source: draft-ietf-iri-3987bis/draft-ietf-iri-3987bis.xml @ 65

Last change on this file since 65 was 65, checked in by duerst@…, 9 years ago

adding subsubsections to the ireg-name subsection for better structure

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1<?xml version="1.0"?>
2<!DOCTYPE rfc SYSTEM "rfc2629.dtd" [
3<!ENTITY rfc1738 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.1738.xml">
4<!ENTITY rfc2045 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2045.xml">
5<!ENTITY rfc2119 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml">
6<!ENTITY rfc2130 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2130.xml">
7<!ENTITY rfc2141 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2141.xml">
8<!ENTITY rfc2192 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2192.xml">
9<!ENTITY rfc2277 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2277.xml">
10<!ENTITY rfc2368 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2368.xml">
11<!ENTITY rfc2384 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2384.xml">
12<!ENTITY rfc2396 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2396.xml">
13<!ENTITY rfc2397 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2397.xml">
14<!ENTITY rfc2616 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2616.xml">
15<!ENTITY rfc2640 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2640.xml">
16<!ENTITY rfc3490 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3490.xml">
17<!ENTITY rfc3491 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3491.xml">
18<!ENTITY rfc3629 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3629.xml">
19<!ENTITY rfc3986 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3986.xml">
20<!ENTITY rfc3987 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3987.xml">
21<!ENTITY rfc5890 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5890.xml">
22<!ENTITY rfc5891 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5891.xml">
23<!ENTITY rfc6055 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.6055.xml">
24]>
25<?rfc strict='yes'?>
26
27<?xml-stylesheet type='text/css' href='rfc2629.css' ?>
28<?xml-stylesheet type='text/xsl' href='rfc2629.xslt' ?>
29<?rfc symrefs='yes'?>
30<?rfc sortrefs='yes'?>
31<?rfc iprnotified="no" ?>
32<?rfc toc='yes'?>
33<?rfc compact='yes'?>
34<?rfc subcompact='no'?>
35<rfc ipr="pre5378Trust200902" docName="draft-ietf-iri-3987bis-05" category="std" xml:lang="en" obsoletes="3987">
36<front>
37<title abbrev="IRIs">Internationalized Resource Identifiers (IRIs)</title>
38
39  <author initials="M.J." surname="Duerst" fullname='Martin Duerst'>
40    <!-- (Note: Please write "Duerst" with u-umlaut wherever
41      possible, for example as "D&#252;rst" in XML and HTML) -->
42  <organization abbrev="Aoyama Gakuin University">Aoyama Gakuin University</organization>
43  <address>
44  <postal>
45  <street>5-10-1 Fuchinobe</street>
46  <city>Sagamihara</city>
47  <region>Kanagawa</region>
48  <code>229-8558</code>
49  <country>Japan</country>
50  </postal>
51  <phone>+81 42 759 6329</phone>
52  <facsimile>+81 42 759 6495</facsimile>
53  <email>duerst@it.aoyama.ac.jp</email>
54  <uri>http://www.sw.it.aoyama.ac.jp/D%C3%BCrst/<!-- (Note: This is the percent-encoded form of an IRI)--></uri>
55  </address>
56</author>
57
58<author initials="M.L." surname="Suignard" fullname="Michel Suignard">
59   <organization>Unicode Consortium</organization>
60   <address>
61   <postal>
62   <street></street>
63   <street>P.O. Box 391476</street>
64   <city>Mountain View</city>
65   <region>CA</region>
66   <code>94039-1476</code>
67   <country>U.S.A.</country>
68   </postal>
69   <phone>+1-650-693-3921</phone>
70   <email>michel@unicode.org</email>
71   <uri>http://www.suignard.com</uri>
72   </address>
73</author>
74<author initials="L." surname="Masinter" fullname="Larry Masinter">
75   <organization>Adobe</organization>
76   <address>
77   <postal>
78   <street>345 Park Ave</street>
79   <city>San Jose</city>
80   <region>CA</region>
81   <code>95110</code>
82   <country>U.S.A.</country>
83   </postal>
84   <phone>+1-408-536-3024</phone>
85   <email>masinter@adobe.com</email>
86   <uri>http://larry.masinter.net</uri>
87   </address>
88</author>
89
90<date year="2011" month="March" day="29"/>
91<area>Applications</area>
92<workgroup>Internationalized Resource Identifiers (iri)</workgroup>
93<keyword>IRI</keyword>
94<keyword>Internationalized Resource Identifier</keyword>
95<keyword>UTF-8</keyword>
96<keyword>URI</keyword>
97<keyword>URL</keyword>
98<keyword>IDN</keyword>
99<keyword>LEIRI</keyword>
100
101<abstract>
102<t>This document defines the Internationalized Resource Identifier
103(IRI) protocol element, as an extension of the Uniform Resource
104Identifier (URI).  An IRI is a sequence of characters from the
105Universal Character Set (Unicode/ISO 10646). Grammar and processing
106rules are given for IRIs and related syntactic forms.</t>
107
108<t>In addition, this document provides named additional rule sets
109for processing otherwise invalid IRIs, in a way that supports
110other specifications that wish to mandate common behavior for
111'error' handling. In particular, rules used in some XML languages
112(LEIRI) and web applications are given.</t>
113
114<t>Defining IRI as new protocol element (rather than updating or
115extending the definition of URI) allows independent orderly
116transitions: other protocols and languages that use URIs must
117explicitly choose to allow IRIs.</t>
118
119<t>Guidelines are provided for the use and deployment of IRIs and
120related protocol elements when revising protocols, formats, and
121software components that currently deal only with URIs.</t>
122
123</abstract>
124  <note title='RFC Editor: Please remove the next paragraph before publication.'>
125    <t>This document is intended to update RFC 3987 and move towards IETF
126    Draft Standard.  For discussion and comments on this
127    draft, please join the IETF IRI WG by subscribing to the mailing
128    list public-iri@w3.org. For a list of open issues, please see
129    the issue tracker of the WG at http://trac.tools.ietf.org/wg/iri/trac/report/1.
130    For a list of individual edits, please see the change history at
131    http://trac.tools.ietf.org/wg/iri/trac/log/draft-ietf-iri-3987bis.</t>
132</note>
133</front>
134<middle>
135
136<section title="Introduction">
137
138<section title="Overview and Motivation" anchor="overview">
139
140<t>A Uniform Resource Identifier (URI) is defined in <xref
141target="RFC3986"/> as a sequence of characters chosen from a limited
142subset of the repertoire of US-ASCII <xref target="ASCII"/>
143characters.</t>
144
145<t>The characters in URIs are frequently used for representing words
146of natural languages.  This usage has many advantages: Such URIs are
147easier to memorize, easier to interpret, easier to transcribe, easier
148to create, and easier to guess. For most languages other than English,
149however, the natural script uses characters other than A - Z. For many
150people, handling Latin characters is as difficult as handling the
151characters of other scripts is for those who use only the Latin
152alphabet. Many languages with non-Latin scripts are transcribed with
153Latin letters. These transcriptions are now often used in URIs, but
154they introduce additional difficulties.</t>
155
156<t>The infrastructure for the appropriate handling of characters from
157additional scripts is now widely deployed in operating system and
158application software. Software that can handle a wide variety of
159scripts and languages at the same time is increasingly common. Also,
160an increasing number of protocols and formats can carry a wide range of
161characters.</t>
162
163<t>URIs are used both as a protocol element (for transmission and
164processing by software) and also a presentation element (for display
165and handling by people who read, interpret, coin, or guess them). The
166transition between these roles is more difficult and complex when
167dealing with the larger set of characters than allowed for URIs in
168<xref target="RFC3986"/>. </t>
169
170<t>This document defines the protocol element called Internationalized
171Resource Identifier (IRI), which allow applications of URIs to be
172extended to use resource identifiers that have a much wider repertoire
173of characters. It also provides corresponding "internationalized"
174versions of other constructs from <xref target="RFC3986"/>, such as
175URI references. The syntax of IRIs is defined in <xref
176target="syntax"/>.
177</t>
178
179<t>Using characters outside of A - Z in IRIs adds a number of
180difficulties. <xref target="Bidi"/> discusses the special case of
181bidirectional IRIs using characters from scripts written
182right-to-left.  <xref target="equivalence"/> discusses various forms
183of equivalence between IRIs. <xref target="IRIuse"/> discusses the use
184of IRIs in different situations.  <xref target="guidelines"/> gives
185additional informative guidelines.  <xref target="security"/>
186discusses IRI-specific security considerations.</t>
187
188  <t>When originally defining IRIs, several design alternatives were considered.
189    Historically interested readers can find an overview in Appendix A of <xref target="RFC3987"/>.
190  For some additional background on the design of URIs and IRIs, please also see
191    <xref target="Gettys"/>.</t>
192</section> <!-- overview -->
193
194<section title="Applicability" anchor="Applicability">
195
196<t>IRIs are designed to allow protocols and software that deal with
197URIs to be updated to handle IRIs. A "URI scheme" (as defined by <xref
198target="RFC3986"/> and registered through the IANA process defined in
199<xref target="RFC4395bis"/> also serves as an "IRI scheme". Processing of
200IRIs is accomplished by extending the URI syntax while retaining (and
201not expanding) the set of "reserved" characters, such that the syntax
202for any URI scheme may be uniformly extended to allow non-ASCII
203characters. In addition, following parsing of an IRI, it is possible
204to construct a corresponding URI by first encoding characters outside
205of the allowed URI range and then reassembling the components.
206</t>
207
208<t>Practical use of IRIs forms in place of URIs forms depends on the
209following conditions being met:</t>
210
211<t><list style="hanging">
212   
213<t hangText="a.">A protocol or format element MUST be explicitly designated to be
214  able to carry IRIs. The intent is to avoid introducing IRIs into
215  contexts that are not defined to accept them.  For example, XML
216  schema <xref target="XMLSchema"/> has an explicit type "anyURI" that
217  includes IRIs and IRI references. Therefore, IRIs and IRI references
218  can be in attributes and elements of type "anyURI".  On the other
219  hand, in the <xref target="RFC2616"/> definition of HTTP/1.1, the
220  Request URI is defined as a URI, which means that direct use of IRIs
221  is not allowed in HTTP requests.</t>
222
223<t hangText="b.">The protocol or format carrying the IRIs MUST have a
224  mechanism to represent the wide range of characters used in IRIs,
225  either natively or by some protocol- or format-specific escaping
226  mechanism (for example, numeric character references in <xref
227  target="XML1"/>).</t>
228
229<t hangText="c.">The URI scheme definition, if it explicitly allows a
230  percent sign ("%") in any syntactic component, SHOULD define the
231  interpretation of sequences of percent-encoded octets (using "%XX"
232  hex octets) as octet from sequences of UTF-8 encoded strings; this
233  is recommended in the guidelines for registering new schemes, <xref
234  target="RFC4395bis"/>.  For example, this is the practice for IMAP URLs
235  <xref target="RFC2192"/>, POP URLs <xref target="RFC2384"/> and the
236  URN syntax <xref target="RFC2141"/>). Note that use of
237  percent-encoding may also be restricted in some situations, for
238  example, URI schemes that disallow percent-encoding might still be
239  used with a fragment identifier which is percent-encoded (e.g.,
240  <xref target="XPointer"/>). See <xref target="UTF8use"/> for further
241  discussion.</t>
242</list></t>
243
244</section> <!-- applicability -->
245
246<section title="Definitions" anchor="sec-Definitions">
247 
248<t>The following definitions are used in this document; they follow the
249terms in <xref target="RFC2130"/>, <xref target="RFC2277"/>, and
250<xref target="ISO10646"/>.</t>
251<t><list style="hanging">
252   
253<t hangText="character:">A member of a set of elements used for the
254    organization, control, or representation of data. For example,
255    "LATIN CAPITAL LETTER A" names a character.</t>
256   
257<t hangText="octet:">An ordered sequence of eight bits considered as a
258    unit.</t>
259   
260<t hangText="character repertoire:">A set of characters (set in the
261    mathematical sense).</t>
262   
263<t hangText="sequence of characters:">A sequence of characters (one
264    after another).</t>
265   
266<t hangText="sequence of octets:">A sequence of octets (one after
267    another).</t>
268   
269<t hangText="character encoding:">A method of representing a sequence
270    of characters as a sequence of octets (maybe with variants). Also,
271    a method of (unambiguously) converting a sequence of octets into a
272    sequence of characters.</t>
273   
274<t hangText="charset:">The name of a parameter or attribute used to
275    identify a character encoding.</t>
276   
277<t hangText="UCS:">Universal Character Set. The coded character set
278    defined by ISO/IEC 10646 <xref target="ISO10646"/> and the Unicode
279    Standard <xref target="UNIV6"/>.</t>
280   
281<t hangText="IRI reference:">Denotes the common usage of an
282    Internationalized Resource Identifier. An IRI reference may be
283    absolute or relative.  However, the "IRI" that results from such a
284    reference only includes absolute IRIs; any relative IRI references
285    are resolved to their absolute form.  Note that in <xref
286    target="RFC2396"/> URIs did not include fragment identifiers, but
287    in <xref target="RFC3986"/> fragment identifiers are part of
288    URIs.</t>
289   
290<t hangText="URL:">The term "URL" was originally used <xref
291   target="RFC1738"/> for roughly what is now called a "URI".  Books,
292   software and documentation often refers to URIs and IRIs using the
293   "URL" term. Some usages restrict "URL" to those URIs which are not
294   URNs. Because of the ambiguity of the term using the term "URL" is
295   NOT RECOMMENDED in formal documents.</t>
296
297<t hangText="LEIRI (Legacy Extended IRI) processing:">  This term was used in
298   various XML specifications to refer
299   to strings that, although not valid IRIs, were acceptable input to
300   the processing rules in <xref target="LEIRIspec" />.</t>
301
302<t hangText="(Web Address, Hypertext Reference, HREF):"> These terms have been
303   added in this document for convenience, to allow other
304   specifications to refer to those strings that, although not valid
305   IRIs, are acceptable input to the processing rules in <xref
306   target="webaddress"/>. This usage corresponds to the parsing rules
307   of some popular web browsing applications.
