2005 USENIX Annual Technical Conference USENIX Association 179 Peer-to-Peer Communication Across Network Address Translators

J’fais des trous, des petits trous... toujours des petits trous- S. Gainsbourg Network Address Translation (NAT) causes well-known difculties for peer-to-peer (P2P) communication, since the peers involved may not be reachable at any globally valid IP address. Several NAT traversal techniques are known, but their documentation is slim, and data about their robustness or relative merits is slimmer. This paper documents and analyzes one of the simplest but most ro-bust and practical NAT traversal techniques, commonly known as hole punching. Hole punching is moderately well-understood for UDP communication, but we show how it can be reliably used to set up peer-to-peer TCP streams as well. After gathering data on the reliability of this technique on a wide variety of deployed NATs, we nd that about 82 % of the NATs tested support hole punching for UDP, and about 64 % support hole punching for TCP streams. As NAT vendors become increasingly conscious of the needs of important P2P applications such as Voice over IP and online gaming protocols, support for hole punching is likely to increase in the future.