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IETF Mobile IP Working Group                            David B. Johnson
INTERNET-DRAFT                                           Rice University
                                                      Charles E. Perkins
                                                   Nokia Research Center
                                                              Jari Arkko
                                                                Ericsson
                                                             1 June 2002


                        Mobility Support in IPv6
                   <draft-ietf-mobileip-ipv6-18.txt>


Status of This Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note
   that other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents, valid for a maximum of six
   months, and may be updated, replaced, or obsoleted by other documents
   at any time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.



   This document specifies the operation the IPv6 Internet with mobile
   computers.  Each mobile node is always identified by its home
   address, regardless of its current point of attachment to the
   Internet.  While situated away from its home, a mobile node is also
   associated with a care-of address, which provides information about
   the mobile node's current location.  IPv6 packets addressed to a
   mobile node's home address are transparently routed to its care-of
   address.  The protocol enables IPv6 nodes to cache the binding of
   a mobile node's home address with its care-of address, and to then
   send any packets destined for the mobile node directly to it at this
   care-of address.  To support this operation, Mobile IPv6 defines a
   new IPv6 protocol and a new destination option.  All IPv6 nodes,
   whether mobile or stationary, MUST support communications with mobile
   nodes.







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                                Contents


Status of This Memo                                                    i

Abstract                                                               i

 1. Introduction                                                       1

 2. Comparison with Mobile IP for IPv4                                 2

 3. Terminology                                                        3
     3.1. General Terms . . . . . . . . . . . . . . . . . . . . . .    3
     3.2. Mobile IPv6 Terms . . . . . . . . . . . . . . . . . . . .    4

 4. Overview of Mobile IPv6                                            7
     4.1. Basic Operation . . . . . . . . . . . . . . . . . . . . .    7
     4.2. New IPv6 Protocols  . . . . . . . . . . . . . . . . . . .    9
     4.3. New IPv6 Destination Options  . . . . . . . . . . . . . .   10
     4.4. New IPv6 ICMP Messages  . . . . . . . . . . . . . . . . .   10
     4.5. Conceptual Data Structures  . . . . . . . . . . . . . . .   11

 5. Overview of Mobile IPv6 Security                                  12
     5.1. Binding Updates to Home Agents  . . . . . . . . . . . . .   12
     5.2. Binding Updates to Correspondent Nodes  . . . . . . . . .   12
           5.2.1. Node Keys . . . . . . . . . . . . . . . . . . . .   13
           5.2.2. Nonces  . . . . . . . . . . . . . . . . . . . . .   13
           5.2.3. Cookies . . . . . . . . . . . . . . . . . . . . .   14
           5.2.4. Cryptographic Functions . . . . . . . . . . . . .   14
           5.2.5. Return Routability Procedure  . . . . . . . . . .   14
           5.2.6. Applying Return Routability for Correspondent
                          Bindings . . . . . . . . . . . . . . . . .  18
           5.2.7. Updating Node Keys and Nonces . . . . . . . . . .   20
           5.2.8. Preventing Replay Attacks . . . . . . . . . . . .   20
     5.3. Payload Packets . . . . . . . . . . . . . . . . . . . . .   21

 6. New IPv6 Protocols, Message Types, and Destination Option         21
     6.1. Mobility Header . . . . . . . . . . . . . . . . . . . . .   21
           6.1.1. Format  . . . . . . . . . . . . . . . . . . . . .   22
           6.1.2. Binding Refresh Request (BRR) Message . . . . . .   23
           6.1.3. Home Test Init (HoTI) Message . . . . . . . . . .   24
           6.1.4. Care-of Test Init (CoTI) Message  . . . . . . . .   26
           6.1.5. Home Test (HoT) Message . . . . . . . . . . . . .   27
           6.1.6. Care-of Test (CoT) Message  . . . . . . . . . . .   28
           6.1.7. Binding Update (BU) Message . . . . . . . . . . .   29
           6.1.8. Binding Acknowledgement (BA) Message  . . . . . .   32
           6.1.9. Binding Error (BE) Message  . . . . . . . . . . .   34
     6.2. Mobility Options  . . . . . . . . . . . . . . . . . . . .   35
           6.2.1. Format  . . . . . . . . . . . . . . . . . . . . .   35



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           6.2.2. Pad1  . . . . . . . . . . . . . . . . . . . . . .   36
           6.2.3. PadN  . . . . . . . . . . . . . . . . . . . . . .   37
           6.2.4. Unique Identifier . . . . . . . . . . . . . . . .   37
           6.2.5. Alternate Care-of Address . . . . . . . . . . . .   38
           6.2.6. Nonce Indices . . . . . . . . . . . . . . . . . .   38
           6.2.7. Binding Authorization Data  . . . . . . . . . . .   39
           6.2.8. Binding Refresh Advice  . . . . . . . . . . . . .   39
     6.3. Home Address Destination Option . . . . . . . . . . . . .   40
     6.4. Routing Header type 2 . . . . . . . . . . . . . . . . . .   42
           6.4.1. Routing Header Packet format  . . . . . . . . . .   42
     6.5. ICMP Home Agent Address Discovery Request Message . . . .   43
     6.6. ICMP Home Agent Address Discovery Reply Message . . . . .   45
     6.7. ICMP Mobile Prefix Solicitation Message Format  . . . . .   47
     6.8. ICMP Mobile Prefix Advertisement Message Format . . . . .   49

 7. Modifications to IPv6 Neighbor Discovery                          51
     7.1. Modified Router Advertisement Message Format  . . . . . .   51
     7.2. Modified Prefix Information Option Format . . . . . . . .   52
     7.3. New Advertisement Interval Option Format  . . . . . . . .   54
     7.4. New Home Agent Information Option Format  . . . . . . . .   55
     7.5. Changes to Sending Router Advertisements  . . . . . . . .   57
     7.6. Changes to Sending Router Solicitations . . . . . . . . .   58

 8. Requirements for Types of IPv6 Nodes                              59
     8.1. General Requirements for All IPv6 Nodes . . . . . . . . .   59
     8.2. Route Optimization Requirements for All IPv6 Nodes  . . .   59
     8.3. Requirements for All IPv6 Routers . . . . . . . . . . . .   60
     8.4. Requirements for IPv6 Home Agents . . . . . . . . . . . .   61
     8.5. Requirements for IPv6 Mobile Nodes  . . . . . . . . . . .   62

 9. Correspondent Node Operation                                      63
     9.1. Conceptual Data Structures  . . . . . . . . . . . . . . .   63
     9.2. Receiving Packets from a Mobile Node  . . . . . . . . . .   64
           9.2.1. Processing Mobility Header (MH) Messages  . . . .   64
           9.2.2. Receiving Packets with Home Address Destination
                          Option . . . . . . . . . . . . . . . . . .  65
     9.3. Return Routability Procedure  . . . . . . . . . . . . . .   66
           9.3.1. Receiving Home Test Init Messages . . . . . . . .   66
           9.3.2. Receiving Care-of Test Init Messages  . . . . . .   66
           9.3.3. Sending Home Test Messages  . . . . . . . . . . .   67
           9.3.4. Sending Care-of Test Messages . . . . . . . . . .   67
     9.4. Processing Bindings . . . . . . . . . . . . . . . . . . .   67
           9.4.1. Receiving Binding Updates . . . . . . . . . . . .   67
           9.4.2. Requests to Cache a Binding . . . . . . . . . . .   69
           9.4.3. Requests to Delete a Binding  . . . . . . . . . .   69
           9.4.4. Sending Binding Acknowledgements  . . . . . . . .   70
           9.4.5. Sending Binding Refresh Requests  . . . . . . . .   71
           9.4.6. Sending Binding Error Messages  . . . . . . . . .   71
     9.5. Cache Replacement Policy  . . . . . . . . . . . . . . . .   71
     9.6. Sending Packets to a Mobile Node  . . . . . . . . . . . .   72
     9.7. Receiving ICMP Error Messages . . . . . . . . . . . . . .   73



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10. Home Agent Operation                                              74
    10.1. Conceptual Data Structures  . . . . . . . . . . . . . . .   74
    10.2. Primary Care-of Address Registration  . . . . . . . . . .   75
    10.3. Primary Care-of Address De-Registration . . . . . . . . .   79
    10.4. Intercepting Packets for a Mobile Node  . . . . . . . . .   80
    10.5. Tunneling Intercepted Packets to a Mobile Node  . . . . .   81
    10.6. Handling Reverse Tunneled Packets from a Mobile Node  . .   83
    10.7. Protecting Return Routability Packets . . . . . . . . . .   83
    10.8. Receiving Router Advertisement Messages . . . . . . . . .   84
    10.9. Dynamic Home Agent Address Discovery  . . . . . . . . . .   85
          10.9.1. Aggregate List of Home Network Prefixes . . . . .   87
          10.9.2. Scheduling Prefix Deliveries to the Mobile Node .   89
          10.9.3. Sending Advertisements to the Mobile Node . . . .   90
          10.9.4. Lifetimes for Changed Prefixes  . . . . . . . . .   91

11. Mobile Node Operation                                             91
    11.1. Conceptual Data Structures  . . . . . . . . . . . . . . .   91
    11.2. Packet Processing . . . . . . . . . . . . . . . . . . . .   93
          11.2.1. Sending Packets While Away from Home  . . . . . .   93
          11.2.2. Interaction with Outbound IPsec Processing  . . .   96
          11.2.3. Receiving Packets While Away from Home  . . . . .   97
          11.2.4. Routing Multicast Packets . . . . . . . . . . . .   99
    11.3. Home Agent and Prefix Management  . . . . . . . . . . . .  100
          11.3.1. Receiving Local Router Advertisement Messages . .  100
          11.3.2. Dynamic Home Agent Address Discovery  . . . . . .  101
          11.3.3. Sending Mobile Prefix Solicitations . . . . . . .  102
          11.3.4. Receiving Mobile Prefix Advertisements  . . . . .  103
    11.4. Movement  . . . . . . . . . . . . . . . . . . . . . . . .  104
          11.4.1. Movement Detection  . . . . . . . . . . . . . . .  104
          11.4.2. Forming New Care-of Addresses . . . . . . . . . .  107
          11.4.3. Using Multiple Care-of Addresses  . . . . . . . .  108
    11.5. Return Routability Procedure  . . . . . . . . . . . . . .  109
          11.5.1. Sending Home and Care-of Test Init Messages . . .  109
          11.5.2. Receiving Return Routability Messages . . . . . .  109
          11.5.3. Retransmitting in the Return Routability Procedure 111
          11.5.4. Rate Limiting for Return Routability Procedure  .  111
    11.6. Processing Bindings . . . . . . . . . . . . . . . . . . .  111
          11.6.1. Sending Binding Updates to the Home Agent . . . .  111
          11.6.2. Correspondent Binding Procedure . . . . . . . . .  114
          11.6.3. Receiving Binding Acknowledgements  . . . . . . .  117
          11.6.4. Receiving Binding Refresh Requests  . . . . . . .  118
          11.6.5. Receiving Binding Error Messages  . . . . . . . .  119
          11.6.6. Forwarding from a Previous Care-of Address  . . .  120
          11.6.7. Returning Home  . . . . . . . . . . . . . . . . .  121
          11.6.8. Retransmitting Binding Updates  . . . . . . . . .  123
          11.6.9. Rate Limiting Binding Updates . . . . . . . . . .  124
    11.7. Receiving ICMP Error Messages . . . . . . . . . . . . . .  124

12. Protocol Constants                                               125

13. IANA Considerations                                              126



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14. Security Considerations                                          127
    14.1. Threats . . . . . . . . . . . . . . . . . . . . . . . . .  127
    14.2. Features  . . . . . . . . . . . . . . . . . . . . . . . .  129
    14.3. Binding Updates to Home Agent . . . . . . . . . . . . . .  130
    14.4. Binding Updates to Correspondent Nodes  . . . . . . . . .  132
          14.4.1. Overview  . . . . . . . . . . . . . . . . . . . .  132
          14.4.2. Offered Protection  . . . . . . . . . . . . . . .  132
          14.4.3. Comparison to Regular IPv6 Communications . . . .  133
          14.4.4. Return Routability Replays  . . . . . . . . . . .  135
          14.4.5. Return Routability Denial-of-Service  . . . . . .  135
    14.5. Tunneling via the Home Agent  . . . . . . . . . . . . . .  136
    14.6. Home Address Destination Option . . . . . . . . . . . . .  137
    14.7. Type 2 Routing Header . . . . . . . . . . . . . . . . . .  138

Acknowledgements                                                     138

References                                                           140

 A. State Machine for the Correspondent Binding Procedure            142
     A.1. Main State Machine  . . . . . . . . . . . . . . . . . . .  143
     A.2. Return Routability Procedure  . . . . . . . . . . . . . .  149

 B. Changes from Previous Version of the Draft                       153

 C. Future Extensions                                                154
     C.1. Piggybacking  . . . . . . . . . . . . . . . . . . . . . .  154
     C.2. Triangular Routing and Unverified Home Addresses  . . . .  154
     C.3. New Authorization Methods beyond Return Routability . . .  155
     C.4. Security and Dynamically Generated Home Addresses . . . .  155
     C.5. Remote Home Address Configuration . . . . . . . . . . . .  155

Chairs' Addresses                                                    157

Authors' Addresses                                                   157




















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1. Introduction

   This document specifies how the IPv6 Internet operates with mobile
   computers.  Without specific support for mobility in IPv6 [11],
   packets destined to a mobile node would not be able to reach it while
   the mobile node is away from its home link.  In order to continue
   communication in spite of its movement, a mobile node could change
   its IP address each time it moves to a new link, but the mobile
   node would then not be able to maintain transport and higher-layer
   connections when it changes location.  Mobility support in IPv6 is
   particularly important, as mobile computers are likely to account for
   a majority or at least a substantial fraction of the population of
   the Internet during the lifetime of IPv6.

   The protocol defined in this document, known as Mobile IPv6, allows
   a mobile node to move from one link to another without changing the
   mobile node's IP address.  A mobile node is always addressable by
   its "home address", an IP address assigned to the mobile node within
   its home subnet prefix on its home link.  Packets may be routed to
   the mobile node using this address regardless of the mobile node's
   current point of attachment to the Internet.  The mobile node may
   also continue to communicate with other nodes (stationary or mobile)
   after moving to a new link.  The movement of a mobile node away from
   its home link is thus transparent to transport and higher-layer
   protocols and applications.

   The Mobile IPv6 protocol is just as suitable for mobility across
   homogeneous media as for mobility across heterogeneous media.  For
   example, Mobile IPv6 facilitates node movement from one Ethernet
   segment to another as well as it facilitates node movement from an
   Ethernet segment to a wireless LAN cell, with the mobile node's IP
   address remaining unchanged in spite of such movement.

   One can think of the Mobile IPv6 protocol as solving the
   network-layer mobility management problem.  Some mobility management
   applications -- for example, handover among wireless transceivers,
   each of which covers only a very small geographic area -- have been
   solved using link-layer techniques.  For example, in many current
   wireless LAN products, link-layer mobility mechanisms allow a
   "handover" of a mobile node from one cell to another, reestablishing
   link-layer connectivity to the node in each new location.

   Mobile IP does not attempt to solve all general problems related to
   the use of mobile computers or wireless networks.  In particular,
   this protocol does not attempt to solve:

    -  Handling links with partial reachability, or unidirectional
       connectivity, such as are often found in wireless networks (but
       see Section 11.4.1).

    -  Access control on a link being visited by a mobile node.



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    -  Local or hierarchical forms of mobility management (similar to
       many current link-layer mobility management solutions).

    -  Assistance for adaptive applications

    -  Mobile routers

    -  Service Discovery

    -  Distinguishing between packets lost due to bit errors vs.
       network congestion


2. Comparison with Mobile IP for IPv4

   The design of Mobile IP support in IPv6 (Mobile IPv6) represents a
   natural combination of the experiences gained from the development of
   Mobile IP support in IPv4 (Mobile IPv4) [20, 21, 22], together with
   the opportunities provided by IPv6.  Mobile IPv6 thus shares many
   features with Mobile IPv4, but is integrated into IP and provides
   many improvements.  This section summarizes the major differences
   between Mobile IPv4 and Mobile IPv6:

    -  There is no longer any need to deploy special routers as
       "foreign agents" as are used in Mobile IPv4.  In Mobile IPv6,
       mobile nodes make use of IPv6 features, to operate in any
       location without any special support required from the local
       router.

    -  Support for what is known in Mobile IPv4 as "route
       optimization" [23] is now built in as a fundamental part
       of the protocol, rather than being added on as an optional set
       of extensions that may not be supported by all nodes as in
       Mobile IPv4.

    -  Mobile IPv6 route optimization can operate securely even without
       pre-arranged security associations.  It is expected that route
       optimization can be deployed on a global scale between all mobile
       nodes and correspondent nodes.

    -  Support is also integrated into Mobile IPv6 for allowing route
       optimization to coexist efficiently with routers that perform
       "ingress filtering" [24].  Both the current care-of address and
       the home address can be carried in packets, allowing packets to
       pass normally through ingress filtering routers.

    -  In Mobile IPv6, the mobile node does not have to tunnel multicast
       packets to its home agent.  The inclusion of the home address in
       packets is compatible with multicast routing that is based in
       part on the packet's Source Address.




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    -  The movement detection mechanism in Mobile IPv6 provides
       bidirectional confirmation of a mobile node's ability to
       communicate with its default router in its current location.

    -  Most packets sent to a mobile node while away from home in
       Mobile IPv6 are sent using an IPv6 Routing header rather than IP
       encapsulation, reducing the amount of resulting overhead compared
       to Mobile IPv4.

    -  Mobile IPv6 is decoupled from any particular link layer, as it
       uses IPv6 Neighbor Discovery [12] instead of ARP. This also
       improves the robustness of the protocol.

    -  The use of IPv6 encapsulation (and the Routing header) removes
       the need in Mobile IPv6 to manage "tunnel soft state".

    -  The dynamic home agent address discovery mechanism in Mobile IPv6
       returns a single reply to the mobile node.  The directed
       broadcast approach used in IPv4 returns separate replies from
       each home agent.


3. Terminology

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [2].


3.1. General Terms

      IP            Internet Protocol Version 6 (IPv6).

      node          A device that implements IP.

      router        A node that forwards IP packets not explicitly
                    addressed to itself.

      host          Any node that is not a router.

      link          A communication facility or medium over which nodes
                    can communicate at the link layer, such as an
                    Ethernet (simple or bridged).  A link is the layer
                    immediately below IP.

      interface     A node's attachment to a link.

      subnet prefix
                    A bit string that consists of some number of initial
                    bits of an IP address.




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      interface identifier
                    A number used to identify a node's interface on a
                    link.  The interface identifier is the remaining
                    low-order bits in the node's IP address after the
                    subnet prefix.

      link-layer address
                    A link-layer identifier for an interface, such as
                    IEEE 802 addresses on Ethernet links.

      packet        An IP header plus payload.

      security association
                    A security object shared between two nodes which
                    includes the data mutually agreed on for operation
                    of some cryptographic algorithm (typically including
                    a key).

      security policy database
                    A database of rules that describe what security
                    associations should be applied for different kinds
                    of packets.

      destination option Destination options are carried by the IPv6
                    Destination Options extension header.  Mobile IPv6
                    defines one new destination option, the Home Address
                    destination option.


3.2. Mobile IPv6 Terms

      home address  An IP address assigned to a mobile node, used as the
                    permanent address of the mobile node.  This address
                    is within the mobile node's home link.  Standard IP
                    routing mechanisms will deliver packets destined for
                    a mobile node's home address to its home link.

      home subnet prefix
                    The IP subnet prefix corresponding to a mobile
                    node's home address.

      home link     The link on which a mobile node's home subnet prefix
                    is defined.

      mobile node   A node that can change its point of attachment from
                    one link to another, while still being reachable via
                    its home address.

      movement      A change in a mobile node's point of attachment to
                    the Internet such that it is no longer connected to
                    the same link as it was previously.  If a mobile



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                    node is not currently attached to its home link, the
                    mobile node is said to be "away from home".

      correspondent node
                    A peer node with which a mobile node is
                    communicating.  The correspondent node may be either
                    mobile or stationary.

      foreign subnet prefix
                    Any IP subnet prefix other than the mobile node's
                    home subnet prefix.

      foreign link  Any link other than the mobile node's home link.

      care-of address
                    An IP address associated with a mobile node while
                    visiting a foreign link; the subnet prefix of this
                    IP address is a foreign subnet prefix.  Among the
                    multiple care-of addresses that a mobile node may
                    have at any given time (e.g., with different subnet
                    prefixes), the one registered with the mobile node's
                    home agent is called its "primary" care-of address.

      home agent    A router on a mobile node's home link with which
                    the mobile node has registered its current care-of
                    address.  While the mobile node is away from home,
                    the home agent intercepts packets on the home
                    link destined to the mobile node's home address,
                    encapsulates them, and tunnels them to the mobile
                    node's registered care-of address.

      binding       The association of the home address of a mobile node
                    with a care-of address for that mobile node, along
                    with the remaining lifetime of that association.

      binding procedure
                    A binding procedure is initiated by the mobile node
                    to inform either a correspondent node or the mobile
                    node's home agent of the current binding of the
                    mobile node.

      binding authorization
                    Binding procedure needs to be authorized to allow
                    the recipient to believe that the sender has the
                    right to specify a new binding.

      return routability procedure
                    The return routability procedure authorizes binding
                    procedures by the use of a cryptographic cookie
                    exchange.




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      correspondent binding procedure
                    A return routability procedure followed by a
                    binding procedure, run between the mobile node and a
                    correspondent node.

      home binding procedure
                    A binding procedure between the mobile node and its
                    home agent, authorized by the use of IPsec.

      nonce         Nonces are random numbers used internally by the
                    correspondent node in the creation of cookies
                    related to the return routability procedure.  The
                    nonces are not specific to a mobile node, and are
                    kept secret within the correspondent node, only used
                    as one input in the creation of the cookies.

      cookie        Cookies are numbers that are used by mobile nodes
                    and correspondent nodes in the return routability
                    procedure.

      care-of cookie
                    A cookie sent directly to the mobile node's claimed
                    care-of address from the correspondent node.

      home cookie   A cookie sent to the mobile node's claimed home
                    address from the correspondent node.

      mobile cookie
                    A cookie sent to the correspondent node from the
                    mobile node, and later returned to the mobile node.
                    Mobile cookies are produced randomly.  There are two
                    kinds of mobile cookies:  the HoT cookie and the CoT
                    cookie.

      CoT cookie
                    A cookie sent by the mobile node to the the
                    correspondent node in the CoTI message, to be
                    returned to the mobile node in the CoT message.

      HoT cookie
                    A cookie sent by the mobile node to the the
                    correspondent node in the HoTI message, to be
                    returned to the mobile node in the HoT message.

      nonce index
                    The mobile node uses a particular set of cookies in
                    the return routability procedure.  The cookies have
                    been produced using a particular set of nonces.  A
                    nonce index is used to indicate which nonces have
                    been used, without revealing the nonces themselves.




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      binding key
                    a key used for authenticating binding cache
                    management messages.

      binding security association
                    a security association established specifically
                    for the purpose of producing and verifying
                    authentication data passed with a Binding
                    Authorization Data option.


4. Overview of Mobile IPv6

4.1. Basic Operation

   A mobile node is always addressable at its home address, whether it
   is currently attached to its home link or is away from home.  While
   a mobile node is at home, packets addressed to its home address
   are routed to it using conventional Internet routing mechanisms in
   the same way as if the node were stationary.  The subnet prefix of
   a mobile node's home address is one of the subnet prefixes of the
   mobile node's home link.  Packets addressed to the mobile node will
   therefore be routed to its home link.

   While a mobile node is attached to some foreign link away from home,
   it is also addressable at one or more care-of addresses.  A care-of
   address is an IP address associated with a mobile node while visiting
   a particular foreign link.  The subnet prefix of a mobile node's
   care-of address is one of the subnet prefixes on the foreign link.
   As long as the mobile node stays in this location, packets addressed
   to this care-of address will be routed to the mobile node.

   The association between a mobile node's home address and care-of
   address is known as a "binding" for the mobile node.  A mobile node
   typically acquires its care-of address through stateless [13] or
   stateful (e.g., DHCPv6 [25]) Address Autoconfiguration, according
   to the methods of IPv6 Neighbor Discovery [12].  Other methods
   of acquiring a care-of address are also possible, but are beyond
   the scope of this document.  The operation of the mobile node is
   specified in Section 11.

   While away from home, a mobile node registers one of its care-of
   addresses with a router on its home link, requesting this router to
   function as the "home agent" for the mobile node.  The mobile node
   performs this binding registration by sending a "Binding Update"
   message to the home agent.  The home agent replies to the mobile
   node by returning a "Binding Acknowledgement" message.  The care-of
   address associated with this binding registration is known as the
   mobile node's "primary care-of address".  The mobile node's home
   agent thereafter uses proxy Neighbor Discovery to intercept any
   IPv6 packets addressed to the mobile node's home address (or home



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   addresses) on the home link.  Each intercepted packet is tunneled
   to the mobile node's primary care-of address.  This tunneling
   is performed using IPv6 encapsulation [15], with the outer IPv6
   header addressed to the mobile node's primary care-of address.  The
   operation of the home agent is specified in Section 10.

   Any node communicating with a mobile node is referred to in this
   document as a "correspondent node" of the mobile node, and may itself
   be either a stationary node or a mobile node.  The operation of the
   correspondent node is specified in Section 9.  Mobile nodes can
   inform the correspondent nodes of the current location of the mobile
   node.  This happens through the correspondent binding procedure.  As
   a part of this procedure, a return routability test is performed
   in order to authorize the establishment of the binding.  This is
   specified in Sections 5.2.5 and 5.2.6.  When sending a packet to
   any IPv6 destination, a node checks its cached bindings for an
   entry for the packet's destination address.  If a cached binding
   for this destination address is found, the node uses a new type of
   IPv6 Routing header [11] (see section 6.4) to route the packet to
   the mobile node by way of the care-of address indicated in this
   binding.  If, instead, the sending node has no cached binding for
   this destination address, the node sends the packet normally (with
   no Routing header), and the packet is subsequently intercepted and
   tunneled by the mobile node's home agent as described above.  When a
   mobile node receives a packet tunneled to it in this manner, it can
   use this as an indication that the correspondent node has no binding
   for the mobile node.  The mobile node can then establish a binding
   with the correspondent node.

   It is expected that correspondent nodes usually will route packets
   directly to the mobile node's care-of address, so that the home agent
   is rarely involved with packet transmission to the mobile node.  This
   is important for scalability and reliability, and for minimizing
   overall network load.  Routing packets directly to the mobile node's
   care-of address also eliminates congestion at the mobile node's home
   agent and home link.  In addition, the impact of any possible failure
   of the home agent, the home link, or intervening networks leading to
   or from the home link is reduced, since these nodes and links are not
   involved in the delivery of most packets to the mobile node.

   Mobile IPv6 defines a new IPv6 destination option.  When a mobile
   node sends a packet while away from home, it could generally use
   a tunnel via the home agent to send this packet.  However, if the
   correspondent node in question has a binding for this mobile node,
   the mobile node can deliver packets directly to the correspondent
   node.  The mobile node sets the Source Address in the packet's IPv6
   header to one of its current care-of addresses, and include a "Home
   Address" destination option in the packet, giving the mobile node's
   home address.  By also including the Home Address destination option
   in each packet, the sending mobile node can communicate its home
   address to the correspondent node receiving this packet.  This makes



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   the use of the care-of address transparent above the Mobile IPv6
   support level (e.g., at the transport layer).

   It is possible that while a mobile node is away from home, some nodes
   on its home link may be reconfigured.  The router that was operating
   as the mobile node's home agent can be replaced by a different
   router serving the same role.  In this case, the mobile node may not
   know the IP address of its own home agent.  Mobile IPv6 provides a
   mechanism, known as "dynamic home agent address discovery", that
   allows a mobile node to dynamically discover the IP address of a
   home agent on its home link with which it may register its (primary)
   care-of address while away from home.  The mobile node sends an ICMP
   "Home Agent Address Discovery Request" message to the "Mobile IPv6
   Home-Agents" anycast address for its own home subnet prefix [16] and
   thus reaches one of the routers on its home link currently operating
   as a home agent.  This home agent then returns an ICMP "Home Agent
   Address Discovery Reply" message to the mobile node, including a list
   of home agents on the home link.  This procedure is specified in
   Sections 10.9 and 11.3.2.

   When a mobile node arrives to a new link and allocates a new care-of
   address, it is desirable for packets arriving at the previous care-of
   address to still reach the mobile node.  The mobile node may still
   accept packets at the previous address, if it is still attached to
   the previous link as well as the new one.  This is discussed in
   Section 11.4.3.  If the mobile node is no longer attached to the
   previous link, procedures described in Section 11.6.6 may be used to
   establish temporary tunneling from the previous link.


4.2. New IPv6 Protocols

   Mobile IPv6 defines a new IPv6 protocol, using the Mobility Header
   (see Section 6.1).  This Header is used to carry the following
   messages:

      Home Test Init
         Home Test
         Care-of Test Init
         Care-of Test

         These four messages are used to initiate the return routability
         procedure from the mobile node to a correspondent node.  This
         ensures authorization of subsequent Binding Updates, as
         described in Section 5.2.5.  The format of the messages are
         defined in Sections 6.1.3 through 6.1.6.

      Binding Update

         A Binding Update message is used by a mobile node to notify
         a correspondent node or the mobile node's home agent of its



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         current binding.  The Binding Update sent to the mobile node's
         home agent to register its primary care-of address is marked as
         a "home registration".  The Binding Update message is described
         in detail in Section 6.1.7.

      Binding Acknowledgement

         A Binding Acknowledgement message is used to acknowledge
         receipt of a Binding Update, if an acknowledgement was
         requested in the Binding Update.  The Binding Acknowledgement
         message is described in detail in Section 6.1.8.

      Binding Refresh Request

         A Binding Refresh Request message is used to request a mobile
         node to re-establish its binding with the correspondent node.
         This message is typically used when the cached binding is in
         active use but the binding's lifetime is close to expiration.
         The correspondent node may use, for instance, recent traffic
         and open transport layer connections as an indication of active
         use.  The Binding Refresh Request message is described in
         detail in Section 6.1.2.

      Binding Error

         The Binding Error message is used by the correspondent node to
         signal an error related to mobility, such as an inappropriate
         attempt to use the Home Address destination option without
         an existing binding.  This message is described in detail in
         Section 6.1.9.


4.3. New IPv6 Destination Options

   Mobile IPv6 defines a new IPv6 destination option, the Home
   Address destination option.  This option is described in detail in
   Section 6.3.


