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Versions: 00 01 02 03 04 05 06 07                                       
Network Working Group                                         J. Klensin
Internet-Draft                                          February 6, 2008
Expires: August 9, 2008

    Internationalizing Domain Names for Applications (IDNA): Issues,
                       Explanation, and Rationale

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   A recent IAB report identified issues that have been raised with
   Internationalized Domain Names (IDNs).  Some of these issues require
   tuning of the existing protocols and the tables on which they depend.
   Based on intensive discussion by an informal design team, this
   document provides an overview some of the proposals that are being
   made, provides explanatory material for them and then further
   explains some of the issues that have been encountered.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Context and Overview . . . . . . . . . . . . . . . . . . .  4
     1.2.  Discussion Forum . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Objectives . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.4.  Applicability and Function of IDNA . . . . . . . . . . . .  5
     1.5.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  6
       1.5.1.  Documents and Standards  . . . . . . . . . . . . . . .  6
       1.5.2.  Terminology about Characters and Character Sets  . . .  6
       1.5.3.  DNS-related Terminology  . . . . . . . . . . . . . . .  7
       1.5.4.  Terminology Specific to IDNA . . . . . . . . . . . . .  7
       1.5.5.  Punycode is an Algorithm, not a Name . . . . . . . . . 10
       1.5.6.  Other Terminology Issues . . . . . . . . . . . . . . . 11
   2.  The Original (2003) IDNA Model . . . . . . . . . . . . . . . . 12
     2.1.  Proposed label . . . . . . . . . . . . . . . . . . . . . . 13
     2.2.  Permitted Character Identification . . . . . . . . . . . . 13
     2.3.  Character Mappings . . . . . . . . . . . . . . . . . . . . 13
     2.4.  Registry Restrictions  . . . . . . . . . . . . . . . . . . 13
     2.5.  Punycode Conversion  . . . . . . . . . . . . . . . . . . . 14
     2.6.  Lookup or Insertion in the Zone  . . . . . . . . . . . . . 14
   3.  The Revised IDNA Model . . . . . . . . . . . . . . . . . . . . 14
     3.1.  Localization: The Role of the Local System, Local
           Preprocessing, and the User Interface  . . . . . . . . . . 14
     3.2.  IDN Processing in the IDNA200X Model . . . . . . . . . . . 16
       3.2.1.  Summary of Effects . . . . . . . . . . . . . . . . . . 16
       3.2.2.  Protocols  . . . . . . . . . . . . . . . . . . . . . . 16
   4.  IDNA200X Document List . . . . . . . . . . . . . . . . . . . . 16
   5.  Permitted Characters: An Inclusion List  . . . . . . . . . . . 17
     5.1.  A Tiered Model of Permitted Characters and Labels  . . . . 17
       5.1.1.  PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . . 17
       5.1.2.  DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 19
       5.1.3.  UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 19
     5.2.  Registration Policy  . . . . . . . . . . . . . . . . . . . 20
     5.3.  Layered Restrictions: Tables, Context, Registration,
           Applications . . . . . . . . . . . . . . . . . . . . . . . 20
   6.  Issues that Constrain Possible Solutions . . . . . . . . . . . 20
     6.1.  Display and Network Order  . . . . . . . . . . . . . . . . 20
     6.2.  Entry and Display in Applications  . . . . . . . . . . . . 22
     6.3.  Linguistic Expectations: Ligatures, Digraphs, and
           Alternate Character Forms  . . . . . . . . . . . . . . . . 23
     6.4.  Case Mapping and Related Issues  . . . . . . . . . . . . . 25
     6.5.  Right-to-left Text . . . . . . . . . . . . . . . . . . . . 26
   7.  IDNs and the Robustness Principle  . . . . . . . . . . . . . . 26
   8.  Front-end and User Interface Processing  . . . . . . . . . . . 27
   9.  Migration and Version Synchronization  . . . . . . . . . . . . 29
     9.1.  Design Criteria  . . . . . . . . . . . . . . . . . . . . . 29
       9.1.1.  General IDNA Validity Criteria . . . . . . . . . . . . 29

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       9.1.2.  Labels in Registration . . . . . . . . . . . . . . . . 31
       9.1.3.  Labels in Resolution (Lookup)  . . . . . . . . . . . . 32
     9.2.  More Flexibility in User Agents  . . . . . . . . . . . . . 33
     9.3.  The Question of Prefix Changes . . . . . . . . . . . . . . 34
       9.3.1.  Conditions Requiring a Prefix Change . . . . . . . . . 34
       9.3.2.  Conditions Not Requiring a Prefix Change . . . . . . . 35
       9.3.3.  Implications of Prefix Changes . . . . . . . . . . . . 35
     9.4.  Stringprep Changes and Compatibility . . . . . . . . . . . 36
     9.5.  The Symbol Question  . . . . . . . . . . . . . . . . . . . 37
     9.6.  Migration Between Unicode Versions: Unassigned Code
           Points . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     9.7.  Other Compatibility Issues . . . . . . . . . . . . . . . . 38
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 39
   11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 39
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 40
     12.1. IDNA Character Registry  . . . . . . . . . . . . . . . . . 40
     12.2. IDNA Context Registry  . . . . . . . . . . . . . . . . . . 40
     12.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 40
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 40
   14. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 42
     14.1. Version -01  . . . . . . . . . . . . . . . . . . . . . . . 42
     14.2. Version -02  . . . . . . . . . . . . . . . . . . . . . . . 42
     14.3. Version -03  . . . . . . . . . . . . . . . . . . . . . . . 42
     14.4. Version -04  . . . . . . . . . . . . . . . . . . . . . . . 43
     14.5. Version -05  . . . . . . . . . . . . . . . . . . . . . . . 43
     14.6. Version -06  . . . . . . . . . . . . . . . . . . . . . . . 43
     14.7. Version -07  . . . . . . . . . . . . . . . . . . . . . . . 44
   Appendix A.  The Contextual Rules Registry . . . . . . . . . . . . 44
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 46
     15.2. Informative References . . . . . . . . . . . . . . . . . . 48
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 49
   Intellectual Property and Copyright Statements . . . . . . . . . . 50

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1.  Introduction

1.1.  Context and Overview

   A recent IAB report [RFC4690] identified issues that have been raised
   with Internationalized Domain Names (IDNs) and the associated
   standards.  Those standards are known as Internationalized Domain
   Names in Applications (IDNA), taken from the name of the highest
   level standard within that group (see Section 1.5).  Based on
   discussion of those issues and their impact, some of these standards
   now require tuning the existing protocols and the tables on which
   they depend.  This document further explains, based on the results of
   some intensive discussions by an informal design team, on a mailing
   list, and in broader discussions, some of the issues that have been
   encountered.  It also provides an overview of the proposals that are
   being made and explanatory material for them.  Additional explanatory
   material for other proposals will appear with the associated

   This document begins with a discussion of the original and new IDNA
   models and the general differences in strategy between the original
   version of IDNA and the proposed new version.  It continues with a
   description of specific changes that are needed and issues that the
   design must address, including some that were not explicitly
   addressed in RFC 4690.

1.2.  Discussion Forum

   [[anchor4: RFC Editor: please remove this section.]]

