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Versions: 00 01 02 03 04 05 06 07 08 rfc4290               Informational
Network Working Group                                         J. Klensin
Internet-Draft                                              May 17, 2005
Expires: November 18, 2005

 Suggested Practices for Registration of Internationalized Domain Names

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document explores the issues in registration of
   internationalized domain names (IDNs).  The basic IDN definition
   potentially allows a very large number of possible characters in
   domain names, and this richness may lead to serious user confusion
   about similar-looking names.  To avoid this confusion, it is
   necessary for the IDN registration process to impose rules that
   disallow some otherwise-valid name combinations.  This document
   suggests a set of mechanisms that registries might use to define and

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   implement such rules, including adaptation of methods developed for
   Chinese, Japanese, and Korean domain names to other languages and

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1   Background . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2   The Nature and Status of these Recommendations . . . . . .  4
     1.3   Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
       1.3.1   Languages and Scripts  . . . . . . . . . . . . . . . .  5
       1.3.2   Characters, Variants, Registrations, and Other
               Issues . . . . . . . . . . . . . . . . . . . . . . . .  6
       1.3.3   Confusion, Fraud, and Cybersquatting . . . . . . . . .  8
     1.4   A Review of the JET Guidelines . . . . . . . . . . . . . .  8
       1.4.1   JET Model  . . . . . . . . . . . . . . . . . . . . . .  8
       1.4.2   Reserved Names and Label Packages  . . . . . . . . . .  9
     1.5   Languages, Scripts, and Variants . . . . . . . . . . . . .  9
       1.5.1   Languages and Scripts  . . . . . . . . . . . . . . . .  9
       1.5.2   Variant Selection  . . . . . . . . . . . . . . . . . . 11
     1.6   Variants are not a Universal Remedy  . . . . . . . . . . . 13
     1.7   Reservations and Exclusions  . . . . . . . . . . . . . . . 13
       1.7.1   Sequence Exclusions for Valid Characters . . . . . . . 13
       1.7.2   Character Pairing Issues . . . . . . . . . . . . . . . 13
     1.8   The Registration Bundle  . . . . . . . . . . . . . . . . . 14
       1.8.1   Definitions and Structure  . . . . . . . . . . . . . . 14
       1.8.2   Application of the Registration Bundle . . . . . . . . 14
   2.  Some Implications of This Approach . . . . . . . . . . . . . . 15
   3.  Required Modifications to JET Model Needed Under Some of
       the Models Above . . . . . . . . . . . . . . . . . . . . . . . 16
   4.  Conclusions and Recommendations About the General Approach . . 17
   5.  A Model Table Format . . . . . . . . . . . . . . . . . . . . . 18
   6.  A Model Label Registration Procedure: "CreateBundle" . . . . . 19
     6.1   Description of the CreateBundle Mechanism  . . . . . . . . 19
     6.2   The "no-variants" Case . . . . . . . . . . . . . . . . . . 20
     6.3   CreateBundle and Nameprep Mapping  . . . . . . . . . . . . 21
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   8.  Internationalization Considerations  . . . . . . . . . . . . . 22
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   10.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23
   11.   References . . . . . . . . . . . . . . . . . . . . . . . . . 24
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 25
       Intellectual Property and Copyright Statements . . . . . . . . 26

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1.  Introduction

1.1  Background

   The IDNA (Internationalized Domain Names in Applications)
   specification [RFC3490] defines the basic model for encoding non-
   ASCII strings in the DNS, and additional specifications ([RFC3491],
   [RFC3492]) define the mechanisms and tables needed to support it.  As
   work on these specifications neared completion, it became apparent
   that it would be desirable for registries to impose additional
   restrictions on the names that could actually be registered (e.g.,
   see [IESG-IDN] and [ICANN-IDN]) as a means of reducing potential
   confusion among characters that were similar in some way.  This
   document explores these IDN (international domain name) registration
   issues and suggests a set of mechanisms that IDN registries might
   use.  Registration restrictions are part of a long tradition.  For
   example, while the original DNS specifications [RFC1035] permitted
   any string of octets to be used in a DNS label, they also recommended
   the use of a much more restricted subset, one that was derived from
   the much older "hostname" rules [RFC0952] and defined by the "LDH"
   convention (for the three permitted types of characters, letters,
   digits, and the hyphen).  Enforcement of those restricted rules in
   registrations was the responsibility of the registry or domain
   administrator.  They were not embedded in the DNS protocol itself,
   although some applications protocols, notably those concerned with
   electronic mail, did impose and then enforce similar rules.

   If there are no constraints on registration in a zone, people can
   register characters that increase the risk of misunderstandings,
   cybersquatting, and other forms of confusion.  That a similar
   situation existed even before the introduction of IDNA is exemplified
   by domain names such as example.com and examp1e.com (note that the
   latter domain contains the digit "1" instead of the letter "l").

   For non-ASCII names (so-called "internationalized domain names" or
   "IDNs"), the problem was more complicated than that which led to the
   LDH (hostname) rules.  In the earlier situation, all protocols,
   hosts, and DNS zones used ASCII exclusively in practice, so the LDH
   restriction could reasonably be applied uniformly across the
   Internet.  With the introduction of a very large character
   repertoire, and with different geographical and political locations
   and languages having requirements for different collections of
   characters, the optimal registration restrictions became, not a
   global matter, but ones that were different in different areas and,
   hence, in different DNS zones.

   For some human languages, there are characters and/or strings that
   have equivalent or near-equivalent usages.  If someone is allowed to

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   register a name with such a character or string, the registry might
   want to automatically associate all of the names that have the same
   meaning with the registered name.  The registry might also decide
   whether the names that are associated with, or generated by, one
   registration should, as a group or individually, go into the zone or
   be blocked from registration by different parties.

