Ok, I see what you mean then by 6to4 already supporting what v4x proposed, but that path wasn't taken either. It's a special feature with rare usage rather than the minimum default way of doing v6.
You see it as a special feature with rare usage because it competed with native and lost. People overwhelmingly preferred deploying v6 natively instead of deploying 6to4. Obviously more people would be using it if it was the only option, but if you're trying to come up with an alternative to the way v6 went then "an approach that v6 took but which nobody wanted to use when native v6 was the alternative" might not be a very promising place to start.
So actually I take it back, 6to4 was pretty different from this ipv4x idea. I misunderstood and thought a v6-enabled ISP would give you a 2002: under 6to4, which you could later subdivide.
Ignoring that, say 6to4 were like v4x... The idea that those 2 years showed that native was preferable, that was among a small minority who even cared about v6 to begin with (0.25% at the time). This was a primary, not a final.
Or, ISPs that want to do so can use one of their own v4 addresses to get the corresponding /48 6to4 prefix, split it up and hand that out to customers while handling the v4 packets themselves.
Looking at the 15 or so years afterwards shows native v6 deployment at 50% of the Internet, and approximately nobody interested in deploying 6to4 outside of these posts where people keep reinventing it but with different names and an assertion that if only the IETF had thought of this, everybody would have loved it. I think that's decent evidence that they wouldn't.
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. I can understand why, big part because of v4's fragmented routes, and also /8 haves&have-nots. 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.
Also, all those compromise ideas had NAT staying at least on day 1. 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. Others recommend a router firewall. Not sure what IETF's initial position was on this, but they later recommended that residential routers give the user a clear choice between closed vs open. v4x with its NAT would just default to closed. In reality, I've never seen a consumer router clearly say what it does with inbound v6 traffic.
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.