





Network Working Group                                   R. Wakikawa, Ed.
Request for Comments: 5648                                    Toyota ITC
Category: Standards Track                                 V. Devarapalli
                                                                Wichorus
                                                             G. Tsirtsis
                                                                Qualcomm
                                                                T. Ernst
                                                                   INRIA
                                                               K. Nagami
                                                           INTEC NetCore
                                                            October 2009


                Multiple Care-of Addresses Registration

Abstract

   According to the current Mobile IPv6 specification, a mobile node may
   have several care-of addresses but only one, called the primary
   care-of address, can be registered with its home agent and the
   correspondent nodes.  However, for matters of cost, bandwidth, delay,
   etc, it is useful for the mobile node to get Internet access through
   multiple accesses simultaneously, in which case the mobile node would
   be configured with multiple active IPv6 care-of addresses.  This
   document proposes extensions to the Mobile IPv6 protocol to register
   and use multiple care-of addresses.  The extensions proposed in this
   document can be used by mobile routers using the NEMO (Network
   Mobility) Basic Support protocol as well.

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 and License 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
   3. Protocol Overview ...............................................4
   4. Mobile IPv6 Extensions .........................................10
      4.1. Binding Cache Structure and Binding Update List ...........10
      4.2. Binding Update Message ....................................10
      4.3. Binding Identifier Mobility Option ........................11
      4.4. New Status Values for Binding Acknowledgement .............13
   5. Mobile Node Operation ..........................................14
      5.1. Management of Care-of Address(es) and Binding
           Identifier(s) .............................................14
      5.2. Binding Registration ......................................15
      5.3. Bulk Registration .........................................16
      5.4. Binding De-Registration ...................................16
      5.5. Returning Home with Complete Binding
           De-Registration: Using a Single Interface .................17
           5.5.1. Using Only the Interface Attached to the
                  Home Link ..........................................17
           5.5.2. Using Only the Interface Attached to the
                  Visited Link .......................................17
      5.6. Returning Home: Simultaneous Home and Visited Link
           Operation .................................................18
           5.6.1. Problems of Simultaneous Home and Foreign
                  Attachments ........................................18
           5.6.2. Overview and Approach ..............................18
           5.6.3. Home Binding Support ...............................19
           5.6.4. Sending Packets from the Home Link .................20
           5.6.5. Leaving from the Home Link .........................20
      5.7. Receiving Binding Acknowledgement .........................21
      5.8. Receiving Binding Refresh Request .........................22
      5.9. Bootstrapping .............................................22
   6. Home Agent and Correspondent Node Operation ....................22
      6.1. Searching Binding Cache with Binding Identifier ...........22
      6.2. Processing Binding Update .................................23
      6.3. Sending a Binding Acknowledgement for Home Link
           Registration ..............................................25
      6.4. Sending Binding Refresh Request ...........................27
      6.5. Receiving Packets from Mobile Node ........................27
   7. Network Mobility Applicability .................................27
   8. DSMIPv6 Applicability ..........................................27
      8.1. IPv4 Care-of Address Registration .........................28
      8.2. IPv4 Home Address Management ..............................29



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   9. IPsec and IKEv2 Interaction ....................................30
      9.1. Use of Care-of Address in the IKEv2 Exchange ..............31
      9.2. Transport Mode IPsec-Protected Messages ...................31
      9.3. Tunnel Mode IPsec-Protected Messages ......................31
           9.3.1. Tunneled Home Test Init and Home Test Messages .....31
           9.3.2. Tunneled Payload Traffic ...........................32
   10. Security Considerations .......................................33
   11. IANA Considerations ...........................................34
   12. Acknowledgements ..............................................35
   13. References ....................................................35
      13.1. Normative References .....................................35
      13.2. Informative References ...................................35

1.  Introduction

   A mobile node may use various types of network interfaces to obtain
   durable and wide area network connectivity.  This has increasingly
   become true with mobile nodes having multiple interfaces, such as
   802.2, 802.11, 802.16, cellular radios, etc.  The motivations for and
   benefits of using multiple points of attachment are discussed in
   [MOTIVATION].  When a mobile node with multiple interfaces uses
   Mobile IPv6 [RFC3775] for mobility management, it cannot use its
   multiple interfaces to send and receive packets while taking
   advantage of session continuity provided by Mobile IPv6.  This is
   because Mobile IPv6 allows the mobile node to bind only one care-of
   address at a time with its home address.  See [MIP6ANALYSIS] for a
   further analysis of using multiple interfaces and addresses with
   Mobile IPv6.

   This document proposes extensions to Mobile IPv6 to allow a mobile
   node to register multiple care-of addresses for a home address and
   create multiple binding cache entries.  A new Binding Identification
   (BID) number is created for each binding the mobile node wants to
   create and is sent in the Binding Update.  The home agent that
   receives this Binding Update creates a separate binding for each BID.
   The BID information is stored in the corresponding binding cache
   entry.  The BID information can now be used to identify individual
   bindings.  The same extensions can also be used in Binding Updates
   sent to the correspondent nodes.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].






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   Terms used in this document are defined in [RFC3775], [RFC3753], and
   [RFC4885].  In addition to or as a replacement of these, the
   following terms are defined or redefined:

   Binding Identification Number (BID)

      The BID is an identification number used to distinguish multiple
      bindings registered by the mobile node.  Assignment of distinct
      BIDs allows a mobile node to register multiple binding cache
      entries for a given home address.  BIDs assigned to the same home
      address must not be duplicated at the same time.  The value zero
      is reserved for future extensions.  Each BID is generated and
      managed by a mobile node.  The BID is stored in the Binding Update
      List and is sent by the mobile node in the Binding Update.  A
      mobile node may change the value of a BID at any time according to
      its administrative policy -- for instance, to protect its privacy.
      An implementation must carefully assign the BID so as to keep
      using the same BID for the same binding even when the status of
      the binding is changed.  More details can be found in Section 5.1.

   Binding Identifier Mobility Option

      The Binding Identifier mobility option is used to carry the BID
      information.

   Bulk Registration

      A mobile node can register multiple bindings at once by sending a
      single Binding Update.  A mobile node can also replace some or all
      of the bindings available at the home agent with the new bindings
      by using the bulk registration.  Bulk registration is supported
      only for home registration (i.e., with the home agent) as
      explained in Section 5.3.  A mobile node must not perform the bulk
      registration mechanism described in this specification with a
      correspondent node.

3.  Protocol Overview

   A new extension called the Binding Identification number (BID) is
   introduced to distinguish between multiple bindings pertaining to the
   same home address.  If a mobile node configures several IPv6 global
   addresses on one or more of its interfaces, it can register these
   addresses with its home agent as care-of addresses.  If the mobile
   node wants to register multiple bindings, it MUST generate a BID for
   each care-of address and store the BID in the Binding Update List.  A
   mobile node can manipulate each binding independently by using the
   BIDs.  The mobile node then registers its care-of addresses by
   sending a Binding Update with a Binding Identifier mobility option.



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   The BID is included in the Binding Identifier mobility option.  After
   receiving the Binding Update with a Binding Identifier mobility
   option, the home agent MUST copy the BID from the Binding Identifier
   mobility option to the corresponding field in the binding cache
   entry.  If there is an existing binding cache entry for the mobile
   node, and if the BID in the Binding Update does not match the one
   with the existing entry, the home agent MUST create a new binding
   cache entry for the new care-of address and BID.  The mobile node can
   either register multiple care-of addresses at once in a single
   Binding Update or independently in individual Binding Updates.

   If the mobile host wishes to register its binding with a
   correspondent node, it must perform return routability operations as
   described in [RFC3775].  This includes managing a Care-of Keygen
   token per care-of address and exchanging Care-of Test Init and Care-
   of Test messages with the correspondent node for each care-of
   address.  The mobile node MAY use the same BID that it used with the
   home agent for a particular care-of address.  For protocol
   simplicity, bulk registration to correspondent nodes is not supported
   in this document.  This is because the return routability mechanism
   introduced in [RFC3775] cannot be easily extended to verify multiple
   care-of addresses stored in a single Binding Update.

   Figure 1 illustrates the configuration where the mobile node obtains
   multiple care-of addresses at foreign links.  The mobile node can
   utilize all the care-of addresses.  In Figure 1, the home address of
   the mobile node (MN) is 2001:db8::EUI.  The mobile node has 3
   different interfaces and possibly acquires care-of addresses 1-3
   (CoA1, CoA2, CoA3).  The mobile node assigns BID1, BID2, and BID3 to
   each care-of address.





















