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Internationalized Resource Identifiers                         M. Duerst
(iri)                                           Aoyama Gakuin University
Internet-Draft                                               M. Suignard
Obsoletes: 3987 (if approved)                         Unicode Consortium
Intended status: Standards Track                             L. Masinter
Expires: July 12, 2012                                             Adobe
                                                         January 9, 2012


             Internationalized Resource Identifiers (IRIs)
                       draft-ietf-iri-3987bis-09

Abstract

   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.

   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.

   This document is part of a set of documents intended to replace RFC
   3987.

RFC Editor: Please remove the next paragraph before publication.

   This (and several companion documents) are intended to obsolete RFC
   3987, and also move towards IETF Draft Standard.  For discussion and
   comments on these drafts, please join the IETF IRI WG by subscribing
   to the mailing list public-iri@w3.org, archives at
   http://lists.w3.org/archives/public/public-iri/.  For a list of open
   issues, please see the issue tracker of the WG at
   http://trac.tools.ietf.org/wg/iri/trac/report/1.  For a list of
   individual edits, please see the change history at
   http://trac.tools.ietf.org/wg/iri/trac/log/draft-ietf-iri-3987bis.

Status of this Memo

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



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   it for publication as an RFC or to translate it into languages other
   than English.













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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.   Overview and Motivation . . . . . . . . . . . . . . . . .  5
     1.2.   Applicability . . . . . . . . . . . . . . . . . . . . . .  6
     1.3.   Definitions . . . . . . . . . . . . . . . . . . . . . . .  7
     1.4.   Notation  . . . . . . . . . . . . . . . . . . . . . . . .  8
   2.  IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     2.1.   Summary of IRI Syntax . . . . . . . . . . . . . . . . . .  9
     2.2.   ABNF for IRI References and IRIs  . . . . . . . . . . . . 10
   3.  Processing IRIs and related protocol elements  . . . . . . . . 13
     3.1.   Converting to UCS . . . . . . . . . . . . . . . . . . . . 13
     3.2.   Parse the IRI into IRI components . . . . . . . . . . . . 13
     3.3.   General percent-encoding of IRI components  . . . . . . . 14
     3.4.   Mapping ireg-name . . . . . . . . . . . . . . . . . . . . 14
       3.4.1.  Mapping using Percent-Encoding . . . . . . . . . . . . 14
       3.4.2.  Mapping using Punycode . . . . . . . . . . . . . . . . 14
       3.4.3.  Additional Considerations  . . . . . . . . . . . . . . 15
     3.5.   Mapping query components  . . . . . . . . . . . . . . . . 16
     3.6.   Mapping IRIs to URIs  . . . . . . . . . . . . . . . . . . 16
   4.  Converting URIs to IRIs  . . . . . . . . . . . . . . . . . . . 16
     4.1.   Examples  . . . . . . . . . . . . . . . . . . . . . . . . 18
   5.  Use of IRIs  . . . . . . . . . . . . . . . . . . . . . . . . . 19
     5.1.   Limitations on UCS Characters Allowed in IRIs . . . . . . 19
     5.2.   Software Interfaces and Protocols . . . . . . . . . . . . 20
     5.3.   Format of URIs and IRIs in Documents and Protocols  . . . 20
     5.4.   Use of UTF-8 for Encoding Original Characters . . . . . . 20
     5.5.   Relative IRI References . . . . . . . . . . . . . . . . . 22
   6.  Legacy Extended IRIs (LEIRIs)  . . . . . . . . . . . . . . . . 22
     6.1.   Legacy Extended IRI Syntax  . . . . . . . . . . . . . . . 23
     6.2.   Conversion of Legacy Extended IRIs to IRIs  . . . . . . . 23
     6.3.   Characters Allowed in Legacy Extended IRIs but not in
            IRIs  . . . . . . . . . . . . . . . . . . . . . . . . . . 23
   7.  URI/IRI Processing Guidelines (Informative)  . . . . . . . . . 25
     7.1.   URI/IRI Software Interfaces . . . . . . . . . . . . . . . 25
     7.2.   URI/IRI Entry . . . . . . . . . . . . . . . . . . . . . . 26
     7.3.   URI/IRI Transfer between Applications . . . . . . . . . . 26
     7.4.   URI/IRI Generation  . . . . . . . . . . . . . . . . . . . 27
     7.5.   URI/IRI Selection . . . . . . . . . . . . . . . . . . . . 27
     7.6.   Display of URIs/IRIs  . . . . . . . . . . . . . . . . . . 28
     7.7.   Interpretation of URIs and IRIs . . . . . . . . . . . . . 28
     7.8.   Upgrading Strategy  . . . . . . . . . . . . . . . . . . . 29
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 30
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 30
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 31
   11. Main Changes Since RFC 3987  . . . . . . . . . . . . . . . . . 32
     11.1.  Split out Bidi, processing guidelines, comparison
            sections  . . . . . . . . . . . . . . . . . . . . . . . . 32



