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rfc:rfc5894

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

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

Abstract

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

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc5894.

Klensin Informational [Page 1] RFC 5894 IDNA Rationale August 2010

Copyright Notice

 Copyright (c) 2010 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

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

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

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1. Introduction

1.1. Context and Overview

 Internationalized Domain Names in Applications (IDNA) is a collection
 of standards that allow client applications to convert some mnemonic
 strings expressed in Unicode to an ASCII-compatible encoding form
 ("ACE") that is a valid DNS label containing only LDH syntax (see the
 Definitions document [RFC5890]).  The specific form of ACE label used
 by IDNA is called an "A-label".  A client can look up an exact
 A-label in the existing DNS, so A-labels do not require any
 extensions to DNS, upgrades of DNS servers, or updates to low-level
 client libraries.  An A-label is recognizable from the prefix "xn--"
 before the characters produced by the Punycode algorithm [RFC3492];
 thus, a user application can identify an A-label and convert it into
 Unicode (or some local coded character set) for display.
 On the registry side, IDNA allows a registry to offer
 Internationalized Domain Names (IDNs) for registration as A-labels.
 A registry may offer any subset of valid IDNs, and may apply any
 restrictions or bundling (grouping of similar labels together in one
 registration) appropriate for the context of that registry.
 Registration of labels is sometimes discussed separately from lookup,
 and it is subject to a few specific requirements that do not apply to
 lookup.
 DNS clients and registries are subject to some differences in
 requirements for handling IDNs.  In particular, registries are urged
 to register only exact, valid A-labels, while clients might do some
 mapping to get from otherwise-invalid user input to a valid A-label.
 The first version of IDNA was published in 2003 and is referred to
 here as IDNA2003 to contrast it with the current version, which is
 known as IDNA2008 (after the year in which IETF work started on it).
 IDNA2003 consists of four documents: the IDNA base specification
 [RFC3490], Nameprep [RFC3491], Punycode [RFC3492], and Stringprep
 [RFC3454].  The current set of documents, IDNA2008, is not dependent
 on any of the IDNA2003 specifications other than the one for Punycode
 encoding.  References to "IDNA2008", "these specifications", or
 "these documents" are to the entire IDNA2008 set listed in a separate
 Definitions document [RFC5890].  The characters that are valid in
 A-labels are identified from rules listed in the Tables document
 [RFC5892], but validity can be derived from the Unicode properties of
 those characters with a very few exceptions.
 Traditionally, DNS labels are matched case-insensitively (as
 described in the DNS specifications [RFC1034][RFC1035]).  That
 convention was preserved in IDNA2003 by a case-folding operation that

Klensin Informational [Page 4] RFC 5894 IDNA Rationale August 2010

 generally maps capital letters into lowercase ones.  However, if case
 rules are enforced from one language, another language sometimes
 loses the ability to treat two characters separately.  Case-
 insensitivity is treated slightly differently in IDNA2008.
 IDNA2003 used Unicode version 3.2 only.  In order to keep up with new
 characters added in new versions of Unicode, IDNA2008 decouples its
 rules from any particular version of Unicode.  Instead, the
 attributes of new characters in Unicode, supplemented by a small
 number of exception cases, determine how and whether the characters
 can be used in IDNA labels.
 This document provides informational context for IDNA2008, including
 terminology, background, and policy discussions.  It contains no
 normative material; specifications for conformance to the IDNA2008
 protocols appears entirely in the other documents in the series.

1.2. Terminology

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

1.2.1. DNS "Name" Terminology

 In the context of IDNs, the DNS term "name" has introduced some
 confusion as people speak of DNS labels in terms of the words or
 phrases of various natural languages.  Historically, many of the
 "names" in the DNS have been mnemonics to identify some particular
 concept, object, or organization.  They are typically rooted in some
 language because most people think in language-based ways.  But,
 because they are mnemonics, they need not obey the orthographic
 conventions of any language: it is not a requirement that it be
 possible for them to be "words".
 This distinction is important because the reasonable goal of an IDN
 effort is not to be able to write the great Klingon (or language of
 one's choice) novel in DNS labels but to be able to form a usefully
 broad range of mnemonics in ways that are as natural as possible in a
 very broad range of scripts.

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1.2.2. New Terminology and Restrictions

 IDNA2008 introduces new terminology.  Precise definitions are
 provided in the Definitions document for the terms U-label, A-Label,
 LDH label (to which all valid pre-IDNA hostnames conformed), Reserved
 LDH label (R-LDH label), XN-label, Fake A-label, and Non-Reserved LDH
 label (NR-LDH label).
 In addition, the term "putative label" has been adopted to refer to a
 label that may appear to meet certain definitional constraints but
 has not yet been sufficiently tested for validity.
 These definitions are also illustrated in Figure 1 of the Definitions
 document.  R-LDH labels contain "--" in the third and fourth
 character positions from the beginning of the label.  In IDNA-aware
 applications, only a subset of these reserved labels is permitted to
 be used, namely the A-label subset.  A-labels are a subset of the
 R-LDH labels that begin with the case-insensitive string "xn--".
 Labels that bear this prefix but that are not otherwise valid fall
 into the "Fake A-label" category.  The Non-Reserved labels (NR-LDH
 labels) are implicitly valid since they do not bear any resemblance
 to the labels specified by IDNA.
 The creation of the Reserved-LDH category is required for three
 reasons:
 o  to prevent confusion with pre-IDNA coding forms;
 o  to permit future extensions that would require changing the
    prefix, no matter how unlikely those might be (see Section 7.4);
    and
 o  to reduce the opportunities for attacks via the Punycode encoding
    algorithm itself.
 As with other documents in the IDNA2008 set, this document uses the
 term "registry" to describe any zone in the DNS.  That term, and the
 terms "zone" or "zone administration", are interchangeable.

1.3. Objectives

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

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 o  Fix a very small number of code point categorizations that have
    turned out to cause problems in the communities that use those
    code points.
 o  Reduce the dependency on mapping, in favor of valid A-labels.
    This will result in pre-mapped forms that are not valid IDNA
    labels appearing less often in various contexts.
 o  Fix some details in the bidirectional code point handling
    algorithms.

1.4. Applicability and Function of IDNA

 The IDNA specification solves the problem of extending the repertoire
 of characters that can be used in domain names to include a large
 subset of the Unicode repertoire.
 IDNA does not extend DNS.  Instead, the applications (and, by
 implication, the users) continue to see an exact-match lookup
 service.  Either there is a single name that matches exactly (subject
 to the base DNS requirement of case-insensitive ASCII matching) or
 there is no match.  This model has served the existing applications
 well, but it requires, with or without internationalized domain
 names, that users know the exact spelling of the domain names that
 are to be typed into applications such as web browsers and mail user
 agents.  The introduction of the larger repertoire of characters
 potentially makes the set of misspellings larger, especially given
 that in some cases the same appearance, for example on a business
 card, might visually match several Unicode code points or several
 sequences of code points.
 The IDNA standard does not require any applications to conform to it,
 nor does it retroactively change those applications.  An application
 can elect to use IDNA in order to support IDNs while maintaining
 interoperability with existing infrastructure.  For applications that
 want to use non-ASCII characters in public DNS domain names, IDNA is
 the only option that is defined at the time this specification is
 published.  Adding IDNA support to an existing application entails
 changes to the application only, and leaves room for flexibility in
 front-end processing and more specifically in the user interface (see
 Section 6).
 A great deal of the discussion of IDN solutions has focused on
 transition issues and how IDNs will work in a world where not all of
 the components have been updated.  Proposals that were not chosen by
 the original IDN Working Group would have depended on updating user
 applications, DNS resolvers, and DNS servers in order for a user to
 apply an internationalized domain name in any form or coding

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 acceptable under that method.  While processing must be performed
 prior to or after access to the DNS, IDNA requires no changes to the
 DNS protocol, any DNS servers, or the resolvers on users' computers.
 IDNA allows the graceful introduction of IDNs not only by avoiding
 upgrades to existing infrastructure (such as DNS servers and mail
 transport agents), but also by allowing some limited use of IDNs in
 applications by using the ASCII-encoded representation of the labels
 containing non-ASCII characters.  While such names are user-
 unfriendly to read and type, and hence not optimal for user input,
 they can be used as a last resort to allow rudimentary IDN usage.
 For example, they might be the best choice for display if it were
 known that relevant fonts were not available on the user's computer.
 In order to allow user-friendly input and output of the IDNs and
 acceptance of some characters as equivalent to those to be processed
 according to the protocol, the applications need to be modified to
 conform to this specification.
 This version of IDNA uses the Unicode character repertoire for
 continuity with the original version of IDNA.

