unison/doc/tutorial/conceptual-overview.texi

@c ========================================================================
@c Begin document body here
@c ========================================================================

@c ========================================================================
@c PART:  Conceptual Overview
@c ========================================================================
@c The below chapters are under the major heading "Getting Started"
@c This is similar to the Latex \part command
@c ========================================================================
@c Some Conceptual Overview
@c ========================================================================
@node Conceptual Overveiw
@chapter Conceptual Overview

@menu
* Key Abstractions::
@end menu

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, abstractions and idioms in the
system.  Much of this may appear transparently obvious to some, but we
recommend taking the time to read through this chapter 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 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 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 Node!class
In 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.  Developers are expected to specialize the 
@code{Node} in the object-oriented programming sense to create new computing
device models.  In this tutorial, we will use a specialization of class 
@code{Node} called @code{InternetNode}.  As you might expect, the
@code{InternetNode} is a class that represents a host in the Internet sense,
and automatically provides core IPv4 networking protocols.

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 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 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 @code{Node}s to drive simulations in the simulated world.

@cindex Application!class
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 @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} in the object-oriented programming sense to create new
applications.  In this tutorial, we will use specializations of class 
@code{Application} called @code{UdpEchoClient} and @code{UdpEchoServer}.
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 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} and @code{PointToPointChannel} 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 ns-3 the @emph{net device} abstraction covers both the software 
driver and the simulated hardware.  A net device is ``attached'' to 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 specialized version
of the @code{NetDevice} called the @code{CsmaNetDevice} 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}.

@subsection Topology Helpers
@cindex helper
@cindex topology
@cindex helper|topology
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 @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 ns-3 we provide what we call @emph{topology helpers} to make this as easy 
as possible.  For example, will take several distinct 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 higher level 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 compose those distinct operations into an easy to use model.

@c ========================================================================
@c A First ns-3 script
@c ========================================================================
@node A First ns-3 Script
@chapter A First ns-3 script
@cindex first script
If you downloaded the system as was suggested above, you will have a release
of 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/  tutorial/  waf*
  build/   LICENSE    regression/    scratch/  utils/     waf.bat*
  doc/     ns3/       RELEASE_NOTES  src/      VERSION    wscript
@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 the script line by line.

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 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 style as described in INSERT LINK TO CODING STYLE PAGE ON WEB.
The emacs mode line above makes it easier to get the formatting correct if you
use the emacs editor.

The 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 ns-3
distribution.

@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

@section 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 ns-3 files
is placed in a directory called @code{ns3} to help with 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 perfectly, you will 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 if you look in the directory @code{build/debug/ns-3}
you will find the four module includes shown above.

@section Ns3 Namespace
The next line in the @code{first.cc} script is a namespace statement:

@verbatim
  using namespace ns3;
@end verbatim

The ns-3 project is implemented in a C++ namespace called @code{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++ @code{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 @code{ns3::} 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 the std namespace and 
a @code{using namespace std;} statement.

@section 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 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 @code{core} module, so go ahead and expand that 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.

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 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 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 
a message 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.

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

@verbatim
    NodeContainer nodes;
    nodes.Create (2);
@end verbatim

Let's find the documentation for the first object before we continue.  Another
way to get into the documentation system is via the @code{Classes} tab in the 
HTML 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 ns-3 classes.  Scroll down, looking for
@code{ns3::NodeContainer}.  When you find the class, go ahead and select it to
go to the Doxygen.

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 just declares a
NodeContainer which we call @code{nodes}.  The second line calls the
@code{Create} method on the @code{nodes} object and asks the container to 
create two nodes.  As described in the Doxygen, the container calls down into
the ns-3 system to create two @code{Node} objects and stores the pointers
internally.

The nodes as they stand at this point 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 is a single point-to-point link between two nodes
so we'll construct one here.

@subsection 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, this corresponds roughly to peripheral cards and network cables.  
Typically these two things are intimately tied together and one cannot expect
to interchange Ethernet devices and wireless channels, for example.  Our 
Topology Helpers follow this intimate coupling and therefore you will use a
single @code{PointToPointHelper} to configure and connect ns-3 
@code{PointToPointNetDevice} and @code{PointToPointChannel} objects in this 
script.

The next three lines in the @code{first.cc} 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 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.

@subsection 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 send data at five
megabits per second over the channel which has a two millisecond transmission
delay.

