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rfc:rfc3987

Network Working Group M. Duerst Request for Comments: 3987 W3C Category: Standards Track M. Suignard

                                                 Microsoft Corporation
                                                          January 2005
           Internationalized Resource Identifiers (IRIs)

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

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.
 The approach of defining a new protocol element was chosen instead of
 extending or changing the definition of URIs.  This was done in order
 to allow a clear distinction and to avoid incompatibilities with
 existing software.  Guidelines are provided for the use and
 deployment of IRIs in various protocols, formats, and software
 components that currently deal with URIs.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Overview and Motivation  . . . . . . . . . . . . . . . .  3
     1.2.  Applicability  . . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Definitions  . . . . . . . . . . . . . . . . . . . . . .  4
     1.4.  Notation . . . . . . . . . . . . . . . . . . . . . . . .  5
 2.  IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Summary of IRI Syntax  . . . . . . . . . . . . . . . . .  6
     2.2.  ABNF for IRI References and IRIs . . . . . . . . . . . .  7

Duerst & Suignard Standards Track [Page 1] RFC 3987 Internationalized Resource Identifiers January 2005

 3.  Relationship between IRIs and URIs . . . . . . . . . . . . . . 10
     3.1.  Mapping of IRIs to URIs  . . . . . . . . . . . . . . . . 10
     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 . . . . . . . . . . . . . . . . . . . 18
     4.3.  Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . 19
     4.4.  Examples . . . . . . . . . . . . . . . . . . . . . . . . 19
 5.  Normalization and Comparison . . . . . . . . . . . . . . . . . 21
     5.1.  Equivalence  . . . . . . . . . . . . . . . . . . . . . . 22
     5.2.  Preparation for Comparison . . . . . . . . . . . . . . . 22
     5.3.  Comparison Ladder  . . . . . . . . . . . . . . . . . . . 23
           5.3.1.  Simple String Comparison . . . . . . . . . . . . 23
           5.3.2.  Syntax-Based Normalization . . . . . . . . . . . 24
           5.3.3.  Scheme-Based Normalization . . . . . . . . . . . 27
           5.3.4.  Protocol-Based Normalization . . . . . . . . . . 28
 6.  Use of IRIs  . . . . . . . . . . . . . . . . . . . . . . . . . 29
     6.1.  Limitations on UCS Characters Allowed in IRIs  . . . . . 29
     6.2.  Software Interfaces and Protocols  . . . . . . . . . . . 29
     6.3.  Format of URIs and IRIs in Documents and Protocols . . . 30
     6.4.  Use of UTF-8 for Encoding Original Characters .. . . . . 30
     6.5.  Relative IRI References  . . . . . . . . . . . . . . . . 32
 7.  URI/IRI Processing Guidelines (informative)  . . . . . . . . . 32
     7.1.  URI/IRI Software Interfaces  . . . . . . . . . . . . . . 32
     7.2.  URI/IRI Entry  . . . . . . . . . . . . . . . . . . . . . 33
     7.3.  URI/IRI Transfer between Applications  . . . . . . . . . 33
     7.4.  URI/IRI Generation . . . . . . . . . . . . . . . . . . . 34
     7.5.  URI/IRI Selection  . . . . . . . . . . . . . . . . . . . 34
     7.6.  Display of URIs/IRIs . . . . . . . . . . . . . . . . . . 35
     7.7.  Interpretation of URIs and IRIs  . . . . . . . . . . . . 36
     7.8.  Upgrading Strategy . . . . . . . . . . . . . . . . . . . 36
 8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 37
 9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 39
 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     10.1. Normative References . . . . . . . . . . . . . . . . . . 40
     10.2. Informative References . . . . . . . . . . . . . . . . . 41
 A.  Design Alternatives  . . . . . . . . . . . . . . . . . . . . . 44
     A.1.  New Scheme(s)  . . . . . . . . . . . . . . . . . . . . . 44
     A.2.  Character Encodings Other Than UTF-8 . . . . . . . . . . 44
     A.3.  New Encoding Convention  . . . . . . . . . . . . . . . . 44
     A.4.  Indicating Character Encodings in the URI/IRI  . . . . . 45
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 45
 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 46

Duerst & Suignard Standards Track [Page 2] RFC 3987 Internationalized Resource Identifiers January 2005

1. Introduction

1.1. Overview and Motivation

 A Uniform Resource Identifier (URI) is defined in [RFC3986] 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.  This 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 those who use only
 the Latin alphabet.  Many languages with non-Latin scripts are
 transcribed with Latin letters.  These 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
 common.  Also, increasing numbers of protocols and formats 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 [RFC3986], 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 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.

