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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: December 30, 2002                                   M. Suignard
                                                    Microsoft Corporation
                                                             July 1, 2002


               Internationalized Resource Identifiers (IRI)
                           draft-duerst-iri-01

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 December 30, 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.

NOTE

    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].

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 IRI Equivalence and Normalization  . . . . . . . . . . . . . .  9
    3.  Relationship between IRIs and URIs . . . . . . . . . . . . . . 10
    3.1 Mapping of IRIs to URIs  . . . . . . . . . . . . . . . . . . . 11
    3.2 Converting URIs to IRIs  . . . . . . . . . . . . . . . . . . . 12
    4.  Bidirectional IRIs for Right-to-left Languages . . . . . . . . 13
    4.1 Bidi IRI Structure . . . . . . . . . . . . . . . . . . . . . . 14
    4.2 Visual Rendering of Bidi IRIs  . . . . . . . . . . . . . . . . 14
    4.3 Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . . . . 15
    5.  Use of IRIs  . . . . . . . . . . . . . . . . . . . . . . . . . 15
    5.1 Limitations on UCS Character Allowed in IRI  . . . . . . . . . 15
    5.2 Software Interfaces and Protocols  . . . . . . . . . . . . . . 16
    5.3 Format of URIs and IRIs in Documents and Protocols . . . . . . 17
    5.4 Relative IRI References  . . . . . . . . . . . . . . . . . . . 17
    6.  URI/IRI Processing Guidelines (informative)  . . . . . . . . . 17
    6.1 URI/IRI Software Interfaces  . . . . . . . . . . . . . . . . . 18
    6.2 URI/IRI Entry  . . . . . . . . . . . . . . . . . . . . . . . . 18
    6.3 URI/IRI Generation . . . . . . . . . . . . . . . . . . . . . . 19
    6.4 URI/IRI Selection  . . . . . . . . . . . . . . . . . . . . . . 19
    6.5 Display of URIs/IRIs . . . . . . . . . . . . . . . . . . . . . 20
    6.6 Interpretation of URIs and IRIs  . . . . . . . . . . . . . . . 20
    6.7 Upgrading Strategy . . . . . . . . . . . . . . . . . . . . . . 21
    7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
    8.  Change log . . . . . . . . . . . . . . . . . . . . . . . . . . 22
    9.  Acknowlegdements . . . . . . . . . . . . . . . . . . . . . . . 23
        References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 26



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        Full Copyright Statement . . . . . . . . . . . . . . . . . . . 27


















































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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.

1.2 Applicability

    IRIs are designed to be compatible with recent recommendations on URI
    syntax [RFC2718].  The compatibility is provided by providing a well
    defined and deterministic mapping from the IRI character sequence to
    the functionally equivalent URI character sequence.  Practical use of
    IRIs (or IRI references) in place of URIs (or URI references) depends
    on the following conditions being met:

       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 example, XML schema [XMLSchema] has an explicit type



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          "anyURI" that designates the use of 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], [RFC2277] and [ISO10646]:

       character: A member of a set of elements used for the
          organization, control, or representation of data.  For example,
          "LATIN CAPITAL LETTER A" names a character.

       octet: an ordered sequence of eight bits considered as a unit

       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.

       code point: A placeholder for a character in a character encoding,
          for example to encode additional characters in future versions
          of the character encoding.



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       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] and [IDNURI]), 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 5.1.

    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
    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.



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    The following rules are different from [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     = iserver | ireg_name
           ireg_name      = 1*( iunreserved | escaped | "$" | "," |
                               ";" | ":" | "@" | "&" | "=" | "+" )
           iserver        = [ [ userinfo "@" ] ihostport ]
           iuserinfo      = *( iunreserved | escaped |
                              ";" | ":" | "&" | "=" | "+" | "$" | "," )
           ihostport      = ihost [ ":" port ]
           ihost          = ihostname | IPv4address | IPv6reference
           ihostname      = << as specified by [IDNA] >>
           ipath_segments = isegment *( "/" isegment )
           isegment       = *ipchar *( ";" iparam )
           iparam         = *ipchar
           ipchar         = iunreserved | escaped |
                             ":" | "@" | "&" | "=" | "+" | "$" | ","
           iquery         = *iric
           ifragment      = *iric
           iric           = reserved | iunreserved | escaped
           iunreserved    = ichar | unreserved
           ichar          = << allowed character of the UCS [ISO10646] >> |
space | delims | unwise

    Note that the space character and various delimiters are allowed in
    IRIs and IRI references.  This is further discussed in Section 5.1.














