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Versions: (draft-soliman-v4v6-mipv6) 00 01 02 03 04 05 06 draft-ietf-mext-nemo-v4traversal

MIP6 Working Group                                  Hesham Soliman (Ed.)
INTERNET-DRAFT                                      Elevate Technologies
Expires: May 2008
                                                          November, 2007



                          Mobile IPv6 support for dual stack
                              Hosts and Routers (DSMIPv6)
                     draft-ietf-mip6-nemo-v4traversal-06.txt

Status of this memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/lid-abstracts.txt

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   http://www.ietf.org/shadow.html

   This document is a submission of the IETF MIP6 WG. Comments should be
   directed to the IPv6 WG mailing list, mip6@ietf.org.


Abstract

   The current Mobile IPv6 and NEMO specifications support IPv6 only.
   This specification extends those standards to allow the registration
   of IPv4 addresses and prefixes, respectively, and the transport of
   both IPv4 and IPv6 packets over the tunnel to the HA. This
   specification also allows the Mobile Node to roam over both IPv6 and
   IPv4, including the case where Network Address Translation is present
   on the path between the mobile node and its home agent.







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   Table of Contents

  1. Introduction.....................................................2
  1.1 Motivation for using Mobile IPv6 only...........................3
     1.2 Scenarios considered by this specification...................4
  2. Solution overview................................................5
  2.1. Home Agent Address Discovery...................................5
     2.2. Mobile Prefix Solicitations and Advertisements..............6
  2.3. Binding management.............................................7
  2.3.1 Foreign network supports IPv6.................................7
  2.3.2 Foreign network supports IPv4 only............................8
  2.3.2.2 Visited network supports IPv4 only (private addresses)......9
  2.4. Route optimization............................................10
  2.5. Dynamic IPv4 home address allocation..........................10
  3. Extensions and modifications to Mobile IPv6.....................10
  3.1. Binding update extensions.....................................10
  3.1.1 IPv4 home address option.....................................10
        3.1.2 The IPv4 Care-of Address option........................11
        3.1.3 The Binding Update Message extensions..................12
  3.2. Binding Acknowledgement extensions............................12
  3.2.1 IPv4 address acknowledgement option..........................12
        3.2.2 The NAT detection option...............................14
        3.2.3 Extensions to the Binding Acknowledgement message......14
  4. Protocol operation..............................................15
     4.1. Tunnelling formats.........................................15
     4.2. NAT detection and traversal................................16
     4.3. NAT Keepalives.............................................18
  4.4. Mobile node operation.........................................19
        4.4.1 Sending packets from a visited network.................20
        4.4.2 Movement detection in IPv4-only networks...............21
  4.5. Home agent operations.........................................21
        4.5.1 Sending packets to the mobile node.....................23
  4.6. Correspondent node operations.................................23
  5. Security considerations.........................................23
     5.1 Handover interactions for IPsec and IKE.....................24
  6. Protocol constants..............................................25
  7. Acknowledgements................................................25
  8. IANA considerations.............................................26
  9. References......................................................26
  10. Contributors...................................................27
  Author's Address...................................................27


1. Introduction

   Mobile IPv6 [MIPv6] and [NEMO] allow mobile nodes to move within the
   Internet while maintaining reachability and ongoing sessions, using
   an IPv6 home address or prefix. However, since IPv6 is not widely
   deployed, it is unlikely that mobile nodes will use IPv6 addresses
   only for their connections. It is reasonable to assume that mobile
   nodes will, for a long time, need an IPv4 home address that can be



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   used by upper layers. It is also reasonable to assume that mobile
   nodes will move to networks that might not support IPv6 and would
   therefore need the capability to support an IPv4 Care-of Address.
   Hence, this specification extends Mobile IPv6 capabilities to allow
   dual stack mobile nodes to request that their home agent (also dual
   stacked) tunnel IPv4/IPv6 packets addressed to their home addresses,
   as well as IPv4/IPv6 care-of address(es).

   Using this specification, mobile nodes would only need Mobile IPv6
   and [NEMO] to manage mobility while moving within the Internet; hence
   eliminating the need to run two mobility management protocols
   simultaneously. This specification provides the extensions needed in
   order to allow IPv6 mobility only to be used by dual stack mobile
   nodes.

   This specification will also consider cases where a mobile node moves
   into a private IPv4 network and gets configured with a private IPv4
   Care-of Address. In those scenarios, the mobile node needs to be able
   to traverse the IPv4 NAT in order to communicate with the Home Agent.
   IPv4 NAT traversal for Mobile IPv6 is presented in this
   specification.

   In this specification, the term mobile node refers to both a mobile
   host and mobile router unless the discussion is specific to either
   hosts or routers. Similarly, we use the term home address to reflect
   an address/prefix format.

   In this specification, extensions are defined for the binding update
   and binding acknowledgement. It should be noted that all these
   extensions apply to cases where the mobile node communicates with a
   Mobility Anchor Point (MAP) as defined in [HMIPv6]. The requirements
   on the MAP are identical to those stated for the home agent, although
   it is unlikely that NAT traversal would be needed with a MAP as it is
   expected to be in the same address domain.

1.1 Motivation for using Mobile IPv6 only

   IPv6 offers a number of improvements over today's IPv4, primarily due
   to its large address space. Mobile IPv6 offers a number of
   improvements over Mobile IPv4, mainly due to capabilities inherited
   from IPv6. For instance, route optimization and Dynamic home agent
   discovery can only be achieved with Mobile IPv6.

   One of the advantages of the large address space provided by IPv6 is
   that it allows mobile nodes to obtain a globally unique care-of
   address wherever they are. Hence, there is no need for Network
   Address Translator (NAT) traversal techniques designed for Mobile
   IPv4. This allows Mobile IPv6 to be a significantly simpler and more
   bandwidth efficient mobility management protocol. At the same time,
   during the transition towards IPv6, NAT traversal for existing




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   private IPv4 networks needs to be considered. This specification
   introduces NAT traversal for this purpose.

   The above benefits make the case for using Mobile IPv6 only for dual
   stack mobile nodes in order to allow for a long lasting mobility
   solution and minimize the need to changing the mobility stack due to
   the introduction of IPv6 within a deployed network.

1.2 Scenarios considered by this specification

   In [SNRIO] several scenarios that illustrate potential
   incompatibilities for mobile nodes using Mobile IPv6 were discussed.
   Some of the problems associated with mobility and transition issues
   were presented in [MIP-PB]. This specification considers a subset of
   the scenarios in [SNRIO], which address all the problems discussed in
   [MIP-PB]. The scenarios considered in this specification are listed
   below.

