Network Working Group M. Duerst

Request for Comments: 3987 W3C

Category: Standards Track M. Suignard

Microsoft Corporation

January 2005

Internationalized Resource Identifiers (IRIs)

Status of This Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the \"Internet

Official Protocol Standards\" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2005).

Abstract

This document defines a new protocol element, the Internationalized

Resource Identifier (IRI), as a complement to the Uniform Resource

Identifier (URI). An IRI is a sequence of characters from the

Universal Character Set (Unicode/ISO 10646). A mapping from IRIs to

URIs is defined, which means that IRIs can be used instead of URIs,

where appropriate, to identify resources.

The approach of defining a new protocol element was chosen instead of

extending or changing the definition of URIs. This was done in order

to allow a clear distinction and to avoid incompatibilities with

existing software. Guidelines are provided for the use and

deployment of IRIs in various protocols, formats, and software

components that currently deal with URIs.

Table of Contents

1\. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1. Overview and Motivation . . . . . . . . . . . . . . . . 3

1.2. Applicability . . . . . . . . . . . . . . . . . . . . . 3

1.3. Definitions . . . . . . . . . . . . . . . . . . . . . . 4

1.4. Notation . . . . . . . . . . . . . . . . . . . . . . . . 5

2\. IRI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1. Summary of IRI Syntax . . . . . . . . . . . . . . . . . 6

2.2. ABNF for IRI References and IRIs . . . . . . . . . . . . 7

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RFC 3987 Internationalized Resource Identifiers January 2005

3\. Relationship between IRIs and URIs . . . . . . . . . . . . . . 10

3.1. Mapping of IRIs to URIs . . . . . . . . . . . . . . . . 10

3.2. Converting URIs to IRIs . . . . . . . . . . . . . . . . 14

3.2.1. Examples . . . . . . . . . . . . . . . . . . . . 15

4\. Bidirectional IRIs for Right-to-Left Languages. . . . . . . . 16

4.1. Logical Storage and Visual Presentation . . . . . . . . 17

4.2. Bidi IRI Structure . . . . . . . . . . . . . . . . . . . 18

4.3. Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . 19

4.4. Examples . . . . . . . . . . . . . . . . . . . . . . . . 19

5\. Normalization and Comparison . . . . . . . . . . . . . . . . . 21

5.1. Equivalence . . . . . . . . . . . . . . . . . . . . . . 22

5.2. Preparation for Comparison . . . . . . . . . . . . . . . 22

5.3. Comparison Ladder . . . . . . . . . . . . . . . . . . . 23

5.3.1. Simple String Comparison . . . . . . . . . . . . 23

5.3.2. Syntax-Based Normalization . . . . . . . . . . . 24

5.3.3. Scheme-Based Normalization . . . . . . . . . . . 27

5.3.4. Protocol-Based Normalization . . . . . . . . . . 28

6\. Use of IRIs . . . . . . . . . . . . . . . . . . . . . . . . . 29

6.1. Limitations on UCS Characters Allowed in IRIs . . . . . 29

6.2. Software Interfaces and Protocols . . . . . . . . . . . 29

6.3. Format of URIs and IRIs in Documents and Protocols . . . 30

6.4. Use of UTF-8 for Encoding Original Characters .. . . . . 30

6.5. Relative IRI References . . . . . . . . . . . . . . . . 32

7\. URI/IRI Processing Guidelines (informative) . . . . . . . . . 32

7.1. URI/IRI Software Interfaces . . . . . . . . . . . . . . 32

7.2. URI/IRI Entry . . . . . . . . . . . . . . . . . . . . . 33

7.3. URI/IRI Transfer between Applications . . . . . . . . . 33

7.4. URI/IRI Generation . . . . . . . . . . . . . . . . . . . 34

7.5. URI/IRI Selection . . . . . . . . . . . . . . . . . . . 34

7.6. Display of URIs/IRIs . . . . . . . . . . . . . . . . . . 35

7.7. Interpretation of URIs and IRIs . . . . . . . . . . . . 36

7.8. Upgrading Strategy . . . . . . . . . . . . . . . . . . . 36

8\. Security Considerations . . . . . . . . . . . . . . . . . . . 37

9\. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 39

10\. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40

10.1. Normative References . . . . . . . . . . . . . . . . . . 40

10.2. Informative References . . . . . . . . . . . . . . . . . 41

A. Design Alternatives . . . . . . . . . . . . . . . . . . . . . 44

A.1. New Scheme(s) . . . . . . . . . . . . . . . . . . . . . 44

A.2. Character Encodings Other Than UTF-8 . . . . . . . . . . 44

A.3. New Encoding Convention . . . . . . . . . . . . . . . . 44

A.4. Indicating Character Encodings in the URI/IRI . . . . . 45

Authors\' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 45

Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 46

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RFC 3987 Internationalized Resource Identifiers January 2005

1\. Introduction

1.1. Overview and Motivation

A Uniform Resource Identifier (URI) is defined in \[RFC3986\] as a

sequence of characters chosen from a limited subset of the repertoire

of US-ASCII \[ASCII\] characters.

The characters in URIs are frequently used for representing words of

natural languages. This usage has many advantages: Such URIs are

easier to memorize, easier to interpret, easier to transcribe, easier

to create, and easier to guess. For most languages other than

English, however, the natural script uses characters other than A -

Z. For many people, handling Latin characters is as difficult as

handling the characters of other scripts is for those who use only

the Latin alphabet. Many languages with non-Latin scripts are

transcribed with Latin letters. These transcriptions are now often

used in URIs, but they introduce additional ambiguities.

The infrastructure for the appropriate handling of characters from

local scripts is now widely deployed in local versions of operating

system and application software. Software that can handle a wide

variety of scripts and languages at the same time is increasingly

common. Also, increasing numbers of protocols and formats can carry

a wide range of characters.

This document defines a new protocol element called Internationalized

Resource Identifier (IRI) by extending the syntax of URIs to a much

wider repertoire of characters. It also defines \"internationalized\"

versions corresponding to other constructs from \[RFC3986\], such as

URI references. The syntax of IRIs is defined in section 2, and the

relationship between IRIs and URIs in section 3.

Using characters outside of A - Z in IRIs brings some difficulties.

Section 4 discusses the special case of bidirectional IRIs, section 5

various forms of equivalence between IRIs, and section 6 the use of

IRIs in different situations. Section 7 gives additional informative

guidelines, and section 8 security considerations.

1.2. Applicability

IRIs are designed to be compatible with recommendations for new URI

schemes \[RFC2718\]. The compatibility is provided by specifying a

well-defined and deterministic mapping from the IRI character

sequence to the functionally equivalent URI character sequence.

Practical use of IRIs (or IRI references) in place of URIs (or URI

references) depends on the following conditions being met:

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a\. A protocol or format element should be explicitly designated to

be able to carry IRIs. The intent is not to introduce IRIs into

contexts that are not defined to accept them. For example, XML

schema \[XMLSchema\] has an explicit type \"anyURI\" that includes

IRIs and IRI references. Therefore, IRIs and IRI references can

be in attributes and elements of type \"anyURI\". On the other

hand, in the HTTP protocol \[RFC2616\], the Request URI is defined

as a URI, which means that direct use of IRIs is not allowed in

HTTP requests.

b\. The protocol or format carrying the IRIs should have a mechanism

to represent the wide range of characters used in IRIs, either

natively or by some protocol- or format-specific escaping

mechanism (for example, numeric character references in \[XML1\]).

c\. The URI corresponding to the IRI in question has to encode

original characters into octets using UTF-8. For new URI

schemes, this is recommended in \[RFC2718\]. It can apply to a

whole scheme (e.g., IMAP URLs \[RFC2192\] and POP URLs \[RFC2384\],

or the URN syntax \[RFC2141\]). It can apply to a specific part of

a URI, such as the fragment identifier (e.g., \[XPointer\]). It

can apply to a specific URI or part(s) thereof. For details,

please see section 6.4.

1.3. Definitions

The following definitions are used in this document; they follow the

terms in \[RFC2130\], \[RFC2277\], and \[ISO10646\].

character: A member of a set of elements used for the organization,

control, or representation of data. For example, \"LATIN CAPITAL

LETTER A\" names a character.

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

character repertoire: A set of characters (in the mathematical

sense).

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

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

character encoding: A method of representing a sequence of characters

as a sequence of octets (maybe with variants). Also, a method of

(unambiguously) converting a sequence of octets into a sequence of

characters.

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

character encoding.

UCS: Universal Character Set. The coded character set defined by

ISO/IEC 10646 \[ISO10646\] and the Unicode Standard \[UNIV4\].

IRI reference: Denotes the common usage of an Internationalized

Resource Identifier. An IRI reference may be absolute or

relative. However, the \"IRI\" that results from such a reference

only includes absolute IRIs; any relative IRI references are

resolved to their absolute form. Note that in \[RFC2396\] URIs did

not include fragment identifiers, but in \[RFC3986\] fragment

identifiers are part of URIs.

running text: Human text (paragraphs, sentences, phrases) with syntax

according to orthographic conventions of a natural language, as

opposed to syntax defined for ease of processing by machines

(e.g., markup, programming languages).

protocol element: Any portion of a message that affects processing of

that message by the protocol in question.

presentation element: A presentation form corresponding to a protocol

element; for example, using a wider range of characters.

create (a URI or IRI): With respect to URIs and IRIs, the term is

used for the initial creation. This may be the initial creation

of a resource with a certain identifier, or the initial exposition

of a resource under a particular identifier.

generate (a URI or IRI): With respect to URIs and IRIs, the term is

used when the IRI is generated by derivation from other

information.

1.4. Notation

RFCs and Internet Drafts currently do not allow any characters

outside the US-ASCII repertoire. Therefore, this document uses

various special notations to denote such characters in examples.

In text, characters outside US-ASCII are sometimes referenced by

using a prefix of \'U+\', followed by four to six hexadecimal digits.

To represent characters outside US-ASCII in examples, this document

uses two notations: \'XML Notation\' and \'Bidi Notation\'.

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XML Notation uses a leading \'&#x\', a trailing \';\', and the

hexadecimal number of the character in the UCS in between. For

example, &#x44F; stands for CYRILLIC CAPITAL LETTER YA. In this

notation, an actual \'&\' is denoted by \'&amp;\'.

Bidi Notation is used for bidirectional examples: Lowercase letters

stand for Latin letters or other letters that are written left to

right, whereas uppercase letters represent Arabic or Hebrew letters

that are written right to left.

To denote actual octets in examples (as opposed to percent-encoded

octets), the two hex digits denoting the octet are enclosed in \"\<\"

and \"\>\". For example, the octet often denoted as 0xc9 is denoted

here as \<c9\>.