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     11.2.  Major restructuring of IRI processing model . . . . . . . 32
       11.2.1. OLD WAY  . . . . . . . . . . . . . . . . . . . . . . . 32
       11.2.2. NEW WAY  . . . . . . . . . . . . . . . . . . . . . . . 33
       11.2.3. Extension of Syntax  . . . . . . . . . . . . . . . . . 33
       11.2.4. More to be added . . . . . . . . . . . . . . . . . . . 33
     11.3.  Change Log  . . . . . . . . . . . . . . . . . . . . . . . 33
       11.3.1. Changes after draft-ietf-iri-3987bis-01  . . . . . . . 33
       11.3.2. Changes from draft-duerst-iri-bis-07 to
               draft-ietf-iri-3987bis-00  . . . . . . . . . . . . . . 34
       11.3.3. Changes from -06 to -07 of draft-duerst-iri-bis  . . . 34
     11.4.  Changes from -00 to -01 . . . . . . . . . . . . . . . . . 34
     11.5.  Changes from -05 to -06 of draft-duerst-iri-bis-00  . . . 34
     11.6.  Changes from -04 to -05 of draft-duerst-iri-bis . . . . . 34
     11.7.  Changes from -03 to -04 of draft-duerst-iri-bis . . . . . 34
     11.8.  Changes from -02 to -03 of draft-duerst-iri-bis . . . . . 35
     11.9.  Changes from -01 to -02 of draft-duerst-iri-bis . . . . . 35
     11.10. Changes from -00 to -01 of draft-duerst-iri-bis . . . . . 35
     11.11. Changes from RFC 3987 to -00 of draft-duerst-iri-bis  . . 35
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     12.1.  Normative References  . . . . . . . . . . . . . . . . . . 35
     12.2.  Informative References  . . . . . . . . . . . . . . . . . 36
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 39





























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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 composed out of a very limited repertoire of characters;
   this design choice was made to support global transcription([RFC3986]
   section 1.2.1.).  Reliable transition between a URI (as an abstract
   protocol element composed of a sequence of characters) and a
   presentation of that URI (written on a napkin, read out loud) and
   back is relatively straightforward, because of the limited repertoire
   of characters used.  IRIs are designed to satisfy a different set of
   use requirements; in particular, to allow IRIs to be written in ways
   that are more meaningful to their users, even at the expense of
   global transcribability.  However, ensuring reliability of the
   transition between an IRI and its presentation and back is more
   difficult and complex when dealing with the larger set of Unicode
   characters.  For example, Unicode supports multiple ways of encoding
   complex combinations of characters and accents, with multiple
   character sequences that can result in the same presentation.

   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.



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   Within this document, Section 5 discusses the use of IRIs in
   different situations.  Section 7 gives additional informative
   guidelines.  Section 9 discusses IRI-specific security
   considerations.

   This specification is part of a collection of specifications intended
   to replace [RFC3987].  [Bidi] discusses the special case of
   bidirectional IRIs using characters from scripts written right-to-
   left.  [Equivalence] gives guidelines for applications wishing to
   determine if two IRIs are equivalent, as well as defining some
   equivalence methods.  [RFC4395bis] updates the URI scheme
   registration guidelines and procedures to note that every URI scheme
   is also automatically an IRI scheme and to allow scheme definitions
   to be directly described in terms of Unicode characters.

1.2.  Applicability

   IRIs are designed to allow protocols and software that deal with URIs
   to be updated to handle IRIs.  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 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



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      in the guidelines for registering new schemes, [RFC4395bis].  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 5.4 for further discussion.

1.3.  Definitions

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

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

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

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

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

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

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

   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 [UNIV6].

   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.






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   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 6.2.