   All of the following scenarios assume that both the mobile node and
   the Home Agent are IPv4 and IPv6-enabled and that only Mobile IPv6 is
   used between the mobile node and the Home Agent. We also assume that
   the Home Agent is always reachable through a globally unique IPv4
   address. Finally, it's important to note that the following scenarios
   are not mutually exclusive.

   Scenario 1: IPv4-only foreign network

   In this scenario, a mobile node is connected to an IPv4-only foreign
   network. The mobile node can only configure an IPv4 Care-of Address.

   Scenario 2: Mobile node behind a NAT

   In this scenario, the mobile node is in a private IPv4 foreign
   network that has a NAT device connecting it to the Internet. If the
   Home Agent is located outside the NAT device, the mobile node will
   need a NAT traversal mechanism to communicate with the Home Agent.

   Scenario 3: Home Agent behind a NAT

   In this scenario, the communication between the mobile node and the
   Home Agent is further complicated by the fact that the Home Agent is
   located within a private IPv4 network. However, in this scenario, we
   assume that the Home Agent is allocated a globally unique IPv4
   address. Such address might not be physically configured on the Home
   Agent interface. Instead, it is associated with the Home Agent on the
   NAT device, which allows the Home Agent to be reachable through
   address or port mapping.







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   Scenario 4: Use Of IPv4-only applications

   In this scenario, the mobile node may be located in an IPv4, IPv6 or
   a dual network. However, the mobile node might be communicating with
   an IPv4-only node. In this case, the mobile node would need a stable
   IPv4 address for its application. The alternative to using an IPv4
   address is the use of protocol translators; however, end-to-end
   communication with IPv4 is preferred to the use of protocol
   translators.

   The mobile node may also be communicating with an IPv4-only
   application that requires an IPv4 address.

   The cases above illustrate the need for a stable IPv4 home address to
   be allocated to the mobile node. This is done using an IPv4 home
   address. Since running Mobile IPv4 and Mobile IPv6 simultaneously is
   problematic (as illustrated in [MIP-PB]), this scenario adds a
   requirement on Mobile IPv6 to support IPv4 home addresses.

2. Solution overview

   In order to allow Mobile IPv6 to be used by dual stack mobile nodes,
   the following needs to be done:

   - Mobile nodes should be able to use an IPv4 and IPv6 home or care-of
   address simultaneously and update their home agents accordingly.

   - Mobile nodes need to be able to know the IPv4 address of the home
   agent as well as its IPv6 address. There is no need for IPv4 prefix
   discovery however.

   - Mobile nodes need to be able to detect the presence of a NAT device
   and traverse such device in order to communicate with the Home Agent
   in a secure manner.

   This section presents an overview of the extensions required in order
   to allow mobile nodes to use Mobile IPv6 only for IP mobility
   management.

2.1. Home Agent Address Discovery

   Dynamic Home Agent Address Discovery (DHAAD) was defined in [MIPv6]
   to allow mobile nodes to discover their home agents by appending a
   well-known anycast interface identifier to their home link's prefix.
   However, this mechanism is based on IPv6-anycast routing. If a mobile
   node is located in an IPv4-only foreign network, it cannot rely on
   native IPv6 routing. In this scenario, the solution for discovering
   the home agent's IPv4 address is through the Domain Name System
   (DNS). If the MN is attached to an IPv6-only or dual stack network,
   it may also use procedures defined in [INTBOOT] to discover Home
   Agent information. Note that the use of [INTBOOT] cannot give the



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   mobile node information that allows it to continue to communicate
   with the Home Agent if, for example, the mobile node moved from an
   IPv6-enabled network to an IPv4-only network. In such scenario, the
   mobile node would need to discover the IPv4 address of its home agent
   through the DNS.

   For DNS lookup by name, the mobile node should be configured with the
   name of the home agent.  When the mobile node needs to discover a
   home agent, it sends a DNS request with QNAME set to the configured
   name.  An example is "ha1.example.com".  If a home agent has an IPv4
   and IPv6 address, the corresponding DNS record should be configured
   with both 'AAAA' and 'A' records. Accordingly the DNS reply will
   contain 'AAAA' and 'A' records.

   For DNS lookup by service, the SRV record defined in [BOOT] is
   reused.  For instance, if the service name is "mip6ha" and the
   protocol name is "ipv6" for the SRV record, the mobile node SHOULD
   send a DNS request with the QNAME set to "mip6ha.ipv6.example.com".
   The response should contain the home agent's FQDN(s) and may have the
   corresponding 'AAAA' and 'A' records enclosed as well.

   If multiple home agents reside on the home link, each configured with
   a public IPv4 addresses, then the operation above applies. In case
   the home agents are behind a NAT box, there are two options, 1)
   configure a public IPv4 address for each home agent on the NAT box,
   2) configure one public address and make the home agents share the
   public address.  In either case, the correct DNS entries can be
   configured.  Another possible solution is to designate one home agent
   on the home link for v4 traversal. The NAT device should associate
   that home agent with the public IPv4 address configured on it for v4
   traversal. In all cases above, both the 'AAAA' and 'A' records
   returned for a particular name MUST correspond to the same physical
   home agent; otherwise the mobile node will not be able to bind its
   addresses correctly.

2.2. Mobile Prefix Solicitations and Advertisements

   According to [MIPv6], the mobile node can send a Mobile Prefix
   Solicitation and receive a Mobile Prefix Advertisement containing all
   prefixes advertised on the home link.

   A dual stack mobile node MAY send a Mobile Prefix Solicitation
   message encapsulated in IPv4 (i.e. IPv6 in IPv4) in the case where
   the mobile node has no access to IPv6 within the local network.
   Securing such messages would require the mobile node to have security
   association with the home agent, using IPsec (AH or ESP) and based on
   the mobile node's IPv4 care-of address as described in [MIPv6]. Since
   the mobile node needs to encapsulate all IPv6 traffic sent to the
   home agent into IPv4 while located in an IPv4-only visited network,
   such SA would affect all packets if the selectors were based on the
   information in the outer header. That is, the SA selectors being the



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   protocol number (protocol is always IP in IP), as well as, source and
   destination addresses are all common to all packets. If this effect
   is not desired, the mobile node can base the SA on the information in
   the inner header (i.e. using the home agent's IPv6 address, the
   mobile node's home address and the ICMP protocol number). Such
   security association would use transport mode ESP protection.

2.3. Binding management

   A dual stack mobile node will need to update its home agent with its
   care-of address. If a mobile node has an IPv4 and an IPv6 home
   address it will need to create a binding cache entry for each
   address. The format of the IP packet carrying the binding update and
   acknowledgement messages will vary depending on whether the mobile
   node has access to IPv6 in the visited network. There are three
   different scenarios to consider with respect to the visited network:

   A. The visited network has IPv6 connectivity and provides the mobile
      node with a care-of address (in a stateful or stateless manner).