In this document, the key words \"MUST\", \"MUST NOT\", \"REQUIRED\",

\"SHALL\", \"SHALL NOT\", \"SHOULD\", \"SHOULD NOT\", \"RECOMMENDED\",
\"MAY\",

and \"OPTIONAL\" are to be interpreted as described in \[RFC2119\].

2\. IRI Syntax

This section defines the syntax of Internationalized Resource

Identifiers (IRIs).

As with URIs, an IRI is defined as a sequence of characters, not as a

sequence of octets. This definition accommodates the fact that IRIs

may be written on paper or read over the radio as well as stored or

transmitted digitally. The same IRI may be represented as different

sequences of octets in different protocols or documents if these

protocols or documents use different character encodings (and/or

transfer encodings). Using the same character encoding as the

containing protocol or document ensures that the characters in the

IRI can be handled (e.g., searched, converted, displayed) in the same

way as the rest of the protocol or document.

2.1. Summary of IRI Syntax

IRIs are defined similarly to URIs in \[RFC3986\], but the class of

unreserved characters is extended by adding the characters of the UCS

(Universal Character Set, \[ISO10646\]) beyond U+007F, subject to the

limitations given in the syntax rules below and in section 6.1.

Otherwise, the syntax and use of components and reserved characters

is the same as that in \[RFC3986\]. All the operations defined in

\[RFC3986\], such as the resolution of relative references, can be

applied to IRIs by IRI-processing software in exactly the same way as

they are for URIs by URI-processing software.

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Characters outside the US-ASCII repertoire are not reserved and

therefore MUST NOT be used for syntactical purposes, such as to

delimit components in newly defined schemes. For example, U+00A2,

CENT SIGN, is not allowed as a delimiter in IRIs, because it is in

the \'iunreserved\' category. This is similar to the fact that it is

not possible to use \'-\' as a delimiter in URIs, because it is in the

\'unreserved\' category.

2.2. ABNF for IRI References and IRIs

Although it might be possible to define IRI references and IRIs

merely by their transformation to URI references and URIs, they can

also be accepted and processed directly. Therefore, an ABNF

definition for IRI references (which are the most general concept and

the start of the grammar) and IRIs is given here. The syntax of this

ABNF is described in \[RFC2234\]. Character numbers are taken from the

UCS, without implying any actual binary encoding. Terminals in the

ABNF are characters, not bytes.

The following grammar closely follows the URI grammar in \[RFC3986\],

except that the range of unreserved characters is expanded to include

UCS characters, with the restriction that private UCS characters can

occur only in query parts. The grammar is split into two parts:

Rules that differ from \[RFC3986\] because of the above-mentioned

expansion, and rules that are the same as those in \[RFC3986\]. For

rules that are different than those in \[RFC3986\], the names of the

non-terminals have been changed as follows. If the non-terminal

contains \'URI\', this has been changed to \'IRI\'. Otherwise, an \'i\'

has been prefixed.

The following rules are different from those in \[RFC3986\]:

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

\[ \"#\" ifragment \]

ihier-part = \"//\" iauthority ipath-abempty

/ ipath-absolute

/ ipath-rootless

/ ipath-empty

IRI-reference = IRI / irelative-ref

absolute-IRI = scheme \":\" ihier-part \[ \"?\" iquery \]

irelative-ref = irelative-part \[ \"?\" iquery \] \[ \"#\" ifragment \]

irelative-part = \"//\" iauthority ipath-abempty

/ ipath-absolute

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/ ipath-noscheme

/ ipath-empty

iauthority = \[ iuserinfo \"@\" \] ihost \[ \":\" port \]

iuserinfo = \*( iunreserved / pct-encoded / sub-delims / \":\" )

ihost = IP-literal / IPv4address / ireg-name

ireg-name = \*( iunreserved / pct-encoded / sub-delims )

ipath = ipath-abempty ; begins with \"/\" or is empty

/ ipath-absolute ; begins with \"/\" but not \"//\"

/ ipath-noscheme ; begins with a non-colon segment

/ ipath-rootless ; begins with a segment

/ ipath-empty ; zero characters

ipath-abempty = \*( \"/\" isegment )

ipath-absolute = \"/\" \[ isegment-nz \*( \"/\" isegment ) \]

ipath-noscheme = isegment-nz-nc \*( \"/\" isegment )

ipath-rootless = isegment-nz \*( \"/\" isegment )

ipath-empty = 0\<ipchar\>

isegment = \*ipchar

isegment-nz = 1\*ipchar

isegment-nz-nc = 1\*( iunreserved / pct-encoded / sub-delims

/ \"@\" )

; non-zero-length segment without any colon \":\"

ipchar = iunreserved / pct-encoded / sub-delims / \":\"

/ \"@\"

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

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

iunreserved = ALPHA / DIGIT / \"-\" / \".\" / \"\_\" / \"\~\" / ucschar

ucschar = %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF

/ %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD

/ %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD

/ %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD

/ %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD

/ %xD0000-DFFFD / %xE1000-EFFFD

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

Some productions are ambiguous. The \"first-match-wins\" (a.k.a.

\"greedy\") algorithm applies. For details, see \[RFC3986\].

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The following rules are the same as those in \[RFC3986\]:

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

port = \*DIGIT

IP-literal = \"\[\" ( IPv6address / IPvFuture ) \"\]\"

IPvFuture = \"v\" 1\*HEXDIG \".\" 1\*( unreserved / sub-delims / \":\" )

IPv6address = 6( h16 \":\" ) ls32

/ \"::\" 5( h16 \":\" ) ls32

/ \[ h16 \] \"::\" 4( h16 \":\" ) ls32

/ \[ \*1( h16 \":\" ) h16 \] \"::\" 3( h16 \":\" ) ls32

/ \[ \*2( h16 \":\" ) h16 \] \"::\" 2( h16 \":\" ) ls32

/ \[ \*3( h16 \":\" ) h16 \] \"::\" h16 \":\" ls32

/ \[ \*4( h16 \":\" ) h16 \] \"::\" ls32

/ \[ \*5( h16 \":\" ) h16 \] \"::\" h16

/ \[ \*6( h16 \":\" ) h16 \] \"::\"

h16 = 1\*4HEXDIG

ls32 = ( h16 \":\" h16 ) / IPv4address

IPv4address = dec-octet \".\" dec-octet \".\" dec-octet \".\" dec-octet

dec-octet = DIGIT ; 0-9

/ %x31-39 DIGIT ; 10-99

/ \"1\" 2DIGIT ; 100-199

/ \"2\" %x30-34 DIGIT ; 200-249

/ \"25\" %x30-35 ; 250-255

pct-encoded = \"%\" HEXDIG HEXDIG

unreserved = ALPHA / DIGIT / \"-\" / \".\" / \"\_\" / \"\~\"

reserved = gen-delims / sub-delims

gen-delims = \":\" / \"/\" / \"?\" / \"#\" / \"\[\" / \"\]\" / \"@\"

sub-delims = \"!\" / \"\$\" / \"&\" / \"\'\" / \"(\" / \")\"

/ \"\*\" / \"+\" / \",\" / \";\" / \"=\"

This syntax does not support IPv6 scoped addressing zone identifiers.

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3\. Relationship between IRIs and URIs

IRIs are meant to replace URIs in identifying resources for

protocols, formats, and software components that use a UCS-based

character repertoire. These protocols and components may never need

to use URIs directly, especially when the resource identifier is used

simply for identification purposes. However, when the resource

identifier is used for resource retrieval, it is in many cases

necessary to determine the associated URI, because currently most

retrieval mechanisms are only defined for URIs. In this case, IRIs

can serve as presentation elements for URI protocol elements. An

example would be an address bar in a Web user agent. (Additional

rationale is given in section 3.1.)

3.1. Mapping of IRIs to URIs

This section defines how to map an IRI to a URI. Everything in this

section also applies to IRI references and URI references, as well as

to components thereof (for example, fragment identifiers).

This mapping has two purposes:

Syntaxical. Many URI schemes and components define additional

syntactical restrictions not captured in section 2.2.

Scheme-specific restrictions are applied to IRIs by converting

IRIs to URIs and checking the URIs against the scheme-specific

restrictions.

Interpretational. URIs identify resources in various ways. IRIs also

identify resources. When the IRI is used solely for

identification purposes, it is not necessary to map the IRI to a

URI (see section 5). However, when an IRI is used for resource

retrieval, the resource that the IRI locates is the same as the

one located by the URI obtained after converting the IRI according

to the procedure defined here. This means that there is no need

to define resolution separately on the IRI level.

Applications MUST map IRIs to URIs by using the following two steps.

Step 1. Generate a UCS character sequence from the original IRI

format. This step has the following three variants,

depending on the form of the input:

a\. If the IRI is written on paper, read aloud, or otherwise

represented as a sequence of characters independent of

any character encoding, represent the IRI as a sequence

of characters from the UCS normalized according to

Normalization Form C (NFC, \[UTR15\]).

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b\. If the IRI is in some digital representation (e.g., an

octet stream) in some known non-Unicode character

encoding, convert the IRI to a sequence of characters

from the UCS normalized according to NFC.

c\. If the IRI is in a Unicode-based character encoding (for

example, UTF-8 or UTF-16), do not normalize (see section

5.3.2.2 for details). Apply step 2 directly to the

encoded Unicode character sequence.

Step 2. For each character in \'ucschar\' or \'iprivate\', apply steps

2.1 through 2.3 below.

2.1. Convert the character to a sequence of one or more octets

using UTF-8 \[RFC3629\].

2.2. Convert each octet to %HH, where HH is the hexadecimal

notation of the octet value. Note that this is identical

to the percent-encoding mechanism in section 2.1 of

\[RFC3986\]. To reduce variability, the hexadecimal notation

SHOULD use uppercase letters.

2.3. Replace the original character with the resulting character

sequence (i.e., a sequence of %HH triplets).

The above mapping from IRIs to URIs produces URIs fully conforming to

\[RFC3986\]. The mapping is also an identity transformation for URIs

and is idempotent; applying the mapping a second time will not

change anything. Every URI is by definition an IRI.

Systems accepting IRIs MAY convert the ireg-name component of an IRI

as follows (before step 2 above) for schemes known to use domain

names in ireg-name, if the scheme definition does not allow

percent-encoding for ireg-name:

Replace the ireg-name part of the IRI by the part converted using the

ToASCII operation specified in section 4.1 of \[RFC3490\] on each

dot-separated label, and by using U+002E (FULL STOP) as a label

separator, with the flag UseSTD3ASCIIRules set to TRUE, and with the

flag AllowUnassigned set to FALSE for creating IRIs and set to TRUE

otherwise.

