Internet DRAFT - draft-duerst-iri

draft-duerst-iri




Network Working Group                                          M. Duerst
Internet-Draft                                                       W3C
Expires: May 31, 2005                                        M. Suignard
                                                   Microsoft Corporation
                                                       November 30, 2004


             Internationalized Resource Identifiers (IRIs)
                          draft-duerst-iri-11

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on May 31, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

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



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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1   Overview and Motivation  . . . . . . . . . . . . . . . . .  4
     1.2   Applicability  . . . . . . . . . . . . . . . . . . . . . .  4
     1.3   Definitions  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.4   Notation . . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.  IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.1   Summary of IRI Syntax  . . . . . . . . . . . . . . . . . .  7
     2.2   ABNF for IRI References and IRIs . . . . . . . . . . . . .  8
   3.  Relationship between IRIs and URIs . . . . . . . . . . . . . . 10
     3.1   Mapping of IRIs to URIs  . . . . . . . . . . . . . . . . . 11
     3.2   Converting URIs to IRIs  . . . . . . . . . . . . . . . . . 14
       3.2.1   Examples . . . . . . . . . . . . . . . . . . . . . . . 15
   4.  Bidirectional IRIs for Right-to-left Languages . . . . . . . . 17
     4.1   Logical Storage and Visual Presentation  . . . . . . . . . 17
     4.2   Bidi IRI Structure . . . . . . . . . . . . . . . . . . . . 18
     4.3   Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . . 20
     4.4   Examples . . . . . . . . . . . . . . . . . . . . . . . . . 20
   5.  Normalization and Comparison . . . . . . . . . . . . . . . . . 22
     5.1   Equivalence  . . . . . . . . . . . . . . . . . . . . . . . 22
     5.2   Preparation for Comparison . . . . . . . . . . . . . . . . 23
     5.3   Comparison Ladder  . . . . . . . . . . . . . . . . . . . . 23
       5.3.1   Simple String Comparison . . . . . . . . . . . . . . . 24
       5.3.2   Syntax-based Normalization . . . . . . . . . . . . . . 25
       5.3.3   Scheme-based Normalization . . . . . . . . . . . . . . 27
       5.3.4   Protocol-based Normalization . . . . . . . . . . . . . 29
   6.  Use of IRIs  . . . . . . . . . . . . . . . . . . . . . . . . . 29
     6.1   Limitations on UCS Characters Allowed in IRIs  . . . . . . 29
     6.2   Software Interfaces and Protocols  . . . . . . . . . . . . 30
     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  . . . . . . . . . . 34
     7.4   URI/IRI Generation . . . . . . . . . . . . . . . . . . . . 34
     7.5   URI/IRI Selection  . . . . . . . . . . . . . . . . . . . . 35
     7.6   Display of URIs/IRIs . . . . . . . . . . . . . . . . . . . 35
     7.7   Interpretation of URIs and IRIs  . . . . . . . . . . . . . 36



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     7.8   Upgrading Strategy . . . . . . . . . . . . . . . . . . . . 36
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 37
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 39
   10.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 39
   11.   References . . . . . . . . . . . . . . . . . . . . . . . . . 39
   11.1  Normative References . . . . . . . . . . . . . . . . . . . . 39
   11.2  Non-normative References . . . . . . . . . . . . . . . . . . 41
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 43
   A.  Design Alternatives  . . . . . . . . . . . . . . . . . . . . . 43
     A.1   New Scheme(s)  . . . . . . . . . . . . . . . . . . . . . . 43
     A.2   Other Character Encodings than UTF-8 . . . . . . . . . . . 44
     A.3   New Encoding Convention  . . . . . . . . . . . . . . . . . 44
     A.4   Indicating Character Encodings in the URI/IRI  . . . . . . 44
       Intellectual Property and Copyright Statements . . . . . . . . 45





































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1.  Introduction

1.1  Overview and Motivation

   A Uniform Resource Identifier (URI) is defined in [RFCYYYY] as a
   sequence of characters chosen from a limited subset of the repertoire
   of US-ASCII [ASCII] characters.

