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Network Working Group                                         J. Klensin
Internet-Draft                                         February 15, 2004
Expires: August 15, 2004

  Registration of Internationalized Domain Names: Overview and Method

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 15, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   IETF has introduced standards-track mechanisms to enable the use of
   "internationalized", i.e., non-ASCII, names in the DNS and
   applications that use it.  This has led, in turn, to concerns that
   characters with similar meanings or appearance could cause user
   confusion and opportunities for deliberate deception and fraud.  Part
   of this problem can be addressed by limiting, on a per-zone (or
   per-registry) basis, the specific characters that can be used to be a
   subset of the list allowed by the standard and by creating
   "reservations" of labels that might create confusion with those that
   are permitted.  The model for doing this for languages that use
   characters that originated with Chinese has been extensively
   developed in another document.  This document discusses some of the
   issues in that design and relates them to considerations and

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   mechanisms that might be appropriate for other languages and scripts,
   especially those involving alphabetic characters.

   In particular, it describes some suggested practices for registering
   internationalized domain names (IDNs) in a zone. Before accepting
   such registrations of domain names into a zone, the zone's registry
   should decide which codepoints in the Unicode character set the zone
   will accept. The registry should also decide whether particular
   characters in a registered domain name should cause registration of
   multiple equivalent domain names; these domain names might be added
   to the zone or blocked from registration. This document also
   describes how to handle character variants in registering IDNs, and
   how to publish tables that list the character variants.

   This document is intended to supply a basis for adapting methods
   developed for Chinese, Japanese, and Korean to other languages and
   scripts.  If these adaptations are made carefully and with due
   consideration for local issues, the likelihood of problematic DNS
   registrations with be significantly reduced.  A specific method is
   introduced that should be applicable (directly, or with minor
   modifications), to many scripts.

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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.1   Background . . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.2   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  5
   1.2.1 Characters, variants, registrations, and other issues  . . .  5
   1.2.2 Confusion, fraud, and cybersquatting . . . . . . . . . . . .  6
   1.3   A Review of the JET Guidelines . . . . . . . . . . . . . . .  6
   1.3.1 JET model  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   1.3.2 Reserved Names and Label Packages  . . . . . . . . . . . . .  8
   1.4   Languages, Scripts, and Variants . . . . . . . . . . . . . .  8
   1.4.1 Languages and Scripts  . . . . . . . . . . . . . . . . . . .  8
   1.4.2 Variant Selection  . . . . . . . . . . . . . . . . . . . . . 10
   1.5   Reservations and Exclusions  . . . . . . . . . . . . . . . . 11
   1.5.1 Sequence Exclusions for Valid Characters . . . . . . . . . . 11
   1.5.2 Character Pairing Issues . . . . . . . . . . . . . . . . . . 11
   1.6   The Registration Bundle  . . . . . . . . . . . . . . . . . . 11
   1.6.1 Definitions and Structure  . . . . . . . . . . . . . . . . . 11
   1.6.2 Application of the Registration Bundle . . . . . . . . . . . 12
   2.    Some Implications of this Approach . . . . . . . . . . . . . 13
   3.    Required Modifications to JET Model Needed Under Some of
         the Models Above . . . . . . . . . . . . . . . . . . . . . . 14
   4.    Conclusions and Recommendations about the General
         Approach . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   5.    A Model Table format . . . . . . . . . . . . . . . . . . . . 15
   6.    A Model Registration Procedure --"CreateBundle"  . . . . . . 16
   6.1   Description of CreateBundle  . . . . . . . . . . . . . . . . 16
   7.    Security Considerations  . . . . . . . . . . . . . . . . . . 17
   8.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
         References . . . . . . . . . . . . . . . . . . . . . . . . . 18
         Author's Address . . . . . . . . . . . . . . . . . . . . . . 19
         Intellectual Property and Copyright Statements . . . . . . . 20

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1. Introduction

1.1 Background

   Once work on the basic model for encoding non-ASCII strings in the
   DNS with IDNA ([1], [2], [3]) was nearing completion, it became clear
   that it would be desirable for registries to impose additional
   restrictions on the names that could actually be registered (e.g.,
   see [6]) as a means of reducing potential confusion among characters
   that were similar in some way.  These restrictions were, in many
   respects, part of a long tradition.  For example, while the original
   DNS specifications [4] 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 [7]
   and defined by the "LDH" (for "letter digit hyphen", the three
   permitted types of characters) convention. 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, imposed and enforced 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. A similar situation
   existed even before the introduction of IDNA as 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 different locations and languages considering different
   characters important, 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
   register a name with such a character or string, the registry might
   want to automatically associate all the names that have the same
   meaning with the registered name. The registry can also decide if the
   names that came from one registration should go into the zone, be
   blocked from other people registering them, or a combination of these
   two actions.

