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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 RFC 3987

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
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   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.



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   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].































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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





















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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:




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      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.




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      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



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   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.











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   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    = <US-ASCII coded character 20 hexadecimal>
          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



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         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.




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      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.




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      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



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         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



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         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



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   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.



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   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



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   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



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   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



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   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



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   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



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   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,
                <http://www.ietf.org/internet-drafts/draft-duerst-iri-
                bidi-00.txt>.

   [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, <http://www.w3.org/TR/charmod>.

   [Duer97]     Duerst, M., "The Properties and Promizes of UTF-8",
                Proc. 11th International Unicode Conference, San Jose ,
                September 1997, <http://www.ifi.unizh.ch/mml/mduerst/
                papers/PDF/IUC11-UTF-8.pdf>.




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   [Duer01]     Duerst, M., "Internationalized Resource Identifiers:
                From Specification to Testing", Proc. 19th International
                Unicode Conference, San Jose , September 2001, <http://
                www.w3.org/2001/Talks/0912-IUC-IRI/paper.html>.

   [HTML4]      Raggett, D., Le Hors, A. and I. Jacobs, "HTML 4.01
                Specification", World Wide Web Consortium
                Recommendation, December 1999, <http://www.w3.org/TR/
                REC-html40/appendix/notes.html#h-B.2>.

   [IDN-URI]    Duerst, M., "Internationalized Domain Names in URIs and
                IRIs", draft-ietf-idn-uri-01 (work in progress),
                November 2001, <http://www.ietf.org/internet-drafts/
                draft-ietf-idn-uri-01.txt>.

   [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.




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   [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, <http://
                www.unicode.org/unicode/reports/tr15/tr15-21.html>.

   [UNIXML]     Duerst, M. and A. Freytag, "Unicode in XML and other
                Markup Languages", Unicode Technical Report #20, World
                Wide Web Consortium Note, Februar 2002, <http://
                www.w3.org/TR/unicode-xml/>.

   [W3CIRI]     "Internationalization - URIs and other identifiers",
                <http://www.w3.org/International/O-URL-and-ident.html>.

   [XLink]      DeRose, S., Maler, E. and D. Orchard, "XML Linking
                Language (XLink) Version 1.0", World Wide Web Consortium
                Recommendation, June 2001, <http://www.w3.org/TR/xlink/
                #link-locators>.

   [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, <http://www.w3.org/TR/REC-
                xml#sec-external-ent>.

   [XMLSchema]  Biron, P. and A. Malhotra, "XML Schema Part 2:
                Datatypes", World Wide Web Consortium Recommendation,
                May 2001, <http://www.w3.org/TR/xmlschema-2/#anyURI>.






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Authors' Addresses

   Martin Duerst (Note: Please write "Duerst" with u-umlaut wherever
                  possible, for example as "D&#252;rst 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

























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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

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