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Network Working Group                                         J. Klensin
Internet-Draft                                        September 12, 2008
Intended status: Standards Track
Expires: March 16, 2009


  Internationalized Domain Names for Applications (IDNA): Definitions,
                        Background and Rationale
                  draft-ietf-idnabis-rationale-02.txt

Status of this Memo

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   This Internet-Draft will expire on March 16, 2009.

Abstract

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







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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 . . . . . . . . . 11
       1.5.6.  Other Terminology Issues . . . . . . . . . . . . . . . 11
     1.6.  Comprehensibility of IDNA Mechanisms and Processing  . . . 12
   2.  The Revised IDNA Model . . . . . . . . . . . . . . . . . . . . 13
   3.  Processing in IDNA2008 . . . . . . . . . . . . . . . . . . . . 14
   4.  IDNA2008 Document List . . . . . . . . . . . . . . . . . . . . 14
   5.  Permitted Characters: An Inclusion List  . . . . . . . . . . . 15
     5.1.  A Tiered Model of Permitted Characters and Labels  . . . . 15
       5.1.1.  PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . . 15
       5.1.2.  DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 17
       5.1.3.  UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 18
     5.2.  Registration Policy  . . . . . . . . . . . . . . . . . . . 18
     5.3.  Layered Restrictions: Tables, Context, Registration,
           Applications . . . . . . . . . . . . . . . . . . . . . . . 19
   6.  Issues that Constrain Possible Solutions . . . . . . . . . . . 19
     6.1.  Display and Network Order  . . . . . . . . . . . . . . . . 19
     6.2.  Entry and Display in Applications  . . . . . . . . . . . . 20
     6.3.  Linguistic Expectations: Ligatures, Digraphs, and
           Alternate Character Forms  . . . . . . . . . . . . . . . . 21
     6.4.  Case Mapping and Related Issues  . . . . . . . . . . . . . 24
     6.5.  Right to Left Text . . . . . . . . . . . . . . . . . . . . 25
   7.  IDNs and the Robustness Principle  . . . . . . . . . . . . . . 25
   8.  Front-end and User Interface Processing  . . . . . . . . . . . 26
   9.  Relationship to IDNA2003 and Earlier Versions of Unicode . . . 28
     9.1.  Summary of Major Changes from IDNA2003 . . . . . . . . . . 29
     9.2.  Migration and Version Synchronization  . . . . . . . . . . 29
       9.2.1.  Design Criteria  . . . . . . . . . . . . . . . . . . . 29
       9.2.2.  More Flexibility in User Agents  . . . . . . . . . . . 33
       9.2.3.  The Question of Prefix Changes . . . . . . . . . . . . 34
       9.2.4.  Stringprep Changes and Compatibility . . . . . . . . . 36
       9.2.5.  The Symbol Question  . . . . . . . . . . . . . . . . . 37
       9.2.6.  Migration Between Unicode Versions: Unassigned
               Code Points  . . . . . . . . . . . . . . . . . . . . . 38
       9.2.7.  Other Compatibility Issues . . . . . . . . . . . . . . 39
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 39
   11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 40



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   12. Internationalization Considerations  . . . . . . . . . . . . . 40
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 41
     13.1. IDNA Character Registry  . . . . . . . . . . . . . . . . . 41
     13.2. IDNA Context Registry  . . . . . . . . . . . . . . . . . . 41
     13.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 41
   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 42
   15. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 43
     15.1. Changes between Version -00 and Version -01 of
           draft-ietf-idnabis-rationale . . . . . . . . . . . . . . . 43
     15.2. Version -02  . . . . . . . . . . . . . . . . . . . . . . . 44
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 44
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 44
     16.2. Informative References . . . . . . . . . . . . . . . . . . 46
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 47
   Intellectual Property and Copyright Statements . . . . . . . . . . 48




































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1.  Introduction

1.1.  Context and Overview

   Several years have passed since the original protocol for
   Internationalized Domain Names (IDNs) was completed and deployed.
   During that time, a number of issues have arisen, including a subset
   of those described in a recent IAB report [RFC4690] and the need to
   update the system to deal with newer versions of Unicode.  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).  Some tuning of the existing protocols and
   the tables on which they depend is now required.  Where it is
   important to understanding of the revised protocols, this document
   further explains the issues that have been encountered.  It also
   provides an overview of the new IDNA model and explanatory material
   for it.  Additional explanatory material for the specific components
   of the proposals will appear with the associated documents.

1.2.  Discussion Forum

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

   This work is being discussed in the IETF "idnabis" Working Group and
   on the mailing list idna-update@alvestrand.no

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" 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 is possible even under the original rules where only ASCII
   characters were permitted.  However, one can hope, through the
   application of different techniques at different points (see
   Section 5.3), to keep problems to an acceptable minimum.  One
   consequence of this general objective is that the desire of some user
   or marketing community to use a particular string --whether the
   reason is to try to write sentences of particular languages in the
   DNS, to express a facsimile of the symbol for a brand, or for some



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   other purpose-- is not a primary goal within the context of
   applications in the domain name space.

1.4.  Applicability and Function of IDNA

   The IDNA standard does not require any applications to conform to it,
   nor does it retroactively change those applications.  An application
   can elect to use IDNA in order to support IDN while maintaining
   interoperability with existing infrastructure.  If an application
   wants to use non-ASCII characters in domain names, IDNA is the only
   currently-defined option.  Adding IDNA support to an existing
   application entails changes to the application only, and leaves room
   for flexibility in front-end processing and more specifically in the
   user interface (see Section 8).

   A great deal of the discussion of IDN solutions has focused on
   transition issues and how IDNs will work in a world where not all of
   the components have been updated.  Proposals that were not chosen by
   the original IDN Working Group would depend on user applications,
   resolvers, and DNS servers being updated in order for a user to apply
   an internationalized domain name in any form or coding acceptable
   under that method.  While processing must be performed prior to or
   after access to the DNS, no changes are needed to the DNS protocol or
   any DNS servers or the resolvers on user's computers.

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

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

   IDNA allows the graceful introduction of IDNs not only by avoiding
   upgrades to existing infrastructure (such as DNS servers and mail
   transport agents), but also by allowing some rudimentary use of IDNs
   in applications by using the ASCII representation of the non-ASCII
   name labels.  While such names are user-unfriendly to read and type,
   and hence not optimal for user input, they can be used as a last



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   resort to allow rudimentary IDN usage.  For example, they might be
   the best choice for display if it were known that relevant fonts were
   not available on the user's computer.  In order to allow user-
   friendly input and output of the IDNs and acceptance of some
   characters as equivalent to those to be processed according to the
   protocol, the applications need to be modified to conform to this
   specification.

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

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 "IDNA2008" is used to refer to a new version of IDNA as
   described in this document and in the documents described in
   Section 4.  References to "these specifications" are to the entire
   set.

