DHCPv6-PD for NEMO: DHCPv6 Prefix Delegation for Network Mobility (NEMO)

Internet Engineering Task Force (IETF)                          R. Droms
Request for Comments: 6276                                    P. Thubert
Category: Standards Track                                          Cisco
ISSN: 2070-1721                                                F. Dupont
                                             Internet Systems Consortium
                                                               W. Haddad
                                                                Ericsson
                                                            C. Bernardos
                                                                    UC3M
                                                               July 2011


          DHCPv6 Prefix Delegation for Network Mobility (NEMO)

Abstract

   One aspect of network mobility support is the assignment of a prefix
   or prefixes to a mobile router for use on the links in the mobile
   network.  This document specifies how DHCPv6 prefix delegation can be
   used for this configuration task.  The mobile router plays the role
   of requesting router, while the home agent assumes the role of
   delegating router.  When the mobile router is outside its home
   network, the mobile router also assumes the role of DHCPv6 relay
   agent, co-located with the requesting router function.

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/rfc6276.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  DHCPv6 Prefix Delegation of Mobile Network Prefixes  . . . . .  4
     3.1.  Exchanging DHCPv6 Messages When the Mobile Router Is
           Not at Home  . . . . . . . . . . . . . . . . . . . . . . .  5
       3.1.1.  Relay Agent Configuration  . . . . . . . . . . . . . .  7
       3.1.2.  Transmission of DHCPv6 Messages  . . . . . . . . . . .  8
       3.1.3.  Receipt of DHCPv6 Messages . . . . . . . . . . . . . .  8
     3.2.  Exchanging DHCPv6 Messages When the Mobile Router Is
           at Home  . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  Selecting a Home Agent That Provides DHCPv6PD  . . . . . .  9
     3.4.  Minimizing DHCPv6PD Messages . . . . . . . . . . . . . . . 10
     3.5.  Other DHCPv6 Functions . . . . . . . . . . . . . . . . . . 10
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   5.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 13

1.  Introduction

   One aspect of network mobility support is the assignment of a prefix
   or prefixes to a mobile router for use on the links in Network
   Mobility (NEMO).  DHCPv6 prefix delegation (DHCPv6PD) [RFC3633] can
   be used for this configuration task.

   The model of operation of DHCPv6PD for prefix delegation is as
   follows [RFC3633].  A delegating router is provided IPv6 prefixes to
   be delegated to requesting routers.  A requesting router requests
   prefix(es) from the delegating router.  The delegating router chooses
   prefix(es) for delegation, and responds with prefix(es) to the
   requesting router.  The requesting router is then responsible for the
   delegated prefix(es).  Note that DHCPv6 options for prefix delegation
   defined in [RFC3633] have been defined for general use across
   routers, and not only for mobile routers running the NEMO Basic
   Support protocol [RFC3963].

   To use DHCPv6PD as a prefix assignment mechanism in mobile networks,
   when the mobile router is located at home, the home agent assumes the
   role of the delegating router and the mobile router assumes the role
   of the requesting router.  However, when the mobile router is away
   from home, in addition to the roles when the mobile router is located
   at home, the mobile router also assumes the role of a DHCPv6 relay
   agent co-located with the requesting router function.

   The DHCPv6PD server running at the home agent is provisioned with
   prefixes to be assigned using any of the prefix assignment mechanisms
   described in the DHCPv6PD specification [RFC3633].

2.  Terminology

   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 RFC 2119 [RFC2119].

   The following terms used in this document are defined in the IPv6
   Addressing Architecture document [RFC4291]:

      Link-Local Unicast address

      Link-Local Scope Multicast address

   The following terms used in this document are defined in the Mobile
   IPv6 specification [RFC6275]:

      Home Agent (HA)

      Home Link

      Home Address (HoA)

      Care-of Address (CoA)

      Binding Update (BU)

      Binding Acknowledgement (BA)

   The following terms used in this document are defined in the Mobile
   Network terminology document [RFC4885]:

      Mobile Router (MR)

      Mobile Network (NEMO)

      Mobile Network Prefix (MNP)
   The following terms used in this document are defined in the DHCPv6
   [RFC3315] and DHCPv6 prefix delegation [RFC3633] specifications:

      Delegating Router (DR; acts as a DHCPv6 server)

      Requesting Router (RR; acts as a DHCPv6 client)

      DHCPv6 Relay Agent (DRA)

   The following acronym is used in this document:

      DHCPv6PD: DHCPv6 Prefix Delegation

3.  DHCPv6 Prefix Delegation of Mobile Network Prefixes

   The NEMO Basic Support protocol [RFC3963] extends the Mobile IPv6
   protocol [RFC6275] to enable network mobility.  With the NEMO Basic
   Support protocol, a mobile router uses Mobile IPv6 to establish and
   maintain a session with its home agent and uses bidirectional
   tunneling between the mobile router and the home agent to provide a
   path through which nodes attached to links in the mobile network can
   maintain connectivity with nodes not in the NEMO.

