port-based bandwidth throttling

[Leigh Orf]

Is it possible to throttle the bandwidth of traffic using a specific
port or range of ports? Say I want to limit the total outgoing traffic
on ports 12345-12456 to 100 kB/s. Or limit outgoing ftp-data (port
20) traffic to 200 kB/s (not using ftp software throttling). Is there
a kernel-based way to do tihs? I've looked at shapecfg but the docs
didn't help me much, and what I've seen of the QoS stuff goes over my
head. Some examples/pointers would be great.

Leigh Orf