You have a couple of options for monitoring the network. TCPdump will dump packets in raw text as they are seen, ttt will provide a plot of throughput of various protocols and senders, and a program that Don Smith wrote as an extension to TCP will also plot the load (with some filtering).
This program is really better suited for data collection and post processing but with the right filtering it can be quite handy when looking for an anomolous condition. "man tcpdump" for details.
This program looks pretty good but has a couple of drawbacks. Notably, it wants to rescale the plot as data scrolls off the side of the screen (it shows you several seconds worth of data "as it's happening"). Man ttt for details. This comes with the ports collection and is installed on bennett (the comp290 file server).
Contact Don for details.