Conceptual Overview¶
The first thing we need to do before actually starting to look at or write ns-3 code is to explain a few core concepts and abstractions in the system. Much of this may appear transparently obvious to some, but we recommend taking the time to read through this section just to ensure you are starting on a firm foundation.
Key Abstractions¶
In this section, we’ll review some terms that are commonly used in networking, but have a specific meaning in ns-3.
Node¶
In Internet jargon, a computing device that connects to a network is called a host or sometimes an end system. Because ns-3 is a network simulator, not specifically an Internet simulator, we intentionally do not use the term host since it is closely associated with the Internet and its protocols. Instead, we use a more generic term also used by other simulators that originates in Graph Theory — the node.
In ns-3 the basic computing device abstraction is called the
node. This abstraction is represented in C++ by the class Node. The
Node class provides methods for managing the representations of
computing devices in simulations.
You should think of a Node as a computer to which you will add
functionality. One adds things like applications, protocol stacks and
peripheral cards with their associated drivers to enable the computer to do
useful work. We use the same basic model in ns-3.
Application¶
Typically, computer software is divided into two broad classes. System Software organizes various computer resources such as memory, processor cycles, disk, network, etc., according to some computing model. System software usually does not use those resources to complete tasks that directly benefit a user. A user would typically run an application that acquires and uses the resources controlled by the system software to accomplish some goal.
Often, the line of separation between system and application software is made
at the privilege level change that happens in operating system traps.
In ns-3 there is no real concept of operating system and especially
no concept of privilege levels or system calls. We do, however, have the
idea of an application. Just as software applications run on computers to
perform tasks in the “real world,” ns-3 applications run on
ns-3 Nodes to drive simulations in the simulated world.
In ns-3 the basic abstraction for a user program that generates some
activity to be simulated is the application. This abstraction is represented
in C++ by the class Application. The Application class provides
methods for managing the representations of our version of user-level
applications in simulations. Developers are expected to specialize the
Application class in the object-oriented programming sense to create new
applications. In this tutorial, we will use specializations of class
Application called UdpEchoClientApplication and
UdpEchoServerApplication. As you might expect, these applications
compose a client/server application set used to generate and echo simulated
network packets
Channel¶
In the real world, one can connect a computer to a network. Often the media
over which data flows in these networks are called channels. When
you connect your Ethernet cable to the plug in the wall, you are connecting
your computer to an Ethernet communication channel. In the simulated world
of ns-3, one connects a Node to an object representing a
communication channel. Here the basic communication subnetwork abstraction
is called the channel and is represented in C++ by the class Channel.
The Channel class provides methods for managing communication
subnetwork objects and connecting nodes to them. Channels may also be
specialized by developers in the object oriented programming sense. A
Channel specialization may model something as simple as a wire. The
specialized Channel can also model things as complicated as a large
Ethernet switch, or three-dimensional space full of obstructions in the case
of wireless networks.
We will use specialized versions of the Channel called
CsmaChannel, PointToPointChannel and WifiChannel in this
tutorial. The CsmaChannel, for example, models a version of a
communication subnetwork that implements a carrier sense multiple
access communication medium. This gives us Ethernet-like functionality.
Net Device¶
It used to be the case that if you wanted to connect a 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 ns-3 the net device abstraction covers both the software
driver and the simulated hardware. A net device is “installed” in a
Node in order to enable the Node to communicate with other
Nodes in the simulation via Channels. Just as in a real
computer, a Node may be connected to more than one Channel via
multiple NetDevices.
The net device abstraction is represented in C++ by the class NetDevice.
The NetDevice class provides methods for managing connections to
Node and Channel objects; and may be specialized by developers
in the object-oriented programming sense. We will use the several specialized
versions of the NetDevice called CsmaNetDevice,
PointToPointNetDevice, and WifiNetDevice in this tutorial.
Just as an Ethernet NIC is designed to work with an Ethernet network, the
CsmaNetDevice is designed to work with a CsmaChannel; the
PointToPointNetDevice is designed to work with a
PointToPointChannel and a WifiNetNevice is designed to work with
a WifiChannel.
