Maybe I'm misunderstanding you, but one of the points of the article in that you can represent IPv4 addresses in IPv6. In other words, IPv4 is a subset of IPv6.
If I'm on an ipv6-only host and blast UDP at ::ffff:1.2.3.4, they should get delivered to 1.2.3.4, no?
The actual, real-world problem with 4 being a subrange of 6 is that 4-only hosts are blissfully unaware that the super-range exists, so have no mechanism to send packets there. This is of course where you're right about NAT64 and the state requirements.
> If I'm on an ipv6-only host and blast UDP at ::ffff:1.2.3.4, they should get delivered to 1.2.3.4, no?
No, it won't necessarily! That's precisely the problem. Until NAT64 was introduced it was in fact impossible for an IPv6 router to deliver your packet to the IPv4 host 1.2.3.4. NAT64 still isn't mandatory (and likely never will be), so if you're writing software you can't assume those packets will get through even if you have an IPv6 network connection with a default route.
NAT64 didn't come about until long after IPv6 was finalized, and NAT64 support from default-route IPv6 routers is still is not mandatory. That's why we have this mess with dual-stack hosts: you cannot safely assume that your IPv6 router is willing to deal with the IPv4 world on your behalf.
The ::ffff:1.2.3.4 address space does, in fact, date back to the early days of IPv6 (it came from RFC 2765, about one year after IPv6 was finalized), but it was not meant for letting IPv6 clients share a single IPv4 address -- it was only for servers which for some reason had their own IPv4 address but couldn't speak IPv4. Yeah, back in the early 2000s people thought this problem might happen.
The IPv6 committee was viciously hostile to NATs. The way they saw it, NATs were the problem that made IPv6 necessary, so no way were they going to allow any NATs to pollute their precious IPv6. If that meant that the whole world had to run two separate internets (IPv4 and IPv6) for the rest of eternity just to keep the IPv6 network puritanically NAT-free, then so be it!
It took them more than a decade to realize how stupid this mindset was.
Ah! T see what you mean. Thanks for clarifying and correcting me.
I possibly have some sympathy with the anti-NAT view taken at the time, even if it ended up being the wrong thing to do it hindsight. Adding more mandatory complexity to implementors would have harmed adoption rates, and I've seen some weird edge cases with NAT64 - it's not necessarily a trivial thing to implement correctly.
Yes, but having two entirely separate internets, like we do today, is much more complex than any amount of NATting!
I fault the IPv6 proponents for not forseeing our current situation. DJB saw it with crystal clarity in 2001. Lots of people warned them that this would happen.
::ffff:0:0/96 is for representing v4 addresses in v6 APIs. If you tell the kernel you want to send a packet to ::ffff:1.2.3.4, you're actually telling it you want to send a (v4!) packet to 1.2.3.4, you're just doing it using an AF_INET6 socket rather than an AF_INET one.
Since packets aren't APIs, you should never see ::ffff:0:0/96 in packets on the wire. A v6-only host can't use this prefix to send v4 packets to v4 hosts.
(What would the source address of those packets even be?)
If I'm on an ipv6-only host and blast UDP at ::ffff:1.2.3.4, they should get delivered to 1.2.3.4, no?
The actual, real-world problem with 4 being a subrange of 6 is that 4-only hosts are blissfully unaware that the super-range exists, so have no mechanism to send packets there. This is of course where you're right about NAT64 and the state requirements.