IDNA2008 mapping: Mapping Characters for Internationalized Domain Names in Applications (IDNA) 2008
How to normalize user-typed domain names before they reach the IDNA2008 protocol.
Resnick, P. and P. Hoffman, "Mapping Characters for Internationalized Domain Names in Applications (IDNA) 2008", RFC 5895, DOI 10.17487/RFC5895, September 2010, <https://www.rfc-editor.org/info/rfc5895>.
@misc{rfc5895,
series = {Request for Comments},
number = 5895,
howpublished = {RFC 5895},
publisher = {RFC Editor},
doi = {10.17487/RFC5895},
url = {https://www.rfc-editor.org/info/rfc5895},
author = {P. Resnick and P. Hoffman},
title = {{Mapping Characters for Internationalized Domain Names in Applications (IDNA) 2008}},
pagetotal = 7,
year = 2010,
month = sep,
} RFC text
Contents
Independent Submission P. Resnick
Request for Comments: 5895 Qualcomm Incorporated
Category: Informational P. Hoffman
ISSN: 2070-1721 VPN Consortium
September 2010
Mapping Characters for
Internationalized Domain Names in Applications (IDNA) 2008
Abstract
In the original version of the Internationalized Domain Names in
Applications (IDNA) protocol, any Unicode code points taken from user
input were mapped into a set of Unicode code points that "made
sense", and then encoded and passed to the domain name system (DNS).
The IDNA2008 protocol (described in RFCs 5890, 5891, 5892, and 5893)
presumes that the input to the protocol comes from a set of
"permitted" code points, which it then encodes and passes to the DNS,
but does not specify what to do with the result of user input. This
document describes the actions that can be taken by an implementation
between receiving user input and passing permitted code points to the
new IDNA protocol.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5895.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
1. Introduction
This document describes the operations that can be applied to user
input in order to get it into a form that is acceptable by the
Internationalized Domain Names in Applications (IDNA) protocol
[IDNA2008protocol]. It includes a general implementation procedure
for mapping.
It should be noted that this document does not specify the behavior
of a protocol that appears "on the wire". It describes an operation
that is to be applied to user input in order to prepare that user
input for use in an "on the network" protocol. As unusual as this
may be for a document concerning Internet protocols, it is necessary
to describe this operation for implementors who may have designed
around the original IDNA protocol (herein referred to as IDNA2003),
which conflates this user-input operation into the protocol.
It is very important to note that there are many potential valid
mappings of characters from user input. The mapping described in
this document is the basis for other mappings, and is not likely to
be useful without modification. Any useful mapping will have
features designed to reduce the surprise for users and is likely to
be slightly (or sometimes radically) different depending on the
locale of the user, the type of input being used (such as typing,
copy-and-paste, voice, and so on), the type of application used, etc.
Although most common mappings will probably produce similar results
for the same input, there will be subtle differences between
applications.
1.1. The Dividing Line between User Interface and Protocol
The user interface to applications is much more complicated than most
network implementers think. When we say "the user enters an
internationalized domain name in the application", we are talking
about a very complex process that encompasses everything from the
user formulating the name and deciding which symbols to use to
express that name, to the user entering the symbols into the computer
using some input method (be it a keyboard, a stylus, or even a voice
recognition program), to the computer interpreting that input (be it
keyboard scan codes, a graphical representation, or digitized sounds)
into some representation of those symbols, through finally
normalizing those symbols into a particular character repertoire in
an encoding recognizable to IDNA processes and the domain name
system.
Considerations for a user interface for internationalized domain
names involves taking into account culture, context, and locale for
any given user. A simple and well-known example is the lowercasing
of the letter LATIN CAPITAL LETTER I (U+0049) when it is used in the
Turkish and other languages. A capital "I" in Turkish is properly
lowercased to a LATIN SMALL LETTER DOTLESS I (U+0131), not to a LATIN
SMALL LETTER I (U+0069). This lowercasing is clearly dependent on
the locale of the system and/or the locale of the user. Using a
single context-free mapping without considering the user interface
properties has the potential of doing exactly the wrong thing for the
user.
