Internet Engineering Task Force (IETF) M. Davis
Request for Comments: 6497 Google
Category: Informational A. Phillips
ISSN: 2070-1721 Lab126
Y. Umaoka
IBM
C. Falk
Infinite Automata
February 2012
BCP 47 Extension T - Transformed Content
Abstract
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
http://www.rfc-editor.org/info/rfc6497.
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
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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]
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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 (which corresponds to the Cyrillic )
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)
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]
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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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
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.
Davis, et al. Informational [Page 4]
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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.
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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
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:
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 ,
RFC, FYI, BCP