start the logging chapter of the manual

doc/manual/source/logging.rst

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 .. include:: replace.txt
 
+.. heading hierarchy:
+   ------------- Chapter
+   ************* Section (#.#)
+   ============= Subsection (#.#.#)
+   ############# Paragraph (no number)
+
 Logging
 -------
 
-*This chapter not yet written.  For now, the ns-3 tutorial contains logging
-information.*
+The |ns3| logging facility can be used to monitor or debug the progress
+of simulation programs.  Logging output can be enabled by program statements
+in your ``main()`` program or by the use of the ``NS_LOG`` environment variable.
+
+Logging statements are not compiled into optimized builds of |ns3|.  To use
+logging, one must build the (default) debug build of |ns3|.
+
+The project makes no guarantee about whether logging output will remain 
+the same over time.  Users are cautioned against building simulation output
+frameworks on top of logging code, as the output and the way the output
+is enabled may change over time. 
+
+Logging overview
+****************
+
+|ns3| logging statements are typically used to log various program
+execution events, such as the occurrence of simulation events or the
+use of a particular function.
+
+For example, this code snippet is from ``Ipv4L3Protocol::IsDestinationAddress()``:
+
+::
+
+        if (address == iaddr.GetBroadcast ())
+          {
+            NS_LOG_LOGIC ("For me (interface broadcast address)");
+            return true;
+          }
+
+If logging has been enabled for the ``Ipv4L3Protocol`` component at a level
+of ``LOGIC`` or above (see below about logging levels), the statement
+will be printed out; otherwise, it will be suppressed.
+
+Logging levels
+==============
+
+The following levels are defined; each level will enable the levels above
+it, with the ``ALL`` level being most verbose:
+
+# ``LOG_NONE``:  the default, no logging
+# ``LOG_ERROR``:  serious error messages only
+# ``LOG_WARN``:  warning messages
+# ``LOG_DEBUG``:  for use in debugging
+# ``LOG_FUNCTION``: function tracing
+# ``LOG_LOGIC``:  control flow tracing within functions
+# ``LOG_ALL``:  print everything
+
+A special logging level will cause logging output to unconditionally
+appear on ``std::clog``, regardless of whether the user has explicitly enabled 
+logging.
+This macro, ``NS_LOG_UNCOND()``, can be used like a kind of ``printf()`` in
+your code.  An example can be found in ``scratch/scratch-simulator.cc``:
+
+::
+
+   NS_LOG_UNCOND ("Scratch Simulator");
+
+Logging prefixes
+================
+
+This section still needs documentation; bug 1496 is open on this:
+
+::
+
+   $ NS_LOG="*=all|prefix_all" ./waf --run scratch-simulator
+   Scratch Simulator
+   ScratchSimulator:main(): [ERROR] error message
+   ScratchSimulator:main(): [WARN] warn message
+   ScratchSimulator:main(): [DEBUG] debug message
+   ScratchSimulator:main(): [INFO] info message
+   ScratchSimulator:main(function)
+   ScratchSimulator:main(): [LOGIC] logic message
+
+Enabling logging output
+=======================
+
+There are two ways that users typically control logging output.  The
+first is by setting an ``NS_LOG`` environment variable; e.g.:
+
+::
+
+   NS_LOG="*" ./waf --run first
+
+will run the first tutorial program with all logging output.  This can
+be made more granular by selecting individual components:
+
+::
+
+   NS_LOG="Ipv4L3Protocol" ./waf --run first
+  
+The second way to enable this is to use explicit statements in your
+program, such as in the first tutorial program:
+
+::
+   
+   int
+   main (int argc, char *argv[])
+   {
+     LogComponentEnable ("UdpEchoClientApplication", LOG_LEVEL_INFO);
+     LogComponentEnable ("UdpEchoServerApplication", LOG_LEVEL_INFO);
+
+Some helpers have special methods to enable the logging of all components
+in a module (across different compilation units, but logically grouped
+together such as the |ns3| wifi code:
+
+::
+
+   WifiHelper wifiHelper;
+   wifiHelper.EnableLogComponents (); 
+
+
+How to add logging to your code
+*******************************
+
+To add logging to your code, please follow the below steps:
+
+1) Put ``NS_LOG_COMPONENT_DEFINE`` macro outside of namespace ns3
+
+Create a unique string identifier (usually based on the name of the file)
+and register it with a macro call such as follows:
+
+::
+
+   NS_LOG_COMPONENT_DEFINE ("Ipv4L3Protocol");
+
+   namespace ns3 {
+   ...
+
+The macro was carefully written to permit inclusion either within or
+outside of namespace ns3, and usage will vary across the codebase, but
+the original intent was to register this outside of namespace ns3.
+
+2) Add logging statements to your functions and function bodies.
+
+There are a couple of guidelines on this:
+
+* Do not add function logging in operators or explicit copy constructors, 
+  since these will cause infinite recursion and stack overflow.
+* Use the ``NS_LOG_FUNCTION_NOARGS()`` variant for static methods only. When 
+  a non-static member function has no arguments, just log it as 
+  ``NS_LOG_FUNCTION (this)``
+* Make sure that you test that your logging changes do not break the code; 
+  running some example programs with all log components turned on (e.g. 
+  ``NS_LOG="*"``) is one way to test this.
+