Tweaking ns-3
*************

Using the Logging Module
========================

We have already taken a brief look at the ns-3 logging module while going over the first.cc script.  We will now take a closer look and see what kind of use-cases the logging subsystem was designed to cover.

Logging Overview
----------------

Many large systems support some kind of message logging facility, and ns-3 is not an exception.  In some cases, only error messages are logged to the ``operator console'' (which is typically stderr in Unix- based systems).  In other systems, warning messages may be output as well as more detailed informational messages.  In some cases, logging facilities are used to output debug messages which can quickly turn the output into a blur.

Ns-3 takes the view that all of these verbosity levels are useful and we provide a selectable, multi-level approach to message logging.  Logging can be disabled completely, enabled on a component-by-component basis, or enabled globally; and it provides selectable verbosity levels.  The ns-3 log module provides a straightforward, relatively easy to use way to get useful information out of your simulation.

You should understand that we do provide a general purpose mechanism --- tracing --- to get data out of your models which should be preferred for simulation output (see the tutorial section Using the Tracing System for more details on our tracing system).  Logging should be preferred for debugging information, warnings, error messages, or any time you want to easily get a quick message out of your scripts or models.

There are currently seven levels of log messages of increasing verbosity defined in the system.

  * NS_LOG_ERROR --- Log error messages;

  * NS_LOG_WARN --- Log warning messages;

  * NS_LOG_DEBUG --- Log relatively rare, ad-hoc debugging messages;

  * NS_LOG_INFO --- Log informational messages about program progress;

  * NS_LOG_FUNCTION --- Log a message describing each function called;

  * NS_LOG_LOGIC -- Log messages describing logical flow within a function;

  * NS_LOG_ALL --- Log everything.

We also provide an unconditional logging level that is always displayed, irrespective of logging levels or component selection.

  * NS_LOG_UNCOND -- Log the associated message unconditionally.

Each level can be requested singly or cumulatively; and logging can be set up using a shell environment variable (NS_LOG) or by logging system function call.  As was seen earlier in the tutorial, the logging system has Doxygen documentation and now would be a good time to peruse the Logging Module documentation if you have not done so.

Now that you have read the documentation in great detail, let's use some of that knowledge to get some interesting information out of the scratch/myfirst.cc example script you have already built.

Enabling Logging
----------------

Let's use the NS_LOG environment variable to turn on some more logging, but first, just to get our bearings, go ahead and run the last script just as you did previously,

  ./waf --run scratch/myfirst

You should see the now familiar output of the first ns-3 example program

  Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  'build' finished successfully (0.413s)
  Sent 1024 bytes to 10.1.1.2
  Received 1024 bytes from 10.1.1.1
  Received 1024 bytes from 10.1.1.2

It turns out that the ``Sent'' and ``Received'' messages you see above are actually logging messages from the UdpEchoClientApplication and UdpEchoServerApplication.  We can ask the client application, for example, to print more information by setting its logging level via the NS_LOG environment variable.

I am going to assume from here on that you are using an sh-like shell that uses the``VARIABLE=value'' syntax.  If you are using a csh-like shell, then you will have to convert my examples to the ``setenv VARIABLE value'' syntax required by those shells.

Right now, the UDP echo client application is responding to the following line of code in scratch/myfirst.cc,

  LogComponentEnable("UdpEchoClientApplication", LOG_LEVEL_INFO);

This line of code enables the LOG_LEVEL_INFO level of logging.  When we pass a logging level flag, we are actually enabling the given level and all lower levels.  In this case, we have enabled NS_LOG_INFO, NS_LOG_DEBUG, NS_LOG_WARN and NS_LOG_ERROR.  We can increase the logging level and get more information without changing the script and recompiling by setting the NS_LOG environment variable like this:

  export NS_LOG=UdpEchoClientApplication=level_all

This sets the shell environment variable NS_LOG to the string,

  UdpEchoClientApplication=level_all

The left hand side of the assignment is the name of the logging component we want to set, and the right hand side is the flag we want to use.  In this case, we are going to turn on all of the debugging levels for the application.  If you run the script with NS_LOG set this way, the ns-3 logging system will pick up the change and you should see the following output:

  Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build
  Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  'build' finished successfully (0.404s)
  UdpEchoClientApplication:UdpEchoClient()
  UdpEchoClientApplication:SetDataSize(1024)
  UdpEchoClientApplication:StartApplication()
  UdpEchoClientApplication:ScheduleTransmit()
  UdpEchoClientApplication:Send()
  Sent 1024 bytes to 10.1.1.2
  Received 1024 bytes from 10.1.1.1
  UdpEchoClientApplication:HandleRead(0x6241e0, 0x624a20)
  Received 1024 bytes from 10.1.1.2
  UdpEchoClientApplication:StopApplication()
  UdpEchoClientApplication:DoDispose()
  UdpEchoClientApplication:~UdpEchoClient()

The additional debug information provided by the application is from the NS_LOG_FUNCTION level.  This shows every time a function in the application is called during script execution.  Note that there are no requirements in the ns-3 system that models must support any particular logging functionality.  The decision regarding how much information is logged is left to the individual model developer.  In the case of the echo applications, a good deal of log output is available.

You can now see a log of the function calls that were made to the application. If you look closely you will notice a single colon between the string UdpEchoClientApplication and the method name where you might have expected a C++ scope operator (::).  This is intentional.

The name is not actually a class name, it is a logging component name.  When there is a one-to-one correspondence between a source file and a class, this will generally be the class name but you should understand that it is not actually a class name, and there is a single colon there instead of a double colon to remind you in a relatively subtle way to conceptually separate the logging component name from the class name.

It turns out that in some cases, it can be hard to determine which method actually generates a log message.  If you look in the text above, you may wonder where the string ``Received 1024 bytes from 10.1.1.2'' comes from.  You can resolve this by OR'ing the prefix_func level into the NS_LOG environment variable.  Try doing the following,

  export 'NS_LOG=UdpEchoClientApplication=level_all|prefix_func'

Note that the quotes are required since the vertical bar we use to indicate an OR operation is also a Unix pipe connector.

Now, if you run the script you will see that the logging system makes sure that every message from the given log component is prefixed with the component name.

  Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  'build' finished successfully (0.417s)
  UdpEchoClientApplication:UdpEchoClient()
  UdpEchoClientApplication:SetDataSize(1024)
  UdpEchoClientApplication:StartApplication()
  UdpEchoClientApplication:ScheduleTransmit()
  UdpEchoClientApplication:Send()
  UdpEchoClientApplication:Send(): Sent 1024 bytes to 10.1.1.2
  Received 1024 bytes from 10.1.1.1
  UdpEchoClientApplication:HandleRead(0x6241e0, 0x624a20)
  UdpEchoClientApplication:HandleRead(): Received 1024 bytes from 10.1.1.2
  UdpEchoClientApplication:StopApplication()
  UdpEchoClientApplication:DoDispose()
  UdpEchoClientApplication:~UdpEchoClient()

You can now see all of the messages coming from the UDP echo client application are identified as such.  The message ``Received 1024 bytes from 10.1.1.2'' is now clearly identified as coming from the echo client application.  The remaining message must be coming from the UDP echo server application.  We can enable that component by entering a colon separated list of components in the NS_LOG environment variable.

  export 'NS_LOG=UdpEchoClientApplication=level_all|prefix_func:
                 UdpEchoServerApplication=level_all|prefix_func'

Warning:  You will need to remove the newline after the : in the example text above which is only there for document formatting purposes.

Now, if you run the script you will see all of the log messages from both the echo client and server applications.  You may see that this can be very useful in debugging problems.

  Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  'build' finished successfully (0.406s)
  UdpEchoServerApplication:UdpEchoServer()
  UdpEchoClientApplication:UdpEchoClient()
  UdpEchoClientApplication:SetDataSize(1024)
  UdpEchoServerApplication:StartApplication()
  UdpEchoClientApplication:StartApplication()
  UdpEchoClientApplication:ScheduleTransmit()
  UdpEchoClientApplication:Send()
  UdpEchoClientApplication:Send(): Sent 1024 bytes to 10.1.1.2
  UdpEchoServerApplication:HandleRead(): Received 1024 bytes from 10.1.1.1
  UdpEchoServerApplication:HandleRead(): Echoing packet
  UdpEchoClientApplication:HandleRead(0x624920, 0x625160)
  UdpEchoClientApplication:HandleRead(): Received 1024 bytes from 10.1.1.2
  UdpEchoServerApplication:StopApplication()
  UdpEchoClientApplication:StopApplication()
  UdpEchoClientApplication:DoDispose()
  UdpEchoServerApplication:DoDispose()
  UdpEchoClientApplication:~UdpEchoClient()
  UdpEchoServerApplication:~UdpEchoServer()

