[ns] Throughput calculation

[Hafida Fouta <961L325403@stud.alakhawayn.ma>]

Dear all,
I have a problem on finding a throughput in ns network.
I used this piece of code in the finish() procedure

set nbytes [$tcp_s1  set ack_]
set time [expr $stop_time - $start_time
set thruput [expr $nbytes * $pktsize * 8 / $time ]
 puts " throughput = $thruput"             

but i got an error message saying that 
start_time and stop_time are not recognized

Can any one of you help me in this problem?

thanx in advance