rfc3987.txt

Network Working Group                                          M. Duerst
Request for Comments: 3987                                           W3C
Category: Standards Track                                    M. Suignard
                                                   Microsoft Corporation
                                                            January 2005


             Internationalized Resource Identifiers (IRIs)

Status of This Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

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

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

Table of Contents

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




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   3.  Relationship between IRIs and URIs . . . . . . . . . . . . . . 10
       3.1.  Mapping of IRIs to URIs  . . . . . . . . . . . . . . . . 10
       3.2.  Converting URIs to IRIs  . . . . . . . . . . . . . . . . 14
             3.2.1.  Examples . . . . . . . . . . . . . . . . . . . . 15
   4.  Bidirectional IRIs for Right-to-Left Languages.  . . . . . . . 16
       4.1.  Logical Storage and Visual Presentation  . . . . . . . . 17
       4.2.  Bidi IRI Structure . . . . . . . . . . . . . . . . . . . 18
       4.3.  Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . 19
       4.4.  Examples . . . . . . . . . . . . . . . . . . . . . . . . 19
   5.  Normalization and Comparison . . . . . . . . . . . . . . . . . 21
       5.1.  Equivalence  . . . . . . . . . . . . . . . . . . . . . . 22
       5.2.  Preparation for Comparison . . . . . . . . . . . . . . . 22
       5.3.  Comparison Ladder  . . . . . . . . . . . . . . . . . . . 23
             5.3.1.  Simple String Comparison . . . . . . . . . . . . 23
             5.3.2.  Syntax-Based Normalization . . . . . . . . . . . 24
             5.3.3.  Scheme-Based Normalization . . . . . . . . . . . 27
             5.3.4.  Protocol-Based Normalization . . . . . . . . . . 28
   6.  Use of IRIs  . . . . . . . . . . . . . . . . . . . . . . . . . 29
       6.1.  Limitations on UCS Characters Allowed in IRIs  . . . . . 29
       6.2.  Software Interfaces and Protocols  . . . . . . . . . . . 29
       6.3.  Format of URIs and IRIs in Documents and Protocols . . . 30
       6.4.  Use of UTF-8 for Encoding Original Characters .. . . . . 30
       6.5.  Relative IRI References  . . . . . . . . . . . . . . . . 32
   7.  URI/IRI Processing Guidelines (informative)  . . . . . . . . . 32
       7.1.  URI/IRI Software Interfaces  . . . . . . . . . . . . . . 32
       7.2.  URI/IRI Entry  . . . . . . . . . . . . . . . . . . . . . 33
       7.3.  URI/IRI Transfer between Applications  . . . . . . . . . 33
       7.4.  URI/IRI Generation . . . . . . . . . . . . . . . . . . . 34
       7.5.  URI/IRI Selection  . . . . . . . . . . . . . . . . . . . 34
       7.6.  Display of URIs/IRIs . . . . . . . . . . . . . . . . . . 35
       7.7.  Interpretation of URIs and IRIs  . . . . . . . . . . . . 36
       7.8.  Upgrading Strategy . . . . . . . . . . . . . . . . . . . 36
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 37
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 39
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
       10.1. Normative References . . . . . . . . . . . . . . . . . . 40
       10.2. Informative References . . . . . . . . . . . . . . . . . 41
   A.  Design Alternatives  . . . . . . . . . . . . . . . . . . . . . 44
       A.1.  New Scheme(s)  . . . . . . . . . . . . . . . . . . . . . 44
       A.2.  Character Encodings Other Than UTF-8 . . . . . . . . . . 44
       A.3.  New Encoding Convention  . . . . . . . . . . . . . . . . 44
       A.4.  Indicating Character Encodings in the URI/IRI  . . . . . 45
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 45
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 46







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1.  Introduction

1.1.  Overview and Motivation

   A Uniform Resource Identifier (URI) is defined in [RFC3986] as a
   sequence of characters chosen from a limited subset of the repertoire
   of US-ASCII [ASCII] characters.

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

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

   This document defines a new protocol element called Internationalized
   Resource Identifier (IRI) by extending the syntax of URIs to a much
   wider repertoire of characters.  It also defines "internationalized"
   versions corresponding to other constructs from [RFC3986], such as
   URI references.  The syntax of IRIs is defined in section 2, and the
   relationship between IRIs and URIs in section 3.

