.. include:: replace.txt
.. highlight:: cpp

.. _Conceptual Overview:

Conceptual Overview
-------------------

The first thing we need to do before actually starting to look at or write
|ns3| 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 |ns3|.

Node
++++
In Internet jargon, a computing device that connects to a network is called
a *host* or sometimes an *end system*.  Because |ns3| 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 |ns3| 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 |ns3|.

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 |ns3| 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," |ns3| applications run on
|ns3| ``Nodes`` to drive simulations in the simulated world.

In |ns3| 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 |ns3|, 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 computer 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,
they 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 |ns3| 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 ``WifiNetDevice`` 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 |ns3| 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 |ns3|, we provide what we call *topology helpers* to make
this as easy as possible.  For example, it may take many distinct
|ns3| 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 |ns3| in a directory called ``workspace`` under your home
directory.  Change into that release directory, and you should find a
directory structure something like the following:

.. sourcecode:: bash

  AUTHORS        CMakeLists.txt   examples   RELEASE_NOTES.md  testpy.supp
  bindings       contrib          LICENSE    scratch           utils
  build-support  CONTRIBUTING.md  ns3        src               utils.py
  CHANGES.md     doc              README.md  test.py           VERSION

Change into the ``examples/tutorial`` 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.

Copyright
+++++++++
The |ns3| simulator is licensed using the GNU General Public
License version 2.  You will see the appropriate GNU legalese at the head of every file
in the |ns3| distribution.  Often you will see a copyright notice for
one of the institutions involved in the |ns3| project above the GPL
text and an author listed below.

::

  /*
   * SPDX-License-Identifier: GPL-2.0-only
   */

Module Includes
+++++++++++++++
The code proper starts with a number of include statements.

::

  #include "ns3/core-module.h"
  #include "ns3/network-module.h"
  #include "ns3/internet-module.h"
  #include "ns3/point-to-point-module.h"
  #include "ns3/applications-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 |ns3| 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, ns3 will place public
header files in an ``ns3`` directory under the appropriate
``build/debug`` or ``build/optimized`` directory depending on your
configuration.  CMake 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 run the following command from the top-level directory:

.. sourcecode:: bash

  $ ./ns3 configure -d debug --enable-examples --enable-tests

in order to configure the project to perform debug builds that include
examples and tests.  You will also have called

.. sourcecode:: bash

  $ ./ns3 build

to build the project.  So now if you look in the directory
``../../build/include/ns3`` you will find the four module include files shown
above (among many other header files).  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 |ns3| project is implemented in a C++ namespace called
``ns3``.  This groups all |ns3|-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 |ns3|
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 |ns3| 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
<https://www.nsnam.org>`_, you will find a link to "Documentation" in the navigation bar.  If you select this link, you will be
taken to our documentation page. There
is a link to "Latest Release" that will take you to the documentation
for the latest stable release of |ns3|.
If you select the "API Documentation" link, you will be
taken to the |ns3| API 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" in the |ns3| navigation tree.  If you expand ``Modules``
you will see a list of |ns3| module documentation.  The concept of
module here ties directly into the module include files discussed above.  The |ns3| logging subsystem is discussed in the :ref:`UsingLogging` section, so
we'll get to it later in this tutorial, but you can find out about the above
statement by looking at the ``Core`` module, then expanding the
``Debugging tools`` book, and then selecting the ``Logging`` page.  Click
on ``Logging``.

You should now be looking at the Doxygen documentation for the Logging module.
In the list of ``Macros``'s 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
|ns3| script is just a C++ program.

The next line sets the time resolution to one nanosecond, which happens
to be the default value:

::

    Time::SetResolution(Time::NS);

The resolution is the smallest time value that can be represented (as well as
the smallest representable difference between two time values).
You can change the resolution exactly once.  The mechanism enabling this
flexibility is somewhat memory hungry, so once the resolution has been
set explicitly we release the memory, preventing further updates.   (If
you don't set the resolution explicitly, it will default to one nanosecond,
and the memory will be released when the simulation starts.)

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
|ns3| ``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
|ns3| 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 |ns3| 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 |ns3|
``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 |ns3| 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 propagation 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 single
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 |ns3| 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 |ns3| 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
|ns3| 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));
    serverApps.Stop(Seconds(10));

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.  If you want, you can set the "Port" ``Attribute``
to another value later using ``SetAttribute``.

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
``NodeContainer`` 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 that takes the result of ``nodes.Get(1)`` (which returns a smart
pointer to a node object --- ``Ptr<Node>``) and uses that in a constructor
for an unnamed ``NodeContainer`` that is then passed to ``Install``.
If you are ever at a loss to find a particular method signature in C++ code
that compiles and runs just fine, look for these kinds of implicit conversions.

