draft-duerst-iri-03.txt   draft-duerst-iri-04.txt 
Network Working Group M. Duerst Network Working Group M. Duerst
Internet-Draft W3C Internet-Draft W3C
Expires: August 31, 2003 M. Suignard Expires: December 28, 2003 M. Suignard
Microsoft Corporation Microsoft Corporation
March 2, 2003 June 29, 2003
Internationalized Resource Identifiers (IRIs) Internationalized Resource Identifiers (IRIs)
draft-duerst-iri-03 draft-duerst-iri-04
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
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
skipping to change at page 1, line 33 skipping to change at page 1, line 33
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at http:// The list of current Internet-Drafts can be accessed at http://
www.ietf.org/ietf/1id-abstracts.txt. www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on August 31, 2003. This Internet-Draft will expire on December 28, 2003.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2003). 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 [RFCYYYY]. An
IRI is a sequence of characters from the Universal Character Set IRI is a sequence of characters from the Universal Character Set
[ISO10646]. A mapping from IRIs to URIs is defined, which means that [ISO10646]. A mapping from IRIs to URIs is defined, which means that
IRIs can be used instead of URIs where appropriate to identify IRIs can be used instead of URIs where appropriate to identify
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.
NOTE NOTE
This document is a product of the Internationalization Working Group This document is a product of the Internationalization Working Group
(I18N WG) of the World Wide Web Consortium (W3C). For general (I18N WG) of the World Wide Web Consortium (W3C). For general
discussion, please use the www-international@w3.org mailing list discussion, please use the public-iri@w3.org mailing list (publicly
(publicly archived at http://lists.w3.org/Archives/Public/www- archived at http://lists.w3.org/Archives/Public/public-iri/). An
international/). For more information on the topic of this document, issues list for this document is maintained at http://www.w3.org/
please also see [W3CIRI] and [Duerst01]. International/iri-edit#issues. For more information on the topic of
this document, please also see [W3CIRI] and [Duerst01].
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
1.4 Notation . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.4 Notation . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 7 2. IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1 Summary of IRI Syntax . . . . . . . . . . . . . . . . . . . 7 2.1 Summary of IRI Syntax . . . . . . . . . . . . . . . . . . . 7
2.2 ABNF for IRI References and IRIs . . . . . . . . . . . . . . 7 2.2 ABNF for IRI References and IRIs . . . . . . . . . . . . . . 7
2.3 IRI Equivalence and Normalization . . . . . . . . . . . . . 10 3. Relationship between IRIs and URIs . . . . . . . . . . . . . 10
3. Relationship between IRIs and URIs . . . . . . . . . . . . . 11 3.1 Mapping of IRIs to URIs . . . . . . . . . . . . . . . . . . 10
3.1 Mapping of IRIs to URIs . . . . . . . . . . . . . . . . . . 12 3.2 Converting URIs to IRIs . . . . . . . . . . . . . . . . . . 12
3.2 Converting URIs to IRIs . . . . . . . . . . . . . . . . . . 14 3.2.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4. Bidirectional IRIs for Right-to-left Languages . . . . . . . 15
4. Bidirectional IRIs for Right-to-left Languages . . . . . . . 16 4.1 Logical Storage and Visual Presentation . . . . . . . . . . 16
4.1 Logical Storage and Visual Presentation . . . . . . . . . . 17 4.2 Bidi IRI Structure . . . . . . . . . . . . . . . . . . . . . 16
4.2 Bidi IRI Structure . . . . . . . . . . . . . . . . . . . . . 17 4.3 Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . . . 17
4.3 Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . . . 18
4.4 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.4 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5. Use of IRIs . . . . . . . . . . . . . . . . . . . . . . . . 20 5. IRI Equivalence and Comparison . . . . . . . . . . . . . . . 19
5.1 Limitations on UCS Characters Allowed in IRIs . . . . . . . 20 5.1 Simple String Comparison . . . . . . . . . . . . . . . . . . 20
5.2 Software Interfaces and Protocols . . . . . . . . . . . . . 21 5.2 Conversion to URIs . . . . . . . . . . . . . . . . . . . . . 20
5.3 Format of URIs and IRIs in Documents and Protocols . . . . . 21 5.3 Normalization . . . . . . . . . . . . . . . . . . . . . . . 20
5.4 Relative IRI References . . . . . . . . . . . . . . . . . . 22 5.4 Preferred Forms . . . . . . . . . . . . . . . . . . . . . . 21
6. URI/IRI Processing Guidelines (informative) . . . . . . . . 22 6. Use of IRIs . . . . . . . . . . . . . . . . . . . . . . . . 22
6.1 URI/IRI Software Interfaces . . . . . . . . . . . . . . . . 22 6.1 Limitations on UCS Characters Allowed in IRIs . . . . . . . 22
6.2 URI/IRI Entry . . . . . . . . . . . . . . . . . . . . . . . 23 6.2 Software Interfaces and Protocols . . . . . . . . . . . . . 22
6.3 URI/IRI Transfer Between Applications . . . . . . . . . . . 23 6.3 Format of URIs and IRIs in Documents and Protocols . . . . . 23
6.4 URI/IRI Generation . . . . . . . . . . . . . . . . . . . . . 24 6.4 Use of UTF-8 for Encoding Original Characters . . . . . . . 23
6.5 URI/IRI Selection . . . . . . . . . . . . . . . . . . . . . 24 6.5 Relative IRI References . . . . . . . . . . . . . . . . . . 24
6.6 Display of URIs/IRIs . . . . . . . . . . . . . . . . . . . . 25 7. URI/IRI Processing Guidelines (informative) . . . . . . . . 24
6.7 Interpretation of URIs and IRIs . . . . . . . . . . . . . . 25 7.1 URI/IRI Software Interfaces . . . . . . . . . . . . . . . . 24
6.8 Upgrading Strategy . . . . . . . . . . . . . . . . . . . . . 26 7.2 URI/IRI Entry . . . . . . . . . . . . . . . . . . . . . . . 25
7. Security Considerations . . . . . . . . . . . . . . . . . . 27 7.3 URI/IRI Transfer Between Applications . . . . . . . . . . . 26
8. Issues List . . . . . . . . . . . . . . . . . . . . . . . . 28 7.4 URI/IRI Generation . . . . . . . . . . . . . . . . . . . . . 26
9. Change log . . . . . . . . . . . . . . . . . . . . . . . . . 28 7.5 URI/IRI Selection . . . . . . . . . . . . . . . . . . . . . 27
9.1 Changes from -02 to -03 . . . . . . . . . . . . . . . . . . 28 7.6 Display of URIs/IRIs . . . . . . . . . . . . . . . . . . . . 27
9.2 Changes from -01 to -02 . . . . . . . . . . . . . . . . . . 29 7.7 Interpretation of URIs and IRIs . . . . . . . . . . . . . . 28
9.3 Changes from -00 to -01 . . . . . . . . . . . . . . . . . . 29 7.8 Upgrading Strategy . . . . . . . . . . . . . . . . . . . . . 28
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29 8. Security Considerations . . . . . . . . . . . . . . . . . . 29
Normative References . . . . . . . . . . . . . . . . . . . . 30 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
Non-normative References . . . . . . . . . . . . . . . . . . 31 Normative References . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 33 Non-normative References . . . . . . . . . . . . . . . . . . 32
Full Copyright Statement . . . . . . . . . . . . . . . . . . 34 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 34
Full Copyright Statement . . . . . . . . . . . . . . . . . . 35
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 [RFCYYYY] 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
easier to memorize, easier to interpret, easier to transcribe, easier easier to memorize, easier to interpret, easier to transcribe, easier
to create, and easier to guess. For most languages other than to create, and easier to guess. For most languages other than
English, however, the natural script uses characters other than A-Z. English, however, the natural script uses characters other than A-Z.
For many people, handling Latin characters is as difficult as For many people, handling Latin characters is as difficult as
handling the characters of other scripts is for people who use only handling the characters of other scripts is for people who use only
the Latin alphabet. Many languages with non-Latin scripts do have the Latin alphabet. Many languages with non-Latin scripts have
transcriptions to Latin letters and such transcriptions are now often transcriptions to Latin letters. Such transcriptions are now often
used in URIs, but they introduce additional ambiguities. used in URIs, but they introduce additional ambiguities.
The infrastructure for the appropriate handling of characters from The infrastructure for the appropriate handling of characters from
local scripts is now widely deployed in local versions of operating local scripts is now widely deployed in local versions of operating
system and application software. Software that can handle a wide system and application software. Software that can handle a wide
variety of scripts and languages at the same time is increasingly variety of scripts and languages at the same time is increasingly
widespread. Also, there are increasing numbers of protocols and widespread. Also, there are increasing numbers of protocols and
formats that can carry a wide range of characters. formats that can carry a wide range of characters.
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. [RFCYYYY], 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.
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 for
syntax [RFC2718]. The compatibility is provided by providing a well new URI schemes [RFC2718]. The compatibility is provided by
defined and deterministic mapping from the IRI character sequence to providing a well defined and deterministic mapping from the IRI
the functionally equivalent URI character sequence. Practical use of character sequence to the functionally equivalent URI character
IRIs (or IRI references) in place of URIs (or URI references) depends sequence. Practical use of IRIs (or IRI references) in place of URIs
on the following conditions being met: (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 example, XML schema [XMLSchema] has an explicit type them. For example, XML schema [XMLSchema] has an explicit type
"anyURI" that designates the use of IRIs. "anyURI" that designates the use of IRIs.
b) The protocol or format carrying the IRIs should have a b) The protocol or format carrying the IRIs should have a
mechanism to represent the wide range of characters used in mechanism to represent the wide range of characters used in
IRIs, either natively or by some protocol- or format-specific IRIs, either natively or by some protocol- or format-specific
escaping mechanism (for example numeric character references in escaping mechanism (for example numeric character references in
[XML1]). [XML1]).
c) Either by definition for all the URIs of a specific URI scheme, c) The URI corresponding to the IRI in question has to encode
or a specific part of a URI (Reference), such as the fragment original characters into octets using UTF-8. For new URI
identifier, or at least for some specific URIs of a given schemes, this is recommended in [RFC2718]. It can apply to a
scheme, the encoding of non-ASCII characters should be based on whole scheme (e.g. IMAP URLs [RFC2192] and POP URLs [RFC2384],
UTF-8. For new URI schemes, this is recommended in [RFC2718]. or the URN syntax [RFC2141]). It can apply to a specific part
This allows IRIs to be used with the URN syntax [RFC2141] as of an URI, such as the fragment identifier (e.g. [XPointer]).
well as recent URL scheme definitions based on UTF-8, such as It can apply to a specific URI or part(s) thereoff. For
IMAP URLs [RFC2192] and POP URLs [RFC2384]. details, please see Section 6.4.
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
have to be %-escaped. In these situations, the ability of IRIs to
directly represent a wide character repertoire cannot be used.
For example, for a document with a URI of
http://www.example.org/r%C3%A9sum%C3%A9.html, it is possible to
construct a corresponding IRI (in XML notation, see Section 1.4):
http://www.example.org/résumé.html (é stands for the
e-acute character, and is the UTF-8 encoded and escaped
representation of that character). On the other hand, for a document
with an URI of http://www.example.org/r%E9sum%E9.html, the escaped
octets cannot be converted to actual characters in an IRI, because
the escaping is based on iso-8859-1 rather than UTF-8.
