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

Network Working Group                                          M. Duerst
Internet-Draft                                                       W3C
Expires: August 31, 2003                                     M. Suignard
                                                    Microsoft Corporation
                                                            March 2, 2003


              Internationalized Resource Identifiers (IRIs)
                           draft-duerst-iri-03

Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.

    Internet-Drafts are working documents of the Internet Engineering
    Task Force (IETF), its areas, and its working groups.  Note that
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    Drafts.

    Internet-Drafts are draft documents valid for a maximum of six months
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    material or to cite them other than as "work in progress."

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    www.ietf.org/ietf/1id-abstracts.txt.

    The list of Internet-Draft Shadow Directories can be accessed at
    http://www.ietf.org/shadow.html.

    This Internet-Draft will expire on August 31, 2003.

Copyright Notice

    Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

    This document defines a new protocol element, the Internationalized
    Resource Identifier (IRI), as a complement to the URI [RFC2396].  An
    IRI is a sequence of characters from the Universal Character Set
    [ISO10646].  A mapping from IRIs to URIs is defined, which means that
    IRIs can be used instead of URIs where appropriate to identify
    resources.

    The approach of defining a new protocol element was chosen, instead
    of extending or changing the definition of URIs, to allow a clear
    distinction and to avoid incompatibilities with existing software.



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

NOTE

    This document is a product of the Internationalization Working Group
    (I18N WG) of the World Wide Web Consortium (W3C).  For general
    discussion, please use the www-international@w3.org mailing list
    (publicly archived at http://lists.w3.org/Archives/Public/www-
    international/).  For more information on the topic of this document,
    please also see [W3CIRI] and [Duerst01].

Table of Contents

    1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  4
    1.1   Overview and Motivation  . . . . . . . . . . . . . . . . . .  4
    1.2   Applicability  . . . . . . . . . . . . . . . . . . . . . . .  4
    1.3   Definitions  . . . . . . . . . . . . . . . . . . . . . . . .  5
    1.4   Notation . . . . . . . . . . . . . . . . . . . . . . . . . .  6
    2.    IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . .  7
    2.1   Summary of IRI Syntax  . . . . . . . . . . . . . . . . . . .  7
    2.2   ABNF for IRI References and IRIs . . . . . . . . . . . . . .  7
    2.3   IRI Equivalence and Normalization  . . . . . . . . . . . . . 10
    3.    Relationship between IRIs and URIs . . . . . . . . . . . . . 11
    3.1   Mapping of IRIs to URIs  . . . . . . . . . . . . . . . . . . 12
    3.2   Converting URIs to IRIs  . . . . . . . . . . . . . . . . . . 14
    3.2.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 15
    4.    Bidirectional IRIs for Right-to-left Languages . . . . . . . 16
    4.1   Logical Storage and Visual Presentation  . . . . . . . . . . 17
    4.2   Bidi IRI Structure . . . . . . . . . . . . . . . . . . . . . 17
    4.3   Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . . . 18
    4.4   Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 18
    5.    Use of IRIs  . . . . . . . . . . . . . . . . . . . . . . . . 20
    5.1   Limitations on UCS Characters Allowed in IRIs  . . . . . . . 20
    5.2   Software Interfaces and Protocols  . . . . . . . . . . . . . 21
    5.3   Format of URIs and IRIs in Documents and Protocols . . . . . 21
    5.4   Relative IRI References  . . . . . . . . . . . . . . . . . . 22
    6.    URI/IRI Processing Guidelines (informative)  . . . . . . . . 22
    6.1   URI/IRI Software Interfaces  . . . . . . . . . . . . . . . . 22
    6.2   URI/IRI Entry  . . . . . . . . . . . . . . . . . . . . . . . 23
    6.3   URI/IRI Transfer Between Applications  . . . . . . . . . . . 23
    6.4   URI/IRI Generation . . . . . . . . . . . . . . . . . . . . . 24
    6.5   URI/IRI Selection  . . . . . . . . . . . . . . . . . . . . . 24
    6.6   Display of URIs/IRIs . . . . . . . . . . . . . . . . . . . . 25
    6.7   Interpretation of URIs and IRIs  . . . . . . . . . . . . . . 25
    6.8   Upgrading Strategy . . . . . . . . . . . . . . . . . . . . . 26
    7.    Security Considerations  . . . . . . . . . . . . . . . . . . 27



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    8.    Issues List  . . . . . . . . . . . . . . . . . . . . . . . . 28
    9.    Change log . . . . . . . . . . . . . . . . . . . . . . . . . 28
    9.1   Changes from -02 to -03  . . . . . . . . . . . . . . . . . . 28
    9.2   Changes from -01 to -02  . . . . . . . . . . . . . . . . . . 29
    9.3   Changes from -00 to -01  . . . . . . . . . . . . . . . . . . 29
    10.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29
          Normative References . . . . . . . . . . . . . . . . . . . . 30
          Non-normative References . . . . . . . . . . . . . . . . . . 31
          Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 33
          Full Copyright Statement . . . . . . . . . . . . . . . . . . 34









































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1. Introduction

1.1 Overview and Motivation

    A URI is defined in [RFC2396] as a sequence of characters chosen from
    a limited subset of the repertoire of US-ASCII characters.

    The characters in URIs are frequently used for representing words of
    natural languages.  Such usage has many advantages: such URIs are
    easier to memorize, easier to interpret, easier to transcribe, easier
    to create, and easier to guess.  For most languages other than
    English, however, the natural script uses characters other than A-Z.
    For many people, handling Latin characters is as difficult as
    handling the characters of other scripts is for people who use only
    the Latin alphabet.  Many languages with non-Latin scripts do have
    transcriptions to Latin letters and such transcriptions are now often
    used in URIs, but they introduce additional ambiguities.

