Network Working Group M. Duerst Internet-Draft W3C/Keio University Expires: October 16, 2002 M. Suignard Microsoft Corporation April 17, 2002 Internationalized Resource Identifiers (IRI) draft-duerst-iri-00 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 October 16, 2002. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract This document defines a new protocol element, the Internationalized Resource Identifier (IRI), as a complement to the URI [RFC2396]. An IRI is a sequence of characters from the Universal Character Set [ISO10646]. A mapping from IRIs to URIs is defined, which means that IRIs can be used instead of URIs where appropriate to identify resources. The approach of defining a new protocol element was chosen, instead of extending or changing the definition of URIs, to allow a clear distinction and to avoid incompatibilities with existing software. Duerst & Suignard Expires October 16, 2002 [Page 1] Internet-Draft Internationalized Resource Identifiers April 2002 Guidelines for the use and deployment of IRIs in various protocols, formats, and software components that now deal with URIs are provided. Section 1 introduces concepts, definitions, and the scope of this specification. Section 2 discusses the IRI syntax and conversion between IRIs and URIs. Section 3 deals with limitations on characters appropriate for use in IRIs, and with processing of IRIs. Section 4 discusses software requirements for IRIs from an operational viewpoint. NOTE This draft replaces draft-masinter-url-i18n-08.txt. This document is a product of the Internationalization Working Group (I18N WG) of the World Wide Web Consortium (W3C). For general discussion, please use the www-i18n-comments@w3.org mailing list (publicly archived at http://lists.w3.org/Archives/Public/www-i18n-comments/). For more information on the topic of this document, please also see [W3CIRI] and [Duer01]. Duerst & Suignard Expires October 16, 2002 [Page 2] Internet-Draft Internationalized Resource Identifiers April 2002 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 Overview and Motivation . . . . . . . . . . . . . . . . . . 4 1.2 Applicability . . . . . . . . . . . . . . . . . . . . . . . 4 1.3 Definitions . . . . . . . . . . . . . . . . . . . . . . . . 5 2. IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1 Summary of IRI syntax . . . . . . . . . . . . . . . . . . . 6 2.2 ABNF for IRI References and IRIs . . . . . . . . . . . . . . 6 2.3 Mapping of IRIs to URIs . . . . . . . . . . . . . . . . . . 8 2.3.1 When to convert from IRIs to URIs . . . . . . . . . . . . . 10 2.4 Converting URIs to IRIs . . . . . . . . . . . . . . . . . . 10 3. Considerations for use of IRIs . . . . . . . . . . . . . . . 11 3.1 IRI Character Limitations . . . . . . . . . . . . . . . . . 11 3.2 Bidirectional IRIs for right-to-left languages . . . . . . . 13 3.3 Processing IRIs . . . . . . . . . . . . . . . . . . . . . . 13 4. Software requirements . . . . . . . . . . . . . . . . . . . 14 4.1 URI/IRI software interfaces . . . . . . . . . . . . . . . . 14 4.2 URI/IRI entry . . . . . . . . . . . . . . . . . . . . . . . 14 4.3 URI/IRI generation . . . . . . . . . . . . . . . . . . . . . 15 4.4 URI/IRI selection . . . . . . . . . . . . . . . . . . . . . 16 4.5 Display of URIs/IRIs . . . . . . . . . . . . . . . . . . . . 16 4.6 Interpretation of URI/IRIs . . . . . . . . . . . . . . . . . 17 4.7 Transportation of URI/IRIs in document formats and protocols 18 5. Upgrading strategy . . . . . . . . . . . . . . . . . . . . . 18 6. Security considerations . . . . . . . . . . . . . . . . . . 19 7. Acknowlegdements . . . . . . . . . . . . . . . . . . . . . . 20 References . . . . . . . . . . . . . . . . . . . . . . . . . 20 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 23 Full Copyright Statement . . . . . . . . . . . . . . . . . . 24 Duerst & Suignard Expires October 16, 2002 [Page 3] Internet-Draft Internationalized Resource Identifiers April 2002 1. Introduction 1.1 Overview and Motivation A URI is defined in [RFC2396] as a sequence of characters chosen from a limited subset of the repertoire of US-ASCII characters. The characters in URIs are frequently used for representing words of natural languages. Such usage has many advantages: such URIs are easier to memorize, easier to interpret, easier to transcribe, easier to create, and easier to guess. For most languages other than English, however, the natural script uses characters other than A-Z. For many people, handling Latin characters is as difficult as handling the characters of other scripts is for people who use only the Latin alphabet. Many languages with non-Latin scripts do have transcriptions to Latin letters and such transcriptions are now often used in URIs, but they introduce additional ambiguities. The infrastructure for the appropriate handling of characters from local scripts is now widely deployed in local versions of operating system and application software. Software that can handle a wide variety of scripts and languages at the same time is increasingly widespread. Also, there are increasing numbers of protocols and formats that can carry a wide range of characters. This document defines a new protocol element, called IRI (Internationalized Resource Identifier), by extending the syntax of URIs to a much wider repertoire of characters. It also defines "internationalized" versions corresponding to other constructs from [RFC2396], such as URI references. Using characters outside of A-Z in IRIs brings with it some difficulties; a discussion of potential problems and workarounds can be found in the later sections of this document. URIs often contain Internet host names embedded within them. There is an ongoing discussion of internationalization and host names; the specific issues of the relationship of IRIs and possible future "internationalized" host names are not discussed here. (See [IDN- URI] for a separate proposal.) 