draft-duerst-iri-00.txt   draft-duerst-iri-01.txt 
Network Working Group M. Duerst Network Working Group M. Duerst
Internet-Draft W3C/Keio University Internet-Draft W3C/Keio University
Expires: October 16, 2002 M. Suignard Expires: December 30, 2002 M. Suignard
Microsoft Corporation Microsoft Corporation
April 17, 2002 July 1, 2002
Internationalized Resource Identifiers (IRI) Internationalized Resource Identifiers (IRI)
draft-duerst-iri-00 draft-duerst-iri-01
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
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and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on October 16, 2002. This Internet-Draft will expire on December 30, 2002.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved. Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract Abstract
This document defines a new protocol element, the Internationalized This document defines a new protocol element, the Internationalized
Resource Identifier (IRI), as a complement to the URI [RFC2396]. An Resource Identifier (IRI), as a complement to the URI [RFC2396]. An
IRI is a sequence of characters from the Universal Character Set IRI is a sequence of characters from the Universal Character Set
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resources. resources.
The approach of defining a new protocol element was chosen, instead The approach of defining a new protocol element was chosen, instead
of extending or changing the definition of URIs, to allow a clear of extending or changing the definition of URIs, to allow a clear
distinction and to avoid incompatibilities with existing software. distinction and to avoid incompatibilities with existing software.
Guidelines for the use and deployment of IRIs in various protocols, Guidelines for the use and deployment of IRIs in various protocols,
formats, and software components that now deal with URIs are formats, and software components that now deal with URIs are
provided. provided.
Section 1 introduces concepts, definitions, and the scope of this
specification. Section 2 discusses the IRI syntax and conversion
between IRIs and URIs. Section 3 deals with limitations on
characters appropriate for use in IRIs, and with processing of IRIs.
Section 4 discusses software requirements for IRIs from an
operational viewpoint.
NOTE NOTE
This draft replaces draft-masinter-url-i18n-08.txt. This document is This document is a product of the Internationalization Working Group
a product of the Internationalization Working Group (I18N WG) of the (I18N WG) of the World Wide Web Consortium (W3C). For general
World Wide Web Consortium (W3C). For general discussion, please use discussion, please use the www-i18n-comments@w3.org mailing list
the www-i18n-comments@w3.org mailing list (publicly archived at (publicly archived at http://lists.w3.org/Archives/Public/www-i18n-
http://lists.w3.org/Archives/Public/www-i18n-comments/). For more comments/). For more information on the topic of this document,
information on the topic of this document, please also see [W3CIRI] please also see [W3CIRI] and [Duer01].
and [Duer01].
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Overview and Motivation . . . . . . . . . . . . . . . . . . 4 1.1 Overview and Motivation . . . . . . . . . . . . . . . . . . . 4
1.2 Applicability . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 Applicability . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Definitions . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 6 2. IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1 Summary of IRI syntax . . . . . . . . . . . . . . . . . . . 6 2.1 Summary of IRI Syntax . . . . . . . . . . . . . . . . . . . . 6
2.2 ABNF for IRI References and IRIs . . . . . . . . . . . . . . 6 2.2 ABNF for IRI References and IRIs . . . . . . . . . . . . . . . 6
2.3 Mapping of IRIs to URIs . . . . . . . . . . . . . . . . . . 8 2.3 IRI Equivalence and Normalization . . . . . . . . . . . . . . 9
2.3.1 When to convert from IRIs to URIs . . . . . . . . . . . . . 10 3. Relationship between IRIs and URIs . . . . . . . . . . . . . . 10
2.4 Converting URIs to IRIs . . . . . . . . . . . . . . . . . . 10 3.1 Mapping of IRIs to URIs . . . . . . . . . . . . . . . . . . . 11
3. Considerations for use of IRIs . . . . . . . . . . . . . . . 11 3.2 Converting URIs to IRIs . . . . . . . . . . . . . . . . . . . 12
3.1 IRI Character Limitations . . . . . . . . . . . . . . . . . 11 4. Bidirectional IRIs for Right-to-left Languages . . . . . . . . 13
3.2 Bidirectional IRIs for right-to-left languages . . . . . . . 13 4.1 Bidi IRI Structure . . . . . . . . . . . . . . . . . . . . . . 14
3.3 Processing IRIs . . . . . . . . . . . . . . . . . . . . . . 13 4.2 Visual Rendering of Bidi IRIs . . . . . . . . . . . . . . . . 14
4. Software requirements . . . . . . . . . . . . . . . . . . . 14 4.3 Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . . . . 15
4.1 URI/IRI software interfaces . . . . . . . . . . . . . . . . 14 5. Use of IRIs . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2 URI/IRI entry . . . . . . . . . . . . . . . . . . . . . . . 14 5.1 Limitations on UCS Character Allowed in IRI . . . . . . . . . 15
4.3 URI/IRI generation . . . . . . . . . . . . . . . . . . . . . 15 5.2 Software Interfaces and Protocols . . . . . . . . . . . . . . 16
4.4 URI/IRI selection . . . . . . . . . . . . . . . . . . . . . 16 5.3 Format of URIs and IRIs in Documents and Protocols . . . . . . 17
4.5 Display of URIs/IRIs . . . . . . . . . . . . . . . . . . . . 16 5.4 Relative IRI References . . . . . . . . . . . . . . . . . . . 17
4.6 Interpretation of URI/IRIs . . . . . . . . . . . . . . . . . 17 6. URI/IRI Processing Guidelines (informative) . . . . . . . . . 17
4.7 Transportation of URI/IRIs in document formats and protocols 18 6.1 URI/IRI Software Interfaces . . . . . . . . . . . . . . . . . 18
5. Upgrading strategy . . . . . . . . . . . . . . . . . . . . . 18 6.2 URI/IRI Entry . . . . . . . . . . . . . . . . . . . . . . . . 18
6. Security considerations . . . . . . . . . . . . . . . . . . 19 6.3 URI/IRI Generation . . . . . . . . . . . . . . . . . . . . . . 19
7. Acknowlegdements . . . . . . . . . . . . . . . . . . . . . . 20 6.4 URI/IRI Selection . . . . . . . . . . . . . . . . . . . . . . 19
References . . . . . . . . . . . . . . . . . . . . . . . . . 20 6.5 Display of URIs/IRIs . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 23 6.6 Interpretation of URIs and IRIs . . . . . . . . . . . . . . . 20
Full Copyright Statement . . . . . . . . . . . . . . . . . . 24 6.7 Upgrading Strategy . . . . . . . . . . . . . . . . . . . . . . 21
7. Security Considerations . . . . . . . . . . . . . . . . . . . 21
8. Change log . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9. Acknowlegdements . . . . . . . . . . . . . . . . . . . . . . . 23
References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 26
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 27
1. Introduction 1. Introduction
1.1 Overview and Motivation 1.1 Overview and Motivation
A URI is defined in [RFC2396] as a sequence of characters chosen from A URI is defined in [RFC2396] as a sequence of characters chosen from
a limited subset of the repertoire of US-ASCII characters. a limited subset of the repertoire of US-ASCII characters.
The characters in URIs are frequently used for representing words of The characters in URIs are frequently used for representing words of
natural languages. Such usage has many advantages: such URIs are natural languages. Such usage has many advantages: such URIs are
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This document defines a new protocol element, called IRI This document defines a new protocol element, called IRI
(Internationalized Resource Identifier), by extending the syntax of (Internationalized Resource Identifier), by extending the syntax of
URIs to a much wider repertoire of characters. It also defines URIs to a much wider repertoire of characters. It also defines
"internationalized" versions corresponding to other constructs from "internationalized" versions corresponding to other constructs from
[RFC2396], such as URI references. [RFC2396], such as URI references.
Using characters outside of A-Z in IRIs brings with it some Using characters outside of A-Z in IRIs brings with it some
difficulties; a discussion of potential problems and workarounds can difficulties; a discussion of potential problems and workarounds can
be found in the later sections of this document. be found in the later sections of this document.
URIs often contain Internet host names embedded within them. There
is an ongoing discussion of internationalization and host names; the
specific issues of the relationship of IRIs and possible future
"internationalized" host names are not discussed here. (See [IDN-
URI] for a separate proposal.)
1.2 Applicability 1.2 Applicability
IRIs are designed to be compatible with recent recommendations on URI IRIs are designed to be compatible with recent recommendations on URI
syntax [RFC2718]. Practical use of IRIs (or IRI references) in place syntax [RFC2718]. The compatibility is provided by providing a well
of URIs (or URI references) depends on the following conditions being defined and deterministic mapping from the IRI character sequence to
met: the functionally equivalent URI character sequence. Practical use of
IRIs (or IRI references) in place of URIs (or URI references) depends
on the following conditions being met:
a. The protocol or format element used should be explicitly a. The protocol or format element used should be explicitly
designated to carry IRIs. That is, the intent is not to designated to carry IRIs. That is, the intent is not to
introduce IRIs into contexts that are not defined to accept introduce IRIs into contexts that are not defined to accept
them. For examlpe, XML schema [XMLSchema] has an explicit type them. For example, XML schema [XMLSchema] has an explicit type
"anyURI" that can be used to designate IRIs. "anyURI" that designates the use of IRIs.
b. The protocol or format carrying the IRIs must have a mechanism b. The protocol or format carrying the IRIs must have a mechanism
to represent the wide range of characters used in IRIs, either to represent the wide range of characters used in IRIs, either
natively or by some protocol- or format-specific escaping natively or by some protocol- or format-specific escaping
mechanism (for example numeric character references in [XML1]). mechanism (for example numeric character references in [XML1]).
c. Either by definition for all the URIs of a specific URI c. Either by definition for all the URIs of a specific URI
scheme, or at least for some specific URIs, the encoding of scheme, or at least for some specific URIs, the encoding of
non-ASCII characters has to be based on UTF-8. For new URI non-ASCII characters has to be based on UTF-8. For new URI
schemes, this is recommended in [RFC2718]. This allows IRIs to schemes, this is recommended in [RFC2718]. This allows IRIs to
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a piece of a URI (reference), such as the fragment identifier. a piece of a URI (reference), such as the fragment identifier.
