only meaningful for fragments. Also don't bother to byte-swap the
ip_id when we do generate it; it is only used at the receiver as a
nonce. I tried several different permutations of this code with no
measurable difference to each other or to the unmodified version, so
I've settled on the one for which gcc seems to generate the best code.
(If anyone cares to microoptimize this differently for an architecture
where it actually matters, feel free.)
Suggested by: Steve Bellovin's paper in IMW'02
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