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With a firewall, the application knows its public-facing ipv6 address/src port number without STUN. A stateful firewall does not alter packets. It can open the

With a firewall, the application knows its public-facing ipv6 address/src port number without STUN. A stateful firewall does not alter packets. It can open the firewall simply by sending something. If it can communicate its public-facing ipv6 address/src port number to the remote side using a SIP proxy (ok, for this signalling, you need a relay), it can receive traffic. No TURN needed either. What scenario do you have in mind?

simiones 3y ago
If the two peers are each behind its own stateful firewall (as would be common with something like BitTorrent), then you still need some 3rd party server accessible to both of them that either relays traffic between them, or at least allows them to negotiate the port pair that they'll communicate on (so that the one which will accept the TCP connection can send a TCP SYN with the other's source port as destination). The second option may not even work with more paranoid firewalls, which might not allow TCP SYN packets on existing connections.