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

1.2.  Applicability

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




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

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

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

1.3.  Definitions

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

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

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

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

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

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

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





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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: Denotes the common usage of an Internationalized
      Resource Identifier.  An IRI reference may be absolute or
      relative.  However, the "IRI" that results from such a reference
      only includes absolute IRIs; any relative IRI references are
      resolved to their absolute form.  Note that in [RFC2396] URIs did
      not include fragment identifiers, but in [RFC3986] fragment
      identifiers are part of URIs.

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

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

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

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

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

1.4.  Notation

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

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

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






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

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

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

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

2.  IRI Syntax

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

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

2.1.  Summary of IRI Syntax

   IRIs are defined similarly to URIs in [RFC3986], but the class of
   unreserved characters is extended by adding the characters of the UCS
   (Universal Character Set, [ISO10646]) beyond U+007F, subject to the
   limitations given in the syntax rules below and in section 6.1.

   Otherwise, the syntax and use of components and reserved characters
   is the same as that in [RFC3986].  All the operations defined in
   [RFC3986], such as the resolution of relative references, can be
   applied to IRIs by IRI-processing software in exactly the same way as
   they are for URIs by URI-processing software.




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   Characters outside the US-ASCII repertoire are not reserved and
   therefore MUST NOT be used for syntactical purposes, such as to
   delimit components in newly defined schemes.  For example, U+00A2,
   CENT SIGN, is not allowed as a delimiter in IRIs, because it is in
   the 'iunreserved' category. This is similar to the fact that it is
   not possible to use '-' as a delimiter in URIs, because it is in the
   'unreserved' category.

2.2.  ABNF for IRI References and IRIs

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

   The following grammar closely follows the URI grammar in [RFC3986],
   except that the range of unreserved characters is expanded to include
   UCS characters, with the restriction that private UCS characters can
   occur only in query parts.  The grammar is split into two parts:
   Rules that differ from [RFC3986] because of the above-mentioned
   expansion, and rules that are the same as those in [RFC3986].  For
   rules that are different than those in [RFC3986], the names of the
   non-terminals have been changed as follows.  If the non-terminal
   contains 'URI', this has been changed to 'IRI'.  Otherwise, an 'i'
   has been prefixed.

   The following rules are different from those in [RFC3986]:

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

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

   IRI-reference  = IRI / irelative-ref

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

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

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



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                  / ipath-noscheme
                  / ipath-empty

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

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

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

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

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

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

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

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

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

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

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

   Some productions are ambiguous.  The "first-match-wins" (a.k.a.
   "greedy") algorithm applies.  For details, see [RFC3986].




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   The following rules are the same as those in [RFC3986]:

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

   port           = *DIGIT

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

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

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

   h16            = 1*4HEXDIG
   ls32           = ( h16 ":" h16 ) / IPv4address

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

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

   pct-encoded    = "%" HEXDIG HEXDIG

   unreserved     = ALPHA / DIGIT / "-" / "." / "_" / "~"
   reserved       = gen-delims / sub-delims
   gen-delims     = ":" / "/" / "?" / "#" / "[" / "]" / "@"
   sub-delims     = "!" / "$" / "&" / "'" / "(" / ")"
                  / "*" / "+" / "," / ";" / "="

   This syntax does not support IPv6 scoped addressing zone identifiers.











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3.  Relationship between IRIs and URIs

   IRIs are meant to replace URIs in identifying resources for
   protocols, formats, and software components that use a UCS-based
   character repertoire.  These protocols and components may never need
   to use URIs directly, especially when the resource identifier is used
   simply for identification purposes.  However, when the resource
   identifier is used for resource retrieval, it is in many cases
   necessary to determine the associated URI, because currently most
   retrieval mechanisms are only defined for URIs.  In this case, IRIs
   can serve as presentation elements for URI protocol elements.  An
   example would be an address bar in a Web user agent.  (Additional
   rationale is given in section 3.1.)

3.1.  Mapping of IRIs to URIs

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

   This mapping has two purposes:

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

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

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

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

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



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

            c. If the IRI is in a Unicode-based character encoding (for
               example, UTF-8 or UTF-16), do not normalize (see section
               5.3.2.2 for details).  Apply step 2 directly to the
               encoded Unicode character sequence.

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

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

       2.2.  Convert each octet to %HH, where HH is the hexadecimal
             notation of the octet value.  Note that this is identical
             to the percent-encoding mechanism in section 2.1 of
             [RFC3986].  To reduce variability, the hexadecimal notation
             SHOULD use uppercase letters.

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

   The above mapping from IRIs to URIs produces URIs fully conforming to
   [RFC3986].  The mapping is also an identity transformation for URIs
   and is idempotent;  applying the mapping a second time will not
   change anything.  Every URI is by definition an IRI.

   Systems accepting IRIs MAY convert the ireg-name component of an IRI
   as follows (before step 2 above) for schemes known to use domain
   names in ireg-name, if the scheme definition does not allow
   percent-encoding for ireg-name:

   Replace the ireg-name part of the IRI by the part converted using the
   ToASCII operation specified in section 4.1 of [RFC3490] on each
   dot-separated label, and by using U+002E (FULL STOP) as a label
   separator, with the flag UseSTD3ASCIIRules set to TRUE, and with the
   flag AllowUnassigned set to FALSE for creating IRIs and set to TRUE
   otherwise.










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   The ToASCII operation may fail, but this would mean that the IRI
   cannot be resolved.  This conversion SHOULD be used when the goal is
   to maximize interoperability with legacy URI resolvers.  For example,
   the IRI

   "http://r&#xE9;sum&#xE9;.example.org"

   may be converted to

   "http://xn--rsum-bpad.example.org"

   instead of

   "http://r%C3%A9sum%C3%A9.example.org".

   An IRI with a scheme that is known to use domain names in ireg-name,
   but where the scheme definition does not allow percent-encoding for
   ireg-name, meets scheme-specific restrictions if either the
   straightforward conversion or the conversion using the ToASCII
   operation on ireg-name result in an URI that meets the scheme-
   specific restrictions.

   Such an IRI resolves to the URI obtained after converting the IRI and
   uses the ToASCII operation on ireg-name.  Implementations do not have
   to do this conversion as long as they produce the same result.

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

   Note: The uniform treatment of the whole IRI in step 2 is important
      to make processing independent of URI scheme.  See [Gettys] for an
      in-depth discussion.

   Note: In practice, whether the general mapping (steps 1 and 2) or the
      ToASCII operation of [RFC3490] is used for ireg-name will not be
      noticed if mapping from IRI to URI and resolution is tightly
      integrated (e.g., carried out in the same user agent).  But





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      conversion using [RFC3490] may be able to better deal with
      backwards compatibility issues in case mapping and resolution are
      separated, as in the case of using an HTTP proxy.

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

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

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

   Note: Some older software transcoding to UTF-8 may produce illegal
      output for some input, in particular for characters outside the
      BMP (Basic Multilingual Plane).  As an example, for the IRI with
      non-BMP characters (in XML Notation):
      "http://example.com/&#x10300;&#x10301;&#x10302";



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      which contains the first three letters of the Old Italic alphabet,
      the correct conversion to a URI is
      "http://example.com/%F0%90%8C%80%F0%90%8C%81%F0%90%8C%82"

3.2.  Converting URIs to IRIs

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

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

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

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

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

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

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

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

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

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





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

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

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

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

3.2.1.  Examples

   This section shows various examples of converting URIs to IRIs.  Each
   example shows the result after each of the steps 1 through 5 is
   applied.  XML Notation is used for the final result.  Octets are
   denoted by "<" followed by two hexadecimal digits followed by ">".

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

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

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

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

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

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

   The following example contains the sequence "%FC", which might
   represent U+00FC, LATIN SMALL LETTER U WITH DIAERESIS, in the
   iso-8859-1 character encoding.  (It might represent other characters
   in other character encodings.  For example, the octet <fc> in



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   iso-8859-5 represents U+045C, CYRILLIC SMALL LETTER KJE.)  Because
   <fc> is not part of a strictly legal UTF-8 sequence, it is
   re-percent-encoded in step 3.

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

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

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

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

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

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

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

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

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

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

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

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

4.  Bidirectional IRIs for Right-to-Left Languages

   Some UCS characters, such as those used in the Arabic and Hebrew
   scripts, have an inherent right-to-left (rtl) writing direction.
   IRIs containing these characters (called bidirectional IRIs or Bidi
   IRIs) require additional attention because of the non-trivial





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   relation between logical representation (used for digital
   representation and for reading/spelling) and visual representation
   (used for display/printing).

