Network Working Group                                         J. Klensin
Internet-Draft                                             March 6, 2009
Intended status: Informational
Expires: September 7, 2009

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

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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Context and Overview . . . . . . . . . . . . . . . . . . .  4
     1.2.  Discussion Forum . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
       1.3.1.  Documents and Standards  . . . . . . . . . . . . . . .  5
       1.3.2.  DNS "Name" Terminology . . . . . . . . . . . . . . . .  5
       1.3.3.  New Terminology and Restrictions . . . . . . . . . . .  6
     1.4.  Objectives . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.5.  Applicability and Function of IDNA . . . . . . . . . . . .  7
     1.6.  Comprehensibility of IDNA Mechanisms and Processing  . . .  8
   2.  Processing in IDNA2008 . . . . . . . . . . . . . . . . . . . .  9
   3.  Permitted Characters: An Inclusion List  . . . . . . . . . . . 10
     3.1.  A Tiered Model of Permitted Characters and Labels  . . . . 10
       3.1.1.  PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . . 11
       3.1.2.  Characters Valid Only in Context With Others . . . . . 11  Rules and Their Application  . . . . . . . . . . . 12
       3.1.3.  DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 12
       3.1.4.  UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 13
     3.2.  Registration Policy  . . . . . . . . . . . . . . . . . . . 14
     3.3.  Layered Restrictions: Tables, Context, Registration,
           Applications . . . . . . . . . . . . . . . . . . . . . . . 14
   4.  Issues that Constrain Possible Solutions . . . . . . . . . . . 15
     4.1.  Display and Network Order  . . . . . . . . . . . . . . . . 15
     4.2.  Entry and Display in Applications  . . . . . . . . . . . . 16
     4.3.  Linguistic Expectations: Ligatures, Digraphs, and
           Alternate Character Forms  . . . . . . . . . . . . . . . . 17
     4.4.  Case Mapping and Related Issues  . . . . . . . . . . . . . 20
     4.5.  Right to Left Text . . . . . . . . . . . . . . . . . . . . 20
   5.  IDNs and the Robustness Principle  . . . . . . . . . . . . . . 21

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   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.  General 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
     7.3.  More Flexibility in User Agents  . . . . . . . . . . . . . 30
     7.4.  The Question of Prefix Changes . . . . . . . . . . . . . . 31
       7.4.1.  Conditions Requiring a Prefix Change . . . . . . . . . 32
       7.4.2.  Conditions Not Requiring a Prefix Change . . . . . . . 32
       7.4.3.  Implications of Prefix Changes . . . . . . . . . . . . 33
     7.5.  Stringprep Changes and Compatibility . . . . . . . . . . . 33
     7.6.  The Symbol Question  . . . . . . . . . . . . . . . . . . . 34
     7.7.  Migration Between Unicode Versions: Unassigned Code
           Points . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     7.8.  Other Compatibility Issues . . . . . . . . . . . . . . . . 37
   8.  Name Server Considerations . . . . . . . . . . . . . . . . . . 37
     8.1.  Processing Non-ASCII Strings . . . . . . . . . . . . . . . 37
     8.2.  DNSSEC Authentication of IDN Domain Names  . . . . . . . . 38
     8.3.  Root and other DNS Server Considerations . . . . . . . . . 38
   9.  Internationalization Considerations  . . . . . . . . . . . . . 39
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 39
     10.1. IDNA Character Registry  . . . . . . . . . . . . . . . . . 39
     10.2. IDNA Context Registry  . . . . . . . . . . . . . . . . . . 39
     10.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 39
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 40
     11.1. General Security Issues with IDNA  . . . . . . . . . . . . 40
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 40
   13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 41
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 41
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 41
     14.2. Informative References . . . . . . . . . . . . . . . . . . 42
   Appendix A.  Change Log  . . . . . . . . . . . . . . . . . . . . . 44
     A.1.  Changes between Version -00 and Version -01 of
           draft-ietf-idnabis-rationale . . . . . . . . . . . . . . . 44
     A.2.  Version -02  . . . . . . . . . . . . . . . . . . . . . . . 45
     A.3.  Version -03  . . . . . . . . . . . . . . . . . . . . . . . 45
     A.4.  Version -04  . . . . . . . . . . . . . . . . . . . . . . . 46
     A.5.  Version -05  . . . . . . . . . . . . . . . . . . . . . . . 46
     A.6.  Version -06  . . . . . . . . . . . . . . . . . . . . . . . 46
     A.7.  Version -07  . . . . . . . . . . . . . . . . . . . . . . . 47
     A.8.  Version -08  . . . . . . . . . . . . . . . . . . . . . . . 47
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 47

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1.  Introduction

1.1.  Context and Overview

   The original standards for Internationalized Domain Names (IDNs) were
   completed and deployed starting in 2003.  Those standards are known
   as Internationalized Domain Names in Applications (IDNA), taken from
   the name of the highest level standard within the group, RFC 3490
   [RFC3490].  After those standards were deployed, a number of issues
   arose that led to a call for a new version of the IDNA protocol and
   the associated tables, including a subset of those described in a
   recent IAB report [RFC4690] and the need to update the system to deal
   with newer versions of Unicode.  This document further explains the
   issues that have been encountered when they are important to
   understanding of the revised protocols.  It also provides an overview
   of the new IDNA model and explanatory material for it.  Additional
   explanatory material for the specific components of the proposals
   appears with the associated documents.

   This document and the associated ones are written from the
   perspective of an IDNA-aware user, application, or implementation.
   While they may reiterate fundamental DNS rules and requirements for
   the convenience of the reader, they make no attempt to be
   comprehensive about DNS principles and should not be considered as a
   substitute for a thorough understanding of the DNS protocols and

   A good deal of the background material that appeared in RFC 3490
   [RFC3490] has been removed from this update.  That material is either
   of historical interest only or has been covered from a more recent
   perspective in RFC 4690 [RFC4690].

   This document is not normative.  The information it provides is
   intended to make the rules, tables, and protocol easier to understand
   and to provide overview information and suggestions for zone
   administrators and others who need to make policy, deployment, and
   similar decisions about IDNs.

1.2.  Discussion Forum

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

   IDNA2008 is being discussed in the IETF "idnabis" Working Group and
   on the mailing list

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1.3.  Terminology

   Terminology that is critical for understanding this document and the
   rest of the documents that make up IDNA2008, appears in
   [IDNA2008-Defs].  That document also contains roadmap to the IDNA2008
   document collection.  No attempt should be made to understand this
   document without the definitions and concepts that appear there.

1.3.1.  Documents and Standards

   This document uses the term "IDNA2003" to refer to the set of
   standards that make up and support the version of IDNA published in
   2003, i.e., those commonly known as the IDNA base specification
   [RFC3490], Nameprep [RFC3491], Punycode [RFC3492], and Stringprep
   [RFC3454].  In this document, those names are used to refer,
   conceptually, to the individual documents, with the base IDNA
   specification called just "IDNA".

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

1.3.2.  DNS "Name" Terminology

   These documents depart from historical DNS terminology and usage in
   one important respect.  Over the years, the community has talked very
   casually about "names" in the DNS, beginning with calling it "the
   domain name system".  That terminology is fine in the very precise
   sense that the identifiers of the DNS do provide names for objects
   and addresses.  But, in the context of IDNs, the term has introduced
   some confusion, confusion that has increased further as people have
   begun to speak of DNS labels in terms of the words or phrases of
   various natural languages.

