WDIFF

draft-duerst-iri-03.txt --> draft-duerst-iri-04.txt

Network Working Group M. Duerst
Internet-Draft W3C
Expires: August 31, December 28, 2003 M. Suignard
Microsoft Corporation
March 2,
June 29, 2003

Internationalized Resource Identifiers (IRIs)
draft-duerst-iri-03
draft-duerst-iri-04

Status of this Memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at http://
www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

This Internet-Draft will expire on August 31, December 28, 2003.

Abstract

This document defines a new protocol element, the Internationalized
Resource Identifier (IRI), as a complement to the URI [RFC2396]. [RFCYYYY]. An
IRI is a sequence of characters from the Universal Character Set
[ISO10646]. A mapping from IRIs to URIs is defined, which means that
IRIs can be used instead of URIs where appropriate to identify
resources.

The approach of defining a new protocol element was chosen, instead
of extending or changing the definition of URIs, to allow a clear
distinction and to avoid incompatibilities with existing software.

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Guidelines for the use and deployment of IRIs in various protocols,
formats, and software components that now deal with URIs are
provided.

NOTE

This document is a product of the Internationalization Working Group
(I18N WG) of the World Wide Web Consortium (W3C). For general
discussion, please use the www-international@w3.org public-iri@w3.org mailing list (publicly
archived at http://lists.w3.org/Archives/Public/www-
international/). http://lists.w3.org/Archives/Public/public-iri/). An
issues list for this document is maintained at http://www.w3.org/
International/iri-edit#issues. For more information on the topic of
this document, please also see [W3CIRI] and [Duerst01].

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Overview and Motivation . . . . . . . . . . . . . . . . . . 4
1.2 Applicability . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Definitions . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Notation . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 7 6
2.1 Summary of IRI Syntax . . . . . . . . . . . . . . . . . . . 7
2.2 ABNF for IRI References and IRIs . . . . . . . . . . . . . . 7
2.3 IRI Equivalence and Normalization . . . . . . . . . . . . . 10
3. Relationship between IRIs and URIs . . . . . . . . . . . . . 11 10
3.1 Mapping of IRIs to URIs . . . . . . . . . . . . . . . . . . 12 10
3.2 Converting URIs to IRIs . . . . . . . . . . . . . . . . . . 14 12
3.2.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 15 14
4. Bidirectional IRIs for Right-to-left Languages . . . . . . . 16 15
4.1 Logical Storage and Visual Presentation . . . . . . . . . . 17 16
4.2 Bidi IRI Structure . . . . . . . . . . . . . . . . . . . . . 17 16
4.3 Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . . . 18 17
4.4 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5. Use of IRIs . . . . . . . . . IRI Equivalence and Comparison . . . . . . . . . . . . . . . 20 19
5.1 Limitations on UCS Characters Allowed in IRIs Simple String Comparison . . . . . . . 20
5.2 Software Interfaces and Protocols . . . . . . . . . . . 20
5.2 Conversion to URIs . . 21
5.3 Format of URIs and IRIs in Documents and Protocols . . . . . 21
5.4 Relative IRI References . . . . . . . . . . . . . . 20
5.3 Normalization . . . . 22
6. URI/IRI Processing Guidelines (informative) . . . . . . . . 22
6.1 URI/IRI Software Interfaces . . . . . . . . . . . 20
5.4 Preferred Forms . . . . . 22
6.2 URI/IRI Entry . . . . . . . . . . . . . . . . . 21
6. Use of IRIs . . . . . . 23
6.3 URI/IRI Transfer Between Applications . . . . . . . . . . . 23
6.4 URI/IRI Generation . . . . . . . 22
6.1 Limitations on UCS Characters Allowed in IRIs . . . . . . . 22
6.2 Software Interfaces and Protocols . . . . . . . 24
6.5 URI/IRI Selection . . . . . . 22
6.3 Format of URIs and IRIs in Documents and Protocols . . . . . 23
6.4 Use of UTF-8 for Encoding Original Characters . . . . . . . 23
6.5 Relative IRI References . . . 24
6.6 Display of URIs/IRIs . . . . . . . . . . . . . . . 24
7. URI/IRI Processing Guidelines (informative) . . . . . 25
6.7 Interpretation of URIs and IRIs . . . 24
7.1 URI/IRI Software Interfaces . . . . . . . . . . . 25
6.8 Upgrading Strategy . . . . . 24
7.2 URI/IRI Entry . . . . . . . . . . . . . . . . 26
7. Security Considerations . . . . . . . 25
7.3 URI/IRI Transfer Between Applications . . . . . . . . . . . 27 26

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8. Issues List . .

7.4 URI/IRI Generation . . . . . . . . . . . . . . . . . . . . . 26
7.5 URI/IRI Selection . 28
9. Change log . . . . . . . . . . . . . . . . . . . . 27
7.6 Display of URIs/IRIs . . . . . 28
9.1 Changes from -02 to -03 . . . . . . . . . . . . . . . 27
7.7 Interpretation of URIs and IRIs . . . 28
9.2 Changes from -01 to -02 . . . . . . . . . . . 28
7.8 Upgrading Strategy . . . . . . . 29
9.3 Changes from -00 to -01 . . . . . . . . . . . . . . 28
8. Security Considerations . . . . . . . . . . . . . . . . . . 29
10.
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29 30
Normative References . . . . . . . . . . . . . . . . . . . . 30 31
Non-normative References . . . . . . . . . . . . . . . . . . 31 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 33 34
Full Copyright Statement . . . . . . . . . . . . . . . . . . 34 35

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1. Introduction

1.1 Overview and Motivation

A URI is defined in [RFC2396] [RFCYYYY] as a sequence of characters chosen from
a limited subset of the repertoire of US-ASCII characters.

The characters in URIs are frequently used for representing words of
natural languages. Such usage has many advantages: such URIs are
easier to memorize, easier to interpret, easier to transcribe, easier
to create, and easier to guess. For most languages other than
English, however, the natural script uses characters other than A-Z.
For many people, handling Latin characters is as difficult as
handling the characters of other scripts is for people who use only
the Latin alphabet. Many languages with non-Latin scripts do have
transcriptions to Latin letters and such letters. Such transcriptions are now often
used in URIs, but they introduce additional ambiguities.

The infrastructure for the appropriate handling of characters from
local scripts is now widely deployed in local versions of operating
system and application software. Software that can handle a wide
variety of scripts and languages at the same time is increasingly
widespread. Also, there are increasing numbers of protocols and
formats that can carry a wide range of characters.

This document defines a new protocol element, called IRI
(Internationalized Resource Identifier), by extending the syntax of
URIs to a much wider repertoire of characters. It also defines
"internationalized" versions corresponding to other constructs from
[RFC2396],
[RFCYYYY], such as URI references.

Using characters outside of A-Z in IRIs brings with it some
difficulties; a discussion of potential problems and workarounds can
be found in the later sections of this document.

1.2 Applicability

IRIs are designed to be compatible with recent recommendations on for
new URI
syntax schemes [RFC2718]. The compatibility is provided by
providing a well defined and deterministic mapping from the IRI
character sequence to the functionally equivalent URI character
sequence. Practical use of IRIs (or IRI references) in place of URIs
(or URI references) depends on the following conditions being met:

a) The protocol or format element used should be explicitly
designated to carry IRIs. That is, the intent is not to
introduce IRIs into contexts that are not defined to accept
them. For example, XML schema [XMLSchema] has an explicit type

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"anyURI" that designates the use of IRIs.

b) The protocol or format carrying the IRIs should have a
mechanism to represent the wide range of characters used in
IRIs, either natively or by some protocol- or format-specific
escaping mechanism (for example numeric character references in
[XML1]).

c) Either by definition for all the URIs of a specific URI scheme,
or a specific part of a The URI (Reference), such as the fragment
identifier, or at least for some specific URIs of a given
scheme, corresponding to the encoding of non-ASCII IRI in question has to encode
original characters should be based on into octets using UTF-8. For new URI
schemes, this is recommended in [RFC2718].
This allows IRIs It can apply to be used with the URN syntax [RFC2141] as
well as recent URL a
whole scheme definitions based on UTF-8, such as (e.g. IMAP URLs [RFC2192] and POP URLs [RFC2384].

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, [RFC2384],
or the ability of IRIs URN syntax [RFC2141]). It can apply to
directly represent a wide character repertoire cannot be used.

For example, for a document with a URI specific part
of
http://www.example.org/r%C3%A9sum%C3%A9.html, it is possible an URI, such as the fragment identifier (e.g. [XPointer]).
It can apply to
construct a corresponding IRI (in XML notation, specific URI or part(s) thereoff. For
details, please see Section 1.4):
http://www.example.org/r&#xe9;sum&#xe9;.html (&#xe9; stands for the
e-acute character, and is 6.4.

1.3 Definitions

The following definitions are used in this document; they follow the UTF-8 encoded
terms in [RFC2130], [RFC2277] and escaped
representation [ISO10646]:

character: A member of that character). On the other hand, for a document
with an URI set 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

The following definitions are used in this document; they follow the
terms in [RFC2130], [RFC2277] and [ISO10646]:

character: A member of a set of elements used for elements used for the
organization, control, or representation of data. For example,
"LATIN CAPITAL LETTER A" names a character.

octet: an An ordered sequence of eight bits considered as a unit

character repertoire: A set of characters (in the mathematical
sense)

sequence of characters: A sequence (one after another) of
characters

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sequence of octets: A sequence (one after another) of octets

(character) encoding: A method of representing a sequence of
characters as a sequence of octets (maybe with variants). A
method of (unambiguously) converting a sequence of octets into
a sequence of characters.

code point: A placeholder for a character in a character encoding,
for example to encode additional characters in future versions
of the character encoding.

charset: The name of a parameter or attribute used to identify a
character encoding.

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UCS: Universal Character Set; the coded character set defined by
[ISO10646] and [UNIV3]. [UNIV4].