   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.

   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
      information.

   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
      characters.

   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
      component".)

   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.

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.




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   To represent characters outside US-ASCII in examples, this document
   uses 'XML 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 '&'.

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

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


2.  IRI Syntax

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

   As with URIs, an IRI is defined as a sequence of characters, not as a
   sequence of octets.  This definition accommodates the fact that IRIs
   may be written on paper or read over the radio as well as stored or
   transmitted digitally.  The same IRI 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.

2.1.  Summary of IRI Syntax

   The IRI syntax extends the URI syntax in [RFC3986] by extending the
   class of unreserved characters, primarily 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 5.1.

   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.




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   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 rule <pct-form> has been introduced in order
   to be able to reference it from other parts of the document.

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

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

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

   IRI-reference  = IRI / irelative-ref

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

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



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   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 / ":"
                  / "@"

   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




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   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].

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

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

   port           = *DIGIT

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

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

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

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

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

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

   pct-encoded    = "%" HEXDIG HEXDIG

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

   This syntax does not support IPv6 scoped addressing zone identifiers.





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3.  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
   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 or changed.

   An IRI or IRI reference is a sequence of characters from the UCS.
   For input from presentations (written on paper, read aloud) or
   translation from other representations (a text stream using a legacy
   character encoding), convert the input to Unicode.  Note that some
   character encodings or transcriptions can be converted to or
   represented by more than one sequence of Unicode characters.  Ideally
   the resulting IRI would use a normalized form, such as Unicode
   Normalization Form C [UTR15], since that ensures a stable, consistent
   representation that is most likely to produce the intended results.
   Previous versions of this specification required normalization at
   this step.  However, attempts to require normalization in other
   protocols have met with strong enough resistance that requiring
   normalization here was considered impractical.  Implementers and
   users are cautioned that, while denormalized character sequences are
   valid, they might be difficult for other users or processes to
   reproduce and might lead to unexpected results.

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 is a set of parsed IRI components.






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3.3.  General percent-encoding of IRI components

   Except as noted in the following subsections, IRI components are
   mapped to the equivalent URI components 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 anywhere in a valid URI 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

3.4.1.  Mapping using Percent-Encoding

   The ireg-name component SHOULD be converted according to the general
   procedure for percent-encoding of IRI components described in
   Section 3.3.

   For example, the IRI
   "http://r&#xE9;sum&#xE9;.example.org"
   will be converted to
   "http://r%C3%A9sum%C3%A9.example.org".

   This conversion for ireg-name is in line with Section 3.2.2 of
   [RFC3986], which does not mandate a particular registered name lookup
   technology.  For further background, see [RFC6055] and [Gettys].

3.4.2.  Mapping using Punycode

   The ireg-name component MAY also be converted as follows:

   If there are any sequences of <pct-encoded>, and their corresponding



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   octets all represent valid UTF-8 octet sequences, then convert these
   back to Unicode character sequences.  (If any <pct-encoded> sequences
   are not valid UTF-8 octet sequences, then leave the entire field as
   is without any change, since punycode encoding would not succeed.)

   Replace the ireg-name part of the IRI by the part converted using the
   Domain Name Lookup procedure (Subsections 5.3 to 5.5) of [RFC5891].
   on each dot-separated label, and by using U+002E (FULL STOP) as a
   label separator.  This procedure 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
   "http://r&#xE9;sum&#xE9;.example.org"
   MAY be converted to
   "http://xn--rsum-bad.example.org"
   .

   This conversion for ireg-name will be better able to deal with legacy
   infrastructure that cannot handle percent-encoding in domain names.

3.4.3.  Additional Considerations

   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;.example.org would lead to an IRI of
      http://validator.w3.org/check?uri=http%3A%2F%2Fr&#xE9;sum&#xE9;.
      example.org, which would convert to a URI of
      http://validator.w3.org/check?uri=http%3A%2F%2Fr%C3%A9sum%C3%A9.
      example.org.  The server-side implementation is responsible for
      making the necessary conversions to be able to retrieve the Web
      page.

   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
      "http://www.example.org/red%09ros&#xE9;#red" (in XML notation) is
      converted to



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      "http://www.example.org/red%09ros%C3%A9#red", not to something
      like
      "http%3A%2F%2Fwww.example.org%2Fred%2509ros%C3%A9%23red".

