Network Working Group A. Phillips, Ed.
Internet-Draft Quest Software
Expires: March 27, 2006 M. Davis, Ed.
IBM
September 23, 2005
Matching Tags for the Identification of Languages
draft-ietf-ltru-matching-04
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes different mechanisms for comparing, matching,
and evaluating language tags. Possible algorithms for language
negotiation and content selection are described.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The Language Range . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Basic Language Range . . . . . . . . . . . . . . . . . . . 4
2.1.1 Matching . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.2 Lookup . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Extended Language Range . . . . . . . . . . . . . . . . . 7
2.2.1 Extended Range Matching . . . . . . . . . . . . . . . 8
2.2.2 Extended Range Lookup . . . . . . . . . . . . . . . . 9
2.2.3 Distance Metric Scheme . . . . . . . . . . . . . . . . 10
2.3 Meaning of Language Tags and Ranges . . . . . . . . . . . 12
2.4 Choosing Between Alternate Matching Schemes . . . . . . . 13
2.5 Considerations for Private Use Subtags . . . . . . . . . . 14
2.6 Length Considerations in Matching . . . . . . . . . . . . 14
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
4. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5. Security Considerations . . . . . . . . . . . . . . . . . . . 19
6. Character Set Considerations . . . . . . . . . . . . . . . . . 20
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.1 Normative References . . . . . . . . . . . . . . . . . . . 21
7.2 Informative References . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 23
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . 25
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1. Introduction
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
language used when presenting or requesting information.
Information about a user's language preferences commonly needs to be
identified so that appropriate processing can be applied. For
example, the user's language preferences in a browser can be used to
select web pages appropriately. A choice of language preference can
also be used to select among tools (such as dictionaries) to assist
in the processing or understanding of content in different languages.
Given a set of language identifiers, such as those defined in [draft-
registry], various mechanisms can be envisioned for performing
language negotiation and tag matching. The suitability of a
particular mechanism to a particular application depends on the needs
of that application.
This document defines several mechanisms for matching and filtering
natural language content identified using Language Tags [draft-
registry]. It also defines the syntax (called a "language range")
associated with each of these mechanisms for specifying user language
preferences.
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. The Language Range
Language Tags [draft-registry] are used to identify the language of
some information item or content. Applications that use language
tags are often faced with the problem of identifying sets of content
that share certain language attributes. For example, HTTP 1.1
[RFC2616] describes language ranges in its discussion of the Accept-
Language header (Section 14.4), which is used for selecting content
from servers based on the language of that content.
When selecting content according to its language, it is useful to
have a mechanism for identifying sets of language tags that share
specific attributes. This allows users to select or filter content
based on specific requirements. Such an identifier is called a
"Language Range".
2.1 Basic Language Range
A basic language range identifies the set of content whose language
tags begin with the same sequence of subtags. Basic language ranges
are described in [RFC3066] and HTTP 1.1 [RFC2616] (where they are
referred to as simply a "language range"). A basic language range is
identified by its 'language-range' tag, by adapting the
ABNF[RFC2234bis] from HTTP/1.1 [RFC2616] :
language-range = language-tag / "*"
language-tag = 1*8[alphanum] *["-" 1*8alphanum]
alphanum = ALPHA / DIGIT
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
a semantic relationship between language tags that share the same
prefix. While this is often the case, it is not always true.
In particular, the set of language tags that match a specific
language-range might not all be mutually intelligible. Matching a
language-range (prefix) to various language tags does not mean that
it is always true that if a user understands a language identified by
a certain tag, then this user will also understand all of the
languages for which this tag is a prefix. The use of prefixes (and
thus basic language ranges) simply allows the use of a prefix if this
is the case.
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.
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When working with tags and ranges users SHOULD also note the
following:
1. Private-use and Extension subtags are normally orthogonal to
language tag fallback. Implementations SHOULD ignore
unrecognized private-use and extension subtags when performing
language tag fallback. 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.