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], [RFC2277] and [ISO10646]: terms in [RFC2130], [RFC2277] and [ISO10646]:
character: A member of a set of elements used for the character: A member of a set of elements used for the
organization, control, or representation of data. For example, organization, control, or representation of data. For example,
"LATIN CAPITAL LETTER A" names a character. "LATIN CAPITAL LETTER A" names a character.
octet: an ordered sequence of eight bits considered as a unit 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
characters as a sequence of octets (maybe with variants). A characters as a sequence of octets (maybe with variants). A
method of (unambiguously) converting a sequence of octets into method of (unambiguously) converting a sequence of octets into
a sequence of characters. a sequence of characters.
code point: A placeholder for a character in a character encoding, code point: A placeholder for a character in a character encoding,
for example to encode additional characters in future versions for example to encode additional characters in future versions
of the character encoding. of the character encoding.
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a sequence of characters. a sequence of characters.
code point: A placeholder for a character in a character encoding, code point: A placeholder for a character in a character encoding,
for example to encode additional characters in future versions for example to encode additional characters in future versions
of the character encoding. of the character encoding.
charset: The name of a parameter or attribute used to identify a charset: The name of a parameter or attribute used to identify a
character encoding. character encoding.
UCS: Universal Character Set; the coded character set defined by UCS: Universal Character Set; the coded character set defined by
[ISO10646] and [UNIV3]. [ISO10646] and [UNIV4].
IRI reference: The term "IRI reference" denotes the common usage IRI reference: The term "IRI reference" denotes the common usage
of an internationalized resource identifier. An IRI reference of an internationalized resource identifier. An IRI reference
may be absolute or relative, and may have additional may be absolute or relative. However, the "IRI" that results
information attached in the form of a fragement identifier. from such a reference only includes absolute IRIs; any relative
However, the "IRI" that results from such a reference only IRIs are resolved to their absolute form. Note that in
includes the absolute IRI after the fragment identifier (if [RFC2396], URIs did not include fragment identifiers, but in
any) is removed and after any relative IRI is resolved to its [RFCYYYY], fragment identifiers are part of URIs.
absolute form.
1.4 Notation 1.4 Notation
RFCs and Internet Drafts currently do not allow any characters RFCs and Internet Drafts currently do not allow any characters
outside the US-ASCII repertoire. Therefore, this document uses outside the US-ASCII repertoire. Therefore, this document uses
various special notations to denote such characters. various special notations to denote such characters in examples.
In text, characters outside US-ASCII are sometimes referenced by In text, characters outside US-ASCII are sometimes referenced by
using a prefix of 'U+', followed by four to six hexadecimal digits. using a prefix of 'U+', followed by four to six hexadecimal digits.
To represent characters outside US-ASCII in examples, this document To represent characters outside US-ASCII in examples, this document
uses two notations called 'XML Notation' and 'Bidi Notation'. uses two notations called 'XML Notation' and 'Bidi Notation'.
XML Notation uses leading '&#x', trailing ';', and the hexadecimal XML Notation uses leading '&#x', trailing ';', and the hexadecimal
number of the character in the UCS in between. Example: я number of the character in the UCS in between. Example: я
stands for CYRILLIC CAPITAL LETTER YA. In this notation, an actual stands for CYRILLIC CAPITAL LETTER YA. In this notation, an actual
'&' is denoted by '&amp'. '&' is denoted by '&'.
Bidi Notation is used for bidirectional examples: lower case ASCII Bidi Notation is used for bidirectional examples: lower case ASCII
letters stand for Latin letters or other letters that are written letters stand for Latin letters or other letters that are written
left-to-right, whereas upper case letters represent Arabic or Hebrew left-to-right, whereas upper case letters represent Arabic or Hebrew
letters that are written right-to-left. letters that are written right-to-left.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. IRI Syntax 2. IRI Syntax
This section defines the syntax of Internationalized Resource This section defines the syntax of Internationalized Resource
Identifiers (IRIs). Identifiers (IRIs).
As with URIs, an IRI is defined as a sequence of characters, not as a As with URIs, an IRI is defined as a sequence of characters, not as a
sequence of octets. This definition accommodates the fact that IRIs 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 stored or transmitted digitally. 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/ these protocols or documents use different character encodings (and/
or transfer encodings). Using the same character encoding as the or 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 [RFCYYYY], but the class of
[RFC2732] and [IDNURI]), but the class of unreserved characters is unreserved characters is extended by adding the characters of the UCS
extended by adding the characters of the UCS (Universal Character (Universal Character Set, [ISO10646]) beyond U+0080, subject to the
Set, [ISO10646]) beyond U+0080, subject to the limitations given in limitations given in the syntax rules below and in Section 6.1.
the syntax rules below and in 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 [RFCYYYY]. All the operations defined in
[RFC2396], such as the resolution of relative URIs, can be applied to [RFCYYYY], 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.
Note: [RFC2396]: Uniform Resource Identifiers (URI): Generic Syntax" Characters outside the US-ASCII range are not reserved and therefore
is being revised as [RFC2396bis]. The syntax used in this document MUST NOT be used for syntactical purposes such as to delimit
includes bug fixes from [RFC2396bis]. components in newly defined schemes. As an example, it is not
allowed to use U+00A2, CENT SIGN, as a delimiter in IRIs, because it
Characters outside the US-ASCII range MUST NOT be used for is in the 'iunreserved' category, in the same way as it is not
syntactical purposes such as to delimit components in newly defined possible to use '-' as a delimiter, because it is in the 'unreserved'
schemes. As an example, it is not allowed to use U+00A2, CENT SIGN, category in URIs.
as a delimiter in IRIs, because it is in the 'iunreserved' category,
in the same way as it is not possible to use '-' as a delimiter,
because it is in the 'unreserved' category in URIs.
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 URI references and URIs, they can also be by their transformation to URI references and URIs, they can also be
accepted and processed directly. Therefore, an ABNF definition for accepted and processed directly. Therefore, an ABNF definition for
IRI references (which are the most general concept and the start of IRI references (which are the most general concept and the start of
the grammar) and IRIs is given here. The syntax of this ABNF is the grammar) and IRIs is given here. The syntax of this ABNF is
described in [RFC2234]. Character numbers are taken from the UCS, described in [RFC2234]. Character numbers are taken from the UCS,
without implying any actual binary encoding. Terminals in the ABNF without implying any actual binary encoding. Terminals in the ABNF
are characters, not bytes. are characters, not bytes.
The following rules are different from [RFC2396]: The following rules are different from [RFCYYYY]:
absolute-IRI-reference = absolute-IRI [ "#" ifragment ] IRI-reference = IRI / relative-IRI
IRI-reference = [ absolute-IRI / relative-IRI ] IRI = scheme ":" ihier-part [ "?" iquery ] [ "#" ifragment ]
[ "#" ifragment ]
absolute-IRI = scheme ":" ( ihier-part / iopaque-part )
relative-IRI = [ inet-path / iabs-path / irel-path ]
[ "?" iquery ]
ihier-part = [ inet-path / iabs-path ] [ "?" iquery ] absolute-IRI = scheme ":" ihier-part [ "?" iquery ]
iopaque-part = iric-no-slash *iric
iric-no-slash = iunreserved / escaped / "[" / "]" / ";" / "?" / relative-IRI = ihier-part [ "?" iquery ] [ "#" ifragment ]
":" / "@" / "&" / "=" / "+" / "$" / "," ihier-part = inet-path / iabs-path / irel-path
inet-path = "//" iauthority [ iabs-path ] inet-path = "//" iauthority [ iabs-path ]
iabs-path = "/" ipath-segments
irel-path = irel-segment [ iabs-path ]
irel-segment = 1*( iunreserved / escaped / ";" / iabs-path = "/" ipath-segments
"@" / "&" / "=" / "+" / "$" / "," )
iauthority = iserver / ireg-name irel-path = ipath-segments
ireg-name = 1*( iunreserved / escaped / ";" / iauthority = [ iuserinfo "@" ] ihost [ ":" port ]
":" / "@" / "&" / "=" / "+" / "$" / "," )
iserver = [ [ iuserinfo "@" ] ihostport ]
iuserinfo = *( iunreserved / escaped / ";" / iuserinfo = *( iunreserved / escaped / ";" /
":" / "&" / "=" / "+" / "$" / "," ) ":" / "&" / "=" / "+" / "$" / "," )
ihostport = ihost [ ":" port ] ihost = [ IPv6reference / IPv4address / ihostname ]
ihost = IPv6reference / IPv4address / ihostname
ihostname = idomainlabel iqualified
iqualified = *( "." idomainlabel ) [ "." ]
ihostname = idomainlabel [ iqualified]
iqualified = *( "." idomainlabel ) [ "." itoplabel [ "." ] ]
idomainlabel = <<See following production rules>> idomainlabel = <<See following production rules>>
itoplabel = <<See following production rules>>
ipath = [ iabs-path / iopaque-part ]
ipath-segments = isegment *( "/" isegment ) ipath-segments = isegment *( "/" isegment )
isegment = *ipchar isegment = *ipchar
ipchar = iunreserved / escaped / ";" / ipchar = iunreserved / escaped / ";" /
":" / "@" / "&" / "=" / "+" / "$" / "," ":" / "@" / "&" / "=" / "+" / "$" / ","
iquery = *( ipchar / iprivate / "/" / "?" ) iquery = *( ipchar / iprivate / "/" / "?" )
ifragment = *( ipchar / "/" / "?" ) ifragment = *( ipchar / "/" / "?" )
iric = reserved / iunreserved / escaped iric = reserved / iunreserved / escaped
iunreserved = unreserved / ucschar / iadditional
iadditional = "<" / ">" / DQUOTE / SP / "{" / "}" / iunreserved = unreserved / ucschar
"|" / "\" / "^" / "`"
ucschar = %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF / ucschar = %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF /
/ %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD / %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD
/ %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD / %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD
/ %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD / %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD
/ %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD / %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD
/ %xD0000-DFFFD / %xE1000-EFFFD / %xD0000-DFFFD / %xE1000-EFFFD
iprivate = %xE000-F8FF / %xF0000-FFFFD / %x100000-10FFFD iprivate = %xE000-F8FF / %xF0000-FFFFD / %x100000-10FFFD
The 'idomainlabel' and 'itoplabel' production rules are as follows: The 'idomainlabel' production rule is as follows:
The values 'idomainlabel' and 'itoplabel' are defined as a string of The value 'idomainlabel' is defined as a string of 'ucschar' obeying
'ucschar' obeying the following rules: the following rules:
a) Given a string of 'ucschar' values, the ToASCII operation a) Given a string of 'ucschar' values, the ToASCII operation
[RFCXXXX] is performed on that string with the flag [RFC3490] is performed on that string with the flag
UseSTD3ASCIIRules set to TRUE and the flag AllowUnassigned set UseSTD3ASCIIRules set to TRUE and the flag AllowUnassigned set
to FALSE for creating IRIs and set to TRUE otherwise. to FALSE for creating IRIs and set to TRUE otherwise.
b) ToASCII is successful and results in a string conforming to b) ToASCII is successful and results in a string conforming to
'domainlabel' for 'idomainlabel' and 'toplabel' for 'itoplabel' 'domainlabel' (see below).
(see below for 'domainlabel' and 'toplabel').
Note that the space character and various delimiters are allowed in
IRIs and IRI references. This is further discussed in Section 5.1.
