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Network Working Group                                         J. Klensin
Internet-Draft                                             June 18, 2009
Intended status: Informational
Expires: December 20, 2009

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

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.  This document may contain material
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   available before November 10, 2008.  The person(s) controlling the
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Copyright Notice

   Copyright (c) 2009 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
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.


   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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Context and Overview . . . . . . . . . . . . . . . . . . .  4
     1.2.  Discussion Forum . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
       1.3.1.  Documents and Standards  . . . . . . . . . . . . . . .  5
       1.3.2.  DNS "Name" Terminology . . . . . . . . . . . . . . . .  5
       1.3.3.  New Terminology and Restrictions . . . . . . . . . . .  6
     1.4.  Objectives . . . . . . . . . . . . . . . . . . . . . . . .  7
     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  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.  Issues that Constrain Possible Solutions . . . . . . . . . . . 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  . . . . . . . . . . . . . . . . 17
     4.4.  Case Mapping and Related Issues  . . . . . . . . . . . . . 18
     4.5.  Right to Left Text . . . . . . . . . . . . . . . . . . . . 19
   5.  IDNs and the Robustness Principle  . . . . . . . . . . . . . . 20

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   6.  Front-end and User Interface Processing for Lookup . . . . . . 20
   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 . . . . . . . . . . . 27
     7.3.  More Flexibility in User Agents  . . . . . . . . . . . . . 28
     7.4.  The Question of Prefix Changes . . . . . . . . . . . . . . 30
       7.4.1.  Conditions Requiring a Prefix Change . . . . . . . . . 30
       7.4.2.  Conditions Not Requiring a Prefix Change . . . . . . . 31
       7.4.3.  Implications of Prefix Changes . . . . . . . . . . . . 31
     7.5.  Stringprep Changes and Compatibility . . . . . . . . . . . 31
     7.6.  The Symbol Question  . . . . . . . . . . . . . . . . . . . 32
     7.7.  Migration Between Unicode Versions: Unassigned Code
           Points . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     7.8.  Other Compatibility Issues . . . . . . . . . . . . . . . . 35
   8.  Name Server Considerations . . . . . . . . . . . . . . . . . . 35
     8.1.  Processing Non-ASCII Strings . . . . . . . . . . . . . . . 36
     8.2.  DNSSEC Authentication of IDN Domain Names  . . . . . . . . 36
     8.3.  Root and other DNS Server Considerations . . . . . . . . . 37
   9.  Internationalization Considerations  . . . . . . . . . . . . . 37
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 37
     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  . . . . . . . . . . . . . . . . . . . . . . . 44
     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
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 47

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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 Unicode
   mnemonics 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

   [[anchor3: Note in draft: The above discussion, and the rest of the
   text in this section, are very informal.  In particular, the term
   "A-label" is used to refer to some things that don't meet all of the
   tests for A-labels.  I have tightened it somewhat from the suggested
   text I received, but not very much.  Is the current form ok with

   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

   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.  The documents that made up both versions are
   listed in Section 1.3.1.  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 case-insensitive [RFC1034][RFC1035].
   That pattern was preserved in IDNA2003, but if case rules are

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   enforced from one language, another language sometimes loses the
   ability to treat two characters separately.  Case-sensitivity 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 determines how and whether
   the characters can be used in IDNA labels.

   This document provides informational context for IDNA2008, including
   terminology, background, and policy discussions.

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.  Documents and Standards

   This document uses the term "IDNA2003" to refer to the set of
   standards published in 2003 to define IDNA: the IDNA base
   specification [RFC3490], Nameprep [RFC3491], Punycode [RFC3492], and
   Stringprep [RFC3454].

   The term "IDNA2008" is used to refer to a new version of IDNA.
   IDNA2008 is not dependent on any of the IDNA2003 specifications other
   than the one for Punycode encoding.  References to "these
   specifications" or "these documents" are to the entire IDNA2008 set
   listed in [IDNA2008-Defs].