    The infrastructure for the appropriate handling of characters from
    local scripts is now widely deployed in local versions of operating
    system and application software.  Software that can handle a wide
    variety of scripts and languages at the same time is increasingly
    widespread.  Also, there are increasing numbers of protocols and
    formats that can carry a wide range of characters.

    This document defines a new protocol element, called IRI
    (Internationalized Resource Identifier), by extending the syntax of
    URIs to a much wider repertoire of characters.  It also defines
    "internationalized" versions corresponding to other constructs from
    [RFC2396], such as URI references.

    Using characters outside of A-Z in IRIs brings with it some
    difficulties; a discussion of potential problems and workarounds can
    be found in the later sections of this document.

1.2 Applicability

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

       a) The protocol or format element used should be explicitly
          designated to carry IRIs.  That is, the intent is not to
          introduce IRIs into contexts that are not defined to accept
          them.  For example, XML schema [XMLSchema] has an explicit type



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          "anyURI" that designates the use of IRIs.

       b) The protocol or format carrying the IRIs 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) Either by definition for all the URIs of a specific URI scheme,
          or a specific part of a URI (Reference), such as the fragment
          identifier, or at least for some specific URIs of a given
          scheme, the encoding of non-ASCII characters should be based on
          UTF-8.  For new URI schemes, this is recommended in [RFC2718].
          This allows IRIs to be used with the URN syntax [RFC2141] as
          well as recent URL scheme definitions based on UTF-8, such as
          IMAP URLs [RFC2192] and POP URLs [RFC2384].

    In cases and for pieces where an encoding other than UTF-8 is used,
    and for raw binary data encoded in URIs (see [RFC2397]), the octets
    have to be %-escaped.  In these situations, the ability of IRIs to
    directly represent a wide character repertoire cannot be used.

    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/résumé.html (é stands for the
    e-acute character, and is the UTF-8 encoded and escaped
    representation of that character).  On the other hand, for a document
    with an URI of http://www.example.org/r%E9sum%E9.html, the escaped
    octets cannot be converted to actual characters in an IRI, because
    the escaping is based on iso-8859-1 rather than UTF-8.

1.3 Definitions

    The following definitions are used in this document; they follow the
    terms in [RFC2130], [RFC2277] and [ISO10646]:

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

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

       character repertoire: A set of characters (in the mathematical
          sense)

       sequence of characters: A sequence (one after another) of
          characters



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       sequence of octets: A sequence (one after another) of octets

       (character) encoding: A method of representing a sequence of
          characters as a sequence of octets (maybe with variants).  A
          method of (unambiguously) converting a sequence of octets into
          a sequence of characters.

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

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

       UCS: Universal Character Set; the coded character set defined by
          [ISO10646] and [UNIV3].

       IRI reference: The term "IRI reference" denotes the common usage
          of an internationalized resource identifier.  An IRI reference
          may be absolute or relative, and may have additional
          information attached in the form of a fragement identifier.
          However, the "IRI" that results from such a reference only
          includes the absolute IRI after the fragment identifier (if
          any) is removed and after any relative IRI is resolved to its
          absolute form.


1.4 Notation

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

    In 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 '&amp'.

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



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2. IRI Syntax

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

    As with URIs, an IRI is defined as a sequence of characters, not as a
    sequence of octets.  This definition accommodates the fact that IRIs
    may be written on paper or read over the radio as well as being
    transmitted over the network.  The same IRI may be represented as
    different sequences of octets in different protocols or documents if
    these protocols or documents use different character encodings (and/
    or transfer encodings).  Using the same character encoding as the
    containing protocol or document assures that the characters in the
    IRI can be handled (searched, converted, displayed,...) in the same
    way as the rest of the protocol or document.

2.1 Summary of IRI Syntax

    IRIs are defined similarly to URIs in [RFC2396] (as modified by
    [RFC2732] and [IDNURI]), but the class of unreserved characters is
    extended by adding the characters of the UCS (Universal Character
    Set, [ISO10646]) beyond U+0080, subject to the limitations given in
    the syntax rules below and in Section 5.1.

    Otherwise, the syntax and use of components and reserved characters
    is the same as that in [RFC2396].  All the operations defined in
    [RFC2396], such as the resolution of relative URIs, can be applied to
    IRIs by IRI-processing software in exactly the same way as this is
    done to URIs by URI-processing software.

    Note: [RFC2396]: Uniform Resource Identifiers (URI): Generic Syntax"
    is being revised as [RFC2396bis].  The syntax used in this document
    includes bug fixes from [RFC2396bis].

    Characters outside the US-ASCII range MUST NOT be used for
    syntactical purposes such as to delimit components in newly defined
    schemes.  As an example, it is not allowed to use U+00A2, CENT SIGN,
    as a delimiter 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



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    described in [RFC2234].  Character numbers are taken from the UCS,
    without implying any actual binary encoding.  Terminals in the ABNF
    are characters, not bytes.