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 [Unicode51] is a coded character set containing almost
   100,000 characters as of the current version.  A single Unicode code
   point is denoted by "U+" followed by four to six hexadecimal digits,
   while a range of Unicode code points is denoted by two four to six
   digit hexadecimal numbers separated by "..", with no prefixes.

   ASCII means US-ASCII [ASCII], a coded character set containing 128
   characters associated with code points in the range 0000..007F.
   Unicode may be thought of as an extension of ASCII; it includes all
   the ASCII characters and associates them with equivalent code points.

   "Letters" are, informally, generalizations from the ASCII and common-
   sense understanding of that term, i.e., characters that are used to
   write text that are not digits, symbols, or punctuation.  Formally,
   they are characters with a Unicode General Category value starting in



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   "L" (see Section 4.5 of [Unicode51]).

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" is used to describe the combination of operations
   performed by this protocol and those actually performed by a DNS
   resolver.  The process of placing an entry into the DNS is referred
   to as "registration", similar to common contemporary usage in other
   contexts.  Consequently, any DNS zone administration is described as
   a "registry", regardless of the actual administrative arrangements or
   level in the DNS tree.  A note about that relationship is included in
   the text below where it seems particularly significant.

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

   The base DNS specifications [RFC1034] [RFC1035] discuss "domain
   names" and "host names", but many people and sections of these
   specifications use the terms interchangeably.  Lack of clarity about
   that terminology has contributed to confusion about intent in some
   cases.  This document generally uses the term "domain name".  When it
   refers to, e.g., host name syntax restrictions, it explicitly cites
   the relevant defining documents.  The remaining definitions in this
   subsection are essentially a review.

   A label is an individual component of a domain name.  Labels are
   usually shown separated by dots; for example, the domain name
   "www.example.com" is composed of three labels: "www", "example", and
   "com".  (The zero-length root label described in RFC 1123 [RFC1123],
   which can be explicit as in "www.example.com." or implicit as in
   "www.example.com", is not considered in this specification.)  IDNA
   extends the set of usable characters in labels that are treated as
   text (as distinct from the binary string labels discussed in RFC 1035
   and RFC 2181 [RFC2181] and the bitstring ones described in RFC 2673
   [RFC2673]).  For the rest of this document and in the related ones,
   the term "label" is shorthand for "text label", and "every label"
   means "every text label".

1.5.4.  Terminology Specific to IDNA

   This section defines some terminology to reduce dependence on terms
   and definitions that have been problematic in the past.





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1.5.4.1.  Terms for IDN Label Codings

1.5.4.1.1.  IDNA-valid strings, A-label, and U-label

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

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

   o  An "A-label" is the ASCII-Compatible Encoding (ACE, see
      Section 1.5.4.5) form of an IDNA-valid string.  It must be a
      complete label: IDNA is defined for labels, not for parts of them
      and not for complete domain names.  This means, by definition,
      that every A-label will begin with the IDNA ACE prefix, "xn--",
      followed by a string that is a valid output of the Punycode
      algorithm and hence a maximum of 59 ASCII characters in length.
      The prefix and string together must conform to all requirements
      for a label that can be stored in the DNS including conformance to
      the rules for the preferred form described in RFC 1034, RFC 1035,
      and RFC 1123.

   o  A "U-label" is an IDNA-valid string of Unicode characters,
      including at least one non-ASCII character, expressed in a
      standard Unicode Encoding Form -- normally UTF-8 in an Internet
      transmission context -- and subject to the constraint below.
      Conversions between U-labels and A-labels are performed according
      to the "Punycode" specification [RFC3492], adding or removing the
      ACE prefix (see Section 1.5.4.5) as needed.

   To be valid, U-labels and A-labels must obey an important symmetry
   constraint.  While that constraint may be tested in any of several
   ways, an A-label must be capable of being produced by conversion from
   a U-label and a U-label must be capable of being produced by
   conversion from an A-label.  Among other things, this implies that
   both U-labels and A-labels must be strings in Unicode NFC
   [Unicode-UAX15] normalized form.  These strings MUST contain only
   characters specified elsewhere in this document and its companion
   documents, and only in the contexts indicated as appropriate.

   Any rules or conventions that apply to DNS labels in general, such as
   rules about lengths of strings, apply to whichever of the U-label or
   A-label would be more restrictive.  For the U-label, constraints
   imposed by existing protocols and their presentation forms make the



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   length restriction apply to the length in octets of the UTF-8 form of
   those labels (which will always be greater than or equal to the
   length in code points).  The exception to this, of course, is that
   the restriction to ASCII characters does not apply to the U-label.

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

   o  not A-labels or

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

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

   o  to prevent confusion with pre-IDNA coding forms;

   o  to permit future extensions that would require changing the
      prefix, no matter how unlikely those might be (see Section 9.2.3);
      and

   o  to reduce the opportunities for attacks via the encoding system.


1.5.4.2.  LDH-label and Internationalized Label

   In the hope of further clarifying discussions about IDNs, these
   specifications use the term "LDH-label" strictly to refer to an all-
   ASCII label that obeys the preferred syntax (often known as
   "hostname" (from RFC 952 [RFC0952]) or "LDH") conventions and that is
   not an IDN.  It should be stressed that an A-label obeys the
   "hostname" rules and is sometimes described as "LDH-conformant" or in
   similar language but that it is not an LDH-label as used in this
   document.






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1.5.4.3.  Internationalized Domain Name

   An "internationalized domain name" (IDN) is a domain name that may
   contain any mixture of LDH-labels, A-labels, or U-labels.  This
   implies that every conventional domain name is an IDN (which implies
   that it is possible for a domain name to be an IDN without it
   containing any non-ASCII characters).  Just as has been the case with
   ASCII names, some DNS zone administrators may impose restrictions,
   beyond those imposed by DNS or IDNA, on the characters or strings
   that may be registered as labels in their zones.  Because of the
   diversity of characters that can be used in a U-label and the
   confusion they might cause, such restrictions are mandatory for IDN
   registries and zones even though the particular restrictions are not
   part of these specifications.  Because these restrictions, commonly
   known as "registry restrictions", only affect what can be registered
   and not lookup processing, they have no effect on the syntax or
   semantics of DNS protocol messages; a query for a name that matches
   no records will yield the same response regardless of the reason why
   it is not in the zone.  Clients issuing queries or interpreting
   responses cannot be assumed to have any knowledge of zone-specific
   restrictions or conventions.  See Section 5.2.

   "Internationalized label" is used when a term is needed to refer to a
   single label of an IDN, i.e., one that might be any of an LDH-label,
   A-label, or U-label.  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.

1.5.4.4.  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.

1.5.4.5.  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".





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1.5.4.6.  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() or getaddrinfo() standard C library functions;
   the part of an email address following the at-sign (@) in the
   parameter to the SMTP MAIL or RCPT commands or the "From:" field of
   an email message header; and the host portion of the URI in the src
   attribute of an HTML <IMG> tag.  General text that just happens to
   contain a domain name is not a domain name slot.  For example, a
   domain name appearing in the plain text body of an email message is
   not occupying a domain name slot.

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

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

1.5.5.  Punycode is an Algorithm, not a Name

   There has been some confusion about whether a "Punycode string" does
   or does not include the ACE prefix and about whether it is required
   that such strings could have been the output of the ToASCII operation
   (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.



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   Historically, many, perhaps most, of the "names" in the DNS have been
   mnemonics to identify some particular concept, object, or
   organization.  They are typically derived from, or rooted in, some
   language because most people think in language-based ways.  But,
   because they are mnemonics, they need not obey the orthographic
   conventions of any language: it is not a requirement that it be
   possible for them to be "words".

   This distinction is important because the reasonable goal of an IDN
   effort is not to be able to write the great Klingon (or language of
   one's choice) novel in DNS labels but to be able to form a usefully
   broad range of mnemonics in ways that are as natural as possible in a
   very broad range of scripts.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.6.  Comprehensibility of IDNA Mechanisms and Processing

   One of the major goals of this work is to improve the general
   understanding of how IDNA works and what characters are permitted and
   what happens to them.  Comprehensibility and predictability to users
   and registrants are themselves important motivations and design goals
   for this effort.  The effort includes some new terminology and a
   revised and extended model, both covered in this section, and some
   more specific protocol, processing, and table modifications.  Details
   of the latter appear in other documents (see Section 4).

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

   A protocol such as IDNA can only assume that such operations as data
   entry and reconciliation of differences in character forms are
   possible.  It may make some recommendations about how display might
   work when characters and fonts are not available, but they can only
   be general recommendations and, because display functions are rarely
   controlled by the types of applications that would call upon IDNA,
   will rarely be very effective.




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   However, shifting responsibility for character mapping and other
   adjustments from the protocol (where it was located in IDNA2003) to
   the user interface or processing before invoking IDNA raises issues
   about both what that processing should do and about compatibility for
   references prepared in an IDNA2003 context.  Those issues are
   discussed in Section 8.

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

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


2.  The Revised 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



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   those changes need to be deployed all at the same time.

   [[anchor17: This paragraph is somewhat redundant with material
   above.It will be dropped in -03 if there are not strong arguments for
   keeping it here.]]


3.  Processing in IDNA2008

   These specifications separate Domain Name Registration and Lookup in
   the protocol specification.  Doing so reflects current practice in
   which per-registry restrictions and special processing are applied at
   registration time but not during lookup.  Even more important in the
   longer term, it facilitates incremental addition of permitted
   character groups to avoid freezing on one particular version of
   Unicode.

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



4.  IDNA2008 Document List

   [[anchor19: This section will need to be extensively revised or
   removed before publication.]]

   The following documents are being produced as part of the IDNA2008
   effort.

   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
      [IDNA2008-Protocol].

   o  A document describing the "Bidi problem" with Stringprep and
      proposing a solution [IDNA2008-Bidi].

   o  A specification of the categories and rules that identify the code
      points allowed in a U-label, based on Unicode 5.0 code
      assignments.  See Section 5 and [IDNA2008-Tables].

   o  One or more documents containing guidance and suggestions for
      registries (in this context, those responsible for establishing
      policies for any zone file in the DNS, not only those at the top



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      or second level).  The documents in this category may not be IETF
      products and may be prepared and completed asynchronously with
      those described above.


5.  Permitted Characters: An Inclusion List

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

   The information given in this section is provided to make the rules,
   tables, and protocol easier to understand.  It is not normative.  The
   normative generating rules appear in [IDNA2008-Tables] and the rules
   that actually determine what labels can be registered or looked up
   are in [IDNA2008-Protocol].

5.1.  A Tiered Model of Permitted Characters and Labels

   Moving to an inclusion model requires respecifying the list of
   characters that are permitted in IDNs.  In IDNA2003, the role and
   utility of characters are independent of context and fixed forever
   (or until the standard is replaced).  Making completely context-
   independent rules globally has proven impractical because some
   characters, especially those that are called "Join_Controls" in
   Unicode, are needed to make reasonable use of some scripts but have
   no visible effect(s) in others.  Of necessity, IDNA2003 prohibited
   those types of characters entirely.  But the restrictions were much
   too severe to permit an adequate range of mnemonics for terminology
   based on some languages.  The requirement to support those characters
   but limit their use to very specific contexts was reinforced by the
   observation that handling of particular characters across the
   languages that use a script, or the use of similar or identical-
   looking characters in different scripts, is less well understood than
   many people believed it was several years ago.

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

5.1.1.  PROTOCOL-VALID

   Characters identified as "PROTOCOL-VALID" (often abbreviated
   "PVALID") are, in general, permitted by IDNA for all uses in IDNs.
   Their use may be restricted by rules about the context in which they



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   appear or by other rules that apply to the entire label in which they
   are to be embedded.  For example, any label that contains a character
   in this category that has a "right-to-left" property must be used in
   context with the "Bidi" rules (see [IDNA2008-Bidi]).

   The term "PROTOCOL-VALID" is used to stress the fact that the
   presence of a character in this category does not imply that a given
   registry need accept registrations containing any of the characters
   in the category.  Registries are still expected to apply judgment
   about labels they will accept and to maintain rules consistent with
   those judgments (see [IDNA2008-Protocol] and Section 5.3).

   Characters that are placed in the "PROTOCOL-VALID" category are never
   removed from it unless the code points themselves are removed from
   Unicode (such removal would be inconsistent with the Unicode
   stability principles (see [Unicode51], Appendix F) and hence should
   never occur).

   [[anchor21: Placeholder: Does this topic or comment need additional
   discussion or explanation?]]

5.1.1.1.  Contextual Rules

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

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






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5.1.1.2.  Rules and Their Application

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

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

   Because it is easier to identify these characters than to know that
   they are actually needed in IDNs or how to establish exactly the
   right rules for each one, a rule may have a null value in a given
   version of the tables.  Characters associated with null rules 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.

   The description of the syntax of the rules, and the rules themselves,
   appears in [IDNA2008-Tables].

5.1.2.  DISALLOWED

   Some characters are sufficiently problematic for use in IDNs that
   they should be excluded for both registration and lookup (i.e., IDNA-
   conforming applications performing name lookup should verify that
   these characters are absent; if they are present, the label strings
   should be rejected rather than converted to A-labels and looked up.

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

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

   [[anchor23: Note in Draft: the permanence of DISALLOWED was still
   under discussion in the WG when this draft was posted.  The text
   above reflects the editor's opinion about the emerging consensus but
   is subject to change as the discussion continues.]]




