Key words for use in RFCs to Indicate Requirement Levels
The information below is for an old version of the document that is already published as an RFC.
This is an older version of an Internet-Draft that was ultimately published as RFC 2119.
|Scott O. Bradner
|2020-01-21 (Latest revision 1996-09-02)
|Internet Engineering Task Force (IETF)
|Intended RFC status
|Became RFC 2119 (Best Current Practice)
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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 This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its Working Groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress.'' To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). 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 - firstname.lastname@example.org Bradner [Page 2] Internet-Draft RFC Key Words August 1996 Bradner [Page 3]