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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: March 28, 2005                                      M. Suignard
                                                    Microsoft Corporation
                                                       September 27, 2004


              Internationalized Resource Identifiers (IRIs)
                           draft-duerst-iri-10

Status of this Memo

    This document is an Internet-Draft and is subject to all provisions
    of section 3 of RFC 3667.  By submitting this Internet-Draft, each
    author represents that any applicable patent or other IPR claims of
    which he or she is aware have been or will be disclosed, and any of
    which he or she become aware will be disclosed, in accordance with
    RFC 3668.

    Internet-Drafts are working documents of the Internet Engineering
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    This Internet-Draft will expire on March 28, 2005.

Copyright Notice

    Copyright (C) The Internet Society (2004).

Abstract

    This document defines a new protocol element, the Internationalized
    Resource Identifier (IRI), as a complement to the Uniform Resource
    Identifier (URI).  An IRI is a sequence of characters from the
    Universal Character Set (Unicode/ISO 10646).  A mapping from IRIs to
    URIs is defined, which means that IRIs can be used instead of URIs
    where appropriate to identify resources.



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    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.
    Guidelines for the use and deployment of IRIs in various protocols,
    formats, and software components that now deal with URIs are
    provided.

Table of Contents

    1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
      1.1   Overview and Motivation  . . . . . . . . . . . . . . . . .  4
      1.2   Applicability  . . . . . . . . . . . . . . . . . . . . . .  5
      1.3   Definitions  . . . . . . . . . . . . . . . . . . . . . . .  5
      1.4   Notation . . . . . . . . . . . . . . . . . . . . . . . . .  6
    2.  IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  7
      2.1   Summary of IRI Syntax  . . . . . . . . . . . . . . . . . .  7
      2.2   ABNF for IRI References and IRIs . . . . . . . . . . . . .  8
    3.  Relationship between IRIs and URIs . . . . . . . . . . . . . . 11
      3.1   Mapping of IRIs to URIs  . . . . . . . . . . . . . . . . . 11
      3.2   Converting URIs to IRIs  . . . . . . . . . . . . . . . . . 14
        3.2.1   Examples . . . . . . . . . . . . . . . . . . . . . . . 16
    4.  Bidirectional IRIs for Right-to-left Languages . . . . . . . . 17
      4.1   Logical Storage and Visual Presentation  . . . . . . . . . 17
      4.2   Bidi IRI Structure . . . . . . . . . . . . . . . . . . . . 19
      4.3   Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . . 20
      4.4   Examples . . . . . . . . . . . . . . . . . . . . . . . . . 20
    5.  IRI Equivalence and Comparison . . . . . . . . . . . . . . . . 22
      5.1   Simple String Comparison . . . . . . . . . . . . . . . . . 22
      5.2   Conversion to URIs . . . . . . . . . . . . . . . . . . . . 23
      5.3   Normalization  . . . . . . . . . . . . . . . . . . . . . . 23
      5.4   Preferred Forms  . . . . . . . . . . . . . . . . . . . . . 24
    6.  Use of IRIs  . . . . . . . . . . . . . . . . . . . . . . . . . 25
      6.1   Limitations on UCS Characters Allowed in IRIs  . . . . . . 25
      6.2   Software Interfaces and Protocols  . . . . . . . . . . . . 25
      6.3   Format of URIs and IRIs in Documents and Protocols . . . . 26
      6.4   Use of UTF-8 for Encoding Original Characters  . . . . . . 26
      6.5   Relative IRI References  . . . . . . . . . . . . . . . . . 28
    7.  URI/IRI Processing Guidelines (informative)  . . . . . . . . . 28
      7.1   URI/IRI Software Interfaces  . . . . . . . . . . . . . . . 28
      7.2   URI/IRI Entry  . . . . . . . . . . . . . . . . . . . . . . 28
      7.3   URI/IRI Transfer Between Applications  . . . . . . . . . . 29
      7.4   URI/IRI Generation . . . . . . . . . . . . . . . . . . . . 30
      7.5   URI/IRI Selection  . . . . . . . . . . . . . . . . . . . . 30
      7.6   Display of URIs/IRIs . . . . . . . . . . . . . . . . . . . 31
      7.7   Interpretation of URIs and IRIs  . . . . . . . . . . . . . 31
      7.8   Upgrading Strategy . . . . . . . . . . . . . . . . . . . . 32
    8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 33
    9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 34