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The ToASCII operation may fail, but this would mean that the IRI

cannot be resolved. This conversion SHOULD be used when the goal is

to maximize interoperability with legacy URI resolvers. For example,

the IRI

\"http://r&#xE9;sum&#xE9;.example.org\"

may be converted to

\"http://xn\--rsum-bpad.example.org\"

instead of

\"http://r%C3%A9sum%C3%A9.example.org\".

An IRI with a scheme that is known to use domain names in ireg-name,

but where the scheme definition does not allow percent-encoding for

ireg-name, meets scheme-specific restrictions if either the

straightforward conversion or the conversion using the ToASCII

operation on ireg-name result in an URI that meets the scheme-

specific restrictions.

Such an IRI resolves to the URI obtained after converting the IRI and

uses the ToASCII operation on ireg-name. Implementations do not have

to do this conversion as long as they produce the same result.

Note: The difference between variants b and c in step 1 (using

normalization with NFC, versus not using any normalization)

accounts for the fact that in many non-Unicode character

encodings, some text cannot be represented directly. For example,

the word \"Vietnam\" is natively written \"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.

Note: The uniform treatment of the whole IRI in step 2 is important

to make processing independent of URI scheme. See \[Gettys\] for an

in-depth discussion.

Note: In practice, whether the general mapping (steps 1 and 2) or the

ToASCII operation of \[RFC3490\] is used for ireg-name will not be

noticed if mapping from IRI to URI and resolution is tightly

integrated (e.g., carried out in the same user agent). But

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conversion using \[RFC3490\] may be able to better deal with

backwards compatibility issues in case mapping and resolution are

separated, as in the case of using an HTTP proxy.

Note: Internationalized Domain Names may be contained in parts of an

IRI other than the ireg-name part. It is the responsibility of

scheme-specific implementations (if the Internationalized Domain

Name is part of the scheme syntax) or of server-side

implementations (if the Internationalized Domain Name is part of

\'iquery\') to apply the necessary conversions at the appropriate

point. Example: Trying to validate the Web page at

http://r&#xE9;sum&#xE9;.example.org would lead to an IRI of

http://validator.w3.org/check?uri=http%3A%2F%2Fr&#xE9;sum&#xE9;.

example.org, which would convert to a URI of

http://validator.w3.org/check?uri=http%3A%2F%2Fr%C3%A9sum%C3%A9.

example.org. The server side implementation would be responsible

for making the necessary conversions to be able to retrieve the

Web page.

Systems accepting IRIs MAY also deal with the printable characters in

US-ASCII that are not allowed in URIs, namely \"\<\", \"\>\", \'\"\',
space,

\"{\", \"}\", \"\|\", \"\\\", \"\^\", and \"\`\", in step 2 above. If
these

characters are found but are not converted, then the conversion

SHOULD fail. Please note that the number sign (\"#\"), the percent

sign (\"%\"), and the square bracket characters (\"\[\", \"\]\") are not
part

of the above list and MUST NOT be converted. Protocols and formats

that have used earlier definitions of IRIs including these characters

MAY require percent-encoding of these characters as a preprocessing

step to extract the actual IRI from a given field. This

preprocessing MAY also be used by applications allowing the user to

enter an IRI.

Note: In this process (in step 2.3), characters allowed in URI

references and existing percent-encoded sequences are not encoded

further. (This mapping is similar to, but different from, the

encoding applied when arbitrary content is included in some part

of a URI.) For example, an IRI of

\"http://www.example.org/red%09ros&#xE9;#red\" (in XML notation) is

converted to

\"http://www.example.org/red%09ros%C3%A9#red\", not to something

like

\"http%3A%2F%2Fwww.example.org%2Fred%2509ros%C3%A9%23red\".

Note: Some older software transcoding to UTF-8 may produce illegal

output for some input, in particular for characters outside the

BMP (Basic Multilingual Plane). As an example, for the IRI with

non-BMP characters (in XML Notation):

\"http://example.com/&#x10300;&#x10301;&#x10302\";

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which contains the first three letters of the Old Italic alphabet,

the correct conversion to a URI is

\"http://example.com/%F0%90%8C%80%F0%90%8C%81%F0%90%8C%82\"

3.2. Converting URIs to IRIs

In some situations, converting a URI into an equivalent IRI may be

desirable. This section gives a procedure for this conversion. The

conversion described in this section will always result in an IRI

that maps back to the URI used as an input for the conversion (except

for potential case differences in percent-encoding and for potential

percent-encoded unreserved characters). However, the IRI resulting

from this conversion may not be exactly the same as the original IRI

(if there ever was one).

URI-to-IRI conversion removes percent-encodings, but not all

percent-encodings can be eliminated. There are several reasons for

this:

1\. Some percent-encodings are necessary to distinguish percent-

encoded and unencoded uses of reserved characters.

2\. Some percent-encodings cannot be interpreted as sequences of

UTF-8 octets.

(Note: The octet patterns of UTF-8 are highly regular.

Therefore, there is a very high probability, but no guarantee,

that percent-encodings that can be interpreted as sequences of

UTF-8 octets actually originated from UTF-8. For a detailed

discussion, see \[Duerst97\].)

3\. The conversion may result in a character that is not appropriate

in an IRI. See sections 2.2, 4.1, and 6.1 for further details.

Conversion from a URI to an IRI is done by using the following steps

(or any other algorithm that produces the same result):

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

2\. Convert all percent-encodings (\"%\" followed by two hexadecimal

digits) to the corresponding octets, except those corresponding

to \"%\", characters in \"reserved\", and characters in US-ASCII not

allowed in URIs.

3\. Re-percent-encode any octet produced in step 2 that is not part

of a strictly legal UTF-8 octet sequence.

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4\. Re-percent-encode all octets produced in step 3 that in UTF-8

represent characters that are not appropriate according to

sections 2.2, 4.1, and 6.1.

5\. Interpret the resulting octet sequence as a sequence of characters

encoded in UTF-8.

This procedure will convert as many percent-encoded characters as

possible to characters in an IRI. Because there are some choices

when step 4 is applied (see section 6.1), results may vary.

Conversions from URIs to IRIs MUST NOT use any character encoding

other than UTF-8 in steps 3 and 4, even if it might be possible to

guess from the context that another character encoding than UTF-8 was

used in the URI. For example, the URI

\"http://www.example.org/r%E9sum%E9.html\" might with some guessing be

interpreted to contain two e-acute characters encoded as iso-8859-1.

It must not be converted to an IRI containing these e-acute

characters. Otherwise, in the future the IRI will be mapped to

\"http://www.example.org/r%C3%A9sum%C3%A9.html\", which is a different

URI from \"http://www.example.org/r%E9sum%E9.html\".

3.2.1. Examples

This section shows various examples of converting URIs to IRIs. Each

example shows the result after each of the steps 1 through 5 is

applied. XML Notation is used for the final result. Octets are

denoted by \"\<\" followed by two hexadecimal digits followed by \"\>\".

The following example contains the sequence \"%C3%BC\", which is a

strictly legal UTF-8 sequence, and which is converted into the actual

character U+00FC, LATIN SMALL LETTER U WITH DIAERESIS (also known as

u-umlaut).

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

2\. http://www.example.org/D\<c3\>\<bc\>rst

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

4\. http://www.example.org/D\<c3\>\<bc\>rst

5\. http://www.example.org/D&#xFC;rst

The following example contains the sequence \"%FC\", which might

represent U+00FC, LATIN SMALL LETTER U WITH DIAERESIS, in the

iso-8859-1 character encoding. (It might represent other characters

in other character encodings. For example, the octet \<fc\> in

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iso-8859-5 represents U+045C, CYRILLIC SMALL LETTER KJE.) Because

\<fc\> is not part of a strictly legal UTF-8 sequence, it is

re-percent-encoded in step 3.

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

2\. http://www.example.org/D\<fc\>rst

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

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

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

The following example contains \"%e2%80%ae\", which is the percent-

encoded UTF-8 character encoding of U+202E, RIGHT-TO-LEFT OVERRIDE.

Section 4.1 forbids the direct use of this character in an IRI.

Therefore, the corresponding octets are re-percent-encoded in step 4.

This example shows that the case (upper- or lowercase) of letters

used in percent-encodings may not be preserved. The example also

contains a punycode-encoded domain name label (xn\--99zt52a), which is

not converted.

1\. http://xn\--99zt52a.example.org/%e2%80%ae

2\. http://xn\--99zt52a.example.org/\<e2\>\<80\>\<ae\>

3\. http://xn\--99zt52a.example.org/\<e2\>\<80\>\<ae\>

4\. http://xn\--99zt52a.example.org/%E2%80%AE

5\. http://xn\--99zt52a.example.org/%E2%80%AE

Implementations with scheme-specific knowledge MAY convert

punycode-encoded domain name labels to the corresponding characters

by using the ToUnicode procedure. Thus, for the example above, the

label \"xn\--99zt52a\" may be converted to U+7D0D U+8C46 (Japanese

Natto), leading to the overall IRI of

\"http://&#x7D0D;&#x8C46;.example.org/%E2%80%AE\".

4\. Bidirectional IRIs for Right-to-Left Languages

Some UCS characters, such as those used in the Arabic and Hebrew

scripts, have an inherent right-to-left (rtl) writing direction.

IRIs containing these characters (called bidirectional IRIs or Bidi

IRIs) require additional attention because of the non-trivial

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relation between logical representation (used for digital

representation and for reading/spelling) and visual representation

(used for display/printing).

Because of the complex interaction between the logical

representation, the visual representation, and the syntax of a Bidi

IRI, a balance is needed between various requirements. The main

requirements are

1\. user-predictable conversion between visual and logical

representation;

2\. the ability to include a wide range of characters in various

parts of the IRI; and

3\. minor or no changes or restrictions for implementations.

4.1. Logical Storage and Visual Presentation

When stored or transmitted in digital representation, bidirectional

IRIs MUST be in full logical order and MUST conform to the IRI syntax

rules (which includes the rules relevant to their scheme). This

ensures that bidirectional IRIs can be processed in the same way as

other IRIs.

Bidirectional IRIs MUST be rendered by using the Unicode

Bidirectional Algorithm \[UNIV4\], \[UNI9\]. Bidirectional IRIs MUST be

rendered in the same way as they would be if they were in a

left-to-right embedding; i.e., as if they were preceded by U+202A,

LEFT-TO-RIGHT EMBEDDING (LRE), and followed by U+202C, POP

DIRECTIONAL FORMATTING (PDF). Setting the embedding direction can

also be done in a higher-level protocol (e.g., the dir=\'ltr\'

attribute in HTML).

