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Individual Submission                                         L-J. Liman
Internet-Draft                                                Autonomica
Updates: 1123 (if approved)                                     J. Abley
Intended status: Standards Track                                   ICANN
Expires: October 24, 2010                                 April 22, 2010


                  Top Level Domain Name Specification
                        draft-liman-tld-names-02

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 October 24, 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



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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.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  TLD Label Syntax Specification . . . . . . . . . . . . . . . .  6
   4.  Policy Considerations  . . . . . . . . . . . . . . . . . . . .  7
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Change History  . . . . . . . . . . . . . . . . . . . 11
     A.1.  draft-liman-tld-names-02 . . . . . . . . . . . . . . . . . 11
     A.2.  draft-liman-tld-names-01 . . . . . . . . . . . . . . . . . 11
     A.3.  draft-liman-tld-names-00 . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12






























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1.  Introduction

   The syntax of Top-Level Domain (TLD) labels in the Domain Name System
   (DNS) was specified somewhat imprecisely in [RFC1123] which required
   that TLD names be "alphabetic".  This is commonly interpreted as
   excluding the hyphen character and numeric digits from TLD labels.
   This restriction does not accommodate the US-ASCII encoding of
   Internationalised Domain Names (IDNs), as specified in
   [I-D.ietf-idnabis-defs].  A more detailed discussion of the existing
   specifications can be found in Section 2.

   This document updates the definition of allowable top-level domain
   names to support IDNs but places some restrictions on the choice of
   IDN labels, consistent with the existing specification for US-ASCII
   TLD labels.  See Section 3 for the updated specification.

   This document focuses narrowly on the issue of allowable 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 TLD labels, and that many considerations
   external to the IETF are included in that wider policy.  See
   Section 4 for more discussion of policy considerations.



























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2.  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, making two additional changes to
   the syntax:

   1.  '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]

   2.  '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]

   Neither [RFC0952] nor [RFC1123] explicitly states the reasons for
   these restrictions.  It might be supposed that human factors
   considerations were involved; [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 are often assumed in deployed software,
   and that changing the rules should be undertaken with caution.

   The Internationalised Domain Names in Applications (IDNA) 2008
   specification 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 [I-D.ietf-idnabis-defs].  Valid A-Labels always contain
   non-alphabetic characters.

   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 TLD DNS labels.  To
   make the change as small as possible, we restrict the U-label form of



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   the label in ways functionally compatible with the restrictions (from
   [RFC0952] and [RFC1123]) on US-ASCII labels for TLDs.  This
   restriction will not enable new A-label TLDs to function with
   existing software that checks DNS top-level labels for conformance
   with the alphabetic restriction.  It merely makes the same
   traditional rule work in a similar way for IDNA top-level labels.













































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3.  TLD Label Syntax Specification

   This document relaxes the existing specification to allow TLD labels
   to be well-formed A-Labels, but places restrictions on the
   corresponding U-Labels.

   The ABNF expression that matches a valid TLD label is as follows:

             tldlabel = 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

   Restricted-A-label is an A-Label as defined in
   [I-D.ietf-idnabis-defs] converted from (and convertible to) a U-Label
   that is consistent with the definition in [I-D.ietf-idnabis-defs] and
   that is further restricted to contain only Unicode characters with
   the derived property value PVALID and of General Category "L" or
   "Mn".  Note that "L" contains several sub-categories, and that many
   characters of the "L" category do not have the derived property value
   PVALID.  Most emphatically, this specification does not allow use of
   any character in a Restricted-A-Label that has derived property value
   of CONTEXT.

   More specifically, a Restricted-A-Label consists of one or more
   Unicode characters such that all of the following statements are
   true:

   1.  the label is a valid A-Label according to
       [I-D.ietf-idnabis-defs];

   2.  the derived property value of all codepoints, as defined by
       [I-D.ietf-idnabis-defs], is PVALID;

   3.  the general category of all codepoints, 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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4.  Policy Considerations

   This document provides a technical specification for TLD label
   syntax; it does not encapsulate a complete policy under which TLD
   names may be 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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5.  IANA Considerations

   This document makes no requests of the IANA.

   This section needs IANA review. :-)














































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6.  Security Considerations

   This document is believed to have limited security implications.

   The creation of new TLDs has the potential to conflict with software
   which (for example) does not accommodate TLD labels which did not
   exist at the time the software was written.  Such software problems
   might in turn lead to security vulnerabilities; e.g., in the event
   that 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 TLDs prior to the publication of this document.

   The issue of characters that can be confused with each other is a
   risk, but it is discussed at length in the Security Considerations
   section of [I-D.ietf-idnabis-defs].


































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7.  References

7.1.  Normative References

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

   [I-D.ietf-idnabis-defs]
              Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              draft-ietf-idnabis-defs-13 (work in progress),
              January 2010.

7.2.  Informative References

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


































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Appendix A.  Change History

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

A.1.  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.

A.2.  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.3.  draft-liman-tld-names-00

   First cut.  Prompted by Olafur Gudmundsson and Tina Dam.































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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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