IRI reference: The term "IRI reference" denotes the common usage
of an internationalized resource identifier. An IRI reference
may be absolute or relative, and may have additional
information attached in the form of a fragement identifier. relative. However, the "IRI" that results
from such a reference only includes the absolute IRI after the fragment identifier (if
any) is removed and after IRIs; any relative IRI is
IRIs are resolved to its their absolute form. Note that in
[RFC2396], URIs did not include fragment identifiers, but in
[RFCYYYY], fragment identifiers are part of URIs.

1.4 Notation

RFCs and Internet Drafts currently do not allow any characters
outside the US-ASCII repertoire. Therefore, this document uses
various special notations to denote such characters. characters in examples.

In text, characters outside US-ASCII are sometimes referenced by
using a prefix of 'U+', followed by four to six hexadecimal digits.

To represent characters outside US-ASCII in examples, this document
uses two notations called 'XML Notation' and 'Bidi Notation'.

XML Notation uses leading '&#x', trailing ';', and the hexadecimal
number of the character in the UCS in between. Example: &#x44f; &#x44F;
stands for CYRILLIC CAPITAL LETTER YA. In this notation, an actual
'&' is denoted by '&amp'. '&amp;'.

Bidi Notation is used for bidirectional examples: lower case ASCII
letters stand for Latin letters or other letters that are written
left-to-right, whereas upper case letters represent Arabic or Hebrew
letters that are written right-to-left.

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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

This section defines the syntax of Internationalized Resource
Identifiers (IRIs).

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
may be written on paper or read over the radio as well as being
stored or transmitted over the network. digitally. The same IRI may be represented as
different sequences of octets in different protocols or documents if

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these protocols or documents use different character encodings (and/
or transfer encodings). Using the same character encoding as the
containing protocol or document assures that the characters in the
IRI can be handled (searched, converted, displayed,...) in the same
way as the rest of the protocol or document.

2.1 Summary of IRI Syntax

IRIs are defined similarly to URIs in [RFC2396] (as modified by
[RFC2732] and [IDNURI]), [RFCYYYY], but the class of
unreserved characters is extended by adding the characters of the UCS
(Universal Character Set, [ISO10646]) beyond U+0080, subject to the
limitations given in the syntax rules below and in Section 5.1. 6.1.

Otherwise, the syntax and use of components and reserved characters
is the same as that in [RFC2396]. [RFCYYYY]. All the operations defined in
[RFC2396],
[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
done to URIs by URI-processing software.

Note: [RFC2396]: Uniform Resource Identifiers (URI): Generic Syntax"
is being revised as [RFC2396bis]. The syntax used in this document
includes bug fixes from [RFC2396bis].

Characters outside the US-ASCII range are not reserved and therefore
MUST NOT be used for syntactical purposes such as to delimit
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
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

While it might be possible to define IRI references and IRIs merely
by their transformation to URI references and URIs, they can also be
accepted and processed directly. Therefore, an ABNF definition for
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

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described in [RFC2234]. Character numbers are taken from the UCS,
without implying any actual binary encoding. Terminals in the ABNF
are characters, not bytes.

The following rules are different from [RFC2396]:

absolute-IRI-reference = absolute-IRI [ "#" ifragment ] [RFCYYYY]:

IRI-reference = [ absolute-IRI IRI / relative-IRI

IRI = scheme ":" ihier-part [ "?" iquery ] [ "#" ifragment ]

absolute-IRI = scheme ":" ( ihier-part / iopaque-part )
relative-IRI = [ inet-path / iabs-path / irel-path ] [ "?" iquery ]

ihier-part

relative-IRI = [ inet-path / iabs-path ] ihier-part [ "?" iquery ]
iopaque-part = iric-no-slash *iric

iric-no-slash = iunreserved / escaped / "[" / "]" / ";" / "?" /
":" / "@" / "&" / "=" / "+" [ "#" ifragment ]

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ihier-part = inet-path / "$" iabs-path / "," irel-path

inet-path = "//" iauthority [ iabs-path ]

iabs-path = "/" ipath-segments

irel-path = irel-segment [ iabs-path ]

irel-segment = 1*( iunreserved / escaped / ";" /
"@" / "&" / "=" / "+" / "$" / "," ) ipath-segments

iauthority = iserver / ireg-name

ireg-name = 1*( iunreserved / escaped / ";" /
":" / "@" / "&" / "=" / "+" / "$" / "," )

iserver = [ [ iuserinfo "@" ] ihostport ihost [ ":" port ]

iuserinfo = *( iunreserved / escaped / ";" /
":" / "&" / "=" / "+" / "$" / "," )

ihostport = ihost [ ":" port ]

ihost = [ IPv6reference / IPv4address / ihostname ]

ihostname = idomainlabel [ iqualified] iqualified

iqualified = *( "." idomainlabel ) [ "." itoplabel [ "." ] ]

idomainlabel = <<See following production rules>>
itoplabel = <<See following production rules>>

ipath = [ iabs-path / iopaque-part ]

ipath-segments = isegment *( "/" isegment )

isegment = *ipchar

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ipchar = iunreserved / escaped / ";" /
":" / "@" / "&" / "=" / "+" / "$" / ","

iquery = *( ipchar / iprivate / "/" / "?" )

ifragment = *( ipchar / "/" / "?" )

iric = reserved / iunreserved / escaped

iunreserved = unreserved / ucschar / iadditional
iadditional = "<" / ">" / DQUOTE / SP / "{" / "}" /
"|" / "\" / "^" / "`"

ucschar = %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF /
/ %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD
/ %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD
/ %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD
/ %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD
/ %xD0000-DFFFD / %xE1000-EFFFD

iprivate = %xE000-F8FF / %xF0000-FFFFD / %x100000-10FFFD

The 'idomainlabel' and 'itoplabel' production rules are rule is as follows:
The values value 'idomainlabel' and 'itoplabel' are is defined as a string of 'ucschar' obeying
the following rules:

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a) Given a string of 'ucschar' values, the ToASCII operation
[RFCXXXX]
[RFC3490] is performed on that string with the flag
UseSTD3ASCIIRules set to TRUE and the flag AllowUnassigned set
to FALSE for creating IRIs and set to TRUE otherwise.

b) ToASCII is successful and results in a string conforming to
'domainlabel' for 'idomainlabel' and 'toplabel' for 'itoplabel' (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. below).

The following are the same as [RFC2396bis]: [RFCYYYY]:

scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )

port = *DIGIT

domainlabel = alphanum [ 0*61( alphanum | "-" ) alphanum ]
toplabel = alpha [ 0*61( alphanum | "-" ) alphanum ]

alphanum = ALPHA / DIGIT

IPv4address = dec-octet 3( "." dec-octet ) "." dec-octet "." dec-octet

dec-octet = DIGIT / ; 0-9
/ ( %x31-39 DIGIT ) / ; 10-99
/ ( "1" 2*DIGIT 2DIGIT ) / ; 100-199
/ ( "2" %x30-34 DIGIT ) / ; 200-249
/ ( "25" %x30-35 ) ; 250-255

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IPv6reference = "[" IPv6address "]"

IPv6address = ( 7( 6( h4 ":" ) h4 ) ls32
/
( "::" 0*6( 5( h4 ":" ) ls32
/ [ h4 ] ) /
( h4 "::" 0*5( 4( h4 ":" ) [ h4 ] ) ls32
/
( [ *1( h4 ":" ) h4 ] "::" 0*4( 3( h4 ":" ) ls32
/ [ h4 ] ) /
( h4 2( ":" h4 ) "::" 0*3( *2( h4 ":" ) [ h4 ] ) /
( h4 3( ":" h4 ) "::" 0*2( 2( h4 ":" ) [ h4 ] ) ls32
/
( h4 4( ":" h4 ) "::" 0*1( [ *3( h4 ":" ) [ h4 ] ) /
( 6( "::" h4 ":" ) IPv4address )/
( "::" 0*5( ls32
/ [ *4( h4 ":" ) IPv4address )/
( h4 ] "::" 0*4( ls32
/ [ *5( h4 ":" ) IPv4address )/
( h4 ":" h4 ] "::" 0*3( h4 ":" ) IPv4address )/
(
/ [ *6( h4 2( ":" h4 ) h4 ] "::" 0*2(

h4 ":" ) IPv4address )/ = 1*4HEXDIG

ls32 = ( h4 3( ":" h4 ) "::" 0*1( h4 ":" ) / IPv4address )

h4 = 1*4HEXDIG

reserved = "/" / "?" / "#" / "[" / "]" / ";" / "/" / "?" /
":" / "@" / "&" / "=" / "+" / "$" / ","

unreserved = ALPHA / DIGIT / mark

mark = "-" / "_" / "." / "!" / "~" / "*" / "'" /

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"(" / ")"

escaped = "%" HEXDIG HEXDIG

2.3 IRI Equivalence

3. Relationship between IRIs and Normalization

There is no general rule or procedure to decide whether two arbitrary URIs

IRIs are equivalent or not (i.e. refer to the same resource or not).
Two IRIs that look almost the same may refer meant to different resources.
Two IRIs that look completely different may refer to, replace URIs in identifying resources for
protocols, formats and resolve to,
the same resource.

In some scenarios software components which use a definite answer UCS-based
character repertoire. These protocols and components may never need
to use URIs directly, especially when the question of IRI
equivalence is needed that resource identifier is independent of the scheme used and
always can be calculated quickly and without accessing a network. An
example of such a case might be 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
simply for identification purposes. However, when the same as
being byte-by-byte equivalent if the character encoding resource
identifier is used for both IRIs resource retrieval, it is in many cases
necessary to determine the same. As associated URI because most retrieval
mechanisms currently only are defined for URIs. (Additional
rationale is given in Section 3.1.)

3.1 Mapping of IRIs to URIs

This section defines how to map an example,
http://example.org/~user, http://example.org/%7euser, IRI to a URI. Everything in this
section applies also to IRI references and
http://example.org/%7Euser would URI references, as well as
components thereof (for example fragment identifiers).

This mapping has two purposes:

a) Syntactical: Many URI schemes and components define additional
syntactical restrictions not captured in Section 2.2. Such
restrictions can be equivalent under this
definition. In such a case, the comparison function MUST NOT map the
IRIs applied to URIs, because such a mapping would create something different
under this equivalence relationship.