3.5.  Mapping query components

   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 mapping from an IRI to URI is accomplished 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.


4.  Converting URIs to IRIs

   In some situations, for presentation and further processing, it is
   desirable to convert a URI into an equivalent IRI without unnecessary
   percent encoding.  Of course, every URI is already an IRI in its own
   right without any conversion.  This section gives one possible
   procedure for URI to IRI mapping.

   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
      octets.

      (Note: The octet patterns of UTF-8 are highly regular.  Therefore,
      there is a very high probability, but no guarantee, that percent-



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      encodings that can be interpreted as sequences of UTF-8 octets
      actually originated from UTF-8.  For a detailed discussion, see
      [Duerst97].)

   3. The conversion may result in a character that is not appropriate
      in an IRI.  See Section 2.2, and Section 5.1 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
   steps:

   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 and Section 5.1.

   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 5.1), results may vary.

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



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4.1.  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
   u-umlaut).

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

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

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

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

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

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

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

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

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

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

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

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

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

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



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   corresponding octets are re-percent-encoded in step 4.  This example
   shows that the case (upper- or lowercase) of letters used in percent-
   encodings may not be preserved.  The example also contains a
   punycode-encoded domain name label (xn--99zt52a), which is not
   converted.

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

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

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

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

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

   6. http://&#x7D0D;&#x8C46;.example.org/%E2%80%AE

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


5.  Use of IRIs

5.1.  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.  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].




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   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.

5.2.  Software Interfaces and Protocols

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

   Intermediate software interfaces between IRI-capable components and
   URI-only components MUST map the IRIs 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.

5.3.  Format of URIs and IRIs in Documents and Protocols

   Document formats that transport URIs may have to be upgraded to allow
   the transport of IRIs.  In cases where the document as a whole has a
   native character encoding, IRIs MUST also be encoded in this
   character encoding and converted accordingly by a parser or
   interpreter.  IRI characters not expressible in the native character
   encoding SHOULD be escaped by using the escaping conventions of the
   document format if such conventions are available.  Alternatively,
   they MAY be percent-encoded according to Section 3.6.  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
   [CharMod].

5.4.  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



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   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 [RFC4395bis].
   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
   IRIs.

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

   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



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   narrower restrictions imposed by the corresponding URI scheme
   specification.

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

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

5.5.  Relative IRI References

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


6.  Legacy Extended IRIs (LEIRIs)

   For historic reasons, some formats have allowed variants of IRIs that
   are somewhat less restricted in syntax.  This section provides a
   definition and a name (Legacy Extended IRI or LEIRI) for these
   variants for easier reference.  These variants have to be used with
   care; they require further processing before being fully
   interchangeable as IRIs.  New protocols and formats SHOULD NOT use
   Legacy Extended IRIs.  Even where Legacy Extended IRIs are allowed,
   only IRIs fully conforming to the syntax definition in Section 2.2
   SHOULD be created, generated, and used.  The provisions in this
   section also apply to Legacy Extended IRI references.







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6.1.  Legacy Extended IRI Syntax

   The syntax of Legacy Extended IRIs is the same as that for IRIs,
   except that ucschar is redefined as follows:

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

   The restriction on bidirectional formatting characters in [Bidi] is
   lifted.  The iprivate production becomes redundant.

   Likewise, the syntax for Legacy Extended IRI references (LEIRI
   references) is the same as that for IRI references with the above
   redefinition of ucschar applied.

   Formats that use Legacy Extended IRIs or Legacy Extended IRI
   references MAY further restrict the characters allowed therein,
   either implicitly by the fact that the format as such does not allow
   some characters, or explicitly.  An example of a character not
   allowed implicitly may be the NUL character (U+0000).  However, all
   the characters allowed in IRIs MUST still be allowed.

6.2.  Conversion of Legacy Extended IRIs to IRIs

   To convert a Legacy Extended IRI (reference) to an IRI (reference),
   each character allowed in a Legacy Extended IRI (reference) but not
   allowed in an IRI (reference) (see Section 6.3) MUST be percent-
   encoded by applying steps 2.1 to 2.3 of Section 3.6.

6.3.  Characters Allowed in Legacy Extended IRIs but not in IRIs

   This section provides a list of the groups of characters and code
   points that are allowed in Legacy Extedend IRIs, 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 not to use them.