Topology Helpers¶
In a real network, you will find host computers with added (or built-in)
NICs. In ns-3 we would say that you will find Nodes with
attached NetDevices. In a large simulated network you will need to
arrange many connections between Nodes, NetDevices and
Channels.
Since connecting NetDevices to Nodes, NetDevices
to Channels, assigning IP addresses, etc., are such common tasks
in ns-3, we provide what we call topology helpers to make
this as easy as possible. For example, it may take many distinct
ns-3 core operations to create a NetDevice, add a MAC address,
install that net device on a Node, configure the node’s protocol stack,
and then connect the NetDevice to a Channel. Even more
operations would be required to connect multiple devices onto multipoint
channels and then to connect individual networks together into internetworks.
We provide topology helper objects that combine those many distinct operations
into an easy to use model for your convenience.
A First ns-3 Script¶
If you downloaded the system as was suggested above, you will have a release
of ns-3 in a directory called workspace under your home
directory. Change into that release directory, and you should find a
directory structure something like the following:
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.
Boilerplate¶
The first line in the file is an emacs mode line. This tells emacs about the formatting conventions (coding style) we use in our source code.
/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
This is always a somewhat controversial subject, so we might as well get it
out of the way immediately. The ns-3 project, like most large
projects, has adopted a coding style to which all contributed code must
adhere. If you want to contribute your code to the project, you will
eventually have to conform to the ns-3 coding standard as described
in the file doc/contributing/source/coding-style.rst or shown on the project web page
here.
We recommend that you, well, just get used to the look and feel of ns-3 code and adopt this standard whenever you are working with our code. All of the development team and contributors have done so with various amounts of grumbling. The emacs mode line above makes it easier to get the formatting correct if you use the emacs editor.
The ns-3 simulator is licensed using the GNU General Public License version 2. You will see the appropriate GNU legalese at the head of every file in the ns-3 distribution. Often you will see a copyright notice for one of the institutions involved in the ns-3 project above the GPL text and an author listed below.
/*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation;
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*/
Module Includes¶
The code proper starts with a number of include statements.
#include "ns3/core-module.h"
#include "ns3/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 ns-3 include files is placed in a directory called
ns3 (under the build directory) during the build process to help avoid
include file name collisions. The ns3/core-module.h file corresponds
to the ns-3 module you will find in the directory src/core in your
downloaded release distribution. If you list this directory you will find a
large number of header files. When you do a build, 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:
$ ./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
$ ./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 ns-3 project is implemented in a C++ namespace called
ns3. This groups all ns-3-related declarations in a scope
outside the global namespace, which we hope will help with integration with
other code. The C++ using statement introduces the ns-3
namespace into the current (global) declarative region. This is a fancy way
of saying that after this declaration, you will not have to type ns3::
scope resolution operator before all of the ns-3 code in order to use
it. If you are unfamiliar with namespaces, please consult almost any C++
tutorial and compare the ns3 namespace and usage here with instances of
the std namespace and the using namespace std; statements you
will often find in discussions of cout and streams.
Logging¶
The next line of the script is the following,
NS_LOG_COMPONENT_DEFINE ("FirstScriptExample");
We will use this statement as a convenient place to talk about our Doxygen documentation system. If you look at the project web site, ns-3 project, you will find a link to “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 ns-3. If you select the “API Documentation” link, you will be taken to the ns-3 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 ns-3 navigation tree. If you expand Modules
you will see a list of ns-3 module documentation. The concept of
module here ties directly into the module include files discussed above. The ns-3 logging subsystem is discussed in the Using the Logging Module 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 ns-3 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
ns-3 Node objects that will represent the computers in the
simulation.