It follows from the above IRIs by noting that IRIs SHOULD NOT are
only valid if they map to syntactically valid URIs. This means
that such syntactical restrictions do not have to be modified when being

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transported.

For actual resolution, differences in escaping (except for defined
again on the
escaping of reserved characters) MUST always result IRI level.

b) Interpretational: URIs identify resources in various ways.
IRIs also identify resources. When 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 IRI is used solely for
identification purposes, it is not necessary to be
tested, map the escaping of both IRIs IRI to be compared has to be aligned,
an URI (see Section 5). However, when an IRI is used for example
resource retrieval, the resource that the IRI locates is the
same as the one located by the URI obtained after converting both
the IRI according to the procedure defined here. This means
that there is no need to define resolution separately on the
IRI level.

Applications MUST map IRIs to URIs (see Section 3.1) and
making sure that using the case following two steps.

Step 1) This step generates a UCS-based encoding from the original
IRI format. This step has three variants, depending on the
form of the hexadecimal characters in input.

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Variant A) If the %-
escape IRI is always the same. Such conversions MUST only be done written on paper or read out loud,
or otherwise represented as a sequence of characters
independent of any encoding: Represent the
fly, without changing the original IRI.

Specific schemes and resolution mechanisms may define additional
equivalences. For IRI as a specific scheme, two IRIs that e.g. differ only
by case may be equivalent. However, this document does not deal with
scheme-specific issues.

The Unicode Standard [UNIV3] defines various equivalences between
sequences
sequence of characters for various purposes. Unicode Standard Annex
#15 [UTR15] defines various Normalization Forms for these
equivalences. IRIs SHOULD be created using from the UCS normalized according
to Normalization Form C
(NFC). Equivalence of IRIs MUST rely on (NFC, [UTR15]).

Variant B) If 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 IRI is in some digital representation
(e.g. 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
Notation) is octet stream) in some known non-Unicode
encoding: Convert the IRI to a sequence of characters
from the UCS normalized according to NFC. On

Variant C) If the other hand, http://www.example.org/
re&#x301;sume&#x301;.html IRI is not in NFC. The former uses precombined
e-acute characters, an Unicode-based encoding (for
example UTF-8 or UTF-16): Do not normalize. Move
directly to Step 2.

Step 2) If the later uses 'e' characters followed by
combining acute accents, both are defined as canonically equivalent
in [UNIV3].

Various IRI schemes may allow contains an 'ihostname' part, replace this
'ihostname' part by the usage of International Domain Names
(IDN) [RFCXXXX]. When in use in IRIs, those names SHOULD be
validated part converted using the ToASCII
operation defined specified in [RFCXXXX], Section 4.1 of [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 may be applied
when mapping an IRI into an URI, see Section 3.1.

3. Relationship between IRIs flag
UseSTD3ASCIIRules set to TRUE and URIs

IRIs are meant the flag AllowUnassigned set
to replace URIs in identifying resources FALSE for
protocols, formats and software components which use a UCS-based

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character repertoire. These protocols creating IRIs and components may never need set to use URIs directly, especially when the resource identifier is used
simply for identification purposes. However, when the resource
identifier is used for resource retrieval, it TRUE otherwise.

Step 3) For each character that is disallowed in many cases
necessary to determine the associated URI because most retrieval
mechanisms currently only are defined for URIs. (Additional
rationale is given in Section 3.1.)

3.1 Mapping references,
apply steps 1) through 3) below. The disallowed characters
consist of IRIs to URIs

This section defines how to map an IRI all non-ASCII characters allowed in IRIs.

1) Convert the character to a URI. Everything in this
section applies also sequence of one or more octets
using UTF-8 [RFCXXXX].

2) Convert each octet to IRI references and URI references, as well as
components thereof (for example fragment identifiers). %HH, where HH is the hexadecimal
notation of the octet value. Note: This mapping has two purposes:

a) Syntactical: Many URI schemes and components define additional
syntactical restrictions not captured is identical to
the escaping mechanism in Section 2.2. Such
restrictions can be applied to IRIs 2.4.1 of [RFCYYYY].
Note: To reduce variability, the hexadecimal notation
SHOULD use upper case letters.

3) Replace the original character by noting the resulting character
sequence (i.e. a sequence of %HH triplets).

Note that IRIs are the ToASCII operation in Step 2) may fail, but only valid if they map to syntactically valid URIs. This means
that such syntactical restrictions do the
IRI does not have conform to be defined
again on the IRI level.

b) Interpretational: URIs identify resources rules in various ways. Section 2.2.

Note: For backwards compatibility with implementations of previous
drafts of this specification, infrastructure accepting IRIs MAY also identify resources. When
deal with 'ihostname' parts escaped according to Step 3) rather than
Step 2). For example, Step 2) converts the IRI is used simply for
identification purposes, it is not necessary
http://r&#xE9;sum&#xE9;.example.org to map the IRI
http://xn--rsum-bpad.example.org. For backwards compatibility,
http://r%C3%A9sum%C3%A9.example.org would also be converted to
an URI (see Section 2.3). However, when
http://xn--rsum-bpad.example.org.

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Note that Internationalized Domain Names may be contained in parts of
an IRI is used for
resource retrieval, other than the resource 'ihostname' part.

Note that the IRI locates is the
same as the one located by the in this process (in step 3.3), characters allowed in URI obtained after converting
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 according of
http://www.example.org/red%09ros&#xE9;#red (in XML notation) is
converted to the procedure defined here. This means
http://www.example.org/red%09ros%C3%A9#red, not to something like
http%3A%2F%2Fwww.example.org%2Fred%2509ros%C3%A9%23red.

Note that there is no need some older software transcoding to define resolution separately on UTF-8 may produce
illegal output for some input, in particular for characters outside
the
IRI level.

Applications MUST map IRIs to URIs using BMP (Basic Multilingual Plane). As an example, for the following two steps.

Step 1) This step generates a UCS-based encoding from the original
IRI format. This step has with non-BMP characters (in XML Notation):
http://example.com/&#x10300;&#x10301;&#x10301;
(the first three variants, depending on the
form letters of the input.

Variant A) If Old Italic alphabet) the IRI is written on paper or read out loud,
or otherwise represented as correct
conversion to a sequence of characters
independent of any encoding: Represent the IRI as URI is:
http://example.com/%F0%90%8C%80%F0%90%8C%81%F0%90%8C%82

The above mapping produces a
sequence URI fully conforming to [RFCYYYY] out of characters from the UCS normalized according
to Normalization Form C (NFC, [UTR15]).

Variant B) If the IRI
each IRI. The mapping is in some digital representation
(e.g. also an octet stream) in some non-Unicode encoding:
Convert identity transformation for URIs
and is idempotent -- applying the IRI to mapping a sequence second time will not
change anything. Every URI is therefore by definition an IRI.

Note: Earlier drafts of characters from the UCS

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normalized according to NFC.

Variant C) If this specification allowed the space
character and various delimiters in IRIs and IRI is references. The
full list of these characters was: "<", ">", '"', Space, "{", "}",
"|", "\", "^", and "`", i.e. all printable characters in an Unicode-based encoding (for
example UTF-8 or UTF-16): Do US-ASCII
that are not normalize. Move
directly to Step 2.

Step 2) allowed in URIs. For each character that is disallowed backwards compatibility,
implementations MAY also include these characters in URI references,
apply steps 1) through step 3) below. The disallowed above.
If such characters
consist of all non-ASCII characters, plus are found but are not converted, then the excluded
characters listed in Section 2.4 of [RFC2396], except for
conversion SHOULD fail. Please note that the number sign (#) and ("#"), the
percent sign (%) ("%"), and the square bracket characters re-allowed in [RFC2732].

1) Convert ("[", "]") are
not part of the character to a sequence above list, and MUST not be converted. Protocols and
formats that have used earlier definitions of one or more octets
using UTF-8 [RFC2279].

2) Convert each octet IRIs including these
characters MAY require unescaping of these characters as a
preprocessing step to %HH, where HH is extract the hexadecimal
notation of actual IRI from a given field.
Such preprocessing MAY also be used by applications allowing the octet value. Note: This is identical user
to
the escaping mechanism enter an IRI.

3.2 Converting URIs to IRIs

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
conversion. The conversion described in Section 2.4.1 of [RFC2396].
Note: To reduce variability, this section will always

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result in an IRI which maps back to the hexadecimal notation
SHOULD use upper case letters.

3) Replace URI that was used as an input
for the original character by conversion (except for potential case differences in escape
sequences). However, the IRI resulting character
sequence (i.e. a sequence of %HH triplets).

Note that in from this process (in step 2.3), characters allowed in conversion may not
be exactly the same as the original IRI (if there ever was one).

URI
references and existing to IRI conversion removes escape sequences, but not all escaping
can be eliminated. There are several reasons for this:

a) Some escape sequences are not necessary to distinguish escaped further.
(This mapping and
unescaped uses of reserved characters.

b) Some escape sequences cannot be interpreted as sequences of
UTF-8 octets.

(Note: The octet patterns of UTF-8 are highly regular.
Therefore, there is similar to, a very high probability, but different from, the escaping applied
when including arbitrary content into some part no guarantee,
that escape sequences that can be interpreted as sequences of
UTF-8 octets actually originated from UTF-8. For a detailed
discussion, see [Duerst97].)

c) The conversion may result in a character that is not
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
any other algorithm that produces the same result):

1) Represent the URI as a sequence of octets in US-ASCII.

2) Replace any punycode-encoded domainlabel in the URI by the
result of the ToUnicode function represented as UTF-8.

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.

5) Re-escape all octets produced in step 3) that in UTF-8
represent characters that are not appropriate according to
Section 4.1 and Section 6.1.

6) Interpret the resulting octet sequence as a sequence of
characters encoded in UTF-8.

This procedure will convert as many escaped non-ASCII characters as
possible to characters in an IRI. Because there are some choices

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when applying step 5) (see Section 6.1), results may vary.