@subsection InternetStackHelper
We have nodes and devices configured, but we don't have any protocol stacks
installed on our nodes.  The next two lines of code 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.

@subsection 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 in the
code).  

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

@verbatim
    Ipv4AddressHelper address;
    address.SetBase ("10.1.1.0", "255.255.255.0");
@end verbatim

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.

The next line of code,

@verbatim
    Ipv4InterfaceContainer interfaces = address.Assign (devices);
@end verbatim

performs the actual address assignment.  In ns-3 we make the association
between an IP address and a device using an @code{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 @code{Ipv4Interface} objects.  The 
@code{Ipv4InterfaceContainer} provides this functionality.

Now we have a point-to-point network made with IP addresses assigned.  What
we need at this point are applications to generate traffic.

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

@subsection UdpEchoServerHelper
The following lines of code in our example script, @code{first.cc}, are used
to set up a UDP echo server application on one of the nodes we have previously
created and connected using a point-to-point link.  

@verbatim
    UdpEchoServerHelper echoServer;
    echoServer.SetPort (9);

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

The first line of code in the above snippet declares the 
@code{UdpEchoServerHelper}.  As usual, this isn't the application itself, it
is an object to help us create the actual applications.  The second line
that has the @code{SetPort} call, is used to tell the helper to assign the
value nine to the ``Port'' attribute when creating 
@code{UdpEchoServerApplication} objects.

Similar to many other helper objects, the @code{UdpEchoServerHelper} object 
has an @code{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 @code{Install} method takes a
@code{NodeContainter} as a parameter just as the other @code{Install} methods
we have seen.  This is actually what is passed to the method even though it 
doesn't look so in this case.  It will be worth a slight digression
to undersand what is going on here.

@subsubsection Implicit Conversions
This is an example of a C++ @code{implicit conversion} at work.  Many
programmers use implicit conversions without even realizing it since they are
sometimes so intuitive.  For example, in the following code,

@verbatim
  int i = 1;
  double d = 2.;
  if (n == d) ...
@end verbatim

@cindex standard conversion
the integer (1) is implicitly converted to a double (1.) before the comparison
takes place.  This conversion is performed using what is known as a C++ 
@emph{standard conversion}.

C++ will try to determine a sequence of type conversions to use in order to
convert an argument in a function call to the type of a corresponding paramter
in a method declaration.  The method call in this case is,

@verbatim
    ApplicationContainer serverApps = echoServer.Install (nodes.Get (1));
@end verbatim

If you look at the declaration of the Install method in the source (or the
Doxygen) you will see,

@verbatim
  ApplicationContainer 
  UdpEchoServerHelper::Install (NodeContainer c)
  {
    ...
  }
@end verbatim

Now, the @code{Get} method of the @code{NodeContainer} returns a smart pointer
to a node object:

@verbatim
  Ptr<Node> 
  NodeContainer::Get (uint32_t i) const
  {
   ...
  }
@end verbatim

C++ notices that it needs to convert a type @code{Ptr<Node>} into a type
@code{NodeContainer}.  It turns out that @code{NodeContainer} has a
constructor that takes a @code{Ptr<Node>} as a parameter.

@verbatim
  NodeContainer::NodeContainer (Ptr<Node> node)
  {
   ...
  }
@end verbatim

The C++ compiler implicitly provides the @code{Ptr<Node>} returned from the
@code{NodeContainer::Get} to the above constructor for @code{NodeContainer} to 
make a new @code{NodeContainer} to pass to the @code{Install} method of the
@code{UdpEchoServerHelper} object.

Taking advantage of these implicit conversion sequences can allow you to 
avoid doing a lot of typing.  The result is also fairly intuitive if you
don't pay close attention to the called method signatures.  If you are
one of those folks that pays close attention, it does have the downside of
sometimes being surprising when you, for example, look for the method 
in @code{UdpEchoServerHelper} named @code{Install} that takes a parameter
@code{Ptr<Node>} and cannot find it.

@subsection UdpEchoServerHelper Continued
Returning to the example script, we are looking at the following code snippet.

@verbatim
    UdpEchoServerHelper echoServer;
    echoServer.SetPort (9);

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

We now see that @code{echoServer.Install} is going to install an 
@code{UdpEchoServerApplication} on the node found at index number one of the
@code{NodeContainer} we use to manage our nodes.  @code{Install} will return
a container that has all of the applications (one in this case since we passed
a @code{NodeContainer} containing one node) made by the helper.  