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

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

   This document defines a new protocol element, called
   Internationalized Resource Identifier (IRI), by extending the syntax
   of URIs to a much wider repertoire of characters.  It also defines
   "internationalized" versions corresponding to other constructs from
   [RFCYYYY], such as URI references.  The syntax of IRIs is defined in
   Section 2, and the relationship between IRIs and URIs in Section 3.

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

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:



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   a) The protocol or format element where IRIs are used should be
      explicitly designated to be able to carry IRIs.  That is, the
      intent is not to introduce IRIs into contexts that are not defined
      to accept them.  For example, XML schema [XMLSchema] has an
      explicit type "anyURI" that includes IRIs and IRI references.
      Therefore, IRIs and IRI references can be in attributes and
      elements of type "anyURI".  On the other hand, in the HTTP
      protocol [RFC2616], the Request URI is defined as an URI, which
      means that direct use of IRIs is not allowed in HTTP requests.

   b) The protocol or format carrying the IRIs should have a mechanism
      to represent the wide range of characters used in IRIs, either
      natively or by some protocol- or format-specific escaping
      mechanism (for example numeric character references in [XML1]).

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


1.3  Definitions

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

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

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

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

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

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

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





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

   UCS: Universal Character Set; the coded character set defined by ISO/
      IEC 10646 [ISO10646] and the Unicode Standard [UNIV4].

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

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

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

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

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

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


1.4  Notation

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

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

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

   XML Notation uses leading '&#x', trailing ';', and the hexadecimal



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   number of the character in the UCS in between.  Example: я
   stands for CYRILLIC CAPITAL LETTER YA.  In this notation, an actual
   '&' is denoted by '&'.

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

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

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

2.  IRI Syntax

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

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

2.1  Summary of IRI Syntax

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

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

   Characters outside the US-ASCII repertoire are not reserved and



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   therefore MUST NOT be used for syntactical purposes such as to
   delimit components in newly defined schemes.  As an example, it is
   not allowed to use U+00A2, CENT SIGN, as a delimiter in IRIs, because
   it is in the 'iunreserved' category, in the same way as it is not
   possible to use '-' as a delimiter, because it is in the 'unreserved'
   category in URIs.

2.2  ABNF for IRI References and IRIs

   While it might be possible to define IRI references and IRIs merely
   by their transformation to URI references and URIs, they can also be
   accepted and processed directly.  Therefore, an ABNF definition for
   IRI references (which are the most general concept and the start of
   the grammar) and IRIs is given here.  The syntax of this ABNF is
   described in [RFC2234].  Character numbers are taken from the UCS,
   without implying any actual binary encoding.  Terminals in the ABNF
   are characters, not bytes.

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

   The following rules are different from [RFCYYYY]:

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

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

      IRI-reference  = IRI / irelative-ref

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

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

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



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                     / 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 [RFCYYYY].

   The following are the same as in [RFCYYYY]:



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      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.

3.  Relationship between IRIs and URIs

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



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   identifier is used for resource retrieval, it is in many cases
   necessary to determine the associated URI because most retrieval
   mechanisms currently only are defined for URIs.  In this case, IRIs
   can serve as presentation elements for URI protocol elements.  An
   example would be an address bar in a Web user agent.  (Additional
   rationale is given in Section 3.1.)

3.1  Mapping of IRIs to URIs

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

   This mapping has two purposes:

   a) Syntactical: Many URI schemes and components define additional
      syntactical restrictions not captured in Section 2.2.
      Scheme-specific restrictions are applied to IRIs by converting
      IRIs to URIs and checking the URIs against the scheme-specific
      restrictions.

   b) Interpretational: URIs identify resources in various ways.  IRIs
      also identify resources.  When the IRI is used solely for
      identification purposes, it is not necessary to map the IRI to a
      URI (see Section 5).  However, when an IRI is used for resource
      retrieval, the resource that the IRI locates is the same as the
      one located by the URI obtained after converting the IRI according
      to the procedure defined here.  This means that there is no need
      to define resolution separately on the IRI level.

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

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

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

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





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      Variant C) If the IRI is in an Unicode-based character encoding
         (for example UTF-8 or UTF-16): Do not normalize (see Section
         5.3.2.2 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 [RFCYYYY].  To
         reduce variability, the hexadecimal notation SHOULD use upper
         case letters.