Conversions from URIs to IRIs MUST NOT use any other encoding than
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
URI. As an example, the URI http://www.example.org/r%E9sum%E9.html
might with some guessing be interpreted to contain two e-acute
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
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.

3.2.1 Examples

This section shows various examples of converting URIs to IRIs. The
notation <hh> is used to denote octets outside those that can be
represented in this document. Each example shows the result after
applying each of the steps 1) to 6). XML Notation is used for the
final result.

The following example contains the sequence '%C3%BC', which is a
strictly legal UTF-8 sequence, and which is converted into the actual
character U+00FC LATIN SMALL LETTER U WITH DIAERESIS (also known as
u-umlaut).

1) http://www.example.org/D%C3%BCrst

2) http://www.example.org/D%C3%BCrst

3) http://www.example.org/D<c3><bc>rst

4) http://www.example.org/D<c3><bc>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
represent U+00FC LATIN SMALL LETTER U WITH DIAERESIS in the
iso-8859-1 encoding. (It might represent other characters in other
encodings. For example, the octet <FC> in iso-8859-5 represents
U+045C CYRILLIC SMALL LETTER KJE.) Because <FC> is not part of a
strictly legal UTF-8 sequence, it is re-escaped in step 2).

1) http://www.example.org/D%FCrst

2) http://www.example.org/D%FCrst

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3) http://www.example.org/D<FC>rst

4) http://www.example.org/D%FCrst

5) http://www.example.org/D%FCrst

6) http://www.example.org/D%FCrst

The following example contains '%e2%80%ae', which is the escaped
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
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://xn--99zt52a.example.org/%e2%80%ae

2) http://<E7><B4><8D><E8><B1><86>.example.org/%e2%80%ae

3) http://<E7><B4><8D><E8><B1><86>.example.org/<E2><80><AE>

4) http://<E7><B4><8D><E8><B1><86>.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

Some UCS characters, such as those used in the Arabic and Hebrew
script, have an inherent right-to-left (rtl) writing direction. IRIs
containing such characters (called bidirectional IRIs or Bidi IRIs)
require additional attention because of the non-trivial relation
between logical representation (used for digital representation as
well as when reading/spelling) and visual representation (used for
display/printing).

Because of the complex interaction between the logical
representation, the visual representation, and the syntax of a Bidi
IRI, a balance is needed between various requirements. The main
requirements are:

1) user-predictable conversion between visual and logical
representation;

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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

When stored or transmitted in digital representation, bidirectional
IRIs MUST be in full logical order, and MUST conform to the IRI
syntax rules (which includes the rules relevant to their scheme).
This assures that bidirectional IRIs can be processed in the same way
as other IRIs.

When rendered, bidirectional IRIs MUST be rendered using the Unicode
Bidirectional Algorithm [UNIV4], [UNI9]. Bidirectional IRIs MUST be
rendered with an overall left-to-right (ltr) direction.

In text with a left-to-right base directionality or embedding (as
used for e.g. English or Cyrillic), the Unicode Bidirectional
Algorithm will automatically use an overall ltr direction for the
IRI. In text with a rtl base directionality or embedding (as used
e.g. for Arabic or Hebrew), setting a URI.) For
example, an different embedding direction
for the IRI of
http://www.example.org/red%09ros&#xe9;#<red> (in XML notation) is
converted to
http://www.example.org/red%09ros%C3%A9#%3Cred%3E, needed. Setting the embedding direction can be done
in a higher-order protocol (e.g. the dir='ltr' attribute in HTML).
If this is not to something
like
http%3A%2F%2Fwww.example.org%2Fred%2509ros%C3%A9%23red.

Note that some older software transcoding to UTF-8 may produce
illegal output for some input, available (e.g. in particular for characters outside plain text), setting the BMP (Basic Multilingual Plane). As an example, for 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 following IRI with non-BMP
itself.

IRIs MUST NOT contain bidirectional formatting characters (in XML Notation):
http://example.com/
(the first three letters of (LRM, RLM,
LRE, RLE, LRO, RLO, and PDF). They affect the Old Italic alphabet) visual rendering of
the correct
conversion IRI, but do not themselves appear visually. It would therefore
not be possible to a URI is:
http://example.com/%F0%90%8C%80%F0%90%8C%81%F0%90%8C%82 correctly input an IRI with such characters.

4.2 Bidi IRI Structure

The above mapping produces a URI fully conforming to [RFC2396] (as
amended by [RFC2732] Unicode Bidirectional Algorithm is designed mainly for running
text. To make sure that it does not affect the rendering of
bidirectional IRIs too much, some restrictions on bidirectional IRIs
are necessary. These restrictions are given in terms of delimiters
(structural characters, mostly punctuation such as '@', '.', ':',
'/') and [IDNURI]) out components (usually consisting mostly of each IRI. letters and
digits).

The mapping is
also an identity transformation following syntax rules from Section 2.2 correspond to components
for URIs and is idempotent -- the purpose of Bidi behavior: iuserinfo, isegment, ihostname,

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applying the mapping a second time will not change anything. Every
URI is therefore by definition an IRI.

Note: For backwards compatibility with infrastructure

iquery, and ifragment.

Specifications that does not
implement define the updates syntax of [IDNURI], converters MAY also convert the
'ihostname' part any of an IRI using the ToASCII operation specified in
Section 4.1 of [RFCXXXX] between Step 1 above components
MAY divide them further and Step 2. Note that the
ToASCII operation may fail. Note that Internationalized Domain Names
may be contained in define smaller parts of an IRI other than the 'ihostname' part.

3.2 Converting URIs to IRIs

In some situations, it may be desirable to try to convert a URI into
an equivalent IRI. This section gives a procedure components
according to do such a
conversion. The conversion described in this section will always
result in an IRI which maps back to the URI that was used as document. As an input
for the conversion (except for potential case differences in escape
sequences). However, example, the IRI resulting from this conversion may not
be exactly restrictions of
[RFC3490] on bidirectional domain names correspond to treating each
label of the same domain name as a component. Even where the original IRI (if there ever was one).

URI to IRI conversion removes escape sequences, but components
are not all escaping
can defined formally, it may be eliminated. There are several reasons for this:

a) Some escape sequences are necessary helpful to distinguish escaped and
unescaped uses of reserved characters.

b) Some escape sequences cannot be interpreted as sequences think about some
syntax in terms of
UTF-8 octets.

(Note: Due components and to apply the relevant restrictions.
For example, for the regularities usual name/value syntax in the octet patterns of UTF-8,
there query parts, it is
convenient to treat each name and each value as a very high probability, but no guarantee, that escape
sequences that component. As
another example, the extensions in a resource name can be interpreted treated as sequences of UTF-8 octets
actually originated from UTF-8.
separate components.

For a detailed discussion, see
[Duerst97].)

c) each component, the following restrictions apply:

1) A component SHOULD NOT not use both right-to-left and left-to-
right characters.

2) A component using right-to-left characters SHOULD start and end
with right-to-left characters.

The conversion may result in a character above restrictions are given as shoulds, rather than as musts.
For IRIs that is are never presented visually, they are not
appropriate relevant.
However, for IRIs in general, they are very important to insure
consistent conversion between visual presentation and logical
representation, in both directions.

In some components, the above restrictions may actually be strictly
enforced. For example, [RFC3490] requires that these restrictions
apply to the labels of the host name part of an IRI. See Section 5.1 In some other
components, for further details.

Conversion from a URI to an IRI is done using the example path components, following steps (or
any these restrictions
may not be too difficult. For other algorithm that produces components, such as parts of the same result):

1) Represent
query part, it may be very difficult to enforce the URI as a sequence restrictions,
because the values of octets in US-ASCII.

2) Convert all hexadecimal escapes (% followed by two hexadecimal
digits) except those corresponding query parameters may be arbitrary character
sequences.

If the above restrictions cannot be satisfied otherwise, the affected
component can always be mapped to '#' and '%' and
characters URI notation as described in 'reserved', to
Section 3.1. Please note that the corresponding octets.

3) Re-escape any octet produced whole component needs to be mapped
(see also Example 9 below).

4.3 Input of Bidi IRIs

Bidi input methods MUST generate Bidi IRIs in step 2) logical order while
rendering them according to Section 4.1. During input, rendering
SHOULD be updated after every new character that is not part of a input to avoid
end user confusion.

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strictly legal UTF-8 octet sequence.

4) Re-escape all octets produced in step 2) that in UTF-8
represent characters that are not appropriate according to
Section 4.1 and Section 5.1.

5) Interpret the resulting octet sequence as a sequence of
characters encoded in UTF-8.

4.4 Examples

This procedure will convert as many escaped non-ASCII characters as
possible to characters section gives examples of bidirectional IRIs, in an IRI. Because there are some choices
when applying step 4) (see Section 5.1), results may differ.

Conversions from URIs to Bidi Notation.
It shows legal IRIs MUST NOT use any other encoding than
UTF-8 in steps 3) with the relationship between logical and 4) above, even if it might be possible from
context to guess that another encoding than UTF-8 was used visual
representation, and explains how certain phenomena in the
URI. As an example, the URI http://www.example.org/r%E9sum%E9.html,
which this
relationship may look strange to somebody not familiar with some guesses might be interpreted
bidirectional behavior, but familiar to contain two e-acute
characters encoded as iso-8859-1, must users of Arabic and Hebrew.
It also shows what happens if the restrictions given in Section 4.2
are not followed. The examples below can be converted to an IRI
containing these e-acute characters. Otherwise, seen at [BidiEx], in
Arabic, Hebrew, and Bidi Notation variants.

To read the IRI will bidi text in the
future be mapped examples, read the visual representation
from left to http://www.example.org/r%C3%A9sum%C3%A9.html,
which is right until you encounter a different URI from http://www.example.org/r%E9sum%E9.html.

3.2.1 Examples

This section shows various examples block of converting URIs rtl text. Read the
rtl block (including slashes and other special characters) from right
to IRIs. The
notation <hh> left, then continue at the next unread ltr character.

