[ns] how to produce different length packets, thanks

["xiaohui JIN" <xhjin@mcn.xidian.edu.cn>]

hi,experts:
    i am simulating a TCP connection, i need to produce different length packets, some of them r 400 bytes, and others r 800 bytes.
and i had looked up all *.cc&*.h under ~ns/. and found out these files relate with packetsize(size_)( i think so)
i had changed tcp.cc to produce different length packets.
such as 
" if (delay_bind(varName, localName, "packetSize_", &size_ , tracer)) return TCL_OK;"
i changed it to
" if (delay_bind(varName, localName, "packetSize_", 400 , tracer)) return TCL_OK;"
the packets' length had changed.
but when i change it to
"float randomnum = Random::uniform();
 int oldsize_ = size_;
 if (randomnum <= 0.4) {size_ = 400;printf(" %f %d %d \n",randomnum,size_,&size_);}
 else {size_ = 800; printf(" %f %d %d\n",randomnum, size_,&size_);}
if (delay_bind(varName, localName, "packetSize_", &size_ , tracer)) return TCL_OK;
"
it didnt work.all the packets r lengthed 800 bytes.
why?
thanks for u help!


Regard

JIN xiaohui