   This work is being discussed on the mailing list

1.3.  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" (LDH) 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.  Our
   objective is to preserve that balance while expanding the character
   repertoire to include extended versions of Roman-derived scripts and
   scripts that are not Roman in origin.  No work of this sort will be
   able to completely eliminate sources of visual or textual confusion:
   such confusion exists even under the original rules.  However, one

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   can hope, through the application of different techniques at
   different points (see Section 5.3), 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.

1.4.  Applicability and Function of IDNA

   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 8).

   A great deal of the discussion of IDN solutions has focused on
   transition issues and how IDN 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 depend on user applications,
   resolvers, and DNS servers being updated in order for a user to apply
   an internationalized domain name in any form or coding acceptable
   under that method.  While processing must be performed prior to or
   after access to the DNS, no changes are needed to the DNS protocol or
   any DNS servers or the resolvers on user's computers.

   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.

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   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 representation of the non-ASCII
   name labels.  While such names are user-unfriendly to read and type,
   and hence not optimal for user input, they allow (for instance)
   replying to email and clicking on URLs even though the domain name
   displayed is incomprehensible to the user.  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

   IDNA uses the Unicode character repertoire, which avoids the
   significant delays that would be inherent in waiting for a different
   and specific character sets to be defined for IDN purposes,
   presumably by some other standards developing organization.

1.5.  Terminology

1.5.1.  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 "IDNA200X" is used to refer to a new version of IDNA as
   described in this document and in the documents described in
   Section 4.  References to "these specifications" are to the entire

1.5.2.  Terminology about Characters and Character Sets

   A code point is an integer value associated with a character in a
   coded character set.

   Unicode [Unicode50] is a coded character set containing almost
   100,000 characters as of the current version.  A single Unicode code
   point is denoted by "U+" followed by four to six hexadecimal digits,
   while a range of Unicode code points is denoted by two four to six
   digit hexadecimal numbers separated by "..", with no prefixes.

   ASCII means US-ASCII [ASCII], a coded character set containing 128
   characters associated with code points in the range 0000..007F.

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   Unicode may be thought of as an extension of ASCII: it includes all
   the ASCII characters and associates them with equivalent code points.

   "Letters" are, informally, generalizations from the ASCII and common-
   sense understanding of that term, i.e., characters that are used to
   write text that are not symbols or punctuation.  Formally, they are
   characters with a Unicode General Category value starting in "L" (see
   Section 4.5 of [Unicode50]).

1.5.3.  DNS-related Terminology

   When discussing the DNS, this document generally assumes the
   terminology used in the DNS specifications [RFC1034] [RFC1035].  The
   terms "lookup" and "resolution" are used interchangeably and the
   process or application component that performs DNS resolution is
   called a "resolver".  The process of placing an entry into the DNS is
   referred to as "registration" paralleling common contemporary usage
   in other contexts.

   The term "LDH code points" is defined in this document to mean the
   code points associated with ASCII letters, digits, and the hyphen-
   minus; that is, U+002D, 0030..0039, 0041..005A, and 0061..007A. "LDH"
   is an abbreviation for "letters, digits, hyphen".

   The base DNS specifications [RFC1034] [RFC1035] discuss "domain
   names" and "host names", but many people and sections of these
   specifications use the terms interchangeably.  Further, because those
   documents were not terribly clear, many people who are sure they know
   the exact definitions of each of these terms disagree on the
   definitions.  This document generally uses the term "domain name".
   When it refers to, e.g., host name syntax restrictions, it explicitly
   cites the relevant defining documents.  The remaining definitions in
   this subsection are essentially a review.

   A label is an individual component of a domain name.  Labels are
   usually shown separated by dots; for example, the domain name
   "www.example.com" is composed of three labels: "www", "example", and
   "com".  (The zero-length root label described in [RFC1123], which can
   be explicit as in "www.example.com." or implicit as in
   "www.example.com", is not considered a label in this specification.)
   IDNA extends the set of usable characters in labels that are text.
   For the rest of this document, the term "label" is shorthand for
   "text label", and "every label" means "every text label".

1.5.4.  Terminology Specific to IDNA

   Some of the terminology used in describing IDNs in the IDNA2003
   context has been a source of confusion.  This section defines some

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   new terminology to reduce dependence on the problematic terms and
   definitions that appears in RFC 3490.  Terms for IDN Label Codings  IDNA-valid strings, A-label, and U-label

   To improve clarity, this document introduces three new terms in this
   subsection.  In the next, it defines a historical one to be slightly
   more precise for IDNA contexts.

   o  A string is "IDNA-valid" if it meets all of the requirements of
      these specifications for an IDNA label.  IDNA-valid strings may
      appear in either of two forms, defined immediately below.  It is
      expected that specific reference will be made to the form
      appropriate to any context in which the distinction is important.

   o  An "A-label" is the ASCII-Compatible Encoding (ACE, see
      Section form of an IDNA-valid string.  It must be a
      complete label: IDNA is defined for labels, not for parts of them
      and not for complete domain names.  This means, by definition,
      that every A-label will begin with the IDNA ACE prefix, "xn--",
      followed by a string that is a valid output of the Punycode
      algorithm and hence a maximum of 59 ASCII characters in length.
      The prefix and string together must conform to all requirements
      for a label that can be stored in the DNS including conformance to
      the LDH ("host name") rule described in RFC 1034, RFC 1123 and

   o  A "U-label" is an IDNA-valid string of Unicode characters,
      expressed in a standard Unicode Encoding Form, normally UTF-8, and
      subject to the constraint below.  Conversions between valid
      U-labels and valid A-labels is performed according to the
      specification in [RFC3492], adding or removing the ACE prefix (see
      Section as needed.

   To be valid, U-labels and A-labels must obey an important symmetry
   constraint.  While that constraint may be tested in any of several
   ways, an A-label must be capable of being produced by conversion from
   a U-label and a U-label must be capable of being produced by
   conversion from an A-label.  Among other things, this implies that
   both U-labels and A-labels must represent strings in normalized form.
   These strings MUST contain only characters specified elsewhere in
   this document and its companion documents, and only in the contexts
   indicated as appropriate.

   Any rules or conventions that apply to DNS labels in general, such as
   rules about lengths of strings, apply to whichever of the U-label or

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   A-label would be more restrictive.  For the U-label, constraints
   imposed by existing protocols and their presentation forms make the
   length restriction apply to the length in octets of the UTF-8 form of
   those labels (which will always be greater than or equal to the
   length in code points).  The exception to this, of course, is that
   the restriction to ASCII characters does not apply to the U-label.