4.4. New IPv6 ICMP Messages

   Mobile IPv6 also introduces four new ICMP message types, two for use
   in the dynamic home agent address discovery mechanism, and two for
   renumbering and mobile configuration mechanisms.  As discussed in
   general in Section 4.1, the following two new ICMP message types are
   used for home agent address discovery:

    -  Home Agent Address Discovery Request, described in Section 6.5.

    -  Home Agent Address Discovery Reply, described in Section 6.6.




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   The next two message types are used for network renumbering
   and address configuration on the mobile node, as described in
   Section 10.9.1:

    -  Mobile Prefix Solicitation, described in Section 6.7.

    -  Mobile Prefix Advertisement, described in Section Section 10.9.3.


4.5. Conceptual Data Structures

   This document describes the Mobile IPv6 protocol in terms of the
   following three conceptual data structures:

      Binding Cache

         A cache, maintained by each IPv6 node, of bindings for other
         nodes.  A separate Binding Cache is maintained by each IPv6
         node for each of its IPv6 addresses.  When sending a packet,
         the Binding Cache is searched before the Neighbor Discovery
         conceptual Destination Cache [12].

         The Binding Cache for any one of a node's IPv6 addresses may
         contain at most one entry for each mobile node home address.
         The contents of all of a node's Binding Cache entries are
         cleared when it reboots.

         Binding Cache entries are marked either as "home registration"
         entries or "correspondent registration" entries.  Home
         registration entries are deleted when its binding lifetime
         expires, while other entries may be replaced at any time
         through a local cache replacement policy.

      Binding Update List

         A list, maintained by each mobile node, recording information
         for each Binding Update sent by this mobile node, for which the
         Lifetime sent in that Binding Update has not yet expired.  The
         Binding Update List includes all bindings sent by the mobile
         node:  those to correspondent nodes, those to the mobile node's
         home agent, and those to a home agent on the link on which the
         mobile node's previous care-of address is located.

      Home Agents List

         A list, maintained by each home agent and each mobile node,
         recording information about each home agent from which this
         node has recently received a Router Advertisement in which the
         Home Agent (H) bit is set.  This list is similar to the Default
         Router List conceptual data structure maintained by each host
         for Neighbor Discovery [12].



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         Each home agent maintains a separate Home Agents List for each
         link on which it is serving as a home agent; this list is used
         by a home agent in the dynamic home agent address discovery
         mechanism.  Each mobile node, while away from home, also
         maintains a Home Agents List, to enable it to notify a home
         agent on its previous link when it moves to a new link.


5. Overview of Mobile IPv6 Security

   This specification provides a number of security features.  These
   include the protection of Binding Updates both to home agents and
   correspondent nodes, and the protection of tunnels, home address
   information, and routing instructions in data packets.


5.1. Binding Updates to Home Agents

   Signaling between the mobile node and the home agent requires message
   integrity, correct ordering and replay protection.

   The mobile node and the home agent must have an security association
   to protect this signaling.  Authentication Header (AH) or
   Encapsulating Security Payload (ESP) can be used for integrity
   protection.  For ESP we require that a non-null authentication
   algorithm is applied.

   Mobile IPv6 provides its own ordering mechanism inside the Binding
   Update and Acknowledgement messages.  A sequence number field is
   used, as described in Section 6.1.7.

   In order to protect messages exchanged between the mobile node and
   the home agent with IPsec, appropriate security policy database
   entries must be created.  We need to avoid the possibility that a
   mobile node could use its security association to send a Binding
   Update on behalf of another mobile node using the same home agent.
   In order to do this, the security policy database entries MUST
   unequivocally identify a single SA for any given home address and
   home agent.  In order for the home address of the mobile node to be
   visible when the policy check is made, the mobile node MUST use the
   Home Address destination option in Binding Updates sent to the home
   agent.


5.2. Binding Updates to Correspondent Nodes

   Binding Updates to correspondent nodes can be protected by using data
   exchanged during the return routability procedure.  We will first
   discuss a number of preliminary concepts such as node keys, nonces,
   and cookies and the cryptographic functions used.  Section 5.2.5
   outlines the basic return routability procedure.  Section 5.2.6 shows



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   how the results of this procedure are used to authorize a Binding
   Update to a correspondent node.  Finally, Sections 5.2.7 and 5.2.8
   discuss some additional issues.


5.2.1. Node Keys

   Each correspondent node has a secret key, Kcn.  The correspondent
   node uses this key to verify that the cookies it receives in messages
   are those which it has created itself.  This key does not need to be
   shared with any other entity.

   A correspondent node can generate a fresh Kcn each time that it boots
   to avoid the need for secure persistent storage for Kcn.  Kcn can be
   either a fixed value or regularly updated.  Procedures for updating
   Kcn are discussed later in Section 5.2.7.

   Kcn consists of 20 octets.


5.2.2. Nonces

   Each correspondent node also generates nonces at regular intervals,
   for example every few minutes.  The nonces should be generated by
   using a random number generator that is known to have good randomness
   properties [1].  A correspondent node may use the same Kcn and nonce
   with all the mobiles it is in communication with, so that it does not
   need to generate and store a new nonce when a new mobile contacts it.

   Each nonce is identified by a nonce index.  Nonce indices are
   16-bit values that are e.g.  incremented each time a new nonce is
   created.  The index value is communicated in the protocol, so that if
   a nonce is replaced by new nonce during the run of a protocol, the
   correspondent node can distinguish messages that should be checked
   against the old nonce from messages that should be checked against
   the new nonce.  Strictly speaking, indices are not necessary in the
   authentication, but allow the correspondent node to efficiently find
   the nonce value that it used in creating a cookie.

   Correspondent nodes keep both the current nonce and a small set of
   old nonces.  Older values can be discarded, and messages using them
   will be rejected as replays.

   The specific nonce index values can not be used by mobile nodes to
   determine the validity of the nonce.  Expected validity times for
   the nonces values and the procedures for updating them are discussed
   later in Section 5.2.7.

   Nonce is an octet string of any length.  The recommended length is
   64-bit.




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5.2.3. Cookies

   Three different types of cookies are used in the protocol:

    -  Two mobile cookies are sent to the correspondent node from the
       mobile node, and later returned to the mobile node.  The mobile
       cookies are produced randomly, and used to verify that the
       response matches the request, and to ensure that parties who have
       not seen the request can not spoof responses.  One of the mobile
       cookies is sent in the HoTI message, and returned in the HoT
       message.  This is called the HoT cookie.  The other mobile cookie
       is sent in the CoTI message, and returned in the CoT message.
       This is called the CoT cookie.

    -  A home cookie sent to the mobile node from the correspondent node
       via the home agent.  Home cookies are produced cryptographically
       from nonces.

    -  A care-of cookie is similar to a home cookie, but sent directly
       to the mobile node from the correspondent node.

   A newly generated random number is typically used for each request
   that carries a mobile cookie.

   Home and care-of cookies are produced by the correspondent node, and
   they are based on the currently active secret keys and nonces of the
   correspondent node as well as the home or care-of address.  Such a
   cookie is valid as long as both the secret key and the nonce used to
   create it are valid.


5.2.4. Cryptographic Functions

   MAC_K(m) denotes a Message Authentication Code computed on message
   m with key K. In this specification, HMAC SHA1 function [26, 19] is
   used to compute these codes.

   Hash(m) denotes a hash of message m.  In this specification, the
   function used to compute the hash is SHA1 [19].


5.2.5. Return Routability Procedure

   The return routability signaling happens as follows:

     Mobile node                 Home agent           Correspondent node
          |                                                     |
          |  1a.                                                |
          |  Home Test Init(HoTI)                               |
          |  Src = home address,                                |
          |  Dst = correspondent     |                          |



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          |  Parameters:             |                          |
          |     - HoT cookie         |                          |
          |------------------------->|------------------------->|
          |                          |                          |
          |                                                     |
          |  1b.                                                |
          |  Care-of Test Init(CoTI)                            |
          |  Src = care-of address                              |
          |  Dst = correspondent                                |
          |  Parameters:                                        |
          |     - CoT cookie                                    |
          |---------------------------------------------------->|
          |                                                     |
          |                              2a.                    |
          |                              Home Test (HoT)        |
          |                              Src = correspondent,   |
          |                              Dst = home address     |
          |                              Parameters:            |
          |                               - HoT cookie          |
          |                          |    - home cookie         |
          |                          |    - home nonce index    |
          |<-------------------------|<-------------------------|
          |                          |                          |
          |                                                     |
          |                              2b.                    |
          |                              Care-of Test(CoT)      |
          |                              Src = correspondent,   |
          |                              Dst = care-of address  |
          |                              Parameters:            |
          |                               - CoT cookie          |
          |                               - care-of cookie      |
          |                               - care-of nonce index |
          |<----------------------------------------------------|
          |                                                     |

   The Home and Care-of Test Init messages are sent at the same
   time.  The correspondent node returns the Home and Care-of Test
   messages as quickly as possible, and perhaps nearly simultaneously,
   requiring very little processing.  Those four messages form the
   return routability procedure.  Due to the nearly simultaneous
   message delivery, the return routability procedure completes in about
   roundtrip between the mobile node and the correspondent.

      1a.  Home Test Init

         A mobile node sends a Home Test Init message to the
         correspondent node to acquire the home cookie.  The contents of
         the message can be summarized as follows:

           Src = home address
           Dst = correspondent



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           Parameters:
            - HoT cookie

         This message conveys the mobile node's home address to the
         correspondent node.  The mobile node also sends along a 64 bit
         HoT cookie that the correspondent node must return later.  The
         Home Test Init message is reverse tunneled through the home
         agent.

      1b.  Care-of Test Init

         The mobile node sends a Care-of Test Init message to the
         correspondent node to acquire the care-of cookie.  The contents
         of this message can be summarized as follows:

           Src = care-of address
           Dst = correspondent
           Parameters:
            - CoT cookie

         The second message conveys the mobile node's care-of address
         to the correspondent node.  The mobile node also sends along
         a 64 bit CoT cookie that the correspondent node must return
         later.  The Care-of Test Init message is sent directly to the
         correspondent node.

      2a.  Home Test

         This message is sent in response to a Home Test Init message.
         The contents of the message are:

           Src = correspondent
           Dst = home address
           Parameters:
           - HoT cookie
           - home cookie
           - home nonce index

         When the correspondent node receives the Home Test Init
         message, it generates a 64-bit home cookie as follows:

             home cookie = First64(MAC_Kcn(home address | nonce))

         The home cookie is formed from the first 64 bits of the MAC
         result.  The message is sent to the mobile node via the home
         agent; the protocol relies on the assumption that the route
         between the home agent and the mobile node is secure.  The home
         cookie also acts as a challenge to test that the mobile can
         receive messages sent to its home address.  Kcn is used in the
         production of home cookie in order to allow the correspondent
         node to verify that it generated the home and care-of cookies,



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         without forcing the correspondent node to remember a list of
         all cookies it has handed out.

         The HoT cookie from the mobile node is returned in the Home
         Test message, to ensure that the message comes from a node on
         the route between the home agent and the correspondent node.

         The home nonce index is delivered to the mobile node to later
         allow the correspondent node to efficiently find the nonce
         value that it used in creating the home cookie.

      2b.  Care-of Test

         This message is sent in response to a Care-of Test Init
         message.  The contents of the message are:

           Src = correspondent
           Dst = care-of address
           Parameters:
           - CoT cookie
           - care-of cookie
           - care-of nonce index

         The correspondent node also sends a challenge to the mobile's
         care-of address.  When the correspondent node receives the
         Care-of Test Init message, it generates a 64-bit care-of cookie
         as follows:

             care-of cookie = First64(MAC_Kcn(care-of address | nonce))

         The cookie is formed from the first 64 bits of the MAC result.
         The cookie is sent directly to the mobile node at its care-of
         address.  The CoT cookie from the from CoTI message is returne
         to ensure that the message comes from a node on the route to
         the correspondent node.

         The care-of nonce index is provided to identify the nonce used
         for the care-of cookie.  The home and care-of nonce indices are
         often the same in the Home and Care-of Test messages.

   When the mobile node has received both the Home and Care-of Test
   messages, the return routability procedure is complete.  As a result,
   the mobile node has the means to prove its authority to send a
   Binding Update to the correspondent node.  The mobile node hashes
   together the challenges to form a 16 octet session key Kbu:

    Kbu = Hash(home cookie | care-of cookie)

   Note that the correspondent node does not create any state specific
   to the mobile node, until it receives the Binding Update from that
   mobile node.  The correspondent node is unaware of the session



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   key Kbu, though it can recreate Kbu if it is presented the right
   addresses and nonce indices.


5.2.6. Applying Return Routability for Correspondent Bindings

   After the above procedure has completed, the mobile node can supply a
   Binding Update to the correspondent node.  An overview of the binding
   procedure is shown below.

     Mobile Node                                     Correspondent node
         |                                                     |
         | 1. Binding Update                                   |
         |    Src = care-of address, Dst = correspondent       |
         |    Parameters:                                      |
         |    - home address                                   |
         |    - a MAC                                          |
         |    - home nonce index                               |
         |    - care-of nonce index                            |
         |    - sequence number                                |
         |    - ...                                            |
         |---------------------------------------------------->|
         |                                                     |
         |                          2. Binding Acknowledgement |
         |                             (if requested)          |
         |                             Src = correspondent,    |
         |                             Dst = care-of address   |
         |                             Parameters:             |
         |                             - sequence number       |
         |                             - a MAC                 |
         |                             - ...                   |
         |<----------------------------------------------------|
         |                                                     |

   Message 1 actually creates a binding, and message 2 is optional.  The
   correspondent binding procedure consists of the return routability
   procedure followed by the messages 1 and 2.

      1.  Binding Update

         The mobile node uses the created session key Kbu to authorize
         the Binding Update.  The contents of the message are as
         follows:

               Src = care-of address
               Dst = correspondent
               Parameters:
               - home address
               - MAC_Kbu(care-of address | correspondent node address | BU)
               - home nonce index
               - care-of nonce index



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               - sequence number
               - ...

         The Binding Update message contains Nonce Index option, so that
         the correspondent node knows which home and care-of nonces to
         use to recompute the session key.  "BU" is the content of the
         Binding Update message, excluding (1) the IP header, (2) any
         extension headers between the IP header the Mobility Header,
         and (3) the Authenticator field inside the Binding Update.  The
         first 96 bits from the MAC result are used as the Authenticator
         field.  A sequence number will be used to match an eventual
         acknowledgement with this message.  The sequence numbers
         start from a random value.  The three dots represent all the
         remaining (not security related) information in the message.

         Once the correspondent node has verified the MAC, it can create
         a binding cache entry for the mobile.

      2.  Binding Acknowledgement

         The Binding Update is optionally acknowledged by the
         correspondent node.  The contents of the message are as
         follows:

           Src = correspondent
           Dst = care-of address
           Parameters:
           - sequence number
           - MAC_Kbu(care-of address | correspondent node address | BA)
           - ...

         The Binding Acknowledgement contains the same sequence number
         as the Binding Update did.  "BA" is the content of the Binding
         Acknowledgement message, excluding (1) the IP header, (2)
         any extension headers between the IP header the Mobility
         Header, and (3) the Authenticator field inside the Binding
         Acknowledgement.  The first 96 bits from the MAC result are
         used as the Authenticator field.  The three dots represent
         all the remaining (not security related) information in the
         message.

   Bindings established with correspondent nodes using the return
   routability procedure MUST NOT exceed MAX_RR_BINDING_LIFE seconds.

   The value in the Source Address field in the IPv6 header carrying
   the Binding Update message is normally also the care-of address
   which is used in the binding.  However, a different care-of address
   MAY be specified by including an Alternate Care-of Address mobility
   option in the Binding Update message (see Section 6.2.5).  When such
   message is sent to the correspondent node and the return routability
   procedure is used as the authorization method, the Care-of Test Init



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   and Care-of Test messages MUST have been performed for the address in
   the Alternate Care-of Address option (not the Source Address).  The
   nonce indices MAC value MUST be based on information gained in this
   test.


5.2.7. Updating Node Keys and Nonces

   An update of Kcn can be done at the same time as an update of a
   nonce, so that the nonce index identifies both the nonce and the key.
   Old Kcn values have to be therefore remembered as long as old nonce
   values.

   Before sending a Binding Update, the mobile node has to wait for both
   the Home and Care-of Cookies to arrive.  Due to resource limitations,
   rapid deletion of bindings, or reboots it can not be guaranteed that
   the cookies are still fresh and acceptable when the correspondent
   node uses them in the processing of the Binding Update.  If the
   cookies have become too old, the correspondent node replies with
   an an error code in the Binding Acknowledgement.  The mobile node
   can then retry the return routability procedure.  However, it is
   recommended that correspondent nodes try to keep these cookies
   acceptable as long as possible and SHOULD NOT accept them beyond
   MAX_COOKIE_LIFE seconds.

   Given that the cookies are normally expected to be usable for
   some time, the mobile node MAY use them beyond a single run of the
   return routability procedure.  A fast moving mobile node may reuse
   a recent Home Cookie from a correspondent node when moving to a new
   location, and just acquire a new Care-of Cookie to show routability
   in the new location.  While this does not save roundtrips due to the
   parallel nature of the home and care-of return routability tests, the
   roundtrip through the home agent may be longer, and consequently this
   optimization is often useful.  A mobile node that has multiple home
   addresses, may also use the same Care-of Cookie for Binding Updates
   concerning all of these addresses.


5.2.8. Preventing Replay Attacks

   The return routability procedure also protects the participants
   against replayed Binding Updates through the use of the sequence
   number and a MAC. Care must be taken when removing bindings at
   the correspondent node, however.  Correspondent nodes must retain
   bindings and the associated sequence number information at least as
   long as the nonces used in the authorization of the binding are still
   valid.  The correspondent node can, for instance, change the nonce
   often enough to ensure that the nonces used when removed entries
   were created are no longer valid.  If many such deletions occur
   the correspondent node can batch them together to avoid having to
   increment the nonce index too often.



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5.3. Payload Packets

   Payload packets exchanged with mobile nodes can be protected in the
   usual manner, in the same way as stationary hosts can protect them.
   However, Mobile IPv6 introduces the Home Address destination option,
   a Routing Header, and tunneling headers in the payload packets.  In
   the following we define the security measures taken to protect these,
   and to prevent their use in attacks against other parties.

   This specification limits the use of the Home Address destination
   option to the situation where the correspondent node already has a
   binding cache entry for the given home address.  This avoids the use
   of the Home Address option in attacks described in Section 14.1.  We
   also allow the option to be used when the packet containing it has
   been protected by IPsec.

   Mobile IPv6 uses a Mobile IPv6 specific type of a Routing Header.
   This type provides the necessary functionality but does not open
   vulnerabilities discussed in Section 14.1.

   Tunnels between the mobile node and the home agent are protected by
   ensuring proper use of source addresses, and optional cryptographic
   protection.  The mobile node verifies that the outer IP address
   corresponds to its home agent.  The home agent verifies that the
   outer IP address corresponds to the current location of the mobile
   node (Binding Updates sent to the home agents are secure).  These
   measures protect the tunnels against vulnerabilities discussed in
   Section 14.1.

   For tunneled traffic to and from the mobile node, encapsulating
   the traffic inside IPsec AH or ESP offers an optional mechanism to
   protect the integrity and confidentiality of the traffic against
   on-path attackers.


6. New IPv6 Protocols, Message Types, and Destination Option

6.1. Mobility Header

   The Mobility Header is used by mobile nodes, correspondent nodes, and
   home agents in all messaging related to the creation and management
   of bindings.  Sections 6.1.2 through 6.1.9 describe the message types
   used in this protocol.  These sections also define which addresses to
   use in the IPv6 header in these messages.










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6.1.1. Format

   The Mobility Header is identified by a Next Header value of TBD in
   the immediately preceding header, and has the following format:

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Payload Proto |  Header Len   |            MH Type            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Checksum            |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
    |                                                               |
    .                                                               .
    .                       Message Data                            .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Payload Proto

         8-bit selector.  Identifies the type of header immediately
         following the Mobility Header.  Uses the same values as the
         IPv6 Next Header field [11].

         This field is intended to be used by a future specification
         of piggybacking binding messages on payload packets (see
         Section C.1).

         Implementations conforming to this specification SHOULD set the
         payload protocol type to NO_NXTHDR (59 decimal).

      Header Len

         8-bit unsigned integer.  Length of the Mobility Header in units
         of 8 octets, including the the Payload Proto, MH Type, Header
         Len, Checksum, and Message Data fields.

         We require that the Mobility Header length is a multiple of 8
         octets.

      MH Type

         16-bit selector.  Identifies the particular mobility message
         in question.  Current values are specified in Sections 6.1.2
         to 6.1.9.  An unrecognized MH Type field causes an error to be
         sent to the source.

      Checksum

         16-bit unsigned integer.  This field contains the checksum of
         the Mobility Header.  The checksum is calculated from the octet
         string consisting of a "pseudo-header" followed by the entire



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         Mobility Header starting with the Payload Proto field.  The
         checksum is the 16-bit one's complement of the one's complement
         sum of this string.

         The pseudo-header contains IPv6 header fields, as specified
         in Section 8.1 of [11].  The Next Header value used in the
         pseudo-header is TBD. The addresses used in the pseudo-header
         are the addresses that appear in the Source and Destination
         Address fields in the IPv6 packet carrying the Mobility Header.
         For computing the checksum, the checksum field is set to zero.

      Message Data

         A variable length field containing the data specific to the
         indicated Mobility Header type.

   Mobile IPv6 also defines a number of "mobility options" for use
   within these messages; if included, any options MUST appear after
   the fixed portion of the message data specified in this document.
   The presence of such options will be indicated by the Header Len
   field within the message.  When the Header Len is greater than the
   length required for the message specified here, the remaining octets
   are interpreted as mobility options.  These options include padding
   options that can be used to ensure that other options are aligned
   properly, and that the total length of the message is divisible by
   8.  The encoding and format of defined options are described in
   Section 6.2.

   Alignment requirements for the Mobility Header are the same as for
   any IPv6 protocol Header.  That is, they MUST be aligned on an
   8-octet boundary.


6.1.2. Binding Refresh Request (BRR) Message

   The Binding Refresh Request (BRR) message is used to request a
   mobile node's binding from the mobile node.  It is sent according
   to the rules in Section 9.4.5.  When a mobile node receives a
   packet containing a Binding Refresh Request message and there
   already exists a Binding Update List entry for the source of the
   Binding Refresh Request, it MAY start a return routability procedure
   (see Section 5.2) if it believes the amount of traffic with the
   correspondent justifies the use of route optimization.  Note that
   the mobile node SHOULD NOT respond to Binding Refresh Requests from
   previously unknown correspondent nodes due to Denial-of-Service
   concerns.








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   The Binding Refresh Request message uses the MH Type value 0.  When
   this value is indicated in the MH Type field, the format of the
   Message Data field in the Mobility Header is as follows:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |          Reserved             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Mobility options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Reserved

         16-bit field reserved for future use.  The value MUST be
         initialized to zero by the sender, and MUST be ignored by the
         receiver.

      Mobility options

         Variable-length field of such length that the complete Mobility
         Header is an integer multiple of 8 octets long.  Contains one
         or more TLV-encoded mobility options.  The encoding and format
         of defined options are described in Section 6.2.  The receiver
         MUST ignore and skip any options which it does not understand.

         There MAY be additional information, associated with this
         Binding Refresh Request message, that need not be present in
         all Binding Requests sent.  This use of mobility options also
         allows for future extensions to the format of the Binding
         Refresh Request message to be defined.  The following options
         are valid in a Binding Refresh Request message:

          -  Unique Identifier Option

          -  Binding Authorization option

   If no actual options are present in this message, no padding is
   necessary and the Header Length field will be set to 1.


6.1.3. Home Test Init (HoTI) Message

   A mobile node uses the Home Test Init (HoTI) message to initiate
   the return routability procedure and request a home cookie from a
   correspondent node (see Section 11.5.1).  The Home Test Init message
   uses the MH Type value 1.  When this value is indicated in the MH





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   Type field, the format of the Message Data field in the Mobility
   Header is as follows:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                          HoT cookie                           +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                       Mobility Options                        .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Reserved

         16-bit field reserved for future use.  This value MUST be
         initialized to zero by the sender, and MUST be ignored by the
         receiver.

      HoT cookie

         64-bit field which contains a random value, the HoT cookie.

      Mobility options

         Variable-length field of such length that the complete Mobility
         Header is an integer multiple of 8 octets long.  Contains
         one or more TLV-encoded mobility options.  The receiver MUST
         ignore and skip any options which it does not understand.  This
         specification does not define any options valid for the Home
         Test Init message.

   If no actual options are present in this message, no padding is
   necessary and the Header Length field will be set to 2.

   This message is sent with the Source Address set to the home
   address of the mobile node, and the Destination Address set to the
   correspondent node's address.  The message is tunneled through the
   home agent when the mobile node is away from home.  Such tunneling
   SHOULD employ IPsec ESP in tunnel mode between the home agent and
   the mobile node.  This protection is indicated by the IPsec policy
   data base.  The protection of Home Test Init messages is unrelated
   to the requirement to protect regular payload traffic, which MAY
   use such tunnels as well.  A packet that includes a Home Test Init
   message MUST NOT include a Home Address destination option between
   the Mobility Header and the preceding IPv6 header.




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6.1.4. Care-of Test Init (CoTI) Message

   A mobile node uses the Care-of Test Init (CoTI) message to initiate
   the return routability procedure and request a care-of cookie from
   a correspondent node (see Section 11.5.1).  The Care-of Test Init
   message uses the MH Type value 2.  When this value is indicated
   in the MH Type field, the format of the Message Data field in the
   Mobility Header is as follows:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |          Reserved             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                          CoT cookie                           +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Mobility Options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Reserved

         16-bit field reserved for future use.  The value MUST be
         initialized to zero by the sender, and MUST be ignored by the
         receiver.

      CoT cookie

         64-bit field which contains a random value, the CoT cookie.

      Mobility options

         Variable-length field of such length that the complete Mobility
         Header is an integer multiple of 8 octets long.  Contains
         one or more TLV-encoded mobility options.  The receiver MUST
         ignore and skip any options which it does not understand.  This
         specification does not define any options valid for the Care-of
         Test Init message.

   If no actual options are present in this message, no padding is
   necessary and the Header Length field will be set to 2.

   This message is always sent with the Source Address set to the
   care-of address of the mobile node, and is sent directly to the
   correspondent node.  A packet that includes a Care-of Test Init
   message MUST NOT include a Home Address destination option between
   the Mobility Header and the preceding IPv6 header.




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6.1.5. Home Test (HoT) Message

   The Home Test (HoT) message is a response to the HoTI message,
   and is sent from the correspondent node to the mobile node (see
   Section 5.2.5).  The Home Test message uses the MH Type value 3.
   When this value is indicated in the MH Type field, the format of the
   Message Data field in the Mobility Header is as follows:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |      Home Nonce Index         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                           HoT cookie                          +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                          Home Cookie                          +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Mobility options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Home Nonce Index

         This field will be echoed back by the mobile node to the
         correspondent node in a subsequent binding update.

      HoT cookie

         64-bit field which contains the HoT cookie.

      Home Cookie

         This field contains the 64 bit home cookie in the return
         routability procedure; it is the first of two cookies which
         are to be processed to form a key which is then used to
         authenticate a binding update.

      Mobility options

         Variable-length field of such length that the complete Mobility
         Header is an integer multiple of 8 octets long.  Contains
         one or more TLV-encoded mobility options.  The receiver MUST
         ignore and skip any options which it does not understand.  This
         specification does not define any options valid for the Home
         Test message.




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   If no actual options are present in this message, no padding is
   necessary and the Header Length field will be set to 3.  This
   message is always sent with the Destination Address set to the home
   address of the mobile node, Source Address set to the address of the
   correspondent node, and is tunneled through the home agent when the
   mobile node is away from home.  Note that the Home Test message is
   always sent to the home address of the mobile node, even when there
   is an existing binding for the mobile node.  The tunneling between
   the home agent and the mobile node SHOULD employ IPsec ESP in tunnel
   mode.  This protection is indicated by the IPsec policy data base.


6.1.6. Care-of Test (CoT) Message

   The Care-of Test (CoT) message is a response to the CoTI message,
   and is sent from the correspondent node to the mobile node (see
   Section 11.5.2).  The Care-of Test message uses the MH Type value 4.
   When this value is indicated in the MH Type field, the format of the
   Message Data field in the Mobility Header is as follows:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |      Care-of Nonce Index      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                          CoT cookie                           +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                        Care-of Cookie                         +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Mobility Options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Reserved

         The two 16-bit fields are reserved for future use.  These
         values MUST be initialized to zero by the sender, and MUST be
         ignored by the receiver.

      Care-of Nonce Index

         This field will be echoed back by the mobile node to the
         correspondent node in a subsequent binding update.






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      CoT cookie

         64-bit field which contains the CoT cookie.

      Care-of Cookie

         This field contains the 64 bit care-of cookie in the return
         routability procedure; it is the second of two cookies which
         are to be processed to form a key which is then used to
         authenticate a binding update.

      Mobility options

         Variable-length field of such length that the complete Mobility
         Header is an integer multiple of 8 octets long.  Contains
         one or more TLV-encoded mobility options.  The receiver MUST
         ignore and skip any options which it does not understand.  This
         specification does not define any options valid for the Care-of
         Test message.

   The CoT message is always sent with the Source Address set to the
   address of the correspondent node, and the Destination Address set to
   the care-of address of the mobile node; it is sent directly to the
   mobile node.  If no actual options are present in this message, no
   padding is necessary and the Header Len field will be set to 3.


6.1.7. Binding Update (BU) Message

   The Binding Update (BU) message is used by a mobile node to notify
   other nodes of a new care-of address for itself.  A packet containing
   a Binding Update message is sent with the Source Address set to the
   care-of address of the mobile node and the Destination Address set to
   the correspondent node's address.

   The Binding Update message uses the MH Type value 5.  When this value
   is indicated in the MH Type field, the format of the Message Data
   field in the Mobility Header is as follows:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |          Sequence #           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |A|H|S|D|L|      Reserved       |           Lifetime            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Mobility options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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      Acknowledge (A)

         The Acknowledge (A) bit is set by the sending mobile node to
         request a Binding Acknowledgement (Section 6.1.8) be returned
         upon receipt of the Binding Update.

      Home Registration (H)

         The Home Registration (H) bit is set by the sending mobile
         node to request that the receiving node should act as this
         node's home agent.  The destination of the packet carrying this
         message MUST be that of a router sharing the same subnet prefix
         as the home address of the mobile node in the binding.

      Single Address Only (S)

         If the `S' bit is set, the mobile node requests that the home
         agent make no changes to any other Binding Cache entry except
         for the particular one containing the home address specified
         in the Home Address destination option.  This disables home
         agent processing for other related addresses, as is described
         in Section 10.2.

