@node Tracing
@chapter Tracing

The tracing subsystem is one of the most important mechanisms to understand in
@command{ns-3}.  In most cases, @command{ns-3} users will have a brilliant idea
for some new and improved networking feature.  In order to verify that this
idea works, the researcher will make changes to an existing system and then run
experiments to see how the new feature behaves by gathering statistics
that capture the behavior of the feature.

In other words, the whole point of running a simulation is to generate output for
further study.  In @command{ns-3}, the subsystem that enables a researcher to do
this is the tracing subsystem.

@menu
* Tracing Motivation::
* Overview::
* Using the Tracing API::
* Tracing implementation details::
@end menu

@node Tracing Motivation
@section Motivation

There are many ways to get information out of a program.  The most straightforward
way is to just directly print the information to the standard output, as in,

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

This is workable in small environments, but as your simulations get more and more
complicated, you end up with more and more prints and the task of parsing and 
performing computations on the output begins to get harder and harder.

Another thing to consider is that every time a new tidbit is needed, the software
core must be edited and another print introduced.  There is no standardized way
to control all of this output, so the amount of output tends to grow without 
bounds.  Eventually, the bandwidth required for simply outputting this information
begins to limit the running time of the simulation.  The output files grow to 
enormous sizes and parsing them becomes a problem.

@command{ns-3} provides a simple mechanism for logging and providing some control 
over output via @emph{Log Components}, but the level of control is not very fine
grained at all.  The logging module is a relatively blunt instrument.

It is desirable to have a facility that allows one to reach into the core system
and only get the information required without having to change and recompile the
core system.  Even better would be a system that notified the user when an item
of interest changed or an interesting event happened.

The @command{ns-3} tracing system is designed to work along those lines and is 
well-integrated with the Attribute and Config substems allowing for relatively
simple use scenarios.

@node Overview
@section Overview

The tracing subsystem relies heavily on the @code{ns-3} Callback and Attribute
mechanisms.  You should read and understand the corresponding sections of the 
manual before attempting to understand the tracing system.

The ns-3 tracing system is built on the concepts of independent tracing sources
and tracing sinks; along with 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.  A trace source might also indicate 
when an interesting state change happens in a model.  For example, the congestion
window of a TCP model is a prime candidate for a trace source.

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 source.
The entities that consume trace information are called trace sinks.  Trace sources 
are generators of events and trace sinks are consumers. 

This explicit division allows for large numbers of trace sources to be scattered
around the system in places which model authors believe might be useful.  Unless
a user connects a trace sink to one of these sources, nothing is output.  This
arrangement allows relatively unsophisticated users to attach new types of sinks
to existing tracing sources, without requiring editing and recompiling the core 
or models of the simulator.  

There can be zero or more consumers of trace events generated by a trace source.  
One can think of a trace source as a kind of point-to-multipoint information link.  

The ``transport protocol'' for this conceptual point-to-multipoint link is an 
@code{ns-3} @code{Callback}.

Recall from the Callback Section that callback facility is a way to allow two
modules in the system to communicate via function calls while at the same time
decoupling the calling function from the called class completely.  This is the
same requirement as outlined above for the tracing system.

Basically, a trace source @emph{is} a callback to which multiple
functions may be registered.  When a trace sink expresses
interest in receiving trace events, it adds a callback to a list of callbacks
held by the trace source.  When an interesting event happens, the trace source
invokes its @code{operator()} providing zero or more parameters.  This tells
the source to go through its list of callbacks invoking each one in turn.  In
this way, the parameter(s) are communicated to the trace sinks, which are just
functions.

@subsection The Simplest Example

It will be useful to go walk a quick example just to reinforce what we've said.

@smallformat
@example
  #include ``ns3/object.h''
  #include ``ns3/uinteger.h''
  #include ``ns3/traced-value.h''
  #include ``ns3/trace-source-accessor.h''
  
  #include <iostream>
  
  using namespace ns3;
@end example
@end smallformat

The first thing to do is include the required files.  As mentioned above, the
trace system makes heavy use of the Object and Attribute systems.  The first
two includes bring in the declarations for those systems.  The file, 
@code{traced-value.h} brings in the required declarations for tracing data
that obeys value semantics.  

