Network Working Group M. Duerst
Internet-Draft W3C
Expires: January 17, 2005 M. Suignard
Microsoft Corporation
July 19, 2004
Internationalized Resource Identifiers (IRIs)
draft-duerst-iri-09
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document defines a new protocol element, the Internationalized
Resource Identifier (IRI), as a complement to the Uniform Resource
Identifier (URI). An IRI is a sequence of characters from the
Universal Character Set (Unicode/ISO 10646). 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
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of extending or changing the definition of URIs, to allow a clear
distinction and to avoid incompatibilities with existing software.
Guidelines for the use and deployment of IRIs in various protocols,
formats, and software components that now deal with URIs are
provided.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Overview and Motivation . . . . . . . . . . . . . . . . . 4
1.2 Applicability . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Definitions . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Notation . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1 Summary of IRI Syntax . . . . . . . . . . . . . . . . . . 7
2.2 ABNF for IRI References and IRIs . . . . . . . . . . . . . 8
3. Relationship between IRIs and URIs . . . . . . . . . . . . . . 11
3.1 Mapping of IRIs to URIs . . . . . . . . . . . . . . . . . 11
3.2 Converting URIs to IRIs . . . . . . . . . . . . . . . . . 14
3.2.1 Examples . . . . . . . . . . . . . . . . . . . . . . . 16
4. Bidirectional IRIs for Right-to-left Languages . . . . . . . . 17
4.1 Logical Storage and Visual Presentation . . . . . . . . . 17
4.2 Bidi IRI Structure . . . . . . . . . . . . . . . . . . . . 19
4.3 Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . . 20
4.4 Examples . . . . . . . . . . . . . . . . . . . . . . . . . 20
5. IRI Equivalence and Comparison . . . . . . . . . . . . . . . . 22
5.1 Simple String Comparison . . . . . . . . . . . . . . . . . 22
5.2 Conversion to URIs . . . . . . . . . . . . . . . . . . . . 23
5.3 Normalization . . . . . . . . . . . . . . . . . . . . . . 23
5.4 Preferred Forms . . . . . . . . . . . . . . . . . . . . . 24
6. Use of IRIs . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.1 Limitations on UCS Characters Allowed in IRIs . . . . . . 25
6.2 Software Interfaces and Protocols . . . . . . . . . . . . 25
6.3 Format of URIs and IRIs in Documents and Protocols . . . . 26
6.4 Use of UTF-8 for Encoding Original Characters . . . . . . 26
6.5 Relative IRI References . . . . . . . . . . . . . . . . . 27
7. URI/IRI Processing Guidelines (informative) . . . . . . . . . 27
7.1 URI/IRI Software Interfaces . . . . . . . . . . . . . . . 28
7.2 URI/IRI Entry . . . . . . . . . . . . . . . . . . . . . . 28
7.3 URI/IRI Transfer Between Applications . . . . . . . . . . 29
7.4 URI/IRI Generation . . . . . . . . . . . . . . . . . . . . 29
7.5 URI/IRI Selection . . . . . . . . . . . . . . . . . . . . 30
7.6 Display of URIs/IRIs . . . . . . . . . . . . . . . . . . . 31
7.7 Interpretation of URIs and IRIs . . . . . . . . . . . . . 31
7.8 Upgrading Strategy . . . . . . . . . . . . . . . . . . . . 32
8. Security Considerations . . . . . . . . . . . . . . . . . . . 33
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 34
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11. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
11.1 Normative References . . . . . . . . . . . . . . . . . . . . 35
11.2 Non-normative References . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 38
A. Design Alternatives . . . . . . . . . . . . . . . . . . . . . 39
A.1 New Scheme(s) . . . . . . . . . . . . . . . . . . . . . . 39
A.2 Other Character Encodings than UTF-8 . . . . . . . . . . . 39
A.3 New Encoding Convention . . . . . . . . . . . . . . . . . 39
A.4 Indicating Character Encodings in the URI/IRI . . . . . . 40
Intellectual Property and Copyright Statements . . . . . . . . 41
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1. Introduction
1.1 Overview and Motivation
A Uniform Resource Identifier (URI) is defined in [RFCYYYY] as a
sequence of characters chosen from a limited subset of the repertoire
of US-ASCII [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 have
transcriptions to Latin 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
Internationalized Resource Identifier (IRI), by extending the syntax
of URIs to a much wider repertoire of characters. It also defines
"internationalized" versions corresponding to other constructs from
[RFCYYYY], such as URI references. The syntax of IRIs is defined in
Section 2, and the relationship between IRIs and URIs in Section 3.
Using characters outside of A-Z in IRIs brings with it some
difficulties. Section 4 discusses the special case of bidirectional
IRIs, Section 5 various forms of equivalence between IRIs, and
Section 6 the use of IRIs in different situations. Section 7 gives
additional informative guidelines, and Section 8 security
considerations.
For discussion of this document, please use the public-iri@w3.org
mailing list (publicly archived at
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].
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1.2 Applicability
IRIs are designed to be compatible with recent recommendations for
new URI schemes [RFC2718]. The compatibility is provided by
specifying 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 where IRIs are used should be
explicitly designated to be able 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 "anyURI" that includes IRIs and IRI references.
Therefore, IRIs and IRI references can be in attributes and
elements of type "anyURI". On the other hand, in the HTTP
protocol [RFC2616], the Request URI is defined as an URI, which
means that direct use of IRIs is not allowed in HTTP requests.
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) The URI corresponding to the IRI in question has to encode
original characters into octets using UTF-8. For new URI schemes,
this is recommended in [RFC2718]. It can apply to a whole scheme
(e.g. IMAP URLs [RFC2192] and POP URLs [RFC2384], or the URN
syntax [RFC2141]). It can apply to a specific part of a URI, such
as the fragment identifier (e.g. [XPointer]). It can apply to a
specific URI or part(s) thereof. For details, please see Section
6.4.
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 the organization,
control, or representation of data. For example, "LATIN CAPITAL
LETTER A" names a character.
octet: An ordered sequence of eight bits considered as a unit
character repertoire: A set of characters (in the mathematical sense)
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sequence of characters: A sequence (one after another) of characters
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.
charset: The name of a parameter or attribute used to identify a
character encoding.
