MEXT Working Group F. Dupont
Internet-Draft ISC
Intended status: Experimental W. Haddad
Expires: August 21, 2008 Ericsson Research
February 18, 2008
DHCPv6 Relay Agents and NEMO
draft-dupont-mext-dhcrelay-00.txt
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Abstract
The IPv6 network mobility (NEMO) configuration relies on a prefix
delegation service to fulfill its task. Such service has already
been described in two different proposals, one is based on DHCPv6 and
the other extends NEMO signaling.
However, both failed to gather consensus. This memo describes how
DHCPv6 Relay Agents can be used in order to provide the missing
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flexibility and pave the way for a solution.
1. Introduction
One aspect of network mobility support is the assignment of a prefix
or a set of prefixes to a Mobile Router (MR) for use on the links in
the Mobile Network. For this purpose, two solutions have been
proposed. The first one (described in [NEMOdhc]) uses DHPCv6
[RFC3315] Prefix Delegation [RFC3633] in the tunnel between the MR
and its Home Agent (HA), while the second [NEMOpd] is an adhoc
extension of NEMO signaling.
While DHCPv6 Prefix Delegation is the standard way to provide the
Prefix Delegation service, using it directly over the MR-HA tunnel is
far from immediate and a source of complexities. In an attempt to
avoid these complexities, the second proposal did not reuse the
DHCPv6 Prefix Delegation framework but failed to gain a rough
consensus as it went against the DHCPv6 mainstream.
In order to improve this situation, the use of DHCPv6 Relay Agents
has been suggested but was never written down and thus, had a limited
effect. This document aims to explain how a clever use of DHCPv6
Relay Agents can inject the desired flexibility to extend the
applicability of DHCPv6 in general and DHCPv6 Prefix Delegation in
particular to environments with very different constraints than in
the original design.
2. A short description of DHCPv6 Relay Agent function
The main function of DHCPv6 Relay Agents is to allow DHCPv6 Servers
to be located anywhere in the network and not only on the same link
than their Clients. But DHCPv6 extends it with the support of
multiple Relays, the resulting topology from a Client is a tree with:
- the Client as the tree root
- Relays as intermediate nodes
- Servers as leaves
Relays forward messages from downstream Clients or Relays to upstream
Servers or Relays, using unicast and/or multicast, and forward
responses back in the other way.
DHCPv6 Relay Agents use two specific messages (Relay-forward and
Relay-repl). These messages share a common header:
- a message type
- a hop-count (to detect loops)
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- a link-address (to identify the downstream link)
- a peer-address (to identity the downstream Client or Relay)
To help the identification of the downstream link, the Relay may
insert an Interface-id option which will be reflected back by Servers
in Relay-repl messages. The content of this option is opaque, i.e.,
Servers do not parse it.
On another side, for the identification of DHCPv6 Clients, Relay
Agents may insert Subscriber-id [RFC4580] or Remote-id [RFC4649]
options in Relay-forward messages.
3. Flexibility introduced by the use of DHCPv6 Relay Agents
DHCPv6 mandates the Client to use link-local address and multicast to
communicate with an onlink Server or Relay. Such design makes sense
for global address assignments but is a very annoying constraint in
the MR-HA context.
Since a Relay does not have such constraint, the idea is to co-locate
a Relay in the MR at the MR end of the tunnel.
4. Transport of DHCPv6 messages
DHCPv6 messages can be easily encapsulated, in fact relaying
encapsulates recursively DHCPv6 messages in Relay-message options.
But when reusing DHCPv6 code, the Relay function is the easiest,
i.e., when flexibility is needed to support transport of DHCPv6
messages in a not standard way the solution is to use (again) Relays.
Consequently, the main argument of NEMO Prefix Delegation [NEMOpd],
the overhead to run signaling then prefix delegation after each
movement, no more stands as DHCPv6 can be piggy-backed into the
mobility signaling.
5. Final recommendation
If the use of DHCPv6 Relays introduces flexibility, it shoud
nevertheless be mentioned that this is not a reason to enforce their
use. It follows that the wording should be enough accurate in order
to keep the choice between a Relay or another entity. For instance,
the DHCPv6 Prefix Delegation for NEMO [NEMOdhc] is right when it
allows a Relay or a Server in the Home Agent.
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6. Acknowledgments
The idea described in this document is not new so we are sure it is
not our own idea...
7. Security Considerations
This memo proposes some ways to improve code and security analysis
sharing for the Prefix Delegation service so should indirectly help
security.
8. IANA Considerations
None.
9. References
9.1. Normative References
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
9.2. Informative References
[NEMOdhc] Droms, R. and P. Thubert, "DHCPv6 Prefix Delegation for
NEMO", draft-ietf-nemo-dhcpv6-pd-03.txt (work in
progress), December 2007.
[NEMOpd] Thubert, P. and TJ. Kniveton, "Mobile Network Prefix
Delegation", draft-ietf-nemo-prefix-delegation-02.txt
(work in progress), August 2007.
[RFC4580] Volz, B., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6) Relay Agent Subscriber-ID Option", RFC 4580,
June 2006.
[RFC4649] Volz, B., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6) Relay Agent Remote-ID Option", RFC 4649,
August 2006.
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Authors' Addresses
Francis Dupont
ISC
Email: Francis.Dupont@fdupont.fr
Wassim Haddad
Ericsson Research
Email: Wassim.Haddad@ericsson.com
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