   To date, the best-developed system for handling registration
   restrictions for IDNs is the JET Guidelines for Chinese, Japanese,
   and Korean [RFC3743], the so-called "CJK" languages.  That system is
   limited to those languages and, in particular, to their common script
   base.  Those languages are also the best-known and most widely-used
   ones in the world whose writing system is constructed on
   "ideographic" or "pictographic" principles.  This document explores
   the principles behind the JET guidelines.  It then examines some of
   the issues that might arise in trying to adapt them to alphabetic
   languages, i.e., ones who characters primarily represent sounds,
   rather than meanings.

   This document describes five things:

   1.  The general background and considerations for non-ASCII scripts
       in names.  Just as the JET Guidelines contain some suggestions
       that may not be applicable to alphabetic scripts, some of the
       suggestions here, especially the more specific ones, may be
       applicable to some scripts and not others

   2.  Suggested practices for describing character variants

   3.  A method for using a zone's character variants to determine which
       names should be associated with a registration

   4.  A format for publishing a zone's table of character variants.
       Such tables are referred to below simply as "the table".

   5.  A model algorithm for name registration given the presence of
       language tables.

1.2  The Nature and Status of these Recommendations

   The document makes recommendations for consideration by registries
   and, where relevant, those who coordinate them and use their
   services.  None of the recommendations are intended to be normative.
   Indeed, the intent of the document is to illustrate a framework from
   which variations to meet the needs of particular registries and their
   processing of particular languages can be developed.  Of course, if
   registries make similar decisions and utilize similar tools, it may
   reduce costs and confusion -- both between registries and for users

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   and registrars who have relationships with more than one domain.

1.3  Terminology

1.3.1  Languages and Scripts

   This document uses the term "language" in what may be, to many
   readers, an odd way.  Neither this specification, nor IDNA, nor the
   DNS are directly concerned with natural language, but only about the
   characters that make up a given label.  In some respects, the term
   "script", as used in the character coding community, might be more
   appropriate.  However, different subsets of the same script may be
   used with different languages and the same language may be written
   using different characters (or even completely different scripts) in
   different locations, so that term is not precisely correct either.
   Long-standing confusion has also resulted by the fact that most
   scripts are, informally at least, named after one of the languages
   written in them: "Chinese" describes both a language and a collection
   of characters also used in writing Japanese, Korean, and, at least
   historically, some other languages; "Latin" describes both a
   language, the characters used to write that language, and, often
   characters used to write a number of contemporary languages that are
   derived from or similar to those used to write Latin; the script used
   to write the Arabic language is called "Arabic" but is also used
   (typically with some additions or deletions) to write a number of
   other languages, and so on.  Situations in which a script has a
   clearly-defined name independent of the name of a language are the
   exception, rather than the rule; examples include Hangul, used to
   write Korean, Katakana and Hiragana, used to write Japanese, and a
   few others.  And some scholars have historically used "Roman" or
   "Roman-derived" in an attempt to distinguish between a script and the
   Latin language.

   The term "language" is hence used in this document in the informal
   sense of a written language and is defined, for this purpose, by the
   characters used to write it.  In this context, a "language" is
   defined by the combination of a code (see Section 1.4.1) and an
   authority that has chosen to use that code and establish a character-
   listing for it.  Authorities are normally TLD registries (see
   Section 7 and [IANA-language-registry]), but it is expected that they
   will find appropriate experts and that advice from language and
   script experts selected by international neutral bodies will also
   become part of the registration system.  In addition, as discussed
   below in Section 7, registries may conclude that the best interests
   of registrants, stakeholders, and the Internet community would be
   served by constructing "language tables" that mix scripts and
   characters in ways that conform to no known language.  Conventions
   should be developed for such registrations that do not misleadingly

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   reflect specific language codes.

1.3.2  Characters, Variants, Registrations, and Other Issues

   1.  Characters in this document are given as their Unicode codepoints
       in U+xxxx format, with their official names, or both.

   2.  The following terms are used in this document.

       *  A "string" is an sequence of one or more characters.

       *  This document discusses characters that may have equivalent or
          near-equivalent characters or strings.  The "base character"
          is the character that has zero or more equivalents.  In the
          JET Guidelines, base characters are referred to as "valid
          characters".  In a table with variants, as described in
          Section 5, the base characters occupy the first column.
          Normally (and always if the recommendation of Section 6.3 is
          adopted) the base characters will be the characters that
          appear in registration requests from registrants; all other
          character will be considered to make the registration attempt

       *  The "variant(s)" are the character(s) and/or string(s) that
          are treated as equivalent to the base character.  Note that
          these might not be true equivalent characters: a particular
          original character may be a base character with a mapping to a
          particular variant character, but that variant character may
          not have a mapping to the original base character and, indeed,
          the variant character may not appear in the base character
          list, and hence may not be valid for use in a registration.
          Usually, characters or strings to be designated as variants
          are considered either equivalent or sufficiently similar (by
          some registry-specific definition) that confusion between them
          and the base character might occur.

       *  The "base registration" is the single name that the registrant
          requested from the registry.  The JET Guidelines use the term
          "label string" for this name.

       *  A label (or "name") is described as "registered" if it is
          actually entered into a domain (i.e., a zone file) by the
          registry, so that it can be accessed and resolved using
          standard DNS tools.  The JET Guidelines describe a
          "registered" label as "activated".  However, some domains use
          a slightly different registration logic in which a name can be
          registered with the registrar, if one is involved, and with
          the registry but not actually entered into the zone file until

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          an additional activation or delegation step occurs.  This
          document does not make that distinction, but is compatible
          with it.