    The following rules are different from [RFC2396]:

        absolute-IRI-reference  = absolute-IRI [ "#" ifragment ]

        IRI-reference  = [ absolute-IRI / relative-IRI ]
                            [ "#" ifragment ]
        absolute-IRI   = scheme ":" ( ihier-part / iopaque-part )
        relative-IRI   = [ inet-path / iabs-path / irel-path ]
                            [ "?" iquery ]

        ihier-part     = [ inet-path / iabs-path ] [ "?" iquery ]
        iopaque-part   = iric-no-slash *iric

        iric-no-slash  = iunreserved / escaped / "[" / "]" / ";" / "?" /
                         ":" / "@" / "&" / "=" / "+" / "$" / ","

        inet-path      = "//" iauthority [ iabs-path ]
        iabs-path      = "/"  ipath-segments
        irel-path      = irel-segment [ iabs-path ]

        irel-segment   = 1*( iunreserved / escaped / ";" /
                            "@" / "&" / "=" / "+" / "$" / "," )

        iauthority     = iserver / ireg-name

        ireg-name      = 1*( iunreserved / escaped / ";" /
                            ":"  / "@" / "&" / "=" /  "+" / "$" / "," )

        iserver        = [ [ iuserinfo "@" ] ihostport ]
        iuserinfo      = *( iunreserved / escaped / ";" /
                           ":" / "&" / "=" / "+" / "$" / "," )

        ihostport      = ihost [ ":" port ]
        ihost          = IPv6reference / IPv4address / ihostname

        ihostname      = idomainlabel [ iqualified]
        iqualified     = *( "." idomainlabel ) [ "." itoplabel [ "." ] ]
        idomainlabel   = <<See following production rules>>
        itoplabel      = <<See following production rules>>

        ipath          = [ iabs-path / iopaque-part ]
        ipath-segments = isegment *( "/" isegment )
        isegment       = *ipchar




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        ipchar         = iunreserved / escaped / ";" /
                         ":" / "@" / "&" / "=" / "+" / "$" / ","
        iquery         = *( ipchar / iprivate / "/" / "?" )
        ifragment      = *( ipchar / "/" / "?" )

        iric           = reserved / iunreserved / escaped
        iunreserved    = unreserved / ucschar / iadditional
        iadditional    = "<" / ">" / DQUOTE / SP / "{" / "}" /
                          "|" / "\" / "^" / "`"
        ucschar        = %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF /
                         / %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD
                         / %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD
                         / %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD
                         / %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD
                         / %xD0000-DFFFD / %xE1000-EFFFD
        iprivate       = %xE000-F8FF / %xF0000-FFFFD / %x100000-10FFFD

    The 'idomainlabel' and 'itoplabel' production rules are as follows:
    The values 'idomainlabel' and 'itoplabel' are defined as a string of
    'ucschar' obeying the following rules:

       a) Given a string of 'ucschar' values, the ToASCII operation
          [RFCXXXX] is performed on that string with the flag
          UseSTD3ASCIIRules set to TRUE and the flag AllowUnassigned set
          to FALSE for creating IRIs and set to TRUE otherwise.

       b) ToASCII is successful and results in a string conforming to
          'domainlabel' for 'idomainlabel' and 'toplabel' for 'itoplabel'
          (see below for 'domainlabel' and 'toplabel').

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

    The following are the same as [RFC2396bis]:

        scheme        = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
        port          = *DIGIT
        domainlabel   = alphanum [ 0*61( alphanum | "-" ) alphanum ]
        toplabel      = alpha    [ 0*61( alphanum | "-" ) alphanum ]
        alphanum      = ALPHA / DIGIT

        IPv4address   = dec-octet 3( "." dec-octet )
        dec-octet     = DIGIT /                         ; 0-9
                        ( %x31-39 DIGIT ) /             ; 10-99
                        ( "1" 2*DIGIT ) /               ; 100-199
                        ( "2" %x30-34 DIGIT ) /         ; 200-249
                        ( "25" %x30-35 )                ; 250-255




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        IPv6reference = "[" IPv6address "]"
        IPv6address   = (                       7( h4 ":" )   h4   ) /
                        (                "::" 0*6( h4 ":" ) [ h4 ] ) /
                        ( h4             "::" 0*5( h4 ":" ) [ h4 ] ) /
                        ( h4    ":" h4   "::" 0*4( h4 ":" ) [ h4 ] ) /
                        ( h4 2( ":" h4 ) "::" 0*3( h4 ":" ) [ h4 ] ) /
                        ( h4 3( ":" h4 ) "::" 0*2( h4 ":" ) [ h4 ] ) /
                        ( h4 4( ":" h4 ) "::" 0*1( h4 ":" ) [ h4 ] ) /
                        (                       6( h4 ":" ) IPv4address )/
                        (                "::" 0*5( h4 ":" ) IPv4address )/
                        ( h4             "::" 0*4( h4 ":" ) IPv4address )/
                        ( h4    ":" h4   "::" 0*3( h4 ":" ) IPv4address )/
                        ( h4 2( ":" h4 ) "::" 0*2( h4 ":" ) IPv4address )/
                        ( h4 3( ":" h4 ) "::" 0*1( h4 ":" ) IPv4address )

        h4            = 1*4HEXDIG
        reserved      = "[" / "]" / ";" / "/" / "?" /
                              ":" / "@" / "&" / "=" / "+" / "$" / ","
        unreserved    = ALPHA / DIGIT / mark
        mark          = "-" / "_" / "." / "!" / "~" / "*" / "'" /
                        "(" / ")"

        escaped       = "%" HEXDIG HEXDIG


2.3 IRI Equivalence and Normalization

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

    In some scenarios 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 might be 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 would not be equivalent under this
    definition.  In such a case, the comparison function MUST NOT map the
    IRIs to URIs, because such a mapping would create something different
    under this equivalence relationship.

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



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    transported.

    For actual resolution, differences in escaping (except for the
    escaping of reserved characters) MUST always result in the same
    resource.  For example, http://example.org/~user,
    http://example.org/%7euser and http://example.org/%7Euser must
    resolve to the same resource.  If this kind of equivalence is to be
    tested, the escaping of both IRIs to be compared has to be aligned,
    for example by converting both IRIs to URIs (see Section 3.1) and
    making sure that the case of the hexadecimal characters in the %-
    escape is always the same.  Such conversions MUST only be done on the
    fly, without changing the original IRI.

