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rfc:rfc4076

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.

Chown, et al. Informational [Page 1] 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.

Chown, et al. Informational [Page 2] 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.

Chown, et al. Informational [Page 3] 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.

Chown, et al. Informational [Page 4] RFC 4076 Renumbering for Stateless DHCPv6 May 2005

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.

Chown, et al. Informational [Page 5] 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.

Chown, et al. Informational [Page 6] RFC 4076 Renumbering for Stateless DHCPv6 May 2005

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

Chown, et al. Informational [Page 7] RFC 4076 Renumbering for Stateless DHCPv6 May 2005

Full Copyright Statement

 Copyright (C) The Internet Society (2005).
 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/SHE 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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Acknowledgement

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

Chown, et al. Informational [Page 8]

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