One document matched: draft-ietf-ltru-matching-11.txt
Differences from draft-ietf-ltru-matching-10.txt
Network Working Group A. Phillips, Ed.
Internet-Draft Yahoo! Inc
Obsoletes: 3066 (if approved) M. Davis, Ed.
Expires: September 5, 2006 Google
March 4, 2006
Matching of Language Tags
draft-ietf-ltru-matching-11
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes different mechanisms for comparing and
matching language tags. Possible algorithms for language negotiation
or content selection, filtering, and lookup are described. This
document, in combination with RFC 3066bis (Ed.: 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. Basic Language Range . . . . . . . . . . . . . . . . . . . 4
2.2. Extended Language Range . . . . . . . . . . . . . . . . . 5
2.3. The Language Priority List . . . . . . . . . . . . . . . . 5
3. Types of Matching . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Choosing a Type of Matching . . . . . . . . . . . . . . . 7
3.2. Filtering . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2.1. Basic Filtering . . . . . . . . . . . . . . . . . . . 9
3.2.2. Extended Filtering . . . . . . . . . . . . . . . . . . 10
3.3. Lookup . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4. Other Considerations . . . . . . . . . . . . . . . . . . . . . 15
4.1. Choosing Language Ranges . . . . . . . . . . . . . . . . . 15
4.2. Meaning of Language Tags and Ranges . . . . . . . . . . . 16
4.3. Considerations for Private Use Subtags . . . . . . . . . . 16
4.4. Length Considerations for Language Ranges . . . . . . . . 17
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
6. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7. Security Considerations . . . . . . . . . . . . . . . . . . . 20
8. Character Set Considerations . . . . . . . . . . . . . . . . . 21
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Normative References . . . . . . . . . . . . . . . . . . . 22
9.2. Informative References . . . . . . . . . . . . . . . . . . 22
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 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 or in some
specific set of information items or "content".
One use for language identifiers, such as those defined in
[RFC3066bis], is to select content by matching the associated
language tags to a user's language preferences.
This document defines a syntax (called a language range (Section 2))
for specifying items in the user's list of language preferences
(called a language priority list (Section 2.3)), as well as several
schemes for selecting or filtering sets of content by comparing the
content's language tags to the user's preferences. Applications,
protocols, or specifications will have varying needs and requirements
that affect the choice of a suitable matching scheme. Depending on
the choice of scheme, there are various options left to the
implementation. Protocols that implement a matching scheme either
need to specify each particular choice or indicate the options that
are left to the implementation to decide.
This document is divided into three main sections. One describes how
to indicate a user's preferences using language ranges. Then a
section describes various schemes for matching these ranges to a set
of language tags. There is also a section that deals with various
practical considerations that apply to implementing and using these
schemes.
This document, in combination with [RFC3066bis] (Ed.: 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 one such mechanism in its discussion of
the Accept-Language header (Section 14.4), which is used when
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".
There are different types of language range, whose specific
attributes vary according to their application. Language ranges are
similar to language tags: they consist of a sequence of subtags
separated by hyphens. In a language range, each subtag MUST either
be a sequence of ASCII alphanumeric characters or the single
character '*' (%2A, ASTERISK). The character '*' is a "wildcard"
that matches any sequence of subtags. The meaning and uses of
wildcards vary according to the type of 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.
2.1. Basic Language Range
A "basic language range" describes a user's language preference as a
specific, uninterrupted, sequence of subtags. Each range consists of
a sequence of alphanumeric subtags separated by hyphens. The basic
language range is defined by the following ABNF [RFC4234]:
language-range = (1*8ALPHA *("-" 1*8alphanum)) / "*"
alphanum = ALPHA / DIGIT
Basic language ranges (originally described by HTTP/1.1 [RFC2616] and
later [RFC3066]) have the same syntax as an [RFC3066] language tag or
are the single character "*". They differ from the language tags
defined in [RFC3066bis] only in that there is no requirement that
they be "well-formed" or be validated against the IANA Language
Subtag Registry (although such ill-formed ranges will probably not
match anything). (Note that the ABNF [RFC4234] in [RFC2616] is
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incorrect, since it disallows the use of digits anywhere in the
'language-range': this is mentioned in the errata)
Use of a basic language range seems to 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 and users should note
that the set of language tags that match a specific language range
may not represent mutually intelligible languages.