UCS: Universal Character Set; the coded character set defined by ISO/
IEC 10646 [ISO10646] and the Unicode Standard [UNIV4].
IRI reference: The term "IRI reference" denotes the common usage of
an Internationalized Resource Identifier. An IRI reference may be
absolute or relative. However, the "IRI" that results from such a
reference only includes absolute IRIs; any relative IRIs are
resolved to their absolute form. Note that in [RFC2396], URIs did
not include fragment identifiers, but in [RFCYYYY], fragment
identifiers are part of URIs.
running text: Human text (paragraphs, sentences, phrases) with syntax
according to orthographic conventions of a natural language, as
opposed to syntax defined for ease of processing by machines
(markup, programming languages,...).
protocol element: Any portion of a message which affects processing
of that message by the protocol in question.
presentation element: Presentation form corresponding to a protocol
element, for example using a wider range of characters.
create (an URI or IRI): With respect to URIs and IRIs, the word
'create' is used for the initial creation. This may be the
initial creation of a resource with a certain name, or the initial
exposition of a resource under a particular name.
generate (an URI or IRI): With respect to URIs and IRIs, the word
'generate' is used when the IRI is generated by derivation from
other information.
1.4 Notation
RFCs and Internet Drafts currently do not allow any characters
outside the US-ASCII repertoire. Therefore, this document uses
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various special notations to denote such 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: я
stands for CYRILLIC CAPITAL LETTER YA. In this notation, an actual
'&' is denoted by '&'.
Bidi Notation is used for bidirectional examples: lower case 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.
To denote actual octets in examples (as opposed to percent-encoded
octets), the two hex digits denoting the octet are enclosed in "<"
and ">". For example, the octet often denoted as 0xc9 is denoted
here as <c9>.
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 digitally. The same IRI may be represented as
different sequences of octets in different protocols or documents if
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 [RFCYYYY], but the class of
unreserved characters is extended by adding the characters of the UCS
(Universal Character Set, [ISO10646]) beyond U+007F, subject to the
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limitations given in the syntax rules below and in Section 6.1.
Otherwise, the syntax and use of components and reserved characters
is the same as that in [RFCYYYY]. All the operations defined in
[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.
Characters outside the US-ASCII repertoire 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
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 grammar closely follows the URI grammar in [RFCYYYY],
except that the range of unreserved characters is expanded to include
UCS characters, with the restriction that private UCS characters can
occur only in query parts and not elsewhere. The grammar is split
into two parts, rules that differ from [RFCYYYY] because of the
above-mentioned expansion, and rules that are the same as in
[RFCYYYY]. For rules that are different than in [RFCYYYY], the names
of the non-terminals have been changed as follows: If the
non-terminal contains 'URI', this has been changed to 'IRI'.
Otherwise, an 'i' has been prefixed.
The following rules are different from [RFCYYYY]:
IRI = scheme ":" ihier-part [ "?" iquery ]
[ "#" ifragment ]
ihier-part = "//" iauthority ipath-abempty
/ ipath-abs
/ ipath-rootless
/ ipath-empty
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IRI-reference = IRI / relative-IRI
absolute-IRI = scheme ":" ihier-part [ "?" iquery ]
relative-IRI = irelative-part [ "?" iquery ] [ "#" ifragment ]
irelative-part = "//" iauthority ipath-abempty
/ ipath-abs
/ ipath-noscheme
/ ipath-empty
iauthority = [ iuserinfo "@" ] ihost [ ":" port ]
iuserinfo = *( iunreserved / pct-encoded / sub-delims / ":" )
ihost = IP-literal / IPv4address / ireg-name
ireg-name = *( iunreserved / pct-encoded / sub-delims )
ipath = ipath-abempty ; begins with "/" or is empty
/ ipath-abs ; begins with "/" but not "//"
/ ipath-noscheme ; begins with a non-colon segment
/ ipath-rootless ; begins with a segment
/ ipath-empty ; zero characters
ipath-abempty = *( "/" isegment )
ipath-abs = "/" [ isegment-nz *( "/" isegment ) ]
ipath-noscheme = isegment-nzc *( "/" isegment )
ipath-rootless = isegment-nz *( "/" isegment )
ipath-empty = 0<ipchar>
isegment = *ipchar
isegment-nz = 1*ipchar
isegment-nzc = 1*( iunreserved / pct-encoded / sub-delims
/ "@" )
ipchar = iunreserved / pct-encoded / sub-delims / ":"
/ "@"
iquery = *( ipchar / iprivate / "/" / "?" )
ifragment = *( ipchar / "/" / "?" )
iunreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" / ucschar
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
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/ %xD0000-DFFFD / %xE1000-EFFFD
iprivate = %xE000-F8FF / %xF0000-FFFFD / %x100000-10FFFD
Some productions are ambiguous. The "first-match-wins" (a.k.a.
"greedy") algorithm applies. For details, see [RFCYYYY].
The following are the same as in [RFCYYYY]:
scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
port = *DIGIT
IP-literal = "[" ( IPv6address / IPvFuture ) "]"
IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims
/ ":" )
IPv6address = 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32
/ [ h16 ] "::" 4( h16 ":" ) ls32
/ [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
/ [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
/ [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
/ [ *4( h16 ":" ) h16 ] "::" ls32
/ [ *5( h16 ":" ) h16 ] "::" h16
/ [ *6( h16 ":" ) h16 ] "::"
h16 = 1*4HEXDIG
ls32 = ( h16 ":" h16 ) / IPv4address
IPv4address = dec-octet "." dec-octet "." dec-octet
"." dec-octet
dec-octet = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255
pct-encoded = "%" HEXDIG HEXDIG
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
reserved = gen-delims / sub-delims
gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "="
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3. Relationship between IRIs and URIs
IRIs are meant to replace URIs in identifying resources for
protocols, formats and software components which use a UCS-based
character repertoire. These protocols and components may never need
to use URIs directly, especially when the resource identifier is used
simply for identification purposes. However, when the resource
identifier is used for resource retrieval, it is in many cases
necessary to determine the associated URI because most retrieval
mechanisms currently only are defined for URIs. In this case, IRIs
can serve as presentation elements for URI protocol elements. An
example would be an address bar in a Web user agent. (Additional
rationale is given in Section 3.1.)