In cases and for pieces where an encoding other than UTF-8 is used, In cases and for pieces where an encoding other than UTF-8 is used,
and for raw binary data encoded in URIs (see [RFC2397]), the octets and for raw binary data encoded in URIs (see [RFC2397]), the octets
have to be %-escaped. In these situations, the ability of IRIs to have to be %-escaped. In these situations, the ability of IRIs to
directly represent a wide character repertoire cannot be used. directly represent a wide character repertoire cannot be used.
1.3 Definitions 1.3 Definitions
The following definitions are used in this document; they follow the The following definitions are used in this document; they follow the
terms in [RFC2130] and [RFC2277]: terms in [RFC2130], [RFC2277] and [ISO10646]:
character: An abstract object with a separate identity. For character: A member of a set of elements used for the
example, "LATIN CAPITAL LETTER A" names a character. organization, control, or representation of data. For example,
"LATIN CAPITAL LETTER A" names a character.
octet: 8 bits octet: an ordered sequence of eight bits considered as a unit
character repertoire: A set of characters (in the mathematical character repertoire: A set of characters (in the mathematical
sense) sense)
sequence of characters: A sequence (one after another) of sequence of characters: A sequence (one after another) of
characters characters
sequence of octets: A sequence (one after another) of octets sequence of octets: A sequence (one after another) of octets
(character) encoding: A method of representing a sequence of (character) encoding: A method of representing a sequence of
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sequence of octets. This definition accommodates the fact that IRIs sequence of octets. This definition accommodates the fact that IRIs
may be written on paper or read over the radio as well as being may be written on paper or read over the radio as well as being
transmitted over the network. The same IRI may be represented as transmitted over the network. The same IRI may be represented as
different sequences of octets in different protocols or documents if different sequences of octets in different protocols or documents if
these protocols or documents use different character encodings and/or these protocols or documents use different character encodings and/or
transfer encodings. Using the same character encoding as the transfer encodings. Using the same character encoding as the
containing protocol or document assures that the characters in the containing protocol or document assures that the characters in the
IRI can be handled (searched, converted, displayed,...) in the same IRI can be handled (searched, converted, displayed,...) in the same
way as the rest of the protocol or document. way as the rest of the protocol or document.
2.1 Summary of IRI syntax 2.1 Summary of IRI Syntax
IRIs are defined similarly to URIs in [RFC2396] (as modified by IRIs are defined similarly to URIs in [RFC2396] (as modified by
[RFC2732]), but the class of unreserved characters is extended by [RFC2732] and [IDNURI]), but the class of unreserved characters is
adding all the characters of the UCS (Universal Character Set, extended by adding all the characters of the UCS (Universal Character
[ISO10646]) beyond U+0080, subject to the limitations given in Set, [ISO10646]) beyond U+0080, subject to the limitations given in
Section 3. Section 5.1.
Otherwise, the syntax and use of components and reserved characters Otherwise, the syntax and use of components and reserved characters
is the same as that in [RFC2396]. All the operations defined in is the same as that in [RFC2396]. All the operations defined in
[RFC2396], such as the resolution of relative URIs, can be applied to [RFC2396], such as the resolution of relative URIs, can be applied to
IRIs by IRI-processing software in exactly the same way as this is IRIs by IRI-processing software in exactly the same way as this is
done to URIs by URI-processing software. done to URIs by URI-processing software.
Characters outside the US-ASCII range MUST NOT be used for Characters outside the US-ASCII range MUST NOT be used for
syntactical purposes such as to delimit components in newly defined syntactical purposes such as to delimit components in newly defined
schemes. As an example, it is not allowed to use U+00A2, CENT SIGN, schemes. As an example, it is not allowed to use U+00A2, CENT SIGN,
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is in the 'unreserved' category. is in the 'unreserved' category.
2.2 ABNF for IRI References and IRIs 2.2 ABNF for IRI References and IRIs
While it might be possible to define IRI references and IRIs merely While it might be possible to define IRI references and IRIs merely
by their transformation to URIs, they can also be accepted and by their transformation to URIs, they can also be accepted and
processed directly. Therefore, an ABNF definition for IRI references processed directly. Therefore, an ABNF definition for IRI references
(which are the most general concept and the start of the grammar) and (which are the most general concept and the start of the grammar) and
IRIs is given here. IRIs is given here.
The following rules are different form [RFC2396]: The following rules are different from [RFC2396]:
IRI-reference = [ absoluteIRI | relativeIRI ] [ "#" ifragment ] IRI-reference = [ absoluteIRI | relativeIRI ] [ "#" ifragment ]
absoluteIRI = scheme ":" ( ihier_part | iopaque_part ) absoluteIRI = scheme ":" ( ihier_part | iopaque_part )
relativeIRI = ( inet_path | iabs_path | irel_path ) relativeIRI = ( inet_path | iabs_path | irel_path )
[ "?" iquery ] [ "?" iquery ]
ihier_part = ( inet_path | iabs_path ) [ "?" iquery ] ihier_part = ( inet_path | iabs_path ) [ "?" iquery ]
iopaque_part = iric_no_slash *iric iopaque_part = iric_no_slash *iric
iric_no_slash = iunreserved | escaped | ";" | "?" | ":" | "@" | iric_no_slash = iunreserved | escaped | ";" | "?" | ":" | "@" |
"&" | "=" | "+" | "$" | "," "&" | "=" | "+" | "$" | ","
inet_path = "//" iauthority [ iabs_path ] inet_path = "//" iauthority [ iabs_path ]
iabs_path = "/" ipath_segments iabs_path = "/" ipath_segments
irel_path = irel_segment [ iabs_path ] irel_path = irel_segment [ iabs_path ]
irel_segment = 1*( iunreserved | escaped | irel_segment = 1*( iunreserved | escaped |
";" | "@" | "&" | "=" | "+" | "$" | "," ) ";" | "@" | "&" | "=" | "+" | "$" | "," )
iauthority = server | ireg_name iauthority = iserver | ireg_name
ireg_name = 1*( iunreserved | escaped | "$" | "," | ireg_name = 1*( iunreserved | escaped | "$" | "," |
";" | ":" | "@" | "&" | "=" | "+" ) ";" | ":" | "@" | "&" | "=" | "+" )
iserver = [ [ userinfo "@" ] ihostport ]
iuserinfo = *( iunreserved | escaped |
";" | ":" | "&" | "=" | "+" | "$" | "," )
ihostport = ihost [ ":" port ]
ihost = ihostname | IPv4address | IPv6reference
ihostname = << as specified by [IDNA] >>
ipath_segments = isegment *( "/" isegment ) ipath_segments = isegment *( "/" isegment )
isegment = *ipchar *( ";" iparam ) isegment = *ipchar *( ";" iparam )
iparam = *ipchar iparam = *ipchar
ipchar = iunreserved | escaped | ipchar = iunreserved | escaped |
":" | "@" | "&" | "=" | "+" | "$" | "," ":" | "@" | "&" | "=" | "+" | "$" | ","
iquery = *iric iquery = *iric
ifragment = *iric ifragment = *iric
iric = reserved | iunreserved | escaped iric = reserved | iunreserved | escaped
iunreserved = ichar | unreserved iunreserved = ichar | unreserved
ichar = << character of the UCS [ISO10646] of beyond ichar = << allowed character of the UCS [ISO10646] >> | space | delims | unwise
U+009F, subject to the limitations in
Section 3.1. >> | space | delims | unwise
Note that the space character and various delimiters are allowed in Note that the space character and various delimiters are allowed in
IRIs and IRI references. This is further discussed in section 3.1, IRIs and IRI references. This is further discussed in Section 5.1.
point b.