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    10.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 34
    11.   References . . . . . . . . . . . . . . . . . . . . . . . . . 35
    11.1  Normative References . . . . . . . . . . . . . . . . . . . . 35
    11.2  Non-normative References . . . . . . . . . . . . . . . . . . 36
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 38
    A.  Design Alternatives  . . . . . . . . . . . . . . . . . . . . . 39
      A.1   New Scheme(s)  . . . . . . . . . . . . . . . . . . . . . . 39
      A.2   Other Character Encodings than UTF-8 . . . . . . . . . . . 40
      A.3   New Encoding Convention  . . . . . . . . . . . . . . . . . 40
      A.4   Indicating Character Encodings in the URI/IRI  . . . . . . 40
        Intellectual Property and Copyright Statements . . . . . . . . 41








































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1.  Introduction

1.1  Overview and Motivation

    A Uniform Resource Identifier (URI) is defined in [RFCYYYY] as a
    sequence of characters chosen from a limited subset of the repertoire
    of US-ASCII [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
    Internationalized Resource Identifier (IRI), 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.  The syntax of IRIs is defined in
    Section 2, and the relationship between IRIs and URIs in Section 3.

    Using characters outside of A-Z in IRIs brings with it some
    difficulties.  Section 4 discusses the special case of bidirectional
    IRIs, Section 5 various forms of equivalence between IRIs, and
    Section 6 the use of IRIs in different situations.  Section 7 gives
    additional informative guidelines, and Section 8 security
    considerations.

    For discussion of this document, 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].




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1.2  Applicability

    IRIs are designed to be compatible with 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 where IRIs are used should be
       explicitly designated to be able 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 "anyURI" that includes IRIs and IRI references.
       Therefore, IRIs and IRI references can be in attributes and
       elements of type "anyURI".  On the other hand, in the HTTP
       protocol [RFC2616], the Request URI is defined as an URI, which
       means that direct use of IRIs is not allowed in HTTP requests.

    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)





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    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 ISO/
       IEC 10646 [ISO10646] and the Unicode Standard [UNIV4].

    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 IRI references
       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,...).

    protocol element: Any portion of a message which affects processing
       of that message by the protocol in question.

    presentation element: Presentation form corresponding to a protocol
       element, for example using a wider range of characters.

    create (an URI or IRI): With respect to URIs and IRIs, the word
       'create' is used for the initial creation.  This may be the
       initial creation of a resource with a certain identifier, or the
       initial exposition of a resource under a particular identifier.

    generate (an URI or IRI): With respect to URIs and IRIs, the word
       'generate' is used when the IRI is generated by derivation from
       other information.


1.4  Notation

    RFCs and Internet Drafts currently do not allow any characters
    outside the US-ASCII repertoire.  Therefore, this document uses



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    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 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 percent-encoded
    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).

    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



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    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 references, 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 repertoire 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 grammar closely follows the URI grammar in [RFCYYYY],
    except that the range of unreserved characters is expanded to include
    UCS characters, with the restriction that private UCS characters can
    occur only in query parts and not elsewhere.  The grammar is split
    into two parts, rules that differ from [RFCYYYY] because of the
    above-mentioned expansion, and rules that are the same as in
    [RFCYYYY].  For rules that are different than in [RFCYYYY], the names
    of the non-terminals have been changed as follows: If the
    non-terminal contains 'URI', this has been changed to 'IRI'.
    Otherwise, an 'i' has been prefixed.

