Vocabulary

This standard uses the words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY and OPTIONAL as described in RFC 2119.

Here are some reminders from RFC 2119:

MUST

This word, or the terms REQUIRED or SHALL, mean that the definition is an absolute requirement of the specification.

MUST NOT

This phrase, or the terms PROHIBITED or SHALL NOT, mean that the definition is an absolute prohibition of the specification.

SHOULD

This word, or the adjective RECOMMENDED, mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understod and carefully weighted before choosing a different course.

SHOULD NOT

This phrase, or the phrase NOT RECOMMENDED, mean 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 the case carefully weighed before implementing any behavior described with this label.

MAY

This word or the adjective OPTIONAL, mean that an item is truly optional. One may choose to include the item because a particular circumstance requires it or because it causes an interesting enhancement. An implementation which does not comply to an OPTIONAL item MUST be prepared to be transformed to comply at any time.