1.3.2.  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,

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   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.

1.3.3.  New Terminology and Restrictions

   These documents introduce new terminology, and precise definitions,
   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 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 trigger any
   resemblance to IDNA-landr NR-LDH-labels.

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

   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);

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

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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
      update for implementations to adopt codepoints from new Unicode

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

   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 codepoint handling

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 (subject to the
   base DNS requirement of case-insensitive ASCII matching) name 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 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

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   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
   prior to or after access to the DNS, IDNA requires no changes to the
   DNS protocol or any DNS servers or the resolvers on user's 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 rudimentary 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
   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.

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   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, the user agent may have to add special logic
   to avoid or reduce loss of information.

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

   IDNA2008 shifts responsibility for character mapping and other
   adjustments from the protocol (where it was located in IDNA2003) to
   pre-processing before invoking IDNA.  The intent is that this change
   leads to greater usage of fully-valid A-Labels in display, transit
   and storage, which should aid comprehensibility.  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. [[anchor9: Fix section reference.]]

2.  Processing in IDNA2008

   These specifications separate Domain Name Registration and Lookup in
   the protocol specification.  This separation reflects current
   practice in which per-registry 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, unless it is included as part of a Unicode property-based
   rule or in rare cases included individually by an exception.  When an

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   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
   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 that are
   used 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


   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]).

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   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
   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.


   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).  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 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
   with Latin or Cyrillic script might not be actively aware that the
   characters exist, much less about the consequences of embedding them
   in labels drawn from those scripts.

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   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.

   Examples of descriptions of typical rules, stated informally and in
   English, include "Must follow a character from Script XYZ", "Must
   occur only if the entire label is in Script ABC", "Must occur only if
   the previous and subsequent characters have the DFG property".

   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 [IDNA2008-Tables].
   [[anchor12: ???  Section number would be good here.]]  That document
   also creates a registry for future rules.


   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

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   code point into Unicode and classify it as "PROTOCOL-VALID" or for
   the IETF to accept the considerable costs of an incompatible change
   and replace the relevant RFC with one containing appropriate

   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

   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.


   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 if such characters were permitted to be looked up, for
   example, and were later assigned, but subject to some set of
   contextual rules, un-updated instances of IDNA-aware software might
   permit lookup of labels containing the previously-unassigned
   characters while updated versions of IDNA-aware software might
   restrict their use in lookup, depending on the contextual rules.  It
   should be clear that under no circumstance should an UNASSIGNED
   character be permitted in a label to be registered as part of a
   domain name.

3.2.  Registration Policy

   While these recommendations cannot and should not define registry
   policies, registries should develop and apply additional restrictions
   as needed to reduce confusion and other problems.  For example, it is

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   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 3843 [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].

   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 and that even a trivial, "anything 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 issues associated with
   IDNs.  Instead, the specifications define a variety of approaches.
   The character tables are the first mechanism, protocol rules about
   how those characters are applied or restricted in context are the
   second, and those two in combination constitute the limits of what
   can be done in the protocol.  As discussed in the previous section
   (Section 3.2), registries are expected to restrict what they permit
   to be registered, devising and using rules that are designed to
   optimize the balance between confusion and risk on the one hand and
   maximum expressiveness in mnemonics on the other.

   In addition, there is an important role for user 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.

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4.  Issues that Constrain Possible Solutions

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
   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

   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

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4.2.  Entry and Display in Applications

   Applications can accept and display domain names using any character
   set or character coding system.  That is, 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 and ugly
   and malicious variations on them are not easily detected by users.
   Where possible, they should thus only be exposed when they are
   absolutely needed.  Because IDN labels can be rendered either as
   A-labels or U-labels, the application may reasonably have an option
   for the user to select the preferred method of display.  Rendering
   the U-label should normally be the default.

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

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

   Where a protocol or document format allows IDNs, labels should be in
   whatever character encoding and escape mechanism the protocol or
   document format uses 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
   [[anchor15: ??  Verify this]] in [IDNA2008-Defs]) already have the
   capacity for handling domain names in the ASCII charset.  Thus,

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   A-labels can inherently be handled by those protocols.

