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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
Expires: August 15, 2004                                     M. Suignard
                                                    Microsoft Corporation
                                                        February 15, 2004


              Internationalized Resource Identifiers (IRIs)
                           draft-duerst-iri-06

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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    Drafts.

    Internet-Drafts are draft documents valid for a maximum of six months
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    The list of Internet-Draft Shadow Directories can be accessed at
    http://www.ietf.org/shadow.html.

    This Internet-Draft will expire on August 15, 2004.

Copyright Notice

    Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

    This document defines a new protocol element, the Internationalized
    Resource Identifier (IRI), as a complement to the URI [RFCYYYY].  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 public-iri@w3.org mailing list (publicly
    archived at http://lists.w3.org/Archives/Public/public-iri/).  An
    issues list for this document is maintained at http://www.w3.org/
    International/iri-edit#issues.  For more information on the topic of
    this document, please also see [W3CIRI] and [Duerst01].

Table of Contents

    1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  4
    1.1   Overview and Motivation  . . . . . . . . . . . . . . . . . .  4
    1.2   Applicability  . . . . . . . . . . . . . . . . . . . . . . .  4
    1.3   Definitions  . . . . . . . . . . . . . . . . . . . . . . . .  5
    1.4   Notation . . . . . . . . . . . . . . . . . . . . . . . . . .  6
    2.    IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . .  6
    2.1   Summary of IRI Syntax  . . . . . . . . . . . . . . . . . . .  7
    2.2   ABNF for IRI References and IRIs . . . . . . . . . . . . . .  7
    3.    Relationship between IRIs and URIs . . . . . . . . . . . . .  9
    3.1   Mapping of IRIs to URIs  . . . . . . . . . . . . . . . . . . 10
    3.2   Converting URIs to IRIs  . . . . . . . . . . . . . . . . . . 13
    3.2.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 14
    4.    Bidirectional IRIs for Right-to-left Languages . . . . . . . 15
    4.1   Logical Storage and Visual Presentation  . . . . . . . . . . 16
    4.2   Bidi IRI Structure . . . . . . . . . . . . . . . . . . . . . 17
    4.3   Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . . . 18
    4.4   Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 18
    5.    IRI Equivalence and Comparison . . . . . . . . . . . . . . . 20
    5.1   Simple String Comparison . . . . . . . . . . . . . . . . . . 20
    5.2   Conversion to URIs . . . . . . . . . . . . . . . . . . . . . 21
    5.3   Normalization  . . . . . . . . . . . . . . . . . . . . . . . 21
    5.4   Preferred Forms  . . . . . . . . . . . . . . . . . . . . . . 22
    6.    Use of IRIs  . . . . . . . . . . . . . . . . . . . . . . . . 23
    6.1   Limitations on UCS Characters Allowed in IRIs  . . . . . . . 23
    6.2   Software Interfaces and Protocols  . . . . . . . . . . . . . 23
    6.3   Format of URIs and IRIs in Documents and Protocols . . . . . 23
    6.4   Use of UTF-8 for Encoding Original Characters  . . . . . . . 24
    6.5   Relative IRI References  . . . . . . . . . . . . . . . . . . 25
    7.    URI/IRI Processing Guidelines (informative)  . . . . . . . . 25
    7.1   URI/IRI Software Interfaces  . . . . . . . . . . . . . . . . 25
    7.2   URI/IRI Entry  . . . . . . . . . . . . . . . . . . . . . . . 26
    7.3   URI/IRI Transfer Between Applications  . . . . . . . . . . . 26



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    7.4   URI/IRI Generation . . . . . . . . . . . . . . . . . . . . . 27
    7.5   URI/IRI Selection  . . . . . . . . . . . . . . . . . . . . . 27
    7.6   Display of URIs/IRIs . . . . . . . . . . . . . . . . . . . . 28
    7.7   Interpretation of URIs and IRIs  . . . . . . . . . . . . . . 28
    7.8   Upgrading Strategy . . . . . . . . . . . . . . . . . . . . . 29
    8.    Security Considerations  . . . . . . . . . . . . . . . . . . 30
    9.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31
          Normative References . . . . . . . . . . . . . . . . . . . . 32
          Non-normative References . . . . . . . . . . . . . . . . . . 32
          Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 35
          Full Copyright Statement . . . . . . . . . . . . . . . . . . 36








































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1. Introduction

1.1 Overview and Motivation

    A URI is defined in [RFCYYYY] 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 have
    transcriptions to Latin letters.  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
    [RFCYYYY], 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 for
    new URI schemes [RFC2718].  The compatibility is provided by
    specifying 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 should 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) The URI corresponding to the IRI in question has to encode
          original characters into octets using UTF-8.  For new URI
          schemes, this is recommended in [RFC2718].  It can apply to a
          whole scheme (e.g.  IMAP URLs [RFC2192] and POP URLs [RFC2384],
          or the URN syntax [RFC2141]).  It can apply to a specific part
          of a URI, such as the fragment identifier (e.g.  [XPointer]).
          It can apply to a specific URI or part(s) thereof.  For
          details, please see Section 6.4.


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.

       charset: The name of a parameter or attribute used to identify a
          character encoding.

       UCS: Universal Character Set; the coded character set defined by
          [ISO10646] and [UNIV4].




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       IRI reference: The term "IRI reference" denotes the common usage
          of an internationalized resource identifier.  An IRI reference
          may be absolute or relative.  However, the "IRI" that results
          from such a reference only includes absolute IRIs; any relative
          IRIs are resolved to their absolute form.  Note that in
          [RFC2396], URIs did not include fragment identifiers, but in
          [RFCYYYY], fragment identifiers are part of URIs.

       running text: Human text (paragraphs, sentences, phrases) with
          syntax according to orthographic conventions of a natural
          language, as opposed to syntax defined for ease of processing
          by machines (markup, programming languages,...).


