draft-ietf-ltru-matching-15.txt   rfc4647.txt 
Network Working Group A. Phillips, Ed. Network Working Group A. Phillips, Ed.
Internet-Draft Yahoo! Inc. Request for Comments: 4647 Yahoo! Inc.
Obsoletes: 3066 (if approved) M. Davis, Ed. BCP: 47 M. Davis, Ed.
Expires: December 24, 2006 Google Obsoletes: 3066 Google
June 22, 2006 Category: Best Current Practice September 2006
Matching of Language Tags Matching of Language Tags
draft-ietf-ltru-matching-15
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2006). Copyright (C) The Internet Society (2006).
Abstract Abstract
This document describes a syntax, called a "language-range", for This document describes a syntax, called a "language-range", for
specifying items in a user's list of language preferences. It also specifying items in a user's list of language preferences. It also
describes different mechanisms for comparing and matching these to describes different mechanisms for comparing and matching these to
language tags. Two kinds of matching mechanisms, filtering and language tags. Two kinds of matching mechanisms, filtering and
lookup, are defined. Filtering produces a (potentially empty) set of lookup, are defined. Filtering produces a (potentially empty) set of
language tags, whereas lookup produces a single language tag. language tags, whereas lookup produces a single language tag.
Possible applications include language negotiation or content Possible applications include language negotiation or content
selection. This document, in combination with RFC 3066bis (Ed.: selection. This document, in combination with RFC 4646, replaces RFC
replace "3066bis" with the RFC number assigned to 3066, which replaced RFC 1766.
draft-ietf-ltru-registry-14), replaces RFC 3066, which replaced RFC
1766.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction ....................................................3
2. The Language Range . . . . . . . . . . . . . . . . . . . . . . 4 2. The Language Range ..............................................3
2.1. Basic Language Range . . . . . . . . . . . . . . . . . . . 4 2.1. Basic Language Range .......................................4
2.2. Extended Language Range . . . . . . . . . . . . . . . . . 5 2.2. Extended Language Range ....................................4
2.3. The Language Priority List . . . . . . . . . . . . . . . . 5 2.3. The Language Priority List .................................5
3. Types of Matching . . . . . . . . . . . . . . . . . . . . . . 7 3. Types of Matching ...............................................6
3.1. Choosing a Matching Scheme . . . . . . . . . . . . . . . . 7 3.1. Choosing a Matching Scheme .................................6
3.2. Implementation Considerations . . . . . . . . . . . . . . 8 3.2. Implementation Considerations ..............................7
3.3. Filtering . . . . . . . . . . . . . . . . . . . . . . . . 9 3.3. Filtering ..................................................8
3.3.1. Basic Filtering . . . . . . . . . . . . . . . . . . . 10 3.3.1. Basic Filtering .....................................9
3.3.2. Extended Filtering . . . . . . . . . . . . . . . . . . 11 3.3.2. Extended Filtering .................................10
3.4. Lookup . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.4. Lookup ....................................................12
3.4.1. Default Values . . . . . . . . . . . . . . . . . . . . 14 3.4.1. Default Values .....................................14
4. Other Considerations . . . . . . . . . . . . . . . . . . . . . 16 4. Other Considerations ...........................................15
4.1. Choosing Language Ranges . . . . . . . . . . . . . . . . . 16 4.1. Choosing Language Ranges ..................................15
4.2. Meaning of Language Tags and Ranges . . . . . . . . . . . 17 4.2. Meaning of Language Tags and Ranges .......................16
4.3. Considerations for Private Use Subtags . . . . . . . . . . 17 4.3. Considerations for Private-Use Subtags ....................17
4.4. Length Considerations for Language Ranges . . . . . . . . 18 4.4. Length Considerations for Language Ranges .................17
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 5. Security Considerations ........................................17
6. Security Considerations . . . . . . . . . . . . . . . . . . . 20 6. Character Set Considerations ...................................17
7. Character Set Considerations . . . . . . . . . . . . . . . . . 21 7. References .....................................................18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 7.1. Normative References ......................................18
8.1. Normative References . . . . . . . . . . . . . . . . . . . 22 7.2. Informative References ....................................18
8.2. Informative References . . . . . . . . . . . . . . . . . . 22 Appendix A. Acknowledgements ......................................19
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . . . 25
1. Introduction 1. Introduction
Human beings on our planet have, past and present, used a number of Human beings on our planet have, past and present, used a number of
languages. There are many reasons why one would want to identify the languages. There are many reasons why one would want to identify the
language used when presenting or requesting information. language used when presenting or requesting information.
Applications, protocols, or specifications that use language Applications, protocols, or specifications that use language
identifiers, such as the language tags defined in [RFC3066bis], identifiers, such as the language tags defined in [RFC4646],
sometimes need to match language tags to a user's language sometimes need to match language tags to a user's language
preferences. preferences.
This document defines a syntax (called a language range (Section 2)) This document defines a syntax (called a language range (Section 2))
for specifying items in the user's list of language preferences for specifying items in the user's list of language preferences
(called a language priority list (Section 2.3)), as well as several (called a language priority list (Section 2.3)), as well as several
schemes for selecting or filtering sets of language tags by comparing schemes for selecting or filtering sets of language tags by comparing
the language tags to the user's preferences. Applications, the language tags to the user's preferences. Applications,
protocols, or specifications will have varying needs and requirements protocols, or specifications will have varying needs and requirements
that affect the choice of a suitable matching scheme. that affect the choice of a suitable matching scheme.
