Dynamic Host Congiguration                                      T. Chown
Internet-Draft                                 University of Southampton
Expires: September 6, August 30, 2004                                       S. Venaas
                                                        A. Vijayabhaskar
                                                  Hewlett-Packard STSD-I
                                                              March 8, 2004

             Renumbering Requirements for Stateless DHCPv6

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   RFC 3668.

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   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   IPv6 hosts using Stateless Address Autoconfiguration are able to
   automatically configure their IPv6 address and default router
   settings.  However, further settings are not available.  If such
   hosts wish to automatically configure their DNS, NTP or other
   specific settings the stateless variant of the Dynamic Host
   Configuration Protocol for IPv6 (DHCPv6) could be used.  This
   combination of Stateless Address Autoconfiguration and stateless
   DHCPv6 could be used quite commonly in IPv6 networks.  However, hosts
   using such a combination currently have no means by which to be
   informed of changes in stateless DHCPv6 option settings, e.g.  the
   addition of a new NTP server address, changes in DNS search paths, or
   full site renumbering.  This document is presented as a problem
   statement from which a solution should be proposed in a subsequent

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Renumbering Scenarios  . . . . . . . . . . . . . . . . . . . .  4
     3.1   Site renumbering . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2   Changes to a DHCPv6-assigned setting . . . . . . . . . . . . .  4
   4.  Renumbering Requirements . . . . . . . . . . . . . . . . . . .  4
   5.  Considerations in choosing a solution  . . . . . . . . . . . .  5
   6.  Solution Space . . . . . . . . . . . . . . . . . . . . . . . .  5
   7.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   7.  6
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  5  6
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  6
   10.   Normative References . . . . . . . . . . . . . . . . . . . . .  6
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  6  7
       Intellectual Property and Copyright Statements . . . . . . . .  7  8

1.  Introduction

   IPv6 hosts using Stateless Address Autoconfiguration [1] are able to
   automatically configure their IPv6 address and default router
   settings.  While Stateless Address Autoconfiguration for IPv6 allows
   automatic configuration of these settings, it does not provide a
   mechanism for additional, non IP-address settings to be automatically

   The full version of the Dynamic Host Configuration Protocol for IPv6
   (DHCPv6) [2] is designed to provide both stateful address assignment
   to IPv6 hosts, as well as additional (non IP-address) configuration
   including DNS, NTP and other specific settings.  A full stateful
   DHCPv6 server allocates the addresses and maintains the clients
   bindings to keep track of client leases.

   If hosts using Stateless Address Autoconfiguration for IPv6 wish to
   automatically configure their DNS, NTP or other specific settings the
   stateless variant [3] of DHCPv6 could be used.  The stateless variant
   of DHCPv6 is more lightweight.  It does not do address assignment,
   instead it only provides additional configuration parameters like DNS
   resolver addresses.  It does not maintain state about the information
   assigned to clients;  the additional parameters do not have an
   explicit life-time associated with them in the same way that IP
   addresses do, and hence the DHCPv6 server does not need to maintain
   the state of the clients.

   This combination of Stateless Address Autoconfiguration and stateless
   DHCPv6 could be used quite commonly in IPv6 networks.  In the absence
   of an alternative method for DNS, NTP and other options to be
   automatically configured, it may become the most common combination
   for statelessly configuring hosts.

2.  Problem Statement

   A problem however lies in the ability, or lack of ability, of clients
   using this combination to be informed of (or to deduce) changes in
   DHCPv6 assigned settings.

   While a DHCPv6 server unicasts Reconfigure message to individual
   clients to trigger the clients to intiate Information-request/reply
   configuration exchanges to update their configuration settings, the
   stateless variant of DHCPv6 cannot use the Reconfigure mechanism
   because it does not maintain a list of IP addresses (leases) to send
   the unicast messages to.

   Thus events including the following cannot be handled:

   o  Full site renumbering

   o  DNS server change of address

   o  NTP server change of address

   o  Changes in DNS search paths

   It would be highly desirable that a host using the combination of
   Stateless Address Autoconfiguration and stateless DHCPv6 could handle
   a renumbering or reconfiguration event, whether planned or unplanned
   by the network administrator.

3.  Renumbering Scenarios

   There are two main scenarios for changes to DHCPv6-assigned settings,
   that would require the client to initiate an Information-request/
   reply exchange to update the configuration.

3.1  Site renumbering

   One of the fundamental principles of IPv6 is that sites receive their
   IPv6 address allocations from an ISP using provider assigned (PA)
   address space.  There is currently no provider independent (PI)
   address space in IPv6.  A site wishing to change ISP must thus
   renumber its network.  Any such site renumbering will require hosts
   to reconfigure both their own address and default router settings as
   well as their stateless DHCPv6-assigned settings.

