@c ========================================================================

@c Begin document body here

@c ========================================================================

@c ========================================================================

@c Conceptual Overview

@c ========================================================================

@node Conceptual Overview

@chapter Conceptual Overview

@menu

* Key Abstractions::

@end menu

The first thing we need to do before actually starting to look at or write

@command{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.

@node Key Abstractions

@section Key Abstractions

In this section, we'll review some terms that are commonly used in

networking, but have a specific meaning in @command{ns-3}.

@subsection Node

@cindex Node

In Internet jargon, a computing device that connects to a network is called

a @emph{host} or sometimes an @emph{end system}.  Because @command{ns-3} is a 

@emph{network} simulator, not specifically an @emph{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 @emph{node}.

@cindex class Node

In @command{ns-3} the basic computing device abstraction is called the 

node.  This abstraction is represented in C++ by the class @code{Node}.  The 

@code{Node} class provides methods for managing the representations of 

computing devices in simulations.

You should think of a @code{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 @command{ns-3}.

@subsection Application

@cindex Application

Typically, computer software is divided into two broad classes.  @emph{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 @emph{application} that acquires

and uses the resources controlled by the system software to accomplish some

goal.  

@cindex system call

Often, the line of separation between system and application software is made

at the privilege level change that happens in operating system traps.

In @command{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,'' @command{ns-3} applications run on

@command{ns-3} @code{Node}s to drive simulations in the simulated world.

@cindex class Application

In @command{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 @code{Application}.  The @code{Application} class provides 

methods for managing the representations of our version of user-level 

applications in simulations.  Developers are expected to specialize the

@code{Application} class in the object-oriented programming sense to create new

applications.  In this tutorial, we will use specializations of class 

@code{Application} called @code{UdpEchoClientApplication} and 

@code{UdpEchoServerApplication}.  As you might expect, these applications 

compose a client/server application set used to generate and echo simulated 

network packets 

@subsection Channel

@cindex 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 @emph{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 @command{ns-3} one connects a @code{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 @code{Channel}.  

The @code{Channel} class provides methods for managing communication 

subnetwork objects and connecting nodes to them.  They may also be specialized

by developers in the object oriented programming sense.  A @code{Channel}

specialization may model something as simple as a wire.  The specialized 

@code{Channel} can also model things as complicated as a large Ethernet

switch, or three-dimensional space in the case of wireless networks.

We will use specialized versions of the @code{Channel} called

@code{CsmaChannel}, @code{PointToPointChannel} and @code{WifiChannel} in this

tutorial.  The @code{CsmaChannel}, for example, models a version of a 

communication subnetwork that implements a @emph{carrier sense multiple 

access} communication medium.  This gives us Ethernet-like functionality.  

@subsection Net Device

@cindex NetDevice

@cindex Ethernet

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 @emph{peripheral card} that needed to be installed in

your computer.  These cards were called Network Interface Cards, or 

@emph{NIC}s.  Today most computers come with the network controller 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 

@emph{device}.  Devices are controlled using @emph{device drivers}, and network

devices (NICs) are controlled using @emph{network device drivers}

collectively known as @emph{net devices}.  In Unix and Linux you refer

to these net devices by names such as @emph{eth0}.

In @command{ns-3} the @emph{net device} abstraction covers both the software 

driver and the simulated hardware.  A net device is ``installed'' in a 

@code{Node} in order to enable the @code{Node} to communicate with other 

@code{Node}s in the simulation via @code{Channel}s.  Just as in a real

computer, a @code{Node} may be connected to more than one @code{Channel} via

multiple @code{NetDevice}s.

The net device abstraction is represented in C++ by the class @code{NetDevice}.

The @code{NetDevice} class provides methods for managing connections to 

@code{Node} and @code{Channel} objects; and may be specialized by developers

in the object-oriented programming sense.  We will use the several specialized

versions of the @code{NetDevice} called @code{CsmaNetDevice},

@code{PointToPointNetDevice}, and @code{WifiNetDevice} in this tutorial.

Just as an Ethernet NIC is designed to work with an Ethernet network, the

@code{CsmaNetDevice} is designed to work with a @code{CsmaChannel}; the

@code{PointToPointNetDevice} is designed to work with a 

@code{PointToPointChannel} and a @code{WifiNetNevice} is designed to work with

a @code{WifiChannel}.

@subsection Topology Helpers

@cindex helper

@cindex topology

@cindex topology helper

In a real network, you will find host computers with added (or built-in)

NICs.  In @command{ns-3} we would say that you will find @code{Nodes} with 

attached @code{NetDevices}.  In a large simulated network you will need to 

arrange many connections between @code{Node}s, @code{NetDevice}s and 

@code{Channel}s.