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

    The Unicode Standard [UNIV3] defines various equivalences between
    sequences of characters for various purposes.  Unicode Standard Annex
    #15 [UTR15] defines various Normalization Forms for these
    equivalences.  IRIs SHOULD be created using Normalization Form C
    (NFC).  Equivalence of IRIs MUST rely on the assumtion that IRIs are
    appropriately pre-normalized, rather than applying normalization when
    comparing two IRIs, except when converting from a non-UCS-based
    encoding to an UCS-based encoding, where a normalizing transcoder
    using NFC MUST be used for interoperability.

    As an example, http://www.example.org/r&#xe9;sum&#xe9;.html (in XML
    Notation) is in NFC.  On the other hand, http://www.example.org/
    re&#x301;sume&#x301;.html is not in NFC.  The former uses precombined
    e-acute characters, the later uses 'e' characters followed by
    combining acute accents, both are defined as canonically equivalent
    in [UNIV3].

    Various IRI schemes may allow the usage of International Domain Names
    (IDN) [RFCXXXX].  When in use in IRIs, those names SHOULD be
    validated using the ToASCII operation defined in [RFCXXXX], 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).  However, this conversion may be applied
    when mapping an IRI into an URI, see Section 3.1.

3. Relationship between IRIs and URIs

    IRIs are meant to replace URIs in identifying resources for
    protocols, formats and software components which use a UCS-based



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    character repertoire.  These protocols and components may never need
    to use URIs directly, especially when the resource identifier is used
    simply for identification purposes.  However, when the resource
    identifier is used for resource retrieval, it is in many cases
    necessary to determine the associated URI because most retrieval
    mechanisms currently only are defined for URIs.  (Additional
    rationale is given in Section 3.1.)

3.1 Mapping of IRIs to URIs

    This section defines how to map an IRI to a URI.  Everything in this
    section applies also to IRI references and URI references, as well as
    components thereof (for example fragment identifiers).

    This mapping has two purposes:

       a) Syntactical:  Many URI schemes and components define additional
          syntactical restrictions not captured in Section 2.2.  Such
          restrictions can be applied to IRIs by noting that IRIs are
          only valid if they map to syntactically valid URIs.  This means
          that such syntactical restrictions do not have to be defined
          again on the IRI level.

       b) Interpretational:  URIs identify resources in various ways.
          IRIs also identify resources.  When the IRI is used simply for
          identification purposes, it is not necessary to map the IRI to
          an URI (see Section 2.3).  However, when an IRI is used for
          resource retrieval, the resource that the IRI locates is the
          same as the one located by the URI obtained after converting
          the IRI according to the procedure defined here.  This means
          that there is no need to define resolution separately on the
          IRI level.

    Applications MUST map IRIs to URIs using the following two steps.

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

             Variant A) If the IRI is written on paper or read out loud,
                or otherwise represented as a sequence of characters
                independent of any encoding: Represent the IRI as a
                sequence of characters from the UCS normalized according
                to Normalization Form C (NFC, [UTR15]).

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



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                normalized according to NFC.

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

       Step 2) For each character that is disallowed in URI references,
          apply steps 1) through 3) below.  The disallowed characters
          consist of all non-ASCII characters, plus the excluded
          characters listed in Section 2.4 of [RFC2396], except for the
          number sign (#) and percent sign (%) and the square bracket
          characters re-allowed in [RFC2732].

             1) Convert the character to a sequence of one or more octets
                using UTF-8 [RFC2279].

             2) Convert each octet to %HH, where HH is the hexadecimal
                notation of the octet value.  Note: This is identical to
                the escaping mechanism in Section 2.4.1 of [RFC2396].
                Note: To reduce variability, the hexadecimal notation
                SHOULD use upper case letters.

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

    Note that in this process (in step 2.3), characters allowed in URI
    references and existing escape sequences are not escaped further.
    (This mapping is similar to, but different from, the escaping applied
    when including arbitrary content into some part of a URI.) 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#%3Cred%3E, not to something
    like
    http%3A%2F%2Fwww.example.org%2Fred%2509ros%C3%A9%23red.

    Note that 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/
    (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

    The above mapping produces a URI fully conforming to [RFC2396] (as
    amended by [RFC2732] and [IDNURI]) out of each IRI.  The mapping is
    also an identity transformation for URIs and is idempotent --



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    applying the mapping a second time will not change anything.  Every
    URI is therefore by definition an IRI.

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

3.2 Converting URIs to IRIs

    In some situations, it may be desirable to try to convert a URI into
    an equivalent IRI.  This section gives a procedure to do such a
    conversion.  The conversion described in this section will always
    result in an IRI which maps back to the URI that was used as an input
    for the conversion (except for potential case differences in escape
    sequences).  However, the IRI resulting from this conversion may not
    be exactly the same as the original IRI (if there ever was one).

    URI to IRI conversion removes escape sequences, but not all escaping
    can be eliminated.  There are several reasons for this:

       a) Some escape sequences are necessary to distinguish escaped and
          unescaped uses of reserved characters.

       b) Some escape sequences cannot be interpreted as sequences of
          UTF-8 octets.

          (Note: Due to the regularities in the octet patterns of UTF-8,
          there is a very high probability, but no guarantee, that escape
          sequences that can be interpreted as sequences of UTF-8 octets
          actually originated from UTF-8.  For a detailed discussion, see
          [Duerst97].)

       c) The conversion may result in a character that is not
          appropriate in an IRI.  See Section 5.1 for further details.

