Internationalized Resource                                     M. Duerst
Identifiers (iri)                               Aoyama Gakuin University
Internet-Draft                                               M. Suignard
Obsoletes: RFC 3987                                   Unicode Consortium
(if approved)                                                L. Masinter
Intended status: Standards Track                                   Adobe
Expires: August 2, 2010                                 January 29, 2010

             Internationalized Resource Identifiers (IRIs)


   This document defines the Internationalized Resource Identifier (IRI)
   protocol element, as an extension of the Uniform Resource Identifier
   (URI).  An IRI is a sequence of characters from the Universal
   Character Set (Unicode/ISO 10646).  Grammar and processing rules are
   given for IRIs and related syntactic forms.

   In addition, this document provides named additional rule sets for
   processing otherwise invalid IRIs, in a way that supports other
   specifications that wish to mandate common behavior for 'error'
   handling.  In particular, rules used in some XML languages (LEIRI)
   and web applications are given.

   Defining IRI as new protocol element (rather than updating or
   extending the definition of URI) allows independent orderly
   transitions: other protocols and languages that use URIs must
   explicitly choose to allow IRIs.

   Guidelines are provided for the use and deployment of IRIs and
   related protocol elements when revising protocols, formats, and
   software components that currently deal only with URIs.

   [RFC Editor: Please remove this paragraph before publication.]  This
   document is intended to update RFC 3987 and move towards IETF Draft
   Standard.  This version is essentially identical to
   draft-duerst-iri-bis-07.txt, and is submitted as an initial draft to
   start WG discussions.  For discussion and comments on this draft,
   please join the IETF IRI WG by subscribing to the mailing list

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Overview and Motivation  . . . . . . . . . . . . . . . . .  5
     1.2.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  6
     1.3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  6
     1.4.  Notation . . . . . . . . . . . . . . . . . . . . . . . . .  9
   2.  IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     2.1.  Summary of IRI Syntax  . . . . . . . . . . . . . . . . . . 10
     2.2.  ABNF for IRI References and IRIs . . . . . . . . . . . . . 10
   3.  Processing IRIs and related protocol elements  . . . . . . . . 13
     3.1.  Converting to UCS  . . . . . . . . . . . . . . . . . . . . 14
     3.2.  Parse the IRI into IRI components  . . . . . . . . . . . . 14
     3.3.  General percent-encoding of IRI components . . . . . . . . 15
     3.4.  Mapping ireg-name  . . . . . . . . . . . . . . . . . . . . 15
     3.5.  Mapping query components . . . . . . . . . . . . . . . . . 17
     3.6.  Mapping IRIs to URIs . . . . . . . . . . . . . . . . . . . 17
     3.7.  Converting URIs to IRIs  . . . . . . . . . . . . . . . . . 17
       3.7.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . 19
   4.  Bidirectional IRIs for Right-to-Left Languages . . . . . . . . 20
     4.1.  Logical Storage and Visual Presentation  . . . . . . . . . 21
     4.2.  Bidi IRI Structure . . . . . . . . . . . . . . . . . . . . 22
     4.3.  Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . . 23
     4.4.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 23
   5.  Normalization and Comparison . . . . . . . . . . . . . . . . . 25
     5.1.  Equivalence  . . . . . . . . . . . . . . . . . . . . . . . 25
     5.2.  Preparation for Comparison . . . . . . . . . . . . . . . . 26
     5.3.  Comparison Ladder  . . . . . . . . . . . . . . . . . . . . 27
       5.3.1.  Simple String Comparison . . . . . . . . . . . . . . . 27
       5.3.2.  Syntax-Based Normalization . . . . . . . . . . . . . . 28
       5.3.3.  Scheme-Based Normalization . . . . . . . . . . . . . . 31
       5.3.4.  Protocol-Based Normalization . . . . . . . . . . . . . 32
   6.  Use of IRIs  . . . . . . . . . . . . . . . . . . . . . . . . . 33
     6.1.  Limitations on UCS Characters Allowed in IRIs  . . . . . . 33
     6.2.  Software Interfaces and Protocols  . . . . . . . . . . . . 33
     6.3.  Format of URIs and IRIs in Documents and Protocols . . . . 33
     6.4.  Use of UTF-8 for Encoding Original Characters  . . . . . . 34
     6.5.  Relative IRI References  . . . . . . . . . . . . . . . . . 36
   7.  Liberal handling of otherwise invalid IRIs . . . . . . . . . . 36
     7.1.  LEIRI processing . . . . . . . . . . . . . . . . . . . . . 36
     7.2.  Web Address processing . . . . . . . . . . . . . . . . . . 36
     7.3.  Characters not allowed in IRIs . . . . . . . . . . . . . . 38
   8.  URI/IRI Processing Guidelines (Informative)  . . . . . . . . . 40
     8.1.  URI/IRI Software Interfaces  . . . . . . . . . . . . . . . 40
     8.2.  URI/IRI Entry  . . . . . . . . . . . . . . . . . . . . . . 41
     8.3.  URI/IRI Transfer between Applications  . . . . . . . . . . 42
     8.4.  URI/IRI Generation . . . . . . . . . . . . . . . . . . . . 42
     8.5.  URI/IRI Selection  . . . . . . . . . . . . . . . . . . . . 43

