Package iftop

Command line tool that displays bandwidth usage on an interface

http://www.ex-parrot.com/~pdw/iftop/

iftop does for network usage what top(1) does for CPU usage. It listens to
network traffic on a named interface and displays a table of current bandwidth
usage by pairs of hosts. Handy for answering the question "why is our ADSL link
so slow?".

System Administration (Section 8)
iftop
iftop listens to network traffic on a named interface, or on the first interface it can find which looks like an external interface if none is specified, and...