| Network Working Group | T. Hansen |
| Internet-Draft | AT&T Laboratories |
| Intended status: Informational | D. Crocker |
| Expires: August 29, 2016 | Brandenburg InternetWorking |
| February 26, 2016 |
Non-Normative Synonyms in RFCs
draft-hansen-nonkeywords-non2119-04
Specifications in RFCs contain normative keywords, as defined in RFC 2119, to signify requirements, permission or prohibitions. These include MUST, SHOULD and MAY, which are commonly recorded in all CAPITALS (but need not be). The RFC 2119 words are sometimes also used with non-normative meaning; this non-normative usage can be confusing and it is better to restrict the RFC 2119 words to be used solely as normative directives.
Happily, natural languages permit variation in phrasing, so that meaning can be retained without use of this otherwise-normative vocabulary. For such situations, this document provides some alternatives to the normative vocabulary of RFC 2119.
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To indicate a degree of requirement, permission or prohibition for an aspect of a specification, words such as MUST, SHOULD and MAY are defined as normative vocabulary in the formal aspects of the RFC series [RFC2119]. However it is also natural to use them non-normatively, in a narrative fashion. Even when this carries no obvious potential confusion, such as within RFCs that do not invoke the conventions of RFC 2119, non-normative use of these words in RFCs invites confusion for the reader; their normative meaning is too deeply ingrained in the culture of the RFC series.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are meant to reference their occurrence in [RFC2119]. The words are not used normatively here.
Fortunately, there are other words readily available, in lieu of the RFC 2119 words, when a non-normative meaning is intended. These alternatives, or their equivalents, can be used instead of the normatively-encumbered vocabulary.
| RFC 2119 Word | When Used With This Meaning | Some Alternative Word(s) |
|---|---|---|
| MUST, REQUIRED, SHALL | indicates that something is essential | needs to, necessary |
| SHOULD, RECOMMENDED | indicates that something is strongly urged | ought to, encouraged, suggested |
| MAY, OPTIONAL | indicates the possibility or capability of performing an action | can, might |
| indicates permission to perform an action | is allowed to, is permitted to |
Because the word "NOT" (or "not") only takes on a special meaning when it is combined with one of the RFC 2119 normative words, the word "not" can be freely used with any of the above suggestions and not be taken to have any separate RFC 2119 connotation. For example, "ought not" is non-normative, while "should not" and "SHOULD NOT" might be normative in the RFC 2119 sense.
By using these alternative wordings, authors of RFCs and internet drafts can avoid the possibility of vocabulary (that is sometimes used normatively) being misinterpreted.
Note that the above list of synonyms is not meant to be exhaustive; other non-RFC-2119-normative words can, of course, also be used at the author's discretion.
Authors who follow these guidelines might want to incorporate a declaration about usage at the beginning of their document.
[Note to RFC Editor: please remove this paragraph before publication.] This document can be discussed on the ietf@ietf.org mailing list.
The comments from Ran Atkinson are gratefully acknowledged.
This document has no IANA considerations.
The RFC 2119 terms are frequently used to specify behavior with security implications. The effects on security of changing something from a "MUST" to a "needs to", or vice versa, can be very subtle, as one can have normative meaning and the other does not. Document authors need to take the time to consider the effects of using non-normative verbiage as specified in this document instead of the normative verbiage from RFC 2119.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |