Internet DRAFT - draft-wing-dhc-dns-reconfigure


DHC Working Group                                               P. Patil
Internet-Draft                                                     Cisco
Intended status: Standards Track                            M. Boucadair
Expires: March 17, 2014                                   France Telecom
                                                                 D. Wing
                                                                T. Reddy
                                                      September 13, 2013

                     DHCPv6 Dynamic Reconfiguration


   This specification extends DHCPv6 so that a DHCPv6 Relay Agent can
   dynamically indicate end host connectivity to a DHCPv6 Server.  This
   information is also triggered by any change in connectivity type
   provided to the host.  The DHCPv6 server uses this information as an
   input to its decision-making about configuration parameters to be
   conveyed to that host.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on March 17, 2014.

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   ( in effect on the date of
   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Problem Statement: Focus on DNS Reconfiguration . . . . . . .   3
   4.  Host Connectivity Status Option . . . . . . . . . . . . . . .   4
   5.  DHCPv6 Relay Agent Behavior . . . . . . . . . . . . . . . . .   5
     5.1.  Relay Forward . . . . . . . . . . . . . . . . . . . . . .   5
     5.2.  Reconfigure Request . . . . . . . . . . . . . . . . . . .   6
   6.  DHCPv6 Server Behavior  . . . . . . . . . . . . . . . . . . .   6
     6.1.  Relay Forward . . . . . . . . . . . . . . . . . . . . . .   6
     6.2.  Reconfigure Request . . . . . . . . . . . . . . . . . . .   6
   7.  Host Tracking . . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Some networks are expected to support IPv4-only, dual-stack, and
   IPv6-only hosts at the same time.  Due to devices capabilities and
   available connectivity types, providing generic configuration from a
   DHCP server to connected hosts is sub-optimal in most cases, and may
   even break functionality in some cases.  Network infrastructure is
   usually well equipped to be aware of single/dual-stack nature of
   hosts.  The network can also track and detect transitions from single
   to dual-stack or vice-versa.

   This specification describes a DHCPv6 extension for relay agents to
   indicate host characteristics pertaining to host connectivity to
   DHCPv6 servers.  The information passed by a relay is generic and a
   DHCPv6 server can interpret and process this information to make a
   more informed decision on the configuration parameters that a client
   is to receive.

   The DHCPv6 server can either be configured or have built-in logic to
   use this information as desired, which is outside the scope of this

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   Section 3 describes a typical problem that can be addressed using the
   mechanism described in this specification.  A DHCPv6 server makes a
   decision on priority of DNS servers to be sent back to the client
   based on host connectivity characteristics provided by the relay

   While the host stack can be upgraded to send this information to the
   DHCPv6 server on its own, a generalized upgrade of all DHCPv6 client
   implementations on all operating systems is extremely difficult.

      [DISCUSSION NOTE: A companion solution could be to define a
      container that can be used to return per-AF specific configuration
      parameters to the client.  In such a scheme, the server blindly
      returns all pieces of configuration and it is up to the client to
      make use of appropriate set of parameters according to its
      available connectivity.  This alternative assumes an update of
      dhcp client.  This approach can be seen as complimentary to the
      one defined in this specification.  The document will be updated
      to reflect consensus of the WG on whether the additional option is
      to be specified.]

2.  Terminology

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

   Dual-Stack host: Denotes a host that is configured with both an IPv4
   address and IPv6 prefix and is reachable using both IPv4 and IPv6

3.  Problem Statement: Focus on DNS Reconfiguration

   Default address selection rules specified in [RFC6724] prefers IPv6
   over IPv4.  If a dual-stack host is configured to use a DNS64 server
   [RFC6147], it will send its DNS queries to that DNS64 server which
   will synthesize a AAAA response if no A record is found.  Thus, the
   dual-stack host will always use IPv6 if a DNS lookup was involved,
   even if IPv4 could have been used more optimally.

   In some deployments, if NAT44 [RFC3022] and NAT64 [RFC6146] are
   deployed on the same network, it is preferable to use NAT44 over
   NAT64 because of scale, performance and application incompatibility
   issues (e.g., FTP) [RFC6384].  At the same time, native IPv6 can
   still be preferred over IPv4.

