Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words
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B. Leiba · 2017-05
Abstract
RFC 8174 clarifies that the key words defined in RFC 2119 (MUST, SHOULD, MAY, etc.) apply only when written in uppercase and that lowercase versions of these words do not carry the RFC 2119 normative meaning. Authors are recommended to include a boilerplate statement at the start of their document explicitly invoking RFC 2119 and RFC 8174 to make this distinction unambiguous.
Why This RFC Matters
RFC 2119 key words are the normative language backbone of nearly every IETF standard. The ambiguity between uppercase MUST and lowercase 'must' caused interpretation disputes in implementations. RFC 8174 resolves this by clarifying that only capitalized forms carry normative force, and provides a standard boilerplate text. Updated boilerplate referencing both RFC 2119 and RFC 8174 now appears in most new IETF RFCs, making it a ubiquitous meta-standard for RFC document authoring.