The following describe the allowed characters of the UCS [ISO10646]
using the UCS-4 encoding notation for these characters:
U+00A0-U+D7FF
U+F900-U+FDCF
U+FDF0-U+FFEF
U+10000-U+1FFFD
U+20000-U+2FFFD
U+30000-U+3FFFD
U+40000-U+4FFFD
U+50000-U+5FFFD
U+60000-U+6FFFD
U+70000-U+7FFFD
U+80000-U+8FFFD
U+90000-U+9FFFD
U+A0000-U+AFFFD
U+B0000-U+BFFFD
U+C0000-U+CFFFD
U+D0000-U+DFFFD
U+E1000-U+EFFFD
The following are the same as [RFC2396] as modified by [RFC2732]: The following are the same as [RFC2396] as modified by [RFC2732]:
reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" | reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" |
"$" | "," | "[" | "]" "$" | "," | "[" | "]"
unreserved = alphanum | mark unreserved = alphanum | mark
mark = "-" | "_" | "." | "!" | "~" | "*" | "'" | mark = "-" | "_" | "." | "!" | "~" | "*" | "'" |
"(" | ")" "(" | ")"
escaped = "%" HEXDIG HEXDIG escaped = "%" hex hex
server = [ [ userinfo "@" ] hostport ] hex = digit | "A" | "B" | "C" | "D" | "E" | "F" |
userinfo = *( unreserved | escaped | "a" | "b" | "c" | "d" | "e" | "f"
";" | ":" | "&" | "=" | "+" | "$" | "," )
hostport = host [ ":" port ]
host = hostname | IPv4address | IPv6reference
IPv6reference = "[" IPv6address "]" IPv6reference = "[" IPv6address "]"
hostname = *( domainlabel "." ) toplabel [ "." ]
domainlabel = alphanum | alphanum *( alphanum | "-" ) alphanum
toplabel = alpha | alpha *( alphanum | "-" ) alphanum
IPv6address = hexpart [ ":" IPv4address ] IPv6address = hexpart [ ":" IPv4address ]
IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT
hexpart = hexseq | hexseq "::" [ hexseq ] | "::" hexpart = hexseq | hexseq "::" [ hexseq ] | "::"
[ hexseq ] [ hexseq ]
hexseq = hex4 *( ":" hex4) hexseq = hex4 *( ":" hex4)
hex4 = 1*4HEXDIG hex4 = 1*4hex
port = *DIGIT port = *DIGIT
scheme = alpha *( alpha | digit | "+" | "-" | "." ) scheme = alpha *( alpha | digit | "+" | "-" | "." )
alphanum = alpha | digit alphanum = alpha | digit
alpha = lowalpha | upalpha alpha = lowalpha | upalpha
lowalpha = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" | lowalpha = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" |
"j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" | "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" |
"s" | "t" | "u" | "v" | "w" | "x" | "y" | "z" "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z"
upalpha = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" | upalpha = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" |
"J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" | "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" |
"S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z" "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z"
space = <US-ASCII coded character 20 hexadecimal> digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
"8" | "9"
space = << US-ASCII coded character 20 hexadecimal >>
delims = "<" | ">" | "#" | "%" | <"> delims = "<" | ">" | "#" | "%" | <">
unwise = "{" | "}" | "|" | "\" | "^" | "`" unwise = "{" | "}" | "|" | "\" | "^" | "`"
2.3 Mapping of IRIs to URIs 2.3 IRI Equivalence and Normalization
There is no general rule or procedure to decide whether two arbitrary
IRIs are equivalent or not (i.e. refer to the same resource or not).
Two IRIs that look almost the same may refer to different resources.
Two IRIs that look completely different may refer to, and resolve to,
the same resource.
In some scenarios, such as XML Namespaces ([XMLNamespace]), a
definite answer to the question of IRI equivalence is needed that is
independent of the scheme used and always can be calculated quickly
and without accessing a network. In such cases, two IRIs SHOULD be
defined as equivalent if and only if they are character-by-character
equivalent (which is the same as byte-by-byte equivalent if the
character encoding for both IRIs is the same). In such a case, the
comparison function MUST NOT map the IRIs to URIs.
It follows from the above that IRIs SHOULD NOT be modified when being
transported.
For actual resolution, differences in escaping (except for the
escaping of reserved characters) MUST always result in the same
resource. For example, foo://example.com/XML, foo://example.com/
XM%4C, and foo://example.com/XM%4c must resolve to the same resource.
If this kind of equivalence is to be tested, the escaping of both
IRIs to be compared has to be aligned, for example by converting both
IRIs to URIs (see Section 3.1) and making sure that the case of the
hexadecimal characters in the %-escape is always the same. Such
conversions MUST only be done on the fly, without changing the
original IRI.
Specific schemes and resolution mechanisms may define additional
equivalences. For a specific scheme, two IRIs that e.g. differ only
by case may be equivalent. However, this document does not deal with
scheme-specific issues.
The Unicode Standard [UNIV3] defines various equivalences between
sequences of characters for various purposes. Unicode Standard Annex
#15 [UNI15] defines various Normalization Forms for these
equivalences. IRIs SHOULD be created using the Normalization Form C
(NFC). When an IRI is created in an UCS-based encoding without the
end-user being aware of or interested in Unicode normalization
issues, the IRI MUST be created using the normalization form NFC.
Equivalence of IRIs MUST rely on the IRIs being appropriately pre-
normalized, rather than applying normalization, except when
converting from a non-UCS-based encoding to an UCS-based encoding,
where a normalizing transcoder using NFC MUST be used.
Various IRI schemes may allow the usage of International Domain Names
(IDN) [IDNA]. When in use in IRIs, those names SHOULD be validated
using the rules defined by [Nameprep]. An IRI containing an invalid
IDN cannot successfully be resolved. For legibility purposes, IDN
components of IRIs SHOULD not be converted into ASCII Compatible
Encoding (ACE). However, this conversion may be applied when mapping
an IRI into an URI, see Section 3.1.
3. Relationship between IRIs and URIs
IRIs are meant to replace URIs in identifying resources for
protocols, formats and software components which use a UCS-based
character repertoire. These protocols and components may never need
to use URIs directly, especially when the resource identifier is used
simply for identification purposes. However, when the resource
identifier is used for resource retrieval, it is in many cases
necessary to determine the associated URI because most retrieval
mechanisms currently only are defined for URIs. (Additional
rationale is given in Section 3.1.)
3.1 Mapping of IRIs to URIs
This section defines how to map an IRI to a URI. Everything in this This section defines how to map an IRI to a URI. Everything in this
section applies also to IRI references and URI references, as well as section applies also to IRI references and URI references, as well as
components thereof (for example fragment identifiers). components thereof (for example fragment identifiers).
This mapping has two purposes: This mapping has two purposes:
a) Syntactical: Many URI schemes and components define additional a) Syntactical: Many URI schemes and components define additional
syntactical restrictions not captured in Section 2.2. Such syntactical restrictions not captured in Section 2.2. Such
restrictions can be applied to IRIs by noting that IRIs are restrictions can be applied to IRIs by noting that IRIs are
only valid if they map to syntactically valid URIs. This means only valid if they map to syntactically valid URIs. This means
that such syntactical restrictions do not have to be defined that such syntactical restrictions do not have to be defined
again on the IRI level. again on the IRI level.
b) Interpretational: URIs identify resources in various ways. b) Interpretational: URIs identify resources in various ways.
IRIs also identify resources. The resource that an IRI IRIs also identify resources. When the IRI is used simply for
identifies is the same as the one identified by the URI indentification purposes, it is not necessary to map the IRI to
obtained after converting the IRI according to the procedure an URI (see Section 2.3). However, when an IRI is used for
defined here. This means that there is no need to define the resource retrieval, the resource that the IRI locates is the
association between identifier and resource again on the IRI same as the one located by the URI obtained after converting
the IRI according to the procedure defined here. This means
that there is no need to define resolution again on the IRI
level. level.
This mapping is accomplished in two parts. Part A) is skipped if the This mapping is accomplished in two steps.
input is already in a UCS-based encoding (for example UTF-8 or UTF-
16). In that case, it is assumed that the IRI is already in NFC.
Part A) This part has three variants, depending on where the input
comes from.
Variant 1) a) Start with an IRI written on paper or read out
loud, or otherwise represented as a sequence of
characters independent of any encoding. b) Represent the
IRI characters as a sequence of characters from the UCS.
c) Normalize the character sequence according to
Normalization Form C (NFC), as defined in [UNI15]. (See
further discussion in Section 3.1.)
Note: In practice, steps b) and c) will often be Step 1) This step generates a UCS-based encoding from the original
performed together, for example by using a keyboard or IRI format. This step has three variants, depending on the
other input mechanism that is designed to produce NFC. form of the input.
Variant 2) a) Start with an IRI in some digital Variant A) If the IRI is written on paper or read out loud,
representation (e.g. an octet stream) in some non- or otherwise represented as a sequence of characters
Unicode encoding. b) Represent the IRI characters as a independent of any encoding: Represent the IRI as a
sequence of characters from the UCS. c) Normalize the sequence of characters from the UCS normalized according
character sequence according to Normalization Form C, as to Normalization Form C (NFC, [UNI15]).
defined in [UNI15]. (See further discussion in Section
3.1.)
Note: In practice, steps b) and c) will often be Variant B) If the IRI is in some digital representation
performed together, for example by using a transcoder (e.g. an octet stream) in some non-Unicode encoding:
that produces output in NFC. Convert the IRI to a sequence of characters from the UCS
normalized according to NFC.
Variant 3) a) Start with an IRI in an Unicode-based encoding Variant C) If the IRI is in an Unicode-based encoding (for
(for example UTF-8 or UTF-16). Move directly to Part 2. example UTF-8 or UTF-16): Do not normalize. Move
It is assumed that the IRI is already in NFC. directly to Step 2.
