Network Working Group A. Phillips, Ed.
Internet-Draft Quest Software
Obsoletes: 3066 (if approved) M. Davis, Ed.
Expires: June 10, 2006 IBM
December 7, 2005
Matching of Language Tags
draft-ietf-ltru-matching-08
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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 or content selection, filtering, and lookup 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. Basic Language Range . . . . . . . . . . . . . . . . . . . 4
2.2. Extended Language Range . . . . . . . . . . . . . . . . . 5
2.3. The Language Priority List . . . . . . . . . . . . . . . . 7
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 . . . . . . . 11
3.2.3. Scored Filtering . . . . . . . . . . . . . . . . . . . 11
3.3. Lookup . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4. Other Considerations . . . . . . . . . . . . . . . . . . . . . 18
4.1. Choosing Language Ranges . . . . . . . . . . . . . . . . . 18
4.2. Meaning of Language Tags and Ranges . . . . . . . . . . . 19
4.3. Considerations for Private Use Subtags . . . . . . . . . . 20
4.4. Length Considerations in Matching . . . . . . . . . . . . 21
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
6. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7. Security Considerations . . . . . . . . . . . . . . . . . . . 25
8. Character Set Considerations . . . . . . . . . . . . . . . . . 26
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9.1. Normative References . . . . . . . . . . . . . . . . . . . 27
9.2. Informative References . . . . . . . . . . . . . . . . . . 27
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
Intellectual Property and Copyright Statements . . . . . . . . . . 30
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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.
This document defines a syntax (called a language range (Section 2))
for specifying a user's language preferences, as well as several
schemes for selecting or filtering content by comparing language
ranges to the language tags [RFC3066bis] used to identify the natural
language of that content. 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 choose a particular option
or indicate that the particular options is left to the specific
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 in order to select specific content. 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".
Language tags and thus language ranges are to be treated as case-
insensitive: there exist conventions for the capitalization of some
of the subtags, but these MUST NOT be taken to carry meaning.
Matching of language tags to language ranges MUST be done in a case-
insensitive manner as well.
2.1. Basic Language Range
A "basic language range" identifies the set of content whose language
tags begin with the same sequence of subtags. Each range consists of
a sequence of alphanumeric subtags separated by hyphens. The basic
language range is defined by the following the ABNF[RFC4234]:
language-range = language-tag / "*"
language-tag = 1*8[alphanum] *["-" 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).
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 be mutually intelligible.
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2.2. 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
use a more fine-grained matching scheme that takes advantage of the
internal structure of language tags. This allows 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.
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:
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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 = "a"-"w" / "y"-"z" / "A"-"W" / "Y"-"Z" / "0"-"9"
; 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
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A field not present in the middle of an extended language range is
treated as if the field contained a "*". Implementations that
normalize extended language ranges SHOULD expand missing fields to be
"*" so that the semantic meaning of the language range is clear to
the user. At the same time, multiple wildcards in a row are
redundant and implementations SHOULD collapse these to a single
wildcard when normalizing the range (for brevity). For example, both
the range "sl-nedis" and the range "sl-*-*-nedis" are equivalent to
and should be normalized as "sl-*-nedis".
2.3. The Language Priority List
When users specify a language preference they 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 "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 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]. A simple list of ranges, i.e. one that
contains no weighting information, is considered to be in descending
order of priority.
The various matching operations described in this document include
considerations for using a language priority list. This document
does not define any syntax for a language priority list; defining
such a syntax is the responsibility of the protocol, application, or
implementation that uses it. When given as examples in this
document, language priority lists will be shown as a quoted sequence
of ranges separated by semi-colons, like this: "en; fr; zh-Hant"
(which would be read as "English before French before Chinese as
written in the Traditional script").
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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 two basic types of matching scheme: those that produce zero
or more information items (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 ranges in the language priority list represent the
_least_ specific content one will accept as a match, while for lookup
operations the language ranges represent the _most_ specific content.
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.
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 result in failure of the operation, an
empty value, returning some protocol defined or implementation
defined default, or returning i-default [RFC2277].
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 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.
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This document describes four types of matching (three types of
filtering, plus the lookup scheme):
1. Basic Filtering (Section 3.2.1) is used to match content using
basic language ranges (Section 2.1).
2. Extended Range Filtering (Section 3.2.2) is used to match content
using extended language ranges (Section 2.2).
3. Scored Filtering (Section 3.2.3) produces an ordered set of
content using 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.
4. Lookup (Section 3.3) is used when each request needs to produce
_exactly_ one piece of content. For example, if process were to
insert a human readable error message into a protocol header, it
might select the text based on the user's language preference.