308   ISSUE: Need to find a good name/abbreviation for these.</t>
309   
310<t hangText="running text:">Human text (paragraphs, sentences,
311   phrases) with syntax according to orthographic conventions of a
312   natural language, as opposed to syntax defined for ease of
313   processing by machines (e.g., markup, programming languages).</t>
314   
315<t hangText="protocol element:">Any portion of a message that affects
316    processing of that message by the protocol in question.</t>
317   
318<t hangText="presentation element:">A presentation form corresponding
319    to a protocol element; for example, using a wider range of
320    characters.</t>
321   
322<t hangText="create (a URI or IRI):">With respect to URIs and IRIs,
323     the term is used for the initial creation. This may be the
324     initial creation of a resource with a certain identifier, or the
325     initial exposition of a resource under a particular
326     identifier.</t>
327   
328<t hangText="generate (a URI or IRI):">With respect to URIs and IRIs,
329     the term is used when the identifier is generated by derivation
330     from other information.</t>
331
332<t hangText="parsed URI component:">When a URI processor parses a URI
333   (following the generic syntax or a scheme-specific syntax, the result
334   is a set of parsed URI components, each of which has a type
335   (corresponding to the syntactic definition) and a sequence of URI
336   characters.  </t>
337
338<t hangText="parsed IRI component:">When an IRI processor parses
339   an IRI directly, following the general syntax or a scheme-specific
340   syntax, the result is a set of parsed IRI components, each of
341   which has a type (corresponding to the syntactice definition)
342   and a sequence of IRI characters. (This definition is analogous
343   to "parsed URI component".)</t>
344
345<t hangText="IRI scheme:">A URI scheme may also be known as
346   an "IRI scheme" if the scheme's syntax has been extended to
347   allow non-US-ASCII characters according to the rules in this
348   document.</t>
349
350</list></t>
351</section> <!-- definitions -->
352<section title="Notation" anchor="sec-Notation">
353     
354<t>RFCs and Internet Drafts currently do not allow any characters
355outside the US-ASCII repertoire. Therefore, this document uses various
356special notations to denote such characters in examples.</t>
357     
358<t>In text, characters outside US-ASCII are sometimes referenced by
359using a prefix of 'U+', followed by four to six hexadecimal
360digits.</t>
361
362<t>To represent characters outside US-ASCII in examples, this document
363uses two notations: 'XML Notation' and 'Bidi Notation'.</t>
364
365<t>XML Notation uses a leading '&amp;#x', a trailing ';', and the
366hexadecimal number of the character in the UCS in between. For
367example, &amp;#x44F; stands for CYRILLIC CAPITAL LETTER YA. In this
368notation, an actual '&amp;' is denoted by '&amp;amp;'.</t>
369
370<t>Bidi Notation is used for bidirectional examples: Lower case
371letters stand for Latin letters or other letters that are written left
372to right, whereas upper case letters represent Arabic or Hebrew
373letters that are written right to left.</t>
374
375<t>To denote actual octets in examples (as opposed to percent-encoded
376octets), the two hex digits denoting the octet are enclosed in "&lt;"
377and "&gt;".  For example, the octet often denoted as 0xc9 is denoted
378here as &lt;c9&gt;.</t>
379
380<t> In this document, the key words "MUST", "MUST NOT", "REQUIRED",
381"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
382and "OPTIONAL" are to be interpreted as described in <xref
383target="RFC2119"/>.</t>
384
385</section> <!-- notation -->
386</section> <!-- introduction -->
387
388<section title="IRI Syntax" anchor="syntax">
389<t>This section defines the syntax of Internationalized Resource
390Identifiers (IRIs).</t>
391
392<t>As with URIs, an IRI is defined as a sequence of characters, not as
393a sequence of octets. This definition accommodates the fact that IRIs
394may be written on paper or read over the radio as well as stored or
395transmitted digitally.  The same IRI might be represented as different
396sequences of octets in different protocols or documents if these
397protocols or documents use different character encodings (and/or
398transfer encodings).  Using the same character encoding as the
399containing protocol or document ensures that the characters in the IRI
400can be handled (e.g., searched, converted, displayed) in the same way
401as the rest of the protocol or document.</t>
402
403<section title="Summary of IRI Syntax" anchor="summary">
404
405<t>IRIs are defined by extending the URI syntax in <xref
406target="RFC3986"/>, but extending the class of unreserved characters
407by adding the characters of the UCS (Universal Character Set, <xref
408target="ISO10646"/>) beyond U+007F, subject to the limitations given
409in the syntax rules below and in <xref target="limitations"/>.</t>
410
411<t>The syntax and use of components and reserved characters is the
412same as that in <xref target="RFC3986"/>. Each "URI scheme" thus also
413functions as an "IRI scheme", in that scheme-specific parsing rules
414for URIs of a scheme are be extended to allow parsing of IRIs using
415the same parsing rules.</t>
416
417<t>All the operations defined in <xref target="RFC3986"/>, such as the
418resolution of relative references, can be applied to IRIs by
419IRI-processing software in exactly the same way as they are for URIs
420by URI-processing software.</t>
421
422<t>Characters outside the US-ASCII repertoire MUST NOT be reserved and
423therefore MUST NOT be used for syntactical purposes, such as to
424delimit components in newly defined schemes. For example, U+00A2, CENT
425SIGN, is not allowed as a delimiter in IRIs, because it is in the
426'iunreserved' category. This is similar to the fact that it is not
427possible to use '-' as a delimiter in URIs, because it is in the
428'unreserved' category.</t>
429
430</section> <!-- summary -->
431<section title="ABNF for IRI References and IRIs" anchor="abnf">
432
433<t>An ABNF definition for IRI references (which are the most general
434concept and the start of the grammar) and IRIs is given here. The
435syntax of this ABNF is described in <xref target="STD68"/>. Character
436numbers are taken from the UCS, without implying any actual binary
437encoding. Terminals in the ABNF are characters, not octets.</t>
438
439<t>The following grammar closely follows the URI grammar in <xref
440target="RFC3986"/>, except that the range of unreserved characters is
441expanded to include UCS characters, with the restriction that private
442UCS characters can occur only in query parts. The grammar is split
443into two parts: Rules that differ from <xref target="RFC3986"/>
444because of the above-mentioned expansion, and rules that are the same
445as those in <xref target="RFC3986"/>. For rules that are different
446than those in <xref target="RFC3986"/>, the names of the non-terminals
447have been changed as follows. If the non-terminal contains 'URI', this
448has been changed to 'IRI'. Otherwise, an 'i' has been prefixed.
449The rule &lt;pct-form> has been introduced in order to be able to reference it from other parts of
450the document.</t>
451
452<!--
453for line length measuring in artwork (max 72 chars, three chars at start):
454      1         2         3         4         5         6         7
455456789012345678901234567890123456789012345678901234567890123456789012
456-->
457<figure>
458<preamble>The following rules are different from those in <xref target="RFC3986"/>:</preamble>
459<artwork>
460IRI            = scheme ":" ihier-part [ "?" iquery ]
461                 [ "#" ifragment ]
462
463ihier-part     = "//" iauthority ipath-abempty
464               / ipath-absolute
465               / ipath-rootless
466               / ipath-empty
467
468IRI-reference  = IRI / irelative-ref
469
470absolute-IRI   = scheme ":" ihier-part [ "?" iquery ]
471
472irelative-ref  = irelative-part [ "?" iquery ] [ "#" ifragment ]
473
474irelative-part = "//" iauthority ipath-abempty
475               / ipath-absolute
476               / ipath-noscheme
477               / ipath-empty
478
479iauthority     = [ iuserinfo "@" ] ihost [ ":" port ]
480iuserinfo      = *( iunreserved / pct-form / sub-delims / ":" )
481ihost          = IP-literal / IPv4address / ireg-name
482
483pct-form       = pct-encoded
484
485ireg-name      = *( iunreserved / sub-delims )
486
487ipath          = ipath-abempty   ; begins with "/" or is empty
488               / ipath-absolute  ; begins with "/" but not "//"
489               / ipath-noscheme  ; begins with a non-colon segment
490               / ipath-rootless  ; begins with a segment
491               / ipath-empty     ; zero characters
492
493ipath-abempty  = *( path-sep isegment )
494ipath-absolute = path-sep [ isegment-nz *( path-sep isegment ) ]
495ipath-noscheme = isegment-nz-nc *( path-sep isegment )
496ipath-rootless = isegment-nz *( path-sep isegment )
497ipath-empty    = 0&lt;ipchar&gt;
498path-sep       = "/"
499
500isegment       = *ipchar
501isegment-nz    = 1*ipchar
502isegment-nz-nc = 1*( iunreserved / pct-form / sub-delims
503                     / "@" )
504               ; non-zero-length segment without any colon ":"                     
505
506ipchar         = iunreserved / pct-form / sub-delims / ":"
507               / "@"
508 
509iquery         = *( ipchar / iprivate / "/" / "?" )
510
511ifragment      = *( ipchar / "/" / "?" )
512
513iunreserved    = ALPHA / DIGIT / "-" / "." / "_" / "~" / ucschar
514
515ucschar        = %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF
516               / %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD
517               / %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD
518               / %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD
519               / %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD
520               / %xD0000-DFFFD / %xE1000-EFFFD
521
522iprivate       = %xE000-F8FF / %xE0000-E0FFF / %xF0000-FFFFD
523               / %x100000-10FFFD
524</artwork>
525</figure>
526
527<t>Some productions are ambiguous. The "first-match-wins" (a.k.a. "greedy")
528algorithm applies. For details, see <xref target="RFC3986"/>.</t>
529
530<figure>
531<preamble>The following rules are the same as those in <xref target="RFC3986"/>:</preamble>
532<artwork>
533scheme         = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
534 
535port           = *DIGIT
536 
537IP-literal     = "[" ( IPv6address / IPvFuture  ) "]"
538 
539IPvFuture      = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )
540 
541IPv6address    =                            6( h16 ":" ) ls32
542               /                       "::" 5( h16 ":" ) ls32
543               / [               h16 ] "::" 4( h16 ":" ) ls32
544               / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
545               / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
546               / [ *3( h16 ":" ) h16 ] "::"    h16 ":"   ls32
547               / [ *4( h16 ":" ) h16 ] "::"              ls32
548               / [ *5( h16 ":" ) h16 ] "::"              h16
549               / [ *6( h16 ":" ) h16 ] "::"
550               
551h16            = 1*4HEXDIG
552ls32           = ( h16 ":" h16 ) / IPv4address
553
554IPv4address    = dec-octet "." dec-octet "." dec-octet "." dec-octet
555
556dec-octet      = DIGIT                 ; 0-9
557               / %x31-39 DIGIT         ; 10-99
558               / "1" 2DIGIT            ; 100-199
559               / "2" %x30-34 DIGIT     ; 200-249
560               / "25" %x30-35          ; 250-255
561           
562pct-encoded    = "%" HEXDIG HEXDIG
563
564unreserved     = ALPHA / DIGIT / "-" / "." / "_" / "~"
565reserved       = gen-delims / sub-delims
566gen-delims     = ":" / "/" / "?" / "#" / "[" / "]" / "@"
567sub-delims     = "!" / "$" / "&amp;" / "'" / "(" / ")"
568               / "*" / "+" / "," / ";" / "="
569</artwork></figure>
570
571<t>This syntax does not support IPv6 scoped addressing zone identifiers.</t>
572
573</section> <!-- abnf -->
574
575</section> <!-- syntax -->
576
577<section title="Processing IRIs and related protocol elements" anchor="processing">
578
579<t>IRIs are meant to replace URIs in identifying resources within new
580versions of protocols, formats, and software components that use a
581UCS-based character repertoire.  Protocols and components may use and
582process IRIs directly. However, there are still numerous systems and
583protocols which only accept URIs or components of parsed URIs; that is,
584they only accept sequences of characters within the subset of US-ASCII
585characters allowed in URIs. </t>
586
587<t>This section defines specific processing steps for IRI consumers
588which establish the relationship between the string given and the
589interpreted derivatives. These
590processing steps apply to both IRIs and IRI references (i.e., absolute
591or relative forms); for IRIs, some steps are scheme specific. </t>
592
593<section title="Converting to UCS" anchor="ucsconv"> 
594 
595<t>Input that is already in a Unicode form (i.e., a sequence of Unicode
596 characters or an octet-stream representing a Unicode-based character
597 encoding such as UTF-8 or UTF-16) should be left as is and not
598 normalized (see <xref target="normalization"/>).</t>
599
600  <t>An IRI or IRI reference is a sequence of characters from the UCS.
601    For IRIs that are not already in a Unicode form
602    (as when written on paper, read aloud, or represented in a text stream
603    using a legacy character encoding), convert the IRI to Unicode.
604    Note that some character encodings or transcriptions can be converted
605    to or represented by more than one sequence of Unicode characters.
606    Ideally the resulting IRI would use a normalized form,
607    such as Unicode Normalization Form C <xref target="UTR15"/>
608    (see <xref target='ladder'/> Normalization and Comparison),
609    since that ensures a stable, consistent representation
610    that is most likely to produce the intended results.
611    Implementers and users are cautioned that, while denormalized character sequences are valid,
612    they might be difficult for other users or processes to reproduce
613    and might lead to unexpected results.
614  </t>
615
616<t> In other cases (written on paper, read aloud, or otherwise
617 represented independent of any character encoding) represent the IRI
618 as a sequence of characters from the UCS normalized according to
619 Unicode Normalization Form C (NFC, <xref target="UTR15"/>).</t>
620</section> <!-- ucsconv -->
621
622<section title="Parse the IRI into IRI components">
623
624<t>Parse the IRI, either as a relative reference (no scheme)
625or using scheme specific processing (according to the scheme
626given); the result is a set of parsed IRI components.</t>
627
628<t>NOTE: The result of parsing into components will correspond
629to subtrings of the IRI that may be accessible via an API.
630For example, in <xref target="HTML5"/>, the protocol
631components of interest are SCHEME (scheme), HOST (ireg-name), PORT
632(port), the PATH (ipath after the initial "/"), QUERY (iquery),
633FRAGMENT (ifragment), and AUTHORITY (iauthority).
634</t>
635
636<t>Subsequent processing rules are sometimes used to define other
637syntactic components. For example, <xref target="HTML5"/> defines APIs
638for IRI processing; in these APIs:
639
640<list style="hanging">
641<t hangText="HOSTSPECIFIC"> the substring that follows
642the substring matched by the iauthority production, or the whole
643string if the iauthority production wasn't matched.</t>
644<t hangText="HOSTPORT"> if there is a scheme component and a port
645component and the port given by the port component is different than
646the default port defined for the protocol given by the scheme
647component, then HOSTPORT is the substring that starts with the
648substring matched by the host production and ends with the substring
649matched by the port production, and includes the colon in between the
650two. Otherwise, it is the same as the host component.
651</t>
652</list>
653</t>
654</section> <!-- parse -->
655
656<section title="General percent-encoding of IRI components" anchor="compmapping">
657   
658<t>Except as noted in the following subsections, IRI components are mapped
659to the equivalent URI components by percent-encoding those characters
660not allowed in URIs. Previous processing steps will have removed
661some characters, and the interpretation of reserved characters will
662have already been done (with the syntactic reserved characters outside
663of the IRI component). This mapping is defined for all sequences
664of Unicode characters, whether or not they are valid for the component
665in question. </t>
666   
667<t>For each character which is not allowed anywhere in a valid URI, apply the following steps. </t>
668
669<t><list style="hanging">
670
671<t hangText="Convert to UTF-8">Convert the character to a sequence of
672  one or more octets using UTF-8 <xref target="RFC3629"/>.</t>
673
674<t hangText="Percent encode">Convert each octet of this sequence to %HH,
675   where HH is the hexadecimal notation of the octet value. The
676   hexadecimal notation SHOULD use uppercase letters. (This is the
677   general URI percent-encoding mechanism in Section 2.1 of <xref
678   target="RFC3986"/>.)</t>
679   
680</list></t>
681
682<t>Note that the mapping is an identity transformation for parsed URI
683components of valid URIs, and is idempotent: applying the mapping a
684second time will not change anything.</t>
685</section> <!-- general conversion -->
686
687<section title="Mapping ireg-name" anchor="dnsmapping">
688  <section title='Mapping using Percent-Encoding' anchor='dnspercent'>
689  <t>The ireg-name component SHOULD be converted
690    according to the general procedure for percent-encoding
691    of IRI components described in <xref target="compmapping"/>.</t>
692 
693  <t>For example, the IRI
694    <vspace/>"http://r&amp;#xE9;sum&amp;#xE9;.example.org"<vspace/> will be
695    converted to <vspace/>"http://r%C3%A9sum%C3%A9.example.org".</t>
696
697  <t>This conversion for ireg-name is in line with Section 3.2.2
698    of <xref target="RFC3986"/>, which does not mandate
699    a particular registered name lookup technology. For further background,
700    see <xref target="RFC6055"/> and <xref target="Gettys"/>.</t>
701</section>
702<section title="Mapping using Punycode">
703  <t>The ireg-name component MAY also be converted as follows:</t>
704
705<t>Replace the ireg-name part of the IRI by the part converted using
706the Domain Name Lookup procedure (Subsections 5.3 to 5.5) of <xref target="RFC5891"/>.
707  on each dot-separated label, and by using U+002E
708(FULL STOP) as a label separator.
709This procedure may fail, but this would mean that the IRI cannot be resolved.
