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RFC 5894

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

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Top   ToC   RFC5894 - Page 1
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


   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
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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
   ( 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  Contextual Restrictions  . . . . . . . . . . . . . 11  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

   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
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   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

   o  to prevent confusion with pre-IDNA coding forms;

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

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

   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
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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

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

   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
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   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


   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
Top   ToC   RFC5894 - Page 11
   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.


   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.  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

   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.  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.


   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).
Top   ToC   RFC5894 - Page 13
   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

   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.


   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
Top   ToC   RFC5894 - Page 14
   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.
Top   ToC   RFC5894 - Page 15
3.3.  Layered Restrictions: Tables, Context, Registration, and

   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 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.
Top   ToC   RFC5894 - Page 16
   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  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

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,
Top   ToC   RFC5894 - Page 17
   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

   All protocols that use domain name slots (see Section 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
Top   ToC   RFC5894 - Page 18
   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.
Top   ToC   RFC5894 - Page 19
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
Top   ToC   RFC5894 - Page 20
   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-
Top   ToC   RFC5894 - Page 21
   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
Top   ToC   RFC5894 - Page 22
   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

(page 22 continued on part 2)

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