          As specified in the IDNA Standard, the name actually placed in
          the zone file is always the internal ("punycode") form.  There
          is no provision for actually entering any other form of an IDN
          into the DNS.  It remains controversial, with different
          registrars and registries having adopted different policies,
          as to whether the registration, as submitted by the
          registrant, is in the form of
             The native-script name, either in UTF-8 or in some coding
             specified by the registrar.
             The internal-form ("punycode") name.
             Both forms of the name together, so that the registrar and
             registry can verify the intended translation.
          If some variant system is used, it is almost certain to be
          necessary that the native-script form of the requested name be
          available to the registry.

       *  A "registration bundle" is the set of all labels that comes
          from expanding the base characters for a single name into
          their variants.  The presence of a label in a registration
          bundle does not imply that it is registered.  In the JET
          Guidelines, a registration bundle is called an "IDN Package".

       *  A "reserved label" is a label in a registration bundle that is
          not actually registered.

       *  A "registry" is the administrative authority for a DNS zone.
          That is, the registry is the body that enforces, and typically
          makes, policies that are used in a particular zone in the DNS.

       *  "Coded Character Set" ("CCS") is a term for a list of
          characters and the code positions assigned to them.  ASCII and
          Unicode are CCSs.

       *  A "language" is something spoken by humans, independent of how
          it is written or coded.  ISO Standard 639 and IETF BCP 47 (RFC
          3066) [RFC3066] list and define codes for identifying

       *  A "script" is a collection of characters (glyphs, independent
          of coding) that are used together, typically to represent one
          or more languages.  Note that the script for one language may
          heavily overlap the script for another.  This does not imply
          that they have identical scripts.

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       *  "Charset" is an IETF-invented term to describe, more or less,
          the combination of a script, a CCS that encodes that script,
          and rules for serializing the bytes when those are stored on a
          computer or transmitted over the network.

   The last four of these definitions are redundant with, but
   deliberately somewhat less precise than, the definitions in
   [RFC3536], which also provides sources.  The two sets of definitions
   are intended to be consistent.

1.3.3  Confusion, Fraud, and Cybersquatting

   The term "confusion" is used very generically in this document to
   cover the entire range from accidental user misperception of the
   relationship between characters with some characteristic in common
   (typically appearance, sound, or meaning) to cybersquatting and
   [other] deliberate fraudulent attempts to exploit those relationships
   or others based on the nature of the characters.

1.4  A Review of the JET Guidelines

1.4.1  JET Model

   In the JET Guidelines model, a prospective registrant approaches the
   registry for a zone (perhaps through an intermediate registrar) with
   a candidate base registration -- a proposed name to be registered --
   and a list of languages in which that name is to be interpreted.  The
   languages are defined according to the fairly high-resolution coding
   of [RFC3066] -- Chinese as used on the mainland of the People's
   Republic of China ("zh-cn") can, at registry option, consist of a
   somewhat different list of characters (code points) and be
   represented by a separate table compared to Chinese as used in Taiwan

   The design of the JET Guidelines took one important constraint as a
   basis: IDNA was treated as a firm standard.  A procedure that
   modified some portion of the IDNA functions, or was a variant on
   them, was considered a violation of those standards and should not be
   encouraged (or, probably, even permitted).

   Each registry is expected to construct (or obtain) a table for each
   language it considers relevant and appropriate.  These tables list,
   for the particular zone, the characters permitted for that language.
   If a character does not appear as a "valid code point" (called a
   "base character" in the rest of this document) in that table, then a
   name containing it cannot be registered.  If multiple languages are
   listed for the registration, then the character must appear in the
   tables for each of those languages.

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   The tables may also contain columns that specify alternate or variant
   forms of the valid character.  If these variants appear, they are
   used to synthesize labels that are alternatives to the original one.
   These labels are all reserved and can be registered or "activated"
   (placed into the DNS) only by the action or request of the original
   registrant; some (the "preferred variant labels") are typically
   registered automatically.  The zone is expected to establish
   appropriate policies for situations in which the variant forms of one
   label conflict with already-reserved or already-registered labels.

   Most of these concepts were introduced because of concerns about
   specific issues with CJK characters, beginning from the requirement
   that the use of Simplified Chinese by some registrants and
   Traditional Chinese by others not be permitted to create confusion or
   opportunities for fraud.  While they may be applicable to registry
   tables constructed for alphabetic scripts, the translation should be
   done with care, since many analogies are not exact.

   Some of the important issues are discussed in the sections that
   follow.  The JET model may be considered as a carefully-worked-out,
   but specialized to CJK characters, variation on the model and method
   presented by the rest of this document.  Other languages or scripts
   may require other variations.

1.4.2  Reserved Names and Label Packages

   A basic assumption of the JET model is that, if the evolution of
   specific characters or the properties of Unicode ([Unicode],
   [Unicode32]) or IDNA cause two strings to appear similar enough to
   cause confusion, either or both should be registered by the same
   party or one of them should become unregisterable.  The definition of
   "appear similar enough" will differ for different cultures and
   circumstances -- and hence DNS zones -- but the principle is fairly
   general.  In the JET model, all of the "variant" strings are
   identified, some are registered into the DNS automatically, and
   others are simply reserved and can be registered, if at all, only by
   the original registrant.  Other zones might find other policies
   appropriate.  For example, a zone might conclude that having similar
   strings registered in the DNS was undesirable.  If so, the list of
   variant labels would be used only to build a list of names that would
   be reserved and prohibited from being registered.

