source: draft-ietf-iri-3987bis/draft-ietf-iri-3987bis.xml @ 64

Last change on this file since 64 was 64, checked in by duerst@…, 9 years ago

added explanation for why conversion to %-encoding is desirable for ireg-names

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File size: 123.3 KB
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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):
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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
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
702
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
719<t><list style="hanging"> 
720
721<t hangText="Note:">Domain Names may appear in parts of an IRI other
722than the ireg-name part.  It is the responsibility of scheme-specific
723implementations (if the Internationalized Domain Name is part of the
724scheme syntax) or of server-side implementations (if the
725Internationalized Domain Name is part of 'iquery') to apply the
726necessary conversions at the appropriate point. Example: Trying to
727validate the Web page at<vspace/>
728http://r&amp;#xE9;sum&amp;#xE9;.example.org would lead to an IRI of
729<vspace/>http://validator.w3.org/check?uri=http%3A%2F%2Fr&amp;#xE9;sum&amp;#xE9;.<vspace/>example.org,
730which would convert to a URI
731of<vspace/>http://validator.w3.org/check?uri=http%3A%2F%2Fr%C3%A9sum%C3%A9.<vspace/>example.org.
732The server-side implementation is responsible for making the
733necessary conversions to be able to retrieve the Web page.</t>
734
735<t hangText="Note:">In this process, characters allowed in URI
736references and existing percent-encoded sequences are not encoded further.
737(This mapping is similar to, but different from, the encoding applied
738when arbitrary content is included in some part of a URI.)
739
740For example, an IRI of
741<vspace/>"http://www.example.org/red%09ros&amp;#xE9;#red"
742(in XML notation) is converted to
743<vspace/>"http://www.example.org/red%09ros%C3%A9#red", not to
744something like
745<vspace/>"http%3A%2F%2Fwww.example.org%2Fred%2509ros%C3%A9%23red".
746((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?))
747
748</t>
749
750</list></t>
751</section> <!-- dnsmapping -->
752
753<section title="Mapping query components" anchor="querymapping">
754
755<t>((NOTE: SEE ISSUES LIST))
756
757For compatibility with existing deployed HTTP infrastructure,
758the following special case applies for schemes "http" and "https"
759and IRIs whose origin has a document charset other than one which
760is UCS-based (e.g., UTF-8 or UTF-16). In such a case, the "query"
761component of an IRI is mapped into a URI by using the document
762charset rather than UTF-8 as the binary representation before
763pct-encoding. This mapping is not applied for any other scheme
764or component.</t>
765
766</section> <!-- querymapping -->
767
768<section title="Mapping IRIs to URIs" anchor="mapping">
769
770<t>The canonical mapping from a IRI to URI is defined by applying the
771mapping above (from IRI to URI components) and then reassembling a URI
772from the parsed URI components using the original punctuation that
773delimited the IRI components. </t>
774
775</section> <!-- mapping -->
776
777<section title="Converting URIs to IRIs" anchor="URItoIRI">
778
779<t>In some situations, for presentation and further processing,
780it is desirable to convert a URI into an equivalent IRI in which
781natural characters are represented directly rather than
782percent encoded. Of course, every URI is already an IRI in
783its own right without any conversion, and in general there
784This section gives one such procedure for this conversion.
785</t>
786
787<t>
788The conversion described in this section, if given a valid URI, will
789result in an IRI that maps back to the URI used as an input for the
790conversion (except for potential case differences in percent-encoding
791and for potential percent-encoded unreserved characters).
792
793However, the IRI resulting from this conversion may differ
794from the original IRI (if there ever was one).</t> 
795
796<t>URI-to-IRI conversion removes percent-encodings, but not all
797percent-encodings can be eliminated. There are several reasons for
798this:</t>
799
800<t><list style="hanging">
801
802<t hangText="1.">Some percent-encodings are necessary to distinguish
803    percent-encoded and unencoded uses of reserved characters.</t>
804
805<t hangText="2.">Some percent-encodings cannot be interpreted as sequences
806    of UTF-8 octets.<vspace blankLines="1"/>
807    (Note: The octet patterns of UTF-8 are highly regular.
808    Therefore, there is a very high probability, but no guarantee,
809    that percent-encodings that can be interpreted as sequences of UTF-8
810    octets actually originated from UTF-8. For a detailed discussion,
811    see <xref target="Duerst97"/>.)</t>
812
813<t hangText="3.">The conversion may result in a character that is not
814    appropriate in an IRI. See <xref target="abnf"/>, <xref target="visual"/>,
815      and <xref target="limitations"/> for further details.</t>
816
817<t hangText="4.">IRI to URI conversion has different rules for
818    dealing with domain names and query parameters.</t>
819
820</list></t>
821
822<t>Conversion from a URI to an IRI MAY be done by using the following
823steps:
824
825<list style="hanging">
826<t hangText="1.">Represent the URI as a sequence of octets in
827       US-ASCII.</t>
828
829<t hangText="2.">Convert all percent-encodings ("%" followed by two
830      hexadecimal digits) to the corresponding octets, except those
831      corresponding to "%", characters in "reserved", and characters
832      in US-ASCII not allowed in URIs.</t> 
833
834<t hangText="3.">Re-percent-encode any octet produced in step 2 that
835      is not part of a strictly legal UTF-8 octet sequence.</t>
836
837
838<t hangText="4.">Re-percent-encode all octets produced in step 3 that
839      in UTF-8 represent characters that are not appropriate according
840      to <xref target="abnf"/>, <xref target="visual"/>, and <xref
841      target="limitations"/>.</t> 
842
843<t hangText="5.">Interpret the resulting octet sequence as a sequence
844      of characters encoded in UTF-8.</t>
845
846<t hangText="6.">URIs known to contain domain names in the reg-name
847      component SHOULD convert punycode-encoded domain name labels to
848      the corresponding characters using the ToUnicode procedure. </t>
849</list></t>
850
851<t>This procedure will convert as many percent-encoded characters as
852possible to characters in an IRI. Because there are some choices when
853step 4 is applied (see <xref target="limitations"/>), results may
854vary.</t>
855
856<t>Conversions from URIs to IRIs MUST NOT use any character
857encoding other than UTF-8 in steps 3 and 4, even if it might be
858possible to guess from the context that another character encoding
859than UTF-8 was used in the URI.  For example, the URI
860"http://www.example.org/r%E9sum%E9.html" might with some guessing be
861interpreted to contain two e-acute characters encoded as
862iso-8859-1. It must not be converted to an IRI containing these
863e-acute characters. Otherwise, in the future the IRI will be mapped to
864"http://www.example.org/r%C3%A9sum%C3%A9.html", which is a different
865URI from "http://www.example.org/r%E9sum%E9.html".</t>
866
867<section title="Examples">
868
869<t>This section shows various examples of converting URIs to IRIs.
870Each example shows the result after each of the steps 1 through 6 is
871applied. XML Notation is used for the final result.  Octets are
872denoted by "&lt;" followed by two hexadecimal digits followed by
873"&gt;".</t>
874
875<t>The following example contains the sequence "%C3%BC", which is a
876strictly legal UTF-8 sequence, and which is converted into the actual
877character U+00FC, LATIN SMALL LETTER U WITH DIAERESIS (also known as
878u-umlaut).
879
880<list style="hanging">
881<t hangText="1.">http://www.example.org/D%C3%BCrst</t>
882<t hangText="2.">http://www.example.org/D&lt;c3&gt;&lt;bc&gt;rst</t>
883<t hangText="3.">http://www.example.org/D&lt;c3&gt;&lt;bc&gt;rst</t>
884<t hangText="4.">http://www.example.org/D&lt;c3&gt;&lt;bc&gt;rst</t>
885<t hangText="5.">http://www.example.org/D&amp;#xFC;rst</t>
886<t hangText="6.">http://www.example.org/D&amp;#xFC;rst</t>
887</list>
888</t>
889
890<t>The following example contains the sequence "%FC", which might
891represent U+00FC, LATIN SMALL LETTER U WITH DIAERESIS, in
892the<vspace/>iso-8859-1 character encoding.  (It might represent other
893characters in other character encodings. For example, the octet
894&lt;fc&gt; in iso-8859-5 represents U+045C, CYRILLIC SMALL LETTER
895KJE.)  Because &lt;fc&gt; is not part of a strictly legal UTF-8
896sequence, it is re-percent-encoded in step 3.
897
898
899<list style="hanging">
900<t hangText="1.">http://www.example.org/D%FCrst</t>
901<t hangText="2.">http://www.example.org/D&lt;fc&gt;rst</t>
902<t hangText="3.">http://www.example.org/D%FCrst</t>
903<t hangText="4.">http://www.example.org/D%FCrst</t>
904<t hangText="5.">http://www.example.org/D%FCrst</t>
905<t hangText="6.">http://www.example.org/D%FCrst</t>
906</list>
907</t>
908
909<t>The following example contains "%e2%80%ae", which is the percent-encoded<vspace/>UTF-8
910character encoding of U+202E, RIGHT-TO-LEFT OVERRIDE. <xref target="visual"/>
911forbids the direct use of this character in an IRI. Therefore, the
912corresponding octets are re-percent-encoded in step 4. This example shows
913that the case (upper- or lowercase) of letters used in percent-encodings may not be preserved.
914The example also contains a punycode-encoded domain name label (xn--99zt52a),
915which is not converted.
916
917<list style="hanging">
918<t hangText="1.">http://xn--99zt52a.example.org/%e2%80%ae</t>
919<t hangText="2.">http://xn--99zt52a.example.org/&lt;e2&gt;&lt;80&gt;&lt;ae&gt;</t>
920<t hangText="3.">http://xn--99zt52a.example.org/&lt;e2&gt;&lt;80&gt;&lt;ae&gt;</t>
921<t hangText="4.">http://xn--99zt52a.example.org/%E2%80%AE</t>
922<t hangText="5.">http://xn--99zt52a.example.org/%E2%80%AE</t>
923<t hangText="6.">http://&amp;#x7D0D;&amp;#x8C46;.example.org/%E2%80%AE</t>
924</list></t>
925
926<t>Note that the label "xn--99zt52a" is converted to U+7D0D U+8C46
927(Japanese Natto). ((EDITOR NOTE: There is some inconsistency in this note.))</t>
928
929</section> <!-- examples -->
930</section> <!-- URItoIRI -->
931</section> <!-- processing -->
932<section title="Bidirectional IRIs for Right-to-Left Languages" anchor="Bidi">
933
934<t>Some UCS characters, such as those used in the Arabic and Hebrew
935scripts, have an inherent right-to-left (rtl) writing direction. IRIs
936containing these characters (called bidirectional IRIs or Bidi IRIs)
937require additional attention because of the non-trivial relation
938between logical representation (used for digital representation and
939for reading/spelling) and visual representation (used for
940display/printing).</t>
941
942<t>Because of the complex interaction between the logical representation,
943the visual representation, and the syntax of a Bidi IRI, a balance is
944needed between various requirements.