This document describes: how to indicate a user's preferences using This document describes how to indicate a user's preferences using
language ranges; three schemes for matching these ranges to a set of language ranges, three schemes for matching these ranges to a set of
language tags; and the various practical considerations that apply to language tags, and the various practical considerations that apply to
implementing and using these schemes. implementing and using these schemes.
This document, in combination with [RFC3066bis] (Ed.: replace This document, in combination with [RFC4646], replaces [RFC3066],
"3066bis" globally in this document with the RFC number assigned to which replaced [RFC1766].
draft-ietf-ltru-registry-14), replaces [RFC3066], which replaced
[RFC1766].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
2. The Language Range 2. The Language Range
Language tags [RFC3066bis] are used to help identify languages, Language tags [RFC4646] are used to help identify languages, whether
whether spoken, written, signed, or otherwise signaled, for the spoken, written, signed, or otherwise signaled, for the purpose of
purpose of communication. Applications, protocols, or specifications communication. Applications, protocols, or specifications that use
that use language tags are often faced with the problem of language tags are often faced with the problem of identifying sets of
identifying sets of content that share certain language attributes. content that share certain language attributes. For example,
For example, HTTP/1.1 [RFC2616] describes one such mechanism in its HTTP/1.1 [RFC2616] describes one such mechanism in its discussion of
discussion of the Accept-Language header (Section 14.4), which is the Accept-Language header (Section 14.4), which is used when
used when selecting content from servers based on the language of selecting content from servers based on the language of that content.
that content.
It is, thus, useful to have a mechanism for identifying sets of It is, thus, useful to have a mechanism for identifying sets of
language tags that share specific attributes. This allows users to language tags that share specific attributes. This allows users to
select or filter the language tags based on specific requirements. select or filter the language tags based on specific requirements.
Such an identifier is called a "language range". Such an identifier is called a "language range".
There are different types of language range, whose specific There are different types of language range, whose specific
attributes vary according to their application. Language ranges are attributes vary according to their application. Language ranges are
similar to language tags: they consist of a sequence of subtags similar to language tags: they consist of a sequence of subtags
separated by hyphens. In a language range, each subtag MUST either separated by hyphens. In a language range, each subtag MUST either
be a sequence of ASCII alphanumeric characters or the single be a sequence of ASCII alphanumeric characters or the single
character '*' (%2A, ASTERISK). The character '*' is a "wildcard" character '*' (%x2A, ASTERISK). The character '*' is a "wildcard"
that matches any sequence of subtags. The meaning and uses of that matches any sequence of subtags. The meaning and uses of
wildcards vary according to the type of language range. wildcards vary according to the type of language range.
Language tags and thus language ranges are to be treated as case- Language tags and thus language ranges are to be treated as case-
insensitive: there exist conventions for the capitalization of some insensitive: there exist conventions for the capitalization of some
of the subtags, but these MUST NOT be taken to carry meaning. 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- Matching of language tags to language ranges MUST be done in a case-
insensitive manner. insensitive manner.
2.1. Basic Language Range 2.1. Basic Language Range
A "basic language range" has the same syntax as an [RFC3066] language A "basic language range" has the same syntax as an [RFC3066] language
tag or is the single character "*". The basic language range was tag or is the single character "*". The basic language range was
originally described by HTTP/1.1 [RFC2616] and later [RFC3066]. It originally described by HTTP/1.1 [RFC2616] and later [RFC3066]. It
is defined by the following ABNF [RFC4234]: is defined by the following ABNF [RFC4234]:
language-range = (1*8ALPHA *("-" 1*8alphanum)) / "*" language-range = (1*8ALPHA *("-" 1*8alphanum)) / "*"
alphanum = ALPHA / DIGIT alphanum = ALPHA / DIGIT
A basic language range differs from the language tags defined in A basic language range differs from the language tags defined in
[RFC3066bis] only in that there is no requirement that it be "well- [RFC4646] only in that there is no requirement that it be "well-
formed" or be validated against the IANA Language Subtag Registry. formed" or be validated against the IANA Language Subtag Registry.
Such ill-formed ranges will probably not match anything. Note that Such ill-formed ranges will probably not match anything. Note that
the ABNF [RFC4234] in [RFC2616] is incorrect, since it disallows the the ABNF [RFC4234] in [RFC2616] is incorrect, since it disallows the
use of digits anywhere in the 'language-range' (see: use of digits anywhere in the 'language-range' (see [RFC2616errata]).
[RFC2616errata]).
2.2. Extended Language Range 2.2. Extended Language Range
Occasionally users will wish to select a set of language tags based Occasionally, users will wish to select a set of language tags based
on the presence of specific subtags. An "extended language range" on the presence of specific subtags. An "extended language range"
describes a user's language preference as an ordered sequence of describes a user's language preference as an ordered sequence of
subtags. For example, a user might wish to select all language tags subtags. For example, a user might wish to select all language tags
that contain the region subtag 'CH' (Switzerland). Extended language that contain the region subtag 'CH' (Switzerland). Extended language
ranges are useful for specifying a particular sequence of subtags ranges are useful for specifying a particular sequence of subtags
that appear in the set of matching tags without having to specify all that appear in the set of matching tags without having to specify all
of the intervening subtags. of the intervening subtags.
An extended language range can be represented by the following ABNF: An extended language range can be represented by the following ABNF:
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language range, where it matches any sequence of subtags that might language range, where it matches any sequence of subtags that might
occur in that position in a language tag. However, wildcards outside occur in that position in a language tag. However, wildcards outside
the first position are ignored by Extended Filtering (see Section the first position are ignored by Extended Filtering (see Section
3.2.2). The use or absence of one or more wildcards cannot be taken 3.2.2). The use or absence of one or more wildcards cannot be taken
to imply that a certain number of subtags will appear in the matching to imply that a certain number of subtags will appear in the matching
set of language tags. set of language tags.
