File Transfer problem with iChat

I didn't transfer files much with iChat, so I haven't noticed this until today. I can't receive a file from someone, so I tried to send something to a few people as a test, and none works, so it had to be on my end. I can't send files dragging it to the text box where I type, or using the "send file..." item in the buddies menu. I suspect this is a port issue, but how do I test a port? What port is needed for file transfer/direct connection? And finally, how do I open the port if it's not open?

Using OS X 10.4.2, all the latest updates, behind an AirPort Extreme router, using DSL.