      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 Legacy Extended IRI to
      be interpreted as two or more separate IRIs.

      Delimiters "<" (U+003C), ">" (U+003E), and '"' (U+0022): Appendix
      C of [RFC3986] suggests the use of double-quotes
      ("http://example.com/") and angle brackets (<http://example.com/>)



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      as delimiters for URIs in plain text.  These conventions are often
      used, and also apply to IRIs.  Legacy Extended IRIs using these
      characters will be 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 as a Japanese Yen symbol.  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.  In case a Legacy Extended IRI with such a
      character is used in such a context, the Legacy Extended IRI will
      be interpreted piecemeal.

      The controls (C0 controls, DEL, and C1 controls, #x0 - #x1F #x7F -
      #x9F): There is no way to transmit these characters reliably
      except potentially in electronic form.  Even when in electronic
      form, some software components might 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 may allow malicious users to
      manipulate the display of a Legacy Extended IRI and its context.

      Bidi formatting characters (U+200E, U+200F, U+202A-202E): These
      characters affect the display ordering of characters.  Displayed
      Legacy Extended IRIs containing these characters cannot be
      converted back to electronic form (logical order) unambiguously.
      These characters may allow malicious users to manipulate the
      display of a Legacy Extended IRI and its context.

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

      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.




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

      Non-characters (U+FDD0-FDEF, U+1FFFE-1FFFF, U+2FFFE-2FFFF,
      U+3FFFE-3FFFF, U+4FFFE-4FFFF, U+5FFFE-5FFFF, U+6FFFE-6FFFF,
      U+7FFFE-7FFFF, U+8FFFE-8FFFF, U+9FFFE-9FFFF, U+AFFFE-AFFFF,
      U+BFFFE-BFFFF, U+CFFFE-CFFFF, U+DFFFE-DFFFF, U+EFFFE-EFFFF,
      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.

   For reference, we here also list the code points and code units not
   even allowed in Legacy Extended IRIs:

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


7.  URI/IRI Processing Guidelines (Informative)

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

7.1.  URI/IRI Software Interfaces

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

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

   The transfer from URI-only to IRI-capable components requires no
   mapping, although the conversion described in Section 4 above may be



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

7.2.  URI/IRI Entry

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

   A person viewing a visual presentation of an IRI (as a sequence of
   glyphs, in some order, in some visual display) 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
   components.

   For the input of IRIs with right-to-left characters, please see
   [Bidi].

7.3.  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.



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   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
   URIs.

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

7.4.  URI/IRI Generation

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

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

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

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

7.5.  URI/IRI Selection

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

   In these cases, it is recommended to avoid choosing IRIs that are
   easily confused.  For example, for US-ASCII, the lower-case ell ("l")



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

   Outside 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.

7.6.  Display of URIs/IRIs

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

   For display of Bidi IRIs, please see [Bidi].

7.7.  Interpretation of URIs and IRIs

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



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

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

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

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

7.8.  Upgrading Strategy

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

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

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

   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.




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   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 5.4.


8.  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.
   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".


9.  Security Considerations

   The security considerations discussed in [RFC3986] also apply to



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   IRIs.  In addition, the following issues require particular care for
   IRIs.

   Incorrect encoding or decoding can lead to security problems.  For
   example, some UTF-8 decoders do not check against overlong byte
   sequences.  See [UTR36] Section 3 for details.

   There are serious difficulties with relying on a human to verify that
   a an IRI (whether presented visually or aurally) is the same as
   another IRI or is the one intended.  These problems exist with ASCII-
   only URIs (bl00mberg.com vs. bloomberg.com) but are strongly
   exacerbated when using the much larger character repertoire of
   Unicode.  For details, see Section 2 of [UTR36].  Using
   administrative and technical means to reduce the availability of such
   exploits is possible, but they are difficult to eliminate altogether.
   User agents SHOULD NOT rely on visual or perceptual comparison or
   verification of IRIs as a means of validating or assuring safety,
   correctness or appropriateness of an IRI.  Other means of presenting
   users with the validity, safety, or appropriateness of visited sites
   are being developed in the browser community as an alternative means
   of avoiding these difficulties.

   Besides the large character repertoire of Unicode, reasons for
   confusion include different forms of normalization and different
   normalization expectations, use of percent-encoding with various
   legacy encodings, and bidirectionality issues.  See also [Bidi].

