Internet Engineering Task Force M. Davis
Internet-Draft Google
Intended status: Informational A. Phillips
Expires: December 18, 2011 Lab126
Y. Umaoka
IBM
June 16, 2011
BCP 47 Extension T
draft-davis-t-langtag-ext-00
Abstract
This document specifies an Extension to BCP 47 which provides subtags
for specifying the source language or script of transformed text,
including text that has been transliterated, transcribed, or
translated. It also provides for additional information used for
identification.
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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This Internet-Draft will expire on December 18, 2011.
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. BCP47 Required Information . . . . . . . . . . . . . . . . . . 3
2.1. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.1. Canonicalization . . . . . . . . . . . . . . . . . . . 8
2.2. Registration Form . . . . . . . . . . . . . . . . . . . . 9
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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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 specifying the source of a text transformation,
including text that has been transliterated, transcribed, or
translated. The "singleton" identifier for this extension is 't'.
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. There are mechanisms for specifying variant
subtags for special purposes. However, these variants are
insufficient for specifying text transformations, including text that
has been transliterated, transcribed, or translated. That is, for
fully specifying such text, it is important to specify the source
language and/or script. In addition, it may also be important to
specify a particular specification for the transformation.
For example, if one is transcribing the names of Italian or Russian
cities on a map for Japanese users, each name will need to be
transliterated into katakana using rules appropriate for the source
language and target languages. When tagging such data, it is
important to be able to indicate not only the resulting content
language ("ja" in this case), but also the source language.
Transforms such as transliteration may vary depending not only on the
basis of the source and target script, but also language. Thus the
Russian <U+041F U+0443 U+0442 U+0438 U+043D> (which corresponds to
the Cyrillic <PE, U, TE, I, EN>) transliterates into "Putin" in
English but "Poutine" in French. The identifier may need to indicate
a desired mechanical transformation in an API, or may need to tag
data that has been converted (mechanically or by hand) according to a
transliteration method.
Such identification is accomplished by using the 't' extension
defined in this document. This extension is formed by the 't'
singleton followed by a sequence of subtags that would form a
language tag defined by [BCP47]. This allows for the source language
or script to be specified to the degree of precision required. There
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are restrictions on the sequence of subtags. They MUST form a
regular, valid, canonical language tag, and MUST neither include
extensions nor private use sequences introduced by the singleton 'x'.
Where only the script is relevant (such as identifying a script-
script transliteration) then 'und' is used for the primary language
subtag.
For example:
+---------------------+---------------------------------------------+
| Language Tag | Description |
+---------------------+---------------------------------------------+
| ja-t-it | The content is Japanese, transformed from |
| | Italian. |
| ja-Kana-t-it | The content is Japanese Katakana, |
| | transformed from Italian. |
| und-Latn-t-und-cyrl | The content is in the Latin script, |
| | transformed from the Cyrillic script. |
+---------------------+---------------------------------------------+
Note that the sequence of subtags governed by 't' cannot contain a
singleton (a single-character subtag), because that would start a new
extension. For example, the tag "ja-t-i-ami" does not indicate that
the source is in "i-ami", because "i-ami" is not a regular language
tag in [BCP47]. That tag would express an empty 't' extension
followed by an 'i' extension.
In addition, it is sometimes necessary to indicate additional
information, such as the mechanism used to do the transformation,
optionally including the version of the mechanism. The mechanism can
be supplied by using the 'm0' separator. The format of such a 't'
extension is thus:
"t-<language-tag>-m0-<mechanism>".
(The full format reserves some additional syntax for future
expansion, as described below.)
The transform <mechanism> is a series of subtags that indicate the
specification used for the transformation, such as "UNGEGN" for the
the United Nations Group of Experts on Geographical Names
transliterations and transcriptions.
For example:
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+------------------------------------+------------------------------+
| Language Tag | Description |
+------------------------------------+------------------------------+
| und-Cyrl-t-und-latn-m0-ungegn-2007 | the content is in Cyrillic, |
| | transformed from Latn, |
| | according to a UNGEGN |
| | specification dated 2007. |
+------------------------------------+------------------------------+
The separator subtags such as 'm0' were chosen because they are
short, visually distinctive, and cannot occur in a language subtag
(outside of an extension and after 'x'), thus eliminating the
potential for collision or confusion with the source language tag.
The subtags that are valid after in the 't' extension are provided by
Section 3 [1] 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 tags
and subtags, are restricted to the ASCII letters and digits, are not
case sensitive, and do not exceed eight characters in length.
EDITORIAL NOTE: This new facility has been accepted by the Unicode
CLDR committee for incorporation into the next version of Unicode
CLDR, parallel with the structure of the 'u' extension [RFC6067], for
which it is already the maintaining authority. The data and
specification will be available by the time this internet draft has
been approved.
