<- RFC Index (3901..4000)
RFC 3987
Network Working Group M. Duerst
Request for Comments: 3987 W3C
Category: Standards Track M. Suignard
Microsoft Corporation
January 2005
Internationalized Resource Identifiers (IRIs)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
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 of
extending or changing the definition of URIs. This was done in order
to allow a clear distinction and to avoid incompatibilities with
existing software. Guidelines are provided for the use and
deployment of IRIs in various protocols, formats, and software
components that currently deal with URIs.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Overview and Motivation . . . . . . . . . . . . . . . . 3
1.2. Applicability . . . . . . . . . . . . . . . . . . . . . 3
1.3. Definitions . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Notation . . . . . . . . . . . . . . . . . . . . . . . . 5
2. IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Summary of IRI Syntax . . . . . . . . . . . . . . . . . 6
2.2. ABNF for IRI References and IRIs . . . . . . . . . . . . 7
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3. Relationship between IRIs and URIs . . . . . . . . . . . . . . 10
3.1. Mapping of IRIs to URIs . . . . . . . . . . . . . . . . 10
3.2. Converting URIs to IRIs . . . . . . . . . . . . . . . . 14
3.2.1. Examples . . . . . . . . . . . . . . . . . . . . 15
4. Bidirectional IRIs for Right-to-Left Languages. . . . . . . . 16
4.1. Logical Storage and Visual Presentation . . . . . . . . 17
4.2. Bidi IRI Structure . . . . . . . . . . . . . . . . . . . 18
4.3. Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . 19
4.4. Examples . . . . . . . . . . . . . . . . . . . . . . . . 19
5. Normalization and Comparison . . . . . . . . . . . . . . . . . 21
5.1. Equivalence . . . . . . . . . . . . . . . . . . . . . . 22
5.2. Preparation for Comparison . . . . . . . . . . . . . . . 22
5.3. Comparison Ladder . . . . . . . . . . . . . . . . . . . 23
5.3.1. Simple String Comparison . . . . . . . . . . . . 23
5.3.2. Syntax-Based Normalization . . . . . . . . . . . 24
5.3.3. Scheme-Based Normalization . . . . . . . . . . . 27
5.3.4. Protocol-Based Normalization . . . . . . . . . . 28
6. Use of IRIs . . . . . . . . . . . . . . . . . . . . . . . . . 29
6.1. Limitations on UCS Characters Allowed in IRIs . . . . . 29
6.2. Software Interfaces and Protocols . . . . . . . . . . . 29
6.3. Format of URIs and IRIs in Documents and Protocols . . . 30
6.4. Use of UTF-8 for Encoding Original Characters .. . . . . 30
6.5. Relative IRI References . . . . . . . . . . . . . . . . 32
7. URI/IRI Processing Guidelines (informative) . . . . . . . . . 32
7.1. URI/IRI Software Interfaces . . . . . . . . . . . . . . 32
7.2. URI/IRI Entry . . . . . . . . . . . . . . . . . . . . . 33
7.3. URI/IRI Transfer between Applications . . . . . . . . . 33
7.4. URI/IRI Generation . . . . . . . . . . . . . . . . . . . 34
7.5. URI/IRI Selection . . . . . . . . . . . . . . . . . . . 34
7.6. Display of URIs/IRIs . . . . . . . . . . . . . . . . . . 35
7.7. Interpretation of URIs and IRIs . . . . . . . . . . . . 36
7.8. Upgrading Strategy . . . . . . . . . . . . . . . . . . . 36
8. Security Considerations . . . . . . . . . . . . . . . . . . . 37
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 39
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
10.1. Normative References . . . . . . . . . . . . . . . . . . 40
10.2. Informative References . . . . . . . . . . . . . . . . . 41
A. Design Alternatives . . . . . . . . . . . . . . . . . . . . . 44
A.1. New Scheme(s) . . . . . . . . . . . . . . . . . . . . . 44
A.2. Character Encodings Other Than UTF-8 . . . . . . . . . . 44
A.3. New Encoding Convention . . . . . . . . . . . . . . . . 44
A.4. Indicating Character Encodings in the URI/IRI . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 45
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 46
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1. Introduction
1.1. Overview and Motivation
A Uniform Resource Identifier (URI) is defined in [RFC3986] 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. This 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 those who use only
the Latin alphabet. Many languages with non-Latin scripts are
transcribed with Latin letters. These 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
common. Also, increasing numbers of protocols and formats 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 [RFC3986], 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 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.
