6man Working Group                                          A. Matsumoto
Internet-Draft                                               T. Fujisaki
Intended status: Standards Track                                 J. Kato                                     NTT
Expires: January 17, February 24, 2013                                            NTT                                      T. Chown
                                               University of Southampton
                                                           July 16,
                                                         August 23, 2012

           Distributing Address Selection Policy using DHCPv6


   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 parameters and policy table, and thus
   control the address selection behavior of nodes in their site.

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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

   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-rfc3484bis] 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",
   document are to be interpreted as described in [RFC2119].

1.2.  Terminology

   This document uses the terminology defined in [RFC2460] and the
   DHCPv6 specification defined in [RFC3315]

2.  Address Selection options

   The Address Selection option provides the address selection policy
   table, and some other configuration parameters.

   A address selection option contains zero or more policy table
   options.  Multiple policy table options in a Policy Table option
   constitute a single policy table.

   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 (TBD).

   option-len:  The total length of the Reserved field, A, P flags, and
        POLICY TABLE OPITONS in octets.

   Reserved:  Reserved field.  Server MUST set this value to zero and
        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 the
        section 2.1 in RFC 3484 revision [I-D.ietf-6man-rfc3484bis].  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 MAY NOT automatically
        add rows to the policy table.

   P:   Privacy Preference flag.  This flag toggles the Privacy
        Preference flag at client hosts, which is described in the
        section 5 in RFC 3484 revision [I-D.ietf-6man-rfc3484bis].  If
        this flag is set to 1, it does not change client host behavior,
        that is, a client SHOULD prefer temporary addresses.  If set to
        0, the Privacy Preference flag is disabled, and a client SHOULD
        prefer public addresses.

   POLICY TABLE OPTIONS:  Zero or more Address Selection Policy Table
        options described 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_TABLE      |         option-len            |
      |    label      |  precedence   |   prefix-len  |               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
      |                                                               |
      |                   prefix   (variable length)                  |
      |                                                               |
      |                                                               |
      .                    Prefix Specific options                    .
      .                                                               .

            Figure 2: Address Selection Policy Table option format

   option-code:  OPTION_ADDRSEL_TABLE (TBD).

   option-len:  The total length of the label field, precedence field,
        prefix-len field, prefix field, and DASP options field in

   label:  An 8-bit unsigned integer; this value is used to make a
        combination of source address prefixes and destination address

   precedence:  An 8-bit unsigned integer; this value is used for
        sorting destination 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.

   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.

   Prefix Specific options:  Options specific to this particular Address
        Selection Policy option.  This includes, but not limited to,
        zero or one Zone Index option that specify the zone index of the
        prefix in this option.

   The format of the Zone Index 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_ZONE        |         option-len          |
      |                          zone-index                           |

                 Figure 3: Zone Index option format

   option-code:  OPTION_ADDRSEL_ZONE (TBD).

   option-len:  4.

   zone-index:  The zone-index field is an 32-bit unsigned integer, and
        used to specify the zone for scoped addresses.  The zone-index
        is defined in RFC 3493 [RFC3493] as 'scope ID'.

3.  Appearance of the Address Selection options

   The Address Selection options MUST NOT appear in any messages other
   than the following ones: Solicit, Advertise, Request, Renew, Rebind,
   Reconfigure, Information-Request, and Reply.

4.  Processing the Policy Table option

   This section describes how to process received Policy Table 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.  So, basically, it should be up to the node's
   administrator how to deal with the received policy information in the
   way described below.

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 implementation SHOULD provide the following choices to the

   a) It receives distributed policy table, and replaces the existing
      policy tables with that.
   b) It preserves the default policy table, or manually configured

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 policies  Multi-interface situation

   The policy table, and other parameters specified in this document are
   node-global information by its nature.  So, the node cannot use
   multiple received policies at the same time.  In other words, once
   the received policy from one source is merged with one from another
   source, the effect of both policy is are more or less changed.  The
   policy table is defined as a whole, so the slightest addition/deletion addition/
   deletion from the policy table brings a change in semantics of the

   It also should be noted that, when a node is single-homed and has
   only one upstream line, adopting a received that absence of the distributed policy table does from a
   certain network interface should not
   degrade be treated as absence of policy
   itself, because it may mean preference for the security level. default address
   selection policy.

   Under the above assumptions, how to handle multiple received policies policy is
   specified below.

   A node MAY use Address Selection options by default in any of the
   following two cases:

   1: The Address Selection option host is delivered across single-homed, where the only secure,
      trusted channel. host belongs to one
      administrative network domain exclusively usually through one
      active network interface.
   2: The Address Selection option delivery is not secured, but the node host implements some advanced heuristics to deal with multiple
      received policy, which is single-homed.

   In other cases the node MUST NOT use Policy Table options unless outside the
   node is specifically configured to do so.

   Discussion: scope of this document.

   The secure trusted channel does above restrictions do not necessarily mean a
      prioritized route in the routing table.  So, such a situation
      could happen that the traffic goes through a non-secure, non-
      trusted channel and the host follows the delivered policy preclude implementations from a
      secure, truested channel.  However, providing
   configuration options to enable this policy is not for
      optimization of traffic and resources at the local network and the
      hosts, but for implementing the option on a certain network policy to the hosts in the

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

   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
   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_ADDRSEL ,
   space as defined in section "DHCPv6 Options" of RFC 3315.

8.  References

8.1.  Normative References

              Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol version 6
              (IPv6)", draft-ietf-6man-rfc3484bis-06 (work in progress),
              June 2012.

              Gont, F., "A method for Generating Stable Privacy-Enhanced
              Addresses with IPv6 Stateless Address Autoconfiguration
              (SLAAC)", draft-ietf-6man-stable-privacy-addresses-00
              (work in progress), May 2012.

   [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

              Chown, T. and A. Matsumoto, "Considerations for IPv6
              Address Selection Policy Changes",
              draft-ietf-6man-addr-select-considerations-04 (work in
              progress), October 2011.

              Matsumoto, A., Fujisaki, T., and R. Hiromi, "Solution
              approaches for address-selection problems",
              draft-ietf-6man-addr-select-sol-03 (work in progress),
              March 2010.

   [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.

   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 NT Lab
   3-9-11 Midori-Cho
   Musashino-shi, Tokyo  180-8585

   Phone: +81 422 59 3334
   Email: arifumi@nttv6.net

   Tomohiro Fujisaki
   NTT PF NT Lab
   3-9-11 Midori-Cho
   Musashino-shi, Tokyo  180-8585

   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

   Phone: +81 422 59 2939
   Email: kato@syce.net
   Tim Chown
   University of Southampton
   Southampton, Hampshire  SO17 1BJ
   United Kingdom

   Email: tjc@ecs.soton.ac.uk