RFC 2119 is a RFC
describing Key words for use in RFCs
to Indicate Requirement Levels
We refer you to the full text.
updates RFC 2119 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: (Terms on the same line imply the same meaning)
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 implementers where the method is not required for interoperability.
These terms are frequently used to specify behavior with security implications. The effects on security of not implementing a MUST
, 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 implementers will not have had the benefit of the experience and discussion that produced the specification.
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