[ns] A question about trace file?

[K Sun]

Hello,
    I am trying to develop a new agent on ns. However, I meet some 
problem in the trace file. It looks, in the trace file, arrivals of 
my control message packets could not be recorded in the trace files.
I believe such a problem is caused by my fault. So, I would like to 
know where the first char "r", "+" and "-" of each line in trace file 
has been decided. It seems, before Trace::format, this char has been 
decided before. However, where is the related processing? Is there 
anybody who can give me a clue? 

    Thanks a lot!

    Sun Kai