| Internationalized Resource Identifiers (iri) | M.J. Duerst |
| Internet-Draft | Aoyama Gakuin University |
| Obsoletes: 3987 (if approved) | M.L. Suignard |
| Intended status: Standards Track | Unicode Consortium |
| Expires: February 15, 2012 | L. Masinter |
| Adobe | |
| August 14, 2011 |
Internationalized Resource Identifiers (IRIs)
draft-ietf-iri-3987bis-07
This document defines the Internationalized Resource Identifier (IRI) protocol element, as an extension of the Uniform Resource Identifier (URI). An IRI is a sequence of characters from the Universal Character Set (Unicode/ISO 10646). Grammar and processing rules are given for IRIs and related syntactic forms.
In addition, this document provides named additional rule sets for processing otherwise invalid IRIs, in a way that supports other specifications that wish to mandate common behavior for 'error' handling. In particular, rules used in some XML languages (LEIRI) and web applications are given.
Defining IRI as new protocol element (rather than updating or extending the definition of URI) allows independent orderly transitions: other protocols and languages that use URIs must explicitly choose to allow IRIs.
Guidelines are provided for the use and deployment of IRIs and related protocol elements when revising protocols, formats, and software components that currently deal only with URIs.
This (and several companion documents) are intended to obsolete RFC 3987, and also move towards IETF Draft Standard. 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 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 February 15, 2012.
Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved.
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A Uniform Resource Identifier (URI) is defined in [RFC3986] as a sequence of characters chosen from a limited subset of the repertoire of US-ASCII [ASCII] characters.
The characters in URIs are frequently used for representing words of natural languages. This usage has many advantages: Such URIs are easier to memorize, easier to interpret, easier to transcribe, easier to create, and easier to guess. For most languages other than English, however, the natural script uses characters other than A - Z. For many people, handling Latin characters is as difficult as handling the characters of other scripts is for those who use only the Latin alphabet. Many languages with non-Latin scripts are transcribed with Latin letters. These transcriptions are now often used in URIs, but they introduce additional difficulties.
The infrastructure for the appropriate handling of characters from additional scripts is now widely deployed in operating system and application software. Software that can handle a wide variety of scripts and languages at the same time is increasingly common. Also, an increasing number of protocols and formats can carry a wide range of characters.
URIs are used both as a protocol element (for transmission and processing by software) and also a presentation element (for display and handling by people who read, interpret, coin, or guess them). The transition between these roles is more difficult and complex when dealing with the larger set of characters than allowed for URIs in [RFC3986].
This document defines the protocol element called Internationalized Resource Identifier (IRI), which allow applications of URIs to be extended to use resource identifiers that have a much wider repertoire of characters. It also provides corresponding "internationalized" versions of other constructs from [RFC3986], such as URI references. The syntax of IRIs is defined in Section 2.
Using characters outside of A - Z in IRIs adds a number of difficulties. Section 4 discusses the use of IRIs in different situations. Section 7 gives additional informative guidelines. Section 9 discusses IRI-specific security considerations.
[Bidi] discusses the special case of bidirectional IRIs using characters from scripts written right-to-left. [Equivalence] gives guidelines for applications wishing to determine if two IRIs are equivalent, as well as defining some equivalence methods. [RFC4395bis] updates the URI scheme registration guidelines and proceedures 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.
When originally defining IRIs, several design alternatives were considered. Historically interested readers can find an overview in Appendix A of [RFC3987]. For some additional background on the design of URIs and IRIs, please also see [Gettys].
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:
The following definitions are used in this document; they follow the terms in [RFC2130], [RFC2277], and [ISO10646].
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 '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. In this notation, an actual '&' is denoted by '&'.
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].
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.
The IRI syntax extends the URI syntax in [RFC3986] by extending the class of unreserved characters, primarily 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 4.1.
The syntax and use of components and reserved characters is the same as that in [RFC3986]. Each "URI scheme" thus also functions as an "IRI scheme", in that scheme-specific parsing rules for URIs of a scheme are be extended to allow parsing of IRIs using the same parsing rules.
