





Internet Engineering Task Force (IETF)                       R. Wakikawa
Request for Comments: 5844                                    Toyota ITC
Category: Standards Track                                  S. Gundavelli
ISSN: 2070-1721                                                    Cisco
                                                                May 2010


                   IPv4 Support for Proxy Mobile IPv6

Abstract

   This document specifies extensions to the Proxy Mobile IPv6 protocol
   for adding IPv4 protocol support.  The scope of IPv4 protocol support
   is two-fold: 1) enable IPv4 home address mobility support to the
   mobile node, and 2) allow the mobility entities in the Proxy Mobile
   IPv6 domain to exchange signaling messages over an IPv4 transport
   network.

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/rfc5844.

Copyright Notice

   Copyright (c) 2010 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
   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.





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Table of Contents

   1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Stated Assumptions . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Relevance to Dual-Stack Mobile IPv6  . . . . . . . . . . .  5
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  6
     2.1.  Conventions  . . . . . . . . . . . . . . . . . . . . . . .  6
     2.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  IPv4 Home Address Mobility Support . . . . . . . . . . . . . .  8
     3.1.  Local Mobility Anchor Considerations . . . . . . . . . . .  9
       3.1.1.  Extensions to Binding Cache Entry  . . . . . . . . . .  9
       3.1.2.  Signaling Considerations . . . . . . . . . . . . . . . 10
       3.1.3.  Routing Considerations for the Local Mobility
               Anchor . . . . . . . . . . . . . . . . . . . . . . . . 15
       3.1.4.  ECN and Payload Fragmentation Considerations . . . . . 16
     3.2.  Mobile Access Gateway Considerations . . . . . . . . . . . 17
       3.2.1.  Extensions to Binding Update List Entry  . . . . . . . 17
       3.2.2.  Extensions to Mobile Node's Policy Profile . . . . . . 17
       3.2.3.  Signaling Considerations . . . . . . . . . . . . . . . 17
       3.2.4.  Routing Considerations for the Mobile Access
               Gateway  . . . . . . . . . . . . . . . . . . . . . . . 21
     3.3.  Mobility Options and Status Codes  . . . . . . . . . . . . 22
       3.3.1.  IPv4 Home Address Request Option . . . . . . . . . . . 22
       3.3.2.  IPv4 Home Address Reply Option . . . . . . . . . . . . 23
       3.3.3.  IPv4 Default-Router Address Option . . . . . . . . . . 25
       3.3.4.  IPv4 DHCP Support Mode Option  . . . . . . . . . . . . 25
       3.3.5.  Status Codes . . . . . . . . . . . . . . . . . . . . . 26
     3.4.  Supporting DHCP-Based Address Configuration  . . . . . . . 27
       3.4.1.  DHCP Server Co-Located with the Mobile Access
               Gateway  . . . . . . . . . . . . . . . . . . . . . . . 28
       3.4.2.  DHCP Relay Agent Co-Located with the Mobile Access
               Gateway  . . . . . . . . . . . . . . . . . . . . . . . 31
       3.4.3.  Common DHCP Considerations . . . . . . . . . . . . . . 33
   4.  IPv4 Transport Support . . . . . . . . . . . . . . . . . . . . 35
     4.1.  Local Mobility Anchor Considerations . . . . . . . . . . . 37
       4.1.1.  Extensions to Binding Cache Entry  . . . . . . . . . . 37
       4.1.2.  Extensions to Mobile Node's Policy Profile . . . . . . 37
       4.1.3.  Signaling Considerations . . . . . . . . . . . . . . . 37
       4.1.4.  Routing Considerations . . . . . . . . . . . . . . . . 39
     4.2.  Mobile Access Gateway Considerations . . . . . . . . . . . 40
       4.2.1.  Extensions to Binding Update List Entry  . . . . . . . 40
       4.2.2.  Signaling Considerations . . . . . . . . . . . . . . . 40
     4.3.  IPsec Considerations . . . . . . . . . . . . . . . . . . . 43
       4.3.1.  PBU and PBA  . . . . . . . . . . . . . . . . . . . . . 43
       4.3.2.  Payload Packet . . . . . . . . . . . . . . . . . . . . 43
   5.  Protocol Configuration Variables . . . . . . . . . . . . . . . 44
     5.1.  Local Mobility Anchor - Configuration Variables  . . . . . 44
     5.2.  Mobile Access Gateway - Configuration Variables  . . . . . 44



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   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 45
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 46
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 46
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 47
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 47
     10.2. Informative References . . . . . . . . . . . . . . . . . . 48

1.  Overview

   The transition from IPv4 to IPv6 is a long process, and during this
   period of transition, both the protocols will be enabled over the
   same network infrastructure.  Thus, it is reasonable to assume that a
   mobile node in a Proxy Mobile IPv6 domain may operate in an IPv4-
   only, IPv6-only, or dual-stack mode, and the network between the
   mobile access gateway and a local mobility anchor may be an IPv4 or
   an IPv6 network.  It is also reasonable to expect the same mobility
   infrastructure in the Proxy Mobile IPv6 domain to provide mobility to
   the mobile nodes operating in IPv4, IPv6, or in dual mode and whether
   the transport network is IPv4 or IPv6 network.  The motivation and
   scope of IPv4 support in Mobile IPv6 is summarized in [RFC4977], and
   all those requirements apply to Proxy Mobile IPv6 protocol as well.

   The Proxy Mobile IPv6 protocol [RFC5213] specifies a mechanism for
   providing IPv6 home address mobility support to a mobile node in a
   Proxy Mobile IPv6 domain.  The protocol requires IPv6 transport
   network between the mobility entities.  The extensions defined in
   this document specify IPv4 support to the Proxy Mobile IPv6 protocol
   [RFC5213].

   The scope of IPv4 support in Proxy Mobile IPv6 includes the support
   for the following two features:

   o  IPv4 Home Address Mobility Support: A mobile node that is dual-
      stack or IPv4-only enabled will be able to obtain an IPv4 address
      and be able to use that address from any of the access networks in
      that Proxy Mobile IPv6 domain.  The mobile node is not required to
      be allocated or assigned an IPv6 address to enable IPv4 home
      address support.

   o  IPv4 Transport Network Support: The mobility entities in the Proxy
      Mobile IPv6 domain will be able to exchange Proxy Mobile IPv6
      signaling messages over an IPv4 transport.

   These two features, the IPv4 home address mobility support and the
   IPv4 transport support features, are independent of each other, and
   deployments may choose to enable either one or both of these features
   as required.



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   Figure 1 shows a typical Proxy Mobile IPv6 domain with an IPv4
   transport network and with IPv4 enabled mobile nodes.  The terms used
   in this illustration are explained in the Terminology section.

               +----+                +----+
               |LMA1|                |LMA2|
               +----+                +----+
   IPv4-LMAA  -> |          IPv4-LMAA-> | <-- LMAA
                 |                      |
                 \\                    //\\
                  \\                  //  \\
                   \\                //    \\
                +---\\------------- //------\\----+
               (     \\  IPv4/IPv6 //        \\    )
               (      \\  Network //          \\   )
                +------\\--------//------------\\-+
                        \\      //              \\
                         \\    //                \\
                          \\  //                  \\
         IPv4-Proxy-CoA --> |                      | <-- Proxy-CoA
                         +----+                 +----+
                         |MAG1|-----{MN2}       |MAG2|
                         +----+    |            +----+
        (MN-HoA)           |       |              | <-- (MN-HoA)
        (IPv4-MN-HoA) -->  |   (IPv4-MN-HoA)      | <-- (IPv4-MN-HoA)
                         {MN1}                   {MN3}

               Figure 1: IPv4 Support for Proxy Mobile IPv6

1.1.  Stated Assumptions

   The following are the system and configuration requirements from the
   mobility entities in the Proxy Mobile IPv6 domain for supporting the
   extensions defined in this document.

   o  Both the mobility entities, the local mobility anchor and the
      mobile access gateway are dual-stack (IPv4/IPv6) enabled.
      Irrespective of the type of transport network (IPv4 or IPv6)
      separating these two entities, the mobility signaling is always
      based on Proxy Mobile IPv6 protocol [RFC5213].

   o  A deployment where a mobile access gateway uses an IPv4 private
      address with NAT [RFC3022] translation devices in the path to a
      local mobility anchor is not supported by this specification.







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   o  The mobile node can be operating in IPv4-only, IPv6-only or in
      dual mode.  Based on the enabled configuration for a mobile node,
      the mobile node should be able to obtain IPv4-only, IPv6-only, or
      both IPv4 and IPv6 addresses for its interface and furthermore
      achieve mobility support for those addresses.

   o  For enabling IPv4 home address mobility support to a mobile node,
      it is not required that the IPv6 home address mobility support
      need be enabled.  However, the respective protocol(s) support,
      such as IPv4 or IPv6 packet forwarding, must be enabled on the
      access link between the mobile node and the mobile access gateway.

   o  The mobile node can obtain an IPv4 address for its attached
      interface.  Based on the type of link, it may be able to acquire
      its IPv4 address configuration using standard IPv4 address
      configuration mechanisms such as DHCP [RFC2131], IP Control
      Protocol (IPCP) [RFC1332], Internet Key Exchange Protocol version
      2 (IKEv2) [RFC4306], or static address configuration.  However,
      the details on how IPCP or IKEv2 can be used for address delivery
      are outside the scope of this document.

   o  The mobile node's IPv4 home subnet is typically a shared address
      space.  It is not for the exclusive use of any one mobile node.
      There can be multiple mobile nodes that are assigned IPv4
      addresses from the same subnet.

   o  The mobile access gateway is the IPv4 default router for the
      mobile node on its access link.  It will be in the forwarding path
      for the mobile node's data traffic.  Additionally, as specified in
      Section 6.9.3 of [RFC5213], all the mobile access gateways in the
      Proxy Mobile IPv6 domain MUST use the same link-layer address on
      any of the access links wherever the mobile node attaches.

1.2.  Relevance to Dual-Stack Mobile IPv6

   IPv4 support for Mobile IPv6 is specified in the Dual-Stack Mobile
   IPv6 specification [RFC5555].  This document leverages some of the
   approaches, messaging options, and processing logic defined in that
   document for extending IPv4 support to Proxy Mobile IPv6, except with
   deviation in some aspects for obvious reasons of supporting a
   network-based mobility model.  The following are some of the related
   considerations.

   o  The Binding Update message flag 'F' and the NAT Detection Option
      defined in Sections 3.1.3 and 3.2.2 of [RFC5555] are used by this
      specification in Proxy Binding Update and Proxy Binding
      Acknowledgement messages.  Their sole purpose is to allow forcing




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      of UDP encapsulation between a mobile access gateway and a local
      mobility anchor in situations similar to those discussed in
      Sections 4.1 and 4.4.1 of [RFC5555].

   o  The necessary extensions to the conceptual data structures,
      Binding Cache entry and Binding Update List entry, for storing the
      state related to the IPv4 support defined in [RFC5555], will all
      be needed and relevant for this document.

   o  In Mobile IPv6 [RFC3775] and in Dual-Stack Mobile IPv6 [RFC5555],
      IPsec security associations (SAs) are specific to a single mobile
      node; they use the identifier visible to upper-layer protocols
      (HoA/IPv4-HoA) as traffic selector; and the IKE/IPsec SAs need to
      be updated when the mobile node moves.

      In Proxy Mobile IPv6 (both [RFC5213] and this document), the IPsec
      SAs are specific to the mobile access gateway (and used for a
      potentially large number of mobile nodes); they use the locators
      used for routing (Proxy-CoA/IPv4-Proxy-CoA) as traffic selectors;
      and they are not updated when the mobile node moves.