2. Implementations that choose not to interpret one or more private-
use or extension subtags SHOULD NOT remove or modify these
extensions in content that they are processing. When a language
tag instance is to be used in a specific, known protocol, and is
not being passed through to other protocols, language tags MAY be
filtered to remove subtags and extensions that are not supported
by that protocol. Such filtering SHOULD be avoided, if possible,
since it removes information that might be relevant if services
on the other end of the protocol would make use of that
information.
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 are included in a matching or
filtering process that utilizes the one of the schemes described
in this document, then the implementation SHOULD canonicalize the
language tags and/or ranges before performing the matching. Note
that language tag processors that claim to be "well-formed"
processors as defined in [draft-registry] generally fall into
this category.
There are two matching schemes that are commonly associated with
basic language ranges: matching and lookup.
Note that neither matching nor lookup using basic language ranges
attempt 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. The choice of
subtags in both the language tag and language range may affect the
results produced as a result.
2.1.1 Matching
Language tag matching is used to select all content that matches a
given prefix. In matching, the language range represents the least
specific tag which is an acceptable match and every piece of content
that matches is returned.
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For example, if an application is applying a style to all content in
a document in a particular language, it might use language tag
matching to select the content to which the style is applied.
A language-range matches a language-tag if it exactly equals the tag,
or if it exactly equals a prefix of the tag such that the first
character following the prefix is "-". (That is, the language-range
"de-de" matches the language tag "de-DE-1996", but not the language
tag "de-Deva".)
The special range "*" matches any tag. A protocol which uses
language ranges MAY specify additional rules about the semantics of
"*"; for instance, HTTP/1.1 specifies that the range "*" matches only
languages not matched by any other range within an "Accept-Language"
header.
2.1.2 Lookup
Content lookup is used to select the single information item that
best matches the language range for a given request. In lookup, the
language range represents the most specific tag which is an
acceptable match and only the closest matching item is returned.
For example, if an application inserts some dynamic content into a
document, returning an empty string if there is no exact match is not
an option. Instead, the application "falls back" until it finds a
suitable piece of content to insert.
When performing lookup, the language range is progressively truncated
from the end until a matching piece of content is located. For
example, starting with the range "zh-Hant-CN-x-wadegile", the lookup
would progressively search for content as shown below:
Range to match: zh-Hant-CN-x-wadegile
1. zh-Hant-CN-x-wadegile
2. zh-Hant-CN
3. zh-Hant
4. zh
5. (default content or the empty tag)
Figure 2: Default Fallback Pattern Example
This scheme allows some flexibility in finding content. It also
typically provides better results when data is not available at a
specific level of tag granularity or is sparsely populated (than if
the default language for the system or content were used).
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2.2 Extended Language Range
Prefix matching using a Basic Language Range, as described above, is
not always the most appropriate way to access the information
contained in language tags when selecting or filtering content. Some
applications might wish to define a more granular matching scheme and
such a matching scheme requires the ability to specify the various
attributes of a language tag in the language range. An extended
language range can be represented by the following ABNF:
extended-language-range = range ; a range
/ privateuse ; private use tag
/ grandfathered ; grandfathered registrations
range = (language
["-" script]
["-" region]
*("-" variant)
*("-" extension)
["-" privateuse])
language = (2*3ALPHA [ extlang ]) ; shortest ISO 639 code
/ 4ALPHA ; reserved for future use
/ 5*8ALPHA ; registered language subtag
/ "*" ; ... or wildcard
extlang = *2("-" 3ALPHA) ("-" ( 3ALPHA / "*"))
; reserved for future use
; wildcard can only appear
; at the end
script = 4ALPHA ; ISO 15924 code
/ "*" ; or wildcard
region = 2ALPHA ; ISO 3166 code
/ 3DIGIT ; UN M.49 code
/ "*" ; ... or wildcard
variant = 5*8alphanum ; registered variants
/ (DIGIT 3alphanum) ;
/ "*" ; ... or wildcard
extension = singleton *("-" (2*8alphanum)) [ "-*" ]
; extension subtags
; wildcard can only appear
; at the end
singleton = %x41-57 / %x59-5A / %x61-77 / %x79-7A / DIGIT
; "a"-"w" / "y"-"z" / "A"-"W" / "Y"-"Z" / "0"-"9"
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; Single letters: x/X is reserved for private use
privateuse = ("x"/"X") 1*("-" (1*8alphanum))
grandfathered = 1*3ALPHA 1*2("-" (2*8alphanum))
; grandfathered registration
; Note: i is the only singleton
; that starts a grandfathered tag
alphanum = (ALPHA / DIGIT) ; letters and numbers
In an extended language range, the identifier takes the form of a
series of subtags which must consist of well-formed subtags or the
special subtag "*". For example, the language range "en-*-US"
specifies a primary language of 'en', followed by any script subtag,
followed by the region subtag 'US'.
