| Network Working Group | P. Saint-Andre |
| Internet-Draft | Cisco Systems, Inc. |
| Obsoletes: 2119 (if approved) | August 29, 2011 |
| Intended status: Best Current Practice | |
| Expires: March 01, 2012 |
Conformance Terms to Indicate Requirement Levels
draft-saintandre-2119bis-00
In many protocol specifications and related documents, special conformance terms (e.g., the uppercase words "MUST", "SHOULD", and "MAY") are often used to signify requirement levels. This document defines these conformance terms and describes how they are to be interpreted in documents produced within the Internet Standards Process. If approved, this document obsoletes RFC 2119 and changes its status to Historic.
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In many protocol specifications and related documents, special conformance terms (e.g., the uppercase words "MUST", "SHOULD", and "MAY") are often used to signify requirement levels. This document defines these conformance terms and describes how they are to be interpreted in documents produced within the Internet Standards Process [BCP9]. If approved, this document obsoletes RFC 2119 and changes its status to Historic.
The discussion venue for this document is the <ietf@ietf.org> mailing list, for which archives and subscription information are available at <https://www.ietf.org/mailman/listinfo/ietf>.
[[ NOTE TO RFC EDITOR: Upon publication, please remove the foregoing sentence. ]]
This term means that the feature or behavior is an absolute requirement of the specification, so that an implementation has an unconditional obligation to implement the feature or to behave as defined. The terms "SHALL" and "REQUIRED" are equivalent to "MUST".
This term means that the feature or behavior is an absolute prohibition of the specification, so that an implementation has an unconditional obligation to not implement the feature or to not behave as defined. The term "SHALL NOT" is equivalent to "MUST NOT".
This term means that the feature or behavior is a limited requirement of the specification, so that an implementation has a conditional obligation to implement the feature or to behave as defined, unless there is a strong, explicitly described reason not to do so in particular circumstances. Those who implement the specification or deploy conformant technologies need to understand and carefully weigh the full implications of violating the requirement before doing so. The term "RECOMMENDED" is equivalent to "SHOULD".
This term means that the feature or behavior is a limited prohibition of the specification, so that an implementation has a conditional obligation to not implement the feature or to not behave as defined, unless there is a strong, explicitly described reason to do so in particular circumstances. Those who implement the specification or deploy conformant technologies need to understand and carefully weigh the full implications of violating the prohibition before doing so. The term "NOT RECOMMENDED" is equivalent to "SHOULD NOT".
This term means that the feature or behavior is truly a matter of preference. One implementation can choose to implement the feature or behavior whereas another implementation can choose not to, without any resulting harm to interoperability. An implementation that does not implement the feature or behavior needs to interoperate with another implementation that does do so, although perhaps with reduced functionality. Likewise, an implementation that implements the feature or behavior needs to interoperate with another implementation that does not do so (except, of course, with respect to the defined feature or behavior). The term "OPTIONAL" is equivalent to "MAY".
The conformance terms defined in this document ought to be used judiciously. In particular, the absolute and limited requirements and prohibitions ought be used only to specify features and behaviors that are necessary for interoperability, or to forbid features and behaviors that have the potential to cause significant harm. For example, such terms are not to be used to impose a particular method on implementers if the method is not necessary for interoperability.
When it is not appropriate to use the conformance terms, authors can use a variety of alternative words and phrases, such as: "need to", "has to", or "mandatory" instead of "MUST"; "ought to", "strongly encouraged to", or "suggested" instead of "SHOULD"; and "might", "can", or "discretionary" instead of "MAY". To prevent confusion, authors are strongly encouraged to use these alternative words and phrases instead of the lowercase versions of the conformance terms whenever possible, and to use the conformance terms only in their uppercase versions.
In order for the requirements force of the conformance terms to apply, authors who follow the guidelines specified herein need to incorporate this sentence near the beginning of their documents:
[[ NOTE TO RFC EDITOR: Upon publication, please change "XXXX" to the number assigned to this document. ]]
The conformance terms defined in this document are frequently used to specify features and behaviors that have security implications. The effects on security of not implementing a "MUST" or a "SHOULD", or of doing something the specification says "MUST NOT" or "SHOULD NOT" be done, can be very subtle. Authors are strongly encouraged to elaborate the security implications of not conforming to requirements and recommendations, since many implementers do not have the benefit of the experience and discussion that produced the specification.
This document requests no actions of the IANA.
This document borrows text from [RFC2119]; Scott Bradner, the author of that document, is gratefully acknowledged.
| [BCP9] | Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |