ns-3 Tutorial (html version)

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:

• ns-3 Doxygen/Manual: Documentation of the public APIs of the simulator
• Tutorial (this document)
• Reference Manual: Reference Manual
• ns-3 wiki

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

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:

• ns-3 Doxygen/Manual: Documentation of the public APIs of the simulator
• Tutorial (this document)
• Reference Manual: Reference Manual
• ns-3 wiki

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 not an extension of ns-2; it is a new simulator. The two simulators are both written in C++ but ns-3 is a new simulator that does not support the ns-2 APIs. Some models from ns-2 have already been ported from ns-2 to ns-3. The project will continue to maintain ns-2 while ns-3 is being built, and will study transition and integration mechanisms.
• Ns-3 is open-source, and the project strives to maintain an open environment for researchers to contribute and share their software.

1.1 For ns-2 Users

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.

1.2 Contributing

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

• Open source licensing based on GNU GPLv2 compatibility;
• wiki;
• Contributed Code page, similar to ns-2's popular Contributed Code page;
• src/contrib directory (we will host your contributed code);
• Open bug tracker;
• 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.

1.3 Tutorial Organization

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

• Try to download and build a copy;
• Try to run a few sample programs;
• Look at simulation output, and try to adjust it.

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

2. Resources

2.1 The Web

There are several important resources of which any ns-3 user must be aware. The main web site is located at http://www.nsnam.org and provides access to basic information about the ns-3 system. Detailed documentation is available through the main web site at http://www.nsnam.org/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.

2.2 Mercurial

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

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

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

2.3 Waf

Once you have source code downloaded to your local system, you will need to compile that source to produce usable programs. Just as in the case of source code management, there are many tools available to perform this function. Probably the most will 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.

2.4 Development Environment

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.

2.5 Socket Programming

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

3. Getting Started

3.1 Downloading ns-3

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 download a tarball release at http://www.nsnam.org/releases/, or you can work with repositories using Mercurial.

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". The current development snapshot (unreleased) of ns-3 may be found at: http://code.nsnam.org/ns-3-dev/. Official releases of ns-3 will be numbered as ns-3.<release> with any requred hotfixes added as minor release numbers. For example, a second hotfix to a hypothetical release nine of ns-3 would be numbered ns-3.9.2.

The current development snapshot (unreleased) of ns-3 may be found at: http://code.nsnam.org/ns-3-dev/. The developers attempt to keep this repository in a consistent, working state but it is 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-3.2) 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.

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 the development version of ns-3 by typing the following into your Linux shell (assuming you have installed Mercurial):

  cd
  mkdir repos
  cd repos
  hg clone http://code.nanam.org/ns-3-dev

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

  destination directory: ns-3-dev
  requesting all changes
  adding changesets
  adding manifests
  adding file changes
  added 3276 changesets with 12301 changes to 1353 files
  594 files updated, 0 files merged, 0 files removed, 0 files unresolved

After the clone command completes, you should have a directory called ns-3-dev under your ~/repos directory, the contents of which should look something like the following:

  AUTHORS  examples/  README         samples/  utils/   waf.bat*
  build/   LICENSE    regression/    scratch/  VERSION  wscript
  doc/     ns3/       RELEASE_NOTES  src/      waf*

Similarly, if working from a released version instead, you can simply

  cd
  mkdir repos
  wget http://www.nsnam.org/releases/ns-3.2.tar.bz2
  bunzip2 ns-3.2.tar.bz2
  tar xvf ns-3.2.tar

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

3.2 Building ns-3

We use Waf to build the ns-3 project. The first thing you will need to do is to configure the build. For reasons that will become clear later, we are going to work with debug builds in the tutorial. To explain to Waf that it should do debug builds you will need to execute the following command,

  ./waf -d debug 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 compiler version            : ok Version 4.0.1 
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 compiler could create programs : ok  
Checking for compiler could create shared libs : ok  
Checking for compiler could create static libs : ok  
Checking for flags -O2 -DNDEBUG                : ok  
Checking for flags -g -DDEBUG                  : ok  
Checking for flags -g3 -O0 -DDEBUG             : ok  
Checking for flags -Wall                       : ok  
Checking for g++                               : ok  
Checking for -Wno-error=deprecated-declarations compilation flag support : no 
Checking for header stdlib.h                   : ok  
Checking for header stdlib.h                   : ok  
Checking for header signal.h                   : ok  
Checking for library rt                        : not found 
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 package gtk+-2.0 >= 2.12          : not found 
Checking for library sqlite3                   : ok  
Checking for package goocanvas gthread-2.0     : not found 
Checking for program python                    : ok /usr/local/bin/python 
Checking for Python version >= 2.3             : ok 2.4.3 
Checking for library python2.4                 : not found 
Checking for library python2.4                 : not found 
Checking for library python24                  : not found 
Checking for program python2.4-config          : not found 
Checking for header Python.h                   : not found 
Checking for program diff                      : ok /usr/bin/diff 
Checking for program hg                        : ok /opt/local/bin/hg 
---- Summary of optional NS-3 features:
Threading Primitives        : enabled
Real Time Simulator         : enabled
GtkConfigStore              : not enabled (library 'gtk+-2.0 >= 2.12' not found)
SQlite stats data output    : enabled
Network Simulation Cradle   : not enabled (--enable-nsc configure option not given)
Python Bindings             : not enabled (Python development headers not found.)
Configuration finished successfully; project is now ready to build. 

