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NETLMM Working Group                                         R. Wakikawa
Internet-Draft                                                Toyota ITC
Intended status: Standards Track                           S. Gundavelli
Expires: September 25, 2009                                        Cisco
                                                          March 24, 2009


                   IPv4 Support for Proxy Mobile IPv6
              draft-ietf-netlmm-pmip6-ipv4-support-10.txt

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 25, 2009.

Copyright Notice

   Copyright (c) 2009 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
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   (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.







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Abstract

   This document specifies extensions to 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. 2) allowing the mobility entities in the Proxy Mobile IPv6
   domain to exchange signaling messages over an IPv4 transport network.












































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Table of Contents

   1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Stated Assumptions . . . . . . . . . . . . . . . . . . . .  6
     1.2.  Relevance to Dual-Stack Mobile IPv6  . . . . . . . . . . .  7

   2.  Conventions & Terminology  . . . . . . . . . . . . . . . . . .  9
     2.1.  Conventions  . . . . . . . . . . . . . . . . . . . . . . .  9
     2.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  9

   3.  IPv4 Home Address Mobility Support . . . . . . . . . . . . . . 11
     3.1.  Local Mobility Anchor Considerations . . . . . . . . . . . 12
       3.1.1.  Extensions to Binding Cache Entry  . . . . . . . . . . 12
       3.1.2.  Signaling Considerations . . . . . . . . . . . . . . . 12
       3.1.3.  Routing Considerations for the Local Mobility
               Anchor . . . . . . . . . . . . . . . . . . . . . . . . 18
     3.2.  Mobile Access Gateway Considerations . . . . . . . . . . . 19
       3.2.1.  Extensions to Binding Update List Entry  . . . . . . . 19
       3.2.2.  Extensions to Mobile Node's Policy Profile . . . . . . 19
       3.2.3.  Signaling Considerations . . . . . . . . . . . . . . . 19
       3.2.4.  Routing Considerations for the Mobile Access
               Gateway  . . . . . . . . . . . . . . . . . . . . . . . 23
     3.3.  Mobility Options and Status Codes  . . . . . . . . . . . . 24
       3.3.1.  IPv4 Default-Router Address Option . . . . . . . . . . 24
       3.3.2.  IPv4 DHCP Support Mode . . . . . . . . . . . . . . . . 25
       3.3.3.  Status Codes . . . . . . . . . . . . . . . . . . . . . 26
     3.4.  Supporting DHCP-Based Address Configuration  . . . . . . . 26
       3.4.1.  DHCP Server co-located with the Mobile Access
               Gateway  . . . . . . . . . . . . . . . . . . . . . . . 27
       3.4.2.  DHCP Relay Agent co-located with the Mobile Access
               Gateway  . . . . . . . . . . . . . . . . . . . . . . . 30

   4.  IPv4 Transport Support . . . . . . . . . . . . . . . . . . . . 33
     4.1.  Local Mobility Anchor Considerations . . . . . . . . . . . 34
       4.1.1.  Extensions to Binding Cache Entry  . . . . . . . . . . 34
       4.1.2.  Extensions to Mobile Node's Policy Profile . . . . . . 35
       4.1.3.  Signaling Considerations . . . . . . . . . . . . . . . 35
       4.1.4.  Routing Considerations . . . . . . . . . . . . . . . . 38
     4.2.  Mobile Access Gateway Considerations . . . . . . . . . . . 39
       4.2.1.  Extensions to Binding Update List Entry  . . . . . . . 39
       4.2.2.  Signaling Considerations . . . . . . . . . . . . . . . 40

   5.  Protocol Configuration Variables . . . . . . . . . . . . . . . 43
     5.1.  Local Mobility Anchor - Configuration Variables  . . . . . 43
     5.2.  Mobile Access Gateway - Configuration Variables  . . . . . 43

   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 45




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   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 46

   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 47

   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 47

   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 47
     10.2. Informative References . . . . . . . . . . . . . . . . . . 48

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49








































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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 in dual-stack mode and additionally 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 when the network between the local mobility anchor and the mobile
   access gateway is an IPv4 or an IPv6 network.  The motivation and
   scope of IPv4 support in Mobile IPv6 is summarized in [RFC-4977] and
   all those requirements apply to Proxy Mobile IPv6 protocol as well.

   The Proxy Mobile IPv6 protocol [RFC-5213] 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 extends IPv4 support to the Proxy Mobile IPv6 protocol
   [RFC-5213].

   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 has an IPv4
      stack 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 for enabling 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 and furthermore the
      mobile access gateway may be using an IPv4 private address and
      with NAT [RFC-3022] translation devices on the path to the local
      mobility anchor.

   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 any one or both of these features as
   required.







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               +----+                +----+
               |LMA1|                |LMA2|
               +----+                +----+
   IPv4-LMAA1 -> |                      | <-- LMAA2
                 |                      |
                 \\                    //\\
                (NAT)                 //  \\
                   \\                //    \\
                +---\\------------- //------\\----+
               (     \\  IPv4/IPv6 //        \\    )
               (      \\  Network //          \\   )
                +------\\--------//------------\\-+
                        \\      //              \\
                         \\    //                \\
                          \\  //                  \\
         IPv4-Proxy-CoA1--> |                      | <-- Proxy-CoA2
                         +----+                 +----+
                         |MAG1|-----{MN2}       |MAG2|
                         +----+    |            +----+
        (IPv6 MN-HoA1)     |       |               | <-- (IPv6 MN-HoA2)
        (IPv4-MN-HoA1) --> |   (IPv4-MN-HoA2)      | <-- (IPv4-MN-HoA3)
                         {MN1}                   {MN3}



               Figure 1: IPv4 support for Proxy Mobile IPv6

1.1.  Stated Assumptions

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

   o  The local mobility anchor and the mobile access gateway are both
      IPv4 and 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 [RFC-5213].

   o  The local mobility anchor and the mobile access gateway MUST be
      configured with IPv6 globally unique addresses, even when they are
      in IPv4-only network.  These addresses can be of the type Unique
      Local IPv6 Unicast Address [RFC-4193], IPv6 Global Unicast Address
      [RFC-3587] or IPv4-mapped IPv6 address [RFC-4291].  When using
      IPv4 transport, it is not required that there is IPv6 routing
      enabled between the local mobility anchor and the mobile access
      gateway.  However, they must be able to receive any IPv6 packets
      sent to the configured IPv6 addresses, after removing the outer
      IPv4 encapsulation header.



