$" character.  This indicates to the
       config system that the segment is the name of an object type, so a GetObject call should be made  looking
       for  that  type.   It  turns  out  that  the  MobilityHelper  used  in third.cc arranges to Aggregate, or
       associate, a mobility model to each of the wireless Nodes.  When you add  the  "$"  you  are  asking  for
       another  Object  that  has  presumably  been  previously  aggregated.  You can think of this as switching
       pointers from the original Ptr<Node> as specified by "/NodeList/7" to its associated mobility  model  ---
       which  is of type ns3::MobilityModel.  If you are familiar with GetObject, we have asked the system to do
       the following:

          Ptr<MobilityModel> mobilityModel = node->GetObject<MobilityModel> ()

       We are now at the last Object in the path, so we turn our attention to the  Attributes  of  that  Object.
       The  MobilityModel  class defines an Attribute called "CourseChange".  You can see this by looking at the
       source code in src/mobility/model/mobility-model.cc and searching for  "CourseChange"  in  your  favorite
       editor.  You should find

          .AddTraceSource ("CourseChange",
                           "The value of the position and/or velocity vector changed",
                           MakeTraceSourceAccessor (&MobilityModel::m_courseChangeTrace),
                           "ns3::MobilityModel::CourseChangeCallback")

       which should look very familiar at this point.

       If  you  look for the corresponding declaration of the underlying traced variable in mobility-model.h you
       will find

          TracedCallback<Ptr<const MobilityModel> > m_courseChangeTrace;

       The type declaration TracedCallback identifies m_courseChangeTrace as a special list  of  Callbacks  that
       can  be  hooked  using  the  Config  functions  described  above.   The typedef for the callback function
       signature is also defined in the header file:

          typedef void (* CourseChangeCallback)(Ptr<const MobilityModel> * model);

       The MobilityModel class is designed to be a base class providing  a  common  interface  for  all  of  the
       specific  subclasses.   If  you  search down to the end of the file, you will see a method defined called
       NotifyCourseChange():

          void
          MobilityModel::NotifyCourseChange (void) const
          {
            m_courseChangeTrace(this);
          }

       Derived classes will call into this method whenever they do a course change  to  support  tracing.   This
       method  invokes  operator() on the underlying m_courseChangeTrace, which will, in turn, invoke all of the
       registered Callbacks, calling all of the trace sinks that have registered interest in the trace source by
       calling a Config function.

       So,  in  the  third.cc  example  we  looked  at,  whenever  a  course  change  is  made  in  one  of  the
       RandomWalk2dMobilityModel instances installed, there will be a NotifyCourseChange() call which  calls  up
       into  the MobilityModel base class.  As seen above, this invokes operator() on m_courseChangeTrace, which
       in turn, calls any registered trace sinks.  In the example, the only code registering an interest was the
       code  that  provided  the  Config  path.   Therefore, the CourseChange function that was hooked from Node
       number seven will be the only Callback called.

       The final piece of the puzzle is the "context".  Recall that we saw an output looking something like  the
       following from third.cc:

          /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.27897, y =
          2.22677

       The first part of the output is the context.  It is simply the path through which the config code located
       the trace source.  In the case we have been looking at there can be any number of trace  sources  in  the
       system corresponding to any number of nodes with mobility models.  There needs to be some way to identify
       which trace source is actually the one that fired  the  Callback.   The  easy  way  is  to  connect  with
       Config::Connect, instead of Config::ConnectWithoutContext.

   Finding Sources
       The  first  question  that inevitably comes up for new users of the Tracing system is, "Okay, I know that
       there must be trace sources in the simulation core, but  how  do  I  find  out  what  trace  sources  are
       available to me?"

       The  second question is, "Okay, I found a trace source, how do I figure out the Config path to use when I
       connect to it?"

       The third question is, "Okay, I found a trace source and the Config path, how do I figure  out  what  the
       return type and formal arguments of my callback function need to be?"

       The fourth question is, "Okay, I typed that all in and got this incredibly bizarre error message, what in
       the world does it mean?"

       We'll address each of these in turn.

   Available Sources
       Okay, I know that there must be trace sources in the simulation core, but how do I find  out  what  trace
       sources are available to me?

       The  answer  to  the first question is found in the ns-3 API documentation.  If you go to the project web
       site, ns-3 project, you will find a link to "Documentation" in the navigation bar.  If  you  select  this
       link, you will be taken to our documentation page. There is a link to "Latest Release" that will take you
       to the documentation for the latest stable release of ns-3.  If you select the "API Documentation"  link,
       you will be taken to the ns-3 API documentation page.

       In the sidebar you should see a hierachy that begins

       • ns-3

          • ns-3 Documentation

          • All TraceSources

          • All Attributes

          • All GlobalValues

       The  list  of  interest  to us here is "All TraceSources".  Go ahead and select that link.  You will see,
       perhaps not too surprisingly, a list of all of the trace sources available in ns-3.

       As an example, scroll down to ns3::MobilityModel.  You will find an entry for

          CourseChange: The value of the position and/or velocity vector changed

       You should recognize this as the trace source we used in the third.cc example.  Perusing this  list  will
       be helpful.

   Config Paths
       Okay, I found a trace source, how do I figure out the Config path to use when I connect to it?

       If  you  know  which  object you are interested in, the "Detailed Description" section for the class will
       list all available trace sources.  For example, starting from the list of "All  TraceSources,"  click  on
       the  ns3::MobilityModel  link,  which  will  take  you  to the documentation for the MobilityModel class.
       Almost at the top of the page is a one line brief description of the class, ending in a  link  "More...".
       Click on this link to skip the API summary and go to the "Detailed Description" of the class.  At the end
       of the description will be (up to) three lists:

       • Config Paths: a list of typical Config paths for this class.

       • Attributes: a list of all attributes supplied by this class.

       • TraceSources: a list of all TraceSources available from this class.

       First we'll discuss the Config paths.

       Let's assume that you have just found the "CourseChange" trace source in the "All TraceSources" list  and
       you  want  to  figure  out  how  to connect to it.  You know that you are using (again, from the third.cc
       example) an ns3::RandomWalk2dMobilityModel.  So either click on the class name in the "All  TraceSources"
       list,  or  find ns3::RandomWalk2dMobilityModel in the "Class List".  Either way you should now be looking
       at the "ns3::RandomWalk2dMobilityModel Class Reference" page.

       If you now scroll down to the "Detailed Description" section, after the summary list of class methods and
       attributes  (or  just click on the "More..." link at the end of the class brief description at the top of
       the page) you will see the overall documentation for the class.  Continuing  to  scroll  down,  find  the
       "Config Paths" list:
          Config Paths

          ns3::RandomWalk2dMobilityModel  is  accessible  through  the  following  paths  with  Config::Set  and
          Config::Connect:

          • "/NodeList/[i]/$ns3::MobilityModel/$ns3::RandomWalk2dMobilityModel"

       The documentation tells you how to get to the RandomWalk2dMobilityModel Object.  Compare the string above
       with the string we actually used in the example code:

          "/NodeList/7/$ns3::MobilityModel"

       The  difference  is  due  to  the  fact  that  two GetObject calls are implied in the string found in the
       documentation.  The first, for $ns3::MobilityModel will query the aggregation for the  base  class.   The
       second implied GetObject call, for $ns3::RandomWalk2dMobilityModel, is used to cast the base class to the
       concrete implementation class.  The documentation shows both of these operations for you.  It  turns  out
       that the actual trace source you are looking for is found in the base class.

       Look further down in the "Detailed Description" section for the list of trace sources.  You will find
          No TraceSources are defined for this type.

          TraceSources defined in parent class ``ns3::MobilityModel``CourseChange: The value of the position and/or velocity vector changed.

            Callback signature: ns3::MobilityModel::CourseChangeCallback

       This  is  exactly  what  you  need  to know.  The trace source of interest is found in ns3::MobilityModel
       (which you knew anyway).  The interesting thing this bit of API Documentation tells you is that you don't
       need  that  extra  cast  in the config path above to get to the concrete class, since the trace source is
       actually in the base class.  Therefore the additional GetObject is not required and you  simply  use  the
       path:

          "/NodeList/[i]/$ns3::MobilityModel"

       which perfectly matches the example path:

          "/NodeList/7/$ns3::MobilityModel"

       As  an  aside, another way to find the Config path is to grep around in the ns-3 codebase for someone who
       has already figured it out.  You should always try to copy someone else's working code before  you  start
       to write your own.  Try something like:

          $ find . -name '*.cc' | xargs grep CourseChange | grep Connect

       and   you   may   find   your   answer   along   with   working   code.    For  example,  in  this  case,
       src/mobility/examples/main-random-topology.cc has something just waiting for you to use:

          Config::Connect ("/NodeList/*/$ns3::MobilityModel/CourseChange",
            MakeCallback (&CourseChange));

       We'll return to this example in a moment.

   Callback Signatures
       Okay, I found a trace source and the Config path, how do I figure out what the  return  type  and  formal
       arguments of my callback function need to be?

       The  easiest way is to examine the callback signature typedef, which is given in the "Callback signature"
       of the trace source in the "Detailed Description" for the class, as shown above.

       Repeating the "CourseChange" trace source entry from ns3::RandomWalk2dMobilityModel we have:

          • CourseChange: The value of the position and/or velocity vector changed.

            Callback signature: ns3::MobilityModel::CourseChangeCallback

       The callback signature is given as a link to the relevant typedef, where we find
          typedef void (* CourseChangeCallback)(const std::string context, Ptr<const MobilityModel> * model);

          TracedCallback signature for course change notifications.

          If the callback is connected using ConnectWithoutContext omit the context argument from the signature.

          Parameters:
              [in] context The context string supplied by the Trace source.
              [in] model The MobilityModel which is changing course.

       As above, to see this in use grep around in the ns-3 codebase for an example.  The  example  above,  from
       src/mobility/examples/main-random-topology.cc,   connects   the   "CourseChange"   trace  source  to  the
       CourseChange function in the same file:

          static void
          CourseChange (std::string context, Ptr<const MobilityModel> model)
          {
            ...
          }

       Notice that this function:

       • Takes a "context" string argument, which we'll describe in a minute.  (If  the  callback  is  connected
         using the ConnectWithoutContext function the context argument will be omitted.)