710In such cases, if the domain name conversion fails, then the
711entire IRI conversion fails. Processors that have no mechanism for
712signalling a failure MAY instead substitute an otherwise
713invalid host name, although such processing SHOULD be avoided.</t>
714
715<t>For example, the IRI
716<vspace/>"http://r&amp;#xE9;sum&amp;#xE9;.example.org"<vspace/> MAY be
717converted to <vspace/>"http://xn--rsum-bad.example.org"<vspace/>.</t>
718</section>
719  <section title="Additional Considerations">
720<t><list style="hanging"> 
721
722<t hangText="Note:">Domain Names may appear in parts of an IRI other
723than the ireg-name part.  It is the responsibility of scheme-specific
724implementations (if the Internationalized Domain Name is part of the
725scheme syntax) or of server-side implementations (if the
726Internationalized Domain Name is part of 'iquery') to apply the
727necessary conversions at the appropriate point. Example: Trying to
728validate the Web page at<vspace/>
729http://r&amp;#xE9;sum&amp;#xE9;.example.org would lead to an IRI of
730<vspace/>http://validator.w3.org/check?uri=http%3A%2F%2Fr&amp;#xE9;sum&amp;#xE9;.<vspace/>example.org,
731which would convert to a URI
732of<vspace/>http://validator.w3.org/check?uri=http%3A%2F%2Fr%C3%A9sum%C3%A9.<vspace/>example.org.
733The server-side implementation is responsible for making the
734necessary conversions to be able to retrieve the Web page.</t>
735
736<t hangText="Note:">In this process, characters allowed in URI
737references and existing percent-encoded sequences are not encoded further.
738(This mapping is similar to, but different from, the encoding applied
739when arbitrary content is included in some part of a URI.)
740
741For example, an IRI of
742<vspace/>"http://www.example.org/red%09ros&amp;#xE9;#red"
743(in XML notation) is converted to
744<vspace/>"http://www.example.org/red%09ros%C3%A9#red", not to
745something like
746<vspace/>"http%3A%2F%2Fwww.example.org%2Fred%2509ros%C3%A9%23red".
747((DESIGN QUESTION: What about e.g. http://r%C3%A9sum%C3%A9.example.org in an IRI? Will that get converted to punycode, or not?))
748
749</t>
750
751</list></t>
752</section>
753</section> <!-- dnsmapping -->
754
755<section title="Mapping query components" anchor="querymapping">
756
757<t>((NOTE: SEE ISSUES LIST))
758
759For compatibility with existing deployed HTTP infrastructure,
760the following special case applies for schemes "http" and "https"
761and IRIs whose origin has a document charset other than one which
762is UCS-based (e.g., UTF-8 or UTF-16). In such a case, the "query"
763component of an IRI is mapped into a URI by using the document
764charset rather than UTF-8 as the binary representation before
765pct-encoding. This mapping is not applied for any other scheme
766or component.</t>
767
768</section> <!-- querymapping -->
769
770<section title="Mapping IRIs to URIs" anchor="mapping">
771
772<t>The canonical mapping from a IRI to URI is defined by applying the
773mapping above (from IRI to URI components) and then reassembling a URI
774from the parsed URI components using the original punctuation that
775delimited the IRI components. </t>
776
777</section> <!-- mapping -->
778
779<section title="Converting URIs to IRIs" anchor="URItoIRI">
780
781<t>In some situations, for presentation and further processing,
782it is desirable to convert a URI into an equivalent IRI in which
783natural characters are represented directly rather than
784percent encoded. Of course, every URI is already an IRI in
785its own right without any conversion, and in general there
786This section gives one such procedure for this conversion.
787</t>
788
789<t>
790The conversion described in this section, if given a valid URI, will
791result in an IRI that maps back to the URI used as an input for the
792conversion (except for potential case differences in percent-encoding
793and for potential percent-encoded unreserved characters).
794
795However, the IRI resulting from this conversion may differ
796from the original IRI (if there ever was one).</t> 
797
798<t>URI-to-IRI conversion removes percent-encodings, but not all
799percent-encodings can be eliminated. There are several reasons for
800this:</t>
801
802<t><list style="hanging">
803
804<t hangText="1.">Some percent-encodings are necessary to distinguish
805    percent-encoded and unencoded uses of reserved characters.</t>
806
807<t hangText="2.">Some percent-encodings cannot be interpreted as sequences
808    of UTF-8 octets.<vspace blankLines="1"/>
809    (Note: The octet patterns of UTF-8 are highly regular.
810    Therefore, there is a very high probability, but no guarantee,
811    that percent-encodings that can be interpreted as sequences of UTF-8
812    octets actually originated from UTF-8. For a detailed discussion,
813    see <xref target="Duerst97"/>.)</t>
814
815<t hangText="3.">The conversion may result in a character that is not
816    appropriate in an IRI. See <xref target="abnf"/>, <xref target="visual"/>,
817      and <xref target="limitations"/> for further details.</t>
818
819<t hangText="4.">IRI to URI conversion has different rules for
820    dealing with domain names and query parameters.</t>
821
822</list></t>
823
824<t>Conversion from a URI to an IRI MAY be done by using the following
825steps:
826
827<list style="hanging">
828<t hangText="1.">Represent the URI as a sequence of octets in
829       US-ASCII.</t>
830
831<t hangText="2.">Convert all percent-encodings ("%" followed by two
832      hexadecimal digits) to the corresponding octets, except those
833      corresponding to "%", characters in "reserved", and characters
834      in US-ASCII not allowed in URIs.</t> 
835
836<t hangText="3.">Re-percent-encode any octet produced in step 2 that
837      is not part of a strictly legal UTF-8 octet sequence.</t>
838
839
840<t hangText="4.">Re-percent-encode all octets produced in step 3 that
841      in UTF-8 represent characters that are not appropriate according
842      to <xref target="abnf"/>, <xref target="visual"/>, and <xref
843      target="limitations"/>.</t> 
844
845<t hangText="5.">Interpret the resulting octet sequence as a sequence
846      of characters encoded in UTF-8.</t>
847
848<t hangText="6.">URIs known to contain domain names in the reg-name
849      component SHOULD convert punycode-encoded domain name labels to
850      the corresponding characters using the ToUnicode procedure. </t>
851</list></t>
852
853<t>This procedure will convert as many percent-encoded characters as
854possible to characters in an IRI. Because there are some choices when
855step 4 is applied (see <xref target="limitations"/>), results may
856vary.</t>
857
858<t>Conversions from URIs to IRIs MUST NOT use any character
859encoding other than UTF-8 in steps 3 and 4, even if it might be
860possible to guess from the context that another character encoding
861than UTF-8 was used in the URI.  For example, the URI
862"http://www.example.org/r%E9sum%E9.html" might with some guessing be
863interpreted to contain two e-acute characters encoded as
864iso-8859-1. It must not be converted to an IRI containing these
865e-acute characters. Otherwise, in the future the IRI will be mapped to
866"http://www.example.org/r%C3%A9sum%C3%A9.html", which is a different
867URI from "http://www.example.org/r%E9sum%E9.html".</t>
868
869<section title="Examples">
870
871<t>This section shows various examples of converting URIs to IRIs.
872Each example shows the result after each of the steps 1 through 6 is
873applied. XML Notation is used for the final result.  Octets are
874denoted by "&lt;" followed by two hexadecimal digits followed by
875"&gt;".</t>
876
877<t>The following example contains the sequence "%C3%BC", which is a
878strictly legal UTF-8 sequence, and which is converted into the actual
879character U+00FC, LATIN SMALL LETTER U WITH DIAERESIS (also known as
880u-umlaut).
881
882<list style="hanging">
883<t hangText="1.">http://www.example.org/D%C3%BCrst</t>
884<t hangText="2.">http://www.example.org/D&lt;c3&gt;&lt;bc&gt;rst</t>
885<t hangText="3.">http://www.example.org/D&lt;c3&gt;&lt;bc&gt;rst</t>
886<t hangText="4.">http://www.example.org/D&lt;c3&gt;&lt;bc&gt;rst</t>
887<t hangText="5.">http://www.example.org/D&amp;#xFC;rst</t>
888<t hangText="6.">http://www.example.org/D&amp;#xFC;rst</t>
889</list>
890</t>
891
892<t>The following example contains the sequence "%FC", which might
893represent U+00FC, LATIN SMALL LETTER U WITH DIAERESIS, in
894the<vspace/>iso-8859-1 character encoding.  (It might represent other
895characters in other character encodings. For example, the octet
896&lt;fc&gt; in iso-8859-5 represents U+045C, CYRILLIC SMALL LETTER
897KJE.)  Because &lt;fc&gt; is not part of a strictly legal UTF-8
898sequence, it is re-percent-encoded in step 3.
899
900
901<list style="hanging">
902<t hangText="1.">http://www.example.org/D%FCrst</t>
903<t hangText="2.">http://www.example.org/D&lt;fc&gt;rst</t>
904<t hangText="3.">http://www.example.org/D%FCrst</t>
905<t hangText="4.">http://www.example.org/D%FCrst</t>
906<t hangText="5.">http://www.example.org/D%FCrst</t>
907<t hangText="6.">http://www.example.org/D%FCrst</t>
908</list>
909</t>
910
911<t>The following example contains "%e2%80%ae", which is the percent-encoded<vspace/>UTF-8
912character encoding of U+202E, RIGHT-TO-LEFT OVERRIDE. <xref target="visual"/>
913forbids the direct use of this character in an IRI. Therefore, the
914corresponding octets are re-percent-encoded in step 4. This example shows
915that the case (upper- or lowercase) of letters used in percent-encodings may not be preserved.
916The example also contains a punycode-encoded domain name label (xn--99zt52a),
917which is not converted.
918
919<list style="hanging">
920<t hangText="1.">http://xn--99zt52a.example.org/%e2%80%ae</t>
921<t hangText="2.">http://xn--99zt52a.example.org/&lt;e2&gt;&lt;80&gt;&lt;ae&gt;</t>
922<t hangText="3.">http://xn--99zt52a.example.org/&lt;e2&gt;&lt;80&gt;&lt;ae&gt;</t>
923<t hangText="4.">http://xn--99zt52a.example.org/%E2%80%AE</t>
924<t hangText="5.">http://xn--99zt52a.example.org/%E2%80%AE</t>
925<t hangText="6.">http://&amp;#x7D0D;&amp;#x8C46;.example.org/%E2%80%AE</t>
926</list></t>
927
928<t>Note that the label "xn--99zt52a" is converted to U+7D0D U+8C46
929(Japanese Natto). ((EDITOR NOTE: There is some inconsistency in this note.))</t>
930
931</section> <!-- examples -->
932</section> <!-- URItoIRI -->
933</section> <!-- processing -->
934<section title="Bidirectional IRIs for Right-to-Left Languages" anchor="Bidi">
935
936<t>Some UCS characters, such as those used in the Arabic and Hebrew
937scripts, have an inherent right-to-left (rtl) writing direction. IRIs
938containing these characters (called bidirectional IRIs or Bidi IRIs)
939require additional attention because of the non-trivial relation
940between logical representation (used for digital representation and
941for reading/spelling) and visual representation (used for
942display/printing).</t>
943
944<t>Because of the complex interaction between the logical representation,
945the visual representation, and the syntax of a Bidi IRI, a balance is
946needed between various requirements.
947The main requirements are<list style="hanging">
948<t hangText="1.">user-predictable conversion between visual and
949    logical representation;</t>
950<t hangText="2.">the ability to include a wide range of characters
951    in various parts of the IRI; and</t>
952<t hangText="3.">minor or no changes or restrictions for
953      implementations.</t>
954</list></t>
955
956<section title="Logical Storage and Visual Presentation" anchor="visual">
957
958<t>When stored or transmitted in digital representation, bidirectional
959IRIs MUST be in full logical order and MUST conform to the IRI syntax
960rules (which includes the rules relevant to their scheme). This
961ensures that bidirectional IRIs can be processed in the same way as
962other IRIs.</t> <t>Bidirectional IRIs MUST be rendered by using the
963Unicode Bidirectional Algorithm <xref target="UNIV6"/>, <xref
964target="UNI9"/>.  Bidirectional IRIs MUST be rendered in the same way
965as they would be if they were in a left-to-right embedding; i.e., as
966if they were preceded by U+202A, LEFT-TO-RIGHT EMBEDDING (LRE), and
967followed by U+202C, POP DIRECTIONAL FORMATTING (PDF).  Setting the
968embedding direction can also be done in a higher-level protocol (e.g.,
969the dir='ltr' attribute in HTML).</t> 
970
971<t>There is no requirement to use the above embedding if the display
972is still the same without the embedding. For example, a bidirectional
973IRI in a text with left-to-right base directionality (such as used for
974English or Cyrillic) that is preceded and followed by whitespace and
975strong left-to-right characters does not need an embedding.  Also, a
976bidirectional relative IRI reference that only contains strong
977right-to-left characters and weak characters and that starts and ends
978with a strong right-to-left character and appears in a text with
979right-to-left base directionality (such as used for Arabic or Hebrew)
980and is preceded and followed by whitespace and strong characters does
981not need an embedding.</t>
982
983<t>In some other cases, using U+200E, LEFT-TO-RIGHT MARK (LRM), may be
984sufficient to force the correct display behavior.  However, the
985details of the Unicode Bidirectional algorithm are not always easy to
986understand. Implementers are strongly advised to err on the side of
987caution and to use embedding in all cases where they are not
988completely sure that the display behavior is unaffected without the
989embedding.</t>
990
991<t>The Unicode Bidirectional Algorithm (<xref target="UNI9"/>, section
9924.3) permits higher-level protocols to influence bidirectional
993rendering. Such changes by higher-level protocols MUST NOT be used if
994they change the rendering of IRIs.</t> 
995
996<t>The bidirectional formatting characters that may be used before or
997after the IRI to ensure correct display are not themselves part of the
998IRI.  IRIs MUST NOT contain bidirectional formatting characters (LRM,
999RLM, LRE, RLE, LRO, RLO, and PDF). They affect the visual rendering of
1000the IRI but do not appear themselves. It would therefore not be
1001possible to input an IRI with such characters correctly.</t>
1002
1003</section> <!-- visual -->
1004<section title="Bidi IRI Structure" anchor="bidi-structure">
1005
1006<t>The Unicode Bidirectional Algorithm is designed mainly for running
1007text.  To make sure that it does not affect the rendering of
1008bidirectional IRIs too much, some restrictions on bidirectional IRIs
1009are necessary. These restrictions are given in terms of delimiters
1010(structural characters, mostly punctuation such as "@", ".", ":",
1011and<vspace/>"/") and components (usually consisting mostly of letters
1012and digits).</t>
1013
1014<t>The following syntax rules from <xref target="abnf"/> correspond to
1015components for the purpose of Bidi behavior: iuserinfo, ireg-name,
1016isegment, isegment-nz, isegment-nz-nc, ireg-name, iquery, and
1017ifragment.</t>
1018
1019<t>Specifications that define the syntax of any of the above
1020components MAY divide them further and define smaller parts to be
1021components according to this document. As an example, the restrictions
1022of <xref target="RFC3490"/> on bidirectional domain names correspond
1023to treating each label of a domain name as a component for schemes
1024with ireg-name as a domain name.  Even where the components are not
1025defined formally, it may be helpful to think about some syntax in
1026terms of components and to apply the relevant restrictions.  For
1027example, for the usual name/value syntax in query parts, it is
1028convenient to treat each name and each value as a component. As
1029another example, the extensions in a resource name can be treated as
1030separate components.</t>
1031
1032<t>For each component, the following restrictions apply:</t>
1033<t>
1034<list style="hanging">
1035
1036<t hangText="1.">A component SHOULD NOT use both right-to-left and
1037  left-to-right characters.</t>
1038
1039<t hangText="2.">A component using right-to-left characters SHOULD
1040  start and end with right-to-left characters.</t>
1041
1042</list></t>
1043
1044<t>The above restrictions are given as "SHOULD"s, rather than as
1045"MUST"s.  For IRIs that are never presented visually, they are not
1046relevant.  However, for IRIs in general, they are very important to
1047ensure consistent conversion between visual presentation and logical
1048representation, in both directions.</t>
1049
1050<t><list style="hanging">
1051
1052<t hangText="Note:">In some components, the above restrictions may
1053  actually be strictly enforced.  For example, <xref
1054  target="RFC3490"></xref> requires that these restrictions apply to
1055  the labels of a host name for those schemes where ireg-name is a
1056  host name.  In some other components (for example, path components)
1057  following these restrictions may not be too difficult.  For other
1058  components, such as parts of the query part, it may be very
1059  difficult to enforce the restrictions because the values of query
1060  parameters may be arbitrary character sequences.</t>
1061
1062</list></t>
1063
1064<t>If the above restrictions cannot be satisfied otherwise, the
1065affected component can always be mapped to URI notation as described
1066in <xref target="compmapping"/>. Please note that the whole component
1067has to be mapped (see also Example 9 below).</t>
1068
1069</section> <!-- bidi-structure -->
1070
1071<section title="Input of Bidi IRIs" anchor="bidiInput">
1072
1073<t>Bidi input methods MUST generate Bidi IRIs in logical order while
1074rendering them according to <xref target="visual"/>.  During input,
1075rendering SHOULD be updated after every new character is input to
1076avoid end-user confusion.</t>
1077
1078</section> <!-- bidiInput -->
1079
1080<section title="Examples">
1081
1082<t>This section gives examples of bidirectional IRIs, in Bidi
1083Notation.  It shows legal IRIs with the relationship between logical
1084and visual representation and explains how certain phenomena in this
1085relationship may look strange to somebody not familiar with
1086bidirectional behavior, but familiar to users of Arabic and Hebrew. It
1087also shows what happens if the restrictions given in <xref
1088target="bidi-structure"/> are not followed. The examples below can be
1089seen at <xref target="BidiEx"/>, in Arabic, Hebrew, and Bidi Notation
1090variants.</t>
1091
1092<t>To read the bidi text in the examples, read the visual
1093representation from left to right until you encounter a block of rtl
1094text. Read the rtl block (including slashes and other special
1095characters) from right to left, then continue at the next unread ltr
1096character.</t>
1097
1098<t>Example 1: A single component with rtl characters is inverted:
1099<vspace/>Logical representation:
1100"http://ab.CDEFGH.ij/kl/mn/op.html"<vspace/>Visual representation:
1101"http://ab.HGFEDC.ij/kl/mn/op.html"<vspace/> Components can be read
1102one by one, and each component can be read in its natural
1103direction.</t>
1104
1105<t>Example 2: More than one consecutive component with rtl characters
1106is inverted as a whole: <vspace/>Logical representation:
1107"http://ab.CDE.FGH/ij/kl/mn/op.html"<vspace/>Visual representation:
1108"http://ab.HGF.EDC/ij/kl/mn/op.html"<vspace/> A sequence of rtl
1109components is read rtl, in the same way as a sequence of rtl words is
1110read rtl in a bidi text.</t>
1111
1112<t>Example 3: All components of an IRI (except for the scheme) are
1113rtl.  All rtl components are inverted overall: <vspace/>Logical
1114representation:
1115"http://AB.CD.EF/GH/IJ/KL?MN=OP;QR=ST#UV"<vspace/>Visual
1116representation: "http://VU#TS=RQ;PO=NM?LK/JI/HG/FE.DC.BA"<vspace/> The
1117whole IRI (except the scheme) is read rtl. Delimiters between rtl
1118components stay between the respective components; delimiters between
1119ltr and rtl components don't move.</t>
1120
1121<t>Example 4: Each of several sequences of rtl components is inverted
1122on its own: <vspace/>Logical representation:
1123"http://AB.CD.ef/gh/IJ/KL.html"<vspace/>Visual representation:
1124"http://DC.BA.ef/gh/LK/JI.html"<vspace/> Each sequence of rtl
1125components is read rtl, in the same way as each sequence of rtl words
1126in an ltr text is read rtl.</t>
1127
1128<t>Example 5: Example 2, applied to components of different kinds:
1129<vspace/>Logical representation: "http://ab.cd.EF/GH/ij/kl.html"
1130<vspace/>Visual representation:
1131"http://ab.cd.HG/FE/ij/kl.html"<vspace/> The inversion of the domain
1132name label and the path component may be unexpected, but it is
1133consistent with other bidi behavior.  For reassurance that the domain
1134component really is "ab.cd.EF", it may be helpful to read aloud the
1135visual representation following the bidi algorithm. After
1136"http://ab.cd." one reads the RTL block "E-F-slash-G-H", which
1137corresponds to the logical representation.