   B. The mobile node can only configure a globally unique IPv4 address
      in the visited network.
   C. The mobile node can only configure a private IPv4 address in the
      visited network.

2.3.1 Foreign network supports IPv6

   In this case, the mobile node is able to configure a globally unique
   IPv6 address. The mobile node will send a binding update to the IPv6
   address of its home agent, as defined in [MIPv6]. The binding update
   MAY include the IPv4 home address option introduced in this document.
   After receiving the binding update, the home agent creates two
   binding cache entries, one for the mobile node's IPv4 home address,
   and another for the mobile node's IPv6 home address. Both entries
   will point to the mobile node's IPv6 care-of address. Hence, whenever
   a packet is addressed to the mobile node's IPv4 or IPv6 home
   addresses, it will be tunneled in IPv6 to the mobile node's IPv6
   care-of address included in the binding update. Effectively, the
   mobile node establishes two different tunnels, one for its IPv4
   traffic (IPv4 in IPv6) and one for its IPv6 traffic (IPv6 in IPv6)
   with a single binding update. The security implications of this
   mechanism are discussed in the security considerations section.

   In this scenario, the only addition to [MIPv6] is the inclusion of
   the IPv4 home address option in the binding update message.

   After accepting the binding update and creating the corresponding
   binding cache entries, the home agent MUST send a binding
   acknowledgement to the mobile node as defined in [MIPv6]. In
   addition, if the binding update included an IPv4 home address option,
   the binding acknowledgement MUST include the IPv4 address



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   acknowledgment option as described later in this specification. This
   option informs the mobile node whether the binding was accepted for
   the IPv4 home address. If this option is not included in the binding
   acknowledgement and the IPv4 home address option was included in the
   binding update, the mobile node MUST assume that the home agent does
   not support the IPv4 home address option and therefore SHOULD NOT
   include the option in future binding updates to that home agent
   address.

   The routing header in the binding update MUST include the mobile
   node's IPv6 home address as specified in [MIPv6].

   When a mobile node acquires both IPv4 and IPv6 care-of addresses at
   foreign network, it SHOULD prioritize IPv6 care-of address for MIP6
   binding registration. The mobile node MUST NOT register both IPv4 and
   IPv6 care-of addresses to its home agent.

2.3.2 Foreign network supports IPv4 only

   If the mobile node is in a foreign network that only supports IPv4,
   it needs to detect whether a NAT is in its communication path to the
   home agent. This is done while exchanging the binding update and
   acknowledgement messages as shown later in this document. If no NAT
   is detected between the mobile node and the home agent, the mobile
   node assumes that it is in a foreign network that supports IPv4
   public addresses. Otherwise, the mobile node assumes that private
   addresses are used in the foreign network. Note that this assumption
   is only valid for the purposes of the signaling presented in this
   specification. A mobile node SHOULD NOT assume that its IPv4 address
   is globally unique if a NAT device was not detected. The operations
   of both cases are discussed below.

2.3.2.1 Foreign network supports IPv4 only (public addresses)

   In this scenario the mobile node will need to tunnel IPv6 packets
   containing the binding update to the home agent's IPv4 address. The
   mobile node uses the IPv4 address it gets from the foreign network as
   a source address in the outer header. The binding update will contain
   the mobile node's IPv6 home address. However, since the care-of
   address in this scenario is the mobile node's IPv4 address, the
   mobile node MUST include its IPv4 care-of address in the IPv6 packet.
   The IPv4 address is represented in the IPv4 Care-of address option
   defined in this specification.  If the mobile node had an IPv4 home
   address, it MUST also include the IPv4 home address option described
   in this specification.

   After accepting the binding update, the home agent MUST create a new
   binding cache entry for the mobile node's IPv6 home address. If an
   IPv4 home address option were included, the home agent MUST create
   another entry for that address. All entries MUST point to the mobile
   node's IPv4 care-of address. Hence, all packets addressed to the



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   mobile node's home address(es) (IPv4 or IPv6) will be encapsulated in
   an IPv4 header that includes the home agent's IPv4 address in the
   source address field and the mobile node's IPv4 care-of address in
   the destination address field.

   After accepting the binding updates and creating the corresponding
   entries, the home agent MUST send a binding acknowledgement as
   specified in [MIPv6]. In addition, if the binding update included an
   IPv4 home address option, the binding acknowledgement MUST include
   the IPv4 address acknowledgment option as described later in this
   specification. The binding acknowledgement is encapsulated to the
   IPv4 care-of address.

2.3.2.2 Visited network supports IPv4 only (private addresses)

   In this scenario the mobile node will need to tunnel IPv6 packets
   containing the binding update to the home agent's IPv4 address. In
   order to traverse the NAT device, IPv6 packets are tunneled UDP and
   IPv4. The UDP port allocated for the home agent is TBD.

   The mobile node uses the IPv4 address it gets from the visited
   network as a source address in the IPv4 header. The binding update
   will contain the mobile node's IPv6 home address.

   After accepting the binding update, the home agent MUST create a new
   binding cache entry for the mobile node's IPv6 home address. If an
   IPv4 home address option were included, the home agent MUST create
   another entry for that address. All entries MUST point to the mobile
   node's IPv4 care-of address included in the binding update message.
   In addition, the tunnel used MUST indicate UDP encapsulation for NAT
   traversal. Hence, all packets addressed to the mobile node's home
   address(es) (IPv4 or IPv6) will be encapsulated in UDP then
   encapsulated in an IPv4 header that includes the home agent's IPv4
   address in the source address field and the mobile node's IPv4 care-
   of address in the destination address field.

   After accepting the binding updates and creating the corresponding
   entries, the home agent MUST send a binding acknowledgement as
   specified in [MIPv6]. In addition, if the binding update included an
   IPv4 home address option, the binding acknowledgement MUST include
   the IPv4 address acknowledgment option as described later in this
   specification. The binding acknowledgement is encapsulated in UDP
   then IPv4 with the home agent's IPv4 address in the source address
   field and the mobile node's IPv4 care-of address in the destination
   field. The IPv4 address in the destination field of the IP v4 packet
   is the source address received in the IPv4 header containing the
   binding update message. The inner IPv6 packet will contain the home
   agent's IPv6 address as a source address and the mobile node's IPv6
   home address in the destination address field.





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   The mobile node needs to maintain the NAT bindings for its current
   IPv4 care-of address. This is done through sending the binding update
   regularly to the home agent.

2.4. Route optimization

   Route optimization, as specified in [MIPv6] will operate in an
   identical manner for dual stack mobile nodes when they are located in
   a visited network that provides IPv6 addresses to the mobile node.
   However, when located in an IPv4-only network, route optimization
   will not be possible due to the difficulty of performing the care-of
   address test. Therefore, mobile nodes will need to communicate
   through the home agent.

