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Versions: 00 01 02 03 04 05 06 07                                       
Network Working Group                                    J. Klensin, Ed.
Internet-Draft                                         November 18, 2007
Expires: May 21, 2008

  Internationalizing Domain Names for Applications (IDNA): Issues and

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   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 . . . . . . . . . . . . . . . 10
   2.  The Original (2003) IDNA Model . . . . . . . . . . . . . . . . 11
     2.1.  Proposed label . . . . . . . . . . . . . . . . . . . . . . 12
     2.2.  Permitted Character Identification . . . . . . . . . . . . 12
     2.3.  Character Mappings . . . . . . . . . . . . . . . . . . . . 12
     2.4.  Registry Restrictions  . . . . . . . . . . . . . . . . . . 12
     2.5.  Punycode Conversion  . . . . . . . . . . . . . . . . . . . 13
     2.6.  Lookup or Insertion in the Zone  . . . . . . . . . . . . . 13
   3.  A Revised IDNA Model . . . . . . . . . . . . . . . . . . . . . 13
     3.1.  Localization: The Role of the Local System and User
           Interface  . . . . . . . . . . . . . . . . . . . . . . . . 13
     3.2.  IDN Processing in the IDNA200x Model . . . . . . . . . . . 14
       3.2.1.  Summary of Effects . . . . . . . . . . . . . . . . . . 14
   4.  IDNA200x Document List . . . . . . . . . . . . . . . . . . . . 15
   5.  Permitted Characters: An Inclusion List  . . . . . . . . . . . 15
     5.1.  A Tiered Model of Permitted Characters and Labels  . . . . 15
       5.1.1.  ALWAYS . . . . . . . . . . . . . . . . . . . . . . . . 16
       5.1.2.  MAYBE  . . . . . . . . . . . . . . . . . . . . . . . . 17
       5.1.3.  CONTEXTUAL RULE REQUIRED . . . . . . . . . . . . . . . 18
       5.1.4.  NEVER  . . . . . . . . . . . . . . . . . . . . . . . . 18
     5.2.  Layered Restrictions: Tables, Context, Registration,
           Applications . . . . . . . . . . . . . . . . . . . . . . . 19
     5.3.  A New Character List -- History  . . . . . . . . . . . . . 19
     5.4.  Understanding New Issues and Constraints . . . . . . . . . 20
     5.5.  ALWAYS, MAYBE, and Contextual Rules  . . . . . . . . . . . 20
   6.  Issues that Any Solution Must Address  . . . . . . . . . . . . 21
     6.1.  Display and Network Order  . . . . . . . . . . . . . . . . 21
     6.2.  Entry and Display in Applications  . . . . . . . . . . . . 22
     6.3.  The Ligature and Digraph Problem . . . . . . . . . . . . . 23
     6.4.  Right-to-left Text . . . . . . . . . . . . . . . . . . . . 25
   7.  IDNs and the Robustness Principle  . . . . . . . . . . . . . . 25
   8.  Migration and Version Synchronization  . . . . . . . . . . . . 26
     8.1.  Design Criteria  . . . . . . . . . . . . . . . . . . . . . 26
     8.2.  More Flexibility in User Agents  . . . . . . . . . . . . . 29
     8.3.  The Question of Prefix Changes . . . . . . . . . . . . . . 31

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       8.3.1.  Conditions requiring a prefix change . . . . . . . . . 31
       8.3.2.  Conditions not requiring a prefix change . . . . . . . 31
     8.4.  Stringprep Changes and Compatibility . . . . . . . . . . . 32
     8.5.  The Symbol Question  . . . . . . . . . . . . . . . . . . . 33
     8.6.  Other Compatibility Issues . . . . . . . . . . . . . . . . 33
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 34
   10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 34
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 34
     11.1. IDNA Permitted Character Registry  . . . . . . . . . . . . 34
     11.2. IDNA Context Registry  . . . . . . . . . . . . . . . . . . 34
     11.3. IANA Repository of TLD IDN Practices . . . . . . . . . . . 35
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 35
   13. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 36
     13.1. Version -01  . . . . . . . . . . . . . . . . . . . . . . . 36
     13.2. Version -02  . . . . . . . . . . . . . . . . . . . . . . . 36
     13.3. Version -03  . . . . . . . . . . . . . . . . . . . . . . . 37
     13.4. Version -04  . . . . . . . . . . . . . . . . . . . . . . . 37
     13.5. Version -05  . . . . . . . . . . . . . . . . . . . . . . . 37
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 37
     14.2. Informative References . . . . . . . . . . . . . . . . . . 39
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 40
   Intellectual Property and Copyright Statements . . . . . . . . . . 41

