Network Working Group                                           R. Droms
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                        December 9, 2011
Expires: June 11, 2012

              Modification to Default Value of SOL_MAX_RT


   This document updates RFC 3315 by redefining the default value for
   SOL_MAX_RT and defining an option through which a DHCPv6 server can
   override the client's default value for SOL_MAX_RT with a new value.

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Internet-Draft              Abbreviated Title              December 2011

1.  Introduction

   Section 5.5 of the DHCPv6 specification [RFC3315] defines the default
   value of SOL_MAX_RT to be 120 seconds.  In some circumstances, this
   default will lead to an unacceptably high volume of aggregated
   traffic at a DHCPv6 server.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Update to RC 3315

   This document changes section 5.5 of RFC 3315 as follows:

      SOL_MAX_RT      120 secs  Max Solicit timeout value

      SOL_MAX_RT     3600 secs  Max Solicit timeout value

   With this change, a DHCPv6 client that does not receive a
   satisfactory response will send Solicit messages with the same
   initial frequency and exponential backoff as specified in RFC 3315.
   However, the long term behavior of these DHCPv6 clients will be to
   send a Solicit message every 3600 seconds rather than every 120
   seconds, significantly reducing the aggregated traffic at the DHCPv6

   The change to SOL_MAX_RT is in response to DHCPv6 message rates
   observed at a DHCPv6 server in a deployment in which many DHCPv6
   clients are sending Solicit messages but the DHCPv6 server has been
   configured not to respond to those Solicit messages.  RFC 3315 was
   written with the expectation that the 'M' and 'O' flags in NDP
   [RFC2461] would control the use of DHCPv6 by hosts.  However, the
   current definition of the 'M' and 'O' flags in RFC 4861 [RFC4861]
   does not explicitly preclude the use of DHCPv6 by a host.  Some
   devices are specified to initiate DHCPv6 even if RAs are received
   with the 'M' and 'O' bits set to 0.  In some circumstances, it is
   desirable to enable the assignment of IPv6 addresses through DHCPv6
   to some nodes on a link but not to others, which cannot be
   implemented through the 'M' and 'O' bits.

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Internet-Draft              Abbreviated Title              December 2011

3.  SOL_MAX_RT option

   A DHCPv6 server sends the SOL_MAX_RT option to a client to override
   the default value of SOL_MAX_RT.  One use for the SOL_MAX_RT option
   is to set a longer value for SOL_MAX_RT, which reduces the Solicit
   traffic from a client that has not received any IPv6 addresses.

   The format of the SOL_MAX_RT option is:

       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-code          |         option-len            |
      |                       SOL_MAX_RT value                        |

        option-code          OPTION_SOL_MAX_RT (TBD).

        option-len           4.

        SOL_MAX_RT value     Overriding value for SOL_MAX_RT in seconds.

                                 Figure 1

   If the DHCPv6 server declines to assign any addresses to a client in
   an IA_NA or IA_TA option, it MAY include a SOL_MAX_RT option in the
   appropriate options field along with a Status Code option indicating

   If a DHCPv6 client receives an IA_NA or IA_TA option containing a
   SOL_MAX_RT option, the client MUST set its internal SOL_MAX_RT
   parameter to the value contained in the SOL_MAX_RT option.

4.  Security Considerations

   This document introduces one security considerations beyond those
   described in RFC 3315.  A malicious DHCPv6 server might cause a
   client to set its SOL_MAX_RT parameter to an arbitrarily high value
   with the SOL_MAX_RT option.  Assuming the client also receives a
   response from a valid DHCPv6 server, the large value for SOL_MAX_RT
   will not have any effect.

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Internet-Draft              Abbreviated Title              December 2011

5.  IANA Considerations

   IANA is requested to assign an options code from the "DHCP Option
   Codes" Registry for OPTION_SOL_MAX_RT.

6.  Normative References

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

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

   [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.

Author's Address

   Ralph Droms
   Cisco Systems
   1414 Massachusetts Avenue
   Boxborough, MA  01719

   Phone: +1 978 936 1674

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