Which RFC defines the key words MUST, SHOULD, and MAY to indicate requirement levels in standards?

• A. RFC 2119
• B. RFC 1035
• C. RFC 2616
• D. RFC 791

Answer: (A)

Standards use specific keywords to express requirement levels, and their meanings are standardized so everyone interprets them the same way. RFC 2119 defines these terms, specifying that MUST is an absolute requirement, SHOULD is a strong recommendation with possible exceptions, and MAY is an allowable option. This consistent language lets authors declare what is mandatory, what should be considered, and what may be chosen by implementers, reducing ambiguity across documents. The other referenced RFCs cover particular technologies—DNS in one, HTTP in another, IPv4 in another—and do not define how these normative terms should be interpreted. So the authoritative source for these requirement levels is RFC 2119.