      2.3) Replace the original character with the resulting character
         sequence (i.e., a sequence of %HH triplets).

   The above mapping from IRIs to URIs produces URIs fully conforming to
   [RFCYYYY].  The mapping is also an identity transformation for URIs
   and is idempotent -- applying the mapping a second time will not
   change anything.  Every URI is by definition an IRI.

   Infrastructure accepting IRIs MAY convert the ireg-name component of
   an IRI as follows (before Step 2 above) for schemes that are known to
   use domain names in ireg-name, but where the scheme definition does
   not allow percent-encoding for ireg-name: Replace the ireg-name part
   of the IRI by the part converted using the ToASCII operation
   specified in Section 4.1 of [RFC3490] on each dot-separated label,
   and using U+002E (FULL STOP) as a label separator, with the flag
   UseSTD3ASCIIRules set to TRUE and the flag AllowUnassigned set to
   FALSE for creating IRIs and set to TRUE otherwise.  The ToASCII
   operation may fail, but this would mean that the IRI cannot be
   resolved.  This conversion SHOULD be used when the goal is to
   maximize interoperability with legacy URI resolvers.  For example,
   the IRI
   http://r&#xE9;sum&#xE9;.example.org may be converted to
   http://xn--rsum-bpad.example.org instead of
   http://r%C3%A9sum%C3%A9.example.org.

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



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   obtained after converting the IRI including using the ToASCII
   operation on ireg-name.  Implementations do not need to do this
   conversion as long as they produce the same result.

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

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

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

   Note: Internationalized Domain Names may be contained in parts of an
      IRI other than the ireg-name part.  It is the responsibility of
      scheme-specific implementations (if the Internationalized Domain
      Name is part of the scheme syntax) or of server-side
      implementations (if the Internationalized Domain Name is part of
      'iquery') to apply the necessary conversions at the appropriate
      point.  Example: Trying to validate the Web page at
      http://r&#xE9;sum&#xE9;.example.org would lead to an IRI of
      http://validator.w3.org/check?uri=http%3A%2F%2Fr&#xE9;sum&#xE9;.
      example.org, which would convert to a URI of
      http://validator.w3.org/check?uri=http%3A%2F%2Fr%C3%A9sum%C3%A9.
      example.org.  The server side implementation would be responsible
      to do the necessary conversions in order to be able to retrieve
      the Web page.

   Infrastructure accepting IRIs MAY also deal with the printable
   characters in US-ASCII that are not allowed in URIs, namely "<", ">",
   '"', Space, "{", "}", "|", "\", "^", and "`", in Step 2 above.  If
   such characters are found but are not converted, then the conversion
   SHOULD fail.  Please note that the number sign ("#"), the percent
   sign ("%"), and the square bracket characters ("[", "]") are not part



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   of the above list, and MUST NOT be converted.  Protocols and formats
   that have used earlier definitions of IRIs including these characters
   MAY require percent-encoding of these characters as a preprocessing
   step to extract the actual IRI from a given field.  Such
   preprocessing MAY also be used by applications allowing the user to
   enter an IRI.

   Note: In this process (in Step 2.3), characters allowed in URI
      references as well as existing percent-encoded sequences are not
      encoded further.  (This mapping is similar to, but different from,
      the encoding applied when including arbitrary content into some
      part of a URI.) For example, an IRI of
      http://www.example.org/red%09ros&#xE9;#red (in XML notation) is
      converted to
      http://www.example.org/red%09ros%C3%A9#red, not to something like
      http%3A%2F%2Fwww.example.org%2Fred%2509ros%C3%A9%23red.

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


3.2  Converting URIs to IRIs

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

   URI to IRI conversion removes percent-encodings, but not all
   percent-encodings can be eliminated.  There are several reasons for
   this:

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

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




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      (Note: The octet patterns of UTF-8 are highly regular.  Therefore,
      there is a very high probability, but no guarantee, that
      percent-encodings that can be interpreted as sequences of UTF-8
      octets actually originated from UTF-8.  For a detailed discussion,
      see [Duerst97].)

   c) The conversion may result in a character that is not appropriate
      in an IRI.  See Section 2.2, Section 4.1, and Section 6.1 for
      further details.