2.2. Extended Language Range
Occasionally users will wish to select a set of language tags based
on the presence of specific subtags. An "extended language range"
describes a user's language preference as an ordered sequence of
subtags. For example, a user might wish to select all language tags
that contain the region subtag 'CH' (Switzerland). Extended language
ranges are useful in specifying a particular sequence of subtags that
appear in the set of matching tags without having to specify all of
the intervening subtags.
An extended language range can be represented by the following ABNF:
extended-language-range = (1*8ALPHA / "*")
*("-" (1*8alphanum / "*"))
Figure 2: Extended Language Range
The wildcard subtag '*' can occur in any position in the extended
language range, where it matches any sequence of subtags that might
occur in that position in a language tag. However, wildcards outside
the first position in an extended language range are ignored by most
matching schemes. Use of one or more wildcards SHOULD NOT be taken
to imply that a certain number of subtags will appear in the matching
set of language tags.
Implementations that specify basic ranges MAY map extended language
ranges to basic language ranges: if the first subtag is a "*" then
the entire range is treated as "*", otherwise each wildcard subtag is
removed. For example, if the language range were "en-*-US", then the
range would be mapped to "en-US".
2.3. The Language Priority List
A user's language preferences will often need to specify more than
one language range and thus users often need to specify a prioritized
list of language ranges in order to best reflect their language
preferences. This is especially true for speakers of minority
languages. A speaker of Breton in France, for example, may specify
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"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 user may wish to fall back from Skolt Sami to
Northern Sami to Finnish.
A "language priority list" is 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. This document
does not define the syntax for a language priority list; defining
such a syntax is the responsibility of the protocol, application, or
specification that uses it. When given as examples in this document,
language priority lists will be shown as a quoted sequence of ranges
separated by commas, like this: "en, fr, zh-Hant" (which would be
read as "English before French before Chinese as written in the
Traditional script").
A simple list of ranges is considered to be in descending order of
priority. Other language priority lists provide "quality weights"
for the language ranges in order to specify the relative priority of
the user's language preferences. An example of this would be the use
of "q" values in the syntax of the "Accept-Language" header (defined
in [RFC2616], Section 14.4, and [RFC3282]).
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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 several different matching
schemes, as well as the considerations for choosing between them.
Protocols and specifications SHOULD clearly indicate the particular
mechanism used in selecting or matching language tags.
There are several types of matching scheme. This document presents
two types: those that produce zero or more information items (called
"filtering") and those that produce a single information item for a
given request (called "lookup").
Implementations or protocols MAY use different matching schemes from
the ones described in this document, as long as those mechanisms are
clearly specified.
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 are suited to different kinds of processing within
a particular application or protocol.
Language tag matching is a tool, and does not by itself specify a
complete procedure for the use of language tags. Such procedures are
intimately tied to the application protocol in which they occur.
When specifying a protocol operation using matching, the protocol
MUST specify:
o Which type(s) of language tag matching it uses
o Whether the operation returns a single result (lookup) or a
possibly empty set of results (filtering)
o For lookup, what the result is when no matching tag is found. For
instance, a protocol might define the result as failure of the
operation, an empty value, returning some protocol defined or
implementation defined default, or returning i-default [RFC2277].
This document describes three types of matching:
1. Basic Filtering (Section 3.2.1) matches a language priority list
consisting of basic language ranges (Section 2.1) to sets of
language tags.
2. Extended Filtering (Section 3.2.2) matches a language priority
list consisting of extended language ranges (Section 2.2) to sets
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of language tags.
3. Lookup (Section 3.3) matches a language priority list consisting
of basic language ranges to sets of language tags to find the one
_exact_ language tag that best matches the range.
Filtering can be used to produce a set of results (such as a
collection of documents) by comparing the user's preferences to
language tags associated with the set of content. For example, when
performing a search, one might use filtering to limit the results to
items tagged as being in the French language. Filtering can also be
used when deciding whether to perform a language-sensitive process 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.
Lookup produces the single result that best matches the user's
preferences, so it is useful in cases in which only a single item can
be returned. For example, if a process were to insert a human
readable error message into a protocol header, it might select the
text based on the user's language priority list. Since the process
can return only one item, it must choose a single item and it must
return some item, even if none of the content's language tags match
the language priority list supplied by the user.
