6man Working Group A. Matsumoto
Internet-Draft T. Fujisaki
Intended status: Standards Track J. Kato
Expires: August 18, 2012 NTT
T. Chown
University of Southampton
February 15, 2012
Distributing Address Selection Policy using DHCPv6
draft-ietf-6man-addr-select-opt-02.txt
Abstract
RFC 3484 defines default address selection mechanisms for IPv6 that
allow nodes to select appropriate address when faced with multiple
source and/or destination addresses to choose between. The RFC 3484
allowed for the future definition of methods to administratively
configure the address selection policy information. This document
defines a new DHCPv6 option for such configuration, allowing a site
administrator to distribute address selection policy overriding the
default address selection policy table, and thus control the address
selection behavior of nodes in their site.
Status of this Memo
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This Internet-Draft will expire on August 18, 2012.
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1. Introduction
RFC 3484 [RFC3484] describes default algorithms for selecting an
address when a node has multiple destination and/or source addresses
to choose from by using an address selection policy. In Section 2 of
RFC 3484, it is suggested that the default policy table may be
administratively configured to suit the specific needs of a site.
This specification defines a new DHCPv6 option for such
configuration.
Some problems have been identified with the default RFC 3484 address
selection policy [RFC5220]. It is unlikely that any default policy
will suit all scenarios, and thus mechanisms to control the source
address selection policy will be necessary. Requirements for those
mechanisms are described in [RFC5221], while solutions are discussed
in [I-D.ietf-6man-addr-select-sol] and
[I-D.ietf-6man-addr-select-considerations]. Those documents have
helped shape the improvements in the default address selection
algorithm [I-D.ietf-6man-rfc3484-revise] as well as the DHCPv6 option
defined in this specification.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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1.2. Terminology
This document uses the terminology defined in [RFC2460] and the
DHCPv6 specification defined in [RFC3315]
2. Address Selection Policy Option
The Address Selection Policy Option provides the policy table for
address selection rules as described in RFC 3484 and in
[I-D.ietf-6man-rfc3484-revise].
Each end node is expected to configure its policy table, as described
in RFC 3484, using the Address Selection Policy option information as
described in the section below on processing the option.
The format of the 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 |z| reserved | prefix-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Prefix (Variable Length) +-+-+-+-+-+-+-+-+
| | suboption-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| suboption-zone-index (if present (z = 1)) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
[Fig. 1]
Fields:
option-code: OPTION_DASP (TBD)
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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.
z bit: 'zone-index' bit. If z bit is set to 1, 32 bit zone-index
value is included right after the "prefix-len" field, and
"Prefix" value continues after the "zone-index" field. If z bit
is 0, "Prefix" value continues right after the "prefix-len"
value.
reserved: 6-bit reserved field. Initialized to zero by sender, and
ignored by receiver.
suboption-len: 'suboption-len' specifies the length of the suboption
fields in bytes. Currently, the only defined suboption is zone-
index, described as 'suboption-zone-index'.
suboption-zone-index: If the z-bit is set to 1, this field is
inserted between "prefix-len" field and "Prefix" field. The
zone-index field is an 32-bit unsigned integer and used to
specify zones for scoped addresses. The zone-index is defined
in RFC 3493 [RFC3493] as 'scope ID'.
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.
Prefix: A variable-length field containing an IP address or the
prefix of an IP address. An IPv4-mapped address [RFC4291] must
be used to represent an IPv4 address as a prefix value. The
Prefix should be truncated on the byte boundary. So the length
of this field should be between 0 and 16 bytes.
Multiple Address Selection Policy options MAY appear in a DHCPv6
message. They MUST be treated in the way that they constitute a
single policy table.
3. Appearance of the Address Selection Policy Option
The Address Selection Policy option MUST NOT appear in any messages
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other than the following ones: Solicit, Advertise, Request, Renew,
Rebind, Reconfigure, Information-Request, and Reply.
4. Processing the Address Selection Policy Option
This section describes how to process received Address Selection
Policy Options at the DHCPv6 client.
This option's concept is to serve as a hint for a node about how to
behave in the network. So, basically, it should be up to the node's
administrator how to make use of or even ignore the received policy
information.
4.1. Handling of the local policy table
RFC 3484 defines the default policy table. Also, a user is usually
able to configure the policy table to satisfy his requirement.
The client node SHOULD provide the following choices:
a) It receives distributed policy table, and replaces the existing
policy tables with that.
b) It preserves the default policy table, or manually configured
policy.
4.2. Handling of the stale policy table
When the information from the DHCP server goes stale, the policy
received form the DHCP server should be removed and the default
policy should be restored.
The received information can be considered stale in several cases,
such as, when the interface goes down, the DHCP server does not
respond for a certain amount of time, and the Information Refresh
Time is expired.
