Port monitoring on Tru64

Does anyone know a way to monitor traffic on a
particular port with the tools that come standard, or
with additional software?  I am trying to use tcpdump
on a particular port, but I am gettig no data.  I am
currently just doing a tcpdump on the entire
interface.  We have traffic coming in a particular
port that is crashing the system, so I'd like to know
anything I can about it (preferably , which
users/accounts are coming in).

Thanks 

5.1A PK6 DS20E

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