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Network Working Group M. Duerst
Internet-Draft Aoyama Gakuin University
Obsoletes: RFC 3987 M. Suignard
(if approved) Unicode Consortium
Intended status: Standards Track L. Masinter
Expires: April 29, 2010 Adobe
October 26, 2009
Internationalized Resource Identifiers (IRIs)
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. This document may contain material
from IETF Documents or IETF Contributions published or made publicly
available before November 10, 2008. The person(s) controlling the
copyright in some of this material may not have granted the IETF
Trust the right to allow modifications of such material outside the
IETF Standards Process. Without obtaining an adequate license from
the person(s) controlling the copyright in such materials, this
document may not be modified outside the IETF Standards Process, and
derivative works of it may not be created outside the IETF Standards
Process, except to format it for publication as an RFC or to
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Internet-Drafts are working documents of the Internet Engineering
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Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
This document defines the Internationalized Resource Identifier (IRI)
protocol element, as an extension of the Uniform Resource Identifier
(URI). An IRI is a sequence of characters from the Universal
Character Set (Unicode/ISO 10646). Grammar and processing rules are
given for IRIs and related syntactic forms.
In addition, this document provides named additional rule sets for
processing otherwise invalid IRIs, in a way that supports other
specifications that wish to mandate common behavior for 'error'
handling. In particular, rules used in some XML languages (LEIRI)
and web applications are given.
Defining IRI as new protocol element (rather than updating or
extending the definition of URI) allows independent orderly
transitions: other protocols and languages that use URIs must
explicitly choose to allow IRIs.
Guidelines are provided for the use and deployment of IRIs and
related protocol elements when revising protocols, formats, and
software components that currently deal only with URIs.
[RFC Editor: Please remove this paragraph before publication.] This
document is intended to update RFC 3987 and move towards IETF Draft
Standard. This is an interim version in preparation for the IRI BOF
at IETF 76 in Hiroshima. For discussion and comments on this draft,
please use the mailing list.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Overview and Motivation . . . . . . . . . . . . . . . . . 5
1.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Definitions . . . . . . . . . . . . . . . . . . . . . . . 6
1.4. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 9
2. IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1. Summary of IRI Syntax . . . . . . . . . . . . . . . . . . 10
2.2. ABNF for IRI References and IRIs . . . . . . . . . . . . . 10
3. Processing IRIs and related protocol elements . . . . . . . . 13
3.1. Converting to UCS . . . . . . . . . . . . . . . . . . . . 14
3.2. Parse the IRI into IRI components . . . . . . . . . . . . 14
3.3. General percent-encoding of IRI components . . . . . . . . 15
3.4. Mapping ireg-name . . . . . . . . . . . . . . . . . . . . 15
3.5. Mapping query components . . . . . . . . . . . . . . . . . 17
3.6. Mapping IRIs to URIs . . . . . . . . . . . . . . . . . . . 17
3.7. Converting URIs to IRIs . . . . . . . . . . . . . . . . . 17
3.7.1. Examples . . . . . . . . . . . . . . . . . . . . . . . 19
4. Bidirectional IRIs for Right-to-Left Languages . . . . . . . . 20
4.1. Logical Storage and Visual Presentation . . . . . . . . . 21
4.2. Bidi IRI Structure . . . . . . . . . . . . . . . . . . . . 22
4.3. Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . . 23
4.4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 23
5. Normalization and Comparison . . . . . . . . . . . . . . . . . 25
5.1. Equivalence . . . . . . . . . . . . . . . . . . . . . . . 25
5.2. Preparation for Comparison . . . . . . . . . . . . . . . . 26
5.3. Comparison Ladder . . . . . . . . . . . . . . . . . . . . 27
5.3.1. Simple String Comparison . . . . . . . . . . . . . . . 27
5.3.2. Syntax-Based Normalization . . . . . . . . . . . . . . 28
5.3.3. Scheme-Based Normalization . . . . . . . . . . . . . . 31
5.3.4. Protocol-Based Normalization . . . . . . . . . . . . . 32
6. Use of IRIs . . . . . . . . . . . . . . . . . . . . . . . . . 33
6.1. Limitations on UCS Characters Allowed in IRIs . . . . . . 33
6.2. Software Interfaces and Protocols . . . . . . . . . . . . 33
6.3. Format of URIs and IRIs in Documents and Protocols . . . . 33
6.4. Use of UTF-8 for Encoding Original Characters . . . . . . 34
6.5. Relative IRI References . . . . . . . . . . . . . . . . . 36
7. Liberal handling of otherwise invalid IRIs . . . . . . . . . . 36
7.1. LEIRI processing . . . . . . . . . . . . . . . . . . . . . 36
7.2. Web Address processing . . . . . . . . . . . . . . . . . . 36
7.3. Characters not allowed in IRIs . . . . . . . . . . . . . . 38
8. URI/IRI Processing Guidelines (Informative) . . . . . . . . . 40
8.1. URI/IRI Software Interfaces . . . . . . . . . . . . . . . 40
8.2. URI/IRI Entry . . . . . . . . . . . . . . . . . . . . . . 41
8.3. URI/IRI Transfer between Applications . . . . . . . . . . 42
8.4. URI/IRI Generation . . . . . . . . . . . . . . . . . . . . 42
8.5. URI/IRI Selection . . . . . . . . . . . . . . . . . . . . 43
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8.6. Display of URIs/IRIs . . . . . . . . . . . . . . . . . . . 43
8.7. Interpretation of URIs and IRIs . . . . . . . . . . . . . 44
8.8. Upgrading Strategy . . . . . . . . . . . . . . . . . . . . 44
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 45
10. Security Considerations . . . . . . . . . . . . . . . . . . . 46
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 47
12. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 48
13. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 50
13.1. Changes from -06 to this document . . . . . . . . . . . . 50
13.1.1. OLD WAY . . . . . . . . . . . . . . . . . . . . . . . 50
13.1.2. NEW WAY . . . . . . . . . . . . . . . . . . . . . . . 51
13.2. Changes from -05 to -06 . . . . . . . . . . . . . . . . . 51
13.3. Changes from -04 to -05 . . . . . . . . . . . . . . . . . 51
13.4. Changes from -03 to -04 . . . . . . . . . . . . . . . . . 51
13.5. Changes from -02 to -03 . . . . . . . . . . . . . . . . . 51
13.6. Changes from -01 to -02 . . . . . . . . . . . . . . . . . 52
13.7. Changes from -00 to -01 . . . . . . . . . . . . . . . . . 52
13.8. Changes from RFC 3987 to -00 . . . . . . . . . . . . . . . 52
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 52
14.1. Normative References . . . . . . . . . . . . . . . . . . . 52
14.2. Informative References . . . . . . . . . . . . . . . . . . 53
Appendix A. Design Alternatives . . . . . . . . . . . . . . . . . 55
A.1. New Scheme(s) . . . . . . . . . . . . . . . . . . . . . . 56
A.2. Character Encodings Other Than UTF-8 . . . . . . . . . . . 56
A.3. New Encoding Convention . . . . . . . . . . . . . . . . . 56
A.4. Indicating Character Encodings in the URI/IRI . . . . . . 57
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 57
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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 difficulties.
