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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 RFC 5894

Network Working Group                                         J. Klensin
Internet-Draft                                          January 11, 2010
Intended status: Informational
Expires: July 15, 2010


  Internationalized Domain Names for Applications (IDNA): Background,
                       Explanation, and Rationale
                  draft-ietf-idnabis-rationale-17.txt

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 Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on July 15, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Context and Overview . . . . . . . . . . . . . . . . . . .  4
     1.2.  Discussion Forum . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
       1.3.1.  DNS "Name" Terminology . . . . . . . . . . . . . . . .  5
       1.3.2.  New Terminology and Restrictions . . . . . . . . . . .  6
     1.4.  Objectives . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.5.  Applicability and Function of IDNA . . . . . . . . . . . .  7
     1.6.  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  . . . . . . . . . . . . . . . . . . . 13
     3.3.  Layered Restrictions: Tables, Context, Registration,
           Applications . . . . . . . . . . . . . . . . . . . . . . . 14
   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  . . . . . . . . . . . . . . . . 18
     4.4.  Case Mapping and Related Issues  . . . . . . . . . . . . . 20
     4.5.  Right to Left Text . . . . . . . . . . . . . . . . . . . . 21
   5.  IDNs and the Robustness Principle  . . . . . . . . . . . . . . 21
   6.  Front-end and User Interface Processing for Lookup . . . . . . 22
   7.  Migration from IDNA2003 and Unicode Version Synchronization  . 24
     7.1.  Design Criteria  . . . . . . . . . . . . . . . . . . . . . 24
       7.1.1.  Summary and Discussion of IDNA Validity Criteria . . . 24
       7.1.2.  Labels in Registration . . . . . . . . . . . . . . . . 25
       7.1.3.  Labels in Lookup . . . . . . . . . . . . . . . . . . . 26
     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 Non-Joiner  . 28
       7.2.3.  Character Changes and the Need for Transition  . . . . 28
       7.2.4.  Transition Strategies  . . . . . . . . . . . . . . . . 29
     7.3.  Elimination of Character Mapping . . . . . . . . . . . . . 30
     7.4.  The Question of Prefix Changes . . . . . . . . . . . . . . 30
       7.4.1.  Conditions Requiring a Prefix Change . . . . . . . . . 31
       7.4.2.  Conditions Not Requiring a Prefix Change . . . . . . . 31
       7.4.3.  Implications of Prefix Changes . . . . . . . . . . . . 32
     7.5.  Stringprep Changes and Compatibility . . . . . . . . . . . 32



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     7.6.  The Symbol Question  . . . . . . . . . . . . . . . . . . . 33
     7.7.  Migration Between Unicode Versions: Unassigned Code
           Points . . . . . . . . . . . . . . . . . . . . . . . . . . 34
     7.8.  Other Compatibility Issues . . . . . . . . . . . . . . . . 36
   8.  Name Server Considerations . . . . . . . . . . . . . . . . . . 36
     8.1.  Processing Non-ASCII Strings . . . . . . . . . . . . . . . 36
     8.2.  Root and other DNS Server Considerations . . . . . . . . . 37
   9.  Internationalization Considerations  . . . . . . . . . . . . . 37
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38
     10.1. IDNA Character Registry  . . . . . . . . . . . . . . . . . 38
     10.2. IDNA Context Registry  . . . . . . . . . . . . . . . . . . 38
     10.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 38
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 38
     11.1. General Security Issues with IDNA  . . . . . . . . . . . . 38
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 39
   13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 39
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 40
     14.2. Informative References . . . . . . . . . . . . . . . . . . 41
   Appendix A.  Change Log  . . . . . . . . . . . . . . . . . . . . . 43
     A.1.  Changes between Version -00 and Version -01 of
           draft-ietf-idnabis-rationale . . . . . . . . . . . . . . . 43
     A.2.  Version -02  . . . . . . . . . . . . . . . . . . . . . . . 44
     A.3.  Version -03  . . . . . . . . . . . . . . . . . . . . . . . 44
     A.4.  Version -04  . . . . . . . . . . . . . . . . . . . . . . . 45
     A.5.  Version -05  . . . . . . . . . . . . . . . . . . . . . . . 45
     A.6.  Version -06  . . . . . . . . . . . . . . . . . . . . . . . 45
     A.7.  Version -07  . . . . . . . . . . . . . . . . . . . . . . . 46
     A.8.  Version -08  . . . . . . . . . . . . . . . . . . . . . . . 46
     A.9.  Version -09  . . . . . . . . . . . . . . . . . . . . . . . 46
     A.10. Version -10  . . . . . . . . . . . . . . . . . . . . . . . 47
     A.11. Version -11  . . . . . . . . . . . . . . . . . . . . . . . 47
     A.12. Version -12  . . . . . . . . . . . . . . . . . . . . . . . 48
     A.13. Version -13  . . . . . . . . . . . . . . . . . . . . . . . 48
     A.14. Version -14  . . . . . . . . . . . . . . . . . . . . . . . 48
     A.15. Version -15  . . . . . . . . . . . . . . . . . . . . . . . 49
     A.16. Version -16  . . . . . . . . . . . . . . . . . . . . . . . 49
     A.17. Version -17  . . . . . . . . . . . . . . . . . . . . . . . 49
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 49












