Internationalized Resource Identifiers (IRIs)
draft-ietf-iri-3987bis-12

Abstract

This document defines the Internationalized Resource Identifier (IRI) protocol element, as an extension of the Uniform Resource Identifier (URI). An IRI is a sequence of characters from the Universal Character Set (Unicode/ISO 10646). Grammar and processing rules are given for IRIs and related syntactic forms.

Defining IRI as a new protocol element (rather than updating or extending the definition of URI) allows independent orderly transitions: protocols and languages that use URIs must explicitly choose to allow IRIs.

Guidelines are provided for the use and deployment of IRIs and related protocol elements when revising protocols, formats, and software components that currently deal only with URIs.

This document is part of a set of documents intended to replace RFC 3987.

RFC Editor: Please remove the next paragraph before publication.

This document, and several companion documents, are intended to obsolete RFC 3987. For discussion and comments on these drafts, please join the IETF IRI WG by subscribing to the mailing list public-iri@w3.org, archives at http://lists.w3.org/archives/public/public-iri/. For a list of open issues, please see the issue tracker of the WG at http://trac.tools.ietf.org/wg/iri/trac/report/1. For a list of individual edits, please see the change history at http://trac.tools.ietf.org/wg/iri/trac/log/draft-ietf-iri-3987bis.

This document is available in (line-printer ready) plaintext ASCII and PDF. It is also available in HTML from http://www.sw.it.aoyama.ac.jp/2012/pub/draft-ietf-iri-3987bis-12.html, and in UTF-8 plaintext from http://www.sw.it.aoyama.ac.jp/2012/pub/draft-ietf-iri-3987bis-12.utf8.txt. While all these versions are identical in their technical content, the HTML, PDF, and UTF-8 plaintext versions show non-Unicode characters directly. This often makes it easier to understand examples, and readers are therefore advised to consult these versions in preference or as a supplement to the ASCII version.

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”

This Internet-Draft will expire on January 17, 2013.

Copyright Notice

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Table of Contents

1.  Introduction
    1.1.  Overview and Motivation
    1.2.  Applicability
    1.3.  Definitions
    1.4.  Notation
2.  IRI Syntax
    2.1.  Summary of IRI Syntax
    2.2.  ABNF for IRI References and IRIs
3.  Processing IRIs and related protocol elements
    3.1.  Converting to UCS
    3.2.  Parse the IRI into IRI components
    3.3.  General percent-encoding of IRI components
    3.4.  Mapping ireg-name
        3.4.1.  Mapping using Percent-Encoding
        3.4.2.  Mapping using Punycode
        3.4.3.  Additional Considerations
    3.5.  Mapping query components
    3.6.  Mapping IRIs to URIs
4.  Converting URIs to IRIs
    4.1.  Examples
5.  Use of IRIs
    5.1.  Limitations on UCS Characters Allowed in IRIs
    5.2.  Software Interfaces and Protocols
    5.3.  Format of URIs and IRIs in Documents and Protocols
    5.4.  Use of UTF-8 for Encoding Original Characters
    5.5.  Relative IRI References
6.  Legacy Extended IRIs (LEIRIs)
    6.1.  Legacy Extended IRI Syntax
    6.2.  Conversion of Legacy Extended IRIs to IRIs
    6.3.  Characters Allowed in Legacy Extended IRIs but not in IRIs
7.  URI/IRI Processing Guidelines (Informative)
    7.1.  URI/IRI Software Interfaces
    7.2.  URI/IRI Entry
    7.3.  URI/IRI Transfer between Applications
    7.4.  URI/IRI Generation
    7.5.  URI/IRI Selection
    7.6.  Display of URIs/IRIs
    7.7.  Interpretation of URIs and IRIs
    7.8.  Upgrading Strategy
8.  IANA Considerations
9.  Security Considerations
10.  Acknowledgements
11.  Main Changes Since RFC 3987
    11.1.  Split out Bidi, processing guidelines, comparison sections
    11.2.  Major restructuring of IRI processing model
        11.2.1.  OLD WAY
        11.2.2.  NEW WAY
        11.2.3.  Extension of Syntax
        11.2.4.  More to be added
    11.3.  Change Log
        11.3.1.  Changes after draft-ietf-iri-3987bis-01
        11.3.2.  Changes from draft-duerst-iri-bis-07 to draft-ietf-iri-3987bis-00
        11.3.3.  Changes from -06 to -07 of draft-duerst-iri-bis
    11.4.  Changes from -00 to -01
    11.5.  Changes from -05 to -06 of draft-duerst-iri-bis-00
    11.6.  Changes from -04 to -05 of draft-duerst-iri-bis
    11.7.  Changes from -03 to -04 of draft-duerst-iri-bis
    11.8.  Changes from -02 to -03 of draft-duerst-iri-bis
    11.9.  Changes from -01 to -02 of draft-duerst-iri-bis
    11.10.  Changes from -00 to -01 of draft-duerst-iri-bis
    11.11.  Changes from RFC 3987 to -00 of draft-duerst-iri-bis
12.  References
    12.1.  Normative References
    12.2.  Informative References
§  Authors' Addresses

1.  Introduction

1.1.  Overview and Motivation

A Uniform Resource Identifier (URI) is defined in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) as a sequence of characters chosen from a limited subset of the repertoire of US-ASCII [ASCII] (American National Standards Institute, “Coded Character Set -- 7-bit American Standard Code for Information Interchange,” 1986.) 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 script. Many languages with non-Latin scripts are transcribed with Latin letters. These transcriptions are now often used in URIs, but they introduce additional difficulties.

The infrastructure for the appropriate handling of characters from additional scripts is now widely deployed in operating system and application software. Software that can handle a wide variety of scripts and languages at the same time is increasingly common. Also, an increasing number of protocols and formats can carry a wide range of characters.

URIs are composed out of a very limited repertoire of characters; this design choice was made to support global transcription (see [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) section 1.2.1.). Reliable transition between a URI (as an abstract protocol element composed of a sequence of characters) and a presentation of that URI (written on a napkin, read out loud) and back is relatively straightforward, because of the limited repertoire of characters used. IRIs are designed to satisfy a different set of use requirements; in particular, to allow IRIs to be written in ways that are more meaningful to their users, even at the expense of global transcribability. However, ensuring reliability of the transition between an IRI and its presentation and back is more difficult and complex when dealing with the larger set of Unicode characters. For example, Unicode supports multiple ways of encoding complex combinations of characters and accents, with multiple character sequences that can result in the same presentation.

This document defines the protocol element called Internationalized Resource Identifier (IRI), which allows applications of URIs to be extended to use resource identifiers that have a much wider repertoire of characters. It also provides corresponding "internationalized" versions of other constructs from [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.), such as URI references. The syntax of IRIs is defined in Section 2 (IRI Syntax).