      Duplicate Address Detection (D)

         The Duplicate Address Detection (D) bit is set by the sending
         mobile node to request that the receiving node (the mobile
         node's home agent) perform Duplicate Address Detection [13]
         on the mobile node's home link for the home address in this
         binding.  This bit is only valid when the Home Registration (H)
         and Acknowledge (A) bits are also set, and MUST NOT be set
         otherwise.

      Link-Local Address Compatibility (L)

         The Link-Local Address Compatibility (L) bit is set when the
         home address reported by the mobile node has the same interface
         identifier (IID) as the mobile node's link-local address.

      Reserved

         These fields are unused.  They MUST be initialized to zero by
         the sender and MUST be ignored by the receiver.

      Sequence #

         A 16-bit number used by the receiving node to sequence Binding
         Updates and by the sending node to match a returned Binding
         Acknowledgement with this Binding Update.





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      Lifetime

         16-bit unsigned integer.  The number of time units remaining
         before the binding MUST be considered expired.  A value of all
         one bits (0xffffffff) indicates infinity.  A value of zero
         indicates that the Binding Cache entry for the mobile node MUST
         be deleted.  One time unit is 16 seconds.

      Mobility options

         Variable-length field of such length that the complete Mobility
         Header is an integer multiple of 8 octets long.  Contains one
         or more TLV-encoded mobility options.  The encoding and format
         of defined options are described in Section 6.2.  The receiver
         MUST ignore and skip any options which it does not understand.

         The following options are valid in a Binding Update message:

          -  Unique Identifier option

          -  Binding Authorization Data option

          -  Nonce Indices option.

          -  Alternate Care-of Address option

   If no actual options are present in this message, no padding is
   necessary and the Header Len field will be set to 4.

   A Binding Update to the home agent MUST include the Home Address
   destination option if the Source Address field in the IPv6 header is
   not the home address of the mobile node.

   The care-of address MUST NOT be any IPv6 address which is prohibited
   for use within a Routing Header; thus multicast addresses, the
   unspecified address, loop-back address, and link-local addresses
   are excluded.  Binding Updates indicating any such excluded care-of
   address MUST be silently discarded.

   The deletion of a binding can be indicated by setting the Lifetime
   field to 0 or by setting the care-of address equal to the home
   address.  Correspondent nodes SHOULD NOT expire the binding cache
   entry before the lifetime expires, if any application hosted by the
   correspondent node is still likely to require communication with the
   mobile node.  A binding cache entry that is deallocated prematurely
   might cause subsequent packets to be dropped from the mobile node,
   if they contain the Home Address destination option.  This situation
   is recoverable, since an error message is sent to the mobile node;
   however, it causes unnecessary delay in the communications.





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6.1.8. Binding Acknowledgement (BA) Message

   The Binding Acknowledgement message is used to acknowledge receipt
   of a Binding Update message (Section 6.1.7).  When a node receives
   a packet containing a Binding Update message, with this node being
   the destination of the packet, this node MUST return a Binding
   Acknowledgement to the mobile node, if the Acknowledge (A) bit is
   set in the the Binding Update.  The Binding Acknowledgement message
   is sent to the Source Address of the Binding Update message which
   is being acknowledged.  The packet includes a Routing header if
   the Source Address was not the home address of the mobile node, as
   described in Sections 10.2 and 9.4.4.  The Source Address of the
   Binding Acknowledgement is the Destination Address from the Binding
   Update.

   The Binding Acknowledgement message has the MH Type value 6.  When
   this value is indicated in the MH Type field, the format of the
   Message Data field in the Mobility Header is as follows:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |    Status     |   Reserved    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Sequence #          |           Lifetime            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Mobility options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Reserved

         These fields are unused.  They MUST be initialized to zero by
         the sender and MUST be ignored by the receiver.

      Status

         8-bit unsigned integer indicating the disposition of the
         Binding Update.  Values of the Status field less than 128
         indicate that the Binding Update was accepted by the receiving
         node.  Values greater than or equal to 128 indicate that
         the Binding Update was rejected by the receiving node.  The
         following Status values are currently defined:

              0   Binding Update accepted
            128   Reason unspecified
            129   Administratively prohibited
            130   Insufficient resources
            131   Home registration not supported
            132   Not home subnet



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            133   Not home agent for this mobile node
            134   Duplicate Address Detection failed
            135   Sequence number out of window
            136   Route optimization unnecessary due to low traffic
            137   Invalid authenticator
            138   Expired Home Nonce Index
            139   Expired Care-of Nonce Index

         Up-to-date values of the Status field are to be specified in
         the IANA registry of assigned numbers [18].

      Sequence #

         The Sequence Number in the Binding Acknowledgement is copied
         from the Sequence Number field in the Binding Update.  It used
         by the mobile node in matching this Acknowledgement with an
         outstanding Binding Update.

      Lifetime

         The granted lifetime, in time units of 4 seconds, for which
         this node SHOULD retain the entry for this mobile node in its
         Binding Cache.  A value of all one bits (0xffffffff) indicates
         infinity.

         The value of this field is undefined if the Status field
         indicates that the Binding Update was rejected.

      Mobility options

         Variable-length field of such length that the complete Mobility
         Header is an integer multiple of 8 octets long.  Contains one
         or more TLV-encoded mobility options.  The encoding and format
         of defined options are described in Section  6.2.  The receiver
         MUST ignore and skip any options which it does not understand.

         There MAY be additional information, associated with this
         Binding Acknowledgement message, that need not be present
         in all Binding Acknowledgements sent.  This use of mobility
         options also allows for future extensions to the format of the
         Binding Acknowledgement message to be defined.  The following
         options are valid for the Binding Acknowledgement message:

          -  Binding Authorization Data option

          -  Binding Refresh Advice option

   If no options are present in this message, 4 bytes of padding is
   necessary and the Header Len field will be set to 2.





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   The Binding Acknowledgement is sent to the source address of the
   Binding Update message, regardless of whether the Binding Update
   succeeded or failed.  No Routing Headers are added to the message.


6.1.9. Binding Error (BE) Message

   The Binding Error (BE) message is used by the correspondent node to
   signal an error related to mobility, such as an inappropriate attempt
   to use the Home Address destination option without an existing
   binding.  A packet containing a Binding Error message is sent to the
   source address of the offending packet.  For instance, in the case
   of the Home Address destination option error, the packet is the one
   that contained the Home Address destination option and therefore
   the Binding Error message is sent to the care-of address of the
   mobile node.  The source address of the Binding Error message is the
   correspondent node's address.

   The Binding Error message uses the MH Type value 7.  When this value
   is indicated in the MH Type field, the format of the Message Data
   field in the Mobility Header is as follows:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |     Status    |   Reserved    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                                                               |
    +                          Home Address                         +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    .                        Mobility Options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Status

         8-bit unsigned integer indicating the reason for this message.
         The following such Status values are currently defined:

              1   Home Address destination option used without a binding
              2   Received message had an unknown value for the MH Type
                  field







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      Reserved

         A 8-bit field reserved for future use.  The value MUST be
         initialized to zero by the sender, and MUST be ignored by the
         receiver.

      Home Address

         The home address that was contained in the Home Address
         destination option.  The mobile node uses this information to
         determine which binding does not exist, in cases where the
         mobile node has several home addresses.

      Mobility options

         Variable-length field of such length that the complete Mobility
         Header is an integer multiple of 8 octets long.  Contains one
         or more TLV-encoded mobility options.  The receiver MUST ignore
         and skip any options which it does not understand.

         There MAY be additional information, associated with this
         Binding Error message, that need not be present in all Binding
         Error messages sent.  This use of mobility options also allows
         for future extensions to the format of the Binding Error
         message to be defined.  The encoding and format of defined
         options are described in Section 6.2.  This specification does
         not define any options valid for the Binding Error message.

   If no actual options are present in this message, no padding is
   necessary and the Header Len field will be set to 3.


6.2. Mobility Options

6.2.1. Format

   In order to allow optional fields that may not be needed in every use
   of any given Mobility Header, and to allow future extensions to the
   format of these messages to be defined, the Mobility Header messages
   defined in this document can include one or more mobility options.

   Such options are included in the data portion of the message itself,
   after the fixed portion of the message data specified in the message
   subsections of section 6.1.

   The presence of such options will be indicated by the Header Len of
   the Mobility Header.







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   These options are encoded within the remaining space of the message
   data for that message, using a type-length-value (TLV) format as
   follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Option Type  |  Option Len   |   Option Data...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Option Type

         8-bit identifier of the type of mobility option.  When
         processing a Mobility Header containing an option for which
         the Option Type value is not recognized by the receiver,
         the receiver MUST quietly ignore and skip over the option,
         correctly handling any remaining options in the message.

      Option Len

         8-bit unsigned integer.  Length of this mobility option, in
         octets.  The Option Len does not include the length of the
         Option Type and Option Len fields.

      Option Data

         A variable length field that contains data specific to the
         option.

   The following subsections specify the Option types which are
   currently defined for use in the Mobility Header.

   Implementations MUST silently ignore any mobility options that they
   do not understand.


6.2.2. Pad1

   The Pad1 option does not have any alignment requirements.  Its format
   is as follows:

     0
     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |       0       |
    +-+-+-+-+-+-+-+-+

   NOTE! the format of the Pad1 option is a special case -- it has
   neither Option Len nor Option Data fields.





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   The Pad1 option is used to insert one octet of padding in the
   Mobility Options area of a Mobility Header.  If more than one octet
   of padding is required, the PadN option, described next, should be
   used rather than multiple Pad1 options.


6.2.3. PadN

   The PadN option does not have any alignment requirements.  Its format
   is as follows:

     0                   1
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
    |       1       | Option Len | Option Data
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -

   The PadN option is used to insert two or more octets of padding in
   the Mobility Options area of a Mobility Header message.  For N octets
   of padding, the Option Len field contains the value N-2, and the
   Option Data consists of N-2 zero-valued octets.  Option data MUST be
   ignored by the receiver.


6.2.4. Unique Identifier

   The Unique Identifier option has the alignment requirement of 2n.
   Its format is as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    Type = 2   |  Length = 2   |       Unique Identifier       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Unique Identifier option is valid only in Binding Refresh
   Request, and Binding Update messages.  The Unique Identifier field
   contains a 16-bit value that serves to uniquely identify a Binding
   Request among those sent by this Source Address, and to allow the
   Binding Update to identify the specific Binding Refresh Request to
   which it responds.













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6.2.5. Alternate Care-of Address

   The Alternate Care-of Address option has an alignment requirement of
   8n+6.  Its format is as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |   Type = 3    |  Length = 16  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                                                               |
    +                   Alternate Care-of Address                   +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Alternate Care-of Address option is valid only in Binding Update
   message.  The Alternate Care-of Address field contains an address to
   use as the care-of address for the binding, rather than using the
   Source Address of the packet as the care-of address.


6.2.6. Nonce Indices

   The Nonce Indices option has an alignment requirement of 2n.  Its
   format is as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |    Type = 4   |   Length = 4  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Home Nonce Index      |     Care-of Nonce Index       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Nonce Indices option is valid only in the Binding Update message,
   and only when present together with an Binding Authorization Data
   option.

   The Home Nonce Index field tells the correspondent node that receives
   the message which of the challenge values (Ni) are to be used to
   authenticate the Binding Update.

   The Care-of Nonce Index field tells the correspondent node that
   receives the message which of the challenge values (Nj) are to be
   used to authenticate the Binding Update.





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6.2.7. Binding Authorization Data

   The Binding Authorization Data option has an alignment requirement of
   4n+2.  Its format is as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |    Type = 5   |     Len       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                         Authenticator                         |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Binding Authorization Data option is valid only in the Binding
   Refresh Request, Binding Update, and Binding Acknowledgment messages.

   The Option Len field contains the length of the authenticator in
   octets.

   The Authenticator field contains a cryptographic value which can be
   used to determine that the message in question comes from the right
   authority.  Rules for calculating this value depend on the used
   authorization procedure.  This specification gives the rules only for
   the return routability procedure.  For this procedure, this option
   can only appear in a Binding Update message and rules for calculating
   the Authenticator value are described in Section 6.1.7.


6.2.8. Binding Refresh Advice

   The Binding Refresh Advice option has an alignment requirement of 2n.
   Its format is as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |    Type = 7   |   Length = 2  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Refresh Interval        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Binding Refresh Advice option is only valid in the Binding
   Acknowledgement message, and only on Acknowledgements sent from
   the mobile node's home agent in reply to a home registration.  The
   Refresh Interval is measured in seconds, and indicates how long
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   home agent.  The Refresh Interval MUST be set to indicate a smaller
   time interval than the Lifetime value of the Binding Acknowledgement.


6.3. Home Address Destination Option

   The Home Address destination option is used in a packet sent by a
   mobile node while away from home, to inform the recipient of the
   mobile node's home address.

   Multicast addresses, link-local addresses, loopback addresses, IPv4
   mapped addresses, and the unspecified address, MUST NOT be used
   within a Home Address option.  The Home Address Option MUST NOT
   appear more than once in any given packet, except inside the payload
   part of the packet if tunneling is involved.

   The Home Address option is encoded in type-length-value (TLV) format
   as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |  Option Type  | Option Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                          Home Address                         +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Option Type

         201 = 0xC9

      Option Length

         8-bit unsigned integer.  Length of the option, in octets,
         excluding the Option Type and Option Length fields.  This field
         MUST be set to 16.

      Home Address

         The home address of the mobile node sending the packet.

   IPv6 requires that options appearing in a Hop-by-Hop Options
   header or Destination Options header be aligned in a packet so that
   multi-octet values within the Option Data field of each option fall
   on natural boundaries (i.e., fields of width n octets are placed at



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   an integer multiple of n octets from the start of the header, for
   n = 1, 2, 4, or 8) [11].  The alignment requirement [11] for the Home
   Address option is 8n+6.

   The three highest-order bits of the Option Type are encoded to
   indicate specific processing of the option [11].  For the Home
   Address option, these three bits are set to 110, indicating that any
   IPv6 node processing this option that does not recognize the Option
   Type must discard the packet and, only if the packet's Destination
   Address was not a multicast address, return an ICMP Parameter
   Problem, Code 2, message to the packet's Source Address; and that the
   data within the option cannot change en-route to the packet's final
   destination.

   A packet MUST NOT contain more than one Home Address option, except
   that an encapsulated packet [15] MAY contain a separate Home Address
   option associated with each encapsulating IP header.

   The Home Address option MUST be placed as follows:

    -  After the Routing Header, if that header is present

    -  Before the Fragment Header, if that header is present

    -  Before the AH Header or ESP Header, if either one of those
       headers is present

   The inclusion of a Home Address destination option in a packet
   affects the receiving node's processing of only this single packet;
   no state is created or modified in the receiving node as a result
   of receiving a Home Address option in a packet.  In particular, the
   presence of a Home Address option in a received packet MUST NOT alter
   the contents of the receiver's Binding Cache and MUST NOT cause any
   changes in the routing of subsequent packets sent by this receiving
   node.



















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6.4. Routing Header type 2

   Mobile IPv6 uses a Routing header to carry the Home Address for
   packets sent from a correspondent node to a mobile node.  The Care of
   Address of the mobile node is carried in the IPv6 destination field.

   The new Routing header uses a different type than defined for
   "regular" IPv6 source routing, enabling firewalls to apply different
   rules to source routed packets than to MIPv6.  This Routing header
   type (Type 2) is restricted to carry only one IPv6 address.  All IPv6
   nodes which process this Routing header MUST verify that the address
   contained within is the node's own home address in order to prevent
   packets from being forwarded outside the node.


6.4.1. Routing Header Packet format

   The Type 2 Routing header has the following format:

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Next Header  | Hdr Ext Len=2 | Routing Type=2|Segments Left=1|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            Reserved                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                                                               |
    +                         Home Address                          +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Next Header

         8-bit selector.  Identifies the type of header immediately
         following the Routing header.  Uses the same values as the IPv6
         Next Header field [11].

      Hdr Ext Len

         8-bit unsigned integer.  Length of the Routing header in
         8-octet units, not including the first 8 octets.  For the Type
         2 Routing header, Hdr Ext Len is always 2.

      Routing Type

         8-bit unsigned integer that contains the value 2.






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      Segments Left

         8-bit unsigned integer.  Number of route segments remaining;
         i.e., number of explicitly listed intermediate nodes still to
         be visited before reaching the final destination.  Packets
         transmitted through an interface have Segments left is always 1
         in this type of Routing header.

      Reserved

         32-bit reserved field.  Initialized to zero for transmission,
         and ignored on reception.

      Home Address

         The Home Address of the destination Mobile Node.

   The ordering rules for extension headers in an IPv6 packet are
   described in Section 4.1 of [11].  The new Routing header (Type 2)
   defined for Mobile IPv6 follows the same ordering as other routing
   headers.  If more than one Routing header (e.g., both a Type 0 and a
   Type 2 Routing header are present), the Type 2 Routing header should
   follow all other Routing headers.  Otherwise the order of routing
   headers is independent of their type and follows [11].

   In addition, the general procedures defined by IPv6 for Routing
   headers suggest that a received Routing header MAY be automatically
   "reversed" to construct a Routing header for use in any response
   packets sent by upper-layer protocols, if the received packet is
   authenticated [6].  This MUST NOT be done automatically for Type 2
   Routing headers.


6.5. ICMP Home Agent Address Discovery Request Message

   The ICMP Home Agent Address Discovery Request message is used by a
   mobile node to initiate the dynamic home agent address discovery
   mechanism, as described in Section 11.3.2.  The mobile node sends
   a Home Agent Address Discovery Request message to the "Mobile IPv6
   Home-Agents" anycast address for its own home subnet prefix [16].

    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      |     Code      |            Checksum           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Identifier           |            Reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






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      Type

         150 <To Be Assigned by IANA>

      Code

         0

      Checksum

         The ICMP checksum [14].

      Identifier

         An identifier to aid in matching Home Agent Address Discovery
         Reply messages to this Home Agent Address Discovery Request
         message.

      Reserved

         This field is unused.  It MUST be initialized to zero by the
         sender and MUST be ignored by the receiver.

   The Source Address of the Home Agent Address Discovery Request
   message packet MUST be one of the mobile node's current care-of
   addresses.  The home agent MUST then return the Home Agent Address
   Discovery Reply message directly to the Source Address chosen by the
   mobile node.  Note that, at the time of performing this dynamic home
   agent address discovery, it is likely that the mobile node is not
   registered with any home agent within the specified anycast group.
























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6.6. ICMP Home Agent Address Discovery Reply Message

   The ICMP Home Agent Address Discovery Reply message is used by a home
   agent to respond to a mobile node that uses the dynamic home agent
   address discovery mechanism, as described in Section 10.9.  One of
   the home agents on the home link responds to the mobile node with a
   Home Agent Address Discovery Reply message, providing a list of the
   routers on the mobile node's home link serving as home agents.

    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      |     Code      |            Checksum           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Identifier          |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   +                            Reserved                           +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   .                                                               .
   .                      Home Agent Addresses                     .
   .                                                               .
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         151 <To Be Assigned by IANA>

      Code

         0

      Checksum

         The ICMP checksum [14].

      Identifier

         The identifier from the invoking Home Agent Address Discovery
         Request message.

      Reserved

         This field is unused.  It MUST be initialized to zero by the
         sender and MUST be ignored by the receiver.




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      Home Agent Addresses

         A list of addresses of home agents on the home link for the
         mobile node.  The number of addresses present in the list is
         indicated by the remaining length of the IPv6 packet carrying
         the Home Agent Address Discovery Reply message.
















































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6.7. ICMP Mobile Prefix Solicitation Message Format

   The ICMP Mobile Prefix Solicitation Message is sent by a mobile node
   to its home agent while it is away from home.  The purpose of the
   message is to solicit a Mobile Prefix Advertisement from the home
   agent, which will allow the mobile node to gather prefix information
   about its home network.  This information can be used to configure
   home address(es) by stateless address autoconfiguration [13],
   or update address(es) according to changes in prefix information
   supplied by the home agent.

   The Mobile Prefix Solicitation is similar to the Router Solicitation
   used in Neighbor Discovery [12], except it is routed from the mobile
   node on the visited network to the home agent on the home network by
   usual unicast routing rules.

    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      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Identifier           |            Reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP Fields:

      Source Address

         The mobile node's care-of address.

      Destination Address

         The address of the mobile node's home agent.  This home agent
         must be on the link which the mobile node wishes to learn
         prefix information about.

      Hop Limit

         Set to an initial hop limit value, similarly to any other
         unicast packet sent by the mobile node.

      Authentication Header

         If a Security Association for the IP Authentication Header
         exists between the sender and the destination address, then the
         sender SHOULD include this header.  [subject to change]

   ICMP Fields:






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      Type

         152 <To Be Assigned by IANA>

      Code

         0

      Checksum

         The ICMP checksum [14].

      Identifier

         An identifier to aid in matching a future Mobile Prefix
         Advertisement message to this Mobile Prefix Solicitation
         message.

      Reserved

         This field is unused.  It MUST be initialized to zero by the
         sender and MUST be ignored by the receiver.

   If the mobile node receives a Mobile Prefix Advertisement Message
   from its home agent (see section 6.8), and the advertisement does not
   contain any authentication data, the mobile node MAY nevertheless
   send a Binding Update message to its home agent using its new home
   address which has been formed from the newly advertised prefix
   information.  If there are security concerns that would inhibit
   responding to unauthenticated advertisements, then the mobile node
   SHOULD send a Mobile Prefix Solicitation message to its home agent,
   with a nonzero Identifier value that can be used to match a future
   advertisement with the solicitation.

   The mobile node SHOULD include authentication data along with any
   Mobile Prefix Solicitation message that it sends to the home agent.


















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6.8. ICMP Mobile Prefix Advertisement Message Format

   A home agent will send a Mobile Prefix Advertisement message to a
   mobile node to distribute prefix information about the home link
   while the mobile node is traveling away from the home network.  This
   will occur in response to a Mobile Prefix Solicitation with an
   Advertisement, or by an unsolicited Advertisement sent according to
   the rules in Section 10.9.1.

    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      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Identifier           |   Options ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP Fields:

      Source Address
                    The home agent's address as the mobile node would
                    expect to see it (i.e., same network prefix)

      Destination Address
                    If this message is a response to a Mobile Prefix
                    Solicitation, this field contains the Source Address
                    field from that packet.  For unsolicited messages,
                    the mobile node's care-of address SHOULD be used.
                    Note that unsolicited messages can only be sent if
                    the mobile node is currently registered with the
                    home agent.

      Authentication Header
                    An AH header MUST be included unless the mobile node
                    has yet to configure a home address.

   ICMP Fields:

      Type

         153 <To Be Assigned by IANA>

      Code

         0

      Checksum

         The ICMP checksum [14].





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      Identifier

         An identifier to aid in matching this Mobile Prefix
         Advertisement message to a previous Mobile Prefix Solicitation
         message.

   Options:

      Prefix Information

         Each message contains one or more Prefix Information options.
         Each option carries the prefix(es) that the mobile node
         should use to configure its home address(es).  Section 10.9.1
         describes which prefixes should be advertised to the mobile
         node.

         The Prefix Information option is defined in Section 4.6.2
         of [12], with modifications defined in Section 7.2 of this
         specification.  The home agent MUST use this modified Prefix
         Information option to send the aggregate list of home network
         prefixes as defined in Section 10.9.1.

   The Mobile Prefix Advertisement sent by the home agent MAY include
   the Source Link-layer Address option defined in RFC 2461 [12], or the
   Advertisement Interval option specified in Section 7.3.

   Future versions of this protocol may define new option types.  Mobile
   nodes MUST silently ignore any options they do not recognize and
   continue processing the message.

   If the Advertisement is sent in response to a Mobile Prefix
   Solicitation, the home agent MUST copy the Identifier value from that
   message into the Identifier field of the Advertisement.

   The home agent MUST NOT send more than one Mobile Prefix
   Advertisement message per second to any mobile node.


















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7. Modifications to IPv6 Neighbor Discovery

7.1. Modified Router Advertisement Message Format

   Mobile IPv6 modifies the format of the Router Advertisement
   message [12] by the addition of a single flag bit to indicate that
   the router sending the Advertisement message is serving as a home
   agent on this link.  The format of the Router Advertisement message
   is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Cur Hop Limit |M|O|H| Reserved|       Router Lifetime         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Reachable Time                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Retrans Timer                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Options ...
   +-+-+-+-+-+-+-+-+-+-+-+-

   This format represents the following changes over that originally
   specified for Neighbor Discovery [12]:

      Home Agent (H)

         The Home Agent (H) bit is set in a Router Advertisement to
         indicate that the router sending this Router Advertisement is
         also functioning as a Mobile IP home agent on this link.

      Reserved

         Reduced from a 6-bit field to a 5-bit field to account for the
         addition of the Home Agent (H) bit.

















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7.2. Modified Prefix Information Option Format

   Mobile IPv6 requires knowledge of a router's global address for two
   reasons:

    -  To allow a home agent (a router) to learn the address of all
       other home agents on the link for which it is providing home
       agent service, for use in building its Home Agents List as
       part of the dynamic home agent address discovery mechanism
       (Sections 10.9 and 11.3.2).

    -  To allow a mobile node to send a Binding Update to a router on
       the link on which its previous care-of address is located, for
       purposes of establishing forwarding from this previous care-of
       address to its new care-of address (Section 11.6.6).

   However, Neighbor Discovery [12] only advertises a router's
   link-local address, by requiring this address to be used as the IP
   Source Address of each Router Advertisement.

   Mobile IPv6 extends Neighbor Discovery to allow a router to easily
   and efficiently advertise its global address, by the addition of a
   single flag bit in the format of a Prefix Information option for
   use in Router Advertisement messages.  The format of the Prefix
   Information option is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     | Prefix Length |L|A|R|Reserved1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Valid Lifetime                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Preferred Lifetime                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Reserved2                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                            Prefix                             +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This format represents the following changes over that originally
   specified for Neighbor Discovery [12]:






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      Router Address (R)

         1-bit router address flag.  When set, indicates that the
         Prefix field, in addition to advertising the indicated prefix,
         contains a complete IP address assigned to the sending router.
         This router IP address has the same scope and conforms to the
         same lifetime values as the advertised prefix.  This use of
         the Prefix field is compatible with its use in advertising
         the prefix itself, since prefix advertisement uses only the
         leading number Prefix bits specified by the Prefix Length
         field.  Interpretation of this flag bit is thus independent
         of the processing required for the On-Link (L) and Autonomous
         Address-Configuration (A) flag bits.

      Reserved1

         Reduced from a 6-bit field to a 5-bit field to account for the
         addition of the Router Address (R) bit.

   In a solicited Router Advertisement, a home agent MUST, and all other
   routers SHOULD, include at least one Prefix Information option with
   the Router Address (R) bit set.  Neighbor Discovery specifies that,
   if including all options in a Router Advertisement causes the size of
   the Advertisement to exceed the link MTU, multiple Advertisements can
   be sent, each containing a subset of the options [12].  In this case,
   at least one of these multiple Advertisements being sent instead
   of a single larger solicited Advertisement, MUST include a Prefix
   Information option with the Router Address (R) bit set.

   All routers SHOULD include at least one Prefix Information option
   with the Router Address (R) bit set, in each unsolicited multicast
   Router Advertisement that they send.  If multiple Advertisements
   are being sent instead of a single larger unsolicited multicast
   Advertisement, at least one of these multiple Advertisements SHOULD
   include a Prefix Information option with the Router Address (R) bit
   set.


















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7.3. New Advertisement Interval Option Format

   Mobile IPv6 defines a new Advertisement Interval option, used in
   Router Advertisement messages to advertise the interval at which the
   sending router sends unsolicited multicast Router Advertisements.
   The format of the Advertisement Interval option is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Advertisement Interval                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         7

      Length

         8-bit unsigned integer.  The length of the option (including
         the type and length fields) in units of 8 octets.  The value of
         this field MUST be 1.

      Reserved

         This field is unused.  It MUST be initialized to zero by the
         sender and MUST be ignored by the receiver.

      Advertisement Interval

         32-bit unsigned integer.  The maximum time, in milliseconds,
         between successive unsolicited router Router Advertisement
         messages sent by this router on this network interface.  Using
         the conceptual router configuration variables defined by
         Neighbor Discovery [12], this field MUST be equal to the value
         MaxRtrAdvInterval, expressed in milliseconds.

   Routers MAY include this option in their Router Advertisements.  A
   mobile node receiving a Router Advertisement containing this option
   SHOULD utilize the specified Advertisement Interval for that router
   in its movement detection algorithm, as described in Section 11.4.1.

   This option MUST be silently ignored for other Neighbor Discovery
   messages.








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7.4. New Home Agent Information Option Format

   Mobile IPv6 defines a new Home Agent Information option, used in
   Router Advertisement messages sent by a home agent to advertise
   information specific to this router's functionality as a home agent.
   The format of the Home Agent Information option is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Home Agent Preference     |      Home Agent Lifetime      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         8

      Length

         8-bit unsigned integer.  The length of the option (including
         the type and length fields) in units of 8 octets.  The value of
         this field MUST be 1.

      Reserved

         This field is unused.  It MUST be initialized to zero by the
         sender and MUST be ignored by the receiver.

      Home Agent Preference

         16-bit signed, twos-complement integer.  The preference for
         the home agent sending this Router Advertisement, for use in
         ordering the addresses returned to a mobile node in the Home
         Agent Addresses field of a Home Agent Address Discovery Reply
         message.  Higher values mean more preferable.  If this option
         is not included in a Router Advertisement in which the Home
         Agent (H) bit is set, the preference value for this home agent
         SHOULD be considered to be 0.  Values greater than 0 indicate a
         home agent more preferable than this default value, and values
         less than 0 indicate a less preferable home agent.

         The manual configuration of the Home Agent Preference value
         is described in Section 8.4.  In addition, the sending home
         agent MAY dynamically set the Home Agent Preference value, for
         example basing it on the number of mobile nodes it is currently
         serving or on its remaining resources for serving additional
         mobile nodes; such dynamic settings are beyond the scope of
         this document.  Any such dynamic setting of the Home Agent
         Preference, however, MUST set the preference appropriately,



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         relative to the default Home Agent Preference value of 0 that
         may be in use by some home agents on this link (i.e., a home
         agent not including a Home Agent Information option in its
         Router Advertisements will be considered to have a Home Agent
         Preference value of 0).

      Home Agent Lifetime

         16-bit unsigned integer.  The lifetime associated with the
         home agent in units of seconds.  The default value is the same
         as the Router Lifetime, as specified in the main body of the
         Router Advertisement message.  The maximum value corresponds
         to 18.2 hours.  A value of 0 MUST NOT be used.  The Home Agent
         Lifetime applies only to this router's usefulness as a home
         agent; it does not apply to information contained in other
         message fields or options.