1.2 Applicability IRIs are designed to be compatible with recent recommendations on URI syntax [RFC2718]. Practical use of IRIs (or IRI references) in place of URIs (or URI references) depends on the following conditions being met: Duerst & Suignard Expires October 16, 2002 [Page 4] Internet-Draft Internationalized Resource Identifiers April 2002 a. The protocol or format element used should be explicitly designated to carry IRIs. That is, the intent is not to introduce IRIs into contexts that are not defined to accept them. For examlpe, XML schema [XMLSchema] has an explicit type "anyURI" that can be used to designate IRIs. b. The protocol or format carrying the IRIs must have a mechanism to represent the wide range of characters used in IRIs, either natively or by some protocol- or format-specific escaping mechanism (for example numeric character references in [XML1]). c. Either by definition for all the URIs of a specific URI scheme, or at least for some specific URIs, the encoding of non-ASCII characters has to be based on UTF-8. For new URI schemes, this is recommended in [RFC2718]. This allows IRIs to be used with the URN syntax [RFC2141] as well as recent URL scheme definitions based on UTF-8, such as IMAP URLs [RFC2192] and POP URLs [RFC2384]. This condition may also apply to only a piece of a URI (reference), such as the fragment identifier. In cases and for pieces where an encoding other than UTF-8 is used, and for raw binary data encoded in URIs (see [RFC2397]), the octets have to be %-escaped. In these situations, the ability of IRIs to directly represent a wide character repertoire cannot be used. 1.3 Definitions The following definitions are used in this document; they follow the terms in [RFC2130] and [RFC2277]: character: An abstract object with a separate identity. For example, "LATIN CAPITAL LETTER A" names a character. octet: 8 bits character repertoire: A set of characters (in the mathematical sense) sequence of characters: A sequence (one after another) of characters sequence of octets: A sequence (one after another) of octets (character) encoding: A method of representing a sequence of characters as a sequence of octets (maybe with variants). A method of (unambiguously) converting a sequence of octets into a sequence of characters. Duerst & Suignard Expires October 16, 2002 [Page 5] Internet-Draft Internationalized Resource Identifiers April 2002 code point: A placeholder for a character in a character encoding, for example to encode additional characters in future versions of the character encoding. charset: The name of a parameter or attribute used to identify a character encoding. 2. IRI Syntax This section defines the syntax of Internationalized Resource Identifiers (IRIs). As with URIs, an IRI is defined as a sequence of characters, not as a sequence of octets. This definition accommodates the fact that IRIs may be written on paper or read over the radio as well as being transmitted over the network. The same IRI may be represented as different sequences of octets in different protocols or documents if these protocols or documents use different character encodings and/or transfer encodings. Using the same character encoding as the containing protocol or document assures that the characters in the IRI can be handled (searched, converted, displayed,...) in the same way as the rest of the protocol or document. 2.1 Summary of IRI syntax IRIs are defined similarly to URIs in [RFC2396] (as modified by [RFC2732]), but the class of unreserved characters is extended by adding all the characters of the UCS (Universal Character Set, [ISO10646]) beyond U+0080, subject to the limitations given in Section 3. Otherwise, the syntax and use of components and reserved characters is the same as that in [RFC2396]. All the operations defined in [RFC2396], such as the resolution of relative URIs, can be applied to IRIs by IRI-processing software in exactly the same way as this is done to URIs by URI-processing software. Characters outside the US-ASCII range MUST NOT be used for syntactical purposes such as to delimit components in newly defined schemes. As an example, it is not allowed to use U+00A2, CENT SIGN, as a delimiter, because it is in the 'iunreserved' category, in the same way as it is not possible to use '-' as a delimiter, because it is in the 'unreserved' category. 2.2 ABNF for IRI References and IRIs While it might be possible to define IRI references and IRIs merely Duerst & Suignard Expires October 16, 2002 [Page 6] Internet-Draft Internationalized Resource Identifiers April 2002 by their transformation to URIs, they can also be accepted and processed directly. Therefore, an ABNF definition for IRI references (which are the most general concept and the start of the grammar) and IRIs is given here. The following rules are different form [RFC2396]: IRI-reference = [ absoluteIRI | relativeIRI ] [ "#" ifragment ] absoluteIRI = scheme ":" ( ihier_part | iopaque_part ) relativeIRI = ( inet_path | iabs_path | irel_path ) [ "?" iquery ] ihier_part = ( inet_path | iabs_path ) [ "?" iquery ] iopaque_part = iric_no_slash *iric iric_no_slash = iunreserved | escaped | ";" | "?" | ":" | "@" | "&" | "=" | "+" | "$" | "," inet_path = "//" iauthority [ iabs_path ] iabs_path = "/" ipath_segments irel_path = irel_segment [ iabs_path ] irel_segment = 1*( iunreserved | escaped | ";" | "@" | "&" | "=" | "+" | "$" | "," ) iauthority = server | ireg_name ireg_name = 1*( iunreserved | escaped | "$" | "," | ";" | ":" | "@" | "&" | "=" | "+" ) ipath_segments = isegment *( "/" isegment ) isegment = *ipchar *( ";" iparam ) iparam = *ipchar ipchar = iunreserved | escaped | ":" | "@" | "&" | "=" | "+" | "$" | "," iquery = *iric ifragment = *iric iric = reserved | iunreserved | escaped iunreserved = ichar | unreserved ichar = << character of the UCS [ISO10646] of beyond U+009F, subject to the limitations in Section 3.1. >> | space | delims | unwise Note that the space character and various delimiters are allowed in IRIs and IRI references. This is further discussed in section 3.1, point b. Duerst & Suignard Expires October 16, 2002 [Page 7] Internet-Draft Internationalized Resource Identifiers April 2002 The following are the same as [RFC2396] as modified by [RFC2732]: reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" | "$" | "," | "[" | "]" unreserved = alphanum | mark mark = "-" | "_" | "." | "!" | "~" | "*" | "'" | "(" | ")" escaped = "%" HEXDIG HEXDIG server = [ [ userinfo "@" ] hostport ] userinfo = *( unreserved | escaped | ";" | ":" | "&" | "=" | "+" | "$" | "," ) hostport = host [ ":" port ] host = hostname | IPv4address | IPv6reference IPv6reference = "[" IPv6address "]" hostname = *( domainlabel "." ) toplabel [ "." ] domainlabel = alphanum | alphanum *( alphanum | "-" ) alphanum toplabel = alpha | alpha *( alphanum | "-" ) alphanum IPv6address = hexpart [ ":" IPv4address ] IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT hexpart = hexseq | hexseq "::" [ hexseq ] | "::" [ hexseq ] hexseq = hex4 *( ":" hex4) hex4 = 1*4HEXDIG port = *DIGIT scheme = alpha *( alpha | digit | "+" | "-" | "." ) alphanum = alpha | digit alpha = lowalpha | upalpha lowalpha = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z" upalpha = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" | "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" | "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z" space = delims = "<" | ">" | "#" | "%" | <"> unwise = "{" | "}" | "|" | "\" | "^" | "`" 2.3 Mapping of IRIs to URIs This section defines how to map an IRI to a URI. Everything in this section applies also to IRI references and URI references, as well as components thereof (for example fragment identifiers). This mapping has two purposes: a) Syntactical: Many URI schemes and components define additional syntactical restrictions not captured in Section 2.2. Such Duerst & Suignard Expires October 16, 2002 [Page 8] Internet-Draft Internationalized Resource Identifiers April 2002 restrictions can be applied to IRIs by noting that IRIs are only valid if they map to syntactically valid URIs. This means that such syntactical restrictions do not have to be defined again on the IRI level. b) Interpretational: URIs identify resources in various ways. IRIs also identify resources. The resource that an IRI identifies is the same as the one identified by the URI obtained after converting the IRI according to the procedure defined here. This means that there is no need to define the association between identifier and resource again on the IRI level. This mapping is accomplished in two parts. Part A) is skipped if the input is already in a UCS-based encoding (for example UTF-8 or UTF- 16). In that case, it is assumed that the IRI is already in NFC. Part A) This part has three variants, depending on where the input comes from. Variant 1) a) Start with an IRI written on paper or read out loud, or otherwise represented as a sequence of characters independent of any encoding. b) Represent the IRI characters as a sequence of characters from the UCS. c) Normalize the character sequence according to Normalization Form C (NFC), as defined in [UNI15]. (See further discussion in Section 3.1.) Note: In practice, steps b) and c) will often be performed together, for example by using a keyboard or other input mechanism that is designed to produce NFC. Variant 2) a) Start with an IRI in some digital representation (e.g. an octet stream) in some non- Unicode encoding. b) Represent the IRI characters as a sequence of characters from the UCS. c) Normalize the character sequence according to Normalization Form C, as defined in [UNI15]. (See further discussion in Section 3.1.) Note: In practice, steps b) and c) will often be performed together, for example by using a transcoder that produces output in NFC. Variant 3) a) Start with an IRI in an Unicode-based encoding (for example UTF-8 or UTF-16). Move directly to Part 2. It is assumed that the IRI is already in NFC. Duerst & Suignard Expires October 16, 2002 [Page 9] Internet-Draft Internationalized Resource Identifiers April 2002 Part B) For each character that is disallowed in URI references, apply steps a) through c) below. The disallowed characters consist of all non-ASCII characters, plus the excluded characters listed in Section 2.4 of [RFC2396], except for the number sign (#) and percent sign (%) and the square bracket characters re-allowed in [RFC2732]. 1) Convert the character to a sequence of one or more octets using UTF-8 [RFC2279]. 2) Convert each octet to %HH, where HH is the hexadecimal notation of the octet value. Note: This is identical to the escaping mechanism in Section 2.4.1 of [RFC2396]. 