source: draft-ietf-iri-3987bis/draft-ietf-iri-3987bis.xml @ 68

Last change on this file since 68 was 68, checked in by duerst@…, 9 years ago

fixed a typo in the Ack section

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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  <section title='Mapping using Percent-Encoding' anchor='dnspercent'>
689  <t>The ireg-name component SHOULD be converted
690    according to the general procedure for percent-encoding
691    of IRI components described in <xref target="compmapping"/>.</t>
692 
693  <t>For example, the IRI
694    <vspace/>"http://r&amp;#xE9;sum&amp;#xE9;.example.org"<vspace/> will be
695    converted to <vspace/>"http://r%C3%A9sum%C3%A9.example.org".</t>
696
697  <t>This conversion for ireg-name is in line with Section 3.2.2
698    of <xref target="RFC3986"/>, which does not mandate
699    a particular registered name lookup technology. For further background,
700    see <xref target="RFC6055"/> and <xref target="Gettys"/>.</t>
701</section>
702<section title="Mapping using Punycode" anchor='dnspunycode'>
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  <t>This conversion for ireg-name will be better able to deal with legacy
719    infrastructure that cannot handle percent-encoding in domain names.</t>
720</section>
721  <section title="Additional Considerations">
722<t><list style="hanging"> 
723
724<t hangText="Note:">Domain Names may appear in parts of an IRI other
725than the ireg-name part.  It is the responsibility of scheme-specific
726implementations (if the Internationalized Domain Name is part of the
727scheme syntax) or of server-side implementations (if the
728Internationalized Domain Name is part of 'iquery') to apply the
729necessary conversions at the appropriate point. Example: Trying to
730validate the Web page at<vspace/>
731http://r&amp;#xE9;sum&amp;#xE9;.example.org would lead to an IRI of
732<vspace/>http://validator.w3.org/check?uri=http%3A%2F%2Fr&amp;#xE9;sum&amp;#xE9;.<vspace/>example.org,
733which would convert to a URI
734of<vspace/>http://validator.w3.org/check?uri=http%3A%2F%2Fr%C3%A9sum%C3%A9.<vspace/>example.org.
735The server-side implementation is responsible for making the
736necessary conversions to be able to retrieve the Web page.</t>
737
738<t hangText="Note:">In this process, characters allowed in URI
739references and existing percent-encoded sequences are not encoded further.
740(This mapping is similar to, but different from, the encoding applied
741when arbitrary content is included in some part of a URI.)
742
743For example, an IRI of
744<vspace/>"http://www.example.org/red%09ros&amp;#xE9;#red"
745(in XML notation) is converted to
746<vspace/>"http://www.example.org/red%09ros%C3%A9#red", not to
747something like
748<vspace/>"http%3A%2F%2Fwww.example.org%2Fred%2509ros%C3%A9%23red".</t>
749
750</list></t>
751</section>
752</section> <!-- dnsmapping -->
753
754<section title="Mapping query components" anchor="querymapping">
755
756<t>((NOTE: SEE ISSUES LIST))
757
758For compatibility with existing deployed HTTP infrastructure,
759the following special case applies for schemes "http" and "https"
760and IRIs whose origin has a document charset other than one which
761is UCS-based (e.g., UTF-8 or UTF-16). In such a case, the "query"
762component of an IRI is mapped into a URI by using the document
763charset rather than UTF-8 as the binary representation before
764pct-encoding. This mapping is not applied for any other scheme
765or component.</t>
766
767</section> <!-- querymapping -->
768
769<section title="Mapping IRIs to URIs" anchor="mapping">
770
771<t>The canonical mapping from a IRI to URI is defined by applying the
772mapping above (from IRI to URI components) and then reassembling a URI
773from the parsed URI components using the original punctuation that
774delimited the IRI components. </t>
775
776</section> <!-- mapping -->
777
778<section title="Converting URIs to IRIs" anchor="URItoIRI">
779
780<t>In some situations, for presentation and further processing,
781it is desirable to convert a URI into an equivalent IRI in which
782natural characters are represented directly rather than
783percent encoded. Of course, every URI is already an IRI in
784its own right without any conversion, and in general there
785This section gives one such procedure for this conversion.
786</t>
787
788<t>
789The conversion described in this section, if given a valid URI, will
790result in an IRI that maps back to the URI used as an input for the
791conversion (except for potential case differences in percent-encoding
792and for potential percent-encoded unreserved characters).
793
794However, the IRI resulting from this conversion may differ
795from the original IRI (if there ever was one).</t> 
796
797<t>URI-to-IRI conversion removes percent-encodings, but not all
798percent-encodings can be eliminated. There are several reasons for
799this:</t>
800
801<t><list style="hanging">
802
803<t hangText="1.">Some percent-encodings are necessary to distinguish
804    percent-encoded and unencoded uses of reserved characters.</t>
805
806<t hangText="2.">Some percent-encodings cannot be interpreted as sequences
807    of UTF-8 octets.<vspace blankLines="1"/>
808    (Note: The octet patterns of UTF-8 are highly regular.
809    Therefore, there is a very high probability, but no guarantee,
810    that percent-encodings that can be interpreted as sequences of UTF-8
811    octets actually originated from UTF-8. For a detailed discussion,
812    see <xref target="Duerst97"/>.)</t>
813
814<t hangText="3.">The conversion may result in a character that is not
815    appropriate in an IRI. See <xref target="abnf"/>, <xref target="visual"/>,
816      and <xref target="limitations"/> for further details.</t>
817
818<t hangText="4.">IRI to URI conversion has different rules for
819    dealing with domain names and query parameters.</t>
820
821</list></t>
822
823<t>Conversion from a URI to an IRI MAY be done by using the following
824steps:
825
826<list style="hanging">
827<t hangText="1.">Represent the URI as a sequence of octets in
828       US-ASCII.</t>
829
830<t hangText="2.">Convert all percent-encodings ("%" followed by two
831      hexadecimal digits) to the corresponding octets, except those
832      corresponding to "%", characters in "reserved", and characters
833      in US-ASCII not allowed in URIs.</t> 
834
835<t hangText="3.">Re-percent-encode any octet produced in step 2 that
836      is not part of a strictly legal UTF-8 octet sequence.</t>
837
838
839<t hangText="4.">Re-percent-encode all octets produced in step 3 that
840      in UTF-8 represent characters that are not appropriate according
841      to <xref target="abnf"/>, <xref target="visual"/>, and <xref
842      target="limitations"/>.</t> 
843
844<t hangText="5.">Interpret the resulting octet sequence as a sequence
845      of characters encoded in UTF-8.</t>
846
847<t hangText="6.">URIs known to contain domain names in the reg-name
848      component SHOULD convert punycode-encoded domain name labels to
849      the corresponding characters using the ToUnicode procedure. </t>
850</list></t>
851
852<t>This procedure will convert as many percent-encoded characters as
853possible to characters in an IRI. Because there are some choices when
854step 4 is applied (see <xref target="limitations"/>), results may
855vary.</t>
856
857<t>Conversions from URIs to IRIs MUST NOT use any character
858encoding other than UTF-8 in steps 3 and 4, even if it might be
859possible to guess from the context that another character encoding
860than UTF-8 was used in the URI.  For example, the URI
861"http://www.example.org/r%E9sum%E9.html" might with some guessing be
862interpreted to contain two e-acute characters encoded as
863iso-8859-1. It must not be converted to an IRI containing these
864e-acute characters. Otherwise, in the future the IRI will be mapped to
865"http://www.example.org/r%C3%A9sum%C3%A9.html", which is a different
866URI from "http://www.example.org/r%E9sum%E9.html".</t>
867
868<section title="Examples">
869
870<t>This section shows various examples of converting URIs to IRIs.
871Each example shows the result after each of the steps 1 through 6 is
872applied. XML Notation is used for the final result.  Octets are
873denoted by "&lt;" followed by two hexadecimal digits followed by
874"&gt;".</t>
875
876<t>The following example contains the sequence "%C3%BC", which is a
877strictly legal UTF-8 sequence, and which is converted into the actual
878character U+00FC, LATIN SMALL LETTER U WITH DIAERESIS (also known as
879u-umlaut).
880
881<list style="hanging">
882<t hangText="1.">http://www.example.org/D%C3%BCrst</t>
883<t hangText="2.">http://www.example.org/D&lt;c3&gt;&lt;bc&gt;rst</t>
884<t hangText="3.">http://www.example.org/D&lt;c3&gt;&lt;bc&gt;rst</t>
885<t hangText="4.">http://www.example.org/D&lt;c3&gt;&lt;bc&gt;rst</t>
886<t hangText="5.">http://www.example.org/D&amp;#xFC;rst</t>
887<t hangText="6.">http://www.example.org/D&amp;#xFC;rst</t>
888</list>
889</t>
890
891<t>The following example contains the sequence "%FC", which might
892represent U+00FC, LATIN SMALL LETTER U WITH DIAERESIS, in
893the<vspace/>iso-8859-1 character encoding.  (It might represent other
894characters in other character encodings. For example, the octet
895&lt;fc&gt; in iso-8859-5 represents U+045C, CYRILLIC SMALL LETTER
896KJE.)  Because &lt;fc&gt; is not part of a strictly legal UTF-8
897sequence, it is re-percent-encoded in step 3.
898
899
900<list style="hanging">
901<t hangText="1.">http://www.example.org/D%FCrst</t>
902<t hangText="2.">http://www.example.org/D&lt;fc&gt;rst</t>
903<t hangText="3.">http://www.example.org/D%FCrst</t>
904<t hangText="4.">http://www.example.org/D%FCrst</t>
905<t hangText="5.">http://www.example.org/D%FCrst</t>
906<t hangText="6.">http://www.example.org/D%FCrst</t>
907</list>
908</t>
909
910<t>The following example contains "%e2%80%ae", which is the percent-encoded<vspace/>UTF-8
911character encoding of U+202E, RIGHT-TO-LEFT OVERRIDE. <xref target="visual"/>
912forbids the direct use of this character in an IRI. Therefore, the
913corresponding octets are re-percent-encoded in step 4. This example shows
914that the case (upper- or lowercase) of letters used in percent-encodings may not be preserved.
915The example also contains a punycode-encoded domain name label (xn--99zt52a),
916which is not converted.
917
918<list style="hanging">
919<t hangText="1.">http://xn--99zt52a.example.org/%e2%80%ae</t>
920<t hangText="2.">http://xn--99zt52a.example.org/&lt;e2&gt;&lt;80&gt;&lt;ae&gt;</t>
921<t hangText="3.">http://xn--99zt52a.example.org/&lt;e2&gt;&lt;80&gt;&lt;ae&gt;</t>
922<t hangText="4.">http://xn--99zt52a.example.org/%E2%80%AE</t>
923<t hangText="5.">http://xn--99zt52a.example.org/%E2%80%AE</t>
924<t hangText="6.">http://&amp;#x7D0D;&amp;#x8C46;.example.org/%E2%80%AE</t>
925</list></t>
926
927<t>Note that the label "xn--99zt52a" is converted to U+7D0D U+8C46
928(Japanese Natto). ((EDITOR NOTE: There is some inconsistency in this note.))</t>
929
930</section> <!-- examples -->
931</section> <!-- URItoIRI -->
932</section> <!-- processing -->
933<section title="Bidirectional IRIs for Right-to-Left Languages" anchor="Bidi">
934
935<t>Some UCS characters, such as those used in the Arabic and Hebrew
936scripts, have an inherent right-to-left (rtl) writing direction. IRIs
937containing these characters (called bidirectional IRIs or Bidi IRIs)
938require additional attention because of the non-trivial relation
939between logical representation (used for digital representation and
940for reading/spelling) and visual representation (used for
941display/printing).</t>
942
943<t>Because of the complex interaction between the logical representation,
944the visual representation, and the syntax of a Bidi IRI, a balance is
945needed between various requirements.