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:

Duerst & Suignard Standards Track [Page 3] RFC 3987 Internationalized Resource Identifiers January 2005

 a.  A protocol or format element should be explicitly designated to
     be able to carry IRIs.  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 a 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).
 sequence of characters: A sequence of characters (one after another).
 sequence of octets: A sequence of octets (one after another).
 character encoding: A method of representing a sequence of characters
    as a sequence of octets (maybe with variants).  Also, a method of
    (unambiguously) converting a sequence of octets into a sequence of
    characters.

Duerst & Suignard Standards Track [Page 4] RFC 3987 Internationalized Resource Identifiers January 2005

 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: 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 [RFC3986] 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
    (e.g., markup, programming languages).
 protocol element: Any portion of a message that affects processing of
    that message by the protocol in question.
 presentation element: A presentation form corresponding to a protocol
    element; for example, using a wider range of characters.
 create (a URI or IRI): With respect to URIs and IRIs, the term 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 (a URI or IRI): With respect to URIs and IRIs, the term 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
 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: 'XML Notation' and 'Bidi Notation'.

Duerst & Suignard Standards Track [Page 5] RFC 3987 Internationalized Resource Identifiers January 2005

 XML Notation uses a leading '&#x', a trailing ';', and the
 hexadecimal number of the character in the UCS in between.  For
 example, я stands for CYRILLIC CAPITAL LETTER YA.  In this
 notation, an actual '&' is denoted by '&'.
 Bidi Notation is used for bidirectional examples: Lowercase letters
 stand for Latin letters or other letters that are written left to
 right, whereas uppercase 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>.
 In this document, the key words "MUST", "MUST NOT", "REQUIRED",
 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY",
 and "OPTIONAL" 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 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 ensures that the characters in the
 IRI can be handled (e.g., 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 [RFC3986], but the class of
 unreserved characters is extended by adding the characters of the UCS
 (Universal Character Set, [ISO10646]) beyond U+007F, subject to the
 limitations given in the syntax rules below and in section 6.1.
 Otherwise, the syntax and use of components and reserved characters
 is the same as that in [RFC3986].  All the operations defined in
 [RFC3986], such as the resolution of relative references, can be
 applied to IRIs by IRI-processing software in exactly the same way as
 they are for URIs by URI-processing software.

Duerst & Suignard Standards Track [Page 6] RFC 3987 Internationalized Resource Identifiers January 2005

 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.  For example, U+00A2,
 CENT SIGN, is not allowed as a delimiter in IRIs, because it is in
 the 'iunreserved' category. This is similar to the fact that it is
 not possible to use '-' as a delimiter in URIs, because it is in the
 'unreserved' category.

2.2. ABNF for IRI References and IRIs

 Although 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 [RFC3986],
 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.  The grammar is split into two parts:
 Rules that differ from [RFC3986] because of the above-mentioned
 expansion, and rules that are the same as those in [RFC3986].  For
 rules that are different than those in [RFC3986], 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 those in [RFC3986]:
 IRI            = scheme ":" ihier-part [ "?" iquery ]
                       [ "#" ifragment ]
 ihier-part     = "//" iauthority ipath-abempty
                / ipath-absolute
                / ipath-rootless
                / ipath-empty
 IRI-reference  = IRI / irelative-ref
 absolute-IRI   = scheme ":" ihier-part [ "?" iquery ]
 irelative-ref  = irelative-part [ "?" iquery ] [ "#" ifragment ]
 irelative-part = "//" iauthority ipath-abempty
                     / ipath-absolute

Duerst & Suignard Standards Track [Page 7] RFC 3987 Internationalized Resource Identifiers January 2005

                / 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
                / %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 [RFC3986].

Duerst & Suignard Standards Track [Page 8] RFC 3987 Internationalized Resource Identifiers January 2005

 The following rules are the same as those in [RFC3986]:
 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     = "!" / "$" / "&" / "'" / "(" / ")"
                / "*" / "+" / "," / ";" / "="
 This syntax does not support IPv6 scoped addressing zone identifiers.

Duerst & Suignard Standards Track [Page 9] RFC 3987 Internationalized Resource Identifiers January 2005

3. Relationship between IRIs and URIs

 IRIs are meant to replace URIs in identifying resources for
 protocols, formats, and software components that 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 currently most
 retrieval mechanisms are only 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 also applies to IRI references and URI references, as well as
 to components thereof (for example, fragment identifiers).
 This mapping has two purposes:
 Syntaxical. 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.
 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 by using the following two steps.
 Step 1.  Generate a UCS character sequence from the original IRI
          format.  This step has the following three variants,
          depending on the form of the input:
          a. If the IRI is written on paper, read aloud, 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]).

Duerst & Suignard Standards Track [Page 10] RFC 3987 Internationalized Resource Identifiers January 2005

          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.
          c. If the IRI is in a Unicode-based character encoding (for
             example, UTF-8 or UTF-16), do not normalize (see section
             5.3.2.2 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
           [RFC3986].  To reduce variability, the hexadecimal notation
           SHOULD use uppercase 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
 [RFC3986].  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.
 Systems accepting IRIs MAY convert the ireg-name component of an IRI
 as follows (before step 2 above) for schemes known to use domain
 names in ireg-name, if 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 by using U+002E (FULL STOP) as a label
 separator, with the flag UseSTD3ASCIIRules set to TRUE, and with the
 flag AllowUnassigned set to FALSE for creating IRIs and set to TRUE
 otherwise.

