path: root/doc/rfc/rfc5555.txt

Network Working Group                                    H. Soliman, Ed.
Request for Comments: 5555                          Elevate Technologies
Category: Standards Track                                      June 2009


          Mobile IPv6 Support for Dual Stack Hosts and Routers

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

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
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   The current Mobile IPv6 and Network Mobility (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 home agent.  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 ....................................................3
      1.1. Requirements Notation ......................................4
      1.2. Motivation for Using Mobile IPv6 Only ......................4
      1.3. Scenarios Considered by This Specification .................4
   2. Solution Overview ...............................................6
      2.1. Home Agent Address Discovery ...............................6
      2.2. Mobile Prefix Solicitation and Advertisement ...............7
      2.3. Binding Management .........................................8
           2.3.1. Foreign Network Supports IPv6 .......................8
           2.3.2. Foreign Network Supports IPv4 Only ..................9
      2.4. Route Optimization ........................................11
      2.5. Dynamic IPv4 Home Address Allocation ......................11
   3. Extensions and Modifications to Mobile IPv6 ....................11
      3.1. Binding Update Extensions .................................11
           3.1.1. IPv4 Home Address Option ...........................11
           3.1.2. The IPv4 Care-of Address Option ....................13
           3.1.3. The Binding Update Message Extensions ..............13
      3.2. Binding Acknowledgement Extensions ........................14
           3.2.1. IPv4 Address Acknowledgement Option ................14
           3.2.2. The NAT Detection Option ...........................16
   4. Protocol Operation .............................................17
      4.1. Tunnelling Formats ........................................17
           4.1.1. Tunnelling Impacts on Transport and MTU ............18
      4.2. NAT Detection .............................................19
      4.3. NAT Keepalives ............................................21
      4.4. Mobile Node Operation .....................................22
           4.4.1. Selecting a Care-of Address ........................22
           4.4.2. Sending Binding Updates ............................23
           4.4.3. Sending Packets from a Visited Network .............25
           4.4.4. Movement Detection in IPv4-Only Networks ...........26
      4.5. Home Agent Operation ......................................26
           4.5.1. Sending Packets to the Mobile Node .................28
      4.6. Correspondent Node Operation ..............................29
   5. Security Considerations ........................................29
      5.1. Handover Interactions for IPsec and IKE ...................30
      5.2. IKE Negotiation Messages between the Mobile Node
           and Home Agent ............................................33
           5.2.1. IKEv2 Operation for Securing DSMIPv6 Signaling .....33
           5.2.2. IKEv2 Operation for Securing Data over IPv4 ........36
   6. Protocol Constants .............................................38
   7. Acknowledgements ...............................................38
   8. IANA Considerations ............................................38
   9. References .....................................................39
      9.1. Normative References ......................................39
      9.2. Informative References ....................................40
   10. Contributors ..................................................41



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1.  Introduction

   Mobile IPv6 [RFC3775] and NEMO [RFC3963] 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
   initially use only IPv6 addresses for their connections.  It is
   reasonable to assume that mobile nodes will, for a long time, need an
   IPv4 home address that can be 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 [RFC3963] 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 dual stack mobile nodes to use IPv6 mobility only.

   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 these 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 a 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.  Note that both mobile host and
   router functionality have already been defined in [RFC3775] and
   [RFC3963], respectively.  This specification does not change those
   already defined behaviors, nor does it extend the specific types of
   hosts and router support already defined, with the following two
   exceptions: (i) allowing the mobile node to communicate with its home
   agent even over IPv4 networks, and (ii) allowing the use of IPv4 home
   addresses and prefixes.

   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 [RFC5380].  The





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   requirements on the MAP are identical to those stated for the home
   agent; however, 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.  Requirements Notation

   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 [RFC2119].

1.2.  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 [RFC3344], 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
   private IPv4 networks needs to be considered.  This specification
   introduces NAT traversal for this purpose.

   The above benefits make the case for using only Mobile IPv6 for dual
   stack mobile nodes, as it allows for a long-lasting mobility
   solution.  The use of Mobile IPv6 for dual stack mobility eliminates
   the need for changing the mobility solution due to the introduction
   of IPv6 within a deployed network.

1.3.  Scenarios Considered by This Specification

   There are several scenarios that illustrate potential
   incompatibilities for mobile nodes using Mobile IPv6.  Some of the
   problems associated with mobility and transition issues were
   presented in [RFC4977].  This specification considers the scenarios
   that address all the problems discussed in [RFC4977].  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





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   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.

   It should be noted that [RFC5389] highlights issues with some types
   of NATs that act as generic Application Level Gateways (ALGs) and
   rewrite any 32-bit field containing the NAT's public IP addresses.
   This specification will not support such NATs.

   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.  The address might not be physically configured on the home
   agent interface.  Instead, it is associated with the home agent on
   the Network Address Port Translation (NAPT) device, which allows the
   home agent to be reachable through address or port mapping.

   Scenario 4: Use of IPv4-only applications

   In this scenario, the mobile node may be located in an IPv4, IPv6, or
   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 to use 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 the allocation of a stable
   IPv4 home address to the mobile node.  This is done using an IPv4
   home address.  Since running Mobile IPv4 and Mobile IPv6



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   simultaneously is problematic (as illustrated in [RFC4977]), this
   scenario adds a requirement on Mobile IPv6 to support IPv4 home
   addresses.