   A different way to look at these terms, which may be more clear to
   some readers, is that U-labels, A-labels, and LDH-labels (see the
   next subsection) are disjoint categories that, together, make up the
   forms of legitimate strings for use in domain names that describe
   hosts.  Of the three, only A-labels and LDH-labels can actually
   appear in DNS zone files or queries; U-labels can appear, along with
   the other two, in presentation and user interface forms and in
   selected protocols other than those of the DNS itself.  Strings that
   do not conform to the rules for one of these three categories and, in
   particular, strings that contain "-" in the third or fourth character
   position but are:

   o  not A-labels or

   o  that cannot be processed as U-labels or A-labels as described in
      these specifications,

   are invalid as labels in domain names that identify Internet hosts or
   similar resources.  This restriction on strings containing "--" is
   required for three reasons:

   o  to prevent confusion with pre-IDNA coding forms;

   o  to permit future extensions that would require changing the
      prefix, no matter how unlikely those might be (see Section 9.3);

   o  and to reduce the opportunities for attacks on the encoding
      system.  LDH-label and Internationalized Label

   In the hope of further clarifying discussions about IDNs, these
   specifications use the term "LDH-label" strictly to refer to an all-
   ASCII label that obeys the "hostname" (LDH) conventions and that is
   not an IDN.  In other words, only "U-label" and "A-label" refer to
   IDNs LDH-labels are not IDNs.  "Internationalized label" is used when
   a term is needed to refer to any of the three categories.  There are
   some standardized DNS label formats, such as those for service
   location (SRV) records [RFC2782] that do not fall into any of the
   three categories and hence are not internationalized labels.

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Internet-Draft             IDNA200X Rationale              February 2008  Equivalence

   In IDNA, equivalence of labels is defined in terms of the A-labels.
   If the A-labels are equal in a case-independent comparison, then the
   labels are considered equivalent, no matter how they are represented.
   Traditional LDH labels already have a notion of equivalence: within
   that list of characters, upper case and lower case are considered
   equivalent.  The IDNA notion of equivalence is an extension of that
   older notion.  Equivalent labels in IDNA are treated as alternate
   forms of the same label, just as "foo" and "Foo" are treated as
   alternate forms of the same label.  ACE Prefix

   The "ACE prefix" is defined in this document to be a string of ASCII
   characters "xn--" that appears at the beginning of every A-label.
   "ACE" stands for "ASCII-Compatible Encoding".  Domain Name Slot

   A "domain name slot" is defined in this document to be a protocol
   element or a function argument or a return value (and so on)
   explicitly designated for carrying a domain name.  Examples of domain
   name slots include: the QNAME field of a DNS query; the name argument
   of the gethostbyname() library function; the part of an email address
   following the at-sign (@) in the From: field of an email message
   header; and the host portion of the URI in the src attribute of an
   HTML <IMG> tag.  General text that just happens to contain a domain
   name is not a domain name slot.  For example, a domain name appearing
   in the plain text body of an email message is not occupying a domain
   name slot.

   An "IDN-aware domain name slot" is defined in this document to be a
   domain name slot explicitly designated for carrying an
   internationalized domain name as defined in this document.  The
   designation may be static (for example, in the specification of the
   protocol or interface) or dynamic (for example, as a result of
   negotiation in an interactive session).

   An "IDN-unaware domain name slot" is defined in this document to be
   any domain name slot that is not an IDN-aware domain name slot.
   Obviously, this includes any domain name slot whose specification
   predates IDNA.

1.5.5.  Punycode is an Algorithm, not a Name

   There has been some confusion about whether a "Punycode string" does
   or does not include the prefix and about whether it is required that

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   such strings could have been the output of ToASCII (see RFC 3490,
   Section 4 [RFC3490]).  This specification discourages the use of the
   term "Punycode" to describe anything but the encoding method and
   algorithm of [RFC3492].  The terms defined above are preferred as
   much more clear than terms such as "Punycode string".

1.5.6.  Other Terminology Issues

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

   An "internationalized domain name" (IDN) is a domain name that may
   contain any mixture of LDH-labels, A-labels, or U-labels.  This
   implies that every conventional domain name is an IDN (which implies
   that it is possible for a domain name to be an IDN without it
   containing any non-ASCII characters).  Just as has been the case with
   ASCII names, some DNS zone administrators may impose restrictions,
   beyond those imposed by DNS or IDNA, on the characters or strings
   that may be registered as labels in their zones.  Because of the
   diversity of characters that can be used in a U-label and the
   confusion they might cause, such restrictions are mandatory for IDN
   registries and zones even though the particular restrictions are not
   part of these specifications.  Because these restrictions, commonly
   known as "registry restrictions", only affect what can be registered
   and not resolution processing, they have no effect on the syntax or
   semantics of DNS protocol messages; a query for a name that matches
   no records will yield the same response regardless of the reason why

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   it is not in the zone.  Clients issuing queries or interpreting
   responses cannot be assumed to have any knowledge of zone-specific
   restrictions or conventions.  See Section 5.2.

2.  The Original (2003) IDNA Model

   IDNA is a client-side protocol, i.e., almost all of the processing is
   performed by the client.  The strings that appear in, and are
   resolved by, the DNS conform to the traditional rules for the naming
   of hosts, and consist of ASCII letters, digits, and hyphens.  This
   approach permits IDNA to be deployed without modifications to the DNS
   itself.  That, in turn, avoids both having to upgrade the entire
   Internet to support IDNs and needing to incur the unknown risks to
   deployed systems of DNS structural or design changes especially if
   those changes need to be deployed all at the same time.

   This section contains a summary of the model underlying IDNA2003.  It
   is approximate and is not a substitute for reading and understanding
   the actual specification document [RFC3490] and the documents on
   which it depends.  The summary is not intended to be completely
   balanced.  It emphasizes some characteristics of IDNA2003 that are
   particularly important to understanding the nature of the proposed

   The original IDNA specifications have the logical flow in domain name
   registration and resolution outlined in the balance of this section.
   They are not defined this way; instead, the steps are presented here
   for convenience in comparison to what is being proposed in this
   document and the associated ones.  In particular, IDNA2003 does not
   make as strong a distinction between procedures for registration and
   those for resolution as the ones suggested in Section 3 and
   Section 5.1.

   The IDNA2003 specification explicitly includes the equivalents of the
   steps in Section 2.2, Section 2.3, and Section 2.5 below.  While the
   other steps are present --either inside the protocol or presumed to
   be performed before or after it-- they are not discussed explicitly.
   That omission has been a source of confusion.  Another source has
   been the definition of IDNA2003 as an algorithm, expressed partially
   in prose and partially in pseudo code and tables.  The steps below
   follow the more traditional IETF practice: the functions are
   specified, rather than the algorithms.  The breakdown into steps is
   for clarity of explanation; any implementation that produces the same
   result with the same inputs is conforming.

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2.1.  Proposed label

   The registrant submits a request for an IDN or the user attempts to
   look up an IDN.  The registrant or user typically produces the
   request string by keyboard entry of a character sequence.  That
   sequence is validated only on the basis of its displayed appearance,
   without knowledge of the character coding used for its internal
   representation or other local details of the way the operating system
   processes it.  This string is converted to Unicode if necessary.
   IDNA2003 assumes that the conversion is straightforward enough not to
   be considered by the protocol.

2.2.  Permitted Character Identification

   The Unicode string is examined to prohibit characters that IDNA does
   not permit in input.  The list of excluded characters is quite
   limited because IDNA2003 permits almost all Unicode characters to be
   used as input, with many of them mapped into others.

2.3.  Character Mappings

   The label string is processed through the Nameprep [RFC3491] profile
   of the Stringprep [RFC3454] tables and procedure.  Among other
   things, these procedures apply the Unicode normalization procedure
   NFKC [Unicode-UAX15] which converts compatibility characters to their
   base forms and resolves the different ways in which some characters
   can be represented in Unicode into a canonical form.  In IDNA2003,
   one-way case mapping was also performed, partially simulating the
   query-time folding operation that the DNS provides for ASCII strings.