Duerst & Suignard Standards Track [Page 11] RFC 3987 Internationalized Resource Identifiers January 2005

 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,
 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 and
 uses the ToASCII operation on ireg-name.  Implementations do not have
 to do this conversion as long as they produce the same result.
 Note: The difference between variants b and c in step 1 (using
    normalization with NFC, versus not using any normalization)
    accounts for the fact that in many non-Unicode character
    encodings, some text cannot be represented directly. For example,
    the word "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).
    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 is important
    to make processing independent of URI scheme.  See [Gettys] for an
    in-depth discussion.
 Note: In practice, whether the general mapping (steps 1 and 2) or the
    ToASCII operation of [RFC3490] is used for ireg-name 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

Duerst & Suignard Standards Track [Page 12] RFC 3987 Internationalized Resource Identifiers January 2005

    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
    for making the necessary conversions to be able to retrieve the
    Web page.
 Systems accepting IRIs MAY also deal with the printable characters in
 US-ASCII that are not allowed in URIs, namely "<", ">", '"', space,
 "{", "}", "|", "\", "^", and "`", in step 2 above.  If these
 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.  This
 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 and existing percent-encoded sequences are not encoded
    further.  (This mapping is similar to, but different from, the
    encoding applied when arbitrary content is included in 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 IRI with
    non-BMP characters (in XML Notation):
    "http://example.com/&#x10300;&#x10301;&#x10302";

Duerst & Suignard Standards Track [Page 13] RFC 3987 Internationalized Resource Identifiers January 2005

    which contains 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, converting a URI into an equivalent IRI may be
 desirable.  This section gives a procedure for this conversion.  The
 conversion described in this section will always result in an IRI
 that maps back to the URI 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:
 1.  Some percent-encodings are necessary to distinguish percent-
     encoded and unencoded uses of reserved characters.
 2.  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].)
 3.  The conversion may result in a character that is not appropriate
     in an IRI.  See sections 2.2, 4.1, and 6.1 for further details.
 Conversion from a URI to an IRI is done by 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) to the corresponding octets, except those corresponding
     to "%", characters in "reserved", and characters in US-ASCII not
     allowed in URIs.
 3.  Re-percent-encode any octet produced in step 2 that is not part
     of a strictly legal UTF-8 octet sequence.

Duerst & Suignard Standards Track [Page 14] RFC 3987 Internationalized Resource Identifiers January 2005

 4. Re-percent-encode all octets produced in step 3 that in UTF-8
    represent characters that are not appropriate according to
    sections 2.2, 4.1, and 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 step 4 is applied (see section 6.1), results may vary.
 Conversions from URIs to IRIs MUST NOT use any character encoding
 other than UTF-8 in steps 3 and 4, even if it might be possible to
 guess from the context that another character encoding than UTF-8 was
 used in the URI.  For 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, in the future the IRI will 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.  Each
 example shows the result after each of the steps 1 through 5 is
 applied.  XML Notation is used for the final result.  Octets are
 denoted by "<" followed by two hexadecimal digits followed by ">".
 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

Duerst & Suignard Standards Track [Page 15] RFC 3987 Internationalized Resource Identifiers January 2005

 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 lowercase) of letters
 used in percent-encodings 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
 by 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
 scripts, have an inherent right-to-left (rtl) writing direction.
 IRIs containing these characters (called bidirectional IRIs or Bidi
 IRIs) require additional attention because of the non-trivial

Duerst & Suignard Standards Track [Page 16] RFC 3987 Internationalized Resource Identifiers January 2005

 relation between logical representation (used for digital
 representation and for 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; and
 3.  minor or no changes or restrictions for implementations.

4.1. Logical Storage and Visual Presentation

 When stored or transmitted in digital representation, bidirectional
 IRIs MUST be in full logical order and MUST conform to the IRI syntax
 rules (which includes the rules relevant to their scheme). This
 ensures that bidirectional IRIs can be processed in the same way as
 other IRIs.
 Bidirectional IRIs MUST be rendered by using the Unicode
 Bidirectional Algorithm [UNIV4], [UNI9].  Bidirectional IRIs MUST be
 rendered in the same way as they would be if they were in a
 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 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 right-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.

Duerst & Suignard Standards Track [Page 17] RFC 3987 Internationalized Resource Identifiers January 2005

 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 ensure correct display are not themselves 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 appear themselves.  It would
 therefore not be possible to input an IRI with such characters
 correctly.

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
 "/") 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 schemes with ireg-name as 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.