All the operations defined in [RFC3986], such as the resolution of relative references, can be applied to IRIs by IRI-processing software in exactly the same way as they are for URIs by URI-processing software.
Characters outside the US-ASCII repertoire MUST NOT be reserved and therefore MUST NOT be used for syntactical purposes, such as to delimit components in newly defined schemes. For example, U+00A2, CENT SIGN, is not allowed as a delimiter in IRIs, because it is in the 'iunreserved' category. This is similar to the fact that it is not possible to use '-' as a delimiter in URIs, because it is in the 'unreserved' category.
An ABNF definition for IRI references (which are the most general concept and the start of the grammar) and IRIs is given here. The syntax of this ABNF is described in [STD68]. Character numbers are taken from the UCS, without implying any actual binary encoding. Terminals in the ABNF are characters, not octets.
The following grammar closely follows the URI grammar in [RFC3986], except that the range of unreserved characters is expanded to include UCS characters, with the restriction that private UCS characters can occur only in query parts. The grammar is split into two parts: Rules that differ from [RFC3986] because of the above-mentioned expansion, and rules that are the same as those in [RFC3986]. For rules that are different than those in [RFC3986], the names of the non-terminals have been changed as follows. If the non-terminal contains 'URI', this has been changed to 'IRI'. Otherwise, an 'i' has been prefixed. The 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]:
IRI = scheme ":" ihier-part [ "?" iquery ]
[ "#" ifragment ]
ihier-part = "//" iauthority ipath-abempty
/ ipath-absolute
/ ipath-rootless
/ ipath-empty
IRI-reference = IRI / irelative-ref
absolute-IRI = scheme ":" ihier-part [ "?" iquery ]
irelative-ref = irelative-part [ "?" iquery ] [ "#" ifragment ]
irelative-part = "//" iauthority ipath-abempty
/ ipath-absolute
/ ipath-noscheme
/ ipath-empty
iauthority = [ iuserinfo "@" ] ihost [ ":" port ]
iuserinfo = *( iunreserved / pct-form / sub-delims / ":" )
ihost = IP-literal / IPv4address / ireg-name
pct-form = pct-encoded
ireg-name = *( iunreserved / sub-delims )
ipath = ipath-abempty ; begins with "/" or is empty
/ ipath-absolute ; begins with "/" but not "//"
/ ipath-noscheme ; begins with a non-colon segment
/ ipath-rootless ; begins with a segment
/ ipath-empty ; zero characters
ipath-abempty = *( path-sep isegment )
ipath-absolute = path-sep [ isegment-nz *( path-sep isegment ) ]
ipath-noscheme = isegment-nz-nc *( path-sep isegment )
ipath-rootless = isegment-nz *( path-sep isegment )
ipath-empty = 0<ipchar>
path-sep = "/"
isegment = *ipchar
isegment-nz = 1*ipchar
isegment-nz-nc = 1*( iunreserved / pct-form / sub-delims
/ "@" )
; non-zero-length segment without any colon ":"
ipchar = iunreserved / pct-form / sub-delims / ":"
/ "@"
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].
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.
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.
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 resource identifiers that are not already in a Unicode form (as when written on paper, read aloud, or represented in a text stream using a legacy character encoding), convert the IRI 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], since that ensures a stable, consistent representation that is most likely to produce the intended results. 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.
In other cases (written on paper, read aloud, or otherwise represented independent of any character encoding) represent the IRI as a sequence of characters from the UCS normalized according to Unicode Normalization Form C (NFC, [UTR15]).
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.
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.
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.
The ireg-name component SHOULD be converted according to the general procedure for percent-encoding of IRI components described in Section 3.3.
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], which does not mandate a particular registered name lookup technology. For further background, see [RFC6055] and [Gettys].
The ireg-name component MAY also be converted as follows:
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]. 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"
MAY be 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.
For compatibility with existing deployed HTTP infrastructure, the following special case applies for schemes "http" and "https" and IRIs whose origin has a document charset other than one which is UCS-based (e.g., UTF-8 or UTF-16). In such a case, the "query" component of an IRI is mapped into a URI by using the document charset rather than UTF-8 as the binary representation before pct-encoding. This mapping is not applied for any other scheme or component.