946The main requirements are<list style="hanging">
947<t hangText="1.">user-predictable conversion between visual and
948    logical representation;</t>
949<t hangText="2.">the ability to include a wide range of characters
950    in various parts of the IRI; and</t>
951<t hangText="3.">minor or no changes or restrictions for
952      implementations.</t>
953</list></t>
954
955<section title="Logical Storage and Visual Presentation" anchor="visual">
956
957<t>When stored or transmitted in digital representation, bidirectional
958IRIs MUST be in full logical order and MUST conform to the IRI syntax
959rules (which includes the rules relevant to their scheme). This
960ensures that bidirectional IRIs can be processed in the same way as
961other IRIs.</t> <t>Bidirectional IRIs MUST be rendered by using the
962Unicode Bidirectional Algorithm <xref target="UNIV6"/>, <xref
963target="UNI9"/>.  Bidirectional IRIs MUST be rendered in the same way
964as they would be if they were in a left-to-right embedding; i.e., as
965if they were preceded by U+202A, LEFT-TO-RIGHT EMBEDDING (LRE), and
966followed by U+202C, POP DIRECTIONAL FORMATTING (PDF).  Setting the
967embedding direction can also be done in a higher-level protocol (e.g.,
968the dir='ltr' attribute in HTML).</t> 
969
970<t>There is no requirement to use the above embedding if the display
971is still the same without the embedding. For example, a bidirectional
972IRI in a text with left-to-right base directionality (such as used for
973English or Cyrillic) that is preceded and followed by whitespace and
974strong left-to-right characters does not need an embedding.  Also, a
975bidirectional relative IRI reference that only contains strong
976right-to-left characters and weak characters and that starts and ends
977with a strong right-to-left character and appears in a text with
978right-to-left base directionality (such as used for Arabic or Hebrew)
979and is preceded and followed by whitespace and strong characters does
980not need an embedding.</t>
981
982<t>In some other cases, using U+200E, LEFT-TO-RIGHT MARK (LRM), may be
983sufficient to force the correct display behavior.  However, the
984details of the Unicode Bidirectional algorithm are not always easy to
985understand. Implementers are strongly advised to err on the side of
986caution and to use embedding in all cases where they are not
987completely sure that the display behavior is unaffected without the
988embedding.</t>
989
990<t>The Unicode Bidirectional Algorithm (<xref target="UNI9"/>, section
9914.3) permits higher-level protocols to influence bidirectional
992rendering. Such changes by higher-level protocols MUST NOT be used if
993they change the rendering of IRIs.</t> 
994
995<t>The bidirectional formatting characters that may be used before or
996after the IRI to ensure correct display are not themselves part of the
997IRI.  IRIs MUST NOT contain bidirectional formatting characters (LRM,
998RLM, LRE, RLE, LRO, RLO, and PDF). They affect the visual rendering of
999the IRI but do not appear themselves. It would therefore not be
1000possible to input an IRI with such characters correctly.</t>
1001
1002</section> <!-- visual -->
1003<section title="Bidi IRI Structure" anchor="bidi-structure">
1004
1005<t>The Unicode Bidirectional Algorithm is designed mainly for running
1006text.  To make sure that it does not affect the rendering of
1007bidirectional IRIs too much, some restrictions on bidirectional IRIs
1008are necessary. These restrictions are given in terms of delimiters
1009(structural characters, mostly punctuation such as "@", ".", ":",
1010and<vspace/>"/") and components (usually consisting mostly of letters
1011and digits).</t>
1012
1013<t>The following syntax rules from <xref target="abnf"/> correspond to
1014components for the purpose of Bidi behavior: iuserinfo, ireg-name,
1015isegment, isegment-nz, isegment-nz-nc, ireg-name, iquery, and
1016ifragment.</t>
1017
1018<t>Specifications that define the syntax of any of the above
1019components MAY divide them further and define smaller parts to be
1020components according to this document. As an example, the restrictions
1021of <xref target="RFC3490"/> on bidirectional domain names correspond
1022to treating each label of a domain name as a component for schemes
1023with ireg-name as a domain name.  Even where the components are not
1024defined formally, it may be helpful to think about some syntax in
1025terms of components and to apply the relevant restrictions.  For
1026example, for the usual name/value syntax in query parts, it is
1027convenient to treat each name and each value as a component. As
1028another example, the extensions in a resource name can be treated as
1029separate components.</t>
1030
1031<t>For each component, the following restrictions apply:</t>
1032<t>
1033<list style="hanging">
1034
1035<t hangText="1.">A component SHOULD NOT use both right-to-left and
1036  left-to-right characters.</t>
1037
1038<t hangText="2.">A component using right-to-left characters SHOULD
1039  start and end with right-to-left characters.</t>
1040
1041</list></t>
1042
1043<t>The above restrictions are given as "SHOULD"s, rather than as
1044"MUST"s.  For IRIs that are never presented visually, they are not
1045relevant.  However, for IRIs in general, they are very important to
1046ensure consistent conversion between visual presentation and logical
1047representation, in both directions.</t>
1048
1049<t><list style="hanging">
1050
1051<t hangText="Note:">In some components, the above restrictions may
1052  actually be strictly enforced.  For example, <xref
1053  target="RFC3490"></xref> requires that these restrictions apply to
1054  the labels of a host name for those schemes where ireg-name is a
1055  host name.  In some other components (for example, path components)
1056  following these restrictions may not be too difficult.  For other
1057  components, such as parts of the query part, it may be very
1058  difficult to enforce the restrictions because the values of query
1059  parameters may be arbitrary character sequences.</t>
1060
1061</list></t>
1062
1063<t>If the above restrictions cannot be satisfied otherwise, the
1064affected component can always be mapped to URI notation as described
1065in <xref target="compmapping"/>. Please note that the whole component
1066has to be mapped (see also Example 9 below).</t>
1067
1068</section> <!-- bidi-structure -->
1069
1070<section title="Input of Bidi IRIs" anchor="bidiInput">
1071
1072<t>Bidi input methods MUST generate Bidi IRIs in logical order while
1073rendering them according to <xref target="visual"/>.  During input,
1074rendering SHOULD be updated after every new character is input to
1075avoid end-user confusion.</t>
1076
1077</section> <!-- bidiInput -->
1078
1079<section title="Examples">
1080
1081<t>This section gives examples of bidirectional IRIs, in Bidi
1082Notation.  It shows legal IRIs with the relationship between logical
1083and visual representation and explains how certain phenomena in this
1084relationship may look strange to somebody not familiar with
1085bidirectional behavior, but familiar to users of Arabic and Hebrew. It
1086also shows what happens if the restrictions given in <xref
1087target="bidi-structure"/> are not followed. The examples below can be
1088seen at <xref target="BidiEx"/>, in Arabic, Hebrew, and Bidi Notation
1089variants.</t>
1090
1091<t>To read the bidi text in the examples, read the visual
1092representation from left to right until you encounter a block of rtl
1093text. Read the rtl block (including slashes and other special
1094characters) from right to left, then continue at the next unread ltr
1095character.</t>
1096
1097<t>Example 1: A single component with rtl characters is inverted:
1098<vspace/>Logical representation:
1099"http://ab.CDEFGH.ij/kl/mn/op.html"<vspace/>Visual representation:
1100"http://ab.HGFEDC.ij/kl/mn/op.html"<vspace/> Components can be read
1101one by one, and each component can be read in its natural
1102direction.</t>
1103
1104<t>Example 2: More than one consecutive component with rtl characters
1105is inverted as a whole: <vspace/>Logical representation:
1106"http://ab.CDE.FGH/ij/kl/mn/op.html"<vspace/>Visual representation:
1107"http://ab.HGF.EDC/ij/kl/mn/op.html"<vspace/> A sequence of rtl
1108components is read rtl, in the same way as a sequence of rtl words is
1109read rtl in a bidi text.</t>
1110
1111<t>Example 3: All components of an IRI (except for the scheme) are
1112rtl.  All rtl components are inverted overall: <vspace/>Logical
1113representation:
1114"http://AB.CD.EF/GH/IJ/KL?MN=OP;QR=ST#UV"<vspace/>Visual
1115representation: "http://VU#TS=RQ;PO=NM?LK/JI/HG/FE.DC.BA"<vspace/> The
1116whole IRI (except the scheme) is read rtl. Delimiters between rtl
1117components stay between the respective components; delimiters between
1118ltr and rtl components don't move.</t>
1119
1120<t>Example 4: Each of several sequences of rtl components is inverted
1121on its own: <vspace/>Logical representation:
1122"http://AB.CD.ef/gh/IJ/KL.html"<vspace/>Visual representation:
1123"http://DC.BA.ef/gh/LK/JI.html"<vspace/> Each sequence of rtl
1124components is read rtl, in the same way as each sequence of rtl words
1125in an ltr text is read rtl.</t>
1126
1127<t>Example 5: Example 2, applied to components of different kinds:
1128<vspace/>Logical representation: "http://ab.cd.EF/GH/ij/kl.html"
1129<vspace/>Visual representation:
1130"http://ab.cd.HG/FE/ij/kl.html"<vspace/> The inversion of the domain
1131name label and the path component may be unexpected, but it is
1132consistent with other bidi behavior.  For reassurance that the domain
1133component really is "ab.cd.EF", it may be helpful to read aloud the
1134visual representation following the bidi algorithm. After
1135"http://ab.cd." one reads the RTL block "E-F-slash-G-H", which
1136corresponds to the logical representation.