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   To date, the best-developed system for handling registration
   restrictions for IDNs is the JET Guidelines for Chinese, Japanese,
   and Korean [5], the so-called "CJK" languages.  That system is
   limited to those languages and, in particular, to their common script
   base.   This document explores the principles behind those guidelines
   and some of the issues that might arise in trying to adapt them to
   alphabetic languages.

   This document describes five things:
   o  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
   o  Suggested practices for describing character variants
   o  A method for using a zone's character variants to determine which
      names should be associated with a registration
   o  A format for publishing a zone's table of character variants
   o  A model algorithm for name registration given the presence of
      language tables.

1.2 Terminology

1.2.1 Characters, variants, registrations, and other issues

   1.   Characters in this document are given as their Unicode
        codepoints on U+xxxx format or with their official names.
   2.   The following terms are used in this document.
   3.   A "string" is an sequence of one or more characters.
   4.   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
   5.   The "variant(s)" are the character(s) and/or string(s) that are
        equivalent to the base character.  Note that these might not be
        true equivalent characters: a base character might have a
        mapping to a particular variant character, but that variant
        character does not have to have a mapping to the base character.
        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.
   6.   The "base registration" is the single name that the registrant
        requested from the registry.
   7.   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

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        DNS tools.  The JET Guidelines describe a "registered" label as
   8.   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".
   9.   A "reserved label" is a label in a registration bundle that is
        not actually registered.
   10.  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.
   11.  A coded character set ("CCS"): A term for a list of characters
        and the code positions assigned to them.  ASCII and Unicode are
   12.  A language: Something spoken by humans, independent of how it is
        written or coded.  ISO Standard 639 and IETF BCP 47 (RFC 3066)
        [8] list and define codes for identifying languages.
   13.  Script: 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 without their having
        identical scripts.
   14.  Charset: 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 [12],
   which also provides sources.  The two sets of definitions are
   intended to be consistent.

1.2.2 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

1.3 A Review of the JET Guidelines

1.3.1 JET model

   In the JET Guidelines model, a prospective registrant approaches the
   registry for a zone (perhaps through an intermediate registrar) with

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   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 [8] -- Chinese as used on the mainland of the People's Republic of
   China ("zh-cn") can, at registry option, be coded differently and
   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.

   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 contructed for alphabetic scripts, the transfer 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 specialized variation on
   the model and method presented by the rest of this document.  Other
   languages or scripts may require other variations

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1.3.2 Reserved Names and Label Packages

   A basic assumption of the JET model is that, if the properties of
   Unicode [9], [10], IDNA, or the evolution of specific characters,
   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 placed into the DNS
   automatically, and others are simply reserved and can be activated,
   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 not able to be registered.

1.4 Languages, Scripts, and Variants

1.4.1 Languages and Scripts

   Conversations about scripts -- collections of characters associated
   with particular languages -- are common when discussing character
   sets and codes.   But the boundaries between one script and another
   are not well-defined.  The Unicode Standard [9][10], 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 (Cf. [11]), with certain
   character-symbols appearing 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 almost always have
   the same interpretation in each one and, at least for the subset of
   characters that actually are ideographs, 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 specfy multiple languages, to verify that the
   characters in the label string were valid for each, and then to
   generate variant labels using each language in turn.  For many
   alphabetic languages, it may make sense 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

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   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
      presumably prohibit any characters that did not appear in all
      three languages).
   o  One table each, but with a "one label, one language" rule for the
   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.

   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.

   It is also worth stressing, as the JET Guidelines do, that no tables
   or systems of this type -- even if identified with languages as a
   means of defining or describing those tables -- 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 those 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

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   with, e.g., confusion caused by similar-looking characters.

1.4.2 Variant Selection

   The area of character variants is rife with problems. 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, LATIN SMALL LETTER O WITH DIAERESIS and
   LATIN SMALL LETTER O WITH STROKE are variants of each other, while in
   other regions, most people would think that LATIN SMALL LETTER O WITH
   DIAERESIS 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 in
   IDNA, and it remains, at the time of this writing, questionable how
   widely those tables will be accepted among users of Chinese from
   areas of the world other than those represented by the group that
   created them.

   Thus, the first thing a registry should ask is whether or not any of
   the characters that they want to use 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 is 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.

   The list of character variants used in a zone should be stable.
   Although it is possible to add variants for characters later, doing
   so can cause confusing with registrants.