    Conversion from a URI to an IRI is done using the following steps (or
    any other algorithm that produces the same result):

       1) Represent the URI as a sequence of octets in US-ASCII.

       2) Convert all hexadecimal escapes (% followed by two hexadecimal
          digits) except those corresponding to '#' and '%' and
          characters in 'reserved', to the corresponding octets.

       3) Re-escape any octet produced in step 2) that is not part of a



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          strictly legal UTF-8 octet sequence.

       4) Re-escape all octets produced in step 2) that in UTF-8
          represent characters that are not appropriate according to
          Section 4.1 and Section 5.1.

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

    This procedure will convert as many escaped non-ASCII characters as
    possible to characters in an IRI.  Because there are some choices
    when applying step 4) (see Section 5.1), results may differ.

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

3.2.1 Examples

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

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

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

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

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

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

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

    The following example contains the sequence '%FC', which might
    represent U+00FC LATIN SMALL LETTER U WITH DIAERESIS in the iso-8859-



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    1 encoding.  (It might represent other characters in other encodings.
    For example, the octet <FC> in iso-8859-5 represents U+045C CYRILLIC
    SMALL LETTER KJE.) Because <FC> is not part of a strictly legal UTF-8
    sequence, it is re-escaped in step 2).

       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 escaped UTF-
    8 encoding of U+202E, RIGHT-TO-LEFT OVERRIDE.  Section 4.1 forbids
    the direct use of this character in an IRI.  Therefore, the
    corresponding octets are re-escaped in step 3).  This example shows
    that the case (upper or lower) of letters used in escapes may not be
    preserved.

       1) http://www.example.org/%e2%80%ae

       2) http://www.example.org/<E2><80><AE>

       3) http://www.example.org/<E2><80><AE>

       4) http://www.example.org/%E2%80%AE

       5) http://www.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 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



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    characters in various parts of the IRI; (3) no or not too big changes
    or restrictions for implementations.

4.1 Logical Storage and Visual Presentation

    In their internal digital representation, i.e.  stored or transmitted
    for resolution, bidirectional IRIs MUST be in full logical order, and
    MUST conform directly 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 [UNIV3], [UNI9].  Bidirectional IRIs MUST be
    rendered with an overall left-to-right direction.

    In text with a left-to-right base directionality or embedding (e.g
    English, Cyrillic), the Unicode Bidirectional Algorithm will
    automatically use an overall left-to-right direction for the IRI.  In
    text with a right-to-left base directionality or embedding (e.g.
    Arabic or Hebrew), some kind of embedding is needed.  This may be
    Unicode bidi formatting codes (LRE before the IRI, and PDF after the
    IRI, both not part of the IRI itself) or equivalent features of a
    higher-order protocol (e.g.  the dir='ltr' attribute in HTML).

    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 itself appear visually.  It would therefore not
    be possible to again correctly input an IRI with such characters.

4.2 Bidi IRI Structure

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

    The following syntax rules from Section 2.2 correspond to components
    for the purpose of Bidi behavior: iopaquepart, irelsegment, iregname,
    iuserinfo, isegment, iparam, ihostname, 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
    [RFCXXXX] on bidirectional domain names correspond to treating each
    label of the domain name as a component.  Even where the components



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    are not defined formally, it may be helpful to think about some
    syntax in terms of components and to apply the relevant restrictions.
    For example, for the usual name/value syntax in query parts, it is
    convenient to treat each name and each value as a component.  As
    another example, the extensions in a resource name can be treated as
    separate components.

    For each component, the following restrictions apply:

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

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

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

    In some components, the above restrictions may actually be strictly
    enforced.  For example, [RFCXXXX] requires that these restrictions
    apply to the labels of the host name part of an IRI.  In some other
    components, for example path components, following these restrictions
    may not be too difficult.  For other components, such as parts of the
    query part, it may be very difficult to enforce the restrictions,
    because the values of query parameters may be arbitrary character
    sequences.

    In order to satisfy the above restrictions, the affected component
    can 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



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    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.

    Example 1: A single component with right-to-left (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.
    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.

    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,



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    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
    escaped:
    logical representation: http://ab.cd.ef/GH%31/%32IJ/KL.html,
    visual representation (Hebrew): http://ab.cd.ef/LK/JI%32/%31HG.html,
    visual representation (Arabic): http://ab.cd.ef/LK/JI32%/31%HG.html.
    Depending on whether the upper-case letters represent Arabic or
    Hebrew, the visual representation is different.

5. Use of IRIs

5.1 Limitations on UCS Characters Allowed in IRIs

    This section discusses the limitations on characters and character
    sequences usable for IRIs.  The considerations in this section are
    relevant when creating IRIs and when converting from URIs to IRIs.

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

          (Note: In accordance with URI practice, generic IRI software
          cannot and should not check for such limitations.)

       b) In the URI syntax, characters that are likely to be used to
          delimit URIs in text and print ("space", "delims", and
          "unwise") were excluded.  They are included in the IRI syntax
          (with the exception of '%', which cannot be used directly, and
          '#', which is used in IRI references), for the following
          reasons:

             1) The syntax includes many other characters that are not
                appropriate in many cases.




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             2) Some implementation practice already allows them in URI
                references (for example spaces in fragment identifiers).

             3) It is very convenient in some cases, for example for
                XPointers in XML attributes.

             4) Considering context is already necessary in the case of
                URIs, for example for "&amp;" in XML.

          However, these characters should be avoided where possible.
          Whenever there is a chance that an IRI will be used in a
          component where these characters can be harmful, they should be
          escaped from the start.

       c) The UCS contains many areas of characters for which there are
          strong visual look-alikes.  Because of the likelihood of
          transcription errors, these also should be avoided.  This
          includes the full-width equivalents of ASCII characters, half-
          width Katakana characters for Japanese, and many others.  This
          also includes many look-alikes of "space", "delims", and
          "unwise", characters excluded in [RFC2396].