1.5. Comprehensibility of IDNA Mechanisms and Processing

 One goal of IDNA2008, which is aided by the main goal of reducing the
 dependency on mapping, is to improve the general understanding of how
 IDNA works and what characters are permitted and what happens to
 them.  Comprehensibility and predictability to users and registrants
 are important design goals for this effort.  End-user applications
 have an important role to play in increasing this comprehensibility.
 Any system that tries to handle international characters encounters
 some common problems.  For example, a User Interface (UI) cannot
 display a character if no font containing that character is
 available.  In some cases, internationalization enables effective
 localization while maintaining some global uniformity but losing some
 universality.
 It is difficult to even make suggestions as to how end-user
 applications should cope when characters and fonts are not available.
 Because display functions are rarely controlled by the types of
 applications that would call upon IDNA, such suggestions will rarely
 be very effective.
 Conversion between local character sets and normalized Unicode, if
 needed, is part of this set of user interface issues.  Those
 conversions introduce complexity in a system that does not use
 Unicode as its primary (or only) internal character coding system.
 If a label is converted to a local character set that does not have

Klensin Informational [Page 8] RFC 5894 IDNA Rationale August 2010

 all the needed characters, or that uses different character-coding
 principles, the user interface program may have to add special logic
 to avoid or reduce loss of information.
 The major difficulty may lie in accurately identifying the incoming
 character set and applying the correct conversion routine.  Even more
 difficult, the local character coding system could be based on
 conceptually different assumptions than those used by Unicode (e.g.,
 choice of font encodings used for publications in some Indic
 scripts).  Those differences may not easily yield unambiguous
 conversions or interpretations even if each coding system is
 internally consistent and adequate to represent the local language
 and script.
 IDNA2008 shifts responsibility for character mapping and other
 adjustments from the protocol (where it was located in IDNA2003) to
 pre-processing before invoking IDNA itself.  The intent is that this
 change will lead to greater usage of fully-valid A-Labels or U-labels
 in display, transit, and storage, which should aid comprehensibility
 and predictability.  A careful look at pre-processing raises issues
 about what that pre-processing should do and at what point
 pre-processing becomes harmful; how universally consistent
 pre-processing algorithms can be; and how to be compatible with
 labels prepared in an IDNA2003 context.  Those issues are discussed
 in Section 6 and in the Mapping document [IDNA2008-Mapping].

2. Processing in IDNA2008

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

3. Permitted Characters: An Inclusion List

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

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 This section provides an overview of the model used to establish the
 algorithm and character lists of the Tables document [RFC5892] and
 describes the names and applicability of the categories used there.
 Note that the inclusion of a character in the PROTOCOL-VALID category
 group (Section 3.1.1) does not imply that it can be used
 indiscriminately; some characters are associated with contextual
 rules that must be applied as well.
 The information given in this section is provided to make the rules,
 tables, and protocol easier to understand.  The normative generating
 rules that correspond to this informal discussion appear in the
 Tables document, and the rules that actually determine what labels
 can be registered or looked up are in the Protocol document.

3.1. A Tiered Model of Permitted Characters and Labels

 Moving to an inclusion model involves a new specification for the
 list of characters that are permitted in IDNs.  In IDNA2003,
 character validity is independent of context and fixed forever (or
 until the standard is replaced).  However, globally context-
 independent rules have proved to be impractical because some
 characters, especially those that are called "Join_Controls" in
 Unicode, are needed to make reasonable use of some scripts but have
 no visible effect in others.  IDNA2003 prohibited those types of
 characters entirely by discarding them.  We now have a consensus that
 under some conditions, these "joiner" characters are legitimately
 needed to allow useful mnemonics for some languages and scripts.  In
 general, context-dependent rules help deal with characters (generally
 characters that would otherwise be prohibited entirely) that are used
 differently or perceived differently across different scripts, and
 allow the standard to be applied more appropriately in cases where a
 string is not universally handled the same way.
 IDNA2008 divides all possible Unicode code points into four
 categories: PROTOCOL-VALID, CONTEXTUAL RULE REQUIRED, DISALLOWED, and
 UNASSIGNED.

3.1.1. PROTOCOL-VALID

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

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 characters in the category.  Registries are still expected to apply
 judgment about labels they will accept and to maintain rules
 consistent with those judgments (see the Protocol document [RFC5891]
 and Section 3.3).
 Characters that are placed in the PROTOCOL-VALID category are
 expected to never be removed from it or reclassified.  While
 theoretically characters could be removed from Unicode, such removal
 would be inconsistent with the Unicode stability principles (see
 UTR 39: Unicode Security Mechanisms [Unicode52], Appendix F) and
 hence should never occur.

3.1.2. CONTEXTUAL RULE REQUIRED

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

3.1.2.1. Contextual Restrictions

 Characters with contextual restrictions are identified as CONTEXTUAL
 RULE REQUIRED and are associated with a rule.  The rule defines
 whether the character is valid in a particular string, and also
 whether the rule itself is to be applied on lookup as well as
 registration.
 A distinction is made between characters that indicate or prohibit
 joining and ones similar to them (known as CONTEXT-JOINER or
 CONTEXTJ) and other characters requiring contextual treatment
 (CONTEXT-OTHER or CONTEXTO).  Only the former require full testing at
 lookup time.
 It is important to note that these contextual rules cannot prevent
 all uses of the relevant characters that might be confusing or
 problematic.  What they are expected to do is to confine
 applicability of the characters to scripts (and narrower contexts)
 where zone administrators are knowledgeable enough about the use of
 those characters to be prepared to deal with them appropriately.

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

3.1.2.2. Rules and Their Application

 Rules have descriptions such as "Must follow a character from Script
 XYZ", "Must occur only if the entire label is in Script ABC", or
 "Must occur only if the previous and subsequent characters have the
 DFG property".  The actual rules may be DEFINED or NULL.  If present,
 they may have values of "True" (character may be used in any position
 in any label), "False" (character may not be used in any label), or
 may be a set of procedural rules that specify the context in which
 the character is permitted.
 Because it is easier to identify these characters than to know that
 they are actually needed in IDNs or how to establish exactly the
 right rules for each one, a rule may have a null value in a given
 version of the tables.  Characters associated with null rules are not
 permitted to appear in putative labels for either registration or
 lookup.  Of course, a later version of the tables might contain a
 non-null rule.
 The actual rules and their descriptions are in Sections 2 and 3 of
 the Tables document [RFC5892].  That document also specifies the
 creation of a registry for future rules.

3.1.3. DISALLOWED

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

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 Of course, this category would include code points that had been
 removed entirely from Unicode should such removals ever occur.
 Characters that are placed in the DISALLOWED category are expected to
 never be removed from it or reclassified.  If a character is
 classified as DISALLOWED in error and the error is sufficiently
 problematic, the only recourse would be either to introduce a new
 code point into Unicode and classify it as PROTOCOL-VALID or for the
 IETF to accept the considerable costs of an incompatible change and
 replace the relevant RFC with one containing appropriate exceptions.
 There is provision for exception cases but, in general, characters
 are placed into DISALLOWED if they fall into one or more of the
 following groups:
 o  The character is a compatibility equivalent for another character.
    In slightly more precise Unicode terms, application of
    Normalization Form KC (NFKC) to the character yields some other
    character.
 o  The character is an uppercase form or some other form that is
    mapped to another character by Unicode case folding.
 o  The character is a symbol or punctuation form or, more generally,
    something that is not a letter, digit, or a mark that is used to
    form a letter or digit.