   Route optimization will not be possible for IPv4 traffic. That is,
   traffic addressed to the mobile node's IPv4 home address. This is
   similar to using Mobile IPv4, therefore there is no reduction of
   features resulting from using this specification.

2.5. Dynamic IPv4 home address allocation

   It is possible to allow for the mobile node's IPv4 home address to be
   allocated dynamically. This is done by including 0.0.0.0 in the IPv4
   home address option included in the binding update. The home agent
   SHOULD allocate an IPv4 address to the mobile node and include it in
   the IPv4 address acknowledgement option sent to the mobile node. In
   this case, the lifetime of the binding is bound to the minimum of the
   lifetimes of the IPv6 binding and the lease time of the IPv4 home
   address.

3. Extensions and modifications to Mobile IPv6

   This section highlights the protocol and implementation additions
   required to support this specification.

3.1. Binding update extensions

3.1.1 IPv4 home address option

   This option is included in the Mobility Header including the binding
   update message sent from the mobile node to a home agent or Mobility
   Anchor Point. The alignment requirement for this option is 4n.

       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      |Pref       |P|    Reserved     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     IPv4 home address                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   Type                 TBD

   Length               6

   Pref                 The length of the prefix allocated to the mobile
                        node. If only a single address is allocated,
                        this field MUST be set to 32. In the first
                        binding update requesting a prefix, the field
                        contains the prefix length requested. However,
                        in the following binding updates, this field
                        must contain the length of the prefix allocated.
                        A value of zero is invalid and MUST be
                        considered an error.

   P                    A flag indicating, when set, that the mobile
                        node requests a mobile network prefix. This flag
                        is only relevant for new requests, and must be
                        ignored for binding refreshes.

   Reserved             This field is reserved for future use. It MUST
                        be set to zero by the sender and ignored by the
                        receiver.

   IPv4 home address    The mobile node's IPv4 home address that should
                        be defended by the home agent. This field could
                        contain any unicast IPv4 address (public or
                        private) that was assigned to the mobile node.
                        The value 0.0.0.0 is used to request an IPv4
                        home address from the home agent. A mobile node
                        may choose to use this option to request a
                        prefix by setting the address to the All Zeroes
                        and setting the P flag. The mobile node could
                        then form an IPv4 home address based on the
                        allocated prefix. Alternatively, the mobile node
                        may use two different options, one for
                        requesting an address (Static or Dynamic) and
                        another for requesting a prefix.

3.1.2 The IPv4 Care-of Address option

   This option is included in the Mobility Header including the binding
   update message sent from the mobile node to a home agent or Mobility
   Anchor Point. The alignment requirement for this option is 4n.










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       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              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     IPv4 Care-of address                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type                 TBD

   Length               6

   Reserved             This field is set to zero by the sender and
                        ignored by the receiver.

   IPv4 care-of address This field contains the mobile node's IPv4 care-
                        of address. The IPv4 care-of address is used
                        when the mobile node is located in an IPv4-only
                        network.

3.1.3 The Binding Update Message extensions

   This specification extends the Binding Update message with two new
   flags. The flags are shown and described below.

       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
                                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                      |          Sequence #           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |A|H|L|K|M|R|F|T|  Reserved     |           Lifetime            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   F                    When set, this flag indicates a request for
                        forcing UDP encapsulation regardless of whether
                        a NAT is present on the path between the mobile
                        node and the home agent.

   T                    When set, this flag indicates that the mobile
                        node requests the use of the TLV-format for
                        encapsulating IPv6 in IPv4. The TLV-format is
                        described later in this document.

3.2. Binding Acknowledgement extensions

3.2.1 IPv4 address acknowledgement option

   This option is included in the Mobility Header including the binding
   acknowledgement message sent from the home agent or Mobility Anchor
   Point to the mobile node. This option indicates whether a binding



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   cache entry was created for the mobile node's IPv4 address.
   Additionally, this option can include an IPv4 home address in the
   case of Dynamic IPv4 home address configuration (i.e. if the
   unspecified IPv4 address was included in the binding update). The
   alignment requirement for this option is 4n.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |    Length     |   Status      | Pref    | Res |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      IPv4 home address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type                 TBD

   Length               6

   Status               Indicates success or failure for the IPv4 home
                        address binding. Values from 0 to 127 indicate
                        success. Higher values indicate failure. The
                        following values are reserved:
                                0   Success
                                128 Failure, reason unspecified
                                129 Administratively prohibited
                                130 Incorrect IPv4 home address
                                131 Invalid IPv4 address
                                132 Dynamic IPv4 home address
                                    assignment not available
                                133 Prefix allocation unauthorized


   Pref                 The prefix length of the address allocated. This
                        field is only valid in case of success and MUST
                        be set to zero and ignored in case of failure.
                        This field overrides what the mobile node
                        requested (if not equal to the requested
                        length).

   Res                  This field is reserved for future use. It MUST
                        be set to zero by the sender and ignored by the
                        receiver.

   IPv4 home address    The IPv4 home address that the home agent will
                        use in the binding cache entry. This could be a
                        public or private address. This field MUST
                        contain the mobile node's IPv4 home address.
                        If the address were dynamically allocated the
                        home agent will add the address to inform the
                        mobile node. Otherwise, if the address were
                        statically allocated to the mobile node, the



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                        home agent will copy it from the binding update
                        message.

3.2.2 The NAT detection option

   This option is sent from the home agent to the mobile node to
   indicate whether a NAT was in the path. This option MAY also include
   a suggested NAT binding refresh time for the mobile node. The
   alignment requirement for this option is 4n. If a NAT is detected,
   this option MUST be sent by the home agent.

       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     |F|          Reserved           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Refresh time                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type                 TBD

   Length               6

   F                    This flag indicates to the mobile node that UDP
                        encapsulation is required. When set, this flag
                        indicates that the mobile node MUST use UDP
                        encapsulation even if a NAT is not located
                        between the mobile node and home agent.

   Reserved             This field is reserved for future use. It MUST
                        be set to zero by the sender and ignored by the
                        receiver.

   Refresh time         A suggested time (in seconds) for the mobile
                        node to refresh the NAT binding. If set to zero,
                        it is ignored. If this field is set to the all
                        1s it means that keepalives are not needed, i.e.
                        no NAT was detected.

3.2.3 Extensions to the Binding Acknowledgement message

   This specification extends the binding acknowledgement message with a
   new flag. The new flag is shown and described below.

