BCP 47 Extension U
draft-davis-u-langtag-ext-03

Abstract

This document specifies an Extension to BCP 47 which 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

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Copyright Notice

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Table of Contents

1.  Introduction
    1.1.  Requirements Language
2.  BCP47 Required Information
    2.1.  Summary
        2.1.1.  Canonicalization
    2.2.  Registration Form
3.  Acknowledgements
4.  IANA Considerations
5.  Security Considerations
6.  References
    6.1.  Normative References
    6.2.  Informative References
§  Authors' Addresses

1.  Introduction

[BCP47] (Davis, M., Ed., “Tags for the Identification of Language (BCP47),” September 2009.) 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

2.  BCP47 Required Information

Language tags, as defined by [BCP47] (Davis, M., Ed., “Tags for the Identification of Language (BCP47),” September 2009.), 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:

The specification of extension subtags is provided by Section 3, Key Type Definitions of Unicode Technical Standard #35: Unicode Locale Data Markup Language (Davis, M., “Unicode Technical Standard #35: Locale Data Markup Language (LDML),” December 2007.) [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 (Davis, M., Ed., “Tags for the Identification of Language (BCP47),” September 2009.) [BCP47]) is also a well-formed locale identifier.

LDML (Davis, M., “Unicode Technical Standard #35: Locale Data Markup Language (LDML),” December 2007.) [UTS35] specifies a canonical representation. 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 structure of the Unicode locale extension 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 (Davis, M., “Unicode Technical Standard #35: Locale Data Markup Language (LDML),” December 2007.) [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.

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] (Davis, M., “Unicode Technical Standard #35: Locale Data Markup Language (LDML),” December 2007.). The following is a summary of that definition (for details see Section 3):

• 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.
• 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:

• The base language tag "de-DE" (German as used in Germany), exactly as defined by [BCP47] (Davis, M., Ed., “Tags for the Identification of Language (BCP47),” September 2009.) using subtags from the IANA Language Subtag Registry.
• The singleton 'u', identifying this extension.
• The attribute 'attr', which is an example for illustration (no attributes were defined at the time this document was published).
• The keyword 'co-phonebk', consisting to the key 'co' (Collation) and the type 'phonebk' (Phonebook collation order).

Only the first occurrence of an attributes 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.

With successive versions of [UTS35] (Davis, M., “Unicode Technical Standard #35: Locale Data Markup Language (LDML),” December 2007.), additional attributes, keys, and types MAY be defined. 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] (Davis, M., “Unicode Technical Standard #35: Locale Data Markup Language (LDML),” December 2007.). 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. Inside the "core.zip" 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 (Registration Form).

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:

• http://unicode.org/repos/cldr/tags/release-1-7-2/common/bcp47/
• http://unicode.org/repos/cldr/tags/release-1-8/common/bcp47/

Some data in the CLDR data files might require reference to LDML (Davis, M., “Unicode Technical Standard #35: Locale Data Markup Language (LDML),” December 2007.) [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] (Davis, M., Ed., “Tags for the Identification of Language (BCP47),” September 2009.), 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] (International Organization for Standardization, “ISO/IEC 646:1991, Information technology -- ISO 7-bit coded character set for information interchange.,” 1991.) order (that is, numbers before letters, with letters sorted as lowercase US-ASCII code points). The canonical order of keywords is in [US‑ASCII] (International Organization for Standardization, “ISO/IEC 646:1991, Information technology -- ISO 7-bit coded character set for information interchange.,” 1991.) order by key. The order 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 keyword.

2.2.  Registration Form

Per RFC 5646, Section 3.7 (Davis, M., Ed., “Tags for the Identification of Language (BCP47),” September 2009.) [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.
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
%%

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

This document will require IANA to insert the record in Section 2.2 (Registration Form) into the Language Extensions Registry, according to Section 3.7. Extensions and the Extensions Registry of "Tags for Identifying Languages" in [BCP47] (Davis, M., Ed., “Tags for the Identification of Language (BCP47),” September 2009.). Per Section 5.2 of [BCP47] (Davis, M., Ed., “Tags for the Identification of Language (BCP47),” September 2009.), 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] (Davis, M., Ed., “Tags for the Identification of Language (BCP47),” September 2009.). See RFC 5646, Section 6, Security Considerations (Davis, M., Ed., “Tags for the Identification of Language (BCP47),” September 2009.) [BCP47].

6.  References

6.1. Normative References

6.2. Informative References

Authors' Addresses