3.1.4. UNASSIGNED

 For convenience in processing and table-building, code points that do
 not have assigned values in a given version of Unicode are treated as
 belonging to a special UNASSIGNED category.  Such code points are
 prohibited in labels to be registered or looked up.  The category
 differs from DISALLOWED in that code points are moved out of it by
 the simple expedient of being assigned in a later version of Unicode
 (at which point, they are classified into one of the other categories
 as appropriate).
 The rationale for restricting the processing of UNASSIGNED characters
 is simply that the properties of such code points cannot be
 completely known until actual characters are assigned to them.  For
 example, assume that an UNASSIGNED code point were included in a
 label to be looked up.  Assume that the code point was later assigned
 to a character that required some set of contextual rules.  With that
 combination, un-updated instances of IDNA-aware software might permit
 lookup of labels containing the previously unassigned characters
 while updated versions of the software might restrict use of the same

Klensin Informational [Page 13] RFC 5894 IDNA Rationale August 2010

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

3.2. Registration Policy

 While these recommendations cannot and should not define registry
 policies, registries should develop and apply additional restrictions
 as needed to reduce confusion and other problems.  For example, it is
 generally believed that labels containing characters from more than
 one script are a bad practice although there may be some important
 exceptions to that principle.  Some registries may choose to restrict
 registrations to characters drawn from a very small number of
 scripts.  For many scripts, the use of variant techniques such as
 those as described in the JET specification for the CJK script
 [RFC3743] and its generalization [RFC4290], and illustrated for
 Chinese by the tables provided by the Chinese Domain Name Consortium
 [RFC4713] may be helpful in reducing problems that might be perceived
 by users.
 In general, users will benefit if registries only permit characters
 from scripts that are well-understood by the registry or its
 advisers.  If a registry decides to reduce opportunities for
 confusion by constructing policies that disallow characters used in
 historic writing systems or characters whose use is restricted to
 specialized, highly technical contexts, some relevant information may
 be found in Section 2.4 (Specific Character Adjustments) of Unicode
 Identifier and Pattern Syntax [Unicode-UAX31], especially Table 4
 (Candidate Characters for Exclusion from Identifiers), and Section
 3.1 (General Security Profile for Identifiers) in Unicode Security
 Mechanisms [Unicode-UTS39].
 The requirement (in Section 4.1 of the Protocol document [RFC5891])
 that registration procedures use only U-labels and/or A-labels is
 intended to ensure that registrants are fully aware of exactly what
 is being registered as well as encouraging use of those canonical
 forms.  That provision should not be interpreted as requiring that
 registrants need to provide characters in a particular code sequence.
 Registrant input conventions and management are part of registrant-
 registrar interactions and relationships between registries and
 registrars and are outside the scope of these standards.
 It is worth stressing that these principles of policy development and
 application apply at all levels of the DNS, not only, e.g., top level
 domain (TLD) or second level domain (SLD) registrations.  Even a
 trivial, "anything is permitted that is valid under the protocol"
 policy is helpful in that it helps users and application developers
 know what to expect.

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

    Applications
 The character rules in IDNA2008 are based on the realization that
 there is no single magic bullet for any of the security,
 confusability, or other issues associated with IDNs.  Instead, the
 specifications define a variety of approaches.  The character tables
 are the first mechanism, protocol rules about how those characters
 are applied or restricted in context are the second, and those two in
 combination constitute the limits of what can be done in the
 protocol.  As discussed in the previous section (Section 3.2),
 registries are expected to restrict what they permit to be
 registered, devising and using rules that are designed to optimize
 the balance between confusion and risk on the one hand and maximum
 expressiveness in mnemonics on the other.
 In addition, there is an important role for user interface programs
 in warning against label forms that appear problematic given their
 knowledge of local contexts and conventions.  Of course, no approach
 based on naming or identifiers alone can protect against all threats.

4. Application-Related Issues

4.1. Display and Network Order

 Domain names are always transmitted in network order (the order in
 which the code points are sent in protocols), but they may have a
 different display order (the order in which the code points are
 displayed on a screen or paper).  When a domain name contains
 characters that are normally written right to left, display order may
 be affected although network order is not.  It gets even more
 complicated if left-to-right and right-to-left labels are adjacent to
 each other within a domain name.  The decision about the display
 order is ultimately under the control of user agents -- including Web
 browsers, mail clients, hosted Web applications and many more --
 which may be highly localized.  Should a domain name abc.def, in
 which both labels are represented in scripts that are written right
 to left, be displayed as fed.cba or cba.fed?  Applications that are
 in deployment today are already diverse, and one can find examples of
 either choice.
 The picture changes once again when an IDN appears in an
 Internationalized Resource Identifier (IRI) [RFC3987].  An IRI or
 internationalized email address contains elements other than the
 domain name.  For example, IRIs contain protocol identifiers and
 field delimiter syntax such as "http://" or "mailto:" while email
 addresses contain the "@" to separate local parts from domain names.

Klensin Informational [Page 15] RFC 5894 IDNA Rationale August 2010

 An IRI in network order begins with "http://" followed by domain
 labels in network order, thus "http://abc.def".
 User interface programs are not required to display and allow input
 of IRIs directly but often do so.  Implementers have to choose
 whether the overall direction of these strings will always be left to
 right (or right to left) for an IRI or email address.  The natural
 order for a user typing a domain name on a right-to-left system is
 fed.cba.  Should the right-to-left (RTL) user interface reverse the
 entire domain name each time a domain name is typed?  Does this
 change if the user types "http://" right before typing a domain name,
 thus implying that the user is beginning at the beginning of the
 network-order IRI?  Experience in the 1980s and 1990s with mixing
 systems in which domain name labels were read in network order (left
 to right) and those in which those labels were read right to left
 would predict a great deal of confusion.
 If each implementation of each application makes its own decisions on
 these issues, users will develop heuristics that will sometimes fail
 when switching applications.  However, while some display order
 conventions, voluntarily adopted, would be desirable to reduce
 confusion, such suggestions are beyond the scope of these
 specifications.

4.2. Entry and Display in Applications

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

Klensin Informational [Page 16] RFC 5894 IDNA Rationale August 2010

 and in the headers and the body content in HTTP.  It is important to
 remember that domain names appear both in domain name slots and in
 the content that is passed over protocols, and it would be helpful if
 protocols explicitly define what their domain name slots are.
 In protocols and document formats that define how to handle
 specification or negotiation of charsets, labels can be encoded in
 any charset allowed by the protocol or document format.  If a
 protocol or document format only allows one charset, the labels must
 be given in that charset.  Of course, not all charsets can properly
 represent all labels.  If a U-label cannot be displayed in its
 entirety, the only choice (without loss of information) may be to
 display the A-label.
 Where a protocol or document format allows IDNs, labels should be in
 whatever character encoding and escape mechanism the protocol or
 document format uses in the local environment.  This provision is
 intended to prevent situations in which, e.g., UTF-8 domain names
 appear embedded in text that is otherwise in some other character
 coding.
 All protocols that use domain name slots (see Section 2.3.2.6 in the
 Definitions document [RFC5890]) already have the capacity for
 handling domain names in the ASCII charset.  Thus, A-labels can
 inherently be handled by those protocols.
 IDNA2008 does not specify required mappings between one character or
 code point and others.  An extended discussion of mapping issues
 appears in Section 6 and specific recommendations appear in the
 Mapping document [IDNA2008-Mapping].  In general, IDNA2008 prohibits
 characters that would be mapped to others by normalization or other
 rules.  As examples, while mathematical characters based on Latin
 ones are accepted as input to IDNA2003, they are prohibited in
 IDNA2008.  Similarly, uppercase characters, double-width characters,
 and other variations are prohibited as IDNA input although mapping
 them as needed in user interfaces is strongly encouraged.
 Since the rules in the Tables document [RFC5892] have the effect that
 only strings that are not transformed by NFKC are valid, if an
 application chooses to perform NFKC normalization before lookup, that
 operation is safe since this will never make the application unable
 to look up any valid string.  However, as discussed above, the
 application cannot guarantee that any other application will perform
 that mapping, so it should be used only with caution and for informed
 users.

