Behind NAT to behind NAT connection

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.

I know if I initiate a connection from the phones to a server then I can push data back down that connection to the phone (ie send it back from the same port that received the message to the same ip and port that it was received from). So I can easily communicate between the 2 devices by connecting both phones to the server. Then sending data to the server and having it re-routed back to the phones. This bypasses any NAT traversal issues I may come up against.

However I would rather just use the server to point the 2 devices at each other and then let them communicate directly. How would I go about doing 开发者_StackOverflow中文版this? Is it possible without using something like uPnP?

Any help would be much appreicated!

Edit: I found this document http://www.brynosaurus.com/pub/net/p2pnat/ It looks like hairpin translation is what I'm after but it doesn't look to be widely supported. I wonder how good mobile ISP's support for UPnP is?

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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.

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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.