Internet DRAFT - draft-venaas-dhc-dual-stack-merge
draft-venaas-dhc-dual-stack-merge
Internet Engineering Task Force S. Venaas
Internet Draft University of Southampton
Expiration Date: January 2006
T. Chown
University of Southampton
July 2005
Dual-stack clients and merging of data from DHCPv4 and DHCPv6
draft-venaas-dhc-dual-stack-merge-00.txt
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Abstract
A node may have support for communications using both IPv4 and IPv6
protocols. Such a node may wish to obtain both IPv4 and IPv6
configuration settings via the Dynamic Host Configuration Protocol
(DHCP). This can be done by using the IPv4 and the IPv6 DHC
protocols respectively. This document considers mechanisms that
allow such a node to make use of the configuration data from both
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protocols to obtain the desired common configuration.
Table of Contents
1. Introduction ............................................... 2
2. Tools for merging .......................................... 3
2.1. Host prefers IPv4 or IPv6 .............................. 3
2.2. Dual-stack or both DHC protocols client option ......... 3
2.3. DUID and integrated DHCPv4/v6 server ................... 3
2.4. DHCPv6 option telling dual-stack client to use DHCPv4 .. 4
2.5. IPv4-mapped addresses in DHCPv6 options ................ 4
3. Solutions .................................................. 4
3.1. Use of preference rules ................................ 4
3.2. Lists of mixed addresses ............................... 5
3.3. Issues not solved ...................................... 6
4. Security Considerations .................................... 6
5. Informative References ..................................... 6
Authors' Addresses ............................................. 7
Intellectual Property and Copyright Statements ................. 7
1. Introduction
The original specification of the Dynamic Host Configuration Protocol
(DHCP) was made with only IPv4 in mind. That specification has been
ubsequently revised, up to the latest version of DHCP [RFC 2131].
With the arrival of IPv6, a new DHCP specification for IPv6 has been
designed, and published as DHCPv6 [RFC 3315].
These protocols allow nodes to communicate via IPv4 or IPv6 to
retrieve configuration settings for operation in a managed
environment. While an IPv6 node may acquire address-related
configuration settings via IPv6 stateless address autoconfiguration
[RFC 2462], such a node may wish to use stateless DHCPv6 [RFC 3736]
for other administratively configured options, such as DNS or NTP.
In early IPv6 deployments, a dual-stack mode of operation is
typically used. There will thus be nodes that require both IPv4 and
IPv6 configuration settings. At the same time there may be IPv4-only
and IPv6-only nodes using these protocols. Issues related to this
have been described in [ISSUES]. This document discusses how to
resolve these issues.
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This initial revision does not attempt to describe any complete
solutions, but rather serve as a discussion point by describing some
of the possible methods that may be of use.
In this document, we refer to DHCP for IPv4 [1] as DHCPv4 and DHCP
for IPv6 [RFC 3315] as DHCPv6.
2. Tools for merging
There are a number of different tools or methods that can be of use
in ensuring that IPv4-only, IPv6-only and dual-stack hosts each get
the info they need from DHCP4, DHCPv6 or a combination of the two.
2.1. Host prefers IPv4 or IPv6
The idea is that a dual-stack host may obtain information from both
DHCPv4 and DHCPv6 but will prefer one of them. So if a single valued
option is received from both it can use the preferred one. For a set
(or unordered list) it might use only the preferred or mix them,
while for an ordered list it should probably use all, but put the
preferred first. The preference could be manually configured on the
host or obtained via either DHCPv4 or DHCPv6. The option would only
be needed for one of them.
2.2. Dual-stack or both DHC protocols client option
Host could use a new DHCP option to tell DHCP server (v4 or v6) that
it is dual-stack and have or will request configuration for the other
protocol. This can indicate to the server what information it needs
to return to the client.
2.3. DUID and integrated DHCPv4/v6 server
DHCPv6 [RFC 3315] uses a DHCP Unique Identifier (DUID). A client
requesting both IPv4 and IPv6, should use the same DUID for the two
requests, see [3315IDV4] for using DUID with IPv4. If e.g. client
requests DHCPv4 first, then when it makes the DHCPv6 request, the
server knows what info the client previously learnt through DHCPv4
and can leave that out from the DHCPv6 reply. We are not sure
whether this can be done if multiple integrated servers are deployed.
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2.4. DHCPv6 option telling dual-stack client to use DHCPv4
A new option could be used by DHCPv6 server to tell a dual-stack
client to request IPv4 information even if it has IPv4 addresses
(tells client to use DHCPINFORM).
2.5. IPv4-mapped addresses in DHCPv6 options
DHCPv6 options could contain IPv4 addresses written as IPv4-mapped
IPv6 addresses.
3. Solutions
We will now discuss how the above tools might be used to solve some
of the issues in [ISSUES].
3.1. Use of preference rules
A simple preference rule as in 2.1 might be sufficient in many cases.
The perhaps most difficult problem is where the option is a list of
values, and one wishes to have a mix of IPv4 and IPv6 addresses where
one does not want to list all of one IP type before the other, or if
one is preferred to the other in most cases but not always. Lists of
mixed addresses are discussed in the next section.