The original version of IDNA conflated user interface processing and
protocol. It took whatever characters the user produced in whatever
encoding the application used, assumed some conversion to Unicode
code points, and then without regard to context, locale, or anything
about the user's intentions, mapped them into a particular set of
other characters, and then re-encoded them in Punycode, in order to
have the entire operation be contained within the protocol. Ignoring
context, locale, and user preference in the IDNA protocol made life
significantly less complicated for the application developer, but at
the expense of violating the principle of "least user surprise" for
consumers and producers of domain names.
In IDNA2008, the dividing line between "user interface" and
"protocol" is clear. The IDNA2008 specification defines the protocol
part of IDNA: it explicitly does not deal with the user interface.
Mappings such as the one described in this document explicitly deal
with the user interface and not the protocol. That is, a mapping is
only to be applied before a string of characters is treated as a
domain name (in the "user interface") and is never to be applied
during domain name processing (in the "protocol").
1.2. The Design of This Mapping
The user interface mapping in this document is a set of expansions to
IDNA2008 that are meant to be sensible and friendly and mostly
obvious to people throughout the world when using typical
applications with domain names that are entered by hand. It is also
designed to let applications be mostly backwards compatible with
IDNA2003. By definition, it cannot meet all of those design goals
for all people, and in fact is known to fail on some of those goals
for quite large populations of people.
A good mapping in the real world might use the "sensible and friendly
and mostly obvious" design goal but come up with a different
algorithm. Many algorithms will have results that are close to what
is described here, but will differ in assumptions about the users'
way of thinking or typing. Having said that, it is likely that some
mappings will be significantly different. For example, a mapping
might apply to a spoken user interface instead of a typed one.
Another example is that a mapping might be different for users that
are typing than for users that are copying-and-pasting from different
applications. Yet another example is that a user interface that
allows typed input that is transliterated from Latin characters could
have very different mappings than one that applies to typing in other
character sets; this would be typical in a Pinyin input method for
Chinese characters.
2. The General Procedure
This section defines a general algorithm that applications ought to
implement in order to produce Unicode code points that will be valid
under the IDNA protocol. An application might implement the full
mapping as described below, or it can choose a different mapping.
This mapping is very general and was designed to be acceptable to the
widest user community, but as stated above, it does not take into
account any particular context, culture, or locale.
The general algorithm that an application (or the input method
provided by an operating system) ought to use is relatively
straightforward:
1. Uppercase characters are mapped to their lowercase equivalents by
using the algorithm for mapping case in Unicode characters. This
step was chosen because the output will behave more like ASCII
host names behave.
2. Fullwidth and halfwidth characters (those defined with
Decomposition Types <wide> and <narrow>) are mapped to their
decomposition mappings as shown in the Unicode character
database. This step was chosen because many input mechanisms,
particularly in Asia, do not allow you to easily enter characters
in the form used by IDNA2008. Even if they do allow the correct
character form, the user might not know which form they are
entering.
3. All characters are mapped using Unicode Normalization Form C
(NFC). This step was chosen because it maps combinations of
combining characters into canonical composed form. As with the
fullwidth/halfwidth mapping, users are not generally aware of the
particular form of characters that they are entering, and
IDNA2008 requires that only the canonical composed forms from NFC
be used.
4. [IDNA2008protocol] is specified such that the protocol acts on
the individual labels of the domain name. If an implementation
of this mapping is also performing the step of separation of the
parts of a domain name into labels by using the FULL STOP
character (U+002E), the IDEOGRAPHIC FULL STOP character (U+3002)
can be mapped to the FULL STOP before label separation occurs.