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                       +----+
                       | CN |
                       +--+-+
                          |
                      +---+------+          +----+
               +------+ Internet |----------+ HA |
               |      +----+---+-+          +--+-+
           CoA2|           |   |               |   Home Link
            +--+--+        |   |         ------+------
            |  MN +--------+   |
            +--+--+ CoA1       |
           CoA3|               |
               +---------------+

        Binding Cache Database:
           home agent's binding (Proxy neighbor advertisement is active)
                 binding [2001:db8::EUI  BID1 care-of address1]
                 binding [2001:db8::EUI  BID2 care-of address2]
                 binding [2001:db8::EUI  BID3 care-of address3]
           correspondent node's binding
                 binding [2001:db8::EUI  BID1 care-of address1]
                 binding [2001:db8::EUI  BID2 care-of address2]
                 binding [2001:db8::EUI  BID3 care-of address3]

           Figure 1: Multiple Care-of Addresses Registration

   If the mobile node decides to act as a regular mobile node compliant
   with [RFC3775], it sends a Binding Update without any Binding
   Identifier mobility options.  The receiver of the Binding Update
   deletes all the bindings registered with a BID and registers only a
   single binding for the mobile node.  Note that the mobile node can
   continue using the BID even if it has only a single binding that is
   active.

   Binding cache lookup is done based on the home address and BID
   information if a BID is available.  This is different from RFC 3775,
   where only the home address is used for binding cache lookup.
   Binding cache lookup is operated for either protocol signaling or
   data packets.  For protocol signaling such as a Binding Update, BID
   should be always carried by a BID sub-option in a protocol signaling.
   Therefore, a correspondent binding cache that matches the specified
   BID MUST be found from the binding cache database.  On the other
   hand, for the data packets, no BID information is carried in a
   packet.  The binding cache lookup may involve policy or flow filters
   to retrieve a correspondent BID per packet in cases where some policy
   or flow filters are used to direct a certain packet or flow to a
   particular care-of address.  However, the binding cache lookup using
   policy or flow filters is out of scope for this document.  If no such



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   mechanism is available and no BID is found for a packet, a node
   SHOULD use the binding that was last verified by receiving data
   packets or signaling from the mobile node.  In case the binding cache
   lookup for data packets, using the combination of home address and
   BID, does not return a valid binding cache entry, the home agent
   SHOULD perform the lookup based on only the home address as described
   in [RFC3775].

   In any case, to avoid problems with upper-layer protocols and TCP in
   particular, a single packet flow as identified by the 5-tuple SHOULD
   only be sent to a single care-of address at a time.

   The mobile node may return to the home link through one of its
   interfaces.  There are two options possible for the mobile node when
   it returns home.  Sections 5.5.1 and 5.6 describe the returning-home
   procedures in more detail.

   1.  The mobile node uses only the interface with which it attaches to
       the home link and takes back full ownership of its HoA (home
       address) on the home link.  This is illustrated in Figure 2.  It
       de-registers all bindings with the home agent related to all
       care-of addresses.  The interfaces still attached to the visited
       link(s) are no longer going to be receiving any encapsulated
       traffic from the home agent.  On the other hand, the mobile node
       can continue communicating with the correspondent nodes from the
       other interfaces attached to foreign links by using route
       optimization.  Even if the mobile node is attached to the home
       link, it can still send Binding Updates for other active care-of
       addresses (CoA1 and CoA2) to correspondent nodes.  Since the
       correspondent node has bindings, packets are routed from and to
       each care-of address directly.




















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                       +----+
                       | CN |
                       +--+-+
                          |
                      +---+------+          +----+
               +------+ Internet |----------+ HA |
               |      +----+-----+          +--+-+
           CoA2|           |                   |   Home Link
            +--+--+        |             --+---+------
            |  MN +--------+               |
            +--+--+ CoA1                   |
               |                           |
               +---------------------------+

        Binding Cache Database:
           home agent's binding
                 none
           correspondent node's binding
                 binding [2001:db8::EUI  BID1 care-of address1]
                 binding [2001:db8::EUI  BID2 care-of address2]

       Figure 2: Using Only an Interface Attached to the Home Link

   2.  The mobile node may simultaneously use both the interface
       attached to the home link and the interfaces still attached to
       the visited link(s) as shown in Figure 3.  There are two possible
       topologies, depending on whether or not the home agent is the
       only router on the home link.  The operation of Neighbor
       Discovery [RFC4861] is different in the two topologies.  More
       details can be found in Section 5.6.  The home agent and the
       correspondent node have the binding entries listed in Figure 3 in
       their binding cache database in both topologies.  The home agent
       also knows that the mobile node is attached to the home link.
       All the traffic from the Internet is intercepted by the home
       agent first and routed to either the interface attached to the
       home link or to one of the foreign links.  How the home agent
       decides to route a particular flow to the interface attached to
       the home link or foreign link is out of scope for this document.













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      Topology-a)
                       +----+
                       | CN |
                       +--+-+
                          |
                      +---+------+          +----+
               +------+ Internet |----------+ HA |
               |      +----+-----+          +--+-+
           CoA2|           |                   |   Home Link
            +--+--+        |             --+---+------
            |  MN +--------+               |
            +--+--+ CoA1                   |
               |                           |
               +---------------------------+


      Topology-b)
                       +----+
                       | CN |
                       +--+-+
                          |
                      +---+------+    Router    +----+
               +------+ Internet |-------R      | HA |
               |      +----+-----+       |      +--+-+
           CoA2|           |             |         |   Home Link
            +--+--+        |           --+-+-------+------
            |  MN +--------+               |
            +--+--+ CoA1                   |
               |                           |
               +---------------------------+

        Binding Cache Database:
           home agent's binding
                 binding [2001:db8::EUI  BID1 care-of address1]
                 binding [2001:db8::EUI  BID2 care-of address2]
           correspondent node's binding
                 binding [2001:db8::EUI  BID1 care-of address1]
                 binding [2001:db8::EUI  BID2 care-of address2]

          Figure 3: Simultaneous Home and Visited Link Operation

   This specification keeps backwards compatibility with [RFC3775].  If
   a receiver (either home agent or correspondent node) does not support
   this specification, it does not understand the Binding Identifier
   mobility option.  The receiver skips the unknown mobility option
   (i.e., the Binding Identifier mobility option) and processes the
   Binding Update as defined in [RFC3775].  In order to keep backwards
   compatibility with [RFC3775], when a mobile node sends a Binding



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   Update message with extensions described in this document, the
   receiver needs to reflect the Binding Identifier mobility option in
   the Binding Acknowledgement.  If the mobile node finds no Binding
   Identifier mobility options in the received Binding Acknowledgement,
   it assumes the other end node does not support this specification.
   In such case, the mobile node needs to fall back to the legacy
   [RFC3775]-compliant mobile node.  If it is the home registration, the
   mobile node MAY try to discover another home agent that supports the
   Binding Identifier mobility option for the home registration.

4.  Mobile IPv6 Extensions

   This section summarizes the extensions to Mobile IPv6 that are
   necessary to manage multiple bindings.

4.1.  Binding Cache Structure and Binding Update List

   The BID is required to be stored in the binding cache and Binding
   Update List structure.

   The sequence number value MUST be shared among all the Binding Update
   List entries related to Binding Updates sent to a particular home
   agent or correspondent node.  Whenever a mobile node sends either an
   individual or a bulk Binding Update, the sequence number is
   incremented.  When a home agent receives an individual Binding
   Update, it should update the sequence number for all the bindings for
   a particular mobile node, with the sequence number in the received
   Binding Update.

4.2.  Binding Update Message

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

                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                       |          Sequence #           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |A|H|L|K|M|R|P|F|T|O| Reserved  |           Lifetime            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       .                                                               .
       .                        Mobility options                       .
       .                                                               .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 4: Binding Update Message




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   Overwrite (O) flag

      When this flag is set, all the binding cache entries for a mobile
      node are replaced by new entries registering with this Binding
      Update message.  This flag is only used when the BID mobility
      option is carried with the Binding Update.

   Reserved

      6-bit Reserved field.