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

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

   2) Convert all percent-encodings (% followed by two hexadecimal
      digits) except those corresponding to '%', characters in
      'reserved', and characters in US-ASCII not allowed in URIs, to the
      corresponding octets.

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

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

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

   This procedure will convert as many percent-encoded characters as
   possible to characters in an IRI.  Because there are some choices
   when applying Step 4 (see Section 6.1), results may vary.

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

3.2.1  Examples

   This section shows various examples of converting URIs to IRIs.  Each



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   example shows the result after applying each of the Steps 1 to 5.
   XML Notation is used for the final result.

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

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

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

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

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

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

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

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

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

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

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

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

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






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

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

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

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

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

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

4.  Bidirectional IRIs for Right-to-left Languages

   Some UCS characters, such as those used in the Arabic and Hebrew
   script, have an inherent right-to-left (rtl) writing direction.  IRIs
   containing such characters (called bidirectional IRIs or Bidi IRIs)
   require additional attention because of the non-trivial relation
   between logical representation (used for digital representation as
   well as when reading/spelling) and visual representation (used for
   display/printing).

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

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

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

   3) minor or no changes or restrictions for implementations.


4.1  Logical Storage and Visual Presentation

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



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

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

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

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

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

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



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   (structural characters, mostly punctuation such as '@', '.', ':',
   '/') and components (usually consisting mostly of letters and
   digits).

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

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

   For each component, the following restrictions apply:

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

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

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

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

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



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   (see also Example 9 below).

4.3  Input of Bidi IRIs

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

4.4  Examples

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

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

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

   Example 2: More than one consecutive component with rtl characters is
   inverted as a whole:
   logical representation: http://ab.CDE.FGH/ij/kl/mn/op.html
   visual representation: http://ab.HGF.EDC/ij/kl/mn/op.html
   A sequence of rtl components is read rtl, in the same way as a
   sequence of rtl words is read rtl in a bidi text.

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

   Example 4: Several sequences of rtl components are each inverted on
   their own:



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

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

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

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

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

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

   Example 10 (allowed, but not recommended):
   logical representation: http://ab.CDEFGH.123/kl/mn/op.html
   visual representation: http://ab.123.HGFEDC/kl/mn/op.html



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   Components consisting of only numbers are allowed (it would be rather
   difficult to prohibit them), but may interact with adjacent RTL
   components in ways that are not easy to predict.

5.  Normalization and Comparison

      Note: The structure and much of the material for this section is
      taken from section 6 of [RFCYYYY]; the differences are due to the
      specifics of IRIs.

   One of the most common operations on IRIs is simple comparison:
   determining if two IRIs are equivalent without using the IRIs or the
   mapped URIs to access their respective resource(s).  A comparison is
   performed every time 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.

   IRI comparison is performed in respect to some particular purpose,
   and implementations with differing 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 a
   variety of 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

   Since IRIs exist to identify resources, presumably they should be
   considered equivalent when they identify the same resource.  However,
   such a definition of equivalence is not of much practical use, since
   there is no way for an implementation to compare two resources that
   are not under its own control.  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 applicationdependent versions of equivalence.

   Even though it is possible to determine that two IRIs are equivalent,
   IRI comparison is not sufficient to determine if 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



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   respective target IRIs before comparison.  When IRIs are being
   compared for the purpose of selecting (or avoiding) 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 [RFCYYYY], 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 parsing the protocol or format.  Examples of such
   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) all get 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 not based 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

   A variety of methods are used in practice 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 those
   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.



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5.3.1  Simple String Comparison

   If two IRIs, 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
   and when a definitive answer to the question of IRI equivalence is
   needed that is independent of the scheme used and 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
   of strings for equality is normally based on pairwise comparison of
   the characters that make up the strings, starting from the first and
   proceeding until both strings are exhausted and all characters found
   to be equal, a pair of characters compares unequal, or one of the
   strings is exhausted before the other.

   Such character comparisons require 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 be 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 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 IRIs 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 choose to 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 consistent enough to negate any
   efficiency that might be obtained from further normalization.







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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.
   Such 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.