Note the trailing portion of the above output. Some ns-3 options are not enabled by default or require support from the underlying system. For instance, to enable Python bindings, Python development headers must be installed on the host machine, and they were not found in the above example, so Python scripting will not be supported in the resulting build. For this tutorial, we will focus on the non-optional pieces of ns-3.

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

  ./waf

(Hint: if you have a multicore machine, use the "-j JOBS" option to speed up your build, where JOBS is the number of cores) You will see many Waf status messages displayed as the system compiles. The most important is the last one:

  Compilation finished successfully

3.3 Testing ns-3

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.

  ~/repos/ns-3-dev > ./waf check
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully
  PASS AddressHelper
  PASS Wifi
  PASS DcfManager
  
  ...

  PASS Object
  PASS Ptr
  PASS Callback
  ~/repos/ns-3-dev >

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 reference output files stored in a central location. To run the regression tests, you provide Waf with the regression flag.

  ./waf --regression

Waf will verify that the current files in the ns-3 distribution are built and will then look for trace files in the aforementioned centralized location. If your tool chain includes Mercurial, the regression tests will be downloaded from a repository at code.nsnam.org. If you do not have Mercurial installed, the reference traces will be downloaded from a tarball located in the releases section of www.nsnam.org. The particular name of the reference trace location is built from the ns-3 version located in the VERSION file, so don't change that string yourself unless you know what you are doing. (Warning: The ns-3.2 release requires you to be online when you run regression tests because it synchronizes the trace directory with an online repository).

Once the reference traces are downloaded to your local machine, 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 just downloaded. If they are identical, the regression tests report a PASS status. If the regression tests pass, you should see something like,

  ~/repos/ns-3-dev > ./waf --regression
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully 
  ========== Running Regression Tests ==========
  Synchronizing reference traces using Mercurial.
  Pulling http://code.nsnam.org/ns-3-dev-ref-traces from repo.
  Skipping csma-bridge: Python bindings not available.
  SKIP test-csma-bridge
  PASS test-csma-broadcast
  PASS test-csma-multicast
  PASS test-csma-one-subnet
  PASS test-csma-packet-socket
  PASS test-realtime-udp-echo
  PASS test-simple-error-model
  PASS test-simple-global-routing
  PASS test-simple-point-to-point-olsr
  PASS test-tcp-large-transfer
  PASS test-udp-echo
  PASS test-wifi-wired-bridging

  ~/repos/ns-3-dev >

If a regression tests 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. Python regression tests will be SKIPped if Python bindings are not built.

3.4 Running a Script

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

  ./waf --run hello-simulator

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

4. Conceptual Overview

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

4.1 Key Abstractions

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

4.1.1 Node

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

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

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

4.1.2 Application

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

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

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

4.1.3 Channel

In the real world, one can connect a computer to a network. Often the media over which data flows in these netowrks 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.

4.1.4 Net Device

It used to be the case that if you wanted to connect a computers to a network, you had to buy a specific kind of network cable and a hardware device called (in PC terminology) a peripheral card that needed to be installed in your computer. If the peripheral card implemented some networking function, 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.

4.1.5 Topology Helpers

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

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

4.2 A First ns-3 Script

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

  AUTHORS  examples/  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.

4.2.1 Boilerplate

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

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

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

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

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

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

4.2.2 Module Includes

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.

4.2.3 Ns3 Namespace

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

  using namespace ns3;

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

4.2.4 Logging

The next line of the script is the following,

  NS_LOG_COMPONENT_DEFINE ("FirstScriptExample");

We will use this statement as a convenient place to talk about our Doxygen documentation system. If you look at the project web site, ns-3 project, you will find a link to "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.

4.2.5 Main Function

The next lines of the script you will find are,

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

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

The next 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.

4.2.6 Topology Helpers

4.2.6.1 NodeContainer

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

    NodeContainer nodes;
    nodes.Create (2);

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

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

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

4.2.6.2 PointToPointHelper

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

The next three lines in the script are,

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

The first line

    PointToPointHelper pointToPoint;

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

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

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

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

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

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

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

4.2.6.3 NetDeviceContainer

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

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

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

After executing the 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.

4.2.6.4 InternetStackHelper

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

    InternetStackHelper stack;
    stack.Install (nodes);

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

4.2.6.5 Ipv4AddressHelper

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

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

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

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

The next line of code,

    Ipv4InterfaceContainer interfaces = address.Assign (devices);

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

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

4.2.7 Applications

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

4.2.7.1 UdpEchoServerHelper

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

    UdpEchoServerHelper echoServer (9);

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

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

4.2.7.2 UdpEchoClientHelper

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

    UdpEchoClientHelper echoClient (interfaces.GetAddress (1), 9);
    echoClient.SetAttribute ("MaxPackets", UintegerValue (1));
    echoClient.SetAttribute ("Interval", TimeValue (Seconds (1.)));
    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 coorespond to the IP address of the zeroth node in the nodes container. The first interface in the interfaces container cooresponds 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).

