6. Use of IRIs
6.1. Limitations on UCS Characters Allowed in IRIs
This section discusses limitations on characters and character
sequences usable for IRIs beyond those given in section 2.2 and
section 4.1. The considerations in this section are relevant when
IRIs are created and when URIs are converted to IRIs.
a. The repertoire of characters allowed in each IRI component is
limited by the definition of that component. For example, the
definition of the scheme component does not allow characters
(Note: In accordance with URI practice, generic IRI software
cannot and should not check for such limitations.)
b. The UCS contains many areas of characters for which there are
strong visual look-alikes. Because of the likelihood of
transcription errors, these also should be avoided. This
includes the full-width equivalents of Latin characters,
half-width Katakana characters for Japanese, and many others. It
also includes many look-alikes of "space", "delims", and
"unwise", characters excluded in [RFC3491].
Additional information is available from [UNIXML]. [UNIXML] is
written in the context of running text rather than in that of
identifiers. Nevertheless, it discusses many of the categories of
characters not appropriate for IRIs.
6.2. Software Interfaces and Protocols
Although an IRI is defined as a sequence of characters, software
interfaces for URIs typically function on sequences of octets or
other kinds of code units. Thus, software interfaces and protocols
MUST define which character encoding is used.
Intermediate software interfaces between IRI-capable components and
URI-only components MUST map the IRIs per section 3.1, when
transferring from IRI-capable to URI-only components. This mapping
SHOULD be applied as late as possible. It SHOULD NOT be applied
between components that are known to be able to handle IRIs.
6.3. Format of URIs and IRIs in Documents and Protocols
Document formats that transport URIs may have to be upgraded to allow
the transport of IRIs. In cases where the document as a whole has a
native character encoding, IRIs MUST also be encoded in this
character encoding and converted accordingly by a parser or
interpreter. IRI characters not expressible in the native character
encoding SHOULD be escaped by using the escaping conventions of the
document format if such conventions are available. Alternatively,
they MAY be percent-encoded according to section 3.1. For example, in
HTML or XML, numeric character references SHOULD be used. If a
document as a whole has a native character encoding and that
character encoding is not UTF-8, then IRIs MUST NOT be placed into
the document in the UTF-8 character encoding.
Note: Some formats already accommodate IRIs, although they use
different terminology. HTML 4.0 [HTML4] defines the conversion from
IRIs to URIs as error-avoiding behavior. XML 1.0 [XML1], XLink
[XLink], XML Schema [XMLSchema], and specifications based upon them
allow IRIs. Also, it is expected that all relevant new W3C formats
and protocols will be required to handle IRIs [CharMod].
6.4. Use of UTF-8 for Encoding Original Characters
This section discusses details and gives examples for point c) in
section 1.2. To be able to use IRIs, the URI corresponding to the
IRI in question has to encode original characters into octets by
using UTF-8. This can be specified for all URIs of a URI scheme or
can apply to individual URIs for schemes that do not specify how to
encode original characters. It can apply to the whole URI, or only
to some part. For background information on encoding characters into
URIs, see also section 2.5 of [RFC3986].
For new URI schemes, using UTF-8 is recommended in [RFC2718].
Examples where UTF-8 is already used are the URN syntax [RFC2141],
IMAP URLs [RFC2192], and POP URLs [RFC2384]. On the other hand,
because the HTTP URL scheme does not specify how to encode original
characters, only some HTTP URLs can have corresponding but different
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.
This means that for most URI schemes, there is no need to upgrade
their scheme definition in order for them to work with IRIs. The
main case where upgrading makes sense is when a scheme definition, or
a particular component of a scheme, is strictly limited to the use of
US-ASCII characters with no provision to include non-ASCII
characters/octets via percent-encoding, or if a scheme definition
currently uses highly scheme-specific provisions for the encoding of
non-ASCII characters. An example of this is the mailto: scheme
This specification does not upgrade any scheme specifications in any
way; this has to be done separately. Also, note that there is no
such thing as an "IRI scheme"; all IRIs use URI schemes, and all URI
schemes can be used with IRIs, even though in some cases only by
using URIs directly as IRIs, without any conversion.
