[Docs] [txt|pdf] [draft-ietf-idnabi...] [Diff1] [Diff2]

INFORMATIONAL

Internet Engineering Task Force (IETF)                        J. Klensin
Request for Comments: 5894                                   August 2010
Category: Informational
ISSN: 2070-1721


        Internationalized Domain Names for Applications (IDNA):
                 Background, Explanation, and Rationale

Abstract

   Several years have passed since the original protocol for
   Internationalized Domain Names (IDNs) was completed and deployed.
   During that time, a number of issues have arisen, including the need
   to update the system to deal with newer versions of Unicode.  Some of
   these issues require tuning of the existing protocols and the tables
   on which they depend.  This document provides an overview of a
   revised system and provides explanatory material for its components.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are 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/rfc5894.

















Klensin                       Informational                     [Page 1]

RFC 5894                     IDNA Rationale                  August 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.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Context and Overview . . . . . . . . . . . . . . . . . . .  4
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
       1.2.1.  DNS "Name" Terminology . . . . . . . . . . . . . . . .  5
       1.2.2.  New Terminology and Restrictions . . . . . . . . . . .  6
     1.3.  Objectives . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.4.  Applicability and Function of IDNA . . . . . . . . . . . .  7
     1.5.  Comprehensibility of IDNA Mechanisms and Processing  . . .  8
   2.  Processing in IDNA2008 . . . . . . . . . . . . . . . . . . . .  9
   3.  Permitted Characters: An Inclusion List  . . . . . . . . . . .  9
     3.1.  A Tiered Model of Permitted Characters and Labels  . . . . 10
       3.1.1.  PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . . 10
       3.1.2.  CONTEXTUAL RULE REQUIRED . . . . . . . . . . . . . . . 11
         3.1.2.1.  Contextual Restrictions  . . . . . . . . . . . . . 11
         3.1.2.2.  Rules and Their Application  . . . . . . . . . . . 12
       3.1.3.  DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 12
       3.1.4.  UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 13
     3.2.  Registration Policy  . . . . . . . . . . . . . . . . . . . 14




Klensin                       Informational                     [Page 2]

RFC 5894                     IDNA Rationale                  August 2010


     3.3.  Layered Restrictions: Tables, Context, Registration, and
           Applications . . . . . . . . . . . . . . . . . . . . . . . 15
   4.  Application-Related Issues . . . . . . . . . . . . . . . . . . 15
     4.1.  Display and Network Order  . . . . . . . . . . . . . . . . 15
     4.2.  Entry and Display in Applications  . . . . . . . . . . . . 16
     4.3.  Linguistic Expectations: Ligatures, Digraphs, and
           Alternate Character Forms  . . . . . . . . . . . . . . . . 19
     4.4.  Case Mapping and Related Issues  . . . . . . . . . . . . . 20
     4.5.  Right-to-Left Text . . . . . . . . . . . . . . . . . . . . 21
   5.  IDNs and the Robustness Principle  . . . . . . . . . . . . . . 22
   6.  Front-end and User Interface Processing for Lookup . . . . . . 22
   7.  Migration from IDNA2003 and Unicode Version Synchronization  . 25
     7.1.  Design Criteria  . . . . . . . . . . . . . . . . . . . . . 25
       7.1.1.  Summary and Discussion of IDNA Validity Criteria . . . 25
       7.1.2.  Labels in Registration . . . . . . . . . . . . . . . . 26
       7.1.3.  Labels in Lookup . . . . . . . . . . . . . . . . . . . 27
     7.2.  Changes in Character Interpretations . . . . . . . . . . . 28
       7.2.1.  Character Changes: Eszett and Final Sigma  . . . . . . 28
       7.2.2.  Character Changes: Zero Width Joiner and Zero
               Width Non-Joiner . . . . . . . . . . . . . . . . . . . 29
       7.2.3.  Character Changes and the Need for Transition  . . . . 29
       7.2.4.  Transition Strategies  . . . . . . . . . . . . . . . . 30
     7.3.  Elimination of Character Mapping . . . . . . . . . . . . . 31
     7.4.  The Question of Prefix Changes . . . . . . . . . . . . . . 31
       7.4.1.  Conditions Requiring a Prefix Change . . . . . . . . . 31
       7.4.2.  Conditions Not Requiring a Prefix Change . . . . . . . 32
       7.4.3.  Implications of Prefix Changes . . . . . . . . . . . . 32
     7.5.  Stringprep Changes and Compatibility . . . . . . . . . . . 33
     7.6.  The Symbol Question  . . . . . . . . . . . . . . . . . . . 33
     7.7.  Migration between Unicode Versions: Unassigned Code
           Points . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     7.8.  Other Compatibility Issues . . . . . . . . . . . . . . . . 36
   8.  Name Server Considerations . . . . . . . . . . . . . . . . . . 37
     8.1.  Processing Non-ASCII Strings . . . . . . . . . . . . . . . 37
     8.2.  Root and Other DNS Server Considerations . . . . . . . . . 37
   9.  Internationalization Considerations  . . . . . . . . . . . . . 38
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38
     10.1. IDNA Character Registry  . . . . . . . . . . . . . . . . . 38
     10.2. IDNA Context Registry  . . . . . . . . . . . . . . . . . . 39
     10.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 39
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 39
     11.1. General Security Issues with IDNA  . . . . . . . . . . . . 39
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 39
   13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 40
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 40
     14.2. Informative References . . . . . . . . . . . . . . . . . . 41




Klensin                       Informational                     [Page 3]

RFC 5894                     IDNA Rationale                  August 2010


1.  Introduction

1.1.  Context and Overview

   Internationalized Domain Names in Applications (IDNA) is a collection
   of standards that allow client applications to convert some mnemonic
   strings expressed in Unicode to an ASCII-compatible encoding form
   ("ACE") that is a valid DNS label containing only LDH syntax (see the
   Definitions document [RFC5890]).  The specific form of ACE label used
   by IDNA is called an "A-label".  A client can look up an exact
   A-label in the existing DNS, so A-labels do not require any
   extensions to DNS, upgrades of DNS servers, or updates to low-level
   client libraries.  An A-label is recognizable from the prefix "xn--"
   before the characters produced by the Punycode algorithm [RFC3492];
   thus, a user application can identify an A-label and convert it into
   Unicode (or some local coded character set) for display.

   On the registry side, IDNA allows a registry to offer
   Internationalized Domain Names (IDNs) for registration as A-labels.
   A registry may offer any subset of valid IDNs, and may apply any
   restrictions or bundling (grouping of similar labels together in one
   registration) appropriate for the context of that registry.
   Registration of labels is sometimes discussed separately from lookup,
   and it is subject to a few specific requirements that do not apply to
   lookup.

   DNS clients and registries are subject to some differences in
   requirements for handling IDNs.  In particular, registries are urged
   to register only exact, valid A-labels, while clients might do some
   mapping to get from otherwise-invalid user input to a valid A-label.

   The first version of IDNA was published in 2003 and is referred to
   here as IDNA2003 to contrast it with the current version, which is
   known as IDNA2008 (after the year in which IETF work started on it).
   IDNA2003 consists of four documents: the IDNA base specification
   [RFC3490], Nameprep [RFC3491], Punycode [RFC3492], and Stringprep
   [RFC3454].  The current set of documents, IDNA2008, is not dependent
   on any of the IDNA2003 specifications other than the one for Punycode
   encoding.  References to "IDNA2008", "these specifications", or
   "these documents" are to the entire IDNA2008 set listed in a separate
   Definitions document [RFC5890].  The characters that are valid in
   A-labels are identified from rules listed in the Tables document
   [RFC5892], but validity can be derived from the Unicode properties of
   those characters with a very few exceptions.

   Traditionally, DNS labels are matched case-insensitively (as
   described in the DNS specifications [RFC1034][RFC1035]).  That
   convention was preserved in IDNA2003 by a case-folding operation that



Klensin                       Informational                     [Page 4]

RFC 5894                     IDNA Rationale                  August 2010


   generally maps capital letters into lowercase ones.  However, if case
   rules are enforced from one language, another language sometimes
   loses the ability to treat two characters separately.  Case-
   insensitivity is treated slightly differently in IDNA2008.

   IDNA2003 used Unicode version 3.2 only.  In order to keep up with new
   characters added in new versions of Unicode, IDNA2008 decouples its
   rules from any particular version of Unicode.  Instead, the
   attributes of new characters in Unicode, supplemented by a small
   number of exception cases, determine how and whether the characters
   can be used in IDNA labels.

   This document provides informational context for IDNA2008, including
   terminology, background, and policy discussions.  It contains no
   normative material; specifications for conformance to the IDNA2008
   protocols appears entirely in the other documents in the series.

1.2.  Terminology

   Terminology for IDNA2008 appears in the Definitions document
   [RFC5890].  That document also contains a road map to the IDNA2008
   document collection.  No attempt should be made to understand this
   document without the definitions and concepts that appear there.

1.2.1.  DNS "Name" Terminology

   In the context of IDNs, the DNS term "name" has introduced some
   confusion as people speak of DNS labels in terms of the words or
   phrases of various natural languages.  Historically, many of the
   "names" in the DNS have been mnemonics to identify some particular
   concept, object, or organization.  They are typically rooted in some
   language because most people think in language-based ways.  But,
   because they are mnemonics, they need not obey the orthographic
   conventions of any language: it is not a requirement that it be
   possible for them to be "words".

   This distinction is important because the reasonable goal of an IDN
   effort is not to be able to write the great Klingon (or language of
   one's choice) novel in DNS labels but to be able to form a usefully
   broad range of mnemonics in ways that are as natural as possible in a
   very broad range of scripts.










Klensin                       Informational                     [Page 5]

RFC 5894                     IDNA Rationale                  August 2010


1.2.2.  New Terminology and Restrictions

   IDNA2008 introduces new terminology.  Precise definitions are
   provided in the Definitions document for the terms U-label, A-Label,
   LDH label (to which all valid pre-IDNA hostnames conformed), Reserved
   LDH label (R-LDH label), XN-label, Fake A-label, and Non-Reserved LDH
   label (NR-LDH label).

   In addition, the term "putative label" has been adopted to refer to a
   label that may appear to meet certain definitional constraints but
   has not yet been sufficiently tested for validity.

   These definitions are also illustrated in Figure 1 of the Definitions
   document.  R-LDH labels contain "--" in the third and fourth
   character positions from the beginning of the label.  In IDNA-aware
   applications, only a subset of these reserved labels is permitted to
   be used, namely the A-label subset.  A-labels are a subset of the
   R-LDH labels that begin with the case-insensitive string "xn--".
   Labels that bear this prefix but that are not otherwise valid fall
   into the "Fake A-label" category.  The Non-Reserved labels (NR-LDH
   labels) are implicitly valid since they do not bear any resemblance
   to the labels specified by IDNA.

   The creation of the Reserved-LDH category is required for three
   reasons:

   o  to prevent confusion with pre-IDNA coding forms;

   o  to permit future extensions that would require changing the
      prefix, no matter how unlikely those might be (see Section 7.4);
      and

   o  to reduce the opportunities for attacks via the Punycode encoding
      algorithm itself.