4.3.  Binding Identifier Mobility Option

   The Binding Identifier mobility option is included in the Binding
   Update, Binding Acknowledgement, Binding Refresh Request, and Care-of
   Test Init and Care-of Test messages.  The Binding Identifier mobility
   option has an alignment requirement of 2n if the Care-of Address
   field is not present.  Otherwise, it has the alignment requirement of
   8n + 2.

                            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 = 35   |     Length    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Binding ID (BID)        |     Status    |H|   Reserved  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------------------------+
       +                                                               +
       :                 IPv4 or IPv6 care-of address (CoA)            :
       +                                                               +
       +---------------------------------------------------------------+

                       Figure 5: BID Mobility Option

   Type

      Type value for Binding Identifier is 35.

   Length

      8-bit unsigned integer.  Length of the option, in octets,
      excluding the Type and Length fields.  It MUST be set to either 4,
      8, or 20 depending on the Care-of Address field.  When the care-of
      address is not carried by this option, the length value MUST be
      set to 4.  If the IPv4 care-of address is stored in the Care-of
      Address field, the length MUST be 8.  Otherwise, the length value
      MUST be set to 20 for IPv6 care-of addresses.




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   Binding ID (BID)

      The BID that is assigned to the binding indicated by the care-of
      address in the Binding Update or the Binding Identifier mobility
      option.  The BID is a 16-bit unsigned integer.  The value of zero
      is reserved and SHOULD NOT be used.

   Status

      The Status field is an 8-bit unsigned integer.  When the Binding
      Identifier mobility option is included in a Binding
      Acknowledgement, this field overwrites the Status field in the
      Binding Acknowledgement only for this BID.  If this field is set
      to zero, the receiver ignores this field and uses the registration
      status stored in the Binding Acknowledgement message.  The
      receiver MUST ignore this field if the Binding Identifier mobility
      option is not carried within either the Binding Acknowledgement or
      the Care-of Test messages.  The possible status codes are the same
      as the status codes of the Binding Acknowledgement.  This Status
      field is also used to carry error information related to the
      care-of address test in the Care-of Test message.

   Simultaneous Home and Foreign Binding (H) flag

      This flag indicates that the mobile node registers multiple
      bindings to the home agent while it is attached to the home link.
      This flag is valid only for a Binding Update sent to the home
      agent.

   Reserved

      7-bit Reserved field.  The value MUST be initialized to zero by
      the sender, and SHOULD be ignored by the receiver.

   Care-of Address

      If a Binding Identifier mobility option is included in a Binding
      Update for the home registration, either IPv4 or IPv6 care-of
      addresses for the corresponding BID can be stored in this field.
      For the binding registration to correspondent nodes (i.e., route
      optimization), only IPv6 care-of addresses can be stored in this
      field.  If no address is specified in this field, the length of
      this field MUST be zero (i.e., not appear in the option).  If the
      option is included in any messages other than a Binding Update,
      the length of this field MUST also be zero.






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4.4.  New Status Values for Binding Acknowledgement

   New status values for the Status field in a Binding Acknowledgement
   are defined for handling the multiple care-of addresses registration:

   MCOA NOTCOMPLETE (4)

      In bulk registration, not all the Binding Identifier mobility
      options were successfully registered.  Some of them were rejected.
      The error status value of the failed mobility option is
      individually stored in the Status field of the Binding Identifier
      mobility option.

   MCOA RETURNHOME WO/NDP (5)

      When a mobile node returns home, it MUST NOT use the Neighbor
      Discovery Protocol (NDP) for the home address on the home link.
      This is explained in more detail in Section 5.6.

   MCOA MALFORMED (164)

      Registration failed because the Binding Identifier mobility option
      was not formatted correctly.  This value is used in the following
      cases:

      *  when the wrong length value is specified (neither 4, 8, nor 20)
         in the Length field of the Binding Identifier mobility option.

      *  when a unicast routable address is not specified in the Care-of
         Address field of the Binding Identifier mobility option.

      *  when a care-of address does not appear in the Care-of Address
         field of the Binding Identifier mobility option stored in an
         IPsec Encapsulating Security Payload (ESP)-protected Binding
         Update.

   MCOA NON-MCOA BINDING EXISTS (165)

      Indicates that a bootstrapping multiple care-of addresses
      registration was performed without the 'O' flag set.

   MCOA UNKOWN COA (167)

      Indicates that a Binding Identifier mobility option did not
      include a Care-of Address field and that the receiver has no
      record for the Binding ID indicated in the same option.





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   MCOA PROHIBITED (166)

      Implies that the multiple care-of addresses registration is
      administratively prohibited.

   MCOA BULK REGISTRATION PROHIBITED (168)

      Bulk binding registration is either not permitted or not
      supported.  Note that the bulk registration is an optional
      procedure and might not be available on a home agent.

   MCOA SIMULTANEOUS HOME AND FOREIGN PROHIBITED (169)

      Simultaneous home and foreign attachment is neither supported nor
      permitted.

5.  Mobile Node Operation

5.1.  Management of Care-of Address(es) and Binding Identifier(s)

   There are two cases when a mobile node might acquire several care-of
   addresses.  A mixture of the two cases is also possible.  Note that a
   mobile node can use BID regardless of the number of interfaces and
   care-of addresses.  Whether or not a mobile node uses BID is
   determined by a local configuration.

   1.  A mobile node is using several physical network interfaces and
       acquires a care-of address on each of its interfaces.

   2.  A mobile node uses a single physical network interface but
       receives advertisements for multiple prefixes on the link to
       which the interface is attached.  This will result in the mobile
       node configuring several global addresses on the interface from
       each of the announced prefixes.

   The difference between the above two cases is only in the number of
   physical network interfaces and is therefore irrelevant in this
   document.  What is of significance is the fact that the mobile node
   has several addresses it can use as care-of addresses.

   A mobile node assigns a BID to each care-of address when it wants to
   register them simultaneously with its home address.  The BID MUST be
   unique for a given home address.  The value is an integer between 1
   and 65535.  A zero value SHOULD NOT be used as a BID.  If a mobile
   node has only one care-of address, the assignment of a BID is not
   needed until it has multiple care-of addresses with which to
   register, at which time all of the care-of addresses MUST be mapped
   to BIDs.



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   When a mobile node registers a given BID for the first time, it MUST
   include the Care-of Address field in the Binding Identifier mobility
   option.  For any subsequent registrations that either re-register or
   de-register the same BID, the MN need not include the Care-of Address
   field in the Binding Identifier mobility option.

5.2.  Binding Registration

   For the multiple care-of addresses registration, the mobile node MUST
   include a Binding Identifier mobility option(s) in the Binding Update
   as shown in Figure 6.

   When IPsec ESP is used for protecting the Binding Update, a care-of
   address MUST be carried in an alternate Care-of Address mobility
   option as described in [RFC4877].  However, in this specification,
   the care-of address MUST be carried in the Care-of Address field of
   the Binding Identifier mobility option.  In order to save bits of the
   Binding Update, the alternate Care-of Address option MUST NOT be
   included.

   For binding registration to a correspondent node, the mobile node
   MUST have both active Home and Care-of Keygen tokens for Kbm (binding
   management key; see Section 5.2.5 of [RFC3775]) before sending the
   Binding Update.  The care-of Keygen tokens MUST be maintained for
   each care-of address that the mobile node wants to register to the
   correspondent node.  The Binding Update to the correspondent node is
   protected by the Binding Authorization Data mobility option that is
   placed after the Binding Identifier mobility option.

             IPv6 header (src=Care-of Address, dst=Home Agent Address)
                  IPv6 Home Address Option
                  ESP Header*
                  Mobility header
                      Binding Update
                     Mobility Options
                        Binding Identifier mobility option
                        Binding Authorization mobility option+
      (*) if necessary, for home registration
      (+) if necessary, for route optimization

           Figure 6: Binding Update for Binding Registration

   If the mobile node wants to replace existing registered bindings on
   the home agent with the single binding in the sent Binding Update, it
   sets the 'O' flag.  If the 'O' flag is not set, then the binding will
   be added to existing bindings in the home agent.  The single binding
   will be registered with the assigned BID.  Section 6.2 describes this
   registration procedure in detail.