1138</t>
1139
1140<t>Example 6: Same as Example 5, with more rtl components:
1141<vspace/>Logical representation:
1142"http://ab.CD.EF/GH/IJ/kl.html"<vspace/>Visual representation:
1143"http://ab.JI/HG/FE.DC/kl.html"<vspace/> The inversion of the domain
1144name labels and the path components may be easier to identify because
1145the delimiters also move.</t>
1146
1147<t>Example 7: A single rtl component includes digits: <vspace/>Logical
1148representation: "http://ab.CDE123FGH.ij/kl/mn/op.html"<vspace/>Visual
1149representation: "http://ab.HGF123EDC.ij/kl/mn/op.html"<vspace/>
1150Numbers are written ltr in all cases but are treated as an additional
1151embedding inside a run of rtl characters. This is completely
1152consistent with usual bidirectional text.</t>
1153
1154<t>Example 8 (not allowed): Numbers are at the start or end of an rtl
1155component:<vspace/>Logical representation:
1156"http://ab.cd.ef/GH1/2IJ/KL.html"<vspace/>Visual representation:
1157"http://ab.cd.ef/LK/JI1/2HG.html"<vspace/> The sequence "1/2" is
1158interpreted by the bidi algorithm as a fraction, fragmenting the
1159components and leading to confusion. There are other characters that
1160are interpreted in a special way close to numbers; in particular, "+",
1161"-", "#", "$", "%", ",", ".", and ":".</t>
1162
1163<t>Example 9 (not allowed): The numbers in the previous example are
1164percent-encoded: <vspace/>Logical representation:
1165"http://ab.cd.ef/GH%31/%32IJ/KL.html",<vspace/>Visual representation:
1166"http://ab.cd.ef/LK/JI%32/%31HG.html"</t>
1167
1168<t>Example 10 (allowed but not recommended): <vspace/>Logical
1169representation: "http://ab.CDEFGH.123/kl/mn/op.html"<vspace/>Visual
1170representation: "http://ab.123.HGFEDC/kl/mn/op.html"<vspace/>
1171Components consisting of only numbers are allowed (it would be rather
1172difficult to prohibit them), but these may interact with adjacent RTL
1173components in ways that are not easy to predict.</t>
1174
1175<t>Example 11 (allowed but not recommended): <vspace/>Logical
1176representation: "http://ab.CDEFGH.123ij/kl/mn/op.html"<vspace/>Visual
1177representation: "http://ab.123.HGFEDCij/kl/mn/op.html"<vspace/>
1178Components consisting of numbers and left-to-right characters are
1179allowed, but these may interact with adjacent RTL components in ways
1180that are not easy to predict.</t>
1181</section><!-- examples -->
1182</section><!-- bidi -->
1183
1184<section title="Normalization and Comparison" anchor="equivalence">
1185
1186<t><list style="hanging"><t hangText="Note:">The structure and much of
1187  the material for this section is taken from section 6 of <xref
1188  target="RFC3986"></xref>; the differences are due to the specifics
1189  of IRIs.</t></list></t>
1190
1191<t>One of the most common operations on IRIs is simple comparison:
1192Determining whether two IRIs are equivalent, without using the IRIs to
1193access their respective resource(s). A comparison is performed
1194whenever a response cache is accessed, a browser checks its history to
1195color a link, or an XML parser processes tags within a
1196namespace. Extensive normalization prior to comparison of IRIs may be
1197used by spiders and indexing engines to prune a search space or reduce
1198duplication of request actions and response storage.</t>
1199
1200<t>IRI comparison is performed for some particular purpose. Protocols
1201or implementations that compare IRIs for different purposes will often
1202be subject to differing design trade-offs in regards to how much
1203effort should be spent in reducing aliased identifiers. This section
1204describes various methods that may be used to compare IRIs, the
1205trade-offs between them, and the types of applications that might use
1206them.</t>
1207
1208<section title="Equivalence">
1209
1210<t>Because IRIs exist to identify resources, presumably they should be
1211considered equivalent when they identify the same resource. However,
1212this definition of equivalence is not of much practical use, as there
1213is no way for an implementation to compare two resources to determine
1214if they are "the same" unless it has full knowledge or control of
1215them. For this reason, determination of equivalence or difference of
1216IRIs is based on string comparison, perhaps augmented by reference to
1217additional rules provided by URI scheme definitions.  We use the terms
1218"different" and "equivalent" to describe the possible outcomes of such
1219comparisons, but there are many application-dependent versions of
1220equivalence.</t>
1221
1222<t>Even when it is possible to determine that two IRIs are equivalent,
1223IRI comparison is not sufficient to determine whether two IRIs
1224identify different resources. For example, an owner of two different
1225domain names could decide to serve the same resource from both,
1226resulting in two different IRIs. Therefore, comparison methods are
1227designed to minimize false negatives while strictly avoiding false
1228positives.</t>
1229
1230<t>In testing for equivalence, applications should not directly
1231compare relative references; the references should be converted to
1232their respective target IRIs before comparison. When IRIs are compared
1233to select (or avoid) a network action, such as retrieval of a
1234representation, fragment components (if any) should be excluded from
1235the comparison.</t>
1236
1237<t>Applications using IRIs as identity tokens with no relationship to
1238a protocol MUST use the Simple String Comparison (see <xref
1239target="stringcomp"></xref>).  All other applications MUST select one
1240of the comparison practices from the Comparison Ladder (see <xref
1241target="ladder"></xref>.</t>
1242</section> <!-- equivalence -->
1243
1244
1245<section title="Preparation for Comparison">
1246<t>Any kind of IRI comparison REQUIRES that any additional contextual
1247processing is first performed, including undoing higher-level
1248escapings or encodings in the protocol or format that carries an
1249IRI. This preprocessing is usually done when the protocol or format is
1250parsed.</t>
1251
1252<t>Examples of contextual preprocessing steps are described in <xref
1253target="LEIRIHREF"/>. </t>
1254
1255<t>Examples of such escapings or encodings are entities and
1256numeric character references in <xref target="HTML4"></xref> and <xref
1257target="XML1"></xref>. As an example,
1258"http://example.org/ros&amp;eacute;" (in HTML),
1259"http://example.org/ros&amp;#233;" (in HTML or XML), and
1260<vspace/>"http://example.org/ros&amp;#xE9;" (in HTML or XML) are all
1261resolved into what is denoted in this document (see <xref
1262target="sec-Notation"></xref>) as "http://example.org/ros&amp;#xE9;"
1263(the "&amp;#xE9;" here standing for the actual e-acute character, to
1264compensate for the fact that this document cannot contain non-ASCII
1265characters).</t>
1266
1267<t>Similar considerations apply to encodings such as Transfer Codings
1268in HTTP (see <xref target="RFC2616"></xref>) and Content Transfer
1269Encodings in MIME (<xref target="RFC2045"></xref>), although in these
1270cases, the encoding is based not on characters but on octets, and
1271additional care is required to make sure that characters, and not just
1272arbitrary octets, are compared (see <xref
1273target="stringcomp"></xref>).</t>
1274
1275</section> <!-- preparation -->
1276
1277<section title="Comparison Ladder" anchor="ladder">
1278
1279<t>In practice, a variety of methods are used to test IRI
1280equivalence. These methods fall into a range distinguished by the
1281amount of processing required and the degree to which the probability
1282of false negatives is reduced. As noted above, false negatives cannot
1283be eliminated. In practice, their probability can be reduced, but this
1284reduction requires more processing and is not cost-effective for all
1285applications.</t>
1286
1287
1288<t>If this range of comparison practices is considered as a ladder,
1289the following discussion will climb the ladder, starting with
1290practices that are cheap but have a relatively higher chance of
1291producing false negatives, and proceeding to those that have higher
1292computational cost and lower risk of false negatives.</t>
1293
1294<section title="Simple String Comparison" anchor="stringcomp">
1295
1296<t>If two IRIs, when considered as character strings, are identical,
1297then it is safe to conclude that they are equivalent.  This type of
1298equivalence test has very low computational cost and is in wide use in
1299a variety of applications, particularly in the domain of parsing. It
1300is also used when a definitive answer to the question of IRI
1301equivalence is needed that is independent of the scheme used and that
1302can be calculated quickly and without accessing a network. An example
1303of such a case is XML Namespaces (<xref
1304target="XMLNamespace"></xref>).</t>
1305
1306
1307<t>Testing strings for equivalence requires some basic precautions.
1308This procedure is often referred to as "bit-for-bit" or
1309"byte-for-byte" comparison, which is potentially misleading. Testing
1310strings for equality is normally based on pair comparison of the
1311characters that make up the strings, starting from the first and
1312proceeding until both strings are exhausted and all characters are
1313found to be equal, until a pair of characters compares unequal, or
1314until one of the strings is exhausted before the other.</t>
1315
1316<t>This character comparison requires that each pair of characters be
1317put in comparable encoding form. For example, should one IRI be stored
1318in a byte array in UTF-8 encoding form and the second in a UTF-16
1319encoding form, bit-for-bit comparisons applied naively will produce
1320errors. It is better to speak of equality on a character-for-character
1321rather than on a byte-for-byte or bit-for-bit basis.  In practical
1322terms, character-by-character comparisons should be done codepoint by
1323codepoint after conversion to a common character encoding form.
1324
1325When comparing character by character, the comparison function MUST
1326NOT map IRIs to URIs, because such a mapping would create additional
1327spurious equivalences. It follows that an IRI SHOULD NOT be modified
1328when being transported if there is any chance that this IRI might be
1329used in a context that uses Simple String Comparison.</t>
1330
1331
1332<t>False negatives are caused by the production and use of IRI
1333aliases. Unnecessary aliases can be reduced, regardless of the
1334comparison method, by consistently providing IRI references in an
1335already normalized form (i.e., a form identical to what would be
1336produced after normalization is applied, as described below).