   As with other documents in the IDNA2008 set, this document uses the
   term "registry" to describe any zone in the DNS.  That term, and the
   terms "zone" or "zone administration", are interchangeable.

1.3.  Objectives

   These are the main objectives in revising IDNA.

   o  Use a more recent version of Unicode and allow IDNA to be
      independent of Unicode versions, so that IDNA2008 need not be
      updated for implementations to adopt code points from new Unicode
      versions.




Klensin                       Informational                     [Page 6]

RFC 5894                     IDNA Rationale                  August 2010


   o  Fix a very small number of code point categorizations that have
      turned out to cause problems in the communities that use those
      code points.

   o  Reduce the dependency on mapping, in favor of valid A-labels.
      This will result in pre-mapped forms that are not valid IDNA
      labels appearing less often in various contexts.

   o  Fix some details in the bidirectional code point handling
      algorithms.

1.4.  Applicability and Function of IDNA

   The IDNA specification solves the problem of extending the repertoire
   of characters that can be used in domain names to include a large
   subset of the Unicode repertoire.

   IDNA does not extend DNS.  Instead, the applications (and, by
   implication, the users) continue to see an exact-match lookup
   service.  Either there is a single name that matches exactly (subject
   to the base DNS requirement of case-insensitive ASCII matching) or
   there is no match.  This model has served the existing applications
   well, but it requires, with or without internationalized domain
   names, that users know the exact spelling of the domain names that
   are to be typed into applications such as web browsers and mail user
   agents.  The introduction of the larger repertoire of characters
   potentially makes the set of misspellings larger, especially given
   that in some cases the same appearance, for example on a business
   card, might visually match several Unicode code points or several
   sequences of code points.

   The IDNA standard does not require any applications to conform to it,
   nor does it retroactively change those applications.  An application
   can elect to use IDNA in order to support IDNs while maintaining
   interoperability with existing infrastructure.  For applications that
   want to use non-ASCII characters in public DNS domain names, IDNA is
   the only option that is defined at the time this specification is
   published.  Adding IDNA support to an existing application entails
   changes to the application only, and leaves room for flexibility in
   front-end processing and more specifically in the user interface (see
   Section 6).

   A great deal of the discussion of IDN solutions has focused on
   transition issues and how IDNs will work in a world where not all of
   the components have been updated.  Proposals that were not chosen by
   the original IDN Working Group would have depended on updating user
   applications, DNS resolvers, and DNS servers in order for a user to
   apply an internationalized domain name in any form or coding



Klensin                       Informational                     [Page 7]

RFC 5894                     IDNA Rationale                  August 2010


   acceptable under that method.  While processing must be performed
   prior to or after access to the DNS, IDNA requires no changes to the
   DNS protocol, any DNS servers, or the resolvers on users' computers.

   IDNA allows the graceful introduction of IDNs not only by avoiding
   upgrades to existing infrastructure (such as DNS servers and mail
   transport agents), but also by allowing some limited use of IDNs in
   applications by using the ASCII-encoded representation of the labels
   containing non-ASCII characters.  While such names are user-
   unfriendly to read and type, and hence not optimal for user input,
   they can be used as a last resort to allow rudimentary IDN usage.
   For example, they might be the best choice for display if it were
   known that relevant fonts were not available on the user's computer.
   In order to allow user-friendly input and output of the IDNs and
   acceptance of some characters as equivalent to those to be processed
   according to the protocol, the applications need to be modified to
   conform to this specification.

   This version of IDNA uses the Unicode character repertoire for
   continuity with the original version of IDNA.

1.5.  Comprehensibility of IDNA Mechanisms and Processing

   One goal of IDNA2008, which is aided by the main goal of reducing the
   dependency on mapping, is to improve the general understanding of how
   IDNA works and what characters are permitted and what happens to
   them.  Comprehensibility and predictability to users and registrants
   are important design goals for this effort.  End-user applications
   have an important role to play in increasing this comprehensibility.

   Any system that tries to handle international characters encounters
   some common problems.  For example, a User Interface (UI) cannot
   display a character if no font containing that character is
   available.  In some cases, internationalization enables effective
   localization while maintaining some global uniformity but losing some
   universality.

   It is difficult to even make suggestions as to how end-user
   applications should cope when characters and fonts are not available.
   Because display functions are rarely controlled by the types of
   applications that would call upon IDNA, such suggestions will rarely
   be very effective.

   Conversion between local character sets and normalized Unicode, if
   needed, is part of this set of user interface issues.  Those
   conversions introduce complexity in a system that does not use
   Unicode as its primary (or only) internal character coding system.
   If a label is converted to a local character set that does not have



Klensin                       Informational                     [Page 8]

RFC 5894                     IDNA Rationale                  August 2010


   all the needed characters, or that uses different character-coding
   principles, the user interface program may have to add special logic
   to avoid or reduce loss of information.

   The major difficulty may lie in accurately identifying the incoming
   character set and applying the correct conversion routine.  Even more
   difficult, the local character coding system could be based on
   conceptually different assumptions than those used by Unicode (e.g.,
   choice of font encodings used for publications in some Indic
   scripts).  Those differences may not easily yield unambiguous
   conversions or interpretations even if each coding system is
   internally consistent and adequate to represent the local language
   and script.

   IDNA2008 shifts responsibility for character mapping and other
   adjustments from the protocol (where it was located in IDNA2003) to
   pre-processing before invoking IDNA itself.  The intent is that this
   change will lead to greater usage of fully-valid A-Labels or U-labels
   in display, transit, and storage, which should aid comprehensibility
   and predictability.  A careful look at pre-processing raises issues
   about what that pre-processing should do and at what point
   pre-processing becomes harmful; how universally consistent
   pre-processing algorithms can be; and how to be compatible with
   labels prepared in an IDNA2003 context.  Those issues are discussed
   in Section 6 and in the Mapping document [IDNA2008-Mapping].

2.  Processing in IDNA2008

   IDNA2008 separates Domain Name Registration and Lookup in the
   protocol specification (RFC 5891, Sections 4 and 5 [RFC5891]).
   Although most steps in the two processes are similar, the separation
   reflects current practice in which per-registry (DNS zone)
   restrictions and special processing are applied at registration time
   but not during lookup.  Another significant benefit is that
   separation facilitates incremental addition of permitted character
   groups to avoid freezing on one particular version of Unicode.

   The actual registration and lookup protocols for IDNA2008 are
   specified in the Protocol document.

3.  Permitted Characters: An Inclusion List

   IDNA2008 adopts the inclusion model.  A code point is assumed to be
   invalid for IDN use unless it is included as part of a Unicode
   property-based rule or, in rare cases, included individually by an
   exception.  When an implementation moves to a new version of Unicode,
   the rules may indicate new valid code points.




Klensin                       Informational                     [Page 9]

RFC 5894                     IDNA Rationale                  August 2010


   This section provides an overview of the model used to establish the
   algorithm and character lists of the Tables document [RFC5892] and
   describes the names and applicability of the categories used there.
   Note that the inclusion of a character in the PROTOCOL-VALID category
   group (Section 3.1.1) does not imply that it can be used
   indiscriminately; some characters are associated with contextual
   rules that must be applied as well.

   The information given in this section is provided to make the rules,
   tables, and protocol easier to understand.  The normative generating
   rules that correspond to this informal discussion appear in the
   Tables document, and the rules that actually determine what labels
   can be registered or looked up are in the Protocol document.

3.1.  A Tiered Model of Permitted Characters and Labels

   Moving to an inclusion model involves a new specification for the
   list of characters that are permitted in IDNs.  In IDNA2003,
   character validity is independent of context and fixed forever (or
   until the standard is replaced).  However, globally context-
   independent rules have proved to be impractical because some
   characters, especially those that are called "Join_Controls" in
   Unicode, are needed to make reasonable use of some scripts but have
   no visible effect in others.  IDNA2003 prohibited those types of
   characters entirely by discarding them.  We now have a consensus that
   under some conditions, these "joiner" characters are legitimately
   needed to allow useful mnemonics for some languages and scripts.  In
   general, context-dependent rules help deal with characters (generally
   characters that would otherwise be prohibited entirely) that are used
   differently or perceived differently across different scripts, and
   allow the standard to be applied more appropriately in cases where a
   string is not universally handled the same way.

   IDNA2008 divides all possible Unicode code points into four
   categories: PROTOCOL-VALID, CONTEXTUAL RULE REQUIRED, DISALLOWED, and
   UNASSIGNED.

3.1.1.  PROTOCOL-VALID

   Characters identified as PROTOCOL-VALID (often abbreviated PVALID)
   are permitted in IDNs.  Their use may be restricted by rules about
   the context in which they appear or by other rules that apply to the
   entire label in which they are to be embedded.  For example, any
   label that contains a character in this category that has a
   "right-to-left" property must be used in context with the Bidi rules
   [RFC5893].  The term PROTOCOL-VALID is used to stress the fact that
   the presence of a character in this category does not imply that a
   given registry need accept registrations containing any of the



Klensin                       Informational                    [Page 10]

RFC 5894                     IDNA Rationale                  August 2010


   characters in the category.  Registries are still expected to apply
   judgment about labels they will accept and to maintain rules
   consistent with those judgments (see the Protocol document [RFC5891]
   and Section 3.3).

   Characters that are placed in the PROTOCOL-VALID category are
   expected to never be removed from it or reclassified.  While
   theoretically characters could be removed from Unicode, such removal
   would be inconsistent with the Unicode stability principles (see
   UTR 39: Unicode Security Mechanisms [Unicode52], Appendix F) and
   hence should never occur.

3.1.2.  CONTEXTUAL RULE REQUIRED

   Some characters may be unsuitable for general use in IDNs but
   necessary for the plausible support of some scripts.  The two most
   commonly cited examples are the ZERO WIDTH JOINER and ZERO WIDTH
   NON-JOINER characters (ZWJ, U+200D and ZWNJ, U+200C), but other
   characters may require special treatment because they would otherwise
   be DISALLOWED (typically because Unicode considers them punctuation
   or special symbols) but need to be permitted in limited contexts.
   Other characters are given this special treatment because they pose
   exceptional danger of being used to produce misleading labels or to
   cause unacceptable ambiguity in label matching and interpretation.

3.1.2.1.  Contextual Restrictions

   Characters with contextual restrictions are identified as CONTEXTUAL
   RULE REQUIRED and are associated with a rule.  The rule defines
   whether the character is valid in a particular string, and also
   whether the rule itself is to be applied on lookup as well as
   registration.

   A distinction is made between characters that indicate or prohibit
   joining and ones similar to them (known as CONTEXT-JOINER or
   CONTEXTJ) and other characters requiring contextual treatment
   (CONTEXT-OTHER or CONTEXTO).  Only the former require full testing at
   lookup time.

   It is important to note that these contextual rules cannot prevent
   all uses of the relevant characters that might be confusing or
   problematic.  What they are expected to do is to confine
   applicability of the characters to scripts (and narrower contexts)
   where zone administrators are knowledgeable enough about the use of
   those characters to be prepared to deal with them appropriately.