3.1 Mapping of IRIs to URIs
This section defines how to map an IRI to a URI. Everything in this
section applies also to IRI references and 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.
Scheme-specific restrictions are applied to IRIs by converting
IRIs to URIs and checking the URIs against the scheme-specific
restrictions.
b) Interpretational: URIs identify resources in various ways. IRIs
also identify resources. When the IRI is used solely for
identification purposes, it is not necessary to map the IRI to a
URI (see Section 5). However, when an IRI is used for resource
retrieval, the resource that the IRI locates is the same as the
one located by the URI obtained after converting the IRI according
to the procedure defined here. This means that there is no need
to define resolution separately on the IRI level.
Applications MUST map IRIs to URIs using the following two steps.
Step 1) This step generates a UCS character sequence from the
original IRI format. This step has three variants, depending on
the form of the input.
Variant A) If the IRI is written on paper or read out loud, or
otherwise represented as a sequence of characters independent
of any character encoding: Represent the IRI as a sequence of
characters from the UCS normalized according to Normalization
Form C (NFC, [UTR15]).
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Variant B) If the IRI is in some digital representation (e.g. an
octet stream) in some known non-Unicode character encoding:
Convert the IRI to a sequence of characters from the UCS
normalized according to NFC.
Variant C) If the IRI is in an Unicode-based character encoding
(for example UTF-8 or UTF-16): Do not normalize. Apply Step 2
directly to the encoded Unicode character sequence.
Step 2) For each character in 'ucschar' or 'iprivate', apply Steps
2.1 through 2.3 below.
2.1) Convert the character to a sequence of one or more octets
using UTF-8 [RFC3629].
2.2) Convert each octet to %HH, where HH is the hexadecimal
notation of the octet value. Note that this is identical to
the percent-encoding mechanism in Section 2.1 of [RFCYYYY]. To
reduce variability, the hexadecimal notation SHOULD use upper
case letters.
2.3) Replace the original character by the resulting character
sequence (i.e. a sequence of %HH triplets).
The above mapping from IRIs to URIs produces URIs fully conforming to
[RFCYYYY]. The mapping is also an identity transformation for URIs
and is idempotent -- applying the mapping a second time will not
change anything. Every URI is by definition an IRI.
Infrastructure accepting IRIs MAY convert the ireg-name component of
an IRI as follows (before Step 2 above) for schemes that are known to
use domain names in ireg-name, but where the scheme definition does
not allow percent-encoding for ireg-name: Replace the ireg-name part
of the IRI by the part converted using the ToASCII operation
specified in Section 4.1 of [RFC3490] on each dot-separated label,
and using U+002E (FULL STOP) as a label separator, with the flag
UseSTD3ASCIIRules set to TRUE and the flag AllowUnassigned set to
FALSE for creating IRIs and set to TRUE otherwise. The ToASCII
operation may fail, but this would mean that the IRI cannot be
resolved. This conversion SHOULD be used when the goal is to
maximize interoperability with legacy URI resolvers. For example,
the IRI
http://résumé.example.org may be converted to
http://xn--rsum-bpad.example.org instead of
http://r%C3%A9sum%C3%A9.example.org.
An IRI with a scheme that is known to use domain names in ireg-name,
but where the scheme definition does not allow percent-encoding for
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ireg-name, meets scheme-specific restrictions if either the
straightforward conversion or the conversion using the ToASCII
operation on ireg-name result in an URI that meets the
scheme-specific restrictions. An IRI with a scheme that is known to
use domain names in ireg-name, but where the scheme definition does
not allow percent-encoding for ireg-name, resolves to the URI
obtained after converting the IRI including using the ToASCII
operation on ireg-name. Implementations do not need to do this
conversion as long as they produce the same result.
Note: The difference between Variants B and C in Step 1 (Variant B
using normalization with NFC while Variant C not using any
normalization) is to account for the fact that in many non-Unicode
character encodings, some text cannot be represented directly.
For example, Vietnam is natively written "Việt Nam"
(containing a LATIN SMALL LETTER E WITH CIRCUMFLEX AND DOT BELOW)
in NFC, but a direct transcoding from the windows-1258 character
encoding leads to "Việt Nam" (containing a LATIN SMALL
LETTER E WITH CIRCUMFLEX followed by a COMBINING DOT BELOW),
whereas direct transcoding of other 8-bit encodings of Vietnamese
may lead to other representations.
Note: The uniform treatment of the whole IRI in Step 2 above is
important to not make processing dependent on URI scheme. See
[Gettys] for an in-depth discussion.
Note: In practice, the difference above will not be noticed if
mapping from IRI to URI and resolution is tightly integrated (e.g.
carried out in the same user agent). But conversion using
[RFC3490] may be able to better deal with backwards compatibility
issues in case mapping and resolution are separated, as in the
case of using an HTTP proxy.
Note: Internationalized Domain Names may be contained in parts of an
IRI other than the ireg-name part. It is the responsibility of
scheme-specific implementations (if the Internationalized Domain
Name is part of the scheme syntax) or of server-side
implementations (if the Internationalized Domain Name is part of
'iquery') to apply the necessary conversions at the appropriate
point. Example: Trying to validate the Web page at
http://résumé.example.org would lead to an IRI of
http://validator.w3.org/check?uri=http%3A%2F%2Frésumé.
example.org, which would convert to a URI of
http://validator.w3.org/check?uri=http%3A%2F%2Fr%C3%A9sum%C3%A9.
example.org. The server side implementation would be responsible
to do the necessary conversions in order to be able to retrieve
the Web page.
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Infrastructure accepting IRIs MAY also deal with the printable
characters in US-ASCII that are not allowed in URIs, namely "<", ">",
'"', Space, "{", "}", "|", "\", "^", and "`", in Step 2 above. If
such characters are found but are not converted, then the conversion
SHOULD fail. Please note that the number sign ("#"), the percent
sign ("%"), and the square bracket characters ("[", "]") are not part
of the above list, and MUST NOT be converted. Protocols and formats
that have used earlier definitions of IRIs including these characters
MAY require percent-encoding of these characters as a preprocessing
step to extract the actual IRI from a given field. Such
preprocessing MAY also be used by applications allowing the user to
enter an IRI.