Within this document, Section 5 (Use of IRIs) discusses the use of IRIs in different situations. Section 7 (URI/IRI Processing Guidelines (Informative)) gives additional informative guidelines. Section 9 (Security Considerations) discusses IRI-specific security considerations.

This specification is part of a collection of specifications intended to replace [RFC3987] (Dürst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.). [Bidi] (Dürst, M., Masinter, L., and A. Allawi (عادل علاوي), “Guidelines for Internationalized Resource Identifiers with Bi-directional Characters (Bidi IRIs),” March 2012.) discusses the special case of bidirectional IRIs, IRIs using characters from scripts written right-to-left. [Equivalence] (Masinter, L. and M. Dürst, “Equivalence and Canonicalization of Internationalized Resource Identifiers (IRIs),” March 2012.) gives guidelines for applications wishing to determine if two IRIs are equivalent, as well as defining some equivalence methods. [RFC4395bis] (Hansen, T., Hardie, T., and L. Masinter, “Guidelines and Registration Procedures for New URI/IRI Schemes,” December 2011.) updates the URI scheme registration guidelines and procedures to note that every URI scheme is also automatically an IRI scheme and to allow scheme definitions to be directly described in terms of Unicode characters.

1.2.  Applicability

IRIs are designed to allow protocols and software that deal with URIs to be updated to handle IRIs. Processing of IRIs is accomplished by extending the URI syntax while retaining (and not expanding) the set of "reserved" characters, such that the syntax for any URI scheme may be extended to allow non-ASCII characters. In addition, following parsing of an IRI, it is possible to construct a corresponding URI by first encoding characters outside of the allowed URI range and then reassembling the components.

Practical use of IRIs forms in place of URIs forms depends on the following conditions being met:

a.

A protocol or format element MUST be explicitly designated to be able to carry IRIs. The intent is to avoid introducing IRIs into contexts that are not defined to accept them. For example, XML schema [XMLSchema] (Biron, P. and A. Malhotra, “XML Schema Part 2: Datatypes Second Edition,” October 2004.) has an explicit type "anyURI" that includes IRIs and IRI references. Therefore, IRIs and IRI references can be used in attributes and elements of type "anyURI". On the other hand, in HTTP/1.1 ([RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.)) , the Request URI is defined as a URI, which means that direct use of IRIs is not allowed in HTTP requests.

b.

The protocol or format carrying the IRIs MUST have a mechanism to represent the wide range of characters used in IRIs, either natively or by some protocol- or format-specific escaping mechanism (for example, numeric character references in [XML1] (Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and F. Yergeau, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.)).

c.

The URI scheme definition, if it explicitly allows a percent sign ("%") in any syntactic component, SHOULD define the interpretation of sequences of percent-encoded octets (using "%XX" hex octets) as octets from sequences of UTF-8 encoded characters; this is recommended in the guidelines for registering new schemes, [RFC4395bis] (Hansen, T., Hardie, T., and L. Masinter, “Guidelines and Registration Procedures for New URI/IRI Schemes,” December 2011.). For example, this is the practice for IMAP URLs [RFC2192] (Newman, C., “IMAP URL Scheme,” September 1997.), POP URLs [RFC2384] (Gellens, R., “POP URL Scheme,” August 1998.) and the URN syntax [RFC2141] (Moats, R., “URN Syntax,” May 1997.)). Note that use of percent-encoding may also be restricted in some situations, for example, URI schemes that disallow percent-encoding might still be used with a fragment identifier which is percent-encoded (e.g., [XPointer] (Grosso, P., Maler, E., Marsh, J., and N. Walsh, “XPointer Framework,” March 2003.)). See Section 5.4 (Use of UTF-8 for Encoding Original Characters) for further discussion.

1.3.  Definitions

The following definitions are used in this document; they follow the terms in [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,” April 1997.), [RFC2277] (Alvestrand, H., “IETF Policy on Character Sets and Languages,” January 1998.), and [ISO10646] (International Organization for Standardization, “ISO/IEC 10646:2011: Information Technology - Universal Multiple-Octet Coded Character Set (UCS),” March 20011.).

character:

A member of a set of elements used for the organization, control, or representation of data. For example, "LATIN CAPITAL LETTER A" names a character.

octet:

An ordered sequence of eight bits considered as a unit.

character repertoire:

A set of characters (set in the mathematical 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.

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] (International Organization for Standardization, “ISO/IEC 10646:2011: Information Technology - Universal Multiple-Octet Coded Character Set (UCS),” March 20011.) and the Unicode Standard [UNIV6] (The Unicode Consortium, “The Unicode Standard, Version 6.1.0 (Mountain View, CA, The Unicode Consortium, 2012, ISBN 978-1-936213-02-3),” 2012.).

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] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifiers (URI): Generic Syntax,” August 1998.) URIs did not include fragment identifiers, but in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) fragment identifiers are part of URIs.

LEIRI (Legacy Extended IRI):

This term is used in various XML specifications to refer to strings that, although not valid IRIs, are acceptable input to the processing rules in Section 6.2 (Conversion of Legacy Extended IRIs to IRIs).

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.

create (a URI or IRI):

With respect to URIs and IRIs, the term is used for the initial creation. This may be the initial creation of a resource with a certain identifier, or the initial exposition of a resource under a particular identifier.

generate (a URI or IRI):

With respect to URIs and IRIs, the term is used when the identifier is generated by derivation from other information.

parsed URI component:

When a URI processor parses a URI (following the generic syntax or a scheme-specific syntax, the result is a set of parsed URI components, each of which has a type (corresponding to the syntactic definition) and a sequence of URI characters.

parsed IRI component:

When an IRI processor parses an IRI directly, following the general syntax or a scheme-specific syntax, the result is a set of parsed IRI components, each of which has a type (corresponding to the syntactic definition) and a sequence of IRI characters. (This definition is analogous to "parsed URI component".)

IRI scheme:

A URI scheme may also be known as an "IRI scheme" if the scheme's syntax has been extended to allow non-US-ASCII characters according to the rules in this document.

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 for 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 a document format that is limited to US-ASCII, this document uses 'XML Notation'. XML Notation uses a leading '&#x', a trailing ';', and the hexadecimal number of the character in the UCS in between. For example, Я stands for CYRILLIC CAPITAL LETTER YA (Я). In this notation, an actual '&' is denoted by '&'. This notation is only used in the ASCII version(s) of this document, because in the other versions, non-ASCII characters are used directly.

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

2.  IRI Syntax

This section defines the syntax of Internationalized Resource Identifiers (IRIs).

As with URIs, an IRI is defined as a sequence of characters, not as a sequence of octets. This definition accommodates the fact that IRIs may be written on paper or read over the radio as well as stored or transmitted digitally. The same IRI might be represented as different sequences of octets in different protocols or documents if these protocols or documents use different character encodings (and/or transfer encodings). Using the same character encoding as the containing protocol or document ensures that the characters in the IRI can be handled (e.g., searched, converted, displayed) in the same way as the rest of the protocol or document.

