Conceptual Overview
*******************

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

Key Abstractions
================

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

Node
----

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

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

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

Application
-----------

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

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

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

Channel
-------

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

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

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

Net Device
----------

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

Topology Helpers
----------------

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

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

A First ns-3 Script
===================

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

  AUTHORS  examples/  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.

Boilerplate
-----------

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

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

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

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

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

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

Module Includes
---------------

The code proper starts with a number of include statements.

  #include "ns3/core-module.h"
  #include "ns3/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.

Ns3 Namespace
-------------

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

  using namespace ns3;

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

Logging
-------

The next line of the script is the following,

  NS_LOG_COMPONENT_DEFINE ("FirstScriptExample");

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

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.

Topology Helpers
----------------

NodeContainer
.............

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

    NodeContainer nodes;
    nodes.Create (2);

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

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

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

PointToPointHelper
..................

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

The next three lines in the script are,

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

The first line,

    PointToPointHelper pointToPoint;

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

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

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

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

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

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

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

NetDeviceContainer
..................

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

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

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

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

InternetStackHelper
...................

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

    InternetStackHelper stack;
    stack.Install (nodes);

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

Ipv4AddressHelper
.................

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

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

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

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

The next line of code,

    Ipv4InterfaceContainer interfaces = address.Assign (devices);

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

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

Applications
------------

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

UdpEchoServerHelper
...................

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

    UdpEchoServerHelper echoServer (9);

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

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

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

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

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

Simulator
---------

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

    Simulator::Run ();

When we previously called the methods,

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

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

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

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

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

    Simulator::Destroy ();
    return 0;
  }

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/myfirst.cc

Now build your first example script using waf:

  ./waf

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

  Entering directory `repos/ns-3-allinone-dev/ns-3-dev/build'
  [563/648] cxx: scratch/myfirst.cc -> build/debug/scratch/myfirst_3.o
  [646/648] cxx_link: build/debug/scratch/myfirst_3.o -> build/debug/scratch/myfirst
  Build finished successfully (00:00:02)

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

  ./waf --run scratch/myfirst

You should see some output:

  Entering directory `repos/ns-3-allinone-dev/ns-3-dev/build'
  Build finished successfully (00:00:00)
  Sent 1024 bytes to 10.1.1.2
  Received 1024 bytes from 10.1.1.1
  Received 1024 bytes from 10.1.1.2

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

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   bindings python                          files
drwxr-xr-x   doc                                      files
drwxr-xr-x   examples                                 files
drwxr-xr-x   ns3                                      files
drwxr-xr-x   regression                               files
drwxr-xr-x   samples                                  files
drwxr-xr-x   scratch                                  files
drwxr-xr-x   src                                      files
drwxr-xr-x   utils                                    files
-rw-r--r-- 2009-03-24 00:51 -0700 505   .hgignore     file | revisions | annotate
-rw-r--r-- 2009-03-24 00:51 -0700 1682  .hgtags       file | revisions | annotate
-rw-r--r-- 2009-03-24 00:51 -0700 686   AUTHORS       file | revisions | annotate
-rw-r--r-- 2009-03-24 00:51 -0700 14893 CHANGES.html  file | revisions | annotate
-rw-r--r-- 2009-03-24 00:51 -0700 17987 LICENSE       file | revisions | annotate
-rw-r--r-- 2009-03-24 00:51 -0700 3742  README        file | revisions | annotate
-rw-r--r-- 2009-03-24 00:51 -0700 13505 RELEASE_NOTES file | revisions | annotate
-rw-r--r-- 2009-03-24 00:51 -0700 6     VERSION       file | revisions | annotate
-rw-r--r-- 2009-03-24 00:51 -0700 9257  regression.py file | revisions | annotate
-rwxr-xr-x 2009-03-24 00:51 -0700 81285 waf           file | revisions | annotate
-rwxr-xr-x 2009-03-24 00:51 -0700 28    waf.bat       file | revisions | annotate
-rw-r--r-- 2009-03-24 00:51 -0700 26270 wscript       file | revisions | annotate
-rw-r--r-- 2009-03-24 00:51 -0700 6636  wutils.py     file | revisions | annotate

The source code is mainly in the src directory.  You can view source code either by clicking on the directory name or by clicking on the files link to the right of the directory name.  If you click on the src directory you be taken to the lising of the src subdirectories.  If you click on core subdirectory, you will find a list of files.  The first file you will find (as of this writing) is abort.h.  If you click on abort.h link, you will be sent to the source file for abort.h.

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