What is the "best" IP/port signaling protocol? XMPP? SIP? It would have a naming scheme which can work without DNS and would handle direct IP/port "calls" without going thru a signaling server.

Such "IP phones" would use UPNP on domestic NATS to forward their port.

Which signaling protocols are able to handle IP/port handover via their signaling servers (in the various mobile network "roaming" contexts)?