   Scenario 5: IPv6 and IPv4-enabled networks

   In this scenario, the mobile node should prefer the use of an IPv6
   care-of address for either its IPv6 or IPv4 home address.  Normal
   IP-in-IP tunnelling should be used in this scenario as described in
   [RFC3775].  Under rare exceptions, where IP-in-IP tunnelling for IPv6
   does not allow the mobile node to reach the home agent, the mobile
   node follows the sending algorithm described in Section 4.4.1.  UDP
   tunnelling in IPv6 networks is proposed in this document as a last-
   resort mechanism when reachability cannot be achieved through normal
   IP-in-IP tunnelling.  It should not be viewed as a normal mode of
   operation and should not be used as a first resort.

2.  Solution Overview

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

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

   o  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.

   o  Mobile nodes need to be able to detect the presence of a NAT
      device and traverse it in order to communicate with the home
      agent.

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

2.1.  Home Agent Address Discovery

   Dynamic Home Agent Address Discovery (DHAAD) is defined in [RFC3775]
   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 (MN) 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



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   stack network, it may also use procedures defined in [CHOWDHURY] to
   discover home agent information.  Note that the use of [CHOWDHURY]
   cannot give the mobile node information that allows it to communicate
   with the home agent if the mobile node is located in an IPv4-only
   network.  In this scenario, the mobile node needs 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 [RFC5026] is
   reused.  For instance, if the service name is "mip6" and the protocol
   name is "ipv6" in the SRV record, the mobile node SHOULD send a DNS
   request with the QNAME set to "_mip6._ipv6.example.com".  The
   response should contain the home agent's FQDN(s) and may include the
   corresponding 'AAAA' and 'A' records as well.

   If multiple home agents reside on the home link, each configured with
   a public IPv4 address, then the operation above applies.  The correct
   DNS entries can be configured accordingly.

2.2.  Mobile Prefix Solicitation and Advertisement

   According to [RFC3775], 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 these messages requires the mobile node to have a security
   association with the home agent, using IPsec and based on the mobile
   node's IPv4 care-of address as described in [RFC3775] and [RFC4877].

   [RFC3775] requires the mobile node to include the home address option
   in the solicitation message sent to the home agent.  If the mobile
   node is located in an IPv4 network, it will not be assigned an IPv6
   address to include in the source address.  In this case, the mobile
   node MUST use its home address in the source address field of the
   IPv6 packet, in addition to using the home address option as expected
   by [RFC3775].





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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:

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

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

   o  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 [RFC3775].  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
   address, the home agent will tunnel it in IPv6 to the mobile node's
   IPv6 care-of address that is 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.

   In this scenario, this document extends [RFC3775] by including the
   IPv4 home address option in the binding update message.  Furthermore,
   if the network supports both IPv4 and IPv6, or if the mobile node is
   experiencing problems with IP-in-IP tunnelling, this document
   proposes some mitigating actions as described in Section 4.4.1.

   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 [RFC3775].  In
   addition, if the binding update included an IPv4 home address option,
   the binding acknowledgement MUST include the IPv4 address
   acknowledgment option as described in Section 3.2.1.  This option



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   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.

   When a mobile node acquires both IPv4 and IPv6 care-of addresses at
   the foreign network, it SHOULD prioritize the IPv6 care-of address
   for its MIPv6 binding as described in Section 4.4.1.

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.  NAT
   detection is needed for the purposes of the signaling presented in
   this specification.

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 is 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
   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 [RFC3775].  In addition, if the binding update included



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   an IPv4 home address option, the binding acknowledgement MUST include
   the IPv4 address acknowledgment option as described in Section 3.2.1.
   The binding acknowledgement is encapsulated to the IPv4 care-of
   address, which was included in the source address field of the IPv4
   header encapsulating the binding update.

2.3.2.2.  Foreign 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 using UDP
   and IPv4.  The UDP port allocated for the home agent is 4191
   (dsmipv6).

   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 is 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 source address of the
   IPv4 header that encapsulated 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 and 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.  Note that the home
   agent MUST store the source UDP port numbers contained in the packet
   carrying the binding update in order to be able to forward packets to
   the mobile node.

   After accepting the binding updates and creating the corresponding
   entries, the home agent MUST send a binding acknowledgement as
   specified in [RFC3775].  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
   and then in 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
   IPv4 packet is the source address that was 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 [RFC3775], 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 and
   while communicating with an IPv6-enabled correspondent node.
   However, when located in an IPv4-only network, or when using the IPv4
   home address to communicate with an IPv4 correspondent node, route
   optimization will not be possible due to the difficulty of performing
   the return-routability test.  In this specification, UDP
   encapsulation is only used between the mobile node and its home
   agent.  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 that is 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.





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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      |Prefix-len |P|    Reserved     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     IPv4 home address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 1: IPv4 Home Address Option

   Type

      29

   Length

      6

   Prefix-len

      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 All Zeroes and setting
      the P flag.  The mobile node could then form an IPv4 home address



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      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.