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    The following describe the allowed characters of the UCS [ISO10646]
    using the UCS-4 encoding notation for these characters:

          U+00A0-U+D7FF
          U+F900-U+FDCF
          U+FDF0-U+FFEF
          U+10000-U+1FFFD
          U+20000-U+2FFFD
          U+30000-U+3FFFD
          U+40000-U+4FFFD
          U+50000-U+5FFFD
          U+60000-U+6FFFD
          U+70000-U+7FFFD
          U+80000-U+8FFFD
          U+90000-U+9FFFD
          U+A0000-U+AFFFD
          U+B0000-U+BFFFD
          U+C0000-U+CFFFD
          U+D0000-U+DFFFD
          U+E1000-U+EFFFD































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    The following are the same as [RFC2396] as modified by [RFC2732]:

           reserved      = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" |
                           "$" | "," | "[" | "]"
           unreserved    = alphanum | mark
           mark          = "-" | "_" | "." | "!" | "~" | "*" | "'" |
                           "(" | ")"
           escaped       = "%" hex hex
           hex           = digit | "A" | "B" | "C" | "D" | "E" | "F" |
                                   "a" | "b" | "c" | "d" | "e" | "f"
           IPv6reference = "[" IPv6address "]"
           IPv6address   = hexpart [ ":" IPv4address ]
           IPv4address   = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT
           hexpart       = hexseq | hexseq "::" [ hexseq ] | "::"
                           [ hexseq ]
           hexseq        = hex4 *( ":" hex4)
           hex4          = 1*4hex
           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"
           digit         = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
                           "8" | "9"
           space         = << US-ASCII coded character 20 hexadecimal >>
           delims        = "<" | ">" | "#" | "%" | <">
           unwise        = "{" | "}" | "|" | "\" | "^" | "`"


2.3 IRI Equivalence and Normalization

    There is no general rule or procedure to decide whether two arbitrary
    IRIs are equivalent or not (i.e.  refer to the same resource or not).
    Two IRIs that look almost the same may refer to different resources.
    Two IRIs that look completely different may refer to, and resolve to,
    the same resource.

    In some scenarios, such as XML Namespaces  ([XMLNamespace]), a
    definite answer to the question of IRI equivalence is needed that is
    independent of the scheme used and always can be calculated quickly
    and without accessing a network.  In such cases, two IRIs SHOULD be
    defined as equivalent if and only if they are character-by-character
    equivalent (which is the same as byte-by-byte equivalent if the



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    character encoding for both IRIs is the same).  In such a case, the
    comparison function MUST NOT map the IRIs to URIs.

    It follows from the above that IRIs SHOULD NOT be modified when being
    transported.

    For actual resolution, differences in escaping (except for the
    escaping of reserved characters) MUST always result in the same
    resource.  For example, foo://example.com/XML, foo://example.com/
    XM%4C, and foo://example.com/XM%4c must resolve to the same resource.
    If this kind of equivalence is to be tested, the escaping of both
    IRIs to be compared has to be aligned, for example by converting both
    IRIs to URIs (see Section 3.1) and making sure that the case of the
    hexadecimal characters in the %-escape is always the same.  Such
    conversions MUST only be done on the fly, without changing the
    original IRI.

    Specific schemes and resolution mechanisms may define additional
    equivalences.  For a specific scheme, two IRIs that e.g.  differ only
    by case may be equivalent.  However, this document does not deal with
    scheme-specific issues.

    The Unicode Standard [UNIV3] defines various equivalences between
    sequences of characters for various purposes.  Unicode Standard Annex
    #15 [UNI15] defines various Normalization Forms for these
    equivalences.  IRIs SHOULD be created using the Normalization Form C
    (NFC).  When an IRI is created in an UCS-based encoding without the
    end-user being aware of or interested in Unicode normalization
    issues, the IRI MUST be created using the normalization form NFC.
    Equivalence of IRIs MUST rely on the IRIs being appropriately pre-
    normalized, rather than applying normalization, except when
    converting from a non-UCS-based encoding to an UCS-based encoding,
    where a normalizing transcoder using NFC MUST be used.

