Further Key Words for Use in RFCs to Indicate Requirement Levels
Richard Barnes, S. Kent, Eric Rescorla · 2013
RFC 2119 defines a standard set of key words for describing requirements of a specification. Many IETF documents have found that these words cannot accurately capture the nuanced requirements of their specification. This document defines additional key words that can be used to address alternative requirements scenarios. Authors who follow these guidelines should incorporate this phrase near the beginning of their document: The key words "MUST (BUT WE KNOW YOU WON’T)", "SHOULD CONSIDER",