4.3. Processing multiple received policy tables
The policy table is node-global information by its nature. So, the
node cannot use multiple received policy tables at the same time. In
other words, once the received policy from one source is merged with
another source, the policy is more or less changed. The policy table
is defined as a whole, so the slightest addition/deletion from the
policy table brings a change in semantics of the policy.
It also should be noted that, when a node is single-homed and has
only one upstream line, adopting a received policy table does not
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degrade the security level.
Under the above assumptions, we specify how to handle multiple
received policy tables below.
A node MAY use OPTION_DASP in any of the following two cases:
1: The address selection option is delivered across the only secure,
trusted channel.
2: The address selection option delivery is not secured, but the node
is single-homed.
In other cases the node MUST NOT use OPTION_DASP unless the node is
specifically configured to do so.
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 the DHCP message. DHCPv6 clients need
to convert this label to a representation specified by each
implementation (e.g., string).
o Currently, the label and precedence values are defined as 8-bit
unsigned integers. In almost all cases, this value will be
enough.
o The maximum number of address selection rules that may be conveyed
in one DHCPv6 message depends on the prefix length of each rule
and the maximum DHCPv6 message size defined in RFC 3315. It is
possible to carry over 3,000 rules in one DHCPv6 message (maximum
UDP message size). However, it should not be expected that DHCP
clients, servers and relay agents can handle UDP fragmentation.
So, the number of the options and the total size of the options
should be taken care of.
o Since the number of selection rules could be large, an
administrator configuring the policy to be distributed should
consider the resulting DHCPv6 message size.
6. Security Considerations
A rogue DHCPv6 server could issue bogus address selection policies to
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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. Alternatively, an IPv6 transition
mechanism might be preferred over native IPv6, even if it is
available. To guard against such attacks, a legitimate DHCPv6 server
should be communicated through a secure, trusted channel, such as a
channel protected by IPsec, SEND and DHCP authentication, as
described in section 21 of RFC 3315,
Another threat is about privacy concern. As in the security
consideration section of RFC 3484, at least a part of, the address
selection policy stored in a host can be leaked by a packet from a
remote host. This issue will not be degraded regardless of the
introduction of this option, or regardless of whether the host is
multihomed or not.
7. IANA Considerations
IANA is requested to assign option codes to OPTION_DASP from the
option-code space as defined in section "DHCPv6 Options" of RFC 3315.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
8.2. Informative References
[I-D.ietf-6man-addr-select-considerations]
Chown, T. and A. Matsumoto, "Considerations for IPv6
Address Selection Policy Changes",
draft-ietf-6man-addr-select-considerations-04 (work in
progress), October 2011.
[I-D.ietf-6man-addr-select-sol]
Matsumoto, A., Fujisaki, T., and R. Hiromi, "Solution
approaches for address-selection problems",
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draft-ietf-6man-addr-select-sol-03 (work in progress),
March 2010.
[I-D.ietf-6man-rfc3484-revise]
Matsumoto, A., Kato, J., Fujisaki, T., and T. Chown,
"Update to RFC 3484 Default Address Selection for IPv6",
draft-ietf-6man-rfc3484-revise-05 (work in progress),
October 2011.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6",
RFC 3493, February 2003.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
[RFC5220] Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
"Problem Statement for Default Address Selection in Multi-
Prefix Environments: Operational Issues of RFC 3484
Default Rules", RFC 5220, July 2008.
[RFC5221] Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
"Requirements for Address Selection Mechanisms", RFC 5221,
July 2008.
Appendix A. Past 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, in general case,
it is hard to use this value.
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o Since we got a comment that some implementations use 32-bit
integers for zone index value, we extended the bit length of the
'zone index' field. However, as described above, there might be
few cases to specify 'zone index' in policy distribution, we
defined this field as optional, controlled by a flag.
o There may be some demands to control the use of special address
types such as the temporary addresses described in RFC4941
[RFC4941], address assigned by DHCPv6 and so on. (e.g., informing
not to use a temporary address when it communicate within the an
organization's network). It is possible to indicate the type of
addresses using reserved field value.
Authors' Addresses
Arifumi Matsumoto
NTT SI Lab
3-9-11 Midori-Cho
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 3334
Email: arifumi@nttv6.net
Tomohiro Fujisaki
NTT PF Lab
3-9-11 Midori-Cho
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 7351
Email: fujisaki@nttv6.net
Jun-ya Kato
NTT SI Lab
3-9-11 Midori-Cho
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 2939
Email: kato@syce.net
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Tim Chown
University of Southampton
Southampton, Hampshire SO17 1BJ
United Kingdom
Email: tjc@ecs.soton.ac.uk
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