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   o  The mobile node can be operating in IPv4-only, IPv6-only or in
      dual mode.  Based on what is enabled for a mobile node, it should
      be able to obtain IPv4-only, IPv6-only or both IPv4 and IPv6
      address(es) 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
      needs to 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 [RFC-2131], IPCP [RFC-1332],
      IKEv2 [RFC-4306] or static address configuration.

   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 [RFC-5213], 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 Dual-Stack Mobile IPv6
   specification [ID-DSMIP6].  This document to most part leverages 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.  Following are some of the related
   considerations.

   o  The messaging options, IPv4 Home Address, IPv4 Address
      Acknowledgement, IPv4 Care-of Address option defined in [ID-
      DSMIP6] for use in Binding Update and Binding Acknowledgement
      messages are used by this specification to be carried in Proxy
      Binding Update and Proxy Binding Acknowledgement messages.

   o  The extensions needed to the conceptual data structures, Binding
      Cache entry and Binding Update List entry, for storing the state



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      related to the IPv4 support defined in [ID-DSMIP6], will all be
      needed and relevant for this document.

   o  The NAT traversal logic specified in [ID-DSMIP6] for detecting the
      on-path NAT devices is valid for this specification as well.

   o  The tunneling considerations specified in [ID-DSMIP6] for
      supporting IPv4 transport is relevant for this document as well.

   If a given home agent [RFC-3775] implementation has support for both
   Dual-stack Mobile IPv6 [ID-DSMIP6] and local mobility anchor function
   [RFC-5213], when extending IPv4 support as specified in this document
   the above common functions and the related considerations have to be
   reused for Proxy Mobile IPv6 signaling flows.





































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2.  Conventions & 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 [RFC-2119].

2.2.  Terminology

   All the mobility related terms used in this document are to be
   interpreted as defined in the Mobile IPv6 specification [RFC-3775]
   and Proxy Mobile IPv6 specification [RFC-5213].  In addition this
   document introduces the following terms.

   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.  However, if the configured address is a private IPv4
      address and with a NAT device in the path to the local mobility
      anchor, the care-of address as seen by the local mobility anchor
      will be the address allocated by the NAT device for that flow.

   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 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



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      It is a procedure for partial de-registration of all the addresses
      that belong to one address family, i.e., de-registration of either
      IPv4 home address, or all of the 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

      IPv4-or-IPv6-over-IPv4-UDP

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

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

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

























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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 interface and be able to retain that
   address configuration even after changing its point of attachment in
   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 [RFC-1332], IKEv2 [RFC-4306] or DHCP [RFC-2131], 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, however
   there needs to be some interactions between these two protocol flows.
   Section 3.4 identifies these interactions for supporting DHCP based
   address configuration.

   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 for 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
   an 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.




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   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 an IPv4 home address from different subnets.  In such
   scenario, there will be multiple Binding Cache entries for the mobile
   node on the local mobility anchor.  All the address (IPv4/IPv6)
   assigned to a given interface will be managed as part of one mobility
   session, as specified in Section 5.4 of [RFC-5213].

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 the [ID-DSMIP6] and is
      presented here for completeness.

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

3.1.2.  Signaling Considerations


3.1.2.1.  Processing Proxy Binding Updates

   The processing rules specified in Section 5.3 of [RFC-5213] are
   applied for processing the received Proxy Binding Update message.
   However, if the received Proxy Binding Update message has an IPv4
   Home Address option [ID-DSMIP6], the following considerations MUST be
   applied additionally.

   o  If there is an IPv4 Home Address option [ID-DSMIP6] present in the
      received Proxy Binding Update message, but if there is no Home
      Network Prefix option [RFC-5213] present in the request, the local
      mobility anchor MUST NOT reject the request as specified in
      Section 5.3.1 of [RFC-5213].  At least one instance of any of
      these two options MUST be present.  If there is not a single
      instance of any of these two options present in the request, the
      local mobility anchor MUST reject the request and send a Proxy
      Binding Acknowledgement message with Status field set to
      MISSING_HOME_NETWORK_PREFIX_OPTION (Missing mobile node's home
      network prefix option) [RFC-5213].



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   o  If there is at least one instance of Home Network Prefix option
      [RFC-5213] present in the received Proxy Binding Update message,
      but either if it is known from the mobile node's policy profile
      that the mobile node is not authorized for IPv6 service or if IPv6
      routing 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_HOME_NETWORK_PREFIX (mobile node not
      authorized for the requesting IPv6 home network prefix).

   o  If there are more than one instance of the IPv4 Home Address
      option present in the request, then the local mobility anchor MUST
      reject the request and send a Proxy Binding Acknowledgement
      message with the Status field set to
      MULTIPLE_IPV4_HOME_ADDRESS_ASSIGNMENT_NOT_SUPPORTED (multiple IPv4
      home address assignment not supported).

   o  If the prefix request(P) flag in the IPv4 Home Address option is
      set to a value of 1, then the local mobility anchor MUST reject
      the request and send a Proxy Binding Acknowledgement message with
      the Status field set to IPV4_PREFIX_ASSIGNMENT_NOT_SUPPORTED (IPv4
      prefix assignment not supported).

   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 [RFC-5213] with a NON_ZERO prefix value, or, if there is
         an IPv4 Home Address option [ID-DSMIP6] with the IPv4 address
         in the option set to ALL_ZERO, considerations from Section
         5.4.1 of [RFC-5213] MUST be applied.

