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Versions: 00 01 02 03 04 RFC 6067
Internet Engineering Task Force M. Davis
Internet-Draft Google
Intended status: Informational A. Phillips
Expires: February 27, 2011 Lab126
Y. Umaoka
IBM
August 26, 2010
BCP 47 Extension U
draft-davis-u-langtag-ext-04
Abstract
This document specifies an Extension to BCP 47 which provides subtags
that specify language and/or locale-based behavior or refinements to
language tags, according to work done by the Unicode Consortium.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on February 27, 2011.
Copyright Notice
Copyright (c) 2010 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3
2. BCP47 Required Information . . . . . . . . . . . . . . . . . . 3
2.1. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1. Canonicalization . . . . . . . . . . . . . . . . . . . 6
2.2. Registration Form . . . . . . . . . . . . . . . . . . . . . 6
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
[BCP47] permits the definition and registration of language tag
extensions "that contain a language component and are compatible with
applications that understand language tags". This document defines
an extension for identifying Unicode locale-based variations using
language tags. The "singleton" identifier for this extension is 'u'.
1.1. Requirements Language
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.
2. BCP47 Required Information
Language tags, as defined by [BCP47], are useful for identifying the
language of content. They are also used as locale identifiers (or
can be mapped to locales) in many operating environments and APIs.
However, many locale identifiers also require additional "tailorings"
or options for specific values within a language, culture, region, or
other variation. This extension provides a mechanism for using these
additional tailorings within language tags for general interchange.
The Unicode Consortium defines a standardized, structured set of
locale data and identifiers for locale data in the "Common Locale
Data Repository" or "CLDR". The maintaining authority for the
extension defined by this document is the Unicode Consortium:
+---------------+---------------------------------------------------+
| Item | Value |
+---------------+---------------------------------------------------+
| Name | Unicode Consortium |
| Contact Email | cldr@unicode.org |
| Discussion | cldr-users@unicode.org |
| List Email | |
| URL Location | cldr.unicode.org |
| Specification | Unicode Technical Standard #35 Unicode Locale |
| | Data Markup Language (LDML), |
| | http://unicode.org/reports/tr35/ |
| Section | Section 3 Unicode Language and Locale Identifiers |
+---------------+---------------------------------------------------+
The specification of extension subtags is provided by Section 3, Key
Type Definitions of Unicode Technical Standard #35: Unicode Locale
Data Markup Language [UTS35]. As required by BCP 47, subtags follow
the language tag ABNF and other rules for the formation of language
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tags and subtags, are restricted to the ASCII letters and digits, are
not case sensitive, and do not exceed eight characters in length.
Note that any "well-formed" language tag (see RFC 5646, Section 2.2.9
[BCP47]) is also a well-formed locale identifier.
LDML [UTS35] specifies a canonical representation. LDML is available
over the Internet and at no cost, and is available via a royalty-free
license at http://unicode.org/copyright.html. LDML is versioned, and
each version of LDML is numbered, dated, and stable. Extension
subtags, once defined by LDML, are never retracted or change in
meaning in a substantial way.
The structure of the Unicode locale extension is determined by the
Unicode CLDR Technical Committee, in accordance with the policies and
procedures in http://www.unicode.org/consortium/tc-procedures.html,
and subject to the Unicode Consortium Policies on
http://www.unicode.org/policies/policies.html.
Changes that can be made by successive versions of LDML [UTS35] by
the Unicode Consortium without requiring a new RFC include: the
allocation of new attributes, keywords, and types; clarifications or
non-material changes to an existing attribute, keyword, or type; and
compatible extensions to the overall syntactic structure of
attributes, keywords, and types. A new RFC would be required for
material changes to an existing attribute, keyword, or type, or an
incompatible change to the overall syntactic structure of attributes,
keywords, and types; however, such a change would be contrary to the
policies of the Unicode Consortium, and thus is not anticipated.
2.1. Summary
The subtags available for use in the 'u' extension consist of a set
of attributes, keys, and types. Attributes, keys, types, and their
respective meanings are defined in Section 3 (Unicode Language and
Locale Identifiers) of [UTS35]. The following is a summary of that
definition:
o An 'attribute' is a subtag with a length of three to eight
characters following the singleton and preceding any 'keyword'
sequences. No attributes were defined at the time of this
document's publication.
o A 'keyword' is a sequence of subtags consisting of a 'key' subtag,
followed by zero or more 'type' subtags (so a 'key' might appear
alone and not be accompanied by a 'type' subtag). A 'key' MUST
NOT appear more than once in a language tag's extension string.
The order of the 'type' subtags within a 'keyword' is sometimes
significant to their interpretation.
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A. A 'key' is a subtag with a length of exactly two characters.
Each 'key' is followed by zero or more 'type' subtags.
B. A 'type' is a subtag with a length of three to eight
characters following a key. 'Type' subtags are specific to a
particular 'key' and the order of the 'type' subtags MAY be
significant to the interpretation of the 'keyword'.
For example, the language tag "de-DE-u-attr-co-phonebk" consists of:
o The base language tag "de-DE" (German as used in Germany), exactly
as defined by [BCP47] using subtags from the IANA Language Subtag
Registry.
o The singleton 'u', identifying this extension.
o The attribute 'attr', which is an example for illustration (no
attributes were defined at the time this document was published).
o The keyword 'co-phonebk', consisting to the key 'co' (Collation)
and the type 'phonebk' (Phonebook collation order).