   Historically, many, perhaps most, of the "names" in the DNS have been
   mnemonics to identify some particular concept, object, or
   organization.  They are typically derived from, or 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

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   broad range of mnemonics in ways that are as natural as possible in a
   very broad range of scripts.

1.3.3.  New Terminology and Restrictions

   These documents introduce new terminology, and precise definitions,
   for the terms "U-label", "A-Label", LDH-label (to which all valid
   pre-IDNA host names conformed), Reserved-LDH-label (R-LDH-label), XN-
   label, Fake-A-Label, and Non-Reserved-LDH-label (NR-LDH-label).

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

   These definitions are illustrated in Figure 1 of the Definitions
   Document [IDNA2008-Defs].  R-LDH-labels contain "--" in the third and
   fourth character from the beginning of the label.  In IDNA-aware
   applications, only a subset of these reserved labels is permitted to
   be used, namely the A-label subset.  A-labels are a subset of the
   R-LDH-labels that begin with the case-insensitive (?) string "xn--".
   Labels that bear this prefix but which are not otherwise valid fall
   into the "Fake-A-label" category.  The non-reserved labels (NR-LDH-
   labels) are implicitly valid since they do not trigger any
   resemblance to IDNA-landr NR-LDH-labels.

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

   o  to prevent confusion with pre-IDNA coding forms;

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

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

1.4.  Objectives

   The intent of the IDNA revision effort, and hence of this document
   and the associated ones, is to increase the usability and
   effectiveness of internationalized domain names (IDNs) while
   preserving or strengthening the integrity of references that use
   them.  The original "hostname" character definitions (see, e.g.,
   [RFC0810]) struck a balance between the creation of useful mnemonics
   and the introduction of parsing problems or general confusion in the
   contexts in which domain names are used.  The objective of IDNA2008
   is to preserve that balance while expanding the character repertoire

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   to include extended versions of Roman-derived scripts and scripts
   that are not Roman in origin.  No work of this sort is able to
   completely eliminate sources of visual or textual confusion: such
   confusion is possible even under the original host naming rules where
   only ASCII characters were permitted.  However, through the
   application of different techniques at different points (see
   Section 3.3), it should be possible to keep problems to an acceptable
   minimum.  One consequence of this general objective is that the
   desire of some user or marketing community to use a particular string
   --whether the reason is to try to write sentences of particular
   languages in the DNS, to express a facsimile of the symbol for a
   brand, or for some other purpose-- is not a primary goal within the
   context of applications in the domain name space.

1.5.  Applicability and Function of IDNA

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

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

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

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

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   to apply an internationalized domain name in any form or coding
   acceptable under that method.  While processing must be performed
   prior to or after access to the DNS, IDNA requires no changes to the
   DNS protocol or any DNS servers or the resolvers on user's computers.

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

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

1.6.  Comprehensibility of IDNA Mechanisms and Processing

   One of the major goals of this work 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 themselves important motivations and design goals
   for this effort.  The effort includes some new terminology and a
   revised and extended model, both covered in this section, and some
   more specific protocol, processing, and table modifications.  Details
   of the latter appear in other documents (see [IDNA2008-Defs]).

   Several issues are inherent in the application of IDNs and, indeed,
   almost any other system that tries to handle international characters
   and concepts.  They range from the apparently trivial --e.g., one
   cannot display a character for which one does not have a font
   available locally-- to the more complex and subtle.  Many people have
   observed that internationalization is just a tool to enable effective
   localization while permitting some global uniformity.  Issues of
   display, of exactly how various strings and characters are entered,
   and so on are inherently issues about localization and user interface

   A protocol such as IDNA can only assume that such operations as data
   entry and reconciliation of differences in character forms are
   possible.  It may make some recommendations about how display might
   work when characters and fonts are not available, but they can only

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   be general recommendations and, because display functions are rarely
   controlled by the types of applications that would call upon IDNA,
   will rarely be very effective.

   However, shifting responsibility for character mapping and other
   adjustments from the protocol (where it was located in IDNA2003) to
   the user interface or processing before invoking IDNA raises issues
   about both what that processing should do and about compatibility for
   references prepared in an IDNA2003 context.  Those issues are
   discussed in Section 6.

   Operations for converting between local character sets and normalized
   Unicode are part of this general set of user interface issues.  The
   conversion is obviously not required at all in a Unicode-native
   system that maintains all strings in Normalization Form C (NFC).
   (See [Unicode-UAX15] for precise definitions of NFC and NFKC if
   needed.)  It may, however, involve some complexity in a system that
   is not Unicode-native, especially if the elements of the local
   character set do not map exactly and unambiguously into Unicode
   characters or do so in a way that is not completely stable over time.
   Perhaps more important, if a label being converted to a local
   character set contains Unicode characters that have no correspondence
   in that character set, the application may have to apply special,
   locally-appropriate, methods to avoid or reduce loss of information.

   Depending on the system involved, the major difficulty may not lie in
   the mapping but in accurately identifying the incoming character set
   and then applying the correct conversion routine.  If a local
   operating system uses one of the ISO 8859 character sets or an
   extensive national or industrial system such as GB18030 [GB18030] or
   BIG5 [BIG5], one must correctly identify the character set in use
   before converting to Unicode even though those character coding
   systems are substantially or completely Unicode-compatible (i.e., all
   of the code points in them have an exact and unique mapping to
   Unicode code points).  It may be even more difficult when the
   character coding system in local use is based on conceptually
   different assumptions than those used by Unicode about, e.g., 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.

2.  Processing in IDNA2008

   These specifications separate Domain Name Registration and Lookup in
   the protocol specification.  Doing so reflects current practice in
   which per-registry restrictions and special processing are applied at

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   registration time but not during lookup.  Even more important in the
   longer term, it facilitates incremental addition of permitted
   character groups to avoid freezing on one particular version of

   The actual registration and lookup protocols for IDNA2008 are
   specified in [IDNA2008-Protocol].

3.  Permitted Characters: An Inclusion List

   This section provides an overview of the model used to establish the
   algorithm and character lists of [IDNA2008-Tables] and describes the
   names and applicability of the categories used there.  Note that the
   inclusion of a character in the first category group (Section 3.1.1)
   does not imply that it can be used indiscriminately; some characters
   are associated with contextual rules that must be applied as well.

   The information given in this section is provided to make the rules,
   tables, and protocol easier to understand.  The normative generating
   rules that correspond to this informal discussion appear in
   [IDNA2008-Tables] and the rules that actually determine what labels
   can be registered or looked up are in [IDNA2008-Protocol].

3.1.  A Tiered Model of Permitted Characters and Labels

   Moving to an inclusion model requires respecifying the list of
   characters that are permitted in IDNs.  In IDNA2003, the role and
   utility of characters are independent of context and fixed forever
   (or until the standard is replaced).  Making completely context-
   independent rules globally has proven 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(s) in others.  IDNA2003 prohibited those types of
   characters entirely.  But the restrictions led to a consensus that
   under some conditions, these "joiner" characters were legitimately
   needed to allow useful mnemonics for some languages and scripts.  The
   requirement to support those characters but limit their use to very
   specific contexts was reinforced by the observation that handling of
   particular characters across the languages that use a script, or the
   use of similar or identical-looking characters in different scripts,
   is more complex than many people believed it was several years ago.