The infrastructure for the appropriate handling of characters from
additional scripts is now widely deployed in operating system and
application software. Software that can handle a wide variety of
scripts and languages at the same time is increasingly common. Also,
an increasing number of protocols and formats can carry a wide range
of characters.
URIs are used both as a protocol element (for transmission and
processing by software) and also a presentation element (for display
and handling by people who read, interpret, coin, or guess them).
The transition between these roles is more difficult and complex when
dealing with the larger set of characters than allowed for URIs in
This document defines the protocol element called Internationalized
Resource Identifier (IRI), which allow applications of URIs to be
extended to use resource identifiers that have a much wider
repertoire of characters. It also provides corresponding
"internationalized" versions of other constructs from [RFC3986], such
as URI references. The syntax of IRIs is defined in Section 2.
Using characters outside of A - Z in IRIs adds a number of
difficulties. Section 4 discusses the special case of bidirectional
IRIs using characters from scripts written right-to-left. Section 5
discusses various forms of equivalence between IRIs. Section 6
discusses the use of IRIs in different situations. Section 8 gives
additional informative guidelines. Section 10 discusses IRI-specific
security considerations.
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1.2. Applicability
IRIs are designed to allow protocols and software that deal with URIs
to be updated to handle IRIs. A "URI scheme" (as defined by
[RFC3986] and registered through the IANA process defined in
[RFC4395] also serves as an "IRI scheme". Processing of IRIs is
accomplished by extending the URI syntax while retaining (and not
expanding) the set of "reserved" characters, such that the syntax for
any URI scheme may be uniformly extended to allow non-ASCII
characters. In addition, following parsing of an IRI, it is possible
to construct a corresponding URI by first encoding characters outside
of the allowed URI range and then reassembling the components.
Practical use of IRIs forms in place of URIs forms depends on the
following conditions being met:
a. A protocol or format element MUST be explicitly designated to be
able to carry IRIs. The intent is to avoid introducing 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 [RFC2616] definition of HTTP/1.1, 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 MUST 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 scheme definition, if it explicitly allows a percent sign
("%") in any syntactic component, SHOULD define the interpretation
of sequences of percent-encoded octets (using "%XX" hex octets) as
octet from sequences of UTF-8 encoded strings; this is recommended
in the guidelines for registering new schemes, [RFC4395]. For
example, this is the practice for IMAP URLs [RFC2192], POP URLs
[RFC2384] and the URN syntax [RFC2141]). Note that use of
percent-encoding may also be restricted in some situations, for
example, URI schemes that disallow percent-encoding might still be
used with a fragment identifier which is percent-encoded (e.g.,
[XPointer]). See Section 6.4 for further discussion.
1.3. Definitions
The following definitions are used in this document; they follow the
terms in [RFC2130], [RFC2277], and [ISO10646].
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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 (set in the mathematical
sequence of characters: A sequence of characters (one after
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.
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.
URL: The term "URL" was originally used [RFC1738] for roughly what
is now called a "URI". Books, software and documentation often
refers to URIs and IRIs using the "URL" term. Some usages
restrict "URL" to those URIs which are not URNs. Because of the
ambiguity of the term using the term "URL" is NOT RECOMMENDED in
formal documents.
LEIRI (Legacy Extended IRI) processing: This term was used in
various XML specifications to refer to strings that, although not
valid IRIs, were acceptable input to the processing rules in
Section 7.1.
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(Web Address, Hypertext Reference, HREF): These terms have been
added in this document for convenience, to allow other
specifications to refer to those strings that, although not valid
IRIs, are acceptable input to the processing rules in Section 7.2.
This usage corresponds to the parsing rules of some popular web
browsing applications. ISSUE: Need to find a good name/
abbreviation for these.
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 identifier is generated by derivation from other
parsed URI component: When a URI processor parses a URI (following
the generic syntax or a scheme-specific syntax, the result is a
set of parsed URI components, each of which has a type
(corresponding to the syntactic definition) and a sequence of URI
parsed IRI component: When an IRI processor parses an IRI directly,
following the general syntax or a scheme-specific syntax, the
result is a set of parsed IRI components, each of which has a type
(corresponding to the syntactice definition) and a sequence of IRI
characters. (This definition is analogous to "parsed URI
IRI scheme: A URI scheme may also be known as an "IRI scheme" if the
scheme's syntax has been extended to allow non-US-ASCII characters
according to the rules in this document.
