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For a pdf version of this tutorial, see http://www.nsnam.org/docs/tutorial.pdf.
This is an ns-3 tutorial.
Primary documentation for the ns-3 project is available in
four forms:
This document is written in GNU Texinfo and is to be maintained in revision
control on the ns-3 code server. Both PDF and HTML versions should
be available on the server. Changes to the document should be discussed on
the ns-developers@isi.edu mailing list.
This software is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
This software 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, see http://www.gnu.org/licenses/.
| 1. Introduction | ||
| 2. Resources | ||
| 3. Getting Started | ||
| 4. Conceptual Overview | ||
| 5. Tweaking ns-3 | ||
| 6. Building Topologies |
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| 1.1 For ns-2 Users | ||
| 1.2 Contributing | ||
| 1.3 Tutorial Organization |
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.
Primary documentation for the ns-3 project is available in four
forms:
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 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.
Ns-3 is open-source, and the project strives to maintain an
open environment for researchers to contribute and share their software.
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For those familiar with ns-2, the most visible outward change when moving to
ns-3 is the choice of scripting language. Ns-2 is
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. The results of some simulations can be visualized by
nam, but new animators are under 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 ascii 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?" The answer is that it depends. ns-3 does not have
all of the models that ns-2 currently has, but on the other hand, ns-3
does have new capabilities (such as handling multiple interfaces on nodes
correctly, use of IP addressing and more alignment with Internet
protocols and designs, more detailed 802.11 models, etc.). ns-2
models can usually be ported to ns-3 (a porting guide is under
development). There is active development on multiple fronts for
ns-3. The ns-3 developers believe (and certain early users
have proven) that ns-3 is ready for active use, and should be an
attractive alternative for users looking to start new simulation projects.
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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:
src/contrib directory (we will host your contributed code);
Ns-3 developers will gladly help potential contributors to get
started with the simulator (please contact one of us).
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.
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The tutorial assumes that new users might initially follow a path such as the following:
As a result, we have tried to organize the tutorial along the above broad sequences of events.
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| 2.1 The Web | ||
| 2.2 Mercurial | ||
| 2.3 Waf | ||
| 2.4 Development Environment | ||
| 2.5 Socket Programming |
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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/documents.html. 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.
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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.
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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, and will
only have to understand a tiny and intuitively obvious subset of Python in
order to extend the system in most cases.
For those interested in the gory details of Waf, the main web site can be found at http://freehackers.org/~tnagy/waf.html.
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As mentioned above, scripting in ns-3 is done in C++ or Python.
As of ns-3.2, 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,
either make or autotools, using Waf instead.
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 supports development in the Cygwin environment for
these users. See http://www.cygwin.com/
for details on downloading (MinGW is presently not supported).
Cygwin provides many of the popular Linux system commands.
It can, however, sometimes be problematic due to the way it actually does its
emulation, and sometimes interactions with other Windows software can cause
problems.
If you do use Cygwin or MinGW; and use Logitech products, we will save you quite a bit of heartburn right off the bat and encourage you to take a look at the MinGW FAQ.
Search for “Logitech” and read the FAQ entry, “why does make often
crash creating a sh.exe.stackdump file when I try to compile my source code.”
Believe it or not, the Logitech Process Monitor insinuates itself into
every DLL in the system when it is running. It can cause your Cygwin or
MinGW DLLs to die in mysterious ways and often prevents debuggers from
running. Beware of Logitech software when using Cygwin.
Another alternative to Cygwin is to install a virtual machine environment such as VMware server and install a Linux virtual machine.
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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 Practical 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.
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| 3.1 Downloading ns-3 | ||
| 3.2 Building ns-3 | ||
| 3.3 Testing ns-3 | ||
| 3.4 Running a Script |
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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. We are also going to assume that you have
Mercurial and Waf installed and running on the target system as described in
the Getting Started section of the ns-3 web site:
http://www.nsnam.org/getting_started.html.
The ns-3 code is available in Mercurial repositories on the server
code.nsnam.org. You can also download a tarball release at
http://www.nsnam.org/releases/, 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 subystems of ns-3 for you. We
recommend that you begin your ns-3 adventures in this environment
as it can really simplify your life at this point.
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One practice is to create a directory called repos in one's home
directory under which one can keep local Mercurial repositories.