2.1.  Summary of IRI Syntax

The IRI syntax extends the URI syntax in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) by extending the class of unreserved characters, primarily by adding the characters of the UCS (Universal Character Set, [ISO10646] (International Organization for Standardization, “ISO/IEC 10646:2011: Information Technology - Universal Multiple-Octet Coded Character Set (UCS),” March 20011.)) beyond U+007F, subject to the limitations given in the syntax rules below and in Section 5.1 (Limitations on UCS Characters Allowed in IRIs).

The syntax and use of components and reserved characters is the same as that in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.). Each URI scheme thus also functions as an IRI scheme, in that scheme-specific parsing rules for URIs of a scheme are extended to allow parsing of IRIs using the same parsing rules.

All the operations defined in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.), such as the resolution of relative references, can be applied to IRIs by IRI-processing software in exactly the same way as they are for URIs by URI-processing software.

Characters outside the US-ASCII repertoire MUST NOT be reserved and therefore MUST NOT be used for syntactical purposes, such as to delimit components in newly defined schemes. For example, U+00A2, CENT SIGN, is not allowed as a delimiter in IRIs, because it is in the 'iunreserved' category. This is similar to the fact that it is not possible to use '-' as a delimiter in URIs, because it is in the 'unreserved' category.

2.2.  ABNF for IRI References and IRIs

An ABNF definition for IRI references (which are the most general concept and the start of the grammar) and IRIs is given here. The syntax of this ABNF is described in [STD68] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.). Character numbers are taken from the UCS, without implying any actual binary encoding. Terminals in the ABNF are characters, not octets.

The following grammar closely follows the URI grammar in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.), 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] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) because of the above-mentioned expansion, and rules that are the same as those in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.). For rules that are different than those in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.), 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 rule <pct-form> has been introduced in order to be able to reference it from other parts of the document.

The following rules are different from those in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.):

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

ihier-part     = "//" iauthority ipath-abempty
               / ipath-absolute
               / ipath-rootless
               / ipath-empty

IRI-reference  = IRI / irelative-ref

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

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

irelative-part = "//" iauthority ipath-abempty
               / ipath-absolute
               / ipath-noscheme
               / ipath-empty

iauthority     = [ iuserinfo "@" ] ihost [ ":" port ]
iuserinfo      = *( iunreserved / pct-form / sub-delims / ":" )
ihost          = IP-literal / IPv4address / ireg-name

pct-form       = pct-encoded

ireg-name      = *( iunreserved / sub-delims )

ipath          = ipath-abempty   ; begins with "/" or is empty
               / ipath-absolute  ; begins with "/" but not "//"
               / ipath-noscheme  ; begins with a non-colon segment
               / ipath-rootless  ; begins with a segment
               / ipath-empty     ; zero characters

ipath-abempty  = *( path-sep isegment )
ipath-absolute = path-sep [ isegment-nz *( path-sep isegment ) ]
ipath-noscheme = isegment-nz-nc *( path-sep isegment )
ipath-rootless = isegment-nz *( path-sep isegment )
ipath-empty    = ""
path-sep       = "/"

isegment       = *ipchar
isegment-nz    = 1*ipchar
isegment-nz-nc = 1*( iunreserved / pct-form / sub-delims
                     / "@" )
               ; non-zero-length segment without any colon ":"

ipchar         = iunreserved / pct-form / sub-delims / ":"
               / "@"

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

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

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

ucschar        = %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF
               / %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD
               / %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD
               / %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD
               / %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD
               / %xD0000-DFFFD / %xE1000-EFFFD

iprivate       = %xE000-F8FF / %xE0000-E0FFF / %xF0000-FFFFD
               / %x100000-10FFFD

Some productions are ambiguous. The "first-match-wins" (a.k.a. "greedy") algorithm applies. For details, see [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.).

The following rules are the same as those in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.):

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

port           = *DIGIT

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

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

IPv6address    =                            6( h16 ":" ) ls32
               /                       "::" 5( h16 ":" ) ls32
               / [               h16 ] "::" 4( h16 ":" ) ls32
               / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
               / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
               / [ *3( h16 ":" ) h16 ] "::"    h16 ":"   ls32
               / [ *4( h16 ":" ) h16 ] "::"              ls32
               / [ *5( h16 ":" ) h16 ] "::"              h16
               / [ *6( h16 ":" ) h16 ] "::"

h16            = 1*4HEXDIG
ls32           = ( h16 ":" h16 ) / IPv4address

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

dec-octet      = DIGIT                 ; 0-9
               / %x31-39 DIGIT         ; 10-99
               / "1" 2DIGIT            ; 100-199
               / "2" %x30-34 DIGIT     ; 200-249
               / "25" %x30-35          ; 250-255

pct-encoded    = "%" HEXDIG HEXDIG

unreserved     = ALPHA / DIGIT / "-" / "." / "_" / "~"
reserved       = gen-delims / sub-delims
gen-delims     = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims     = "!" / "$" / "&" / "'" / "(" / ")"
               / "*" / "+" / "," / ";" / "="

This syntax does not support IPv6 scoped addressing zone identifiers.

3.  Processing IRIs and related protocol elements

IRIs are meant to replace URIs in identifying resources within new versions of protocols, formats, and software components that use a UCS-based character repertoire. Protocols and components may use and process IRIs directly. However, there are still numerous systems and protocols which only accept URIs or components of parsed URIs; that is, they only accept sequences of characters within the subset of US-ASCII characters allowed in URIs.

This section defines specific processing steps for IRI consumers which establish the relationship between the string given and the interpreted derivatives. These processing steps apply to both IRIs and IRI references (i.e., absolute or relative forms); for IRIs, some steps are scheme specific.

3.1.  Converting to UCS

Input that is already in a Unicode form (i.e., a sequence of Unicode characters or an octet-stream representing a Unicode-based character encoding such as UTF-8 or UTF-16) should be left as is and not normalized or changed.

An IRI or IRI reference is a sequence of characters from the UCS. For input from presentations (written on paper, read aloud) or translation from other representations (a text stream using a legacy character encoding), convert the input to Unicode. Note that some character encodings or transcriptions can be converted to or represented by more than one sequence of Unicode characters. Ideally the resulting IRI would use a normalized form, such as Unicode Normalization Form C [UTR15] (Davis, M. and M. Dürst, “Unicode Normalization Forms,” March 2008.), since that ensures a stable, consistent representation that is most likely to produce the intended results. Previous versions of this specification required normalization at this step. However, attempts to require normalization in other protocols have met with strong enough resistance that requiring normalization here was considered impractical. Implementers and users are cautioned that, while denormalized character sequences are valid, they might be difficult for other users or processes to reproduce and might lead to unexpected results.

3.2.  Parse the IRI into IRI components

Parse the IRI, either as a relative reference (no scheme) or using scheme specific processing (according to the scheme given); the result is a set of parsed IRI components.

