Internet Engineering Task Force M. Davis
Internet-Draft Google
Intended status: Informational A. Phillips
Expires: March 28, 2012 Lab126
Y. Umaoka
IBM
C. Falk
Infinite Automata
September 25, 2011
BCP 47 Extension T - Transformed Content
draft-davis-t-langtag-ext-06
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 March 28, 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 March 28, 2012 [Page 1]
Internet-Draft BCP 47 Extension T September 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. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Structure . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Canonicalization . . . . . . . . . . . . . . . . . . . . . 7
2.4. BCP47 Registration Form . . . . . . . . . . . . . . . . . 8
2.5. Field Definitions . . . . . . . . . . . . . . . . . . . . 8
2.6. Registration of Field Subtags . . . . . . . . . . . . . . 10
2.7. Registration of Additional Fields . . . . . . . . . . . . 10
2.8. Committee Responses to Registration Proposals . . . . . . 10
2.9. Machine-Readable Data . . . . . . . . . . . . . . . . . . 11
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
5. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Normative References . . . . . . . . . . . . . . . . . . . 14
6.2. Informative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
Davis, et al. Expires March 28, 2012 [Page 2]
Internet-Draft BCP 47 Extension T September 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 content that has been
transformed, including text that has been transliterated,
transcribed, or translated, or in some other way influenced by the
source. It may be used in queries to request content that has been
transformed. 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 content that has undergone
transformations, including content that has been transliterated,
transcribed, or translated. The correct interpretation of the
content may depend upon knowledge of the conventions used for the
transformation.
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.
Transforms such as transliterations may vary depending not only on
the basis of the source and target script, but also on the source and
target 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.
In addition, many different conventions have arisen for how to
transform text, even between the same languages and scripts. For
example, "Gaddafi" is commonly transliterated from Arabic to English
as any of (G/Q/K/Kh)a(d/dh/dd/dhdh/th/zz)af(i/y). Some examples of
standardized conventions used for transcribing or transliterating
text include:
a. United Nations Group of Experts on Geographical Names (UNGEGN)
b. US Library of Congress (LOC)
Davis, et al. Expires March 28, 2012 [Page 3]
Internet-Draft BCP 47 Extension T September 2011
c. US Board on Geographic Names (BGN)
d. Korean Ministry of Culture, Sports and Tourism (MCST)
e. International Organization for Standardization (ISO)
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").
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. Overview
Identification of transformed content 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. |
Davis, et al. Expires March 28, 2012 [Page 4]
Internet-Draft BCP 47 Extension T September 2011
| 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.
The t extension is not intended for use in structured data that
already provides separate source and target language identifiers.
For example, this is the case in localization interchange formats
such as XLIFF. In such cases, it would be inappropriate to use "ja-
t-it" for the target language tag because the source language tag
"it" would already be present in the data. Instead one would use the
language tag "ja".
As noted earlier, it is sometimes necessary to indicate additional
information about a 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 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.
Davis, et al. Expires March 28, 2012 [Page 5]
Internet-Draft BCP 47 Extension T September 2011
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 substantially changed in meaning.
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 |
+---------------+---------------------------------------------------+
2.2. Structure
The subtags in the 't' extension are of the following form:
Davis, et al. Expires March 28, 2012 [Page 6]
Internet-Draft BCP 47 Extension T September 2011
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
tag.
d. The order of the fields in a t extension is not significant. The
order of subtags within a field 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. The order of subtags
within a field is significant, and MUST NOT be changed in the process
of canonicalizing.
Davis, et al. Expires March 28, 2012 [Page 7]
Internet-Draft BCP 47 Extension T September 2011
2.4. BCP47 Registration Form
Per RFC 5646, Section 3.7 [BCP47]:
%%
Identifier: t
Description: Specifying Transformed Content
Comments: Subtags for the identification of content that has been
transformed, including but not limited to:
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
Assignment 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.
Assignments 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.
o The allocation of subtags valid after a field separator subtag.
o The addition of subtag aliases and descriptions.
o The modification of subtag descriptions.