1.4 Notation

    RFCs and Internet Drafts currently do not allow any characters
    outside the US-ASCII repertoire.  Therefore, this document uses
    various special notations to denote such characters in examples.

    In text, characters outside US-ASCII are sometimes referenced by
    using a prefix of 'U+', followed by four to six hexadecimal digits.

    To represent characters outside US-ASCII in examples, this document
    uses two notations called 'XML Notation' and 'Bidi Notation'.

    XML Notation uses leading '&#x', trailing ';', and the hexadecimal
    number of the character in the UCS in between.  Example: я
    stands for CYRILLIC CAPITAL LETTER YA.  In this notation, an actual
    '&' is denoted by '&'.

    Bidi Notation is used for bidirectional examples: lower case ASCII
    letters stand for Latin letters or other letters that are written
    left-to-right, whereas upper case letters represent Arabic or Hebrew
    letters that are written right-to-left.

    To denote actual octets in examples (as opposed to escaped octets),
    the two hex digits denoting the octet are enclosed in "<" and ">".
    For example, the octet often denoted as 0xc9 is denoted here as <c9>.

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
    document are to be interpreted as described in [RFC2119].

2. IRI Syntax

    This section defines the syntax of Internationalized Resource
    Identifiers (IRIs).



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    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
    stored or transmitted digitally.  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 [RFCYYYY], but the class of
    unreserved characters is extended by adding the characters of the UCS
    (Universal Character Set, [ISO10646]) beyond U+007F, subject to the
    limitations given in the syntax rules below and in Section 6.1.

    Otherwise, the syntax and use of components and reserved characters
    is the same as that in [RFCYYYY].  All the operations defined in
    [RFCYYYY], 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 are not reserved and therefore
    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 in IRIs, 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 in URIs.

2.2 ABNF for IRI References and IRIs

    While it might be possible to define IRI references and IRIs merely
    by their transformation to URI references and 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 syntax of this ABNF is
    described in [RFC2234].  Character numbers are taken from the UCS,
    without implying any actual binary encoding.  Terminals in the ABNF
    are characters, not bytes.

    The following rules are different from [RFCYYYY]:

       IRI           = scheme ":" ["//" iauthority] ipath ["?" iquery]
                       ["#" ifragment]




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       IRI-reference = IRI / relative-IRI

       relative-IRI  = ["//" iauthority] ipath ["?" iquery]
                       ["#" ifragment]

       absolute-IRI  = scheme ":" ["//" iauthority] ipath ["?" iquery]

       iauthority    = [ iuserinfo "@" ] ihost [ ":" port ]

       iuserinfo     = *( iunreserved / pct-encoded / sub-delims
                           / ":" )

       ihost         = IP-literal / IPv4address / ireg-name

       ireg-name     = 0*255( iunreserved / pct-encoded / sub-delims )

       ipath         = isegment *( "/" isegment )

       isegment      = *ipchar

       iquery        = *( ipchar / iprivate / "/" / "?" )

       ifragment     = *( ipchar / "/" / "?" )

       ipchar        = iunreserved / pct-encoded / sub-delims / ":"
                      / "@"

       iunreserved   = ALPHA / DIGIT / "-" / "." / "_" / "~" / ucschar

       ucschar       = %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF /
                     / %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD
                     / %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD
                     / %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD
                     / %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD
                     / %xD0000-DFFFD / %xE1000-EFFFD

       iprivate      = %xE000-F8FF / %xF0000-FFFFD / %x100000-10FFFD

    Some productions ambiguous.  The "first-match-wins" (a.k.a.
    "greedy") algorithm applies.  For details, see [RFCYYYY].

    The following are the same as [RFCYYYY]:

       scheme        = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )

       port          = *DIGIT

       IP-literal    = "[" ( IPv6address | IPvFuture  ) "]"



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       IPvFuture     = "v" HEXDIG "." 1*( unreserved / sub-delims / ":" )

       IPv6address   =                          6( h4 ":" ) ls32
                     /                     "::" 5( h4 ":" ) ls32
                     / [              h4 ] "::" 4( h4 ":" ) ls32
                     / [ *1( h4 ":" ) h4 ] "::" 3( h4 ":" ) ls32
                     / [ *2( h4 ":" ) h4 ] "::" 2( h4 ":" ) ls32
                     / [ *3( h4 ":" ) h4 ] "::"    h4 ":"   ls32
                     / [ *4( h4 ":" ) h4 ] "::"             ls32
                     / [ *5( h4 ":" ) h4 ] "::"             h4
                     / [ *6( h4 ":" ) h4 ] "::"

       h4            = 1*4HEXDIG

       ls32          = ( h4 ":" h4 ) / IPv4address

       IPv4address   = dec-octet "." dec-octet "." dec-octet "." dec-octet

       dec-octet     = DIGIT                 ; 0-9
                     / %x31-39 DIGIT         ; 10-99
                     / "1" 2DIGIT            ; 100-199
                     / "2" %x30-34 DIGIT     ; 200-249
                     / "25" %x30-35          ; 250-255

       pct-encoded   = "%" HEXDIG HEXDIG

       unreserved    = ALPHA / DIGIT / "-" / "." / "_" / "~"

       reserved      = gen-delims / sub-delims

       gen-delims    = ":" / "/" / "?" / "#" / "[" / "]" / "@"

       sub-delims    = "!" / "$" / "&" / "'" / "(" / ")"
                     / "*" / "+" / "," / ";" / "="


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
    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.)




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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 solely for
          identification purposes, it is not necessary to map the IRI to
          a URI (see Section 5).  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 separately on the
          IRI level.

    Applications MUST map IRIs to URIs using the following 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, [UTR15]).

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

             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



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          consist of all non-ASCII characters allowed in IRIs.