   A DHCPv6 Relay Agent can observe host characteristics on a network to
   determine if a host is IPv4-only, dual-stack or IPv6-only and also

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   detect transitions from single to dual-stack or vice-versa.  This
   information can be used by the DHCPv6 Relay Agent to influence the
   DHCPv6 Server to send appropriately prioritized DNS Servers to the
   client.  The DHCPv6 server can implement the following based on
   connectivity information received from the relay agent.

   o  IPv6-only transition to Dual-Stack: In case a host is IPv6-only,
      it is provided with a DNS64 server.  When transitioning to dual-
      stack, an IPv4 DNS server is assigned as a consequence of
      obtaining an IPv4 Address.  The DHCPv6 Relay Agent can detect this
      and send a RECONFIGURE_REQUEST message [RFC6977] to the DHCPv6
      Server indicating that the host needs to be provided with a
      regular DNS server.  In lieu of this mechanism, the host would
      continue to use the DNS64 server until the host stack

   o  Dual-Stack to IPv6-only: In case a host is dual-stack, it is
      provided with a regular DNS server followed by DNS64 server.  When
      transitioning to IPv6-only, the DHCPv6 Relay Agent can detect this
      change and send a RECONFIGURE_REQUEST message to the DHCPv6 server
      indicating that the host needs to be assigned a DNS64 server only.
      In lieu of this mechanism, the host would continue to use the
      regular DNS Server which is inaccessible and eventually time out
      to fail over to the DNS64 Server.  The host will take additional
      time to fully initialize causing delays in connection.

4.  Host Connectivity Status Option

   The option (Figure 1) includes an 8-bit status code that indicates
   specific host connectivity characteristics.  The option can be
   included by a DHCPv6 Relay Agent in RELAY-FORW and RECONFIGURE-

       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_HOST_CONNECTIVITY    |          option-len           |
       |    status     |                                               |

       option-code   OPTION_HOST_CONNECTIVITY (TBA).
       option-len    1.
       status        8-bit integer value carrying the connectivity status
                     of a host. The following codes are defined:
                     |Value| Name               |

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                     |  1  | IPv4_TO_DUAL_STACK |
                     |  2  | IPv6_TO_DUAL_STACK |
                     |  3  | DUAL_STACK_TO_IPv4 |
                     |  4  | DUAL_STACK_TO_IPv6 |

       Figure 1: Relay Agent Host Connectivity Option message format

   o  IPv4_TO_DUAL_STACK: Host is transitioning from IPv4-Only to Dual-
      Stack mode.

   o  IPv6_TO_DUAL_STACK: Host is transitioning from IPv6-Only to Dual-
      Stack mode.

   o  DUAL_STACK_TO_IPv4: Host is transitioning from Dual-Stack to
      IPv4-Only mode.

   o  DUAL_STACK_TO_IPv6: Host is transitioning from Dual-Stack to
      IPv6-Only mode.

5.  DHCPv6 Relay Agent Behavior

   DHCPv6 relay agents that implement this specification MUST be
   configurable for tracking host connectivity and inserting the

   To be able to notify details of hosts' connectivity, a relay agent
   must be able to track host connectivity.  A Relay Agent can detect
   host connectivity type using mechanisms discussed in Section 7.  The
   Relay Agent then includes this information in the appropriate DHCPv6

   Relay agents need to maintain connectivity state of each host it can
   track.  This ensures that notifications to the DHCPv6 server,
   especially DHCPv6 RECONFIGURE_REQUEST, are accurately sent when there
   is a change in status.  If a relay agent loses state due to some
   reason (e.g., during restart events), it will build state again using
   the mechanisms described in Section 7 and then send appropriate
   notifications to the server.  Such notifications are redundant and a
   DHCPv6 Server can choose to ignore such redundant notifications from
   the relay agent.  Redundant notifications are also possible when
   relay agents are deployed in fault tolerant mode.

5.1.  Relay Forward

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   DHCPv6 relay agents that implement this specification MAY include the
   option OPTION_HOST_CONNECTIVITY in the RELAY_FORW to indicate status
   of host connectivity.

5.2.  Reconfigure Request

   DHCPv6 relay agents that implement this specification MUST be
   configurable for sending the RECONFIGURE_REQUEST message.  The relay
   agent generates a Reconfigure-Request [RFC6977] anytime status of
   host connectivity changes by including OPTION_HOST_CONNECTIVITY in
   the request.