Klensin                       Informational                    [Page 11]

RFC 5894                     IDNA Rationale                  August 2010


   For example, a registry dealing with an Indic script that requires
   ZWJ and/or ZWNJ as part of the writing system is expected to
   understand where the characters have visible effect and where they do
   not and to make registration rules accordingly.  By contrast, a
   registry dealing primarily with Latin or Cyrillic script might not be
   actively aware that the characters exist, much less about the
   consequences of embedding them in labels drawn from those scripts and
   therefore should avoid accepting registrations containing those
   characters, at least in labels using characters from the Latin or
   Cyrillic scripts.

3.1.2.2.  Rules and Their Application

   Rules have descriptions such as "Must follow a character from Script
   XYZ", "Must occur only if the entire label is in Script ABC", or
   "Must occur only if the previous and subsequent characters have the
   DFG property".  The actual rules may be DEFINED or NULL.  If present,
   they may have values of "True" (character may be used in any position
   in any label), "False" (character may not be used in any label), or
   may be a set of procedural rules that specify the context in which
   the character is permitted.

   Because it is easier to identify these characters than to know that
   they are actually needed in IDNs or how to establish exactly the
   right rules for each one, a rule may have a null value in a given
   version of the tables.  Characters associated with null rules are not
   permitted to appear in putative labels for either registration or
   lookup.  Of course, a later version of the tables might contain a
   non-null rule.

   The actual rules and their descriptions are in Sections 2 and 3 of
   the Tables document [RFC5892].  That document also specifies the
   creation of a registry for future rules.

3.1.3.  DISALLOWED

   Some characters are inappropriate for use in IDNs and are thus
   excluded for both registration and lookup (i.e., IDNA-conforming
   applications performing name lookup should verify that these
   characters are absent; if they are present, the label strings should
   be rejected rather than converted to A-labels and looked up.  Some of
   these characters are problematic for use in IDNs (such as the
   FRACTION SLASH character, U+2044), while some of them (such as the
   various HEART symbols, e.g., U+2665, U+2661, and U+2765, see
   Section 7.6) simply fall outside the conventions for typical
   identifiers (basically letters and numbers).





Klensin                       Informational                    [Page 12]

RFC 5894                     IDNA Rationale                  August 2010


   Of course, this category would include code points that had been
   removed entirely from Unicode should such removals ever occur.

   Characters that are placed in the DISALLOWED category are expected to
   never be removed from it or reclassified.  If a character is
   classified as DISALLOWED in error and the error is sufficiently
   problematic, the only recourse would be either to introduce a new
   code point into Unicode and classify it as PROTOCOL-VALID or for the
   IETF to accept the considerable costs of an incompatible change and
   replace the relevant RFC with one containing appropriate exceptions.

   There is provision for exception cases but, in general, characters
   are placed into DISALLOWED if they fall into one or more of the
   following groups:

   o  The character is a compatibility equivalent for another character.
      In slightly more precise Unicode terms, application of
      Normalization Form KC (NFKC) to the character yields some other
      character.

   o  The character is an uppercase form or some other form that is
      mapped to another character by Unicode case folding.

   o  The character is a symbol or punctuation form or, more generally,
      something that is not a letter, digit, or a mark that is used to
      form a letter or digit.

3.1.4.  UNASSIGNED

   For convenience in processing and table-building, code points that do
   not have assigned values in a given version of Unicode are treated as
   belonging to a special UNASSIGNED category.  Such code points are
   prohibited in labels to be registered or looked up.  The category
   differs from DISALLOWED in that code points are moved out of it by
   the simple expedient of being assigned in a later version of Unicode
   (at which point, they are classified into one of the other categories
   as appropriate).

   The rationale for restricting the processing of UNASSIGNED characters
   is simply that the properties of such code points cannot be
   completely known until actual characters are assigned to them.  For
   example, assume that an UNASSIGNED code point were included in a
   label to be looked up.  Assume that the code point was later assigned
   to a character that required some set of contextual rules.  With that
   combination, un-updated instances of IDNA-aware software might permit
   lookup of labels containing the previously unassigned characters
   while updated versions of the software might restrict use of the same




Klensin                       Informational                    [Page 13]

RFC 5894                     IDNA Rationale                  August 2010


   label in lookup, depending on the contextual rules.  It should be
   clear that under no circumstance should an UNASSIGNED character be
   permitted in a label to be registered as part of a domain name.

3.2.  Registration Policy

   While these recommendations cannot and should not define registry
   policies, registries should develop and apply additional restrictions
   as needed to reduce confusion and other problems.  For example, it is
   generally believed that labels containing characters from more than
   one script are a bad practice although there may be some important
   exceptions to that principle.  Some registries may choose to restrict
   registrations to characters drawn from a very small number of
   scripts.  For many scripts, the use of variant techniques such as
   those as described in the JET specification for the CJK script
   [RFC3743] and its generalization [RFC4290], and illustrated for
   Chinese by the tables provided by the Chinese Domain Name Consortium
   [RFC4713] may be helpful in reducing problems that might be perceived
   by users.

   In general, users will benefit if registries only permit characters
   from scripts that are well-understood by the registry or its
   advisers.  If a registry decides to reduce opportunities for
   confusion by constructing policies that disallow characters used in
   historic writing systems or characters whose use is restricted to
   specialized, highly technical contexts, some relevant information may
   be found in Section 2.4 (Specific Character Adjustments) of Unicode
   Identifier and Pattern Syntax [Unicode-UAX31], especially Table 4
   (Candidate Characters for Exclusion from Identifiers), and Section
   3.1 (General Security Profile for Identifiers) in Unicode Security
   Mechanisms [Unicode-UTS39].

   The requirement (in Section 4.1 of the Protocol document [RFC5891])
   that registration procedures use only U-labels and/or A-labels is
   intended to ensure that registrants are fully aware of exactly what
   is being registered as well as encouraging use of those canonical
   forms.  That provision should not be interpreted as requiring that
   registrants need to provide characters in a particular code sequence.
   Registrant input conventions and management are part of registrant-
   registrar interactions and relationships between registries and
   registrars and are outside the scope of these standards.

   It is worth stressing that these principles of policy development and
   application apply at all levels of the DNS, not only, e.g., top level
   domain (TLD) or second level domain (SLD) registrations.  Even a
   trivial, "anything is permitted that is valid under the protocol"
   policy is helpful in that it helps users and application developers
   know what to expect.



Klensin                       Informational                    [Page 14]

RFC 5894                     IDNA Rationale                  August 2010


3.3.  Layered Restrictions: Tables, Context, Registration, and
      Applications

   The character rules in IDNA2008 are based on the realization that
   there is no single magic bullet for any of the security,
   confusability, or other issues associated with IDNs.  Instead, the
   specifications define a variety of approaches.  The character tables
   are the first mechanism, protocol rules about how those characters
   are applied or restricted in context are the second, and those two in
   combination constitute the limits of what can be done in the
   protocol.  As discussed in the previous section (Section 3.2),
   registries are expected to restrict what they permit to be
   registered, devising and using rules that are designed to optimize
   the balance between confusion and risk on the one hand and maximum
   expressiveness in mnemonics on the other.

   In addition, there is an important role for user interface programs
   in warning against label forms that appear problematic given their
   knowledge of local contexts and conventions.  Of course, no approach
   based on naming or identifiers alone can protect against all threats.

4.  Application-Related Issues

4.1.  Display and Network Order

   Domain names are always transmitted in network order (the order in
   which the code points are sent in protocols), but they may have a
   different display order (the order in which the code points are
   displayed on a screen or paper).  When a domain name contains
   characters that are normally written right to left, display order may
   be affected although network order is not.  It gets even more
   complicated if left-to-right and right-to-left labels are adjacent to
   each other within a domain name.  The decision about the display
   order is ultimately under the control of user agents -- including Web
   browsers, mail clients, hosted Web applications and many more --
   which may be highly localized.  Should a domain name abc.def, in
   which both labels are represented in scripts that are written right
   to left, be displayed as fed.cba or cba.fed?  Applications that are
   in deployment today are already diverse, and one can find examples of
   either choice.

   The picture changes once again when an IDN appears in an
   Internationalized Resource Identifier (IRI) [RFC3987].  An IRI or
   internationalized email address contains elements other than the
   domain name.  For example, IRIs contain protocol identifiers and
   field delimiter syntax such as "http://" or "mailto:" while email
   addresses contain the "@" to separate local parts from domain names.




Klensin                       Informational                    [Page 15]

RFC 5894                     IDNA Rationale                  August 2010


   An IRI in network order begins with "http://" followed by domain
   labels in network order, thus "http://abc.def".

   User interface programs are not required to display and allow input
   of IRIs directly but often do so.  Implementers have to choose
   whether the overall direction of these strings will always be left to
   right (or right to left) for an IRI or email address.  The natural
   order for a user typing a domain name on a right-to-left system is
   fed.cba.  Should the right-to-left (RTL) user interface reverse the
   entire domain name each time a domain name is typed?  Does this
   change if the user types "http://" right before typing a domain name,
   thus implying that the user is beginning at the beginning of the
   network-order IRI?  Experience in the 1980s and 1990s with mixing
   systems in which domain name labels were read in network order (left
   to right) and those in which those labels were read right to left
   would predict a great deal of confusion.

   If each implementation of each application makes its own decisions on
   these issues, users will develop heuristics that will sometimes fail
   when switching applications.  However, while some display order
   conventions, voluntarily adopted, would be desirable to reduce
   confusion, such suggestions are beyond the scope of these
   specifications.

4.2.  Entry and Display in Applications

   Applications can accept and display domain names using any character
   set or character coding system.  The IDNA protocol does not
   necessarily affect the interface between users and applications.  An
   IDNA-aware application can accept and display internationalized
   domain names in two formats: as the internationalized character
   set(s) supported by the application (i.e., an appropriate local
   representation of a U-label) and as an A-label.  Applications may
   allow the display of A-labels, but are encouraged not to do so except
   as an interface for special purposes, possibly for debugging, or to
   cope with display limitations.  In general, they should allow, but
   not encourage, user input of A-labels.  A-labels are opaque and ugly,
   and malicious variations on them are not easily detected by users.
   Where possible, they should thus only be exposed when they are
   absolutely needed.  Because IDN labels can be rendered either as
   A-labels or U-labels, the application may reasonably have an option
   for the user to select the preferred method of display.  Rendering
   the U-label should normally be the default.

   Domain names are often stored and transported in many places.  For
   example, they are part of documents such as mail messages and web
   pages.  They are transported in many parts of many protocols, such as
   both the control commands of SMTP and associated message body parts,



Klensin                       Informational                    [Page 16]

RFC 5894                     IDNA Rationale                  August 2010


   and in the headers and the body content in HTTP.  It is important to
   remember that domain names appear both in domain name slots and in
   the content that is passed over protocols, and it would be helpful if
   protocols explicitly define what their domain name slots are.

   In protocols and document formats that define how to handle
   specification or negotiation of charsets, labels can be encoded in
   any charset allowed by the protocol or document format.  If a
   protocol or document format only allows one charset, the labels must
   be given in that charset.  Of course, not all charsets can properly
   represent all labels.  If a U-label cannot be displayed in its
   entirety, the only choice (without loss of information) may be to
   display the A-label.

