When the xbox is searching for games, it broadcasts 42 bytes of data over UDP, from a source address of 0.0.0.1 to the IPv4 broadcast address 126.96.36.199 on port 3074. Pretty standard LAN play with UDP. (If anyone wants this capture PM me, but it's pretty useless).
I'm assuming that when it gets a reply to this is when the exchange occurs. However I can't test this because I don't have two 360s, nor do I have the network hardware to do a proper capture.
However, if anyone well-enough equipped were to take some captures and post them, others could analyze them.
Edited by freyyr890, 02 July 2008 - 01:46 AM.