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5.3.  Bulk Registration

   Bulk registration is an optimization for binding multiple care-of
   addresses to a home address using a single Binding Update.  This is
   very useful if the mobile node, for instance, does not want to send a
   lot of signaling messages through an interface where the bandwidth is
   scarce.  This document specifies bulk registration only for the
   mobile node's home registration.  A mobile node performing bulk
   registration with a correspondent node is out of scope.

   To use bulk registration, the mobile node includes a Binding
   Identifier mobility option for each BID it wants to register in the
   same Binding Update message.  As with single registrations (see
   Section 5.1), the Care-of Address field is included for each BID
   registered for the first time.  This is shown in Figure 7.  The rest
   of the fields and options in the Binding Update (such as Lifetime,
   Sequence Number, and the flags in the Binding Update) are common
   across all care-of addresses.

           IPv6 header (src=Care-of Address, dst=Home Agent Address)
                IPv6 Home Address Option
                ESP Header
                Mobility header
                    Binding Update
                   Mobility Options
                      Binding Identifier1 (including Care-of Address)
                      Binding Identifier2 (including Care-of Address)
                      Binding Identifier3 (no Care-of Address)
                      Binding IdentifierN (no Care-of Address)

                              :

           Figure 7: Binding Update for Bulk Registration

   As with regular registrations, if the mobile node wants to replace
   existing registered bindings on the home agent with the multiple
   bindings in the sent Binding Update, it sets the 'O' flag in the
   Binding Update; otherwise, the bindings are added to the existing
   bindings in the home agent.

5.4.  Binding De-Registration

   When a mobile node decides to delete all the bindings for its home
   address, it sends a regular de-registration Binding Update with
   lifetime set to zero as defined in [RFC3775].  The Binding Identifier
   mobility option is not required.





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   If a mobile node wants to delete a particular binding(s) from its
   home agent and correspondent nodes, the mobile node sends a Binding
   Update with lifetime set to zero and includes a Binding Identifier
   mobility option(s) with the BID(s) it wants to de-register.  The
   receiver will remove only the care-of address(es) that match(es) the
   specified BID(s).  Since de-registration attempts to remove a BID
   that already exists, the Care-of Address field in each Binding
   Identifier option can be omitted by the sender as defined in Section
   5.1.

5.5.  Returning Home with Complete Binding De-Registration: Using a
      Single Interface

   The mobile node may return to the home link by attaching to the home
   link through one of its interfaces.  When the mobile node wants to
   return home, it should be configured with information on what
   interface it needs to use.

5.5.1.  Using Only the Interface Attached to the Home Link

   The mobile node returns home and de-registers all the bindings it has
   with the home agent, as shown in Figure 2 and as defined in
   [RFC3775].  After the de-registration step, all the packets routed by
   the home agent are only forwarded to the interface attached to the
   home link, even if there are other active interfaces attached to the
   visited link(s).  While the mobile node de-registers all the bindings
   from the home agent, it may continue registering, to the
   correspondent node, bindings for interfaces attached to visited links
   as shown in Figure 2.

5.5.2.  Using Only the Interface Attached to the Visited Link

   The mobile node returns home physically but shuts down the interface
   attached to the home link.  As a result, a mobile node does not
   return home even though it attaches to the home link by one of the
   interfaces.  Before shutting down the interface, any binding for the
   care-of address previously associated with the interface should be
   deleted as defined in Section 5.4.

   In this scenario, despite the fact that the mobile node is connected
   to its home link, all of its traffic is sent and received via the
   home agent and its foreign links.









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5.6.  Returning Home: Simultaneous Home and Visited Link Operation

5.6.1.  Problems of Simultaneous Home and Foreign Attachments

   The mobile node returns home and continues using all the interfaces
   attached to both foreign and home links as shown in Figure 3.

   In [RFC3775], the home agent intercepts packets meant for the mobile
   node using proxy Neighbor Discovery [RFC4861] while the mobile node
   is away from the home link.  When the mobile node returns home, the
   home agent deletes the binding cache and stops proxying for the home
   address so that a mobile node can configure its home address on the
   interface attached to the home link.  In this specification, a mobile
   node may return home and configure the home address on the interface
   attached to the home link, but still use the interfaces attached to
   the foreign links.  In this case, a possible conflict arises when
   both the home agent and the mobile node try to defend the home
   address.  If the home agent stops proxying for the home address, the
   packets are always routed to the interface attached to the home link
   and are never routed to the interfaces attached to the visited links.
   Deployments making use of multiple care-of addresses are required to
   avoid configuration conflict between the home agent and the mobile
   node, while still allowing the simultaneous use of home and foreign
   links.  The following describes the mechanism for achieving this.

5.6.2.  Overview and Approach

   The home agent MUST intercept all the packets meant for the mobile
   node, whether or not the mobile node is attached to the home link,
   and decide whether to send the traffic directly to the home address
   on the link or tunnel to the care-of address.

   Two scenarios are illustrated in Figure 3, depending on whether or
   not the home agent is the only router at the home link.  The
   difference is on who defends the home address by (Proxy) Neighbor
   Discovery on the home link.

   1.  Mobile node defends the home address by the regular Neighbor
       Discovery protocol (illustrated as topology-a in Figure 3).  The
       home agent is the only router on the home link.  Therefore, the
       home agent is capable of intercepting packets without relying on
       the proxy Neighbor Discovery protocol, and the mobile node can
       manage the neighbor cache entry of the home address on the home
       link as a regular IPv6 node.  However, there is one limitation of
       this scenario.  If a correspondent node is located at the home
       link, the home agent may not intercept the packets destined to





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       the mobile node.  These packets are routed only via the home
       link, but this is the most optimal path for the mobile node to
       communicate with nodes on the home link.

   2.  If there are routers other than the home agent on the home link,
       then it cannot be guaranteed that all packets meant for the
       mobile node are routed to the home agent.  In this case, the
       mobile node MUST NOT operate the Neighbor Discovery protocol for
       the home address on the home link.  This allows the home agent to
       keep using proxy Neighbor Discovery, and thus it keeps receiving
       all the packets sent to the mobile node's home address.  If the
       home agent, according to its local policy, needs to deliver
       packets to the mobile node over the home link, an issue arises
       with respect to how the home agent discovers the mobile node's
       link local address.  This specification uses the Mobility Header
       Link-Layer Address option defined in [RFC5568] in order to carry
       the mobile node's link-layer address in the Binding Update.
       Likewise, the mobile node would also know the link-layer address
       of the default router address to send packets from the home link
       without Neighbor Discovery.  The link-layer address is used to
       transmit packets from and to the mobile node on the home link.
       The packets are transmitted without the Neighbor Discovery
       protocol by constructing the link-layer header manually.  This
       operation is similar to Mobile IPv6 [RFC3775] when a mobile node
       sends a de-registration Binding Update to the home agent's link-
       layer address in the operation for returning home.

5.6.3.  Home Binding Support

   When the home binding is used, the mobile node MUST send a
   registering Binding Update with a Binding Identifier mobility option
   with the 'H' flag set.  The lifetime MUST be set to a non-zero
   lifetime of the home binding, and the Care-of Address field MUST be
   set to the home address.  The mobile node registers only one home
   binding at a time, even if it attaches to the home link by multiple
   interfaces.

   The mobile node SHOULD include the Mobility Header Link-Layer Address
   option [RFC5568] to notify the mobile node's link-layer address to
   the home agent, too.  The option code of the Mobility Header Link-
   Layer Address option MUST be set to '2' (link-layer address of the
   mobile node).  This link-layer address is required for the home agent
   to send the Binding Acknowledgement and to forward the mobile node's
   packet.

   According to [RFC3775], the mobile node MUST start responding to
   Neighbor Solicitation for its home address right after it sends the
   de-registration Binding Update to the home agent.  However, in this



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   specification, the mobile node MUST NOT respond to Neighbor
   Solicitation before receiving a Binding Acknowledgement, since the
   home agent may continue proxying for the home address.  If the mobile
   node receives [MCOA RETURNHOME WO/NDP (5)] status value in the
   received Binding Acknowledgment, it MUST NOT respond to Neighbor
   Solicitation even after the Binding Acknowledgement.

   The management of the home binding is the same as the binding
   management described in this specification.  The home binding can be
   included in a bulk binding registration (Section 5.3).  The MN SHOULD
   refresh the lifetime of the home binding by sending appropriate
   Binding Updates as with any other binding.

