Network Working Group A. Phillips, Ed. Internet-Draft Yahoo! Inc Obsoletes: 3066 (if approved) M. Davis, Ed. Expires: August
10,27, 2006 Google February 6,23, 2006 Matching of Language Tags draft-ietf-ltru-matching-09draft-ietf-ltru-matching-10 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 10,27, 2006. Copyright Notice Copyright (C) The Internet Society (2006). 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(Ed.: replace "3066bis" with the RFC number assigned to draft-ietf-ltru-registry-14), replaces RFC 3066, which replaced RFC 1766. 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 . . . . . . . . . . . . . . . . 75 3. Types of Matching . . . . . . . . . . . . . . . . . . . . . . 87 3.1. Choosing a Type of Matching . . . . . . . . . . . . . . . 87 3.2. Filtering . . . . . . . . . . . . . . . . . . . . . . . . 98 3.2.1. Filtering withBasic Language RangesFiltering . . . . . . . . . . . 11 3.2.2. Filtering with Extended Language Ranges. . . . . . . 11 3.2.3. Scored Filtering. 9 3.2.2. Extended Filtering . . . . . . . . . . . . . . . . . . 1110 3.3. Lookup . . . . . . . . . . . . . . . . . . . . . . . . . . 1510 4. Other Considerations . . . . . . . . . . . . . . . . . . . . . 1914 4.1. Choosing Language Ranges . . . . . . . . . . . . . . . . . 1914 4.2. Meaning of Language Tags and Ranges . . . . . . . . . . . 2015 4.3. Considerations for Private Use Subtags . . . . . . . . . . 2115 4.4. Length Considerations in Matching . . . . . . . . . . . . 2215 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 2417 6. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 2518 7. Security Considerations . . . . . . . . . . . . . . . . . . . 2619 8. Character Set Considerations . . . . . . . . . . . . . . . . . 2720 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 2821 9.1. Normative References . . . . . . . . . . . . . . . . . . . 2821 9.2. Informative References . . . . . . . . . . . . . . . . . . 2821 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 2922 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 3023 Intellectual Property and Copyright Statements . . . . . . . . . . 3124 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 processinginformation or understanding of contentin different languages. Given asome specific set of information items or "content". One use for language identifiers, such as those defined in [RFC3066bis], various mechanisms can be envisioned for performingis to select content by matching the associated language negotiation and tag matching.tags to a user's language preferences. This document defines a syntax (called a language range (Section 2)) for specifying aitems in the user's language preferences,preferences (called a language priority list (Section 2.3)), as well as several schemes for selecting or filtering sets of content by comparing language ranges tothe content's language tags [RFC3066bis] usedto identifythe natural language of that content.user's preferences. Applications, protocols, or specifications will have varying needs and requirements that affect the choice of a suitable matching scheme. Depending on the choice of scheme, there are various options left to the implementation. Protocols that implement a matching scheme either need to specify each particular choice or indicate the options that are left to the implementation to decide. This document is divided into three main sections. One describes how to indicate a user's preferences using language ranges. Then a section describes various schemes for matching these ranges to a set of language tags in order to select specific content.tags. There is also a section that deals with various practical considerations that apply to implementing and using these schemes. This document, in combination with [RFC3066bis] (Ed.: replace "3066bis" globally in this document with the RFC number assigned to draft-ietf-ltru-registry-14), replaces [RFC3066], which replaced [RFC1766]. The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2. The Language Range Language Tags [RFC3066bis] are used to identify the language of some information item or content. Applications or protocols that use language tags are often faced with the problem of identifying sets of content that share certain language attributes. For example, HTTP/1.1 [RFC2616] describes one such mechanism in its discussion of the Accept-Language header (Section 14.4), which is used when selecting content from servers based on the language of that content. When selecting content according to its language, it is useful to have a mechanism for identifying sets of language tags that share specific attributes. This allows users to select or filter content based on specific requirements. Such an identifier is called a "Language Range". There are different types of language range, whose specific attributes vary to match their application. Language ranges are similar in structure andcontent to language tags: they consist of alphanumeric "subtags"a sequence of subtags separated by hyphens, plushyphens. In a speciallanguage range, each subtag consistingMUST either be a sequence of ASCII alphanumeric characters or the single character "*"'*' (%2A, ASTERISK), whichASTERISK). The character '*' is used in ranges asa "wildcard", that is, a value"wildcard" that matches any subtag.sequence of subtags. Restrictions on the meaning and use of wildcards vary according to the type of language range. Language tags and thus language ranges are to be treated as case- insensitive: there exist conventions for the capitalization of some of the subtags, but these MUST NOT be taken to carry meaning. Matching of language tags to language ranges MUST be done in a case- insensitive manner as well.manner. 