   Independently of the characters chosen (see next subsection), the
   approach is to divide the characters that appear in Unicode into
   three categories:

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   Characters identified as "PROTOCOL-VALID" (often abbreviated
   "PVALID") are, in general, permitted by IDNA for all uses in IDNs.
   Their use may be restricted by rules about the context in which they
   appear or by other rules that apply to the entire label in which they
   are to be embedded.  For example, any label that contains a character
   in this category that has a "right-to-left" property must be used in
   context with the "Bidi" rules (see [IDNA2008-Bidi]).

   The term "PROTOCOL-VALID" is used to stress the fact that the
   presence of a character in this category does not imply that a given
   registry need accept registrations containing any of the characters
   in the category.  Registries are still expected to apply judgment
   about labels they will accept and to maintain rules consistent with
   those judgments (see [IDNA2008-Protocol] and Section 3.3).

   Characters that are placed in the "PROTOCOL-VALID" category are
   expected to never be removed from it or reclassified.  While
   theoretically characters could be removed from Unicode, such removal
   would be inconsistent with the Unicode stability principles (see
   [Unicode51], Appendix F) and hence should never occur.

3.1.2.  Characters Valid Only in Context With Others

   Some characters may be unsuitable for general use in IDNs but
   necessary for the plausible support of some scripts.  The two most
   commonly-cited examples are the zero-width joiner and non-joiner
   characters (ZWJ, U+200D and ZWNJ, U+200C), but provisions for
   unambiguous labels may require that other characters be restricted to
   particular contexts.  For example, the ASCII hyphen is not permitted
   to start or end a label, whether that label contains non-ASCII
   characters or not.  Contextual Restrictions

   These characters must not appear in IDNs without additional
   restrictions, typically because they have no visible consequences in
   most scripts but affect format or presentation in a few others or
   because they are combining characters that are safe for use only in
   conjunction with particular characters or scripts.  In order to
   permit them to be used at all, they are specially identified as
   "CONTEXTUAL RULE REQUIRED" and, when adequately understood,
   associated with a rule.  In addition, the rule will define whether it
   is to be applied on lookup as well as registration.  A distinction is
   made between characters that indicate or prohibit joining (known as
   "CONTEXT-JOINER" or "CONTEXTJ") and other characters requiring
   contextual treatment ("CONTEXT-OTHER" or "CONTEXTO").  Only the

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   former require full testing at lookup time.

   It is important to note that these contextual rules cannot prevent
   all uses of the relevant characters that might be confusing or
   problematic.  What they are expected do is to confine applicability
   of the characters to scripts (and narrower contexts) where zone
   administrators are knowledgeable enough about the use of those
   characters to be prepared to deal with them appropriately.  For
   example, a registry dealing with an Indic script that requires ZWJ
   and/or ZWNJ as part of the writing system is expected to understand
   where the characters have visible effect and where they do not and to
   make registration rules accordingly.  By contrast, a registry dealing
   with Latin or Cyrillic script might not be actively aware that the
   characters exist, much less about the consequences of embedding them
   in labels drawn from those scripts.  Rules and Their Application

   The actual rules may be DEFINED or NULL.  If present, they may have
   values of "True" (character may be used in any position in any
   label), "False" (character may not be used in any label), or may be a
   set of procedural rules that specify the context in which the
   character is permitted.

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

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

   The description of the syntax of the rules, and the rules themselves,
   appears in [IDNA2008-Tables]. [[anchor11: ???  Section number would
   be good here.]]


   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

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   these characters are problematic for use in IDNs (such as the
   FRACTION SLASH character, U+2044), while some of them (such as the
   various HEART symbols, e.g., U+2665, U+2661, and U+2765, see
   Section 7.6) simply fall outside the conventions for typical
   identifiers (basically letters and numbers).

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

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

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

   o  The character is a compatibility equivalent for another character.
      In slightly more precise Unicode terms, application of
      normalization method NFKC to the character yields some other

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

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


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

   The rationale for restricting the processing of UNASSIGNED characters
   is simply that if such characters were permitted to be looked up, for
   example, and were later assigned, but subject to some set of

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   contextual rules, un-updated instances of IDNA-aware software might
   permit lookup of labels containing the previously-unassigned
   characters while updated versions of IDNA-aware software might
   restrict their use in lookup, depending on the contextual rules.  It
   should be clear that under no circumstance should an UNASSIGNED
   character be permitted in a label to be registered as part of a
   domain name.

3.2.  Registration Policy

   While these recommendations cannot and should not define registry
   policies, registries should develop and apply additional restrictions
   as needed to reduce confusion and other problems.  For example, it is
   generally believed that labels containing characters from more than
   one script are a bad practice although there may be some important
   exceptions to that principle.  Some registries may choose to restrict
   registrations to characters drawn from a very small number of
   scripts.  For many scripts, the use of variant techniques such as
   those as described in RFC 3843 [RFC3743] and RFC 4290 [RFC4290], and
   illustrated for Chinese by the tables described in RFC 4713 [RFC4713]
   may be helpful in reducing problems that might be perceived by users.

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

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

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

   The essence of the character rules in IDNA2008 is based on the
   realization that there is no single magic bullet for any of the
   issues associated with a multiscript DNS.  Instead, the
   specifications define a variety of approaches that, together,
   constitute multiple lines of defense against ambiguity in identifiers
   and loss of referential integrity.  The actual character tables are
   the first mechanism, protocol rules about how those characters are

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   applied or restricted in context are the second, and those two in
   combination constitute the limits of what can be done by a protocol
   alone.  As discussed in the previous section (Section 3.2),
   registries are expected to restrict what they permit to be
   registered, devising and using rules that are designed to optimize
   the balance between confusion and risk on the one hand and maximum
   expressiveness in mnemonics on the other.

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

4.  Issues that Constrain Possible Solutions

4.1.  Display and Network Order

   The correct treatment of domain names requires a clear distinction
   between Network Order (the order in which the code points are sent in
   protocols) and Display Order (the order in which the code points are
   displayed on a screen or paper).  The order of labels in a domain
   name that contains characters that are normally written right to left
   is discussed in [IDNA2008-Bidi].  In particular, there are questions
   about the order in which labels are displayed if left to right and
   right to left labels are adjacent to each other, especially if there
   are also multiple consecutive appearances of one of the types.  The
   decision about the display order is ultimately under the control of
   user agents --including web browsers, mail clients, and the like--
   which may be highly localized.  Even when formats are specified by
   protocols, the full composition of an Internationalized Resource
   Identifier (IRI) [RFC3987] 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.  User agents are not required
   to use those protocol-based forms directly but often do so.  While
   display, parsing, and processing within a label is specified by the
   normative documents in the IDNA2008 collection, the relationship
   between fully-qualified domain names and internationalized labels is
   unchanged from the base DNS specifications.  Comments in this
   document about such full domain names are explanatory or examples of
   what might be done and must not be considered normative.

   Questions remain about protocol constraints implying that the overall
   direction of these strings will always be left to right (or right to
   left) for an IRI or email address, or if they even should conform to
   such rules.  These questions also have several possible answers.

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   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?  An IRI for clear text web access would, in network order,
   begin with "http://" and the characters will appear as
   "http://abc.def" -- but what does this suggest about the display
   order?  When entering a URI to many browsers, it may be possible to
   provide only the domain name and leave the "http://" to be filled in
   by default, assuming no tail (an approach that does not work for
   protocols other than HTTP or whatever is chosen as the default).  The
   natural display order for the typed domain name on a right to left
   system is  Does this change if a protocol identifier, tail,
   and the corresponding delimiters are specified?