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

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

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

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

5.1.3.  UNASSIGNED

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

5.2.  Registration Policy

   While these recommendations cannot and should not define registry
   policies, registries SHOULD develop and apply additional restrictions
   to reduce confusion and other problems.  For example, it is generally
   believed that labels containing characters from more than one script
   are a bad practice although there may be some important exceptions to
   that principle.  Some registries may choose to restrict registrations
   to characters drawn from a very small number of scripts.  For many
   scripts, the use of variant techniques such as those as described in
   [RFC3743] and [RFC4290], and illustrated for Chinese by the tables
   described in RFC 4713 [RFC4713] may be helpful in reducing problems
   that might be perceived by users.  It is worth stressing that these
   principles of policy development and application apply at all levels
   of the DNS, not only, e.g., TLD registrations and that even a
   trivial, "anything permitted that is valid under the protocol" policy
   is helpful in that it helps users and application developers know
   what to expect..






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

   The essence of the character rules in IDNA2008 is based on the
   realization that there is no magic bullet for any of the issues
   associated with a multiscript DNS.  Instead, the specifications
   define a variety of approaches that, together, constitute multiple
   lines of defense against ambiguity in identifiers and loss of
   referential integrity.  The actual character tables are the first
   mechanism, protocol rules about how those characters are applied or
   restricted in context are the second, and those two in combination
   constitute the limits of what can be done by a protocol alone.  As
   discussed in the previous section (Section 5.2), registries are
   expected to restrict what they permit to be registered, devising and
   using rules that are designed to optimize the balance between
   confusion and risk on the one hand and maximum expressiveness in
   mnemonics on the other.

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


6.  Issues that Constrain Possible Solutions

6.1.  Display and Network Order

   The correct treatment of domain names requires a clear distinction
   between Network Order (the order in which the code points are sent in
   protocols) and Display Order (the order in which the code points are
   displayed on a screen or paper).  The order of labels in a domain
   name that contains characters that are normally written right to left
   is discussed in [IDNA2008-Bidi].  In particular, there are questions
   about the order in which labels are displayed if left to right and
   right to left labels are adjacent to each other, especially if there
   are also multiple consecutive appearances of one of the types.  The
   decision about the display order is ultimately under the control of
   user agents --including web browsers, mail clients, and the like--
   which may be highly localized.  Even when formats are specified by
   protocols, the full composition of an Internationalized Resource
   Identifier (IRI) [RFC3987] or Internationalized Email address
   contains elements other than the domain name.  For example, IRIs
   contain protocol identifiers and field delimiter syntax such as
   "http://" or "mailto:" while email addresses contain the "@" to
   separate local parts from domain names.  User agents are not required
   to use those protocol-based forms directly but often do so.  While
   display, parsing, and processing within a label is specified by the
   IDNA protocol and the associated documents, the relationship between



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   fully-qualified domain names and internationalized labels is
   unchanged from the base DNS specifications.  Comments here about such
   full domain names are explanatory or examples of what might be done
   and must not be considered normative.

   Questions remain about protocol constraints implying that the overall
   direction of these strings will always be left to right (or right to
   left) for an IRI or email address, or if they even should conform to
   such rules.  These questions also have several possible answers.
   Should a domain name abc.def, in which both labels are represented in
   scripts that are written right to left, be displayed as fed.cba or
   cba.fed?  An IRI for clear text web access would, in network order,
   begin with "http://" and the characters will appear as
   "http://abc.def" -- but what does this suggest about the display
   order?  When entering a URI to many browsers, it may be possible to
   provide only the domain name and leave the "http://" to be filled in
   by default, assuming no tail (an approach that does not work for
   other protocols).  The natural display order for the typed domain
   name on a right to left system is fed.cba.  Does this change if a
   protocol identifier, tail, and the corresponding delimiters are
   specified?

   While logic, precedent, and reality suggest that these are questions
   for user interface design, not IETF protocol specifications,
   experience in the 1980s and 1990s with mixing systems in which domain
   name labels were read in network order (left to right) and those in
   which those labels were read right to left would predict a great deal
   of confusion, and heuristics that sometimes fail, if each
   implementation of each application makes its own decisions on these
   issues.

   It should be obvious that any revision of IDNA, including the current
   one, must be clear about the network (transmission on the wire) order
   of characters in labels and for the labels in complete (fully-
   qualified) domain names.  In order to prevent user confusion and, in
   particular, to reduce the chances for inconsistent transcription of
   domain names from printed form, 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 or specified by the operating
   system, 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



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   domain names in two formats: the internationalized character set(s)
   supported by the application (i.e., an appropriate local
   representation of a U-label), and as an A-label.  Applications MAY
   allow the display of A-labels, but are encouraged to not do so except
   as an interface for special purposes, possibly for debugging, or to
   cope with display limitations.  In general, they SHOULD allow, but
   not encourage, user input of that label form.  A-labels are opaque
   and ugly, and, where possible, should thus only be exposed to users
   and in contexts in which they are absolutely needed.  Because IDN
   labels can be rendered either as A-labels or U-labels, the
   application may reasonably have an option for the user to select the
   preferred method of display; if it does, rendering the U-label should
   normally be the default.

   Domain names are often stored and transported in many places.  For
   example, they are part of documents such as mail messages and web
   pages.  They are transported in many parts of many protocols, such as
   both the control commands and the RFC 2822 body parts of SMTP, and
   the headers and the body content in HTTP.  It is important to
   remember that domain names appear both in domain name slots and in
   the content that is passed over protocols.

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

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

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

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

   Users often have expectations about character matching or equivalence
   that are based on their languages and the orthography of those



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   languages.  These expectations may not be consistent with forms or
   actions that can be naturally accommodated in a character coding
   system, especially if multiple languages are written using the same
   script but using different conventions.  A Norwegian user might
   expect a label with the ae-ligature to be treated as the same label
   as one using the Swedish spelling with a-umlaut even though applying
   that mapping to English would be astonishing to users.  A user in
   German might expect a label with an o-umlaut and a label that had
   "oe" substituted, but was otherwise the same, treated as equivalent
   even though that substitution would be a clear error in Swedish.  A
   Chinese user might expect automatic matching of Simplified and
   Traditional Chinese characters, but applying that matching for Korean
   or Japanese text would create considerable confusion.  For that
   matter, an English user might expect "theater" and "theatre" to
   match.

   Related issues arise because there are a number of languages written
   with alphabetic scripts in which single phonemes are written using
   two characters, termed a "digraph", for example, the "ph" in
   "pharmacy" and "telephone".  (Note that characters paired in this
   manner can also appear consecutively without forming a digraph, as in
   "tophat".)  Certain digraphs are normally indicated typographically
   by setting the two characters closer together than they would be if
   used consecutively to represent different phonemes.  Some digraphs
   are fully joined as ligatures (strictly designating setting totally
   without intervening white space, although the term is sometimes
   applied to close set pairs).  An example of this may be seen when the
   word "encyclopaedia" is set with a U+00E6 LATIN SMALL LIGATURE AE
   (and some would not consider that word correctly spelled unless the
   ligature form was used or the "a" was dropped entirely).  When these
   ligature and digraph forms have the same interpretation across all
   languages that use a given script, application of Unicode
   normalization generally resolves the differences and causes them to
   match.  When they have different interpretations, any requirements
   for matching must utilize other methods or users must be educated to
   understand that matching will not occur.