                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                       |    Status     |K|R|T| Reserved|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Sequence #          |           Lifetime            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   T                    This flag indicates, when set, that the sender



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                        of the binding acknowledgement supports the TLV-
                        tunnel format.

4. Protocol operation

   This section presents the protocol operation and processing for the
   messages presented above. In addition, this section introduces the
   NAT detection and traversal mechanism used by this specification.

4.1. Tunnelling formats

   This speacification allows two different types of UDP-based
   tunnelling formats to be used between the mobile node and its home
   agent or MAP. The two formats are vanilla UDP encapsulation and TLV-
   header UDP encapsulation. A vanilla UDP encapsulation format means
   the following order of headers:
   IP (v4 or v6)
   UDP
   IP (v4 or v6)
   Other headers

   When using this format the receiver would parse the version field
   following the UDP header in order to determine whether the following
   header is IPv4 or IPv6. The rest of the headers are processed
   normally. The above order of headers does not take IPsec headers into
   account as they may be placed in different parts of the packet. The
   above format MUST be supported by all implementations of this
   specification and MUST always be used to send the binding update
   message.

   A TLV-header UDP encapsulation is represented by the following order
   of headers:
   IP (v4 or v6)
   UDP
   TLV-header
   Other headers

   The receiver of the TLV-header UDP encapsulated packet expects the
   TLV-header to follow UDP. The TLV header contains the type of the
   following message and its length. Hence, the TLV header can carry
   traffic other than IP.

   The mobile node negotiates the format for tunnelling payload traffic
   during the binding exchange. If a mobile node prefers to use the TLV-
   header UDP encapsulation, it sets the T flag in the binding update
   sent to the home agent or MAP. If the receiver of the binding update
   supports the TLV-header format, it SHOULD set the T flag in the
   binding acknowledgement message. Otherwise, the T flag is cleared.
   The setting of the T flag in the binding acknowledgement indicates to
   the mobile node that it must use the TLV-header UDP encapsulation
   format for all packets sent for the duration of the binding or until



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   a new binding update is sent. Each binding update may renegotiate the
   tunnelling format. To avoid interoperability issues, the sender of
   the binding acknowledgement MUST not set the T flag unless it was set
   in the binding update sent from the mobile node.

   The TLV-header format is shown below.

       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           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type               This field indicates the type of the payload
                      following this header.

   Length             This field indicates the length of the payload
                      following the header, excluding the TLV-header
                      itself.

   Reserved           This field MUST be set to zero by the sender and
                      ignored by the receiver.

   The following values are assigned to the Type field, other values may
   be assigned in future:

   1         IPv4
   2         IPv6
   3         IPsec
   4         GRE

   In addition to the UDP-based tunnelling formats described above, this
   specification allows for standard IP in IP tunnelling. This can take
   place by tunnelling IPv4 in IPv6 or IP v6 in IPv4. However, whenever
   a NAT a detected, the mobile node will default to UDP-based
   encapsulation. The mobile node can request to always use UDP
   encapsulation by setting the F flag in the binding update. If the
   home agent does not agree to such request, it MUST reject the binding
   update with the new Status code:
               144  Cannot force UDP encapsulation
   Alternatively, the home agent can force UDP encapsulation by setting
   the F flag in the NAT detection option included in the binding
   acknowledgement.

4.2. NAT detection and traversal

   NAT detection is done when the initial binding update message is sent
   from the mobile node to the home agent. When located in an IPv4-only
   foreign link, the mobile node sends the binding update message
   encapsulated in UDP and IPv4. The source address of the IPv6 packet
   is the mobile node's IPv6 home address. The destination address is



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   the IPv6 address of the home agent. The IPv4 header contains the IPv4
   care-of address in the source address field and the IPv4 address of
   the home agent in the destination address field.

   When the home agent receives the encapsulated binding update it
   compares the IPv4 address of the source address field in the IPv4
   header with the IPv4 address in the source address of the IPv6
   header. If the two addresses match, no NAT device was in the path.
   Otherwise, a NAT device was in the path and the NAT detection option
   is included in the binding acknowledgement. The binding
   acknowledgement, and all future packets, are then encapsulated in UDP
   and IPv4. The source address in the IPv4 header is the IPv4 address
   of the home agent. The destination address is the IPv4 address
   received in the IPv4 header encapsulating the binding update (this
   address will be different from the IPv4 care-of address when a NAT is
   in the path). The source port in the packet is the home agent's
   source port. The destination port is the source port received in the
   binding update message. Note that the home agent stores the port
   numbers and associates them with the mobile node's tunnel in order to
   forward future packets.

   Upon receiving the binding acknowledgement with the NAT detection
   option, the mobile node sets the tunnel to the home agent to UDP
   encapsulation. Hence, all future packets to the home agent are
   tunneled in UDP and IPv4. For all tunneled IPv6 packets, the source
   address in the IPv6 header is the mobile node's IPv6 home address and
   the destination address is the correspondent node's IPv6 address. All
   tunneled IPv4 packets will contain the mobile node's IPv4 home
   address in the source address field of the inner IPv4 packet and the
   correspondent node's IPv4 address in the destination address field.
   The outer IPv4 header is the same whether the inner packet is IPv4 or
   IPv6.

   If no NAT device was detected in the path between the mobile node and
   the home agent then IPv6 packets are tunneled in an IPv4 header,
   unless the home agent forces UDP encapsulation using the F flag. The
   content of the inner and outer headers are identical to the UDP
   encapsulation case.

   A mobile node MUST always tunnel binding updates in UDP when located
   in an IPv4-only network. Essentially, this process allows for
   perpetual NAT detection. Similarly, the home agent MUST encapsulate
   binding acknowledgements in a UDP header whenever the binding update
   is encapsulated in UDP.

   In conclusion, the packet formats for the binding update and
   acknowledgement messages are shown below:

   A. Binding update received by the home agent:

              IPv4 header (src=V4ADDR, dst=HA_V4ADDR)



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                 UDP header
                   IPv6 header (src=V6HOA, dst=HAADDR)
                      ESP-header
                      Mobility header
                          BU [IPv4 HAO]
                            IPv4 CoA option

   Where V4ADDR is either the IPv4 care-of address or the address
   provided by the NAT device. V6HOA is the IPv6 home address of the
   mobile node. The binding update MAY also contain the IPv4 home
   address option IPv4 HAO.

   B. Binding acknowledgement sent by the home agent:
              IPv4 header (src= HA_V4ADDR, dst=V4ADDR)
                  UDP header
                   IPv6 header (src=HAADDR, dst=V6HOA)
                          V6HOA (IPv6 home address)
                      Mobility header
                          BA ([IPv4 ACK], NAT DET)

   Where V6HOA is IPv6 home address of the mobile node. The IPv4 ACK is
   the IPv4 address acknowledgement option, which is only included if
   the IPv4 home address option were present in the BU. The NAT DET is
   the NAT detection option, which MUST be present in the binding
   acknowledgement message if the binding update was encapsulated in
   UDP.

