Note: The following is an excerpt from the official IETF RFC. It is provided here as a convenience and is not authoritative. Refer to the original document as the authoritative reference.
Abstract
RFC2119 - Key words for use in RFCs to Indicate Requirement Levels specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.
Introduction
RFC2119 - Key words for use in RFCs to Indicate Requirement Levels specifies common key words, such as “MUST”, “SHOULD”, and “MAY”, that may be used in protocol specifications. It says that the key words “are often capitalized,” which has caused confusion about how to interpret non-capitalized words such as “must” and “should”.