       • Has  the  MobilityModel  supplied  as  the  last argument (or only argument if ConnectWithoutContext is
         used).

       • Returns void.

       If, by chance, the callback signature hasn't been documented, and there are no  examples  to  work  from,
       determining  the  right callback function signature can be, well, challenging to actually figure out from
       the source code.

       Before embarking on a walkthrough of the code, I'll be kind and just tell you a simple way to figure this
       out:  The  return  value  of  your  callback  will  always  be  void.   The  formal  parameter list for a
       TracedCallback can be found from the template parameter list in the declaration.   Recall  that  for  our
       current example, this is in mobility-model.h, where we have previously found:

          TracedCallback<Ptr<const MobilityModel> > m_courseChangeTrace;

       There  is  a  one-to-one  correspondence  between  the template parameter list in the declaration and the
       formal arguments of the callback function.  Here, there is one template parameter, which is  a  Ptr<const
       MobilityModel>.   This  tells  you  that  you  need  a  function  that returns void and takes a Ptr<const
       MobilityModel>.  For example:

          void
          CourseChange (Ptr<const MobilityModel> model)
          {
            ...
          }

       That's all you need if you want to Config::ConnectWithoutContext.  If you want a  context,  you  need  to
       Config::Connect and use a Callback function that takes a string context, then the template arguments:

          void
          CourseChange (const std::string context, Ptr<const MobilityModel> model)
          {
            ...
          }

       If you want to ensure that your CourseChangeCallback function is only visible in your local file, you can
       add the keyword static and come up with:

          static void
          CourseChange (const std::string path, Ptr<const MobilityModel> model)
          {
            ...
          }

       which is exactly what we used in the third.cc example.

   Implementation
       This section is entirely optional.  It is going to be a bumpy ride, especially for those unfamiliar  with
       the details of templates.  However, if you get through this, you will have a very good handle on a lot of
       the ns-3 low level idioms.

       So, again, let's figure out what signature of callback function is required for the "CourseChange"  trace
       source.  This is going to be painful, but you only need to do this once.  After you get through this, you
       will be able to just look at a TracedCallback and understand it.

       The first thing we need to look at is the declaration of the  trace  source.   Recall  that  this  is  in
       mobility-model.h, where we have previously found:

          TracedCallback<Ptr<const MobilityModel> > m_courseChangeTrace;

       This  declaration  is  for  a  template.   The template parameter is inside the angle-brackets, so we are
       really interested in finding out what that TracedCallback<> is.  If you have  absolutely  no  idea  where
       this might be found, grep is your friend.

       We  are  probably  going to be interested in some kind of declaration in the ns-3 source, so first change
       into the src directory.  Then, we know this declaration is going to have to be in  some  kind  of  header
       file, so just grep for it using:

          $ find . -name '*.h' | xargs grep TracedCallback

       You'll see 303 lines fly by (I piped this through wc to see how bad it was).  Although that may seem like
       a lot, that's not really a lot.  Just pipe the output through more and start scanning through it.  On the
       first page, you will see some very suspiciously template-looking stuff.

          TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::TracedCallback ()
          TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::ConnectWithoutContext (c ...
          TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::Connect (const CallbackB ...
          TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::DisconnectWithoutContext ...
          TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::Disconnect (const Callba ...
          TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (void) const ...
          TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1) const ...
          TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ...
          TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ...
          TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ...
          TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ...
          TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ...
          TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ...

       It  turns  out that all of this comes from the header file traced-callback.h which sounds very promising.
       You can then take a look at mobility-model.h and see that there is a line which confirms this hunch:

          #include "ns3/traced-callback.h"

       Of course, you could have gone at this from the other direction and started by looking at the includes in
       mobility-model.h  and  noticing the include of traced-callback.h and inferring that this must be the file
       you want.

       In either case, the next step is to take a look  at  src/core/model/traced-callback.h  in  your  favorite
       editor to see what is happening.

       You will see a comment at the top of the file that should be comforting:
          An  ns3::TracedCallback  has  almost  exactly  the  same  API as a normal ns3::Callback but instead of
          forwarding calls to a single function (as an ns3::Callback normally does),  it  forwards  calls  to  a
          chain of ns3::Callback.

       This should sound very familiar and let you know you are on the right track.

       Just after this comment, you will find

          template<typename T1 = empty, typename T2 = empty,
                   typename T3 = empty, typename T4 = empty,
                   typename T5 = empty, typename T6 = empty,
                   typename T7 = empty, typename T8 = empty>
          class TracedCallback
          {
            ...

       This  tells  you  that  TracedCallback  is a templated class.  It has eight possible type parameters with
       default values.  Go back and compare this with the declaration you are trying to understand:

          TracedCallback<Ptr<const MobilityModel> > m_courseChangeTrace;

       The typename T1 in the templated class declaration corresponds to the  Ptr<const  MobilityModel>  in  the
       declaration  above.   All  of the other type parameters are left as defaults.  Looking at the constructor
       really doesn't tell you much.  The one place where you have seen a connection made between your  Callback
       function  and  the  tracing  system is in the Connect and ConnectWithoutContext functions.  If you scroll
       down, you will see a ConnectWithoutContext method here:

          template<typename T1, typename T2,
                   typename T3, typename T4,
                   typename T5, typename T6,
                   typename T7, typename T8>
          void
          TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::ConnectWithoutContext ...
          {
            Callback<void,T1,T2,T3,T4,T5,T6,T7,T8> cb;
            cb.Assign (callback);
            m_callbackList.push_back (cb);
          }

       You are now in the belly of the beast.  When the template is instantiated for the declaration above,  the
       compiler will replace T1 with Ptr<const MobilityModel>.

          void
          TracedCallback<Ptr<const MobilityModel>::ConnectWithoutContext ... cb
          {
            Callback<void, Ptr<const MobilityModel> > cb;
            cb.Assign (callback);
            m_callbackList.push_back (cb);
          }

       You  can  now see the implementation of everything we've been talking about.  The code creates a Callback
       of the right type and assigns your function to it.  This is the equivalent of the  pfi  =  MyFunction  we
       discussed  at  the  start  of this section.  The code then adds the Callback to the list of Callbacks for
       this source.  The only thing left is to look at the definition of Callback.  Using the same grep trick as
       we  used  to  find TracedCallback, you will be able to find that the file ./core/callback.h is the one we
       need to look at.

       If you look down through the file, you will see a lot of probably almost incomprehensible template  code.
       You will eventually come to some API Documentation for the Callback template class, though.  Fortunately,
       there is some English:
          Callback template class.

          This class template implements the Functor Design Pattern. It  is  used  to  declare  the  type  of  a
          Callback:

          • the first non-optional template argument represents the return type of the callback.

          • the  reminaining (optional) template arguments represent the type of the subsequent arguments to the
            callback.

          • up to nine arguments are supported.

       We are trying to figure out what the

          Callback<void, Ptr<const MobilityModel> > cb;

       declaration means.  Now we are in a  position  to  understand  that  the  first  (non-optional)  template
       argument,  void,  represents  the  return type of the Callback.  The second (optional) template argument,
       Ptr<const MobilityModel> represents the type of the first argument to the callback.

       The Callback in question is your function to receive the trace events.  From this you can infer that  you
       need a function that returns void and takes a Ptr<const MobilityModel>.  For example,

          void
          CourseChangeCallback (Ptr<const MobilityModel> model)
          {
            ...
          }

       That's  all  you  need  if you want to Config::ConnectWithoutContext.  If you want a context, you need to
       Config::Connect and use a Callback function that takes a string context.  This  is  because  the  Connect
       function will provide the context for you.  You'll need:

          void
          CourseChangeCallback (std::string context, Ptr<const MobilityModel> model)
          {
            ...
          }

       If  you want to ensure that your CourseChangeCallback is only visible in your local file, you can add the
       keyword static and come up with:

          static void
          CourseChangeCallback (std::string path, Ptr<const MobilityModel> model)
          {
            ...
          }

       which is exactly what we used in the third.cc example.  Perhaps you should now go  back  and  reread  the
       previous section (Take My Word for It).

       If you are interested in more details regarding the implementation of Callbacks, feel free to take a look
       at the ns-3 manual.  They are one of the most frequently used constructs in the low-level parts of  ns-3.
       It is, in my opinion, a quite elegant thing.

   TracedValues
       Earlier  in  this  section,  we  presented  a  simple  piece  of code that used a TracedValue<int32_t> to
       demonstrate the basics of the tracing code.  We just glossed over the what a TracedValue  really  is  and
       how to find the return type and formal arguments for the callback.

       As  we  mentioned,  the file, traced-value.h brings in the required declarations for tracing of data that
       obeys value semantics.  In general, value semantics just means  that  you  can  pass  the  object  itself
       around,  rather  than  passing the address of the object.  We extend that requirement to include the full
       set of assignment-style operators that are pre-defined for plain-old-data (POD) types:

                                   ┌─────────────────────────────────────┬─────────────┐
                                   │operator= (assignment)               │             │
                                   ├─────────────────────────────────────┼─────────────┤
                                   │operator*=operator/=  │
                                   ├─────────────────────────────────────┼─────────────┤
                                   │operator+=operator-=  │
                                   ├─────────────────────────────────────┼─────────────┤
                                   │operator++ (both prefix and postfix) │             │
                                   ├─────────────────────────────────────┼─────────────┤
                                   │operator-- (both prefix and postfix) │             │
                                   └─────────────────────────────────────┴─────────────┘

                                   │operator<<=operator>>= │
                                   ├─────────────────────────────────────┼─────────────┤
                                   │operator&=operator|=  │
                                   ├─────────────────────────────────────┼─────────────┤
                                   │operator%=operator^=  │
                                   └─────────────────────────────────────┴─────────────┘

       What this all really means is that you will be able to trace all changes made using those operators to  a
       C++ object which has value semantics.

       The  TracedValue<>  declaration  we  saw  above  provides the infrastructure that overloads the operators
       mentioned above and drives the callback process.  On use of any of the operators above with a TracedValue
       it  will  provide  both  the  old  and the new value of that variable, in this case an int32_t value.  By
       inspection of the TracedValue declaration, we know the trace sink function  will  have  arguments  (const
       int32_t oldValue, const int32_t newValue).  The return type for a TracedValue callback function is always
       void, so the expected callback signature will be:

          void (* TracedValueCallback)(const int32_t oldValue, const int32_t newValue);

       The .AddTraceSource in the GetTypeId method provides the "hooks" used for connecting the trace source  to
       the  outside  world  through  the  Config  system.   We  already  discussed  the first three agruments to
       AddTraceSource: the Attribute name for the  Config  system,  a  help  string,  and  the  address  of  the
       TracedValue class data member.