Hint: we will assume you do this later in the tutorial. If you adopt
that approach, you can get a copy of ns-3-allinone by typing the
following into your Linux shell (assuming you have installed Mercurial):
cd mkdir repos cd repos hg clone http://code.nsnam.org/ns-3-allinone
As the hg (Mercurial) command executes, you should see something like the following displayed,
destination directory: ns-3-allinone requesting all changes adding changesets adding manifests adding file changes added 26 changesets with 40 changes to 7 files 7 files updated, 0 files merged, 0 files removed, 0 files unresolved
After the clone command completes, you should have a directory called
ns-3-allinone under your ~/repos directory, the contents of which should
look something like the following:
build.py* constants.py dist.py* download.py* README util.py
Notice that you really just downloaded some Python scripts. The next step
will be to use those scripts to download and build the ns-3
distribution of your choice.
If you go to the following link: http://code.nsnam.org/,
you will see a number of repositories. Many are the private repositories of
the ns-3 development team. The repositories of interest to you will
be prefixed with “ns-3”. Official releases of ns-3 will be
numbered as ns-3.<release>.<hotfix>. For example, a second hotfix to a
still hypothetical release nine of ns-3 would be numbered as
ns-3.9.2.
We have had a regression testing framework in place since the first release.
For each release, a set of output files that define “good behavior” are saved.
These known good output files are called reference traces and are associated
with a given release by name. For example, in http://code.nsnam.org/
you will find a repository named ns-3.1 which is the first stable release
of ns-3. You will also find a separate repository named
ns-3.1-ref-traces that holds the reference traces for the ns-3.1
release. It is crucial to keep these files consistent if you want to do any
regression testing of your repository. This is a good idea to do at least once
to verify everything has built correctly.
The current development snapshot (unreleased) of ns-3 may be found
at http://code.nsnam.org/ns-3-dev/ and the associated reference traces
may be found at http://code.nsnam.org/ns-3-dev-ref-traces/. 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.
Since the release numbers are going to be changing, I will stick with the more constant ns-3-dev here in the tutorial, but you can replace the string “ns-3-dev” with your choice of release (e.g., ns-3.4 and ns-3.4-ref-traces) in the text below. You can find the latest version of the code either by inspection of the repository list or by going to the “Getting Started” web page and looking for the latest release identifier.
Go ahead and change into the ns-3-allinone directory you created when
you cloned that repository. We are now going to use the download.py
script to pull down the various pieces of ns-3 you will be using/
Go ahead and type the following into your shell (remember you can substitute
the name of your chosen release number instead of ns-3-dev – like
"ns-3.4" and "ns-3.4-ref-traces" if you want to work with a
stable release).
./download.py -n ns-3-dev -r ns-3-dev-ref-traces
As the hg (Mercurial) command executes, you should see something like the following,
#
# Get NS-3
#
Cloning ns-3 branch
=> hg clone http://code.nsnam.org/ns-3-dev ns-3-dev
requesting all changes
adding changesets
adding manifests
adding file changes
added 4292 changesets with 15368 changes to 1671 files
823 files updated, 0 files merged, 0 files removed, 0 files unresolved
This is output by the download script as it fetches the actual ns-3
code from the repository. Next, you should see something like,
#
# Get the regression traces
#
Synchronizing reference traces using Mercurial.
=> hg clone http://code.nsnam.org/ns-3-dev-ref-traces ns-3-dev-ref-traces
requesting all changes
adding changesets
adding manifests
adding file changes
added 79 changesets with 1102 changes to 222 files
206 files updated, 0 files merged, 0 files removed, 0 files unresolved
This is the download script fetching the reference trace files for you. The download script is smart enough to know that on some platforms various pieces of ns-3 are not supported. On your platform you may not see some of these pieces come down. However, on most platforms, the process should continue with something like,
#
# Get PyBindGen
#
Required pybindgen version: 0.10.0.630
Trying to fetch pybindgen; this will fail if no network connection is available. Hit Ctrl-C to skip.
=> bzr checkout -rrevno:630 https://launchpad.net/pybindgen pybindgen
Fetch was successful.
This was the download script getting the Python bindings generator for you. Next you should see (modulo platform variations) something along the lines of,
#
# Get NSC
#
Required NSC version: nsc-0.5.0
Retrieving nsc from https://secure.wand.net.nz/mercurial/nsc
=> hg clone https://secure.wand.net.nz/mercurial/nsc nsc
requesting all changes
adding changesets
adding manifests
adding file changes
added 270 changesets with 17375 changes to 14991 files
10614 files updated, 0 files merged, 0 files removed, 0 files unresolved
This part of the process is the script downloading the Network Simulation Cradle for you.
After the clone command completes, you should have several new directories
under ~/repos/ns-3-allinone:
build.py* constants.pyc download.py* ns-3-dev-ref-traces/ pybindgen/ util.py constants.py dist.py* ns-3-dev/ nsc/ README util.pyc
Go ahead and change into ns-3-dev under your ~/repos/ns-3-allinone
directory. You should see something like the following there:
AUTHORS examples/ regression/ scratch/ waf* bindings/ LICENSE regression.py src/ waf.bat* CHANGES.html ns3/ RELEASE_NOTES utils/ wscript doc/ README samples/ VERSION wutils.py
You are now ready to build the ns-3 distribution.
