Hole punching
http://www.rfc-base.org/txt/rfc-5389.txt
https://tools.ietf.org/html/rfc5245
https://tools.ietf.org/html/rfc5780
https://tools.ietf.org/html/rfc5626
https://tools.ietf.org/html/rfc5128
https://tools.ietf.org/id/draft-takeda-symmetric-nat-traversal-00.txt
http://stackoverflow.com/questions/12359502/udp-hole-punching-not-going-through-on-3g
Best resource, but I've probably bled it dry: http://www.brynosaurus.com/pub/net/p2pnat/