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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 the Protocol document [RFC5891].  However,
 those changes will be local ones only -- local to environments in
 which users will clearly understand that the character forms are
 equivalent.  For use in interchanges among systems, it appears to be
 much more important that U-labels and A-labels can be mapped back and
 forth without loss of information.
 One specific, and very important, instance of this strategy arises
 with case folding.  In the ASCII-only DNS, names are looked up and
 matched in a case-independent way, but no actual case folding occurs.
 Names can be placed in the DNS in either uppercase or lowercase form
 (or any mixture of them) and that form is preserved, returned in
 queries, and so on.  IDNA2003 approximated that behavior for
 non-ASCII strings by performing case folding at registration time
 (resulting in only lowercase IDNs in the DNS) and when names were
 looked up.
 As suggested earlier in this section, it appears to be desirable to
 do as little character mapping as possible as long as Unicode works
 correctly (e.g., Normalization Form C (NFC) mapping to resolve
 different codings for the same character is still necessary although
 the specifications require that it be performed prior to invoking the
 protocol) in order to make the mapping between A-labels and U-labels
 idempotent.  Case mapping is not an exception to this principle.  If
 only lowercase characters can be registered in the DNS (i.e., be
 present in a U-label), then IDNA2008 should prohibit uppercase
 characters as input even though user interfaces to applications
 should probably map those characters.  Some other considerations
 reinforce this conclusion.  For example, in ASCII case mapping for
 individual characters, uppercase(character) is always equal to
 uppercase(lowercase(character)).  That may not be true with IDNs.  In
 some scripts that use case distinctions, there are a few characters
 that do not have counterparts in one case or the other.  The
 relationship between uppercase and lowercase may even be language-
 dependent, with different languages (or even the same language in
 different areas) expecting different mappings.  User interface
 programs can meet the expectations of users who are accustomed to the
 case-insensitive DNS environment by performing case folding prior to
 IDNA processing, but the IDNA procedures themselves should neither
 require such mapping nor expect them when they are not natural to the
 localized environment.

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4.3. Linguistic Expectations: Ligatures, Digraphs, and Alternate

    Character Forms
 Users have expectations about character matching or equivalence that
 are based on their own languages and the orthography of those
 languages.  These expectations may not always be met in a global
 system, especially if multiple languages are written using the same
 script but using different conventions.  Some examples:
 o  A Norwegian user might expect a label with the ae-ligature to be
    treated as the same label as one using the Swedish spelling with
    a-diaeresis even though applying that mapping to English would be
    astonishing to users.
 o  A German user might expect a label with an o-umlaut and a label
    that had "oe" substituted, but was otherwise the same, to be
    treated as equivalent even though that substitution would be a
    clear error in Swedish.
 o  A Chinese user might expect automatic matching of Simplified and
    Traditional Chinese characters, but applying that matching for
    Korean or Japanese text would create considerable confusion.
 o  An English user might expect "theater" and "theatre" to match.
 A number of languages use alphabetic scripts in which single phonemes
 are written using two characters, termed a "digraph", for example,
 the "ph" in "pharmacy" and "telephone".  (Such characters can also
 appear consecutively without forming a digraph, as in "tophat".)
 Certain digraphs may be indicated typographically by setting the two
 characters closer together than they would be if used consecutively
 to represent different phonemes.  Some digraphs are fully joined as
 ligatures.  For example, the word "encyclopaedia" is sometimes set
 with a U+00E6 LATIN SMALL LIGATURE AE.  When ligature and digraph
 forms have the same interpretation across all languages that use a
 given script, application of Unicode normalization generally resolves
 the differences and causes them to match.  When they have different
 interpretations, matching must utilize other methods, presumably
 chosen at the registry level, or users must be educated to understand
 that matching will not occur.
 The nature of the problem can be illustrated by many words in the
 Norwegian language, where the "ae" ligature is the 27th letter of a
 29-letter extended Latin alphabet.  It is equivalent to the 28th
 letter of the Swedish alphabet (also containing 29 letters),
 U+00E4 LATIN SMALL LETTER A WITH DIAERESIS, for which an "ae" cannot
 be substituted according to current orthographic standards.  That
 character (U+00E4) is also part of the German alphabet where, unlike

Klensin Informational [Page 19] RFC 5894 IDNA Rationale August 2010

 in the Nordic languages, the two-character sequence "ae" is usually
 treated as a fully acceptable alternate orthography for the "umlauted
 a" character.  The inverse is however not true, and those two
 characters cannot necessarily be combined into an "umlauted a".  This
 also applies to another German character, the "umlauted o"
 (U+00F6 LATIN SMALL LETTER O WITH DIAERESIS) which, for example,
 cannot be used for writing the name of the author "Goethe".  It is
 also a letter in the Swedish alphabet where, like the "a with
 diaeresis", it cannot be correctly represented as "oe" and in the
 Norwegian alphabet, where it is represented, not as "o with
 diaeresis", but as "slashed o", U+00F8.
 Some of the ligatures that have explicit code points in Unicode were
 given special handling in IDNA2003 and now pose additional problems
 in transition.  See Section 7.2.
 Additional cases with alphabets written right to left are described
 in Section 4.5.
 Matching and comparison algorithm selection often requires
 information about the language being used, context, or both --
 information that is not available to IDNA or the DNS.  Consequently,
 IDNA2008 makes no attempt to treat combined characters in any special
 way.  A registry that is aware of the language context in which
 labels are to be registered, and where that language sometimes (or
 always) treats the two-character sequences as equivalent to the
 combined form, should give serious consideration to applying a
 "variant" model [RFC3743][RFC4290] or to prohibiting registration of
 one of the forms entirely, to reduce the opportunities for user
 confusion and fraud that would result from the related strings being
 registered to different parties.

4.4. Case Mapping and Related Issues

 In the DNS, ASCII letters are stored with their case preserved.
 Matching during the query process is case-independent, but none of
 the information that might be represented by choices of case has been
 lost.  That model has been accidentally helpful because, as people
 have created DNS labels by catenating words (or parts of words) to
 form labels, case has often been used to distinguish among components
 and make the labels more memorable.
 Since DNS servers do not get involved in parsing IDNs, they cannot do
 case-independent matching.  Thus, keeping the cases separate in
 lookup or registration, and doing matching at the server, is not
 feasible with IDNA or any similar approach.  Matching of characters
 that are considered to differ only by case must be done, if desired,
 by programs invoking IDNA lookup even though it wasn't done by ASCII-

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 only DNS clients.  That situation was recognized in IDNA2003 and
 nothing in IDNA2008 fundamentally changes it or could do so.  In
 IDNA2003, all characters are case folded and mapped by clients in a
 standardized step.
 Even in scripts that generally support case distinctions, some
 characters do not have uppercase forms.  For example, the Unicode
 case-folding operation maps Greek Final Form Sigma (U+03C2) to the
 medial form (U+03C3) and maps Eszett (German Sharp S, U+00DF) to
 "ss".  Neither of these mappings is reversible because the uppercase
 of U+03C3 is the uppercase Sigma (U+03A3) and "ss" is an ASCII
 string.  IDNA2008 permits, at the risk of some incompatibility,
 slightly more flexibility in this area by avoiding case folding and
 treating these characters as themselves.  Approaches to handling one-
 way mappings are discussed in Section 7.2.
 Because IDNA2003 maps Final Sigma and Eszett to other characters, and
 the reverse mapping is never possible, neither Final Sigma nor Eszett
 can be represented in the ACE form of IDNA2003 IDN nor in the native
 character (U-label) form derived from it.  With IDNA2008, both
 characters can be used in an IDN and so the A-label used for lookup
 for any U-label containing those characters is now different.  See
 Section 7.1 for a discussion of what kinds of changes might require
 the IDNA prefix to change; after extended discussions, the IDNABIS
 Working Group came to consensus that the change for these characters
 did not justify a prefix change.

4.5. Right-to-Left Text

 In order to be sure that the directionality of right-to-left text is
 unambiguous, IDNA2003 required that any label in which right-to-left
 characters appear both starts and ends with them and that it does not
 include any characters with strong left-to-right properties (that
 excludes other alphabetic characters but permits European digits).
 Any other string that contains a right-to-left character and does not
 meet those requirements is rejected.  This is one of the few places
 where the IDNA algorithms (both in IDNA2003 and in IDNA2008) examine
 an entire label, not just individual characters.  The algorithmic
 model used in IDNA2003 rejects the label when the final character in
 a right-to-left string requires a combining mark in order to be
 correctly represented.
 That prohibition is not acceptable for writing systems for languages
 written with consonantal alphabets to which diacritical vocalic
 systems are applied, and for languages with orthographies derived
 from them where the combining marks may have different functionality.
 In both cases, the combining marks can be essential components of the
 orthography.  Examples of this are Yiddish, written with an extended

Klensin Informational [Page 21] RFC 5894 IDNA Rationale August 2010

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

5. IDNs and the Robustness Principle

 The "Robustness Principle" is often stated as "Be conservative about
 what you send and liberal in what you accept" (see, e.g., Section
 1.2.2 of the applications-layer Host Requirements specification
 [RFC1123]).  This principle applies to IDNA.  In applying the
 principle to registries as the source ("sender") of all registered
 and useful IDNs, registries are responsible for being conservative
 about what they register and put out in the Internet.  For IDNs to
 work well, zone administrators (registries) must have and require
 sensible policies about what is registered -- conservative policies
 -- and implement and enforce them.
 Conversely, lookup applications are expected to reject labels that
 clearly violate global (protocol) rules (no one has ever seriously
 claimed that being liberal in what is accepted requires being
 stupid).  However, once one gets past such global rules and deals
 with anything sensitive to script or locale, it is necessary to
 assume that garbage has not been placed into the DNS, i.e., one must
 be liberal about what one is willing to look up in the DNS rather
 than guessing about whether it should have been permitted to be
 registered.
 If a string cannot be successfully found in the DNS after the lookup
 processing described here, it makes no difference whether it simply
 wasn't registered or was prohibited by some rule at the registry.
 Application implementers should be aware that where DNS wildcards are
 used, the ability to successfully resolve a name does not guarantee
 that it was actually registered.