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

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

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

   3.  minor or no changes or restrictions for implementations.

4.1.  Logical Storage and Visual Presentation

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

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

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






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

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

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

4.2.  Bidi IRI Structure

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

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

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





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   For each component, the following restrictions apply:

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

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

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

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

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

4.3.  Input of Bidi IRIs

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

4.4.  Examples

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





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

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

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

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

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

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

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



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

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

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

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

5.  Normalization and Comparison

      Note: The structure and much of the material for this section is
      taken from section 6 of [RFC3986]; the differences are due to the
      specifics of IRIs.

   One of the most common operations on IRIs is simple comparison:
   Determining whether two IRIs are equivalent without using the IRIs or
   the mapped URIs to access their respective resource(s).  A comparison
   is performed whenever a response cache is accessed, a browser checks
   its history to color a link, or an XML parser processes tags within a
   namespace.  Extensive normalization prior to comparison of IRIs may
   be used by spiders and indexing engines to prune a search space or
   reduce duplication of request actions and response storage.






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

5.1.  Equivalence

   Because IRIs exist to identify resources, presumably they should be
   considered equivalent when they identify the same resource.  However,
   this definition of equivalence is not of much practical use, as there
   is no way for an implementation to compare two resources unless it
   has full knowledge or control of them. For this reason, determination
   of equivalence or difference of IRIs is based on string comparison,
   perhaps augmented by reference to additional rules provided by URI
   scheme definitions.  We use the terms "different" and "equivalent" to
   describe the possible outcomes of such comparisons, but there are
   many application-dependent versions of equivalence.

   Even though it is possible to determine that two IRIs are equivalent,
   IRI comparison is not sufficient to determine whether two IRIs
   identify different resources.  For example, an owner of two different
   domain names could decide to serve the same resource from both,
   resulting in two different IRIs.  Therefore, comparison methods are
   designed to minimize false negatives while strictly avoiding false
   positives.

   In testing for equivalence, applications should not directly compare
   relative references; the references should be converted to their
   respective target IRIs before comparison.  When IRIs are compared to
   select (or avoid) a network action, such as retrieval of a
   representation, fragment components (if any) should be excluded from
   the comparison.

   Applications using IRIs as identity tokens with no relationship to a
   protocol MUST use the Simple String Comparison (see section 5.3.1).
   All other applications MUST select one of the comparison practices
   from the Comparison Ladder (see section 5.3 or, after IRI-to-URI
   conversion, select one of the comparison practices from the URI
   comparison ladder in [RFC3986], section 6.2)

5.2.  Preparation for Comparison

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



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   escapings or encodings are entities and numeric character references
   in [HTML4] and [XML1].  As an example,
   "http://example.org/ros&eacute;" (in HTML),
   "http://example.org/ros&#233"; (in HTML or XML), and
   "http://example.org/ros&#xE9"; (in HTML or XML) are all resolved into
   what is denoted in this document (see section 1.4) as
   "http://example.org/ros&#xE9"; (the "&#xE9;" here standing for the
   actual e-acute character, to compensate for the fact that this
   document cannot contain non-ASCII characters).

   Similar considerations apply to encodings such as Transfer Codings in
   HTTP (see [RFC2616]) and Content Transfer Encodings in MIME
   ([RFC2045]), although in these cases, the encoding is based not on
   characters but on octets, and additional care is required to make
   sure that characters, and not just arbitrary octets, are compared
   (see section 5.3.1).

5.3.  Comparison Ladder

   In practice, a variety of methods are used, to test IRI equivalence.
   These methods fall into a range distinguished by the amount of
   processing required and the degree to which the probability of false
   negatives is reduced.  As noted above, false negatives cannot be
   eliminated.  In practice, their probability can be reduced, but this
   reduction requires more processing and is not cost-effective for all
   applications.

   If this range of comparison practices is considered as a ladder, the
   following discussion will climb the ladder, starting with practices
   that are cheap but have a relatively higher chance of producing false
   negatives, and proceeding to those that have higher computational
   cost and lower risk of false negatives.