      *  If there is an IPv4 Home Address option [ID-DSMIP6] 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
      [RFC-5213] present in the request, considerations from Section
      5.3.2 of [RFC-5213] MUST also be applied.





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   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 [RFC-5213], (point 13), to determine if the
      request is re-registration or a de-registration request and the
      respective considerations from the below sections 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 and with the request to extend 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.

3.1.2.2.  Initial Binding Registration (New Mobility Session)


   o  If there is an IPv4 Home Address option [ID-DSMIP6] 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 option [ID-DSMIP6] with the IPv4
      address in the option set to a NON_ZERO value, the local mobility
      anchor before accepting the request MUST ensure the address is
      topologically anchored on the local mobility anchor and
      furthermore 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).  It MUST also include the IPv4
      Address Acknowledgement option [ID-DSMIP6] 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 Address
      Acknowledgement option [ID-DSMIP6] 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 [ID-
      DSMIP6], there should still be only one Binding Cache entry that



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      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 bi-directional tunnel
      to the mobile access gateway and 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 4.0.

   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 [RFC-5213] MUST be
   applied.


3.1.2.4.  Binding Lifetime Extension (After handoff)

   o  If there is no Home Network Prefix option(s) [RFC-5213] 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 option [ID-
      DSMIP6] 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
      bi-directional tunnel for carrying the IPv4 payload traffic (if
      there are no other mobile nodes for which the tunnel is being



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      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 bi-directional tunnel to
      the mobile access gateway that sent the request (if there is no
      existing bi-directional 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.

   o  The required forwarding state identified in Section 5.3.6 of [RFC-
      5213] is for IPv6 payload traffic.  Those considerations apply for
      IPv4 payload traffic as well.  However, if IPv4 transport is in
      use, considerations from Section 4.0 MUST be applied.

3.1.2.5.  Binding De-Registration

   All the considerations from Section 5.3.5 of [RFC-5213] MUST be
   applied.  Additionally, for removing the IPv4 state as part of the
   Binding Cache entry deletion, the IPv4 host route and the dynamically
   created bi-directional 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 with respect to those addresses MUST be deleted.

3.1.2.6.  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 [RFC-5213].
   Additionally, the following considerations MUST be applied.

   o  Section 5.3.6 of [RFC-5213] requires the local mobility anchor to
      include at least one instance of Home Network Prefix option [RFC-
      5213] 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 option [ID-
      DSMIP6] and did 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 [RFC-5213]
      or the IPv4 Address Acknowledgement option [ID-DSMIP6] present in
      the Proxy Binding Acknowledgement message.





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   o  The IPv4 Address Acknowledgement option [ID-DSMIP6] 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 Address Acknowledgement option [ID-DSMIP6]
          corresponding to the IPv4 Home Address option [ID-DSMIP6]
          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.

      2.  For all other cases, there MUST be an IPv4 Address
          Acknowledgement option for carrying 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
          allocated address.  The status field value in the option must
          be set to 0 (Success).

   o  The IPv4 Default-Router Address option MUST be present, if the
      Status field value in the Proxy Binding Acknowledgement message is
      set to 0 (Proxy Binding Update Accepted).  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 [RFC-5213] uses the Home Network Prefix option [RFC-5213]
   as the key parameter for identifying the Binding Cache entry.
   However, when there is not a single Home Network Prefix option with
   NON_ZERO value present in the request, but if there an IPv4 Home
   Address option with a NON_ZERO value present in the request, the
   following considerations MUST be applied.

   o  The search rules specified in section 5.4.1.1 of [RFC-5213], 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  These rules specified in section 5.4.1.1 of [RFC-5213], 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



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      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 request.

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
      be able to receive packets that are sent to the mobile node's IPv4
      home address.  In order for it to receive those packets, it MUST
      advertise a connected route in to the Routing Infrastructure for
      the mobile node's IPv4 home address or for its home subnet.  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 correspondent node with the
      destination address matching the mobile node's IPv4 home address,
      the local mobility anchor MUST forward the packet through the bi-
      directional 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 LMA to 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.






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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 Acknowledgment 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 [RFC-5213] MUST be
   extended with the following additional fields.


   Extensions to the mandatory section of the policy profile:

   o  This field identifies all the IP protocol 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








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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 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
   [RFC-5213] MUST be applied with the following exceptions.

   o  The IPv4 Home Address option [ID-DSMIP6] 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 option [ID-DSMIP6] with the
         IPv4 home address and the prefix length fields in the option
         set to that specific address and its prefix length.
         Furthermore, the (P) flag in the option MUST be set to 0.

      *  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 ALL_ZERO value.  Furthermore, the (P) flag in the
         option MUST be set to 0.  This essentially 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 [RFC-5213].  However, the Home Network
      Prefix option(s) [RFC-5213] 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 for carrying the signaling messages, the
      related considerations from section 4.0 MUST be applied
      additionally.







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3.2.3.2.  Receiving Proxy Binding Acknowledgement

   All the considerations from section 6.9.1.2 of [RFC-5213] 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_HOME_ADDRESS(The
      mobile node is not authorized for IPv4 home address), the mobile
      access gateway SHOULD NOT send a Proxy Binding Update message
      including the IPv4 Home Address option [ID-DSMIP6] till an
      administrative action is taken.

   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 for the
      same address again, but MAY request the local mobility anchor to
      do the assignment of address by including exactly one instance of
      IPv4 Home Address option [ID-DSMIP6] with the IPv4 home address
      and the prefix length fields in the option set to ALL_ZERO value.

   o  If there is no IPv4 Address Acknowledgement option [ID-DSMIP6]
      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 Address Acknowledgement Option [ID-
      DSMIP6] set to a value that indicates that the request was
      rejected by the local mobility anchor, the mobile access gateway
      MUST NOT enable forwarding for that specific IPv4 home address.

