    Network Working Group                                           T. Chown
    Request for Comments: 4076                     University of Southampton
    Category: Informational                                        S. Venaas
                                                                     UNINETT
                                                            A. Vijayabhaskar
                                       Cisco Systems (India) Private Limited
                                                                    May 2005

                    Renumbering Requirements for Stateless
             Dynamic Host Configuration Protocol for IPv6 (DHCPv6)

    Status of This Memo

       This memo provides information for the Internet community.  It does
       not specify an Internet standard of any kind.  Distribution of this
       memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2005).

    Abstract

       IPv6 hosts using Stateless Address Autoconfiguration are able to
       configure their IPv6 address and default router settings
       automatically.  However, further settings are not available.  If
       these hosts wish to configure their DNS, NTP, or other specific
       settings automatically, 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 this 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, 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.

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    RFC 4076            Renumbering for Stateless DHCPv6            May 2005

    Table of Contents

       1.  Introduction ...................................................2
       2.  Problem Statement ..............................................3
       3.  Renumbering Scenarios ..........................................3
           3.1.  Site Renumbering .........................................4
           3.2.  Changes to a DHCPv6-assigned Setting .....................4
       4.  Renumbering Requirements .......................................4
       5.  Considerations in Choosing a Solution ..........................4
       6.  Solution Space .................................................5
       7.  Summary ........................................................5
       8.  Security Considerations ........................................6
       9.  Acknowledgements ...............................................6
       10. References .....................................................6
           10.1. Normative References .....................................6
           10.2. Informative References ...................................6

    1.  Introduction

       IPv6 hosts using Stateless Address Autoconfiguration [2] are able to
       configure their IPv6 address and default router settings
       automatically.  Although 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 configured
       automatically.

       The full version of the Dynamic Host Configuration Protocol for IPv6
       (DHCPv6) [3] 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
       configure their DNS, NTP, or other specific settings automatically,
       the stateless variant [4] of DHCPv6 could be used.  This variant is
       more lightweight.  It does not do address assignment; instead, it
       only provides additional configuration parameters, such as DNS
       resolver addresses.  It does not maintain dynamic state about the
       information assigned to clients, and therefore there is no need to
       maintain dynamic per-client state on the server.

       This combination of Stateless Address Autoconfiguration and stateless
       DHCPv6 could be used quite commonly in IPv6 networks.

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    RFC 4076            Renumbering for Stateless DHCPv6            May 2005

    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 messages to individual
       clients to trigger them to initiate 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  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 could extend beyond Stateless
       DHCPv6, since only IP address options have a lifetime; i.e., there is
       no mechanism even in the full DHCPv6 that "expires" old information
       or otherwise forces 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.

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    RFC 4076            Renumbering for Stateless DHCPv6            May 2005

    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.  Therefore, a site changing its ISP must
       renumber its network.  Any such site renumbering will require hosts
       to reconfigure both their own address and default router settings and
       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 and is moved to a new network (prefix), or if 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 whereby the hosts are 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 planned or unplanned, from the perspective of
       the network administrator.  There is already work underway toward
       understanding the planned renumbering [5] scenario for IPv6 networks.
       However, there is currently no mechanism in stateless DHCPv6 for
       handling planned renumbering events.

    5.  Considerations in Choosing a Solution

       A number of considerations could be listed for a desirable solution:

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

       o  Security is 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.

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    6.  Solution Space

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

       o  Add 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 in order
          to be realised.

       o  Convey 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 handle unplanned events to some
          extent at the expense of the additional request traffic.  The
          selection of recommended lifetime values/ranges would be the
          subject of future work.

       o  Use some form of Router Advertisement (RA) [1] as a hint to
          request new stateless DHCPv6-assigned settings.  Using only an
          observed new RA 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 Goals of Detecting Network
          Attachment (DNA) in IPv6 [6]).

       o  Change the semantics of the 'O' flag in RAs [2] so that toggling
          its value may trigger an Information-request message.

       There will also be conditions under which a client should send an
       Information-request, such as reconnection to a link.  Recommendations
       for these cases are outside the scope of this document, but we expect
       ongoing work in the DNA WG (as scoped in Goals of Detecting Network
       Attachment (DNA) in IPv6 [6]) 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 learned through
       stateless DHCPv6.  A short list of candidate solutions is presented,
       which the authors hope will be expanded upon in subsequent documents.

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    RFC 4076            Renumbering for Stateless DHCPv6            May 2005

    8.  Security Considerations

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

       The issues of maintaining appropriate security through a renumbering
       event are outside the scope of this document (if 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, Bernie Volz, and other
       individuals on the DHC mail list for their comments on this document,
       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]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
            for IP Version 6 (IPv6)", RFC 2461, December 1998.

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

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

       [4]  Droms, R., "Stateless Dynamic Host Configuration Protocol (DHCP)
            Service for IPv6", RFC 3736, April 2004.

    10.2.  Informative References

       [5]  Baker, F., Lear, E. and R. Droms, "Procedures for Renumbering an
            IPv6 Network without a Flag Day", Work in Progress, July 2004.

       [6]  Choi, J., "Goals of Detecting Network Attachment (DNA) in IPv6",
            Work in Progress, October 2004.

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    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 Kalusivalingam
       Cisco Systems (India) Private Limited
       9, Brunton Road
       Bangalore  560025
       India

       EMail: vibhaska@cisco.com

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    RFC 4076            Renumbering for Stateless DHCPv6            May 2005

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