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== Some more thinking about requirements in specifications

[[Aristotle Pagaltzis http://plasmasturm.org/]]'s comment on my
[[previous entry SpecMustVsShould]] prodded me into doing some
more thinking about this, and [[his entry on RFC 2119 usage
http://plasmasturm.org/log/445/]] has persuaded me that there
are actually four degrees of requirements that it's useful to
put into specifications.

Put simply, I'd say that they are:

* allowed: you can do this, and some people will (so be prepared to cope
  with it).
* recommended: we think that you should do this.
* should: not doing this causes problems for people and other programs,
  but we have to admit that they won't actually break if you don't do this.
* must: your program won't work if you don't do this.

The IETF MAY is the first, the IETF MUST is the fourth, and I think that
the IETF SHOULD sort of covers both the second and the third. I've now
been convinced that there is a real difference between the second and
the third cases, and I think it's important to distinguish between them
in the name of honesty (if we're honest, we have a better chance of
getting people to do should's because we aren't devaluing them; a should
is a mark of something fairly important).

(Whether one should use the IETF SHOULD for merely recommended
things is one of those debatable issues. The language of [[RFC 2119
http://rfc.net/rfc2119.html]] appears to allow it, but there's [[an
argument http://plasmasturm.org/log/445/]] that a mere recommended
practice should just be a MAY if you have to use only the three IETF
levels.)

One can argue about recommended versus allowed, but pragmatically I
think that it's important to differentiate between what's allowed and
what's preferred. I want specifications to be able to steer implementors
towards making the right choice as well as warn them about legal things
that other implementations will do.

(Possibly I am out on left field on all of this, though. I don't have
much experience in trying to write specifications for anyone but
myself.)