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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'.
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: Lower case letters
stand for Latin letters or other letters that are written left to
right, whereas upper case letters represent Arabic or Hebrew letters
that are written right to left.
To denote actual octets in examples (as opposed to percent-encoded
octets), the two hex digits denoting the octet are enclosed in "<"
and ">". For example, the octet often denoted as 0xc9 is denoted
here as <c9>.
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
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 might 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.
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2.1. Summary of IRI Syntax
IRIs are defined by extending the URI syntax in [RFC3986], but
extending the class of unreserved characters 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.
The syntax and use of components and reserved characters is the same
as that in [RFC3986]. Each "URI scheme" thus also functions as an
"IRI scheme", in that scheme-specific parsing rules for URIs of a
scheme are be extended to allow parsing of IRIs using the same
parsing rules.
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.
Characters outside the US-ASCII repertoire MUST NOT be 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
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 [STD68]. Character numbers are
taken from the UCS, without implying any actual binary encoding.
Terminals in the ABNF are characters, not octets.
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]:
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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
/ ipath-noscheme
/ ipath-empty
iauthority = [ iuserinfo "@" ] ihost [ ":" port ]
iuserinfo = *( iunreserved / pct-form / sub-delims / ":" )
ihost = IP-literal / IPv4address / ireg-name
pct-form = pct-encoded
ireg-name = *( iunreserved / 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 = *( path-sep isegment )
ipath-absolute = path-sep [ isegment-nz *( path-sep isegment ) ]
ipath-noscheme = isegment-nz-nc *( path-sep isegment )
ipath-rootless = isegment-nz *( path-sep isegment )
ipath-empty = 0<ipchar>
path-sep = "/"
isegment = *ipchar
isegment-nz = 1*ipchar
isegment-nz-nc = 1*( iunreserved / pct-form / sub-delims
/ "@" )
; non-zero-length segment without any colon ":"
ipchar = iunreserved / pct-form / sub-delims / ":"
/ "@"
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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 / %xE0000-E0FFF / %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.
3. Processing IRIs and related protocol elements
IRIs are meant to replace URIs in identifying resources within new
versions of protocols, formats, and software components that use a
UCS-based character repertoire. Protocols and components may use and
process IRIs directly. However, there are still numerous systems and
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protocols which only accept URIs or components of parsed URIs; that
is, they only accept sequences of characters within the subset of US-
ASCII characters allowed in URIs.
This section defines specific processing steps for IRI consumers
which establish the relationship between the string given and the
interpreted derivatives. These processing steps apply to both IRIs
and IRI references (i.e., absolute or relative forms); for IRIs, some
steps are scheme specific.
3.1. Converting to UCS
Input that is already in a Unicode form (i.e., a sequence of Unicode
characters or an octet-stream representing a Unicode-based character
encoding such as UTF-8 or UTF-16) should be left as is and not
normalized (see (see Section
If the IRI or IRI reference is an octet stream in some known non-
Unicode character encoding, convert the IRI to a sequence of
characters from the UCS; this sequence SHOULD also be normalized
according to Unicode Normalization Form C (NFC, [UTR15]). In this
case, retain the original character encoding as the "document
In other cases (written on paper, read aloud, or otherwise
represented independent of any character encoding) represent the IRI
as a sequence of characters from the UCS normalized according to
Unicode Normalization Form C (NFC, [UTR15]).
3.2. Parse the IRI into IRI components
Parse the IRI, either as a relative reference (no scheme) or using
scheme specific processing (according to the scheme given); the
result resulting in a set of parsed IRI components. (NOTE: FIX
NOTE: The result of parsing into components will correspond result in
a correspondence of subtrings of the IRI according to the part
matched. For example, in [HTML5], the protocol components of
interest are SCHEME (scheme), HOST (ireg-name), PORT (port), the PATH
(ipath after the initial "/"), QUERY (iquery), FRAGMENT (ifragment),
and AUTHORITY (iauthority).
Subsequent processing rules are sometimes used to define other
syntactic components. For example, [HTML5] defines APIs for IRI
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processing; in these APIs:
HOSTSPECIFIC the substring that follows the substring matched by the
iauthority production, or the whole string if the iauthority
production wasn't matched.
HOSTPORT if there is a scheme component and a port component and the
port given by the port component is different than the default
port defined for the protocol given by the scheme component, then
HOSTPORT is the substring that starts with the substring matched
by the host production and ends with the substring matched by the
port production, and includes the colon in between the two.
Otherwise, it is the same as the host component.
3.3. General percent-encoding of IRI components
For most IRI components, it is possible to map the IRI component to
an equivalent URI component by percent-encoding those characters not
allowed in URIs. Previous processing steps will have removed some
characters, and the interpretation of reserved characters will have
already been done (with the syntactic reserved characters outside of
the IRI component). This mapping is defined for all sequences of
Unicode characters, whether or not they are valid for the component
in question.
For each character which is not allowed in a valid URI (NOTE: WHAT IS
THE RIGHT REFERENCE HERE), apply the following steps.
Convert to UTF-8 Convert the character to a sequence of one or more
octets using UTF-8 [RFC3629].
Percent encode Convert each octet of this sequence to %HH, where HH
is the hexadecimal notation of the octet value. The hexadecimal
notation SHOULD use uppercase letters. (This is the general URI
percent-encoding mechanism in Section 2.1 of [RFC3986].)
Note that the mapping is an identity transformation for parsed URI
components of valid URIs, and is idempotent: applying the mapping a
second time will not change anything.