2.1. Basic Language Range A "basic language range" identifies the set of content whoselanguage tags that all 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 ABNF[RFC4234]:ABNF [RFC4234]: language-range = language-tag(1*8ALPHA *("-" 1*8alphanum)) / "*" 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). (Note that the ABNF [RFC4234] in [RFC2616] is incorrect, since it disallows the use of digits anywhere in the 'language-range': this is mentioned in the errata) Use of a basic language range seems to imply that there is a semantic relationship between language tags that share the same prefix. While this is often the case, it is not always true and users should note that the set of language tags that match a specific language-rangelanguage range may not berepresent mutually intelligible.intelligible languages. 2.2. Extended Language Range ABasic Language Range does not always provide the most appropriate waylanguage ranges allow users to specify a user's preferences. Sometimes it is beneficial to use a more fine-grained matching scheme that takes advantage of the internal structureset of language tags. This allowstags that share the usersame initial subtags. Occasionally users will wish to specify, for example,select a set of language tags based on the valuepresence of aspecific field insubtags. For example, a language tag oruser might wish to indicate which values are of interest in filtering or selecting the content. In an extendedselect all language range, the identifier takestags that contains the form ofregion subtag 'CH'. Extended language ranges are useful in specifying a series of subtags which MUST consistparticular sequence of well-formedsubtags or the special subtag "*". For example,that appear in the language range "en-*-US" specifies a primary languageset of matching tags without having to specify all of 'en', followed by any script subtag, followed bythe region subtag 'US'.intervening subtags. An extended language range can be represented by the following ABNF: extended-language-range = range ; a range / privateuse ; a private-use range(1*8ALPHA / grandfathered ; a grandfathered registration 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(1*8alphanum / "*")) ; 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 ;Figure 2: Extended Language Range The wildcard can only appear ; at the end singleton = %x41-57 / %x59-5A / %x61-77 / %x79-7A / DIGIT ; single letters (except for "x") or digits privateuse = "x" 1*("-" (1*8alphanum)) grandfathered = 1*3ALPHA 1*2("-" (2*8alphanum)) ; grandfathered registration ; Note: I is the only singleton ; that starts a grandfathered tag alphanum = (ALPHA / DIGIT) ; letters and numbers A field not presentsubtag '*' MAY occur in any position in the middle of anextended language range is treated as if the field contained a "*". Implementationsrange, where it matches any sequence of subtags that normalizemight occur in that position in a language tag. However wildcards outside the first position in an extended language rangesrange are ignored by most matching schemes. Use of multiple wildcards SHOULD expand missing fields toNOT be "*" sotaken to imply that the semantic meaninga certain number of subtags will appear in the matching set of language range is cleartags. Implementations that specify basic ranges MAY map extended language ranges to basic language ranges: if the user. At the same time, multiple wildcards in a row are redundant and implementations SHOULD collapse these tofirst subtag is a single wildcard when normalizing"*" then the entire range (for brevity).is treated as "*" (which matches the default content), otherwise each wildcard subtag is removed. For example, bothif the language range "sl-nedis" andwere "en-*-US", then the range "sl-*-*-nedis" are equivalent to and shouldwould be normalized as "sl-*-nedis".mapped to "en-US". 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,semicolons, like this: "en; fr; zh-Hant" (which would be read as "English before French before Chinese as written in the Traditional script"). Where a language priority list provides "quality weights" for the language ranges, such as the use of Q weights in the syntax of the "Accept-Language" header (defined in [RFC2616], Section 14.4, and [RFC3282]), language ranges without a weight are given values equal to the value of the previous language range (processing from first to last). If the first language range has no weight, it is given a value of 1.0. Then language ranges with zero weights are removed. For example, "fr, en;q=0.5, de, it" becomes "fr;q=1.0, en;q=0.5, de;q=0.5, it;q=0.5". The language priority list is then sorted from highest priority to lowest, with language ranges that share the same weights remain in the same order as in the original language priority list. 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 basicseveral types of matching scheme:scheme. This document presents two types: those that produce zero or more information items (called "filtering") and those that produce a single information item for a given request (called "lookup"). A key difference between these two types ofImplementations or protocols MAY use different matching scheme is thatschemes than the language rangesones described in the language priority list represent the _least_ specific content one will acceptthis document, as a match, while for lookup operations the language ranges represent the _most_ specific content.long as those mechanisms are clearly specified. 