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Internationalized Domain Names in Applications (IDNA): Registry Restrictions and Recommendations

Document Type Active Internet-Draft (individual in art area)
Authors Dr. John C. Klensin , Asmus Freytag
Last updated 2023-08-18 (Latest revision 2020-07-13)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Stream WG state (None)
Document shepherd Pete Resnick
Shepherd write-up Show Last changed 2019-08-30
IESG IESG state AD Evaluation
Action Holder
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Murray Kucherawy
Send notices to (None)
IANA IANA review state Version Changed - Review Needed
Network Working Group                                         J. Klensin
Updates: 5890, 5891, 5894 (if approved)                       A. Freytag
Intended status: Standards Track                             ASMUS, Inc.
Expires: January 14, 2021                                  July 13, 2020

    Internationalized Domain Names in Applications (IDNA): Registry
                    Restrictions and Recommendations


   The IDNA specifications for internationalized domain names combine
   rules that determine the labels that are allowed in the DNS without
   violating the protocol itself and an assignment of responsibility,
   consistent with earlier specifications, for determining the labels
   that are allowed in particular zones.  Conformance to IDNA by
   registries and other implementations requires both parts.  Experience
   strongly suggests that the language describing those responsibilities
   was insufficiently clear to promote safe and interoperable use of the
   specifications and that more details and discussion of circumstances
   would have been helpful.  Without making any substantive changes to
   IDNA, this specification updates two of the core IDNA documents (RFCs
   5890 and 5891) and the IDNA explanatory document (RFC 5894) to
   provide that guidance and to correct some technical errors in the

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 14, 2021.

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Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Registry Restrictions in IDNA2008 . . . . . . . . . . . . . .   4
   3.  Progressive Subsets of Allowed Characters . . . . . . . . . .   5
   4.  Considerations for Domains Operated Primarily for the
       Financial Benefit of the Registry Owner or Operator
       Organization  . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Other corrections and updates . . . . . . . . . . . . . . . .   9
     5.1.  Updates to RFC 5890 . . . . . . . . . . . . . . . . . . .   9
     5.2.  Updates to RFC 5891 . . . . . . . . . . . . . . . . . . .  10
   6.  Related Discussions . . . . . . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     10.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  15
     A.1.  Changes from version -00 (2017-03-11) to -01  . . . . . .  15
     A.2.  Changes from version -01 (2017-09-12) to -02  . . . . . .  15
     A.3.  Changes from version -02 (2019-07-06) to -03  . . . . . .  16
     A.4.  Changes from version -03 (2019-07-22) to -04  . . . . . .  16
     A.5.  Changes from version -04 (2019-08-02) to -05  . . . . . .  16
     A.6.  Changes from version -05 (2019-08-29) to -06  . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   Parts of the specifications for Internationalized Domain Names in
   Applications (IDNA) [RFC5890] [RFC5891] [RFC5894] (collectively
   known, along with RFC 5892 [RFC5892], RFC 5893 [RFC5893] and updates
   to them, as "IDNA2008" (or just "IDNA") impose a requirement that

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   domain name system (DNS) registries restrict the characters they
   allow in domain name labels (see Section 2 below), and the contents
   and structure of those labels.  That requirement and restriction are
   consistent with the "duty to serve the community" described in the
   original specification for DNS naming and authority [RFC1591].  The
   restrictions are intended to limit the permitted characters and
   strings to those for which the registries or their advisers have a
   thorough understanding and for which they are willing to take

   That provision is centrally important because it recognized that
   historical relationships and variations among scripts and writing
   systems, the continuing evolution of those systems, differences in
   the uses of characters among languages (and locations) that use the
   same script, and so on make it impossible for a single list of
   characters and simple rules to be able to generate an "if we use
   these, we will be safe from confusion and various attacks" guideline.

   Instead, the algorithm and rules of RFCs 5891 and 5892 eliminate many
   of the most dangerous and otherwise problematic cases, but cannot
   eliminate the need for registries and registrars to understand what
   they are doing and taking responsibility for the decisions they make.