There is no requirement to use the above embedding if the display is

still the same without the embedding. For example, a bidirectional

IRI in a text with left-to-right base directionality (such as used

for English or Cyrillic) that is preceded and followed by whitespace

and strong left-to-right characters does not need an embedding.

Also, a bidirectional relative IRI reference that only contains

strong right-to-left characters and weak characters and that starts

and ends with a strong right-to-left character and appears in a text

with right-to-left base directionality (such as used for Arabic or

Hebrew) and is preceded and followed by whitespace and strong

characters does not need an embedding.

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In some other cases, using U+200E, LEFT-TO-RIGHT MARK (LRM), may be

sufficient to force the correct display behavior. However, the

details of the Unicode Bidirectional algorithm are not always easy to

understand. Implementers are strongly advised to err on the side of

caution and to use embedding in all cases where they are not

completely sure that the display behavior is unaffected without the

embedding.

The Unicode Bidirectional Algorithm (\[UNI9\], section 4.3) permits

higher-level protocols to influence bidirectional rendering. Such

changes by higher-level protocols MUST NOT be used if they change the

rendering of IRIs.

The bidirectional formatting characters that may be used before or

after the IRI to ensure correct display are not themselves part of

the IRI. IRIs MUST NOT contain bidirectional formatting characters

(LRM, RLM, LRE, RLE, LRO, RLO, and PDF). They affect the visual

rendering of the IRI but do not appear themselves. It would

therefore not be possible to input an IRI with such characters

correctly.

4.2. Bidi IRI Structure

The Unicode Bidirectional Algorithm is designed mainly for running

text. To make sure that it does not affect the rendering of

bidirectional IRIs too much, some restrictions on bidirectional IRIs

are necessary. These restrictions are given in terms of delimiters

(structural characters, mostly punctuation such as \"@\", \".\", \":\",
and

\"/\") and components (usually consisting mostly of letters and

digits).

The following syntax rules from section 2.2 correspond to components

for the purpose of Bidi behavior: iuserinfo, ireg-name, isegment,

isegment-nz, isegment-nz-nc, ireg-name, iquery, and ifragment.

Specifications that define the syntax of any of the above components

MAY divide them further and define smaller parts to be components

according to this document. As an example, the restrictions of

\[RFC3490\] on bidirectional domain names correspond to treating each

label of a domain name as a component for schemes with ireg-name as a

domain name. Even where the components are not defined formally, it

may be helpful to think about some syntax in terms of components and

to apply the relevant restrictions. For example, for the usual

name/value syntax in query parts, it is convenient to treat each name

and each value as a component. As another example, the extensions in

a resource name can be treated as separate components.

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For each component, the following restrictions apply:

1\. A component SHOULD NOT use both right-to-left and left-to-right

characters.

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

with right-to-left characters.

The above restrictions are given as shoulds, rather than as musts.

For IRIs that are never presented visually, they are not relevant.

However, for IRIs in general, they are very important to ensure

consistent conversion between visual presentation and logical

representation, in both directions.

Note: In some components, the above restrictions may actually be

strictly enforced. For example, \[RFC3490\] requires that these

restrictions apply to the labels of a host name for those schemes

where ireg-name is a host name. In some other components (for

example, path components) following these restrictions may not be

too difficult. For other components, such as parts of the query

part, it may be very difficult to enforce the restrictions because

the values of query parameters may be arbitrary character

sequences.

If the above restrictions cannot be satisfied otherwise, the affected

component can always be mapped to URI notation as described in

section 3.1. Please note that the whole component has to be mapped

(see also Example 9 below).

4.3. Input of Bidi IRIs

Bidi input methods MUST generate Bidi IRIs in logical order while

rendering them according to section 4.1. During input, rendering

SHOULD be updated after every new character is input to avoid end-

user confusion.

4.4. Examples

This section gives examples of bidirectional IRIs, in Bidi Notation.

It shows legal IRIs with the relationship between logical and visual

representation and explains how certain phenomena in this

relationship may look strange to somebody not familiar with

bidirectional behavior, but familiar to users of Arabic and Hebrew.

It also shows what happens if the restrictions given in section 4.2

are not followed. The examples below can be seen at \[BidiEx\], in

Arabic, Hebrew, and Bidi Notation variants.

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To read the bidi text in the examples, read the visual representation

from left to right until you encounter a block of rtl text. Read the

rtl block (including slashes and other special characters) from right

to left, then continue at the next unread ltr character.

Example 1: A single component with rtl characters is inverted:

Logical representation: \"http://ab.CDEFGH.ij/kl/mn/op.html\"

Visual representation: \"http://ab.HGFEDC.ij/kl/mn/op.html\"

Components can be read one by one, and each component can be read in

its natural direction.

Example 2: More than one consecutive component with rtl characters is

inverted as a whole:

Logical representation: \"http://ab.CDE.FGH/ij/kl/mn/op.html\"

Visual representation: \"http://ab.HGF.EDC/ij/kl/mn/op.html\"

A sequence of rtl components is read rtl, in the same way as a

sequence of rtl words is read rtl in a bidi text.

Example 3: All components of an IRI (except for the scheme) are rtl.

All rtl components are inverted overall:

Logical representation: \"http://AB.CD.EF/GH/IJ/KL?MN=OP;QR=ST#UV\"

Visual representation: \"http://VU#TS=RQ;PO=NM?LK/JI/HG/FE.DC.BA\"

The whole IRI (except the scheme) is read rtl. Delimiters between

rtl components stay between the respective components; delimiters

between ltr and rtl components don\'t move.

Example 4: Each of several sequences of rtl components is inverted on

its own:

Logical representation: \"http://AB.CD.ef/gh/IJ/KL.html\"

Visual representation: \"http://DC.BA.ef/gh/LK/JI.html\"

Each sequence of rtl components is read rtl, in the same way as each

sequence of rtl words in an ltr text is read rtl.

Example 5: Example 2, applied to components of different kinds:

Logical representation: \"http://ab.cd.EF/GH/ij/kl.html\"

Visual representation: \"http://ab.cd.HG/FE/ij/kl.html\"

The inversion of the domain name label and the path component may be

unexpected, but it is consistent with other bidi behavior. For

reassurance that the domain component really is \"ab.cd.EF\", it may be

helpful to read aloud the visual representation following the bidi

algorithm. After \"http://ab.cd.\" one reads the RTL block

\"E-F-slash-G-H\", which corresponds to the logical representation.

Example 6: Same as Example 5, with more rtl components:

Logical representation: \"http://ab.CD.EF/GH/IJ/kl.html\"

Visual representation: \"http://ab.JI/HG/FE.DC/kl.html\"

The inversion of the domain name labels and the path components may

be easier to identify because the delimiters also move.

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Example 7: A single rtl component includes digits:

Logical representation: \"http://ab.CDE123FGH.ij/kl/mn/op.html\"

Visual representation: \"http://ab.HGF123EDC.ij/kl/mn/op.html\"

Numbers are written ltr in all cases but are treated as an additional

embedding inside a run of rtl characters. This is completely

consistent with usual bidirectional text.

Example 8 (not allowed): Numbers are at the start or end of an rtl

component:

Logical representation: \"http://ab.cd.ef/GH1/2IJ/KL.html\"

Visual representation: \"http://ab.cd.ef/LK/JI1/2HG.html\"

The sequence \"1/2\" is interpreted by the bidi algorithm as a

fraction, fragmenting the components and leading to confusion. There

are other characters that are interpreted in a special way close to

numbers; in particular, \"+\", \"-\", \"#\", \"\$\", \"%\", \",\",
\".\", and \":\".

Example 9 (not allowed): The numbers in the previous example are

percent-encoded:

Logical representation: \"http://ab.cd.ef/GH%31/%32IJ/KL.html\",

Visual representation (Hebrew): \"http://ab.cd.ef/%31HG/LK/JI%32.html\"

Visual representation (Arabic): \"http://ab.cd.ef/31%HG/%LK/JI32.html\"

Depending on whether the uppercase letters represent Arabic or

Hebrew, the visual representation is different.

Example 10 (allowed but not recommended):

Logical representation: \"http://ab.CDEFGH.123/kl/mn/op.html\"

Visual representation: \"http://ab.123.HGFEDC/kl/mn/op.html\"

Components consisting of only numbers are allowed (it would be rather

difficult to prohibit them), but these may interact with adjacent RTL

components in ways that are not easy to predict.

5\. Normalization and Comparison

Note: The structure and much of the material for this section is

taken from section 6 of \[RFC3986\]; the differences are due to the

specifics of IRIs.

One of the most common operations on IRIs is simple comparison:

Determining whether two IRIs are equivalent without using the IRIs or

the mapped URIs to access their respective resource(s). A comparison

is performed whenever a response cache is accessed, a browser checks

its history to color a link, or an XML parser processes tags within a

namespace. Extensive normalization prior to comparison of IRIs may

be used by spiders and indexing engines to prune a search space or

reduce duplication of request actions and response storage.

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IRI comparison is performed for some particular purpose. Protocols

or implementations that compare IRIs for different purposes will

often be subject to differing design trade-offs in regards to how

much effort should be spent in reducing aliased identifiers. This

section describes various methods that may be used to compare IRIs,

the trade-offs between them, and the types of applications that might

use them.

5.1. Equivalence

Because IRIs exist to identify resources, presumably they should be

considered equivalent when they identify the same resource. However,

this definition of equivalence is not of much practical use, as there

is no way for an implementation to compare two resources unless it

has full knowledge or control of them. For this reason, determination

of equivalence or difference of IRIs is based on string comparison,

perhaps augmented by reference to additional rules provided by URI

scheme definitions. We use the terms \"different\" and \"equivalent\" to

describe the possible outcomes of such comparisons, but there are

many application-dependent versions of equivalence.

Even though it is possible to determine that two IRIs are equivalent,

IRI comparison is not sufficient to determine whether two IRIs

identify different resources. For example, an owner of two different

domain names could decide to serve the same resource from both,

resulting in two different IRIs. Therefore, comparison methods are

designed to minimize false negatives while strictly avoiding false

positives.

In testing for equivalence, applications should not directly compare

relative references; the references should be converted to their

respective target IRIs before comparison. When IRIs are compared to

select (or avoid) a network action, such as retrieval of a

representation, fragment components (if any) should be excluded from

the comparison.

Applications using IRIs as identity tokens with no relationship to a

protocol MUST use the Simple String Comparison (see section 5.3.1).