5.6.4.  Sending Packets from the Home Link

   o  When the mobile node receives the Binding Acknowledgement with the
      status value 'Binding Update Accepted' and the BID option, it can
      configure its home address to the interface attached to the home
      link and start operating Neighbor Discovery for the home address
      on the home link.  Packets can be transmitted from and to the
      mobile node as if the mobile node were a regular IPv6 node.

   o  If the mobile node receives the status [MCOA RETURNHOME WO/NDP] in
      the Binding Acknowledgement, it MUST NOT operate Neighbor
      Discovery for the home address.  When the mobile node sends
      packets from the interface attached to the home link, it MUST
      learn the link-layer address of the next hop (i.e., default router
      of the mobile node).  A mobile node learns the default router's
      link-layer address from a Source Link-Layer Address option in
      Router Advertisements.  The mobile node sends packets directly to
      the default router's link-layer address.  This is done by
      constructing the packet to include a link-layer header with the
      learned link-layer address of the default router.  The home agent
      also forwards the packet to the mobile node on the home link by
      using the mobile node's link-layer address.  The link-layer
      address SHOULD be cached when the home agent receives the
      de-registration Binding Update message.  Note that the default
      router MUST NOT cache the mobile node's link-layer address in the
      neighbor cache when it forwards the packet from the mobile node to
      the home agent.

5.6.5.  Leaving from the Home Link

   When the mobile node detaches from the home link, it SHOULD
   immediately send a Binding Update for one of the active care-of
   addresses with the 'H' flag unset.  When the 'H' flag of the BID
   option is unset in any Binding Update, the home agent stops
   forwarding the mobile node's packets to the home link.



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5.7.  Receiving Binding Acknowledgement

   The verification of a Binding Acknowledgement is the same as Mobile
   IPv6 (Section 11.7.3 of [RFC3775]).  The operation for sending a
   Binding Acknowledgement is described in Section 6.2.

   If a mobile node includes a Binding Identifier mobility option in a
   Binding Update with the 'A' flag set, a Binding Acknowledgement
   SHOULD carry a Binding Identifier mobility option.  According to
   [RFC3775], the receiver of the Binding Update ignores unknown
   mobility options and processes the Binding Update without the unknown
   mobility option.  Therefore, if no such mobility option is included
   in the Binding Acknowledgement in response to a Binding Update for a
   multiple care-of addresses registration, this indicates that the
   originating node of the Binding Acknowledgement does not support
   processing the Binding Identifier mobility option regardless of
   status value.  In such case, the receiver of the Binding Update may
   create a regular binding.  The mobile node then SHOULD no longer
   attempt a multiple care-of addresses registration with that node.  If
   this occurs with home registration, the mobile node MAY attempt to
   discover another home agent that supports the Binding Identifier
   mobility option for the home registration.

   If a Binding Identifier mobility option is present in the received
   Binding Acknowledgement, the mobile node checks the Status field in
   the option.  If the status value in the Binding Identifier mobility
   option is zero, the mobile node uses the value in the Status field of
   the Binding Acknowledgement.  Otherwise, it uses the value in the
   Status field of the Binding Identifier mobility option.

   If the status code is greater than or equal to 128, the mobile node
   starts relevant operations according to the error code.  Otherwise,
   the mobile node assumes that the originator (home agent or
   correspondent node) successfully registered the binding information
   and BID for the mobile node.

   o  If the status value is [MCOA PROHIBITED], the mobile node MUST
      stop registering multiple bindings with the node that sent the
      Binding Acknowledgement.

   o  If the status value is [MCOA BULK REGISTRATION PROHIBITED], the
      mobile node needs to stop using bulk registrations with the node
      that sent the Binding Acknowledgement.  It should assume that none
      of the attempted registrations were successful.

   o  If [MCOA MALFORMED] is specified, it indicates that the Binding
      Identifier mobility option is formatted wrong, presumably due to a
      programming error or major packet corruption.



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   o  If [MCOA NON-MCOA BINDING EXISTS] is specified, it means that
      there is a non-MCoA binding entry in the receiver.  The mobile
      node MUST set 'O' flag so that all the registered bindings are
      replaced by an MCoA registration as described in Section 5.9.

   o  If [MCOA UNKNOWN COA] is specified, it means that the mobile node
      sent a Binding Identifier mobility option without a Care-of
      Address field, but the receiver could not find an entry for the
      BID indicated.  If the mobile node is trying to de-register a BID,
      it need not do anything further.  If the mobile node is trying to
      refresh a binding, it SHOULD send a Binding Identifier mobility
      option including the Care-of Address field.

5.8.  Receiving Binding Refresh Request

   The verification of a Binding Refresh Request is the same as in
   Mobile IPv6 (Section 11.7.4 of [RFC3775]).  The operation of sending
   a Binding Refresh Request is described in Section 6.4.

   If a mobile node receives a Binding Refresh Request with a Binding
   Identifier mobility option, it indicates that the node sending the
   Binding Refresh Request message is requesting that the mobile node
   send a new Binding Update for the BID.  The mobile node SHOULD then
   send a Binding Update at least for the respective binding, as
   described in Sections 5.2 and 5.3.

5.9.  Bootstrapping

   When a mobile node bootstraps and registers multiple bindings for the
   first time, it MUST set the 'O' flag in the Binding Update message.
   If old bindings still exist at the home agent, the mobile node has no
   knowledge of which bindings still exist at the home agent.  This
   scenario happens when a mobile node reboots and loses state regarding
   the registrations.  If the 'O' flag is set, all the bindings are
   replaced by the new binding(s).

6.  Home Agent and Correspondent Node Operation

6.1.  Searching Binding Cache with Binding Identifier

   If either a correspondent node or a home agent has multiple bindings
   for a mobile node in their binding cache database, it can use any of
   the bindings to communicate with the mobile node.  This section
   explains how to retrieve the desired binding for the binding
   management.  This document does not provide any mechanism to select
   the suitable binding for forwarding data packets.





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   A node that is either a correspondent node or a home agent SHOULD use
   both the home address and the BID as the search key of the binding
   cache if it knows the corresponding BID (for example, when processing
   signaling messages).  In the example below, if a node searches the
   binding with the home address and BID2, it gets binding2 for this
   mobile node.

           binding1 [2001:db8::EUI,  care-of address1,  BID1]
           binding2 [2001:db8::EUI,  care-of address2,  BID2]
           binding3 [2001:db8::EUI,  care-of address3,  BID3]

                 Figure 8: Searching the Binding Cache

   The node learns the BID when it receives a Binding Identifier
   mobility option.  At that time, the node MUST look up its binding
   cache database with the home address and the BID retrieved from the
   Binding Update.  If the node does not know the BID, it searches for a
   binding with only the home address.  In such a case, the first
   matched binding is found.  If the node does not desire to use
   multiple bindings for a mobile node, it can simply ignore the BID.

6.2.  Processing Binding Update

   If a Binding Update does not contain a Binding Identifier mobility
   option, its processing is the same as in [RFC3775].  If the receiver
   already has multiple bindings for the home address, it MUST replace
   all the existing bindings with the received binding.  If the
   [RFC3775] Binding Update is for de-registration, the receiver MUST
   delete all existing bindings from its binding cache.

   If the Binding Update contains Binding Identifier mobility option(s),
   it is first validated according to Section 9.5.1 of [RFC3775].  Then
   the receiver processes the Binding Identifier mobility option(s) as
   described in the following steps.

   o  The length value is examined.  The length value MUST be either 4,
      8, or 20 depending on the Care-of Address field.  If the length is
      incorrect, the receiver MUST reject the Binding Update and return
      the status value set to [MCOA MALFORMED].

   o  When the length value is either 8 or 20, the care-of address MUST
      be present in the Binding Identifier mobility option.  If the
      unicast routable address [RFC3775] is not present in the Care-of
      Address field, the receiver MUST reject the Binding Identifier
      mobility option and return the status value set to [MCOA
      MALFORMED].