Since connecting @code{NetDevice}s to @code{Node}s, @code{NetDevice}s

to @code{Channel}s, assigning IP addresses,  etc., are such common tasks

in @command{ns-3}, we provide what we call @emph{topology helpers} to make 

this as easy as possible.  For example, will take several distinct 

@command{ns-3} core operations to create a NetDevice, add a MAC address, 

connect the net device to a @code{Node}, configure the protocol stack, and 

then connect the @code{NetDevice} to a @code{Channel}.  More operations would

be required to connect multiple devices onto multipoint channels and then to 

connect networks together into internetworks.  We use topology helper objects

to combine those many distinct operations into an easy to use model.

@c ========================================================================

@c A First ns-3 script

@c ========================================================================

@node A First ns-3 Script

@section A First ns-3 script

@cindex first script

If you downloaded the system as was suggested above, you will have a release

of @command{ns-3} in a directory called @code{repos} under your home 

directory.  Change into that release directory, and you should find a 

directory structure something like the following:

@verbatim

  AUTHORS  examples/  README         samples/  utils/   waf.bat*

  build/   LICENSE    regression/    scratch/  VERSION  wscript

  doc/     ns3/       RELEASE_NOTES  src/      waf*

@end verbatim

@cindex first.cc

Change into the examples directory.  You should see a file named 

@code{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.

@subsection 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.  

@verbatim

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

@end verbatim

This is always a somewhat controversial subject, so we might as well get it

out of the way immediately.  The @code{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 @command{ns-3} coding standard as described 

in the file @code{doc/codingstd.txt}.  We recommend that you, well, just get 

used to the look and feel of @code{ns-3} code and adopt this standard whenever

you are working with our code.  All of the development team have done so.

The emacs mode line above makes it easier to get the formatting correct if you

use the emacs editor.

The @command{ns-3} simulator is licentsed using the GNU General Public 

License.  You will see the appropriate GNU legalese at the head of every file 

in the @command{ns-3} distribution.  Often you will see a copyright notice for

one of the institutions involved in the @code{ns-3} project and an author

listed.

@verbatim

  /*

   * 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

   */

@end verbatim

@subsection Module Includes

The code proper starts with a number of include statements.  To help our high

level 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, in turn, includes 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 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 

@command{ns-3} files is placed in a directory called @code{ns3} (under the 

build directory) to help avoid include file name collisions.

@verbatim

  #include "ns3/core-module.h"

  #include "ns3/simulator-module.h"

  #include "ns3/node-module.h"

  #include "ns3/helper-module.h"

@end verbatim

The @code{ns3/core-module.h} file corresponds to the ns-3 module you will find

in the directory @code{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 @code{ns3} directory

under the appropriate @code{build/debug} or @code{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 following

this tutorial religiously, you will already have done a

@verbatim

  ./waf -d debug configure

@end verbatim

to configure the project to perform debug builds.  You will also have done a

@verbatim

  ./waf

@end verbatim

to build the project.  So now if you look in the directory 

@code{build/debug/ns-3} you will find the four module include files shown 

above.  You can take a look at the contents to find that these files will

include all of the public includes in the respective modules.

@subsection Ns3 Namespace

The next line in the @code{first.cc} script is a namespace declaration.

@verbatim

  using namespace ns3;

@end verbatim

The @command{ns-3} project is implemented in a C++ namespace called 

@code{ns3}.  This groups all @command{ns-3}-related declarations in a scope

outside the global namespace, which we hope will help with integration with 

other code.  The C++ @code{using} statement introduces the @code{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 @code{ns3::}

scope resolution operator before all of the @code{ns-3} code in order to use

it.  If you are unfamiliar with namespaces, please consult almost any C++ 

tutorial and compare the @code{ns3} namespace and usage here with instances of

the @code{std} namespace and the @code{using namespace std;} statements you 

will often find in discussions of @code{cout} and streams.

@subsection Logging

The next line of the script is the following,

@verbatim

  NS_LOG_COMPONENT_DEFINE ("FirstScriptExample");

@end verbatim

We will use this statement as a convenient place to talk about our Doxygen

documentation system.  If you look at the project web site, 

@uref{http://www.nsnam.org,,ns-3 project}, you will find a link to ``Other

Documentation'' in the navigation bar.  If you select this link, you will be

taken to our documentation page.  You will find links to our development

@code{ns-3-dev} documentation as well as that for our latest release.  If you

select the @code{HTML} link you will be taken to the Doxygen documentation for

that version.  

Along the left side, you will find a graphical representation of the structure

of the documentation.  A good place to start is the @code{NS-3 Modules} 

``book.''  If you expand @code{Modules} you will see a list of @command{ns-3}

module documentation.  The concept of module here ties directly into the 

module include files discussed above.  It turns out that the @command{ns-3}

logging subsystem is part of the @code{core} module, so go ahead and expand 

that documentation node.  Now, open the @code{Debugging} book and then select 

the @code{Logging} page.

You should now be looking at the Doxygen documentation for the Logging module.

In the list of @code{#define}s you will see @code{NS_LOG_COMPONENT_DEFINE}.  