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The process for downloading ns-3 via tarball is simpler than the
Mercurial process since all of the pieces are pre-packaged for you. You just
have to pick a release, download it and decompress it.
As mentioned above, one practice is to create a directory called repos
in one's home directory under which one can keep local Mercurial repositories.
One could also keep a tarballs directory. Hint: the tutorial
will assume you downloaded into a repos directory, so remember the
placekeeper. If you adopt the tarballs 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 tarballs cd tarballs wget http://www.nsnam.org/releases/ns-allinone-3.4.tar.bz2 tar xjf ns-3.4.tar.bz2
If you change into the directory ns-allinone-3.4 you should see a
number of files:
build.py* ns-3.4-RC2/ nsc-0.5.0/ util.py constants.py ns-3.4-RC2-ref-traces/ pybindgen-0.10.0.630/
You are now ready to build the ns-3 distribution.
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The first time you build the ns-3 project you should build using the
allinone environment. This will get the project configured for you
in the most commonly useful way.
Change into the directory you created in the download section above. If you
downloaded using Mercurial you should have a directory called
ns-3-allinone under your ~/repos directory. If you downloaded
using a tarball you should have a directory called something like
ns-3-allinone-3.4 under your ~/tarballs directory. Take a deep
breath and type the following:
./build.py
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 magic words:
Build finished successfully (00:02:37) Leaving directory `./ns-3-dev'
Once the project has built you can say goodbye to your old friends, the
ns-3-allinone scripts. You got what you needed from them and will now
interact directly with Waf and we do it in the ns-3-dev directory and
not in the ns-3-allinone directory. Go ahead and change into the
ns-3-dev directory (or the directory for the appropriate release you
downloaded.
cd ns-3-dev
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We use Waf to configure and build the ns-3 project. 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 do
make an optimized build. To explain to Waf that it should do optimized
builds you will need to execute the following command,
./waf -d optimized configure
This runs Waf out of the local directory (which is provided as a convenience for you). As the build system checks for various dependencies you should see output that looks similar to the following,
Checking for program g++ : ok /usr/bin/g++ Checking for program cpp : ok /usr/bin/cpp Checking for program ar : ok /usr/bin/ar Checking for program ranlib : ok /usr/bin/ranlib Checking for g++ : ok Checking for program pkg-config : ok /usr/bin/pkg-config Checking for regression reference traces : ok ../ns-3-dev-ref-traces (guessed) Checking for -Wno-error=deprecated-declarations support : yes Checking for header stdlib.h : ok Checking for header signal.h : ok Checking for header pthread.h : ok Checking for high precision time implementation : 128-bit integer Checking for header stdint.h : ok Checking for header inttypes.h : ok Checking for header sys/inttypes.h : not found Checking for library rt : ok Checking for header netpacket/packet.h : ok Checking for header linux/if_tun.h : ok Checking for pkg-config flags for GTK_CONFIG_STORE : ok Package libxml-2.0 was not found in the pkg-config search path. Perhaps you should add the directory containing `libxml-2.0.pc' to the PKG_CONFIG_PATH environment variable No package 'libxml-2.0' found Checking for pkg-config flags for LIBXML2 : not found Checking for library sqlite3 : ok Checking for NSC location : ok ../nsc (guessed) Checking for library dl : ok Checking for NSC supported architecture x86_64 : ok Package goocanvas was not found in the pkg-config search path. Perhaps you should add the directory containing `goocanvas.pc' to the PKG_CONFIG_PATH environment variable No package 'goocanvas' found Checking for pkg-config flags for MOBILITY_VISUALIZER : not found Checking for program python : ok /usr/bin/python Checking for Python version >= 2.3 : ok 2.5.2 Checking for library python2.5 : ok Checking for program python2.5-config : ok /usr/bin/python2.5-config Checking for header Python.h : ok Checking for -fvisibility=hidden support : yes Checking for pybindgen location : ok ../pybindgen (guessed) Checking for Python module pybindgen : ok Checking for pybindgen version : ok 0.10.0.630 Checking for Python module pygccxml : ok Checking for pygccxml version : ok 0.9.5 Checking for program gccxml : ok /usr/local/bin/gccxml Checking for gccxml version : ok 0.9.0 Checking for program sudo : ok /usr/bin/sudo Checking for program hg : ok /usr/bin/hg Checking for program valgrind : ok /usr/bin/valgrind ---- Summary of optional NS-3 features: Threading Primitives : enabled Real Time Simulator : enabled Emulated Net Device : enabled Tap Bridge : enabled GtkConfigStore : enabled XmlIo : not enabled (library 'libxml-2.0 >= 2.7' not found) SQlite stats data output : enabled Network Simulation Cradle : enabled Python Bindings : enabled Python API Scanning Support : enabled Use sudo to set suid bit : not enabled (option --enable-sudo not selected) Configuration finished successfully (00:00:02); project is now ready to build.