             1) Convert the character to a sequence of one or more octets
                using UTF-8 [RFC3629].

             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 [RFCYYYY].  To
                reduce variability, the hexadecimal notation SHOULD use
                upper case letters.

             3) Replace the original character by the resulting character
                sequence (i.e.  a sequence of %HH triplets).

    The above mapping from IRIs to URIs produces URIs fully conforming to
    [RFCYYYY].  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 by definition an IRI.

    Infrastructure accepting IRIs MAY also convert the ireg-name
    component of an IRI as follows (before step 2 above) if it knows that
    the scheme in question uses domain names: Replace the iregname part
    of the IRI by the part converted using the ToASCII operation
    specified in Section 4.1 of [RFC3490], with the flag
    UseSTD3ASCIIRules set to TRUE and the flag AllowUnassigned set to
    FALSE for creating IRIs and set to TRUE otherwise.  The ToASCII
    operation may fail, but this would mean that the IRI cannot be
    resolved.  For example, the IRI
    http://r&#xE9;sum&#xE9;.example.org may be converted to
    http://xn--rsum-bpad.example.org instead of
    http://r%C3%A9sum%C3%A9.example.org.

       Note: The uniform treatment of the whole IRI in step 2) above is
          important to not make processing dependent on URI scheme.  See
          [Gettys] for an in-depth discussion.

       Note: In practice, the difference above will not be noticed if
          mapping from IRI to URI and resolution is tightly integrated
          (e.g.  carried out in the same user agent).  But conversion
          using [RFC3490] may be able to better deal with backwards
          compatibility issues in case mapping and resolution are
          separated, as in the case of using an HTTP proxy.

       Note: Internationalized Domain Names may be contained in parts of
          an IRI other than the ireg-name part.  It is the responsibility
          of scheme-specific implementations (if the Internationalized
          Domain Name is part of the scheme syntax) or of server-side
          implementations (if the Internationalized Domain Name is part



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          of 'iquery') to apply the necessary conversions at the
          appropriate point.  Example: Trying to validate the Web page at
          http://r&#xE9;sum&#xE9;.example.org would lead to an IRI of
          http://validator.w3.org/
          check?uri=http%3A%2F%2Fr&#xE9;sum&#xE9;.example.org, which
          would convert to a URI of
          http://validator.w3.org/
          check?uri=http%3A%2F%2Fr%C3%A9sum%C3%A9.example.org.  The
          server side implementation would be responsible to do the
          necessary conversions in order to be able to retrieve the Web
          page.

    Infrastructure accepting IRIs MAY also deal with the printable
    characters in US-ASCII that are not allowed in URIs, namely "<", ">",
    '"', Space, "{", "}", "|", "\", "^", and "`", in step 2) above.  If
    such characters are found but are not converted, then the conversion
    SHOULD fail.  Please note that the number sign ("#"), the percent
    sign ("%"), and the square bracket characters ("[", "]") are not part
    of the above list, and MUST NOT be converted.  Protocols and formats
    that have used earlier definitions of IRIs including these characters
    MAY require unescaping of these characters as a preprocessing step to
    extract the actual IRI from a given field.  Such preprocessing MAY
    also be used by applications allowing the user to enter an IRI.

       Note: In this process (in step 2.3), characters allowed in URI
          references as well as 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.) For example, an IRI of
          http://www.example.org/red%09ros&#xE9;#red (in XML notation) is
          converted to
          http://www.example.org/red%09ros%C3%A9#red, not to something
          like
          http%3A%2F%2Fwww.example.org%2Fred%2509ros%C3%A9%23red.

       Note: Some older software transcoding to UTF-8 may produce illegal
          output for some input, in particular for characters outside the
          BMP (Basic Multilingual Plane).  As an example, for the
          following IRI with non-BMP characters (in XML Notation):
          http://example.com/&#x10300;&#x10301;&#x10302;
          (the first three letters of the Old Italic alphabet) the
          correct conversion to a URI is:
          http://example.com/%F0%90%8C%80%F0%90%8C%81%F0%90%8C%82








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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
    result in an IRI which maps back to the URI that was used as an input
    for the conversion (except for potential case differences in escape
    sequences).  However, the IRI resulting from this conversion may not
    be exactly the same as the original IRI (if there ever was one).

    URI to IRI conversion removes escape sequences, but not all escaping
    can be eliminated.  There are several 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: The octet patterns of UTF-8 are highly regular.
          Therefore, 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 [Duerst97].)

       c) The conversion may result in a character that is not
          appropriate in an IRI.  See Section 6.1 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) except those corresponding to '%', characters in
          'reserved', and characters in US-ASCII not allowed in URIs, to
          the corresponding octets.

       3) Re-escape any octet produced in step 2) that is not part of a
          strictly legal UTF-8 octet sequence.

       4) Re-escape all octets produced in step 3) that in UTF-8
          represent characters that are not appropriate according to
          Section 4.1 and Section 6.1.

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




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    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 6.1), results may vary.

    Conversions from URIs to IRIs MUST NOT use any other encoding than
    UTF-8 in steps 3) and 4) above, even if it might be possible from
    context to guess that another encoding than UTF-8 was used in the
    URI.  As an example, the URI http://www.example.org/r%E9sum%E9.html
    might with some guessing be interpreted to contain two e-acute
    characters encoded as iso-8859-1.  It must not be converted to an IRI
    containing these e-acute characters.  Otherwise, the IRI will in the
    future be mapped to http://www.example.org/r%C3%A9sum%C3%A9.html,
    which is a different URI than http://www.example.org/r%E9sum%E9.html.

3.2.1 Examples

    This section shows various examples of converting URIs to IRIs.  The
    notation <hh> is used to denote octets outside those that can be
    represented in this document.  Each example shows the result after
    applying each of the steps 1) to 5).  XML Notation is used for the
    final result.

