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dhc Working Group                                               R. Droms
Internet-Draft                                                     Cisco
Intended status: Standards Track                               T. Narten
Expires: September 3, 2009                                           IBM
                                                           March 2, 2009


       Default Router and Prefix Advertisement Options for DHCPv6
              draft-droms-dhc-dhcpv6-default-router-00.txt

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Abstract

   In some IPv6 deployments, there is a requirement to communicate a
   list of default routers and advertised prefixes to a host through



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   DHCP.  This document defines DHCP options to carry that information.


1.  Introduction

   In many IPv6 deployments, particularly in edge networks, end devices
   obtain configuration information about default routers, on-link
   prefixes and addresses from Router Advertisements as defined in
   Neighbor Discovery.  In some deployments, however, there is a strong
   desire not to use Router Advertisements at all and to perform all
   configuration via DHCP [RFC3315].  For example, network
   administration may want to control all host configuration through a
   single centralized DHCP service in order to more closely manage which
   addresses are assigned and in use.  In such an environment, network
   administration may prefer to manage all host configuration aspects
   through a single on-the-wire protocol rather than a combination of
   the Neighbor Discovery (ND) protocol [RFC4861] and DHCP.  In
   addition, most existing IPv4 deployments are already use DHCP
   exclusively for configuration.  In deploying IPv6, an operator may
   wish to continue a DHCP-only configuration approach in order to
   minimize the number of gratuitous operational changes needed to
   deploy IPv6.

   When DHCP was originally defined, it was felt that configuration
   information concerning default routers and prefixes was best learned
   exclusively via Router Advertisements and (unlike DHCPv4) no DHCPv6
   options were defined to carry such information.  This document
   recognizes that there are deployment scenarios where an operator may
   want to disable the use of Router Advertisements completely and rely
   exclusively on DHCP for all configuration.

   This document defines two DHCP options, the Default Router option and
   the Prefix Information option, which carry information a host needs
   to use IPv6 on a link where no information is provided by ND.

   These options are not targeted towards networks characterized by a
   wide variety of end devices running diverse software chosen by end
   users (e.g., laptops, PDAs, etc.).  Rather, the options are targeted
   towards networks that are typically under a single administrative
   control where the operator has significant control over the number
   and kinds of devices that connect to the network.  As an example,
   broadband deployments may include an access network on which all
   devices run software controlled by the operator.


2.  Terminology

   The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,



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   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL in this document are to be
   interpreted as described in RFC 2119 [RFC2119].

   The following terms are used throughout this document:

   o  DHCP - Dynamic Host Configuration Protocol for IPv6 [RFC3315]

   o  ND - Neighbor Discovery protocol [RFC4861]

   Additional terms used in the description of ND and DHCP in this
   documents are defined in RFC 4861 and RFC 3315.


3.  Requirements and Design Considerations

   In planning and deploying IPv6, some network operators have expressed
   the following requirements for operating an IPv6 network:

   o  IPv4 works with a just a single configuration protocol, DHCPv4; it
      should be possible to deploy IPv6 using just one configuration
      protocol.

   o  For consistency and ease of management, DHCPv6 should carry the
      same information and enable the same mode of operation as DHCPv4.

   o  Use of Router Advertisements provides identical configuration of
      default routers and prefixes for all hosts on a link, while some
      deployments require that different classes or groups of hosts be
      configured with different default routers and prefixes.

   o  Misconfigured Router Advertisements immediately cause connectivity
      disruptions and can come from any router as a side effect of other
      changes in configuration or even simple attachment to a link.
      DHCP service is typically provided by a centralized service
      composed of fewer managed components, so DHCP server
      misconfiguration is less likely than delivery of misconfigured
      Router Advertisements.

   In addition to the information currently carried by DHCP, a host
   requires, at a minimum, a default router and an on-link prefix to
   enable IPv6 operation.

   The DHCP options to carry default router and prefix information
   should, wherever possible, reuse the conceptual variables, router
   selection and other mechanisms from ND.  The intention is to allow
   IPv6 implementations to share data structures and code between ND and
   DHCP, treating information received through either source in a
   uniform way.



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   The host's use of the information in the DHCP options should follow
   the model of other DHCP information.  There will only be one DHCP
   server providing default router and prefix information for an
   interface, and the server will provide all of the relevant
   information with each update.


