An IPv6 router with a stateful firewall blocking incoming connections could have just the same issues with timeouts, I'd imagine. Switching to IPv6 doesn't just mean that anyone can make a P2P connection to anyone else (even STUN needs a third-party server to coordinate the two peers).
(D)TLS session resumption (I'm not sure if their "Connection IDs" are that or something similar) seems like the most foolproof solution to this scenario, assuming that the remote host can support it.
Not if the end user isn't in control of the firewall. (And if they were, then they could just forward dedicated ports for the devices they need.) It might not be as bad as the CGNAT situation, but there are plenty of big WANs that can't be reconfigured at will.
I have firewalls in v4 and v6 networks which don’t do any natting (well other than some 6-4 between them). They track sessions for security purposes, and they time them out for both security and memory reasons.
>An IPv6 router with a stateful firewall blocking incoming connections could have just the same issues with timeouts, I'd imagine.
You'd be surprised... PCP (Port Control Protocol) implemented by large vendors such as Cisco and Apple are able to punch through a firewall for up to 24 hours in a single session.
(D)TLS session resumption (I'm not sure if their "Connection IDs" are that or something similar) seems like the most foolproof solution to this scenario, assuming that the remote host can support it.