Network Working Group M. Duerst
Internet-Draft W3C
Expires: April 25, 2004 M. Suignard
Microsoft Corporation
October 26, 2003
Internationalized Resource Identifiers (IRIs)
draft-duerst-iri-05
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on April 25, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines a new protocol element, the Internationalized
Resource Identifier (IRI), as a complement to the URI [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 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].
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 . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1 Summary of IRI Syntax . . . . . . . . . . . . . . . . . . . 7
2.2 ABNF for IRI References and IRIs . . . . . . . . . . . . . . 7
3. Relationship between IRIs and URIs . . . . . . . . . . . . . 10
3.1 Mapping of IRIs to URIs . . . . . . . . . . . . . . . . . . 10
3.2 Converting URIs to IRIs . . . . . . . . . . . . . . . . . . 13
3.2.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4. Bidirectional IRIs for Right-to-left Languages . . . . . . . 16
4.1 Logical Storage and Visual Presentation . . . . . . . . . . 16
4.2 Bidi IRI Structure . . . . . . . . . . . . . . . . . . . . . 17
4.3 Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . . . 18
4.4 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5. IRI Equivalence and Comparison . . . . . . . . . . . . . . . 20
5.1 Simple String Comparison . . . . . . . . . . . . . . . . . . 20
5.2 Conversion to URIs . . . . . . . . . . . . . . . . . . . . . 21
5.3 Normalization . . . . . . . . . . . . . . . . . . . . . . . 21
5.4 Preferred Forms . . . . . . . . . . . . . . . . . . . . . . 22
6. Use of IRIs . . . . . . . . . . . . . . . . . . . . . . . . 22
6.1 Limitations on UCS Characters Allowed in IRIs . . . . . . . 23
6.2 Software Interfaces and Protocols . . . . . . . . . . . . . 23
6.3 Format of URIs and IRIs in Documents and Protocols . . . . . 23
6.4 Use of UTF-8 for Encoding Original Characters . . . . . . . 24
6.5 Relative IRI References . . . . . . . . . . . . . . . . . . 25
7. URI/IRI Processing Guidelines (informative) . . . . . . . . 25
7.1 URI/IRI Software Interfaces . . . . . . . . . . . . . . . . 25
7.2 URI/IRI Entry . . . . . . . . . . . . . . . . . . . . . . . 26
7.3 URI/IRI Transfer Between Applications . . . . . . . . . . . 26
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7.4 URI/IRI Generation . . . . . . . . . . . . . . . . . . . . . 27
7.5 URI/IRI Selection . . . . . . . . . . . . . . . . . . . . . 27
7.6 Display of URIs/IRIs . . . . . . . . . . . . . . . . . . . . 28
7.7 Interpretation of URIs and IRIs . . . . . . . . . . . . . . 28
7.8 Upgrading Strategy . . . . . . . . . . . . . . . . . . . . . 29
8. Security Considerations . . . . . . . . . . . . . . . . . . 30
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31
Normative References . . . . . . . . . . . . . . . . . . . . 32
Non-normative References . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 35
Full Copyright Statement . . . . . . . . . . . . . . . . . . 36
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1. Introduction
1.1 Overview and Motivation
A URI is defined in [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 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 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
[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 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 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) 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)
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
[ISO10646] and [UNIV4].
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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,...).
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 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 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.
To denote actual octets in examples (as opposed to escaped 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).