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



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   current orthographic standards.

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

   Some of the ligatures that have explicit code points in Unicode were
   given special handling in IDNA2003 and now pose additional problems
   as people argue that they should have been treated differently to
   preserve important information.  For example, the German character
   Eszett (Sharp S, U+00DF) is retained as itself by NFKC but case-
   folded by Stringprep to "ss", but the closely-related, but less
   frequently seen, character "Long S T" (U+FB05) is a compatibility
   character that is mapped out by NFKC.  Unless exceptions are made,
   both will be treated as DISALLOWED by IDNA2008.  But there is
   significant interest in an exception, especially for Eszett.
   Depending on what the exception was, making it would either raise
   some backward compatibility problems with IDNA2003 or create an
   unusual special case that would highlight differences in preferred
   orthography between German as written in Germany and German as
   written in some other countries, notably Switzerland.  Additional
   discussion of issues with Eszett appear in Section 9.2.7.

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

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

   Just as with the examples of different-looking characters that may be
   assumed to be the same, it is in general impossible to deal with
   these situations in a system such as IDNA -- or with Unicode
   normalization generally -- since determining what to do requires



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   information about the language being used, context, or both.
   Consequently, these specifications make no attempt to treat these
   combined characters in any special way.  However, their existence
   provides a prime example of a situation in which a registry that is
   aware of the language context in which labels are to be registered,
   and where that language sometimes (or always) treats the two-
   character sequences as equivalent to the combined form, should give
   serious consideration to applying a "variant" model [RFC3743]
   [RFC4290] to reduce the opportunities for user confusion and fraud
   that would result from the related strings being registered to
   different parties.

6.4.  Case Mapping and Related Issues

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

   The solution of keeping the characters separate but doing matching
   independent of case is not feasible with an IDNA-like model because
   the matching would then have to be done on the server rather than
   have characters mapped on the client.  That situation was recognized
   in IDNA2003 and nothing in IDNA2008 fundamentally changes it or could
   do so.  In IDNA2003, all upper-case characters are mapped to lower-
   case ones and, in general, all code points that represent alternate
   forms of the same character are mapped to that character (including
   mapping Greek final form sigma to the medial form).  IDNA2008
   permits, at the risk of some incompatibility, slightly more
   flexibility in this area.  That additional flexibility still does not
   solve the problem with final form sigma and other characters that
   Unicode treats as completely separate characters that match only
   under casemapping if at all.  Many people now believe these should be
   handled as separate characters so information about them can be
   preserved in the transformations to A-labels and back.  However
   making a change to permit that behavior would create a situation in
   which the same string, valid in both protocols, would be interpreted
   differently by IDNA2003 and IDNA2008.  In principle, that would
   violate one of the conditions discussed in Section 9.2.3.1 and hence
   require a prefix change.  Of course, if a prefix change were made (at
   the costs discussed in Section 9.2.3.3) there would be several
   options, including, if desired, assigning the characer to the
   CONTEXTUAL RULE REQUIRED category and requiring that it only be used
   in carefully-selected contexts.




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6.5.  Right to Left Text

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

   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).  The new
   rules for right to left scripts are described in [IDNA2008-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, not only must the protocol be
   carefully designed and implemented, but zone administrators
   (registries) must have and require sensible policies about what is
   registered -- conservative policies -- and implement and enforce
   them.

   Conversely, lookup applications 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
   registered.




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   As mentioned elsewhere, if a string cannot be successfully found in
   the DNS after the lookup processing described here, it makes no
   difference whether it simply wasn't registered or was prohibited by
   some rule.

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


8.  Front-end and User Interface Processing

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

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



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   lower-case (there are also a few characters that are given special-
   case mapping treatment in Stringprep).

   Other issues in domain name identification and processing arise
   because IDNA2003 specified that several other characters be treated
   as equivalent to the ASCII period (dot, full stop) character used as
   a label separator.  If a string that might be a domain name appears
   in an arbitrary context (such as running text), it is difficult, even
   with only ASCII characters, to know whether an actual domain name (or
   a protocol parameter like a URI) is present and where it starts and
   ends.  When using Unicode, this gets even more difficult if treatment
   of certain special characters (like the dot that separates labels in
   a domain name) depends on context (e.g., prior knowledge of whether
   the string represents a domain name or not).  That knowledge is not
   available if the primary heuristic for identifying the presence of
   domain names in strings depends on the presence of dots separating
   groups of characters with no intervening spaces.
   [[anchor27: Above text is a substitute for an earlier (pre -01)
   version and is hoped to be more clear.  Comments and improvements
   welcome.]]

   As discussed elsewhere in this document, the IDNA2008 model removes
   all of these mappings and interpretations, including the equivalence
   of different forms of dots, from the protocol, discouraging such
   mappings and leaving them, when necessary, to local processing.  This
   should not be taken to imply that local processing is optional or can
   be avoided entirely.  Instead, unless the program context is such
   that it is known that any IDNs that appear will be either U-labels or
   A-labels, or that other forms can safely be rejected, some local
   processing of apparent domain name strings will be required, both to
   maintain compatibility with IDNA2003 and to prevent user
   astonishment.  Such local processing, while not specified in this
   document or the associated ones, will generally take one of two
   forms:

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

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



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      by IDNA2003.

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

      In these cases, we should avoid trying to tell implementers what
      they should do, if only because they are quite likely (and for
      good reason) to ignore us.  We would assume that they would map
      characters that the intuitions of their users would suggest be
      mapped and would hope that they would do that mapping as early as
      possible, storing A-label or U-label forms in files and
      transporting only those forms between systems.  One can imagine
      switches about whether some sorts of mappings occur, warnings
      before applying them or, in a slightly more extreme version of the
      approach taken in Internet Explorer version 7 (IE7), systems that
      utterly refuse to handle "strange" characters at all if they
      appear in U-label form.  None of those local decisions are a
      threat to interoperability as long as (i) only U-labels and
      A-labels are used in interchange with systems outside the local
      environment, (ii) no character that would be valid in a U-label as
      itself is mapped to something else, (iii) any local mappings are
      applied as a preprocessing step (or, for conversions from U-labels
      or A-labels to presentation forms, postprocessing), not as part of
      IDNA processing proper, and (iv) appropriate consideration is
      given to labels that might have entered the environment in
      conformance to IDNA2003. [[anchor28: Placeholder: there have been
      suggestions that this text be removed entirely.  Comments (or
      improved text) welcome.]]

   In either case, it is vital that user interface designs and, where
   the interfaces are not sufficient, users, be aware that the only
   forms of domain names that this protocol anticipates will resolve
   globally or compare equal when crude methods (i.e., those not
   conforming to Section 1.5.4.4) are used are those in which all
   native-script labels are in U-label form.  Forms that assume mapping
   will occur, especially forms that were not valid under IDNA2003, may
   or may not function in predictable ways across all implementations.


