ns-3-tutorial - Online in the Cloud

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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 controllled, reproducible environment, and to learn about how networks work.
Users will note that the available model set in ns-3 focuses on modeling how Internet
protocols and networks work, but ns-3 is not limited to Internet systems; several users
are using ns-3 to model non-Internet-based systems.

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

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

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

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

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

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

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

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

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

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

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

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

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

· Open source licensing based on GNU GPLv2 compatibility

· 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-3.22.tar.bz2
$ tar xjf ns-allinone-3.22.tar.bz2

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

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

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

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

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

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

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

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

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

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

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

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

There are a few configuration targets available:

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

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

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

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

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

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

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

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

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

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

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

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

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

$ ./bake.py check

You should see something like the following,

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

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

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

Next, try to download the software:

$ ./bake.py download

should yield something like:

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

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

$ ls
netanim-3.105 ns-3.22 pybindgen-0.16.0.886

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

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

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

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

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

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

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

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

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

Leaving directory `./ns-3.22'

Regarding the portion about modules not built:

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

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

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

$ ./bake.py build

and you should see something like:

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

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

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

$ ./bake.py show

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

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

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

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

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

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

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

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

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

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

$ ./waf

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

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

$ ./build.py --disable-python

will result in

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

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

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

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

Here are a few more introductory tips about Waf.

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

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

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

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

$ ./waf --help

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

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

$ ./waf --build-profile=debug

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

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

$ ./waf configure --out=foo

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Even better is to define this as a shell function:

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

$ waff build

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

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

$ cd scratch
$ waff build

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

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

$ ./test.py -c core

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

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

This is the important message.

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

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

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

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

...

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

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

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

$ ./waf --run hello-simulator

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

Hello Simulator

Congratulations! You are now an ns-3 user!

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

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

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

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

$ ./waf

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

$ ./waf --cwd=...

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

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

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

CONCEPTUAL OVERVIEW

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

$ ./waf

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

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

using namespace ns3;

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

Logging
The next line of the script is the following,

NS_LOG_COMPONENT_DEFINE ("FirstScriptExample");

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

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

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

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

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

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

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

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

Time::SetResolution (Time::NS);

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

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

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

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

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

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

NodeContainer nodes;
nodes.Create (2);

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

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

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

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

The next three lines in the script are,

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

The first line,

PointToPointHelper pointToPoint;

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

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

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

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

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

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

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

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

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

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

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

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

InternetStackHelper stack;
stack.Install (nodes);

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

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

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

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

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

The next line of code,

Ipv4InterfaceContainer interfaces = address.Assign (devices);

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

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

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

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

UdpEchoServerHelper echoServer (9);

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Simulator::Run ();

When we previously called the methods,

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

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

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

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

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

Simulator::Destroy ();
return 0;
}

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

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

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

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

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

· FlowMonitor - periodic check for lost packets

· RIPng - periodic broadcast of routing tables update

· etc.

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

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

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

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

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

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

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

Now build your first example script using waf:

$ ./waf

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

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

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

$ ./waf --run scratch/myfirst

You should see some output:

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

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

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

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

summary | shortlog | changelog | graph | tags | files

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

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

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

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

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

TWEAKING

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

$ ./waf --run scratch/myfirst

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

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

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

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

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

LogComponentEnable("UdpEchoClientApplication", LOG_LEVEL_INFO);

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

$ export NS_LOG=UdpEchoClientApplication=level_all

This sets the shell environment variable NS_LOG to the string,

UdpEchoClientApplication=level_all

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

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

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

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

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

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

$ export 'NS_LOG=UdpEchoClientApplication=level_all|prefix_func'

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

NS_LOG_COMPONENT_DEFINE ("FirstScriptExample");

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

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

NS_LOG_INFO ("Creating Topology");

right before the lines,

NodeContainer nodes;
nodes.Create (2);

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

$ ./waf
$ export NS_LOG=

Now, if you run the script,

$ ./waf --run scratch/myfirst

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

$ export NS_LOG=FirstScriptExample=info

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

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

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

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

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

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

...
}

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

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

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

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

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

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

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

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

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

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

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

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

...

NodeContainer nodes;
nodes.Create (2);

PointToPointHelper pointToPoint;

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

...

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

$ export 'NS_LOG=UdpEchoServerApplication=level_all|prefix_time'

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

...

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

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

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

Try,

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

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

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

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

You should now see

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

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

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

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

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

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

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

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

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

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

The ns-3 tracing system is built on the concepts of independent tracing sources and
tracing sinks, and a uniform mechanism for connecting sources to sinks. Trace sources are
entities that can signal events that happen in a simulation and provide access to
interesting underlying data. For example, a trace source could indicate when a packet is
received by a net device and provide access to the packet contents for interested trace
sinks.

Trace sources are not useful by themselves, they must be "connected" to other pieces of
code that actually do something useful with the information provided by the sink. Trace
sinks are consumers of the events and data provided by the trace sources. For example,
one could create a trace sink that would (when connected to the trace source of the
previous example) print out interesting parts of the received packet.

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

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

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

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

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

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

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

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

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

$ ./waf --run scratch/myfirst

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

pointToPoint.EnablePcapAll ("myfirst");

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

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

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

$ ./waf --run scratch/myfirst

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

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

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

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

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

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

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

BUILDING TOPOLOGIES

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

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

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

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

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

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

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

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

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

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

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

using namespace ns3;

NS_LOG_COMPONENT_DEFINE ("SecondScriptExample");

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

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

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

bool verbose = true;
uint32_t nCsma = 3;

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

cmd.Parse (argc, argv);

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

nCsma = nCsma == 0 ? 1 : nCsma;

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

NodeContainer p2pNodes;
p2pNodes.Create (2);

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

UdpEchoServerHelper echoServer (9);

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

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

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

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

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

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

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

Ipv4GlobalRoutingHelper::PopulateRoutingTables ();

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

You should see the contents of the pcap file displayed:

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

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

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

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

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

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

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

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

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

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

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

This exchange is seen in the following lines,

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

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

2.007803 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024

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

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

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

2.013921 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024

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

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

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

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

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

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

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

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

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

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

You should now see,

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

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

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

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

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

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

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

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

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

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

$ rm *.pcap
$ rm *.tr

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

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

you will see the following output:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The network topology illustration follows:

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

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

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

using namespace ns3;

NS_LOG_COMPONENT_DEFINE ("ThirdScriptExample");

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

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

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

cmd.Parse (argc,argv);

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

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

NodeContainer p2pNodes;
p2pNodes.Create (2);

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

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

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

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

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

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

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

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

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

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

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

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

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

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

phy.SetChannel (channel.Create ());

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

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

NqosWifiMacHelper mac = NqosWifiMacHelper::Default ();

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

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

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

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

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

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

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

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

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

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

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

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

MobilityHelper mobility;

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

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

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

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

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

mobility.Install (wifiStaNodes);

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

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

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

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

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

Ipv4AddressHelper address;

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

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

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

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

UdpEchoServerHelper echoServer (9);

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

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

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

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

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

Ipv4GlobalRoutingHelper::PopulateRoutingTables ();

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

Simulator::Stop (Seconds (10.0));

We create just enough tracing to cover all three networks:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Again, you should see some familiar looking contents:

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

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

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

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

Again, you should see some familiar looking contents:

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

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

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

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

You should see some familiar looking contents:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

TRACING

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Trace sources are not useful by themselves; they must be connected to other pieces of code
that actually do something useful with the information provided by the source. The
entities that consume trace information are called trace sinks. Trace sources are
generators of data and trace sinks are consumers. This explicit division allows for large
numbers of trace sources to be scattered around the system in places which model authors
believe might be useful. Inserting trace sources introduces a very small execution
overhead.

There can be zero or more consumers of trace events generated by a trace source. One can
think of a trace source as a kind of point-to-multipoint information link. Your code
looking for trace events from a particular piece of core code could happily coexist with
other code doing something entirely different from the same information.

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

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

Callbacks
The goal of the Callback system in ns-3 is to allow one piece of code to call a function
(or method in C++) without any specific inter-module dependency. This ultimately means
you need some kind of indirection -- you treat the address of the called function as a
variable. This variable is called a pointer-to-function variable. The relationship
between function and pointer-to-function is really no different that that of object and
pointer-to-object.

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

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

(But read the C++-FAQ Section 33 before writing code like this!) What you get from this
is a variable named simply pfi that is initialized to the value 0. If you want to
initialize this pointer to something meaningful, you have to have a function with a
matching signature. In this case, you could provide a function that looks like:

int MyFunction (int arg) {}

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

pfi = MyFunction;

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

int result = (*pfi) (1234);

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

int result = pfi (1234);

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

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

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

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

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

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

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

#include <iostream>

using namespace ns3;

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

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

Since the tracing system is integrated with Attributes, and Attributes work with Objects,
there must be an ns-3 Object for the trace source to live in. The next code snippet
declares and defines a simple Object we can work with.

class MyObject : public Object
{
public:
static TypeId GetTypeId (void)
{
static TypeId tid = TypeId ("MyObject")
.SetParent (Object::GetTypeId ())
.AddConstructor<MyObject> ()
.AddTraceSource ("MyInteger",
"An integer value to trace.",
MakeTraceSourceAccessor (&MyObject::m_myInt),
"ns3::Traced::Value::Int32Callback")
;
return tid;
}

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

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

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

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

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

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

Sure enough, continuing through fourth.cc we see:

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

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

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

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

myObject->m_myInt = 1234;
}

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

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

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

Let's ignore the bit about context for now.

Finally, the line assigning a value to m_myInt:

myObject->m_myInt = 1234;

should be interpreted as an invocation of operator= on the member variable m_myInt with
the integer 1234 passed as a parameter.

Since m_myInt is a TracedValue, this operator is defined to execute a callback that
returns void and takes two integer values as parameters --- an old value and a new value
for the integer in question. That is exactly the function signature for the callback
function we provided --- IntTrace.

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

If you now build and run this example,

$ ./waf --run fourth

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

Traced 0 to 1234

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

Connect with Config
The TraceConnectWithoutContext call shown above in the simple example is actually very
rarely used in the system. More typically, the Config subsystem is used to select a trace
source in the system using what is called a Config path. We saw an example of this in the
previous section where we hooked the "CourseChange" event when we were experimenting with
third.cc.

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

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

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

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

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

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

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

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

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

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

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

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

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

The leading "/" character in the path refers to a so-called namespace. One of the
predefined namespaces in the config system is "NodeList" which is a list of all of the
nodes in the simulation. Items in the list are referred to by indices into the list, so
"/NodeList/7" refers to the eighth Node in the list of nodes created during the simulation
(recall indices start at 0'). This reference is actually a ``Ptr<Node>` and so is a
subclass of an ns3::Object.

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

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

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

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

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

which should look very familiar at this point.

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

TracedCallback<Ptr<const MobilityModel> > m_courseChangeTrace;

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

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

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

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

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

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

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

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

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

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

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

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

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

We'll address each of these in turn.

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

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

In the sidebar you should see a hierachy that begins

· ns-3

· ns-3 Documentation

· All TraceSources

· All Attributes

· All GlobalValues

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

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

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

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

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

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

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

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

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

First we'll discuss the Config paths.

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

If you now scroll down to the "Detailed Description" section, after the summary list of
class methods and attributes (or just click on the "More..." link at the end of the class
brief description at the top of the page) you will see the overall documentation for the
class. Continuing to scroll down, find the "Config Paths" list:
Config Paths

ns3::RandomWalk2dMobilityModel is accessible through the following paths with
Config::Set and Config::Connect:

· "/NodeList/[i]/$ns3::MobilityModel/$ns3::RandomWalk2dMobilityModel"

The documentation tells you how to get to the RandomWalk2dMobilityModel Object. Compare
the string above with the string we actually used in the example code:

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

The difference is due to the fact that two GetObject calls are implied in the string found
in the documentation. The first, for $ns3::MobilityModel will query the aggregation for
the base class. The second implied GetObject call, for $ns3::RandomWalk2dMobilityModel,
is used to cast the base class to the concrete implementation class. The documentation
shows both of these operations for you. It turns out that the actual trace source you are
looking for is found in the base class.

Look further down in the "Detailed Description" section for the list of trace sources.
You will find
No TraceSources are defined for this type.