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. in the example above, this library was not found and the corresponding feature was not enabled. There is a feature to use sudo to set the suid bit of certain programs. This was not enabled by default.
Now go ahead and switch back to the debug build.
./waf -d debug configure
The build system is now configured and you can build the debug versions of
the ns-3 programs by simply typing,
./waf
Some waf commands are meaningful during the build phase and some commands are valid
in the configuration 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. This is a configuration command, and so you could have run the following
command
./waf -d debug --enable-sudo configure
If you had done this, waf would have run sudo to change the socket creator programs 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.
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.
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You can run the unit tests of the ns-3 distribution by running the
“check” command,
./waf check
You should see a report from each unit test that executes indicating that the test has passed.
Entering directory `repos/ns-3-allinone/ns-3-dev/build' Build finished successfully (00:00:00) -- Running NS-3 C++ core unit tests... PASS AddressHelper PASS Wifi PASS DcfManager ... PASS Object PASS Ptr PASS Callback -- Running NS-3 Python bindings unit tests... ........... ---------------------------------------------------------------------- Ran 11 tests in 0.003s OK
This command is typically run by users to quickly verify that an
ns-3 distribution has built correctly.
You can also run our regression test suite to ensure that your distribution and
tool chain have produced binaries that generate output that is identical to
known-good reference output files. You downloaded these reference traces to
your machine during the download process above. (Warning: The ns-3.2
and ns-3.3 releases do not use the ns-3-allinone environment
and require you to be online when you run regression tests because they
dynamically synchronize the reference traces directory with an online
repository immediately prior to the run).
During regression testing Waf will run a number of tests that generate what we call trace files. The content of these trace files are compared with the reference traces. If they are identical, the regression tests report a PASS status. If a regression test fails you will see a FAIL indication along with a pointer to the offending trace file and its associated reference trace file along with a suggestion on diff parameters and options in order to see what has gone awry. If the error was discovered in a pcap file, it will be useful to convert the pcap files to text using tcpdump prior to comparison.
Some regression tests wmay be SKIPped if the required support is not present.
To run the regression tests, you provide Waf with the regression flag.
./waf --regression
You should see messages indicating that many tests are being run and are passing.
Entering directory `repos/ns-3-allinone/ns-3-dev/build' [647/669] regression-test (test-csma-bridge) [648/669] regression-test (test-csma-broadcast) [649/669] regression-test (test-csma-multicast) [650/669] regression-test (test-csma-one-subnet) PASS test-csma-multicast [651/669] regression-test (test-csma-packet-socket) PASS test-csma-bridge ... Regression testing summary: PASS: 22 of 22 tests passed Build finished successfully (00:00:23)
If you want to take a look at an example of what might be checked during a regression test, you can do the following:
cd build/debug/regression/traces/second.ref tcpdump -nn -tt -r second-2-0.pcap
The output should be clear to anyone who is familiar with tcpdump or net sniffers. We'll have much more to say on pcap files later in this tutorial.
Remember to cd back into the top-level ns-3 directory
after you are done:
cd ../../../../..
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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 then executes the program, which produces the following output.
Hello Simulator
Congratulations. You are now an ns-3 user.
If you want to run programs under another tool such as gdb or valgrind, see this wiki entry.
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| 4.1 Key Abstractions | ||
| 4.2 A First ns-3 Script |
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.
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In this section, we'll review some terms that are commonly used in
networking, but have a specific meaning in ns-3.
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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.
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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
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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.
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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, theys 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.
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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.
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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/ README samples/ utils/ waf.bat* build/ LICENSE regression/ scratch/ VERSION wscript doc/ ns3/ RELEASE_NOTES src/ waf*
Change into the examples 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.
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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 */
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The code proper starts with a number of include statements.
#include "ns3/core-module.h" #include "ns3/simulator-module.h" #include "ns3/node-module.h" #include "ns3/helper-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 configure
in order to configure the project to perform debug builds. You will also have done a
./waf
to build the project. So now if you look in the directory
build/debug/ns-3 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.
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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.
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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 “APIs (Doxygen)” in the navigation bar. If you select this link, you will be taken to our 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.” 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. It turns out that the ns-3
logging subsystem is part of the core module, 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 #defines 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.
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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 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.