2.4.  Registry Restrictions

   Registries at all levels of the DNS, not just the top level, are
   expected to establish policies about the labels that may be
   registered and for the processes associated with that action (see the
   discussion of guidelines and statements in [RFC4690]).  Such
   restrictions have always existed in the DNS and have always been
   applied at registration time, with the most notable example being
   enforcement of the hostname (LDH) convention itself.  For IDNs, the
   restrictions to be applied are not an IETF matter except insofar as
   they derive from restrictions imposed by application protocols (e.g.,
   email has always required a more restricted syntax for domain names
   than the restrictions of the DNS itself).  Because these are
   restrictions on what can be registered, it is not generally necessary
   that they be global.  If a name is not found on resolution, it is not
   relevant whether it could have been registered; only that it was not
   registered.  Registry restrictions might include prohibition of
   mixed-script labels or restrictions on labels permitted in a zone if

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   certain other labels are already present.  The "variant" systems
   discussed in [RFC3743] and [RFC4290] are examples of fairly
   sophisticated registry restriction models.  The various sets of ICANN
   IDN Guidelines [ICANN-Guidelines] also suggest restrictions that
   might sensibly be imposed.

   The string produced by the above steps is checked and processed as
   appropriate to local registry restrictions.  Application of those
   registry restrictions may result in the rejection of some labels or
   the application of special restrictions to others.

2.5.  Punycode Conversion

   The resulting label (in Unicode code point character form) is
   processed with the Punycode algorithm [RFC3492] and converted to a
   form suitable for storage in the DNS (the "xn--..." form).

2.6.  Lookup or Insertion in the Zone

   For registration, the Punycode-encoded label is then placed in the
   DNS by insertion into a zone.  For lookup, that label is processed
   according to normal DNS query procedures [RFC1035].

3.  The Revised IDNA Model

   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 Section 4).

3.1.  Localization: The Role of the Local System, Local Preprocessing,
      and the User Interface

   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

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   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
   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 8.

   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).  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., how
   different presentation or combining forms are handled, such as
   proposals now being developed for Tamil.  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.

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3.2.  IDN Processing in the IDNA200X Model

3.2.1.  Summary of Effects

   Separating Domain Name Registration and Resolution in the protocol
   specification has one substantive impact.  With IDNA2003, the tests
   and steps made in these two parts of the protocol are essentially
   identical.  Separating them reflects current practice in which per-
   registry restrictions and special processing are applied at
   registration time but not on resolution.  Even more important in the
   longer term, it allows incremental addition of permitted character
   groups to avoid freezing on one particular version of Unicode.

3.2.2.  Protocols

   The actual registration and lookup protocols for IDNA200X are
   specified in [IDNA200X-Protocol].

4.  IDNA200X Document List

   [[anchor21: This section will need to be extensively revised or
   removed before publication.]]

   The following documents are being produced as part of the IDNA200X

   o  A revised version of this document, containing an overview,
      rationale, and conformance conditions.

   o  A separate document, drawn from material in early versions of this
      one, that explicitly updates and replaces RFC 3490 but which has
      most rationale material from that document moved to this one

   o  A document describing the "Bidi problem" with Stringprep and
      proposing a solution [IDNA200X-Bidi].

   o  A specification of the categories and rules that identify the code
      points allowed in a U-label, based on Unicode 5.0 code
      assignments.  See Section 5 and [IDNA200X-Tables].

   o  One or more documents containing guidance and suggestions for
      registries (in this context, those responsible for establishing
      policies for any zone file in the DNS, not only those at the top
      or second level).  The documents in this category may not be IETF
      products and may be prepared and completed asynchronously with

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      those described above.

5.  Permitted Characters: An Inclusion List

   This section provides an overview of the model used to establish the
   algorithm and character lists of [IDNA200X-Tables] and describes the
   names and applicability of the categories used there.  Note that the
   inclusion of a character in the first category group 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.  It is not normative.  The
   normative generating rules appear in [IDNA200X-Tables] and the rules
   that actually determine what labels can be registered or looked up
   are in [IDNA200X-Protocol].

5.1.  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.
   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 become invisible characters in others.  IDNA2003
   prohibited some characters entirely to avoid dealing with those
   issues -- restrictions that were much too severe for mnemonics based
   on some languages.  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 less well understood than
   many people believed it was several years ago.

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


   Characters identified as "PROTOCOL-VALID" 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 group that has a "right
   to left" property must be used in context with the "Bidi" rules.

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   The term "PROTOCOL-VALID", is used to stress the fact that the
   presence of a character in this category does not imply that a given
   registry need accept registrations containing any of the characters
   in the category.  Registries are still expected to apply judgment
   about labels they will accept and to maintain rules consistent with
   those judgments (see [IDNA200X-Protocol] and Section 5.3).

   Characters that are placed in the "PROTOCOL-VALID" category are never
   removed from it unless the code points themselves are removed from
   Unicode (such removal would be inconsistent with the Unicode
   stability principles (see [Unicode50], Appendix F) and hence should
   never occur).  Contextual Rules

   Characters in the PROTOCOL-VALID category may actually 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 (ZWNJ, U+200C, and ZWJ,
   U+200D), 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.

   These characters must not appear in IDNs without additional
   restrictions, typically because they are invisible 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.  Only rules associated with characters that
   indicate or prohibit joining are fully tested at lookup time.  Rules and Their Application

   The actual rules may be present or absent.  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
   an extended regular expression that specifies the context in which
   the character is permitted.

   Examples of descriptions of typical rules 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 DEF property".

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   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 MUST
   NOT appear in putative labels for either registration or lookup.  Of
   course, a later version of the tables might contain a non-null rule.

   [[anchor24: Definition of regular expression language to be


   Some characters are sufficiently problematic for use in IDNs that
   they should be excluded for both registration and lookup (i.e.,
   conforming applications performing name resolution 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.

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

   Characters that are placed in the "DISALLOWED" category are never
   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 to introduce a new code point into Unicode and
   classify it as "PROTOCOL-VALID".

   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 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 MUST
   NOT appear in labels to be registered or looked up.  The category

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   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.

5.2.  Registration Policy

   These recommendations do not address, but registries SHOULD develop
   and apply addition restrictions 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 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 [RFC3743] and
   [RFC4290] may be helpful in reducing problems that might be perceived
   by users.

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

   The essence of the character rules in IDNA200X is based on the
   realization that there is no 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 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.
   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.

6.  Issues that Constrain Possible Solutions

6.1.  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 is discussed in [IDNA200X-Bidi].  There are, however, also
   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

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   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
   IDNA protocol and the associated documents, the relationship between
   fully-qualified domain names and internationalized labels is
   unchanged from the base DNS specifications.  Comments here 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.
   Should a domain name abc.def, in which both labels are represented in
   scripts that are written right-to-left, be displayed as fed.cba or
   cba.fed?  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
   other protocols).  The natural display order for the typed domain
   name on a right-to-left system is fed.cba.  Does this change if a
   protocol identifier, tail, and the corresponding delimiters are

   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

   It should be obvious that any revision of IDNA must be more clear
   about the distinction between network and display order for complete
   (fully-qualified) domain names, as well as simply for individual
   labels, than the original specification was.  It is likely that some
   strong suggestions should be made about display order as well.

