GNU is not trying to push a single implementation. They are pushing an open protocol - SIP, and getting market share requires at least one compelling implementation. XMPP sounds like good open competition to SIP - especially if it uses just one port. STUN doesn't *not* solve the NAT problem unless you are behind a single simple consumer router. In many ways, it makes it worse since it tries to map out the entire route. Sip proxies would solve the NAT problem if they were easier for non SIP gurus to configure. (And the embedded PBX solutions are generally that easy.)
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GNU is not trying to push a single implementation. They are pushing an open protocol - SIP, and getting market share requires at least one compelling implementation. XMPP sounds like good open competition to SIP - especially if it uses just one port. STUN doesn't *not* solve the NAT problem unless you are behind a single simple consumer router. In many ways, it makes it worse since it tries to map out the entire route. Sip proxies would solve the NAT problem if they were easier for non SIP gurus to configure. (And the embedded PBX solutions are generally that easy.)