    The following example contains the sequence '%C3%BC', which is a
    strictly legal UTF-8 sequence, and which is converted into the actual
    character U+00FC LATIN SMALL LETTER U WITH DIAERESIS (also known as
    u-umlaut).

       1) http://www.example.org/D%C3%BCrst

       2) http://www.example.org/D<c3><bc>rst

       3) http://www.example.org/D<c3><bc>rst

       4) http://www.example.org/D<c3><bc>rst

       5) http://www.example.org/D&#xFC;rst

    The following example contains the sequence '%FC', which might
    represent U+00FC LATIN SMALL LETTER U WITH DIAERESIS in the
    iso-8859-1 encoding.  (It might represent other characters in other
    encodings.  For example, the octet <fc> in iso-8859-5 represents
    U+045C CYRILLIC SMALL LETTER KJE.) Because <fc> is not part of a
    strictly legal UTF-8 sequence, it is re-escaped in step 3).

       1) http://www.example.org/D%FCrst

       2) http://www.example.org/D<fc>rst




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       3) http://www.example.org/D%FCrst

       4) http://www.example.org/D%FCrst

       5) http://www.example.org/D%FCrst

    The following example contains '%e2%80%ae', which is the escaped
    UTF-8 encoding of U+202E, RIGHT-TO-LEFT OVERRIDE.  Section 4.1
    forbids the direct use of this character in an IRI.  Therefore, the
    corresponding octets are re-escaped in step 4).  This example shows
    that the case (upper or lower) of letters used in escapes may not be
    preserved.  The example also contains a punycode-encoded domain name
    label (xn--99zt52a), which is not converted.

       1) http://xn--99zt52a.example.org/%e2%80%ae

       2) http://xn--99zt52a.example.org/<e2><80><ae>

       3) http://xn--99zt52a.example.org/<e2><80><ae>

       4) http://xn--99zt52a.example.org/%E2%80%AE

       5) http://xn--99zt52a.example.org/%E2%80%AE

    Implementations with scheme-specific knowledge MAY convert punycode-
    encoded domain name labels to the corresponding characters using the
    ToUnicode procedure.  Thus, for the example above, the label xn--
    99zt52a may be converted to U+7D0D U+8C46 (Japanese Natto), leading
    to the overall IRI of
    http://&#x7D0D;&#x8C46;.example.org/%E2%80%AE

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 (rtl) 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).

    Because of the complex interaction between the logical
    representation, the visual representation, and the syntax of a Bidi
    IRI, a balance is needed between various requirements.  The main
    requirements are:

       1) user-predictable conversion between visual and logical
          representation;



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       2) the ability to include a wide range of characters in various
          parts of the IRI;

       3) minor or no changes or restrictions for implementations.


4.1 Logical Storage and Visual Presentation

    When stored or transmitted in digital representation, bidirectional
    IRIs MUST be in full logical order, and MUST conform to the IRI
    syntax rules (which includes the rules relevant to their scheme).
    This assures that bidirectional IRIs can be processed in the same way
    as other IRIs.

    When rendered, bidirectional IRIs MUST be rendered using the Unicode
    Bidirectional Algorithm [UNIV4], [UNI9].  Bidirectional IRIs MUST be
    rendered in the same way as they would be rendered if they were in an
    left-to-right embedding, i.e.  as if they were preceded by U+202A,
    LEFT-TO-RIGHT EMBEDDING (LRE), and followed by U+202C, POP
    DIRECTIONAL FORMATTING (PDF).  Setting the embedding direction can
    also be done in a higher-order protocol (e.g.  the dir='ltr'
    attribute in HTML).

    There is no requirement to actually use the above embedding if the
    display is still the same without the embedding.  For example, a
    bidirectional IRI in a text with left-to-right base directionality
    (such as used for English or Cyrillic) that is preceded and followed
    by whitespace and  strong left-to-right characters does not need an
    embedding.  Also, a bidirectional relative IRI that only contains
    strong right-to-left characters and weak characters and that starts
    and ends with a strong rigth-to-left character and appears in a text
    with right-to-left base directionality (such as used for Arabic or
    Hebrew) and is preceded and followed by whitespace and strong
    characters does not need an embedding.

    In some other cases, using U+200E, LEFT-TO-RIGHT MARK (LRM) may be
    sufficient to force the correct display behavior.  However, the
    details of the Unicode Bidirectional algorithm are not always easy to
    understand.  Implementers are strongly advised to err on the side of
    caution and to use embedding in all cases where they are not
    completely sure that the display behavior is unaffected without the
    embedding.

    The Unicode Bidirectional Algorithm ([UNI9], Section 4.3) permits
    higher-level protocols to influence bidirectional rendering.  Such
    changes by higher-level protocols MUST NOT be used if they change the
    rendering of IRIs.




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    The bidirectional formatting characters that may be used before or
    after the IRI to assure correct display are themselves not part of
    the IRI.  IRIs MUST NOT contain bidirectional formatting characters
    (LRM, RLM, LRE, RLE, LRO, RLO, and PDF).  They affect the visual
    rendering of the IRI, but do not themselves appear visually.  It
    would therefore not be possible to correctly input an IRI with such
    characters.

4.2 Bidi IRI Structure

    The Unicode Bidirectional Algorithm is designed mainly for running
    text.  To make sure that it does not affect the rendering of
    bidirectional IRIs too much, some restrictions on bidirectional IRIs
    are necessary.  These restrictions are given in terms of delimiters
    (structural characters, mostly punctuation such as '@', '.', ':',
    '/') and components (usually consisting mostly of letters and
    digits).

    The following syntax rules from Section 2.2 correspond to components
    for the purpose of Bidi behavior: iuserinfo, isegment, ireg-name,
    iquery, and ifragment.