2.3. The Language Priority List 2.3. The Language Priority List
A user's language preferences will often need to specify more than A user's language preferences will often need to specify more than
one language range and thus users often need to specify a prioritized one language range, and thus users often need to specify a
list of language ranges in order to best reflect their language prioritized list of language ranges in order to best reflect their
preferences. This is especially true for speakers of minority language preferences. This is especially true for speakers of
languages. A speaker of Breton in France, for example, can specify minority languages. A speaker of Breton in France, for example, can
"br" followed by "fr", meaning that if Breton is available, it is specify "br" followed by "fr", meaning that if Breton is available,
preferred, but otherwise French is the best alternative. It can get it is preferred, but otherwise French is the best alternative. It
more complex: a different user might want to fall back from Skolt can get more complex: a different user might want to fall back from
Sami to Northern Sami to Finnish. Skolt Sami to Northern Sami to Finnish.
A "language priority list" is a prioritized or weighted list of A "language priority list" is a prioritized or weighted list of
language ranges. One well known example of such a list is the language ranges. One well-known example of such a list is the
"Accept-Language" header defined in RFC 2616 [RFC2616] (see Section "Accept-Language" header defined in RFC 2616 [RFC2616] (see Section
14.4) and RFC 3282 [RFC3282]. 14.4) and RFC 3282 [RFC3282].
The various matching operations described in this document include The various matching operations described in this document include
considerations for using a language priority list. This document considerations for using a language priority list. This document
does not define the syntax for a language priority list; defining does not define the syntax for a language priority list; defining
such a syntax is the responsibility of the protocol, application, or such a syntax is the responsibility of the protocol, application, or
specification that uses it. When given as examples in this document, specification that uses it. When given as examples in this document,
language priority lists will be shown as a quoted sequence of ranges language priority lists will be shown as a quoted sequence of ranges
separated by commas, like this: "en, fr, zh-Hant" (which is read separated by commas, like this: "en, fr, zh-Hant" (which is read
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1. Basic Filtering (Section 3.3.1) matches a language priority list 1. Basic Filtering (Section 3.3.1) matches a language priority list
consisting of basic language ranges (Section 2.1) to sets of consisting of basic language ranges (Section 2.1) to sets of
language tags. language tags.
2. Extended Filtering (Section 3.3.2) matches a language priority 2. Extended Filtering (Section 3.3.2) matches a language priority
list consisting of extended language ranges (Section 2.2) to sets list consisting of extended language ranges (Section 2.2) to sets
of language tags. of language tags.
3. Lookup (Section 3.4) matches a language priority list consisting 3. Lookup (Section 3.4) matches a language priority list consisting
of basic language ranges to sets of language tags to find the one of basic language ranges to sets of language tags to find the one
_exact_ language tag that best matches the range. exact language tag that best matches the range.
Filtering can be used to produce a set of results (such as a Filtering can be used to produce a set of results (such as a
collection of documents) by comparing the user's preferences to a set collection of documents) by comparing the user's preferences to a set
of language tags. For example, when performing a search, filtering of language tags. For example, when performing a search, filtering
can be used to limit the results to items tagged as being in the can be used to limit the results to items tagged as being in the
French language. Filtering can also be used when deciding whether to French language. Filtering can also be used when deciding whether to
perform a language-sensitive process on some content. For example, a perform a language-sensitive process on some content. For example, a
process might cause paragraphs whose language tag matched the process might cause paragraphs whose language tag matched the
language range "nl" (Dutch) to be displayed in italics within a language range "nl" (Dutch) to be displayed in italics within a
document. document.
Lookup produces the single result that best matches the user's Lookup produces the single result that best matches the user's
preferences from the list of available tags, so it is useful in cases preferences from the list of available tags, so it is useful in cases
in which a single item is required (and for which only a single item in which a single item is required (and for which only a single item
can be returned). For example, if a process were to insert a human can be returned). For example, if a process were to insert a human-
readable error message into a protocol header, it might select the readable error message into a protocol header, it might select the
text based on the user's language priority list. Since the process text based on the user's language priority list. Since the process
can return only one item, it is forced to choose a single item and it can return only one item, it is forced to choose a single item and it
has to return some item, even if none of the content's language tags has to return some item, even if none of the content's language tags
match the language priority list supplied by the user. match the language priority list supplied by the user.
3.2. Implementation Considerations 3.2. Implementation Considerations
Language tag matching is a tool, and does not by itself specify a Language tag matching is a tool, and does not by itself specify a
complete procedure for the use of language tags. Such procedures are complete procedure for the use of language tags. Such procedures are
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o For lookup, what the default item is (or the sequence of o For lookup, what the default item is (or the sequence of
operations or configuration information used to determine the operations or configuration information used to determine the
default) when no matching tag is found. For instance, a protocol default) when no matching tag is found. For instance, a protocol
might define the result as failure of the operation, an empty might define the result as failure of the operation, an empty
value, returning some protocol defined or implementation defined value, returning some protocol defined or implementation defined
default, or returning i-default [RFC2277]. default, or returning i-default [RFC2277].
Applications, protocols, and specifications are not required to Applications, protocols, and specifications are not required to
validate or understand any of the semantics of the language tags or validate or understand any of the semantics of the language tags or
ranges or of the subtags in them, nor do they require access to the ranges or of the subtags in them, nor do they require access to the
IANA Language Subtag Registry (see Section 3 in [RFC3066bis]). This IANA Language Subtag Registry (see Section 3 in [RFC4646]). This
simplifies implementation. simplifies implementation.
