Network Working Group A. Phillips, Ed.
Internet-Draft Quest Software
Obsoletes: 3066 (if approved) M. Davis, Ed.
Expires: May 20, 2006 IBM
November 16, 2005
Matching Tags for the Identification of Languages
draft-ietf-ltru-matching-06
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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. This document, in
combination with RFC 3066bis (replace "3066bis" with the RFC number
assigned to draft-ietf-ltru-registry-14), replaces RFC 3066, which
replaced RFC 1766.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The Language Range . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Lists of Language Ranges . . . . . . . . . . . . . . . . . 4
2.2. Basic Language Range . . . . . . . . . . . . . . . . . . . 4
2.3. Extended Language Range . . . . . . . . . . . . . . . . . 5
3. Types of Matching . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Choosing a Type of Matching . . . . . . . . . . . . . . . 8
3.2. Filtering . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2.1. Filtering with Basic Language Ranges . . . . . . . . . 10
3.2.2. Filtering with Extended Language Ranges . . . . . . . 10
3.2.3. Distance Metric Filtering . . . . . . . . . . . . . . 11
3.3. Lookup . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4. Other Considerations . . . . . . . . . . . . . . . . . . . . . 16
4.1. Meaning of Language Tags and Ranges . . . . . . . . . . . 16
4.2. Considerations for Private Use Subtags . . . . . . . . . . 17
4.3. Length Considerations in Matching . . . . . . . . . . . . 17
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
6. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7. Security Considerations . . . . . . . . . . . . . . . . . . . 22
8. Character Set Considerations . . . . . . . . . . . . . . . . . 23
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
9.1. Normative References . . . . . . . . . . . . . . . . . . . 24
9.2. Informative References . . . . . . . . . . . . . . . . . . 24
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
Intellectual Property and Copyright Statements . . . . . . . . . . 27
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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. Language preferences 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
[RFC3066bis], various mechanisms can be envisioned for performing
language negotiation and tag matching. Applications, protocols, or
specifications will have varying needs and requirements that will
affect the choice of a suitable mechanism. Protocols and
specifications SHOULD clearly indicate the particular mechanism used
in selecting or matching language tags.
This document defines several mechanisms for matching, selecting, or
filtering content whose natural language is identified using Language
Tags [RFC3066bis], as well as the syntax (called a "language range")
associated with each of these mechanisms for specifying the user's
language preferences.
This document, in combination with [RFC3066bis] (replace "3066bis"
globally in this document with the RFC number assigned to
draft-ietf-ltru-registry-14), replaces [RFC3066], which replaced
[RFC1766].
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 [RFC3066bis] are used to identify the language of some
information item or content. Applications or protocols 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. Lists of Language Ranges
When users specify a language preference they often need to specify a
prioritized list of language ranges in order to best reflect their
language requirements for the matching operation. This is especially
true for speakers of minority languages. A speaker of Breton in
France, for example, may specify "be" followed by "fr", meaning that
if Breton is available, it is preferred, but otherwise French is the
best alternative. It can get more complex: a speaker may wish to
fallback from Skolt Sami to Northern Sami to Finnish.
A "Language Priority List" consists of a prioritized or weighted list
of language ranges. One well known example of such a list is the
"Accept-Language" header defined in RFC 2616 [RFC2616] (see Section
14.4) and RFC 3282 [RFC3282]. The various matching operations
described in this document include considerations for using a
language priority list.
2.2. Basic Language Range
A "Basic Language Range" identifies the set of content whose language
tags begin with the same sequence of subtags. A basic language range
is identified by its 'language-range' tag, by adapting the
ABNF[RFC4234] 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
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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
language-range may not be mutually intelligible.
Basic language ranges were originally described in [RFC3066] and HTTP
1.1 [RFC2616] (where they are referred to as simply a "language
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
A Basic Language Range does not always provide the most appropriate
way to specify a user's preferences. Sometimes it is beneficial to
define a more granular matching scheme that takes advantage of the
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
indicate which values are of interest in filtering or selecting the
content.
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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'.
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"
; Single letters: x/X is reserved for private use
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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
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").
When working with tags and ranges users SHOULD note the following:
1. Private-use and Extension subtags are normally orthogonal to
language tag fallback. Implementations or specifications that
use a lookup (Section 3.3) matching scheme 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. Applications, specifications, or protocols 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 [RFC3066bis] generally fall into this
category.
There are several matching algorithms or schemes which can be applied
when matching extended language ranges to language tags.
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3. Types of Matching
Matching language ranges to language tags can be done in a number of
different ways. This section describes the different types of
matching scheme, as well as the considerations for choosing between
them.
There are two basic types of matching scheme: those that produce an
open-ended set of content (called "filtering") and those that produce
a single information item for a given request (called "lookup").