Since it can return only one item, it must choose a single item
and it must return some item, even if no content matches the
language priority list supplied by the user.
Most types of matching in this document are designed so that
implementations are not required to validate or understand any of the
semantics of the subtags supplied and, except for scored filtering,
they do not need access to the IANA Language Subtag Registry (see
Section 3 in [RFC3066bis]). This simplifies and speeds the
performance of implementations.
If an implementation canonicalizes either ranges or tags, then the
implementation will require the IANA Language Subtag Registry
information for that purpose. Implementations MAY 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-CN" is more 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
language priority list. It is called "filtering" because this set of
content may contain no items at all or it may return an arbitrarily
large number of matching items--as many as match the language range
used to specify the items, thus filtering out the non-matching
content.
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In filtering, the language range represents the _least_ specific
(that is, the fewest number of subtags) language 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 priority list
consists of the range "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 2.3) 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 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.
Filtering can produce either an ordered or an 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 is selected by the language priority list (or not). A search
application, on the other hand, probably would want to order the
results.
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 measurement is called a "distance metric". A distance metric
assigns a numeric value to the comparison of a language tag to a
language range that 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 indicates
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 they
might use the numeric values to order the results.
3.2.1. Filtering with Basic Language Ranges
When filtering using basic language ranges, each basic language range
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in the language priority list is considered in turn, according to
priority. A particular language tag matches a language range 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 "*" 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.
3.2.2. Filtering with Extended Language Ranges
When filtering using extended language ranges, 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).
Subtags not specified, including those at the end of the language
range, are assigned the wildcard value "*". This makes each range
into a prefix much like that used in basic language range matching.
For example, the extended language range "de-*-DE" matches all of the
following tags because the unspecified variant field is expanded to
"*":
de-DE
de-Latn-DE
de-Latf-DE
de-DE-x-goethe
de-Latn-DE-1996
3.2.3. Scored Filtering
Both basic and extended language range filtering produce simple
boolean matches between a language range and a language tag.
Sometimes it may be useful to provide an array of results with
different levels of matching, for example, sorting results based on
the overall "quality" of the match. Scored (or "distance metric")
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filtering provides a way to generate these quality values.
As with the other forms of filtering, the process considers each
language range in the language priority list in order of priority.
Each extended language range and language tag MUST first be
canonicalized by mapping grandfathered and obsolete tags into modern
equivalents. This requires the information in the IANA Language
Subtag Registry (see Section 3 of [RFC3066bis]).
The language range and each language tag it is to be compared to are
then transformed into a "quintuple" consisting of five "elements" in
the form (language, script, country, variant, extension).
Any extended language subtags are considered part of the language
"element". For example, the language element for the tag "zh-cmn-
Hans" would be "zh-cmn".
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. The different handling of private-use
sequences prevents a range such as "x-twain" from matching all
possible tags, while a range such as "en-US-x-twain" would closely
match nearly all tags for English as used in the United States.
Language subtags 'und', 'mul', and the script subtag 'Zyyy' are
converted to "*": these subtag values represent undetermined,
multiple, or private-use values which are consistent with the use of
the wildcard.
For language tags that have no script subtag but whose language
subtag's record in the IANA Language Subtag Registry contains the
field "Suppress-Script", the script element in the quintuple MUST be
set to the script subtag in the Suppress-Script field. This is
necessary because [RFC3066bis] strongly recommends that users not use
this subtag to form language tags and this document recommends that
users not use them to form ranges. For example, if the script were
not expanded in this manner, a range such as "de-DE" would produce a
more-distant score for content that happened to be labeled
"de-Latn-DE" than users would expect that it should. Note that
languages which have a "Suppress-Script" field in the registry are
predominantly written in a single script.
Any remaining missing components in the language tag are set to "*";
thus an empty language tag becomes the quintuple ("*", "*", "*", "*",
"*"). Missing components in the 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
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pattern "en-US" becomes the quintuple ("en","*","US","","").