3.1. Choosing a Type of Matching Applications, protocols, and specifications are faced with the decision of what type of matching to use. Sometimes, different styles of matching 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 define the result inas failure of the operation, an empty value, returning some protocol defined or implementation defined default, or returning i-default [RFC2277]. This document describes three types of matching: 1. Basic Filtering (Section 3.2.1) matches a language priority list consisting of basic language ranges (Section 2.1) to sets of language tags. 2. Extended Filtering (Section 3.2.2) matches a language priority list consisting of extended language ranges (Section 2.2) to sets of language tags. 3. Lookup (Section 3.3) matches a language priority list consisting of basic language ranges to sets of language tags find the _exactly_ one language tag that best matches the range. Both types of filtering can be used to produce a set of results (such as a collection of documents).documents) by comparing the user's preferences to language tags associated with the set of content. For example, if usingwhen performing a search engine,search, one might use filtering to limit the results to documents tagged as being written in French. It canThey might 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. 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)Lookup produces an ordered set of content using extended language ranges. It SHOULD be used whenthe quality of the match within a specific language range is important, as when presentingsingle result that best matches a listgiven set of documents resulting from a search. 4. Lookup (Section 3.3)user preferences, so it is used when each request needs to produce _exactly_ one piece of content.useful in cases in which only a single item can be returned. For example, if a process were to insert a human readable error message into a protocol header, it might select the text based on the user's language preference.priority list. Since itthe process can return only one item, it must choose a single item and it must return some item, even if no contentcontent's language tag matches the language priority list supplied by the user. MostThe types of matching in this document are designed so that implementations are not required to validate or understand any of the semantics of the language tags or ranges or of the subtags supplied and, except for scored filtering, they do not needin them. None of them require access to the IANA Language Subtag Registry (see Section 3 in [RFC3066bis]). This simplifies and speeds the performance of implementations. Regardless of the matching scheme chosen, protocols and implementations MAY canonicalize language tags and ranges by mapping grandfathered and obsolete tags or subtags into modern equivalents. If an implementation canonicalizes either ranges or tags, then the implementation will require the IANA Language Subtag Registry information for that purpose. Implementations MAY also use semantic information external to the registry when matching tags. For example, the primary language subtags 'nn' (Nynorsk Norwegian) and 'nb' (Bokmal Norwegian) might both be usefully matched to the more general subtag 'no' (Norwegian). Or an implementation might infer that content labeled "zh-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 contentlanguage tags that matches a given language priority list.list and return the associated content. It is called "filtering" because this set of content maymight contain no items at all or it maymight return an arbitrarily large number of matching items: as many items as match the language priority list, thus "filtering out" the non-matching items. In filtering, the language range represents the _least_ specific (that is, the fewest number of subtags) language tag which is an acceptable match. That is, allAll of the language tags in the matching set of filtered contenttags will have an equal or greater number of subtags than the language range. Every non-wildcard subtag in the language range will appear in every one of the matching language tags. For example, if the language priority list consists of the range "de-CH", one might see matching content with the tagtags such as "de-CH-1996" but one will never see a match with thetag "de".such as "de" (because the 'CH' subtag is missing). If the language priority list (see Section 2.3) contains more than one range, the content returned is typically ordered in descending level of preference. Some examples of applications where filtering might be appropriate include: o Applying a style to sections of a document in a particular set of languages. o Displaying the set of documents containing a particular set of keywords written in a specific set of languages. o Selecting all email items written in a specific set of languages. Filtering can produceThe content returned MAY either anbe ordered or anunordered set of results. For example, applying formattingaccording to a document based on the language of specific pieces of content does not requirethe content to be ordered. It is sufficient to know whether a specific piece of content is selected bypriority in the language priority list (or not). A search application, on the(and other hand, probably would wantcriteria), according to order the results. If an ordered set is desired, as described above, thenthe application or protocolneeds to determine the relative "quality"of the match between differentapplication or protocol. 