HI, I tried to simulate a CBR/RTP src/rcvr connection. This is how I set it up. set cbr0 [new Agent/CBR/RTP] $cbr0 set interval_ 100ms $ns attach-agent $src $cbr0 set rcvr [new Agent/CBR/RTP] $ns attach-agent $dst $rcvr $ns connect $cbr0 $rcvr ns crashed at RTPSession:srcid(). I appriciate anyone has a sample script that works. Karu.