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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 5.2), 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 or even a
   particularly important one.

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 the user interface.

   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 use
   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.

   IDNA allows the graceful introduction of IDNs not only by avoiding

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   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, the applications need to be
   modified to conform to this specification.

   IDNA uses the Unicode character repertoire, which avoids the
   significant delays that would be inherent in waiting for a different
   and specific character set 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 possible new version of
   IDNA without specifying which particular documents would be affected.
   While more common IETF usage might refer to the successor document(s)
   as "IDNAbis", this document uses that term, and similar ones, to
   refer to successors to the individual documents, e.g., "IDNAbis" is a
   synonym for the specific successor to RFC3490, or "RFC3490bis".  See
   also Section 4.

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 tens of
   thousands of characters.  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 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 00..7F. Unicode

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

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, 30..39, 41..5A, and 61..7A. "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.  In this document the term "domain name" is used in
   general.  This document explicitly cites those documents whenever
   referring to the host name syntax restrictions defined therein.  The
   remaining definitions in this subsection are essentially a review.

   A label is an individual part 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
   new terminology to reduce dependence on the problematic terms and
   definitions that appears in RFC 3490.

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Internet-Draft             IDNA200X Rationale              November 2007  Terms for IDN Label Codings  IDNA-valid strings, A-label, and U-label

   To improve clarity, this document introduces three new terms.  A
   string is "IDNA-valid" if it meets all of the requirements of this
   specification for an IDNA label.  It may be either an "A-label" or a
   "U-label", and it is expected that specific reference will be made to
   the form appropriate to any context in which the distinction is
   important.  An "A-label" is the ASCII-Compatible Encoding (ACE) form
   of an IDNA-valid string.  It must be a complete label and valid as
   the output of ToASCII, regardless of how it is actually produced.
   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 rule.  A "U-label" is an IDNA-valid string of
   Unicode-coded characters that is a valid output of performing
   ToUnicode on an A-label, again regardless of how the label is
   actually produced.  A Unicode string that cannot be generated by
   decoding a valid A-label is not a valid U-label.  [IDNA200X-protocol]
   specifies the conversions between U-labels and A-labels.

   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
   A-label would be more restrictive.  The exception to this, of course,
   is that the restriction to ASCII characters does not apply to the

   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 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 those two, in presentation
   and user interface forms and in selected protocols other than the DNS
   ones themselves.  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.

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Internet-Draft             IDNA200X Rationale              November 2007  LDH-label and Internationalized Label

   In the hope of further clarifying discussions about IDNs, this
   document uses 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, the categories "U-label", "A-label", and "LDH-
   label" are disjoint, with only the first two referring to IDNs.  When
   such a term is needed, an "internationalized label" is one that is a
   member of the union of those 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.  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

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   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
   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 just
   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 one or more A-labels or U-labels, as appropriate, instead of
   LDH labels.  This implies that every conventional domain name is an

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   IDN (which implies that it is possible for a name to be an IDN
   without it containing any non-ASCII characters).  This document does
   not attempt to define an "internationalized host name".  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.  Such restrictions 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 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.

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

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   been 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.

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.,

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   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
   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.  A 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 and 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

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   available locally-- to the more complex and subtle.  Many people have
   observed that internationalization is just a tool to permit effective
   localization while permitting some global uniformity.  Issues of
   display, of exactly how various strings and characters are entered,
   and so on are inherently issues about localization and user interface

   A protocol such as IDNA can only assume that such operations as data
   entry 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.