   Home agents MAY include this option in their Router Advertisements.
   This option MUST NOT be included in a Router Advertisement in which
   the Home Agent (H) bit (see Section 7.1) is not set.  If this option
   is not included in a Router Advertisement in which the Home Agent (H)
   bit is set, the lifetime for this home agent MUST be considered to
   be the same as the Router Lifetime in the Router Advertisement.
   If multiple Advertisements are being sent instead of a single
   larger unsolicited multicast Advertisement, all of the multiple
   Advertisements with the Router Address (R) bit set MUST include this
   option with the same contents, otherwise this option MUST be omitted
   from all Advertisements.

   This option MUST be silently ignored for other Neighbor Discovery
   messages.

   If both the Home Agent Preference and Home Agent Lifetime are set
   to their default values specified above, this option SHOULD NOT be
   included in the Router Advertisement messages sent by this home
   agent.


















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7.5. Changes to Sending Router Advertisements

   The Neighbor Discovery protocol specification [12] limits routers to
   a minimum interval of 3 seconds between sending unsolicited multicast
   Router Advertisement messages from any given network interface
   (limited by MinRtrAdvInterval and MaxRtrAdvInterval), stating that:

      "Routers generate Router Advertisements frequently enough
      that hosts will learn of their presence within a few
      minutes, but not frequently enough to rely on an absence
      of advertisements to detect router failure; a separate
      Neighbor Unreachability Detection algorithm provides failure
      detection."

   This limitation, however, is not suitable to providing timely
   movement detection for mobile nodes.  Mobile nodes detect their
   own movement by learning the presence of new routers as the mobile
   node moves into wireless transmission range of them (or physically
   connects to a new wired network), and by learning that previous
   routers are no longer reachable.  Mobile nodes MUST be able to
   quickly detect when they move to a link served by a new router, so
   that they can acquire a new care-of address and send Binding Updates
   to register this care-of address with their home agent and to notify
   correspondent nodes as needed.

   Thus, to provide good support for mobile nodes, Mobile IPv6 relaxes
   this limit such that routers MAY send unsolicited multicast Router
   Advertisements more frequently.  In particular, on network interfaces
   where the router is expecting to provide service to visiting mobile
   nodes (e.g., wireless network interfaces), or on which it is serving
   as a home agent to one or more mobile nodes (who may return home and
   need to hear its Advertisements), the router SHOULD be configured
   with a smaller MinRtrAdvInterval value and MaxRtrAdvInterval value,
   to allow sending of unsolicited multicast Router Advertisements more
   often.  Recommended values for these limits are:

    -  MinRtrAdvInterval       0.05 seconds

    -  MaxRtrAdvInterval       1.5 seconds

   Use of these modified limits MUST be configurable, and specific
   knowledge of the type of network interface in use SHOULD be taken
   into account in configuring these limits for each network interface.

   When sending unsolicited multicast Router Advertisements more
   frequently than the standard limit on unsolicited multicast
   Advertisement frequency, the sending router need not include all
   options in each of these Advertisements, but it SHOULD include at
   least one Prefix Information option with the Router Address (R) bit
   set (Section 7.2) in each.




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7.6. Changes to Sending Router Solicitations

   In addition to the limit on routers sending unsolicited multicast
   Router Advertisement messages (Section 7.5), Neighbor Discovery
   defines limits on nodes sending Router Solicitation messages, such
   that a node SHOULD send no more than 3 Router Solicitations, and that
   these 3 transmissions SHOULD be spaced at least 4 seconds apart.
   However, these limits prevent a mobile node from finding a new
   default router (and thus a new care-of address) quickly as it moves
   about.

   Mobile IPv6 relaxes this limit such that, while a mobile node is away
   from home, it MAY send Router Solicitations more frequently.  The
   following limits for sending Router Solicitations are recommended for
   mobile nodes while away from home:

    -  A mobile node that is not configured with any current care-of
       address (e.g., the mobile node has moved since its previous
       care-of address was configured), MAY send more than the defined
       Neighbor Discovery limit of MAX_RTR_SOLICITATIONS Router
       Solicitations.

    -  The rate at which a mobile node sends Router Solicitations MUST
       be limited, although a mobile node MAY send Router Solicitations
       more frequently than the defined Neighbor Discovery limit of
       RTR_SOLICITATION_INTERVAL seconds.  The minimum interval MUST
       be configurable, and specific knowledge of the type of network
       interface in use SHOULD be taken into account in configuring this
       limit for each network interface.  A recommended minimum interval
       is 1 second.

    -  After sending at most MAX_RTR_SOLICITATIONS Router Solicitations,
       a mobile node MUST reduce the rate at which it sends subsequent
       Router Solicitations.  Subsequent Router Solicitations SHOULD
       be sent using a binary exponential backoff mechanism, doubling
       the interval between consecutive Router Solicitations, up to a
       maximum interval.  The maximum interval MUST be configurable and
       SHOULD be chosen appropriately based on the characteristics of
       the type of network interface in use.

    -  While still searching for a new default router and care-of
       address, a mobile node MUST NOT increase the rate at which it
       sends Router Solicitations unless it has received a positive
       indication (such as from lower network layers) that it has moved
       to a new link.  After successfully acquiring a new care-of
       address, the mobile node SHOULD also increase the rate at which
       it will send Router Solicitations when it next begins searching
       for a new default router and care-of address.

    -  A mobile node that is currently configured with a care-of address
       SHOULD NOT send Router Solicitations to the default router



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       on its current link, until its movement detection algorithm
       (Section 11.4.1) determines that it has moved and that its
       current care-of address might no longer be valid.


8. Requirements for Types of IPv6 Nodes

   Mobile IPv6 places some special requirements on the functions
   provided by different types of IPv6 nodes.  This section summarizes
   those requirements, identifying the functionality each requirement is
   intended to support.

   The requirements are set for the following groups of nodes:

    -  All IPv6 nodes.

    -  All IPv6 nodes with support for route optimization.

    -  All IPv6 routers.

    -  All Mobile IPv6 home agents.

    -  All Mobile IPv6 mobile nodes.

   It is outside the scope of this specification to specify which
   of these groups are mandatory in IPv6.  We only describe what is
   mandatory for a node that supports, for instance, route optimization.
   Other specifications are expected to define the extent of IPv6.


8.1. General Requirements for All IPv6 Nodes

   Any IPv6 node may at any time be a correspondent node of a mobile
   node, either sending a packet to a mobile node or receiving a
   packet from a mobile node.  The following requirements are necessary
   for every IPv6 node (whether host or router, whether mobile or
   stationary), since otherwise communications may be impossible:

    -  The node MUST be able to validate a Home Address option received
       in any IPv6 packet as described in Section 9.2.2.

    -  The node MUST be able to send a Binding Error message as
       described in Section 9.4.6.


8.2. Route Optimization Requirements for All IPv6 Nodes

   The ability of a correspondent node to participate in route
   optimization is essential for the efficient operation of the IPv6
   Internet, beneficial for robustness and reduction of jitter and
   latency, and necessary to avoid congestion in the home network.



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   The following requirements apply to all nodes that support route
   optimization:

    -  The node MUST be able to participate in a return routability
       procedure (Section 9.3).

    -  The node MUST be able to process Binding Update messages
       (Section 9.4).

    -  The node MUST be able to return a Binding Acknowledgement message
       (Section 6.1.8).

    -  The node MUST be able to maintain a Binding Cache of the
       bindings received in accepted Binding Updates, as described in
       Sections 9.1 and 9.5.

    -  The node MUST be able to insert a Routing Header type 2 into
       packets to be sent to a mobile node, as described in Section 9.6.

    -  The node SHOULD be able to interpret ICMP messages as described
       in Section 9.7.


8.3. Requirements for All IPv6 Routers

   All IPv6 routers, even those not serving as a home agent for
   Mobile IPv6, have an effect on how well mobile nodes can communicate:

    -  Every IPv6 router SHOULD be able to send an Advertisement
       Interval option (Section 7.3) in each of its Router
       Advertisements [12], to aid movement detection by mobile nodes
       (as in Section 11.4.1).  The use of this option in Router
       Advertisements MUST be configurable.

    -  Every IPv6 router SHOULD be able to support sending unsolicited
       multicast Router Advertisements at the faster rate described in
       Section 7.5.  The use of this faster rate MUST be configurable.

    -  Each router SHOULD include at least one prefix with the `R' bit
       set and with its full IP address in its router advertisements (as
       described in Section 7.2).

    -  Filtering routers SHOULD support different rules for Type 0 and
       Type 2 Routing headers (see Section 6.4) so that filtering of
       source routed packets (Type 0) will not necessarily limit MIPv6
       traffic which is delivered via Type 2 Routing headers.








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8.4. Requirements for IPv6 Home Agents

   In order for a mobile node to operate correctly while away from home,
   at least one IPv6 router on the mobile node's home link must function
   as a home agent for the mobile node.  The following additional
   requirements apply to all IPv6 routers capable of serving as a home
   agent:

    -  Every home agent MUST be able to maintain an entry in its Binding
       Cache for each mobile node for which it is serving as the home
       agent (Section 10.1).  Each such Binding Cache entry records the
       mobile node's binding with its primary care-of address and is
       marked as a "home registration" (Section 10.2).

    -  Every home agent MUST be able to intercept packets (using
       proxy Neighbor Discovery [12]) addressed to a mobile node for
       which it is currently serving as the home agent, on that mobile
       node's home link, while the mobile node is away from home
       (Section 10.4).

    -  Every home agent MUST be able to encapsulate [15] such
       intercepted packets in order to tunnel them to the primary
       care-of address for the mobile node indicated in its binding in
       the home agent's Binding Cache (Section 10.5).

    -  Every home agent MUST support decapsulating [15] reverse tunneled
       packets sent to it from a mobile node's home address.  Every home
       agent MUST also check that the source address in the tunneled
       packets corresponds to the currently registered location of the
       mobile node (Section 10.6).

    -  Every home agent MUST be able to return a Binding Acknowledgement
       message in response to a Binding Update option received with the
       Acknowledge (A) bit set (Section 10.2).

    -  Every home agent MUST maintain a separate Home Agents List for
       each link on which it is serving as a home agent, as described in
       Section 4.5.

    -  Every home agent MUST be able to accept packets addressed to
       the "Mobile IPv6 Home-Agents" anycast address for the subnet
       on which it is serving as a home agent [16], and MUST be
       able to participate in dynamic home agent address discovery
       (Section 10.9).

    -  Every home agent SHOULD support a configuration mechanism to
       allow a system administrator to manually set the value to be sent
       by this home agent in the Home Agent Preference field of the Home
       Agent Information Option in Router Advertisements that it sends
       (Section 7.4).




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    -  Every home agent SHOULD support sending ICMP Mobile Prefix
       Advertisements (Section 6.8), and SHOULD respond to Mobile Prefix
       Solicitations (Section 6.7).  This behavior MUST be configurable,
       so that home agents can be configured to avoid sending such
       Prefix Advertisements according to the needs of the network
       administration in the home domain.


8.5. Requirements for IPv6 Mobile Nodes

   Finally, the following requirements apply to all IPv6 nodes capable
   of functioning as mobile nodes:

    -  Every IPv6 mobile node MUST be able to perform IPv6 encapsulation
       and decapsulation [15].

    -  Every IPv6 mobile node MUST support the return routability
       procedure (Section 5.2.5).

    -  Every IPv6 mobile node MUST be able to send Binding Update
       messages, as specified in Sections 11.6.1, 11.6.2, and 11.6.6.

    -  Every IPv6 mobile node MUST be able to receive and process
       Binding Acknowledgement messages, as specified in Section 11.6.3.

    -  Every IPv6 mobile node MUST maintain a Binding Update List in
       which it records the IP address of each other node to which it
       has sent a Binding Update, for which the Lifetime sent in that
       binding has not yet expired (Section 11.1).

    -  Every IPv6 mobile node MUST support receiving a Binding Refresh
       Request (Section 6.1.2), by responding with a Binding Update
       message.

    -  Every IPv6 mobile node MUST support sending packets containing a
       Home Address option (Section 11.2.1).

    -  Every IPv6 mobile node MUST maintain a Home Agents List, as
       described in Section 4.5.

    -  Every mobile node MUST support receiving Mobile Prefix
       Advertisements (Section 11.3.4) and reconfiguring its home
       address based on the prefix information contained therein.

    -  Every IPv6 mobile node SHOULD support use of the dynamic
       home agent address discovery mechanism, as described in
       Section 11.3.2.







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9. Correspondent Node Operation

   This section explains the special processing required for the return
   routability and binding procedures, as well as to manage the binding
   cache, handle ICMP messages and send packets to a mobile node.


9.1. Conceptual Data Structures

   Each IPv6 node maintains a Binding Cache of bindings for other nodes.
   A separate Binding Cache SHOULD be maintained by each IPv6 node for
   each of its IPv6 addresses.  The Binding Cache MAY be implemented in
   any manner consistent with the external behavior described in this
   document, for example by being combined with the node's Destination
   Cache as maintained by Neighbor Discovery [12].  When sending a
   packet, the Binding Cache is searched before the Neighbor Discovery
   conceptual Destination Cache [12] (i.e., any Binding Cache entry for
   this destination SHOULD take precedence over any Destination Cache
   entry for the same destination).

   Each Binding Cache entry conceptually contains the following fields:

    -  The home address of the mobile node for which this is the Binding
       Cache entry.  This field is used as the key for searching the
       Binding Cache for the destination address of a packet being sent.
       If the destination address of the packet matches the home address
       in the Binding Cache entry, this entry SHOULD be used in routing
       that packet.

    -  The care-of address for the mobile node indicated by the home
       address field in this Binding Cache entry.  If the destination
       address of a packet being routed by a node matches the home
       address in this entry, the packet SHOULD be routed to this
       care-of address.  This is described in Section 9.6 for packets
       originated by this node, and in Section 10.5 if this node is the
       mobile node's home agent and the packet was intercepted by it on
       the home link.

    -  A lifetime value, indicating the remaining lifetime for this
       Binding Cache entry.  The lifetime value is initialized from
       the Lifetime field in the Binding Update that created or last
       modified this Binding Cache entry.  Once the lifetime of this
       entry expires, the entry MUST be deleted from the Binding Cache.

    -  A flag indicating whether or not this Binding Cache entry is a
       "home registration" entry.

    -  The maximum value of the Sequence Number field received in
       previous Binding Updates for this mobile node home address.
       The Sequence Number field is 16 bits long, and all comparisons




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       between Sequence Number values MUST be performed modulo 2**15 as
       explained in Section 9.4.1.

    -  Recent usage information for this Binding Cache entry, as needed
       to implement the cache replacement policy in use in the Binding
       Cache and to assist in determining whether a Binding Refresh
       Request should be sent when the lifetime of this entry nears
       expiration.

   Binding Cache entries not marked as "home registrations" MAY be
   replaced at any time by any reasonable local cache replacement policy
   but SHOULD NOT be unnecessarily deleted.  The Binding Cache for any
   one of a node's IPv6 addresses may contain at most one entry for
   each mobile node home address.  The contents of a node's Binding
   Cache MUST NOT be changed in response to a Home Address option in
   a received packet.  The contents of all of a node's Binding Cache
   entries, for each of its IPv6 addresses, MUST be cleared when the
   node reboots.


9.2. Receiving Packets from a Mobile Node

   Packets sent by a mobile node with either a Home Address destination
   option or a Mobility Header (or both) require special processing at
   the correspondent node as explained below.


9.2.1. Processing Mobility Header (MH) Messages

   All IPv6 correspondent nodes MUST observe the following rules when
   processing Mobility Header messages:

    1. If an MH message of unknown type is received (Section 6.1, the
       correspondent node SHOULD issue a Binding Error message to the
       packet's Source Address with Status field set to 2.  Finally, the
       correspondent node MUST discard the packet.

    2. If the "Next Header" field is not NO_NXTHDR (59 decimal), the
       packet MUST be silently discarded.

    3. The checksum must be verified as per Section 6.1.

   Subsequent checks depend on the particular Mobility Header message,
   as specified in Sections 9.3 and 9.4.  Subsequent checks depend
   on the particular Mobility Header message.  There are two types
   of Mobility Header messages.  The return routability procedure
   (Section 9.3) is used to verify liveness of the mobile node at both
   its home address as well as its care-of address.  These liveness
   probes are used to secure binding updates.





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   The other type of Mobility Header messages are directly concerned
   with managing bindings (Section 9.4).


9.2.2. Receiving Packets with Home Address Destination Option

   If the correspondent node has a Binding Cache Entry for the home
   address of a mobile node, packets sent by the mobile node MAY include
   a Home Address destination option.  The correspondent node MUST
   process the option in a manner consistent with exchanging the Home
   Address field from the Home Address option into the IPv6 header and
   replacing the original value of the Source Address field there.
   After all IPv6 options have been processed, it MUST be possible to
   process the packet without the knowledge that it came originally from
   a care-of address or that a Home Address option was used.

   Due to the threat of reflection attacks, this specification requires
   that packets containing a Home Address option MUST be dropped if
   there is no corresponding Binding Cache Entry for the given home
   address and the packet was not protected by IPsec.  A corresponding
   Binding Cache Entry MUST have the currently registered care-of
   address equal to the source address of the packet.  A packet that
   contains a Binding Update message and a Home Address option is
   considered to pass the above tests if the Binding Update successfully
   creates or updates a Binding Cache Entry.

   If the packet is dropped due the above tests, the correspondent nodes
   SHOULD send the Binding Error message to the source address of the
   packet that contained the Home Address option (see Section 6.1.9).
   The Status field in this message should be set to 1.  These messages
   SHOULD be rate-limited.

   No additional authentication of the Home Address option is
   required, except that if the IPv6 header of a packet is covered
   by authentication, then that authentication MUST also cover the
   Home Address option; this coverage is achieved automatically by the
   definition of the Option Type code for the Home Address option, since
   it indicates that the data within the option cannot change en-route
   to the packet's final destination, and thus the option is included in
   the authentication computation.  By requiring that any authentication
   of the IPv6 header also cover the Home Address option, the security
   of the Source Address field in the IPv6 header is not compromised by
   the presence of a Home Address option.  Security issues related to
   the Home Address option are discussed further in Section 5.  When
   attempting to verify authentication data in a packet that contains
   a Home Address option, the receiving node MUST make the calculation
   as if the care-of address were present in the Home Address option,
   and the home address were present in the source IPv6 address field
   of the IPv6 header.  This conforms with the calculation specified in
   section 11.2.2.




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9.3. Return Routability Procedure

   This subsection specifies actions taken by a correspondent node
   during the return routability procedure.


9.3.1. Receiving Home Test Init Messages

   Upon receiving a Home Test Init message, the correspondent node
   verifies the following:

    -  MH Type field for this message is 1.

    -  The Header Extension Length field MUST be greater than or equal
       to the length specified in Section 6.1.3.

    -  The packet MUST NOT include a Home Address destination option.

   In preparation for sending the corresponding Home Test Message,
   the correspondent node checks that it has the necessary material
   to engage in a return routability procedure, as specified in
   Section 5.2.  For that procedure, the correspondent node MUST have a
   secret Kcn and a nonce Nj.  If it does not have this material yet,
   it MUST produce it before continuing with the return routability
   procedure.

   Section 9.3.3 specifies further processing.


9.3.2. Receiving Care-of Test Init Messages

   Upon receiving a Care-of Test Init message, the correspondent node
   verifies the following:

    -  MH Type field for this message is 2.

    -  The Header Extension Length field MUST be greater than or equal
       to the length specified in Section 6.1.4.

    -  The packet MUST NOT include a Home Address destination option.

   In preparation for sending the corresponding Care-of Test Message,
   the correspondent node checks that it has the necessary material
   to engage in a return routability procedure, as specified in
   Section 5.2.  For that procedure, the correspondent node MUST have a
   secret Kcn and a nonce Nl.  If it does not have this material yet,
   it MUST produce it before continuing with the return routability
   procedure.

   Section 9.3.4 specifies further processing.




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9.3.3. Sending Home Test Messages

   Unless already created, the correspondent node creates a "Home
   Cookie" and an associated "Home Nonce Index".  It then creates a Home
   Test message (Section 6.1.5) and sends it to the mobile node at the
   latter's home address.


9.3.4. Sending Care-of Test Messages

   Unless already created, the correspondent node creates a "Care-of
   Cookie" and an associated "Care-of Nonce Index".  It then creates a
   Care-of Test message (Section 6.1.6) and sends it to the mobile node
   at the latter's care-of address.


9.4. Processing Bindings

   This section explains how the correspondent node processes the
   binding cache messages.  These messages are:

    -  Binding Update

    -  Binding Refresh Request

    -  Binding Acknowledgement

    -  Binding Error


9.4.1. Receiving Binding Updates

   Before accepting a Binding Update message, the receiving node MUST
   validate the Binding Update according to the following tests:

    -  The packet MUST contain a Home Address option.

    -  The Header Len field in the Binding Update option is greater than
       or equal to the length specified in Section 6.1.7.

    -  The Sequence Number field in the Binding Update message is
       greater than the Sequence Number received in the previous Binding
       Update for this home address, if any.

       This Sequence Number comparison MUST be performed modulo 2**16,
       i.e., the number is a free running counter represented modulo
       65536.  A Sequence Number in a received Binding Update is
       considered less than or equal to the last received number if
       its value lies in the range of the last received number and the
       preceding 32767 values, inclusive.  For example, if the last
       received sequence number was 15, then messages with sequence



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       numbers 0 through 15, as well as 32784 through 65535, would be
       considered less than or equal.

   When the return routability procedure is used as an authorization
   method, the following are also required:

    -  The Home and Care-of Nonce Index values in the Nonce Indices
       mobility option are recognized by the correspondent node.  As
       described in Section 5.2, the correspondent node discards Nonce
       values that are too old.

    -  The correspondent node MUST re-generate the Home Cookie and the
       Care-of Cookie from the information contained in the packet.
       It then generates the session key Kbu and uses it to verify
       the authenticator field in the Binding Update as specified in
       Section 6.1.7.  Note that a care-of address different from the
       Source Address MAY have been specified by including an Alternate
       Care-of Address mobility option in the Binding Update message.
       When such message is received and the return routability
       procedure is used as an authorization method, the correspondent
       node MUST verify the authenticator by using the address within
       the Alternate Care-of Address in the calculations.

    -  The Binding Authorization Data option MUST be present, and its
       contents MUST be satisfy rules presented in Section 5.2.6.

   If the mobile node sends a sequence number which is not greater than
   the sequence number from the last successful Binding Update, then the
   receiving node MUST send back a Binding Acknowledgement with status
   code 141, and the last accepted sequence number in the Sequence
   Number field of the Binding Acknowledgement.

   If the mobile node sends a Home or Care-of Nonce Index value which is
   no longer recognized by the correspondent node, then the receiving
   node MUST send back a Binding Acknowledgement with status code 144 or
   145, respectively.

   Any Binding Update which fails to satisfy all of these tests for
   any reason other than insufficiency of the Sequence Number or Nonce
   Indices MUST be silently ignored, and the packet carrying the Binding
   Update MUST be discarded.

   In this section, the care-of address refers to the IPv6 address,
   which was originally located in the IPv6 header when the packet was
   transmitted by the mobile node.

   If the Binding Update is valid according to the tests above, then the
   Binding Update is processed further as follows:

    -  If the Lifetime specified in the Binding Update is nonzero and
       the specified Care-of Address is not equal to the home address



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       for the binding, then this is a request to cache a binding for
       the mobile node.  If the Home Registration (H) bit is set in the
       Binding Update, the Binding Update is processed according to the
       procedure specified in Section 10.2; otherwise, it is processed
       according to the procedure specified in Section 9.4.2.

    -  If the Lifetime specified in the Binding Update is zero or the
       specified Care-of Address matches the home address for the
       binding, then this is a request to delete the mobile node's
       cached binding.  If the Home Registration (H) bit is set in the
       Binding Update, the Binding Update is processed according to the
       procedure specified in Section 10.3; otherwise, it is processed
       according to the procedure specified in Section 9.4.3.


9.4.2. Requests to Cache a Binding

   This section describes the processing of a valid Binding Update that
   requests a node to cache a mobile node's binding, for which the Home
   Registration (H) bit is not set in the Binding Update.

   In this case, the receiving node SHOULD create a new entry in its
   Binding Cache for this mobile node, or update its existing Binding
   Cache entry for this mobile node, if such an entry already exists.
   The lifetime for the Binding Cache entry is initialized from the
   Lifetime field specified in the Binding Update, although this
   lifetime MAY be reduced by the node caching the binding; the lifetime
   for the Binding Cache entry MUST NOT be greater than the Lifetime
   value specified in the Binding Update.  Any Binding Cache entry MUST
   be deleted after the expiration of its lifetime.

   The Sequence Number value received from a mobile node in a Binding
   Update is stored by a correspondent node in its Binding Cache entry
   for that mobile node.  If the receiving correspondent node has no
   Binding Cache entry for the sending mobile node, it MUST accept any
   Sequence Number value in a received Binding Update from this mobile
   node.


9.4.3. Requests to Delete a Binding

   This section describes the processing of a valid Binding Update that
   requests a node to delete a mobile node's binding from its Binding
   Cache, for which the Home Registration (H) bit is not set in the
   Binding Update.

   Any existing binding for the mobile node MUST be deleted.  A Binding
   Cache entry for the mobile node MUST NOT be created in response to
   receiving the Binding Update.





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   If the binding cache entry was created by use of return routability
   nonces, the correspondent node MUST ensure that the same nonces are
   not used again with the particular home and care-of address.  If
   both nonces are still valid, the correspondent node has to remember
   the particular combination of nonce indexes, addresses, and sequence
   number as illegal, until at least one of the nonces has become too
   old.


9.4.4. Sending Binding Acknowledgements

   When any node receives a packet containing a Binding Update message
   in which the Acknowledge (A) bit is set, it MUST return a Binding
   Acknowledgement message acknowledging receipt of the Binding Update.
   If the node accepts the Binding Update and creates or updates an
   entry in its Binding Cache for this binding, the Status field in the
   Binding Acknowledgement MUST be set to a value less than 128; if, on
   the other hand the Binding Update is accepted and the `A' bit is not
   set, the node SHOULD NOT send a Binding Acknowledgement.  If the node
   rejects the Binding Update and does not create or update an entry for
   this binding, a Binding Acknowledgement MUST be sent even if the `A'
   bit was not set, and the Status field in the Binding Acknowledgement
   MUST be set to a value greater than or equal to 128.  Specific values
   for the Status field are described in Section 6.1.8 and in the IANA
   registry of assigned numbers [18].

   The packet in which the Binding Acknowledgement is returned
   MUST meet the specific authentication requirements for Binding
   Acknowledgements, defined in Section 5.2.  Furthermore, if the packet
   is to be sent to the mobile node at any address other than the mobile
   node's home address, it MUST be sent using a Routing header (even if
   the binding was rejected).  The intermediate IP address, to which
   the packet will be delivered immediately before the home address, is
   determined as follows:

    -  Whenever the Binding Update is accepted with a nonzero lifetime,
       the routing header will be constructed using the care-of address
       as described in Section 9.6.

    -  Otherwise, if the Source IP Address of the packet containing
       the Binding Update, is legal for inclusion in a Routing Header,
       the routing header will be constructed using that IP address.
       Note that multicast addresses, link-local addresses, loopback
       addresses, IPv4 mapped addresses, and the unspecified address,
       MUST NOT be used within a Routing Header for the Binding
       Acknowledgement.

   Otherwise, if the Binding Update has a zero lifetime but the Source
   IP address is not allowable for use within the Routing Header,
   the Binding Acknowledgment MUST be sent to the mobile node's home
   address.



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   Entries in a node's Binding Cache MUST be deleted when their lifetime
   expires.


9.4.5. Sending Binding Refresh Requests

   If a Binding Cache entry being deleted is still in active use
   in sending packets to a mobile node, the next packet sent to the
   mobile node will be routed normally to the mobile node's home link.
   Communication with the mobile node continues, but the tunneling
   from the home network creates additional overhead and latency in
   delivering packets to the mobile node.

   If the sender knows that the Binding Cache entry is still in active
   use, it MAY send a Binding Refresh Request message to the mobile node
   in an attempt to avoid this overhead and latency due to deleting and
   recreating the Binding Cache entry.  The Binding Refresh Request
   message is sent in the same way as any packet addressed to the mobile
   node (Section 9.6).

   The correspondent node MAY retransmit Binding Refresh Request
   messages provided that rate limitation is applied.  The correspondent
   node SHOULD stop retransmitting when it receive a Home Test Init
   message, as the mobile node is responsible for retransmissions during
   the return routability procedure.


9.4.6. Sending Binding Error Messages

   If the correspondent node receives a packet with a Home Address
   destination option it MUST verify that it has a binding for that
   mobile node.  Specifically, it MUST have a binding entry for the
   mobile node's home address (as obtained from the Home Address option)
   at the mobile node's care-of address (from the IP source address of
   the packet).  If the correspondent node does not find such a binding
   entry, it MUST discard the packet.  It MUST also return a Binding
   Error message (Section 6.1.9), subject to rate limiting in the same
   manner as is done for ICMPv6 messages [14].


9.5. Cache Replacement Policy

   Conceptually, a node maintains a separate timer for each entry in its
   Binding Cache.  When creating or updating a Binding Cache entry in
   response to a received and accepted Binding Update, the node sets the
   timer for this entry to the specified Lifetime period.  Any entry in
   a node's Binding Cache MUST be deleted after the expiration of the
   Lifetime specified in the Binding Update from which the entry was
   created or last updated.





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   Each node's Binding Cache will, by necessity, have a finite size.
   A node MAY use any reasonable local policy for managing the space
   within its Binding Cache, except that any entry marked as a "home
   registration" (Section 10.2) MUST NOT be deleted from the cache until
   the expiration of its lifetime period.  When such "home registration"
   entries are deleted, the home agent MUST also cease intercepting
   packets on the mobile node's home link addressed to the mobile node
   (Section 10.4), just as if the mobile node had de-registered its
   primary care-of address (see Section 10.3).

   When attempting to add a new "home registration" entry in response
   to a Binding Update with the Home Registration (H) bit set, if no
   sufficient space can be found, the node MUST reject the Binding
   Update and MUST return a Binding Acknowledgement to the sending
   mobile node, in which the Status field is set to 131 (insufficient
   resources).  When otherwise attempting to add a new entry to its
   Binding Cache, a node MAY, if needed, choose to drop any entry
   already in its Binding Cache, other than "home registration"
   entries, in order to make space for the new entry.  For example, a
   "least-recently used" (LRU) strategy for cache entry replacement
   among entries not marked as "home registrations" is likely to
   work well unless the size of the Binding Cache is substantially
   insufficient.