4.2.8 Simulator

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

    Simulator::Run ();

When we previously called the methods,

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

we actually scheduled events in the simulator at 1.0 seconds, 2.0 seconds and 10.0 seconds. When Simulator::Run is called, the ssytem 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;
  }

4.2.9 Building Your Script

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

  ~/repos/ns-3-dev > cp examples/first.cc scratch/

and then build it using waf,

  ~/repos/ns-3-dev > ./waf
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  [467/511] cxx: scratch/first.cc -> build/debug/scratch/first_1.o
  [468/511] cxx: scratch/multiple-sources/simple-main.cc -> build/debug/scratch/multiple-sources/simple-main_2.o
  [469/511] cxx: scratch/multiple-sources/simple-simulation.cc -> build/debug/scratch/multiple-sources/simple-simulation_2.o
  [470/511] cxx: scratch/simple.cc -> build/debug/scratch/simple_3.o
  [508/511] cxx_link: build/debug/scratch/first_1.o -> build/debug/scratch/first
  Compilation finished successfully 
  ~/repos/ns-3-dev >

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 direcory):

  ~/repos/ns-3-dev > ./waf --run scratch/first
  Entering directory `/home/craigdo/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
  ~/repos/ns-3-dev >

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.

4.3 Ns-3 Source Code

Now that you have used some of the ns-3 helpers you may want to have a look at some of the source code that implements that functionality. The most recent code can be browsed on our web server at the following link: http://code.nsnam.org/?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         doc             manifest 
drwxr-xr-x         examples        manifest 
drwxr-xr-x         ns3             manifest 
drwxr-xr-x         regression      manifest 
drwxr-xr-x         samples         manifest 
drwxr-xr-x         scratch         manifest 
drwxr-xr-x         src             manifest 
drwxr-xr-x         tutorial        manifest 
drwxr-xr-x         utils           manifest 
-rw-r--r-- 135     .hgignore       file | revisions | annotate 
-rw-r--r-- 891     .hgtags         file | revisions | annotate 
-rw-r--r-- 441     AUTHORS         file | revisions | annotate 
-rw-r--r-- 17987   LICENSE         file | revisions | annotate 
-rw-r--r-- 4948    README          file | revisions | annotate 
-rw-r--r-- 4917    RELEASE_NOTES   file | revisions | annotate 
-rw-r--r-- 7       VERSION         file | revisions | annotate 
-rwxr-xr-x 99143   waf             file | revisions | annotate 
-rwxr-xr-x 28      waf.bat         file | revisions | annotate 
-rw-r--r-- 30584   wscript         file | revisions | annotate 

The source code is mainly in the src directory. You can view source code by clicking on the manifest link to the right of the directory name. If you click on the manifest link to the right of the src directory you will find a subdirectory. If you click on the manifest link next to the core subdirectory in under src, you will find a list of files. The first file you will find is assert.h. If you click on the file link, you will be sent to the source file for assert.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.

5. Tweaking ns-3

5.1 Using the Logging Module

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

5.1.1 Logging Overview

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

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

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

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

• NS_LOG_ERROR -- Log error messages;
• NS_LOG_WARN -- Log warning messages;
• NS_LOG_DEBUG -- Log relatively rare, ad-hoc debugging messages;
• NS_LOG_INFO -- Log informational messages about program progress;
• NS_LOG_FUNCTION -- Log a message describing each function called;
• NS_LOG_LOGIC - Log messages describing logical flow within a function;
• NS_LOG_ALL -- Log everything.

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

• NS_LOG_UNCOND - Log the associated message unconditionally.

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 first.cc example script you have already built.

5.1.2 Enabling Logging

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,

  ~/repos/ns-3-dev > ./waf --run scratch/first
  Entering directory `/home/craigdo/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
  ~/repos/ns-3-dev >

It turns out that the "Sent" and "Received" messages 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 first.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:

  ~/repos/ns-3-dev > 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:

  ~/repos/ns-3-dev > ./waf --run scratch/first
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully
  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(0x62c640, 0x62cd70)
  Received 1024 bytes from 10.1.1.2
  UdpEchoClientApplication:StopApplication()
  UdpEchoClientApplication:DoDispose()
  UdpEchoClientApplication:~UdpEchoClient()
  ~/repos/ns-3-dev >

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 come 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.

  ~/repos/ns-3-dev > ./waf --run scratch/first
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully
  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(0x62c710, 0x62ce40)
  UdpEchoClientApplication:HandleRead(): Received 1024 bytes from 10.1.1.2
  UdpEchoClientApplication:StopApplication()
  UdpEchoClientApplication:DoDispose()
  UdpEchoClientApplication:~UdpEchoClient()
  ~/repos/ns-3-dev >

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'

Note that you will need to remove the newline after the : in the example text above.

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.