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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
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 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
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 [RFCYYYY]:
IRI-reference = IRI / relative-IRI
IRI = scheme ":" ihier-part [ "?" iquery ] [ "#" ifragment ]
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absolute-IRI = scheme ":" ihier-part [ "?" iquery ]
relative-IRI = ihier-part [ "?" iquery ] [ "#" ifragment ]
ihier-part = inet-path / iabs-path / irel-path
inet-path = "//" iauthority [ iabs-path ]
iabs-path = "/" ipath-segments
irel-path = ipath-segments
iauthority = [ iuserinfo "@" ] ihost [ ":" port ]
iuserinfo = *( iunreserved / escaped / ";" /
":" / "&" / "=" / "+" / "$" / "," )
ihost = [ IPv6reference / IPv4address / ihostname ]
ihostname = idomainlabel iqualified
iqualified = *( "." idomainlabel ) [ "." ]
idomainlabel = <<See following production rules>>
ipath-segments = ipath-segment *( "/" ipath-segment )
ipath-segment = *ipchar
ipchar = iunreserved / escaped / ";" /
":" / "@" / "&" / "=" / "+" / "$" / ","
iquery = *( ipchar / iprivate / "/" / "?" )
ifragment = *( ipchar / "/" / "?" )
iric = reserved / iunreserved / escaped
iunreserved = unreserved / 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
/ %xD0000-DFFFD / %xE1000-EFFFD
iprivate = %xE000-F8FF / %xF0000-FFFFD / %x100000-10FFFD
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The 'idomainlabel' production rule is as follows:
The value 'idomainlabel' is defined as a string of 'ucschar' obeying
the following rules:
a) Given a string of 'ucschar' values, the ToASCII operation
[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. (Note: This means that its output
conforms to 'domainlabel' as defined below.)
The following are the same as [RFCYYYY]:
scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
port = *DIGIT
domainlabel = alphanum [ 0*61( alphanum | "-" ) alphanum ]
alphanum = ALPHA / DIGIT
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
IPv6reference = "[" IPv6address "]"
IPv6address = 6( h4 ":" ) ls32
/ "::" 5( h4 ":" ) ls32
/ [ h4 ] "::" 4( h4 ":" ) ls32
/ [ *1( h4 ":" ) h4 ] "::" 3( h4 ":" ) ls32
/ [ *2( h4 ":" ) h4 ] "::" 2( h4 ":" ) ls32
/ [ *3( h4 ":" ) h4 ] "::" h4 ":" ls32
/ [ *4( h4 ":" ) h4 ] "::" ls32
/ [ *5( h4 ":" ) h4 ] "::" h4
/ [ *6( h4 ":" ) h4 ] "::"
h4 = 1*4HEXDIG
ls32 = ( h4 ":" h4 ) / IPv4address
reserved = "/" / "?" / "#" / "[" / "]" / ";" /
":" / "@" / "&" / "=" / "+" / "$" / ","
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unreserved = ALPHA / DIGIT / mark
mark = "-" / "_" / "." / "!" / "~" / "*" / "'" /
"(" / ")"
escaped = "%" HEXDIG HEXDIG
3. Relationship between IRIs and URIs
IRIs are meant to replace URIs in identifying resources for
protocols, formats and software components which use a UCS-based
character repertoire. These protocols and components may never need
to use URIs directly, especially when the resource identifier is used
simply for identification purposes. However, when the resource
identifier is used for resource retrieval, it is in many cases
necessary to determine the associated URI because most retrieval
mechanisms currently only are defined for URIs. (Additional
rationale is given in Section 3.1.)
3.1 Mapping of IRIs to URIs
This section defines how to map an IRI to a URI. Everything in this
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. Such
restrictions can be applied to IRIs by noting that IRIs are
only valid if they map to syntactically valid URIs. This means
that such syntactical restrictions do not have to be defined
again on the IRI level.
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-based encoding from the original
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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 encoding: Represent the IRI as a
sequence of characters from the UCS normalized according
to Normalization Form C (NFC, [UTR15]).
Variant B) If the IRI is in some digital representation
(e.g. an octet stream) in some known non-Unicode
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 encoding (for
example UTF-8 or UTF-16): Do not normalize. Move
directly to Step 2.
Step 2) If the IRI contains an 'ihostname' part, replace this
'ihostname' part by the part converted using the ToASCII
operation specified in Section 4.1 of [RFC3490], with the flag
UseSTD3ASCIIRules set to TRUE and the flag AllowUnassigned set
to FALSE for creating IRIs and set to TRUE otherwise. The
ToASCII operation may fail, but only if the IRI does not
conform to the rules in Section 2.2.
Step 3) For each character that is disallowed in URI references,
apply steps 1) through 3) below. The disallowed characters
consist of all non-ASCII characters allowed in IRIs.
1) Convert the character to a sequence of one or more octets
using UTF-8 [RFCXXXX].