       The  final  string argument, "ns3::Traced::Value::Int32" in the example, is the name of a typedef for the
       callback function signature.  We require these signatures to be defined, and  give  the  fully  qualified
       type  name to AddTraceSource, so the API documentation can link a trace source to the function signature.
       For TracedValue the signature is straightforward; for TracedCallbacks we've already  seen  the  API  docs
       really help.

   Real Example
       Let's do an example taken from one of the best-known books on TCP around.  "TCP/IP Illustrated, Volume 1:
       The Protocols," by W. Richard Stevens is a classic.  I just flipped the book open and ran across  a  nice
       plot  of  both  the  congestion window and sequence numbers versus time on page 366.  Stevens calls this,
       "Figure 21.10. Value of cwnd and send sequence number while  data  is  being  transmitted."   Let's  just
       recreate the cwnd part of that plot in ns-3 using the tracing system and gnuplot.

   Available Sources
       The first thing to think about is how we want to get the data out.  What is it that we need to trace?  So
       let's consult "All Trace Sources" list to see what we have to work with.  Recall that this  is  found  in
       the ns-3 API Documentation.  If you scroll through the list, you will eventually find:
          ns3::TcpNewRenoCongestionWindow: The TCP connection's congestion window

          • SlowStartThreshold: TCP slow start threshold (bytes)

       It    turns    out    that    the    ns-3    TCP    implementation    lives    (mostly)   in   the   file
       src/internet/model/tcp-socket-base.cc  while  congestion  control  variants  are   in   files   such   as
       src/internet/model/tcp-newreno.cc.   If  you  don't  know  this  a priori, you can use the recursive grep
       trick:

          $ find . -name '*.cc' | xargs grep -i tcp

       You will find page after page of instances of tcp pointing you to that file.

       Bringing up the class documentation for TcpNewReno and skipping to the list of TraceSources you will find
          TraceSourcesCongestionWindow: The TCP connnection's congestion window

            Callback signature:  ns3::Traced::Value::Uint322Callback

       Clicking on the callback typedef link we see the signature you now know to expect:

          typedef void(* ns3::Traced::Value::Int32Callback)(const int32_t oldValue, const int32_t newValue)

       You should now understand this code completely.   If  we  have  a  pointer  to  the  TcpNewReno,  we  can
       TraceConnect  to  the "CongestionWindow" trace source if we provide an appropriate callback target.  This
       is the same kind of trace source that we saw in the simple example at the start of this  section,  except
       that  we  are  talking about uint32_t instead of int32_t.  And we know that we have to provide a callback
       function with that signature.

   Finding Examples
       It's always best to try and find working code laying around that you can  modify,  rather  than  starting
       from  scratch.   So  the  first  order  of  business  now  is  to  find  some code that already hooks the
       "CongestionWindow" trace source and see if we can modify it.  As usual, grep is your friend:

          $ find . -name '*.cc' | xargs grep CongestionWindow

       This  will  point  out  a  couple  of  promising   candidates:   examples/tcp/tcp-large-transfer.cc   and
       src/test/ns3tcp/ns3tcp-cwnd-test-suite.cc.

       We  haven't  visited  any of the test code yet, so let's take a look there.  You will typically find that
       test   code   is   fairly   minimal,   so    this    is    probably    a    very    good    bet.     Open
       src/test/ns3tcp/ns3tcp-cwnd-test-suite.cc in your favorite editor and search for "CongestionWindow".  You
       will find,

          ns3TcpSocket->TraceConnectWithoutContext ("CongestionWindow",
            MakeCallback (&Ns3TcpCwndTestCase1::CwndChange, this));

       This should look very familiar to you.  We mentioned above that if we had a pointer to the TcpNewReno, we
       could TraceConnect to the "CongestionWindow" trace source.  That's exactly what we have here; so it turns
       out that this line of code does exactly what we want.  Let's go ahead and extract the code we  need  from
       this  function  (Ns3TcpCwndTestCase1::DoRun (void)).  If you look at this function, you will find that it
       looks just like an ns-3 script.  It turns out that is exactly what it is.  It is a script run by the test
       framework,  so we can just pull it out and wrap it in main instead of in DoRun.  Rather than walk through
       this, step, by step, we have provided the file that results from porting this test back to a native  ns-3
       script -- examples/tutorial/fifth.cc.

   Dynamic Trace Sources
       The  fifth.cc  example demonstrates an extremely important rule that you must understand before using any
       kind of trace source: you must ensure that the target of a Config::Connect command exists  before  trying
       to  use  it.   This  is no different than saying an object must be instantiated before trying to call it.
       Although this may seem obvious when stated this way, it does trip up many people trying to use the system
       for the first time.

       Let's  return  to  basics  for  a  moment.  There are three basic execution phases that exist in any ns-3
       script.  The first phase is sometimes called "Configuration Time" or "Setup Time," and exists during  the
       period when the main function of your script is running, but before Simulator::Run is called.  The second
       phase is sometimes called "Simulation Time" and exists during the  time  period  when  Simulator::Run  is
       actively  executing  its events.  After it completes executing the simulation, Simulator::Run will return
       control back to the main function.  When this happens, the script enters what can be called the "Teardown
       Phase," which is when the structures and objects created during setup are taken apart and released.

       Perhaps  the  most  common  mistake  made  in  trying to use the tracing system is assuming that entities
       constructed dynamically during simulation time are available during configuration time.   In  particular,
       an  ns-3  Socket is a dynamic object often created by Applications to communicate between Nodes.  An ns-3
       Application always has a "Start Time" and a "Stop Time" associated with it.   In  the  vast  majority  of
       cases,  an  Application  will not attempt to create a dynamic object until its StartApplication method is
       called at some "Start Time".  This is to ensure that the simulation is completely configured  before  the
       app tries to do anything (what would happen if it tried to connect to a Node that didn't exist yet during
       configuration time?).  As a result, during the configuration phase you can't connect a trace source to  a
       trace sink if one of them is created dynamically during the simulation.

       The two solutions to this connundrum are

       1. Create  a simulator event that is run after the dynamic object is created and hook the trace when that
          event is executed; or

       2. Create the dynamic object at configuration time, hook it then, and give the object to  the  system  to
          use during simulation time.

       We  took  the  second  approach  in  the fifth.cc example.  This decision required us to create the MyApp
       Application, the entire purpose of which is to take a Socket as a parameter.

   Walkthrough: fifth.cc
       Now, let's take a look at the example program we constructed by dissecting the  congestion  window  test.
       Open examples/tutorial/fifth.cc in your favorite editor.  You should see some familiar looking code:

          /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
          /*
           * This program is free software; you can redistribute it and/or modify
           * it under the terms of the GNU General Public License version 2 as
           * published by the Free Software Foundation;
           *
           * This program is distributed in the hope that it will be useful,
           * but WITHOUT ANY WARRANTY; without even the implied warranty of
           * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
           * GNU General Public License for more details.
           *
           * You should have received a copy of the GNU General Public License
           * along with this program; if not, write to the Free Software
           * Foundation, Include., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
           */

          #include <fstream>
          #include "ns3/core-module.h"
          #include "ns3/network-module.h"
          #include "ns3/internet-module.h"
          #include "ns3/point-to-point-module.h"
          #include "ns3/applications-module.h"

          using namespace ns3;

          NS_LOG_COMPONENT_DEFINE ("FifthScriptExample");

       This  has all been covered, so we won't rehash it.  The next lines of source are the network illustration
       and a comment addressing the problem described above with Socket.

          // ===========================================================================
          //
          //         node 0                 node 1
          //   +----------------+    +----------------+
          //   |    ns-3 TCP    |    |    ns-3 TCP    |
          //   +----------------+    +----------------+
          //   |    10.1.1.1    |    |    10.1.1.2    |
          //   +----------------+    +----------------+
          //   | point-to-point |    | point-to-point |
          //   +----------------+    +----------------+
          //           |                     |
          //           +---------------------+
          //                5 Mbps, 2 ms
          //
          //
          // We want to look at changes in the ns-3 TCP congestion window.  We need
          // to crank up a flow and hook the CongestionWindow attribute on the socket
          // of the sender.  Normally one would use an on-off application to generate a
          // flow, but this has a couple of problems.  First, the socket of the on-off
          // application is not created until Application Start time, so we wouldn't be
          // able to hook the socket (now) at configuration time.  Second, even if we
          // could arrange a call after start time, the socket is not public so we
          // couldn't get at it.
          //
          // So, we can cook up a simple version of the on-off application that does what
          // we want.  On the plus side we don't need all of the complexity of the on-off
          // application.  On the minus side, we don't have a helper, so we have to get
          // a little more involved in the details, but this is trivial.
          //
          // So first, we create a socket and do the trace connect on it; then we pass
          // this socket into the constructor of our simple application which we then
          // install in the source node.
          // ===========================================================================
          //

       This should also be self-explanatory.

       The next part is the declaration of the MyApp Application that we put together to allow the Socket to  be
       created at configuration time.

          class MyApp : public Application
          {
          public:

            MyApp ();
            virtual ~MyApp();

            void Setup (Ptr<Socket> socket, Address address, uint32_t packetSize,
              uint32_t nPackets, DataRate dataRate);

          private:
            virtual void StartApplication (void);
            virtual void StopApplication (void);

            void ScheduleTx (void);
            void SendPacket (void);

            Ptr<Socket>     m_socket;
            Address         m_peer;
            uint32_t        m_packetSize;
            uint32_t        m_nPackets;
            DataRate        m_dataRate;
            EventId         m_sendEvent;
            bool            m_running;
            uint32_t        m_packetsSent;
          };

       You   can   see   that   this   class  inherits  from  the  ns-3  Application  class.   Take  a  look  at
       src/network/model/application.h if you are interested in what is inherited.  The MyApp class is obligated
       to  override  the  StartApplication  and StopApplication methods.  These methods are automatically called
       when MyApp is required to start and stop sending data during the simulation.