TraceSources defined in parent class ``ns3::MobilityModel``

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

Callback signature: ns3::MobilityModel::CourseChangeCallback

This is exactly what you need to know. The trace source of interest is found in
ns3::MobilityModel (which you knew anyway). The interesting thing this bit of API
Documentation tells you is that you don't need that extra cast in the config path above to
get to the concrete class, since the trace source is actually in the base class.
Therefore the additional GetObject is not required and you simply use the path:

"/NodeList/[i]/$ns3::MobilityModel"

which perfectly matches the example path:

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

As an aside, another way to find the Config path is to grep around in the ns-3 codebase
for someone who has already figured it out. You should always try to copy someone else's
working code before you start to write your own. Try something like:

$ find . -name '*.cc' | xargs grep CourseChange | grep Connect

and you may find your answer along with working code. For example, in this case,
src/mobility/examples/main-random-topology.cc has something just waiting for you to use:

Config::Connect ("/NodeList/*/$ns3::MobilityModel/CourseChange",
MakeCallback (&CourseChange));

We'll return to this example in a moment.

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

The easiest way is to examine the callback signature typedef, which is given in the
"Callback signature" of the trace source in the "Detailed Description" for the class, as
shown above.

Repeating the "CourseChange" trace source entry from ns3::RandomWalk2dMobilityModel we
have:

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

Callback signature: ns3::MobilityModel::CourseChangeCallback

The callback signature is given as a link to the relevant typedef, where we find
typedef void (* CourseChangeCallback)(const std::string context, Ptr<const
MobilityModel> * model);

TracedCallback signature for course change notifications.

If the callback is connected using ConnectWithoutContext omit the context argument from
the signature.

Parameters:
[in] context The context string supplied by the Trace source.
[in] model The MobilityModel which is changing course.

As above, to see this in use grep around in the ns-3 codebase for an example. The example
above, from src/mobility/examples/main-random-topology.cc, connects the "CourseChange"
trace source to the CourseChange function in the same file:

static void
CourseChange (std::string context, Ptr<const MobilityModel> model)
{
...
}

Notice that this function:

· Takes a "context" string argument, which we'll describe in a minute. (If the callback
is connected using the ConnectWithoutContext function the context argument will be
omitted.)

· Has the MobilityModel supplied as the last argument (or only argument if
ConnectWithoutContext is used).

· Returns void.

If, by chance, the callback signature hasn't been documented, and there are no examples to
work from, determining the right callback function signature can be, well, challenging to
actually figure out from the source code.

Before embarking on a walkthrough of the code, I'll be kind and just tell you a simple way
to figure this out: The return value of your callback will always be void. The formal
parameter list for a TracedCallback can be found from the template parameter list in the
declaration. Recall that for our current example, this is in mobility-model.h, where we
have previously found:

TracedCallback<Ptr<const MobilityModel> > m_courseChangeTrace;

There is a one-to-one correspondence between the template parameter list in the
declaration and the formal arguments of the callback function. Here, there is one
template parameter, which is a Ptr<const MobilityModel>. This tells you that you need a
function that returns void and takes a Ptr<const MobilityModel>. For example:

void
CourseChange (Ptr<const MobilityModel> model)
{
...
}

That's all you need if you want to Config::ConnectWithoutContext. If you want a context,
you need to Config::Connect and use a Callback function that takes a string context, then
the template arguments:

void
CourseChange (const std::string context, Ptr<const MobilityModel> model)
{
...
}

If you want to ensure that your CourseChangeCallback function is only visible in your
local file, you can add the keyword static and come up with:

static void
CourseChange (const std::string path, Ptr<const MobilityModel> model)
{
...
}

which is exactly what we used in the third.cc example.

Implementation
This section is entirely optional. It is going to be a bumpy ride, especially for those
unfamiliar with the details of templates. However, if you get through this, you will have
a very good handle on a lot of the ns-3 low level idioms.

So, again, let's figure out what signature of callback function is required for the
"CourseChange" trace source. This is going to be painful, but you only need to do this
once. After you get through this, you will be able to just look at a TracedCallback and
understand it.

The first thing we need to look at is the declaration of the trace source. Recall that
this is in mobility-model.h, where we have previously found:

TracedCallback<Ptr<const MobilityModel> > m_courseChangeTrace;

This declaration is for a template. The template parameter is inside the angle-brackets,
so we are really interested in finding out what that TracedCallback<> is. If you have
absolutely no idea where this might be found, grep is your friend.

We are probably going to be interested in some kind of declaration in the ns-3 source, so
first change into the src directory. Then, we know this declaration is going to have to
be in some kind of header file, so just grep for it using:

$ find . -name '*.h' | xargs grep TracedCallback

You'll see 303 lines fly by (I piped this through wc to see how bad it was). Although
that may seem like a lot, that's not really a lot. Just pipe the output through more and
start scanning through it. On the first page, you will see some very suspiciously
template-looking stuff.

TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::TracedCallback ()
TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::ConnectWithoutContext (c ...
TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::Connect (const CallbackB ...
TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::DisconnectWithoutContext ...
TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::Disconnect (const Callba ...
TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (void) const ...
TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1) const ...
TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ...
TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ...
TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ...
TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ...
TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ...
TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ...

It turns out that all of this comes from the header file traced-callback.h which sounds
very promising. You can then take a look at mobility-model.h and see that there is a line
which confirms this hunch:

#include "ns3/traced-callback.h"

Of course, you could have gone at this from the other direction and started by looking at
the includes in mobility-model.h and noticing the include of traced-callback.h and
inferring that this must be the file you want.

In either case, the next step is to take a look at src/core/model/traced-callback.h in
your favorite editor to see what is happening.

You will see a comment at the top of the file that should be comforting:
An ns3::TracedCallback has almost exactly the same API as a normal ns3::Callback but
instead of forwarding calls to a single function (as an ns3::Callback normally does),
it forwards calls to a chain of ns3::Callback.

This should sound very familiar and let you know you are on the right track.

Just after this comment, you will find

template<typename T1 = empty, typename T2 = empty,
typename T3 = empty, typename T4 = empty,
typename T5 = empty, typename T6 = empty,
typename T7 = empty, typename T8 = empty>
class TracedCallback
{
...

This tells you that TracedCallback is a templated class. It has eight possible type
parameters with default values. Go back and compare this with the declaration you are
trying to understand:

TracedCallback<Ptr<const MobilityModel> > m_courseChangeTrace;

The typename T1 in the templated class declaration corresponds to the Ptr<const
MobilityModel> in the declaration above. All of the other type parameters are left as
defaults. Looking at the constructor really doesn't tell you much. The one place where
you have seen a connection made between your Callback function and the tracing system is
in the Connect and ConnectWithoutContext functions. If you scroll down, you will see a
ConnectWithoutContext method here:

template<typename T1, typename T2,
typename T3, typename T4,
typename T5, typename T6,
typename T7, typename T8>
void
TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::ConnectWithoutContext ...
{
Callback<void,T1,T2,T3,T4,T5,T6,T7,T8> cb;
cb.Assign (callback);
m_callbackList.push_back (cb);
}

You are now in the belly of the beast. When the template is instantiated for the
declaration above, the compiler will replace T1 with Ptr<const MobilityModel>.

void
TracedCallback<Ptr<const MobilityModel>::ConnectWithoutContext ... cb
{
Callback<void, Ptr<const MobilityModel> > cb;
cb.Assign (callback);
m_callbackList.push_back (cb);
}

You can now see the implementation of everything we've been talking about. The code
creates a Callback of the right type and assigns your function to it. This is the
equivalent of the pfi = MyFunction we discussed at the start of this section. The code
then adds the Callback to the list of Callbacks for this source. The only thing left is
to look at the definition of Callback. Using the same grep trick as we used to find
TracedCallback, you will be able to find that the file ./core/callback.h is the one we
need to look at.

If you look down through the file, you will see a lot of probably almost incomprehensible
template code. You will eventually come to some API Documentation for the Callback
template class, though. Fortunately, there is some English:
Callback template class.

This class template implements the Functor Design Pattern. It is used to declare the
type of a Callback:

· the first non-optional template argument represents the return type of the callback.

· the reminaining (optional) template arguments represent the type of the subsequent
arguments to the callback.

· up to nine arguments are supported.

We are trying to figure out what the

Callback<void, Ptr<const MobilityModel> > cb;

declaration means. Now we are in a position to understand that the first (non-optional)
template argument, void, represents the return type of the Callback. The second
(optional) template argument, Ptr<const MobilityModel> represents the type of the first
argument to the callback.

The Callback in question is your function to receive the trace events. From this you can
infer that you need a function that returns void and takes a Ptr<const MobilityModel>.
For example,

void
CourseChangeCallback (Ptr<const MobilityModel> model)
{
...
}

That's all you need if you want to Config::ConnectWithoutContext. If you want a context,
you need to Config::Connect and use a Callback function that takes a string context. This
is because the Connect function will provide the context for you. You'll need:

void
CourseChangeCallback (std::string context, Ptr<const MobilityModel> model)
{
...
}

If you want to ensure that your CourseChangeCallback is only visible in your local file,
you can add the keyword static and come up with:

static void
CourseChangeCallback (std::string path, Ptr<const MobilityModel> model)
{
...
}

which is exactly what we used in the third.cc example. Perhaps you should now go back and
reread the previous section (Take My Word for It).

If you are interested in more details regarding the implementation of Callbacks, feel free
to take a look at the ns-3 manual. They are one of the most frequently used constructs in
the low-level parts of ns-3. It is, in my opinion, a quite elegant thing.

TracedValues
Earlier in this section, we presented a simple piece of code that used a
TracedValue<int32_t> to demonstrate the basics of the tracing code. We just glossed over
the what a TracedValue really is and how to find the return type and formal arguments for
the callback.

As we mentioned, the file, traced-value.h brings in the required declarations for tracing
of data that obeys value semantics. In general, value semantics just means that you can
pass the object itself around, rather than passing the address of the object. We extend
that requirement to include the full set of assignment-style operators that are
pre-defined for plain-old-data (POD) types:

┌─────────────────────────────────┬─────────────┐
│operator= (assignment) │ │
├─────────────────────────────────┼─────────────┤
│operator*= │ operator/= │
├─────────────────────────────────┼─────────────┤
│operator+= │ operator-= │
├─────────────────────────────────┼─────────────┤
│operator++ (both prefix and │ │
│postfix) │ │
├─────────────────────────────────┼─────────────┤
│operator-- (both prefix and │ │
│postfix) │ │
├─────────────────────────────────┼─────────────┤
│operator<<= │ operator>>= │
├─────────────────────────────────┼─────────────┤
│operator&= │ operator|= │
├─────────────────────────────────┼─────────────┤
│operator%= │ operator^= │
└─────────────────────────────────┴─────────────┘

What this all really means is that you will be able to trace all changes made using those
operators to a C++ object which has value semantics.

The TracedValue<> declaration we saw above provides the infrastructure that overloads the
operators mentioned above and drives the callback process. On use of any of the operators
above with a TracedValue it will provide both the old and the new value of that variable,
in this case an int32_t value. By inspection of the TracedValue declaration, we know the
trace sink function will have arguments (const int32_t oldValue, const int32_t newValue).
The return type for a TracedValue callback function is always void, so the expected
callback signature will be:

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

The .AddTraceSource in the GetTypeId method provides the "hooks" used for connecting the
trace source to the outside world through the Config system. We already discussed the
first three agruments to AddTraceSource: the Attribute name for the Config system, a help
string, and the address of the TracedValue class data member.

The final string argument, "ns3::Traced::Value::Int32" in the example, is the name of a
typedef for the callback function signature. We require these signatures to be defined,
and give the fully qualified type name to AddTraceSource, so the API documentation can
link a trace source to the function signature. For TracedValue the signature is
straightforward; for TracedCallbacks we've already seen the API docs really help.

Real Example
Let's do an example taken from one of the best-known books on TCP around. "TCP/IP
Illustrated, Volume 1: The Protocols," by W. Richard Stevens is a classic. I just flipped
the book open and ran across a nice plot of both the congestion window and sequence
numbers versus time on page 366. Stevens calls this, "Figure 21.10. Value of cwnd and
send sequence number while data is being transmitted." Let's just recreate the cwnd part
of that plot in ns-3 using the tracing system and gnuplot.