Part B) For each character that is disallowed in URI references, Step 2) For each character that is disallowed in URI references,
apply steps a) through c) below. The disallowed characters apply steps 1) through 3) below. The disallowed characters
consist of all non-ASCII characters, plus the excluded consist of all non-ASCII characters, plus the excluded
characters listed in Section 2.4 of [RFC2396], except for the characters listed in Section 2.4 of [RFC2396], except for the
number sign (#) and percent sign (%) and the square bracket number sign (#) and percent sign (%) and the square bracket
characters re-allowed in [RFC2732]. characters re-allowed in [RFC2732].
1) Convert the character to a sequence of one or more octets 1) Convert the character to a sequence of one or more octets
using UTF-8 [RFC2279]. using UTF-8 [RFC2279].
2) Convert each octet to %HH, where HH is the hexadecimal 2) Convert each octet to %HH, where HH is the hexadecimal
notation of the octet value. Note: This is identical to notation of the octet value. Note: This is identical to
the escaping mechanism in Section 2.4.1 of [RFC2396]. the escaping mechanism in Section 2.4.1 of [RFC2396].
3) Replace the original character by the resulting character 3) Replace the original character by the resulting character
sequence. sequence.
Note that in this process (in step B3), characters allowed in URI Note that in this process (in step 2.3), characters allowed in URI
references and existing escape sequences are not escaped further. references and existing escape sequences are not escaped further.
(This mapping is similar to, but different from, the escaping applied (This mapping is similar to, but different from, the escaping applied
when including arbitrary content into some part of a URI.) when including arbitrary content into some part of a URI.)
The above mapping produces a URI fully conforming to [RFC2396] out of The above mapping produces a URI fully conforming to [RFC2396] (as
each IRI. The mapping is also an identity transformation for URIs amended by [RFC2732] and [IDNURI]) out of each IRI. The mapping is
and is idempotent--applying the mapping a second time will not change also an identity transformation for URIs and is idempotent --
anything. Every URI is therefore by definition an IRI. Section 2.3 applying the mapping a second time will not change anything. Every
gives details about when exactly to convert from an IRI to an URI. URI is therefore by definition an IRI.
2.3.1 When to convert from IRIs to URIs
The mapping from IRIs to URIs SHOULD only be applied when necessary, Note: For backwards compatibility with infrastructure that does not
and as late as possible. implement the updates of [IDNURI], converters MAY also convert the
'ihostname' part of an IRI using the ToASCII operation specified in
Section 4.1 of [IDNA] between Step 1 and Step 2. Note that the
ToASCII operation may fail. Note that Internationalized Domain Names
may be contained in parts of an IRI other than the 'ihostname' part.
2.4 Converting URIs to IRIs 3.2 Converting URIs to IRIs
In some situations, it may be desirable to try to convert a URI into In some situations, it may be desirable to try to convert a URI into
an equivalent IRI. This section gives a procedure to do such a an equivalent IRI. This section gives a procedure to do such a
conversion. In general, the IRI to URI mapping is many-to-one, so conversion. The conversion described in this section will always
the conversion is not invertible. The conversion described in this give an IRI which maps back to the URI that was used as an input for
section will always give an IRI which maps back to the URI that was the conversion, but perhaps not exactly the original IRI (if there
used as an input for the conversion, but perhaps not exactly the ever was one).
original IRI (if there ever was one). In general, URI to IRI
conversion removes escape sequences, but not all escaping can be URI to IRI conversion removes escape sequences, but not all escaping
eliminated. There are many reasons for this: can be eliminated. There are many reasons for this:
a. Some escape sequences are necessary to distinguish escaped and a. Some escape sequences are necessary to distinguish escaped and
unescaped uses of reserved characters. unescaped uses of reserved characters.
b. Some escape sequences cannot be interpreted as sequences of b. Some escape sequences cannot be interpreted as sequences of
UTF-8 octets. UTF-8 octets.
(Note: Due to the regularities in the octet patterns of UTF-8, (Note: Due to the regularities in the octet patterns of UTF-8,
there is a very high probability, but no guarantee, that escape there is a very high probability, but no guarantee, that escape
sequences that can be interpreted as sequences of UTF-8 octets sequences that can be interpreted as sequences of UTF-8 octets
actually originated from UTF-8. For a detailed discussion of actually originated from UTF-8. For a detailed discussion, see
the odds, see [Duer97].) [Duer97].)
c. The conversion may result in a character that is not c. The conversion may result in a character that is not
appropriate in an IRI. See section 3.1 for further details. appropriate in an IRI. See Section 5.1 for further details.
Conversion from a URI to an IRI is done using the following steps (or Conversion from a URI to an IRI is done using the following steps (or
any other algorithm that produces the same result): any other algorithm that produces the same result):
1) Represent the URI as a sequence of octets in US-ASCII. 1) Represent the URI as a sequence of octets in US-ASCII.
2) Convert all hexadecimal escapes (% followed by two hexadecimal 2) Convert all hexadecimal escapes (% followed by two hexadecimal
digits) of %80 and higher to the corresponding octets. digits) of %80 and higher to the corresponding octets.
3) Re-escape any octets that are not part of a strictly legal UTF- 3) Re-escape any octets that are not part of a strictly legal UTF-
8 octet sequence. 8 octet sequence.
4) Re-escape all octets that in UTF-8 reperesent characters that 4) Re-escape all octets that in UTF-8 represent characters that
are not appropriate according to Section 3.1. are not appropriate according to Section 5.1.
5) Interpret the resulting octet sequence as a sequence of
characters encoded in UTF-8.
This procedure will convert as many escaped non-ASCII characters as This procedure will convert as many escaped non-ASCII characters as
possible to characters in an IRI. Because there are some choices possible to characters in an IRI. Because there are some choices
when applying step 3) (see Section 3.1), results may differ. when applying step 4) (see Section 5.1), results may differ.
3. Considerations for use of IRIs 4. Bidirectional IRIs for Right-to-left Languages
3.1 IRI Character Limitations Some UCS characters, such as those used in the Arabic and Hebrew
script, have an inherent right-to-left writing direction. IRIs
containing such characters (called bidirectional IRIs or Bidi IRIs)
require additional attention because of the non-trivial relation
between logical representation (used for digital representation as
well as when reading/spelling) and visual representation (used for
display/printing).
Not all characters of the UCS are appropriate for use as resource 4.1 Bidi IRI Structure
identifiers. This section discusses the limitations on characters
and character sequences usable for IRIs. The considerations in this
section are relevant when creating IRIs and when converting from URIs
to IRIs.
Because of the large and increasing number of characters in the UCS IRIs have an inherent structure that distinguishes structural
and the large number of situations where IRIs can be used, it is characters (usually punctuation such as '@', '.', ':', '/', and so
impossible to give general rules for which characters should be on) called delimiters and payload components (usually consisting
avoided. The following considerations are relevant: mostly of letters and digits).
ISSUE: Exact definition of components.
In their internal digital representation, i.e. stored or transmitted
for resolution, bidirectional IRIs MUST be in full logical order both
for the overall structure as well as for the individual components.
They MUST conform directly to the IRI syntax rules (which includes
the rules relevant to their scheme). This is necessary to make sure
that bidirectional IRIs can be processed in the same way as other
IRIs.
The components have the following restrictions:
1) A component MUST NOT not use both right-to-left and left-to-
right characters.
2) A component MUST NOT contain bidirectional formatting
characters.
3) A component using right-to-left characters MUST NOT use any
other class of characters (e.g. neutrals or numbers).
Note: Restrictions 1) and 2) are not very severe, in that they do not
overly restrict useful identifiers. Also, trying to remove it would
make it impossible for humans to predict the logical sequence of
characters inside a single component. On the other hand, it would be
very desirable to remove or at least soften restriction 3).
Otherwise, it is impossible to combine Arabic or Hebrew letters with
numbers, or to use a hyphen between two subcomponents of an Arabic
component to avoid the cursive connection of the two subcomponents.
To a certain extent, softening this restriction should be easily
possible by adding additional formatting characters in well defined
ways similar to the provisions in Section 4.2. Feedback on this
issue is particularly welcome.
4.2 Visual Rendering of Bidi IRIs
Bidirectional IRIs MUST be rendered visually by rendering each
component and each structural character from left to right. They
MUST render each component according to its natural direction (i.e.
left-to-right for components with left-to-right characters, right-to-
left for components with right-to-left characters).
ISSUE: The alternative is to display a series of right-to-left
components in their natural (right-to-left) order. This has the
advantage that it will often be easier for native people to read the
components in the right order. The restrictions on individual
components change. In some cases, the correct visual rendering is
automatic (i.e. exactly the same as with the Unicode algorithm), and
so in these cases, no bidi formatting characters have to be added.
In a textual context, i.e. assuming rendering by the Unicode
bidirectional algorithm, the visual rendering backing store is done
as follows:
The visual representation uses some of the following Bidi formatting
characters described by using a XML-style entity notation:
&lrm; U+200E LEFT-TO-RIGHT MARK
&rlm; U+200F RIGHT-TO-LEFT MARK
&lre; U+202A LEFT-TO-RIGHT EMBEDDING
&rle; U+202B RIGHT-TO-LEFT EMBEDDING
&pdf; U+202C POP DIRECTIONAL FORMATTING
&lro; U+202D LEFT-TO-RIGHT OVERRIDE
&rlo; U+202E RIGHT-TO-LEFT OVERRIDE
Each component with right-to-left characters is preceded and
followed by an &lrm;. This left-to-right mark provides a left-
to-right context to intervening syntactic characters.