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

                       Figure 2: The IPv4 CoA Option

   Type

      32

   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 one new
   flag.  The flag is shown and described below.








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

                     Figure 3: Binding Update Message

   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.  This flag may be set
      by the mobile node if it is required to use UDP encapsulation
      regardless of the presence of a NAT.  This flag SHOULD NOT be set
      when the mobile node is configured with an IPv6 care-of address --
      with the exception of the scenario mentioned in Section 4.4.1.

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
   cache entry was created for the mobile node's IPv4 address.
   Additionally, this option includes 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-len   |Res|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      IPv4 home address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 4: IPv4 Address Acknowledgement Option









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   Type

      30

   Length

      6

   Status

      Indicates success or failure for the IPv4 home address binding.
      Values from 0 to 127 indicate success.  Higher values indicate
      failure.

   Pref-len

      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 is
      statically allocated to the mobile node, the home agent will copy
      it from the binding update message.

   The following values are allocated for the status field:

   o  0 Success

   o  128 Failure, reason unspecified

   o  129 Administratively prohibited

   o  130 Incorrect IPv4 home address

   o  131 Invalid IPv4 address




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   o  132 Dynamic IPv4 home address assignment not available

   o  133 Prefix allocation unauthorized

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.  This might
   be useful for scenarios where the mobile node is known to be moving
   within the home agent's administrative domain and, therefore, the NAT
   timeout is known (through configuration) to the home agent.  Section
   3.5 of [RFC5405] discusses issues with NAT timeout in some detail.

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

                    Figure 5: The NAT Detection Option

   Type

      31

   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.  This flag SHOULD NOT be set when the
      mobile node is assigned an IPv6 care-of address -- with the
      exception of accommodating the scenarios discussed in
      Section 4.4.1.







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   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 all 1s, it means that keepalives are not needed, i.e., no NAT
      was detected.  The home agent MUST be configured with a default
      value for the refresh time.  The recommended value is outlined in
      Section 6.

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 specification allows the mobile node to use various tunnelling
   formats depending on its location and the visited network's
   capabilities.  The mobile node can tunnel IPv6 in IPv4, IPv4 in IPv6,
   or use UDP encapsulation to tunnel IPv6 in IPv4.  Naturally, this
   specification also supports tunnelling IPv6 in IPv6 [RFC2473].

   This specification allows UDP-based tunnelling to be used between the
   mobile node and its home agent or MAP.  A UDP encapsulation format
   means the following order of headers:

      IPv4/v6

      UDP

      IP (v4 or v6)

      Other headers

   Note that the use of UDP encapsulation for IPv6 care-of addresses
   SHOULD NOT be done except in the circumstances highlighted in Section
   4.4.1.

   When using this format, the receiver parses 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



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   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.

   UDP tunnelling can also encapsulate an Encapsulating Security Payload
   (ESP) header as shown below:

      IPv4/v6

      UDP

      ESP

      IP (v4 or v6)

      Other headers

   The negotiation of the secure tunnel format described above is
   discussed in Section 5.2.  The receiver of a UDP tunnel detects
   whether or not an ESP header is present based on the UDP port used.

4.1.1.  Tunnelling Impacts on Transport and MTU

   Changing the tunnel format may occur due to movement of the mobile
   node from one network to another.  This can impact the link and path
   MTU, which may affect the amount of bandwidth available to the
   applications.  The mobile node may use Path MTU Discovery (PMTUD) as
   specified in [RFC4459].

   To accommodate traffic that uses Explicit Congestion Notification
   (ECN), it is RECOMMENDED that the ECN and Differentiated Services
   Code Point (DSCP) information be copied between the inner and outer
   header as defined in [RFC3168] and [RFC2983].  It is RECOMMENDED that
   the full-functionality option defined in Section 9.1.1 of [RFC3168]
   be used to deal with ECN.

   Note that some implementations may not be able to use ECN over the
   UDP tunnel.  This is due to the lack of access to ECN bits in the UDP
   API on most platforms.  However, this issue can be avoided if UDP
   encapsulation is done in the kernel.

   Note that, when using UDP encapsulation, the Time to Live (TTL) field
   must be decremented in the same manner as when IP-in-IP encapsulation
   is used.






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4.2.  NAT Detection

   This section deals with NAT detection for the purpose of
   encapsulating packets between the mobile node and the home agent when
   the mobile node is present in a private IPv4 network.  Mobile IPv6
   uses IKEv2 to establish the IPsec security association (SA) between
   the mobile node and the home agent.  IKEv2 has its own NAT detection
   mechanism.  However, IKEv2's NAT detection is only used for the
   purpose of setting up the IPsec SA for secure traffic.  The
   interactions between the two NAT traversal mechanisms are described
   in Section 5.

   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
   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 included in the IPv4 care-of address
   option.  If the two addresses match, no NAT device was in the path.
   Otherwise, a NAT 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





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   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:

   Binding update received by the home agent:

      IPv4 header (src=V4ADDR, dst=HA_V4ADDR)

      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.

