[ns] overhead problem

["imran ali" <aliimran@rediffmail.com>]

dear sir/madam
i have to calculate overhead in packets for a 
TCP traffic . suppose i have source as the initiater
of the traffic and i want to put its value in over. i have tried 

set over [$source set overhead_]

and also

set over [$source set sizeof(hdr_tcp)]

but it is not giving result
can u tell me how overhead in a packet is calculated
thanx in adv



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