3.4. Mapping ireg-name
Schemes that allow non-ASCII based characters in the reg-name (ireg-
name) position MUST convert the ireg-name component of an IRI as
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-
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separated label, and by using U+002E (FULL STOP) as a label
separator, with the flag UseSTD3ASCIIRules set to FALSE, and with the
flag AllowUnassigned set to FALSE. The ToASCII operation may fail,
but this would mean that the IRI cannot be resolved. In such cases,
if the domain name conversion fails, then the entire IRI conversion
fails. Processors that have no mechanism for signalling a failure
MAY instead substitute an otherwise invalid host name, although such
processing SHOULD be avoided.
For example, the IRI
MAY be converted to
; conversion to percent-encoded form, e.g.,
"", MUST NOT be performed.
Note: Domain Names may appear 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://r&#xE9;sum&#xE9; would lead to an IRI of;sum&#xE9;., which would convert to a URI of The server-side implementation is responsible for
making the necessary conversions to be able to retrieve the Web
Note: In this process, 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
";#red" (in XML notation) is
converted to
"", not to something
((DESIGN QUESTION: What about e.g. in an IRI? Will that get
converted to punycode, or not?))
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3.5. Mapping query components
((NOTE: SEE ISSUES LIST)) For compatibility with existing deployed
HTTP infrastructure, the following special case applies for schemes
"http" and "https" and IRIs whose origin has a document charset other
than one which is UCS-based (e.g., UTF-8 or UTF-16). In such a case,
the "query" component of an IRI is mapped into a URI by using the
document charset rather than UTF-8 as the binary representation
before pct-encoding. This mapping is not applied for any other
scheme or component.
3.6. Mapping IRIs to URIs
The canonical mapping from a IRI to URI is defined by applying the
mapping above (from IRI to URI components) and then reassembling a
URI from the parsed URI components using the original punctuation
that delimited the IRI components.
3.7. Converting URIs to IRIs
In some situations, for presentation and further processing, it is
desirable to convert a URI into an equivalent IRI in which natural
characters are represented directly rather than percent encoded. Of
course, every URI is already an IRI in its own right without any
conversion, and in general there This section gives one such
procedure for this conversion.
The conversion described in this section, if given a valid URI, will
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 differ from 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
(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
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3. The conversion may result in a character that is not appropriate
in an IRI. See Section 2.2, Section 4.1, and Section 6.1 for
further details.
4. IRI to URI conversion has different rules for dealing with domain
names and query parameters.
Conversion from a URI to an IRI MAY be done by using the following
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.
4. Re-percent-encode all octets produced in step 3 that in UTF-8
represent characters that are not appropriate according to
Section 2.2, Section 4.1, and Section 6.1.
5. Interpret the resulting octet sequence as a sequence of characters
encoded in UTF-8.
6. URIs known to contain domain names in the reg-name component
SHOULD convert punycode-encoded domain name labels to the
corresponding characters using the ToUnicode procedure.
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
"" 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
"", which is a different
URI from "".
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3.7.1. Examples
This section shows various examples of converting URIs to IRIs. Each
example shows the result after each of the steps 1 through 6 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
The following example contains the sequence "%FC", which might
iso-8859-1 character encoding. (It might represent other characters
in other character encodings. For example, the octet <fc> in iso-
8859-5 represents U+045C, CYRILLIC SMALL LETTER KJE.) Because <fc>
is not part of a strictly legal UTF-8 sequence, it is re-percent-
encoded in step 3.
The following example contains "%e2%80%ae", which is the percent-
UTF-8 character encoding of U+202E, RIGHT-TO-LEFT OVERRIDE.
Section 4.1 forbids the direct use of this character in an IRI.
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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.
6. http://&#x7D0D;&#x8C46;
Note that the label "xn--99zt52a" is converted to U+7D0D U+8C46
(Japanese Natto). ((EDITOR NOTE: There is some inconsistency in this
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
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
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.
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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-
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.
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
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
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4.2. Bidi IRI Structure
The Unicode Bidirectional Algorithm is designed mainly for running
text. To make sure that it does not affect the rendering of
bidirectional IRIs too much, some restrictions on bidirectional IRIs
are necessary. These restrictions are given in terms of delimiters
(structural characters, mostly punctuation such as "@", ".", ":", and
"/") and components (usually consisting mostly of letters and
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.
For each component, the following restrictions apply:
1. A component SHOULD NOT use both right-to-left and left-to-right
2. A component using right-to-left characters SHOULD start and end
with right-to-left characters.
The above restrictions are given as "SHOULD"s, rather than as
"MUST"s. 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
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If the above restrictions cannot be satisfied otherwise, the affected
component can always be mapped to URI notation as described in
Section 3.3. 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.
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
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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: ""
Visual representation: ""
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 "", it may be
helpful to read aloud the visual representation following the bidi
algorithm. After "" one reads the RTL block
"E-F-slash-G-H", which corresponds to the logical representation.
Example 6: Same as Example 5, with more rtl components:
Logical representation: "http://ab.CD.EF/GH/IJ/kl.html"
Visual representation: "http://ab.JI/HG/FE.DC/kl.html"
The inversion of the domain name labels and the path components may
be easier to identify because the delimiters also move.
Example 7: A single rtl component 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
Logical representation: ""
Visual representation: ""
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
Logical representation: "",
Visual representation: ""
Example 10 (allowed but not recommended):
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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.
Example 11 (allowed but not recommended):
Logical representation: "http://ab.CDEFGH.123ij/kl/mn/op.html"
Visual representation: "http://ab.123.HGFEDCij/kl/mn/op.html"
Components consisting of numbers and left-to-right characters are
allowed, 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
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.
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 to determine
if they are "the same" 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
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of such comparisons, but there are many application-dependent
versions of equivalence.
Even when 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
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.
5.2. Preparation for Comparison
Any kind of IRI comparison REQUIRES that any additional contextual
processing is first performed, including undoing higher-level
escapings or encodings in the protocol or format that carries an IRI.