URI schemes can impose restrictions on the syntax of scheme-specific
URIs; i.e., URIs that are admissible under the generic URI syntax
[RFC3986] may not be admissible due to narrower syntactic constraints
imposed by a URI scheme specification. URI scheme definitions cannot
broaden the syntactic restrictions of the generic URI syntax;
otherwise, it would be possible to generate URIs that satisfied the
scheme-specific syntactic constraints without satisfying the
syntactic constraints of the generic URI syntax. However, additional
syntactic constraints imposed by URI scheme specifications are
applicable to IRI, as the corresponding URI resulting from the
mapping defined in section 3.1 MUST be a valid URI under the
syntactic restrictions of generic URI syntax and any narrower
restrictions imposed by the corresponding URI scheme specification.
The requirement for the use of UTF-8 applies to all parts of a URI
(with the potential exception of the ireg-name part; see section
3.1). However, it is possible that the capability of IRIs to
represent a wide range of characters directly is used just in some
parts of the IRI (or IRI reference). The other parts of the IRI may
only contain US-ASCII characters, or they may not be based on UTF-8.
They may be based on another character encoding, or they may directly
encode raw binary data (see also [RFC2397]).
For example, it is possible to have a URI reference of
"http://www.example.org/r%E9sum%E9.xml#r%C3%A9sum%C3%A9", where the
document name is encoded in iso-8859-1 based on server settings, but
where the fragment identifier is encoded in UTF-8 according to
[XPointer]. The IRI corresponding to the above URI would be (in XML
Similar considerations apply to query parts. The functionality of
IRIs (namely, to be able to include non-ASCII characters) can only be
used if the query part is encoded in UTF-8.
6.5. Relative IRI References
Processing of relative IRI references against a base is handled
straightforwardly; the algorithms of [RFC3986] can be applied
directly, treating the characters additionally allowed in IRI
references in the same way that unreserved characters are in URI
7. URI/IRI Processing Guidelines (Informative)
This informative section provides guidelines for supporting IRIs in
the same software components and operations that currently process
URIs: Software interfaces that handle URIs, software that allows
users to enter URIs, software that creates or generates URIs,
software that displays URIs, formats and protocols that transport
URIs, and software that interprets URIs. These may all require
modification before functioning properly with IRIs. The
considerations in this section also apply to URI references and IRI
7.1. URI/IRI Software Interfaces
Software interfaces that handle URIs, such as URI-handling APIs and
protocols transferring URIs, need interfaces and protocol elements
that are designed to carry IRIs.
In case the current handling in an API or protocol is based on
US-ASCII, UTF-8 is recommended as the character encoding for IRIs, as
it is compatible with US-ASCII, is in accordance with the
recommendations of [RFC2277], and makes converting to URIs easy. In
any case, the API or protocol definition must clearly define the
character encoding to be used.
The transfer from URI-only to IRI-capable components requires no
mapping, although the conversion described in section 3.2 above may
be performed. It is preferable not to perform this inverse
conversion when there is a chance that this cannot be done correctly.
7.2. URI/IRI Entry
Some components allow users to enter URIs into the system by typing
or dictation, for example. This software must be updated to allow
for IRI entry.
A person viewing a visual representation of an IRI (as a sequence of
glyphs, in some order, in some visual display) or hearing an IRI will
use an entry method for characters in the user's language to input
the IRI. Depending on the script and the input method used, this may
be a more or less complicated process.
The process of IRI entry must ensure, as much as possible, that the
restrictions defined in section 2.2 are met. This may be done by
choosing appropriate input methods or variants/settings thereof, by
appropriately converting the characters being input, by eliminating
characters that cannot be converted, and/or by issuing a warning or
error message to the user.
As an example of variant settings, input method editors for East
Asian Languages usually allow the input of Latin letters and related
characters in full-width or half-width versions. For IRI input, the
input method editor should be set so that it produces half-width
Latin letters and punctuation and full-width Katakana.
An input field primarily or solely used for the input of URIs/IRIs
may allow the user to view an IRI as it is mapped to a URI. Places
where the input of IRIs is frequent may provide the possibility for
viewing an IRI as mapped to a URI. This will help users when some of
the software they use does not yet accept IRIs.
An IRI input component interfacing to components that handle URIs,
but not IRIs, must map the IRI to a URI before passing it to these
For the input of IRIs with right-to-left characters, please see
7.3. URI/IRI Transfer between Applications
Many applications, particularly mail user agents, try to detect URIs
appearing in plain text. For this, they use some heuristics based on
URI syntax. They then allow the user to click on such URIs and
retrieve the corresponding resource in an appropriate (usually
Such applications have to be upgraded to use the IRI syntax as a base
for heuristics. In particular, a non-ASCII character should not be
taken as the indication of the end of an IRI. Such applications also
have to make sure that they correctly convert the detected IRI from
the character encoding of the document or application where the IRI
appears to the character encoding used by the system-wide IRI
invocation mechanism, or to a URI (according to section 3.1) if the
system-wide invocation mechanism only accepts URIs.