Note: In this process (in Step 2.3), characters allowed in URI
references as well as existing percent-encoded sequences are not
encoded further. (This mapping is similar to, but different from,
the encoding applied when including arbitrary content into some
part of a URI.) For example, an IRI of
http://www.example.org/red%09rosé#red (in XML notation) is
converted to
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: Some older software transcoding to UTF-8 may produce illegal
output for some input, in particular for characters outside the
BMP (Basic Multilingual Plane). As an example, for the following
IRI with non-BMP characters (in XML Notation):
http://example.com/𐌀𐌁𐌂
(the first three letters of the Old Italic alphabet) the correct
conversion to a URI is:
http://example.com/%F0%90%8C%80%F0%90%8C%81%F0%90%8C%82
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 this section will always
result in an IRI which maps back to the URI that was used as an input
for the conversion (except for potential case differences in
percent-encoding and for potential percent-encoded unreserved
characters). However, the IRI resulting from this conversion may not
be exactly the same as the original IRI (if there ever was one).
URI to IRI conversion removes percent-encodings, but not all
percent-encodings can be eliminated. There are several reasons for
this:
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a) Some percent-encodings are necessary to distinguish
percent-encoded and unencoded uses of reserved characters.
b) Some percent-encodings cannot be interpreted as sequences of UTF-8
octets.
(Note: The octet patterns of UTF-8 are highly regular. Therefore,
there is a very high probability, but no guarantee, that
percent-encodings 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 2.2, Section 4.1, and 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) Convert all percent-encodings (% 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.
3) Re-percent-encode any octet produced in Step 2 that is not part of
a strictly legal UTF-8 octet sequence.
4) Re-percent-encode all octets produced in Step 3 that in UTF-8
represent characters that are not appropriate according to Section
2.2, Section 4.1, and Section 6.1.
5) Interpret the resulting octet sequence as a sequence of characters
encoded in UTF-8.
This procedure will convert as many percent-encoded characters as
possible to characters in an IRI. Because there are some choices
when applying Step 4 (see Section 6.1), results may vary.
Conversions from URIs to IRIs MUST NOT use any other character
encoding than UTF-8 in Steps 3 and 4 above, even if it might be
possible from context to guess that another character 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
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http://www.example.org/r%C3%A9sum%C3%A9.html, which is a different
URI than http://www.example.org/r%E9sum%E9.html.
3.2.1 Examples
This section shows various examples of converting URIs to IRIs. Each
example shows the result after applying each of the Steps 1 to 5.
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><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/Dürst
The following example contains the sequence '%FC', which might
represent U+00FC LATIN SMALL LETTER U WITH DIAERESIS in the
iso-8859-1 character encoding. (It might represent other characters
in other character 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-percent-encoded in Step 3.
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 following example contains '%e2%80%ae', which is the
percent-encoded
UTF-8 character 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-percent-encoded in Step 4. This
example shows that the case (upper or lower) of letters used in
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percent-encodes may not be preserved. The example also contains a
punycode-encoded domain name label (xn--99zt52a), which is not
converted.
1) http://xn--99zt52a.example.org/%e2%80%ae
2) http://xn--99zt52a.example.org/<e2><80><ae>
3) http://xn--99zt52a.example.org/<e2><80><ae>
4) http://xn--99zt52a.example.org/%E2%80%AE
5) http://xn--99zt52a.example.org/%E2%80%AE
Implementations with scheme-specific knowledge MAY convert
punycode-encoded domain name labels to the corresponding characters
using the ToUnicode procedure. Thus, for the example above, the
label xn--99zt52a may be converted to U+7D0D U+8C46 (Japanese Natto),
leading to the overall IRI of
http://納豆.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;
2) the ability to include a wide range of characters in various parts
of the IRI;
3) minor or no changes or restrictions for implementations.
4.1 Logical Storage and Visual Presentation
When stored or transmitted in digital representation, bidirectional
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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 in the same way as they would be rendered if they were in an
left-to-right embedding, i.e. as if they were preceded by U+202A,
LEFT-TO-RIGHT EMBEDDING (LRE), and followed by U+202C, POP
DIRECTIONAL FORMATTING (PDF). Setting the embedding direction can
also be done in a higher-level protocol (e.g. the dir='ltr'
attribute in HTML).
There is no requirement to actually use the above embedding if the
display is still the same without the embedding. For example, a
bidirectional IRI in a text with left-to-right base directionality
(such as used for English or Cyrillic) that is preceded and followed
by whitespace and strong left-to-right characters does not need an
embedding. Also, a bidirectional relative IRI that only contains
strong right-to-left characters and weak characters and that starts
and ends with a strong rigth-to-left character and appears in a text
with right-to-left base directionality (such as used for Arabic or
Hebrew) and is preceded and followed by whitespace and strong
characters does not need an embedding.
In some other cases, using U+200E, LEFT-TO-RIGHT MARK (LRM) may be
sufficient to force the correct display behavior. However, the
details of the Unicode Bidirectional algorithm are not always easy to
understand. Implementers are strongly advised to err on the side of
caution and to use embedding in all cases where they are not
completely sure that the display behavior is unaffected without the
embedding.
The Unicode Bidirectional Algorithm ([UNI9], Section 4.3) permits
higher-level protocols to influence bidirectional rendering. Such
changes by higher-level protocols MUST NOT be used if they change the
rendering of IRIs.
The bidirectional formatting characters that may be used before or
after the IRI to assure correct display are themselves not part of
the IRI. IRIs MUST NOT contain bidirectional formatting characters
(LRM, RLM, LRE, RLE, LRO, RLO, and PDF). They affect the visual
rendering of the IRI, but do not themselves appear visually. It
would therefore not be possible to correctly input an IRI with such
characters.
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4.2 Bidi IRI Structure
The 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 components (usually consisting mostly of letters and
digits).
The following syntax rules from Section 2.2 correspond to components
for the purpose of Bidi behavior: iuserinfo, ireg-name, isegment,
isegment-nz, isegment-nzc, ireg-name, iquery, and ifragment.
