I can't stop a ftp transfer

[���±� <sgna@dokdo.dongguk.ac.kr>]

Dear  ns users,

I'm sorry for bothering you with the same questions.
I tried to stop a ftp transfer in NS 2.1b3  at given time as follows.
But the source just goes on sending. 
The below scripts nomally work in telnet source.
 
set tcp0 [$ns create-connection TCP $node_s1 TCPSink $node_r1  0]
$tcp0  set window_ 30
$tcp0  set packetSize_ 1500
set ftp0 [$tcp0 attach-source FTP]

$ns at 0.0   "$ftp0 start"
$ns at 20.0  "$ftp0 stop"

Please let me know the solution.
Thanks.
 
- SeungGu Na -