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IETF RFC 7078
Last modified on Wednesday, January 8th, 2014
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Internet Engineering Task Force (IETF) A. Matsumoto
Request for Comments: 7078 T. Fujisaki
Category: Standards Track NTT
ISSN: 2070-1721 T. Chown
University of Southampton
January 2014
Distributing Address Selection Policy Using DHCPv6
 Abstract
RFC 6724 defines default address selection mechanisms for IPv6 that
allow nodes to select an appropriate address when faced with multiple
source and/or destination addresses to choose between. RFC 6724
allows 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 parameters and policy table, and thus
allowing the administrator to control the address selection behavior
of nodes in their site.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7078.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
Matsumoto, et al. Standards Track PAGE 1 
RFC 7078 DHCPv6 Address Selection Policy Opt January 2014
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
1. Introduction
[RFC 6724] 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. This specification
defines a new DHCPv6 option for configuring the default policy table.
Some problems were identified with the default address selection
policy as originally defined in [RFC 3484]. As a result, RFC 3484 was
updated and obsoleted by [RFC 6724]. While this update corrected a
number of issues identified from operational experience, 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
[RFC 5221], while solutions are discussed in [ADDR-SEL]. Those
documents have helped shape the improvements in the default address
selection algorithm in [RFC 6724] as well as the requirements for the
DHCPv6 option defined in this specification.
This option's concept is to serve as a hint for a node about how to
behave in the network. Ultimately, while the node's administrator
can control how to deal with the received policy information, the
implementation SHOULD follow the method described below uniformly to
ease troubleshooting and to reduce operational costs.
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 [RFC 2119].
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RFC 7078 DHCPv6 Address Selection Policy Opt January 2014
1.2. Terminology
This document uses the terminology defined in [RFC 2460] and the
DHCPv6 specification defined in [RFC 3315]
2. Address Selection Options
The Address Selection option provides the address selection policy
table and some other configuration parameters.
An Address Selection option contains zero or more policy table
options. Multiple policy table options in an Address Selection
option constitute a single policy table. When an Address Selection
option does not contain a policy table option, it may be used to just
convey the A and P flags for Automatic Row Additions and Privacy
Preference, respectively.
The format of the Address Selection 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_ADDRSEL | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |A|P| |
+-+-+-+-+-+-+-+-+ POLICY TABLE OPTIONS |
| (variable length) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Address Selection Option Format
option-code: OPTION_ADDRSEL (84).
option-len: The total length of the Reserved field, A and P flags,
and POLICY TABLE OPTIONS in octets.
Reserved: Reserved field. The server MUST set this value to 0, and
the client MUST ignore its content.
A: Automatic Row Addition flag. This flag toggles the Automatic
Row Addition flag at client hosts, which is described in
Section 2.1 of [RFC 6724]. If this flag is set to 1, it does not
change client host behavior; that is, a client MAY automatically
add additional site-specific rows to the policy table. If set
to 0, the Automatic Row Addition flag is disabled, and a client
SHOULD NOT automatically add rows to the policy table. If the
option contains a POLICY TABLE option, this flag is meaningless,
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RFC 7078 DHCPv6 Address Selection Policy Opt January 2014
and automatic row addition SHOULD NOT be performed against the
distributed policy table. This flag SHOULD be set to 0 only
when the Automatic Row Addition at client hosts is harmful for
site-specific reasons.
P: Privacy Preference flag. This flag toggles the Privacy
Preference flag on client hosts, which is described in Section 5
of [RFC 6724]. If this flag is set to 1, it does not change
client host behavior; that is, a client will prefer temporary
addresses [RFC 4941]. If set to 0, the Privacy Preference flag
is disabled, and a client will prefer public addresses. This
flag SHOULD be set to 0 only when the temporary addresses should
not be preferred for site-specific reasons.
POLICY TABLE OPTIONS: Zero or more Address Selection Policy
Table options, as described below. This option corresponds to a
row in the policy table defined in Section 2.1 of [RFC 6724].
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_ADDRSEL_TABLE | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| label | precedence | prefix-len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| prefix (variable length) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Address Selection Policy Table Option Format
option-code: OPTION_ADDRSEL_TABLE (85).
option-len: The total length of the label field, precedence field,
prefix-len field, and prefix field.
label: An 8-bit unsigned integer; this value is for correlation of
source address prefixes and destination address prefixes. This
field is used to deliver a label value in the [RFC 6724] policy
table.
precedence: An 8-bit unsigned integer; this value is used for
sorting destination addresses. This field is used to deliver a
precedence value in the [RFC 6724] policy table.
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RFC 7078 DHCPv6 Address Selection Policy Opt January 2014
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. If
an option with a prefix length greater than 128 is included, the
whole Address Selection option MUST be ignored.
prefix: A variable-length field containing an IP address or the
prefix of an IP address. An IPv4-mapped address [RFC 4291] must
be used to represent an IPv4 address as a prefix value.
