Re: "the world is clinging stubbornly to IPv4"
Changing the version number just singles it's not plain'ol IPv4. For existing devices, response would be much the same if an IPv4 box recieved a v6 packet. If it's dual stack, it can process it, if it isn't.. It can be an interesting exercise to see how correctly an IP stack has been implemented. Or firewalls, which can be why simply blocking all ICMP is sometimes a bad thing.
But again it's about deciding what the objective for reinventing the Internet is. Just expanding address space from 32 to 64bit is one thing, but IPv6 went so much further in it's 'design by committee' approach to include a slew of extra features, often unwanted and problematic. But existing v4 devices carry on working as normal because they see 4 in the version field. If they see 5, they could reply with an ICMP type 3 code 2 packet.. But that's really host/application behaviour.
But it's also a very old proposal dating back to the 'Simple Internet Protocol' which was discussed here:-
https://www.rfc-editor.org/rfc/rfc1454.txt
SIP is simply IP with larger addresses and fewer options. Its main advantage is that it is even simpler that IPv4 to process.
from 1993 where SIP and other proposals were compared because address space depletion was recognised very early on. Some years later, we ended up with v6 and a different SIP..
Biting the hand that feeds IT