   Binding acknowledgement sent by the home agent:

      IPv4 header (src= HA_V4ADDR, dst=V4ADDR)

      UDP header

      IPv6 header (src=HAADDR, dst=V6HOA)




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      ESP header

      Mobility header

      BA ([IPv4 ACK], NAT DET)

   Where V6HOA is the 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 is 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 (see Section 6).  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 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., one that only cares about the session continuity
   aspect of Mobile IP) does not need to refresh the NAT binding.  In
   this case, the mobile node would only be able to initiate
   communication with other nodes.  However, this is likely to imply
   that the mobile node will need to send a binding update before
   initiating communication after a long idle period as it is likely to
   be assigned a different port and IPv4 address by the NAT when it
   initiates communication.  Hence, an implementation may choose, for
   the sake of simplicity, to always maintain the NAT bindings even when
   it does not need reachability.




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   Note that keepalives are also needed by IKEv2 over UDP port 4500.
   This is needed for IKE (Internet Key Exchange Protocol) dead-peer
   detection, which is not handled by DSMIPv6 keepalives.

4.4.  Mobile Node Operation

   In addition to the operations specified in [RFC3775] and [RFC3963],
   this specification requires mobile nodes to be able to support an
   IPv4 home address.  This specification also requires the mobile node
   to choose an IPv4 or an IPv6 care-of address.  We first discuss
   care-of address selection, then continue with binding management and
   transmission of normal traffic.

4.4.1.  Selecting a Care-of Address

   When a mobile node is in a dual stacked, visited network, it will
   have a choice between an IPv4 and an IPv6 care-of address.  The
   mobile node SHOULD prefer the IPv6 care-of address and bind it to its
   home address(es).  If a mobile node attempted to bind the IPv6 care-
   of address to its home address(es) and the binding update timed out,
   the mobile node SHOULD:

   o  Resend the binding update using the exponential back-off algorithm
      described in [RFC3775].

   o  If after three attempts, in total, a binding acknowledgement was
      not received, the mobile node SHOULD send a new binding update
      using the IPv4 care-of address.  The exponential backoff algorithm
      described in [RFC3775] should be used for re-transmission of the
      binding update if needed.

   This procedure should be used to avoid scenarios where IPv6
   connectivity may not be as reliable as IPv4.  This unreliability may
   take place during early deployments of IPv6 or may simply be due to
   temporary outages affecting IPv6 routing.

   It is RECOMMENDED that upon movement, the mobile node not change the
   IP address family chosen for the previous binding update unless the
   mobile node is aware that it has moved to a different administrative
   domain where previous problems with IPv6 routing may not be present.
   Repeating the above procedure upon every movement can cause
   significant degradation of the mobile node's applications'
   performance due to extended periods of packet losses after handover,
   if the routing outage is still in effect.

   When using an IPv4 care-of address and IP-in-IP encapsulation, if the
   mobile node implementation is made aware by upper layers of
   persistent packet losses, it may attempt to resend the binding update



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   with the F flag set, requesting UDP encapsulation for all packets.
   This may avoid packet losses due to situations where local
   firewalling policies prevent the use of IP-in-IP encapsulation.

   The effect of this address selection mechanism is to allow the
   following preferences in the absence of NAT:

   1. IPv6

   2. IPv4 (using IP-in-IP or UDP encapsulation if a NAT is detected)

   3. UDP encapsulation when IP-in-IP is not allowed by the local
      domain.

4.4.2.  Sending Binding Updates

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

   o  The IPv6 packet is encapsulated in UDP.

   o  The source address in the IPv4 header is the mobile node's IPv4
      care-of address.

   o  The destination address in the IPv4 header is the home agent's
      IPv4 address.

   o  The source address in the IPv6 header is the mobile node's IPv6
      home address.

   o  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).

   o  If the mobile node wishes to use UDP encapsulation only, it must
      set the F flag in the binding update message.

   o  The IPv6 packet MUST be authenticated as per [RFC3775], 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 [RFC3775].
   In addition, if the mobile node has an IPv4 home address or needs



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   one, it MUST include the IPv4 home address option in the mobility
   header.  If the mobile node already has a static IPv4 home address,
   this 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 0.0.0.0 address in the IPv4 home address option.

   In addition to the rules in [RFC3775], the mobile node should follow
   the care-of address selection guidelines in Section 4.4.1.

   When the mobile node receives a binding acknowledgement from the home
   agent, it follows the rules in [RFC3775] and [RFC3963].  In addition,
   the following actions MUST be made:

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

   o  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.

   o  If the NAT detection option is present, the mobile node MUST
      tunnel future packets in UDP and IPv4.  This MUST be indicated in
      the binding update list.

   o  If no IPv4 address acknowledgement option is 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.

   o  If an IPv4 address acknowledgement option is 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.

4.4.2.1.  Removing Bindings

   Mobile nodes will remove bindings from the home agent's binding cache
   whenever they move to the home link, or simply when mobility support
   is not needed.

   Deregistering the IPv6 home address is described in [RFC3775].  The
   same mechanism applies in this specification.  Mobile nodes may
   remove the binding for only the IPv4 home address by sending a
   binding update that does not include the IPv4 home address option.



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   Upon receiving this binding update, the home agent will replace the
   existing cache entries with the content of the new message.  This
   ensures that the IPv4 home address binding is removed while
   maintaining an IPv6 binding.

