Internationalized Domain Names for Applications (IDNA): Background, Explanation, and Rationale
RFC 5894

• Status
• IESG evaluation record
• IESG writeups
• Email expansions
• History

• Versions
• 17

Internet Engineering Task Force (IETF)                        J. Klensin
Request for Comments: 5894                                   August 2010
Category: Informational
ISSN: 2070-1721

        Internationalized Domain Names for Applications (IDNA):
                 Background, Explanation, and Rationale

Abstract

   Several years have passed since the original protocol for
   Internationalized Domain Names (IDNs) was completed and deployed.
   During that time, a number of issues have arisen, including the need
   to update the system to deal with newer versions of Unicode.  Some of
   these issues require tuning of the existing protocols and the tables
   on which they depend.  This document provides an overview of a
   revised system and provides explanatory material for its components.

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 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).  Not all documents
   approved by the IESG are 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/rfc5894.

Klensin                       Informational                     [Page 1]
RFC 5894                     IDNA Rationale                  August 2010

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.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Context and Overview . . . . . . . . . . . . . . . . . . .  4
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
       1.2.1.  DNS "Name" Terminology . . . . . . . . . . . . . . . .  5
       1.2.2.  New Terminology and Restrictions . . . . . . . . . . .  6
     1.3.  Objectives . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.4.  Applicability and Function of IDNA . . . . . . . . . . . .  7
     1.5.  Comprehensibility of IDNA Mechanisms and Processing  . . .  8
   2.  Processing in IDNA2008 . . . . . . . . . . . . . . . . . . . .  9
   3.  Permitted Characters: An Inclusion List  . . . . . . . . . . .  9
     3.1.  A Tiered Model of Permitted Characters and Labels  . . . . 10
       3.1.1.  PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . . 10
       3.1.2.  CONTEXTUAL RULE REQUIRED . . . . . . . . . . . . . . . 11
         3.1.2.1.  Contextual Restrictions  . . . . . . . . . . . . . 11
         3.1.2.2.  Rules and Their Application  . . . . . . . . . . . 12
       3.1.3.  DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 12
       3.1.4.  UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 13
     3.2.  Registration Policy  . . . . . . . . . . . . . . . . . . . 14

Klensin                       Informational                     [Page 2]
RFC 5894                     IDNA Rationale                  August 2010

     3.3.  Layered Restrictions: Tables, Context, Registration, and
           Applications . . . . . . . . . . . . . . . . . . . . . . . 15
   4.  Application-Related Issues . . . . . . . . . . . . . . . . . . 15
     4.1.  Display and Network Order  . . . . . . . . . . . . . . . . 15
     4.2.  Entry and Display in Applications  . . . . . . . . . . . . 16
     4.3.  Linguistic Expectations: Ligatures, Digraphs, and
           Alternate Character Forms  . . . . . . . . . . . . . . . . 19