   While logic, precedent, and reality suggest that these are questions
   for user interface design, not IETF protocol specifications,
   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, and heuristics that sometimes fail, if each
   implementation of each application makes its own decisions on these

   Any version of IDNA, including the current one, must be written in
   terms of the network (transmission on the wire) order of characters
   in labels and for the labels in complete (fully-qualified) domain
   names and must be quite precise about those relationships.  While
   some strong suggestions about display order would be desirable to
   reduce the chances for inconsistent transcription of domain names
   from printed form, such suggestions are beyond the scope of these

4.2.  Entry and Display in Applications

   Applications can accept domain names using any character set or sets
   desired by the application developer, specified by the operating
   system, or dictated by other constraints, and can display domain
   names in any character set or character coding system.  That is, the
   IDNA protocol does not affect the interface between users and

   An IDNA-aware application can accept and display internationalized
   domain names in two formats: the internationalized character set(s)
   supported by the application (i.e., an appropriate local
   representation of a U-label), and as an A-label.  Applications may
   allow the display of A-labels, but are encouraged to not do so except
   as an interface for special purposes, possibly for debugging, or to
   cope with display limitations.  In general, they should allow, but
   not encourage, user input of that label form.  A-labels are opaque

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   and ugly and malicious variations on them are not easily detected by
   users.  Where possible, they should thus only be exposed to users and
   in contexts in which 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; if it does, rendering the U-label should normally be the

   Domain names are often stored and transported in many places.  For
   example, they are part of documents such as mail messages and web
   pages.  They are transported in many parts of many protocols, such as
   both the control commands of SMTP and associated the message body
   parts, and in the headers and the body content in HTTP.  It is
   important to remember that domain names appear both in domain name
   slots and in the content that is passed over protocols.

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

   In any place where a protocol or document format allows transmission
   of the characters in internationalized labels, labels should be
   transmitted using whatever character encoding and escape mechanism
   the protocol or document format uses at that place.  This provision
   is intended to prevent situations in which, e.g., UTF-8 domain names
   appear embedded in text that is otherwise in some other character

   All protocols that use domain name slots (See Section
   [[anchor14: ??  Verify this]] in [IDNA2008-Defs]) already have the
   capacity for handling domain names in the ASCII charset.  Thus,
   A-labels can inherently be handled by those protocols.

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

   [[anchor15: There is some internal redundancy and repetition in the
   material in this section.  Specific suggestions about to reduce or
   eliminate redundant text would be appreciated.  If no such
   suggestions are received before -07 is posted, this note will be

   Users often have expectations about character matching or equivalence

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   that are based on their own languages and the orthography of those
   languages.  These expectations may not be consistent with forms or
   actions that can be naturally accommodated in a character coding
   system, especially if multiple languages are written using the same
   script but using different conventions.  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.  A
   user in German might expect a label with an o-umlaut and a label that
   had "oe" substituted, but was otherwise the same, treated as
   equivalent even though that substitution would be a clear error in
   Swedish.  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.  For that matter, an English user might expect "theater"
   and "theatre" to match.

   Related issues arise because there are a number of languages written
   with alphabetic scripts in which single phonemes are written using
   two characters, termed a "digraph", for example, the "ph" in
   "pharmacy" and "telephone".  (Note that characters paired in this
   manner can also appear consecutively without forming a digraph, as in
   "tophat".)  Certain digraphs are normally 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 (strictly designating setting totally
   without intervening white space, although the term is sometimes
   applied to close set pairs).  An example of this may be seen when the
   word "encyclopaedia" is set with a U+00E6 LATIN SMALL LIGATURE AE
   (and some would not consider that word correctly spelled unless the
   ligature form was used or the "a" was dropped entirely).  When these
   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, any requirements
   for matching must utilize other methods, presumably at the registry
   level, or users must be educated to understand that matching will not

   Difficulties arise from the fact that a given ligature may be a
   completely optional typographic convenience for representing a
   digraph in one language (as in the above example with some spelling
   conventions), while in another language it is a single character that
   may not always be correctly representable by a two-letter sequence
   (as in the above example with different spelling conventions).  This
   can be illustrated by many words in the Norwegian language, where the
   "ae" ligature is the 27th letter of a 29-letter extended Latin
   alphabet.  It is equivalent to the 28th letter of the Swedish

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   alphabet (also containing 29 letters), U+00E4 LATIN SMALL LETTER A
   WITH DIAERESIS, for which an "ae" cannot be substituted according to
   current orthographic standards.

   That character (U+00E4) is also part of the German alphabet where,
   unlike in the Nordic languages, the two-character sequence "ae" is
   usually treated as a fully acceptable alternate orthography for the
   "umlauted a" character.  The inverse is however not true, and those
   two characters cannot necessarily be combined into an "umlauted a".
   This also applies to another German character, the "umlauted o"
   (U+00F6 LATIN SMALL LETTER O WITH DIAERESIS) which, for example,
   cannot be used for writing the name of the author "Goethe".  It is
   also a letter in the Swedish alphabet where, like the "a with
   diaeresis", it cannot be correctly represented as "oe" and in the
   Norwegian alphabet, where it is represented, not as "o with
   diaeresis", but as "slashed o", U+00F8.

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

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

   Whether ligatures and digraphs are to be treated as a sequence of
   characters or as a single standalone one constitute a problem that
   cannot be resolved solely by operating on scripts.  They are,
   however, a key concern in the IDN context.  Their satisfactory
   resolution will require support in policies set by registries, which
   therefore need to be particularly mindful not just of this specific
   issue, but of all other related matters that cannot be dealt with on
   an exclusively algorithmic and global basis.

   Just as with the examples of different-looking characters that may be
   assumed to be the same, it is in general impossible to deal with
   these situations in a system such as IDNA -- or with Unicode
   normalization generally -- since determining what to do requires
   information about the language being used, context, or both.
   Consequently, these specifications make no attempt to treat these
   combined characters in any special way.  However, their existence
   provides a prime example of a situation in which a registry that is
   aware of the language context in which labels are to be registered,
   and where that language sometimes (or always) treats the two-
   character sequences as equivalent to the combined form, should give
   serious consideration to applying a "variant" model [RFC3743]
   [RFC4290], or to prohibiting registration of one the forms entirely,
   to reduce the opportunities for user confusion and fraud that would
   result from the related strings being registered to different

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

4.4.  Case Mapping and Related Issues

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

   The solution of keeping the characters separate but doing matching
   independent of case is not feasible with IDNA or any IDNA-like model
   because the matching would then have to be done on the server rather
   than have characters mapped on the client.  That situation was
   recognized in IDNA2003 and nothing in these specifications
   fundamentally changes it or could do so.  In IDNA2003, all characters
   are case-folded and mapped.  That results in upper-case characters
   being mapped to lower-case ones and in some other transformations of
   alternate forms of characters, especially those that do not have (or
   did not have) upper-case forms.  For example, Greek Final Form Sigma
   (U+03C2) is mapped to the medial form (U+03C3) and Eszett (German
   Sharp S, U+00DF) is mapped to "ss".  Neither of these mappings is
   reversible because the upper case of U+03C3 is the Upper Case Sigma
   (U+03A3) and "ss" is an ASCII string.  IDNA2008 permits, at the risk
   of some incompatibility, slightly more flexibility in this area by
   avoid case folding and treating these characters as themselves.
   Approaches to handling that incompatibility are discussed in
   Section 7.2.  Although information is lost in IDNA2003's ToASCII
   operation so that, in some sense, neither Final Sigma nor Eszett can
   be represented in an IDN at all, its guarantee of mapping when those
   characters are used as input can be interpreted as violating one of
   the conditions discussed in Section 7.4.1 and hence requiring a
   prefix change.  The consensus was to not make a prefix change in
   spite of this issue.  Of course, had a prefix change been made (at
   the costs discussed in Section 7.4.3) there would have been several
   options, including, if desired, assignment of the character to the
   CONTEXTUAL RULE REQUIRED category and requiring that it only be used
   in carefully-selected contexts.