   Note that the mobile node cannot remove the IPv6 home address binding
   while maintaining an IPv4 home address binding.

   A binding update message with a lifetime of zero will remove all
   bindings for the mobile node.

4.4.3.  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 [RFC3775].  In cases where
   IP-in-IP encapsulation is not providing connectivity to the home
   agent, the mobile node may choose to encapsulate in UDP as suggested
   in Section 4.4.1.  However, this encapsulation of IPv6 traffic should
   be used as a last resort, as described.  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

   Here, the UDP header is only used if a NAT has been 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





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

4.4.4.  Movement Detection in IPv4-Only Networks

   [RFC3775] describes movement detection mostly based on IPv6-specific
   triggers and Neighbor Discovery [RFC4861] information.  These
   triggers are not available in an IPv4-only network.  Hence, a mobile
   node located in an IPv4-only network SHOULD use [RFC4436] for
   guidance on movement-detection mechanisms in IPv4-only networks.

   The mobile node detects that it's in an IPv4-only network when the
   IPv6 movement-detection algorithm fails to configure an IPv6 address.

   This specification does not support mobile nodes returning home while
   using IPv4.  That is, the IPv4 support is only defined for mobile
   nodes that are in a visited network.

4.5.  Home Agent Operation

   In addition to the home agent specification in [RFC3775] and
   [RFC3963], 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 presence 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 is 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 [RFC3775] and [RFC3963].  In addition, the following checks
   MUST be done:



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   o  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.

   o  If the F flag in the binding update is 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.
      UDP encapsulation SHOULD NOT be used when the mobile node is
      located in an IPv6-enabled link, with the exception of the
      scenarios outlined in Section 4.4.1.

   o  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.

   o  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 is requested, the home agent
      SHOULD allocate a prefix with the requested length; if prefix
      allocation (of any length) is not possible, the home agent MUST
      indicate failure of the operation with the appropriate error code.

   o  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.

   o  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 set up 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 [RFC3775] and [RFC3963].
   Note that the routing header MUST always contain the IPv6 home
   address as specified in [RFC3775].

   If the care-of address of the mobile node is 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 is detected,
   the home agent MUST then encapsulate the packet in UDP and in an IPv4



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

4.5.1.  Sending Packets to the Mobile Node

   The home agent follows the rules specified in [RFC3775] for sending
   IPv6 packets to mobile nodes located in IPv6 networks.  When sending
   IPv4 packets to 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 uses the following format:

      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 is 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



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

4.6.  Correspondent Node Operation

   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 [RFC3775], which requires one binding update per home address.
   However, like [RFC3775], 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 the mobile node has an IPv6 home address and the
   right credentials for sending an authentic binding update for the
   IPv6 address.

   This specification requires the use of IKEv2 as the default mechanism
   for dynamic keying.

   In cases where this specification is used for NAT traversal, it is
   important to note that it has the same vulnerabilities associated
   with [RFC3519].  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, is possible.

   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



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   as described earlier in Section 4.2.  However, UDP SHOULD 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 [RFC3948].  During the IKE negotiation with the home
   agent, if the mobile node and home agent support the use of port
   4500, the mobile node MUST establish the security association over
   port 4500, regardless of the presence of a NAT.  This is done to
   avoid switching between ports 500 and 4500 and the potential traffic
   disruption resulting from this switch.

   Handovers within private IPv4 networks or from IPv6 to IPv4 networks
   will impact 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.  The details of
   the IKE negotiations and messages are illustrated in Section 5.2.

5.1.  Handover Interactions for IPsec and IKE

   After the mobile node detects movement, it configures a new care-of
   address.  If the mobile node is in an IPv4-only network, it removes
   binding update list entries for correspondent nodes, since route
   optimisation cannot be supported.  This may cause inbound packet
   losses, as remote correspondent nodes are unaware of such movement.
   To avoid confusion in the correspondent node, the mobile node SHOULD
   deregister its binding with each correspondent node by sending a
   deregistration binding update.  The deregistration binding update
   message is tunnelled to the home agent and onto the correspondent
   node.  This is done after the mobile node updates the home agent with
   its new location as discussed below.

   The mobile node sends the binding update message to the home agent.
   If the mobile node is in an IPv6-enabled network, the binding update
   SHOULD be sent without IPv4/UDP encapsulation, unless UDP
   encapsulation is needed as described in Section 4.4.1.  If the mobile
   node is in an IPv4-only network, then -- after IPsec processing of
   the binding update (BU) message -- it encapsulates the BU in UDP/IPv4
   as discussed in Sections 4.2 and 4.4.  In order to be able to send
   the binding update while in an IPv4-only network, the mobile node
   needs to use the new IPv4 care-of address in the outer header, which
   is different from the care-of address used in the existing tunnel.
   This should be done without permanently updating the tunnel within
   the mobile node's implementation in order to allow the mobile node to
   receive packets on the old care-of address until the binding
   acknowledgement is received.  The method used to achieve this effect
   is implementation dependent and is outside the scope of this
   specification.  This implies that the IP forwarding function (which



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   selects the interface or tunnel through which a packet is sent) is
   not based solely on the destination address: some IPv6 packets
   destined to the home agent are sent via the existing tunnel, while
   BUs are sent using the new care-of address.  Since BUs are protected
   by IPsec, the forwarding function cannot necessarily determine the
   correct treatment from the packet headers.  Thus, the DSMIPv6
   implementation has to attach additional information to BUs, and this
   information has to be preserved after IPsec processing and made
   available to the forwarding function or to DSMIP extensions included
   in the forwarding function.  Depending on the mobile node's
   implementation, meeting this requirement may require changes to the
   IPsec implementation.