1.5  Languages, Scripts, and Variants

1.5.1  Languages and Scripts

   Conversations about scripts -- collections of characters associated
   with particular languages -- are common when discussing character

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   sets and codes.  But the boundaries between one script and another
   are not well-defined.  The Unicode Standard ([Unicode], [Unicode32]),
   for example, does not define them at all, even though it is
   structured in terms of usually-related blocks of characters.  The
   issue is complicated by the common origin of most alphabetic scripts
   in contemporary use in the world today (see, for example, [Drucker]).
   Because of that history, certain characters (or, more precisely,
   symbols representing characters) appear in the scripts associated
   with multiple languages, sometimes with very different sounds or
   meanings.  This differs from the CJK situation in which, if a
   character appears in more than one of the relevant languages, it will
   usually have the same interpretation in each one.  For the subset of
   characters that actually are ideographs or pictographs, pronunciation
   is expected to vary widely while meaning is preserved.  At least in
   part because of that similarity of meaning, it made sense in the JET
   case to permit a registration to specify multiple languages, to
   verify that the characters in the label string (the requested "Base
   registration") were valid for each, and then to generate variant
   labels using each language in turn.  For many alphabetic languages,
   it may be more sensible to prohibit the label string submitted for
   registration from being associated with more than one language.
   Indeed, "one label, one language" has been suggested as an important
   barrier against common sources of "look-alike" confusion.  For
   example, the imposition of that rule in a zone would prevent the
   insertion of a few Greek or Cyrillic characters with shapes identical
   to the Latin ones into what was otherwise a Latin-based string.  For
   a particular table, the list of valid characters may be thought of as
   the script associated with the relevant language, with the
   understanding that the table design does not prevent the same
   character from appearing in the tables for multiple languages.

   Indeed, this notion of a locally, and specifically-identified, script
   can be turned around: while the tables are referred to as "language
   tables", they are associated with languages only insofar as thinking
   about the character structure and word forms associated with a given
   language helps to inform the construction of a table.  A country like
   Finland, for example, might select among

   o  One table each for Finnish, Swedish, and English characters and
      conventions, permitting a string to be registered in one, two, or
      all three languages (although a three-language registration would
      necessarily prohibit any characters that did not appear in all
      three languages since the label would make little sense

   o  One table each, but with a "one label, one language" rule for the

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   o  A combined table based on the observation that all three writing
      systems were based on Roman characters and that the possibilities
      for confusion that were of interest to the registry would not be
      reduced by "language" differentiation.  This option raises an
      interesting issue about language labeling as described in
      Section 1.4.1, see the discussion in Section 7, below.

   Regardless of what decisions were made about those languages and
   scripts, if they also decided to permit registrations of labels
   containing Cyrillic characters, they might have a separate table for
   them.  That table might contain some Roman-derived characters (either
   as base characters or as variants) just as some CJK tables do.  See
   also Section 2, below.

   As the JET Guidelines stress, no tables or systems of this type --
   even if identified with a language as a means of defining or
   describing the table -- can assure linguistic or even syntactic
   correctness of labels with regard to that language.  That level of
   assurance may not be possible without human intervention or at least
   dictionary lookups of complete proposed labels.  It may even not be
   desirable to attempt that level of correctness (see Section 2).

   Of course, if any language-based tests or constraints, including "one
   label, one language", are to be applied to limit the associated
   sources of confusion, each zone must have a table for each language
   in which it expects to accept registrations; the notion of a single
   combined table for the zone is, in the general case, simply
   unworkable.  One could use a single table for the zone if the intent
   were to impose only minimal restrictions, e.g., to force alphabetic
   and numeric characters only and exclude symbols and punctuation.
   That type of restriction might be useful in eliminating some
   problems, such as those of unreadable labels, but would be unlikely
   to be very helpful with, e.g., confusion caused by similar-looking

1.5.2  Variant Selection

   The area of character variants is rife with difficulties (and perhaps
   opportunities).  There is no universal agreement about which base
   characters have variants, or if they do, what those variants are.
   For example, in some regions of the world and in some languages,
   WITH STROKE (U+00F8) are variants of each other, while in other
   regions, most people would think that LATIN SMALL LETTER O WITH
   STROKE has no variants.  In some cases, the list of variants is
   difficult to enumerate.  For example, it required several years for
   the Chinese language community to create variant tables for use with
   IDNA, and it remains, at the time of this writing, questionable how

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   widely those tables will be accepted among users of Chinese from
   areas of the world other than those represented by the groups that
   created them.

   Thus, the first thing a registry should ask is whether or not any of
   the characters that they want to permit to be used have variants.  If
   not, the registry's work is much simpler.  This is not to say that a
   registry should ignore variants if they exist: adding variants after
   a registry has started to take registrations will be nearly as
   difficult administratively as removing characters from the list of
   acceptable characters.  That is, if a registry later decides that two
   characters are variants of each other, and there are actively-used
   names in the zones that differ only on the new variants, the registry
   might have to transfer ownership of one of the names to a different
   owner, using some process that is certain to be controversial.

   This situation in likely to be much easier for areas and zones that
   use characters that previously did not occur in the DNS at all than
   it will be for zones in which non-English labels have been registered
   in ASCII characters for some time, presumably because the language of
   interest uses additional "Latin" characters with some conventions
   when only ASCII is available.  In the former case, the rules and
   conventions can be established before any registrations occur.  In
   the latter, there may be conflicts or opportunities for confusion
   between existing registrations and now-permitted Roman-based
   characters that do not appear in ASCII.  For example, a domain name
   might exist today that uses the name of a city in Canada spelled as
   "Montreal".  If the zone in which it occurs changes its rules to
   permit the use of the character LATIN SMALL LETTER E WITH ACUTE
   (U+00E9), does the name of the city, spelled (correctly) using that
   character, conflict with the existing domain name registration?
   Certainly, if both are permitted, and permitted to be registered by
   separate parties, there are many opportunities for confusion.

   Of course, zone managers should inform all current registrants when
   the registration policy for the zone changes.  This includes the time
   at which IDN characters are allowed in the zone the first time, when
   additional characters are permitted later, and, if it is necessary to
   change character variant tables, when that occurs.

