Internet DRAFT - draft-saintandre-2119bis
draft-saintandre-2119bis
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.
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This Internet-Draft will expire on March 1, 2012.
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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
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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 <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
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".
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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.
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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.
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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.
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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
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