Another solution could be to use FQDNs as option values whenever
possible. Then DHCPv4 and DHCPv6 might simply specify the same FQDN
where the fqdn is registered in the DNS with both IPv4 and IPv6
addresses. The preference would then be determined by the host's
destination address selection rules. Some sites deploying IPv6
choose initially to use different FQDNs for IPv6, in which case this
would not work.
The preference rule is not sufficient if say IPv6 is generally
preferred, but IPv4 should preferred in some cases. One way of doing
this, could be to have client prefer IPv6, and make the DHCPv6 server
omit IPv6 info for options where IPv4 is preferred. The server could
do this if by use of 2.2 it knows that the client also will get the
IPv4 information. An IPv6-only client, or one not requesting IPv4
configuration, should still get all the IPv6 options. The
administrator may manually configure a DHCPv6 server to omit some
IPv6 config for clients that also obtain IPv4 information. A
combined DHCPv4 and DHCPv6 server might be able to determine this
automatically. With different servers it might help to have a single
combined admin interface.
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One issue with the above is that the server must only omit options if
it knows for sure that client will request and successfully obtain
both IPv4 and IPv6 information. There are two ways this might be
done. One is that server is told by client that it uses both (2.2),
possibly combined with 2.4 where server tells client to request the
other. Another possibly safer way is to make use of the DUID (2.3)
so that server knows that the client that previously made a DHCPv6
request, now makes a DHCPv4 request. The latter should work if a
client generally prefering one protocol, uses DHCP for the preferred
protocol last.
3.2. Lists of mixed addresses
As we said previously, the most difficult problem is when one has a
list of values, and one wishes to have a mix of IPv4 and IPv6
addresses where one does not want to list all of one IP type before
the other. We are not sure if this is necessary to solve. If it is,
the easiest solution might be to use IPv4-mapped addresses as in 2.5,
so that a mixed list of IPv4-mapped IPv6 addresses and other IPv6
addresses can be passed in a DHCPv6 option. If this is done it might
be useful to have an option as described in 2.2 that tells the server
that the client is dual-stack. One should not pass mapped addresses
to an IPv6-only host.
Another way of solving this would be to somehow leave holes in the
IPv6 list, using some special IPv6 address to indicate where the IPv4
addresses returned from DHCPv4 should be placed in the list. We
don't think this is a good solution, but it could be done provided
the server knows client will or has asked for DHCPv4 information, and
that it knows what IPv4 info the client has or will be given.
Another issue with using a simple preference for lists, is that if a
server is dual-stack with both IPv4 and IPv6 addresses, one may not
wish to have both the addresses in the list. E.g. if one has a
nameserver with IPv4 address a4 and IPv6 address a6, and another with
IPv4 address b4, one may not want the list "a6, a4, b4", but rather
"a6, b4". Whether this is a problem may depend on whether the list
is processed sequentially and how long timeout there is before trying
the next in the list. If an integrated DHCPv4 and DHCPv6 server
knows that a client has previously got the list "a6" via say DHCPv6,
it could choose to omit "a4" when the same client makes a DHCPv4
query. It can detect that it is the same client using DUID as in
2.3. However if there are multiple integrated servers the two
requests may go to different servers. Another alternative could be
to use the option in 2.2.
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3.3. Issues not solved
There are many issues in [ISSUES] that are not tackled by the above.
We have not looked at the issue of different people managing DHCP4
and DHCPv6 or the case where the node is statically configured with
information for one protocol while using DHCP for the other. Another
issue is what to do when initially only one IP protocol is enabled,
and the other is enabled later. There are other issues not
sufficiently tackled as well, we suggest reading [ISSUES] for the
full details. The methods presented here are just some preliminary
ideas. Through discussion in the DHC WG we will try to come up with
solutions that can resolve the issues. It may however not be
possible to come up with a complete solution to all of them.
4. Security Considerations
We are not aware of any new security issues as a result of any of the
described options, but this needs to be considered.
5. Informative References
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC 2462] S. Thomson, T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[RFC 3315] R. Droms, Ed., J. Bound, B. Volz, T. Lemon, C. Perkins,
M. Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[RFC 3736] R. Droms, "Stateless Dynamic Host Configuration Protocol
(DHCP) Service for IPv6", RFC 3736, April 2004.
[ISSUES] T. Chown, S. Venaas, C. Strauf, "DHCP: IPv4 and IPv6
Dual-Stack Issues", work-in-progress,
draft-ietf-dhc-dual-stack-03, July 2005.
[3315IDV4] T. Lemon, B. Sommerfeld, "Node-Specific Client
Identifiers for DHCPv4", work-in-progress,
draft-ietf-dhc-3315id-for-v4-04.txt, February 2005.
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Authors' Addresses
Stig Venaas
University of Southampton
School of Electronics and Computer Science
Southampton, Hampshire SO17 1BJ
United Kingdom
EMail: sv@ecs.soton.ac.uk
Tim Chown
University of Southampton
School of Electronics and Computer Science
Southampton, Hampshire SO17 1BJ
United Kingdom
EMail: tjc@ecs.soton.ac.uk
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