6. Front-end and User Interface Processing for Lookup

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

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 While there are strong arguments for any domain name that is placed
 "on the wire" -- transmitted between systems -- to be in the zero-
 ambiguity forms of A-labels, it is inevitable that programs that
 process domain names will encounter U-labels or variant forms.
 An application that implements the IDNA protocol [RFC5891] will
 always take any user input and convert it to a set of Unicode code
 points.  That user input may be acquired by any of several different
 input methods, all with differing conversion processes to be taken
 into consideration (e.g., typed on a keyboard, written by hand onto
 some sort of digitizer, spoken into a microphone and interpreted by a
 speech-to-text engine, etc.).  The process of taking any particular
 user input and mapping it into a Unicode code point may be a simple
 one: if a user strikes the "A" key on a US English keyboard, without
 any modifiers such as the "Shift" key held down, in order to draw a
 Latin small letter A ("a"), many (perhaps most) modern operating
 system input methods will produce to the calling application the code
 point U+0061, encoded in a single octet.
 Sometimes the process is somewhat more complicated: a user might
 strike a particular set of keys to represent a combining macron
 followed by striking the "A" key in order to draw a Latin small
 letter A with a macron above it.  Depending on the operating system,
 the input method chosen by the user, and even the parameters with
 which the application communicates with the input method, the result
 might be the code point U+0101 (encoded as two octets in UTF-8 or
 UTF-16, four octets in UTF-32, etc.), the code point U+0061 followed
 by the code point U+0304 (again, encoded in three or more octets,
 depending upon the encoding used) or even the code point U+FF41
 followed by the code point U+0304 (and encoded in some form).  These
 examples leave aside the issue of operating systems and input methods
 that do not use Unicode code points for their character set.
 In every case, applications (with the help of the operating systems
 on which they run and the input methods used) need to perform a
 mapping from user input into Unicode code points.
 IDNA2003 used a model whereby input was taken from the user, mapped
 (via whatever input method mechanisms were used) to a set of Unicode
 code points, and then further mapped to a set of Unicode code points
 using the Nameprep profile [RFC3491].  In this procedure, there are
 two separate mapping steps: first, a mapping done by the input method
 (which might be controlled by the operating system, the application,
 or some combination) and then a second mapping performed by the
 Nameprep portion of the IDNA protocol.  The mapping done in Nameprep
 includes a particular mapping table to re-map some characters to
 other characters, a particular normalization, and a set of prohibited
 characters.

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 Note that the result of the two-step mapping process means that the
 mapping chosen by the operating system or application in the first
 step might differ significantly from the mapping supplied by the
 Nameprep profile in the second step.  This has advantages and
 disadvantages.  Of course, the second mapping regularizes what gets
 looked up in the DNS, making for better interoperability between
 implementations that use the Nameprep mapping.  However, the
 application or operating system may choose mappings in their input
 methods, which when passed through the second (Nameprep) mapping
 result in characters that are "surprising" to the end user.
 The other important feature of IDNA2003 is that, with very few
 exceptions, it assumes that any set of Unicode code points provided
 to the Nameprep mapping can be mapped into a string of Unicode code
 points that are "sensible", even if that means mapping some code
 points to nothing (that is, removing the code points from the
 string).  This allowed maximum flexibility in input strings.
 The present version of IDNA (IDNA2008) differs significantly in
 approach from the original version.  First and foremost, it does not
 provide explicit mapping instructions.  Instead, it assumes that the
 application (perhaps via an operating system input method) will do
 whatever mapping it requires to convert input into Unicode code
 points.  This has the advantage of giving flexibility to the
 application to choose a mapping that is suitable for its user given
 specific user requirements, and avoids the two-step mapping of the
 original protocol.  Instead of a mapping, IDNA2008 provides a set of
 categories that can be used to specify the valid code points allowed
 in a domain name.
 In principle, an application ought to take user input of a domain
 name and convert it to the set of Unicode code points that represent
 the domain name the user intends.  As a practical matter, of course,
 determining user intent is a tricky business, so an application needs
 to choose a reasonable mapping from user input.  That may differ
 based on the particular circumstances of a user, depending on locale,
 language, type of input method, etc.  It is up to the application to
 make a reasonable choice.

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7. Migration from IDNA2003 and Unicode Version Synchronization

7.1. Design Criteria

 As mentioned above and in the IAB review and recommendations for IDNs
 [RFC4690], two key goals of the IDNA2008 design are:
 o  to enable applications to be agnostic about whether they are being
    run in environments supporting any Unicode version from 3.2
    onward.
 o  to permit incrementally adding new characters, character groups,
    scripts, and other character collections as they are incorporated
    into Unicode, doing so without disruption and, in the long term,
    without "heavy" processes (an IETF consensus process is required
    by the IDNA2008 specifications and is expected to be required and
    used until significant experience accumulates with IDNA operations
    and new versions of Unicode).

7.1.1. Summary and Discussion of IDNA Validity Criteria

 The general criteria for a label to be considered valid under IDNA
 are (the actual rules are rigorously defined in the Protocol
 [RFC5891] and Tables [RFC5892] documents):
 o  The characters are "letters", marks needed to form letters,
    numerals, or other code points used to write words in some
    language.  Symbols, drawing characters, and various notational
    characters are intended to be permanently excluded.  There is no
    evidence that they are important enough to Internet operations or
    internationalization to justify expansion of domain names beyond
    the general principle of "letters, digits, and hyphen".
    (Additional discussion and rationale for the symbol decision
    appears in Section 7.6.)
 o  Other than in very exceptional cases, e.g., where they are needed
    to write substantially any word of a given language, punctuation
    characters are excluded.  The fact that a word exists is not proof
    that it should be usable in a DNS label, and DNS labels are not
    expected to be usable for multiple-word phrases (although they are
    certainly not prohibited if the conventions and orthography of a
    particular language cause that to be possible).
 o  Characters that are unassigned (have no character assignment at
    all) in the version of Unicode being used by the registry or
    application are not permitted, even on lookup.  The issues
    involved in this decision are discussed in Section 7.7.

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 o  Any character that is mapped to another character by a current
    version of NFKC is prohibited as input to IDNA (for either
    registration or lookup).  With a few exceptions, this principle
    excludes any character mapped to another by Nameprep [RFC3491].
 The principles above drive the design of rules that are specified
 exactly in the Tables document.  Those rules identify the characters
 that are valid under IDNA.  The rules themselves are normative, and
 the tables are derived from them, rather than vice versa.