   Operations for converting between local character sets and Unicode
   are part of this general set of user interface issues.  The
   conversion is obviously not required at all in a Unicode-native
   system where no conversion is required.  It may, however, involve
   some complexity in one that is not, especially if the elements of the
   local character set do not map exactly and unambiguously into Unicode
   characters and do so in a way that is 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.  It may be
   especially 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.  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.

3.2.  IDN Processing in the IDNA200x Model

   [[anchor20: Placeholder ???  Do we need a summary of the two parts

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

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   longer term, it allows incremental addition of permitted character
   groups to avoid freezing on one particular version of Unicode.

4.  IDNA200x Document List

   [[anchor22: 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 list of code points allowed in a U-label, based on Unicode 5.0
      code assignments.  See Section 5.

   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 all be
      IETF products and may be prepared and completed asynchronously
      with those described above.

5.  Permitted Characters: An Inclusion List

   This section describes 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 one of the first three categories does not imply
   that it can be used indiscriminately; some characters are associated
   with contextual rules that must be applied as well.

5.1.  A Tiered Model of Permitted Characters and Labels

   Moving to an inclusion model requires a new list of characters that
   are permitted in IDNs.  In IDNA2003, the role and utility of

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   characters are independent of context and fixed forever.  Making
   those rules globally has proven impractical, partially because
   handling of particular characters across the languages that use a
   script, or the use of similar or identical-looking characters in
   different scripts, are less well understood than many people believed
   several years ago.  Conversely, IDNA2003 prohibited some characters
   entirely to avoid dealing with some of the issues discussed here --
   restrictions that were much too severe for mnemonics based on some

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

5.1.1.  ALWAYS

   Characters identified as "ALWAYS" are permitted for all uses in IDNs,
   but may be associated with contextual restrictions (for example, any
   character in this group that has a "right to left" property must be
   used in context with the "Bidi" rules).  The presence of a character
   in this category implies that it has been examined and determined to
   be appropriate for IDN use, and that it is well-understood that
   contextual protocol restrictions in addition to those already
   specified, such as rules about the use of given characters, are not
   required.  That, in turn, indicates that the script community
   relevant to that character, reflecting appropriate authorities for
   all of the known languages that use that script, has agreed that the
   script and its components are sufficiently well understood.  This
   subsection discusses characters, rather than scripts, because it is
   explicitly understood that a script community may decide to include
   some characters of the script and not others.

   Because of this condition, which requires evaluation by individual
   script communities of the characters suitable for use in IDNs (not
   just, e.g., the general stability of the scripts in which those
   characters are embedded) it is not feasible to define the boundary
   point between this category and the next one by general properties of
   the characters, such as the Unicode property lists.

   Despite its name, the presence of a character on this list 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 5.2).

   Characters that are placed in the "ALWAYS" category are never removed
   from it unless the code points themselves are removed from Unicode (a

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   condition that may never occur).

5.1.2.  MAYBE

   Characters that are used to write the languages of the world and that
   are thought of broadly as "letters" rather than, e.g., symbols or
   punctuation, and that have not been placed in the "ALWAYS" or "NEVER"
   categories (see Section 5.1.4 for the latter) belong to the "MAYBE"
   category.  As implied above, the collection of scripts and characters
   in "MAYBE" has not yet been reviewed and finally approved by the
   script community.  It is possible that they may be appropriate for
   general use only when special contextual rules (tests on the entire
   label or on adjacent characters) are identified and specified.

   In general and for maximum safety, registries SHOULD confine
   themselves to characters from the "ALWAYS" category.  However, if a
   registry is permitting registrations only in a small number of
   scripts the usage of which it is familiar with to develop rules that
   are safe in its own environment -- it may be entirely appropriate for
   it permit registrations that use characters from the "MAYBE"
   categories as well as the "ALWAYS" one.

   Applications are expected to not treat "ALWAYS" and "MAYBE"
   differently with regard to name resolution ("lookup").  They may
   choose to provide warnings to users when labels or fully-qualified
   names containing characters in the "MAYBE" categories are to be
   presented to users.