      This means the IPsec processing for Mobile IPv6 and Proxy Mobile
      IPv6 (whether IPv6-only or dual-stack) is very different.

   o  The tunneling considerations specified in [RFC5555] for supporting
      IPv4 transport are relevant for this document as well.

2.  Conventions and Terminology

2.1.  Conventions

   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].

2.2.  Terminology

   All the mobility related terms used in this document are to be
   interpreted as defined in the Mobile IPv6 specification [RFC3775] and
   Proxy Mobile IPv6 specification [RFC5213].  In addition, this
   document introduces the following terms.










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   IPv4 Proxy Care-of Address (IPv4-Proxy-CoA)

      The IPv4 address that is configured on the egress-interface of the
      mobile access gateway.  When using IPv4 transport, this address
      will be the registered care-of address in the mobile node's
      Binding Cache entry and will also be the transport-endpoint of the
      tunnel between the local mobility anchor and a mobile access
      gateway.

   IPv4 Local Mobility Anchor Address (IPv4-LMAA)

      The IPv4 address that is configured on the egress-interface of the
      local mobility anchor.  When using IPv4 transport, the mobile
      access gateway sends the Proxy Binding Update messages to this
      address and will be the transport-endpoint of the tunnel between
      the local mobility anchor and the mobile access gateway.

   Mobile Node's IPv4 Home Address (IPv4-MN-HoA)

      The IPv4 home address assigned to the mobile node's attached
      interface.  This address is topologically anchored at the mobile
      node's local mobility anchor.  The mobile node configures this
      address on its attached interface.  If the mobile node connects to
      the Proxy Mobile IPv6 domain via multiple interfaces each of the
      interfaces are assigned a unique IPv4 address.  All the IPv6 home
      network prefixes and the IPv4 home address assigned to a given
      interface of a mobile node will be managed under one mobility
      session.

   Selective De-registration

      A procedure for partial de-registration of all the addresses that
      belong to one address family, i.e., de-registration of either the
      IPv4 home address or one or more of the assigned IPv6 home network
      prefixes.

   Encapsulation Modes

      This document uses the following terms when referring to the
      different encapsulation modes.

      IPv4-or-IPv6-over-IPv6

         IPv4 or IPv6 packet carried as a payload of an IPv6 packet

      IPv4-or-IPv6-over-IPv4

         IPv4 or IPv6 packet carried as a payload of an IPv4 packet



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      IPv4-or-IPv6-over-IPv4-UDP

         IPv4 or IPv6 packet carried as a payload in an IPv4 packet with
         a UDP header

      IPv4-or-IPv6-over-IPv4-UDP-TLV

         IPv4 or IPv6 packet carried as a payload in an IPv4 packet with
         UDP and TLV headers

      IPv4-or-IPv6-over-IPv4-GRE

         IPv4 or IPv6 packet carried as a payload in an IPv4 packet with
         a Generic Routing Encapsulation (GRE) header (but no UDP or TLV
         header)

3.  IPv4 Home Address Mobility Support

   The IPv4 home address mobility support essentially enables a mobile
   node in a Proxy Mobile IPv6 domain to obtain IPv4 home address
   configuration for its attached interfaces and be able to retain that
   address configuration even after performing a handoff anywhere within
   that Proxy Mobile IPv6 domain.  This section describes the protocol
   operation and the required extensions to Proxy Mobile IPv6 protocol
   for extending IPv4 home address mobility support.

   When an IPv4-enabled or a dual-stack-enabled mobile node attaches to
   the Proxy Mobile IPv6 domain, the mobile access gateway on the access
   link where the mobile node is attached will identify the mobile node
   and will initiate the Proxy Mobile IPv6 signaling with the mobile
   node's local mobility anchor.  The mobile access gateway will follow
   the signaling considerations specified in Section 3.2 for requesting
   IPv4 home address mobility support.  Upon the completion of the
   signaling, the local mobility anchor and the mobile access gateway
   will establish the required routing states for allowing the mobile
   node to use its IPv4 home address from its current point of
   attachment.

   The mobile node on the access link using any of the standard IPv4
   address configuration mechanisms supported on that access link, such
   as IPCP [RFC1332], IKEv2 [RFC4306], or DHCP [RFC2131], will be able
   to obtain an IPv4 home address (IPv4-MN-HoA) for its attached
   interface.  Although the address configuration mechanisms for
   delivering the address configuration to the mobile node is
   independent of the Proxy Mobile IPv6 protocol operation, there needs
   to be some interaction between these two protocol flows.  Section 3.4
   identifies these interactions for supporting DHCP-based address
   configuration.



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   The support for IPv4 home address mobility is not dependent on the
   IPv6 home address mobility support.  It is not required that the IPv6
   home address mobility support needs to be enabled for providing IPv4
   home address mobility support.  A mobile node will be able to obtain
   IPv4-only, IPv6-only, or dual IPv4/IPv6 address configuration for its
   attached interface.  The mobile node's policy profile will determine
   if the mobile node is entitled to both the protocol versions or a
   single protocol version.  Based on the policy, only those protocols
   will be enabled on the access link.  Furthermore, if the mobile node,
   after obtaining the address configuration on its interface, performs
   a handoff, either by changing its point of attachment over the same
   interface or to a different interface, the network will ensure the
   mobile node will be able to use the same IPv4 address configuration
   after the handoff.

   Additionally, if the mobile node connects to the Proxy Mobile IPv6
   domain, through multiple interfaces and simultaneously through
   different access networks, each of the connected interfaces will
   obtain a unique IPv4 home address.  In such a scenario, there will be
   multiple Binding Cache entries for the mobile node on the local
   mobility anchor.  All the addresses (IPv4/IPv6) assigned to a given
   interface will be managed as part of one mobility session, as
   specified in Section 5.4 of [RFC5213].

3.1.  Local Mobility Anchor Considerations

3.1.1.  Extensions to Binding Cache Entry

   To support this feature, the conceptual Binding Cache entry data
   structure maintained by the local mobility anchor needs to include
   the following parameters.

   o  The IPv4 home address assigned to the mobile node's interface and
      registered by the mobile access gateway.  The IPv4 home address
      entry also includes the corresponding subnet mask.  It is to be
      noted that this parameter is defined in [RFC5555] and is presented
      here for completeness.

   o  The IPv4 default router address assigned to the mobile node.












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3.1.2.  Signaling Considerations

3.1.2.1.  Processing Proxy Binding Updates

   The processing rules specified in Section 5.3 of [RFC5213] are
   applied for processing the received Proxy Binding Update message.
   However, if the received Proxy Binding Update message has an IPv4
   Home Address Request option, the following considerations MUST be
   applied additionally.

   o  If there is an IPv4 Home Address Request option (Section 3.3.1)
      present in the received Proxy Binding Update message, but no Home
      Network Prefix option [RFC5213] present in the received Proxy
      Binding Update message, the local mobility anchor MUST NOT reject
      the request as specified in Section 5.3.1 of [RFC5213].  At least
      one instance of either of these two options, either the IPv4 Home
      Address Request option or the Home Network Prefix option, MUST be
      present.  If there is not a single instance of either of these two
      options present in the request, the local mobility anchor MUST
      reject the request and send a Proxy Binding Acknowledgement
      message with the Status field set to
      MISSING_HOME_NETWORK_PREFIX_OPTION (missing the mobile node's home
      network prefix option) [RFC5213].

   o  If there is at least one instance of the Home Network Prefix
      option [RFC5213] present in the received Proxy Binding Update
      message, but it is known from the mobile node's policy profile
      that the mobile node is not authorized for IPv6 service, or IPv6
      routing in not enabled in the home network, the local mobility
      anchor MUST reject the request and send a Proxy Binding
      Acknowledgement message with the Status field set to
      NOT_AUTHORIZED_FOR_IPV6_MOBILITY_SERVICE (mobile node not
      authorized for IPv6 mobility service; see Section 3.3.5).

   o  If there is an IPv4 Home Address Request option present in the
      received Proxy Binding Update message, but it is known from the
      mobile node's policy profile that the mobile node is not
      authorized for IPv4 service, or if IPv4 routing is not enabled in
      the home network, the local mobility anchor MUST reject the
      request and send a Proxy Binding Acknowledgement message with the
      Status field set to NOT_AUTHORIZED_FOR_IPV4_MOBILITY_SERVICE
      (mobile node not authorized for IPv4 mobility service; see
      Section 3.3.5).

   o  If there is more than one instance of the IPv4 Home Address
      Request option present in the request, then the local mobility
      anchor MUST reject the request and send a Proxy Binding




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      Acknowledgement message with the Status field set to
      MULTIPLE_IPV4_HOME_ADDRESS_ASSIGNMENT_NOT_SUPPORTED (multiple IPv4
      home address assignments not supported; see Section 3.3.5).

   o  For associating the received Proxy Binding Update message to an
      existing mobility session, the local mobility anchor MUST perform
      the Binding Cache entry existence test by applying the following
      considerations.

      *  If there is at least one instance of the Home Network Prefix
         option [RFC5213] with a NON_ZERO prefix value, or, if there is
         an IPv4 Home Address Request option with the IPv4 address in
         the option set to ALL_ZERO, considerations from Section 5.4.1
         of [RFC5213] MUST be applied.

      *  If there is an IPv4 Home Address Request option present in the
         request with the IPv4 address value in the option set to a
         NON_ZERO value, considerations from Section 3.1.2.7 MUST be
         applied.

   o  If there is no existing Binding Cache entry that can be associated
      with the request, the local mobility anchor MUST consider this
      request as an initial binding registration request, and
      considerations from Section 3.1.2.2 MUST be applied.
      Additionally, if there are one or more Home Network Prefix options
      [RFC5213] present in the request, considerations from Section
      5.3.2 of [RFC5213] MUST also be applied.

   o  If there exists a Binding Cache entry that can be associated with
      the request, the local mobility anchor MUST apply considerations
      from Section 5.3.1 of [RFC5213], (point 13), to determine if the
      request is a re-registration or a de-registration request.  If the
      request is a re-registration request, considerations from
      Section 3.1.2.3 MUST be applied, and if it is a de-registration
      request, considerations from Section 3.1.2.5 MUST be applied.

   o  If there exists a Binding Cache entry that can be associated with
      the request and if it is determined that the request is a re-
      registration request for extending an IPv4 home address mobility
      support to the existing IPv6-only mobility session, considerations
      from Section 3.1.2.2 MUST be applied with respect to IPv4 support.










