keywords 4 cards
When do the words MUST, SHOULD, and MAY carry their RFC 2119 meaning?
Only when they appear in ALL CAPITALS (per RFC 8174 / BCP 14). Lowercase is just prose.
MUST / REQUIRED / SHALL
What does SHOULD actually license an implementer to do?
Deviate — but only for a specific, well-understood reason whose full implications have been weighed. Not "ignore", not "treat as MUST".
SHOULD / RECOMMENDED
If my implementation skips an OPTIONAL feature, what must it still do?
Be prepared to interoperate with an implementation that includes it (and vice versa). You can't require a peer to have made the same optional choice.
MAY / OPTIONAL
"The key words … are to be interpreted as described in BCP 14 … when, and only when, they appear in all capitals."
RFC 8174 added the "only when in all capitals" clause; RFC 2119 defined the keywords originally.
MUST / REQUIRED / SHALL
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