    Various IRI schemes may allow the usage of International Domain Names
    (IDN) [IDNA].  When in use in IRIs, those names SHOULD be validated
    using the rules defined by [Nameprep].  An IRI containing an invalid
    IDN cannot successfully be resolved.  For legibility purposes, IDN
    components of IRIs SHOULD not be converted into ASCII Compatible
    Encoding (ACE).  However, this conversion may be applied when mapping
    an IRI into an URI, see Section 3.1.

3. Relationship between IRIs and URIs

    IRIs are meant to replace URIs in identifying resources for
    protocols, formats and software components which use a UCS-based
    character repertoire.  These protocols and components may never need
    to use URIs directly, especially when the resource identifier is used



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    simply for identification purposes.  However, when the resource
    identifier is used for resource retrieval, it is in many cases
    necessary to determine the associated URI because most retrieval
    mechanisms currently only are defined for URIs.  (Additional
    rationale is given in Section 3.1.)

3.1 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
          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.  When the IRI is used simply for
          indentification purposes, it is not necessary to map the IRI to
          an URI (see Section 2.3).  However, when an IRI is used for
          resource retrieval, the resource that the IRI locates is the
          same as the one located by the URI obtained after converting
          the IRI according to the procedure defined here.  This means
          that there is no need to define resolution again on the IRI
          level.

    This mapping is accomplished in two steps.

       Step 1) This step generates a UCS-based encoding from the original
          IRI format.  This step has three variants, depending on the
          form of the input.

             Variant A) If the IRI is written on paper or read out loud,
                or otherwise represented as a sequence of characters
                independent of any encoding: Represent the IRI as a
                sequence of characters from the UCS normalized according
                to Normalization Form C (NFC, [UNI15]).

             Variant B) If the IRI is in some digital representation
                (e.g.  an octet stream) in some non-Unicode encoding:
                Convert the IRI to a sequence of characters from the UCS
                normalized according to NFC.




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             Variant C) If the IRI is in an Unicode-based encoding (for
                example UTF-8 or UTF-16): Do not normalize.  Move
                directly to Step 2.

       Step 2) For each character that is disallowed in URI references,
          apply steps 1) through 3) 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 2.3), 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] (as
    amended by [RFC2732] and [IDNURI]) 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.

    Note: For backwards compatibility with infrastructure that does not
    implement the updates of [IDNURI], converters MAY also convert the
    'ihostname' part of an IRI using the ToASCII operation specified in
    Section 4.1 of [IDNA] between Step 1 and Step 2.  Note that the
    ToASCII operation may fail.  Note that Internationalized Domain Names
    may be contained in parts of an IRI other than the 'ihostname' part.

3.2 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.  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).




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    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.

       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, see
          [Duer97].)

       c.  The conversion may result in a character that is not
          appropriate in an IRI.  See Section 5.1 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 represent characters that
          are not appropriate according to Section 5.1.

       5) Interpret the resulting octet sequence as a sequence of
          characters encoded in UTF-8.

    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 4) (see Section 5.1), results may differ.

4. 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).



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4.1 Bidi IRI Structure

    IRIs have an inherent structure that distinguishes structural
    characters (usually punctuation such as '@', '.', ':', '/', and so
    on) called delimiters and payload components (usually consisting
    mostly of letters and digits).

    ISSUE: Exact definition of components.

    In their internal digital representation, i.e.  stored or transmitted
    for resolution, bidirectional IRIs MUST be in full logical order both
    for the overall structure as well as for the individual components.
    They MUST conform directly to the IRI syntax rules (which includes
    the rules relevant to their scheme).  This is necessary to make sure
    that bidirectional IRIs can be processed in the same way as other
    IRIs.

    The components have the following restrictions:

       1) A component MUST NOT not use both right-to-left and left-to-
          right characters.

       2) A component MUST NOT contain bidirectional formatting
          characters.

       3) A component using right-to-left characters MUST NOT use any
          other class of characters (e.g.  neutrals or numbers).