Specifications that define the syntax of any of the above components
MAY divide them further and define smaller parts to be components
according to this document. As an example, the restrictions of
[RFC3490] on bidirectional domain names correspond to treating each
label of a domain name as a component for those schemes where
ireg-name is a domain name. Even where the components are not
defined formally, it may be helpful to think about some syntax in
terms of components and to apply the relevant restrictions. For
example, for the usual name/value syntax in query parts, it is
convenient to treat each name and each value as a component. As
another example, the extensions in a resource name can be treated as
separate components.
For each component, the following restrictions apply:
1) A component SHOULD 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 are given as shoulds, rather than as musts.
For IRIs that are never presented visually, they are not relevant.
However, for IRIs in general, they are very important to insure
consistent conversion between visual presentation and logical
representation, in both directions.
Note: In some components, the above restrictions may actually be
strictly enforced. For example, [RFC3490] requires that these
restrictions apply to the labels of a host name for those schemes
where ireg-name is a host name. In some other components, for
example path components, following these restrictions may not be
too difficult. For other components, such as parts of the query
part, it may be very difficult to enforce the restrictions,
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because the values of query parameters may be arbitrary character
sequences.
If the above restrictions cannot be satisfied otherwise, the affected
component can always be mapped to URI notation as described in
Section 3.1. Please note that the 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 logical order while
rendering them according to Section 4.1. During input, rendering
SHOULD be updated after every new character that is input to avoid
end user confusion.
4.4 Examples
This section gives examples of bidirectional IRIs, in Bidi Notation.
It shows legal IRIs with the relationship between logical and visual
representation, and explains how certain phenomena in this
relationship may look strange to somebody not familiar with
bidirectional behavior, but familiar to 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 seen at [BidiEx], in
Arabic, Hebrew, and Bidi Notation variants.
To read the bidi text in the examples, read the visual representation
from left to right until you encounter a block of rtl text. Read the
rtl block (including slashes and other special characters) from right
to left, then continue at the next unread ltr character.
Example 1: A single component with rtl characters is inverted:
logical representation: http://ab.CDEFGH.ij/kl/mn/op.html
visual representation: http://ab.HGFEDC.ij/kl/mn/op.html
Components can be read one-by-one, and each component can be read in
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 same way as a
sequence of rtl words is read rtl in a bidi text.
Example 3: All components of an IRI (except for the 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
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The whole IRI (except the scheme) is read rtl. Delimiters between
rtl components stay between the respective components; delimiters
between ltr and 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 read rtl, in the same way as each
sequence 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 path component may be
unexpected, but is consistent with other bidi behavior. For
reassurance that the domain component really is "ab.cd.EF", it may be
helpful to read aloud the visual representation following the bidi
algorithm. After "http://ab.cd." one reads the RTL block
"E-F-slash-G-H", which corresponds to the logical representation.
Example 6: Same as example 5, with more rtl components:
logical representation: http://ab.CD.EF/GH/IJ/kl.html
visual representation: http://ab.JI/HG/FE.DC/kl.html
The inversion 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
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
percent-encoded:
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
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Depending on whether the upper-case letters represent Arabic or
Hebrew, the visual representation is different.
Example 10 (allowed, but not recommended):
logical representation: http://ab.CDEFGH.123/kl/mn/op.html
visual representation: http://ab.123.HGFEDC/kl/mn/op.html
Components consisting of only numbers are allowed (it would be rather
difficult to prohibit them), but may interact with adjacent RTL
components in ways that are not easy to predict.
5. IRI Equivalence and Comparison
This section discusses IRI Equivalence and Comparison similar to
Section 6, "Normalization and Comparison", in [RFCYYYY]. This
section focuses on the main issues and on aspects that are different
from [RFCYYYY]; Section 6 of [RFCYYYY] is recommended background
reading.
There is no general rule or procedure to decide whether two arbitrary
IRIs are equivalent or not (i.e. whether they refer to the same
resource or not). Two IRIs that look almost the same may refer to
different resources. Two IRIs that look completely different may
refer to the same resource. Each specification or application that
uses IRIs has to decide on the appropriate criterion for IRI
equivalence.
5.1 Simple String Comparison
In some scenarios a definite answer to the question of IRI
equivalence is needed that is independent of the scheme used and
always can be calculated quickly and without accessing a network. An
example of such a case is XML Namespaces ([XMLNamespace]). In such
cases, two IRIs SHOULD be defined as equivalent if and only if they
are character-by-character equivalent. This is the same as being
byte-by-byte equivalent if the character encoding for both IRIs is
the same. As an example,
http://example.org/~user, http://example.org/%7euser, and
http://example.org/%7Euser are not equivalent under this definition.
When comparing character-by-character, the comparison function MUST
NOT map IRIs to URIs, because such a mapping would create additional
spurious equivalences.
It follows that IRIs SHOULD NOT be modified when being transported if
there is any chance that this IRI might be used as an identifier in
the way explained above. When an IRI is used as an identifier in
scenarios that depend upon character-by-character equivalence,
creators of IRIs should take additional care to avoid IRIs that only
differ in their use of percent-escaping. As an example, using both
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http://example.org/~user and http://example.org/%7Euser to identify
XML Namespaces is a bad idea.
5.2 Conversion to URIs
For actual resolution, differences in percent-encoding (except for
the percent-encoding of reserved characters) MUST always result in
the same resource. For example, http://example.org/~user,
http://example.org/%7euser and http://example.org/%7Euser must
resolve to the same resource.
If this kind of equivalence is to be tested, the percent-encoding of
both IRIs to be compared has to be aligned, for example by converting
both IRIs to URIs (see Section 3.1), eliminating escape differences
in the resulting URIs, and making sure that the case of the
hexadecimal characters in the percent-encodeing is always the same
(preferably upper case). If the IRI is to be passed to another
application, or used further in some other way, its original form
MUST be preserved; the conversion described here should be performed
only for the purpose of local comparison.
Additional, similar equivalences are possible based on knowledge
about the generic URI/IRI syntax, such as the fact that the scheme
part is case-insensitive.
5.3 Normalization
The Unicode Standard [UNIV4] defines various equivalences between
sequences of characters for various purposes. Unicode Standard Annex
#15 [UTR15] defines various Normalization Forms for these
equivalences, in particular Normalization Form C (NFC, Canonical
Decomposition, followed by Canonical Composition) and Normalization
Form KC (NFKC, Compatibility Decomposition, followed by Canonical
Composition).