The clipboard is another frequently used way to transfer URIs and
IRIs from one application to another. On most platforms, the
clipboard is able to store and transfer text in many languages and
scripts. Correctly used, the clipboard transfers characters, not
bytes, which will do the right thing with IRIs.
7.4. URI/IRI Generation
Systems that offer resources through the Internet, where those
resources have logical names, sometimes automatically generate URIs
for the resources they offer. For example, some HTTP servers can
generate a directory listing for a file directory and then respond to
the generated URIs with the files.
Many legacy character encodings are in use in various file systems.
Many currently deployed systems do not transform the local character
representation of the underlying system before generating URIs.
For maximum interoperability, systems that generate resource
identifiers should make the appropriate transformations. For
example, if a file system contains a file named
"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].
7.5. URI/IRI Selection
In some cases, resource owners and publishers have control over the
IRIs used to identify their resources. This control is mostly
executed by controlling the resource names, such as file names,
In these cases, it is recommended to avoid choosing IRIs that are
easily confused. For example, for US-ASCII, the lower-case ell ("l")
is easily confused with the digit one ("1"), and the upper-case oh
("O") is easily confused with the digit zero ("0"). Publishers
should avoid confusing users with "br0ken" or "1ame" identifiers.
Outside the US-ASCII repertoire, there are many more opportunities
for confusion; a complete set of guidelines is too lengthy to include
here. As long as names are limited to characters from a single
script, native writers of a given script or language will know best
when ambiguities can appear, and how they can be avoided. What may
look ambiguous to a stranger may be completely obvious to the average
native user. On the other hand, in some cases, the UCS contains
variants for compatibility reasons; for example, for typographic
purposes. These should be avoided wherever possible. Although there
may be exceptions, newly created resource names should generally be
in NFKC [UTR15] (which means that they are also in NFC).
As an example, the UCS contains the "fi" ligature at U+FB01 for
compatibility reasons. Wherever possible, IRIs should use the two
letters "f" and "i" rather than the "fi" ligature. An example where
the latter may be used is in the query part of an IRI for an explicit
search for a word written containing the "fi" ligature.
In certain cases, there is a chance that characters from different
scripts look the same. The best known example is the similarity of
the Latin "A", the Greek "Alpha", and the Cyrillic "A". To avoid
such cases, only IRIs should be created where all the characters in a
single component are used together in a given language. This usually
means that all of these characters will be from the same script, but
there are languages that mix characters from different scripts (such
as Japanese). This is similar to the heuristics used to distinguish
between letters and numbers in the examples above. Also, for Latin,
Greek, and Cyrillic, using lowercase letters results in fewer
ambiguities than using uppercase letters would.
7.6. Display of URIs/IRIs
In situations where the rendering software is not expected to display
non-ASCII parts of the IRI correctly using the available layout and
font resources, these parts should be percent-encoded before being
For display of Bidi IRIs, please see section 4.1.
7.7. Interpretation of URIs and IRIs
Software that interprets IRIs as the names of local resources should
accept IRIs in multiple forms and convert and match them with the
appropriate local resource names.
First, multiple representations include both IRIs in the native
character encoding of the protocol and also their URI counterparts.
Second, it may include URIs constructed based on character encodings
other than UTF-8. These URIs may be produced by user agents that do
not conform to this specification and that use legacy character
encodings to convert non-ASCII characters to URIs. Whether this is
necessary, and what character encodings to cover, depends on a number
of factors, such as the legacy character encodings used locally and
the distribution of various versions of user agents. For example,
software for Japanese may accept URIs in Shift_JIS and/or EUC-JP in
addition to UTF-8.
Third, it may include additional mappings to be more user-friendly
and robust against transmission errors. These would be similar to
how some servers currently treat URIs as case insensitive or perform
additional matching to account for spelling errors. For characters
beyond the US-ASCII repertoire, this may, for example, include
ignoring the accents on received IRIs or resource names. Please note
that such mappings, including case mappings, are language dependent.
It can be difficult to identify a resource unambiguously if too many
mappings are taken into consideration. However, percent-encoded and
not percent-encoded parts of IRIs can always be clearly
distinguished. Also, the regularity of UTF-8 (see [Duerst97]) makes
the potential for collisions lower than it may seem at first.