The following are the same as [RFC2396bis]: The following are the same as [RFCYYYY]:
scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." ) scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
port = *DIGIT port = *DIGIT
domainlabel = alphanum [ 0*61( alphanum | "-" ) alphanum ]
toplabel = alpha [ 0*61( alphanum | "-" ) alphanum ]
alphanum = ALPHA / DIGIT
IPv4address = dec-octet 3( "." dec-octet ) domainlabel = alphanum [ 0*61( alphanum | "-" ) alphanum ]
dec-octet = DIGIT / ; 0-9
( %x31-39 DIGIT ) / ; 10-99
( "1" 2*DIGIT ) / ; 100-199
( "2" %x30-34 DIGIT ) / ; 200-249
( "25" %x30-35 ) ; 250-255
IPv6reference = "[" IPv6address "]"
IPv6address = ( 7( h4 ":" ) h4 ) /
( "::" 0*6( h4 ":" ) [ h4 ] ) /
( h4 "::" 0*5( h4 ":" ) [ h4 ] ) /
( h4 ":" h4 "::" 0*4( h4 ":" ) [ h4 ] ) /
( h4 2( ":" h4 ) "::" 0*3( h4 ":" ) [ h4 ] ) /
( h4 3( ":" h4 ) "::" 0*2( h4 ":" ) [ h4 ] ) /
( h4 4( ":" h4 ) "::" 0*1( h4 ":" ) [ h4 ] ) /
( 6( h4 ":" ) IPv4address )/
( "::" 0*5( h4 ":" ) IPv4address )/
( h4 "::" 0*4( h4 ":" ) IPv4address )/
( h4 ":" h4 "::" 0*3( h4 ":" ) IPv4address )/
( h4 2( ":" h4 ) "::" 0*2( h4 ":" ) IPv4address )/
( h4 3( ":" h4 ) "::" 0*1( h4 ":" ) IPv4address )
h4 = 1*4HEXDIG alphanum = ALPHA / DIGIT
reserved = "[" / "]" / ";" / "/" / "?" /
":" / "@" / "&" / "=" / "+" / "$" / ","
unreserved = ALPHA / DIGIT / mark
mark = "-" / "_" / "." / "!" / "~" / "*" / "'" /
"(" / ")"
escaped = "%" HEXDIG HEXDIG IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
2.3 IRI Equivalence and Normalization dec-octet = DIGIT ; 0-9
/ ( %x31-39 DIGIT ) ; 10-99
/ ( "1" 2DIGIT ) ; 100-199
/ ( "2" %x30-34 DIGIT ) ; 200-249
/ ( "25" %x30-35 ) ; 250-255
There is no general rule or procedure to decide whether two arbitrary IPv6reference = "[" IPv6address "]"
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 a definite answer to the question of IRI IPv6address = 6( h4 ":" ) ls32
equivalence is needed that is independent of the scheme used and / "::" 5( h4 ":" ) ls32
always can be calculated quickly and without accessing a network. An / [ h4 ] "::" 4( h4 ":" ) ls32
example of such a case might be XML Namespaces ([XMLNamespace]). In / [ *1( h4 ":" ) h4 ] "::" 3( h4 ":" ) ls32
such cases, two IRIs SHOULD be defined as equivalent if and only if / [ *2( h4 ":" ) h4 ] "::" 2( h4 ":" ) ls32
they are character-by-character equivalent. This is the same as / [ *3( h4 ":" ) h4 ] "::" h4 ":" ls32
being byte-by-byte equivalent if the character encoding for both IRIs / [ *4( h4 ":" ) h4 ] "::" ls32
is the same. As an example, / [ *5( h4 ":" ) h4 ] "::" h4
http://example.org/~user, http://example.org/%7euser, and / [ *6( h4 ":" ) h4 ] "::"
http://example.org/%7Euser would not be equivalent under this
definition. In such a case, the comparison function MUST NOT map the
IRIs to URIs, because such a mapping would create something different
under this equivalence relationship.
It follows from the above that IRIs SHOULD NOT be modified when being h4 = 1*4HEXDIG
transported.
For actual resolution, differences in escaping (except for the ls32 = ( h4 ":" h4 ) / IPv4address
escaping of reserved characters) MUST always result in the same
resource. For example, http://example.org/~user,
http://example.org/%7euser and http://example.org/%7Euser 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 reserved = "/" / "?" / "#" / "[" / "]" / ";" /
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 unreserved = ALPHA / DIGIT / mark
sequences of characters for various purposes. Unicode Standard Annex
#15 [UTR15] defines various Normalization Forms for these
equivalences. IRIs SHOULD be created using Normalization Form C
(NFC). Equivalence of IRIs MUST rely on the assumtion that IRIs are
appropriately pre-normalized, rather than applying normalization when
comparing two IRIs, except when converting from a non-UCS-based
encoding to an UCS-based encoding, where a normalizing transcoder
using NFC MUST be used for interoperability.
As an example, http://www.example.org/r&#xe9;sum&#xe9;.html (in XML mark = "-" / "_" / "." / "!" / "~" / "*" / "'" /
Notation) is in NFC. On the other hand, http://www.example.org/ "(" / ")"
re&#x301;sume&#x301;.html is not in NFC. The former uses precombined
e-acute characters, the later uses 'e' characters followed by
combining acute accents, both are defined as canonically equivalent
in [UNIV3].
Various IRI schemes may allow the usage of International Domain Names escaped = "%" HEXDIG HEXDIG
(IDN) [RFCXXXX]. When in use in IRIs, those names SHOULD be
validated using the ToASCII operation defined in [RFCXXXX], with the
flags "UseSTD3ASCIIRules" and "AllowUnassigned". 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 3. Relationship between IRIs and URIs
IRIs are meant to replace URIs in identifying resources for IRIs are meant to replace URIs in identifying resources for
protocols, formats and software components which use a UCS-based protocols, formats and software components which use a UCS-based
character repertoire. These protocols and components may never need character repertoire. These protocols and components may never need
to use URIs directly, especially when the resource identifier is used to use URIs directly, especially when the resource identifier is used
simply for identification purposes. However, when the resource simply for identification purposes. However, when the resource
identifier is used for resource retrieval, it is in many cases identifier is used for resource retrieval, it is in many cases
necessary to determine the associated URI because most retrieval necessary to determine the associated URI because most retrieval
skipping to change at page 12, line 28 skipping to change at page 10, line 36
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. When the IRI is used simply for IRIs also identify resources. When the IRI is used solely for
identification purposes, it is not necessary to map the IRI to identification purposes, it is not necessary to map the IRI to
an URI (see Section 2.3). However, when an IRI is used for an URI (see Section 5). However, when an IRI is used for
resource retrieval, the resource that the IRI locates is the resource retrieval, the resource that the IRI locates is the
same as the one located by the URI obtained after converting same as the one located by the URI obtained after converting
the IRI according to the procedure defined here. This means the IRI according to the procedure defined here. This means
that there is no need to define resolution separately on the that there is no need to define resolution separately on the
IRI level. IRI level.
Applications MUST map IRIs to URIs using the following two steps. Applications MUST map IRIs to URIs using the following two steps.
Step 1) This step generates a UCS-based encoding from the original Step 1) This step generates a UCS-based encoding from the original
IRI format. This step has three variants, depending on the IRI format. This step has three variants, depending on the
form of the input. form of the input.
Variant A) If the IRI is written on paper or read out loud, Variant A) If the IRI is written on paper or read out loud,
or otherwise represented as a sequence of characters or otherwise represented as a sequence of characters
independent of any encoding: Represent the IRI as a independent of any encoding: Represent the IRI as a
sequence of characters from the UCS normalized according sequence of characters from the UCS normalized according
to Normalization Form C (NFC, [UTR15]). to Normalization Form C (NFC, [UTR15]).
Variant B) If the IRI is in some digital representation Variant B) If the IRI is in some digital representation
(e.g. an octet stream) in some non-Unicode encoding: (e.g. an octet stream) in some known non-Unicode
Convert the IRI to a sequence of characters from the UCS encoding: Convert the IRI to a sequence of characters
normalized according to NFC. from the UCS normalized according to NFC.
Variant C) If the IRI is in an Unicode-based encoding (for Variant C) If the IRI is in an Unicode-based encoding (for
example UTF-8 or UTF-16): Do not normalize. Move example UTF-8 or UTF-16): Do not normalize. Move
directly to Step 2. directly to Step 2.
Step 2) For each character that is disallowed in URI references, Step 2) If the IRI contains an 'ihostname' part, replace this
'ihostname' part by the part converted using the ToASCII
operation specified in Section 4.1 of [RFC3490], with the flag
UseSTD3ASCIIRules set to TRUE and the flag AllowUnassigned set
to FALSE for creating IRIs and set to TRUE otherwise.
Step 3) For each character that is disallowed in URI references,
apply steps 1) through 3) 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 allowed in IRIs.
characters listed in Section 2.4 of [RFC2396], except for the
number sign (#) and percent sign (%) and the square bracket
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 [RFCXXXX].
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 [RFCYYYY].
Note: To reduce variability, the hexadecimal notation Note: To reduce variability, the hexadecimal notation
SHOULD use upper case letters. SHOULD use upper case letters.
3) Replace the original character by the resulting character 3) Replace the original character by the resulting character
sequence (i.e. a sequence of %HH triplets). sequence (i.e. a sequence of %HH triplets).
Note that in this process (in step 2.3), characters allowed in URI Note that the ToASCII operation in Step 2) may fail, but only if the
references and existing escape sequences are not escaped further. IRI does not conform to the rules in Section 2.2.
(This mapping is similar to, but different from, the escaping applied
when including arbitrary content into some part of a URI.) For Note: For backwards compatibility with implementations of previous
example, an IRI of drafts of this specification, infrastructure accepting IRIs MAY also
http://www.example.org/red%09ros&#xe9;#<red> (in XML notation) is deal with 'ihostname' parts escaped according to Step 3) rather than
Step 2). For example, Step 2) converts the IRI
http://r&#xE9;sum&#xE9;.example.org to
http://xn--rsum-bpad.example.org. For backwards compatibility,
http://r%C3%A9sum%C3%A9.example.org would also be converted to
http://xn--rsum-bpad.example.org.
Note that Internationalized Domain Names may be contained in parts of
an IRI other than the 'ihostname' part.
Note that in this process (in step 3.3), characters allowed in URI
references as well as existing escape sequences are not escaped
further. (This mapping is similar to, but different from, the
escaping applied when including arbitrary content into some part of a
URI.) For example, an IRI of
http://www.example.org/red%09ros&#xE9;#red (in XML notation) is
converted to converted to
http://www.example.org/red%09ros%C3%A9#%3Cred%3E, not to something http://www.example.org/red%09ros%C3%A9#red, not to something like
like
http%3A%2F%2Fwww.example.org%2Fred%2509ros%C3%A9%23red. http%3A%2F%2Fwww.example.org%2Fred%2509ros%C3%A9%23red.
Note that some older software transcoding to UTF-8 may produce Note that some older software transcoding to UTF-8 may produce
illegal output for some input, in particular for characters outside illegal output for some input, in particular for characters outside
the BMP (Basic Multilingual Plane). As an example, for the following the BMP (Basic Multilingual Plane). As an example, for the following
IRI with non-BMP characters (in XML Notation): IRI with non-BMP characters (in XML Notation):
http://example.com/ http://example.com/&#x10300;&#x10301;&#x10301;
(the first three letters of the Old Italic alphabet) the correct (the first three letters of the Old Italic alphabet) the correct
conversion to a URI is: conversion to a URI is:
http://example.com/%F0%90%8C%80%F0%90%8C%81%F0%90%8C%82 http://example.com/%F0%90%8C%80%F0%90%8C%81%F0%90%8C%82
The above mapping produces a URI fully conforming to [RFC2396] (as The above mapping produces a URI fully conforming to [RFCYYYY] out of
amended by [RFC2732] and [IDNURI]) out of each IRI. The mapping is each IRI. The mapping is also an identity transformation for URIs
also an identity transformation for URIs and is idempotent -- and is idempotent -- applying the mapping a second time will not
applying the mapping a second time will not change anything. Every change anything. Every URI is therefore by definition an IRI.