1.2. Applicability
IRIs are designed to be compatible with 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:
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a. A protocol or format element should be explicitly designated to
be able to carry IRIs. 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 a 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).
sequence of characters: A sequence of characters (one after another).
sequence of octets: A sequence of octets (one after another).
character encoding: A method of representing a sequence of characters
as a sequence of octets (maybe with variants). Also, a method of
(unambiguously) converting a sequence of octets into a sequence of
characters.
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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: 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 IRI references are
resolved to their absolute form. Note that in [RFC2396] URIs did
not include fragment identifiers, but in [RFC3986] 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
(e.g., markup, programming languages).
protocol element: Any portion of a message that affects processing of
that message by the protocol in question.
presentation element: A presentation form corresponding to a protocol
element; for example, using a wider range of characters.
create (a URI or IRI): With respect to URIs and IRIs, the term is
used for the initial creation. This may be the initial creation
of a resource with a certain identifier, or the initial exposition
of a resource under a particular identifier.
generate (a URI or IRI): With respect to URIs and IRIs, the term 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
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: 'XML Notation' and 'Bidi Notation'.
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XML Notation uses a leading '&#x', a trailing ';', and the
hexadecimal number of the character in the UCS in between. For
example, я stands for CYRILLIC CAPITAL LETTER YA. In this
notation, an actual '&' is denoted by '&'.
Bidi Notation is used for bidirectional examples: Lowercase letters
stand for Latin letters or other letters that are written left to
right, whereas uppercase 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>.
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" 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 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 ensures that the characters in the
IRI can be handled (e.g., 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 [RFC3986], 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 [RFC3986]. All the operations defined in
[RFC3986], such as the resolution of relative references, can be
applied to IRIs by IRI-processing software in exactly the same way as
they are for URIs by URI-processing software.
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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. For example, U+00A2,
CENT SIGN, is not allowed as a delimiter in IRIs, because it is in
the 'iunreserved' category. This is similar to the fact that it is
not possible to use '-' as a delimiter in URIs, because it is in the
'unreserved' category.
2.2. ABNF for IRI References and IRIs
Although 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 [RFC3986],
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. The grammar is split into two parts:
Rules that differ from [RFC3986] because of the above-mentioned
expansion, and rules that are the same as those in [RFC3986]. For
rules that are different than those in [RFC3986], 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 those in [RFC3986]:
IRI = scheme ":" ihier-part [ "?" iquery ]
[ "#" ifragment ]
ihier-part = "//" iauthority ipath-abempty
/ ipath-absolute
/ ipath-rootless
/ ipath-empty
IRI-reference = IRI / irelative-ref
absolute-IRI = scheme ":" ihier-part [ "?" iquery ]
irelative-ref = irelative-part [ "?" iquery ] [ "#" ifragment ]
irelative-part = "//" iauthority ipath-abempty
/ ipath-absolute
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/ 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-absolute ; 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-absolute = "/" [ isegment-nz *( "/" isegment ) ]
ipath-noscheme = isegment-nz-nc *( "/" isegment )
ipath-rootless = isegment-nz *( "/" isegment )
ipath-empty = 0<ipchar>
isegment = *ipchar
isegment-nz = 1*ipchar
isegment-nz-nc = 1*( iunreserved / pct-encoded / sub-delims
/ "@" )
; non-zero-length segment without any colon ":"
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
/ %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 [RFC3986].
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The following rules are the same as those in [RFC3986]:
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 = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "="
This syntax does not support IPv6 scoped addressing zone identifiers.
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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 that 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 currently most
retrieval mechanisms are only 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 also applies to IRI references and URI references, as well as
to components thereof (for example, fragment identifiers).
This mapping has two purposes:
Syntaxical. 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.