It would probably be good to look for the ``Detailed Description'' of the 

logging module now to get a feel for the overall operation and then look at

the specific @code{NS_LOG_COMPONENT_DEFINE} documentation.  I won't duplicate

the documentation here, but to summarize, this line declares a logging 

component called @code{FirstScriptExample} that allows you to enable and 

disable console message logging by reference to the name.

@subsection Main Function

The next lines of the script you will find are:

@verbatim

    int

  main (int argc, char *argv[])

  {

@end verbatim

This is just the declaration of the main function of your program.  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 @command{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:

@verbatim

    LogComponentEnable("UdpEchoClientApplication", LOG_LEVEL_INFO);

    LogComponentEnable("UdpEchoServerApplication", LOG_LEVEL_INFO);

@end verbatim

If you have read over the Logging component documentation you will see that

there are a number of levels of 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.

Now we will get directly to the business of creating a topology and running 

a simulation.  We will use the topology helper objects to make this job as

easy as possible.

@subsection Topology Helpers

@subsubsection NodeContainer

The next two lines of code in our script will actually create the 

@command{ns-3} @code{Node} objects that will represent the computers in the 

simulation.  

@verbatim

    NodeContainer nodes;

    nodes.Create (2);

@end verbatim

Let's find the documentation for the @code{NodeContainer} class before we

continue.  Another way to get into the documentation for a given class is via 

the @code{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 @code{Classes} 

tab.  You should see a new set of tabs appear, one of which is 

@code{Class List}.  Under that tab you will see a list of all of the 

@command{ns-3} classes.  Scroll down, looking for @code{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 @code{Node}.  This

represents a computer to which we are going to add things like protocol stacks,

applications and peripheral cards.  The @code{NodeContainer} topology helper

provides a convenient way to create, manage and access any @code{Node} objects

that we create in order to run a simulation.  The first line above just 

declares a NodeContainer which we call @code{nodes}.  The second line calls the

@code{Create} method on the @code{nodes} object that asks the container to 

create two nodes.  As described in the Doxygen, the container calls down into

the @command{ns-3} system proper to create two @code{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.

@subsubsection 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 @code{NetDevice} and the @code{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 @code{PointToPointHelper} to configure and connect @command{ns-3}

@code{PointToPointNetDevice} and @code{PointToPointChannel} objects in this 

script.

The next three lines in the script are,

@verbatim

    PointToPointHelper pointToPoint;

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

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

@end verbatim

The first line 

@verbatim

    PointToPointHelper pointToPoint;

@end verbatim

creates a @code{PointToPointHelper} object on the stack.  From a high-level 

perspective the next line,

@verbatim

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

@end verbatim

tells the @code{PointToPointHelper} object to use the value ``5mbps''

(five megabits per second) as the ``DataRate'' when it creates a 

@code{PointToPointNetDevice} object.

From a more detailed perspective, the string ``DataRate'' corresponds

to what we call an @code{Attribute} of the @code{PointToPointNetDevice}.

If you look at the Doxygen for class @code{ns3::PointToPointNetDevice} and 

find the documentation for the @code{GetTypeId} method, you will find a list

of  @code{Attributes} defined for the device.  Among these is the ``DataRate''

attribute.  Most user-visible @command{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 @code{PointToPointNetDevice} we find a

``Delay'' attribute associated with the @code{PointToPointChannel}.  The 

final line,

@verbatim

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

@end verbatim

tells the @code{PointToPointHelper} to use the value ``2ms'' (two milliseconds)

as the value of the transmission delay of every point to point channel it 

creates.

@subsubsection NetDeviceContainer

At this point in the script, we have a @code{NodeContainer} that contains

two nodes.  We have a @code{PointToPointHelper} that is primed and ready to 

make @code{PointToPointNetDevices} and wire @code{PoiintToPointChannel} objects

between them.  Just as we used the @code{NodeContainer} topology helper object

to create the @code{Node}s for our simulation, we will ask the 

@code{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,

@verbatim

    NetDeviceContainer devices;

    devices = pointToPoint.Install (nodes);

@end verbatim

will finish configuring the devices and channel.  The first line declares the 

device container mentioned above and the second does the heavy lifting.  The 

@code{Install} method of the @code{PointToPointHelper} takes a 

@code{NodeContainer} as a parameter.  Internally, a @code{NetDeviceContainer} 

is created.  For each node in the @code{NodeContainer} (there must be exactly 

two for a point-to-point link) a @code{PointToPointNetDevice} is created and 

saved in the device container.  A @code{PointToPointChannel} is created and 

the two @code{PointToPointNetDevices} are attached.  When objects are created

by the @code{PointToPointHelper}, the attributes previously set in the helper

are used to initialize the corresponding attributes in the created objects.

After executing the the @code{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.

@subsubsection 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.

@verbatim

    InternetStackHelper stack;

    stack.Install (nodes);

@end verbatim

The @code{InternetStackHelper} is a topology helper that is to internet stacks

what the @code{PointToPointHelper} is to point-to-point net devices.  The

@code{Install} method takes a @code{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.

@subsubsection 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, @code{first.cc},

@verbatim

    Ipv4AddressHelper address;