  ~/repos/ns-3-dev > ./waf --run scratch/first
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully
  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(0x62c760, 0x62ce90)
  UdpEchoClientApplication:HandleRead(): Received 1024 bytes from 10.1.1.2
  UdpEchoServerApplication:StopApplication()
  UdpEchoClientApplication:StopApplication()
  UdpEchoClientApplication:DoDispose()
  UdpEchoServerApplication:DoDispose()
  UdpEchoClientApplication:~UdpEchoClient()
  UdpEchoServerApplication:~UdpEchoServer()
  ~/repos/ns-3-dev >

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'

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

  ~/repos/ns-3-dev > ./waf --run scratch/first
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully
  0ns UdpEchoServerApplication:UdpEchoServer()
  0ns UdpEchoClientApplication:UdpEchoClient()
  1000000000ns UdpEchoServerApplication:StartApplication()
  2000000000ns UdpEchoClientApplication:StartApplication()
  2000000000ns UdpEchoClientApplication:ScheduleTransmit()
  2000000000ns UdpEchoClientApplication:Send()
  2000000000ns UdpEchoClientApplication:Send(): Sent 1024 bytes to 10.1.1.2
  2003686400ns UdpEchoServerApplication:HandleRead(): Received 1024 bytes 
    from 10.1.1.1
  2003686400ns UdpEchoServerApplication:HandleRead(): Echoing packet
  2007372800ns UdpEchoClientApplication:HandleRead(0x62c8c0, 0x62d020)
  2007372800ns UdpEchoClientApplication:HandleRead(): Received 1024 bytes 
    from 10.1.1.2
  10000000000ns UdpEchoServerApplication:StopApplication()
  10000000000ns UdpEchoClientApplication:StopApplication()
  UdpEchoClientApplication:DoDispose()
  UdpEchoServerApplication:DoDispose()
  UdpEchoClientApplication:~UdpEchoClient()
  UdpEchoServerApplication:~UdpEchoServer()
  ~/repos/ns-3-dev >

You can see that the constructor for the UdpEchoServer was called at a simulation time of 0 nanoseconds. This is actually happening before the simulation starts. The same for the UdpEchoClient constructor.

Recall that the 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 772 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,

  ~/repos/ns-3-dev > ./waf --run scratch/first >& log.out

I personally use this 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.

5.1.3 Adding Logging to your Code

You can add new logging to your simulations by making calls to the log component via several macros. Let's do so in the first.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,

  NS_LOG_INFO ("Creating Topology");

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

  ~/repos/ns-3-dev > export NS_LOG=

Now, if you run the script, 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,

  ~/repos/ns-3-dev > export NS_LOG=FirstScriptExample=info

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

  ~/repos/ns-3-dev > ./waf --run scratch/first
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully
  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
  ~/repos/ns-3-dev >

5.2 Using Command Line Arguments

5.2.1 Overriding Default Attributes

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

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

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

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

    ...
  }

This simple two line snippet is actually very useful by itself. It opens the door to the ns-3 global variable and attribute systems. Go ahead and add that two lines of code to the 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,

  ~/repos/ns-3-dev > ./waf --run "scratch/first --PrintHelp"

This will ask Waf to run the scratch/first 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,

  ~/repos/ns-3-dev > ./waf --run ``scratch/first --PrintHelp''
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully
  --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.
  ~/repos/ns-3-dev >

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/first --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,

  ~/repos/ns-3-dev > ./waf --run scratch/first
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully
  0ns UdpEchoServerApplication:UdpEchoServer()
  1000000000ns UdpEchoServerApplication:StartApplication()
  Sent 1024 bytes to 10.1.1.2
  2257324218ns Received 1024 bytes from 10.1.1.1
  2257324218ns Echoing packet
  Received 1024 bytes from 10.1.1.2
  10000000000ns UdpEchoServerApplication:StopApplication()
  UdpEchoServerApplication:DoDispose()
  UdpEchoServerApplication:~UdpEchoServer()
  ~/repos/ns-3-dev >

Recall that the last time we looked at the simulation time at which the packet was received by the echo server, it was at 2.0036864 seconds. Now it is receiving the packet at about 2.257 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/first --ns3::PointToPointNetDevice::DataRate=5Mbps"

This will set the default value of the DataRate attribute back to five megabits per second. To get the original behavior of the script back, we will have to set the speed-of-light delay of the channel. We can ask the command line system to print out the Attributes of the channel just like we did the net device:

  ./waf --run "scratch/first --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/first
    --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:

  Compilation finished successfully
  0ns UdpEchoServerApplication:UdpEchoServer()
  1000000000ns UdpEchoServerApplication:StartApplication()
  Sent 1024 bytes to 10.1.1.2
  2003686400ns Received 1024 bytes from 10.1.1.1
  2003686400ns Echoing packet
  Received 1024 bytes from 10.1.1.2
  10000000000ns UdpEchoServerApplication:StopApplication()
  UdpEchoServerApplication:DoDispose()
  UdpEchoServerApplication:~UdpEchoServer()

Note that the packet is again received by the server at 2.0036864 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/first 
    --ns3::PointToPointNetDevice::DataRate=5Mbps 
    --ns3::PointToPointChannel::Delay=2ms 
    --ns3::UdpEchoClient::MaxPackets=2"

5.2.2 Hooking Your Own Values

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

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

  ~/repos/ns-3-dev > ./waf --run "scratch/first --PrintHelp"
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully
  --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
  ~/repos/ns-3-dev >

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,

  ~/repos/ns-3-dev > ./waf --run "scratch/first --nPackets=2"
  Entering directory `/home/craigdo/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
  Sent 1024 bytes to 10.1.1.2
  Received 1024 bytes from 10.1.1.1
  Received 1024 bytes from 10.1.1.2
  ~/repos/ns-3-dev >

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.