5.3.1.  Simple String Comparison

   If two IRIs, when considered as character strings, are identical,
   then it is safe to conclude that they are equivalent.  This type of
   equivalence test has very low computational cost and is in wide use
   in a variety of applications, particularly in the domain of parsing.
   It is also used when a definitive answer to the question of IRI
   equivalence is needed that is independent of the scheme used and that
   can be calculated quickly and without accessing a network.  An
   example of such a case is XML Namespaces ([XMLNamespace]).

   Testing strings for equivalence requires some basic precautions. This
   procedure is often referred to as "bit-for-bit" or "byte-for-byte"
   comparison, which is potentially misleading.  Testing strings for
   equality is normally based on pair comparison of the characters that



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   make up the strings, starting from the first and proceeding until
   both strings are exhausted and all characters are found to be equal,
   until a pair of characters compares unequal, or until one of the
   strings is exhausted before the other.

   This character comparison requires that each pair of characters be
   put in comparable encoding form.  For example, should one IRI be
   stored in a byte array in UTF-8 encoding form and the second in a
   UTF-16 encoding form, bit-for-bit comparisons applied naively will
   produce errors.  It is better to speak of equality on a
   character-for-character rather than on a byte-for-byte or bit-for-bit
   basis.  In practical terms, character-by-character comparisons should
   be done codepoint by codepoint after conversion to a common character
   encoding form.  When comparing character by character, the comparison
   function MUST NOT map IRIs to URIs, because such a mapping would
   create additional spurious equivalences.  It follows that an IRI
   SHOULD NOT be modified when being transported if there is any chance
   that this IRI might be used as an identifier.

   False negatives are caused by the production and use of IRI aliases.
   Unnecessary aliases can be reduced, regardless of the comparison
   method, by consistently providing IRI references in an already
   normalized form (i.e., a form identical to what would be produced
   after normalization is applied, as described below). Protocols and
   data formats often limit some IRI comparisons to simple string
   comparison, based on the theory that people and implementations will,
   in their own best interest, be consistent in providing IRI
   references, or at least be consistent enough to negate any efficiency
   that might be obtained from further normalization.

5.3.2.  Syntax-Based Normalization

   Implementations may use logic based on the definitions provided by
   this specification to reduce the probability of false negatives. This
   processing is moderately higher in cost than character-for-character
   string comparison.  For example, an application using this approach
   could reasonably consider the following two IRIs equivalent:

      example://a/b/c/%7Bfoo%7D/ros&#xE9;
      eXAMPLE://a/./b/../b/%63/%7bfoo%7d/ros%C3%A9

   Web user agents, such as browsers, typically apply this type of IRI
   normalization when determining whether a cached response is
   available.  Syntax-based normalization includes such techniques as
   case normalization, character normalization, percent-encoding
   normalization, and removal of dot-segments.





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5.3.2.1.  Case Normalization

   For all IRIs, the hexadecimal digits within a percent-encoding
   triplet (e.g., "%3a" versus "%3A") are case-insensitive and therefore
   should be normalized to use uppercase letters for the digits A - F.

   When an IRI uses components of the generic syntax, the component
   syntax equivalence rules always apply; namely, that the scheme and
   US-ASCII only host are case insensitive and therefore should be
   normalized to lowercase.  For example, the URI
   "HTTP://www.EXAMPLE.com/" is equivalent to "http://www.example.com/".
   Case equivalence for non-ASCII characters in IRI components that are
   IDNs are discussed in section 5.3.3.  The other generic syntax
   components are assumed to be case sensitive unless specifically
   defined otherwise by the scheme.

   Creating schemes that allow case-insensitive syntax components
   containing non-ASCII characters should be avoided. Case normalization
   of non-ASCII characters can be culturally dependent and is always a
   complex operation.  The only exception concerns non-ASCII host names
   for which the character normalization includes a mapping step derived
   from case folding.

5.3.2.2.  Character Normalization

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

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

   As an example, "http://www.example.org/r&#xE9;sum&#xE9;.html" (in XML
   Notation) is in NFC.  On the other hand,
   "http://www.example.org/re&#x301;sume&#x301;.html" is not in NFC.



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   The former uses precombined e-acute characters, and the latter uses
   "e" characters followed by combining acute accents.  Both usages are
   defined as canonically equivalent in [UNIV4].