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     8.6.  Display of URIs/IRIs . . . . . . . . . . . . . . . . . . . 43
     8.7.  Interpretation of URIs and IRIs  . . . . . . . . . . . . . 44
     8.8.  Upgrading Strategy . . . . . . . . . . . . . . . . . . . . 44
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 45
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 46
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 47
   12. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 48
   13. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 50
     13.1. Changes from draft-duerst-iri-bis-07 to
           draft-ietf-iri-3987bis-00  . . . . . . . . . . . . . . . . 50
     13.2. Changes from -06 to -07 of draft-duerst-iri-bis  . . . . . 50
       13.2.1. OLD WAY  . . . . . . . . . . . . . . . . . . . . . . . 50
       13.2.2. NEW WAY  . . . . . . . . . . . . . . . . . . . . . . . 51
     13.3. Changes from -05 to -06 of draft-duerst-iri-bis  . . . . . 51
     13.4. Changes from -04 to -05 of draft-duerst-iri-bis  . . . . . 51
     13.5. Changes from -03 to -04 of draft-duerst-iri-bis  . . . . . 51
     13.6. Changes from -02 to -03 of draft-duerst-iri-bis  . . . . . 52
     13.7. Changes from -01 to -02 of draft-duerst-iri-bis  . . . . . 52
     13.8. Changes from -00 to -01 of draft-duerst-iri-bis  . . . . . 52
     13.9. Changes from RFC 3987 to -00 of draft-duerst-iri-bis . . . 52
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 52
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 52
     14.2. Informative References . . . . . . . . . . . . . . . . . . 53
   Appendix A.  Design Alternatives . . . . . . . . . . . . . . . . . 56
     A.1.  New Scheme(s)  . . . . . . . . . . . . . . . . . . . . . . 56
     A.2.  Character Encodings Other Than UTF-8 . . . . . . . . . . . 56
     A.3.  New Encoding Convention  . . . . . . . . . . . . . . . . . 56
     A.4.  Indicating Character Encodings in the URI/IRI  . . . . . . 57
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 57

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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 difficulties.

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

   URIs are used both as a protocol element (for transmission and
   processing by software) and also a presentation element (for display
   and handling by people who read, interpret, coin, or guess them).
   The transition between these roles is more difficult and complex when
   dealing with the larger set of characters than allowed for URIs in

   This document defines the protocol element called Internationalized
   Resource Identifier (IRI), which allow applications of URIs to be
   extended to use resource identifiers that have a much wider
   repertoire of characters.  It also provides corresponding
   "internationalized" versions of other constructs from [RFC3986], such
   as URI references.  The syntax of IRIs is defined in Section 2.

   Using characters outside of A - Z in IRIs adds a number of
   difficulties.  Section 4 discusses the special case of bidirectional
   IRIs using characters from scripts written right-to-left.  Section 5
   discusses various forms of equivalence between IRIs.  Section 6
   discusses the use of IRIs in different situations.  Section 8 gives
   additional informative guidelines.  Section 10 discusses IRI-specific
   security considerations.

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1.2.  Applicability

   IRIs are designed to allow protocols and software that deal with URIs
   to be updated to handle IRIs.  A "URI scheme" (as defined by
   [RFC3986] and registered through the IANA process defined in
   [RFC4395] also serves as an "IRI scheme".  Processing of IRIs is
   accomplished by extending the URI syntax while retaining (and not
   expanding) the set of "reserved" characters, such that the syntax for
   any URI scheme may be uniformly extended to allow non-ASCII
   characters.  In addition, following parsing of an IRI, it is possible
   to construct a corresponding URI by first encoding characters outside
   of the allowed URI range and then reassembling the components.

   Practical use of IRIs forms in place of URIs forms depends on the
   following conditions being met:

   a. A protocol or format element MUST be explicitly designated to be
      able to carry IRIs.  The intent is to avoid introducing 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 [RFC2616] definition of HTTP/1.1, 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 MUST 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 scheme definition, if it explicitly allows a percent sign
      ("%") in any syntactic component, SHOULD define the interpretation
      of sequences of percent-encoded octets (using "%XX" hex octets) as
      octet from sequences of UTF-8 encoded strings; this is recommended
      in the guidelines for registering new schemes, [RFC4395].  For
      example, this is the practice for IMAP URLs [RFC2192], POP URLs
      [RFC2384] and the URN syntax [RFC2141]).  Note that use of
      percent-encoding may also be restricted in some situations, for
      example, URI schemes that disallow percent-encoding might still be
      used with a fragment identifier which is percent-encoded (e.g.,
      [XPointer]).  See Section 6.4 for further discussion.

1.3.  Definitions

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

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   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 (set in the mathematical

   sequence of characters:  A sequence of characters (one after

   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.

   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.

   URL:  The term "URL" was originally used [RFC1738] for roughly what
      is now called a "URI".  Books, software and documentation often
      refers to URIs and IRIs using the "URL" term.  Some usages
      restrict "URL" to those URIs which are not URNs.  Because of the
      ambiguity of the term using the term "URL" is NOT RECOMMENDED in
      formal documents.

   LEIRI (Legacy Extended IRI) processing:  This term was used in
      various XML specifications to refer to strings that, although not
      valid IRIs, were acceptable input to the processing rules in
      Section 7.1.

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   (Web Address, Hypertext Reference, HREF):  These terms have been
      added in this document for convenience, to allow other
      specifications to refer to those strings that, although not valid
      IRIs, are acceptable input to the processing rules in Section 7.2.
      This usage corresponds to the parsing rules of some popular web
      browsing applications.  ISSUE: Need to find a good name/
      abbreviation for these.