1137</t>
1138
1139<t>Example 6: Same as Example 5, with more rtl components:
1140<vspace/>Logical representation:
1141"http://ab.CD.EF/GH/IJ/kl.html"<vspace/>Visual representation:
1142"http://ab.JI/HG/FE.DC/kl.html"<vspace/> The inversion of the domain
1143name labels and the path components may be easier to identify because
1144the delimiters also move.</t>
1145
1146<t>Example 7: A single rtl component includes digits: <vspace/>Logical
1147representation: "http://ab.CDE123FGH.ij/kl/mn/op.html"<vspace/>Visual
1148representation: "http://ab.HGF123EDC.ij/kl/mn/op.html"<vspace/>
1149Numbers are written ltr in all cases but are treated as an additional
1150embedding inside a run of rtl characters. This is completely
1151consistent with usual bidirectional text.</t>
1152
1153<t>Example 8 (not allowed): Numbers are at the start or end of an rtl
1154component:<vspace/>Logical representation:
1155"http://ab.cd.ef/GH1/2IJ/KL.html"<vspace/>Visual representation:
1156"http://ab.cd.ef/LK/JI1/2HG.html"<vspace/> The sequence "1/2" is
1157interpreted by the bidi algorithm as a fraction, fragmenting the
1158components and leading to confusion. There are other characters that
1159are interpreted in a special way close to numbers; in particular, "+",
1160"-", "#", "$", "%", ",", ".", and ":".</t>
1161
1162<t>Example 9 (not allowed): The numbers in the previous example are
1163percent-encoded: <vspace/>Logical representation:
1164"http://ab.cd.ef/GH%31/%32IJ/KL.html",<vspace/>Visual representation:
1165"http://ab.cd.ef/LK/JI%32/%31HG.html"</t>
1166
1167<t>Example 10 (allowed but not recommended): <vspace/>Logical
1168representation: "http://ab.CDEFGH.123/kl/mn/op.html"<vspace/>Visual
1169representation: "http://ab.123.HGFEDC/kl/mn/op.html"<vspace/>
1170Components consisting of only numbers are allowed (it would be rather
1171difficult to prohibit them), but these may interact with adjacent RTL
1172components in ways that are not easy to predict.</t>
1173
1174<t>Example 11 (allowed but not recommended): <vspace/>Logical
1175representation: "http://ab.CDEFGH.123ij/kl/mn/op.html"<vspace/>Visual
1176representation: "http://ab.123.HGFEDCij/kl/mn/op.html"<vspace/>
1177Components consisting of numbers and left-to-right characters are
1178allowed, but these may interact with adjacent RTL components in ways
1179that are not easy to predict.</t>
1180</section><!-- examples -->
1181</section><!-- bidi -->
1182
1183<section title="Normalization and Comparison" anchor="equivalence">
1184
1185<t><list style="hanging"><t hangText="Note:">The structure and much of
1186  the material for this section is taken from section 6 of <xref
1187  target="RFC3986"></xref>; the differences are due to the specifics
1188  of IRIs.</t></list></t>
1189
1190<t>One of the most common operations on IRIs is simple comparison:
1191Determining whether two IRIs are equivalent, without using the IRIs to
1192access their respective resource(s). A comparison is performed
1193whenever a response cache is accessed, a browser checks its history to
1194color a link, or an XML parser processes tags within a
1195namespace. Extensive normalization prior to comparison of IRIs may be
1196used by spiders and indexing engines to prune a search space or reduce
1197duplication of request actions and response storage.</t>
1198
1199<t>IRI comparison is performed for some particular purpose. Protocols
1200or implementations that compare IRIs for different purposes will often
1201be subject to differing design trade-offs in regards to how much
1202effort should be spent in reducing aliased identifiers. This section
1203describes various methods that may be used to compare IRIs, the
1204trade-offs between them, and the types of applications that might use
1205them.</t>
1206
1207<section title="Equivalence">
1208
1209<t>Because IRIs exist to identify resources, presumably they should be
1210considered equivalent when they identify the same resource. However,
1211this definition of equivalence is not of much practical use, as there
1212is no way for an implementation to compare two resources to determine
1213if they are "the same" unless it has full knowledge or control of
1214them. For this reason, determination of equivalence or difference of
1215IRIs is based on string comparison, perhaps augmented by reference to
1216additional rules provided by URI scheme definitions.  We use the terms
1217"different" and "equivalent" to describe the possible outcomes of such
1218comparisons, but there are many application-dependent versions of
1219equivalence.</t>
1220
1221<t>Even when it is possible to determine that two IRIs are equivalent,
1222IRI comparison is not sufficient to determine whether two IRIs
1223identify different resources. For example, an owner of two different
1224domain names could decide to serve the same resource from both,
1225resulting in two different IRIs. Therefore, comparison methods are
1226designed to minimize false negatives while strictly avoiding false
1227positives.</t>
1228
1229<t>In testing for equivalence, applications should not directly
1230compare relative references; the references should be converted to
1231their respective target IRIs before comparison. When IRIs are compared
1232to select (or avoid) a network action, such as retrieval of a
1233representation, fragment components (if any) should be excluded from
1234the comparison.</t>
1235
1236<t>Applications using IRIs as identity tokens with no relationship to
1237a protocol MUST use the Simple String Comparison (see <xref
1238target="stringcomp"></xref>).  All other applications MUST select one
1239of the comparison practices from the Comparison Ladder (see <xref
1240target="ladder"></xref>.</t>
1241</section> <!-- equivalence -->
1242
1243
1244<section title="Preparation for Comparison">
1245<t>Any kind of IRI comparison REQUIRES that any additional contextual
1246processing is first performed, including undoing higher-level
1247escapings or encodings in the protocol or format that carries an
1248IRI. This preprocessing is usually done when the protocol or format is
1249parsed.</t>
1250
1251<t>Examples of contextual preprocessing steps are described in <xref
1252target="LEIRIHREF"/>. </t>
1253
1254<t>Examples of such escapings or encodings are entities and
1255numeric character references in <xref target="HTML4"></xref> and <xref
1256target="XML1"></xref>. As an example,
1257"http://example.org/ros&amp;eacute;" (in HTML),
1258"http://example.org/ros&amp;#233;" (in HTML or XML), and
1259<vspace/>"http://example.org/ros&amp;#xE9;" (in HTML or XML) are all
1260resolved into what is denoted in this document (see <xref
1261target="sec-Notation"></xref>) as "http://example.org/ros&amp;#xE9;"
1262(the "&amp;#xE9;" here standing for the actual e-acute character, to
1263compensate for the fact that this document cannot contain non-ASCII
1264characters).</t>
1265
1266<t>Similar considerations apply to encodings such as Transfer Codings
1267in HTTP (see <xref target="RFC2616"></xref>) and Content Transfer
1268Encodings in MIME (<xref target="RFC2045"></xref>), although in these
1269cases, the encoding is based not on characters but on octets, and
1270additional care is required to make sure that characters, and not just
1271arbitrary octets, are compared (see <xref
1272target="stringcomp"></xref>).</t>
1273
1274</section> <!-- preparation -->
1275
1276<section title="Comparison Ladder" anchor="ladder">
1277
1278<t>In practice, a variety of methods are used to test IRI
1279equivalence. These methods fall into a range distinguished by the
1280amount of processing required and the degree to which the probability
1281of false negatives is reduced. As noted above, false negatives cannot
1282be eliminated. In practice, their probability can be reduced, but this
1283reduction requires more processing and is not cost-effective for all
1284applications.</t>
1285
1286
1287<t>If this range of comparison practices is considered as a ladder,
1288the following discussion will climb the ladder, starting with
1289practices that are cheap but have a relatively higher chance of
1290producing false negatives, and proceeding to those that have higher
1291computational cost and lower risk of false negatives.</t>
1292
1293<section title="Simple String Comparison" anchor="stringcomp">
1294
1295<t>If two IRIs, when considered as character strings, are identical,
1296then it is safe to conclude that they are equivalent.  This type of
1297equivalence test has very low computational cost and is in wide use in
1298a variety of applications, particularly in the domain of parsing. It
1299is also used when a definitive answer to the question of IRI
1300equivalence is needed that is independent of the scheme used and that
1301can be calculated quickly and without accessing a network. An example
1302of such a case is XML Namespaces (<xref
1303target="XMLNamespace"></xref>).</t>
1304
1305
1306<t>Testing strings for equivalence requires some basic precautions.
1307This procedure is often referred to as "bit-for-bit" or
1308"byte-for-byte" comparison, which is potentially misleading. Testing
1309strings for equality is normally based on pair comparison of the
1310characters that make up the strings, starting from the first and
1311proceeding until both strings are exhausted and all characters are
1312found to be equal, until a pair of characters compares unequal, or
1313until one of the strings is exhausted before the other.</t>
1314
1315<t>This character comparison requires that each pair of characters be
1316put in comparable encoding form. For example, should one IRI be stored
1317in a byte array in UTF-8 encoding form and the second in a UTF-16
1318encoding form, bit-for-bit comparisons applied naively will produce
1319errors. It is better to speak of equality on a character-for-character
1320rather than on a byte-for-byte or bit-for-bit basis.  In practical
1321terms, character-by-character comparisons should be done codepoint by
1322codepoint after conversion to a common character encoding form.
1323
1324When comparing character by character, the comparison function MUST
1325NOT map IRIs to URIs, because such a mapping would create additional
1326spurious equivalences. It follows that an IRI SHOULD NOT be modified
1327when being transported if there is any chance that this IRI might be
1328used in a context that uses Simple String Comparison.</t>
1329
1330
1331<t>False negatives are caused by the production and use of IRI
1332aliases. Unnecessary aliases can be reduced, regardless of the
1333comparison method, by consistently providing IRI references in an
1334already normalized form (i.e., a form identical to what would be
1335produced after normalization is applied, as described below).
1336Protocols and data formats often limit some IRI comparisons to simple
1337string comparison, based on the theory that people and implementations
1338will, in their own best interest, be consistent in providing IRI
1339references, or at least be consistent enough to negate any efficiency
1340that might be obtained from further normalization.</t>
1341</section> <!-- stringcomp -->
1342
1343<section title="Syntax-Based Normalization">
1344
1345<figure><preamble>Implementations may use logic based on the
1346definitions provided by this specification to reduce the probability
1347of false negatives. This processing is moderately higher in cost than
1348character-for-character string comparison. For example, an application
1349using this approach could reasonably consider the following two IRIs
1350equivalent:</preamble>
1351
1352<artwork>
1353   example://a/b/c/%7Bfoo%7D/ros&amp;#xE9;
1354   eXAMPLE://a/./b/../b/%63/%7bfoo%7d/ros%C3%A9
1355</artwork></figure>
1356
1357<t>Web user agents, such as browsers, typically apply this type of IRI
1358normalization when determining whether a cached response is
1359available. Syntax-based normalization includes such techniques as case
1360normalization, character normalization, percent-encoding
1361normalization, and removal of dot-segments.</t>
1362
1363<section title="Case Normalization">
1364
1365<t>For all IRIs, the hexadecimal digits within a percent-encoding
1366triplet (e.g., "%3a" versus "%3A") are case-insensitive and therefore
1367should be normalized to use uppercase letters for the digits A-F.</t>
1368
1369<t>When an IRI uses components of the generic syntax, the component
1370syntax equivalence rules always apply; namely, that the scheme and
1371US-ASCII only host are case insensitive and therefore should be
1372normalized to lowercase. For example, the URI
1373"HTTP://www.EXAMPLE.com/" is equivalent to
1374"http://www.example.com/". Case equivalence for non-ASCII characters
1375in IRI components that are IDNs are discussed in <xref
1376target="schemecomp"></xref>.  The other generic syntax components are
1377assumed to be case sensitive unless specifically defined otherwise by
1378the scheme.</t>
1379
1380<t>Creating schemes that allow case-insensitive syntax components
1381containing non-ASCII characters should be avoided. Case normalization
1382of non-ASCII characters can be culturally dependent and is always a
1383complex operation. The only exception concerns non-ASCII host names
1384for which the character normalization includes a mapping step derived
1385from case folding.</t>
1386
1387</section> <!-- casenorm -->
1388
1389<section title="Character Normalization" anchor="normalization">
1390
1391<t>The Unicode Standard <xref target="UNIV6"></xref> defines various
1392equivalences between sequences of characters for various
1393purposes. Unicode Standard Annex #15 <xref target="UTR15"></xref>
1394defines various Normalization Forms for these equivalences, in
1395particular Normalization Form C (NFC, Canonical Decomposition,
1396followed by Canonical Composition) and Normalization Form KC (NFKC,
1397Compatibility Decomposition, followed by Canonical Composition).</t>
1398
1399<t> IRIs already in Unicode MUST NOT be normalized before parsing or
1400interpreting. In many non-Unicode character encodings, some text
1401cannot be represented directly. For example, the word "Vietnam" is
1402natively written "Vi&amp;#x1EC7;t Nam" (containing a LATIN SMALL
1403LETTER E WITH CIRCUMFLEX AND DOT BELOW) in NFC, but a direct
1404transcoding from the windows-1258 character encoding leads to
1405"Vi&amp;#xEA;&amp;#x323;t Nam" (containing a LATIN SMALL LETTER E WITH
1406CIRCUMFLEX followed by a COMBINING DOT BELOW). Direct transcoding of
1407other 8-bit encodings of Vietnamese may lead to other
1408representations.</t>
1409
1410<t>Equivalence of IRIs MUST rely on the assumption that IRIs are
1411appropriately pre-character-normalized rather than apply character
1412normalization when comparing two IRIs. The exceptions are conversion
1413from a non-digital form, and conversion from a non-UCS-based character
1414encoding to a UCS-based character encoding. In these cases, NFC or a
1415normalizing transcoder using NFC MUST be used for interoperability. To
1416avoid false negatives and problems with transcoding, IRIs SHOULD be
1417created by using NFC. Using NFKC may avoid even more problems; for
1418example, by choosing half-width Latin letters instead of full-width
1419ones, and full-width instead of half-width Katakana.</t>
1420
1421
1422<t>As an example,
1423"http://www.example.org/r&amp;#xE9;sum&amp;#xE9;.html" (in XML
1424Notation) is in NFC. On the other hand,
1425"http://www.example.org/re&amp;#x301;sume&amp;#x301;.html" is not in
1426NFC.</t>
1427
1428<t>The former uses precombined e-acute characters, and the latter uses
1429"e" characters followed by combining acute accents. Both usages are
1430defined as canonically equivalent in <xref target="UNIV6"></xref>.</t>
1431
1432<t><list style="hanging">
1433
1434<t hangText="Note:">
1435Because it is unknown how a particular sequence of characters is being
1436treated with respect to character normalization, it would be
1437inappropriate to allow third parties to normalize an IRI
1438arbitrarily. This does not contradict the recommendation that when a
1439resource is created, its IRI should be as character normalized as
1440possible (i.e., NFC or even NFKC). This is similar to the
1441uppercase/lowercase problems.  Some parts of a URI are case
1442insensitive (for example, the domain name). For others, it is unclear
1443whether they are case sensitive, case insensitive, or something in
1444between (e.g., case sensitive, but with a multiple choice selection if
1445the wrong case is used, instead of a direct negative result).  The
1446best recipe is that the creator use a reasonable capitalization and,
1447when transferring the URI, capitalization never be
1448changed.</t></list></t>
1449
1450<t>Various IRI schemes may allow the usage of Internationalized Domain
1451Names (IDN) <xref target="RFC5890"/> either in the ireg-name
1452part or elsewhere. Character Normalization also applies to IDNs, as
1453discussed in <xref target="schemecomp"/>.</t>
1454</section> <!-- charnorm -->
1455
1456<section title="Percent-Encoding Normalization">
1457
1458<t>The percent-encoding mechanism (Section 2.1 of <xref
1459target="RFC3986"></xref>) is a frequent source of variance among
1460otherwise identical IRIs. In addition to the case normalization issue
1461noted above, some IRI producers percent-encode octets that do not
1462require percent-encoding, resulting in IRIs that are equivalent to
1463their nonencoded counterparts. These IRIs should be normalized by
1464decoding any percent-encoded octet sequence that corresponds to an
1465unreserved character, as described in section 2.3 of <xref
1466target="RFC3986"></xref>.</t>
1467
1468<t>For actual resolution, differences in percent-encoding (except for
1469the percent-encoding of reserved characters) MUST always result in the
1470same resource.  For example, "http://example.org/~user",
1471"http://example.org/%7euser", and "http://example.org/%7Euser", must
1472resolve to the same resource.</t>
1473
1474<t>If this kind of equivalence is to be tested, the percent-encoding
1475of both IRIs to be compared has to be aligned; for example, by
1476converting both IRIs to URIs (see Section 3.1), eliminating escape
1477differences in the resulting URIs, and making sure that the case of
1478the hexadecimal characters in the percent-encoding is always the same
1479(preferably upper case). If the IRI is to be passed to another
1480application or used further in some other way, its original form MUST
1481be preserved.  The conversion described here should be performed only
1482for local comparison.</t>
1483
1484</section> <!-- pctnorm -->
1485
1486<section title="Path Segment Normalization">
1487
1488<t>The complete path segments "." and ".." are intended only for use
1489within relative references (Section 4.1 of <xref
1490target="RFC3986"></xref>) and are removed as part of the reference
1491resolution process (Section 5.2 of <xref target="RFC3986"></xref>).