   Starting/Stopping Applications
       It is worthwhile to spend a bit of time explaining how events actually get started in the  system.   This
       is  another fairly deep explanation, and can be ignored if you aren't planning on venturing down into the
       guts of the system.  It is useful, however, in that the discussion touches on  how  some  very  important
       parts  of  ns-3  work and exposes some important idioms.  If you are planning on implementing new models,
       you probably want to understand this section.

       The most common way to start pumping events is to start an Application.  This is done as  the  result  of
       the following (hopefully) familar lines of an ns-3 script:

          ApplicationContainer apps = ...
          apps.Start (Seconds (1.0));
          apps.Stop (Seconds (10.0));

       The  application  container  code  (see src/network/helper/application-container.h if you are interested)
       loops through its contained applications and calls,

          app->SetStartTime (startTime);

       as a result of the apps.Start call and

          app->SetStopTime (stopTime);

       as a result of the apps.Stop call.

       The ultimate result of these calls is that we want to have the simulator automatically  make  calls  into
       our  Applications  to  tell  them  when  to start and stop.  In the case of MyApp, it inherits from class
       Application and overrides StartApplication, and StopApplication.  These are the functions  that  will  be
       called  by  the  simulator  at  the  appropriate  time.   In  the  case  of  MyApp  you  will  find  that
       MyApp::StartApplication does the initial Bind, and Connect on the socket, and then starts data flowing by
       calling  MyApp::SendPacket.   MyApp::StopApplication  stops  generating packets by cancelling any pending
       send events then closes the socket.

       One of the nice things about ns-3 is that you can completely ignore the  implementation  details  of  how
       your  Application  is  "automagically"  called  by  the simulator at the correct time.  But since we have
       already ventured deep into ns-3 already, let's go for it.

       If you look at src/network/model/application.cc  you  will  find  that  the  SetStartTime  method  of  an
       Application  just  sets  the member variable m_startTime and the SetStopTime method just sets m_stopTime.
       From there, without some hints, the trail will probably end.

       The key to picking up the trail again is to know that there is a global list of all of the nodes  in  the
       system.   Whenever  you  create  a  node  in  a simulation, a pointer to that Node is added to the global
       NodeList.

       Take  a  look  at  src/network/model/node-list.cc  and  search  for  NodeList::Add.   The  public  static
       implementation calls into a private implementation called NodeListPriv::Add.  This is a relatively common
       idom in ns-3.  So, take a look at NodeListPriv::Add.  There you will find,

          Simulator::ScheduleWithContext (index, TimeStep (0), &Node::Initialize, node);

       This tells you that whenever a Node is created in a simulation, as a side-effect, a call to  that  node's
       Initialize  method  is  scheduled for you that happens at time zero.  Don't read too much into that name,
       yet.  It doesn't mean that the Node is going to start  doing  anything,  it  can  be  interpreted  as  an
       informational  call  into  the  Node  telling  it  that the simulation has started, not a call for action
       telling the Node to start doing something.

       So, NodeList::Add indirectly schedules a call to Node::Initialize at time zero to advise a new Node  that
       the simulation has started.  If you look in src/network/model/node.h you will, however, not find a method
       called Node::Initialize.  It turns out that the Initialize method is inherited from  class  Object.   All
       objects  in the system can be notified when the simulation starts, and objects of class Node are just one
       kind of those objects.

       Take a look at src/core/model/object.cc next and search for Object::Initialize.   This  code  is  not  as
       straightforward  as  you  might  have  expected  since  ns-3  Objects  support  aggregation.  The code in
       Object::Initialize then loops through all of the objects that have been  aggregated  together  and  calls
       their  DoInitialize  method.   This  is  another  idiom that is very common in ns-3, sometimes called the
       "template design pattern.": a public non-virtual API method, which stays constant across implementations,
       and  that  calls a private virtual implementation method that is inherited and implemented by subclasses.
       The names are typically something like MethodName for the public API and  DoMethodName  for  the  private
       API.

       This  tells  us  that we should look for a Node::DoInitialize method in src/network/model/node.cc for the
       method that will continue our trail.  If you locate the code, you will find a method that  loops  through
       all  of  the  devices in the Node and then all of the applications in the Node calling device->Initialize
       and application->Initialize respectively.

       You may already know that classes Device and Application both inherit from class Object and so  the  next
       step  will  be  to  look  at  what  happens  when  Application::DoInitialize  is  called.  Take a look at
       src/network/model/application.cc and you will find:

          void
          Application::DoInitialize (void)
          {
            m_startEvent = Simulator::Schedule (m_startTime, &Application::StartApplication, this);
            if (m_stopTime != TimeStep (0))
              {
                m_stopEvent = Simulator::Schedule (m_stopTime, &Application::StopApplication, this);
              }
            Object::DoInitialize ();
          }

       Here, we finally come to the end of the trail.  If you have kept it all straight, when you  implement  an
       ns-3   Application,   your   new   application   inherits  from  class  Application.   You  override  the
       StartApplication and StopApplication methods and provide mechanisms for starting and stopping the flow of
       data  out  of  your  new  Application.  When a Node is created in the simulation, it is added to a global
       NodeList.  The act of adding a Node to this NodeList causes a simulator event to be  scheduled  for  time
       zero  which  calls  the  Node::Initialize method of the newly added Node to be called when the simulation
       starts.  Since a Node inherits from Object, this calls the Object::Initialize method on the  Node  which,
       in  turn,  calls  the  DoInitialize  methods on all of the Objects aggregated to the Node (think mobility
       models).  Since the Node Object has overridden DoInitialize, that method is called  when  the  simulation
       starts.   The  Node::DoInitialize  method  calls the Initialize methods of all of the Applications on the
       node.  Since Applications are also Objects, this causes Application::DoInitialize  to  be  called.   When
       Application::DoInitialize  is  called,  it  schedules events for the StartApplication and StopApplication
       calls on the Application.  These calls are designed  to  start  and  stop  the  flow  of  data  from  the
       Application

       This  has  been  another  fairly  long  journey,  but it only has to be made once, and you now understand
       another very deep piece of ns-3.

   The MyApp Application
       The MyApp Application needs a constructor and a destructor, of course:

          MyApp::MyApp ()
            : m_socket (0),
              m_peer (),
              m_packetSize (0),
              m_nPackets (0),
              m_dataRate (0),
              m_sendEvent (),
              m_running (false),
              m_packetsSent (0)
          {
          }

          MyApp::~MyApp()
          {
            m_socket = 0;
          }

       The existence of the next bit of code is the whole reason why we wrote  this  Application  in  the  first
       place.

          void
          MyApp::Setup (Ptr<Socket> socket, Address address, uint32_t packetSize,
                               uint32_t nPackets, DataRate dataRate)
          {
            m_socket = socket;
            m_peer = address;
            m_packetSize = packetSize;
            m_nPackets = nPackets;
            m_dataRate = dataRate;
          }

       This  code  should be pretty self-explanatory.  We are just initializing member variables.  The important
       one from the perspective of tracing is  the  Ptr<Socket>  socket  which  we  needed  to  provide  to  the
       application  during  configuration  time.   Recall  that we are going to create the Socket as a TcpSocket
       (which is implemented by TcpNewReno) and hook its "CongestionWindow" trace source before  passing  it  to
       the Setup method.

          void
          MyApp::StartApplication (void)
          {
            m_running = true;
            m_packetsSent = 0;
            m_socket->Bind ();
            m_socket->Connect (m_peer);
            SendPacket ();
          }

       The  above code is the overridden implementation Application::StartApplication that will be automatically
       called by the simulator to start our Application running at the appropriate time.  You can  see  that  it
       does  a  Socket  Bind operation.  If you are familiar with Berkeley Sockets this shouldn't be a surprise.
       It performs the required work on the local side  of  the  connection  just  as  you  might  expect.   The
       following  Connect will do what is required to establish a connection with the TCP at Address m_peer.  It
       should now be clear why we need to defer a lot of this to simulation time, since the Connect is going  to
       need  a  fully  functioning network to complete.  After the Connect, the Application then starts creating
       simulation events by calling SendPacket.

       The next bit of code explains to the Application how to stop creating simulation events.

          void
          MyApp::StopApplication (void)
          {
            m_running = false;

            if (m_sendEvent.IsRunning ())
              {
                Simulator::Cancel (m_sendEvent);
              }

            if (m_socket)
              {
                m_socket->Close ();
              }
          }

       Every time a simulation event is scheduled, an Event is created.  If the Event is  pending  execution  or
       executing,  its  method IsRunning will return true.  In this code, if IsRunning() returns true, we Cancel
       the event which removes it from the simulator event queue.  By doing this, we break the chain  of  events
       that the Application is using to keep sending its Packets and the Application goes quiet.  After we quiet
       the Application we Close the socket which tears down the TCP connection.

       The socket is actually deleted in the destructor when the m_socket = 0 is  executed.   This  removes  the
       last reference to the underlying Ptr<Socket> which causes the destructor of that Object to be called.

       Recall  that  StartApplication  called  SendPacket  to  start  the  chain  of  events  that describes the
       Application behavior.

          void
          MyApp::SendPacket (void)
          {
            Ptr<Packet> packet = Create<Packet> (m_packetSize);
            m_socket->Send (packet);

            if (++m_packetsSent < m_nPackets)
              {
                ScheduleTx ();
              }
          }

       Here, you see that SendPacket does just that.  It creates a Packet and then does a  Send  which,  if  you
       know Berkeley Sockets, is probably just what you expected to see.

       It  is  the  responsibility  of the Application to keep scheduling the chain of events, so the next lines
       call ScheduleTx to schedule another transmit event (a SendPacket) until the Application  decides  it  has
       sent enough.

          void
          MyApp::ScheduleTx (void)
          {
            if (m_running)
              {
                Time tNext (Seconds (m_packetSize * 8 / static_cast<double> (m_dataRate.GetBitRate ())));
                m_sendEvent = Simulator::Schedule (tNext, &MyApp::SendPacket, this);
              }
          }

       Here,  you  see that ScheduleTx does exactly that.  If the Application is running (if StopApplication has
       not been called) it will schedule a new event, which calls SendPacket again.  The alert reader will  spot
       something that also trips up new users.  The data rate of an Application is just that.  It has nothing to
       do with the data rate of an underlying Channel.  This is the rate at which the Application produces bits.
       It  does  not  take  into  account  any  overhead  for  the various protocols or channels that it uses to
       transport the data.  If you set the data rate of an Application to the same data rate as your  underlying
       Channel you will eventually get a buffer overflow.