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

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   characters appear both starts and ends with them, not include any
   characters with strong left to right properties (which excludes other
   alphabetic characters but permits European digits), and rejects any
   other string that contains a right to left character.  This is one of
   the few places where the IDNA algorithms (both in IDNA2003 and in
   IDAN2008) are required to examine an entire label, not just
   individual characters.  The algorithmic model used in IDNA2003
   rejects the label when the final character in a right to left string
   requires a combining mark in order to be correctly represented.

   That prohibition is not acceptable for writing systems for languages
   written with consonantal alphabets to which diacritical vocalic
   systems are applied, and for languages with orthographies derived
   from them where the combining marks may have different functionality.
   In both cases the combining marks can be essential components of the
   orthography.  Examples of this are Yiddish, written with an extended
   Hebrew script, and Dhivehi (the official language of Maldives) which
   is written in the Thaana script (which is, in turn, derived from the
   Arabic script).  IDNA2008 removes the restriction on final combining
   characters with a new set of rules for right to left scripts and
   their characters.  Those new rules are specified in [IDNA2008-Bidi].

5.  IDNs and the Robustness Principle

   The model of IDNs described in this document can be seen as a
   particular instance of the "Robustness Principle" that has been so
   important to other aspects of Internet protocol design.  This
   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]).  For
   IDNs to work well, not only must the protocol be carefully designed
   and implemented, but 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

   As mentioned elsewhere, if a string cannot be successfully found in
   the DNS after the lookup processing described here, it makes no

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   difference whether it simply wasn't registered or was prohibited by
   some rule at the registry.  Applications should, however, be
   sensitive to the fact that, because of the possibility of DNS
   wildcards, the ability to successfully resolve a name does not
   guarantee that it was actually registered.

   If lookup applications, as a user interface (UI) or other local
   matter, decide to warn about some strings that are valid under the
   global rules but that they perceive as dangerous, that is their
   prerogative and we can only hope that the market (and maybe
   regulators) will reinforce the good choices and discourage the poor
   ones.  In this context, a lookup application that decides a string
   that is valid under the protocol is dangerous and refuses to look it
   up is in violation of the protocols; one that is willing to look
   something up, but warns against it, is exercising a local choice.

6.  Front-end and User Interface Processing for Lookup

   Domain names may be identified and processed in many contexts.  They
   may be typed in by users either by themselves or embedded in an
   identifier structured for a particular protocol or class of protocols
   such a email addresses, URIs, or IRIs.  They may occur in running
   text or be processed by one system after being provided in another.
   Systems may wish to try to normalize URLs so as to determine (or
   guess) whether a reference is valid or two references point to the
   same object without actually looking the objects up and comparing
   them (that is necessary, not just a choice, for URI types that are
   not intended to be resolved).  Some of these goals may be more easily
   and reliably satisfied than others.  While there are strong arguments
   for any domain name that is placed "on the wire" -- transmitted
   between systems -- to be in the zero-ambiguity forms of A-labels, it
   is inevitable that programs that process domain names will encounter
   U-labels or variant forms.

   One source of such forms will be labels created under IDNA2003
   because that protocol allowed labels that were transformed from
   native-character format by mapping some characters into others before
   conversion into ACE ("xn--...") format.  One consequence of the
   transformations was that, when the ToUnicode and ToASCII operations
   of IDNA2003 were applied, ToUnicode(ToASCII(original-label)) often
   did not produce the original label.  IDNA2008 explicitly defines
   A-labels and U-labels as different forms of the same abstract label,
   forms that are stable when conversions are performed between them
   (without mappings).  A different way of explaining this is that there
   are, today, domain names in files on the Internet that use characters
   that cannot be represented directly in, or recovered from, (A-label)
   domain names but for which interpretations are provided by IDNA2003.

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   There are two major categories of such characters, those that are
   removed by NFKC normalization and those upper-case characters that
   are mapped to lower-case (there are also a few characters that are
   given special-case mapping treatment in Stringprep, including lower-
   case characters that are case-folded into other lower-case characters
   or strings).

   Other issues in domain name identification and processing arise
   because IDNA2003 specified that several other characters be treated
   as equivalent to the ASCII period (dot, full stop) character used as
   a label separator.  If a string that might be a domain name appears
   in an arbitrary context (such as running text), it is difficult, even
   with only ASCII characters, to know whether an actual domain name (or
   a protocol parameter like a URI) is present and where it starts and
   ends.  When using Unicode, this gets even more difficult if treatment
   of certain special characters (like the dot that separates labels in
   a domain name) depends on context (e.g., prior knowledge of whether
   the string represents a domain name or not).  That knowledge is not
   available if the primary heuristic for identifying the presence of
   domain names in strings depends on the presence of dots separating
   groups of characters with no intervening spaces.

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

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

      For this case, the best practice will usually be to apply NFKC and
      case-mapping (or, perhaps better yet, Stringprep itself), plus
      dot-mapping where appropriate, to the domain name string prior to
      applying IDNA.  That practice will not only yield a reasonable

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      compromise of user experience with protocol requirements but will
      be almost completely compatible with the various forms permitted
      by IDNA2003.

   o  Highly Localized Preprocessing.
      Unlike the case above, there will be some situations in which
      software will be highly localized for a particular environment and
      carefully adapted to the expectations of users in that
      environment.  The many discussions about using the Internet to
      preserve and support local cultures suggest that these cases may
      be more common in the future than they have been so far.

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

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

   User interfaces involving Latin-based scripts should take special
   care when considering how to handle case mapping because small
   differences in label strings may cause behavior that is astonishing
   to users.  Because case-insensitive comparison is done for ASCII

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   strings by DNS-servers, an all-ASCII label is treated as case-
   insensitive.  However, if even one of the characters of that string
   is replaced by one that requires the label to be given IDN treatment
   (e.g., by adding a diacritical mark), then the label effectively
   becomes case-sensitive because only lower-case characters are
   permitted in IDNs.  This suggests that case mapping for Latin-based
   scripts (and possibly other scripts with case distinctions) as a
   preprocessing matter in applications may be wise to prevent user
   astonishment, but, since all applications may not do this and
   ambiguity in transport is not desirable, the that case-dependent
   forms should not be stored in files.

   The comments above apply only in operations that look up names or
   interpret files.  There are several reasons why registration
   activities should require final names and verification of those names
   by the would-be registrant.

7.  Migration from IDNA2003 and Unicode Version Synchronization

7.1.  Design Criteria

   As mentioned above and in RFC 4690, two key goals of the IDNA2008
   design are to enable applications to be agnostic about whether they
   are being run in environments supporting any Unicode version from 3.2
   onward and to permit incrementally adding new characters, character
   groups, scripts, and other character collections as they are
   incorporated into Unicode, without disruption and, in the long term,
   without "heavy" processes such as those involving IETF consensus.
   (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.)  The
   mechanisms that support this are outlined above and elsewhere in the
   IDNA2008 document set, but this section reviews them in a context
   that may be more helpful to those who need to understand the approach
   and make plans for it.

