This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.
The following 'Verified' errata have been incorporated in this document: EID 493, EID 494, EID 495, EID 496, EID 498, EID 499, EID 500, EID 5101

    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", "NOT
           RECOMMENDED",  "MAY", and "OPTIONAL" in this document are to
           be interpreted as described in RFC 2119.

    EID 499 (Verified) is as follows:

    Section: Abstract

    Original Text:

           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.

    Corrected Text:

           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 RFC 2119.

    Notes:

    The phrase "NOT RECOMMENDED" is missing from this sentence.

       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", means that the
       definition is an absolute requirement of the specification.

    EID 493 (Verified) is as follows:

    Section: 1

    Original Text:

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

    Corrected Text:

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

    Notes:

    EID 498 (Verified) is as follows:

    Section: 1

    Original Text:

    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.

    Corrected Text:

    4. SHOULD NOT   This phrase, or the phrase "NOT RECOMMENDED", means 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.

    Notes:

    EID 500 (Verified) is as follows:

    Section: 1

    Original Text:

    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.

    Corrected Text:

    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.

    Notes:

    EID 495 (Verified) is as follows:

    Section: 1

    Original Text:

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

    Corrected Text:

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

    Notes:

    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).

    EID 5101 (Verified) is as follows:

    Section: 5

    Original Text:

    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.)

    Corrected Text:

    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).

    Notes:

    Full stop should appear outside the parentheses in the last sentence.

    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 retransmissions)  For

    EID 494 (Verified) is as follows:

    Section: 6

    Original Text:

    (e.g., limiting retransmisssions)

    Corrected Text:

    (e.g., limiting retransmissions)

    Notes:

    EID 496 (Verified) is as follows:

    Section: 6

    Original Text:

       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.

    Corrected Text:

       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 retransmissions).  For example, they
       must not be used to try to impose a particular method on
       implementors where the method is not required for interoperability.

    Notes:

       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.

    9. Author's Address

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

          phone - +1 617 495 3864

          email - sob@harvard.edu