3) Replace the original character by the resulting character sequence. Note that in this process (in step B3), characters allowed in URI references and existing escape sequences are not escaped further. (This mapping is similar to, but different from, the escaping applied when including arbitrary content into some part of a URI.) The above mapping produces a URI fully conforming to [RFC2396] out of each IRI. The mapping is also an identity transformation for URIs and is idempotent--applying the mapping a second time will not change anything. Every URI is therefore by definition an IRI. Section 2.3 gives details about when exactly to convert from an IRI to an URI. 2.3.1 When to convert from IRIs to URIs The mapping from IRIs to URIs SHOULD only be applied when necessary, and as late as possible. 2.4 Converting URIs to IRIs In some situations, it may be desirable to try to convert a URI into an equivalent IRI. This section gives a procedure to do such a conversion. In general, the IRI to URI mapping is many-to-one, so the conversion is not invertible. The conversion described in this section will always give an IRI which maps back to the URI that was used as an input for the conversion, but perhaps not exactly the original IRI (if there ever was one). In general, URI to IRI conversion removes escape sequences, but not all escaping can be eliminated. There are many reasons for this: a. Some escape sequences are necessary to distinguish escaped and unescaped uses of reserved characters. Duerst & Suignard Expires October 16, 2002 [Page 10] Internet-Draft Internationalized Resource Identifiers April 2002 b. Some escape sequences cannot be interpreted as sequences of UTF-8 octets. (Note: Due to the regularities in the octet patterns of UTF-8, there is a very high probability, but no guarantee, that escape sequences that can be interpreted as sequences of UTF-8 octets actually originated from UTF-8. For a detailed discussion of the odds, see [Duer97].) c. The conversion may result in a character that is not appropriate in an IRI. See section 3.1 for further details. Conversion from a URI to an IRI is done using the following steps (or any other algorithm that produces the same result): 1) Represent the URI as a sequence of octets in US-ASCII. 2) Convert all hexadecimal escapes (% followed by two hexadecimal digits) of %80 and higher to the corresponding octets. 3) Re-escape any octets that are not part of a strictly legal UTF- 8 octet sequence. 4) Re-escape all octets that in UTF-8 reperesent characters that are not appropriate according to Section 3.1. This procedure will convert as many escaped non-ASCII characters as possible to characters in an IRI. Because there are some choices when applying step 3) (see Section 3.1), results may differ. 3. Considerations for use of IRIs 3.1 IRI Character Limitations Not all characters of the UCS are appropriate for use as resource identifiers. This section discusses the limitations on characters and character sequences usable for IRIs. The considerations in this section are relevant when creating IRIs and when converting from URIs to IRIs. Because of the large and increasing number of characters in the UCS and the large number of situations where IRIs can be used, it is impossible to give general rules for which characters should be avoided. The following considerations are relevant: a. The repertoire of characters allowed in each IRI component is limited by the definition of that component. For example, the definition of host names in URIs does not currently allow hex Duerst & Suignard Expires October 16, 2002 [Page 11] Internet-Draft Internationalized Resource Identifiers April 2002 escapes, or "_", or many other punctuation characters. This specification does not relax those limits, and so IRIs currently may not contain any non-ASCII characters in host names. This specification likewise does not extended the scheme component beyond US-ASCII. (Note: In accordance with URI practice, generic IRI software cannot and should not check for such limitations.) b. In the URI syntax, characters that are likely to be used to delimit URIs in text and print ("space", "delims", and "unwise") were excluded. They are included in the IRI syntax, for the following reasons: 1) The syntax includes many other characters that are not appropriate in many cases. 2) Some implementation practice already allows them in URI references (for example spaces in fragment identifiers). 3) It is very convenient in some cases, for example for XPointers in XML attributes. 4) Considering context is already necessary in the case of URIs, for example for "&" in XML. However, these characters should be used carefully. Whenever there is a chance that an IRI will be used in a component where these characters can be harmful, they should be escaped. c. The UCS contains many areas of "characters" which have no well-established way of inputting them. These should be avoided. Characters that fall into this category include Dingbats, Mathematical and other symbols, ligatures and presentation forms. d. The UCS contains many areas of characters for which there are strong visual look-alikes. Because of the likelihood of transcription errors, these also should be avoided. This includes the full-width equivalents of ASCII characters, half- width Katakana characters for Japanese, and many others. This also includes many look-alikes of "space", "delims", and "unwise", characters excluded in [RFC2396]. e. Characters with no visual representation may not be interoperably entered. Control characters MUST NOT be used. This includes the traditional ranges of control characters (U+0000-U+001F and U+007F-U+009F) as well as other cases such Duerst & Suignard Expires October 16, 2002 [Page 12] Internet-Draft Internationalized Resource Identifiers April 2002 as plane-14 language tag characters. f. Some code points are reserved for private use or for special encoding purposes. They are not interoperable. Code points reserved for private use MUST NOT be used. Code points reserved for surrogates MUST NOT be used. g. Where there exist duplicate ways of encoding a certain character as visible to the user, Normalization Form C as defined in [UNI15] MUST be used. Additional information is available from [UNIXML]. Although this is written in a different context, it discusses many of the categories of characters and code points not appropriate for IRIs. For reasons of transcribability, many characters have been excluded from IRIs above. These can nevertheless be encoded in an IRI if necessary. They have to be escaped using the procedure in Section 2.3. For example, a space can always be encoded in a URI and in an IRI as %20. A non-breaking space (U+00A0) must be encoded as %C2%A0. 