3.2.1. Basic Filtering When filtering using basic language tags andranges, each basic language range in the language range. This measurementpriority list is called a "distance metric". A distance metric assigns a numeric valueconsidered in turn, according to the comparison of apriority. A particular language tag tomatches a language range that represents the 'distance' betweenif it exactly equals the two. A distance of zero means that they are identical, a small distance indicates that they are very similar, andtag, or if it exactly equals 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 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 suchprefix of the tag such that the first character following the prefix is "-". (That is,For example, the language-range "de-de" matches the language tag "de-DE-1996","de-DE- 1996", but not the language tag "de-Deva".)tags "de-Deva" or "de-Latn-DE". The special range "*" in a language priority list matches any tag. A protocol which uses language ranges MAY specify additional rules about the semantics of "*"; for instance, HTTP/1.1 [RFC2616] specifies that the range "*" matches only languages not matched by any other range within an "Accept-Language" header. 3.2.2. Filtering withExtended Language RangesFiltering 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 extendedA particular language range areis compared to each language tag using the corresponding subtagsfollowing process: Compare the first subtag in the extended language tag being examined. Theto the first subtag fromin the language tag in a case insensitive manner. If the first subtag in the range is considered to match if"*", it exactlymatches any value. Otherwise the two values must match or the correspondingoverall match fails. Take each non-wildcard subtag in the language range and compare it to the next subtag in the language tag in turn until a matching subtag is found or the range'slangauge tag is exhausted. If the end of the language tag is found first, the match fails. If a match is found, this step is repeated with the next non-wildcard subtag hasin the value "*" (which matches all subtags, includinglanguage range (and beginning with the empty subtag).next subtag in the language tag) until the list of subtags in the language range is exhausted or the match fails. Subtags not specified, including those at the end of the language range, are thus treated as if assigned the wildcard value "*". This makes each range into a prefixExtended filtering works, therefore, much like that used inbasic language range matching.filtering. For example, the extended language range "de-*-DE" matches all of the following tags, in part because the unspecified variant, extension, and private-use subtags are expanded to "*":tags: 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 useful3.3. Lookup Lookup is used to provide an array of results with different levels of matching, for example, sorting results based onselect the overall "quality" ofsingle language tag that best matches the match. Scored (or "distance metric") filtering provideslanguage priority list for a way to generate these quality values. As with the other forms of filtering,given request and return the process considersassociated content. When performing lookup, each language range in the language priority list is considered in order ofturn, according to priority. Each extendedBy contrast with filtering, each language range and language tag MUST first be canonicalized by mapping grandfathered and obsolete tags into modern equivalents. This requires the information inrepresents the IANA Language Subtag Registry (see Section 3 of [RFC3066bis]). The language range and each language_most_ specific tag itwhich is to be compared to are then transformed intoan acceptable match. The first content found with a "quintuple" consisting of five "elements" inmatching tag, according to the form (language, script, country, variant, extension). Any extended language subtags areuser's priority, is considered part ofthe language "element".closest match and is the content returned. For example, if the language element forrange is "de-ch", a lookup operation might produce content with the tag "zh-cmn- Hans" would be "zh-cmn". Private-use subtag sequences are considered part oftags "de" or "de-CH" but never one with the language "element"tag "de-CH-1996". Usually if in the initial position inno content matches the tag and part ofrequest, the variant "element""default" content is returned. For example, if not. The different handling of private-use sequences prevents a range such as "x-twain" from matching all possible tags, whilean application inserts some dynamic content into a range such as "en-US-x-twain" would closelydocument, returning an empty string if there is no exact match nearly all tags for English as used in the United States. Language subtags 'und', 'mul', andis not an option. Instead, the script subtag 'Zyyy' are converted to "*": these subtag values represent undetermined, multiple, or private-use values which are consistentapplication "falls back" until it finds a matching language tag associated with the usea suitable piece 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 setcontent to insert. Examples of lookup might include: o Selection of a template containing the script