This field is padded with zeros up to the nearest octet boundary
when prefix-len is not divisible by 8. This can be expressed
using the following equation: (prefix-len + 7)/8
So, the length of this field should be between 0 and 16 bytes.
For example, the prefix 2001:db8::/60 would be encoded with a
prefix-len of 60; the prefix would be 8 octets and would contain
octets 20 01 0d b8 00 00 00 00.
3. Processing the Address Selection Option
This section describes how to process a received Address Selection
option at the DHCPv6 client.
This option's concept is to serve as a hint for a node about how to
behave in the network. Ultimately, while the node's administrator
can control how to deal with the received policy information, the
implementation SHOULD follow the method described below uniformly to
ease troubleshooting and to reduce operational costs.
3.1. Handling Local Configurations
[RFC 6724] defines two flags (A and P) and the default policy table.
Also, users are usually able to configure the flags and the policy
table to satisfy their own requirements.
The client implementation SHOULD provide the following choices to the
user.
(a) replace the existing flags and active policy table with the
DHCPv6 distributed flags and policy table.
(b) preserve the existing flags and active policy table, whether
this be the default policy table or the user configured policy.
Choice (a) SHOULD be the default, i.e., that the policy table is not
explicitly configured by the user.
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RFC 7078 DHCPv6 Address Selection Policy Opt January 2014
3.2. Handling Stale Distributed Flags and Policy Table
When the information from the DHCP server goes stale, the flags and
the policy table received from the DHCP server SHOULD be deprecated.
The local configuration SHOULD be restored when the DHCP-supplied
configuration has been deprecated. In order to implement this, a
host can retain the local configuration even after the flags and the
policy table is updated by the distributed flags and policy table.
The received information can be considered stale in several cases,
e.g., when the interface goes down, the DHCP server does not respond
for a certain amount of time, or the Information Refresh Time has
expired.
3.3. Handling Multiple Interfaces
The policy table, and other parameters specified in this document,
are node-global information by their nature. One reason being that
the outbound interface is usually chosen after destination address
selection. So a host cannot make use of multiple address selection
policies even if they are stored per interface.
The policy table is defined as a whole, so the slightest addition/
deletion from the policy table brings a change in the semantics of
the policy.
It also should be noted that the absence of a DHCP-distributed policy
from a certain network interface should not infer that the network
administrator does not care about address selection policy at all,
because it may mean there is a preference to use the default address
selection policy. So, it should be safe to assume that the default
address selection policy should be used where no overriding policy is
provided.
Under the above assumptions, we can specify how to handle received
policy as follows.
In the absence of distributed policy for a certain network interface,
the default address selection policy SHOULD be used. A node should
use Address Selection options by default in any of the following two
cases:
1: A single-homed host SHOULD use default address selection options,
where the host belongs exclusively to one administrative network
domain, usually through one active network interface.
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RFC 7078 DHCPv6 Address Selection Policy Opt January 2014
2: Hosts that use advanced heuristics to deal with multiple received
policies that are defined outside the scope of this document
SHOULD use Address Selection options.
Implementations MAY provide configuration options to enable this
protocol on a per-interface basis.
Implementations MAY store distributed address selection policies per
interface. They can be used effectively on implementations that
adopt per-application interface selection.
4. 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
SHOULD convert this label to a representation appropriate for the
local implementation (e.g., string).
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.
Network administrators SHOULD consider local limitations to the
maximum DHCPv6 message size that can be reliably transported via
their specific local infrastructure to end nodes; therefore, they
SHOULD consider the number of options, the total size of the
options, and the resulting DHCPv6 message size when defining their
policy table.
5. Security Considerations
A rogue DHCPv6 server could issue bogus 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, incur additional network charges, or be
misdirected to an attacker's machine. 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 communicate through a secure, trusted channel, such as
a channel protected by IPsec, Secure Neighbor Discovery (SEND), and
DHCP authentication, as described in Section 21 of [RFC 3315]. A
commonly used alternative mitigation is to employ DHCP snooping at
Layer 2.
Matsumoto, et al. Standards Track PAGE 7
RFC 7078 DHCPv6 Address Selection Policy Opt January 2014
Another threat surrounds the potential privacy concern as described
in the security considerations section of [RFC 6724], whereby an
attacker can send packets with different source addresses to a
destination to solicit different source addresses in the responses
from that destination. This issue will not be modified by the
introduction of this option, regardless of whether or not the host is
multihomed.
6. IANA Considerations
IANA has assigned option codes to OPTION_ADDRSEL (84) and
OPTION_ADDRSEL_TABLE (85) from the "DHCP Option Codes" registry
(http://www.iana.org/assignments/dhcpv6-parameters/).
