Internationalized Resource Identifiers (IRIs)

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 composed out of a very limited repertoire of characters; this design choice was made to support global transcription([RFC3986] 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 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.¶

Within this document, Section 5 discusses the use of IRIs in different situations. Section 7 gives additional informative guidelines. Section 9 discusses IRI-specific security considerations.¶

This specification is part of a collection of specifications intended to replace [RFC3987]. [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 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.¶

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.¶

a.

A protocol or format element MUST be explicitly designated to be able to carry IRIs. The intent is to avoid introducing IRIs into contexts that are not defined to accept them. For example, XML schema [XMLSchema] has an explicit type "anyURI" that includes IRIs and IRI references. Therefore, IRIs and IRI references can be in attributes and elements of type "anyURI". On the other hand, in the [RFC2616] definition of HTTP/1.1, the Request URI is defined as a URI, which means that direct use of IRIs is not allowed in HTTP requests.

b.

The protocol or format carrying the IRIs MUST have a mechanism to represent the wide range of characters used in IRIs, either natively or by some protocol- or format-specific escaping mechanism (for example, numeric character references in [XML1]).

c.

The URI scheme definition, if it explicitly allows a percent sign ("%") in any syntactic component, SHOULD define the interpretation of sequences of percent-encoded octets (using "%XX" hex octets) as octet from sequences of UTF-8 encoded strings; this is recommended in the guidelines for registering new schemes, [RFC4395bis]. For example, this is the practice for IMAP URLs [RFC2192], POP URLs [RFC2384] and the URN syntax [RFC2141]). Note that use of percent-encoding may also be restricted in some situations, for example, URI schemes that disallow percent-encoding might still be used with a fragment identifier which is percent-encoded (e.g., [XPointer]). See Section 5.4 for further discussion.

The following definitions are used in this document; they follow the terms in [RFC2130], [RFC2277], and [ISO10646].¶

character:

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

octet:

An ordered sequence of eight bits considered as a unit.

character repertoire:

A set of characters (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] and the Unicode Standard [UNIV6].

IRI reference:

Denotes the common usage of an Internationalized Resource Identifier. An IRI reference may be absolute or relative. However, the "IRI" that results from such a reference only includes absolute IRIs; any relative IRI references are resolved to their absolute form. Note that in [RFC2396] URIs did not include fragment identifiers, but in [RFC3986] fragment identifiers are part of URIs.

LEIRI (Legacy Extended IRI) processing:

This term was used in various XML specifications to refer to strings that, although not valid IRIs, were acceptable input to the processing rules in Section 6.2.

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 syntactice 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.

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].¶

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 5.1.¶

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    = ""
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.¶

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], 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. ¶

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.¶

Convert to UTF-8

Convert the character to a sequence of one or more octets using UTF-8 [RFC3629].

Percent encode

Convert each octet of this sequence to %HH, where HH is the hexadecimal notation of the octet value. The hexadecimal notation SHOULD use uppercase letters. (This is the general URI percent-encoding mechanism in Section 2.1 of [RFC3986].)

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

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

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.¶

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/D&#xFC;rst

6.

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

The following example contains the sequence "%FC", which might represent U+00FC, LATIN SMALL LETTER U WITH DIAERESIS, in the
iso-8859-1 character encoding. (It might represent other characters in other character encodings. For example, the octet <fc> in 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 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. ¶

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

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.¶

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].

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] 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]. [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/r&#xE9;sum&#xE9;.html" ("&#xE9;" stands for the e-acute character, and "%C3%A9" is the UTF-8 encoded and percent-encoded representation of that character). On the other hand, for a document with a URI of "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#r&#xE9;sum&#xE9;".¶

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

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.¶

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] 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.¶

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) MUST be percent-encoded by applying the steps in Section 3.3.¶

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] 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] 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.

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 4 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 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 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 "r&#xE9;sum&#xE9;.html", a server should expose this as "r%C3%A9sum%C3%A9.html" in a URI, which allows use of "r&#xE9;sum&#xE9;.html" in an IRI, even if locally the file name is kept in a character encoding other than UTF-8.¶

This recommendation particularly applies to HTTP servers. For FTP servers, similar considerations apply; see [RFC2640].¶

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 5.4.¶

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: ¶

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)

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

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

• 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.

• Updated references.
• Changed IPR text to pre5378Trust200902.

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

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

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

• 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).

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

12.1. Normative References

[ASCII]

American National Standards Institute, “Coded Character Set -- 7-bit American Standard Code for Information Interchange”, ANSI X3.4, 1986.

[ISO10646]

International Organization for Standardization, “ISO/IEC 10646:2011: Information Technology - Universal Multiple-Octet Coded Character Set (UCS)”, ISO Standard 10646, March 20011, <http://standards.iso.org/ittf/PubliclyAvailableStandards/c051273_ISO_IEC_10646_2011(E).zip>.

[RFC2119]

Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.

[RFC3491]

Hoffman, P. and M. Blanchet, “Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)”, RFC 3491, March 2003.

[RFC3629]

Yergeau, F., “UTF-8, a transformation format of ISO 10646”, STD 63, RFC 3629, November 2003.

[RFC3986]

Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax”, STD 66, RFC 3986, January 2005.

[RFC5890]

Klensin, J., “Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework”, RFC 5890, August 2010.

[RFC5891]

Klensin, J., “Internationalized Domain Names in Applications (IDNA): Protocol”, RFC 5891, August 2010.

[RFC5892]

Faltstrom, P., “The Unicode Code Points and Internationalized Domain Names for Applications (IDNA)”, RFC 5892, August 2010.

[STD68]

Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, STD 68, RFC 5234, January 2008.

