I think we should fly up one level and discuss a hypothetical
world in which addresses in A000::/3 are deemed to be mutable
in flight between bits 3 and 47 inclusive. See what it does
to TCP, SCTP and IPSEC for example.
Well, break them... The TCP/UDP checksum should be easy enough to fix,
IPsec AH not much harder. The real problem is that if I have a session
with a001::1 and suddenly packets start coming in from a002::1, how do
I know these belong to the same session? This can be fixed by making
the bottom 64/80 bits should be globally unique, or by informing the
other side of all possible values that may appear in those 45 bits
beforehand.