If the overall context (base directionality) is right-to-left,
the identifier is preceded by an &lre; and followed by a &pdf;.
This makes sure that the components of the identifier are
rendered in left-to-right order. This may also be done by
using the equivalent features of a higher-order protocol (e.g.
by using the dir='ltr' attribute in HTML).
4.3 Input of Bidi IRIs
Bidi input methods MUST generate Bidi IRIs in logical order while
rendering them according to Section 4.2. During input, rendering
should be updated after every new character that is input to avoid
end user confusion.
5. Use of IRIs
5.1 Limitations on UCS Character Allowed in IRI
This section discusses the limitations on characters and character
sequences usable for IRIs. The considerations in this section are
relevant when creating IRIs and when converting from URIs to IRIs.
a. The repertoire of characters allowed in each IRI component is a. The repertoire of characters allowed in each IRI component is
limited by the definition of that component. For example, the limited by the definition of that component. For example, the
definition of host names in URIs does not currently allow hex definition of the scheme component does not allow characters
escapes, or "_", or many other punctuation characters. This beyond US-ASCII.
specification does not relax those limits, and so IRIs
currently may not contain any non-ASCII characters in host
names. This specification likewise does not extended the
scheme component beyond US-ASCII.
(Note: In accordance with URI practice, generic IRI software (Note: In accordance with URI practice, generic IRI software
cannot and should not check for such limitations.) cannot and should not check for such limitations.)
b. In the URI syntax, characters that are likely to be used to b. In the URI syntax, characters that are likely to be used to
delimit URIs in text and print ("space", "delims", and delimit URIs in text and print ("space", "delims", and
"unwise") were excluded. They are included in the IRI syntax, "unwise") were excluded. They are included in the IRI syntax,
for the following reasons: for the following reasons:
1) The syntax includes many other characters that are not 1) The syntax includes many other characters that are not
skipping to change at page 12, line 34 skipping to change at page 16, line 35
3) It is very convenient in some cases, for example for 3) It is very convenient in some cases, for example for
XPointers in XML attributes. XPointers in XML attributes.
4) Considering context is already necessary in the case of 4) Considering context is already necessary in the case of
URIs, for example for "&" in XML. URIs, for example for "&" in XML.
However, these characters should be used carefully. Whenever However, these characters should be used carefully. Whenever
there is a chance that an IRI will be used in a component where there is a chance that an IRI will be used in a component where
these characters can be harmful, they should be escaped. these characters can be harmful, they should be escaped.
c. The UCS contains many areas of "characters" which have no c. The UCS contains many areas of characters for which there are
well-established way of inputting them. These should be
avoided. Characters that fall into this category include
Dingbats, Mathematical and other symbols, ligatures and
presentation forms.
d. The UCS contains many areas of characters for which there are
strong visual look-alikes. Because of the likelihood of strong visual look-alikes. Because of the likelihood of
transcription errors, these also should be avoided. This transcription errors, these also should be avoided. This
includes the full-width equivalents of ASCII characters, half- includes the full-width equivalents of ASCII characters, half-
width Katakana characters for Japanese, and many others. This width Katakana characters for Japanese, and many others. This
also includes many look-alikes of "space", "delims", and also includes many look-alikes of "space", "delims", and
"unwise", characters excluded in [RFC2396]. "unwise", characters excluded in [RFC2396].
e. Characters with no visual representation may not be Additional information is available from [UNIXML]. Although [UNIXML]
interoperably entered. Control characters MUST NOT be used. is written in a different context, it discusses many of the
This includes the traditional ranges of control characters categories of characters and code points not appropriate for IRIs.
(U+0000-U+001F and U+007F-U+009F) as well as other cases such
as plane-14 language tag characters.
f. Some code points are reserved for private use or for special 5.2 Software Interfaces and Protocols
encoding purposes. They are not interoperable. Code points
reserved for private use MUST NOT be used. Code points
reserved for surrogates MUST NOT be used.
g. Where there exist duplicate ways of encoding a certain Although an IRI is defined as a sequence of characters, software
character as visible to the user, Normalization Form C as interfaces for URIs typically function on sequences of octets. Thus,
defined in [UNI15] MUST be used. software interfaces and protocols MUST define which character
encoding is used.
Additional information is available from [UNIXML]. Although this is Intermediate software interfaces between IRI-capable components and
written in a different context, it discusses many of the categories URI-only components MUST map the IRIs as per Section 3.1, when
of characters and code points not appropriate for IRIs. transferring from IRI-capable to URI-only components. Such a mapping
SHOULD be applied as late as possible. It should not be applied
between components that are known to be able to handle IRIs.
For reasons of transcribability, many characters have been excluded 5.3 Format of URIs and IRIs in Documents and Protocols
from IRIs above. These can nevertheless be encoded in an IRI if
necessary. They have to be escaped using the procedure in Section
2.3. For example, a space can always be encoded in a URI and in an
IRI as %20. A non-breaking space (U+00A0) must be encoded as %C2%A0.
3.2 Bidirectional IRIs for right-to-left languages Document formats that transport URIs may need to be upgraded to allow
the transport of IRIs. In those cases where the document as a whole
has a native character encoding, IRIs MUST also be encoded in this
encoding, and converted accordingly by a parser or interpreter. IRI
characters that are not expressible in the native encoding SHOULD be
escaped using the escaping conventions of the document format if such
conventions are available. Alternatively, they MAY be escaped
according to Section 3.1. For example, in HTML, XML, or SGML,
numeric character references should be used. If a document as a
whole has a native character encoding, and that character encoding is
not UTF-8, then IRIs MUST NOT be placed into the document in the UTF-
8 character encoding.
Some UCS characters, such as those used in the Arabic and Hebrew Note: Some formats already accommodate IRIs, although they use
script, have an inherent right-to-left writing direction. IRIs different terminology. HTML 4.0 [HTML4] defines the conversion from
containing such characters (called bidirectional IRIs or Bidi IRIs) IRIs to URIs as error-avoiding behavior. XML 1.0 [XML1], XLink
require additional attention because of the non-trivial relation [XLink], and XML Schema [XMLSchema] and specifications based upon
between logical representation (used for digital representation as them allow IRIs. Also, it is expected that all relevant new W3C
well as when reading/spelling) and visual representation (used for formats and protocols will be required to handle IRIs [CharMod].
display/printing). This document does not address bidi-specific
issues. A proposal for addressing these issues can be found in
[Bidi].
3.3 Processing IRIs 5.4 Relative IRI References
Processing of relative forms of IRIs against a base is handled Processing of relative forms of IRIs against a base is handled
straightforwardly; the algorithms of RFC 2396 may be applied straightforwardly; the algorithms of RFC 2396 may be applied
directly, treating the characters additionally allowed in IRIs in the directly, treating the characters additionally allowed in IRIs in the
same way as unreserved characters in URIs. Other processing same way as unreserved characters in URIs.
operations on IRIs and IRI references similarly work analogous to
their URI complements.
Such processing and mapping to URIs is commutative, which means that
the same result is obtained independent of whether the processing or
the mapping is done first. If both IRIs and URIs are involved in
processing, the IRI parts SHOULD be preserved as long as possible.
For example, it is possible to create an absolute IRI from a relative
IRI and a URI base. When IRIs are compared, they SHOULD temporarily
be mapped to URIs to eliminate potential differences in the degree of
escaping.
4. Software requirements 6. URI/IRI Processing Guidelines (informative)
This section explains the issues and difficulties in supporting IRIs This informative section provides guidelines for supporting IRIs in
in the same software components and operations that currently process the same software components and operations that currently process
URIs: software interfaces that handle URIs, software that allows URIs: software interfaces that handle URIs, software that allows
users to enter URIs, software that generates URIs, software that users to enter URIs, software that generates URIs, software that
displays URIs, formats and protocols that transport URIs, and displays URIs, formats and protocols that transport URIs, and
software that interprets URIs. These may all require more or less software that interprets URIs. These may all require more or less
modification before functioning properly with IRIs. The modification before functioning properly with IRIs. The
considerations in this section also apply to URI references and IRI considerations in this section also apply to URI references and IRI
references. references.
4.1 URI/IRI software interfaces 6.1 URI/IRI Software Interfaces
Software interfaces that handle URIs, such as URI-handling APIs and Software interfaces that handle URIs, such as URI-handling APIs and
protocols transferring URIs, need interfaces and protocol elements protocols transferring URIs, need interfaces and protocol elements
that are designed to carry IRIs. that are designed to carry IRIs.
Note that although an IRI is defined as a sequence of characters,
software interfaces for URIs typically function on sequences of
octets. Thus, it is necessary to define clearly which character
encoding is used.
In case the current handling in an API or protocol is based on US- In case the current handling in an API or protocol is based on US-
ASCII, UTF-8 is recommended as the encoding for IRIs, because this is ASCII, UTF-8 is recommended as the encoding for IRIs, because this is
compatible with US-ASCII, is in accordance with the recommendations compatible with US-ASCII, is in accordance with the recommendations
of [RFC2277], and makes it easy to convert to URIs where necessary. of [RFC2277], and makes it easy to convert to URIs where necessary.
In any case, the encoding used must not be left undefined. In any case, the encoding used must not be left undefined.