    Specifications that define the syntax of any of the above components
    MAY divide them further and define smaller parts to be components
    according to this document.  As an example, the restrictions of
    [RFC3490] on bidirectional domain names correspond to treating each
    label of a domain name as a component for those schemes where ireg-
    name is a domain name.  Even where the components are not defined
    formally, it may be helpful to think about some syntax in terms of
    components and to apply the relevant restrictions.  For example, for
    the usual name/value syntax in query parts, it is convenient to treat
    each name and each value as a component.  As another example, the
    extensions in a resource name can be treated as separate components.

    For each component, the following restrictions apply:

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

       2) A component using right-to-left characters SHOULD start and end
          with right-to-left characters.

    The above restrictions are given as shoulds, rather than as musts.
    For IRIs that are never presented visually, they are not relevant.
    However, for IRIs in general, they are very important to insure
    consistent conversion between visual presentation and logical
    representation, in both directions.




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       Note: In some components, the above restrictions may actually be
          strictly enforced.  For example, [RFC3490] requires that these
          restrictions apply to the labels of a host name for those
          schemes where ireg-name is a host name.  In some other
          components, for example path components, following these
          restrictions may not be too difficult.  For other components,
          such as parts of the query part, it may be very difficult to
          enforce the restrictions, because the values of query
          parameters may be arbitrary character sequences.

    If the above restrictions cannot be satisfied otherwise, the affected
    component can always be mapped to URI notation as described in
    Section 3.1.  Please note that the whole component needs to be mapped
    (see also Example 9 below).

4.3 Input of Bidi IRIs

    Bidi input methods MUST generate Bidi IRIs in logical order while
    rendering them according to Section 4.1.  During input, rendering
    SHOULD be updated after every new character that is input to avoid
    end user confusion.

4.4 Examples

    This section gives examples of bidirectional IRIs, in Bidi Notation.
    It shows legal IRIs with the relationship between logical and visual
    representation, and explains how certain phenomena in this
    relationship may look strange to somebody not familiar with
    bidirectional behavior, but familiar to users of Arabic and Hebrew.
    It also shows what happens if the restrictions given in Section 4.2
    are not followed.  The examples below can be seen at [BidiEx], in
    Arabic, Hebrew, and Bidi Notation variants.

    To read the bidi text in the examples, read the visual representation
    from left to right until you encounter a block of rtl text.  Read the
    rtl block (including slashes and other special characters) from right
    to left, then continue at the next unread ltr character.

    Example 1: A single component with rtl characters is inverted:
    logical representation: http://ab.CDEFGH.ij/kl/mn/op.html
    visual representation: http://ab.HGFEDC.ij/kl/mn/op.html
    Components can be read one-by-one, and each component can be read in
    its natural direction.

    Example 2: More than one consecutive component with rtl characters is
    inverted as a whole:
    logical representation: http://ab.CDE.FGH/ij/kl/mn/op.html
    visual representation: http://ab.HGF.EDC/ij/kl/mn/op.html



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    A sequence of rtl components is read rtl, in the same way as a
    sequence of rtl words is read rtl in a bidi text.

    Example 3: All components of an IRI (except for the scheme) are rtl.
    All rtl components are inverted overall:
    logical representation: http://AB.CD.EF/GH/IJ/KL?MN=OP;QR=ST#UV
    visual representation: http://VU#TS=RQ;PO=NM?LK/JI/HG/FE.DC.BA
    The whole IRI (except the scheme) is read rtl.  Delimiters between
    rtl components stay between the respective components; delimiters
    between ltr and rtl components don't move.

    Example 4: Several sequences of rtl components are each inverted on
    their own:
    logical representation: http://AB.CD.ef/gh/IJ/KL.html
    visual representation: http://DC.BA.ef/gh/LK/JI.html
    Each sequence of rtl components is read rtl, in the same way as each
    sequence of rtl words in an ltr text is read rtl.

    Example 5: Example 2, applied to components of different kinds:
    logical representation: http://ab.cd.EF/GH/ij/kl.html
    visual representation: http://ab.cd.HG/FE/ij/kl.html
    The inversion of the domain name label and the path component may be
    unexpected, but is consistent with other bidi behavior.  For
    reassurance that the domain component really is "ab.cd.EF", it may be
    helpful to read aloud the visual representation following the bidi
    algorithm.  After "http://ab.cd." one reads the RTL block "E-F-slash-
    G-H", which corresponds to the logical representation.

    Example 6: Same as example 5, with more rtl components:
    logical representation: http://ab.CD.EF/GH/IJ/kl.html
    visual representation: http://ab.JI/HG/FE.DC/kl.html
    The inversion of the domain name labels and the path components may
    be easier to identify because the delimiters also move.

    Example 7: A single rtl component with included digits:
    logical representation: http://ab.CDE123FGH.ij/kl/mn/op.html
    visual representation: http://ab.HGF123EDC.ij/kl/mn/op.html
    Numbers are written ltr in all cases, but are treated as an
    additional embedding inside a run of rtl characters.  This is
    completely consistent with usual bidirectional text.

    Example 8 (not allowed): Numbers at the start or end of a rtl
    component:
    logical representation: http://ab.cd.ef/GH1/2IJ/KL.html
    visual representation: http://ab.cd.ef/LK/JI1/2HG.html
    The sequence '1/2' is interpreted by the bidi algorithm as a
    fraction, fragmenting the components and leading to confusion.  There
    are other characters that are interpreted in a special way close to



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    numbers, in particular '+', '-', '#', '$', '%', ',', '.', and ':'.

    Example 9 (not allowed): The numbers in the previous example are
    escaped:
    logical representation: http://ab.cd.ef/GH%31/%32IJ/KL.html,
    visual representation (Hebrew): http://ab.cd.ef/LK/JI%32/%31HG.html
    visual representation (Arabic): http://ab.cd.ef/LK/JI32%/31%HG.html
    Depending on whether the upper-case letters represent Arabic or
    Hebrew, the visual representation is different.

