Internet Engineering Task Force M. Davis
Internet-Draft Google
Intended status: Informational A. Phillips
Expires: January 12, 2012 Lab126
Y. Umaoka
IBM
C. Falk
Infinite Automata
July 11, 2011
BCP 47 Extension T
draft-davis-t-langtag-ext-02
Abstract
This document specifies an Extension to BCP 47 which provides subtags
for specifying the source language or script of transformed content,
including content that has been transliterated, transcribed, or
translated, or in some other way influenced by the source. 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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 12, 2012.
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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
Davis, et al. Expires January 12, 2012 [Page 1]
Internet-Draft BCP 47 Transform Extension July 2011
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. BCP47 Required Information . . . . . . . . . . . . . . . . . . 4
2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Structure . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Canonicalization . . . . . . . . . . . . . . . . . . . . . 7
2.4. BCP47 Registration Form . . . . . . . . . . . . . . . . . 7
2.5. Field Definitions . . . . . . . . . . . . . . . . . . . . 7
2.6. Registration of Field Subtags . . . . . . . . . . . . . . 9
2.7. Machine-Readable Data . . . . . . . . . . . . . . . . . . 10
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Normative References . . . . . . . . . . . . . . . . . . . 12
6.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Davis, et al. Expires January 12, 2012 [Page 2]
Internet-Draft BCP 47 Transform Extension July 2011
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, or in some other way influenced by the source. The
"singleton" identifier for this extension is 't'.
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 content
that has been transliterated, transcribed, or translated. That is,
for fully specifying such content, it is important to specify the
source language and/or script. In addition, it may also be important
to identify a particular specification for the transformation.
For example, suppose that Italian or Russian cities on a map, are
transcribed for Japanese users. Each name needs to be transliterated
into katakana using rules appropriate for the specific source and
target language. 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. 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 specific source and target
language. 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 on 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 could be used to
indicate a desired mechanical transformation in an API, or could be
used to tag data that has been converted (mechanically or by hand)
according to a transliteration method.
The usage of this extension is not limited to formal transformations,
and may include other instances where the content is in some other
way influenced by the source. For example, this extension could be
used to designate a request for a speech recognizer that is tailored
specifically for 2nd-language speakers who are 1st-language speakers
of a particular language (e.g. a recognizer for "English spoken with
a Chinese accent").
Davis, et al. Expires January 12, 2012 [Page 3]
Internet-Draft BCP 47 Transform Extension July 2011
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
2.1. Introduction
Identification of transforms can be done 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 as defined by [BCP47]. This allows for the source
language or script to be specified to the degree of precision
required. There 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.
It is sometimes necessary to indicate additional information about
the transformation. This additional information is optionally
supplied after the source in a series of one or more fields, where
each field consists of a field separator subtag followed by one or
more non-separator subtags. Each field separator subtag consists of
Davis, et al. Expires January 12, 2012 [Page 4]
Internet-Draft BCP 47 Transform Extension July 2011
a single letter followed by a single digit.
A transformation mechanism is an optional field that indicates the
specification used for the transformation, such as "UNGEGN" for the
the United Nations Group of Experts on Geographical Names
transliterations and transcriptions. It uses the 'm0' field
separator followed by certain subtags.
For example:
+------------------------------------+------------------------------+
| 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 field 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 field subtags are defined 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 maintaining authority for the 't' extension is the Unicode
Consortium:
Davis, et al. Expires January 12, 2012 [Page 5]
Internet-Draft BCP 47 Transform Extension July 2011
+---------------+---------------------------------------------------+
| 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 |
+---------------+---------------------------------------------------+
2.2. Structure
The subtags in the 't' extension are of the following form:
t_ext= "t" // Extension
("-" lang *("-" field) // Source + optional field(s)
/ 1*("-" field)) // Field(s) only (no source)
lang= language // BCP47, with restrictions
["-" script]
["-" region]
*("-" variant)
field= sep 1*("-" 3*8alphanum) // With restrictions
sep= ALPHA DIGIT // Subtag separators
alphanum= ALPHA / DIGIT
where <language>, <script>, <region>, and <variant> rules are
specified in [BCP47], <ALPHA> and <DIGIT> rules - in [RFC5234].
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
tag MAY be omitted: some field values do not require it.
c. There is optionally a sequence of fields, where each field has a
separator followed by a sequence of one or more subtags. Two
identical field separators MUST NOT be present in the language
Davis, et al. Expires January 12, 2012 [Page 6]
Internet-Draft BCP 47 Transform Extension July 2011
tag.
d. The order of the subtags in a t extension is significant (see
Section 2.3 Canonicalization).
e. The 't' subtag fields are defined by Section 3 [1] of Unicode
Technical Standard #35: Unicode Locale Data Markup Language
[UTS35].
2.3. 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
ordered by the separators, alphabetically.
2.4. BCP47 Registration Form
Per RFC 5646, Section 3.7 [BCP47]:
%%
Identifier: t
Description: Transform Specification
Comments: Subtags for the identification of content 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
%%
2.5. Field Definitions
The structure of 't' field 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:
o The allocation of new field separator subtags for use after the
't' extension.
Davis, et al. Expires January 12, 2012 [Page 7]
Internet-Draft BCP 47 Transform Extension July 2011
o The allocation of subtags valid after a field separator subtag.
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.
