UDP and TCP Hole Punching: Peer-to-Peer Communication Across Network Address Translators - P2P

Peer-to-Peer Communication Across Network Address Translators
Technical paper explaining circumvention of of NAT restrictions on Peer to Peer traffic. There was a Technetcast or codecon lecture on this a while back, I'll try to find it.

Hole punching is a general-purpose technique for establishing peer-to-peer connections in the presence of NAT. As long as the NATs involved meet certain behavioral requirements, hole punching works consistently and robustly for both TCP and UDP communication, and can be implemented by ordinary applications with no special privileges or specific network topology information. Hole punching fully preserves the transparency that is one of the most important hallmarks and attractions of NAT, and works even with multiple levels of NAT--though certain corner case situations require hairpin translation, a NAT feature not yet widely implemented.