   The way in which the IDNA2008 specifications expressed these
   requirements may have under emphasized the intention that they
   actually are requirements.  Section of the Definitions
   document [RFC5890] mentions the need for the restrictions, indicates
   that they are mandatory, and points the reader to section 4.3 of the
   Protocol document [RFC5891], which in turn points to Section 3.2 of
   the Rationale document [RFC5894], with each document providing
   further detail, discussion, and clarification.

   At the same time, the Internet has evolved significantly since the
   management assumptions for the DNS were established with RFC 1591 and
   earlier.  In particular, the management and use of domain names have
   gone through several transformations.  Recounting of those changes is
   beyond the scope of this document but one of them has had significant
   practical impact on the degree to which the requirement for registry
   knowledge and responsibility is observed in practice.  When RFC 1591
   was written, the assumption was that domains at all levels of the DNS
   would be operated in the best interest of the registrants in the
   domain and of the Internet as a whole.  There were no notions about
   domains being operated for a profit, much less with a business model
   that made them more profitable the more names that could be
   registered (or even, under some circumstances, reserved and not
   registered).  At the time RFC 1591 was written, there was also no
   notion that domains would be considered more successful based on the
   number of names registered and delegated from them.  While rarely

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   reflected in the DNS protocols, the distinction between domains
   operated primarily as a revenue source of the organizations operating
   the registry and ones that are operated for, e.g., use within an
   enterprise or otherwise as a service have become very important
   today.  See Section 4 for a discussion on how those issues affect
   this specification.

   This specification is intended to unify and clarify these
   requirements for registry decisions and responsibility and to
   emphasize the importance of registry restrictions at all levels of
   the DNS.  It also makes a specific recommendation for character
   repertoire subsetting that is intermediate between the code points
   allowed by RFCs 5891 and 5892 and those allowed by individual
   registries.  It does not alter the basic IDNA2008 protocols and rules
   themselves in any way.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Registry Restrictions in IDNA2008

   As mentioned above, IDNA2008 specifies that the registries for each
   zone in the DNS that supports IDN labels are required to develop and
   apply their own rules to restrict the allowable labels, including
   limiting characters they allow to be used in labels in that zone.
   The chosen list MUST be a subset of the collection of code points
   specified as "PVALID", "CONTEXTJ", and "CONTEXTO" by the rules
   established by the protocols themselves.  Labels containing any
   characters from the two CONTEXT categories or any characters that are
   normally part of a script written right to left [RFC5893] require
   that additional rules, specified in the protocols and known as
   "contextual rules" and "bidi rules", be applied.  The entire
   collection of rules and restrictions required by the IDNA2008
   protocols themselves are known as "protocol restrictions".

   As mentioned above, registries may apply (and generally are required
   to apply) additional rules to further restrict the list of permitted
   code points, contextual rules (perhaps applied to normally PVALID
   code points) that apply additional restrictions, and/or restrictions
   on labels as distinct from code points.  The most obvious of those
   restrictions include provisions for restricting suggested new
   registrations based on conflicts with labels already registered in
   the zone, so as to avoid homograph attacks [Gabrilovich2002] and
   other issues.  The specifications of what constitutes such conflicts,
   as well as the definition of "conflict" based on the properties of
   the labels in question, is the responsibility of each registry.  They
   further include prohibitions on code points and labels that are not

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   consistent with the intended function of the zone, the subtree in
   which the zone is embedded (see Section 3), or limitations on where
   allowable code points may be placed in a label.

   These per-registry (or per-zone) rules are commonly known as
   "registry restrictions" to distinguish them from the protocol
   restrictions described above.  By necessity, protocol restrictions
   are somewhat generic, having to cater both to the union of the needs
   for all zones as well as to the desires of the most permissive zones.
   In consequence, additional registry restrictions are essential to
   provide for the necessary security in the face of the tremendous
   variations and differences in writing systems and their ongoing
   evolution and development, as well as the human ability to recognize
   and distinguish characters in different scripts around the world and
   under different circumstances.