1492However, some implementations may incorrectly assume that reference
1493resolution is not necessary when the reference is already an IRI, and
1494thus fail to remove dot-segments when they occur in non-relative
1495paths.  IRI normalizers should remove dot-segments by applying the
1496remove_dot_segments algorithm to the path, as described in Section
14975.2.4 of <xref target="RFC3986"></xref>.</t>
1498
1499</section> <!-- pathnorm -->
1500</section> <!-- ladder -->
1501
1502<section title="Scheme-Based Normalization" anchor="schemecomp">
1503
1504<t>The syntax and semantics of IRIs vary from scheme to scheme, as
1505described by the defining specification for each
1506scheme. Implementations may use scheme-specific rules, at further
1507processing cost, to reduce the probability of false negatives. For
1508example, because the "http" scheme makes use of an authority
1509component, has a default port of "80", and defines an empty path to be
1510equivalent to "/", the following four IRIs are equivalent:</t>
1511
1512<figure><artwork>
1513   http://example.com
1514   http://example.com/
1515   http://example.com:/
1516   http://example.com:80/</artwork></figure>
1517
1518<t>In general, an IRI that uses the generic syntax for authority with
1519an empty path should be normalized to a path of "/". Likewise, an
1520explicit ":port", for which the port is empty or the default for the
1521scheme, is equivalent to one where the port and its ":" delimiter are
1522elided and thus should be removed by scheme-based normalization. For
1523example, the second IRI above is the normal form for the "http"
1524scheme.</t>
1525
1526<t>Another case where normalization varies by scheme is in the
1527handling of an empty authority component or empty host
1528subcomponent. For many scheme specifications, an empty authority or
1529host is considered an error; for others, it is considered equivalent
1530to "localhost" or the end-user's host. When a scheme defines a default
1531for authority and an IRI reference to that default is desired, the
1532reference should be normalized to an empty authority for the sake of
1533uniformity, brevity, and internationalization. If, however, either the
1534userinfo or port subcomponents are non-empty, then the host should be
1535given explicitly even if it matches the default.</t>
1536
1537<t>Normalization should not remove delimiters when their associated
1538component is empty unless it is licensed to do so by the scheme
1539specification. For example, the IRI "http://example.com/?" cannot be
1540assumed to be equivalent to any of the examples above. Likewise, the
1541presence or absence of delimiters within a userinfo subcomponent is
1542usually significant to its interpretation.  The fragment component is
1543not subject to any scheme-based normalization; thus, two IRIs that
1544differ only by the suffix "#" are considered different regardless of
1545the scheme.</t>
1546 
1547<t>Some IRI schemes allow the usage of Internationalized Domain
1548Names (IDN) <xref target='RFC5890'></xref> either in their ireg-name
1549part or elswhere. When in use in IRIs, those names SHOULD
1550conform to the definition of U-Label in <xref
1551target='RFC5890'></xref>. An IRI containing an invalid IDN cannot
1552successfully be resolved. For legibility purposes, they
1553SHOULD NOT be converted into ASCII Compatible Encoding (ACE).</t>
1554
1555<t>Scheme-based normalization may also consider IDN
1556components and their conversions to punycode as equivalent. As an
1557example, "http://r&amp;#xE9;sum&amp;#xE9;.example.org" may be
1558considered equivalent to
1559"http://xn--rsum-bpad.example.org".</t><t>Other scheme-specific
1560normalizations are possible.</t>
1561
1562</section> <!-- schemenorm -->
1563
1564<section title="Protocol-Based Normalization">
1565
1566<t>Substantial effort to reduce the incidence of false negatives is
1567often cost-effective for web spiders. Consequently, they implement
1568even more aggressive techniques in IRI comparison. For example, if
1569they observe that an IRI such as</t>
1570
1571<figure><artwork>
1572   http://example.com/data</artwork></figure>
1573<t>redirects to an IRI differing only in the trailing slash</t>
1574<figure><artwork>
1575   http://example.com/data/</artwork></figure>
1576
1577<t>they will likely regard the two as equivalent in the future.  This
1578kind of technique is only appropriate when equivalence is clearly
1579indicated by both the result of accessing the resources and the common
1580conventions of their scheme's dereference algorithm (in this case, use
1581of redirection by HTTP origin servers to avoid problems with relative
1582references).</t>
1583
1584</section> <!-- protonorm -->
1585</section> <!-- equivalence -->
1586</section> 
1587
1588<section title="Use of IRIs" anchor="IRIuse">
1589
1590<section title="Limitations on UCS Characters Allowed in IRIs" anchor="limitations">
1591
1592<t>This section discusses limitations on characters and character
1593sequences usable for IRIs beyond those given in <xref target="abnf"/>
1594and <xref target="visual"/>. The considerations in this section are
1595relevant when IRIs are created and when URIs are converted to
1596IRIs.</t>
1597
1598<t>
1599
1600<list style="hanging"><t hangText="a.">The repertoire of characters allowed
1601    in each IRI component is limited by the definition of that component.
1602    For example, the definition of the scheme component does not allow
1603    characters beyond US-ASCII.
1604    <vspace blankLines="1"/>
1605    (Note: In accordance with URI practice, generic IRI
1606    software cannot and should not check for such limitations.)</t>
1607
1608<t hangText="b.">The UCS contains many areas of characters for which
1609    there are strong visual look-alikes. Because of the likelihood of
1610    transcription errors, these also should be avoided. This includes
1611    the full-width equivalents of Latin characters, half-width
1612    Katakana characters for Japanese, and many others. It also
1613    includes many look-alikes of "space", "delims", and "unwise",
1614    characters excluded in <xref target="RFC3491"/>.</t>
1615   
1616</list>
1617</t>
1618
1619<t>Additional information is available from <xref target="UNIXML"/>.
1620    <xref target="UNIXML"/> is written in the context of running text
1621    rather than in that of identifiers. Nevertheless, it discusses
1622    many of the categories of characters not appropriate for IRIs.</t>
1623</section> <!-- limitations -->
1624
1625<section title="Software Interfaces and Protocols">
1626
1627<t>Although an IRI is defined as a sequence of characters, software
1628interfaces for URIs typically function on sequences of octets or other
1629kinds of code units. Thus, software interfaces and protocols MUST
1630define which character encoding is used.</t>
1631
1632<t>Intermediate software interfaces between IRI-capable components and
1633URI-only components MUST map the IRIs per <xref target="mapping"/>,
1634when transferring from IRI-capable to URI-only components.
1635
1636This mapping SHOULD be applied as late as possible. It SHOULD NOT be
1637applied between components that are known to be able to handle IRIs.</t>
1638</section> <!-- software -->
1639
1640<section title="Format of URIs and IRIs in Documents and Protocols">
1641
1642<t>Document formats that transport URIs may have to be upgraded to allow
1643the transport of IRIs. In cases where the document as a whole
1644has a native character encoding, IRIs MUST also be encoded in this
1645character encoding and converted accordingly by a parser or interpreter.
1646
1647IRI characters not expressible in the native character encoding SHOULD
1648be escaped by using the escaping conventions of the document format if
1649such conventions are available. Alternatively, they MAY be
1650percent-encoded according to <xref target="mapping"/>. For example, in
1651HTML or XML, numeric character references SHOULD be used. If a
1652document as a whole has a native character encoding and that character
1653encoding is not UTF-8, then IRIs MUST NOT be placed into the document
1654in the UTF-8 character encoding.</t>
1655
1656<t>((UPDATE THIS NOTE)) Note: Some formats already accommodate IRIs,
1657although they use different terminology. HTML 4.0 <xref
1658target="HTML4"/> defines the conversion from IRIs to URIs as
1659error-avoiding behavior. XML 1.0 <xref target="XML1"/>, XLink <xref
1660target="XLink"/>, XML Schema <xref target="XMLSchema"/>, and
1661specifications based upon them allow IRIs. Also, it is expected that
1662all relevant new W3C formats and protocols will be required to handle
1663IRIs <xref target="CharMod"/>.</t>
1664
1665</section> <!-- format -->
1666
1667<section title="Use of UTF-8 for Encoding Original Characters" anchor="UTF8use">
1668
1669<t>This section discusses details and gives examples for point c) in
1670<xref target="Applicability"/>. To be able to use IRIs, the URI
1671corresponding to the IRI in question has to encode original characters
1672into octets by using UTF-8.  This can be specified for all URIs of a
1673URI scheme or can apply to individual URIs for schemes that do not
1674specify how to encode original characters.  It can apply to the whole
1675URI, or only to some part. For background information on encoding
1676characters into URIs, see also Section 2.5 of <xref
1677target="RFC3986"/>.</t>
1678
1679<t>For new URI schemes, using UTF-8 is recommended in <xref
1680target="RFC4395bis"/>.  Examples where UTF-8 is already used are the URN
1681syntax <xref target="RFC2141"/>, IMAP URLs <xref target="RFC2192"/>,
1682and POP URLs <xref target="RFC2384"/>.  On the other hand, because the
1683HTTP URI scheme does not specify how to encode original characters,
1684only some HTTP URLs can have corresponding but different IRIs.</t>
1685
1686<t>For example, for a document with a URI
1687of<vspace/>"http://www.example.org/r%C3%A9sum%C3%A9.html", it is
1688possible to construct a corresponding IRI (in XML notation, see <xref
1689target="sec-Notation"/>):
1690"http://www.example.org/r&amp;#xE9;sum&amp;#xE9;.html" ("&amp;#xE9;"
1691stands for the e-acute character, and "%C3%A9" is the UTF-8 encoded
1692and percent-encoded representation of that character). On the other
1693hand, for a document with a URI of
1694"http://www.example.org/r%E9sum%E9.html", the percent-encoding octets
1695cannot be converted to actual characters in an IRI, as the
1696percent-encoding is not based on UTF-8.</t>
1697
1698<t>For most URI schemes, there is no need to upgrade their scheme
1699definition in order for them to work with IRIs.  The main case where
1700upgrading makes sense is when a scheme definition, or a particular
1701component of a scheme, is strictly limited to the use of US-ASCII
1702characters with no provision to include non-ASCII characters/octets
1703via percent-encoding, or if a scheme definition currently uses highly
1704scheme-specific provisions for the encoding of non-ASCII characters.