   Where a protocol or document format allows IDNs, labels should be in
   whatever character encoding and escape mechanism the protocol or
   document format uses in the local environment.  This provision is
   intended to prevent situations in which, e.g., UTF-8 domain names
   appear embedded in text that is otherwise in some other character
   coding.

   All protocols that use domain name slots (see Section 2.3.2.6 in the
   Definitions document [RFC5890]) already have the capacity for
   handling domain names in the ASCII charset.  Thus, A-labels can
   inherently be handled by those protocols.

   IDNA2008 does not specify required mappings between one character or
   code point and others.  An extended discussion of mapping issues
   appears in Section 6 and specific recommendations appear in the
   Mapping document [IDNA2008-Mapping].  In general, IDNA2008 prohibits
   characters that would be mapped to others by normalization or other
   rules.  As examples, while mathematical characters based on Latin
   ones are accepted as input to IDNA2003, they are prohibited in
   IDNA2008.  Similarly, uppercase characters, double-width characters,
   and other variations are prohibited as IDNA input although mapping
   them as needed in user interfaces is strongly encouraged.

   Since the rules in the Tables document [RFC5892] have the effect that
   only strings that are not transformed by NFKC are valid, if an
   application chooses to perform NFKC normalization before lookup, that
   operation is safe since this will never make the application unable
   to look up any valid string.  However, as discussed above, the
   application cannot guarantee that any other application will perform
   that mapping, so it should be used only with caution and for informed
   users.






Klensin                       Informational                    [Page 17]

RFC 5894                     IDNA Rationale                  August 2010


   In many cases, these prohibitions should have no effect on what the
   user can type as input to the lookup process.  It is perfectly
   reasonable for systems that support user interfaces to perform some
   character mapping that is appropriate to the local environment.  This
   would normally be done prior to actual invocation of IDNA.  At least
   conceptually, the mapping would be part of the Unicode conversions
   discussed above and in the Protocol document [RFC5891].  However,
   those changes will be local ones only -- local to environments in
   which users will clearly understand that the character forms are
   equivalent.  For use in interchanges among systems, it appears to be
   much more important that U-labels and A-labels can be mapped back and
   forth without loss of information.

   One specific, and very important, instance of this strategy arises
   with case folding.  In the ASCII-only DNS, names are looked up and
   matched in a case-independent way, but no actual case folding occurs.
   Names can be placed in the DNS in either uppercase or lowercase form
   (or any mixture of them) and that form is preserved, returned in
   queries, and so on.  IDNA2003 approximated that behavior for
   non-ASCII strings by performing case folding at registration time
   (resulting in only lowercase IDNs in the DNS) and when names were
   looked up.

   As suggested earlier in this section, it appears to be desirable to
   do as little character mapping as possible as long as Unicode works
   correctly (e.g., Normalization Form C (NFC) mapping to resolve
   different codings for the same character is still necessary although
   the specifications require that it be performed prior to invoking the
   protocol) in order to make the mapping between A-labels and U-labels
   idempotent.  Case mapping is not an exception to this principle.  If
   only lowercase characters can be registered in the DNS (i.e., be
   present in a U-label), then IDNA2008 should prohibit uppercase
   characters as input even though user interfaces to applications
   should probably map those characters.  Some other considerations
   reinforce this conclusion.  For example, in ASCII case mapping for
   individual characters, uppercase(character) is always equal to
   uppercase(lowercase(character)).  That may not be true with IDNs.  In
   some scripts that use case distinctions, there are a few characters
   that do not have counterparts in one case or the other.  The
   relationship between uppercase and lowercase may even be language-
   dependent, with different languages (or even the same language in
   different areas) expecting different mappings.  User interface
   programs can meet the expectations of users who are accustomed to the
   case-insensitive DNS environment by performing case folding prior to
   IDNA processing, but the IDNA procedures themselves should neither
   require such mapping nor expect them when they are not natural to the
   localized environment.




Klensin                       Informational                    [Page 18]

RFC 5894                     IDNA Rationale                  August 2010


4.3.  Linguistic Expectations: Ligatures, Digraphs, and Alternate
      Character Forms

   Users have expectations about character matching or equivalence that
   are based on their own languages and the orthography of those
   languages.  These expectations may not always be met in a global
   system, especially if multiple languages are written using the same
   script but using different conventions.  Some examples:

   o  A Norwegian user might expect a label with the ae-ligature to be
      treated as the same label as one using the Swedish spelling with
      a-diaeresis even though applying that mapping to English would be
      astonishing to users.

   o  A German user might expect a label with an o-umlaut and a label
      that had "oe" substituted, but was otherwise the same, to be
      treated as equivalent even though that substitution would be a
      clear error in Swedish.

   o  A Chinese user might expect automatic matching of Simplified and
      Traditional Chinese characters, but applying that matching for
      Korean or Japanese text would create considerable confusion.

   o  An English user might expect "theater" and "theatre" to match.

   A number of languages use alphabetic scripts in which single phonemes
   are written using two characters, termed a "digraph", for example,
   the "ph" in "pharmacy" and "telephone".  (Such characters can also
   appear consecutively without forming a digraph, as in "tophat".)
   Certain digraphs may be indicated typographically by setting the two
   characters closer together than they would be if used consecutively
   to represent different phonemes.  Some digraphs are fully joined as
   ligatures.  For example, the word "encyclopaedia" is sometimes set
   with a U+00E6 LATIN SMALL LIGATURE AE.  When ligature and digraph
   forms have the same interpretation across all languages that use a
   given script, application of Unicode normalization generally resolves
   the differences and causes them to match.  When they have different
   interpretations, matching must utilize other methods, presumably
   chosen at the registry level, or users must be educated to understand
   that matching will not occur.

   The nature of the problem can be illustrated by many words in the
   Norwegian language, where the "ae" ligature is the 27th letter of a
   29-letter extended Latin alphabet.  It is equivalent to the 28th
   letter of the Swedish alphabet (also containing 29 letters),
   U+00E4 LATIN SMALL LETTER A WITH DIAERESIS, for which an "ae" cannot
   be substituted according to current orthographic standards.  That
   character (U+00E4) is also part of the German alphabet where, unlike



Klensin                       Informational                    [Page 19]

RFC 5894                     IDNA Rationale                  August 2010


   in the Nordic languages, the two-character sequence "ae" is usually
   treated as a fully acceptable alternate orthography for the "umlauted
   a" character.  The inverse is however not true, and those two
   characters cannot necessarily be combined into an "umlauted a".  This
   also applies to another German character, the "umlauted o"
   (U+00F6 LATIN SMALL LETTER O WITH DIAERESIS) which, for example,
   cannot be used for writing the name of the author "Goethe".  It is
   also a letter in the Swedish alphabet where, like the "a with
   diaeresis", it cannot be correctly represented as "oe" and in the
   Norwegian alphabet, where it is represented, not as "o with
   diaeresis", but as "slashed o", U+00F8.

   Some of the ligatures that have explicit code points in Unicode were
   given special handling in IDNA2003 and now pose additional problems
   in transition.  See Section 7.2.

   Additional cases with alphabets written right to left are described
   in Section 4.5.

   Matching and comparison algorithm selection often requires
   information about the language being used, context, or both --
   information that is not available to IDNA or the DNS.  Consequently,
   IDNA2008 makes no attempt to treat combined characters in any special
   way.  A registry that is aware of the language context in which
   labels are to be registered, and where that language sometimes (or
   always) treats the two-character sequences as equivalent to the
   combined form, should give serious consideration to applying a
   "variant" model [RFC3743][RFC4290] or to prohibiting registration of
   one of the forms entirely, to reduce the opportunities for user
   confusion and fraud that would result from the related strings being
   registered to different parties.

4.4.  Case Mapping and Related Issues

   In the DNS, ASCII letters are stored with their case preserved.
   Matching during the query process is case-independent, but none of
   the information that might be represented by choices of case has been
   lost.  That model has been accidentally helpful because, as people
   have created DNS labels by catenating words (or parts of words) to
   form labels, case has often been used to distinguish among components
   and make the labels more memorable.

   Since DNS servers do not get involved in parsing IDNs, they cannot do
   case-independent matching.  Thus, keeping the cases separate in
   lookup or registration, and doing matching at the server, is not
   feasible with IDNA or any similar approach.  Matching of characters
   that are considered to differ only by case must be done, if desired,
   by programs invoking IDNA lookup even though it wasn't done by ASCII-



Klensin                       Informational                    [Page 20]

RFC 5894                     IDNA Rationale                  August 2010


   only DNS clients.  That situation was recognized in IDNA2003 and
   nothing in IDNA2008 fundamentally changes it or could do so.  In
   IDNA2003, all characters are case folded and mapped by clients in a
   standardized step.

   Even in scripts that generally support case distinctions, some
   characters do not have uppercase forms.  For example, the Unicode
   case-folding operation maps Greek Final Form Sigma (U+03C2) to the
   medial form (U+03C3) and maps Eszett (German Sharp S, U+00DF) to
   "ss".  Neither of these mappings is reversible because the uppercase
   of U+03C3 is the uppercase Sigma (U+03A3) and "ss" is an ASCII
   string.  IDNA2008 permits, at the risk of some incompatibility,
   slightly more flexibility in this area by avoiding case folding and
   treating these characters as themselves.  Approaches to handling one-
   way mappings are discussed in Section 7.2.

   Because IDNA2003 maps Final Sigma and Eszett to other characters, and
   the reverse mapping is never possible, neither Final Sigma nor Eszett
   can be represented in the ACE form of IDNA2003 IDN nor in the native
   character (U-label) form derived from it.  With IDNA2008, both
   characters can be used in an IDN and so the A-label used for lookup
   for any U-label containing those characters is now different.  See
   Section 7.1 for a discussion of what kinds of changes might require
   the IDNA prefix to change; after extended discussions, the IDNABIS
   Working Group came to consensus that the change for these characters
   did not justify a prefix change.

4.5.  Right-to-Left Text

   In order to be sure that the directionality of right-to-left text is
   unambiguous, IDNA2003 required that any label in which right-to-left
   characters appear both starts and ends with them and that it does not
   include any characters with strong left-to-right properties (that
   excludes other alphabetic characters but permits European digits).
   Any other string that contains a right-to-left character and does not
   meet those requirements is rejected.  This is one of the few places
   where the IDNA algorithms (both in IDNA2003 and in IDNA2008) examine
   an entire label, not just individual characters.  The algorithmic
   model used in IDNA2003 rejects the label when the final character in
   a right-to-left string requires a combining mark in order to be
   correctly represented.

   That prohibition is not acceptable for writing systems for languages
   written with consonantal alphabets to which diacritical vocalic
   systems are applied, and for languages with orthographies derived
   from them where the combining marks may have different functionality.
   In both cases, the combining marks can be essential components of the
   orthography.  Examples of this are Yiddish, written with an extended



Klensin                       Informational                    [Page 21]

RFC 5894                     IDNA Rationale                  August 2010


   Hebrew script, and Dhivehi (the official language of Maldives), which
   is written in the Thaana script (which is, in turn, derived from the
   Arabic script).  IDNA2008 removes the restriction on final combining
   characters with a new set of rules for right-to-left scripts and
   their characters.  Those new rules are specified in the Bidi document
   [RFC5893].