   The requirements for Network Mobility [RFC4885] include the ability
   of the mobile router to receive delegated prefixes that can then be
   assigned to links in the mobile network.  DHCPv6PD can be used to
   meet this requirement for prefix delegation.

   To use DHCPv6PD for mobile networks, when the mobile router is
   located at home, the home agent assumes the role of the delegating
   router and the mobile router assumes the role of the requesting
   router.  However, when the mobile router is away from home, in
   addition to the roles when the mobile router is located at home, the
   mobile router also assumes the role of a DHCPv6 relay agent
   co-located with the requesting router function.

   When the mobile router is not at home, the home agent and the mobile
   router exchange DHCPv6PD protocol messages as specified in [RFC6275].
   This means that the messages sent by the mobile router MUST include
   the Home Address destination option and messages sent by the home
   agent MUST make use of a Routing Header type 2.  See Figure 1 for the
   deployment topologies when the MR is at home and when it is visiting
   a foreign network.

                  ------                ------
                  | MR |----------------| HA |
                  |(RR)| (home network) |(DR)|
                  ------                ------

              -------    /-----------\     ------
              | MR  |----|  Internet |-----| HA |
              |(RR) |    \-----------/     |(DR)|
              |(DRA)|                      ------
              -------
         (visited network)

   Figure 1: Deployment topologies of the use of DHCPv6PD for delegation
                        of Mobile Network Prefixes

   The DHCPv6PD server is provisioned with prefixes to be assigned using
   any of the prefix assignment mechanisms described in the DHCPv6PD
   specifications.  Other updates to the home agent data structures
   required as a side effect of prefix delegation are specified by the
   particular network mobility protocol.  For example, in the case of
   NEMO Basic Network Mobility Support [RFC3963], the HA would add an
   entry in its binding cache registering the delegated prefix to the
   mobile router to which the prefix was delegated.

3.1.  Exchanging DHCPv6 Messages When the Mobile Router Is Not at Home

   The case when the mobile router is away from home is described in
   this section.  Section 3.2 describes the protocol operation for the
   case when the mobile router is attached to its home link.

   The mobile router MUST register at the home agent (i.e., by sending a
   Binding Update to the home agent) before initiating a DHCPv6 message
   exchange for prefix delegation.  The mobile router MUST use implicit
   BU signaling, since the mobile router may not have yet requested any
   prefixes.

   If the mobile router does not have any active delegated prefixes
   (with unexpired leases), the mobile router MUST initiate a DHCPv6
   message exchange with a DHCPv6 Solicit message as described in
   Section 17 of [RFC3315] and Section 11.1 of [RFC3633].  The
   delegating router at the home agent responds with an Advertise
   message.  Then, the mobile router MUST request a set of prefixes by
   sending a Request message.  The delegating router includes the
   delegated prefixes in a Reply message.  Note that in this case, the
   mobile router has previously sent a Binding Update to the home agent
   without knowing yet the set of prefixes that it can use as mobile
   network prefixes.  The home agent, upon reception of the implicit
   Binding Update from the mobile router, MUST select (in case this was
   not pre-configured already) the prefixes that would then be delegated
   to the mobile router via DHCPv6PD.  The home agent, once the DHCPv6
   signaling has been completed, MUST add an entry in its binding cache
   including the delegated prefixes.

   In case the mobile router has one or more active delegated prefixes
   -- for example, as if the mobile router reboots or the mobile network
   prefix(es) currently used by the mobile router is about to expire --
   the mobile router MUST initiate a DHCPv6 message exchange with a
   DHCPv6 Rebind message as described in Section 18.1.2 of [RFC3315] and
   Section 12.1 of [RFC3633].

