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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.












Bradner                  Best Current Practice                  [Page 2]


RFC 2119                     RFC Key Words                    March 1997


9. Author's Address

      Scott Bradner
      Harvard University
      1350 Mass. Ave.
      Cambridge, MA 02138

      phone - +1 617 495 3864

      email - sob@harvard.edu









































Bradner                  Best Current Practice                  [Page 3]

=========================================================================





Internet Engineering Task Force (IETF)                          B. Leiba
Request for Comments: 8174                           Huawei Technologies
BCP: 14                                                         May 2017
Updates: 2119
Category: Best Current Practice
ISSN: 2070-1721


       Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words

Abstract

   RFC 2119 specifies common key words that may be used in protocol
   specifications.  This document aims to reduce the ambiguity by
   clarifying that only UPPERCASE usage of the key words have the
   defined special meanings.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8174.





















Leiba                     Best Current Practice                 [Page 1]


RFC 8174                 RFC 2119 Clarification                 May 2017


Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Clarifying Capitalization of Key Words  . . . . . . . . . . .   3
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Introduction

   RFC 2119 specifies common key words, such as "MUST", "SHOULD", and
   "MAY", that may be used in protocol specifications.  It says that the
   key words "are often capitalized," which has caused confusion about
   how to interpret non-capitalized words such as "must" and "should".

   This document updates RFC 2119 by clarifying that only UPPERCASE
   usage of the key words have the defined special meanings.  This
   document is part of BCP 14.

















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RFC 8174                 RFC 2119 Clarification                 May 2017


2.  Clarifying Capitalization of Key Words

   The following change is made to [RFC2119]:

   === OLD ===
   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.


   === NEW ===
   In many IETF documents, several words, when they are in all capitals
   as shown below, are used to signify the requirements in the
   specification.  These capitalized words can bring significant clarity
   and consistency to documents because their meanings are well defined.
   This document defines how those words are interpreted in IETF
   documents when the words are in all capitals.

   o  These words can be used as defined here, but using them is not
      required.  Specifically, normative text does not require the use
      of these key words.  They are used for clarity and consistency
      when that is what's wanted, but a lot of normative text does not
      use them and is still normative.

   o  The words have the meanings specified herein only when they are in
      all capitals.

   o  When these words are not capitalized, they have their normal
      English meanings and are not affected by this document.

   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", "NOT RECOMMENDED",
      "MAY", and "OPTIONAL" in this document are to be interpreted as
      described in BCP 14 [RFC2119] [RFC8174] when, and only when, they
      appear in all capitals, as shown here.

   === END ===





Leiba                     Best Current Practice                 [Page 3]


RFC 8174                 RFC 2119 Clarification                 May 2017


3.  IANA Considerations

   This document does not require any IANA actions.

4.  Security Considerations

   This document is purely procedural; there are no related security
   considerations.

5.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

Author's Address

   Barry Leiba
   Huawei Technologies

   Phone: +1 646 827 0648
   Email: barryleiba@computer.org
   URI:   http://internetmessagingtechnology.org/



























Leiba                     Best Current Practice                 [Page 4]


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