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
"en-US" MAY be considered to be equivalent to the range "en-*-US".
This also means that multiple wildcards can be collapsed (so that
"en-*-*-US" is equivalent to "en-*-US").
There are several matching algorithms or schemes which can be applied
when matching extended language ranges to language tags.
2.2.1 Extended Range Matching
In extended range matching, the subtags in a language tag are
compared to the corresponding subtags in the extended language range.
A subtag is considered to match if it exactly matches the
corresponding subtag in the range or the range contains a subtag with
the value "*" (which matches all subtags, including the empty
subtag). Extended Range Matching is an extension of basic matching
(Section 2.1.1): the language range represents the least specific tag
which is an acceptable match.
By default all extensions and their subtags are ignored for extended
language range matching.
Private use subtags MAY be specified in the language range and MUST
NOT be ignored when matching.
Subtags not specified, including those at the end of the language
range, are assigned the value "*". This makes each range into a
prefix much like that used in basic language range matching. For
example, the extended language range "zh-*-CN" matches all of the
following tags because the unspecified variant field is expanded to
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"*":
zh-Hant-CN
zh-CN
zh-Hans-CN
zh-CN-x-wadegile
zh-Latn-CN-boont
zh-cmn-Hans-CN-x-wadegile
2.2.2 Extended Range Lookup
In extended range lookup, the subtags in a language tag are compared
to the corresponding subtags in the extended language range. The
subtag is considered to match if it exactly matches the corresponding
subtag in the range or the range contains a subtag with the value "*"
(which matches all subtags, including the empty subtag). Extended
language range lookup is an extension of basic lookup
(Section 2.1.2): the language range represents the most specific tag
which will form an acceptable match.
Subtags not specified are assigned the value "*" prior to performing
tag matching. Unlike in extended range matching, however, fields at
the end of the range MUST NOT be expanded in this manner. For
example, "en-US" MUST NOT be considered to be the same as the range
"en-US-*". This allows ranges to be specific. The "*" wildcard MUST
be used at the end of the range to indicate that all tags with the
range as a prefix are allowable matches. That is, the range "zh-*"
matches the tags "zh-Hant" and "zh-Hant-CN", while the range "zh"
matches neither of those tags.
The wildcard "*" at the end of a range SHOULD be considered to match
any private use subtag sequences (making extended language range
lookup function exactly like extended range matching Section 2.2.1).
By default all extensions and their subtags SHOULD be ignored for
extended language range lookup. Private use subtags MAY be specified
in the language range and MUST NOT be ignored when performing lookup.
The wildcard "*" at the end of a range SHOULD be considered to match
any private use subtag sequences in addition to variants.