1337Protocols and data formats often limit some IRI comparisons to simple
1338string comparison, based on the theory that people and implementations
1339will, in their own best interest, be consistent in providing IRI
1340references, or at least be consistent enough to negate any efficiency
1341that might be obtained from further normalization.</t>
1342</section> <!-- stringcomp -->
1343
1344<section title="Syntax-Based Normalization">
1345
1346<figure><preamble>Implementations may use logic based on the
1347definitions provided by this specification to reduce the probability
1348of false negatives. This processing is moderately higher in cost than
1349character-for-character string comparison. For example, an application
1350using this approach could reasonably consider the following two IRIs
1351equivalent:</preamble>
1352
1353<artwork>
1354   example://a/b/c/%7Bfoo%7D/ros&amp;#xE9;
1355   eXAMPLE://a/./b/../b/%63/%7bfoo%7d/ros%C3%A9
1356</artwork></figure>
1357
1358<t>Web user agents, such as browsers, typically apply this type of IRI
1359normalization when determining whether a cached response is
1360available. Syntax-based normalization includes such techniques as case
1361normalization, character normalization, percent-encoding
1362normalization, and removal of dot-segments.</t>
1363
1364<section title="Case Normalization">
1365
1366<t>For all IRIs, the hexadecimal digits within a percent-encoding
1367triplet (e.g., "%3a" versus "%3A") are case-insensitive and therefore
1368should be normalized to use uppercase letters for the digits A-F.</t>
1369
1370<t>When an IRI uses components of the generic syntax, the component
1371syntax equivalence rules always apply; namely, that the scheme and
1372US-ASCII only host are case insensitive and therefore should be
1373normalized to lowercase. For example, the URI
1374"HTTP://www.EXAMPLE.com/" is equivalent to
1375"http://www.example.com/". Case equivalence for non-ASCII characters
1376in IRI components that are IDNs are discussed in <xref
1377target="schemecomp"></xref>.  The other generic syntax components are
1378assumed to be case sensitive unless specifically defined otherwise by
1379the scheme.</t>
1380
1381<t>Creating schemes that allow case-insensitive syntax components
1382containing non-ASCII characters should be avoided. Case normalization
1383of non-ASCII characters can be culturally dependent and is always a
1384complex operation. The only exception concerns non-ASCII host names
1385for which the character normalization includes a mapping step derived
1386from case folding.</t>
1387
1388</section> <!-- casenorm -->
1389
1390<section title="Character Normalization" anchor="normalization">
1391
1392<t>The Unicode Standard <xref target="UNIV6"></xref> defines various
1393equivalences between sequences of characters for various
1394purposes. Unicode Standard Annex #15 <xref target="UTR15"></xref>
1395defines various Normalization Forms for these equivalences, in
1396particular Normalization Form C (NFC, Canonical Decomposition,
1397followed by Canonical Composition) and Normalization Form KC (NFKC,
1398Compatibility Decomposition, followed by Canonical Composition).</t>
1399
1400<t> IRIs already in Unicode MUST NOT be normalized before parsing or
1401interpreting. In many non-Unicode character encodings, some text
1402cannot be represented directly. For example, the word "Vietnam" is
1403natively written "Vi&amp;#x1EC7;t Nam" (containing a LATIN SMALL
1404LETTER E WITH CIRCUMFLEX AND DOT BELOW) in NFC, but a direct
1405transcoding from the windows-1258 character encoding leads to
1406"Vi&amp;#xEA;&amp;#x323;t Nam" (containing a LATIN SMALL LETTER E WITH
1407CIRCUMFLEX followed by a COMBINING DOT BELOW). Direct transcoding of
1408other 8-bit encodings of Vietnamese may lead to other
1409representations.</t>
1410
1411<t>Equivalence of IRIs MUST rely on the assumption that IRIs are
1412appropriately pre-character-normalized rather than apply character
1413normalization when comparing two IRIs. The exceptions are conversion
1414from a non-digital form, and conversion from a non-UCS-based character
1415encoding to a UCS-based character encoding. In these cases, NFC or a
1416normalizing transcoder using NFC MUST be used for interoperability. To
1417avoid false negatives and problems with transcoding, IRIs SHOULD be
1418created by using NFC. Using NFKC may avoid even more problems; for
1419example, by choosing half-width Latin letters instead of full-width
1420ones, and full-width instead of half-width Katakana.</t>
1421
1422
1423<t>As an example,
1424"http://www.example.org/r&amp;#xE9;sum&amp;#xE9;.html" (in XML
1425Notation) is in NFC. On the other hand,
1426"http://www.example.org/re&amp;#x301;sume&amp;#x301;.html" is not in
1427NFC.</t>
1428
1429<t>The former uses precombined e-acute characters, and the latter uses
1430"e" characters followed by combining acute accents. Both usages are
1431defined as canonically equivalent in <xref target="UNIV6"></xref>.</t>
1432
1433<t><list style="hanging">
1434
1435<t hangText="Note:">
1436Because it is unknown how a particular sequence of characters is being
1437treated with respect to character normalization, it would be
1438inappropriate to allow third parties to normalize an IRI
1439arbitrarily. This does not contradict the recommendation that when a
1440resource is created, its IRI should be as character normalized as
1441possible (i.e., NFC or even NFKC). This is similar to the
1442uppercase/lowercase problems.  Some parts of a URI are case
1443insensitive (for example, the domain name). For others, it is unclear
1444whether they are case sensitive, case insensitive, or something in
1445between (e.g., case sensitive, but with a multiple choice selection if
1446the wrong case is used, instead of a direct negative result).  The
1447best recipe is that the creator use a reasonable capitalization and,
1448when transferring the URI, capitalization never be
1449changed.</t></list></t>
1450
1451<t>Various IRI schemes may allow the usage of Internationalized Domain
1452Names (IDN) <xref target="RFC5890"/> either in the ireg-name
1453part or elsewhere. Character Normalization also applies to IDNs, as
1454discussed in <xref target="schemecomp"/>.</t>
1455</section> <!-- charnorm -->
1456
1457<section title="Percent-Encoding Normalization">
1458
1459<t>The percent-encoding mechanism (Section 2.1 of <xref
1460target="RFC3986"></xref>) is a frequent source of variance among
1461otherwise identical IRIs. In addition to the case normalization issue
1462noted above, some IRI producers percent-encode octets that do not
1463require percent-encoding, resulting in IRIs that are equivalent to
1464their nonencoded counterparts. These IRIs should be normalized by
1465decoding any percent-encoded octet sequence that corresponds to an
1466unreserved character, as described in section 2.3 of <xref
1467target="RFC3986"></xref>.</t>
1468
1469<t>For actual resolution, differences in percent-encoding (except for
1470the percent-encoding of reserved characters) MUST always result in the
1471same resource.  For example, "http://example.org/~user",
1472"http://example.org/%7euser", and "http://example.org/%7Euser", must
1473resolve to the same resource.</t>
1474
1475<t>If this kind of equivalence is to be tested, the percent-encoding
1476of both IRIs to be compared has to be aligned; for example, by
1477converting both IRIs to URIs (see Section 3.1), eliminating escape
1478differences in the resulting URIs, and making sure that the case of
1479the hexadecimal characters in the percent-encoding is always the same
1480(preferably upper case). If the IRI is to be passed to another
1481application or used further in some other way, its original form MUST
1482be preserved.  The conversion described here should be performed only
1483for local comparison.</t>
1484
1485</section> <!-- pctnorm -->
1486
1487<section title="Path Segment Normalization">
1488
1489<t>The complete path segments "." and ".." are intended only for use
1490within relative references (Section 4.1 of <xref
1491target="RFC3986"></xref>) and are removed as part of the reference
1492resolution process (Section 5.2 of <xref target="RFC3986"></xref>).
1493However, some implementations may incorrectly assume that reference
1494resolution is not necessary when the reference is already an IRI, and
1495thus fail to remove dot-segments when they occur in non-relative
1496paths.  IRI normalizers should remove dot-segments by applying the
1497remove_dot_segments algorithm to the path, as described in Section
14985.2.4 of <xref target="RFC3986"></xref>.</t>
1499
1500</section> <!-- pathnorm -->
1501</section> <!-- ladder -->
1502
1503<section title="Scheme-Based Normalization" anchor="schemecomp">
1504
1505<t>The syntax and semantics of IRIs vary from scheme to scheme, as
1506described by the defining specification for each
1507scheme. Implementations may use scheme-specific rules, at further
1508processing cost, to reduce the probability of false negatives. For
1509example, because the "http" scheme makes use of an authority
1510component, has a default port of "80", and defines an empty path to be
1511equivalent to "/", the following four IRIs are equivalent:</t>
1512
1513<figure><artwork>
1514   http://example.com
1515   http://example.com/
1516   http://example.com:/
1517   http://example.com:80/</artwork></figure>
1518
1519<t>In general, an IRI that uses the generic syntax for authority with
1520an empty path should be normalized to a path of "/". Likewise, an
1521explicit ":port", for which the port is empty or the default for the
1522scheme, is equivalent to one where the port and its ":" delimiter are
1523elided and thus should be removed by scheme-based normalization. For
1524example, the second IRI above is the normal form for the "http"
1525scheme.</t>
1526
1527<t>Another case where normalization varies by scheme is in the
1528handling of an empty authority component or empty host
1529subcomponent. For many scheme specifications, an empty authority or
1530host is considered an error; for others, it is considered equivalent
1531to "localhost" or the end-user's host. When a scheme defines a default
1532for authority and an IRI reference to that default is desired, the
1533reference should be normalized to an empty authority for the sake of
1534uniformity, brevity, and internationalization. If, however, either the
1535userinfo or port subcomponents are non-empty, then the host should be
1536given explicitly even if it matches the default.</t>
1537
1538<t>Normalization should not remove delimiters when their associated
1539component is empty unless it is licensed to do so by the scheme
1540specification. For example, the IRI "http://example.com/?" cannot be
1541assumed to be equivalent to any of the examples above. Likewise, the
1542presence or absence of delimiters within a userinfo subcomponent is
1543usually significant to its interpretation.  The fragment component is
1544not subject to any scheme-based normalization; thus, two IRIs that
1545differ only by the suffix "#" are considered different regardless of
1546the scheme.</t>
1547 
1548<t>Some IRI schemes allow the usage of Internationalized Domain
1549Names (IDN) <xref target='RFC5890'></xref> either in their ireg-name
1550part or elswhere. When in use in IRIs, those names SHOULD
1551conform to the definition of U-Label in <xref
1552target='RFC5890'></xref>. An IRI containing an invalid IDN cannot
1553successfully be resolved. For legibility purposes, they
1554SHOULD NOT be converted into ASCII Compatible Encoding (ACE).</t>
1555
1556<t>Scheme-based normalization may also consider IDN
1557components and their conversions to punycode as equivalent. As an
1558example, "http://r&amp;#xE9;sum&amp;#xE9;.example.org" may be
1559considered equivalent to
1560"http://xn--rsum-bpad.example.org".</t><t>Other scheme-specific
1561normalizations are possible.</t>
1562
1563</section> <!-- schemenorm -->
1564
1565<section title="Protocol-Based Normalization">
1566
1567<t>Substantial effort to reduce the incidence of false negatives is
1568often cost-effective for web spiders. Consequently, they implement
1569even more aggressive techniques in IRI comparison. For example, if
1570they observe that an IRI such as</t>
1571
1572<figure><artwork>
1573   http://example.com/data</artwork></figure>
1574<t>redirects to an IRI differing only in the trailing slash</t>
1575<figure><artwork>
1576   http://example.com/data/</artwork></figure>
1577
1578<t>they will likely regard the two as equivalent in the future.  This
1579kind of technique is only appropriate when equivalence is clearly
1580indicated by both the result of accessing the resources and the common
1581conventions of their scheme's dereference algorithm (in this case, use
1582of redirection by HTTP origin servers to avoid problems with relative
1583references).</t>
1584
1585</section> <!-- protonorm -->
1586</section> <!-- equivalence -->
1587</section> 
1588
1589<section title="Use of IRIs" anchor="IRIuse">
1590
1591<section title="Limitations on UCS Characters Allowed in IRIs" anchor="limitations">
1592
1593<t>This section discusses limitations on characters and character
1594sequences usable for IRIs beyond those given in <xref target="abnf"/>
1595and <xref target="visual"/>. The considerations in this section are
1596relevant when IRIs are created and when URIs are converted to
1597IRIs.</t>
1598
1599<t>
1600
1601<list style="hanging"><t hangText="a.">The repertoire of characters allowed
1602    in each IRI component is limited by the definition of that component.
1603    For example, the definition of the scheme component does not allow
1604    characters beyond US-ASCII.
1605    <vspace blankLines="1"/>
1606    (Note: In accordance with URI practice, generic IRI
1607    software cannot and should not check for such limitations.)</t>
1608
1609<t hangText="b.">The UCS contains many areas of characters for which
1610    there are strong visual look-alikes. Because of the likelihood of
1611    transcription errors, these also should be avoided. This includes
1612    the full-width equivalents of Latin characters, half-width
1613    Katakana characters for Japanese, and many others. It also
1614    includes many look-alikes of "space", "delims", and "unwise",
1615    characters excluded in <xref target="RFC3491"/>.</t>
1616   
1617</list>
1618</t>
1619
1620<t>Additional information is available from <xref target="UNIXML"/>.
1621    <xref target="UNIXML"/> is written in the context of running text
1622    rather than in that of identifiers. Nevertheless, it discusses
1623    many of the categories of characters not appropriate for IRIs.</t>
1624</section> <!-- limitations -->
1625
1626<section title="Software Interfaces and Protocols">
1627
1628<t>Although an IRI is defined as a sequence of characters, software
1629interfaces for URIs typically function on sequences of octets or other
1630kinds of code units. Thus, software interfaces and protocols MUST
1631define which character encoding is used.</t>
1632
1633<t>Intermediate software interfaces between IRI-capable components and
1634URI-only components MUST map the IRIs per <xref target="mapping"/>,
1635when transferring from IRI-capable to URI-only components.
1636
1637This mapping SHOULD be applied as late as possible. It SHOULD NOT be
1638applied between components that are known to be able to handle IRIs.</t>
1639</section> <!-- software -->
1640
1641<section title="Format of URIs and IRIs in Documents and Protocols">
1642
1643<t>Document formats that transport URIs may have to be upgraded to allow
1644the transport of IRIs. In cases where the document as a whole
1645has a native character encoding, IRIs MUST also be encoded in this
1646character encoding and converted accordingly by a parser or interpreter.
1647
1648IRI characters not expressible in the native character encoding SHOULD
1649be escaped by using the escaping conventions of the document format if
1650such conventions are available. Alternatively, they MAY be
1651percent-encoded according to <xref target="mapping"/>. For example, in
1652HTML or XML, numeric character references SHOULD be used. If a
1653document as a whole has a native character encoding and that character
1654encoding is not UTF-8, then IRIs MUST NOT be placed into the document
1655in the UTF-8 character encoding.</t>
1656
1657<t>((UPDATE THIS NOTE)) Note: Some formats already accommodate IRIs,
1658although they use different terminology. HTML 4.0 <xref
1659target="HTML4"/> defines the conversion from IRIs to URIs as
1660error-avoiding behavior. XML 1.0 <xref target="XML1"/>, XLink <xref
1661target="XLink"/>, XML Schema <xref target="XMLSchema"/>, and
1662specifications based upon them allow IRIs. Also, it is expected that
1663all relevant new W3C formats and protocols will be required to handle
1664IRIs <xref target="CharMod"/>.</t>
1665
1666</section> <!-- format -->
1667
1668<section title="Use of UTF-8 for Encoding Original Characters" anchor="UTF8use">
1669
1670<t>This section discusses details and gives examples for point c) in
1671<xref target="Applicability"/>. To be able to use IRIs, the URI
1672corresponding to the IRI in question has to encode original characters
1673into octets by using UTF-8.  This can be specified for all URIs of a
1674URI scheme or can apply to individual URIs for schemes that do not
1675specify how to encode original characters.  It can apply to the whole
1676URI, or only to some part. For background information on encoding
1677characters into URIs, see also Section 2.5 of <xref
1678target="RFC3986"/>.</t>
1679
1680<t>For new URI schemes, using UTF-8 is recommended in <xref
1681target="RFC4395bis"/>.  Examples where UTF-8 is already used are the URN
1682syntax <xref target="RFC2141"/>, IMAP URLs <xref target="RFC2192"/>,
1683and POP URLs <xref target="RFC2384"/>.  On the other hand, because the
1684HTTP URI scheme does not specify how to encode original characters,
1685only some HTTP URLs can have corresponding but different IRIs.</t>
1686
1687<t>For example, for a document with a URI
1688of<vspace/>"http://www.example.org/r%C3%A9sum%C3%A9.html", it is
1689possible to construct a corresponding IRI (in XML notation, see <xref
1690target="sec-Notation"/>):
1691"http://www.example.org/r&amp;#xE9;sum&amp;#xE9;.html" ("&amp;#xE9;"
1692stands for the e-acute character, and "%C3%A9" is the UTF-8 encoded
1693and percent-encoded representation of that character). On the other
1694hand, for a document with a URI of
1695"http://www.example.org/r%E9sum%E9.html", the percent-encoding octets
1696cannot be converted to actual characters in an IRI, as the
1697percent-encoding is not based on UTF-8.</t>
1698
1699<t>For most URI schemes, there is no need to upgrade their scheme
1700definition in order for them to work with IRIs.  The main case where
1701upgrading makes sense is when a scheme definition, or a particular
1702component of a scheme, is strictly limited to the use of US-ASCII
1703characters with no provision to include non-ASCII characters/octets
1704via percent-encoding, or if a scheme definition currently uses highly
1705scheme-specific provisions for the encoding of non-ASCII characters.