Only the first occurrence of an attribute or key conveys meaning in a
language tag. When interpreting tags containing the Unicode locale
extension, duplicate attributes or keywords are ignored in the
following way: ignore any attribute that has already appeared in the
tag and ignore any keyword whose key has already occurred in the tag.
Successive versions of [UTS35] could define additional attributes,
keys, and types. Once defined, attributes, keys, and types will
never be removed.
Beginning with CLDR version 1.7.2, machine-readable files are
available listing the valid attributes, keys, and types for each
successive version of [UTS35]. These releases are listed on
http://cldr.unicode.org/index/downloads. Each release has an
associated data directory of the form
"http://unicode.org/Public/cldr/<version>", where "<version>" is
replaced by the release number. For example, for version 1.7.2, the
"core.zip" file is located at
http://unicode.org/Public/cldr/1.7.2/core.zip [1]. Inside the
"core.zip" file, the path "common/bcp47" contains the data files
defining the valid attributes, keys, and types. The most recent
version is always identified by the version "latest" and can be
accessed by the URL in Section 2.2.
To get the version information in XML when working with the data
files, the XML parser must be validating. When the 'core.zip' file
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is unzipped, the 'dtd' directory will be at the same level as the
'bcp47' directory; that is required for correct validation. For each
release after CLDR 1.8, types introduced in that release are also
marked in the data files by the XML attribute "since", such as in the
following example:
<type name="adp" since="1.9"/>
The data is also currently maintained in a source code repository,
with each release tagged, for viewing directly without unzipping.
For example, see:
o http://unicode.org/repos/cldr/tags/release-1-7-2/common/bcp47/
o http://unicode.org/repos/cldr/tags/release-1-8/common/bcp47/
Some data in the CLDR data files might require reference to LDML
[UTS35]. For specific information, see Appendix Q in that document.
For example, LDML reserves the type 'codepoints' to define specific
code point ranges in Unicode for specific purposes.
2.1.1. Canonicalization
As required by [BCP47], the use of uppercase or lowercase letters is
not significant in the subtags used in this extension. The canonical
form for all subtags in the extension is lowercase. The canonical
order of attributes is in [US-ASCII] order (that is, numbers before
letters, with letters sorted as lowercase US-ASCII code points). The
canonical order of keywords is in [US-ASCII] order by key. The order
of subtags within a keyword is significant; the meaning of this
extension is altered if those subtags are rearranged. Thus, the
canonical form of the extension never reorders the subtags within a
keyword.
2.2. Registration Form
Per RFC 5646, Section 3.7 [BCP47]:
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%%
Identifier: u
Description: Unicode Locale
Comments: Subtags for the identification of language and cultural
variations. Used to set behavior in locale APIs. Data is
located in the "common/bcp47" directory inside the referenced
URL. Unicode Technical Standard #35 (LDML) provides additional
reference material defining the keys and values.
Added: 2010-mm-dd
RFC: [TBD]
Authority: Unicode Consortium
Contact_Email: cldr-contact@unicode.org
Mailing_List: cldr-users@unicode.org
URL: http://www.unicode.org/Public/cldr/latest/core.zip
%%
3. Acknowledgements
Thanks to John Emmons and the rest of the Unicode CLDR Technical
Committee for their work in developing the BCP 47 subtags for LDML.
Thanks also to Doug Ewell, for his many suggestions for improvements
to this document.
4. IANA Considerations
This document will require IANA to insert the record in Section 2.2
into the Language Extensions Registry, according to Section 3.7.
Extensions and the Extensions Registry of "Tags for Identifying
Languages" in [BCP47]. Per Section 5.2 of [BCP47], there might be
occasional (rare) requests by the Unicode Consortium (the "Authority"
listed in the record) for maintenance of this record. Changes that
can be submitted to IANA without the publication of a new RFC are
limited to modification of the Comments, Contact_Email, Mailing_List,
and URL fields. Any such requested changes MUST use the domain
'unicode.org' in any new addresses or URIs, MUST explicitly cite this
document (so that IANA can reference these requirements), and MUST
originate from the 'unicode.org' domain. The domain or authority can
only be changed via a new RFC.
This document does not require IANA to create or maintain a new
registry or otherwise impact IANA.
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5. Security Considerations
The security considerations for this extension are the same as those
for [BCP47]. See RFC 5646, Section 6, Security Considerations
[BCP47].
6. References
6.1. Normative References
[BCP47] Davis, M., Ed., "Tags for the Identification of Language
(BCP47)", September 2009.
[US-ASCII]
International Organization for Standardization, "ISO/IEC
646:1991, Information technology -- ISO 7-bit coded
character set for information interchange.", 1991.
[UTS35] Davis, M., "Unicode Technical Standard #35: Locale Data
Markup Language (LDML)", December 2007,
<http://www.unicode.org/reports/tr35/>.
Section 3: http://unicode.org/reports/
tr35/#Unicode_Language_and_Locale_Identifiers
Appendix Q: http://unicode.org/reports/
tr35/#Locale_Extension_Key_and_Type_Data
6.2. Informative References
[ldml-registry]
"Registry for Common Locale Data Repository tag elements",
September 2009.
URIs
[1] <http://unicode.org/Public/cldr/1.7.2/>
Authors' Addresses
Mark Davis
Google
Email: mark@macchiato.com
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Addison Phillips
Lab126
Email: addison@lab126.com
Yoshito Umaoka
IBM
Email: yoshito_umaoka@us.ibm.com
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