"Unlike most bandwidth monitoring tools, which display network
usage per IP address or per protocol, NetHogs monitors the network
and presents bandwidth usage per application. Thus you can see how
much bandwidth the RSS aggregator, browser, software updater, and
even IRC and IM clients are using.
"NetHogs is available through the software repositories for most
distributions, or you can download the compressed tarball and
install from source. When it's installed, open a terminal, switch
to the root user, and type nethogs. You should see each application
name and its PID along with how much data it has sent and received,
similar the output of the top command. The information is updated
in real time so you don't have to relaunch nethogs every time you
launch a new application.
"By default the refresh rate is 1 second, but you can change it
anything you like with the -d command option. Also by default, the
transfer is shown in KBps (kilobytes per second) but this too can
be changed. While nethogs is running, press the m key to cycle
through the available options for displaying the data transfer. The
options are KBps, Bps, and MBps."