Intermediate software interfaces between IRI-capable components and
URI-only components MUST map the IRIs as per section 2.3 above, when
transferring from IRI-capable to URI-only components. However, such
a mapping SHOULD be applied as late as possible. It should not be
applied between components that are known to be able to handle IRIs.
The transfer from URI-only to IRI-capable components requires no The transfer from URI-only to IRI-capable components requires no
mapping, although the conversion described in section 2.4 above may mapping, although the conversion described in Section 3.2 above may
be performed. It is preferable not to perform this inverse be performed. It is preferable not to perform this inverse
conversion when there is a chance that this cannot be done correctly. conversion when there is a chance that this cannot be done correctly.
4.2 URI/IRI entry 6.2 URI/IRI Entry
There are components that allow users to enter URIs into the system, There are components that allow users to enter URIs into the system,
for example, by typing or dictation. This software must be updated for example, by typing or dictation. This software must be updated
to allow for IRI entry. to allow for IRI entry.
A person viewing a visual representation of an IRI (as a sequence of A person viewing a visual representation of an IRI (as a sequence of
glyphs, in some order, in some visual display) or hearing an IRI, glyphs, in some order, in some visual display) or hearing an IRI,
will use a entry method for characters in the user's language to will use a entry method for characters in the user's language to
input the IRI. Depending on the script and the input method used, input the IRI. Depending on the script and the input method used,
this may be a more or less complicated process. this may be a more or less complicated process.
The process of IRI entry must assure, as far as possible, that the The process of IRI entry must assure, as far as possible, that the
limitations defined in Section 3.1 are met. This may be done by restrictions defined in Section 2.2 are met. This may be done by
choosing appropriate input methods or variants/settings thereof, by choosing appropriate input methods or variants/settings thereof, by
appropriately converting the characters being input, by eliminating appropriately converting the characters being input, by eliminating
characters that cannot be converted, and/or by issuing a warning or characters that cannot be converted, and/or by issuing a warning or
error message to the user. error message to the user.
An input field primarily or only used for the input of URIs/IRIs An input field primarily or only used for the input of URIs/IRIs
should allow the user to view an IRI as converted to a URI. Places should allow the user to view an IRI as converted to a URI. Places
where the input of IRIs is frequent should provide the possibility where the input of IRIs is frequent should provide the possibility
for viewing an IRI as converted to a URI. This will help users when for viewing an IRI as converted to a URI. This will help users when
some of the software they use does not yet accept IRIs. some of the software they use does not yet accept IRIs.
An IRI input component that interfaces to components that handle An IRI input component that interfaces to components that handle
URIs, but not IRIs, must escape the IRI before passing it to such a URIs, but not IRIs, must escape the IRI before passing it to such a
component. component.
For the input of IRIs with right-to-left characters, please see For the input of IRIs with right-to-left characters, please see
[Bidi]. Section 4.
4.3 URI/IRI generation 6.3 URI/IRI Generation
Systems that are offering resources through the Internet, where those Systems that are offering resources through the Internet, where those
resources have logical names, sometimes automatically generate URIs resources have logical names, sometimes automatically generate URIs
for the resources they offer. For example, some HTTP servers can for the resources they offer. For example, some HTTP servers can
generate a directory listing for a file directory, and then respond generate a directory listing for a file directory, and then respond
to the generated URIs with the files. to the generated URIs with the files.
Many legacy character encodings are in use in various file systems. Many legacy character encodings are in use in various file systems.
Many currently deployed systems do not transform the local character Many currently deployed systems do not transform the local character
representation of the underlying system before generating URIs. representation of the underlying system before generating URIs.
skipping to change at page 16, line 6 skipping to change at page 19, line 29
use IRIs converted to URIs in cases where it cannot be expected that use IRIs converted to URIs in cases where it cannot be expected that
the recipient is able to handle IRIs. Due to the way most user the recipient is able to handle IRIs. Due to the way most user
agents currently work, native IRIs, encoded in UTF-8, may be used if agents currently work, native IRIs, encoded in UTF-8, may be used if
the recipient announces that it can interpret UTF-8. This requires the recipient announces that it can interpret UTF-8. This requires
that the whole page is sent as UTF-8. If this is not possible, that the whole page is sent as UTF-8. If this is not possible,
escaping can always be used. escaping can always be used.
This recommendation in particular applies to HTTP servers. For FTP This recommendation in particular applies to HTTP servers. For FTP
servers, similar considerations apply, see in particular [RFC2640]. servers, similar considerations apply, see in particular [RFC2640].
4.4 URI/IRI selection 6.4 URI/IRI Selection
In some cases, resource owners and publishers have control over the In some cases, resource owners and publishers have control over the
IRIs used to identify their resources. Such control is mostly IRIs used to identify their resources. Such control is mostly
executed by controlling the resource names, such as file names, executed by controlling the resource names, such as file names,
directly. directly.
In such cases, it is recommended to avoid choosing IRIs that are In such cases, it is recommended to avoid choosing IRIs that are
easily confused. For example, for US-ASCII, the lower-case ell "l" easily confused. For example, for US-ASCII, the lower-case ell "l"
is easily confused with the digit one "1", and the upper-case oh "O" is easily confused with the digit one "1", and the upper-case oh "O"
is easily confused with the digit zero "0". Publishers should avoid is easily confused with the digit zero "0". Publishers should avoid
skipping to change at page 16, line 31 skipping to change at page 20, line 5
here. As long as names are limited to characters from a single here. As long as names are limited to characters from a single
script, native writers of a given script or language will know best script, native writers of a given script or language will know best
when ambiguities can appear, and how they can be avoided. What may when ambiguities can appear, and how they can be avoided. What may
look ambiguous to a stranger may be completely obvious to the average look ambiguous to a stranger may be completely obvious to the average
native user. On the other hand, in some cases, the UCS contains native user. On the other hand, in some cases, the UCS contains
variants for compatibility reasons, for example for typographic variants for compatibility reasons, for example for typographic
purposes. These should be avoided wherever possible. Although there purposes. These should be avoided wherever possible. Although there
may be exceptions, in general newly created resource names should be may be exceptions, in general newly created resource names should be
in NFKC [UNI15] (which means that they are also in NFC). in NFKC [UNI15] (which means that they are also in NFC).
Note that the limitations defined in Section 3.1 and the
recommendations given here are of a different nature. The
limitations defined in Section 3.1 are necessary to avoid duplicate
encodings that are artifacts of digital representation and that the
user has no way to distinguish visually. On the other hand, in a
given context, an identifier such as "BOX0021" can be completely
appropriate, and it is impossible to find an algorithm that
distinguishes the appropriate from the confusing identifiers.
In certain cases, there is a chance that letters from different In certain cases, there is a chance that letters from different
scripts look the same. The best known example is the Latin 'A', the scripts look the same. The best known example is the Latin 'A', the
Greek 'Alpha', and the Cyrillic 'A'. To avoid such cases, only IRIs Greek 'Alpha', and the Cyrillic 'A'. To avoid such cases, only IRIs
should be generated where all the letters in a single component are should be generated where all the letters in a single component are
from the same script. This is similar to the heuristics used to from the same script. This is similar to the heuristics used to
distinguish between letters and numbers in the examples above. Also, distinguish between letters and numbers in the examples above. Also,
for the above three scripts, using lower-case letters results in for the above three scripts, using lower-case letters results in
fewer ambiguities than using upper-case letters. fewer ambiguities than using upper-case letters.
4.5 Display of URIs/IRIs 6.5 Display of URIs/IRIs
Many systems contain software that presents URIs to users as part of
the system's user interface (sometimes presenting 'friendly' URIs,
such as a shortened or more legible substring of the URI). This
section applies to this presentation, as well as to the strategy for
printing URIs in magazines, newspapers, or reading them over the
radio.
Software that displays identifiers to users should follow a general
principle: "Don't display something to a user that the user would not
be able to enter." The consequences of this principle require
judgement about the availability of software that implements the
entry methods described in Section 3.2.
a) In situations where a viewer is not likely to have software
that implements non-ASCII character entry (as described in
Section 3.1), or where it can be expected that only a limited
range of non-ASCII characters can be entered, any part of an
IRI containing characters outside the range allowed in
[RFC2396] or any additions SHOULD be escaped before being
displayed.
b) In situations where a viewer _is_ likely to have such software, In situations where the rendering software is not expected to display
IRIs SHOULD be displayed directly. non-ASCII parts of the IRI correctly using the available layout and
font resources, these parts should be escaped before being displayed.
For display of Bidi IRIs, please see [Bidi]. For display of Bidi IRIs, please see Section 4.2.
4.6 Interpretation of URI/IRIs 6.6 Interpretation of URIs and IRIs
Software that interprets IRIs as the names of local resources should Software that interprets IRIs as the names of local resources should
accept IRIs in multiple forms, and convert and match them with the accept IRIs in multiple forms, and convert and match them with the
appropriate local resource names. appropriate local resource names.
First, multiple representations include both IRIs in the native First, multiple representations include both IRIs in the native
character encoding of the protocol and also their URI counterparts. character encoding of the protocol and also their URI counterparts.
Second, it may include URIs constructed based on other character Second, it may include URIs constructed based on other character
encodings than UTF-8. Such URIs may be produced by user agents that encodings than UTF-8. Such URIs may be produced by user agents that
skipping to change at page 18, line 15 skipping to change at page 21, line 7
the accents on received IRIs or resource names where appropriate. the accents on received IRIs or resource names where appropriate.