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3.1.2.2.  Initial Binding Registration (New Mobility Session)

   o  If there is an IPv4 Home Address Request option present in the
      Proxy Binding Update message with the IPv4 address value in the
      option set to ALL_ZERO, the local mobility anchor MUST allocate an
      IPv4 home address to the mobile node and associate it with the new
      mobility session created for that mobile node.

   o  If there is an IPv4 Home Address Request option with the IPv4
      address in the option set to a NON_ZERO value, the local mobility
      anchor, before accepting the request, MUST ensure that the address
      is topologically anchored on the local mobility anchor and
      furthermore that the mobile node is authorized to use that
      address.  If the mobile node is not authorized for that specific
      address, the local mobility anchor MUST reject the request and
      send a Proxy Binding Acknowledgement message with the Status field
      set to NOT_AUTHORIZED_FOR_IPV4_HOME_ADDRESS (mobile node not
      authorized for the requesting IPv4 address; see Section 3.3.5).
      It MUST also include the IPv4 Home Address Reply option
      (Section 3.3.2). in the reply with the Status field value in the
      option set to 129 (Administratively prohibited).

   o  If the local mobility anchor is unable to allocate an IPv4 address
      due to lack of resources, it MUST reject the request and send a
      Proxy Binding Acknowledgement message with Status field set to 130
      (Insufficient resources).  It MUST also include the IPv4 Home
      Address Reply option in the reply with the Status field value in
      the option set to 128 (Failure, reason unspecified).

   o  Upon accepting the request, the local mobility anchor MUST create
      a Binding Cache entry for this mobility session.  However, if the
      request also contains one or more Home Network Prefix options
      [RFC5213], there should still be only one Binding Cache entry that
      should be created for this mobility session.  The created Binding
      Cache entry MUST be used for managing both IPv4 and IPv6 home
      address bindings.  The fields in the Binding Cache entry MUST be
      updated with the accepted values for that session.

   o  The local mobility anchor MUST establish a bidirectional tunnel to
      the mobile access gateway with the encapsulation mode set to the
      negotiated mode for carrying the IPv4 payload traffic.  When using
      IPv6 transport, the encapsulation mode is IPv4-or-IPv6-over-IPv6
      (IPv4 or IPv6 packet carried as a payload of an IPv6 packet).
      When using IPv4 transport, the encapsulation mode is as specified
      in Section 4.






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   o  The local mobility anchor MUST create an IPv4 host route (or a
      platform-specific equivalent function that sets up the forwarding)
      for tunneling the packets received for the mobile node's home
      address associated with this mobility session.

   o  The local mobility anchor MUST send the Proxy Binding
      Acknowledgement message with the Status field set to 0 (Proxy
      Binding Update accepted).  The message MUST be constructed as
      specified in Section 3.1.2.6.

3.1.2.3.  Binding Lifetime Extension (No Handoff)

   All the considerations from Section 5.3.3 of [RFC5213] MUST be
   applied.

3.1.2.4.  Binding Lifetime Extension (after Handoff)

   o  If there is no Home Network Prefix option [RFC5213] present in the
      request, but if the Binding Cache entry associated with this
      request has IPv6 home network prefix(es), the local mobility
      anchor MUST consider this as a request to extend lifetime only for
      the IPv4 home address and not for the IPv6 home network
      prefix(es).  Hence, the local mobility anchor SHOULD release all
      the IPv6 home network prefix(es) assigned to that mobile node and
      for that specific attached interface.  Similar considerations
      apply for the case where there is no IPv4 Home Address Request
      option present in the request, but if the Binding Cache entry
      associated with that request has both IPv4 home address and IPv6
      home network prefix(es).

   o  The local mobility anchor MUST remove the previously created IPv4
      host route (or the forwarding state) and the dynamically created
      bidirectional tunnel for carrying the IPv4 payload traffic (if
      there are no other mobile nodes for which the tunnel is being
      used).  This will remove the routing state towards the mobile
      access gateway where the mobile node was anchored prior to the
      handoff.

   o  The local mobility anchor MUST create a bidirectional tunnel to
      the mobile access gateway that sent the request (if there is no
      existing bidirectional tunnel) and with the encapsulation mode set
      to the negotiated mode for carrying the IPv4 payload traffic.  An
      IPv4 host route for tunneling the packets received for the mobile
      node's IPv4 home address MUST also be added.







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   o  The required forwarding state identified in Section 5.3.6 of
      [RFC5213] is for IPv6 payload traffic.  Those considerations apply
      for IPv4 payload traffic as well.  However, if IPv4 transport is
      in use, considerations from Section 4 MUST be applied.

3.1.2.5.  Binding De-Registration

   All the considerations from Section 5.3.5 of [RFC5213] MUST be
   applied.  Additionally, to remove the IPv4 state as part of the
   Binding Cache entry deletion, the IPv4 host route and the dynamically
   created bidirectional tunnel for carrying the IPv4 payload traffic
   (if there are no other mobile nodes for which the tunnel is being
   used) MUST be removed.  However, if the request is for a selective
   de-registration (IPv4 home address only, or all the IPv6 home network
   prefixes), the Binding Cache entry MUST NOT be deleted, only the
   respective states related to those addresses MUST be deleted.

3.1.2.6.  Constructing the Proxy Binding Acknowledgement Message

   When sending the Proxy Binding Acknowledgement message to the mobile
   access gateway, the local mobility anchor MUST construct the message
   as specified in Section 5.3.6 of [RFC5213].  Additionally, the
   following considerations MUST be applied.

   o  Section 5.3.6 of [RFC5213] requires the local mobility anchor to
      include at least one instance of the Home Network Prefix option
      [RFC5213] in the Proxy Binding Acknowledgement message that it
      sends to the mobile access gateway.  However, if the received
      Proxy Binding Update message has only the IPv4 Home Address
      Request option and does not contain the Home Network Prefix
      option(s), then the local mobility anchor MUST NOT include any
      Home Network Prefix option(s) in the reply.  However, there MUST
      be at least one instance of either the Home Network Prefix option
      [RFC5213] or the IPv4 Home Address Reply option present in the
      Proxy Binding Acknowledgement message.

   o  The IPv4 Home Address Reply option MUST be present in the Proxy
      Binding Acknowledgement message.

      1.  If the Status field is set to a value greater than or equal to
          128, i.e., if the Proxy Binding Update is rejected, then there
          MUST be an IPv4 Home Address Reply option corresponding to the
          IPv4 Home Address Request option present in the request and
          with the IPv4 address value and the prefix length fields in
          the option set to the corresponding values in the request.
          The Status field value in the option must be set to the
          specific error code.




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      2.  For all other cases, there MUST be an IPv4 Home Address Reply
          option to carry the IPv4 home address assigned for that
          mobility session and with the value in the option set to the
          allocated IPv4 address.  The prefix length in the option MUST
          be set to the prefix length of the mobile node's IPv4 home
          network.  The Status field value in the option must be set to
          0 (Success).

   o  The IPv4 Default-Router Address option (Section 3.3.3) MUST be
      present, if the Status field value in the Proxy Binding
      Acknowledgement message is set to 0 (Proxy Binding Update
      accepted) [RFC5213].  Otherwise, the option MUST NOT be present.
      If the option is present, the default router address in the option
      MUST be set to the mobile node's default router address.

3.1.2.7.  Binding Cache Entry Lookup Considerations

   The Binding Cache entry lookup considerations specified in Section
   5.4.1.1 of [RFC5213] uses the Home Network Prefix option [RFC5213] as
   the key parameter for identifying the Binding Cache entry.  However,
   when there is not a single Home Network Prefix option with a NON_ZERO
   value present in the request, but there is an IPv4 Home Address
   option with a NON_ZERO value present in the request, then the
   following considerations MUST be applied.

   o  The search rules specified in Section 5.4.1.1 of [RFC5213], which
      primarily uses IPv6 home network prefix set as the search key, are
      equally valid when using a single IPv4 home address as the key.
      When applying those considerations, instead of the IPv6 home
      network prefix(es), the IPv4 home address from the IPv4 Home
      Address option present in the request MUST be used as the search
      key.

   o  The rules specified in Section 5.4.1.1 of [RFC5213] assume the
      presence of one or more IPv6 home network prefixes in the received
      request and also in the Binding Cache entry.  But, when using the
      IPv4 home address as the search key, these considerations MUST
      always assume just one single IPv4 home address, both in the
      request and also in the Binding Cache entry.

3.1.3.  Routing Considerations for the Local Mobility Anchor

   Intercepting Packets Sent to the Mobile Node's IPv4 Home Address:

   o  When the local mobility anchor is serving a mobile node, it MUST
      advertise a connected route into the Routing Infrastructure for
      the mobile node's IPv4 home address or for its home subnet, in
      order to receive packets that are sent to the mobile node's IPv4



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      home address.  This essentially enables IPv4 routers in that
      network to detect the local mobility anchor as the last-hop router
      for that subnet.

   Forwarding Packets to the Mobile Node:

   o  On receiving a packet from a corresponding node with the
      destination address matching the mobile node's IPv4 home address,
      the local mobility anchor MUST forward the packet through the
      bidirectional tunnel setup for that mobile node.

   o  The format of the tunneled packet when payload protection is not
      enabled:

        IPv6 header (src= LMAA, dst= Proxy-CoA       /* Tunnel Header */
           IPv4 header (src= CN, dst= IPv4-MN-HOA )  /* Packet Header */
              Upper-layer protocols                  /* Packet Content*/

      Figure 2: Tunneled Packets from the Local Mobility Anchor (LMA) to
                       the Mobile Access Gateway (MAG)

   Forwarding Packets Sent by the Mobile Node:

   o  All the reverse tunneled packets that the local mobility anchor
      receives from the mobile access gateway, after removing the tunnel
      header, MUST be routed to the destination specified in the inner
      IPv4 packet header.  These routed packets will have the Source
      Address field set to the mobile node's IPv4 home address.

3.1.4.  ECN and Payload Fragmentation Considerations

   The Explicit Congestion Notification (ECN) considerations specified
   in Section 5.6.3 of [RFC5213] apply for the IPv4 payload packets as
   well.  The mobility agents at the tunnel entry and exit points MUST
   handle ECN information as specified in that document.

   The mobility agents at the tunnel entry and exit points MUST apply
   the IP packet fragmentation considerations as specified in Section 7
   of [RFC2473]; additionally, they MUST apply the considerations
   related to tunnel error processing and reporting as specified in
   Section 8 of [RFC2473].










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3.2.  Mobile Access Gateway Considerations

3.2.1.  Extensions to Binding Update List Entry

   To support the IPv4 home address mobility feature, the conceptual
   Binding Update List entry data structure needs to be extended with
   the following additional fields.

   o  The IPv4 home address assigned to the mobile node's attached
      interface.  This IPv4 home address may have been statically
      configured in the mobile node's policy profile, or, may have been
      dynamically allocated by the local mobility anchor.  The IPv4 home
      address entry also includes the corresponding subnet mask.

   o  The IPv4 default router address of the mobile node.  This is
      acquired from the mobile node's local mobility anchor through the
      received Proxy Binding Acknowledgement message.

3.2.2.  Extensions to Mobile Node's Policy Profile

   To support the IPv4 home address mobility support feature, the mobile
   node's policy profile, specified in Section 6.2 of [RFC5213], MUST be
   extended with the following additional fields.

   Extensions to the mandatory section of the policy profile:

   o  This field identifies all the IP versions for which the home
      address mobility support needs to be extended to the mobile node.
      The supported modes are IPv4-only, IPv6-only, and dual IPv4/IPv6.


   Extensions to the optional section of the policy profile:

   o  The IPv4 home address assigned to the mobile node's attached
      interface.  The specific details on how the network maintains the
      association between the address and the attached interface is
      outside the scope of this document.  This address field also
      includes the corresponding subnet mask.

3.2.3.  Signaling Considerations

3.2.3.1.  Mobile Node Attachment and Initial Binding Registration

   After detecting a new mobile node on its access link, the mobile
   access gateway on the access link MUST determine if IPv4 home address
   mobility support needs to be enabled for that mobile node.  The
   mobile node's policy profile identifies the supported modes (IPv4-
   only, IPv6-only, or dual IPv4/IPv6) for that mobile node for which



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   the mobile service needs to be enabled.  Based on those policy
   considerations and from other triggers such as from the network, if
   it is determined that IPv4 home address mobility support needs to be
   enabled for the mobile node, considerations from Section 6.9.1.1 of
   [RFC5213] MUST be applied with the following exceptions.

   o  The IPv4 Home Address Request option MUST be present in the Proxy
      Binding Update message.