[UNIV6]

The Unicode Consortium, “The Unicode Standard, Version 6.0.0 (Mountain View, CA, The Unicode Consortium, 2011, ISBN 978-1-936213-01-6)”, October 2010.

[UTR15]

Davis, M. and M. Duerst, “Unicode Normalization Forms”, Unicode Standard Annex #15, March 2008, <http://www.unicode.org/unicode/reports/tr15/tr15-23.html>.

12.2. Informative References

[Bidi]

Duerst, M. and L. Masinter, “Guidelines for Internationalized Resource Identifiers with Bi-directional Characters (Bidi IRIs)”, Internet-Draft draft-ietf-iri-bidi-guidelines-00 (work in progress), August 2011.

[CharMod]

Duerst, M., Yergeau, F., Ishida, R., Wolf, M., and T. Texin, “Character Model for the World Wide Web: Resource Identifiers”, World Wide Web Consortium Candidate Recommendation, November 2004, <http://www.w3.org/TR/charmod-resid>.

[Duerst97]

Duerst, M., “The Properties and Promises of UTF-8”, Proc. 11th International Unicode Conference, San Jose, September 1997, <http://www.ifi.unizh.ch/mml/mduerst/papers/PDF/IUC11-UTF-8.pdf>.

[Equivalence]

Masinter, L. and M. Duerst, “Equivalence and Canonicalization of Internationalized Resource Identifiers (IRIs)”, Internet-Draft draft-ietf-iri-comparison-00 (work in progress), August 2011.

[Gettys]

Gettys, J., “URI Model Consequences”, <http://www.w3.org/DesignIssues/ModelConsequences>.

[HTML4]

Raggett, D., Le Hors, A., and I. Jacobs, “HTML 4.01 Specification”, World Wide Web Consortium Recommendation, December 1999, <http://www.w3.org/TR/html401/appendix/notes.html#h-B.2>.

[LEIRI]

Thompson, H., Tobin, R., and N. Walsh, “Legacy extended IRIs for XML resource identification”, World Wide Web Consortium Note, November 2008, <http://www.w3.org/TR/leiri/>.

[RFC2045]

Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies”, RFC 2045, November 1996.

[RFC2130]

Weider, C., Preston, C., Simonsen, K., Alvestrand, H., Atkinson, R., Crispin, M., and P. Svanberg, “The Report of the IAB Character Set Workshop held 29 February - 1 March, 1996”, RFC 2130, April 1997.

[RFC2141]

Moats, R., “URN Syntax”, RFC 2141, May 1997.

[RFC2192]

Newman, C., “IMAP URL Scheme”, RFC 2192, September 1997.

[RFC2277]

Alvestrand, H., “IETF Policy on Character Sets and Languages”, BCP 18, RFC 2277, January 1998.

[RFC2368]

Hoffman, P., Masinter, L., and J. Zawinski, “The mailto URL scheme”, RFC 2368, July 1998.

[RFC2384]

Gellens, R., “POP URL Scheme”, RFC 2384, August 1998.

[RFC2396]

Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifiers (URI): Generic Syntax”, RFC 2396, August 1998.

[RFC2397]

Masinter, L., “The "data" URL scheme”, RFC 2397, August 1998.

[RFC2616]

Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2616, June 1999.

[RFC2640]

Curtin, B., “Internationalization of the File Transfer Protocol”, RFC 2640, July 1999.

[RFC3987]

Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs)”, RFC 3987, January 2005.

[RFC4395bis]

Hansen, T., Hardie, T., and L. Masinter, “Guidelines and Registration Procedures for New URI/IRI Schemes”, Internet-Draft draft-ietf-iri-4395bis-irireg-03 (work in progress), July 2011.

[RFC6055]

Thaler, D., Klensin, J., and S. Cheshire, “IAB Thoughts on Encodings for Internationalized Domain Names”, RFC 6055, February 2011.

[RFC6082]

Whistler, K., Adams, G., Duerst, M., Presuhn, R., and J. Klensin, “Deprecating Unicode Language Tag Characters: RFC 2482 is Historic”, RFC 6082, November 2010.

[UNIXML]

Duerst, M. and A. Freytag, “Unicode in XML and other Markup Languages”, Unicode Technical Report #20, World Wide Web Consortium Note, June 2003, <http://www.w3.org/TR/unicode-xml/>.

[UTR36]

Davis, M. and M. Suignard, “Unicode Security Considerations”, Unicode Technical Report #36, August 2010, <http://unicode.org/reports/tr36/>.

[XLink]

DeRose, S., Maler, E., and D. Orchard, “XML Linking Language (XLink) Version 1.0”, World Wide Web Consortium REC-xlink-20010627, June 2001, <http://www.w3.org/TR/xlink/#link-locators>.

[XML1]

Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and F. Yergeau, “Extensible Markup Language (XML) 1.0 (Forth Edition)”, World Wide Web Consortium REC-xml-20081126, August 2006, <http://www.w3.org/TR/REC-xml>.

[XMLNamespace]

Bray, T., Hollander, D., Layman, A., and R. Tobin, “Namespaces in XML (Second Edition)”, World Wide Web Consortium REC-xml-names-20091208, August 2006, <http://www.w3.org/TR/REC-xml-names>.

[XMLSchema]

Biron, P. and A. Malhotra, “XML Schema Part 2: Datatypes”, World Wide Web Consortium REC-xmlschema-2-20041028, May 2001, <http://www.w3.org/TR/xmlschema-2/#anyURI>.

[XPointer]

Grosso, P., Maler, E., Marsh, J., and N. Walsh, “XPointer Framework”, World Wide Web Consortium REC-xptr-framework-20030325, March 2003, <http://www.w3.org/TR/xptr-framework/#escaping>.