   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 Address Acknowledgement Option [ID-DSMIP6] set to a
      value that indicates that the request was accepted by the local
      mobility anchor, the mobile access gateway MUST establish a bi-
      directional tunnel to the local mobility anchor (if there is no
      existing bi-directional tunnel to that local mobility anchor) and
      with the encapsulation mode set to IPv4-or-IPv6-over-IPv6 (IPv4 or
      IPv6 packet carried as a payload of an IPv6 packet).



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      Considerations from Section 5.6.1 of [RFC-5213] MUST be applied
      for managing the dynamically created bi-directional tunnel.
      However, when using IPv4 transport, the encapsulation mode MUST be
      set to the negotiated encapsulation mode, as specified in Section
      4 of this specification.

   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 bi-directional 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 MAY configure
      this address on its interface and respond to any ARP requests sent
      by the mobile node for resolving the hardware address of the
      default-router.  It MAY also use this address as the source
      address for any datagrams sent to the mobile node and originated
      by the mobile access gateway itself.  It MAY also use this address
      in the DHCP Router option [RFC-2132] in the DHCP messages.

   o  If there is an IPv4 DHCP Support Mode option 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 for extending the lifetime
   of a mobility session or for de-registering the mobility session, the
   respective considerations from [RFC-5213] 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
      option [ID-DSMIP6] 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.  The (P) flag in the option MUST be set to 0.

   o  If there was no IPv4 home address requested in the initial Proxy
      Binding Update message, but if it is determined that the IPv4 home
      address MUST be requested subsequently, then there MUST be exactly
      one instance of the IPv4 Home Address option [ID-DSMIP6] present



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      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(s) [RFC-5213].  It MUST include exactly one instance of
      the IPv4 Home Address option [ID-DSMIP6] 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) [RFC-5213] MUST NOT be present
      if the same option(s) was not present in the initial Proxy Binding
      Update message.  Otherwise considerations from [RFC-5213] with
      respect to this option MUST be applied.

3.2.4.  Routing Considerations for the Mobile Access Gateway


   o  On receiving a packet from the bi-directional 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  Considerations from Section 6.10.3 of [RFC-5213] MUST be applied
      with respect the local routing and on the use of
      EnableMAGLocalRouting flag.

   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 bi-directional tunnel established with the local
      mobility anchor.  The encapsulation considerations specified in
      section 3.1.3 MUST be applied.  However, before 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.






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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 Default-Router Address Option

   A new option, IPv4 Default-Router Address Option is defined for using
   it in the Proxy Binding Acknowledgment message [RFC-5213] sent by the
   local mobility anchor to the mobile access gateway.  This option can
   be used for sending 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 3: IPv4 Default-Router Address Option

      Type

         IANA

      Length

         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 8-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






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

3.3.2.  IPv4 DHCP Support Mode

   A new option, IPv4 DHCP Support Mode Option is defined for using it
   in the Proxy Binding Acknowledgment message [RFC-5213] sent by the
   local mobility anchor to the mobile access gateway.  This option can
   be used for notifying the mobile access gateway, if 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 4: IPv4 DHCP Support Mode Option

      Type

         IANA

      Length

         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 if 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



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         should act as a DHCP Server.

3.3.3.  Status Codes

   This document defines the following new Status values for use in the
   Proxy Binding Acknowledgement message [RFC-5213].  These values are
   to be allocated from the same numbering space, as defined in Section
   6.1.8 of [RFC-3775].


   NOT_AUTHORIZED_FOR_IPV4_HOME_ADDRESS: IANA

      Mobile node not authorized for the requesting IPv4 home address

   NOT_AUTHORIZED_FOR_IPV6_HOME_NETWORK_PREFIX: IANA

      Mobile node not authorized for the requesting IPv6 home network
      prefix(es).

   MULTIPLE_IPV4_HOME_ADDRESS_ASSIGNMENT_NOT_SUPPORTED

      Multiple IPv4 home address assignment not supported

   IPV4_PREFIX_ASSIGNMENT_NOT_SUPPORTED

      IPv4 prefix assignment 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:





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   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 can be
      either the IPv4 Proxy Care-of Address or 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 potentially an IPv4
      private address [RFC-1918] 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  The DHCP server identifies the DHCP client either from the client
      identifier or the client hardware address (chaddr) [RFC-2131].  A
      mobile node in a Proxy Mobile IPv6 domain may present any of these
      identifiers to the DHCP server as long as the identifier remains
      the same through out the mobile node's attachment in that Proxy
      Mobile IPv6 domain.  If the client hardware address is used as the
      identifier and if the mobile node performs an handoff between two
      interfaces, this hardware identifier will change and the DHCP
      server will not be able to identify the mobile node.  Thus, it is
      recommended that the DHCP client on the mobile node, which
      performs mobility session handoff across interfaces in a Proxy
      Mobile IPv6 domain, is configured to use a stable client
      identifier that does not change during the active life of its DHCP
      session.

   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 (Ex: 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
    |       |        |
    * DHCPDISCOVER (Step-1) and Proxy Mobile IPv6 signaling
    * (Step-2 to Step-4) may happen in parallel and in no specific order
    * Tunnel/Route setup(Step-4) and the subsequent steps will happen in
    * in the specified order.


    Figure 5: Overview of DHCP Server located at Mobile Access Gateway


   Initial IPv4 Home Address Assignment:

   o  When the mobile node sends a DHCPDISCOVER message [RFC-2131], the
      DHCP server 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.  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 support for the mobile node on that
      access link.  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  After a 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 [RFC-2131] to the mobile
      node.  The offered address will be the mobile node's IPv4 home
      address, assigned by the local mobility anchor.  The server
      address, 'siaddr' field of the DHCPOFFER message, will be set to
      the mobile node's default-router address.  The DHCPOFFER message
      will be sent to the mobile node just as specified in [RFC-2131].

   o  If the mobile node sends the DHCPREQUEST message, the DHCP server
      will send DHCPACK message, as per [RFC-2131].