3.3.  General percent-encoding of IRI components

Except as noted in the following subsections, IRI components are mapped to the equivalent URI components by percent-encoding those characters not allowed in URIs. Previous processing steps will have removed some characters, and the interpretation of reserved characters will have already been done (with the syntactic reserved characters outside of the IRI component). This mapping is defined for all sequences of Unicode characters, whether or not they are valid for the component in question.

For each character which is not allowed anywhere in a valid URI apply the following steps.

Convert to UTF-8:

Convert the character to a sequence of one or more octets using UTF-8 [STD63] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.).

Percent encode:

Convert each octet of this sequence to %HH, where HH is the hexadecimal notation of the octet value. The hexadecimal notation SHOULD use uppercase letters. (This is the general URI percent-encoding mechanism in Section 2.1 of [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.).)

Note that the mapping is an identity transformation for parsed URI components of valid URIs, and is idempotent: applying the mapping a second time will not change anything.

3.4.  Mapping ireg-name

The mapping from <ireg-name> to a <reg-name> requires a choice between one of the two methods described below.

3.4.1.  Mapping using Percent-Encoding

The ireg-name component SHOULD be converted according to the general procedure for percent-encoding of IRI components described in Section 3.3 (General percent-encoding of IRI components).

For example, the IRI
"http://résumé.example.org"
will be converted to
"http://r%C3%A9sum%C3%A9.example.org".

This conversion for ireg-name is in line with Section 3.2.2 of [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.), which does not mandate a particular registered name lookup technology. For further background, see [RFC6055] (Thaler, D., Klensin, J., and S. Cheshire, “IAB Thoughts on Encodings for Internationalized Domain Names,” February 2011.) and [Gettys] (Gettys, J., “URI Model Consequences,” .).

3.4.2.  Mapping using Punycode

In situations where it is certain that <ireg-name> is intended to be used as a domain name to be processed by Domain Name Lookup (as per [RFC5891] (Klensin, J., “Internationalized Domain Names in Applications (IDNA): Protocol,” August 2010.)), an alternative method MAY be used, converting <ireg-name> as follows:

If there are any sequences of <pct-encoded>, and their corresponding octets all represent valid UTF-8 octet sequences, then convert these back to Unicode character sequences. (If any <pct-encoded> sequences are not valid UTF-8 octet sequences, then leave the entire field as is without any change, since punycode encoding would not succeed.)

Replace the ireg-name part of the IRI by the part converted using the Domain Name Lookup procedure (Subsections 5.3 to 5.5) of [RFC5891] (Klensin, J., “Internationalized Domain Names in Applications (IDNA): Protocol,” August 2010.). on each dot-separated label, and by using U+002E (FULL STOP) as a label separator. This procedure may fail, but this would mean that the IRI cannot be resolved. In such cases, if the domain name conversion fails, then the entire IRI conversion fails. Processors that have no mechanism for signalling a failure MAY instead substitute an otherwise invalid host name, although such processing SHOULD be avoided.

For example, the IRI
"http://résumé.example.org"
is converted to
"http://xn--rsum-bad.example.org".

This conversion for ireg-name will be better able to deal with legacy infrastructure that cannot handle percent-encoding in domain names.

3.4.3.  Additional Considerations

Note:

Domain Names may appear in parts of an IRI other than the ireg-name part. It is the responsibility of scheme-specific implementations (if the Internationalized Domain Name is part of the scheme syntax) or of server-side implementations (if the Internationalized Domain Name is part of 'iquery') to apply the necessary conversions at the appropriate point. Example: Trying to validate the Web page at http://résumé.example.org would lead to an IRI of
http://validator.w3.org/check?uri=http%3A%2F%2Frésumé
.example.org, which would convert to a URI of
http://validator.w3.org/check?uri=http%3A%2F%2Fr%C3%A9sum%C3%A9.
example.org. The server-side implementation is responsible for making the necessary conversions to be able to retrieve the Web page.

Note:

In this process, characters allowed in URI references and existing percent-encoded sequences are not encoded further. (This mapping is similar to, but different from, the encoding applied when arbitrary content is included in some part of a URI.) For example, an IRI of "http://www.example.org/red%09rosé#red" 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".

3.5.  Mapping query components

For compatibility with existing deployed HTTP infrastructure, the following special case applies for the schemes "http" and "https" when an IRI is found in a document whose charset is not based on UCS (e.g., not UTF-8 or UTF-16). In such a case, the "query" component of an IRI is mapped into a URI by using the document charset rather than UTF-8 as the binary representation before pct-encoding. This mapping is not applied for any other scheme or component.

3.6.  Mapping IRIs to URIs

The mapping from an IRI to URI is accomplished by applying the mapping above (from IRI to URI components) and then reassembling a URI from the parsed URI components using the original punctuation that delimited the IRI components.

4.  Converting URIs to IRIs

In some situations, for presentation and further processing, it is desirable to convert a URI into an equivalent IRI without unnecessary percent encoding. Of course, every URI is already an IRI in its own right without any conversion. This section gives one possible procedure for URI to IRI mapping.

The conversion described in this section, if given a valid URI, will result in an IRI that maps back to the URI used as an input for the conversion (except for potential case differences in percent-encoding and for potential percent-encoded unreserved characters). However, the IRI resulting from this conversion may differ from the original IRI (if there ever was one).

URI-to-IRI conversion removes percent-encodings, but not all percent-encodings can be eliminated. There are several reasons for this:

1.

Some percent-encodings are necessary to distinguish percent-encoded and unencoded uses of reserved characters.

2.

Some percent-encodings cannot be interpreted as sequences of UTF-8 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] (Dürst, M., “The Properties and Promises of UTF-8,” September 1997.).)

3.

The conversion may result in a character that is not appropriate in an IRI. See Section 2.2 (ABNF for IRI References and IRIs), and Section 5.1 (Limitations on UCS Characters Allowed in IRIs) for further details.

4.

IRI to URI conversion has different rules for dealing with domain names and query parameters.

Conversion from a URI to an IRI MAY be done by using the following steps:

1.

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

2.

Convert all percent-encodings ("%" followed by two hexadecimal digits) to the corresponding octets, except those corresponding to "%", characters in "reserved", and characters in US-ASCII not allowed in URIs.

3.

Re-percent-encode any octet produced in step 2 that is not part of a strictly legal UTF-8 octet sequence.

4.

Re-percent-encode all octets produced in step 3 that in UTF-8 represent characters that are not appropriate according to Section 2.2 (ABNF for IRI References and IRIs) and Section 5.1 (Limitations on UCS Characters Allowed in IRIs).

5.

Interpret the resulting octet sequence as a sequence of characters encoded in UTF-8.

6.

URIs known to contain domain names in the reg-name component SHOULD convert punycode-encoded domain name labels to the corresponding characters using the ToUnicode procedure.

This procedure will convert as many percent-encoded characters as possible to characters in an IRI. Because there are some choices when step 4 is applied (see Section 5.1 (Limitations on UCS Characters Allowed in IRIs)), 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".