945The main requirements are<list style="hanging">
946<t hangText="1.">user-predictable conversion between visual and
947    logical representation;</t>
948<t hangText="2.">the ability to include a wide range of characters
949    in various parts of the IRI; and</t>
950<t hangText="3.">minor or no changes or restrictions for
951      implementations.</t>
952</list></t>
953
954<section title="Logical Storage and Visual Presentation" anchor="visual">
955
956<t>When stored or transmitted in digital representation, bidirectional
957IRIs MUST be in full logical order and MUST conform to the IRI syntax
958rules (which includes the rules relevant to their scheme). This
959ensures that bidirectional IRIs can be processed in the same way as
960other IRIs.</t> <t>Bidirectional IRIs MUST be rendered by using the
961Unicode Bidirectional Algorithm <xref target="UNIV6"/>, <xref
962target="UNI9"/>.  Bidirectional IRIs MUST be rendered in the same way
963as they would be if they were in a left-to-right embedding; i.e., as
964if they were preceded by U+202A, LEFT-TO-RIGHT EMBEDDING (LRE), and
965followed by U+202C, POP DIRECTIONAL FORMATTING (PDF).  Setting the
966embedding direction can also be done in a higher-level protocol (e.g.,
967the dir='ltr' attribute in HTML).</t> 
968
969<t>There is no requirement to use the above embedding if the display
970is still the same without the embedding. For example, a bidirectional
971IRI in a text with left-to-right base directionality (such as used for
972English or Cyrillic) that is preceded and followed by whitespace and
973strong left-to-right characters does not need an embedding.  Also, a
974bidirectional relative IRI reference that only contains strong
975right-to-left characters and weak characters and that starts and ends
976with a strong right-to-left character and appears in a text with
977right-to-left base directionality (such as used for Arabic or Hebrew)
978and is preceded and followed by whitespace and strong characters does
979not need an embedding.</t>
980
981<t>In some other cases, using U+200E, LEFT-TO-RIGHT MARK (LRM), may be
982sufficient to force the correct display behavior.  However, the
983details of the Unicode Bidirectional algorithm are not always easy to
984understand. Implementers are strongly advised to err on the side of
985caution and to use embedding in all cases where they are not
986completely sure that the display behavior is unaffected without the
987embedding.</t>
988
989<t>The Unicode Bidirectional Algorithm (<xref target="UNI9"/>, section
9904.3) permits higher-level protocols to influence bidirectional
991rendering. Such changes by higher-level protocols MUST NOT be used if
992they change the rendering of IRIs.</t> 
993
994<t>The bidirectional formatting characters that may be used before or
995after the IRI to ensure correct display are not themselves part of the
996IRI.  IRIs MUST NOT contain bidirectional formatting characters (LRM,
997RLM, LRE, RLE, LRO, RLO, and PDF). They affect the visual rendering of
998the IRI but do not appear themselves. It would therefore not be
999possible to input an IRI with such characters correctly.</t>
1000
1001</section> <!-- visual -->
1002<section title="Bidi IRI Structure" anchor="bidi-structure">
1003
1004<t>The Unicode Bidirectional Algorithm is designed mainly for running
1005text.  To make sure that it does not affect the rendering of
1006bidirectional IRIs too much, some restrictions on bidirectional IRIs
1007are necessary. These restrictions are given in terms of delimiters
1008(structural characters, mostly punctuation such as "@", ".", ":",
1009and<vspace/>"/") and components (usually consisting mostly of letters
1010and digits).</t>
1011
1012<t>The following syntax rules from <xref target="abnf"/> correspond to
1013components for the purpose of Bidi behavior: iuserinfo, ireg-name,
1014isegment, isegment-nz, isegment-nz-nc, ireg-name, iquery, and
1015ifragment.</t>
1016
1017<t>Specifications that define the syntax of any of the above
1018components MAY divide them further and define smaller parts to be
1019components according to this document. As an example, the restrictions
1020of <xref target="RFC3490"/> on bidirectional domain names correspond
1021to treating each label of a domain name as a component for schemes
1022with ireg-name as a domain name.  Even where the components are not
1023defined formally, it may be helpful to think about some syntax in
1024terms of components and to apply the relevant restrictions.  For
1025example, for the usual name/value syntax in query parts, it is
1026convenient to treat each name and each value as a component. As
1027another example, the extensions in a resource name can be treated as
1028separate components.</t>
1029
1030<t>For each component, the following restrictions apply:</t>
1031<t>
1032<list style="hanging">
1033
1034<t hangText="1.">A component SHOULD NOT use both right-to-left and
1035  left-to-right characters.</t>
1036
1037<t hangText="2.">A component using right-to-left characters SHOULD
1038  start and end with right-to-left characters.</t>
1039
1040</list></t>
1041
1042<t>The above restrictions are given as "SHOULD"s, rather than as
1043"MUST"s.  For IRIs that are never presented visually, they are not
1044relevant.  However, for IRIs in general, they are very important to
1045ensure consistent conversion between visual presentation and logical
1046representation, in both directions.</t>
1047
1048<t><list style="hanging">
1049
1050<t hangText="Note:">In some components, the above restrictions may
1051  actually be strictly enforced.  For example, <xref
1052  target="RFC3490"></xref> requires that these restrictions apply to
1053  the labels of a host name for those schemes where ireg-name is a
1054  host name.  In some other components (for example, path components)
1055  following these restrictions may not be too difficult.  For other
1056  components, such as parts of the query part, it may be very
1057  difficult to enforce the restrictions because the values of query
1058  parameters may be arbitrary character sequences.</t>
1059
1060</list></t>
1061
1062<t>If the above restrictions cannot be satisfied otherwise, the
1063affected component can always be mapped to URI notation as described
1064in <xref target="compmapping"/>. Please note that the whole component
1065has to be mapped (see also Example 9 below).</t>
1066
1067</section> <!-- bidi-structure -->
1068
1069<section title="Input of Bidi IRIs" anchor="bidiInput">
1070
1071<t>Bidi input methods MUST generate Bidi IRIs in logical order while
1072rendering them according to <xref target="visual"/>.  During input,
1073rendering SHOULD be updated after every new character is input to
1074avoid end-user confusion.</t>
1075
1076</section> <!-- bidiInput -->
1077
1078<section title="Examples">
1079
1080<t>This section gives examples of bidirectional IRIs, in Bidi
1081Notation.  It shows legal IRIs with the relationship between logical
1082and visual representation and explains how certain phenomena in this
1083relationship may look strange to somebody not familiar with
1084bidirectional behavior, but familiar to users of Arabic and Hebrew. It
1085also shows what happens if the restrictions given in <xref
1086target="bidi-structure"/> are not followed. The examples below can be
1087seen at <xref target="BidiEx"/>, in Arabic, Hebrew, and Bidi Notation
1088variants.</t>
1089
1090<t>To read the bidi text in the examples, read the visual
1091representation from left to right until you encounter a block of rtl
1092text. Read the rtl block (including slashes and other special
1093characters) from right to left, then continue at the next unread ltr
1094character.</t>
1095
1096<t>Example 1: A single component with rtl characters is inverted:
1097<vspace/>Logical representation:
1098"http://ab.CDEFGH.ij/kl/mn/op.html"<vspace/>Visual representation:
1099"http://ab.HGFEDC.ij/kl/mn/op.html"<vspace/> Components can be read
1100one by one, and each component can be read in its natural
1101direction.</t>
1102
1103<t>Example 2: More than one consecutive component with rtl characters
1104is inverted as a whole: <vspace/>Logical representation:
1105"http://ab.CDE.FGH/ij/kl/mn/op.html"<vspace/>Visual representation:
1106"http://ab.HGF.EDC/ij/kl/mn/op.html"<vspace/> A sequence of rtl
1107components is read rtl, in the same way as a sequence of rtl words is
1108read rtl in a bidi text.</t>
1109
1110<t>Example 3: All components of an IRI (except for the scheme) are
1111rtl.  All rtl components are inverted overall: <vspace/>Logical
1112representation:
1113"http://AB.CD.EF/GH/IJ/KL?MN=OP;QR=ST#UV"<vspace/>Visual
1114representation: "http://VU#TS=RQ;PO=NM?LK/JI/HG/FE.DC.BA"<vspace/> The
1115whole IRI (except the scheme) is read rtl. Delimiters between rtl
1116components stay between the respective components; delimiters between
1117ltr and rtl components don't move.</t>
1118
1119<t>Example 4: Each of several sequences of rtl components is inverted
1120on its own: <vspace/>Logical representation:
1121"http://AB.CD.ef/gh/IJ/KL.html"<vspace/>Visual representation:
1122"http://DC.BA.ef/gh/LK/JI.html"<vspace/> Each sequence of rtl
1123components is read rtl, in the same way as each sequence of rtl words
1124in an ltr text is read rtl.</t>
1125
1126<t>Example 5: Example 2, applied to components of different kinds:
1127<vspace/>Logical representation: "http://ab.cd.EF/GH/ij/kl.html"
1128<vspace/>Visual representation:
1129"http://ab.cd.HG/FE/ij/kl.html"<vspace/> The inversion of the domain
1130name label and the path component may be unexpected, but it is
1131consistent with other bidi behavior.  For reassurance that the domain
1132component really is "ab.cd.EF", it may be helpful to read aloud the
1133visual representation following the bidi algorithm. After
1134"http://ab.cd." one reads the RTL block "E-F-slash-G-H", which
1135corresponds to the logical representation.
1136</t>
1137
1138<t>Example 6: Same as Example 5, with more rtl components:
1139<vspace/>Logical representation:
1140"http://ab.CD.EF/GH/IJ/kl.html"<vspace/>Visual representation:
1141"http://ab.JI/HG/FE.DC/kl.html"<vspace/> The inversion of the domain
1142name labels and the path components may be easier to identify because
1143the delimiters also move.</t>
1144
1145<t>Example 7: A single rtl component includes digits: <vspace/>Logical
1146representation: "http://ab.CDE123FGH.ij/kl/mn/op.html"<vspace/>Visual
1147representation: "http://ab.HGF123EDC.ij/kl/mn/op.html"<vspace/>
1148Numbers are written ltr in all cases but are treated as an additional
1149embedding inside a run of rtl characters. This is completely
1150consistent with usual bidirectional text.</t>
1151
1152<t>Example 8 (not allowed): Numbers are at the start or end of an rtl
1153component:<vspace/>Logical representation:
1154"http://ab.cd.ef/GH1/2IJ/KL.html"<vspace/>Visual representation:
1155"http://ab.cd.ef/LK/JI1/2HG.html"<vspace/> The sequence "1/2" is
1156interpreted by the bidi algorithm as a fraction, fragmenting the
1157components and leading to confusion. There are other characters that
1158are interpreted in a special way close to numbers; in particular, "+",
1159"-", "#", "$", "%", ",", ".", and ":".</t>
1160
1161<t>Example 9 (not allowed): The numbers in the previous example are
1162percent-encoded: <vspace/>Logical representation:
1163"http://ab.cd.ef/GH%31/%32IJ/KL.html",<vspace/>Visual representation:
1164"http://ab.cd.ef/LK/JI%32/%31HG.html"</t>
1165
1166<t>Example 10 (allowed but not recommended): <vspace/>Logical
1167representation: "http://ab.CDEFGH.123/kl/mn/op.html"<vspace/>Visual
1168representation: "http://ab.123.HGFEDC/kl/mn/op.html"<vspace/>
1169Components consisting of only numbers are allowed (it would be rather
1170difficult to prohibit them), but these may interact with adjacent RTL
1171components in ways that are not easy to predict.</t>
1172
1173<t>Example 11 (allowed but not recommended): <vspace/>Logical
1174representation: "http://ab.CDEFGH.123ij/kl/mn/op.html"<vspace/>Visual
1175representation: "http://ab.123.HGFEDCij/kl/mn/op.html"<vspace/>
1176Components consisting of numbers and left-to-right characters are
1177allowed, but these may interact with adjacent RTL components in ways
1178that are not easy to predict.</t>
1179</section><!-- examples -->
1180</section><!-- bidi -->
1181
1182<section title="Normalization and Comparison" anchor="equivalence">
1183
1184<t><list style="hanging"><t hangText="Note:">The structure and much of
1185  the material for this section is taken from section 6 of <xref
1186  target="RFC3986"></xref>; the differences are due to the specifics
1187  of IRIs.</t></list></t>
1188
1189<t>One of the most common operations on IRIs is simple comparison:
1190Determining whether two IRIs are equivalent, without using the IRIs to
1191access their respective resource(s). A comparison is performed
1192whenever a response cache is accessed, a browser checks its history to
1193color a link, or an XML parser processes tags within a
1194namespace. Extensive normalization prior to comparison of IRIs may be
1195used by spiders and indexing engines to prune a search space or reduce
1196duplication of request actions and response storage.</t>
1197
1198<t>IRI comparison is performed for some particular purpose. Protocols
1199or implementations that compare IRIs for different purposes will often
1200be subject to differing design trade-offs in regards to how much
1201effort should be spent in reducing aliased identifiers. This section
1202describes various methods that may be used to compare IRIs, the
1203trade-offs between them, and the types of applications that might use
1204them.</t>
1205
1206<section title="Equivalence">
1207
1208<t>Because IRIs exist to identify resources, presumably they should be
1209considered equivalent when they identify the same resource. However,
1210this definition of equivalence is not of much practical use, as there
1211is no way for an implementation to compare two resources to determine
1212if they are "the same" unless it has full knowledge or control of
1213them. For this reason, determination of equivalence or difference of
1214IRIs is based on string comparison, perhaps augmented by reference to
1215additional rules provided by URI scheme definitions.  We use the terms
1216"different" and "equivalent" to describe the possible outcomes of such
1217comparisons, but there are many application-dependent versions of
