Phase one has us use something in tcp headers, maybe something in the reserved area, to set a number. It's OK for that to just be maybe 1 to 4 even, or something small depending upon bit size requirements.
Since old systems won't use that reserved header space to determine anything, they'll ignore it. And new systems will exclusively use unroutable address space, like 240 being discussed here, for routing.
So old systems will drop the 240/ address space, but new systems will route it, as it will be 2.240.x.x.x. So only compliant systems will see the new address space ; the rest won't.
It won't help old systems, but it will mean new systems only using old address space, can speak to old systems, without breaking them.
Everyone loves sensible change, so unlike ipv6, ipv8 will only take 20 years! (What's ipv6 been out for, 25 years and not adopted yet?)
Rather than changing TCP, we could just make web browsers query SRV records to see what port to connect to. Then you could host 65535 websites on the same IP address, without requiring any software in the middle to check Host/ALPN/SNI. (The web is moving to UDP anyway with HTTP/3. Not saying the web is the only important part of the Internet, but it's a big one.)
IPv6 is kind of over the major adoption hurdle of rewriting all software to understand it. Nearly all software understands it. I'm not sure anyone really has the appetite for that again.
People like to make fun of ipv6 adoption rates but I'm pretty sure we'll all be dead and dust before srv becomes much more than a niche curiosity. It has similar chicken and egg problems but much less financial motivation behind it.
Phase one has us use something in tcp headers, maybe something in the reserved area, to set a number. It's OK for that to just be maybe 1 to 4 even, or something small depending upon bit size requirements.
Since old systems won't use that reserved header space to determine anything, they'll ignore it. And new systems will exclusively use unroutable address space, like 240 being discussed here, for routing.
So old systems will drop the 240/ address space, but new systems will route it, as it will be 2.240.x.x.x. So only compliant systems will see the new address space ; the rest won't.
It won't help old systems, but it will mean new systems only using old address space, can speak to old systems, without breaking them.
Everyone loves sensible change, so unlike ipv6, ipv8 will only take 20 years! (What's ipv6 been out for, 25 years and not adopted yet?)