   In many languages there are two variants for a character, but one
   variant is strongly preferred.  A registry might only allow the base
   registration in the preferred form, or it might allow any form for
   the base registration.  If the variant tables are created carefully,
   the resulting bundles will be the same, but some registries will give
   special status to the base registration such as its appearance in
   "Whois" databases.

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1.6  Variants are not a Universal Remedy

   It is worth stressing that there are many obvious opportunities for
   confusion which variant systems, by virtue of being based on
   processing of individual characters, cannot address.  For example, if
   a language can be written with more than one script, or
   transliterations of the language into another script are common,
   variant models are insufficient to prevent conflicting registration
   of the related forms.  Avoiding those types of problems would require
   different mechanisms, perhaps based on phonetic or natural language
   processing techniques for the entire proposed base registration.

1.7  Reservations and Exclusions

1.7.1  Sequence Exclusions for Valid Characters

   The JET Guidelines are based on processing only single characters.
   Any processing of pairs or longer sequences of characters are left to
   what that document describes as "additional processing" -- procedures
   specifically permitted by the Guidelines but defined by a registry in
   addition to the variant table processing specified in the Guidelines
   themselves.  A different zone, with different needs, could use a
   modified version of the table structure, or different types of
   additional processing, to prohibit, as well as accept, particular
   sequences of characters by marking them as invalid.  Other
   modifications or extensions might be designed to prevent certain
   letters from appearing at the beginning or end of labels.  The use of
   regular expressions in the "valid characters" column might be one way
   to implement these types of restrictions, but there has been no
   experience so far with that approach.

   In particular, in some scripts derived from Roman characters,
   sequences that have historically been typographically represented by
   single "ligature" or "digraph" characters may also be represented by
   the separate characters (e.g., "ae" for U+00E6 or "ij" for U+0133).
   If it is desired to either prohibit these, or to treat them as
   variants, some extensions to the single-character JET model may be
   needed (as may be some careful thinking about IDNA (especially
   nameprep), since some of these combinations are excluded there).

1.7.2  Character Pairing Issues

   Some character pairings -- the use of a character form (glyph) in one
   language and a different form with the same properties in a related
   one -- closely approximate the issues with mapping between
   Traditional and Simplified Chinese although the history is different.
   For example, it might be useful to have "o" with a stroke (U+00F8) as
   a variant for "o" with diaeresis above it (U+00F6) (and the

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   equivalent upper-case pair) in a Swedish table, and vice versa in a
   Norwegian one, or to prohibit one of these characters entirely in
   each table.  In a German table, U+00F8 would presumably be
   prohibited, while U+00F6 might have "oe" as a variant.  Obviously, if
   the relevant language of registration is unknown, this type of
   variant matching cannot be applied in any sensible way.

1.8  The Registration Bundle

1.8.1  Definitions and Structure

   As one of its critical innovations, the JET model defines an "IDN
   package", known in this document as a "registration bundle", which
   consists of the primary registered string (which is used as the name
   of the bundle), the information about the language table(s) used, the
   variant labels for that string, and indications of which of those
   labels are registered in the relevant zone file ("activated" in the
   JET terminology).  Registration bundles are also atomic -- one can
   not add or remove variant labels from one without unregistering the
   entire package.  A label exists in only one registration bundle at a
   time; if a new label is registered that would generate a variant that
   matches one that appears in an existing package, that variant simply
   is not included in the second package.  A subsequent deregistration
   of the first package does not cause the variant to be added to the
   second.  While it might be possible to change this in other models,
   the JET conclusion was that other options would be far too complex to
   implement and operate and would cause many new types of name

1.8.2  Application of the Registration Bundle

   A registry has three options for how to handle the case where the
   registration bundle is non-trivial, i.e., contains more than one
   label.  The policy options are:

   o  Register and resolve all labels in the zone, making the zone
      information identical to that of the registered label.  This
      option will cause end users to be able to find names with variants
      more easily, but will result in larger zone files.  For some
      language tables, the zone file could become so large that it could
      negatively affect the ability of the registry to perform name
      resolution.  If the base registration contains several characters
      that have equivalents, the owner could end up having to take care
      of large numbers of zones.  For instance, if DIGIT ONE is a
      variant of LATIN SMALL LETTER L, the owner of the domain name all-
      lollypops.example.com will have to manage 32 zones.  If the intent
      is to keep the contents of those zones identical, the owner may
      then face a significant administrative problem.  If other concerns

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      dictate short times to live and absolute consistency of DNS
      responses, the challenges may be nearly impossible.

   o  Block all labels other than the registered label so they cannot be
      registered in the future.  This option does not increase the size
      of the zone file and provides maximum safety against false
      positives, but it may cause end users to not be able to find names
      with variants that they would expect.  If the base registration
      contains characters that have equivalents, Internet users who do
      not know what base characters were used in the registration will
      not know what character to type in to get a DNS response.  For
      instance, if DIGIT ONE is a variant of LATIN SMALL LETTER L, and
      LATIN SMALL LETTER L is a variant of DIGIT ONE, the user who sees
      "pale.example.com" will not know whether to type a "1" or a "l"
      after the "pa" in the first label.

   o  Resolve some labels and block some other labels.  This option is
      likely to cause the most confusion with users because including
      some variants will cause a name to be found, but using other
      variants will cause the name to be not found.  For example, even
      if people understood that DIGIT ONE and LATIN SMALL LETTER L were
      variants, a typical DNS user wouldn't know which character to type
      because they wouldn't know whether this pair were used to register
      or block the labels.  However, this option can be used to balance
      the desires of the name owner (that every possible attempt to
      enter their name will work) with the desires of the zone
      administrator (to make the zone more manageable and possibly to be
      compensated for greater amounts of work needed for a single
      registration).  For many circumstances, it may be the most
      attractive option.