9.  Relationship to IDNA2003 and Earlier Versions of Unicode






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9.1.  Summary of Major Changes from IDNA2003

   1.   Update base character set from Unicode 3.2 to Unicode version-
        agnostic.

   2.   Separate the definitions for the "registration" and "lookup"
        activities.

   3.   Disallow symbol and punctuation characters except where special
        exceptions are necessary.

   4.   Remove the mapping and normalization steps from the protocol and
        have them instead done by the applications themselves, possibly
        in a local fashion, before invoking the protocol.

   5.   Change the way that the protocol specifies which characters are
        allowed in labels from "humans decide what the table of
        codepoints contains" to "decision about codepoints are based on
        Unicode properties plus a small exclusion list created by
        humans".

   6.   Introduce the new concept of characters that can be used only in
        specific contexts.

   7.   Allow typical words and names in languages such as Dhivehi and
        Yiddish to be expressed.

   8.   Make bidirectional domain names (delimited strings of labels,
        not just labels standing on their own) display in a non-
        surprising fashion whether they appear in obvious domain name
        contexts or as part of running text in paragraphs.

   9.   Remove the dot separator from the mandatory part of the
        protocol.

   10.  Make some currently-valid labels that are not actually IDNA
        labels invalid.

9.2.  Migration and Version Synchronization

9.2.1.  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 or, subsequent to this version,
   "heavy" processes such as formation of an IETF WG.  The mechanisms



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   that support this are outlined above, but this section reviews them
   in a context that may be more helpful to those who need to understand
   the approach and make plans for it.

9.2.1.1.  General IDNA Validity Criteria

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

   o  The characters are "letters", marks needed to form letters,
      numerals, or other code points used to write words in some
      language.  Symbols, drawing characters, and various notational
      characters are permanently excluded -- some because they are
      actively dangerous in URI, IRI, or similar contexts and others
      because there is no evidence that they are important enough to
      Internet operations or internationalization to justify inclusion
      and the complexities that would come with it (additional
      discussion and rationale for the symbol decision appears in
      Section 9.2.5).

   o  Other than in very exceptional cases, e.g., where they are needed
      to write substantially any word of a given language, punctuation
      characters are excluded as well.  The fact that a word exists is
      not proof that it should be usable in a DNS label and DNS labels
      are not expected to be usable for multiple-word phrases (although
      they are certainly not prohibited if the conventions and
      orthography of a particular language cause that to be possible).
      Even for English, very common constructions -- contractions like
      "don't" or "it's", names that are written with apostrophes such as
      "O'Reilly" or characters for which apostrophes are common
      substitutes, and words whose usually-preferred spellings retain
      diacritical marks from earlier forms -- cannot be represented in
      DNS labels.

   o  Characters that are unassigned (have no character assignment at
      all) in the version of Unicode being used by the registry or
      application are not permitted, even on lookup.  There are at least
      two reasons for this.  Tests involving the context of characters
      (e.g., some characters being permitted only adjacent to ones of
      specific types but otherwise invisible or very problematic for
      other reasons) and integrity tests on complete labels are needed.
      Unassigned code points cannot be permitted because one cannot
      determine whether particular code points will require contextual
      rules (and what those rules should be) before characters are
      assigned to them and the properties of those characters fully
      understood.  Second, Unicode specifies that an unassigned code
      point normalizes and case folds to itself.  If the code point is
      later assigned to a character, and particularly if the newly-



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      assigned code point has a combining class that determines its
      placement relative to other combining characters, it could
      normalize to some other code point or sequence, creating confusion
      and/or violating other rules listed here.

   o  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 lookup).  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 (described in more
   precise form in [IDNA2008-Tables]).  The principles are not just an
   interpretation of the tables.

9.2.1.2.  Labels in Registration

   Anyone entering a label into a DNS zone must properly validate that
   label -- i.e., be sure that the criteria for that label are met -- in
   order for applications to work as intended.  This principle is not
   new: for example, zone administrators are expected to verify that
   names meet "hostname" [RFC0952] or special service location formats
   [RFC2782] where necessary for the expected applications.  For zones
   that will contain IDNs, support for Unicode version-independence
   requires restrictions on all strings placed in the zone.  In
   particular, for such zones:

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

   o  The Unicode tables (i.e., tables of code points, character
      classes, and properties) and IDNA tables (i.e., tables of
      contextual rules such as those 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.

   [[anchor31: Note in draft: the above text was changed significantly
   between -00 and -01 to clearly restrict its scope to zones supporting
   IDNA and to eliminate comments about labels containing "--" in the
   third and forth positions but with different prefixes.  There appears
   to be consensus that more extensive rules belong in a "best
   practices" document about appropriate DNS labels, but that document
   is not in-scope for the IDNABIS WG.]]



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   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.

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

9.2.1.3.  Labels in Lookup

   Anyone looking up a label in a DNS zone

   o  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.

   o  MUST validate the characters in labels to be looked up only to the
      extent of determining that the U-label does not contain either
      code points prohibited by IDNA (categorized as "DISALLOWED") or
      code points that are unassigned in its version of Unicode.

   o  MUST validate the label itself for conformance with a small number
      of whole-label rules, notably verifying that there are no leading
      combining marks, that the "bidi" conditions are met if right to
      left characters appear, that any required contextual rules are
      available and that, if such rules are associated with Joiner
      Controls, they are tested.

   o  MUST NOT validate other contextual rules about characters,
      including mixed-script label prohibitions, although such rules MAY
      be used to influence presentation decisions in the user interface.

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

   An application or client that looks processes names according to this
   protocol and then resolves them in the DNS will be able to locate any



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   name that is validly registered, as long as its version of the
   Unicode-associated tables is sufficiently up-to-date to interpret all
   of the characters in the label.  It SHOULD distinguish, in its
   messages to users, between "label contains an unallocated code point"
   and other types of lookup failures.  A failure on the basis of an old
   version of Unicode may lead the user to a desire to upgrade to a
   newer version, but will have no other ill effects (this is consistent
   with behavior in the transition to the DNS when some hosts could not
   yet handle some forms of names or record types).

9.2.2.  More Flexibility in User Agents

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

   Since the rules in [IDNA2008-Tables] provide that only strings that
   are stable under NFKC are valid, if it is convenient for an
   application to perform NFKC normalization before lookup, that
   operation is safe since this will never make the application unable
   to look up any valid string.

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

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




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   As suggested earlier in this section, it appears to be desirable to
   do as little character mapping as possible consistent with having
   Unicode work correctly (e.g., NFC mapping to resolve different
   codings for the same character is still necessary although the
   specifications require that it be performed prior to invoking the
   protocol) and to make the mapping between A-labels and U-labels
   idempotent.  Case-mapping is not an exception to this principle.  If
   only lower case characters can be registered in the DNS (i.e., be
   present in a U-label), then IDNA2008 should prohibit upper-case
   characters as input.  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) expecting different
   mappings.  Of course, the expectations of users who are accustomed to
   a case-insensitive DNS environment will probably be well-served if
   user agents perform case mapping prior to IDNA processing, but the
   IDNA procedures themselves should neither require such mapping nor
   expect them when they are not natural to the localized environment.