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

   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 completely optional typographic convenience
   for representing a digraph in one language (as in the above example
   with some spelling conventions), while in another language it is a
   single character that may not always be correctly representable by a
   two-letter sequence (as in the above example with different spelling
   conventions).  This 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

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   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
   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,
   these specifications make 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 the forms entirely, to reduce the opportunities for user
   confusion and fraud that would result from the related strings being
   registered to different parties.

   [[anchor16: Placeholder: A discussion of the Arabic digit issue
   should go here once it is resolved in some appropriate way.]]

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

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   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 these specifications 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 avoid 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, that in some sense means that
   neither Final Sigma nor Eszett can be represented in a IDNA2003 IDN.
   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; this case is clearly
   worth discussing but the WG came to consensus not to make a prefix
   change anyway.

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

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   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.

   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

   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

   [[anchor18: Note in Draft: While this section has been revised in
   version -10 to improve clarity, a significant revision is expected
   once the discussions of mapping stabilize.]]

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   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.

   This section discusses these mapping and transformation issues among
   names, contrasting IDNA2003 and IDNA2008 behavior.  The discussion
   applies only in operations that look up names or interpret files.
   There are several reasons why registration activities should require
   final names and verification of those names by the would-be

   One source of label forms that are neither A-labels nor U-labels will
   be labels created under IDNA2003.  That protocol allowed labels that
   were transformed from native-character format by mapping some
   characters into others before conversion into A-label format.  One
   consequence of the transformations was that conversion from the
   A-label format back to native characters often did not produce the
   original label.  IDNA2008 explicitly defines A-labels and U-labels as
   different forms of the same abstract label, forms that are stable
   when conversions are performed between them (without mappings).

   A different way of explaining this is that there are, today, domain
   names in files on the Internet that use characters that cannot be
   represented directly in, or recovered from, (A-label) domain names
   but for which interpretations were provided by IDNA2003).  There are
   two major categories of characters irreversibly remapped by
   Stringprep, those that are removed by NFKC normalization and those
   upper-case characters that are mapped to lower-case (there are also a
   few characters that are given special-case mapping treatment,
   including lower-case characters that are case-folded into other
   lower-case characters or strings and those that are simply

   Other issues in domain name identification and processing arise
   because IDNA2003 specified that several other characters be treated
   as equivalent to the ASCII period (dot, full stop) character used as
   a label separator.  If a string that might be a domain name appears

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   in an arbitrary context (such as running text), it is difficult, even
   with only ASCII characters, to know whether an actual domain name (or
   a protocol parameter like a URI) is present and where it starts and
   ends.  When using Unicode, this gets even more difficult if treatment
   of certain special characters (like the dot that separates labels in
   a domain name) depends on context (e.g., prior knowledge of whether
   the string represents a domain name or not).  That knowledge is not
   available if the primary heuristic for identifying the presence of
   domain names in strings depends on the presence of dots separating
   groups of characters with no intervening spaces.

   [[anchor19: Placeholder: In serial efforts to move the mapping model
   out of the protocol and leave it unspecified here, this paragraph has
   become a complete botch.  Rewrite when the mapping plan stabilizes.]]
   The IDNA2008 model removes all of these mappings and interpretations,
   including the equivalence of different forms of dots, from the
   protocol, discouraging such mappings and leaving them, when
   necessary, to local processing.  This should not be taken to imply
   that local processing is optional or can be avoided entirely, even if
   doing so might have been desirable in a world without IDNA2003 IDNs
   in files and archives.  Instead, unless the program context is such
   that it is known that any IDNs that appear will contain either
   U-label or A-label forms, or that other forms can safely be rejected,
   some local processing of apparent domain name strings will be
   required, both to maintain compatibility with IDNA2003 and to prevent
   user astonishment.  Such local processing, while not specified in
   this document or the associated ones, will generally take one of two

   o  Generic Preprocessing.
      When the context in which the program or system that processes
      domain names operates is global, a reasonable balance must be
      found that is sensitive to the broad range of local needs and
      assumptions while, at the same time, not sacrificing the needs of
      one language, script, or user population to those of another.

