Direct P2P connection

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+3 Q:

Direct P2P connection

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I'm rather new to the topic of network programming, never having done anything but basic TCP/UDP on a single local machine. Now I'm developing an application that will need P2P network support. More specifically I will need the application to connnect and communicate across the internet preferably without the use of a server to do the matchmaking between the clients.

I'm aware and assuming that almost all users are behind a router which complicates the process since neither clients will be able to initialize a direct connection to the other.

I know UPnP is an option to allow port forwarding without having the users configure this manually, but as of now this is not an option. Is they any way to achieve my goal or will I need that server?

+2 A:

Ignoring UPnP (which only works with some routers, unfortunately), and no central server, I'm not sure it would be possible to create a direct connection when both users are behind a NAT.

Kitsune 2009-07-26 01:02:58

I'm aware of the NET punching /hole punching tecnique, but it requires a server.

Qua 2009-07-26 01:07:54

I missed that link in your post the first time I went through. Without having a central server (to help negotiate the connection), I don't believe it's possible.

Kitsune 2009-07-26 01:11:21

What sad world are we living in, when two computers cannot connect to eachother directly without having special configured hardware or a middleman to help them.

Qua 2009-07-26 01:13:34

That is why we need IPv6.

bortzmeyer 2009-07-30 08:19:49

IPv6 would help a lot, it should make the UDP trick a lot easier, while still giving the same security.

Kitsune 2009-07-30 15:28:40

+3 A:

Check out the P2P channel with WCF: http://msdn.microsoft.com/en-us/library/cc297274.aspx

It works quite well.

codekaizen 2009-07-26 01:05:04

And this works without a server and UPnP?

Qua 2009-07-26 01:08:36

Yes. It uses the Peer Naming Resolution and IPV6.

codekaizen 2009-07-26 01:17:25

IPv6 basically doesn't use NAT as there is a big enough address space anyway. But most ISPs don't support it yet. And consumer NAT need updated firmware to know to just bridge IPv6 connections.

ewanm89 2009-07-26 01:22:18

Right, but with Teredo and IPV6 tunneling, you don't need to worry about network support.

codekaizen 2009-07-26 02:10:11

Do you've a link to any articles/papers discussing those teqniques with relation to .NET?

Qua 2009-07-26 03:39:12

WCF Peer channel takes care of it for you using Windows PNRP and Peer Networking. It's pretty robust and fairly simple to grab a WCF Peer Channel demo/sample and try it yourself.

codekaizen 2009-07-26 06:30:09

IPv6 tunneling is not possible in all cases, and the point is we are trying to minimise user configuration. Not maximise it.

ewanm89 2009-07-26 10:13:04

@ewanm89 have you used Peer Channel? I've never had a lot of configuring to do... and never had problems with IPv6 tunneling. In v2.0 of Peer-to-peer it just worked.

codekaizen 2009-07-26 19:48:15

+3 A:

You'll need a server to exchange IP address and such. As the other thread literally points out, the only way of guaranteeing a connection is to proxy through a server. Most peer to peer systems use UPnP and NAT Hole Punching (this method needs a server relaying port information and only works with UDP) to establish a connection in most cases.

NAT Hole Punching works by both clients establishing a connection to a server, then the both try to connect directly to a port that the other has relayed to the other. Most UDP NAT remember the IP address and port for a short time, so although the data never made it to the other end (not that this matters with UDP) the other client will try to connect a few moments later to that report as the NAT would expect the reply.

ewanm89 2009-07-26 01:18:00

You mention that IPv6 avoids the problem of NAT, does this mean that if I by that address format I can avoid this whole trouble of NAT blocking direct connections?

Qua 2009-07-26 01:50:06

Needs a fully routable IPv6 address both ends.

ewanm89 2009-07-26 10:13:41

+1 A:

Well to avoid a server to "matchmake" you could do what skype does and set up some peers as relays to others behind NATs. You wll always need some sort of boot strapping mechanism, so a centralized server will probably play into your system somehow (depending on what your developing, of course).

Junier 2009-07-26 01:34:04

+4 A:

A very good reading, made just for you :-), is RFC 5128, "State of Peer-to-Peer (P2P) Communication across Network Address Translators (NATs)".

bortzmeyer 2009-07-30 08:19:12

Thanks! That is indead a very good article.

Qua 2009-08-27 03:00:08

ansaurus

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