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6.2.  Entry and Display in Applications

   Applications can accept domain names using any character set or sets
   desired by the application developer, and can display domain names in
   any charset.  That is, the IDNA protocol does not affect the
   interface between users and applications.

   An IDNA-aware application can accept and display internationalized
   domain names in two formats: the internationalized character set(s)
   supported by the application (i.e., an appropriate local
   representation of a U-label), and as an A-label.  Applications MAY
   allow the display and user input of A-labels, but are not encouraged
   to do so except as an interface for special purposes, possibly for
   debugging, or to cope with display limitations.  A-labels are opaque
   and ugly, and, where possible, should thus only be exposed to users
   who absolutely need them.  Because IDN labels can be rendered either
   as the 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 default.

   Domain names are often stored and transported in many places.  For
   example, they are part of documents such as mail messages and web
   pages.  They are transported in many parts of many protocols, such as
   both the control commands and the RFC 2822 body parts of SMTP, and
   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.

   All protocols that use domain name slots already have the capacity
   for handling domain names in the ASCII charset.  Thus, A-labels can
   inherently be handled by those protocols.

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

   Users often have expectations about character matching or equivalence
   that are based on their languages, and the orthography of those
   languages, rather than anything 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-umlaut 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 or users must be educated to
   understand that matching will not occur.

   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

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   "ae" ligature is the 27th letter of a 29-letter extended Latin
   alphabet.  It is equivalent to the 28th letter of the Swedish
   alphabet (also containing 29 letters), U+00E4 LATIN SMALL LETTER A
   WITH DIAERESIS, for which an "ae" cannot be substituted according to
   current orthographic standards.

   That character (U+00E4) is also part of the German alphabet where,
   unlike in the Nordic languages, the two-character sequence "ae" is
   usually treated as a fully acceptable alternate orthography.  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, in parallel to the "umlauted a", it cannot be
   correctly represented as "oe" and in the Norwegian alphabet, where it
   is represented, not as "umlauted o", 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.
   For example, the German character Eszett (Sharp S, U+00DF) is
   retained as itself by NFKC but mapped by Stringprep to "ss", but the
   closely-related, but less frequently seen, character "Long S T"
   (U+FB05) is a compatibility character that is mapped out by NFKC.
   Unless exceptions are made, both will be treated as DISALLOWED by
   IDNA200X. But there is significant interest in an exception,
   especially for Eszett.  Depending on what the exception was, making
   it would either raise some backward compatibility problems with
   IDNA2003 or create an unusual special case that would highlight
   differences in preferred orthography between German as written in
   Germany and German as written in some other countries, notably
   Switzerland.  Additional discussion of issues with Eszett appear in
   Section 9.7.

   Additional cases with alphabets written right-to-left are described
   in Section 6.5.

   These issues with 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 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

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   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] to reduce the opportunities for user confusion and fraud
   that would result from the related strings being registered to
   different parties.

6.4.  Case Mapping and Related Issues

   Traditionally in the DNS, ASCII letters have been stored with their
   case preserved.  Matching during the query process has been 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 an 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 IDNA200X fundamentally changes it or could
   do so.  In IDNA, all upper-case characters are mapped to lower-case
   ones and, in general, all code points that represent alternate forms
   of the same character are mapped to that character (including mapping
   Greek final form sigma to the lower case sigma character).  IDNA200X
   permits, at the risk of some incompatibility, slightly more
   flexibility in this area.  That additional flexibility still does not
   solve the problem with final form sigma and other characters that
   Unicode treats as completely separate characters that match only
   under casemapping if at all.  Many people now believe these should be
   handled as separate characters so information about them can be
   preserved in the transformations to A-labels and back.  However
   making a change to permit that behavior would create a situation in
   which the same string, valid in both protocols, would be interpreted
   differently by IDNA2003 and IDNA200X. That would violate one of the
   conditions discussed in Section 9.3.1 and hence require a prefix
   change.  Of course, if a prefix change were made (at the costs
   discussed in Section 9.3.3) there would be several options,
   including, if desired, assigning the characer to the CONTEXTUAL RULE
   REQUIRED category and requiring that it only be used in carefully-

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   selected contexts.

6.5.  Right-to-left Text

   In order to be sure that the directionality of right-to-left text is
   unambiguous, IDNA2003 required that any label in which right-to-left
   characters appear both starts and ends with them, may 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 old and new) are
   required to look at an entire label, not just at individual
   characters.  Unfortunately, the algorithmic model used in IDNA2003
   fails when the final character in a right-to-left string requires a
   combining mark in order to be correctly represented.  The mark will
   be the final code point in the string but is not identified with the
   right-to-left character attribute and Stringprep therefore rejects
   the string.

   This problem manifests itself in languages written with consonantal
   alphabets to which diacritical vocalic systems are applied, and in
   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).  Other
   languages are still being investigated, but the new rules for right
   to left scripts are described in [IDNA200X-Bidi].

7.  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., RFC 1123, Section 1.2.2
   [RFC1123]).  For IDNs to work well, registries must have or require
   sensible policies about what is registered -- conservative policies
   -- and implement and enforce them.  Registries, registrars, or other
   actors who do not do so, or who get too liberal, too greedy, or too
   weird may deserve punishment that will primarily be meted out in the
   marketplace or by consumer protection rules and legislation.  One can
   debate whether or not "punishment by browser vendor" is an effective
   marketplace tool, but it falls into the general category of
   approaches being discussed here.  In any event, the Protocol Police
   (an important, although mythical, Internet mechanism for enforcing

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   protocol conformance) are going to be worth about as much here as
   they usually are -- i.e., very little -- simply because, unlike the
   marketplace and legal and regulatory mechanisms, they have no
   enforcement power.

   Conversely, resolvers can (and SHOULD or maybe MUST) 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 above, if a string doesn't resolve, it makes no
   difference whether it simply wasn't registered or was prohibited by
   some rule.

   If resolvers, 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 reward the good
   choices and punish the bad ones.  In this context, a resolver 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.

8.  Front-end and User Interface Processing

   Domain names may be identified and processed in many contexts.  They
   may be typed in by users either by themselves or as part of URIs or
   IRIs.  They may occur in running text or be processed by one system
   after being provided in another.  They 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.  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 minimum-ambiguity forms
   of A-labels, U-labels, or LDH-labels, it is inevitable that programs
   that process domain names will encounter variant forms.  One source
   of such forms will be labels created under IDNA2003.  Because of the
   way that protocol was specified, there are a significant number of
   domain names in files on the Internet that use characters that cannot
   be represented directly in domain names but for which interpretations

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   are provided.  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

   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 domain name appears in an arbitrary context
   (such as running text), one may be faced with the requirement to know
   that a string is a domain name in order to adjust for the different
   forms of dots but also to have traditional dots to recognize that a
   string is a domain name -- an obvious contradiction.