   A DHPCv6 relay agent function [RFC3315] MUST be used at the mobile
   router.  This relay agent function is co-located in the mobile router
   with the DHCPv6 client function (see Figure 2).  The DHCPv6 signaling
   between the mobile router and the home agent is exchanged between the
   DHCPv6 relay agent in the mobile router and the DHCPv6 server on the
   home agent.  DHCPv6 messages from the mobile router to the home agent
   are unicast packets sent from the unicast home address of the mobile
   router to the global unicast address of the home agent, and therefore
   the Home Address destination option MUST be used.  DHCPv6 replies
   from the home agent to the mobile router MUST be sent using the
   Routing Header type 2, as specified in [RFC6275].  The DHCPv6 client
   in the mobile router MUST hand any outbound DHCPv6 messages to the
   co-located relay agent.  Responses from the DHCPv6 server are
   delivered to the relay agent function in the mobile router, which
   MUST extract the encapsulated message and deliver it to the DHCPv6
   client in the mobile router.

     -----------------------------                  --------
     |            MR             |                  |  HA  |
     | (RR)                (DRA) |                  | (DR) |
     ----------------------------                   --------
         |                   |       Binding Update    |
         |                   |------------------------>|
         |                   |       (HoA, CoA)        |
         |                   |                         |
         |                   |       Binding Ack       |
         |                   |<------------------------|
         |                   |                         |
         | DHCPv6 Solicit    |   DHCPv6 Solicit        |
         |..................>|--=====================->|
         |                   |                         |
         |  DHCPv6 Advertise |       DHCPv6 Advertise  |
         |<..................|<-=====================--|
         |                   |                         |
         | DHCPv6 Request    |       DHCPv6 Request    |
         |..................>|--=====================->|
         |                   |                         |
         |      DHCPv6 Reply |       DHCPv6 Reply      |
         |<..................|<-=====================--|
         |                   | (Mobile Network Prefix) |
         |                   |                         |

    Figure 2: Signaling sequence when the mobile router is not at home

   Note that a mobile router using DHCPv6PD to obtain the set of
   prefixes to be used as mobile network prefixes cannot derive its home
   address from one of its mobile network prefix(es) (as the mobile
   router does not know them before registering to the home agent).
   Therefore, the mobile router MUST assign its home address from the
   prefix on its Home Link.

3.1.1.  Relay Agent Configuration

   The use of the relay agent function in the mobile router allows the
   mobile router to unicast DHCPv6 messages to the DHCPv6 server.  The
   relay agent MUST be configured with the address of the DHCPv6 server.
   For the purposes of this specification, the relay agent assumes that
   the home agent for the mobile router hosts the DHCPv6 server.
   Therefore, the mobile router MUST configure the DHCPv6 relay agent to
   forward DHCPv6 messages to the home agent.

   The DHCPv6 specification supports in certain scenarios the use of
   unicast between the client and the server.  However, its use presents
   some difficulties, as the client has to first receive a Server
   Unicast option (Section 22.12 of [RFC3315]) from the server, which
   means that a Solicit/Advertise message exchange is required in
   advance.  That signaling exchange would require the presence of a
   relay agent on the mobile router, and therefore little gain would be
   achieved in this case from the use of the Server Unicast option.

3.1.2.  Transmission of DHCPv6 Messages

   When the DHCPv6 client in the mobile router sends a message, it MUST
   hand the message to the DHCPv6 relay agent in the mobile router.  The
   way in which the message is passed to the DHCP relay agent is beyond
   the scope of this document.  The relay agent encapsulates the message
   from the client according to [RFC3315] in a Relay-forward message and
   sends the resulting DHCPv6 message to the home agent.  The relay
   agent sets the fields in the Relay-forward message as follows:

   msg-type       RELAY-FORW

   hop-count      1

   link-address   The home address of the mobile router

   peer-address   The home address of the mobile router

   options        MUST include a "Relay Message option" [RFC3315]; MAY
                  include other options added by the relay agent.

3.1.3.  Receipt of DHCPv6 Messages

   Messages from the DHCPv6 server will be returned to the DHCPv6 relay
   agent, with the message for the DHCPv6 client encapsulated in the
   Relay Message option [RFC3315] in a Relay-reply message.  The relay
   agent function MUST extract the message for the client from the Relay
   Message option and hand the message to the DHCPv6 client in the
   mobile router.  The way in which the message is passed to the client
   is beyond the scope of this document.

3.2.  Exchanging DHCPv6 Messages When the Mobile Router Is at Home

   When the mobile router is on its home link, the home agent MUST use
   the home link to exchange DHCPv6PD messages with the mobile router
   (Figure 3).  In this case, the DHCPv6 co-located relay function MUST
   be disabled.  It is the responsibility of the implementation to
   determine when the mobile router is on its home link.  The Home Link
   Detection mechanism is described in Section 11.5.2 of [RFC6275].