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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") which is a valid DNS label containing only letters, digits,
   and hyphens.  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 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, are 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
   [IDNA2008-Defs].  The characters that are valid in A-labels are
   identified from rules listed in the Tables document
   [IDNA2008-Tables], but validity can be derived from the Unicode
   properties of those characters with a very few exceptions.

   Traditionally, DNS labels are matched case-insensitively
   [RFC1034][RFC1035].  That convention was preserved in IDNA2003 by a
   case-folding operation that generally maps capital letters into



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   lower-case 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.  Discussion Forum

   [[ RFC Editor: please remove this section. ]]

   IDNA2008 is being discussed in the IETF "idnabis" Working Group and
   on the mailing list idna-update@alvestrand.no

1.3.  Terminology

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

1.3.1.  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.



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1.3.2.  New Terminology and Restrictions

   IDNA2008 introduces new terminology.  Precise definitions are
   provided (in [IDNA2008-Defs]), for the terms "U-label", "A-Label",
   LDH-label (to which all valid pre-IDNA host names 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 [IDNA2008-Defs].  R-LDH-labels contain "--" in the third and
   fourth character 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 which 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.

1.4.  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.




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   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 order that the pre-mapped
      forms (which are not valid IDNA labels) tend to appear less often
      in various contexts, in favor of valid A-labels.

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

1.5.  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 exactly-matching name (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 IDN while maintaining
   interoperability with existing infrastructure.  If an application
   wants to use non-ASCII characters in public DNS domain names, IDNA is
   the only currently-defined option.  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 of
   user applications, DNS resolvers, and DNS servers in order for a user
   to apply an internationalized domain name in any form or coding
   acceptable under that method.  While processing must be performed



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   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.

1.6.  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 UI cannot display a character
   if no font for 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 for end-user applications to
   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.

   Converting between local character sets and normalized Unicode, if
   needed, is part of this set of user agent issues.  This conversion
   introduces complexity in a system that is not Unicode-native.  If a
   label is converted to a local character set that does not have all
   the needed characters, or that uses different character-coding
   principles, the user agent may have to add special logic to avoid or
   reduce loss of information.




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   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 a
   IDNA2003 context.  Those issues are discussed in Section 6 and in the
   separate document [IDNA2008-Mapping].


2.  Processing in IDNA2008

   IDNA2008 separates Domain Name Registration and Lookup in the
   protocol specification.  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 [IDNA2008-Protocol].


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.

   This section provides an overview of the model used to establish the
   algorithm and character lists of [IDNA2008-Tables] and describes the



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   names and applicability of the categories used there.  Note that the
   inclusion of a character in the first 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
   [IDNA2008-Tables] and the rules that actually determine what labels
   can be registered or looked up are in [IDNA2008-Protocol].

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 (see [IDNA2008-Bidi]).

   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 characters
   in the category.  Registries are still expected to apply judgment



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   about labels they will accept and to maintain rules consistent with
   those judgments (see [IDNA2008-Protocol] 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
   [Unicode51], 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 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 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.

   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



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   consequences of embedding them in labels drawn from those scripts and
   therefore should avoid accepting registrations containing those
   characters, at least in Latin or Cyrillic-script labels.

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
   [IDNA2008-Tables].  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).

   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



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   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 method NFKC to the character yields some other
      character.

   o  The character is an upper-case form or some other form that is
      mapped to another character by Unicode casefolding.

   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
   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



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   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 RFC 3743 [RFC3743] and RFC 4290 [RFC4290], and
   illustrated for Chinese by the tables described in RFC 4713 [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", Table 4
   "Candidate Characters for Exclusion from Identifiers" of
   [Unicode-UAX31] and Section 3.1.  "General Security Profile for
   Identifiers" in [Unicode-Security].