The types of matching in this document are designed so that
implementations are not required to validate or understand any of the
semantics of the language tags or ranges or of the subtags in them.
None of them require access to the IANA Language Subtag Registry (see
Section 3 in [RFC3066bis]). This simplifies implementation of these
schemes. An implementation MAY choose to check if either the
language ranges or language tags being matched are "well-formed" or
"valid" (see [RFC3066bis], Section 2.2.9) and MAY choose not to
process invalid ranges.
Regardless of the matching scheme chosen, protocols and
implementations MAY canonicalize language tags and ranges by mapping
grandfathered and obsolete tags or subtags into modern equivalents.
If an implementation canonicalizes either ranges or tags, then the
implementation will require the IANA Language Subtag Registry
information for that purpose. Implementations MAY also use semantic
information external to the registry when matching tags. 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-Hans" (Chinese as written in the Simplified
script) is more likely to match the range "zh-CN" (Chinese as used in
China, where the Simplified script is predominant) than equivalent
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content labeled "zh-TW" (Chinese as used in Taiwan, where the
Traditional script is predominant).
3.2. Filtering
Filtering is used to select the set of language tags that matches a
given language priority list and return the associated content. It
is called "filtering" because this set might contain no items at all
or it might return an arbitrarily large number of matching items: as
many items as match the language priority list, thus "filtering out"
the non-matching items.
In filtering, each language range represents the _least_ specific
language tag (that is, the language tag with fewest number of
subtags) which is an acceptable match. All of the language tags in
the matching set of tags will have an equal or greater number of
subtags than the language range. Every non-wildcard subtag in the
language range will appear in every one of the matching language
tags. For example, if the language priority list consists of the
range "de-CH", one might see tags such as "de-CH-1996" but one will
never see a tag such as "de" (because the 'CH' subtag is missing).
If the language priority list (see Section 2.3) contains more than
one range, the content returned is typically ordered in descending
level of preference, but it MAY be unordered, according to the needs
of the application or protocol.
Some examples of applications where filtering might be appropriate
include:
o Applying a style to sections of a document in a particular set of
languages.
o Displaying the set of documents containing a particular set of
keywords written in a specific set of languages.
o Selecting all email items written in a specific set of languages.
o Selecting audio files spoken in a particular language.
3.2.1. Basic Filtering
When filtering using basic language ranges, each basic language range
in the language priority list is considered in turn, according to
priority. A particular language tag matches a language range if, in
a case-insensitive comparison, it exactly equals the tag, or if it
exactly equals a prefix of the tag such that the first character
following the prefix is "-". For example, the language-range "de-de"
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matches the language tag "de-DE-1996", but not the language tags "de-
Deva" or "de-Latn-DE".
The special range "*" in a language priority list matches any tag. A
protocol which uses language ranges MAY specify additional rules
about the semantics of "*"; for instance, HTTP/1.1 [RFC2616]
specifies that the range "*" matches only languages not matched by
any other range within an "Accept-Language" header.
Basic filtering is identical to the type of matching described in
[RFC3066], Section 2.5 (Language-range).
3.2.2. Extended Filtering
When filtering using extended language ranges, each extended language
range in the language priority list is considered in turn, according
to priority. A particular language range is compared to each
language tag using the following process:
Compare the first subtag in the extended language tag to the first
subtag in the language tag in a case insensitive manner. If the
first subtag in the range is "*", it matches any value. Otherwise
the two values must match or the overall match fails.
Take each non-wildcard subtag in the language range and compare it in
a case-insensitive manner to the next subtag in the language tag. If
the range's subtag exactly matches the tag's subtag, proceed to the
next non-wildcard subtag in the language range (and beginning with
the next subtag in the language tag) until the list of subtags in the
language range is exhausted or the match fails. If the tag's subtag
is a "singleton" (a single letter or digit, which, in this case,
includes the private-use subtag 'x') and the range's subtag does not
match or if the language tag's list of subtags is exhausted, the
match fails. If the language range's list of subtags is exhausted,
the match succeeds.
