draft-ietf-ltru-matching-06.txt   draft-ietf-ltru-matching-07.txt 
Network Working Group A. Phillips, Ed. Network Working Group A. Phillips, Ed.
Internet-Draft Quest Software Internet-Draft Quest Software
Obsoletes: 3066 (if approved) M. Davis, Ed. Obsoletes: 3066 (if approved) M. Davis, Ed.
Expires: May 20, 2006 IBM Expires: May 22, 2006 IBM
November 16, 2005 November 18, 2005
Matching Tags for the Identification of Languages Matching of Language Tags
draft-ietf-ltru-matching-06 draft-ietf-ltru-matching-07
Status of this Memo Status of this Memo
By submitting this Internet-Draft, each author represents that any By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79. aware will be disclosed, in accordance with Section 6 of BCP 79.
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This Internet-Draft will expire on May 20, 2006. This Internet-Draft will expire on May 22, 2006.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2005).
Abstract Abstract
This document describes different mechanisms for comparing, matching, This document describes different mechanisms for comparing, matching,
and evaluating language tags. Possible algorithms for language and evaluating language tags. Possible algorithms for language
negotiation and content selection are described. This document, in negotiation and content selection are described. This document, in
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assigned to draft-ietf-ltru-registry-14), replaces RFC 3066, which assigned to draft-ietf-ltru-registry-14), replaces RFC 3066, which
replaced RFC 1766. replaced RFC 1766.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The Language Range . . . . . . . . . . . . . . . . . . . . . . 4 2. The Language Range . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Lists of Language Ranges . . . . . . . . . . . . . . . . . 4 2.1. Lists of Language Ranges . . . . . . . . . . . . . . . . . 4
2.2. Basic Language Range . . . . . . . . . . . . . . . . . . . 4 2.2. Basic Language Range . . . . . . . . . . . . . . . . . . . 4
2.3. Extended Language Range . . . . . . . . . . . . . . . . . 5 2.3. Extended Language Range . . . . . . . . . . . . . . . . . 5
3. Types of Matching . . . . . . . . . . . . . . . . . . . . . . 8 2.4. Choosing a Language Range . . . . . . . . . . . . . . . . 6
3.1. Choosing a Type of Matching . . . . . . . . . . . . . . . 8 3. Types of Matching . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Filtering . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. Choosing a Type of Matching . . . . . . . . . . . . . . . 9
3.2.1. Filtering with Basic Language Ranges . . . . . . . . . 10 3.2. Filtering . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2.2. Filtering with Extended Language Ranges . . . . . . . 10 3.2.1. Filtering with Basic Language Ranges . . . . . . . . . 11
3.2.3. Distance Metric Filtering . . . . . . . . . . . . . . 11 3.2.2. Filtering with Extended Language Ranges . . . . . . . 11
3.3. Lookup . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2.3. Scored Filtering . . . . . . . . . . . . . . . . . . . 12
4. Other Considerations . . . . . . . . . . . . . . . . . . . . . 16 3.3. Lookup . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1. Meaning of Language Tags and Ranges . . . . . . . . . . . 16 4. Other Considerations . . . . . . . . . . . . . . . . . . . . . 18
4.2. Considerations for Private Use Subtags . . . . . . . . . . 17 4.1. Meaning of Language Tags and Ranges . . . . . . . . . . . 18
4.3. Length Considerations in Matching . . . . . . . . . . . . 17 4.2. Considerations for Private Use Subtags . . . . . . . . . . 19
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 4.3. Length Considerations in Matching . . . . . . . . . . . . 19
6. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
7. Security Considerations . . . . . . . . . . . . . . . . . . . 22 6. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8. Character Set Considerations . . . . . . . . . . . . . . . . . 23 7. Security Considerations . . . . . . . . . . . . . . . . . . . 24
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24 8. Character Set Considerations . . . . . . . . . . . . . . . . . 25
9.1. Normative References . . . . . . . . . . . . . . . . . . . 24 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
9.2. Informative References . . . . . . . . . . . . . . . . . . 24 9.1. Normative References . . . . . . . . . . . . . . . . . . . 26
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 25 9.2. Informative References . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . . . . 27 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28
Intellectual Property and Copyright Statements . . . . . . . . . . 29
1. Introduction 1. Introduction
Human beings on our planet have, past and present, used a number of Human beings on our planet have, past and present, used a number of
languages. There are many reasons why one would want to identify the languages. There are many reasons why one would want to identify the
language used when presenting or requesting information. language used when presenting or requesting information.
Information about a user's language preferences commonly needs to be Information about a user's language preferences commonly need to be
identified so that appropriate processing can be applied. For identified so that appropriate processing can be applied. For
example, the user's language preferences in a browser can be used to example, the user's language preferences in a browser can be used to
select web pages appropriately. Language preferences can also be select web pages appropriately. Language preferences can also be
used to select among tools (such as dictionaries) to assist in the used to select among tools (such as dictionaries) to assist in the
processing or understanding of content in different languages. processing or understanding of content in different languages.
Given a set of language identifiers, such as those defined in Given a set of language identifiers, such as those defined in
[RFC3066bis], various mechanisms can be envisioned for performing [RFC3066bis], various mechanisms can be envisioned for performing
language negotiation and tag matching. Applications, protocols, or language negotiation and tag matching. Applications, protocols, or
specifications will have varying needs and requirements that will specifications will have varying needs and requirements that affect
affect the choice of a suitable mechanism. Protocols and the choice of a suitable mechanism.
specifications SHOULD clearly indicate the particular mechanism used
in selecting or matching language tags.
This document defines several mechanisms for matching, selecting, or This document defines several mechanisms for matching, selecting, or
filtering content whose natural language is identified using Language filtering content whose natural language is identified using Language
Tags [RFC3066bis], as well as the syntax (called a "language range") Tags [RFC3066bis], as well as the syntax (called a "language range")
associated with each of these mechanisms for specifying the user's associated with each of these mechanisms for specifying the user's
language preferences. language preferences.
