Provided by: ns3-doc_3.27+dfsg-1_all 
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 tree
• ns-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 controlled, 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
• wiki
• Contributed 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-3.27.tar.bz2
$ tar xjf ns-allinone-3.27.tar.bz2
If you change into the directory ns-allinone-3.27 you should see a number of files and directories:
$ ls
bake constants.py ns-3.27 README
build.py netanim-3.108 pybindgen-0.17.0.post58+ngcf00cc0 util.py
You are now ready to build the base ns-3 distribution and may skip ahead to the section on building ns-3.
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-3.27: 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-3.27: 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-3.27.
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-3.27
Next, we’ll 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 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 setuptools - OK
>> Searching for system dependency pygoocanvas - OK
>> Searching for system dependency pygraphviz - OK
>> Searching for system dependency python-dev - OK
>> Searching for system dependency libxml2-dev - OK
>> Searching for system dependency clang-dev - OK
>> Downloading click-ns-3.25 - OK
>> Downloading BRITE - OK
>> Searching for system dependency qt4 - OK
>> Downloading nsc-0.5.3 - OK
>> Searching for system dependency g++ - OK
>> Downloading castxml - OK
>> Downloading openflow-ns-3.25 - OK
>> Downloading netanim-3.108 - OK
>> Downloading pygccxml-1.9.1 - OK
>> Downloading pygccxml - OK
>> Downloading pybindgen-ns3.27-castxml (target directory:pybindgen) - OK
>> Downloading ns-3.27 - OK
The above suggests that seven sources have been downloaded. Check the source directory now and type ls;
one should see:
$ ls
BRITE netanim-3.108 openflow-ns-3.25 pygccxml-1.9.1
castxml ns-3.27 pybindgen v1.9.1.tar.gz
click-ns-3.25 nsc-0.5.3 pygccxml
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-3.27
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-3.27/ns-3.27/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
internet-apps lr-wpan lte
mesh mobility mpi
netanim (no Python) network nix-vector-routing
olsr openflow (no Python) point-to-point
point-to-point-layout propagation sixlowpan
spectrum stats tap-bridge
test (no Python) topology-read traffic-control
uan virtual-net-device visualizer
wave wifi wimax
Modules not built (see ns-3 tutorial for explanation):
brite click
Regarding the portion about modules not built:
Modules not built (see ns-3 tutorial for explanation):
brite click
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 BRITE - OK
>> Building nsc-0.5.3 - OK
>> Building click-ns-3.25 - OK
...
>> Building ns-3.27 - OK
Hint: you can also perform both steps, download and build, by calling ``bake.py deploy``.
There may be failures to build all components, but the build will proceed anyway if the component is
optional. For example, a common issue at the moment is that castxml may not build via the bake build
tool on all platforms; in this case, the line will show:
>> Building castxml - Problem
> Problem: Optional dependency, module "castxml" failed
This may reduce the functionality of the final build.
However, bake will continue since "castxml" is not an essential dependency.
For more information call bake with -v or -vvv, for full verbose mode.
However, castxml is only needed if one wants to generate updated Python bindings, and most users do not
need to do so (or to do so until they are more involved with ns-3 changes), so such warnings might be
safely ignored for now.
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 configure --build-profile=optimized --enable-examples --enable-tests
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
BRITE Integration : not enabled (BRITE not enabled (see option --with-brite))
Build directory : build
Build examples : enabled
Build tests : enabled
Emulated Net Device : enabled (<netpacket/packet.h> include not detected)
Emulation FdNetDevice : not enabled (needs netpacket/packet.h)
File descriptor NetDevice : enabled
GNU Scientific Library (GSL) : enabled
GtkConfigStore : enabled
MPI Support : enabled
NS-3 Click Integration : not enabled (nsclick not enabled (see option --with-nsclick))
NS-3 OpenFlow Integration : not enabled (Required boost libraries not found, missing: system, signals, filesystem)
Network Simulation Cradle : not enabled (NSC not found (see option --with-nsc))
PlanetLab FdNetDevice : not enabled (PlanetLab operating system not detected (see option --force-planetlab))
PyViz visualizer : enabled
Python Bindings : enabled
Real Time Simulator : enabled (librt is not available)
SQlite stats data output : enabled
Tap Bridge : not enabled (<linux/if_tun.h> include not detected)
Tap FdNetDevice : not enabled (needs linux/if_tun.h)
Threading Primitives : enabled
Use sudo to set suid bit : not enabled (option --enable-sudo not selected)
XmlIo : 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.” Finally, to reprint this summary of which optional features are enabled, use the
--check-config option to waf.
Now go ahead and switch back to the debug build that includes the examples and tests.
$ ./waf clean
$ ./waf configure --build-profile=debug --enable-examples --enable-tests
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.
A command exists for checking which profile is currently active for an already configured project:
$ ./waf --check-profile
Waf: Entering directory \`/path/to/ns-3-allinone/ns-3.27/build'
Build profile: debug
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.
The build profile controls the use of logging, assertions, and compiler optimization:
──────────────────────────────────────────────────────────────────────────────────────────────────────────
Feature Build Profile
──────────────────────────────────────────────────────────────────────────────────────────────────────────
debug release optimized
──────────────────────────────────────────────────────────────────────────────────────────────────────────
Enabled Features NS3_BUILD_PROFILE_DEBUG NS3_BUILD_PROFILE_RELEASE NS3_BUILD_PROFILE_OPTIMIZED
NS_LOG...