Subtags not specified, including those at the end of the language
range, are thus treated as if assigned the wildcard value "*". Much
like basic filtering, extended filtering selects content with
arbitrarily long tags that share the same initial subtags as the
language range. In addition extended filtering selects content with
any intermediate subtags unspecified in the language range. For
example, the extended language range "de-*-DE" matches all of the
following tags:
de-DE
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de-Latn-DE
de-Latf-DE
de-de
de-DE-x-goethe
de-Latn-DE-1996
The same range does not match any of the following tags for the
reasons shown:
de (missing 'DE')
de-x-DE (singleton 'x' occurs before 'DE')
de-Deva ('Deva' not equal to 'DE')
Note: The structure of language tags defined by [RFC3066bis] defines
each type of subtag (language, script, region, and so forth)
according to position, size, and content. This means that subtags in
a language range can only match specific types of subtags in a
language tag. For example, a subtag such as 'Latn' is always a
script subtag (unless it follows a singleton) while a subtag such as
'nedis' can only match the equivalent variant subtag.
3.3. Lookup
Lookup is used to select the single language tag that best matches
the language priority list for a given request and return the
associated content. When performing lookup, each language range in
the language priority list is considered in turn, according to
priority. By contrast with filtering, each language range represents
the _most_ specific tag which is an acceptable match. The first
content found with a matching tag, according to the user's priority,
is considered the closest match and is the content returned. For
example, if the language range is "de-ch", a lookup operation might
produce content with the tags "de" or "de-CH" but never one with the
tag "de-CH-1996". Usually if no content matches the request, the
"default" content 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
matching language tag associated with a suitable piece of content to
insert. Examples of lookup might include:
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o Selection of a template containing the text for an automated email
response.
o Selection of a item containing some text for inclusion in a
particular Web page.
o Selection of a string of text for inclusion in an error log.
o Selection of an audio file to play as a prompt in a phone system.
In the lookup scheme, the language range is progressively truncated
from the end until a matching piece of content is located. Single
letter or digit subtags (including both the letter 'x' which
introduces private-use sequences, and the subtags that introduce
extensions) are removed at the same time as their closest trailing
subtag. For example, starting with the range "zh-Hant-CN-x-private1-
private2", the lookup progressively searches for content as shown
below:
Range to match: zh-Hant-CN-x-private1-private2
1. zh-Hant-CN-x-private1-private2
2. zh-Hant-CN-x-private1
3. zh-Hant-CN
4. zh-Hant
5. zh
6. (default content)
Figure 3: Example of a Lookup Fallback Pattern
This allows some flexibility in finding a match. For example, lookup
provides better results for cases in which content is not available
that exactly matches the user request than if the default language
for the system or content were returned immediately. Not every
specific level of tag granularity is usually available or language
content may be sparsely populated. "Falling back" through the subtag
sequence provides more opportunity to find a match between available
language tags and the user's request.
The default behavior when no tag matches the language priority list
is implementation defined. An implementation might, for example,
return content:
o with no language tag
o of a non-linguistic nature, such as an image or sound
o with an empty language tag value, in cases where the protocol
permits the empty value (see, for example, "xml:lang" in [XML10],
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which indicates that the element contains non-linguistic content)
o in a particular language designated for the bit of content being
selected
o labelled with the tag "i-default" (see [RFC2277])
When performing lookup using a language priority list, the
progressive search MUST process each language range in the list
before finding the default content or empty tag.
One common way for an application or implementation to provide for a
default 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 as a whole, 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. (search for the default content)
a. ja-JP
b. ja
c. (implementation defined default)
Figure 4: Lookup Using a Language Priority List
Implementations SHOULD ignore extensions and unrecognized private-use
subtags when performing lookup, since these subtags are usually
orthogonal to the user's request.
The special language range "*" matches any language tag. In the
lookup scheme, this range does not convey enough information by
itself to determine which content is most appropriate, since it
matches everything. If the language range "*" is followed by other
language ranges, it SHOULD be skipped. If the language range "*" is
the only one in the language priority list or if no other language
range follows, the default content SHOULD be returned.
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
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operations). Wildcard values in an extended language range normally
match any value that occurs in that position in a language tag.
Since only one item can be returned for any given lookup request,
wildcards in a language range have to be processed in a consistent
manner or the same request will produce widely varying results.
Implementations that accept extended language ranges MUST define
which content is returned when more than one item matches the
extended language range.