1218equivalence.</t>
1219
1220<t>Even when it is possible to determine that two IRIs are equivalent,
1221IRI comparison is not sufficient to determine whether two IRIs
1222identify different resources. For example, an owner of two different
1223domain names could decide to serve the same resource from both,
1224resulting in two different IRIs. Therefore, comparison methods are
1225designed to minimize false negatives while strictly avoiding false
1226positives.</t>
1227
1228<t>In testing for equivalence, applications should not directly
1229compare relative references; the references should be converted to
1230their respective target IRIs before comparison. When IRIs are compared
1231to select (or avoid) a network action, such as retrieval of a
1232representation, fragment components (if any) should be excluded from
1233the comparison.</t>
1234
1235<t>Applications using IRIs as identity tokens with no relationship to
1236a protocol MUST use the Simple String Comparison (see <xref
1237target="stringcomp"></xref>).  All other applications MUST select one
1238of the comparison practices from the Comparison Ladder (see <xref
1239target="ladder"></xref>.</t>
1240</section> <!-- equivalence -->
1241
1242
1243<section title="Preparation for Comparison">
1244<t>Any kind of IRI comparison REQUIRES that any additional contextual
1245processing is first performed, including undoing higher-level
1246escapings or encodings in the protocol or format that carries an
1247IRI. This preprocessing is usually done when the protocol or format is
1248parsed.</t>
1249
1250<t>Examples of contextual preprocessing steps are described in <xref
1251target="LEIRIHREF"/>. </t>
1252
1253<t>Examples of such escapings or encodings are entities and
1254numeric character references in <xref target="HTML4"></xref> and <xref
1255target="XML1"></xref>. As an example,
1256"http://example.org/ros&amp;eacute;" (in HTML),
1257"http://example.org/ros&amp;#233;" (in HTML or XML), and
1258<vspace/>"http://example.org/ros&amp;#xE9;" (in HTML or XML) are all
1259resolved into what is denoted in this document (see <xref
1260target="sec-Notation"></xref>) as "http://example.org/ros&amp;#xE9;"
1261(the "&amp;#xE9;" here standing for the actual e-acute character, to
1262compensate for the fact that this document cannot contain non-ASCII
1263characters).</t>
1264
1265<t>Similar considerations apply to encodings such as Transfer Codings
1266in HTTP (see <xref target="RFC2616"></xref>) and Content Transfer
1267Encodings in MIME (<xref target="RFC2045"></xref>), although in these
1268cases, the encoding is based not on characters but on octets, and
1269additional care is required to make sure that characters, and not just
1270arbitrary octets, are compared (see <xref
1271target="stringcomp"></xref>).</t>
1272
1273</section> <!-- preparation -->
1274
1275<section title="Comparison Ladder" anchor="ladder">
1276
1277<t>In practice, a variety of methods are used to test IRI
1278equivalence. These methods fall into a range distinguished by the
1279amount of processing required and the degree to which the probability
1280of false negatives is reduced. As noted above, false negatives cannot
1281be eliminated. In practice, their probability can be reduced, but this
1282reduction requires more processing and is not cost-effective for all
1283applications.</t>
1284
1285
1286<t>If this range of comparison practices is considered as a ladder,
1287the following discussion will climb the ladder, starting with
1288practices that are cheap but have a relatively higher chance of
1289producing false negatives, and proceeding to those that have higher
1290computational cost and lower risk of false negatives.</t>
1291
1292<section title="Simple String Comparison" anchor="stringcomp">
1293
1294<t>If two IRIs, when considered as character strings, are identical,
1295then it is safe to conclude that they are equivalent.  This type of
1296equivalence test has very low computational cost and is in wide use in
1297a variety of applications, particularly in the domain of parsing. It
1298is also used when a definitive answer to the question of IRI
1299equivalence is needed that is independent of the scheme used and that
1300can be calculated quickly and without accessing a network. An example
1301of such a case is XML Namespaces (<xref
1302target="XMLNamespace"></xref>).</t>
1303
1304
1305<t>Testing strings for equivalence requires some basic precautions.
1306This procedure is often referred to as "bit-for-bit" or
1307"byte-for-byte" comparison, which is potentially misleading. Testing
1308strings for equality is normally based on pair comparison of the
1309characters that make up the strings, starting from the first and
1310proceeding until both strings are exhausted and all characters are
1311found to be equal, until a pair of characters compares unequal, or
1312until one of the strings is exhausted before the other.</t>
1313
1314<t>This character comparison requires that each pair of characters be
1315put in comparable encoding form. For example, should one IRI be stored
1316in a byte array in UTF-8 encoding form and the second in a UTF-16
1317encoding form, bit-for-bit comparisons applied naively will produce
1318errors. It is better to speak of equality on a character-for-character
1319rather than on a byte-for-byte or bit-for-bit basis.  In practical
1320terms, character-by-character comparisons should be done codepoint by
1321codepoint after conversion to a common character encoding form.
1322
1323When comparing character by character, the comparison function MUST
1324NOT map IRIs to URIs, because such a mapping would create additional
1325spurious equivalences. It follows that an IRI SHOULD NOT be modified
1326when being transported if there is any chance that this IRI might be
1327used in a context that uses Simple String Comparison.</t>
1328
1329
1330<t>False negatives are caused by the production and use of IRI
1331aliases. Unnecessary aliases can be reduced, regardless of the
1332comparison method, by consistently providing IRI references in an
1333already normalized form (i.e., a form identical to what would be
1334produced after normalization is applied, as described below).
1335Protocols and data formats often limit some IRI comparisons to simple
1336string comparison, based on the theory that people and implementations
1337will, in their own best interest, be consistent in providing IRI
1338references, or at least be consistent enough to negate any efficiency
1339that might be obtained from further normalization.</t>
1340</section> <!-- stringcomp -->
1341
1342<section title="Syntax-Based Normalization">
1343
1344<figure><preamble>Implementations may use logic based on the
1345definitions provided by this specification to reduce the probability
1346of false negatives. This processing is moderately higher in cost than
1347character-for-character string comparison. For example, an application
1348using this approach could reasonably consider the following two IRIs
1349equivalent:</preamble>
1350
1351<artwork>
1352   example://a/b/c/%7Bfoo%7D/ros&amp;#xE9;
1353   eXAMPLE://a/./b/../b/%63/%7bfoo%7d/ros%C3%A9
1354</artwork></figure>
1355
1356<t>Web user agents, such as browsers, typically apply this type of IRI
1357normalization when determining whether a cached response is
1358available. Syntax-based normalization includes such techniques as case
1359normalization, character normalization, percent-encoding
1360normalization, and removal of dot-segments.</t>
1361
1362<section title="Case Normalization">
1363
1364<t>For all IRIs, the hexadecimal digits within a percent-encoding
1365triplet (e.g., "%3a" versus "%3A") are case-insensitive and therefore
1366should be normalized to use uppercase letters for the digits A-F.</t>
1367
1368<t>When an IRI uses components of the generic syntax, the component
1369syntax equivalence rules always apply; namely, that the scheme and
1370US-ASCII only host are case insensitive and therefore should be
1371normalized to lowercase. For example, the URI
1372"HTTP://www.EXAMPLE.com/" is equivalent to
1373"http://www.example.com/". Case equivalence for non-ASCII characters
1374in IRI components that are IDNs are discussed in <xref
1375target="schemecomp"></xref>.  The other generic syntax components are
1376assumed to be case sensitive unless specifically defined otherwise by
1377the scheme.</t>
1378
1379<t>Creating schemes that allow case-insensitive syntax components
1380containing non-ASCII characters should be avoided. Case normalization
1381of non-ASCII characters can be culturally dependent and is always a
1382complex operation. The only exception concerns non-ASCII host names
1383for which the character normalization includes a mapping step derived
1384from case folding.</t>
1385
1386</section> <!-- casenorm -->
1387
1388<section title="Character Normalization" anchor="normalization">
1389
1390<t>The Unicode Standard <xref target="UNIV6"></xref> defines various
1391equivalences between sequences of characters for various
1392purposes. Unicode Standard Annex #15 <xref target="UTR15"></xref>
1393defines various Normalization Forms for these equivalences, in
1394particular Normalization Form C (NFC, Canonical Decomposition,
1395followed by Canonical Composition) and Normalization Form KC (NFKC,
1396Compatibility Decomposition, followed by Canonical Composition).</t>
1397
1398<t> IRIs already in Unicode MUST NOT be normalized before parsing or
1399interpreting. In many non-Unicode character encodings, some text
1400cannot be represented directly. For example, the word "Vietnam" is
1401natively written "Vi&amp;#x1EC7;t Nam" (containing a LATIN SMALL
1402LETTER E WITH CIRCUMFLEX AND DOT BELOW) in NFC, but a direct
1403transcoding from the windows-1258 character encoding leads to
1404"Vi&amp;#xEA;&amp;#x323;t Nam" (containing a LATIN SMALL LETTER E WITH
1405CIRCUMFLEX followed by a COMBINING DOT BELOW). Direct transcoding of
1406other 8-bit encodings of Vietnamese may lead to other
1407representations.</t>
1408
1409<t>Equivalence of IRIs MUST rely on the assumption that IRIs are
1410appropriately pre-character-normalized rather than apply character
1411normalization when comparing two IRIs. The exceptions are conversion
1412from a non-digital form, and conversion from a non-UCS-based character
1413encoding to a UCS-based character encoding. In these cases, NFC or a
1414normalizing transcoder using NFC MUST be used for interoperability. To
1415avoid false negatives and problems with transcoding, IRIs SHOULD be
1416created by using NFC. Using NFKC may avoid even more problems; for
1417example, by choosing half-width Latin letters instead of full-width
1418ones, and full-width instead of half-width Katakana.</t>
1419
1420
1421<t>As an example,
1422"http://www.example.org/r&amp;#xE9;sum&amp;#xE9;.html" (in XML
1423Notation) is in NFC. On the other hand,
1424"http://www.example.org/re&amp;#x301;sume&amp;#x301;.html" is not in
1425NFC.</t>
1426
1427<t>The former uses precombined e-acute characters, and the latter uses
1428"e" characters followed by combining acute accents. Both usages are
1429defined as canonically equivalent in <xref target="UNIV6"></xref>.</t>
1430
1431<t><list style="hanging">
1432
1433<t hangText="Note:">
1434Because it is unknown how a particular sequence of characters is being
1435treated with respect to character normalization, it would be
1436inappropriate to allow third parties to normalize an IRI
1437arbitrarily. This does not contradict the recommendation that when a
1438resource is created, its IRI should be as character normalized as
1439possible (i.e., NFC or even NFKC). This is similar to the
1440uppercase/lowercase problems.  Some parts of a URI are case
1441insensitive (for example, the domain name). For others, it is unclear
1442whether they are case sensitive, case insensitive, or something in
1443between (e.g., case sensitive, but with a multiple choice selection if
1444the wrong case is used, instead of a direct negative result).  The
1445best recipe is that the creator use a reasonable capitalization and,
1446when transferring the URI, capitalization never be
1447changed.</t></list></t>
1448
1449<t>Various IRI schemes may allow the usage of Internationalized Domain
1450Names (IDN) <xref target="RFC5890"/> either in the ireg-name
1451part or elsewhere. Character Normalization also applies to IDNs, as
1452discussed in <xref target="schemecomp"/>.</t>
1453</section> <!-- charnorm -->
1454
1455<section title="Percent-Encoding Normalization">
1456
1457<t>The percent-encoding mechanism (Section 2.1 of <xref
1458target="RFC3986"></xref>) is a frequent source of variance among
1459otherwise identical IRIs. In addition to the case normalization issue
1460noted above, some IRI producers percent-encode octets that do not
1461require percent-encoding, resulting in IRIs that are equivalent to
1462their nonencoded counterparts. These IRIs should be normalized by
1463decoding any percent-encoded octet sequence that corresponds to an
1464unreserved character, as described in section 2.3 of <xref
1465target="RFC3986"></xref>.</t>
1466
1467<t>For actual resolution, differences in percent-encoding (except for
1468the percent-encoding of reserved characters) MUST always result in the
1469same resource.  For example, "http://example.org/~user",
1470"http://example.org/%7euser", and "http://example.org/%7Euser", must
1471resolve to the same resource.</t>
1472
1473<t>If this kind of equivalence is to be tested, the percent-encoding
1474of both IRIs to be compared has to be aligned; for example, by
1475converting both IRIs to URIs (see Section 3.1), eliminating escape
1476differences in the resulting URIs, and making sure that the case of
1477the hexadecimal characters in the percent-encoding is always the same
1478(preferably upper case). If the IRI is to be passed to another
1479application or used further in some other way, its original form MUST
1480be preserved.  The conversion described here should be performed only
1481for local comparison.</t>
1482
1483</section> <!-- pctnorm -->
1484
1485<section title="Path Segment Normalization">
1486
1487<t>The complete path segments "." and ".." are intended only for use
1488within relative references (Section 4.1 of <xref
1489target="RFC3986"></xref>) and are removed as part of the reference
1490resolution process (Section 5.2 of <xref target="RFC3986"></xref>).