7.8. Upgrading Strategy
Where this recommendation places further constraints on software for
which many instances are already deployed, it is important to
introduce upgrades carefully and to be aware of the various
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
8. Security Considerations
The security considerations discussed in [RFC3986] also apply to
IRIs. In addition, the following issues require particular care for
Incorrect encoding or decoding can lead to security problems. In
particular, some UTF-8 decoders do not check against overlong byte
sequences. As an example, a "/" is encoded with the byte 0x2F both
in UTF-8 and in US-ASCII, but some UTF-8 decoders also wrongly
interpret the sequence 0xC0 0xAF as a "/". A sequence such as
"%C0%AF.." may pass some security tests and then be interpreted as
"/.." in a path if UTF-8 decoders are fault-tolerant, if conversion
and checking are not done in the right order, and/or if reserved
characters and unreserved characters are not clearly distinguished.
There are various ways in which "spoofing" can occur with IRIs.
"Spoofing" means that somebody may add a resource name that looks the
same or similar to the user, but that points to a different resource.
The added resource may pretend to be the real resource by looking
very similar but may contain all kinds of changes that may be
difficult to spot and that can cause all kinds of problems. Most
spoofing possibilities for IRIs are extensions of those for URIs.
Spoofing can occur for various reasons. First, a user's
normalization expectations or actual normalization when entering an
IRI or transcoding an IRI from a legacy character encoding do not
match the normalization used on the server side. Conceptually, this
is no different from the problems surrounding the use of
case-insensitive web servers. For example, a popular web page with a
mixed-case name ("http://big.example.com/PopularPage.html") might be
"spoofed" by someone who is able to create
"http://big.example.com/popularpage.html". However, the use of
unnormalized character sequences, and of additional mappings for user
convenience, may increase the chance for spoofing. Protocols and
servers that allow the creation of resources with names that are not
normalized are particularly vulnerable to such attacks. This is an
inherent security problem of the relevant protocol, server, or
resource and is not specific to IRIs, but it is mentioned here for
Spoofing can occur in various IRI components, such as the domain name
part or a path part. For considerations specific to the domain name
part, see [RFC3491]. For the path part, administrators of sites that
allow independent users to create resources in the same sub area may
have to be careful to check for spoofing.
Spoofing can occur because in the UCS many characters look very
similar. Details are discussed in Section 7.5. Again, this is very
similar to spoofing possibilities on US-ASCII, e.g., using "br0ken"
or "1ame" URIs.
Spoofing can occur when URIs with percent-encodings based on various
character encodings are accepted to deal with older user agents. In
some cases, particularly for Latin-based resource names, this is
usually easy to detect because UTF-8-encoded names, when interpreted
and viewed as legacy character encodings, produce mostly garbage.
When concurrently used character encodings have a similar structure
but there are no characters that have exactly the same encoding,
detection is more difficult.
Spoofing can occur with bidirectional IRIs, if the restrictions in
section 4.2 are not followed. The same visual representation may be
interpreted as different logical representations, and vice versa. It
is also very important that a correct Unicode bidirectional
implementation be used.
We would like to thank Larry Masinter for his work as coauthor of
many earlier versions of this document (draft-masinter-url-i18n-xx).
The discussion on the issue addressed here started a long time ago.
There was a thread in the HTML working group in August 1995 (under
the topic of "Globalizing URIs") and in the www-international mailing
list in July 1996 (under the topic of "Internationalization and
URLs"), and there were ad-hoc meetings at the Unicode conferences in
September 1995 and September 1997.
Many thanks go to Francois Yergeau, Matitiahu Allouche, Roy Fielding,
Tim Berners-Lee, Mark Davis, M.T. Carrasco Benitez, James Clark, Tim
Bray, Chris Wendt, Yaron Goland, Andrea Vine, Misha Wolf, Leslie
Daigle, Ted Hardie, Bill Fenner, Margaret Wasserman, Russ Housley,
Makoto MURATA, Steven Atkin, Ryan Stansifer, Tex Texin, Graham Klyne,
Bjoern Hoehrmann, Chris Lilley, Ian Jacobs, Adam Costello, Dan
Oscarson, Elliotte Rusty Harold, Mike J. Brown, Roy Badami, Jonathan
Rosenne, Asmus Freytag, Simon Josefsson, Carlos Viegas Damasio, Chris
Haynes, Walter Underwood, and many others for help with understanding
the issues and possible solutions, and with getting the details
This document is a product of the Internationalization Working Group
(I18N WG) of the World Wide Web Consortium (W3C). Thanks to the
members of the W3C I18N Working Group and Interest Group for their
contributions and their work on [CharMod]. Thanks also go to the
members of many other W3C Working Groups for adopting IRIs, and to
the members of the Montreal IAB Workshop on Internationalization and
Localization for their review.