      *  If the mobile access gateway learns the mobile node's IPv4 home
         address either from its policy profile or from other means, the
         mobile access gateway MAY ask the local mobility anchor to
         allocate that specific address by including exactly one
         instance of the IPv4 Home Address Request option with the IPv4
         home address and the prefix length fields in the option set to
         that specific IPv4 address and the prefix length of the
         corresponding home network.

      *  The mobile access gateway MAY also ask the local mobility
         anchor for dynamic IPv4 home address allocation.  It can
         include exactly one instance of the IPv4 Home Address option
         with the IPv4 home address and the prefix length fields in the
         option set to the ALL_ZERO value.  Furthermore, the (P) flag in
         the option MUST be set to 0.  This serves as a request to the
         local mobility anchor for the IPv4 home address allocation.

   o  The Proxy Binding Update message MUST be constructed as specified
      in Section 6.9.1.5 of [RFC5213].  However, the Home Network Prefix
      option(s) [RFC5213] MUST be present in the Proxy Binding Update
      only if IPv6 home address mobility support also needs to be
      enabled for the mobile node.  Otherwise, the Home Network Prefix
      option(s) MUST NOT be present.

   o  When using IPv4 transport to carry the signaling messages, the
      related considerations from Section 4 MUST be applied
      additionally.

3.2.3.2.  Receiving Proxy Binding Acknowledgement

   All the considerations from Section 6.9.1.2 of [RFC5213] MUST be
   applied with the following exceptions.

   o  If the received Proxy Binding Acknowledgement message has the
      Status field value set to NOT_AUTHORIZED_FOR_IPV4_MOBILITY_SERVICE
      (The mobile node is not authorized for IPv4 mobility service), the
      mobile access gateway SHOULD NOT send a Proxy Binding Update
      message including a IPv4 Home Address Request option until an
      administrative action is taken.



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   o  If the received Proxy Binding Acknowledgement message has the
      Status field value set to NOT_AUTHORIZED_FOR_IPV4_HOME_ADDRESS
      (The mobile node is not authorized for the requesting IPv4 home
      address), the mobile access gateway SHOULD NOT request the same
      IPv4 address again, but MAY request the local mobility anchor to
      perform the address assignment by including exactly one instance
      of the IPv4 Home Address Request option with the IPv4 home address
      and the prefix length fields in the option set to the ALL_ZERO
      value.

   o  If the received Proxy Binding Acknowledgement message has the
      Status field value set to NOT_AUTHORIZED_FOR_IPV6_MOBILITY_SERVICE
      (The mobile node is not authorized for IPv6 mobility service), the
      mobile access gateway SHOULD NOT send a Proxy Binding Update
      message including any Home Network Prefix option(s) until an
      administrative action is taken.

   o  If there is no IPv4 Home Address Reply option present in the
      received Proxy Binding Acknowledgement message, the mobile access
      gateway MUST NOT enable IPv4 support for the mobile node and the
      rest of the considerations from this section can be skipped.

   o  If the received Proxy Binding Acknowledgement message has the
      Status field value in the IPv4 Home Address Reply option set to a
      value that indicates that the request was rejected by the local
      mobility anchor, the mobile access gateway MUST NOT enable IPv4
      mobility support.

   o  If the received Proxy Binding Acknowledgement message has the
      Status field value set to 0 (Proxy Binding Update accepted), the
      mobile access gateway MUST update a Binding Update List entry for
      that mobile node.  The entry MUST be updated with the assigned
      IPv4 home address and other accepted registration values.

   o  If the received Proxy Binding Acknowledgement message has the
      Status field value set to 0 (Proxy Binding Update accepted) and
      has the IPv4 Home Address Reply option set to a value that
      indicates that the request was accepted by the local mobility
      anchor, the mobile access gateway MUST establish a bidirectional
      tunnel to the local mobility anchor (if there is no existing
      bidirectional tunnel to that local mobility anchor) and with the
      encapsulation mode set to IPv4-or-IPv6-over-IPv6 (an IPv4 or IPv6
      packet carried as a payload of an IPv6 packet).  Considerations
      from Section 5.6.1 of [RFC5213] MUST be applied for managing the
      dynamically created bidirectional tunnel.  However, when using
      IPv4 transport, the encapsulation mode MUST be set to the
      negotiated encapsulation mode, as specified in Section 4 of this
      document.



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   o  The mobile access gateway MUST set up the route for forwarding the
      IPv4 packets received from the mobile node (using its IPv4 home
      address) through the bidirectional tunnel set up for that mobile
      node.

   o  The default router address MUST be obtained from the IPv4 Default-
      Router Address option present in the received Proxy Binding
      Acknowledgement message.  The mobile access gateway SHOULD
      configure this address on its interface and respond to any Address
      Resolution Protocol (ARP) requests sent by the mobile node to
      resolve the hardware address of the default router.  However,
      since the link between the mobile access gateway and the mobile
      node is a point-to-point link, implementations will be able
      receive any packets sent to the default router address without
      having to explicitly configure the default router address on its
      interface.  The mobile access gateway MAY also use the default
      router address as the source address for any datagrams sent to the
      mobile node and originated by the mobile access gateway itself.
      It MUST also use this address in the DHCP Router option [RFC2132]
      in the DHCP messages.

   o  If there is an IPv4 DHCP Support Mode option (Section 3.3.4)
      present in the received Proxy Binding Acknowledgement message and
      if the (S) flag in the option is set to a value of (1), then the
      mobile access gateway MUST function as a DHCP server for the
      mobile node.  If either the (S) flag in the option is set to a
      value of (0), or if the option is not present in the request, then
      the mobile access gateway MUST function as a DHCP Relay for the
      mobile node.

3.2.3.3.  Binding Re-Registration and De-Registrations

   When sending a Proxy Binding Update either to extend the lifetime of
   a mobility session or to de-register the mobility session, the
   respective considerations from [RFC5213] MUST be applied.
   Furthermore, the following additional considerations MUST also be
   applied.

   o  If there is an IPv4 home address assigned to the mobility session,
      then there MUST be exactly one instance of the IPv4 Home Address
      Request option present in the Proxy Binding Update message.  The
      IPv4 home address and the prefix length fields in the option MUST
      be set to that specific address and its corresponding subnet-mask
      length.

   o  If there was no IPv4 home address requested in the initial Proxy
      Binding Update message, but it is determined that the IPv4 home
      address MUST be requested subsequently, then there MUST be exactly



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      one instance of the IPv4 Home Address Request option present in
      the Proxy Binding Update message.  The IPv4 home address in the
      option MUST be set to either ALL_ZERO or to a specific address
      that is being requested.

   o  For performing selective de-registration of IPv4 home address but
      still retaining the mobility session with all the IPv6 home
      network prefixes, the Proxy Binding Update message with the
      lifetime value of (0) MUST NOT include any IPv6 Home Network
      Prefix options [RFC5213].  It MUST include exactly one instance of
      the IPv4 Home Address Request option with the IPv4 home address
      and the prefix length fields in the option set to the IPv4 home
      address that is being de-registered.  Similarly, for selective de-
      registration of all the IPv6 home network prefixes, the Proxy
      Binding Update message MUST NOT include the IPv4 Home address
      option, it MUST include a Home Network Prefix option for each of
      the assigned home network prefixes assigned for that mobility
      session and with the prefix value in the option set to that
      respective prefix value.

   o  The Home Network Prefix option(s) [RFC5213] MUST NOT be present if
      the same option(s) was not present in the initial Proxy Binding
      Update message.  Otherwise, considerations from [RFC5213] with
      respect to this option MUST be applied.

   o  If at any point the mobile access gateway fails to extend the
      binding lifetime with the local mobility anchor for the mobile
      node's IPv4 address, it MUST remove any forwarding state set up
      for the mobile node's IPv4 home address.

3.2.4.  Routing Considerations for the Mobile Access Gateway

   o  On receiving a packet from the bidirectional tunnel established
      with the mobile node's local mobility anchor, the mobile access
      gateway MUST remove the outer header before forwarding the packet
      to the mobile node.

   o  On receiving a packet from a mobile node connected to its access
      link, the packet MUST be forwarded to the local mobility anchor
      through the bidirectional tunnel established with the local
      mobility anchor.  However, when the EnableMAGLocalRouting flag is
      set, considerations from Section 6.10.3 of [RFC5213] MUST be
      applied with respect to local routing.

   o  When forwarding the packet through the bidirectional tunnel, the
      encapsulation considerations as specified in Section 3.1.3 MUST be
      applied (except that the source and destination addresses fields
      in the outer encapsulation header are reversed).  However, before



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      forwarding the packet, the mobile access gateway MUST ensure the
      source address in the received packet is the address allocated for
      that mobile node and that there is an active binding on the local
      mobility anchor for that mobile node.

   o  The mobile access gateway SHOULD use the Proxy ARP [RFC0925] to
      reply to ARP Requests that it receives from the mobile node
      seeking address resolutions for the destinations on the mobile
      node's home subnet.  When receiving an ARP Request, the mobile
      access gateway SHOULD examine the target IP address of the
      Request, and if this IP address matches the mobile node's IPv4
      home subnet, it SHOULD transmit a Proxy ARP Reply.  However, on
      certain types of links, the mobile node does not use ARP for
      address resolutions, instead it forwards all the packets to the
      mobile access gateway.  On such types of links, the mobile access
      gateway is not required to support the Proxy ARP function.  At the
      same time, implementations not supporting the Proxy ARP function
      on links where the mobile node uses ARP for seeking address
      resolutions for the destinations on the mobile node's home subnet
      will result in communication failure.

3.3.  Mobility Options and Status Codes

   To support the IPv4 home address mobility feature, this specification
   defines the following new options and status codes.

3.3.1.  IPv4 Home Address Request Option

   A new option, the IPv4 Home Address Request option, is defined for
   use with the Proxy Binding Update message sent by the mobile access
   gateway to the local mobility anchor.  This option is used to request
   IPv4 home address assignment for the mobile node.

   The IPv4 Home Address Request option has an alignment requirement of
   4n.  Its format is as follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |   Length      |Prefix-len |      Reserved     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     IPv4 home address                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 3: IPv4 Home Address Request Option






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      Type

         36

      Length

         An 8-bit unsigned integer indicating the length of the option
         in octets, excluding the Type and Length fields.  This field
         MUST be set to (6).

      Prefix-len

         This 6-bit unsigned integer indicating the prefix length of the
         mobile node's IPv4 home network corresponding to the IPv4 home
         address contained in the option.

      Reserved

         This 10-bit field is unused for now.  The value MUST be
         initialized to (0) by the sender and MUST be ignored by the
         receiver.

      IPv4 home address

         This 4-byte field containing the IPv4 home address that is
         being requested.  The value of 0.0.0.0 is used to request that
         the local mobility anchor perform the address allocation.

3.3.2.  IPv4 Home Address Reply Option

   A new option, the IPv4 Home Address Reply option, is defined for use
   in the Proxy Binding Acknowledgement message sent by the local
   mobility anchor to the mobile access gateway.  This option can be
   used to send the assigned mobile node's IPv4 home address.

   The IPv4 Home Address Reply option has an alignment requirement of
   4n.  Its format is as follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |    Length     |   Status      |Pref-len   |Res|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      IPv4 home address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 4: IPv4 Home Address Reply Option




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      Type

         37

      Length

         An 8-bit unsigned integer indicating the length of the option
         in octets, excluding the Type and Length fields.  This field
         MUST be set to (6).