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 by using the following two steps.
Step 1. Generate a UCS character sequence from the original IRI
format. This step has the following three variants,
depending on the form of the input:
a. If the IRI is written on paper, read aloud, 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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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.
c. If the IRI is in a Unicode-based character encoding (for
example, UTF-8 or UTF-16), do not normalize (see section
5.3.2.2 for details). 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
[RFC3986]. To reduce variability, the hexadecimal notation
SHOULD use uppercase letters.
2.3. Replace the original character with the resulting character
sequence (i.e., a sequence of %HH triplets).
The above mapping from IRIs to URIs produces URIs fully conforming to
[RFC3986]. 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.
Systems accepting IRIs MAY convert the ireg-name component of an IRI
as follows (before step 2 above) for schemes known to use domain
names in ireg-name, if 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 by using U+002E (FULL STOP) as a label
separator, with the flag UseSTD3ASCIIRules set to TRUE, and with the
flag AllowUnassigned set to FALSE for creating IRIs and set to TRUE
otherwise.
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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
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.
Such an IRI resolves to the URI obtained after converting the IRI and
uses the ToASCII operation on ireg-name. Implementations do not have
to do this conversion as long as they produce the same result.
Note: The difference between variants b and c in step 1 (using
normalization with NFC, versus not using any normalization)
accounts for the fact that in many non-Unicode character
encodings, some text cannot be represented directly. For example,
the word "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).
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 is important
to make processing independent of URI scheme. See [Gettys] for an
in-depth discussion.
Note: In practice, whether the general mapping (steps 1 and 2) or the
ToASCII operation of [RFC3490] is used for ireg-name 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
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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
for making the necessary conversions to be able to retrieve the
Web page.
Systems 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 these
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. This
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 and existing percent-encoded sequences are not encoded
further. (This mapping is similar to, but different from, the
encoding applied when arbitrary content is included in 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 IRI with
non-BMP characters (in XML Notation):
"http://example.com/𐌀𐌁𐌂";
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which contains 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, converting a URI into an equivalent IRI may be
desirable. This section gives a procedure for this conversion. The
conversion described in this section will always result in an IRI
that maps back to the URI 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:
1. Some percent-encodings are necessary to distinguish percent-
encoded and unencoded uses of reserved characters.
2. 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].)
3. The conversion may result in a character that is not appropriate
in an IRI. See sections 2.2, 4.1, and 6.1 for further details.
Conversion from a URI to an IRI is done by 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) to the corresponding octets, except those corresponding
to "%", characters in "reserved", and characters in US-ASCII not
allowed in URIs.
3. Re-percent-encode any octet produced in step 2 that is not part
of a strictly legal UTF-8 octet sequence.
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4. Re-percent-encode all octets produced in step 3 that in UTF-8
represent characters that are not appropriate according to
sections 2.2, 4.1, and 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 step 4 is applied (see section 6.1), results may vary.
Conversions from URIs to IRIs MUST NOT use any character encoding
other than UTF-8 in steps 3 and 4, even if it might be possible to
guess from the context that another character encoding than UTF-8 was
used in the URI. For 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, in the future the IRI will 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. Each
example shows the result after each of the steps 1 through 5 is
applied. XML Notation is used for the final result. Octets are
denoted by "<" followed by two hexadecimal digits followed by ">".
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
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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 lowercase) of letters
used in percent-encodings 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
by 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
scripts, have an inherent right-to-left (rtl) writing direction.
IRIs containing these characters (called bidirectional IRIs or Bidi
IRIs) require additional attention because of the non-trivial
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relation between logical representation (used for digital
representation and for 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; and
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
ensures that bidirectional IRIs can be processed in the same way as
other IRIs.
Bidirectional IRIs MUST be rendered by using the Unicode
Bidirectional Algorithm [UNIV4], [UNI9]. Bidirectional IRIs MUST be
rendered in the same way as they would be if they were in a
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 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 reference that only contains
strong right-to-left characters and weak characters and that starts
and ends with a strong right-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.
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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 ensure correct display are not themselves 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 appear themselves. It would
therefore not be possible to input an IRI with such characters
correctly.