1705An example of this is the mailto: scheme <xref target="RFC2368"/>.</t>
1706
1707<t>This specification updates the IANA registry of URI schemes to note
1708their applicability to IRIs, see <xref target="iana"/>.  All IRIs use
1709URI schemes, and all URIs with URI schemes can be used as IRIs, even
1710though in some cases only by using URIs directly as IRIs, without any
1711conversion.</t>
1712
1713<t>Scheme definitions can impose restrictions on the syntax of
1714scheme-specific URIs; i.e., URIs that are admissible under the generic
1715URI syntax <xref target="RFC3986"/> may not be admissible due to
1716narrower syntactic constraints imposed by a URI scheme
1717specification. URI scheme definitions cannot broaden the syntactic
1718restrictions of the generic URI syntax; otherwise, it would be
1719possible to generate URIs that satisfied the scheme-specific syntactic
1720constraints without satisfying the syntactic constraints of the
1721generic URI syntax. However, additional syntactic constraints imposed
1722by URI scheme specifications are applicable to IRI, as the
1723corresponding URI resulting from the mapping defined in <xref
1724target="mapping"/> MUST be a valid URI under the syntactic
1725restrictions of generic URI syntax and any narrower restrictions
1726imposed by the corresponding URI scheme specification.</t>
1727
1728<t>The requirement for the use of UTF-8 generally applies to all parts
1729of a URI.  However, it is possible that the capability of IRIs to
1730represent a wide range of characters directly is used just in some
1731parts of the IRI (or IRI reference). The other parts of the IRI may
1732only contain US-ASCII characters, or they may not be based on
1733UTF-8. They may be based on another character encoding, or they may
1734directly encode raw binary data (see also <xref
1735target="RFC2397"/>). </t>
1736
1737<t>For example, it is possible to have a URI reference
1738of<vspace/>"http://www.example.org/r%E9sum%E9.xml#r%C3%A9sum%C3%A9",
1739where the document name is encoded in iso-8859-1 based on server
1740settings, but where the fragment identifier is encoded in UTF-8 according
1741to <xref target="XPointer"/>. The IRI corresponding to the above
1742URI would be (in XML notation)<vspace/>"http://www.example.org/r%E9sum%E9.xml#r&amp;#xE9;sum&amp;#xE9;".</t>
1743
1744<t>Similar considerations apply to query parts. The functionality
1745of IRIs (namely, to be able to include non-ASCII characters) can
1746only be used if the query part is encoded in UTF-8.</t>
1747
1748</section> <!-- utf8 -->
1749
1750<section title="Relative IRI References">
1751<t>Processing of relative IRI references against a base is handled
1752straightforwardly; the algorithms of <xref target="RFC3986"/> can
1753be applied directly, treating the characters additionally allowed
1754in IRI references in the same way that unreserved characters are in URI
1755references.</t>
1756
1757</section> <!-- relative -->
1758</section> <!-- IRIuse -->
1759
1760<section title="Liberal Handling of Otherwise Invalid IRIs" anchor="LEIRIHREF">
1761
1762<t>(EDITOR NOTE: This Section may move to an appendix.)
1763 
1764Some technical specifications and widely-deployed software have
1765allowed additional variations and extensions of IRIs to be used in
1766syntactic components. This section describes two widely-used
1767preprocessing agreements. Other technical specifications may wish to
1768reference a syntactic component which is "a valid IRI or a string that
1769will map to a valid IRI after this preprocessing algorithm". These two
1770variants are known as <xref target="LEIRI">Legacy Extended IRI or
1771LEIRI</xref>, and <xref target="HTML5">Web Address</xref>).
1772</t>
1773
1774<t>Future technical specifications SHOULD NOT allow conforming
1775producers to produce, or conforming content to contain, such forms,
1776as they are not interoperable with other IRI consuming software.</t>
1777
1778<section title="LEIRI Processing"  anchor="LEIRIspec">
1779  <t>This section defines Legacy Extended IRIs (LEIRIs).
1780    The syntax of Legacy Extended IRIs is the same as that for &lt;IRI-reference>,
1781    except that the ucschar production is replaced by the leiri-ucschar production:</t>
1782<figure>
1783
1784<artwork>
1785  leiri-ucschar  = " " / "&lt;" / "&gt;" / '"' / "{" / "}" / "|"
1786                   / "\" / "^" / "`" / %x0-1F / %x7F-D7FF
1787                   / %xE000-FFFD / %x10000-10FFFF
1788</artwork>
1789
1790<postamble>
1791  Among other extensions, processors based on this specification also
1792  did not enforce the restriction on bidirectional formatting
1793  characters in <xref target="visual"></xref>, and the iprivate
1794  production becomes redundant.</postamble>
1795</figure>
1796
1797<t>To convert a string allowed as a LEIRI to an IRI, each character
1798allowed in leiri-ucschar but not in ucschar must be percent-encoded
1799using <xref target="compmapping"/>.</t>
1800</section> <!-- leiriproc -->
1801
1802<section title="Web Address Processing" anchor="webaddress">
1803
1804<t>Many popular web browsers have taken the approach of being quite
1805liberal in what is accepted as a "URL" or its relative
1806forms. This section describes their behavior in terms of a preprocessor
1807which maps strings into the IRI space for subsequent parsing and
1808interpretation as an IRI.</t>
1809
1810<t>In some situations, it might be appropriate to describe the syntax
1811that a liberal consumer implementation might accept as a "Web
1812Address" or "Hypertext Reference" or "HREF". However,
1813technical specifications SHOULD restrict the syntactic form allowed by compliant producers
1814to the IRI or IRI reference syntax defined in this document
1815even if they want to mandate this processing.</t>
1816
1817<t>
1818Summary:
1819<list style="symbols">
1820   <t>Leading and trailing whitespace is removed.</t>
1821   <t>Some additional characters are removed.</t>
1822   <t>Some additional characters are allowed and escaped (as with LEIRI).</t>
1823   <t>If interpreting an IRI as a URI, the pct-encoding of the query
1824   component of the parsed URI component depends on operational
1825   context.</t>
1826</list>
1827</t>
1828
1829<t>Each string provided may have an associated charset (called
1830the HREF-charset here); this defaults to UTF-8.
1831For web browsers interpreting HTML, the document
1832charset of a string is determined:
1833
1834<list style="hanging">
1835<t hangText="If the string came from a script (e.g. as an argument to
1836 a method)">The HRef-charset is the script's charset.</t>
1837
1838<t hangText="If the string came from a DOM node (e.g. from an
1839  element)">The node has a Document, and the HRef-charset is the
1840  Document's character encoding.</t>
1841
1842<t hangText="If the string had a HRef-charset defined when the string was
1843created or defined">The HRef-charset is as defined.</t>
1844
1845</list></t>
1846
1847<t>If the resulting HRef-charset is a unicode based character encoding
1848(e.g., UTF-16), then use UTF-8 instead.</t>
1849
1850
1851<figure>
1852<preamble>The syntax for Web Addresses is obtained by replacing the 'ucschar',
1853  pct-form, path-sep, and ifragment rules with the href-ucschar, href-pct-form, href-path-sep,
1854  and href-ifragment
1855  rules below. In addition, some characters are stripped.</preamble>
1856
1857<artwork type='abnf'>
1858  href-ucschar   = " " / "&lt;" / "&gt;" / DQUOTE / "{" / "}" / "|"
1859                 / "\" / "^" / "`" / %x0-1F / %x7F-D7FF
1860                 / %xE000-FFFD / %x10000-10FFFF
1861  href-pct-form  = pct-encoded / "%"
1862  href-path-sep  = "/" / "\"
1863  href-ifragment = *( ipchar / "/" / "?" / "#" )  ; adding "#"
1864  href-strip     = &lt;to be done&gt;
1865</artwork>
1866
1867<postamble>
1868(NOTE: NEED TO FIX THESE SETS TO MATCH HTML5; NOT SURE ABOUT NEXT SENTENCE)
1869browsers did not enforce the restriction on bidirectional formatting
1870  characters in <xref target="visual"></xref>, and the iprivate
1871  production becomes redundant.</postamble>
1872</figure>
1873
1874<t>'Web Address processing' requires the following additional
1875preprocessing steps:
1876
1877<list style="numbers">
1878
1879<t>Leading and trailing instances of space (U+0020),
1880CR (U+000A), LF (U+000D), and TAB (U+0009) characters are removed.</t>
1881
1882<t>strip all characters in href-strip.</t>
1883  <t>Percent-encode all characters in href-ucschar not in ucschar.</t>
1884  <t>Replace occurrences of "%" not followed by two hexadecimal digits by "%25".</t>
1885  <t>Convert backslashes ('\') matching href-path-sep to forward slashes ('/').</t>
1886</list></t>
1887</section> <!-- webaddress -->
1888
1889<section title="Characters Not Allowed in IRIs" anchor="notAllowed">
1890
1891<t>This section provides a list of the groups of characters and code
1892points that are allowed by LEIRI or HREF but are not allowed in IRIs or are
1893allowed in IRIs only in the query part. For each group of characters,
1894advice on the usage of these characters is also given, concentrating
1895on the reasons for why they are excluded from IRI use.</t>
1896
1897<t>
1898
1899<list><t>Space (U+0020): Some formats and applications use space as a
1900delimiter, e.g. for items in a list. Appendix C of <xref
1901target="RFC3986"></xref> also mentions that white space may have to be
1902added when displaying or printing long URIs; the same applies to long
1903IRIs. This means that spaces can disappear, or can make the what is
1904intended as a single IRI or IRI reference to be treated as two or more
1905separate IRIs.</t>
1906
1907<t>Delimiters "&lt;" (U+003C), "&gt;" (U+003E), and '"' (U+0022):
1908Appendix C of <xref target="RFC3986"></xref> suggests the use of
1909double-quotes ("http://example.com/") and angle brackets
1910(&lt;http://example.com/&gt;) as delimiters for URIs in plain
1911text. These conventions are often used, and also apply to IRIs.  Using
1912these characters in strings intended to be IRIs would result in the
1913IRIs being cut off at the wrong place.</t>
1914
1915<t>Unwise characters "\" (U+005C), "^" (U+005E), "`"
1916(U+0060), "{" (U+007B), "|" (U+007C), and "}" (U+007D): These
1917characters originally have been excluded from URIs because the
1918respective codepoints are assigned to different graphic characters in
1919some 7-bit or 8-bit encoding. Despite the move to Unicode, some of
1920these characters are still occasionally displayed differently on some
1921systems, e.g. U+005C may appear as a Japanese Yen symbol on some
1922systems. Also, the fact that these characters are not used in URIs or
1923IRIs has encouraged their use outside URIs or IRIs in contexts that
1924may include URIs or IRIs. If a string with such a character were used
1925as an IRI in such a context, it would likely be interpreted
1926piecemeal.</t>
1927
1928<t>The controls (C0 controls, DEL, and C1 controls, #x0 - #x1F #x7F -
1929#x9F): There is generally no way to transmit these characters reliably
1930as text outside of a charset encoding.  Even when in encoded form,
1931many software components silently filter out some of these characters,
1932or may stop processing alltogether when encountering some of
1933them. These characters may affect text display in subtle, unnoticable
1934ways or in drastic, global, and irreversible ways depending on the
1935hardware and software involved. The use of some of these characters
1936would allow malicious users to manipulate the display of an IRI and
1937its context in many situations.</t>
1938
1939<t>Bidi formatting characters (U+200E, U+200F, U+202A-202E): These
1940characters affect the display ordering of characters. If IRIs were
1941allowed to contain these characters and the resulting visual display
1942transcribed. they could not be converted back to electronic form
1943(logical order) unambiguously. These characters, if allowed in IRIs,
1944might allow malicious users to manipulate the display of IRI and its
1945context.</t>
1946
1947<t>Specials (U+FFF0-FFFD): These code points provide functionality
1948beyond that useful in an IRI, for example byte order identification,
1949annotation, and replacements for unknown characters and objects. Their
1950use and interpretation in an IRI would serve no purpose and might lead