URI is therefore by definition an IRI.
Note: For backwards compatibility with infrastructure that does not Note: Earlier drafts of this specification allowed the space
implement the updates of [IDNURI], converters MAY also convert the character and various delimiters in IRIs and IRI references. The
'ihostname' part of an IRI using the ToASCII operation specified in full list of these characters was: "<", ">", '"', Space, "{", "}",
Section 4.1 of [RFCXXXX] between Step 1 and Step 2. Note that the "|", "\", "^", and "`", i.e. all printable characters in US-ASCII
ToASCII operation may fail. Note that Internationalized Domain Names that are not allowed in URIs. For backwards compatibility,
may be contained in parts of an IRI other than the 'ihostname' part. implementations MAY also include these characters in step 3) above.
If such characters are found but are not converted, then the
conversion SHOULD fail. Please note that the number sign ("#"), the
percent sign ("%"), and the square bracket characters ("[", "]") are
not part of the above list, and MUST not be converted. Protocols and
formats that have used earlier definitions of IRIs including these
characters MAY require unescaping of these characters as a
preprocessing step to extract the actual IRI from a given field.
Such preprocessing MAY also be used by applications allowing the user
to enter an IRI.
3.2 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. The conversion described in this section will always conversion. The conversion described in this section will always
result in an IRI which maps back to the URI that was used as an input result in an IRI which maps back to the URI that was used as an input
for the conversion (except for potential case differences in escape for the conversion (except for potential case differences in escape
sequences). However, the IRI resulting from this conversion may not sequences). However, the IRI resulting from this conversion may not
be exactly the same as the original IRI (if there ever was one). be exactly the same as the original IRI (if there ever was one).
URI to IRI conversion removes escape sequences, but not all escaping URI to IRI conversion removes escape sequences, but not all escaping
can be eliminated. There are several reasons for this: can be eliminated. There are several 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: The octet patterns of UTF-8 are highly regular.
there is a very high probability, but no guarantee, that escape Therefore, there is a very high probability, but no guarantee,
sequences that can be interpreted as sequences of UTF-8 octets that escape sequences that can be interpreted as sequences of
actually originated from UTF-8. For a detailed discussion, see UTF-8 octets actually originated from UTF-8. For a detailed
[Duerst97].) discussion, see [Duerst97].)
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 5.1 for further details. appropriate in an IRI. See Section 6.1 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) Replace any punycode-encoded domainlabel in the URI by the
digits) except those corresponding to '#' and '%' and result of the ToUnicode function represented as UTF-8.
characters in 'reserved', to the corresponding octets.
3) Re-escape any octet produced in step 2) that is not part of a 3) Convert all hexadecimal escapes (% followed by two hexadecimal
digits) except those corresponding to '%', characters in
'reserved', and characters in US-ASCII not allowed in URIs, to
the corresponding octets.
4) Re-escape any octet produced in step 3) that is not part of a
strictly legal UTF-8 octet sequence. strictly legal UTF-8 octet sequence.
4) Re-escape all octets produced in step 2) that in UTF-8 5) Re-escape all octets produced in step 3) that in UTF-8
represent characters that are not appropriate according to represent characters that are not appropriate according to
Section 4.1 and Section 5.1. Section 4.1 and Section 6.1.
5) Interpret the resulting octet sequence as a sequence of 6) Interpret the resulting octet sequence as a sequence of
characters encoded in UTF-8. 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 4) (see Section 5.1), results may differ. when applying step 5) (see Section 6.1), results may vary.
Conversions from URIs to IRIs MUST NOT use any other encoding than Conversions from URIs to IRIs MUST NOT use any other encoding than
UTF-8 in steps 3) and 4) above, even if it might be possible from UTF-8 in steps 2), 4) and 5) above, even if it might be possible from
context to guess that another encoding than UTF-8 was used in the context to guess that another encoding than UTF-8 was used in the
URI. As an example, the URI http://www.example.org/r%E9sum%E9.html, URI. As an example, the URI http://www.example.org/r%E9sum%E9.html
which with some guesses might be interpreted to contain two e-acute might with some guessing be interpreted to contain two e-acute
characters encoded as iso-8859-1, must not be converted to an IRI characters encoded as iso-8859-1. It must not be converted to an IRI
containing these e-acute characters. Otherwise, the IRI will in the containing these e-acute characters. Otherwise, the IRI will in the
future be mapped to http://www.example.org/r%C3%A9sum%C3%A9.html, future be mapped to http://www.example.org/r%C3%A9sum%C3%A9.html,
which is a different URI from http://www.example.org/r%E9sum%E9.html. which is a different URI from http://www.example.org/r%E9sum%E9.html.
3.2.1 Examples 3.2.1 Examples
This section shows various examples of converting URIs to IRIs. The This section shows various examples of converting URIs to IRIs. The
notation <hh> is used to denote octets outside those that can be notation <hh> is used to denote octets outside those that can be
represented in this document. Each example shows the result after represented in this document. Each example shows the result after
applying each of the steps 1) to 5). XML Notation is used for the applying each of the steps 1) to 6). XML Notation is used for the
final result. final result.
The following example contains the sequence '%C3%BC', which is a The following example contains the sequence '%C3%BC', which is a
strictly legal UTF-8 sequence, and which is converted into the actual strictly legal UTF-8 sequence, and which is converted into the actual
character U+00FC LATIN SMALL LETTER U WITH DIAERESIS (also known as character U+00FC LATIN SMALL LETTER U WITH DIAERESIS (also known as
u-umlaut). u-umlaut).
1) http://www.example.org/D%C3%BCrst 1) http://www.example.org/D%C3%BCrst
2) http://www.example.org/D<c3><bc>rst 2) http://www.example.org/D%C3%BCrst
3) http://www.example.org/D<c3><bc>rst 3) http://www.example.org/D<c3><bc>rst
4) http://www.example.org/D<c3><bc>rst 4) http://www.example.org/D<c3><bc>rst
5) http://www.example.org/D&#xfc;rst 5) http://www.example.org/D<c3><bc>rst
6) http://www.example.org/D&#xFC;rst
The following example contains the sequence '%FC', which might The following example contains the sequence '%FC', which might
represent U+00FC LATIN SMALL LETTER U WITH DIAERESIS in the iso-8859- represent U+00FC LATIN SMALL LETTER U WITH DIAERESIS in the
1 encoding. (It might represent other characters in other encodings. iso-8859-1 encoding. (It might represent other characters in other
For example, the octet <FC> in iso-8859-5 represents U+045C CYRILLIC encodings. For example, the octet <FC> in iso-8859-5 represents
SMALL LETTER KJE.) Because <FC> is not part of a strictly legal UTF-8 U+045C CYRILLIC SMALL LETTER KJE.) Because <FC> is not part of a
sequence, it is re-escaped in step 2). strictly legal UTF-8 sequence, it is re-escaped in step 2).
1) http://www.example.org/D%FCrst 1) http://www.example.org/D%FCrst
2) http://www.example.org/D<FC>rst 2) http://www.example.org/D%FCrst
3) http://www.example.org/D<FC>rst
3) http://www.example.org/D%FCrst
4) http://www.example.org/D%FCrst 4) http://www.example.org/D%FCrst
5) http://www.example.org/D%FCrst 5) http://www.example.org/D%FCrst
The following example contains '%e2%80%ae', which is the escaped UTF- 6) http://www.example.org/D%FCrst
8 encoding of U+202E, RIGHT-TO-LEFT OVERRIDE. Section 4.1 forbids
the direct use of this character in an IRI. Therefore, the The following example contains '%e2%80%ae', which is the escaped
corresponding octets are re-escaped in step 3). This example shows UTF-8 encoding of U+202E, RIGHT-TO-LEFT OVERRIDE. Section 4.1
forbids the direct use of this character in an IRI. Therefore, the
corresponding octets are re-escaped in step 5). This example shows
that the case (upper or lower) of letters used in escapes may not be that the case (upper or lower) of letters used in escapes may not be
preserved. preserved. The example also contains a punycode-encoded domain name
label (xn--99zt52a), which is converted to the corresponding
characters U+7D0D U+8C46 (Japanese Natto).
1) http://www.example.org/%e2%80%ae 1) http://xn--99zt52a.example.org/%e2%80%ae
2) http://www.example.org/<E2><80><AE> 2) http://<E7><B4><8D><E8><B1><86>.example.org/%e2%80%ae
3) http://www.example.org/<E2><80><AE> 3) http://<E7><B4><8D><E8><B1><86>.example.org/<E2><80><AE>
4) http://www.example.org/%E2%80%AE 4) http://<E7><B4><8D><E8><B1><86>.example.org/<E2><80><AE>
5) http://www.example.org/%E2%80%AE 5) http://<E7><B4><8D><E8><B1><86>.example.org/%E2%80%AE
6) http://&#x7D0D;&#x8C46;.example.org/%E2%80%AE
4. Bidirectional IRIs for Right-to-left Languages 4. Bidirectional IRIs for Right-to-left Languages
Some UCS characters, such as those used in the Arabic and Hebrew Some UCS characters, such as those used in the Arabic and Hebrew
script, have an inherent right-to-left writing direction. IRIs script, have an inherent right-to-left (rtl) writing direction. IRIs
containing such characters (called bidirectional IRIs or Bidi IRIs) containing such characters (called bidirectional IRIs or Bidi IRIs)
require additional attention because of the non-trivial relation require additional attention because of the non-trivial relation
between logical representation (used for digital representation as between logical representation (used for digital representation as
well as when reading/spelling) and visual representation (used for well as when reading/spelling) and visual representation (used for
display/printing). display/printing).
Because of the complex interaction between the logical Because of the complex interaction between the logical
representation, the visual representation, and the syntax of a Bidi representation, the visual representation, and the syntax of a Bidi
IRI, a balance is needed between various requirements. The main IRI, a balance is needed between various requirements. The main
requirements are (1) user-predictable conversion between visual and requirements are:
logical representation; (2) the ability to include a wide range of
characters in various parts of the IRI; (3) no or not too big changes 1) user-predictable conversion between visual and logical
or restrictions for implementations. representation;
2) the ability to include a wide range of characters in various
parts of the IRI;
3) no or not too big changes or restrictions for implementations.
4.1 Logical Storage and Visual Presentation 4.1 Logical Storage and Visual Presentation
In their internal digital representation, i.e. stored or transmitted When stored or transmitted in digital representation, bidirectional
for resolution, bidirectional IRIs MUST be in full logical order, and IRIs MUST be in full logical order, and MUST conform to the IRI
MUST conform directly to the IRI syntax rules (which includes the syntax rules (which includes the rules relevant to their scheme).
rules relevant to their scheme). This assures that bidirectional This assures that bidirectional IRIs can be processed in the same way
IRIs can be processed in the same way as other IRIs. as other IRIs.
When rendered, bidirectional IRIs MUST be rendered using the Unicode When rendered, bidirectional IRIs MUST be rendered using the Unicode
Bidirectional Algorithm [UNIV3], [UNI9]. Bidirectional IRIs MUST be Bidirectional Algorithm [UNIV4], [UNI9]. Bidirectional IRIs MUST be
rendered with an overall left-to-right direction. rendered with an overall left-to-right (ltr) direction.
