Schaller: The long journey towards good free video conferencing

As for SIP/P2P/etc. that's not how it works. Everything into the home network is blocked by default, everything going out is allowed. Hence once you need to allow further ports aside the control connection you need to tell the firewall to allow it in. As there's no such protocol, the firewall does inspection. If your protocol is encrypted (like e.g. BitTorrent nowadays) or not supported, then you can just call out but you can't be called. You cannot do a simple "allow P2P switch" on the firewall unless you allow all incoming traffic. Those applications do not use fixed ports. Of course you can whitelist single ports and then do a configuration dance like the port forwarding one on IPv4. But that won't fly with the plug'n'play applications that just work on IPv4 because we have STUN/ICE and the like.

(Funny enough the NAT traversal between Windows BitTorrent clients on IPv4 and unfirewalled IPv6 hosts happens through Teredo tunneling.)