This preprocessing is usually done when the protocol or format is
Examples of contextual preprocessing steps are described in
Section 7.
Examples of such escapings or encodings are entities and numeric
character references in [HTML4] and [XML1]. As an example,
";" (in HTML),
";" (in HTML or XML), and
";" (in HTML or XML) are all resolved into
what is denoted in this document (see Section 1.4) as
";" (the "&#xE9;" 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
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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
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 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
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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 in a context that uses Simple String
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:
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. 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://" is equivalent to "".
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
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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. 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
IRIs already in Unicode MUST NOT be normalized before parsing or
interpreting. In many non-Unicode character encodings, some text
cannot be represented directly. For example, the word "Vietnam" is
natively written "Vi&#x1EC7;t 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 "Vi&#xEA;&#x323;t 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.
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, ";sum&#xE9;.html" (in XML
Notation) is in NFC. On the other hand,
";sume&#x301;.html" is not in NFC.
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].
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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 (for example, the 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. 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 nonencoded
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, "",
"", and "", 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 upper case). If the IRI is to be passed to another
application or used further in some other way, its original form MUST
be preserved. The conversion described here should be performed only
for local comparison.
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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:
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"
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,
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
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specification. For example, the IRI "" 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.
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://r&#xE9;sum&#xE9;" may be considered equivalent to
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
redirects to an IRI differing only in the trailing slash
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
common conventions of their scheme's dereference algorithm (in this
case, use of redirection by HTTP origin servers to avoid problems
with relative references).
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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 3.6, 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.
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
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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 3.6. 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.
((UPDATE THIS NOTE)) 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
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 [RFC4395].
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 URI scheme does not specify how to encode original
characters, only some HTTP URLs can have corresponding but different
For example, for a document with a URI of
"", it is possible to
construct a corresponding IRI (in XML notation, see Section 1.4):
";sum&#xE9;.html" ("&#xE9;" 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 "", the
percent-encoding octets cannot be converted to actual characters in
an IRI, as the percent-encoding is not based on UTF-8.
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
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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 updates the IANA registry of URI schemes to note
their applicability to IRIs, see Section 9. All IRIs use URI
schemes, and all URIs with URI schemes can be used as IRIs, even
though in some cases only by using URIs directly as IRIs, without any
Scheme definitions 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 3.6 MUST be a valid URI
under the syntactic restrictions of generic URI syntax and any
narrower restrictions imposed by the corresponding URI scheme
The requirement for the use of UTF-8 generally applies to all parts
of a URI. 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
"", 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
[XPointer]. The IRI corresponding to the above URI would be (in XML
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.
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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
7. Liberal handling of otherwise invalid IRIs
(EDITOR NOTE: This Section may move to an appendix.) Some technical
specifications and widely-deployed software have allowed additional
variations and extensions of IRIs to be used in syntactic components.
This section describes two widely-used preprocessing agreements.
Other technical specifications may wish to reference a syntactic
component which is "a valid IRI or a string that will map to a valid
IRI after this preprocessing algorithm". These two variants are
known as Legacy Extended IRI or LEIRI [LEIRI], and Web Address
Future technical specifications SHOULD NOT allow conforming producers
to produce, or conforming content to contain, such forms, as they are
not interoperable with other IRI consuming software.
7.1. LEIRI processing
This section defines Legacy Extended IRIs (LEIRIs). The syntax of
Legacy Extended IRIs is the same as that for IRIs, except that the
ucschar production is replaced by the leiri-ucschar production:
leiri-ucschar = " " / "<" / ">" / '"' / "{" / "}" / "|"
/ "\" / "^" / "`" / %x0-1F / %x7F-D7FF
/ %xE000-FFFD / %x10000-10FFFF
Among other extensions, processors based on this specification also
did not enforce the restriction on bidirectional formatting
characters in Section 4.1, and the iprivate production becomes
To convert a string allowed as a LEIRI to an IRI, each character
allowed in leiri-ucschar but not in ucschar must be percent-encoded
using Section 3.3.
7.2. Web Address processing
Many popular web browsers have taken the approach of being quite
liberal in what is accepted as a "URL" or its relative forms. This
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section describes their behavior in terms of a preprocessor which
maps strings into the IRI space for subsequent parsing and
interpretation as an IRI.
In some situations, it might be appropriate to describe the syntax
that a liberal consumer implementation might accept as a "Web
Address" or "Hypertext Reference" or "HREF". However, technical
specifications SHOULD restrict the syntactic form allowed by
compliant producers to the IRI or IRI reference syntax defined in
this document even if they want to mandate this processing.
o Leading and trailing whitespace is removed.
o Some additional characters are removed.
o Some additional characters are allowed and escaped (as with
o If interpreting an IRI as a URI, the pct-encoding of the query
component of the parsed URI component depends on operational
Each string provided may have an associated charset (called the HREF-
charset here); this defaults to UTF-8. For web browsers interpreting
HTML, the document charset of a string is determined:
If the string came from a script (e.g. as an argument to a method)
The HRef-charset is the script's charset.
If the string came from a DOM node (e.g. from an element) The node
has a Document, and the HRef-charset is the Document's character
If the string had a HRef-charset defined when the string was created
or defined The HRef-charset is as defined.
If the resulting HRef-charset is a unicode based character encoding
(e.g., UTF-16), then use UTF-8 instead.
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The syntax for Web Addresses is obtained by replacing the 'ucschar',
pct-form, and path-sep rules with the href-ucschar, href-pct-form,
and href-path-sep rules below. In addition, some characters are
href-ucschar = " " / "<" / ">" / '"' / "{" / "}" / "|"
/ "\" / "^" / "`" / %x0-1F / %x7F-D7FF
/ %xE000-FFFD / %x10000-10FFFF
href-pct-form = pct-encoded | "%"
href-path-sep = "/" | "\"
href-strip =
SENTENCE) browsers did not enforce the restriction on bidirectional
formatting characters in Section 4.1, and the iprivate production
becomes redundant.
