> NAT traversal requires a server accessible to both parties, correct? If both parties are behind a NAT, you need a STUN server to help set up UDP hole punchin

> NAT traversal requires a server accessible to both parties, correct? If both parties are behind a NAT, you need a STUN server to help set up UDP hole punching. STUN servers are cheap to run, as it is a stateless UDP protocol. I imagine a single server could handle tens of millions of concurrent users, or more if the STUN client is not aggressive. There are enough public STUN servers that I think an open source project could ship with a big list of them and it'd be functional. (The two peers don't need to be using the same STUN server.) There is also pwnat, which uses a neat trick to connect two parties behind a NAT without a third server necessary. > If there was an open-source library that managed this well I have a feeling it would be used everywhere. Check out telehash.org for one attempt to make a p2p overlay of the regular Internet as an open source library. ZeroTier, as mentioned in the link, has all the code necessary to also provide a p2p library. (It's p2p VPN software right now, but I imagine someone could separate out the guts if that hasn't been done already.)