It is also sometimes useful to be able to see the simulation time at which a log message is generated.  You can do this by ORing in the prefix_time bit.

  export 'NS_LOG=UdpEchoClientApplication=level_all|prefix_func|prefix_time:
                 UdpEchoServerApplication=level_all|prefix_func|prefix_time'

Again, you will have to remove the newline above.  If you run the script now, you should see the following output:

  Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  'build' finished successfully (0.418s)
  0s UdpEchoServerApplication:UdpEchoServer()
  0s UdpEchoClientApplication:UdpEchoClient()
  0s UdpEchoClientApplication:SetDataSize(1024)
  1s UdpEchoServerApplication:StartApplication()
  2s UdpEchoClientApplication:StartApplication()
  2s UdpEchoClientApplication:ScheduleTransmit()
  2s UdpEchoClientApplication:Send()
  2s UdpEchoClientApplication:Send(): Sent 1024 bytes to 10.1.1.2
  2.00369s UdpEchoServerApplication:HandleRead(): Received 1024 bytes from 10.1.1.1
  2.00369s UdpEchoServerApplication:HandleRead(): Echoing packet
  2.00737s UdpEchoClientApplication:HandleRead(0x624290, 0x624ad0)
  2.00737s UdpEchoClientApplication:HandleRead(): Received 1024 bytes from 10.1.1.2
  10s UdpEchoServerApplication:StopApplication()
  10s UdpEchoClientApplication:StopApplication()
  UdpEchoClientApplication:DoDispose()
  UdpEchoServerApplication:DoDispose()
  UdpEchoClientApplication:~UdpEchoClient()
  UdpEchoServerApplication:~UdpEchoServer()

You can see that the constructor for the UdpEchoServer was called at a simulation time of 0 seconds.  This is actually happening before the simulation starts, but the time is displayed as zero seconds.  The same is true for the UdpEchoClient constructor message.

Recall that the scratch/first.cc script started the echo server application at one second into the simulation.  You can now see that the StartApplication method of the server is, in fact, called at one second. You can also see that the echo client application is started at a simulation time of two seconds as we requested in the script.

You can now follow the progress of the simulation from the ScheduleTransmit call in the client that calls Send to the HandleRead callback in the echo server application.  Note that the elapsed time for the packet to be sent across the point-to-point link is 3.69 milliseconds.  You see the echo server logging a message telling you that it has echoed the packet and then, after another channel delay, you see the echo client receive the echoed packet in its HandleRead method.

There is a lot that is happening under the covers in this simulation that you are not seeing as well.  You can very easily follow the entire process by turning on all of the logging components in the system.  Try setting the NS_LOG variable to the following,

  export 'NS_LOG=*=level_all|prefix_func|prefix_time'

The asterisk above is the logging component wildcard.  This will turn on all of the logging in all of the components used in the simulation.  I won't reproduce the output here (as of this writing it produces 1265 lines of output for the single packet echo) but you can redirect this information into a file and look through it with your favorite editor if you like,

  ./waf --run scratch/myfirst > log.out 2>&1

I personally use this extremely verbose version of logging when I am presented with a problem and I have no idea where things are going wrong.  I can follow the progress of the code quite easily without having to set breakpoints and step through code in a debugger.  I can just edit up the output in my favorite editor and search around for things I expect, and see things happening that I don't expect.  When I have a general idea about what is going wrong, I transition into a debugger for a fine-grained examination of the problem.  This kind of output can be especially useful when your script does something completely unexpected. If you are stepping using a debugger you may miss an unexpected excursion completely.  Logging the excursion makes it quickly visible.

Adding Logging to your Code
---------------------------

You can add new logging to your simulations by making calls to the log component via several macros.  Let's do so in the myfirst.cc script we have in the scratch directory.