   Trace Sinks
       The  whole point of this exercise is to get trace callbacks from TCP indicating the congestion window has
       been updated.  The next piece of code implements the corresponding trace sink:

          static void
          CwndChange (uint32_t oldCwnd, uint32_t newCwnd)
          {
            NS_LOG_UNCOND (Simulator::Now ().GetSeconds () << "\t" << newCwnd);
          }

       This should be very familiar to you now, so we won't dwell on the details.  This function just  logs  the
       current  simulation  time  and  the new value of the congestion window every time it is changed.  You can
       probably imagine that you could load the resulting output into a graphics program (gnuplot or Excel)  and
       immediately see a nice graph of the congestion window behavior over time.

       We  added a new trace sink to show where packets are dropped.  We are going to add an error model to this
       code also, so we wanted to demonstrate this working.

          static void
          RxDrop (Ptr<const Packet> p)
          {
            NS_LOG_UNCOND ("RxDrop at " << Simulator::Now ().GetSeconds ());
          }

       This trace sink will be connected to the "PhyRxDrop" trace source of the point-to-point NetDevice.   This
       trace  source  fires  when a packet is dropped by the physical layer of a NetDevice.  If you take a small
       detour to the source (src/point-to-point/model/point-to-point-net-device.cc) you will see that this trace
       source     refers     to     PointToPointNetDevice::m_phyRxDropTrace.     If    you    then    look    in
       src/point-to-point/model/point-to-point-net-device.h for this member variable, you will find that  it  is
       declared  as  a TracedCallback<Ptr<const Packet> >.  This should tell you that the callback target should
       be a function that returns void and takes a single parameter which is a Ptr<const  Packet>  (assuming  we
       use ConnectWithoutContext) -- just what we have above.

   Main Program
       The following code should be very familiar to you by now:

          int
          main (int argc, char *argv[])
          {
            NodeContainer nodes;
            nodes.Create (2);

            PointToPointHelper pointToPoint;
            pointToPoint.SetDeviceAttribute ("DataRate", StringValue ("5Mbps"));
            pointToPoint.SetChannelAttribute ("Delay", StringValue ("2ms"));

            NetDeviceContainer devices;
            devices = pointToPoint.Install (nodes);

       This  creates  two nodes with a point-to-point channel between them, just as shown in the illustration at
       the start of the file.

       The next few lines of code show something new.  If we trace a connection that behaves perfectly, we  will
       end  up  with  a  monotonically increasing congestion window.  To see any interesting behavior, we really
       want to introduce link errors which will  drop  packets,  cause  duplicate  ACKs  and  trigger  the  more
       interesting behaviors of the congestion window.

       ns-3  provides  ErrorModel  objects  which  can be attached to Channels.  We are using the RateErrorModel
       which allows us to introduce errors into a Channel at a given rate.

          Ptr<RateErrorModel> em = CreateObject<RateErrorModel> ();
          em->SetAttribute ("ErrorRate", DoubleValue (0.00001));
          devices.Get (1)->SetAttribute ("ReceiveErrorModel", PointerValue (em));

       The above code instantiates a RateErrorModel Object, and we set the "ErrorRate" Attribute to the  desired
       value.   We  then  set  the  resulting  instantiated  RateErrorModel  as  the  error  model  used  by the
       point-to-point NetDevice.  This will give us some  retransmissions  and  make  our  plot  a  little  more
       interesting.

          InternetStackHelper stack;
          stack.Install (nodes);

          Ipv4AddressHelper address;
          address.SetBase ("10.1.1.0", "255.255.255.252");
          Ipv4InterfaceContainer interfaces = address.Assign (devices);

       The  above  code should be familiar.  It installs internet stacks on our two nodes and creates interfaces
       and assigns IP addresses for the point-to-point devices.

       Since we are using TCP, we need something on the destination Node to receive TCP  connections  and  data.
       The PacketSink Application is commonly used in ns-3 for that purpose.

          uint16_t sinkPort = 8080;
          Address sinkAddress (InetSocketAddress(interfaces.GetAddress (1), sinkPort));
          PacketSinkHelper packetSinkHelper ("ns3::TcpSocketFactory",
            InetSocketAddress (Ipv4Address::GetAny (), sinkPort));
          ApplicationContainer sinkApps = packetSinkHelper.Install (nodes.Get (1));
          sinkApps.Start (Seconds (0.));
          sinkApps.Stop (Seconds (20.));

       This should all be familiar, with the exception of,

          PacketSinkHelper packetSinkHelper ("ns3::TcpSocketFactory",
            InetSocketAddress (Ipv4Address::GetAny (), sinkPort));

       This   code   instantiates   a   PacketSinkHelper  and  tells  it  to  create  sockets  using  the  class
       ns3::TcpSocketFactory.  This class implements a  design  pattern  called  "object  factory"  which  is  a
       commonly  used mechanism for specifying a class used to create objects in an abstract way.  Here, instead
       of having to create the objects themselves, you provide the PacketSinkHelper a string  that  specifies  a
       TypeId  string  used  to  create  an  object  which can then be used, in turn, to create instances of the
       Objects created by the factory.

       The remaining parameter tells the Application which address and port it should Bind to.

       The next two lines of code will create the socket and connect the trace source.

          Ptr<Socket> ns3TcpSocket = Socket::CreateSocket (nodes.Get (0),
            TcpSocketFactory::GetTypeId ());
          ns3TcpSocket->TraceConnectWithoutContext ("CongestionWindow",
            MakeCallback (&CwndChange));

       The first statement calls the static member function Socket::CreateSocket and  provides  a  Node  and  an
       explicit  TypeId  for  the object factory used to create the socket.  This is a slightly lower level call
       than the PacketSinkHelper call above, and uses an explicit C++ type instead  of  one  referred  to  by  a
       string.  Otherwise, it is conceptually the same thing.

       Once  the TcpSocket is created and attached to the Node, we can use TraceConnectWithoutContext to connect
       the CongestionWindow trace source to our trace sink.

       Recall that we coded an Application so we could take that Socket we just made (during configuration time)
       and use it in simulation time.  We now have to instantiate that Application.  We didn't go to any trouble
       to create a helper to manage the Application so we are going to have to create and install it "manually".
       This is actually quite easy:

          Ptr<MyApp> app = CreateObject<MyApp> ();
          app->Setup (ns3TcpSocket, sinkAddress, 1040, 1000, DataRate ("1Mbps"));
          nodes.Get (0)->AddApplication (app);
          app->Start (Seconds (1.));
          app->Stop (Seconds (20.));

       The first line creates an Object of type MyApp -- our Application.  The second line tells the Application
       what Socket to use, what address to connect to, how much data to send at each send event, how  many  send
       events to generate and the rate at which to produce data from those events.

       Next,  we  manually  add  the MyApp Application to the source Node and explicitly call the Start and Stop
       methods on the Application to tell it when to start and stop doing its thing.

       We need to actually do the connect from the receiver point-to-point NetDevice drop event  to  our  RxDrop
       callback now.

          devices.Get (1)->TraceConnectWithoutContext("PhyRxDrop", MakeCallback (&RxDrop));

       It  should  now  be  obvious  that  we  are  getting a reference to the receiving Node NetDevice from its
       container and connecting the trace source defined by the attribute "PhyRxDrop"  on  that  device  to  the
       trace sink RxDrop.

       Finally, we tell the simulator to override any Applications and just stop processing events at 20 seconds
       into the simulation.

            Simulator::Stop (Seconds(20));
            Simulator::Run ();
            Simulator::Destroy ();

            return 0;
          }

       Recall that as soon as Simulator::Run is called, configuration time ends,  and  simulation  time  begins.
       All  of the work we orchestrated by creating the Application and teaching it how to connect and send data
       actually happens during this function call.

       As soon as Simulator::Run returns, the simulation is complete and we enter the teardown phase.   In  this
       case,  Simulator::Destroy  takes  care  of  the  gory  details and we just return a success code after it
       completes.

   Running fifth.cc
       Since we have provided the file fifth.cc for you, if you have built  your  distribution  (in  debug  mode
       since  it  uses NS_LOG -- recall that optimized builds optimize out NS_LOG) it will be waiting for you to
       run.

          $ ./waf --run fifth
          Waf: Entering directory `/home/craigdo/repos/ns-3-allinone-dev/ns-3-dev/build'
          Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone-dev/ns-3-dev/build'
          'build' finished successfully (0.684s)
          1       536
          1.0093  1072
          1.01528 1608
          1.02167 2144
          ...
          1.11319 8040
          1.12151 8576
          1.12983 9112
          RxDrop at 1.13696
          ...

       You can probably see immediately a downside of using prints of any kind in your  traces.   We  get  those
       extraneous  waf  messages  printed all over our interesting information along with those RxDrop messages.
       We will remedy that soon, but I'm sure you can't wait to see the results of  all  of  this  work.   Let's
       redirect that output to a file called cwnd.dat:

          $ ./waf --run fifth > cwnd.dat 2>&1

       Now  edit  up  "cwnd.dat" in your favorite editor and remove the waf build status and drop lines, leaving
       only the traced data (you could also comment out the TraceConnectWithoutContext("PhyRxDrop", MakeCallback
       (&RxDrop)); in the script to get rid of the drop prints just as easily.

       You can now run gnuplot (if you have it installed) and tell it to generate some pretty pictures:

          $ gnuplot
          gnuplot> set terminal png size 640,480
          gnuplot> set output "cwnd.png"
          gnuplot> plot "cwnd.dat" using 1:2 title 'Congestion Window' with linespoints
          gnuplot> exit

       You  should  now  have  a  graph of the congestion window versus time sitting in the file "cwnd.png" that
       looks like:
         [image]

   Using Mid-Level Helpers
       In the previous section, we showed how to hook a trace source and get hopefully  interesting  information
       out  of  a  simulation.   Perhaps  you  will  recall  that we called logging to the standard output using
       std::cout a "blunt instrument" much earlier in this chapter.  We also wrote about how it  was  a  problem
       having  to parse the log output in order to isolate interesting information.  It may have occurred to you
       that we just spent a lot of time implementing an example that exhibits all of the problems we purport  to
       fix with the ns-3 tracing system!  You would be correct.  But, bear with us.  We're not done yet.