Example 1: A single component with rtl characters is used to denote octets outside those that inverted:
logical representation: http://ab.CDEFGH.ij/kl/mn/op.html
visual representation: http://ab.HGFEDC.ij/kl/mn/op.html
Components can be
represented in this document. Each example shows the result after
applying read one-by-one, and each component can be read in
its natural direction.

Example 2: More than one consecutive component with rtl characters is
inverted as a whole:
logical representation: http://ab.CDE.FGH/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 steps 1) to 5). XML Notation same way as a
sequence of rtl words is used read rtl in a bidi text.

Example 3: All components of an IRI (except for the
final result. scheme) are rtl.
All rtl components are inverted overall:
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
The following example contains whole IRI (except the sequence '%C3%BC', which scheme) is a
strictly legal UTF-8 sequence, read rtl. Delimiters between
rtl components stay between the respective components; delimiters
between ltr and which rtl components don't move.

Example 4: Several sequences of rtl components are each inverted on
their own:
logical representation: http://AB.CD.ef/gh/IJ/KL.html
visual representation: http://DC.BA.ef/gh/LK/JI.html
Each sequence of rtl components is converted into read rtl, in the actual
character U+00FC LATIN SMALL LETTER U WITH DIAERESIS (also known same way as
u-umlaut).

1) http://www.example.org/D%C3%BCrst

2) 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

5) http://www.example.org/D&#xfc;rst

The following example contains the each
sequence '%FC', which might
represent U+00FC LATIN SMALL LETTER U WITH DIAERESIS of rtl words in an ltr text is read rtl.

Example 5: Example 2, applied to components of different kinds:
logical representation: http://ab.cd.EF/GH/ij/kl.html
visual representation: http://ab.cd.HG/FE/ij/kl.html
The inversion of the domain name label and the iso-8859- path component may be

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1 encoding. (It might represent other characters in

unexpected, but is consistent with other encodings. bidi behavior. For example,
reassurance that the octet <FC> in iso-8859-5 represents U+045C CYRILLIC
SMALL LETTER KJE.) Because <FC> domain component really is not part of a strictly legal UTF-8
sequence, "ab.cd.EF", it is re-escaped in step 2).

1) http://www.example.org/D%FCrst

2) http://www.example.org/D<FC>rst

3) http://www.example.org/D%FCrst

4) http://www.example.org/D%FCrst

5) http://www.example.org/D%FCrst

The may be
helpful to read aloud the visual representation following example contains '%e2%80%ae', the bidi
algorithm. After "http://ab.cd." one reads the RTL block "E-F-slash-
G-H", which is corresponds to the escaped UTF-
8 encoding logical representation.

Example 6: Same as example 5, with more rtl components:
logical representation: http://ab.CD.EF/GH/IJ/kl.html
visual representation: http://ab.JI/HG/FE.DC/kl.html
The inversion of U+202E, RIGHT-TO-LEFT OVERRIDE. Section 4.1 forbids the direct use of this character in an IRI. Therefore, domain name labels and the
corresponding octets path components may
be easier to identify because the delimiters also move.

Example 7: A single rtl component with included digits:
logical representation: http://ab.CDE123FGH.ij/kl/mn/op.html
visual representation: http://ab.HGF123EDC.ij/kl/mn/op.html
Numbers are re-escaped written ltr in step 3). all cases, but are treated as an
additional embedding inside a run of rtl characters. This example shows
that is
completely consistent with usual bidirectional text.

Example 8 (not allowed): Numbers at the case (upper start or lower) end of letters used in escapes may not be
preserved.

1) http://www.example.org/%e2%80%ae

2) http://www.example.org/<E2><80><AE>

3) http://www.example.org/<E2><80><AE>

4) http://www.example.org/%E2%80%AE

5) http://www.example.org/%E2%80%AE

4. Bidirectional IRIs for Right-to-left Languages

Some UCS characters, such a rtl
component:
logical representation: http://ab.cd.ef/GH1/2IJ/KL.html
visual representation: http://ab.cd.ef/LK/JI1/2HG.html
The sequence '1/2' is interpreted by the bidi algorithm as those used in a
fraction, fragmenting the Arabic components and Hebrew
script, have an inherent right-to-left writing direction. IRIs
containing such leading to confusion. There
are other characters (called bidirectional IRIs or Bidi IRIs)
require additional attention because of that are interpreted in a special way close to
numbers, in particular '+', '-', '#', '$', '%', ',', '.', and ':'.

Example 9 (not allowed): The numbers in the non-trivial relation
between previous example are
escaped:
logical representation: http://ab.cd.ef/GH%31/%32IJ/KL.html,
visual representation (used for digital representation as
well as when reading/spelling) and (Hebrew): http://ab.cd.ef/LK/JI%32/%31HG.html
visual representation (used for
display/printing).

Because of the complex interaction between (Arabic): http://ab.cd.ef/LK/JI32%/31%HG.html
Depending on whether the logical
representation, upper-case letters represent Arabic or
Hebrew, the visual representation, representation is different.

5. IRI Equivalence and Comparison

This section discusses IRI Equivalence and Comparison similar to
Section 6, "Normalization and Comparison", in [RFCYYYY]. This
section focusses on the syntax main issues and on aspects that are different
from [RFCYYYY]; Section 6 of a Bidi
IRI, a balance [RFCYYYY] is needed between various requirements. The main
requirements recommended background
reading.

There is no general rule or procedure to decide whether two arbitrary
IRIs are (1) user-predictable conversion between visual and
logical representation; (2) equivalent or not (i.e. whether they refer to the ability same
resource or not). Two IRIs that look almost the same may refer to include a wide range of
different resources. Two IRIs that look completely different may

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characters in various parts of

refer to the IRI; (3) no or not too big changes
or restrictions for implementations.

4.1 Logical Storage and Visual Presentation

In their internal digital representation, i.e. stored same resource. Each specification or transmitted
for resolution, bidirectional application that
uses IRIs MUST be in full logical order, and
MUST conform directly has to decide on the appropriate criterion for IRI syntax rules (which includes the
rules relevant
equivalence.

5.1 Simple String Comparison

In some scenarios a definite answer to their scheme). This assures the question of IRI
equivalence is needed that bidirectional
IRIs is independent of the scheme used and
always can be processed in 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 way as other IRIs.

When rendered, bidirectional IRIs MUST be rendered using being
byte-by-byte equivalent if the Unicode
Bidirectional Algorithm [UNIV3], [UNI9]. Bidirectional character encoding for both IRIs MUST be
rendered with is
the same. As an overall left-to-right direction. example,
http://example.org/~user, http://example.org/%7euser, and
http://example.org/%7Euser are not equivalent under this definition.
In text with such a left-to-right base directionality or embedding (e.g
English, Cyrillic), the Unicode Bidirectional Algorithm will
automatically use an overall left-to-right direction for case, the IRI. In
text with comparison function MUST NOT map IRIs to URIs,
because such a right-to-left base directionality or embedding (e.g.
Arabic or Hebrew), some kind of embedding mapping would create additional spurious equivalences.

It follows that IRIs SHOULD NOT be modified when being transported if
there is needed. This may any chance that this IRI might be
Unicode bidi formatting codes (LRE before the IRI, and PDF after used as an identifier in
the
IRI, both not part of way explained above.

5.2 Conversion to URIs

For actual resolution, differences in escaping (except for the IRI itself) or equivalent features
escaping of a
higher-order protocol (e.g. the dir='ltr' attribute in HTML).

IRIs reserved characters) MUST NOT contain bidirectional formatting characters (LRM, RLM,
LRE, RLE, LRO, RLO, always result in the same
resource. For example, http://example.org/~user,
http://example.org/%7euser and PDF). They affect http://example.org/%7Euser must
resolve to the visual rendering same resource.

If this kind of
the IRI, but do not itself appear visually. It would therefore not
be possible to again correctly input an IRI with such characters.

4.2 Bidi IRI Structure

The Unicode Bidirectional Algorithm equivalence is designed mainly for running
text. To make sure that it does not affect to be tested, the rendering escaping of
bidirectional IRIs too much, some restrictions on bidirectional both
IRIs
are necessary. These restrictions are given in terms of delimiters
(structural characters, mostly punctuation such as
'@', '.', ':', '/') and components (usually consisting mostly of
letters and digits).

The following syntax rules from Section 2.2 correspond to components be compared has to be aligned, for the purpose of Bidi behavior: iopaquepart, irelsegment, iregname,
iuserinfo, isegment, iparam, ihostname, iquery, example by converting both
IRIs to URIs (see Section 3.1) and ifragment.

Specifications making sure that define the syntax of any case of the above components
MAY divide them further and define smaller parts to
hexadecimal characters in the %-escape is always the same (preferably
upper case). For comparison, such conversions MUST only be components
according to this document. As an example, done on
the restrictions of
[RFCXXXX] fly, while retaining the original IRI.

Additional, similar equivalences are possible based on bidirectional domain names correspond to treating each
label of knowledge
about the domain name generic URI/IRI syntax, such as a component. Even where the components 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

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are not defined formally, it may be helpful to think about some
syntax in terms

Decomposition, followed by Canonical Composition) and Normalization
Form KC (NFKC, Compatibility Decomposition, followed by Canonical
Composition).

Equivalence of components 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 apply the relevant restrictions.
For example, an UCS-
based encoding. In these cases, NFC or a normalizing transcoder
using NFC MUST be used for the usual name/value syntax 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 query parts, it NFC. On the other hand, http://www.example.org/
re&#x301;sume&#x301;.html is
convenient 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 treat each name and each value as be canonically
equivalent in [UNIV4].

Because we do not know how a component. As
another example, 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 extensions in recommendation that if you create a resource name can resource, and an IRI for that
resource, you try to be treated as
separate components.

For each component, normalized as possible (i.e. NFKC if
possible). This is similar to the following restrictions apply:

1) A component SHOULD NOT not use both right-to-left and left-to-
right characters.

2) A component using right-to-left characters SHOULD start and end
with right-to-left characters.