However, designers of applications, protocols, or specifications are However, designers of applications, protocols, or specifications are
encouraged to use the information from the IANA Language Subtag encouraged to use the information from the IANA Language Subtag
Registry to support canonicalizing language tags and ranges in order Registry to support canonicalizing language tags and ranges in order
to map grandfathered and obsolete tags or subtags into modern to map grandfathered and obsolete tags or subtags into modern
equivalents. equivalents.
Applications, protocols, or specifications that canonicalize ranges Applications, protocols, or specifications that canonicalize ranges
MUST either perform matching operations with both the canonical and MUST either perform matching operations with both the canonical and
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each tag for the purposes of comparison. each tag for the purposes of comparison.
Note that canonicalizing language ranges makes certain operations Note that canonicalizing language ranges makes certain operations
impossible. For example, an implementation that canonicalizes the impossible. For example, an implementation that canonicalizes the
language range "art-lojban" (artificial language, lojban variant) to language range "art-lojban" (artificial language, lojban variant) to
use the more modern "jbo" (Lojban) cannot be used to select just the use the more modern "jbo" (Lojban) cannot be used to select just the
items with the older tag. items with the older tag.
Applications, protocols, or specifications that use basic ranges Applications, protocols, or specifications that use basic ranges
might sometimes receive extended language ranges instead. An might sometimes receive extended language ranges instead. An
application, protocol, or specification MUST choose to: a) map application, protocol, or specification MUST choose to a) map
extended language ranges to basic ranges using the algorithm below, extended language ranges to basic ranges using the algorithm below,
b) reject any extended language ranges in the language priority list b) reject any extended language ranges in the language priority list
that are not valid basic language ranges, or c) treat each extended that are not valid basic language ranges, or c) treat each extended
language range as if it were a basic language range, which will have language range as if it were a basic language range, which will have
the same result as ignoring them, since these ranges will not match the same result as ignoring them, since these ranges will not match
any valid language tags. any valid language tags.
An extended language range is mapped to a basic language range as An extended language range is mapped to a basic language range as
follows: if the first subtag is a '*' then the entire range is follows: if the first subtag is a '*' then the entire range is
treated as "*", otherwise each wildcard subtag is removed. For treated as "*", otherwise each wildcard subtag is removed. For
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configuration choices. configuration choices.
3.3. Filtering 3.3. Filtering
Filtering is used to select the set of language tags that matches a Filtering is used to select the set of language tags that matches a
given language priority list. It is called "filtering" because this given language priority list. It is called "filtering" because this
set might contain no items at all or it might return an arbitrarily 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 large number of matching items: as many items as match the language
priority list, thus "filtering out" the non-matching items. priority list, thus "filtering out" the non-matching items.
In filtering, each language range represents the _least_ specific In filtering, each language range represents the least specific
language tag (that is, the language tag with fewest number of language tag (that is, the language tag with fewest number of
subtags) which is an acceptable match. All of the language tags in subtags) that is an acceptable match. All of the language tags in
the matching set of tags will have an equal or greater number of the matching set of tags will have an equal or greater number of
subtags than the language range. Every non-wildcard subtag in the subtags than the language range. Every non-wildcard subtag in the
language range will appear in every one of the matching language language range will appear in every one of the matching language
tags. For example, if the language priority list consists of the tags. For example, if the language priority list consists of the
range "de-CH" (German as used in Switzerland), one might see tags range "de-CH" (German as used in Switzerland), one might see tags
such as "de-CH-1996" (German as used in Switzerland, orthography of such as "de-CH-1996" (German as used in Switzerland, orthography of
1996) but one will never see a tag such as "de" (because the 'CH' 1996) but one will never see a tag such as "de" (because the 'CH'
subtag is missing). subtag is missing).
If the language priority list (see Section 2.3) contains more than If the language priority list (see Section 2.3) contains more than
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particular language tag if, in a case-insensitive comparison, it particular language tag if, in a case-insensitive comparison, it
exactly equals the tag, or if it exactly equals a prefix of the tag exactly equals the tag, or if it exactly equals a prefix of the tag
such that the first character following the prefix is "-". For such that the first character following the prefix is "-". For
example, the language-range "de-de" (German as used in Germany) example, the language-range "de-de" (German as used in Germany)
matches the language tag "de-DE-1996" (German as used in Germany, matches the language tag "de-DE-1996" (German as used in Germany,
orthography of 1996), but not the language tags "de-Deva" (German as orthography of 1996), but not the language tags "de-Deva" (German as
written in the Devanagari script) or "de-Latn-DE" (German, Latin written in the Devanagari script) or "de-Latn-DE" (German, Latin
script, as used in Germany). script, as used in Germany).
The special range "*" in a language priority list matches any tag. A The special range "*" in a language priority list matches any tag. A
protocol which uses language ranges MAY specify additional rules protocol that uses language ranges MAY specify additional rules about
about the semantics of "*"; for instance, HTTP/1.1 [RFC2616] the semantics of "*"; for instance, HTTP/1.1 [RFC2616] specifies that
specifies that the range "*" matches only languages not matched by the range "*" matches only languages not matched by any other range
any other range within an "Accept-Language" header. within an "Accept-Language" header.
Basic filtering is identical to the type of matching described in Basic filtering is identical to the type of matching described in
[RFC3066], Section 2.5 (Language-range). [RFC3066], Section 2.5 (Language-range).