Duerst & Suignard Standards Track [Page 18] RFC 3987 Internationalized Resource Identifiers January 2005

 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 ensure
 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 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 has 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 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.

Duerst & Suignard Standards Track [Page 19] RFC 3987 Internationalized Resource Identifiers January 2005

 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"
 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: Each of several sequences of rtl components is inverted on
 its 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 it 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.

Duerst & Suignard Standards Track [Page 20] RFC 3987 Internationalized Resource Identifiers January 2005

 Example 7: A single rtl component includes 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 are at the start or end of an 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"
 Depending on whether the uppercase 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 these may interact with adjacent RTL
 components in ways that are not easy to predict.

5. Normalization and Comparison

    Note: The structure and much of the material for this section is
    taken from section 6 of [RFC3986]; the differences are due to the
    specifics of IRIs.
 One of the most common operations on IRIs is simple comparison:
 Determining whether two IRIs are equivalent without using the IRIs or
 the mapped URIs to access their respective resource(s).  A comparison
 is performed whenever a response cache is accessed, a browser checks
 its history to color a link, or an XML parser processes tags within a
 namespace.  Extensive normalization prior to comparison of IRIs may
 be used by spiders and indexing engines to prune a search space or
 reduce duplication of request actions and response storage.

Duerst & Suignard Standards Track [Page 21] RFC 3987 Internationalized Resource Identifiers January 2005

 IRI comparison is performed for some particular purpose.  Protocols
 or implementations that compare IRIs for different purposes will
 often be subject to differing design trade-offs in regards to how
 much effort should be spent in reducing aliased identifiers.  This
 section describes various methods that may be used to compare IRIs,
 the trade-offs between them, and the types of applications that might
 use them.

5.1. Equivalence

 Because IRIs exist to identify resources, presumably they should be
 considered equivalent when they identify the same resource.  However,
 this definition of equivalence is not of much practical use, as there
 is no way for an implementation to compare two resources unless it
 has full knowledge or control of them. For this reason, determination
 of equivalence or difference of IRIs is based on string comparison,
 perhaps augmented by reference to additional rules provided by URI
 scheme definitions.  We use the terms "different" and "equivalent" to
 describe the possible outcomes of such comparisons, but there are
 many application-dependent versions of equivalence.
 Even though it is possible to determine that two IRIs are equivalent,
 IRI comparison is not sufficient to determine whether two IRIs
 identify different resources.  For example, an owner of two different
 domain names could decide to serve the same resource from both,
 resulting in two different IRIs.  Therefore, comparison methods are
 designed to minimize false negatives while strictly avoiding false
 positives.
 In testing for equivalence, applications should not directly compare
 relative references; the references should be converted to their
 respective target IRIs before comparison.  When IRIs are compared to
 select (or avoid) a network action, such as retrieval of a
 representation, fragment components (if any) should be excluded from
 the comparison.
 Applications using IRIs as identity tokens with no relationship to a
 protocol MUST use the Simple String Comparison (see section 5.3.1).
 All other applications MUST select one of the comparison practices
 from the Comparison Ladder (see section 5.3 or, after IRI-to-URI
 conversion, select one of the comparison practices from the URI
 comparison ladder in [RFC3986], section 6.2)

5.2. Preparation for Comparison

 Any kind of IRI comparison REQUIRES that all escapings or encodings
 in the protocol or format that carries an IRI are resolved.  This is
 usually done when the protocol or format is parsed.  Examples of such

Duerst & Suignard Standards Track [Page 22] RFC 3987 Internationalized Resource Identifiers January 2005

 escapings or encodings are entities and numeric character references
 in [HTML4] and [XML1].  As an example,
 "http://example.org/ros&eacute;" (in HTML),
 "http://example.org/ros&#233"; (in HTML or XML), and
 "http://example.org/ros&#xE9"; (in HTML or XML) are all resolved into
 what is denoted in this document (see section 1.4) as
 "http://example.org/ros&#xE9"; (the "&#xE9;" here standing for the
 actual e-acute character, to compensate for the fact that this
 document cannot contain non-ASCII characters).
 Similar considerations apply to encodings such as Transfer Codings in
 HTTP (see [RFC2616]) and Content Transfer Encodings in MIME
 ([RFC2045]), although in these cases, the encoding is based not on
 characters but on octets, and additional care is required to make
 sure that characters, and not just arbitrary octets, are compared
 (see section 5.3.1).

5.3. Comparison Ladder

 In practice, a variety of methods are used, to test IRI equivalence.
 These methods fall into a range distinguished by the amount of
 processing required and the degree to which the probability of false
 negatives is reduced.  As noted above, false negatives cannot be
 eliminated.  In practice, their probability can be reduced, but this
 reduction requires more processing and is not cost-effective for all
 applications.
 If this range of comparison practices is considered as a ladder, the
 following discussion will climb the ladder, starting with practices
 that are cheap but have a relatively higher chance of producing false
 negatives, and proceeding to those that have higher computational
 cost and lower risk of false negatives.