   Upon receiving the binding update message encapsulated in UDP/IPv4,
   the home agent processes it as follows.  In order to allow the
   DSMIPv6 implementation in the home agent to detect the presence of a
   NAT on the path to the mobile node, it needs to compare the outer
   IPv4 source address with the IPv4 address in the IPv4 care-of address
   option.  This implies that the information in the outer header will
   be preserved after IPsec processing and made available to the DSMIPv6
   implementation in the home agent.  Depending on the home agent's
   implementation, meeting this requirement may require changes to the
   IPsec implementation.

   The home agent updates its tunnel mode security association to
   include the mobile node's care-of address as the remote-tunnel header
   address and 4500 as the port number.  The IPv4 address and port
   number are likely to be wrong; the mobile node provides the correct
   information in a separate exchange as described below.  When the
   mobile node is located in a private IPv4 network (which is detected
   as described above), the new address and port number are allocated by
   the NAT.  The home agent will also enable or disable UDP
   encapsulation for outgoing ESP packets for the purpose of NAT
   traversal.

   If the Key Management Mobility Capability (K) bit was set in the
   binding update, and the home agent supports this feature, the home
   agent updates its IKE security associations to include the mobile
   node's care-of address as the peer address and 4500 as the port
   number.  The home agent may also need to change NAT traversal fields
   in the IKE_SA to enable the dynamic update of the IP address and port
   number, based on the reception of authenticated IKE messages or
   authenticated packets using tunnel mode ESP.  The dynamic updates are
   described in Section 2.23 of [RFC4306].  As described above, when the
   mobile node is located in a private IPv4 network, the address and
   port number used for IPsec and IKE traffic is not yet known by the
   home agent at this point.




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   The mobile node updates the IKE SA in one of two ways.  If the K flag
   was set in the binding acknowledgement message, the mobile node
   SHOULD send an empty informational message, which results in the IKE
   module in the home agent dynamically updating the SA information.
   The IKE implementation in the home agent is REQUIRED to support this
   feature.  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.

   It is important to note that the mobile node's IPv4 care-of address
   seen by the DSMIPv6 module in the home agent upon receiving the
   binding update may differ from the IPv4 care-of address seen by the
   IKE module and the care-of address used for forwarding IPsec tunnel
   mode traffic.  Hence, it is probable that different modules in the
   home agent will have a different care-of address that should be used
   for encapsulating traffic to the mobile node.

   After successfully processing the binding update, the home agent
   sends the binding acknowledgement to the mobile node's care-of
   address as received in the outer header of the packet containing the
   binding update.  Note that if the BU was rejected, the binding
   acknowledgement (BAck) is sent to the same address from which the BU
   was received.  This may require special treatment in IP forwarding
   and/or IPsec processing that resembles the sending of BUs in the
   mobile node (described above).

   Upon receiving the binding acknowledgement, the mobile node updates
   its local tunnel mode security association information to include the
   tunnel header IP source address, which is the mobile node's address,
   and the tunnel header IP destination, which is the home agent's
   address.  The mobile node may also need to enable or disable UDP
   encapsulation for outgoing ESP packets for the purpose of NAT
   traversal and the sending of keepalives.

   The mobile node MAY use MOBIKE [RFC4555] 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 MUST 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.







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5.2.  IKE Negotiation Messages between the Mobile Node and Home Agent

   This specification defines a number of possible data encapsulation
   formats, depending on the mobile node's connectivity to the visited
   network.  When connected to an IPv6-enabled network, the tunnelling
   formats are clear.  However, when connected to an IPv4-only network,
   care should be taken when negotiating the IKE association and the
   consequential tunnelling formats used for secure and insecure
   traffic.  This section illustrates the IKE message exchange between
   the mobile node and home agent when the mobile node is located in an
   IPv4-only network.  Two different IKE negotiations are considered:

   o  IKEv2 operation for securing DSMIPv6 signaling.

   o  IKEv2 operation for securing data over IPv4

5.2.1.  IKEv2 Operation for Securing DSMIPv6 Signaling

   A mobile node connected to an IPv4-only network SHOULD follow the
   procedures described below in order to establish an SA for the
   protection of binding update and binding acknowledgement messages.
   Note that V4ADDR refers to either the mobile node's care-of address
   in the visited link or the public address allocated to the mobile
   node by the NAT.