5.  IDNs and the Robustness Principle

   The "Robustness Principle" is often stated as "Be conservative about
   what you send and liberal in what you accept" (see, e.g., Section
   1.2.2 of the applications-layer Host Requirements specification
   [RFC1123]).  This principle applies to IDNA.  In applying the
   principle to registries as the source ("sender") of all registered
   and useful IDNs, registries are responsible for being conservative
   about what they register and put out in the Internet.  For IDNs to
   work well, zone administrators (registries) must have and require
   sensible policies about what is registered -- conservative policies
   -- and implement and enforce them.

   Conversely, lookup applications are expected to reject labels that
   clearly violate global (protocol) rules (no one has ever seriously
   claimed that being liberal in what is accepted requires being
   stupid).  However, once one gets past such global rules and deals
   with anything sensitive to script or locale, it is necessary to
   assume that garbage has not been placed into the DNS, i.e., one must
   be liberal about what one is willing to look up in the DNS rather
   than guessing about whether it should have been permitted to be
   registered.

   If a string cannot be successfully found in the DNS after the lookup
   processing described here, it makes no difference whether it simply
   wasn't registered or was prohibited by some rule at the registry.
   Application implementers should be aware that where DNS wildcards are
   used, the ability to successfully resolve a name does not guarantee
   that it was actually registered.

6.  Front-end and User Interface Processing for Lookup

   Domain names may be identified and processed in many contexts.  They
   may be typed in by users themselves or embedded in an identifier such
   as an email address, URI, or IRI.  They may occur in running text or
   be processed by one system after being provided in another.  Systems
   may try to normalize URLs to determine (or guess) whether a reference
   is valid or if two references point to the same object without
   actually looking the objects up (comparison without lookup is
   necessary for URI types that are not intended to be resolved).  Some
   of these goals may be more easily and reliably satisfied than others.



Klensin                       Informational                    [Page 22]

RFC 5894                     IDNA Rationale                  August 2010


   While there are strong arguments for any domain name that is placed
   "on the wire" -- transmitted between systems -- to be in the zero-
   ambiguity forms of A-labels, it is inevitable that programs that
   process domain names will encounter U-labels or variant forms.

   An application that implements the IDNA protocol [RFC5891] will
   always take any user input and convert it to a set of Unicode code
   points.  That user input may be acquired by any of several different
   input methods, all with differing conversion processes to be taken
   into consideration (e.g., typed on a keyboard, written by hand onto
   some sort of digitizer, spoken into a microphone and interpreted by a
   speech-to-text engine, etc.).  The process of taking any particular
   user input and mapping it into a Unicode code point may be a simple
   one: if a user strikes the "A" key on a US English keyboard, without
   any modifiers such as the "Shift" key held down, in order to draw a
   Latin small letter A ("a"), many (perhaps most) modern operating
   system input methods will produce to the calling application the code
   point U+0061, encoded in a single octet.

   Sometimes the process is somewhat more complicated: a user might
   strike a particular set of keys to represent a combining macron
   followed by striking the "A" key in order to draw a Latin small
   letter A with a macron above it.  Depending on the operating system,
   the input method chosen by the user, and even the parameters with
   which the application communicates with the input method, the result
   might be the code point U+0101 (encoded as two octets in UTF-8 or
   UTF-16, four octets in UTF-32, etc.), the code point U+0061 followed
   by the code point U+0304 (again, encoded in three or more octets,
   depending upon the encoding used) or even the code point U+FF41
   followed by the code point U+0304 (and encoded in some form).  These
   examples leave aside the issue of operating systems and input methods
   that do not use Unicode code points for their character set.

   In every case, applications (with the help of the operating systems
   on which they run and the input methods used) need to perform a
   mapping from user input into Unicode code points.

   IDNA2003 used a model whereby input was taken from the user, mapped
   (via whatever input method mechanisms were used) to a set of Unicode
   code points, and then further mapped to a set of Unicode code points
   using the Nameprep profile [RFC3491].  In this procedure, there are
   two separate mapping steps: first, a mapping done by the input method
   (which might be controlled by the operating system, the application,
   or some combination) and then a second mapping performed by the
   Nameprep portion of the IDNA protocol.  The mapping done in Nameprep
   includes a particular mapping table to re-map some characters to
   other characters, a particular normalization, and a set of prohibited
   characters.



Klensin                       Informational                    [Page 23]

RFC 5894                     IDNA Rationale                  August 2010


   Note that the result of the two-step mapping process means that the
   mapping chosen by the operating system or application in the first
   step might differ significantly from the mapping supplied by the
   Nameprep profile in the second step.  This has advantages and
   disadvantages.  Of course, the second mapping regularizes what gets
   looked up in the DNS, making for better interoperability between
   implementations that use the Nameprep mapping.  However, the
   application or operating system may choose mappings in their input
   methods, which when passed through the second (Nameprep) mapping
   result in characters that are "surprising" to the end user.

   The other important feature of IDNA2003 is that, with very few
   exceptions, it assumes that any set of Unicode code points provided
   to the Nameprep mapping can be mapped into a string of Unicode code
   points that are "sensible", even if that means mapping some code
   points to nothing (that is, removing the code points from the
   string).  This allowed maximum flexibility in input strings.

   The present version of IDNA (IDNA2008) differs significantly in
   approach from the original version.  First and foremost, it does not
   provide explicit mapping instructions.  Instead, it assumes that the
   application (perhaps via an operating system input method) will do
   whatever mapping it requires to convert input into Unicode code
   points.  This has the advantage of giving flexibility to the
   application to choose a mapping that is suitable for its user given
   specific user requirements, and avoids the two-step mapping of the
   original protocol.  Instead of a mapping, IDNA2008 provides a set of
   categories that can be used to specify the valid code points allowed
   in a domain name.

   In principle, an application ought to take user input of a domain
   name and convert it to the set of Unicode code points that represent
   the domain name the user intends.  As a practical matter, of course,
   determining user intent is a tricky business, so an application needs
   to choose a reasonable mapping from user input.  That may differ
   based on the particular circumstances of a user, depending on locale,
   language, type of input method, etc.  It is up to the application to
   make a reasonable choice.













Klensin                       Informational                    [Page 24]

RFC 5894                     IDNA Rationale                  August 2010


7.  Migration from IDNA2003 and Unicode Version Synchronization

7.1.  Design Criteria

   As mentioned above and in the IAB review and recommendations for IDNs
   [RFC4690], two key goals of the IDNA2008 design are:

   o  to enable applications to be agnostic about whether they are being
      run in environments supporting any Unicode version from 3.2
      onward.

   o  to permit incrementally adding new characters, character groups,
      scripts, and other character collections as they are incorporated
      into Unicode, doing so without disruption and, in the long term,
      without "heavy" processes (an IETF consensus process is required
      by the IDNA2008 specifications and is expected to be required and
      used until significant experience accumulates with IDNA operations
      and new versions of Unicode).

7.1.1.  Summary and Discussion of IDNA Validity Criteria

   The general criteria for a label to be considered valid under IDNA
   are (the actual rules are rigorously defined in the Protocol
   [RFC5891] and Tables [RFC5892] documents):

   o  The characters are "letters", marks needed to form letters,
      numerals, or other code points used to write words in some
      language.  Symbols, drawing characters, and various notational
      characters are intended to be permanently excluded.  There is no
      evidence that they are important enough to Internet operations or
      internationalization to justify expansion of domain names beyond
      the general principle of "letters, digits, and hyphen".
      (Additional discussion and rationale for the symbol decision
      appears in Section 7.6.)

   o  Other than in very exceptional cases, e.g., where they are needed
      to write substantially any word of a given language, punctuation
      characters are excluded.  The fact that a word exists is not proof
      that it should be usable in a DNS label, and DNS labels are not
      expected to be usable for multiple-word phrases (although they are
      certainly not prohibited if the conventions and orthography of a
      particular language cause that to be possible).

   o  Characters that are unassigned (have no character assignment at
      all) in the version of Unicode being used by the registry or
      application are not permitted, even on lookup.  The issues
      involved in this decision are discussed in Section 7.7.




Klensin                       Informational                    [Page 25]

RFC 5894                     IDNA Rationale                  August 2010


   o  Any character that is mapped to another character by a current
      version of NFKC is prohibited as input to IDNA (for either
      registration or lookup).  With a few exceptions, this principle
      excludes any character mapped to another by Nameprep [RFC3491].

   The principles above drive the design of rules that are specified
   exactly in the Tables document.  Those rules identify the characters
   that are valid under IDNA.  The rules themselves are normative, and
   the tables are derived from them, rather than vice versa.

7.1.2.  Labels in Registration

   Any label registered in a DNS zone must be validated -- i.e., the
   criteria for that label must be met -- in order for applications to
   work as intended.  This principle is not new.  For example, since the
   DNS was first deployed, zone administrators have been expected to
   verify that names meet "hostname" requirements [RFC0952] where those
   requirements are imposed by the expected applications.  Other
   applications contexts, such as the later addition of special service
   location formats [RFC2782] imposed new requirements on zone
   administrators.  For zones that will contain IDNs, support for
   Unicode version-independence requires restrictions on all strings
   placed in the zone.  In particular, for such zones (the exact rules
   appear in Section 4 of the Protocol document [RFC5891]):

   o  Any label that appears to be an A-label, i.e., any label that
      starts in "xn--", must be valid under IDNA, i.e., they must be
      valid A-labels, as discussed in Section 2 above.

   o  The Unicode tables (i.e., tables of code points, character
      classes, and properties) and IDNA tables (i.e., tables of
      contextual rules such as those that appear in the Tables
      document), must be consistent on the systems performing or
      validating labels to be registered.  Note that this does not
      require that tables reflect the latest version of Unicode, only
      that all tables used on a given system are consistent with each
      other.

   Under this model, registry tables will need to be updated (both the
   Unicode-associated tables and the tables of permitted IDN characters)
   to enable a new script or other set of new characters.  The registry
   will not be affected by newer versions of Unicode, or newly
   authorized characters, until and unless it wishes to support them.
   The zone administrator is responsible for verifying validity for IDNA
   as well as its local policies -- a more extensive set of checks than
   are required for looking up the labels.  Systems looking up or





Klensin                       Informational                    [Page 26]

RFC 5894                     IDNA Rationale                  August 2010


   resolving DNS labels, especially IDN DNS labels, must be able to
   assume that applicable registration rules were followed for names
   entered into the DNS.