All other applications MUST select one of the comparison practices

from the Comparison Ladder (see section 5.3 or, after IRI-to-URI

conversion, select one of the comparison practices from the URI

comparison ladder in \[RFC3986\], section 6.2)

5.2. Preparation for Comparison

Any kind of IRI comparison REQUIRES that all escapings or encodings

in the protocol or format that carries an IRI are resolved. This is

usually done when the protocol or format is parsed. Examples of such

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escapings or encodings are entities and numeric character references

in \[HTML4\] and \[XML1\]. As an example,

\"http://example.org/ros&eacute;\" (in HTML),

\"http://example.org/ros&#233\"; (in HTML or XML), and

\"http://example.org/ros&#xE9\"; (in HTML or XML) are all resolved into

what is denoted in this document (see section 1.4) as

\"http://example.org/ros&#xE9\"; (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

sure that characters, and not just arbitrary octets, are compared

(see section 5.3.1).

5.3. Comparison Ladder

In practice, a variety of methods are used, to test IRI equivalence.

These methods fall into a range distinguished by the amount of

processing required and the degree to which the probability of false

negatives is reduced. As noted above, false negatives cannot be

eliminated. In practice, their probability can be reduced, but this

reduction requires more processing and is not cost-effective for all

applications.

If this range of comparison practices is considered as a ladder, the

following discussion will climb the ladder, starting with practices

that are cheap but have a relatively higher chance of producing false

negatives, and proceeding to those that have higher computational

cost and lower risk of false negatives.

5.3.1. Simple String Comparison

If two IRIs, when considered as character strings, are identical,

then it is safe to conclude that they are equivalent. This type of

equivalence test has very low computational cost and is in wide use

in a variety of applications, particularly in the domain of parsing.

It is also used when a definitive answer to the question of IRI

equivalence is needed that is independent of the scheme used and that

can be calculated quickly and without accessing a network. An

example of such a case is XML Namespaces (\[XMLNamespace\]).

Testing strings for equivalence requires some basic precautions. This

procedure is often referred to as \"bit-for-bit\" or \"byte-for-byte\"

comparison, which is potentially misleading. Testing strings for

equality is normally based on pair comparison of the characters that

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make up the strings, starting from the first and proceeding until

both strings are exhausted and all characters are found to be equal,

until a pair of characters compares unequal, or until one of the

strings is exhausted before the other.

This character comparison requires that each pair of characters be

put in comparable encoding form. For example, should one IRI be

stored in a byte array in UTF-8 encoding form and the second in a

UTF-16 encoding form, bit-for-bit comparisons applied naively will

produce errors. It is better to speak of equality on a

character-for-character rather than on a byte-for-byte or bit-for-bit

basis. In practical terms, character-by-character comparisons should

be done codepoint by codepoint after conversion to a common character

encoding form. When comparing character by character, the comparison

function MUST NOT map IRIs to URIs, because such a mapping would

create additional spurious equivalences. It follows that an IRI

SHOULD NOT be modified when being transported if there is any chance

that this IRI might be used as an identifier.

False negatives are caused by the production and use of IRI aliases.

Unnecessary aliases can be reduced, regardless of the comparison

method, by consistently providing IRI references in an already

normalized form (i.e., a form identical to what would be produced

after normalization is applied, as described below). Protocols and

data formats often limit some IRI comparisons to simple string

comparison, based on the theory that people and implementations will,

in their own best interest, be consistent in providing IRI

references, or at least be consistent enough to negate any efficiency

that might be obtained from further normalization.

5.3.2. Syntax-Based Normalization

Implementations may use logic based on the definitions provided by

this specification to reduce the probability of false negatives. This

processing is moderately higher in cost than character-for-character

string comparison. For example, an application using this approach

could reasonably consider the following two IRIs equivalent:

example://a/b/c/%7Bfoo%7D/ros&#xE9;

eXAMPLE://a/./b/../b/%63/%7bfoo%7d/ros%C3%A9

Web user agents, such as browsers, typically apply this type of IRI

normalization when determining whether a cached response is

available. Syntax-based normalization includes such techniques as

case normalization, character normalization, percent-encoding

normalization, and removal of dot-segments.

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5.3.2.1. Case Normalization

For all IRIs, the hexadecimal digits within a percent-encoding

triplet (e.g., \"%3a\" versus \"%3A\") are case-insensitive and
therefore

should be normalized to use uppercase letters for the digits A - F.

When an IRI uses components of the generic syntax, the component

syntax equivalence rules always apply; namely, that the scheme and

US-ASCII only host are case insensitive and therefore should be

normalized to lowercase. For example, the URI

\"HTTP://www.EXAMPLE.com/\" is equivalent to
\"http://www.example.com/\".

Case equivalence for non-ASCII characters in IRI components that are

IDNs are discussed in section 5.3.3. The other generic syntax

components are assumed to be case sensitive unless specifically

defined otherwise by the scheme.

Creating schemes that allow case-insensitive syntax components

containing non-ASCII characters should be avoided. Case normalization

of non-ASCII characters can be culturally dependent and is always a

complex operation. The only exception concerns non-ASCII host names

for which the character normalization includes a mapping step derived

from case folding.

5.3.2.2. Character Normalization

The Unicode Standard \[UNIV4\] defines various equivalences between

sequences of characters for various purposes. Unicode Standard Annex

#15 \[UTR15\] defines various Normalization Forms for these

equivalences, in particular Normalization Form C (NFC, Canonical

Decomposition, followed by Canonical Composition) and Normalization

Form KC (NFKC, Compatibility Decomposition, followed by Canonical

Composition).

Equivalence of IRIs MUST rely on the assumption that IRIs are

appropriately pre-character-normalized rather than apply character

normalization when comparing two IRIs. The exceptions are conversion

from a non-digital form, and conversion from a non-UCS-based

character encoding to a UCS-based character encoding. In these cases,

NFC or a normalizing transcoder using NFC MUST be used for

interoperability. To avoid false negatives and problems with

transcoding, IRIs SHOULD be created by using NFC. Using NFKC may

avoid even more problems; for example, by choosing half-width Latin

letters instead of full-width ones, and full-width instead of

half-width Katakana.

As an example, \"http://www.example.org/r&#xE9;sum&#xE9;.html\" (in XML

Notation) is in NFC. On the other hand,

\"http://www.example.org/re&#x301;sume&#x301;.html\" is not in NFC.

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The former uses precombined e-acute characters, and the latter uses

\"e\" characters followed by combining acute accents. Both usages are

defined as canonically equivalent in \[UNIV4\].

Note: Because it is unknown how a particular sequence of characters

is being treated with respect to character normalization, it would

be inappropriate to allow third parties to normalize an IRI

arbitrarily. This does not contradict the recommendation that

when a resource is created, its IRI should be as character

normalized as possible (i.e., NFC or even NFKC). This is similar

to the uppercase/lowercase problems. Some parts of a URI are case

insensitive (domain name). For others, it is unclear whether they

are case sensitive, case insensitive, or something in between

(e.g., case sensitive, but with a multiple choice selection if the

wrong case is used, instead of a direct negative result). The

best recipe is that the creator use a reasonable capitalization

and, when transferring the URI, capitalization never be changed.

Various IRI schemes may allow the usage of Internationalized Domain

Names (IDN) \[RFC3490\] either in the ireg-name part or elsewhere.

Character Normalization also applies to IDNs, as discussed in section

5.3.3.

5.3.2.3. Percent-Encoding Normalization

The percent-encoding mechanism (section 2.1 of \[RFC3986\]) is a

frequent source of variance among otherwise identical IRIs. In

addition to the case normalization issue noted above, some IRI

producers percent-encode octets that do not require percent-encoding,

resulting in IRIs that are equivalent to their non encoded

counterparts. These IRIs should be normalized by decoding any

percent-encoded octet sequence that corresponds to an unreserved

character, as described in section 2.3 of \[RFC3986\].

For actual resolution, differences in percent-encoding (except for

the percent-encoding of reserved characters) MUST always result in

the same resource. For example, \"http://example.org/\~user\",

\"http://example.org/%7euser\", and \"http://example.org/%7Euser\", must

resolve to the same resource.

If this kind of equivalence is to be tested, the percent-encoding of

both IRIs to be compared has to be aligned; for example, by

converting both IRIs to URIs (see section 3.1), eliminating escape

differences in the resulting URIs, and making sure that the case of

the hexadecimal characters in the percent-encoding is always the same

(preferably uppercase). If the IRI is to be passed to another

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application or used further in some other way, its original form MUST

be preserved. The conversion described here should be performed only

for local comparison.

5.3.2.4. Path Segment Normalization

The complete path segments \".\" and \"..\" are intended only for use

within relative references (section 4.1 of \[RFC3986\]) and are removed

as part of the reference resolution process (section 5.2 of

\[RFC3986\]). However, some implementations may incorrectly assume

that reference resolution is not necessary when the reference is

already an IRI, and thus fail to remove dot-segments when they occur

in non-relative paths. IRI normalizers should remove dot-segments by

applying the remove_dot_segments algorithm to the path, as described

in section 5.2.4 of \[RFC3986\].

5.3.3. Scheme-Based Normalization

The syntax and semantics of IRIs vary from scheme to scheme, as

described by the defining specification for each scheme.

Implementations may use scheme-specific rules, at further processing

cost, to reduce the probability of false negatives. For example,

because the \"http\" scheme makes use of an authority component, has a

default port of \"80\", and defines an empty path to be equivalent to

\"/\", the following four IRIs are equivalent:

http://example.com

http://example.com/

http://example.com:/

http://example.com:80/

In general, an IRI that uses the generic syntax for authority with an

empty path should be normalized to a path of \"/\". Likewise, an

explicit \":port\", for which the port is empty or the default for the

scheme, is equivalent to one where the port and its \":\" delimiter are

elided and thus should be removed by scheme-based normalization. For

example, the second IRI above is the normal form for the \"http\"

scheme.

Another case where normalization varies by scheme is in the handling

of an empty authority component or empty host subcomponent. For many

scheme specifications, an empty authority or host is considered an

error; for others, it is considered equivalent to \"localhost\" or the

end-user\'s host. When a scheme defines a default for authority and

an IRI reference to that default is desired, the reference should be

normalized to an empty authority for the sake of uniformity, brevity,

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and internationalization. If, however, either the userinfo or port

subcomponents are non-empty, then the host should be given explicitly

even if it matches the default.

Normalization should not remove delimiters when their associated

component is empty unless it is licensed to do so by the scheme

specification. For example, the IRI \"http://example.com/?\" cannot be

assumed to be equivalent to any of the examples above. Likewise, the

presence or absence of delimiters within a userinfo subcomponent is

usually significant to its interpretation. The fragment component is

not subject to any scheme-based normalization; thus, two IRIs that

differ only by the suffix \"#\" are considered different regardless of

the scheme.

Some IRI schemes may allow the usage of Internationalized Domain

Names (IDN) \[RFC3490\] either in their ireg-name part or elsewhere.