9.2.3.  The Question of Prefix Changes

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

9.2.3.1.  Conditions Requiring a Prefix Change

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

   1.  The conversion of an A-label to Unicode (i.e., a U-label) yields
       one string under IDNA2003 (RFC3490) and a different string under
       IDNA2008.

   2.  An input string that is valid under IDNA2003 and also valid under
       IDNA2008 yields two different A-labels with the different
       versions of IDNA.  This condition is believed to be essentially



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       equivalent to the one above.

       Note, however, that if the input string is valid under one
       version and not valid under the other, this condition does not
       apply.  See the first item in Section 9.2.3.2, below.

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

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

9.2.3.2.  Conditions Not Requiring a Prefix Change

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

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

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

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

   Of course, because these specifications do not involve changes to
   Stringprep or Nameprep, the third condition above and part of the
   second are moot.

9.2.3.3.  Implications of Prefix Changes

   While it might be possible to make a prefix change, the costs of such
   a change are considerable.  Even if they wanted to do so, all
   registries could not convert all IDNA2003 ("xn--") registrations to a
   new form at the same time and synchronize that change with
   applications supporting lookup.  Unless all existing registrations
   were simply to be declared invalid, and perhaps even then, systems
   that needed to support both labels with old prefixes and labels with
   new ones would first process a putative label under the IDNA2008
   rules and try to look it up and then, if it were not found, would



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   process the label under IDNA2003 rules and look it up again.  That
   process could significantly slow down all processing that involved
   IDNs in the DNS especially since, in principle, a fully-qualified
   name could contain a mixture of labels that were registered with the
   old and new prefixes, a situation that would make the use of DNS
   caching very difficult.  In addition, looking up the same input
   string as two separate A-labels would create some potential for
   confusion and attacks, since they could, in principle, map to
   different targets and then resolve to different DNS label nodes.

   Consequently, a prefix change is to be avoided if at all possible,
   even if it means accepting some IDNA2003 decisions about character
   distinctions as irreversible.

9.2.4.  Stringprep Changes and Compatibility

   Concerns have been expressed about problems for non-DNS uses of
   Stringprep being caused by changes to the specification intended to
   improve the handling of IDNs, most notably as this might affect
   identification and authentication protocols.  Section 9.2.3, above,
   essentially also applies in this context.  The proposed new inclusion
   tables [IDNA2008-Tables], the reduction in the number of characters
   permitted as input for registration or lookup (Section 5), and even
   the proposed changes in handling of right to left strings
   [IDNA2008-Bidi] either give interpretations to strings prohibited
   under IDNA2003 or prohibit strings that IDNA2003 permitted.  Strings
   that are valid under both IDNA2003 and IDNA2008, 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 [IDNA2008-Tables] are unlikely to have any effect at all,
   especially not on registries and registrations that follow rules
   already in existence when this work started.




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9.2.5.  The Symbol Question

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

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

   o  Most Unicode names for letters are, in most cases, fairly
      intuitive, unambiguous and recognizable to users of the relevant
      script.  Symbol names are more problematic because there may be no
      general agreement on whether a particular glyph matches a symbol;
      there are no uniform conventions for naming; variations such as
      outline, solid, and shaded forms may or may not exist; and so on.
      As just one example, consider a "heart" symbol as it might appear
      in a logo that might be read as "I love...".  While the user might
      read such a logo as "I love..." or "I heart...", considerable
      knowledge of the coding distinctions made in Unicode is needed to
      know that there more than one "heart" character (e.g., U+2665,
      U+2661, and U+2765) and how to describe it.  These issues are of
      particular importance if strings are expected to be understood or
      transcribed by the listener after being read out loud.
      [[anchor33: The above paragraph remains controversial as to
      whether it is valid.  The WG will need to make a decision if this
      section is not dropped entirely.]]

   o  As a simplified example of this, assume one wanted to use a
      "heart" or "star" symbol in a label.  This is problematic because
      the those names are ambiguous in the Unicode system of naming (the
      actual Unicode names require far more qualification).  A user or
      would-be registrant has no way to know --absent careful study of
      the code tables-- whether it is ambiguous (e.g., where there are
      multiple "heart" characters) or not.  Conversely, the user seeing
      the hypothetical label doesn't know whether to read it --try to
      transmit it to a colleague by voice-- as "heart", as "love", as
      "black heart", or as any of the other examples below.





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

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

   [[anchor34: Note in Draft: Should the above section be significantly
   trimmed or eliminated?]]

9.2.6.  Migration Between Unicode Versions: Unassigned Code Points

   In IDNA2003, labels containing unassigned code points are looked up
   on the theory that, if they appear in labels and can be mapped and
   then resolved, the relevant standards must have changed and the
   registry has properly allocated only assigned values.

   In this specification, strings containing unassigned code points MUST
   NOT be either looked up or registered.  There are several reasons for
   this, with the most important ones being:

   o  It cannot be known with sufficient reliability in advance that a
      code point that was not previously assigned will not be assigned
      to a compatibility character.  In IDNA2003, since there is no
      direct dependency on NFKC (Stringprep's tables are based on NFKC,
      but IDNA2003 depends only on Stringprep), allocation of a
      compatibility character might produce some odd situations, but it
      would not be a problem.  In IDNA2008, where compatibility
      characters are generally assigned to DISALLOWED, permitting
      strings containing unassigned characters to be looked up would
      permit violating the principle that characters in DISALLOWED are
      not looked up.





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

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

9.2.7.  Other Compatibility Issues

   The existing (2003) IDNA model includes several odd artifacts of the
   context in which it was developed.  Many, if not all, of these are
   potential avenues for exploits, especially if the registration
   process permits "source" names (names that have not been processed
   through IDNA and nameprep) to be registered.  As one example, since
   the character Eszett, used in German, is mapped by IDNA2003 into the
   sequence "ss" rather than being retained as itself or prohibited, a
   string containing that character but that is otherwise in ASCII is
   not really an IDN (in the U-label sense defined above) at all.  After
   Nameprep maps the Eszett out, the result is an ASCII string and so
   does not get an xn-- prefix, but the string that can be displayed to
   a user appears to be an IDN.  The proposed IDNA2008 eliminates this
   artifact.  A character is either permitted as itself or it is
   prohibited; special cases that make sense only in a particular
   linguistic or cultural context can be dealt with as localization
   matters where appropriate.