   Note: Because it is unknown how a particular sequence of characters
      is being treated with respect to character normalization, it would
      be inappropriate to allow third parties to normalize an IRI
      arbitrarily.  This does not contradict the recommendation that
      when a resource is created, its IRI should be as character
      normalized as possible (i.e., NFC or even NFKC).  This is similar
      to the uppercase/lowercase problems.  Some parts of a URI are case
      insensitive (domain name).  For others, it is unclear whether they
      are case sensitive, case insensitive, or something in between
      (e.g., case sensitive, but with a multiple choice selection if the
      wrong case is used, instead of a direct negative result).  The
      best recipe is that the creator use a reasonable capitalization
      and, when transferring the URI, capitalization never be changed.

   Various IRI schemes may allow the usage of Internationalized Domain
   Names (IDN) [RFC3490] either in the ireg-name part or elsewhere.
   Character Normalization also applies to IDNs, as discussed in section
   5.3.3.

5.3.2.3.  Percent-Encoding Normalization

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

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

   If this kind of equivalence is to be tested, the percent-encoding of
   both IRIs to be compared has to be aligned; for example, by
   converting both IRIs to URIs (see section 3.1), eliminating escape
   differences in the resulting URIs, and making sure that the case of
   the hexadecimal characters in the percent-encoding is always the same
   (preferably uppercase).  If the IRI is to be passed to another





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

5.3.2.4.  Path Segment Normalization

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

5.3.3.  Scheme-Based Normalization

   The syntax and semantics of IRIs vary from scheme to scheme, as
   described by the defining specification for each scheme.
   Implementations may use scheme-specific rules, at further processing
   cost, to reduce the probability of false negatives.  For example,
   because the "http" scheme makes use of an authority component, has a
   default port of "80", and defines an empty path to be equivalent to
   "/", the following four IRIs are equivalent:

      http://example.com
      http://example.com/
      http://example.com:/
      http://example.com:80/

   In general, an IRI that uses the generic syntax for authority with an
   empty path should be normalized to a path of "/".  Likewise, an
   explicit ":port", for which the port is empty or the default for the
   scheme, is equivalent to one where the port and its ":" delimiter are
   elided and thus should be removed by scheme-based normalization.  For
   example, the second IRI above is the normal form for the "http"
   scheme.

   Another case where normalization varies by scheme is in the handling
   of an empty authority component or empty host subcomponent.  For many
   scheme specifications, an empty authority or host is considered an
   error; for others, it is considered equivalent to "localhost" or the
   end-user's host.  When a scheme defines a default for authority and
   an IRI reference to that default is desired, the reference should be
   normalized to an empty authority for the sake of uniformity, brevity,





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   and internationalization.  If, however, either the userinfo or port
   subcomponents are non-empty, then the host should be given explicitly
   even if it matches the default.

   Normalization should not remove delimiters when their associated
   component is empty unless it is licensed to do so by the scheme
   specification.  For example, the IRI "http://example.com/?" cannot be
   assumed to be equivalent to any of the examples above.  Likewise, the
   presence or absence of delimiters within a userinfo subcomponent is
   usually significant to its interpretation.  The fragment component is
   not subject to any scheme-based normalization; thus, two IRIs that
   differ only by the suffix "#" are considered different regardless of
   the scheme.

   Some IRI schemes may allow the usage of Internationalized Domain
   Names (IDN) [RFC3490] either in their ireg-name part or elsewhere.
   When in use in IRIs, those names SHOULD be validated by using the
   ToASCII operation defined in [RFC3490], with the flags
   "UseSTD3ASCIIRules" and "AllowUnassigned".  An IRI containing an
   invalid IDN cannot successfully be resolved.  Validated IDN
   components of IRIs SHOULD be character normalized by using the
   Nameprep process [RFC3491]; however, for legibility purposes, they
   SHOULD NOT be converted into ASCII Compatible Encoding (ACE).

   Scheme-based normalization may also consider IDN components and their
   conversions to punycode as equivalent.  As an example,
   "http://r&#xE9;sum&#xE9;.example.org" may be considered equivalent to
   "http://xn--rsum-bpad.example.org".

   Other scheme-specific normalizations are possible.

