DHC Working Group T. Fujisaki
Internet-Draft A. Matsumoto
Expires: June 4, 2006 J. Kato
S. Niinobe
NTT
Dec 2005
Distributing Default Address Selection Policy using DHCPv6
draft-fujisaki-dhc-addr-select-opt-01.txt
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Copyright (C) The Internet Society (2005).
Abstract
This document describes a new DHCPv6 option for distributing default
address selection policy information to a client. With this option,
site administrators can distribute address selection policy to
control the node's address selection behavior.
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1. Introduction
RFC3484 [RFC3484] describes algorithms for selecting a default
address when a node has multiple destination and/or source addresses
by using an address selection policy. Administrators can control the
node's default address selection behavior by distributing the policy.
Practical usages are described in [A. Matsumoto]. This document
describes an option for distributing default address selection policy
information using DHCPv6.
2. Terminology
This document uses the terminology defined in [RFC2460] and the DHCP
specification defined in [RFC3315]
3. Default Address Selection Policy Option
The Default Address Selection Policy Option provides policy
information for address selection rules. Specifically, it transmits
a set of IPv6 source and destination address prefixes and some
parameters that are used to control address selection as described in
RFC 3484.
Each end node is expected to configure its policy table, as described
in RFC 3484, in a manner consistent with the Default Address
Selection Policy option information.
The format of the Default Address Selection Policy option is given
below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_DASP | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| label | precedence | zone-index | prefix-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Prefix (Variable Length) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| label | precedence | zone-index | prefix-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Prefix (Variable Length) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| label | precedence | zone-index | prefix-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Prefix (Variable Length) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
[Fig. 1]
Fields:
option-code: OPTION_DASP (TBD)
option-len: The total length of the label fields, precedence fields,
zone-index fields, prefix-len fields, and prefix fields in
octets.
label: An 8-bit unsigned integer; this value is used to make a
combination of source address prefixes and destination address
prefixes.
precedence: An 8-bit unsigned integer; this value is used for sorting
destination addresses.
zone-index: An 8-bit unsigned integer; this value is used to specify
zones for scoped addresses.
prefix-len: An 8-bit unsigned integer; the number of leading bits in
the prefix that are valid. The value ranges from 0 to 128. The
Prefix field is 0, 4, 8, 12, or 16 octets, depending on the
length.
Prefix: A variable-length field containing an IP address or the
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prefix of an IP address. IPv4-mapped address [mapped] must be
used to represent an IPv4 address as a prefix value.
4. Appearance of this Option
The Default Address Selection Policy option MUST NOT appear in any
messages other than the following ones : Solicit, Advertise, Request,
Renew, Rebind, Information-Request, and Reply.
5. Implementation Considerations
o The value 'label' is passed as an unsigned integer, but there is
no special meaning for the value, that is whether it is a large or
small number. It is used to select a preferred source address
prefix corresponding to a destination address prefix by matching
the same label value within this DHCP message. DHCPv6 clients
need to convert this label to a representation specified by each
implementation (e.g., string).
o Currently, the value label, precedence, and zone indices are
defined as 8-bit unsigned integers. In almost all cases, this
value will be enough.
o The 'precedence' is used to sort destination addresses. There
might be some cases where precedence values will conflict when a
client already has a selection policy configured or a client
receives multiple policies from multiple DHCP servers (e.g., when
a home gateway in a user network is connected to multiple upstream
ISPs). In such cases, manual configuration of the policy will be
necessary.
6. Discussion
o The 'zone index' value is used to specify a particular zone for
scoped addresses. This can be used effectively to control address
selection in the site scope (e.g., to tell a node to use a
specified source address corresponding to a site-scoped multicast
address). However, in some cases such as a link-local scope
address, the value specifying one zone is only meaningful locally
within that node. There might be some cases where the
administrator knows which clients are on the network and wants
specific interfaces to be used though. However, it is hard to use
this value in general case.
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7. Security Considerations
A rogue DHCPv6 server could issue bogus default address selection
policies to a client. This might lead to incorrect address selection
by the client, and the affected packets might be blocked at an
outgoing ISP because of ingress filtering.
To guard against such attacks, both DCHP clients and servers SHOULD
use DHCP authentication, as described in section 21 of RFC 3315,
"Authentication of DHCP messages."
8. References
[A. Matsumoto]
Matsumoto, A., Fujisaki, T., and J. Kato, "Practical
Usages of Default Address Selection Policy Distribution",
draft-arifumi-ipv6-policy-dist-00.txt-00.txt. (Work In
Progress) (work in progress), Dec 1 2005.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
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Authors' Addresses
Tomohiro Fujisaki
NTT PF Lab
3-9-11 Midori-Cho
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 7351
Email: fujisaki.tomohiroi@lab.ntt.co.jp
Arifumi Matsumoto
NTT PF Lab
3-9-11 Midori-Cho
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 3334
Email: arifumi@nttv6.net
Jun-ya Kato
NTT PF Lab
3-9-11 Midori-Cho
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 2939
Email: kato@syce.net
Shirou Niinobe
NTT PF Lab
3-9-11 Midori-Cho
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 4949
Email: nin@syce.net
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