   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 identifier is generated by derivation from other

   parsed URI component:  When a URI processor parses a URI (following
      the generic syntax or a scheme-specific syntax, the result is a
      set of parsed URI components, each of which has a type
      (corresponding to the syntactic definition) and a sequence of URI

   parsed IRI component:  When an IRI processor parses an IRI directly,
      following the general syntax or a scheme-specific syntax, the
      result is a set of parsed IRI components, each of which has a type
      (corresponding to the syntactice definition) and a sequence of IRI
      characters.  (This definition is analogous to "parsed URI

   IRI scheme:  A URI scheme may also be known as an "IRI scheme" if the
      scheme's syntax has been extended to allow non-US-ASCII characters
      according to the rules in this document.

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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'.

   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: Lower case letters
   stand for Latin letters or other letters that are written left to
   right, whereas upper case letters represent Arabic or Hebrew letters
   that are written right to left.

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

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   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 might 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.

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2.1.  Summary of IRI Syntax

   IRIs are defined by extending the URI syntax in [RFC3986], but
   extending the class of unreserved characters 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.

   The syntax and use of components and reserved characters is the same
   as that in [RFC3986].  Each "URI scheme" thus also functions as an
   "IRI scheme", in that scheme-specific parsing rules for URIs of a
   scheme are be extended to allow parsing of IRIs using the same
   parsing rules.

   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.

   Characters outside the US-ASCII repertoire MUST NOT be 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

   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 [STD68].  Character numbers are
   taken from the UCS, without implying any actual binary encoding.
   Terminals in the ABNF are characters, not octets.

   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]:

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

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

   IRI-reference  = IRI / irelative-ref

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

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

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

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

   pct-form       = pct-encoded

   ireg-name      = *( iunreserved / 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  = *( path-sep isegment )
   ipath-absolute = path-sep [ isegment-nz *( path-sep isegment ) ]
   ipath-noscheme = isegment-nz-nc *( path-sep isegment )
   ipath-rootless = isegment-nz *( path-sep isegment )
   ipath-empty    = 0<ipchar>
   path-sep       = "/"

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

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

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   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 / %xE0000-E0FFF / %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.

3.  Processing IRIs and related protocol elements

   IRIs are meant to replace URIs in identifying resources within new
   versions of protocols, formats, and software components that use a
   UCS-based character repertoire.  Protocols and components may use and
   process IRIs directly.  However, there are still numerous systems and

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   protocols which only accept URIs or components of parsed URIs; that
   is, they only accept sequences of characters within the subset of US-
   ASCII characters allowed in URIs.

   This section defines specific processing steps for IRI consumers
   which establish the relationship between the string given and the
   interpreted derivatives.  These processing steps apply to both IRIs
   and IRI references (i.e., absolute or relative forms); for IRIs, some
   steps are scheme specific.

3.1.  Converting to UCS

   Input that is already in a Unicode form (i.e., a sequence of Unicode
   characters or an octet-stream representing a Unicode-based character
   encoding such as UTF-8 or UTF-16) should be left as is and not
   normalized (see (see Section

   If the IRI or IRI reference is an octet stream in some known non-
   Unicode character encoding, convert the IRI to a sequence of
   characters from the UCS; this sequence SHOULD also be normalized
   according to Unicode Normalization Form C (NFC, [UTR15]).  In this
   case, retain the original character encoding as the "document
   DO, CHANGE? )

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

3.2.  Parse the IRI into IRI components

   Parse the IRI, either as a relative reference (no scheme) or using
   scheme specific processing (according to the scheme given); the
   result resulting in a set of parsed IRI components.  (NOTE: FIX

   NOTE: The result of parsing into components will correspond result in
   a correspondence of subtrings of the IRI according to the part
   matched.  For example, in [HTML5], the protocol components of
   interest are SCHEME (scheme), HOST (ireg-name), PORT (port), the PATH
   (ipath after the initial "/"), QUERY (iquery), FRAGMENT (ifragment),
   and AUTHORITY (iauthority).

   Subsequent processing rules are sometimes used to define other
   syntactic components.  For example, [HTML5] defines APIs for IRI

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   processing; in these APIs:

   HOSTSPECIFIC  the substring that follows the substring matched by the
      iauthority production, or the whole string if the iauthority
      production wasn't matched.

   HOSTPORT  if there is a scheme component and a port component and the
      port given by the port component is different than the default
      port defined for the protocol given by the scheme component, then
      HOSTPORT is the substring that starts with the substring matched
      by the host production and ends with the substring matched by the
      port production, and includes the colon in between the two.
      Otherwise, it is the same as the host component.

3.3.  General percent-encoding of IRI components

   For most IRI components, it is possible to map the IRI component to
   an equivalent URI component by percent-encoding those characters not
   allowed in URIs.  Previous processing steps will have removed some
   characters, and the interpretation of reserved characters will have
   already been done (with the syntactic reserved characters outside of
   the IRI component).  This mapping is defined for all sequences of
   Unicode characters, whether or not they are valid for the component
   in question.

   For each character which is not allowed in a valid URI (NOTE: WHAT IS
   THE RIGHT REFERENCE HERE), apply the following steps.

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

   Percent encode  Convert each octet of this sequence to %HH, where HH
      is the hexadecimal notation of the octet value.  The hexadecimal
      notation SHOULD use uppercase letters.  (This is the general URI
      percent-encoding mechanism in Section 2.1 of [RFC3986].)

   Note that the mapping is an identity transformation for parsed URI
   components of valid URIs, and is idempotent: applying the mapping a
   second time will not change anything.