Please note that such mappings, including case mappings, are Please note that such mappings, including case mappings, are
language-dependent. language-dependent.
It can be difficult to unambiguously identify a resource if too many It can be difficult to unambiguously identify a resource if too many
mappings are taken into consideration. However, escaped and non- mappings are taken into consideration. However, escaped and non-
escaped parts of IRIs can always clearly be distinguished. Also, the escaped parts of IRIs can always clearly be distinguished. Also, the
regularity of UTF-8 (see [Duer97] makes the potential for collisions regularity of UTF-8 (see [Duer97] makes the potential for collisions
lower than it may seem at first sight. lower than it may seem at first sight.
4.7 Transportation of URI/IRIs in document formats and protocols 6.7 Upgrading Strategy
Document formats that transport URIs may need to be upgraded to allow
the transport of IRIs. In those cases where the document as a whole
has a native character encoding, IRIs SHOULD also be encoded in this
encoding, and converted accordingly by a parser or interpreter. IRI
characters that are not expressible in the native encoding SHOULD be
escaped according to Section 2.2, or MAY be escaped in another way if
the document format provides a way to do this. For example, in HTML,
XML, or SGML, numeric character references can be used. If a
document as a whole has a native character encoding, and that
character encoding is not UTF-8, then IRIs MUST NOT be placed into
the document in the UTF-8 character encoding.
Please note that some formats already accomodate IRIs, although they
use different terminology. HTML 4.0 [HTML4] defines the conversion
from IRIs to URIs as error-avoiding behavior. XML 1.0 [XML1], XLink
[XLink], and XML Schema [XMLSchema] and specifications based upon
them allow IRIs. Also, it is expected that all relevant new W3C
formats and protocols will be required to handle IRIs [CharMod].
5. Upgrading strategy
As this recommendation places further constraints on software for As this recommendation places further constraints on software for
which many instances are already deployed, it is important to which many instances are already deployed, it is important to
introduce upgrades carefully, and to be aware of the various introduce upgrades carefully, and to be aware of the various
interdependencies. interdependencies.
If IRIs cannot be interpreted correctly, they should not be generated If IRIs cannot be interpreted correctly, they should not be generated
or transported. This suggests that upgrading URI interpreting or transported. This suggests that upgrading URI interpreting
software to accept IRIs should have highest priority. software to accept IRIs should have highest priority.
skipping to change at page 19, line 23 skipping to change at page 21, line 42
is known to transport them safely. is known to transport them safely.
Display software should be upgraded only after upgraded entry Display software should be upgraded only after upgraded entry
software has been widely deployed to the population that will see the software has been widely deployed to the population that will see the
displayed result. displayed result.
These recommendations, when taken together, will allow for the These recommendations, when taken together, will allow for the
extension from URIs to IRIs in order to handle scripts other than extension from URIs to IRIs in order to handle scripts other than
ASCII while minimizing interoperability problems. ASCII while minimizing interoperability problems.
6. Security considerations 7. Security Considerations
If IRI entry software normalizes the characters entered, but the Incorrect escaping or unescaping can lead to security problems. In
resource names on the interpreting side are not normalized particular, some UTF-8 decoders do not check against overlong byte
accordingly, and the interpreting software does not take this into sequences. As an example, a '/' is encoded with the byte 0x2F both
account, there is a possibility of "spoofing". Similar possibilities in UTF-8 and in ASCII, but some UTF-8 decoders also wrongly interpret
turn up when interpreting software accepts URIs in various native the sequence 0xC0 0xAF as a '/'. A sequence such as '%C0%AF..' may
encodings or allows accents and similar things to be ignored. pass some security tests and then be interpreted as '/..' in a path
if UTF-8 decoders are fault-tolerant, if conversion and checking are
not done in the right order, and/or if reserved characters and
unreserved characters are not clearly distinguished.
There are various ways in which "spoofing" can occur with IRIs.
"Spoofing" means that somebody may add a resource name that looks the "Spoofing" means that somebody may add a resource name that looks the
same or similar to the user while actually being different, or a same or similar to the user, but points to a different resource. The
resource name that contains the same characters, but in a different added resource may pretend to be the real resource by looking very
encoding. The added resource may pretend to be the real resource by similar, but may contain all kinds of changes that may be difficult
looking very similar, but may contain all kinds of changes that may to spot but can cause all kinds of problems. Most spoofing
be difficult to spot but can cause all kinds of problems. possibilities for IRIs are extensions of those for URIs.
Conceptually, this is no different from the problems surrounding the Spoofing can occur for various reasons. A first reason is that
use of case-insensitive web servers. For example, a popular web page normalization expectations of a user or actual normalization when
with a mixed case name (http://big.site/PopularPage.html) might be entering an IRI do not match the normalization used on the server
"spoofed" by someone who obtains access to (http://big.site/ side. Conceptually, this is no different from the problems
popularpage.html). surrounding the use of case-insensitive web servers. For example, a
popular web page with a mixed case name (http://big.site/
PopularPage.html) might be "spoofed" by someone who obtains access to
http://big.site/popularpage.html. However, the introduction of
character normalization, and of additional mappings for user
convenience, may increase the chance for spoofing.
However, the introduction of character normalization, of additional Spoofing can occur due to the fact that in the UCS, there are many
mappings for user convenience, and of mappings for various encodings characters that look very similar. Details are discussed in Section
may increase the number of spoofing possibilities. In some cases, in 6.4. Again, this is very similar to spoofing possibilities on US-
particular for Latin-based resource names, this is usually easy to ASCII, e.g. using 'br0ken' or '1ame' URIs.
detect because UTF-8-encoded names, when interpreted and viewed as
legacy encodings, produce mostly garbage. In other cases, when
concurrently used encodings have a similar structure, but there are
no characters that have exactly the same encoding, detection is more
difficult. A good example may be the concurrent use of Shift_JIS and
EUC-JP on a Japanese server.
Administrators of large sites which allow independent users to create Spoofing can occur when URIs in various encodings are accepted to
subareas may need to be careful that the aliasing rules do not create deal with older user agents. In some cases, in particular for Latin-
chances for spoofing. based resource names, this is usually easy to detect because UTF-8-
encoded names, when interpreted and viewed as legacy encodings,
produce mostly garbage. In other cases, when concurrently used
encodings have a similar structure, but there are no characters that
have exactly the same encoding, detection is more difficult.
7. Acknowlegdements Spoofing can occur in various IRI components, such as the domain name
part or a path part. For considerations specific to the domain name
part, see [Nameprep]. For the path part, administrators of sites
which allow independent users to create resources in the same subarea
may need to be careful to check for spoofing.
We would like to thank Larry Masinter for his work as co-author of 8. Change log
many earlier versions of this document.
Changes from -00 to -01
- Re-integrated the section on Bidi, some issues left.
- Integrated IDN, changed syntax (host, userinfo,....).
- Moved some text around, marked some as informational.
- Made a clear distinction of IRI use for identification only and
for resource resolution.
- Fixed various details in wording, spelling,...
9. Acknowlegdements
We would like to thank Larry Masinter for his work as coauthor of
many earlier versions of this document (draft-masinter-url-i18n-xx).
The issue addressed here has been discussed at numerous times over The issue addressed here has been discussed at numerous times over
the last years; for example, there was a thread in the HTML working the last years; for example, there was a thread in the HTML working
group in August 1995 (under the topic of "Globalizing URIs") in the group in August 1995 (under the topic of "Globalizing URIs") in the
www-international mailing list in July 1996 (under the topic of www-international mailing list in July 1996 (under the topic of
"Internationalization and URLs"), and ad-hoc meetings at the Unicode "Internationalization and URLs"), and ad-hoc meetings at the Unicode
conferences in September 1995 and September 1997. conferences in September 1995 and September 1997.
Thanks to Francois Yergeau, Chris Wendt, Yaron Goland, Graham Klyne, Thanks to Francois Yergeau, Chris Wendt, Yaron Goland, Graham Klyne,
Roy Fielding, Tim Berners-Lee, M.T. Carrasco Benitez, James Clark, Roy Fielding, Tim Berners-Lee, M.T. Carrasco Benitez, James Clark,
skipping to change at page 20, line 37 skipping to change at page 23, line 37
Bjoern Hoehrmann, Dan Oscarson, and many others for help with Bjoern Hoehrmann, Dan Oscarson, and many others for help with
understanding the issues and possible solutions. Thanks also to the understanding the issues and possible solutions. Thanks also to the
members of the W3C I18N Working Group and Interest Group for their members of the W3C I18N Working Group and Interest Group for their
contributions and their work on [CharMod], to the members of many contributions and their work on [CharMod], to the members of many
other W3C WGs for adopting the ideas, and to the members of the other W3C WGs for adopting the ideas, and to the members of the
Montreal IAB Workshop on Internationalization and Localization for Montreal IAB Workshop on Internationalization and Localization for
their review. their review.
References References
[Bidi] Duerst, M., "Internet Identifiers and Bidirectionality", [CharMod] Duerst, M., Yergeau, F., Ishida, R., Wolf, M.,
draft-duerst-iri-bidi-00 (work in progress), July 2001, Freytag, A. and T. Texin, "Character Model for the
<http://www.ietf.org/internet-drafts/draft-duerst-iri- World Wide Web", World Wide Web Consortium Working
bidi-00.txt>. Draft, April 2002, <http://www.w3.org/TR/charmod>.