One field is initially specified in [UTS35]: the transform mechanism.
That field is summarized here:
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".
A language tag with the t extension MAY be used to request a specific
Davis, et al. Expires January 12, 2012 [Page 8]
Internet-Draft BCP 47 Transform Extension July 2011
transform of content. In such a case, the recipient SHOULD return
content that corresponds as closely as feasible to the requested
transform, including the specification of the mechanism. For
example, if the request is ja-t-it-m0-xxx-v21a-2007, and the
recipient has content corresponding to both ja-t-it-m0-xxx-v21a-2007
and ja-t-it-m0-xxx-v21a-2009, then the 2007 version would be
preferred. As is the case for language matching as discussed in
[BCP47], different implementations MAY have different measures of
"closeness".
2.6. Registration of Field Subtags
Registration of transform mechanisms is requested by filing a ticket
at cldr.unicode.org [2]. The proposal in the ticket MUST contain the
following information:
+-------------+-----------------------------------------------------+
| Item | Description |
+-------------+-----------------------------------------------------+
| Subtag | The proposed mechanism subtag (or subtag sequence). |
| Description | A description of the proposed mechanism; that |
| | description MUST be sufficient to distinguish it |
| | from other mechanisms in use. |
| Version | If versioning for the mechanism is not done |
| | according to date, then a description of the |
| | versioning conventions used for the mechanism. |
+-------------+-----------------------------------------------------+
Proposals for clarifications of descriptions or additional aliases
may also be requested by filing a ticket.
The committee MAY define a template for submissions that requests
more information, if it is found that such information would be
useful in evaluating proposals.
The committee MUST post each proposal publicly within 2 weeks after
reception, to allow for comments. The committee must respond
publicly to each proposal within 4 weeks after reception.
The response MAY:
o request more information or clarification
o accept the proposal, optionally with modifications to the subtag
or description
o reject the proposal, because of significant objections raised on
the mailing list or due to problems with constraints in this
Davis, et al. Expires January 12, 2012 [Page 9]
Internet-Draft BCP 47 Transform Extension July 2011
document or in [UTS35]
Accepted tickets result an a new entry in the machine-readable CLDR
BCP47 data, or in the case of a clarified description, modifications
to the description attribute value for an existing entry.
2.7. Machine-Readable Data
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. The discription
field is in the process of being added to CLDR.
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 [3]. The most recent
version is always identified by the version "latest" and can be
accessed by the URL in Section 2.4.
Inside the "core.zip" file, the directory "common/bcp47" contains the
data files listing the valid attributes, keys, and types for each
successive version of [UTS35]. Each data file list the keys and
types relevant to that topic. For example, mechanism.xml contains
the subtags (types) for the t mechanisms.
The XML structure lists the keys, such as <key name="co"
alias="collation">, with subelements for the types, such as <type
name="phonebk" alias="phonebook"/>. The currently defined attributes
for the mechanisms include:
+-------------+-------------------------------+---------------------+
| Attribute | Description | Examples |
+-------------+-------------------------------+---------------------+
| name | The name of the mechanism, | UNGEGN, ALALC |
| | limited to 3-8 characters (or | |
| | sequences of them). | |
Davis, et al. Expires January 12, 2012 [Page 10]
Internet-Draft BCP 47 Transform Extension July 2011
| description | A description of the name, | United Nations |
| | with all and only that | Group of Experts on |
| | information necessary to | Geographical Names; |
| | distinguish one name from | American Library |
| | others with which it might be | Association-Library |
| | confused. Descriptions are | of Congress |
| | not intended to provide | |
| | general background | |
| | information. | |
| since | Indicates the first version | 1.9, 2.0.1 |
| | of CLDR where the name | |
| | appears. | |
| alias | Alternative name of the key | |
| | or type, not limited in | |
| | number of characters. | |
| | Aliases are intended for | |
| | backwards compatibility, not | |
| | to provide all possible | |
| | alternate names or | |
| | designations | |
+-------------+-------------------------------+---------------------+
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/
For more information, see
http://cldr.unicode.org/index/bcp47-extension.
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.
Davis, et al. Expires January 12, 2012 [Page 11]
Internet-Draft BCP 47 Transform Extension July 2011
4. IANA Considerations
This document will require IANA to insert the record in Section 2.4
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.
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. and A. Phillips, Ed., "Tags for the
Identification of Language (BCP47)", September 2009.
[RFC5234] Crocker, Ed., "Augmented BNF for Syntax Specifications:
ABNF", 2008.
[RFC6067] Davis, M., Ed., Phillips, A., Ed., and Y. Yoshito, Ed.,
"BCP 47 Extension U", September 2010.
[UTS35] Davis, M., "Unicode Technical Standard #35: Locale Data
Markup Language (LDML)", December 2007,
<http://www.unicode.org/reports/tr35/>.
6.2. Informative References
[ldml-registry]
"Registry for Common Locale Data Repository tag elements",
Davis, et al. Expires January 12, 2012 [Page 12]
Internet-Draft BCP 47 Transform Extension July 2011
September 2009.
URIs
[1] <http://unicode.org/reports/tr35/>
[2] <http://cldr.unicode.org/>
[3] <http://unicode.org/Public/cldr/1.7.2/>
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
Courtney Falk
Infinite Automata
Email: court@infiauto.com
Davis, et al. Expires January 12, 2012 [Page 13]