7. References
7.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC 6724] Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, September 2012.
7.2. Informative References
[ADDR-SEL] Chown, T., Ed., and A. Matsumoto, Ed., "Considerations for
IPv6 Address Selection Policy Changes", Work in Progress,
April 2013.
[RFC 2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC 3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC 4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC 4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
Matsumoto, et al. Standards Track PAGE 8
RFC 7078 DHCPv6 Address Selection Policy Opt January 2014
[RFC 5220] 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.
[RFC 5221] Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
"Requirements for Address Selection Mechanisms", RFC 5221,
July 2008.
Matsumoto, et al. Standards Track PAGE 9
RFC 7078 DHCPv6 Address Selection Policy Opt January 2014
Appendix A. Acknowledgements
The authors would like to thank to Dave Thaler, Pekka Savola, Remi
Denis-Courmont, Francois-Xavier Le Bail, Ole Troan, Bob Hinden,
Dmitry Anipko, Ray Hunter, Rui Paulo, Brian E. Carpenter, Tom Petch,
and the members of 6man's address selection design team for their
invaluable contributions to this document.
Appendix B. Examples
[RFC 5220] gives several cases where address selection problems
happen. This section contains some examples for solving those cases
by using the DHCP option defined in this text to update the hosts'
policy table in a network, accordingly. There is also some
discussion of example policy tables in Sections 10.3 to 10.7 of RFC
6724.
B.1. Ingress Filtering Problem
In the case described in Section 2.1.2 of [RFC 5220], the following
policy table should be distributed when the Router performs static
routing and directs the default route to ISP1 as per Figure 2. By
putting the same label value to all IPv6 addresses (::/0) and the
local subnet (2001:db8:1000:1::/64), a host picks a source address in
this subnet to send a packet via the default route.
Prefix Precedence Label
::1/128 50 0
::/0 40 1
2001:db8:1000:1::/64 45 1
2001:db8:8000:1::/64 45 14
::ffff:0:0/96 35 4
2002::/16 30 2
2001::/32 5 5
fc00::/7 3 13
::/96 1 3
fec0::/10 1 11
3ffe::/16 1 12
B.2. Half-Closed Network Problem
In the case described in Section 2.1.3 of [RFC 5220], the following
policy table should be distributed. By splitting the closed network
prefix (2001:db8:8000::/36) from all IPv6 addresses (::/0) and giving
different labels, the closed network prefix will only be used when
packets are destined for the closed network.
Matsumoto, et al. Standards Track PAGE 10
RFC 7078 DHCPv6 Address Selection Policy Opt January 2014
Prefix Precedence Label
::1/128 50 0
::/0 40 1
2001:db8:8000::/36 45 14
::ffff:0:0/96 35 4
2002::/16 30 2
2001::/32 5 5
fc00::/7 3 13
::/96 1 3
fec0::/10 1 11
3ffe::/16 1 12
B.3. IPv4 or IPv6 Prioritization
In the case described in Section 2.2.1 of [RFC 5220], the following
policy table should be distributed to prioritize IPv6. This case is
also described in [RFC 6724].
Prefix Precedence Label
::1/128 50 0
::/0 40 1
::ffff:0:0/96 100 4
2002::/16 30 2
2001::/32 5 5
fc00::/7 3 13
::/96 1 3
fec0::/10 1 11
3ffe::/16 1 12
B.4. ULA or Global Prioritization
In the case described in Section 2.2.3 of [RFC 5220], the following
policy table should be distributed, or the Automatic Row Addition
flag should be set to 1. By splitting the Unique Local Address (ULA)
in this site (fc12:3456:789a::/48) from all IPv6 addresses (::/0) and
giving it higher precedence, the ULA will be used to connect to
servers in the same site.
Matsumoto, et al. Standards Track PAGE 11
RFC 7078 DHCPv6 Address Selection Policy Opt January 2014
Prefix Precedence Label
::1/128 50 0
fc12:3456:789a::/48 45 14
::/0 40 1
::ffff:0:0/96 35 4
2002::/16 30 2
2001::/32 5 5
fc00::/7 3 13
::/96 1 3
fec0::/10 1 11
3ffe::/16 1 12
Authors' Addresses
Arifumi Matsumoto
NTT NT Lab
3-9-11 Midori-Cho
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 3334
EMail: arifumi@nttv6.net
Tomohiro Fujisaki
NTT NT Lab
3-9-11 Midori-Cho
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 7351
EMail: fujisaki@nttv6.net
Tim Chown
University of Southampton
Southampton, Hampshire SO17 1BJ
United Kingdom
EMail: tjc@ecs.soton.ac.uk
Matsumoto, et al. Standards Track PAGE 12
RFC TOTAL SIZE: 25094 bytes
PUBLICATION DATE: Wednesday, January 8th, 2014
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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