Recall that we have defined a logging component in that script:

  NS_LOG_COMPONENT_DEFINE ("FirstScriptExample");

You now know that you can enable all of the logging for this component by setting the NS_LOG environment variable to the various levels.  Let's go ahead and add some logging to the script.  The macro used to add an informational level log message is NS_LOG_INFO.  Go ahead and add one (just before we start creating the nodes) that tells you that the script is ``Creating Topology.''  This is done as in this code snippet,

Open scratch/myfirst.cc in your favorite editor and add the line,

  NS_LOG_INFO ("Creating Topology");

right before the lines,

  NodeContainer nodes;
  nodes.Create (2);

Now build the script using waf and clear the NS_LOG variable to turn off the torrent of logging we previously enabled:

  ./waf
  export NS_LOG=

Now, if you run the script,

  ./waf --run scratch/myfirst

you will not see your new message since its associated logging component (FirstScriptExample) has not been enabled.  In order to see your message you will have to enable the FirstScriptExample logging component with a level greater than or equal to NS_LOG_INFO.  If you just want to see this particular level of logging, you can enable it by,

  export NS_LOG=FirstScriptExample=info

If you now run the script you will see your new ``Creating Topology'' log message,

  Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  'build' finished successfully (0.404s)
  Creating Topology
  Sent 1024 bytes to 10.1.1.2
  Received 1024 bytes from 10.1.1.1
  Received 1024 bytes from 10.1.1.2

Using Command Line Arguments
============================

Overriding Default Attributes
-----------------------------

Another way you can change how ns-3 scripts behave without editing and building is via command line arguments.  We provide a mechanism to parse command line arguments and automatically set local and global variables based on those arguments.

The first step in using the command line argument system is to declare the command line parser.  This is done quite simply (in your main program) as in the following code,

    int
  main (int argc, char *argv[])
  {
    ...

    CommandLine cmd;
    cmd.Parse (argc, argv);

    ...
  }

This simple two line snippet is actually very useful by itself.  It opens the door to the ns-3 global variable and Attribute systems.  Go ahead and add that two lines of code to the scratch/myfirst.cc script at the start of main.  Go ahead and build the script and run it, but ask the script for help in the following way,

  ./waf --run "scratch/myfirst --PrintHelp"

This will ask Waf to run the scratch/myfirst script and pass the command line argument --PrintHelp to the script.  The quotes are required to sort out which program gets which argument.  The command line parser will now see the --PrintHelp argument and respond with,

  Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  'build' finished successfully (0.413s)
  TcpL4Protocol:TcpStateMachine()
  CommandLine:HandleArgument(): Handle arg name=PrintHelp value=
  --PrintHelp: Print this help message.
  --PrintGroups: Print the list of groups.
  --PrintTypeIds: Print all TypeIds.
  --PrintGroup=[group]: Print all TypeIds of group.
  --PrintAttributes=[typeid]: Print all attributes of typeid.
  --PrintGlobals: Print the list of globals.

Let's focus on the --PrintAttributes option.  We have already hinted at the ns-3 Attribute system while walking through the first.cc script.  We looked at the following lines of code,

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

and mentioned that DataRate was actually an Attribute of the PointToPointNetDevice.  Let's use the command line argument parser to take a look at the Attributes of the PointToPointNetDevice.  The help listing says that we should provide a TypeId.  This corresponds to the class name of the class to which the Attributes belong.  In this case it will be ns3::PointToPointNetDevice.  Let's go ahead and type in,

  ./waf --run "scratch/myfirst --PrintAttributes=ns3::PointToPointNetDevice"

The system will print out all of the Attributes of this kind of net device. Among the Attributes you will see listed is,

  --ns3::PointToPointNetDevice::DataRate=[32768bps]:
    The default data rate for point to point links

This is the default value that will be used when a PointToPointNetDevice is created in the system.  We overrode this default with the Attribute setting in the PointToPointHelper above.  Let's use the default values for the point-to-point devices and channels by deleting the SetDeviceAttribute call and the SetChannelAttribute call from the myfirst.cc we have in the scratch directory.

Your script should now just declare the PointToPointHelper and not do any set operations as in the following example,

  ...

  NodeContainer nodes;
  nodes.Create (2);

  PointToPointHelper pointToPoint;

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

  ...

