TitleMapping Characters for Internationalized Domain Names in Applications (IDNA) 2008
AuthorP. Resnick, P. Hoffman
DateSeptember 2010
Format:TXT, HTML

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


   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

RFC 5895                      IDNA Mapping                September 2010

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

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

RFC 5895                      IDNA Mapping                September 2010

   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

   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

   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

RFC 5895                      IDNA Mapping                September 2010

   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

   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

RFC 5895                      IDNA Mapping                September 2010

   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

   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>.

RFC 5895                      IDNA Mapping                September 2010

   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.

RFC 5895                      IDNA Mapping                September 2010

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,

   [Unicode-UAX15]     The Unicode Consortium, "Unicode Standard Annex
                       #15: Unicode Normalization Forms, Revision 31",
                       September 2009, <http://www.unicode.org/reports/

   [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).

Authors' Addresses

   Peter W. Resnick
   Qualcomm Incorporated
   5775 Morehouse Drive
   San Diego, CA  92121-1714

   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

   Phone: 1-831-426-9827
   EMail: paul.hoffman@vpnc.org