10.1. Normative References
[ASCII] American National Standards Institute, "Coded
Character Set -- 7-bit American Standard Code for
Information Interchange", ANSI X3.4, 1986.
[ISO10646] International Organization for Standardization,
"ISO/IEC 10646:2003: Information Technology -
Universal Multiple-Octet Coded Character Set (UCS)",
ISO Standard 10646, December 2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications
(IDNA)", RFC 3490, March 2003.
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)", RFC
3491, March 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter,
"Uniform Resource Identifier (URI): Generic Syntax",
STD 66, RFC 3986, January 2005.
[UNI9] Davis, M., "The Bidirectional Algorithm", Unicode
Standard Annex #9, March 2004,
[UNIV4] The Unicode Consortium, "The Unicode Standard, Version
4.0.1, defined by: The Unicode Standard, Version 4.0
(Reading, MA, Addison-Wesley, 2003. ISBN
0-321-18578-1), as amended by Unicode 4.0.1
Appendix A. Design Alternatives
This section shortly summarizes major design alternatives and the
reasons for why they were not chosen.
Appendix A.1. New Scheme(s)
Introducing new schemes (for example, httpi:, ftpi:,...) or a new
metascheme (e.g., i:, leading to URI/IRI prefixes such as i:http:,
i:ftp:,...) was proposed to make IRI-to-URI conversion scheme
dependent or to distinguish between percent-encodings resulting from
IRI-to-URI conversion and percent-encodings from legacy character
New schemes are not needed to distinguish URIs from true IRIs (i.e.,
IRIs that contain non-ASCII characters). The benefit of being able
to detect the origin of percent-encodings is marginal, as UTF-8 can
be detected with very high reliability. Deploying new schemes is
extremely hard, so not requiring new schemes for IRIs makes
deployment of IRIs vastly easier. Making conversion scheme dependent
is highly inadvisable and would be encouraged by separate schemes for
IRIs. Using a uniform convention for conversion from IRIs to URIs
makes IRI implementation orthogonal to the introduction of actual new
Appendix A.2. Character Encodings Other Than UTF-8
At an early stage, UTF-7 was considered as an alternative to UTF-8
when IRIs are converted to URIs. UTF-7 would not have needed
percent-encoding and in most cases would have been shorter than
Using UTF-8 avoids a double layering and overloading of the use of
the "+" character. UTF-8 is fully compatible with US-ASCII and has
therefore been recommended by the IETF, and is being used widely.
UTF-7 has never been used much and is now clearly being discouraged.
Requiring implementations to convert from UTF-8 to UTF-7 and back
would be an additional implementation burden.
Appendix A.3. New Encoding Convention
Instead of using the existing percent-encoding convention of URIs,
which is based on octets, the idea was to create a new encoding
convention; for example, to use "%u" to introduce UCS code points.
Using the existing octet-based percent-encoding mechanism does not
need an upgrade of the URI syntax and does not need corresponding
Appendix A.4. Indicating Character Encodings in the URI/IRI
Some proposals suggested indicating the character encodings used in
an URI or IRI with some new syntactic convention in the URI itself,
similar to the "charset" parameter for e-mails and Web pages. As an
example, the label in square brackets in
"http://www.example.org/ros[iso-8859-1]é"; indicated that the
following "é"; had to be interpreted as iso-8859-1.
If UTF-8 is used exclusively, an upgrade to the URI syntax is not
needed. It avoids potentially multiple labels that have to be copied
correctly in all cases, even on the side of a bus or on a napkin,
leading to usability problems (and being prohibitively annoying).
Exclusively using UTF-8 also reduces transcoding errors and
Martin Duerst (Note: Please write "Duerst" with u-umlaut wherever
possible, for example as "Dürst" in XML and
World Wide Web Consortium
Fujisawa, Kanagawa 252-8520
Phone: +81 466 49 1170
Fax: +81 466 49 1171
(Note: This is the percent-encoded form of an IRI.)
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 882-8080
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