subtagtext for an automated email response. o Selection of a item containing some text for inclusion 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 (see Section 4.1) recommends that users not use them to form ranges. Languages which havea "Suppress-Script" fieldparticular Web page. o Selection of a string of text for inclusion in an error log. In the registry are predominantly written in that single script, makinglookup scheme, the subtag redundant in forming alanguage tag or range. Thus ifrange is progressively truncated from the script were not expanded in this manner,end until a matching piece of content is located. For example, starting with the range such"zh-Hant-CN-x-private", the lookup progressively searches for content as "de-DE" would produceshown 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) Figure 3: Example of a Lookup Fallback Pattern This scheme allows some flexibility in finding a more-distant scorematch. For example, lookup provides better results for cases in which content is not available that happened to be labeled "de-Latn-DE"exactly matches the user request than users would expect that it should. Any remaining missing components inif the default 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 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: 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) Where a language priority list follows the syntax of the "Accept- Language" header defined in [RFC2616] (see Section 14.4) and [RFC3282], language ranges without a Q value are given values equal to the value of the previous language range in the list (processing from first to last). If the first language range has no Q value, it is given a value of 1.0. Language ranges with Q values of zero are removed. For example, "fr, en;q=0.5, de, it" becomes "fr;q=1.0,en;q=0.5,de;q=0.5,it;q=0.5". The distance values given above are then divided by the Q values. For example, if that language tag "fr-FR" has a distance of 384 from a language range with a Q value of 0.8, then the resulting distance is 480 (384 div 0.8). Implementations or protocols MAY use different weighting systems than the ones described above, as long as the weightings and weighting mechanisms are clearly specified. Thus, for example, an implementation or protocol could give all language tags with missing Q values a value of 1.0, or give the distance value 1000 to a language mismatch. They MAY also use more sophisticated weights that depend on the values of the corresponding elements. For example, an implementation might give a small distance to the difference closely related subtags. 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 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 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 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. Implementations MAY also map extended language ranges to basic language ranges: if the first subtag is a "*" then the entire range is treated as "*" (which matches the default content), otherwise each wildcard subtag is removed. For example, if the language range were "en-*-US", then the range would be mapped to "en-US". Where a language priority list contains Q values as in the syntax of the "Accept-Language" header defined in [RFC2616] (see Section 14.4) and [RFC3282], language tags without a Q value are given values equal to the value of the previous language tag (processing from first to last). If the first language tag has no Q value, it is given a value of 1.0. Then language tags with zero Q values are removed. For example, "fr, en;q=0.5, de, it" becomes "fr;q=1.0, en;q=0.5, de;q=0.5, it;q=0.5". The language priority list is then sorted from highest priority to lowest, whereby any two language tags with the same Q values are remain in the same order as in the original language priority list. This list is then traversed as described above in doing lookup. Implementations or protocols MAY use different lookup mechanisms systems than the ones described above, as long as those mechanisms are clearly specified. 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 isfor 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 inthe language range, the moresystem or 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 interfere with filtering (Section 3), since they appear at the endwere returned immediately. Not every specific level of thetag and will match all prefixes. When working with language tags andgranularity is usually available or language ranges note that: o Private-use and Extension subtags are normally orthogonalcontent may be sparsely populated, so "falling back" through the subtag sequence provides more opportunity to language tag fallback. Implementations or specifications that usefind a lookup (Section 3.3) matching scheme often ignore unrecognized private-usematch between available language tags and extension subtagsthe user's request. The default behavior when performing languageno tag fallback. In addition, since these subtags are always at the end ofmatches the sequence of subtags, their use inlanguage tags normally doesn't interferepriority list is implementation defined. An implementation might, for example, return content with the use of ranges that omit them in the filtering (Section 3.2) matching schemes described below. However, they do interfereno language tag; might supply content with filtering when used inan empty language ranges and SHOULD be avoidedtag value (the built-in attribute xml:lang in ranges