I'm looking for a a good reference on all the techniques to tunnel
data between two computers on the internet that may be behind NAT or
Firewalled routers.

For example, I know the technique where both machines start sending
UDP packets to each other on the same port, which works about 75% of
the time.

I also know about Skype's system of using 3rd party reflectors to
stand between two machines.

What I am hoping for is a review of all the current techniques, how
often they work, and what problems they have.

It could be an open source project, an academic paper, or just
someone's web page. I'm just interesting in getting the info.

Thanks!

Hi bigjosh2,

As you know, there is always a trade-off between being open and
secure. Itz wonderful if we can get adequate security while
encouraging p2p.

This reference is a publication which outlines p2p in the presence of
NAT/Firewall. Its very interesting and talks about traversal
techniques for NAT.
http://www.tml.tkk.fi/Publications/C/18/hu.pdf

This reference talks all about Skype and its internal operations!. 
http://mnet.cs.nthu.edu.tw/paper/Chance/041125.pdf

This reference is not very great but just outlines the method of
configuring windows xp firewall for bittorrents.
http://www.vladd44.com/torrent/firewall.php

This is post is a good summary of most of the techniques and the UDP
stuff is explained better!
http://slashdot.org/articles/04/11/23/1755226.shtml

This is about ipv6 and outlines the basics about ipv6 and it
highlights why p2p using ipv6 is left to user discretion.
http://www.ipv6style.jp/en/tech/20050704/index.shtml

Hope this will give you some info which you need!

Thanks!

Use teredo which is integrated into each Windows XP.

see below a 6WIND/LIP6 implementation:
  http://www-rp.lip6.fr/teredo/

See the Microsoft documentation to enable it:
http://www.microsoft.com/technet/prodtechnol/winxppro/maintain/teredo.mspx

Thanks to Teredo, you'll be able to support any p2p easily through
NATs and Firewall.