Behind NAT to behind NAT connection

Question

I\'ve come across an interesting problem. Basically I have 2 mobile phones that are both behind NATs. I want to communicate directly between the 2 devices using UDP.

Answer 1

You need a mediator server, so the clients can tell where they are. Then one opens a server by uPNP, and the other connects to it.

Answer 2

What you're looking for is UDP hole punching, see e.g. http://en.wikipedia.org/wiki/UDP_hole_punching

The basic idea is simple, you tell each endpoint the ports to use, and they start sending udp packets. The NAT'ing devices will set up a traversal rule when they see the first outgoing packet, and then the next attempt from the other end will match this traversal rule.