NodeContainer nodes;
nodes.Create (2);
Let’s find the documentation for the NodeContainer class before we
continue. Another way to get into the documentation for a given class is via
the Classes tab in the Doxygen pages. If you still have the Doxygen
handy, just scroll up to the top of the page and select the Classes
tab. You should see a new set of tabs appear, one of which is
Class List. Under that tab you will see a list of all of the
ns-3 classes. Scroll down, looking for ns3::NodeContainer.
When you find the class, go ahead and select it to go to the documentation for
the class.
You may recall that one of our key abstractions is the Node. This
represents a computer to which we are going to add things like protocol stacks,
applications and peripheral cards. The NodeContainer topology helper
provides a convenient way to create, manage and access any Node objects
that we create in order to run a simulation. The first line above just
declares a NodeContainer which we call nodes. The second line calls the
Create method on the nodes object and asks the container to
create two nodes. As described in the Doxygen, the container calls down into
the ns-3 system proper to create two Node objects and stores
pointers to those objects internally.
The nodes as they stand in the script do nothing. The next step in constructing a topology is to connect our nodes together into a network. The simplest form of network we support is a single point-to-point link between two nodes. We’ll construct one of those links here.
PointToPointHelper¶
We are constructing a point to point link, and, in a pattern which will become
quite familiar to you, we use a topology helper object to do the low-level
work required to put the link together. Recall that two of our key
abstractions are the NetDevice and the Channel. In the real
world, these terms correspond roughly to peripheral cards and network cables.
Typically these two things are intimately tied together and one cannot expect
to interchange, for example, Ethernet devices and wireless channels. Our
Topology Helpers follow this intimate coupling and therefore you will use a
single PointToPointHelper to configure and connect ns-3
PointToPointNetDevice and PointToPointChannel objects in this
script.
The next three lines in the script are,
PointToPointHelper pointToPoint;
pointToPoint.SetDeviceAttribute ("DataRate", StringValue ("5Mbps"));
pointToPoint.SetChannelAttribute ("Delay", StringValue ("2ms"));
The first line,
PointToPointHelper pointToPoint;
instantiates a PointToPointHelper object on the stack. From a
high-level perspective the next line,
pointToPoint.SetDeviceAttribute ("DataRate", StringValue ("5Mbps"));
tells the PointToPointHelper object to use the value “5Mbps”
(five megabits per second) as the “DataRate” when it creates a
PointToPointNetDevice object.
From a more detailed perspective, the string “DataRate” corresponds
to what we call an Attribute of the PointToPointNetDevice.
If you look at the Doxygen for class ns3::PointToPointNetDevice and
find the documentation for the GetTypeId method, you will find a list
of Attributes defined for the device. Among these is the “DataRate”
Attribute. Most user-visible ns-3 objects have similar lists of
Attributes. We use this mechanism to easily configure simulations without
recompiling as you will see in a following section.
Similar to the “DataRate” on the PointToPointNetDevice you will find a
“Delay” Attribute associated with the PointToPointChannel. The
final line,
pointToPoint.SetChannelAttribute ("Delay", StringValue ("2ms"));
tells the PointToPointHelper to use the value “2ms” (two milliseconds)
as the value of the 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 ns-3 system actually remembers all of the IP addresses allocated and will generate a fatal error if you accidentally cause the same address to be generated twice (which is a very hard to debug error, by the way).
The next line of code,
Ipv4InterfaceContainer interfaces = address.Assign (devices);
performs the actual address assignment. In ns-3 we make the
association between an IP address and a device using an Ipv4Interface
object. Just as we sometimes need a list of net devices created by a helper
for future reference we sometimes need a list of Ipv4Interface objects.
The Ipv4InterfaceContainer provides this functionality.
Now we have a point-to-point network built, with stacks installed and IP addresses assigned. What we need at this point are applications to generate traffic.
Applications¶
Another one of the core abstractions of the ns-3 system is the
Application. In this script we use two specializations of the core
ns-3 class Application called UdpEchoServerApplication
and UdpEchoClientApplication. Just as we have in our previous
explanations, we use helper objects to help configure and manage the
underlying objects. Here, we use UdpEchoServerHelper and
UdpEchoClientHelper objects to make our lives easier.