7.1.3.  Labels in Lookup

   Any application processing a label through IDNA so it can be looked
   up in a DNS zone is required to (the exact rules appear in Section 5
   of the Protocol document [RFC5891]):

   o  Maintain IDNA and Unicode tables that are consistent with regard
      to versions, i.e., unless the application actually executes the
      classification rules in the Tables document [RFC5892], its IDNA
      tables must be derived from the version of Unicode that is
      supported more generally on the system.  As with registration, the
      tables need not reflect the latest version of Unicode, but they
      must be consistent.

   o  Validate the characters in labels to be looked up only to the
      extent of determining that the U-label does not contain
      "DISALLOWED" code points or code points that are unassigned in its
      version of Unicode.

   o  Validate the label itself for conformance with a small number of
      whole-label rules.  In particular, it must verify that:

      *  there are no leading combining marks,

      *  the Bidi conditions are met if right-to-left characters appear,

      *  any required contextual rules are available, and

      *  any contextual rules that are associated with joiner characters
         (and CONTEXTJ characters more generally) are tested.

   o  Do not reject labels based on other contextual rules about
      characters, including mixed-script label prohibitions.  Such rules
      may be used to influence presentation decisions in the user
      interface, but not to avoid looking up domain names.

   To further clarify the rules about handling characters that require
   contextual rules, note that one can have a context-required character
   (i.e., one that requires a rule), but no rule.  In that case, the
   character is treated the same way DISALLOWED characters are treated,
   until and unless a rule is supplied.  That state is more or less
   equivalent to "the idea of permitting this character is accepted in
   principle, but it won't be permitted in practice until consensus is
   reached on a safe way to use it".



Klensin                       Informational                    [Page 27]

RFC 5894                     IDNA Rationale                  August 2010


   The ability to add a rule more or less exempts these characters from
   the prohibition against reclassifying characters from DISALLOWED to
   PVALID.

   And, obviously, "no rule" is different from "have a rule, but the
   test either succeeds or fails".

   Lookup applications that follow these rules, rather than having their
   own criteria for rejecting lookup attempts, are not sensitive to
   version incompatibilities with the particular zone registry
   associated with the domain name except for labels containing
   characters recently added to Unicode.

   An application or client that processes names according to this
   protocol and then resolves them in the DNS will be able to locate any
   name that is registered, as long as those registrations are valid
   under IDNA and its version of the IDNA tables is sufficiently up to
   date to interpret all of the characters in the label.  Messages to
   users should distinguish between "label contains an unallocated code
   point" and other types of lookup failures.  A failure on the basis of
   an old version of Unicode may lead the user to a desire to upgrade to
   a newer version, but will have no other ill effects (this is
   consistent with behavior in the transition to the DNS when some hosts
   could not yet handle some forms of names or record types).

7.2.  Changes in Character Interpretations

   As a consequence of the elimination of mapping, the current version
   of IDNA changes the interpretation of a few characters relative to
   its predecessors.  This subsection outlines the issues and discusses
   possible transition strategies.

7.2.1.  Character Changes: Eszett and Final Sigma

   In those scripts that make case distinctions, there are a few
   characters for which an obvious and unique uppercase character has
   not historically been available to match a lowercase one, or vice
   versa.  For those characters, the mappings used in constructing the
   Stringprep tables for IDNA2003, performed using the Unicode
   toCaseFold operation (see Section 5.18 of the Unicode Standard
   [Unicode52]), generate different characters or sets of characters.
   Those operations are not reversible and lose even more information
   than traditional uppercase or lowercase transformations, but are more
   useful than those transformations for comparison purposes.  Two
   notable characters of this type are the German character Eszett
   (Sharp S, U+00DF) and the Greek Final Form Sigma (U+03C2).  The
   former is case folded to the ASCII string "ss", the latter to a
   medial (lowercase) Sigma (U+03C3).



Klensin                       Informational                    [Page 28]

RFC 5894                     IDNA Rationale                  August 2010


7.2.2.  Character Changes: Zero Width Joiner and Zero Width Non-Joiner

   IDNA2003 mapped both ZERO WIDTH JOINER (ZWJ, U+200D) and ZERO WIDTH
   NON-JOINER (ZWNJ, U+200C) to nothing, effectively dropping these
   characters from any label in which they appeared and treating strings
   containing them as identical to strings that did not.  As discussed
   in Section 3.1.2 above, those characters are essential for writing
   many reasonable mnemonics for certain scripts.  However, treating
   them as valid in IDNA2008, even with contextual restrictions, raises
   approximately the same problem as exists with Eszett and Final Sigma:
   strings that were valid under IDNA2003 have different interpretations
   as labels, and different A-labels, than the same strings under this
   newer version.

7.2.3.  Character Changes and the Need for Transition

   The decision to eliminate mandatory and standardized mappings,
   including case folding, from the IDNA2008 protocol in order to make
   A-labels and U-labels idempotent made these characters problematic.
   If they were to be disallowed, important words and mnemonics could
   not be written in orthographically reasonable ways.  If they were to
   be permitted as distinct characters, there would be no information
   loss and registries would have more flexibility, but IDNA2003 and
   IDNA2008 lookups might result in different A-labels.

   With the understanding that there would be incompatibility either way
   but a judgment that the incompatibility was not significant enough to
   justify a prefix change, the Working Group concluded that Eszett and
   Final Form Sigma should be treated as distinct and Protocol-Valid
   characters.

   Since these characters are interpreted in different ways under the
   older and newer versions of IDNA, transition strategies and policies
   will be necessary.  Some actions can reasonably be taken by
   applications' client programs (those that perform lookup operations
   or cause them to be performed), but because of the diversity of
   situations and uses of the DNS, much of the responsibility will need
   to fall on registries.

   Registries, especially those maintaining zones for third parties,
   must decide how to introduce a new service in a way that does not
   create confusion or significantly weaken or invalidate existing
   identifiers.  This is not a new problem; registries were faced with
   similar issues when IDNs were introduced (potentially, and especially
   for Latin-based scripts, in conflict with existing labels that had
   been rendered in ASCII characters by applying more or less
   standardized conventions) and when other new forms of strings have
   been permitted as labels.



Klensin                       Informational                    [Page 29]

RFC 5894                     IDNA Rationale                  August 2010


7.2.4.  Transition Strategies

   There are several approaches to the introduction of new characters or
   changes in interpretation of existing characters from their mapped
   forms in the earlier version of IDNA.  The transition issue is
   complicated because the forms of these labels after the
   ToUnicode(ToASCII()) translation in IDNA2003 not only remain valid
   but do not provide strong indications of what the registrant
   intended: a string containing "ss" could have simply been intended to
   be that string or could have been intended to contain an Eszett; a
   string containing lowercase Sigma could have been intended to contain
   Final Sigma (one might make heuristic guesses based on position in a
   string, but the long tradition of forming labels by concatenating
   words makes such heuristics unreliable), and strings that do not
   contain ZWJ or ZWNJ might have been intended to contain them.
   Without any preference or claim to completeness, some of these, all
   of which have been used by registries in the past for similar
   transitions, are:

   1.  Do not permit use of the newly available character at the
       registry level.  This might cause lookup failures if a domain
       name were to be written with the expectation of the IDNA2003
       mapping behavior, but would eliminate any possibility of false
       matches.

   2.  Hold a "sunrise"-like arrangement in which holders of labels
       containing "ss" in the Eszett case, lowercase Sigma in that case,
       or that might have contained ZWJ or ZWNJ in context, are given
       priority (and perhaps other benefits) for registering the
       corresponding string containing Eszett, Final Sigma, or the
       appropriate zero-width character respectively.

   3.  Adopt some sort of "variant" approach in which registrants obtain
       labels with both character forms.

   4.  Adopt a different form of "variant" approach in which
       registration of additional strings that would produce the same
       A-label if interpreted according to IDNA2003 is either not
       permitted at all or permitted only by the registrant who already
       has one of the names.

   5.  Ignore the issue and assume that the marketplace or other
       mechanisms will sort things out.

   In any event, a registry (at any level of the DNS tree) that chooses
   to permit labels to be registered that contains these characters, or
   considers doing so, will have to address the relationship with
   existing, possibly conflicting, labels in some way, just as



Klensin                       Informational                    [Page 30]

RFC 5894                     IDNA Rationale                  August 2010


   registries that already had a considerable number of labels did when
   IDNs were first introduced.

7.3.  Elimination of Character Mapping

   As discussed at length in Section 6, IDNA2003, via Nameprep (see
   Section 7.5), mapped many characters into related ones.  Those
   mappings no longer exist as requirements in IDNA2008.  These
   specifications strongly prefer that only A-labels or U-labels be used
   in protocol contexts and as much as practical more generally.
   IDNA2008 does anticipate situations in which some mapping at the time
   of user input into lookup applications is appropriate and desirable.
   The issues are discussed in Section 6 and specific recommendations
   are made in the Mapping document [IDNA2008-Mapping].

7.4.  The Question of Prefix Changes

   The conditions that would have required a change in the IDNA ACE
   prefix ("xn--", used in IDNA2003) were of great concern to the
   community.  A prefix change would have clearly been necessary if the
   algorithms were modified in a manner that would have created serious
   ambiguities during subsequent transition in registrations.  This
   section summarizes the working group's conclusions about the
   conditions under which a change in the prefix would have been
   necessary and the implications of such a change.

7.4.1.  Conditions Requiring a Prefix Change

   An IDN prefix change would have been needed if a given string would
   be looked up or otherwise interpreted differently depending on the
   version of the protocol or tables being used.  This IDNA upgrade
   would have required a prefix change if, and only if, one of the
   following four conditions were met:

   1.  The conversion of an A-label to Unicode (i.e., a U-label) would
       have yielded one string under IDNA2003 and a different string
       under IDNA2008.

   2.  In a significant number of cases, an input string that was valid
       under IDNA2003 and also valid under IDNA2008 would have yielded
       two different A-labels with the different versions.  This
       condition is believed to be essentially equivalent to the one
       above except for a very small number of edge cases that were not
       found to justify a prefix change (see Section 7.2).

       Note that if the input string was valid under one version and not
       valid under the other, this condition would not apply.  See the
       first item in Section 7.4.2, below.



Klensin                       Informational                    [Page 31]

RFC 5894                     IDNA Rationale                  August 2010


   3.  A fundamental change was made to the semantics of the string that
       would be inserted in the DNS, e.g., if a decision were made to
       try to include language or script information in the encoding in
       addition to the string itself.

   4.  A sufficiently large number of characters were added to Unicode
       so that the Punycode mechanism for block offsets would no longer
       reference the higher-numbered planes and blocks.  This condition
       is unlikely even in the long term and certain not to arise in the
       next several years.

7.4.2.  Conditions Not Requiring a Prefix Change

   As a result of the principles described above, none of the following
   changes required a new prefix:

   1.  Prohibition of some characters as input to IDNA.  Such a
       prohibition might make names that were previously registered
       inaccessible, but did not change those names.

   2.  Adjustments in IDNA tables or actions, including normalization
       definitions, that affected characters that were already invalid
       under IDNA2003.

   3.  Changes in the style of the IDNA definition that did not alter
       the actions performed by IDNA.

7.4.3.  Implications of Prefix Changes

   While it might have been possible to make a prefix change, the costs
   of such a change are considerable.  Registries could not have
   converted all IDNA2003 ("xn--") registrations to a new form at the
   same time and synchronize that change with applications supporting
   lookup.  Unless all existing registrations were simply to be declared
   invalid (and perhaps even then), systems that needed to support both
   labels with old prefixes and labels with new ones would be required
   to first process a putative label under the IDNA2008 rules and try to
   look it up and then, if it were not found, would be required to
   process the label under IDNA2003 rules and look it up again.  That
   process would probably have significantly slowed down all processing
   that involved IDNs in the DNS, especially since a fully-qualified
   name might contain a mixture of labels that were registered with the
   old and new prefixes.  That would have made DNS caching very
   difficult.  In addition, looking up the same input string as two
   separate A-labels would have created some potential for confusion and
   attacks, since the labels could map to different targets and then
   resolve to different entries in the DNS.