   If the node sends a packet to a destination for which it has dropped
   the entry from its Binding Cache, the packet will be routed through
   the mobile node's home link.  The mobile node can detect this, and
   establish a new binding if necessary.


9.6. Sending Packets to a Mobile Node

   Before sending any packet, the sending node SHOULD examine its
   Binding Cache for an entry for the destination address to which the
   packet is being sent.  If the sending node has a Binding Cache entry
   for this address, the sending node SHOULD use a Routing header to
   route the packet to this mobile node (the destination node) by way
   of the care-of address in the binding recorded in that Binding Cache
   entry.  For example, assuming use of a Type 2 Routing header (see
   Section 6.4), if no other use of a Routing header is involved in
   the routing of this packet, the mobile node sets the fields in the
   packet's IPv6 header and Routing header as follows:

    -  The Destination Address in the packet's IPv6 header is set to
       the mobile node's care-of address copied from the Binding Cache
       entry.

    -  The Routing header is initialized to contain a single route
       segment, with an Address of the mobile node's home address (the
       original destination address to which the packet was being sent).




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   Following the definition of a Type 2 Routing header in Section 6.4,
   this packet will be routed to the mobile node's care-of address,
   where it will be delivered to the mobile node (the mobile node has
   associated the care-of address with its network interface).

   Note that following the above conceptual model in an implementation
   creates some additional requirements for path MTU discovery since the
   layer that decides the packet size (e.g., TCP and applications using
   UDP) needs to be aware of the size of the headers added by the IP
   layer on the sending node.

   The IP layer will insert the routing header before performing IPsec
   processing.  The IPsec Security Policy Database will be consulted
   based on the IP source address and the final IP destination (which
   will be in the routing header).  The definition of AH ensures that
   the AH calculation is done on the packet in the form it will have on
   the receiver after advancing the routing header.

   If, instead, the sending node has no Binding Cache entry for the
   destination address to which the packet is being sent, the sending
   node simply sends the packet normally, with no Routing header.  If
   the destination node is not a mobile node (or is a mobile node that
   is currently at home), the packet will be delivered directly to this
   node and processed normally by it.  If, however, the destination node
   is a mobile node that is currently away from home, the packet will
   be intercepted by the mobile node's home agent and tunneled (using
   IPv6 encapsulation [15]) to the mobile node's current primary care-of
   address, as described in Section 10.5.  The mobile node MAY then send
   a Binding Update to the sending node, as described in Section 11.6.2,
   allowing the sending node to create a Binding Cache entry for its use
   in sending subsequent packets to this mobile node.


9.7. Receiving ICMP Error Messages

   When the correspondent node has a Binding Cache entry for a mobile
   node, all traffic destined to the mobile node goes directly to the
   current care-of address of the mobile node using a Routing header.
   Any ICMP error message caused by packets on their way to the care-of
   address will be returned in the normal manner to the correspondent
   node.

   On the other hand, if the correspondent node has no Binding Cache
   entry for the mobile node, the packet will be routed through the
   MN's home link.  Any ICMP error message caused by the packet on its
   way to the mobile node while in the tunnel, will be transmitted
   to the mobile node's home agent.  By the definition of IPv6
   encapsulation [15], the home agent MUST relay certain ICMP error
   messages back to the original sender of the packet, which in this
   case is the correspondent node.




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   Likewise, if a packet for a mobile node arrives at the mobile node's
   previous link and is intercepted there by a home agent for the mobile
   node's previous care-of address (see Section 11.6.6), the packet will
   be tunneled to the mobile node's new care-of address.  As above, any
   ICMP error message caused by the packet while in this tunnel will
   be returned to that home agent, which MUST relay certain ICMP error
   messages back to the correspondent node [15].  The relayed packet
   MUST NOT contain a routing header entry with the care-of address of
   the mobile node.

   Thus, in all cases, any meaningful ICMP error messages caused by
   packets from a correspondent node to a mobile node will be returned
   to the correspondent node.  If the correspondent node receives
   persistent ICMP Destination Unreachable messages after sending
   packets to a mobile node based on an entry in its Binding Cache, the
   correspondent node SHOULD delete this Binding Cache entry.


10. Home Agent Operation

10.1. Conceptual Data Structures

   Each home agent MUST maintain a Binding Cache and Home Agents List.

   The rules for maintaining a Binding Cache are same for home
   agents and correspondent nodes, and have already been described in
   Section 9.1.  In addition, if an entry in a node's Binding Cache
   for which the node is serving as a home agent is marked as a "home
   registration" entry, it MUST NOT be deleted by the home agent until
   the expiration of its binding lifetime.

   The Home Agents List is maintained by each home agent (as well as
   each mobile node), recording information about each home agent from
   which this node has received a Router Advertisement in which the Home
   Agent (H) bit is set, for which the remaining lifetime for this list
   entry (defined below) has not yet expired.  The home agents list is
   thus similar to the Default Router List conceptual data structure
   maintained by each host for Neighbor Discovery [12], although the
   Home Agents List MAY be implemented in any manner consistent with the
   external behavior described in this document.

   Each home agent maintains a separate Home Agents List for each link
   on which it is serving as a home agent; this list is used by a home
   agent in the dynamic home agent address discovery mechanism.  Each
   mobile node, while away from home, also maintains a Home Agents
   List, to enable it to notify a home agent on its previous link when
   it moves to a new link; a mobile node MAY maintain a separate Home
   Agents List for each link on which it has a home agent, or it MAY
   maintain a single list for all links.  Each Home Agents List entry
   conceptually contains the following fields:




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    -  The link-local IP address of a router on the link, that this
       node currently believes is operating as a home agent for that
       link.  A new entry is created or an existing entry is updated
       in the Home Agents List in response to receipt of a valid
       Router Advertisement in which the Home Agent (H) bit is set.
       The link-local address of the home agent is learned through
       the Source Address of the Router Advertisements received from
       it [12].

    -  One or more global IP addresses for this home agent, learned
       through Prefix Information options with the Router Address (R)
       bit set, received in Router Advertisements from this link-local
       address.  Global addresses for the router in a Home Agents List
       entry MUST be deleted once the prefix associated with that
       address is no longer valid [12].

          Are there interactions with the new Router Advertisement
          stuff?

    -  The remaining lifetime of this Home Agents List entry.  If a Home
       Agent Information Option is present in a Router Advertisement
       received from a home agent, the lifetime of the Home Agents List
       entry representing that home agent is initialized from the Home
       Agent Lifetime field in the option; otherwise, the lifetime
       is initialized from the Router Lifetime field in the received
       Router Advertisement.  The Home Agents List entry lifetime is
       decremented until it reaches zero, at which time this entry MUST
       be deleted from the Home Agents List.

    -  The preference for this home agent; higher values indicate a more
       preferable home agent.  The preference value is taken from the
       Home Agent Preference field (a signed, twos-complement integer)
       in the received Router Advertisement, if the Router Advertisement
       contains a Home Agent Information Option, and is otherwise set
       to the default value of 0.  A home agent uses this preference
       in ordering the Home Agents List returned in an ICMP Home
       Agent Address Discovery message in response to a mobile node's
       initiation of dynamic home agent address discovery.  A mobile
       node uses this preference in determining which of the home agents
       on its previous link to notify when it moves to a new link.


10.2. Primary Care-of Address Registration

   This section describes the processing of a valid Binding Update that
   requests the receiving node to serve as its home agent, registering
   its primary care-of address.

   To begin processing the Binding Update, the home agent MUST perform
   the following sequence of tests:




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    -  If the node is not a router that implements home agent
       functionality, then the node MUST reject the Binding Update
       and MUST return a Binding Acknowledgement to the mobile node,
       in which the Status field is set to 132 (home registration not
       supported).

    -  Else, if the home address for the binding (the Home Address field
       in the packet's Home Address option) is not an on-link IPv6
       address with respect to the home agent's current Prefix List,
       then the home agent MUST reject the Binding Update and SHOULD
       return a Binding Acknowledgement to the mobile node, in which the
       Status field is set to 133 (not home subnet).

    -  Else, if the home agent chooses to reject the Binding Update for
       any other reason (e.g., insufficient resources to serve another
       mobile node as a home agent), then the home agent SHOULD return a
       Binding Acknowledgement to the mobile node, in which the Status
       field is set to an appropriate value to indicate the reason for
       the rejection.

    -  A Home Address destination option MUST be present in the message.

    -  Finally, if the Duplicate Address Detection (D) bit is set in the
       Binding Update, this home agent MUST perform Duplicate Address
       Detection [13] on the mobile node's home link for the link-local
       address associated with the home address in this binding, before
       returning the Binding Acknowledgement.  This ensures that no
       other node on the home link was using the mobile node's home
       address when the Binding Update arrived

   If home agent accepts the Binding Update, it MUST then create a
   new entry in its Binding Cache for this mobile node, or update its
   existing Binding Cache entry, if such an entry already exists.  The
   Home Address field as received in the Home Address option provides
   the home address of the mobile node.  The care-of address for this
   Binding Cache entry is determined as follows:

    -  If the Alternate Care-of Address option is present, the care-of
       address is the address in that option.

    -  Otherwise, the care-of address is the the Source Address field in
       the packet's IPv6 header.

   The home agent MUST mark this Binding Cache entry as a "home
   registration" to indicate that the node is serving as a home
   agent for this binding.  Binding Cache entries marked as a "home
   registration" MUST be excluded from the normal cache replacement
   policy used for the Binding Cache (Section 9.5) and MUST NOT be
   removed from the Binding Cache until the expiration of the Lifetime
   period.




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   When the 'D' bit is set, the address used for Duplicate Address
   Detection SHOULD be the mobile node's link-local address.  Normal
   processing for Duplicate Address Detection specifies that, in
   certain cases, the node SHOULD delay sending the initial Neighbor
   Solicitation message of Duplicate Address Detection by a random
   delay between 0 and MAX_RTR_SOLICITATION_DELAY [12, 13]; however, in
   this case, the home agent SHOULD NOT perform such a delay.  If this
   Duplicate Address Detection fails, then the home agent MUST reject
   the Binding Update and MUST return a Binding Acknowledgement to the
   mobile node, in which the Status field is set to 138 (Duplicate
   Address Detection failed).  When the home agent sends a successful
   Binding Acknowledgement to the mobile node, in response to a Binding
   Update with the `D' bit set, the home agent assures to the mobile
   node that its home address will continue to be kept unique by the
   home agent at least as long as the lifetime granted for that home
   address binding is not over.

   If the `S' bit field in the Binding Update is zero, The home agent
   creates or updates Binding Cache entries for each of possibly
   several home addresses.  The set of such home addresses is formed
   by replacing the routing prefix for the given home address with
   all other routing prefixes on the mobile node's home link that are
   supported by the home agent processing the Binding Update.  The home
   agent creates such a separate primary care-of address registration
   for each such home address.  Note that the same considerations for
   Duplicate Address Detection apply for each affected home address.

   The specific addresses which are to be tested before accepting the
   Binding Update, and later to be defended by performing Duplicate
   Address Detection, depend on the settings of the `S' and `L' bits, as
   follows:

    -  S=0 & L=0:  Defend all non link-local unicast addresses possible
       on link.

    -  S=0 & L=1:  Defend all non link-local unicast addresses possible
       on link and the derived link-local.

    -  S=1 & L=0:  Defend the given address.

    -  S=1 & L=1:  Defend both the given non link-local unicast (home)
       address and the derived link-local.

   The lifetime of the Binding Cache entry depends on a number of
   factors:

    -  The lifetime for the Binding Cache entry MUST NOT be greater
       than the remaining valid lifetime for the subnet prefix in the
       mobile node's home address specified with the Binding Update,
       and MUST NOT be greater than the Lifetime value specified in the
       Binding Update.  The remaining valid lifetime for this prefix is



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       determined by the home agent based on its own Prefix List entry
       for this prefix [12].

    -  However, if the `S' bit field in the Binding Update is zero, the
       lifetime for the each Binding Cache entry MUST NOT be greater
       than the minimum remaining valid lifetime for all subnet prefixes
       on the mobile node's home link.  If the value of the Lifetime
       field specified by the mobile node in its Binding Update is
       greater than this prefix lifetime, the home agent MUST decrease
       the binding lifetime to less than or equal to the prefix valid
       lifetime.

    -  The home agent MAY further decrease the specified lifetime for
       the binding, for example based on a local policy.  The resulting
       lifetime is stored by the home agent in the Binding Cache entry,
       and this Binding Cache entry MUST be deleted by the home agent
       after the expiration of this lifetime.

   Regardless of the setting of the `A' bit in the Binding Update, the
   home agent MUST return a Binding Acknowledgement to the mobile node,
   constructed as follows:

    -  The Status field MUST be set to a value 0, indicating success.

    -  The Sequence Number field MUST be copied from the Sequence Number
       given in the Binding Update.

    -  The Lifetime field MUST be set to the remaining lifetime for
       the binding as set by the home agent in its "home registration"
       Binding Cache entry for the mobile node, as described above.

    -  The Refresh field MUST be set to a value less than or equal to
       the Lifetime value being returned in the Binding Update.  If the
       home agent stores the Binding Cache entry in nonvolatile storage
       (that survives the crash or other failure of the home agent),
       then the Refresh field SHOULD be set to the same value as the
       Lifetime field; otherwise, the home agent MAY set the Refresh
       field to a value less than the Lifetime field, to indicate that
       the mobile node SHOULD attempt to refresh its home registration
       at the indicated shorter interval (although the home agent will
       still retain the registration for the Lifetime period, even if
       the mobile node does not refresh its registration within the
       Refresh period).

   The rules for selecting the Destination IP address (and possibly
   Routing Header construction) for the Binding Acknowledgement to the
   mobile node are the same as in section 9.4.4.

   In addition, the home agent MUST follow the procedure defined in
   Section 10.4 to intercept packets on the mobile node's home link
   addressed to the mobile node, while the home agent is serving as



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   the home agent for this mobile node.  The home agent MUST also be
   prepared to accept reverse tunneled packets from the new care-of
   address of the mobile node, as described in Section 10.6.  Finally,
   the home agent MUST also propagate new home network prefixes, as
   described in Section 10.9.1.


10.3. Primary Care-of Address De-Registration

   When a node receives a Binding Update, it MUST validate it and
   determine the type of Binding Update according to the steps described
   in Section 9.4.1.  This section describes the processing of a valid
   Binding Update that requests the receiving node to no longer serve as
   its home agent, de-registering its primary care-of address.

   To begin processing the Binding Update, the home agent MUST perform
   the following test:

    -  If the receiving node has no entry marked as a "home
       registration" in its Binding Cache for this mobile node, then
       this node MUST reject the Binding Update and SHOULD return a
       Binding Acknowledgement to the mobile node, in which the Status
       field is set to 137 (not home agent for this mobile node).

   If the home agent does not reject the Binding Update as described
   above, then it MUST delete any existing entry in its Binding Cache
   for this mobile node, and proceed as follows.

   The home agent MUST return a Binding Acknowledgement to the mobile
   node, constructed as follows:

    -  The Status field MUST be set to a value 0, indicating success.

    -  The Sequence Number field MUST be copied from the Sequence Number
       given in the Binding Update.

    -  The Lifetime field MUST be set to zero.

    -  The Refresh field MUST be set to zero.

   In addition, the home agent MUST stop intercepting packets on
   the mobile node's home link that are addressed to the mobile node
   (Section 10.4).

   The rules for selecting the Destination IP address (and possibly
   Routing Header construction) for the Binding Acknowledgement to the
   mobile node are the same as in section 9.4.4.







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10.4. Intercepting Packets for a Mobile Node

   While a node is serving as the home agent for mobile node it MUST
   attempt to intercept packets on the mobile node's home link that are
   addressed to the mobile node, and MUST tunnel each intercepted packet
   to the mobile node using IPv6 encapsulation [15].

   In order to do this, when a node begins serving as the home agent
   it MUST multicast onto the home link a Neighbor Advertisement
   message [12] on behalf of the mobile node.  Specifically, the home
   agent performs the following steps:

    -  The home agent examines the value of the `S' bit in the received
       Binding Update message.  If this bit is nonzero, the following
       step is carried out only for the individual home address
       specified for this binding.  If, instead, this bit is zero, then
       the following step is carried out for each address for the mobile
       node formed from the interface identifier in the mobile node's
       home address in this binding (the remaining low-order bits in
       the address after the configured subnet prefix), together with
       each one of the subnet prefixes currently considered by the home
       agent to be on-link (including both the link-local and site-local
       prefix).

    -  For each specific IP address for the mobile node determined
       in the first step above, the home agent sends a Neighbor
       Advertisement message [12] to the all-nodes multicast address
       on the home link, to advertise the home agent's own link-layer
       address for this IP address on behalf of the mobile node.

       All fields in each such Neighbor Advertisement message SHOULD be
       set in the same way they would be set by the mobile node itself
       if sending this Neighbor Advertisement while at home [12], with
       the following exceptions:

        *  The Target Address in the Neighbor Advertisement message MUST
           be set to the specific IP address for the mobile node.

        *  The Advertisement MUST include a Target Link-layer Address
           option specifying the home agent's link-layer address.

        *  The Router (R) bit in the Advertisement MUST be set to zero.

        *  The Solicited Flag (S) in the Advertisement MUST NOT be set,
           since it was not solicited by any Neighbor Solicitation
           message.

        *  The Override Flag (O) in the Advertisement MUST be set,
           indicating that the Advertisement SHOULD override any
           existing Neighbor Cache entry at any node receiving it.




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   Any node on the home link receiving one of the Neighbor Advertisement
   messages described above will thus update its Neighbor Cache to
   associate the mobile node's address with the home agent's link
   layer address, causing it to transmit any future packets for the
   mobile node normally destined to this address instead to the mobile
   node's home agent.  Since multicasting on the local link (such as
   Ethernet) is typically not guaranteed to be reliable, the home
   agent MAY retransmit this Neighbor Advertisement message up to
   MAX_ADVERT_REXMIT times to increase its reliability.  It is still
   possible that some nodes on the home link will not receive any of
   these Neighbor Advertisements, but these nodes will eventually be
   able to detect the link-layer address change for the mobile node's
   home address, through use of Neighbor Unreachability Detection [12].

   While a node is serving as a home agent for some mobile node (it
   still has a "home registration" entry for this mobile node in its
   Binding Cache), the home agent uses IPv6 Neighbor Discovery [12] to
   intercept unicast packets on the home link addressed to the mobile
   node's home address.  In order to intercept packets in this way, the
   home agent MUST act as a proxy for this mobile node, and reply to any
   received Neighbor Solicitation messages for it.  When a home agent
   receives a Neighbor Solicitation message, it MUST check if the Target
   Address specified in the message matches the home address of any
   mobile node for which it has a Binding Cache entry marked as a "home
   registration".  (Note that Binding Update messages with the `S' bit
   set to zero will result in multiple Binding Cache entries, so checks
   on all these entries necessarily include all possible home addresses
   for the mobile node).

   If such an entry exists in the home agent's Binding Cache, the home
   agent MUST reply to the Neighbor Solicitation message with a Neighbor
   Advertisement message, giving the home agent's own link-layer address
   as the link-layer address for the specified Target Address.  In
   addition, the Router (R) bit in the Advertisement MUST be set to
   zero.  Acting as a proxy in this way allows other nodes on the mobile
   node's home link to resolve the mobile node's IPv6 home address, and
   allows the home agent to defend these addresses on the home link for
   Duplicate Address Detection [12].


10.5. Tunneling Intercepted Packets to a Mobile Node

   For any packet sent to a mobile node from the mobile node's home
   agent (for which the home agent is the original sender of the
   packet), the home agent is operating as a correspondent node of
   the mobile node for this packet and the procedures described in
   Section 9.6 apply.  The home agent (as a correspondent node) uses a
   Routing header to route the packet to the mobile node by way of the
   care-of address in the home agent's Binding Cache (the mobile node's
   primary care-of address, in this case).




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   While the mobile node is away from home and this node is acting
   as the mobile node's home agent, the home agent intercepts any
   packets on the home link addressed to the mobile node's home
   address (including addresses formed from other on-link prefixes,
   if the 'S' bit was zero in the Binding Update), as described in
   Section 10.4.  The home agent cannot use a Routing header to forward
   these intercepted packets to the mobile node, since it cannot modify
   the packet in flight without invalidating any existing IPv6 AH [5] or
   ESP [6] header present in the packet.

   In order to forward each intercepted packet to the mobile node, the
   home agent MUST tunnel the packet to the mobile node using IPv6
   encapsulation [15]; the tunnel entry point node is the home agent,
   and the tunnel exit point node is the primary care-of address as
   registered with the home agent.  When a home agent encapsulates
   an intercepted packet for forwarding to the mobile node, the home
   agent sets the Source Address in the new tunnel IP header to the
   home agent's own IP address, and sets the Destination Address
   in the tunnel IP header to the mobile node's primary care-of
   address.  When received by the mobile node (using its primary care-of
   address), normal processing of the tunnel header [15] will result in
   decapsulation and processing of the original packet by the mobile
   node.

   However, packets addressed to the mobile node's link-local address
   MUST NOT be tunneled to the mobile node.  Instead, such a packet MUST
   be discarded, and the home agent SHOULD return an ICMP Destination
   Unreachable, Code 3, message to the packet's Source Address (unless
   this Source Address is a multicast address).  Packets addressed to
   the mobile node's site-local address SHOULD be tunneled to the mobile
   node by default, but this behavior MUST be configurable to disable
   it; currently, the exact definition and semantics of a "site" and a
   site-local address are incompletely defined in IPv6, and this default
   behavior might change at some point in the future.

   Tunneling of multicast packets to a mobile node follows similar
   limitations to those defined above for unicast packets addressed to
   the mobile node's link-local and site-local addresses.  Multicast
   packets addressed to a multicast address with link-local scope [3],
   to which the mobile node is subscribed, MUST NOT be tunneled
   to the mobile node; such packets SHOULD be silently discarded
   (after delivering to other local multicast recipients).  Multicast
   packets addressed to a multicast address with scope larger
   than link-local but smaller than global (e.g., site-local and
   organization-local) [3], to which the mobile node is subscribed,
   SHOULD be tunneled to the mobile node by default, but this behavior
   MUST be configurable to disable it; this default behavior might
   change at some point in the future as the definition of these scopes
   become more completely defined in IPv6.





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   Before tunneling a packet to the mobile node, the home agent MUST
   perform any IPsec processing as indicated by the security policy data
   base.


10.6. Handling Reverse Tunneled Packets from a Mobile Node

   Unless a binding has been established between the mobile node and a
   correspondent node, traffic from the mobile node to the correspondent
   node goes through a reverse tunnel.  This tunnel extends between the
   mobile node and the home agent.  Home agents MUST support reverse
   tunneling as follows:

    -  The tunneled traffic arrives to the home agent using IPv6
       encapsulation [15].

    -  The tunnel entry point is the primary care-of address as
       registered with the home agent and the tunnel exit point is the
       home agent.

    -  When a home agent decapsulates a tunneled packet from the mobile
       node, the home agent verifies that the Source Address in the
       tunnel IP header is the mobile node's primary care-of address.

   Reverse tunneled packets MAY be discarded unless accompanied by a
   valid AH or ESP header, depending on the security policies used by
   the home agent.  In any case, the home agent MUST check that the
   source address in the tunneled packets corresponds to the currently
   registered location of the mobile node, as otherwise any node in the
   Internet could send traffic through the home agent and escape ingress
   filtering limitations.

   The support for authenticated reverse tunneling allows the home agent
   to protect the home network and correspondent nodes from malicious
   nodes masquerading as a mobile node, even if they know the current
   location of the real mobile node.


10.7. Protecting Return Routability Packets

   The return routability procedure described in Section 5 assumes that
   the confidentiality of the Home Test Init and Home Test messages is
   protected as it is tunneled from the home agent to the mobile node.
   Therefore, the home agent MUST support IPsec ESP for the protection
   of packets belonging to the return routability procedure.  Support
   for a non-null encryption transform MUST be available.  In this case
   it isn't necessary to distinguish between different kinds of packets
   within the return routability procedure.






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   The above protection SHOULD be turned on and used with all mobile
   nodes.  The use is controlled by configuration of the IPsec security
   policy database both at the mobile node and at the home agent.

   As described earlier, the Binding Update and Binding Acknowledgement
   messages require protection between the home agent and the mobile
   node.  These messages and the return routability messages employ
   the same protocol from the point of view of the security policy
   database, the Mobility Header.  One way to set up the security policy
   database is to have one rule for the Mobility Header traffic between
   the mobile node and the home agent addresses, and an optional rule
   following it for Mobility Header traffic between the mobile node and
   any other address.


10.8. Receiving Router Advertisement Messages

   For each link on which a router provides service as a home agent,
   the router maintains a Home Agents List recording information
   about all other home agents on that link.  This list is used in
   the dynamic home agent address discovery mechanism, described in
   Section 10.9.  The information for the list is learned through
   receipt of the periodic unsolicited multicast Router Advertisements,
   in a manner similar to the Default Router List conceptual data
   structure maintained by each host for Neighbor Discovery [12].  In
   the construction of the Home Agents List, the Router Advertisements
   are from each other home agent on the link, and the Home Agent (H)
   bit is set in them.

   On receipt of a valid Router Advertisement, as defined in the
   processing algorithm specified for Neighbor Discovery [12], the home
   agent performs the following steps, in addition to any steps already
   required of it by Neighbor Discovery:

    -  If the Home Agent (H) bit in the Router Advertisement is not set,
       check to see if the sending node has an entry in the current Home
       Agents List.  If it does, delete the corresponding entry.  In any
       case all of the following steps are skipped.

    -  Otherwise, extract the Source Address from the IP header of the
       Router Advertisement.  This is the link-local IP address on this
       link of the home agent sending this Advertisement [12].

    -  Determine from the Router Advertisement the preference for this
       home agent.  If the Router Advertisement contains a Home Agent
       Information Option, then the preference is taken from the Home
       Agent Preference field in the option; otherwise, the default
       preference of 0 MUST be used.

    -  Determine from the Router Advertisement the lifetime for
       this home agent.  If the Router Advertisement contains a Home



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       Agent Information Option, then the lifetime is taken from
       the Home Agent Lifetime field in the option; otherwise, the
       lifetime specified by the Router Lifetime field in the Router
       Advertisement SHOULD be used.

    -  If the link-local address of the home agent sending this
       Advertisement is already present in this home agent's Home
       Agents List and the received home agent lifetime value is zero,
       immediately delete this entry in the Home Agents List.

    -  Otherwise, if the link-local address of the home agent sending
       this Advertisement is already present in the receiving home
       agent's Home Agents List, reset its lifetime and preference to
       the values determined above.

    -  If the link-local address of the home agent sending this
       Advertisement, as determined above, is not already present in
       the Home Agents List maintained by the receiving home agent, and
       the lifetime for the sending home agent, as determined above,
       is non-zero, create a new entry in the list, and initialize its
       lifetime and preference to the values determined above.

    -  If the Home Agents List entry for the link-local address of
       the home agent sending this Advertisement was not deleted as
       described above, determine any global address(es) of the home
       agent based on each Prefix Information option received in
       this Advertisement in which the Router Address (R) bit is set
       (Section 7.2).  For each such global address determined from this
       Advertisement, add this global address to the list of global
       addresses for this home agent in this Home Agents List entry.

   A home agent SHOULD maintain an entry in its Home Agents List for
   each such valid home agent address until that entry's lifetime
   expires, after which time the entry MUST be deleted.


10.9. Dynamic Home Agent Address Discovery

   A mobile node, while away from home, MAY use the dynamic home agent
   address discovery mechanism in section 11.3.2 to attempt to discover
   the address of one or more routers serving as home agents on its home
   link.  This discovery might become necessary, for example, if some
   nodes on its home link have been reconfigured while the mobile node
   has been away from home, such that the router that was operating as
   the mobile node's home agent has been replaced by a different router
   serving this role.

   As described in Section 11.3.2, a mobile node attempts dynamic
   home agent address discovery by sending an ICMP Home Agent Address
   Discovery Request message to the "Mobile IPv6 Home-Agents" anycast
   address [16] for its home IP subnet prefix, using its care-of address



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   as the Source Address of the packet.  A home agent receiving such a
   Home Agent Address Discovery Request message that is serving this
   subnet (the home agent is configured with this anycast address on one
   of its network interfaces) SHOULD return an ICMP Home Agent Address
   Discovery Reply message to the mobile node (at its care-of address
   that was used as the Source Address of the Request message), with the
   Source Address of the Reply packet set to one of the global unicast
   addresses of the home agent.  The Home Agent Addresses field in the
   Reply message is constructed as follows:

    -  The Home Agent Addresses field SHOULD contain one global IP
       address for each home agent currently listed in this home
       agent's own Home Agents List (Section 4.5).  However, if this
       home agent's own global IP address would be placed in the list
       (as described below) as the first entry in the list, then this
       home agent SHOULD NOT include its own address in the Home Agent
       Addresses field in the Reply message.  Not placing this home
       agent's own IP address in the list will cause the receiving
       mobile node to consider this home agent as the most preferred
       home agent; otherwise, this home agent will be considered to be
       preferred in its order given by its place in the list returned.

    -  The IP addresses in the Home Agent Addresses field SHOULD be
       listed in order of decreasing preference value, based either
       on the respective advertised preference from a Home Agent
       Information option or on the default preference of 0 if no
       preference is advertised (or on the configured home agent
       preference for this home agent itself).  The home agent with
       the highest preference SHOULD be listed first in the Home Agent
       Addresses field, and the home agent with the lowest preference
       SHOULD be listed last.

    -  Among home agents with equal preference, their IP addresses
       in the Home Agent Addresses field SHOULD be listed in an
       order randomized with respect to other home agents with equal
       preference, each time a Home Agent Address Discovery Reply
       message is returned by this home agent.

    -  For each entry in this home agent's Home Agents List, if more
       than one global IP address is associated with this list entry,
       then one of these global IP addresses SHOULD be selected
       to include in the Home Agent Addresses field in the Reply
       message.  As described in Section 4.5, one Home Agents List
       entry, identified by the home agent's link-local address,
       exists for each home agent on the link; associated with that
       list entry is one or more global IP addresses for this home
       agent, learned through Prefix Information options with the
       Router Address (R) bit is set, received in Router Advertisements
       from this link-local address.





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       The selected global IP address for each home agent to include in
       forming the Home Agent Addresses field in the Reply message MUST
       be the global IP address of the respective home agent sharing a
       prefix with the Destination IP address of the Request message;
       if no such global IP address is known for some home agent, an
       entry for that home agent MUST NOT be included in the Home Agent
       Addresses field in the Reply message.