For example, the range "*-US" matches all of the following tags:
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en-US
en-Latn-US
en-US-r-extends (extensions are ignored)
fr-US
For example, the range "en-*-US" matches _none_ of the following
tags:
fr-US
en (missing region US)
en-Latn (missing region US)
en-Latn-US-scouse (variant field is present)
For example, the range "en-*" matches all of the following tags:
en-Latn
en-Latn-US
en-Latn-US-scouse
en-US
en-scouse
Note that the ability to be specific in extended range lookup can
make this matching scheme a more appropriate replacement for basic
matching than the extended range matching scheme.
2.2.3 Distance Metric Scheme
Both Basic and Extended Language Ranges produce simple boolean
matches. Some applications may benefit by providing an array of
results with different levels of matching, for example, sorting
results based on the overall "quality" of the match.
This type of matching is sometimes called a "distance metric". A
distance metric assigns a pair of language tags a numeric value
representing the 'distance' between the two. A distance of zero
means that they are identical, a small distance indicates that they
are very similar, and a large distance indicated that they are very
different. Using a distance metric, implementations can, for
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example, allow users to select a threshold distance for a match to be
successful or a filter to be applied.
The first step in the process is to normalize the extended language
range and the language tags to be matched to it by canonicalizing
them, mapping grandfathered and obsolete tags into modern
equivalents.
The language range and the language tags are then transformed into
quintuples of elements of the form (language, script, country,
variant, extension). Any extended language subtags are considered
part of the language element; private use subtag sequences are
considered part of the language element if in the initial position in
the tag and part of the variant element if not. Language subtags
'und', 'mul', and the script subtag 'Zyyy' are converted to "*".
Missing components in the language-tag are set to "*"; thus a "*"
pattern becomes the quintuple ("*", "*", "*", "*", "*"). Missing
components in the extended language-range are handled similarly to
extended range lookup: missing internal subtags are expanded to "*".
Missing end subtags are expanded as the empty string. Thus a pattern
"en-US" becomes the quintuple ("en","*","US","","").
Here are some examples of language-tags and their quintuples:
en-US ("en","*","US","*","*")
sr-Latn ("sr,"Latn","*","*","*")
zh-cmn-Hant ("zh-cmn","Hant","*","*","*")
x-foo ("x-foo","*","*","*","*")
en-x-foo ("en","*","*","x-foo","*")
i-default ("i-default","*","*","*","*")
sl-Latn-IT-roazj ("sl","Latn","IT","rozaj","*")
zh-r-wadegile ("zh","*","*","*","r-wadegile") // hypothetical
Each language-range/language-tag pair being matched or filtered is
assigned a distance value, whereby small values indicate better
matches and large values indicate worse ones. The distance between
the pair is the sum of the distances for each of the corresponding
elements of the quintuple. If the elements are identical or one is
'*', then the distance value between them is zero. Otherwise, it is
given by the following table:
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256 language mismatch
128 script mismatch
32 region mismatch
4 variant mismatch
1 extension mismatch
A value of 0 is a perfect match; 421 is no match at all. Different
threshold values might be appropriate for different applications and
implementations will probably allow users to choose the most
appropriate selection value, ranking the selections based on score.
Examples of various tag's distances from the range "en-US":
"fr" 256 (language mismatch, region match)
"en-GB" 384 (language, region mismatch)
"en-Latn-US" 0 (all fields match)
"en-Brai" 32 (region mismatch)
"en-US-x-foo" 4 (variant mismatch: range is the empty string)
"en-US-r-wadegile" 1 (extension mismatch: range is the empty string)
Implementations may want to use more sophisticated weights that
depend on the values of the corresponding elements. For example,
depending on the domain, an implemenation might give a small distance
to the difference between the language subtag 'no' and the closely
related language subtags 'nb' or 'nn'; or between the script subtags
'Kata' and 'Hira'; or between the region subtags 'US' and 'UM'.
2.3 Meaning of Language Tags and Ranges
A language tag defines a language as spoken (or written, signed or
otherwise signaled) by human beings for communication of information
to other human beings.