    Example 10 (allowed, but not recommended):
    logical representation: http://ab.CDEFGH.123/kl/mn/op.html
    visual representation: http://ab.123.HGFEDC/kl/mn/op.html
    Components consisting of only numbers are allowed (it would be rather
    difficult to prohibit them), but may interact with adjacent RTL
    components in ways that are not easy to predict.

5. IRI Equivalence and Comparison

    This section discusses IRI Equivalence and Comparison similar to
    Section 6, "Normalization and Comparison", in [RFCYYYY].  This
    section focuses on the main issues and on aspects that are different
    from [RFCYYYY]; Section 6 of [RFCYYYY] is recommended background
    reading.

    There is no general rule or procedure to decide whether two arbitrary
    IRIs are equivalent or not (i.e.  whether they 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 the same resource.  Each specification or application that
    uses IRIs has to decide on the appropriate criterion for IRI
    equivalence.

5.1 Simple String Comparison

    In some scenarios 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.  An
    example of such a case is XML Namespaces ([XMLNamespace]).  In such
    cases, two IRIs SHOULD be defined as equivalent if and only if they
    are character-by-character equivalent.  This is the same as being
    byte-by-byte equivalent if the character encoding for both IRIs is
    the same.  As an example,
    http://example.org/~user, http://example.org/%7euser, and
    http://example.org/%7Euser are not equivalent under this definition.
    In such a case, the comparison function MUST NOT map IRIs to URIs,
    because such a mapping would create additional spurious equivalences.




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    It follows that IRIs SHOULD NOT be modified when being transported if
    there is any chance that this IRI might be used as an identifier in
    the way explained above.

5.2 Conversion to URIs

    For actual resolution, differences in escaping (except for the
    escaping of reserved characters) MUST always result in the same
    resource.  For example, http://example.org/~user,
    http://example.org/%7euser and http://example.org/%7Euser 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 (preferably
    upper case).  For comparison, such conversions MUST only be done on
    the fly, while retaining the original IRI.

    Additional, similar equivalences are possible based on knowledge
    about the generic URI/IRI syntax, such as the fact that the scheme
    part is case-insensitive.

5.3 Normalization

    The Unicode Standard [UNIV4] defines various equivalences between
    sequences of characters for various purposes.  Unicode Standard Annex
    #15 [UTR15] defines various Normalization Forms for these
    equivalences, in particular Normalization Form C (NFC, Canonical
    Decomposition, followed by Canonical Composition) and Normalization
    Form KC (NFKC, Compatibility Decomposition, followed by Canonical
    Composition).

    Equivalence of IRIs MUST rely on the assumption that IRIs are
    appropriately pre-normalized, rather than applying normalization when
    comparing two IRIs.  The exceptions are conversion from a non-digital
    form, and conversion from a non-UCS-based encoding to an UCS-based
    encoding.  In these cases, NFC or a normalizing transcoder using NFC
    MUST be used for interoperability.  To avoid false negatives and
    problems with transcoding, IRIs SHOULD be created using NFC.  Using
    NFKC may avoid even more problems, for example by choosing half-width
    Latin letters instead of full-width, and full-width Katakana instead
    of half-width.

    As an example, http://www.example.org/r&#xE9;sum&#xE9;.html (in XML
    Notation) is in NFC.  On the other hand, http://www.example.org/
    re&#x301;sume&#x301;.html is not in NFC.  The former uses precombined
    e-acute characters, the later uses 'e' characters followed by



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    combining acute accents.  Both usages are defined to be canonically
    equivalent in [UNIV4].

          Because it is unknow how a particular field is being treated
          with respect to text normalization, it would be inappropriate
          to allow third parties to normalize an IRI arbitrarily.  This
          does not contradict the recommendation that when a resource is
          created, and an IRI for that resource, you try to be as
          normalized as possible (i.e.  NFC or even NFKC).  This is
          similar to the upper-case/lower-case problems in URIs.  Some
          parts of a URI are case-insensitive (domain name).  For others,
          it is unclear whether they are case-sensitive or case-
          insensitive, or something in between (e.g.  case-sensitive, but
          if the wrong case is used, a multiple choice selection is
          provided instead of a direct negative result).  The best recipe
          is that the generator uses a reasonable capitalization, and
          when transfering the URI, that capitalization is never changed.

    Various IRI schemes may allow the usage of International Domain Names
    (IDN) [RFC3490].  When in use in IRIs, those names SHOULD be
    validated using the ToASCII operation defined in [RFC3490], with the
    flags "UseSTD3ASCIIRules" and "AllowUnassigned".  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).

5.4 Preferred Forms

    The following are the preferred forms for IRIs when generated:

       -  Always provide the URI scheme in lowercase characters.

       -  Only perform percent-escaping where it is essential.

       -  Always use uppercase A-through-F characters when percent-
          escaping.

       -  Always provide the hostname, if any, in the form produced when
          applying nameprep [RFC3491].  This in particular includes using
          lowercase characters rather than uppercase characters where
          applicable.

       -  Where possible, provide IRI components in NFKC or NFC.

       -  Prevent /./ and /../ from appearing in non-relative URI paths.

       -  For schemes that define an empty path to be equivalent to a
          path of "/", use "/".



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6. Use of IRIs

6.1 Limitations on UCS Characters Allowed in IRIs

    This section discusses 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.

       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) 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 [RFC3491].

    Additional information is available from [UNIXML].  [UNIXML] is
    written in the context of running text rather than in the context of
    identifiers.  Nevertheless, it discusses many of the categories of
    characters not appropriate for IRIs.

6.2 Software Interfaces and Protocols

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

    Intermediate software interfaces between IRI-capable components and
    URI-only components MUST map the IRIs 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.