    Note: Restrictions 1) and 2) are not very severe, in that they do not
    overly restrict useful identifiers.  Also, trying to remove it would
    make it impossible for humans to predict the logical sequence of
    characters inside a single component.  On the other hand, it would be
    very desirable to remove or at least soften restriction 3).
    Otherwise, it is impossible to combine Arabic or Hebrew letters with
    numbers, or to use a hyphen between two subcomponents of an Arabic
    component to avoid the cursive connection of the two subcomponents.
    To a certain extent, softening this restriction should be easily
    possible by adding additional formatting characters in well defined
    ways similar to the provisions in Section 4.2.  Feedback on this
    issue is particularly welcome.

4.2 Visual Rendering of Bidi IRIs

    Bidirectional IRIs MUST be rendered visually by rendering each
    component and each structural character from left to right.  They
    MUST render each component according to its natural direction (i.e.
    left-to-right for components with left-to-right characters, right-to-
    left for components with right-to-left characters).



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    ISSUE: The alternative is to display a series of right-to-left
    components in their natural (right-to-left) order.  This has the
    advantage that it will often be easier for native people to read the
    components in the right order.  The restrictions on individual
    components change.  In some cases, the correct visual rendering is
    automatic (i.e.  exactly the same as with the Unicode algorithm), and
    so in these cases, no bidi formatting characters have to be added.

    In a textual context, i.e.  assuming rendering by the Unicode
    bidirectional algorithm, the visual rendering backing store is done
    as follows:

    The visual representation uses some of the following Bidi formatting
    characters described by using a XML-style entity notation:

           &lrm;    U+200E     LEFT-TO-RIGHT MARK
           &rlm;    U+200F     RIGHT-TO-LEFT MARK
           &lre;    U+202A     LEFT-TO-RIGHT EMBEDDING
           &rle;    U+202B     RIGHT-TO-LEFT EMBEDDING
           &pdf;    U+202C     POP DIRECTIONAL FORMATTING
           &lro;    U+202D     LEFT-TO-RIGHT OVERRIDE
           &rlo;    U+202E     RIGHT-TO-LEFT OVERRIDE

          Each component with right-to-left characters is preceded and
          followed by an &lrm;.  This left-to-right mark provides a left-
          to-right context to intervening syntactic characters.

          If the overall context (base directionality) is right-to-left,
          the identifier is preceded by an &lre; and followed by a &pdf;.
          This makes sure that the components of the identifier are
          rendered in left-to-right order.  This may also be done by
          using the equivalent features of a higher-order protocol (e.g.
          by using the dir='ltr' attribute in HTML).


4.3 Input of Bidi IRIs

    Bidi input methods MUST generate Bidi IRIs in logical order while
    rendering them according to Section 4.2.  During input, rendering
    should be updated after every new character that is input to avoid
    end user confusion.

5. Use of IRIs

5.1 Limitations on UCS Character Allowed in IRI

    This section discusses the limitations on characters and character
    sequences usable for IRIs.  The considerations in this section are



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    relevant when creating IRIs and when converting from URIs to IRIs.

       a.  The repertoire of characters allowed in each IRI component is
          limited by the definition of that component.  For example, the
          definition of the scheme component does not allow characters
          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 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].

    Additional information is available from [UNIXML].  Although [UNIXML]
    is written in a different context, it discusses many of the
    categories of characters and code points not appropriate for IRIs.

5.2 Software Interfaces and Protocols

    Although an IRI is defined as a sequence of characters, software
    interfaces for URIs typically function on sequences of octets.  Thus,
    software interfaces and protocols MUST define which character



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    encoding is used.

    Intermediate software interfaces between IRI-capable components and
    URI-only components MUST map the IRIs as per Section 3.1, when
    transferring from IRI-capable to URI-only components.  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.

5.3 Format of URIs and IRIs in Documents 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 MUST 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 using the escaping conventions of the document format if such
    conventions are available.  Alternatively, they MAY be escaped
    according to Section 3.1.  For example, in HTML, XML, or SGML,
    numeric character references should 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.

    Note: Some formats already accommodate 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].

5.4 Relative IRI References

    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.

6. URI/IRI Processing Guidelines (informative)

    This informative section provides guidelines for 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.



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6.1 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.

    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.

    The transfer from URI-only to IRI-capable components requires no
    mapping, although the conversion described in Section 3.2 above may
    be performed.  It is preferable not to perform this inverse
    conversion when there is a chance that this cannot be done correctly.