4.1.  Examples

This section shows various examples of converting URIs to IRIs. Each example shows the result after each of the steps 1 through 6 is applied. XML Notation is used for the final result. Octets are denoted by "<" followed by two hexadecimal digits followed by ">".

The following example contains the sequence "%C3%BC", which is a strictly legal UTF-8 sequence, and which is converted into the actual character U+00FC, LATIN SMALL LETTER U WITH DIAERESIS (also known as u-umlaut).

1.

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

2.

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

3.

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

4.

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

5.

http://www.example.org/Dürst

6.

http://www.example.org/Dürst

The following example contains the sequence "%FC", which might represent U+00FC, LATIN SMALL LETTER U WITH DIAERESIS, in the
iso-8859-1 character encoding. (It might represent other characters in other character encodings. For example, the octet <fc> in iso-8859-5 represents U+045C, CYRILLIC SMALL LETTER KJE.) Because <fc> is not part of a strictly legal UTF-8 sequence, it is re-percent-encoded in step 3.

1.

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

2.

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

3.

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

4.

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

5.

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

6.

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. The direct use of this character is forbidden 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

6.

http://納豆.example.org/%E2%80%AE

Note that the label "xn--99zt52a" is converted to U+7D0D U+8C46 (Japanese Natto, 納豆). ((EDITOR NOTE: There is some inconsistency in this note.))

5.  Use of IRIs

5.1.  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 (ABNF for IRI References and IRIs). 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] (Hoffman, P. and M. Blanchet, “Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN),” March 2003.).

c.

At the start of a component, the use of combining marks is strongly discouraged. As an example, a COMBINING TILDE OVERLAY (U+0334) would be very confusing at the start of a <isegment>. Combined with the preceeding '/', it might look like a solidus with combining tilde overlay, but IRI processing software will parse and process the '/' separately.

d.

The ZERO WIDTH NON-JOINER (U+200C) and ZERO WIDTH JOINER (U+200D) are invisible in most contexts, but are crucial in some very limited contexts. Appendix A of [RFC5892] (Fältström, P., “The Unicode Code Points and Internationalized Domain Names for Applications (IDNA),” August 2010.) contains contextual restrictions for these and some other characters. The use of these characters are strongly discouraged except in the relevant contexts.

Additional information is available from [UNIXML] (Dürst, M. and A. Freytag, “Unicode in XML and other Markup Languages,” June 2003.). [UNIXML] (Dürst, M. and A. Freytag, “Unicode in XML and other Markup Languages,” June 2003.) 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.

5.2.  Software Interfaces and Protocols

Although an IRI is defined as a sequence of characters, software interfaces for URIs typically function on sequences of octets or other kinds of code units. Thus, software interfaces and protocols MUST define which character encoding is used.

Intermediate software interfaces between IRI-capable components and URI-only components MUST map the IRIs per Section 3.6 (Mapping IRIs to URIs), 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.

5.3.  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 3.6 (Mapping IRIs to URIs). For example, in HTML or XML, numeric character references SHOULD be used. If a document as a whole has a native character encoding and that character encoding is not UTF-8, then IRIs MUST NOT be placed into the document in the UTF-8 character encoding.

((UPDATE THIS NOTE)) Note: Some formats already accommodate IRIs, although they use different terminology. HTML 4.0 [HTML4] (Raggett, D., Le Hors, A., and I. Jacobs, “HTML 4.01 Specification,” December 1999.) defines the conversion from IRIs to URIs as error-avoiding behavior. XML 1.0 [XML1] (Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and F. Yergeau, “Extensible Markup Language (XML) 1.0 (Fifth Edition),” November 2008.), XLink [XLink] (DeRose, S., Maler, E., Orchard, D., and N. Walsh, “XML Linking Language (XLink) Version 1.1,” May 2010.), XML Schema [XMLSchema] (Biron, P. and A. Malhotra, “XML Schema Part 2: Datatypes Second Edition,” October 2004.), 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] (Dürst, M., Yergeau, F., Ishida, R., Wolf, M., and T. Texin, “Character Model for the World Wide Web 1.0: Resource Identifiers,” November 2004.).

5.4.  Use of UTF-8 for Encoding Original Characters

This section discusses details and gives examples for point c) in Section 1.2 (Applicability). 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] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.).

For new URI schemes, using UTF-8 is recommended in [RFC4395bis] (Hansen, T., Hardie, T., and L. Masinter, “Guidelines and Registration Procedures for New URI/IRI Schemes,” December 2011.). Examples where UTF-8 is already used are the URN syntax [RFC2141] (Moats, R., “URN Syntax,” May 1997.), IMAP URLs [RFC2192] (Newman, C., “IMAP URL Scheme,” September 1997.), POP URLs [RFC2384] (Gellens, R., “POP URL Scheme,” August 1998.), XMPP URLs [RFC5122] (Saint-Andre, P., “Internationalized Resource Identifiers (IRIs) and Uniform Resource Identifiers (URIs) for the Extensible Messaging and Presence Protocol (XMPP),” February 2008.), and the 'mailto:' scheme [RFC6068] (Dürst, M., Masinter, L., and J. Zawinski, “The 'mailto' URI Scheme,” October 2010.). On the other hand, because the HTTP URI scheme does not specify how to encode original characters, only some HTTP URLs can have corresponding but different 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 : "http://www.example.org/résumé.html" ("é" is the e-acute character, and "%C3%A9" is the UTF-8 encoded and percent-encoded representation of that character). On the other hand, for a document with a URI of "http://www.example.org/r%E9sum%E9.html", the percent-encoded octets cannot be converted to actual characters in an IRI, as the percent-encoding is not based on UTF-8.

For most URI schemes, there is no need to upgrade their scheme definition in order for them to work with IRIs. The main case where 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.

This specification updates the IANA registry of URI schemes to note their applicability to IRIs, see Section 8 (IANA Considerations). All IRIs use URI schemes, and all URIs with URI schemes can be used as IRIs, even though in some cases only by using URIs directly as IRIs, without any conversion.

Scheme definitions can impose restrictions on the syntax of scheme-specific URIs; i.e., URIs that are admissible under the generic URI syntax [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) may not be admissible due to narrower syntactic constraints imposed by a URI scheme specification. URI scheme definitions cannot broaden the syntactic restrictions of the generic URI syntax; otherwise, it would be possible to generate URIs that satisfied the scheme-specific syntactic constraints without satisfying the syntactic constraints of the generic URI syntax. However, additional syntactic constraints imposed by URI scheme specifications are applicable to IRI, as the corresponding URI resulting from the mapping defined in Section 3.6 (Mapping IRIs to URIs) 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 generally applies to all parts of a URI. However, it is possible that the capability of IRIs to represent a wide range of characters directly is used just in some parts of the IRI (or IRI reference). The other parts of the IRI may only contain US-ASCII characters, or they may not be based on UTF-8. They may be based on another character encoding, or they may directly encode raw binary data (see also [RFC2397] (Masinter, L., “The "data" URL scheme,” August 1998.)).