3.2 Bidirectional IRIs for right-to-left languages Some UCS characters, such as those used in the Arabic and Hebrew script, have an inherent right-to-left writing direction. IRIs containing such characters (called bidirectional IRIs or Bidi IRIs) require additional attention because of the non-trivial relation between logical representation (used for digital representation as well as when reading/spelling) and visual representation (used for display/printing). This document does not address bidi-specific issues. A proposal for addressing these issues can be found in [Bidi]. 3.3 Processing IRIs Processing of relative forms of IRIs against a base is handled straightforwardly; the algorithms of RFC 2396 may be applied directly, treating the characters additionally allowed in IRIs in the same way as unreserved characters in URIs. Other processing operations on IRIs and IRI references similarly work analogous to their URI complements. Such processing and mapping to URIs is commutative, which means that the same result is obtained independent of whether the processing or the mapping is done first. If both IRIs and URIs are involved in processing, the IRI parts SHOULD be preserved as long as possible. For example, it is possible to create an absolute IRI from a relative IRI and a URI base. When IRIs are compared, they SHOULD temporarily Duerst & Suignard Expires October 16, 2002 [Page 13] Internet-Draft Internationalized Resource Identifiers April 2002 be mapped to URIs to eliminate potential differences in the degree of escaping. 4. Software requirements This section explains the issues and difficulties in supporting IRIs in the same software components and operations that currently process URIs: software interfaces that handle URIs, software that allows users to enter URIs, software that generates URIs, software that displays URIs, formats and protocols that transport URIs, and software that interprets URIs. These may all require more or less modification before functioning properly with IRIs. The considerations in this section also apply to URI references and IRI references. 4.1 URI/IRI software interfaces Software interfaces that handle URIs, such as URI-handling APIs and protocols transferring URIs, need interfaces and protocol elements that are designed to carry IRIs. Note that although an IRI is defined as a sequence of characters, software interfaces for URIs typically function on sequences of octets. Thus, it is necessary to define clearly which character encoding is used. In case the current handling in an API or protocol is based on US- ASCII, UTF-8 is recommended as the encoding for IRIs, because this is compatible with US-ASCII, is in accordance with the recommendations of [RFC2277], and makes it easy to convert to URIs where necessary. In any case, the encoding used must not be left undefined. Intermediate software interfaces between IRI-capable components and URI-only components MUST map the IRIs as per section 2.3 above, when transferring from IRI-capable to URI-only components. However, such a mapping SHOULD be applied as late as possible. It should not be applied between components that are known to be able to handle IRIs. The transfer from URI-only to IRI-capable components requires no mapping, although the conversion described in section 2.4 above may be performed. It is preferable not to perform this inverse conversion when there is a chance that this cannot be done correctly. 4.2 URI/IRI entry There are components that allow users to enter URIs into the system, for example, by typing or dictation. This software must be updated to allow for IRI entry. Duerst & Suignard Expires October 16, 2002 [Page 14] Internet-Draft Internationalized Resource Identifiers April 2002 A person viewing a visual representation of an IRI (as a sequence of glyphs, in some order, in some visual display) or hearing an IRI, will use a entry method for characters in the user's language to input the IRI. Depending on the script and the input method used, this may be a more or less complicated process. The process of IRI entry must assure, as far as possible, that the limitations defined in Section 3.1 are met. This may be done by choosing appropriate input methods or variants/settings thereof, by appropriately converting the characters being input, by eliminating characters that cannot be converted, and/or by issuing a warning or error message to the user. An input field primarily or only used for the input of URIs/IRIs should allow the user to view an IRI as converted to a URI. Places where the input of IRIs is frequent should provide the possibility for viewing an IRI as converted to a URI. This will help users when some of the software they use does not yet accept IRIs. An IRI input component that interfaces to components that handle URIs, but not IRIs, must escape the IRI before passing it to such a component. For the input of IRIs with right-to-left characters, please see [Bidi]. 