[c-nsp] traffic monitoring?

[Shaun]

I'm looking for a software that will run on a linux server and monitor 
traffic and keep accounting totals per ip and per port (and protocol would 
be nice too).  Basically I would like to be able to look at the statistics 
and see if a certain ip is sending a abnormal about of smtp traffic 
(spamming, mailing).  I could sworn I ran across a piece of software like 
this in the past but I have yet to find anything.

Anybody know of any open source tools out their that can do this?

-- 

~Shaun