6.3 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



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    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 or XML, 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].

6.4 Use of UTF-8 for Encoding Original Characters

    This section discusses details and gives examples for point c) in
    Section 1.2.  In order to be able to use IRIs, the URI corresponding
    to the IRI in question has to encode original characters into octets
    using UTF-8.  This can be specified for all URIs of a URI scheme, or
    can apply to individual URIs for schemes that do not specify how to
    encode original characters.  It can apply to the whole URI, or only
    some part.

    For new URI schemes, using UTF-8 is recommended in [RFC2718].
    Examples where this is already used are the URN syntax [RFC2141],
    IMAP URLs [RFC2192], and POP URLs [RFC2384].  On the other hand,
    because the HTTP URL scheme does not specify how to encode original
    characters, only some HTTP URLs can have corresponding but different
    IRIs.

    For example, for a document with a URI of
    http://www.example.org/r%C3%A9sum%C3%A9.html, it is possible to
    construct a corresponding IRI (in XML notation, see Section 1.4):
    http://www.example.org/r&#xE9;sum&#xE9;.html (&#xE9; stands for the
    e-acute character, and %C3%A9 is the UTF-8 encoded and escaped
    representation of that character).  On the other hand, for a document
    with a URI of http://www.example.org/r%E9sum%E9.html, the escaped
    octets cannot be converted to actual characters in an IRI, because
    the escaping is not based on UTF-8.

    The requirement for the use of UTF-8 applies to all parts of a URI.
    However, it is possible that the capability of IRIs to represent a
    wide range of characters directly is used just in some parts of the
    IRI (or IRI reference).  The other parts of the IRI may only contain
    ASCII characters, or they may not be based on UTF-8.  They may be



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    based on another encoding, or they may directly encode raw binary
    data (see also [RFC2397]).

    For example, it is possible to have a URI reference of
    http://www.example.org/r%E9sum%E9.xml#r%C3%A9sum%C3%A9, where the
    document name is encoded in iso-8859-1 based on server settings, but
    the fragment identifier is encoded in UTF-8 according to [XPointer].
    The IRI corresponding to the above URI would be (in XML notation)
    http://www.example.org/r%E9sum%E9.xml#r&#xE9;sum&#xE9;.

    Similar considerations apply to query parts.  The functionality of
    IRIs (namely to be able to include non-ASCII characters) can only be
    used if the query part is encoded in UTF-8.

6.5 Relative IRI References

    Processing of relative forms of IRIs against a base is handled
    straightforwardly; the algorithms of [RFCYYYY] can be applied
    directly, treating the characters additionally allowed in IRIs in the
    same way as unreserved characters in URIs.

7. 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.

7.1 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 API or protocol definition must clearly define the
    encoding to be used.

    The transfer from URI-only to IRI-capable components requires no
    mapping, although the conversion described in Section 3.2 above may



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    be performed.  It is preferable not to perform this inverse
    conversion when there is a chance that this cannot be done correctly.

7.2 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.

    As an example of variant settings, input method editors for East
    Asian Languages usually allow the input of Latin letters and related
    characters in full-width or half-width versions.  For IRI input, the
    input method editor should be set so that it produces half-width
    Latin letters, and full-width Katakana.

    An input field primarily or only used for the input of URIs/IRIs may
    allow the user to view an IRI as mapped to a URI.  Places where the
    input of IRIs is frequent may provide the possibility for viewing an
    IRI as mapped 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 map the IRI to a URI before passing it to
    such a component.

    For the input of IRIs with right-to-left characters, please see
    Section 4.3.

7.3 URI/IRI Transfer Between Applications

    Many applications, in particular many mail user agents, try to detect
    URIs appearing in plain text.  For this, they use some heuristics
    based on URI syntax.  They then allow the user to click on such URIs
    and retrieve the corresponding resource in an appropriate (usually
    scheme-dependent) application.



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    Such applications have to be upgraded to use the IRI syntax rather
    than the URI syntax as a base for heuristics.  In particular, a non-
    ASCII character should not be taken as the indication of the end of
    an IRI.  Such applications also have to make sure that they correctly
    convert the detected IRI from the encoding of the document or
    application where the IRI appears to the encoding used by the system-
    wide IRI invocation mechanism, or to a URI (according to Section 3.1)
    if the system-wide invocation mechanism only accepts URIs.

    The clipboard is another frequently used way to transfer URIs and
    IRIs from one application to another.  On most platforms, the
    clipboard is able to store and transfer text in many languages and
    scripts.  Correctly used, the clipboard transfers characters, not
    bytes, which will do the right thing with IRIs.

7.4 URI/IRI Generation

    Systems that offer 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.  For example,
    if a file system contains a file named r&#xE9;sum&#xE9;.html, a
    server should expose this as r%C3%A9sum%C3%A9.html in a URI, which
    allows to use r&#xE9;sum&#xE9;.html in an IRI, even if the file name
    locally is kept in an encoding other than UTF-8.

    This recommendation in particular applies to HTTP servers.  For FTP
    servers, similar considerations apply, see in particular [RFC2640].

7.5 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



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    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 [UTR15] (which means that they are also in NFC).

    As an example, the UCS contains the 'fi' ligature at U+FB01 for
    compatibility reasons.  Wherever possible, IRIs should use the two
    letters 'f' and 'i' rather than the 'fi' ligature.  An example where
    the latter may be used is in the query part of an IRI for an explicit
    search for a word written containing the 'fi' ligature.

    In certain cases, there is a chance that characters 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 characters in a single component
    are used together in a given language.  This usually means that all
    these characters will be from the same script, but there are
    languages that mix characters from different scripts (such as
    Japanese).  This is similar to the heuristics used to distinguish
    between letters and numbers in the examples above.  Also, for Latin,
    Greek, and Cyrillic, using lower-case letters results in fewer
    ambiguities than using upper-case letters.