    Additional information is available from [UNIXML].  [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 and code points not appropriate for IRIs.

5.2 Software Interfaces and Protocols

    Although an IRI is defined as a sequence of characters, software
    interfaces for URIs typically function on sequences of octets 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 as per Section 3.1, when
    transferring from IRI-capable to URI-only components.  Such a mapping
    SHOULD be applied as late as possible.  It should not be applied
    between components that are known to be able to handle IRIs.

5.3 Format of URIs and IRIs in Documents and Protocols

    Document formats that transport URIs may need to be upgraded to allow
    the transport of IRIs.  In those cases where the document as a whole
    has a native character encoding, IRIs MUST also be encoded in this
    encoding, and converted accordingly by a parser or interpreter.  IRI
    characters that are not expressible in the native encoding SHOULD be
    escaped using the escaping conventions of the document format if such



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    conventions are available.  Alternatively, they MAY be escaped
    according to Section 3.1.  For example, in HTML, XML, or SGML,
    numeric character references should be used.  If a document as a
    whole has a native character encoding, and that character encoding is
    not UTF-8, then IRIs MUST NOT be placed into the document in the UTF-
    8 character encoding.

    Note: Some formats already accommodate IRIs, although they use
    different terminology.  HTML 4.0 [HTML4] defines the conversion from
    IRIs to URIs as error-avoiding behavior.  XML 1.0 [XML1], XLink
    [XLink], and XML Schema [XMLSchema] and specifications based upon
    them allow IRIs.  Also, it is expected that all relevant new  W3C
    formats and protocols will be required to handle IRIs [CharMod].

5.4 Relative IRI References

    Processing of relative forms of IRIs against a base is handled
    straightforwardly; the algorithms of RFC 2396 may be applied
    directly, treating the characters additionally allowed in IRIs in the
    same way as unreserved characters in URIs.

6. URI/IRI Processing Guidelines (informative)

    This informative section provides guidelines for supporting IRIs in
    the same software components and operations that currently process
    URIs: software interfaces that handle URIs, software that allows
    users to enter URIs, software that generates URIs, software that
    displays URIs, formats and protocols that transport URIs, and
    software that interprets URIs.  These may all require more or less
    modification before functioning properly with IRIs.  The
    considerations in this section also apply to URI references and IRI
    references.

6.1 URI/IRI Software Interfaces

    Software interfaces that handle URIs, such as URI-handling APIs and
    protocols transferring URIs, need interfaces and protocol elements
    that are designed to carry IRIs.

    In case the current handling in an API or protocol is based on US-
    ASCII, UTF-8 is recommended as the encoding for IRIs, because this is
    compatible with US-ASCII, is in accordance with the recommendations
    of [RFC2277], and makes it easy to convert to URIs where necessary.
    In any case, the encoding used must not be left undefined.

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



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    conversion when there is a chance that this cannot be done correctly.

6.2 URI/IRI Entry

    There are components that allow users to enter URIs into the system,
    for example, by typing or dictation.  This software must be updated
    to allow for IRI entry.

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

    The process of IRI entry must assure, as far as possible, that the
    restrictions defined in Section 2.2 are met.  This may be done by
    choosing appropriate input methods or variants/settings thereof, by
    appropriately converting the characters being input, by eliminating
    characters that cannot be converted, and/or by issuing a warning or
    error message to the user.

    As an example of variant settings, input method editors for East
    Asian Languages usually allow to input Latin letters and related
    characters in full-width or half-width versions.  For IRI input, the
    input method editor should be set to half-width input, in order to
    produce US-ASCII characters where possible.

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

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

6.3 URI/IRI Transfer Between Applications

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




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

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

6.4 URI/IRI Generation

    Systems that are offering resources through the Internet, where those
    resources have logical names, sometimes automatically generate URIs
    for the resources they offer.  For example, some HTTP servers can
    generate a directory listing for a file directory, and then respond
    to the generated URIs with the files.

    Many legacy character encodings are in use in various file systems.
    Many currently deployed systems do not transform the local character
    representation of the underlying system before generating URIs.

    For maximum interoperability, systems that generate resource
    identifiers should do the appropriate transformations.  They should
    use IRIs converted to URIs in cases where it cannot be expected that
    the recipient is able to handle IRIs.  Due to the way most user
    agents currently work, native IRIs, encoded in UTF-8, may be used if
    the recipient announces that it can interpret UTF-8.  This requires
    that the whole page is sent as UTF-8.  If this is not possible,
    escaping can always be used.

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

6.5 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"



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    is easily confused with the digit one "1", and the upper-case oh "O"
    is easily confused with the digit zero "0".  Publishers should avoid
    confusing users with "br0ken" or "1ame" identifiers.

    Outside of the US-ASCII range, there are many more opportunities for
    confusion; a complete set of guidelines is too lengthy to include
    here.  As long as names are limited to characters from a single
    script, native writers of a given script or language will know best
    when ambiguities can appear, and how they can be avoided.  What may
    look ambiguous to a stranger may be completely obvious to the average
    native user.  On the other hand, in some cases, the UCS contains
    variants for compatibility reasons, for example for typographic
    purposes.  These should be avoided wherever possible.  Although there
    may be exceptions, in general newly created resource names should be
    in NFKC [UTR15] (which means that they are also in NFC).

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

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

6.6 Display of URIs/IRIs

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

    For display of Bidi IRIs, please see Section 4.1.

6.7 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.




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    First, multiple representations include both IRIs in the native
    character encoding of the protocol and also their URI counterparts.