5.3 Using the Tracing System

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

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

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

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

• For basic tasks, the tracing system should allow the user to generate standard tracing for popular tracing sources, and to customize which objects generate the tracing;
• Intermediate users must be able to extend the tracing system to modify the output format generated, or to insert new tracing sources, without modifying the core of the simulator;
• Advanced users can modify the simulator core to add new tracing sources and sinks.

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

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

The rationale for this explicit division is to allow users to attach new types of sinks to existing tracing sources, without requiring editing and recompilation of the 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.

5.3.1 ASCII Tracing

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

Let's just jump right in and add some ASCII tracing output to our first.cc script. The first thing you need to do is to add the following code to the script just before the call to Simulator::Run ().

  std::ofstream ascii;
  ascii.open ("first.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 "first.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.

Since we have used a std::ofstream object, we also need to include the appropriate header. Add the following line to the script (I typically add it above the ns-3 includes):

  #include <fstream>

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

  ./waf --run scratch/first

Just as you have seen previously, you may see some messages from Waf and then the "Compilation finished successfully" with some number of messages from the running program.

When it ran, the program will have created a file named first.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 first.tr in your favorite editor.

5.3.1.1 Parsing Ascii Traces

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

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

• +: An enqueue operation occurred on the device queue;
• -: A dequeue operation occurred on the device queue;
• d: A packet was dropped, typically because the queue was full;
• r: A packet was received by the net device.

Let's take a more detailed view of the first line in the trace file. I'll break it down into sections (indented for clarity) with a 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 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 encapulated 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/Rx 
  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 (/Rx). 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.

5.3.2 PCAP Tracing

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

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

  PointToPointHelper::EnablePcapAll ("first");

Go ahead and insert this line of code after the ASCII tracing code we just added to scratch/first.cc. Notice that we only passed the string "first," and not "first.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 "first-0-0.pcap" and "first.1-0.pcap" which are the pcap traces for node 0-device 0 and node 1-device 1, 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/first

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

5.3.2.1 Reading output with tcpdump

The easiest thing to do at this point will be to use tcpdump to look at the pcap files. Output from dumping both files is shown below:

  ~/repos/ns-3-dev > /usr/sbin/tcpdump -r first-0-0.pcap -nn -tt
  reading from file first-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
  ~/repos/ns-3-dev > /usr/sbin/tcpdump -r first-1-0.pcap -nn -tt
  reading from file first-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
  ~/repos/ns-3-dev >

You can see in the dump of "first-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 (of "first-1-0.pcap") you can see that packet being received at 2.257324 seconds. You see the packet being echoed 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.

5.3.2.2 Reading output with Wireshark

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.

6. Building Topologies

6.1 Building a Bus Network Topology

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

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

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

We provide an example script in our examples 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.

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 thee "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

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

  #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"
  
  using namespace ns3;
  
  NS_LOG_COMPONENT_DEFINE ("SecondScriptExample");

The main program begins by enabling the UdpEchoClientApplication and UdpEchoServerApplication logging components at INFO level so we can see some output when we run the example. This should be entirely familiar to you so far.

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

Next, 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.

  uint32_t nCsma = 3;
  CommandLine cmd;
  cmd.AddValue ("nCsma", "Number of \"extra\" CSMA nodes/devices", nCsma);
  cmd.Parse (argc,argv);

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 delare 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.

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.

  CsmaHelper csma;

  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 the 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 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 ();

The remainder of the script should be very familiar to you. We just enable pcap tracing, run the simulation and exit the script. Notice that enabling pcap tracing using the CSMA helper is done in the same way as for the pcap tracing with the point-to-point helper.

    PointToPointHelper::EnablePcapAll ("second");
    CsmaHelper::EnablePcapAll ("second");
  
    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/
  ./waf
  ./waf --run scratch/second

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

  ~/repos/ns-3-dev > ./waf --run scratch/second
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully
  Sent 1024 bytes to 10.1.2.4
  Received 1024 bytes from 10.1.1.1
  Received 1024 bytes from 10.1.2.4
  ~/repos/ns-3-dev >

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 a number of trace files:

  ~/repos/ns-3-dev > ls *.pcap
  second-0-0.pcap  second-1-1.pcap  second-3-0.pcap
  second-1-0.pcap  second-2-0.pcap  second-4-0.pcap
  ~/repos/ns-3-dev >

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. There are no other devices on node zero so this is the only trace from that node.

Now look at second-1-0.pcap and second-1-1.pcap. The former is the pcap trace for device zero on node one and the latter is the trace file for device one on node one. If you refer back to the topology illustrration at the start of the section, you will see that node one is the node that has both a point-to-point device and a CSMA device, so we should expect two pcap traces for that node.

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.