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 [XPointer] (Grosso, P., Maler, E., Marsh, J., and N. Walsh, “XPointer Framework,” March 2003.). The IRI corresponding to the above URI would be
"http://www.example.org/r%E9sum%E9.xml#résumé".

Similar considerations apply to query parts. The functionality of IRIs (namely, to be able to include non-ASCII characters) can only be used if the query part is encoded in UTF-8.

5.5.  Relative IRI References

Processing of relative IRI references against a base is handled straightforwardly; the algorithms of [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) can be applied directly, treating the characters additionally allowed in IRI references in the same way that unreserved characters are treated in URI references.

6.  Legacy Extended IRIs (LEIRIs)

In some cases, there have been formats which have used a protocol element which is a variant of the IRI definition; these variants have usually been somewhat less restricted in syntax. This section provides a definition and a name (Legacy Extended IRI or LEIRI) for one of these variants used widely in XML-based protocols. This variant has to be used with care; it requires further processing before being fully interchangeable as IRIs. New protocols and formats SHOULD NOT use Legacy Extended IRIs. Even where Legacy Extended IRIs are allowed, only IRIs fully conforming to the syntax definition in Section 2.2 (ABNF for IRI References and IRIs) SHOULD be created, generated, and used. The provisions in this section also apply to Legacy Extended IRI references.

6.1.  Legacy Extended IRI Syntax

This section defines Legacy Extended IRIs (LEIRIs). The syntax of Legacy Extended IRIs is the same as that for <IRI-reference>, except that the ucschar production is replaced by the leiri-ucschar production:

leiri-ucschar  = " " / "<" / ">" / '"' / "{" / "}" / "|"
               / "\" / "^" / "`" / %x0-1F / %x7F-D7FF
               / %xE000-FFFD / %x10000-10FFFF

The restriction on bidirectional formatting characters in [Bidi] (Dürst, M., Masinter, L., and A. Allawi (عادل علاوي), “Guidelines for Internationalized Resource Identifiers with Bi-directional Characters (Bidi IRIs),” March 2012.) is lifted. The iprivate production becomes redundant.

Likewise, the syntax for Legacy Extended IRI references (LEIRI references) is the same as that for IRI references with the above replacement of ucschar with leiri-ucschar.

6.2.  Conversion of Legacy Extended IRIs to IRIs

To convert a Legacy Extended IRI (reference) to an IRI (reference), each character allowed in a Legacy Extended IRI (reference) but not allowed in an IRI (reference) (see Section 6.3 (Characters Allowed in Legacy Extended IRIs but not in IRIs)) MUST be percent-encoded by applying the steps in Section 3.3 (General percent-encoding of IRI components).

6.3.  Characters Allowed in Legacy Extended IRIs but not in IRIs

This section provides a list of the groups of characters and code points that are allowed in Legacy Extedend IRIs, but are not allowed in IRIs or are allowed in IRIs only in the query part. For each group of characters, advice on the usage of these characters is also given, concentrating on the reasons for why not to use them.

Space (U+0020): Some formats and applications use space as a delimiter, e.g., for items in a list. Appendix C of [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) also mentions that white space may have to be added when displaying or printing long URIs; the same applies to long IRIs. Spaces might disappear, or a single Legacy Extended IRI might incorrectly be interpreted as two or more separate ones.

Delimiters "<" (U+003C), ">" (U+003E), and '"' (U+0022): Appendix C of [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) suggests the use of double-quotes ("http://example.com/") and angle brackets (<http://example.com/>) as delimiters for URIs in plain text. These conventions are often used, and also apply to IRIs. Legacy Extended IRIs using these characters might be cut off at the wrong place.

Unwise characters "\" (U+005C), "^" (U+005E), "`" (U+0060), "{" (U+007B), "|" (U+007C), and "}" (U+007D): These characters originally were excluded from URIs because the respective codepoints are assigned to different graphic characters in some 7-bit or 8-bit encoding. Despite the move to Unicode, some of these characters are still occasionally displayed differently on some systems, e.g., U+005C as a Japanese Yen symbol. Also, the fact that these characters are not used in URIs or IRIs has encouraged their use outside URIs or IRIs in contexts that may include URIs or IRIs. In case a Legacy Extended IRI with such a character is used in such a context, the Legacy Extended IRI will be interpreted piecemeal.

The controls (C0 controls, DEL, and C1 controls, #x0 - #x1F #x7F - #x9F): There is no way to transmit these characters reliably except potentially in electronic form. Even when in electronic form, some software components might silently filter out some of these characters, or may stop processing alltogether when encountering some of them. These characters may affect text display in subtle, unnoticable ways or in drastic, global, and irreversible ways depending on the hardware and software involved. The use of some of these characters may allow malicious users to manipulate the display of a Legacy Extended IRI and its context.

Bidi formatting characters (U+200E, U+200F, U+202A-202E): These characters affect the display ordering of characters. Displayed Legacy Extended IRIs containing these characters cannot be converted back to electronic form (logical order) unambiguously. These characters may allow malicious users to manipulate the display of a Legacy Extended IRI and its context.

Specials (U+FFF0-FFFD): These code points provide functionality beyond that useful in a Legacy Extended IRI, for example byte order identification, annotation, and replacements for unknown characters and objects. Their use and interpretation in a Legacy Extended IRI serves no purpose and may lead to confusing display variations.

Private use code points (U+E000-F8FF, U+F0000-FFFFD, U+100000-10FFFD): Display and interpretation of these code points is by definition undefined without private agreement. Therefore, these code points are not suited for use on the Internet. They are not interoperable and may have unpredictable effects.

Tags (U+E0000-E0FFF): These characters provide a way to language tag in Unicode plain text. They are not appropriate for Legacy Extended IRIs because language information in identifiers cannot reliably be input, transmitted (e.g., on a visual medium such as paper), or recognized.

Non-characters (U+FDD0-FDEF, U+1FFFE-1FFFF, U+2FFFE-2FFFF, U+3FFFE-3FFFF, U+4FFFE-4FFFF, U+5FFFE-5FFFF, U+6FFFE-6FFFF, U+7FFFE-7FFFF, U+8FFFE-8FFFF, U+9FFFE-9FFFF, U+AFFFE-AFFFF, U+BFFFE-BFFFF, U+CFFFE-CFFFF, U+DFFFE-DFFFF, U+EFFFE-EFFFF, U+FFFFE-FFFFF, U+10FFFE-10FFFF): These code points are defined as non-characters. Applications may use some of them internally, but are not prepared to interchange them.

For reference, we here also list the code points and code units not even allowed in Legacy Extended IRIs:

Surrogate code units (D800-DFFF): These do not represent Unicode codepoints.

Non-characters (U+FFFE-FFFF): These are not allowed in XML nor LEIRIs.

