Principles for Unicode Code Point Inclusion in Labels in the DNS
RFC 6912

Document Type RFC - Informational (April 2013; No errata)
Last updated 2013-04-16
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Internet Architecture Board (IAB)                            A. Sullivan
Request for Comments: 6912                                     Dyn, Inc.
Category: Informational                                        D. Thaler
ISSN: 2070-1721                                                Microsoft
                                                              J. Klensin

                                                              O. Kolkman
                                                              NLnet Labs
                                                              April 2013

    Principles for Unicode Code Point Inclusion in Labels in the DNS

Abstract

   Internationalized Domain Names in Applications (IDNA) makes available
   to DNS zone administrators a very wide range of Unicode code points.
   Most operators of zones should probably not permit registration of
   U-labels using the entire range.  This is especially true of zones
   that accept registrations across organizational boundaries, such as
   top-level domains and, most importantly, the root.  It is
   unfortunately not possible to generate algorithms to determine
   whether permitting a code point presents a low risk.  This memo
   presents a set of principles that can be used to guide the decision
   of whether a Unicode code point may be wisely included in the
   repertoire of permissible code points in a U-label in a zone.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Architecture Board (IAB)
   and represents information that the IAB has deemed valuable to
   provide for permanent record.  It represents the consensus of the
   Internet Architecture Board (IAB).  Documents approved for
   publication by the IAB are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6912.

Sullivan, et al.              Informational                     [Page 1]
RFC 6912             DNS Zone Code Point Principles           April 2013

Copyright Notice

   Copyright (c) 2013 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
   (http://trustee.ietf.org/license-info) 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  More-Restrictive Rules Going Up the DNS Tree  . . . . . .   6
   3.  Principles Applicable to All Zones  . . . . . . . . . . . . .   6
     3.1.  Longevity Principle . . . . . . . . . . . . . . . . . . .   6
     3.2.  Least Astonishment Principle  . . . . . . . . . . . . . .   6
     3.3.  Contextual Safety Principle . . . . . . . . . . . . . . .   7
   4.  Principles Applicable to All Public Zones . . . . . . . . . .   7
     4.1.  Conservatism Principle  . . . . . . . . . . . . . . . . .   7
     4.2.  Inclusion Principle . . . . . . . . . . . . . . . . . . .   7
     4.3.  Simplicity Principle  . . . . . . . . . . . . . . . . . .   7
     4.4.  Predictability Principle  . . . . . . . . . . . . . . . .   8
     4.5.  Stability Principle . . . . . . . . . . . . . . . . . . .   8
   5.  Principle Specific to the Root Zone . . . . . . . . . . . . .   8
     5.1.  Letter Principle  . . . . . . . . . . . . . . . . . . . .   8
   6.  Confusion and Context . . . . . . . . . . . . . . . . . . . .   9
   7.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   10. IAB Members at the Time of Approval . . . . . . . . . . . . .  10
   11. Informative References  . . . . . . . . . . . . . . . . . . .  10

Sullivan, et al.              Informational                     [Page 2]
RFC 6912             DNS Zone Code Point Principles           April 2013

1.  Introduction

   Operators of a DNS zone need to set policies around what Unicode code
   points are allowed in labels in that zone.  Typically there are a
   number of important goals to consider when constructing such
   policies.  These include, for instance, avoiding possible visual
   confusability between two labels, avoiding possible confusion between
   Fully Qualified Domain Names (FQDNs) and IP address literals,
   accessibility to the disabled (see "Web Content Accessibility
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