Network Working Group S. Bradner Internet-Draft Harvard University August 1996 Key words for use in RFCs to Indicate Requirement Levels 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 of 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 itself. 1. MUST This word, or the adjective "REQUIRED", means that the definition is an absolute requirement of the specification. 2. MUST NOT This phrase 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 or even useful, but the full implications should be understood and Bradner [Page 1] ^L Internet-Draft RFC Key Words August 1996 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. 6. Security Considerations These terms are frequently used to specify options or behavior in a way to minimize security risks but security itself is not addressed in this memo. 7. Acknowledgments The definitions of these terms are an amalgam of definitions taken from a number of RFCs. 8. Author's Address Scott Bradner Harvard University 1350 Mass. Ave. Cambridge, MA 02138 phone - +1 617 495 3864 email - sob@harvard.edu Bradner [Page 2]