The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
NOT, "RECOMMEND", "MAY", and "OPTIONAL" in this document are to be interpreted as described
in [RFC 2119].
- 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 phrase "SHALL NOT", means that the definition is an absolute
prohibition of the specification.
- This word, or the adjective "RECOMMENDED", means that there may exist valid
reasons in particular circumstances to ignore a particular item, but the full
implications must be understood and carefully weighed before choosing a
- SHOULD NOT
- This phrase, or the phrase "NOT RECOMMENDED", means 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
- This word, or the adjective "OPTIONAL", means that an item is truly optional.
One vendor may choose to include the item because a particular marketplace
requires it or because the vendor feels that it enhances the product while
another vendor may omit the same item. An implementation which does not include
a particular option must be prepared to interoperate with another implementation
which does include the option, though perhaps with reduced functionality. In the
same vein an implementation which does include a particular option must be
prepared to interoperate with another implementation which does not include the
option (except, of course, for the feature the option provides).