This document, in combination with [RFC3066bis] (replace "3066bis" This document, in combination with [RFC3066bis] (replace "3066bis"
globally in this document with the RFC number assigned to globally in this document with the RFC number assigned to
draft-ietf-ltru-registry-14), replaces [RFC3066], which replaced draft-ietf-ltru-registry-14), replaces [RFC3066], which replaced
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The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
2. The Language Range 2. The Language Range
Language Tags [RFC3066bis] are used to identify the language of some Language Tags [RFC3066bis] are used to identify the language of some
information item or content. Applications or protocols that use information item or content. Applications or protocols that use
language tags are often faced with the problem of identifying sets of language tags are often faced with the problem of identifying sets of
content that share certain language attributes. For example, HTTP content that share certain language attributes. For example,
1.1 [RFC2616] describes language ranges in its discussion of the HTTP/1.1 [RFC2616] describes language ranges in its discussion of the
Accept-Language header (Section 14.4), which is used for selecting Accept-Language header (Section 14.4). These are to be used when
content from servers based on the language of that content. selecting content from servers based on the language of that content.
When selecting content according to its language, it is useful to When selecting content according to its language, it is useful to
have a mechanism for identifying sets of language tags that share have a mechanism for identifying sets of language tags that share
specific attributes. This allows users to select or filter content specific attributes. This allows users to select or filter content
based on specific requirements. Such an identifier is called a based on specific requirements. Such an identifier is called a
"Language Range". "Language Range".
Language tags and thus language ranges are to be treated as case
insensitive: there exist conventions for the capitalization of some
of the subtags, but these MUST NOT be taken to carry meaning.
Matching of language tags to language ranges MUST be done in a case
insensitive manner as well.
2.1. Lists of Language Ranges 2.1. Lists of Language Ranges
When users specify a language preference they often need to specify a When users specify a language preference they often need to specify a
prioritized list of language ranges in order to best reflect their prioritized list of language ranges in order to best reflect their
language requirements for the matching operation. This is especially language preferences. This is especially true for speakers of
true for speakers of minority languages. A speaker of Breton in minority languages. A speaker of Breton in France, for example, may
France, for example, may specify "be" followed by "fr", meaning that specify "be" followed by "fr", meaning that if Breton is available,
if Breton is available, it is preferred, but otherwise French is the it is preferred, but otherwise French is the best alternative. It
best alternative. It can get more complex: a speaker may wish to can get more complex: a speaker may wish to fall back from Skolt Sami
fallback from Skolt Sami to Northern Sami to Finnish. to Northern Sami to Finnish.
A "Language Priority List" consists of a prioritized or weighted list A "Language Priority List" consists of a prioritized or weighted list
of language ranges. One well known example of such a list is the of language ranges. One well known example of such a list is the
"Accept-Language" header defined in RFC 2616 [RFC2616] (see Section "Accept-Language" header defined in RFC 2616 [RFC2616] (see Section
14.4) and RFC 3282 [RFC3282]. The various matching operations 14.4) and RFC 3282 [RFC3282].
described in this document include considerations for using a
language priority list. The various matching operations described in this document include
considerations for using a language priority list. When given as
examples in this document, language priority lists will be shown as a
quoted sequence of ranges separated by semi-colons, like this: "en;
fr; zh-Hant" (which would be read as "English before French before
Chinese as written in the Traditional script").
2.2. Basic Language Range 2.2. Basic Language Range
A "Basic Language Range" identifies the set of content whose language A "Basic Language Range" identifies the set of content whose language
tags begin with the same sequence of subtags. A basic language range tags begin with the same sequence of subtags. A basic language range
is identified by its 'language-range' tag, by adapting the is identified by its 'language range' tag, by adapting the
ABNF[RFC4234] from HTTP/1.1 [RFC2616] : ABNF[RFC4234] from HTTP/1.1 [RFC2616] :
language-range = language-tag / "*" language-range = language-tag / "*"
language-tag = 1*8[alphanum] *["-" 1*8alphanum] language-tag = 1*8[alphanum] *["-" 1*8alphanum]
alphanum = ALPHA / DIGIT alphanum = ALPHA / DIGIT
That is, a language-range has the same syntax as a language-tag or is That is, a language-range has the same syntax as a language-tag or is
the single character "*". Basic Language Ranges imply that there is the single character "*". Basic Language Ranges imply that there is
a semantic relationship between language tags that share the same a semantic relationship between language tags that share the same
prefix. While this is often the case, it is not always true and prefix. While this is often the case, it is not always true and
users should note that the set of language tags that match a specific users should note that the set of language tags that match a specific
language-range may not be mutually intelligible. language-range may not be mutually intelligible.
Basic language ranges were originally described in [RFC3066] and HTTP Basic language ranges were originally described in [RFC3066] and
1.1 [RFC2616] (where they are referred to as simply a "language HTTP/1.1 [RFC2616] (where they are referred to as simply a "language
range"). range").
Users SHOULD avoid subtags that add no distinguishing value to a
language range. For example, script subtags SHOULD NOT be used to
form a language range with language subtags which have a matching
Suppress-Script field in their registry record. Thus the language
range "en-Latn" is probably inappropriate in most cases (because the
vast majority English documents are written in the Latin script and
thus the 'en' language subtag has a Suppress-Script field for 'Latn'
in the registry).
Language tags and thus language ranges are to be treated as case
insensitive: there exist conventions for the capitalization of some
of the subtags, but these MUST NOT be taken to carry meaning.
Matching of language tags to language ranges MUST be done in a case
insensitive manner.
When working with tags and ranges, note that extensions and most
private use subtags are generally orthogonal to language tag fallback
and users SHOULD avoid using these subtags in language ranges, since
they will often interfere with the selection of available language
content. Since these subtags are always at the end of the sequence
of subtags, they don't normally interfere with the use of prefixes
for matching in the schemes described below.
Note that when working with basic language ranges, no attempt is made
to process the semantics of the tags or ranges in any way. The
language tag and language range are compared in a case insensitive
manner using basic string processing. Thus the choice of subtags in
both the language tag and language range may affect the results
produced as a result.
