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Individual Submission                                         L-J. Liman
Internet-Draft                                                Autonomica
Updates: 1123 (if approved)                                     J. Abley
Intended status: Standards Track                                   ICANN
Expires: April 21, 2011                                 October 18, 2010


                  Top Level Domain Name Specification
                        draft-liman-tld-names-04

Abstract

   The syntax for allowed Top-Level Domain (TLD) labels in the Domain
   Name System (DNS) is not clearly applicable to the encoding of
   Internationalised Domain Names (IDNs) as TLDs.

   This document provides a concise specification of TLD label syntax
   based on existing syntax documentation, extended minimally to
   accommodate IDNs.

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 http://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 April 21, 2011.

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



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   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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  TLD Label Syntax Specification . . . . . . . . . . . . . . . .  7
   5.  Policy Considerations  . . . . . . . . . . . . . . . . . . . .  8
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Change History  . . . . . . . . . . . . . . . . . . . 13
     A.1.  draft-liman-tld-names-04 . . . . . . . . . . . . . . . . . 13
     A.2.  draft-liman-tld-names-03 . . . . . . . . . . . . . . . . . 13
     A.3.  draft-liman-tld-names-02 . . . . . . . . . . . . . . . . . 13
     A.4.  draft-liman-tld-names-01 . . . . . . . . . . . . . . . . . 13
     A.5.  draft-liman-tld-names-00 . . . . . . . . . . . . . . . . . 13
     A.6.  Version Identification Tag . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14

























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1.  Introduction

   The syntax of TLD labels ("TLD DNS-Labels", as defined in Section 2)
   is specified in [RFC1123], where such labels are asserted to be
   "alphabetic" within a section of that document entitled "DISCUSSION".
   This can be interpreted as requiring that the hyphen character ("-")
   and numeric digits be excluded from TLD DNS-Labels.  Such a
   restriction would not accommodate the US-ASCII encoding of
   Internationalised Domain Names (IDNs), as specified in [RFC5890].  A
   more detailed discussion of the existing specifications can be found
   in Section 3.

   This document extends the syntax of allowable TLD DNS-Labels to
   support IDNs, but places some restrictions on the choice of IDN
   labels.  These restrictions are intended to be consistent with the
   existing specification for US-ASCII TLD DNS-Labels.  See Section 4
   for the updated specification.

   This document focuses narrowly on the issue of allowable DNS-Labels
   in TLDs and does not (and is not intended to) make any other changes
   or clarifications to existing domain name syntax rules.

   It is carefully noted that the specification in this document is not
   the only factor in choosing suitable TLD DNS-Labels, and that many
   considerations external to the IETF are included in that wider
   policy.  See Section 5 for more discussion of policy considerations.

























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2.  Definitions

   The term DNS-Label is used in this document to have precisely the
   same meaning as the term "label", as introduced in [RFC1034], section
   3.1.  A DNS-Label denotes one node in a DNS tree.  A DNS-Label is
   zero to 63 octets in length.  The term "DNS-Label" refers exclusively
   to the "wire format" of the label, and not to any presentation format
   of the label.

   A Top-Level Domain (TLD) DNS-Label is the right-most ("highest-
   level") DNS-Label in a fully-qualified domain name.

   The terms A-Label and U-Label are used in this document as defined in
   [RFC5890].





































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3.  Background

   [RFC0952] defines a host name as follows:

      'A "name" ... is a text string up to 24 characters drawn from the
      alphabet (A-Z), digits (0-9), minus sign (-), and period (.).
      Note that periods are only allowed when they serve to delimit
      components of "domain style names".  (See RFC-921, "Domain Name
      System Implementation Schedule", for background).  No blank or
      space characters are permitted as part of a name.  No distinction
      is made between upper and lower case.  The first character must be
      an alpha character.  The last character must not be a minus sign
      or period.'  [Unnumbered section titled "ASSUMPTIONS", first
      paragraph]

   [RFC1123] reaffirms this definition, but makes one change to the
   syntax:

      'The syntax of a legal Internet host name was specified in RFC-952
      [DNS:4].  One aspect of host name syntax is hereby changed: the
      restriction on the first character is relaxed to allow either a
      letter or a digit.  Host software MUST support this more liberal
      syntax.'  [Section 2.1]

   In addition, the DISCUSSION section of Section 2.1 says:

      'However, a valid host name can never have the dotted-decimal form
      #.#.#.#, since at least the highest-level component label will be
      alphabetic.'  [Section 2.1]

   Some implementers may have understood the above phrase 'will be
   alphabetic' to be a protocol restriction.

   Neither [RFC0952] nor [RFC1123] explicitly states the reasons for
   these restrictions.  It might be supposed that human factors were a
   consideration; [RFC1123] appears to suggest that one of the reasons
   was to prevent confusion between dotted-decimal IPv4 addresses and
   host domain names.  In any case, it is reasonable to believe that the
   restrictions have been assumed in some deployed software, and that
   changes to the rules should be undertaken with caution.

   The Internationalised Domain Names in Applications 2008 specification
   (IDNA2008) [RFC5891] [RFC5892] provides a protocol for encoding
   Unicode strings in DNS-Labels.  The Unicode string used by
   applications is known as a U-Label; its corresponding encoding in the
   DNS is known as an A-Label.  The terms A-Label and U-Label are used
   in this document as defined in [RFC5890].  Valid A-Labels always
   contain non-alphabetic characters.