7.1.2. Labels in Registration

 Any label registered in a DNS zone must be validated -- i.e., the
 criteria for that label must be met -- in order for applications to
 work as intended.  This principle is not new.  For example, since the
 DNS was first deployed, zone administrators have been expected to
 verify that names meet "hostname" requirements [RFC0952] where those
 requirements are imposed by the expected applications.  Other
 applications contexts, such as the later addition of special service
 location formats [RFC2782] imposed new requirements on zone
 administrators.  For zones that will contain IDNs, support for
 Unicode version-independence requires restrictions on all strings
 placed in the zone.  In particular, for such zones (the exact rules
 appear in Section 4 of the Protocol document [RFC5891]):
 o  Any label that appears to be an A-label, i.e., any label that
    starts in "xn--", must be valid under IDNA, i.e., they must be
    valid A-labels, as discussed in Section 2 above.
 o  The Unicode tables (i.e., tables of code points, character
    classes, and properties) and IDNA tables (i.e., tables of
    contextual rules such as those that appear in the Tables
    document), must be consistent on the systems performing or
    validating labels to be registered.  Note that this does not
    require that tables reflect the latest version of Unicode, only
    that all tables used on a given system are consistent with each
    other.
 Under this model, registry tables will need to be updated (both the
 Unicode-associated tables and the tables of permitted IDN characters)
 to enable a new script or other set of new characters.  The registry
 will not be affected by newer versions of Unicode, or newly
 authorized characters, until and unless it wishes to support them.
 The zone administrator is responsible for verifying validity for IDNA
 as well as its local policies -- a more extensive set of checks than
 are required for looking up the labels.  Systems looking up or

Klensin Informational [Page 26] RFC 5894 IDNA Rationale August 2010

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

7.1.3. Labels in Lookup

 Any application processing a label through IDNA so it can be looked
 up in a DNS zone is required to (the exact rules appear in Section 5
 of the Protocol document [RFC5891]):
 o  Maintain IDNA and Unicode tables that are consistent with regard
    to versions, i.e., unless the application actually executes the
    classification rules in the Tables document [RFC5892], its IDNA
    tables must be derived from the version of Unicode that is
    supported more generally on the system.  As with registration, the
    tables need not reflect the latest version of Unicode, but they
    must be consistent.
 o  Validate the characters in labels to be looked up only to the
    extent of determining that the U-label does not contain
    "DISALLOWED" code points or code points that are unassigned in its
    version of Unicode.
 o  Validate the label itself for conformance with a small number of
    whole-label rules.  In particular, it must verify that:
  • there are no leading combining marks,
  • the Bidi conditions are met if right-to-left characters appear,
  • any required contextual rules are available, and
  • any contextual rules that are associated with joiner characters

(and CONTEXTJ characters more generally) are tested.

 o  Do not reject labels based on other contextual rules about
    characters, including mixed-script label prohibitions.  Such rules
    may be used to influence presentation decisions in the user
    interface, but not to avoid looking up domain names.
 To further clarify the rules about handling characters that require
 contextual rules, note that one can have a context-required character
 (i.e., one that requires a rule), but no rule.  In that case, the
 character is treated the same way DISALLOWED characters are treated,
 until and unless a rule is supplied.  That state is more or less
 equivalent to "the idea of permitting this character is accepted in
 principle, but it won't be permitted in practice until consensus is
 reached on a safe way to use it".

Klensin Informational [Page 27] RFC 5894 IDNA Rationale August 2010

 The ability to add a rule more or less exempts these characters from
 the prohibition against reclassifying characters from DISALLOWED to
 PVALID.
 And, obviously, "no rule" is different from "have a rule, but the
 test either succeeds or fails".
 Lookup applications that follow these rules, rather than having their
 own criteria for rejecting lookup attempts, are not sensitive to
 version incompatibilities with the particular zone registry
 associated with the domain name except for labels containing
 characters recently added to Unicode.
 An application or client that processes names according to this
 protocol and then resolves them in the DNS will be able to locate any
 name that is registered, as long as those registrations are valid
 under IDNA and its version of the IDNA tables is sufficiently up to
 date to interpret all of the characters in the label.  Messages to
 users should distinguish between "label contains an unallocated code
 point" and other types of lookup failures.  A failure on the basis of
 an old version of Unicode may lead the user to a desire to upgrade to
 a newer version, but will have no other ill effects (this is
 consistent with behavior in the transition to the DNS when some hosts
 could not yet handle some forms of names or record types).

7.2. Changes in Character Interpretations

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

7.2.1. Character Changes: Eszett and Final Sigma

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

Klensin Informational [Page 28] RFC 5894 IDNA Rationale August 2010

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

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

7.2.3. Character Changes and the Need for Transition

 The decision to eliminate mandatory and standardized mappings,
 including case folding, from the IDNA2008 protocol in order to make
 A-labels and U-labels idempotent made these characters problematic.
 If they were to be disallowed, important words and mnemonics could
 not be written in orthographically reasonable ways.  If they were to
 be permitted as distinct characters, there would be no information
 loss and registries would have more flexibility, but IDNA2003 and
 IDNA2008 lookups might result in different A-labels.
 With the understanding that there would be incompatibility either way
 but a judgment that the incompatibility was not significant enough to
 justify a prefix change, the Working Group concluded that Eszett and
 Final Form Sigma should be treated as distinct and Protocol-Valid
 characters.
 Since these characters are interpreted in different ways under the
 older and newer versions of IDNA, transition strategies and policies
 will be necessary.  Some actions can reasonably be taken by
 applications' client programs (those that perform lookup operations
 or cause them to be performed), but because of the diversity of
 situations and uses of the DNS, much of the responsibility will need
 to fall on registries.
 Registries, especially those maintaining zones for third parties,
 must decide how to introduce a new service in a way that does not
 create confusion or significantly weaken or invalidate existing
 identifiers.  This is not a new problem; registries were faced with
 similar issues when IDNs were introduced (potentially, and especially
 for Latin-based scripts, in conflict with existing labels that had
 been rendered in ASCII characters by applying more or less
 standardized conventions) and when other new forms of strings have
 been permitted as labels.

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7.2.4. Transition Strategies

 There are several approaches to the introduction of new characters or
 changes in interpretation of existing characters from their mapped
 forms in the earlier version of IDNA.  The transition issue is
 complicated because the forms of these labels after the
 ToUnicode(ToASCII()) translation in IDNA2003 not only remain valid
 but do not provide strong indications of what the registrant
 intended: a string containing "ss" could have simply been intended to
 be that string or could have been intended to contain an Eszett; a
 string containing lowercase Sigma could have been intended to contain
 Final Sigma (one might make heuristic guesses based on position in a
 string, but the long tradition of forming labels by concatenating
 words makes such heuristics unreliable), and strings that do not
 contain ZWJ or ZWNJ might have been intended to contain them.
 Without any preference or claim to completeness, some of these, all
 of which have been used by registries in the past for similar
 transitions, are:
 1.  Do not permit use of the newly available character at the
     registry level.  This might cause lookup failures if a domain
     name were to be written with the expectation of the IDNA2003
     mapping behavior, but would eliminate any possibility of false
     matches.
 2.  Hold a "sunrise"-like arrangement in which holders of labels
     containing "ss" in the Eszett case, lowercase Sigma in that case,
     or that might have contained ZWJ or ZWNJ in context, are given
     priority (and perhaps other benefits) for registering the
     corresponding string containing Eszett, Final Sigma, or the
     appropriate zero-width character respectively.
 3.  Adopt some sort of "variant" approach in which registrants obtain
     labels with both character forms.
 4.  Adopt a different form of "variant" approach in which
     registration of additional strings that would produce the same
     A-label if interpreted according to IDNA2003 is either not
     permitted at all or permitted only by the registrant who already
     has one of the names.
 5.  Ignore the issue and assume that the marketplace or other
     mechanisms will sort things out.
 In any event, a registry (at any level of the DNS tree) that chooses
 to permit labels to be registered that contains these characters, or
 considers doing so, will have to address the relationship with
 existing, possibly conflicting, labels in some way, just as

Klensin Informational [Page 30] RFC 5894 IDNA Rationale August 2010

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

7.3. Elimination of Character Mapping

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

7.4. The Question of Prefix Changes

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

7.4.1. Conditions Requiring a Prefix Change

 An IDN prefix change would have been needed if a given string would
 be looked up or otherwise interpreted differently depending on the
 version of the protocol or tables being used.  This IDNA upgrade
 would have required a prefix change if, and only if, one of the
 following four conditions were met:
 1.  The conversion of an A-label to Unicode (i.e., a U-label) would
     have yielded one string under IDNA2003 and a different string
     under IDNA2008.
 2.  In a significant number of cases, an input string that was valid
     under IDNA2003 and also valid under IDNA2008 would have yielded
     two different A-labels with the different versions.  This
     condition is believed to be essentially equivalent to the one
     above except for a very small number of edge cases that were not
     found to justify a prefix change (see Section 7.2).
     Note that if the input string was valid under one version and not
     valid under the other, this condition would not apply.  See the
     first item in Section 7.4.2, below.

Klensin Informational [Page 31] RFC 5894 IDNA Rationale August 2010

 3.  A fundamental change was made to the semantics of the string that
     would be inserted in the DNS, e.g., if a decision were made to
     try to include language or script information in the encoding in
     addition to the string itself.
 4.  A sufficiently large number of characters were added to Unicode
     so that the Punycode mechanism for block offsets would no longer
     reference the higher-numbered planes and blocks.  This condition
     is unlikely even in the long term and certain not to arise in the
     next several years.

