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So Skype and other p2p type applications often work by what is called hole punching (see simple guide here) to get two clients connected to each other that are both behind firewalls which block incoming connections.
uPnP is also used to get around the user manually having to forward ports. How exactly is it different to the kind of hole punching described above? I assume it must operate differently as most routers have the setting to turn it on/off, whereas I see no way one could stop the above type of UDP hole punching (aside blocking outgoing connections and incoming established/related).
Second, if say Skype/bittorrent can set up and direct connection between A-B with hole punching, does that mean it doesn't need upnp enable on the router?
So to summarise: 1)UPnP: protocol for letting devices/applications internal to a network tell the router to forward a given port (without the user having done so manually) 2) UDP punching: no ports are forwarded, but the router A accepts traffic from B because it tracks that outbound traffic from A to B has already occured (even though that first packet never reached B etc) and vice versa for router B. All this is negotiated by some third-party server which both A and B are connected to. – fpghost – 2013-07-09T07:44:01.867
So providing the firewall allows Skype outbound access there would be no need to enable UPnP on a router which Skype was behind in order for it to establish a A-B direct connection; it could just establish this with UDP hole punching instead? – fpghost – 2013-07-09T07:46:38.800
You are correct. – LawrenceC – 2013-07-09T10:49:18.350