'Web Address processing' requires the following additional
preprocessing steps:
1. Leading and trailing instances of space (U+0020), CR (U+000A), LF
(U+000D), and TAB (U+0009) characters are removed.
2. strip all characters in href-strip.
3. Percent-encode all characters in href-ucschar not in ucschar.
4. Replace occurrences of "%" not followed by two hexadecimal digits
by "%25".
5. Convert backslashes ('\') matching href-path-sep to forward
slashes ('/').
7.3. Characters not allowed in IRIs
This section provides a list of the groups of characters and code
points that are allowed by LEIRI or HREF but are not allowed in IRIs
or are allowed in IRIs only in the query part. For each group of
characters, advice on the usage of these characters is also given,
concentrating on the reasons for why they are excluded from IRI use.
Space (U+0020): Some formats and applications use space as a
delimiter, e.g. for items in a list. Appendix C of [RFC3986] also
mentions that white space may have to be added when displaying or
printing long URIs; the same applies to long IRIs. This means
that spaces can disappear, or can make the what is intended as a
single IRI or IRI reference to be treated as two or more separate
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Delimiters "<" (U+003C), ">" (U+003E), and '"' (U+0022): Appendix
C of [RFC3986] suggests the use of double-quotes
("") and angle brackets (<>)
as delimiters for URIs in plain text. These conventions are often
used, and also apply to IRIs. Using these characters in strings
intended to be IRIs would result in the IRIs being cut off at the
wrong place.
Unwise characters "\" (U+005C), "^" (U+005E), "`" (U+0060), "{"
(U+007B), "|" (U+007C), and "}" (U+007D): These characters
originally have been excluded from URIs because the respective
codepoints are assigned to different graphic characters in some
7-bit or 8-bit encoding. Despite the move to Unicode, some of
these characters are still occasionally displayed differently on
some systems, e.g. U+005C may appear as a Japanese Yen symbol on
some systems. Also, the fact that these characters are not used
in URIs or IRIs has encouraged their use outside URIs or IRIs in
contexts that may include URIs or IRIs. If a string with such a
character were used as an IRI in such a context, it would likely
be interpreted piecemeal.
The controls (C0 controls, DEL, and C1 controls, #x0 - #x1F #x7F -
#x9F): There is generally no way to transmit these characters
reliably as text outside of a charset encoding. Even when in
encoded form, many software components silently filter out some of
these characters, or may stop processing alltogether when
encountering some of them. These characters may affect text
display in subtle, unnoticable ways or in drastic, global, and
irreversible ways depending on the hardware and software involved.
The use of some of these characters would allow malicious users to
manipulate the display of an IRI and its context in many
Bidi formatting characters (U+200E, U+200F, U+202A-202E): These
characters affect the display ordering of characters. If IRIs
were allowed to contain these characters and the resulting visual
display transcribed. they could not be converted back to
electronic form (logical order) unambiguously. These characters,
if allowed in IRIs, might allow malicious users to manipulate the
display of IRI and its context.
Specials (U+FFF0-FFFD): These code points provide functionality
beyond that useful in an IRI, for example byte order
identification, annotation, and replacements for unknown
characters and objects. Their use and interpretation in an IRI
would serve no purpose and might lead to confusing display
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Private use code points (U+E000-F8FF, U+F0000-FFFFD, U+100000-
10FFFD): Display and interpretation of these code points is by
definition undefined without private agreement. Therefore, these
code points are not suited for use on the Internet. They are not
interoperable and may have unpredictable effects.
Tags (U+E0000-E0FFF): These characters provide a way to language
tag in Unicode plain text. They are not appropriate for IRIs
because language information in identifiers cannot reliably be
input, transmitted (e.g. on a visual medium such as paper), or
Non-characters (U+FDD0-FDEF, U+1FFFE-1FFFF, U+2FFFE-2FFFF,
U+FFFFE-FFFFF, U+10FFFE-10FFFF): These code points are defined as
non-characters. Applications may use some of them internally, but
are not prepared to interchange them.
LEIRI preprocessing disallowed some code points and code units:
Surrogate code units (D800-DFFF): These do not represent Unicode
8. 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
8.1. 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
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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 3.7 above may
be performed. It is preferable not to perform this inverse
conversion unless it is certain this can be done correctly.
8.2. 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
might 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
For the input of IRIs with right-to-left characters, please see
Section 4.3.
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8.3. URI/IRI Transfer between Applications
Many applications (for example, mail user agents) try to detect URIs
appearing in plain text. For this, they use some heuristics based on
URI syntax. They then allow the user to click on such URIs and
retrieve the corresponding resource in an appropriate (usually
scheme-dependent) application.
Such applications would need to be upgraded, in order 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 would need 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 3.6) if the system-wide invocation mechanism only accepts
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
octets, which will do the right thing with IRIs.
8.4. 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 "r&#xE9;sum&#
xE9;.html", a server should expose this as "r%C3%A9sum%C3%A9.html" in
a URI, which allows use of "r&#xE9;sum&#xE9;.html" in an IRI, even if
locally the file name is kept in a character encoding other than
This recommendation particularly applies to HTTP servers. For FTP
servers, similar considerations apply; see [RFC2640].
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8.5. 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,
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, IRIs should only 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.
8.6. 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
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For display of Bidi IRIs, please see Section 4.1.
8.7. 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.
8.8. 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
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.
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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.
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.
9. IANA Considerations
RFC Editor and IANA note: Please Replace RFC XXXX with the number of
this document when it issues as an RFC.