Go ahead and build the new script with Waf (./waf) and let's go back and enable some logging from the UDP echo server application and turn on the time prefix.

  export 'NS_LOG=UdpEchoServerApplication=level_all|prefix_time'

If you run the script, you should now see the following output,

  Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  'build' finished successfully (0.405s)
  0s UdpEchoServerApplication:UdpEchoServer()
  1s UdpEchoServerApplication:StartApplication()
  Sent 1024 bytes to 10.1.1.2
  2.25732s Received 1024 bytes from 10.1.1.1
  2.25732s Echoing packet
  Received 1024 bytes from 10.1.1.2
  10s UdpEchoServerApplication:StopApplication()
  UdpEchoServerApplication:DoDispose()
  UdpEchoServerApplication:~UdpEchoServer()

Recall that the last time we looked at the simulation time at which the packet was received by the echo server, it was at 2.00369 seconds.

  2.00369s UdpEchoServerApplication:HandleRead(): Received 1024 bytes from 10.1.1.1

Now it is receiving the packet at 2.25732 seconds.  This is because we just dropped the data rate of the PointToPointNetDevice down to its default of 32768 bits per second from five megabits per second.

If we were to provide a new DataRate using the command line, we could speed our simulation up again.  We do this in the following way, according to the formula implied by the help item:

  ./waf --run "scratch/myfirst --ns3::PointToPointNetDevice::DataRate=5Mbps"

This will set the default value of the DataRate Attribute back to five megabits per second.  Are you surprised by the result?  It turns out that in order to get the original behavior of the script back, we will have to set the speed-of-light delay of the channel as well.  We can ask the command line system to print out the Attributes of the channel just like we did for the net device:

  ./waf --run "scratch/myfirst --PrintAttributes=ns3::PointToPointChannel"

We discover the Delay Attribute of the channel is set in the following way:

  --ns3::PointToPointChannel::Delay=[0ns]:
    Transmission delay through the channel

We can then set both of these default values through the command line system,

  ./waf --run "scratch/myfirst
    --ns3::PointToPointNetDevice::DataRate=5Mbps
    --ns3::PointToPointChannel::Delay=2ms"

in which case we recover the timing we had when we explicitly set the DataRate and Delay in the script:

  Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  'build' finished successfully (0.417s)
  0s UdpEchoServerApplication:UdpEchoServer()
  1s UdpEchoServerApplication:StartApplication()
  Sent 1024 bytes to 10.1.1.2
  2.00369s Received 1024 bytes from 10.1.1.1
  2.00369s Echoing packet
  Received 1024 bytes from 10.1.1.2
  10s UdpEchoServerApplication:StopApplication()
  UdpEchoServerApplication:DoDispose()
  UdpEchoServerApplication:~UdpEchoServer()

Note that the packet is again received by the server at 2.00369 seconds.  We could actually set any of the Attributes used in the script in this way. In particular we could set the UdpEchoClient Attribute MaxPackets to some other value than one.

How would you go about that?  Give it a try.  Remember you have to comment out the place we override the default Attribute and explicitly set MaxPackets in the script.  Then you have to rebuild the script.  You will also have to find the syntax for actually setting the new default attribute value using the command line help facility.  Once you have this figured out you should be able to control the number of packets echoed from the command line.  Since we're nice folks, we'll tell you that your command line should end up looking something like,

  ./waf --run "scratch/myfirst
    --ns3::PointToPointNetDevice::DataRate=5Mbps
    --ns3::PointToPointChannel::Delay=2ms
    --ns3::UdpEchoClient::MaxPackets=2"

Hooking Your Own Values
-----------------------

You can also add your own hooks to the command line system.  This is done quite simply by using the AddValue method to the command line parser.

Let's use this facility to specify the number of packets to echo in a completely different way.  Let's add a local variable called nPackets to the main function.  We'll initialize it to one to match our previous default behavior.  To allow the command line parser to change this value, we need to hook the value into the parser.  We do this by adding a call to AddValue.  Go ahead and change the scratch/myfirst.cc script to start with the following code,

  int
  main (int argc, char *argv[])
  {
    uint32_t nPackets = 1;

    CommandLine cmd;
    cmd.AddValue("nPackets", "Number of packets to echo", nPackets);
    cmd.Parse (argc, argv);

    ...

Scroll down to the point in the script where we set the MaxPackets Attribute and change it so that it is set to the variable nPackets instead of the constant 1 as is shown below.

  echoClient.SetAttribute ("MaxPackets", UintegerValue (nPackets));

Now if you run the script and provide the --PrintHelp argument, you should see your new User Argument listed in the help display.