Available Sources
The first thing to think about is how we want to get the data out. What is it that we
need to trace? So let's consult "All Trace Sources" list to see what we have to work
with. Recall that this is found in the ns-3 API Documentation. If you scroll through the
list, you will eventually find:
ns3::TcpNewReno

· CongestionWindow: The TCP connection's congestion window

· SlowStartThreshold: TCP slow start threshold (bytes)

It turns out that the ns-3 TCP implementation lives (mostly) in the file
src/internet/model/tcp-socket-base.cc while congestion control variants are in files such
as src/internet/model/tcp-newreno.cc. If you don't know this a priori, you can use the
recursive grep trick:

$ find . -name '*.cc' | xargs grep -i tcp

You will find page after page of instances of tcp pointing you to that file.

Bringing up the class documentation for TcpNewReno and skipping to the list of
TraceSources you will find
TraceSources

· CongestionWindow: The TCP connnection's congestion window

Callback signature: ns3::Traced::Value::Uint322Callback

Clicking on the callback typedef link we see the signature you now know to expect:

typedef void(* ns3::Traced::Value::Int32Callback)(const int32_t oldValue, const int32_t newValue)

You should now understand this code completely. If we have a pointer to the TcpNewReno,
we can TraceConnect to the "CongestionWindow" trace source if we provide an appropriate
callback target. This is the same kind of trace source that we saw in the simple example
at the start of this section, except that we are talking about uint32_t instead of
int32_t. And we know that we have to provide a callback function with that signature.

Finding Examples
It's always best to try and find working code laying around that you can modify, rather
than starting from scratch. So the first order of business now is to find some code that
already hooks the "CongestionWindow" trace source and see if we can modify it. As usual,
grep is your friend:

$ find . -name '*.cc' | xargs grep CongestionWindow

This will point out a couple of promising candidates: examples/tcp/tcp-large-transfer.cc
and src/test/ns3tcp/ns3tcp-cwnd-test-suite.cc.

We haven't visited any of the test code yet, so let's take a look there. You will
typically find that test code is fairly minimal, so this is probably a very good bet.
Open src/test/ns3tcp/ns3tcp-cwnd-test-suite.cc in your favorite editor and search for
"CongestionWindow". You will find,

ns3TcpSocket->TraceConnectWithoutContext ("CongestionWindow",
MakeCallback (&Ns3TcpCwndTestCase1::CwndChange, this));

This should look very familiar to you. We mentioned above that if we had a pointer to the
TcpNewReno, we could TraceConnect to the "CongestionWindow" trace source. That's exactly
what we have here; so it turns out that this line of code does exactly what we want.
Let's go ahead and extract the code we need from this function (Ns3TcpCwndTestCase1::DoRun
(void)). If you look at this function, you will find that it looks just like an ns-3
script. It turns out that is exactly what it is. It is a script run by the test
framework, so we can just pull it out and wrap it in main instead of in DoRun. Rather
than walk through this, step, by step, we have provided the file that results from porting
this test back to a native ns-3 script -- examples/tutorial/fifth.cc.

Dynamic Trace Sources
The fifth.cc example demonstrates an extremely important rule that you must understand
before using any kind of trace source: you must ensure that the target of a
Config::Connect command exists before trying to use it. This is no different than saying
an object must be instantiated before trying to call it. Although this may seem obvious
when stated this way, it does trip up many people trying to use the system for the first
time.

Let's return to basics for a moment. There are three basic execution phases that exist in
any ns-3 script. The first phase is sometimes called "Configuration Time" or "Setup
Time," and exists during the period when the main function of your script is running, but
before Simulator::Run is called. The second phase is sometimes called "Simulation Time"
and exists during the time period when Simulator::Run is actively executing its events.
After it completes executing the simulation, Simulator::Run will return control back to
the main function. When this happens, the script enters what can be called the "Teardown
Phase," which is when the structures and objects created during setup are taken apart and
released.

Perhaps the most common mistake made in trying to use the tracing system is assuming that
entities constructed dynamically during simulation time are available during configuration
time. In particular, an ns-3 Socket is a dynamic object often created by Applications to
communicate between Nodes. An ns-3 Application always has a "Start Time" and a "Stop
Time" associated with it. In the vast majority of cases, an Application will not attempt
to create a dynamic object until its StartApplication method is called at some "Start
Time". This is to ensure that the simulation is completely configured before the app
tries to do anything (what would happen if it tried to connect to a Node that didn't exist
yet during configuration time?). As a result, during the configuration phase you can't
connect a trace source to a trace sink if one of them is created dynamically during the
simulation.

The two solutions to this connundrum are

1. Create a simulator event that is run after the dynamic object is created and hook the
trace when that event is executed; or

2. Create the dynamic object at configuration time, hook it then, and give the object to
the system to use during simulation time.

We took the second approach in the fifth.cc example. This decision required us to create
the MyApp Application, the entire purpose of which is to take a Socket as a parameter.

Walkthrough: fifth.cc
Now, let's take a look at the example program we constructed by dissecting the congestion
window test. Open examples/tutorial/fifth.cc in your favorite editor. You should see
some familiar looking code:

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

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

using namespace ns3;

NS_LOG_COMPONENT_DEFINE ("FifthScriptExample");

This has all been covered, so we won't rehash it. The next lines of source are the
network illustration and a comment addressing the problem described above with Socket.

// ===========================================================================
//
// node 0 node 1
// +----------------+ +----------------+
// | ns-3 TCP | | ns-3 TCP |
// +----------------+ +----------------+
// | 10.1.1.1 | | 10.1.1.2 |
// +----------------+ +----------------+
// | point-to-point | | point-to-point |
// +----------------+ +----------------+
// | |
// +---------------------+
// 5 Mbps, 2 ms
//
//
// We want to look at changes in the ns-3 TCP congestion window. We need
// to crank up a flow and hook the CongestionWindow attribute on the socket
// of the sender. Normally one would use an on-off application to generate a
// flow, but this has a couple of problems. First, the socket of the on-off
// application is not created until Application Start time, so we wouldn't be
// able to hook the socket (now) at configuration time. Second, even if we
// could arrange a call after start time, the socket is not public so we
// couldn't get at it.
//
// So, we can cook up a simple version of the on-off application that does what
// we want. On the plus side we don't need all of the complexity of the on-off
// application. On the minus side, we don't have a helper, so we have to get
// a little more involved in the details, but this is trivial.
//
// So first, we create a socket and do the trace connect on it; then we pass
// this socket into the constructor of our simple application which we then
// install in the source node.
// ===========================================================================
//

This should also be self-explanatory.

The next part is the declaration of the MyApp Application that we put together to allow
the Socket to be created at configuration time.

class MyApp : public Application
{
public:

MyApp ();
virtual ~MyApp();

void Setup (Ptr<Socket> socket, Address address, uint32_t packetSize,
uint32_t nPackets, DataRate dataRate);

private:
virtual void StartApplication (void);
virtual void StopApplication (void);

void ScheduleTx (void);
void SendPacket (void);

Ptr<Socket> m_socket;
Address m_peer;
uint32_t m_packetSize;
uint32_t m_nPackets;
DataRate m_dataRate;
EventId m_sendEvent;
bool m_running;
uint32_t m_packetsSent;
};

You can see that this class inherits from the ns-3 Application class. Take a look at
src/network/model/application.h if you are interested in what is inherited. The MyApp
class is obligated to override the StartApplication and StopApplication methods. These
methods are automatically called when MyApp is required to start and stop sending data
during the simulation.

Starting/Stopping Applications
It is worthwhile to spend a bit of time explaining how events actually get started in the
system. This is another fairly deep explanation, and can be ignored if you aren't
planning on venturing down into the guts of the system. It is useful, however, in that
the discussion touches on how some very important parts of ns-3 work and exposes some
important idioms. If you are planning on implementing new models, you probably want to
understand this section.

The most common way to start pumping events is to start an Application. This is done as
the result of the following (hopefully) familar lines of an ns-3 script:

ApplicationContainer apps = ...
apps.Start (Seconds (1.0));
apps.Stop (Seconds (10.0));

The application container code (see src/network/helper/application-container.h if you are
interested) loops through its contained applications and calls,

app->SetStartTime (startTime);

as a result of the apps.Start call and

app->SetStopTime (stopTime);

as a result of the apps.Stop call.

The ultimate result of these calls is that we want to have the simulator automatically
make calls into our Applications to tell them when to start and stop. In the case of
MyApp, it inherits from class Application and overrides StartApplication, and
StopApplication. These are the functions that will be called by the simulator at the
appropriate time. In the case of MyApp you will find that MyApp::StartApplication does
the initial Bind, and Connect on the socket, and then starts data flowing by calling
MyApp::SendPacket. MyApp::StopApplication stops generating packets by cancelling any
pending send events then closes the socket.

One of the nice things about ns-3 is that you can completely ignore the implementation
details of how your Application is "automagically" called by the simulator at the correct
time. But since we have already ventured deep into ns-3 already, let's go for it.

If you look at src/network/model/application.cc you will find that the SetStartTime method
of an Application just sets the member variable m_startTime and the SetStopTime method
just sets m_stopTime. From there, without some hints, the trail will probably end.

The key to picking up the trail again is to know that there is a global list of all of the
nodes in the system. Whenever you create a node in a simulation, a pointer to that Node
is added to the global NodeList.

Take a look at src/network/model/node-list.cc and search for NodeList::Add. The public
static implementation calls into a private implementation called NodeListPriv::Add. This
is a relatively common idom in ns-3. So, take a look at NodeListPriv::Add. There you
will find,

Simulator::ScheduleWithContext (index, TimeStep (0), &Node::Initialize, node);

This tells you that whenever a Node is created in a simulation, as a side-effect, a call
to that node's Initialize method is scheduled for you that happens at time zero. Don't
read too much into that name, yet. It doesn't mean that the Node is going to start doing
anything, it can be interpreted as an informational call into the Node telling it that the
simulation has started, not a call for action telling the Node to start doing something.

So, NodeList::Add indirectly schedules a call to Node::Initialize at time zero to advise a
new Node that the simulation has started. If you look in src/network/model/node.h you
will, however, not find a method called Node::Initialize. It turns out that the
Initialize method is inherited from class Object. All objects in the system can be
notified when the simulation starts, and objects of class Node are just one kind of those
objects.

Take a look at src/core/model/object.cc next and search for Object::Initialize. This code
is not as straightforward as you might have expected since ns-3 Objects support
aggregation. The code in Object::Initialize then loops through all of the objects that
have been aggregated together and calls their DoInitialize method. This is another idiom
that is very common in ns-3, sometimes called the "template design pattern.": a public
non-virtual API method, which stays constant across implementations, and that calls a
private virtual implementation method that is inherited and implemented by subclasses.
The names are typically something like MethodName for the public API and DoMethodName for
the private API.

This tells us that we should look for a Node::DoInitialize method in
src/network/model/node.cc for the method that will continue our trail. If you locate the
code, you will find a method that loops through all of the devices in the Node and then
all of the applications in the Node calling device->Initialize and application->Initialize
respectively.

You may already know that classes Device and Application both inherit from class Object
and so the next step will be to look at what happens when Application::DoInitialize is
called. Take a look at src/network/model/application.cc and you will find:

void
Application::DoInitialize (void)
{
m_startEvent = Simulator::Schedule (m_startTime, &Application::StartApplication, this);
if (m_stopTime != TimeStep (0))
{
m_stopEvent = Simulator::Schedule (m_stopTime, &Application::StopApplication, this);
}
Object::DoInitialize ();
}

Here, we finally come to the end of the trail. If you have kept it all straight, when you
implement an ns-3 Application, your new application inherits from class Application. You
override the StartApplication and StopApplication methods and provide mechanisms for
starting and stopping the flow of data out of your new Application. When a Node is
created in the simulation, it is added to a global NodeList. The act of adding a Node to
this NodeList causes a simulator event to be scheduled for time zero which calls the
Node::Initialize method of the newly added Node to be called when the simulation starts.
Since a Node inherits from Object, this calls the Object::Initialize method on the Node
which, in turn, calls the DoInitialize methods on all of the Objects aggregated to the
Node (think mobility models). Since the Node Object has overridden DoInitialize, that
method is called when the simulation starts. The Node::DoInitialize method calls the
Initialize methods of all of the Applications on the node. Since Applications are also
Objects, this causes Application::DoInitialize to be called. When
Application::DoInitialize is called, it schedules events for the StartApplication and
StopApplication calls on the Application. These calls are designed to start and stop the
flow of data from the Application

This has been another fairly long journey, but it only has to be made once, and you now
understand another very deep piece of ns-3.