       One  of the most important things we want to do is to is to have the ability to easily control the amount
       of output coming out of the simulation; and we also want to save those data to a file  so  we  can  refer
       back  to  it later.  We can use the mid-level trace helpers provided in ns-3 to do just that and complete
       the picture.

       We provide a script that writes the cwnd change and drop events developed in the example fifth.cc to disk
       in  separate  files.   The  cwnd changes are stored as a tab-separated ASCII file and the drop events are
       stored in a PCAP file.  The changes to make this happen are quite small.

   Walkthrough: sixth.cc
       Let's  take  a  look   at   the   changes   required   to   go   from   fifth.cc   to   sixth.cc.    Open
       examples/tutorial/sixth.cc  in  your  favorite  editor.   You  can  see the first change by searching for
       CwndChange.  You will find that we have changed the signatures for the  trace  sinks  and  have  added  a
       single line to each sink that writes the traced information to a stream representing a file.

          static void
          CwndChange (Ptr<OutputStreamWrapper> stream, uint32_t oldCwnd, uint32_t newCwnd)
          {
            NS_LOG_UNCOND (Simulator::Now ().GetSeconds () << "\t" << newCwnd);
            *stream->GetStream () << Simulator::Now ().GetSeconds () << "\t" << oldCwnd << "\t" << newCwnd << std::endl;
          }

          static void
          RxDrop (Ptr<PcapFileWrapper> file, Ptr<const Packet> p)
          {
            NS_LOG_UNCOND ("RxDrop at " << Simulator::Now ().GetSeconds ());
            file->Write(Simulator::Now(), p);
          }

       We  have  added  a  "stream" parameter to the CwndChange trace sink.  This is an object that holds (keeps
       safely alive) a C++ output stream.  It turns out that this is a very simple object, but one that  manages
       lifetime  issues  for the stream and solves a problem that even experienced C++ users run into.  It turns
       out that the copy constructor for std::ostream is marked private.  This means that std::ostreams  do  not
       obey  value  semantics  and  cannot be used in any mechanism that requires the stream to be copied.  This
       includes the ns-3 callback system, which as you may recall, requires objects that obey  value  semantics.
       Further notice that we have added the following line in the CwndChange trace sink implementation:

          *stream->GetStream () << Simulator::Now ().GetSeconds () << "\t" << oldCwnd << "\t" << newCwnd << std::endl;

       This would be very familiar code if you replaced *stream->GetStream () with std::cout, as in:

          std::cout << Simulator::Now ().GetSeconds () << "\t" << oldCwnd << "\t" << newCwnd << std::endl;

       This  illustrates  that  the  Ptr<OutputStreamWrapper> is really just carrying around a std::ofstream for
       you, and you can use it here like any other output stream.

       A similar situation happens in RxDrop except that the object being passed around (a Ptr<PcapFileWrapper>)
       represents  a PCAP file.  There is a one-liner in the trace sink to write a timestamp and the contents of
       the packet being dropped to the PCAP file:

          file->Write(Simulator::Now(), p);

       Of course, if we have objects representing the two files, we need to create them somewhere and also cause
       them  to  be  passed  to the trace sinks.  If you look in the main function, you will find new code to do
       just that:

          AsciiTraceHelper asciiTraceHelper;
          Ptr<OutputStreamWrapper> stream = asciiTraceHelper.CreateFileStream ("sixth.cwnd");
          ns3TcpSocket->TraceConnectWithoutContext ("CongestionWindow", MakeBoundCallback (&CwndChange, stream));

          ...

          PcapHelper pcapHelper;
          Ptr<PcapFileWrapper> file = pcapHelper.CreateFile ("sixth.pcap", std::ios::out, PcapHelper::DLT_PPP);
          devices.Get (1)->TraceConnectWithoutContext("PhyRxDrop", MakeBoundCallback (&RxDrop, file));

       In the first section of the code snippet above, we are creating the ASCII trace file, creating an  object
       responsible  for  managing  it  and  using a variant of the callback creation function to arrange for the
       object to be passed to the sink.  Our ASCII trace helpers provide a rich set of functions to  make  using
       text  (ASCII)  files  easy.  We are just going to illustrate the use of the file stream creation function
       here.

       The CreateFileStream function is basically going to instantiate a std::ofstream object and create  a  new
       file  (or  truncate  an existing file).  This std::ofstream is packaged up in an ns-3 object for lifetime
       management and copy constructor issue resolution.

       We then take this ns-3 object representing the file and pass it to  MakeBoundCallback().   This  function
       creates  a  callback just like MakeCallback(), but it "binds" a new value to the callback.  This value is
       added as the first argument to the callback before it is called.

       Essentially, MakeBoundCallback(&CwndChange, stream)  causes  the  trace  source  to  add  the  additional
       "stream"  parameter to the front of the formal parameter list before invoking the callback.  This changes
       the required signature of the CwndChange sink to match the one shown above, which  includes  the  "extra"
       parameter Ptr<OutputStreamWrapper> stream.

       In  the second section of code in the snippet above, we instantiate a PcapHelper to do the same thing for
       our PCAP trace file that we did with the AsciiTraceHelper. The line of code,

          Ptr<PcapFileWrapper> file = pcapHelper.CreateFile ("sixth.pcap",
          "w", PcapHelper::DLT_PPP);

       creates a PCAP file named "sixth.pcap" with file mode "w".  This means that the  new  file  is  truncated
       (contents  deleted)  if  an existing file with that name is found.  The final parameter is the "data link
       type" of the new PCAP file.  These are the same as the PCAP library data link types defined in  bpf.h  if
       you  are  familar  with  PCAP.   In  this  case, DLT_PPP indicates that the PCAP file is going to contain
       packets prefixed with point to point headers.  This is  true  since  the  packets  are  coming  from  our
       point-to-point  device  driver.  Other common data link types are DLT_EN10MB (10 MB Ethernet) appropriate
       for csma devices and DLT_IEEE802_11 (IEEE 802.11) appropriate for wifi devices.   These  are  defined  in
       src/network/helper/trace-helper.h  if  you  are  interested  in seeing the list.  The entries in the list
       match those in bpf.h but we duplicate them to avoid a PCAP source dependence.

       A ns-3 object representing the PCAP file is returned from CreateFile and used in a bound callback exactly
       as it was in the ASCII case.

       An  important  detour:  It  is important to notice that even though both of these objects are declared in
       very similar ways,

          Ptr<PcapFileWrapper> file ...
          Ptr<OutputStreamWrapper> stream ...

       The underlying objects are entirely different.  For example, the Ptr<PcapFileWrapper> is a smart  pointer
       to  an ns-3 Object that is a fairly heavyweight thing that supports Attributes and is integrated into the
       Config system.  The Ptr<OutputStreamWrapper>, on the other hand,  is  a  smart  pointer  to  a  reference
       counted  object  that  is  a  very lightweight thing.  Remember to look at the object you are referencing
       before making any assumptions about the "powers" that object may have.

       For example, take a look at src/network/utils/pcap-file-wrapper.h in the distribution and notice,

          class PcapFileWrapper : public Object

       that  class  PcapFileWrapper  is  an  ns-3  Object  by  virtue  of  its  inheritance.    Then   look   at
       src/network/model/output-stream-wrapper.h and notice,

          class OutputStreamWrapper : public
          SimpleRefCount<OutputStreamWrapper>

       that  this  object  is  not  an  ns-3  Object at all, it is "merely" a C++ object that happens to support
       intrusive reference counting.

       The point here is that just because you read Ptr<something> it does not necessarily mean  that  something
       is an ns-3 Object on which you can hang ns-3 Attributes, for example.

       Now, back to the example.  If you build and run this example,

          $ ./waf --run sixth

       you  will  see  the  same  messages  appear as when you ran "fifth", but two new files will appear in the
       top-level directory of your ns-3 distribution.

          sixth.cwnd  sixth.pcap

       Since "sixth.cwnd" is an ASCII text file, you can view it with cat or your favorite file viewer.

          1       0       536
          1.0093  536     1072
          1.01528 1072    1608
          1.02167 1608    2144
          ...
          9.69256 5149    5204
          9.89311 5204    5259

       You have a tab separated file with a timestamp, an old congestion window  and  a  new  congestion  window
       suitable  for  directly importing into your plot program.  There are no extraneous prints in the file, no
       parsing or editing is required.

       Since "sixth.pcap" is a PCAP file, you can fiew it with tcpdump.

          reading from file sixth.pcap, link-type PPP (PPP)
          1.136956 IP 10.1.1.1.49153 > 10.1.1.2.8080: Flags [.], seq 17177:17681, ack 1, win 32768, options [TS val 1133 ecr 1127,eol], length 504
          1.403196 IP 10.1.1.1.49153 > 10.1.1.2.8080: Flags [.], seq 33280:33784, ack 1, win 32768, options [TS val 1399 ecr 1394,eol], length 504
          ...
          7.426220 IP 10.1.1.1.49153 > 10.1.1.2.8080: Flags [.], seq 785704:786240, ack 1, win 32768, options [TS val 7423 ecr 7421,eol], length 536
          9.630693 IP 10.1.1.1.49153 > 10.1.1.2.8080: Flags [.], seq 882688:883224, ack 1, win 32768, options [TS val 9620 ecr 9618,eol], length 536

       You have a PCAP file with the packets that were dropped in the simulation.  There are  no  other  packets
       present in the file and there is nothing else present to make life difficult.

       It's  been a long journey, but we are now at a point where we can appreciate the ns-3 tracing system.  We
       have pulled important events out of the middle of a TCP implementation and a device  driver.   We  stored
       those  events  directly  in files usable with commonly known tools.  We did this without modifying any of
       the core code involved, and we did this in only 18 lines of code:

          static void
          CwndChange (Ptr<OutputStreamWrapper> stream, uint32_t oldCwnd, uint32_t newCwnd)
          {
            NS_LOG_UNCOND (Simulator::Now ().GetSeconds () << "\t" << newCwnd);
            *stream->GetStream () << Simulator::Now ().GetSeconds () << "\t" << oldCwnd << "\t" << newCwnd << std::endl;
          }

          ...