3.  Progressive Subsets of Allowed Characters

   The algorithm and rules of RFCs 5891 and 5892 determine the set of
   code points that are possible for inclusion in domain name labels;
   registries MUST NOT permit code points in labels unless they are part
   of that set.  Labels that contain code points that are normally
   written from right to left MUST also conform to the requirements of
   RFC 5893.  Each registry that intends to allow IDN registrations MUST
   then determine the strict subset of that set of code points that will
   be allowed by that registry.  It SHOULD also consider additional
   rules, including contextual and whole label restrictions that provide
   further protection for registrants and users.  For example, the
   widely-used principle that bars labels containing characters from
   more than one script is not an IDNA2008 requirement.  It has been
   adopted by many registries but there may be circumstances in which is
   it not required or appropriate.

   In formulating their own rules, registries should normally consult
   carefully-developed consensus recommendations about global maximum
   repertoires to be used such as the ICANN Maximal Starting Repertoire
   4 (MSR-4) for the Development of Label Generation Rules for the Root
   Zone [ICANN-MSR4] (or its successor documents).  Additional
   recommendations of similar quality about particular scripts or
   languages exist, including, but not limited to, the RFCs for Cyrillic
   [RFC5992], Arabic Language [RFC5564], or script-based repertoires
   from the approved ICANN Root Zone Label Generation Rules (LGR-3)
   [ICANN-LGR3] (or its successor documents).  Many of these
   recommendations also cover rules about relationships among code
   points that may be particularly important for complex scripts.  They
   also interact with recommendations about how labels that appear to be
   the same should be handled.

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   It is the responsibility of the registry to determine which, if any,
   of those recommendations are applicable and to further subset or
   extend them as needed.  For example, several of the recommendations
   are designed for the root zone and therefore exclude digits and
   U+002D HYPHEN-MINUS; this restriction is not generally appropriate
   for other zones.  On the other hand, some zones may be designed to
   not cater for all users of a given script, but perhaps only for the
   needs of selected languages, in which case a more selective
   repertoire may be appropriate.

   In making these determinations, a registry SHOULD follow the IAB
   guidance in RFC 6912 [RFC6912].  Those guidelines include a number of
   principles for use in making decisions about allowable code points.
   In addition, that document notes that the closer a particular zone is
   to the root, the more restrictive the space of permitted labels
   should be.  RFC 5894 provides some suggestions for any registry that
   may decide to reduce opportunities for confusion or attacks by
   constructing policies that disallow characters used in historic
   writing systems (whether these be archaic scripts or extensions of
   modern scripts for historic or obsolete orthographies) or characters
   whose use is restricted to specialized, or highly technical contexts.
   These suggestions were among the principles guiding the design of
   ICANN's Maximal Starting Repertoires (MSR) [LGR-Procedure].

   A registry decision to allow only those code points in the full
   repertoire of the MSR (plus digits and hyphen) would already avoid a
   number of issues inherent in a more permissive policy such as "use
   anything permitted by IDNA2008", while still supporting the native
   languages and scripts for the vast majority of users today.  However,
   it is unlikely, by itself, to fully satisfy the mandate set out above
   for three reasons.

   1.  The MSR, like the set of code points permissible under IDNA2008
       itself, was conceived merely as a boundary condition on
       permissible letter code points (it excludes digits and the
       hyphen).  It was always intended to be used as a starting point
       for setting registry policy, with the expectation that some of
       the code points in the MSR would not be included in the final
       registry policy, whether for lack of actual usage, or for being
       inherently problematic.

   2.  It was recognized that many scripts require contextual rules for
       many more code points than are covered by CONTEXTO or CONTEXTJ
       rules defined in IDNA2008.  This is particularly true for
       combining marks, typically used to encode diacritics, tone marks,
       vowel signs and the like.  While, theoretically, any combining
       mark may occur in any context in Unicode, in practice rendering
       and other software that users rely on in viewing or entering

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       labels will not support arbitrary combining sequences, or indeed
       arbitrary combinations of code points, in the case of complex

       Contextual rules are needed in order to limit allowable code
       point sequences to those that can be expected to be rendered
       reliably.  Identifying those requires knowledge about the way
       code points are used in a script, whence the mandate for
       registries to only support code points they understand.  In this,
       some of the other recommendations, such as the Informational RFCs
       for specific scripts (e.g., Cyrillic [RFC5992]) or languages
       (e.g., Arabic [RFC5564] or Chinese [RFC4713]), or the Root Zone
       LGRs developed by ICANN, may provide useful guidance.