The MyApp Application
The MyApp Application needs a constructor and a destructor, of course:

MyApp::MyApp ()
: m_socket (0),
m_peer (),
m_packetSize (0),
m_nPackets (0),
m_dataRate (0),
m_sendEvent (),
m_running (false),
m_packetsSent (0)
{
}

MyApp::~MyApp()
{
m_socket = 0;
}

The existence of the next bit of code is the whole reason why we wrote this Application in
the first place.

void
MyApp::Setup (Ptr<Socket> socket, Address address, uint32_t packetSize,
uint32_t nPackets, DataRate dataRate)
{
m_socket = socket;
m_peer = address;
m_packetSize = packetSize;
m_nPackets = nPackets;
m_dataRate = dataRate;
}

This code should be pretty self-explanatory. We are just initializing member variables.
The important one from the perspective of tracing is the Ptr<Socket> socket which we
needed to provide to the application during configuration time. Recall that we are going
to create the Socket as a TcpSocket (which is implemented by TcpNewReno) and hook its
"CongestionWindow" trace source before passing it to the Setup method.

void
MyApp::StartApplication (void)
{
m_running = true;
m_packetsSent = 0;
m_socket->Bind ();
m_socket->Connect (m_peer);
SendPacket ();
}

The above code is the overridden implementation Application::StartApplication that will be
automatically called by the simulator to start our Application running at the appropriate
time. You can see that it does a Socket Bind operation. If you are familiar with
Berkeley Sockets this shouldn't be a surprise. It performs the required work on the local
side of the connection just as you might expect. The following Connect will do what is
required to establish a connection with the TCP at Address m_peer. It should now be clear
why we need to defer a lot of this to simulation time, since the Connect is going to need
a fully functioning network to complete. After the Connect, the Application then starts
creating simulation events by calling SendPacket.

The next bit of code explains to the Application how to stop creating simulation events.

void
MyApp::StopApplication (void)
{
m_running = false;

if (m_sendEvent.IsRunning ())
{
Simulator::Cancel (m_sendEvent);
}

if (m_socket)
{
m_socket->Close ();
}
}

Every time a simulation event is scheduled, an Event is created. If the Event is pending
execution or executing, its method IsRunning will return true. In this code, if
IsRunning() returns true, we Cancel the event which removes it from the simulator event
queue. By doing this, we break the chain of events that the Application is using to keep
sending its Packets and the Application goes quiet. After we quiet the Application we
Close the socket which tears down the TCP connection.

The socket is actually deleted in the destructor when the m_socket = 0 is executed. This
removes the last reference to the underlying Ptr<Socket> which causes the destructor of
that Object to be called.

Recall that StartApplication called SendPacket to start the chain of events that describes
the Application behavior.

void
MyApp::SendPacket (void)
{
Ptr<Packet> packet = Create<Packet> (m_packetSize);
m_socket->Send (packet);

if (++m_packetsSent < m_nPackets)
{
ScheduleTx ();
}
}

Here, you see that SendPacket does just that. It creates a Packet and then does a Send
which, if you know Berkeley Sockets, is probably just what you expected to see.

It is the responsibility of the Application to keep scheduling the chain of events, so the
next lines call ScheduleTx to schedule another transmit event (a SendPacket) until the
Application decides it has sent enough.

void
MyApp::ScheduleTx (void)
{
if (m_running)
{
Time tNext (Seconds (m_packetSize * 8 / static_cast<double> (m_dataRate.GetBitRate ())));
m_sendEvent = Simulator::Schedule (tNext, &MyApp::SendPacket, this);
}
}

Here, you see that ScheduleTx does exactly that. If the Application is running (if
StopApplication has not been called) it will schedule a new event, which calls SendPacket
again. The alert reader will spot something that also trips up new users. The data rate
of an Application is just that. It has nothing to do with the data rate of an underlying
Channel. This is the rate at which the Application produces bits. It does not take into
account any overhead for the various protocols or channels that it uses to transport the
data. If you set the data rate of an Application to the same data rate as your underlying
Channel you will eventually get a buffer overflow.

Trace Sinks
The whole point of this exercise is to get trace callbacks from TCP indicating the
congestion window has been updated. The next piece of code implements the corresponding
trace sink:

static void
CwndChange (uint32_t oldCwnd, uint32_t newCwnd)
{
NS_LOG_UNCOND (Simulator::Now ().GetSeconds () << "\t" << newCwnd);
}

This should be very familiar to you now, so we won't dwell on the details. This function
just logs the current simulation time and the new value of the congestion window every
time it is changed. You can probably imagine that you could load the resulting output
into a graphics program (gnuplot or Excel) and immediately see a nice graph of the
congestion window behavior over time.

We added a new trace sink to show where packets are dropped. We are going to add an error
model to this code also, so we wanted to demonstrate this working.

static void
RxDrop (Ptr<const Packet> p)
{
NS_LOG_UNCOND ("RxDrop at " << Simulator::Now ().GetSeconds ());
}

This trace sink will be connected to the "PhyRxDrop" trace source of the point-to-point
NetDevice. This trace source fires when a packet is dropped by the physical layer of a
NetDevice. If you take a small detour to the source
(src/point-to-point/model/point-to-point-net-device.cc) you will see that this trace
source refers to PointToPointNetDevice::m_phyRxDropTrace. If you then look in
src/point-to-point/model/point-to-point-net-device.h for this member variable, you will
find that it is declared as a TracedCallback<Ptr<const Packet> >. This should tell you
that the callback target should be a function that returns void and takes a single
parameter which is a Ptr<const Packet> (assuming we use ConnectWithoutContext) -- just
what we have above.

Main Program
The following code should be very familiar to you by now:

int
main (int argc, char *argv[])
{
NodeContainer nodes;
nodes.Create (2);

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

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

This creates two nodes with a point-to-point channel between them, just as shown in the
illustration at the start of the file.

The next few lines of code show something new. If we trace a connection that behaves
perfectly, we will end up with a monotonically increasing congestion window. To see any
interesting behavior, we really want to introduce link errors which will drop packets,
cause duplicate ACKs and trigger the more interesting behaviors of the congestion window.

ns-3 provides ErrorModel objects which can be attached to Channels. We are using the
RateErrorModel which allows us to introduce errors into a Channel at a given rate.

Ptr<RateErrorModel> em = CreateObject<RateErrorModel> ();
em->SetAttribute ("ErrorRate", DoubleValue (0.00001));
devices.Get (1)->SetAttribute ("ReceiveErrorModel", PointerValue (em));

The above code instantiates a RateErrorModel Object, and we set the "ErrorRate" Attribute
to the desired value. We then set the resulting instantiated RateErrorModel as the error
model used by the point-to-point NetDevice. This will give us some retransmissions and
make our plot a little more interesting.

InternetStackHelper stack;
stack.Install (nodes);

Ipv4AddressHelper address;
address.SetBase ("10.1.1.0", "255.255.255.252");
Ipv4InterfaceContainer interfaces = address.Assign (devices);

The above code should be familiar. It installs internet stacks on our two nodes and
creates interfaces and assigns IP addresses for the point-to-point devices.

Since we are using TCP, we need something on the destination Node to receive TCP
connections and data. The PacketSink Application is commonly used in ns-3 for that
purpose.

uint16_t sinkPort = 8080;
Address sinkAddress (InetSocketAddress(interfaces.GetAddress (1), sinkPort));
PacketSinkHelper packetSinkHelper ("ns3::TcpSocketFactory",
InetSocketAddress (Ipv4Address::GetAny (), sinkPort));
ApplicationContainer sinkApps = packetSinkHelper.Install (nodes.Get (1));
sinkApps.Start (Seconds (0.));
sinkApps.Stop (Seconds (20.));

This should all be familiar, with the exception of,

PacketSinkHelper packetSinkHelper ("ns3::TcpSocketFactory",
InetSocketAddress (Ipv4Address::GetAny (), sinkPort));

This code instantiates a PacketSinkHelper and tells it to create sockets using the class
ns3::TcpSocketFactory. This class implements a design pattern called "object factory"
which is a commonly used mechanism for specifying a class used to create objects in an
abstract way. Here, instead of having to create the objects themselves, you provide the
PacketSinkHelper a string that specifies a TypeId string used to create an object which
can then be used, in turn, to create instances of the Objects created by the factory.

The remaining parameter tells the Application which address and port it should Bind to.

The next two lines of code will create the socket and connect the trace source.

Ptr<Socket> ns3TcpSocket = Socket::CreateSocket (nodes.Get (0),
TcpSocketFactory::GetTypeId ());
ns3TcpSocket->TraceConnectWithoutContext ("CongestionWindow",
MakeCallback (&CwndChange));

The first statement calls the static member function Socket::CreateSocket and provides a
Node and an explicit TypeId for the object factory used to create the socket. This is a
slightly lower level call than the PacketSinkHelper call above, and uses an explicit C++
type instead of one referred to by a string. Otherwise, it is conceptually the same
thing.

Once the TcpSocket is created and attached to the Node, we can use
TraceConnectWithoutContext to connect the CongestionWindow trace source to our trace sink.

Recall that we coded an Application so we could take that Socket we just made (during
configuration time) and use it in simulation time. We now have to instantiate that
Application. We didn't go to any trouble to create a helper to manage the Application so
we are going to have to create and install it "manually". This is actually quite easy:

Ptr<MyApp> app = CreateObject<MyApp> ();
app->Setup (ns3TcpSocket, sinkAddress, 1040, 1000, DataRate ("1Mbps"));
nodes.Get (0)->AddApplication (app);
app->Start (Seconds (1.));
app->Stop (Seconds (20.));

The first line creates an Object of type MyApp -- our Application. The second line tells
the Application what Socket to use, what address to connect to, how much data to send at
each send event, how many send events to generate and the rate at which to produce data
from those events.

Next, we manually add the MyApp Application to the source Node and explicitly call the
Start and Stop methods on the Application to tell it when to start and stop doing its
thing.

We need to actually do the connect from the receiver point-to-point NetDevice drop event
to our RxDrop callback now.

devices.Get (1)->TraceConnectWithoutContext("PhyRxDrop", MakeCallback (&RxDrop));

It should now be obvious that we are getting a reference to the receiving Node NetDevice
from its container and connecting the trace source defined by the attribute "PhyRxDrop" on
that device to the trace sink RxDrop.

Finally, we tell the simulator to override any Applications and just stop processing
events at 20 seconds into the simulation.

Simulator::Stop (Seconds(20));
Simulator::Run ();
Simulator::Destroy ();

return 0;
}

Recall that as soon as Simulator::Run is called, configuration time ends, and simulation
time begins. All of the work we orchestrated by creating the Application and teaching it
how to connect and send data actually happens during this function call.

As soon as Simulator::Run returns, the simulation is complete and we enter the teardown
phase. In this case, Simulator::Destroy takes care of the gory details and we just return
a success code after it completes.

Running fifth.cc
Since we have provided the file fifth.cc for you, if you have built your distribution (in
debug mode since it uses NS_LOG -- recall that optimized builds optimize out NS_LOG) it
will be waiting for you to run.

$ ./waf --run fifth
Waf: Entering directory `/home/craigdo/repos/ns-3-allinone-dev/ns-3-dev/build'
Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone-dev/ns-3-dev/build'
'build' finished successfully (0.684s)
1 536
1.0093 1072
1.01528 1608
1.02167 2144
...
1.11319 8040
1.12151 8576
1.12983 9112
RxDrop at 1.13696
...

You can probably see immediately a downside of using prints of any kind in your traces.
We get those extraneous waf messages printed all over our interesting information along
with those RxDrop messages. We will remedy that soon, but I'm sure you can't wait to see
the results of all of this work. Let's redirect that output to a file called cwnd.dat:

$ ./waf --run fifth > cwnd.dat 2>&1

Now edit up "cwnd.dat" in your favorite editor and remove the waf build status and drop
lines, leaving only the traced data (you could also comment out the
TraceConnectWithoutContext("PhyRxDrop", MakeCallback (&RxDrop)); in the script to get rid
of the drop prints just as easily.