          AsciiTraceHelper asciiTraceHelper;
          Ptr<OutputStreamWrapper> stream = asciiTraceHelper.CreateFileStream ("sixth.cwnd");
          ns3TcpSocket->TraceConnectWithoutContext ("CongestionWindow", MakeBoundCallback (&CwndChange, stream));

          ...

          static void
          RxDrop (Ptr<PcapFileWrapper> file, Ptr<const Packet> p)
          {
            NS_LOG_UNCOND ("RxDrop at " << Simulator::Now ().GetSeconds ());
            file->Write(Simulator::Now(), p);
          }

          ...

          PcapHelper pcapHelper;
          Ptr<PcapFileWrapper> file = pcapHelper.CreateFile ("sixth.pcap", "w", PcapHelper::DLT_PPP);
          devices.Get (1)->TraceConnectWithoutContext("PhyRxDrop", MakeBoundCallback (&RxDrop, file));

   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 BuildingTopologies, 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 when it 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.

   Device Helpers
   PCAP
       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, bool explicitFilename) = 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.

   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 API Documentation 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");

   Filenames
       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
       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,
                                            bool explicitFilename) = 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, bool explicitFilename = false);
          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.

   Methods
          void EnableAscii (std::string prefix, Ptr<NetDevice> nd, bool explicitFilename = false);
          void EnableAscii (Ptr<OutputStreamWrapper> stream, Ptr<NetDevice> nd);

          void EnableAscii (std::string prefix, std::string ndName, bool explicitFilename = false);
          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 EnableAsciiAll (std::string prefix);
          void EnableAsciiAll (Ptr<OutputStreamWrapper> stream);

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

       You  are  encouraged  to  peruse  the  API  Documentation for class AsciiTraceHelperForDevice 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);

       • The first four methods also include a default parameter called explicitFilename that operate similar to
         equivalent parameters in the PCAP case.

         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.  We have already seen this
         in the "cwnd" example above:

            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 suffix 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.

   Filenames
       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.

       Several  of  the  methods  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 take a prefix and enable tracing on a
       single device.

   Protocol Helpers
   PCAP
       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,
                                               bool explicitFilename) = 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, bool explicitFilename = false);

       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.

   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, bool explicitFilename = false);
          void EnablePcapIpv4 (std::string prefix, std::string ipv4Name, uint32_t interface, bool explicitFilename = false);
          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, bool explicitFilename);
          void EnablePcapIpv4All (std::string prefix);

       You  are  encouraged  to  peruse the API Documentation 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");

   Filenames
       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".

       Several  of  the  methods  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 take a prefix and enable tracing on a
       single device.

   ASCII
       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,
                                                bool explicitFilename) = 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, bool explicitFilename = false);
          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.

   Methods
          void EnableAsciiIpv4 (std::string prefix, Ptr<Ipv4> ipv4, uint32_t interface, bool explicitFilename = false);
          void EnableAsciiIpv4 (Ptr<OutputStreamWrapper> stream, Ptr<Ipv4> ipv4, uint32_t interface);

          void EnableAsciiIpv4 (std::string prefix, std::string ipv4Name, uint32_t interface, bool explicitFilename = false);
          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 EnableAsciiIpv4All (std::string prefix);
          void EnableAsciiIpv4All (Ptr<OutputStreamWrapper> stream);

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

       You are encouraged to peruse the API  Documentation  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.

   Filenames
       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".

       Several  of  the  methods  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 take a prefix and enable tracing on a
       single device.

   Summary
       ns-3 includes an extremely rich environment allowing users at several levels to customize  the  kinds  of
       information that can be extracted from simulations.

       There  are  high-level  helper functions that allow users to simply control the collection of pre-defined
       outputs to a fine granularity.  There are mid-level helper functions to allow more sophisticated users to
       customize  how information is extracted and saved; and there are low-level core functions to allow expert
       users to alter the system to present new and previously unexported information in  a  way  that  will  be
       immediately accessible to users at higher levels.

       This  is a very comprehensive system, and we realize that it is a lot to digest, especially for new users
       or those not intimately familiar with C++ and its idioms.  We do  consider  the  tracing  system  a  very
       important  part  of  ns-3  and so recommend becoming as familiar as possible with it.  It is probably the
       case that understanding the rest of the ns-3 system will be quite  simple  once  you  have  mastered  the
       tracing system

DATA COLLECTION

       Our  final  tutorial chapter introduces some components that were added to ns-3 in version 3.18, and that
       are still under development.  This tutorial section is also a work-in-progress.

   Motivation
       One of the main points of running simulations is to generate output data, either for research purposes or
       simply  to  learn about the system.  In the previous chapter, we introduced the tracing subsystem and the
       example sixth.cc. from which PCAP or ASCII trace files are generated.  These traces are valuable for data
       analysis  using a variety of external tools, and for many users, such output data is a preferred means of
       gathering data (for analysis by external tools).

       However, there are also use cases for more than trace file generation, including the following:

       • generation of data that does not map well to PCAP or  ASCII  traces,  such  as  non-packet  data  (e.g.
         protocol state machine transitions),

       • large  simulations  for  which  the  disk I/O requirements for generating trace files is prohibitive or
         cumbersome, and

       • the need for online  data reduction or computation, during  the  course  of  the  simulation.   A  good
         example  of  this is to define a termination condition for the simulation, to tell it when to stop when
         it has received enough data to form a narrow-enough confidence interval around  the  estimate  of  some
         parameter.

       The  ns-3  data  collection  framework  is  designed  to  provide  these  additional  capabilities beyond
       trace-based output.  We recommend that the reader interested in this topic consult the ns-3 Manual for  a
       more  detailed  treatment  of  this  framework;  here,  we  summarize with an example program some of the
       developing capabilities.

   Example Code
       The tutorial example examples/tutorial/seventh.cc resembles the sixth.cc example we previously  reviewed,
       except for a few changes.  First, it has been enabled for IPv6 support with a command-line option:

          CommandLine cmd;
          cmd.AddValue ("useIpv6", "Use Ipv6", useV6);
          cmd.Parse (argc, argv);

       If  the  user  specifies  useIpv6,  option, the program will be run using IPv6 instead of IPv4.  The help
       option, available on all ns-3 programs that support the CommandLine object as shown above, can be invoked
       as follows (please note the use of double quotes):

          ./waf --run "seventh --help"

       which produces:

          ns3-dev-seventh-debug [Program Arguments] [General Arguments]

          Program Arguments:
              --useIpv6:  Use Ipv6 [false]

          General Arguments:
              --PrintGlobals:              Print the list of globals.
              --PrintGroups:               Print the list of groups.
              --PrintGroup=[group]:        Print all TypeIds of group.
              --PrintTypeIds:              Print all TypeIds.
              --PrintAttributes=[typeid]:  Print all attributes of typeid.
              --PrintHelp:                 Print this help message.

       This  default  (use  of  IPv4,  since  useIpv6  is false) can be changed by toggling the boolean value as
       follows:

          ./waf --run "seventh --useIpv6=1"

       and have a look at the pcap generated, such as with tcpdump:

          tcpdump -r seventh.pcap -nn -tt

       This has been a short digression into IPv6 support and  the  command  line,  which  was  also  introduced
       earlier   in   this   tutorial.    For   a   dedicated   example   of  command  line  usage,  please  see
       src/core/examples/command-line-example.cc.

       Now back to data collection.  In the examples/tutorial/ directory, type the following  command:  diff  -u
       sixth.cc seventh.cc, and examine some of the new lines of this diff:

          +  std::string probeType;
          +  std::string tracePath;
          +  if (useV6 == false)
          +    {
             ...
          +      probeType = "ns3::Ipv4PacketProbe";
          +      tracePath = "/NodeList/*/$ns3::Ipv4L3Protocol/Tx";
          +    }
          +  else
          +    {
             ...
          +      probeType = "ns3::Ipv6PacketProbe";
          +      tracePath = "/NodeList/*/$ns3::Ipv6L3Protocol/Tx";
          +    }
           ...
          +   // Use GnuplotHelper to plot the packet byte count over time
          +   GnuplotHelper plotHelper;
          +
          +   // 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");
          +
          +   // 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);
          +
          +   // Use FileHelper to write out the packet byte count over time
          +   FileHelper fileHelper;
          +
          +   // Configure the file to be written, and the formatting of output data.
          +   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");
          +
          +   // Specify the probe type, probe path (in configuration namespace), and
          +   // probe output trace source ("OutputBytes") to write.
          +   fileHelper.WriteProbe (probeType,
          +                          tracePath,
          +                          "OutputBytes");
          +
              Simulator::Stop (Seconds (20));
              Simulator::Run ();
              Simulator::Destroy ();

       The careful reader will have noticed, when testing the IPv6 command line attribute above, that seventh.cc
       had created a number of new output files:

          seventh-packet-byte-count-0.txt
          seventh-packet-byte-count-1.txt
          seventh-packet-byte-count.dat
          seventh-packet-byte-count.plt
          seventh-packet-byte-count.png
          seventh-packet-byte-count.sh

       These were created by the additional statements introduced above; in particular, by a GnuplotHelper and a
       FileHelper.   This data was produced by hooking the data collection components to ns-3 trace sources, and
       marshaling the data into a formatted gnuplot and into a formatted text file.  In the next sections, we'll
       review each of these.

   GnuplotHelper
       The  GnuplotHelper  is  an  ns-3  helper  object  aimed  at  the  production of gnuplot plots with as few
       statements as possible, for common cases.  It hooks ns-3 trace sources with data types supported  by  the
       data  collection  system.   Not  all  ns-3 trace sources data types are supported, but many of the common
       trace types are, including TracedValues with plain old data (POD) types.

       Let's look at the output produced by this helper:

          seventh-packet-byte-count.dat
          seventh-packet-byte-count.plt
          seventh-packet-byte-count.sh

       The first is a gnuplot data file with a series of space-delimited  timestamps  and  packet  byte  counts.
       We'll  cover  how  this  particular  data output was configured below, but let's continue with the output
       files.  The file seventh-packet-byte-count.plt is a gnuplot plot file, that can  be  opened  from  within
       gnuplot.   Readers  who  understand  gnuplot syntax can see that this will produce a formatted output PNG
       file named seventh-packet-byte-count.png.   Finally, a small  shell  script  seventh-packet-byte-count.sh
       runs  this plot file through gnuplot to produce the desired PNG (which can be viewed in an image editor);
       that is, the command:

          sh seventh-packet-byte-count.sh

       will yield seventh-packet-byte-count.png.  Why wasn't this PNG produced in the first place?   The  answer
       is  that  by  providing the plt file, the user can hand-configure the result if desired, before producing
       the PNG.