In text with a left-to-right base directionality or embedding (e.g In text with a left-to-right base directionality or embedding (as
English, Cyrillic), the Unicode Bidirectional Algorithm will used for e.g. English or Cyrillic), the Unicode Bidirectional
automatically use an overall left-to-right direction for the IRI. In Algorithm will automatically use an overall ltr direction for the
text with a right-to-left base directionality or embedding (e.g. IRI. In text with a rtl base directionality or embedding (as used
Arabic or Hebrew), some kind of embedding is needed. This may be e.g. for Arabic or Hebrew), setting a different embedding direction
Unicode bidi formatting codes (LRE before the IRI, and PDF after the for the IRI is needed. Setting the embedding direction can be done
IRI, both not part of the IRI itself) or equivalent features of a in a higher-order protocol (e.g. the dir='ltr' attribute in HTML).
higher-order protocol (e.g. the dir='ltr' attribute in HTML). If this is not available (e.g. in plain text), setting the embedding
is done with Unicode bidi formatting codes, i.e. U+202A, LEFT-TO-
RIGHT EMBEDDING (LRE) before the IRI, and U+202C, POP DIRECTIONAL
FORMATTING (PDF) after the IRI, both not being part of the IRI
itself.
IRIs MUST NOT contain bidirectional formatting characters (LRM, RLM, IRIs MUST NOT contain bidirectional formatting characters (LRM, RLM,
LRE, RLE, LRO, RLO, and PDF). They affect the visual rendering of LRE, RLE, LRO, RLO, and PDF). They affect the visual rendering of
the IRI, but do not itself appear visually. It would therefore not the IRI, but do not themselves appear visually. It would therefore
be possible to again correctly input an IRI with such characters. not be possible to correctly input an IRI with such characters.
4.2 Bidi IRI Structure 4.2 Bidi IRI Structure
The Unicode Bidirectional Algorithm is designed mainly for running The Unicode Bidirectional Algorithm is designed mainly for running
text. To make sure that it does not affect the rendering of text. To make sure that it does not affect the rendering of
bidirectional IRIs too much, some restrictions on bidirectional IRIs bidirectional IRIs too much, some restrictions on bidirectional IRIs
are necessary. These restrictions are given in terms of delimiters are necessary. These restrictions are given in terms of delimiters
(structural characters, mostly punctuation such as (structural characters, mostly punctuation such as '@', '.', ':',
'@', '.', ':', '/') and components (usually consisting mostly of '/') and components (usually consisting mostly of letters and
letters and digits). digits).
The following syntax rules from Section 2.2 correspond to components The following syntax rules from Section 2.2 correspond to components
for the purpose of Bidi behavior: iopaquepart, irelsegment, iregname, for the purpose of Bidi behavior: iuserinfo, isegment, ihostname,
iuserinfo, isegment, iparam, ihostname, iquery, and ifragment. iquery, and ifragment.
Specifications that define the syntax of any of the above components Specifications that define the syntax of any of the above components
MAY divide them further and define smaller parts to be components MAY divide them further and define smaller parts to be components
according to this document. As an example, the restrictions of according to this document. As an example, the restrictions of
[RFCXXXX] on bidirectional domain names correspond to treating each [RFC3490] on bidirectional domain names correspond to treating each
label of the domain name as a component. Even where the components label of the domain name as a component. Even where the components
are not defined formally, it may be helpful to think about some are not defined formally, it may be helpful to think about some
syntax in terms of components and to apply the relevant restrictions. syntax in terms of components and to apply the relevant restrictions.
For example, for the usual name/value syntax in query parts, it is For example, for the usual name/value syntax in query parts, it is
convenient to treat each name and each value as a component. As convenient to treat each name and each value as a component. As
another example, the extensions in a resource name can be treated as another example, the extensions in a resource name can be treated as
separate components. separate components.
For each component, the following restrictions apply: For each component, the following restrictions apply:
skipping to change at page 18, line 26 skipping to change at page 17, line 33
2) A component using right-to-left characters SHOULD start and end 2) A component using right-to-left characters SHOULD start and end
with right-to-left characters. with right-to-left characters.
The above restrictions are given as shoulds, rather than as musts. The above restrictions are given as shoulds, rather than as musts.
For IRIs that are never presented visually, they are not relevant. For IRIs that are never presented visually, they are not relevant.
However, for IRIs in general, they are very important to insure However, for IRIs in general, they are very important to insure
consistent conversion between visual presentation and logical consistent conversion between visual presentation and logical
representation, in both directions. representation, in both directions.
In some components, the above restrictions may actually be strictly In some components, the above restrictions may actually be strictly
enforced. For example, [RFCXXXX] requires that these restrictions enforced. For example, [RFC3490] requires that these restrictions
apply to the labels of the host name part of an IRI. In some other apply to the labels of the host name part of an IRI. In some other
components, for example path components, following these restrictions components, for example path components, following these restrictions
may not be too difficult. For other components, such as parts of the may not be too difficult. For other components, such as parts of the
query part, it may be very difficult to enforce the restrictions, query part, it may be very difficult to enforce the restrictions,
because the values of query parameters may be arbitrary character because the values of query parameters may be arbitrary character
sequences. sequences.
In order to satisfy the above restrictions, the affected component If the above restrictions cannot be satisfied otherwise, the affected
can be mapped to URI notation as described in Section 3.1. Please component can always be mapped to URI notation as described in
note that the whole component needs to be mapped (see also Example 9 Section 3.1. Please note that the whole component needs to be mapped
below). (see also Example 9 below).
4.3 Input of Bidi IRIs 4.3 Input of Bidi IRIs
Bidi input methods MUST generate Bidi IRIs in logical order while Bidi input methods MUST generate Bidi IRIs in logical order while
rendering them according to Section 4.1. During input, rendering rendering them according to Section 4.1. During input, rendering
should be updated after every new character that is input to avoid SHOULD be updated after every new character that is input to avoid
end user confusion. end user confusion.
4.4 Examples 4.4 Examples
This section gives examples of bidirectional IRIs, in Bidi Notation. This section gives examples of bidirectional IRIs, in Bidi Notation.
It shows legal IRIs with the relationship between logical and visual It shows legal IRIs with the relationship between logical and visual
representation, and explains how certain phenomena in this representation, and explains how certain phenomena in this
relationship may look strange to somebody not familiar with relationship may look strange to somebody not familiar with
bidirectional behavior, but familiar to users of Arabic and Hebrew. bidirectional behavior, but familiar to users of Arabic and Hebrew.
It also shows what happens if the restrictions given in Section 4.2 It also shows what happens if the restrictions given in Section 4.2
are not followed. The examples below can be seen at [BidiEx], in are not followed. The examples below can be seen at [BidiEx], in
Arabic, Hebrew, and Bidi Notation variants. Arabic, Hebrew, and Bidi Notation variants.
Example 1: A single component with right-to-left (rtl) characters is To read the bidi text in the examples, read the visual representation
inverted: from left to right until you encounter a block of rtl text. Read the
logical representation: http://ab.CDEFGH.ij/kl/mn/op.html, rtl block (including slashes and other special characters) from right
visual representation: http://ab.HGFEDC.ij/kl/mn/op.html. to left, then continue at the next unread ltr character.
Example 1: A single component with rtl characters is inverted:
logical representation: http://ab.CDEFGH.ij/kl/mn/op.html
visual representation: http://ab.HGFEDC.ij/kl/mn/op.html
Components can be read one-by-one, and each component can be read in Components can be read one-by-one, and each component can be read in
its natural direction. its natural direction.
Example 2: More than one consecutive component with rtl characters is Example 2: More than one consecutive component with rtl characters is
inverted as a whole: inverted as a whole:
logical representation: http://ab.CDE.FGH/ij/kl/mn/op.html, logical representation: http://ab.CDE.FGH/ij/kl/mn/op.html
visual representation: http://ab.HGF.EDC/ij/kl/mn/op.html. visual representation: http://ab.HGF.EDC/ij/kl/mn/op.html
A sequence of rtl components is read rtl, in the same way as a A sequence of rtl components is read rtl, in the same way as a
sequence of rtl words is read rtl in a bidi text. sequence of rtl words is read rtl in a bidi text.
Example 3: All components of an IRI (except for the scheme) are rtl. Example 3: All components of an IRI (except for the scheme) are rtl.
All rtl components are inverted overall: All rtl components are inverted overall:
logical representation: http://AB.CD.EF/GH/IJ/KL?MN=OP;QR=ST#UV, logical representation: http://AB.CD.EF/GH/IJ/KL?MN=OP;QR=ST#UV
visual representation: http://VU#TS=RQ;PO=NM?LK/JI/HG/FE.DC.BA. visual representation: http://VU#TS=RQ;PO=NM?LK/JI/HG/FE.DC.BA
The whole IRI (except the scheme) is read rtl. Delimiters between The whole IRI (except the scheme) is read rtl. Delimiters between
rtl components stay between the respective components; delimiters rtl components stay between the respective components; delimiters
between ltr and rtl components don't move. between ltr and rtl components don't move.
Example 4: Several sequences of rtl components are each inverted on Example 4: Several sequences of rtl components are each inverted on
their own: their own:
logical representation: http://AB.CD.ef/gh/IJ/KL.html, logical representation: http://AB.CD.ef/gh/IJ/KL.html
visual representation: http://DC.BA.ef/gh/LK/JI.html. visual representation: http://DC.BA.ef/gh/LK/JI.html
Each sequence of rtl components is read rtl, in the same way as each Each sequence of rtl components is read rtl, in the same way as each
sequence of rtl words in an ltr text is read rtl. sequence of rtl words in an ltr text is read rtl.
Example 5: Example 2, applied to components of different kinds: Example 5: Example 2, applied to components of different kinds:
logical representation: http://ab.cd.EF/GH/ij/kl.html, logical representation: http://ab.cd.EF/GH/ij/kl.html
visual representation: http://ab.cd.HG/FE/ij/kl.html. visual representation: http://ab.cd.HG/FE/ij/kl.html
The inversion of the domain name label and the path component may be The inversion of the domain name label and the path component may be
unexpected, but is consistent with other bidi behavior. unexpected, but is consistent with other bidi behavior. For
reassurance that the domain component really is "ab.cd.EF", it may be
helpful to read aloud the visual representation following the bidi
algorithm. After "http://ab.cd." one reads the RTL block "E-F-slash-
G-H", which corresponds to the logical representation.
Example 6: Same as example 5, with more rtl components: Example 6: Same as example 5, with more rtl components:
logical representation: http://ab.CD.EF/GH/IJ/kl.html, logical representation: http://ab.CD.EF/GH/IJ/kl.html
visual representation: http://ab.JI/HG/FE.DC/kl.html. visual representation: http://ab.JI/HG/FE.DC/kl.html
The inversion of the domain name labels and the path components may The inversion of the domain name labels and the path components may
be easier to identify because the delimiters also move. be easier to identify because the delimiters also move.
Example 7: A single rtl component with included digits: Example 7: A single rtl component with included digits:
logical representation: http://ab.CDE123FGH.ij/kl/mn/op.html, logical representation: http://ab.CDE123FGH.ij/kl/mn/op.html
visual representation: http://ab.HGF123EDC.ij/kl/mn/op.html. visual representation: http://ab.HGF123EDC.ij/kl/mn/op.html
Numbers are written ltr in all cases, but are treated as an Numbers are written ltr in all cases, but are treated as an
additional embedding inside a run of rtl characters. This is additional embedding inside a run of rtl characters. This is
completely consistent with usual bidirectional text. completely consistent with usual bidirectional text.
