2nd suggested revision for MUST/SHOULD

[Keith Moore <[e-mail]>]

several points:

I see no inherent problem with copying the 2119 definitions for 
MUST/SHOULD.  However I assume that much of the existing text is 
based on the 1123 definitions of these terms, so changing might
require examining every occurance of these terms.  Fortunately,
the 1123, 2119, and proposed definitions of these terms are quite
similar.  And I do see value in consistency with 2119, though
RFCs which deviate from the 2119 definitions are not uncommon.

However I wonder if the definitions of "fully conforming" 
vs. "partially conforming" may still be useful, regardless of which
definitions of MUST/SHOULD are used.  OTOH, I do have some 
reservations about defining "fully/partially conforming" in this way 
because (if memory serves) SHOULD [NOT] are sometimes used to avoid 
having to specify the behavior of each particular kind of MTA 
configuration.  An SMTP implementation that, for whatever valid 
reason, doesn't perform every single one of the functions of an MTA 
(submission, delivery, relaying, aliasing, list expansion, etc) 
should perhaps be "fully conforming" as long as it meets the 
SHOULD [NOT] requirements for the functions that it does perform.

I also think that it's worthwhile to explain 

- why SHOULD or even MUST might specify something that degrades
interoperability with noncomforming implementations of this or
a previous specification.  (interoperability with conforming
implementations is paramount)

- that the spec is deliberately tightened as compared to previous
versions; pre-existing implementations are expected to require
some changes

no matter which definitions of MUST/SHOULD/MAY are used.

personally, I don't think that consistency with 2119 is so important 
that it necessiates changing the document at this late date.
but if it helps us get consensus on the document, it might be worth it.

so at the risk of bogging things down more, here is a 2nd concrete 
proposal.  the paragraphs not in 2119 are labelled [A], [B], and [C]
for easy reference in DRUMS discussion, not for inclusion in the
RFC.


The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
"OPTIONAL" in this document are to be interpreted as follows:

1. MUST   This word, or the terms "REQUIRED" or "SHALL", mean that the
   definition is an absolute requirement of the specification.

2. MUST NOT   This phrase, or the phrase "SHALL NOT", mean that the
   definition is an absolute prohibition of the specification.

3. SHOULD   This word, or the adjective "RECOMMENDED", mean that there
   may exist valid reasons in particular circumstances to ignore a
   particular item, but the full implications must be understood and
   carefully weighed before choosing a different course.

4. SHOULD NOT   This phrase, or the phrase "NOT RECOMMENDED" mean that
   there may exist valid reasons in particular circumstances when the
   particular behavior is acceptable or even useful, but the full
   implications should be understood and the case carefully weighed
   before implementing any behavior described with this label.

5. MAY   This word, or the adjective "OPTIONAL", mean that an item is
   truly optional.  One vendor may choose to include the item because a
   particular marketplace requires it or because the vendor feels that
   it enhances the product while another vendor may omit the same item.
   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.)

[A] An implementation that does not adhere to each of the "MUST" and "MUST 
NOT" requirements is not considered to be conforming to this specification.
An implementation that adheres to each of the "MUST" and "MUST NOT"
requirements, but does not adhere to each of the "SHOULD" and "SHOULD NOT"
requirements, is considered to be partially conforming to this 
specification.  An implementation that adheres to all requirements is 
considered to be fully conforming.  There is no intention or expectation 
that partially or fully conforming implementations of this specification 
will interoperate with nonconforming implementations of either this 
specification or its predecessors.

[B] "MUST" and "MUST NOT" are generally used when the requirement is
necessary to ensure robust interoperation between conforming
implementations.  "SHOULD" and "SHOULD NOT" are also used to
describe cases where the requirement is necessary for robust
interoperation between (fully or partially) conforming
implementations, but when it is recognized that there may be
valid reasons for a particular implementation (or a particular
type of implementation) to not follow the requirement.

[C] In some cases this document imposes "MUST" and/or "SHOULD"
requirements which deviate from a significant body of current practice
at the time of this writing.  In such cases these requirements are
imposed when experience indicates that following such requirements
is likely to lead, in practice, to better interoperation, or
smoother operation of the Internet email infrastructure.   Though
this document does attempt to ensure that (fully or partially)
implementations of this specification will interoperate with
(fully or partially) conforming implementations of its predecessors,
it does not attempt to legitimize common deviations from those
specifications, and it does impose new requirements.  Existing
implementations of SMTP will need some changes if they are to
conform to this new specification.