1706An example of this is the mailto: scheme <xref target="RFC2368"/>.</t>
1707
1708<t>This specification updates the IANA registry of URI schemes to note
1709their applicability to IRIs, see <xref target="iana"/>.  All IRIs use
1710URI schemes, and all URIs with URI schemes can be used as IRIs, even
1711though in some cases only by using URIs directly as IRIs, without any
1712conversion.</t>
1713
1714<t>Scheme definitions can impose restrictions on the syntax of
1715scheme-specific URIs; i.e., URIs that are admissible under the generic
1716URI syntax <xref target="RFC3986"/> may not be admissible due to
1717narrower syntactic constraints imposed by a URI scheme
1718specification. URI scheme definitions cannot broaden the syntactic
1719restrictions of the generic URI syntax; otherwise, it would be
1720possible to generate URIs that satisfied the scheme-specific syntactic
1721constraints without satisfying the syntactic constraints of the
1722generic URI syntax. However, additional syntactic constraints imposed
1723by URI scheme specifications are applicable to IRI, as the
1724corresponding URI resulting from the mapping defined in <xref
1725target="mapping"/> MUST be a valid URI under the syntactic
1726restrictions of generic URI syntax and any narrower restrictions
1727imposed by the corresponding URI scheme specification.</t>
1728
1729<t>The requirement for the use of UTF-8 generally applies to all parts
1730of a URI.  However, it is possible that the capability of IRIs to
1731represent a wide range of characters directly is used just in some
1732parts of the IRI (or IRI reference). The other parts of the IRI may
1733only contain US-ASCII characters, or they may not be based on
1734UTF-8. They may be based on another character encoding, or they may
1735directly encode raw binary data (see also <xref
1736target="RFC2397"/>). </t>
1737
1738<t>For example, it is possible to have a URI reference
1739of<vspace/>"http://www.example.org/r%E9sum%E9.xml#r%C3%A9sum%C3%A9",
1740where the document name is encoded in iso-8859-1 based on server
1741settings, but where the fragment identifier is encoded in UTF-8 according
1742to <xref target="XPointer"/>. The IRI corresponding to the above
1743URI would be (in XML notation)<vspace/>"http://www.example.org/r%E9sum%E9.xml#r&amp;#xE9;sum&amp;#xE9;".</t>
1744
1745<t>Similar considerations apply to query parts. The functionality
1746of IRIs (namely, to be able to include non-ASCII characters) can
1747only be used if the query part is encoded in UTF-8.</t>
1748
1749</section> <!-- utf8 -->
1750
1751<section title="Relative IRI References">
1752<t>Processing of relative IRI references against a base is handled
1753straightforwardly; the algorithms of <xref target="RFC3986"/> can
1754be applied directly, treating the characters additionally allowed
1755in IRI references in the same way that unreserved characters are in URI
1756references.</t>
1757
1758</section> <!-- relative -->
1759</section> <!-- IRIuse -->
1760
1761<section title="Liberal Handling of Otherwise Invalid IRIs" anchor="LEIRIHREF">
1762
1763<t>(EDITOR NOTE: This Section may move to an appendix.)
1764 
1765Some technical specifications and widely-deployed software have
1766allowed additional variations and extensions of IRIs to be used in
1767syntactic components. This section describes two widely-used
1768preprocessing agreements. Other technical specifications may wish to
1769reference a syntactic component which is "a valid IRI or a string that
1770will map to a valid IRI after this preprocessing algorithm". These two
1771variants are known as <xref target="LEIRI">Legacy Extended IRI or
1772LEIRI</xref>, and <xref target="HTML5">Web Address</xref>).
1773</t>
1774
1775<t>Future technical specifications SHOULD NOT allow conforming
1776producers to produce, or conforming content to contain, such forms,
1777as they are not interoperable with other IRI consuming software.</t>
1778
1779<section title="LEIRI Processing"  anchor="LEIRIspec">
1780  <t>This section defines Legacy Extended IRIs (LEIRIs).
1781    The syntax of Legacy Extended IRIs is the same as that for &lt;IRI-reference>,
1782    except that the ucschar production is replaced by the leiri-ucschar production:</t>
1783<figure>
1784
1785<artwork>
1786  leiri-ucschar  = " " / "&lt;" / "&gt;" / '"' / "{" / "}" / "|"
1787                   / "\" / "^" / "`" / %x0-1F / %x7F-D7FF
1788                   / %xE000-FFFD / %x10000-10FFFF
1789</artwork>
1790
1791<postamble>
1792  Among other extensions, processors based on this specification also
1793  did not enforce the restriction on bidirectional formatting
1794  characters in <xref target="visual"></xref>, and the iprivate
1795  production becomes redundant.</postamble>
1796</figure>
1797
1798<t>To convert a string allowed as a LEIRI to an IRI, each character
1799allowed in leiri-ucschar but not in ucschar must be percent-encoded
1800using <xref target="compmapping"/>.</t>
1801</section> <!-- leiriproc -->
1802
1803<section title="Web Address Processing" anchor="webaddress">
1804
1805<t>Many popular web browsers have taken the approach of being quite
1806liberal in what is accepted as a "URL" or its relative
1807forms. This section describes their behavior in terms of a preprocessor
1808which maps strings into the IRI space for subsequent parsing and
1809interpretation as an IRI.</t>
1810
1811<t>In some situations, it might be appropriate to describe the syntax
1812that a liberal consumer implementation might accept as a "Web
1813Address" or "Hypertext Reference" or "HREF". However,
1814technical specifications SHOULD restrict the syntactic form allowed by compliant producers
1815to the IRI or IRI reference syntax defined in this document
1816even if they want to mandate this processing.</t>
1817
1818<t>
1819Summary:
1820<list style="symbols">
1821   <t>Leading and trailing whitespace is removed.</t>
1822   <t>Some additional characters are removed.</t>
1823   <t>Some additional characters are allowed and escaped (as with LEIRI).</t>
1824   <t>If interpreting an IRI as a URI, the pct-encoding of the query
1825   component of the parsed URI component depends on operational
1826   context.</t>
1827</list>
1828</t>
1829
1830<t>Each string provided may have an associated charset (called
1831the HREF-charset here); this defaults to UTF-8.
1832For web browsers interpreting HTML, the document
1833charset of a string is determined:
1834
1835<list style="hanging">
1836<t hangText="If the string came from a script (e.g. as an argument to
1837 a method)">The HRef-charset is the script's charset.</t>
1838
1839<t hangText="If the string came from a DOM node (e.g. from an
1840  element)">The node has a Document, and the HRef-charset is the
1841  Document's character encoding.</t>
1842
1843<t hangText="If the string had a HRef-charset defined when the string was
1844created or defined">The HRef-charset is as defined.</t>
1845
1846</list></t>
1847
1848<t>If the resulting HRef-charset is a unicode based character encoding
1849(e.g., UTF-16), then use UTF-8 instead.</t>
1850
1851
1852<figure>
1853<preamble>The syntax for Web Addresses is obtained by replacing the 'ucschar',
1854  pct-form, path-sep, and ifragment rules with the href-ucschar, href-pct-form, href-path-sep,
1855  and href-ifragment
1856  rules below. In addition, some characters are stripped.</preamble>
1857
1858<artwork type='abnf'>
1859  href-ucschar   = " " / "&lt;" / "&gt;" / DQUOTE / "{" / "}" / "|"
1860                 / "\" / "^" / "`" / %x0-1F / %x7F-D7FF
1861                 / %xE000-FFFD / %x10000-10FFFF
1862  href-pct-form  = pct-encoded / "%"
1863  href-path-sep  = "/" / "\"
1864  href-ifragment = *( ipchar / "/" / "?" / "#" )  ; adding "#"
1865  href-strip     = &lt;to be done&gt;
1866</artwork>
1867
1868<postamble>
1869(NOTE: NEED TO FIX THESE SETS TO MATCH HTML5; NOT SURE ABOUT NEXT SENTENCE)
1870browsers did not enforce the restriction on bidirectional formatting
1871  characters in <xref target="visual"></xref>, and the iprivate
1872  production becomes redundant.</postamble>
1873</figure>
1874
1875<t>'Web Address processing' requires the following additional
1876preprocessing steps:
1877
1878<list style="numbers">
1879
1880<t>Leading and trailing instances of space (U+0020),
1881CR (U+000A), LF (U+000D), and TAB (U+0009) characters are removed.</t>
1882
1883<t>strip all characters in href-strip.</t>
1884  <t>Percent-encode all characters in href-ucschar not in ucschar.</t>
1885  <t>Replace occurrences of "%" not followed by two hexadecimal digits by "%25".</t>
1886  <t>Convert backslashes ('\') matching href-path-sep to forward slashes ('/').</t>
1887</list></t>
1888</section> <!-- webaddress -->
1889
1890<section title="Characters Not Allowed in IRIs" anchor="notAllowed">
1891
1892<t>This section provides a list of the groups of characters and code
1893points that are allowed by LEIRI or HREF but are not allowed in IRIs or are
1894allowed in IRIs only in the query part. For each group of characters,
1895advice on the usage of these characters is also given, concentrating
1896on the reasons for why they are excluded from IRI use.</t>
1897
1898<t>
1899
1900<list><t>Space (U+0020): Some formats and applications use space as a
1901delimiter, e.g. for items in a list. Appendix C of <xref
1902target="RFC3986"></xref> also mentions that white space may have to be
1903added when displaying or printing long URIs; the same applies to long
1904IRIs. This means that spaces can disappear, or can make the what is
1905intended as a single IRI or IRI reference to be treated as two or more
1906separate IRIs.</t>
1907
1908<t>Delimiters "&lt;" (U+003C), "&gt;" (U+003E), and '"' (U+0022):
1909Appendix C of <xref target="RFC3986"></xref> suggests the use of
1910double-quotes ("http://example.com/") and angle brackets
1911(&lt;http://example.com/&gt;) as delimiters for URIs in plain
1912text. These conventions are often used, and also apply to IRIs.  Using
1913these characters in strings intended to be IRIs would result in the
1914IRIs being cut off at the wrong place.</t>
1915
1916<t>Unwise characters "\" (U+005C), "^" (U+005E), "`"
1917(U+0060), "{" (U+007B), "|" (U+007C), and "}" (U+007D): These
1918characters originally have been excluded from URIs because the
1919respective codepoints are assigned to different graphic characters in
1920some 7-bit or 8-bit encoding. Despite the move to Unicode, some of
1921these characters are still occasionally displayed differently on some
1922systems, e.g. U+005C may appear as a Japanese Yen symbol on some
1923systems. Also, the fact that these characters are not used in URIs or
1924IRIs has encouraged their use outside URIs or IRIs in contexts that
1925may include URIs or IRIs. If a string with such a character were used
1926as an IRI in such a context, it would likely be interpreted
1927piecemeal.</t>
1928
1929<t>The controls (C0 controls, DEL, and C1 controls, #x0 - #x1F #x7F -
1930#x9F): There is generally no way to transmit these characters reliably
1931as text outside of a charset encoding.  Even when in encoded form,
1932many software components silently filter out some of these characters,
1933or may stop processing alltogether when encountering some of
1934them. These characters may affect text display in subtle, unnoticable
1935ways or in drastic, global, and irreversible ways depending on the
1936hardware and software involved. The use of some of these characters
1937would allow malicious users to manipulate the display of an IRI and
1938its context in many situations.</t>
1939
1940<t>Bidi formatting characters (U+200E, U+200F, U+202A-202E): These
1941characters affect the display ordering of characters. If IRIs were
1942allowed to contain these characters and the resulting visual display
1943transcribed. they could not be converted back to electronic form
1944(logical order) unambiguously. These characters, if allowed in IRIs,
1945might allow malicious users to manipulate the display of IRI and its
1946context.</t>
1947
1948<t>Specials (U+FFF0-FFFD): These code points provide functionality
1949beyond that useful in an IRI, for example byte order identification,
1950annotation, and replacements for unknown characters and objects. Their
1951use and interpretation in an IRI would serve no purpose and might lead
1952to confusing display variations.</t>
1953
1954<t>Private use code points (U+E000-F8FF, U+F0000-FFFFD,
1955U+100000-10FFFD): Display and interpretation of these code points is
1956by definition undefined without private agreement. Therefore, these
1957code points are not suited for use on the Internet. They are not
1958interoperable and may have unpredictable effects.</t>
1959
1960<t>Tags (U+E0000-E0FFF): These characters provide a way to language
1961tag in Unicode plain text. They are not appropriate for IRIs because
1962language information in identifiers cannot reliably be input,
1963transmitted (e.g. on a visual medium such as paper), or
1964recognized.</t>
1965
1966<t>Non-characters (U+FDD0-FDEF, U+1FFFE-1FFFF, U+2FFFE-2FFFF,
1967U+3FFFE-3FFFF, U+4FFFE-4FFFF, U+5FFFE-5FFFF, U+6FFFE-6FFFF,
1968U+7FFFE-7FFFF, U+8FFFE-8FFFF, U+9FFFE-9FFFF, U+AFFFE-AFFFF,
1969U+BFFFE-BFFFF, U+CFFFE-CFFFF, U+DFFFE-DFFFF, U+EFFFE-EFFFF,
1970U+FFFFE-FFFFF, U+10FFFE-10FFFF): These code points are defined as
1971non-characters. Applications may use some of them internally, but are
1972not prepared to interchange them.</t>
1973
1974</list></t>
1975
1976<t>LEIRI preprocessing disallowed some code points and
1977code units:
1978
1979<list><t>Surrogate code units (D800-DFFF): These do not represent
1980Unicode codepoints.</t></list></t>
1981</section> <!-- notallowed -->
1982</section> <!-- lieirihref -->
1983 
1984<section title="URI/IRI Processing Guidelines (Informative)" anchor="guidelines">
1985
1986<t>This informative section provides guidelines for supporting IRIs in
1987the same software components and operations that currently process
1988URIs: Software interfaces that handle URIs, software that allows users
1989to enter URIs, software that creates or generates URIs, software that
1990displays URIs, formats and protocols that transport URIs, and software
1991that interprets URIs. These may all require modification before
1992functioning properly with IRIs. The considerations in this section
1993also apply to URI references and IRI references.</t>
1994
1995<section title="URI/IRI Software Interfaces">
1996<t>Software interfaces that handle URIs, such as URI-handling APIs and
1997protocols transferring URIs, need interfaces and protocol elements
1998that are designed to carry IRIs.</t>
1999
2000<t>In case the current handling in an API or protocol is based on
2001US-ASCII, UTF-8 is recommended as the character encoding for IRIs, as
2002it is compatible with US-ASCII, is in accordance with the
2003recommendations of <xref target="RFC2277"/>, and makes converting to
2004URIs easy. In any case, the API or protocol definition must clearly
2005define the character encoding to be used.</t>
2006
2007<t>The transfer from URI-only to IRI-capable components requires no
2008mapping, although the conversion described in <xref
2009target="URItoIRI"/> above may be performed. It is preferable not to
2010perform this inverse conversion unless it is certain this can be done
2011correctly.</t>
2012</section>
2013
2014<section title="URI/IRI Entry">
2015
2016<t>Some components allow users to enter URIs into the system
2017by typing or dictation, for example. This software must be updated to allow
2018for IRI entry.</t>
2019
2020<t>A person viewing a visual representation of an IRI (as a sequence
2021of glyphs, in some order, in some visual display) or hearing an IRI
2022will use an entry method for characters in the user's language to
2023input the IRI. Depending on the script and the input method used, this
2024may be a more or less complicated process.</t>
2025
2026<t>The process of IRI entry must ensure, as much as possible, that the
2027restrictions defined in <xref target="abnf"/> are met. This may be
2028done by choosing appropriate input methods or variants/settings
2029thereof, by appropriately converting the characters being input, by
2030eliminating characters that cannot be converted, and/or by issuing a
2031warning or error message to the user.</t>
2032
2033<t>As an example of variant settings, input method editors for East
2034Asian Languages usually allow the input of Latin letters and related
2035characters in full-width or half-width versions. For IRI input, the
2036input method editor should be set so that it produces half-width Latin
2037letters and punctuation and full-width Katakana.</t>
2038
2039<t>An input field primarily or solely used for the input of URIs/IRIs
2040might allow the user to view an IRI as it is mapped to a URI.  Places
2041where the input of IRIs is frequent may provide the possibility for
2042viewing an IRI as mapped to a URI. This will help users when some of
2043the software they use does not yet accept IRIs.</t>
2044
2045<t>An IRI input component interfacing to components that handle URIs,
2046but not IRIs, must map the IRI to a URI before passing it to these
2047components.</t>
2048
2049<t>For the input of IRIs with right-to-left characters, please see
2050<xref target="bidiInput"></xref>.</t>
2051</section>
2052
2053<section title="URI/IRI Transfer between Applications">
2054
2055<t>Many applications (for example, mail user agents) try to detect
2056URIs appearing in plain text. For this, they use some heuristics based
2057on URI syntax. They then allow the user to click on such URIs and
2058retrieve the corresponding resource in an appropriate (usually
2059scheme-dependent) application.</t>
2060
2061<t>Such applications would need to be upgraded, in order to use the
2062IRI syntax as a base for heuristics. In particular, a non-ASCII
2063character should not be taken as the indication of the end of an IRI.