   3.  Third, because of the widely accepted practice of limiting any
       given label to a single script, a universal repertoire, such as
       the MSR, would have to be divided on a per-script basis into
       subrepertoires to make it useful, with some of those repertoires
       overlapping, for example, in the case of East Asian shared usage
       of the Han ideographs.

   Registries choosing to make exceptions -- allow code points that
   recommendations such as the MSR do not allow -- should make such
   decisions only with great care and only if they have considerable
   understanding of, and great confidence in, their appropriateness.
   The obvious exception from the MSR would be to allow digits and the
   hyphen.  Neither were allowed by the MSR, but only because they are
   not allowed in the Root Zone.

   Nothing in this document permits a registry to allow code points or
   labels that are disallowed or otherwise prohibited by IDNA2008.

4.  Considerations for Domains Operated Primarily for the Financial
    Benefit of the Registry Owner or Operator Organization

   As discussed in the Introduction (Section 1), the distributed
   administrative structure of the DNS today can be described by
   dividing zones into two categories depending on how they are
   administered and for whom.  These categories are not precise -- some
   zones may not fall neatly into one category or the other -- but are
   useful in understanding the practical applicability of this
   specification.  They are:

      Zones operating primarily or exclusively within a country,
      organization, or enterprise and responsible to the Internet users
      in that country or the management of the organization or
      enterprise.  DNS operations, including registrations and
      delegations, will typically occur in support of the purpose of

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      that country, organization or enterprise rather than being its
      primary purpose.

      Zones operating primarily as all or part of a business of selling
      names for the financial benefit of entities responsible for the
      registry.  For these domains, most delegations of subdomains are
      to entities with little or no affiliation with the registry
      operator other than contractual agreements about operation of
      those subdomains.  These zones are often known as "public domains"
      or with similar terms, but those terms often have other semantics
      and may not cover all cases.  In particular, a country code domain
      operated primarily in the interest of registrants and Internet
      users and in service to the broader Internet community is often
      considered a "public domain" but would fall into the first
      category, not the second.

   Rules requiring strict registry responsibility, including either
   thorough understanding of scripts and related issues in domain name
   labels being considered for registration or local naming rules that
   have the same effect, typically come naturally to registries for
   zones of the first type.  Registration of labels that would prove
   problematic for any reason hurts the relevant organization or
   enterprise or its customers or users within the relevant country and
   more broadly.  More generally, there are strong incentives to be
   extremely conservative about labels that might be registered and few,
   if any, incentives favoring adventures into labels that might be
   considered clever, much less ones that are hard to type, render, or,
   where it is relevant to users, remember correctly.

   By contrast, in a zone in which the profits are derived exclusively,
   or almost exclusively, from selling or reserving (including
   "blocking") names, there may be perceived incentives to register
   whatever names would-be registrants "want" or fears that any
   restrictions will cut into the available namespace.  In such
   situations, restrictions are unlikely to be applied unless they meet
   at least one of two criteria: (i) they are easy to apply and can be
   applied algorithmically or otherwise automatically and/or (ii) there
   is clear evidence that the particular label would cause harm.

   As suggested above, the two categories above are not precise.  In
   particular, there may be domains that, despite being set up to
   operate to produce revenue about actual costs, are sufficiently
   conservative about their operations to more closely resemble the
   first group in practice than the second one.

   The requirement of IDNA that is discussed at length elsewhere in this
   specification stands: IDNA (and IDNs generally) would work better and
   Internet users would be better protected and more secure if

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   registries and registrars (of any type) confined their registrations
   to scripts and code point sequences that they understood thoroughly.
   While the IETF rarely gives advice to those who choose to violate
   IETF Standards, some advice to zones in the second category above may
   be in order.  That advice is that significant conservatism in what is
   allowed to be registered, even for reservation purposes, and even
   more conservatism about what labels are actually entered into zones
   and delegated, is the best option for the Internet and its users.  If
   practical considerations do not allow that much conservatism, then it
   is desirable to consult and utilize the many lists and tables that
   have been, and continue to be, developed to advise on what might be
   sensible for particular scripts and languages.  These include ICANN's
   twin efforts of creating per-script Root Zone Label Generation Rules
   [RZ-LGR-3] and Second Level Reference Label Generation Rules
   [SL-REF-LGR] (the latter of which may be per language).  They also
   include other lists of code points or code point relationships that
   may be particularly problematic and that should be treated with extra
   caution or prohibited entirely such as the proposed "troublesome
   character" list [Freytag-troublesome].  See also Section 6 below.

