monitoring internet traffic

[Geoffrey Leach <geoff@xxxxxxxxxx>]

My Internet connection is via a satellite service that throttles the  
connection once I exceed a number of Mb. I'd like to monitor this so as  
to avoid unpleasant surprises. I think that what I need here is a  
specialized proxy. Can anyone point me to a proxy that either does such  
a thing or is sufficiently simple to be modifiable by a non-guru? Perl  
preferred, FWIW.