[RFC Home] [TEXT|PDF|HTML] [Tracker] [IPR] [Errata] [Info page]

Errata Exist
Internet Engineering Task Force (IETF)                          M. Davis
Request for Comments: 6497                                        Google
Category: Informational                                      A. Phillips
ISSN: 2070-1721                                                   Lab126
                                                               Y. Umaoka
                                                                 C. Falk
                                                       Infinite Automata
                                                           February 2012

                BCP 47 Extension T - Transformed Content


   This document specifies an Extension to BCP 47 that 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 document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Davis, et al.                 Informational                     [Page 1]

RFC 6497                   BCP 47 Extension T              February 2012

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
      1.1. Requirements Language ......................................4
   2. BCP 47 Required Information .....................................4
      2.1. Overview ...................................................4
      2.2. Structure ..................................................6
      2.3. Canonicalization ...........................................7
      2.4. BCP 47 Registration Form ...................................8
      2.5. Field Definitions ..........................................8
      2.6. Registration of Field Subtags .............................10
      2.7. Registration of Additional Fields .........................11
      2.8. Committee Responses to Registration Proposals .............11
      2.9. Machine-Readable Data .....................................11
   3. Acknowledgements ...............................................14
   4. IANA Considerations ............................................14
   5. Security Considerations ........................................14
   6. References .....................................................14
      6.1. Normative References ......................................14
      6.2. Informative References ....................................15

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'.

Davis, et al.                 Informational                     [Page 2]

RFC 6497                   BCP 47 Extension T              February 2012

   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

   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

   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)

   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

Davis, et al.                 Informational                     [Page 3]

RFC 6497                   BCP 47 Extension T              February 2012

   specifically for second-language speakers who are first-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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  BCP 47 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 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

   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.

Davis, et al.                 Informational                     [Page 4]

RFC 6497                   BCP 47 Extension T              February 2012

   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
   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 Latin,      |
   |                                    | 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 of Unicode Technical
   Standard #35: Unicode Locale Data Markup Language (LDML) [UTS35], the
   main specification for the Unicode Common Locale Data Repository
   (CLDR) project.  That section also defines the parallel 'u' extension
   [RFC6067], for which the Unicode Consortium is also the maintaining
   authority.  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.

Davis, et al.                 Informational                     [Page 5]

RFC 6497                   BCP 47 Extension T              February 2012

   The LDML specification 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

   The maintaining authority for the 't' extension is the Unicode

   | 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                 ; BCP 47, with restrictions
             ["-" script]
             ["-" region]
             *("-" variant)

   field     = fsep 1*("-" 3*8alphanum) ; With restrictions

   fsep      = ALPHA DIGIT              ; Subtag separators
   alphanum  = ALPHA / DIGIT

   where <language>, <script>, <region>, and <variant> rules are
   specified in [BCP47], and <ALPHA> and <DIGIT> rules in [RFC5234].

Davis, et al.                 Informational                     [Page 6]

RFC 6497                   BCP 47 Extension T              February 2012

   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

   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 of Unicode
       Technical Standard #35: Unicode Locale Data Markup Language

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.                 Informational                     [Page 7]

RFC 6497                   BCP 47 Extension T              February 2012

2.4.  BCP 47 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: 2011-12-16
   RFC: RFC 6497
   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

   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

Davis, et al.                 Informational                     [Page 8]

RFC 6497                   BCP 47 Extension T              February 2012

   At the time this document was published, one field separator subtag
   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
       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 only consist of a sequence of digits of the form YYYY,
          YYYYMM, or YYYYMMDD

       *  it SHOULD be as short as possible

       Note: The format is related to that of [RFC3339], but is not the
       same.  The RFC 3339 full-date won't work because it uses hyphens.
       The offset ("Z") is not used because the date is a publication
       date (aka 'floating date').  For more information, see
       Section 3.3 ("Floating Time") of [W3C-TimeZones].

   c.  Examples:

       *  20110623 represents June 23, 2011.

       *  There are three 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

Davis, et al.                 Informational                     [Page 9]

RFC 6497                   BCP 47 Extension T              February 2012

       *  Suppose that the BGN transliteration specification for
          Cyrillic to Latin had three versions, dated June 11, 1999;
          Dec 30, 1999; and May 1, 2011.  In that case, the
          corresponding first two DATE subtags would require the months
          to be distinctive (199906 and 199912), but the last subtag
          would only require the year (2011).

   d.  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 sub-versions 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 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 http://cldr.unicode.org/.  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.

Davis, et al.                 Informational                    [Page 10]

RFC 6497                   BCP 47 Extension T              February 2012

   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
   http://cldr.unicode.org/.  The proposal in the ticket MUST contain a
   full description of the proposed field semantics and subtag syntax,
   and MUST 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
   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
   BCP 47 data or, in the case of a clarified description, modifications
   to the description attribute value for an existing entry.

2.9.  Machine-Readable Data

   Beginning with CLDR version 1.7.2, machine-readable files are
   available listing the data defined for BCP 47 extensions for each
   successive version of [UTS35].  The data in these files is used for
   testing the validity of subtags for the 't' extension and for the 'u'
   extension [RFC6067], for which the Unicode Consortium is also the
   maintaining authority.  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

Davis, et al.                 Informational                    [Page 11]

RFC 6497                   BCP 47 Extension T              February 2012

   "core.zip" file is located at
   http://unicode.org/Public/cldr/1.7.2/core.zip.  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 lists the keys and
   types relevant to that topic.

   The XML structure lists the keys, such as <key extension="t"
   name="m0" 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 mechanisms

   | 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.                 Informational                    [Page 12]

RFC 6497                   BCP 47 Extension T              February 2012

     <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"

   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; this 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

Davis, et al.                 Informational                    [Page 13]

RFC 6497                   BCP 47 Extension T              February 2012

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

   IANA has inserted 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" [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.

5.  Security Considerations

   The security considerations for this extension are the same as those
   for [BCP47].  See RFC 5646, Section 6, Security Considerations

6.  References

6.1.  Normative References

   [BCP47]    Phillips, A., Ed., and M. Davis, Ed., "Tags for
              Identifying Languages", BCP 47, RFC 5646, September 2009.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", STD 68, RFC 5234,
              January 2008.

   [UTS35]    Davis, M., "Unicode Technical Standard #35: Locale Data
              Markup Language (LDML)", February 2012,

Davis, et al.                 Informational                    [Page 14]

RFC 6497                   BCP 47 Extension T              February 2012

6.2.  Informative References

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, July 2002.

   [RFC6067]  Davis, M., Phillips, A., and Y. Umaoka, "BCP 47
              Extension U", RFC 6067, December 2010.

              Phillips, Ed., "W3C Working Group Note: Working with Time
              Zones", July 2011,

Authors' Addresses

   Mark Davis

   EMail: mark@macchiato.com

   Addison Phillips

   EMail: addison@lab126.com

   Yoshito Umaoka

   EMail: yoshito_umaoka@us.ibm.com

   Courtney Falk
   Infinite Automata

   EMail: court@infiauto.com

Davis, et al.                 Informational                    [Page 15]