4.  Design Overview and Client Behaviors

   In general, the use of these DHCP options follows the design and use
   of Router Advertisements, as defined in RFC 4861, as closely as
   possible.  Unless otherwise specified, the host uses the information
   from the DHCP Default Router and Prefix Information options in the
   same way as defined for information derived from Router
   Advertisements as defined in RFC 4861.  Some explicit references to
   text in RFC 4861 are included here for clarity.

4.1.  Conceptual Data Structures and Sending Algorithm,

   A host that receives the DHCP Default Router and Prefix Information
   options inserts the information from the option in the Prefix List
   and Default Router list, which are defined in section 5.1 of RFC
   4861.

   A host continues to use the Conceptual Sending Algorithm defined in
   section 5.2 of RFC 4861 when the Prefix List and the Default Router
   list are populated with prefixes and default router information
   derived from DHCP options.

4.2.  Host Specification

   A host that receives the DHCP Default Router and Prefix Information
   options follows the Host Specification defined in section 6.3 of RFC
   4861, with some specific exceptions as defined below.

   The host maintains the Host Variables as defined in section 6.3.2.  A
   host processes information received through the DHCP Default Router
   and Prefix Information options somewhat differently than information
   received through ND, as specified in section 6.3.4 of RFC 4861.  The
   differences are described in the following paragraphs.

   The host takes the Router Address and Router Lifetime from a received
   DHCP Default Router option and processes it as specified in section
   6.3.4 of RFC 4861.  The option is required to contain a valid address
   from the router interface on the link to which the host is connected.

   Any prefixes received in DHCP Prefix Information options are defined
   to be on-link and to be processed as specified in section 6.3.4 of



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   RFC 4861.


5.  Option Formats

   This section defines the information that will be carried in the DHCP
   Default Router and Prefix Information options.  The exact format of
   the options is TBD.

5.1.  DHCP Default Router option

   The DHCP Default Router option carries the following information:

   o  Address of the interface for a default router

   o  Source link-layer address for the interface (opt)

   o  Router lifetime

   Multiple DHCP Default Router options can appear in a DHCP message.  A
   DHCP client indicates that it requires Default Router information by
   including the DHCP Default Router option code in an Option Request
   option send to the server.

5.2.  DHCP Prefix Information option

   The DHCP Prefix Information option carries the following information:

   o  Prefix

   o  Prefix length

   o  Valid lifetime

   o  Preferred lifetime

   Multiple DHCP Prefix Information options can appear in a DHCP
   message.  A DHCP client indicates that it requires Prefix Information
   by including the DHCP Prefix Information option code in an Option
   Request option send to the server.


6.  Discussion and Open Issues

   MTU, Cur Hop Limit, Reachable Time and Retrans Timer can be defined
   in a separate DHCP option(s) if there is demand.

   Timing for renewal of information received through these options is



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   TBD.  There are two alternatives:

   o  Host is responsible for contacting server before any timers such
      as preferred lifetime or router lifetime expire.

   o  Server provides timer values, similar to T1 and T2, in the options

   Default router and prefix information are considered independent and
   one may be received through Router Advertisements while the other is
   received through DHCP.

   Conflicting or overlapping information received through Router
   Advertisements and through DHCP is considered a misconfiguration and
   is out-of-scope of this document.  For example, if a host receives a
   list of default routers through DHCP and a Router Advertisement with
   a non-zero lifetime, the result is undefined.

   Note that the IPv6 stateless address autoconfiguration specification
   [RFC4862] allows a host to initiate a DHCP message exchange if the
   host receives no Router Advertisements.  Therefore, the information
   carried in the Default Router and Prefix Information options allows
   for hosts to use IPv6 on a link where no Router Advertisements are
   transmitted.

   Any prefixes received through Prefix Information options are assumed
   to be on-link.


7.  Security Considerations

   An adversarial DHCP server could use these options to divert IP
   traffic to mount a man-in-the-middle attack.  Mitigation of attacks
   through DHCP is discussed in RFC 3315.


8.  IANA Considerations

   IANA is requested to assign option codes to the Default Router and
   Prefix Information options from the DHCPv6 Option Code registry.


9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.




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   [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.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

9.2.  Informative References

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.


Authors' Addresses

   Ralph Droms
   Cisco
   1414 Massachusetts Avenue
   Boxborough, MA  01719
   USA

   Phone: +1 978.936.1674
   Email: rdroms@cisco.com


   Thomas Narten
   IBM

   Email: narten@us.ibm.com





















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