2) Convert each octet to %HH, where HH is the hexadecimal
notation of the octet value. Note: This is identical to
the escaping mechanism in Section 2.4.1 of [RFCYYYY]. To
reduce variability, the hexadecimal notation SHOULD use
upper case letters.
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 also deal with 'ihostname' parts
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escaped according to Step 3) rather than Step 2). For example, Step
2) converts the IRI
http://résumé.example.org to
http://xn--rsum-bpad.example.org. For backward compatibility,
http://r%C3%A9sum%C3%A9.example.org would also be converted to
http://xn--rsum-bpad.example.org.
Infrastructure accepting IRIs MAY also deal with the printable
characters in US-ASCII that are not allowed in URIs, namely "<", ">",
'"', Space, "{", "}", "|", "\", "^", and "`", in step 3) above. If
such characters are found but are not converted, then the conversion
SHOULD fail. Please note that the number sign ("#"), the percent
sign ("%"), and the square bracket characters ("[", "]") are not part
of the above list, and MUST NOT be converted. Protocols and formats
that have used earlier definitions of IRIs including these characters
MAY require unescaping of these characters as a preprocessing step to
extract the actual IRI from a given field. Such preprocessing MAY
also be used by applications allowing the user to enter an IRI.
Internationalized Domain Names may be contained in parts of an
IRI other than the 'ihostname' part. In this case, Step 2) is
not used, but Step 3) is applied. This is important to
maintain uniform treatment of URIs. See [Gettys] for an in-
depth discussion. 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.
In this process (in step 3.3), characters allowed in URI
references as well as existing escape sequences are not escaped
further. (This mapping is similar to, but different from, the
escaping applied when including arbitrary content into some
part of a URI.) For example, an IRI of
http://www.example.org/red%09rosé#red (in XML notation) is
converted to
http://www.example.org/red%09ros%C3%A9#red, not to something
like
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http%3A%2F%2Fwww.example.org%2Fred%2509ros%C3%A9%23red.
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 escape
sequences). 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 escape sequences, but not all escaping
can be eliminated. There are several reasons for this:
a) Some escape sequences are necessary to distinguish escaped 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 a very high probability, but 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) Apply the ToUnicode operation to each 'domainlabel' in the
'hostname' part (if there is one), representing the output as
UTF-8.
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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
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
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4) http://www.example.org/D<c3><bc>rst
5) http://www.example.org/D<c3><bc>rst
6) 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 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
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://納豆.example.org/%E2%80%AE
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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
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 (such
as used for 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 (such as used for Arabic
or Hebrew), setting a different embedding direction for the IRI is
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 available (e.g. in plain text), setting the embedding is done
with Unicode bidi formatting codes, i.e. U+202A, LEFT-TO-RIGHT
EMBEDDING (LRE) before the IRI, and U+202C, POP DIRECTIONAL
FORMATTING (PDF) after the IRI, both not being part of the IRI
itself.
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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.
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, ipath-segment,
ihostname, 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 the domain name as a component. 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 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.
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
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IRI. 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, 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.
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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 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
escaped:
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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.
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.
In such a case, the comparison function MUST NOT map IRIs to URIs,
because such a mapping would create additional spurious equivalences.
It follows that IRIs SHOULD NOT be modified when being transported if
there is any chance that this IRI might be used as an identifier in
the way explained above.
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5.2 Conversion to URIs
For actual resolution, differences in escaping (except for the
escaping of reserved characters) MUST always result in the same
resource. For example, http://example.org/~user,
http://example.org/%7euser and http://example.org/%7Euser must
resolve to the same resource.
If this kind of equivalence is to be tested, the escaping of both
IRIs to be compared has to be aligned, for example by converting both
IRIs to URIs (see Section 3.1) and making sure that the case of the
hexadecimal characters in the %-escape is always the same (preferably
upper case). For comparison, such conversions MUST only be done on
the fly, while retaining the original IRI.
Additional, similar equivalences are possible based on knowledge
about the generic URI/IRI syntax, such as the fact that the scheme
part is case-insensitive.
