Re: there is compat mode
The fundamental incompatibility is that IPv4 can only address 32 bits worth of different hosts. As soon as you have 2^32+1 hosts (and we have far more than that hiding behind NAT) an IPv4 host can't address it and your network becomes fragmented.
The original logic was that there was enough time available to roll out IPv6 before the IPv4 address space was exhausted so the requirement for compatibility was simply that IPv6 hosts could talk to IPv4 hosts for as long as it took for them to be upgraded.
Of course, what actually happened is that most people did nothing at all, and those that did engage with IPv6 spent most of their time arguing about it rather than making progress. It's a bit like Brexit in that respect - all these years later and not much has happened.
You could make IPv6 more compatible with IPv4 for the longer run by, for example, virtualising the IPv4 address space: with the cooperation of DNS and IPv6 relay services, an IPv4 host does a gethostbyname and if the target has only an IPv6 address, the DNS service allocates a temporary IPv4 address which it returns to the requester and signals the relay service to gateway any IPv4 traffic to that address to the IPv6 host. This would allow IPv4 systems to talk to IPv6 systems in perpetuity. In the reverse direction, the IPv4 host has an IPv6 address in DNS with a prefix that causes the incoming traffic to be routed via the relay service. You could also do it in other ways, but they all require the active participation of ISPs who haven't exactly been that keen even to roll out IPv6-capable routers to their customers until recently. But if you want people to simply move to IPv6, why give them more excuses to delay?
I first got involved in IPv6 more than 25 years ago (it has really been that long) and I gave up in despair shortly afterwards when I saw the (lack of) direction it was taking. There have been a number of opportunities to change course over that time, but we are now where we are and there's not much choice but to plough on. The implementations are done, they work, there isn't going to be a Plan B.
Biting the hand that feeds IT