5.3.1. Simple String Comparison

 If two IRIs, when considered as character strings, are identical,
 then it is safe to conclude that they are equivalent.  This type of
 equivalence test has very low computational cost and is in wide use
 in a variety of applications, particularly in the domain of parsing.
 It is also used when a definitive answer to the question of IRI
 equivalence is needed that is independent of the scheme used and that
 can be calculated quickly and without accessing a network.  An
 example of such a case is XML Namespaces ([XMLNamespace]).
 Testing strings for equivalence requires some basic precautions. This
 procedure is often referred to as "bit-for-bit" or "byte-for-byte"
 comparison, which is potentially misleading.  Testing strings for
 equality is normally based on pair comparison of the characters that

Duerst & Suignard Standards Track [Page 23] RFC 3987 Internationalized Resource Identifiers January 2005

 make up the strings, starting from the first and proceeding until
 both strings are exhausted and all characters are found to be equal,
 until a pair of characters compares unequal, or until one of the
 strings is exhausted before the other.
 This character comparison requires that each pair of characters be
 put in comparable encoding form.  For example, should one IRI be
 stored in a byte array in UTF-8 encoding form and the second in a
 UTF-16 encoding form, bit-for-bit comparisons applied naively will
 produce errors.  It is better to speak of equality on a
 character-for-character rather than on a byte-for-byte or bit-for-bit
 basis.  In practical terms, character-by-character comparisons should
 be done codepoint by codepoint after conversion to a common character
 encoding form.  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 an IRI
 SHOULD NOT be modified when being transported if there is any chance
 that this IRI might be used as an identifier.
 False negatives are caused by the production and use of IRI aliases.
 Unnecessary aliases can be reduced, regardless of the comparison
 method, by consistently providing IRI references in an already
 normalized form (i.e., a form identical to what would be produced
 after normalization is applied, as described below). Protocols and
 data formats often limit some IRI comparisons to simple string
 comparison, based on the theory that people and implementations will,
 in their own best interest, be consistent in providing IRI
 references, or at least be consistent enough to negate any efficiency
 that might be obtained from further normalization.

5.3.2. Syntax-Based Normalization

 Implementations may use logic based on the definitions provided by
 this specification to reduce the probability of false negatives. This
 processing is moderately higher in cost than character-for-character
 string comparison.  For example, an application using this approach
 could reasonably consider the following two IRIs equivalent:
    example://a/b/c/%7Bfoo%7D/ros&#xE9;
    eXAMPLE://a/./b/../b/%63/%7bfoo%7d/ros%C3%A9
 Web user agents, such as browsers, typically apply this type of IRI
 normalization when determining whether a cached response is
 available.  Syntax-based normalization includes such techniques as
 case normalization, character normalization, percent-encoding
 normalization, and removal of dot-segments.

Duerst & Suignard Standards Track [Page 24] RFC 3987 Internationalized Resource Identifiers January 2005

5.3.2.1. Case Normalization

 For all IRIs, the hexadecimal digits within a percent-encoding
 triplet (e.g., "%3a" versus "%3A") are case-insensitive and therefore
 should be normalized to use uppercase letters for the digits A - F.
 When an IRI uses components of the generic syntax, the component
 syntax equivalence rules always apply; namely, that the scheme and
 US-ASCII only host are case insensitive and therefore should be
 normalized to lowercase.  For example, the URI
 "HTTP://www.EXAMPLE.com/" is equivalent to "http://www.example.com/".
 Case equivalence for non-ASCII characters in IRI components that are
 IDNs are discussed in section 5.3.3.  The other generic syntax
 components are assumed to be case sensitive unless specifically
 defined otherwise by the scheme.
 Creating schemes that allow case-insensitive syntax components
 containing non-ASCII characters should be avoided. Case normalization
 of non-ASCII characters can be culturally dependent and is always a
 complex operation.  The only exception concerns non-ASCII host names
 for which the character normalization includes a mapping step derived
 from case folding.

5.3.2.2. Character 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-character-normalized rather than apply character
 normalization when comparing two IRIs.  The exceptions are conversion
 from a non-digital form, and conversion from a non-UCS-based
 character encoding to a 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 by using NFC.  Using NFKC may
 avoid even more problems; for example, by choosing half-width Latin
 letters instead of full-width ones, and full-width instead of
 half-width Katakana.
 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.