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

   o  Any time the mobile node goes into the DHCP RENEWING state [RFC-
      2131], 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 'server
      identifier' field 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 a different DHCP server (After
   Handoff):

   When the mobile node goes into the DHCP RENEWING state [RFC-2131], 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 updates the DHCP
   server address and uses the address of the DHCP server that is co-
   located with the new mobile access gateway.  Any of the following
   approaches can be adopted to ensure the mobile node uses the DHCP
   server on the attached link.


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




          Figure 6: Address renewal with a different DHCP server

   o  If a fixed address such as the IPv4 default-router address of the
      mobile node is used as the DHCP server Id on any of the links in
      that Proxy Mobile IPv6 domain, the DHCPREQUEST message sent by the
      mobile node for renewing the address will be received by the new
      mobile access gateway on the attached link.  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 IPv6 signaling or



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      is unable to acquire the same IPv4 address as requested by the
      mobile node, it will send a DHCPNACK message [RFC-2131] to the
      mobile node, as shown in Figure 6-1).


   Additional Operation:

   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.

3.4.2.  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 7 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 (IPv4HoA)
    |       |========|      | 3. Tunnel/Route Setup*
    |------>|-------------->| 4. DHCPDISCOVER (IPv4HoA) via DHCP-R
    |<------|<--------------| 5. DHCPOFFER (IPv4 HoA) via DHCP-R
    |------>|-------------->| 6. DHCPREQUEST (IPv4 HoA) via DHCP-R
    |<------|<--------------| 7. DHCPACK via DHCP-R
    |       |        |
    * The Proxy Mobile IPv6 signaling (Step-1 to Step-2) and the
      DHCPDISCOVER phase (Step-4) may occur in any order. However,
      the DHCPOFFER (Step-5) and the following steps will occur in
      the specified order and after the Tunnel/Route Setup (Step-3).


   Figure 7: Overview of the DHCP relay located at mobile access gateway


   Initial IPv4 Home Address Assignment:

   o  When the mobile access gateway receives a DHCPDISCOVER message
      from a mobile node, it can 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 has to complete the
      Proxy Mobile IPv6 signaling with the local mobility anchor by
      sending a Proxy Binding Update message.




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   o  If the Proxy Binding Update message is rejected by the local
      mobility anchor for any reason, the mobile access gateway will
      discard the DHCPDISCOVER messages from the mobile node.

   o  If the Proxy Binding Update message is accepted by the local
      mobility anchor and if there is an assigned IPv4 home address for
      the mobile node, the DHCP messages will be forwarded to the DHCP
      server.

   o  The DHCP relay agent on the mobile access gateway will add the
      DHCP relay agent information option [RFC-3046] 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 needs 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 needs to
      have a route towards the DHCP server.


   IPv4 Home Address Renewal to the same DHCP server: (No Handover)

   o  When the DHCP client goes into the DHCP RENEW STATE [RFC-2131], it
      directly unicasts DHCPREQUEST message to the DHCP server.  The
      DHCP relay agent may not detect these unicasted DHCPREQUEST
      messages for renewing the address.  Thus, any of the following
      approaches can be adopted.

      *  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.





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      *  The DHCP relay agent can use the DHCP Server Identifier
         Override Sub-option [RFC-5107] to be in path for all the DHCP
         message flows.  The DHCP client uses the DHCP server address
         which 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  The DHCP Relay agent on detecting the DHCPREQUEST message from the
      mobile node, will verify if the address in the DHCPREQUEST message
      is the address assigned by the local mobility anchor for that
      mobile node.  If the requesting IPv4 home address is not
      registered to the local mobility anchor, the mobile access gateway
      will drop the packet.  Once the address verification is
      successfully completed, the DHCP relay agent will forward the
      DHCPREQUEST message to the DHCP server.  However, the mobile
      access gateway is neither required to perform this check, nor
      intercept the DCHPREQUEST message from the mobile node, if the
      DHCP server is co-located with the local mobility anchor, as that
      check will be performed by the local mobility anchor in such
      scenario.  The mobile access gateway can determine the co-location
      of the DHCP server and the local mobility anchor by comparing the
      source address of the DHCPOFFER message with the address of the
      mobile node's local mobility anchor.


   IPv4 Address Release Triggers from the mobile node:

   o  The mobile node may release its IPv4 home address at any time by
      sending the DHCPRELEASE message [RFC-2131].  When the mobile
      access gateway detects the DHCPRELEASE message sent by the mobile
      node and for releasing its assigned IPv4 home address, the mobile
      access gateway should consider this as a trigger for de-
      registering the mobile node's IPv4 home address.  It MUST apply
      the considerations specified in section 3.2.3.3 for performing the
      de-registration procedure.  However, this operation should not
      release any IPv6 home network prefix(es) assigned to the mobile
      node.


   Additional Operations:

   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.



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4.  IPv4 Transport Support

   The Proxy Mobile IPv6 specification [RFC-5213] requires the signaling
   messages exchanged between the local mobility anchor and the mobile
   access gateway to be over an IPv6 transport.  The extensions defined
   in this section allow the exchange of signaling messages over an IPv4
   transport when the local mobility anchor and the mobile access
   gateway are separated by an IPv4 network and are reachable using only
   IPv4 addresses.


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



                     Figure 8: 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.  Additionally,
      both entities are also IPv6 enabled and have configured IPv6
      addresses on their interfaces, as specified in [RFC-5213], but are
      reachable only over an IPv4 transport.

   o  The mobile access gateway can be potentially in a private IPv4
      network behind a NAT [RFC-3022] device, with a private IPv4
      address configured on its egress interface.  But, the local
      mobility anchor must not be behind a NAT and must be using a
      globally routable IPv4 address.  However, both the local mobility
      anchor and the mobile access gateway can be in the same private
      IPv4 routing domain, i.e., when both are configured with private
      IPv4 addresses and with no need for NAT translation between them.

   o  The IPv6 address configuration requirement on the mobile access
      gateway does not imply there needs to be IPv6 routing enabled



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      between the local mobility anchor and the mobile access gateway.
      It just requires each of the mobile access gateways and local
      mobility anchors in a Proxy Mobile IPv6 domain to be configured
      with a globally unique IPv6 address.

   o  The Proxy Mobile IPv6 signaling messages exchanged between the
      local mobility anchor and the mobile access gateway for
      negotiating the IPv4 transport will be encapsulated and carried as
      IPv4 packets.  However, these signaling messages are fundamentally
      IPv6 messages using the mobility header and the related semantics
      as specified in base Proxy Mobile IPv6 specification [RFC-5213],
      but carried as a payload in an IPv4 packet.  The supported
      encapsulation modes for the signaling messages are either native
      IPv4 or IPv4 with UDP header.