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.
In some situations, for presentation and further processing, it is desirable to convert a URI into an equivalent IRI in which natural characters are represented directly rather than percent encoded. Of course, every URI is already an IRI in its own right without any conversion, and in general there This section gives one such procedure for this conversion.
The conversion described in this section, if given a valid URI, will result in an IRI that maps back to the URI used as an input for the conversion (except for potential case differences in percent-encoding and for potential percent-encoded unreserved characters). However, the IRI resulting from this conversion may differ from the original IRI (if there ever was one).
URI-to-IRI conversion removes percent-encodings, but not all percent-encodings can be eliminated. There are several reasons for this:
Conversion from a URI to an IRI MAY be done by using the following steps:
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 4.1), 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".
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).
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.
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 forbiddin 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.
Note that the label "xn--99zt52a" is converted to U+7D0D U+8C46 (Japanese Natto). ((EDITOR NOTE: There is some inconsistency in this note.))
This section discusses limitations on characters and character sequences usable for IRIs beyond those given in Section 2.2. The considerations in this section are relevant when IRIs are created and when URIs are converted to IRIs.
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.
Although an IRI is defined as a sequence of characters, software interfaces for URIs typically function on sequences of octets or other kinds of code units. Thus, software interfaces and protocols MUST define which character encoding is used.
Intermediate software interfaces between IRI-capable components and URI-only components MUST map the IRIs per Section 3.6, when transferring from IRI-capable to URI-only components. This mapping SHOULD be applied as late as possible. It SHOULD NOT be applied between components that are known to be able to handle IRIs.
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. For example, in HTML or XML, numeric character references SHOULD be used. If a document as a whole has a native character encoding and that character encoding is not UTF-8, then IRIs MUST NOT be placed into the document in the UTF-8 character encoding.
((UPDATE THIS NOTE)) Note: Some formats already accommodate IRIs, although they use different terminology. HTML 4.0 [HTML4] defines the conversion from IRIs to URIs as error-avoiding behavior. XML 1.0 [XML1], XLink [XLink], XML Schema [XMLSchema], and specifications based upon them allow IRIs. Also, it is expected that all relevant new W3C formats and protocols will be required to handle IRIs [CharMod].
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 [RFC4395bis]. Examples where UTF-8 is already used are the URN syntax [RFC2141], IMAP URLs [RFC2192], and POP URLs [RFC2384]. On the other hand, because the HTTP URI scheme does not specify how to encode original characters, only some HTTP URLs can have corresponding but different IRIs.
For example, for a document with a URI of
"http://www.example.org/r%C3%A9sum%C3%A9.html", it is possible to construct a corresponding IRI (in XML notation, see Section 1.4): "http://www.example.org/résumé.html" ("é" stands for the e-acute character, and "%C3%A9" is the UTF-8 encoded and percent-encoded representation of that character). On the other hand, for a document with a URI of "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.
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 updates the IANA registry of URI schemes to note their applicability to IRIs, see Section 8. 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] may not be admissible due to narrower syntactic constraints imposed by a URI scheme specification. URI scheme definitions cannot broaden the syntactic restrictions of the generic URI syntax; otherwise, it would be possible to generate URIs that satisfied the scheme-specific syntactic constraints without satisfying the syntactic constraints of the generic URI syntax. However, additional syntactic constraints imposed by URI scheme specifications are applicable to IRI, as the corresponding URI resulting from the mapping defined in Section 3.6 MUST be a valid URI under the syntactic restrictions of generic URI syntax and any narrower restrictions imposed by the corresponding URI scheme 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]).
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]. The IRI corresponding to the above URI would be (in XML notation)
"http://www.example.org/r%E9sum%E9.xml#résumé".
Similar considerations apply to query parts. The functionality of IRIs (namely, to be able to include non-ASCII characters) can only be used if the query part is encoded in UTF-8.
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.
Some technical specifications and widely-deployed software have allowed additional variations and extensions of IRIs to be used in syntactic components.