You can now run gnuplot (if you have it installed) and tell it to generate some pretty
pictures:

$ gnuplot
gnuplot> set terminal png size 640,480
gnuplot> set output "cwnd.png"
gnuplot> plot "cwnd.dat" using 1:2 title 'Congestion Window' with linespoints
gnuplot> exit

You should now have a graph of the congestion window versus time sitting in the file
"cwnd.png" loading="lazy" that looks like:
[image]

Using Mid-Level Helpers
In the previous section, we showed how to hook a trace source and get hopefully
interesting information out of a simulation. Perhaps you will recall that we called
logging to the standard output using std::cout a "blunt instrument" much earlier in this
chapter. We also wrote about how it was a problem having to parse the log output in order
to isolate interesting information. It may have occurred to you that we just spent a lot
of time implementing an example that exhibits all of the problems we purport to fix with
the ns-3 tracing system! You would be correct. But, bear with us. We're not done yet.

One of the most important things we want to do is to is to have the ability to easily
control the amount of output coming out of the simulation; and we also want to save those
data to a file so we can refer back to it later. We can use the mid-level trace helpers
provided in ns-3 to do just that and complete the picture.

We provide a script that writes the cwnd change and drop events developed in the example
fifth.cc to disk in separate files. The cwnd changes are stored as a tab-separated ASCII
file and the drop events are stored in a PCAP file. The changes to make this happen are
quite small.

Walkthrough: sixth.cc
Let's take a look at the changes required to go from fifth.cc to sixth.cc. Open
examples/tutorial/sixth.cc in your favorite editor. You can see the first change by
searching for CwndChange. You will find that we have changed the signatures for the trace
sinks and have added a single line to each sink that writes the traced information to a
stream representing a file.

static void
CwndChange (Ptr<OutputStreamWrapper> stream, uint32_t oldCwnd, uint32_t newCwnd)
{
NS_LOG_UNCOND (Simulator::Now ().GetSeconds () << "\t" << newCwnd);
*stream->GetStream () << Simulator::Now ().GetSeconds () << "\t" << oldCwnd << "\t" << newCwnd << std::endl;
}

static void
RxDrop (Ptr<PcapFileWrapper> file, Ptr<const Packet> p)
{
NS_LOG_UNCOND ("RxDrop at " << Simulator::Now ().GetSeconds ());
file->Write(Simulator::Now(), p);
}

We have added a "stream" parameter to the CwndChange trace sink. This is an object that
holds (keeps safely alive) a C++ output stream. It turns out that this is a very simple
object, but one that manages lifetime issues for the stream and solves a problem that even
experienced C++ users run into. It turns out that the copy constructor for std::ostream
is marked private. This means that std::ostreams do not obey value semantics and cannot
be used in any mechanism that requires the stream to be copied. This includes the ns-3
callback system, which as you may recall, requires objects that obey value semantics.
Further notice that we have added the following line in the CwndChange trace sink
implementation:

*stream->GetStream () << Simulator::Now ().GetSeconds () << "\t" << oldCwnd << "\t" << newCwnd << std::endl;

This would be very familiar code if you replaced *stream->GetStream () with std::cout, as
in:

std::cout << Simulator::Now ().GetSeconds () << "\t" << oldCwnd << "\t" << newCwnd << std::endl;

This illustrates that the Ptr<OutputStreamWrapper> is really just carrying around a
std::ofstream for you, and you can use it here like any other output stream.

A similar situation happens in RxDrop except that the object being passed around (a
Ptr<PcapFileWrapper>) represents a PCAP file. There is a one-liner in the trace sink to
write a timestamp and the contents of the packet being dropped to the PCAP file:

file->Write(Simulator::Now(), p);

Of course, if we have objects representing the two files, we need to create them somewhere
and also cause them to be passed to the trace sinks. If you look in the main function,
you will find new code to do just that:

AsciiTraceHelper asciiTraceHelper;
Ptr<OutputStreamWrapper> stream = asciiTraceHelper.CreateFileStream ("sixth.cwnd");
ns3TcpSocket->TraceConnectWithoutContext ("CongestionWindow", MakeBoundCallback (&CwndChange, stream));

...

PcapHelper pcapHelper;
Ptr<PcapFileWrapper> file = pcapHelper.CreateFile ("sixth.pcap", std::ios::out, PcapHelper::DLT_PPP);
devices.Get (1)->TraceConnectWithoutContext("PhyRxDrop", MakeBoundCallback (&RxDrop, file));

In the first section of the code snippet above, we are creating the ASCII trace file,
creating an object responsible for managing it and using a variant of the callback
creation function to arrange for the object to be passed to the sink. Our ASCII trace
helpers provide a rich set of functions to make using text (ASCII) files easy. We are
just going to illustrate the use of the file stream creation function here.

The CreateFileStream function is basically going to instantiate a std::ofstream object and
create a new file (or truncate an existing file). This std::ofstream is packaged up in an
ns-3 object for lifetime management and copy constructor issue resolution.

We then take this ns-3 object representing the file and pass it to MakeBoundCallback().
This function creates a callback just like MakeCallback(), but it "binds" a new value to
the callback. This value is added as the first argument to the callback before it is
called.

Essentially, MakeBoundCallback(&CwndChange, stream) causes the trace source to add the
additional "stream" parameter to the front of the formal parameter list before invoking
the callback. This changes the required signature of the CwndChange sink to match the one
shown above, which includes the "extra" parameter Ptr<OutputStreamWrapper> stream.

In the second section of code in the snippet above, we instantiate a PcapHelper to do the
same thing for our PCAP trace file that we did with the AsciiTraceHelper. The line of
code,

Ptr<PcapFileWrapper> file = pcapHelper.CreateFile ("sixth.pcap",
"w", PcapHelper::DLT_PPP);

creates a PCAP file named "sixth.pcap" with file mode "w". This means that the new file
is truncated (contents deleted) if an existing file with that name is found. The final
parameter is the "data link type" of the new PCAP file. These are the same as the PCAP
library data link types defined in bpf.h if you are familar with PCAP. In this case,
DLT_PPP indicates that the PCAP file is going to contain packets prefixed with point to
point headers. This is true since the packets are coming from our point-to-point device
driver. Other common data link types are DLT_EN10MB (10 MB Ethernet) appropriate for csma
devices and DLT_IEEE802_11 (IEEE 802.11) appropriate for wifi devices. These are defined
in src/network/helper/trace-helper.h if you are interested in seeing the list. The
entries in the list match those in bpf.h but we duplicate them to avoid a PCAP source
dependence.

A ns-3 object representing the PCAP file is returned from CreateFile and used in a bound
callback exactly as it was in the ASCII case.

An important detour: It is important to notice that even though both of these objects are
declared in very similar ways,

Ptr<PcapFileWrapper> file ...
Ptr<OutputStreamWrapper> stream ...

The underlying objects are entirely different. For example, the Ptr<PcapFileWrapper> is a
smart pointer to an ns-3 Object that is a fairly heavyweight thing that supports
Attributes and is integrated into the Config system. The Ptr<OutputStreamWrapper>, on the
other hand, is a smart pointer to a reference counted object that is a very lightweight
thing. Remember to look at the object you are referencing before making any assumptions
about the "powers" that object may have.

For example, take a look at src/network/utils/pcap-file-wrapper.h in the distribution and
notice,

class PcapFileWrapper : public Object

that class PcapFileWrapper is an ns-3 Object by virtue of its inheritance. Then look at
src/network/model/output-stream-wrapper.h and notice,

class OutputStreamWrapper : public
SimpleRefCount<OutputStreamWrapper>

that this object is not an ns-3 Object at all, it is "merely" a C++ object that happens to
support intrusive reference counting.

The point here is that just because you read Ptr<something> it does not necessarily mean
that something is an ns-3 Object on which you can hang ns-3 Attributes, for example.

Now, back to the example. If you build and run this example,

$ ./waf --run sixth

you will see the same messages appear as when you ran "fifth", but two new files will
appear in the top-level directory of your ns-3 distribution.

sixth.cwnd sixth.pcap

Since "sixth.cwnd" is an ASCII text file, you can view it with cat or your favorite file
viewer.

1 0 536
1.0093 536 1072
1.01528 1072 1608
1.02167 1608 2144
...
9.69256 5149 5204
9.89311 5204 5259

You have a tab separated file with a timestamp, an old congestion window and a new
congestion window suitable for directly importing into your plot program. There are no
extraneous prints in the file, no parsing or editing is required.

Since "sixth.pcap" is a PCAP file, you can fiew it with tcpdump.

reading from file sixth.pcap, link-type PPP (PPP)
1.136956 IP 10.1.1.1.49153 > 10.1.1.2.8080: Flags [.], seq 17177:17681, ack 1, win 32768, options [TS val 1133 ecr 1127,eol], length 504
1.403196 IP 10.1.1.1.49153 > 10.1.1.2.8080: Flags [.], seq 33280:33784, ack 1, win 32768, options [TS val 1399 ecr 1394,eol], length 504
...
7.426220 IP 10.1.1.1.49153 > 10.1.1.2.8080: Flags [.], seq 785704:786240, ack 1, win 32768, options [TS val 7423 ecr 7421,eol], length 536
9.630693 IP 10.1.1.1.49153 > 10.1.1.2.8080: Flags [.], seq 882688:883224, ack 1, win 32768, options [TS val 9620 ecr 9618,eol], length 536

You have a PCAP file with the packets that were dropped in the simulation. There are no
other packets present in the file and there is nothing else present to make life
difficult.

It's been a long journey, but we are now at a point where we can appreciate the ns-3
tracing system. We have pulled important events out of the middle of a TCP implementation
and a device driver. We stored those events directly in files usable with commonly known
tools. We did this without modifying any of the core code involved, and we did this in
only 18 lines of code:

static void
CwndChange (Ptr<OutputStreamWrapper> stream, uint32_t oldCwnd, uint32_t newCwnd)
{
NS_LOG_UNCOND (Simulator::Now ().GetSeconds () << "\t" << newCwnd);
*stream->GetStream () << Simulator::Now ().GetSeconds () << "\t" << oldCwnd << "\t" << newCwnd << std::endl;
}

...

AsciiTraceHelper asciiTraceHelper;
Ptr<OutputStreamWrapper> stream = asciiTraceHelper.CreateFileStream ("sixth.cwnd");
ns3TcpSocket->TraceConnectWithoutContext ("CongestionWindow", MakeBoundCallback (&CwndChange, stream));

...

static void
RxDrop (Ptr<PcapFileWrapper> file, Ptr<const Packet> p)
{
NS_LOG_UNCOND ("RxDrop at " << Simulator::Now ().GetSeconds ());
file->Write(Simulator::Now(), p);
}

...

PcapHelper pcapHelper;
Ptr<PcapFileWrapper> file = pcapHelper.CreateFile ("sixth.pcap", "w", PcapHelper::DLT_PPP);
devices.Get (1)->TraceConnectWithoutContext("PhyRxDrop", MakeBoundCallback (&RxDrop, file));

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

Perhaps you will recall seeing some of these variations:

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

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

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

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

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

┌────────────────┬──────┬───────┐
│ │ PCAP │ ASCII │
└────────────────┴──────┴───────┘

│Device Helper │ │ │
├────────────────┼──────┼───────┤
│Protocol Helper │ │ │
└────────────────┴──────┴───────┘

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

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

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

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

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

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

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

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

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

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

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

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

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

You are encouraged to peruse the API Documentation for class PcapHelperForDevice to find
the details of these methods; but to summarize ...

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

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

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

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

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

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

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

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

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

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

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

helper.EnablePcapAll ("prefix");

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

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

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

Finally, two of the methods shown above,

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

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

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

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

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

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

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

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

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

void EnableAscii (std::string prefix, Ptr<NetDevice> nd, bool explicitFilename = false);
void EnableAscii (Ptr<OutputStreamWrapper> stream, Ptr<NetDevice> nd);

will call the device implementation of EnableAsciiInternal directly, providing either a
valid prefix or stream. All other public ASCII tracing methods will build on these
low-level functions to provide additional user-level functionality. What this means to
the user is that all device helpers in the system will have all of the ASCII trace methods
available; and these methods will all work in the same way across devices if the devices
implement EnablAsciiInternal correctly.