Example 8 (not allowed): Numbers at the start or end of a rtl Example 8 (not allowed): Numbers at the start or end of a rtl
component: component:
logical representation: http://ab.cd.ef/GH1/2IJ/KL.html, logical representation: http://ab.cd.ef/GH1/2IJ/KL.html
visual representation: http://ab.cd.ef/LK/JI1/2HG.html. visual representation: http://ab.cd.ef/LK/JI1/2HG.html
The sequence '1/2' is interpreted by the bidi algorithm as a The sequence '1/2' is interpreted by the bidi algorithm as a
fraction, fragmenting the components and leading to confusion. There fraction, fragmenting the components and leading to confusion. There
are other characters that are interpreted in a special way close to are other characters that are interpreted in a special way close to
numbers, in particular '+', '-', '#', '$', '%', ',', '.', and ':'. numbers, in particular '+', '-', '#', '$', '%', ',', '.', and ':'.
Example 9 (not allowed): The numbers in the previous example are Example 9 (not allowed): The numbers in the previous example are
escaped: escaped:
logical representation: http://ab.cd.ef/GH%31/%32IJ/KL.html, logical representation: http://ab.cd.ef/GH%31/%32IJ/KL.html,
visual representation (Hebrew): http://ab.cd.ef/LK/JI%32/%31HG.html, visual representation (Hebrew): http://ab.cd.ef/LK/JI%32/%31HG.html
visual representation (Arabic): http://ab.cd.ef/LK/JI32%/31%HG.html. visual representation (Arabic): http://ab.cd.ef/LK/JI32%/31%HG.html
Depending on whether the upper-case letters represent Arabic or Depending on whether the upper-case letters represent Arabic or
Hebrew, the visual representation is different. Hebrew, the visual representation is different.
5. Use of IRIs 5. IRI Equivalence and Comparison
5.1 Limitations on UCS Characters Allowed in IRIs This section discusses IRI Equivalence and Comparison similar to
Section 6, "Normalization and Comparison", in [RFCYYYY]. This
section focusses on the main issues and on aspects that are different
from [RFCYYYY]; Section 6 of [RFCYYYY] is recommended background
reading.
This section discusses the limitations on characters and character There is no general rule or procedure to decide whether two arbitrary
IRIs are equivalent or not (i.e. whether they 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 the same resource. Each specification or application that
uses IRIs has to decide on the appropriate criterion for IRI
equivalence.
5.1 Simple String Comparison
In some scenarios 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. An
example of such a case is XML Namespaces ([XMLNamespace]). In such
cases, two IRIs SHOULD be defined as equivalent if and only if they
are character-by-character equivalent. This is the same as being
byte-by-byte equivalent if the character encoding for both IRIs is
the same. As an example,
http://example.org/~user, http://example.org/%7euser, and
http://example.org/%7Euser are not equivalent under this definition.
In such a case, the comparison function MUST NOT map IRIs to URIs,
because such a mapping would create additional spurious equivalences.
It follows that IRIs SHOULD NOT be modified when being transported if
there is any chance that this IRI might be used as an identifier in
the way explained above.
5.2 Conversion to URIs
For actual resolution, differences in escaping (except for the
escaping of reserved characters) MUST always result in the same
resource. For example, http://example.org/~user,
http://example.org/%7euser and http://example.org/%7Euser 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 (preferably
upper case). For comparison, such conversions MUST only be done on
the fly, while retaining the original IRI.
Additional, similar equivalences are possible based on knowledge
about the generic URI/IRI syntax, such as the fact that the scheme
part is case-insensitive.
5.3 Normalization
The Unicode Standard [UNIV4] defines various equivalences between
sequences of characters for various purposes. Unicode Standard Annex
#15 [UTR15] defines various Normalization Forms for these
equivalences, in particular Normalization Form C (NFC, Canonical
Decomposition, followed by Canonical Composition) and Normalization
Form KC (NFKC, Compatibility Decomposition, followed by Canonical
Composition).
Equivalence of IRIs MUST rely on the assumption that IRIs are
appropriately pre-normalized, rather than applying normalization when
comparing two IRIs. The exceptions are convertsion from a non-
digital form, and conversion from a non-UCS-based encoding to an UCS-
based encoding. In these cases, NFC or a normalizing transcoder
using NFC MUST be used for interoperability. To avoid false
negatives and problems with transcoding, IRIs SHOULD be created using
NFC. Using NFKC will avoid even more problems.
As an example, http://www.example.org/r&#xE9;sum&#xE9;.html (in XML
Notation) is in NFC. On the other hand, http://www.example.org/
re&#x301;sume&#x301;.html is not in NFC. The former uses precombined
e-acute characters, the later uses 'e' characters followed by
combining acute accents. Both usages are defined to be canonically
equivalent in [UNIV4].
Because we do not know how a particular field is treated with respect
to text normalization, it would be inappropriate to allow third
parties to normalize an IRI arbitrarily. This does not contradict
the recommendation that if you create a resource, and an IRI for that
resource, you try to be as normalized as possible (i.e. NFKC if
possible). This is similar to the upper-case/lower-case problems in
URIs. Some parts of an URI are case-insensitive (domain name). For
others, it is unclear whether they are case-sensitive or case-
insensitive, or something in between (e.g. case-sensitive, but if
you use the wrong case, may not directly get a result, but rather a
'Multiple choices'). The best recipe we have there is that the
generator uses a reasonable capitalization, and when transfering the
URI, you do not change capitalization.
Various IRI schemes may allow the usage of International Domain Names
(IDN) [RFC3490]. When in use in IRIs, those names SHOULD be
validated using the ToASCII operation defined in [RFC3490], with the
flags "UseSTD3ASCIIRules" and "AllowUnassigned". 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 is applied when
mapping an IRI into an URI, see Section 3.1.
5.4 Preferred Forms
The following are the preferred forms for IRIs when generated:
- Always provide the URI scheme in lowercase characters.
- Only perform percent-escaping where it is essential.
- Always use uppercase A-through-F characters when percent-
escaping.
- Always provide the hostname, if any, in the form produced when
applying [RFC3491]. This in particular includes using
lowercase characters rather than uppercase characters where
applicable.
- Where possible, provide IRI components in NFKC or NFC.
- Prevent /./ and /../ from appearing in non-relative URI paths.
6. Use of IRIs
6.1 Limitations on UCS Characters Allowed in IRIs
This section discusses limitations on characters and character
sequences usable for IRIs. The considerations in this section are sequences usable for IRIs. The considerations in this section are
relevant when creating IRIs and when converting from URIs to IRIs. 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 the scheme component does not allow characters definition of the scheme component does not allow characters
beyond US-ASCII. 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) The UCS contains many areas of characters for which there are
delimit URIs in text and print ("space", "delims", and
"unwise") were excluded. They are included in the IRI syntax
(with the exception of '%', which cannot be used directly, and
'#', which is used in IRI references), for the following
reasons:
1) The syntax includes many other characters that are not
appropriate in many cases.
2) Some implementation practice already allows them in URI
references (for example spaces in fragment identifiers).
3) It is very convenient in some cases, for example for
XPointers in XML attributes.
4) Considering context is already necessary in the case of
URIs, for example for "&amp;" in XML.
However, these characters should be avoided where possible.
Whenever there is a chance that an IRI will be used in a
component where these characters can be harmful, they should be
escaped from the start.
c) 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 [RFC3491].
Additional information is available from [UNIXML]. [UNIXML] is Additional information is available from [UNIXML]. [UNIXML] is
written in the context of running text rather than in the context of written in the context of running text rather than in the context of
identifiers. Nevertheless, it discusses many of the categories of identifiers. Nevertheless, it discusses many of the categories of
characters and code points not appropriate for IRIs. characters and code points not appropriate for IRIs.
5.2 Software Interfaces and Protocols 6.2 Software Interfaces and Protocols
Although an IRI is defined as a sequence of characters, software Although an IRI is defined as a sequence of characters, software
interfaces for URIs typically function on sequences of octets or interfaces for URIs typically function on sequences of octets or
other kinds of code units. Thus, software interfaces and protocols other kinds of code units. Thus, software interfaces and protocols
MUST define which character encoding is used. MUST define which character encoding is used.
Intermediate software interfaces between IRI-capable components and Intermediate software interfaces between IRI-capable components and
URI-only components MUST map the IRIs as per Section 3.1, when URI-only components MUST map the IRIs per Section 3.1, when
transferring from IRI-capable to URI-only components. Such a mapping transferring from IRI-capable to URI-only components. Such a mapping
SHOULD be applied as late as possible. It should not be applied SHOULD be applied as late as possible. It should not be applied
between components that are known to be able to handle IRIs. between components that are known to be able to handle IRIs.
5.3 Format of URIs and IRIs in Documents and Protocols 6.3 Format of URIs and IRIs in Documents and Protocols
Document formats that transport URIs may need to be upgraded to allow 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 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 has a native character encoding, IRIs MUST also be encoded in this
encoding, and converted accordingly by a parser or interpreter. IRI encoding, and converted accordingly by a parser or interpreter. IRI
characters that are not expressible in the native encoding SHOULD be characters that are not expressible in the native encoding SHOULD be
escaped using the escaping conventions of the document format if such escaped using the escaping conventions of the document format if such
conventions are available. Alternatively, they MAY be escaped conventions are available. Alternatively, they MAY be escaped
according to Section 3.1. For example, in HTML, XML, or SGML, according to Section 3.1. For example, in HTML or XML, numeric
numeric character references should be used. If a document as a character references SHOULD be used. If a document as a whole has a
whole has a native character encoding, and that character encoding is native character encoding, and that character encoding is not UTF-8,
not UTF-8, then IRIs MUST NOT be placed into the document in the UTF- then IRIs MUST NOT be placed into the document in the UTF-8 character
8 character encoding. encoding.
Note: Some formats already accommodate IRIs, although they use Note: Some formats already accommodate IRIs, although they use
different terminology. HTML 4.0 [HTML4] defines the conversion from different terminology. HTML 4.0 [HTML4] defines the conversion from
IRIs to URIs as error-avoiding behavior. XML 1.0 [XML1], XLink IRIs to URIs as error-avoiding behavior. XML 1.0 [XML1], XLink
[XLink], and XML Schema [XMLSchema] and specifications based upon [XLink], and XML Schema [XMLSchema] and specifications based upon
them allow IRIs. Also, it is expected that all relevant new W3C them allow IRIs. Also, it is expected that all relevant new W3C
formats and protocols will be required to handle IRIs [CharMod]. formats and protocols will be required to handle IRIs [CharMod].
5.4 Relative IRI References 6.4 Use of UTF-8 for Encoding Original Characters
This section discusses details and gives examples for point c) in
Section 1.2. In order to be able to use IRIs, the URI corresponding
to the IRI in question has to encode original characters into octets
using UTF-8. This can be specified for all URIs of an URI scheme, or
can apply to individual URIs for schemes that do not specify how to
encode original characters. It can apply to the whole URI, or only
some part.
For new URI schemes, using UTF-8 is recommended in [RFC2718].
Examples where this is already used are the URN syntax [RFC2141],
IMAP URLs [RFC2192], and POP URLs [RFC2384]. On the other hand, the
HTTP URL scheme does not specify how to encode original characters,
and therefore IRIs only can be used for some HTTP URLs.