3.3.2. Extended Filtering 3.3.2. Extended Filtering
Extended filtering compares extended language ranges to language Extended filtering compares extended language ranges to language
tags. Each extended language range in the language priority list is tags. Each extended language range in the language priority list is
considered in turn, according to priority. A language range matches considered in turn, according to priority. A language range matches
a particular language tag if their list of subtags match. To a particular language tag if each respective list of subtags matches.
determine a match: To determine a match:
1. Split both the extended language range and the language tag being 1. Split both the extended language range and the language tag being
compared into a list of subtags by dividing on the hyphen (%2D) compared into a list of subtags by dividing on the hyphen (%x2D)
character. Two subtags match if either they are the same when character. Two subtags match if either they are the same when
compared case-insensitively or the language range's subtag is the compared case-insensitively or the language range's subtag is the
wildcard '*'. wildcard '*'.
2. Begin with the first subtag in each list. If the first subtag in 2. Begin with the first subtag in each list. If the first subtag in
the range does not match the first subtag in the tag, the overall the range does not match the first subtag in the tag, the overall
match fails. Otherwise, move to the next subtag in both the match fails. Otherwise, move to the next subtag in both the
range and the tag. range and the tag.
3. While there are more subtags left in the language range's list: 3. While there are more subtags left in the language range's list:
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(or its synonym "de-DE") matches all of the following tags: (or its synonym "de-DE") matches all of the following tags:
de-DE (German, as used in Germany) de-DE (German, as used in Germany)
de-de (German, as used in Germany) de-de (German, as used in Germany)
de-Latn-DE (Latin script) de-Latn-DE (Latin script)
de-Latf-DE (Fraktur variant of Latin script) de-Latf-DE (Fraktur variant of Latin script)
de-DE-x-goethe (private use subtag) de-DE-x-goethe (private-use subtag)
de-Latn-DE-1996 (orthography of 1996) de-Latn-DE-1996 (orthography of 1996)
de-Deva-DE (Devanagari script) de-Deva-DE (Devanagari script)
The same range does not match any of the following tags for the The same range does not match any of the following tags for the
reasons shown: reasons shown:
de (missing 'DE') de (missing 'DE')
de-x-DE (singleton 'x' occurs before 'DE') de-x-DE (singleton 'x' occurs before 'DE')
de-Deva ('Deva' not equal to 'DE') de-Deva ('Deva' not equal to 'DE')
Note: [RFC3066bis] defines each type of subtag (language, script, Note: [RFC4646] defines each type of subtag (language, script,
region, and so forth) according to position, size, and content. This region, and so forth) according to position, size, and content. This
means that subtags in a language range can only match specific types 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' 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 is always a script subtag (unless it follows a singleton) while a
subtag such as 'nedis' can only match the equivalent variant subtag. subtag such as 'nedis' can only match the equivalent variant subtag.
Two-letter subtags in initial position have a different type Two-letter subtags in the initial position have a different type
(language) than two-letter subtags in later positions (region). This (language) than two-letter subtags in later positions (region). This
is the reason why a wildcard in the extended language range is is the reason why a wildcard in the extended language range is
significant in the first position but is ignored in all other significant in the first position but is ignored in all other
positions. positions.
3.4. Lookup 3.4. Lookup
Lookup is used to select the single language tag that best matches Lookup is used to select the single language tag that best matches
the language priority list for a given request. When performing the language priority list for a given request. When performing
lookup, each language range in the language priority list is lookup, each language range in the language priority list is
considered in turn, according to priority. By contrast with considered in turn, according to priority. By contrast with
filtering, each language range represents the _most_ specific tag filtering, each language range represents the most specific tag that
which is an acceptable match. The first matching tag found, is an acceptable match. The first matching tag found, according to
according to the user's priority, is considered the closest match and the user's priority, is considered the closest match and is the item
is the item returned. For example, if the language range is "de-ch", returned. For example, if the language range is "de-ch", a lookup
a lookup operation can produce content with the tags "de" or "de-CH" operation can produce content with the tags "de" or "de-CH" but never
but never content with the tag "de-CH-1996". If no language tag content with the tag "de-CH-1996". If no language tag matches the
matches the request, the "default" value is returned. request, the "default" value is returned.
For example, if an application inserts some dynamic content into a For example, if an application inserts some dynamic content into a
document, returning an empty string if there is no exact match is not document, returning an empty string if there is no exact match is not
an option. Instead, the application "falls back" until it finds a an option. Instead, the application "falls back" until it finds a
matching language tag associated with a suitable piece of content to matching language tag associated with a suitable piece of content to
insert. Some applications of lookup include: insert. Some applications of lookup include:
o Selection of a template containing the text for an automated email o Selection of a template containing the text for an automated email
response. response.
o Selection of a item containing some text for inclusion in a o Selection of an item containing some text for inclusion in a
particular Web page. particular Web page.
o Selection of a string of text for inclusion in an error log. o Selection of a string of text for inclusion in an error log.