2.3. Extended Language Range 2.3. Extended Language Range
A Basic Language Range does not always provide the most appropriate A Basic Language Range does not always provide the most appropriate
way to specify a user's preferences. Sometimes it is beneficial to way to specify a user's preferences. Sometimes it is beneficial to
define a more granular matching scheme that takes advantage of the use a more granular matching scheme that takes advantage of the
internal structure of language tags, by allowing the user to specify, internal structure of language tags, by allowing the user to specify,
for example, the value of a specific field in a language tag or to for example, the value of a specific field in a language tag or to
indicate which values are of interest in filtering or selecting the indicate which values are of interest in filtering or selecting the
content. content.
In an extended language range, the identifier takes the form of a In an extended language range, the identifier takes the form of a
series of subtags which must consist of well-formed subtags or the series of subtags which MUST consist of well-formed subtags or the
special subtag "*". For example, the language range "en-*-US" special subtag "*". For example, the language range "en-*-US"
specifies a primary language of 'en', followed by any script subtag, specifies a primary language of 'en', followed by any script subtag,
followed by the region subtag 'US'. followed by the region subtag 'US'.
An extended language range can be represented by the following ABNF: An extended language range can be represented by the following ABNF:
extended-language-range = range ; a range extended-language-range = range ; a range
/ privateuse ; private use tag / privateuse ; private-use tag
/ grandfathered ; grandfathered registrations / grandfathered ; grandfathered registrations
range = (language range = (language
["-" script] ["-" script]
["-" region] ["-" region]
*("-" variant) *("-" variant)
*("-" extension) *("-" extension)
["-" privateuse]) ["-" privateuse])
language = (2*3ALPHA [ extlang ]) ; shortest ISO 639 code language = (2*3ALPHA [ extlang ]) ; shortest ISO 639 code
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/ "*" ; ... or wildcard / "*" ; ... or wildcard
extension = singleton *("-" (2*8alphanum)) [ "-*" ] extension = singleton *("-" (2*8alphanum)) [ "-*" ]
; extension subtags ; extension subtags
; wildcard can only appear ; wildcard can only appear
; at the end ; at the end
singleton = %x41-57 / %x59-5A / %x61-77 / %x79-7A / DIGIT singleton = %x41-57 / %x59-5A / %x61-77 / %x79-7A / DIGIT
; "a"-"w" / "y"-"z" / "A"-"W" / "Y"-"Z" / "0"-"9" ; "a"-"w" / "y"-"z" / "A"-"W" / "Y"-"Z" / "0"-"9"
; Single letters: x/X is reserved for private use ; Single letters: x/X is reserved for private use
privateuse = ("x"/"X") 1*("-" (1*8alphanum)) privateuse = ("x"/"X") 1*("-" (1*8alphanum))
grandfathered = 1*3ALPHA 1*2("-" (2*8alphanum)) grandfathered = 1*3ALPHA 1*2("-" (2*8alphanum))
; grandfathered registration ; grandfathered registration
; Note: i is the only singleton ; Note: I is the only singleton
; that starts a grandfathered tag ; that starts a grandfathered tag
alphanum = (ALPHA / DIGIT) ; letters and numbers alphanum = (ALPHA / DIGIT) ; letters and numbers
A field not present in the middle of an extended language range MAY A field not present in the middle of an extended language range MAY
be treated as if the field contained a "*". For example, the range be treated as if the field contained a "*". For example, the range
"en-US" MAY be considered to be equivalent to the range "en-*-US". "en-US" MAY be considered to be equivalent to the range "en-*-US".
This also means that multiple wildcards can be collapsed (so that This also means that multiple wildcards can be collapsed (so that
"en-*-*-US" is equivalent to "en-*-US"). "en-*-*-US" is equivalent to "en-*-US").
2.4. Choosing a Language Range
Users indicate their language preferences via the choice of a
language range or the set of language ranges in the language priority
list. The type of matching will affect what the best choice is for
given user. In addition, user's should be aware that, when working
with language ranges, most matching schemes make no attempt to
process the semantic meaning of the subtags. The language tag and
language range (or their subtags) are usually compared in a case
insensitive manner using basic string processing. Thus the choice of
subtags in both the language tag and language range may affect the
results produced.
Users SHOULD avoid subtags that add no distinguishing value to a
language range. For example, script subtags SHOULD NOT be used to
form a language range with language subtags which have a matching
Suppress-Script field in their registry record. Thus the language
range "en-Latn" is probably inappropriate in most cases (because the
vast majority of English documents are written in the Latin script
and thus the 'en' language subtag has a Suppress-Script field for
'Latn' in the registry).
When working with tags and ranges, note that extensions and most
private-use subtags are orthogonal to language tag fallback and users
SHOULD avoid using these subtags in language ranges, since they will
often interfere with the selection of available language content.
Since these subtags are always at the end of the sequence of subtags,
they don't normally interfere with the use of prefixes for the
filtering schemes described below in Section 3.
When working with tags and ranges users SHOULD note the following: When working with tags and ranges users SHOULD note the following:
1. Private-use and Extension subtags are normally orthogonal to 1. Private-use and Extension subtags are normally orthogonal to
language tag fallback. Implementations or specifications that language tag fallback. Implementations or specifications that
use a lookup (Section 3.3) matching scheme SHOULD ignore use a lookup (Section 3.3) matching scheme SHOULD ignore
unrecognized private-use and extension subtags when performing unrecognized private-use and extension subtags when performing
language tag fallback. Since these subtags are always at the end language tag fallback. Since these subtags are always at the end
of the sequence of subtags, they don't normally interfere with of the sequence of subtags, they don't normally interfere with
the use of prefixes for matching in the schemes described below. the use of prefixes for matching in the schemes described below.
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3. Some applications of language tags might want or need to consider 3. Some applications of language tags might want or need to consider
extensions and private-use subtags when matching tags. If extensions and private-use subtags when matching tags. If
extensions and private-use subtags are included in a matching or extensions and private-use subtags are included in a matching or
filtering process that utilizes the one of the schemes described filtering process that utilizes the one of the schemes described
in this document, then the implementation SHOULD canonicalize the in this document, then the implementation SHOULD canonicalize the
language tags and/or ranges before performing the matching. Note language tags and/or ranges before performing the matching. Note
that language tag processors that claim to be "well-formed" that language tag processors that claim to be "well-formed"
processors as defined in [RFC3066bis] generally fall into this processors as defined in [RFC3066bis] generally fall into this
category. category.
