I wonder if something like HTTP connection upgrade would have been possible with ipv6-ipv4, maybe something like imagine Machine 1 with ips 1.1.1.1 and 11::11 and machine 2 with ips 2.2.2.2 and 22::22.
When machine 2 receives a packet from 1.1.1.1 at 2.2.2.2 it sends a ipv6 ping-like packet to the ipv4-mapped address ::ffff:1.1.1.1 saying something like "hey you can also contact me at 22::22 and if machine 1 undertands then it can try to use the new address for the following packets.
I can see how it would be hard to secure this operation.
When machine 2 receives a packet from 1.1.1.1 at 2.2.2.2 it sends a ipv6 ping-like packet to the ipv4-mapped address ::ffff:1.1.1.1 saying something like "hey you can also contact me at 22::22 and if machine 1 undertands then it can try to use the new address for the following packets.
I can see how it would be hard to secure this operation.