Try,

  ./waf --run "scratch/myfirst --PrintHelp"

  Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  'build' finished successfully (0.403s)
  --PrintHelp: Print this help message.
  --PrintGroups: Print the list of groups.
  --PrintTypeIds: Print all TypeIds.
  --PrintGroup=[group]: Print all TypeIds of group.
  --PrintAttributes=[typeid]: Print all attributes of typeid.
  --PrintGlobals: Print the list of globals.
  User Arguments:
      --nPackets: Number of packets to echo

If you want to specify the number of packets to echo, you can now do so by setting the --nPackets argument in the command line,

  ./waf --run "scratch/myfirst --nPackets=2"

You should now see

  Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  'build' finished successfully (0.404s)
  0s UdpEchoServerApplication:UdpEchoServer()
  1s UdpEchoServerApplication:StartApplication()
  Sent 1024 bytes to 10.1.1.2
  2.25732s Received 1024 bytes from 10.1.1.1
  2.25732s Echoing packet
  Received 1024 bytes from 10.1.1.2
  Sent 1024 bytes to 10.1.1.2
  3.25732s Received 1024 bytes from 10.1.1.1
  3.25732s Echoing packet
  Received 1024 bytes from 10.1.1.2
  10s UdpEchoServerApplication:StopApplication()
  UdpEchoServerApplication:DoDispose()
  UdpEchoServerApplication:~UdpEchoServer()

You have now echoed two packets.  Pretty easy, isn't it?

You can see that if you are an ns-3 user, you can use the command line argument system to control global values and Attributes.  If you are a model author, you can add new Attributes to your Objects and they will automatically be available for setting by your users through the command line system.  If you are a script author, you can add new variables to your scripts and hook them into the command line system quite painlessly.

Using the Tracing System
========================

The whole point of simulation is to generate output for further study, and the ns-3 tracing system is a primary mechanism for this.  Since ns-3 is a C++ program, standard facilities for generating output from C++ programs could be used:

  #include <iostream>
  ...
  int main ()
  {
    ...
    std::cout << "The value of x is " << x << std::endl;
    ...
  }

You could even use the logging module to add a little structure to your solution.  There are many well-known problems generated by such approaches and so we have provided a generic event tracing subsystem to address the issues we thought were important.

The basic goals of the ns-3 tracing system are:

  * For basic tasks, the tracing system should allow the user to generate standard tracing for popular tracing sources, and to customize which objects generate the tracing;

  * Intermediate users must be able to extend the tracing system to modify the output format generated, or to insert new tracing sources, without modifying the core of the simulator;

  * Advanced users can modify the simulator core to add new tracing sources and sinks.

The ns-3 tracing system is built on the concepts of independent tracing sources and tracing sinks, and a uniform mechanism for connecting sources to sinks.  Trace sources are entities that can signal events that happen in a simulation and provide access to interesting underlying data. For example, a trace source could indicate when a packet is received by a net device and provide access to the packet contents for interested trace sinks.

Trace sources are not useful by themselves, they must be ``connected'' to other pieces of code that actually do something useful with the information provided by the sink.  Trace sinks are consumers of the events and data provided by the trace sources.  For example, one could create a trace sink that would (when connected to the trace source of the previous example) print out interesting parts of the received packet.

The rationale for this explicit division is to allow users to attach new types of sinks to existing tracing sources, without requiring editing and recompilation of the core of the simulator.  Thus, in the example above, a user could define a new tracing sink in her script and attach it to an existing tracing source defined in the simulation core by editing only the user script.

In this tutorial, we will walk through some pre-defined sources and sinks and show how they may be customized with little user effort.  See the ns-3 manual or how-to sections for information on advanced tracing configuration including extending the tracing namespace and creating new tracing sources.

ASCII Tracing
-------------

Ns-3 provides helper functionality that wraps the low-level tracing system to help you with the details involved in configuring some easily understood packet traces.  If you enable this functionality, you will see output in a ASCII files --- thus the name.  For those familiar with ns-2 output, this type of trace is analogous to the out.tr generated by many scripts.