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   o  When multiple Binding Identifier mobility options are present in
      the Binding Update, it is treated as bulk registration.  If the
      receiving node is a correspondent node, it MUST reject the Binding
      Update and return the status value set to [MCOA BULK REGISTRATION
      PROHIBITED] in the binding Acknowledgement.

   o  If the Lifetime field in the Binding Update is set to zero, the
      receiving node deletes the binding entry that corresponds to the
      BID in the Binding Identifier mobility option.  If the receiving
      node does not have an appropriate binding for the BID, it MUST
      reject the Binding Update and send a Binding Acknowledgement with
      status set to 133 [not home agent for this mobile node].

   o  If the 'O' flag is set in the de-registering Binding Update, it is
      ignored.  If the 'H' flag is set, the home agent stores a home
      address in the Care-of Address field of the binding cache entry.
      The home agent MUST follow the descriptions described in Section
      5.6.

   o  If the Lifetime field is not set to zero, the receiving node
      registers a binding with the specified BID as a mobile node's
      binding.  The care-of address is obtained from the Binding Update
      packet as follows:

      *  If the length value of the Binding Identifier mobility option
         is 20, the care-of address is the IPv6 address copied from the
         Care-of Address field in the Binding Identifier mobility
         option.

      *  When the length value is 8, the address MUST be the IPv4 valid
         address.  How to obtain an IPv4 care-of address is described in
         Section 8.

      *  When the length value is 4 and the Binding Identifier is
         present in the binding cache, the receiving node MUST update
         the associated binding entry.  Otherwise, the receiving node
         MUST reject that Binding Identifier mobility option and send a
         Binding Acknowledgement with the status for that Binding
         Identifier mobility option set to [MCOA UNKNOWN].

   o  Once the care-of address(es) have been retrieved from the Binding
      Update, the receiving nodes create new binding(s).

      *  If the 'O' flag is set in the Binding Update, the receiving
         node removes all the existing bindings and registers the
         received binding(s).





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      *  If the 'O' flag is unset in the Binding Update and the receiver
         has a regular binding that does not have a BID for the mobile
         node, it must not process the Binding Update.  The receiver
         should send a Binding Acknowledgement with status set to [MCOA
         NON-MCOA BINDING EXISTS].

      *  If the receiver already has a binding with the same BID but
         different care-of address, it MUST update the binding and
         respond with a Binding Acknowledgement with status set to 0
         [Binding Update accepted].

      *  If the receiver does not have a binding entry for the BID, it
         registers a new binding for the BID and responds with a Binding
         Acknowledgement with status set to 0 [Binding Update accepted].

   If all the above operations are successfully completed and the 'A'
   flag is set in the Binding Update, a Binding Acknowledgement
   containing the Binding Identifier mobility options MUST be sent to
   the mobile node.  Whenever a Binding Acknowledgement is sent, all the
   Binding Identifier mobility options stored in the Binding Update MUST
   be copied to the Binding Acknowledgement except the Status field.
   The Care-of Address field in each Binding Identifier mobility option,
   however, MAY be omitted, because the mobile node can match a
   corresponding Binding Update List entry using the BID.

   When a correspondent node sends a Binding Acknowledgement, the status
   value MUST always be stored in the Status field of the Binding
   Acknowledgement and the Status field of the Binding Identifier
   mobility option MUST always be set to zero.

   When the home agent sends a Binding Acknowledgement, the status value
   can be stored in the Status field of either a Binding Acknowledgement
   or a Binding Identifier mobility option.  If the status value is
   specific to one of the bindings in the bulk registration, the status
   value MUST be stored in the Status field in the corresponding Binding
   Identifier mobility option.  In this case, the Status field of the
   Binding Acknowledgement MUST be set to [MCOA NOTCOMPLETE], so that
   the receiver can examine the Status field of each Binding Identifier
   mobility option for further operations.  Otherwise, the Status field
   of the Binding Identifier mobility option MUST be set to zero and the
   home agent Status field of the Binding Acknowledgement is used.

6.3.  Sending a Binding Acknowledgement for Home Link Registration

   The operations described in this section are related to returning
   home with simultaneous use of home and foreign links.





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   o  When the home agent sends the Binding Acknowledgement after
      successfully processing the home binding registration, it MUST set
      the status value to either 0 [Binding Update Accepted] or [MCOA
      RETURNHOME WO/NDP (5)] in the Status field of the Binding
      Acknowledgment, depending on home agent configuration at the home
      link.  The new values are:

      *  Binding Update Accepted (0): The Neighbor Discovery protocol is
         permitted for the home address at the home link.  This is the
         regular returning home operation of [RFC3775].

      *  MCOA RETURNHOME WO/NDP (5): The Neighbor Discovery protocol is
         prohibited for the home address at the home link.

         The respective Binding Identifier mobility options need to be
         included in the Binding Acknowledgement.

   o  If the Binding Update is rejected, the appropriate error value
      MUST be set in the Status field.  In this case, the home agent
      operation is the same as in [RFC3775].

   o  Only if the home agent is the only router in the home link MAY it
      turn off Neighbor Discovery for the requested home address and
      respond with the [Binding Update Accepted] status value to the
      mobile node.  Since the mobile node will not reply to Neighbor
      Solicitation for the home address before receiving the Binding
      Acknowledgement, the home agent SHOULD use the link-layer address
      carried by the Mobility Header Link-Layer Address option [RFC5568]
      in the received Binding Update.  After the completion of the home
      binding registration, the mobile node starts regular Neighbor
      Discovery operations for the home address on the home link.  The
      neighbor cache entry for the home address is created by the
      regular exchange of Neighbor Solicitation and Neighbor
      Advertisement.

   o  If the home agent is not the only router in the home link, the
      home agent returns [MCOA RETURNHOME WO/NDP] value in the Status
      field of the Binding Identifier mobility option.  The home agent
      learns the mobile node's link-layer address by receiving the
      Mobility Header Link-Layer Address option carried by the Binding
      Update.  It stores the link-layer address as a neighbor cache
      entry for the mobile node so that it can send the packets to the
      mobile node's link-layer address.

   o  Note that the use of proxy Neighbor Discovery is an easier way to
      intercept the mobile nodes' packets instead of IP routing in some
      deployment scenarios.  Therefore, even if a home agent is the only




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      router, it is an implementation and operational choice whether the
      home agent returns [Binding Update Accepted] or [MCOA RETURNHOME
      WO/NDP].

   o  If the BID option is not included in the Binding Acknowledgement,
      the home agent might not recognize the home registration.  The
      home agent might have processed the home registration Binding
      Update as a regular de-registration, as described in [RFC3775],
      and deleted all the registered binding cache entries for the
      mobile node.  Thus, the mobile node SHOULD stop using the
      interface attached to the foreign link and use only the interface
      attached to the home link.

6.4.  Sending Binding Refresh Request

   When a node (home agent or correspondent node) sends a Binding
   Refresh Request for a particular binding created with the BID, the
   node SHOULD include the Binding Identifier mobility option in the
   Binding Refresh Request.  The node MAY include multiple Binding
   Identifier mobility options if there are multiple bindings that need
   to be refreshed.

6.5.  Receiving Packets from Mobile Node

   When a node receives packets with a Home Address destination option
   from a mobile node, it MUST check that the care-of address that
   appears in the Source Address field of the IPv6 header is equal to
   one of the care-of addresses in the binding cache entry.  If no
   binding is found, the packets MUST be discarded.  The node MUST also
   send a Binding Error message as specified in [RFC3775].  This
   verification MUST NOT be done for a Binding Update.

7.  Network Mobility Applicability

   The binding management mechanisms are the same for a mobile host that
   uses Mobile IPv6 and for a mobile router that is using the NEMO Basic
   Support protocol [RFC3963].  Therefore, the extensions described in
   this document can also be used to support a mobile router with
   multiple care-of addresses.  [RFC4980] contains an analysis of NEMO
   multihoming.

8.  DSMIPv6 Applicability

   Dual Stack Mobile IPv6 (DSMIPv6) [RFC5555] extends Mobile IPv6 to
   register an IPv4 care-of address instead of the IPv6 care-of address
   when the mobile node is attached to an IPv4-only access network.  It
   also allows the mobile node to acquire an IPv4 home address in




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   addition to an IPv6 home address for use with IPv4-only correspondent
   nodes.  This section describes how the multiple care-of addresses
   registration works with IPv4 care-of and home addresses.

8.1.  IPv4 Care-of Address Registration

   The mobile node can use the extensions described in the document to
   register multiple care-of addresses, even if some of the care-of
   addresses are IPv4 addresses.