5.  Other corrections and updates

   After the initial IDNA2008 documents were published (and RFC 5892 was
   updated for Unicode 6.0 by RFC 6452 [RFC6452]) several errors or
   instances of confusing text were noted.  For the convenience of the
   community, the relevant corrections for RFCs 5890 and 5891 are noted
   below and update the corresponding documents.  There are no errata
   for RFC 5893 or 5894 as of the date this document was published.
   Because further updates to RFC 5892 would require addressing other
   pending issues, the outstanding erratum for that document is not
   considered here.  For consistency with the original documents,
   references to Unicode 5.0 are preserved in this document.

5.1.  Updates to RFC 5890

   The outstanding errata against RFC 5890 (Errata ID 4695, 4696, 4823,
   and 4824 [RFC-Editor-5890Errata]) are all associated with the same
   issue, the number of Unicode characters that can be associated with a
   maximum-length (63 octet) A-label.  In retrospect and contrary to
   some of the suggestions in the errata, that value should not be
   expressed in octets because RFC 5890 and the other IDNA 2008
   documents are otherwise careful to not specify Unicode encoding forms
   but, instead, work exclusively with Unicode code points.
   Consequently the relevant material in RFC 5890 should be corrected as


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      Old:  expansion of the A-label form to a U-label may produce
         strings that are much longer than the normal 63 octet DNS limit
         (potentially up to 252 characters).

      New:  expansion of the A-label form to a U-label may produce
         strings that are much longer than the normal 63 octet DNS limit
         (See Section 4.2).

      Comment:  If the length limit is going to be a source of confusion
         or careful calculations, it should appear in only one place.

   Section 4.2

      Old:  Because A-labels (the form actually used in the DNS) are
         potentially much more compressed than UTF-8 (and UTF-8 is, in
         general, more compressed that UTF-16 or UTF-32), U-labels that
         obey all of the relevant symmetry (and other) constraints of
         these documents may be quite a bit longer, potentially up to
         252 characters (Unicode code points).

      New:  A-labels (the form actually used in the DNS) and the
         Punycode algorithm used as part of the process to produce them
         [RFC3492] are strings that are potentially much more compressed
         than any standard Unicode Encoding Form.  A 63 octet A-label
         cannot represent more than 58 Unicode code points (four octet
         overhead and the requirement that at least one character lie
         outside the ASCII range) but implementations allocating buffer
         space for the conversion should allow significantly more space
         (i.e., extra octets) depending on the encoding form they are

5.2.  Updates to RFC 5891

   Errata ID 3969: Improve reference for combining marks.  There is only
      one erratum for RFC 5891, Errata ID 3969 [RFC5891Erratum].
      Combining marks are explained in the cited section, but not, as
      the text indicates, exactly defined.

      Old:  The Unicode string MUST NOT begin with a combining mark or
         combining character (see The Unicode Standard, Section 2.11
         [UnicodeA] for an exact definition).

      New:  The Unicode string MUST NOT begin with a combining mark or
         combining character (see The Unicode Standard, Section 2.11
         [UnicodeA] for an explanation and Section 3.6, definition D52
         [UnicodeB]) for an exact definition).

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      Comment:  When RFC 5891 is actually updated, the references in the
         text should be updated to the current version of Unicode and
         the section numbers checked.

6.  Related Discussions

   This document is one of a series of measures that have been suggested
   to address IDNA issues raised in other documents and discussions.
   Those other discussions and associated documents include suggested
   mechanisms for dealing with combining sequences and single-code point
   characters with the same appearance, ones that normalization neither
   combines nor decomposes as IDNA2008 assumed.  That topic was
   discussed further in [IDNA-Unicode] and in the IAB response to that
   issue [IAB-2015].  Those and other documents also discuss issues with
   IDNA and character graphemes for which abstractions exist in Unicode
   in precomposed form but that can be generated from combining
   sequences.  Another approach is a suggested registry of code points
   known to be problematic [Freytag-troublesome].  In combination, the
   various discussions of combining sequences and non-decomposing
   characters may lay the foundation for an actual update to the IDNA
   code points document [RFC5892].  Such an update would presumably also
   address the existing errata against that document.