   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 IPv4 tunnel established between the local mobility anchor and
      the mobile access gateway (with any of the supported encapsulation
      modes over IPv4 transport) will be used for carrying the mobile
      node's IPv4 and IPv6 traffic.  The following are the outer headers
      based on the negotiated encapsulation mode.

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

      *  IPv4-UDP (Payload packet carried in an IPv4 packet with UDP
         header).  If payload protection using IPsec is enabled for the
         tunneled traffic, the ESP header follows the outer tunnel
         header, as specified in [RFC-3948].

      *  IPv4-UDP-TLV (Payload packet carried in an IPv4 packet with UDP
         and TLV header).  Refer to [ID-GREKEY-NEGO].  If payload
         protection using IPsec is enabled for the tunneled traffic, the
         ESP header follows the outer tunnel header.

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 [RFC-5213] MUST be
   extended with the following additional parameters.  It is to be noted
   that all of these parameters are specified in [ID-DSMIP6] and also
   required here in the present usage context, and are presented here



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   only for completeness.

   o  The IPv4 Proxy Care-of Address configured on the mobile access
      gateway that sent the Proxy Binding Update message.  This address
      can be obtained from the IPv4 Care-of Address option [ID-DSMIP6],
      present in the received 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 ALL_ZERO value.

   o  The IPv4 NAT translated address of the mobile access gateway.  If
      the mobile access gateway is not behind a NAT [RFC-3022], this
      address will be the same as the address configured on the egress
      interface of the mobile access gateway.  This address can be
      obtained from the IPv4 header of the received Proxy Binding Update
      message.  However, if the received Proxy Binding Update message is
      not sent as an IPv4 packet, this field in the Binding Cache entry
      MUST be set to ALL_ZERO value.

   o  The source UDP port, if the Proxy Binding Update was received in
      an IPv4 packet with UDP header.

   o  The destination UDP port, if the Proxy Binding Update was received
      in an IPv4 packet with UDP header.

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 [RFC-5213] 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.

4.1.3.1.  Processing Proxy Binding Updates

   o  If the received Proxy Binding Update message was sent encapsulated
      in IPv4 or IPv4-UDP header, the message MUST be authenticated
      after removing the outer encapsulation (IPv4 or IPv4-UDP) header.
      Considerations from Section 4 of [RFC-5213] MUST be applied for
      authenticating and authorizing the request.





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   o  All the considerations from Section 5.3.1 of [RFC-5213] MUST be
      applied on the encapsulated Proxy Binding Update message, after
      removing the outer encapsulation (IPv4 or IPv4-UDP) header.

   o  If there is an IPv4 Care-of Address option [ID-DSMIP6] present in
      the request and if the outer encapsulation header is IPv4-UDP,
      then the NAT presence detection procedure specified in Section
      4.1.3.3 MUST be used for detecting the NAT in the path.

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

      *  If the received Proxy Binding Update message was sent with IPv4
         encapsulated header, then the encapsulation mode for the bi-
         directional tunnel MUST be set to IPv4.  Otherwise, the
         following considerations apply.

      *  If NAT is not detected on the path and 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 [ID-GREKEY-NEGO] in the Proxy Binding Update
         message is set to value of 1, then the encapsulation mode MUST
         be set to IPv4-or-IPv6-over-IPv4-UDP-TLV.

      *  If NAT is detected on the path, or 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-or-IPv6-over-
         IPv4-UDP.  Otherwise the encapsulation mode MUST be set to
         IPv4-or-IPv6-over-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 [RFC-5213].  However, if



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   the received Proxy Binding Update message was encapsulated in an IPv4
   packet or as a payload in the UDP header of an IPv4 packet, the
   following additional considerations MUST be applied.

   o  The Proxy Binding Acknowledgement message MUST be encapsulated in
      an IPv4 packet.  However, if the received Proxy Binding Update
      message was sent encapsulated in the UDP header of an IPv4 packet,
      then the Proxy Binding Acknowledgement message MUST be
      encapsulated in the UDP header of an IPv4 packet.

   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 the mobile access gateway and the local mobility anchor are
      using globally routable IPv4 addresses and if there is a security
      association that is based of IPv4 addresses, then the encapsulated
      IPv4 packet (containing the IPv6 Proxy Binding Acknowledgement)
      MUST be protected using IPsec ESP [RFC-4301] mode.  There is no
      need to apply IPsec ESP header to the IPv6 packet.  In all other
      cases, the Proxy Binding Acknowledgement message MUST be protected
      using IPsec prior to the IPv4 or IPv4-UDP encapsulation.

   o  The NAT Detection option [ID-DSMIP6] MUST be present only if there
      is an IPv4 Care-of Address option [ID-DSMIP6] present in the
      received Proxy Binding Update message and if the NAT detection
      procedure resulted in detecting a NAT on path.  However, if the
      received Proxy Binding Update message was not sent encapsulated in
      IPv4 UDP header, then the option MUST NOT be present.
      Furthermore, in all other cases, the option MUST NOT be present.

   o  The IPv4 DHCP Support Mode option MAY be present.  If this option
      is not present, the mobile access gateway will enable the default
      behavior and function as a DHCP Relay for the mobile node.

   o  The format of the Proxy Binding Acknowledgement message
      encapsulated in an IPv4 packet and protected using IPv6 security
      association.  The UDP header MUST be present only if the received
      Proxy Binding Update message was sent with IPv4-UDP encapsulation
      header.


