ipfw is the Mac OS X firewall, which has a number of interesting features and is woefully underused.
Bandwidth Throttling Notes
These are taken from a post I saw somewhere (can’t remember where):
sudo ipfw pipe 1 config bw 15KByte/s
creates a pipe that only allows up to 15KB/s to go through.
sudo ipfw add 1 pipe 1 src-port 80
will attach that pipe to the outgoing traffic on port 80, effectively limiting the outgoing traffic of the web server.
sudo ipfw delete 1
will remove the pipe from the port.