1491However, some implementations may incorrectly assume that reference
1492resolution is not necessary when the reference is already an IRI, and
1493thus fail to remove dot-segments when they occur in non-relative
1494paths.  IRI normalizers should remove dot-segments by applying the
1495remove_dot_segments algorithm to the path, as described in Section
14965.2.4 of <xref target="RFC3986"></xref>.</t>
1497
1498</section> <!-- pathnorm -->
1499</section> <!-- ladder -->
1500
1501<section title="Scheme-Based Normalization" anchor="schemecomp">
1502
1503<t>The syntax and semantics of IRIs vary from scheme to scheme, as
1504described by the defining specification for each
1505scheme. Implementations may use scheme-specific rules, at further
1506processing cost, to reduce the probability of false negatives. For
1507example, because the "http" scheme makes use of an authority
1508component, has a default port of "80", and defines an empty path to be
1509equivalent to "/", the following four IRIs are equivalent:</t>
1510
1511<figure><artwork>
1512   http://example.com
1513   http://example.com/
1514   http://example.com:/
1515   http://example.com:80/</artwork></figure>
1516
1517<t>In general, an IRI that uses the generic syntax for authority with
1518an empty path should be normalized to a path of "/". Likewise, an
1519explicit ":port", for which the port is empty or the default for the
1520scheme, is equivalent to one where the port and its ":" delimiter are
1521elided and thus should be removed by scheme-based normalization. For
1522example, the second IRI above is the normal form for the "http"
1523scheme.</t>
1524
1525<t>Another case where normalization varies by scheme is in the
1526handling of an empty authority component or empty host
1527subcomponent. For many scheme specifications, an empty authority or
1528host is considered an error; for others, it is considered equivalent
1529to "localhost" or the end-user's host. When a scheme defines a default
1530for authority and an IRI reference to that default is desired, the
1531reference should be normalized to an empty authority for the sake of
1532uniformity, brevity, and internationalization. If, however, either the
1533userinfo or port subcomponents are non-empty, then the host should be
1534given explicitly even if it matches the default.</t>
1535
1536<t>Normalization should not remove delimiters when their associated
1537component is empty unless it is licensed to do so by the scheme
1538specification. For example, the IRI "http://example.com/?" cannot be
1539assumed to be equivalent to any of the examples above. Likewise, the
1540presence or absence of delimiters within a userinfo subcomponent is
1541usually significant to its interpretation.  The fragment component is
1542not subject to any scheme-based normalization; thus, two IRIs that
1543differ only by the suffix "#" are considered different regardless of
1544the scheme.</t>
1545 
1546<t>Some IRI schemes allow the usage of Internationalized Domain
1547Names (IDN) <xref target='RFC5890'></xref> either in their ireg-name
1548part or elswhere. When in use in IRIs, those names SHOULD
1549conform to the definition of U-Label in <xref
1550target='RFC5890'></xref>. An IRI containing an invalid IDN cannot
1551successfully be resolved. For legibility purposes, they
1552SHOULD NOT be converted into ASCII Compatible Encoding (ACE).</t>
1553
1554<t>Scheme-based normalization may also consider IDN
1555components and their conversions to punycode as equivalent. As an
1556example, "http://r&amp;#xE9;sum&amp;#xE9;.example.org" may be
1557considered equivalent to
1558"http://xn--rsum-bpad.example.org".</t><t>Other scheme-specific
1559normalizations are possible.</t>
1560
1561</section> <!-- schemenorm -->
1562
1563<section title="Protocol-Based Normalization">
1564
1565<t>Substantial effort to reduce the incidence of false negatives is
1566often cost-effective for web spiders. Consequently, they implement
1567even more aggressive techniques in IRI comparison. For example, if
1568they observe that an IRI such as</t>
1569
1570<figure><artwork>
1571   http://example.com/data</artwork></figure>
1572<t>redirects to an IRI differing only in the trailing slash</t>
1573<figure><artwork>
1574   http://example.com/data/</artwork></figure>
1575
1576<t>they will likely regard the two as equivalent in the future.  This
1577kind of technique is only appropriate when equivalence is clearly
1578indicated by both the result of accessing the resources and the common
1579conventions of their scheme's dereference algorithm (in this case, use
1580of redirection by HTTP origin servers to avoid problems with relative
1581references).</t>
1582
1583</section> <!-- protonorm -->
1584</section> <!-- equivalence -->
1585</section> 
1586
1587<section title="Use of IRIs" anchor="IRIuse">
1588
1589<section title="Limitations on UCS Characters Allowed in IRIs" anchor="limitations">
1590
1591<t>This section discusses limitations on characters and character
1592sequences usable for IRIs beyond those given in <xref target="abnf"/>
1593and <xref target="visual"/>. The considerations in this section are
1594relevant when IRIs are created and when URIs are converted to
1595IRIs.</t>
1596
1597<t>
1598
1599<list style="hanging"><t hangText="a.">The repertoire of characters allowed
1600    in each IRI component is limited by the definition of that component.
1601    For example, the definition of the scheme component does not allow
1602    characters beyond US-ASCII.
1603    <vspace blankLines="1"/>
1604    (Note: In accordance with URI practice, generic IRI
1605    software cannot and should not check for such limitations.)</t>
1606
1607<t hangText="b.">The UCS contains many areas of characters for which
1608    there are strong visual look-alikes. Because of the likelihood of
1609    transcription errors, these also should be avoided. This includes
1610    the full-width equivalents of Latin characters, half-width
1611    Katakana characters for Japanese, and many others. It also
1612    includes many look-alikes of "space", "delims", and "unwise",
1613    characters excluded in <xref target="RFC3491"/>.</t>
1614   
1615</list>
1616</t>
1617
1618<t>Additional information is available from <xref target="UNIXML"/>.
1619    <xref target="UNIXML"/> is written in the context of running text
1620    rather than in that of identifiers. Nevertheless, it discusses
1621    many of the categories of characters not appropriate for IRIs.</t>
1622</section> <!-- limitations -->
1623
1624<section title="Software Interfaces and Protocols">
1625
1626<t>Although an IRI is defined as a sequence of characters, software
1627interfaces for URIs typically function on sequences of octets or other
1628kinds of code units. Thus, software interfaces and protocols MUST
1629define which character encoding is used.</t>
1630
1631<t>Intermediate software interfaces between IRI-capable components and
1632URI-only components MUST map the IRIs per <xref target="mapping"/>,
1633when transferring from IRI-capable to URI-only components.
1634
1635This mapping SHOULD be applied as late as possible. It SHOULD NOT be
1636applied between components that are known to be able to handle IRIs.</t>
1637</section> <!-- software -->
1638
1639<section title="Format of URIs and IRIs in Documents and Protocols">
1640
1641<t>Document formats that transport URIs may have to be upgraded to allow
1642the transport of IRIs. In cases where the document as a whole
1643has a native character encoding, IRIs MUST also be encoded in this
1644character encoding and converted accordingly by a parser or interpreter.
1645
1646IRI characters not expressible in the native character encoding SHOULD
1647be escaped by using the escaping conventions of the document format if
1648such conventions are available. Alternatively, they MAY be
1649percent-encoded according to <xref target="mapping"/>. For example, in
1650HTML or XML, numeric character references SHOULD be used. If a
1651document as a whole has a native character encoding and that character
1652encoding is not UTF-8, then IRIs MUST NOT be placed into the document
1653in the UTF-8 character encoding.</t>
1654
1655<t>((UPDATE THIS NOTE)) Note: Some formats already accommodate IRIs,
1656although they use different terminology. HTML 4.0 <xref
1657target="HTML4"/> defines the conversion from IRIs to URIs as
1658error-avoiding behavior. XML 1.0 <xref target="XML1"/>, XLink <xref
1659target="XLink"/>, XML Schema <xref target="XMLSchema"/>, and
1660specifications based upon them allow IRIs. Also, it is expected that
1661all relevant new W3C formats and protocols will be required to handle
1662IRIs <xref target="CharMod"/>.</t>
1663
1664</section> <!-- format -->
1665
1666<section title="Use of UTF-8 for Encoding Original Characters" anchor="UTF8use">
1667
1668<t>This section discusses details and gives examples for point c) in
1669<xref target="Applicability"/>. To be able to use IRIs, the URI
1670corresponding to the IRI in question has to encode original characters
1671into octets by using UTF-8.  This can be specified for all URIs of a
1672URI scheme or can apply to individual URIs for schemes that do not
1673specify how to encode original characters.  It can apply to the whole
1674URI, or only to some part. For background information on encoding
1675characters into URIs, see also Section 2.5 of <xref
1676target="RFC3986"/>.</t>
1677
1678<t>For new URI schemes, using UTF-8 is recommended in <xref
1679target="RFC4395bis"/>.  Examples where UTF-8 is already used are the URN
1680syntax <xref target="RFC2141"/>, IMAP URLs <xref target="RFC2192"/>,
1681and POP URLs <xref target="RFC2384"/>.  On the other hand, because the
1682HTTP URI scheme does not specify how to encode original characters,
1683only some HTTP URLs can have corresponding but different IRIs.</t>
1684
1685<t>For example, for a document with a URI
1686of<vspace/>"http://www.example.org/r%C3%A9sum%C3%A9.html", it is
1687possible to construct a corresponding IRI (in XML notation, see <xref
1688target="sec-Notation"/>):
1689"http://www.example.org/r&amp;#xE9;sum&amp;#xE9;.html" ("&amp;#xE9;"
1690stands for the e-acute character, and "%C3%A9" is the UTF-8 encoded
1691and percent-encoded representation of that character). On the other
1692hand, for a document with a URI of
1693"http://www.example.org/r%E9sum%E9.html", the percent-encoding octets
1694cannot be converted to actual characters in an IRI, as the
1695percent-encoding is not based on UTF-8.</t>
1696
1697<t>For most URI schemes, there is no need to upgrade their scheme
1698definition in order for them to work with IRIs.  The main case where
1699upgrading makes sense is when a scheme definition, or a particular
1700component of a scheme, is strictly limited to the use of US-ASCII
1701characters with no provision to include non-ASCII characters/octets
1702via percent-encoding, or if a scheme definition currently uses highly
1703scheme-specific provisions for the encoding of non-ASCII characters.