2064Such applications also would need to make sure that they correctly
2065convert the detected IRI from the character encoding of the document
2066or application where the IRI appears, to the character encoding used
2067by the system-wide IRI invocation mechanism, or to a URI (according to
2068<xref target="mapping"/>) if the system-wide invocation mechanism only
2069accepts URIs.</t>
2070
2071<t>The clipboard is another frequently used way to transfer URIs and
2072IRIs from one application to another. On most platforms, the clipboard
2073is able to store and transfer text in many languages and scripts.
2074Correctly used, the clipboard transfers characters, not octets, which
2075will do the right thing with IRIs.</t>
2076</section>
2077
2078<section title="URI/IRI Generation">
2079
2080<t>Systems that offer resources through the Internet, where those
2081resources have logical names, sometimes automatically generate URIs
2082for the resources they offer. For example, some HTTP servers can
2083generate a directory listing for a file directory and then respond to
2084the generated URIs with the files.</t>
2085
2086<t>Many legacy character encodings are in use in various file systems.
2087Many currently deployed systems do not transform the local character
2088representation of the underlying system before generating URIs.</t>
2089
2090<t>For maximum interoperability, systems that generate resource
2091identifiers should make the appropriate transformations. For example,
2092if a file system contains a file named
2093"r&amp;#xE9;sum&amp;#xE9;.html", a server should expose this as
2094"r%C3%A9sum%C3%A9.html" in a URI, which allows use of
2095"r&amp;#xE9;sum&amp;#xE9;.html" in an IRI, even if locally the file
2096name is kept in a character encoding other than UTF-8.
2097</t>
2098
2099<t>This recommendation particularly applies to HTTP servers. For FTP
2100servers, similar considerations apply; see <xref target="RFC2640"/>.</t>
2101</section>
2102
2103<section title="URI/IRI Selection" anchor="selection">
2104<t>In some cases, resource owners and publishers have control over the
2105IRIs used to identify their resources. This control is mostly
2106executed by controlling the resource names, such as file names,
2107directly.</t>
2108
2109<t>In these cases, it is recommended to avoid choosing IRIs that are
2110easily confused. For example, for US-ASCII, the lower-case ell ("l") is
2111easily confused with the digit one ("1"), and the upper-case oh ("O") is
2112easily confused with the digit zero ("0"). Publishers should avoid
2113confusing users with "br0ken" or "1ame" identifiers.</t>
2114
2115<t>Outside the US-ASCII repertoire, there are many more opportunities for
2116confusion; a complete set of guidelines is too lengthy to include
2117here. As long as names are limited to characters from a single script,
2118native writers of a given script or language will know best when
2119ambiguities can appear, and how they can be avoided. What may look
2120ambiguous to a stranger may be completely obvious to the average
2121native user. On the other hand, in some cases, the UCS contains
2122variants for compatibility reasons; for example, for typographic purposes.
2123These should be avoided wherever possible. Although there may be exceptions,
2124newly created resource names should generally be in NFKC
2125<xref target="UTR15"></xref> (which means that they are also in NFC).</t>
2126
2127<t>As an example, the UCS contains the "fi" ligature at U+FB01
2128for compatibility reasons.
2129Wherever possible, IRIs should use the two letters "f" and "i" rather
2130than the "fi" ligature. An example where the latter may be used is
2131in the query part of an IRI for an explicit search for a word written
2132containing the "fi" ligature.</t>
2133
2134<t>In certain cases, there is a chance that characters from different
2135scripts look the same. The best known example is the similarity of the
2136Latin "A", the Greek "Alpha", and the Cyrillic "A". To avoid such
2137cases, IRIs should only be created where all the characters in a
2138single component are used together in a given language. This usually
2139means that all of these characters will be from the same script, but
2140there are languages that mix characters from different scripts (such
2141as Japanese).  This is similar to the heuristics used to distinguish
2142between letters and numbers in the examples above. Also, for Latin,
2143Greek, and Cyrillic, using lowercase letters results in fewer
2144ambiguities than using uppercase letters would.</t>
2145</section>
2146
2147<section title="Display of URIs/IRIs" anchor="display">
2148<t>
2149In situations where the rendering software is not expected to display
2150non-ASCII parts of the IRI correctly using the available layout and font
2151resources, these parts should be percent-encoded before being displayed.</t>
2152
2153<t>For display of Bidi IRIs, please see <xref target="visual"/>.</t>
2154</section>
2155
2156<section title="Interpretation of URIs and IRIs">
2157<t>Software that interprets IRIs as the names of local resources should
2158accept IRIs in multiple forms and convert and match them with the
2159appropriate local resource names.</t>
2160
2161<t>First, multiple representations include both IRIs in the native
2162character encoding of the protocol and also their URI counterparts.</t>
2163
2164<t>Second, it may include URIs constructed based on character
2165encodings other than UTF-8. These URIs may be produced by user agents that do
2166not conform to this specification and that use legacy character encodings to
2167convert non-ASCII characters to URIs. Whether this is necessary, and what
2168character encodings to cover, depends on a number of factors, such as
2169the legacy character encodings used locally and the distribution of
2170various versions of user agents. For example, software for Japanese
2171may accept URIs in Shift_JIS and/or EUC-JP in addition to UTF-8.</t>
2172
2173<t>Third, it may include additional mappings to be more user-friendly
2174and robust against transmission errors. These would be similar to how
2175some servers currently treat URIs as case insensitive or perform
2176additional matching to account for spelling errors. For characters
2177beyond the US-ASCII repertoire, this may, for example, include
2178ignoring the accents on received IRIs or resource names. Please note
2179that such mappings, including case mappings, are language
2180dependent.</t>
2181
2182<t>It can be difficult to identify a resource unambiguously if too
2183many mappings are taken into consideration. However, percent-encoded
2184and not percent-encoded parts of IRIs can always be clearly distinguished.
2185Also, the regularity of UTF-8 (see <xref target="Duerst97"/>) makes the
2186potential for collisions lower than it may seem at first.</t>
2187</section>
2188
2189<section title="Upgrading Strategy">
2190<t>Where this recommendation places further constraints on software
2191for which many instances are already deployed, it is important to
2192introduce upgrades carefully and to be aware of the various
2193interdependencies.</t>
2194
2195<t>If IRIs cannot be interpreted correctly, they should not be created,
2196generated, or transported. This suggests that upgrading URI interpreting
2197software to accept IRIs should have highest priority.</t>
2198
2199<t>On the other hand, a single IRI is interpreted only by a single or
2200very few interpreters that are known in advance, although it may be
2201entered and transported very widely.</t>
2202
2203<t>Therefore, IRIs benefit most from a broad upgrade of software to be
2204able to enter and transport IRIs. However, before an
2205individual IRI is published, care should be taken to upgrade the corresponding
2206interpreting software in order to cover the forms expected to be
2207received by various versions of entry and transport software.</t>
2208
2209<t>The upgrade of generating software to generate IRIs instead of using a
2210local character encoding should happen only after the service is upgraded
2211to accept IRIs. Similarly, IRIs should only be generated when the service
2212accepts IRIs and the intervening infrastructure and protocol is known
2213to transport them safely.</t>
2214
2215<t>Software converting from URIs to IRIs for display should be upgraded
2216only after upgraded entry software has been widely deployed to the
2217population that will see the displayed result.</t>
2218
2219
2220<t>Where there is a free choice of character encodings, it is often
2221possible to reduce the effort and dependencies for upgrading to IRIs
2222by using UTF-8 rather than another encoding. For example, when a new
2223file-based Web server is set up, using UTF-8 as the character encoding
2224for file names will make the transition to IRIs easier. Likewise, when
2225a new Web form is set up using UTF-8 as the character encoding of the
2226form page, the returned query URIs will use UTF-8 as the character
2227encoding (unless the user, for whatever reason, changes the character
2228encoding) and will therefore be compatible with IRIs.</t>
2229
2230
2231<t>These recommendations, when taken together, will allow for the
2232extension from URIs to IRIs in order to handle characters other than
2233US-ASCII while minimizing interoperability problems. For
2234considerations regarding the upgrade of URI scheme definitions, see
2235<xref target="UTF8use"/>.</t>
2236
2237</section>
2238</section> <!-- guidelines -->
2239
2240<section title="IANA Considerations" anchor="iana">
2241
2242<t>RFC Editor and IANA note: Please Replace RFC XXXX with the
2243number of this document when it issues as an RFC. </t>
2244
2245<t>IANA maintains a registry of "URI schemes". A "URI scheme" also
2246serves an "IRI scheme". </t>
2247
2248<t>To clarify that the URI scheme registration process also applies to
2249IRIs, change the description of the "URI schemes" registry
2250header to say "[RFC4395] defines an IANA-maintained registry of URI
2251Schemes. These registries include the Permanent and Provisional URI
2252Schemes.  RFC XXXX updates this registry to designate that schemes may
2253also indicate their usability as IRI schemes.</t>
2254
2255<t> Update "per RFC 4395" to "per RFC 4395 and RFC XXXX".
2256</t>
2257
2258</section> <!-- IANA -->
2259   
2260<section title="Security Considerations" anchor="security">
2261<t>The security considerations discussed in <xref target="RFC3986"/>
2262also apply to IRIs. In addition, the following issues require
2263particular care for IRIs.</t>
2264<t>Incorrect encoding or decoding can lead to security problems.
2265For example, some UTF-8 decoders do not check against overlong
2266byte sequences. See <xref target='UTR36'/> Section 3 for details.</t>
2267
2268  <t>There are serious difficulties with relying on a human to verify that a
2269    an IRI (whether presented visually or aurally)
2270    is the same as another IRI or is the one intended.
2271    These problems exist with ASCII-only URIs (bl00mberg.com vs. bloomberg.com)
2272    but are strongly exacerbated when using the much larger character repertoire of Unicode.
2273    For details, see Section 2 of <xref target='UTR36'/>.
2274    Using administrative and technical means to reduce the availability
2275    of such exploits is possible, but they are difficult to eliminate altogether.
2276    User agents SHOULD NOT rely on visual or perceptual comparison or verification of IRIs
2277    as a means of validating or assuring safety, correctness or appropriateness of an IRI.
2278    Other means of presenting users with the validity, safety, or appropriateness
2279    of visited sites are being developed in the browser community
2280    as an alternative means of avoiding these difficulties.</t>
2281
2282<t>Besides the large character repertoire of Unicode, reasons for
2283  confusion include different forms of normalization and different normalization
2284  expectations, use of percent-encoding with various legacy encodings,
2285  and bidirectionality issues. See also <xref target='UTR36'/>.</t>
2286
2287<t>Confusion can occur in various IRI components, such as the
2288domain name part or the path part, or between IRI components. For considerations specific
2289to the domain name part, see <xref target="RFC5890"/>. For considerations specific to
2290particular protocols or schemes, see the security sections of the relevant specifications
2291and registration templates.
2292Administrators of sites that allow independent
2293users to create resources in the same sub area have to be careful.
2294Details are discussed in <xref target="selection"/>.</t>
2295
2296<t>Confusion can occur with bidirectional IRIs, if the restrictions
2297in <xref target="bidi-structure"/> are not followed. The same visual
2298representation may be interpreted as different logical representations,
2299and vice versa. It is also very important that a correct Unicode bidirectional
2300implementation be used.</t>
2301  <t>The characters additionally allowed in Legacy Extended IRIs
2302    introduce additional security issues. For details, see <xref target='notAllowed'/>.</t>
2303</section><!-- security -->
2304
2305<section title="Acknowledgements">
2306<t>This document was derived from <xref target="RFC3987"/>; the acknowledgments from
2307that specification still apply.</t>
2308<t>We would like to thank Ian Hickson, Michael Sperberg-McQueen,
2309  and Dan Connolly for their work on HyperText References, and Norman Walsh, Richard Tobin,
2310  Henry S. Thomson, John Cowan, Paul Grosso, and the XML Core Working Group of the W3C for their work on LEIRIs.</t>
2311<t>In addition, this document was influenced by contributions from (in no particular order) Chris
2312  Lilley, Bjoern Hoehrmann,
2313Felix Sasaki, Jeremy Carroll, Frank Ellermann, Michael Everson, Cary Karp, Matitiahu Allouche,
2314Richard Ishida, Addison Phillips, Jonathan Rosenne, Najib Tounsi, Debbie Garside, Mark Davis, Sarmad
2315Hussain, Ted Hardie, Konrad Lanz, Thomas Roessler, Lisa Dusseault, Julian Reschke, Giovanni
2316Campagna, Anne van Kesteren, Mark Nottingham, Erik van der Poel, Marcin Hanclik, Marcos Caceres, Roy
2317Fielding, Greg Wilkins, Pieter Hintjens, Daniel R. Tobias, Marko Martin, Maciej Stanchowiak, Wil
2318Tan, Yui Naruse, Michael A. Puls II, Dave Thaler, Tom Perch, John Klensin, Shawn Steele, Peter
2319Saint-Andre, Geoffrey Sneddon, Chris Weber, Alex Melnikov, Slim Amamou, SM, Tim Berners-Lee, Yaron
2320Goland, Sam Ruby, Adam Barth, Abdulrahman I. ALGhadir, Aharon Lanin, Thomas Milo, Murray Sargent,
2321Marc Blanchet, and Mykyta Yevstifeyev.</t>
2322</section>
2323
2324<section title="Main Changes Since RFC 3987">
2325  <t>This section describes the main changes since <xref target="RFC3987"></xref>.</t>
2326  <section title="Major restructuring of IRI processing model" anchor="forkChanges">
2327    <t>Major restructuring of IRI processing model to make scheme-specific translation
2328      necessary to handle IDNA requirements and for consistency with web implementations. </t>
2329    <t>Starting with IRI, you want one of:
2330      <list style="hanging">
2331        <t hangText="a"> IRI components (IRI parsed into UTF8 pieces)</t>
2332        <t hangText="b"> URI components (URI parsed into ASCII pieces, encoded correctly) </t>
2333        <t hangText="c"> whole URI  (for passing on to some other system that wants whole URIs) </t>
2334      </list></t>
2335   
2336    <section title="OLD WAY">
2337      <t><list style="numbers">
2338       
2339        <t>Pct-encoding on the whole thing to a URI.
2340          (c1) If you want a (maybe broken) whole URI, you might
2341          stop here.</t>
2342       
2343        <t>Parsing the URI into URI components.
2344          (b1) If you want (maybe broken) URI components, stop here.</t>
2345       
2346        <t> Decode the components (undoing the pct-encoding).
2347          (a) if you want IRI components, stop here.</t>
2348       
2349        <t> reencode:  Either using a different encoding some components
2350          (for domain names, and query components in web pages, which
2351          depends on the component, scheme and context), and otherwise
2352          using pct-encoding.
2353          (b2) if you want (good) URI components, stop here.</t>
2354       
2355        <t> reassemble the reencoded components.
2356          (c2) if you want a (*good*) whole URI stop here.</t>
2357      </list>
2358       
2359      </t>
2360     
2361    </section>
2362   
2363    <section title="NEW WAY">
2364      <t>
2365        <list style="numbers">
2366         
2367          <t> Parse the IRI into IRI components using the generic syntax.
2368            (a) if you want IRI components, stop here.</t>
2369         
2370          <t> Encode each components, using pct-encoding, IDN encoding, or
2371            special query part encoding depending on the component
2372            scheme or context. (b) If you want URI components, stop here.</t>
2373          <t> reassemble the a whole URI from URI components.
2374            (c) if you want a whole URI stop here.</t>
2375        </list></t>
2376    </section>
2377    <section title="Extension of Syntax">
2378      <t>Added the tag range (U+E0000-E0FFF) to the iprivate production.