7.6 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 4.1.

7.7 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.



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    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 matching 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
    regularity of UTF-8 (see [Duerst97]) makes the potential for
    collisions lower than it may seem at first sight.

7.8 Upgrading Strategy

    Where 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



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    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.

8. 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
    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 and 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, or when transcoding an IRI from a legacy encoding,
    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 is able to create http://big.site/
    popularpage.html.  However, the introduction of character
    normalization, and of additional mappings for user convenience, may
    increase the chance for spoofing.  Protocols and servers that allow
    the creation of resources with unnormalized names, and resources with
    names that are not normalized, are particularly vulnerable to such
    attacks.  This is an inherent security problem of the relevant
    protocol, server, or resource, and not specific to IRIs, but
    mentioned here for completeness.



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    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 [RFC3491].  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.

    Spoofing can occur because in the UCS, there are many characters that
    look very similar.  Details are discussed in Section 7.5.  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 with bidirectional IRIs, if the restrictions in
    Section 4.2 are not followed.  The same visual representation may be
    interpreted as different logical representations, and vice versa.  It
    is also very important that a correct Unicode bidirectional
    implementation is used.

9. Acknowledgements

    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 discussion on the issue addressed here has started a long time
    ago.  There was a thread in the HTML working group in August 1995
    (under the topic of "Globalizing URIs") and 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, Matti Allouche, Roy Fielding, Tim
    Berners-Lee, Mark Davis, M.T.  Carrasco Benitez, James Clark, Tim
    Bray, Chris Wendt, Yaron Goland, Andrea Vine, Misha Wolf, Leslie
    Daigle, Ted Hardie, Makoto MURATA, Steven Atkin, Ryan Stansifer, Tex
    Texin, Graham Klyne, Bjoern Hoehrmann, Chris Lilley, Ian Jacobs, Adam
    Costello, Dan Oscarson, Elliotte Rusty Harold, Mike J.  Brown, Andrea
    Vine, Roy Badami, Simon Josefsson, Carlos Viegas Damasio, and many
    others for help with understanding the issues and possible solutions,
    and getting the details right.  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



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    adopting the ideas, and to the members of the Montreal IAB Workshop
    on Internationalization and Localization for their review.

Normative References

    [ISO10646]  International Organization for Standardization,
                "Information Technology - Universal Multiple-Octet Coded
                Character Set (UCS) - Part 1: Architecture and Basic
                Multilingual Plane - Part 2: Supplementary Planes", ISO
                Standard 10646, with amendment, July 2002.

    [RFC2234]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
                Specifications: ABNF", RFC 2234, November 1997.

    [RFC3490]   Faltstrom, P., Hoffman, P. and A. Costello,
                "Internationalizing Domain Names in Applications (IDNA)",
                RFC 3490, March 2003, <http://www.ietf.org/rfc/
                rfc3490.txt>.

    [RFC3491]   Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
                Profile for Internationalized Domain Names (IDN)", RFC
                3491, March 2003.

    [RFC3629]   Yergeau, F., "UTF-8, a transformation format of ISO
                10646", STD 63, RFC 3629, November 2003, <http://
                www.ietf.org/rfc/rfc3629.txt>.

    [RFCYYYY]   Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
                Resource Identifier (URI): Generic Syntax", draft-
                fielding-uri-rfc2396bis-03.txt (work in progress), June
                2003.

    [UTR15]     Davis, M. and M. Duerst, "Unicode Normalization Forms",
                Unicode Standard Annex #15, March 2001, <http://
                www.unicode.org/unicode/reports/tr15/tr15-21.html>.

Non-normative References

    [BidiEx]        "Examples of bidirectional IRIs", <http://www.w3.org/
                    International/iri-edit/BidiExamples>.

    [CharMod]       Duerst, M., Yergeau, F., Ishida, R., Wolf, M. and T.
                    Texin, "Character Model for the World Wide Web",
                    World Wide Web Consortium Working Draft, August 2003,
                    <http://www.w3.org/TR/charmod>.

    [Duerst97]      Duerst, M., "The Properties and Promises of UTF-8",
                    Proc. 11th International Unicode Conference, San Jose



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                    , September 1997, <http://www.ifi.unizh.ch/mml/
                    mduerst/papers/PDF/IUC11-UTF-8.pdf>.

    [Duerst01]      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>.

    [Gettys]        Gettys, J., "URI Model Consequences", <http://
                    www.w3.org/DesignIssues/ModelConsequences>.

    [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>.

    [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.

    [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.

    [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



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                    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.

    [UNIV4]         The Unicode Consortium, "The Unicode Standard,
                    Version 4.0", Addison-Wesley, Reading, MA , 2003.

    [UNI9]          Davis, M., "The Bidirectional Algorithm", Unicode
                    Standard Annex #9, March 2002, <http://
                    www.unicode.org/unicode/reports/tr9>.

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

    [W3CIRI]        Duerst, M., "Internationalization - URIs and other
                    identifiers", World Wide Web Consortium Note,
                    September 2002, <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>.

    [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>.

    [XPointer]      Grosso, P., Maler, E., Marsh, J. and N. Walsh,
                    "XPointer Framework", World Wide Web Consortium
                    Recommendation, March 2003, <http://www.w3.org/TR/
                    xptr-framework/#escaping>.



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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.)
    World Wide Web Consortium
    200 Technology Square
    Cambridge, MA  02139
    U.S.A.

    Phone: +1 617 253 5509
    Fax:   +1 617 258 5999
    EMail: mailto: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 (2004).  All Rights Reserved.

    This document and translations of it may be copied and furnished to
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    or assist in its implementation may be prepared, copied, published
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Acknowledgement

    Funding for the RFC Editor function is currently provided by the
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