   Of course, zone managers should inform all current registrants when
   the registration policy for the zone changes. This includes when IDN
   characters are allowed in the zone the first time, when characters
   are added later, and when character variant tables change.

   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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1.5 Reservations and Exclusions

1.5.1 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 Guildlines 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.

   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" (U+00E6) or "ij" (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).

1.5.2 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
   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.

1.6 The Registration Bundle

1.6.1 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

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   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

1.6.2 Application of the Registration Bundle

   A registry has three options for how to handle the case where the
   registration bundle has more than one label. The policy options are:

   1.  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
       number 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.
   2.  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 don't know what the base characters 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 no know whether to type a
       "1" or a "l" after the "pa" in the first label.
   3.  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

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       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 allocating
       variants or blocking variants. 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 have actually
   been quite common.  In general, in those languages, words contain at
   least one vowel and do not have embedded numbers. The more one moves
   toward "language"-based registry restrictions, the less it is going
   to be possible to construct labels out of fanciful strings. Such
   strings may make very good identifiers, while being terrible
   candidates for "words".  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, but, given the embedded
   digits and lack of vowels, would fail even the most superficial of
   tests for valid Engish word forms.

   Interestingly, if one is 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
   effectively equivalent to this one have historically been used to
   restrict registrations in some country-code top level domains. On the
   other hand, if look-alike characters are a concern, even that type of
   rule (or restriction) would still not avoid the need for 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.

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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.
   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 sublabels, each in a
      different language,  with some particular delimiter used to define
      the boundaries of the sublabels.

4. Conclusions and Recommendations about the General Approach

   Thinking about the implications of the use in DNS labels of the full
   range of characters permitted by IDNA has led multiple groups to the
   conclusion that some restrictions, on a per-registry or per-zone
   basis, 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.  It appears that the best way to approach such
   restrictions involves drawing from the language and culture of the
   community of registrants and users in the relevant zone: if
   particular characters are likely to be 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 use the package in a way that confused his or her
   own users, the possibility of turning potential confusion into a
   hostile attack would be considerably reduced.

   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

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   constructing identifiers.

   The discussion above provides many options that could be selected,
   defined, and applied in different types in different registries
   (zones).  Registrars 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 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, which is also called the "base character".
   If the base character has any variants, it 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 beging 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
   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)

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   The table would look like:
   # An example of a table
   U+2237|U+003A-U+003A # Note that the variant is a string
   Implementors 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 exactly four octets
   representing the character value) will fail with tables that use
   characters whose value is greater than 0xFFFF.

6. A Model Registration Procedure --"CreateBundle"

   This procedure has three inputs:
   o  the proposed base registration
   o  the language for the proposed base registration
   o  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 CreateBundle process
       described below.
   Note that the process must be executed only once. The process must
   not be run on any output of the process, only on the proposed base

6.1 Description of CreateBundle

   The CreateBundle process determines if a registration bundle can be
   created and, if so, fills that bundle only with valid labels.

   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 in the CreateBundle process 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

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       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 ToASCII for the base registration. If
       ToASCII fails for the base registration, the system must stop and
       not register any of the label (that is, it must not register
       either the base registration or any created labels, even if those
       labels would have passed ToASCII). If ToASCII succeeds, add the
       result to 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, put the label into the registration bundle. Otherwise, do
       not process this label from the temporary bundle any further; it
       will not go into the registration bundle.
   5.  The result is the registration bundle with the base registration
       and possibly other labels. Finish.

7. 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 that can
   be imposed on a DNS zone ("registry") and some possible tools for
   implementing restrictions of that sort.  No plausible set of
   restrictions will eliminate all problems and sources of confusion:
   for example, it has often been pointed out that the characters
   digit-one ("1") and lower case L ("l") can easily be confused in some
   fonts used to display ASCII.   But, to the degree to which security
   may be aided by sensible risk reduction, these techniques may be

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8. 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
   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.  While the initial version of that document indicated that
   multiple languages (or multiple language tables) for a single zone
   were infeasible, more recent versions [13] 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 permission.

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


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

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

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

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

   [5]   Seng, J., Ed., Klensin, J., Ed., Rickard, W., Ed., Konishi, K.,
         Huang, K., Qian, H. and Y. Ko, "International Domain Names
         Registration and Administration Guidelines for Chinese,
         Japanese, and Korean", draft-jseng-idn-admin-05.txt (work in
         progress), June 2003.

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   [6]   Internet Engineering Steering Group, IETF, "IESG Statement on
         IDN", IESG Statement IDNstatement.txt, February 2003.

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

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

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

   [10]  The Unicode Consortium, "Unicode Standard Annex #28", March

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

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

   [13]  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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