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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.
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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.
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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
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.
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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.
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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.
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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.
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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. You can, if desired, set the “Port” Attribute
to another value later.
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.
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. 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 implicilty declared
a simulation event (the application stop event) to be executed at ten seconds,
the simulation will last at least ten seconds.
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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.)));
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).
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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
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. Then it will run the event scheduled for t=2.0 seconds which
will start the echo client application. 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, 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 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;
}
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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/first.cc into
the scratch directory.
~/repos/ns-3-dev > cp examples/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.
Entering directory `repos/ns-3-allinone-dev/ns-3-dev/build' [563/648] cxx: scratch/myfirst.cc -> build/debug/scratch/myfirst_3.o [646/648] cxx_link: build/debug/scratch/myfirst_3.o -> build/debug/scratch/myfirst Build finished successfully (00:00:02)
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:
Entering directory `repos/ns-3-allinone-dev/ns-3-dev/build' Build finished successfully (00:00:00) 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.
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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/?sort=lastchange. If you click on the bold
repository names on the left of the page, you will see changelogs for
these repositories, and links to the manifest. From the manifest
links, one can browse the source tree.
The top-level directory for one of our repositories will look something like:
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 regression files drwxr-xr-x samples files drwxr-xr-x scratch files drwxr-xr-x src files drwxr-xr-x utils files -rw-r--r-- 2009-03-24 00:51 -0700 505 .hgignore file | revisions | annotate -rw-r--r-- 2009-03-24 00:51 -0700 1682 .hgtags file | revisions | annotate -rw-r--r-- 2009-03-24 00:51 -0700 686 AUTHORS file | revisions | annotate -rw-r--r-- 2009-03-24 00:51 -0700 14893 CHANGES.html file | revisions | annotate -rw-r--r-- 2009-03-24 00:51 -0700 17987 LICENSE file | revisions | annotate -rw-r--r-- 2009-03-24 00:51 -0700 3742 README file | revisions | annotate -rw-r--r-- 2009-03-24 00:51 -0700 13505 RELEASE_NOTES file | revisions | annotate -rw-r--r-- 2009-03-24 00:51 -0700 6 VERSION file | revisions | annotate -rw-r--r-- 2009-03-24 00:51 -0700 9257 regression.py file | revisions | annotate -rwxr-xr-x 2009-03-24 00:51 -0700 81285 waf file | revisions | annotate -rwxr-xr-x 2009-03-24 00:51 -0700 28 waf.bat file | revisions | annotate -rw-r--r-- 2009-03-24 00:51 -0700 26270 wscript file | revisions | annotate -rw-r--r-- 2009-03-24 00:51 -0700 6636 wutils.py file | revisions | annotate
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 be taken to the lising of the src subdirectories. If you
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 abort.h link, you will be sent to the source file for abort.h.
Our example scripts are in the examples directory. The source code for
the helpers we have used in this chapter can be found in the
src/helpers 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.
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| 5.1 Using the Logging Module | ||
| 5.2 Using Command Line Arguments | ||
| 5.3 Using the Tracing System |
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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.
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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.
We also provide an unconditional logging level that is always displayed, irrespective of logging levels or component selection.
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.
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Let's use the NS_LOG environment variable to turn on some more logging, but to get our bearings, go ahead and run the script just as you did previously,
./waf --run scratch/myfirst
You should see the now familiar output of the first ns-3 example
program
Entering directory `repos/ns-3-dev/build' Compilation finished successfully 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 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:
Entering directory `repos/ns-3-allinone/ns-3-dev/build' Build finished successfully (00:00:00) UdpEchoClientApplication:UdpEchoClient() UdpEchoClientApplication:StartApplication() UdpEchoClientApplication:ScheduleTransmit() UdpEchoClientApplication:Send() Sent 1024 bytes to 10.1.1.2 Received 1024 bytes from 10.1.1.1 UdpEchoClientApplication:HandleRead(0x638180, 0x6389b0) 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. Note 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 ORing 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.
Entering directory `repos/ns-3-allinone/ns-3-dev/build' Build finished successfully (00:00:00) UdpEchoClientApplication:UdpEchoClient() 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(0x638180, 0x6389b0) 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.
Entering directory `repos/ns-3-allinone/ns-3-dev/build' Build finished successfully (00:00:00) UdpEchoServerApplication:UdpEchoServer() UdpEchoClientApplication:UdpEchoClient() 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(0x638320, 0x638b50) 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:
Entering directory `repos/ns-3-allinone/ns-3-dev/build' Build finished successfully (00:00:00) 0s UdpEchoServerApplication:UdpEchoServer() 0s UdpEchoClientApplication:UdpEchoClient() 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(0x638490, 0x638cc0) 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. The same for the UdpEchoClient constructor.