Changes to the syntax or meaning of the 't' extension would require a
new RFC that obsoletes this document; such an RFC would break
stability, and would thus be contrary to the policies of the Unicode
Consortium.
At the time this document was published, one field was 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
Davis, et al. Expires March 28, 2012 [Page 8]
Internet-Draft BCP 47 Extension T September 2011
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, but it
SHOULD only be used where necessary. If it does occur:
* it MUST occur as the final subtag in the field
* it MUST NOT be the only subtag in the field
* it MUST consist of a sequence of digits of the form YYYY,
YYYYMM, or YYYYMMDD
* it SHOULD be as short as possible
Examples:
* 20110623 represents June 23rd, 2011.
* There are 3 dated versions of the UNGEGN transliteration
specification for Hebrew to Latin. They can be represented by
the following language tags:
+ und-Hebr-t-und-Latn-m0-ungegn-1972
+ und-Hebr-t-und-Latn-m0-ungegn-1977
+ und-Hebr-t-und-Latn-m0-ungegn-2007
* 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".
Davis, et al. Expires March 28, 2012 [Page 9]
Internet-Draft BCP 47 Extension T September 2011
A language tag with the t extension MAY be used to request a specific
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 and
ja-t-it-m0-xxx-v21b-2009, then the v21a 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.
2.7. Registration of Additional Fields
In the event that it proves necessary to add an additional field
(such as 'm2'), it can be requested by filing a ticket at
cldr.unicode.org [2]. The proposal in the ticket MUST contain a full
description of the proposed field semantics and subtag syntax, and
MUST be conform to the ABNF syntax for "field" presented in
Section 2.2.
2.8. Committee Responses to Registration Proposals
The committee MUST post each proposal publicly within 2 weeks after
reception, to allow for comments. The committee must respond
Davis, et al. Expires March 28, 2012 [Page 10]
Internet-Draft BCP 47 Extension T September 2011
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
document or in [UTS35]
Accepted tickets result in 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.
2.9. 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 description
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 extension="t"
name="m0" alias="collation" description="Transliteration extension
mechanism">, with subelements for the types, such as <type
name="ungegn" description="United Nations Group of Experts on
Geographical Names"/>. The currently defined attributes for the
Davis, et al. Expires March 28, 2012 [Page 11]
Internet-Draft BCP 47 Extension T September 2011
mechanisms include:
+-------------+-------------------------------+---------------------+
| Attribute | Description | Examples |
+-------------+-------------------------------+---------------------+
| name | The name of the mechanism, | UNGEGN, ALALC |
| | limited to 3-8 characters (or | |
| | sequences of them). | |
| 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. (Required for new | |
| | items.) | |
| 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. (Optional) | |
+-------------+-------------------------------+---------------------+
The file for the transform extension is "transform.xml". The initial
version of that file contains the following information.
Davis, et al. Expires March 28, 2012 [Page 12]
Internet-Draft BCP 47 Extension T September 2011
<key extension="t" name="m0" description=
"Transliteration extension mechanism"/>
<type name="ungegn" description=
"United Nations Group of Experts on Geographical Names"/>
<type name="alaloc" description=
"American Library Association-Library of Congress"/>
<type name="bgn" description=
"US Board on Geographic Names"/>
<type name="mcst" description=
"Korean Ministry of Culture, Sports and Tourism"/>
<type name="iso" description=
"International Organization for Standardization"/>
<type name="din" description=
"Deutsches Institut fuer Normung"/>
<type name="gost" description=
"Euro-Asian Council for Standardization, Metrology
and Certification"/>
</key>
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 March 28, 2012 [Page 13]
Internet-Draft BCP 47 Extension T September 2011
4. IANA Considerations
This document will require IANA to insert the record of 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. Umaoka, 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 March 28, 2012 [Page 14]
Internet-Draft BCP 47 Extension T September 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 March 28, 2012 [Page 15]