3.4.  Mapping ireg-name

   Schemes that allow non-ASCII based characters in the reg-name (ireg-
   name) position MUST convert the ireg-name component of an IRI as

   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-

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   separated label, and by using U+002E (FULL STOP) as a label
   separator, with the flag UseSTD3ASCIIRules set to FALSE, and with the
   flag AllowUnassigned set to FALSE.  The ToASCII operation may fail,
   but this would mean that the IRI cannot be resolved.  In such cases,
   if the domain name conversion fails, then the entire IRI conversion
   fails.  Processors that have no mechanism for signalling a failure
   MAY instead substitute an otherwise invalid host name, although such
   processing SHOULD be avoided.

   For example, the IRI
   MAY be converted to
   ; conversion to percent-encoded form, e.g.,
   "", MUST NOT be performed.

   Note:  Domain Names may appear 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; would lead to an IRI of;sum&#xE9;., which would convert to a URI of  The server-side implementation is responsible for
      making the necessary conversions to be able to retrieve the Web

   Note:  In this process, 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
      ";#red" (in XML notation) is
      converted to
      "", not to something
      ((DESIGN QUESTION: What about e.g. in an IRI?  Will that get
      converted to punycode, or not?))

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3.5.  Mapping query components

   ((NOTE: SEE ISSUES LIST)) For compatibility with existing deployed
   HTTP infrastructure, the following special case applies for schemes
   "http" and "https" and IRIs whose origin has a document charset other
   than one which is UCS-based (e.g., UTF-8 or UTF-16).  In such a case,
   the "query" component of an IRI is mapped into a URI by using the
   document charset rather than UTF-8 as the binary representation
   before pct-encoding.  This mapping is not applied for any other
   scheme or component.

3.6.  Mapping IRIs to URIs

   The canonical mapping from a IRI to URI is defined by applying the
   mapping above (from IRI to URI components) and then reassembling a
   URI from the parsed URI components using the original punctuation
   that delimited the IRI components.

3.7.  Converting URIs to IRIs

   In some situations, for presentation and further processing, it is
   desirable to convert a URI into an equivalent IRI in which natural
   characters are represented directly rather than percent encoded.  Of
   course, every URI is already an IRI in its own right without any
   conversion, and in general there This section gives one such
   procedure for this conversion.

   The conversion described in this section, if given a valid URI, will
   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 differ from 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

      (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

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

   4. IRI to URI conversion has different rules for dealing with domain
      names and query parameters.

   Conversion from a URI to an IRI MAY be done by using the following

   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.

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

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

   6. URIs known to contain domain names in the reg-name component
      SHOULD convert punycode-encoded domain name labels to the
      corresponding characters using the ToUnicode procedure.

   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
   "" 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
   "", which is a different
   URI from "".

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3.7.1.  Examples

   This section shows various examples of converting URIs to IRIs.  Each
   example shows the result after each of the steps 1 through 6 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







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







   The following example contains "%e2%80%ae", which is the percent-
   UTF-8 character encoding of U+202E, RIGHT-TO-LEFT OVERRIDE.
   Section 4.1 forbids the direct use of this character in an IRI.

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   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.






   6. http://&#x7D0D;&#x8C46;

   Note that the label "xn--99zt52a" is converted to U+7D0D U+8C46
   (Japanese Natto).  ((EDITOR NOTE: There is some inconsistency in this

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
   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

   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.

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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-
   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.

   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

   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

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4.2.  Bidi IRI Structure

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

   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.

   For each component, the following restrictions apply:

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

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

   The above restrictions are given as "SHOULD"s, rather than as
   "MUST"s.  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

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   If the above restrictions cannot be satisfied otherwise, the affected
   component can always be mapped to URI notation as described in
   Section 3.3.  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.

   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

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   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: ""
   Visual representation: ""
   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 "", it may be
   helpful to read aloud the visual representation following the bidi
   algorithm.  After "" one reads the RTL block
   "E-F-slash-G-H", which corresponds to the logical representation.

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

   Example 7: A single rtl component 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
   Logical representation: ""
   Visual representation: ""
   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
   Logical representation: "",
   Visual representation: ""

   Example 10 (allowed but not recommended):

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   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.

   Example 11 (allowed but not recommended):
   Logical representation: "http://ab.CDEFGH.123ij/kl/mn/op.html"
   Visual representation: "http://ab.123.HGFEDCij/kl/mn/op.html"
   Components consisting of numbers and left-to-right characters are
   allowed, 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
   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.

   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 to determine
   if they are "the same" 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

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   of such comparisons, but there are many application-dependent
   versions of equivalence.

   Even when 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

   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.

5.2.  Preparation for Comparison

   Any kind of IRI comparison REQUIRES that any additional contextual
   processing is first performed, including undoing higher-level
   escapings or encodings in the protocol or format that carries an IRI.
   This preprocessing is usually done when the protocol or format is

   Examples of contextual preprocessing steps are described in
   Section 7.

   Examples of such escapings or encodings are entities and numeric
   character references in [HTML4] and [XML1].  As an example,
   ";" (in HTML),
   ";" (in HTML or XML), and
   ";" (in HTML or XML) are all resolved into
   what is denoted in this document (see Section 1.4) as
   ";" (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

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   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

   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 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

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   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 in a context that uses Simple String

   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:


   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.  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://" is equivalent to "".
   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

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   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.  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

   IRIs already in Unicode MUST NOT be normalized before parsing or
   interpreting.  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.

   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, ";sum&#xE9;.html" (in XML
   Notation) is in NFC.  On the other hand,
   ";sume&#x301;.html" is not in NFC.

   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].

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   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 (for example, the 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.  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 nonencoded
   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, "",
   "", and "", must
   resolve to the same resource.

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

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   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:

   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"

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

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   specification.  For example, the IRI "" 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.

