| 6man Working Group | A.M. Matsumoto |
| Internet-Draft | T.F. Fujisaki |
| Intended status: Standards Track | NTT |
| Expires: February 08, 2014 | T.C. Chown |
| University of Southampton | |
| August 07, 2013 |
Distributing Address Selection Policy using DHCPv6
draft-ietf-6man-addr-select-opt-11.txt
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 to control the address selection behavior of nodes in their site.
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[RFC6724] 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 6724, 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 were identified with the default address selection policy as originally defined in [RFC3484]. As a result, RFC 3484 was updated and obsoleted by [RFC6724]. While this update corrected a number of issues identifed 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 [RFC5221], while solutions are discussed in [I-D.ietf-6man-addr-select-considerations]. Those documents have helped shape the improvements in the default address selection algorithm in [RFC6724] as well as the requirements for the DHCPv6 option defined in this specification.
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].
This document uses the terminology defined in [RFC2460] and the DHCPv6 specification defined in [RFC3315]
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.
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
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
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.
[RFC6724] defines two flags (A, 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.
Choice (a) SHOULD be the default, i.e. that the policy table is not explictly configured by the user.
When the information from the DHCP server goes stale, the policy received from the DHCP server SHOULD be deprecated.
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, and the Information Refresh Time is expired.
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:
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.
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, SEND and DHCP authentication, as described in section 21 of [RFC3315]. A commonly used alternative mitigation is to employ DHCP snooping at Layer 2.
Another threat surrounds the potential privacy concern as described in the security considerations section of [RFC6724], 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 the host is multihomed or not.
IANA is requested to assign option codes to OPTION_ADDRSEL and OPTION_ADDRSEL_TABLE from the "DHCP Option Codes" registry (http://www.iana.org/assignments/dhcpv6-parameters/dhcpv6-parameters.xml).
| [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. |
| [RFC6724] | Thaler, D., Draves, R., Matsumoto, A. and T. Chown, "Default Address Selection for Internet Protocol Version 6 (IPv6)", RFC 6724, September 2012. |
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.
[RFC5220] 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.
In the case described in section 2.1.2 of [RFC5220], the following policy table should be distributed, when 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
In the case described in section 2.1.3 of [RFC5220], 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.
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
In the case described in section 2.2.1 of [RFC5220], the following policy table should be distributed to prioritize IPv6. This case is also described in [RFC6724]
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
In the case described in section 2.2.3 of [RFC5220], the following policy table should be distributed, or Automatic Row Addition flag should be set to 1. By splitting the ULA in this site (fc12:3456:789a::/48) from all IPv6 addresses (::/0) and giving it higher precendence, the ULA will be used to connect to servers in the same site.
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