There are several matching algorithms or schemes which can be applied
when matching extended language ranges to language tags.
3. Types of Matching 3. Types of Matching
Matching language ranges to language tags can be done in a number of Matching language ranges to language tags can be done in a number of
different ways. This section describes the different types of different ways. This section describes the different types of
matching scheme, as well as the considerations for choosing between matching scheme, as well as the considerations for choosing between
them. them. Protocols and specifications SHOULD clearly indicate the
particular mechanism used in selecting or matching language tags.
There are two basic types of matching scheme: those that produce an There are two basic types of matching scheme: those that produce an
open-ended set of content (called "filtering") and those that produce open-ended set of content (called "filtering") and those that produce
a single information item for a given request (called "lookup"). a single information item for a given request (called "lookup").
A key difference between these two types of matching scheme is that A key difference between these two types of matching scheme is that
the language range for filtering operations is always the _least_ the language range for filtering operations is always the _least_
specific tag one will accept as a match, while for lookup operations specific tag one will accept as a match, while for lookup operations
the language range is always the _most_ specific tag. the language range is always the _most_ specific tag.
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collection of documents). For example, if using a search engine, one collection of documents). For example, if using a search engine, one
might use filtering to limit the results to documents written in might use filtering to limit the results to documents written in
French. It can also be used when deciding whether to perform some French. It can also be used when deciding whether to perform some
processing that is language sensitive on some content. For example, processing that is language sensitive on some content. For example,
a process might cause paragraphs whose language tag matched the a process might cause paragraphs whose language tag matched the
language range "nl" to be displayed in italics within a document. language range "nl" to be displayed in italics within a document.
This document describes three types of filtering: This document describes three types of filtering:
1. Basic Filtering (Section 3.2.1) is used to match content using 1. Basic Filtering (Section 3.2.1) is used to match content using
basic language rangesSection 2.2. It is compatible with basic language ranges (Section 2.2). It is compatible with
implementations that do not produce extended language ranges. implementations that do not produce extended language ranges.
2. Extended Range Filtering (Section 3.2.2) is used to match content 2. Extended Range Filtering (Section 3.2.2) is used to match content
using extended language rangesSection 2.3. Newer implementations using extended language ranges (Section 2.3). Newer
SHOULD use this form of filtering in preference to basic implementations SHOULD use this form of filtering in preference
filtering. to basic filtering.
3. Scored Filtering (Section 3.2.3) produces an ordered set of 3. Scored Filtering (Section 3.2.3) produces an ordered set of
content using either basic or extended language ranges. It content using either basic or extended language ranges. It
should be used when the quality of the match within a specific SHOULD be used when the quality of the match within a specific
language range is important, as when presenting a list of language range is important, as when presenting a list of
documents resulting from a search. documents resulting from a search.
Lookup (Section 3.3) is used when each request MUST produce exactly Lookup (Section 3.3) is used when each request MUST produce exactly
one piece of content. For example, a Web server might use the one piece of content. For example, a Web server might use the
Accept-Language HTTP header to choose which language to return a Accept-Language HTTP header to choose which language to return a
custom 404 page in: since it can return only one page, it must choose custom 404 page in: since it can return only one page, it must choose
a single item and it must return some item, even if no content a single item and it must return some item, even if no content
matches the language ranges supplied by the user. matches the language ranges supplied by the user.
Most types of matching in this document are designed so that Most types of matching in this document are designed so that
implementations do not have to examine the values of the subtags implementations do not have to examine the values of the subtags
supplied and, except for scored filtering, they do not need access to supplied and, except for scored filtering, they do not need access to
the Language Subtag Registry nor do they require the use of valid the Language Subtag Registry nor do they require the use of valid
subtags in either language tags or language ranges. This has great subtags in either language tags or language ranges. This has great
benefit for speed and simplicity of implementation. benefit for speed and simplicity of implementation.
Implementations might also wish to use semantic information external Implementations might also wish to use semantic information external
to the langauge tags when performing fallback. For example, the to the language tags when performing fallback. For example, the
primary language subtags 'nn' (Nynorsk Norwegian) and 'nb' (Bokmal primary language subtags 'nn' (Nynorsk Norwegian) and 'nb' (Bokmal
Norwegian) might both be usefully matched to the more general subtag Norwegian) might both be usefully matched to the more general subtag
'no' (Norwegian). Or an implementation might infer that content 'no' (Norwegian). Or an implementation might infer that content
labeled "zh-CN" is morely likely to match the range "zh-Hans" than labeled "zh-CN" is more likely to match the range "zh-Hans" than
equivalent content labeled "zh-TW". equivalent content labeled "zh-TW".
3.2. Filtering 3.2. Filtering
Filtering is used to select the set of content that matches a given Filtering is used to select the set of content that matches a given
prefix. It is called "filtering" because this set of content may prefix. It is called "filtering" because this set of content may
contain no items at all or it may return an arbitrary number of contain no items at all or it may return an arbitrary number of
matching items--as many as match the language range used to specify matching items--as many as match the language range used to specify
the items, thus filtering out the non-matching content. the items, thus filtering out the non-matching content.
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Some examples where filtering might be appropriate include: Some examples where filtering might be appropriate include:
o Applying a style to sections of a document in a particular o Applying a style to sections of a document in a particular
language range. language range.
o Displaying the set of documents containing a particular set of o Displaying the set of documents containing a particular set of
keywords written in a specific language. keywords written in a specific language.
o Selecting all email items written in specific range of languages. o Selecting all email items written in specific range of languages.
Filtering can produce either ordered or unordered set of results. Filtering can produce either an ordered or an unordered set of
For example, applying formatting to a document based on the language results. For example, applying formatting to a document based on the
of specific pieces of content does not require the content to be language of specific pieces of content does not require the content
ordered. It is sufficient to know whether a specific piece of to be ordered. It is sufficient to know whether a specific piece of
content matches or does not match. A search application, on the content matches or does not match. A search application, on the
other hand, probably would put the results into a priority order. other hand, probably would put the results into a priority order.
