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Versions: (RFC 5891) 00 01 02 03 04 05 Draft is active
In: IESG_Evaluation
Network Working Group                                         J. Klensin
Updates: 5890, 5891, 5894 (if approved)                       A. Freytag
Intended status: Standards Track                             ASMUS, Inc.
Expires: March 16, 2018                               September 12, 2017

    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 some specific examples would
   have been helpful.  Without making any substantive changes to IDNA,
   this specification updates two of the core IDNA documents (RFC 5980
   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 https://datatracker.ietf.org/drafts/current/.

   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 March 16, 2018.

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Copyright Notice

   Copyright (c) 2017 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
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Registry Restrictions in IDNA2008 . . . . . . . . . . . . . .   3
   3.  Progressive Subsets of Allowed Characters . . . . . . . . . .   4
   4.  Other corrections and updates . . . . . . . . . . . . . . . .   6
     4.1.  Updates to RFC 5890 . . . . . . . . . . . . . . . . . . .   7
     4.2.  Updates to RFC 5891 . . . . . . . . . . . . . . . . . . .   8
   5.  Related Discussions . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  12
     A.1.  Changes from version -00 (2017-03-11) to -01  . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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
   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 "trustee for the community" requirements of 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

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   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 RFC 5981 and 5982 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 obscured 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.

   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 intermediate between the code points allowed by
   RFC 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 smaller than the collection of code points
   specified as "PVALID", "CONTEXTJ", and "CONTEXTO" by the rules
   established by the protocols themselves.  The latter two categories,
   and labels containing any characters that are normally part of a

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   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.  The most obvious of those restrictions include provisions
   for restricting suggested new registrations based on conflicts with
   labels already registered in the zone and specifications of what
   constitutes such conflicts based on the properties of the labels in
   question.  They further include prohibitions on code points and
   labels that are not consistent with the intended function of the zone
   or the subtree in which it is embedded (see Section 3) or limitations
   on where in a label allowable code points may be placed.

   These per-registry (or per-zone) rules are commonly known as
   "registry restrictions" to distinguish them from the protocol
   restrictions described above.  By necessity, the latter are somewhat
   generic, having to cater both to the union of the needs for all
   zones, as well as to 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, 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 RFC 5891 and 5892 set an absolute upper
   bound on the code points that can be used in domain name labels;
   registries MUST NOT include code points unless they are allowed by
   those rules.  Each registry that intends to allow IDN registrations
   MUST then determine which code points 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, as Section 4.4 of RFC 5890 indicates, 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

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   repertoires to be used such as the ICANN Maximal Starting Repertoire
   2 (MSR-2) for the Development of Label Generation Rules for the Root
   Zone [ICANN-MSR2] (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] or Arabic Language [RFC5564] or script-based repertoires
   from the approved ICANN Root Zone Label Generation Rules (LGR-1)
   [ICANN-LGR1] (or its successor documents).

   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 [LGR-Procedure].

   Particularly for a zone for which all labels to be delegated are not
   for the use of the same organization or enterprise, 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 like "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 an upper bound 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

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       points in the MSR would not be included in the final registry
       policy, whether for lack of actual usage, or for being inherently

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

       Contextual rules are required 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 and 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.  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

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   community, the relevant corrections for RFC 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.

4.1.  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


      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.  [[CREF1: Do we need a
         reference for this here??]]  A 63 octet A-label cannot

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         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 depending
         on the encoding form they are using.

4.2.  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
         [Unicode] 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
         [Unicode] for an explanation and Section 3.6, definition D52)
         for an exact definition).

      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.

5.  Related Discussions

   This document is one of a series of measures that have been suggested
   to address IDNA issues raised in other documents, including
   mechanisms for dealing with combining sequences and single-code point
   characters with the same appearance that normalization neither
   combines nor decomposes as IDNA2008 assumed [IDNA-Unicode], including
   the IAB response to that issue [IAB-2015], and to take a higher-level
   view of issues, demands, and proposals for new uses of the DNS.
   Those documents also include a discussion of issues with IDNA and
   character graphemes for which abstractions exist in Unicode in
   precomposed form but that can be generated from combining sequences
   and a suggested registry of code points known to be problematic
   [Freytag-troublesome].  The discussion of combining sequences and
   non-decomposing characters is intended to lay the foundation for an
   actual update to the IDNA code points document [RFC5892].  Such an
   update will presumably also address the existing errata against that

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6.  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.

7.  Acknowledgments

   Many thanks to Patrik Faltstrom who provided an important review on
   the initial version.

8.  IANA Considerations

   [[CREF2: 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.

9.  References

9.1.  Normative References

              ICANN, "Root Zone Label Generation Rules (LGR-1)", June
              2015, <https://www.icann.org/resources/pages/

              ICANN, "Maximal Starting Repertoire Version 2 (MSR-2) for
              the Development of Label Generation Rules for the Root
              Zone", April 2015,

   [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,

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   [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,

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

9.2.  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-

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

              Klensin, J. and P. Falstrom, "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, <https://www.rfc-editor.org/info/rfc4713>.

   [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, <https://www.rfc-editor.org/info/rfc5564>.

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

   [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,

   [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, <https://www.rfc-editor.org/info/rfc6452>.

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   [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,

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 4 with updates to respond to errata.

   o  Added Section 5 to discuss relationships to other documents.

   o  Modified the Abstract to note specifically updated documents.

   o  Several small editorial changes and corrections.

Authors' Addresses

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

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

   Asmus Freytag
   ASMUS, Inc.

   Email: asmus@unicode.org

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