[CharMod] Duerst, M., Yergeau, F., Ishida, R., Wolf, M., Freytag,
A. and T. Texin, "Character Model for the World Wide
Web", World Wide Web Consortium Working Draft, February
2002, <http://www.w3.org/TR/charmod>.
[Duer97] Duerst, M., "The Properties and Promizes of UTF-8", [Duer97] Duerst, M., "The Properties and Promises of UTF-8",
Proc. 11th International Unicode Conference, San Jose , Proc. 11th International Unicode Conference, San Jose
September 1997, <http://www.ifi.unizh.ch/mml/mduerst/ , September 1997, <http://www.ifi.unizh.ch/mml/
papers/PDF/IUC11-UTF-8.pdf>. mduerst/papers/PDF/IUC11-UTF-8.pdf>.
[Duer01] Duerst, M., "Internationalized Resource Identifiers: [Duer01] Duerst, M., "Internationalized Resource Identifiers:
From Specification to Testing", Proc. 19th International From Specification to Testing", Proc. 19th
Unicode Conference, San Jose , September 2001, <http:// International Unicode Conference, San Jose ,
www.w3.org/2001/Talks/0912-IUC-IRI/paper.html>. September 2001, <http://www.w3.org/2001/Talks/0912-
IUC-IRI/paper.html>.
[HTML4] Raggett, D., Le Hors, A. and I. Jacobs, "HTML 4.01 [HTML4] Raggett, D., Le Hors, A. and I. Jacobs, "HTML 4.01
Specification", World Wide Web Consortium Specification", World Wide Web Consortium
Recommendation, December 1999, <http://www.w3.org/TR/ Recommendation, December 1999, <http://www.w3.org/TR/
REC-html40/appendix/notes.html#h-B.2>. REC-html40/appendix/notes.html#h-B.2>.
[IDN-URI] Duerst, M., "Internationalized Domain Names in URIs and [IDNURI] Duerst, M., "Internationalized Domain Names in URIs",
IRIs", draft-ietf-idn-uri-01 (work in progress), draft-ietf-idn-uri-02.txt (work in progress), July
November 2001, <http://www.ietf.org/internet-drafts/ 2002, <http://www.ietf.org/internet-drafts/draft-
draft-ietf-idn-uri-01.txt>. ietf-idn-uri-02.txt>.
[IDNA] Faltstrom, P., Hoffman, P. and A. Faltstrom,
"Internationalizing Domain Names in Applications
(IDNA)", draft-ietf-idn-idna-09.txt (work in
progress), May 2002, <http://www.ietf.org/internet-
drafts/draft-ietf-idn-idna-09.txt>.
[ISO10646] International Organization for Standardization, [ISO10646] International Organization for Standardization,
"Information Technology - Universal Multiple-Octet Coded "Information Technology - Universal Multiple-Octet
Character Set (UCS) - Part 1: Architecture and Basic Coded Character Set (UCS) - Part 1: Architecture and
Multilingual Plane", ISO Standard 10646-1, with Basic Multilingual Plane", ISO Standard 10646-1, with
amendments, October 2000. amendments, October 2000.
[Nameprep] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names", draft-
ietf-idn-nameprep-10.txt (work in progress), May
2002, <http://www.ietf.org/internet-drafts/draft-
ietf-idn-nameprep-10.txt>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2130] Weider, C., Preston, C., Simonsen, K., Alvestrand, H., [RFC2130] Weider, C., Preston, C., Simonsen, K., Alvestrand,
Atkinson, R., Crispin, M. and P. Svanberg, "The Report H., Atkinson, R., Crispin, M. and P. Svanberg, "The
of the IAB Character Set Workshop held 29 February - 1 Report of the IAB Character Set Workshop held 29
March, 1996", RFC 2130, April 1997. February - 1 March, 1996", RFC 2130, April 1997.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997. [RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC2192] Newman, C., "IMAP URL Scheme", RFC 2192, September 1997. [RFC2192] Newman, C., "IMAP URL Scheme", RFC 2192, September
1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998. Languages", BCP 18, RFC 2277, January 1998.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO [RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998. 10646", RFC 2279, January 1998.
[RFC2384] Gellens, R., "POP URL Scheme", RFC 2384, August 1998. [RFC2384] Gellens, R., "POP URL Scheme", RFC 2384, August 1998.
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform [RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter,
Resource Identifiers (URI): Generic Syntax", RFC 2396, "Uniform Resource Identifiers (URI): Generic Syntax",
August 1998. RFC 2396, August 1998.
[RFC2397] Masinter, L., "The "data" URL scheme", RFC 2397, August [RFC2397] Masinter, L., "The "data" URL scheme", RFC 2397,
1998. August 1998.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., [RFC2616] Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Masinter, L., Leach, P. and T. Berners-Lee,
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
June 1999.
[RFC2640] Curtin, B., "Internationalization of the File Transfer [RFC2640] Curtin, B., "Internationalization of the File
Protocol", RFC 2640, July 1999. Transfer Protocol", RFC 2640, July 1999.
[RFC2718] Masinter, L., Alvestrand, H., Zigmond, D. and R. Petke, [RFC2718] Masinter, L., Alvestrand, H., Zigmond, D. and R.
"Guidelines for new URL Schemes", RFC 2718, November Petke, "Guidelines for new URL Schemes", RFC 2718,
1999. November 1999.
[RFC2732] Hinden, R., Carpenter, B. and L. Masinter, "Format for [RFC2732] Hinden, R., Carpenter, B. and L. Masinter, "Format
Literal IPv6 Addresses in URL's", RFC 2732, December for Literal IPv6 Addresses in URL's", RFC 2732,
1999. December 1999.
[UNIV3] The Unicode Consortium, "The Unicode Standard Version [UNIV3] The Unicode Consortium, "The Unicode Standard Version
3.0", Addison-Wesley, Reading, MA , 2000. 3.0", Addison-Wesley, Reading, MA , 2000.
[UNI15] Davis, M. and M. Duerst, "Unicode Normalization Forms", [UNI15] Davis, M. and M. Duerst, "Unicode Normalization
Unicode Standard Annex #15, March 2001, <http:// Forms", Unicode Standard Annex #15, March 2001,
www.unicode.org/unicode/reports/tr15/tr15-21.html>. <http://www.unicode.org/unicode/reports/tr15/tr15-
21.html>.
[UNIXML] Duerst, M. and A. Freytag, "Unicode in XML and other [UNIXML] Duerst, M. and A. Freytag, "Unicode in XML and other
Markup Languages", Unicode Technical Report #20, World Markup Languages", Unicode Technical Report #20,
Wide Web Consortium Note, Februar 2002, <http:// World Wide Web Consortium Note, Februar 2002, <http:/
www.w3.org/TR/unicode-xml/>. /www.w3.org/TR/unicode-xml/>.
[W3CIRI] "Internationalization - URIs and other identifiers", [W3CIRI] "Internationalization - URIs and other identifiers",
<http://www.w3.org/International/O-URL-and-ident.html>. <http://www.w3.org/International/O-URL-and-
ident.html>.
[XLink] DeRose, S., Maler, E. and D. Orchard, "XML Linking [XLink] DeRose, S., Maler, E. and D. Orchard, "XML Linking
Language (XLink) Version 1.0", World Wide Web Consortium Language (XLink) Version 1.0", World Wide Web
Recommendation, June 2001, <http://www.w3.org/TR/xlink/ Consortium Recommendation, June 2001, <http://
#link-locators>. www.w3.org/TR/xlink/#link-locators>.
[XML1] Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler, [XML1] Bray, T., Paoli, J., Sperberg-McQueen, C. and E.
"Extensible Markup Language (XML) 1.0 (Second Edition)", Maler, "Extensible Markup Language (XML) 1.0 (Second
World Wide Web Consortium Recommendation, including Edition)", World Wide Web Consortium Recommendation,
Erratum 26 at http://www.w3.org/XML/xml-V10-2e- including Erratum 26 at http://www.w3.org/XML/xml-
errata#E26, October 2000, <http://www.w3.org/TR/REC- V10-2e-errata#E26, October 2000, <http://www.w3.org/
xml#sec-external-ent>. TR/REC-xml#sec-external-ent>.
[XMLNamespace] Bray, T., Hollander, D. and A. Layman, "Namespaces in
XML", World Wide Web Consortium Recommendation,
January 1999, <http://www.w3.org/TR/REC-xml#sec-
external-ent>.
[XMLSchema] Biron, P. and A. Malhotra, "XML Schema Part 2: [XMLSchema] Biron, P. and A. Malhotra, "XML Schema Part 2:
Datatypes", World Wide Web Consortium Recommendation, Datatypes", World Wide Web Consortium Recommendation,
May 2001, <http://www.w3.org/TR/xmlschema-2/#anyURI>. May 2001, <http://www.w3.org/TR/xmlschema-2/#anyURI>.
Authors' Addresses Authors' Addresses
Martin Duerst (Note: Please write "Duerst" with u-umlaut wherever Martin Duerst (Note: Please write "Duerst" with u-umlaut wherever
possible, for example as "D&#252;rst in XML and HTML.) possible, for example as "D&#252;rst in XML and HTML.)
W3C/Keio University W3C/Keio University
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