NS_ASSERT...
──────────────────────────────────────────────────────────────────────────────────────────────────────────
Code Wrapper Macro NS_BUILD_DEBUG(code) NS_BUILD_RELEASE(code) NS_BUILD_OPTIMIZED(code)
──────────────────────────────────────────────────────────────────────────────────────────────────────────
Compiler Flags -O0 -ggdb -g3 -O3 -g0 -O3 -g -fstrict-overflow
-fomit-frame-pointer -march=native
┌────────────────────┬─────────────────────────┬───────────────────────────┬─────────────────────────────┐
│ │ │ │ │
--
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 computer 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 UsingLogging section, so
we’ll get to it later in this tutorial, but you can find out about the above statement by looking at the
Core module, then expanding the Debugging tools book, and then selecting the Logging page. Click on
Logging.
You should now be looking at the Doxygen documentation for the Logging module. In the list of Macros’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 propagation
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 will 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"
A natural question to arise at this point is how to learn about the existence of all of these attributes.
Again, the command line help facility has a feature for this. If we ask for command line help we should
see:
$ ./waf --run "scratch/myfirst --PrintHelp"
myfirst [Program Arguments] [General Arguments]
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.
If you select the “PrintGroups” argument, you should see a list of all registered TypeId groups. The
group names are aligned with the module names in the source directory (although with a leading capital
letter). Printing out all of the information at once would be too much, so a further filter is available
to print information on a per-group basis. So, focusing again on the point-to-point module:
./waf --run "scratch/myfirst --PrintGroup=PointToPoint"
TypeIds in group PointToPoint:
ns3::PointToPointChannel
ns3::PointToPointNetDevice
ns3::PointToPointRemoteChannel
ns3::PppHeader
and from here, one can find the possible TypeId names to search for attributes, such as in the
--PrintAttributes=ns3::PointToPointChannel example shown above.
Another way to find out about attributes is through the ns-3 Doxygen; there is a page that lists out all
of the registered attributes in the simulator.
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 2.017607 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 Wi-Fi 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 Install 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 Wi-Fi 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. WifiMacHelper object is used to set MAC parameters.
WifiHelper wifi;
wifi.SetRemoteStationManager ("ns3::AarfWifiManager");
WifiMacHelper mac;
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. “QosSupported”
Attribute is set to false by default for WifiMacHelper objects. 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 Wi-Fi devices of these stations:
NetDeviceContainer staDevices;
staDevices = wifi.Install (phy, mac, wifiStaNodes);
We have configured Wi-Fi 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 WifiMacHelper to reflect the
requirements of the AP.
mac.SetType ("ns3::ApWifiMac",
"Ssid", SsidValue (ssid));
In this case, the WifiMacHelper is going to create MAC layers of the “ns3::ApWifiMac”, the latter
specifying that a MAC instance configured as an AP should be created. We do not change the default
setting of “QosSupported” Attribute, so it remains 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 Wi-Fi 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 Wi-Fi 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
Wi-Fi 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 Wi-Fi 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 left 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 Wi-Fi 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 Wi-Fi 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::ProcessEstablished () there is no log message for the reception of a SYN+ACK in
ESTABLISHED state. You could simply add one, changing the code. Here is the original:
/* Received a packet upon ESTABLISHED state. This function is mimicking the
role of tcp_rcv_established() in tcp_input.c in Linux kernel. */
void
TcpSocketBase::ProcessEstablished (Ptr<Packet> packet, const TcpHeader& tcpHeader)
{
NS_LOG_FUNCTION (this << tcpHeader);
...
else if (tcpflags == (TcpHeader::SYN | TcpHeader::ACK))
{ // No action for received SYN+ACK, it is probably a duplicated packet
}
...
To log the SYN+ACK case, you can add a new NS_LOG_LOGIC in the if statement body:
/* Received a packet upon ESTABLISHED state. This function is mimicking the
role of tcp_rcv_established() in tcp_input.c in Linux kernel. */
void
TcpSocketBase::ProcessEstablished (Ptr<Packet> packet, const TcpHeader& tcpHeader)
{
NS_LOG_FUNCTION (this << tcpHeader);
...
else if (tcpflags == (TcpHeader::SYN | TcpHeader::ACK))
{ // No action for received SYN+ACK, it is probably a duplicated packet
NS_LOG_LOGIC ("TcpSocketBase " << this << " ignoring SYN+ACK");
}
...
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 need 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 ())
.SetGroupName ("MyGroup")
.AddConstructor<MyObject> ()
.AddTraceSource ("MyInteger",
"An integer value to trace.",
MakeTraceSourceAccessor (&MyObject::m_myInt),
"ns3::TracedValueCallback::Int32")
;
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 traceSink for this TracedValue will
need the signature
void (* traceSink)(int32_t oldValue, 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)(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 (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 (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 remaining (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^=
┌──────────────────────────────────────┬─────────────┐
│ │ │
--
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> ()
.SetGroupName ("Stats")
.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
COPYRIGHT
2018, ns-3 project
ns-3.27 1516823160 NS-3-TUTORIAL(1)