Duerst & Suignard Standards Track [Page 25] RFC 3987 Internationalized Resource Identifiers January 2005

 The former uses precombined e-acute characters, and the latter uses
 "e" characters followed by combining acute accents.  Both usages are
 defined as canonically equivalent in [UNIV4].
 Note: Because it is unknown how a particular sequence of characters
    is being treated with respect to character 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 character
    normalized as possible (i.e., NFC or even NFKC).  This is similar
    to the uppercase/lowercase problems.  Some parts of a URI are case
    insensitive (domain name).  For others, it is unclear whether they
    are case sensitive, case insensitive, or something in between
    (e.g., case sensitive, but with a multiple choice selection if the
    wrong case is used, instead of a direct negative result).  The
    best recipe is that the creator use a reasonable capitalization
    and, when transferring the URI, capitalization never be changed.
 Various IRI schemes may allow the usage of Internationalized Domain
 Names (IDN) [RFC3490] either in the ireg-name part or elsewhere.
 Character Normalization also applies to IDNs, as discussed in section
 5.3.3.

5.3.2.3. Percent-Encoding Normalization

 The percent-encoding mechanism (section 2.1 of [RFC3986]) is a
 frequent source of variance among otherwise identical IRIs.  In
 addition to the case normalization issue noted above, some IRI
 producers percent-encode octets that do not require percent-encoding,
 resulting in IRIs that are equivalent to their non encoded
 counterparts.  These IRIs should be normalized by decoding any
 percent-encoded octet sequence that corresponds to an unreserved
 character, as described in section 2.3 of [RFC3986].
 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 uppercase).  If the IRI is to be passed to another

Duerst & Suignard Standards Track [Page 26] RFC 3987 Internationalized Resource Identifiers January 2005

 application or used further in some other way, its original form MUST
 be preserved.  The conversion described here should be performed only
 for local comparison.

5.3.2.4. Path Segment Normalization

 The complete path segments "." and ".." are intended only for use
 within relative references (section 4.1 of [RFC3986]) and are removed
 as part of the reference resolution process (section 5.2 of
 [RFC3986]).  However, some implementations may incorrectly assume
 that reference resolution is not necessary when the reference is
 already an IRI, and thus fail to remove dot-segments when they occur
 in non-relative paths.  IRI normalizers should remove dot-segments by
 applying the remove_dot_segments algorithm to the path, as described
 in section 5.2.4 of [RFC3986].

5.3.3. Scheme-Based Normalization

 The syntax and semantics of IRIs vary from scheme to scheme, as
 described by the defining specification for each scheme.
 Implementations may use scheme-specific rules, at further processing
 cost, to reduce the probability of false negatives.  For example,
 because the "http" scheme makes use of an authority component, has a
 default port of "80", and defines an empty path to be equivalent to
 "/", the following four IRIs are equivalent:
    http://example.com
    http://example.com/
    http://example.com:/
    http://example.com:80/
 In general, an IRI that uses the generic syntax for authority with an
 empty path should be normalized to a path of "/".  Likewise, an
 explicit ":port", for which the port is empty or the default for the
 scheme, is equivalent to one where the port and its ":" delimiter are
 elided and thus should be removed by scheme-based normalization.  For
 example, the second IRI above is the normal form for the "http"
 scheme.
 Another case where normalization varies by scheme is in the handling
 of an empty authority component or empty host subcomponent.  For many
 scheme specifications, an empty authority or host is considered an
 error; for others, it is considered equivalent to "localhost" or the
 end-user's host.  When a scheme defines a default for authority and
 an IRI reference to that default is desired, the reference should be
 normalized to an empty authority for the sake of uniformity, brevity,

Duerst & Suignard Standards Track [Page 27] RFC 3987 Internationalized Resource Identifiers January 2005

 and internationalization.  If, however, either the userinfo or port
 subcomponents are non-empty, then the host should be given explicitly
 even if it matches the default.
 Normalization should not remove delimiters when their associated
 component is empty unless it is licensed to do so by the scheme
 specification.  For example, the IRI "http://example.com/?" cannot be
 assumed to be equivalent to any of the examples above.  Likewise, the
 presence or absence of delimiters within a userinfo subcomponent is
 usually significant to its interpretation.  The fragment component is
 not subject to any scheme-based normalization; thus, two IRIs that
 differ only by the suffix "#" are considered different regardless of
 the scheme.
 Some IRI schemes may allow the usage of Internationalized Domain
 Names (IDN) [RFC3490] either in their ireg-name part or elsewhere.
 When in use in IRIs, those names SHOULD be validated by using the
 ToASCII operation defined in [RFC3490], with the flags
 "UseSTD3ASCIIRules" and "AllowUnassigned".  An IRI containing an
 invalid IDN cannot successfully be resolved.  Validated IDN
 components of IRIs SHOULD be character normalized by using the
 Nameprep process [RFC3491]; however, for legibility purposes, they
 SHOULD NOT be converted into ASCII Compatible Encoding (ACE).
 Scheme-based normalization may also consider IDN components and their
 conversions to punycode as equivalent.  As an example,
 "http://r&#xE9;sum&#xE9;.example.org" may be considered equivalent to
 "http://xn--rsum-bpad.example.org".
 Other scheme-specific normalizations are possible.

