NAT Traversal

This is the original STUN algorithm.
One situation where it fails is the following:

               STUN
             server
               |
              NAT
               |
       ------------------
       |                |
      UA1              UA2

If the NAT does not support hairpinning, then your algorithm will not
work.

[http://tools.ietf.org/html/draft-jennings-behave-test-results-04
shows that most NATs today do not support UDP hairpinning, and
http://www.guha.cc/saikat/pub/imc05-tcpnat.pdf reports that most do not
support TCP hairpinning.]

This situation can occur when the two UAs belong to the same company,
or both are in the same hotel, or both use the same service provider
and the service provider has a NAT in front of its entire network, etc.

ICE does to things to solve this problem. First of all, UA1 and UA2 exchange
their local addresses and ports as well as their STUN-learned addresses.
They then test to see which path works. Second, they also exchange
TURN addresses, which serve as a backup in case everything else fails.

David Barrett's algorithm, if I understand it correctly, would work in
this situation. (As far as I can tell, David's is a simplified version
of ICE).