    The following rules are different from [RFCYYYY]:

       IRI            = scheme ":" ihier-part [ "?" iquery ]
                     [ "#" ifragment ]

       ihier-part     = "//" iauthority ipath-abempty
                      / ipath-absolute
                      / ipath-rootless
                      / ipath-empty




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       IRI-reference  = IRI / irelative-ref

       absolute-IRI   = scheme ":" ihier-part [ "?" iquery ]

       irelative-ref  = irelative-part [ "?" iquery ] [ "#" ifragment ]

       irelative-part = "//" iauthority ipath-abempty
                   / ipath-absolute
                      / ipath-noscheme
                      / ipath-empty

       iauthority     = [ iuserinfo "@" ] ihost [ ":" port ]
       iuserinfo      = *( iunreserved / pct-encoded / sub-delims / ":" )
       ihost          = IP-literal / IPv4address / ireg-name

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

       ipath          = ipath-abempty   ; begins with "/" or is empty
                      / ipath-absolute  ; begins with "/" but not "//"
                      / ipath-noscheme  ; begins with a non-colon segment
                      / ipath-rootless  ; begins with a segment
                      / ipath-empty     ; zero characters

       ipath-abempty  = *( "/" isegment )
       ipath-absolute = "/" [ isegment-nz *( "/" isegment ) ]
       ipath-noscheme = isegment-nz-nc *( "/" isegment )
       ipath-rootless = isegment-nz *( "/" isegment )
       ipath-empty    = 0<ipchar>

       isegment       = *ipchar
       isegment-nz    = 1*ipchar
       isegment-nz-nc = 1*( iunreserved / pct-encoded / sub-delims
                            / "@" )
                      ; non-zero-length segment without any colon ":"

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

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

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

       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



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                      / %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD
                      / %xD0000-DFFFD / %xE1000-EFFFD

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

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

    The following are the same as in [RFCYYYY]:

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

       port           = *DIGIT

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

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

       IPv6address    =                            6( h16 ":" ) ls32
                      /                       "::" 5( h16 ":" ) ls32
                      / [               h16 ] "::" 4( h16 ":" ) ls32
                      / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
                      / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
                      / [ *3( h16 ":" ) h16 ] "::"    h16 ":"   ls32
                      / [ *4( h16 ":" ) h16 ] "::"              ls32
                      / [ *5( h16 ":" ) h16 ] "::"              h16
                      / [ *6( h16 ":" ) h16 ] "::"

       h16             = 1*4HEXDIG
       ls32           = ( h16 ":" h16 ) / 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     = "!" / "$" / "&" / "'" / "(" / ")"
                      / "*" / "+" / "," / ";" / "="



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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.  In this case, IRIs
    can serve as presentation elements for URI protocol elements.  An
    example would be an address bar in a Web user agent.  (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.
       Scheme-specific restrictions are applied to IRIs by converting
       IRIs to URIs and checking the URIs against the scheme-specific
       restrictions.

    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 character sequence 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 character encoding: Represent the IRI as a sequence of
          characters from the UCS normalized according to Normalization
          Form C (NFC, [UTR15]).



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       Variant B) If the IRI is in some digital representation (e.g.  an
          octet stream) in some known non-Unicode character 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 character encoding
          (for example UTF-8 or UTF-16): Do not normalize (see Section
          5.3 for details).  Apply Step 2 directly to the encoded Unicode
          character sequence.

    Step 2) For each character in 'ucschar' or 'iprivate', apply Steps
       2.1 through 2.3 below.

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

       2.2) Convert each octet to %HH, where HH is the hexadecimal
          notation of the octet value.  Note that this is identical to
          the percent-encoding mechanism in Section 2.1 of [RFCYYYY].  To
          reduce variability, the hexadecimal notation SHOULD use upper
          case letters.

       2.3) Replace the original character with 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 convert the ireg-name component of
    an IRI as follows (before Step 2 above) for schemes that are known to
    use domain names in ireg-name, but where the scheme definition does
    not allow percent-encoding for ireg-name: Replace the ireg-name part
    of the IRI by the part converted using the ToASCII operation
    specified in Section 4.1 of [RFC3490] on each dot-separated label,
    and using U+002E (FULL STOP) as a label separator, 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.  This conversion SHOULD be used when the goal is to
    maximize interoperability with legacy URI resolvers.  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.