6.  DHCPv6 Server Behavior

   A DHCPv6 Server that supports OPTION_HOST_CONNECTIVITY may either
   have specific configuration or built-in logic to process information
   available in the option and send configuration parameters in DHCPv6
   responses.  How the server consumes and acts on the information
   obtained in the option are outside the scope of this document.

   The DHCPv6 server may use this connectivity information, if
   available, in addition to other relay agent option data, other
   options included in the DHCPv6 client messages, server configuration,
   and physical network topology information in order to assign
   appropriate configuration to the client.

   The server MUST ignore the option if it doesn't recognize the status
   in the OPTION_HOST_CONNECTIVITY option.  The server SHOULD maintain
   the latest status received from the relay agent.  The server can use
   this state to match against subsequent notifications and only further
   process if there is change in status.  A relay agent could, for
   reasons such as restart, fault-tolerant mode etc, send redundant
   notifications and matching of status at the server will avoid
   unnecessary processing and message exchanges.

6.1.  Relay Forward

   Upon receiving a RELAY-FORW message containing
   OPTION_HOST_CONNECTIVITY, the server can send appropriate
   configuration in the RELAY-REPLY response.  The server MUST NOT
   return this option in a RELAY-REPLY message.

6.2.  Reconfigure Request

   Upon receiving a RECONIFURE-REQUEST message containing an
   OPTION_HOST_CONNECTIVITY option, the server MUST follow the mechanism
   described in [RFC6977] to create and send Reconfigure message.  The
   server MUST NOT return this option in a RECONFIGURE-REPLY message.

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7.  Host Tracking

   Relay Agents can actively keep track of all IPv4/IPv6 addresses and
   associated lease times assigned to hosts via the respective DHCP
   servers.  Relay Agents can therefore detect transitions from single
   to dual-stack and vice-versa efficiently.  In addition to this
   technique, relay agents closest to the client can detect transitions
   using snooping mechanisms.  Network devices today use mechanisms such
   as ARP and NDP snooping (bindings learnt by snooping all NDP traffic,
   NS, NA, RS, RA) to determine host characteristics such as IPv4/IPv6 -
   MAC - DUID bindings.  IPv4/IPv6 and MAC counters are also used to
   determine host liveliness.

   First hop devices that implement first hop security also track IP
   address bindings and determine binding updates such as temporary
   addresses, deprecated addresses, etc.  Existing work such as
   [I-D.ietf-savi-dhcp] and [I-D.levy-abegnoli-savi-plbt] also aim to
   active current host bindings, all of which can be leveraged to track
   host addresses.

   These mechanisms help determine if a particular IP address family is
   inactive, has reverted to using a single stack even though it
   initially had dual-stack capabilities and detect active dual-stack
   usage after long periods of single-stack activity.

   Other techniques to track host connectivity can be envisaged.  It is
   out of scope of this document to provide an exhaustive list of host
   tracking techniques.

8.  Security Considerations

   This document describes an application of the mechanism specified in
   [RFC6977].  Host tracking mechanisms MUST be reliable.  If a relay is
   compromised, it may be used to force an uncompromised server abuse
   clients by triggering repetitive reconfigurations.  Security
   considerations described in [RFC6977] are applicable to this

9.  IANA Considerations

   IANA is requested to assign the following new DHCPv6 Option Code in
   the registry maintained in


10.  References

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10.1.  Normative References

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

   [RFC6384]  van Beijnum, I., "An FTP Application Layer Gateway (ALG)
              for IPv6-to-IPv4 Translation", RFC 6384, October 2011.

   [RFC6724]  Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, September 2012.

   [RFC6977]  Boucadair, M. and X. Pougnard, "Triggering DHCPv6
              Reconfiguration from Relay Agents", RFC 6977, July 2013.

10.2.  Informative References

              Bi, J., Wu, J., Yao, G., and F. Baker, "SAVI Solution for
              DHCP", draft-ietf-savi-dhcp-18 (work in progress), June

              Levy-Abegnoli, E., "Preference Level based Binding Table",
              draft-levy-abegnoli-savi-plbt-02 (work in progress), March

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022, January

   [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.

   [RFC3646]  Droms, R., "DNS Configuration options for Dynamic Host
              Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
              December 2003.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, April 2011.

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              April 2011.

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Authors' Addresses

   Prashanth Patil
   Cisco Systems, Inc.


   Mohamed Boucadair
   France Telecom
   Rennes  35000


   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, California  95134


   Tirumaleswar Reddy
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103


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