   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;" may be considered equivalent to

   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

   redirects to an IRI differing only in the trailing slash

   they will likely regard the two as equivalent in the future.  This
   kind of technique is only appropriate when equivalence is clearly
   indicated by both the result of accessing the resources and the
   common conventions of their scheme's dereference algorithm (in this
   case, use of redirection by HTTP origin servers to avoid problems
   with relative references).

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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 3.6, 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.

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

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   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 3.6.  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.

   ((UPDATE THIS NOTE)) 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

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 [RFC4395].
   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 URI scheme does not specify how to encode original
   characters, only some HTTP URLs can have corresponding but different

   For example, for a document with a URI of
   "", it is possible to
   construct a corresponding IRI (in XML notation, see Section 1.4):
   ";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 "", the
   percent-encoding octets cannot be converted to actual characters in
   an IRI, as the percent-encoding is not based on UTF-8.

   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

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   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 updates the IANA registry of URI schemes to note
   their applicability to IRIs, see Section 9.  All IRIs use URI
   schemes, and all URIs with URI schemes can be used as IRIs, even
   though in some cases only by using URIs directly as IRIs, without any

   Scheme definitions 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 3.6 MUST be a valid URI
   under the syntactic restrictions of generic URI syntax and any
   narrower restrictions imposed by the corresponding URI scheme

   The requirement for the use of UTF-8 generally applies to all parts
   of a URI.  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
   "", 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
   [XPointer].  The IRI corresponding to the above URI would be (in XML

   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.

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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

7.  Liberal handling of otherwise invalid IRIs

   (EDITOR NOTE: This Section may move to an appendix.)  Some technical
   specifications and widely-deployed software have allowed additional
   variations and extensions of IRIs to be used in syntactic components.
   This section describes two widely-used preprocessing agreements.
   Other technical specifications may wish to reference a syntactic
   component which is "a valid IRI or a string that will map to a valid
   IRI after this preprocessing algorithm".  These two variants are
   known as Legacy Extended IRI or LEIRI [LEIRI], and Web Address

   Future technical specifications SHOULD NOT allow conforming producers
   to produce, or conforming content to contain, such forms, as they are
   not interoperable with other IRI consuming software.

7.1.  LEIRI processing

   This section defines Legacy Extended IRIs (LEIRIs).  The syntax of
   Legacy Extended IRIs is the same as that for IRIs, except that the
   ucschar production is replaced by the leiri-ucschar production:

     leiri-ucschar  = " " / "<" / ">" / '"' / "{" / "}" / "|"
                      / "\" / "^" / "`" / %x0-1F / %x7F-D7FF
                      / %xE000-FFFD / %x10000-10FFFF

   Among other extensions, processors based on this specification also
   did not enforce the restriction on bidirectional formatting
   characters in Section 4.1, and the iprivate production becomes

   To convert a string allowed as a LEIRI to an IRI, each character
   allowed in leiri-ucschar but not in ucschar must be percent-encoded
   using Section 3.3.

7.2.  Web Address processing

   Many popular web browsers have taken the approach of being quite
   liberal in what is accepted as a "URL" or its relative forms.  This

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   section describes their behavior in terms of a preprocessor which
   maps strings into the IRI space for subsequent parsing and
   interpretation as an IRI.

   In some situations, it might be appropriate to describe the syntax
   that a liberal consumer implementation might accept as a "Web
   Address" or "Hypertext Reference" or "HREF".  However, technical
   specifications SHOULD restrict the syntactic form allowed by
   compliant producers to the IRI or IRI reference syntax defined in
   this document even if they want to mandate this processing.


   o  Leading and trailing whitespace is removed.

   o  Some additional characters are removed.

   o  Some additional characters are allowed and escaped (as with

   o  If interpreting an IRI as a URI, the pct-encoding of the query
      component of the parsed URI component depends on operational

   Each string provided may have an associated charset (called the HREF-
   charset here); this defaults to UTF-8.  For web browsers interpreting
   HTML, the document charset of a string is determined:

   If the string came from a script (e.g. as an argument to a method)
      The HRef-charset is the script's charset.

   If the string came from a DOM node (e.g. from an element)  The node
      has a Document, and the HRef-charset is the Document's character

   If the string had a HRef-charset defined when the string was created
   or defined  The HRef-charset is as defined.

   If the resulting HRef-charset is a unicode based character encoding
   (e.g., UTF-16), then use UTF-8 instead.

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   The syntax for Web Addresses is obtained by replacing the 'ucschar',
   pct-form, and path-sep rules with the href-ucschar, href-pct-form,
   and href-path-sep rules below.  In addition, some characters are

     href-ucschar  = " " / "<" / ">" / '"' / "{" / "}" / "|"
                      / "\" / "^" / "`" / %x0-1F / %x7F-D7FF
                      / %xE000-FFFD / %x10000-10FFFF
     href-pct-form = pct-encoded | "%"
     href-path-sep = "/" | "\"
     href-strip    =

   SENTENCE) browsers did not enforce the restriction on bidirectional
   formatting characters in Section 4.1, and the iprivate production
   becomes redundant.

   'Web Address processing' requires the following additional
   preprocessing steps:

   1.  Leading and trailing instances of space (U+0020), CR (U+000A), LF
       (U+000D), and TAB (U+0009) characters are removed.

   2.  strip all characters in href-strip.