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
"/") 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-nz-nc, 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 schemes with ireg-name as 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.
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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 ensure
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 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 has 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 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.
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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"
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: Each of several sequences of rtl components is inverted on
its 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 it 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.
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Example 7: A single rtl component includes 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 are at the start or end of an 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/%31HG/LK/JI%32.html"
Visual representation (Arabic): "http://ab.cd.ef/31%HG/%LK/JI32.html"
Depending on whether the uppercase 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 these may interact with adjacent RTL
components in ways that are not easy to predict.
5. Normalization and Comparison
Note: The structure and much of the material for this section is
taken from section 6 of [RFC3986]; the differences are due to the
specifics of IRIs.
One of the most common operations on IRIs is simple comparison:
Determining whether two IRIs are equivalent without using the IRIs or
the mapped URIs to access their respective resource(s). A comparison
is performed whenever a response cache is accessed, a browser checks
its history to color a link, or an XML parser processes tags within a
namespace. Extensive normalization prior to comparison of IRIs may
be used by spiders and indexing engines to prune a search space or
reduce duplication of request actions and response storage.
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IRI comparison is performed for some particular purpose. Protocols
or implementations that compare IRIs for different purposes will
often be subject to differing design trade-offs in regards to how
much effort should be spent in reducing aliased identifiers. This
section describes various methods that may be used to compare IRIs,
the trade-offs between them, and the types of applications that might
use them.
5.1. Equivalence
Because IRIs exist to identify resources, presumably they should be
considered equivalent when they identify the same resource. However,
this definition of equivalence is not of much practical use, as there
is no way for an implementation to compare two resources unless it
has full knowledge or control of them. For this reason, determination
of equivalence or difference of IRIs is based on string comparison,
perhaps augmented by reference to additional rules provided by URI
scheme definitions. We use the terms "different" and "equivalent" to
describe the possible outcomes of such comparisons, but there are
many application-dependent versions of equivalence.
Even though it is possible to determine that two IRIs are equivalent,
IRI comparison is not sufficient to determine whether two IRIs
identify different resources. For example, an owner of two different
domain names could decide to serve the same resource from both,
resulting in two different IRIs. Therefore, comparison methods are
designed to minimize false negatives while strictly avoiding false
positives.
In testing for equivalence, applications should not directly compare
relative references; the references should be converted to their
respective target IRIs before comparison. When IRIs are compared to
select (or avoid) a network action, such as retrieval of a
representation, fragment components (if any) should be excluded from
the comparison.
Applications using IRIs as identity tokens with no relationship to a
protocol MUST use the Simple String Comparison (see section 5.3.1).
All other applications MUST select one of the comparison practices
from the Comparison Ladder (see section 5.3 or, after IRI-to-URI
conversion, select one of the comparison practices from the URI
comparison ladder in [RFC3986], section 6.2)
5.2. Preparation for Comparison
Any kind of IRI comparison REQUIRES that all escapings or encodings
in the protocol or format that carries an IRI are resolved. This is
usually done when the protocol or format is parsed. Examples of such
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escapings or encodings are entities and numeric character references
in [HTML4] and [XML1]. As an example,
"http://example.org/rosé" (in HTML),
"http://example.org/rosé"; (in HTML or XML), and
"http://example.org/rosé"; (in HTML or XML) are all resolved into
what is denoted in this document (see section 1.4) as
"http://example.org/rosé"; (the "é" here standing for the
actual e-acute character, to compensate for the fact that this
document cannot contain non-ASCII characters).
Similar considerations apply to encodings such as Transfer Codings in
HTTP (see [RFC2616]) and Content Transfer Encodings in MIME
([RFC2045]), although in these cases, the encoding is based not on
characters but on octets, and additional care is required to make
sure that characters, and not just arbitrary octets, are compared
(see section 5.3.1).
5.3. Comparison Ladder
In practice, a variety of methods are used, to test IRI equivalence.
These methods fall into a range distinguished by the amount of
processing required and the degree to which the probability of false
negatives is reduced. As noted above, false negatives cannot be
eliminated. In practice, their probability can be reduced, but this
reduction requires more processing and is not cost-effective for all
applications.