1704An example of this is the mailto: scheme <xref target="RFC2368"/>.</t>
1705
1706<t>This specification updates the IANA registry of URI schemes to note
1707their applicability to IRIs, see <xref target="iana"/>.  All IRIs use
1708URI schemes, and all URIs with URI schemes can be used as IRIs, even
1709though in some cases only by using URIs directly as IRIs, without any
1710conversion.</t>
1711
1712<t>Scheme definitions can impose restrictions on the syntax of
1713scheme-specific URIs; i.e., URIs that are admissible under the generic
1714URI syntax <xref target="RFC3986"/> may not be admissible due to
1715narrower syntactic constraints imposed by a URI scheme
1716specification. URI scheme definitions cannot broaden the syntactic
1717restrictions of the generic URI syntax; otherwise, it would be
1718possible to generate URIs that satisfied the scheme-specific syntactic
1719constraints without satisfying the syntactic constraints of the
1720generic URI syntax. However, additional syntactic constraints imposed
1721by URI scheme specifications are applicable to IRI, as the
1722corresponding URI resulting from the mapping defined in <xref
1723target="mapping"/> MUST be a valid URI under the syntactic
1724restrictions of generic URI syntax and any narrower restrictions
1725imposed by the corresponding URI scheme specification.</t>
1726
1727<t>The requirement for the use of UTF-8 generally applies to all parts
1728of a URI.  However, it is possible that the capability of IRIs to
1729represent a wide range of characters directly is used just in some
1730parts of the IRI (or IRI reference). The other parts of the IRI may
1731only contain US-ASCII characters, or they may not be based on
1732UTF-8. They may be based on another character encoding, or they may
1733directly encode raw binary data (see also <xref
1734target="RFC2397"/>). </t>
1735
1736<t>For example, it is possible to have a URI reference
1737of<vspace/>"http://www.example.org/r%E9sum%E9.xml#r%C3%A9sum%C3%A9",
1738where the document name is encoded in iso-8859-1 based on server
1739settings, but where the fragment identifier is encoded in UTF-8 according
1740to <xref target="XPointer"/>. The IRI corresponding to the above
1741URI would be (in XML notation)<vspace/>"http://www.example.org/r%E9sum%E9.xml#r&amp;#xE9;sum&amp;#xE9;".</t>
1742
1743<t>Similar considerations apply to query parts. The functionality
1744of IRIs (namely, to be able to include non-ASCII characters) can
1745only be used if the query part is encoded in UTF-8.</t>
1746
1747</section> <!-- utf8 -->
1748
1749<section title="Relative IRI References">
1750<t>Processing of relative IRI references against a base is handled
1751straightforwardly; the algorithms of <xref target="RFC3986"/> can
1752be applied directly, treating the characters additionally allowed
1753in IRI references in the same way that unreserved characters are in URI
1754references.</t>
1755
1756</section> <!-- relative -->
1757</section> <!-- IRIuse -->
1758
1759<section title="Liberal Handling of Otherwise Invalid IRIs" anchor="LEIRIHREF">
1760
1761<t>(EDITOR NOTE: This Section may move to an appendix.)
1762 
1763Some technical specifications and widely-deployed software have
1764allowed additional variations and extensions of IRIs to be used in
1765syntactic components. This section describes two widely-used
1766preprocessing agreements. Other technical specifications may wish to
1767reference a syntactic component which is "a valid IRI or a string that
1768will map to a valid IRI after this preprocessing algorithm". These two
1769variants are known as <xref target="LEIRI">Legacy Extended IRI or
1770LEIRI</xref>, and <xref target="HTML5">Web Address</xref>).
1771</t>
1772
1773<t>Future technical specifications SHOULD NOT allow conforming
1774producers to produce, or conforming content to contain, such forms,
1775as they are not interoperable with other IRI consuming software.</t>
1776
1777<section title="LEIRI Processing"  anchor="LEIRIspec">
1778  <t>This section defines Legacy Extended IRIs (LEIRIs).
1779    The syntax of Legacy Extended IRIs is the same as that for &lt;IRI-reference>,
1780    except that the ucschar production is replaced by the leiri-ucschar production:</t>
1781<figure>
1782
1783<artwork>
1784  leiri-ucschar  = " " / "&lt;" / "&gt;" / '"' / "{" / "}" / "|"
1785                   / "\" / "^" / "`" / %x0-1F / %x7F-D7FF
1786                   / %xE000-FFFD / %x10000-10FFFF
1787</artwork>
1788
1789<postamble>
1790  Among other extensions, processors based on this specification also
1791  did not enforce the restriction on bidirectional formatting
1792  characters in <xref target="visual"></xref>, and the iprivate
1793  production becomes redundant.</postamble>
1794</figure>
1795
1796<t>To convert a string allowed as a LEIRI to an IRI, each character
1797allowed in leiri-ucschar but not in ucschar must be percent-encoded
1798using <xref target="compmapping"/>.</t>
1799</section> <!-- leiriproc -->
1800
1801<section title="Web Address Processing" anchor="webaddress">
1802
1803<t>Many popular web browsers have taken the approach of being quite
1804liberal in what is accepted as a "URL" or its relative
1805forms. This section describes their behavior in terms of a preprocessor
1806which maps strings into the IRI space for subsequent parsing and
1807interpretation as an IRI.</t>
1808
1809<t>In some situations, it might be appropriate to describe the syntax
1810that a liberal consumer implementation might accept as a "Web
1811Address" or "Hypertext Reference" or "HREF". However,
1812technical specifications SHOULD restrict the syntactic form allowed by compliant producers
1813to the IRI or IRI reference syntax defined in this document
1814even if they want to mandate this processing.</t>
1815
1816<t>
1817Summary:
1818<list style="symbols">
1819   <t>Leading and trailing whitespace is removed.</t>
1820   <t>Some additional characters are removed.</t>
1821   <t>Some additional characters are allowed and escaped (as with LEIRI).</t>
1822   <t>If interpreting an IRI as a URI, the pct-encoding of the query
1823   component of the parsed URI component depends on operational
1824   context.</t>
1825</list>
1826</t>
1827
1828<t>Each string provided may have an associated charset (called
1829the HREF-charset here); this defaults to UTF-8.
1830For web browsers interpreting HTML, the document
1831charset of a string is determined:
1832
1833<list style="hanging">
1834<t hangText="If the string came from a script (e.g. as an argument to
1835 a method)">The HRef-charset is the script's charset.</t>
1836
1837<t hangText="If the string came from a DOM node (e.g. from an
1838  element)">The node has a Document, and the HRef-charset is the
1839  Document's character encoding.</t>
1840
1841<t hangText="If the string had a HRef-charset defined when the string was
1842created or defined">The HRef-charset is as defined.</t>
1843
1844</list></t>
1845
1846<t>If the resulting HRef-charset is a unicode based character encoding
1847(e.g., UTF-16), then use UTF-8 instead.</t>
1848
1849
1850<figure>
1851<preamble>The syntax for Web Addresses is obtained by replacing the 'ucschar',
1852  pct-form, path-sep, and ifragment rules with the href-ucschar, href-pct-form, href-path-sep,
1853  and href-ifragment
1854  rules below. In addition, some characters are stripped.</preamble>
1855
1856<artwork type='abnf'>
1857  href-ucschar   = " " / "&lt;" / "&gt;" / DQUOTE / "{" / "}" / "|"
1858                 / "\" / "^" / "`" / %x0-1F / %x7F-D7FF
1859                 / %xE000-FFFD / %x10000-10FFFF
1860  href-pct-form  = pct-encoded / "%"
1861  href-path-sep  = "/" / "\"
1862  href-ifragment = *( ipchar / "/" / "?" / "#" )  ; adding "#"
1863  href-strip     = &lt;to be done&gt;
1864</artwork>
1865
1866<postamble>
1867(NOTE: NEED TO FIX THESE SETS TO MATCH HTML5; NOT SURE ABOUT NEXT SENTENCE)
1868browsers did not enforce the restriction on bidirectional formatting
1869  characters in <xref target="visual"></xref>, and the iprivate
1870  production becomes redundant.</postamble>
1871</figure>
1872
1873<t>'Web Address processing' requires the following additional
1874preprocessing steps:
1875
1876<list style="numbers">
1877
1878<t>Leading and trailing instances of space (U+0020),
1879CR (U+000A), LF (U+000D), and TAB (U+0009) characters are removed.</t>
1880
1881<t>strip all characters in href-strip.</t>
1882  <t>Percent-encode all characters in href-ucschar not in ucschar.</t>
1883  <t>Replace occurrences of "%" not followed by two hexadecimal digits by "%25".</t>
1884  <t>Convert backslashes ('\') matching href-path-sep to forward slashes ('/').</t>
1885</list></t>
1886</section> <!-- webaddress -->
1887
1888<section title="Characters Not Allowed in IRIs" anchor="notAllowed">
1889
1890<t>This section provides a list of the groups of characters and code
1891points that are allowed by LEIRI or HREF but are not allowed in IRIs or are
1892allowed in IRIs only in the query part. For each group of characters,
1893advice on the usage of these characters is also given, concentrating
1894on the reasons for why they are excluded from IRI use.</t>
1895
1896<t>
1897
1898<list><t>Space (U+0020): Some formats and applications use space as a
1899delimiter, e.g. for items in a list. Appendix C of <xref
1900target="RFC3986"></xref> also mentions that white space may have to be
1901added when displaying or printing long URIs; the same applies to long
1902IRIs. This means that spaces can disappear, or can make the what is
1903intended as a single IRI or IRI reference to be treated as two or more
1904separate IRIs.</t>
1905
1906<t>Delimiters "&lt;" (U+003C), "&gt;" (U+003E), and '"' (U+0022):
1907Appendix C of <xref target="RFC3986"></xref> suggests the use of
1908double-quotes ("http://example.com/") and angle brackets
1909(&lt;http://example.com/&gt;) as delimiters for URIs in plain
1910text. These conventions are often used, and also apply to IRIs.  Using
1911these characters in strings intended to be IRIs would result in the
1912IRIs being cut off at the wrong place.</t>
1913
1914<t>Unwise characters "\" (U+005C), "^" (U+005E), "`"
1915(U+0060), "{" (U+007B), "|" (U+007C), and "}" (U+007D): These
1916characters originally have been excluded from URIs because the
1917respective codepoints are assigned to different graphic characters in
1918some 7-bit or 8-bit encoding. Despite the move to Unicode, some of
1919these characters are still occasionally displayed differently on some
1920systems, e.g. U+005C may appear as a Japanese Yen symbol on some
1921systems. Also, the fact that these characters are not used in URIs or
1922IRIs has encouraged their use outside URIs or IRIs in contexts that
1923may include URIs or IRIs. If a string with such a character were used
1924as an IRI in such a context, it would likely be interpreted
1925piecemeal.</t>
1926
1927<t>The controls (C0 controls, DEL, and C1 controls, #x0 - #x1F #x7F -
1928#x9F): There is generally no way to transmit these characters reliably
1929as text outside of a charset encoding.  Even when in encoded form,
1930many software components silently filter out some of these characters,
1931or may stop processing alltogether when encountering some of
1932them. These characters may affect text display in subtle, unnoticable
1933ways or in drastic, global, and irreversible ways depending on the
1934hardware and software involved. The use of some of these characters
1935would allow malicious users to manipulate the display of an IRI and
1936its context in many situations.</t>
1937
1938<t>Bidi formatting characters (U+200E, U+200F, U+202A-202E): These
1939characters affect the display ordering of characters. If IRIs were
1940allowed to contain these characters and the resulting visual display
1941transcribed. they could not be converted back to electronic form
1942(logical order) unambiguously. These characters, if allowed in IRIs,
1943might allow malicious users to manipulate the display of IRI and its
1944context.</t>
1945
1946<t>Specials (U+FFF0-FFFD): These code points provide functionality
1947beyond that useful in an IRI, for example byte order identification,
1948annotation, and replacements for unknown characters and objects. Their
1949use and interpretation in an IRI would serve no purpose and might lead
1950to confusing display variations.</t>
1951
1952<t>Private use code points (U+E000-F8FF, U+F0000-FFFFD,
1953U+100000-10FFFD): Display and interpretation of these code points is
1954by definition undefined without private agreement. Therefore, these
1955code points are not suited for use on the Internet. They are not
1956interoperable and may have unpredictable effects.</t>
1957
1958<t>Tags (U+E0000-E0FFF): These characters provide a way to language
1959tag in Unicode plain text. They are not appropriate for IRIs because
1960language information in identifiers cannot reliably be input,
1961transmitted (e.g. on a visual medium such as paper), or
1962recognized.</t>
1963
1964<t>Non-characters (U+FDD0-FDEF, U+1FFFE-1FFFF, U+2FFFE-2FFFF,
1965U+3FFFE-3FFFF, U+4FFFE-4FFFF, U+5FFFE-5FFFF, U+6FFFE-6FFFF,
1966U+7FFFE-7FFFF, U+8FFFE-8FFFF, U+9FFFE-9FFFF, U+AFFFE-AFFFF,
1967U+BFFFE-BFFFF, U+CFFFE-CFFFF, U+DFFFE-DFFFF, U+EFFFE-EFFFF,
1968U+FFFFE-FFFFF, U+10FFFE-10FFFF): These code points are defined as
1969non-characters. Applications may use some of them internally, but are
1970not prepared to interchange them.</t>
1971
1972</list></t>
1973
1974<t>LEIRI preprocessing disallowed some code points and
1975code units:
1976
1977<list><t>Surrogate code units (D800-DFFF): These do not represent
1978Unicode codepoints.</t></list></t>
1979</section> <!-- notallowed -->
1980</section> <!-- lieirihref -->
1981 
1982<section title="URI/IRI Processing Guidelines (Informative)" anchor="guidelines">
1983
1984<t>This informative section provides guidelines for supporting IRIs in
1985the same software components and operations that currently process
1986URIs: Software interfaces that handle URIs, software that allows users
1987to enter URIs, software that creates or generates URIs, software that