In general, value semantics just means that you can pass the object around,
not an address.  In order to use value semantics at all you have to have an
object with an associated copy constructor and assignment operator
available.  We extend the requirements to talk about the set of operators
that are pre-defined for plain-old-data (POD) types.  Operator=, operator++,
operator--, operator+, operator==, etc.

What this all means is that you will be able to trace changes to an object
made using those operators.

@smallformat
@example
  class MyObject : public Object
  {
  public:
    static TypeId GetTypeId (void)
    {
      static TypeId tid = TypeId ("MyObject")
        .SetParent (Object::GetTypeId ())
        .AddConstructor<MyObject> ()
        .AddTraceSource ("MyInteger",
                         "An integer value to trace.",
                         MakeTraceSourceAccessor (&MyObject::m_myInt))
        ;
      return tid;
    }
  
    MyObject () {}
    TracedValue<uint32_t> m_myInt;
  };
@end example
@end smallformat

Since the tracing system is integrated with Attributes, and Attributes work
with Objects, there must be an @command{ns-3} @code{Object} for the trace source
to live in.  The two important lines of code are the @code{.AddTraceSource} and 
the @code{TracedValue} declaration.

The @code{.AddTraceSource} provides the ``hooks'' used for connecting the trace
source to the outside world.  The @code{TracedValue} declaration provides the
infrastructure that overloads the operators mentioned above and drives the callback 
process.

@smallformat
@example
  void
  IntTrace (Int oldValue, Int newValue)
  {
    std::cout << ``Traced `` << oldValue << `` to `` << newValue << std::endl;
  }
@end example
@end smallformat

This is the definition of the trace sink.  It corresponds directly to a callback
function.  This function will be called whenever one of the operators of the
@code{TracedValue} is executed.

@smallformat
@example
  int
  main (int argc, char *argv[])
  {
    Ptr<MyObject> myObject = CreateObject<MyObject> ();
  
    myObject->TraceConnectWithoutContext ("MyInteger", MakeCallback(&IntTrace));

    myObject->m_myInt = 1234;
  }
@end example
@end smallformat

In this snippet, the first thing that needs to be done is to create the object
in which the trace source lives.

The next step, the @code{TraceConnectWithoutContext}, forms the connection
between the trace source and the trace sink.  Notice the @code{MakeCallback}
template function.  Recall from the Callback section that this creates the
specialized functor responsible for providing the overloaded @code{operator()}
used to ``fire'' the callback.  The overloaded operators (++, --, etc.) will
use this @code{operator()} to actually invoke the callback. The 
@code{TraceConnectWithoutContext}, takes a string parameter that provides 
the name of the Attribute assigned to the trace source.  Let's ignore the bit
about context for now since it is not important yet.

Finally, the line,

@verbatim
   myObject->m_myInt = 1234;
@end verbatim

should be interpreted as an invocation of @code{operator=} on the member 
variable @code{m_myInt} with the integer @code{1234} passed as a parameter.
It turns out that this operator is defined (by @code{TracedValue}) to execute
a callback that returns void and takes two integer values as parameters -- 
an old value and a new value for the integer in question.  That is exactly 
the function signature for the callback function we provided -- @code{IntTrace}.

To summarize, a trace source is, in essence, a variable that holds a list of
callbacks.  A trace sink is a function used as the target of a callback.  The
Attribute and object type information systems are used to provide a way to 
connect trace sources to trace sinks.  The act of ``hitting'' a trace source
is executing an operator on the trace source which fires callbacks.  This 
results in the trace sink callbacks registering interest in the source being 
called with the parameters provided by the source.

@subsection Using the Config Subsystem to Connect to Trace Sources

The @code{TraceConnectWithoutContext} call shown above in the simple example is
actually very rarely used in the system.  More typically, the @code{Config}
subsystem is used to allow selecting a trace source in the system using what is
called a @emph{config path}.