Methods
void EnableAscii (std::string prefix, Ptr<NetDevice> nd, bool explicitFilename = false);
void EnableAscii (Ptr<OutputStreamWrapper> stream, Ptr<NetDevice> nd);

void EnableAscii (std::string prefix, std::string ndName, bool explicitFilename = false);
void EnableAscii (Ptr<OutputStreamWrapper> stream, std::string ndName);

void EnableAscii (std::string prefix, NetDeviceContainer d);
void EnableAscii (Ptr<OutputStreamWrapper> stream, NetDeviceContainer d);

void EnableAscii (std::string prefix, NodeContainer n);
void EnableAscii (Ptr<OutputStreamWrapper> stream, NodeContainer n);

void EnableAsciiAll (std::string prefix);
void EnableAsciiAll (Ptr<OutputStreamWrapper> stream);

void EnableAscii (std::string prefix, uint32_t nodeid, uint32_t deviceid, bool explicitFilename);
void EnableAscii (Ptr<OutputStreamWrapper> stream, uint32_t nodeid, uint32_t deviceid);

You are encouraged to peruse the API Documentation for class AsciiTraceHelperForDevice to
find the details of these methods; but to summarize ...

· There are twice as many methods available for ASCII tracing as there were for PCAP
tracing. This is because, in addition to the PCAP-style model where traces from each
unique node/device pair are written to a unique file, we support a model in which trace
information for many node/device pairs is written to a common file. This means that the
<prefix>-<node>-<device> file name generation mechanism is replaced by a mechanism to
refer to a common file; and the number of API methods is doubled to allow all
combinations.

· Just as in PCAP tracing, you can enable ASCII tracing on a particular (node, net-device)
pair by providing a Ptr<NetDevice> to an EnableAscii method. The Ptr<Node> is implicit
since the net device must belong to exactly one Node. For example,

Ptr<NetDevice> nd;
...
helper.EnableAscii ("prefix", nd);

· The first four methods also include a default parameter called explicitFilename that
operate similar to equivalent parameters in the PCAP case.

In this case, no trace contexts are written to the ASCII trace file since they would be
redundant. The system will pick the file name to be created using the same rules as
described in the PCAP section, except that the file will have the suffix .tr instead of
.pcap.

· If you want to enable ASCII tracing on more than one net device and have all traces sent
to a single file, you can do that as well by using an object to refer to a single file.
We have already seen this in the "cwnd" example above:

Ptr<NetDevice> nd1;
Ptr<NetDevice> nd2;
...
Ptr<OutputStreamWrapper> stream = asciiTraceHelper.CreateFileStream ("trace-file-name.tr");
...
helper.EnableAscii (stream, nd1);
helper.EnableAscii (stream, nd2);

In this case, trace contexts are written to the ASCII trace file since they are required
to disambiguate traces from the two devices. Note that since the user is completely
specifying the file name, the string should include the ,tr suffix for consistency.

· You can enable ASCII tracing on a particular (node, net-device) pair by providing a
std::string representing an object name service string to an EnablePcap method. The
Ptr<NetDevice> is looked up from the name string. Again, the <Node> is implicit since
the named net device must belong to exactly one Node. For example,

Names::Add ("client" ...);
Names::Add ("client/eth0" ...);
Names::Add ("server" ...);
Names::Add ("server/eth0" ...);
...
helper.EnableAscii ("prefix", "client/eth0");
helper.EnableAscii ("prefix", "server/eth0");

This would result in two files named ``prefix-client-eth0.tr`` and
``prefix-server-eth0.tr`` with traces for each device in the
respective trace file. Since all of the ``EnableAscii`` functions
are overloaded to take a stream wrapper, you can use that form as
well::

Names::Add ("client" ...);
Names::Add ("client/eth0" ...);
Names::Add ("server" ...);
Names::Add ("server/eth0" ...);
...
Ptr<OutputStreamWrapper> stream = asciiTraceHelper.CreateFileStream ("trace-file-name.tr");
...
helper.EnableAscii (stream, "client/eth0");
helper.EnableAscii (stream, "server/eth0");

This would result in a single trace file called trace-file-name.tr that contains all of
the trace events for both devices. The events would be disambiguated by trace context
strings.

· You can enable ASCII tracing on a collection of (node, net-device) pairs by providing a
NetDeviceContainer. For each NetDevice in the container the type is checked. For each
device of the proper type (the same type as is managed by the device helper), tracing is
enabled. Again, the <Node> is implicit since the found net device must belong to
exactly one Node. For example,

NetDeviceContainer d = ...;
...
helper.EnableAscii ("prefix", d);

This would result in a number of ASCII trace files being created,
each of which follows the ``<prefix>-<node id>-<device id>.tr``
convention.

Combining all of the traces into a single file is accomplished similarly to the examples
above:

NetDeviceContainer d = ...;
...
Ptr<OutputStreamWrapper> stream = asciiTraceHelper.CreateFileStream ("trace-file-name.tr");
...
helper.EnableAscii (stream, d);

· You can enable ASCII tracing on a collection of (node, net-device) pairs by providing a
NodeContainer. For each Node in the NodeContainer its attached NetDevices are iterated.
For each NetDevice attached to each Node in the container, the type of that device is
checked. For each device of the proper type (the same type as is managed by the device
helper), tracing is enabled.

NodeContainer n;
...
helper.EnableAscii ("prefix", n);

This would result in a number of ASCII trace files being created, each of which follows
the <prefix>-<node id>-<device id>.tr convention. Combining all of the traces into a
single file is accomplished similarly to the examples above.

· You can enable PCAP tracing on the basis of Node ID and device ID as well as with
explicit Ptr. Each Node in the system has an integer Node ID and each device connected
to a Node has an integer device ID.

helper.EnableAscii ("prefix", 21, 1);

Of course, the traces can be combined into a single file as shown above.

· Finally, you can enable PCAP tracing for all devices in the system, with the same type
as that managed by the device helper.

helper.EnableAsciiAll ("prefix");

This would result in a number of ASCII trace files being created, one for every device
in the system of the type managed by the helper. All of these files will follow the
<prefix>-<node id>-<device id>.tr convention. Combining all of the traces into a single
file is accomplished similarly to the examples above.

Filenames
Implicit in the prefix-style method descriptions above is the construction of the complete
filenames by the implementation method. By convention, ASCII traces in the ns-3 system
are of the form <prefix>-<node id>-<device id>.tr

As previously mentioned, every Node in the system will have a system-assigned Node id; and
every device will have an interface index (also called a device id) relative to its node.
By default, then, an ASCII trace file created as a result of enabling tracing on the first
device of Node 21, using the prefix "prefix", would be prefix-21-1.tr.

You can always use the ns-3 object name service to make this more clear. For example, if
you use the object name service to assign the name "server" to Node 21, the resulting
ASCII trace file name will automatically become, prefix-server-1.tr and if you also assign
the name "eth0" to the device, your ASCII trace file name will automatically pick this up
and be called prefix-server-eth0.tr.

Several of the methods have a default parameter called explicitFilename. When set to
true, this parameter disables the automatic filename completion mechanism and allows you
to create an explicit filename. This option is only available in the methods which take a
prefix and enable tracing on a single device.

Protocol Helpers
PCAP
The goal of these mixins is to make it easy to add a consistent PCAP trace facility to
protocols. We want all of the various flavors of PCAP tracing to work the same across all
protocols, so the methods of these helpers are inherited by stack helpers. Take a look at
src/network/helper/trace-helper.h if you want to follow the discussion while looking at
real code.

In this section we will be illustrating the methods as applied to the protocol Ipv4. To
specify traces in similar protocols, just substitute the appropriate type. For example,
use a Ptr<Ipv6> instead of a Ptr<Ipv4> and call EnablePcapIpv6 instead of EnablePcapIpv4.

The class PcapHelperForIpv4 provides the high level functionality for using PCAP tracing
in the Ipv4 protocol. Each protocol helper enabling these methods must implement a single
virtual method inherited from this class. There will be a separate implementation for
Ipv6, for example, but the only difference will be in the method names and signatures.
Different method names are required to disambiguate class Ipv4 from Ipv6 which are both
derived from class Object, and methods that share the same signature.

virtual void EnablePcapIpv4Internal (std::string prefix,
Ptr<Ipv4> ipv4,
uint32_t interface,
bool explicitFilename) = 0;

The signature of this method reflects the protocol and interface-centric view of the
situation at this level. All of the public methods inherited from class PcapHelperForIpv4
reduce to calling this single device-dependent implementation method. For example, the
lowest level PCAP method,

void EnablePcapIpv4 (std::string prefix, Ptr<Ipv4> ipv4, uint32_t interface, bool explicitFilename = false);

will call the device implementation of EnablePcapIpv4Internal directly. All other public
PCAP tracing methods build on this implementation to provide additional user-level
functionality. What this means to the user is that all protocol helpers in the system
will have all of the PCAP trace methods available; and these methods will all work in the
same way across protocols if the helper implements EnablePcapIpv4Internal correctly.

Methods
These methods are designed to be in one-to-one correspondence with the Node- and
NetDevice- centric versions of the device versions. Instead of Node and NetDevice pair
constraints, we use protocol and interface constraints.

Note that just like in the device version, there are six methods:

void EnablePcapIpv4 (std::string prefix, Ptr<Ipv4> ipv4, uint32_t interface, bool explicitFilename = false);
void EnablePcapIpv4 (std::string prefix, std::string ipv4Name, uint32_t interface, bool explicitFilename = false);
void EnablePcapIpv4 (std::string prefix, Ipv4InterfaceContainer c);
void EnablePcapIpv4 (std::string prefix, NodeContainer n);
void EnablePcapIpv4 (std::string prefix, uint32_t nodeid, uint32_t interface, bool explicitFilename);
void EnablePcapIpv4All (std::string prefix);

You are encouraged to peruse the API Documentation for class PcapHelperForIpv4 to find the
details of these methods; but to summarize ...

· You can enable PCAP tracing on a particular protocol/interface pair by providing a
Ptr<Ipv4> and interface to an EnablePcap method. For example,

Ptr<Ipv4> ipv4 = node->GetObject<Ipv4> ();
...
helper.EnablePcapIpv4 ("prefix", ipv4, 0);

· You can enable PCAP tracing on a particular node/net-device pair by providing a
std::string representing an object name service string to an EnablePcap method. The
Ptr<Ipv4> is looked up from the name string. For example,

Names::Add ("serverIPv4" ...);
...
helper.EnablePcapIpv4 ("prefix", "serverIpv4", 1);

· You can enable PCAP tracing on a collection of protocol/interface pairs by providing an
Ipv4InterfaceContainer. For each Ipv4 / interface pair in the container the protocol
type is checked. For each protocol of the proper type (the same type as is managed by
the device helper), tracing is enabled for the corresponding interface. For example,

NodeContainer nodes;
...
NetDeviceContainer devices = deviceHelper.Install (nodes);
...
Ipv4AddressHelper ipv4;
ipv4.SetBase ("10.1.1.0", "255.255.255.0");
Ipv4InterfaceContainer interfaces = ipv4.Assign (devices);
...
helper.EnablePcapIpv4 ("prefix", interfaces);

· You can enable PCAP tracing on a collection of protocol/interface pairs by providing a
NodeContainer. For each Node in the NodeContainer the appropriate protocol is found.
For each protocol, its interfaces are enumerated and tracing is enabled on the resulting
pairs. For example,

NodeContainer n;
...
helper.EnablePcapIpv4 ("prefix", n);

· You can enable PCAP tracing on the basis of Node ID and interface as well. In this
case, the node-id is translated to a Ptr<Node> and the appropriate protocol is looked up
in the node. The resulting protocol and interface are used to specify the resulting
trace source.

helper.EnablePcapIpv4 ("prefix", 21, 1);

· Finally, you can enable PCAP tracing for all interfaces in the system, with associated
protocol being the same type as that managed by the device helper.

helper.EnablePcapIpv4All ("prefix");

Filenames
Implicit in all of the method descriptions above is the construction of the complete
filenames by the implementation method. By convention, PCAP traces taken for devices in
the ns-3 system are of the form "<prefix>-<node id>-<device id>.pcap". In the case of
protocol traces, there is a one-to-one correspondence between protocols and Nodes. This
is because protocol Objects are aggregated to Node Objects. Since there is no global
protocol id in the system, we use the corresponding Node id in file naming. Therefore
there is a possibility for file name collisions in automatically chosen trace file names.
For this reason, the file name convention is changed for protocol traces.