7.4.2. Conditions Not Requiring a Prefix Change

 As a result of the principles described above, none of the following
 changes required a new prefix:
 1.  Prohibition of some characters as input to IDNA.  Such a
     prohibition might make names that were previously registered
     inaccessible, but did not change those names.
 2.  Adjustments in IDNA tables or actions, including normalization
     definitions, that affected characters that were already invalid
     under IDNA2003.
 3.  Changes in the style of the IDNA definition that did not alter
     the actions performed by IDNA.

7.4.3. Implications of Prefix Changes

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

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

7.5. Stringprep Changes and Compatibility

 The Nameprep specification [RFC3491], a key part of IDNA2003, is a
 profile of Stringprep [RFC3454].  While Nameprep is a Stringprep
 profile specific to IDNA, Stringprep is used by a number of other
 protocols.  Were Stringprep to have been modified by IDNA2008, those
 changes to improve the handling of IDNs could cause problems for
 non-DNS uses, most notably if they affected identification and
 authentication protocols.  Several elements of IDNA2008 give
 interpretations to strings prohibited under IDNA2003 or prohibit
 strings that IDNA2003 permitted.  Those elements include the new
 inclusion information in the Tables document [RFC5892], the reduction
 in the number of characters permitted as input for registration or
 lookup (Section 3), and even the changes in handling of right-to-left
 strings as described in the Bidi document [RFC5893].  IDNA2008 does
 not use Nameprep or Stringprep at all, so there are no side-effect
 changes to other protocols.
 It is particularly important to keep IDNA processing separate from
 processing for various security protocols because some of the
 constraints that are necessary for smooth and comprehensible use of
 IDNs may be unwanted or undesirable in other contexts.  For example,
 the criteria for good passwords or passphrases are very different
 from those for desirable IDNs: passwords should be hard to guess,
 while domain names should normally be easily memorable.  Similarly,
 internationalized Small Computer System Interface (SCSI) identifiers
 and other protocol components are likely to have different
 requirements than IDNs.

7.6. The Symbol Question

 One of the major differences between this specification and the
 original version of IDNA is that IDNA2003 permitted non-letter
 symbols of various sorts, including punctuation and line-drawing
 symbols, in the protocol.  They were always discouraged in practice.
 In particular, both the "IESG Statement" about IDNA and all versions
 of the ICANN Guidelines specify that only language characters be used
 in labels.  This specification disallows symbols entirely.  There are
 several reasons for this, which include:
 1.  As discussed elsewhere, the original IDNA specification assumed
     that as many Unicode characters as possible should be permitted,
     directly or via mapping to other characters, in IDNs.  This

Klensin Informational [Page 33] RFC 5894 IDNA Rationale August 2010

     specification operates on an inclusion model, extrapolating from
     the original "hostname" rules (LDH, see the Definitions document
     [RFC5890]) -- which have served the Internet very well -- to a
     Unicode base rather than an ASCII base.
 2.  Symbol names are more problematic than letters because there may
     be no general agreement on whether a particular glyph matches a
     symbol; there are no uniform conventions for naming; variations
     such as outline, solid, and shaded forms may or may not exist;
     and so on.  As just one example, consider a "heart" symbol as it
     might appear in a logo that might be read as "I love...".  While
     the user might read such a logo as "I love..." or "I heart...",
     considerable knowledge of the coding distinctions made in Unicode
     is needed to know that there is more than one "heart" character
     (e.g., U+2665, U+2661, and U+2765) and how to describe it.  These
     issues are of particular importance if strings are expected to be
     understood or transcribed by the listener after being read out
     loud.
 3.  Design of a screen reader used by blind Internet users who must
     listen to renderings of IDN domain names and possibly reproduce
     them on the keyboard becomes considerably more complicated when
     the names of characters are not obvious and intuitive to anyone
     familiar with the language in question.
 4.  As a simplified example of this, assume one wanted to use a
     "heart" or "star" symbol in a label.  This is problematic because
     those names are ambiguous in the Unicode system of naming (the
     actual Unicode names require far more qualification).  A user or
     would-be registrant has no way to know -- absent careful study of
     the code tables -- whether it is ambiguous (e.g., where there are
     multiple "heart" characters) or not.  Conversely, the user seeing
     the hypothetical label doesn't know whether to read it -- try to
     transmit it to a colleague by voice -- as "heart", as "love", as
     "black heart", or as any of the other examples below.
 5.  The actual situation is even worse than this.  There is no
     possible way for a normal, casual, user to tell the difference
     between the hearts of U+2665 and U+2765 and the stars of U+2606
     and U+2729 without somehow knowing to look for a distinction.  We
     have a white heart (U+2661) and few black hearts.  Consequently,
     describing a label as containing a heart is hopelessly ambiguous:
     we can only know that it contains one of several characters that
     look like hearts or have "heart" in their names.  In cities where
     "Square" is a popular part of a location name, one might well
     want to use a square symbol in a label as well and there are far
     more squares of various flavors in Unicode than there are hearts
     or stars.

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

7.7. Migration between Unicode Versions: Unassigned Code Points

 In IDNA2003, labels containing unassigned code points are looked up
 on the assumption that, if they appear in labels and can be mapped
 and then resolved, the relevant standards must have changed and the
 registry has properly allocated only assigned values.
 In the IDNA2008 protocol, strings containing unassigned code points
 must not be either looked up or registered.  In summary, the status
 of an unassigned character with regard to the DISALLOWED,
 PROTOCOL-VALID, and CONTEXTUAL RULE REQUIRED categories cannot be
 evaluated until a character is actually assigned and known.  There
 are several reasons for this, with the most important ones being:
 o  Tests involving the context of characters (e.g., some characters
    being permitted only adjacent to others of specific types) and
    integrity tests on complete labels are needed.  Unassigned code
    points cannot be permitted because one cannot determine whether
    particular code points will require contextual rules (and what
    those rules should be) before characters are assigned to them and
    the properties of those characters fully understood.
 o  It cannot be known in advance, and with sufficient reliability,
    whether a newly assigned code point will be associated with a
    character that would be disallowed by the rules in the Tables
    document [RFC5892] (such as a compatibility character).  In
    IDNA2003, since there is no direct dependency on NFKC (many of the
    entries in Stringprep's tables are based on NFKC, but IDNA2003
    depends only on Stringprep), allocation of a compatibility
    character might produce some odd situations, but it would not be a
    problem.  In IDNA2008, where compatibility characters are
    DISALLOWED unless character-specific exceptions are made,
    permitting strings containing unassigned characters to be looked
    up would violate the principle that characters in DISALLOWED are
    not looked up.
 o  The Unicode Standard specifies that an unassigned code point
    normalizes (and, where relevant, case folds) to itself.  If the
    code point is later assigned to a character, and particularly if
    the newly assigned code point has a combining class that

Klensin Informational [Page 35] RFC 5894 IDNA Rationale August 2010

    determines its placement relative to other combining characters,
    it could normalize to some other code point or sequence.
 It is possible to argue that the issues above are not important and
 that, as a consequence, it is better to retain the principle of
 looking up labels even if they contain unassigned characters because
 all of the important scripts and characters have been coded as of
 Unicode 5.2 (or even earlier), and hence unassigned code points will
 be assigned only to obscure characters or archaic scripts.
 Unfortunately, that does not appear to be a safe assumption for at
 least two reasons.  First, much the same claim of completeness has
 been made for earlier versions of Unicode.  The reality is that a
 script that is obscure to much of the world may still be very
 important to those who use it.  Cultural and linguistic preservation
 principles make it inappropriate to declare the script of no
 importance in IDNs.  Second, we already have counterexamples, e.g.,
 in the relationships associated with new Han characters being added
 (whether in the BMP or in Unicode Plane 2).
 Independent of the technical transition issues identified above, it
 can be observed that any addition of characters to an existing script
 to make it easier to use or to better accommodate particular
 languages may lead to transition issues.  Such additions may change
 the preferred form for writing a particular string, changes that may
 be reflected, e.g., in keyboard transition modules that would
 necessarily be different from those for earlier versions of Unicode
 where the newer characters may not exist.  This creates an inherent
 transition problem because attempts to access labels may use either
 the old or the new conventions, requiring registry action whether or
 not the older conventions were used in labels.  The need to consider
 transition mechanisms is inherent to evolution of Unicode to better
 accommodate writing systems and is independent of how IDNs are
 represented in the DNS or how transitions among versions of those
 mechanisms occur.  The requirement for transitions of this type is
 illustrated by the addition of Malayalam Chillu in Unicode 5.1.0.