   As discussed elsewhere in this document, the IDNA200X model is to
   remove all of these mappings and interpretations, including the
   equivalence of different forms of dots, from the protocol, leaving
   such mappings to local processing.  This should not be taken to imply
   that local processing is optional or can be avoided entirely.
   Instead, unless the program context is such that it is known that any
   IDNs that appear will be either U-labels or A-labels, some local
   processing of apparent domain name strings will be required, both to
   maintain compatibility with IDNA2003 and to prevent user
   astonishment.  Such local processing, while not specified in this
   document or the associated ones, will generally take one of two

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

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

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

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      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 do, 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.  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), 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.

9.  Migration and Version Synchronization

9.1.  Design Criteria

   As mentioned above and in RFC 4690, two key goals of this work 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 permitted scripts and other character
   collections without disruption.  The mechanisms that support this are
   outlined above, 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.

9.1.1.  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:

   o  The characters are "letters", numerals, or otherwise used to write
      words in some language.  Symbols, drawing characters, and various
      notational characters are permanently excluded -- some because
      they are actively dangerous in URI, IRI, or similar contexts and
      others because there is no evidence that they are important enough
      to Internet operations or internationalization to justify
      inclusion and the complexities that would come with it (additional

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      discussion and rationale for the symbol decision appears in
      Section 9.5).

      If strings are read out loud, rather than seen on paper, there are
      opportunities for considerable confusion between the name of a
      symbol (and a single symbol may have multiple names) and the
      symbol itself.

   o  As a simplified example of this, assume one wanted to use a
      "heart" or "star" symbol in a label.  This is problematic because
      the 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
      and describing a label containing a heart symbol is hopelessly
      ambiguous.  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  Unlike font and style variations in language (and "mathematical")
      characters, identification of compatibility encodings and the
      application of NFKC is of no help here.  All of these symbols (and
      many other pairs and triples) are treated as valid, independent,
      non-reducible, code points.

   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 not prohibited if the conventions and orthography of a
      particular language cause that to be possible).

   o  Characters that are unassigned in the version of Unicode being
      used by the registry or application are not permitted, even on
      resolution (lookup).  There are at least two reasons for this.
      First, unlike the conditions contemplated in IDNA2003 (except for

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      right-to-left text), we now understand that tests involving the
      context of characters (e.g., some characters being permitted only
      adjacent to other ones of specific types) and integrity tests on
      complete labels will be needed.  Unassigned code points cannot be
      permitted because one cannot determine the contextual rules that
      particular code points will require before characters are assigned
      to them and the properties of those characters fully understood.
      Second, Unicode specifies that an unassigned code point normalizes
      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

   o  Any character that is mapped to another character by Nameprep2003
      or by a current version of NFKC is prohibited as input to IDNA
      (for either registration or resolution).  Implementers of user
      interfaces to applications are free to make those conversions when
      they consider them suitable for their operating system
      environments, context, or users.

   Tables used to identify the characters that are IDNA-valid are
   expected to be driven by the principles above.  The principles are
   not just an interpretation of the tables.

9.1.2.  Labels in Registration

   Anyone entering a label into a DNS zone must properly validate that
   label -- i.e., be sure that the criteria for an A-label are met -- in
   order for Unicode version-independence to be possible.  In

   o  Any label that contains hyphens as its third and fourth characters
      MUST be IDNA-valid.  This implies that, (i) if the third and
      fourth characters are hyphens, the first and second ones MUST be
      "xn" until and unless this specification is updated to permit
      other prefixes and (ii) labels starting in "xn--" MUST be valid
      A-labels, as discussed in Section 3 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 described above), MUST be
      consistent on the systems performing or validating labels to be
      registered.  Note that this does not require that tables reflect
      the latest version of Unicode, only that all tables used on a
      given system are consistent with each other.

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   Systems looking up or resolving DNS labels MUST be able to assume
   that those rules were followed on registration.

9.1.3.  Labels in Resolution (Lookup)

   Anyone looking up a label in a DNS zone MUST

   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
      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.  No attempt should be made to validate
      other contextual rules about characters, including mixed-script
      label prohibitions, although such rules MAY be used to influence
      presentation decisions in the user interface.

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

   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 registration side is also responsible --under the
   protocol and to registrants and users-- for much more careful
   checking than is expected of applications systems that look names up,
   both checking as required by the protocol and checking required by
   whatever policies it develops for minimizing risks due to confusable
   characters and sequences and preserving language or script integrity.

   An application or client that looks names up in the DNS will be able
   to resolve 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.  It SHOULD

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   distinguish, in its messages to users, 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).

9.2.  More Flexibility in User Agents

   One key philosophical difference between IDNA2003 and this proposal
   is that the former provided mappings for many characters into others.
   These mappings were not reversible: the original string could not be
   recovered from the form stored in the DNS and, probably as a
   consequence, users became confused about what characters were valid
   for IDNs and which ones were not.  Too many times, the answer to the
   question "can this character be used in an IDN" was "it depends on
   exactly what you mean by 'used'".

   IDNA200X does not perform these mappings but, instead, prohibits the
   characters that would be mapped to others.  As examples, while
   mathematical characters based on Latin ones are accepted as input to
   IDNA2003, they are prohibited in IDNA200X. Similarly, double-width
   characters and other variations are prohibited as IDNA input.

   Since the rules in [IDNA200X-Tables] provide that only strings that
   are stable under NFKC are valid, if it is convenient for an
   application to perform NFKC normalization before lookup, that
   operation is safe since this will never make the application unable
   to look up any valid string.

   In many cases these prohibitions should have no effect on what the
   user can type at resolution time: it is perfectly reasonable for
   systems that support user interfaces at lookup time, to perform some
   character mapping that is appropriate to the local environment prior
   to actual invocation of IDNA as part of the Unicode conversions of
   [IDNA200X-Protocol] above.  However, those changes will be local ones
   only -- local to environments in which users will clearly understand
   that the character forms are equivalent.  For use in interchange
   among systems, it appears to be much more important that U-labels and
   A-labels can be mapped back and forth without loss of information.

   One specific, and very important, instance of this change in 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 attempted to simulate that behavior by

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   performing case-mapping 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) 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., present in a U-label), then
   IDNA200X should prohibit upper-case characters as input.  Some other
   considerations reinforce this conclusion.  For example, an essential
   element of the ASCII case-mapping functions is that
   uppercase(character) must be equal to
   uppercase(lowercase(character)).  That requirement may not be
   satisfied with IDNs.  The relationship between upper case and lower
   case may even be language-dependent, with different languages (or
   even the same language in different areas) using 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 mapping prior to IDNA processing, but the IDNA
   procedures themselves should neither require such mapping nor expect
   it when it isn't natural to the localized environment.

9.3.  The Question of Prefix Changes

   The conditions that would require a change in the IDNA "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.

9.3.1.  Conditions Requiring a Prefix Change

   An IDN prefix change is needed if a given string would resolve or
   otherwise be 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

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   2.  An input string that is valid under IDNA2003 and also valid under
       IDNA200X yields two different A-labels with the different
       versions of IDNA.  This condition is believed to be essentially
       equivalent to the one above.

       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 9.3.2, 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.

9.3.2.  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 Stringprep tables or IDNA actions, including
       normalization definitions, that affect characters that were
       already invalid under IDNA2003.

   3.  Changes in the style of definitions of Stringprep or Nameprep
       that do not alter the actions performed by them.

