Dynamic Host Congiguration                                      T. Chown
Internet-Draft                                 University of Southampton
Expires: August 30, 2004 April 25, 2005                                        S. Venaas
                                                                 UNINETT
                                                        A. Vijayabhaskar
                                                  Hewlett-Packard STSD-I
                                                              March
                                           Cisco Systems (India) Private
                                                                 Limited
                                                        October 25, 2004

             Renumbering Requirements for Stateless DHCPv6
             draft-ietf-dhc-stateless-dhcpv6-renumbering-01
             draft-ietf-dhc-stateless-dhcpv6-renumbering-02

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

Abstract

   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 a change 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
   document.

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  . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  6
   10.   References . . . . . . . . . . . . . . . . . . . . . . . . .  6
   10.1  Normative References . . . . . . . . . . . . . . . . . . . .  6
   10.2  Informative References . . . . . . . . . . . . . . . . . . .  7
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  7
       Intellectual Property and Copyright Statements . . . . . . . .  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
   configured.

   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; clients, hence there is no need to maintain per-client
   state on the additional parameters do not have an
   explicit life-time associated with them server.  In other words, all clients can be given the
   same information, in the same way that IP
   addresses do, and hence the DHCPv6 server does information in Router
   Advertisements is not need to maintain
   the state of the clients. client-specific.

   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.  Note that in DHCPv6, Reconfigure messages
   must be unicast; multicast is not allowed.

   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  A change 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.

   Note that the scope of the problem can also be seen to extend beyond
   Stateless DHCPv6, since only IP address options have a lifetime, i.e.
   there is no mechanism even in the full DHCPv6 to "expire" old
   information or otherwise force a client to recheck that new/updated
   information is available.  However, with full DHCPv6, a node may
   learn of updates to non-address options when renewing its address
   lease.

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 changing its 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
   events.

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

5.  Considerations in choosing a solution

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

   o  It  The solution should support planned renumbering; it is desirable to support
      that it also supports unplanned renumbering.

   o  Security is important; e.g., avoiding denialof service attacks
      mounted through Reconfigure messages sent from an attacker. important.  No new security concerns should be
      introduced to Stateless DHCPv6 by the solution.

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

   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
      realised.

   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.
      The selection of recommended lifetime values/ranges would be the
      subject of future work.

   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.

   There will also be conditions under which a client should also send
   an Information-request, such as reconnection to a link.  Such
   specific recommendations are outside the scope of this document but
   we expect ongoing work in the Detecting Network Attachment (DNA) WG
   (as scoped in IPv6 DNA Goals [5]) to yield recommendations.

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.

   The issues of maintaining appropriate security through a renumbering
   event are outside the scope of this document (in the case where
   specific servers within the network are being added or removed,
   firewall configurations and ACLs, for example, will need to reflect
   this).  However, this is an important area for further work.

9.  Acknowledgements

   The authors would like to thank Ralph Droms and Droms, Bermie Volz and other
   individuals on the DHC mail list for their comments on this draft.

10 draft, as
   well as colleagues on the 6NET project.  We also thank the review
   comments, particularly those from Thomas Narten.

10.  References

10.1  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., "Stateless Dynamic Host Configuration Protocol (DHCP)
        Service for IPv6", RFC 3736, April 2004.

10.2  Informative References

   [4]  Baker, F., Lear, E. and R. Droms, "Procedures for Renumbering an
        IPv6 Network without a Flag Day",
        draft-baker-ipv6-renumber-procedure-01
        draft-ietf-v6ops-renumbering-procedure-01 (work in progress),
        October 2003.
        July 2004.

   [5]  Choi, J., "Detecting Network Attachment in IPv6 Goals",
        draft-ietf-dna-goals-00
        draft-ietf-dna-goals-03 (work in progress), June October 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
   UNINETT
   Trondheim  NO 7465
   Norway

   EMail: venaas@uninett.no

   Vijayabhaskar A K
   Hewlett-Packard STSD-I
   29, Cunningham Kalusivalingam
   Cisco Systems (India) Private Limited
   9, Brunton Road
   Bangalore  560052  560025
   India

   EMail: vijayak@india.hp.com vibhaska@cisco.com

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