as[XML10] permits the empty value); might be 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 inparticular language designated for the bit of content that they are processing.being selected; or it might select the tag "i-default" (see [RFC2277]). When performing lookup using a language tag instance ispriority list, the progressive search MUST proceed to be usedconsider each language range in the list before finding the default content or empty tag. One common way for an application or implementation to provide for a specific, known protocol, anddefault is not being passed throughto other protocols,allow a specific language tags MAY be filteredrange to remove subtags and extensions that are not supported by that protocol. Such filtering SHOULDbe avoided,set as the default for a specific type of request. This language range is then treated as if possible, sinceit removes information that might be relevantwere appended to services onthe otherend of the protocol that would make use of that information. o Some applicationslanguage 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 tags might want or need to consider4. zh 5. (search for the default content) a. ja-JP b. ja c. (implementation defined default) Figure 4: Lookup Using a Language Priority List Implementations SHOULD ignore extensions and unrecognized private-use subtags when matching tags. If extensions and private-useperforming lookup, since these subtags are included in a matching or filtering process that utilizes one ofusually orthogonal to the schemes described inuser's request. The special language range "*" matches any language tag. In the lookup scheme, this document, thenrange does not convey enough information by itself to determine which content is most appropriate, since it matches everything. If the implementation SHOULD canonicalizelanguage 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 tags and/orpriority list might contain one or more extended language ranges before performing(as, for example, when the matching. Note thatsame language tag processors that claim to be "well-formed" processorspriority list is used as defined in [RFC3066bis] generally fall into this category. 4.2. Meaning of Language Tagsinput for both lookup and Ranges Selecting content using language ranges requires some understanding by users of what they are selecting. Afiltering operations). Wildcard values in an extended language tag orrange identifiesnormally match any value that occurs in that position in a language as spoken (or written, signed or otherwise signaled) by human beingstag. Since only one item can be returned for communication of information to other human beings. Ifany given lookup request, wildcards in a language tag B contains language tag A asrange have to be processed in a prefix, then B is typically "narrower"consistent manner or "more specific" than A. For example, "zh- Hant-TW"the same request will produce widely varying results. Implementations that accept extended language ranges MUST define which content is returned when more specificthan "zh-Hant". This relationship is not guaranteed in all cases: specifically, languages that begin withone item matches the same sequence of subtags are NOT guaranteed to be mutually intelligible, although they might be.extended language range. For example, an implementation could return the matching tag "az" shares a prefix with both "az-Latn" (Azerbaijani written usingthat is first in ASCII-order. If the Latin script)language range were "*-CH" and "az-Arab" (Azerbaijani written using the Arabic script). A person fluent in one script might not be able to readthe other, even thoughset of tags included "de-CH", "fr-CH", and "it-CH", then the text mighttag "de-CH" would be otherwise identical. Content tagged as "az" most probably is written in just one script and thus might notreturned. Another example would be intelligiblefor an implementation to a reader familiar withmap the other script. Variant subtags in particular seem to represent specific divisions in mutual understanding, since they often encode dialects or other idiosyncratic variations within a language. They also seemextended language ranges to represent relatively low divisionsbasic ranges. 4. Other Considerations When working with a high chance of at least limited understanding, although this depends on the specific variant in question. The relationship between thelanguage tagranges and matching schemes, there are some additional points that may influence the information it relates to is defined by the standard describingchoice of either. 4.1. Choosing Language Ranges Users indicate their language preferences via the context in which it appears. Accordingly, this section can only give possible exampleschoice of its usage: o Fora single information object, the associatedlanguage tags might be interpreted asrange or the setlist of languages that are necessary forlanguage ranges in a complete comprehensionlanguage priority list. The type of matching affects what the complete object. Example: Plain text documents. o For an aggregationbest choice is for a given user. Most matching schemes make no attempt to process the semantic meaning of information objects,the associatedsubtags. The 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 purposerange (or its subtags) is usually compared in a case-insensitive manner to provide alternatives,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 associatedlanguage tags couldrange, the more content the range will match. Most