Equivalence of IRIs MUST rely on the assumption that IRIs are
appropriately pre-normalized, rather than applying normalization when
comparing two IRIs. The exceptions are conversion from a non-digital
form, and conversion from a non-UCS-based character encoding to an
UCS-based character encoding. In these cases, NFC or a normalizing
transcoder using NFC MUST be used for interoperability. To avoid
false negatives and problems with transcoding, IRIs SHOULD be created
using NFC. Using NFKC may avoid even more problems, for example by
choosing half-width Latin letters instead of full-width, and
full-width Katakana instead of half-width.
As an example, http://www.example.org/résumé.html (in XML
Notation) is in NFC. On the other hand,
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http://www.example.org/résumé.html is not in NFC. The
former uses precombined e-acute characters, the later uses 'e'
characters followed by combining acute accents. Both usages are
defined to be canonically equivalent in [UNIV4].
Note: Because it is unknown how a particular field is being treated
with respect to text normalization, it would be inappropriate to
allow third parties to normalize an IRI arbitrarily. This does
not contradict the recommendation that when a resource is created,
its IRI should be as normalized as possible (i.e. NFC or even
NFKC). This is similar to the upper-case/lower-case problems in
URIs. Some parts of a URI are case-insensitive (domain name).
For others, it is unclear whether they are case-sensitive or
case-insensitive, or something in between (e.g. case-sensitive,
but if the wrong case is used, a multiple choice selection is
provided instead of a direct negative result). The best recipe is
that the creator uses a reasonable capitalization, and when
transferring the URI, that capitalization is never changed.
Various IRI schemes may allow the usage of International Domain Names
(IDN) [RFC3490]. When in use in IRIs, those names SHOULD be
validated using the ToASCII operation defined in [RFC3490], with the
flags "UseSTD3ASCIIRules" and "AllowUnassigned". An IRI containing
an invalid IDN cannot successfully be resolved. For legibility
purposes, IDN components of IRIs SHOULD NOT be converted into ASCII
Compatible Encoding (ACE).
5.4 Preferred Forms
The following are the preferred forms for IRIs when created:
- Always provide the URI scheme in lowercase characters.
- Only perform percent-encoding where it is essential.
- Always use uppercase A-through-F characters when percent-encoding.
- For those schemes where ireg-name is a domain name, always provide
the individual labels, in the form produced when applying nameprep
[RFC3491]. This in particular includes using lowercase characters
rather than uppercase characters where applicable. Also, always
use US-ASCII '.' as a separator.
- Where possible, provide IRI components in NFKC or NFC.
- Prevent /./ and /../ from appearing in non-relative URI paths.
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- For schemes that define an empty path to be equivalent to a path
of "/", use "/".
6. Use of IRIs
6.1 Limitations on UCS Characters Allowed in IRIs
This section discusses limitations on characters and character
sequences usable for IRIs beyond those given in Section 2.2 and
Section 4.1. The considerations in this section are relevant when
creating IRIs and when converting from URIs to IRIs.
a) The repertoire of characters allowed in each IRI component is
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 should not check for such limitations.)
b) The UCS contains many areas of characters 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 of Latin characters, half-width
Katakana characters for Japanese, and many others. This also
includes many look-alikes of "space", "delims", and "unwise",
characters excluded in [RFC3491].
Additional information is available from [UNIXML]. [UNIXML] is
written in the context of running text rather than in the context of
identifiers. Nevertheless, it discusses many of the categories of
characters not appropriate for IRIs.
6.2 Software Interfaces and Protocols
Although an IRI is defined as a sequence of characters, software
interfaces for URIs typically function on sequences of octets or
other kinds of code units. Thus, software interfaces and protocols
MUST define which character encoding is used.
Intermediate software interfaces between IRI-capable components and
URI-only components MUST map the IRIs per Section 3.1, when
transferring from IRI-capable to URI-only components. Such a mapping
SHOULD be applied as late as possible. It SHOULD NOT be applied
between components that are known to be able to handle IRIs.
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6.3 Format of URIs and IRIs in Documents and Protocols
Document formats that transport URIs may need to be upgraded to allow
the transport of IRIs. In those cases where the document as a whole
has a native character encoding, IRIs MUST also be encoded in this
character encoding, and converted accordingly by a parser or
interpreter. IRI characters that are not expressible in the native
character encoding SHOULD be escaped using the escaping conventions
of the document format if such conventions are available.
Alternatively, they MAY be percent-encoded according to Section 3.1.
For example, in HTML or XML, numeric character references SHOULD be
used. If a document as a whole has a native character encoding, and
that character encoding is not UTF-8, then IRIs MUST NOT be placed
into the document in the UTF-8 character encoding.
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 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 UTF-8 for Encoding Original Characters
This section discusses details and gives examples for point c) in
Section 1.2. In order to be able to use IRIs, the URI corresponding
to the IRI in question has to encode original characters into octets
using UTF-8. This can be specified for all URIs of a URI scheme, or
can apply to individual URIs for schemes that do not specify how to
encode original characters. It can apply to the whole URI, or only
some part. For background information on encoding characters into
URIs, see also Section 2.5 of [RFCYYYY].
For new URI schemes, using UTF-8 is recommended in [RFC2718].
Examples where UTF-8 is already used are the URN syntax [RFC2141],
IMAP URLs [RFC2192], and POP URLs [RFC2384]. On the other hand,
because the HTTP URL scheme does not specify how to encode original
characters, only some HTTP URLs can have corresponding but different
IRIs.
For example, for a document with a URI of
http://www.example.org/r%C3%A9sum%C3%A9.html, it is possible to
construct a corresponding IRI (in XML notation, see Section 1.4):
http://www.example.org/résumé.html (é stands for the
e-acute character, and %C3%A9 is the UTF-8 encoded and
percent-encoded representation of that character). On the other
hand, for a document with a URI of
http://www.example.org/r%E9sum%E9.html, the percent-encoding octets
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cannot be converted to actual characters in an IRI, because the
percent-encoding is not based on UTF-8.