   At a much higher-level, discussions are ongoing to consider issues,
   demands, and proposals for new uses of the DNS.

7.  Security Considerations

   As discussed in IAB recommendations about internationalized domain
   names [RFC4690], [RFC6912], and elsewhere, poor choices of strings
   for DNS labels can lead to opportunities for attacks, user confusion,
   and other issues less directly related to security.  This document
   clarifies the importance of registries carefully establishing design
   policies for the labels they will allow and that having such policies
   and taking responsibility for them is a requirement, not an option.
   If that clarification is useful in practice, the result should be an
   improvement in security.

8.  Acknowledgments

   Many thanks to Patrik Faltstrom who provided an important review on
   the initial version, to Jaap Akkerhuis, Don Eastlake, Barry Leiba,
   and Alessandro Vesely who did reviews that improved the text and to
   Pete Resnick who acted as document shepherd and did an additional
   careful review.

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9.  IANA Considerations

   [[CREF1: RFC Editor: Please remove this section before publication.]]

   This memo includes no requests to or actions for IANA.  In
   particular, it does not contain any provisions that would alter any
   IDNA-related registries or tables.

10.  References

10.1.  Normative References

              ICANN, "Root Zone Label Generation Rules (LGR-1)", July

              ICANN, "Maximal Starting Repertoire Version 4 (MSR-4) for
              the Development of Label Generation Rules for the Root
              Zone", January 2019,

   [RFC1591]  Postel, J., "Domain Name System Structure and Delegation",
              RFC 1591, DOI 10.17487/RFC1591, March 1994,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891,
              DOI 10.17487/RFC5891, August 2010,

              "RFC 5891, "Internationalized Domain Names in Applications
              (IDNA): Protocol"", Errata ID 3969, April 2014,

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   [RFC5893]  Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts
              for Internationalized Domain Names for Applications
              (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010,

   [RFC5894]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Background, Explanation, and
              Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010,

   [RFC6912]  Sullivan, A., Thaler, D., Klensin, J., and O. Kolkman,
              "Principles for Unicode Code Point Inclusion in Labels in
              the DNS", RFC 6912, DOI 10.17487/RFC6912, April 2013,

10.2.  Informative References

              Freytag, A., Klensin, J., and A. Sullivan, "Those
              Troublesome Characters: A Registry of Unicode Code Points
              Needing Special Consideration When Used in Network
              Identifiers", June 2017, <draft-freytag-troublesome-

              Gabrilovich, E. and A. Gontmakher, "The Homograph Attack",
              Communications of the ACM 45(2):128, February 2002.

              Internet Architecture Board (IAB), "IAB Statement on
              Identifiers and Unicode 7.0.0", February 2015,

              Klensin, J. and P. Faltstrom, "IDNA Update for Unicode
              7.0.0", September 2017, <draft-klensin-idna-5892upd-

              Internet Corporation for Assigned Names and Numbers
              (ICANN), "Procedure to Develop and Maintain the Label
              Generation Rules for the Root Zone in Respect of IDNA
              Labels", March 2013,

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              RFC Editor, "RFC Errata: RFC 5890, "Internationalized
              Domain Names for Applications (IDNA): Definitions and
              Document Framework", August 2010", Note to RFC
              Editor: Please figure out how you would like this
              referenced and make it so., Captured 2017-09-10, 2016,

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,

   [RFC4690]  Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
              Recommendations for Internationalized Domain Names
              (IDNs)", RFC 4690, DOI 10.17487/RFC4690, September 2006,

   [RFC4713]  Lee, X., Mao, W., Chen, E., Hsu, N., and J. Klensin,
              "Registration and Administration Recommendations for
              Chinese Domain Names", RFC 4713, DOI 10.17487/RFC4713,
              October 2006, <>.

   [RFC5564]  El-Sherbiny, A., Farah, M., Oueichek, I., and A. Al-Zoman,
              "Linguistic Guidelines for the Use of the Arabic Language
              in Internet Domains", RFC 5564, DOI 10.17487/RFC5564,
              February 2010, <>.