7.1.1.  General IDNA Validity Criteria

   The general criteria for a putative label, and the collection of
   characters that make it up, to be considered IDNA-valid are (the
   actual rules are rigorously defined in the "Protocol" and "Tables"

   o  The characters are "letters", marks needed to form letters,
      numerals, or other code points used to write words in some
      language.  Symbols, drawing characters, and various notational
      characters are intended to be permanently excluded -- some because

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      they are harmful in URI, IRI, or similar contexts (e.g.,
      characters that appear to be slashes or other reserved URI
      punctuation) and others because 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" and the complexities that would come
      with it (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 as well.  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).
      Even for English, very common constructions -- contractions like
      "don't" or "it's", names that are written with apostrophes such as
      "O'Reilly", or characters for which apostrophes are common
      substitutes cannot be represented in DNS labels.  Words in English
      whose usually-preferred spellings include diacritical marks cannot
      be represented under the original hostname rules, but most can be
      represented if treated as IDNs.

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

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

   Tables used to identify the characters that are IDNA-valid are
   expected to be driven by the principles above, principles that are
   specified exactly in [IDNA2008-Tables]).  The rules given there are
   normative, rather than being just an interpretation of the tables.

7.1.2.  Labels in Registration

   Anyone entering a label into a DNS zone must properly validate that
   label -- i.e., be sure that the criteria for that label are 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" [RFC0952] where necessary for the expected applications.
   Later addition of special service location formats [RFC2782] imposed

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   new requirements on zone administrators for the use of labels that
   conform to the requirements of those formats.  For zones that will
   contain IDNs, support for Unicode version-independence requires
   restrictions on all strings placed in the zone.  In particular, for
   such zones:

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

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

   Under this model, a registry (or entity communicating with a registry
   to accomplish name registrations) will need to update its tables --
   both the Unicode-associated tables and the tables of permitted IDN
   characters -- to enable a new script or other set of new characters.
   It will not be affected by newer versions of Unicode, or newly-
   authorized characters, until and unless it wishes to make those
   registrations.  The zone administrator is also responsible -- under
   the protocol and to registrants and users -- for both checking as
   required by the protocol and verification that whatever policies it
   develops are complied with, whether those policies are for minimizing
   risks due to confusable characters and sequences, for preserving
   language or script integrity, or for other purposes.  Those checking
   and verification procedures are more extensive than those that are is
   expected of applications systems that look names up.

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

7.1.3.  Labels in Lookup

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

   o  Maintain a consistent set of tables, as discussed above.  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 either

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      code points prohibited by IDNA (categorized as "DISALLOWED") 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, notably verifying that there are no leading
      combining marks, that the "bidi" conditions are met if right to
      left characters appear, that any required contextual rules are
      available and that, if such rules are associated with Joiner
      Controls, they are tested.

   o  Avoid validating other contextual rules about characters,
      including mixed-script label prohibitions, although such rules may
      be used to influence presentation decisions in the user interface.
      [[anchor20: Check this, and all similar statements, against
      Protocol when that is finished.]]

   By avoiding applying its own interpretation of which labels are valid
   as a means of rejecting lookup attempts, the lookup application
   becomes less sensitive to version incompatibilities with the
   particular zone registry associated with the domain name.

   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 validly registered, as long as its version of the
   Unicode-associated 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

   [[anchor21: Note in Draft: This subsection is completely new in
   version -04 and has been further tuned in -05 and -06 of this
   document.  It could almost certainly use improvement, although this
   note will be removed if there are not significant suggestions about
   the -06 version.  It also contains some material that is redundant
   with material in other sections.  I have not tried to remove that
   material and will not do so until the WG concludes that this section
   is relatively stable, but would appreciate help in identifying what
   should be removed or how this might be enhanced to contain more of
   that other material. --JcK]]

   In those scripts that make case distinctions, there are a few
   characters for which an obvious and unique upper case character has

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   not historically been available to match a lower case one or vice
   versa.  For those characters, the mappings used in constructing the
   Stringprep tables for IDNA2003, performed using the Unicode CaseFold
   operation (See Section 5.8 of the Unicode Standard [Unicode51]),
   generate different characters or sets of characters.  Those
   operations are not reversible and lose even more information than
   traditional upper case or lower case transformations, but are more
   useful than those transformations for comparison purposes.  Two
   notable characters of this type are the German character Eszett
   (Sharp S, U+00DF) and the Greek Final Form Sigma (U+03C2).  The
   former is case-folded to the ASCII string "ss", the latter to a
   medial (Lower Case) Sigma (U+03C3).

   The decision to eliminate mappings, including case folding, from the
   IDNA2008 protocol in order to make A-labels and U-labels idempotent
   made these characters problematic.  If they were to be disallowed,
   important words and mnemonics could not be written in
   orthographically reasonable ways.  If they were to be permitted as
   characters distinct from the forms produced by case folding, there
   would be no information loss and registries would have maximum
   flexibility, but labels using those characters that were looked up
   according to IDNA2003 rules would be transformed into A-labels using
   their case-mapped variations while lookup according to IDNA2008 rules
   would be based on different A-labels that represented the actual

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

   The decision faces registries, especially registries maintaining
   zones for third parties, with a variation on what has become a
   familiar problem: how to introduce a new service in a way that does
   not create confusion or significantly weaken or invalidate existing

   There have traditionally been several approaches to problems of this
   type.  Without any preference or claim to completeness, these are:

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

   o  Hold a "sunrise"-like arrangement in which holders of the
      previously-mapped labels (labels containing "ss" in the Eszett
      case or ones containing Lower Case Sigma in the Final Sigma case)

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      are given priority (and perhaps other benefits) for registering
      the corresponding string containing the newly-available

   o  Adopt some sort of "variant" approach in which registrants either
      obtained labels with both character forms or one of them was
      blocked from registration by anyone but the registrant of the
      other form.

   In principle, lookup applications could also compensate for the
   difference in interpretation by looking up the string according to
   the interpretation specified in these documents and then, if that
   failed, doing the lookup with the mapping, simulating the IDNA2003
   interpretation.  The risk of false positives is such that this is
   generally to be discouraged unless the application is able to engage
   in a "is this what you meant" dialogue with the end user.

7.3.  More Flexibility in User Agents

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

   Since the rules in [IDNA2008-Tables] have the effect that only
   strings that are not transformed by NFKC are valid, if an application
   chooses to perform NFKC normalization before lookup, that operation
   is safe since this will never make the application unable to look up
   any valid string.  However, as discussed above, the application
   cannot guarantee that any other application will perform that
   mapping, so it should be used only with caution and for informed

   In many cases these prohibitions should have no effect on what the
   user can type as input to the lookup process.  It is perfectly
   reasonable for systems that support user interfaces to perform some
   character mapping that is appropriate to the local environment.  This
   would normally be done prior to actual invocation of IDNA.  At least
   conceptually, the mapping would be part of the Unicode conversions
   discussed above and in [IDNA2008-Protocol].  However, those changes
   will be local ones only -- local to environments in which users will
   clearly understand that the character forms are equivalent.  For use
   in interchange among systems, it appears to be much more important
   that U-labels and A-labels can be mapped back and forth without loss
   of information.