This means that for most URI schemes, there is no need to upgrade
their scheme definition in order for them to work with IRIs. The
main case where upgrading a scheme definition may make sense is when
a scheme definition is limited to the use of US-ASCII characters with
no provision to include non-ASCII characters/octets but a desire to
include such characters, or only with provisions that are highly
scheme-specific. An example of such a scheme might be the mailto:
scheme [RFC2368].
This specification does not upgrade any scheme specifications in any
way, this has to be done separately. Also, it should be noted that
there is no such thing as an "IRI scheme"; all IRIs use URI schemes,
and all URI schemes can be used with IRIs, even though in some cases
only by using URIs directly as IRIs, without any conversion.
The requirement for the use of UTF-8 applies to all parts of a URI
(with the potential exception of the ireg-name part, see Section
3.1). However, it is possible that the capability of IRIs to
represent a wide range of characters directly is used just in some
parts of the IRI (or IRI reference). The other parts of the IRI may
only contain US-ASCII characters, or they may not be based on UTF-8.
They may be based on another character encoding, or they may directly
encode raw binary data (see also [RFC2397]).
For example, it is possible to have a URI reference of
http://www.example.org/r%E9sum%E9.xml#r%C3%A9sum%C3%A9, where the
document name is encoded in iso-8859-1 based on server settings, but
the fragment identifier is encoded in UTF-8 according to [XPointer].
The IRI corresponding to the above URI would be (in XML notation)
http://www.example.org/r%E9sum%E9.xml#résumé.
Similar considerations apply to query parts. The functionality of
IRIs (namely to be able to include non-ASCII characters) can only be
used if the query part is encoded in UTF-8.
6.5 Relative IRI References
Processing of relative forms of IRIs against a base is handled
straightforwardly; the algorithms of [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
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the same software components and operations that currently process
URIs: software interfaces that handle URIs, software that allows
users to enter URIs, software that creates or 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.
7.1 URI/IRI Software Interfaces
Software interfaces that handle URIs, such as URI-handling APIs and
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 character 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 API or protocol definition must
clearly define the character encoding to be 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
conversion when there is a chance that this cannot be done correctly.
7.2 URI/IRI Entry
There are components that allow users to enter URIs into the system,
for 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 the input of Latin letters and related
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characters in full-width or half-width versions. For IRI input, the
input method editor should be set so that it produces half-width
Latin letters and punctuation, and full-width Katakana.
An input field primarily or only used for the input of URIs/IRIs may
allow the user to view an IRI as mapped to a URI. Places where the
input of IRIs is frequent may 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 IRI to a URI before passing it to
such a component.
For the input of IRIs with right-to-left characters, please see
Section 4.3.
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.
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 character encoding of the
document or application where the IRI appears to the character
encoding used by the system-wide IRI invocation mechanism, or to a
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.
7.4 URI/IRI Generation
Systems that 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.
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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. For example,
if a file system contains a file named résumé.html, a
server should expose this as r%C3%A9sum%C3%A9.html in a URI, which
allows to use résumé.html in an IRI, even if the file name
locally is kept in a character encoding other than UTF-8.
This recommendation in particular applies to HTTP servers. For FTP
servers, similar considerations apply, see in particular [RFC2640].
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"
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 repertoire, 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 the 'fi' ligature at U+FB01 for
compatibility reasons. Wherever possible, IRIs should use the two
letters 'f' and 'i' rather than the 'fi' ligature. An example where
the latter may be used is in the query part of an IRI for an explicit
search for a word written 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
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should be created 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.
7.6 Display of URIs/IRIs
In situations where the rendering software is not expected to display
non-ASCII parts of the IRI correctly using the available layout and
font resources, these parts should be percent-encoded before being
displayed.
For display of Bidi IRIs, please see Section 4.1.
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.
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 character
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 US-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, percent-encoded and
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not percent-encoded 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.
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 created,
generated, or transported. This suggests that upgrading URI
interpreting 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.
The upgrade of generating software to generate IRIs instead of using
a local character 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.
Software converting from URIs to IRIs for display should be upgraded
only after upgraded entry software has been widely deployed to the
population that will see the displayed result.
It is often possible to reduce the effort and dependencies for
upgrading to IRIs by using UTF-8 rather than another character
encoding where there is a free choice of character encodings. For
example, when setting up a new file-based Web server, using UTF-8 as
the character encoding for file names will make the transition to
IRIs easier. Likewise, when setting up a new Web form using UTF-8 as
the character encoding of the form page, the returned query URIs will
use UTF-8 as the character encoding (unless the user, for whatever
reason, changes the character encoding) and will therefore be
compatible with IRIs.
These recommendations, when taken together, will allow for the
extension from URIs to IRIs in order to handle characters other than
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US-ASCII while minimizing interoperability problems. For
considerations regarding the upgrade of URI scheme definitions,
please see Section 6.4.
8. Security Considerations
The security considerations discussed in [RFCYYYY] also apply to
IRIs. In addition, the following issues require particular care for
IRIs.
Incorrect encoding or decoding 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 US-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 and 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
entering an IRI, or when transcoding an IRI from a legacy character
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.example.com/PopularPage.html)
might be "spoofed" by someone who is able to create
http://big.example.com/popularpage.html. However, the use of
unnormalized character sequences, and of additional mappings for user
convenience, may increase the chance for spoofing. Protocols and
servers that allow the creation of 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 in various IRI components, such as the domain name
part or a path part. For considerations specific to the domain name
part, see [RFC3491]. For the path part, administrators of sites
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which allow independent users to create resources in the same subarea
may need to be careful to check for spoofing.
Spoofing can occur because in the UCS, there are many characters that
look very similar. Details are discussed in Section 7.5. Again,
this is very similar to spoofing possibilities on US-ASCII, e.g.
using 'br0ken' or '1ame' URIs.
Spoofing can occur when URIs with percent-encodings based on various
character 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 character encodings, produce mostly garbage. In
other cases, when concurrently used character encodings have a
similar structure, but there are no characters that have exactly the
same encoding, detection is more difficult.
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.
9. IANA Considerations
This document has no actions for IANA.
10. 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
and URLs"), and ad-hoc meetings at the Unicode conferences in
September 1995 and September 1997.