A key difference between these two types of matching scheme is that
the language range for filtering operations is always the _least_
specific tag one will accept as a match, while for lookup operations
the language range is always the _most_ specific tag.
3.1. Choosing a Type of Matching
Applications, protocols, and specifications are faced with the
decision of what type of matching to use. Sometimes, different
styles of matching might be suited for different kinds of processing
within a particular application or protocol.
Filtering can be used to produce a set of results (such as a
collection of documents). For example, if using a search engine, one
might use filtering to limit the results to documents written in
French. It can also be used when deciding whether to perform some
processing that is language sensitive on some content. For example,
a process might cause paragraphs whose language tag matched the
language range "nl" to be displayed in italics within a document.
This document describes three types of filtering:
1. Basic Filtering (Section 3.2.1) is used to match content using
basic language rangesSection 2.2. It is compatible with
implementations that do not produce extended language ranges.
2. Extended Range Filtering (Section 3.2.2) is used to match content
using extended language rangesSection 2.3. Newer implementations
SHOULD use this form of filtering in preference to basic
filtering.
3. Scored Filtering (Section 3.2.3) produces an ordered set of
content using either basic or extended language ranges. It
should be used when the quality of the match within a specific
language range is important, as when presenting a list of
documents resulting from a search.
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Lookup (Section 3.3) is used when each request MUST produce exactly
one piece of content. For example, a Web server might use the
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
a single item and it must return some item, even if no content
matches the language ranges supplied by the user.
Most types of matching in this document are designed so that
implementations do not have to examine the values of the subtags
supplied and, except for scored filtering, they do not need access to
the Language Subtag Registry nor do they require the use of valid
subtags in either language tags or language 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 implementation might infer that content
labeled "zh-CN" is morely likely to match the range "zh-Hans" than
equivalent content labeled "zh-TW".
3.2. Filtering
Filtering is used to select the set of content that matches a given
prefix. It is called "filtering" because this set of content may
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
the items, thus filtering out the non-matching content.
In filtering, the language range represents the _least_ specific tag
which is an acceptable match. That is, all of the language tags in
the set of filtered content will have an equal or greater number of
subtags than the language range. For example, if the language range
is "de-CH", one might see matching content with the tag "de-CH-1996"
but one will never see a match with the tag "de".
If the language priority list (see Section 2.1) contains more than
one range, the content returned is typically ordered in descending
level of preference.
Some examples where filtering might be appropriate include:
o Applying a style to sections of a document in a particular
language range.
o Displaying the set of documents containing a particular set of
keywords written in a specific language.
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o Selecting all email items written in specific range of languages.
Filtering can produce either ordered or unordered set of results.
For example, applying formatting to a document based on the language
of specific pieces of content does not require the content to be
ordered. It is sufficient to know whether a specific piece of
content matches or does not match. A search application, on the
other hand, probably would put the results into a priority order.
If an ordered set is desired, as described above, then the
application or protocol needs to determine the relative "quality" of
the match between different language tags and the language range.
This measurment is called a "distance metric". A distance metric
assigns a numeric value to the comparison of each language tag to a
language range and represents 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 example, allow users to select a threshold
distance for a match to be "successful" while filtering or it can use
the numeric value to order the results.
3.2.1. Filtering with Basic Language Ranges
When filtering using a basic language range, the 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.
3.2.2. Filtering with Extended Language Ranges
In the Extended Range Matching scheme, each extended language range
in the language priority list is considered in turn, according to
priority. The subtags in each extended language range are compared
to the corresponding subtags in the language tag being examined. The
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
value "*" (which matches all subtags, including the empty subtag).
Extended Range Matching is an extension of basic matching
(Section 3.2.1): the language range represents the least specific tag
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which is an acceptable match.
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
"*":
zh-Hant-CN
zh-CN
zh-Hans-CN
zh-CN-x-wadegile
zh-Latn-CN-boont
zh-cmn-Hans-CN-x-private
3.2.3. Distance Metric Filtering
Both basic and extended language range filtering produce simple
boolean matches. Sometimes it may be beneficial to provide an array
of results with different levels of matching, for example, sorting
results based on the overall "quality" of the match. Distance metric
filtering provides a way to generate these quality values.
First both the extended language range and the language tags to be
matched to it must be canonicalized by 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 "*".
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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 compared 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:
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 or
protocols. Implementations will usually allow users to choose the
most appropriate selection value, ranking the matched items 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)
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Implementations or protocols sometimes might wish 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'.
3.3. Lookup
Lookup is used to select the single information item that best
matches the language priority list for a given request. In lookup,
each language range in the language priority list represents the
_most_ specific tag which is an acceptable match; only the closest
matching item according the user's priority is returned. For
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
tag "de-CH-1996". Usually if no content matches the request, a
"default" 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. Other examples of lookup might
include:
o Selection of a template containing the text for an automated email
response.
o Selection of a graphic containing text for inclusion in a
particular Web page.
o Selection of a string of text for inclusion in an error log.