When in use in IRIs, those names SHOULD be validated by using the

ToASCII operation defined in \[RFC3490\], with the flags

\"UseSTD3ASCIIRules\" and \"AllowUnassigned\". An IRI containing an

invalid IDN cannot successfully be resolved. Validated IDN

components of IRIs SHOULD be character normalized by using the

Nameprep process \[RFC3491\]; however, for legibility purposes, they

SHOULD NOT be converted into ASCII Compatible Encoding (ACE).

Scheme-based normalization may also consider IDN components and their

conversions to punycode as equivalent. As an example,

\"http://r&#xE9;sum&#xE9;.example.org\" may be considered equivalent to

\"http://xn\--rsum-bpad.example.org\".

Other scheme-specific normalizations are possible.

5.3.4. Protocol-Based Normalization

Substantial effort to reduce the incidence of false negatives is

often cost-effective for web spiders. Consequently, they implement

even more aggressive techniques in IRI comparison. For example, if

they observe that an IRI such as

http://example.com/data

redirects to an IRI differing only in the trailing slash

http://example.com/data/

they will likely regard the two as equivalent in the future. This

kind of technique is only appropriate when equivalence is clearly

indicated by both the result of accessing the resources and the

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common conventions of their scheme\'s dereference algorithm (in this

case, use of redirection by HTTP origin servers to avoid problems

with relative references).

6\. Use of IRIs

6.1. Limitations on UCS Characters Allowed in IRIs

This section discusses limitations on characters and character

sequences usable for IRIs beyond those given in section 2.2 and

section 4.1. The considerations in this section are relevant when

IRIs are created and when URIs are converted to IRIs.

a\. The repertoire of characters allowed in each IRI component is

limited by the definition of that component. For example, the

definition of the scheme component does not allow characters

beyond US-ASCII.

(Note: In accordance with URI practice, generic IRI software

cannot and should not check for such limitations.)

b\. The UCS contains many areas of characters for which there are

strong visual look-alikes. Because of the likelihood of

transcription errors, these also should be avoided. This

includes the full-width equivalents of Latin characters,

half-width Katakana characters for Japanese, and many others. It

also includes many look-alikes of \"space\", \"delims\", and

\"unwise\", characters excluded in \[RFC3491\].

Additional information is available from \[UNIXML\]. \[UNIXML\] is

written in the context of running text rather than in that of

identifiers. Nevertheless, it discusses many of the categories of

characters not appropriate for IRIs.

6.2. Software Interfaces and Protocols

Although an IRI is defined as a sequence of characters, software

interfaces for URIs typically function on sequences of octets or

other kinds of code units. Thus, software interfaces and protocols

MUST define which character encoding is used.

Intermediate software interfaces between IRI-capable components and

URI-only components MUST map the IRIs per section 3.1, when

transferring from IRI-capable to URI-only components. This mapping

SHOULD be applied as late as possible. It SHOULD NOT be applied

between components that are known to be able to handle IRIs.

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6.3. Format of URIs and IRIs in Documents and Protocols

Document formats that transport URIs may have to be upgraded to allow

the transport of IRIs. In cases where the document as a whole has a

native character encoding, IRIs MUST also be encoded in this

character encoding and converted accordingly by a parser or

interpreter. IRI characters not expressible in the native character

encoding SHOULD be escaped by using the escaping conventions of the

document format if such conventions are available. Alternatively,

they MAY be percent-encoded according to section 3.1. For example, in

HTML or XML, numeric character references SHOULD be used. If a

document as a whole has a native character encoding and that

character encoding is not UTF-8, then IRIs MUST NOT be placed into

the document in the UTF-8 character encoding.

Note: Some formats already accommodate IRIs, although they use

different terminology. HTML 4.0 \[HTML4\] defines the conversion from

IRIs to URIs as error-avoiding behavior. XML 1.0 \[XML1\], XLink

\[XLink\], XML Schema \[XMLSchema\], and specifications based upon them

allow IRIs. Also, it is expected that all relevant new W3C formats

and protocols will be required to handle IRIs \[CharMod\].

6.4. Use of UTF-8 for Encoding Original Characters

This section discusses details and gives examples for point c) in

section 1.2. To be able to use IRIs, the URI corresponding to the

IRI in question has to encode original characters into octets by

using UTF-8. This can be specified for all URIs of a URI scheme or

can apply to individual URIs for schemes that do not specify how to

encode original characters. It can apply to the whole URI, or only

to some part. For background information on encoding characters into

URIs, see also section 2.5 of \[RFC3986\].

For new URI schemes, using UTF-8 is recommended in \[RFC2718\].

Examples where UTF-8 is already used are the URN syntax \[RFC2141\],

IMAP URLs \[RFC2192\], and POP URLs \[RFC2384\]. On the other hand,

because the HTTP URL scheme does not specify how to encode original

characters, only some HTTP URLs can have corresponding but different

IRIs.

For example, for a document with a URI of

\"http://www.example.org/r%C3%A9sum%C3%A9.html\", it is possible to

construct a corresponding IRI (in XML notation, see, section 1.4):

\"http://www.example.org/r&#xE9;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

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\"http://www.example.org/r%E9sum%E9.html\", the percent-encoding octets

cannot be converted to actual characters in an IRI, as the

percent-encoding is not based on UTF-8.

This means that for most URI schemes, there is no need to upgrade

their scheme definition in order for them to work with IRIs. The

main case where upgrading makes sense is when a scheme definition, or

a particular component of a scheme, is strictly limited to the use of

US-ASCII characters with no provision to include non-ASCII

characters/octets via percent-encoding, or if a scheme definition

currently uses highly scheme-specific provisions for the encoding of

non-ASCII characters. An example of this is the mailto: scheme

\[RFC2368\].

This specification does not upgrade any scheme specifications in any

way; this has to be done separately. Also, note that there is no

such thing as an \"IRI scheme\"; all IRIs use URI schemes, and all URI

schemes can be used with IRIs, even though in some cases only by

using URIs directly as IRIs, without any conversion.

URI schemes can impose restrictions on the syntax of scheme-specific

URIs; i.e., URIs that are admissible under the generic URI syntax

\[RFC3986\] may not be admissible due to narrower syntactic constraints

imposed by a URI scheme specification. URI scheme definitions cannot

broaden the syntactic restrictions of the generic URI syntax;

otherwise, it would be possible to generate URIs that satisfied the

scheme-specific syntactic constraints without satisfying the

syntactic constraints of the generic URI syntax. However, additional

syntactic constraints imposed by URI scheme specifications are

applicable to IRI, as the corresponding URI resulting from the

mapping defined in section 3.1 MUST be a valid URI under the

syntactic restrictions of generic URI syntax and any narrower

restrictions imposed by the corresponding URI scheme specification.

The requirement for the use of UTF-8 applies to all parts of a URI

(with the potential exception of the ireg-name part; see section

3.1). However, it is possible that the capability of IRIs to

represent a wide range of characters directly is used just in some

parts of the IRI (or IRI reference). The other parts of the IRI may

only contain US-ASCII characters, or they may not be based on UTF-8.

They may be based on another character encoding, or they may directly

encode raw binary data (see also \[RFC2397\]).

For example, it is possible to have a URI reference of

\"http://www.example.org/r%E9sum%E9.xml#r%C3%A9sum%C3%A9\", where the

document name is encoded in iso-8859-1 based on server settings, but

where the fragment identifier is encoded in UTF-8 according to

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\[XPointer\]. The IRI corresponding to the above URI would be (in XML

notation)

\"http://www.example.org/r%E9sum%E9.xml#r&#xE9;sum&#xE9\";.

Similar considerations apply to query parts. The functionality of

IRIs (namely, to be able to include non-ASCII characters) can only be

used if the query part is encoded in UTF-8.

6.5. Relative IRI References

Processing of relative IRI references against a base is handled

straightforwardly; the algorithms of \[RFC3986\] can be applied

directly, treating the characters additionally allowed in IRI

references in the same way that unreserved characters are in URI

references.

7\. URI/IRI Processing Guidelines (Informative)

This informative section provides guidelines for supporting IRIs in

the same software components and operations that currently process

URIs: Software interfaces that handle URIs, software that allows

users to enter URIs, software that creates or generates URIs,

software that displays URIs, formats and protocols that transport

URIs, and software that interprets URIs. These may all require

modification before functioning properly with IRIs. The

considerations in this section also apply to URI references and IRI

references.

7.1. URI/IRI Software Interfaces

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

protocols transferring URIs, need interfaces and protocol elements

that are designed to carry IRIs.

In case the current handling in an API or protocol is based on

US-ASCII, UTF-8 is recommended as the character encoding for IRIs, as

it is compatible with US-ASCII, is in accordance with the

recommendations of \[RFC2277\], and makes converting to URIs easy. In

any case, the API or protocol definition must clearly define the

character encoding to be used.

The transfer from URI-only to IRI-capable components requires no

mapping, although the conversion described in section 3.2 above may

be performed. It is preferable not to perform this inverse

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

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7.2. URI/IRI Entry

Some components allow users to enter URIs into the system by typing

or dictation, for example. This software must be updated to allow

for IRI entry.

A person viewing a visual representation of an IRI (as a sequence of

glyphs, in some order, in some visual display) or hearing an IRI will

use an entry method for characters in the user\'s language to input

the IRI. Depending on the script and the input method used, this may

be a more or less complicated process.

The process of IRI entry must ensure, as much as possible, that the

restrictions defined in section 2.2 are met. This may be done by

choosing appropriate input methods or variants/settings thereof, by

appropriately converting the characters being input, by eliminating

characters that cannot be converted, and/or by issuing a warning or

error message to the user.

As an example of variant settings, input method editors for East

Asian Languages usually allow the input of Latin letters and related

characters in full-width or half-width versions. For IRI input, the

input method editor should be set so that it produces half-width

Latin letters and punctuation and full-width Katakana.

An input field primarily or solely used for the input of URIs/IRIs

may allow the user to view an IRI as it is mapped to a URI. Places

where the input of IRIs is frequent may provide the possibility for

viewing an IRI as mapped to a URI. This will help users when some of

the software they use does not yet accept IRIs.

An IRI input component interfacing to components that handle URIs,

but not IRIs, must map the IRI to a URI before passing it to these

components.

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

section 4.3.

7.3. URI/IRI Transfer between Applications

Many applications, particularly mail user agents, try to detect URIs

appearing in plain text. For this, they use some heuristics based on

URI syntax. They then allow the user to click on such URIs and

retrieve the corresponding resource in an appropriate (usually

scheme-dependent) application.

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Such applications have to be upgraded to use the IRI syntax as a base

for heuristics. In particular, a non-ASCII character should not be

taken as the indication of the end of an IRI. Such applications also

have to make sure that they correctly convert the detected IRI from

the character encoding of the document or application where the IRI

appears to the character encoding used by the system-wide IRI

invocation mechanism, or to a URI (according to section 3.1) if the

system-wide invocation mechanism only accepts URIs.