   The requirement (in Section 4.1 of [IDNA2008-Protocol]) 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., TLD or
   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.

3.3.  Layered Restrictions: Tables, Context, Registration, 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



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

   User agents are not required to display and allow input of IRIs
   directly but often do so.  Implementors 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 R2L user agent 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



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   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: 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 to not 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, 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,
   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



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   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 at that place.  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 2.3.1.6 in
   [IDNA2008-Defs]) 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
   occurs in Section 6 and specific recommendations appear in
   [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, upper-case 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 [IDNA2008-Tables] 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.

   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 [IDNA2008-Protocol].  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 interchange 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



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   matched in a case-independent way, but no actual case-folding occurs.
   Names can be placed in the DNS in either upper or lower case 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
   lower-case 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., 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 lower case characters
   can be registered in the DNS (i.e., be present in a U-label), then
   IDNA2008 should prohibit upper-case 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)
   must be 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 upper case and lower case may even
   be language-dependent, with different languages (or even the same
   language in different areas) expecting different mappings.  User
   agents 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.

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



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      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
   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



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   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.  Case-matching must be
   done, if desired, by IDN clients even though it wasn't done by ASCII-
   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.

   Some characters do not have upper case 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 upper
   case of U+03C3 is the Upper Case 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



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   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 WG 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 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 IDAN2008) 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
   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 [IDNA2008-Bidi].


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.



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   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 implementors 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 either by themselves or embedded in an
   identifier such as email addresses, URIs, or IRIs.  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 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.  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 [IDNA2008-Protocol]
   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.



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   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).  And
   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.

   The original version of the IDNA protocol [RFC3490] 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 specified in [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.

   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 which 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 the original version of the IDNA
   protocol 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



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   input strings.

   The present version of IDNA 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, the current version of IDNA
   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.


7.  Migration from IDNA2003 and Unicode Version Synchronization

7.1.  Design Criteria

   As mentioned above and in RFC 4690, 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 [IDNA2008-Protocol]
   and [IDNA2008-Tables]):



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   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.

   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 [IDNA2008-Tables].  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 the Protocol Document, Section 4 [IDNA2008-Protocol]):




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   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
   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 the
   Protocol Document, Section 5 [IDNA2008-Protocol])

   o  Maintain IDNA and Unicode tables that are consistent with regard
      to versions, i.e., unless the application actually executes the
      classification rules in [IDNA2008-Tables], 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





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      *  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 Controls
         (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".

   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).



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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 upper case character has
   not historically been available to match a lower case one or vice
   versa.  For those characters, the mappings used in constructing the
   Stringprep tables for IDNA2003, performed using the Unicode CaseFold
   operation (See Section 5.8 of the Unicode Standard [Unicode51]),
   generate different characters or sets of characters.  Those
   operations are not reversible and lose even more information than
   traditional upper case or lower case 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 (Lower Case) Sigma (U+03C3).

7.2.2.  Character Changes: Zero-Width Joiner and 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 the current version of IDNA, 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.



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   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 WG 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 character by applying more or less
   standardized conventions) and when other new forms of strings have
   been permitted as labels.

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
   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 lower-case 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.




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   2.  Hold a "sunrise"-like arrangement in which holders of labels
       containing "ss" in the Eszett case, Lower Case 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
   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 [IDNA2008-Mapping].

7.4.  The Question of Prefix Changes

   The conditions that would have required a change in the IDNA ACE
   prefix ("xn--" for the version of IDNA specified in [RFC3490]) 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



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   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 (RFC3490) 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.

   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.



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   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.

   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 [RFC3491] specification, 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 proposed
   new inclusion tables [IDNA2008-Tables], the reduction in the number
   of characters permitted as input for registration or lookup
   (Section 3), and even the proposed changes in handling of right to
   left strings [IDNA2008-Bidi].  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



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   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 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 the original version 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
       specification operates on an inclusion model, extrapolating from
       the original "hostname" rules (LDH, see [IDNA2008-Defs]) -- 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.




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   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.

   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:






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   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
      [IDNA2008-Tables] (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
      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 5.1 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 in, e.g., 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



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   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 the
   older conventions were used in labels or not.  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) at all.  After
   Nameprep maps the Eszett out, the result is an ASCII string and so
   does not get an xn-- prefix, but the string that can be displayed to
   a user appears to be an IDN.  The newer version of the protocol
   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]) are IDN-unaware 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



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   Registration and Lookup in [IDNA2008-Protocol] (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 RFC 2181 [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 (see, e.g., [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,
   so 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.





