Mark Smith writes:
Firewalls could be considered to be performing QA for defined
protocol fields. I agree that reserved fields shouldn't be "QA"'ed for
their default values.
Except that a change from default values can be an excellent indicator
that you are dealing with a software version different from what you
expected (and possibly incompatible).
I can't remember exactly where I saw the
definition, I've understood reserved fields to mean "could change in
the future, don't rely on this default value".
That's what reserved means, but very often "reserved" is accompanied by
"must be zero."