If a language tag B contains language tag A as a prefix, then B is
typically "narrower" or "more specific" than A. For example, "zh-
Hant-TW" is more specific than "zh-Hant".
This relationship is not guaranteed in all cases: specifically,
languages that begin with the same sequence of subtags are NOT
guaranteed to be mutually intelligible, although they might be.
For example, the tag "az" shares a prefix with both "az-Latn"
(Azerbaijani written using the Latin script) and "az-Cyrl"
(Azerbaijani written using the Cyrillic script). A person fluent in
one script might not be able to read the other, even though the text
might be otherwise identical. Content tagged as "az" most probably
is written in just one script and thus might not be intelligible to a
reader familiar with the other script.
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Variant subtags in particular seem to represent specific divisions in
mutual understanding, since they often encode dialects or other
idiosyncratic variations within a language.
The relationship between the language tag and the information it
relates to is defined by the standard describing the context in which
it appears. Accordingly, this section can only give possible
examples of its usage.
o For a single information object, the associated language tags
might be interpreted as the set of languages that are necessary
for a complete comprehension of the complete object. Example:
Plain text documents.
o For an aggregation of information objects, the associated language
tags could be taken as the set of languages used inside components
of that aggregation. Examples: Document stores and libraries.
o For information objects whose purpose is to provide alternatives,
the associated language tags could be regarded as a hint that the
content is provided in several languages, and that one has to
inspect each of the alternatives in order to find its language or
languages. In this case, the presence of multiple tags might not
mean that one needs to be multi-lingual to get complete
understanding of the document. Example: MIME multipart/
alternative.
o In markup languages, such as HTML and XML, language information
can be added to each part of the document identified by the markup
structure (including the whole document itself). For example, one
could write <span lang="FR">C'est la vie.</span> inside a
Norwegian document; the Norwegian-speaking user could then access
a French-Norwegian dictionary to find out what the marked section
meant. If the user were listening to that document through a
speech synthesis interface, this formation could be used to signal
the synthesizer to appropriately apply French text-to-speech
pronunciation rules to that span of text, instead of misapplying
the Norwegian rules.
2.4 Choosing Between Alternate Matching Schemes
Implementations can choose to implement different styles of matching
for different kinds of processing. For example, an implementation
could treat an absent script subtag as a "wildcard" field; thus
"az-AZ" would match "az-AZ", "az-Cyrl-AZ", "az-Latn-AZ", etc. but not
"az" (this is an example of extended range lookup). If one item is
to be chosen, the implementation could pick among those matches based
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on other information, such as the most likely script used in the
language/region in question or the script used by other content
selected.
Because the primary language subtag cannot be absent in a language
tag, the 'und' subtag is sometimes be used as a 'wildcard' in basic
matching. For example, in a query where you want to select all
language tags that contain 'Latn' as the script code and 'AZ' as the
region code, you could use the range "und-Latn-AZ". This requires an
implementation to examine the actual values of the subtags, though,
and users SHOULD NOT assume that the value 'und' will be treated as a
wildcard.
The matching schemes described elsewhere in this document are
designed such that implementations do not have to examine the values
of the subtags supplied and, except for scored matching, they do not
need access to the Language Subtag Registry nor do they require the
use of valid subtags in language tags or ranges. This has great
benefit for speed and simplicity of implementation.
Implementations might also wish to use semantic information external
to the langauge tags when performing fallback. For example, the
primary language subtags 'nn' (Nynorsk Norwegian) and 'nb' (Bokmal
Norwegian) might both be usefully matched to the more general subtag
'no' (Norwegian). Or an application might infer that content labeled
"zh-CN" is morely likely to match the range "zh-Hans" than equivalent
content labeled "zh-TW".
2.5 Considerations for Private Use Subtags
Private-use subtags require private agreement between the parties
that intend to use or exchange language tags that use them and great
caution SHOULD be used in employing them in content or protocols
intended for general use. Private-use subtags are simply useless for
information exchange without prior arrangement.