5.3.4.  Protocol-Based Normalization

   Substantial effort to reduce the incidence of false negatives is
   often cost-effective for web spiders. Consequently, they implement
   even more aggressive techniques in IRI comparison.  For example, if
   they observe that an IRI such as

      http://example.com/data

   redirects to an IRI differing only in the trailing slash

      http://example.com/data/

   they will likely regard the two as equivalent in the future.  This
   kind of technique is only appropriate when equivalence is clearly
   indicated by both the result of accessing the resources and the




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

6.  Use of IRIs

6.1.  Limitations on UCS Characters Allowed in IRIs

   This section discusses limitations on characters and character
   sequences usable for IRIs beyond those given in section 2.2 and
   section 4.1.  The considerations in this section are relevant when
   IRIs are created and when URIs are converted to IRIs.

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

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

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

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

6.2.  Software Interfaces and Protocols

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

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





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6.3.  Format of URIs and IRIs in Documents and Protocols

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

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

6.4.  Use of UTF-8 for Encoding Original Characters

   This section discusses details and gives examples for point c) in
   section 1.2.  To be able to use IRIs, the URI corresponding to the
   IRI in question has to encode original characters into octets by
   using UTF-8.  This can be specified for all URIs of a URI scheme or
   can apply to individual URIs for schemes that do not specify how to
   encode original characters.  It can apply to the whole URI, or only
   to some part.  For background information on encoding characters into
   URIs, see also section 2.5 of [RFC3986].

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

   For example, for a document with a URI of
   "http://www.example.org/r%C3%A9sum%C3%A9.html", it is possible to
   construct a corresponding IRI (in XML notation, see, section 1.4):
   "http://www.example.org/r&#xE9;sum&#xE9;.html" ("&#xE9"; stands for
   the e-acute character, and "%C3%A9" is the UTF-8 encoded and
   percent-encoded representation of that character).  On the other
   hand, for a document with a URI of




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   "http://www.example.org/r%E9sum%E9.html", the percent-encoding octets
   cannot be converted to actual characters in an IRI, as the
   percent-encoding is not based on UTF-8.

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

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

   URI schemes can impose restrictions on the syntax of scheme-specific
   URIs; i.e., URIs that are admissible under the generic URI syntax
   [RFC3986] may not be admissible due to narrower syntactic constraints
   imposed by a URI scheme specification.  URI scheme definitions cannot
   broaden the syntactic restrictions of the generic URI syntax;
   otherwise, it would be possible to generate URIs that satisfied the
   scheme-specific syntactic constraints without satisfying the
   syntactic constraints of the generic URI syntax.  However, additional
   syntactic constraints imposed by URI scheme specifications are
   applicable to IRI, as the corresponding URI resulting from the
   mapping defined in section 3.1 MUST be a valid URI under the
   syntactic restrictions of generic URI syntax and any narrower
   restrictions imposed by the corresponding URI scheme specification.

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

   For example, it is possible to have a URI reference of
   "http://www.example.org/r%E9sum%E9.xml#r%C3%A9sum%C3%A9", where the
   document name is encoded in iso-8859-1 based on server settings, but
   where the fragment identifier is encoded in UTF-8 according to




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   [XPointer]. The IRI corresponding to the above URI would be (in XML
   notation)
   "http://www.example.org/r%E9sum%E9.xml#r&#xE9;sum&#xE9";.

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

6.5.  Relative IRI References

   Processing of relative IRI references against a base is handled
   straightforwardly; the algorithms of [RFC3986] can be applied
   directly, treating the characters additionally allowed in IRI
   references in the same way that unreserved characters are in URI
   references.

7.  URI/IRI Processing Guidelines (Informative)

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

7.1.  URI/IRI Software Interfaces

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

   In case the current handling in an API or protocol is based on
   US-ASCII, UTF-8 is recommended as the character encoding for IRIs, as
   it is compatible with US-ASCII, is in accordance with the
   recommendations of [RFC2277], and makes converting to URIs easy.  In
   any case, the API or protocol definition must clearly define the
   character encoding to be used.

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






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7.2.  URI/IRI Entry

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

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

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

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

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

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

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

7.3.  URI/IRI Transfer between Applications

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






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

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

7.4.  URI/IRI Generation

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

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

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

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

7.5.  URI/IRI Selection

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







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

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

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

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

7.6.  Display of URIs/IRIs

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

   For display of Bidi IRIs, please see section 4.1.