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 't' subtags 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 subtags for use after the 't' extension. A new RFC
would be required for material changes to an existing 't' subtag, or
an incompatible change to the overall syntactic structure of the 't'
extension; however, such a change would be contrary to the policies
of the Unicode Consortium, and thus is not anticipated.
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The maintaining authority for the 't' extension is the Unicode
Consortium:
+---------------+---------------------------------------------------+
| Item | Value |
+---------------+---------------------------------------------------+
| Name | Unicode Consortium |
| Contact Email | cldr-contact@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 |
+---------------+---------------------------------------------------+
2.1. Summary
The following is a summary of the definition for the 't' subtags
defined by Section 3 [1] of Unicode Technical Standard #35: Unicode
Locale Data Markup Language [UTS35], which is relevant for this
specification.
The subtags in the 't' extension are of the following form:
+--------+-------------------------+----------------------------+
| Label | ABNF | Comment |
+--------+-------------------------+----------------------------+
| t_ext= | "t-" | Extension |
| | [lang] | Source |
| | *("-" field) | Optional information |
| lang= | language | [BCP47], with restrictions |
| | ["-" script] | |
| | ["-" region] | |
| | *("-" variant) | |
| field= | sep 1*("-" 3*8alphanum) | With restrictions |
| sep= | 1ALPHA 1DIGIT | Subtag separators |
+--------+-------------------------+----------------------------+
Description and restrictions:
a. The 't' extension MUST have at least one subtag.
b. The 't' extension normally starts with a source language tag,
which MUST be a regular, canonical language tag as specified by
[BCP47]. Tags described by the 'irregular' production in BCP 47
MUST NOT be used to form the language tag. The source language
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tag MAY be omitted: some field values do not require it.
c. There is optionally a sequence of fields, where each field is a
separator followed by a sequence of subtags. Two identical
separators MUST NOT be present.
d. One field is initially specified in [UTS35]: the transform
mechanism.
A. The transform mechanism consists of a sequence of subtags
starting with the 'm0' separator followed by one or more
mechanism subtags. Each mechanism subtag has a length of 3
to 8 alphanumeric characters. The sequence as a whole
provides an identification of the specification for the
transform, such as the mechanism subtag 'UNGEGN' in "und-
Cyrl-t-und-latn-m0-ungegn". In many cases, only one
mechanism subtag is necessary, but multiple subtags MAY be
defined in [UTS35] where necessary.
B. Any purely numeric subtag is a representation of a date in
the Gregorian calendar. It MAY occur in any mechanism field.
If it does occur:
+ it MUST occur as the final subtag in the field,
+ it MUST NOT be the only subtag in the field, and
+ it MUST consist of a sequence of digits of the form YYYY,
YYYYMM, or YYYYMMDD.
For example, 20110623 represents June 23th, 2011. A date
subtag SHOULD only be used where necessary, and then SHOULD
be as short as possible. For example, suppose that the BGN
transliteration specification for Cyrillic to Latin had three
versions, dated June 11th, 1999; Dec 30th, 1999; and May 1st,
2011. In that case, the corresponding first two DATE subtags
would require months to be distinctive (199906 and 199912),
but the last subtag would only require the year (2011).
C. Some mechanisms may use a versioning system that is not
distinguished by date, or not by date alone. In the latter
case, the version will be of a form specified by [UTS35] for
that mechanism. For example, if the mechanism XXX uses
versions of the form v21a, then a tag could look like "ja-t-
it-m0-xxx-v21a". If there are multiple subversions
distinguished by date, then a tag could look like "ja-t-it-
m0-xxx-v21a-2007".
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e. Successive versions of [UTS35] could define additional separator
subtags, and additional subtags for those separators. Once
defined, those subtags will never be removed.
f. The order of the subtags is significant (see Section 2.1.1
Canonicalization).
EDITORIAL NOTE: The following parallels the structure used for the
'u' extension [RFC6067], for which the Unicode Consortium is the
maintaining authority. The data and specification will be available
by the time this internet draft has been approved.
Beginning with CLDR version 1.7.2, machine-readable files are
available listing the data defined for BCP47 extensions 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 [2]. Inside the
"core.zip" file, the path "common/bcp47" contains the data files
defining the data defined for BCP47 extensions. 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
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/
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, with the fields
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ordered by the separators, alphabetically.
2.2. Registration Form
Per RFC 5646, Section 3.7 [BCP47] :
%%
Identifier: t
Description: Transform Specification
Comments: Subtags for the identification of text transforms,
including transliteration, transcription, and translation.
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.
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.
[RFC6067] Davis, M., Ed., "BCP 47 Extension U", September 2010.
[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/reports/tr35/>
[2] <http://unicode.org/Public/cldr/1.7.2/>
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Authors' Addresses
Mark Davis
Google
Email: mark@macchiato.com
Addison Phillips
Lab126
Email: addison@lab126.com
Yoshito Umaoka
IBM
Email: yoshito_umaoka@us.ibm.com
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