TCP NAT traversal

[ueberbill]

We have a (basically P2P) application that uses TCP and we're trying to get it to work when one or both of the machines are behind a NAT box. I've read a lot of literature on how to get UDP through NAT's but they all say that TCP is hard to get working without using a relay for all of the traffic. Does anyone have any experience traversing NAT's with TCP? Is there literature (or, dare I hope, code) out there?

Thanks,
Bill

 

[bcastner]

Are you writing the app, or just using it?

NAT traversal is a problem, but the millions of Kazaa, eDoneky network client, and instant messenger clients show it is not insurmountable.


You can either:

. uPnP enable the application, or make entries to open the ports through the uPnP client for the gateway;

. Use the router's ability to forward ports to specific LAN-side IPs.


The first would allow an applicaton to open the necessary port forwards automaticly; the latter would require user intervention.

 

[ueberbill]

We are actually writing the app and we're looking for a way to make it work with all kinds of NAT configurations without having to communicate with any routing/NAT hardware.
UPnP does look pretty cool, I have one question-
Does it work now? I looked at the UPnP.org site and it didn't seem to say whether or not I could write a UPnP-enabled app and have it work right now.

The problem is we're trying to avoid requiring any special firewall hardware and it looks like we'd have to have a UPnP enabled NAT box if we were going to use UPnP (unless, and it's happened before, I am wrong).

We're basically trying to allow two clients to connect to each other when/if they are behind NAT boxes. We're pretty early in the brainstorming phase, but ideas we've had include:
-Using UDP to tunnel through (as there are myriad methods for making UDP connections through NAT which don't work well for TCP)
-Using a relay (obviously not a great solution as it would be a big slow mess under load)

There are others but I can't seem to locate my notes. How do those lucky Kazaa, eDonkey, and AIM users surmount the NAT TCP problem?

Thanks for your help,
Bill