4.3 URI/IRI generation Systems that are offering resources through the Internet, where those resources have logical names, sometimes automatically generate URIs for the resources they offer. For example, some HTTP servers can generate a directory listing for a file directory, and then respond to the generated URIs with the files. Many legacy character encodings are in use in various file systems. Many currently deployed systems do not transform the local character representation of the underlying system before generating URIs. For maximum interoperability, systems that generate resource identifiers should do the appropriate transformations. They should use IRIs converted to URIs in cases where it cannot be expected that the recipient is able to handle IRIs. Due to the way most user agents currently work, native IRIs, encoded in UTF-8, may be used if the recipient announces that it can interpret UTF-8. This requires that the whole page is sent as UTF-8. If this is not possible, escaping can always be used. This recommendation in particular applies to HTTP servers. For FTP Duerst & Suignard Expires October 16, 2002 [Page 15] Internet-Draft Internationalized Resource Identifiers April 2002 servers, similar considerations apply, see in particular [RFC2640]. 4.4 URI/IRI selection In some cases, resource owners and publishers have control over the IRIs used to identify their resources. Such control is mostly executed by controlling the resource names, such as file names, directly. In such cases, it is recommended to avoid choosing IRIs that are easily confused. For example, for US-ASCII, the lower-case ell "l" is easily confused with the digit one "1", and the upper-case oh "O" is easily confused with the digit zero "0". Publishers should avoid confusing users with "br0ken" or "1ame" identifiers. Outside of the US-ASCII range, there are many more opportunities for confusion; a complete set of guidelines is too lengthy to include here. As long as names are limited to characters from a single script, native writers of a given script or language will know best when ambiguities can appear, and how they can be avoided. What may look ambiguous to a stranger may be completely obvious to the average native user. On the other hand, in some cases, the UCS contains variants for compatibility reasons, for example for typographic purposes. These should be avoided wherever possible. Although there may be exceptions, in general newly created resource names should be in NFKC [UNI15] (which means that they are also in NFC). Note that the limitations defined in Section 3.1 and the recommendations given here are of a different nature. The limitations defined in Section 3.1 are necessary to avoid duplicate encodings that are artifacts of digital representation and that the user has no way to distinguish visually. On the other hand, in a given context, an identifier such as "BOX0021" can be completely appropriate, and it is impossible to find an algorithm that distinguishes the appropriate from the confusing identifiers. In certain cases, there is a chance that letters from different scripts look the same. The best known example is the Latin 'A', the Greek 'Alpha', and the Cyrillic 'A'. To avoid such cases, only IRIs should be generated where all the letters in a single component are from the same script. This is similar to the heuristics used to distinguish between letters and numbers in the examples above. Also, for the above three scripts, using lower-case letters results in fewer ambiguities than using upper-case letters. 4.5 Display of URIs/IRIs Many systems contain software that presents URIs to users as part of Duerst & Suignard Expires October 16, 2002 [Page 16] Internet-Draft Internationalized Resource Identifiers April 2002 the system's user interface (sometimes presenting 'friendly' URIs, such as a shortened or more legible substring of the URI). This section applies to this presentation, as well as to the strategy for printing URIs in magazines, newspapers, or reading them over the radio. Software that displays identifiers to users should follow a general principle: "Don't display something to a user that the user would not be able to enter." The consequences of this principle require judgement about the availability of software that implements the entry methods described in Section 3.2. a) In situations where a viewer is not likely to have software that implements non-ASCII character entry (as described in Section 3.1), or where it can be expected that only a limited range of non-ASCII characters can be entered, any part of an IRI containing characters outside the range allowed in [RFC2396] or any additions SHOULD be escaped before being displayed. b) In situations where a viewer _is_ likely to have such software, IRIs SHOULD be displayed directly. For display of Bidi IRIs, please see [Bidi]. 