      For this case, the best practice will usually be to apply NFKC and
      case-mapping (or, perhaps better yet, Stringprep itself), plus
      dot-mapping where appropriate, to the domain name string prior to
      applying IDNA.  That practice will not only yield a reasonable
      compromise of user experience with protocol requirements but will
      be almost completely compatible with the various forms permitted
      by IDNA2003.

   o  Highly Localized Preprocessing.
      Unlike the case above, there will be some situations in which
      software will be highly localized for a particular environment and
      carefully adapted to the expectations of users in that

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      environment.  The many discussions about using the Internet to
      preserve and support local cultures suggest that these cases may
      be more common in the future than they have been so far.

      In these cases, we should avoid trying to tell implementers what
      they should accept, if only because they are quite likely (and for
      good reason) to ignore us.  We would assume that they would map
      characters that the intuitions of their users would suggest be
      mapped and would hope that they would do that mapping as early as
      possible, storing A-label or U-label forms in files and
      transporting only those forms between systems.  One can imagine
      switches about whether some sorts of mappings occur, warnings
      before applying them or, in a slightly more extreme version of the
      approach taken in Internet Explorer version 7 (IE7), systems that
      utterly refuse to handle "strange" characters at all if they
      appear in U-label form.  None of those local decisions are a
      threat to interoperability as long as (i) only U-labels and
      A-labels are used in interchange with systems outside the local
      environment, (ii) no character that would be valid in a U-label as
      itself is mapped to something else, (iii) any local mappings are
      applied as a preprocessing step (or, for conversions from U-labels
      or A-labels to presentation forms, postprocessing), not as part of
      IDNA processing proper, and (iv) appropriate consideration is
      given to labels that might have entered the environment in
      conformance to IDNA2003.

   In either case, it is vital that user interface designs and, where
   the interfaces are not sufficient, users, be aware that the only
   forms of domain names that this protocol anticipates will resolve
   globally or compare equal when crude methods (i.e., those not
   conforming to the strict definition of label equivalence given in
   [IDNA2008-Defs]) are used are those in which all native-script labels
   are in U-label form.  Forms that assume mapping will occur,
   especially forms that were not valid under IDNA2003, may or may not
   function in predictable ways across all implementations.

   User interfaces involving Latin-based scripts should take special
   care when considering how to handle case mapping because small
   differences in label strings may cause behavior that is astonishing
   to users.  Because case-insensitive comparison is done for ASCII
   strings by DNS-servers, an all-ASCII label is treated as case-
   insensitive.  However, if even one of the characters of that string
   is replaced by one that requires the label to be given IDN treatment
   (e.g., by adding a diacritical mark), then the label effectively
   becomes case-sensitive because only lower-case characters are
   permitted in IDNs.  Preprocessing in applications to handle case
   mapping for Latin-based scripts (and possibly other scripts with case
   distinctions) may be wise to prevent user astonishment.  However, all

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   applications may not do this and ambiguity in transport is not
   desirable.  Consequently the case-dependent forms should not be
   stored in files.

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

   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 IDNA-valid are (the
   actual rules are rigorously defined in the "Protocol" and "Tables"

   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).

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   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 IDNA-valid.  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:

   o  Any label that appears to be an A-label, i.e., any label that
      starts in "xn--", must be IDNA-valid, 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

   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 IDNA-validity as

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   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

   Anyone looking up a label in a DNS zone is required to

   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

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

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

      *  there are no leading combining marks,

      *  the "bidi" conditions are met if right to left characters

      *  any required contextual rules are available, and

      *  any contextual rules that are associated with Joiner Controls
         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.

