Re: redesign
In others words a (presumably transitional but not necessarily) gateway between OLD protocol, and NEW protocol, not a 'compatible protocol'.
Not really. You need to think of both the endpoints, and the network.
For the endpoints I agree, this is little different to an IPv4/v6 situation. No IPv4 system can communicate directly with an IPv6-only one, since it has no way to address messages to it, and the same would be true of IPv4/IPvX, but that's of little significance. Most new systems, at least for some time, would be dual-stack.
The big problem is the network side. Drop an IPv6 system into a pure IPv4 network, and it can't talk to anything. Same is true for an IPv6 subnet whose only external connections are pure IPv4. Unless someone configures gateways or tunnels on the networks you'll remain as an island.
Of course, the IPv6 experts will say that such tunnels are easy to setup, but that's not the point. Someone needs to set them up, configure them, identify peers on all the networks they need to tunnel to, etc. That's why IPv6 hasn't taken off, no-one wants tto do that for every single network they talk to.
If IPv6 had been designed so that its packets could have been processed by IPv4 hosts, even if that 'processing' just meant tossing them to some catchall address with a "hey, you deal with it" tag, it would have been much simpler to setup interworking. A few catchall systems, setup by ISPs, could have been adequate. I could have plugged an IPv6 system into my home IPv4-only network and it could still have got messages to any IPv6 system, albeit in a roundabout way. Make upgrading that simple, and far more people would be running IPv6 now. Eventually those catchall routers would be obsolete and would fade away. Instead, people prefer NAT for the same reason, it works that easily and requires negligeable setup.
Biting the hand that feeds IT