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] (Alvestrand, H., “IETF Policy on Character Sets and Languages,” January 1998.), 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 4 (Converting URIs to IRIs) above may be performed. It is preferable not to perform this inverse conversion unless it is certain this can be done correctly.

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 presentation of an IRI (as a sequence of glyphs, in some order, in some visual display) 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 (ABNF for IRI References and IRIs) are met. This may be done by choosing appropriate input methods or variants/settings thereof, by appropriately converting the characters being input, by eliminating characters that cannot be converted, and/or by issuing a warning or error message to the user.

As an example of variant settings, input method editors for East Asian Languages usually allow the input of Latin letters and related characters in full-width or half-width versions. For IRI input, the input method editor should be set so that it produces half-width Latin letters and punctuation and full-width Katakana.

An input field primarily or solely used for the input of URIs/IRIs might allow the user to view an IRI as it is mapped to a URI. Places where the input of IRIs is frequent may provide the possibility for viewing an IRI as mapped to a URI. This will help users when some of the software they use does not yet accept IRIs.

An IRI input component interfacing to components that handle URIs, but not IRIs, must map the IRI to a URI before passing it to these components.

For the input of IRIs with right-to-left characters, please see [Bidi] (Dürst, M., Masinter, L., and A. Allawi (عادل علاوي), “Guidelines for Internationalized Resource Identifiers with Bi-directional Characters (Bidi IRIs),” March 2012.).

7.3.  URI/IRI Transfer between Applications

Many applications (for example, mail user agents) try to detect URIs appearing in plain text. For this, they use some heuristics based on URI syntax. They then allow the user to click on such URIs and retrieve the corresponding resource in an appropriate (usually scheme-dependent) application.

Such applications would need to be upgraded, in order to use the IRI syntax as a base for heuristics. In particular, a non-ASCII character should not be taken as the indication of the end of an IRI. Such applications also would need to make sure that they correctly convert the detected IRI from the character encoding of the document or application where the IRI appears, to the character encoding used by the system-wide IRI invocation mechanism, or to a URI (according to Section 3.6 (Mapping IRIs to URIs)) 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 octets, which will do the right thing with IRIs.

7.4.  URI/IRI Generation

Systems that offer resources through the Internet, where those resources have logical names, sometimes automatically generate URIs for the resources they offer. For example, some HTTP servers can generate a directory listing for a file directory and then respond to the generated URIs with the files.

Many legacy character encodings are in use in various file systems. Many currently deployed systems do not transform the local character representation of the underlying system before generating URIs.

For maximum interoperability, systems that generate resource identifiers should make the appropriate transformations. For example, if a file system contains a file named "résumé.html", a server should expose this as "r%C3%A9sum%C3%A9.html" in a URI, which allows use of "résumé.html" in an IRI, even if locally the file name is kept in a character encoding other than UTF-8.

This recommendation particularly applies to HTTP servers. For FTP servers, similar considerations apply; see [RFC2640] (Curtin, B., “Internationalization of the File Transfer Protocol,” July 1999.).

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.

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] (Davis, M. and M. Dürst, “Unicode Normalization Forms,” March 2008.) (which means that they are also in NFC).

As an example, the UCS contains the "fi" ligature at U+FB01 for compatibility reasons. Wherever possible, IRIs should use the two letters "f" and "i" rather than the "fi" ligature. An example where the latter may be used is in the query part of an IRI for an explicit search for a word written containing the "fi" ligature.

In certain cases, there is a chance that characters from different scripts look the same. The best known example is the similarity of the Latin "A", the Greek "Alpha", and the Cyrillic "A". To avoid such cases, IRIs should only be created where all the characters in a single component are used together in a given language. This usually means that all of these characters will be from the same script, but there are languages that mix characters from different scripts (such as Japanese). This is similar to the heuristics used to distinguish between letters and numbers in the examples above. Also, for Latin, Greek, and Cyrillic, using lowercase letters results in fewer ambiguities than using uppercase letters would.

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 [Bidi] (Dürst, M., Masinter, L., and A. Allawi (عادل علاوي), “Guidelines for Internationalized Resource Identifiers with Bi-directional Characters (Bidi IRIs),” March 2012.).

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] (Dürst, M., “The Properties and Promises of UTF-8,” September 1997.)) 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.

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 5.4 (Use of UTF-8 for Encoding Original Characters).

8.  IANA Considerations

NOTE: THIS SECTION NEEDS REVIEW AGAINST HAPPIANA WORK.

RFC Editor and IANA note: Please Replace RFC XXXX with the number of this document when it issues as an RFC, and RFC YYYY with the number of the RFC issued for draft-ietf-iri-rfc3987bis.

IANA maintains a registry of "URI schemes". This document attempts to make it clear from the registry that a "URI scheme" also serves an "IRI scheme", and makes several changes to the registry.

The description of the registry should be changed: "RFC 4395 defined an IANA-maintained registry of URI Schemes. RFC XXXX updates this registry to make it clear that the registered values also serve as IRI schemes, as defined in RFC YYYY."

The registry includes schemes marked as Permanent or Provisional. Previously, this was accomplished by having two sections, "Permanent" and "Provisional". However, in order to allow other status ("Historical", and possibly a Proposed status for proposals which have been received but not accepted), the registry should be changed so that the status is indicated in a separate "Status" column, whose values may be "Permanent", "Provisional" or "Historical". Changes in status as well as updates to the entire registration may be accomplished by requests and expert review.

9.  Security Considerations

The security considerations discussed in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) also apply to IRIs. In addition, the following issues require particular care for IRIs.

Incorrect encoding or decoding can lead to security problems. For example, some UTF-8 decoders do not check against overlong byte sequences. See [UTR36] (Davis, M. and M. Suignard, “Unicode Security Considerations,” August 2010.) Section 3 for details.

There are serious difficulties with relying on a human to verify that a an IRI (whether presented visually or aurally) is the same as another IRI or is the one intended. These problems exist with ASCII-only URIs (bl00mberg.com vs. bloomberg.com) but are strongly exacerbated when using the much larger character repertoire of Unicode. For details, see Section 2 of [UTR36] (Davis, M. and M. Suignard, “Unicode Security Considerations,” August 2010.). Using administrative and technical means to reduce the availability of such exploits is possible, but they are difficult to eliminate altogether. User agents SHOULD NOT rely on visual or perceptual comparison or verification of IRIs as a means of validating or assuring safety, correctness or appropriateness of an IRI. Other means of presenting users with the validity, safety, or appropriateness of visited sites are being developed in the browser community as an alternative means of avoiding these difficulties.

Besides the large character repertoire of Unicode, reasons for confusion include different forms of normalization and different normalization expectations, use of percent-encoding with various legacy encodings, and bidirectionality issues. See also [Bidi] (Dürst, M., Masinter, L., and A. Allawi (عادل علاوي), “Guidelines for Internationalized Resource Identifiers with Bi-directional Characters (Bidi IRIs),” March 2012.).