If an ordered set is desired, as described above, then the If an ordered set is desired, as described above, then the
application or protocol needs to determine the relative "quality" of application or protocol needs to determine the relative "quality" of
the match between different language tags and the language range. the match between different language tags and the language range.
This measurment is called a "distance metric". A distance metric This measurement is called a "distance metric". A distance metric
assigns a numeric value to the comparison of each language tag to a assigns a numeric value to the comparison of each language tag to a
language range and represents the 'distance' between the two. A language range and represents the 'distance' between the two. A
distance of zero means that they are identical, a small distance distance of zero means that they are identical, a small distance
indicates that they are very similar, and a large distance indicated indicates that they are very similar, and a large distance indicated
that they are very different. Using a distance metric, that they are very different. Using a distance metric,
implementations can, for example, allow users to select a threshold implementations can, for example, allow users to select a threshold
distance for a match to be "successful" while filtering or it can use distance for a match to be "successful" while filtering or it can use
the numeric value to order the results. the numeric value to order the results.
3.2.1. Filtering with Basic Language Ranges 3.2.1. Filtering with Basic Language Ranges
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in the language priority list is considered in turn, according to in the language priority list is considered in turn, according to
priority. The subtags in each extended language range are compared priority. The subtags in each extended language range are compared
to the corresponding subtags in the language tag being examined. The to the corresponding subtags in the language tag being examined. The
subtag from the range is considered to match if it exactly matches subtag from the range is considered to match if it exactly matches
the corresponding subtag in the tag or the range's subtag has the the corresponding subtag in the tag or the range's subtag has the
value "*" (which matches all subtags, including the empty subtag). value "*" (which matches all subtags, including the empty subtag).
Extended Range Matching is an extension of basic matching Extended Range Matching is an extension of basic matching
(Section 3.2.1): the language range represents the least specific tag (Section 3.2.1): the language range represents the least specific tag
which is an acceptable match. which is an acceptable match.
Private use subtags MAY be specified in the language range and MUST private-use subtags MAY be specified in the language range and MUST
NOT be ignored when matching. NOT be ignored when matching.
Subtags not specified, including those at the end of the language Subtags not specified, including those at the end of the language
range, are assigned the value "*". This makes each range into a range, are assigned the value "*". This makes each range into a
prefix much like that used in basic language range matching. For prefix much like that used in basic language range matching. For
example, the extended language range "zh-*-CN" matches all of the example, the extended language range "de-*-DE" matches all of the
following tags because the unspecified variant field is expanded to following tags because the unspecified variant field is expanded to
"*": "*":
zh-Hant-CN de-DE
zh-CN
zh-Hans-CN de-Latn-DE
zh-CN-x-wadegile de-Latf-DE
zh-Latn-CN-boont de-DE-x-goethe
zh-cmn-Hans-CN-x-private de-Latn-DE-1996
3.2.3. Distance Metric Filtering 3.2.3. Scored Filtering
Both basic and extended language range filtering produce simple Both basic and extended language range filtering produce simple
boolean matches. Sometimes it may be beneficial to provide an array boolean matches. Sometimes it may be beneficial to provide an array
of results with different levels of matching, for example, sorting of results with different levels of matching, for example, sorting
results based on the overall "quality" of the match. Distance metric results based on the overall "quality" of the match. Scored (or
filtering provides a way to generate these quality values. "distance metric") filtering provides a way to generate these quality
values.
First both the extended language range and the language tags to be First both the extended language range and the language tags to be
matched to it must be canonicalized by mapping grandfathered and matched to it must be canonicalized by mapping grandfathered and
obsolete tags into modern equivalents. obsolete tags into modern equivalents.
The language range and the language tags are then transformed into The language range and the language tags are then transformed into
quintuples of elements of the form (language, script, country, quintuples of elements of the form (language, script, country,
variant, extension). Any extended language subtags are considered variant, extension). Any extended language subtags are considered
part of the language element; private use subtag sequences are part of the language element; private-use subtag sequences are
considered part of the language element if in the initial position in considered part of the language element if in the initial position in
the tag and part of the variant element if not. Language subtags the tag and part of the variant element if not. Language subtags
'und', 'mul', and the script subtag 'Zyyy' are converted to "*". 'und', 'mul', and the script subtag 'Zyyy' are converted to "*".
Missing components in the language-tag are set to "*"; thus a "*" Missing components in the language-tag are set to "*"; thus a "*"
pattern becomes the quintuple ("*", "*", "*", "*", "*"). Missing pattern becomes the quintuple ("*", "*", "*", "*", "*"). Missing
components in the extended language-range are handled similarly to components in the extended language-range are handled similarly to
extended range lookup: missing internal subtags are expanded to "*". extended range lookup: missing internal subtags are expanded to "*".
Missing end subtags are expanded as the empty string. Thus a pattern Missing end subtags are expanded as the empty string. Thus a pattern
"en-US" becomes the quintuple ("en","*","US","",""). "en-US" becomes the quintuple ("en","*","US","","").