The above restrictions upper-case/lower-case problems in
URIs. Some parts of an URI are given as shoulds, rather than as musts. case-insensitive (domain name). For IRIs that are never presented visually,
others, it is unclear whether they are not relevant.
However, for IRIs case-sensitive or case-
insensitive, or something in general, they are very important to insure
consistent conversion between visual presentation and logical
representation, in both directions.

In some components, (e.g. case-sensitive, but if
you use the above restrictions wrong case, may actually be strictly
enforced. For example, [RFCXXXX] requires not directly get a result, but rather a
'Multiple choices'). The best recipe we have there is that these restrictions
apply to the labels of
generator uses a reasonable capitalization, and when transfering the host name part of an IRI. In some other
components, for example path components, following these restrictions
may
URI, you do not be too difficult. For other components, such as parts of the
query part, it change capitalization.

Various IRI schemes may be very difficult to enforce the restrictions,
because allow the values usage of query parameters may International Domain Names
(IDN) [RFC3490]. When in use in IRIs, those names SHOULD be arbitrary character
sequences.

In order to satisfy the above restrictions,
validated using the affected component
can be mapped to URI notation as described ToASCII operation defined in Section 3.1. Please
note that [RFC3490], with the whole component needs to
flags "UseSTD3ASCIIRules" and "AllowUnassigned". An IRI containing
an invalid IDN cannot successfully be mapped (see also Example 9
below).

4.3 Input resolved. For legibility
purposes, IDN components of Bidi IRIs

Bidi input methods MUST generate Bidi IRIs in logical order while
rendering them according to Section 4.1. During input, rendering
should SHOULD not be updated after every new character that converted into ASCII
Compatible Encoding (ACE). However, this conversion is input to avoid
end user confusion.

4.4 Examples

This section gives examples of bidirectional IRIs, in Bidi Notation.
It shows legal applied when
mapping an IRI into an URI, see Section 3.1.

5.4 Preferred Forms

The following are the preferred forms for IRIs with when generated:

- Always provide the relationship between logical and visual
representation, and explains how certain phenomena URI scheme in this
relationship may look strange to somebody not familiar with lowercase characters.

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bidirectional behavior, but familiar to users of Arabic and Hebrew.
It also shows what happens

- 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 restrictions given form produced when
applying [RFC3491]. This in Section 4.2
are not followed. The examples below can be seen at [BidiEx], particular includes using
lowercase characters rather than uppercase characters where
applicable.

- Where possible, provide IRI components in
Arabic, Hebrew, NFKC or NFC.

- Prevent /./ and Bidi Notation variants.

Example 1: A single component with right-to-left (rtl) /../ 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 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 character
sequences usable for IRIs. The considerations in this section are
relevant when creating IRIs and when converting from URIs to IRIs.

a) The repertoire of characters allowed in
its natural direction.

Example 2: More than one consecutive each IRI component with rtl characters is
inverted as a whole:
logical representation: http://ab.CDE.FGH/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
limited by the same way as a
sequence definition of rtl words is read rtl in a bidi text.

Example 3: All components that component. For example, the
definition of an the scheme component does not allow characters
beyond US-ASCII.

(Note: In accordance with URI practice, generic IRI (except software
cannot and should not check for the scheme) are rtl.
All rtl components such limitations.)

b) The UCS contains many areas of characters for which there are inverted overall:
logical representation: http://AB.CD.EF/GH/IJ/KL?MN=OP;QR=ST#UV,
strong visual representation: http://VU#TS=RQ;PO=NM?LK/JI/HG/FE.DC.BA.
The whole IRI (except look-alikes. Because of the scheme) is read rtl. Delimiters between
rtl components stay between likelihood of
transcription errors, these also should be avoided. This
includes the respective components; delimiters
between ltr and rtl components don't move.

Example 4: Several sequences full-width equivalents of rtl components are each inverted on
their own:
logical representation: http://AB.CD.ef/gh/IJ/KL.html,
visual representation: http://DC.BA.ef/gh/LK/JI.html.
Each sequence ASCII characters, half-
width Katakana characters for Japanese, and many others. This
also includes many look-alikes of rtl components "space", "delims", and
"unwise", characters excluded in [RFC3491].

Additional information is read rtl, available from [UNIXML]. [UNIXML] is
written in the same way as each
sequence context of rtl words in an ltr running text is read rtl.

Example 5: Example 2, applied to components of different kinds:
logical representation: http://ab.cd.EF/GH/ij/kl.html,
visual representation: http://ab.cd.HG/FE/ij/kl.html.
The inversion rather than in the context of
identifiers. Nevertheless, it discusses many of the domain name label categories of
characters and the path component may be
unexpected, but code points not appropriate for IRIs.

6.2 Software Interfaces and Protocols

Although an IRI is consistent with other bidi behavior.

Example 6: Same defined as example 5, with more rtl components:
logical representation: http://ab.CD.EF/GH/IJ/kl.html,
visual representation: http://ab.JI/HG/FE.DC/kl.html.
The inversion a sequence of the domain name labels and the path components may
be easier to identify because the delimiters also move.

Example 7: A single rtl component with included digits:
logical representation: http://ab.CDE123FGH.ij/kl/mn/op.html, characters, software
interfaces for URIs typically function on sequences of octets or

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visual representation: http://ab.HGF123EDC.ij/kl/mn/op.html.
Numbers are written ltr in all cases, but are treated as an
additional embedding inside a run of rtl characters. This is
completely consistent with usual bidirectional text.

Example 8 (not allowed): Numbers at the start or end of a rtl
component:
logical representation: http://ab.cd.ef/GH1/2IJ/KL.html,
visual representation: http://ab.cd.ef/LK/JI1/2HG.html.
The sequence '1/2' is interpreted by the bidi algorithm as a
fraction, fragmenting the components and leading to confusion. There
are

other characters that are interpreted in a special way close to
numbers, in particular '+', '-', '#', '$', '%', ',', '.', and ':'.

Example 9 (not allowed): The numbers in the previous example are
escaped:
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 (Arabic): http://ab.cd.ef/LK/JI32%/31%HG.html.
Depending on whether the upper-case letters represent Arabic or
Hebrew, the visual representation is different.

5. Use kinds of IRIs

5.1 Limitations on UCS Characters Allowed in IRIs

This section discusses the limitations on characters code units. Thus, software interfaces and protocols
MUST define which character
sequences usable for IRIs. The considerations in this section are
relevant when creating IRIs encoding is used.

Intermediate software interfaces between IRI-capable components and
URI-only components MUST map the IRIs per Section 3.1, when converting
transferring from URIs IRI-capable to IRIs.

a) The repertoire of characters allowed in each IRI component is
limited by the definition of that component. For example, the
definition of the scheme component does not allow characters
beyond US-ASCII.

(Note: In accordance with URI practice, generic IRI software
cannot and URI-only components. Such a mapping
SHOULD be applied as late as possible. It should not check for such limitations.)

b) In the URI syntax, characters be applied
between components that are likely known to be used able to
delimit handle IRIs.

6.3 Format of URIs in text and print ("space", "delims", and
"unwise") were excluded. They are included IRIs in Documents and Protocols

Document formats that transport URIs may need to be upgraded to allow
the IRI syntax
(with the exception transport of '%', which cannot IRIs. In those cases where the document as a whole
has a native character encoding, IRIs MUST also be used directly, and
'#', which is used encoded in this
encoding, and converted accordingly by a parser or interpreter. IRI references), for the following
reasons:

1) The syntax includes many other
characters that are not
appropriate expressible in many cases.

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2) Some implementation practice already allows them the native encoding SHOULD be
escaped using the escaping conventions of the document format if such
conventions are available. Alternatively, they MAY be escaped
according to Section 3.1. For example, in URI HTML or XML, numeric
character references (for example spaces in fragment identifiers).

3) It SHOULD be used. If a document as a whole has a
native character encoding, and that character encoding is very convenient in some cases, for example for
XPointers not UTF-8,
then IRIs MUST NOT be placed into the document in XML attributes.

4) Considering context is the UTF-8 character
encoding.

Note: Some formats already necessary in accommodate IRIs, although they use
different terminology. HTML 4.0 [HTML4] defines the case conversion from
IRIs to URIs as error-avoiding behavior. XML 1.0 [XML1], XLink
[XLink], and XML Schema [XMLSchema] and specifications based upon
them allow IRIs. Also, it is expected that all relevant new W3C
formats and protocols will be required to handle IRIs [CharMod].

6.4 Use of
URIs, UTF-8 for example Encoding Original Characters

This section discusses details and gives examples for "&amp;" point c) in XML.

However, these characters should
Section 1.2. In order to be avoided where possible.
Whenever there is a chance that an able to use IRIs, the URI corresponding
to the IRI will be used in a
component where these question has to encode original characters into octets
using UTF-8. This can be harmful, they should be
escaped from the start.

c) The UCS contains many areas of characters specified for which there are
strong visual look-alikes. Because of the likelihood of
transcription errors, these also should be avoided. This
includes the full-width equivalents all URIs of ASCII characters, half-
width Katakana characters an URI scheme, or
can apply to individual URIs for Japanese, and many others. This
also includes many look-alikes of "space", "delims", and
"unwise", characters excluded in [RFC2396].

Additional information is available from [UNIXML]. [UNIXML] 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
written recommended in [RFC2718].
Examples where this is already used are the context of running text rather than in URN syntax [RFC2141],
IMAP URLs [RFC2192], and POP URLs [RFC2384]. On the context of
identifiers. Nevertheless, it discusses many of other hand, the categories of
characters and code points
HTTP URL scheme does not appropriate for IRIs.

5.2 Software Interfaces specify how to encode original characters,
and Protocols

Although an IRI is defined as therefore IRIs only can be used for some HTTP URLs.

For example, for a sequence document with a URI of characters, software
interfaces

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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 URIs typically function on sequences of octets or
other kinds of code units. Thus, software interfaces the
e-acute character, and protocols
MUST define which character encoding %C3%A9 is used.

Intermediate software interfaces between IRI-capable components the UTF-8 encoded and
URI-only components MUST map escaped
representation of that character). On the IRIs as per Section 3.1, when
transferring from IRI-capable to URI-only components. Such other hand, for a mapping
SHOULD be applied as late as possible. It should not be applied
between components that are known to document
with an URI of http://www.example.org/r%E9sum%E9.html, the escaped
octets cannot be able converted to handle IRIs.