  ~/repos/ns-3-dev > tcpdump -r second-0-0.pcap -nn -tt
  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
  2.007382 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024
  ~/repos/ns-3-dev >

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:

  ~/repos/ns-3-dev > tcpdump -r second-1-0.pcap -nn -tt
  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
  2.003695 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024
  ~/repos/ns-3-dev >

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 headed toward 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 the other device headed for its ultimate destination. Let's then look at second-1-1.pcap and see if its there.

  ~/repos/ns-3-dev > tcpdump -r second-1-1.pcap -nn -tt
  reading from file second-1-1.pcap, link-type EN10MB (Ethernet)
  2.003686 arp who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1
  2.003687 arp reply 10.1.2.4 is-at 00:00:00:00:00:06
  2.003687 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
  2.003691 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4
  2.003691 arp reply 10.1.2.1 is-at 00:00:00:00:00:03
  2.003695 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024
  ~/repos/ns-3-dev >

As you can see, the link type is now "Ethernet." Something new has appeared, though. The bus network needs ARP, the Address Resolution Protocol. The node 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. This exchange is seen in the following lines,

  2.003686 arp who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1
  2.003687 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. We can now look at the pcap trace for the echo server,

  ~/repos/ns-3-dev > tcpdump -r second-4-0.pcap -nn -tt
  reading from file second-4-0.pcap, link-type EN10MB (Ethernet)
  2.003686 arp who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1
  2.003686 arp reply 10.1.2.4 is-at 00:00:00:00:00:06
  2.003690 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
  2.003690 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4
  2.003692 arp reply 10.1.2.1 is-at 00:00:00:00:00:03
  2.003692 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024
  ~/repos/ns-3-dev >

Again, you see that the link type is "Ethernet." The first two entries are the ARP exchange we just explained. The third packet is the echo packet being delivered to its final destination.

The echo server turns the packet around and needs to send it back to the echo client on 10.1.1.1 but it 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. We leave it as an exercise for you to find the entries corresponding to the packet returning back on its way to the client (we have already dumped the traces and you can find them in those tcpdumps above.

Let's take a look at one of the CSMA nodes that wasn't involved in the packet exchange:

  ~/repos/ns-3-dev > tcpdump -r second-2-0.pcap -nn -tt
  reading from file second-2-0.pcap, link-type EN10MB (Ethernet)
  2.003686 arp who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1
  2.003691 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4
  ~/repos/ns-3-dev >

You can see that the CSMA channel is a broadcast medium and so all of the devices see the ARP requests involved in the packet exchange. The remaining pcap trace will be identical to this one.

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 setting the number of "extra" devices to four:

  ~/repos/ns-3-dev > ./waf --run "scratch/second --nCsma=4"
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully
  Sent 1024 bytes to 10.1.2.5
  Received 1024 bytes from 10.1.1.1
  Received 1024 bytes from 10.1.2.5
  ~/repos/ns-3-dev >

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. You can increase the number to your hearts content, but remember that you will get a pcap trace file for every node in the simulation. One thing you can do to keep from getting all of those pcap traces with nothing but ARP exchanges in them is to be more specific about which nodes and devices you want to trace.

Let's take a look at scratch/second.cc and add that code enabling us to be more specific. The file we provided used the EnablePcapAll methods of the helpers to enable pcap on all devices. We now want to use the more specific method, EnablePcap, which takes a node number and device number as parameters. Go ahead and replace the EnablePcapAll calls with the calls below.

  PointToPointHelper::EnablePcap ("second", p2pNodes.Get (0)->GetId (), 0);
  CsmaHelper::EnablePcap ("second", csmaNodes.Get (nCsma)->GetId (), 0);

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.

Now that we have got some trace filtering in place, it is reasonable to start increasing the number of CSMA devices in our simulation. If you build the new script and run the simulation setting nCsma to 100, you will see the following output:

  ~/repos/ns-3-dev > ./waf --run "scratch/second --nCsma=100"
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully
  Sent 1024 bytes to 10.1.2.101
  Received 1024 bytes from 10.1.1.1
  Received 1024 bytes from 10.1.2.101
  ~/repos/ns-3-dev >

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,

  ~/repos/ns-3-dev > ls *.pcap
  second-0-0.pcap  second-101-0.pcap
  ~/repos/ns-3-dev >

you will see that we have, in fact, only created two trace files. 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.

6.2 Building a Wireless Network Topology

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

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

We provide an example script in our examples 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 thee "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");

As has become the norm in this tutorial, the main program begins by enabling the UdpEchoClientApplication and UdpEchoServerApplication logging components at INFO level so we can see some output when we run the simulation.

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

Next, you will see more familiar code that will allow you to change the number of devices on the CSMA and Wifi networks via command line argument.

  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.Parse (argc,argv);

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 delare 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 a NetDeviceContainer to keep track of the CSMA net devices. Then we Install CSMA devices on the selected nodes.

  CsmaHelper csma;

  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 is going to be quite different from the helper-based topology generation we've seen so far, so we're going to take it line-by-line for a while. The next line of code you will see is:

  Ptr<WifiChannel> channel = CreateObject<WifiChannel> ();

Now, I'm not going to explain at this stage precisely what this all means, but hopefully with a very short digression I can give you enough information so that this makes sense.

