RFC2119: Key Words

a standard for requirement levels

Introduction

In many standards-track documents, several all-capitalized words are used to signify the requirements in the specification. This document defines these words as they should be interpreted in IETF documents. –RFC2119

The iAM.AMR project has adopted RFC2119 to define key words when specifying requirement levels.

Usage

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” are to be interpreted as described in RFC2119:

MUST

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”, mean that the definition is an absolute prohibition of the specification.

SHOULD

This word, or the adjective “RECOMMENDED”, mean 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 different course.

SHOULD NOT

This phrase, or the phrase “NOT RECOMMENDED” mean 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 label.

MAY

This word, or the adjective “OPTIONAL”, mean that an item is truly optional. 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).

Limitations

These key words are only applicable when CAPITALIZED. Otherwise, they are to be interpreted as thier lay meaning.