Except that a change from default values can be an excellent
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
the future, don't rely on this default value".
That's what reserved means, but very often "reserved" is accompanied
"must be zero."
Using the current definition of reserved fields as a proxy for
protocol versioning, especially as an indicator of
upward-compatible/non-upward-compatible, is just about the most
disgusting thing I can imagine.
Too bad it's the only alternative for some protocols, of course.
There was a public service commercial in the United States several
years ago that went something like "it's ten o'clock, do you know
where your children are?"
"It's 2003, do you know how you're going to tell other hosts that your
computer is running a new version of this protocol?"