The clipboard is another frequently used way to transfer URIs and

IRIs from one application to another. On most platforms, the

clipboard is able to store and transfer text in many languages and

scripts. Correctly used, the clipboard transfers characters, not

bytes, which will do the right thing with IRIs.

7.4. URI/IRI Generation

Systems that offer resources through the Internet, where those

resources have logical names, sometimes automatically generate URIs

for the resources they offer. For example, some HTTP servers can

generate a directory listing for a file directory and then respond to

the generated URIs with the files.

Many legacy character encodings are in use in various file systems.

Many currently deployed systems do not transform the local character

representation of the underlying system before generating URIs.

For maximum interoperability, systems that generate resource

identifiers should make the appropriate transformations. For

example, if a file system contains a file named

\"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 UTF-8.

This recommendation particularly applies to HTTP servers. For FTP

servers, similar considerations apply; see \[RFC2640\].

7.5. URI/IRI Selection

In some cases, resource owners and publishers have control over the

IRIs used to identify their resources. This control is mostly

executed by controlling the resource names, such as file names,

directly.

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In these cases, it is recommended to avoid choosing IRIs that are

easily confused. For example, for US-ASCII, the lower-case ell (\"l\")

is easily confused with the digit one (\"1\"), and the upper-case oh

(\"O\") is easily confused with the digit zero (\"0\"). Publishers

should avoid confusing users with \"br0ken\" or \"1ame\" identifiers.

Outside the US-ASCII repertoire, there are many more opportunities

for confusion; a complete set of guidelines is too lengthy to include

here. As long as names are limited to characters from a single

script, native writers of a given script or language will know best

when ambiguities can appear, and how they can be avoided. What may

look ambiguous to a stranger may be completely obvious to the average

native user. On the other hand, in some cases, the UCS contains

variants for compatibility reasons; for example, for typographic

purposes. These should be avoided wherever possible. Although there

may be exceptions, newly created resource names should generally be

in NFKC \[UTR15\] (which means that they are also in NFC).

As an example, the UCS contains the \"fi\" ligature at U+FB01 for

compatibility reasons. Wherever possible, IRIs should use the two

letters \"f\" and \"i\" rather than the \"fi\" ligature. An example
where

the latter may be used is in the query part of an IRI for an explicit

search for a word written containing the \"fi\" ligature.

In certain cases, there is a chance that characters from different

scripts look the same. The best known example is the similarity of

the Latin \"A\", the Greek \"Alpha\", and the Cyrillic \"A\". To avoid

such cases, only IRIs should be created where all the characters in a

single component are used together in a given language. This usually

means that all of these characters will be from the same script, but

there are languages that mix characters from different scripts (such

as Japanese). This is similar to the heuristics used to distinguish

between letters and numbers in the examples above. Also, for Latin,

Greek, and Cyrillic, using lowercase letters results in fewer

ambiguities than using uppercase letters would.

7.6. Display of URIs/IRIs

In situations where the rendering software is not expected to display

non-ASCII parts of the IRI correctly using the available layout and

font resources, these parts should be percent-encoded before being

displayed.

For display of Bidi IRIs, please see section 4.1.

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7.7. Interpretation of URIs and IRIs

Software that interprets IRIs as the names of local resources should

accept IRIs in multiple forms and convert and match them with the

appropriate local resource names.

First, multiple representations include both IRIs in the native

character encoding of the protocol and also their URI counterparts.

Second, it may include URIs constructed based on character encodings

other than UTF-8. These URIs may be produced by user agents that do

not conform to this specification and that use legacy character

encodings to convert non-ASCII characters to URIs. Whether this is

necessary, and what character encodings to cover, depends on a number

of factors, such as the legacy character encodings used locally and

the distribution of various versions of user agents. For example,

software for Japanese may accept URIs in Shift_JIS and/or EUC-JP in

addition to UTF-8.

Third, it may include additional mappings to be more user-friendly

and robust against transmission errors. These would be similar to

how some servers currently treat URIs as case insensitive or perform

additional matching to account for spelling errors. For characters

beyond the US-ASCII repertoire, this may, for example, include

ignoring the accents on received IRIs or resource names. Please note

that such mappings, including case mappings, are language dependent.

It can be difficult to identify a resource unambiguously if too many

mappings are taken into consideration. However, percent-encoded and

not percent-encoded parts of IRIs can always be clearly

distinguished. Also, the regularity of UTF-8 (see \[Duerst97\]) makes

the potential for collisions lower than it may seem at first.

7.8. Upgrading Strategy

Where this recommendation places further constraints on software for

which many instances are already deployed, it is important to

introduce upgrades carefully and to be aware of the various

interdependencies.

If IRIs cannot be interpreted correctly, they should not be created,

generated, or transported. This suggests that upgrading URI

interpreting software to accept IRIs should have highest priority.

On the other hand, a single IRI is interpreted only by a single or

very few interpreters that are known in advance, although it may be

entered and transported very widely.

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Therefore, IRIs benefit most from a broad upgrade of software to be

able to enter and transport IRIs. However, before an individual IRI

is published, care should be taken to upgrade the corresponding

interpreting software in order to cover the forms expected to be

received by various versions of entry and transport software.

The upgrade of generating software to generate IRIs instead of using

a local character encoding should happen only after the service is

upgraded to accept IRIs. Similarly, IRIs should only be generated

when the service accepts IRIs and the intervening infrastructure and

protocol is known to transport them safely.

Software converting from URIs to IRIs for display should be upgraded

only after upgraded entry software has been widely deployed to the

population that will see the displayed result.

Where there is a free choice of character encodings, it is often

possible to reduce the effort and dependencies for upgrading to IRIs

by using UTF-8 rather than another encoding. For example, when a new

file-based Web server is set up, using UTF-8 as the character

encoding for file names will make the transition to IRIs easier.

Likewise, when a new Web form is set up using UTF-8 as the character

encoding of the form page, the returned query URIs will use UTF-8 as

the character encoding (unless the user, for whatever reason, changes

the character encoding) and will therefore be compatible with IRIs.

These recommendations, when taken together, will allow for the

extension from URIs to IRIs in order to handle characters other than

US-ASCII while minimizing interoperability problems. For

considerations regarding the upgrade of URI scheme definitions, see

section 6.4.

8\. Security Considerations

The security considerations discussed in \[RFC3986\] also apply to

IRIs. In addition, the following issues require particular care for

IRIs.

Incorrect encoding or decoding can lead to security problems. In

particular, some UTF-8 decoders do not check against overlong byte

sequences. As an example, a \"/\" is encoded with the byte 0x2F both

in UTF-8 and in US-ASCII, but some UTF-8 decoders also wrongly

interpret the sequence 0xC0 0xAF as a \"/\". A sequence such as

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\"%C0%AF..\" may pass some security tests and then be interpreted as

\"/..\" in a path if UTF-8 decoders are fault-tolerant, if conversion

and checking are not done in the right order, and/or if reserved

characters and unreserved characters are not clearly distinguished.

There are various ways in which \"spoofing\" can occur with IRIs.

\"Spoofing\" means that somebody may add a resource name that looks the

same or similar to the user, but that points to a different resource.

The added resource may pretend to be the real resource by looking

very similar but may contain all kinds of changes that may be

difficult to spot and that can cause all kinds of problems. Most

spoofing possibilities for IRIs are extensions of those for URIs.

Spoofing can occur for various reasons. First, a user\'s

normalization expectations or actual normalization when entering an

IRI or transcoding an IRI from a legacy character encoding do not

match the normalization used on the server side. Conceptually, this

is no different from the problems surrounding the use of

case-insensitive web servers. For example, a popular web page with a

mixed-case name (\"http://big.example.com/PopularPage.html\") might be

\"spoofed\" by someone who is able to create

\"http://big.example.com/popularpage.html\". However, the use of

unnormalized character sequences, and of additional mappings for user

convenience, may increase the chance for spoofing. Protocols and

servers that allow the creation of resources with names that are not

normalized are particularly vulnerable to such attacks. This is an

inherent security problem of the relevant protocol, server, or

resource and is not specific to IRIs, but it is mentioned here for

completeness.

Spoofing can occur in various IRI components, such as the domain name

part or a path part. For considerations specific to the domain name

part, see \[RFC3491\]. For the path part, administrators of sites that

allow independent users to create resources in the same sub area may

have to be careful to check for spoofing.

Spoofing can occur because in the UCS many characters look very

similar. Details are discussed in Section 7.5. Again, this is very

similar to spoofing possibilities on US-ASCII, e.g., using \"br0ken\"

or \"1ame\" URIs.

Spoofing can occur when URIs with percent-encodings based on various

character encodings are accepted to deal with older user agents. In

some cases, particularly for Latin-based resource names, this is

usually easy to detect because UTF-8-encoded names, when interpreted

and viewed as legacy character encodings, produce mostly garbage.

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When concurrently used character encodings have a similar structure

but there are no characters that have exactly the same encoding,

detection is more difficult.

Spoofing can occur with bidirectional IRIs, if the restrictions in

section 4.2 are not followed. The same visual representation may be

interpreted as different logical representations, and vice versa. It

is also very important that a correct Unicode bidirectional

implementation be used.

9\. Acknowledgements

We would like to thank Larry Masinter for his work as coauthor of

many earlier versions of this document (draft-masinter-url-i18n-xx).

The discussion on the issue addressed here started a long time ago.

There was a thread in the HTML working group in August 1995 (under

the topic of \"Globalizing URIs\") and in the www-international mailing

list in July 1996 (under the topic of \"Internationalization and

URLs\"), and there were ad-hoc meetings at the Unicode conferences in

September 1995 and September 1997.

Many thanks go to Francois Yergeau, Matitiahu Allouche, Roy Fielding,

Tim Berners-Lee, Mark Davis, M.T. Carrasco Benitez, James Clark, Tim

Bray, Chris Wendt, Yaron Goland, Andrea Vine, Misha Wolf, Leslie

Daigle, Ted Hardie, Bill Fenner, Margaret Wasserman, Russ Housley,

Makoto MURATA, Steven Atkin, Ryan Stansifer, Tex Texin, Graham Klyne,

Bjoern Hoehrmann, Chris Lilley, Ian Jacobs, Adam Costello, Dan

Oscarson, Elliotte Rusty Harold, Mike J. Brown, Roy Badami, Jonathan

Rosenne, Asmus Freytag, Simon Josefsson, Carlos Viegas Damasio, Chris

Haynes, Walter Underwood, and many others for help with understanding

the issues and possible solutions, and with getting the details

right.

This document is a product of the Internationalization Working Group

(I18N WG) of the World Wide Web Consortium (W3C). Thanks to the

members of the W3C I18N Working Group and Interest Group for their

contributions and their work on \[CharMod\]. Thanks also go to the

members of many other W3C Working Groups for adopting IRIs, and to

the members of the Montreal IAB Workshop on Internationalization and

Localization for their review.

Duerst & Suignard Standards Track \[Page 39\]

RFC 3987 Internationalized Resource Identifiers January 2005

10\. References

10.1. Normative References

\[ASCII\] American National Standards Institute, \"Coded

Character Set \-- 7-bit American Standard Code for

Information Interchange\", ANSI X3.4, 1986.

\[ISO10646\] International Organization for Standardization,

\"ISO/IEC 10646:2003: Information Technology -

Universal Multiple-Octet Coded Character Set (UCS)\",

ISO Standard 10646, December 2003.

\[RFC2119\] Bradner, S., \"Key words for use in RFCs to Indicate

Requirement Levels\", BCP 14, RFC 2119, March 1997.

\[RFC2234\] Crocker, D. and P. Overell, \"Augmented BNF for Syntax

Specifications: ABNF\", RFC 2234, November 1997.

\[RFC3490\] Faltstrom, P., Hoffman, P., and A. Costello,

\"Internationalizing Domain Names in Applications

(IDNA)\", RFC 3490, March 2003.

\[RFC3491\] Hoffman, P. and M. Blanchet, \"Nameprep: A Stringprep

Profile for Internationalized Domain Names (IDN)\", RFC

3491, March 2003.

\[RFC3629\] Yergeau, F., \"UTF-8, a transformation format of ISO

10646\", STD 63, RFC 3629, November 2003.

\[RFC3986\] Berners-Lee, T., Fielding, R., and L. Masinter,

\"Uniform Resource Identifier (URI): Generic Syntax\",

STD 66, RFC 3986, January 2005.

\[UNI9\] Davis, M., \"The Bidirectional Algorithm\", Unicode

Standard Annex #9, March 2004,

\<http://www.unicode.org/reports/tr9/tr9-13.html\>.

\[UNIV4\] The Unicode Consortium, \"The Unicode Standard, Version

4.0.1, defined by: The Unicode Standard, Version 4.0

(Reading, MA, Addison-Wesley, 2003. ISBN

0-321-18578-1), as amended by Unicode 4.0.1

(http://www.unicode.org/versions/Unicode4.0.1/)\",

March 2004.

Duerst & Suignard Standards Track \[Page 40\]

RFC 3987 Internationalized Resource Identifiers January 2005

\[UTR15\] Davis, M. and M. Duerst, \"Unicode Normalization

Forms\", Unicode Standard Annex #15, April 2003,

\<http://www.unicode.org/unicode/reports/

tr15/tr15-23.html\>.

10.2. Informative References

\[BidiEx\] \"Examples of bidirectional IRIs\",

\<http://www.w3.org/International/iri-edit/

BidiExamples\>.

\[CharMod\] Duerst, M., Yergeau, F., Ishida, R., Wolf, M., and T.

Texin, \"Character Model for the World Wide Web:

Resource Identifiers\", World Wide Web Consortium

Candidate Recommendation, November 2004,

\<http://www.w3.org/TR/charmod-resid\>.

\[Duerst97\] Duerst, M., \"The Properties and Promises of UTF-8\",

Proc. 11th International Unicode Conference, San Jose

, September 1997,

\<http://www.ifi.unizh.ch/mml/mduerst/papers/

PDF/IUC11-UTF-8.pdf\>.

\[Gettys\] Gettys, J., \"URI Model Consequences\",

\<http://www.w3.org/DesignIssues/ModelConsequences\>.

\[HTML4\] Raggett, D., Le Hors, A., and I. Jacobs, \"HTML 4.01

Specification\", World Wide Web Consortium

Recommendation, December 1999,

\<http://www.w3.org/TR/html401/appendix/

notes.html#h-B.2\>.

\[RFC2045\] Freed, N. and N. Borenstein, \"Multipurpose Internet

Mail Extensions (MIME) Part One: Format of Internet

Message Bodies\", RFC 2045, November 1996.

\[RFC2130\] Weider, C., Preston, C., Simonsen, K., Alvestrand, H.,

Atkinson, R., Crispin, M., and P. Svanberg, \"The

Report of the IAB Character Set Workshop held 29

February - 1 March, 1996\", RFC 2130, April 1997.

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

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

1997\.

\[RFC2277\] Alvestrand, H., \"IETF Policy on Character Sets and

Languages\", BCP 18, RFC 2277, January 1998.

Duerst & Suignard Standards Track \[Page 41\]

RFC 3987 Internationalized Resource Identifiers January 2005

\[RFC2368\] Hoffman, P., Masinter, L., and J. Zawinski, \"The

mailto URL scheme\", RFC 2368, July 1998.

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

\[RFC2396\] Berners-Lee, T., Fielding, R., and L. Masinter,

\"Uniform Resource Identifiers (URI): Generic Syntax\",

RFC 2396, August 1998.

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

August 1998.

\[RFC2616\] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,

Masinter, L., Leach, P., and T. Berners-Lee,

\"Hypertext Transfer Protocol \-- HTTP/1.1\", RFC 2616,

June 1999.

\[RFC2640\] Curtin, B., \"Internationalization of the File Transfer

Protocol\", RFC 2640, July 1999.

\[RFC2718\] Masinter, L., Alvestrand, H., Zigmond, D., and R.

Petke, \"Guidelines for new URL Schemes\", RFC 2718,

November 1999.

\[UNIXML\] Duerst, M. and A. Freytag, \"Unicode in XML and other

Markup Languages\", Unicode Technical Report #20, World

Wide Web Consortium Note, June 2003,

\<http://www.w3.org/TR/unicode-xml/\>.

\[XLink\] DeRose, S., Maler, E., and D. Orchard, \"XML Linking

Language (XLink) Version 1.0\", World Wide Web

Consortium Recommendation, June 2001,

\<http://www.w3.org/TR/xlink/#link-locators\>.

\[XML1\] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E.,

and F. Yergeau, \"Extensible Markup Language (XML) 1.0

(Third Edition)\", World Wide Web Consortium

Recommendation, February 2004,

\<http://www.w3.org/TR/REC-xml#sec-external-ent\>.

\[XMLNamespace\] Bray, T., Hollander, D., and A. Layman, \"Namespaces in

XML\", World Wide Web Consortium Recommendation,

January 1999, \<http://www.w3.org/TR/REC-xml-names\>.

\[XMLSchema\] Biron, P. and A. Malhotra, \"XML Schema Part 2:

Datatypes\", World Wide Web Consortium Recommendation,

May 2001, \<http://www.w3.org/TR/xmlschema-2/#anyURI\>.

Duerst & Suignard Standards Track \[Page 42\]

RFC 3987 Internationalized Resource Identifiers January 2005

\[XPointer\] Grosso, P., Maler, E., Marsh, J. and N. Walsh,

\"XPointer Framework\", World Wide Web Consortium

Recommendation, March 2003,

\<http://www.w3.org/TR/xptr-framework/#escaping\>.

Duerst & Suignard Standards Track \[Page 43\]

RFC 3987 Internationalized Resource Identifiers January 2005

Appendix A. Design Alternatives

This section shortly summarizes major design alternatives and the

reasons for why they were not chosen.

Appendix A.1. New Scheme(s)

Introducing new schemes (for example, httpi:, ftpi:,\...) or a new

metascheme (e.g., i:, leading to URI/IRI prefixes such as i:http:,

i:ftp:,\...) was proposed to make IRI-to-URI conversion scheme

dependent or to distinguish between percent-encodings resulting from

IRI-to-URI conversion and percent-encodings from legacy character

encodings.

New schemes are not needed to distinguish URIs from true IRIs (i.e.,

IRIs that contain non-ASCII characters). The benefit of being able

to detect the origin of percent-encodings is marginal, as UTF-8 can

be detected with very high reliability. Deploying new schemes is

extremely hard, so not requiring new schemes for IRIs makes

deployment of IRIs vastly easier. Making conversion scheme dependent

is highly inadvisable and would be encouraged by separate schemes for

IRIs. Using a uniform convention for conversion from IRIs to URIs

makes IRI implementation orthogonal to the introduction of actual new

schemes.

Appendix A.2. Character Encodings Other Than UTF-8

At an early stage, UTF-7 was considered as an alternative to UTF-8

when IRIs are converted to URIs. UTF-7 would not have needed

percent-encoding and in most cases would have been shorter than

percent-encoded UTF-8.

Using UTF-8 avoids a double layering and overloading of the use of

the \"+\" character. UTF-8 is fully compatible with US-ASCII and has

therefore been recommended by the IETF, and is being used widely.

UTF-7 has never been used much and is now clearly being discouraged.

Requiring implementations to convert from UTF-8 to UTF-7 and back

would be an additional implementation burden.

Appendix A.3. New Encoding Convention

Instead of using the existing percent-encoding convention of URIs,

which is based on octets, the idea was to create a new encoding

convention; for example, to use \"%u\" to introduce UCS code points.

Duerst & Suignard Standards Track \[Page 44\]

RFC 3987 Internationalized Resource Identifiers January 2005

Using the existing octet-based percent-encoding mechanism does not

need an upgrade of the URI syntax and does not need corresponding

server upgrades.

Appendix A.4. Indicating Character Encodings in the URI/IRI

Some proposals suggested indicating the character encodings used in

an URI or IRI with some new syntactic convention in the URI itself,

similar to the \"charset\" parameter for e-mails and Web pages. As an

example, the label in square brackets in

\"http://www.example.org/ros\[iso-8859-1\]&#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

confusion.

Authors\' Addresses

Martin Duerst (Note: Please write \"Duerst\" with u-umlaut wherever

possible, for example as \"D&#252;rst\" in XML and

HTML.)

World Wide Web Consortium

5322 Endo

Fujisawa, Kanagawa 252-8520

Japan

Phone: +81 466 49 1170

Fax: +81 466 49 1171

EMail: duerst@w3.org

URI: http://www.w3.org/People/D%C3%BCrst/

(Note: This is the percent-encoded form of an IRI.)

Michel Suignard

Microsoft Corporation

One Microsoft Way

Redmond, WA 98052

U.S.A.

Phone: +1 425 882-8080

EMail: michelsu@microsoft.com

URI: http://www.suignard.com

Duerst & Suignard Standards Track \[Page 45\]

RFC 3987 Internationalized Resource Identifiers January 2005

Full Copyright Statement

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Duerst & Suignard Standards Track \[Page 46\]