As previously mentioned, every Node in the system will have a system-assigned Node id.
Since there is a one-to-one correspondence between protocol instances and Node instances
we use the Node id. Each interface has an interface id relative to its protocol. We use
the convention "<prefix>-n<node id>-i<interface id>.pcap" for trace file naming in
protocol helpers.

Therefore, by default, a PCAP trace file created as a result of enabling tracing on
interface 1 of the Ipv4 protocol of Node 21 using the prefix "prefix" would be
"prefix-n21-i1.pcap".

You can always use the ns-3 object name service to make this more clear. For example, if
you use the object name service to assign the name "serverIpv4" to the Ptr<Ipv4> on Node
21, the resulting PCAP trace file name will automatically become,
"prefix-nserverIpv4-i1.pcap".

Several of the methods have a default parameter called explicitFilename. When set to
true, this parameter disables the automatic filename completion mechanism and allows you
to create an explicit filename. This option is only available in the methods which take a
prefix and enable tracing on a single device.

ASCII
The behavior of the ASCII trace helpers is substantially similar to the PCAP case. Take a
look at src/network/helper/trace-helper.h if you want to follow the discussion while
looking at real code.

In this section we will be illustrating the methods as applied to the protocol Ipv4. To
specify traces in similar protocols, just substitute the appropriate type. For example,
use a Ptr<Ipv6> instead of a Ptr<Ipv4> and call EnableAsciiIpv6 instead of
EnableAsciiIpv4.

The class AsciiTraceHelperForIpv4 adds the high level functionality for using ASCII
tracing to a protocol helper. Each protocol that enables these methods must implement a
single virtual method inherited from this class.

virtual void EnableAsciiIpv4Internal (Ptr<OutputStreamWrapper> stream,
std::string prefix,
Ptr<Ipv4> ipv4,
uint32_t interface,
bool explicitFilename) = 0;

The signature of this method reflects the protocol- and interface-centric view of the
situation at this level; and also the fact that the helper may be writing to a shared
output stream. All of the public methods inherited from class
PcapAndAsciiTraceHelperForIpv4 reduce to calling this single device- dependent
implementation method. For example, the lowest level ASCII trace methods,

void EnableAsciiIpv4 (std::string prefix, Ptr<Ipv4> ipv4, uint32_t interface, bool explicitFilename = false);
void EnableAsciiIpv4 (Ptr<OutputStreamWrapper> stream, Ptr<Ipv4> ipv4, uint32_t interface);

will call the device implementation of EnableAsciiIpv4Internal directly, providing either
the prefix or the stream. All other public ASCII tracing methods will build on these
low-level functions to provide additional user-level functionality. What this means to
the user is that all device helpers in the system will have all of the ASCII trace methods
available; and these methods will all work in the same way across protocols if the
protocols implement EnablAsciiIpv4Internal correctly.

Methods
void EnableAsciiIpv4 (std::string prefix, Ptr<Ipv4> ipv4, uint32_t interface, bool explicitFilename = false);
void EnableAsciiIpv4 (Ptr<OutputStreamWrapper> stream, Ptr<Ipv4> ipv4, uint32_t interface);

void EnableAsciiIpv4 (std::string prefix, std::string ipv4Name, uint32_t interface, bool explicitFilename = false);
void EnableAsciiIpv4 (Ptr<OutputStreamWrapper> stream, std::string ipv4Name, uint32_t interface);

void EnableAsciiIpv4 (std::string prefix, Ipv4InterfaceContainer c);
void EnableAsciiIpv4 (Ptr<OutputStreamWrapper> stream, Ipv4InterfaceContainer c);

void EnableAsciiIpv4 (std::string prefix, NodeContainer n);
void EnableAsciiIpv4 (Ptr<OutputStreamWrapper> stream, NodeContainer n);

void EnableAsciiIpv4All (std::string prefix);
void EnableAsciiIpv4All (Ptr<OutputStreamWrapper> stream);

void EnableAsciiIpv4 (std::string prefix, uint32_t nodeid, uint32_t deviceid, bool explicitFilename);
void EnableAsciiIpv4 (Ptr<OutputStreamWrapper> stream, uint32_t nodeid, uint32_t interface);

You are encouraged to peruse the API Documentation for class PcapAndAsciiHelperForIpv4 to
find the details of these methods; but to summarize ...

· There are twice as many methods available for ASCII tracing as there were for PCAP
tracing. This is because, in addition to the PCAP-style model where traces from each
unique protocol/interface pair are written to a unique file, we support a model in which
trace information for many protocol/interface pairs is written to a common file. This
means that the <prefix>-n<node id>-<interface> file name generation mechanism is
replaced by a mechanism to refer to a common file; and the number of API methods is
doubled to allow all combinations.

· Just as in PCAP tracing, you can enable ASCII tracing on a particular protocol/interface
pair by providing a Ptr<Ipv4> and an interface to an EnableAscii method. For example,

Ptr<Ipv4> ipv4;
...
helper.EnableAsciiIpv4 ("prefix", ipv4, 1);

In this case, no trace contexts are written to the ASCII trace file since they would be
redundant. The system will pick the file name to be created using the same rules as
described in the PCAP section, except that the file will have the suffix ".tr" instead
of ".pcap".

· If you want to enable ASCII tracing on more than one interface and have all traces sent
to a single file, you can do that as well by using an object to refer to a single file.
We have already something similar to this in the "cwnd" example above:

Ptr<Ipv4> protocol1 = node1->GetObject<Ipv4> ();
Ptr<Ipv4> protocol2 = node2->GetObject<Ipv4> ();
...
Ptr<OutputStreamWrapper> stream = asciiTraceHelper.CreateFileStream ("trace-file-name.tr");
...
helper.EnableAsciiIpv4 (stream, protocol1, 1);
helper.EnableAsciiIpv4 (stream, protocol2, 1);

In this case, trace contexts are written to the ASCII trace file since they are required
to disambiguate traces from the two interfaces. Note that since the user is completely
specifying the file name, the string should include the ",tr" for consistency.

· You can enable ASCII tracing on a particular protocol by providing a std::string
representing an object name service string to an EnablePcap method. The Ptr<Ipv4> is
looked up from the name string. The <Node> in the resulting filenames is implicit since
there is a one-to-one correspondence between protocol instances and nodes, For example,

Names::Add ("node1Ipv4" ...);
Names::Add ("node2Ipv4" ...);
...
helper.EnableAsciiIpv4 ("prefix", "node1Ipv4", 1);
helper.EnableAsciiIpv4 ("prefix", "node2Ipv4", 1);

This would result in two files named "prefix-nnode1Ipv4-i1.tr" and
"prefix-nnode2Ipv4-i1.tr" with traces for each interface in the respective trace file.
Since all of the EnableAscii functions are overloaded to take a stream wrapper, you can
use that form as well:

Names::Add ("node1Ipv4" ...);
Names::Add ("node2Ipv4" ...);
...
Ptr<OutputStreamWrapper> stream = asciiTraceHelper.CreateFileStream ("trace-file-name.tr");
...
helper.EnableAsciiIpv4 (stream, "node1Ipv4", 1);
helper.EnableAsciiIpv4 (stream, "node2Ipv4", 1);

This would result in a single trace file called "trace-file-name.tr" that contains all
of the trace events for both interfaces. The events would be disambiguated by trace
context strings.

· You can enable ASCII tracing on a collection of protocol/interface pairs by providing an
Ipv4InterfaceContainer. For each protocol of the proper type (the same type as is
managed by the device helper), tracing is enabled for the corresponding interface.
Again, the <Node> is implicit since there is a one-to-one correspondence between each
protocol and its node. For example,

NodeContainer nodes;
...
NetDeviceContainer devices = deviceHelper.Install (nodes);
...
Ipv4AddressHelper ipv4;
ipv4.SetBase ("10.1.1.0", "255.255.255.0");
Ipv4InterfaceContainer interfaces = ipv4.Assign (devices);
...
...
helper.EnableAsciiIpv4 ("prefix", interfaces);

This would result in a number of ASCII trace files being created, each of which follows
the <prefix>-n<node id>-i<interface>.tr convention. Combining all of the traces into a
single file is accomplished similarly to the examples above:

NodeContainer nodes;
...
NetDeviceContainer devices = deviceHelper.Install (nodes);
...
Ipv4AddressHelper ipv4;
ipv4.SetBase ("10.1.1.0", "255.255.255.0");
Ipv4InterfaceContainer interfaces = ipv4.Assign (devices);
...
Ptr<OutputStreamWrapper> stream = asciiTraceHelper.CreateFileStream ("trace-file-name.tr");
...
helper.EnableAsciiIpv4 (stream, interfaces);

· You can enable ASCII tracing on a collection of protocol/interface pairs by providing a
NodeContainer. For each Node in the NodeContainer the appropriate protocol is found.
For each protocol, its interfaces are enumerated and tracing is enabled on the resulting
pairs. For example,

NodeContainer n;
...
helper.EnableAsciiIpv4 ("prefix", n);

This would result in a number of ASCII trace files being created, each of which follows
the <prefix>-<node id>-<device id>.tr convention. Combining all of the traces into a
single file is accomplished similarly to the examples above.

· You can enable PCAP tracing on the basis of Node ID and device ID as well. In this
case, the node-id is translated to a Ptr<Node> and the appropriate protocol is looked up
in the node. The resulting protocol and interface are used to specify the resulting
trace source.

helper.EnableAsciiIpv4 ("prefix", 21, 1);

Of course, the traces can be combined into a single file as shown above.

· Finally, you can enable ASCII tracing for all interfaces in the system, with associated
protocol being the same type as that managed by the device helper.

helper.EnableAsciiIpv4All ("prefix");

This would result in a number of ASCII trace files being created, one for every
interface in the system related to a protocol of the type managed by the helper. All of
these files will follow the <prefix>-n<node id>-i<interface.tr convention. Combining
all of the traces into a single file is accomplished similarly to the examples above.

Filenames
Implicit in the prefix-style method descriptions above is the construction of the complete
filenames by the implementation method. By convention, ASCII traces in the ns-3 system
are of the form "<prefix>-<node id>-<device id>.tr"

As previously mentioned, every Node in the system will have a system-assigned Node id.
Since there is a one-to-one correspondence between protocols and nodes we use to node-id
to identify the protocol identity. Every interface on a given protocol will have an
interface index (also called simply an interface) relative to its protocol. By default,
then, an ASCII trace file created as a result of enabling tracing on the first device of
Node 21, using the prefix "prefix", would be "prefix-n21-i1.tr". Use the prefix to
disambiguate multiple protocols per node.

You can always use the ns-3 object name service to make this more clear. For example, if
you use the object name service to assign the name "serverIpv4" to the protocol on Node
21, and also specify interface one, the resulting ASCII trace file name will automatically
become, "prefix-nserverIpv4-1.tr".

Several of the methods have a default parameter called explicitFilename. When set to
true, this parameter disables the automatic filename completion mechanism and allows you
to create an explicit filename. This option is only available in the methods which take a
prefix and enable tracing on a single device.

Summary
ns-3 includes an extremely rich environment allowing users at several levels to customize
the kinds of information that can be extracted from simulations.

There are high-level helper functions that allow users to simply control the collection of
pre-defined outputs to a fine granularity. There are mid-level helper functions to allow
more sophisticated users to customize how information is extracted and saved; and there
are low-level core functions to allow expert users to alter the system to present new and
previously unexported information in a way that will be immediately accessible to users at
higher levels.

This is a very comprehensive system, and we realize that it is a lot to digest, especially
for new users or those not intimately familiar with C++ and its idioms. We do consider
the tracing system a very important part of ns-3 and so recommend becoming as familiar as
possible with it. It is probably the case that understanding the rest of the ns-3 system
will be quite simple once you have mastered the tracing system

DATA COLLECTION

Our final tutorial chapter introduces some components that were added to ns-3 in version
3.18, and that are still under development. This tutorial section is also a
work-in-progress.

Motivation
One of the main points of running simulations is to generate output data, either for
research purposes or simply to learn about the system. In the previous chapter, we
introduced the tracing subsystem and the example sixth.cc. from which PCAP or ASCII trace
files are generated. These traces are valuable for data analysis using a variety of
external tools, and for many users, such output data is a preferred means of gathering
data (for analysis by external tools).