If this range of comparison practices is considered as a ladder, the
following discussion will climb the ladder, starting with practices
that are cheap but have a relatively higher chance of producing false
negatives, and proceeding to those that have higher computational
cost and lower risk of false negatives.
5.3.1. Simple String Comparison
If two IRIs, when considered as character strings, are identical,
then it is safe to conclude that they are equivalent. This type of
equivalence test has very low computational cost and is in wide use
in a variety of applications, particularly in the domain of parsing.
It is also used when a definitive answer to the question of IRI
equivalence is needed that is independent of the scheme used and that
can be calculated quickly and without accessing a network. An
example of such a case is XML Namespaces ([XMLNamespace]).
Testing strings for equivalence requires some basic precautions. This
procedure is often referred to as "bit-for-bit" or "byte-for-byte"
comparison, which is potentially misleading. Testing strings for
equality is normally based on pair comparison of the characters that
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make up the strings, starting from the first and proceeding until
both strings are exhausted and all characters are found to be equal,
until a pair of characters compares unequal, or until one of the
strings is exhausted before the other.
This character comparison requires that each pair of characters be
put in comparable encoding form. For example, should one IRI be
stored in a byte array in UTF-8 encoding form and the second in a
UTF-16 encoding form, bit-for-bit comparisons applied naively will
produce errors. It is better to speak of equality on a
character-for-character rather than on a byte-for-byte or bit-for-bit
basis. In practical terms, character-by-character comparisons should
be done codepoint by codepoint after conversion to a common character
encoding form. 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 an IRI
SHOULD NOT be modified when being transported if there is any chance
that this IRI might be used as an identifier.
False negatives are caused by the production and use of IRI aliases.
Unnecessary aliases can be reduced, regardless of the comparison
method, by consistently providing IRI references in an already
normalized form (i.e., a form identical to what would be produced
after normalization is applied, as described below). Protocols and
data formats often limit some IRI comparisons to simple string
comparison, based on the theory that people and implementations will,
in their own best interest, be consistent in providing IRI
references, or at least be consistent enough to negate any efficiency
that might be obtained from further normalization.
5.3.2. Syntax-Based Normalization
Implementations may use logic based on the definitions provided by
this specification to reduce the probability of false negatives. This
processing is moderately higher in cost than character-for-character
string comparison. For example, an application using this approach
could reasonably consider the following two IRIs equivalent:
example://a/b/c/%7Bfoo%7D/rosé
eXAMPLE://a/./b/../b/%63/%7bfoo%7d/ros%C3%A9
Web user agents, such as browsers, typically apply this type of IRI
normalization when determining whether a cached response is
available. Syntax-based normalization includes such techniques as
case normalization, character normalization, percent-encoding
normalization, and removal of dot-segments.
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5.3.2.1. Case Normalization
For all IRIs, the hexadecimal digits within a percent-encoding
triplet (e.g., "%3a" versus "%3A") are case-insensitive and therefore
should be normalized to use uppercase letters for the digits A - F.
When an IRI uses components of the generic syntax, the component
syntax equivalence rules always apply; namely, that the scheme and
US-ASCII only host are case insensitive and therefore should be
normalized to lowercase. For example, the URI
"HTTP://www.EXAMPLE.com/" is equivalent to "http://www.example.com/".
Case equivalence for non-ASCII characters in IRI components that are
IDNs are discussed in section 5.3.3. The other generic syntax
components are assumed to be case sensitive unless specifically
defined otherwise by the scheme.
Creating schemes that allow case-insensitive syntax components
containing non-ASCII characters should be avoided. Case normalization
of non-ASCII characters can be culturally dependent and is always a
complex operation. The only exception concerns non-ASCII host names
for which the character normalization includes a mapping step derived
from case folding.
5.3.2.2. Character 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-character-normalized rather than apply character
normalization when comparing two IRIs. The exceptions are conversion
from a non-digital form, and conversion from a non-UCS-based
character encoding to a 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 by using NFC. Using NFKC may
avoid even more problems; for example, by choosing half-width Latin
letters instead of full-width ones, and full-width instead of
half-width Katakana.