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
(or one billion nanoseconds). 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 as the packet is sent across the point-to-point link is 3.6864
milliseconds. You see the echo server logging a message telling you that it
has echoed the packet and then, after a 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 974 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 volume of logging quite a bit 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. When I have a general idea about what is going wrong, I transition into a debugger for 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.
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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 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,
Entering directory `repos/ns-3-allinone/ns-3-dev/build' Build finished successfully (00:00:00) 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
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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/first.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,
Entering directory `repos/ns-3-allinone/ns-3-dev/build' Build finished successfully (00:00:00) --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 first.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,
Build finished successfully (00:00:00) 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:
Entering directory `repos/ns-3-allinone/ns-3-dev/build' Build finished successfully (00:00:00) 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 in the script. Then you
have to rebuild the script using the default. You will also have to find the
syntax for actually setting the new default atribute 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"
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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"
Entering directory `repos/ns-3-allinone/ns-3-dev/build'
Build finished successfully (00:00:00)
--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
Entering directory `repos/ns-3-allinone/ns-3-dev/build' Build finished successfully (00:00:00) 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.
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.
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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:
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.
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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.
The first thing you need to do is to add the following include to the top of the script just after the GNU GPL comment:
#include <fstream>
Then, right before the before the call to Simulator::Run (), add the
following lines of code.
std::ofstream ascii;
ascii.open ("myfirst.tr");
PointToPointHelper::EnableAsciiAll (ascii);
The first two lines are just vanilla C++ code to open a stream that will be
written to a file named “myfirst.tr.” See your favorite C++ tutorial if you
are unfamiliar with this code. The last line of code in the snippet above
tells ns-3 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 to the
stream provided. 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 the “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.
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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 two digit reference number on the left side:
00 + 01 2 02 /NodeList/0/DeviceList/0/$ns3::PointToPointNetDevice/TxQueue/Enqueue 03 ns3::PppHeader ( 04 Point-to-Point Protocol: IP (0x0021)) 05 ns3::Ipv4Header ( 06 tos 0x0 ttl 64 id 0 protocol 17 offset 0 flags [none] 07 length: 1052 10.1.1.1 > 10.1.1.2) 08 ns3::UdpHeader ( 09 length: 1032 49153 > 9) 10 Payload (size=1024)
The first line of this expanded trace event (reference number 00) is the
operation. We have a + character, so this corresponds to an
enqueue operation on the transmit queue. The second line (reference 01)
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 line of the example trace (reference 02) 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, $ns3::PointToPointNetDevice tells you what kind of
device is in the zeroth position of the device list for node zero.
Recall that the operation + found at reference 00 meant that an enqueue
operation happened on the transmit queue of the device. This is reflected in
the final segments of the “trace path” which are TxQueue/Enqueue.
The remaining lines in the trace should be fairly intuitive. References 03-04 indicate that the packet is encapsulated in the point-to-point protocol. References 05-07 show that the packet has an IP version four header and has originated from IP address 10.1.1.1 and is destined for 10.1.1.2. References 08-09 show that this packet has a UDP header and, finally, reference 10 shows that the payload is the expected 1024 bytes.
The next line in the trace file shows the same packet being dequeued from the transmit queue on the same node.
The Third line in the trace file shows the packet being received by the net device on the node with the echo server. I have reproduced that event below.
00 r 01 2.25732 02 /NodeList/1/DeviceList/0/$ns3::PointToPointNetDevice/MacRx 03 ns3::PppHeader ( 04 Point-to-Point Protocol: IP (0x0021)) 05 ns3::Ipv4Header ( 06 tos 0x0 ttl 64 id 0 offset 0 flags [none] 07 length: 1052 10.1.1.1 > 10.1.1.2) 08 ns3::UdpHeader ( 09 length: 1032 49153 > 9) 10 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.
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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.
PointToPointHelper::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.
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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 (first-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.
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If you are unfamilar 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.
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| 6.1 Building a Bus Network Topology | ||
| 6.2 Building a Wireless Network Topology |
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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 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/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/simulator-module.h" #include "ns3/node-module.h" #include "ns3/helper-module.h" #include "ns3/global-routing-module.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 node 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.)));
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 a global route
manager to set up the routing tables on nodes. This route manager has a
global function that runs though the nodes created for the simulation and does
the hard work of setting up routing for you.
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:
GlobalRouteManager::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.
PointToPointHelper::EnablePcapAll ("second");
CsmaHelper::EnablePcap ("second", csmaDevices.Get (0), 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 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(0),
which selects the zeroth 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, you have to 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 would type,
cp examples/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
#end verbatim
Since we have set up the UDP echo applications to log just as we did in
@code{first.cc}, you will see similar output when you run the script.