   Confusion can occur in various IRI components, such as the domain
   name part or the path part, or between IRI components.  For
   considerations specific to the domain name part, see [RFC5890].  For
   considerations specific to particular protocols or schemes, see the
   security sections of the relevant specifications and registration
   templates.  Administrators of sites that allow independent users to
   create resources in the same sub area have to be careful.  Details
   are discussed in Section 7.5.

   The characters additionally allowed in Legacy Extended IRIs introduce
   additional security issues.  For details, see Section 6.3.


10.  Acknowledgements

   This document was derived from [RFC3987]; the acknowledgments from
   that specification still apply.

   In addition, this document was influenced by contributions from (in
   no particular order)Norman Walsh, Richard Tobin, Henry S. Thomson,
   John Cowan, Paul Grosso, the XML Core Working Group of the W3C, Chris



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   Lilley, Bjoern Hoehrmann, Felix Sasaki, Jeremy Carroll, Frank
   Ellermann, Michael Everson, Cary Karp, Matitiahu Allouche, Richard
   Ishida, Addison Phillips, Jonathan Rosenne, Najib Tounsi, Debbie
   Garside, Mark Davis, Sarmad Hussain, Ted Hardie, Konrad Lanz, Thomas
   Roessler, Lisa Dusseault, Julian Reschke, Giovanni Campagna, Anne van
   Kesteren, Mark Nottingham, Erik van der Poel, Marcin Hanclik, Marcos
   Caceres, Roy Fielding, Greg Wilkins, Pieter Hintjens, Daniel R.
   Tobias, Marko Martin, Maciej Stanchowiak, Wil Tan, Yui Naruse,
   Michael A. Puls II, Dave Thaler, Tom Petch, John Klensin, Shawn
   Steele, Peter Saint-Andre, Geoffrey Sneddon, Chris Weber, Alex
   Melnikov, Slim Amamou, S. Moonesamy, Tim Berners-Lee, Yaron Goland,
   Sam Ruby, Adam Barth, Abdulrahman I. ALGhadir, Aharon Lanin, Thomas
   Milo, Murray Sargent, Marc Blanchet, and Mykyta Yevstifeyev.


11.  Main Changes Since RFC 3987

   This section describes the main changes since [RFC3987].

11.1.  Split out Bidi, processing guidelines, comparison sections

   Move some components (comparison, bidi, processing) into separate
   documents.

11.2.  Major restructuring of IRI processing model

   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
      URIs)

11.2.1.  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.

11.2.2.  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.

11.2.3.  Extension of Syntax

   Added the tag range (U+E0000-E0FFF) to the iprivate production.  Some
   IRIs generated with the new syntax may fail to pass very strict
   checks relying on the old syntax.  But characters in this range
   should be extremely infrequent anyway.

11.2.4.  More to be added

   TODO: There are more main changes that need to be documented in this
   section.

11.3.  Change Log

   Note to RFC Editor: Please completely remove this section before
   publication.

11.3.1.  Changes after draft-ietf-iri-3987bis-01

   Changes from draft-ietf-iri-3987bis-01 onwards are available as
   changesets in the IETF tools subversion repository at http://
   trac.tools.ietf.org/wg/iri/trac/log/draft-ietf-iri-3987bis/
   draft-ietf-iri-3987bis.xml.





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11.3.2.  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).

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

   Major restructuring of the processing model, see Section 11.2.

11.4.  Changes from -00 to -01

   o  Removed 'mailto:' before mail addresses of authors.

   o  Added "<to be done>" as right side of 'href-strip' rule.  Fixed
      '|' to '/' for alternatives.

11.5.  Changes from -05 to -06 of draft-duerst-iri-bis-00

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

   o  Add Masinter back as another editor.

   o  Masinter integrates HRef material from HTML5 spec.

   o  Rewrite introduction sections to modernize.

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

   o  Updated references.

   o  Changed IPR text to pre5378Trust200902.

11.7.  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
      specials.







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

   o  Updated some references.

   o  Updated Michel Suginard's coordinates.

11.9.  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.

11.10.  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).

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

      Fixed errata (see
      http://www.rfc-editor.org/cgi-bin/errataSearch.pl?rfc=3987).