1951to confusing display variations.</t>
1952
1953<t>Private use code points (U+E000-F8FF, U+F0000-FFFFD,
1954U+100000-10FFFD): Display and interpretation of these code points is
1955by definition undefined without private agreement. Therefore, these
1956code points are not suited for use on the Internet. They are not
1957interoperable and may have unpredictable effects.</t>
1958
1959<t>Tags (U+E0000-E0FFF): These characters provide a way to language
1960tag in Unicode plain text. They are not appropriate for IRIs because
1961language information in identifiers cannot reliably be input,
1962transmitted (e.g. on a visual medium such as paper), or
1963recognized.</t>
1964
1965<t>Non-characters (U+FDD0-FDEF, U+1FFFE-1FFFF, U+2FFFE-2FFFF,
1966U+3FFFE-3FFFF, U+4FFFE-4FFFF, U+5FFFE-5FFFF, U+6FFFE-6FFFF,
1967U+7FFFE-7FFFF, U+8FFFE-8FFFF, U+9FFFE-9FFFF, U+AFFFE-AFFFF,
1968U+BFFFE-BFFFF, U+CFFFE-CFFFF, U+DFFFE-DFFFF, U+EFFFE-EFFFF,
1969U+FFFFE-FFFFF, U+10FFFE-10FFFF): These code points are defined as
1970non-characters. Applications may use some of them internally, but are
1971not prepared to interchange them.</t>
1972
1973</list></t>
1974
1975<t>LEIRI preprocessing disallowed some code points and
1976code units:
1977
1978<list><t>Surrogate code units (D800-DFFF): These do not represent
1979Unicode codepoints.</t></list></t>
1980</section> <!-- notallowed -->
1981</section> <!-- lieirihref -->
1982 
1983<section title="URI/IRI Processing Guidelines (Informative)" anchor="guidelines">
1984
1985<t>This informative section provides guidelines for supporting IRIs in
1986the same software components and operations that currently process
1987URIs: Software interfaces that handle URIs, software that allows users
1988to enter URIs, software that creates or generates URIs, software that
1989displays URIs, formats and protocols that transport URIs, and software
1990that interprets URIs. These may all require modification before
1991functioning properly with IRIs. The considerations in this section
1992also apply to URI references and IRI references.</t>
1993
1994<section title="URI/IRI Software Interfaces">
1995<t>Software interfaces that handle URIs, such as URI-handling APIs and
1996protocols transferring URIs, need interfaces and protocol elements
1997that are designed to carry IRIs.</t>
1998
1999<t>In case the current handling in an API or protocol is based on
2000US-ASCII, UTF-8 is recommended as the character encoding for IRIs, as
2001it is compatible with US-ASCII, is in accordance with the
2002recommendations of <xref target="RFC2277"/>, and makes converting to
2003URIs easy. In any case, the API or protocol definition must clearly
2004define the character encoding to be used.</t>
2005
2006<t>The transfer from URI-only to IRI-capable components requires no
2007mapping, although the conversion described in <xref
2008target="URItoIRI"/> above may be performed. It is preferable not to
2009perform this inverse conversion unless it is certain this can be done
2010correctly.</t>
2011</section>
2012
2013<section title="URI/IRI Entry">
2014
2015<t>Some components allow users to enter URIs into the system
2016by typing or dictation, for example. This software must be updated to allow
2017for IRI entry.</t>
2018
2019<t>A person viewing a visual representation of an IRI (as a sequence
2020of glyphs, in some order, in some visual display) or hearing an IRI
2021will use an entry method for characters in the user's language to
2022input the IRI. Depending on the script and the input method used, this
2023may be a more or less complicated process.</t>
2024
2025<t>The process of IRI entry must ensure, as much as possible, that the
2026restrictions defined in <xref target="abnf"/> are met. This may be
2027done by choosing appropriate input methods or variants/settings
2028thereof, by appropriately converting the characters being input, by
2029eliminating characters that cannot be converted, and/or by issuing a
2030warning or error message to the user.</t>
2031
2032<t>As an example of variant settings, input method editors for East
2033Asian Languages usually allow the input of Latin letters and related
2034characters in full-width or half-width versions. For IRI input, the
2035input method editor should be set so that it produces half-width Latin
2036letters and punctuation and full-width Katakana.</t>
2037
2038<t>An input field primarily or solely used for the input of URIs/IRIs
2039might allow the user to view an IRI as it is mapped to a URI.  Places
2040where the input of IRIs is frequent may provide the possibility for
2041viewing an IRI as mapped to a URI. This will help users when some of
2042the software they use does not yet accept IRIs.</t>
2043
2044<t>An IRI input component interfacing to components that handle URIs,
2045but not IRIs, must map the IRI to a URI before passing it to these
2046components.</t>
2047
2048<t>For the input of IRIs with right-to-left characters, please see
2049<xref target="bidiInput"></xref>.</t>
2050</section>
2051
2052<section title="URI/IRI Transfer between Applications">
2053
2054<t>Many applications (for example, mail user agents) try to detect
2055URIs appearing in plain text. For this, they use some heuristics based
2056on URI syntax. They then allow the user to click on such URIs and
2057retrieve the corresponding resource in an appropriate (usually
2058scheme-dependent) application.</t>
2059
2060<t>Such applications would need to be upgraded, in order to use the
2061IRI syntax as a base for heuristics. In particular, a non-ASCII
2062character should not be taken as the indication of the end of an IRI.
2063Such applications also would need to make sure that they correctly
2064convert the detected IRI from the character encoding of the document
2065or application where the IRI appears, to the character encoding used
2066by the system-wide IRI invocation mechanism, or to a URI (according to
2067<xref target="mapping"/>) if the system-wide invocation mechanism only
2068accepts URIs.</t>
2069
2070<t>The clipboard is another frequently used way to transfer URIs and
2071IRIs from one application to another. On most platforms, the clipboard
2072is able to store and transfer text in many languages and scripts.
2073Correctly used, the clipboard transfers characters, not octets, which
2074will do the right thing with IRIs.</t>
2075</section>
2076
2077<section title="URI/IRI Generation">
2078
2079<t>Systems that offer resources through the Internet, where those
2080resources have logical names, sometimes automatically generate URIs
2081for the resources they offer. For example, some HTTP servers can
2082generate a directory listing for a file directory and then respond to
2083the generated URIs with the files.</t>
2084
2085<t>Many legacy character encodings are in use in various file systems.
2086Many currently deployed systems do not transform the local character
2087representation of the underlying system before generating URIs.</t>
2088
2089<t>For maximum interoperability, systems that generate resource
2090identifiers should make the appropriate transformations. For example,
2091if a file system contains a file named
2092"r&amp;#xE9;sum&amp;#xE9;.html", a server should expose this as
2093"r%C3%A9sum%C3%A9.html" in a URI, which allows use of
2094"r&amp;#xE9;sum&amp;#xE9;.html" in an IRI, even if locally the file
2095name is kept in a character encoding other than UTF-8.
2096</t>
2097
2098<t>This recommendation particularly applies to HTTP servers. For FTP
2099servers, similar considerations apply; see <xref target="RFC2640"/>.</t>
2100</section>
2101
2102<section title="URI/IRI Selection" anchor="selection">
2103<t>In some cases, resource owners and publishers have control over the
2104IRIs used to identify their resources. This control is mostly
2105executed by controlling the resource names, such as file names,
2106directly.</t>
2107
2108<t>In these cases, it is recommended to avoid choosing IRIs that are
2109easily confused. For example, for US-ASCII, the lower-case ell ("l") is
2110easily confused with the digit one ("1"), and the upper-case oh ("O") is
2111easily confused with the digit zero ("0"). Publishers should avoid
2112confusing users with "br0ken" or "1ame" identifiers.</t>
2113
2114<t>Outside the US-ASCII repertoire, there are many more opportunities for
2115confusion; a complete set of guidelines is too lengthy to include
2116here. As long as names are limited to characters from a single script,
2117native writers of a given script or language will know best when
2118ambiguities can appear, and how they can be avoided. What may look
2119ambiguous to a stranger may be completely obvious to the average
2120native user. On the other hand, in some cases, the UCS contains
2121variants for compatibility reasons; for example, for typographic purposes.
2122These should be avoided wherever possible. Although there may be exceptions,
2123newly created resource names should generally be in NFKC
2124<xref target="UTR15"></xref> (which means that they are also in NFC).</t>
2125
2126<t>As an example, the UCS contains the "fi" ligature at U+FB01
2127for compatibility reasons.
2128Wherever possible, IRIs should use the two letters "f" and "i" rather
2129than the "fi" ligature. An example where the latter may be used is
2130in the query part of an IRI for an explicit search for a word written
2131containing the "fi" ligature.</t>
2132
2133<t>In certain cases, there is a chance that characters from different
2134scripts look the same. The best known example is the similarity of the
2135Latin "A", the Greek "Alpha", and the Cyrillic "A". To avoid such
2136cases, IRIs should only be created where all the characters in a
2137single component are used together in a given language. This usually
2138means that all of these characters will be from the same script, but
2139there are languages that mix characters from different scripts (such
2140as Japanese).  This is similar to the heuristics used to distinguish
2141between letters and numbers in the examples above. Also, for Latin,
2142Greek, and Cyrillic, using lowercase letters results in fewer
2143ambiguities than using uppercase letters would.</t>
2144</section>
2145
2146<section title="Display of URIs/IRIs" anchor="display">
2147<t>
2148In situations where the rendering software is not expected to display
2149non-ASCII parts of the IRI correctly using the available layout and font
2150resources, these parts should be percent-encoded before being displayed.</t>
2151
2152<t>For display of Bidi IRIs, please see <xref target="visual"/>.</t>
2153</section>
2154
2155<section title="Interpretation of URIs and IRIs">
2156<t>Software that interprets IRIs as the names of local resources should
2157accept IRIs in multiple forms and convert and match them with the
2158appropriate local resource names.</t>
2159
2160<t>First, multiple representations include both IRIs in the native
2161character encoding of the protocol and also their URI counterparts.</t>
2162
2163<t>Second, it may include URIs constructed based on character
2164encodings other than UTF-8. These URIs may be produced by user agents that do
2165not conform to this specification and that use legacy character encodings to
2166convert non-ASCII characters to URIs. Whether this is necessary, and what
2167character encodings to cover, depends on a number of factors, such as
2168the legacy character encodings used locally and the distribution of
2169various versions of user agents. For example, software for Japanese
2170may accept URIs in Shift_JIS and/or EUC-JP in addition to UTF-8.</t>
2171
2172<t>Third, it may include additional mappings to be more user-friendly
2173and robust against transmission errors. These would be similar to how
2174some servers currently treat URIs as case insensitive or perform
2175additional matching to account for spelling errors. For characters
2176beyond the US-ASCII repertoire, this may, for example, include
2177ignoring the accents on received IRIs or resource names. Please note
2178that such mappings, including case mappings, are language
2179dependent.</t>
2180
2181<t>It can be difficult to identify a resource unambiguously if too
2182many mappings are taken into consideration. However, percent-encoded
2183and not percent-encoded parts of IRIs can always be clearly distinguished.