       The PNG image title states that this plot is a plot of "Packet Byte Count  vs.  Time",  and  that  it  is
       plotting the probed data corresponding to the trace source path:

          /NodeList/*/$ns3::Ipv6L3Protocol/Tx

       Note  the  wild-card  in  the  trace path.  In summary, what this plot is capturing is the plot of packet
       bytes observed at the transmit trace source of the Ipv6L3Protocol object; largely 596-byte  TCP  segments
       in  one  direction,  and 60-byte TCP acks in the other (two node trace sources were matched by this trace
       source).

       How was this configured?  A few statements need to be provided.  First, the GnuplotHelper object must  be
       declared and configured:

          +  // Use GnuplotHelper to plot the packet byte count over time
          +  GnuplotHelper plotHelper;
          +
          +  // 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");

       To  this  point,  an empty plot has been configured.  The filename prefix is the first argument, the plot
       title is the second, the x-axis label the third, and the y-axis label the fourth argument.

       The next step is to configure the data, and here is where the trace source is hooked.  First, note  above
       in the program we declared a few variables for later use:

          +  std::string probeType;
          +  std::string tracePath;
          +  probeType = "ns3::Ipv6PacketProbe";
          +  tracePath = "/NodeList/*/$ns3::Ipv6L3Protocol/Tx";

       We use them here:

          +  // 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);

       The first two arguments are the name of the probe type and the trace source path.  These two are probably
       the hardest to determine when you try to use this framework to plot other traces.  The probe  trace  here
       is   the   Tx  trace  source  of  class  Ipv6L3Protocol.   When  we  examine  this  class  implementation
       (src/internet/model/ipv6-l3-protocol.cc) we can observe:

          .AddTraceSource ("Tx", "Send IPv6 packet to outgoing interface.",
                           MakeTraceSourceAccessor (&Ipv6L3Protocol::m_txTrace))

       This says that Tx is a name for variable m_txTrace, which has a declaration of:

          /**
           * \brief Callback to trace TX (transmission) packets.
           */
          TracedCallback<Ptr<const Packet>, Ptr<Ipv6>, uint32_t> m_txTrace;

       It turns out that this specific trace source signature is supported by a Probe class (what we need  here)
       of class Ipv6PacketProbe.  See the files src/internet/model/ipv6-packet-probe.{h,cc}.

       So,  in  the PlotProbe statement above, we see that the statement is hooking the trace source (identified
       by path string) with a matching ns-3 Probe type of Ipv6PacketProbe.  If we did  not  support  this  probe
       type  (matching  trace  source  signature),  we  could  have  not used this statement (although some more
       complicated lower-level statements could have been used, as described in the manual).

       The Ipv6PacketProbe exports, itself, some trace sources that extract the data out of  the  probed  Packet
       object:

          TypeId
          Ipv6PacketProbe::GetTypeId ()
          {
            static TypeId tid = TypeId ("ns3::Ipv6PacketProbe")
              .SetParent<Probe> ()
              .AddConstructor<Ipv6PacketProbe> ()
              .AddTraceSource ( "Output",
                                "The packet plus its IPv6 object and interface that serve as the output for this probe",
                                MakeTraceSourceAccessor (&Ipv6PacketProbe::m_output))
              .AddTraceSource ( "OutputBytes",
                                "The number of bytes in the packet",
                                MakeTraceSourceAccessor (&Ipv6PacketProbe::m_outputBytes))
            ;
            return tid;
          }

       The  third argument of our PlotProbe statement specifies that we are interested in the number of bytes in
       this packet; specifically, the "OutputBytes" trace source of  Ipv6PacketProbe.   Finally,  the  last  two
       arguments  of  the  statement  provide the plot legend for this data series ("Packet Byte Count"), and an
       optional gnuplot formatting statement (GnuplotAggregator::KEY_BELOW) that we want  the  plot  key  to  be
       inserted below the plot.  Other options include NO_KEY, KEY_INSIDE, and KEY_ABOVE.

   Supported Trace Types
       The following traced values are supported with Probes as of this writing:

                          ┌─────────────────┬─────────────────┬─────────────────────────────────┐
                          │TracedValue type │ Probe type      │ File                            │
                          ├─────────────────┼─────────────────┼─────────────────────────────────┤
                          │double           │ DoubleProbe     │ stats/model/double-probe.h      │
                          ├─────────────────┼─────────────────┼─────────────────────────────────┤
                          │uint8_t          │ Uinteger8Probe  │ stats/model/uinteger-8-probe.h  │
                          ├─────────────────┼─────────────────┼─────────────────────────────────┤
                          │uint16_t         │ Uinteger16Probe │ stats/model/uinteger-16-probe.h │
                          ├─────────────────┼─────────────────┼─────────────────────────────────┤
                          │uint32_t         │ Uinteger32Probe │ stats/model/uinteger-32-probe.h │
                          ├─────────────────┼─────────────────┼─────────────────────────────────┤
                          │bool             │ BooleanProbe    │ stats/model/uinteger-16-probe.h │
                          ├─────────────────┼─────────────────┼─────────────────────────────────┤
                          │ns3::Time        │ TimeProbe       │ stats/model/time-probe.h        │
                          └─────────────────┴─────────────────┴─────────────────────────────────┘

       The following TraceSource types are supported by Probes as of this writing:

     ┌───────────────────────┬────────────────────────┬───────────────┬───────────────────────────────────────────────┐
     │TracedSource type      │ Probe type             │ Probe outputs │ File                                          │
     ├───────────────────────┼────────────────────────┼───────────────┼───────────────────────────────────────────────┤
     │Ptr<const Packet>      │ PacketProbe            │ OutputBytes   │ network/utils/packet-probe.h                  │
     ├───────────────────────┼────────────────────────┼───────────────┼───────────────────────────────────────────────┤
     │Ptr<const     Packet>, │ Ipv4PacketProbe        │ OutputBytes   │ internet/model/ipv4-packet-probe.h            │
     │Ptr<Ipv4>, uint32_t    │                        │               │                                               │
     ├───────────────────────┼────────────────────────┼───────────────┼───────────────────────────────────────────────┤
     │Ptr<const     Packet>, │ Ipv6PacketProbe        │ OutputBytes   │ internet/model/ipv6-packet-probe.h            │
     │Ptr<Ipv6>, uint32_t    │                        │               │                                               │
     ├───────────────────────┼────────────────────────┼───────────────┼───────────────────────────────────────────────┤
     │Ptr<const     Packet>, │ Ipv6PacketProbe        │ OutputBytes   │ internet/model/ipv6-packet-probe.h            │
     │Ptr<Ipv6>, uint32_t    │                        │               │                                               │
     ├───────────────────────┼────────────────────────┼───────────────┼───────────────────────────────────────────────┤
     │Ptr<const     Packet>, │ ApplicationPacketProbe │ OutputBytes   │ applications/model/application-packet-probe.h │
     │const Address&         │                        │               │                                               │
     └───────────────────────┴────────────────────────┴───────────────┴───────────────────────────────────────────────┘

       As can be seen, only a few trace sources are supported, and they are all oriented towards outputting  the
       Packet  size  (in  bytes).   However, most of the fundamental data types available as TracedValues can be
       supported with these helpers.

   FileHelper
       The FileHelper class is just a variation of the previous  GnuplotHelper  example.   The  example  program
       provides formatted output of the same timestamped data, such as follows:

          Time (Seconds) = 9.312e+00    Packet Byte Count = 596
          Time (Seconds) = 9.312e+00    Packet Byte Count = 564

       Two  files  are  provided,  one for node "0" and one for node "1" as can be seen in the filenames.  Let's
       look at the code piece-by-piece:

          +   // Use FileHelper to write out the packet byte count over time
          +   FileHelper fileHelper;
          +
          +   // Configure the file to be written, and the formatting of output data.
          +   fileHelper.ConfigureFile ("seventh-packet-byte-count",
          +                             FileAggregator::FORMATTED);

       The file helper file prefix is the first argument, and a format specifier is next.   Some  other  options
       for formatting include SPACE_SEPARATED, COMMA_SEPARATED, and TAB_SEPARATED.  Users are able to change the
       formatting (if FORMATTED is specified) with a format string such as follows:

          +
          +   // Set the labels for this formatted output file.
          +   fileHelper.Set2dFormat ("Time (Seconds) = %.3e\tPacket Byte Count = %.0f");

       Finally, the trace source of interest must be hooked.  Again, the probeType and  tracePath  variables  in
       this example are used, and the probe's output trace source "OutputBytes" is hooked:

          +
          +   // Specify the probe type, trace source path (in configuration namespace), and
          +   // probe output trace source ("OutputBytes") to write.
          +   fileHelper.WriteProbe (probeType,
          +                          tracePath,
          +                          "OutputBytes");
          +

       The  wildcard  fields  in  this trace source specifier match two trace sources.  Unlike the GnuplotHelper
       example, in which two data series were overlaid on the same plot, here, two separate files are written to
       disk.

   Summary
       Data collection support is new as of ns-3.18, and basic support for providing time series output has been
       added.  The basic pattern described above may be replicated within the scope of support of  the  existing
       probes  and  trace  sources.   More  capabilities including statistics processing will be added in future
       releases.

CONCLUSION

   Futures
       This document is intended as a living document.  We hope and expect it to grow over time  to  cover  more
       and more of the nuts and bolts of ns-3.

       Writing  manual and tutorial chapters is not something we all get excited about, but it is very important
       to the project.  If you are an expert in one of these areas, please  consider  contributing  to  ns-3  by
       providing one of these chapters; or any other chapter you may think is important.

   Closing
       ns-3  is  a  large  and complicated system.  It is impossible to cover all of the things you will need to
       know in one small tutorial.  Readers who want  to  learn  more  are  encouraged  to  read  the  following
       additional documentation:

       • The ns-3 manual

       • The ns-3 model library documentation

       • The ns-3 Doxygen (API documentation)

       • The ns-3 wiki

       -- The ns-3 development team.

AUTHOR

       ns-3 project

       2010, ns-3 project