1988displays URIs, formats and protocols that transport URIs, and software
1989that interprets URIs. These may all require modification before
1990functioning properly with IRIs. The considerations in this section
1991also apply to URI references and IRI references.</t>
1992
1993<section title="URI/IRI Software Interfaces">
1994<t>Software interfaces that handle URIs, such as URI-handling APIs and
1995protocols transferring URIs, need interfaces and protocol elements
1996that are designed to carry IRIs.</t>
1997
1998<t>In case the current handling in an API or protocol is based on
1999US-ASCII, UTF-8 is recommended as the character encoding for IRIs, as
2000it is compatible with US-ASCII, is in accordance with the
2001recommendations of <xref target="RFC2277"/>, and makes converting to
2002URIs easy. In any case, the API or protocol definition must clearly
2003define the character encoding to be used.</t>
2004
2005<t>The transfer from URI-only to IRI-capable components requires no
2006mapping, although the conversion described in <xref
2007target="URItoIRI"/> above may be performed. It is preferable not to
2008perform this inverse conversion unless it is certain this can be done
2009correctly.</t>
2010</section>
2011
2012<section title="URI/IRI Entry">
2013
2014<t>Some components allow users to enter URIs into the system
2015by typing or dictation, for example. This software must be updated to allow
2016for IRI entry.</t>
2017
2018<t>A person viewing a visual representation of an IRI (as a sequence
2019of glyphs, in some order, in some visual display) or hearing an IRI
2020will use an entry method for characters in the user's language to
2021input the IRI. Depending on the script and the input method used, this
2022may be a more or less complicated process.</t>
2023
2024<t>The process of IRI entry must ensure, as much as possible, that the
2025restrictions defined in <xref target="abnf"/> are met. This may be
2026done by choosing appropriate input methods or variants/settings
2027thereof, by appropriately converting the characters being input, by
2028eliminating characters that cannot be converted, and/or by issuing a
2029warning or error message to the user.</t>
2030
2031<t>As an example of variant settings, input method editors for East
2032Asian Languages usually allow the input of Latin letters and related
2033characters in full-width or half-width versions. For IRI input, the
2034input method editor should be set so that it produces half-width Latin
2035letters and punctuation and full-width Katakana.</t>
2036
2037<t>An input field primarily or solely used for the input of URIs/IRIs
2038might allow the user to view an IRI as it is mapped to a URI.  Places
2039where the input of IRIs is frequent may provide the possibility for
2040viewing an IRI as mapped to a URI. This will help users when some of
2041the software they use does not yet accept IRIs.</t>
2042
2043<t>An IRI input component interfacing to components that handle URIs,
2044but not IRIs, must map the IRI to a URI before passing it to these
2045components.</t>
2046
2047<t>For the input of IRIs with right-to-left characters, please see
2048<xref target="bidiInput"></xref>.</t>
2049</section>
2050
2051<section title="URI/IRI Transfer between Applications">
2052
2053<t>Many applications (for example, mail user agents) try to detect
2054URIs appearing in plain text. For this, they use some heuristics based
2055on URI syntax. They then allow the user to click on such URIs and
2056retrieve the corresponding resource in an appropriate (usually
2057scheme-dependent) application.</t>
2058
2059<t>Such applications would need to be upgraded, in order to use the
2060IRI syntax as a base for heuristics. In particular, a non-ASCII
2061character should not be taken as the indication of the end of an IRI.
2062Such applications also would need to make sure that they correctly
2063convert the detected IRI from the character encoding of the document
2064or application where the IRI appears, to the character encoding used
2065by the system-wide IRI invocation mechanism, or to a URI (according to
2066<xref target="mapping"/>) if the system-wide invocation mechanism only
2067accepts URIs.</t>
2068
2069<t>The clipboard is another frequently used way to transfer URIs and
2070IRIs from one application to another. On most platforms, the clipboard
2071is able to store and transfer text in many languages and scripts.
2072Correctly used, the clipboard transfers characters, not octets, which
2073will do the right thing with IRIs.</t>
2074</section>
2075
2076<section title="URI/IRI Generation">
2077
2078<t>Systems that offer resources through the Internet, where those
2079resources have logical names, sometimes automatically generate URIs
2080for the resources they offer. For example, some HTTP servers can
2081generate a directory listing for a file directory and then respond to
2082the generated URIs with the files.</t>
2083
2084<t>Many legacy character encodings are in use in various file systems.
2085Many currently deployed systems do not transform the local character
2086representation of the underlying system before generating URIs.</t>
2087
2088<t>For maximum interoperability, systems that generate resource
2089identifiers should make the appropriate transformations. For example,
2090if a file system contains a file named
2091"r&amp;#xE9;sum&amp;#xE9;.html", a server should expose this as
2092"r%C3%A9sum%C3%A9.html" in a URI, which allows use of
2093"r&amp;#xE9;sum&amp;#xE9;.html" in an IRI, even if locally the file
2094name is kept in a character encoding other than UTF-8.
2095</t>
2096
2097<t>This recommendation particularly applies to HTTP servers. For FTP
2098servers, similar considerations apply; see <xref target="RFC2640"/>.</t>
2099</section>
2100
2101<section title="URI/IRI Selection" anchor="selection">
2102<t>In some cases, resource owners and publishers have control over the
2103IRIs used to identify their resources. This control is mostly
2104executed by controlling the resource names, such as file names,
2105directly.</t>
2106
2107<t>In these cases, it is recommended to avoid choosing IRIs that are
2108easily confused. For example, for US-ASCII, the lower-case ell ("l") is
2109easily confused with the digit one ("1"), and the upper-case oh ("O") is
2110easily confused with the digit zero ("0"). Publishers should avoid
2111confusing users with "br0ken" or "1ame" identifiers.</t>
2112
2113<t>Outside the US-ASCII repertoire, there are many more opportunities for
2114confusion; a complete set of guidelines is too lengthy to include
2115here. As long as names are limited to characters from a single script,
2116native writers of a given script or language will know best when
2117ambiguities can appear, and how they can be avoided. What may look
2118ambiguous to a stranger may be completely obvious to the average
2119native user. On the other hand, in some cases, the UCS contains
2120variants for compatibility reasons; for example, for typographic purposes.
2121These should be avoided wherever possible. Although there may be exceptions,
2122newly created resource names should generally be in NFKC
2123<xref target="UTR15"></xref> (which means that they are also in NFC).</t>
2124
2125<t>As an example, the UCS contains the "fi" ligature at U+FB01
2126for compatibility reasons.
2127Wherever possible, IRIs should use the two letters "f" and "i" rather
2128than the "fi" ligature. An example where the latter may be used is
2129in the query part of an IRI for an explicit search for a word written
2130containing the "fi" ligature.</t>
2131
2132<t>In certain cases, there is a chance that characters from different
2133scripts look the same. The best known example is the similarity of the
2134Latin "A", the Greek "Alpha", and the Cyrillic "A". To avoid such
2135cases, IRIs should only be created where all the characters in a
2136single component are used together in a given language. This usually
2137means that all of these characters will be from the same script, but
2138there are languages that mix characters from different scripts (such
2139as Japanese).  This is similar to the heuristics used to distinguish
2140between letters and numbers in the examples above. Also, for Latin,
2141Greek, and Cyrillic, using lowercase letters results in fewer
2142ambiguities than using uppercase letters would.</t>
2143</section>
2144
2145<section title="Display of URIs/IRIs" anchor="display">
2146<t>
2147In situations where the rendering software is not expected to display
2148non-ASCII parts of the IRI correctly using the available layout and font
2149resources, these parts should be percent-encoded before being displayed.</t>
2150
2151<t>For display of Bidi IRIs, please see <xref target="visual"/>.</t>
2152</section>
2153
2154<section title="Interpretation of URIs and IRIs">
2155<t>Software that interprets IRIs as the names of local resources should
2156accept IRIs in multiple forms and convert and match them with the
2157appropriate local resource names.</t>
2158
2159<t>First, multiple representations include both IRIs in the native
2160character encoding of the protocol and also their URI counterparts.</t>
2161
2162<t>Second, it may include URIs constructed based on character
2163encodings other than UTF-8. These URIs may be produced by user agents that do
2164not conform to this specification and that use legacy character encodings to
2165convert non-ASCII characters to URIs. Whether this is necessary, and what
2166character encodings to cover, depends on a number of factors, such as
2167the legacy character encodings used locally and the distribution of
2168various versions of user agents. For example, software for Japanese
2169may accept URIs in Shift_JIS and/or EUC-JP in addition to UTF-8.</t>
2170
2171<t>Third, it may include additional mappings to be more user-friendly
2172and robust against transmission errors. These would be similar to how
2173some servers currently treat URIs as case insensitive or perform
2174additional matching to account for spelling errors. For characters
2175beyond the US-ASCII repertoire, this may, for example, include
2176ignoring the accents on received IRIs or resource names. Please note
2177that such mappings, including case mappings, are language
2178dependent.</t>
2179
2180<t>It can be difficult to identify a resource unambiguously if too
2181many mappings are taken into consideration. However, percent-encoded
2182and not percent-encoded parts of IRIs can always be clearly distinguished.
2183Also, the regularity of UTF-8 (see <xref target="Duerst97"/>) makes the
2184potential for collisions lower than it may seem at first.</t>
2185</section>
2186
2187<section title="Upgrading Strategy">
2188<t>Where this recommendation places further constraints on software
2189for which many instances are already deployed, it is important to
2190introduce upgrades carefully and to be aware of the various
2191interdependencies.</t>
2192
2193<t>If IRIs cannot be interpreted correctly, they should not be created,
2194generated, or transported. This suggests that upgrading URI interpreting
2195software to accept IRIs should have highest priority.</t>
2196
2197<t>On the other hand, a single IRI is interpreted only by a single or
2198very few interpreters that are known in advance, although it may be
2199entered and transported very widely.</t>
2200
2201<t>Therefore, IRIs benefit most from a broad upgrade of software to be
2202able to enter and transport IRIs. However, before an
2203individual IRI is published, care should be taken to upgrade the corresponding
2204interpreting software in order to cover the forms expected to be
2205received by various versions of entry and transport software.</t>
2206
2207<t>The upgrade of generating software to generate IRIs instead of using a
2208local character encoding should happen only after the service is upgraded
2209to accept IRIs. Similarly, IRIs should only be generated when the service
2210accepts IRIs and the intervening infrastructure and protocol is known
2211to transport them safely.</t>
2212
2213<t>Software converting from URIs to IRIs for display should be upgraded
2214only after upgraded entry software has been widely deployed to the
2215population that will see the displayed result.</t>
2216
2217
2218<t>Where there is a free choice of character encodings, it is often
2219possible to reduce the effort and dependencies for upgrading to IRIs
2220by using UTF-8 rather than another encoding. For example, when a new
2221file-based Web server is set up, using UTF-8 as the character encoding
2222for file names will make the transition to IRIs easier. Likewise, when
2223a new Web form is set up using UTF-8 as the character encoding of the
2224form page, the returned query URIs will use UTF-8 as the character
2225encoding (unless the user, for whatever reason, changes the character
2226encoding) and will therefore be compatible with IRIs.</t>
2227
2228
2229<t>These recommendations, when taken together, will allow for the
2230extension from URIs to IRIs in order to handle characters other than
2231US-ASCII while minimizing interoperability problems. For
2232considerations regarding the upgrade of URI scheme definitions, see
2233<xref target="UTF8use"/>.</t>
2234
2235</section>
2236</section> <!-- guidelines -->
2237
2238<section title="IANA Considerations" anchor="iana">
2239
2240<t>RFC Editor and IANA note: Please Replace RFC XXXX with the
2241number of this document when it issues as an RFC. </t>
2242
2243<t>IANA maintains a registry of "URI schemes". A "URI scheme" also
2244serves an "IRI scheme". </t>
2245
2246<t>To clarify that the URI scheme registration process also applies to
2247IRIs, change the description of the "URI schemes" registry
2248header to say "[RFC4395] defines an IANA-maintained registry of URI
2249Schemes. These registries include the Permanent and Provisional URI
2250Schemes.  RFC XXXX updates this registry to designate that schemes may
2251also indicate their usability as IRI schemes.</t>
2252
2253<t> Update "per RFC 4395" to "per RFC 4395 and RFC XXXX".