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.
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The former uses precombined e-acute characters, and the latter uses
"e" characters followed by combining acute accents. Both usages are
defined as canonically equivalent in [UNIV4].
Note: Because it is unknown how a particular sequence of characters
is being treated with respect to character 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 character
normalized as possible (i.e., NFC or even NFKC). This is similar
to the uppercase/lowercase problems. Some parts of a URI are case
insensitive (domain name). For others, it is unclear whether they
are case sensitive, case insensitive, or something in between
(e.g., case sensitive, but with a multiple choice selection if the
wrong case is used, instead of a direct negative result). The
best recipe is that the creator use a reasonable capitalization
and, when transferring the URI, capitalization never be changed.
Various IRI schemes may allow the usage of Internationalized Domain
Names (IDN) [RFC3490] either in the ireg-name part or elsewhere.
Character Normalization also applies to IDNs, as discussed in section
5.3.3.
5.3.2.3. Percent-Encoding Normalization
The percent-encoding mechanism (section 2.1 of [RFC3986]) is a
frequent source of variance among otherwise identical IRIs. In
addition to the case normalization issue noted above, some IRI
producers percent-encode octets that do not require percent-encoding,
resulting in IRIs that are equivalent to their non encoded
counterparts. These IRIs should be normalized by decoding any
percent-encoded octet sequence that corresponds to an unreserved
character, as described in section 2.3 of [RFC3986].
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-encoding is always the same
(preferably uppercase). If the IRI is to be passed to another
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application or used further in some other way, its original form MUST
be preserved. The conversion described here should be performed only
for local comparison.
5.3.2.4. Path Segment Normalization
The complete path segments "." and ".." are intended only for use
within relative references (section 4.1 of [RFC3986]) and are removed
as part of the reference resolution process (section 5.2 of
[RFC3986]). However, some implementations may incorrectly assume
that reference resolution is not necessary when the reference is
already an IRI, and thus fail to remove dot-segments when they occur
in non-relative paths. IRI normalizers should remove dot-segments by
applying the remove_dot_segments algorithm to the path, as described
in section 5.2.4 of [RFC3986].
5.3.3. Scheme-Based Normalization
The syntax and semantics of IRIs vary from scheme to scheme, as
described by the defining specification for each scheme.
Implementations may use scheme-specific rules, at further processing
cost, to reduce the probability of false negatives. For example,
because the "http" scheme makes use of an authority component, has a
default port of "80", and defines an empty path to be equivalent to
"/", the following four IRIs are equivalent:
http://example.com
http://example.com/
http://example.com:/
http://example.com:80/
In general, an IRI that uses the generic syntax for authority with an
empty path should be normalized to a path of "/". Likewise, an
explicit ":port", for which the port is empty or the default for the
scheme, is equivalent to one where the port and its ":" delimiter are
elided and thus should be removed by scheme-based normalization. For
example, the second IRI above is the normal form for the "http"
scheme.
Another case where normalization varies by scheme is in the handling
of an empty authority component or empty host subcomponent. For many
scheme specifications, an empty authority or host is considered an
error; for others, it is considered equivalent to "localhost" or the
end-user's host. When a scheme defines a default for authority and
an IRI reference to that default is desired, the reference should be
normalized to an empty authority for the sake of uniformity, brevity,
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and internationalization. If, however, either the userinfo or port
subcomponents are non-empty, then the host should be given explicitly
even if it matches the default.
Normalization should not remove delimiters when their associated
component is empty unless it is licensed to do so by the scheme
specification. For example, the IRI "http://example.com/?" cannot be
assumed to be equivalent to any of the examples above. Likewise, the
presence or absence of delimiters within a userinfo subcomponent is
usually significant to its interpretation. The fragment component is
not subject to any scheme-based normalization; thus, two IRIs that
differ only by the suffix "#" are considered different regardless of
the scheme.
Some IRI schemes may allow the usage of Internationalized Domain
Names (IDN) [RFC3490] either in their ireg-name part or elsewhere.