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

   As suggested earlier in this section, it appears to be desirable to
   do as little character mapping as possible consistent with having
   Unicode work correctly (e.g., NFC mapping to resolve different
   codings for the same character is still necessary although the
   specifications require that it be performed prior to invoking the
   protocol) and to make the mapping between A-labels and U-labels
   idempotent.  Case-mapping is not an exception to this principle.  If
   only lower case characters can be registered in the DNS (i.e., be
   present in a U-label), then IDNA2008 should prohibit upper-case
   characters as input (and therefore does so).  Some other
   considerations reinforce this conclusion.  For example, an essential
   element of the ASCII case-mapping functions is that, for individual
   characters, uppercase(character) must be equal to
   uppercase(lowercase(character)).  That requirement may not be
   satisfied with IDNs.  For example, there are some characters in
   scripts that use case distinction that do not have counterparts in
   one case or the other.  The relationship between upper case and lower
   case may even be language-dependent, with different languages (or
   even the same language in different areas) expecting different
   mappings.  Of course, the expectations of users who are accustomed to
   a case-insensitive DNS environment will probably be well-served if
   user agents perform 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.

7.4.  The Question of Prefix Changes

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

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7.4.1.  Conditions Requiring a Prefix Change

   An IDN prefix change is needed if a given string would be looked up
   or otherwise interpreted differently depending on the version of the
   protocol or tables being used.  Consequently, work to update IDNs
   would require a prefix change if, and only if, one of the following
   four conditions were met:

   1.  The conversion of an A-label to Unicode (i.e., a U-label) yields
       one string under IDNA2003 (RFC3490) and a different string under

   2.  An input string that is valid under IDNA2003 and also valid under
       IDNA2008 yields two different A-labels with the different
       versions of IDNA.  This condition is believed to be essentially
       equivalent to the one above except for a very small number of
       edge cases which may not, pragmatically, justify a prefix change
       (See Section 7.2).

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

   3.  A fundamental change is made to the semantics of the string that
       is inserted in the DNS, e.g., if a decision were made to try to
       include language or specific script information in that string,
       rather than having it be just a string of characters.

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

7.4.2.  Conditions Not Requiring a Prefix Change

   In particular, as a result of the principles described above, none of
   the following changes require a new prefix:

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

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

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

7.4.3.  Implications of Prefix Changes

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

   Consequently, a prefix change is to be avoided if at all possible,
   even if it means accepting some IDNA2003 decisions about character
   distinctions as irreversible and/or giving special treatment to edge

7.5.  Stringprep Changes and Compatibility

   The Nameprep [RFC3491] specification, a key part of IDNA2003, is a
   profile of Stringprep [RFC3454].  While Nameprep is a Stringprep
   profile specific to IDNA, Stringprep is used by a number of other
   protocols.  Concerns have been expressed about problems for non-DNS
   uses of Stringprep being caused by changes to the specification
   intended to improve the handling of IDNs, most notably as this might
   affect identification and authentication protocols.  The proposed new
   inclusion tables [IDNA2008-Tables], the reduction in the number of
   characters permitted as input for registration or lookup (Section 3),
   and even the proposed changes in handling of right to left strings
   [IDNA2008-Bidi] either give interpretations to strings prohibited
   under IDNA2003 or prohibit strings that IDNA2003 permitted.  The
   IDNA2008 protocol does not use either 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

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   constraints that are necessary for smooth and comprehensible use of
   IDNs may be unwanted or undesirable in other contexts.  For example,
   the criteria for good passwords or passphrases are very different
   from those for desirable IDNs: passwords should be hard to guess,
   while domain names should normally be easily memorable.  Similarly,
   internationalized SCSI identifiers and other protocol components are
   likely to have different requirements than IDNs.

7.6.  The Symbol Question

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

   o  As discussed elsewhere, the original IDNA specification assumed
      that as many Unicode characters as possible should be permitted,
      directly or via mapping to other characters, in IDNs.  This
      specification operates on an inclusion model, extrapolating from
      the LDH rules -- which have served the Internet very well -- to a
      Unicode base rather than an ASCII base.

   o  Most Unicode names for letters are, in most cases, fairly
      intuitive, unambiguous and recognizable to users of the relevant
      script.  Symbol names are more problematic because there may be no
      general agreement on whether a particular glyph matches a symbol;
      there are no uniform conventions for naming; variations such as
      outline, solid, and shaded forms may or may not exist; and so on.
      As just one example, consider a "heart" symbol as it might appear
      in a logo that might be read as "I love...".  While the user might
      read such a logo as "I love..." or "I heart...", considerable
      knowledge of the coding distinctions made in Unicode is needed to
      know that there more than one "heart" character (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.
      [[anchor22: The above paragraph remains controversial as to
      whether it is valid.  The WG will need to make a decision if this
      section is not dropped entirely.]]

   o  Consider the case of a screen reader used by blind Internet users
      who must listen to renderings of IDN domain names and possibly
      reproduce them on the keyboard.

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   o  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.

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

   o  The consequence of these ambiguities of description and
      dependencies on distinctions that were, or were not, made in
      Unicode codings 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 protocol as described in these documents, strings containing
   unassigned code points must not be either looked up or registered.
   There are several reasons for this, with the most important ones

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   o  It cannot be known in advance, and with sufficient reliability,
      that a code point that was not previously assigned will not be
      assigned to a compatibility character or one that would be
      otherwise disallowed by the rules in [IDNA2008-Tables].  In
      IDNA2003, since there is no direct dependency on NFKC
      (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 assigned to
      DISALLOWED unless character-specific exceptions are made,
      permitting strings containing unassigned characters to be looked
      up would permit violating the principle that characters in
      DISALLOWED are not looked up.

   o  The Unicode Standard specifies that an unassigned code point
      normalizes (and, where relevant, case folds) to itself.  If the
      code point is later assigned to a character, and particularly if
      the newly-assigned code point has a combining class that
      determines its placement relative to other combining characters,
      it could normalize to some other code point or sequence, creating
      confusion and/or violating other rules listed here.

   o  Tests involving the context of characters (e.g., some characters
      being permitted only adjacent to ones of specific types but
      otherwise invisible or very problematic for other reasons) 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  More generally, the status of an unassigned character with regard
      to the DISALLOWED and PROTOCOL-VALID categories, and whether
      contextual rules are required with the latter, cannot be evaluated
      until a character is actually assigned and known.  By contrast,
      characters that are actually DISALLOWED are placed in that
      category only as a consequence of rules applied to known
      properties or per-character evaluation.

   Another way to look at this is that permitting an unassigned
   character to be looked up is nearly equivalent to reclassifying a
   character from DISALLOWED to PROTOCOL-VALID since different systems
   will interpret the character in different ways.

   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

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   Unicode 5.1 and hence unassigned code points will be assigned only to
   obscure characters or archaic scripts.  Unfortunately, that does not
   appear to be a safe assumption for at least two reasons.  First, much
   the same claim of completeness has been made for earlier versions of
   Unicode.  The reality is that a script that is obscure to much of the
   world may still be very important to those who use it.  Cultural and
   linguistic preservation principles make it inappropriate to declare
   the script of no importance in IDNs.  Second, we already have
   counterexamples in, e.g., the relationships associated with new Han
   characters being added (whether in the BMP or in Unicode Plane 2).