      Status

         Indicates success or failure for the IPv4 home address
         assignment.  Values from 0 to 127 indicate success.  Higher
         values (128 to 255) indicate failure.  The following Status
         values are currently allocated by this document:

            0 Success

            128 Failure, reason unspecified

            129 Administratively prohibited

            130 Incorrect IPv4 home address

            131 Invalid IPv4 address

            132 Dynamic IPv4 home address assignment not available

      Prefix-len

         This 6-bit unsigned integer is used to carry the prefix length
         of the mobile node's IPv4 home network corresponding to the
         IPv4 home address contained in the option.

      Reserved (Res)

         This 2-bit field is unused for now.  The value MUST be
         initialized to (0) by the sender and MUST be ignored by the
         receiver.

      IPv4 home address

         This 4-byte field is used to carry the IPv4 home address
         assigned to the mobile node.






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3.3.3.  IPv4 Default-Router Address Option

   A new option, the IPv4 Default-Router Address option, is defined for
   use in the Proxy Binding Acknowledgement message sent by the local
   mobility anchor to the mobile access gateway.  This option can be
   used to send the mobile node's IPv4 default router address.

   The IPv4 Default-Router Address option has an alignment requirement
   of 4n.  Its format is as follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Type     |   Length      |         Reserved (R)          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  IPv4 Default-Router Address                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 5: IPv4 Default-Router Address Option

      Type

         38

      Length

         An 8-bit unsigned integer indicating the length of the option
         in octets, excluding the Type and Length fields.  This field
         MUST be set to (6).

      Reserved (R)

         This 16-bit field is unused for now.  The value MUST be
         initialized to (0) by the sender and MUST be ignored by the
         receiver.

      IPv4 Default-Router Address

         A 4-byte field containing the mobile node's default router
         address.

3.3.4.  IPv4 DHCP Support Mode Option

   A new option, the IPv4 DHCP Support Mode option, is defined for use
   in the Proxy Binding Acknowledgement message sent by the local
   mobility anchor to the mobile access gateway.  This option can be





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   used to notify the mobile access gateway as to whether it should
   function as a DHCP Server or a DHCP Relay for the attached mobile
   node.

   The IPv4 DHCP Support Mode option has no alignment requirement.  Its
   format is as follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Type     |   Length      |    Reserved (R)             |S|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 6: IPv4 DHCP Support Mode Option

      Type

         39

      Length

         An 8-bit unsigned integer indicating the length of the option
         in octets, excluding the Type and Length fields.  This field
         MUST be set to 2.

      Reserved (R)

         This 15-bit field is unused for now.  The value MUST be
         initialized to (0) by the sender and MUST be ignored by the
         receiver.

      DHCP Support Mode (S)

         A 1-bit field that specifies the DHCP support mode.  This flag
         indicates whether the mobile access gateway should function as
         a DHCP Server or a DHCP Relay for the attached mobile node.
         The flag value of (0) indicates the mobile access gateway
         should act as a DHCP Relay, and the flag value of (1) indicates
         it should act as a DHCP Server.

3.3.5.  Status Codes

   This document defines the following new Status values for use in the
   Proxy Binding Acknowledgement message.  These values are to be
   allocated from the same numbering space, as defined in Section 6.1.8
   of [RFC3775].





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   NOT_AUTHORIZED_FOR_IPV4_MOBILITY_SERVICE: 170

      Mobile node not authorized for IPv4 mobility service.

   NOT_AUTHORIZED_FOR_IPV4_HOME_ADDRESS: 171

      Mobile node not authorized for the requesting IPv4 home address.

   NOT_AUTHORIZED_FOR_IPV6_MOBILITY_SERVICE: 172

      Mobile node not authorized for IPv6 mobility service.

   MULTIPLE_IPV4_HOME_ADDRESS_ASSIGNMENT_NOT_SUPPORTED: 173

      Multiple IPv4 home address assignments not supported.

3.4.  Supporting DHCP-Based Address Configuration

   This section explains how DHCP-based address configuration support
   can be enabled for a mobile node in a Proxy Mobile IPv6 domain.  It
   explains the protocol operation, supported DHCP server deployment
   configurations, and the protocol interactions between DHCP agents and
   mobility entities in each of the supported configurations.

   This specification supports the following two DHCP deployment
   configurations.

   o  DHCP relay agent co-located with the mobile access gateway.

   o  DHCP server co-located in the mobile access gateway.

   The following are the configuration requirements:

   o  The DHCP server or the DHCP relay agent configured on the mobile
      access gateway is required to have an IPv4 address for exchanging
      the DHCP messages with the mobile node.  This address is the
      mobile node's default router address provided by the local
      mobility anchor.  Optionally, all the DHCP servers co-located with
      the mobile access gateways in the Proxy Mobile IPv6 domain can be
      configured with a fixed IPv4 address.  This fixed address can be
      an IPv4 private address [RFC1918] that can be used for the DHCP
      protocol communication on any of the access links.  This address
      will be used as the server identifier in the DHCP messages.

   o  A DHCP server identifies a DHCP interface from the contents of the
      DHCP "Client-identifier" option [RFC2132], if present, or from the
      client hardware address (chaddr), as specified in [RFC2131].  Note
      that the name "Client-identifier" is a misnomer as it actually



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      identifies an interface and not the client.  The DHCP server uses
      this identity to identify the interface for which the address is
      assigned.  A mobile node in a Proxy Mobile IPv6 domain, can attach
      to the network through multiple interfaces and can obtain address
      configuration for each of its interfaces.  Additionally, it may
      perform handoffs between its interfaces.  The following are the
      related considerations with respect to the identification
      presented to the DHCP server.

      *  If the mobile node attaches to the Proxy Mobile IPv6 domain
         through multiple physical interfaces, the DHCP server will
         uniquely identify each of those interfaces and will perform
         address assignment.  The DHCP server will identify the
         interface as specified in RFC 2131.  The mobile node SHOULD
         generate and use the "Client-identifier" for each physical
         interface according to [RFC4361].  Any time the mobile node
         performs a handoff of a physical interface to a different
         mobile access gateway, using the same interface, the DHCP
         server will always be able to identify the binding using the
         presented identifier.  The presented identifier (either the
         "Client-identifier" or the hardware address) will remain as the
         primary key for each binding, just as how they are unique in a
         Binding Cache entry.

      *  If the mobile node is capable of performing a handoff between
         interfaces, as per [RFC5213], a "Client-identifier" value MUST
         be used for the attachment point that is not tied to any of the
         physical interfaces.  The identifier MUST be generated
         according to [RFC4361], which guarantees that the identifier is
         stable and unique across all "Client-identifier" values in use
         in the Proxy Mobile IPv6 domain.

   o  All the DHCP servers co-located with the mobile access gateways in
      a Proxy Mobile IPv6 domain can be configured with the same set of
      DHCP option values (e.g., DNS Server, SIP Server, etc.) to ensure
      the mobile node receives the same configuration values on any of
      the access links in that Proxy Mobile IPv6 domain.

3.4.1.  DHCP Server Co-Located with the Mobile Access Gateway

   This section explains the operational sequence of home address
   assignment operation when the DHCP server is co-located with the
   mobile access gateway.








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     MN   MAG(DHCP-S)  LMA
      |------>|        |    1. DHCPDISCOVER
      |       |------->|    2. Proxy Binding Update
      |       |<-------|    3. Proxy Binding Acknowledgement (IPv4 HoA)
      |       |========|    4. Tunnel/Route Setup
      |<------|        |    5. DHCPOFFER  (IPv4 HoA)
      |------>|        |    6. DHCPREQUEST (IPv4 HoA)
      |<------|        |    7. DHCPACK
      |       |        |

    Figure 7: Overview of DHCP Server Located at Mobile Access Gateway

   o  It is possible that the mobile access gateway may have already
      completed the Proxy Mobile IPv6 signaling with the local mobility
      anchor to request both IPv6 home network prefix(es) and IPv4 home
      address assignment prior to Step 1.  In such an event, the Proxy
      Mobile IPv6 signaling steps (Steps 2 to 4) above are not relevant.

   o  It is possible the mobile access gateway may have initially
      completed the Proxy Mobile IPv6 signaling prior to Step 1, but
      only for requesting IPv6 home network prefix(es), and it may later
      request IPv4 home address assignment after detecting the DHCP
      triggers from the mobile node as shown above.

   o  The mobile access gateway may choose to ignore the DHCPDISCOVER
      messages until the Proxy Mobile IPv6 signaling is successfully
      completed, or it may choose to send a delayed response for
      reducing the additional delay waiting for a new DHCPDISCOVER
      message from the mobile node.

   Initial IPv4 Home Address Assignment:

   o  To acquire the mobile node's IPv4 home address from the local
      mobility anchor, the mobile access gateway will initiate Proxy
      Mobile IPv6 signaling with the local mobility anchor.

   o  After the successful completion of the Proxy Mobile IPv6 signaling
      and upon acquiring the mobile node's IPv4 home address from the
      local mobility anchor, the DHCP server on the mobile access
      gateway will send a DHCPOFFER message [RFC2131] to the mobile
      node.  The offered address will be the mobile node's IPv4 home
      address, assigned by the local mobility anchor.  The DHCPOFFER
      message will also have the Subnet Mask option [RFC2132] and Router
      option [RFC2132], with the values in those options set to the
      mobile node's IPv4 home subnet mask and default router address,
      respectively.  Additionally, the Server Identifier option will be
      included and with the value in the option set to the default
      router address.



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   o  If the mobile node sends the DHCPREQUEST message, the DHCP server
      will send DHCPACK message, as per [RFC2131].

   IPv4 Home Address Renewal with the DHCP Server (No Handoff):

   o  Any time the mobile node goes into the DHCP RENEWING state
      [RFC2131], it simply unicasts the DHCPREQUEST message including
      the assigned IPv4 home address in the 'Requested IP Address'
      option.  The DHCPREQUEST is sent to the address specified in the
      Server Identifier option of the previously received DHCPOFFER and
      DHCPACK messages.

   o  The DHCP server will send a DHCPACK to the mobile node to
      acknowledge the assignment of the committed IPv4 address.

   IPv4 Home Address Renewal with the DHCP Server (after Handoff):

   When the mobile node goes into the DHCP RENEWING state [RFC2131], it
   directly unicasts the DHCPREQUEST message to the DHCP server that
   currently provided the DHCP lease.  However, if the mobile node
   changed its point of attachment and is attached to a new mobile
   access gateway, it is required that the mobile node update the DHCP
   server address and use the address of the DHCP server that is co-
   located with the new mobile access gateway.  The following approach
   can be adopted to ensure the mobile node uses the DHCP server on the
   attached link.

     MN   oMAG(DHCP-S) nMAG(DHCP-S)
      |       :        |
    RENEW------------->|    1. DHCPREQUEST (IPv4 HoA)
    BOUND<-------------|    2. DHCPACK (IPv4 HoA) or DHCPNACK
      |       :        |
    *  The use of a fixed DHCP server address on all DHCP servers

              Figure 8: Address Renewal with the DHCP Server

   o  The use of a stable address, either the IPv4 default router
      address of the mobile node or a fixed IPv4 address common in that
      Proxy Mobile IPv6 domain, as the DHCP Server Identifier will
      ensure the DHCPREQUEST message sent by the mobile node to renew
      the address will be received by the new mobile access gateway on
      the attached link.

   o  The mobile access gateway after completing the Proxy Mobile IPv6
      signaling and upon acquiring the IPv4 home address of the mobile
      node will return the address in the DHCPACK message.  However, if
      the mobile access gateway is unable to complete the Proxy Mobile




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      IPv6 signaling or is unable to acquire the same IPv4 address as
      requested by the mobile node, it will send a DHCPNACK message
      [RFC2131] to the mobile node, as shown in Figure 8.

