DHC Working Group                                            T. Fujisaki
Internet-Draft                                              A. Matsumoto
Expires: December 15, 2006                                       J. Kato
                                                              S. Niinobe
                                                                     NTT
                                                           June 13, 2006


       Distributing Default Address Selection Policy using DHCPv6
               draft-fujisaki-dhc-addr-select-opt-02.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes a new DHCPv6 option for distributing default
   address selection policy information defined in RFC3484 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.  However, there are some
   problems with the default address selection policy in RFC3484
   [ID.arifumi-v6ops-addr-select-ps], and administrators can change the
   node's address selection behavior by distributing the policy.
   Practical usages are described in [ID.arifumi-ipv6-policy-dist].
   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.



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   Prefix: A variable-length field containing an IP address or the
        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



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      this value in general case.

   o  We also proposed a policy distribution option using a Router
      Advertisement message defined in RFC2461 [RFC2461].  There was a
      discussion that using DHCPv6 was more suitable to distribute a
      selection policy, because such policy should be distributed under
      the site administrator's centralized control.

   o  There may be some demands to control the use of temporary
      addresses described in RFC3041 [RFC3041] (e.g., informing not to
      use a temporary address when it communicate within the an
      organization's network).  Since a temporary address cannot
      represent as an IPv6 address and its prefix, some semantics to
      specify the temporary address will be necessary to control it
      (such as a flag to indicate a temporary address or a special
      representation for temporary address in prefix field).



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.  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.


Appendix A.  RFC3484 implementation status

   Today, many operating systems implement address selection mechanism
   defined in RFC3484.  Many of them, however, implement the
   specification partially.  We summarize current implementation status
   of RFC 3484 at http://www.nttv6.net/dass/.

9.  References

   [ID.arifumi-ipv6-policy-dist]
              Matsumoto, A., Fujisaki, T., and J. Kato, "Practical



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              Usages of Default Address Selection Policy Distribution",
              draft-arifumi-ipv6-policy-dist-01.txt (Work In Progress)
              (work in progress), June 2006.

   [ID.arifumi-v6ops-addr-select-ps]
              Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
              "Problem Statement of Default Address Selection in Multi-
              prefix Environment: Operational Issues of RFC3484 Default
              Rules", draft-arifumi-v6ops-addr-select-ps-00.txt (Work In
              Progress) (work in progress), June 2006.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461,
              December 1998.

   [RFC3041]  Narten, T. and R. Draves, "Privacy Extensions for
              Stateless Address Autoconfiguration in IPv6", RFC 3041,
              January 2001.

   [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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