For example, one might find something that looks like the following in the system
(taken from @code{examples/tcp-large-transfer.cc})

@smallformat
@example
  void CwndTracer (uint32_t oldval, uint32_t newval) {}

  ...

  Config::ConnectWithoutContext (
    "/NodeList/0/$ns3::TcpL4Protocol/SocketList/0/CongestionWindow", 
    MakeCallback (&CwndTracer));
@end example
@end smallformat

This should look very familiar.  It is the same thing as the previous example,
except that a static member function of class @code{Config} is being called instead 
of a method on @code{Object}; and instead of an @code{Attribute} name, a path is
being provided.

The first thing to do is to read the path backward.  The last segment of the path
must be an @code{Attribute} of an @code{Object}.  In fact, if you had a pointer to
the @code{Object} that has the ``CongestionWindow'' @code{Attribute} handy (call it
@code{theObject}), you could write this just like the previous example:

@smallformat
@example
  void CwndTracer (uint32_t oldval, uint32_t newval) {}

  ...

  theObject->TraceConnectWithoutContext ("CongestionWindow", MakeCallback (&CwndTracer));
@end example
@end smallformat

It turns out that the code for @code{Config::ConnectWithoutContext} does exactly that.
This function takes a path that represents a chain of @code{Object} pointers and follows 
them until it gets to the end of the path and interprets the last segment as an
@code{Attribute} on the last object.  Let's walk through what happens.

The leading ``/'' character in the path refers to a so-called namespace.  One of the
predefined namespaces in the config system is ``NodeList'' which is a list of all of
the nodes in the simulation.  Items in the list are referred to by indices into the 
list, so ``/NodeList/0'' refers to the zeroth node in the list of nodes created by
the simulation.  This node is actually a @code{Ptr<Node>} and so is a subclass of
an @code{ns3::Object}.  

As described in the Object Model section, @code{ns-3} supports an object aggregation
model.  The next path segment begins with the ``$'' character which indicates a
@code{GetObject} call should be made looking for the type that follows.  When a
node is initialized by an @code{InternetStackHelper} a number of interfaces are
aggregated to the node.  One of these is the TCP level four protocol.  The runtime
type of this protocol object is ``ns3::TcpL4Protocol''.  When the @code{GetObject}
is executed, it returns a pointer to the object of this type.

The @code{TcpL4Protocol} class defines an Attribute called ``SocketList'' which is
a list of sockets.  Each socket is actually an @code{ns3::Object} with its own 
@code{Attributes}.  The items in the list of sockets are referred to by index just
as in the NodeList, so ``SocketList/0'' refers to the zeroth socket in the list
of sockets on the zeroth node in the NodeList -- the first node constructed in the
simulation.

This socket, the type of which turns out to be an @code{ns3::TcpSocketImpl} defines
an attribute called ``CongestionWindow'' which is a @code{TracedValue<uint32_t>}.
The @code{Config::ConnectWithoutContext} now does a,

@smallformat
@example
  object->TraceConnectWithoutContext ("CongestionWindow", MakeCallback (&CwndTracer));
@end example
@end smallformat

using the object pointer from ``SocketList/0'' which makes the connection between
the trace source defined in the socket to the callback -- @code{CwndTracer}.

Now, whenever a change is made to the @code{TracedValue<uint32_t>} representing the
congestion window in the TCP socket, the registered callback will be executed and 
the function @code{CwndTracer} will be called printing out the old and new values
of the TCP congestion window.

@node Using the Tracing API
@section Using the Tracing API

  There 
are three levels of interaction with the tracing system:

@itemize @bullet
@item Beginning user can easily control which objects are participating in tracing;
@item Intermediate users can extend the tracing system to modify the output format 
generated or use existing trace sources in different ways, without modifying the 
core of the simulator;
@item Advanced users can modify the simulator core to add new tracing sources and 
sinks. 
@end itemize

@node Tracing implementation details
@section Implementation details