   Bulk registration MUST NOT be used for the initial binding
   registration from an IPv4 care-of address.  This is because the
   Binding Update and Binding Acknowledgement exchange is used to detect
   NAT on the path between the mobile node and the home agent.  So the
   mobile node needs to check for a NAT between each IPv4 care-of
   address and the home agent.

   The Binding Update MUST be sent to the IPv4 home agent address by
   using UDP and IPv4 headers as shown in Figure 9.  It is similar to
   [RFC5555] except that the IPv4 care-of address option MUST NOT be
   used when the BID mobility option is used.

              IPv4 header (src=V4ADDR, dst=HA_V4ADDR)
                UDP Header
                  IPv6 header (src=V6HoA, dst=HAADDR)
                       ESP Header
                       Mobility header
                           -Binding Update
                          Mobility Options
                            - Binding Identifier (IPv4 CoA)
          *V4ADDR, HA_V4ADDR, V6HOA, HAADDR are defined in [RFC5555]

         Figure 9: Initial Binding Update for IPv4 Care-of Address

   If a NAT is not detected, the mobile node can update the IPv4 care-of
   address by using bulk registration.  The mobile node can register the
   IPv4 care-of address along with other IPv4 and IPv6 care-of
   addresses.  Figure 10 shows the Binding Update format when the mobile
   node sends a Binding Update from one of its IPv6 care-of addresses.
   If the mobile node sends a Binding Update from an IPv4 care-of
   address, it MUST follow the format described in Figure 9.  Note that
   the IPv4 care-of address must be registered by a non-bulk binding
   registration whenever it is changed.

   As shown in Figure 9, the IPv4 care-of address will appear in the
   Binding Identifier mobility option.  The IPv4 Care-of Address
   mobility option defined in [RFC5555] MUST always be omitted.  The
   receiver of the Binding Update message for an IPv4 care-of address



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   MUST treat the IPv4 address stored in the Binding Identifier mobility
   option as the one in the IPv4 Care-of Address mobility option of
   [RFC5555].  If the IPv4 address in the Binding Identifier mobility
   option is different from one in the Source Address field in the IPv4
   header of the Binding Update (i.e., V4ADDR in Figure 9), the source
   address is used as an IPv4 care-of address.  Otherwise, the IPv4
   address in the Binding Identifier mobility option is used as an IPv4
   care-of address.

          IPv6 header (src=Care-of Address, dst=Home Agent Address)
               IPv6 Home Address Option
               ESP Header
               Mobility header
                   -Binding Update
                  Mobility Options
                     - Binding Identifier (IPv6/v4 CoA)
                     - Binding Identifier (IPv6/v4 CoA)
                     - ...

     Figure 10: Binding Bulk Registration for an IPv4 Care-of Address

   When the home agent returns a Binding Acknowledgement for the IPv4
   care-of address registration, it SHOULD NOT use the IPv4 Address
   Acknowledgement mobility option and SHOULD use only the Binding
   Identifier mobility option.  The registration status for the IPv4
   care-of address is stored in the Status field of the Binding
   Identifier mobility option.  However, if the home agent needs to
   store the status value specially defined for the IPv4 Address
   Acknowledgement mobility option, it MUST store the status value in
   the IPv4 Address Acknowledgement mobility option and MUST NOT store
   it in the Binding Identifier mobility option.  In such case, the home
   agent MUST include both the IPv4 Address Acknowledgement mobility
   option and the Binding Identifier mobility option.

8.2.  IPv4 Home Address Management

   When the mobile node wants to configure an IPv4 home address in
   addition to the IPv6 home address, it can request one using the IPv4
   Home Address option in the Binding Update.  If the home agent accepts
   the Binding Update, the mobile node can now register multiple care-of
   addresses for the IPv4 home address in addition to the IPv6 home
   address.  The mobile node MUST always use the IPv4 Home Address
   mobility option for any purposes of the IPv4 home address management.
   The same set of care-of addresses will be registered for both IPv6
   and IPv4 home addresses.  The mobile node cannot bind a different set
   of care-of addresses to each home address.





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   According to [RFC5555], the home agent includes the IPv4 Address
   Acknowledgement option in the Binding Acknowledgement only if the
   mobile node had requested an IPv4 home address in the corresponding
   Binding Update.  The IPv4 Address Acknowledgement option MUST be
   present before any Binding Identifier mobility option.  The Status
   field of the IPv4 Address Acknowledgement option contains only the
   error code defined in Section 3.2.1 of [RFC5555].  The home agent
   MUST always include the IPv4 Address Acknowledgement mobility option
   in the Binding Acknowledgement for the IPv4 home address
   registration.

9.  IPsec and IKEv2 Interaction

   Mobile IPv6 [RFC3775] and the NEMO protocol [RFC3963] require the use
   of IPsec to protect signaling messages, including Binding Updates,
   Binding Acknowledgements, and return routability messages.  IPsec may
   also be used to protect all tunneled data traffic.  The Mobile IPv6-
   IKEv2 specification [RFC4877] specifies how IKEv2 can be used to set
   up the required IPsec security associations.  The following
   assumptions were made in [RFC3775], [RFC3963], and [RFC4877] with
   respect to the use of IKEv2 and IPsec.

   o  There is only one primary care-of address per mobile node.

   o  The primary care-of address is stored in the IPsec database for
      tunnel encapsulation and decapsulation.

   o  When the home agent receives a packet from the mobile node, the
      source address is verified against the care-of address in the
      corresponding binding cache entry.  If the packet is a reverse-
      tunneled packet from the mobile node, the care-of address check is
      done against the source address on the outer IPv6 header.  The
      reverse-tunneled packet could either be a tunneled Home Test Init
      message or tunneled data traffic to the correspondent node.

   o  The mobile node runs IKEv2 (or IKEv1) with the home agent using
      the care-of address.  The IKE SA is based on the care-of address
      of the mobile node.

   The above assumptions may not be valid when multiple care-of
   addresses are used by the mobile node.  In the following sections,
   the main issues with the use of multiple care-of addresses with IPsec
   are addressed.








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9.1.  Use of Care-of Address in the IKEv2 Exchange

   For each home address for which the mobile node sets up security
   associations with the home agent, the mobile node must pick one
   care-of address and use that as the source address for all IKEv2
   messages exchanged to create and maintain the IPsec security
   associations associated with the home address.  The resultant IKEv2
   security association is created based on this care-of address.

   If the mobile node needs to change the care-of address, it just sends
   a Binding Update with the care-of address it wants to use, with the
   corresponding Binding Identifier mobility option, and with the 'K'
   bit set.  This will force the home agent to update the IKEv2 security
   association to use the new care-of address.  If the 'K' bit is not
   supported on the mobile node or the home agent, the mobile node MUST
   re-establish the IKEv2 security association with the new care-of
   address.  This will also result in new IPsec security associations
   being set up for the home address.

9.2.  Transport Mode IPsec-Protected Messages

   For Mobile IPv6 signaling message protected using IPsec in transport
   mode, the use of a particular care-of address among multiple care-of
   addresses does not matter for IPsec processing.

   The home agent processes Mobile Prefix Discovery messages with the
   same rules of data packets described in Section 6.5.

9.3.  Tunnel Mode IPsec-Protected Messages

   The use of IPsec in tunnel mode with multiple care-of addresses
   introduces a few issues that require changes to how the mobile node
   and the home agent send and receive tunneled traffic.  The route
   optimization mechanism described in [RFC3775] mandates the use of
   IPsec protection in tunnel mode for the Home Test Init and Home Test
   messages.  The mobile node and the home agent may also choose to
   protect all reverse-tunneled payload traffic with IPsec in tunnel
   mode.  The following sections address multiple care-of address
   support for these two types of messages.

9.3.1.  Tunneled Home Test Init and Home Test Messages

   The mobile node MAY use the same care-of address for all Home Test
   Init messages sent reverse tunneled through the home agent.  The
   mobile node may use the same care-of address irrespective of which
   correspondent node the Home Test Init message is being to.  RFC 3775
   requires the home agent to verify that the mobile node is using the
   care-of address that is in the binding cache entry when it receives a



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   reverse-tunneled Home Test Init message.  If a different address is
   used as the source address, the message is silently dropped by the
   home agent.  This document requires the home agent implementation to
   decapsulate and forward the Home Test Init message as long as the
   source address is one of the care-of addresses in the binding cache
   entry for the mobile node.