When in use in IRIs, those names SHOULD be validated by using the
ToASCII operation defined in [RFC3490], with the flags
"UseSTD3ASCIIRules" and "AllowUnassigned". An IRI containing an
invalid IDN cannot successfully be resolved. Validated IDN
components of IRIs SHOULD be character normalized by using the
Nameprep process [RFC3491]; however, for legibility purposes, they
SHOULD NOT be converted into ASCII Compatible Encoding (ACE).
Scheme-based normalization may also consider IDN components and their
conversions to punycode as equivalent. As an example,
"http://résumé.example.org" may be considered equivalent to
"http://xn--rsum-bpad.example.org".
Other scheme-specific normalizations are possible.
5.3.4. Protocol-Based Normalization
Substantial effort to reduce the incidence of false negatives is
often cost-effective for web spiders. Consequently, they implement
even more aggressive techniques in IRI comparison. For example, if
they observe that an IRI such as
http://example.com/data
redirects to an IRI differing only in the trailing slash
http://example.com/data/
they will likely regard the two as equivalent in the future. This
kind of technique is only appropriate when equivalence is clearly
indicated by both the result of accessing the resources and the
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common conventions of their scheme's dereference algorithm (in this
case, use of redirection by HTTP origin servers to avoid problems
with relative references).
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
IRIs are created and when URIs are converted 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. It
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 that 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. This 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 have to be upgraded to allow
the transport of IRIs. In 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 not expressible in the native character
encoding SHOULD be escaped by 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], 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. To be able to use IRIs, the URI corresponding to the
IRI in question has to encode original characters into octets by
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
to some part. For background information on encoding characters into
URIs, see also section 2.5 of [RFC3986].
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
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"http://www.example.org/r%E9sum%E9.html", the percent-encoding octets
cannot be converted to actual characters in an IRI, as 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 makes sense is when a scheme definition, or
a particular component of a scheme, is strictly limited to the use of
US-ASCII characters with no provision to include non-ASCII
characters/octets via percent-encoding, or if a scheme definition
currently uses highly scheme-specific provisions for the encoding of
non-ASCII characters. An example of this is the mailto: scheme
[RFC2368].
This specification does not upgrade any scheme specifications in any
way; this has to be done separately. Also, note 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.
URI schemes can impose restrictions on the syntax of scheme-specific
URIs; i.e., URIs that are admissible under the generic URI syntax
[RFC3986] may not be admissible due to narrower syntactic constraints
imposed by a URI scheme specification. URI scheme definitions cannot
broaden the syntactic restrictions of the generic URI syntax;
otherwise, it would be possible to generate URIs that satisfied the
scheme-specific syntactic constraints without satisfying the
syntactic constraints of the generic URI syntax. However, additional
syntactic constraints imposed by URI scheme specifications are
applicable to IRI, as the corresponding URI resulting from the
mapping defined in section 3.1 MUST be a valid URI under the
syntactic restrictions of generic URI syntax and any narrower
restrictions imposed by the corresponding URI scheme specification.
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
where the fragment identifier is encoded in UTF-8 according to
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[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 IRI references against a base is handled
straightforwardly; the algorithms of [RFC3986] can be applied
directly, treating the characters additionally allowed in IRI
references in the same way that unreserved characters are in URI
references.
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 creates or generates URIs,
software that displays URIs, formats and protocols that transport
URIs, and software that interprets URIs. These may all require
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, as
it is compatible with US-ASCII, is in accordance with the
recommendations of [RFC2277], and makes converting to URIs easy. 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.
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7.2. URI/IRI Entry
Some components allow users to enter URIs into the system by typing
or dictation, for example. 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 an 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 ensure, as much 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 punctuation and full-width Katakana.
An input field primarily or solely used for the input of URIs/IRIs
may allow the user to view an IRI as it is 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 interfacing to components that handle URIs,
but not IRIs, must map the IRI to a URI before passing it to these
components.
For the input of IRIs with right-to-left characters, please see
section 4.3.
