IPV6_USE_MTU: A new ancillary data item (and socket option)

Originally proposed by Robert Elz in April 2000.
Natural generalization of IPV6_USE_MIN_MTU.
takes an integer.
requires the kernel to send packets at a specified MTU.
would obsolete IPV6_USE_MIN_MTU.
IPV6_USE_MTU(1280) == IPV6_USE_MIN_MTU
Useful when
an application can't divide a single transaction,
it would rather perform PMTU discovery than just sending at the minimum MTU, and
we can't assume the kernel's behavior about PMTU discovery.
e.g. introduce a strict condition to reject forged TOO BIG erros
But
At least every BSD kernel always performs PMTU discovery.
not necessary if we can always rely on the kernel's behavior.
So
would like to hear opinions.