6.2 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.

    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
    restrictions defined in Section 2.2 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
    Section 4.



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6.3 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
    servers, similar considerations apply, see in particular [RFC2640].

6.4 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).



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    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.

6.5 Display of URIs/IRIs

    In situations where the rendering software is not expected to display
    non-ASCII parts of the IRI correctly using the available layout and
    font resources, these parts should be escaped before being displayed.

    For display of Bidi IRIs, please see Section 4.2.

6.6 Interpretation of URIs and 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
    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



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    regularity of UTF-8 (see [Duer97] makes the potential for collisions
    lower than it may seem at first sight.

6.7 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
    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.

7. Security Considerations

    Incorrect escaping or unescaping can lead to security problems.  In
    particular, some UTF-8 decoders do not check against overlong byte
    sequences.  As an example, a '/' is encoded with the byte 0x2F both
    in UTF-8 and in ASCII, but some UTF-8 decoders also wrongly interpret
    the sequence 0xC0 0xAF as a '/'.  A sequence such as '%C0%AF..' may
    pass some security tests and then be interpreted as '/..' in a path
    if UTF-8 decoders are fault-tolerant, if conversion and checking are
    not done in the right order, and/or if reserved characters and



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    unreserved characters are not clearly distinguished.

    There are various ways in which "spoofing" can occur with IRIs.
    "Spoofing" means that somebody may add a resource name that looks the
    same or similar to the user, but points to a different resource.  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.  Most spoofing
    possibilities for IRIs are extensions of those for URIs.

    Spoofing can occur for various reasons.  A first reason is that
    normalization expectations of a user or actual normalization when
    entering an IRI do not match the normalization used on the server
    side.  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, and of additional mappings for user
    convenience, may increase the chance for spoofing.

    Spoofing can occur due to the fact that in the UCS, there are many
    characters that look very similar.  Details are discussed in Section
    6.4.  Again, this is very similar to spoofing possibilities on US-
    ASCII, e.g.  using 'br0ken' or '1ame' URIs.

    Spoofing can occur when URIs in various encodings are accepted to
    deal with older user agents.  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 no characters that
    have exactly the same encoding, detection is more difficult.

    Spoofing can occur in various IRI components, such as the domain name
    part or a path part.  For considerations specific to the domain name
    part, see [Nameprep].  For the path part, administrators of sites
    which allow independent users to create resources in the same subarea
    may need to be careful to check for spoofing.

8. Change log

    Changes from -00 to -01

       -  Re-integrated the section on Bidi, some issues left.

       -  Integrated IDN, changed syntax (host, userinfo,....).




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       -  Moved some text around, marked some as informational.

       -  Made a clear distinction of IRI use for identification only and
          for resource resolution.

       -  Fixed various details in wording, spelling,...


9. Acknowlegdements

    We would like to thank Larry Masinter for his work as coauthor of
    many earlier versions of this document (draft-masinter-url-i18n-xx).

    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

    [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, April 2002, <http://www.w3.org/TR/charmod>.

    [Duer97]        Duerst, M., "The Properties and Promises 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>.

    [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>.



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    [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>.

    [IDNURI]        Duerst, M., "Internationalized Domain Names in URIs",
                    draft-ietf-idn-uri-02.txt (work in progress), July
                    2002, <http://www.ietf.org/internet-drafts/draft-
                    ietf-idn-uri-02.txt>.

    [IDNA]          Faltstrom, P., Hoffman, P. and A. Faltstrom,
                    "Internationalizing Domain Names in Applications
                    (IDNA)", draft-ietf-idn-idna-09.txt (work in
                    progress), May 2002, <http://www.ietf.org/internet-
                    drafts/draft-ietf-idn-idna-09.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.

    [Nameprep]      Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
                    Profile for Internationalized Domain Names", draft-
                    ietf-idn-nameprep-10.txt (work in progress), May
                    2002, <http://www.ietf.org/internet-drafts/draft-
                    ietf-idn-nameprep-10.txt>.

    [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.



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    [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.

    [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,



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                    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>.

    [XMLNamespace]  Bray, T., Hollander, D. and A. Layman, "Namespaces in
                    XML", World Wide Web Consortium Recommendation,
                    January 1999, <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>.


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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