5.3 Normalization
The Unicode Standard [UNIV4] defines various equivalences between
sequences of characters for various purposes. Unicode Standard Annex
#15 [UTR15] defines various Normalization Forms for these
equivalences, in particular Normalization Form C (NFC, Canonical
Decomposition, followed by Canonical Composition) and Normalization
Form KC (NFKC, Compatibility Decomposition, followed by Canonical
Composition).
Equivalence of IRIs MUST rely on the assumption that IRIs are
appropriately pre-normalized, rather than applying normalization when
comparing two IRIs. The exceptions are conversion from a non-digital
form, and conversion from a non-UCS-based encoding to an UCS-based
encoding. In these cases, NFC or a normalizing transcoder using NFC
MUST be used for interoperability. To avoid false negatives and
problems with transcoding, IRIs SHOULD be created using NFC. Using
NFKC 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, 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].
Because it is unknow how a particular field is being treated
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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, and an IRI for that resource, you try to 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 generator uses a reasonable capitalization, and
when transfering 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). However, this conversion is applied when
mapping an IRI into a URI, see Section 3.1.
5.4 Preferred Forms
The following are the preferred forms for IRIs when generated:
- Always provide the URI scheme in lowercase characters.
- Only perform percent-escaping where it is essential.
- Always use uppercase A-through-F characters when percent-
escaping.
- Always provide the hostname, if any, in the form produced when
applying nameprep [RFC3491]. This in particular includes using
lowercase characters rather than uppercase characters where
applicable.
- Where possible, provide IRI components in NFKC or NFC.
- Prevent /./ and /../ from appearing in non-relative URI paths.
6. Use of IRIs
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6.1 Limitations on UCS Characters Allowed in IRIs
This section discusses limitations on characters and character
sequences usable for IRIs. The considerations in this section are
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 ASCII 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.
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
encoding, and converted accordingly by a parser or interpreter. IRI
characters that are not expressible in the native encoding SHOULD be
escaped using the escaping conventions of the document format if such
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conventions are available. Alternatively, they MAY be escaped
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 new URI schemes, using UTF-8 is recommended in [RFC2718].
Examples where this is already used are the URN syntax [RFC2141],
IMAP URLs [RFC2192], and POP URLs [RFC2384]. On the other hand,
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 escaped
representation of that character). On the other hand, for a document
with a URI of http://www.example.org/r%E9sum%E9.html, the escaped
octets cannot be converted to actual characters in an IRI, because
the escaping is not based on UTF-8.
The requirement for the use of UTF-8 applies to all parts of a URI,
with the exception of the ihostname part. However, it is possible
that the capability of IRIs to represent a wide range of characters
directly is used just in some parts of the IRI (or IRI reference).
The other parts of the IRI may only contain ASCII characters, or they
may not be based on UTF-8. They may be based on another encoding, or
they may directly encode raw binary data (see also [RFC2397]).
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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
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.
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 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
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.
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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
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 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-
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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 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.
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 an 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.
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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 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
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.
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 escaped 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
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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.
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
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 a
local encoding should happen only after the service is upgraded to
accept IRIs. Similarly, IRIs should only be generated when the
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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 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 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) might be
"spoofed" by someone who is able to create 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.
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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
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 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 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. 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.
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, Adam
Costello, Dan Oscarson, Elliotte Rusty Harold, Mike J. Brown, Andrea
Vine, Roy Badami, 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
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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.
[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", 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 Identifier (URI): Generic Syntax", draft-
fielding-uri-rfc2396bis-03.txt (work in progress), June
2003.
[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>.
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, August 2003,
<http://www.w3.org/TR/charmod>.
[Duerst97] Duerst, M., "The Properties and Promises of UTF-8",
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Internet-Draft Internationalized Resource Identifiers October 2003
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>.
[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>.
[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
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.
[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.
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Internet-Draft Internationalized Resource Identifiers October 2003
[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.
[UNIV4] The Unicode Consortium, "The Unicode Standard,
Version 4.0", Addison-Wesley, Reading, MA , 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>.
[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>.
[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/
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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
200 Technology Square
Cambridge, MA 02139
U.S.A.
Phone: +1 617 253 5509
Fax: +1 617 258 5999
EMail: mailto: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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