7.8.  Other Compatibility Issues

   The 2003 IDNA model includes several odd artifacts of the context in
   which it was developed.  Many, if not all, of these are potential
   avenues for exploits, especially if the registration process permits
   "source" names (names that have not been processed through IDNA and
   Nameprep) to be registered.  As one example, since the character
   Eszett, used in German, is mapped by IDNA2003 into the sequence "ss"
   rather than being retained as itself or prohibited, a string
   containing that character but that is otherwise in ASCII is not
   really an IDN (in the U-label sense defined above) at all.  After
   Nameprep maps the Eszett out, the result is an ASCII string and so
   does not get an xn-- prefix, but the string that can be displayed to
   a user appears to be an IDN.  The newer version of the protocol
   eliminates this artifact.  A character is either permitted as itself
   or it is prohibited; special cases that make sense only in a
   particular linguistic or cultural context can be dealt with as
   localization matters where appropriate.

8.  Name Server Considerations

8.1.  Processing Non-ASCII Strings

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

   Because of the distinction made between the algorithms for
   Registration and Lookup in [IDNA2008-Protocol] (a domain name
   containing only ASCII codepoints can not be converted to an A-label),

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   there can not be more than one A-label form for any given U-label.

   As specified in RFC 2181 [RFC2181], the DNS protocol explicitly
   allows domain labels to contain octets beyond the ASCII range
   (0000..007F), and this document does not change that.  Note, however,
   that there is no defined interpretation of octets 0080..00FF as
   characters.  If labels containing these octets are returned to
   applications, unpredictable behavior could result.  The A-label form,
   which cannot contain those characters, is the only standard
   representation for internationalized labels in the DNS protocol.

8.2.  DNSSEC Authentication of IDN Domain Names

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

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

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

8.3.  Root and other DNS Server Considerations

   IDNs in A-label form will generally be somewhat longer than current
   domain names, so the bandwidth needed by the root servers is likely
   to go up by a small amount.  Also, queries and responses for IDNs
   will probably be somewhat longer than typical queries historically,
   so EDNS0 [RFC2671] support may be more important (otherwise, queries
   and responses may be forced to go to TCP instead of UDP).

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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, a decision that
   would be impossible in many or most cases.

10.  IANA Considerations

   This section gives an overview of registries required for IDNA.  The
   actual definitions of the first two appear in [IDNA2008-Tables].

10.1.  IDNA Character Registry

   The distinction among the three major categories "UNASSIGNED",
   "DISALLOWED", and "PROTOCOL-VALID" is made by special categories and
   rules that are integral elements of [IDNA2008-Tables].  Convenience
   in programming and validation requires a registry of characters and
   scripts and their categories, updated for each new version of Unicode
   and the characters it contains.  The details of this registry are
   specified in [IDNA2008-Tables].

10.2.  IDNA Context Registry

   For characters that are defined in the IDNA Character Registry list
   as PROTOCOL-VALID but requiring a contextual rule (i.e., the types of
   rule described in Section 3.1.2), IANA will create and maintain a
   list of approved contextual rules.  The details for those rules
   appear in [IDNA2008-Tables].

10.3.  IANA Repository of IDN Practices of TLDs

   This registry, historically described as the "IANA Language Character
   Set Registry" or "IANA Script Registry" (both somewhat misleading
   terms) is maintained by IANA at the request of ICANN.  It is used to

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   provide a central documentation repository of the IDN policies used
   by top level domain (TLD) registries who volunteer to contribute to
   it and is used in conjunction with ICANN Guidelines for IDN use.

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

11.  Security Considerations

11.1.  General Security Issues with IDNA

   This document in the IDNA2008 series 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

12.  Acknowledgments

   The editor and contributors would like to express their thanks to
   those who contributed significant early (pre-WG) review comments,
   sometimes accompanied by text, especially Mark Davis, Paul Hoffman,
   Simon Josefsson, and Sam Weiler.  In addition, some specific ideas
   were incorporated from suggestions, text, or comments about sections
   that were unclear supplied by Vint Cerf, Frank Ellerman, Michael
   Everson, Asmus Freytag, Erik van der Poel, Michel Suignard, and Ken
   Whistler, although, as usual, they bear little or no responsibility
   for the conclusions the editor and contributors reached after
   receiving their suggestions.  Thanks are also due to Vint Cerf,
   Debbie Garside, and Jefsey Morfin for conversations that led to
   considerable improvements in the content of this document.

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

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   for support of that meeting and to the participants for their

   Useful comments and text on the WG versions of the draft were
   received from many participants in the IETF "IDNABIS" WG and a number
   of document changes resulted from mailing list discussions made by
   that group.  Marcos Sanz provided specific analysis and suggestions
   that were exceptionally helpful in refining the text, as did Vint
   Cerf, Mark Davis, Martin Duerst, Ken Whistler, and Andrew Sullivan.

13.  Contributors

   While the listed editor held the pen, this core of this document and
   the initial WG version represents the joint work and conclusions of
   an ad hoc design team consisting of the editor and, in alphabetic
   order, Harald Alvestrand, Tina Dam, Patrik Faltstrom, and Cary Karp.
   In addition, there were many specific contributions and helpful
   comments from those listed in the Acknowledgments section and others
   who have contributed to the development and use of the IDNA

14.  References

14.1.  Normative References

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

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

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

              Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              November 2008, <

              Klensin, J., "Internationalized Domain Names in

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              Applications (IDNA): Protocol", November 2008, <https://

              Faltstrom, P., "The Unicode Code Points and IDNA",
              July 2008, <

              A version of this document is available in HTML format at

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

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

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

              The Unicode Consortium, "The Unicode Standard, Version
              5.1.0", 2008.

              defined by: The Unicode Standard, Version 5.0, Boston, MA,
              Addison-Wesley, 2007, ISBN 0-321-48091-0, as amended by
              Unicode 5.1.0

14.2.  Informative References

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

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

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

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

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

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

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

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

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

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

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

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

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

   [RFC2673]  Crawford, M., "Binary Labels in the Domain Name System",
              RFC 2673, August 1999.

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

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

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

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   [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.

   [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.

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

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

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

Appendix A.  Change Log

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

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

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

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

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

   o  Added a discussion of examples to Section 7.1

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

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

A.2.  Version -02

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

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

   o  Rewrote the material on preprocessing somewhat.

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

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

A.3.  Version -03

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

   Two annotations have been applied to sections that might reasonably
   be considered normative.  One annotation is based on the list of
   sections in Mark Davis's note of 29 September (http://

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   The other is based on an elaboration of John Klensin's response on 7
   October (
   002691.html).  These should just be considered two suggestions to
   illuminate and, one hopes, advance the Working Group's discussions.

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

A.4.  Version -04

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

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

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

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

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

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

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

A.5.  Version -05

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

A.6.  Version -06

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

   o  Started changing uses of "IDNA2008" in running text to "in these
      specifications" or the equivalent.  These documents are titled
      simply "IDNA"; once they are standardized, "the current version"
      may be a more appropriate reference than one containing a year.

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      As discussed on the mailing list, we can and should discuss how to
      refer to these documents at an appropriate time (e.g., when we
      know when we will be finished) but, in the interim, it seems
      appropriate to simply start getting rid of the version-specific
      terminology where it can naturally be removed.

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

   o  Clarified relationship to base DNS specifications.

   o  Consolidated discussion of lookup of unassigned characters.

   o  More editorial fine-tuning.

A.7.  Version -07

   o  Revised terminology by adding terms: NR-LDH-label, Invalid-A-label
      (or False-A-label), R-LDH-label, valid IDNA-label in
      Section 1.3.3.

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

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

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

A.8.  Version -08

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

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

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Author's Address

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

   Phone: +1 617 245 1457

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