notably, script subtags SHOULD NOT be regarded asused to form a hintlanguage range in combination with language subtags that have a matching Suppress-Script field in their registry entry. Thus the contentlanguage range "en-Latn" is providedprobably inappropriate in most cases (because the vast majority of English documents are written in several languages,the Latin script and that one has to inspect each ofthus the alternatives in order to find its'en' language or languages. In this case,subtag has a Suppress-Script field for 'Latn' in the presence of multipleregistry). When working with tags might not meanand ranges, note that one needs to be multi-lingual to get complete understanding of the document. Example: MIME multipart/ alternative. o In markup languages, such as HTMLextensions and XML, language information can be addedmost private-use subtags are orthogonal to each partlanguage tag matching, in that they specify additional attributes of the document identified by the markup structure (including the whole document itself). For example, one could write <span lang="FR">C'est la vie.</span> inside a Norwegian document; the Norwegian-speaking user could then access a French-Norwegian dictionarytext not related to find out whatthe marked section meant. Ifgoals of most matching schemes. Users SHOULD avoid using these subtags in language ranges, since they interfere with the user were listening to that document through a speech synthesis interface, this formation could beselection of available content. When used in language tags (as opposed to signalranges), these subtags normally do not interfere with filtering (Section 3), since they appear at the synthesizer to appropriately apply French text-to-speech pronunciation rules to that span of text, insteadend of misapplyingthe Norwegian rules. 4.3. Considerations for Private Use Subtagstag and will match all prefixes. Private-use and Extension subtags require private agreement between the parties that intendare normally orthogonal to use or exchangelanguage tagstag fallback. Implementations or specifications that use them and great caution SHOULD be used in employing them in content or protocols intended for general use. Private-use subtags are simply useless for information exchange without prior arrangement. The value and semantic meaning ofa lookup (Section 3.3) matching scheme often ignore unrecognized private-use tagsand of theextension subtags used within such awhen performing language tag fallback. In addition, since these subtags are not defined. Matching private-always at the end of the sequence of subtags, their use tags usingin language tags normally doesn't interfere with the use of ranges or extendedthat omit them in the filtering (Section 3.2) matching schemes described below. However, they do interfere with filtering when used in language ranges can result in unpredictable content being returned. 4.4. Length Considerationsand SHOULD be avoided in Matching RFC 3066 [RFC3066] didranges as a result. Applications, specifications, or protocols that choose not provide an upper limit on the size of language tagsto interpret one or ranges. RFC 3066 did define the semantics of particularmore private-use or extension subtags SHOULD NOT remove or modify these extensions in such a waycontent that most language tags or ranges consisted ofthey are processing. When a language and region subtags withtag instance is to be used in a combined total length of upspecific, known protocol, and is not being passed through to six characters. Largerother protocols, language tags and ranges (in terms of bothMAY be filtered to remove subtags and characters) did exist, however. [RFC3066bis] also doesextensions that are not impose a fixed upper limitsupported by that protocol. Such filtering SHOULD be avoided, if possible, since it removes information that might be relevant to services on the numberother end of subtags in a language tag or range (and thus an upper bound onthe sizeprotocol that would make use of either). The syntax inthat document suggests that, depending on the specificinformation. Some applications of language tags might want or range of languages, more subtags (and thus characters) are sometimes necessary as a result. Length considerations and their impact on the selectionneed to consider extensions and processing of tagsprivate-use subtags when matching tags. If extensions and private-use subtags are describedincluded in Section 2.1.1 of that document. An applicationa matching or protocol MAY choose to limit the lengthfiltering process that utilizes one of the language tags or ranges usedschemes described in matching. Any such limitation SHOULD be clearly documented, and such documentationthis document, then the implementation SHOULD includecanonicalize the disposition of any longerlanguage tags orand/or ranges (for example, whether an error value is generated orbefore performing the matching. Note that language tag or range is truncated). If truncation is permitted it MUST NOT permit a subtagprocessors that claim to be divided, since"well-formed" processors as defined in [RFC3066bis] generally fall into this changes the semanticscategory. 