    Second, it may include URIs constructed based on other character
    encodings than UTF-8.  Such URIs may be produced by user agents that
    do not conform to this specification and use legacy encodings to
    convert non-ASCII characters to URIs.  Whether this is necessary, and
    what character encodings to cover, depends on a number of factors,
    such as the legacy character encodings used locally and the
    distribution of various versions of user agents.  For example,
    software for Japanese may accept URIs in Shift_JIS and/or EUC-JP in
    addition to UTF-8.

    Third, it may include additional mappings to be more user-friendly
    and robust against transmission errors.  These would be similar to
    how currently some servers treat URIs as case-insensitive, or perform
    additional matching to account for spelling errors.  For characters
    beyond the ASCII repertoire, this may for example include ignoring
    the accents on received IRIs or resource names where appropriate.
    Please note that such mappings, including case mappings, are
    language-dependent.

    It can be difficult to unambiguously identify a resource if too many
    mappings are taken into consideration.  However, escaped and non-
    escaped parts of IRIs can always clearly be distinguished.  Also, the
    regularity of UTF-8 (see [Duerst97]) makes the potential for
    collisions lower than it may seem at first sight.

6.8 Upgrading Strategy

    Where this recommendation places further constraints on software for
    which many instances are already deployed, it is important to
    introduce upgrades carefully, and to be aware of the various
    interdependencies.

    If IRIs cannot be interpreted correctly, they should not be generated
    or transported.  This suggests that upgrading URI interpreting
    software to accept IRIs should have highest priority.

    On the other hand, a single IRI is interpreted only by a single or
    very few interpreters that are known in advance, while it may be
    entered and transported very widely.

    Therefore, IRIs benefit most from a broad upgrade of software to be
    able to enter and transport IRIs, but before publishing any
    individual IRI, care should be taken to upgrade the corresponding
    interpreting software in order to cover the forms expected to be
    received by various versions of entry and transport software.



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    The upgrade of generating software to generate IRIs instead of a
    local encoding should happen only after the service is upgraded to
    accept IRIs.  Similarly, IRIs should only be generated when the
    service accepts IRIs and the intervening infrastructure and protocol
    is known to transport them safely.

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

    These recommendations, when taken together, will allow for the
    extension from URIs to IRIs in order to handle scripts other than
    ASCII while minimizing interoperability problems.

7. Security Considerations

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

    There are various ways in which "spoofing" can occur with IRIs.
    "Spoofing" means that somebody may add a resource name that looks the
    same or similar to the user, but points to a different resource.  The
    added resource may pretend to be the real resource by looking very
    similar, but may contain all kinds of changes that may be difficult
    to spot but can cause all kinds of problems.  Most spoofing
    possibilities for IRIs are extensions of those for URIs.

    Spoofing can occur for various reasons.  A first reason is that
    normalization expectations of a user or actual normalization when
    entering an IRI do not match the normalization used on the server
    side.  Conceptually, this is no different from the problems
    surrounding the use of case-insensitive web servers.  For example, a
    popular web page with a mixed case name (http://big.site/
    PopularPage.html) might be "spoofed" by someone who is able to create
    http://big.site/popularpage.html.  However, the introduction of
    character normalization, and of additional mappings for user
    convenience, may increase the chance for spoofing.

    Spoofing can occur because in the UCS, there are many characters that
    look very similar.  Details are discussed in Section 6.5.  Again,
    this is very similar to spoofing possibilities on US-ASCII, e.g.



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    using 'br0ken' or '1ame' URIs.

    Spoofing can occur when URIs in various encodings are accepted to
    deal with older user agents.  In some cases, in particular for Latin-
    based resource names, this is usually easy to detect because UTF-8-
    encoded names, when interpreted and viewed as legacy encodings,
    produce mostly garbage.  In other cases, when concurrently used
    encodings have a similar structure, but there are no characters that
    have exactly the same encoding, detection is more difficult.

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

    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.

8. Issues List

       -  Should characters in iadditional be allowed? Under what
          conditions?.

       -  Allign the description in Section 2.3 with the results of W3C
          TAG discussions on issue URIEquivalence.

       -  Adapt depending on how [IDNURI] is integrated into
          [RFC2396bis].


9. Change log

9.1 Changes from -02 to -03

       -  Added an issues list.

       -  Added a paragraph prohibiting conversions from URIs to IRIs not
          based on UTF-8 to Section 3.2.

       -  Introduced iadditional to combine unwise, delims, and space.

       -  Tweaked description and added examples for URI-to-IRI
          conversion.




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       -  Improved syntax rules for hostname part.

       -  Improved description of equivalences in Section 2.3.

       -  Improved description of URI-to-IRI-mapping in Section 3.2.

       -  Changed preferred case when hex-escaping from lower to UPPER.

       -  Fixed various details.


9.2 Changes from -01 to -02

       -  New approach for Bidi section, many examples.

       -  Created idelims, removed '%' and '#'.  Changed userinfo to
          iuserinfo in iserver.

       -  Changed to ABNF defined by [RFC2234].

       -  Included bug fixes from [RFC2396bis].

       -  Additions to Acknowledgements.


9.3 Changes from -00 to -01

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

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

       -  Moved some text around, marked some as informational.

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

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


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).

    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



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    and URLs"), and ad-hoc meetings at the Unicode conferences in
    September 1995 and September 1997.