@verbatim
Entering directory `repos/ns-3-allinone/ns-3-dev/build'
Build finished successfully (00:00:00)
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 two 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 2.007602 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 2.003915 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) 2.003696 arp who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1 2.003707 arp reply 10.1.2.4 is-at 00:00:00:00:00:06 2.003801 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024 2.003811 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4 2.003822 arp reply 10.1.2.1 is-at 00:00:00:00:00:03 2.003915 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,
2.003696 arp who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1 2.003707 arp reply 10.1.2.4 is-at 00:00:00:00:00:06
Then node one, device one goes ahead and sends the echo packet to the UDP echo server at IP address 10.1.2.4.
2.003801 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.
2.003811 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4 2.003822 arp reply 10.1.2.1 is-at 00:00:00:00:00:03
The server then sends the echo back to the forwarding node.
2.003915 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 2.003915 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 2.007602 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,
Entering directory `repos/ns-3-allinone/ns-3-dev/build' Build finished successfully (00:00:00) Sent 1024 bytes to 10.1.2.5 Received 1024 bytes from 10.1.1.1 Received 1024 bytes from 10.1.2.5
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,
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.
PointToPointHelper::EnablePcap ("second", p2pNodes.Get (0)->GetId (), 0);
CsmaHelper::EnablePcap ("second", csmaNodes.Get (nCsma)->GetId (), 0, false);
CsmaHelper::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.
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:
Entering directory `repos/ns-3-allinone/ns-3-dev/build' Build finished successfully (00:00:00) Sent 1024 bytes to 10.1.2.101 Received 1024 bytes from 10.1.1.1 Received 1024 bytes from 10.1.2.101
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-10-0.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) 2.003696 arp who-has 10.1.2.101 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1 2.003811 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.101
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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 directory. This script
builds on the second.cc script and adds a Wifi network. Go ahead and
open examples/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/simulator-module.h" #include "ns3/node-module.h" #include "ns3/helper-module.h" #include "ns3/global-routing-module.h" #include "ns3/wifi-module.h" #include "ns3/mobility-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 objects created layer by the YansWifiPhyHelper
all 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:
WifiHelper wifi = WifiHelper::Default ();
wifi.SetRemoteStationManager ("ns3::AarfWifiManager");
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 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");
wifi.SetMac ("ns3::NqstaWifiMac",
"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 is specified by
Attribute as being of the "ns3::NqstaWifiMac" type. This means that
the MAC will use a “non-QoS station” (nqsta) state machine. 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, 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 WifiHelper to reflect the
requirements of the AP.
wifi.SetMac ("ns3::NqapWifiMac",
"Ssid", SsidValue (ssid),
"BeaconGeneration", BooleanValue (true),
"BeaconInterval", TimeValue (Seconds (2.5)));
In this case, the WifiHelper is going to create MAC layers of the
“ns3::NqapWifiMac” (Non-Qos Access Point) type. We set the
“BeaconGeneration” Attribute to true and also set an interval between
beacons of 2.5 seconds.
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, 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.)));
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.
GlobalRouteManager::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, 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:
PointToPointHelper::EnablePcapAll ("third");
YansWifiPhyHelper::EnablePcap ("third", apDevices.Get (0));
CsmaHelper::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/third.cc scratch/mythird.cc ./waf ./waf --run scratch/mythird
Since we have set up the UDP echo applications just as we did in the
second.cc script, you will see similar output.