The value and semantic meaning of private-use tags and of the subtags
used within such a language tag are not defined. Matching private
use tags using language ranges or extended language ranges can result
in unpredictable content being returned.
2.6 Length Considerations in Matching
RFC 3066 [RFC3066] did not provide an upper limit on the size of
language tags or ranges. RFC 3066 did define the semantics of
particular subtags in such a way that most language tags or ranges
consisted of language and region subtags with a combined total length
of up to six characters. Larger tags and ranges (in terms of both
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subtags and characters) did exist, however.
[draft-registry] also does not impose a fixed upper limit on the
number of subtags in a language tag or range (and thus an upper bound
on the size of either). The syntax in that document suggests that,
depending on the specific language or range of languages, more
subtags (and thus characters) are sometimes necessary as a result.
Length considerations and their impact on the selection and
processing of tags are described in Section 2.1.1 of that document.
A matching implementation MAY choose to limit the length of the
language tags or ranges used in matching. Any such limitation SHOULD
be clearly documented, and such documentation SHOULD include the
disposition of any longer tags or ranges (for example, whether an
error value is generated or the language tag or range is truncated).
If truncation is permitted it MUST NOT permit a subtag to be divided,
since this changes the semantics of the subtag being matched and can
result in false positives or negatives.
Implementations that restrict storage SHOULD consider the impact of
tag or range truncation on the resulting matches. For example,
removing the "*" from the end of an extended language range (see
Section 2.2) can greatly modify the set of returned matches. A
protocol that allows tags or ranges to be truncated at an arbitrary
limit, without giving any indication of what that limit is, has the
potential for causing harm by changing the meaning of values in
substantial ways.
In practice, most tags do not require additional subtags or
substantially more characters. Additional subtags sometimes add
useful distinguishing information, but extraneous subtags interfere
with the meaning, understanding, and especially matching of language
tags. Since language tags or ranges MAY be truncated by an
application or protocol that limits storage, when choosing language
tags or ranges users and applications SHOULD avoid adding subtags
that add no distinguishing value. In particular, users and
implementations SHOULD follow the 'Prefix' and 'Suppress-Script'
fields in the registry (defined in Section 3.6 of [draft-registry]):
these fields provide guidance on when specific additional subtags
SHOULD (and SHOULD NOT) be used.
Implementations MUST support a limit of at least 33 characters. This
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
characters is strongly RECOMMENDED.
The practical limit on tags or ranges derived solely from registered
values is 42 characters. Implementations MUST be able to handle tags
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and ranges of this length. Support for tags and ranges of at least
62 characters in length is RECOMMENDED. Implementations MAY support
longer values, including matching extensive sets of private use or
extension subtags.
Applications or protocols which have to truncate a tag MUST do so by
progressively removing subtags along with their preceding "-" from
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
subtag, that subtag and its preceding "-" MUST also be removed. For
example:
Tag to truncate: zh-Hant-CN-variant1-a-extend1-x-wadegile-private1
1. zh-Hant-CN-variant1-a-extend1-x-wadegile
2. zh-Hant-CN-variant1-a-extend1
3. zh-Hant-CN-variant1
4. zh-Hant-CN
5. zh-Hant
6. zh
Figure 6: Example of Tag Truncation
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3. IANA Considerations
This document presents no new or existing considerations for IANA.
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4. Changes
This is the first version of this document.
The following changes were put into this document since draft-03:
Modified the ABNF to match changes in [draft-registry]
(K.Karlsson)
Matched the references and reference formats to [draft-registry]
(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.
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5. Security Considerations
Language ranges used in content negotiation might be used to infer
the nationality of the sender, and thus identify potential targets
for surveillance. In addition, unique or highly unusual language
ranges or combinations of language ranges might be used to track
specific individual's activities.