   When the home agent tunnels a Home Test message to the mobile node,
   the care-of address used in the outer IPv6 header is not relevant to
   the Home Test message.  So regular IPsec tunnel encapsulation with
   the care-of address known to the IPsec implementation on the home
   agent is sufficient.

9.3.2.  Tunneled Payload Traffic

   When the mobile node sends and receives multiple traffic flows
   protected by IPsec to different care-of addresses, the use of the
   correct care-of address for each flow becomes important.  Support for
   this requires the following two considerations on the home agent.

   o  When the home agent receives a reverse-tunneled payload message
      protected by IPsec in tunnel mode, the source address used in the
      outer IPv6 header is irrelevant to IPsec, since the tunnel mode
      security association is based on the addresses in the inner IPv6
      header.  Therefore, the same IPsec security association can be
      used for payload traffic tunneled from any of the care-of
      addresses.  Note that the care-of address used in the reverse-
      tunneled traffic can be different from the care-of address used as
      the source address in the IKEv2 exchange.  However, this does not
      cause an issue due to the above-mentioned reason.

   o  For tunneled IPsec traffic from the home agent to the mobile node,
      the IPsec implementation on the home agent will not be aware of
      which care-of address to use when performing IPsec tunnel
      encapsulation.  The Mobile IP stack on the home agent, based on
      the binding cache entries created by the mobile node, knows to
      which care-of address the packet belonging to a particular flow
      needs to be tunneled.  The destination address for the outer IP
      header must either be conveyed dynamically per packet to the IPsec
      stack when it performs the encapsulation or the Mobile IPv6 stack
      must get access to the packet after IPsec processing is done and
      modify the destination address.  The first option requires changes
      to the IPsec implementation.  In the second option, there is a
      need for special processing in the forwarding function to replace
      the destination address on the outer header with the correct
      care-of address.  The exact technique to achieve the above is
      implementation specific.




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10.  Security Considerations

   The security considerations for securing the Binding Update and
   Binding Acknowledgement messages with multiple care-of addresses are
   very similar to the security considerations for securing the Binding
   Update and Binding Acknowledgement.  Please see [RFC3775] for more
   information.  The Binding Update and Binding Acknowledgement messages
   with multiple care-of addresses are securely exchanged as described
   in [RFC3775], [RFC4877], and Section 9 of this document.  Additional
   security considerations are described below.

   With simultaneous binding support, it is possible for a malicious
   mobile node to successfully bind a number of victims' addresses as
   valid care-of addresses for the mobile node with its home agent.
   Once these addresses have been bound, the malicious mobile node can
   perform a re-direction attack by instructing the home agent (e.g.,
   setting filtering rules to direct a large file transfer) to tunnel
   packets to the victims' addresses.  Such risk is highlighted in
   [MIP6ANALYSIS].  These attacks are possible because the care-of
   addresses sent by the mobile node in the Binding Update messages are
   not verified by the home agent, i.e., the home agent does not check
   if the mobile node is at the care-of address at which it claims to
   be.  The security model for Mobile IPv6 assumes that there is a trust
   relationship between the mobile node and its home agent.  Any
   malicious attack by the mobile node is traceable by the home agent.
   This acts as a deterrent for the mobile node to launch such attacks.

   Although such a risk exists in Mobile IPv6, the risk level is
   increased when simultaneous multiple care-of address bindings are
   performed.  In Mobile IPv6, a mobile node can only have a single
   care-of address binding per home address at a given time.  However,
   for simultaneous multiple care-of address bindings, a mobile node can
   have more than one care-of address binding per home address at a
   given time.  This implies that a mobile node using simultaneous
   binding support can effectively bind more than a single victim's
   address.  Another difference is the degree of risk involved.  In the
   single care-of address binding case, once the re-direction attack is
   initiated, a malicious mobile node would be unable to use its home
   address for communications (such as to receive control packets
   pertaining to the file transfer).  However, in the simultaneous
   binding support case, a malicious mobile node could bind a valid
   care-of address in addition to multiple victims addresses.  This
   valid care-of address could then be used by the malicious mobile node
   to set up flow filtering rules at its home agent, thereby controlling
   and/or launching new re-direction attacks.






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   Thus, in view of such risks, it is advisable for a home agent to
   employ some form of care-of address verification mechanism before
   using the care-of addresses as a valid routing path to a mobile node.
   These mechanisms are out of scope for this document.

   In the binding registration of Mobile IPv6, a care-of address is
   always verified by its reachability by a home agent.  This
   reachability test may decrease the above risks.  However, when bulk
   registration is used, a home agent cannot verify reachability of
   care-of addresses carried in a Binding Identifier mobility option.
   Therefore, the home agent can choose to reject bulk registration by
   using [MCOA BULK REGISTRATION PROHIBITED] in a Binding
   Acknowledgement.  Alternatively, when a mobile node first registers a
   care-of address, it uses the individual Binding Updates for the first
   appeared care-of address.  During the initial binding registration, a
   home agent can verify the address reachability for that given care-of
   address.  After that, the mobile node uses bulk registration to
   refresh the care-of address.

11.  IANA Considerations

   The following Extension Types have been assigned by IANA:

   o  Binding Identifier mobility option type: (35) has been assigned
      from the same space as the mobility option in [RFC3775].

   o  New Successful Status of Binding Acknowledgement: These status
      codes have been assigned from the same space as the Binding
      Acknowledgement status codes in [RFC3775].

      *  MCOA NOTCOMPLETE (4)

      *  MCOA RETURNHOME WO/NDP (5)

   o  New Unsuccessful Status of Binding Acknowledgement: These status
      codes have also been assigned from the same space as the Binding
      Acknowledgement status codes in [RFC3775].

      *  MCOA MALFORMED (164)

      *  MCOA NON-MCOA BINDING EXISTS (165)

      *  MCOA PROHIBITED (166)

      *  MCOA UNKNOWN COA (167)

      *  MCOA BULK REGISTRATION PROHIBITED (168)




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      *  MCOA SIMULTANEOUS HOME AND FOREIGN PROHIBITED (169)

12.  Acknowledgements

   Ryuji Wakikawa and Thierry Ernst are grateful to Keio University for
   its initial support on this specification at the time when they were
   working there.  In addition, the authors would like to thank Masafumi
   Aramoto, Keigo Aso, Julien Charbon, Tero Kauppinen, Martti Kuparinen,
   Romain Kuntz, Benjamin Lim, Heikki Mahkonen, Nicolas Montavont, and
   Chan-Wah Ng for their discussions and inputs.  Thanks to Susumu
   Koshiba, Hiroki Matutani, Koshiro Mitsuya, Koji Okada, Keisuke
   Uehara, Masafumi Watari, and Jun Murai for earlier work on this
   subject.

13.  References

13.1.  Normative References

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

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

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

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

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

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

   [RFC5568]      Koodli, R., Ed., "Mobile IPv6 Fast Handovers", RFC
                  5568, July 2009.

13.2.  Informative References

   [MOTIVATION]   Ernst, T., Montavont, N., Wakikawa, R., Ng, C., and K.
                  Kuladinithi, "Motivations and Scenarios for Using
                  Multiple Interfaces and Global Addresses", Work in
                  Progress, May 2008.



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   [RFC4980]      Ng, C., Ernst, T., Paik, E., and M. Bagnulo, "Analysis
                  of Multihoming in Network Mobility Support", RFC 4980,
                  October 2007.

   [MIP6ANALYSIS] Montavont, N., Wakikawa, R., Ernst, T., Ng, C., and K.
                  Kuladinithi, "Analysis of Multihoming in Mobile IPv6",
                  Work in Progress, May 2008.

   [RFC3753]      Manner, J., Ed., and M. Kojo, Ed., "Mobility Related
                  Terminology", RFC 3753, June 2004.

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

Authors' Addresses

   Ryuji Wakikawa (Editor)
   TOYOTA InfoTechnology Center Co., Ltd.

   EMail: ryuji.wakikawa@gmail.com (ryuji@jp.toyota-itc.com)


   Vijay Devarapalli
   Wichorus

   EMail: vijay@wichorus.com


   George Tsirtsis
   Qualcomm

   EMail: Tsirtsis@gmail.com


   Thierry Ernst
   INRIA

   EMail: thierry.ernst@inria.fr


   Kenichi Nagami
   INTEC NetCore Inc.

   EMail: nagami@inetcore.com







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