Dynamic Host Congiguration T. Chown
Internet-Draft University of Southampton
Expires: April 25, 2005 S. Venaas
UNINETT
A. Vijayabhaskar
Cisco Systems (India) Private
Limited
October 25, 2004
Renumbering Requirements for Stateless DHCPv6
draft-ietf-dhc-stateless-dhcpv6-renumbering-02
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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
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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, 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
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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, hence there is no need to maintain per-client
state on the server. In other words, all clients can be given the
same information, in the same way that the information in Router
Advertisements is not client-specific.
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 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
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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 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 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
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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.
5. Considerations in choosing a solution
There are a number of considerations that 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.
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.
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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, Bermie Volz and other
individuals on the DHC mail list for their comments on this 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.
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[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-ietf-v6ops-renumbering-procedure-01 (work in progress),
July 2004.
[5] Choi, J., "Detecting Network Attachment in IPv6 Goals",
draft-ietf-dna-goals-03 (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
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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