3.2  Changes to a DHCPv6-assigned setting

   An administrator may need to change one or more stateless
   DHCPv6-assigned settings, e.g.  an NTP server, DNS server, or the DNS
   search path.  This may be required if a new, additional DNS server is
   brought online, is moved to a new network (prefix), or an existing
   server is decommissioned or known to be unavailable.

4.  Renumbering Requirements

   Ideally, any of the above scenarios should be handled automatically
   by the hosts on the network.  For this to be realised, a method is
   required for the hosts to be informed that they should request new
   stateless DHCPv6-assigned setting information.

   The solution to the problem may depend on whether the renumbering or
   configuration change is a planned or unplanned one, from the
   perspective of the network administrator.  There is already work
   underway in understanding the planned renumbering [4] scenario for
   IPv6 networks.  However, there is currently no mechanism in stateless
   DHCPv6 to even handle planned renumbering events.

   The unplanned renumbering event, which may be more common in smaller,
   unmanaged networks, is more difficult to cater for.  Ideally, any
   solution for the problem should consider planned and unplanned

   The solution should also be secure, such that additional security
   concerns are not added to the stateless DHCPv6 networking

5.  Considerations in choosing a solution

   There are a number of considerations that could be listed for a
   desirable solution:

   o  It should support planned renumbering; it is desirable to support
      unplanned renumbering.

   o  Security is important; e.g., avoiding denialof service attacks
      mounted through Reconfigure messages sent from an attacker.

   o  It must be possible to update options even if the network is not

   o  It is desirable to maintain the "stateless" property; i.e., no
      per-client state should need to be kept in the server.

6.  Solution Space

   Solutions should be designed and presented in a separate document.
   An initial, brief set of candidate solutions might include:

   o  Adding a Reconfigure message mechanism that would work in the
      stateless DHCPv6 environment.  This could enable planned or
      unplanned events, but may require a multicast mechanism to be

   o  Conveying a valid lifetime timer to clients for stateless
      DHCPv6-assigned settings.  This could primarily enable planned
      events, but with a small time-out it could to some extent handle
      unplanned events at the expense of the additional request traffic.

   o  Using some form of Router Advertisement as a hint to request new
      stateless DHCPv6-assigned settings.  Using only an observed new
      Router Advertisement prefix as a hint to re-request settings would
      not handle changes that are purely to NTP, DNS or other options.

      Other possible means of detection of network (re)attachment could
      also be used as cues (e.g.  see IPv6 DNA Goals [5]).

   o  Changing semantics of the DHCPv6 'O' flag such that toggling its
      value may trigger an Information-request message.

7.  Summary

   This document presents a problem statement for how IPv6 hosts that
   use the combination of Stateless Address Autoconfiguration and
   stateless DHCPv6 may be informed of renumbering events or other
   changes to the settings that they originally learnt through stateless
   DHCPv6.  A short list of candidate solutions is presented, which the
   authors hope may be expanded upon in subsequent documents.


8.  Security Considerations

   There are no security considerations in this problem statemement per
   se.  However, whatever mechanism is designed or chosen to address
   this problem should avoid the introduction of new security concerns
   for (stateless) DHCPv6.

9.  Acknowledgements

   The authors would like to thank Ralph Droms and Bermie Volz for their
   comments on this draft.

10  Normative References

   [1]  Thomson, S. and T. Narten, "IPv6 Stateless Address
        Autoconfiguration", RFC 2462, December 1998.

   [2]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M.
        Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
        RFC 3315, July 2003.

   [3]  Droms, R., "A Guide to Implementing Stateless DHCPv6 Service",
        draft-ietf-dhc-dhcpv6-stateless-01 (work in progress), October
        2003. "Stateless Dynamic Host Configuration Protocol (DHCP)
        Service for IPv6", RFC 3736, April 2004.

   [4]  Baker, F., Lear, E. and R. Droms, "Procedures for Renumbering an
        IPv6 Network without a Flag Day",
        draft-baker-ipv6-renumber-procedure-01 (work in progress),
        October 2003.

   [5]  Choi, J., "Detecting Network Attachment in IPv6 Goals",
        draft-ietf-dna-goals-00 (work in progress), June 2004.

Authors' Addresses

   Tim Chown
   University of Southampton
   School of Electronics and Computer Science
   Southampton, Hampshire  SO17 1BJ
   United Kingdom

   EMail: tjc@ecs.soton.ac.uk

   Stig Venaas
   Trondheim  NO 7465

   EMail: venaas@uninett.no

   Vijayabhaskar A K
   Hewlett-Packard STSD-I
   29, Cunningham Road
   Bangalore  560052

   EMail: vijayak@india.hp.com

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