RFC 2119 is a RFC describing Key words for use in RFCs to Indicate Requirement Levels

We refer you to the full text.

RFC 8174 by clarifying that only UPPERCASE usage of the key words have the defined special meanings. RFC 8174 document is part of BCP 14.

RFC 2119 includes:

Guidance in the use of these Imperatives#

Imperatives of the type defined in this memo must be used with care and sparingly. In particular, they MUST only be used where it is actually required for interoperation or to limit behavior which has potential for causing harm (e.g., limiting retransmissions) For example, they must not be used to try to impose a particular method on implementors where the method is not required for interoperability.

Security Considerations#

These terms are frequently used to specify behavior with security implications. The effects on security of not implementing a MUST or SHOULD, or doing something the specification says MUST NOT or SHOULD NOT be done may be very subtle. Document authors should take the time to elaborate the security implications of not following recommendations or requirements as most implementors will not have had the benefit of the experience and discussion that produced the specification.

More Information#

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