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RFC 7078 - Distributing Address Selection Policy Using DHCPv6


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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/rfc7078.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   Without obtaining an adequate license from the person(s) controlling
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   than English.

1.  Introduction

   [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.  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 [RFC3484].  As a result, RFC 3484 was
   updated and obsoleted by [RFC6724].  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
   [RFC5221], while solutions are discussed in [ADDR-SEL].  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.

   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 [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.

   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 [RFC6724].  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,

        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 [RFC6724].  If this flag is set to 1, it does not change
        client host behavior; that is, a client will prefer temporary
        addresses [RFC4941].  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 [RFC6724].

       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 [RFC6724] 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 [RFC6724] policy table.

   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 [RFC4291] 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

   [RFC6724] 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.

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.

   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 [RFC3315].  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 [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 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

   [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.

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.

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

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, 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.  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

   [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.

B.1.  Ingress Filtering Problem

   In the case described in Section 2.1.2 of [RFC5220], 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 [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

B.3.  IPv4 or IPv6 Prioritization

   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

B.4.  ULA or Global Prioritization

   In the case described in Section 2.2.3 of [RFC5220], 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.

         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

 

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