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
|ns3| ``Time`` object using a ``Seconds`` cast.  Be aware that
the conversion rules may be controlled by the model author, and C++ has its
own rules, so you can't always just assume that parameters will be happily
converted for you.  The two lines,

::

    serverApps.Start(Seconds(1));
    serverApps.Stop(Seconds(10));

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 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));
    clientApps.Stop(Seconds(10));

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));
    serverApps.Stop(Seconds(10));
    ...
    clientApps.Start(Seconds(2));
    clientApps.Stop(Seconds(10));

we actually scheduled events in the simulator at 1.0 seconds, 2.0 seconds and
two events at 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 (this event may, in turn, schedule many other events).  Then it
will run the event scheduled for t=2.0 seconds which will start the echo client
application.  Again, this event may schedule many more events.  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 (recall the ``MaxPackets``
``Attribute`` was set to one), 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 then 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
|ns3| 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 |ns3| system
took care of the hard part for you.  The remaining lines of our first
|ns3| script, ``first.cc``, do just that:

::

    Simulator::Destroy();
    return 0;
  }

When the simulator will stop?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

|ns3| is a Discrete Event (DE) simulator. In such a simulator, each event is
associated with its execution time, and the simulation proceeds by executing
events in the temporal order of simulation time.  Events may cause future
events to be scheduled (for example, a timer may reschedule itself to
expire at the next interval).

The initial events are usually triggered by each object, e.g., IPv6 will
schedule Router Advertisements, Neighbor Solicitations, etc.,
an Application schedule the first packet sending event, etc.

When an event is processed, it may generate zero, one or more events.
As a simulation executes, events are consumed, but more events may (or may
not) be generated.
The simulation will stop automatically when no further events are in the
event queue, or when a special Stop event is found. The Stop event is
created through the
``Simulator::Stop(stopTime);`` function.

There is a typical case where ``Simulator::Stop`` is absolutely necessary
to stop the simulation: when there is a self-sustaining event.
Self-sustaining (or recurring) events are events that always reschedule
themselves. As a consequence, they always keep the event queue non-empty.

There are many protocols and modules containing recurring events, e.g.:

* FlowMonitor - periodic check for lost packets
* RIPng - periodic broadcast of routing tables update
* etc.

In these cases, ``Simulator::Stop`` is necessary to gracefully stop the
simulation.  In addition, when |ns3| is in emulation mode, the
``RealtimeSimulator`` is used to keep the simulation clock aligned with
the machine clock, and ``Simulator::Stop`` is necessary to stop the
process.

Many of the simulation programs in the tutorial do not explicitly call
``Simulator::Stop``, since the event queue will automatically run out
of events.  However, these programs will also accept a call to
``Simulator::Stop``.  For example, the following additional statement
in the first example program will schedule an explicit stop at 11 seconds:

::

  +  Simulator::Stop(Seconds(11));
     Simulator::Run();
     Simulator::Destroy();
     return 0;
   }

The above will not actually change the behavior of this program, since
this particular simulation naturally ends after 10 seconds.  But if you
were to change the stop time in the above statement from 11 seconds to 1
second, you would notice that the simulation stops before any output is
printed to the screen (since the output occurs around time 2 seconds of
simulation time).

It is important to call ``Simulator::Stop`` *before* calling
``Simulator::Run``; otherwise, ``Simulator::Run`` may never return control
to the main program to execute the stop!

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 ns3.  Let's try it.  Copy ``examples/tutorial/first.cc`` into
the ``scratch`` directory after changing back into the top level directory.

.. sourcecode:: bash

  $ cd ../..
  $ cp examples/tutorial/first.cc scratch/myfirst.cc

Now build your first example script using ns3:

.. sourcecode:: bash

  $ ./ns3 build

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

.. sourcecode:: bash

  Scanning dependencies of target scratch_myfirst
  [  0%] Building CXX object scratch/CMakeFiles/scratch_myfirst.dir/myfirst.cc.o
  [  0%] Linking CXX executable ../../build/scratch/ns3.36.1-myfirst-debug
  Finished executing the following commands:
  cd cmake-cache; cmake --build . -j 7 ; cd ..

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):

.. sourcecode:: bash

  $ ./ns3 run scratch/myfirst

You should see some output:

.. sourcecode:: bash

  At time +2s client sent 1024 bytes to 10.1.1.2 port 9
  At time +2.00369s server received 1024 bytes from 10.1.1.1 port 49153
  At time +2.00369s server sent 1024 bytes to 10.1.1.1 port 49153
  At time +2.00737s client received 1024 bytes from 10.1.1.2 port 9

Here 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 |ns3| helpers you may want to
have a look at some of the source code that implements that functionality.

Our example scripts are in the ``examples`` directory.  If you change to ``examples`` directory,
you will see a list of subdirectories.  One of the files in ``tutorial`` subdirectory is ``first.cc``.  If
you click on ``first.cc`` you will find the code you just walked through.

The source code is mainly in the ``src`` directory.  The core of the simulator
is in the ``src/core/model`` subdirectory.  The first file you will find there
(as of this writing) is ``abort.h``.  If you open that file, you can view
macros for exiting scripts if abnormal conditions are detected.

The source code for the helpers we have used in this chapter can be found in the
``src/applications/helper`` directory.  Feel free to poke around in the directory tree to
get a feel for what is there and the style of |ns3| programs.