I am of the opinion that RFC 2119 is woefully underspecified
In particular, this is overly vague:
“An implementation which does not include a particular option MUST be prepared to interoperate with another implementation which does include the option, though perhaps with reduced functionality. In the same vein an implementation which does include a particular option MUST be prepared to interoperate with another implementation which does not include the option (except, of course, for the feature the option provides.)”
What does it mean to “interoperate”? What constitutes “reduced functionality” as opposed to failing to comply with the standard? Why was “the option” used instead of “the optional item”? What, specifically, is “the feature the option provides”?