> > 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." 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?" Spencer