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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 must be newly-created for IDNA2008, appear in
   [IDNA2008-Tables].  This document describes the registries but 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
   [IDNA2008-Tables].  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, will be convenient
   for programming and validation purposes.  The details of this
   registry are specified in [IDNA2008-Tables].

10.2.  IDNA Context Registry

   IANA will create and maintain 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 rule described in
   Section 3.1.2).  The details for those rules appear in
   [IDNA2008-Tables].

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,



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   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 [IDNA2008-Defs].


12.  Acknowledgments

   The editor and contributors would like to express their thanks to
   those who contributed significant early (pre-WG) 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
   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 WG versions of the draft were
   received from many participants in the IETF "IDNABIS" WG 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



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   the initial WG 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 [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

   [ASCII]    American National Standards Institute (formerly United
              States of America Standards Institute), "USA Code for
              Information Interchange", ANSI X3.4-1968, 1968.

              ANSI X3.4-1968 has been replaced by newer versions with
              slight modifications, but the 1968 version remains
              definitive for the Internet.

   [IDNA2008-Bidi]
              Alvestrand, H. and C. Karp, "An updated IDNA criterion for
              right to left scripts", August 2009, <https://
              datatracker.ietf.org/drafts/draft-ietf-idnabis-bidi/>.

   [IDNA2008-Defs]
              Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              August 2009, <https://datatracker.ietf.org/drafts/
              draft-ietf-idnabis-defs/>.

   [IDNA2008-Protocol]
              Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", August 2009, <https://
              datatracker.ietf.org/drafts/draft-ietf-idnabis-protocol/>.

   [IDNA2008-Tables]
              Faltstrom, P., "The Unicode Code Points and IDNA",
              August 2009, <https://datatracker.ietf.org/drafts/
              draft-ietf-idnabis-tables/>.

              A version of this document is available in HTML format at
              http://stupid.domain.name/idnabis/
              draft-ietf-idnabis-tables-06.html




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

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

   [Unicode-UAX15]
              The Unicode Consortium, "Unicode Standard Annex #15:
              Unicode Normalization Forms", March 2008,
              <http://www.unicode.org/reports/tr15/>.

   [Unicode51]
              The Unicode Consortium, "The Unicode Standard, Version
              5.1.0", 2008.

              defined by: The Unicode Standard, Version 5.0, Boston, MA,
              Addison-Wesley, 2007, ISBN 0-321-48091-0, as amended by
              Unicode 5.1.0
              (http://www.unicode.org/versions/Unicode5.1.0/).

14.2.  Informative References

   [BIG5]     Institute for Information Industry of Taiwan, "Computer
              Chinese Glyph and Character Code Mapping Table, Technical
              Report C-26", 1984.

              There are several forms and variations and a closely-
              related standard, CNS 11643.  See the discussion in
              Chapter 3 of Lunde, K., CJKV Information Processing,
              O'Reilly & Associates, 1999

   [GB18030]  "Chinese National Standard GB 18030-2000: Information
              Technology -- Chinese ideograms coded character set for
              information interchange -- Extension for the basic set.",
              2000.

   [IDNA2008-Mapping]
              Resnick, P., "Mapping Characters in IDNA", August 2009, <h
              ttps://datatracker.ietf.org/drafts/
              draft-ietf-idnabis-mapping/>.

   [RFC0810]  Feinler, E., Harrenstien, K., Su, Z., and V. White, "DoD
              Internet host table specification", RFC 810, March 1982.

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



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   [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.

   [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.

   [RFC2673]  Crawford, M., "Binary Labels in the Domain Name System",
              RFC 2673, 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.



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   [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.

   [Unicode-Security]
              The Unicode Consortium, "Unicode Technical Standard #39:
              Unicode Security Mechanisms", August 2008,
              <http://www.unicode.org/reports/tr39/>.

   [Unicode-UAX31]
              The Unicode Consortium, "Unicode Standard Annex #31:
              Unicode Identifier and Pattern Syntax", March 2008,
              <http://www.unicode.org/reports/tr31/>.

   [Unicode-UTR36]
              The Unicode Consortium, "Unicode Technical Report #36:
              Unicode Security Considerations", July 2008,
              <http://www.unicode.org/reports/tr36/>.