4.6 Interpretation of URI/IRIs Software that interprets IRIs as the names of local resources should accept IRIs in multiple forms, and convert and match them with the appropriate local resource names. First, multiple representations include both IRIs in the native character encoding of the protocol and also their URI counterparts. Second, it may include URIs constructed based on other character encodings than UTF-8. Such URIs may be produced by user agents that do not conform to this specification and use legacy encodings to convert non-ASCII characters to URIs. Whether this is necessary, and what character encodings to cover, depends on a number of factors, such as the legacy character encodings used locally and the distribution of various versions of user agents. For example, software for Japanese may accept URIs in Shift_JIS and/or EUC-JP in addition to UTF-8. Third, it may include additional mappings to be more user-friendly and robust against transmission errors. These would be similar to how currently some servers treat URIs as case-insensitive, or perform additional matchings to account for spelling errors. For characters Duerst & Suignard Expires October 16, 2002 [Page 17] Internet-Draft Internationalized Resource Identifiers April 2002 beyond the ASCII repertoire, this may for example include ignoring the accents on received IRIs or resource names where appropriate. Please note that such mappings, including case mappings, are language-dependent. It can be difficult to unambiguously identify a resource if too many mappings are taken into consideration. However, escaped and non- escaped parts of IRIs can always clearly be distinguished. Also, the regularity of UTF-8 (see [Duer97] makes the potential for collisions lower than it may seem at first sight. 4.7 Transportation of URI/IRIs in document formats and protocols Document formats that transport URIs may need to be upgraded to allow the transport of IRIs. In those cases where the document as a whole has a native character encoding, IRIs SHOULD also be encoded in this encoding, and converted accordingly by a parser or interpreter. IRI characters that are not expressible in the native encoding SHOULD be escaped according to Section 2.2, or MAY be escaped in another way if the document format provides a way to do this. For example, in HTML, XML, or SGML, numeric character references can be used. If a document as a whole has a native character encoding, and that character encoding is not UTF-8, then IRIs MUST NOT be placed into the document in the UTF-8 character encoding. Please note that some formats already accomodate IRIs, although they use different terminology. HTML 4.0 [HTML4] defines the conversion from IRIs to URIs as error-avoiding behavior. XML 1.0 [XML1], XLink [XLink], and XML Schema [XMLSchema] and specifications based upon them allow IRIs. Also, it is expected that all relevant new W3C formats and protocols will be required to handle IRIs [CharMod]. 5. Upgrading strategy As this recommendation places further constraints on software for which many instances are already deployed, it is important to introduce upgrades carefully, and to be aware of the various interdependencies. If IRIs cannot be interpreted correctly, they should not be generated or transported. This suggests that upgrading URI interpreting software to accept IRIs should have highest priority. On the other hand, a single IRI is interpreted only by a single or very few interpreters that are known in advance, while it may be entered and transported very widely. Therefore, IRIs benefit most from a broad upgrade of software to be Duerst & Suignard Expires October 16, 2002 [Page 18] Internet-Draft Internationalized Resource Identifiers April 2002 able to enter and transport IRIs, but before publishing any individual IRI, care should be taken to upgrade the corresponding interpreting software in order to cover the forms expected to be received by various versions of entry and transport software. The upgrade of generating software to generate IRIs instead of a local encoding should happen only after the service is upgraded to accept IRIs. Similarly, IRIs should only be generated when the service accepts IRIs and the intervening infrastructure and protocol is known to transport them safely. Display software should be upgraded only after upgraded entry software has been widely deployed to the population that will see the displayed result. These recommendations, when taken together, will allow for the extension from URIs to IRIs in order to handle scripts other than ASCII while minimizing interoperability problems. 6. Security considerations If IRI entry software normalizes the characters entered, but the resource names on the interpreting side are not normalized accordingly, and the interpreting software does not take this into account, there is a possibility of "spoofing". Similar possibilities turn up when interpreting software accepts URIs in various native encodings or allows accents and similar things to be ignored. "Spoofing" means that somebody may add a resource name that looks the same or similar to the user while actually being different, or a resource name that contains the same characters, but in a different encoding. The added resource may pretend to be the real resource by looking very similar, but may contain all kinds of changes that may be difficult to spot but can cause all kinds of problems. Conceptually, this is no different from the problems surrounding the use of case-insensitive web servers. For example, a popular web page with a mixed case name (http://big.site/PopularPage.html) might be "spoofed" by someone who obtains access to (http://big.site/ popularpage.html). However, the introduction of character normalization, of additional mappings for user convenience, and of mappings for various encodings may increase the number of spoofing possibilities. In some cases, in particular for Latin-based resource names, this is usually easy to detect because UTF-8-encoded names, when interpreted and viewed as legacy encodings, produce mostly garbage. In other cases, when concurrently used encodings have a similar structure, but there are Duerst & Suignard Expires October 16, 2002 [Page 19] Internet-Draft Internationalized Resource Identifiers April 2002 no characters that have exactly the same encoding, detection is more difficult. A good example may be the concurrent use of Shift_JIS and EUC-JP on a Japanese server. Administrators of large sites which allow independent users to create subareas may need to be careful that the aliasing rules do not create chances for spoofing. 