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
such concerns can exist in some cases.
The evaluation of the exact magnitude of the threat, and any possible
countermeasures, is left to each application protocol.
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6. Character Set Considerations
The syntax of language tags and language ranges permit only the
characters A-Z, a-z, 0-9, and HYPHEN-MINUS (%x2D). These characters
are present in most character sets, so presentation of language tags
should not present any character set issues.
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7. References
7.1 Normative References
[ID.ietf-ltru-initial]
Ewell, D., Ed., "Language Tags Initial Registry (work in
progress)", August 2005, <http://www.ietf.org/
internet-drafts/draft-ietf-ltru-initial-04.txt>.
[RFC1327] Hardcastle-Kille, S., "Mapping between X.400(1988) / ISO
10021 and RFC 822", RFC 1327, May 1992.
[RFC1521] Borenstein, N. and N. Freed, "MIME (Multipurpose Internet
Mail Extensions) Part One: Mechanisms for Specifying and
Describing the Format of Internet Message Bodies",
RFC 1521, September 1993.
[RFC2028] Hovey, R. and S. Bradner, "The Organizations Involved in
the IETF Standards Process", BCP 11, RFC 2028,
October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Word Extensions: Character Sets, Languages, and
Continuations", RFC 2231, November 1997.
[RFC2234bis]
Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", draft-crocker-abnf-rfc2234bis-00
(work in progress), March 2005.
[RFC2396] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2860] Carpenter, B., Baker, F., and M. Roberts, "Memorandum of
Understanding Concerning the Technical Work of the
Internet Assigned Numbers Authority", RFC 2860, June 2000.
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[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[draft-registry]
Phillips, A., Ed. and M. Davis, Ed., "Tags for the
Identification of Languages (work in progress)",
August 2005, <http://www.ietf.org/internet-drafts/
draft-ietf-ltru-registry-12.txt>.
7.2 Informative References
[ISO15924]
"ISO 15924:2004. Information and documentation -- Codes
for the representation of names of scripts", January 2004.
[ISO3166-1]
"ISO 3166-1:1997. Codes for the representation of names of
countries and their subdivisions -- Part 1: Country
codes", 1997.
[ISO639-1]
"ISO 639-1:2002. Codes for the representation of names of
languages -- Part 1: Alpha-2 code", 2002.
[ISO639-2]
"ISO 639-2:1998. Codes for the representation of names of
languages -- Part 2: Alpha-3 code, first edition", 1998.
[RFC1766] Alvestrand, H., "Tags for the Identification of
Languages", RFC 1766, March 1995.
[RFC3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, July 2002.
[UN_M.49] Statistics Division, United Nations, "Standard Country or
Area Codes for Statistical Use", UN Standard Country or
Area Codes for Statistical Use, Revision 4 (United Nations
publication, Sales No. 98.XVII.9, June 1999.
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Authors' Addresses
Addison Phillips (editor)
Quest Software
Email: addison dot phillips at quest dot com
Mark Davis (editor)
IBM
Email: mark dot davis at ibm dot com
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Appendix A. Acknowledgements
Any list of contributors is bound to be incomplete; please regard the
following as only a selection from the group of people who have
contributed to make this document what it is today.
The contributors to [draft-registry], [RFC3066] and [RFC1766], each
of which is a precursor to this document, made enormous
contributions directly or indirectly to this document and are
generally responsible for the success of language tags.
The following people (in alphabetical order by family name)
contributed to this document:
Jeremy Carroll, John Cowan, Frank Ellermann, Doug Ewell, Kent
Karlsson, Ira McDonald, M. Patton, Randy Presuhn and many, many
others.
Very special thanks must go to Harald Tveit Alvestrand, who
originated RFCs 1766 and 3066, and without whom this document would
not have been possible.
For this particular document, John Cowan originated the scheme
described in Section 2.2.3. Mark Davis originated the scheme
described in the Section 2.1.2.
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