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 US-ASCII characters should be avoided because such a
   case normalization may be cultural dependant 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



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   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 applying
   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 an UCS-based character encoding.
   In these cases, NFC or a normalizing transcoder using NFC MUST be
   used for interoperability.  To avoid false negatives and problems
   with transcoding, IRIs SHOULD be created using NFC.  Using NFKC may
   avoid even more problems, for example by choosing half-width Latin
   letters instead of full-width, and full-width Katakana instead of
   half-width.

   As an example, http://www.example.org/r&#xE9;sum&#xE9;.html (in XML
   Notation) is in NFC.  On the other hand,
   http://www.example.org/re&#x301;sume&#x301;.html is not in NFC.  The
   former uses precombined e-acute characters, the latter uses 'e'
   characters followed by combining acute accents.  Both usages are
   defined to be canonically equivalent in [UNIV4].

   Note: Because it is unknown how a particular 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 upper-case/lower-case problems in
      character-normalized as possible (i.e.  NFC or even NFKC).  URIs.
      Some parts of a URI are case-insensitive (domain name).  For
      others, it is unclear whether they are case-sensitive or
      case-insensitive, or something in between (e.g.  case-sensitive,
      but if the wrong case is used, a multiple choice selection is
      provided instead of a direct negative result).  The best recipe is
      that the creator uses a reasonable capitalization, and when
      transferring the URI, that capitalization is never changed.

   Various IRI schemes may allow the usage of 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 [RFCYYYY]) is a
   frequent source of variance among otherwise identical IRIs.  In
   addition to the case normalization issue noted above, some IRI



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   producers percent-encode octets that do not require percent-encoding,
   resulting in IRIs that are equivalent to their nonencoded
   counterparts.  Such IRIs should be normalized by decoding any
   percent-encoded octet  sequence that corresponds to an unreserved
   character, as described in Section 2.3 of [RFCYYYY].

   For actual resolution, differences in percent-encoding (except for
   the percent-encoding of reserved characters) MUST always result in
   the same resource.  For example, http://example.org/~user,
   http://example.org/%7euser and http://example.org/%7Euser must
   resolve to the same resource.

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

5.3.2.4  Path Segment Normalization

   The complete path segments "." and ".." are intended only for use
   within relative references (Section 4.1 of [RFCYYYY]) and are removed
   as part of the reference resolution process (Section 5.2 of
   [RFCYYYY]).  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 [RFCYYYY].

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,
   since 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/



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   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", where 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,
   and internationalization.  If, however, either the userinfo or port
   subcomponent is 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 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 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 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.






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5.3.4  Protocol-based Normalization

   Web spiders, for which substantial effort to reduce the incidence of
   false negatives is often cost-effective, are observed to 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
   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
   creating IRIs and when converting from URIs to IRIs.

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

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

   b) The UCS contains many areas of characters for which there are
      strong visual look-alikes.  Because of the likelihood of
      transcription errors, these also should be avoided.  This includes
      the full-width equivalents of Latin characters, half-width
      Katakana characters for Japanese, and many others.  This also
      includes many look-alikes of "space", "delims", and "unwise",
      characters excluded in [RFC3491].

   Additional information is available from [UNIXML].  [UNIXML] is
   written in the context of running text rather than in the context of
   identifiers.  Nevertheless, it discusses many of the categories of



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   characters not appropriate for IRIs.

6.2  Software Interfaces and Protocols

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

   Intermediate software interfaces between IRI-capable components and
   URI-only components MUST map the IRIs per Section 3.1, when
   transferring from IRI-capable to URI-only components.  Such a mapping
   SHOULD be applied as late as possible.  It SHOULD NOT be applied
   between components that are known to be able to handle IRIs.

6.3  Format of URIs and IRIs in Documents and Protocols

   Document formats that transport URIs may need to be upgraded to allow
   the transport of IRIs.  In those cases where the document as a whole
   has a native character encoding, IRIs MUST also be encoded in this
   character encoding, and converted accordingly by a parser or
   interpreter.  IRI characters that are not expressible in the native
   character encoding SHOULD be escaped using the escaping conventions
   of the document format if such conventions are available.
   Alternatively, they MAY be percent-encoded according to Section 3.1.
   For example, in HTML or XML, numeric character references SHOULD be
   used.  If a document as a whole has a native character encoding, and
   that character encoding is not UTF-8, then IRIs MUST NOT be placed
   into the document in the UTF-8 character encoding.