3.4.2.  DHCP Relay Agent Co-Located with the Mobile Access Gateway

   A DHCP relay agent is co-located with each mobile access gateway.  A
   DHCP server is located somewhere in the Proxy Mobile IPv6 domain
   (e.g., is co-located with the local mobility anchor).  Figure 9 shows
   the sequence of IPv4 home address assignment using DHCP Relay.

  MN   MAG(DHCP-R) LMA   DHCP-S
   |       |------->|      | 1. Proxy Binding Update *
   |       |<-------|      | 2. Proxy Binding Acknowledgement (IPv4 HoA)
   |       |========|      | 3. Tunnel/Route Setup*
   |------>|-------------->| 4. DHCPDISCOVER (IPv4 HoA) via DHCP-R
   |<------|<--------------| 5. DHCPOFFER (IPv4 HoA) via DHCP-R
   |------>|-------------->| 6. DHCPREQUEST (IPv4 HoA) via DHCP-R
   |<------|<--------------| 7. DHCPACK (IPv4 HoA) via DHCP-R
   |       |               |

   Figure 9: Overview of the DHCP Relay Located at Mobile Access Gateway

   o  The Proxy Mobile IPv6 signaling (starting at Step 1) and the DHCP
      address configuration (starting at Step 4) may start in any order.
      However, the DHCPOFFER (Step 5) and the immediate steps following
      it will occur in the specified order and only after the Tunnel/
      Route Setup (Step 3).

   o  It is possible the mobile access gateway may have initially
      completed the Proxy Mobile IPv6 signaling with the local mobility
      anchor only to request IPv6 home network prefix(es) and may later
      request IPv4 home address assignment after detecting the DHCP
      triggers from the mobile node (after Step 4).

   o  The mobile access gateway may choose to ignore the DHCPDISCOVER
      messages until the Proxy Mobile IPv6 signaling is successfully
      completed, or it may choose to send a delayed response for
      reducing the additional delay waiting for a new DHCPDISCOVER
      message from the mobile node.

   Initial IPv4 Home Address Assignment:

   o  To acquire the mobile node's IPv4 home address from the local
      mobility anchor, the mobile access gateway will initiate Proxy
      Mobile IPv6 signaling with the local mobility anchor.





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   o  After the successful completion of the Proxy Mobile IPv6 signaling
      and upon acquiring the mobile node's IPv4 home address from the
      local mobility anchor, the mobile access gateway will enable
      forwarding for all the DHCP messages between the mobile node and
      the DHCP server.

   o  The DHCP relay agent on the mobile access gateway will add the
      DHCP Relay Agent Information option [RFC3046] to the DHCPDISCOVER
      message.  The assigned IPv4 home address will be included in the
      Agent Remote ID Sub-option of the DHCP Relay Agent Information
      option.  This sub-option is used as a hint for requesting the DHCP
      server to allocate that specific IPv4 address.

   o  On receiving a DHCPOFFER message from the DHCP server, the mobile
      access gateway will ensure the assigned address is currently
      assigned by the local mobility anchor to that mobile node.  If
      this address is different from what is assigned to the mobile
      node, then the mobile access gateway will drop the DHCPOFFER
      message and an administrative error message will be logged.

   o  When the DHCP messages are sent over administrative boundaries,
      the operators need to ensure these messages are secured.  All the
      DHCP messages relayed by the mobile access gateway can be tunneled
      to the local mobility anchor if needed.  Alternatively, if the
      network in the Proxy Mobile IPv6 domain is secure enough, the
      mobile access gateway can just relay the DHCP messages to the
      server.  To achieve this, all the mobile access gateways need to
      have a route towards the DHCP server.

   IPv4 Home Address Renewal to the same DHCP Server: (No Handoff)

   o  When the DHCP client goes into the DHCP RENEW STATE [RFC2131], it
      directly unicasts DHCPREQUEST messages to the DHCP server.  The
      DHCP relay agent may not detect any changes in the DHCP state.
      For example, if the mobile node releases the IPv4 address, the
      relay agent would not be aware of it.  The following describes
      additional mechanisms for the mobile access gateway to detect any
      changes in the DHCP state.

      *  The DHCP relay agent can intercept all IPv4 DHCP packets
         destined to the set of addresses used within the Proxy Mobile
         IPv6 domain as DHCP addresses.  Since the link between a mobile
         node and a mobile access gateway is the point-to-point link,
         the mobile access gateway will be in path for all the messages.

      *  The DHCP relay agent can use the DHCP Server Identifier
         Override Sub-option [RFC5107] to be in path for all the DHCP
         message flows.  The DHCP client uses the DHCP server address



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         that is overridden by the DHCP relay agent address as a
         destination address of DHCPREQUEST.  The DHCP Server Identifier
         Override Sub-option is recommended only when the fixed DHCP
         relay address is configured on all the mobile access gateways.
         Otherwise, the DHCP relay agent address is changed when the
         mobile node changes the attached mobile access gateway.

   o  However, if the DHCP server is co-located with the local mobility
      anchor, then the DHCP relay agent is not required to intercept the
      unicast DHCP messages between the mobile node and the DHCP server.
      This is because the local mobility anchor will ensure that the
      DHCP state is consistent with the Proxy Mobile IPv6 binding that
      exists for the IPv4 address.

   o  Once the mobile access gateway intercepts the DHCP message from
      the mobile node to the DHCP server, it can verify if the mobile
      node is negotiating the same IPv4 address that the local mobility
      anchor allocated for that mobile node.  If the address in the
      DHCPREQUEST message does not match with the IPv4 address allocated
      for the mobile node, then the mobile access gateway SHOULD drop
      the DHCP message and an administrative error message can be
      logged.

   o  Any time the mobile access gateway detects that the mobile node
      has released its IPv4 address, it can send a Proxy Binding Update
      to the local mobility anchor and de-register the IPv4 mobility
      session.

3.4.3.  Common DHCP Considerations

   The following DHCP-related considerations are common to both the
   supported configuration modes, specified in Sections 3.4.1 and 3.4.2.

   o  When a mobile node sends a DHCPDISCOVER message [RFC2131], the
      DHCP server or the relay agent co-located with the mobile access
      gateway will trigger the mobile access gateway to complete the
      Proxy Mobile IPv6 signaling.  This is the required interaction
      between these two protocols.  The mobile access gateway, on
      receiving this trigger, will check if there is already an assigned
      IPv4 home address for the mobile node, from the local mobility
      anchor.  If there is no assigned IPv4 home address assigned for
      that mobile node, the mobile access gateway will complete the
      Proxy Mobile IPv6 signaling with the local mobility anchor by
      sending a Proxy Binding Update message.

   o  The mobile node needs to be identified by the MN-Identifier, as
      specified in Section 6.6 of [RFC5213].  This identity should be
      associated to the DHCP messages sent by the mobile node.



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   o  The mobile access gateway will drop all the DHCPDISCOVER messages
      until it completes the Proxy Mobile IPv6 signaling.  If the mobile
      access gateway is unable to complete the Proxy Mobile IPv6
      signaling, or, if the local mobility anchor does not assign an
      IPv4 address for the mobile node, the mobile access gateway MUST
      NOT enable IPv4 home address mobility support for the mobile node
      on that access link.

   o  The trigger for initiating Proxy Mobile IPv6 signaling can also be
      delivered to the mobile access gateway as part of a context
      transfer from the previous mobile access gateway, or delivered
      from the other network elements in the radio network, the details
      of which are outside the scope of this document.

   o  The DHCPOFFER message [RFC2131] sent to the mobile node MUST
      include the Subnet Mask option [RFC2132] and the Router option
      [RFC2132].  The values in the Subnet Mask option and Router option
      MUST be set to the mobile node's IPv4 home subnet mask and its
      default router address, respectively.

   o  The DHCPOFFER message [RFC2131] sent to the mobile node MUST
      include the Interface MTU option [RFC2132].  The DHCP servers in
      the Proxy Mobile IPv6 domain MUST be configured to include the
      Interface MTU option.  The MTU value SHOULD reflect the tunnel MTU
      for the bidirectional tunnel between the mobile access gateway and
      the local mobility anchor.

   o  The DHCP lease length allocated to the mobile node's IPv4 home
      address may be different from the binding lifetime at the local
      mobility anchor for that mobile node's session.  It is not
      possible to keep these lifetimes synchronized, and so its not
      required that the configured lifetimes should be kept same in both
      DHCP and Proxy Mobile IPv6.

   o  When the mobile node performs a handoff from one mobile access
      gateway to another, the mobile access gateway on the new link will
      initiate the Proxy Mobile IPv6 signaling with the local mobility
      anchor.  On completing the Proxy Mobile IPv6 signaling, the mobile
      access gateway has the proper IPv4 address state that the local
      mobility anchor has allocated for the mobile node and that can be
      used for supporting DHCP based address configuration on that link.

   o  Any time the mobile node detects a link change event due to
      handoff, or due to other reasons such as re-establishment of the
      link-layer, the following are the mobile node's considerations
      with respect to the DHCP protocol.





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      *  If the mobile node is DNAv4-capable (Detecting Network
         Attachment version 4) [RFC4436] and if it performs DNAv4
         procedures after receiving a link change event, it would always
         detect the same default router on any of the access links in
         that Proxy Mobile IPv6 domain, as the mobile access gateway
         configures a fixed link-layer address on all the access links,
         as per the base Proxy Mobile IPv6 specification [RFC5213].  The
         mobile node will not perform any DHCP operation specifically
         due to this event.

      *  If the mobile node is not DNAv4-capable [RFC4436], after
         receiving the link change event it will enter INIT-REBOOT state
         [RFC2131] and will send a DHCPREQUEST message as specified in
         Section 3.7 of [RFC2131].  The mobile node will obtain the same
         address configuration as before, as the link change does not
         result in any change at the network layer.

   o  The mobile node may release its IPv4 home address at any time by
      sending the DHCPRELEASE message [RFC2131].  When the mobile access
      gateway detects the DHCPRELEASE message sent by the mobile node,
      it should consider this as a trigger for de-registering the mobile
      node's IPv4 home address.  It will apply the considerations
      specified in Section 3.2.3.3 for performing the de-registration
      procedure.  However, this operation MUST NOT release any IPv6 home
      network prefix(es) assigned to the mobile node.

4.  IPv4 Transport Support

   The Proxy Mobile IPv6 specification [RFC5213] requires the signaling
   messages exchanged between the local mobility anchor and the mobile
   access gateway to be over an IPv6 transport.  However, in some cases,
   the local mobility anchor and the mobile access gateway are separated
   by an IPv4 network.

   The normal Proxy Mobile IPv6 specification [RFC5213] can be run over
   an IPv4 transport without any modifications by using a transition
   technology that allows IPv6 hosts to communicate over IPv4 networks.
   For example, the mobile access gateway and the local mobility anchor
   could have a simple configured IPv6-over-IPv4 tunnel.  Instead of
   configured tunnels, various mechanisms for automatic tunneling could
   be used, too.  To these tunnels, Proxy Mobile IPv6 would look just
   like any other application traffic running over IPv6.

   However, treating Proxy Mobile IPv6 just like any other IPv6 traffic
   would mean an extra layer of encapsulation for the mobile node's
   tunneled data traffic, adding 40 octets of overhead for each packet.





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   The extensions defined in this section allow the mobile access
   gateway and the local mobility anchor to communicate over an IPv4
   network without this overhead.