7. Acknowlegdements We would like to thank Larry Masinter for his work as co-author of many earlier versions of this document. The issue addressed here has been discussed at numerous times over the last years; for example, there was a thread in the HTML working group in August 1995 (under the topic of "Globalizing URIs") in the www-international mailing list in July 1996 (under the topic of "Internationalization and URLs"), and ad-hoc meetings at the Unicode conferences in September 1995 and September 1997. Thanks to Francois Yergeau, Chris Wendt, Yaron Goland, Graham Klyne, Roy Fielding, Tim Berners-Lee, M.T. Carrasco Benitez, James Clark, Andrea Vine, Misha Wolf, Leslie Daigle, Makoto MURATA, Tex Texin, Bjoern Hoehrmann, Dan Oscarson, and many others for help with understanding the issues and possible solutions. Thanks also to the members of the W3C I18N Working Group and Interest Group for their contributions and their work on [CharMod], to the members of many other W3C WGs for adopting the ideas, and to the members of the Montreal IAB Workshop on Internationalization and Localization for their review. References [Bidi] Duerst, M., "Internet Identifiers and Bidirectionality", draft-duerst-iri-bidi-00 (work in progress), July 2001, . [CharMod] Duerst, M., Yergeau, F., Ishida, R., Wolf, M., Freytag, A. and T. Texin, "Character Model for the World Wide Web", World Wide Web Consortium Working Draft, February 2002, . [Duer97] Duerst, M., "The Properties and Promizes of UTF-8", Proc. 11th International Unicode Conference, San Jose , September 1997, . Duerst & Suignard Expires October 16, 2002 [Page 20] Internet-Draft Internationalized Resource Identifiers April 2002 [Duer01] Duerst, M., "Internationalized Resource Identifiers: From Specification to Testing", Proc. 19th International Unicode Conference, San Jose , September 2001, . [HTML4] Raggett, D., Le Hors, A. and I. Jacobs, "HTML 4.01 Specification", World Wide Web Consortium Recommendation, December 1999, . [IDN-URI] Duerst, M., "Internationalized Domain Names in URIs and IRIs", draft-ietf-idn-uri-01 (work in progress), November 2001, . [ISO10646] International Organization for Standardization, "Information Technology - Universal Multiple-Octet Coded Character Set (UCS) - Part 1: Architecture and Basic Multilingual Plane", ISO Standard 10646-1, with amendments, October 2000. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2130] Weider, C., Preston, C., Simonsen, K., Alvestrand, H., Atkinson, R., Crispin, M. and P. Svanberg, "The Report of the IAB Character Set Workshop held 29 February - 1 March, 1996", RFC 2130, April 1997. [RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997. [RFC2192] Newman, C., "IMAP URL Scheme", RFC 2192, September 1997. [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, January 1998. [RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 2279, January 1998. [RFC2384] Gellens, R., "POP URL Scheme", RFC 2384, August 1998. [RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998. [RFC2397] Masinter, L., "The "data" URL scheme", RFC 2397, August 1998. Duerst & Suignard Expires October 16, 2002 [Page 21] Internet-Draft Internationalized Resource Identifiers April 2002 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [RFC2640] Curtin, B., "Internationalization of the File Transfer Protocol", RFC 2640, July 1999. [RFC2718] Masinter, L., Alvestrand, H., Zigmond, D. and R. Petke, "Guidelines for new URL Schemes", RFC 2718, November 1999. [RFC2732] Hinden, R., Carpenter, B. and L. Masinter, "Format for Literal IPv6 Addresses in URL's", RFC 2732, December 1999. [UNIV3] The Unicode Consortium, "The Unicode Standard Version 3.0", Addison-Wesley, Reading, MA , 2000. [UNI15] Davis, M. and M. Duerst, "Unicode Normalization Forms", Unicode Standard Annex #15, March 2001, . [UNIXML] Duerst, M. and A. Freytag, "Unicode in XML and other Markup Languages", Unicode Technical Report #20, World Wide Web Consortium Note, Februar 2002, . [W3CIRI] "Internationalization - URIs and other identifiers", . [XLink] DeRose, S., Maler, E. and D. Orchard, "XML Linking Language (XLink) Version 1.0", World Wide Web Consortium Recommendation, June 2001, . [XML1] Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler, "Extensible Markup Language (XML) 1.0 (Second Edition)", World Wide Web Consortium Recommendation, including Erratum 26 at http://www.w3.org/XML/xml-V10-2e- errata#E26, October 2000, . [XMLSchema] Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes", World Wide Web Consortium Recommendation, May 2001, . Duerst & Suignard Expires October 16, 2002 [Page 22] Internet-Draft Internationalized Resource Identifiers April 2002 Authors' Addresses Martin Duerst (Note: Please write "Duerst" with u-umlaut wherever possible, for example as "Dürst in XML and HTML.) W3C/Keio University 5322 Endo Fujisawa 252-8520 Japan Phone: +81 466 49 1170 Fax: +81 466 49 1171 EMail: duerst@w3.org URI: http://www.w3.org/People/D%C3%BCrst/ (Note: This is the escaped form of an IRI.) Michel Suignard Microsoft Corporation One Microsoft Way Redmond, WA 98052 U.S.A. Phone: +1 425 882-8080 EMail: mailto:michelsu@microsoft.com URI: http://www.suignard.com Duerst & Suignard Expires October 16, 2002 [Page 23] Internet-Draft Internationalized Resource Identifiers April 2002 Full Copyright Statement Copyright (C) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS 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. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Duerst & Suignard Expires October 16, 2002 [Page 24]