4.2. Meaning of the subtag being matchedLanguage Tags and can result in false positives or negatives. Applications or protocols that restrict storage SHOULD consider the impactRanges Selecting content using language ranges requires some understanding by users of tag or range truncation on the resulting matches. For example, removing the "*" from the endwhat they are selecting. The meaning of an extendedthe various subtags in a language range are identical to their meaning in a language tag (see Section 2.2) can greatly modify4.2 in [RFC3066bis]), with the set of returned matches. A protocoladdition that allows tags or ranges to be truncated at an arbitrary limit, without givingthe wildcard "*" represents any indicationmatching sequence of what that limit is, has the potentialvalues. 4.3. Considerations 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 extraneousPrivate Use Subtags Private-use subtags interfere withrequire private agreement between the meaning, understanding, and especially matching of language tags. Since language tags or ranges MAY be truncated by an application or protocolparties that limits storage, when choosing language tags or ranges users and applications SHOULD avoid adding subtagsintend to use or exchange language tags that add no distinguishing value. In particular, usersuse them and implementationsgreat caution SHOULD follow the 'Prefix' and 'Suppress-Script' fieldsbe used in the registry (definedemploying them in Section 3.6 of [RFC3066bis]): these fields provide guidance on when specific additionalcontent or protocols intended for general use. Private-use subtags SHOULD (and SHOULD NOT) be used. Implementations MUST support a limitare simply useless for information exchange without prior arrangement. The value and semantic meaning of at least 33 characters. This limit includes at least one subtagprivate-use tags and of each non-extension, non-private use type. When choosing a buffer limit,the subtags used within such a length of at least 42 characters is strongly RECOMMENDED. The practical limit onlanguage tag are not defined. Matching private- use tags using language ranges or extended language ranges derived solely from registered values is 42 characters. Implementations MUST be ablecan result in unpredictable content being returned. 4.4. Length Considerations in Matching Language ranges are very similar to handlelanguage tags and rangesin terms of this length. Support for tagscontent and rangesusage. The same types of at least 62 characters inrestrictions on length is RECOMMENDED. Implementations MAY support longer values, including matching extensive sets of private-use or extension subtags. Applications or protocols which havethat apply to truncate a tag MUST do so by progressively removing subtags along with their preceding "-" from the right side of thelanguage 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 "-" MUSTtags could also be removed. For example: Tagapply 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 Truncationlanguage ranges. Implementation, protocol, and specificiation authors SHOULD apply the considerations in [RFC3066bis] Section 4.3 (Length Considerations) where appropriate to language ranges and language priority lists. 5. IANA Considerations This document presents no new or existing considerations for IANA. 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)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. 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. 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, January 1998. [RFC3066bis] Phillips, A., Ed. and M. Davis, Ed., "Tags for the Identification of Languages", October 2005, <http:// www.ietf.org/internet-drafts/ draft-ietf-ltru-registry-14.txt>. [RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. 9.2. Informative References [RFC1766] Alvestrand, H., "Tags for the Identification of Languages", RFC 1766, March 1995. [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [RFC2616errata] IETF, "HTTP/1.1 Specification Errata", 10 2004, <http://purl.org/NET/http-errata>. [RFC3066] Alvestrand, H., "Tags for the Identification of Languages", BCP 47, RFC 3066, January 2001. [RFC3282] Alvestrand, H., "Content Language Headers", RFC 3282, May 2002. [XML10] Bray (et al), T., "Extensible Markup Language (XML) 1.0", 02 2004. Appendix A. Acknowledgements Any list of contributors is bound to be incomplete; please regard the following as only a selection from the group of people who have contributed to make this document what it is today. The contributors to [RFC3066bis], [RFC3066] and [RFC1766], each of which is a precursor to this document, made enormous contributions directly or indirectly to this document and are generally responsible for the success of language tags. The following people (in alphabetical order by family name) contributed to this document: Harald Alvestrand, Jeremy Carroll, John Cowan, Martin Duerst, Frank Ellermann, Doug Ewell, Marion Gunn, Kent Karlsson, Ira McDonald, M. Patton, Randy Presuhn, Eric van der Poel, Markus Scherer, and many, many others. Very special thanks must go to Harald Tveit Alvestrand, who originated RFCs 1766 and 3066, and without whom this document would not have been possible. For this particular document, John Cowan originated the scheme described in Section 3.2.3.scoring scheme. Mark Davis originated the scheme described in theSection 3.3. Authors' Addresses Addison Phillips (editor) Yahoo! Inc Email: addison at inter dash locale dot com Mark Davis (editor) Google Email: mark dot davis at macchiato dot com Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in 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 made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at email@example.com. Disclaimer of Validity 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 Internet Society.