how to limit cable modem upload speed?

I want to limit the speed at which my linux box will upload to the cable modem, since, above a certain rate, this slows everything down. On the other hand, I want it to work as fast as possible when streaming to the internal network. One card connects to both -- ie to the router, which connects to the cable modem and to everything else on the network. POking around on Google tells me there is a "tc" command, which will slow down all traffic on an interface. But I can't work out if I can tell it to slow down only external traffic, and the documentaiton presumes a level of competence and knowledge far beyond me. -- Andrew Brown What I do: www.darwinwars.com What I'm up to: www.thewormbook.com/helmintholog/