UdpEchoServerHelper¶
The following lines of code in our example script, first.cc, are used
to set up a UDP echo server application on one of the nodes we have previously
created.
UdpEchoServerHelper echoServer (9);
ApplicationContainer serverApps = echoServer.Install (nodes.Get (1));
serverApps.Start (Seconds (1.0));
serverApps.Stop (Seconds (10.0));
The first line of code in the above snippet declares the
UdpEchoServerHelper. As usual, this isn’t the application itself, it
is an object used to help us create the actual applications. One of our
conventions is to place required Attributes in the helper constructor.
In this case, the helper can’t do anything useful unless it is provided with
a port number that the client also knows about. Rather than just picking one
and hoping it all works out, we require the port number as a parameter to the
constructor. The constructor, in turn, simply does a SetAttribute
with the passed value. 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
NodeContainter as a parameter just as the other Install methods
we have seen. This is actually what is passed to the method even though it
doesn’t look so in this case. There is a C++ implicit conversion at
work here 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
ns-3 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 (1.0));
serverApps.Stop (Seconds (10.0));
will cause the echo server application to Start (enable itself) at one
second into the simulation and to Stop (disable itself) at ten seconds
into the simulation. By virtue of the fact that we have 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 (1.0)));
echoClient.SetAttribute ("PacketSize", UintegerValue (1024));
ApplicationContainer clientApps = echoClient.Install (nodes.Get (0));
clientApps.Start (Seconds (2.0));
clientApps.Stop (Seconds (10.0));
For the echo client, however, we need to set five different Attributes.
The first two Attributes are set during construction of the
UdpEchoClientHelper. We pass parameters that are used (internally to
the helper) to set the “RemoteAddress” and “RemotePort” Attributes
in accordance with our convention to make required Attributes parameters
in the helper constructors.
Recall that we used an Ipv4InterfaceContainer to keep track of the IP
addresses we assigned to our devices. The zeroth interface in the
interfaces container is going to correspond to the IP address of the
zeroth node in the nodes container. The first interface in the
interfaces container corresponds to the IP address of the first node
in the nodes container. So, in the first line of code (from above), we
are creating the helper and telling it so set the remote address of the client
to be the IP address assigned to the node on which the server resides. We
also tell it to arrange to send packets to port nine.
The “MaxPackets” Attribute tells the client the maximum number of
packets we allow it to send during the simulation. The “Interval”
Attribute tells the client how long to wait between packets, and the
“PacketSize” Attribute tells the client how large its packet payloads
should be. With this particular combination of Attributes, we are
telling the client to send one 1024-byte packet.
Just as in the case of the echo server, we tell the echo client to Start
and Stop, but here we start the client one second after the server is
enabled (at two seconds into the simulation).
Simulator¶
What we need to do at this point is to actually run the simulation. This is
done using the global function Simulator::Run.
Simulator::Run ();
When we previously called the methods,
serverApps.Start (Seconds (1.0));
serverApps.Stop (Seconds (10.0));
...
clientApps.Start (Seconds (2.0));
clientApps.Stop (Seconds (10.0));
we actually scheduled events in the simulator at 1.0 seconds, 2.0 seconds and
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
ns-3 code) executed, they arranged it so that hooks were inserted in
the simulator to destroy all of the objects that were created. You did not
have to keep track of any of these objects yourself — all you had to do
was to call Simulator::Destroy and exit. The ns-3 system
took care of the hard part for you. The remaining lines of our first
ns-3 script, first.cc, do just that:
Simulator::Destroy ();
return 0;
}
When the simulator will stop?¶
ns-3 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 ns-3 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 (11.0));
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.
$ cd ../..
$ cp examples/tutorial/first.cc scratch/myfirst.cc
Now build your first example script using ns3:
$ ./ns3 build
You should see messages reporting that your myfirst example was built
successfully.
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):
$ ./ns3 run scratch/myfirst
You should see some output:
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 ns-3 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 ns-3 programs.