     IPv4 header (src=IPv4-LMAA, dst=pbu_src_address)
        UDP header (sport=DSMIP_PORT, dport= pbu_sport) /*Optional*/
           /* IPv6 PBA Packet protected with ESP header */


      Figure 9: Proxy Binding Acknowledgment (PBA) Message encapsulated
                                in IPv4 header



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4.1.3.3.  NAT Presence Detection

   When the transport network between the local mobility anchor and the
   mobile access gateway is an IPv4 network and if the received Proxy
   Binding Update message was sent encapsulated in IPv4 UDP header, the
   local mobility anchor performs the NAT Presence Detection as
   specified below.

   On receiving the Proxy Binding Update message encapsulated in an IPv4
   UDP packet, the local mobility anchor, if it detects a NAT on the
   path, will send the Proxy Binding Acknowledgment message with the NAT
   Detection Option.  The presence of this option in the Proxy Binding
   Acknowledgment is an indication to the mobile access gateway about
   the presence of NAT in the path.  On detecting any NAT in the path,
   both the local mobility anchor and the mobile access gateway will set
   the encapsulation mode of the tunnel to IPv4-UDP-based encapsulation.
   The specific details around the NAT detection and the related logic
   are described in DSMIPv6 specification [ID-DSMIP6].

   However, if the value of the configuration variable,
   UseIPv4UDPEncapForSignalingMessages, is set to a value of 0, the
   mobile access gateway will not use IPv4 UDP encapsulation for Proxy
   Binding Update messages and hence the local mobility anchor will not
   perform this NAT Presence Detection procedure on these messages that
   are not sent in IPv4 UDP packet.

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 bi-directional tunnel set up for
      that mobile node.

   o  The format of the tunneled packet is shown below.











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  IPv4 Header (src= IPv4-LMAA, dst= IPv4-Proxy-CoA)] /* Tunnel Header */
    [UDP Header (src port=DSMIPv6, dst port=Z]   /* If UDP encap nego */
      [TLV Header]                               /* If TLV negotiated */
        /* IPv6 or IPv4 Payload Packet */
               IPv6 header (src= CN, dst= MN-HOA)
                          OR
               IPv4 header (src= CN, dst= IPv4 MN-HoA)


               Figure 10: Tunneled IPv4 Packet from LMA to MAG


   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 Considerations

   The ECN considerations specified in Section 5.6.3 of [RFC-5213] 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.

4.1.4.3.  Bi-Directional Tunnel Management

   The Tunnel Management considerations specified in section 5.6.1 of
   [RFC-5213] 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 [RFC-5213] 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.





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   o  The IPv4 address of the mobile access gateway.  This is the
      address configured on the egress interface of the mobile access
      gateway and is registered with the mobile node's local mobility
      anchor as the IPv4 Proxy-CoA.  However, if the mobile access
      gateway is in a private IPv4 network and behind a NAT, the address
      that is registered with the mobile node's local mobility anchor is
      the NAT translated public IPv4 address.

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 [RFC-5213].  However, if the mobile access
   gateway is in an IPv4-only access network, the following additional
   considerations MUST be applied.

   o  The Proxy Binding Update message MUST be encapsulated in an IPv4
      packet.  However, if the value of the configuration variable,
      UseIPv4UDPEncapForSignalingMessages, is set to 1, then the Proxy
      Binding Update message MUST be encapsulated in an UDP header of an
      IPv4 packet.

   o  The IPv4 Care-of Address option [ID-DSMIP6] MUST be present.  The
      IPv4 address in the option MUST be set to the mobile access
      gateway's IPv4-Proxy-CoA.

   o  The packet MUST be constructed as specified in Section 4.2.3.

   o  When sending a Proxy Binding message for extending the lifetime of
      a currently existing mobility session or for de-registering the
      mobility session, the Proxy Binding Update message MUST be
      constructed as the initial request.


   Receiving Proxy Binding Acknowledgement

   o  If the received Proxy Binding Acknowledgement message is
      encapsulated in IPv4 or IPv4 UDP packet, the message MUST be
      authenticated after removing the outer IPv4 or IPv4-UDP header.
      Considerations from Section 4 of [RFC-5213] MUST be applied for
      authenticating and authorizing the message.

   o  All the considerations from Section 6.9.1.2 of [RFC-5213] MUST be
      applied on the encapsulated Proxy Binding Acknowledgement message,
      after removing the outer IPv4 UDP header.

   o  If the Status field indicates Success, the mobile access gateway
      MUST setup a bi-directional tunnel to the local mobility anchor.



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

      *  The encapsulation mode for the bi-directional tunnel MUST be
         set to IPv4.  However, if the value of the configuration
         variable, UseIPv4UDPEncapForSignalingMessages, is set to 1,
         then the following considerations MUST be applied.

      *  If there is a NAT Detection option [ID-DSMIP6] in the received
         Proxy Binding Acknowledgement message and if the value of the
         configuration flag, UseIPv4UDPEncapForSignalingMessages, is set
         to value of 1, then the encapsulation mode for the tunnel MUST
         be set to IPv4-UDP.  Otherwise the encapsulation mode MUST be
         set to IPv4.

      *  If the (T) flag in the Proxy Binding Acknowledgement message is
         set to value of 1, then the encapsulation mode MUST be set to
         IPv4-UDP-TLV.

4.2.2.1.  Constructing the Proxy Binding Update Message

   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  The IPv4 Care-of Address option [ID-DSMIP6] MUST be present.  The
      address MUST be set to the mobile access gateway's IPv4-Proxy-CoA.