Klensin                       Informational                    [Page 32]

RFC 5894                     IDNA Rationale                  August 2010


   Consequently, a prefix change should have been, and was, avoided if
   at all possible, even if it means accepting some IDNA2003 decisions
   about character distinctions as irreversible and/or giving special
   treatment to edge cases.

7.5.  Stringprep Changes and Compatibility

   The Nameprep specification [RFC3491], a key part of IDNA2003, is a
   profile of Stringprep [RFC3454].  While Nameprep is a Stringprep
   profile specific to IDNA, Stringprep is used by a number of other
   protocols.  Were Stringprep to have been modified by IDNA2008, those
   changes to improve the handling of IDNs could cause problems for
   non-DNS uses, most notably if they affected identification and
   authentication protocols.  Several elements of IDNA2008 give
   interpretations to strings prohibited under IDNA2003 or prohibit
   strings that IDNA2003 permitted.  Those elements include the new
   inclusion information in the Tables document [RFC5892], the reduction
   in the number of characters permitted as input for registration or
   lookup (Section 3), and even the changes in handling of right-to-left
   strings as described in the Bidi document [RFC5893].  IDNA2008 does
   not use Nameprep or Stringprep at all, so there are no side-effect
   changes to other protocols.

   It is particularly important to keep IDNA processing separate from
   processing for various security protocols because some of the
   constraints that are necessary for smooth and comprehensible use of
   IDNs may be unwanted or undesirable in other contexts.  For example,
   the criteria for good passwords or passphrases are very different
   from those for desirable IDNs: passwords should be hard to guess,
   while domain names should normally be easily memorable.  Similarly,
   internationalized Small Computer System Interface (SCSI) identifiers
   and other protocol components are likely to have different
   requirements than IDNs.

7.6.  The Symbol Question

   One of the major differences between this specification and the
   original version of IDNA is that IDNA2003 permitted non-letter
   symbols of various sorts, including punctuation and line-drawing
   symbols, in the protocol.  They were always discouraged in practice.
   In particular, both the "IESG Statement" about IDNA and all versions
   of the ICANN Guidelines specify that only language characters be used
   in labels.  This specification disallows symbols entirely.  There are
   several reasons for this, which include:

   1.  As discussed elsewhere, the original IDNA specification assumed
       that as many Unicode characters as possible should be permitted,
       directly or via mapping to other characters, in IDNs.  This



Klensin                       Informational                    [Page 33]

RFC 5894                     IDNA Rationale                  August 2010


       specification operates on an inclusion model, extrapolating from
       the original "hostname" rules (LDH, see the Definitions document
       [RFC5890]) -- which have served the Internet very well -- to a
       Unicode base rather than an ASCII base.

   2.  Symbol names are more problematic than letters because there may
       be no general agreement on whether a particular glyph matches a
       symbol; there are no uniform conventions for naming; variations
       such as outline, solid, and shaded forms may or may not exist;
       and so on.  As just one example, consider a "heart" symbol as it
       might appear in a logo that might be read as "I love...".  While
       the user might read such a logo as "I love..." or "I heart...",
       considerable knowledge of the coding distinctions made in Unicode
       is needed to know that there is more than one "heart" character
       (e.g., U+2665, U+2661, and U+2765) and how to describe it.  These
       issues are of particular importance if strings are expected to be
       understood or transcribed by the listener after being read out
       loud.

   3.  Design of a screen reader used by blind Internet users who must
       listen to renderings of IDN domain names and possibly reproduce
       them on the keyboard becomes considerably more complicated when
       the names of characters are not obvious and intuitive to anyone
       familiar with the language in question.

   4.  As a simplified example of this, assume one wanted to use a
       "heart" or "star" symbol in a label.  This is problematic because
       those names are ambiguous in the Unicode system of naming (the
       actual Unicode names require far more qualification).  A user or
       would-be registrant has no way to know -- absent careful study of
       the code tables -- whether it is ambiguous (e.g., where there are
       multiple "heart" characters) or not.  Conversely, the user seeing
       the hypothetical label doesn't know whether to read it -- try to
       transmit it to a colleague by voice -- as "heart", as "love", as
       "black heart", or as any of the other examples below.

   5.  The actual situation is even worse than this.  There is no
       possible way for a normal, casual, user to tell the difference
       between the hearts of U+2665 and U+2765 and the stars of U+2606
       and U+2729 without somehow knowing to look for a distinction.  We
       have a white heart (U+2661) and few black hearts.  Consequently,
       describing a label as containing a heart is hopelessly ambiguous:
       we can only know that it contains one of several characters that
       look like hearts or have "heart" in their names.  In cities where
       "Square" is a popular part of a location name, one might well
       want to use a square symbol in a label as well and there are far
       more squares of various flavors in Unicode than there are hearts
       or stars.



Klensin                       Informational                    [Page 34]

RFC 5894                     IDNA Rationale                  August 2010


   The consequence of these ambiguities is that symbols are a very poor
   basis for reliable communication.  Consistent with this conclusion,
   the Unicode standard recommends that strings used in identifiers not
   contain symbols or punctuation [Unicode-UAX31].  Of course, these
   difficulties with symbols do not arise with actual pictographic
   languages and scripts which would be treated like any other language
   characters; the two should not be confused.

7.7.  Migration between Unicode Versions: Unassigned Code Points

   In IDNA2003, labels containing unassigned code points are looked up
   on the assumption that, if they appear in labels and can be mapped
   and then resolved, the relevant standards must have changed and the
   registry has properly allocated only assigned values.

   In the IDNA2008 protocol, strings containing unassigned code points
   must not be either looked up or registered.  In summary, the status
   of an unassigned character with regard to the DISALLOWED,
   PROTOCOL-VALID, and CONTEXTUAL RULE REQUIRED categories cannot be
   evaluated until a character is actually assigned and known.  There
   are several reasons for this, with the most important ones being:

   o  Tests involving the context of characters (e.g., some characters
      being permitted only adjacent to others of specific types) and
      integrity tests on complete labels are needed.  Unassigned code
      points cannot be permitted because one cannot determine whether
      particular code points will require contextual rules (and what
      those rules should be) before characters are assigned to them and
      the properties of those characters fully understood.

   o  It cannot be known in advance, and with sufficient reliability,
      whether a newly assigned code point will be associated with a
      character that would be disallowed by the rules in the Tables
      document [RFC5892] (such as a compatibility character).  In
      IDNA2003, since there is no direct dependency on NFKC (many of the
      entries in Stringprep's tables are based on NFKC, but IDNA2003
      depends only on Stringprep), allocation of a compatibility
      character might produce some odd situations, but it would not be a
      problem.  In IDNA2008, where compatibility characters are
      DISALLOWED unless character-specific exceptions are made,
      permitting strings containing unassigned characters to be looked
      up would violate the principle that characters in DISALLOWED are
      not looked up.

   o  The Unicode Standard specifies that an unassigned code point
      normalizes (and, where relevant, case folds) to itself.  If the
      code point is later assigned to a character, and particularly if
      the newly assigned code point has a combining class that



Klensin                       Informational                    [Page 35]

RFC 5894                     IDNA Rationale                  August 2010


      determines its placement relative to other combining characters,
      it could normalize to some other code point or sequence.

   It is possible to argue that the issues above are not important and
   that, as a consequence, it is better to retain the principle of
   looking up labels even if they contain unassigned characters because
   all of the important scripts and characters have been coded as of
   Unicode 5.2 (or even earlier), and hence unassigned code points will
   be assigned only to obscure characters or archaic scripts.
   Unfortunately, that does not appear to be a safe assumption for at
   least two reasons.  First, much the same claim of completeness has
   been made for earlier versions of Unicode.  The reality is that a
   script that is obscure to much of the world may still be very
   important to those who use it.  Cultural and linguistic preservation
   principles make it inappropriate to declare the script of no
   importance in IDNs.  Second, we already have counterexamples, e.g.,
   in the relationships associated with new Han characters being added
   (whether in the BMP or in Unicode Plane 2).

   Independent of the technical transition issues identified above, it
   can be observed that any addition of characters to an existing script
   to make it easier to use or to better accommodate particular
   languages may lead to transition issues.  Such additions may change
   the preferred form for writing a particular string, changes that may
   be reflected, e.g., in keyboard transition modules that would
   necessarily be different from those for earlier versions of Unicode
   where the newer characters may not exist.  This creates an inherent
   transition problem because attempts to access labels may use either
   the old or the new conventions, requiring registry action whether or
   not the older conventions were used in labels.  The need to consider
   transition mechanisms is inherent to evolution of Unicode to better
   accommodate writing systems and is independent of how IDNs are
   represented in the DNS or how transitions among versions of those
   mechanisms occur.  The requirement for transitions of this type is
   illustrated by the addition of Malayalam Chillu in Unicode 5.1.0.

7.8.  Other Compatibility Issues

   The 2003 IDNA model includes several odd artifacts of the context in
   which it was developed.  Many, if not all, of these are potential
   avenues for exploits, especially if the registration process permits
   "source" names (names that have not been processed through IDNA and
   Nameprep) to be registered.  As one example, since the character
   Eszett, used in German, is mapped by IDNA2003 into the sequence "ss"
   rather than being retained as itself or prohibited, a string
   containing that character, but that is otherwise in ASCII, is not
   really an IDN (in the U-label sense defined above).  After Nameprep
   maps out the Eszett, the result is an ASCII string and so it does not



Klensin                       Informational                    [Page 36]

RFC 5894                     IDNA Rationale                  August 2010


   get an xn-- prefix, but the string that can be displayed to a user
   appears to be an IDN.  IDNA2008 eliminates this artifact.  A
   character is either permitted as itself or it is prohibited; special
   cases that make sense only in a particular linguistic or cultural
   context can be dealt with as localization matters where appropriate.

8.  Name Server Considerations

8.1.  Processing Non-ASCII Strings

   Existing DNS servers do not know the IDNA rules for handling
   non-ASCII forms of IDNs, and therefore need to be shielded from them.
   All existing channels through which names can enter a DNS server
   database (for example, master files (as described in RFC 1034) and
   DNS update messages [RFC2136]) could not be IDNA-aware because they
   predate IDNA.  Other sections of this document provide the needed
   shielding by ensuring that internationalized domain names entering
   DNS server databases through such channels have already been
   converted to their equivalent ASCII A-label forms.

   Because of the distinction made between the algorithms for
   Registration and Lookup in Sections 4 and 5 (respectively) of the
   Protocol document [RFC5891] (a domain name containing only ASCII code
   points cannot be converted to an A-label), there cannot be more than
   one A-label form for any given U-label.