10.  Acknowledgments

   The editor and contributors would like to express their thanks to
   those who contributed significant early (pre-WG) review comments,
   sometimes accompanied by text, especially Mark Davis, Paul Hoffman,
   Simon Josefsson, and Sam Weiler.  In addition, some specific ideas
   were incorporated from suggestions, text, or comments about sections
   that were unclear supplied by Frank Ellerman, Michael Everson, Asmus



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   Freytag, Erik van der Poel, Michel Suignard, and Ken Whistler,
   although, as usual, they bear little or no responsibility for the
   conclusions the editor and contributors reached after receiving their
   suggestions.  Thanks are also due to Vint Cerf, Debbie Garside, and
   Jefsey Morphin for conversations that led to considerable
   improvements in the content of this document.

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

   Special thanks are due to Paul Hoffman for permission to extract
   material from his Internet-Draft to form the basis for Section 9.1.

   Useful comments and text on the WG versions of the draft were
   received from many participants in the IETF "IDNABIS" WG and a number
   of document changes resulted from mailing list discussions made by
   that group.  Marcos Sanz provided specific analysis and suggestions
   that were exceptionally helpful in refining the text.


11.  Contributors

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


12.  Internationalization Considerations

   DNS labels and fully-qualified domain names provide mnemonics that
   assist in identifying and referring to resources on the Internet.
   IDNs expand the range of those mnemonics to include those based on
   languages and character sets other than Western European and Roman-



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   derived ones.  But domain "names" are not, in general, words in any
   language.  The recommendations of the IETF policy on character sets
   and languages, BCP 18 [RFC2277] are applicable to situations in which
   language identification is used to provide language-specific
   contexts.  The DNS is, by contrast, global and international and
   ultimately has nothing to do with languages.  Adding languages (or
   similar context) to IDNs generally, or to DNS matching in particular,
   would imply context dependent matching in DNS, which would be a very
   significant change to the DNS protocol itself.  It would also imply
   that users would need to identify the language associated with a
   particular label in order to look that label up, a decision that
   would be impossible in many or most cases.


13.  IANA Considerations

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

13.1.  IDNA Character Registry

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

13.2.  IDNA Context Registry

   For characters that are defined in the IDNA Character Registry list
   as PROTOCOL-VALID but requiring a contextual rule (i.e., the types of
   rule described in Section 5.1.1.1), IANA will create and maintain a
   list of approved contextual rules.  The details for those rules
   appear in [IDNA2008-Tables].

13.3.  IANA Repository of IDN Practices of TLDs

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

   It is not an IETF-managed registry and, while the protocol changes
   specified here may call for some revisions to the tables, these



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   specifications have no direct effect on that registry and no IANA
   action is required as a result.


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

   To help prevent confusion between characters that are visually
   similar, it is suggested that implementations provide visual
   indications where a domain name contains multiple scripts.  Such
   mechanisms can also be used to show when a name contains a mixture of
   simplified and traditional Chinese characters, or to distinguish zero
   and one from O and l.  DNS zone administrators may impose
   restrictions (subject to the limitations identified elsewhere in this
   document) that try to minimize characters that have similar
   appearance or similar interpretations.  It is worth noting that there
   are no comprehensive technical solutions to the problems of
   confusable characters.  One can reduce the extent of the problems in
   various ways, but probably never eliminate it.  Some specific
   suggestions about identification and handling of confusable
   characters appear in a Unicode Consortium publication
   [Unicode-UTR36].

   The registration and lookup models described above and in
   [IDNA2008-Protocol] change the mechanisms available for lookup
   applications 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



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   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
   above.

   Any change to Stringprep or, more broadly, the IETF's model of the
   use of internationalized character strings in different protocols,
   creates some risk of inadvertent changes to those protocols,
   invalidating deployed applications or databases, and so on.  Our
   current hypothesis is that the same considerations that would require
   changing the IDN prefix (see Section 9.2.3.2) 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.

   No mechanism involving names or identifiers alone can protect a wide
   variety of security threats and attacks that are largely independent
   of them including spoofed pages, DNS query trapping and diversion,
   and so on.


15.  Change Log

   [[anchor42: RFC Editor: Please remove this section.]]

15.1.  Changes between Version -00 and Version -01 of
       draft-ietf-idnabis-rationale

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

   o  Rewrote the discussion of Labels in Registration (Section 9.2.1.2)
      and related text in Section 1.5.4.1.1 to narrow its focus and
      remove more general restrictions.  Added a temporary note in line
      to explain the situation.

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



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      controversial assertions.

   o  Added a discussion of examples to Section 9.2.1

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

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

15.2.  Version -02

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

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

   o  Rewrote the material on preprocessing somewhat.

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

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


16.  References

16.1.  Normative References

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

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

   [IDNA2008-Bidi]
              Alvestrand, H. and C. Karp, "An updated IDNA criterion for
              right to left scripts", July 2008, <https://
              datatracker.ietf.org/drafts/draft-ietf-idnabs-bidi/>.

   [IDNA2008-Protocol]
              Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", July 2008, <https://
              datatracker.ietf.org/drafts/draft-ietf-idnabis-protocol/>.



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   [IDNA2008-Tables]
              Faltstrom, P., "The Unicode Code Points and IDNA",
              July 2008, <https://datatracker.ietf.org/drafts/
              draft-ietf-idnabis-tables/>.

              A version of this document is available in HTML format at
              http://stupid.domain.name/idnabis/
              draft-ietf-idnabis-tables-02.html

   [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.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RulesInit]
              Klensin, J., "Internationalizing Domain Names in
              Applications (IDNA): Protocol, Appendix A Contextual Rules
              Table", July 2008, <http://www.ietf.org/internet-drafts/
              draft-ietf-idnabis-protocol-02.txt>.

   [Unicode-UAX15]
              The Unicode Consortium, "Unicode Standard Annex #15:
              Unicode Normalization Forms", March 2008,
              <http://www.unicode.org/reports/tr15/>.

   [Unicode51]
              The Unicode Consortium, "The Unicode Standard, Version
              5.1.0", 2008.

              defined by: The Unicode Standard, Version 5.0, Boston, MA,



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              Addison-Wesley, 2007, ISBN 0-321-48091-0, as amended by
              Unicode 5.1.0
              (http://www.unicode.org/versions/Unicode5.1.0/).

16.2.  Informative References

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

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

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

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

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

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

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

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

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

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

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

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




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   [RFC3743]  Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint
              Engineering Team (JET) Guidelines for Internationalized
              Domain Names (IDN) Registration and Administration for
              Chinese, Japanese, and Korean", RFC 3743, April 2004.

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

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

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

   [RFC4713]  Lee, X., Mao, W., Chen, E., Hsu, N., and J. Klensin,
              "Registration and Administration Recommendations for
              Chinese Domain Names", RFC 4713, October 2006.

   [Unicode-UAX31]
              The Unicode Consortium, "Unicode Standard Annex #31:
              Unicode Identifier and Pattern Syntax", March 2008,
              <http://www.unicode.org/reports/tr31/>.

   [Unicode-UTR36]
              The Unicode Consortium, "Unicode Technical Report #36:
              Unicode Security Considerations", July 2008,
              <http://www.unicode.org/reports/tr36/>.


Author's Address

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

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











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Full Copyright Statement

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