    -  In order to avoid the possibility of the Reply message packet
       being fragmented (or rejected by an intermediate router with an
       ICMP Packet Too Big message [14]), if the resulting total packet
       size containing the complete list of home agents in the Home
       Agent Addresses field would exceed the minimum IPv6 MTU [11], the
       home agent SHOULD reduce the number of home agent IP addresses
       returned in the packet to the number of addresses that will fit
       without exceeding this limit.  The home agent addresses returned
       in the packet SHOULD be those from the complete list with the
       highest preference.


10.9.1. Aggregate List of Home Network Prefixes

   IPv6 provides mechanisms for node configuration when it turns on,
   and in renumbering a subnet, such as when a site switches to a new
   network service provider.  These mechanisms are a part of Neighbor
   Discovery [12] and Address Autoconfiguration [13].

   In renumbering, new prefixes and addresses can be introduced for the
   subnet and old ones can be deprecated and removed.  These mechanisms
   are defined to work while all nodes using the old prefixes are at
   home, connected to the link using these prefixes.  Mobile IPv6
   extends these mechanisms to work also with mobile nodes that are away
   from home when the renumbering takes place.

   Mobile IPv6 arranges to propagate relevant prefix information to the
   mobile node when it is away from home, so that it may be used in
   mobile node home address configuration, and in network renumbering.
   In this mechanism, mobile nodes away from home receive Mobile Prefix
   Advertisements messages with Prefix Information Options, which give
   the valid lifetime and preferred lifetime for available prefixes on
   the home link.

   To avoid possible security attacks from forged Mobile Prefix
   Advertisements all such Advertisements MUST be authenticated to the
   mobile node by its home agent using IPsec [4, 5, 6].

   A mobile node on a remote network SHOULD autoconfigure all of the
   global IP addresses, which it would autoconfigure if it were attached
   to its home network, from network prefixes representing network
   addresses that are served by home agents.  Site-local addresses MAY
   be autoconfigured if the mobile node is roaming in a network on the



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   same site as its home addresses.  Site-local addresses and addresses
   not served by a home agent MUST NOT be autoconfigured, since they are
   unusable in the remote network.

   To support this, the home agent monitors prefixes advertised by
   itself and other home agents routers on the home link, and passes
   this aggregated list of relevant subnet prefixes on to the mobile
   node in Mobile Prefix Advertisements.

   The home agent SHOULD construct the aggregate list of home subnet
   prefixes as follows:

    -  Copy prefix information defined in the home agent's AdvPrefixList
       on the home subnet's interfaces to the aggregate list.  Also
       apply any changes made to the AdvPrefixList on the home agent to
       the aggregate list.

    -  Check valid prefixes received in Router Advertisements
       from the home network for consistency with the home agent's
       AdvPrefixList, as specified in section 6.2.7 of RFC 2461
       (Neighbor Discovery [12]).  Do not update the aggregate list with
       any information from received prefixes that fail this check.

    -  Check Router Advertisements which contain an `H' bit (from other
       home agents) for valid prefixes that are not yet in the aggregate
       list, and if they are usable for autoconfiguration (`A' bit set,
       and prefix length is valid for address autoconfiguration on the
       home subnet) add them and preserve the `L' flag value.  Clear the
       `R' flag and zero the interface-id portion of the prefix field
       to prevent mobile nodes from treating another router's interface
       address as belonging to the home agent.  Treat the lifetimes
       of these prefixes as decrementing in real time, as defined in
       section 6.2.7 of RFC 2461 [12].

    -  Do not perform consistency checks on valid prefixes received in
       Router Advertisements on the home network that do not exist in
       the home agent's AdvPrefixList.  Instead, if the prefixes already
       exist in the aggregate list, update the prefix lifetime fields in
       the aggregate list according to the rules specified for hosts in
       section 6.3.4 of RFC 2461 (Neighbor Discovery [12]) and section
       5.5.3 of RFC 2462 (Stateless Address Autoconfiguration [13]).

    -  If the L flag is set on valid prefixes received in a Router
       Advertisement, and that prefix already exists in the aggregate
       list, set the flag in the aggregate list.  Ignore the flag if it
       is clear.

    -  Delete prefixes from the aggregate list when their valid
       lifetimes expire.





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   The home agent uses the information in the aggregate list to
   construct Mobile Prefix Advertisements.  It may be possible to
   construct an aggregate list by combining information contained in the
   home agent's AdvPrefixList and its Home Agents List used for Dynamic
   Home Agent Address Discovery (Section 11.3.2).


10.9.2. Scheduling Prefix Deliveries to the Mobile Node

   A home agent serving a mobile node will schedule the delivery of new
   prefix information to that mobile node when any of the following
   conditions occur:

   MUST:

    -  The valid or preferred lifetime or the state of the flags changes
       for the prefix of the mobile node's registered home address.

    -  The mobile node requests the information with a Mobile Prefix
       Solicitation (see section 11.3.3).

   MAY:

    -  A new prefix is added to the aggregate list.

    -  The valid or preferred lifetime or the state of the flags changes
       for a prefix which is not used in any binding cache entry for
       this mobile node.

   The home agent uses the following algorithm to determine when to send
   prefix information to the mobile node.

    -  If the mobile node has not received the prefix information within
       the last HomeRtrAdvInterval seconds, then transmit the prefix
       information.  This MAY be done according to a periodically
       scheduled transmission.

    -  If a mobile node sends a solicitation, answer right away.

    -  If a prefix in the aggregate list that matches the mobile node's
       home registration is added, or if its information changes in
       any way that does not cause the mobile node's address to go
       deprecated, ensure that a transmission is scheduled (as described
       below), and calculate RAND_ADV_DELAY in order to randomize the
       time at which the transmission is scheduled.

    -  If a home registration expires, cancel any scheduled
       advertisements to the mobile node.

   Suppose that the home agent already has scheduled the transmission
   of a Router Advertisement to the mobile node.  Then add the data



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   from the existing scheduled transmission to the newly scheduled
   transmission, deleting the previously scheduled transmission event.
   In this case, the home agent does not perform the following algorithm
   to schedule an advertisement to the mobile node.

   Otherwise, the home agent uses the following algorithm to compute
   a fresh value for RAND_ADV_DELAY, the offset from the current time
   for the scheduled transmission.  The computation is expected to
   alleviate bursts of advertisements when prefix information changes.
   In addition, a home agent MAY further reduce the rate of packet
   transmission by further delaying individual advertisements, if needed
   to avoid overwhelming local network resources.

   Calculate the maximum delay for the scheduled Advertisment as
   follows.
       MAX_SCHEDULE_DELAY == min (MAX_PFX_ADV_DELAY, Preferred Lifetime)

   Then compute RAND_ADV_DELAY ==
       MinRtrAdvInt + rand()*(MAX_SCHEDULE_DELAY - MinRtrAdvInt)

   The home agent SHOULD periodically continue to retransmit an
   unsolicited Advertisement to the mobile node, either until it is
   acknowledged by the receipt from the mobile node of a Binding Update
   with a home address matching the new home prefix in the packet,
   or until the home agent receives a Mobile Prefix Solicitation
   from the mobile node.  The home agent MUST wait PREFIX_ADV_TIMEOUT
   before the first retransmission, and double the retransmission wait
   time for every succeeding retransmission, up until a maximum of
   PREFIX_ADV_RETRIES attempts.  If the mobile node's bindings expire
   before the matching Binding Update has been received, then the home
   agent MUST NOT attempt any more retransmissions, even if not all
   PREFIX_ADV_RETRIES have been retransmitted.  After another Binding
   Update is received from the mobile node, and if the mobile node has
   not returned to the home network in the meantime, the home agent
   SHOULD begin the process again of transmitting the unsolicited
   Advertisement.

   If while the home agent is still retransmitting a Mobile Prefix
   Advertisement to the mobile node, another condition as described
   above occurs on the home link causing another Prefix Advertisement to
   be sent to the mobile node, the home agent SHOULD combine any Prefix
   Information options in the unacknowledged Mobile Prefix Advertisement
   into the new Advertisement, discard the old Advertisement, and then
   begin retransmitting the new one.  according to the above algorithm.


10.9.3. Sending Advertisements to the Mobile Node

   When sending a Mobile Prefix Advertisement to the mobile node, the
   home agent MUST construct the packet as follows:




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    -  The Source Address in the packet's IPv6 header MUST be set to
       the home agent's IP address to which the mobile node addressed
       its current home registration, or its default global home agent
       address if no binding exists.

    -  If a security association exists with the mobile node's address,
       the packet MUST be protected by IPsec [4, 5, 6] to guard against
       malicious Mobile Prefix Advertisements.  The IPsec protection
       MUST provide sender authentication and data integrity protection
       covering the Mobile Prefix Advertisement, and MAY provide replay
       protection.

    -  If the advertisement was solicited, it MUST be authenticated and
       destined to the source address of the solicitation.  If it was
       triggered by prefix changes or renumbering, the advertisement's
       destination will be the mobile node's home address in the binding
       which triggered the rule.

    -  The packet MUST be sent as any other unicast IPv6 packet.  If a
       care-of address is used, the packet will be delivered directly.
       If a binding exists, the home agent will send the packet with
       a routing header containing the care-of address, as any other
       packet sent to the mobile node originated by the home agent
       (rather than using IPv6 encapsulation, as would be used by the
       home agent for intercepted packets).


10.9.4. Lifetimes for Changed Prefixes

   As described in Section 10.2, the lifetime returned by the home agent
   in a Binding Acknowledgement MUST be no greater than the remaining
   valid lifetime for the subnet prefix in the mobile node's home
   address.  This limit on the binding lifetime serves to prohibit use
   of a mobile node's home address after it becomes invalid.


11. Mobile Node Operation

11.1. Conceptual Data Structures

   Each mobile node MUST maintain a Binding Update List and Home Agents
   List.

   The rules for maintaining a Home Agents List are same for home agents
   and correspondent nodes, and have been described in Section 10.1.

   The Binding Update List records information for each Binding Update
   sent by this mobile node, for which the Lifetime sent in that Binding
   Update has not yet expired.  The Binding Update List includes all
   bindings sent by the mobile node:  those to correspondent nodes,
   those to the mobile node's home agent, and those to a home agent



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   on the link on which the mobile node's previous care-of address is
   located.  It also contains Binding Updates which are waiting for
   the completion of the return routability procedure before they can
   be sent.  However, for multiple Binding Updates sent to the same
   destination address, the Binding Update List contains only the most
   recent Binding Update (i.e., with the greatest Sequence Number value)
   sent to that destination.  The Binding Update List MAY be implemented
   in any manner consistent with the external behavior described in this
   document.

   Each Binding Update List entry conceptually contains the following
   fields:

    -  The IP address of the node to which a Binding Update was sent.
       If the Binding Update was successfully received by that node
       (e.g., not lost by the network), a Binding Cache entry may have
       been created or updated based on this Binding Update.  The
       Binding Cache entry may still exist, if that node has not deleted
       the entry before its expiration (e.g., to reclaim space in its
       Binding Cache for other entries).

    -  The home address for which that Binding Update was sent.  This
       will be one of the following:

        *  one the mobile node's home addresses for typical Binding
           Updates (Sections 11.6.1 and 11.6.2), or

        *  the mobile node's previous care-of address for Binding
           Updates sent to establish forwarding from the mobile node's
           previous location (Section 11.6.6).

    -  The care-of address sent in that Binding Update.  This value
       is necessary for the mobile node to determine if it has sent a
       Binding Update giving its new care-of address to this destination
       after changing its care-of address.

    -  The initial value of the Lifetime field sent in that Binding
       Update.

    -  The remaining lifetime of that binding.  This lifetime is
       initialized from the Lifetime value sent in the Binding Update
       and is decremented until it reaches zero, at which time this
       entry MUST be deleted from the Binding Update List.

    -  The maximum value of the Sequence Number field sent in previous
       Binding Updates to this destination.  The Sequence Number field
       is 16 bits long, and all comparisons between Sequence Number
       values MUST be performed modulo 2**15 in the manner explained
       already in Section 9.4.1.





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    -  The time at which a Binding Update was last sent to this
       destination, as needed to implement the rate limiting restriction
       for sending Binding Updates.

    -  The state of any retransmissions needed for this Binding Update,
       if the Acknowledge (A) bit was set in this Binding Update.  This
       state includes the time remaining until the next retransmission
       attempt for the Binding Update, and the current state of the
       exponential back-off mechanism for retransmissions.

    -  A flag that, when set, indicates that future Binding Updates
       should not be sent to this destination.  The mobile node sets
       this flag in the Binding Update List entry when it receives an
       ICMP Parameter Problem, Code 1, error message in response to
       a return routability message or Binding Update sent to that
       destination, as described in Section 11.7.

   The Binding Update list also conceptually contains data related to
   running the return routability procedure.  This data is relevant only
   for Binding Updates sent to correspondent nodes.

    -  The time at which a Home Test Init or Care-of Test Init message
       was last sent to this destination, as needed to implement the
       rate limiting restriction for the return routability procedure.

    -  The state of any retransmissions needed for this return
       routability procedure.  This state includes the time remaining
       until the next retransmission attempt and the current state of
       the exponential back-off mechanism for retransmissions.

    -  Mobile cookie values used the Home Test Init and Care-of Test
       Init messages.

    -  Home and care-of cookies received from the correspondent node.

    -  Home and care-of nonce indices received from the correspondent
       node.

    -  The time at which each of the cookies was received from this
       correspondent node, as needed to implement cookie reuse while
       moving.


11.2. Packet Processing

11.2.1. Sending Packets While Away from Home

   While a mobile node is away from home, it continues to use its home
   address, as well as also using one or more care-of addresses.  When
   sending a packet while away from home, a mobile node MAY choose among




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   these in selecting the address that it will use as the source of the
   packet, as follows:

    -  Protocols layered over IP will generally treat the mobile node's
       home address as its IP address for most packets.  For packets
       sent that are part of transport-level connections established
       while the mobile node was at home, the mobile node MUST use
       its home address.  Likewise, for packets sent that are part of
       transport-level connections that the mobile node may still be
       using after moving to a new location, the mobile node SHOULD
       use its home address in this way.  When sending such packets,
       the delivery method depends on whether a binding exists with
       the correspondent node.  If a binding exists, the mobile node
       SHOULD send the packets directly to the correspondent node.
       Otherwise, if a binding does not exist, the mobile node MUST use
       reverse tunneling.  Detailed operation for both of these cases is
       described later in this section.

    -  For short-term communication, particularly for communication that
       may easily be retried if it fails, the mobile node MAY choose
       to directly use one of its care-of addresses as the source of
       the packet, thus not requiring the use of a Home Address option
       in the packet.  An example of this type of communication might
       be DNS queries sent by the mobile node [27, 28].  Using the
       mobile node's care-of address as the source for such queries will
       generally have a lower overhead than using the mobile node's
       home address, since no extra options need be used in either the
       query or its reply, and all packets can be routed normally,
       directly between their source and destination without relying
       on Mobile IP. If the mobile node has no particular knowledge
       that the communication being sent fits within this general type
       of communication, however, the mobile node SHOULD NOT use its
       care-of address as the source of the packet in this way.

   For packets sent by a mobile node while it is at home, no special
   Mobile IP processing is required for sending this packet.  Likewise,
   if the mobile node uses any address other than any of its home
   addresses as the source of a packet sent while away from home no
   special Mobile IP processing is required for sending that packet.  In
   each case, the packet is simply addressed and transmitted in the same
   way as any normal IPv6 packet.

   For packets sent by the mobile node sent while away from home using
   the mobile node's home address as the source, special Mobile IP
   processing of the packet is required.  This can be done in two ways,
   as described above.  These ways are:

      direct delivery

         This is manner of delivering packets does not require going
         through the home network, and typically will enable faster and



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         more reliable transmission.  A mobile node SHOULD arrange to
         supply the home address in a Home Address option, and allowing
         the IPv6 header's Source Address field to be set to one of the
         mobile node's care-of addresses; the correspondent node will
         then use the address supplied in the Home Address option to
         serve the function traditionally done by the Source IP address
         in the IPv6 header.  the mobile node's home address is then
         supplied to higher protocol layers and applications.

         Specifically:

          -  Construct the packet using the mobile node's home address
             as the packet's Source Address, in the same way as if the
             mobile node were at home.

          -  Insert a Home Address option into the packet, with the Home
             Address field copied from the original value of the Source
             Address field in the packet.

          -  Change the Source Address field in the packet's IPv6 header
             to one of the mobile node's care-of addresses.  This will
             typically be the mobile node's current primary care-of
             address, but MUST be a care-of address with a subnet prefix
             that is on-link on the network interface on which the
             mobile node will transmit the packet.

         By using the care-of address as the Source Address in the IPv6
         header, with the mobile node's home address instead in the Home
         Address option, the packet will be able to safely pass through
         any router implementing ingress filtering [24].

      reverse tunneling

         This is the mechanism which tunnels the packets via the home
         agent.  It isn't as efficient as the above mechanism, but is
         needed if there is no binding yet with the correspondent node.
         Specifically:

          -  The packet is sent to the home agent using IPv6
             encapsulation [15].

          -  The Source Address in the tunnel packet is the primary
             care-of address as registered with the home agent.

          -  The Destination Address in the tunnel packet is the home
             agent's address.

         Reverse tunneled packets MAY be protected using a AH or ESP
         header, depending on the security policies used by the home
         agent.  The support for encrypted reverse tunneling allows
         mobile nodes to defeat certain kinds of traffic analysis, and



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         provides a mechanism by which routers on the home network can
         distinguish authorized traffic from other possibly malicious
         traffic.


11.2.2. Interaction with Outbound IPsec Processing

   This section sketches the interaction between outbound Mobile
   IP processing and outbound IP Security (IPsec) processing for
   packets sent by a mobile node while away from home.  Any specific
   implementation MAY use algorithms and data structures other than
   those suggested here, but its processing MUST be consistent with the
   effect of the operation described here and with the relevant IPsec
   specifications.  In the steps described below, it is assumed that
   IPsec is being used in transport mode [4] and that the mobile node is
   using its home address as the source for the packet (from the point
   of view of higher protocol layers or applications, as described in
   Section 11.2.1):

    -  The packet is created by higher layer protocols and applications
       (e.g., by TCP) as if the mobile node were at home and Mobile IP
       were not being used.

    -  As part of outbound packet processing in IP, the packet is
       compared against the IPsec security policy database to determine
       what processing is required for the packet [4].

    -  If IPsec processing is required, the packet is either mapped to
       an existing Security Association (or SA bundle), or a new SA (or
       SA bundle) is created for the packet, according to the procedures
       defined for IPsec.

    -  Since the mobile node is away from home, the mobile is either
       using reverse tunneling or route optimization to reach the
       correspondent node.

       If reverse tunneling is used, the packet is constructed in the
       normal manner and then tunneled through the home agent.

       If route optimization is in use, the mobile node inserts a Home
       Address destination option into the packet, replacing the Source
       Address in the packet's IP header with a care-of address suitable
       for the link on which the packet is being sent, as described in
       Section 11.2.1.  The Destination Options header in which the
       Home Address destination option is inserted MUST appear in the
       packet after the Routing Header, if present, and before the IPsec
       (AH [5] or ESP [6]) header, so that the Home Address destination
       option is processed by the destination node before the IPsec
       header is processed.





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       Finally, once the packet is fully assembled, the necessary IPsec
       authentication (and encryption, if required) processing is
       performed on the packet, initializing the Authentication Data in
       the IPsec header.  The AH authentication data MUST be calculated
       as if the following were true:

        *  the IPv6 source address in the IPv6 header contains the
           mobile node's home address,

        *  the Home Address field of the Home Address destination option
           (section 6.3) contains the new care-of address.

    -  This allows, but does not require, the receiver of the packet
       containing a Home Address destination option to exchange the two
       fields of the incoming packet, simplifying processing for all
       subsequent packet headers.  However, such an exchange is not
       required, as long as the result of the authentication calculation
       remains the same.

   In addition, when using any automated key management protocol [4]
   (such as IKE [9]) to create a new SA (or SA bundle) while away from
   home, a mobile node MUST take special care in its processing of the
   key management protocol.  Otherwise, other nodes with which the
   mobile node must communicate as part of the automated key management
   protocol processing may be unable to correctly deliver packets to
   the mobile node if they and/or the mobile node's home agent do
   not then have a current Binding Cache entry for the mobile node.
   For the default case of using IKE as the automated key management
   protocol [9, 4], such problems can be avoided by the following
   requirements on the use of IKE by a mobile node while away from home:

    -  The mobile node MUST use its care-of address as the Source
       Address of all packets it sends as part of the key management
       protocol (without use of Mobile IP for these packets, as
       suggested in Section 11.2.1).

    -  In addition, for all security associations bound to the mobile
       node's home address established by way of IKE, the mobile node
       MUST include an ISAKMP Identification Payload [8] in the IKE
       exchange, giving the mobile node's home address as the initiator
       of the Security Association [7].


11.2.3. Receiving Packets While Away from Home

   While away from home, a mobile node will receive packets addressed to
   its home address, by one of three methods:

    -  Packets sent by a correspondent node that does not have a Binding
       Cache entry for the mobile node, will be tunneled to the mobile
       node via its home agent.



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    -  Packets sent by a correspondent node that has a Binding Cache
       entry for the mobile node that contains the mobile node's current
       care-of address, will be sent by the correspondent node using
       a type 2 Routing header.  The packet will be addressed to the
       mobile node's care-of address, with the final hop in the Routing
       header directing the packet to the mobile node's home address;
       the processing of this last hop of the Routing header is entirely
       internal to the mobile node, since the care-of address and home
       address are both addresses within the mobile node.

    -  Packets sent by a correspondent node that has a Binding
       Cache entry for the mobile node that contains an out-of-date
       care-of address for the mobile node, will also be sent by the
       correspondent node using a type 2 Routing header, as described
       above.  If the mobile node sent a Binding Update to a home agent
       on the link on which its previous care-of address is located
       (Section 11.6.6), and if this home agent is still serving as a
       home agent for the mobile node's previous care-of address, then
       such a packet will be delivered to the mobile node via this home
       agent.

   For packets received by the first of these methods, the mobile node
   MUST check that the IPv6 source address of the tunnel packet is the
   IP address of its home agent.

   For packets received by either the first or last of these three
   methods, the mobile node SHOULD send a Binding Update to the original
   sender of the packet, as described in Section 11.6.2, subject to
   the rate limiting defined in Section 11.6.9.  The mobile node MUST
   also process the received packet in the manner defined for IPv6
   encapsulation [15], which will result in the encapsulated (inner)
   packet being processed normally by upper-layer protocols within the
   mobile node, as if it had been addressed (only) to the mobile node's
   home address.

   For packets received by the second method above (using a Type 2
   Routing header), the following rules will result in the packet being
   processed normally by upper-layer protocols within the mobile node,
   as if it had been addressed to the mobile node's home address.















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   A node receiving a packet addressed to itself (i.e., one of the
   node's addresses is in the IPv6 destination field) follows the next
   header chain of headers and processes them.  When it encounters
   a Type 2 Routing header during this processing it performs the
   following checks.  If any of these checks fail the node MUST silently
   discard the packet.

    -  The length field in the RH is exactly 2.

    -  The segments left field in the RH is either 0 or 1.  (Values on
       the wire are always 1.  But implementations may process RH in a
       manner the value may become 0 after RH has been processed, but
       before the rest of the packet is processed.)

    -  The Home Address field in the RH is one of the node's home
       addresses, if the segments left field was 1.

   Once the above checks have been performed, the node swaps the IPv6
   destination field with the Home Address field in the RH, decrements
   segments left, and resubmits the packet to IP for processing the
   next header.  Conceptually this follows the same model as in RFC
   2460.  However, in the case of Type 2 Routing header this can be
   simplified since it is known that the packet will not be forwarded to
   a different node.

   The definition of AH requires the sender to calculate the AH
   integrity check value of a routing header in a way as it appears in
   the receiver after it has processed the header.  Since IPsec headers
   follow the Routing Header, any IPsec processing will operate on
   the packet with the home address in the IP destination field and
   segments left being zero.  Thus, the AH calculations at the sender
   and receiver will have an identical view of the packet.


11.2.4. Routing Multicast Packets

   A mobile node that is connected to its home link functions in the
   same way as any other (stationary) node.  Thus, when it is at home,
   a mobile node functions identically to other multicast senders and
   receivers.  This section therefore describes the behavior of a mobile
   node that is not on its home link.

   In order to receive packets sent to some multicast group, a mobile
   node must join that multicast group.  One method by which a mobile
   node MAY join the group is via a (local) multicast router on the
   foreign link being visited.  The mobile node SHOULD use one of its
   care-of addresses that shares a subnet prefix with the multicast
   router, as the source IPv6 address of its multicast group membership
   control messages.  If the multicast applications depend on the
   address of the joining node, the mobile node MAY treat the router as
   a correspondent node and establish a binding with it.  The mobile



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   node can then use the Home Address destination option in the sent
   control messages.

   Alternatively, a mobile node MAY join multicast groups via a
   bi-directional tunnel to its home agent.  The mobile node tunnels its
   multicast group membership control packets to its home agent, and the
   home agent forwards multicast packets down the tunnel to the mobile
   node.

   A mobile node that wishes to send packets to a multicast group
   also has two options:  (1) send directly on the foreign link being
   visited; or (2) send via a tunnel to its home agent.  Because
   multicast routing in general depends upon the Source Address used in
   the IPv6 header of the multicast packet, a mobile node that tunnels a
   multicast packet to its home agent MUST use its home address as the
   IPv6 Source Address of the inner multicast packet.


11.3. Home Agent and Prefix Management

11.3.1. Receiving Local Router Advertisement Messages

   Each mobile node maintains a Home Agents List recording information
   about all home agents from which it receives a Router Advertisement,
   for which the home agent lifetime indicated in that Router
   Advertisement has not yet expired.  This list is used by the mobile
   node to enable it to send a Binding Update to the global unicast
   address of a home agent on its previous link when it moves to a new
   link, as described in Section 11.6.6.  On receipt of a valid Router
   Advertisement, as defined in the processing algorithm specified for
   Neighbor Discovery [12], the mobile node performs the following
   steps, in addition to any steps already required of it by Neighbor
   Discovery.

    -  If the Home Agent (H) bit in the Router Advertisement is not set,
       and the sending node currently has an entry in the node's Home
       Agents List, delete the corresponding entry.  Subsequently, skip
       all of the following steps.

    -  Otherwise, extract the Source Address from the IP header of the
       Router Advertisement.  This is the link-local IP address on this
       link of the home agent sending this Advertisement [12].

    -  Determine from the Router Advertisement the preference for this
       home agent.  If the Router Advertisement contains a Home Agent
       Information Option, then the preference is taken from the Home
       Agent Preference field in the option; otherwise, the default
       preference of 0 MUST be used.

    -  Determine from the Router Advertisement the lifetime for
       this home agent.  If the Router Advertisement contains a Home



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       Agent Information Option, then the lifetime is taken from
       the Home Agent Lifetime field in the option; otherwise, the
       lifetime specified by the Router Lifetime field in the Router
       Advertisement SHOULD be used.

    -  If the link-local address of the home agent sending this
       Advertisement is already present in this mobile node's Home
       Agents List and the received home agent lifetime value is zero,
       immediately delete this entry in the Home Agents List.

    -  Otherwise, if the link-local address of the home agent sending
       this Advertisement is already present in the receiving mobile
       node's Home Agents List, reset its lifetime and preference to the
       values determined above.

    -  If the link-local address of the home agent sending this
       Advertisement, as determined above, is not already present in the
       Home Agents List maintained by the receiving mobile node, and
       the lifetime for the sending home agent, as determined above,
       is non-zero, create a new entry in the list, and initialize its
       lifetime and preference to the values determined above.

    -  If the Home Agents List entry for the link-local address of
       the home agent sending this Advertisement was not deleted as
       described above, determine any global address(es) of the home
       agent based on each Prefix Information option received in
       this Advertisement in which the Router Address (R) bit is set
       (Section 7.2).  For each such global address determined from this
       Advertisement, add this global address to the list of global
       addresses for this home agent in this Home Agents List entry.

   A mobile node SHOULD maintain an entry in its Home Agents List for
   each such valid home agent address until that entry's lifetime
   expires, after which time the entry MUST be deleted.


11.3.2. Dynamic Home Agent Address Discovery

   Sometimes, when the mobile node needs to send a Binding Update to its
   home agent to register its new primary care-of address, as described
   in Section 11.6.1, the mobile node may not know the address of any
   router on its home link that can serve as a home agent for it.  For
   example, some nodes on its home link may have been reconfigured while
   the mobile node has been away from home, such that the router that
   was operating as the mobile node's home agent has been replaced by a
   different router serving this role.

   In this case, the mobile node MAY attempt to discover the address of
   a suitable home agent on its home link.  To do so, the mobile node
   sends an ICMP Home Agent Address Discovery Request message to the
   "Mobile IPv6 Home-Agents" anycast address [16] for its home subnet



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   prefix.  As described in Section 10.9, the home agent on its home
   link that receives this Request message will return an ICMP Home
   Agent Address Discovery Reply message, giving this home agent's own
   global unicast IP address along with a list of the global unicast IP
   address of each other home agent operating on the home link.

   The mobile node, upon receiving this Home Agent Address Discovery
   Reply message, MAY then send its home registration Binding Update to
   the home agent address given as the IP Source Address of the packet
   carrying the Reply message or to any of the unicast IP addresses
   listed in the Home Agent Addresses field in the Reply.  For example,
   if necessary, the mobile node MAY attempt its home registration
   with each of these home agents, in turn, by sending each a Binding
   Update and waiting for the matching Binding Acknowledgement, until
   its registration is accepted by one of these home agents.  In trying
   each of the returned home agent addresses, the mobile node SHOULD try
   each in the order listed in the Home Agent Addresses field in the
   received Home Agent Address Discovery Reply message.  If the home
   agent identified by the Source Address field in the IP header of the
   packet carrying the Home Agent Address Discovery Reply message is
   not listed in the Home Agent Addresses field in the Reply, it SHOULD
   be tried before the first address given in the list; otherwise, it
   SHOULD be tried in its listed order.

   If the mobile node has a current registration with some home agent
   on its home link (the Lifetime for that registration has not yet
   expired), then the mobile node MUST attempt any new registration
   first with that home agent.  If that registration attempt fails
   (e.g., times out or is rejected), the mobile node SHOULD then
   reattempt this registration with another home agent on its home link.
   If the mobile node knows of no other suitable home agent, then it MAY
   attempt the dynamic home agent address discovery mechanism described
   above.

   If, after a mobile node transmits a Home Agent Address Discovery
   Request message to the Home Agents Anycast address, it does not
   receive a corresponding Home Agent Address Discovery Reply message
   within INITIAL_DHAAD_TIMEOUT seconds, the mobile node MAY retransmit
   the same Request message to the same anycast address.  This
   retransmission MAY be repeated up to a maximum of DHAAD_RETRIES
   attempts.  Each retransmission MUST be delayed by twice the time
   interval of the previous retransmission.


