RFC 2119: Key words for use in RFCs to Indicate Requirement Levels
In plain English — editorial summary, not part of the RFC
RFC 2119 defines what the capitalised keywords in a specification actually mean, so "SHOULD" and "MUST" are not read as English opinion but as precise requirement levels. If a spec says it uses RFC 2119 keywords, every capitalised MUST, SHOULD and MAY in it carries the meaning below.
- ·MUST / REQUIRED / SHALL — an absolute requirement of the specification.
- ·MUST NOT / SHALL NOT — an absolute prohibition.
- ·SHOULD / RECOMMENDED — there may be valid reasons to ignore this, but the full implications must be understood and weighed first.
- ·SHOULD NOT / NOT RECOMMENDED — valid reasons may exist to do it anyway, but understand the consequences.
- ·MAY / OPTIONAL — genuinely optional; implementations must interoperate whether or not they include it.
Original abstract from the RFC
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. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.
Document record
- Document ID
- RFC2119
- Published
- March 1997
- Authors
- S. Bradner
- Status
- BEST CURRENT PRACTICE
- Stream
- IETF
- Area
- —
- Pages
- 3
- Also known as
- BCP14
- Updated by:
- RFC 8174
Topics
Referenced by
One later RFC formally updates or obsoletes part of this document.
Related documents
Ranked automatically by shared keywords, IETF area and stream — not by editorial selection.
- RFC 2120Managing the X.500 Root Naming ContextCurrent
March 1997
- RFC 1493Definitions of Managed Objects for BridgesObsoleted
July 1993
- RFC 1490Multiprotocol Interconnect over Frame RelayObsoleted
July 1993
- RFC 1294Multiprotocol Interconnect over Frame RelayObsoleted
January 1992
- RFC 1293Inverse Address Resolution ProtocolObsoleted
January 1992
- RFC 1289DECnet Phase IV MIB ExtensionsObsoleted
December 1991
- RFC 1286Definitions of Managed Objects for BridgesObsoleted
December 1991
- RFC 1285FDDI Management Information BaseUpdated
January 1992
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About this page
The document record above — title, authors, date, status, stream, area, relationships, DOI and errata — is imported verbatim from the public RFC Editor index. The “in plain English” section is editorial: written by The metasystema editorial team, not part of the RFC. Where the two differ, the RFC text governs.
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