Many thanks go to Francois Yergeau, Matitiahu 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, Adam
Costello, Dan Oscarson, Elliotte Rusty Harold, Mike J. Brown, Roy
Badami, Jonathan Rosenne, Asmus Freytag, Simon Josefsson, Carlos
Viegas Damasio, Chris Haynes, Walter Underwood, and many others for
help with understanding the issues and possible solutions, and
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getting the details right.
This document is a product of the Internationalization Working Group
(I18N WG) of the World Wide Web Consortium (W3C). Thanks to the
members of the W3C I18N Working Group and Interest Group for their
contributions and their work on [CharMod]. Thanks also go to the
members of many other W3C Working Groups for adopting IRIs, and to
the members of the Montreal IAB Workshop on Internationalization and
Localization for their review.
11. References
11.1 Normative References
[ASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[ISO10646]
International Organization for Standardization, "ISO/IEC
10646:2003: Information Technology - Universal
Multiple-Octet Coded Character Set (UCS)", ISO Standard
10646, December 2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[RFC3490] Faltstrom, P., Hoffman, P. and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)", RFC
3491, March 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFCYYYY] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax (Note to the RFC
Editor: Please update this reference with the RFC
resulting from draft-fielding-uri-rfc2396bis-xx.txt, and
remove this Note)", draft-fielding-uri-rfc2396bis-05.txt
(work in progress), April 2004.
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[UNI9] Davis, M., "The Bidirectional Algorithm", Unicode Standard
Annex #9, March 2004,
<http://www.unicode.org/reports/tr9/tr9-13.html>.
[UNIV4] The Unicode Consortium, "The Unicode Standard, Version
4.0.1, defined by: The Unicode Standard, Version 4.0
(Reading, MA, Addison-Wesley, 2003. ISBN 0-321-18578-1),
as amended by Unicode 4.0.1
(http://www.unicode.org/versions/Unicode4.0.1/)", March
2004.
[UTR15] Davis, M. and M. Duerst, "Unicode Normalization Forms",
Unicode Standard Annex #15, April 2003,
<http://www.unicode.org/unicode/reports/tr15/tr15-23.html>.
11.2 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. and T.
Texin, "Character Model for the World Wide Web", World
Wide Web Consortium Working Draft, February 2004, <http://
www.w3.org/TR/charmod>.
[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>.
[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>.
[Gettys] Gettys, J., "URI Model Consequences",
<http://www.w3.org/DesignIssues/ModelConsequences>.
[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>.
[RFC2130] Weider, C., Preston, C., Simonsen, K., Alvestrand, H.,
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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 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC2368] Hoffman, P., Masinter, L. and J. Zawinski, "The mailto URL
scheme", RFC 2368, July 1998.
[RFC2384] Gellens, R., "POP URL Scheme", RFC 2384, August 1998.
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", 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.
[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", 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>.
[XML1] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E. and
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F. Yergeau, "Extensible Markup Language (XML) 1.0 (Third
Edition)", World Wide Web Consortium Recommendation,
February 2004,
<http://www.w3.org/TR/REC-xml#sec-external-ent>.
[XMLNamespace]
Bray, T., Hollander, D. and A. Layman, "Namespaces in
XML", World Wide Web Consortium Recommendation, January
1999, <http://www.w3.org/TR/REC-xml-names>.
[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 "Dürst" in XML and HTML.)
World Wide Web Consortium
5322 Endo
Fujisawa, Kanagawa 252-8520
Japan
Phone: +81 466 49 1170
Fax: +81 466 49 1171
EMail: mailto:duerst@w3.org
URI: http://www.w3.org/People/D%C3%BCrst/
(Note: This is the percent-encoded 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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Appendix A. Design Alternatives
This section shortly summarizes major design alternatives and the
reasons for why they were not chosen.
Appendix A.1 New Scheme(s)
Introducing new schemes (for example httpi:, ftpi:,...) or a new
metascheme (e.g. i:, leading to URI/IRI prefixes such as i:http:,
i:ftp:,...) was proposed to make IRI-to-URI conversion
scheme-dependent or to distinguish between percent-encodings
resulting from IRI-to-URI conversion and percent-encodings from
legacy character encodings.
New schemes are not needed to distinguish URIs from true IRIs (i.e.
IRIs that contain non-ASCII characters). The benefit of being able
to detect the origin of percent-encodings is marginal, because UTF-8
can be detected with very high reliability. Deploying new schemes is
extremely hard, so not requiring new schemes for IRIs makes
deployment of IRIs vastly easier. Making conversion scheme-dependent
is highly inadvisable, and would be encouraged by separate schemes
for IRIs. Using an uniform convention for conversion from IRIs to
URIs makes IRI implementation orthogonal to the introduction of
actual new schemes.
Appendix A.2 Other Character Encodings than UTF-8
At an early stage, UTF-7 was considered as an alternative to UTF-8
when converting IRIs to URIs. UTF-7 would not have needed
percent-encoding, and would in most cases have been shorter than
percent-encoded UTF-8.
Using UTF-8 avoids a double layering and overloading of the use of
the "+" character. UTF-8 is fully compatible with US-ASCII, and has
therefore been recommended by the IETF, and is being used widely,
while UTF-7 has never been used much and is now clearly being
discouraged. Requiring implementations to convert from UTF-8 to
UTF-7 and back would be an additional implementation burden.
Appendix A.3 New Encoding Convention
Instead of using the existing percent-encoding convention of URIs,
which is based on octets, the idea was to create a new encoding
convention, for example to use '%u' to introduce UCS code points.
Using the existing octet-based percent-encoding mechanism does not
need an upgrade of the URI syntax, and does not need corresponding
server upgrades.
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Appendix A.4 Indicating Character Encodings in the URI/IRI
Some proposals suggested indicating the character encodings used in
an URI or IRI with some new syntactic convention in the URI itself,
similar to the 'charset' parameter for emails and Web pages. As an
example, the label in square brackets in
http://www.example.org/ros[iso-8859-1]é indicated that the
following é had to be interpreted as iso-8859-1.
Using UTF-8 only does not need an upgrade to the URI syntax. It
avoids potentially multiple labels that have to be copied correctly
in all cases, even on the side of a bus or on a napkin, leading to
usability problems to the extent of being prohibitively annoying.
Using UTF-8 only also reduces transcoding errors and confusions.
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