   Lookup applications that following 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

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

7.2.  Changes in Character Interpretations

   [[anchor22: This subsection will need to be rewritten when the
   mapping decisions stabilize.]]

   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).

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

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

   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

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   identifiers.  This is not a new problem; registries were faced with
   similar issues when IDNs were introduced and when other new forms of
   strings have been permitted as labels.

   There are several approaches to problems of this type.  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:

   o  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.

   o  Hold a "sunrise"-like arrangement in which holders of labels
      containing "ss" in the Eszett case or Lower Case Sigma are given
      priority (and perhaps other benefits) for registering the
      corresponding string containing Eszett or Final Sigma

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

   o  Adopt a different form of "variant" approach in which registration
      of additional names is either not permitted at all or permitted
      only by the registrant who already has one of the names.

7.3.  More Flexibility in User Agents

   [[anchor23: Note in Draft: This section is mapping-related and may
   need to be revised after that issue settles down.]]  Also, it is
   closely related to Section 4.2 and may need to be cross-referenced
   from it or consolidated into it.

   These documents do not specify mappings between one character or code
   point and others.  Instead, IDNA2008 prohibits characters that would
   be mapped to others by normalization, upper case to lower case
   changes, or other rules.  As examples, while mathematical characters
   based on Latin ones are accepted as input to IDNA2003, they are
   prohibited in IDNA2008.  Similarly, double-width characters and other
   variations are prohibited as IDNA input.

   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

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   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
   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.  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.

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7.4.  The Question of Prefix Changes

   The conditions that would require a change in the IDNA ACE prefix
   ("xn--" for the version of IDNA specified in [RFC3490]) have been a
   great concern to the community.  A prefix change would clearly be
   necessary if the algorithms were modified in a manner that would
   create serious ambiguities during subsequent transition in
   registrations.  This section summarizes our conclusions about the
   conditions under which changes in prefix would be necessary and the
   implications of such a change.

7.4.1.  Conditions Requiring a Prefix Change

   An IDN prefix change is needed if a given string would be looked up
   or otherwise interpreted differently depending on the version of the
   protocol or tables being used.  An IDNA upgrade would require 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) yields
       one string under IDNA2003 (RFC3490) and a different string under

   2.  In a significant number of cases, an input string that is valid
       under IDNA2003 and also valid under IDNA2008 yields 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 which may not justify a prefix change
       (See Section 7.2).

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

   3.  A fundamental change is made to the semantics of the string that
       is 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 is added to Unicode so
       that the Punycode mechanism for block offsets can 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.

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7.4.2.  Conditions Not Requiring a Prefix Change

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

   1.  Prohibition of some characters as input to IDNA.  This may make
       names that are now registered inaccessible, but does not change
       those names.

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

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

7.4.3.  Implications of Prefix Changes

   While it might be possible to make a prefix change, the costs of such
   a change are considerable.  Registries could not convert 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 first process a putative label under
   the IDNA2008 rules and try to look it up and then, if it were not
   found, would process the label under IDNA2003 rules and look it up
   again.  That process could significantly slow 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 make DNS caching very difficult.  In
   addition, looking up the same input string as two separate A-labels
   creates 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 is to be 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

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 be modified by IDNA2008, those changes
   to improve the handling of IDNs could cause problems for non-DNS

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   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
   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 more than one "heart" character

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       (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
       [[anchor24: The above paragraph remains controversial as to
       whether it is valid.  The WG will need to make a decision if this
       section is not dropped entirely.]]

   3.  Consider the case 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.

   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 or the 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
       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

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   and then resolved, the relevant standards must have changed and the
   registry has properly allocated only assigned values.