   There are actually two subcategories of MAYBE.  The assignment of a
   character to one or the other represents an estimate of whether the
   character will eventually be treated as "ALWAYS" or "NEVER" (some
   characters may, however, remain in the "MAYBE" categories
   indefinitely).  Since the differences between the "MAYBE"
   subcategories do not affect the protocol, characters may be moved
   back and forth between them as information and knowledge accumulates.  Subcategory MAYBE YES

   These are letter, digit, or letter-like characters that are generally
   presumed to be appropriate in DNS labels, for which no specific in-
   depth script or character evaluation has been performed.  The risk
   with characters in the "MAYBE YES" category is that it may later be
   discovered that contextual rules are required for their safe use with
   labels that otherwise contain characters from arbitrary scripts or
   that the characters themselves may be problematic.

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   These are characters that are not letter-like, but are not excluded
   by some other rule.  Given the general ban on characters other than
   letters and digits, it is likely that they will be moved to "NEVER"
   when their contexts are fully understood by the relevant community.
   However, since characters once moved to "NEVER" cannot be moved back
   out, conservatism about making that classification is in order.


   These characters are unsafe for general use in IDNs, 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, these
   characters are assigned to the category "CONTEXTUAL RULE REQUIRED"
   and, when adequately understood, associated with a rule.  Examples of
   typical rules include "Must follow a character from Script XYZ", "MAY
   occur only if the entire label is in Script ABC", "MAY occur only if
   the previous and subsequent characters have the DEF property".

   Because it is easier to identify these characters than to know that
   they are actually needed in IDNs or how to establish exactly the
   right rules for each one, a character in the CONTEXTUAL RULE REQUIRED
   category may have a null (missing) rule set in a given version of the
   tables.  Such characters 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.

   If there is a rule, it MUST be evaluated and tested on registration
   and SHOULD be evaluated and tested on lookup.  If the test fails, the
   label should not be processed for registration or lookup in the DNS.

5.1.4.  NEVER

   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 includes code points that have been removed
   entirely from Unicode should such characters ever occur.

   Characters that are placed in the "NEVER" category are never removed
   from it or reclassified.  If a character is classified as "NEVER" in
   error and the error is sufficiently problematic, the only recourse is

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   to introduce a new code point into Unicode and classify it as "MAYBE"
   or "ALWAYS" as appropriate.

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

   The essence of the character rules in IDNAbis is that there is no
   magic bullet for any of the issues associated with a multiscript DNS.
   Instead, we need to have a variety of approaches that, together,
   constitute multiple lines of defense.  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 in a protocol
   context.  Registrars 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.

5.3.  A New Character List -- History

   [[anchor29: RFC Editor: please delete this subsection.]]

   A preliminary version of a character list that reflects the above
   categories has been was developed by the contributors to this
   document [IDNA200X-Tables].  An earlier, initial, version was
   developed by going through Unicode 5.0 one block and one character
   class at a time and determining which characters, classes, and blocks
   were clearly acceptable for IDNs, which one were clearly unacceptable
   (e.g., all blocks consisting entirely of compatibility characters and
   non-language symbols were excluded as were a number of character
   classes), and which blocks and classes were in need of further study
   or input from the relevant language communities.  That effort was
   successful, but not at the level of producing a directly-useful
   character table.  Additional iterations on the mailing list and with
   UTC participation largely dropped the use of Unicode blocks and
   focused on character classes, scripts, and properties together with
   understandings gained from other Unicode Consortium efforts.  Those
   iterations have been more successful.  The iterative process has led
   to the conclusion that the best strategy is likely to be a mixed one
   consisting of (i) classification into "ALWAYS" and "MAYBE YES" versus
   "MAYBE NO" and "NEVER" based on Unicode properties and a few
   exceptions and (ii) discrimination between "ALWAYS" and "MAYBE YES"
   and between "MAYBE NO" and "NEVER" based on script community criteria
   about IDN appropriateness will be needed.  An alternative would
   involve an entirely new property specifically associated with
   appropriateness for IDN use, but it is not clear that is either
   necessary or desirable.