There are other characters that are used as "full stops" that one
could consider mapping as label separators, but their use as such
has not been investigated thoroughly. This step was chosen
because some input mechanisms do not allow the user to easily
enter proper label separators. Only the IDEOGRAPHIC FULL STOP
character (U+3002) is added in this mapping because the authors
have not fully investigated the applicability of other characters
and the environments where they should and should not be
considered domain name label separators.
Note that the steps above are ordered.
Definitions for the rules in this algorithm can be found in
[Unicode52]. Specifically:
o Unicode Normalization Form C can be found in Annex #15 of
[Unicode-UAX15].
o In order to map uppercase characters to their lowercase
equivalents (defined in Section 3.13 of [Unicode52]), first map
characters to the "Lowercase_Mapping" property (the "<lower>"
entry in the second column) in
<http://www.unicode.org/Public/UNIDATA/SpecialCasing.txt>, if any.
Then, map characters to the "Simple_Lowercase_Mapping" property
(the fourteenth column) in
<http://www.unicode.org/Public/UNIDATA/UnicodeData.txt>, if any.
o In order to map fullwidth and halfwidth characters to their
decomposition mappings, map any character whose
"Decomposition_Type" (contained in the first part of the sixth
column) in <http://www.unicode.org/Public/UNIDATA/UnicodeData.txt>
is either "<wide>" or "<narrow>" to the "Decomposition_Mapping" of
that character (contained in the second part of the sixth column)
in <http://www.unicode.org/Public/UNIDATA/UnicodeData.txt>.
o The Unicode Character Database [TR44] has useful descriptions of
the contents of these files.
If the mappings in this document are applied to versions of Unicode
later than Unicode 5.2, the later versions of the Unicode Standard
should be consulted.
These form a minimal set of mappings that an application should
strongly consider doing. Of course, there are many others that might
be done.
3. Implementing This Mapping
If you are implementing a mapping for an application or operating
system by using exactly the four steps in Section 2, the authors of
this document have a request: please don't. We mean it. Section 2
does not describe a universal mapping algorithm because, as we said,
there is no universally-applicable mapping algorithm.
If you read the material in Section 2 without reading Section 1, go
back and carefully read all of Section 1; in many ways, Section 1 is
more important than Section 2. Further, you can probably think of
user interface considerations that we did not list in Section 1. If
you did read Section 1 but somehow decided that the algorithm in
Section 2 is completely correct for the intended users of your
application or operating system, you are probably not thinking hard
enough about your intended users.
4. Security Considerations
This document suggests creating mappings that might cause confusion
for some users while alleviating confusion in other users. Such
confusion is not covered in any depth in this document (nor in the
other IDNA-related documents).
5. Acknowledgements
This document is the product of many contributions from numerous
people in the IETF.
6. Normative References
[IDNA2008protocol] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
August 2010.
[TR44] The Unicode Consortium, "Unicode Technical Report
#44: Unicode Character Database", September 2009,
<http://www.unicode.org/reports/tr44/
tr44-4.html>.
[Unicode-UAX15] The Unicode Consortium, "Unicode Standard Annex
#15: Unicode Normalization Forms, Revision 31",
September 2009, <http://www.unicode.org/reports/
tr15/tr15-31.html>.
[Unicode52] The Unicode Consortium. The Unicode Standard,
Version 5.2.0, defined by: "The Unicode Standard,
Version 5.2.0", (Mountain View, CA: The Unicode
Consortium, 2009. ISBN 978-1-936213-00-9).
<http://www.unicode.org/versions/Unicode5.2.0/>.
Authors' Addresses
Peter W. Resnick
Qualcomm Incorporated
5775 Morehouse Drive
San Diego, CA 92121-1714
US
Phone: +1 858 651 4478
EMail: presnick@qualcomm.com
URI: http://www.qualcomm.com/~presnick/
Paul Hoffman
VPN Consortium
127 Segre Place
Santa Cruz, CA 95060
US
Phone: 1-831-426-9827
EMail: paul.hoffman@vpnc.org
AI ELI5
On this page
§1Introduction and Scope
The IDNA2008 protocol defines how domain names with international characters are encoded and validated, but it assumes the input is already in a permitted form. This RFC addresses what happens before the protocol: user interface processing. When a user types or pastes a domain name, applications need to convert it into a form IDNA2008 can handle.