   3.  Percent-encode all characters in href-ucschar not in ucschar.

   4.  Replace occurrences of "%" not followed by two hexadecimal digits
       by "%25".

   5.  Convert backslashes ('\') matching href-path-sep to forward
       slashes ('/').

7.3.  Characters not allowed in IRIs

   This section provides a list of the groups of characters and code
   points that are allowed by LEIRI or HREF but are not allowed in IRIs
   or are allowed in IRIs only in the query part.  For each group of
   characters, advice on the usage of these characters is also given,
   concentrating on the reasons for why they are excluded from IRI use.

      Space (U+0020): Some formats and applications use space as a
      delimiter, e.g. for items in a list.  Appendix C of [RFC3986] also
      mentions that white space may have to be added when displaying or
      printing long URIs; the same applies to long IRIs.  This means
      that spaces can disappear, or can make the what is intended as a
      single IRI or IRI reference to be treated as two or more separate

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      Delimiters "<" (U+003C), ">" (U+003E), and '"' (U+0022): Appendix
      C of [RFC3986] suggests the use of double-quotes
      ("") and angle brackets (<>)
      as delimiters for URIs in plain text.  These conventions are often
      used, and also apply to IRIs.  Using these characters in strings
      intended to be IRIs would result in the IRIs being cut off at the
      wrong place.

      Unwise characters "\" (U+005C), "^" (U+005E), "`" (U+0060), "{"
      (U+007B), "|" (U+007C), and "}" (U+007D): These characters
      originally have been excluded from URIs because the respective
      codepoints are assigned to different graphic characters in some
      7-bit or 8-bit encoding.  Despite the move to Unicode, some of
      these characters are still occasionally displayed differently on
      some systems, e.g.  U+005C may appear as a Japanese Yen symbol on
      some systems.  Also, the fact that these characters are not used
      in URIs or IRIs has encouraged their use outside URIs or IRIs in
      contexts that may include URIs or IRIs.  If a string with such a
      character were used as an IRI in such a context, it would likely
      be interpreted piecemeal.

      The controls (C0 controls, DEL, and C1 controls, #x0 - #x1F #x7F -
      #x9F): There is generally no way to transmit these characters
      reliably as text outside of a charset encoding.  Even when in
      encoded form, many software components silently filter out some of
      these characters, or may stop processing alltogether when
      encountering some of them.  These characters may affect text
      display in subtle, unnoticable ways or in drastic, global, and
      irreversible ways depending on the hardware and software involved.
      The use of some of these characters would allow malicious users to
      manipulate the display of an IRI and its context in many

      Bidi formatting characters (U+200E, U+200F, U+202A-202E): These
      characters affect the display ordering of characters.  If IRIs
      were allowed to contain these characters and the resulting visual
      display transcribed. they could not be converted back to
      electronic form (logical order) unambiguously.  These characters,
      if allowed in IRIs, might allow malicious users to manipulate the
      display of IRI and its context.

      Specials (U+FFF0-FFFD): These code points provide functionality
      beyond that useful in an IRI, for example byte order
      identification, annotation, and replacements for unknown
      characters and objects.  Their use and interpretation in an IRI
      would serve no purpose and might lead to confusing display

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      Private use code points (U+E000-F8FF, U+F0000-FFFFD, U+100000-
      10FFFD): Display and interpretation of these code points is by
      definition undefined without private agreement.  Therefore, these
      code points are not suited for use on the Internet.  They are not
      interoperable and may have unpredictable effects.

      Tags (U+E0000-E0FFF): These characters provide a way to language
      tag in Unicode plain text.  They are not appropriate for IRIs
      because language information in identifiers cannot reliably be
      input, transmitted (e.g. on a visual medium such as paper), or

      Non-characters (U+FDD0-FDEF, U+1FFFE-1FFFF, U+2FFFE-2FFFF,
      U+FFFFE-FFFFF, U+10FFFE-10FFFF): These code points are defined as
      non-characters.  Applications may use some of them internally, but
      are not prepared to interchange them.

   LEIRI preprocessing disallowed some code points and code units:

      Surrogate code units (D800-DFFF): These do not represent Unicode

8.  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

8.1.  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

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   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 3.7 above may
   be performed.  It is preferable not to perform this inverse
   conversion unless it is certain this can be done correctly.

8.2.  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
   might 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

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

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8.3.  URI/IRI Transfer between Applications

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

   Such applications would need to be upgraded, in order 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 would need 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 3.6) if the system-wide invocation mechanism only accepts

   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
   octets, which will do the right thing with IRIs.

8.4.  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

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

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8.5.  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,

   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, IRIs should only 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.

8.6.  Display of URIs/IRIs

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

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   For display of Bidi IRIs, please see Section 4.1.

8.7.  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.

8.8.  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

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

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

   Therefore, IRIs benefit most from a broad upgrade of software to be
   able to enter and transport IRIs.  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.

9.  IANA Considerations

   RFC Editor and IANA note: Please Replace RFC XXXX with the number of
   this document when it issues as an RFC.

   IANA maintains a registry of "URI schemes".  A "URI scheme" also
   serves an "IRI scheme".

   To clarify that the URI scheme registration process also applies to
   IRIs, change the description of the "URI schemes" registry header to
   say "[RFC4395] defines an IANA-maintained registry of URI Schemes.

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   These registries include the Permanent and Provisional URI Schemes.
   RFC XXXX updates this registry to designate that schemes may also
   indicate their usability as IRI schemes.

   Update "per RFC 4395" to "per RFC 4395 and RFC XXXX".

10.  Security Considerations

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

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

   There are various ways in which "spoofing" can occur with IRIs.
   "Spoofing" means that somebody may add a resource name that looks the
   same or similar to the user, but 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 ("") might be
   "spoofed" by someone who is able to create
   "".  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

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   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 8.5.  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.

   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.