C++ is an object oriented programming language. Ns-3 extends the basic C++ object model to implement a number of nifty features. We have seen the Attribute system which is one of the major extensions we have implemented. Another extension is to provide for relatively automatic memory management. Like many systems, ns-3 creates a base class called Object that provides our extensions "for free" to other classes that inherit from our class Object.

In the code snippet above, the right hand side of the expression is a call to a templated C++ function called CreateObject. The template parameter inside the angle brackets basically tells the compiler what class it is we want to instantiate. Our system returns a smart pointer to the object of the class that was created and assigns it to the smart pointer named channel that is declared on the left hand side of the assignment.

The ns-3 smart pointer is also template-based. Here you see that we declare a smart pointer to a WifiChannel which is the type of object that was created in the CreateObject call. The feature of immediate interest here is that we are never going to have to delete the underlying C++ object. It is handled automatically for us. Nice, eh?

The idea to take away from this discussion is that this line of code creates an ns-3 Object that will automatically bring you the benefits of the ns-3 Attribute system we've seen previously. The resulting smart pointer works with the Object to perform memory management automatically for you. If you are interested in more details about low level ns-3 code and exactly what it is doing, you are encouraged to explore the ns-3 manual and our "how-to" documents.

Now, back to the example. The line of code above has created a wireless Wifi channel. This channel model requires that we create and attach other models that describe various behaviors. This provides an accomplished user with even more opportunity to change the way the wireless network behaves without changing the core code.

The first opportunity we have to change the behavior of the wireless network is by providing a propagation delay model. Again, I don't want to devolve this tutorial into a manual on Wifi, but this model describes how the electromagnetic signals are going to propagate. We are going to create the simplest model, the ConstantSpeedPropagationDelayModel that, by default, has the signals propagating at a constant speed -- approximately that of the speed of light in air.

Recall that we created the WifiChannel and assigned it to a smart pointer. One of the features of a smart pointer is that you can use it just as you would a "normal" C++ pointer. The next line of code will create a ConstantSpeedPropagationDelayModel using the CreateObject template function and pass the resulting smart pointer to the chanel model as an unnamed parameter of the WifiChannel SetPropagationDelayModel method. In English, we create a model for propagation speed of electromagnetic signals and tell the wireless channel to use it.

  channel->SetPropagationDelayModel (
    CreateObject<ConstantSpeedPropagationDelayModel> ());

The next lines of code use similar low-level ns-3 methods to create and set a "propagation loss model" for the channel.

  Ptr<LogDistancePropagationLossModel> log =
    CreateObject<LogDistancePropagationLossModel> ();

  log->SetReferenceModel (CreateObject<FriisPropagationLossModel> ());

  channel->SetPropagationLossModel (log);

This snippet is used to tell the channel how it should calculate signal attenuation of waves flowing in the channel. The details of these calcuations are beyond the scope of a tutorial. You are encouraged to explore the Doxygen documentation of classes LogDistancePropagationLossModel and FriisPropagationLossModel if you are interested in the details. As usual, you will find the documentation in the "Classes" tab of the Doxygen documentation.

Now we will return to more familiar ground. We next create a WifiHelper object and set two default attributes that it will use when creating the actual devices.

  WifiHelper wifi;
  wifi.SetPhy ("ns3::WifiPhy");
  wifi.SetRemoteStationManager ("ns3::ArfWifiManager");

The SetPhy method tells the helper the type of physical layer class we want it to instantiate when building Wifi devices. In this case, the script is asking for physical layer models based on the YANS 802.11a model. Again, details are avialable in Doxygen.

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, avialable in Doxygen.

Just as we can vary attributes describing the physical layer, we can do the same for the MAC layer.

  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.

Again, for the next lines of code we are back on familiar ground. This code will Install Wifi net devices on the nodes we have created as STA nodes and will tie them to the WifiChannel. Since we created the channel manually rather than having the helper do it for us, we have to pass it into the helper when we call the Install method.

  NetDeviceContainer staDevices;
  staDevices = wifi.Install (wifiStaNodes, channel);

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 and connect it to the channel in a familiar way.

  NetDeviceContainer apDevices;
  apDevices = wifi.Install (wifiApNode, channel);

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 obejct 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 aranged 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::StaticMobilityModel:

  mobility.SetMobilityModel ("ns3::StaticMobilityModel");
  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 the 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 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 use the same trick as in the second.cc script to only generate pcap traces from the nodes we find interesting. Note that we use the same "formula" to get pcap tracing enabled on Wifi devices as we did on the CSMA and point-to-point devices.