9.3.3.  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, all
   registries could not convert all IDNA2003 ("xn--") registrations to a
   new form at the same time, and the costs would be considerable.
   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 IDNA200X 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

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   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, resolve to different targets.

   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.

9.4.  Stringprep Changes and Compatibility

   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.  Section 9.3, above,
   essentially also applies in this context.  The proposed new inclusion
   tables [IDNA200X-Tables], the reduction in the number of characters
   permitted as input for registration or resolution (Section 5), and
   even the proposed changes in handling of right-to-left strings
   [IDNA200X-Bidi] either give interpretations to strings prohibited
   under IDNA2003 or prohibit strings that IDNA2003 permitted.  Strings
   that are valid under both IDNA2003 and IDNA200X, and the
   corresponding versions of Stringprep, are not changed in
   interpretation.  This protocol does not use either Nameprep or
   Stringprep as specified in IDNA2003.

   It is particularly important to keep IDNA processing separate from
   processing for various security protocols because some of the
   constraints that are necessary for smooth and comprehensible use of
   IDNs may be unwanted or undesirable in other contexts.  For example,
   the criteria for good passwords or passphrases are very different
   from those for desirable IDNs.  Similarly, internationalized SCSI
   identifiers and other protocol components are likely to have
   different requirements than IDNs.

   Perhaps even more important in practice, since most other known uses
   of Stringprep encode or process characters that are already in
   normalized form and expect the use of only those characters that can
   be used in writing words of languages, the changes proposed here and
   in [IDNA200X-Tables] are unlikely to have any effect at all,
   especially not on registries and registrations that follow rules
   already in existence when this work started.

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9.5.  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 bans the 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.

   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.  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.

9.6.  Migration Between Unicode Versions: Unassigned Code Points

   In IDNA2003, labels containing unassigned code points are resolved on
   the theory that, if they appear in labels and can be resolved, the
   relevant standards much have changed and the registry has properly
   allocated only assigned values.  This is the one main provision in
   IDNA2003 for migration to new versions of Unicode.

   In this specification, strings containing unassigned code points MUST

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   NOT be either looked up or registered.  There are several reasons for
   this, with the most important ones being:

   o  It cannot be known with sufficient reliability in advance that a
      code point that was not previously assigned will not be assigned
      to a compatibility character.  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 IDNA200X, where compatibility
      characters are generally assigned to DISALLOWED, permitting
      strings containing unassigned characters to be looked up would
      permit violating the principle that characters in DISALLOWED are
      not looked up.

   o  More generally, the status of an unassigned character with regard
      to the DIALLOWED and PROTOCOL-VALID categories, and whether
      contextual rules are required with the latter cannot be evaluated
      until a character is actually assigned and known.

   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 will have been coded by
   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 was made for Unicode 4.0 and 5.0.  The
   reality is that a script that is obscure to much of the rest of the
   world may still be very important to those who use it and cultural
   preservation principles will 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.

9.7.  Other Compatibility Issues

   The existing (2003) IDNA model has several odd artifacts which occur
   largely by accident.  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 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

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   an IDN.  The proposed IDNA200X 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.

10.  Acknowledgments

   The editor and contributors would like to express their thanks to
   those who contributed significant early 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 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 Morphin for conversations that led to considerable
   improvements in the content of this document.

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

11.  Contributors

   While the listed editor held the pen, this document 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 protocols.

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12.  IANA Considerations

   This section gives an overview of registries required for IDNA.  The
   actual definition of the first one appears in [IDNA200X-Tables].

12.1.  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 [IDNA200X-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 [IDNA200X-Tables].

12.2.  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, IANA will create and maintain a
   list of approved contextual rules, using the the "expert reviewer"
   model.  Unlike usual practice, we recommend that the "expert
   reviewer" be a committee that reflects expertise on the relevant
   scripts, and encourage IANA, the IESG, and IAB to establish liaisons
   and work together with other relevant standards bodies to populate
   that committee and its procedures over the long term.

   A table from which that registry can be initialized, and some further
   discussion, appears in Appendix A.

12.3.  IANA Repository of IDN Practices of TLDs

   This registry, often described as the "IANA Language Character Set
   Registry" or "IANA Script Registry" (both somewhat misleading terms)
   is maintained by IANA at the request of ICANN.  It is used to provide
   a central documentation repository of the IDN policies used by top
   level domain (TLD) registries who volunteer to contribute to it and
   is used in conjunction with ICANN Guidelines for IDN use.

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

13.  Security Considerations

   Security on the Internet partly relies on the DNS.  Thus, any change

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   to the characteristics of the DNS can change the security of much of
   the Internet.

   Domain names are used by users to identify and connect to Internet
   servers.  The security of the Internet is compromised if a user
   entering a single internationalized name is connected to different
   servers based on different interpretations of the internationalized
   domain name.

   When systems use local character sets other than ASCII and Unicode,
   this specification leaves the problem of transcoding between the
   local character set and Unicode up to the application or local
   system.  If different applications (or different versions of one
   application) implement different transcoding rules, they could
   interpret the same name differently and contact different servers.
   This problem is not solved by security protocols like TLS that do not
   take local character sets into account.

   To help prevent confusion between characters that are visually
   similar, it is suggested that implementations provide visual
   indications where a domain name contains multiple scripts.  Such
   mechanisms can also be used to show when a name contains a mixture of
   simplified and traditional Chinese characters, or to distinguish zero
   and one from O and l.  DNS zone administrators may impose
   restrictions (subject to the limitations identified elsewhere in this
   document) that try to minimize characters that have similar
   appearance or similar interpretations.  It is worth noting that there
   are no comprehensive technical solutions to the problems of
   confusable characters.  One can reduce the extent of the problems in
   various ways, but probably never eliminate it.  Some specific
   suggestions about identification and handling of confusable
   characters appear in a Unicode Consortium publication

   The registration and resolution models described above and in
   [IDNA200X-Protocol] change the mechanisms available for applications
   and resolvers to determine the validity of labels they encounter.  In
   some respects, the ability to test is strengthened.  For example,
   putative labels that contain unassigned code points will now be
   rejected, while IDNA2003 permitted them (something that is now
   recognized as a considerable source of risk).  On the other hand, the
   protocol specification no longer assumes that the application that
   looks up a name will be able to determine, and apply, information
   about the protocol version used in registration.  In theory, that may
   increase risk since the application will be able to do less pre-
   lookup validation.  In practice, the protection afforded by that test
   has been largely illusory for reasons explained in RFC 4690 and

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   Any change to Stringprep or, more broadly, the IETF's model of the
   use of internationalized character strings in different protocols,
   creates some risk of inadvertent changes to those protocols,
   invalidating deployed applications or databases, and so on.  Our
   current hypothesis is that the same considerations that would require
   changing the IDN prefix (see Section 9.3.2) are the ones that would,
   e.g., invalidate certificates or hashes that depend on Stringprep,
   but those cases require careful consideration and evaluation.  More
   important, it is not necessary to change Stringprep2003 at all in
   order to make the IDNA changes contemplated here.  It is far
   preferable to create a separate document, or separate profile
   components, for IDN work, leaving the question of upgrading to other
   protocols to experts on them and eliminating any possible
   synchronization dependency between IDNA changes and possible upgrades
   to security protocols or conventions.