    An IRI with a scheme that is known to use domain names in ireg-name,



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    but where the scheme definition does not allow percent-encoding for
    ireg-name, meets scheme-specific restrictions if either the
    straightforward conversion or the conversion using the ToASCII
    operation on ireg-name result in an URI that meets the
    scheme-specific restrictions.  Such an IRI resolves to the URI
    obtained after converting the IRI including using the ToASCII
    operation on ireg-name.  Implementations do not need to do this
    conversion as long as they produce the same result.

    Note: The difference between Variants B and C in Step 1 (Variant B
       using normalization with NFC while Variant C not using any
       normalization) is to account for the fact that in many non-Unicode
       character encodings, some text cannot be represented directly.
       For example, Vietnam is natively written "Vi&#x1EC7;t Nam"
       (containing a LATIN SMALL LETTER E WITH CIRCUMFLEX AND DOT BELOW)
       in NFC, but a direct transcoding from the windows-1258 character
       encoding leads to "Vi&#xEA;&#x323;t Nam" (containing a LATIN SMALL
       LETTER E WITH CIRCUMFLEX followed by a COMBINING DOT BELOW),
       whereas direct transcoding of other 8-bit encodings of Vietnamese
       may lead to other representations.

    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 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



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    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 percent-encoding 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 percent-encoded sequences are not
       encoded further.  (This mapping is similar to, but different from,
       the encoding 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


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
    percent-encoding and for potential percent-encoded unreserved
    characters).  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 percent-encodings, but not all
    percent-encodings can be eliminated.  There are several reasons for
    this:





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    a) Some percent-encodings are necessary to distinguish
       percent-encoded and unencoded uses of reserved characters.

    b) Some percent-encodings 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
       percent-encodings 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 2.2, Section 4.1, and 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 percent-encodings (% 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-percent-encode any octet produced in Step 2 that is not part of
       a strictly legal UTF-8 octet sequence.

    4) Re-percent-encode all octets produced in Step 3 that in UTF-8
       represent characters that are not appropriate according to Section
       2.2, Section 4.1, and Section 6.1.

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

    This procedure will convert as many percent-encoded 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 character
    encoding than UTF-8 in Steps 3 and 4 above, even if it might be
    possible from context to guess that another character 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



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    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.  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 character encoding.  (It might represent other characters
    in other character 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-percent-encoded in Step 3.

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

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

    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
    percent-encoded
    UTF-8 character 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-percent-encoded in Step 4.  This
    example shows that the case (upper or lower) of letters used in



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    percent-encodes 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;

    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



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    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-level 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 reference 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.

    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.





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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, ireg-name, isegment,
    isegment-nz, isegment-nz-nc, 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 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.

    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,



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       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
    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



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

    Example 9 (not allowed): The numbers in the previous example are
    percent-encoded:
    logical representation: http://ab.cd.ef/GH%31/%32IJ/KL.html,
    visual representation (Hebrew): http://ab.cd.ef/%31HG/LK/JI%32.html
    visual representation (Arabic): http://ab.cd.ef/31%HG/%LK/JI32.html



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    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.
    When comparing character-by-character, the comparison function MUST
    NOT map IRIs to URIs, because such a mapping would create additional
    spurious equivalences.

    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.  When an IRI is used as an identifier in
    scenarios that depend upon character-by-character equivalence,
    creators of IRIs should take additional care to avoid IRIs that only
    differ in their use of percent-escaping.  As an example, using both



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    http://example.org/~user and http://example.org/%7Euser to identify
    XML Namespaces is a bad idea.

5.2  Conversion to URIs

    For actual resolution, differences in percent-encoding (except for
    the percent-encoding 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 percent-encoding of
    both IRIs to be compared has to be aligned, for example by converting
    both IRIs to URIs (see Section 3.1), eliminating escape differences
    in the resulting URIs, and making sure that the case of the
    hexadecimal characters in the percent-encoding is always the same
    (preferably upper case).  If the IRI is to be passed to another
    application, or used further in some other way, its original form
    MUST be preserved;  the conversion described here should be performed
    only for the purpose of local comparison.

    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 character encoding to an
    UCS-based character 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,



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    http://www.example.org/re&#x301;sume&#x301;.html is not in NFC.  The
    former uses precombined e-acute characters, the latter uses 'e'
    characters followed by combining acute accents.  Both usages are
    defined to be canonically equivalent in [UNIV4].