2254</t>
2255
2256</section> <!-- IANA -->
2257   
2258<section title="Security Considerations" anchor="security">
2259<t>The security considerations discussed in <xref target="RFC3986"/>
2260also apply to IRIs. In addition, the following issues require
2261particular care for IRIs.</t>
2262<t>Incorrect encoding or decoding can lead to security problems.
2263For example, some UTF-8 decoders do not check against overlong
2264byte sequences. See <xref target='UTR36'/> Section 3 for details.</t>
2265
2266  <t>There are serious difficulties with relying on a human to verify that a
2267    an IRI (whether presented visually or aurally)
2268    is the same as another IRI or is the one intended.
2269    These problems exist with ASCII-only URIs (bl00mberg.com vs. bloomberg.com)
2270    but are strongly exacerbated when using the much larger character repertoire of Unicode.
2271    For details, see Section 2 of <xref target='UTR36'/>.
2272    Using administrative and technical means to reduce the availability
2273    of such exploits is possible, but they are difficult to eliminate altogether.
2274    User agents SHOULD NOT rely on visual or perceptual comparison or verification of IRIs
2275    as a means of validating or assuring safety, correctness or appropriateness of an IRI.
2276    Other means of presenting users with the validity, safety, or appropriateness
2277    of visited sites are being developed in the browser community
2278    as an alternative means of avoiding these difficulties.</t>
2279
2280<t>Besides the large character repertoire of Unicode, reasons for
2281  confusion include different forms of normalization and different normalization
2282  expectations, use of percent-encoding with various legacy encodings,
2283  and bidirectionality issues. See also <xref target='UTR36'/>.</t>
2284
2285<t>Confusion can occur in various IRI components, such as the
2286domain name part or the path part, or between IRI components. For considerations specific
2287to the domain name part, see <xref target="RFC5890"/>. For considerations specific to
2288particular protocols or schemes, see the security sections of the relevant specifications
2289and registration templates.
2290Administrators of sites that allow independent
2291users to create resources in the same sub area have to be careful.
2292Details are discussed in <xref target="selection"/>.</t>
2293
2294<t>Confusion can occur with bidirectional IRIs, if the restrictions
2295in <xref target="bidi-structure"/> are not followed. The same visual
2296representation may be interpreted as different logical representations,
2297and vice versa. It is also very important that a correct Unicode bidirectional
2298implementation be used.</t>
2299  <t>The characters additionally allowed in Legacy Extended IRIs
2300    introduce additional security issues. For details, see <xref target='notAllowed'/>.</t>
2301</section><!-- security -->
2302
2303<section title="Acknowledgements">
2304<t>This document was derived from <xref target="RFC3987"/>; the acknowledgments from
2305that specification still apply.</t>
2306<t>We would like to thank Ian Hickson, Michael Sperberg-McQueen,
2307  and Dan Connolly for their work on HyperText References, and Norman Walsh, Richard Tobin,
2308  Henry S. Thomson, John Cowan, Paul Grosso, and the XML Core Working Group of the W3C for their work on LEIRIs.</t>
2309<t>In addition, this document was influenced by contributions from (in no particular order) Chris
2310  Lilley, Bjoern Hoehrmann,
2311Felix Sasaki, Jeremy Carroll, Frank Ellermann, Michael Everson, Cary Karp, Matitiahu Allouche,
2312Richard Ishida, Addison Phillips, Jonathan Rosenne, Najib Tounsi, Debbie Garside, Mark Davis, Sarmad
2313Hussain, Ted Hardie, Konrad Lanz, Thomas Roessler, Lisa Dusseault, Julian Reschke, Giovanni
2314Campagna, Anne van Kesteren, Mark Nottingham, Erik van der Poel, Marcin Hanclik, Marcos Caceres, Roy
2315Fielding, Greg Wilkins, Pieter Hintjens, Daniel R. Tobias, Marko Martin, Maciej Stanchowiak, Wil
2316Tan, Yui Naruse, Michael A. Puls II, Dave Thaler, Tom Perch, John Klensin, Shawn Steele, Peter
2317Saint-Andre, Geoffrey Sneddon, Chris Weber, Alex Melnikov, Slim Amamou, SM, Tim Berners-Lee, Yaron
2318Goland, Sam Ruby, Adam Barth, Abdulrahman I. ALGhadir, Aharon Lanin, Thomas Milo, Murray Sargent,
2319Marc Blanchet, and Mykyta Yevstifeyev.</t>
2320</section>
2321
2322<section title="Main Changes Since RFC 3987">
2323  <t>This section describes the main changes since <xref target="RFC3987"></xref>.</t>
2324  <section title="Major restructuring of IRI processing model" anchor="forkChanges">
2325    <t>Major restructuring of IRI processing model to make scheme-specific translation
2326      necessary to handle IDNA requirements and for consistency with web implementations. </t>
2327    <t>Starting with IRI, you want one of:
2328      <list style="hanging">
2329        <t hangText="a"> IRI components (IRI parsed into UTF8 pieces)</t>
2330        <t hangText="b"> URI components (URI parsed into ASCII pieces, encoded correctly) </t>
2331        <t hangText="c"> whole URI  (for passing on to some other system that wants whole URIs) </t>
2332      </list></t>
2333   
2334    <section title="OLD WAY">
2335      <t><list style="numbers">
2336       
2337        <t>Pct-encoding on the whole thing to a URI.
2338          (c1) If you want a (maybe broken) whole URI, you might
2339          stop here.</t>
2340       
2341        <t>Parsing the URI into URI components.
2342          (b1) If you want (maybe broken) URI components, stop here.</t>
2343       
2344        <t> Decode the components (undoing the pct-encoding).
2345          (a) if you want IRI components, stop here.</t>
2346       
2347        <t> reencode:  Either using a different encoding some components
2348          (for domain names, and query components in web pages, which
2349          depends on the component, scheme and context), and otherwise
2350          using pct-encoding.
2351          (b2) if you want (good) URI components, stop here.</t>
2352       
2353        <t> reassemble the reencoded components.
2354          (c2) if you want a (*good*) whole URI stop here.</t>
2355      </list>
2356       
2357      </t>
2358     
2359    </section>
2360   
2361    <section title="NEW WAY">
2362      <t>
2363        <list style="numbers">
2364         
2365          <t> Parse the IRI into IRI components using the generic syntax.
2366            (a) if you want IRI components, stop here.</t>
2367         
2368          <t> Encode each components, using pct-encoding, IDN encoding, or
2369            special query part encoding depending on the component
2370            scheme or context. (b) If you want URI components, stop here.</t>
2371          <t> reassemble the a whole URI from URI components.
2372            (c) if you want a whole URI stop here.</t>
2373        </list></t>
2374    </section>
2375    <section title="Extension of Syntax">
2376      <t>Added the tag range (U+E0000-E0FFF) to the iprivate production.
2377        Some IRIs generated with the new syntax may fail to pass very strict checks
2378        relying on the old syntax. But characters in this range should be extremely infrequent
2379        anyway.</t>
2380    </section>
2381    <section title="More to be added"><t>TODO: There are more main changes that need to be
2382      documented in this section.</t></section>
2383</section>
2384
2385<section title="Change Log">
2386
2387<t>Note to RFC Editor: Please completely remove this section before publication.</t>
2388
2389<section title='Changes after draft-ietf-iri-3987bis-01'>
2390    <t>Changes from draft-ietf-iri-3987bis-01 onwards are available as changesets
2391      in the IETF tools subversion repository at
2392      http://trac.tools.ietf.org/wg/iri/trac/log/draft-ietf-iri-3987bis/draft-ietf-iri-3987bis.xml.</t>
2393</section>
2394 
2395<section title='Changes from draft-duerst-iri-bis-07 to draft-ietf-iri-3987bis-00'>
2396     <t>Changed draft name, date, last paragraph of abstract, and titles in change log, and added this section
2397     in moving from draft-duerst-iri-bis-07 (personal submission) to draft-ietf-iri-3987bis-00 (WG document).</t>
2398</section>
2399
2400<section title="Changes from -06 to -07 of draft-duerst-iri-bis">
2401  <t>Major restructuring of the processing model, see <xref target="forkChanges"></xref>.</t>
2402</section>
2403</section>
2404
2405<section title='Changes from -00 to -01'><t><list style="symbols">
2406  <t>Removed 'mailto:' before mail addresses of authors.</t>
2407  <t>Added "&lt;to be done&gt;" as right side of 'href-strip' rule. Fixed '|' to '/' for
2408    alternatives.</t>
2409</list></t>
2410</section>
2411
2412<section title="Changes from -05 to -06 of draft-duerst-iri-bis-00"><t><list style="symbols">
2413<t>Add HyperText Reference, change abstract, acks and references for it</t>
2414<t>Add Masinter back as another editor.</t>
2415<t>Masinter integrates HRef material from HTML5 spec.</t>
2416<t>Rewrite introduction sections to modernize.</t>
2417</list></t>
2418</section>
2419
2420<section title="Changes from -04 to -05 of draft-duerst-iri-bis">
2421  <t><list style="symbols">
2422    <t>Updated references.</t>
2423    <t>Changed IPR text to pre5378Trust200902.</t></list></t>
2424</section>
2425
2426<section title="Changes from -03 to -04 of draft-duerst-iri-bis">
2427  <t><list style="symbols">
2428    <t>Added explicit abbreviation for LEIRIs.</t>
2429    <t>Mentioned LEIRI references.</t>
2430    <t>Completed text in LEIRI section about tag characters and about specials.</t></list></t>
2431</section>
2432
2433<section title="Changes from -02 to -03 of draft-duerst-iri-bis">
2434  <t><list style="symbols">
2435    <t>Updated some references.</t>
2436    <t>Updated Michel Suginard's coordinates.</t></list></t>
2437</section>
2438
2439<section title="Changes from -01 to -02 of draft-duerst-iri-bis">
2440  <t><list style="symbols">
2441    <t>Added tag range to iprivate (issue private-include-tags-115).</t>
2442    <t>Added Specials (U+FFF0-FFFD) to Legacy Extended IRIs.</t></list></t>
2443</section>
2444<section title="Changes from -00 to -01 of draft-duerst-iri-bis">
2445  <t><list style="symbols">
2446    <t>Changed from "IRIs with Spaces/Controls" to "Legacy Extended IRI"
2447      based on input from the W3C XML Core WG.