   [RFC5892]  Faltstrom, P., Ed., "The Unicode Code Points and
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5892, DOI 10.17487/RFC5892, August 2010,

   [RFC5992]  Sharikov, S., Miloshevic, D., and J. Klensin,
              "Internationalized Domain Names Registration and
              Administration Guidelines for European Languages Using
              Cyrillic", RFC 5992, DOI 10.17487/RFC5992, October 2010,

   [RFC6452]  Faltstrom, P., Ed. and P. Hoffman, Ed., "The Unicode Code
              Points and Internationalized Domain Names for Applications
              (IDNA) - Unicode 6.0", RFC 6452, DOI 10.17487/RFC6452,
              November 2011, <>.

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              Internet Corporation for Assigned Names and Numbers, "Root
              Zone Label Generation Rules - LGR-3: Overview and Summary,
              Version 3", July 2019,

              Internet Corporation for Assigned Names and Numbers
              (ICANN), "Second Level Label Generation Rules", 2019,

              The Unicode Consortium, "The Unicode Standard, Version
              12.1", May 2019.

              Section 2.11

              The Unicode Consortium, "The Unicode Standard, Version
              12.1", May 2019.

              Section 3.6, definition D52

Appendix A.  Change Log

   RFC Editor: Please remove this appendix before publication.

A.1.  Changes from version -00 (2017-03-11) to -01

   o  Added Acknowledgments and adjusted references.

   o  Filled in Section 5 with updates to respond to errata.

   o  Added Section 6 to discuss relationships to other documents.

   o  Modified the Abstract to note specifically updated documents.

   o  Several small editorial changes and corrections.

A.2.  Changes from version -01 (2017-09-12) to -02

   After a pause of nearly 34 months due to inability to get this draft
   processed, including nearly a year waiting for a new directorate to
   actually do anything of substance about fundamental IDNA issues, the
   -02 version was posted in the hope of getting a new start.  Specific
   changes include:

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   o  Added a new section, Section 4, and some introductory material to
      address the very practical issue that domains run on a for-profit
      basis are unlikely to follow the very strict "understand what you
      are registering" requirement if they support IDNs at all and
      expect to profit from them.

   o  Added a pointer to draft-klensin-idna-unicode-review to the
      discussion of other work.

   o  Editorial corrections and changes.

A.3.  Changes from version -02 (2019-07-06) to -03

   o  Minor editorial changes in response to shepherd review.

   o  Additional references.

A.4.  Changes from version -03 (2019-07-22) to -04

   o  Editorial changes after AD review and some additional changes to
      improve clarity.

A.5.  Changes from version -04 (2019-08-02) to -05

   o  Small editorial corrections, many to correct glitches found during
      IETF Last Call.

   o  Updated acknowledgments, particularly to reflect reviews in Last

A.6.  Changes from version -05 (2019-08-29) to -06

   Other than some small editorial adjustments, these changes made
   after, and reflect, IESG post-last-call review and comments.  To the
   extent it was possible to do so without making this document
   inconsistent with the other IDNA documents, established IETF,
   Unicode, and ICANN community i18n terminology, or well-established
   IDNA or i18n practices, the first author believes that the document
   responds to all previously-outstanding IESG substantive comments.

   o  Fixed a remaining citation issue with a Unicode document.  This
      version has not been updated to reflect Unicode 13, but the
      document should be adjusted so that all references are
      contemporary at the time of publication.

   o  Added reference to homograph attacks, and slightly adjusted
      discussion of them, per discussion with IESG post-last-call.

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   o  Removed pointer to RFC 5890 from discussion of mixed-script labels
      in Section 3.

   o  Rewrote parts of Section 4 to eliminate the term "for-profit" and
      clarify the issues.

   o  Removed pointer to draft-klensin-idna-unicode-review because RFC
      8753 has been published and is therefore no longer pending /
      parallel work.

   o  Rewrote Section 6 to make the relationships among various
      documents and efforts somewhat more clear.

   o  References to RFCs 5893 and 6912 moved from Informative to

Authors' Addresses

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

   Phone: +1 617 245 1457

   Asmus Freytag
   ASMUS, Inc.


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