Network Working Group                                          A. Durand
Request for Comments: 3901                        SUN Microsystems, Inc.
BCP: 91                                                         J. Ihren
Category: Best Current Practice                               Autonomica
                                                         September 2004


              DNS IPv6 Transport Operational Guidelines

Status of this Memo

  This document specifies an Internet Best Current Practices for the
  Internet Community, and requests discussion and suggestions for
  improvements.  Distribution of this memo is unlimited.

Copyright Notice

  Copyright (C) The Internet Society (2004).

Abstract

  This memo provides guidelines and Best Current Practice for operating
  DNS in a world where queries and responses are carried in a mixed
  environment of IPv4 and IPv6 networks.

1.  Introduction to the Problem of Name Space Fragmentation:
   following the referral chain

  A resolver that tries to look up a name starts out at the root, and
  follows referrals until it is referred to a name server that is
  authoritative for the name.  If somewhere down the chain of referrals
  it is referred to a name server that is only accessible over a
  transport which the resolver cannot use, the resolver is unable to
  finish the task.

  When the Internet moves from IPv4 to a mixture of IPv4 and IPv6 it is
  only a matter of time until this starts to happen.  The complete DNS
  hierarchy then starts to fragment into a graph where authoritative
  name servers for certain nodes are only accessible over a certain
  transport.  The concern is that a resolver using only a particular
  version of IP and querying information about another node using the
  same version of IP can not do it because somewhere in the chain of
  servers accessed during the resolution process, one or more of them
  will only be accessible with the other version of IP.

  With all DNS data only available over IPv4 transport everything is
  simple.  IPv4 resolvers can use the intended mechanism of following
  referrals from the root and down while IPv6 resolvers have to work



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  through a "translator", i.e., they have to use a recursive name
  server on a so-called "dual stack" host as a "forwarder" since they
  cannot access the DNS data directly.

  With all DNS data only available over IPv6 transport everything would
  be equally simple, with the exception of IPv4 recursive name servers
  having to switch to a forwarding configuration.

  However, the second situation will not arise in the foreseeable
  future.  Instead, the transition will be from IPv4 only to a mixture
  of IPv4 and IPv6, with three categories of DNS data depending on
  whether the information is available only over IPv4 transport, only
  over IPv6 or both.

  Having DNS data available on both transports is the best situation.
  The major question is how to ensure that it becomes the norm as
  quickly as possible.  However, while it is obvious that some DNS data
  will only be available over v4 transport for a long time it is also
  obvious that it is important to avoid fragmenting the name space
  available to IPv4 only hosts.  For example, during transition it is
  not acceptable to break the name space that we presently have
  available for IPv4-only hosts.

2.  Terminology

  The phrase "IPv4 name server" indicates a name server available over
  IPv4 transport.  It does not imply anything about what DNS [1,2] data
  is served.  Likewise, "IPv6 [4,5,6] name server" indicates a name
  server available over IPv6 transport.  The phrase "dual-stack name
  server" indicates a name server that is actually configured to run
  both protocols, IPv4 and IPv6, and not merely a server running on a
  system capable of running both but actually configured to run only
  one.

3.  Policy Based Avoidance of Name Space Fragmentation

  Today there are only a few DNS "zones" on the public Internet that
  are available over IPv6 transport, and most of them can be regarded
  as "experimental".  However, as soon as the root and top level
  domains are available over IPv6 transport, it is reasonable to expect
  that it will become more common to have zones served by IPv6 servers.

  Having those zones served only by IPv6-only name server would not be
  a good development, since this will fragment the previously
  unfragmented IPv4 name space and there are strong reasons to find a
  mechanism to avoid it.





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  The recommended approach to maintain name space continuity is to use
  administrative policies, as described in the next section.

4.  DNS IPv6 Transport recommended Guidelines

  In order to preserve name space continuity, the following
  administrative policies are recommended:

     - every recursive name server SHOULD be either IPv4-only or dual
       stack,

        This rules out IPv6-only recursive servers.  However, one might
        design configurations where a chain of IPv6-only name server
        forward queries to a set of dual stack recursive name server
        actually performing those recursive queries.

     - every DNS zone SHOULD be served by at least one IPv4-reachable
       authoritative name server.

        This rules out DNS zones served only by IPv6-only authoritative
        name servers.

  Note: zone validation processes SHOULD ensure that there is at least
  one IPv4 address record available for the name servers of any child
  delegations within the zone.

5.  Security Considerations

  The guidelines described in this memo introduce no new security
  considerations into the DNS protocol or associated operational
  scenarios.

6.  Acknowledgment

  This document is the result of many conversations that happened in
  the DNS community at IETF and elsewhere since 2001.  During that
  period of time, a number of Internet drafts have been published to
  clarify various aspects of the issues at stake.  This document
  focuses on the conclusion of those discussions.

  The authors would like to acknowledge the role of Pekka Savola in his
  thorough review of the document.









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7.  Normative References

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

  [2]  Mockapetris, P., "Domain names - implementation and
       specification", STD 13, RFC 1035, November 1987.

  [3]  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
       9, RFC 2026, October 1996.

  [4]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
       Specification", RFC 2460, December 1998.

  [5]  Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
       Addressing Architecture", RFC 3513, April 2003.

  [6]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, "DNS
       Extensions to Support IP Version 6", RFC 3596, October 2003.

8.  Authors' Addresses

  Alain Durand
  SUN Microsystems, Inc
  17 Network circle UMPK17-202
  Menlo Park, CA, 94025
  USA

  EMail: [email protected]


  Johan Ihren
  Autonomica
  Bellmansgatan 30
  SE-118 47 Stockholm
  Sweden

  EMail: [email protected]













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9.  Full Copyright Statement

  Copyright (C) The Internet Society (2004).

  This document is subject to the rights, licenses and restrictions
  contained in BCP 78, and except as set forth therein, the authors
  retain all their rights.

  This document and the information contained herein are provided on an
  "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/S HE
  REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
  INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
  IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
  THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
  WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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  pertain to the implementation or use of the technology described in
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Acknowledgement

  Funding for the RFC Editor function is currently provided by the
  Internet Society.







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