11.3.3. Sending Mobile Prefix Solicitations

   When a mobile node has a home address that is about to become
   invalid, it sends a Mobile Prefix Solicitation to its home agent
   in an attempt to acquire fresh routing prefix information.  The
   new information also enables the mobile node to participate in




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   renumbering operations affecting the home network, as described in
   section 10.9.1.

   The mobile node SHOULD send a Solicitation to the home agent when
   its home address will become invalid within MaxRtrAdvInterval
   seconds, where this value is acquired in a previous Mobile Prefix
   Advertisement from the home agent.  If no such value is known, the
   value MAX_PFX_ADV_DELAY seconds is used instead (see section 12).

   This solicitation follows the same retransmission rules specified for
   Router Solicitations [12], except that the initial retransmission
   interval is specified to be INITIAL_SOLICIT_TIMER (see section 12).

   As described in Section 11.6.2, Binding Updates sent by the mobile
   node to other nodes MUST use a lifetime no greater than the remaining
   lifetime of its home registration of its primary care-of address.
   The mobile node SHOULD further limit the lifetimes that it sends on
   any Binding Updates to be within the remaining preferred lifetime
   (see Section 10.9.2) for the prefix in its home address.

   When the lifetime for a changed prefix decreases, and the change
   would cause cached bindings at correspondent nodes in the Binding
   Update List to be stored past the newly shortened lifetime, the
   mobile node MUST issue a Binding Update to all such correspondent
   nodes.

   These limits on the binding lifetime serve to prohibit use of a
   mobile node's home address after it becomes invalid.


11.3.4. Receiving Mobile Prefix Advertisements

   Section 10.9.1 describes the operation of a home agent to support
   boot time configuration and renumbering a mobile node's home subnet
   while the mobile node is away from home.  The home agent sends Mobile
   Prefix Advertisement messages to the mobile node while away from
   home, giving "important" Prefix Information options that describe
   changes in the prefixes in use on the mobile node's home link.

   When a mobile node receives a Mobile Prefix Advertisement, it MUST
   validate it according to the following test:

    -  The Source Address of the IP packet carrying the Mobile Prefix
       Advertisement is the same as the home agent address to which the
       mobile node last sent an accepted "home registration" Binding
       Update to register its primary care-of address.  Otherwise, if
       no such registrations have been made, it SHOULD be the mobile
       node's stored home agent address, if one exists.  Otherwise, if
       the mobile node has not yet discovered its home agent's address,
       it MUST NOT accept Mobile Prefix Advertisements.




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    -  The packet MUST be protected by IPsec [4, 5, 6] to guard against
       malicious prefix advertisements, if a security association
       exists (i.e.  unless the mobile node does not yet have a home
       address configured).  The IPsec protection MUST provide sender
       authentication, data integrity protection, and replay protection,
       covering the advertisement.

   Any received Mobile Prefix Advertisement not meeting this test MUST
   be silently discarded.  For advertisements that do not contain
   a solicitation cookie, the mobile node MAY send a solitication
   containing such a cookie before accepting the advertisement for
   further processing.

   For an accepted Mobile Prefix Advertisement, the mobile node MUST
   process the Prefix Information Options as if they arrived in a
   Router Advertisement on the mobile node's home link [12].  Such
   processing may result in the mobile node configuring a new home
   address, although due to separation between preferred lifetime and
   valid lifetime, such changes should not affect most communication
   by the mobile node, in the same way as for nodes that are at home.
   In this case,, the mobile node MUST return a Binding Update, which
   will be viewed by the home agent as an acknowledgement of the
   corresponding Mobile Prefix Advertisement, which it can cease
   transmitting.  In addition, if the method used for this new home
   address configuration would require the mobile node to perform
   Duplicate Address Detection [13] for the new address if the mobile
   node were located at home, then the mobile node MUST set the
   Duplicate Address Detection (D) bit in this Binding Update to its
   home agent, to request the home agent to perform this Duplicate
   Address Detection on behalf of the mobile node.


11.4. Movement

11.4.1. Movement Detection

   The primary movement detection mechanism for Mobile IPv6 defined
   in this section uses the facilities of IPv6 Neighbor Discovery,
   including Router Discovery and Neighbor Unreachability Detection.
   The mobile node SHOULD supplement this mechanism with other
   information whenever it is available to the mobile node (e.g.,
   from lower protocol layers).  The description here is based on the
   conceptual model of the organization and data structures defined by
   Neighbor Discovery [12].

   Mobile nodes SHOULD use Router Discovery to discover new routers and
   on-link subnet prefixes; a mobile node MAY send Router Solicitation
   messages, or MAY wait for unsolicited (periodic) multicast Router
   Advertisement messages, as specified for Router Discovery [12].
   Based on received Router Advertisement messages, a mobile node
   maintains an entry in its Default Router List for each router, and



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   an entry in its Prefix List for each subnet prefix that it currently
   considers to be on-link.  Each entry in these lists has an associated
   invalidation timer value.  While away from home, a mobile node
   typically selects one default router and one subnet prefix to use
   as the subnet prefix in its primary care-of address.  A mobile node
   MAY also have associated additional care-of addresses, using other
   subnet prefixes from its Prefix List.  The method by which a mobile
   node selects and forms a care-of address from the available subnet
   prefixes is described in Section 11.4.2.  The mobile node registers
   its primary care-of address with its home agent, as described in
   Section 11.6.1.

   While a mobile node is away from home, it is important for the mobile
   node to quickly detect when its default router becomes unreachable.
   When this happens, the mobile node SHOULD switch to a new default
   router and potentially to a new primary care-of address.  If, on the
   other hand, the mobile node becomes unreachable from its default
   router, it should attempt to become reachable through some other
   router.  To detect when its default router becomes unreachable, a
   mobile node SHOULD use Neighbor Unreachability Detection.

   For a mobile node to detect when it has become unreachable from its
   default router, the mobile node cannot efficiently rely on Neighbor
   Unreachability Detection alone, since the network overhead would
   be prohibitively high in many cases.  Instead, when a mobile node
   receives any IPv6 packets from its current default router at all,
   irrespective of the source IPv6 address, it SHOULD use that as an
   indication that it is still reachable from the router.

   Since the router SHOULD be sending periodic unsolicited multicast
   Router Advertisement messages, the mobile node will have frequent
   opportunity to check if it is still reachable from its default
   router, even in the absence of other packets to it from the router.
   If Router Advertisements that the mobile node receives include
   an Advertisement Interval option, the mobile node MAY use its
   Advertisement Interval field as an indication of the frequency with
   which it SHOULD expect to continue to receive future Advertisements
   from that router.  This field specifies the minimum rate (the maximum
   amount of time between successive Advertisements) that the mobile
   node SHOULD expect.  If this amount of time elapses without the
   mobile node receiving any Advertisement from this router, the mobile
   node can be sure that at least one Advertisement sent by the router
   has been lost.  It is thus possible for the mobile node to implement
   its own policy for determining the number of Advertisements from
   its current default router it is willing to tolerate losing before
   deciding to switch to a different router from which it may currently
   be correctly receiving Advertisements.

   On some types of network interfaces, the mobile node MAY also
   supplement this monitoring of Router Advertisements, by setting its
   network interface into "promiscuous" receive mode, so that it is able



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   to receive all packets on the link, including those not addressed to
   it at the link layer (i.e., disabling link-level address filtering).
   The mobile node will then be able to detect any packets sent by the
   router, in order to detect reachability from the router.  This use of
   promiscuous mode may be useful on very low bandwidth (e.g., wireless)
   links, but its use MUST be configurable on the mobile node since it
   is likely to consume additional energy resources.

   If the above means do not provide indication that the mobile node
   is still reachable from its current default router (for instance,
   the mobile node receives no packets from the router for a period
   of time), then the mobile node SHOULD attempt to actively probe
   the router with Neighbor Solicitation messages, even if it is not
   otherwise actively sending packets to the router.  If it receives a
   solicited Neighbor Advertisement message in response from the router,
   then the mobile node can deduce that it is still reachable.  It is
   expected that the mobile node will in most cases be able to determine
   its reachability from the router by listening for packets from the
   router as described above, and thus, such extra Neighbor Solicitation
   probes should rarely be necessary.

   With some types of networks, indications about link-layer mobility
   might be obtained from lower-layer protocol or device driver software
   within the mobile node.  However, all link-layer mobility indications
   from lower layers do not necessarily indicate a movement of the
   mobile node to a new link, such that the mobile node would need to
   switch to a new default router and primary care-of address.  For
   example, movement of a mobile node from one cell to another in many
   wireless LANs can be made transparent to the IP level through use of
   a link-layer "roaming" protocol, as long as the different wireless
   LAN cells all operate as part of the same IP link with the same
   subnet prefix.  Upon lower-layer indication of link-layer mobility,
   the mobile node MAY send Router Solicitation messages to determine if
   additional on-link subnet prefixes are available on its new link.

   Such lower-layer information might also be useful to a mobile node in
   deciding to switch its primary care-of address to one of the other
   care-of addresses it has formed from the on-link subnet prefixes
   currently available through different routers from which the mobile
   node is reachable.  For example, a mobile node MAY use signal
   strength or signal quality information (with suitable hysteresis) for
   its link with the available routers to decide when to switch to a new
   primary care-of address using that router rather than its current
   default router (and current primary care-of address).  Even though
   the mobile node's current default router may still be reachable in
   terms of Neighbor Unreachability Detection, the mobile node MAY use
   such lower-layer information to determine that switching to a new
   default router would provide a better connection.






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11.4.2. Forming New Care-of Addresses

   After detecting that it has moved from one link to another (i.e., its
   current default router has become unreachable and it has discovered
   a new default router), a mobile node SHOULD form a new primary
   care-of address using one of the on-link subnet prefixes advertised
   by the new router.  A mobile node MAY form a new primary care-of
   address at any time, except that it MUST NOT do so too frequently.
   Specifically, a mobile node MUST NOT send a Binding Update about a
   new care-of address to its home agent (which is required to register
   the new address as its primary care-of address) more often than once
   per MAX_UPDATE_RATE seconds.

   In addition, after discovering a new on-link subnet prefix, a mobile
   node MAY form a new (non-primary) care-of address using that subnet
   prefix, even when it has not switched to a new default router.  A
   mobile node can have only one primary care-of address at a time
   (which is registered with its home agent), but it MAY have an
   additional care-of address for any or all of the prefixes on its
   current link.  Furthermore, since a wireless network interface may
   actually allow a mobile node to be reachable on more than one link at
   a time (i.e., within wireless transmitter range of routers on more
   than one separate link), a mobile node MAY have care-of addresses
   on more than one link at a time.  The use of more than one care-of
   address at a time is described in Section 11.4.3.

   As described in Section 4, in order to form a new care-of address,
   a mobile node MAY use either stateless [13] or stateful (e.g.,
   DHCPv6 [25]) Address Autoconfiguration.  If a mobile node needs to
   send packets as part of the method of address autoconfiguration,
   it MUST use an IPv6 link-local address rather than its own IPv6
   home address as the Source Address in the IPv6 header of each such
   autoconfiguration packet.

   In some cases, a mobile node may already know a (constant) IPv6
   address that has been assigned to it for its use only while
   visiting a specific foreign link.  For example, a mobile node may be
   statically configured with an IPv6 address assigned by the system
   administrator of some foreign link, for its use while visiting that
   link.  If so, rather than using Address Autoconfiguration to form a
   new care-of address using this subnet prefix, the mobile node MAY use
   its own pre-assigned address as its care-of address on this link.

   After forming a new care-of address, a mobile node MAY perform
   Duplicate Address Detection [13] on that new address to confirm its
   uniqueness.  However, doing so represents a trade-off between safety
   (ensuring that the new address is not used if it is a duplicate
   address) and overhead (performing Duplicate Address Detection
   requires the sending of one or more additional packets over what
   may be, for example, a slow wireless link through which the mobile
   node is connected).  Performing Duplicate Address Detection also in



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   general would cause a delay before the mobile node could use the
   new care-of address, possibly causing the mobile node to be unable
   to continue communication with correspondent nodes for some period
   of time.  For these reasons, a mobile node, after forming a new
   care-of address, MAY begin using the new care-of address without
   performing Duplicate Address Detection.  Furthermore, the mobile node
   MAY continue using the address without performing Duplicate Address
   Detection, although it SHOULD in most cases (e.g., unless network
   bandwidth or battery consumption for communication is of primary
   concern) begin Duplicate Address Detection asynchronously when it
   begins use of the address, allowing the Duplicate Address Detection
   procedure to complete in parallel with normal communication using the
   address.

   In addition, normal processing for Duplicate Address Detection
   specifies that, in certain cases, the node SHOULD delay sending the
   initial Neighbor Solicitation message of Duplicate Address Detection
   by a random delay between 0 and MAX_RTR_SOLICITATION_DELAY [12, 13];
   however, in this case, the mobile node SHOULD NOT perform such a
   delay in its use of Duplicate Address Detection, unless the mobile
   node is initializing after rebooting.


11.4.3. Using Multiple Care-of Addresses

   As described in Section 11.4.2, a mobile node MAY use more than one
   care-of address at a time.  Particularly in the case of many wireless
   networks, a mobile node effectively might be reachable through
   multiple links at the same time (e.g., with overlapping wireless
   cells), on which different on-link subnet prefixes may exist.  A
   mobile node SHOULD select a primary care-of address from among those
   care-of addresses it has formed using any of these subnet prefixes,
   based on the movement detection mechanism in use, as described in
   Section 11.4.1.  When the mobile node selects a new primary care-of
   address, it MUST register it with its home agent by sending it a
   Binding Update with the Home Registration (H) and Acknowledge (A)
   bits set, as described in Section 11.6.1.

   To assist with smooth handovers, a mobile node SHOULD retain
   its previous primary care-of address as a (non-primary) care-of
   address, and SHOULD still accept packets at this address, even after
   registering its new primary care-of address with its home agent.
   This is reasonable, since the mobile node could only receive packets
   at its previous primary care-of address if it were indeed still
   connected to that link.  If the previous primary care-of address was
   allocated using stateful Address Autoconfiguration [25], the mobile
   node may not wish to release the address immediately upon switching
   to a new primary care-of address.






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11.5. Return Routability Procedure

   This section defines the rules that the mobile node must follow
   when performing the return routability procedure.  Appendix A
   specifies also a (non-normative) state-machine that describes the
   same procedure.  Section 11.6.2 describes the rules when the return
   routability procedure needs to be initiated.


11.5.1. Sending Home and Care-of Test Init Messages

   A mobile node that initiates a return routability procedure MUST
   send (in parallel) a Home Test Init message and a Care-of Test Init
   messages.  A Home Test Init message MUST be created as described
   in Section 6.1.3.  A Care-of Test Init message MUST be created as
   described in Section 6.1.4.

   When sending a Home Test Init or Care-of Test Init message the mobile
   node MUST record in its Binding Update List the following fields from
   the messages:

    -  The IP address of the node to which the message was sent.

    -  The home address for which the binding is desired.  This value
       will appear in the Source Address field of the Home Test Init
       message.

    -  The time at which each of these messages was sent.

    -  The mobile cookie used in the messages.


11.5.2. Receiving Return Routability Messages

   Upon receiving a packet carrying a Home Test message, a mobile node
   MUST validate the packet according to the following tests:

    -  The Header Len field in the Mobility Header is greater than or
       equal to the length specified in Section 6.1.5.

    -  The Source Address of the packet belongs to a correspondent
       node for which the mobile node has a Binding Update List entry
       with a state indicating that return routability procedure is in
       progress.

    -  The Binding Update List indicates that no home cookie has been
       received yet.

    -  The Destination Address of the packet has the home address of the
       mobile node, and the packet has been received in a tunnel from
       the home agent.



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    -  The Mobile Cookie field in the message matches the value stored
       in the Binding Update List.

   Any Home Test message not satisfying all of these tests MUST be
   silently ignored.  Otherwise, the mobile node MUST record the Home
   Nonce Index and Home Cookie in the Binding Update List.  If the
   Binding Update List entry does not have a Care-of Cookie, the mobile
   node SHOULD continue waiting for additional messages.

   Upon receiving a packet carrying a Care-of Test message, a mobile
   node MUST validate the packet according to the following tests:

    -  The Header Len field in the Mobility Header is greater than or
       equal to the length specified in Section 6.1.6.

    -  The Source Address of the packet belongs to a correspondent
       node for which the mobile node has a Binding Update List entry
       with a state indicating that return routability procedure is in
       progress.

    -  The Binding Update List indicates that no care-of cookie has been
       received yet.

    -  The Destination Address of the packet is the current care-of
       address of the mobile node.

    -  The Mobile Cookie field in the message matches the value stored
       in the Binding Update List.

   Any Care-of Test message not satisfying all of these tests MUST be
   silently ignored.  Otherwise, the mobile node MUST record the Care-of
   Nonce Index and Care-of Cookie in the Binding Update List.  If the
   Binding Update List entry does not have a Home Cookie, the mobile
   node SHOULD continue waiting for additional messages.

   If after receiving either the Home Test or the Care-of Test message
   and performing the above actions, the Binding Update List entry
   has both the Home and the Care-of Cookies, the return routability
   procedure is complete.  The mobile node SHOULD then proceed with
   sending a Binding Update message as described in Section 11.6.2.

   Correspondent nodes from the time before this specification was
   published may not not support the Mobility Header protocol.  These
   nodes will respond to Home Test Init and Care-of Test Init messages
   with an ICMP Parameter Problem code 1.  The mobile node SHOULD
   take such messages as an indication that the correspondent node
   can not provide route optimization, and revert back to the use of
   bidirectional routing.






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11.5.3. Retransmitting in the Return Routability Procedure

   The mobile node is responsible for retransmissions in the return
   routability procedure.

   When the mobile node sends a Home Test Init or Care-of Test Init
   message, it has to determine a value for the initial retransmission
   timer.  It should use the specified value of INITIAL_BINDACK_TIMEOUT
   for this initial retransmission timer.

   If, after sending either a Home Test Init or Care-of Test Init
   message and the mobile node fails to receive a valid, matching
   Home Test or Care-of Test message within the selected initial
   retransmission interval, the mobile node SHOULD retransmit
   the original message, until a valid answer is received.  The
   retransmissions by the mobile node MUST use an exponential
   back-off process, in which the timeout period is doubled upon each
   retransmission until either the node receives a valid response or the
   timeout period reaches the value MAX_BINDACK_TIMEOUT.


11.5.4. Rate Limiting for Return Routability Procedure

   A mobile node MUST NOT send Home Test Init or Care-of Test
   Init messages to any individual node more often than once per
   MAX_UPDATE_RATE seconds.  After sending MAX_FAST_UPDATES consecutive
   messages to a particular node with the same care-of address, the
   mobile node SHOULD reduce its rate of sending these messages to that
   node, to the rate of SLOW_UPDATE_RATE per second.  The mobile node
   MAY continue to send these messages at this slower rate indefinitely,
   in hopes that the node will eventually be able to complete the return
   routability procedure.


11.6. Processing Bindings

11.6.1. Sending Binding Updates to the Home Agent

   After deciding to change its primary care-of address as described in
   Sections 11.4.1 and 11.4.2, a mobile node MUST register this care-of
   address with its home agent in order to make this its primary care-of
   address.  Also, if the mobile node wants the services of the home
   agent beyond the current registration period, the mobile node MUST
   send a new Binding Update to it well before the expiration of this
   period, even if it is not changing its primary care-of address.

   In both of these situations, the mobile node sends a packet to its
   home agent containing a Binding Update message, with the packet
   constructed as follows:

    -  The Home Registration (H) bit MUST be set in the Binding Update.



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    -  The Acknowledge (A) bit MUST be set in the Binding Update.

    -  The packet MUST contain a Home Address destination option, giving
       the mobile node's home address for the binding.

    -  The care-of address for the binding MUST be used as the Source
       Address in the packet's IPv6 header, unless an Alternate Care-of
       Address mobility option is included in the Binding Update
       message.

    -  The `S' bit is set to the zero to request the mobile node's home
       agent to serve as a home agent for all home addresses for the
       mobile node based on all on-link subnet prefixes on the home
       link; this is the default behavior.  If the mobile node desires
       that only a single home address should be affected by this
       Binding Update, the `S' bit can be set to 1.

    -  If the mobile node's link-local address has the same interface
       identifier (IID) as the home address for which it is supplying a
       new care-of address, then the mobile node SHOULD set the `L' bit.
       If the home address was generated using RFC 3041 [17], then the
       link local address is unlikely to have a compatible IID. In this
       case, the mobile node SHOULD NOT set the 'L' bit.

    -  The value specified in the Lifetime field SHOULD be less than
       or equal to the remaining lifetime of the home address and the
       care-of address specified for the binding.

   The Acknowledge (A) bit in the Binding Update requests the home agent
   to return a Binding Acknowledgement in response to this Binding
   Update.  As described in Section 6.1.8, the mobile node SHOULD
   retransmit this Binding Update to its home agent until it receives
   a matching Binding Acknowledgement.  Once reaching a retransmission
   timeout period of MAX_BINDACK_TIMEOUT, the mobile node SHOULD restart
   the process of delivering the Binding Update, but trying instead the
   next home agent from its Home Agents List (see Section 11.3.2).  If
   there is only one home agent in the Home Agents List, the mobile node
   instead SHOULD continue to periodically retransmit the Binding Update
   at this rate until acknowledged (or until it begins attempting to
   register a different primary care-of address).  See Section 11.6.8
   for information about retransmitting Binding Updates.

   Depending on the value of the Single Address Only (S) bit in the
   Binding Update, the home agent is requested to serve either a single
   home address or all home home addresses for the mobile node.  Until
   the lifetime of this registration expires, the home agent considers
   itself the home agent for each such home address of the mobile node.
   As the set of on-link subnet prefixes on the home link changes over
   time, the home agent changes the set of home addresses for this
   mobile node for which it is serving as the home agent.




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   Each Binding Update MUST be authenticated as coming from the right
   mobile node, as defined in Section 5.1.  The mobile node MUST use its
   home address -- either in the Home Address destination option or in
   the Source Address field of the IPv6 header -- in Binding Updates
   sent to the home agent.  This is necessary in order to allow the
   IPsec policies to be matched with the right home address.

   When sending a Binding Update to its home agent, the mobile node MUST
   also create or update the corresponding Binding Update List entry, as
   specified in Section 11.6.2.

   The last Sequence Number value sent to the home agent in a Binding
   Update is stored by the mobile node.  If the sending mobile node has
   no knowledge of the right Sequence Number value, it may start at any
   value.  If the home agent rejects the value, it sends back a Binding
   Acknowledgement with status code 141, and the last accepted sequence
   number in the Sequence Number field of the Binding Acknowledgement.
   The mobile node MUST store this information and use the next Sequence
   Number value for the next Binding Update it sends.

   If the mobile node has additional home addresses using a different
   interface identifier, then the mobile node SHOULD send an additional
   packet containing a Binding Update to its home agent to register the
   care-of address for each such other home address (or set of home
   addresses sharing an interface identifier).

   While the mobile node is away from home, it relies on the home agent
   to participate in Duplicate Address Detection (DAD) to defend its
   home address against stateless autoconfiguration performed by another
   node.  Therefore, the mobile node SHOULD set the Duplicate Address
   Detection (D) bit based on any requirements for DAD that would apply
   to the mobile node if it were at home [12, 13].  If the mobile
   node's recent Binding Update was accepted by the home agent, and the
   lifetime for that Binding Update has not yet expired, the mobile node
   SHOULD NOT set the `D' bit in the new Binding Update; the home agent
   will already be defending the home address(es) of the mobile node and
   does not need to perform DAD again.

   The home agent will only perform DAD for the mobile node's home
   address when the mobile node has supplied a valid binding between
   its home address and a care-of address.  If some time elapses during
   which the mobile node has no binding at the home agent, it might
   be possible for another node to autoconfigure the mobile node's
   home address.  Therefore, the mobile node MUST treat creation of
   a new binding with the home agent using an existing home address
   the same as creation of a new home address.  In the unlikely event
   that the mobile node's home address is autoconfigured as the IPv6
   address of another network node on the home network, the home agent
   will reply to the mobile node's subsequent Binding Update with a
   Binding Acknowledgement containing a Status of 138, Duplicate Address
   Detection failed.  In this case, the mobile node MUST NOT attempt to



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   re-use the same home address.  It SHOULD continue to register care-of
   addresses for its other home addresses, if any.  The mobile node MAY
   also attempt to acquire a new home address to replace the one for
   which Status 138 was received, for instance by using the techniques
   described in Appendix C.5.


11.6.2. Correspondent Binding Procedure

   When the mobile node is assured that its home address is valid, it
   MAY at any time initiate a correspondent binding procedure with
   the purpose of allowing the correspondent node to cache the mobile
   node's current care-of address.  The mobile node is responsible for
   the initiation and completion of this procedure, as well as any
   retransmissions that may be needed (subject to the rate limiting
   defined in Section 11.6.9).

   This section defines the rules that the mobile node must follow
   when performing the correspondent binding procedure.  Appendix A
   specifies also a (non-normative) state-machine that describes the
   same procedure.

   The mobile node can be assured that its home address is still valid,
   for example, by the home agent's use the `D' bit of Binding Updates
   (see Section 10.2).  In any Binding Update sent by a mobile node,
   the care-of address (either the Source Address in the packet's IPv6
   header or the Care-of Address in the Alternate Care-of Address
   mobility option of the Binding Update) MUST be set to one of the
   care-of addresses currently in use by the mobile node or to the
   mobile node's home address.  A mobile node MAY set the care-of
   address differently for sending Binding Updates to different
   correspondent nodes.

   A mobile node MAY choose to keep its location private from
   certain correspondent nodes, and thus need not initiate the
   return routability procedure, or send new Binding Updates to those
   correspondents.  A mobile node MAY also send a Binding Update to
   such a correspondent node to instruct it to delete any existing
   binding for the mobile node from its Binding Cache, as described in
   Section 6.1.7.  However, all Binding Updates to the correspondent
   node require the successful completion of the return routability
   procedure first, as no other IPv6 nodes are authorized to send
   Binding Updates on behalf of a mobile node.

   If set to one of the mobile node's current care-of addresses (the
   care-of address given MAY differ from the mobile node's primary
   care-of address), the Binding Update requests the correspondent node
   to create or update an entry for the mobile node in the correspondent
   node's Binding Cache in order to record this care-of address for use
   in sending future packets to the mobile node.  In this case, the
   value specified in the Lifetime field sent in the Binding Update



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   SHOULD be less than or equal to the remaining lifetime of the home
   address and the care-of address specified for the binding.

   If, instead, the care-of address is set to the mobile node's home
   address, the Binding Update requests the correspondent node to delete
   any existing Binding Cache entry that it has for the mobile node.

   When a mobile node sends a Binding Update to its home agent
   to register a new primary care-of address (as described in
   Section 11.6.1), the mobile node SHOULD also start a return
   routability procedure to each other node for which an entry exists
   in the mobile node's Binding Update List, as detailed below.  Upon
   successful return routability procedure and after receiving a
   successful Binding Acknowledgement from the Home Agent, a Binding
   Update message is sent to all other nodes.  Thus, other relevant
   nodes are generally kept updated about the mobile node's binding and
   can send packets directly to the mobile node using the mobile node's
   current care-of address.

   The mobile node, however, need not initiate these actions immediately
   after configuring a new care-of address.  For example, the mobile
   node MAY delay initiating the return routability procedure to any
   correspondent node for a short period of time, if it isn't certain
   that there's traffic to the correspondent node.  This is particularly
   useful if the mobile node anticipates that it is not going to stay
   long in this location.

   In addition, when a mobile node receives a packet for which the
   mobile node can deduce that the original sender of the packet either
   has no Binding Cache entry for the mobile node, or a stale entry
   for the mobile node in its Binding Cache, the mobile node SHOULD
   initiate a return routability procedure with the sender, in order to
   finally update the sender's Binding Cache with the current care-of
   address (subject to the rate limiting defined in Section 11.6.9).
   In particular, the mobile node SHOULD initiate a return routability
   procedure in response to receiving a packet that meets all of the
   following tests:

    -  The packet was tunneled using IPv6 encapsulation.

    -  The Destination Address in the tunnel (outer) IPv6 header is
       equal to any of the mobile node's care-of addresses.

    -  The Destination Address in the original (inner) IPv6 header
       is equal to one of the mobile node's home addresses; or this
       Destination Address is equal to one of the mobile node's previous
       care-of addresses for which the mobile node has an entry in its
       Binding Update List, representing an unexpired Binding Update
       sent to a home agent on the link on which its previous care-of
       address is located (Section 11.6.6).




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    -  The Source Address in the tunnel (outer) IPv6 header differs from
       the Source Address in the original (inner) IPv6 header.

   The destination address to which the procedure should be initiated to
   in response to receiving a packet meeting all of the above tests is
   the Source Address in the original (inner) IPv6 header of the packet.
   The home address for which this Binding Update is sent should be the
   Destination Address of the original (inner) packet.

   Binding Updates sent to correspondent nodes are not generally
   required to be acknowledged.  However, if the mobile node wants
   to be sure that its new care-of address has been entered into a
   correspondent node's Binding Cache, the mobile node MAY request an
   acknowledgement by setting the Acknowledge (A) bit in the Binding
   Update.  In this case, however, the mobile node SHOULD NOT continue
   to retransmit the Binding Update once the retransmission timeout
   period has reached MAX_BINDACK_TIMEOUT.

   The mobile node SHOULD create a Binding Update message as follows:

    -  The Source Address of the IPv6 header MUST contain the current
       care-of address of the mobile node.

    -  The Destination Address of the IPv6 header MUST contain the
       address of the correspondent node.

    -  The Mobility Header is constructed according to rules in
       Section 6.1.7, including the authenticator field which is
       calculated based on the received Home and Care-of Cookies.

    -  The Home Address destination option MUST be attached to the
       message, unless the Source Address is the home address of the
       mobile node.

   Each Binding Update MUST a Sequence Number greater than the Sequence
   Number value sent in the previous Binding Update (if any) to the same
   destination address modulo 2**16, as described in Section 9.4.1.
   There is no requirement, however, that the Sequence Number value
   strictly increase by 1 with each new Binding Update sent or received,
   as long as the value stays within the window.  The last Sequence
   Number value sent to a destination in a Binding Update is stored
   by the mobile node in its Binding Update List entry for that
   destination.  If the sending mobile node has no Binding Update List
   entry, the Sequence Number SHOULD start at a random value.  The
   mobile node MUST NOT use the same Sequence Number in two different
   Binding Updates to the same correspondent node, even if the Binding
   Updates provide different care-of addresses.







