I don't think that RFC is quite correct. Both types are set up the same way (the only difference is whether the dest IP of a packet is 6to4 or native) and it was designed to be unmanaged in the scenario we're imagining here. Windows machines configured it automatically if they received a public IP, for example.
But if it is, it's basically saying that people really didn't want to take this approach.
> To whoever wishes the IETF had thought of ipv4x, I'll bet the IETF thought of it and decided they wanted to start with a clean slate in v6.
They obviously did think of it, since v4x and 6to4 take pretty much the same approach. Both of them put extra address bits into the start of the payload. Both of them indicate they've done this with some flag in the header. Both of them send packets through unmodified v4 routers. About the only difference is that v4x consumes the entire 128-bit address space while 6to4 sits in a /16.
Everything that you've said can be done in v4x can also be done with v6 and 6to4 (possibly combined with some of the other transition options), and it doesn't make it any easier either unless you handwave away a bunch of the work that needs to happen.
> But v4x at least had a way to gradually defrag things once it was fully adopted. If you're suggesting staying on 6to4 forever, that would've meant continuing to use vanilla ipv4 between subnets.
6to4 prefixes can be routed natively without needing to be tunnelled inside v4 packets, so you don't need to do that. That's how ISP-side 6to4 would work, but you could also do the same thing in the DFZ. If you're announcing v4 addresses you could also make a BGP announcement for the 6to4 prefixes that correspond to those v4 addresses, allowing packets sent to those prefixes to reach you natively instead of getting tunnelled. Presumably this is about the same as what you were imagining v4x would have done to get rid of vanilla v4.
I don't see how you could gradually defrag things in v4x, unless you include ways that would also work with 6to4.
> Many v6 proponents seem to think your device on any random guest network should be able to receive inbound connections, enabling true P2P everywhere, which implies having no router firewall
A few do, but it's rare. The main point is that the IP of a machine should be the same no matter where you are in the network, and nobody should be using clashing address spaces. None of that implies no firewall.
NAT features heavily in the v6 transition. It's how you handle connectivity to and from v4 hosts that can't do v6. There's just no need for it between two v6 hosts.
