On 26 May 2020, at 19:33, Aikman-Scalese, Anne <AAikman@lrrc.com> wrote:

Jeff, Cheryl, et al – below I paste the IETF definitions that were linked – note, for example, that “should” says “can be ignored” under certain circumstances – not sure anyone in the WG ever had that understanding of “should”.

 

Anne

 

Network Working Group                                         S. Bradner

Request for Comments: 2119                            Harvard University

BCP: 14                                                       March 1997

Category: Best Current Practice

 

 

       

Key words for use in RFCs to Indicate Requirement Levels

 

 

Status of this Memo

 

   This document specifies an Internet Best Current Practices for the

   Internet Community, and requests discussion and suggestions for

   improvements.  Distribution of this memo is unlimited.

 

Abstract

 

   In many standards track documents several words are used to signify

   the requirements in the specification.  These words are often

   capitalized.  This document defines these words as they should be

   interpreted in IETF documents.  Authors who follow these guidelines

   should incorporate this phrase near the beginning of their document:

 

      The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL

      NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and

      "OPTIONAL" in this document are to be interpreted as described in

      RFC 2119.

 

   Note that the force of these words is modified by the requirement

   level of the document in which they are used.

 

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.

 

 

 

 

 

Bradner                  Best Current Practice                  [Page 1]

---

RFC 2119                     RFC Key Words                    March 1997

 

 

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.)

 

6. Guidance in the use of these Imperatives

 

 

   Imperatives of the type defined in this memo must be used with care

   and sparingly.  In particular, they MUST only be used where it is

   actually required for interoperation or to limit behavior which has

   potential for causing harm (e.g., limiting retransmisssions)  For

   example, they must not be used to try to impose a particular method

   on implementors where the method is not required for

   interoperability.

 

7. Security Considerations

 

 

   These terms are frequently used to specify behavior with security

   implications.  The effects on security of not implementing a MUST or

   SHOULD, or doing something the specification says MUST NOT or SHOULD

   NOT be done may be very subtle. Document authors should take the time

   to elaborate the security implications of not following

   recommendations or requirements as most implementors will not have

   had the benefit of the experience and discussion that produced the

   specification.

 

8. Acknowledgments

 

 

   The definitions of these terms are an amalgam of definitions taken

   from a number of RFCs.  In addition, suggestions have been

   incorporated from a number of people including Robert Ullmann, Thomas

   Narten, Neal McBurnett, and Robert Elz.

 

 

 

Anne E. Aikman-Scalese

Of Counsel

520.629.4428 office

520.879.4725 fax

AAikman@lrrc.com

_____________________________

Lewis Roca Rothgerber Christie LLP

One South Church Avenue, Suite 2000

Tucson, Arizona 85701-1611

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Because what matters

to you, matters to us.™

 

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