                  --------                   --------
                  |  MR  |                   |  HA  |
                  | (RR) |                   | (DR) |
                  --------                   --------
                      |                         |
                      |       DHCPv6 Solicit    |
                      |------------------------>|
                      |                         |
                      |       DHCPv6 Advertise  |
                      |<------------------------|
                      |                         |
                      |       DHCPv6 Request    |
                      |------------------------>|
                      |                         |
                      |       DHCPv6 Reply      |
                      |<------------------------|
                      | (Mobile Network Prefix) |
                      |                         |

    Figure 3: Signaling sequence for the case the home agent is at home

3.3.  Selecting a Home Agent That Provides DHCPv6PD

   Not all nodes that are willing to act as a home agent are required to
   provide DHCPv6PD.  Therefore, when selecting a home agent, a mobile
   router that requires DHCPv6PD service MUST identify a home agent that
   will provide the service.  The mobile router can determine if a home
   agent provides DHCPv6PD by initiating a DHCPv6 message exchange
   (i.e., sending a Solicit message) in which the mobile router requests
   delegated prefix(es).  If the home agent does not respond or responds
   but does not delegate any prefix(es) in its response, the mobile
   router assumes that the home agent does not provide DHCPv6PD service.
   The mobile router continues to query all candidate home agents until
   it finds one that provides DHCPv6PD.  Note that in this particular
   case and if the mobile router is away from home, the mobile router
   has to have already performed a Mobile IPv6 registration with the
   home agent it queries.

   Querying a home agent to determine if it provides DHCPv6PD requires
   different operational variables than those recommended by the DHCPv6
   specification.  [RFC3315] recommends that under normal circumstances,
   a host will continue to send DHCPv6 Solicit messages until it
   receives a response (see Section 17 of [RFC3315]), i.e., the Maximum
   Retransmission Duration (MRD) and Maximum Retransmission Count (MRC)
   are both set to zero.  However, a home agent may not respond to the
   Solicit messages from the mobile router because the home agent does
   not support DHCPv6 prefix delegation.  Therefore, when querying a
   home agent to determine if the home agent provides DHCPv6PD service,
   it is RECOMMENDED that MRD and MRC be set to non-zero values so that
   the mobile router discontinues sending Solicit messages to the home
   agent after sending 6 Solicit messages, and conclude that the home
   agent will not provide DHCPv6PD service.  Sending 6 queries provides
   enough reliability for scenarios in which the wireless connectivity
   is lost for a short period after sending the first Binding Update
   message.

   It is RECOMMENDED that the mobile router uses a sequential probing of
   the home agents for DHCPv6PD service.

3.4.  Minimizing DHCPv6PD Messages

   The use DHCPv6PD in a mobile network can be combined with the Rapid
   Commit option [RFC3315] to provide DHCPv6 prefix delegation with a
   two-message exchange between the mobile router and the DHCPv6PD
   delegating router.

3.5.  Other DHCPv6 Functions

   The DHCPv6 messages exchanged between the mobile router and the home
   agent MAY also be used for other DHCPv6 functions in addition to
   DHCPv6PD.  For example, the home agent MAY assign global addresses to
   the mobile router and MAY pass other configuration information such
   as a list of available DNS recursive name servers [RFC3646] to the
   mobile router using the same DHCPv6 messages as used for DHCPv6PD.

   The home agent MAY act as a DHCPv6 relay agent for mobile nodes while
   it acts as a delegating router for mobile routers.

4.  Security Considerations

   This document describes the use of DHCPv6 for prefix delegation in
   mobile networks.  In addition to the security considerations for
   DHCPv6 described in the "Security Considerations" section of the
   DHCPv6 base specification [RFC3315] and the "Security Considerations"
   of the DHCPv6 Prefix Delegation specification [RFC3633], there are
   two aspects that need to be considered.

   First, the NEMO Basic Support specification requires the home agent
   to prevent a mobile router from claiming mobile network prefixes
   belonging to another mobile router.  Upon reception of an implicit
   Binding Update from a mobile router, the home agent MUST only add
   prefixes into the mobile router's Binding Cache Entry if the mobile
   router has a valid DHCPv6 Prefix Delegation lease for said prefixes.
   If the mobile router does not have a valid DHCPv6 Prefix Delegation
   lease, the home agent MUST NOT add any prefixes into the mobile
   router's Binding Cache Entry.  Upon the mobile router obtaining a
   valid DHCPv6 Prefix Delegation lease for a given set of prefixes, the
   home agent MUST add these prefixes to the mobile router's Binding
   Cache Entry.  This avoids the home agent forwarding traffic addressed
   to prefixes that have not been yet delegated to the mobile router.