Applications require a time to ``start'' generating traffic and a time to
``stop.''  These times are set using @code{ApplicationContainer} methods
@code{Start} and @code{Stop}.  These methods take @code{Time} parameters.  
In this case, we use an explicit conversion sequence to take a c++ double
1.0 and convert it to a @code{Time} object using a @code{Seconds ()} cast.
The two lines,

@verbatim
    serverApps.Start (Seconds (1.0));
    serverApps.Stop (Seconds (10.0));
@end verbatim

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

@subsection UdpEchoClientHelper

The echo client application is set up in a method similar to the server.
There is an underlying @code{UdpEchoClientApplication} that is managed by an
@code{UdpEchoClientHelper}.

@verbatim
    UdpEchoClientHelper echoClient;
    echoClient.SetRemote (interfaces.GetAddress (1), 9);
    echoClient.SetAppAttribute ("MaxPackets", UintegerValue (1));
    echoClient.SetAppAttribute ("Interval", TimeValue (Seconds (1.)));
    echoClient.SetAppAttribute ("PacketSize", UintegerValue (1024));

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

For the echo client, we need to set four different attributes.  The first 
attribute is set using the @code{SetRemote} method.  Here we refer back to 
the @code{Ipv4InterfaceContainer} we created to keep track of the IP addresses
we assigned to our devices.

@verbatim
    Ipv4InterfaceContainer interfaces = address.Assign (devices);
@end verbatim

In this case, the zeroth interface in the @code{interfaces} container 
cooresponds to the IP address of the zeroth node in the @code{nodes} container.
The first interface in the @code{interfaces} container cooresponds to the IP
address of the first node in the @code{nodes} container.  So, in the following 
line of code, we are setting the remote address of the client to be the IP
address assigned to the node on which the server resides.  We tell it to send
to port nine.

@verbatim
    echoClient.SetRemote (interfaces.GetAddress (1), 9);
@end verbatim

The ``MaxPackets'' attribute tells the client the maximum number of packets 
it can send.  The ``Interval'' attribute tells the cleint how long to wait 
between packets and the ``PacketSize'' attribute tells the client how large 
its packets should be.  In this case 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 @code{Start}
and @code{Stop}, but here we start the client one second after the server is
enabled (at two seconds into the simulation).

@subsection Simulator
What we need to do at this point is to actually run the simulation.  This is 
done using the @code{Simulator} singleton which is accessed via the global 
function @code{Simulator::Run} 

@verbatim
    Simulator::Run ();
@end verbatim

When we called the methods,

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

we actually scheduled events in the simulator at 1.0 seconds, 2.0 seconds and
10.0 seconds.  When @code{Simulator::Run} is called, it 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 cleint 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
which will be automatically scheduled and which will perform the packet echo
according to the various timing parameters that we have set in the script.

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

All that remains is to clean up after ourselves.  This is done by calling the
global function @code{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 destruct all of the objects that were created.  You do not have
to keep track of all of these objects yourself --- all you have to do is to
call @code{Simulator::Destroy} and exit.  The remaining lines of our first 
ns-3 script, @code{first.cc}, do just that

@verbatim
    Simulator::Destroy ();
    return 0;
  }
@end verbatim

@section 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.  Let's try it.  Copy @code{examples/first.cc} into @code{scratch}:

@verbatim
  ~/repos/ns-3-tutorial > cp examples/first.cc scratch
@end verbatim

and then build it using waf,

@verbatim
  ~/repos/ns-3-tutorial > ./waf
  Entering directory `/home/craigdo/repos/ns-3-tutorial/build'
  [432/477] cxx: scratch/first.cc -> build/debug/scratch/first_2.o
  [433/477] cxx: scratch/simple.cc -> build/debug/scratch/simple_3.o
  [475/477] cxx_link: build/debug/scratch/first_2.o -> build/debug/scratch/first
  Compilation finished successfully
~/repos/ns-3-tutorial >
@end verbatim

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):

@verbatim
  ~/repos/ns-3-tutorial > ./waf --run scratch/first
  Entering directory `/home/craigdo/repos/ns-3-tutorial/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-tutorial >
@end verbatim

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 
note 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 receive the 1024 bytes
from 10.1.1.1.  The echo server silently echoes the packet and you see the
echo client note that it has received its packet back from the server.