   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  The Proxy Binding Update message MUST be protected using IPsec ESP
      [RFC-4301], as specified in [RFC-5213].  The protection MUST be
      applied on the IPv6 packet of the Proxy Binding Update message,
      prior to the IPv4 encapsulation.

   o  The format of the Proxy Binding Update message encapsulated in an
      IPv4 or IPv4-UDP packet with no IPsec protection:


       IPv4 header (src=IPv4-Proxy-CoA, dst=IPv4-LMAA)
          UDP header (sport=ANY, dport= DSMIP_PORT) /*Optional*/
             /* IPv6 PBU Packet protected with ESP header */




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      Figure 11: Proxy Binding Update (PBU) message encapsulated in IPv4
                                  UDP header

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.  However, considerations from Section 6.10.3 of [RFC-
      5213] 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=DSMIPv6, dst port=Z]   /* If UDP encap nego */
        [TLV Header]                             /* If TLV negotiated */
              /* 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

   o  Forwarding Packets received from the bi-directional tunnel:

   o  On receiving a packet from the bi-directional 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.


















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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, even if there is no NAT detected in the path.

      The default value for this flag is set to value of 0, indicating
      that the local mobility anchor MUST NOT accept IPv4 UDP
      encapsulation request when NAT is not detected in the path.

      When the value for this flag is set to value of 1, the local
      mobility anchor MUST accept IPv4 UDP encapsulation request even
      when NAT is not detected in the path.


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.


   UseIPv4UDPEncapForSignalingMessages

      This flag indicates whether or not the mobile access gateway
      should use IPv4-UDP encapsulation mode for the signaling messages.

      The default value for this flag is set to value of 0, indicating
      that the mobile access gateway MUST NOT use IPv4-UDP encapsulation
      mode, but MUST use native IPv4 encapsulation mode for sending the
      Proxy Mobile IPv6 signaling messages.

      When the value for this flag is set to value of 1, the mobile
      access gateway MUST use IPv4-UDP encapsulation mode for sending
      the Proxy Mobile IPv6 signaling messages.

   ForceIPv4UDPEncapsulationSupport



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      This flag indicates whether or not the mobile access gateway
      should request the mobile node's local mobility anchor for forcing
      IPv4 UDP encapsulation support for the mobile node's data traffic,
      even when NAT is not detected in the path.

      The default value for this flag is set to value of 0, indicating
      that the mobile access gateway MUST NOT request the mobile node's
      local mobility anchor for forcing IPv4 UDP encapsulation support
      even when NAT is not detected in path.

      When the value for this flag is set to value of 1, the mobile
      access gateway MUST force the mobile node's local mobility anchor
      for IPv4 UDP encapsulation support.

      This flag is applicable only when the flag
      UseIPv4UDPEncapForSignalingMessages is set to a value of 1.



































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6.  IANA Considerations

   This document defines a new Mobility Header option, IPv4 Default
   Router Address option.  This option is described in Section 3.3.1.
   The Type value for this option needs to be assigned from the same
   number space as allocated for the other mobility options, as defined
   in [RFC-3775].

   This document also defines new status values, used in Proxy Binding
   Acknowledgement message, as described in Section 3.3.2.  These values
   MUST be assigned from the same number space as allocated for other
   Status codes [RFC-3775].  Each of these allocated values MUST be
   greater than 128.






































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7.  Security Considerations

   All the security considerations from the base Proxy Mobile IPv6 [RFC-
   5213], Mobile IPv6 [RFC-3775], and Dual-Stack Mobile IPv6 [ID-DSMIP6]
   apply when using the extensions defined in this document.
   Additionally, the following security considerations need to be
   applied.

   This document defines news 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
   the signaling messages between the local mobility anchor and the
   mobile access gateway.  These signaling messages are fundamentally
   IPv6 messages, but encapsulated in an IPv4 header and routed as IPv4
   packets.  The encapsulated inner IPv6 message is still protected
   using IPsec, using the established security association and this
   offers the same level of security as when the messages are routed
   natively as IPv6 packets.  The use of outer IPv4 header does not
   introduce any new security vulnerabilities.



























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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

   Sangjin Jeong

      sjjeong@etri.re.kr

   Basavaraj Patil

      basavaraj.patil@nsn.com

   Myungki Shin

      myungki.shin@gmail.com


9.  Acknowledgments

   The IPv4 support for Proxy Mobile IPv6 was initially covered in the
   internet-draft [draft-sgundave-mip6-proxymip6-02.txt].  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, Premec Domagoj,
   Ralph Droms, Sammy Touati, Vidya Narayanan, Yingzhe Wu and Zu Qiang
   for their helpful review of this document.


10.  References


10.1.  Normative References

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




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

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

   [RFC-4193] Hinden, R. and Haberman, B., "Unique Local IPv6 Unicast
   Addresses", RFC-4193, October 2005.

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

   [RFC-5213] Gundavelli, S., et.al, "Proxy Mobile IPv6", RFC 5213,
   November 2007.

   [ID-DSMIP6] Soliman, H. et al, "Mobile IPv6 support for dual stack
   Hosts and Routers (DSMIPv6)",
   draft-ietf-mext-nemo-v4traversal-09.txt, February 2009.

   [ID-GREKEY-NEGO] Muhanna, A., Khalil, M., Gundavelli, S., Leung, K.,
   "GRE Key Option for Proxy Mobile IPv6",
   draft-ietf-netlmm-grekey-option-06.txt, February 2009.

10.2.  Informative References

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

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

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

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

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

   [RFC-3587] Hinden, R., Deering, S., and E. Nordmark, "IPv6 Global
   Unicast Address Format", RFC 3587, August 2003.

   [RFC-3948] Huttunen, A. et al, "UDP Encapsulation of IPsec ESP
   Packets", RFC 3948, January 2005.

   [RFC-4301] Kent, S. and K. Seo, "Security Architecture for the



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   Internet Protocol", RFC 4301, December 2005.

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

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

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




Authors' Addresses

   Ryuji Wakikawa
   Toyota ITC / Keio University
   6-6-20 Akasaka, Minato-ku
   Tokyo  107-0052
   Japan

   Phone: +81-3-5561-8276
   Fax:   +81-3-5561-8292
   Email: ryuji@jp.toyota-itc.com


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

   Email: sgundave@cisco.com

















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