Future technical specifications SHOULD NOT allow conforming producers to produce, or conforming content to contain, such forms, as they are not interoperable with other IRI consuming software.
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
Among other extensions, processors based on this specification also did not enforce the restriction on bidirectional formatting characters in [Bidi], and the iprivate production becomes redundant.
To convert a string allowed as a LEIRI to an IRI, each character allowed in leiri-ucschar but not in ucschar must be percent-encoded using Section 3.3.
This section provides a list of the groups of characters and code points that are allowed in some contexts but are not allowed in IRIs or are allowed in IRIs only in the query part. For each group of characters, advice on the usage of these characters is also given, concentrating on the reasons for why they are excluded from IRI use.
LEIRI preprocessing disallowed some code points and code units:
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.
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 3.7 above may be performed. It is preferable not to perform this inverse conversion unless it is certain this can be done correctly.
Some components allow users to enter URIs into the system by typing or dictation, for example. This software must be updated to allow for IRI entry.
A person viewing a visual representation of an IRI (as a sequence of glyphs, in some order, in some visual display) or hearing an IRI will use an entry method for characters in the user's language to input the IRI. Depending on the script and the input method used, this may be a more or less complicated process.
The process of IRI entry must ensure, as much as possible, that the restrictions defined in Section 2.2 are met. This may be done by choosing appropriate input methods or variants/settings thereof, by appropriately converting the characters being input, by eliminating characters that cannot be converted, and/or by issuing a warning or error message to the user.
As an example of variant settings, input method editors for East Asian Languages usually allow the input of Latin letters and related characters in full-width or half-width versions. For IRI input, the input method editor should be set so that it produces half-width Latin letters and punctuation and full-width Katakana.
An input field primarily or solely used for the input of URIs/IRIs might allow the user to view an IRI as it is mapped to a URI. Places where the input of IRIs is frequent may provide the possibility for viewing an IRI as mapped to a URI. This will help users when some of the software they use does not yet accept IRIs.
An IRI input component interfacing to components that handle URIs, but not IRIs, must map the IRI to a URI before passing it to these components.
For the input of IRIs with right-to-left characters, please see [Bidi].
Many applications (for example, mail user agents) try to detect URIs appearing in plain text. For this, they use some heuristics based on URI syntax. They then allow the user to click on such URIs and retrieve the corresponding resource in an appropriate (usually scheme-dependent) application.
Such applications would need to be upgraded, in order to use the IRI syntax as a base for heuristics. In particular, a non-ASCII character should not be taken as the indication of the end of an IRI. Such applications also would need to make sure that they correctly convert the detected IRI from the character encoding of the document or application where the IRI appears, to the character encoding used by the system-wide IRI invocation mechanism, or to a URI (according to Section 3.6) if the system-wide invocation mechanism only accepts 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.
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].
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] (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.
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].
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.
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 4.4.
RFC Editor and IANA note: Please Replace RFC XXXX with the number of this document when it issues as an RFC.
IANA maintains a registry of "URI schemes". A "URI scheme" also serves an "IRI scheme".
To clarify that the URI scheme registration process also applies to IRIs, change the description of the "URI schemes" registry header to say "[RFC4395] defines an IANA-maintained registry of URI Schemes. These registries include the Permanent and Provisional URI Schemes. RFC XXXX updates this registry to designate that schemes may also indicate their usability as IRI schemes.
Update "per RFC 4395" to "per RFC 4395 and RFC XXXX".
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. For example, some UTF-8 decoders do not check against overlong byte sequences. See [UTR36] 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]. 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].
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]. 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.
The characters additionally allowed in Legacy Extended IRIs introduce additional security issues. For details, see Section 6.
This document was derived from [RFC3987]; 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, Bjoern Hoehrmann, 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, 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.
This section describes the main changes since [RFC3987].
Move some components (comparison, bidi, processing) into separate documents.
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:
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.
TODO: There are more main changes that need to be documented in this section.
Note to RFC Editor: Please completely remove this section before publication.
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.
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).
Major restructuring of the processing model, see Section 11.2.