Appendix A.  Change Log

   [[ RFC Editor: Please remove this appendix. ]]

A.1.  Changes between Version -00 and Version -01 of
      draft-ietf-idnabis-rationale

   o  Clarified the U-label definition to note that U-labels must
      contain at least one non-ASCII character.  Also clarified the
      relationship among label types.

   o  Rewrote the discussion of Labels in Registration (Section 7.1.2)
      and related text about IDNA-validity (in the "Defs" document as of
      -04 of this one) to narrow its focus and remove more general
      restrictions.  Added a temporary note in line to explain the
      situation.

   o  Changed the "IDNA uses Unicode" statement to focus on
      compatibility with IDNA2003 and avoid more general or
      controversial assertions.




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   o  Added a discussion of examples to Section 7.1

   o  Made a number of other small editorial changes and corrections
      suggested by Mark Davis.

   o  Added several more discussion anchors and notes and expanded or
      updated some existing ones.

A.2.  Version -02

   o  Trimmed change log, removing information about pre-WG drafts.

   o  Adjusted discussion of Contextual Rules to match the new location
      of the tables and some conceptual material.

   o  Rewrote the material on preprocessing somewhat.

   o  Moved the material about relationships with IDNA2003 to be part of
      a single section on transitions.

   o  Removed several placeholders and made editorial changes in
      accordance with decisions made at IETF 72 in Dublin and not
      disputed on the mailing list.

A.3.  Version -03

   This special update to the Rationale document is intended to try to
   get the discussion of what is normative or not under control.  While
   the IETF does not normally annotate individual sections of documents
   with whether they are normative or not, concerns that we don't know
   which is which, claims that some material is normative that would be
   problematic if so classified, etc., argue that we should at least be
   able to have a clear discussion on the subject.

   Two annotations have been applied to sections that might reasonably
   be considered normative.  One annotation is based on the list of
   sections in Mark Davis's note of 29 September (http://
   www.alvestrand.no/pipermail/idna-update/2008-September/002667.html).
   The other is based on an elaboration of John Klensin's response on 7
   October (http://www.alvestrand.no/pipermail/idna-update/2008-October/
   002691.html).  These should just be considered two suggestions to
   illuminate and, one hopes, advance the Working Group's discussions.

   Some additional editorial changes have been made, but they are
   basically trivial.  In the editor's judgment, it is not possible to
   make significantly more progress with this document until the matter
   of document organization is settled.




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A.4.  Version -04

   o  Definitional and other normative material moved to new document
      (draft-ietf-idnabis-defs).  Version -03 annotations removed.

   o  Material on differences between IDNA2003 and IDNA2008 moved to an
      appendix in Protocol.

   o  Material left over from the origins of this document as a
      preliminary proposal has been removed or rewritten.

   o  Changes made to reflect consensus call results, including removing
      several placeholder notes for discussion.

   o  Added more material, including discussion of historic scripts, to
      Section 3.2 on registration policies.

   o  Added a new section (Section 7.2) to contain specific discussion
      of handling of characters that are interpreted differently in
      input to IDNA2003 and 2008.

   o  Some material, including this section/appendix, rearranged.

A.5.  Version -05

   o  Many small editorial changes, including changes to eliminate the
      last vestiges of what appeared to be 2119 language (upper-case
      MUST, SHOULD, or MAY) and small adjustments to terminology.

A.6.  Version -06

   o  Removed Security Considerations material and pointed to Defs,
      where it now appears as of version 05.

   o  Started changing uses of "IDNA2008" in running text to "in these
      specifications" or the equivalent.  These documents are titled
      simply "IDNA"; once they are standardized, "the current version"
      may be a more appropriate reference than one containing a year.
      As discussed on the mailing list, we can and should discuss how to
      refer to these documents at an appropriate time (e.g., when we
      know when we will be finished) but, in the interim, it seems
      appropriate to simply start getting rid of the version-specific
      terminology where it can naturally be removed.

   o  Additional discussion of mappings, etc., especially for case-
      sensitivity.





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   o  Clarified relationship to base DNS specifications.

   o  Consolidated discussion of lookup of unassigned characters.

   o  More editorial fine-tuning.

A.7.  Version -07

   o  Revised terminology by adding terms: NR-LDH-label, Invalid-A-label
      (or False-A-label), R-LDH-label, valid IDNA-label in
      Section 1.3.2.

   o  Moved the "name server considerations" material to this document
      from Protocol because it is non-normative and not part of the
      protocol itself.

   o  To improve clarity, redid discussion of the reasons why looking up
      unassigned code points is prohibited.

   o  Editorial and other non-substantive corrections to reflect earlier
      errors as well as new definitions and terminology.

