One thing that was not clear to me about the API: shouldn't UDP_SEGMENT
just be automatically determined in the stack from the pmtu? Whats
the motivation for the socket option for this? also AIUI this can be
either a per-socket or a per-packet option?

I decided to let the application explicitly set segment size, to avoid
bugs from the application assuming a different MTU from the one
used in the kernel for segmentation.

With path MTU, it is too easy for a process to incorrectly assume
link MTU or stale path MTU. With the current interface, if a process
tries to assemble segments larger than relevant path MTU, the
send call will fail.

A process may also explicitly want to send a chain of packets
smaller than MTU.