For example, an implementation could return the matching tag that is
first in ASCII-order. If the language range were "*-CH" and the set
of tags included "de-CH", "fr-CH", and "it-CH", then the tag "de-CH"
would be returned. Another possibility would be for an
implementation to map the extended language ranges to basic ranges.
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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. Choosing Language Ranges
Users indicate their language preferences via the choice of a
language range or the list of language ranges in a language priority
list. The type of matching affects what the best choice is for a
user.
Most matching schemes make no attempt to process the semantic meaning
of the subtags and the language range is compared, in a case-
insensitive manner, to each language tag being matched, using basic
string processing. Users SHOULD select language ranges that are
well-formed, valid language tags according to [RFC3066bis]
(substituting wildcards as appropriate in extended language ranges).
Users SHOULD replace tags or subtags which have been deprecated with
the Preferred-Value from the IANA Language Subtag Registry. If the
user is working with content that might use the older form, the user
might include both the new and old forms in a language priority list.
For example, the tag "art-lojban" is deprecated. The subtag 'jbo' is
supposed to be used instead, so the user might use it to form the
language range. Or the user might include both in a language
priority list: "jbo, art-lojban".
Users SHOULD avoid subtags that add no distinguishing value to a
language range. When filtering, the fewer the number of subtags that
appear in the language range, the more content the range will
probably match, while in lookup unnecessary subtags might cause
"better", more-specific content to be skipped in favor of less
specific content. For example, the range "de-Latn-DE" would return
content tagged "de" instead of content tagged "de-DE", even though
the latter is probably a better match.
Many languages are written predominantly in a single script. This is
usually recorded in the Suppress-Script field in that language
subtag's registry entry. For these languages, script subtags SHOULD
NOT be used to form a language range. Thus the language range "en-
Latn" is 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 matching, in that
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they specify additional attributes of the text not related to the
goals of most matching schemes. Users SHOULD avoid using these
subtags in language ranges, since they interfere with the selection
of available content. When used in language tags (as opposed to
ranges), these subtags normally do not interfere with filtering
(Section 3), since they appear at the end of the tag and will match
all prefixes. Lookup (Section 3.3) implementations often ignore
unrecognized private-use and extension subtags when performing
language tag fallback.
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 altered to remove subtags and
extensions that are not supported by that protocol. Such alterations
SHOULD be avoided, if possible, since they remove information that
might be relevant elsewhere that would make use of that information.
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 process that
utilizes 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.
4.2. Meaning of Language Tags and Ranges
Selecting content using language ranges requires some understanding
by users of what they are selecting. The meaning of the various
subtags in a language range are identical to their meaning in a
language tag (see Section 4.2 in [RFC3066bis]), with the addition
that the wildcard "*" represents any matching sequence of values.
4.3. 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
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in unpredictable content being returned.
4.4. Length Considerations for Language Ranges
Language ranges are very similar to language tags in terms of content
and usage. The same types of restrictions on length that apply to
language tags can also apply to language ranges. See [RFC3066bis]
Section 4.3 (Length Considerations).
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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.
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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 a
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
Language tags permit only the characters A-Z, a-z, 0-9, and HYPHEN-
MINUS (%x2D). Language ranges also use the character ASTERISK
(%x2A). These characters are present in most character sets, so
presentation or exchange of language tags or ranges should not be
constrained by 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.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC3066bis]
Phillips, A., Ed. and M. Davis, Ed., "Tags for the
Identification of Languages", October 2005, <http://
www.ietf.org/internet-drafts/
draft-ietf-ltru-registry-14.txt>.
[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.
[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.
[RFC2616errata]
IETF, "HTTP/1.1 Specification Errata", 10 2004,
<http://purl.org/NET/http-errata>.
[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.
[XML10] Bray (et al), T., "Extensible Markup Language (XML) 1.0",
02 2004.
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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:
Harald Alvestrand, Jeremy Carroll, John Cowan, Martin Duerst, Frank
Ellermann, Doug Ewell, Marion Gunn, Kent Karlsson, Ira McDonald, M.
Patton, Randy Presuhn, Eric van der Poel, Markus Scherer, 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.
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Authors' Addresses
Addison Phillips (editor)
Yahoo! Inc
Email: addison at inter dash locale dot com
Mark Davis (editor)
Google
Email: mark dot davis at macchiato dot com
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