    Note: Because it is unknown 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,
       its IRI should 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 creator uses a reasonable capitalization, and when
       transferring 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 created:

    -  Always provide the URI scheme in lowercase characters.

    -  Only perform percent-encoding where it is essential.

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

    -  For those schemes where ireg-name is a domain name, always provide
       the individual labels, in the form produced when applying nameprep
       [RFC3491].  This in particular includes using lowercase characters
       rather than uppercase characters where applicable.  Also, always
       use US-ASCII '.' as a separator.

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

    -  Prevent /./ and /../ from appearing in IRI paths.





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


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 beyond those given in Section 2.2 and
    Section 4.1.  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 Latin 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.





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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
    character encoding, and converted accordingly by a parser or
    interpreter.  IRI characters that are not expressible in the native
    character encoding SHOULD be escaped using the escaping conventions
    of the document format if such conventions are available.
    Alternatively, they MAY be percent-encoded 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 background information on encoding characters into
    URIs, see also Section 2.5 of [RFCYYYY].

    For new URI schemes, using UTF-8 is recommended in [RFC2718].
    Examples where UTF-8 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
    percent-encoded representation of that character).  On the other
    hand, for a document with a URI of
    http://www.example.org/r%E9sum%E9.html, the percent-encoding octets



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    cannot be converted to actual characters in an IRI, because the
    percent-encoding is not based on UTF-8.

    This means that for most URI schemes, there is no need to upgrade
    their scheme definition in order for them to work with IRIs.  The
    main case where upgrading a scheme definition makes sense is when a
    scheme definition, or a particular component of a scheme, is strictly
    limited to the use of US-ASCII characters with no provision to
    include  non-ASCII characters/octets via percent-encoding, or if a
    scheme definition currently uses highly scheme-specific provisions
    for the encoding of non-ASCII characters.  An example of such a
    scheme might be the mailto: scheme [RFC2368].

    This specification does not upgrade any scheme specifications in any
    way, this has to be done separately.  Also, it should be noted that
    there is no such thing as an "IRI scheme"; all IRIs use URI schemes,
    and all URI schemes can be used with IRIs, even though in some cases
    only by using URIs directly as IRIs, without any conversion.

    URI schemes can impose restrictions on the syntax of scheme-specific
    URIs, ie.  URIs that are admissable under the generic URI syntax
    [RFCYYYY] may not be admissable due to narrower syntactic constraints
    imposed by a URI scheme specification.  URI scheme definitions cannot
    broaden the syntactic restrictions of the generic URI syntax,
    otherwise it would be possible to generate URIs that satisfied the
    scheme specific syntactic constraints without satisfying the
    syntactic constraints of the generic URI syntax.  However, additional
    syntactic constraints imposed by URI scheme specifications are
    applicable to IRI since the corresponding URI resulting from the
    mapping defined in Section 3.1 MUST be a valid URI under the
    syntactic restrictions of generic URI syntax and any narrower
    restrictions imposed by the corresponding URI scheme specification.

    The requirement for the use of UTF-8 applies to all parts of a URI
    (with the potential exception of the ireg-name part, see Section
    3.1).  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 US-ASCII characters, or they may not be based on UTF-8.
    They may be based on another character 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;.



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    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 IRI references against a base is handled
    straightforwardly; the algorithms of [RFCYYYY] can be applied
    directly, treating the characters additionally allowed in IRI
    references in the same way as unreserved characters in URI
    references.

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 creates or 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 character 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 character 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
    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.




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    A person viewing a visual representation of an IRI (as a sequence of
    glyphs, in some order, in some visual display) or hearing an IRI,
    will use a entry method for characters in the user's language to
    input the IRI.  Depending on the script and the input method used,
    this may be a more or less complicated process.

    The process of IRI entry must assure, as far as possible, that the
    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 punctuation, 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.

    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 character encoding of the
    document or application where the IRI appears to the character
    encoding used by the system-wide IRI invocation mechanism, or to a
    URI (according to Section 3.1) if the system-wide invocation



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    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 a character 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
    confusing users with "br0ken" or "1ame" identifiers.