2184Also, the regularity of UTF-8 (see <xref target="Duerst97"/>) makes the
2185potential for collisions lower than it may seem at first.</t>
2186</section>
2187
2188<section title="Upgrading Strategy">
2189<t>Where this recommendation places further constraints on software
2190for which many instances are already deployed, it is important to
2191introduce upgrades carefully and to be aware of the various
2192interdependencies.</t>
2193
2194<t>If IRIs cannot be interpreted correctly, they should not be created,
2195generated, or transported. This suggests that upgrading URI interpreting
2196software to accept IRIs should have highest priority.</t>
2197
2198<t>On the other hand, a single IRI is interpreted only by a single or
2199very few interpreters that are known in advance, although it may be
2200entered and transported very widely.</t>
2201
2202<t>Therefore, IRIs benefit most from a broad upgrade of software to be
2203able to enter and transport IRIs. However, before an
2204individual IRI is published, care should be taken to upgrade the corresponding
2205interpreting software in order to cover the forms expected to be
2206received by various versions of entry and transport software.</t>
2207
2208<t>The upgrade of generating software to generate IRIs instead of using a
2209local character encoding should happen only after the service is upgraded
2210to accept IRIs. Similarly, IRIs should only be generated when the service
2211accepts IRIs and the intervening infrastructure and protocol is known
2212to transport them safely.</t>
2213
2214<t>Software converting from URIs to IRIs for display should be upgraded
2215only after upgraded entry software has been widely deployed to the
2216population that will see the displayed result.</t>
2217
2218
2219<t>Where there is a free choice of character encodings, it is often
2220possible to reduce the effort and dependencies for upgrading to IRIs
2221by using UTF-8 rather than another encoding. For example, when a new
2222file-based Web server is set up, using UTF-8 as the character encoding
2223for file names will make the transition to IRIs easier. Likewise, when
2224a new Web form is set up using UTF-8 as the character encoding of the
2225form page, the returned query URIs will use UTF-8 as the character
2226encoding (unless the user, for whatever reason, changes the character
2227encoding) and will therefore be compatible with IRIs.</t>
2228
2229
2230<t>These recommendations, when taken together, will allow for the
2231extension from URIs to IRIs in order to handle characters other than
2232US-ASCII while minimizing interoperability problems. For
2233considerations regarding the upgrade of URI scheme definitions, see
2234<xref target="UTF8use"/>.</t>
2235
2236</section>
2237</section> <!-- guidelines -->
2238
2239<section title="IANA Considerations" anchor="iana">
2240
2241<t>RFC Editor and IANA note: Please Replace RFC XXXX with the
2242number of this document when it issues as an RFC. </t>
2243
2244<t>IANA maintains a registry of "URI schemes". A "URI scheme" also
2245serves an "IRI scheme". </t>
2246
2247<t>To clarify that the URI scheme registration process also applies to
2248IRIs, change the description of the "URI schemes" registry
2249header to say "[RFC4395] defines an IANA-maintained registry of URI
2250Schemes. These registries include the Permanent and Provisional URI
2251Schemes.  RFC XXXX updates this registry to designate that schemes may
2252also indicate their usability as IRI schemes.</t>
2253
2254<t> Update "per RFC 4395" to "per RFC 4395 and RFC XXXX".
2255</t>
2256
2257</section> <!-- IANA -->
2258   
2259<section title="Security Considerations" anchor="security">
2260<t>The security considerations discussed in <xref target="RFC3986"/>
2261also apply to IRIs. In addition, the following issues require
2262particular care for IRIs.</t>
2263<t>Incorrect encoding or decoding can lead to security problems.
2264For example, some UTF-8 decoders do not check against overlong
2265byte sequences. See <xref target='UTR36'/> Section 3 for details.</t>
2266
2267  <t>There are serious difficulties with relying on a human to verify that a
2268    an IRI (whether presented visually or aurally)
2269    is the same as another IRI or is the one intended.
2270    These problems exist with ASCII-only URIs (bl00mberg.com vs. bloomberg.com)
2271    but are strongly exacerbated when using the much larger character repertoire of Unicode.
2272    For details, see Section 2 of <xref target='UTR36'/>.
2273    Using administrative and technical means to reduce the availability
2274    of such exploits is possible, but they are difficult to eliminate altogether.
2275    User agents SHOULD NOT rely on visual or perceptual comparison or verification of IRIs
2276    as a means of validating or assuring safety, correctness or appropriateness of an IRI.
2277    Other means of presenting users with the validity, safety, or appropriateness
2278    of visited sites are being developed in the browser community
2279    as an alternative means of avoiding these difficulties.</t>
2280
2281<t>Besides the large character repertoire of Unicode, reasons for
2282  confusion include different forms of normalization and different normalization
2283  expectations, use of percent-encoding with various legacy encodings,
2284  and bidirectionality issues. See also <xref target='UTR36'/>.</t>
2285
2286<t>Confusion can occur in various IRI components, such as the
2287domain name part or the path part, or between IRI components. For considerations specific
2288to the domain name part, see <xref target="RFC5890"/>. For considerations specific to
2289particular protocols or schemes, see the security sections of the relevant specifications
2290and registration templates.
2291Administrators of sites that allow independent
2292users to create resources in the same sub area have to be careful.
2293Details are discussed in <xref target="selection"/>.</t>
2294
2295<t>Confusion can occur with bidirectional IRIs, if the restrictions
2296in <xref target="bidi-structure"/> are not followed. The same visual
2297representation may be interpreted as different logical representations,
2298and vice versa. It is also very important that a correct Unicode bidirectional
2299implementation be used.</t>
2300  <t>The characters additionally allowed in Legacy Extended IRIs
2301    introduce additional security issues. For details, see <xref target='notAllowed'/>.</t>
2302</section><!-- security -->
2303
2304<section title="Acknowledgements">
2305<t>This document was derived from <xref target="RFC3987"/>; the acknowledgments from
2306that specification still apply.</t>
2307<t>We would like to thank Ian Hickson, Michael Sperberg-McQueen,
2308  and Dan Connolly for their work on HyperText References, and Norman Walsh, Richard Tobin,
2309  Henry S. Thomson, John Cowan, Paul Grosso, and the XML Core Working Group of the W3C for their work on LEIRIs.</t>
2310<t>In addition, this document was influenced by contributions from (in no particular order) Chris
2311  Lilley, Bjoern Hoehrmann,
2312Felix Sasaki, Jeremy Carroll, Frank Ellermann, Michael Everson, Cary Karp, Matitiahu Allouche,
2313Richard Ishida, Addison Phillips, Jonathan Rosenne, Najib Tounsi, Debbie Garside, Mark Davis, Sarmad
2314Hussain, Ted Hardie, Konrad Lanz, Thomas Roessler, Lisa Dusseault, Julian Reschke, Giovanni
2315Campagna, Anne van Kesteren, Mark Nottingham, Erik van der Poel, Marcin Hanclik, Marcos Caceres, Roy
2316Fielding, Greg Wilkins, Pieter Hintjens, Daniel R. Tobias, Marko Martin, Maciej Stanchowiak, Wil
2317Tan, Yui Naruse, Michael A. Puls II, Dave Thaler, Tom Petch, John Klensin, Shawn Steele, Peter
2318Saint-Andre, Geoffrey Sneddon, Chris Weber, Alex Melnikov, Slim Amamou, SM, Tim Berners-Lee, Yaron
2319Goland, Sam Ruby, Adam Barth, Abdulrahman I. ALGhadir, Aharon Lanin, Thomas Milo, Murray Sargent,
2320Marc Blanchet, and Mykyta Yevstifeyev.</t>
2321</section>
2322
2323<section title="Main Changes Since RFC 3987">
2324  <t>This section describes the main changes since <xref target="RFC3987"></xref>.</t>
2325  <section title="Major restructuring of IRI processing model" anchor="forkChanges">
2326    <t>Major restructuring of IRI processing model to make scheme-specific translation
2327      necessary to handle IDNA requirements and for consistency with web implementations. </t>
2328    <t>Starting with IRI, you want one of:
2329      <list style="hanging">
2330        <t hangText="a"> IRI components (IRI parsed into UTF8 pieces)</t>
2331        <t hangText="b"> URI components (URI parsed into ASCII pieces, encoded correctly) </t>
2332        <t hangText="c"> whole URI  (for passing on to some other system that wants whole URIs) </t>
2333      </list></t>
2334   
2335    <section title="OLD WAY">
2336      <t><list style="numbers">
2337       
2338        <t>Pct-encoding on the whole thing to a URI.
2339          (c1) If you want a (maybe broken) whole URI, you might
2340          stop here.</t>
2341       
2342        <t>Parsing the URI into URI components.
2343          (b1) If you want (maybe broken) URI components, stop here.</t>
2344       
2345        <t> Decode the components (undoing the pct-encoding).
2346          (a) if you want IRI components, stop here.</t>
2347       
2348        <t> reencode:  Either using a different encoding some components
2349          (for domain names, and query components in web pages, which
2350          depends on the component, scheme and context), and otherwise
2351          using pct-encoding.
2352          (b2) if you want (good) URI components, stop here.</t>
2353       
2354        <t> reassemble the reencoded components.
2355          (c2) if you want a (*good*) whole URI stop here.</t>
2356      </list>
2357       
2358      </t>
2359     
2360    </section>
2361   
2362    <section title="NEW WAY">
2363      <t>
2364        <list style="numbers">
2365         
2366          <t> Parse the IRI into IRI components using the generic syntax.
2367            (a) if you want IRI components, stop here.</t>
2368         
2369          <t> Encode each components, using pct-encoding, IDN encoding, or
2370            special query part encoding depending on the component
2371            scheme or context. (b) If you want URI components, stop here.</t>
2372          <t> reassemble the a whole URI from URI components.
2373            (c) if you want a whole URI stop here.</t>
2374        </list></t>
2375    </section>
2376    <section title="Extension of Syntax">
2377      <t>Added the tag range (U+E0000-E0FFF) to the iprivate production.
2378        Some IRIs generated with the new syntax may fail to pass very strict checks
2379        relying on the old syntax. But characters in this range should be extremely infrequent
2380        anyway.</t>
2381    </section>
2382    <section title="More to be added"><t>TODO: There are more main changes that need to be
2383      documented in this section.</t></section>
2384</section>
2385
2386<section title="Change Log">
2387
2388<t>Note to RFC Editor: Please completely remove this section before publication.</t>
2389
2390<section title='Changes after draft-ietf-iri-3987bis-01'>
2391    <t>Changes from draft-ietf-iri-3987bis-01 onwards are available as changesets
2392      in the IETF tools subversion repository at
2393      http://trac.tools.ietf.org/wg/iri/trac/log/draft-ietf-iri-3987bis/draft-ietf-iri-3987bis.xml.</t>
2394</section>
2395 
2396<section title='Changes from draft-duerst-iri-bis-07 to draft-ietf-iri-3987bis-00'>
2397     <t>Changed draft name, date, last paragraph of abstract, and titles in change log, and added this section
2398     in moving from draft-duerst-iri-bis-07 (personal submission) to draft-ietf-iri-3987bis-00 (WG document).</t>
2399</section>
2400
2401<section title="Changes from -06 to -07 of draft-duerst-iri-bis">
2402  <t>Major restructuring of the processing model, see <xref target="forkChanges"></xref>.</t>
2403</section>
2404</section>
2405
2406<section title='Changes from -00 to -01'><t><list style="symbols">
2407  <t>Removed 'mailto:' before mail addresses of authors.</t>
2408  <t>Added "&lt;to be done&gt;" as right side of 'href-strip' rule. Fixed '|' to '/' for
2409    alternatives.</t>
2410</list></t>
2411</section>
2412
2413<section title="Changes from -05 to -06 of draft-duerst-iri-bis-00"><t><list style="symbols">
2414<t>Add HyperText Reference, change abstract, acks and references for it</t>
2415<t>Add Masinter back as another editor.</t>
2416<t>Masinter integrates HRef material from HTML5 spec.</t>
2417<t>Rewrite introduction sections to modernize.</t>
2418</list></t>
2419</section>
2420
2421<section title="Changes from -04 to -05 of draft-duerst-iri-bis">
2422  <t><list style="symbols">
2423    <t>Updated references.</t>
2424    <t>Changed IPR text to pre5378Trust200902.</t></list></t>
2425</section>
2426
2427<section title="Changes from -03 to -04 of draft-duerst-iri-bis">
2428  <t><list style="symbols">
2429    <t>Added explicit abbreviation for LEIRIs.</t>
2430    <t>Mentioned LEIRI references.</t>
2431    <t>Completed text in LEIRI section about tag characters and about specials.</t></list></t>
2432</section>
2433
2434<section title="Changes from -02 to -03 of draft-duerst-iri-bis">
2435  <t><list style="symbols">
2436    <t>Updated some references.</t>
2437    <t>Updated Michel Suginard's coordinates.</t></list></t>
2438</section>
2439
2440<section title="Changes from -01 to -02 of draft-duerst-iri-bis">
2441  <t><list style="symbols">
2442    <t>Added tag range to iprivate (issue private-include-tags-115).</t>
2443    <t>Added Specials (U+FFF0-FFFD) to Legacy Extended IRIs.</t></list></t>
2444</section>
2445<section title="Changes from -00 to -01 of draft-duerst-iri-bis">
2446  <t><list style="symbols">
2447    <t>Changed from "IRIs with Spaces/Controls" to "Legacy Extended IRI"
2448      based on input from the W3C XML Core WG.