In the Lookup scheme, 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-private", the lookup
would progressively search for content as shown below:
Range to match: zh-Hant-CN-x-private
1. zh-Hant-CN-x-private
2. zh-Hant-CN
3. zh-Hant
4. zh
5. (default content or the empty tag)
Figure 5: Example of a Lookup Fallback Pattern
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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).
The language range "*" matches any language tag. In the lookup
scheme, this language range does not convey enough information to
determine which content is most appropriate. If this language range
is the only one in the language priority list, it matches the default
content. If this language range is followed by other language
ranges, it should be skipped.
When performing lookup using a language priority list, the
progressive search MUST proceed to consider each language range
before finding the default content or empty tag. The default content
might be content with no language tag (or with an empty value, as
with xml:lang in the XML specification), or it might be a particular
language designated for that bit of content.
One common way to provide for default content is to allow a specific
language range to be set as the default for a specific type of
request. This language range is then treated as if it were appended
to the end of the language priority list, rather than after each item
in the language priority list.
For example, if a particular user's language priority list were
"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
follows:
1. fr-FR
2. fr
3. zh-Hant // next language
4. zh
5. (return default content)
a. ja-JP
b. ja
c. (empty tag or other default content)
Figure 6: Lookup Using a Language Priority List
In some cases, the language priority list might contain one or more
extended language ranges (as, for example, when the same language
priority list is used as input for both lookup and filtering
operations). Wildcard values in an extended language range are
supposed to match any value that occurs in that position in a
language tag. Since only one item can be returned for any given
lookup request, the wildcards must be processed in a predictable
manner (or the same request might produce widely varying results).
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Thus, for each range in the language priority list, the following
rules must be applied to produce a basic language range for use in
the fallback mechanism:
1. If the first subtag in the extended language range is a "*" then
entire range is converted to "*".
2. For each subsequent subtag, if the value is a "*" then that
subtag and its preceeding hyphen are removed.
For example:
*-US becomes *
en-*-US becomes en-US
en-Latn-* becomes en-Latn
Figure 7: Transformation of Extended Language Ranges
For the language priority list "*-US; fr-*-FR; zh-Hant", the fallback
pattern would be:
1. * (skipped)
2. fr-FR
3. fr
4. zh-Hant
5. zh
6. (default content)
Figure 8: Extended Language Range Fallback Example
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4. Other Considerations
When working with language ranges and matching schemes, there are
some additional points that may influence the choice of either.
4.1. Meaning of Language Tags and Ranges
Selecting content using language ranges requires some understanding
by users of what they are selecting. A language tag or range
identifies 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-Arab"
(Azerbaijani written using the Arabic 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.
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.
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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 C'est la vie. 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.
4.2. 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.
4.3. 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
subtags and characters) did exist, however.
[RFC3066bis] 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.
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Length considerations and their impact on the selection and
processing of tags are described in Section 2.1.1 of that document.
An application or protocol 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.
Applications or protocols 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 2.3) 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 [RFC3066bis]):
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
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
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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-Latn-CN-variant1-a-extend1-x-wadegile-private1
1. zh-Latn-CN-variant1-a-extend1-x-wadegile
2. zh-Latn-CN-variant1-a-extend1
3. zh-Latn-CN-variant1
4. zh-Latn-CN
5. zh-Latn
6. zh
Figure 9: Example of Tag Truncation
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5. IANA Considerations
This document presents no new or existing considerations for IANA.
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6. Changes
This is the first version of this document.
The following changes were put into this document since draft-05:
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)
Reorganized the text and broke it into sections (M.Duerst)
Modified occurences of the word "application" to refer to
"applications or protocols" or otherwise be specific (E. van der
Poel)
Removed "Extended Language Range Lookup", merging it with other
text on lookup to form a single scheme. (M.Davis)
Fixed or removed obsolete or dangling references (Ed.)
Added an introduction to section 4 and added one sentence to make
it flow better to the start of section 4.1. (Ed.)
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7. 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 or protocol.
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8. 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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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC3066bis]
Phillips, A., Ed. and M. Davis, Ed., "Tags for the
Identification of Languages", October 2005, .
[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
9.2. Informative References
[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.
[RFC3282] Alvestrand, H., "Content Language Headers", RFC 3282,
May 2002.
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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 [RFC3066bis], [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, Martin Duerst, Frank Ellermann, Doug
Ewell, Marion Gunn, Kent Karlsson, Ira McDonald, M. Patton, Randy
Presuhn, Eric van der Poel, 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 3.2.3. Mark Davis originated the scheme
described in the Section 3.3.
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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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