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TCP simultaneous open is very difficult to use.

In our experience with a deployed P2P system, UDP hole punching is more successful with all the strange NAT boxes deployed. Our success rate is 95.3% now in the wild. As said above, this TCP fallback sounds like an excellent way to get to 97% connection success rate!

Note that there is an upcoming IETF Internet Standard describing UDP hole punching: http://tools.ietf.org/html/draft-ietf-ppsp-peer-protocol-06#...

We've implemented this as LGPL code: svn.tribler.org/libswift/branches/ppsp-03/