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rfc:rfc7078

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.
 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

 [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].

Matsumoto, et al. Standards Track [Page 2] RFC 7078 DHCPv6 Address Selection Policy Opt January 2014

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,

Matsumoto, et al. Standards Track [Page 3] 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 [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.

Matsumoto, et al. Standards Track [Page 4] 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 [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.

Matsumoto, et al. Standards Track [Page 5] 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.

Matsumoto, et al. Standards Track [Page 6] 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 [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.

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 [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.

Matsumoto, et al. Standards Track [Page 8] RFC 7078 DHCPv6 Address Selection Policy Opt January 2014

 [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.

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

 [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.

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 [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.

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]

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