o Selection of an audio file to play as a prompt in a phone system. 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 In the lookup scheme, the language range is progressively truncated
from the end until a matching language tag is located. Single letter from the end until a matching language tag is located. Single letter
or digit subtags (including both the letter 'x' which introduces or digit subtags (including both the letter 'x', which introduces
private-use sequences, and the subtags that introduce extensions) are private-use sequences, and the subtags that introduce extensions) are
removed at the same time as their closest trailing subtag. For removed at the same time as their closest trailing subtag. For
example, starting with the range "zh-Hant-CN-x-private1-private2" example, starting with the range "zh-Hant-CN-x-private1-private2"
(Chinese, Traditional script, China, two private use tags) the lookup (Chinese, Traditional script, China, two private-use tags) the lookup
progressively searches for content as shown below: progressively searches for content as shown below:
Example of a Lookup Fallback Pattern Example of a Lookup Fallback Pattern
Range to match: zh-Hant-CN-x-private1-private2 Range to match: zh-Hant-CN-x-private1-private2
1. zh-Hant-CN-x-private1-private2 1. zh-Hant-CN-x-private1-private2
2. zh-Hant-CN-x-private1 2. zh-Hant-CN-x-private1
3. zh-Hant-CN 3. zh-Hant-CN
4. zh-Hant 4. zh-Hant
5. zh 5. zh
skipping to change at page 15, line 4 skipping to change at page 14, line 15
3.4.1. Default Values 3.4.1. Default Values
Each application, protocol, or specification that uses lookup MUST Each application, protocol, or specification that uses lookup MUST
define the defaulting behavior when no tag matches the language define the defaulting behavior when no tag matches the language
priority list. What this action consists of strongly depends on how priority list. What this action consists of strongly depends on how
lookup is being applied. Some examples of defaulting behavior lookup is being applied. Some examples of defaulting behavior
include: include:
o return an item with no language tag or an item of a non-linguistic o return an item with no language tag or an item of a non-linguistic
nature, such as an image or sound nature, such as an image or sound
o return a null string as the language tag value, in cases where the o return a null string as the language tag value, in cases where the
protocol permits the empty value (see, for example, "xml:lang" in protocol permits the empty value (see, for example, "xml:lang" in
[XML10]) [XML10])
o return a particular language tag designated for the operation o return a particular language tag designated for the operation
o return the language tag "i-default" (see: [RFC2277]) o return the language tag "i-default" (see [RFC2277])
o return an error condition or error message o return an error condition or error message
o return a list of available languages for the user to select from o return a list of available languages for the user to select from
When performing lookup using a language priority list, the When performing lookup using a language priority list, the
progressive search MUST process each language range in the list progressive search MUST process each language range in the list
before seeking or calculating the default. before seeking or calculating the default.
The default value MAY be calculated or include additional searching The default value MAY be calculated or include additional searching
skipping to change at page 16, line 21 skipping to change at page 15, line 29
Users indicate their language preferences via the choice of a Users indicate their language preferences via the choice of a
language range or the list of language ranges in a language priority 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 list. The type of matching affects what the best choice is for a
user. user.
Most matching schemes make no attempt to process the semantic meaning Most matching schemes make no attempt to process the semantic meaning
of the subtags. The language range is compared, in a case- of the subtags. The language range is compared, in a case-
insensitive manner, to each language tag being matched, using basic insensitive manner, to each language tag being matched, using basic
string processing. Users SHOULD select language ranges that are string processing. Users SHOULD select language ranges that are
well-formed, valid language tags according to [RFC3066bis] well-formed, valid language tags according to [RFC4646] (substituting
(substituting wildcards as appropriate in extended language ranges). wildcards as appropriate in extended language ranges).
Applications are encouraged to canonicalize language tags and ranges Applications are encouraged to canonicalize language tags and ranges
by using the Preferred-Value from the IANA Language Subtag Registry by using the Preferred-Value from the IANA Language Subtag Registry
for tags or subtags which have been deprecated. If the user is for tags or subtags that have been deprecated. If the user is
working with content that might use the older form, the user might working with content that might use the older form, the user might
want to include both the new and old forms in a language priority want to include both the new and old forms in a language priority
list. For example, the tag "art-lojban" is deprecated. The subtag 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 '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 form the language range. Or the user might include both in a
language priority list: "jbo, art-lojban". language priority list: "jbo, art-lojban".
Users SHOULD avoid subtags that add no distinguishing value to a Users SHOULD avoid subtags that add no distinguishing value to a
language range. When filtering, the fewer the number of subtags that language range. When filtering, the fewer the number of subtags that
appear in the language range, the more content the range will appear in the language range, the more content the range will
probably match, while in lookup unnecessary subtags can cause probably match, while in lookup unnecessary subtags can cause
"better", more-specific content to be skipped in favor of less "better", more-specific content to be skipped in favor of less
specific content. For example, the range "de-Latn-DE" returns specific content. For example, the range "de-Latn-DE" returns
content tagged "de" instead of content tagged "de-DE", even though content tagged "de" instead of content tagged "de-DE", even though
the latter is probably a better match. the latter is probably a better match.
Whether a subtag adds distinguishing value can depend on the context Whether a subtag adds distinguishing value can depend on the context
of the request. For example, a user who reads both Simplified and of the request. For example, a user who reads both Simplified and
Traditional Chinese, but who prefers Simplified, might use the range Traditional Chinese, but who prefers Simplified, might use the range
"zh" for filtering (matching all items that user can read) but "zh- "zh" for filtering (matching all items that user can read) but
Hans" for lookup (making sure that user gets the preferred form if "zh-Hans" for lookup (making sure that user gets the preferred form
it's available, but the fallback to "zh" will still work). On the if it's available, but the fallback to "zh" will still work). On the
other hand, content in this case ought to be labeled as "zh-Hans" (or other hand, content in this case ought to be labeled as "zh-Hans" (or
"zh-Hant" if that applies) for filtering, while for lookup, if there "zh-Hant" if that applies) for filtering, while for lookup, if there
is either "zh-Hans" content or "zh-Hant" content, one of them (the is either "zh-Hans" content or "zh-Hant" content, one of them (the
one considered 'default') also ought to be made available with the one considered 'default') also ought to be made available with the
simple "zh". Note that the user can create a language priority list simple "zh". Note that the user can create a language priority list
"zh-Hans, zh" that delivers the best possible results for both "zh-Hans, zh" that delivers the best possible results for both
schemes. If the user cannot be sure which scheme is being used (or schemes. If the user cannot be sure which scheme is being used (or
if more than one might be applied to a given request), the user if more than one might be applied to a given request), the user
SHOULD specify the most specific (largest number of subtags) range SHOULD specify the most specific (largest number of subtags) range
first and then supply shorter prefixes later in the list to ensure first and then supply shorter prefixes later in the list to ensure
that filtering returns a complete set of tags. that filtering returns a complete set of tags.
