Right-to-Left Scripts for Internationalized Domain Names for Applications (IDNA)
RFC 5893

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Internet Engineering Task Force (IETF)                H. Alvestrand, Ed.
Request for Comments: 5893                                        Google
Category: Standards Track                                        C. Karp
ISSN: 2070-1721                        Swedish Museum of Natural History
                                                             August 2010

                       Right-to-Left Scripts for
         Internationalized Domain Names for Applications (IDNA)

Abstract

   The use of right-to-left scripts in Internationalized Domain Names
   (IDNs) has presented several challenges.  This memo provides a new
   Bidi rule for Internationalized Domain Names for Applications (IDNA)
   labels, based on the encountered problems with some scripts and some
   shortcomings in the 2003 IDNA Bidi criterion.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in 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/rfc5893.

Copyright Notice

   Copyright (c) 2010 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.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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RFC 5893                   IDNA Right to Left                August 2010

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  Purpose and Applicability  . . . . . . . . . . . . . . . .  2
     1.2.  Background and History . . . . . . . . . . . . . . . . . .  3
     1.3.  Structure of the Rest of This Document . . . . . . . . . .  3
     1.4.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  The Bidi Rule  . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  The Requirement Set for the Bidi Rule  . . . . . . . . . . . .  6
   4.  Examples of Issues Found with RFC 3454 . . . . . . . . . . . .  9
     4.1.  Dhivehi  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  Yiddish  . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.3.  Strings with Numbers . . . . . . . . . . . . . . . . . . . 12
   5.  Troublesome Situations and Guidelines  . . . . . . . . . . . . 12
   6.  Other Issues in Need of Resolution . . . . . . . . . . . . . . 13
   7.  Compatibility Considerations . . . . . . . . . . . . . . . . . 14
     7.1.  Backwards Compatibility Considerations . . . . . . . . . . 14
     7.2.  Forward Compatibility Considerations . . . . . . . . . . . 15
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     10.2. Informative References . . . . . . . . . . . . . . . . . . 17

1.  Introduction

1.1.  Purpose and Applicability

   The purpose of this document is to establish a rule that can be
   applied to Internationalized Domain Name (IDN) labels in Unicode form
   (U-labels) containing characters from scripts that are written from
   right to left.  It is part of the revised IDNA protocol [RFC5891].

   When labels satisfy the rule, and when certain other conditions are
   satisfied, there is only a minimal chance of these labels being
   displayed in a confusing way by the Unicode bidirectional display
   algorithm.

   The other normative documents in the IDNA2008 document set establish
   criteria for valid labels, including listing the permitted
   characters.  This document establishes additional validity criteria
   for labels in scripts normally written from right to left.

   This specification is not intended to place any requirements on
   domain names that do not contain characters from such scripts.

Alvestrand & Karp            Standards Track                    [Page 2]
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1.2.  Background and History

   The "Stringprep" specification [RFC3454], part of IDNA2003, made the