Originally Posted by a Hero
Either you or you're friend have a strict NAT type.
If your connected directly to a router or modem or PC/Laptop turn off your xbox360 and unplug the ethernet cord for 30 seconds, than plug it back in and turn your xbox on.
i tried that but it still wont let me join his game maybe if i mess with my nat settings a little it will help
i know it's my connection because i just got a new service a couple weeks ago and it never did this before