   In all cases, at least the registered label should appear in the
   zone.  It would be almost impossible to describe to name owners why
   the name that they asked for is not in the zone, but some other name
   that they now control is.  By implication, if the requested label is
   already registered, the entire registration request must be rejected.

2.  Some Implications of This Approach

   Historically, DNS labels were considered to be arbitrary identifier
   strings, without any inherent meaning.  Even in ASCII, there was no
   requirement that labels form words.  Labels that could not possibly
   represent words in any Romance or Germanic language (the languages
   that have been written in "Latin" scripts since medieval times or
   earlier) have actually been quite common.  In general, in those
   languages, words contain at least one vowel and do not have embedded
   numbers.  As a result, a string such as "bc345df" cannot possibly be
   a "word" in these languages.  More generally, the more one moves

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   toward "language"-based registry restrictions, the less it is going
   to be possible to construct labels out of fanciful strings.  While
   fanciful strings are terrible candidates for "words", they may make
   very good identifiers.  To take a trivial example using only ASCII
   characters, "rtr32w", "rtr32x", and "rtr32z" might be very good DNS
   labels for a particular zone and application.  However, given the
   embedded digits and lack of vowels, they, like the "bc345df" example
   given above, would fail even the most superficial of tests for valid
   English (or German or French (etc.)) word forms.

   It is worth noting that several DNS experts have suggested that a
   number of problems could be solved by prohibiting meaningful names in
   labels, requiring instead that the labels be random or nonsense
   strings.  If methods similar to those discussed in this document were
   used to force identifiers to be closer to meaningful words in real
   languages, the result would be directly contradictory to those
   "random name" approaches.

   Interestingly, if one were trying to develop an "only words" system,
   a rather different -- but very restrictive -- model could be
   developed using lookups in a dictionary for the relevant language and
   a listing of valid business names for the relevant area.  If a string
   did not appear in either, it would not be permitted to be registered.
   Models that require a prior national business listing (or
   registration) that is identical to the proposed domain name label
   have historically been used to restrict registrations in some
   country-code top level domains, so this is not a new idea.  On the
   other hand, if look-alike characters are a concern, even that type of
   rule (or restriction) would still not avoid the need to consider
   character variants.

   Consequently, registries applying the principles outlined in this
   document should be careful not to apply more severe restrictions than
   are reasonable and appropriate while, at the same time, being aware
   of how difficult it usually is to add restrictions at a later time.

3.  Required Modifications to JET Model Needed Under Some of the Models

   The JET model was designed for CJK characters.  The discussion above
   implies that some extensions to it may be needed to handle the
   characteristics of various alphabetic scripts and the decisions that
   might be made about them in different zones.  Those extensions might
   include facilities to process:

   o  Two-character (or more) sequences, such as ligatures and
      typographic spelling conventions, as variants.

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   o  Regular expressions or some other mechanism for dealing with
      string positions of characters (e.g., characters that must, or
      must not, appear at the beginning or end of strings).

   o  Delimiter breaks to permit multiple languages to be used,
      separately, within the same label.  E.g., is it possible to define
      a label as consisting of two or more components, each in a
      different language,  with some particular delimiter used to define
      the boundaries of the components?

4.  Conclusions and Recommendations About the General Approach

   After examining the implications of the potential use of the full
   range of characters permitted by IDNA in DNS labels, multiple groups,
   included IESG [IESG-IDN] and ICANN [ICANN-IDN] have concluded that
   some restrictions are needed to prevent many forms of user confusion
   about the actual structure of a name or the word, phrase, or term
   that it appears to spell out.  The best way to approach such
   restrictions appears to draw from the language and culture of the
   community of registrants and users in the relevant zone: if
   particular characters are likely to be surprising or unintelligible
   to both of those groups, it is probably wise to not permit them to be
   used in registrations.  Registration restrictions can be carried much
   further than restricting permitted characters to a selected Unicode
   subset.  The idea of a reserved "bundle" of related labels permits
   probably-confusing combinations or sets of characters to be bound
   together, under the control of a single registrant.  While that
   registrant might still use the package in a way that confused his or
   her own users (the approach outlined here will not prevent either
   ill-though-out ideas or stupidity), the possibility of turning
   potential confusion into a hostile attack would be considerably

   At the same time, excessive restrictions may make DNS identifiers
   less useful for their original, intended, purpose: identifying
   particular hosts and similar resources on the network in an orderly
   way.  Registries creating rules and policies about what can be
   registered in particular zones -- whether those are based on the JET
   Guidelines or the suggestions in this document -- should balance the
   need for restrictions against the need for flexibility in
   constructing identifiers.

   The discussion above provides many options that could be selected,
   defined, and applied in different ways in different registries
   (zones).  Registrars and registrants  would almost certainly prefer
   systems in which they can predict, at least to a first order
   approximation, the implications of a particular potential
   registration to ones in which they cannot.  Predictability of that

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   sort probably requires more standards, and less flexibility, than the
   model itself might suggest.

5.  A Model Table Format

   The format of the table is meant to be machine-readable but not
   human-readable.  It is fairly trivial to convert the table into one
   that can be read by people.

   Each character in the table is given in the "U+" notation for Unicode
   characters.  The lines of the table are terminated with either a
   carriage return character (ASCII 0x0D), a linefeed character (ASCII
   0x0A), or a sequence of carriage return followed by linefeed (ASCII
   0x0D 0x0A).  The order of the lines in the table may or may not
   matter, depending on how the table is constructed.

   Comment lines in the table are preceded with a "#" character (ASCII

   Each non-comment line in the table starts with the character that is
   allowed in the registry and expected to be used in registrations,
   which is also called the "base character".  If the base character has
   any variants, the base character is followed by a vertical bar
   character ("|", ASCII 0x7C) and the variant string.  If the base
   character has more than one variant, the variants are separated by a
   colon (":", ASCII 0x3A).  Strings are given with a hyphen ("-", ASCII
   0x2D) between each character.  Comments beginning with a "#" (ASCII
   0x2C), and may be preceded by spaces (" ", ASCII 0x20).

