Monitoring bandwidth utilization with/on NetBSD

[Chris Ross <cross+netbsd%distal.com@localhost>]

  I have a few NetBSD systems of multiple architectures, including a 
gateway machine at the edge of a few LANs.  That edge host is running 
netbsd-5, and using ipfilter.
  Is there an easy way, either within ipf, or with a package in pkgsrc, 
to get reports on the amount of network traffic used, ideally on a 
per-IP-address basis?  I want both to know how much bandwidth I'm using 
on a whole, and which of the machines behind the edge router machine are 
using how much of it.  Various time-slicing options would be useful too, 
but mostly I need large-scale (per day/week/month) numbers, not 
momentary numbers.
  A little google searching didn't show an obvious answer, but I 
apologize if I didn't search long enough and missed something I 
should've been able to find on my own.
  Much thanks.

                                                - Chris