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Versions: 00 01 02 03 04 05 06 07 RFC 6497

Internet Engineering Task Force                                 M. Davis
Internet-Draft                                                    Google
Intended status: Informational                               A. Phillips
Expires: December 24, 2011                                        Lab126
                                                               Y. Umaoka
                                                                     IBM
                                                                 C. Falk
                                                       Infinite Automata
                                                           June 22, 2011


                           BCP 47 Extension T
                      draft-davis-t-langtag-ext-01

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
   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 December 24, 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  3
   2.  BCP47 Required Information . . . . . . . . . . . . . . . . . .  3
     2.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  Structure  . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.3.  Canonicalization . . . . . . . . . . . . . . . . . . . . .  6
     2.4.  BCP47 Registration Form  . . . . . . . . . . . . . . . . .  7
     2.5.  Field Definitions  . . . . . . . . . . . . . . . . . . . .  7
     2.6.  Registration of Field Subtags  . . . . . . . . . . . . . .  8
     2.7.  Machine-Readable Data  . . . . . . . . . . . . . . . . . .  9
   3.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11




























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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'.

   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
   identify 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 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 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.

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



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   language tag 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.

   In addition, 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 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:








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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 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:

   +---------------+---------------------------------------------------+
   | 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/                  |



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   | Section       | Section 3 Unicode Language and Locale Identifiers |
   +---------------+---------------------------------------------------+

2.2.  Structure

   The subtags in the 't' extension are of the following form:

     +--------+-------------------------+----------------------------+
     | Label  | ABNF                    | Comment                    |
     +--------+-------------------------+----------------------------+
     | 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=   | 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
       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 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



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   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 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
   %%

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.

   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



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       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".

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.                       |






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   | Version     | If versioning for the mechanism is not done         |
   |             | according to date, then a description of the        |
   |             | versioning conventions used for the mechanism.      |
   +-------------+-----------------------------------------------------+

   The committee MAY request more information be supplied in tickets in
   the future if such information is found to be useful.

   The committee MUST respond to each proposal within 2 weeks.

   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 an a new entry in the machine-readable CLDR
   BCP47 data.

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.

   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].  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 2.4.

   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



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   '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/


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 2.4
   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].





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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://cldr.unicode.org/>

   [3]  <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


   Courtney Falk
   Infinite Automata

   Email: court@infiauto.com



































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