12.  References

12.1.  Normative References

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

   [ISO10646]
              International Organization for Standardization, "ISO/IEC
              10646:2011: Information Technology - Universal Multiple-
              Octet Coded Character Set (UCS)", ISO Standard 10646,
              March 20011, <http://standards.iso.org/ittf/
              PubliclyAvailableStandards/
              c051273_ISO_IEC_10646_2011(E).zip>.



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

   [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.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.

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

   [UNIV6]    The Unicode Consortium, "The Unicode Standard, Version
              6.0.0 (Mountain View, CA, The Unicode Consortium, 2011,
              ISBN 978-1-936213-01-6)", October 2010.

   [UTR15]    Davis, M. and M. Duerst, "Unicode Normalization Forms",
              Unicode Standard Annex #15, March 2008,
              <http://www.unicode.org/unicode/reports/tr15/
              tr15-23.html>.

12.2.  Informative References

   [Bidi]     Duerst, M. and L. Masinter, "Guidelines for
              Internationalized Resource Identifiers with Bi-directional
              Characters (Bidi IRIs)", draft-ietf-iri-bidi-guidelines-00
              (work in progress), August 2011.

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

   [Duerst97]
              Duerst, M., "The Properties and Promises of UTF-8", Proc.



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              11th International Unicode Conference, San Jose ,
              September 1997, <http://www.ifi.unizh.ch/mml/mduerst/
              papers/PDF/IUC11-UTF-8.pdf>.

   [Equivalence]
              Masinter, L. and M. Duerst, "Equivalence and
              Canonicalization of Internationalized Resource Identifiers
              (IRIs)", draft-ietf-iri-comparison-00 (work in progress),
              August 2011.

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

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

   [LEIRI]    Thompson, H., Tobin, R., and N. Walsh, "Legacy extended
              IRIs for XML resource identification", World Wide Web
              Consortium Note, November 2008,
              <http://www.w3.org/TR/leiri/>.

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




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   [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.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

   [RFC4395bis]
              Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI/IRI Schemes",
              draft-ietf-iri-4395bis-irireg-03 (work in progress),
              July 2011.

   [RFC6055]  Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on
              Encodings for Internationalized Domain Names", RFC 6055,
              February 2011.

   [RFC6082]  Whistler, K., Adams, G., Duerst, M., Presuhn, R., and J.
              Klensin, "Deprecating Unicode Language Tag Characters: RFC
              2482 is Historic", RFC 6082, November 2010.

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

   [UTR36]    Davis, M. and M. Suignard, "Unicode Security
              Considerations", Unicode Technical Report #36,
              August 2010, <http://unicode.org/reports/tr36/>.

   [XLink]    DeRose, S., Maler, E., and D. Orchard, "XML Linking
              Language (XLink) Version 1.0", World Wide Web
              Consortium REC-xlink-20010627, June 2001,
              <http://www.w3.org/TR/xlink/#link-locators>.

   [XML1]     Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Forth
              Edition)", World Wide Web Consortium REC-xml-20081126,
              August 2006, <http://www.w3.org/TR/REC-xml>.

   [XMLNamespace]
              Bray, T., Hollander, D., Layman, A., and R. Tobin,



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              "Namespaces in XML (Second Edition)", World Wide Web
              Consortium REC-xml-names-20091208, August 2006,
              <http://www.w3.org/TR/REC-xml-names>.

   [XMLSchema]
              Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes",
              World Wide Web Consortium REC-xmlschema-2-20041028,
              May 2001, <http://www.w3.org/TR/xmlschema-2/#anyURI>.

   [XPointer]
              Grosso, P., Maler, E., Marsh, J., and N. Walsh, "XPointer
              Framework", World Wide Web Consortium REC-xptr-framework-
              20030325, March 2003,
              <http://www.w3.org/TR/xptr-framework/#escaping>.


Authors' Addresses

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

   Phone: +81 42 759 6329
   Fax:   +81 42 759 6495
   Email: duerst@it.aoyama.ac.jp
   URI:   http://www.sw.it.aoyama.ac.jp/D%C3%BCrst/
          (Note: This is the percent-encoded form of an IRI.)


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

   Phone: +1-650-693-3921
   Email: michel@unicode.org
   URI:   http://www.suignard.com










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   Larry Masinter
   Adobe
   345 Park Ave
   San Jose, CA  95110
   U.S.A.

   Phone: +1-408-536-3024
   Email: masinter@adobe.com
   URI:   http://larry.masinter.net










































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