5.3.4. Protocol-Based Normalization

 Substantial effort to reduce the incidence of false negatives is
 often cost-effective for web spiders. Consequently, they implement
 even more aggressive techniques in IRI comparison.  For example, if
 they observe that an IRI such as
    http://example.com/data
 redirects to an IRI differing only in the trailing slash
    http://example.com/data/
 they will likely regard the two as equivalent in the future.  This
 kind of technique is only appropriate when equivalence is clearly
 indicated by both the result of accessing the resources and the

Duerst & Suignard Standards Track [Page 28] RFC 3987 Internationalized Resource Identifiers January 2005

 common conventions of their scheme's dereference algorithm (in this
 case, use of redirection by HTTP origin servers to avoid problems
 with relative references).

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
 IRIs are created and when URIs are converted 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.  It
     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 that 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.  This mapping
 SHOULD be applied as late as possible.  It SHOULD NOT be applied
 between components that are known to be able to handle IRIs.

Duerst & Suignard Standards Track [Page 29] RFC 3987 Internationalized Resource Identifiers January 2005

6.3. Format of URIs and IRIs in Documents and Protocols

 Document formats that transport URIs may have to be upgraded to allow
 the transport of IRIs.  In 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 not expressible in the native character
 encoding SHOULD be escaped by 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], 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.  To be able to use IRIs, the URI corresponding to the
 IRI in question has to encode original characters into octets by
 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
 to some part.  For background information on encoding characters into
 URIs, see also section 2.5 of [RFC3986].
 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

Duerst & Suignard Standards Track [Page 30] RFC 3987 Internationalized Resource Identifiers January 2005

 "http://www.example.org/r%E9sum%E9.html", the percent-encoding octets
 cannot be converted to actual characters in an IRI, as 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 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 this is the mailto: scheme
 [RFC2368].
 This specification does not upgrade any scheme specifications in any
 way; this has to be done separately.  Also, note 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; i.e., URIs that are admissible under the generic URI syntax
 [RFC3986] may not be admissible 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, as 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
 where the fragment identifier is encoded in UTF-8 according to

Duerst & Suignard Standards Track [Page 31] RFC 3987 Internationalized Resource Identifiers January 2005

 [XPointer]. The IRI corresponding to the above URI would be (in XML
 notation)
 "http://www.example.org/r%E9sum%E9.xml#r&#xE9;sum&#xE9";.
 Similar considerations apply to query parts.  The functionality of
 IRIs (namely, to be able to include non-ASCII characters) can only be
 used if the query part is encoded in UTF-8.

6.5. Relative IRI References

 Processing of relative IRI references against a base is handled
 straightforwardly; the algorithms of [RFC3986] can be applied
 directly, treating the characters additionally allowed in IRI
 references in the same way that unreserved characters are 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
 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, as
 it is compatible with US-ASCII, is in accordance with the
 recommendations of [RFC2277], and makes converting to URIs easy.  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.

Duerst & Suignard Standards Track [Page 32] RFC 3987 Internationalized Resource Identifiers January 2005

7.2. URI/IRI Entry

 Some components allow users to enter URIs into the system by typing
 or dictation, for example.  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 an 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 ensure, as much 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 solely used for the input of URIs/IRIs
 may allow the user to view an IRI as it is 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 interfacing to components that handle URIs,
 but not IRIs, must map the IRI to a URI before passing it to these
 components.
 For the input of IRIs with right-to-left characters, please see
 section 4.3.

7.3. URI/IRI Transfer between Applications

 Many applications, particularly 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.

Duerst & Suignard Standards Track [Page 33] RFC 3987 Internationalized Resource Identifiers January 2005

 Such applications have to be upgraded to use the IRI 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 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 make 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 use of
 "r&#xE9;sum&#xE9;.html" in an IRI, even if locally the file name is
 kept in a character encoding other than UTF-8.
 This recommendation particularly applies to HTTP servers.  For FTP
 servers, similar considerations apply; see [RFC2640].

7.5. URI/IRI Selection

 In some cases, resource owners and publishers have control over the
 IRIs used to identify their resources.  This control is mostly
 executed by controlling the resource names, such as file names,
 directly.

Duerst & Suignard Standards Track [Page 34] RFC 3987 Internationalized Resource Identifiers January 2005

 In these 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 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
 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, newly created resource names should generally 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 similarity of
 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 of 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 lowercase letters results in fewer
 ambiguities than using uppercase letters would.

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.

Duerst & Suignard Standards Track [Page 35] RFC 3987 Internationalized Resource Identifiers January 2005

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 character encodings
 other than UTF-8.  These URIs may be produced by user agents that do
 not conform to this specification and that 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
 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 some servers currently 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.  Please note
 that such mappings, including case mappings, are language dependent.
 It can be difficult to identify a resource unambiguously if too many
 mappings are taken into consideration.  However, percent-encoded and
 not percent-encoded parts of IRIs can always be clearly
 distinguished.  Also, the regularity of UTF-8 (see [Duerst97]) makes
 the potential for collisions lower than it may seem at first.

