Subject: Re: [netmod] extensions and conformance

From: Randy Presuhn <[email-masked]>
Date: Thu, 20 Aug 2015 14:11:24 -0700 (GMT-07:00)

Hi -

>From: Martin Bjorklund <[email-masked]>
>Sent: Aug 19, 2015 11:43 PM
>To: [email-masked]
>Cc: [email-masked]
>Subject: Re: [netmod] extensions and conformance
>
>Randy Presuhn <[email-masked]> wrote:
>> Hi -
>> 
>> >From: Martin Bjorklund <[email-masked]>
>> >Sent: Aug 19, 2015 10:49 AM
>> >To: [email-masked]
>> >Cc: [email-masked]
>> >Subject: Re: [netmod] extensions and conformance
>> ...
>> >> > Juergen suggested this replacement text, which I support.  Maybe it
>> >> > can be improved even more.
>> >> >
>> >> >        If a YANG parser does not support a particular extension, which
>> >> >        appears in a YANG module as an unknown-statement (see Section 13),
>> >> >        the entire unknown-statement MAY be ignored by the parser. Note
>> >> >        that even in this case the semantics associated with the extension
>> >> >        still apply (as if they were part of a description statement).
>> ...
>> >No.  It means that if a server advertises module that uses some
>> >extension to define some behaviour, the server supports that
>> >behavior.  Just as we expect a server to support the text in
>> >description statements.
>> 
>> Sorry, but that interpretation is not supported by the 2119 definition
>> of MAY.
>
>Note that the original text says "if x then MAY y".  Juergen's text
>says the same.  Maybe this should be rephrased...

Under the RFC 2119 definition of "MAY", "if x then MAY y" is exactly
the same as saying "if x then y is OPTIONAL".  It really sounds like
you mean to say "if x and z then MUST y", where "z" provides sufficient
wiggle to permit situations where the augmentation may be absent for
some instances, such as those exhibiting other augmentations.  (I'm
thinking about what one might see in a system with a bunch of interefaces
employing different technologies, but whose management interfaces
are all derived from a single abstract class and have various medium-
specific extensions.)

FWIW there are is another problem with the formulation in question: it's
in terms of the capabilities of a particular Yang *parser*, not the
client or server in which that parser finds itself.

Randy