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   In order to accommodate the wish to express TLD names in scripts
   other than Latin (or rather, the US-ASCII subset of Latin), it is
   necessary to allow non-alphabetic characters in the corresponding TLD
   DNS-Labels.  To minimize changes, the U-label form of a TLD name is
   restricted in ways functionally compatible with the restrictions
   (from [RFC0952] and [RFC1123]) on US-ASCII TLD names, by applying
   rules analogous to those already imposed on US-ASCII TLD DNS-Labels
   to TLD U-labels.

   However, deployed software that checks DNS top-level labels for
   conformance with an alphabetic restriction will not recognize such
   corresponding A-Labels (i.e., U-labels represented in their US-ASCII
   form).






































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4.  TLD Label Syntax Specification

   This document relaxes the existing specification to allow TLD DNS-
   Labels to be well-formed A-Labels, but places restrictions on their
   corresponding U-Labels.  That is, not every well-formed A-Label is a
   valid TLD DNS-Label.

   The ABNF expression that matches a valid TLD DNS-Label is as follows:

             tld-dns-label = traditional-tld-label / idn-label

             traditional-tld-label = 1*63(ALPHA)

             idn-label = Restricted-A-Label

             ALPHA    = %x41-5A / %x61-7A   ; A-Z / a-z

   A Restricted-A-Label is a DNS-Label which satisfies all the following
   conditions:

   1.  the DNS-Label is a valid A-Label according to [RFC5890];

   2.  the derived property value of all code points, as defined by
       [RFC5890], is PVALID;

   3.  the general category of all code points, is one of { Ll, Lo, Lm,
       Mn }.

   This new specification reflects current practice in registration of
   TLD names by the IANA, extended to accommodate IDNs.





















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5.  Policy Considerations

   This document provides a technical specification that limits the set
   of TLD DNS-Labels that are available for assignment; it does not aim
   to encapsulate the full policy framework within which TLD names are
   chosen.

   At the time of writing, the policy under which TLD names are chosen
   is developed and maintained by ICANN in consultation with a wide base
   of stakeholders.  As the Internet continues to grow to serve new user
   communities, applications and services, it is to be expected that the
   corresponding policy will be changed accordingly.







































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6.  IANA Considerations

   While this document makes no requests of the IANA, management of the
   root zone is an IANA function.  This document expands the set of
   strings permitted for delegation from the root zone, and hence
   establishes new limits for the corresponding IANA policy.













































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7.  Security Considerations

   This document is believed to have limited security implications.

   General discussion about the security effects of internationalized
   labels can be found in [RFC5890], section 4.  Those considerations
   apply equally to TLD labels.

   The creation of new TLDs has the potential to conflict with software
   which (for example) predates and correspondingly does not accommodate
   new TLD names.  Such software problems might in turn lead to security
   vulnerabilities, e.g. in the case where a DNS name specified by a
   user is truncated or otherwise misinterpreted, causing an application
   to interact with a different remote host from that which the user
   intended.  It should be noted that this is not a new phenomenon, and
   has been observed following the creation of new (US-ASCII) TLD names
   prior to the publication of this document.

   The issue that some Unicode characters can be confused with each
   other is discussed at length in the Security Considerations section
   of [RFC5890].






























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8.  Acknowledgements

   Tina Dam, Patrik Faltstrom, John Klensin, Thomas Narten and Andrew
   Sullivan contributed text to this document, and their contributions
   are hereby acknowledged.














































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9.  References

9.1.  Normative References

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

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

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

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

9.2.  Informative References

   [RFC0952]  Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
              host table specification", RFC 952, October 1985.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.


























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Appendix A.  Change History

   This section (and sub-sections) should be removed before publication.

A.1.  draft-liman-tld-names-04

   Removed subjective and unverified statements regarding deployed
   software.  Replaced with more generic text.  Polishing a few
   expressions to make them less obtrusive.  Removed confusing paragraph
   after ABNF table.  Updated some references that are now published as
   RFCs.

A.2.  draft-liman-tld-names-03

   More wordsmithing, and explanatory text.  Work on the IANA and the
   security considerations sections.

A.3.  draft-liman-tld-names-02

   Wordsmithing and rearrangement of text following discussions with Joe
   Abley, Tina Dam, Thomas Narten and Andrew Sullivan.  Incorporated
   revised ABNF and associated specification from Patrik Faltstrom.
   Tightened definitions and introduced the term "DNS-Label" to avoid
   ambiguity with various other uses of the word "label".

A.4.  draft-liman-tld-names-01

   Substantial comments and improvements supplied by Thomas Narten and
   John Klensin.  Decided to go for a minimal change approach.  Also
   noted that U-labels have to be letters due to jumping digit problem.
   Rewritten major parts.

A.5.  draft-liman-tld-names-00

   First cut.  Prompted by Olafur Gudmundsson and Tina Dam.

A.6.  Version Identification Tag

   $Id: draft-liman-tld-names.xml,v 1.36 2010/10/19 09:55:48 liman Exp $












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Authors' Addresses

   Lars-Johan Liman
   Autonomica AB
   Franzengatan 5
   SE-112 51 Stockholm
   Sweden

   Email: liman@autonomica.se
   URI:   http://www.autonomica.se/


   Joe Abley
   ICANN
   4676 Admiralty Way
   Suite 330
   Marina del Rey  90292
   USA

   Email: joe.abley@icann.org
   URI:   http://www.icann.org/






























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