    Thanks to Francois Yergeau, Matti Allouche, Roy Fielding, Tim
    Berners-Lee, Mark Davis, M.T.  Carrasco Benitez, James Clark, Tim
    Bray, Chris Wendt, Yaron Goland, Andrea Vine, Misha Wolf, Leslie
    Daigle, Ted Hardie, Makoto MURATA, Steven Atkin, Ryan Stansifer, Tex
    Texin, Graham Klyne, Bjoern Hoehrmann, Chris Lilley, Dan Oscarson,
    Elliotte Rusty Harold, Mike J.  Brown, Carlos Viegas Damasio, and
    many others for help with understanding the issues and possible
    solutions, and getting the details right.  Thanks also to the members
    of the W3C I18N Working Group and Interest Group for their
    contributions and their work on [CharMod], to the members of many
    other W3C WGs for adopting the ideas, and to the members of the
    Montreal IAB Workshop on Internationalization and Localization for
    their review.

Normative References

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

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

    [RFC2279]   Yergeau, F., "UTF-8, a transformation format of ISO
                10646", RFC 2279, January 1998.

    [RFC2396]   Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
                Resource Identifiers (URI): Generic Syntax", RFC 2396,
                August 1998.

    [RFC2732]   Hinden, R., Carpenter, B. and L. Masinter, "Format for
                Literal IPv6 Addresses in URL's", RFC 2732, December
                1999.

    [RFCXXXX]   Faltstrom, P., Hoffman, P. and A. Costello,
                "Internationalizing Domain Names in Applications (IDNA)",
                draft-ietf-idn-idna-14.txt (work in progress), October
                2002, <http://www.ietf.org/internet-drafts/draft-ietf-
                idn-idna-14.txt>.

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



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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.,
                    Freytag, A. and T. Texin, "Character Model for the
                    World Wide Web", World Wide Web Consortium Working
                    Draft, April 2002, <http://www.w3.org/TR/charmod>.

    [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>.

    [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>.

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

    [IDNURI]        Duerst, M., "Internationalized Domain Names in URIs",
                    draft-ietf-idn-uri-03.txt (work in progress),
                    November 2002, <http://www.ietf.org/internet-drafts/
                    draft-ietf-idn-uri-03.txt>.

    [Nameprep]      Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
                    Profile for Internationalized Domain Names", draft-
                    ietf-idn-nameprep-11.txt (work in progress), June
                    2002, <http://www.ietf.org/internet-drafts/draft-
                    ietf-idn-nameprep-11.txt>.

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

    [RFC2130]       Weider, C., Preston, C., Simonsen, K., Alvestrand,
                    H., Atkinson, R., Crispin, M. and P. Svanberg, "The
                    Report of the IAB Character Set Workshop held 29
                    February - 1 March, 1996", RFC 2130, April 1997.

    [RFC2141]       Moats, R., "URN Syntax", RFC 2141, May 1997.

    [RFC2192]       Newman, C., "IMAP URL Scheme", RFC 2192, September



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                    1997.

    [RFC2277]       Alvestrand, H., "IETF Policy on Character Sets and
                    Languages", BCP 18, RFC 2277, January 1998.

    [RFC2384]       Gellens, R., "POP URL Scheme", RFC 2384, August 1998.

    [RFC2396bis]    Berners-Lee, T., Fielding, R. and L. Masinter,
                    "Uniform Resource Identifier (URI): Generic Syntax",
                    Internet-Draft (work in progress), October 2002.

    [RFC2397]       Masinter, L., "The "data" URL scheme", RFC 2397,
                    August 1998.

    [RFC2616]       Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
                    Masinter, L., Leach, P. and T. Berners-Lee,
                    "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
                    June 1999.

    [RFC2640]       Curtin, B., "Internationalization of the File
                    Transfer Protocol", RFC 2640, July 1999.

    [RFC2718]       Masinter, L., Alvestrand, H., Zigmond, D. and R.
                    Petke, "Guidelines for new URL Schemes", RFC 2718,
                    November 1999.

    [UNIV3]         The Unicode Consortium, "The Unicode Standard Version
                    3.0", Addison-Wesley, Reading, MA , 2000.

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

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

    [W3CIRI]        Duerst, M., "Internationalization - URIs and other
                    identifiers", World Wide Web Consortium Note,
                    September 2002, <http://www.w3.org/International/O-
                    URL-and-ident.html>.

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




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    [XML1]          Bray, T., Paoli, J., Sperberg-McQueen, C. and E.
                    Maler, "Extensible Markup Language (XML) 1.0 (Second
                    Edition)", World Wide Web Consortium Recommendation,
                    including Erratum 26 at http://www.w3.org/XML/xml-
                    V10-2e-errata#E26, October 2000, <http://www.w3.org/
                    TR/REC-xml#sec-external-ent>.

    [XMLNamespace]  Bray, T., Hollander, D. and A. Layman, "Namespaces in
                    XML", World Wide Web Consortium Recommendation,
                    January 1999, <http://www.w3.org/TR/REC-xml#sec-
                    external-ent>.

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


Authors' Addresses

    Martin Duerst (Note: Please write "Duerst" with u-umlaut wherever
                   possible, for example as "D&#252;rst in XML and HTML.)
    World Wide Web Consortium
    200 Technology Square
    Cambridge, MA  02139
    U.S.A.

    Phone: +1 617 253 5509
    Fax:   +1 617 258 5999
    EMail: duerst@w3.org
    URI:   http://www.w3.org/People/D%C3%BCrst/
           (Note: This is the escaped form of an IRI.)


    Michel Suignard
    Microsoft Corporation

    One Microsoft Way
    Redmond, WA  98052
    U.S.A.

    Phone: +1 425 882-8080
    EMail: mailto:michelsu@microsoft.com
    URI:   http://www.suignard.com








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Full Copyright Statement

    Copyright (C) The Internet Society (2003).  All Rights Reserved.

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

    Funding for the RFC Editor function is currently provided by the
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