For example, for a document with a URI of
http://www.example.org/r%C3%A9sum%C3%A9.html, it is possible to
construct a corresponding IRI (in XML notation, see Section 1.4):
http://www.example.org/r&#xE9;sum&#xE9;.html (&#xE9; stands for the
e-acute character, and %C3%A9 is the UTF-8 encoded and escaped
representation of that character). On the other hand, for a document
with an URI of http://www.example.org/r%E9sum%E9.html, the escaped
octets cannot be converted to actual characters in an IRI, because
the escaping is not based on UTF-8.
The requirement for the use of UTF-8 applies to all parts of an URI,
with the exception of the ihostname part. However, it is possible
that the capability of IRIs to represent a wide range of characters
directly is used just in some parts of the IRI (or IRI reference).
The other parts of the IRI may only contain ASCII characters, or they
may not be based on UTF-8. They may be based on another encoding, or
they may directly encode raw binary data (see also [RFC2397]).
For example, it is possible to have an URI reference of
http://www.example.org/r%E9sum%E9.xml#r%C3%A9sum%C3%A9, where the
document name is encoded in iso-8859-1 based on server settings, but
the fragment identifier is encoded in UTF-8 according to [XPointer].
The IRI corresponding to the above URI would be (in XML notation)
http://www.example.org/r%E9sum%E9.xml#r&#xE9;sum&#xE9;.
@@@@ add something about query parts
6.5 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 [RFCYYYY] can 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. same way as unreserved characters in URIs.
6. URI/IRI Processing Guidelines (informative) 7. URI/IRI Processing Guidelines (informative)
This informative section provides guidelines for supporting IRIs in This informative section provides guidelines for supporting IRIs 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.
6.1 URI/IRI Software Interfaces 7.1 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.
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 API or protocol definition must clearly define the
encoding to be used.
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 3.2 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.
6.2 URI/IRI Entry 7.2 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
to allow for IRI entry. 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
restrictions defined in Section 2.2 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.
As an example of variant settings, input method editors for East As an example of variant settings, input method editors for East
Asian Languages usually allow to input Latin letters and related Asian Languages usually allow the input of Latin letters and related
characters in full-width or half-width versions. For IRI input, the characters in full-width or half-width versions. For IRI input, the
input method editor should be set to half-width input, in order to input method editor should be set to half-width input, in order to
produce US-ASCII characters where possible. produce US-ASCII characters where possible.
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 mapped to a URI. Places should allow the user to view an IRI as mapped 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 mapped to a URI. This will help users when for viewing an IRI as mapped to a URI. This will help users when
some of the software they use does not yet accept IRIs. 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 map the the IRI to an URI before passing it URIs, but not IRIs, must map the IRI to a URI before passing it to
to such a component. such a 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
Section 4.3. Section 4.3.
6.3 URI/IRI Transfer Between Applications 7.3 URI/IRI Transfer Between Applications
Many applications, in particular many mail user agents, try to detect Many applications, in particular many mail user agents, try to detect
URIs appearing in plain text. For this, they use some heuristics URIs appearing in plain text. For this, they use some heuristics
based on URI syntax. They then allow the user to click on such URIs based on URI syntax. They then allow the user to click on such URIs
and retrieve the corresponding resource in an appropriate (usually and retrieve the corresponding resource in an appropriate (usually
scheme-dependent) application. scheme-dependent) application.
Such applications have to be upgraded to use the IRI syntax rather Such applications have to be upgraded to use the IRI syntax rather
than the URI syntax as a base for heuristics. In particular, a non- than the URI syntax as a base for heuristics. In particular, a non-
ASCII character should not be taken as the indication of the end of ASCII character should not be taken as the indication of the end of
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application where the IRI appears to the encoding used by the system- application where the IRI appears to the encoding used by the system-
wide IRI invocation mechanism, or to an URI (according to Section wide IRI invocation mechanism, or to an URI (according to Section
3.1) if the system-wide invocation mechanism only accepts URIs. 3.1) if the system-wide invocation mechanism only accepts URIs.
The clipboard is another frequently used way to transfer URIs and The clipboard is another frequently used way to transfer URIs and
IRIs from one application to another. On most platforms, the IRIs from one application to another. On most platforms, the
clipboard is able to store and transfer text in many languages and clipboard is able to store and transfer text in many languages and
scripts. Correctly used, the clipboard transfers characters, not scripts. Correctly used, the clipboard transfers characters, not
bytes, which will do the right thing with IRIs. bytes, which will do the right thing with IRIs.
6.4 URI/IRI Generation 7.4 URI/IRI Generation
Systems that are offering resources through the Internet, where those Systems that offer 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.
For maximum interoperability, systems that generate resource For maximum interoperability, systems that generate resource
identifiers should do the appropriate transformations. They should identifiers should do the appropriate transformations. For example,
use IRIs converted to URIs in cases where it cannot be expected that if a file system contains a file named r&#xE9;sum&#xE9;.html, a
the recipient is able to handle IRIs. Due to the way most user server should expose this as r%C3%A9sum%C3%A9.html in an URI, which
agents currently work, native IRIs, encoded in UTF-8, may be used if allows to use r&#xE9;sum&#xE9;.html in an IRI, even if the file name
the recipient announces that it can interpret UTF-8. This requires locally is kept in an encoding other than UTF-8.
that the whole page is sent as UTF-8. If this is not possible,
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].
6.5 URI/IRI Selection 7.5 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
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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 characters in a single component should be generated where all the characters in a single component
are used together in a given language. This usually means that all are used together in a given language. This usually means that all
these characters will be from the same script, but there are these characters will be from the same script, but there are
languages that mix characters from different scripts (such as languages that mix characters from different scripts (such as
Japanese). This is similar to the heuristics used to distinguish Japanese). This is similar to the heuristics used to distinguish
between letters and numbers in the examples above. Also, for Latin, between letters and numbers in the examples above. Also, for Latin,
Greek, and Cyrillic, using lower-case letters results in fewer Greek, and Cyrillic, using lower-case letters results in fewer
ambiguities than using upper-case letters. ambiguities than using upper-case letters.
6.6 Display of URIs/IRIs 7.6 Display of URIs/IRIs
In situations where the rendering software is not expected to display In situations where the rendering software is not expected to display
non-ASCII parts of the IRI correctly using the available layout and non-ASCII parts of the IRI correctly using the available layout and
font resources, these parts should be escaped before being displayed. font resources, these parts should be escaped before being displayed.
For display of Bidi IRIs, please see Section 4.1. For display of Bidi IRIs, please see Section 4.1.
6.7 Interpretation of URIs and IRIs 7.7 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
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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 [Duerst97]) makes the potential for regularity of UTF-8 (see [Duerst97]) makes the potential for
collisions lower than it may seem at first sight. collisions lower than it may seem at first sight.
6.8 Upgrading Strategy 7.8 Upgrading Strategy
Where this recommendation places further constraints on software for Where 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.
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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.
7. Security Considerations 8. Security Considerations
Incorrect escaping or unescaping can lead to security problems. In Incorrect escaping or unescaping can lead to security problems. In
particular, some UTF-8 decoders do not check against overlong byte particular, some UTF-8 decoders do not check against overlong byte
sequences. As an example, a '/' is encoded with the byte 0x2F both sequences. As an example, a '/' is encoded with the byte 0x2F both
in UTF-8 and in ASCII, but some UTF-8 decoders also wrongly interpret in UTF-8 and in ASCII, but some UTF-8 decoders also wrongly interpret
the sequence 0xC0 0xAF as a '/'. A sequence such as '%C0%AF..' may the sequence 0xC0 0xAF as a '/'. A sequence such as '%C0%AF..' may
pass some security tests and then be interpreted as '/..' in a path pass some security tests and then be interpreted as '/..' in a path
if UTF-8 decoders are fault-tolerant, if conversion and checking are if UTF-8 decoders are fault-tolerant, if conversion and checking are
not done in the right order, and/or if reserved characters and not done in the right order, and/or if reserved characters and
unreserved characters are not clearly distinguished. unreserved characters are not clearly distinguished.
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There are various ways in which "spoofing" can occur with IRIs. 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, but points to a different resource. The same or similar to the user, but points to a different resource. The
added resource may pretend to be the real resource by looking very added resource may pretend to be the real resource by looking very
similar, but may contain all kinds of changes that may be difficult similar, but may contain all kinds of changes that may be difficult
to spot but can cause all kinds of problems. Most spoofing to spot but can cause all kinds of problems. Most spoofing
possibilities for IRIs are extensions of those for URIs. possibilities for IRIs are extensions of those for URIs.
Spoofing can occur for various reasons. A first reason is that Spoofing can occur for various reasons. A first reason is that
normalization expectations of a user or actual normalization when normalization expectations of a user or actual normalization when
entering an IRI do not match the normalization used on the server entering an IRI, or when transcoding an IRI from a legacy encoding,
side. Conceptually, this is no different from the problems do not match the normalization used on the server side.
surrounding the use of case-insensitive web servers. For example, a Conceptually, this is no different from the problems surrounding the
popular web page with a mixed case name (http://big.site/ use of case-insensitive web servers. For example, a popular web page
PopularPage.html) might be "spoofed" by someone who is able to create with a mixed case name (http://big.site/PopularPage.html) might be
http://big.site/popularpage.html. However, the introduction of "spoofed" by someone who is able to create http://big.site/
character normalization, and of additional mappings for user popularpage.html. However, the introduction of character
convenience, may increase the chance for spoofing. normalization, and of additional mappings for user convenience, may
increase the chance for spoofing. Protocols and servers that allow
the creation of resources with unnormalized names, and resources with
names that are not normalized, are particularly vulnerable to such
attacks. This is an inherent security problem of the relevant
protocol, server, or resource, and not specific to IRIs, but
mentioned here for completeness.
Spoofing can occur because in the UCS, there are many characters that Spoofing can occur because in the UCS, there are many characters that
look very similar. Details are discussed in Section 6.5. Again, look very similar. Details are discussed in Section 7.5. Again,
this is very similar to spoofing possibilities on US-ASCII, e.g. this is very similar to spoofing possibilities on US-ASCII, e.g.
using 'br0ken' or '1ame' URIs. using 'br0ken' or '1ame' URIs.
Spoofing can occur when URIs in various encodings are accepted to Spoofing can occur when URIs in various encodings are accepted to
deal with older user agents. In some cases, in particular for Latin- deal with older user agents. In some cases, in particular for Latin-
based resource names, this is usually easy to detect because UTF-8- based resource names, this is usually easy to detect because UTF-8-
encoded names, when interpreted and viewed as legacy encodings, encoded names, when interpreted and viewed as legacy encodings,
produce mostly garbage. In other cases, when concurrently used produce mostly garbage. In other cases, when concurrently used
encodings have a similar structure, but there are no characters that encodings have a similar structure, but there are no characters that
have exactly the same encoding, detection is more difficult. have exactly the same encoding, detection is more difficult.
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Spoofing can occur when URIs in various encodings are accepted to Spoofing can occur when URIs in various encodings are accepted to
deal with older user agents. In some cases, in particular for Latin- deal with older user agents. In some cases, in particular for Latin-
based resource names, this is usually easy to detect because UTF-8- based resource names, this is usually easy to detect because UTF-8-
encoded names, when interpreted and viewed as legacy encodings, encoded names, when interpreted and viewed as legacy encodings,
produce mostly garbage. In other cases, when concurrently used produce mostly garbage. In other cases, when concurrently used
encodings have a similar structure, but there are no characters that encodings have a similar structure, but there are no characters that
have exactly the same encoding, detection is more difficult. have exactly the same encoding, detection is more difficult.
