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RFC 4076 - Renumbering Requirements for Stateless Dynamic Host C

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Network Working Group                                           T. Chown
Request for Comments: 4076                     University of Southampton
Category: Informational                                        S. Venaas
                                                        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).


   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

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

   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.

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.

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

   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 the

   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.

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.

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

   EMail: vibhaska@cisco.com

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