Network Working Group P. Saint-Andre Internet-Draft Cisco Systems, Inc. Obsoletes: 2119 (if approved) August 29, 2011 Intended status: BCP Expires: March 1, 2012 Conformance Terms to Indicate Requirement Levels draft-saintandre-2119bis-01 Abstract 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. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on March 1, 2012. Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must Saint-Andre Expires March 1, 2012 [Page 1] Internet-Draft Conformance Terms August 2011 include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. MUST . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. MUST NOT . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.3. SHOULD . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.4. SHOULD NOT . . . . . . . . . . . . . . . . . . . . . . . . 4 2.5. MAY . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Boilerplate . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5 8. Differences from RFC 2119 . . . . . . . . . . . . . . . . . . . 5 9. Informative References . . . . . . . . . . . . . . . . . . . . 6 Appendix A. Changelog . . . . . . . . . . . . . . . . . . . . . . 6 A.1. 00 to 01 . . . . . . . . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6 Saint-Andre Expires March 1, 2012 [Page 2] Internet-Draft Conformance Terms August 2011 1. Introduction 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 mailing list, for which archives and subscription information are available at . [[ NOTE TO RFC EDITOR: Upon publication, please remove the foregoing paragraph. ]] 2. Definitions 2.1. MUST This term means that the feature or behavior is an absolute requirement of the specification, so that an implementation has an obligation to implement the feature or to behave as defined. The terms "SHALL" and "REQUIRED" are equivalent to "MUST". 2.2. MUST NOT This term means that the feature or behavior is an absolute prohibition of the specification, so that an implementation has an obligation to not implement the feature or to not behave as defined. The term "SHALL NOT" is equivalent to "MUST NOT". 2.3. SHOULD This term means that the feature or behavior is a conditional requirement of the specification, so that an implementation has an obligation to implement the feature or to behave as defined unless there is a strong reason why it might be prudent not to do so in particular circumstances. Specification authors are strongly encouraged to clearly describe such reasons, along with the implications of not conforming with the conditional requirement. Those who implement the specification or deploy conformant technologies need to understand and carefully weigh the full implications of not conforming to the conditional requirement before doing so. The term "RECOMMENDED" is equivalent to "SHOULD". Saint-Andre Expires March 1, 2012 [Page 3] Internet-Draft Conformance Terms August 2011 2.4. SHOULD NOT This term means that the feature or behavior is a conditional prohibition of the specification, so that an implementation has an obligation to not implement the feature or to not behave as defined unless there is a strong reason why it might be prudent to do so in particular circumstances. Specification authors are strongly encouraged to clearly describe such reasons, along with the implications of not conforming with the conditional prohibition. Those who implement the specification or deploy conformant technologies need to understand and carefully weigh the full implications of not conforming to the conditional prohibition before doing so. The term "NOT RECOMMENDED" is equivalent to "SHOULD NOT". 2.5. MAY This term means that the feature or behavior is purely discretionary. 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". 3. Usage The conformance terms defined in this document ought to be used judiciously. In particular, the absolute and conditional 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" or "mandatory" instead of "MUST"; "ought to" or "strongly encouraged" instead of "SHOULD"; and "might" or "discretionary" instead of "MAY". To prevent confusion, authors ought to use these alternative words and phrases instead of the lowercase versions of the conformance terms, and to use the conformance terms only in their uppercase versions. Saint-Andre Expires March 1, 2012 [Page 4] Internet-Draft Conformance Terms August 2011 4. Boilerplate 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: The following conformance terms are to be interpreted as described in [RFCXXXX]: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL". [[ NOTE TO RFC EDITOR: Upon publication, please change "XXXX" to the number assigned to this document. ]] 5. Security Considerations 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. Specification authors are strongly encouraged to clearly describe the security implications of not conforming to absolute and conditional requirements, since implementers might not have the benefit of the experience and discussion that produced the specification. 6. IANA Considerations This document requests no actions of the IANA. 7. Acknowledgements This document borrows text from [RFC2119]; Scott Bradner, the author of that document, is gratefully acknowledged. Thanks also to Harald Alvestrand and Julian Reschke for their feedback. 8. Differences from RFC 2119 The following modifications were made from RFC 2119. Saint-Andre Expires March 1, 2012 [Page 5] Internet-Draft Conformance Terms August 2011 o Clarified the definitions to specify that features and behaviors are absolutely required ("MUST"), absolutely prohibited ("MUST NOT"), conditionally required ("SHOULD"), conditionally prohibited ("SHOULD NOT"), or purely discretionary ("MAY"). o Expanded the definitions of "SHOULD" and "SHOULD NOT" to include the notion of conforming unless there is a strong reason to do so, and encouraged specification authors to clearly describe such reasons and the implications of not conforming. o Suggested alternative words and phrases for use when the conformance terms are not appropriate. o Encourage use of the conformance terms only in their uppercase versions. o Modified the title to use the phrase "conformance terms" instead of the phrase "key words". o Modified the boilerplate to include "NOT RECOMMENDED" (erratum #499) and to group similar terms together. 9. Informative References [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. Appendix A. Changelog [[ NOTE TO RFC EDITOR: Upon publication, please delete this entire section. ]] A.1. 00 to 01 o Added section describing differences from RFC 2119. o Changed "limited" to "conditional" in definitions of "SHOULD" and "SHOULD NOT". o Changed "truly a matter of preference" to "purely discretionary" in definition of "MAY". o Recommended that specification authors clearly describe the reasons why it might be prudent to not conform with "SHOULD" and "SHOULD NOT" statements, and the implications of non-conformance. o Adjusted and harmonized wording throughout. Saint-Andre Expires March 1, 2012 [Page 6] Internet-Draft Conformance Terms August 2011 Author's Address Peter Saint-Andre Cisco Systems, Inc. 1899 Wyknoop Street, Suite 600 Denver, CO 80202 USA Phone: +1-303-308-3282 Email: psaintan@cisco.com Saint-Andre Expires March 1, 2012 [Page 7]