4.3. NAT Keepalives

   If a NAT is detected, the mobile node will need to refresh the NAT
   bindings in order to be reachable from the home agent. NAT bindings
   can be refreshed through sending and receiving traffic encapsulated
   in UDP. However, if the mobile node is not active, it will need to
   periodically send a message to the home agent in order to refresh the
   NAT binding. This can be done using the binding update message. The
   binding update/acknowledgement pair will ensure that the NAT bindings
   are refreshed in a reliable manner. There is no way for the mobile
   node to know the exact time of the NAT binding. The default time
   suggested in this specification is NATKATIMEOUT. If the home agent
   suggests a different refresh period in the binding acknowledgement,
   the mobile node SHOULD use the value suggested by the home agent.

   If the refresh time in the NAT detection option in the binding
   acknowledgement is set to the all 1s, the mobile node need not send
   messages to refresh the NAT binding. However, the mobile node may
   still be required to encapsulate traffic in UDP. This scenario may
   take place when a NAT is not detected, but the home agent still
   requires the mobile node to use UDP encapsulation.

   It should be noted that a mobile node that does not need to be
   reachable (i.e. only cares about the session continuity aspect of



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   Mobile IP) does not need to refresh NAT binding. In this case, the
   mobile node would only be able to initiate communication with other
   nodes.

4.4. Mobile node operation

   In addition to the operations specified in [MIPv6] and [NEMO], this
   specification requires mobile nodes to be able to support an IPv4
   home address.

   When sending an IPv6 packet containing a binding update while
   connected to an IPv4-only access network, mobile nodes MUST ensure
   the following:

   - The IPv6 packet is encapsulated in the vanilla UDP encapsulation
     format.
   - The source address in the IPv4 header is the mobile node's IPv4
     care-of address
   - The destination address in the IPv4 header is the home agent's
     IPv4 address.
   - The source address in the IPv6 header is the mobile node's IPv6
     home address.
   - The IPv4 home address option MAY be included in the mobility
     header. This option contains the IPv4 home address. If the mobile
     node did not have a static home address it MAY include the
     unspecified IPv4 address, which acts as a request for a dynamic
     IPv4 home address. Alternatively, one or more IPv4 home address
     options may be included with requests for IPv4 prefixes (i.e. with
     the P flag set.).
   - If the mobile node wishes to use UDP encapsulation only, it should
     set the F flag in the binding update message.
   - If the mobile node prefers to use the TLV-header format, it should
     set the T flag in the binding update.
   - The IPv6 packet MUST be authenticated as per [MIPv6], based on the
     mobile node's IPv6 home address.

   When sending a binding update from a visited network that supports
   IPv6, the mobile node MUST follow the rules specified in [MIPv6]. In
   addition, if the mobile node has an IPv4 home address or needs one,
   it MUST include the IPv4 home address option in the mobility header.
   If the mobile node already has a static IPv4 home address, such
   address MUST be included in the IPv4 home address option. Otherwise,
   if the mobile node needs a dynamic IPv4 address, it MUST include the
   IPv4 unspecified address in the IPv4 home address option.

   When the mobile node receives a binding acknowledgement from the home
   agent, it follows the rules in [MIPv6] and [NEMO]. In addition, the
   following actions MUST be made:

   - If the status field indicated failure with error code 144, the
     mobile node MAY resend the binding update without setting the F



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     flag.
   - If the mobility header includes an IPv4 address acknowledgement
     option indicating success, the mobile node should create two
     entries in its binding update list, one for the IPv6 home address
     and another for the IPv4 home address.
   - If the NAT detection option were present, the mobile node
     MUST tunnel future packets in UDP and IPv4. This MUST be indicated
     in the binding update list.
   - If no IPv4 address acknowledgement option were present, and an
     IPv4 home address option was present in the binding update, the
     mobile node MUST only create one binding update list entry for its
     IPv6 home address. The mobile node MAY include the IPv4 home
     address option in future binding updates.
   - If an IPv4 address acknowledgement option were present and it
     indicates failure for the IPv4 home address binding, the mobile
     node MUST NOT create an entry for that address in its binding
     update list. The mobile node MAY include the IPv4 home address
     option in future binding updates.
   - If the T flag was set in the binding update and the binding
     acknowledgement included a T flag set, the mobile node MUST use the
     TLV-header UDP encapsulation format.

4.4.1 Sending packets from a visited network.

   When the mobile node is located in an IPv6-enabled network it sends
   and receives IPv6 packets as described in [MIPv6]. IPv4 traffic is
   encapsulated in IPv6 packets to the home agent.

   When the mobile node is located in an IPv4 only network, it will send
   IPv6 packets to its home agent according to the following format:

                IPv4 header (src=V4CoA, dst=HA_V4ADDR)
                 [UDP header]
                   IPv6 header (src=V6HoA, dst=CN)
                    Upper layer protocols

   Where the UDP header is only used if a NAT were detected between the
   mobile node and the home agent, or if the home agent forced UDP
   encapsulation. V4CoA is the IPv4 care-of address configured by the
   mobile node in the visited network.

   Similarly, IPv4 packets are sent according to the following format:

                IPv4 header (src=V4CoA, dst=HA_V4ADDR)
                 [UDP header]
                   IPv4 header (src=V4HoA, dst=V4CN)
                    Upper layer protocols

   Where the UDP header is only used if a NAT were detected between the
   mobile node and the home agent, or if the home agent forced UDP
   encapsulation.



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   If the mobile node requested, and the home agent agreed, to use the
   TLV-header UDP encapsulation format, then the TLV-header would be
   used after the UDP header.

4.4.2 Movement detection in IPv4-only networks

   [MIPv6] describes movement detection mostly based on IPv6-specific
   triggers. Such triggers would not be available in an IPv4-only
   network. Hence, a mobile node located in an IPv4-only network SHOULD
   use [DNAv4] for guidance on movement detection mechanisms in IPv4-
   only networks.

4.5. Home agent operations

   In addition to the home agent specification in [MIPv6] and [NEMO],
   the home agent needs to be able to process the IPv4 home address
   option and generate the IPv4 address acknowledgement option. Both
   options are included in the mobility header. Furthermore, the home
   agent MUST be able to detect the presence of a NAT device and
   indicate that in the NAT detection option included in the binding
   acknowledgement.

   A home agent must also act as a proxy for address resolution in IPv4
   for the registered IPv4 home addresses of mobile nodes it is serving.
   Moreover, the administrative domain of the home agent is responsible
   for advertising the routing information of registered IPv4 mobile
   network prefixes of the mobile nodes.