   The use of Legacy Extended IRIs introduces additional security

11.  Acknowledgements

   For contributions to this update, we would like to thank Ian Hickson,
   Michael Sperberg-McQueen, Dan Connolly, Norman Walsh, Richard Tobin,
   Henry S. Thomson, and the XML Core Working Group of the W3C.

   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.

   For contributions to the previous version of this document, RFC 3987,
   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

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   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.

   A definition of HyperText Reference was initially produced by Ian
   Hixson, and further edited by Dan Connolly and C. M. Spergerg-

   Thanks to the Internationalization Working Group (I18N WG) of the
   World Wide Web Consortium (W3C), and 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

12.  Open Issues

   NOTE: The issues noted in this section should be addressed before the
   document is submitted as an RFC.  These issues are not in any
   particular order, and do not necessarily form a complete list of all
   known issues.

   length limits on domain name  See, for example,
      discussion on (that discussion is mostly
      irrelevant now as the "63 octets in UTF-8 per label" restriction
      was dropped)

   Allow generic scheme-independent IRI to URI translation  Previous
      drafts of this specification proposed a generic IRI to URI
      transformation using pct-encoding, and allowed domain name
      translation to be optionally handled by retranslating host names
      from pct-encoding back into Unicode and then into punycode.  This
      draft does not allow that behavior, but this should be fixed to be
      in line with RFC 3986 syntax and to lead implementations towards
      an uniform an long-term URI<->IRI correspondence.  See also

   update URI scheme registry?  This document starts the process of
      making minor changes to the URI scheme registry.  This should be
      handled as an update to RFC 4395.

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   utf8 in HTTP  Not really IRI issue, but some HTTP implementations
      send UTF8 path directly, review.

   handling of \\  Some web applications convert \ to / and others
      don't.  Make this mandatory or disallowed (but not optional), for
      Web Addresses.

   dealing with disallowed IRI characters

   misplaced text  Find a place to note that some older software
      transcoding to UTF-8 may produce illegal output for some input, in
      particular for characters outside the BMP (Basic Multilingual
      Plane).  As an example, for the IRI with non-BMP characters (in
      XML Notation):
      which contains the first three letters of the Old Italic alphabet,
      the correct conversion to a URI is

   Special Query Handling needed?  The percent-encoding handling of
      query components in the HTTP scheme is really unfortunate.  There
      is no good normative advice to give if the percent-encoding is
      delayed until the query-IRI is interpreted.  Could HTML ask
      browsers to percent-encode the form data using the document
      character set BEFORE the query IRI is constructed, and only in the
      case where the document character set isn't Unicode-based and the
      query is being added to http: or https: URIs?  This would give
      more consistent results.  Browsers might have to change their
      behavior in constructing the IRI-with-query-added, but the results
      would be more consistent and fewer bugs, and it wouldn't affect
      interpretation of any existing web pages.  It would remove the
      need to have a normative special case for queries in HTML
      documents, just for http, in a way in which things like
      transcoding etc. wouldn't work well.  You could tell the
      difference between a query URI in the address bar and one created
      via a form because the address bar would always be UTF-8.  The
      browsers might have to change the algorithm for showing the
      address in the adress bar to know how to undo the encoding.

   handling illegal characters  Section 3.3 used to apply only to
      characters in either 'ucschar' or 'iprivate', but then later said
      that systems accepting IRIs MAY also deal with the printable
      characters in US-ASCII that are not allowed in URIs, namely "<",
      ">", '"', space, "{", "}", "|", "\", "^", and "`".  Larry felt
      that this a MAY would result in non-uniform behavior, because some
      systems would produce valid URI components and others wouldn't.
      Non-printable US-ASCII characters should be stripped by most
      software, so if they get to if they're passed on somewhere as IRI

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      characters, encoding them makes sense.  The section also used to
      say "If these characters are found but are not converted, then the
      conversion SHOULD fail." but there is no notion of conversion
      failing -- every string is converted.  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.

   adding single % and hash  Changed the BNF to not match the URI
      document in allowing single % in path but not everywhere, and
      allowing a # in the fragment part.

13.  Change Log

   Note to RFC Editor: Please completely remove this section before

13.1.  Changes from draft-duerst-iri-bis-07 to draft-ietf-iri-3987bis-00

   Changed draft name, date, last paragraph of abstract, and titles in
   change log, and added this section in moving from
   draft-duerst-iri-bis-07 (personal submission) to
   draft-ietf-iri-3987bis-00 (WG document).

13.2.  Changes from -06 to -07 of draft-duerst-iri-bis

   Major restructuring of IRI processing model to make scheme-specific
   translation necessary to handle IDNA requirements and for consistency
   with web implementations.

   Starting with IRI, you want one of:

   a  IRI components (IRI parsed into UTF8 pieces)

   b  URI components (URI parsed into ASCII pieces, encoded correctly)

   c  whole URI (for passing on to some other system that wants whole

13.2.1.  OLD WAY

   1.  Pct-encoding on the whole thing to a URI. (c1) If you want a
       (maybe broken) whole URI, you might stop here.

   2.  Parsing the URI into URI components. (b1) If you want (maybe
       broken) URI components, stop here.

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   3.  Decode the components (undoing the pct-encoding). (a) if you want
       IRI components, stop here.

   4.  reencode: Either using a different encoding some components (for
       domain names, and query components in web pages, which depends on
       the component, scheme and context), and otherwise using pct-
       encoding. (b2) if you want (good) URI components, stop here.

   5.  reassemble the reencoded components. (c2) if you want a (*good*)
       whole URI stop here.