Entering directory `repos/ns-3-allinone-dev/ns-3-dev/build' Build finished successfully (00:00:00) Sent 1024 bytes to 10.1.2.4 Received 1024 bytes from 10.1.3.3 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 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, 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 () [6.0* 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit] IBSS 0.000263 Assoc Request () [6.0 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit] 0.000279 Acknowledgment RA:00:00:00:00:00:07 0.000357 Assoc Response AID(0) :: Succesful 0.000501 Acknowledgment RA:00:00:00:00:00:0a 0.000748 Assoc Request () [6.0 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit] 0.000764 Acknowledgment RA:00:00:00:00:00:08 0.000842 Assoc Response AID(0) :: Succesful 0.000986 Acknowledgment RA:00:00:00:00:00:0a 0.001242 Assoc Request () [6.0 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit] 0.001258 Acknowledgment RA:00:00:00:00:00:09 0.001336 Assoc Response AID(0) :: Succesful 0.001480 Acknowledgment RA:00:00:00:00:00:0a 2.000112 arp who-has 10.1.3.4 (ff:ff:ff:ff:ff:ff) tell 10.1.3.3 2.000128 Acknowledgment RA:00:00:00:00:00:09 2.000206 arp who-has 10.1.3.4 (ff:ff:ff:ff:ff:ff) tell 10.1.3.3 2.000487 arp reply 10.1.3.4 is-at 00:00:00:00:00:0a 2.000659 Acknowledgment RA:00:00:00:00:00:0a 2.002169 IP 10.1.3.3.49153 > 10.1.2.4.9: UDP, length 1024 2.002185 Acknowledgment RA:00:00:00:00:00:09 2.009771 arp who-has 10.1.3.3 (ff:ff:ff:ff:ff:ff) tell 10.1.3.4 2.010029 arp reply 10.1.3.3 is-at 00:00:00:00:00:09 2.010045 Acknowledgment RA:00:00:00:00:00:09 2.010231 IP 10.1.2.4.9 > 10.1.3.3.49153: UDP, length 1024 2.011767 Acknowledgment RA:00:00:00:00:00:0a 2.500000 Beacon () [6.0* 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit] IBSS 5.000000 Beacon () [6.0* 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit] IBSS 7.500000 Beacon () [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) 2.002169 IP 10.1.3.3.49153 > 10.1.2.4.9: UDP, length 1024 2.009771 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) 2.005855 IP 10.1.3.3.49153 > 10.1.2.4.9: UDP, length 1024 2.006084 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) 2.005855 arp who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1 2.005877 arp reply 10.1.2.4 is-at 00:00:00:00:00:06 2.005877 IP 10.1.3.3.49153 > 10.1.2.4.9: UDP, length 1024 2.005980 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4 2.005980 arp reply 10.1.2.1 is-at 00:00:00:00:00:03 2.006084 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. Let's do this by hooking into the
MobilityModel course change trace source. This is usually considered
a fairly advanced topic, but let's just go for it.
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, 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 can easily infer that
this trace path references the seventh node in the 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 (00:00:01) /NodeList/7/$ns3::MobilityModel/CourseChange x = 10, y = 0 /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.41539, y = -0.811313 /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.46199, y = -1.11303 /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.52738, y = -1.46869 /NodeList/7/$ns3::MobilityModel/CourseChange x = 6.67099, y = -1.98503 /NodeList/7/$ns3::MobilityModel/CourseChange x = 5.6835, y = -2.14268 /NodeList/7/$ns3::MobilityModel/CourseChange x = 4.70932, y = -1.91689 Sent 1024 bytes to 10.1.2.4 Received 1024 bytes from 10.1.3.3 Received 1024 bytes from 10.1.2.4 /NodeList/7/$ns3::MobilityModel/CourseChange x = 5.53175, y = -2.48576 /NodeList/7/$ns3::MobilityModel/CourseChange x = 4.58021, y = -2.17821 /NodeList/7/$ns3::MobilityModel/CourseChange x = 4.18915, y = -1.25785 /NodeList/7/$ns3::MobilityModel/CourseChange x = 4.7572, y = -0.434856 /NodeList/7/$ns3::MobilityModel/CourseChange x = 4.62404, y = 0.556238 /NodeList/7/$ns3::MobilityModel/CourseChange x = 4.74127, y = 1.54934 /NodeList/7/$ns3::MobilityModel/CourseChange x = 5.73934, y = 1.48729 /NodeList/7/$ns3::MobilityModel/CourseChange x = 6.18521, y = 0.59219 /NodeList/7/$ns3::MobilityModel/CourseChange x = 6.58121, y = 1.51044 /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.27897, y = 2.22677 /NodeList/7/$ns3::MobilityModel/CourseChange x = 6.42888, y = 1.70014 /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.40519, y = 1.91654 /NodeList/7/$ns3::MobilityModel/CourseChange x = 6.51981, y = 1.45166 /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.34588, y = 2.01523 /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.81046, y = 2.90077 /NodeList/7/$ns3::MobilityModel/CourseChange x = 6.89186, y = 3.29596 /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.46617, y = 2.47732 /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.05492, y = 1.56579 /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.00393, y = 1.25054 /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.00968, y = 1.35768 /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.33503, y = 2.30328 /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.18682, y = 3.29223 /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.96865, y = 2.66873
If you are feeling brave, there is a list of all trace sources in the
ns-3 Doxygen
which you can find in the “Modules” tab.
Under the “core” section, you will find a link to “The list of all trace
sources.” You may find it interesting to try and hook some of these
traces yourself. Additionally in the “Modules” documentation, there is
a link to “The list of all attributes.” You can set the default value of
any of these Attributes via the command line as we have previously
discussed.
We have just scratched the surface of ns-3 in this tutorial, but we
hope we have covered enough to get you started doing useful work.
– The ns-3 development team.
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