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5.4.  Understanding New Issues and Constraints

   The discussion in [IDNA200X-Bidi] illustrates some areas in which
   more work and input is needed.  Other issues are raised by the
   Unicode "presentation form" model and, in particular, by the need for
   zero-width characters in some limited cases to correctly designate
   those forms and by some other issues with combining characters in
   different contexts.  It is expected that, once expert and materially-
   concerned parties are identified to supply contextual rules, such
   problems will be resolved quickly and the questioned collections of
   characters either added to the list of permitted characters or
   permanently excluded.

5.5.  ALWAYS, MAYBE, and Contextual Rules

   As discussed above, characters will be associated with the "ALWAYS"
   or "MAYBE YES" properties if they can plausibly be used in an IDN.
   They are classified as "MAYBE NO" if it appears unlikely that they
   should be used in IDNs but there is uncertainty on that point.  Non-
   language characters and other character codes that can be identified
   as globally inappropriate for IDNs, such as conventional spaces and
   punctuation, will be assigned to "NEVER" (i.e., will never be
   permitted in IDNs).  A character associated with "CONTEXTUAL RULE
   REQUIRED" is acceptable in a label if it is associated with the
   identifier of a contextual rule set and the test implied by the rule
   set is successful.  If no such identifier is present in the version
   of the tables in use, the character is treated as roughly equivalent
   to "NEVER", i.e., it MUST NOT be used in either registration or
   lookup with that version of the tables.  Because a rule set
   identifier may be installed in a later table version, this status is
   obviously not permanent.  This general approach could, obviously, be
   implemented in several ways, not just by the exact arrangements
   suggested above.

   The property and rule sets are used as follows:

   o  Systems supporting domain name resolution SHOULD attempt to
      resolve any label consisting entirely of characters that are in
      the "ALWAYS" or "MAYBE" categories, including those that have not
      been permanently excluded but that have not been classified with
      regard to whether additional restrictions are needed, i.e., they
      are categorized as "MAYBE YES" or "MAYBE NO".  They MUST NOT
      attempt to resolve label strings that contain unassigned character
      positions or those that contain "NEVER" characters.

   o  Systems providing domain name registration functions MUST NOT
      register any label that contains characters classified as "NEVER"
      OR code point positions that are unassigned in the version of

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      Unicode they are using.  If a character in a label has associated
      contextual rules, they MUST NOT register the label unless the
      conditions required by those rules are satisfied.  They SHOULD NOT
      register labels that contain a character assigned to a "MAYBE"

   A procedure for assigning rules to characters with the "MAYBE YES" or
   "MAYBE NO" property, and for assigning (or not) the property to
   characters assigned in future version of Unicode, is outlined under
   Section 11.  A key part of that procedure will be specifications that
   make it possible to add new characters and blocks without long delays
   in implementation.  The procedure will result in an update to
   existing IANA-maintained registries.

6.  Issues that Any Solution Must Address

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
   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.

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   Should a domain name abc.def, in which both labels are represented in
   scripts that are written right-to-left, be displayed as 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.

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.

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   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.

6.3.  The Ligature and Digraph Problem

   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).

   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

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   (as in the above example with different spelling conventions).  This
   can be illustrated by many words in the Norwegian language, where the
   "ae" ligature is the 27th letter of a 29-letter extended Latin
   alphabet.  It is equivalent to the 28th letter of the Swedish
   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.

   Additional cases with alphabets written right-to-left are described
   in Section 6.4.  This constitutes a problem that cannot be resolved
   solely by operating on scripts.  It is, however, a key concern in the
   IDN context.  Its 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

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

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6.4.  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
   protocol conformance) are going to be worth about as much here as
   they usually are -- i.e., very little -- simply because, unlike the

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   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) 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 (if they are properly
   defined); one that is willing to look something up, but warns against
   it, is exercising a UI choice.

8.  Migration and Version Synchronization

8.1.  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.

   1.  The general criteria for a putative label, and the collection of
       characters that make it up, to be considered IDNA-valid are:

       *  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

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          similar contexts and others because there is no evidence that
          they are important enough to Internet operations or
          internationalization to justify large numbers of special cases
          and character-specific handling (additional discussion and
          rationale for the symbol decision appears in Section 8.5).  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.  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).