Let's just jump right in and add some ASCII tracing output to our scratch/myfirst.cc script.  Right before the call to Simulator::Run (), add the following lines of code:

  AsciiTraceHelper ascii;
  pointToPoint.EnableAsciiAll (ascii.CreateFileStream ("myfirst.tr"));

Like in many other ns-3 idioms, this code uses a  helper object to help create ASCII traces.  The second line contains two nested method calls. The ``inside'' method, CreateFileStream() uses an unnamed object idiom to create a file stream object on the stack (without an object  name) and pass it down to the called method.  We'll go into this more in the future, but all you have to know at this point is that you are creating an object representing a file named ``myfirst.tr'' and passing it into ns-3.  You are telling ns-3 to deal with the lifetime issues of the created object and also to deal with problems caused by a little-known (intentional) limitation of C++ ofstream objects relating to copy constructors.

The outside call, to EnableAsciiAll(), tells the helper that you want to enable ASCII tracing on all point-to-point devices in your simulation; and you want the (provided) trace sinks to write out information about packet movement in ASCII format.

For those familiar with ns-2, the traced events are equivalent to the popular trace points that log "+", "-", "d", and "r" events.

You can now build the script and run it from the command line:

  ./waf --run scratch/myfirst

Just as you have seen many times before, you will see some messages from Waf and then ``'build' finished successfully'' with some number of messages from the running program.

When it ran, the program will have created a file named myfirst.tr. Because of the way that Waf works, the file is not created in the local directory, it is created at the top-level directory of the repository by default.  If you want to control where the traces are saved you can use the --cwd option of Waf to specify this.  We have not done so, thus we need to change into the top level directory of our repo and take a look at the ASCII trace file myfirst.tr in your favorite editor.

Parsing Ascii Traces
....................

There's a lot of information there in a pretty dense form, but the first thing to notice is that there are a number of distinct lines in this file.  It may be difficult to see this clearly unless you widen your window considerably.

Each line in the file corresponds to a trace event.  In this case we are tracing events on the transmit queue present in every point-to-point net device in the simulation.  The transmit queue is a queue through which every packet destined for a point-to-point channel must pass. Note that each line in the trace file begins with a lone character (has a space after it).  This character will have the following meaning:

  * +: An enqueue operation occurred on the device queue;

  * -: A dequeue operation occurred on the device queue;

  * d: A packet was dropped, typically because the queue was full;

  * r: A packet was received by the net device.

Let's take a more detailed view of the first line in the trace file.  I'll break it down into sections (indented for clarity) with a two digit reference number on the left side:

  00 +
  01 2
  02 /NodeList/0/DeviceList/0/$ns3::PointToPointNetDevice/TxQueue/Enqueue
  03 ns3::PppHeader (
  04   Point-to-Point Protocol: IP (0x0021))
  05   ns3::Ipv4Header (
  06     tos 0x0 ttl 64 id 0 protocol 17 offset 0 flags [none]
  07     length: 1052 10.1.1.1 > 10.1.1.2)
  08     ns3::UdpHeader (
  09       length: 1032 49153 > 9)
  10       Payload (size=1024)

The first line of this expanded trace event (reference number 00) is the operation.  We have a + character, so this corresponds to an enqueue operation on the transmit queue.  The second line (reference 01) is the simulation time expressed in seconds.  You may recall that we asked the UdpEchoClientApplication to start sending packets at two seconds.  Here we see confirmation that this is, indeed, happening.

The next line of the example trace (reference 02) tell us which trace source originated this event (expressed in the tracing namespace).  You can think of the tracing namespace somewhat like you would a filesystem namespace.  The root of the namespace is the NodeList.  This corresponds to a container managed in the ns-3 core code that contains all of the nodes that are created in a script.  Just as a filesystem may have directories under the root, we may have node numbers in the NodeList.  The string /NodeList/0 therefore refers to the zeroth node in the NodeList which we typically think of as ``node 0''.  In each node there is a list of devices that have been installed.  This list appears next in the namespace. You can see that this trace event comes from DeviceList/0 which is the zeroth device installed in the node.

The next string, $ns3::PointToPointNetDevice tells you what kind of device is in the zeroth position of the device list for node zero. Recall that the operation + found at reference 00 meant that an enqueue operation happened on the transmit queue of the device.  This is reflected in the final segments of the ``trace path'' which are TxQueue/Enqueue.

The remaining lines in the trace should be fairly intuitive.  References 03-04 indicate that the packet is encapsulated in the point-to-point protocol. References 05-07 show that the packet has an IP version four header and has originated from IP address 10.1.1.1 and is destined for 10.1.1.2.  References 08-09 show that this packet has a UDP header and, finally, reference 10 shows that the payload is the expected 1024 bytes.