    Outside of the US-ASCII repertoire, 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



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    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 created 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 percent-encoded 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.

    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 character
    encodings to convert non-ASCII characters to URIs.  Whether this is
    necessary and what character encodings to cover, depends on a number



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    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 US-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, percent-encoded and
    not percent-encoded 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 created,
    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 using
    a local character 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.

    Software converting from URIs to IRIs for display should be upgraded



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    only after upgraded entry software has been widely deployed to the
    population that will see the displayed result.

    It is often possible to reduce the effort and dependencies for
    upgrading to IRIs by using UTF-8 rather than another character
    encoding where there is a free choice of character encodings.  For
    example, when setting up a new file-based Web server, using UTF-8 as
    the character encoding for file names will make the transition to
    IRIs easier.  Likewise, when setting up a new Web form using UTF-8 as
    the character encoding of the form page, the returned query URIs will
    use UTF-8 as the character encoding (unless the user, for whatever
    reason, changes the character encoding) and will therefore be
    compatible with IRIs.

    These recommendations, when taken together, will allow for the
    extension from URIs to IRIs in order to handle characters other than
    US-ASCII while minimizing interoperability problems.  For
    considerations regarding the upgrade of URI scheme definitions,
    please see Section 6.4.

8.  Security Considerations

    The security considerations discussed in [RFCYYYY] also apply to
    IRIs.  In addition, the following issues require particular care for
    IRIs.

    Incorrect encoding or decoding 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 US-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 character
    encoding, do not match the normalization used on the server side.



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    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.example.com/PopularPage.html)
    might be "spoofed" by someone who is able to create
    http://big.example.com/popularpage.html.  However, the use of
    unnormalized character sequences, and of additional mappings for user
    convenience, may increase the chance for spoofing.  Protocols and
    servers that allow the creation of 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.

    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 with percent-encodings based on various
    character 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 character encodings, produce mostly garbage.  In
    other cases, when concurrently used character 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.  IANA Considerations

    This document has no actions for IANA.

10.  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).




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    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.

    Many thanks go to Francois Yergeau, Matitiahu 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, Roy
    Badami, Jonathan Rosenne, Asmus Freytag, Simon Josefsson, Carlos
    Viegas Damasio, Chris Haynes, Walter Underwood, and many others for
    help with understanding the issues and possible solutions, and
    getting the details right.

    This document is a product of the Internationalization Working Group
    (I18N WG) of the World Wide Web Consortium (W3C).  Thanks to the
    members of the W3C I18N Working Group and Interest Group for their
    contributions and their work on [CharMod].  Thanks also go to the
    members of many other W3C Working Groups for adopting IRIs, and to
    the members of the Montreal IAB Workshop on Internationalization and
    Localization for their review.

11.  References

11.1  Normative References

    [ASCII]    American National Standards Institute, "Coded Character
               Set -- 7-bit American Standard Code for Information
               Interchange", ANSI X3.4, 1986.

    [ISO10646]
               International Organization for Standardization, "ISO/IEC
               10646:2003: Information Technology - Universal
               Multiple-Octet Coded Character Set (UCS)", ISO Standard
               10646, December 2003.

    [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

    [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)",



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               RFC 3490, March 2003.

    [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.

    [RFCYYYY]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
               Resource Identifier (URI): Generic Syntax (Note to the RFC
               Editor: Please update this reference with the RFC
               resulting from draft-fielding-uri-rfc2396bis-xx.txt, and
               remove this Note)", draft-fielding-uri-rfc2396bis-07.txt
               (work in progress), April 2004.

    [UNI9]     Davis, M., "The Bidirectional Algorithm", Unicode Standard
               Annex #9, March 2004,
               <http://www.unicode.org/reports/tr9/tr9-13.html>.

    [UNIV4]    The Unicode Consortium, "The Unicode Standard, Version
               4.0.1, defined by: The Unicode Standard, Version 4.0
               (Reading, MA, Addison-Wesley, 2003. ISBN 0-321-18578-1),
               as amended by Unicode 4.0.1
               (http://www.unicode.org/versions/Unicode4.0.1/)", March
               2004.

