Contorle de congestion sur ns-2 par l'extension de eurane

j ai des erreurs sur mon code j'essaye d'implementer le controle de congestion :

set cwndf [open cwnd.tr w]
proc record {} {
global ns cwndf
set time 0.1
set now [$ns now]
set cwnd "$now"
set cwnd "$cwnd [$tcp0 set cwnd_]"
puts $cwndf "$cwnd"
$ns at [expr $now+$time] "record"
}
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this is the error when i run my script:

ns: record: can't read "tcp0": no such variable
while executing
"$tcp0 set cwnd_"
(procedure "record" line 6)
invoked from within
"record"
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!!! thank you