The most important basics of P2P

Question

I've been reading around on the www but just can't get the most important basics of P2P.

The diagram is like this:

[peer1]<-->[dsl-router1]<-->[central server]<-->[dsl-router2]<-->[peer2]

I'm developing a chat software on the central server. Chat messages being transfered thru' the central server well by now, however, I need to make the p2p file sharing feature because the bandwidth (the cable bandwith, not the transfer limit) of the server supposed for transfering chat messages only.

The problem is that, my software on central server knows the IPs and ports of router1 and router2, but not the peer1 and peer2 as these peers are behind the routers and don't have IP addresses.

How to actually transfer some data from peer1 to peer2 and vice versa without having this data passing thru' central server?

(and the worst case is that there is a wireless router between peer and dsl-router)

Answer 1

There are two basic ways of doing this. The new way is to use IGDP (opening a port via uPnP). This is described quite well here:

http://www.codeproject.com/Articles/13285/Using-UPnP-for-Programmatic-Port-Forwardings-and-N

If neither of the two nodes have a router supporting uPnP then another alternative is TCP hole punching, which is not perfect but works quite well in practice. This is described here:

http://www.brynosaurus.com/pub/net/p2pnat/

Answer 2

During some situations, "routers" supplied by the ISP may run on bridge mode, which directly exposes the peer computer on the internet (the computer gets a public internet address). If at least one side has this configuration (or in a similar situation that the peer client is not behind another device), then things should be rather straight forward: simply assign the central server's job to whoever that have this privilege.

In the other case where both peers only have a local address (e.g. 192.168.0.2) assigned to their computers, it would then be rather difficult to get through the routers; clients behind routers are for the most part unreachable from the outside unless they originated the request. Then, one solution to the problem is port forwarding. By doing port forwarding, either through explicitly written rules or UPnP, some ports on the peer computer is exposed to the public internet, as in the first situation where instead of only some ports the entire computer is exposed.

If you are without either of these, then there is no simple way to avoid sending through the central server. Though you could, potentially, find other peers who have the capability to transfer for others.