   As specified in clarifications to the DNS specification [RFC2181],
   the DNS protocol explicitly allows domain labels to contain octets
   beyond the ASCII range (0000..007F), and this document does not
   change that.  However, although the interpretation of octets
   0080..00FF is well-defined in the DNS, many application protocols
   support only ASCII labels and there is no defined interpretation of
   these non-ASCII octets as characters and, in particular, no
   interpretation of case-independent matching for them (e.g., see the
   clarification on DNS case insensitivity [RFC4343]).  If labels
   containing these octets are returned to applications, unpredictable
   behavior could result.  The A-label form, which cannot contain those
   characters, is the only standard representation for internationalized
   labels in the DNS protocol.

8.2.  Root and Other DNS Server Considerations

   IDNs in A-label form will generally be somewhat longer than current
   domain names, so the bandwidth needed by the root servers is likely
   to go up by a small amount.  Also, queries and responses for IDNs
   will probably be somewhat longer than typical queries historically,





Klensin                       Informational                    [Page 37]

RFC 5894                     IDNA Rationale                  August 2010


   so Extension Mechanisms for DNS (EDNS0) [RFC2671] support may be more
   important (otherwise, queries and responses may be forced to go to
   TCP instead of UDP).

9.  Internationalization Considerations

   DNS labels and fully-qualified domain names provide mnemonics that
   assist in identifying and referring to resources on the Internet.
   IDNs expand the range of those mnemonics to include those based on
   languages and character sets other than Western European and Roman-
   derived ones.  But domain "names" are not, in general, words in any
   language.  The recommendations of the IETF policy on character sets
   and languages (BCP 18 [RFC2277]) are applicable to situations in
   which language identification is used to provide language-specific
   contexts.  The DNS is, by contrast, global and international and
   ultimately has nothing to do with languages.  Adding languages (or
   similar context) to IDNs generally, or to DNS matching in particular,
   would imply context-dependent matching in DNS, which would be a very
   significant change to the DNS protocol itself.  It would also imply
   that users would need to identify the language associated with a
   particular label in order to look that label up.  That knowledge is
   generally not available because many labels are not words in any
   language and some may be words in more than one.

10.  IANA Considerations

   This section gives an overview of IANA registries required for IDNA.
   The actual definitions of, and specifications for, the first two,
   which have been newly created for IDNA2008, appear in the Tables
   document [RFC5892].  This document describes the registries, but it
   does not specify any IANA actions.

10.1.  IDNA Character Registry

   The distinction among the major categories "UNASSIGNED",
   "DISALLOWED", "PROTOCOL-VALID", and "CONTEXTUAL RULE REQUIRED" is
   made by special categories and rules that are integral elements of
   the Tables document.  While not normative, an IANA registry of
   characters and scripts and their categories, updated for each new
   version of Unicode and the characters it contains, are convenient for
   programming and validation purposes.  The details of this registry
   are specified in the Tables document.









Klensin                       Informational                    [Page 38]

RFC 5894                     IDNA Rationale                  August 2010


10.2.  IDNA Context Registry

   IANA has created and now maintains a list of approved contextual
   rules for characters that are defined in the IDNA Character Registry
   list as requiring a Contextual Rule (i.e., the types of rules
   described in Section 3.1.2).  The details for those rules appear in
   the Tables document.

10.3.  IANA Repository of IDN Practices of TLDs

   This registry, historically described as the "IANA Language Character
   Set Registry" or "IANA Script Registry" (both somewhat misleading
   terms), is maintained by IANA at the request of ICANN.  It is used to
   provide a central documentation repository of the IDN policies used
   by top level domain (TLD) registries who volunteer to contribute to
   it and is used in conjunction with ICANN Guidelines for IDN use.

   It is not an IETF-managed registry and, while the protocol changes
   specified here may call for some revisions to the tables, IDNA2008
   has no direct effect on that registry and no IANA action is required
   as a result.

11.  Security Considerations

11.1.  General Security Issues with IDNA

   This document is purely explanatory and informational and
   consequently introduces no new security issues.  It would, of course,
   be a poor idea for someone to try to implement from it; such an
   attempt would almost certainly lead to interoperability problems and
   might lead to security ones.  A discussion of security issues with
   IDNA, including some relevant history, appears in the Definitions
   document [RFC5890].

12.  Acknowledgments

   The editor and contributors would like to express their thanks to
   those who contributed significant early (pre-working group) review
   comments, sometimes accompanied by text, Paul Hoffman, Simon
   Josefsson, and Sam Weiler.  In addition, some specific ideas were
   incorporated from suggestions, text, or comments about sections that
   were unclear supplied by Vint Cerf, Frank Ellerman, Michael Everson,
   Asmus Freytag, Erik van der Poel, Michel Suignard, and Ken Whistler.
   Thanks are also due to Vint Cerf, Lisa Dusseault, Debbie Garside, and
   Jefsey Morfin for conversations that led to considerable improvements
   in the content of this document and to several others, including Ben





Klensin                       Informational                    [Page 39]

RFC 5894                     IDNA Rationale                  August 2010


   Campbell, Martin Duerst, Subramanian Moonesamy, Peter Saint-Andre,
   and Dan Winship, for catching specific errors and recommending
   corrections.

   A meeting was held on 30 January 2008 to attempt to reconcile
   differences in perspective and terminology about this set of
   specifications between the design team and members of the Unicode
   Technical Consortium.  The discussions at and subsequent to that
   meeting were very helpful in focusing the issues and in refining the
   specifications.  The active participants at that meeting were (in
   alphabetic order, as usual) Harald Alvestrand, Vint Cerf, Tina Dam,
   Mark Davis, Lisa Dusseault, Patrik Faltstrom (by telephone), Cary
   Karp, John Klensin, Warren Kumari, Lisa Moore, Erik van der Poel,
   Michel Suignard, and Ken Whistler.  We express our thanks to Google
   for support of that meeting and to the participants for their
   contributions.

   Useful comments and text on the working group versions of the working
   draft were received from many participants in the IETF "IDNABIS"
   working group and a number of document changes resulted from mailing
   list discussions made by that group.  Marcos Sanz provided specific
   analysis and suggestions that were exceptionally helpful in refining
   the text, as did Vint Cerf, Martin Duerst, Andrew Sullivan, and Ken
   Whistler.  Lisa Dusseault provided extensive editorial suggestions
   during the spring of 2009, most of which were incorporated.

13.  Contributors

   While the listed editor held the pen, the core of this document and
   the initial working group version represents the joint work and
   conclusions of an ad hoc design team consisting of the editor and, in
   alphabetic order, Harald Alvestrand, Tina Dam, Patrik Faltstrom, and
   Cary Karp.  Considerable material describing mapping principles has
   been incorporated from a draft of the Mapping document
   [IDNA2008-Mapping] by Pete Resnick and Paul Hoffman.  In addition,
   there were many specific contributions and helpful comments from
   those listed in the Acknowledgments section and others who have
   contributed to the development and use of the IDNA protocols.

14.  References

14.1.  Normative References

   [RFC3490]    Faltstrom, P., Hoffman, P., and A. Costello,
                "Internationalizing Domain Names in Applications
                (IDNA)", RFC 3490, March 2003.





Klensin                       Informational                    [Page 40]

RFC 5894                     IDNA Rationale                  August 2010


   [RFC3492]    Costello, A., "Punycode: A Bootstring encoding of
                Unicode for Internationalized Domain Names in
                Applications (IDNA)", RFC 3492, March 2003.

   [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.

   [RFC5893]    Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
                Internationalized Domain Names for Applications (IDNA)",
                RFC 5893, August 2010.

   [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/>.

14.2.  Informative References

   [IDNA2008-Mapping]
                Resnick, P. and P. Hoffman, "Mapping Characters in
                Internationalized Domain Names for Applications (IDNA)",
                Work in Progress, April 2010.

   [RFC0952]    Harrenstien, K., Stahl, M., and E. Feinler, "DoD
                Internet host table specification", RFC 952,
                October 1985.

   [RFC1034]    Mockapetris, P., "Domain names - concepts and
                facilities", STD 13, RFC 1034, November 1987.

   [RFC1035]    Mockapetris, P., "Domain names - implementation and
                specification", STD 13, RFC 1035, November 1987.

   [RFC1123]    Braden, R., "Requirements for Internet Hosts -
                Application and Support", STD 3, RFC 1123, October 1989.

   [RFC2136]    Vixie, P., Thomson, S., Rekhter, Y., and J.  Bound,
                "Dynamic Updates in the Domain Name System (DNS
                UPDATE)", RFC 2136, April 1997.



Klensin                       Informational                    [Page 41]

RFC 5894                     IDNA Rationale                  August 2010


   [RFC2181]    Elz, R. and R. Bush, "Clarifications to the DNS
                Specification", RFC 2181, July 1997.

   [RFC2277]    Alvestrand, H., "IETF Policy on Character Sets and
                Languages", BCP 18, RFC 2277, January 1998.

   [RFC2671]    Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
                RFC 2671, August 1999.

   [RFC2782]    Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
                specifying the location of services (DNS SRV)",
                RFC 2782, February 2000.

   [RFC3454]    Hoffman, P. and M. Blanchet, "Preparation of
                Internationalized Strings ("stringprep")", RFC 3454,
                December 2002.

   [RFC3491]    Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
                Profile for Internationalized Domain Names (IDN)",
                RFC 3491, March 2003.

   [RFC3743]    Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint
                Engineering Team (JET) Guidelines for Internationalized
                Domain Names (IDN) Registration and Administration for
                Chinese, Japanese, and Korean", RFC 3743, April 2004.

   [RFC3987]    Duerst, M. and M. Suignard, "Internationalized Resource
                Identifiers (IRIs)", RFC 3987, January 2005.

   [RFC4290]    Klensin, J., "Suggested Practices for Registration of
                Internationalized Domain Names (IDN)", RFC 4290,
                December 2005.

   [RFC4343]    Eastlake, D., "Domain Name System (DNS) Case
                Insensitivity Clarification", RFC 4343, January 2006.

   [RFC4690]    Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review
                and Recommendations for Internationalized Domain Names
                (IDNs)", RFC 4690, September 2006.

   [RFC4713]    Lee, X., Mao, W., Chen, E., Hsu, N., and J.  Klensin,
                "Registration and Administration Recommendations for
                Chinese Domain Names", RFC 4713, October 2006.








Klensin                       Informational                    [Page 42]

RFC 5894                     IDNA Rationale                  August 2010


   [Unicode-UAX31]
                The Unicode Consortium, "Unicode Standard Annex #31:
                Unicode Identifier and Pattern Syntax, Revision 11",
                September 2009,
                <http://www.unicode.org/reports/tr31/tr31-11.html>.

   [Unicode-UTS39]
                The Unicode Consortium, "Unicode Technical Standard #39:
                Unicode Security Mechanisms, Revision 2", August 2006,
                <http://www.unicode.org/reports/tr39/tr39-2.html>.

Author's Address

   John C Klensin
   1770 Massachusetts Ave, Ste 322
   Cambridge, MA  02140
   USA

   Phone: +1 617 245 1457
   EMail: john+ietf@jck.com































Klensin                       Informational                    [Page 43]


Html markup produced by rfcmarkup 1.107, available from http://tools.ietf.org/tools/rfcmarkup/