IANA maintains a registry of "URI schemes". A "URI scheme" also
serves an "IRI scheme".
To clarify that the URI scheme registration process also applies to
IRIs, change the description of the "URI schemes" registry header to
say "[RFC4395] defines an IANA-maintained registry of URI Schemes.
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These registries include the Permanent and Provisional URI Schemes.
RFC XXXX updates this registry to designate that schemes may also
indicate their usability as IRI schemes.
Update "per RFC 4395" to "per RFC 4395 and RFC XXXX".
10. Security Considerations
The security considerations discussed in [RFC3986] also apply to
IRIs. In addition, the following issues require particular care for
Incorrect encoding or decoding can lead to security problems. In
particular, some UTF-8 decoders do not check against overlong byte
sequences. As an example, a "/" is encoded with the byte 0x2F both
in UTF-8 and in US-ASCII, but some UTF-8 decoders also wrongly
interpret the sequence 0xC0 0xAF as a "/". A sequence such as
"%C0%AF.." may pass some security tests and then be interpreted as
"/.." in a path if UTF-8 decoders are fault-tolerant, if conversion
and checking are not done in the right order, and/or if reserved
characters and unreserved characters are not clearly distinguished.
There are various ways in which "spoofing" can occur with IRIs.
"Spoofing" means that somebody may add a resource name that looks the
same or similar to the user, but 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 ("") might be
"spoofed" by someone who is able to create
"". 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
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Spoofing can occur in various IRI components, such as the domain name
part or a path part. For considerations specific to the domain name
part, see [RFC3491]. For the path part, administrators of sites 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 8.5. 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.
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.
The use of Legacy Extended IRIs introduces additional security
11. Acknowledgements
For contributions to this update, we would like to thank Ian Hickson,
Michael Sperberg-McQueen, Dan Connolly, Norman Walsh, Richard Tobin,
Henry S. Thomson, and the XML Core Working Group of the W3C.
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.
For contributions to the previous version of this document, RFC 3987,
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
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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.
A definition of HyperText Reference was initially produced by Ian
Hixson, and further edited by Dan Connolly and C. M. Spergerg-
Thanks to the Internationalization Working Group (I18N WG) of the
World Wide Web Consortium (W3C), and 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
12. Open Issues
NOTE: The issues noted in this section should be addressed before the
document is submitted as an RFC. These issues are not in any
particular order.
length limits on domain name See, for example,
discussion on (that discussion is mostly
irrelevant now as the "63 octets in UTF-8 per label" restriction
was dropped)
Allow generic scheme-independent IRI to URI translation Previous
drafts of this specification proposed a generic IRI to URI
transformation using pct-encoding, and allowed domain name
translation to be optionally handled by retranslating host names
from pct-encoding back into Unicode and then into punycode. This
draft does not allow that behavior, but this should be fixed to be
in line with RFC 3986 syntax and to lead implementations towards
an uniform an long-term URI<->IRI correspondence. See also
update URI scheme registry? This document starts the process of
making minor changes to the URI scheme registry. This should be
handled as an update to RFC 4395.
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utf8 in HTTP Not really IRI issue, but some HTTP implementations
send UTF8 path directly, review.
handling of \\ Some web applications convert \ to / and others
don't. Make this mandatory or disallowed (but not optional), for
Web Addresses.
dealing with disallowed IRI characters
misplaced text Find a place to note that 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):
which contains the first three letters of the Old Italic alphabet,
the correct conversion to a URI is
Special Query Handling needed? The percent-encoding handling of
query components in the HTTP scheme is really unfortunate. There
is no good normative advice to give if the percent-encoding is
delayed until the query-IRI is interpreted. Could HTML ask
browsers to percent-encode the form data using the document
character set BEFORE the query IRI is constructed, and only in the
case where the document character set isn't Unicode-based and the
query is being added to http: or https: URIs? This would give
more consistent results. Browsers might have to change their
behavior in constructing the IRI-with-query-added, but the results
would be more consistent and fewer bugs, and it wouldn't affect
interpretation of any existing web pages. It would remove the
need to have a normative special case for queries in HTML
documents, just for http, in a way in which things like
transcoding etc. wouldn't work well. You could tell the
difference between a query URI in the address bar and one created
via a form because the address bar would always be UTF-8. The
browsers might have to change the algorithm for showing the
address in the adress bar to know how to undo the encoding.
handling illegal characters Section 3.3 used to apply only to
characters in either 'ucschar' or 'iprivate', but then later said
that systems accepting IRIs MAY also deal with the printable
characters in US-ASCII that are not allowed in URIs, namely "<",
">", '"', space, "{", "}", "|", "\", "^", and "`". Larry felt
that this a MAY would result in non-uniform behavior, because some
systems would produce valid URI components and others wouldn't.
Non-printable US-ASCII characters should be stripped by most
software, so if they get to if they're passed on somewhere as IRI
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characters, encoding them makes sense. The section also used to
say "If these characters are found but are not converted, then the
conversion SHOULD fail." but there is no notion of conversion
failing -- every string is converted. 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.
adding single % and hash Changed the BNF to not match the URI
document in allowing single % in path but not everywhere, and
allowing a # in the fragment part.
13. Change Log
Note to RFC Editor: Please completely remove this section before
13.1. Changes from -06 to this document
Major restructuring of IRI processing model to make scheme-specific
translation necessary to handle IDNA requirements and for consistency
with web implementations.
Starting with IRI, you want one of:
a IRI components (IRI parsed into UTF8 pieces)
b URI components (URI parsed into ASCII pieces, encoded correctly)
c whole URI (for passing on to some other system that wants whole
13.1.1. OLD WAY
1. Pct-encoding on the whole thing to a URI. (c1) If you want a
(maybe broken) whole URI, you might stop here.