   The use of DHCPv6, as described in this document, requires message
   integrity protection and source authentication.  When the mobile
   router is at home, normal DHCPv6 operation is used between the mobile
   router and the home agent and therefore this specification does not
   add any new security issue.  While the mobile router is away from
   home, the IPsec security mechanism mandated by Mobile IPv6 [RFC3776]
   MUST be used to secure the DHCPv6 signaling.  In the following, we
   describe the Security Policy Database (SPD) and Security Association
   Database (SAD) entries necessary to protect the DHCPv6 signaling.  We
   use the same format used by [RFC4877].  The SPD and SAD entries are
   only example configurations.  A particular mobile router
   implementation and a home agent implementation could configure
   different SPD and SAD entries as long as they provide the required
   security of the DHCPv6 signaling messages.

   For the examples described in this document, a mobile router with
   home address "home_address_1", and a home agent with address
   "home_agent_1" are assumed.  If the home address of the mobile router
   changes, the SPD and SAD entries need to be re-created or updated for
   the new home address.

      mobile router SPD-S:
        - IF local_address = home_address_1 &
             remote_address = home_agent_1 & proto = UDP &
             local_port = any & remote_port = DHCP
          Then use SA1 (OUT) and SA2 (IN)

      mobile router SAD:
        - SA1(OUT, spi_a, home_agent_1, ESP, TRANSPORT):
              local_address = home_address_1 &
              remote_address = home_agent_1 &
              proto = UDP & remote_port = DHCP
        - SA2(IN, spi_b, home_address_1, ESP, TRANSPORT):
              local_address = home_agent_1 &
              remote_address = home_address_1 &
              proto = UDP & local_port = DHCP

      home agent SPD-S:
        - IF local_address = home_agent_1 &
             remote_address = homa_address_1 & proto = UDP &
             local_port = DHCP & remote_port = any
          Then use SA2 (OUT) and SA1 (IN)

      home agent SAD:
        - SA2(OUT, spi_b, home_address_1, ESP, TRANSPORT):
              local_address = home_agent_1 &
              remote_address = home_address_1 &
              proto = UDP & local_port = DHCP
        - SA1(IN, spi_a, home_agent_1, ESP, TRANSPORT):
              local_address = home_address_1 &
              remote_address = home_agent_1 &
              proto = UDP & remote_port = DHCP

5.  Acknowledgments

   The authors would like to thank people who have given valuable
   comments on the mailing list.  Specific suggestions from Ryuji
   Wakikawa, George Tsirtsis, Alexandru Petrescu, Vijay Devarapalli, and
   Marcelo Bagnulo were incorporated into this document.

   The authors would like to thank Julien Laganier, Michaela Vanderveen,
   and Jean-Michel Combes for their review of previous versions of this
   document.

6.  References

6.1.  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.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

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

   [RFC3776]  Arkko, J., Devarapalli, V., and F. Dupont, "Using IPsec to
              Protect Mobile IPv6 Signaling Between Mobile Nodes and
              Home Agents", RFC 3776, June 2004.

   [RFC3963]  Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
              Thubert, "Network Mobility (NEMO) Basic Support Protocol",
              RFC 3963, January 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4877]  Devarapalli, V. and F. Dupont, "Mobile IPv6 Operation with
              IKEv2 and the Revised IPsec Architecture", RFC 4877,
              April 2007.

   [RFC6275]  Perkins, C., Johnson, D., and J. Arkko, "Mobility Support
              in IPv6", RFC 6275, July 2011.

6.2.  Informative References

   [RFC4885]  Ernst, T. and H-Y. Lach, "Network Mobility Support
              Terminology", RFC 4885, July 2007.

Authors' Addresses

   Ralph Droms
   Cisco
   1414 Massachusetts Avenue
   Boxborough, MA  01719
   USA

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


   Pascal Thubert
   Cisco
   Village d'Entreprises Green Side
   400, Avenue Roumanille
   Biot - Sophia Antipolis  06410
   FRANCE

   EMail: pthubert@cisco.com


   Francis Dupont
   Internet Systems Consortium

   EMail: fdupont@isc.org


   Wassim Haddad
   Ericsson
   6210 Spine Road
   Boulder, CO  80301
   USA

   Phone: +1 303.473.6963
   EMail: Wassim.Haddad@ericsson.com


   Carlos J. Bernardos
   Universidad Carlos III de Madrid
   Av. Universidad, 30
   Leganes, Madrid  28911
   Spain

   Phone: +34 91624 6236
   EMail: cjbc@it.uc3m.es
   URI:   http://www.it.uc3m.es/cjbc/