            IPv4-Proxy-CoA                      IPv4-LMAA
                   |         + - - - - - - +        |
   +--+          +---+      /               \     +---+          +--+
   |MN|----------|MAG|=====   IPv4  Network  =====|LMA|----------|CN|
   +--+          +---+      \               /     +---+          +--+
                             + - - - - - - +

                     Figure 10: IPv4 Transport Network

   When the local mobility anchor and the mobile access gateway are
   configured and reachable using only IPv4 addresses, the mobile access
   gateway serving a mobile node can potentially send the signaling
   messages over IPv4 transport and register its IPv4 address as the
   care-of address in the mobile node's Binding Cache entry.  An IPv4
   tunnel (with any of the supported encapsulation modes) can be used
   for tunneling the mobile node's data traffic.  The following are the
   key aspects of this feature.

   o  The local mobility anchor and the mobile access gateway are both
      configured and reachable using an IPv4 address of the same scope.

   o  The IPv4 addresses used can be private IPv4 addresses, but it is
      assumed that there is no NAT between the local mobility anchor and
      the mobile access gateway.  However, it is possible to use UDP
      encapsulation if other types of middleboxes are present.

   o  The Mobility Header [RFC3775] is carried inside an IPv4 packet
      with UDP header (IPv4-UDP-MH), using a UDP port number for Proxy
      Mobile IPv6 signaling over IPv4.

   o  The mobile node can be an IPv6, IPv4, or a dual IPv4/IPv6 node and
      the IPv4 transport support specified in this section is agnostic
      to the type of address mobility enabled for that mobile node.

   o  The mobile node's data traffic will be tunneled between the local
      mobility anchor and the mobile access gateway.  There are several
      encapsulation modes available:

      *  IPv4 (IPv4 or IPv6 payload packet carried in an IPv4 packet).
         If payload protection using IPsec is enabled for the tunneled
         traffic, the Encapsulating Security Payload (ESP) header
         follows the outer tunnel header.





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      *  IPv4-UDP (payload packet carried in an IPv4 packet with UDP
         header, using a UDP port number for Proxy Mobile IPv6 data;
         this is different port than is used for signaling).  If payload
         protection using IPsec is enabled, the ESP header follows the
         outer IPv4 header, as explained in Section 4.3.

      *  IPv4-UDP-TLV (payload packet carried in an IPv4 packet with UDP
         and TLV header) and IPv4-GRE (Payload packet carried in an IPv4
         packet with GRE header).  Refer to [GREKEY].  If payload
         protection using IPsec is enabled, the ESP header follows the
         outer IPv4 header, as explained in Section 4.3.

4.1.  Local Mobility Anchor Considerations

4.1.1.  Extensions to Binding Cache Entry

   To support this feature, the conceptual Binding Cache entry data
   structure maintained by the local mobility anchor [RFC5213] MUST be
   extended with the following additional parameters.  It is to be noted
   that all of these parameters are specified in [RFC5555] and also
   required here in the present usage context, and are presented here
   only for completeness.

   o  The IPv4 Proxy Care-of Address configured on the mobile access
      gateway that sent the Proxy Binding Update message.  The address
      MUST be the same as the source address of the received IPv4 packet
      that contains the Proxy Binding Update message.  However, if the
      received Proxy Binding Update message is not sent as an IPv4
      packet, i.e., when using IPv6 transport, this field in the Binding
      Cache entry MUST be set to the ALL_ZERO value.

4.1.2.  Extensions to Mobile Node's Policy Profile

   To support the IPv4 Transport Support feature, the mobile node's
   policy profile, specified in Section 6.2 of [RFC5213], MUST be
   extended with the following additional fields.  These are mandatory
   fields of the policy profile required for supporting this feature.

   o  The IPv4 address of the local mobility anchor (IPv4-LMAA).

4.1.3.  Signaling Considerations

   This section provides the rules for processing the Proxy Mobile IPv6
   signaling messages received over IPv4 transport.







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4.1.3.1.  Processing Proxy Binding Updates

   o  If the Proxy Binding Update message is protected with IPsec ESP,
      IPsec processing happens before the packet is passed to Proxy
      Mobile IPv6.

   o  All the considerations from Section 5.3.1 of [RFC5213] except Step
      1 (about IPsec) MUST be applied on the encapsulated Proxy Binding
      Update message.  Note that the Checksum field in Mobility Header
      MUST be ignored.

   o  Upon accepting the request, the local mobility anchor MUST set up
      an IPv4 bidirectional tunnel to the mobile access gateway.  The
      tunnel endpoint addresses are IPv4-LMAA and the IPv4-Proxy-CoA.
      The encapsulation mode MUST be determined by applying the
      following considerations:

      *  If the (F) flag in the received Proxy Binding Update message is
         set to the value of (1), but if the configuration flag,
         AcceptForcedIPv4UDPEncapsulationRequest, is set to a value of
         (0), then the local mobility anchor MUST reject the request
         with the Status field value set to 129 (Administratively
         prohibited).

      *  If the (T) flag is set to (1), or GRE Key option is included,
         see [GREKEY].

      *  If the (F) flag in the received Proxy Binding Update message is
         set to the value of (1), then the encapsulation mode MUST be
         set to IPv4-UDP.  Otherwise, the encapsulation mode MUST be set
         to IPv4.

   o  The local mobility anchor MUST send the Proxy Binding
      Acknowledgement message with the Status field value set to (0)
      (Proxy Binding Update accepted).  The message MUST be constructed
      as specified in Section 4.1.3.2.

4.1.3.2.  Constructing the Proxy Binding Acknowledgement Message

   The local mobility anchor when sending the Proxy Binding
   Acknowledgement message to the mobile access gateway MUST construct
   the message as specified in Section 5.3.6 of [RFC5213].  However, if
   the Proxy Binding Update message was received over IPv4, the
   following additional considerations MUST be applied.

   o  The IPv6 Header is removed, and the Mobility Header containing the
      Proxy Binding Acknowledgement is encapsulated in UDP (with source
      port set to 5436 and destination port set to the source port of



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      the received Proxy Binding Update message).  The Mobility Header
      Checksum field MUST be set to zero (and the UDP checksum MUST be
      used instead).

   o  The source address in the IPv4 header of the message MUST be set
      to the destination IPv4 address of the received request.

   o  If IPsec ESP is used to protect signaling, the packet is processed
      using transport mode ESP as described in Section 4.3.

   o  Figure 11 shows the format of the Proxy Binding Acknowledgement
      message sent over IPv4 and protected using ESP.

     IPv4 header (src=IPv4-LMAA, dst=pbu_src_address)
       ESP header (in transport mode)
         UDP header (sport=5436, dport=5436)
           Mobility Header (PBA)

       Figure 11: Proxy Binding Acknowledgement (PBA) Message Sent over
                                     IPv4

4.1.4.  Routing Considerations

4.1.4.1.  Forwarding Considerations

   Forwarding Packets to the Mobile Node:

   o  On receiving an IPv4 or an IPv6 packet from a correspondent node
      with the destination address matching any of the mobile node's
      IPv4 or IPv6 home addresses, the local mobility anchor MUST
      forward the packet through the bidirectional tunnel set up for
      that mobile node.

   o  The format of the tunneled packet is shown below.  The IPv4-UDP-
      TLV and IPv4-GRE encapsulation modes are described in [GREKEY].

  IPv4 Header (src=IPv4-LMAA, dst=IPv4-Proxy-CoA)] /* Tunnel Header */
    [UDP Header (src port=5437, dst port=5437]   /* If UDP encap nego */
      /* IPv6 or IPv4 Payload Packet */
      IPv6 header (src=CN, dst=MN-HOA)
        OR
      IPv4 header (src=CN, dst=IPv4-MN-HoA)

      Figure 12: Tunneled IPv4 Packet from LMA to MAG (IPv4 or IPv4-UDP
                             Encapsulation Mode)






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   o  Forwarding Packets Sent by the Mobile Node:

      *  All the reverse tunneled packets (IPv4 and IPv6) that the local
         mobility anchor receives from the mobile access gateway, after
         removing the tunnel header (i.e., the outer IPv4 header along
         with the UDP and TLV header, if negotiated) MUST be routed to
         the destination specified in the inner packet header.  These
         routed packets will have the source address field set to the
         mobile node's home address.

4.1.4.2.  ECN and Payload Fragmentation Considerations

   The ECN considerations specified in Section 5.6.3 of [RFC5213] apply
   for the IPv4 transport tunnels as well.  The mobility agents at the
   tunnel entry and exit points MUST handle ECN information as specified
   in that document.

   The mobility agents at the tunnel entry and exit points MUST apply
   the IP packet fragmentation considerations as specified in [RFC4213].
   Additionally, they MUST also apply the considerations related to
   tunnel error processing and reporting as specified in the same
   specification.

4.1.4.3.  Bidirectional Tunnel Management

   The Tunnel Management considerations specified in Section 5.6.1 of
   [RFC5213] apply for the IPv4 transport tunnels as well, with just one
   difference that the encapsulation mode is different.

4.2.  Mobile Access Gateway Considerations

4.2.1.  Extensions to Binding Update List Entry

   To support the IPv4 Transport Support feature, the conceptual Binding
   Update List entry data structure maintained by the mobile access
   gateway [RFC5213] MUST be extended with the following additional
   parameters.

   o  The IPv4 address of the local mobility anchor.  This address can
      be obtained from the mobile node's policy profile.

4.2.2.  Signaling Considerations

   The mobile access gateway, when sending a Proxy Binding Update
   message to the local mobility anchor, MUST construct the message as
   specified in Section 6.9.1.5 of [RFC5213].  However, if the mobile
   access gateway is in an IPv4-only access network, the following
   additional considerations MUST be applied.



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   o  The Proxy Binding Update message MUST be sent over IPv4 as
      described in Section 4.2.2.1.

   o  Just as specified in [RFC5213], when sending a Proxy Binding
      Update message for extending the lifetime of a currently existing
      mobility session or to de-register the mobility session, the Proxy
      Binding Update message MUST be constructed just as the initial
      request.

   Receiving Proxy Binding Acknowledgement:

   o  If the received Proxy Binding Acknowledgement message is protected
      with IPsec ESP, IPsec processing happens before the packet is
      passed to Proxy Mobile IPv6.  Considerations from Section 4 of
      [RFC5213] MUST be applied to authenticate and authorize the
      message.

   o  All the considerations from Section 6.9.1.2 of [RFC5213] MUST be
      applied on the encapsulated Proxy Binding Acknowledgement message.
      Note that the Checksum field in Mobility Header MUST be ignored.

   o  If the Status field indicates Success, the mobile access gateway
      MUST set up a bidirectional tunnel to the local mobility anchor.

   o  Upon accepting the request, the mobile access gateway MUST set up
      an IPv4 bidirectional tunnel to the local mobility anchor.  The
      tunnel endpoint addresses are the IPv4-Proxy-CoA and the IPv4-
      LMAA.  The encapsulation mode MUST be determined from the below
      considerations:

      *  If the (T) flag is set to (1), or the GRE Key option is
         included, see [GREKEY].

      *  If there is a NAT Detection option [RFC5555] in the received
         Proxy Binding Acknowledgement message, and the (F) flag is set
         to value of (1), the encapsulation mode for the tunnel MUST be
         set to IPv4-UDP.  Otherwise, the encapsulation mode MUST be set
         to IPv4.

4.2.2.1.  Constructing the Proxy Binding Update Message

   o  The IPv6 Header is removed, and the Mobility Header containing the
      Proxy Binding Update message is encapsulated in UDP (with the
      destination port set to 5436).  The Mobility Header Checksum field
      MUST be set to zero (and UDP checksum MUST be used instead).