The RFC emphasizes that there is no universal mapping. Different applications need different rules depending on locale, input method (keyboard, voice, copy-paste), and user expectations. A Turkish user's "I" lowercases differently than an English user's. This document provides a baseline mapping suitable for typical hand-typed input worldwide, but applications should adapt it to their specific users.
Different languages and input methods need different rules. The RFC gives a starting point, but applications should change it based on who is using them.
§2The Division Between User Interface and Protocol
The original IDNA (IDNA2003) mixed user interface processing into the protocol itself, making implementers' lives simpler but resulting in surprising behavior for users. IDNA2008 separates these concerns: the protocol part (RFCs 5890-5893) defines validation and encoding; this RFC defines the user interface mapping part that comes before the protocol.
A user interface for international domain names is complex. It must consider the user's language, the input device, how the computer interprets the input, and how to normalize that input into Unicode characters recognizable by the DNS. A capital "I" in Turkish lowercases to U+0131 (dotless i), not U+0069 (regular i). Simple context-free rules cannot capture all these nuances, but a sensible baseline can handle most cases.
IDNA2008 handles the protocol part ('now it's a valid name'). This RFC handles the user part ('I typed this, clean it up').
§3The Four-Step Mapping Algorithm
The RFC defines four steps, applied in order, to map user input to IDNA2008-compatible code points:
Step 1: Lowercase. Convert uppercase Unicode characters to their lowercase equivalents using the Unicode Lowercase_Mapping property. This matches ASCII domain name behavior and makes things simpler for users who capitalize inconsistently.
Step 2: Decompose full-width and half-width characters. Asian input methods often produce full-width versions of Latin characters (e.g., the full-width Latin 'A', U+FF21) instead of the normal ASCII 'A'. This step converts those using the Unicode decomposition mapping.
Step 3: Apply Unicode Normalization Form C (NFC). NFC composes combining characters (like accents) into single precomposed characters. Users may type an 'e' followed by an accent mark, but IDNA2008 requires the precomposed 'é'. NFC handles this automatically.
Step 4: Map ideographic full stops. The ideographic full stop (U+3002, a dot used in CJK text) can be mapped to the ASCII period (U+002E) for label separation, since some input methods do not easily produce the ASCII period.
Step 1: Make capitals small. Step 2: Convert weird 'A' to regular A. Step 3: Combine accent marks with letters. Step 4: Turn the fancy dot into a regular dot. That's the order to do it in.
§4Implementing Sensible Mapping
The RFC includes a strong caveat: if you are implementing this four-step algorithm exactly, you are probably doing it wrong. The authors recommend that implementers read the introduction section carefully and think about their specific users before adopting this baseline unchanged.
Good mappings for the real world might differ significantly based on assumptions about how users think or type. For example, a mapping for spoken input (speech-to-text) would be very different from one for typed input. A mapping for users copy-pasting from documents might be different from one for users typing manually. The baseline is a starting point, not a finished product.
Useful modifications might include handling Pinyin (romanized Chinese input), applying context-aware case folding for specific languages, or adjusting the mapping based on the domain's registered scripts.
The four steps are a template, not a final answer. Think about your actual users and change the steps if needed.
§5Security and Character Confusion
The RFC notes that mappings can cause confusion for users. Converting uppercase to lowercase, for example, might result in homoglyphs: different characters that look the same after normalization. A capital and lowercase version of a letter might look identical on screen, confusing users about which domain they registered or are accessing.
Security concerns about character confusion are not fully addressed in this RFC or the other IDNA2008 documents. Implementers and administrators should be aware that normalization can create unexpected equivalences and design user interfaces to mitigate confusion where possible.
When characters look too similar after normalization, users might get confused about which domain is which. Be careful about that.