14.  Change Log

   [[anchor39: RFC Editor: Please remove this section.]]

14.1.  Version -01

   Version -01 of this document is a considerable rewrite from -00.
   Many sections have been clarified or extended and several new
   sections have been added to reflect discussions in a number of
   contexts since -00 was issued.

14.2.  Version -02

   o  Corrected several editorial errors including an accidentally-
      introduced misstatement about NFKC.

   o  Extensively revised the document to synchronize its terminology
      with version 03 of [IDNA200X-Tables] and to provide a better
      conceptual framework for its categories and how they are used.
      Added new material to clarify terminology and relationships with
      other efforts.  More subtle changes in this version lay the
      groundwork for separating the document into a conceptual overview
      and a protocol specification for version 03.

14.3.  Version -03

   o  Removed protocol materials to a separate document and incorporated
      rationale and explanation materials from the original
      specification in RFC 3960 into this document.  Cleaned up earlier
      text to reflect a more mature specification and restructured
      several sections and added additional rationale material.

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   o  Strengthened and clarified the A-label / U-label/ LDH-label

   o  Retitled the document to reflect its evolving role.

14.4.  Version -04

   o  Moved more text from "protocol" and further reorganized material.

   o  Provided new material on "Contextual Rule Required.

   o  Improved consistency of terminology, both internally and with the
      "tables" document.

   o  Improved the IANA Considerations section and discussed the
      existing IDNA-related registry.

   o  More small changes to increase consistency.

14.5.  Version -05

   Changed "YES" category back to "ALWAYS" to re-synch with the tables
   document and provide clearer terminology.

14.6.  Version -06

   o  Clarified the prohibitions on strings that look like A-labels but
      are not and on unassigned code points.

   o  Clarified length restrictions on IDN labels.

   o  Revised the terminology definitions to remove the impression of
      circularity and removed invocations of ToASCII and ToUnicode,
      which do not exist in IDNA200X.

   o  Added a new section on front-end processing.

   o  Added a new section to discuss case-mapping.

   o  Extended the discussion of prefix changes to identify the
      implications of making one.

   o  Several more editorial improvements, corrected references, and
      similar adjustments.

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14.7.  Version -07

   o  Added material that specifically defines the format of contextual

   o  Added and altered text after discussions at the 30 January meeting
      (see Section 10) and the follow-up to those discussions.  Among
      the key decisions at that meeting were to eliminate the
      distinction among the valid categories (formerly "ALWAYS", "MAYBE
      YES", and "MAYBE NO"), to adjust the terminology accordingly, and
      to change "CONTEXTUAL RULE REQUIRED" from a separate category in
      this document and the protocol one to a modifier of what is now
      called "PROTOCOL-VALID".  The consequent changes resulted in
      removal of several sections of explanation from this document.

   o  Resynchronized terminology with "protocol" and "tables" documents.

   o  More editorial and typographic corrections.

Appendix A.  The Contextual Rules Registry

   As discussed in Section 12, a registry of rules that define the
   contexts in which particular PROTOCOL-VALID characters, characters
   associated with a requirement for Contextual Information, are
   permitted.  These rules are expressed as tests on the label in which
   the characters appear (all, or any part of, the label may be tested).

   [[anchor47: In version -07 of this document, this appendix is
   something of a placeholder, intended more to illustrate what the
   descriptions of the rules would look like than to contain actual,
   final, rules.]]

   For each character specified as requiring a contextual rule, a rule
   MAY be established with the following data elements:

   1.  The code point associated with the character.

   2.  The name of the character.

   3.  An indication as to whether the code point requires the rule be
       processed at lookup time (this indication is equivalent to the
       difference between "CONTEXTJ" and "CONTEXTO" in the tables
       document [IDNA200X-Tables]).

   4.  A prose description of the contextual rule.

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   5.  A description of the contextual rule using Unicode Regular
       Expression notation.

   6.  An optional comment

   Should there be any conflict between the two statements of a rule,
   the regular expression form MUST be considered normative until the
   registry can be corrected.

   The rules for the characters listed in the Tables document as
   exception cases or Join_Controls and for which rules are being
   defined at this time appear below.
   [[anchor48: Note in draft: This table is not complete and the rule
   entries below are temporarily only examples.]]

      Between two characters from the same script only.  The script must
      be one in which the use of this character causes significant
      visual transformation of one or both of the adjacent characters;
      [[anchor49: ...Regular expression form to be supplied]]

      Between two characters from the same script only.  The script must
      be one in which the use of this character causes significant
      visual transformation of one or both of the adjacent characters;
      [[anchor50: ...Regular expression form to be supplied]]

   00B7; MIDDLE DOT; F;
      Between two 'l' (U+006C) characters only, used to permit the
      Catalan character ela geminada to be expressed;
      [[anchor51: ...Regular expression form to be supplied]]

      Permitted only in contexts in which U+0375 is permitted.  U+0374
      and U+0375 are indicators for numeric use of letters in older
      Greek writing systems.  U+02B9 is relevant because normalization
      maps U+0374 into it.;

      The script of the preceding character and the subsequent
      character, if any, MUST be Hebrew;
      [[anchor52: ...Regular expression form to be supplied]]

      The script of the preceding character and the subsequent
      character, if any, MUST be Hebrew;
      [[anchor53: ...Regular expression form to be supplied]]

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      MUST NOT be at the beginning of the label, and the previous
      character MUST be in Han Script;
      [[anchor54: ...Regular expression form to be supplied]]

      Adjacent characters MUST be Katakana;
      [[anchor55: ...Regular expression form to be supplied]]

   While the information above is to be used to initialize the registry,
   IANA should treat the table format in this Appendix simply as an
   initial, tentative, suggestion.  Subject to review and comment from
   the IESG and any Expert Reviewers, IANA is responsible for, and
   should develop, a format for that registry, or a copy of it
   maintained in parallel, that is convenient for retrieval and machine
   processing and publish the location of that version.

15.  References

15.1.  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", January 2008, <http://

              Klensin, J., "Internationalizing Domain Names in
              Applications (IDNA): Protocol", February 2008, <http://

              Faltstrom, P., "The Unicode Code Points and IDNA",
              February 2008, <http://stupid.domain.name/idnabis/

              A version of this document is available in HTML format at

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

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

   [RFC3491]  Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
              Profile for Internationalized Domain Names (IDN)",
              RFC 3491, 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", 2006,

              The Unicode Consortium, "The Unicode Standard, Version
              3.0", 2000.

              (Reading, MA, Addison-Wesley, 2000.  ISBN 0-201-61633-5).
              Version 3.2 consists of the definition in that book as
              amended by the Unicode Standard Annex #27: Unicode 3.1
              (http://www.unicode.org/reports/tr27/) and by the Unicode
              Standard Annex #28: Unicode 3.2

              The Unicode Consortium, "The Unicode Standard, Version
              4.0", 2003.

              The Unicode Consortium, "The Unicode Standard, Version
              5.0", 2007.

              Boston, MA, USA: Addison-Wesley.  ISBN 0-321-48091-0

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15.2.  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.",

              ICANN, "IDN Implementation Guidelines", 2006,

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

   [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.

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

   [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.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

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   [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 Report #36:
              Unicode Security Considerations", August 2006,

Author's Address

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

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

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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
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