2448      Moved the relevant subsections to the back and promoted them to a section.</t>
2449    <t>Added some text re. Legacy Extended IRIs to the security section.</t>
2450    <t>Added a IANA Consideration Section.</t>
2451    <t>Added this Change Log Section.</t>
2452    <t>Added a section about "IRIs with Spaces/Controls" (converting from a Note in RFC 3987).</t></list></t>
2453</section>
2454<section title="Changes from RFC 3987 to -00 of draft-duerst-iri-bis">
2455  <t><list>
2456    <t>Fixed errata (see http://www.rfc-editor.org/cgi-bin/errataSearch.pl?rfc=3987).</t></list></t>
2457</section>
2458</section>
2459</middle>
2460
2461<back>
2462<references title="Normative References">
2463
2464<reference anchor="ASCII">
2465<front>
2466<title>Coded Character Set -- 7-bit American Standard Code for Information
2467Interchange</title>
2468<author>
2469<organization>American National Standards Institute</organization>
2470</author>
2471<date year="1986"/>
2472</front>
2473<seriesInfo name="ANSI" value="X3.4"/>
2474</reference>
2475
2476<reference anchor="ISO10646">
2477<front>
2478<title>ISO/IEC 10646:2003: Information Technology -
2479Universal Multiple-Octet Coded Character Set (UCS)</title>
2480<author>
2481<organization>International Organization for Standardization</organization>
2482</author>
2483<date month="December" year="2003"/>
2484</front>
2485<seriesInfo name="ISO" value="Standard 10646"/>
2486</reference>
2487
2488&rfc2119;
2489&rfc3490;
2490&rfc3491;
2491&rfc3629;
2492&rfc3986;
2493
2494<reference anchor="STD68">
2495<front>
2496<title abbrev="ABNF">Augmented BNF for Syntax Specifications: ABNF</title>
2497<author initials="D." surname="Crocker" fullname="Dave Crocker"><organization/></author>
2498<author initials="P." surname="Overell" fullname="Paul Overell"><organization/></author>
2499<date month="January" year="2008"/></front>
2500<seriesInfo name="STD" value="68"/><seriesInfo name="RFC" value="5234"/>
2501</reference>
2502 
2503&rfc5890;
2504&rfc5891;
2505
2506<reference anchor="UNIV6">
2507<front>
2508<title>The Unicode Standard, Version 6.0.0 (Mountain View, CA, The Unicode Consortium, 2011, ISBN 978-1-936213-01-6)</title>
2509<author><organization>The Unicode Consortium</organization></author>
2510<date year="2010" month="October"/>
2511</front>
2512</reference>
2513
2514<reference anchor="UNI9" target="http://www.unicode.org/reports/tr9/tr9-13.html">
2515<front>
2516<title>The Bidirectional Algorithm</title>
2517<author initials="M." surname="Davis" fullname="Mark Davis"><organization/></author>
2518<date year="2004" month="March"/>
2519</front>
2520<seriesInfo name="Unicode Standard Annex" value="#9"/>
2521</reference>
2522
2523<reference anchor="UTR15" target="http://www.unicode.org/unicode/reports/tr15/tr15-23.html">
2524<front>
2525<title>Unicode Normalization Forms</title>
2526<author initials="M." surname="Davis" fullname="Mark Davis"><organization/></author>
2527<author initials="M.J." surname="Duerst" fullname="Martin Duerst"><organization/></author>
2528<date year="2008" month="March"/>
2529</front>
2530<seriesInfo name="Unicode Standard Annex" value="#15"/>
2531</reference>
2532
2533</references>
2534
2535<references title="Informative References">
2536
2537<reference anchor="BidiEx" target="http://www.w3.org/International/iri-edit/BidiExamples">
2538<front>
2539<title>Examples of bidirectional IRIs</title>
2540<author><organization/></author>
2541<date year="" month=""/>
2542</front>
2543</reference>
2544
2545<reference anchor="CharMod" target="http://www.w3.org/TR/charmod-resid">
2546<front>
2547<title>Character Model for the World Wide Web: Resource Identifiers</title>
2548<author initials="M." surname="Duerst" fullname="Martin Duerst"><organization/></author>
2549<author initials="F." surname="Yergeau" fullname="Francois Yergeau"><organization/></author>
2550<author initials="R." surname="Ishida" fullname="Richard Ishida"><organization/></author>
2551<author initials="M." surname="Wolf" fullname="Misha Wolf"><organization/></author>
2552<author initials="T." surname="Texin" fullname="Tex Texin"><organization/></author>
2553<date year="2004" month="November" day="25"/>
2554</front>
2555<seriesInfo name="World Wide Web Consortium" value="Candidate Recommendation"/>
2556</reference>
2557
2558<reference anchor="Duerst97" target="http://www.ifi.unizh.ch/mml/mduerst/papers/PDF/IUC11-UTF-8.pdf">
2559<front>
2560<title>The Properties and Promises of UTF-8</title>
2561<author initials="M.J." surname="Duerst" fullname="Martin Duerst"><organization/></author>
2562<date year="1997" month="September"/>
2563</front>
2564<seriesInfo name="Proc. 11th International Unicode Conference, San Jose" value=""/>
2565</reference>
2566
2567<reference anchor="Gettys" target="http://www.w3.org/DesignIssues/ModelConsequences">
2568<front>
2569<title>URI Model Consequences</title>
2570<author initials="J." surname="Gettys" fullname="Jim Gettys"><organization/></author>
2571<date month="" year=""/>
2572</front>
2573</reference>
2574
2575<reference anchor="HTML4" target="http://www.w3.org/TR/html401/appendix/notes.html#h-B.2">
2576<front>
2577<title>HTML 4.01 Specification</title>
2578<author initials="D." surname="Raggett" fullname="Dave Raggett"><organization/></author>
2579<author initials="A." surname="Le Hors" fullname="Arnaud Le Hors"><organization/></author>
2580<author initials="I." surname="Jacobs" fullname="Ian Jacobs"><organization/></author>
2581<date year="1999" month="December" day="24"/>
2582</front>
2583<seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2584</reference>
2585
2586<reference anchor="LEIRI" target="http://www.w3.org/TR/leiri/">
2587<front>
2588<title>Legacy extended IRIs for XML resource identification</title>
2589<author initials="H." surname="Thompson" fullname="Henry Thompson"><organization/></author>
2590<author initials="R." surname="Tobin"    fullname="Richard Tobin"><organization/></author>
2591<author initials="N." surname="Walsh" fullname="Norman Walsh"><organization/></author>
2592  <date year="2008" month="November" day="3"/>
2593
2594</front>
2595<seriesInfo name="World Wide Web Consortium" value="Note"/>
2596</reference>
2597
2598
2599&rfc2045;
2600&rfc2130;
2601&rfc2141;
2602&rfc2192;
2603&rfc2277;
2604&rfc2368;
2605&rfc2384;
2606&rfc2396;
2607&rfc2397;
2608&rfc2616;
2609&rfc1738;
2610&rfc2640;
2611&rfc3987;
2612&rfc6055;
2613<reference anchor='RFC4395bis'>
2614  <front>
2615    <title>Guidelines and Registration Procedures for New URI/IRI Schemes</title>
2616    <author initials='T.' surname='Hansen' fullname="Tony Hansen"><organization/></author>
2617    <author initials='T.' surname='Hardie' fullname="Ted Hardie"><organization/></author>
2618    <author initials='L.' surname='Masinter' fullname="Larry Masinter"><organization/></author>
2619    <date year="2010" month='September' day="30"/>
2620    <workgroup>IRI</workgroup>
2621  </front>
2622  <seriesInfo name="Internet-Draft" value="draft-hansen-iri-4395bis-irireg-00"/>
2623</reference>
2624 
2625 
2626<reference anchor="UNIXML" target="http://www.w3.org/TR/unicode-xml/">
2627<front>
2628<title>Unicode in XML and other Markup Languages</title>
2629<author initials="M.J." surname="Duerst" fullname="Martin Duerst"><organization/></author>
2630<author initials="A." surname="Freytag" fullname="Asmus Freytag"><organization/></author>
2631<date year="2003" month="June" day="18"/>
2632</front>
2633<seriesInfo name="Unicode Technical Report" value="#20"/>
2634<seriesInfo name="World Wide Web Consortium" value="Note"/>
2635</reference>
2636 
2637<reference anchor="UTR36" target="http://unicode.org/reports/tr36/">
2638<front>
2639<title>Unicode Security Considerations</title>
2640<author initials="M." surname="Davis" fullname="Mark Davis"><organization/></author>
2641<author initials="M." surname="Suignard" fullname="Michel Suignard"><organization/></author>
2642<date year="2010" month="August" day="4"/>
2643</front>
2644<seriesInfo name="Unicode Technical Report" value="#36"/>
2645</reference>
2646
2647<reference anchor="XLink" target="http://www.w3.org/TR/xlink/#link-locators">
2648<front>
2649<title>XML Linking Language (XLink) Version 1.0</title>
2650<author initials="S." surname="DeRose" fullname="Steve DeRose"><organization/></author>
2651<author initials="E." surname="Maler" fullname="Eve Maler"><organization/></author>
2652<author initials="D." surname="Orchard" fullname="David Orchard"><organization/></author>
2653<date year="2001" month="June" day="27"/>
2654</front>
2655<seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2656</reference>
2657
2658<reference anchor="XML1" target="http://www.w3.org/TR/REC-xml">
2659  <front>
2660    <title>Extensible Markup Language (XML) 1.0 (Forth Edition)</title>
2661    <author initials="T." surname="Bray" fullname="Tim Bray"><organization/></author>
2662    <author initials="J." surname="Paoli" fullname="Jean Paoli"><organization/></author>
2663    <author initials="C.M." surname="Sperberg-McQueen" fullname="C. M. Sperberg-McQueen">
2664      <organization/></author>
2665    <author initials="E." surname="Maler" fullname="Eve Maler"><organization/></author>
2666    <author initials="F." surname="Yergeau" fullname="Francois Yergeau"><organization/></author>
2667    <date day="16" month="August" year="2006"/>
2668  </front>
2669  <seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2670</reference>
2671
2672<reference anchor="XMLNamespace" target="http://www.w3.org/TR/REC-xml-names">
2673  <front>
2674    <title>Namespaces in XML (Second Edition)</title>
2675    <author initials="T." surname="Bray" fullname="Tim Bray"><organization/></author>
2676    <author initials="D." surname="Hollander" fullname="Dave Hollander"><organization/></author>
2677    <author initials="A." surname="Layman" fullname="Andrew Layman"><organization/></author>
2678    <author initials="R." surname="Tobin" fullname="Richard Tobin"><organization></organization></author>
2679    <date day="16" month="August" year="2006"/>
2680  </front>
2681  <seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2682</reference>
2683
2684<reference anchor="XMLSchema" target="http://www.w3.org/TR/xmlschema-2/#anyURI">
2685<front>
2686<title>XML Schema Part 2: Datatypes</title>
2687<author initials="P." surname="Biron" fullname="Paul Biron"><organization/></author>
2688<author initials="A." surname="Malhotra" fullname="Ashok Malhotra"><organization/></author>
2689<date year="2001" month="May" day="2"/>
2690</front>
2691<seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2692</reference>
2693
2694<reference anchor="XPointer" target="http://www.w3.org/TR/xptr-framework/#escaping">
2695<front>
2696<title>XPointer Framework</title>
2697<author initials="P." surname="Grosso" fullname="Paul Grosso"><organization/></author>
2698<author initials="E." surname="Maler" fullname="Eve Maler"><organization/></author>
2699<author initials="J." surname="Marsh" fullname="Jonathan Marsh"><organization/></author>
2700<author initials="N." surname="Walsh" fullname="Norman Walsh"><organization/></author>
2701<date year="2003" month="March" day="25"/>
2702</front>
2703<seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2704</reference>
2705
2706<reference anchor="HTML5" target="http://www.w3.org/TR/2009/WD-html5-20090423/">
2707<front>
2708<title>A vocabulary and associated APIs for HTML and XHTML</title>
2709<author initials="I." surname="Hickson" fullname="Ian Hickson"><organization>Google, Inc.</organization></author>
2710<author initials="D." surname="Hyatt" fullname="David Hyatt"><organization>Apple, Inc.</organization></author>
2711<date year="2009"  month="April" day="23"/>
2712</front>
2713<seriesInfo name="World Wide Web Consortium" value="Working Draft"/>
2714</reference>
2715
2716</references>
2717
2718</back>
2719</rfc>
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