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, although it may be
 entered and transported very widely.

Duerst & Suignard Standards Track [Page 36] RFC 3987 Internationalized Resource Identifiers January 2005

 Therefore, IRIs benefit most from a broad upgrade of software to be
 able to enter and transport IRIs.  However, before an individual IRI
 is published, 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
 only after upgraded entry software has been widely deployed to the
 population that will see the displayed result.
 Where there is a free choice of character encodings, it is often
 possible to reduce the effort and dependencies for upgrading to IRIs
 by using UTF-8 rather than another encoding.  For example, when a new
 file-based Web server is set up, using UTF-8 as the character
 encoding for file names will make the transition to IRIs easier.
 Likewise, when a new Web form is set up 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, see
 section 6.4.

8. Security Considerations

 The security considerations discussed in [RFC3986] 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

Duerst & Suignard Standards Track [Page 37] RFC 3987 Internationalized Resource Identifiers January 2005

 "%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 that 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 that can cause all kinds of problems.  Most
 spoofing possibilities for IRIs are extensions of those for URIs.
 Spoofing can occur for various reasons.  First, a user's
 normalization expectations or actual normalization when entering an
 IRI or transcoding an IRI from a legacy character encoding do not
 match the normalization used on the server side.  Conceptually, this
 is no different from the problems surrounding the use of
 case-insensitive web servers.  For example, a popular web page with a
 mixed-case name ("http://big.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 is not specific to IRIs, but it is 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 that
 allow independent users to create resources in the same sub area may
 have to be careful to check for spoofing.
 Spoofing can occur because in the UCS many characters 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, particularly 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.

Duerst & Suignard Standards Track [Page 38] RFC 3987 Internationalized Resource Identifiers January 2005

 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 be used.

9. Acknowledgements

 We would like to thank Larry Masinter for his work as coauthor of
 many earlier versions of this document (draft-masinter-url-i18n-xx).
 The discussion on the issue addressed here 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 there were 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, Bill Fenner, Margaret Wasserman, Russ Housley,
 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 with 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.

Duerst & Suignard Standards Track [Page 39] RFC 3987 Internationalized Resource Identifiers January 2005

10. References

10.1. 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)", 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.
 [RFC3986]      Berners-Lee, T., Fielding, R., and L. Masinter,
                "Uniform Resource Identifier (URI): Generic Syntax",
                STD 66, RFC 3986, January 2005.
 [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.

Duerst & Suignard Standards Track [Page 40] RFC 3987 Internationalized Resource Identifiers January 2005

 [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>.

10.2. Informative 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:
                Resource Identifiers", World Wide Web Consortium
                Candidate Recommendation, November 2004,
                <http://www.w3.org/TR/charmod-resid>.
 [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/html401/appendix/
                notes.html#h-B.2>.
 [RFC2045]      Freed, N. and N. Borenstein, "Multipurpose Internet
                Mail Extensions (MIME) Part One: Format of Internet
                Message Bodies", RFC 2045, November 1996.
 [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.

Duerst & Suignard Standards Track [Page 41] RFC 3987 Internationalized Resource Identifiers January 2005

 [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., Frystyk, 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.
 [UNIXML]       Duerst, M. and A. Freytag, "Unicode in XML and other
                Markup Languages", Unicode Technical Report #20, World
                Wide Web Consortium Note, June 2003,
                <http://www.w3.org/TR/unicode-xml/>.
 [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>.

Duerst & Suignard Standards Track [Page 42] RFC 3987 Internationalized Resource Identifiers January 2005

 [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>.

Duerst & Suignard Standards Track [Page 43] RFC 3987 Internationalized Resource Identifiers January 2005

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, as 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 a uniform convention for conversion from IRIs to URIs
 makes IRI implementation orthogonal to the introduction of actual new
 schemes.

Appendix A.2. Character Encodings Other Than UTF-8

 At an early stage, UTF-7 was considered as an alternative to UTF-8
 when IRIs are converted to URIs.  UTF-7 would not have needed
 percent-encoding and in most cases would 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.
 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.

Duerst & Suignard Standards Track [Page 44] RFC 3987 Internationalized Resource Identifiers January 2005

 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 e-mails 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.
 If UTF-8 is used exclusively, an upgrade to the URI syntax is not
 needed.  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 (and being prohibitively annoying).
 Exclusively using UTF-8 also reduces transcoding errors and
 confusion.

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: 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: michelsu@microsoft.com
 URI:   http://www.suignard.com

Duerst & Suignard Standards Track [Page 45] RFC 3987 Internationalized Resource Identifiers January 2005

Full Copyright Statement

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 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Acknowledgement

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Duerst & Suignard Standards Track [Page 46]

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