STUN doesn't work for symmetric NATs and TURN is not a solution (the traffic gets relayed over the server, which defeats the purpose of a peer-to-peer system).

STUN doesn't work for symmetric NATs and TURN is not a solution (the traffic gets relayed over the server, which defeats the purpose of a peer-to-peer system). Port restricted NAT is sufficient: it serves the actual purpose without breaking peer-to-peer systems. Note: I'm currently developing a peer-to-peer app.