Peer-to-Peer Communication Across Network Address Translators
Massachusetts Institute of Technology
baford (at) mit.edu
Caymas Systems, Inc.
srisuresh (at) yahoo.com
dank (at) kegel.com
Presented at the
USENIX Annual Technical Conference, April 2005.
Network Address Translation (NAT)
causes well-known difficulties
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 robust 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 find 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.