RFC 8174 BEST CURRENT PRACTICE PROCESS · TERMINOLOGY
RFC 2119 Clarification: Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words
A short fix that says only ALL-CAPS MUST and SHOULD carry the special RFC 2119 meanings.
RFC EDITOR
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
BIBTEX
@misc{rfc8174,
series = {Request for Comments},
number = 8174,
howpublished = {RFC 8174},
publisher = {RFC Editor},
doi = {10.17487/RFC8174},
url = {https://www.rfc-editor.org/info/rfc8174},
author = {B. Leiba},
title = {{Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words}},
pagetotal = 4,
year = 2017,
month = may,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
Internet Engineering Task Force (IETF) B. Leiba
Request for Comments: 8174 Huawei Technologies
BCP: 14 May 2017
Updates: 2119
Category: Best Current Practice
ISSN: 2070-1721
Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words
Abstract
RFC 2119 specifies common key words that may be used in protocol
specifications. This document aims to reduce the ambiguity by
clarifying that only UPPERCASE usage of the key words have the
defined special meanings.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8174.
Copyright Notice
Copyright (c) 2017 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
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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Clarifying Capitalization of Key Words . . . . . . . . . . . 3
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. Normative References . . . . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4
1. Introduction
RFC 2119 specifies common key words, such as "MUST", "SHOULD", and
"MAY", that may be used in protocol specifications. It says that the
key words "are often capitalized," which has caused confusion about
how to interpret non-capitalized words such as "must" and "should".
This document updates RFC 2119 by clarifying that only UPPERCASE
usage of the key words have the defined special meanings. This
document is part of BCP 14.
2. Clarifying Capitalization of Key Words
The following change is made to [RFC2119]:
=== OLD ===
In many standards track documents several words are used to signify
the requirements in the specification. These words are often
capitalized. This document defines these words as they should be
interpreted in IETF documents. Authors who follow these guidelines
should incorporate this phrase near the beginning of their document:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
=== NEW ===
In many IETF documents, several words, when they are in all capitals
as shown below, are used to signify the requirements in the
specification. These capitalized words can bring significant clarity
and consistency to documents because their meanings are well defined.
This document defines how those words are interpreted in IETF
documents when the words are in all capitals.
o These words can be used as defined here, but using them is not
required. Specifically, normative text does not require the use
of these key words. They are used for clarity and consistency
when that is what's wanted, but a lot of normative text does not
use them and is still normative.
o The words have the meanings specified herein only when they are in
all capitals.
o When these words are not capitalized, they have their normal
English meanings and are not affected by this document.
Authors who follow these guidelines should incorporate this phrase
near the beginning of their document:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in BCP 14 [RFC2119] [RFC8174] when, and only when, they
appear in all capitals, as shown here.
=== END ===
3. IANA Considerations
This document does not require any IANA actions.
4. Security Considerations
This document is purely procedural; there are no related security
considerations.
5. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
Author's Address
Barry Leiba
Huawei Technologies
Phone: +1 646 827 0648
Email: barryleiba@computer.org
URI: http://internetmessagingtechnology.org/
AI ELI5
On this page
PROBLEMRFC 2119 said its key words are 'often capitalized', which left readers unsure how to treat lowercase 'must' and 'should'.
MECHANISMRFC 8174 replaces one paragraph of RFC 2119 so that the key words have their defined meanings only when written in all capitals; otherwise they are ordinary English.
WHY IT MATTERSIt updates RFC 2119 and supplies a new boilerplate sentence that cites BCP 14 and tests for all capitals.
§1The Ambiguity
RFC 2119 defines common key words such as MUST, SHOULD and MAY for protocol specifications. It says they 'are often capitalized', and that wording caused confusion about how to read lowercase 'must' and 'should'.
RFC 8174 updates RFC 2119 to settle this. It is part of BCP 14.
A rulebook said special words were 'often' in capitals. People did not know if small-letter versions counted too.
§2What Changes in RFC 2119
The RFC swaps an OLD paragraph of RFC 2119 for a NEW one. Three points in the new text matter:
- The words can be used as defined, but using them is not required. Normative text does not need the key words, and a lot of normative text does not use them and is still normative.
- The words have the specified meanings only when they are in all capitals.
- When not capitalized, they have their normal English meanings and are not affected by this document.
Capital MUST is a stop sign with a legal meaning. Small 'must' is just a word in a sentence.
§2The Updated Boilerplate
Authors who follow the guidelines should place a phrase near the start of their document. The new phrase adds NOT RECOMMENDED to the list of key words and says they are to be interpreted as described in BCP 14 RFC 2119 and RFC 8174 'when, and only when, they appear in all capitals, as shown here'.
Documents add a sentence that says: the big-letter words are the special ones, and only those.
§4IANA and Security
The document needs no IANA actions. It calls itself purely procedural and says there are no related security considerations.
It only changes how a rule is written down, so it creates no new danger.