Confusion can occur in various IRI components, such as the domain name part or the path part, or between IRI components. For considerations specific to the domain name part, see [RFC5890] (Klensin, J., “Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework,” August 2010.). For considerations specific to particular protocols or schemes, see the security sections of the relevant specifications and registration templates. Administrators of sites that allow independent users to create resources in the same sub area have to be careful. Details are discussed in Section 7.5 (URI/IRI Selection).

The characters additionally allowed in Legacy Extended IRIs introduce additional security issues. For details, see Section 6.3 (Characters Allowed in Legacy Extended IRIs but not in IRIs).

10.  Acknowledgements

This document was derived from [RFC3987] (Dürst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.); the acknowledgments from that specification still apply.

In addition, this document was influenced by contributions from (in no particular order) Norman Walsh, Richard Tobin, Henry S. Thomson, John Cowan, Paul Grosso, the XML Core Working Group of the W3C, Chris Lilley, Björn Höhrmann, Felix Sasaki, Jeremy Carroll, Frank Ellermann, Michael Everson, Cary Karp, Matitiahu Allouche, Richard Ishida, Addison Phillips, Jonathan Rosenne, Najib Tounsi, Debbie Garside, Mark Davis, Sarmad Hussain, Ted Hardie, Konrad Lanz, Thomas Roessler, Lisa Dusseault, Julian Reschke, Giovanni Campagna, Anne van Kesteren, Mark Nottingham, Erik van der Poel, Marcin Hanclik, Marcos Caceres, Roy Fielding, Greg Wilkins, Pieter Hintjens, Daniel R. Tobias, Marko Martin, Maciej Stanchowiak, Wil Tan, Yui Naruse (成瀬ゆい), Michael A. Puls II, Dave Thaler, Tom Petch, John Klensin, Shawn Steele, Peter Saint-Andre, Geoffrey Sneddon, Chris Weber, Alex Melnikov, Slim Amamou, S. Moonesamy, Tim Berners-Lee, Yaron Goland, Sam Ruby, Adam Barth, Abdulrahman I. ALGhadir, Aharon Lanin, Thomas Milo, Murray Sargent, Marc Blanchet, and Mykyta Yevstifeyev.

11.  Main Changes Since RFC 3987

This section describes the main changes since [RFC3987] (Dürst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.).

11.1.  Split out Bidi, processing guidelines, comparison sections

Move some components (comparison, bidi, processing) into separate documents.

11.2.  Major restructuring of IRI processing model

Major restructuring of IRI processing model to make scheme-specific translation necessary to handle IDNA requirements and for consistency with web implementations.

Starting with IRI, you want one of:

a

IRI components (IRI parsed into UTF8 pieces)

b

URI components (URI parsed into ASCII pieces, encoded correctly)

c

whole URI (for passing on to some other system that wants whole URIs)

11.2.1.  OLD WAY

1. Pct-encoding on the whole thing to a URI. (c1) If you want a (maybe broken) whole URI, you might stop here.
2. Parsing the URI into URI components. (b1) If you want (maybe broken) URI components, stop here.
3. Decode the components (undoing the pct-encoding). (a) if you want IRI components, stop here.
4. reencode: Either using a different encoding some components (for domain names, and query components in web pages, which depends on the component, scheme and context), and otherwise using pct-encoding. (b2) if you want (good) URI components, stop here.
5. reassemble the reencoded components. (c2) if you want a (*good*) whole URI stop here.

11.2.2.  NEW WAY

1. Parse the IRI into IRI components using the generic syntax. (a) if you want IRI components, stop here.
2. Encode each components, using pct-encoding, IDN encoding, or special query part encoding depending on the component scheme or context. (b) If you want URI components, stop here.
3. reassemble the a whole URI from URI components. (c) if you want a whole URI stop here.

11.2.3.  Extension of Syntax

Added the tag range (U+E0000-E0FFF) to the iprivate production. Some IRIs generated with the new syntax may fail to pass very strict checks relying on the old syntax. But characters in this range should be extremely infrequent anyway.

11.2.4.  More to be added

TODO: There are more main changes that need to be documented in this section.

11.3.  Change Log

Note to RFC Editor: Please completely remove this section before publication.

11.3.1.  Changes after draft-ietf-iri-3987bis-01

Changes from draft-ietf-iri-3987bis-01 onwards are available as changesets in the IETF tools subversion repository at http://trac.tools.ietf.org/wg/iri/trac/log/draft-ietf-iri-3987bis/draft-ietf-iri-3987bis.xml.

11.3.2.  Changes from draft-duerst-iri-bis-07 to draft-ietf-iri-3987bis-00

Changed draft name, date, last paragraph of abstract, and titles in change log, and added this section in moving from draft-duerst-iri-bis-07 (personal submission) to draft-ietf-iri-3987bis-00 (WG document).

11.3.3.  Changes from -06 to -07 of draft-duerst-iri-bis

Major restructuring of the processing model, see Section 11.2 (Major restructuring of IRI processing model).

11.4.  Changes from -00 to -01

• Removed 'mailto:' before mail addresses of authors.
• Added "<to be done>" as right side of 'href-strip' rule. Fixed '|' to '/' for alternatives.

11.5.  Changes from -05 to -06 of draft-duerst-iri-bis-00

• Add HyperText Reference, change abstract, acks and references for it
• Add Masinter back as another editor.
• Masinter integrates HRef material from HTML5 spec.
• Rewrite introduction sections to modernize.

11.6.  Changes from -04 to -05 of draft-duerst-iri-bis

• Updated references.
• Changed IPR text to pre5378Trust200902.

11.7.  Changes from -03 to -04 of draft-duerst-iri-bis

• Added explicit abbreviation for LEIRIs.
• Mentioned LEIRI references.
• Completed text in LEIRI section about tag characters and about specials.

11.8.  Changes from -02 to -03 of draft-duerst-iri-bis

• Updated some references.
• Updated Michel Suginard's coordinates.

11.9.  Changes from -01 to -02 of draft-duerst-iri-bis

• Added tag range to iprivate (issue private-include-tags-115).
• Added Specials (U+FFF0-FFFD) to Legacy Extended IRIs.

11.10.  Changes from -00 to -01 of draft-duerst-iri-bis

• Changed from "IRIs with Spaces/Controls" to "Legacy Extended IRI" based on input from the W3C XML Core WG. Moved the relevant subsections to the back and promoted them to a section.
• Added some text re. Legacy Extended IRIs to the security section.
• Added a IANA Consideration Section.
• Added this Change Log Section.
• Added a section about "IRIs with Spaces/Controls" (converting from a Note in RFC 3987).

11.11.  Changes from RFC 3987 to -00 of draft-duerst-iri-bis

Fixed errata (see http://www.rfc-editor.org/cgi-bin/errataSearch.pl?rfc=3987).

12.  References

12.1. Normative References

12.2. Informative References

Authors' Addresses