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

6.4  Use of UTF-8 for Encoding Original Characters

   This section discusses details and gives examples for point c) in
   Section 1.2.  In order to be able to use IRIs, the URI corresponding
   to the IRI in question has to encode original characters into octets
   using UTF-8.  This can be specified for all URIs of a URI scheme, or
   can apply to individual URIs for schemes that do not specify how to
   encode original characters.  It can apply to the whole URI, or only
   some part.  For background information on encoding characters into
   URIs, see also Section 2.5 of [RFCYYYY].




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

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

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

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

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




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   The requirement for the use of UTF-8 applies to all parts of a URI
   (with the potential exception of the ireg-name part, see Section
   3.1).  However, it is possible that the capability of IRIs to
   represent a wide range of characters directly is used just in some
   parts of the IRI (or IRI reference).  The other parts of the IRI may
   only contain US-ASCII characters, or they may not be based on UTF-8.
   They may be based on another character encoding, or they may directly
   encode raw binary data (see also [RFC2397]).

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

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

6.5  Relative IRI References

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

7.  URI/IRI Processing Guidelines (informative)

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

7.1  URI/IRI Software Interfaces

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

   In case the current handling in an API or protocol is based on
   US-ASCII, UTF-8 is recommended as the character encoding for IRIs,



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   because this is compatible with US-ASCII, is in accordance with the
   recommendations of [RFC2277], and makes it easy to convert to URIs
   where necessary.  In any case, the API or protocol definition must
   clearly define the character encoding to be used.

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

7.2  URI/IRI Entry

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

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

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

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

   An input field primarily or only used for the input of URIs/IRIs may
   allow the user to view an IRI as mapped to a URI.  Places where the
   input of IRIs is frequent may provide the possibility for viewing an
   IRI as mapped to a URI.  This will help users when some of the
   software they use does not yet accept IRIs.

   An IRI input component that interfaces to components that handle
   URIs, but not IRIs, must map the IRI to a URI before passing it to
   such a component.

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




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7.3  URI/IRI Transfer Between Applications

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

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

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

7.4  URI/IRI Generation

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

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

   For maximum interoperability, systems that generate resource
   identifiers should do the appropriate transformations.  For example,
   if a file system contains a file named r&#xE9;sum&#xE9;.html, a
   server should expose this as r%C3%A9sum%C3%A9.html in a URI, which
   allows to use r&#xE9;sum&#xE9;.html in an IRI, even if the file name
   locally is kept in a character encoding other than UTF-8.

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






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7.5  URI/IRI Selection

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

   In such cases, it is recommended to avoid choosing IRIs that are
   easily confused.  For example, for US-ASCII, the lower-case ell "l"
   is easily confused with the digit one "1", and the upper-case oh "O"
   is easily confused with the digit zero "0".  Publishers should avoid
   confusing users with "br0ken" or "1ame" identifiers.

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

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

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

7.6  Display of URIs/IRIs

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



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   displayed.

   For display of Bidi IRIs, please see Section 4.1.

7.7  Interpretation of URIs and IRIs

   Software that interprets IRIs as the names of local resources should
   accept IRIs in multiple forms, and convert and match them with the
   appropriate local resource names.

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

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

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

   It can be difficult to unambiguously identify a resource if too many
   mappings are taken into consideration.  However, percent-encoded and
   not percent-encoded parts of IRIs can always clearly be
   distinguished.  Also, the regularity of UTF-8 (see [Duerst97]) makes
   the potential for collisions lower than it may seem at first sight.

7.8  Upgrading Strategy

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

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



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

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

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

   Software converting from URIs to IRIs for display should be upgraded
   only after upgraded entry software has been widely deployed to the
   population that will see the displayed result.