Here are some examples of language tags, showing their quintuples as
both language tags and language ranges:
en-US
Tag: (en, *, US, *, *)
Range: (en, *, US, "", "")
sr-Latn
Tag: (sr, Latn, *, *, *)
Range: (sr, Latn, "", "", "")
zh-cmn-Hant
Tag: (zh-cmn, Hant, *, *, *)
Range: (zh-cmn, Hant, "", "", "")
x-foo
Tag: (x-foo, *, *, *, *)
Range: (x-foo, "", "", "", "")
en-x-foo
Tag: (en, *, *, x-foo, *)
Range: (en, *, *, x-foo, "")
i-default
Tag: (i-default, *, *, *, *)
Range: (i-default, "", "", "", "")
sl-Latn-IT-rozaj
Tag: (sl, Latn, IT, rozaj, *)
Range: (sl, Latn, IT, rozaj, "")
zh-r-wadegile (hypothetical)
Tag: (zh, *, *, *, r-wadegile)
Range: (zh, *, *, *, r-wadegile)
Figure 3: Examples of Distance Metric Quintuples
Each pair of quintuples being compared is assigned a distance value,
in which 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 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-FR" 384 (language & region mismatch)
"fr" 256 (language mismatch, region match)
"en-GB" 32 (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)
Note: A variation of this algorithm might vary the scoring used
overall or for specific values. For example, sometimes it might make
sense to use more sophisticated weighting that depends on the values
of the corresponding elements. Thus, depending on the domain, an
implementation might assign a smaller distance to the difference
between closely related subtags (or treat certain values as equal).
Some examples of closely related subtags might be:
Language:
no (Norwegian)
nb (Bokmal Norwegian)
nn (Nynorsk Norwegian)
Script:
Kata (katakana)
Hira (hiragana)
Region:
US (United States of America)
UM (United States Minor Outlying Islands)
Figure 6: Examples of Closely Related Subtags
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3.3. Lookup
Lookup is used to select the single information item that best
matches the language priority list for a given request. When
performing lookup, each language range in the language priority list
is considered in turn, according to priority. By contrast with
filtering, each language ranges represents the _most_ specific tag
which is an acceptable match. The first information item found with
a matching tag, according the user's priority, is considered the
closest match and is the item 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 item containing some 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
progressively searches 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 7: Example of a Lookup Fallback Pattern
This scheme allows some flexibility in finding content. For example,
it provides better results for cases in which data 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, so "falling back" through the
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subtag sequence provides more opportunity to find a match between
available content and the user's request.
The default content is implementation defined. It might be content
with no language tag; might have an empty value (the built-in
attribute xml:lang in [XML10] permits the empty value); might be a
particular language designated for that bit of content; or it might
be content that is labeled with the tag "i-default" (see [RFC2277]).
When performing lookup using a language priority list, the
progressive search MUST proceed to consider 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
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 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 8: 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 the only one in the
language priority list, it matches the default content. If the
language range "*" is followed by other language ranges, it should be
skipped.
In some cases, the language priority list might contain one or more
extended language ranges (as, for example, when the same language
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priority list is used as input for both lookup and filtering
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 content that
is first in ASCII-order. For example, if the language range were
"*-CH" and the set of content included "de-CH", "fr-CH", and "it-CH",
then the content labeled "de-CH" would be returned.
Another way an implementation could address extended language ranges
would be to map them to basic language ranges: if the first subtag is
a "*" then the entire range is treated as "*" (which matches the
default content), otherwise the wildcard subtag is removed. For
example, if the language range were "en-*-US", then the range would
be mapped to "en-US".
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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
given user.
Most matching schemes make no attempt to process the semantic meaning
of the subtags. The language range (or its subtags) is usually
compared in a case-insensitive manner to each language tag being
matched, using basic string processing.
Users SHOULD avoid subtags that add no distinguishing value to a
language range. Generally, the fewer subtags that appear in the
language range, the more content the range will match.
Most notably, script subtags SHOULD NOT be used to form a language
range in combination with language subtags that have a matching
Suppress-Script field in their registry entry. Thus the language
range "en-Latn" is probably inappropriate in most cases (because the
vast majority of English documents are written in the Latin script
and thus the 'en' language subtag has a Suppress-Script field for
'Latn' in the registry).
When working with tags and ranges, note that extensions and most
private-use subtags are orthogonal to language tag matching, in that
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 interefer with filtering
(Section 3), since they appear at the end of the tag and will match
all prefixes.
When working with language tags and language ranges note that:
o Private-use and Extension subtags are normally orthogonal to
language tag fallback. Implementations or specifications that use
a lookup (Section 3.3) matching scheme often ignore unrecognized
private-use and extension subtags when performing language tag
fallback. In addition, since these subtags are always at the end
of the sequence of subtags, their use in language tags normally
doesn't interfere with the use of ranges that omit them in the
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filtering (Section 3.2) matching schemes described below.
However, they do interfere with filtering when used in language
ranges and SHOULD be avoided in ranges as a result.
o 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 to services on the other end of
the protocol that would make use of that information.
o 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 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. 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.
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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. They also seem to
represent relatively low divisions with a high chance of at least
limited understanding, although this depends on the specific variant
in question.
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 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.
4.3. 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
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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.
4.4. 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.
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 2.2) 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
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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
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-07:
Added a mention of "*" to the Character Set Considerations section
(D.Ewell)
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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, .
[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.
[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, 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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