A.8.  Version -08

   o  Slight revision to "contextual" discussion (Section 3.1.2) and
      moving it to a separate subsection, rather than under "PVALID",
      for better parallelism with Tables.  Also reflected Mark's
      comments about the limitations of the approach.

   o  Added placeholder notes as reminders of where references to the
      other documents need Section numbers.  More of these will be added
      as needed (feel free to identify relevant places), but the actual
      section numbers will not be inserted until the documents are
      completely stable, i.e., on their way to the RFC Editor.

A.9.  Version -09

   o  Small editorial changes to clarify transition possibilities.

   o  Small clarification to the description of DNS "exact match".

   o  Added discussion of adding characters to an existing script to the
      discussion of unassigned code point transitions in Section 7.7.

   o  Tightened up the discussion of non-ASCII string processing
      (Section 8.1) slightly.





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   o  Removed some placeholders and comments that have been around long
      enough to be considered acceptable or that no longer seem
      necessary for other reasons.

A.10.  Version -10

   o  Extensive editorial improvements, mostly due to suggestions from
      Lisa Dusseault.

   o  Changes required for the new "mapping" approach and document have,
      in general, not been incorporated despite several suggestions.
      The editor intends to wait until the mapping model is stable, or
      at least until -11 of this document, before trying to incorporate
      those suggestions.

A.11.  Version -11

   o  Several placeholders for additional material or editing have been
      removed since no comments have been received.

   o  Updated references.

   o  Corrected an apparent patching error in Section 1.6 and another
      one in Section 4.3.

   o  Adjusted several sections that had not properly reflected removal
      of the material that is now in the Definitions document and
      removed an unnecessary one.

   o  New material added to Section 3.2 about registration policy issues
      to reflect discussions on the mailing list.

   o  Incorporated mapping material from the former "Architectural
      Principles" of version -01 of the Mapping draft into Section 6 and
      removed most of the prior mapping material and explanations.

   o  Eliminated the former Section 7.3 ("More Flexibility in User
      Agents"), moving its material into Section 4.2.  The replacement
      section is basically a placeholder to retain the mapping issues as
      one of the migration topics.  Note that this item and the previous
      one involve considerable text, so people should check things
      carefully.

   o  Corrected several typographical and editorial errors that don't
      fall into any of the above categories.






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A.12.  Version -12

   o  Got rid of the term "IDNA-valid".  It no longer appears in
      Definitions and we didn't really need the extra term.  Where the
      concept was needed, the text now says "valid under IDNA" or
      equivalent.

   o  Adjusted Acknowledgments to remove Mark Davis's name, per his
      request and advice from IETF Trust Counsel.

   o  Incorporated other changes from WG Last Call.

   o  Small typographical and editorial corrections.

A.13.  Version -13

   o  Substituted in Section numbers to references to other IDNA2008
      documents.

A.14.  Version -14

   This is the version of the document produced to reflect comments on
   IETF Last Call.  For the convenience of those who made comments and
   of the IESG in evaluating them, this section therefore identifies
   non-editorial changes made in response to Last Call comments in
   somewhat more detail than may be usual.

   o  Removed the discussion of DNSSEC after extensive discussion on the
      IETF and IDNABIS lists.

   o  Modified the discussion of prefix changes to make it clear that
      the decisions have been made, rather than still representing open
      issues.  (Dan Winship review, 20091013)

   o  Suggested explicit identification of domain name slots in
      protocols that use IDNA.  Peter Saint-Andre, 20091019.

   o  Several other clarifications as suggested by Peter Saint-Andre,
      20091019.

   o  Several minor editorial corrections per suggestions in Ben
      Campbell's Gen-ART review 20091013.

   o  Typo corrections.







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Internet-Draft               IDNA Rationale                 January 2010


A.15.  Version -15

   o  Rewrote and expanded the "transition" material of Section 7.2.

A.16.  Version -16

   This version contains changes made at IESG request during their
   review.  Some additional comments were logged during or immediately
   before the 7 January 2010 teleconference, so this is not the final
   I-D version.

   o  Altered use of "these documents" and "these specifications" back
      to "IDNA2008", undoing the change made in Appendix A.6.  The
      convolutions became ambiguous in places.

   o  Added a sentence to the Introduction to make the non-normative
      status of this document even more clear and added references to
      7.1.2 and 7.1.3 to point to the more formal definitions.

A.17.  Version -17

   o  Final IESG comments picked up and included.  A few more editorial/
      typographic errors caught and fixed.

   o  Section 4 title adjusted to better match its content.


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















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