   Mobile Node                                      Home Agent
   -----------                                      ----------
   IPv4(source_addr=V4ADDR, dest_addr=HAADDR)
    UDP (500, 500) HDR, SAi1, KEi, Ni
     NAT-D, NAT-D -->

                      <- IPv4(source_addr=HAADDR, dest_addr=V4ADDR)
                               UDP(500,X) HDR, SAr1, KEr, Nr, [CERTREQ]
                                NAT-D, NAT-D

   IPv4(source_addr=V4ADDR, dest_addr=HAADDR)
     UDP (4500,4500) <non-ESP Marker > HDR, SK
     {IDi, [CERT,] [CERTREQ,] [IDr,] AUTH, N(USE_TRANSPORT_MODE),
     SAi2, TSi, TSr}
    -->

                      <-- IPv4(source_addr=HAADDR, dest_addr=V4ADDR)
                              UDP (4500,Y) <non-ESP Marker > HDR, SK
                              {IDr, [CERT,] AUTH, N(USE_TRANSPORT_MODE),
                              SAr2, TSi, TSr}






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   The corresponding Security Policy Database (SPD) entries are shown
   below.

   Mobile node SPD-S:

      IF local_address = home_address_1 &

         remote_address = home_agent_1 &

         proto = MH & local_mh_type = BU &

         remote_mh_type = BAck

     Then use SA ESP transport mode

     Initiate using IDi = user_1 to address home_agent_1

   Home Agent SPD-S:

      IF local_address = home_agent_1 &

         remote_address = home_address_1 &

         proto = MH &

         local_mh_type = BAck &

         remote_mh_type = BU

      Then use SA ESP transport mode

   Where home_address_1 is the mobile node's registered IPv6 home
   address and home_agent_1 is the IP address of the home agent.

   The above should result in BU/BA messages with the following BU
   received by the home agent:

      IPv4 header (src=V4ADDR, dst=HA_V4ADDR)

      UDP header (sport=Z, dport=DSMIPv6)

      IPv6 header (src=V6HOA, dst=HAADDR)

      ESP header in transport mode

      Mobility header

      BU [IPv4 HAO]



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      IPv4 CoA option

      (and others as needed)

   At the home agent, following UDP de-capsulation, the binding update
   is delivered to the IPsec module as shown below:

      IPv6 header (src=V6HOA, dst=HAADDR)

      ESP header in transport mode

      Mobility header

      BU [IPv4 HAO]

      IPv4 CoA option

      (and others as needed)

   In addition, V4ADDR and the sport (Z) need to be passed with the
   packet to ensure correct processing.

   Following IPsec processing, the binding update is delivered to the
   DSMIPv6 home agent module as follows:

      IPv6 header (src=V6HOA, dst=HAADDR)

      Mobility header

      BU [IPv4 HAO]

      IPv4 CoA option

      (and others as needed)

   In addition, V4ADDR and the sport (Z) need to be passed with the
   packet to ensure correct processing.

   The binding acknowledgement sent by the home agent module to the
   IPsec module is as follows:

      IPv6 header (src=HAADDR, dst=V6HOA)

      Mobility header

      BA ([IPv4 ACK], NAT DET)

      (and others as needed)



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   In addition, V4ADDR, the sport from the BU (Z), and an indication
   that UDP encapsulation must be used need to be passed with the packet
   to ensure correct processing.

   The binding acknowledgement sent by the home agent to the mobile node
   is as follows:

      IPv4 header (src= HA_V4ADDR, dst=V4ADDR)

      UDP header (sport=DSMIPv6, dport=Z)

      IPv6 header (src=HAADDR, dst=V6HOA)

      ESP header in transport mode

      Mobility header

      BA ([IPv4 ACK], NAT DET)

5.2.2.  IKEv2 Operation for Securing Data over IPv4

   To secure data traffic when the mobile node is located in an IPv4-
   only network, the mobile node MUST establish a child_SA for that
   purpose.  Note that V4ADDR refers to either the mobile node's care-of
   address in the visited link or the public address allocated to the
   mobile node by the NAT.  The procedure is as follows:

   Mobile Node                                     Home Agent
   -----------                                     ----------
   IPv4(source_addr=V4ADDR, dest_addr=HAADDR)
    UDP (4500,4500) < non-ESP Marker > HDR, SK
     {[N], SA, Ni, [KEi], TSi, TSr}    -->

                        <--IPv4(source_addr=HAADDR, dest_addr=V4ADDR)
                               UDP (4500,Y) < non-ESP Marker > HDR, SK
                                SA, Nr, [KEr], TSi, TSr}

   If no NAT is detected, the encapsulation used will be:

      IPv4 (source_addr=v4CoA, dest_addr=HAAddr)

      ESP

      IP (source_addr=HoA, set_addr=CNAddr)

      Upper_layer_HDR





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   Where IP is either IPv4 or IPv6 and HoA is either the IPv4 HoA or the
   IPv6 HoA.

   If a NAT is detected, the encapsulation used will be:

      IPv4 (source_addr=v4Addr, dest_addr=HAAddr)

      UDP (sport=Y, dport=4500)

      ESP

      IP (source_addr=HoA, set_addr=CNAddr)

      Upper_layer_HDR

   Where v4CoA may be the external IPv4 address of the NAT, IP is either
   an IPv4 or IPv6 header, and HoA is either the IPv4 or the IPv6 HoA.
   The above format shows the packet as seen by the home agent.

   The SPD, whether a NAT is detected or not, is set as follows.  Note
   that this rule is designed to match all data from the MN to nodes
   other than the home agent.  This is done so that this rule does not
   overlap with the earlier rule securing BU/BA signaling between the MN
   and the HA.

