e2e delay

[souhaj]

hello  

i am workin with ns...and i am tring to analyse the trace file..
i have found an awk programm that calculate the end to end delay...
http://mailman.isi.edu/pipermail/ns-users/2007-August/060808.html
however i didn't understand, this part of the program (i know that is calculating the end to end delay : but how this is done) :
 
# CALCULATE DELAY
  if ( start_time[packet_id] == 0 )  start_time[packet_id] = time;
  if (( $1 == "r") &&  ( $35 == "cbr" ) && ( $19=="AGT" )) {  end_time[packet_id] = time;  }
       else {  end_time[packet_id] = -1;  }

 
..............................
....................................
...................
for ( i in end_time )
  {
  start = start_time[i];
  end = end_time[i];
  packet_duration = end - start;
  if ( packet_duration > 0 )  
  {    sum += packet_duration;
       recvnum++;
  }
  }

can you help me..
thanks in advance