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Versions: 02 RFC 2119

Network Working Group                                         S. Bradner
Internet-Draft                                        Harvard University
                                                             August 1996

        Key words for use in RFCs to Indicate Requirement Levels


                    <draft-bradner-key-words-02.txt>

Status of this Memo

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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.  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 adjectives "REQUIRED" or "SHALL", means that
   the definition is an absolute requirement of the specification.

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

3. SHOULD   This word, or the adjective "RECOMMENDED", means 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 means that there may exist valid reasons in
   particular circumstances when the particular behavior is acceptable



Bradner                                                         [Page 1]

Internet-Draft                RFC Key Words                  August 1996


   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", means 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.

6. Security Considerations
   These terms are frequently used to specify options or behavior in a
   way that can effect security risks.  Careful consideration should be
   taken to understand the security implications of any use of these
   imperatives.

7. 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, and Robert Elz.

   8. Author's Address
   Scott Bradner
   Harvard University
   1350 Mass. Ave.
   Cambridge, MA 02138

   phone - +1 617 495 3864

   email - sob@harvard.edu



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