Here are some examples of language-tags and their quintuples: Here are some examples of language tags, showing their quintuples as
both language tags and language ranges:
en-US ("en","*","US","*","*") en-US
Tag: (en, *, US, *, *)
Range: (en, *, US, "", "")
sr-Latn ("sr,"Latn","*","*","*") sr-Latn
Tag: (sr, Latn, *, *, *)
Range: (sr, Latn, "", "", "")
zh-cmn-Hant ("zh-cmn","Hant","*","*","*") zh-cmn-Hant
Tag: (zh-cmn, Hant, *, *, *)
Range: (zh-cmn, Hant, "", "", "")
x-foo ("x-foo","*","*","*","*") x-foo
Tag: (x-foo, *, *, *, *)
Range: (x-foo, "", "", "", "")
en-x-foo ("en","*","*","x-foo","*") en-x-foo
Tag: (en, *, *, x-foo, *)
Range: (en, *, *, x-foo, "")
i-default ("i-default","*","*","*","*") i-default
Tag: (i-default, *, *, *, *)
Range: (i-default, "", "", "", "")
sl-Latn-IT-roazj ("sl","Latn","IT","rozaj","*") sl-Latn-IT-rozaj
Tag: (sl, Latn, IT, rozaj, *)
Range: (sl, Latn, IT, rozaj, "")
zh-r-wadegile ("zh","*","*","*","r-wadegile") // hypothetical zh-r-wadegile (hypothetical)
Tag: (z., *, *, *, r-wadegile)
Range: (z., *, *, *, r-wadegile)
Figure 3: Examples of Distance Metric Quintuples
Each language-range/language-tag pair being compared is assigned a Each language-range/language-tag pair being compared is assigned a
distance value, whereby small values indicate better matches and distance value, whereby small values indicate better matches and
large values indicate worse ones. The distance between the pair is large values indicate worse ones. The distance between the pair is
the sum of the distances for each of the corresponding elements of the sum of the distances for each of the corresponding elements of
the quintuple. If the elements are identical or one is '*', then the the quintuple. If the elements are identical or one is '*', then the
distance value between them is zero. Otherwise, it is given by the distance value between them is zero. Otherwise, it is given by the
following table: following table:
256 language mismatch 256 language mismatch
128 script mismatch 128 script mismatch
32 region mismatch 32 region mismatch
4 variant mismatch 4 variant mismatch
1 extension mismatch 1 extension mismatch
A value of 0 is a perfect match; 421 is no match at all. Different A value of 0 is a perfect match; 421 is no match at all. Different
threshold values might be appropriate for different applications or threshold values might be appropriate for different applications or
protocols. Implementations will usually allow users to choose the protocols. Implementations will usually allow users to choose the
most appropriate selection value, ranking the matched items based on most appropriate selection value, ranking the matched items based on
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1 extension mismatch 1 extension mismatch
A value of 0 is a perfect match; 421 is no match at all. Different A value of 0 is a perfect match; 421 is no match at all. Different
threshold values might be appropriate for different applications or threshold values might be appropriate for different applications or
protocols. Implementations will usually allow users to choose the protocols. Implementations will usually allow users to choose the
most appropriate selection value, ranking the matched items based on most appropriate selection value, ranking the matched items based on
score. score.
Examples of various tag's distances from the range "en-US": Examples of various tag's distances from the range "en-US":
"fr-FR" 384 (language & region mismatch)
"fr" 256 (language mismatch, region match) "fr" 256 (language mismatch, region match)
"en-GB" 384 (language, region mismatch) "en-GB" 32 (region mismatch)
"en-Latn-US" 0 (all fields match) "en-Latn-US" 0 (all fields match)
"en-Brai" 32 (region mismatch) "en-Brai" 32 (region mismatch)
"en-US-x-foo" 4 (variant mismatch: range is the empty string) "en-US-x-foo" 4 (variant mismatch: range is the empty string)
"en-US-r-wadegile" 1 (extension mismatch: range is the empty string) "en-US-r-wadegile" 1 (extension mismatch: range is the empty string)
Implementations or protocols sometimes might wish to use more Implementations or protocols sometimes might wish to use more
sophisticated weights that depend on the values of the corresponding sophisticated weights that depend on the values of the corresponding
elements. For example, depending on the domain, an implemenation elements. For example, depending on the domain, an implementation
might give a small distance to the difference between the language might give a small distance to the difference closely related
subtag 'no' and the closely related language subtags 'nb' or 'nn'; or subtags. Some examples of closely related subtags might be:
between the script subtags 'Kata' and 'Hira'; or between the region
subtags 'US' and 'UM'. Language:
no (Norwegian)
nb (Bokmal Norwegian)
nn (Nynorsk Norwegian)
Script:
Kata (katakana)
Hira (hiragana)
Region:
US (United States of America)
UM (United States Minor Outlying Islands
Figure 6: Examples of Closely Related Subtags
3.3. Lookup 3.3. Lookup
Lookup is used to select the single information item that best Lookup is used to select the single information item that best
matches the language priority list for a given request. In lookup, matches the language priority list for a given request. In lookup,
each language range in the language priority list represents the each language-range in the language priority list represents the
_most_ specific tag which is an acceptable match; only the closest _most_ specific tag which is an acceptable match; only the closest
matching item according the user's priority is returned. For matching item according the user's priority is returned. For
example, if the language range is "de-CH", one might expect to example, if the language range is "de-CH", one might expect to
receive an information item with the tag "de" but never one with the receive an information item with the tag "de" but never one with the
tag "de-CH-1996". Usually if no content matches the request, a tag "de-CH-1996". Usually if no content matches the request, a
"default" item is returned. "default" item is returned.
For example, if an application inserts some dynamic content into a For example, if an application inserts some dynamic content into a
document, returning an empty string if there is no exact match is not document, returning an empty string if there is no exact match is not
an option. Instead, the application "falls back" until it finds a an option. Instead, the application "falls back" until it finds a
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include: include:
o Selection of a template containing the text for an automated email o Selection of a template containing the text for an automated email
response. response.
o Selection of a graphic containing text for inclusion in a o Selection of a graphic containing text for inclusion in a
particular Web page. particular Web page.
o Selection of a string of text for inclusion in an error log. o Selection of a string of text for inclusion in an error log.
In the Lookup scheme, the language range is progressively truncated In the Lookup scheme, the language-range is progressively truncated
from the end until a matching piece of content is located. For from the end until a matching piece of content is located. For
example, starting with the range "zh-Hant-CN-x-private", the lookup example, starting with the range "zh-Hant-CN-x-private", the lookup
would progressively search for content as shown below: would progressively search for content as shown below:
Range to match: zh-Hant-CN-x-private Range to match: zh-Hant-CN-x-private
1. zh-Hant-CN-x-private 1. zh-Hant-CN-x-private
2. zh-Hant-CN 2. zh-Hant-CN
3. zh-Hant 3. zh-Hant
4. zh 4. z.