7.8. Other Compatibility Issues

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

Klensin Informational [Page 36] RFC 5894 IDNA Rationale August 2010

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

8. Name Server Considerations

8.1. Processing Non-ASCII Strings

 Existing DNS servers do not know the IDNA rules for handling
 non-ASCII forms of IDNs, and therefore need to be shielded from them.
 All existing channels through which names can enter a DNS server
 database (for example, master files (as described in RFC 1034) and
 DNS update messages [RFC2136]) could not be IDNA-aware because they
 predate IDNA.  Other sections of this document provide the needed
 shielding by ensuring that internationalized domain names entering
 DNS server databases through such channels have already been
 converted to their equivalent ASCII A-label forms.
 Because of the distinction made between the algorithms for
 Registration and Lookup in Sections 4 and 5 (respectively) of the
 Protocol document [RFC5891] (a domain name containing only ASCII code
 points cannot be converted to an A-label), there cannot be more than
 one A-label form for any given U-label.
 As specified in clarifications to the DNS specification [RFC2181],
 the DNS protocol explicitly allows domain labels to contain octets
 beyond the ASCII range (0000..007F), and this document does not
 change that.  However, although the interpretation of octets
 0080..00FF is well-defined in the DNS, many application protocols
 support only ASCII labels and there is no defined interpretation of
 these non-ASCII octets as characters and, in particular, no
 interpretation of case-independent matching for them (e.g., see the
 clarification on DNS case insensitivity [RFC4343]).  If labels
 containing these octets are returned to applications, unpredictable
 behavior could result.  The A-label form, which cannot contain those
 characters, is the only standard representation for internationalized
 labels in the DNS protocol.

8.2. Root and Other DNS Server Considerations

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

Klensin Informational [Page 37] RFC 5894 IDNA Rationale August 2010

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

9. Internationalization Considerations

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

10. IANA Considerations

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

10.1. IDNA Character Registry

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

Klensin Informational [Page 38] RFC 5894 IDNA Rationale August 2010

10.2. IDNA Context Registry

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

10.3. IANA Repository of IDN Practices of TLDs

 This registry, historically described as the "IANA Language Character
 Set Registry" or "IANA Script Registry" (both somewhat misleading
 terms), is maintained by IANA at the request of ICANN.  It is used to
 provide a central documentation repository of the IDN policies used
 by top level domain (TLD) registries who volunteer to contribute to
 it and is used in conjunction with ICANN Guidelines for IDN use.
 It is not an IETF-managed registry and, while the protocol changes
 specified here may call for some revisions to the tables, IDNA2008
 has no direct effect on that registry and no IANA action is required
 as a result.

11. Security Considerations

11.1. General Security Issues with IDNA

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

12. Acknowledgments

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

Klensin Informational [Page 39] RFC 5894 IDNA Rationale August 2010

 Campbell, Martin Duerst, Subramanian Moonesamy, Peter Saint-Andre,
 and Dan Winship, for catching specific errors and recommending
 corrections.
 A meeting was held on 30 January 2008 to attempt to reconcile
 differences in perspective and terminology about this set of
 specifications between the design team and members of the Unicode
 Technical Consortium.  The discussions at and subsequent to that
 meeting were very helpful in focusing the issues and in refining the
 specifications.  The active participants at that meeting were (in
 alphabetic order, as usual) Harald Alvestrand, Vint Cerf, Tina Dam,
 Mark Davis, Lisa Dusseault, Patrik Faltstrom (by telephone), Cary
 Karp, John Klensin, Warren Kumari, Lisa Moore, Erik van der Poel,
 Michel Suignard, and Ken Whistler.  We express our thanks to Google
 for support of that meeting and to the participants for their
 contributions.
 Useful comments and text on the working group versions of the working
 draft were received from many participants in the IETF "IDNABIS"
 working group and a number of document changes resulted from mailing
 list discussions made by that group.  Marcos Sanz provided specific
 analysis and suggestions that were exceptionally helpful in refining
 the text, as did Vint Cerf, Martin Duerst, Andrew Sullivan, and Ken
 Whistler.  Lisa Dusseault provided extensive editorial suggestions
 during the spring of 2009, most of which were incorporated.

13. Contributors

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

14. References

14.1. Normative References

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

Klensin Informational [Page 40] RFC 5894 IDNA Rationale August 2010

 [RFC3492]    Costello, A., "Punycode: A Bootstring encoding of
              Unicode for Internationalized Domain Names in
              Applications (IDNA)", RFC 3492, March 2003.
 [RFC5890]    Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document
              Framework", RFC 5890, August 2010.
 [RFC5891]    Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.
 [RFC5892]    Faltstrom, P., "The Unicode Code Points and
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5892, August 2010.
 [RFC5893]    Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5893, August 2010.
 [Unicode52]  The Unicode Consortium.  The Unicode Standard, Version
              5.2.0, defined by: "The Unicode Standard, Version
              5.2.0", (Mountain View, CA: The Unicode Consortium,
              2009. ISBN 978-1-936213-00-9).
              <http://www.unicode.org/versions/Unicode5.2.0/>.

14.2. Informative References

 [IDNA2008-Mapping]
              Resnick, P. and P. Hoffman, "Mapping Characters in
              Internationalized Domain Names for Applications (IDNA)",
              Work in Progress, April 2010.
 [RFC0952]    Harrenstien, K., Stahl, M., and E. Feinler, "DoD
              Internet host table specification", RFC 952,
              October 1985.
 [RFC1034]    Mockapetris, P., "Domain names - concepts and
              facilities", STD 13, RFC 1034, November 1987.
 [RFC1035]    Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.
 [RFC1123]    Braden, R., "Requirements for Internet Hosts -
              Application and Support", STD 3, RFC 1123, October 1989.
 [RFC2136]    Vixie, P., Thomson, S., Rekhter, Y., and J.  Bound,
              "Dynamic Updates in the Domain Name System (DNS
              UPDATE)", RFC 2136, April 1997.

Klensin Informational [Page 41] RFC 5894 IDNA Rationale August 2010

 [RFC2181]    Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.
 [RFC2277]    Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, January 1998.
 [RFC2671]    Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
              RFC 2671, August 1999.
 [RFC2782]    Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)",
              RFC 2782, February 2000.
 [RFC3454]    Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.
 [RFC3491]    Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
              Profile for Internationalized Domain Names (IDN)",
              RFC 3491, March 2003.
 [RFC3743]    Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint
              Engineering Team (JET) Guidelines for Internationalized
              Domain Names (IDN) Registration and Administration for
              Chinese, Japanese, and Korean", RFC 3743, April 2004.
 [RFC3987]    Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.
 [RFC4290]    Klensin, J., "Suggested Practices for Registration of
              Internationalized Domain Names (IDN)", RFC 4290,
              December 2005.
 [RFC4343]    Eastlake, D., "Domain Name System (DNS) Case
              Insensitivity Clarification", RFC 4343, January 2006.
 [RFC4690]    Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review
              and Recommendations for Internationalized Domain Names
              (IDNs)", RFC 4690, September 2006.
 [RFC4713]    Lee, X., Mao, W., Chen, E., Hsu, N., and J.  Klensin,
              "Registration and Administration Recommendations for
              Chinese Domain Names", RFC 4713, October 2006.

Klensin Informational [Page 42] RFC 5894 IDNA Rationale August 2010

 [Unicode-UAX31]
              The Unicode Consortium, "Unicode Standard Annex #31:
              Unicode Identifier and Pattern Syntax, Revision 11",
              September 2009,
              <http://www.unicode.org/reports/tr31/tr31-11.html>.
 [Unicode-UTS39]
              The Unicode Consortium, "Unicode Technical Standard #39:
              Unicode Security Mechanisms, Revision 2", August 2006,
              <http://www.unicode.org/reports/tr39/tr39-2.html>.

Author's Address

 John C Klensin
 1770 Massachusetts Ave, Ste 322
 Cambridge, MA  02140
 USA
 Phone: +1 617 245 1457
 EMail: john+ietf@jck.com

Klensin Informational [Page 43]

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