How to initiate direct connection between clients connected to a server

Question

Suppose that I have a server and the clients are connected to the server. The server is accessible through a public ip.

I would like to "forward" the connection so that the clients would be directly connected to each other withouth the server in the middle.

I do not know if this is possible at all and I myself couldn't find a way to do it.

Other assumptions:

• neither client has a public IP, both clients are behind a NAT
• there can be more connections initiated if necessary

I am looking for a strictly software solution, without the need to reconfigure router, open ports, etc. etc.

The reason I would like to achieve this is to reduce the load on server. Once a two clients are associated together there is no real need (except technical one) to continue using the server as a, sort of, proxy. Direct client connection would also reduce the latency of the connection.

Answer 1

Take a look at http://en.wikipedia.org/wiki/UDP_hole_punching. If neither machine has a public IP, and you can't open ports, and you don't want all data to pass through the server, this is probably the only other potential option. If this wouldn't work for you, you're likely stuck with all data going through the server. If you can set up port forwarding, that would make for a better solution, then just use the server to exchange IP and port information (as it exists at the time the connection is established).

Answer 2

A short answer is: it is not possible.

One of the main problems is that router do not know where to redirect the request from the server (or other client). Just a case : You have a router which has multiple devices(computer, cell phone...) behind it. It gets a request and do not know, who wants to get the request.

There is a workaround for that but it is not reliable (Does not work always.) Some Companies use it if it is possible, but they have always an alternative to that (like communication over server) if it fails. It is called nat punching. More details here: http://en.wikipedia.org/wiki/TCP_hole_punching

I do not know for which reason do you need. If you need it for client server connection you can use something like long polling, call back....

Otherwise you need to set the router, or take the route over the server.