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

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

8.  Security Considerations

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

   Incorrect encoding or decoding can lead to security problems.  In
   particular, some UTF-8 decoders do not check against overlong byte
   sequences.  As an example, a '/' is encoded with the byte 0x2F both
   in UTF-8 and in US-ASCII, but some UTF-8 decoders also wrongly
   interpret the sequence 0xC0 0xAF as a '/'.  A sequence such as



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   '%C0%AF..' may pass some security tests and then be interpreted as '/
   ..' in a path if UTF-8 decoders are fault-tolerant, if conversion and
   checking are not done in the right order, and/or if reserved
   characters and unreserved characters are not clearly distinguished.

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

   Spoofing can occur for various reasons.  A first reason is that
   normalization expectations of a user or actual normalization when
   entering an IRI, or when transcoding an IRI from a legacy character
   encoding, do not match the normalization used on the server side.
   Conceptually, this is no different from the problems surrounding the
   use of case-insensitive web servers.  For example, a popular web page
   with a mixed case name (http://big.example.com/PopularPage.html)
   might be "spoofed" by someone who is able to create
   http://big.example.com/popularpage.html.  However, the use of
   unnormalized character sequences, and of additional mappings for user
   convenience, may increase the chance for spoofing.  Protocols and
   servers that allow the creation of resources with names that are not
   normalized are particularly vulnerable to such attacks.  This is an
   inherent security problem of the relevant protocol, server, or
   resource, and not specific to IRIs, but mentioned here for
   completeness.

   Spoofing can occur in various IRI components, such as the domain name
   part or a path part.  For considerations specific to the domain name
   part, see [RFC3491].  For the path part, administrators of sites
   which allow independent users to create resources in the same subarea
   may need to be careful to check for spoofing.

   Spoofing can occur because in the UCS, there are many characters that
   look very similar.  Details are discussed in Section 7.5.  Again,
   this is very similar to spoofing possibilities on US-ASCII, e.g.
   using 'br0ken' or '1ame' URIs.

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



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   same encoding, detection is more difficult.

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

9.  IANA Considerations

   This document has no actions for IANA.

10.  Acknowledgements

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

   The discussion on the issue addressed here has started a long time
   ago.  There was a thread in the HTML working group in August 1995
   (under the topic of "Globalizing URIs") and in the www-international
   mailing list in July 1996 (under the topic of "Internationalization
   and URLs"), and ad-hoc meetings at the Unicode conferences in
   September 1995 and September 1997.

   Many thanks go to Francois Yergeau, Matitiahu Allouche, Roy Fielding,
   Tim Berners-Lee, Mark Davis, M.T.  Carrasco Benitez, James Clark, Tim
   Bray, Chris Wendt, Yaron Goland, Andrea Vine, Misha Wolf, Leslie
   Daigle, Ted Hardie, 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 getting the details right.

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

11.  References

11.1  Normative References

   [ASCII]    American National Standards Institute, "Coded Character



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              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.

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

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

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

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



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11.2  Non-normative References

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

   [CharMod]  Duerst, M., Yergeau, F., Ishida, R., Wolf, M. and T.
              Texin, "Character Model for the World Wide Web", World
              Wide Web Consortium Working Draft, February 2004,
              <http://www.w3.org/TR/charmod>.

   [Duerst97]
              Duerst, M., "The Properties and Promises of UTF-8", Proc.
              11th International Unicode Conference, San Jose ,
              September 1997,
              <http://www.ifi.unizh.ch/mml/mduerst/papers/PDF/
               IUC11-UTF-8.pdf>.

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

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

   [RFC2045]  Freed, N. and N. Freed, "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.

   [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



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              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 1998.

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

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

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

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

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

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

   [XML1]     Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E. and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Third
              Edition)", World Wide Web Consortium Recommendation,
              February 2004,
              <http://www.w3.org/TR/REC-xml#sec-external-ent>.

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

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

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




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Authors' Addresses

   Martin Duerst (Note: Please write "Duerst" with u-umlaut wherever
                 possible, for example as "D&#252;rst" in XML and HTML.)
   World Wide Web Consortium
   5322 Endo
   Fujisawa, Kanagawa  252-8520
   Japan

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


   Michel Suignard
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   U.S.A.

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

Appendix A.  Design Alternatives

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

Appendix A.1  New Scheme(s)

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

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

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Appendix A.2  Other Character Encodings than UTF-8

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

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

Appendix A.3  New Encoding Convention

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

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

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

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

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













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