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   o  The source address in the IPv4 header MUST be set to IPv4-Proxy-
      CoA of the mobile access gateway and the destination address MUST
      be set to the local mobility anchor's IPv4-LMAA.

   o  If the configuration variable ForceIPv4UDPEncapsulationSupport is
      set to value of (1), then the (F) flag in the Proxy Binding Update
      message MUST be set to value of (1).

   o  If IPsec ESP is used to protect signaling, the packet is processed
      using transport mode ESP as described in Section 4.3.

   o  Figure 13 shows the format of the Proxy Binding Update message
      sent over IPv4 and protected using ESP.

     IPv4 header (src=IPv4-Proxy-CoA, dst=IPv4-LMAA)
       ESP header (in transport mode)
         UDP header (sport=5436, dport=5436)
           Mobility Header (PBU)

         Figure 13: Proxy Binding Update (PBU) Message Sent over IPv4

4.2.2.2.  Forwarding Considerations

   Forwarding Packets Sent by the Mobile Node:

   o  On receiving an IPv4 or an IPv6 packet from the mobile node to any
      destination, the mobile access gateway MUST tunnel the packet to
      the local mobility anchor.  The format of the tunneled packet is
      shown below.  The IPv4-UDP-TLV and IPv4-GRE encapsulation modes
      are described in [GREKEY].  However, considerations from Section
      6.10.3 of [RFC5213] MUST be applied with respect the local routing
      and on the use of EnableMAGLocalRouting flag.

 IPv4 Header (src=IPv4-Proxy-CoA, dst=IPv4-LMAA)] /* Tunnel Header */
    [UDP Header (src port=5437, dst port=5437]   /* If UDP encap nego */
      /* IPv6 or IPv4 Payload Packet */
      IPv6 header (src=MN-HOA, dst=CN)
        OR
      IPv4 header (src=IPv4-MN-HOA, dst=CN)

      Figure 14: Tunneled IPv4 Packet from MAG to LMA (IPv4 or IPv4-UDP
                             Encapsulation Mode)









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   Forwarding Packets Received from the Bidirectional Tunnel:

   o  On receiving a packet from the bidirectional tunnel established
      with the mobile node's local mobility anchor, the mobile access
      gateway MUST remove the outer header before forwarding the packet
      to the mobile node.

4.3.  IPsec Considerations

4.3.1.  PBU and PBA

   The following section describes how IPsec is used to protect the
   signaling messages and data packets between the local mobility anchor
   and mobile access gateway when using IPv4 transport.

   The following are the Security Policy Database (SPD) example entries
   to protect PBU and PBA on the local mobility anchor and mobile access
   gateway.

           MAG SPD-S:
             - IF local_address = IPv4-Proxy-CoA_1 &
                  remote_address = IPv4-LMAA_1 & proto = UDP &
                  remote_port = 5436
               Then use SA ESP transport mode

           LMA SPD-S:
             - IF local_address = IPv4-LMAA_1 &
                  remote_address = IPv4-Proxy-CoA_1 & proto = UDP &
                  local_port = 5436
               Then use SA ESP transport mode

4.3.2.  Payload Packet

   The following are the SPD example entries to protect payload packets
   on the local mobility anchor and mobile access gateway.  Note that
   the example SPDs protect all payload packets sent to and from mobile
   nodes.  If an operator needs to apply a different security mechanism
   per mobile node, they need to create a SPD and a SA entry per mobile
   node.












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           MAG SPD-S:
             - IF interface = tunnel to LMAA_1 &
                  local_address != Proxy-CoA_1 &
                  remote_address != LMAA_1 & proto=any
               Then use SA ESP tunnel mode

           LMA SPD-S:
             - IF interface = tunnel to Proxy-CoA_1 &
                  local_address != LMAA_1 &
                  remote_address != Proxy-CoA_1 & proto=any
               Then use SA ESP tunnel mode

   When payload packets are protected by IPsec, payload packets matching
   the SPDs are passed to the IPsec module and encapsulated using the
   tunnel mode ESP.  The tunnel mode ESP encapsulated payload packets
   are then directly sent to the peer mobile access gateway or local
   mobility anchor.  If IPsec is not applied to payload packets, then
   they are encapsulated as shown in Figures 12 and 14.

5.  Protocol Configuration Variables

5.1.  Local Mobility Anchor - Configuration Variables

   The local mobility anchor MUST allow the following variables to be
   configured by the system management.  The configured values for these
   protocol variables MUST survive server reboots and service restarts.

   AcceptForcedIPv4UDPEncapsulationRequest

      This flag indicates whether or not the local mobility anchor
      should accept IPv4 UDP encapsulation request for the mobile node's
      data traffic.  The default value for this flag is set to (0),
      indicating that plain IPv4 encapsulation (without UDP) is used for
      data traffic.

5.2.  Mobile Access Gateway - Configuration Variables

   The mobile access gateway MUST allow the following variables to be
   configured by the system management.  The configured values for these
   protocol variables MUST survive server reboots and service restarts.

   ForceIPv4UDPEncapsulationSupport

      This flag indicates whether or not the mobile access gateway
      should request the mobile node's local mobility anchor to use
      IPv4-UDP encapsulation mode for the mobile node's data traffic.
      The default value for this flag is set to (0), indicating that
      plain IPv4 encapsulation (without UDP) is used for data traffic.



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6.  IANA Considerations

   This document defines four new Mobility Header options: the IPv4 Home
   Address Request option, IPv4 Home Address Reply option, IPv4 Default
   Router Address option, and IPv4 DHCP Support Mode option.  These
   options are described in Sections 3.3.1, 3.3.2, 3.3.3, and 3.3.4,
   respectively.  The Type value for these options has been assigned
   from the same number space as allocated for the other mobility
   options, as defined in [RFC3775].

   The IPv4 Home Address Reply option, described in Section 3.3.2 of
   this document, introduces a new number space, IPv4 Home Address Reply
   status codes.  This document currently reserves the following values.
   Approval of any new status code values are to be made through IANA
   Expert Review.

   o  0 Success

   o  128 Failure, Reason Unspecified

   o  129 Administratively prohibited

   o  130 Incorrect IPv4 home address

   o  131 Invalid IPv4 address

   o  132 Dynamic IPv4 home address assignment not available

   The IPv4 DHCP Support Mode option, described in Section 3.3.4 of this
   document, introduces a new number space, IPv4 DHCP Support Mode
   Flags.  This document reserves the value 0x1 for the (S) flag.
   Approval of flag values are to be made through IANA Expert Review.
   At this point in time, there are no thoughts on what the new flag
   allocations can be, and hence this document leaves this to the
   discretion of the Expert Review.

   This document also defines new Status values, used in Proxy Binding
   Acknowledgement message, as described in Section 3.3.5.  These values
   have been assigned from the same number space as allocated for other
   status codes [RFC3775].  Each of these allocated values is greater
   than 128.

      NOT_AUTHORIZED_FOR_IPV4_MOBILITY_SERVICE: 170

         Mobile node not authorized for IPv4 mobility service.






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      NOT_AUTHORIZED_FOR_IPV4_HOME_ADDRESS: 171

         Mobile node not authorized for the requesting IPv4 home
         address.

      NOT_AUTHORIZED_FOR_IPV6_MOBILITY_SERVICE: 172

         Mobile node not authorized for IPv6 mobility service.

      MULTIPLE_IPV4_HOME_ADDRESS_ASSIGNMENT_NOT_SUPPORTED: 173

         Multiple IPv4 home address assignment not supported.

   IANA has assigned two UDP port numbers, 5436 and 5437, for "pmip6-
   cntl" and "pmip6-data", respectively.

7.  Security Considerations

   All the security considerations from the base Proxy Mobile IPv6
   [RFC5213], Mobile IPv6 [RFC3775], and Dual-Stack Mobile IPv6
   [RFC5555] specifications apply when using the extensions defined in
   this document.  Additionally, the following security considerations
   need to be applied.

   This document defines new mobility options for supporting the IPv4
   Home Address assignment and IPv4 Transport Support features.  These
   options are to be carried in Proxy Binding Update and Proxy Binding
   Acknowledgement messages.  The required security mechanisms specified
   in the base Proxy Mobile IPv6 protocol for protecting these signaling
   messages are sufficient when carrying these mobility options.

   This specification describes the use of IPv4 transport for exchanging
   signaling messages between the local mobility anchor and the mobile
   access gateway.  These can be protected using IPsec as described in
   Section 4.3.

8.  Contributors

   This document reflects discussions and contributions from several
   people (in alphabetical order):

   Kuntal Chowdhury

      kchowdhury@starentnetworks.com

   Vijay Devarapalli

      vijay.devarapalli@azairenet.com



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   Pasi Eronen

      Pasi.Eronen@nokia.com

   Sangjin Jeong

      sjjeong@etri.re.kr

   Basavaraj Patil

      basavaraj.patil@nokia.com

   Myungki Shin

      myungki.shin@gmail.com

9.  Acknowledgements

   The IPv4 support for Proxy Mobile IPv6 was initially covered in
   "Proxy Mobile IPv6" (March 2007).  We would like to thank all the
   authors of the document and acknowledge that initial work.

   Thanks to Alper Yegin, Behcet Sarikaya, Bernard Aboba, Charles
   Perkins, Damic Damjan, Jari Arkko, Joel Hortelius, Jonne Soinnen,
   Julien Laganier, Mohana Jeyatharan, Niklas Nuemann, Pasi Eronen,
   Premec Domagoj, Ralph Droms, Sammy Touati, Vidya Narayanan, Yingzhe
   Wu, and Zu Qiang for their helpful review of this document.

   Also, we would like to thank Spencer Dawkins, Tim Polk, Menachem
   Dodge, Adrian Farrel, and Pekka Savola for their reviews of this
   document as part of the IESG review process.  Finally, special thanks
   to Jouni Korohonen for his support in addressing the IPsec issues.

10.  References

10.1.  Normative References

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

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, March 1997.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, December 1998.



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   [RFC3046]  Patrick, M., "DHCP Relay Agent Information Option",
              RFC 3046, January 2001.

   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
              in IPv6", RFC 3775, June 2004.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

   [RFC4361]  Lemon, T. and B. Sommerfeld, "Node-specific Client
              Identifiers for Dynamic Host Configuration Protocol
              Version Four (DHCPv4)", RFC 4361, February 2006.

   [RFC5107]  Johnson, R., Kumarasamy, J., Kinnear, K., and M. Stapp,
              "DHCP Server Identifier Override Suboption", RFC 5107,
              February 2008.

   [RFC5213]  Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K.,
              and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008.

   [RFC5555]  Soliman, H., "Mobile IPv6 Support for Dual Stack Hosts and
              Routers", RFC 5555, June 2009.

10.2.  Informative References

   [RFC0925]  Postel, J., "Multi-LAN address resolution", RFC 925,
              October 1984.

   [RFC1332]  McGregor, G., "The PPP Internet Protocol Control Protocol
              (IPCP)", RFC 1332, May 1992.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 2005.

   [RFC4436]  Aboba, B., Carlson, J., and S. Cheshire, "Detecting
              Network Attachment in IPv4 (DNAv4)", RFC 4436, March 2006.

   [RFC4977]  Tsirtsis, G. and H. Soliman, "Problem Statement: Dual
              Stack Mobility", RFC 4977, August 2007.




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   [GREKEY]   Muhanna, A., Khalil, M., Gundavelli, S., and K. Leung,
              "GRE Key Option for Proxy Mobile IPv6", Work in Progress,
              May 2009.

Authors' Addresses

   Ryuji Wakikawa
   TOYOTA InfoTechnology Center, U.S.A., Inc.
   465 Bernardo Avenue
   Mountain View, CA  94043
   USA

   EMail: ryuji@us.toyota-itc.com


   Sri Gundavelli
   Cisco
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   EMail: sgundave@cisco.com





























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