   The following is an example of how a table might look.  The entries
   in this table are purposely silly and should not be used by any
   registry as the basis for choosing variants.  For the example, assume
   that the registry:

   o  allows the FOR ALL character (U+2200) with no variants

   o  allows the COMPLEMENT character (U+2201) which has a single
      variant of LATIN CAPITAL LETTER C (U+0043)

   o  allows the PROPORTION character (U+2237) which has one variant
      which is the string COLON (U+003A) COLON (U+003A)

   o  allows the PARTIAL DIFFERENTIAL character (U+2202) which has two
      variants: LATIN SMALL LETTER D (U+0064) and GREEK SMALL LETTER
      DELTA (U+03B4)

   The table contents (after any required header information, see [IANA-
   language-registry] and the discussion in Section 7 below) would look

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   # An example of a table
   U+2237|U+003A-U+003A # Note that the variant is a string
   U+2202|U+0064:U+03B4 # Two variants for the same character

   Implementers of table processors should remember that there are tens
   of thousands of characters whose codepoints are greater than 0xFFFF.
   Thus, any program that assumes that each character in the table is
   represented in exactly six octets ("U", "+", and four octets
   representing the character value) will fail with tables that use
   characters whose value is greater than 0xFFFF.

6.  A Model Label Registration Procedure: "CreateBundle"

   This procedure has three inputs:

   1.  the proposed base registration

   2.  the language for the proposed base registration

   3.  the processing table associated with that language

   The output of the process is either failure (the base registration
   cannot be registered at all), or a registration bundle that contains
   one or more labels ( always including the base registration).  As
   described earlier, the registration bundle should be stored with its
   date of creation so that issues with overlapping elements between
   bundles can later be resolved on a first-come, first-served basis.

   There are two steps to processing the registration:

   1.  Check whether the proposed base registration exists in any
       bundle.  If it does, stop immediately with a failure.

   2.  Process the base registration with the mechanism described as
       "CreateBundle" in Section 6.1, below.

   Note that the process must be executed only once.  The process must
   not be performed on any output of the process, only on the proposed
   base registration.

6.1  Description of the CreateBundle Mechanism

   The CreateBundle mechanism determines whether a registration bundle
   can be created and, if so, populates that bundle with valid labels.

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   During the processing, an "temporary bundle" contains partial labels,
   that is, labels that are being built and are not complete labels.
   The partial labels in the temporary bundle consist of strings.

   The steps are:

   1.  Split the base registration into individual characters, called
       "candidate characters".  Compare every candidate character
       against the base characters in the table.  If any candidate
       character does not exist in the set of base characters, the
       system must stop and not register any names (that is, it must not
       register either the base registration or any labels that would
       have come from character variants).

   2.  Perform the steps in IDNA's ToASCII sequence for the base
       registration.  If ToASCII fails for the base registration, the
       system must stop and not register any label (that is, it must not
       register either the base registration or labels that might have
       been created from variants of characters contained in it).  If
       ToASCII succeeds, place the base registration into the
       registration bundle.

   3.  For every candidate character in the base registration, do the
       1.  Create the set of characters that consists of the candidate
           character and any variants.
       2.  For each character in the set from the previous step,
           duplicate the temporary bundle that resulted from the
           previous candidate character, and add the new character to
           the end of each partial label.
   4.  The temporary bundle now contains zero or more labels that
       consist of Unicode characters.  For every label in the temporary
       bundle, do the following:

       Process the label with ToASCII to see if ToASCII succeeds.  If it
       does, add the label to the registration bundle.  Otherwise, do
       not process this label from the temporary bundle any further; it
       will not go into the registration bundle.

   The result of the processing outlined above is the registration
   bundle with the base registration and possibly other labels.

6.2  The "no-variants" Case

   It is clear that, for many scripts, registries will choose to create
   tables without variants, either because variants are clearly not
   necessary or because they are determined to cause more confusion and

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   overhead than is justified by the circumstances.  For those
   situations the table model of Section 5 becomes a trivial listing of
   base characters and only the first two steps of CreateBundle
   (verifying that all candidate character are in the base ("valid")
   character list and verifying that the resulting characters will
   succeed in the ToASCII operation) are applicable.  Even the second of
   those steps becomes pro forma if the advice in the next subsection is

6.3  CreateBundle and Nameprep Mapping

   One of the functions of Nameprep, and IDNA more generally, is to map
   a large number of Unicode characters (code points) into a smaller
   number to avoid a different, but overlapping, set of confusion
   problems.  For example, when a  non-ASCII script makes distinctions
   between "upper case" and "lower case", nameprep maps the upper case
   characters to the lower case ones in order to simulate the DNS
   protocol's rule that ASCII characters are interpreted in a case-
   insensitive way.  Unicode also contains many code points that are
   typographic variants on each other and the Unicode standard
   explicitly identifies them that way, e.g., forms with different
   widths and code points that designate font variations for
   mathematical uses, and Nameprep maps these onto base characters.

   While having these mapping functions available during lookup may be
   quite helpful to users who type equivalent forms, registrations are
   probably best performed in terms of the IDNA base characters only,
   i.e., those characters that nameprep will not change.  This will have
   two advantages.

   o  Registrants will never find themselves in the rather confusing
      position of having submitted one string for registration and
      finding a different string in the registry database (which could
      otherwise occur even if the relevant language table does not
      contain variants).

   o  Those who are interested in what characters are permitted by a
      given registry will only need to examine the relevant tables,
      rather than simulating the IDNA algorithm to determine the result
      of processing particular characters.