   In order to comply with this specification, the home agent MUST be
   able to find the IPv4 home address of a mobile node when given the
   IPv6 home address. That is, given an IPv6 home address, the home
   agent MUST store the corresponding IPv4 home address if a static one
   is present. If a dynamic address were requested by the mobile node,
   the home agent MUST store that address (associated with the IPv6 home
   address) after it's allocated to the mobile node.

   When the home agent receives a binding update encapsulated in UDP and
   containing the IPv4 home address option, it needs to follow all the
   steps in [MIPv6] and [NEMO]. In addition, the following checks MUST
   be done:

   - If the IPv4 care-of address in the IPv4 CoA option is not the same
     as the IPv4 address in the source address in the IPv4 header then
     a NAT was in the path. This information should be flagged for the
     binding acknowledgement.

   - If the F flag in the binding update were set, the home agent needs
     to determine whether it accepts forcing UDP encapsulation. If it
     does not, the binding acknowledgement is sent with error code 144.




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     UDP encapsulation MUST NOT be used when the mobile node is located
     in an IPv6-enabled link.

   - If the T flag was set in the binding update, the home agent needs
     to determine whether it can accept the TLV-header encapsulation
     format. If it does, it should set the T flag in the binding
     acknowledgement. Otherwise, the home agent MUST NOT set the T flag
     in the binding acknowledgement.

   - If the IPv4 home address option contains a valid unicast IPv4
     address, the home agent MUST check that this address is allocated
     to the mobile node that has the IPv6 home address included in the
     home address option. The same MUST be done for an IPv4 prefix.

   - If the IPv4 home address option contained the unspecified IPv4
     address, the home agent SHOULD dynamically allocate an IPv4 home
     address to the mobile node. If none is available, the home agent
     MUST return error code 132 in the status field of the IPv4 address
     acknowledgement option. If a prefix were requested, the home agent
     MUST allocate a prefix with the requested length; otherwise the
     home agent MUST indicate failure of the operation with the
     appropriate error code.

   - If the binding update is accepted for the IPv4 home address, the
     home agent creates a binding cache entry for the IPv4 home
     address/prefix. The home agent MUST include an IPv4 acknowledgement
     option in the mobility header containing the binding
     acknowledgement.

   If the binding update is accepted for both IPv4 and IPv6 home
   addresses, the home agent creates separate binding cache entries, one
   for each home address. The care-of address is the one included in the
   binding update. If the care-of address is an IPv4 address, the home
   agent MUST setup a tunnel to the IPv4 care-of address of the mobile
   node.

   When sending a binding acknowledgement to the mobile node, the home
   agent constructs the message according to [MIPv6] and [NEMO]. Note
   that the routing header MUST always contain the IPv6 home address as
   specified in [MIPv6].

   If the care-of address of the mobile node were an IPv4 address, the
   home agent includes the mobile node's IPv6 home address in the
   destination address field in the IPv6 header. If a NAT were detected,
   the home agent MUST then encapsulate the packet in UDP and an IPv4
   header. The source address is set to the home agent's IPv4 address
   and the destination address is set to the address received in the
   source address of the IPv4 header encapsulating the binding update.

   After creating a binding cache entry for the mobile node's home
   addresses, all packets sent to the mobile node's home addresses are



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   tunneled by the home agent to the mobile node's care-of address. If a
   NAT were detected, packets are encapsulated in UDP and IPv4.
   Otherwise, if the care-of address is an IPv4 address, and no NAT were
   detected, packets are encapsulated in an IPv4 header.

4.5.1 Sending packets to the mobile node

   The home agent follows the rules specified in [MIPv6] for sending
   IPv6 packets to mobile nodes located in IPv6 networks. When sending
   IPv4 packets to When mobile nodes in an IPv6 network, the home agent
   must encapsulate the IPv4 packets in IPv6.

   When sending IPv6 packets to a mobile node located in an IPv4
   network, the home agent must follow the format negotiated in the
   binding update/acknowledgement exchange. In the absence of a
   negotiated format, the default format that MUST be supported by all
   implementations is:

                IPv4 header (src= HA_V4ADDR, dst= V4ADDR)
                 [UDP header]
                   IPv6 header (src=CN, dst= V6HoA)
                    Upper layer protocols

   Where the UDP header is only included if a NAT were detected between
   the mobile node and the home agent, or if the home agent forced UDP
   encapsulation. V4ADDR is the IPv4 address received in the source
   address field of the IPv4 packet containing the binding update.

   When sending IPv4 packets to a mobile node located in an IPv4
   network, the home agent must follow the format negotiated in the
   binding update/acknowledgement exchange. In the absence of a
   negotiated format, the default format that MUST be supported by all
   implementations is:

                IPv4 header (src= HA_V4ADDR, dst= V4ADDR)
                 [UDP header]
                   IPv4 header (src=V4CN, dst= V4HoA)
                    Upper layer protocols

   Where the UDP header is only included if a NAT were detected between
   the mobile node and home agent, or if the home agent forced UDP
   encapsulation.

4.6. Correspondent node operations

   This specification has no impact on IPv4 or IPv6 correspondent nodes.

5. Security considerations

   This specification allows a mobile node to send one binding update
   for its IPv6 and IPv4 home addresses. This is a slight deviation from



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   [MIPv6] which requires one binding update per home address. However,
   like [MIPv6], the IPsec security association needed to authenticate
   the binding update is still based on the mobile node's IPv6 home
   address. Therefore, in order to authorize the mobile node's IPv4 home
   address binding, the home agent MUST store the IPv4 address
   corresponding to the IPv6 address that is allocated to a mobile node.
   Therefore, it is sufficient for the home agent to know that the IPsec
   verification for the packet containing the binding update was valid
   provided that it knows which IPv4 home address is associated with
   which IPv6 home address. Hence, the security of the IPv4 home address
   binding is the same as the IPv6 binding.

   In effect, associating the mobile node's IPv4 home address with its
   IPv6 home address moves the authorization of the binding update for
   the IPv4 address to the Mobile IPv6 implementation, which infers it
   from the fact that mobile node has an IPv6 home address and the right
   credentials for sending an authentic binding update for such address.

   In cases where this specification is used for NAT traversal, it is
   important to note that it has the same UNSAF vulnerabilities
   associated with RFC 3519. An attacker is able to hijack the mobile
   node's session with the home agent if it can modify the contents of
   the outer IPv4 header. The contents of the header are not
   authenticated and there is no way for the home agent to verify their
   validity. Hence, a man in the middle attack where a change in the
   contents of the IPv4 header can cause a legitimate mobile node's
   traffic to be diverted to an illegitimate receiver independently of
   the authenticity of the binding update message.