2. Parsing the URI into URI components. (b1) If you want (maybe
broken) URI components, stop here.
3. Decode the components (undoing the pct-encoding). (a) if you want
IRI components, stop here.
4. reencode: Either using a different encoding some components (for
domain names, and query components in web pages, which depends on
the component, scheme and context), and otherwise using pct-
encoding. (b2) if you want (good) URI components, stop here.
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5. reassemble the reencoded components. (c2) if you want a (*good*)
whole URI stop here.
13.1.2. NEW WAY
1. Parse the IRI into IRI components using the generic syntax. (a)
if you want IRI components, stop here.
2. Encode each components, using pct-encoding, IDN encoding, or
special query part encoding depending on the component scheme or
context. (b) If you want URI components, stop here.
3. reassemble the a whole URI from URI components. (c) if you want a
whole URI stop here.
13.2. Changes from -05 to -06
o Add HyperText Reference, change abstract, acks and references for
o Add Masinter back as another editor.
o Masinter integrates HRef material from HTML5 spec.
o Rewrite introduction sections to modernize.
13.3. Changes from -04 to -05
o Updated references.
o Changed IPR text to pre5378Trust200902.
13.4. Changes from -03 to -04
o Added explicit abbreviation for LEIRIs.
o Mentioned LEIRI references.
o Completed text in LEIRI section about tag characters and about
13.5. Changes from -02 to -03
o Updated some references.
o Updated Michel Suginard's coordinates.
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13.6. Changes from -01 to -02
o Added tag range to iprivate (issue private-include-tags-115).
o Added Specials (U+FFF0-FFFD) to Legacy Extended IRIs.
13.7. Changes from -00 to -01
o Changed from "IRIs with Spaces/Controls" to "Legacy Extended IRI"
based on input from the W3C XML Core WG. Moved the relevant
subsections to the back and promoted them to a section.
o Added some text re. Legacy Extended IRIs to the security section.
o Added a IANA Consideration Section.
o Added this Change Log Section.
o Added a section about "IRIs with Spaces/Controls" (converting from
a Note in RFC 3987).
13.8. Changes from RFC 3987 to -00
Fixed errata (see
14. References
14.1. Normative References
[ASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
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.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
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[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.
[STD68] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[UNI9] Davis, M., "The Bidirectional Algorithm", Unicode Standard
Annex #9, March 2004,
[UNIV4] The Unicode Consortium, "The Unicode Standard, Version
5.1.0, defined by: The Unicode Standard, Version 5.0
(Boston, MA, Addison-Wesley, 2007. ISBN 0-321-48091-0), as
amended by Unicode 4.1.0
April 2008.
[UTR15] Davis, M. and M. Duerst, "Unicode Normalization Forms",
Unicode Standard Annex #15, March 2008,
14.2. Informative References
[BidiEx] "Examples of bidirectional IRIs",
[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,
Duerst, M., "The Properties and Promises of UTF-8", Proc.
11th International Unicode Conference, San Jose ,
September 1997, <
[Gettys] Gettys, J., "URI Model Consequences",
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Internet-Draft IRIs October 2009
[HTML4] Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01
Specification", World Wide Web Consortium Recommendation,
December 1999,
[HTML5] Hickson, I. and D. Hyatt, "A vocabulary and associated
APIs for HTML and XHTML", World Wide Web
Consortium Working Draft, April 2009,
[LEIRI] Thompson, H., Tobin, R., and N. Walsh, "Legacy extended
IRIs for XML resource identification", World Wide Web
Consortium Note, November 2008,
[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, December 1994.
[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.
[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.
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[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.
[RFC4395] Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
Registration Procedures for New URI Schemes", BCP 35,
RFC 4395, February 2006.
[UNIXML] Duerst, M. and A. Freytag, "Unicode in XML and other
Markup Languages", Unicode Technical Report #20, World
Wide Web Consortium Note, June 2003,
[XLink] DeRose, S., Maler, E., and D. Orchard, "XML Linking
Language (XLink) Version 1.0", World Wide Web
Consortium Recommendation, June 2001,
[XML1] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Forth
Edition)", World Wide Web Consortium Recommendation,
August 2006, <>.
Bray, T., Hollander, D., Layman, A., and R. Tobin,
"Namespaces in XML (Second Edition)", World Wide Web
Consortium Recommendation, August 2006,
Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes",
World Wide Web Consortium Recommendation, May 2001,
Grosso, P., Maler, E., Marsh, J., and N. Walsh, "XPointer
Framework", World Wide Web Consortium Recommendation,
March 2003,
Appendix A. Design Alternatives
This section briefly summarizes some design alternatives considered
earlier and the reasons why they were not chosen.
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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
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
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.
A.3. New Encoding Convention
Instead of using the existing percent-encoding convention of URIs,
which is based on octets, the idea was to create a new encoding
convention; for example, to use "%u" to introduce UCS code points.
Using the existing octet-based percent-encoding mechanism does not
need an upgrade of the URI syntax and does not need corresponding
server upgrades.
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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
"[iso-8859-1]&#xE9;" indicated that the
following "&#xE9;" 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
Authors' Addresses
Martin Duerst (Note: Please write "Duerst" with u-umlaut wherever
possible, for example as "D&amp;#252;rst" in XML and HTML.)
Aoyama Gakuin University
5-10-1 Fuchinobe
Sagamihara, Kanagawa 229-8558
Phone: +81 42 759 6329
Fax: +81 42 759 6495
(Note: This is the percent-encoded form of an IRI.)
Michel Suignard
Unicode Consortium
P.O. Box 391476
Mountain View, CA 94039-1476
Phone: +1-650-693-3921
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Larry Masinter
345 Park Ave
San Jose, CA 95110
Phone: +1-408-536-3024
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