   In the protocol described in these documents, 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
   cannot be evaluated until a character is actually assigned and known.
   There are several reasons for this, with the most important ones

   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,
      that a no newly-assigned code point will 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

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   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 changes may change the
   preferred form for writing a particular string, changes that may be
   reflected, e.g., in keyboard transition modules that would
   necessarily be different from those for earlier versions of Unicode
   where the newer characters may not exist.  This creates an inherent
   transition problem because attempts to access labels may use either
   the old or the new conventions, requiring registry action whether 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

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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
   Registration and Lookup in [IDNA2008-Protocol] (a domain name
   containing only ASCII codepoints can not be converted to an A-label),
   there can not 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.  DNSSEC Authentication of IDN Domain Names

   DNS Security (DNSSEC) [RFC2535] is a method for supplying
   cryptographic verification information along with DNS messages.
   Public Key Cryptography is used in conjunction with digital
   signatures to provide a means for a requester of domain information
   to authenticate the source of the data.  This ensures that it can be
   traced back to a trusted source, either directly or via a chain of
   trust linking the source of the information to the top of the DNS

   IDNA specifies that all internationalized domain names served by DNS
   servers that cannot be represented directly in ASCII MUST use the
   A-label form.  Conversion to A-labels MUST be performed prior to a
   zone being signed by the private key for that zone.  Because of this
   ordering, it is important to recognize that DNSSEC authenticates a
   domain name containing A-labels or conventional LDH-labels, not
   U-labels.  In the presence of DNSSEC, no form of a zone file or query

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   response that contains a U-label may be signed or the signature

   One consequence of this for sites deploying IDNA in the presence of
   DNSSEC is that any special purpose proxies or forwarders used to
   transform user input into IDNs must be earlier in the lookup flow
   than DNSSEC authenticating nameservers for DNSSEC to work.

8.3.  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.

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.

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10.1.  IDNA Character Registry

   The distinction among the major categories "UNASSIGNED",
   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

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, these
   specifications have no direct effect on that registry and no IANA
   action is required as a result.

11.  Security Considerations

11.1.  General Security Issues with IDNA

   This document is purely explanatory and informational and
   consequently introduces no new security issues.  It would, of course,
   be a poor idea for someone to try to implement from it; such an
   attempt would almost certainly lead to interoperability problems and
   might lead to security ones.  A discussion of security issues with
   IDNA, including some relevant history, appears in [IDNA2008-Defs].

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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, especially Mark Davis, 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, although, as usual, they bear little or no responsibility
   for the conclusions the editor and contributors reached after
   receiving their suggestions.  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.

   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

   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, Mark Davis, Martin Duerst, Andrew Sullivan, and Ken Whistler.
   Lisa Dusseault provided extensive editorial suggestions during the
   spring of 2009, most of which were incorporated.

13.  Contributors

   While the listed editor held the pen, the core of this document and
   the initial 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.
   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

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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.

              Alvestrand, H. and C. Karp, "An updated IDNA criterion for
              right to left scripts", July 2008, <https://

              Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              November 2008, <https://datatracker.ietf.org/drafts/

              Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", November 2008, <https://

              Faltstrom, P., "The Unicode Code Points and IDNA",
              July 2008, <https://datatracker.ietf.org/drafts/

              A version of this document is available in HTML format at

   [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.

              The Unicode Consortium, "Unicode Standard Annex #15:
              Unicode Normalization Forms", March 2008,

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              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

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.",

   [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.

   [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.

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

   [RFC2535]  Eastlake, D., "Domain Name System Security Extensions",
              RFC 2535, March 1999.

   [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.

   [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.


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              The Unicode Consortium, "Unicode Technical Standard #39:
              Unicode Security Mechanisms", August 2008,

              The Unicode Consortium, "Unicode Standard Annex #31:
              Unicode Identifier and Pattern Syntax", March 2008,

              The Unicode Consortium, "Unicode Technical Report #36:
              Unicode Security Considerations", July 2008,

Appendix A.  Change Log

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

A.1.  Changes between Version -00 and Version -01 of

   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

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

   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.

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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://
   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.

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.

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   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-

   o  Clarified relationship to base DNS specifications.

   o  Consolidated discussion of lookup of unassigned characters.

   o  More editorial fine-tuning.

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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 1.3.3.

   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.

   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.

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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.

Author's Address

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

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

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