Many languages are written predominantly in a single script. This is Many languages are written predominantly in a single script. This is
usually recorded in the Suppress-Script field in that language usually recorded in the Suppress-Script field in that language
subtag's registry entry. For these languages, script subtags SHOULD subtag's registry entry. For these languages, script subtags SHOULD
NOT be used to form a language range. Thus the language range "en- NOT be used to form a language range. Thus, the language range
Latn" is inappropriate in most cases (because the vast majority of "en-Latn" is inappropriate in most cases (because the vast majority
English documents are written in the Latin script and thus the 'en' of English documents are written in the Latin script and thus the
language subtag has a Suppress-Script field for 'Latn' in the 'en' language subtag has a Suppress-Script field for 'Latn' in the
registry). registry).
When working with tags and ranges, note that extensions and most When working with tags and ranges, note that extensions and most
private-use subtags are orthogonal to language tag matching, in that private-use subtags are orthogonal to language tag matching, in that
they specify additional attributes of the text not related to the they specify additional attributes of the text not related to the
goals of most matching schemes. Users SHOULD avoid using these goals of most matching schemes. Users SHOULD avoid using these
subtags in language ranges, since they interfere with the selection subtags in language ranges, since they interfere with the selection
of available content. When used in language tags (as opposed to of available content. When used in language tags (as opposed to
ranges), these subtags normally do not interfere with filtering ranges), these subtags normally do not interfere with filtering
(Section 3), since they appear at the end of the tag and will match (Section 3), since they appear at the end of the tag and will match
all prefixes. Lookup (Section 3.4) implementations are advised to all prefixes. Lookup (Section 3.4) implementations are advised to
ignore unrecognized private-use and extension subtags when performing ignore unrecognized private-use and extension subtags when performing
language tag fallback. language tag fallback.
4.2. Meaning of Language Tags and Ranges 4.2. Meaning of Language Tags and Ranges
Selecting language tags using language ranges requires some Selecting language tags using language ranges requires some
understanding by users of what they are selecting. The meaning of understanding by users of what they are selecting. The meanings of
the various subtags in a language range are identical to their the various subtags in a language range are identical to their
meaning in a language tag (see Section 4.2 in [RFC3066bis]), with the meanings in a language tag (see Section 4.2 in [RFC4646]), with the
addition that the wildcard "*" represents any matching sequence of addition that the wildcard "*" represents any matching sequence of
values. values.
4.3. Considerations for Private Use Subtags 4.3. Considerations for Private-Use Subtags
Private agreement is necessary between the parties that intend to use Private agreement is necessary between the parties that intend to use
or exchange language tags that contain private-use subtags. Great or exchange language tags that contain private-use subtags. Great
caution SHOULD be used in employing private-use subtags in content or caution SHOULD be used in employing private-use subtags in content or
protocols intended for general use. Private-use subtags are simply protocols intended for general use. Private-use subtags are simply
useless for information exchange without prior arrangement. useless for information exchange without prior arrangement.
The value and semantic meaning of private-use tags and of the subtags The value and semantic meaning of private-use tags and of the subtags
used within such a language tag are not defined. Matching private- used within such a language tag are not defined. Matching private-
use tags using language ranges or extended language ranges can result use tags using language ranges or extended language ranges can result
in unpredictable content being returned. in unpredictable content being returned.
4.4. Length Considerations for Language Ranges 4.4. Length Considerations for Language Ranges
Language ranges are very similar to language tags in terms of content Language ranges are very similar to language tags in terms of content
and usage. The same types of restrictions on length that can be and usage. The same types of restrictions on length that can be
applied to language tags can also be applied to language ranges. See applied to language tags can also be applied to language ranges. See
[RFC3066bis] Section 4.3 (Length Considerations). [RFC4646] Section 4.3 (Length Considerations).
5. IANA Considerations
This document presents no new or existing considerations for IANA.
6. Security Considerations 5. Security Considerations
Language ranges used in content negotiation might be used to infer Language ranges used in content negotiation might be used to infer
the nationality of the sender, and thus identify potential targets the nationality of the sender, and thus identify potential targets
for surveillance. In addition, unique or highly unusual language for surveillance. In addition, unique or highly unusual language
ranges or combinations of language ranges might be used to track a ranges or combinations of language ranges might be used to track a
specific individual's activities. specific individual's activities.
This is a special case of the general problem that anything you send This is a special case of the general problem that anything you send
is visible to the receiving party. It is useful to be aware that is visible to the receiving party. It is useful to be aware that
such concerns can exist in some cases. such concerns can exist in some cases.
The evaluation of the exact magnitude of the threat, and any possible The evaluation of the exact magnitude of the threat, and any possible
countermeasures, is left to each application or protocol. countermeasures, is left to each application or protocol.