  WifiHelper::EnablePcap ("third",
    wifiStaNodes.Get (nWifi - 1)->GetId (), 0);
  CsmaHelper::EnablePcap ("third",
    csmaNodes.Get (nCsma)->GetId (), 0);

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/
  ./waf
  ./waf --run scratch/third

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

  ~/repos/ns-3-dev > ./waf --run scratch/third
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully
  Sent 1024 bytes to 10.1.2.4
  Received 1024 bytes from 10.1.3.3
  Received 1024 bytes from 10.1.2.4
  ~/repos/ns-3-dev >

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 two trace files:

  ~/repos/ns-3-dev > ls *.pcap
  third-4-0.pcap  third-7-0.pcap
  ~/repos/ns-3-dev >

The file "third-4-0.pcap" corresponds to the pcap trace for node four - device zero. This is the CSMA network node that acted as the echo server. Take a look at the tcpdump for this device:

  ~/repos/ns-3-dev > tcpdump -r third-4-0.pcap -nn -tt
  reading from file third-4-0.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
  2.005855 arp reply 10.1.2.4 is-at 00:00:00:00:00:06
  2.005859 IP 10.1.3.3.49153 > 10.1.2.4.9: UDP, length 1024
  2.005859 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4
  2.005861 arp reply 10.1.2.1 is-at 00:00:00:00:00:03
  2.005861 IP 10.1.2.4.9 > 10.1.3.3.49153: UDP, length 1024
  ~/repos/ns-3-dev >

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

Now, take a look at the other trace file, "third-7-0.pcap." This is the trace file for the wireless STA node that acts as the echo client.

  ~/repos/ns-3-dev > tcpdump -r third-7-0.pcap -nn -tt
  reading from file third-7-0.pcap, link-type IEEE802_11 (802.11)
  0.000146 Beacon (ns-3-ssid) ...
  H: 0
  0.000180 Assoc Request (ns-3-ssid) ...
  0.000336 Acknowledgment RA:00:00:00:00:00:07
  0.000454 Assoc Response AID(0) :: Succesful
  0.000514 Acknowledgment RA:00:00:00:00:00:0a
  0.000746 Assoc Request (ns-3-ssid) ...
  0.000902 Acknowledgment RA:00:00:00:00:00:09
  0.001020 Assoc Response AID(0) :: Succesful
  0.001036 Acknowledgment RA:00:00:00:00:00:0a
  0.001219 Assoc Request (ns-3-ssid) ...
  0.001279 Acknowledgment RA:00:00:00:00:00:08
  0.001478 Assoc Response AID(0) :: Succesful
  0.001538 Acknowledgment RA:00:00:00:00:00:0a
  2.000000 arp who-has 10.1.3.4 (ff:ff:ff:ff:ff:ff) tell 10.1.3.3
  2.000172 Acknowledgment RA:00:00:00:00:00:09
  2.000318 arp who-has 10.1.3.4 (ff:ff:ff:ff:ff:ff) tell 10.1.3.3
  2.000581 arp reply 10.1.3.4 is-at 00:00:00:00:00:0a
  2.000597 Acknowledgment RA:00:00:00:00:00:0a
  2.000693 IP 10.1.3.3.49153 > 10.1.2.4.9: UDP, length 1024
  2.002229 Acknowledgment RA:00:00:00:00:00:09
  2.009663 arp who-has 10.1.3.3 (ff:ff:ff:ff:ff:ff) tell 10.1.3.4
  2.009697 arp reply 10.1.3.3 is-at 00:00:00:00:00:09
  2.009869 Acknowledgment RA:00:00:00:00:00:09
  2.011487 IP 10.1.2.4.9 > 10.1.3.3.49153: UDP, length 1024
  2.011503 Acknowledgment RA:00:00:00:00:00:0a
  2.500112 Beacon[|802.11]
  5.000112 Beacon[|802.11]
  7.500112 Beacon[|802.11]
  ~/repos/ns-3-dev >

You can see that the link type is now 802.11 as you would expect. We leave it as an exercise to parse the dump and trace packets across the internetwork.

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/third.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.

  ~/repos/ns-3-dev > ./waf --run scratch/third
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 10, y = 0
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.1304, y = 0.493761
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.70417, y = 1.39837
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.94799, y = 2.05274
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.82597, y = 1.57404
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.3003, y = 0.723347
  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/$ns3::MobilityModel/CourseChange x = 8.74083, y = 1.62109
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.00146, y = 0.655647
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.98731, y = 0.823279
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.50206, y = 1.69766
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.68108, y = 2.26862
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.25992, y = 1.45317
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.55655, y = 0.742346
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.21992, y = 1.68398
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.81273, y = 0.878638
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.83171, y = 1.07256
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.60027, y = 0.0997156
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.45367, y = 0.620978
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.68484, y = 1.26043
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.53659, y = 0.736479
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.51876, y = 0.548502
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.89778, y = 1.47389
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.98984, y = 1.893
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.91524, y = 1.51402
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.98761, y = 1.14054
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.16617, y = 0.570239
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.02954, y = 1.56086
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.09551, y = 2.55868
  ~/repos/ns-3-dev >

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.

Table of Contents

About This Document

This document was generated on September, 23 2008 using texi2html 1.76.

The buttons in the navigation panels have the following meaning:

where the Example assumes that the current position is at Subsubsection One-Two-Three of a document of the following structure:

• 1. Section One
  • 1.1 Subsection One-One
    • ...
  • 1.2 Subsection One-Two
    • 1.2.1 Subsubsection One-Two-One
    • 1.2.2 Subsubsection One-Two-Two
    • 1.2.3 Subsubsection One-Two-Three     <== Current Position
    • 1.2.4 Subsubsection One-Two-Four
  • 1.3 Subsection One-Three
    • ...
  • 1.4 Subsection One-Four

This document was generated on September, 23 2008 using texi2html 1.76.