       *  Characters that are unassigned in the version of Unicode being
          used by the registry or application are not permitted, even on
          resolution (lookup).  This is because, unlike the conditions
          contemplated in IDNA2003 (except for 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

       *  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.

   2.  For registration purposes, the collection of IDNA-valid
       characters will be a growing list.  The conditions for entry to
       the list for a set of characters are (i) that they meet the
       conditions for IDNA-valid characters discussed immediately above
       and (ii) that consensus can be reached about usage and contextual
       rules.  Because it is likely that such consensus cannot be
       reached immediately about the correct contextual rules for some

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       characters -- e.g., the use of invisible ("zero-width")
       characters to modify presentation forms -- some sets of
       characters may be deferred from the IDNA-valid set even if they
       appear in a current version of Unicode.  Of course, characters
       first assigned code points in later versions of Unicode would
       need to be introduced into IDNA only after those code points are

   3.  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 particular:

       *  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

       *  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.

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

   4.  Anyone looking up a label in a DNS zone MUST

       *  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.

       *  Validate 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 "NEVER") or code
          points that are unassigned in its version of Unicode.  No
          attempt should be made to validate 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

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       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

   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 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 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).

8.2.  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

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   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
   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.

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8.3.  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.

8.3.1.  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

   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 8.3.2, 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.

8.3.2.  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:

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   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 do not affect characters that
       have already been invalid under IDNA2003.

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

8.4.  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 8.3, 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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8.5.  The Symbol Question

   [[anchor37: Move this material and integrate with the Symbol
   discussion above???]]

   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 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  Unicode names for letters are fairly intuitive, recognizable to
      uses of the relevant script, and unambiguous.  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 as result,
      symbols are a very poor basis for reliable communications.  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.

8.6.  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
   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.

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9.  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, 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.

10.  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.

11.  IANA Considerations

11.1.  IDNA Permitted Character Registry

   The distinction between "MAYBE" code points and those classified into
   "ALWAYS" and "NEVER" (see Section 5) requires a registry of
   characters and scripts and their categories.  IANA is requested to
   establish that registry, using 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.

11.2.  IDNA Context Registry

   For characters that are defined in the permitted character as
   requiring a contextual rule, IANA will create and maintain a list of
   approved contextual rules, using the registration methods described
   above.  IANA should develop a format for that registry, or a copy of
   it maintained in parallel, that is convenient for retrieval and

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   machine processing and publish the location of that version.

11.3.  IANA Repository of TLD IDN Practices

   This registry is maintained by IANA at the request of ICANN, 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
   effect on that registry and no IANA action is required as a result.

12.  Security Considerations

   Security on the Internet partly relies on the DNS.  Thus, any change
   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 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 adminstrators 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 suggestion
   about identification and handling of confusable characters appear in
   a Unicode Consortium publication [???]

   The registration and resolution models described above and in

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   [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

   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 8.3.2) 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.

13.  Change Log

   [[anchor44: RFC Editor: Please remove this section.]]

13.1.  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.

13.2.  Version -02

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

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   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.

13.3.  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.

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

   o  Retitled the document to reflect its evolving role.

13.4.  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.

13.5.  Version -05

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

14.  References

14.1.  Normative References

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

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              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 IDNA problem in right-to-
              left scripts", July 2007, <http://www.ietf.org/

              Faltstrom, P., "The Unicode Codepoints and IDN",
              November 2007, <http://stupid.domain.name/idnabis/

              A version of this document, is available in HTML format at

              Klensin, J., "Internationalizing Domain Names in
              Applications (IDNA): Protocol", November 2007, <http://

   [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.

   [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.

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   [RFC4290]  Klensin, J., "Suggested Practices for Registration of
              Internationalized Domain Names (IDN)", RFC 4290,
              December 2005.

              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

14.2.  Informative References

              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.

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

   [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.

   [RFC4690]  Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
              Recommendations for Internationalized Domain Names
              (IDNs)", RFC 4690, September 2006.

Author's Address

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

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

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