5.3 Format of URIs and IRIs actual characters in Documents and Protocols

Document formats that transport URIs may need to be upgraded an IRI, because
the escaping is not based on UTF-8.

The requirement for the use of UTF-8 applies to allow all parts of an URI,
with the transport exception of IRIs. In those cases where the document as a whole
has a native character encoding, ihostname part. However, it is possible
that the capability of IRIs MUST also be encoded in this
encoding, and converted accordingly by to represent a parser or interpreter. IRI wide range of characters that are not expressible
directly is used just in some parts of the native encoding SHOULD be
escaped using the escaping conventions IRI (or IRI reference).
The other parts of the document format if such

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conventions are available. Alternatively, IRI may only contain ASCII characters, or they MAY
may not be escaped
according to Section 3.1. For example, in HTML, XML, or SGML,
numeric character references should based on UTF-8. They may be used. If a document as a
whole has a native character based on another encoding, and that character encoding or
they may directly encode raw binary data (see also [RFC2397]).

For example, it is
not UTF-8, then IRIs MUST NOT be placed into 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 the UTF-
8 character encoding.

Note: Some formats already accommodate IRIs, although they use
different terminology. HTML 4.0 [HTML4] defines the conversion from
IRIs to URIs as error-avoiding behavior. XML 1.0 [XML1], XLink
[XLink], and XML Schema [XMLSchema] and specifications iso-8859-1 based upon
them allow IRIs. Also, it on server settings, but
the fragment identifier is expected that all relevant new W3C
formats and protocols will be required encoded in UTF-8 according to handle IRIs [CharMod].

5.4 [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
straightforwardly; the algorithms of RFC 2396 may [RFCYYYY] can be applied
directly, treating the characters additionally allowed in IRIs in the
same way as unreserved characters in URIs.

7. URI/IRI Processing Guidelines (informative)

This informative section provides guidelines for supporting IRIs in
the same software components and operations that currently process
URIs: software interfaces that handle URIs, software that allows
users to enter URIs, software that generates URIs, software that
displays URIs, formats and protocols that transport URIs, and
software that interprets URIs. These may all require more or less
modification before functioning properly with IRIs. The
considerations in this section also apply to URI references and IRI
references.

6.1

7.1 URI/IRI Software Interfaces

Software interfaces that handle URIs, such as URI-handling APIs and

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protocols transferring URIs, need interfaces and protocol elements
that are designed to carry IRIs.

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
compatible with US-ASCII, is in accordance with the recommendations
of [RFC2277], and makes it easy to convert to URIs where necessary.
In any case, the encoding used API or protocol definition must not clearly define the
encoding to be left undefined. used.

The transfer from URI-only to IRI-capable components requires no
mapping, although the conversion described in Section 3.2 above may
be performed. It is preferable not to perform this inverse

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conversion when there is a chance that this cannot be done correctly.

6.2

7.2 URI/IRI Entry

There are components that allow users to enter URIs into the system,
for example, example by typing or dictation. This software must be updated to
allow for IRI entry.

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,
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,
this may be a more or less complicated process.

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
choosing appropriate input methods or variants/settings thereof, by
appropriately converting the characters being input, by eliminating
characters that cannot be converted, and/or by issuing a warning or
error message to the user.

As an example of variant settings, input method editors for East
Asian Languages usually allow to the input of Latin letters and related
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
produce US-ASCII characters where possible.

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
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
some of the software they use does not yet accept IRIs.

An IRI input component that interfaces to components that handle
URIs, but not IRIs, must map the the IRI to an a URI before passing it to

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such a component.

For the input of IRIs with right-to-left characters, please see
Section 4.3.

6.3

7.3 URI/IRI Transfer Between Applications

Many applications, in particular many mail user agents, try to detect
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
and retrieve the corresponding resource in an appropriate (usually
scheme-dependent) application.

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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-
ASCII character should not be taken as the indication of the end of
an IRI. Such applications also have to make sure that they correctly
convert the detected IRI from the encoding of the document or
application where the IRI appears to the encoding used by the system-
wide IRI invocation mechanism, or to an URI (according to Section
3.1) if the system-wide invocation mechanism only accepts URIs.

The clipboard is another frequently used way to transfer URIs and
IRIs from one application to another. On most platforms, the
clipboard is able to store and transfer text in many languages and
scripts. Correctly used, the clipboard transfers characters, not
bytes, which will do the right thing with IRIs.

6.4

7.4 URI/IRI Generation

Systems that are offering offer resources through the Internet, where those
resources have logical names, sometimes automatically generate URIs
for the resources they offer. For example, some HTTP servers can
generate a directory listing for a file directory, and then respond
to the generated URIs with the files.

Many legacy character encodings are in use in various file systems.
Many currently deployed systems do not transform the local character
representation of the underlying system before generating URIs.

For maximum interoperability, systems that generate resource
identifiers should do the appropriate transformations. They For example,
if a file system contains a file named r&#xE9;sum&#xE9;.html, a
server should
use IRIs converted to URIs expose this as r%C3%A9sum%C3%A9.html in cases where it cannot be expected that
the recipient is able to handle IRIs. Due an URI, which
allows to the way most user
agents currently work, native IRIs, encoded use r&#xE9;sum&#xE9;.html in UTF-8, may be used an IRI, even if the recipient announces that it can interpret UTF-8. This requires
that the whole page file name
locally is sent as kept in an encoding other than UTF-8. If this is not possible,
escaping can always be used.

This recommendation in particular applies to HTTP servers. For FTP

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servers, similar considerations apply, see in particular [RFC2640].

6.5

7.5 URI/IRI Selection

In some cases, resource owners and publishers have control over the
IRIs used to identify their resources. Such control is mostly
executed by controlling the resource names, such as file names,
directly.

In such cases, it is recommended to avoid choosing IRIs that are
easily confused. For example, for US-ASCII, the lower-case ell "l"

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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
confusing users with "br0ken" or "1ame" identifiers.

Outside of the US-ASCII range, there are many more opportunities for
confusion; a complete set of guidelines is too lengthy to include
here. As long as names are limited to characters from a single
script, native writers of a given script or language will know best
when ambiguities can appear, and how they can be avoided. What may
look ambiguous to a stranger may be completely obvious to the average
native user. On the other hand, in some cases, the UCS contains
variants for compatibility reasons, for example for typographic
purposes. These should be avoided wherever possible. Although there
may be exceptions, in general newly created resource names should be
in NFKC [UTR15] (which means that they are also in NFC).

As an example, the UCS contains codepoint U+FB01 for the 'fi'
ligature for compatibility reasons. Wherever possible, IRIs should
use the two letters 'f' and 'i' rather than the 'fi' ligature. An
example where the later may be used is in the query part of an IRI
for an explicit search for a word containing the 'fi' ligature.

In certain cases, there is a chance that characters from different
scripts look the same. The best known example is the Latin 'A', the
Greek 'Alpha', and the Cyrillic 'A'. To avoid such cases, only IRIs
should be generated where all the characters in a single component
are used together in a given language. This usually means that all
these characters will be from the same script, but there are
languages that mix characters from different scripts (such as
Japanese). This is similar to the heuristics used to distinguish
between letters and numbers in the examples above. Also, for Latin,
Greek, and Cyrillic, using lower-case letters results in fewer
ambiguities than using upper-case letters.

6.6

7.6 Display of URIs/IRIs

In situations where the rendering software is not expected to display

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non-ASCII parts of the IRI correctly using the available layout and
font resources, these parts should be escaped before being displayed.

For display of Bidi IRIs, please see Section 4.1.

6.7

7.7 Interpretation of URIs and IRIs

Software that interprets IRIs as the names of local resources should
accept IRIs in multiple forms, and convert and match them with the
appropriate local resource names.

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First, multiple representations include both IRIs in the native
character encoding of the protocol and also their URI counterparts.

Second, it may include URIs constructed based on other character
encodings than UTF-8. Such URIs may be produced by user agents that
do not conform to this specification and use legacy encodings to
convert non-ASCII characters to URIs. Whether this is necessary, and
what character encodings to cover, depends on a number of factors,
such as the legacy character encodings used locally and the
distribution of various versions of user agents. For example,
software for Japanese may accept URIs in Shift_JIS and/or EUC-JP in
addition to UTF-8.

Third, it may include additional mappings to be more user-friendly
and robust against transmission errors. These would be similar to
how currently some servers treat URIs as case-insensitive, or perform
additional matching to account for spelling errors. For characters
beyond the ASCII repertoire, this may for example include ignoring
the accents on received IRIs or resource names where appropriate.
Please note that such mappings, including case mappings, are
language-dependent.

It can be difficult to unambiguously identify a resource if too many
mappings are taken into consideration. However, escaped and non-
escaped parts of IRIs can always clearly be distinguished. Also, the
regularity of UTF-8 (see [Duerst97]) makes the potential for
collisions lower than it may seem at first sight.

6.8

7.8 Upgrading Strategy

Where this recommendation places further constraints on software for
which many instances are already deployed, it is important to
introduce upgrades carefully, and to be aware of the various
interdependencies.

If IRIs cannot be interpreted correctly, they should not be generated
or transported. This suggests that upgrading URI interpreting

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software to accept IRIs should have highest priority.

On the other hand, a single IRI is interpreted only by a single or
very few interpreters that are known in advance, while it may be
entered and transported very widely.

Therefore, IRIs benefit most from a broad upgrade of software to be
able to enter and transport IRIs, but before publishing any
individual IRI, care should be taken to upgrade the corresponding
interpreting software in order to cover the forms expected to be
received by various versions of entry and transport software.

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The upgrade of generating software to generate IRIs instead of a
local encoding should happen only after the service is upgraded to
accept IRIs. Similarly, IRIs should only be generated when the
service accepts IRIs and the intervening infrastructure and protocol
is known to transport them safely.