   In this specification the binding update message MUST be protected
   using ESP transport mode. When the mobile node is located in an IPv4-
   only network, the binding update message is encapsulated in UDP as
   described earlier. However, UDP MUST NOT be used to encapsulate the
   binding update message when the mobile node is located in an IPv6-
   enabled network. If protection of payload traffic is needed when the
   mobile node is located in an IPv4-only network, encapsulation is done
   using tunnel mode ESP over port 4500 as described in [TUNSEC].

   Handovers within private IPv4 networks or from IPv6 to IPv4 networks
   will have impacts on the security association between the mobile node
   and the home agent. The following section presents the expected
   behaviour of the mobile node and home agent in those situations.

5.1 Handover interactions for IPsec and IKE

   After the mobile node detects movement it updates its tunnel
   information depending on the network capability. If the new link is
   IPv4-only then the mobile node updates its tunnel to UDP using the
   DSMIPv6 port and the local IPv4 address. Furthermore, the mobile node
   updates its local Security Association information to include its
   local IPv4 address and with the remote address being the home agent's



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   IPv4 address. The mobile node also removes binding update list
   entries for correspondent nodes since route optimisation cannot be
   supported while the mobile node is in an IPv4-only network. This may
   cause inbound packet losses as remote correspondent node are unaware
   of such movement. Following this, the mobile node sends the binding
   update message to the home agent.

   Upon receiving the binding update message, the home agent updates it
   security association locally to include the mobile node's remote
   address as the IP address received in the outer header. When the
   mobile node is located in a private IPv4 network (which is detected
   as described above), this address is the public address allocated by
   the NAT. Based on the setting of the K flag in the binding update,
   the home agent updates its IKE security association to include the
   remote address as that received in the outer header of the binding
   update message. If the mobile node were located in a private IPv4
   network this address is likely to be wrong as the NAT would likely
   allocate a different address to the IKE messages. Hence, the mobile
   node MUST update the IKE SA in one of two ways as follows. The mobile
   node SHOULD send an empty informational message, which results in the
   IKE module in the home agent to dynamically update the SA
   information. Alternatively, the IKE SA should be re-negotiated. Note
   that updating the IKE SA MUST take place after the mobile node has
   sent the binding update and received the acknowledgement from the
   home agent.

   The mobile node MAY use [MOBIKE] to update its IKE SA with the home
   agent. Using MOBIKE requires negotiating this capability with the
   home agent when establishing the SA. In this case, the mobile node
   and the home agent need not update their IPsec SAs locally as this
   step is performed by MOBIKE. Furthermore, the use of MOBIKE allows
   the mobile node to update the SA independently of the binding update
   exchange. Hence, there is no need for the mobile node to wait for a
   binding acknowledgement before performing MOBIKE. The use of MOBIKE
   is OPTIONAL in this specification.

6. Protocol constants

           NATKATIMEOUT      110 seconds

7. Acknowledgements

   Thanks to the following members (in alphabetical order) of the MIP6
   and NEMO Working Groups for their contributions, discussion, and
   review: Jari Arkko, Sri Gundavelli, Wassim Haddad, Conny Larsson,
   Ahmad Muhanna, Vidya Narayanan, Karen Neilsen and Keiichi Shima.
   Thanks to Karen Neilsen, Pasi Eronen and Christian Kaas-Petersen for
   raising the issue of IKEv2 interactions and proposing the solution
   included in this document.





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8. IANA considerations

   The specification requires the following allocations from IANA:
   - A UDP port is needed for the NAT traversal mechanism described in
     section 4.1.
   - The IPv4 home address option described in section 3.1.1 requires an
     option type. This option is included in the Mobility header
     described in [MIPv6].
   - The IPv4 address acknowledgement option described in section 3.2.1
     requires a new option type. This option is included in the Mobility
     header described in [MIPv6].
   - The NAT detection option described in section 3.2.2 requires a new
     option type. This option is included in the Mobility header
     described in [MIPv6].
   - The IPv4 Care-of address option described in section 3.1.2 requires
     an option type. This option is included in the Mobility header
     described in [MIPv6].

9. References

NORMATIVE

   [IPv6] S. Deering and B. Hinden, "Internet Protocol version 6 (IPv6)
          specification", RFC 2460

   [MIPv6] D. Johnson, C. Perkins and J. Arkko, "Mobility Support in
           IPv6", RFC 3775, June 2004.

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

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

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

   [TUNSEC] Huttunen, A., Swander, B., Volpe, V., DiBurro, L. and M.
            Stenberg, "UPD Encapsulation for IPsec ESP Packets", RFC
            3948, January 2005.

INFORMATIVE

   [BOOT] Giaretta, G. (Ed.), Kempf J., and V. Devarapalli, " Mobile
          IPv6 bootstrapping in split scenario", draft-ietf-mip6-
          bootstrapping-split, June 2005, work in progress.

   [HMIPv6] Soliman, H., Castelluccia, C., ElMalki, K., and L. Bellier,
            "Hierarchical Mobile IPv6 Mobility Management (HMIPv6)",



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            Draftietf-mipshop-4140bis-04 November 2007.

   [INTBOOT] K. Chowdhury and A. Yegin, "MIP6-bootstrapping for the
             Integrated Scenario", draft-ietf-mip6-bootstrapping-
             integrated-dhc-02, Work in Progress.

   [MIP-PB]   Tsirtsis, G., and H. Soliman, "Mobility management for
              Dual stack mobile nodes, A Problem Statement",
              RFC 4977, August 2007.

   [MIPv4] C. Perkins, "Mobility Support for IPv4", RFC3344

   [MOBIKE] P. Eronen,"IKEv2 Mobility and Multihoming Protocol (MOBIKE)"
            , RFC 4555, June 2006.

   [SNRIO] Larsson, T., Gustafsson, E., and H. Levkowetz, "Use of MIPv6
           in IPv4 and MIPv4 in IPv6 networks", draft-larsson-v6ops-
           mip-scenarios-01.txt, February 2004.

10. Contributors

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

   Vijay Devarapalli
   E-mail: vijay.devarapalli@azairenet.com

   James Kempf
   E-mail: kempf@docomolabs-usa.com

   Henrik Levkowetz
   E-mail: henrik@levkowetz.com

   Pascal Thubert
   E-mail: pthubert@cisco.com

   George Tsirtsis
   E-mail1: G.Tsirtsis@Qualcomm.com
   E-mail2: tsirtsisg@yahoo.com

   Wakikawa Ryuji
   ryuji@sfc.wide.ad.jp

Author's Address

   Hesham Soliman
   Elevate Technologies
   E-mail: Hesham@elevatemobile.com






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Soliman                                                        [Page 28]


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