   Mobile Node SPD-S:

      IF local_address = home_address &

         remote_address != home_agent &

         proto=any

      Then use SA ESP tunnel mode

      Initiate using IDi = user_1 to address home_agent_1

   home agent SPD-S:

      IF local_address != home_agent &

         remote_address = home_address &

         proto=any

      Then use SA ESP tunnel mode





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   Where home_address is the MN's registered IPv6 or IPv4 home address
   and home_agent is the IPv6 or the IPv4 address of the home agent.

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, discussions, and
   reviews: Jari Arkko, Sri Gundavelli, Wassim Haddad, Alfred Hoenes,
   Conny Larsson, Acee Lindem, Ahmad Muhanna, Vidya Narayanan, Karen
   Nielsen, and Keiichi Shima.  Thanks to Karen Nielsen, Pasi Eronen,
   and Christian Kaas-Petersen for raising the issue of IKEv2
   interactions and proposing the solution included in this document.
   Thanks to Pasi Eronen for many thorough reviews of this document.

8.  IANA Considerations

   IANA has made the following allocations according to this
   specification:

      A UDP port (4191) has been assigned for the NAT traversal
      mechanism described in Section 4.2.

      The IPv4 home address option described in Section 3.1.1 has been
      assigned value 29.  This option is included in the mobility header
      described in [RFC3775].

      The IPv4 address acknowledgement option described in Section 3.2.1
      has been assigned value 29.  This option is included in the
      mobility header described in [RFC3775].

      The NAT detection option described in Section 3.2.2 has been
      assigned a value 31.  This option is included in the mobility
      header described in [RFC3775].

      The IPv4 care-of address option described in Section 3.1.2 has
      been assigned value 32.  This option is included in the mobility
      header described in [RFC3775].

   The status field in the IPv4 home address option has been allocated
   by IANA under the new registry: "DSMIPv6 IPv4 Home Address Option
   Status Codes".






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   The status field values are allocated using the following procedure:

   1. New status field values are allocated through IETF review.  This
      is for all RFC types including standards track, informational, and
      experimental status that originate from the IETF and have been
      approved by the IESG for publication.

   2. Requests for new option type value assignments from outside the
      IETF are only made through the publication of an IETF document,
      per 1 above.  Note also that documents published as Independent
      "RFC Editor contributions" [RFC4844] are not considered to be IETF
      documents.

9.  References

9.1.  Normative References

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

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

   [RFC3168]   Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
               of Explicit Congestion Notification (ECN) to IP", RFC
               3168, September 2001.

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

   [RFC3948]   Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
               Stenberg, "UDP Encapsulation of IPsec ESP Packets", RFC
               3948, January 2005.

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

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

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

   [RFC4555]   Eronen, P., "IKEv2 Mobility and Multihoming Protocol
               (MOBIKE)", RFC 4555, June 2006.




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   [RFC4861]   Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
               "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
               September 2007.

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

   [RFC5026]   Giaretta, G., Ed., Kempf, J., and V. Devarapalli, Ed.,
               "Mobile IPv6 Bootstrapping in Split Scenario", RFC 5026,
               October 2007.

9.2.  Informative References

   [CHOWDHURY] Chowdhury, K. and A. Yegin, "MIP6-bootstrapping for the
               Integrated Scenario", Work in Progress, April 2008.

   [RFC2983]   Black, D., "Differentiated Services and Tunnels", RFC
               2983, October 2000.

   [RFC3344]   Perkins, C., Ed., "IP Mobility Support for IPv4", RFC
               3344, August 2002.

   [RFC3519]   Levkowetz, H. and S. Vaarala, "Mobile IP Traversal of
               Network Address Translation (NAT) Devices", RFC 3519,
               April 2003.

   [RFC4459]   Savola, P., "MTU and Fragmentation Issues with In-the-
               Network Tunneling", RFC 4459, April 2006.

   [RFC4844]   Daigle, L., Ed., and Internet Architecture Board, "The
               RFC Series and RFC Editor", RFC 4844, July 2007.

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

   [RFC5380]   Soliman, H., Castelluccia, C., ElMalki, K., and L.
               Bellier, "Hierarchical Mobile IPv6 (HMIPv6) Mobility
               Management", RFC 5380, October 2008.

   [RFC5389]   Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
               "Session Traversal Utilities for NAT (STUN)", RFC 5389,
               October 2008.

   [RFC5405]   Eggert, L. and G. Fairhurst, "Unicast UDP Usage
               Guidelines for Application Designers", BCP 145, RFC 5405,
               November 2008.




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10. Contributors

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

      Vijay Devarapalli: vijay.devarapalli@azairenet.com

      James Kempf: kempf@docomolabs-usa.com

      Henrik Levkowetz: henrik@levkowetz.com

      Pascal Thubert: pthubert@cisco.com

      George Tsirtsis: G.Tsirtsis@Qualcomm.com

      Ryuji Wakikawa: ryuji@sfc.wide.ad.jp

Author's Address

   Hesham Soliman (editor)
   Elevate Technologies

   EMail: hesham@elevatemobile.com




























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