Display software should be upgraded only after upgraded entry
software has been widely deployed to the population that will see the
displayed result.

These recommendations, when taken together, will allow for the
extension from URIs to IRIs in order to handle scripts other than
ASCII while minimizing interoperability problems.

8. Security Considerations

Incorrect escaping or unescaping can lead to security problems. In
particular, some UTF-8 decoders do not check against overlong byte
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
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
if UTF-8 decoders are fault-tolerant, if conversion and checking are
not done in the right order, and/or if reserved characters and
unreserved characters are not clearly distinguished.

There are various ways in which "spoofing" can occur with IRIs.
"Spoofing" means that somebody may add a resource name that looks the
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
similar, but may contain all kinds of changes that may be difficult
to spot but can cause all kinds of problems. Most spoofing
possibilities for IRIs are extensions of those for URIs.

Spoofing can occur for various reasons. A first reason is that
normalization expectations of a user or actual normalization when

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entering an IRI, or when transcoding an IRI from a legacy encoding,
do not match the normalization used on the server side.
Conceptually, this is no different from the problems surrounding the
use of case-insensitive web servers. For example, a popular web page
with a mixed case name (http://big.site/
PopularPage.html) (http://big.site/PopularPage.html) might be
"spoofed" by someone who is able to create
http://big.site/popularpage.html. http://big.site/
popularpage.html. However, the introduction of character
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
look very similar. Details are discussed in Section 6.5. 7.5. Again,
this is very similar to spoofing possibilities on US-ASCII, e.g.

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using 'br0ken' or '1ame' URIs.

Spoofing can occur when URIs in various encodings are accepted to
deal with older user agents. In some cases, in particular for Latin-
based resource names, this is usually easy to detect because UTF-8-
encoded names, when interpreted and viewed as legacy encodings,
produce mostly garbage. In other cases, when concurrently used
encodings have a similar structure, but there are no characters that
have exactly the same encoding, detection is more difficult.

Spoofing can occur in various IRI components, such as the domain name
part or a path part. For considerations specific to the domain name
part, see [Nameprep]. [RFC3491]. For the path part, administrators of sites
which allow independent users to create resources in the same subarea
may need to be careful to check for spoofing.

Spoofing can occur with bidirectional IRIs, if the restrictions in
Section 4.2 are not followed. The same visual representation may be
interpreted as different logical representations, and vice versa. It
is also very important that a correct Unicode bidirectional
implementation is used.

8. Issues List

- 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.

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- 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. Unicode bidirectional
implementation is used.

9. Acknowledgements

We would like to thank Larry Masinter for his work as coauthor of
many earlier versions of this document (draft-masinter-url-i18n-xx).

The discussion on the issue addressed here has started a long time
ago. There was a thread in the HTML working group in August 1995
(under the topic of "Globalizing URIs") and in the www-international
mailing list in July 1996 (under the topic of "Internationalization

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mailing list in July 1996 (under the topic of "Internationalization
and URLs"), and ad-hoc meetings at the Unicode conferences in
September 1995 and September 1997.

Thanks to Francois Yergeau, Matti Allouche, Roy Fielding, Tim
Berners-Lee, Mark Davis, M.T. Carrasco Benitez, James Clark, Tim
Bray, Chris Wendt, Yaron Goland, Andrea Vine, Misha Wolf, Leslie
Daigle, Ted Hardie, Makoto MURATA, Steven Atkin, Ryan Stansifer, Tex
Texin, Graham Klyne, Bjoern Hoehrmann, Chris Lilley, Ian Jacobs, Dan
Oscarson, Elliotte Rusty Harold, Mike J. Brown, Simon Josefsson,
Carlos Viegas Damasio, and many others for help with understanding
the issues and possible solutions, and getting the details right.
Thanks also to the members of the W3C I18N Working Group and Interest
Group for their contributions and their work on [CharMod], to the
members of many other W3C WGs for adopting the ideas, and to the
members of the Montreal IAB Workshop on Internationalization and
Localization for their review.

Normative References

[ISO10646] International Organization for Standardization,
"Information Technology - Universal Multiple-Octet Coded
Character Set (UCS) - Part 1: Architecture and Basic
Multilingual Plane - Part 2: Supplementary Planes", ISO
Standard 10646, with amendment, July 2002.

[RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.

[RFC2279]

[RFC3490] Faltstrom, P., Hoffman, P. and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003, <http://www.ietf.org/rfc/
rfc3490.txt>.

[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)", RFC
3491, March 2003.

[RFCXXXX] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.

[RFC2396] draft-yergeau-rfc2279bis-05.txt (work in
progress), June 2003, <http://www.ietf.org/internet-
drafts/draft-yergeau-rfc2279bis-05.txt>.

[RFCYYYY] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers Identifier (URI): Generic Syntax", RFC 2396,
August 1998.

[RFC2732] Hinden, R., Carpenter, B. and L. Masinter, "Format for
Literal IPv6 Addresses in URL's", RFC 2732, December
1999.

[RFCXXXX] Faltstrom, P., Hoffman, P. and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
draft-ietf-idn-idna-14.txt draft-
fielding-uri-rfc2396bis-03.txt (work in progress), October
2002, <http://www.ietf.org/internet-drafts/draft-ietf-
idn-idna-14.txt>. June
2003.

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[UTR15] Davis, M. and M. Duerst, "Unicode Normalization Forms",
Unicode Standard Annex #15, March 2001, <http://
www.unicode.org/unicode/reports/tr15/tr15-21.html>.

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Non-normative References

[BidiEx] "Examples of bidirectional IRIs", <http://www.w3.org/
International/iri-edit/BidiExamples>.

[CharMod] Duerst, M., Yergeau, F., Ishida, R., Wolf, M.,
Freytag, A. and T. Texin, "Character Model for the
World Wide Web", World Wide Web Consortium Working
Draft, April 2002, <http://www.w3.org/TR/charmod>.

[Duerst97] Duerst, M., "The Properties and Promises of UTF-8",
Proc. 11th International Unicode Conference, San Jose
, September 1997, <http://www.ifi.unizh.ch/mml/
mduerst/papers/PDF/IUC11-UTF-8.pdf>.

[Duerst01] Duerst, M., "Internationalized Resource Identifiers:
From Specification to Testing", Proc. 19th
International Unicode Conference, San Jose ,
September 2001, <http://www.w3.org/2001/Talks/0912-
IUC-IRI/paper.html>.

[HTML4] Raggett, D., Le Hors, A. and I. Jacobs, "HTML 4.01
Specification", World Wide Web Consortium
Recommendation, December 1999, <http://www.w3.org/TR/
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
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2130] Weider, C., Preston, C., Simonsen, K., Alvestrand,
H., Atkinson, R., Crispin, M. and P. Svanberg, "The
Report of the IAB Character Set Workshop held 29
February - 1 March, 1996", RFC 2130, April 1997.

[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.

[RFC2192] Newman, C., "IMAP URL Scheme", RFC 2192, September

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1997.

[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.

[RFC2384] Gellens, R., "POP URL Scheme", RFC 2384, August 1998.

[RFC2396bis]

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[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter,
"Uniform Resource Identifier Identifiers (URI): Generic Syntax",
Internet-Draft (work in progress), October 2002.
RFC 2396, August 1998.

[RFC2397] Masinter, L., "The "data" URL scheme", RFC 2397,
August 1998.

[RFC2616] Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P. and T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
June 1999.

[RFC2640] Curtin, B., "Internationalization of the File
Transfer Protocol", RFC 2640, July 1999.

[RFC2718] Masinter, L., Alvestrand, H., Zigmond, D. and R.
Petke, "Guidelines for new URL Schemes", RFC 2718,
November 1999.

[UNIV3]

[UNIV4] The Unicode Consortium, "The Unicode Standard Standard,
Version
3.0", 4.0", Addison-Wesley, Reading, MA , 2000. 2003.

[UNI9] Davis, M., "The Bidirectional Algorithm", Unicode
Standard Annex #9, March 2002, <http://
www.unicode.org/unicode/reports/tr9>.

[UNIXML] Duerst, M. and A. Freytag, "Unicode in XML and other
Markup Languages", Unicode Technical Report #20,
World Wide Web Consortium Note, February 2002,
<http://www.w3.org/TR/unicode-xml/>.

[W3CIRI] Duerst, M., "Internationalization - URIs and other
identifiers", World Wide Web Consortium Note,
September 2002, <http://www.w3.org/International/O-
URL-and-ident.html>.

[XLink] DeRose, S., Maler, E. and D. Orchard, "XML Linking
Language (XLink) Version 1.0", World Wide Web
Consortium Recommendation, June 2001, <http://
www.w3.org/TR/xlink/#link-locators>.

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[XML1] Bray, T., Paoli, J., Sperberg-McQueen, C. and E.
Maler, "Extensible Markup Language (XML) 1.0 (Second
Edition)", World Wide Web Consortium Recommendation,
including Erratum 26 at http://www.w3.org/XML/xml-
V10-2e-errata#E26, October 2000, <http://www.w3.org/
TR/REC-xml#sec-external-ent>.

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[XMLNamespace] Bray, T., Hollander, D. and A. Layman, "Namespaces in
XML", World Wide Web Consortium Recommendation,
January 1999, <http://www.w3.org/TR/REC-xml#sec-
external-ent>.

[XMLSchema] Biron, P. and A. Malhotra, "XML Schema Part 2:
Datatypes", World Wide Web Consortium Recommendation,
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

Martin Duerst (Note: Please write "Duerst" with u-umlaut wherever
possible, for example as "D&#252;rst in XML and HTML.)
World Wide Web Consortium
200 Technology Square
Cambridge, MA 02139
U.S.A.

Phone: +1 617 253 5509
Fax: +1 617 258 5999
EMail: duerst@w3.org
URI: http://www.w3.org/People/D%C3%BCrst/
(Note: This is the escaped form of an IRI.)

Michel Suignard
Microsoft Corporation

One Microsoft Way
Redmond, WA 98052
U.S.A.

Phone: +1 425 882-8080
EMail: mailto:michelsu@microsoft.com
URI: http://www.suignard.com

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