7.3. URI/IRI Transfer between Applications
Many applications, particularly mail user agents, try to detect URIs
appearing in plain text. For this, they use some heuristics based on
URI syntax. They then allow the user to click on such URIs and
retrieve the corresponding resource in an appropriate (usually
scheme-dependent) application.
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Such applications have to be upgraded to use the IRI syntax 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.
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 make 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 use of
"résumé.html" in an IRI, even if locally the file name is
kept in a character encoding other than UTF-8.
This recommendation particularly applies to HTTP servers. For FTP
servers, similar considerations apply; see [RFC2640].
7.5. URI/IRI Selection
In some cases, resource owners and publishers have control over the
IRIs used to identify their resources. This control is mostly
executed by controlling the resource names, such as file names,
directly.
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In these 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 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, newly created resource names should generally 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 similarity of
the Latin "A", the Greek "Alpha", and the Cyrillic "A". To avoid
such cases, only IRIs should be created where all the characters in a
single component are used together in a given language. This usually
means that all of 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 lowercase letters results in fewer
ambiguities than using uppercase letters would.
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.
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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 character encodings
other than UTF-8. These URIs may be produced by user agents that do
not conform to this specification and that 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 some servers currently 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. Please note
that such mappings, including case mappings, are language dependent.
It can be difficult to identify a resource unambiguously if too many
mappings are taken into consideration. However, percent-encoded and
not percent-encoded parts of IRIs can always be clearly
distinguished. Also, the regularity of UTF-8 (see [Duerst97]) makes
the potential for collisions lower than it may seem at first.
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, although it may be
entered and transported very widely.
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Therefore, IRIs benefit most from a broad upgrade of software to be
able to enter and transport IRIs. However, before an individual IRI
is published, 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.
Where there is a free choice of character encodings, it is often
possible to reduce the effort and dependencies for upgrading to IRIs
by using UTF-8 rather than another encoding. For example, when a new
file-based Web server is set up, using UTF-8 as the character
encoding for file names will make the transition to IRIs easier.
Likewise, when a new Web form is set up 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
US-ASCII while minimizing interoperability problems. For
considerations regarding the upgrade of URI scheme definitions, see
section 6.4.
8. Security Considerations
The security considerations discussed in [RFC3986] 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
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"%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 that 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 that can cause all kinds of problems. Most
spoofing possibilities for IRIs are extensions of those for URIs.
Spoofing can occur for various reasons. First, a user's
normalization expectations or actual normalization when entering an
IRI or 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 is not specific to IRIs, but it is 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 that
allow independent users to create resources in the same sub area may
have to be careful to check for spoofing.
Spoofing can occur because in the UCS many characters 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, particularly 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.
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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 be 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 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 there were 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, Bill Fenner, Margaret Wasserman, Russ Housley,
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 with 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.
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10. References
10.1. 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.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter,
"Uniform Resource Identifier (URI): Generic Syntax",
STD 66, RFC 3986, January 2005.
[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.
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[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>.
10.2. Informative 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:
Resource Identifiers", World Wide Web Consortium
Candidate Recommendation, November 2004,
<http://www.w3.org/TR/charmod-resid>.
[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/html401/appendix/
notes.html#h-B.2>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, November 1996.
[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.
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[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., Frystyk, 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, June 2003,
<http://www.w3.org/TR/unicode-xml/>.
[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 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>.
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[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>.
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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, as 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 a uniform convention for conversion from IRIs to URIs
makes IRI implementation orthogonal to the introduction of actual new
schemes.
Appendix A.2. Character Encodings Other Than UTF-8
At an early stage, UTF-7 was considered as an alternative to UTF-8
when IRIs are converted to URIs. UTF-7 would not have needed
percent-encoding and in most cases would 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.
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.
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Using the existing octet-based percent-encoding mechanism does not
need an upgrade of the URI syntax and does not need corresponding
server upgrades.
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 e-mails 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.
If UTF-8 is used exclusively, an upgrade to the URI syntax is not
needed. 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 (and being prohibitively annoying).
Exclusively using UTF-8 also reduces transcoding errors and
confusion.
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: 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: michelsu@microsoft.com
URI: http://www.suignard.com
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Full Copyright Statement
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