The next line in the trace file shows the same packet being dequeued from the transmit queue on the same node.

The Third line in the trace file shows the packet being received by the net device on the node with the echo server. I have reproduced that event below.

  00 r
  01 2.25732
  02 /NodeList/1/DeviceList/0/$ns3::PointToPointNetDevice/MacRx
  03   ns3::Ipv4Header (
  04     tos 0x0 ttl 64 id 0 protocol 17 offset 0 flags [none]
  05     length: 1052 10.1.1.1 > 10.1.1.2)
  06     ns3::UdpHeader (
  07       length: 1032 49153 > 9)
  08       Payload (size=1024)

Notice that the trace operation is now r and the simulation time has increased to 2.25732 seconds.  If you have been following the tutorial steps closely this means that you have left the DataRate of the net devices and the channel Delay set to their default values.  This time should be familiar as you have seen it before in a previous section.

The trace source namespace entry (reference 02) has changed to reflect that this event is coming from node 1 (/NodeList/1) and the packet reception trace source (/MacRx).  It should be quite easy for you to follow the progress of the packet through the topology by looking at the rest of the traces in the file.

PCAP Tracing
------------

The ns-3 device helpers can also be used to create trace files in the .pcap format.  The acronym pcap (usually written in lower case) stands for packet capture, and is actually an API that includes the definition of a .pcap file format.  The most popular program that can read and display this format is Wireshark (formerly called Ethereal). However, there are many traffic trace analyzers that use this packet format. We encourage users to exploit the many tools available for analyzing pcap traces.  In this tutorial, we concentrate on viewing pcap traces with tcpdump.

The code used to enable pcap tracing is a one-liner.

  pointToPoint.EnablePcapAll ("myfirst");

Go ahead and insert this line of code after the ASCII tracing code we just added to scratch/myfirst.cc.  Notice that we only passed the string ``myfirst,'' and not ``myfirst.pcap'' or something similar.  This is because the parameter is a prefix, not a complete file name.  The helper will actually create a trace file for every point-to-point device in the simulation.  The file names will be built using the prefix, the node number, the device number and a ``.pcap'' suffix.

In our example script, we will eventually see files named ``myfirst-0-0.pcap'' and ``myfirst-1-0.pcap'' which are the pcap traces for node 0-device 0 and node 1-device 0, respectively.

Once you have added the line of code to enable pcap tracing, you can run the script in the usual way:

  ./waf --run scratch/myfirst

If you look at the top level directory of your distribution, you should now see three log files:  myfirst.tr is the ASCII trace file we have previously examined.  myfirst-0-0.pcap and myfirst-1-0.pcap are the new pcap files we just generated.

Reading output with tcpdump
...........................

The easiest thing to do at this point will be to use tcpdump to look at the pcap files.

  tcpdump -nn -tt -r myfirst-0-0.pcap
  reading from file myfirst-0-0.pcap, link-type PPP (PPP)
  2.000000 IP 10.1.1.1.49153 > 10.1.1.2.9: UDP, length 1024
  2.514648 IP 10.1.1.2.9 > 10.1.1.1.49153: UDP, length 1024

  tcpdump -nn -tt -r myfirst-1-0.pcap
  reading from file myfirst-1-0.pcap, link-type PPP (PPP)
  2.257324 IP 10.1.1.1.49153 > 10.1.1.2.9: UDP, length 1024
  2.257324 IP 10.1.1.2.9 > 10.1.1.1.49153: UDP, length 1024

You can see in the dump of myfirst-0-0.pcap (the client device) that the echo packet is sent at 2 seconds into the simulation.  If you look at the second dump (myfirst-1-0.pcap) you can see that packet being received at 2.257324 seconds.  You see the packet being echoed back at 2.257324 seconds in the second dump, and finally, you see the packet being received back at the client in the first dump at 2.514648 seconds.

Reading output with Wireshark
.............................

If you are unfamiliar with Wireshark, there is a web site available from which you can download programs and documentation:  http://www.wireshark.org/.

Wireshark is a graphical user interface which can be used for displaying these trace files.  If you have Wireshark available, you can open each of the trace files and display the contents as if you had captured the packets using a packet sniffer.