2379        Some IRIs generated with the new syntax may fail to pass very strict checks
2380        relying on the old syntax. But characters in this range should be extremely infrequent
2381        anyway.</t>
2382    </section>
2383    <section title="More to be added"><t>TODO: There are more main changes that need to be
2384      documented in this section.</t></section>
2385</section>
2386
2387<section title="Change Log">
2388
2389<t>Note to RFC Editor: Please completely remove this section before publication.</t>
2390
2391<section title='Changes after draft-ietf-iri-3987bis-01'>
2392    <t>Changes from draft-ietf-iri-3987bis-01 onwards are available as changesets
2393      in the IETF tools subversion repository at
2394      http://trac.tools.ietf.org/wg/iri/trac/log/draft-ietf-iri-3987bis/draft-ietf-iri-3987bis.xml.</t>
2395</section>
2396 
2397<section title='Changes from draft-duerst-iri-bis-07 to draft-ietf-iri-3987bis-00'>
2398     <t>Changed draft name, date, last paragraph of abstract, and titles in change log, and added this section
2399     in moving from draft-duerst-iri-bis-07 (personal submission) to draft-ietf-iri-3987bis-00 (WG document).</t>
2400</section>
2401
2402<section title="Changes from -06 to -07 of draft-duerst-iri-bis">
2403  <t>Major restructuring of the processing model, see <xref target="forkChanges"></xref>.</t>
2404</section>
2405</section>
2406
2407<section title='Changes from -00 to -01'><t><list style="symbols">
2408  <t>Removed 'mailto:' before mail addresses of authors.</t>
2409  <t>Added "&lt;to be done&gt;" as right side of 'href-strip' rule. Fixed '|' to '/' for
2410    alternatives.</t>
2411</list></t>
2412</section>
2413
2414<section title="Changes from -05 to -06 of draft-duerst-iri-bis-00"><t><list style="symbols">
2415<t>Add HyperText Reference, change abstract, acks and references for it</t>
2416<t>Add Masinter back as another editor.</t>
2417<t>Masinter integrates HRef material from HTML5 spec.</t>
2418<t>Rewrite introduction sections to modernize.</t>
2419</list></t>
2420</section>
2421
2422<section title="Changes from -04 to -05 of draft-duerst-iri-bis">
2423  <t><list style="symbols">
2424    <t>Updated references.</t>
2425    <t>Changed IPR text to pre5378Trust200902.</t></list></t>
2426</section>
2427
2428<section title="Changes from -03 to -04 of draft-duerst-iri-bis">
2429  <t><list style="symbols">
2430    <t>Added explicit abbreviation for LEIRIs.</t>
2431    <t>Mentioned LEIRI references.</t>
2432    <t>Completed text in LEIRI section about tag characters and about specials.</t></list></t>
2433</section>
2434
2435<section title="Changes from -02 to -03 of draft-duerst-iri-bis">
2436  <t><list style="symbols">
2437    <t>Updated some references.</t>
2438    <t>Updated Michel Suginard's coordinates.</t></list></t>
2439</section>
2440
2441<section title="Changes from -01 to -02 of draft-duerst-iri-bis">
2442  <t><list style="symbols">
2443    <t>Added tag range to iprivate (issue private-include-tags-115).</t>
2444    <t>Added Specials (U+FFF0-FFFD) to Legacy Extended IRIs.</t></list></t>
2445</section>
2446<section title="Changes from -00 to -01 of draft-duerst-iri-bis">
2447  <t><list style="symbols">
2448    <t>Changed from "IRIs with Spaces/Controls" to "Legacy Extended IRI"
2449      based on input from the W3C XML Core WG.
2450      Moved the relevant subsections to the back and promoted them to a section.</t>
2451    <t>Added some text re. Legacy Extended IRIs to the security section.</t>
2452    <t>Added a IANA Consideration Section.</t>
2453    <t>Added this Change Log Section.</t>
2454    <t>Added a section about "IRIs with Spaces/Controls" (converting from a Note in RFC 3987).</t></list></t>
2455</section>
2456<section title="Changes from RFC 3987 to -00 of draft-duerst-iri-bis">
2457  <t><list>
2458    <t>Fixed errata (see http://www.rfc-editor.org/cgi-bin/errataSearch.pl?rfc=3987).</t></list></t>
2459</section>
2460</section>
2461</middle>
2462
2463<back>
2464<references title="Normative References">
2465
2466<reference anchor="ASCII">
2467<front>
2468<title>Coded Character Set -- 7-bit American Standard Code for Information
2469Interchange</title>
2470<author>
2471<organization>American National Standards Institute</organization>
2472</author>
2473<date year="1986"/>
2474</front>
2475<seriesInfo name="ANSI" value="X3.4"/>
2476</reference>
2477
2478<reference anchor="ISO10646">
2479<front>
2480<title>ISO/IEC 10646:2003: Information Technology -
2481Universal Multiple-Octet Coded Character Set (UCS)</title>
2482<author>
2483<organization>International Organization for Standardization</organization>
2484</author>
2485<date month="December" year="2003"/>
2486</front>
2487<seriesInfo name="ISO" value="Standard 10646"/>
2488</reference>
2489
2490&rfc2119;
2491&rfc3490;
2492&rfc3491;
2493&rfc3629;
2494&rfc3986;
2495
2496<reference anchor="STD68">
2497<front>
2498<title abbrev="ABNF">Augmented BNF for Syntax Specifications: ABNF</title>
2499<author initials="D." surname="Crocker" fullname="Dave Crocker"><organization/></author>
2500<author initials="P." surname="Overell" fullname="Paul Overell"><organization/></author>
2501<date month="January" year="2008"/></front>
2502<seriesInfo name="STD" value="68"/><seriesInfo name="RFC" value="5234"/>
2503</reference>
2504 
2505&rfc5890;
2506&rfc5891;
2507
2508<reference anchor="UNIV6">
2509<front>
2510<title>The Unicode Standard, Version 6.0.0 (Mountain View, CA, The Unicode Consortium, 2011, ISBN 978-1-936213-01-6)</title>
2511<author><organization>The Unicode Consortium</organization></author>
2512<date year="2010" month="October"/>
2513</front>
2514</reference>
2515
2516<reference anchor="UNI9" target="http://www.unicode.org/reports/tr9/tr9-13.html">
2517<front>
2518<title>The Bidirectional Algorithm</title>
2519<author initials="M." surname="Davis" fullname="Mark Davis"><organization/></author>
2520<date year="2004" month="March"/>
2521</front>
2522<seriesInfo name="Unicode Standard Annex" value="#9"/>
2523</reference>
2524
2525<reference anchor="UTR15" target="http://www.unicode.org/unicode/reports/tr15/tr15-23.html">
2526<front>
2527<title>Unicode Normalization Forms</title>
2528<author initials="M." surname="Davis" fullname="Mark Davis"><organization/></author>
2529<author initials="M.J." surname="Duerst" fullname="Martin Duerst"><organization/></author>
2530<date year="2008" month="March"/>
2531</front>
2532<seriesInfo name="Unicode Standard Annex" value="#15"/>
2533</reference>
2534
2535</references>
2536
2537<references title="Informative References">
2538
2539<reference anchor="BidiEx" target="http://www.w3.org/International/iri-edit/BidiExamples">
2540<front>
2541<title>Examples of bidirectional IRIs</title>
2542<author><organization/></author>
2543<date year="" month=""/>
2544</front>
2545</reference>
2546
2547<reference anchor="CharMod" target="http://www.w3.org/TR/charmod-resid">
2548<front>
2549<title>Character Model for the World Wide Web: Resource Identifiers</title>
2550<author initials="M." surname="Duerst" fullname="Martin Duerst"><organization/></author>
2551<author initials="F." surname="Yergeau" fullname="Francois Yergeau"><organization/></author>
2552<author initials="R." surname="Ishida" fullname="Richard Ishida"><organization/></author>
2553<author initials="M." surname="Wolf" fullname="Misha Wolf"><organization/></author>
2554<author initials="T." surname="Texin" fullname="Tex Texin"><organization/></author>
2555<date year="2004" month="November" day="25"/>
2556</front>
2557<seriesInfo name="World Wide Web Consortium" value="Candidate Recommendation"/>
2558</reference>
2559
2560<reference anchor="Duerst97" target="http://www.ifi.unizh.ch/mml/mduerst/papers/PDF/IUC11-UTF-8.pdf">
2561<front>
2562<title>The Properties and Promises of UTF-8</title>
2563<author initials="M.J." surname="Duerst" fullname="Martin Duerst"><organization/></author>
2564<date year="1997" month="September"/>
2565</front>
2566<seriesInfo name="Proc. 11th International Unicode Conference, San Jose" value=""/>
2567</reference>
2568
2569<reference anchor="Gettys" target="http://www.w3.org/DesignIssues/ModelConsequences">
2570<front>
2571<title>URI Model Consequences</title>
2572<author initials="J." surname="Gettys" fullname="Jim Gettys"><organization/></author>
2573<date month="" year=""/>
2574</front>
2575</reference>
2576
2577<reference anchor="HTML4" target="http://www.w3.org/TR/html401/appendix/notes.html#h-B.2">
2578<front>
2579<title>HTML 4.01 Specification</title>
2580<author initials="D." surname="Raggett" fullname="Dave Raggett"><organization/></author>
2581<author initials="A." surname="Le Hors" fullname="Arnaud Le Hors"><organization/></author>
2582<author initials="I." surname="Jacobs" fullname="Ian Jacobs"><organization/></author>
2583<date year="1999" month="December" day="24"/>
2584</front>
2585<seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2586</reference>
2587
2588<reference anchor="LEIRI" target="http://www.w3.org/TR/leiri/">
2589<front>
2590<title>Legacy extended IRIs for XML resource identification</title>
2591<author initials="H." surname="Thompson" fullname="Henry Thompson"><organization/></author>
2592<author initials="R." surname="Tobin"    fullname="Richard Tobin"><organization/></author>
2593<author initials="N." surname="Walsh" fullname="Norman Walsh"><organization/></author>
2594  <date year="2008" month="November" day="3"/>
2595
2596</front>
2597<seriesInfo name="World Wide Web Consortium" value="Note"/>
2598</reference>
2599
2600
2601&rfc2045;
2602&rfc2130;
2603&rfc2141;
2604&rfc2192;
2605&rfc2277;
2606&rfc2368;
2607&rfc2384;
2608&rfc2396;
2609&rfc2397;
2610&rfc2616;
2611&rfc1738;
2612&rfc2640;
2613&rfc3987;
2614&rfc6055;
2615<reference anchor='RFC4395bis'>
2616  <front>
2617    <title>Guidelines and Registration Procedures for New URI/IRI Schemes</title>
2618    <author initials='T.' surname='Hansen' fullname="Tony Hansen"><organization/></author>
2619    <author initials='T.' surname='Hardie' fullname="Ted Hardie"><organization/></author>
2620    <author initials='L.' surname='Masinter' fullname="Larry Masinter"><organization/></author>
2621    <date year="2010" month='September' day="30"/>
2622    <workgroup>IRI</workgroup>
2623  </front>
2624  <seriesInfo name="Internet-Draft" value="draft-hansen-iri-4395bis-irireg-00"/>
2625</reference>
2626 
2627 
2628<reference anchor="UNIXML" target="http://www.w3.org/TR/unicode-xml/">
2629<front>
2630<title>Unicode in XML and other Markup Languages</title>
2631<author initials="M.J." surname="Duerst" fullname="Martin Duerst"><organization/></author>
2632<author initials="A." surname="Freytag" fullname="Asmus Freytag"><organization/></author>
2633<date year="2003" month="June" day="18"/>
2634</front>
2635<seriesInfo name="Unicode Technical Report" value="#20"/>
2636<seriesInfo name="World Wide Web Consortium" value="Note"/>
2637</reference>
2638 
2639<reference anchor="UTR36" target="http://unicode.org/reports/tr36/">
2640<front>
2641<title>Unicode Security Considerations</title>
2642<author initials="M." surname="Davis" fullname="Mark Davis"><organization/></author>
2643<author initials="M." surname="Suignard" fullname="Michel Suignard"><organization/></author>
2644<date year="2010" month="August" day="4"/>
2645</front>
2646<seriesInfo name="Unicode Technical Report" value="#36"/>
2647</reference>
2648
2649<reference anchor="XLink" target="http://www.w3.org/TR/xlink/#link-locators">
2650<front>
2651<title>XML Linking Language (XLink) Version 1.0</title>
2652<author initials="S." surname="DeRose" fullname="Steve DeRose"><organization/></author>
2653<author initials="E." surname="Maler" fullname="Eve Maler"><organization/></author>
2654<author initials="D." surname="Orchard" fullname="David Orchard"><organization/></author>
2655<date year="2001" month="June" day="27"/>
2656</front>
2657<seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2658</reference>
2659
2660<reference anchor="XML1" target="http://www.w3.org/TR/REC-xml">
2661  <front>
2662    <title>Extensible Markup Language (XML) 1.0 (Forth Edition)</title>
2663    <author initials="T." surname="Bray" fullname="Tim Bray"><organization/></author>
2664    <author initials="J." surname="Paoli" fullname="Jean Paoli"><organization/></author>
2665    <author initials="C.M." surname="Sperberg-McQueen" fullname="C. M. Sperberg-McQueen">
2666      <organization/></author>
2667    <author initials="E." surname="Maler" fullname="Eve Maler"><organization/></author>
2668    <author initials="F." surname="Yergeau" fullname="Francois Yergeau"><organization/></author>
2669    <date day="16" month="August" year="2006"/>
2670  </front>
2671  <seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2672</reference>
2673
2674<reference anchor="XMLNamespace" target="http://www.w3.org/TR/REC-xml-names">
2675  <front>
2676    <title>Namespaces in XML (Second Edition)</title>
2677    <author initials="T." surname="Bray" fullname="Tim Bray"><organization/></author>
2678    <author initials="D." surname="Hollander" fullname="Dave Hollander"><organization/></author>
2679    <author initials="A." surname="Layman" fullname="Andrew Layman"><organization/></author>
2680    <author initials="R." surname="Tobin" fullname="Richard Tobin"><organization></organization></author>
2681    <date day="16" month="August" year="2006"/>
2682  </front>
2683  <seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2684</reference>
2685
2686<reference anchor="XMLSchema" target="http://www.w3.org/TR/xmlschema-2/#anyURI">
2687<front>
2688<title>XML Schema Part 2: Datatypes</title>
2689<author initials="P." surname="Biron" fullname="Paul Biron"><organization/></author>
2690<author initials="A." surname="Malhotra" fullname="Ashok Malhotra"><organization/></author>
2691<date year="2001" month="May" day="2"/>
2692</front>
2693<seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2694</reference>
2695
2696<reference anchor="XPointer" target="http://www.w3.org/TR/xptr-framework/#escaping">
2697<front>
2698<title>XPointer Framework</title>
2699<author initials="P." surname="Grosso" fullname="Paul Grosso"><organization/></author>
2700<author initials="E." surname="Maler" fullname="Eve Maler"><organization/></author>
2701<author initials="J." surname="Marsh" fullname="Jonathan Marsh"><organization/></author>
2702<author initials="N." surname="Walsh" fullname="Norman Walsh"><organization/></author>
2703<date year="2003" month="March" day="25"/>
2704</front>
2705<seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2706</reference>
2707
2708<reference anchor="HTML5" target="http://www.w3.org/TR/2009/WD-html5-20090423/">
2709<front>
2710<title>A vocabulary and associated APIs for HTML and XHTML</title>
2711<author initials="I." surname="Hickson" fullname="Ian Hickson"><organization>Google, Inc.</organization></author>
2712<author initials="D." surname="Hyatt" fullname="David Hyatt"><organization>Apple, Inc.</organization></author>
2713<date year="2009"  month="April" day="23"/>
2714</front>
2715<seriesInfo name="World Wide Web Consortium" value="Working Draft"/>
2716</reference>
2717
2718</references>
2719
2720</back>
2721</rfc>
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