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BCP 47 Extension U
RFC 6067

Document Type RFC - Informational (December 2010)
Was draft-davis-u-langtag-ext (individual in app area)
Authors Yoshito Umaoka , Addison Phillips , Mark Davis
Last updated 2020-07-29
RFC stream Internet Engineering Task Force (IETF)
IESG Responsible AD Alexey Melnikov
Send notices to (None)
RFC 6067
Internet Engineering Task Force (IETF)                          M. Davis
Request for Comments: 6067                                        Google
Category: Informational                                      A. Phillips
ISSN: 2070-1721                                                   Lab126
                                                               Y. Umaoka
                                                           December 2010

                           BCP 47 Extension U


   This document specifies an Extension to BCP 47 that provides subtags
   that specify language and/or locale-based behavior or refinements to
   language tags, according to work done by the Unicode Consortium.

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

Copyright Notice

   Copyright (c) 2010 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
   ( 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.

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RFC 6067             BCP 47 Unicode Locale Extension       December 2010

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . . . 2
   2.  BCP 47 Required Information . . . . . . . . . . . . . . . . . . 2
     2.1.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 4
       2.1.1.  Canonicalization  . . . . . . . . . . . . . . . . . . . 5
     2.2.  Registration Form . . . . . . . . . . . . . . . . . . . . . 6
   3.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     6.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     6.2.  Informative References  . . . . . . . . . . . . . . . . . . 7

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 identifying Unicode locale-based variations using
   language tags.  The "singleton" identifier for this extension is 'u'.

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

   Language tags, as defined by [BCP47], are useful for identifying the
   language of content.  They are also used as locale identifiers (or
   can be mapped to locales) in many operating environments and APIs.
   However, many locale identifiers also require additional "tailorings"
   or options for specific values within a language, culture, region, or
   other variation.  This extension provides a mechanism for using these
   additional tailorings within language tags for general interchange.

   The Unicode Consortium defines a standardized, structured set of
   locale data and identifiers for locale data in the "Common Locale
   Data Repository" or "CLDR".  The maintaining authority for the
   extension defined by this document is the Unicode Consortium:

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   | Item          | Value                                             |
   | Name          | Unicode Consortium                                |
   | Contact Email |                          |
   | Discussion    |                            |
   | List Email    |                                                   |
   | URL Location  |                                  |
   | Specification | Unicode Technical Standard #35 Unicode Locale     |
   |               | Data Markup Language (LDML),                      |
   |               |                  |
   | Section       | Section 3 Unicode Language and Locale Identifiers |

   The specification of extension subtags is provided by Section 3, Key
   Type Definitions 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.
   Note that any "well-formed" language tag (see RFC 5646, Section 2.2.9
   [BCP47]) is also a well-formed locale identifier.

   LDML [UTS35] specifies a canonical representation.  LDML is available
   over the Internet and at no cost, and is available via a royalty-free
   license at  LDML is versioned, and
   each version of LDML is numbered, dated, and stable.  Extension
   subtags, once defined by LDML, are never retracted and never change
   in meaning in a substantial way.

   The structure of the Unicode locale extension is determined by the
   Unicode CLDR Technical Committee, in accordance with the policies and
   procedures in,
   and subject to the Unicode Consortium Policies on

   Changes that can be made by successive versions of LDML [UTS35] by
   the Unicode Consortium without requiring a new RFC include: the
   allocation of new attributes, keywords, and types; clarifications or
   non-material changes to an existing attribute, keyword, or type; and
   compatible extensions to the overall syntactic structure of
   attributes, keywords, and types.  A new RFC would be required for
   material changes to an existing attribute, keyword, or type, or an
   incompatible change to the overall syntactic structure of attributes,
   keywords, and types; however, such a change would be contrary to the
   policies of the Unicode Consortium, and thus is not anticipated.

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2.1.  Summary

   The subtags available for use in the 'u' extension consist of a set
   of attributes, keys, and types.  Attributes, keys, types, and their
   respective meanings are defined in Section 3 (Unicode Language and
   Locale Identifiers) of [UTS35].  The following is a summary of that

   o  An 'attribute' is a subtag with a length of three to eight
      characters following the singleton and preceding any 'keyword'
      sequences.  No attributes were defined at the time of this
      document's publication.

   o  A 'keyword' is a sequence of subtags consisting of a 'key' subtag,
      followed by zero or more 'type' subtags (so a 'key' might appear
      alone and not be accompanied by a 'type' subtag).  A 'key' MUST
      NOT appear more than once in a language tag's extension string.
      The order of the 'type' subtags within a 'keyword' is sometimes
      significant to their interpretation.