13.2.2.  NEW WAY

   1.  Parse the IRI into IRI components using the generic syntax. (a)
       if you want IRI components, stop here.

   2.  Encode each components, using pct-encoding, IDN encoding, or
       special query part encoding depending on the component scheme or
       context. (b) If you want URI components, stop here.

   3.  reassemble the a whole URI from URI components. (c) if you want a
       whole URI stop here.

13.3.  Changes from -05 to -06 of draft-duerst-iri-bis

   o  Add HyperText Reference, change abstract, acks and references for

   o  Add Masinter back as another editor.

   o  Masinter integrates HRef material from HTML5 spec.

   o  Rewrite introduction sections to modernize.

13.4.  Changes from -04 to -05 of draft-duerst-iri-bis

   o  Updated references.

   o  Changed IPR text to pre5378Trust200902.

13.5.  Changes from -03 to -04 of draft-duerst-iri-bis

   o  Added explicit abbreviation for LEIRIs.

   o  Mentioned LEIRI references.

   o  Completed text in LEIRI section about tag characters and about

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13.6.  Changes from -02 to -03 of draft-duerst-iri-bis

   o  Updated some references.

   o  Updated Michel Suginard's coordinates.

13.7.  Changes from -01 to -02 of draft-duerst-iri-bis

   o  Added tag range to iprivate (issue private-include-tags-115).

   o  Added Specials (U+FFF0-FFFD) to Legacy Extended IRIs.

13.8.  Changes from -00 to -01 of draft-duerst-iri-bis

   o  Changed from "IRIs with Spaces/Controls" to "Legacy Extended IRI"
      based on input from the W3C XML Core WG.  Moved the relevant
      subsections to the back and promoted them to a section.

   o  Added some text re.  Legacy Extended IRIs to the security section.

   o  Added a IANA Consideration Section.

   o  Added this Change Log Section.

   o  Added a section about "IRIs with Spaces/Controls" (converting from
      a Note in RFC 3987).

13.9.  Changes from RFC 3987 to -00 of draft-duerst-iri-bis

      Fixed errata (see

14.  References

14.1.  Normative References

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

              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

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              Requirement Levels", BCP 14, RFC 2119, March 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.

   [STD68]    Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [UNI9]     Davis, M., "The Bidirectional Algorithm", Unicode Standard
              Annex #9, March 2004,

   [UNIV4]    The Unicode Consortium, "The Unicode Standard, Version
              5.1.0, defined by: The Unicode Standard, Version 5.0
              (Boston, MA, Addison-Wesley, 2007. ISBN 0-321-48091-0), as
              amended by Unicode 4.1.0
              April 2008.

   [UTR15]    Davis, M. and M. Duerst, "Unicode Normalization Forms",
              Unicode Standard Annex #15, March 2008,

14.2.  Informative References

   [BidiEx]   "Examples of bidirectional IRIs",

   [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,


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              Duerst, M., "The Properties and Promises of UTF-8", Proc.
              11th International Unicode Conference, San Jose ,
              September 1997, <

   [Gettys]   Gettys, J., "URI Model Consequences",

   [HTML4]    Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01
              Specification", World Wide Web Consortium Recommendation,
              December 1999,

   [HTML5]    Hickson, I. and D. Hyatt, "A vocabulary and associated
              APIs for HTML and XHTML", World Wide Web
              Consortium Working Draft, April 2009,

   [LEIRI]    Thompson, H., Tobin, R., and N. Walsh, "Legacy extended
              IRIs for XML resource identification", World Wide Web
              Consortium Note, November 2008,

   [RFC1738]  Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
              Resource Locators (URL)", RFC 1738, December 1994.

   [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.

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

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

   [RFC2396]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform

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

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

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., 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.

   [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", BCP 35,
              RFC 4395, February 2006.

   [UNIXML]   Duerst, M. and A. Freytag, "Unicode in XML and other
              Markup Languages", Unicode Technical Report #20, World
              Wide Web Consortium Note, June 2003,

   [XLink]    DeRose, S., Maler, E., and D. Orchard, "XML Linking
              Language (XLink) Version 1.0", World Wide Web
              Consortium Recommendation, June 2001,

   [XML1]     Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Forth
              Edition)", World Wide Web Consortium Recommendation,
              August 2006, <>.

              Bray, T., Hollander, D., Layman, A., and R. Tobin,
              "Namespaces in XML (Second Edition)", World Wide Web
              Consortium Recommendation, August 2006,

              Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes",
              World Wide Web Consortium Recommendation, May 2001,

              Grosso, P., Maler, E., Marsh, J., and N. Walsh, "XPointer
              Framework", World Wide Web Consortium Recommendation,
              March 2003,

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Appendix A.  Design Alternatives

   This section briefly summarizes some design alternatives considered
   earlier and the reasons why they were not chosen.

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

   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

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.

A.3.  New Encoding Convention

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

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

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   server upgrades.

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
   "[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

Authors' Addresses

   Martin Duerst (Note: Please write "Duerst" with u-umlaut wherever
             possible, for example as "D&amp;#252;rst" in XML and HTML.)
   Aoyama Gakuin University
   5-10-1 Fuchinobe
   Sagamihara, Kanagawa  229-8558

   Phone: +81 42 759 6329
   Fax:   +81 42 759 6495
          (Note: This is the percent-encoded form of an IRI.)

   Michel Suignard
   Unicode Consortium
   P.O. Box 391476
   Mountain View, CA  94039-1476

   Phone: +1-650-693-3921

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   Larry Masinter
   345 Park Ave
   San Jose, CA  95110

   Phone: +1-408-536-3024

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