However, there are also use cases for more than trace file generation, including the
following:

· generation of data that does not map well to PCAP or ASCII traces, such as non-packet
data (e.g. protocol state machine transitions),

· large simulations for which the disk I/O requirements for generating trace files is
prohibitive or cumbersome, and

· the need for online data reduction or computation, during the course of the simulation.
A good example of this is to define a termination condition for the simulation, to tell
it when to stop when it has received enough data to form a narrow-enough confidence
interval around the estimate of some parameter.

The ns-3 data collection framework is designed to provide these additional capabilities
beyond trace-based output. We recommend that the reader interested in this topic consult
the ns-3 Manual for a more detailed treatment of this framework; here, we summarize with
an example program some of the developing capabilities.

Example Code
The tutorial example examples/tutorial/seventh.cc resembles the sixth.cc example we
previously reviewed, except for a few changes. First, it has been enabled for IPv6
support with a command-line option:

CommandLine cmd;
cmd.AddValue ("useIpv6", "Use Ipv6", useV6);
cmd.Parse (argc, argv);

If the user specifies useIpv6, option, the program will be run using IPv6 instead of IPv4.
The help option, available on all ns-3 programs that support the CommandLine object as
shown above, can be invoked as follows (please note the use of double quotes):

./waf --run "seventh --help"

which produces:

ns3-dev-seventh-debug [Program Arguments] [General Arguments]

Program Arguments:
--useIpv6: Use Ipv6 [false]

General Arguments:
--PrintGlobals: Print the list of globals.
--PrintGroups: Print the list of groups.
--PrintGroup=[group]: Print all TypeIds of group.
--PrintTypeIds: Print all TypeIds.
--PrintAttributes=[typeid]: Print all attributes of typeid.
--PrintHelp: Print this help message.

This default (use of IPv4, since useIpv6 is false) can be changed by toggling the boolean
value as follows:

./waf --run "seventh --useIpv6=1"

and have a look at the pcap generated, such as with tcpdump:

tcpdump -r seventh.pcap -nn -tt

This has been a short digression into IPv6 support and the command line, which was also
introduced earlier in this tutorial. For a dedicated example of command line usage,
please see src/core/examples/command-line-example.cc.

Now back to data collection. In the examples/tutorial/ directory, type the following
command: diff -u sixth.cc seventh.cc, and examine some of the new lines of this diff:

+ std::string probeType;
+ std::string tracePath;
+ if (useV6 == false)
+ {
...
+ probeType = "ns3::Ipv4PacketProbe";
+ tracePath = "/NodeList/*/$ns3::Ipv4L3Protocol/Tx";
+ }
+ else
+ {
...
+ probeType = "ns3::Ipv6PacketProbe";
+ tracePath = "/NodeList/*/$ns3::Ipv6L3Protocol/Tx";
+ }
...
+ // Use GnuplotHelper to plot the packet byte count over time
+ GnuplotHelper plotHelper;
+
+ // Configure the plot. The first argument is the file name prefix
+ // for the output files generated. The second, third, and fourth
+ // arguments are, respectively, the plot title, x-axis, and y-axis labels
+ plotHelper.ConfigurePlot ("seventh-packet-byte-count",
+ "Packet Byte Count vs. Time",
+ "Time (Seconds)",
+ "Packet Byte Count");
+
+ // Specify the probe type, trace source path (in configuration namespace), and
+ // probe output trace source ("OutputBytes") to plot. The fourth argument
+ // specifies the name of the data series label on the plot. The last
+ // argument formats the plot by specifying where the key should be placed.
+ plotHelper.PlotProbe (probeType,
+ tracePath,
+ "OutputBytes",
+ "Packet Byte Count",
+ GnuplotAggregator::KEY_BELOW);
+
+ // Use FileHelper to write out the packet byte count over time
+ FileHelper fileHelper;
+
+ // Configure the file to be written, and the formatting of output data.
+ fileHelper.ConfigureFile ("seventh-packet-byte-count",
+ FileAggregator::FORMATTED);
+
+ // Set the labels for this formatted output file.
+ fileHelper.Set2dFormat ("Time (Seconds) = %.3e\tPacket Byte Count = %.0f");
+
+ // Specify the probe type, probe path (in configuration namespace), and
+ // probe output trace source ("OutputBytes") to write.
+ fileHelper.WriteProbe (probeType,
+ tracePath,
+ "OutputBytes");
+
Simulator::Stop (Seconds (20));
Simulator::Run ();
Simulator::Destroy ();

The careful reader will have noticed, when testing the IPv6 command line attribute above,
that seventh.cc had created a number of new output files:

seventh-packet-byte-count-0.txt
seventh-packet-byte-count-1.txt
seventh-packet-byte-count.dat
seventh-packet-byte-count.plt
seventh-packet-byte-count.png
seventh-packet-byte-count.sh

These were created by the additional statements introduced above; in particular, by a
GnuplotHelper and a FileHelper. This data was produced by hooking the data collection
components to ns-3 trace sources, and marshaling the data into a formatted gnuplot and
into a formatted text file. In the next sections, we'll review each of these.

GnuplotHelper
The GnuplotHelper is an ns-3 helper object aimed at the production of gnuplot plots with
as few statements as possible, for common cases. It hooks ns-3 trace sources with data
types supported by the data collection system. Not all ns-3 trace sources data types are
supported, but many of the common trace types are, including TracedValues with plain old
data (POD) types.

Let's look at the output produced by this helper:

seventh-packet-byte-count.dat
seventh-packet-byte-count.plt
seventh-packet-byte-count.sh

The first is a gnuplot data file with a series of space-delimited timestamps and packet
byte counts. We'll cover how this particular data output was configured below, but let's
continue with the output files. The file seventh-packet-byte-count.plt is a gnuplot plot
file, that can be opened from within gnuplot. Readers who understand gnuplot syntax can
see that this will produce a formatted output PNG file named
seventh-packet-byte-count.png. Finally, a small shell script
seventh-packet-byte-count.sh runs this plot file through gnuplot to produce the desired
PNG (which can be viewed in an image editor); that is, the command:

sh seventh-packet-byte-count.sh

will yield seventh-packet-byte-count.png. Why wasn't this PNG produced in the first
place? The answer is that by providing the plt file, the user can hand-configure the
result if desired, before producing the PNG.

The PNG image title states that this plot is a plot of "Packet Byte Count vs. Time", and
that it is plotting the probed data corresponding to the trace source path:

/NodeList/*/$ns3::Ipv6L3Protocol/Tx

Note the wild-card in the trace path. In summary, what this plot is capturing is the plot
of packet bytes observed at the transmit trace source of the Ipv6L3Protocol object;
largely 596-byte TCP segments in one direction, and 60-byte TCP acks in the other (two
node trace sources were matched by this trace source).

How was this configured? A few statements need to be provided. First, the GnuplotHelper
object must be declared and configured:

+ // Use GnuplotHelper to plot the packet byte count over time
+ GnuplotHelper plotHelper;
+
+ // Configure the plot. The first argument is the file name prefix
+ // for the output files generated. The second, third, and fourth
+ // arguments are, respectively, the plot title, x-axis, and y-axis labels
+ plotHelper.ConfigurePlot ("seventh-packet-byte-count",
+ "Packet Byte Count vs. Time",
+ "Time (Seconds)",
+ "Packet Byte Count");

To this point, an empty plot has been configured. The filename prefix is the first
argument, the plot title is the second, the x-axis label the third, and the y-axis label
the fourth argument.

The next step is to configure the data, and here is where the trace source is hooked.
First, note above in the program we declared a few variables for later use:

+ std::string probeType;
+ std::string tracePath;
+ probeType = "ns3::Ipv6PacketProbe";
+ tracePath = "/NodeList/*/$ns3::Ipv6L3Protocol/Tx";

We use them here:

+ // Specify the probe type, trace source path (in configuration namespace), and
+ // probe output trace source ("OutputBytes") to plot. The fourth argument
+ // specifies the name of the data series label on the plot. The last
+ // argument formats the plot by specifying where the key should be placed.
+ plotHelper.PlotProbe (probeType,
+ tracePath,
+ "OutputBytes",
+ "Packet Byte Count",
+ GnuplotAggregator::KEY_BELOW);

The first two arguments are the name of the probe type and the trace source path. These
two are probably the hardest to determine when you try to use this framework to plot other
traces. The probe trace here is the Tx trace source of class Ipv6L3Protocol. When we
examine this class implementation (src/internet/model/ipv6-l3-protocol.cc) we can observe:

.AddTraceSource ("Tx", "Send IPv6 packet to outgoing interface.",
MakeTraceSourceAccessor (&Ipv6L3Protocol::m_txTrace))

This says that Tx is a name for variable m_txTrace, which has a declaration of:

/**
* \brief Callback to trace TX (transmission) packets.
*/
TracedCallback<Ptr<const Packet>, Ptr<Ipv6>, uint32_t> m_txTrace;

It turns out that this specific trace source signature is supported by a Probe class (what
we need here) of class Ipv6PacketProbe. See the files
src/internet/model/ipv6-packet-probe.{h,cc}.

So, in the PlotProbe statement above, we see that the statement is hooking the trace
source (identified by path string) with a matching ns-3 Probe type of Ipv6PacketProbe. If
we did not support this probe type (matching trace source signature), we could have not
used this statement (although some more complicated lower-level statements could have been
used, as described in the manual).

The Ipv6PacketProbe exports, itself, some trace sources that extract the data out of the
probed Packet object:

TypeId
Ipv6PacketProbe::GetTypeId ()
{
static TypeId tid = TypeId ("ns3::Ipv6PacketProbe")
.SetParent<Probe> ()
.AddConstructor<Ipv6PacketProbe> ()
.AddTraceSource ( "Output",
"The packet plus its IPv6 object and interface that serve as the output for this probe",
MakeTraceSourceAccessor (&Ipv6PacketProbe::m_output))
.AddTraceSource ( "OutputBytes",
"The number of bytes in the packet",
MakeTraceSourceAccessor (&Ipv6PacketProbe::m_outputBytes))
;
return tid;
}

The third argument of our PlotProbe statement specifies that we are interested in the
number of bytes in this packet; specifically, the "OutputBytes" trace source of
Ipv6PacketProbe. Finally, the last two arguments of the statement provide the plot legend
for this data series ("Packet Byte Count"), and an optional gnuplot formatting statement
(GnuplotAggregator::KEY_BELOW) that we want the plot key to be inserted below the plot.
Other options include NO_KEY, KEY_INSIDE, and KEY_ABOVE.

Supported Trace Types
The following traced values are supported with Probes as of this writing:

┌─────────────────┬─────────────────┬─────────────────────────────────┐
│TracedValue type │ Probe type │ File │
├─────────────────┼─────────────────┼─────────────────────────────────┤
│double │ DoubleProbe │ stats/model/double-probe.h │
├─────────────────┼─────────────────┼─────────────────────────────────┤
│uint8_t │ Uinteger8Probe │ stats/model/uinteger-8-probe.h │
├─────────────────┼─────────────────┼─────────────────────────────────┤
│uint16_t │ Uinteger16Probe │ stats/model/uinteger-16-probe.h │
├─────────────────┼─────────────────┼─────────────────────────────────┤
│uint32_t │ Uinteger32Probe │ stats/model/uinteger-32-probe.h │
├─────────────────┼─────────────────┼─────────────────────────────────┤
│bool │ BooleanProbe │ stats/model/uinteger-16-probe.h │
├─────────────────┼─────────────────┼─────────────────────────────────┤
│ns3::Time │ TimeProbe │ stats/model/time-probe.h │
└─────────────────┴─────────────────┴─────────────────────────────────┘

The following TraceSource types are supported by Probes as of this writing:

┌────────────────────┬────────────────────────┬───────────────┬───────────────────────────────────────────────┐
├────────────────────┼────────────────────────┼───────────────┼───────────────────────────────────────────────┤
├────────────────────┼────────────────────────┼───────────────┼───────────────────────────────────────────────┤
├────────────────────┼────────────────────────┼───────────────┼───────────────────────────────────────────────┤
├────────────────────┼────────────────────────┼───────────────┼───────────────────────────────────────────────┤
├────────────────────┼────────────────────────┼───────────────┼───────────────────────────────────────────────┤
└────────────────────┴────────────────────────┴───────────────┴───────────────────────────────────────────────┘

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.

Use ns-3-tutorial online using onworks.net services