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7.7.  Interpretation of URIs and IRIs

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

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

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

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

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

7.8.  Upgrading Strategy

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

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

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




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

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

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

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

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

8.  Security Considerations

   The security considerations discussed in [RFC3986] also apply to
   IRIs.  In addition, the following issues require particular care for
   IRIs.

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








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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 that points to a different resource.
   The added resource may pretend to be the real resource by looking
   very similar but may contain all kinds of changes that may be
   difficult to spot and that can cause all kinds of problems.  Most
   spoofing possibilities for IRIs are extensions of those for URIs.

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

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

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

   Spoofing can occur when URIs with percent-encodings based on various
   character encodings are accepted to deal with older user agents.  In
   some cases, particularly for Latin-based resource names, this is
   usually easy to detect because UTF-8-encoded names, when interpreted
   and viewed as legacy character encodings, produce mostly garbage.





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   When concurrently used character encodings have a similar structure
   but there are no characters that have exactly the same encoding,
   detection is more difficult.

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

9.  Acknowledgements

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

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

   Many thanks go to Francois Yergeau, Matitiahu Allouche, Roy Fielding,
   Tim Berners-Lee, Mark Davis, M.T. Carrasco Benitez, James Clark, Tim
   Bray, Chris Wendt, Yaron Goland, Andrea Vine, Misha Wolf, Leslie
   Daigle, Ted Hardie, Bill Fenner, Margaret Wasserman, Russ Housley,
   Makoto MURATA, Steven Atkin, Ryan Stansifer, Tex Texin, Graham Klyne,
   Bjoern Hoehrmann, Chris Lilley, Ian Jacobs, Adam Costello, Dan
   Oscarson, Elliotte Rusty Harold, Mike J. Brown, Roy Badami, Jonathan
   Rosenne, Asmus Freytag, Simon Josefsson, Carlos Viegas Damasio, Chris
   Haynes, Walter Underwood, and many others for help with understanding
   the issues and possible solutions, and with getting the details
   right.

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










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10.  References

10.1.  Normative References

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

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

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

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

   [RFC3490]      Faltstrom, P., Hoffman, P., and A. Costello,
                  "Internationalizing Domain Names in Applications
                  (IDNA)", RFC 3490, March 2003.

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

   [RFC3629]      Yergeau, F., "UTF-8, a transformation format of ISO
                  10646", STD 63, RFC 3629, November 2003.

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

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

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







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

10.2.  Informative References

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

   [CharMod]      Duerst, M., Yergeau, F., Ishida, R., Wolf, M., and T.
                  Texin, "Character Model for the World Wide Web:
                  Resource Identifiers", World Wide Web Consortium
                  Candidate Recommendation, November 2004,
                  <http://www.w3.org/TR/charmod-resid>.

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

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

   [HTML4]        Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01
                  Specification", World Wide Web Consortium
                  Recommendation, December 1999,
                  <http://www.w3.org/TR/html401/appendix/
                  notes.html#h-B.2>.

   [RFC2045]      Freed, N. and N. Borenstein, "Multipurpose Internet
                  Mail Extensions (MIME) Part One: Format of Internet
                  Message Bodies", RFC 2045, November 1996.

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

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

   [RFC2192]      Newman, C., "IMAP URL Scheme", RFC 2192, September
                  1997.

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



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   [RFC2368]      Hoffman, P., Masinter, L., and J. Zawinski, "The
                  mailto URL scheme", RFC 2368, July 1998.

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

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

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

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

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

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

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

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

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

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

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




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   [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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Appendix A.  Design Alternatives

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

Appendix A.1.  New Scheme(s)

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

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

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

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

   Using UTF-8 avoids a double layering and overloading of the use of
   the "+" character.  UTF-8 is fully compatible with US-ASCII and has
   therefore been recommended by the IETF, and is being used widely.

   UTF-7 has never been used much and is now clearly being discouraged.
   Requiring implementations to convert from UTF-8 to UTF-7 and back
   would be an additional implementation burden.

Appendix A.3.  New Encoding Convention

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






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

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

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

   If UTF-8 is used exclusively, an upgrade to the URI syntax is not
   needed.  It avoids potentially multiple labels that have to be copied
   correctly in all cases, even on the side of a bus or on a napkin,
   leading to usability problems (and being prohibitively annoying).
   Exclusively using UTF-8 also reduces transcoding errors and
   confusion.

Authors' Addresses

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

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


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

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





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Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.






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