5. (default content or the empty tag) 5. (default content or the empty tag)
Figure 5: Example of a Lookup Fallback Pattern Figure 7: Example of a Lookup Fallback Pattern
This scheme allows some flexibility in finding content. It also This scheme allows some flexibility in finding content. It also
typically provides better results when data is not available at a typically provides better results when data is not available at a
specific level of tag granularity or is sparsely populated (than if specific level of tag granularity or is sparsely populated (than if
the default language for the system or content were used). the default language for the system or content were used).
The language range "*" matches any language tag. In the lookup The language range "*" matches any language tag. In the lookup
scheme, this language range does not convey enough information to scheme, this language range does not convey enough information to
determine which content is most appropriate. If this language range determine which content is most appropriate. If this language range
is the only one in the language priority list, it matches the default is the only one in the language priority list, it matches the default
content. If this language range is followed by other language content. If this language range is followed by other language
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to the end of the language priority list, rather than after each item to the end of the language priority list, rather than after each item
in the language priority list. in the language priority list.
For example, if a particular user's language priority list were For example, if a particular user's language priority list were
"fr-FR; zh-Hant" and the program doing the matching had a default "fr-FR; zh-Hant" and the program doing the matching had a default
language range of "ja-JP", the program would search for content as language range of "ja-JP", the program would search for content as
follows: follows:
1. fr-FR 1. fr-FR
2. fr 2. fr
3. zh-Hant // next language 3. zh-Hant // next language
4. zh 4. z.
5. (return default content) 5. (return default content)
a. ja-JP a. ja-JP
b. ja b. ja
c. (empty tag or other default content) c. (empty tag or other default content)
Figure 6: Lookup Using a Language Priority List Figure 8: Lookup Using a Language Priority List
In some cases, the language priority list might contain one or more In some cases, the language priority list might contain one or more
extended language ranges (as, for example, when the same language extended language ranges (as, for example, when the same language
priority list is used as input for both lookup and filtering priority list is used as input for both lookup and filtering
operations). Wildcard values in an extended language range are operations). Wildcard values in an extended language range are
supposed to match any value that occurs in that position in a supposed to match any value that occurs in that position in a
language tag. Since only one item can be returned for any given language tag. Since only one item can be returned for any given
lookup request, the wildcards must be processed in a predictable lookup request, the wildcards must be processed in a predictable
manner (or the same request might produce widely varying results). manner (or the same request might produce widely varying results).
Thus, for each range in the language priority list, the following Thus, for each range in the language priority list, the following
rules must be applied to produce a basic language range for use in rules must be applied to produce a basic language range for use in
the fallback mechanism: the fallback mechanism:
1. If the first subtag in the extended language range is a "*" then 1. If the first subtag in the extended language range is a "*" then
entire range is converted to "*". entire range is converted to "*".
2. For each subsequent subtag, if the value is a "*" then that 2. For each subsequent subtag, if the value is a "*" then that
subtag and its preceeding hyphen are removed. subtag and its preceding hyphen are removed.
For example: For example:
*-US becomes * *-US becomes *
en-*-US becomes en-US en-*-US becomes en-US
en-Latn-* becomes en-Latn en-Latn-* becomes en-Latn
Figure 7: Transformation of Extended Language Ranges Figure 9: Transformation of Extended Language Ranges
For the language priority list "*-US; fr-*-FR; zh-Hant", the fallback For the language priority list "*-US; fr-*-FR; zh-Hant", the fallback
pattern would be: pattern would be:
1. * (skipped) 1. * (skipped)
2. fr-FR 2. fr-FR
3. fr 3. fr
4. zh-Hant 4. zh-Hant
5. zh 5. z.
6. (default content) 6. (default content)
Figure 8: Extended Language Range Fallback Example Figure 10: Extended Language Range Fallback Example
4. Other Considerations 4. Other Considerations
When working with language ranges and matching schemes, there are When working with language ranges and matching schemes, there are
some additional points that may influence the choice of either. some additional points that may influence the choice of either.
4.1. Meaning of Language Tags and Ranges 4.1. Meaning of Language Tags and Ranges
Selecting content using language ranges requires some understanding Selecting content using language ranges requires some understanding
by users of what they are selecting. A language tag or range by users of what they are selecting. A language tag or range
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4.2. Considerations for Private Use Subtags 4.2. Considerations for Private Use Subtags
Private-use subtags require private agreement between the parties Private-use subtags require private agreement between the parties
that intend to use or exchange language tags that use them and great that intend to use or exchange language tags that use them and great
caution SHOULD be used in employing them in content or protocols caution SHOULD be used in employing them in content or protocols
intended for general use. Private-use subtags are simply useless for intended for general use. Private-use subtags are simply useless for
information exchange without prior arrangement. information exchange without prior arrangement.
The value and semantic meaning of private-use tags and of the subtags The value and semantic meaning of private-use tags and of the subtags
used within such a language tag are not defined. Matching private used within such a language tag are not defined. Matching private-
use tags using language ranges or extended language ranges can result use tags using language ranges or extended language ranges can result
in unpredictable content being returned. in unpredictable content being returned.
4.3. Length Considerations in Matching 4.3. Length Considerations in Matching
RFC 3066 [RFC3066] did not provide an upper limit on the size of RFC 3066 [RFC3066] did not provide an upper limit on the size of
language tags or ranges. RFC 3066 did define the semantics of language tags or ranges. RFC 3066 did define the semantics of
particular subtags in such a way that most language tags or ranges particular subtags in such a way that most language tags or ranges
consisted of language and region subtags with a combined total length consisted of language and region subtags with a combined total length
of up to six characters. Larger tags and ranges (in terms of both of up to six characters. Larger tags and ranges (in terms of both
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Implementations MUST support a limit of at least 33 characters. This Implementations MUST support a limit of at least 33 characters. This
limit includes at least one subtag of each non-extension, non-private limit includes at least one subtag of each non-extension, non-private
use type. When choosing a buffer limit, a length of at least 42 use type. When choosing a buffer limit, a length of at least 42
characters is strongly RECOMMENDED. characters is strongly RECOMMENDED.
The practical limit on tags or ranges derived solely from registered The practical limit on tags or ranges derived solely from registered
values is 42 characters. Implementations MUST be able to handle tags values is 42 characters. Implementations MUST be able to handle tags
and ranges of this length. Support for tags and ranges of at least and ranges of this length. Support for tags and ranges of at least
62 characters in length is RECOMMENDED. Implementations MAY support 62 characters in length is RECOMMENDED. Implementations MAY support
longer values, including matching extensive sets of private use or longer values, including matching extensive sets of private-use or
extension subtags. extension subtags.
Applications or protocols which have to truncate a tag MUST do so by Applications or protocols which have to truncate a tag MUST do so by
progressively removing subtags along with their preceding "-" from progressively removing subtags along with their preceding "-" from
the right side of the language tag until the tag is short enough for the right side of the language tag until the tag is short enough for
the given buffer. If the resulting tag ends with a single-character the given buffer. If the resulting tag ends with a single-character
subtag, that subtag and its preceding "-" MUST also be removed. For subtag, that subtag and its preceding "-" MUST also be removed. For
example: example:
Tag to truncate: zh-Latn-CN-variant1-a-extend1-x-wadegile-private1 Tag to truncate: zh-Latn-CN-variant1-a-extend1-x-wadegile-private1
1. zh-Latn-CN-variant1-a-extend1-x-wadegile 1. zh-Latn-CN-variant1-a-extend1-x-wadegile
2. zh-Latn-CN-variant1-a-extend1 2. zh-Latn-CN-variant1-a-extend1
3. zh-Latn-CN-variant1 3. zh-Latn-CN-variant1
4. zh-Latn-CN 4. zh-Latn-CN
5. zh-Latn 5. zh-Latn
6. zh 6. z.
Figure 9: Example of Tag Truncation Figure 11: Example of Tag Truncation
5. IANA Considerations 5. IANA Considerations
This document presents no new or existing considerations for IANA. This document presents no new or existing considerations for IANA.
6. Changes 6. Changes
This is the first version of this document. This is the first version of this document.
The following changes were put into this document since draft-05: The following changes were put into this document since draft-06:
Modified the ABNF to match changes in [RFC3066bis] (K.Karlsson)
Matched the references and reference formats to [RFC3066bis]
(K.Karlsson)
Various edits, additions, and emendations to deal with changes in
the Last Call of draft-registry as well as cleaning up the text.
Changed from 'defined' to 'identifies' in Section 4.1. (M.Gunn) Changed the document title from the unwieldy "Matching Tags for
the Identification of Languages" to "Matching Language Tags" (Ed.)
Reorganized the text and broke it into sections (M.Duerst) Fixed problems with the distance metric filtering scheme
(Section 3.2.3) examples (in which tags were expanded
incorrectly). (D.Ewell)
Modified occurences of the word "application" to refer to Moved the sentence "Protocols and specifications SHOULD clearly
"applications or protocols" or otherwise be specific (E. van der indicate the particular mechanism used in selecting or matching
Poel) language tags." from the introduction (where there should not be
any normative language) to the start of Section 3. (A.Phillips)
Removed "Extended Language Range Lookup", merging it with other Created section Section 2.4 and moved text there (A.Phillips)
text on lookup to form a single scheme. (M.Davis)
Fixed or removed obsolete or dangling references (Ed.) Modified the examples of closely related subtags in Section 3.2.3
to show what the examples mean (M.Duerst)
Added an introduction to section 4 and added one sentence to make Various spelling and grammatical fixes (D.Ewell)
it flow better to the start of section 4.1. (Ed.)
7. Security Considerations 7. Security Considerations
Language ranges used in content negotiation might be used to infer Language ranges used in content negotiation might be used to infer
the nationality of the sender, and thus identify potential targets the nationality of the sender, and thus identify potential targets
for surveillance. In addition, unique or highly unusual language for surveillance. In addition, unique or highly unusual language
ranges or combinations of language ranges might be used to track ranges or combinations of language ranges might be used to track a
specific individual's activities. specific individual's activities.
This is a special case of the general problem that anything you send This is a special case of the general problem that anything you send
is visible to the receiving party. It is useful to be aware that is visible to the receiving party. It is useful to be aware that
such concerns can exist in some cases. such concerns can exist in some cases.
The evaluation of the exact magnitude of the threat, and any possible The evaluation of the exact magnitude of the threat, and any possible
countermeasures, is left to each application or protocol. countermeasures, is left to each application or protocol.
8. Character Set Considerations 8. Character Set Considerations
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contributed to make this document what it is today. contributed to make this document what it is today.
The contributors to [RFC3066bis], [RFC3066] and [RFC1766], each of The contributors to [RFC3066bis], [RFC3066] and [RFC1766], each of
which is a precursor to this document, made enormous contributions which is a precursor to this document, made enormous contributions
directly or indirectly to this document and are generally responsible directly or indirectly to this document and are generally responsible
for the success of language tags. for the success of language tags.
The following people (in alphabetical order by family name) The following people (in alphabetical order by family name)
contributed to this document: contributed to this document:
Jeremy Carroll, John Cowan, Martin Duerst, Frank Ellermann, Doug Harald Alvestrand, Jeremy Carroll, John Cowan, Martin Duerst, Frank
Ewell, Marion Gunn, Kent Karlsson, Ira McDonald, M. Patton, Randy Ellermann, Doug Ewell, Marion Gunn, Kent Karlsson, Ira McDonald, M.
Presuhn, Eric van der Poel, and many, many others. Patton, Randy Presuhn, Eric van der Poel, and many, many others.
Very special thanks must go to Harald Tveit Alvestrand, who Very special thanks must go to Harald Tveit Alvestrand, who
originated RFCs 1766 and 3066, and without whom this document would originated RFCs 1766 and 3066, and without whom this document would
not have been possible. not have been possible.
For this particular document, John Cowan originated the scheme For this particular document, John Cowan originated the scheme
described in Section 3.2.3. Mark Davis originated the scheme described in Section 3.2.3. Mark Davis originated the scheme
described in the Section 3.3. described in the Section 3.3.
Authors' Addresses Authors' Addresses
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