2449      Moved the relevant subsections to the back and promoted them to a section.</t>
2450    <t>Added some text re. Legacy Extended IRIs to the security section.</t>
2451    <t>Added a IANA Consideration Section.</t>
2452    <t>Added this Change Log Section.</t>
2453    <t>Added a section about "IRIs with Spaces/Controls" (converting from a Note in RFC 3987).</t></list></t>
2454</section>
2455<section title="Changes from RFC 3987 to -00 of draft-duerst-iri-bis">
2456  <t><list>
2457    <t>Fixed errata (see http://www.rfc-editor.org/cgi-bin/errataSearch.pl?rfc=3987).</t></list></t>
2458</section>
2459</section>
2460</middle>
2461
2462<back>
2463<references title="Normative References">
2464
2465<reference anchor="ASCII">
2466<front>
2467<title>Coded Character Set -- 7-bit American Standard Code for Information
2468Interchange</title>
2469<author>
2470<organization>American National Standards Institute</organization>
2471</author>
2472<date year="1986"/>
2473</front>
2474<seriesInfo name="ANSI" value="X3.4"/>
2475</reference>
2476
2477<reference anchor="ISO10646">
2478<front>
2479<title>ISO/IEC 10646:2003: Information Technology -
2480Universal Multiple-Octet Coded Character Set (UCS)</title>
2481<author>
2482<organization>International Organization for Standardization</organization>
2483</author>
2484<date month="December" year="2003"/>
2485</front>
2486<seriesInfo name="ISO" value="Standard 10646"/>
2487</reference>
2488
2489&rfc2119;
2490&rfc3490;
2491&rfc3491;
2492&rfc3629;
2493&rfc3986;
2494
2495<reference anchor="STD68">
2496<front>
2497<title abbrev="ABNF">Augmented BNF for Syntax Specifications: ABNF</title>
2498<author initials="D." surname="Crocker" fullname="Dave Crocker"><organization/></author>
2499<author initials="P." surname="Overell" fullname="Paul Overell"><organization/></author>
2500<date month="January" year="2008"/></front>
2501<seriesInfo name="STD" value="68"/><seriesInfo name="RFC" value="5234"/>
2502</reference>
2503 
2504&rfc5890;
2505&rfc5891;
2506
2507<reference anchor="UNIV6">
2508<front>
2509<title>The Unicode Standard, Version 6.0.0 (Mountain View, CA, The Unicode Consortium, 2011, ISBN 978-1-936213-01-6)</title>
2510<author><organization>The Unicode Consortium</organization></author>
2511<date year="2010" month="October"/>
2512</front>
2513</reference>
2514
2515<reference anchor="UNI9" target="http://www.unicode.org/reports/tr9/tr9-13.html">
2516<front>
2517<title>The Bidirectional Algorithm</title>
2518<author initials="M." surname="Davis" fullname="Mark Davis"><organization/></author>
2519<date year="2004" month="March"/>
2520</front>
2521<seriesInfo name="Unicode Standard Annex" value="#9"/>
2522</reference>
2523
2524<reference anchor="UTR15" target="http://www.unicode.org/unicode/reports/tr15/tr15-23.html">
2525<front>
2526<title>Unicode Normalization Forms</title>
2527<author initials="M." surname="Davis" fullname="Mark Davis"><organization/></author>
2528<author initials="M.J." surname="Duerst" fullname="Martin Duerst"><organization/></author>
2529<date year="2008" month="March"/>
2530</front>
2531<seriesInfo name="Unicode Standard Annex" value="#15"/>
2532</reference>
2533
2534</references>
2535
2536<references title="Informative References">
2537
2538<reference anchor="BidiEx" target="http://www.w3.org/International/iri-edit/BidiExamples">
2539<front>
2540<title>Examples of bidirectional IRIs</title>
2541<author><organization/></author>
2542<date year="" month=""/>
2543</front>
2544</reference>
2545
2546<reference anchor="CharMod" target="http://www.w3.org/TR/charmod-resid">
2547<front>
2548<title>Character Model for the World Wide Web: Resource Identifiers</title>
2549<author initials="M." surname="Duerst" fullname="Martin Duerst"><organization/></author>
2550<author initials="F." surname="Yergeau" fullname="Francois Yergeau"><organization/></author>
2551<author initials="R." surname="Ishida" fullname="Richard Ishida"><organization/></author>
2552<author initials="M." surname="Wolf" fullname="Misha Wolf"><organization/></author>
2553<author initials="T." surname="Texin" fullname="Tex Texin"><organization/></author>
2554<date year="2004" month="November" day="25"/>
2555</front>
2556<seriesInfo name="World Wide Web Consortium" value="Candidate Recommendation"/>
2557</reference>
2558
2559<reference anchor="Duerst97" target="http://www.ifi.unizh.ch/mml/mduerst/papers/PDF/IUC11-UTF-8.pdf">
2560<front>
2561<title>The Properties and Promises of UTF-8</title>
2562<author initials="M.J." surname="Duerst" fullname="Martin Duerst"><organization/></author>
2563<date year="1997" month="September"/>
2564</front>
2565<seriesInfo name="Proc. 11th International Unicode Conference, San Jose" value=""/>
2566</reference>
2567
2568<reference anchor="Gettys" target="http://www.w3.org/DesignIssues/ModelConsequences">
2569<front>
2570<title>URI Model Consequences</title>
2571<author initials="J." surname="Gettys" fullname="Jim Gettys"><organization/></author>
2572<date month="" year=""/>
2573</front>
2574</reference>
2575
2576<reference anchor="HTML4" target="http://www.w3.org/TR/html401/appendix/notes.html#h-B.2">
2577<front>
2578<title>HTML 4.01 Specification</title>
2579<author initials="D." surname="Raggett" fullname="Dave Raggett"><organization/></author>
2580<author initials="A." surname="Le Hors" fullname="Arnaud Le Hors"><organization/></author>
2581<author initials="I." surname="Jacobs" fullname="Ian Jacobs"><organization/></author>
2582<date year="1999" month="December" day="24"/>
2583</front>
2584<seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2585</reference>
2586
2587<reference anchor="LEIRI" target="http://www.w3.org/TR/leiri/">
2588<front>
2589<title>Legacy extended IRIs for XML resource identification</title>
2590<author initials="H." surname="Thompson" fullname="Henry Thompson"><organization/></author>
2591<author initials="R." surname="Tobin"    fullname="Richard Tobin"><organization/></author>
2592<author initials="N." surname="Walsh" fullname="Norman Walsh"><organization/></author>
2593  <date year="2008" month="November" day="3"/>
2594
2595</front>
2596<seriesInfo name="World Wide Web Consortium" value="Note"/>
2597</reference>
2598
2599
2600&rfc2045;
2601&rfc2130;
2602&rfc2141;
2603&rfc2192;
2604&rfc2277;
2605&rfc2368;
2606&rfc2384;
2607&rfc2396;
2608&rfc2397;
2609&rfc2616;
2610&rfc1738;
2611&rfc2640;
2612&rfc3987;
2613&rfc6055;
2614<reference anchor='RFC4395bis'>
2615  <front>
2616    <title>Guidelines and Registration Procedures for New URI/IRI Schemes</title>
2617    <author initials='T.' surname='Hansen' fullname="Tony Hansen"><organization/></author>
2618    <author initials='T.' surname='Hardie' fullname="Ted Hardie"><organization/></author>
2619    <author initials='L.' surname='Masinter' fullname="Larry Masinter"><organization/></author>
2620    <date year="2010" month='September' day="30"/>
2621    <workgroup>IRI</workgroup>
2622  </front>
2623  <seriesInfo name="Internet-Draft" value="draft-hansen-iri-4395bis-irireg-00"/>
2624</reference>
2625 
2626 
2627<reference anchor="UNIXML" target="http://www.w3.org/TR/unicode-xml/">
2628<front>
2629<title>Unicode in XML and other Markup Languages</title>
2630<author initials="M.J." surname="Duerst" fullname="Martin Duerst"><organization/></author>
2631<author initials="A." surname="Freytag" fullname="Asmus Freytag"><organization/></author>
2632<date year="2003" month="June" day="18"/>
2633</front>
2634<seriesInfo name="Unicode Technical Report" value="#20"/>
2635<seriesInfo name="World Wide Web Consortium" value="Note"/>
2636</reference>
2637 
2638<reference anchor="UTR36" target="http://unicode.org/reports/tr36/">
2639<front>
2640<title>Unicode Security Considerations</title>
2641<author initials="M." surname="Davis" fullname="Mark Davis"><organization/></author>
2642<author initials="M." surname="Suignard" fullname="Michel Suignard"><organization/></author>
2643<date year="2010" month="August" day="4"/>
2644</front>
2645<seriesInfo name="Unicode Technical Report" value="#36"/>
2646</reference>
2647
2648<reference anchor="XLink" target="http://www.w3.org/TR/xlink/#link-locators">
2649<front>
2650<title>XML Linking Language (XLink) Version 1.0</title>
2651<author initials="S." surname="DeRose" fullname="Steve DeRose"><organization/></author>
2652<author initials="E." surname="Maler" fullname="Eve Maler"><organization/></author>
2653<author initials="D." surname="Orchard" fullname="David Orchard"><organization/></author>
2654<date year="2001" month="June" day="27"/>
2655</front>
2656<seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2657</reference>
2658
2659<reference anchor="XML1" target="http://www.w3.org/TR/REC-xml">
2660  <front>
2661    <title>Extensible Markup Language (XML) 1.0 (Forth Edition)</title>
2662    <author initials="T." surname="Bray" fullname="Tim Bray"><organization/></author>
2663    <author initials="J." surname="Paoli" fullname="Jean Paoli"><organization/></author>
2664    <author initials="C.M." surname="Sperberg-McQueen" fullname="C. M. Sperberg-McQueen">
2665      <organization/></author>
2666    <author initials="E." surname="Maler" fullname="Eve Maler"><organization/></author>
2667    <author initials="F." surname="Yergeau" fullname="Francois Yergeau"><organization/></author>
2668    <date day="16" month="August" year="2006"/>
2669  </front>
2670  <seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2671</reference>
2672
2673<reference anchor="XMLNamespace" target="http://www.w3.org/TR/REC-xml-names">
2674  <front>
2675    <title>Namespaces in XML (Second Edition)</title>
2676    <author initials="T." surname="Bray" fullname="Tim Bray"><organization/></author>
2677    <author initials="D." surname="Hollander" fullname="Dave Hollander"><organization/></author>
2678    <author initials="A." surname="Layman" fullname="Andrew Layman"><organization/></author>
2679    <author initials="R." surname="Tobin" fullname="Richard Tobin"><organization></organization></author>
2680    <date day="16" month="August" year="2006"/>
2681  </front>
2682  <seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2683</reference>
2684
2685<reference anchor="XMLSchema" target="http://www.w3.org/TR/xmlschema-2/#anyURI">
2686<front>
2687<title>XML Schema Part 2: Datatypes</title>
2688<author initials="P." surname="Biron" fullname="Paul Biron"><organization/></author>
2689<author initials="A." surname="Malhotra" fullname="Ashok Malhotra"><organization/></author>
2690<date year="2001" month="May" day="2"/>
2691</front>
2692<seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2693</reference>
2694
2695<reference anchor="XPointer" target="http://www.w3.org/TR/xptr-framework/#escaping">
2696<front>
2697<title>XPointer Framework</title>
2698<author initials="P." surname="Grosso" fullname="Paul Grosso"><organization/></author>
2699<author initials="E." surname="Maler" fullname="Eve Maler"><organization/></author>
2700<author initials="J." surname="Marsh" fullname="Jonathan Marsh"><organization/></author>
2701<author initials="N." surname="Walsh" fullname="Norman Walsh"><organization/></author>
2702<date year="2003" month="March" day="25"/>
2703</front>
2704<seriesInfo name="World Wide Web Consortium" value="Recommendation"/>
2705</reference>
2706
2707<reference anchor="HTML5" target="http://www.w3.org/TR/2009/WD-html5-20090423/">
2708<front>
2709<title>A vocabulary and associated APIs for HTML and XHTML</title>
2710<author initials="I." surname="Hickson" fullname="Ian Hickson"><organization>Google, Inc.</organization></author>
2711<author initials="D." surname="Hyatt" fullname="David Hyatt"><organization>Apple, Inc.</organization></author>
2712<date year="2009"  month="April" day="23"/>
2713</front>
2714<seriesInfo name="World Wide Web Consortium" value="Working Draft"/>
2715</reference>
2716
2717</references>
2718
2719</back>
2720</rfc>
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