Spoofing can occur in various IRI components, such as the domain name 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 or a path part. For considerations specific to the domain name
part, see [Nameprep]. For the path part, administrators of sites part, see [RFC3491]. For the path part, administrators of sites
which allow independent users to create resources in the same subarea which allow independent users to create resources in the same subarea
may need to be careful to check for spoofing. may need to be careful to check for spoofing.
Spoofing can occur with bidirectional IRIs, if the restrictions in Spoofing can occur with bidirectional IRIs, if the restrictions in
Section 4.2 are not followed. The same visual representation may be Section 4.2 are not followed. The same visual representation may be
interpreted as different logical representations, and vice versa. It interpreted as different logical representations, and vice versa. It
is also very important that a correct Unicode bidirectional is also very important that a correct Unicode bidirectional
implementation is used. implementation is used.
8. Issues List 9. Acknowledgements
- Should characters in iadditional be allowed? Under what
conditions?.
- Allign the description in Section 2.3 with the results of W3C
TAG discussions on issue URIEquivalence.
- Adapt depending on how [IDNURI] is integrated into
[RFC2396bis].
9. Change log
9.1 Changes from -02 to -03
- Added an issues list.
- Added a paragraph prohibiting conversions from URIs to IRIs not
based on UTF-8 to Section 3.2.
- Introduced iadditional to combine unwise, delims, and space.
- Tweaked description and added examples for URI-to-IRI
conversion.
- Improved syntax rules for hostname part.
- Improved description of equivalences in Section 2.3.
- Improved description of URI-to-IRI-mapping in Section 3.2.
- Changed preferred case when hex-escaping from lower to UPPER.
- Fixed various details.
9.2 Changes from -01 to -02
- New approach for Bidi section, many examples.
- Created idelims, removed '%' and '#'. Changed userinfo to
iuserinfo in iserver.
- Changed to ABNF defined by [RFC2234].
- Included bug fixes from [RFC2396bis].
- Additions to Acknowledgements.
9.3 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,...
10. Acknowledgements
We would like to thank Larry Masinter for his work as coauthor of We would like to thank Larry Masinter for his work as coauthor of
many earlier versions of this document (draft-masinter-url-i18n-xx). many earlier versions of this document (draft-masinter-url-i18n-xx).
The discussion on the issue addressed here has started a long time The discussion on the issue addressed here has started a long time
ago. There was a thread in the HTML working group in August 1995 ago. There was a thread in the HTML working group in August 1995
(under the topic of "Globalizing URIs") and in the www-international (under the topic of "Globalizing URIs") and in the www-international
mailing list in July 1996 (under the topic of "Internationalization mailing list in July 1996 (under the topic of "Internationalization
and URLs"), and ad-hoc meetings at the Unicode conferences in and URLs"), and ad-hoc meetings at the Unicode conferences in
September 1995 and September 1997. September 1995 and September 1997.
Thanks to Francois Yergeau, Matti Allouche, Roy Fielding, Tim Thanks to Francois Yergeau, Matti Allouche, Roy Fielding, Tim
Berners-Lee, Mark Davis, M.T. Carrasco Benitez, James Clark, Tim Berners-Lee, Mark Davis, M.T. Carrasco Benitez, James Clark, Tim
Bray, Chris Wendt, Yaron Goland, Andrea Vine, Misha Wolf, Leslie Bray, Chris Wendt, Yaron Goland, Andrea Vine, Misha Wolf, Leslie
Daigle, Ted Hardie, Makoto MURATA, Steven Atkin, Ryan Stansifer, Tex Daigle, Ted Hardie, Makoto MURATA, Steven Atkin, Ryan Stansifer, Tex
Texin, Graham Klyne, Bjoern Hoehrmann, Chris Lilley, Dan Oscarson, Texin, Graham Klyne, Bjoern Hoehrmann, Chris Lilley, Ian Jacobs, Dan
Elliotte Rusty Harold, Mike J. Brown, Carlos Viegas Damasio, and Oscarson, Elliotte Rusty Harold, Mike J. Brown, Simon Josefsson,
many others for help with understanding the issues and possible Carlos Viegas Damasio, and many others for help with understanding
solutions, and getting the details right. Thanks also to the members the issues and possible solutions, and getting the details right.
of the W3C I18N Working Group and Interest Group for their Thanks also to the members of the W3C I18N Working Group and Interest
contributions and their work on [CharMod], to the members of many Group for their contributions and their work on [CharMod], to the
other W3C WGs for adopting the ideas, and to the members of the members of many other W3C WGs for adopting the ideas, and to the
Montreal IAB Workshop on Internationalization and Localization for members of the Montreal IAB Workshop on Internationalization and
their review. Localization for their review.
Normative References Normative References
[ISO10646] International Organization for Standardization, [ISO10646] International Organization for Standardization,
"Information Technology - Universal Multiple-Octet Coded "Information Technology - Universal Multiple-Octet Coded
Character Set (UCS) - Part 1: Architecture and Basic Character Set (UCS) - Part 1: Architecture and Basic
Multilingual Plane - Part 2: Supplementary Planes", ISO Multilingual Plane - Part 2: Supplementary Planes", ISO
Standard 10646, with amendment, July 2002. Standard 10646, with amendment, July 2002.
[RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax [RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997. Specifications: ABNF", RFC 2234, November 1997.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO [RFC3490] Faltstrom, P., Hoffman, P. and A. Costello,
10646", RFC 2279, January 1998. "Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003, <http://www.ietf.org/rfc/
rfc3490.txt>.
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform [RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Resource Identifiers (URI): Generic Syntax", RFC 2396, Profile for Internationalized Domain Names (IDN)", RFC
August 1998. 3491, March 2003.
[RFC2732] Hinden, R., Carpenter, B. and L. Masinter, "Format for [RFCXXXX] Yergeau, F., "UTF-8, a transformation format of ISO
Literal IPv6 Addresses in URL's", RFC 2732, December 10646", draft-yergeau-rfc2279bis-05.txt (work in
1999. progress), June 2003, <http://www.ietf.org/internet-
drafts/draft-yergeau-rfc2279bis-05.txt>.
[RFCXXXX] Faltstrom, P., Hoffman, P. and A. Costello, [RFCYYYY] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
"Internationalizing Domain Names in Applications (IDNA)", Resource Identifier (URI): Generic Syntax", draft-
draft-ietf-idn-idna-14.txt (work in progress), October fielding-uri-rfc2396bis-03.txt (work in progress), June
2002, <http://www.ietf.org/internet-drafts/draft-ietf- 2003.
idn-idna-14.txt>.
[UTR15] Davis, M. and M. Duerst, "Unicode Normalization Forms", [UTR15] Davis, M. and M. Duerst, "Unicode Normalization Forms",
Unicode Standard Annex #15, March 2001, <http:// Unicode Standard Annex #15, March 2001, <http://
www.unicode.org/unicode/reports/tr15/tr15-21.html>. www.unicode.org/unicode/reports/tr15/tr15-21.html>.
Non-normative References Non-normative References
[BidiEx] "Examples of bidirectional IRIs", <http://www.w3.org/ [BidiEx] "Examples of bidirectional IRIs", <http://www.w3.org/
International/iri-edit/BidiExamples>. International/iri-edit/BidiExamples>.
skipping to change at page 31, line 31 skipping to change at page 32, line 35
From Specification to Testing", Proc. 19th From Specification to Testing", Proc. 19th
International Unicode Conference, San Jose , International Unicode Conference, San Jose ,
September 2001, <http://www.w3.org/2001/Talks/0912- September 2001, <http://www.w3.org/2001/Talks/0912-
IUC-IRI/paper.html>. 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>.
[IDNURI] Duerst, M., "Internationalized Domain Names in URIs",
draft-ietf-idn-uri-03.txt (work in progress),
November 2002, <http://www.ietf.org/internet-drafts/
draft-ietf-idn-uri-03.txt>.
[Nameprep] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names", draft-
ietf-idn-nameprep-11.txt (work in progress), June
2002, <http://www.ietf.org/internet-drafts/draft-
ietf-idn-nameprep-11.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, [RFC2130] Weider, C., Preston, C., Simonsen, K., Alvestrand,
H., Atkinson, R., Crispin, M. and P. Svanberg, "The H., Atkinson, R., Crispin, M. and P. Svanberg, "The
Report of the IAB Character Set Workshop held 29 Report of the IAB Character Set Workshop held 29
February - 1 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 [RFC2192] Newman, C., "IMAP URL Scheme", RFC 2192, September
1997. 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.
[RFC2384] Gellens, R., "POP URL Scheme", RFC 2384, August 1998. [RFC2384] Gellens, R., "POP URL Scheme", RFC 2384, August 1998.
[RFC2396bis] Berners-Lee, T., Fielding, R. and L. Masinter, [RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter,
"Uniform Resource Identifier (URI): Generic Syntax", "Uniform Resource Identifiers (URI): Generic Syntax",
Internet-Draft (work in progress), October 2002. RFC 2396, August 1998.
[RFC2397] Masinter, L., "The "data" URL scheme", RFC 2397, [RFC2397] Masinter, L., "The "data" URL scheme", RFC 2397,
August 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, Masinter, L., Leach, P. and T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
June 1999. June 1999.
[RFC2640] Curtin, B., "Internationalization of the File [RFC2640] Curtin, B., "Internationalization of the File
Transfer Protocol", RFC 2640, July 1999. Transfer Protocol", RFC 2640, July 1999.
[RFC2718] Masinter, L., Alvestrand, H., Zigmond, D. and R. [RFC2718] Masinter, L., Alvestrand, H., Zigmond, D. and R.
Petke, "Guidelines for new URL Schemes", RFC 2718, Petke, "Guidelines for new URL Schemes", RFC 2718,
November 1999. November 1999.
[UNIV3] The Unicode Consortium, "The Unicode Standard Version [UNIV4] The Unicode Consortium, "The Unicode Standard,
3.0", Addison-Wesley, Reading, MA , 2000. Version 4.0", Addison-Wesley, Reading, MA , 2003.
[UNI9] Davis, M., "The Bidirectional Algorithm", Unicode [UNI9] Davis, M., "The Bidirectional Algorithm", Unicode
Standard Annex #9, March 2002, <http:// Standard Annex #9, March 2002, <http://
www.unicode.org/unicode/reports/tr9>. www.unicode.org/unicode/reports/tr9>.
[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, Markup Languages", Unicode Technical Report #20,
World Wide Web Consortium Note, February 2002, World Wide Web Consortium Note, February 2002,
<http://www.w3.org/TR/unicode-xml/>. <http://www.w3.org/TR/unicode-xml/>.
skipping to change at page 33, line 21 skipping to change at page 34, line 14
[XMLNamespace] Bray, T., Hollander, D. and A. Layman, "Namespaces in [XMLNamespace] Bray, T., Hollander, D. and A. Layman, "Namespaces in
XML", World Wide Web Consortium Recommendation, XML", World Wide Web Consortium Recommendation,
January 1999, <http://www.w3.org/TR/REC-xml#sec- January 1999, <http://www.w3.org/TR/REC-xml#sec-
external-ent>. 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>.
[XPointer] Grosso, P., Maler, E., Marsh, J. and N. Walsh,
"XPointer Framework", World Wide Web Consortium
Recommendation, March 2003, <http://www.w3.org/TR/
xptr-framework/#escaping>.
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.)
World Wide Web Consortium World Wide Web Consortium
200 Technology Square 200 Technology Square
Cambridge, MA 02139 Cambridge, MA 02139
U.S.A. U.S.A.
Phone: +1 617 253 5509 Phone: +1 617 253 5509
 End of changes. 

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