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.
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.
9. Author's Address
 Scott Bradner
 Harvard University
 1350 Mass. Ave.
 Cambridge, MA 02138
 phone - +1 617 495 3864
 email - sob@harvard.edu