7. Character Set Considerations 6. Character Set Considerations
Language tags permit only the characters A-Z, a-z, 0-9, and HYPHEN- Language tags permit only the characters A-Z, a-z, 0-9, and HYPHEN-
MINUS (%x2D). Language ranges also use the character ASTERISK MINUS (%x2D). Language ranges also use the character ASTERISK
(%x2A). These characters are present in most character sets, so (%x2A). These characters are present in most character sets, so
presentation or exchange of language tags or ranges should not be presentation or exchange of language tags or ranges should not be
constrained by character set issues. constrained by character set issues.
8. References 7. References
8.1. Normative References 7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998. Languages", BCP 18, RFC 2277, January 1998.
[RFC3066bis] [RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for
Phillips, A., Ed. and M. Davis, Ed., "Tags for the Syntax Specifications: ABNF", RFC 4234, October 2005.
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 [RFC4646] Phillips, A., Ed., and M. Davis, Ed., "Tags for
Specifications: ABNF", RFC 4234, October 2005. Identifying Languages", BCP 47, RFC 4646, September
2006.
8.2. Informative References 7.2. Informative References
[RFC1766] Alvestrand, H., "Tags for the Identification of [RFC1766] Alvestrand, H., "Tags for the Identification of
Languages", RFC 1766, March 1995. Languages", RFC 1766, March 1995.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Masinter, L., Leach, P., and T. Berners-Lee,
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
June 1999.
[RFC2616errata] [RFC2616errata] IETF, "HTTP/1.1 Specification Errata", October 2004,
IETF, "HTTP/1.1 Specification Errata", October 2004,
<http://purl.org/NET/http-errata>. <http://purl.org/NET/http-errata>.
[RFC3066] Alvestrand, H., "Tags for the Identification of [RFC3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001. Languages", BCP 47, RFC 3066, January 2001.
[RFC3282] Alvestrand, H., "Content Language Headers", RFC 3282, [RFC3282] Alvestrand, H., "Content Language Headers", RFC 3282,
May 2002. May 2002.
[XML10] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and [XML10] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E.,
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Third and F. Yergeau, "Extensible Markup Language (XML) 1.0
Edition)", World Wide Web Consortium Recommendation, (Third Edition)", World Wide Web Consortium
February 2004, <http://www.w3.org/TR/REC-xml>. Recommendation, February 2004,
<http://www.w3.org/TR/REC-xml>.
Appendix A. Acknowledgments Appendix A. Acknowledgements
Any list of contributors is bound to be incomplete; please regard the Any list of contributors is bound to be incomplete; please regard the
following as only a selection from the group of people who have following as only a selection from the group of people who have
contributed to make this document what it is today. contributed to make this document what it is today.
The contributors to [RFC1766] and [RFC3066], each of which was a The contributors to [RFC1766] and [RFC3066], each of which was a
precursor to this document, contributed greatly to the development of precursor to this document, contributed greatly to the development of
language tag matching, and, in particular, the basic language range language tag matching, and, in particular, the basic language range
and the basic matching scheme. This document was originally part of and the basic matching scheme. This document was originally part of
[RFC3066bis], but was split off before that document's completion. [RFC4646], but was split off before that document's completion.
Thus, directly or indirectly, those acknowledged in [RFC3066bis] also Thus, directly or indirectly, those acknowledged in [RFC4646] also
had a hand in the development of this document, and work done prior had a hand in the development of this document, and work done prior
to the split is acknowledged in that document. to the split is acknowledged in that document.
The following people (in alphabetical order by family name) The following people (in alphabetical order by family name)
contributed to this document: contributed to this document:
Harald Alvestrand, Stephane Bortzmeyer, Jeremy Carroll, Peter Harald Alvestrand, Stephane Bortzmeyer, Jeremy Carroll, Peter
Constable, John Cowan, Mark Crispin, Martin Duerst, Frank Ellermann, Constable, John Cowan, Mark Crispin, Martin Duerst, Frank Ellermann,
Doug Ewell, Debbie Garside, Marion Gunn, Jon Hanna, Kent Karlsson, Doug Ewell, Debbie Garside, Marion Gunn, Jon Hanna, Kent Karlsson,
Erkki Kolehmainen, Jukka Korpela, Ira McDonald, M. Patton, Randy Erkki Kolehmainen, Jukka Korpela, Ira McDonald, M. Patton, Randy
Presuhn, Eric van der Poel, Markus Scherer, Misha Wolf, and many, Presuhn, Eric van der Poel, Markus Scherer, Misha Wolf, and many,
many others. many others.
Very special thanks must go to Harald Tveit Alvestrand, who Very special thanks must go to Harald Tveit Alvestrand, who
originated RFCs 1766 and 3066, and without whom this document would originated RFCs 1766 and 3066, and without whom this document would
not have been possible. not have been possible.
Authors' Addresses Authors' Addresses
Addison Phillips (editor) Addison Phillips (Editor)
Yahoo! Inc. Yahoo! Inc.
Email: addison@inter-locale.com EMail: addison@inter-locale.com
Mark Davis (editor) Mark Davis (Editor)
Google Google
Email: mark.davis@macchiato.com EMail: mark.davis@macchiato.com or mark.davis@google.com
Intellectual Property Statement Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79. found in BCP 78 and BCP 79.
skipping to change at page 25, line 29 skipping to change at page 20, line 45
such proprietary rights by implementers or users of this such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr. http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at this standard. Please address the information to the IETF at
ietf-ipr@ietf.org. ietf-ipr@ietf.org.
Disclaimer of Validity Acknowledgement
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is provided by the IETF
Internet Society. Administrative Support Activity (IASA).
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