    [UTR15]    Davis, M. and M. Duerst, "Unicode Normalization Forms",
               Unicode Standard Annex #15, April 2003,
               <http://www.unicode.org/unicode/reports/tr15/
                tr15-23.html>.

11.2  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, February 2004,
               <http://www.w3.org/TR/charmod>.

    [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>.




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    [Duerst97]
               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>.

    [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>.

    [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.

    [RFC2368]  Hoffman, P., Masinter, L. and J. Zawinski, "The mailto URL
               scheme", RFC 2368, July 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 Transfer
               Protocol", RFC 2640, July 1999.

    [RFC2718]  Masinter, L., Alvestrand, H., Zigmond, D. and R. Petke,



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               "Guidelines for new URL Schemes", RFC 2718, November 1999.

    [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", 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., Maler, E. and
               F. Yergeau, "Extensible Markup Language (XML) 1.0 (Third
               Edition)", World Wide Web Consortium Recommendation,
               February 2004,
               <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-names>.

    [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
    5322 Endo
    Fujisawa, Kanagawa  252-8520
    Japan

    Phone: +81 466 49 1170
    Fax:   +81 466 49 1171
    EMail: mailto:duerst@w3.org
    URI:   http://www.w3.org/People/D%C3%BCrst/
           (Note: This is the percent-encoded 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

Appendix A.  Design Alternatives

    This section shortly summarizes major design alternatives and the
    reasons for why they were not chosen.

Appendix A.1  New Scheme(s)

    Introducing new schemes (for example httpi:, ftpi:,...) or a new
    metascheme (e.g.  i:, leading to URI/IRI prefixes such as i:http:,
    i:ftp:,...) was proposed to make IRI-to-URI conversion
    scheme-dependent or to distinguish between percent-encodings
    resulting from IRI-to-URI conversion and percent-encodings from
    legacy character encodings.

    New schemes are not needed to distinguish URIs from true IRIs (i.e.
    IRIs that contain non-ASCII characters).  The benefit of being able
    to detect the origin of percent-encodings is marginal, because UTF-8
    can be detected with very high reliability.  Deploying new schemes is
    extremely hard, so not requiring new schemes for IRIs makes
    deployment of IRIs vastly easier.  Making conversion scheme-dependent
    is highly inadvisable, and would be encouraged by separate schemes
    for IRIs.  Using an uniform convention for conversion from IRIs to



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    URIs makes IRI implementation orthogonal to the introduction of
    actual new schemes.

Appendix A.2  Other Character Encodings than UTF-8

    At an early stage, UTF-7 was considered as an alternative to UTF-8
    when converting IRIs to URIs.  UTF-7 would not have needed

    percent-encoding, and would in most cases have been shorter than
    percent-encoded UTF-8.

    Using UTF-8 avoids a double layering and overloading of the use of
    the "+" character.  UTF-8 is fully compatible with US-ASCII, and has
    therefore been recommended by the IETF, and is being used widely,
    while UTF-7 has never been used much and is now clearly being
    discouraged.  Requiring implementations to convert from UTF-8 to
    UTF-7 and back would be an additional implementation burden.

Appendix A.3  New Encoding Convention

    Instead of using the existing percent-encoding convention of URIs,
    which is based on octets, the idea was to create a new encoding
    convention, for example to use '%u' to introduce UCS code points.

    Using the existing octet-based percent-encoding mechanism does not
    need an upgrade of the URI syntax, and does not need corresponding
    server upgrades.

Appendix A.4  Indicating Character Encodings in the URI/IRI

    Some proposals suggested indicating the character encodings used in
    an URI or IRI with some new syntactic convention in the URI itself,
    similar to the 'charset' parameter for emails and Web pages.  As an
    example, the label in square brackets in
    http://www.example.org/ros[iso-8859-1]&#xE9; indicated that the
    following &#xE9; had to be interpreted as iso-8859-1.

    Using UTF-8 only does not need an upgrade to the URI syntax.  It
    avoids potentially multiple labels that have to be copied correctly
    in all cases, even on the side of a bus or on a napkin, leading to
    usability problems to the extent of being prohibitively annoying.
    Using UTF-8 only also reduces transcoding errors and confusions.










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