7.  IANA Considerations

   Under ICANN (not IETF) direction and management, the IANA has created
   a registry for language variant tables.  The authoritative
   documentation for that registry is in [IANA-language-registry].
   Since the registry exists and is being managed under ICANN direction,
   the material that follows is a review of the theory of this registry,

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   rather than new instructions for IANA.

   As described above and suggested in the JET Guidelines, the
   registration rules generally require only that

   o  The application be submitted or endorsed by a TLD registry, to
      ensure that someone cares about the particular table.

   o  The table be identified by the submitting (or endorsing) registry,
      a language designation consistent with [RFC3066] or other language
      or script designation acceptable to IANA for this purpose, a
      version number, and a date.

   o  Characters listed in the table be identified by Unicode code
      points, as discussed above.

   o  The table format may correspond to that identified in [RFC3743],
      or in Section 5 above, or may be some variation on those themes
      appropriate to the local processing model (with or without

   This raises some issues that will need to be worked out as
   experiences accumulate.  For example, more standardization of table
   formats would be desirable to make processing by the same computer
   tools for different registries and languages possible.  But it seems
   premature at this time due to differences in languages, processing,
   and requirements and lack of experience with them.  Similarly, if a
   registry concludes that it should use a table that contains
   characters from several scripts, it is not clear how such a table
   should be designated.  If it is identified with a language code
   (either according to [RFC3066] or an independent one registered with
   IANA) that is likely to just introduce more confusion, especially
   given other Internet uses of the language codes.  It appears that
   some other convention will be needed for those cases and should be
   developed (if it has not already been established by the time this
   document is published).

8.  Internationalization Considerations

   This document specifies a model mechanism for registering
   Internationalized Domain Names (IDNs) that can be used to reduce
   confusion among similar-appearing names.  The proposal is designed to
   facilitate internationalization while permitting a balance between
   internationalization concerns and concerns about keeping the Internet
   global and domain name system references unique in the perception of
   the user as well as in practice.

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9.  Security Considerations

   Registration of labels in the DNS that contain essentially
   unrestricted sequences of arbitrary Unicode characters may introduce
   several opportunities for either attacks or simple confusion.  Some
   of these risks, such as confusion about which character (of several
   that look alike) is actually intended, may be associated with the
   presentation form of DNS names.  Others may be linked to databases
   associated with the DNS, e.g., with the difficulty of finding an
   entry in a "Whois file" when it is not clear how to enter, or search
   for, the characters that make up a name.  This document discusses a
   family of restrictions on the names that can be registered.
   Restrictions of the type described can be imposed by a DNS zone
   ("registry").  The document also describes some possible tools for
   implementing such restrictions.

   While the increased number and types of character made available by
   Unicode considerably increases the scale of the potential problems,
   the problems addressed by this document are not new.  No plausible
   set of restrictions will eliminate all problems and sources of
   confusion: for example, it has often been pointed out that, even in
   ASCII, the characters digit-one ("1") and lower case L ("l") can
   easily be confused in some display fonts.  But, to the degree to
   which security may be aided by sensible risk reduction, these
   techniques may be helpful.

10.  Acknowledgements

   Discussions in the process of developing the JET Guidelines were
   vital in developing this document and all of the JET participants are
   consequently acknowledged.  Attempts to explain some of the issues
   uncovered there to, and feedback from, Vint Cerf, Wendy Rickard, and
   members of the ICANN IDN Committee were also helpful in the thinking
   leading up to this document.

   An effort by Paul Hoffman to create a generic specification for
   registration restrictions of this type helped to inspire this
   document, which takes a somewhat different, more language-oriented,
   approach than his initial draft.  While the initial version of that
   draft indicated that multiple languages (or multiple language tables)
   for a single zone were infeasible, more recent versions [I-D.hoffman-
   registration] shifted to inclusion of language-based approaches.  The
   current version of this document incorporates considerable text, and
   even more ideas, from those drafts, with Paul Hoffman's generous

   Feedback from several registry operators (of both country code and
   generic TLDs) including Edmon Chung and Ram Mohan of Afilias, and

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   from ICANN and IANA staff (notably Tina Dam and Theresa Swinehart)
   about issues encountered in registering tables and designing IDN
   implementations resulted in the addition of significant clarifying
   text to the current version of the document.

   The opinions expressed here are, of course, the sole responsibility
   of the author.  Some of those whose ideas and comments are reflected
   in this document may disagree with the conclusions the author has
   drawn from them.

11.  References

   [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.

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

   [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.

              Internet Engineering Steering Group, IETF, "IESG Statement
              on IDN", IESG Statement available from
              February 2003.

              Internet Corporation for Assigned Names and Numbers,
              "Guidelines for the Implementation of Internationalized
              Domain Names, Version 1.0", June 2003.

              Internet Assigned Numbers Authority, "IDN Language Table
              Registry", April 2004.

   [RFC0952]  Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet

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              host table specification", RFC 952, October 1985.

   [RFC3066]  Alvestrand, H., "Tags for the Identification of
              Languages", BCP 47, RFC 3066, January 2001.

   [Unicode]  The Unicode Consortium, "The Unicode Standard -- Version
              3.0", January 2000.

              The Unicode Consortium, "Unicode Standard Annex #28:
              Unicode 3.2"", March 2002.

   [Drucker]  Drucker, J., "The Alphabetic Labyrinth: The Letters in
              History and Imagination", 1995.

   [RFC3536]  Hoffman, P., "Terminology Used in Internationalization in
              the IETF", RFC 3536, May 2003.

              Hoffman, P., "A Method for Registering Internationalized
              Domain Names", draft-hoffman-idn-reg-02.txt (work in
              progress), October 2003.

Author's Address

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

   Phone: +1 617 491 5735
   Email: john-ietf@jck.com

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Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
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Klensin                 Expires November 18, 2005              [Page 26]