      A.  A 'key' is a subtag with a length of exactly two characters.
          Each 'key' is followed by zero or more 'type' subtags.

      B.  A 'type' is a subtag with a length of three to eight
          characters following a 'key'.  'Type' subtags are specific to
          a particular 'key' and the order of the 'type' subtags MAY be
          significant to the interpretation of the 'keyword'.

   For example, the language tag "de-DE-u-attr-co-phonebk" consists of:

   o  The base language tag "de-DE" (German as used in Germany), exactly
      as defined by [BCP47] using subtags from the IANA Language Subtag

   o  The singleton 'u', identifying this extension.

   o  The attribute 'attr', which is an example for illustration (no
      attributes were defined at the time this document was published).

   o  The keyword 'co-phonebk', consisting to the key 'co' (Collation)
      and the type 'phonebk' (Phonebook collation order).

   Only the first occurrence of an attribute or key conveys meaning in a
   language tag.  When interpreting tags containing the Unicode locale
   extension, duplicate attributes or keywords are ignored in the
   following way: ignore any attribute that has already appeared in the
   tag and ignore any keyword whose key has already occurred in the tag.

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   Successive versions of [UTS35] could define additional attributes,
   keys, and types.  Once defined, attributes, keys, and types will
   never be removed.

   Beginning with CLDR version 1.7.2, machine-readable files are
   available listing the valid attributes, keys, and types for each
   successive version of [UTS35].  These releases are listed on  Each release has an
   associated data directory of the form
   "<version>", where "<version>" is
   replaced by the release number.  For example, for version 1.7.2, the
   "" file is located at  Inside the ""
   file, the path "common/bcp47" contains the data files defining the
   valid attributes, keys, and types.  The most recent version is always
   identified by the version "latest" and can be accessed by the URL in
   Section 2.2.

   To get the version information in XML when working with the data
   files, the XML parser must be validating.  When the '' 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:



   Some data in the CLDR data files might require reference to LDML
   [UTS35].  For specific information, see Appendix Q in that document.
   For example, LDML reserves the type 'codepoints' to define specific
   code point ranges in Unicode for specific purposes.

2.1.1.  Canonicalization

   As required by [BCP47], the use of uppercase or lowercase letters is
   not significant in the subtags used in this extension.  The canonical
   form for all subtags in the extension is lowercase.  The canonical
   order of attributes is in [US-ASCII] order (that is, numbers before
   letters, with letters sorted as lowercase US-ASCII code points).  The
   canonical order of keywords is in [US-ASCII] order by key.  The order

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   of subtags within a keyword is significant; the meaning of this
   extension is altered if those subtags are rearranged.  Thus, the
   canonical form of the extension never reorders the subtags within a

2.2.  Registration Form

   Per RFC 5646, Section 3.7 [BCP47]:

   Identifier: u
   Description: Unicode Locale
   Comments: Subtags for the identification of language and cultural
      variations.  Used to set behavior in locale APIs.  Data is
      located in the "common/bcp47" directory inside the referenced
      URL.  Unicode Technical Standard #35 (LDML) provides additional
      reference material defining the keys and values.
      For more details please see
   Added: 2010-09-02
   RFC: RFC 6067
   Authority:     Unicode Consortium

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.

   Thanks also to Doug Ewell, for his many suggestions for improvements
   to this document.

4.  IANA Considerations

   According to this document, IANA has inserted the record in
   Section 2.2 into the Language Extensions Registry, according to
   Section 3.7 (Extensions and the Extensions Registry) of [BCP47],
   "Tags for Identifying Languages".  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 '' in any new addresses or URIs, MUST
   explicitly cite this document (so that IANA can reference these

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   requirements), and MUST originate from the '' 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.

   [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,

                    Section 3:

                    Appendix Q:

6.2.  Informative References

   [ldml-registry]  "Registry for Common Locale Data Repository tag
                    elements", September 2009.

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Authors' Addresses

   Mark Davis


   Addison Phillips


   Yoshito Umaoka


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