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11.6.3. Receiving Binding Acknowledgements

   Upon receiving a packet carrying a Binding Acknowledgement, a mobile
   node MUST validate the packet according to the following tests:

    -  The packet meets the authentication requirements for Binding
       Acknowledgements, defined in Sections 6.1.8 and 5.  That is,
       if the Binding Update was sent to the home agent, underlying
       IPsec protection is used.  If the Binding Update was sent to the
       correspondent node, the authenticator field MUST be present and
       have a valid value.

    -  The Header Len field in the Binding Acknowledgement message is
       greater than or equal to the length specified in Section 6.1.8.

    -  The Sequence Number field matches the Sequence Number sent by the
       mobile node to this destination address in an outstanding Binding
       Update.

   Any Binding Acknowledgement not satisfying all of these tests MUST be
   silently ignored.

   When a mobile node receives a packet carrying a valid Binding
   Acknowledgement, the mobile node MUST examine the Status field as
   follows:

    -  If the Status field indicates that the Binding Update was
       accepted (the Status field is less than 128), then the mobile
       node MUST update the corresponding entry in its Binding Update
       List to indicate that the Binding Update has been acknowledged;
       the mobile node MUST then stop retransmitting the Binding Update.
       In addition, if the value specified in the Lifetime field in the
       Binding Acknowledgement is less than the Lifetime value sent
       in the Binding Update being acknowledged, then the mobile node
       MUST subtract the difference between these two Lifetime values
       from the remaining lifetime for the binding as maintained in the
       corresponding Binding Update List entry (with a minimum value
       for the Binding Update List entry lifetime of 0).  That is, if
       the Lifetime value sent in the Binding Update was L_update, the
       Lifetime value received in the Binding Acknowledgement was L_ack,
       and the current remaining lifetime of the Binding Update List
       entry is L_remain, then the new value for the remaining lifetime
       of the Binding Update List entry should be

          max((L_remain - (L_update - L_ack)), 0)

       where max(X, Y) is the maximum of X and Y. The effect of this
       step is to correctly manage the mobile node's view of the
       binding's remaining lifetime (as maintained in the corresponding
       Binding Update List entry) so that it correctly counts down from
       the Lifetime value given in the Binding Acknowledgement, but with



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       the timer countdown beginning at the time that the Binding Update
       was sent.

       Mobile nodes SHOULD send a new Binding Update well before the
       expiration of this period in order to extend the lifetime and
       not cause a disruption in communications.  This is particularly
       necessary in order to prevent packets from being dropped due
       to the use of the Home Address destination option without an
       existing Binding Cache Entry, and the possibility of clock drift.

    -  If the Status field indicates that the Binding Update was
       rejected (the Status field is greater than or equal to 128), then
       the mobile node MUST delete the corresponding Binding Update List
       entry, and it MUST also stop retransmitting the Binding Update.
       Optionally, the mobile node MAY then take steps to correct the
       cause of the error and retransmit the Binding Update (with a new
       Sequence Number value), subject to the rate limiting restriction
       specified in Section 11.6.9.


11.6.4. Receiving Binding Refresh Requests

   When a mobile node receives a packet containing a Binding Refresh
   Request message and there already exists a Binding Update List
   entry for the source of the Binding Refresh Request, it MAY start
   a return routability procedure (see Section 5) if it believes
   the amount of traffic with the correspondent justifies the use of
   route optimization.  Note that the mobile node SHOULD NOT respond
   Binding Requests from previously unknown correspondent nodes due to
   Denial-of-Service concerns.

   If the return routability procedure completes successfully, a
   Binding Update message SHOULD be sent as described in Section 11.6.2.
   The Lifetime field in this Binding Update SHOULD be set to a new
   lifetime, extending any current lifetime remaining from a previous
   Binding Update sent to this node (as indicated in any existing
   Binding Update List entry for this node), and lifetime SHOULD
   again be less than or equal to the remaining lifetime of the home
   registration and the care-of address specified for the binding.  When
   sending this Binding Update, the mobile node MUST update its Binding
   Update List in the same way as for any other Binding Update sent by
   the mobile node.

   Note, however, that the mobile node MAY choose to delete its binding
   from the sender of the Binding Refresh Request.  In this case, the
   mobile node instead SHOULD return a Binding Update to the sender,
   in which the Lifetime field is set to zero and the care-of address
   (using a Alternate Care-Of Address option) is set to the mobile
   node's home address.





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   If the Binding Refresh Request for which the Binding Update is being
   returned contains a Unique Identifier mobility option, the resulting
   Home Test Init, Care-of Test Init, and Update messages MUST also
   include a Unique Identifier mobility option.  The unique identifier
   in the Option Data field of the Unique Identifier mobility option
   MUST be copied from the unique identifier carried in the Binding
   Refresh Request.


11.6.5. Receiving Binding Error Messages

   When a mobile node receives a packet containing a Binding Error
   message, it should first check if the mobile node has a Binding
   Update List entry for the the source of the Binding Error message.
   If the mobile node does not have such entry, it MUST ignore the
   message.  This is necessary to prevent a waste of resources on e.g.
   return routability procedure due to spoofed Binding Error messages.

   Otherwise, if the message Status field was 1 (Home Address
   destination option used without a binding), the mobile node should
   perform one of the following two actions:

    -  If the mobile node does have a Binding Update List entry but
       has recent upper layer progress information that indicates
       communications with the correspondent node are progressing, it
       MAY ignore the message.  This can be done in order to limit the
       damage that spoofed Binding Error messages can cause to ongoing
       communications.

    -  If the mobile node does have a Binding Update List entry but
       no upper layer progress information, it MUST remove the entry
       and route further communications through the home agent.  It
       MAY also optionally start a return routability procedure (see
       Section 5.2).

   If the message Status field was 2 (received message had an unknown
   value for the MH Type field), the mobile node should perform one of
   the following two actions:

    -  If the mobile node is not expecting an acknowledgement or
       response from the correspondent node, the mobile node SHOULD
       ignore this message.

    -  Otherwise, the mobile node SHOULD cease the use of any extensions
       to this specification.  If no extensions had been used, the
       mobile node should cease the attempt to use route optimization.








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11.6.6. Forwarding from a Previous Care-of Address

   When a mobile node connects to a new link and forms a new care-of
   address, it MAY establish forwarding of packets from a previous
   care-of address to this new care-of address.  To do so, the mobile
   node sends a Binding Update to any home agent on the link on which
   the previous care-of address is located, indicating this previous
   care-of address as the home address for the binding, and giving its
   new care-of address as the binding's care-of address.  Such packet
   forwarding allows packets destined to the mobile node from nodes that
   have not yet learned the mobile node's new care-of address, to be
   forwarded to the mobile node rather than being lost once the mobile
   node is no longer reachable at this previous care-of address.

   This Binding Update is sent to a home agent, albeit a temporary
   one.  Nevertheless, the authentication requirements for Binding
   Updates from a mobile node to its home agent apply, as specified in
   Section 11.6.1.

   In constructing this Binding Update, the mobile node utilizes the
   following specific steps:

    -  The Home Address field in the Home Address destination option
       in the packet carrying the Binding Update MUST be set to the
       previous care-of address for which packet forwarding is being
       established.

    -  The care-of address for the new binding MUST be set to the new
       care-of address to which packets destined to the previous care-of
       address are to be forwarded.  Normally, this care-of address for
       the binding is specified by setting the Source Address of the
       packet carrying the Binding Update, to this address.  However,
       the mobile node MAY instead include an Alternate Care-of Address
       mobility option in the Binding Update message, with its Alternate
       Care-of Address field set to the care-of address for the binding.

    -  The Home Registration (H) bit MUST also be set in this Binding
       Update, to request this home agent to temporarily act as a home
       agent for this previous care-of address.

    -  The Duplicate Address Detection (D) and Link-Local Address
       Compatibility  (L) MUST also be set in this Binding Update.
       If previous care-of address did not have the same interface
       identifier as the mobile link-local address, the mobile node MUST
       NOT use forwarding from a previous care-of address.

   This home agent will thus tunnel packets for the mobile node (packets
   destined to its specified previous care-of address) to its new
   care-of address.  All of the procedures defined for home agent
   operation MUST be followed by this home agent for this registration.
   Note that this home agent does not necessarily know (and need not



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   know) the mobile node's (permanent) home address as part of this
   registration.

   The packet carrying the Binding Update MUST be addressed to
   this home agent's global unicast address.  Normally, this global
   unicast address is learned by the mobile node based on the Router
   Advertisements received by the mobile node (Section 7.2) while
   attached to the link on which this previous care-of address and this
   home agent are located; the mobile node obtains this home agent
   address from its Home Agents List (Section 4.4).  Alternatively,
   the mobile node MAY use dynamic home agent address discovery
   (Section 10.9) to discover the global unicast address of a home agent
   on this previous link, but it SHOULD use an address from its Home
   Agents List if available for the prefix it used to form this previous
   care-of address.

   As with any packet containing a Binding Update (see Section 6.1.7),
   the Binding Update packet to this home agent MUST meet the
   authentication requirements for Binding Updates, defined in
   Section 5.1 and 11.6.1.


11.6.7. Returning Home

   A mobile node detects that it has returned to its home link through
   the movement detection algorithm in use (Section 11.4.1), when the
   mobile node detects that its home subnet prefix is again on-link.
   The mobile node SHOULD then send a Binding Update to its home agent,
   to instruct its home agent to no longer intercept or tunnel packets
   for it.  In this home registration, the mobile node MUST set the
   Acknowledge (A) and Home Registration (H) bits, and set the care-of
   address for the binding to the mobile node's own home address.  The
   mobile node MUST NOT include a Home Address option in this Binding
   Update.

   When sending this Binding Update to its home agent, the mobile
   node must be careful in how it uses Neighbor Solicitation [12] (if
   needed) to learn the home agent's link-layer address, since the home
   agent will be currently configured to defend the mobile node's home
   address for Duplicate Address Detection.  In particular, a Neighbor
   Solicitation from the mobile node using its home address as the
   Source Address would be detected by the home agent as a duplicate
   address.  In many cases, Neighbor Solicitation by the mobile node
   for the home agent's address will not be necessary, since the mobile
   node may have already learned the home agent's link-layer address,
   for example from a Source Link-Layer Address option in the Router
   Advertisement from which it learned that its home address was on-link
   and that the mobile node had thus returned home.

   If the mobile node does Neighbor Solicitation to learn the home
   agent's link-layer address, in this special case of the mobile node



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   returning home, the mobile node MUST multicast the packet, and in
   addition set the Source Address of this Neighbor Solicitation to the
   unspecified address (0:0:0:0:0:0:0:0).  The target of the Neighbor
   Solicitation MUST be set to the home agent's IPv6 address, which is
   known to the mobile node.  The destination IP address MUST be set to
   the Solicited-Node multicast address [3].  The home agent will be
   unable to distinguish this solicitation from a similar packet that
   would only be used for DAD, and it will respond as if for DAD. The
   home agent will send a multicast Neighbor Advertisement back to the
   mobile node with the Solicited flag ('S') set to zero.  The mobile
   node SHOULD accept this advertisement, and set the state of the
   Neighbor Cache entry for the home agent to REACHABLE.

   The mobile node then sends its Binding Update using the home agent's
   link-layer address, instructing its home agent to no longer serve
   as a home agent for it.  By processing this Binding Update, the
   home agent will cease defending the mobile node's home address for
   Duplicate Address Detection and will no longer respond to Neighbor
   Solicitations for the mobile node's home address.  The mobile node
   is then the only node on the link receiving packets at the mobile
   node's home address.  In addition, when returning home prior to the
   expiration of a current binding for its home address, and configuring
   its home address on its network interface on its home link, the
   mobile node MUST NOT perform Duplicate Address Detection on its own
   home address, in order to avoid confusion or conflict with its home
   agent's use of the same address.  If the mobile node returns home
   after the bindings for all of its care-of addresses have expired,
   then it SHOULD perform DAD. It SHOULD also perform DAD for addresses
   which may have been registered with 'D' and 'S' bits set to one.

   After the Mobile Node sends the Binding Update, the Home Agent MUST
   remove the Proxy Neighbor Cache entry for the Mobile Node and MAY
   learn its link-layer address based on the link-layer packet or cached
   information, or if that is not available, it SHOULD send a Neighbor
   Solicitation with the target address equal to the Binding Update's
   source IP address.  The Mobile Node MUST then reply with a unicast
   Neighbor Advertisement to the Home Agent with its link-layer address.
   While the Mobile Node is waiting for a Binding Acknowledgement, it
   MUST NOT respond to any Neighbor Solicitations for its Home Address
   other than those originating from the IP address to which it sent the
   Binding Update.

   After receiving the Binding Acknowledgement for its Binding Update
   to its home agent, the mobile node MUST multicast onto the home
   link (to the all-nodes multicast address) a Neighbor Advertisement
   message [12], to advertise the mobile node's own link-layer address
   for its own home address.  The Target Address in this Neighbor
   Advertisement message MUST be set to the mobile node's home address,
   and the Advertisement MUST include a Target Link-layer Address option
   specifying the mobile node's link-layer address.  The mobile node
   MUST multicast such a Neighbor Advertisement message for each of its



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   home addresses, as defined by the current on-link prefixes, including
   its link-local address and site-local address.  The Solicited
   Flag (S) in these Advertisements MUST NOT be set, since they were
   not solicited by any Neighbor Solicitation message.  The Override
   Flag (O) in these Advertisements MUST be set, indicating that the
   Advertisements SHOULD override any existing Neighbor Cache entries at
   any node receiving them.

   Since multicasting on the local link (such as Ethernet) is typically
   not guaranteed to be reliable, the mobile node MAY retransmit these
   Neighbor Advertisement messages up to MAX_ADVERT_REXMIT times to
   increase their reliability.  It is still possible that some nodes on
   the home link will not receive any of these Neighbor Advertisements,
   but these nodes will eventually be able to recover through use of
   Neighbor Unreachability Detection [12].


11.6.8. Retransmitting Binding Updates

   The mobile node is responsible for retransmissions in the binding
   procedure.

   When the mobile node sends a Binding Update message, it has to
   determine a value for the initial retransmission timer.  If the
   mobile node is changing or updating an existing binding at the home
   agent, it should use the specified value of INITIAL_BINDACK_TIMEOUT
   for this initial retransmission timer.  If on the other hand the
   mobile node does not have an existing binding at the home agent, it
   SHOULD use a value for the initial retransmission timer that is at
   least 1.5 times longer than (RetransTimer * DupAddrDetectTransmits).
   This value is likely to be substantially longer than the otherwise
   specified value of INITIAL_BINDACK_TIMEOUT that would be used by the
   mobile node.  This longer retransmission interval will allow the the
   home agent to complete the DAD procedure which is mandated in this
   case, as detailed in Section 11.6.1.

   If, after sending a Binding Update in which the care-of address has
   changed and the Acknowledge (A) bit is set, a mobile node fails
   to receive a valid, matching Binding Acknowledgement within the
   selected initial retransmission interval, the mobile node SHOULD
   retransmit the Binding Update, until a Binding Acknowledgement is
   received.  Such a retransmitted Binding Update MUST use a Sequence
   Number value greater than that used for the previous transmission of
   this Binding Update.  The retransmissions by the mobile node MUST
   use an exponential back-off process, in which the timeout period
   is doubled upon each retransmission until either the node receives
   a Binding Acknowledgement or the timeout period reaches the value
   MAX_BINDACK_TIMEOUT.






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11.6.9. Rate Limiting Binding Updates

   A mobile node MUST NOT send Binding Update messages for the
   same binding to any individual node more often than once per
   MAX_UPDATE_RATE seconds.  After sending MAX_FAST_UPDATES consecutive
   messages to a particular node with the same care-of address, the
   mobile node SHOULD reduce its rate of sending these messages to that
   node, to the rate of SLOW_UPDATE_RATE per second.  The mobile node
   MAY continue to send these messages at this slower rate indefinitely,
   in hopes that the node will eventually be able to process a Binding
   Update, and begin to route its packets directly to the mobile node at
   its new care-of address.


11.7. Receiving ICMP Error Messages

   Any node receiving a Mobility header that does not recognize the
   protocol SHOULD return an ICMP Parameter Problem, Code 1, message
   to the sender of the packet.  If a node performing the return
   routability procedure or sending a Binding Update receives such an
   ICMP error message in response, it SHOULD record in its Binding
   Update List that future Binding Updates SHOULD NOT be sent to this
   destination.

   Correspondent nodes who have participated in the return routability
   procedure MUST implement the ability to correctly process received
   packets containing a Home Address destination option.  Therefore,
   correctly implemented correspondent nodes should always be able to
   recognize Home Address options.  If a mobile node receives an ICMP
   Parameter Problem, Code 2, message from some node indicating that it
   does not support the Home Address option, the mobile node SHOULD log
   the error and then discard the ICMP message.






















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12. Protocol Constants

      HomeRtrAdvInterval       3,600 seconds
      DHAAD_RETRIES            3 retransmissions
      INITIAL_BINDACK_TIMEOUT  1 second
      INITIAL_DHAAD_TIMEOUT    2 seconds
      INITIAL_SOLICIT_TIMER    2 seconds
      MAX_ADVERT_REXMIT        3 transmissions
      MAX_BINDACK_TIMEOUT      256 seconds
      MAX_COOKIE_LIFE          240 seconds
      MAX_FAST_UPDATES         5 transmissions
      MAX_PFX_ADV_DELAY        1,000 seconds
      MAX_RR_BINDING_LIFE      420 seconds
      MAX_UPDATE_RATE          once per second
      PREFIX_ADV_RETRIES       3 retransmissions
      PREFIX_ADV_TIMEOUT       5 seconds
      SLOW_UPDATE_RATE         once per 10 second interval





































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13. IANA Considerations

   This document defines a new IPv6 protocol, the Mobility Header,
   described in Section 6.1.  This protocol must be assigned a protocol
   number.  The MH Type field in the Mobility Header is used to indicate
   a particular type of a message.  The current message types are
   described in Sections 6.1.2 through 6.1.9, and include the following:

      0       Binding Refresh Request

      1       Home Test Init

      2       Care-of Test Init

      3       Home Test

      4       Care-of Test

      5       Binding Update

      6       Binding Acknowledgement

      7       Binding Error

   Future values of the MH Type can be allocated using standards
   action [10].

   Furthermore, each Mobility Header message may contain mobility
   options as described in Section 6.2.  The current mobility options
   are defined in Sections 6.2.4 through 6.2.5, and include the
   following:

      0       Pad1

      1       PadN

      2       Unique Identifier

      3       Alternate Care-of Address

      4       Nonce Indices

      5       Authorization Data

   Future values of the Option Type can be allocated using standards
   action [10].

   This document also defines a new IPv6 destination option, the Home
   Address option, described in Section 6.3.  This option must be
   assigned an Option Type value.




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   This document also defines a new IPv6 Type 2 Routing Header,
   described in Section 6.4.  The value 2 must be allocated by IANA when
   this specification becomes an RFC.

   In addition, this document defines four ICMP message types, two used
   as part of the dynamic home agent address discovery mechanism and
   two used in lieu of router solicitations and advertisements when the
   mobile node is away from the home link:

    -  The Home Agent Address Discovery Request message, described in
       Section 6.5;

    -  The Home Agent Address Discovery Reply message, described in
       Section 6.6;

    -  The Mobile Prefix Solicitation message, described in Section 6.7;
       and

    -  The Mobile Prefix Advertisement message, described in
       Section 6.8.

   This document also defines two new Neighbor Discovery [12] options,
   which must be assigned Option Type values within the option numbering
   space for Neighbor Discovery messages:

    -  The Advertisement Interval option, described in Section 7.3; and

    -  The Home Agent Information option, described in Section 7.4.


14. Security Considerations

14.1. Threats

   Any mobility solution must protect itself against misuses of
   the mobility features and mechanisms.  In Mobile IPv6, most of
   the potential threats are concerned with false Bindings, usually
   resulting in Denial-of-Service attacks.  Some of the threats also
   pose potential for Man-in-the-Middle, Hijacking, Confidentiality,
   and Impersonation attacks.  The main threats this protocol protects
   against are the following:

    1. Threats involving Binding Updates sent to home agents and
       correspondent nodes.  For instance, an attacker might claim that
       a certain mobile node is currently at a different location than
       it really is.  If a home agent accepts such spoofed information
       sent to it, the mobile node might not get traffic destined to
       it.  Similarly, a malicious (mobile) node might use the home
       address of a victim node in a forged Binding Update sent to a
       correspondent node.




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       These pose threats against confidentiality, integrity, and
       availability.  That is, an attacker might learn the contents
       of packets destined to another node by redirecting the traffic
       to itself.  Furthermore, an attacker might use the redirected
       packets in an attempt to set itself as a Man-in-the-Middle
       between a mobile and a correspondent node.  This would allow the
       attacker to impersonate the mobile node, leading to integrity and
       availability problems.

       A malicious (mobile) node might also send Binding Updates in
       which the care-of address is set to the address of a victim
       node.  If such Binding Updates were accepted, the malicious
       node could lure the correspondent node into sending potentially
       large amounts of data to the victim; the correspondent node's
       replies to messages sent by the malicious mobile node will be
       sent to the victim host or network.  This could be used to
       cause a Distributed Denial-of-Service attack.  For example,
       the correspondent node might be a site that will send a
       high-bandwidth stream of video to anyone who asks for it.  Note
       that the use of flow-control protocols such as TCP does not
       necessarily defend against this type of attack, because the
       attacker can fake the acknowledgements.  Even keeping TCP initial
       sequence numbers secret doesn't help, because the attacker can
       receive the first few segments (including the ISN) at its own
       address, and only then redirect the stream to the victim's
       address.  These types of attacks may also be directed towards
       networks instead of nodes.  Further variations of this threat are
       described elsewhere [29, 30].

       An attacker might also attempt to disrupt a mobile node's
       communications by replaying a Binding Update that the node had
       sent earlier.  If the old Binding Update was accepted, packets
       destined for the mobile node would be sent to its old location
       and not its current location.

       In conclusion, there are Denial-of-Service, Man-in-the-Middle,
       Confidentiality, and Impersonation threats against the
       parties involved in sending legitimate Binding Updates, and
       Denial-of-Service threats against any other party.

    2. Threats associated with payload packets:  Payload packets
       exchanged with mobile nodes are exposed to similar threats as
       regular IPv6 traffic is.  However, Mobile IPv6 introduces the
       Home Address destination option, a new Routing Header type (Type
       2), and uses tunneling headers in the payload packets.  The
       protocol must protect against potential new threats involving the
       use of these mechanisms.

       Third parties become exposed to a reflection threat via the
       Home Address destination option, unless appropriate security
       precautions are followed.  The Home Address destination option



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       could be used to direct response traffic toward a node whose IP
       address appears in the option.  In this case, ingress filtering
       would not catch the forged "return address" [31] [32].

       A similar threat exists with the tunnels between the mobile node
       and the home agent.  An attacker might forge tunnel packets
       between the mobile node and the home agent, making it appear
       that the traffic is coming from the mobile node when it is not.
       Note that an attacker who is able to forge tunnel packets would
       typically be able forge also packets that appear to come directly
       from the mobile node.  This is a not a new threat as such.
       However, it may make it easier for attackers to escape detection
       by avoiding ingress filtering and packet tracing mechanisms.
       Furthermore, spoofed tunnel packets might be used to gain access
       to the home network.

       Finally, a Routing Header could also be used in reflection
       attacks, and in attacks designed to bypass firewalls.
       The generality of the regular Routing Header would allow
       circumvention of IP-address based rules in firewalls.  It would
       also allow reflection of traffic to other nodes.  These threats
       exist with Routing Headers in general, even if the usage that
       Mobile IPv6 requires is safe.

    3. Threats against the Mobile IPv6 security mechanisms themselves:
       An attacker might, for instance, lure the participants into
       executing expensive cryptographic operations or allocating memory
       for the purpose of keeping state.  The victim node would have no
       resources left to handle other tasks.

   As a fundamental service in an IPv6 stack, Mobile IPv6 is expected to
   be deployed in most nodes of the IPv6 Internet.  The above threats
   should therefore be considered in the light of being applicable to
   the whole Internet.


14.2. Features

   This specification provides a number of security features designed to
   mitigate or alleviate the threats listed above.  The main security
   features are the following:

    -  Protection of Binding Updates sent to home agents.

    -  Protection of Binding Updates sent to correspondent nodes.

    -  Protection against reflection attacks that use the Home Address
       destination option.

    -  Protection of tunnels between the mobile node and the home agent.




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    -  Closing Routing Header vulnerabilities.

    -  Mitigating Denial-of-Service threats to the Mobile IPv6 security
       mechanisms themselves.

   Protecting those Binding Updates that are sent to home agents and
   those that are sent to arbitrary correspondent nodes requires very
   different security solutions due to the different situations.  Mobile
   nodes and home agents are expected to be naturally subject to the
   network administration of the home domain.

   Thus, they can and are supposed to have a strong security association
   that can be used to reliably authenticate the exchanged messages.
   See Section 5.1 for the description of the protocol mechanisms,
   and Section 14.3 below for a discussion of the resulting level of
   security.

   It is expected that Mobile IPv6 route optimization will be
   used on a global basis between nodes belonging to different
   administrative domains.  It would be a very demanding task to
   build an authentication infrastructure on this scale.  Furthermore,
   a traditional authentication infrastructure cannot be easily
   used to authenticate IP addresses, because these change often.
   It is not sufficient to just authenticate the mobile nodes.
   Authorization to claim the right to use an address is needed as
   well.  Thus, an "infrastructureless" approach is necessary.  The
   chosen infrastructureless method is described in Section 5.2 and
   Section 14.4 discusses the resulting security level and the design
   rationale of this approach.

   Specific rules guide the use of the Home Address destination option,
   the Routing Header, and the tunneling headers in the payload packets.
   These rules are necessary to remove the vulnerabilities associated
   with their unrestricted use.  The effect of the rules is discussed in
   Sections 14.5, 14.6, and 14.7.

   Denial-of-Service threats against Mobile IPv6 security mechanisms
   themselves concern mainly the Binding Update procedures with
   correspondent nodes.  The protocol has been designed to limit the
   effects of such attacks, as will be described in Section 14.4.5.


14.3. Binding Updates to Home Agent

   Signaling between the mobile node and the home agent requires message
   integrity, correct ordering and replay protection.  This is necessary
   to assure the home agent that a Binding Update is from a legitimate
   mobile node.

   IPsec AH or ESP protects the integrity of the Binding Updates and
   Binding Acknowledgements, by securing Mobility Header messages



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   between the mobile node and the home agent.  However, IPsec can
   easily provide replay protection only if dynamic security association
   establishment is used.  This may not always be possible, and manual
   keying would be preferred in some cases.  IPsec also does not
   guarantee correct ordering of packets, only that they have not been
   replayed.  Because of this, sequence numbers with the Mobile IPv6
   messages ensure correct ordering (see Section 5.1).  However, if
   a home agent reboots and loses its state regarding the sequence
   numbers, replay attacks become possible.  The use of a key management
   mechanism together with IPsec can be used to prevent such replay
   attacks.

   A sliding window scheme is used for the sequence numbers.  The
   protection against replays and reordering attacks without a key
   management mechanism works when the attacker remembers up to a
   maximum of 2**15 Binding Updates.

   The above mechanisms do not show that the care-of address given
   in the Binding Update is correct.  This opens the possibility for
   Denial-of-Service attacks against third parties.  However, since the
   mobile node and home agent have a security association, the home
   agent can always identify an ill-behaving mobile node.  This allows
   the home agent operator to discontinue the mobile node's service, and
   possibly take further actions based on the business relationship with
   the mobile node's owner.

   Note that where forwarding from a previous care-of address is used,
   a router in the visited network must act as a temporary home agent
   for the mobile node.  Nevertheless, the same security requirements
   apply in this case.  That is, a pre-arranged security association
   must exist even with the temporary home agent.  This limits the use
   of the forwarding feature to those networks where such arrangements
   are practical.

   Note that the use of a single pair of manually keyed security
   associations conflicts with the generation of a new home
   addresses [17] for the mobile node, or with the adoption of a
   new home prefix.  This is because IPsec SAs are bound to the used
   addresses.  While certificate-based automatic keying alleviates
   this problem to an extent, it is still necessary to ensure that a
   given mobile node can not send Binding Updates for the address of
   another mobile node.  In general, this leads to the inclusion of
   home addresses in certificates in the Subject AltName field.  This
   again limits the introduction of new addresses without either manual
   or automatic procedures to establish new certificates.  Therefore,
   this specification limits restricts the generation of new home
   addresses (for any reason) to those situations where there already
   exists a security association or certificate for the new address.
   (Section C.4 lists the improvement of security for new addresses as
   one of the future developments for Mobile IPv6.)




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14.4. Binding Updates to Correspondent Nodes

14.4.1. Overview

   The motivation for designing the return routability procedure
   was to have sufficient support for Mobile IPv6, without creating
   significant new security problems.  The goal for this procedure was
   not to protect against attacks that were already possible before the
   introduction of Mobile IPv6.

   The chosen infrastructureless method verifies that the mobile node
   is "live" (that is, it responds to probes) at its home and care-of
   addresses.  Section 5.2 describes the return routability procedure in
   detail.  The procedure uses the following principles:

    -  A cookie exchange verifies that the mobile node is reachable at
       its addresses i.e.  is at least able to transmit and receive
       traffic at both the home and care-of addresses.

    -  The eventual Binding Update is cryptographically bound to the
       exchanged cookies.

    -  Symmetric exchanges are employed to avoid the use of this
       protocol in reflection attacks.  In a symmetric exchange, the
       responses are always sent to the same address as the request was
       sent from.

    -  The correspondent node operates in a stateless manner until it
       receives a fully authorized Binding Update.

    -  Some additional protection is provided by encrypting the tunnels
       between the mobile node and home agent with IPsec ESP. As the
       tunnel transports also the cookie exchanges, this limits the
       ability of attackers to see these cookies.  For instance, this
       prevents attacks launched from the mobile node's current foreign
       link where no link-layer confidentiality is available.

   For further information about the design rationale of the return
   routability procedure, see [29, 30, 33, 32].  The used mechanisms
   have been adopted from these documents.


14.4.2. Offered Protection

   This procedure protects Binding Updates against all attackers
   who are unable to monitor the path between the home agent and the
   correspondent node.  The procedure does not defend against attackers
   who can monitor this path.  Note that such attackers are in any case
   able to mount an active attack against the mobile node when it is
   at its home location.  The possibility of such attacks is not an




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   impediment to the deployment of Mobile IPv6, because these attacks
   are possible regardless of whether Mobile IPv6 is in use.

   This procedure also protects against Denial-of-Service attacks in
   which the attacker pretends to be a mobile, but uses the victim's
   address as the care of address.  This would cause the correspondent
   node to send the victim some unexpected traffic.  The procedure
   defends against these attacks by requiring the participation of the
   node at the care-of address.  Normally, this will be the mobile node.

   The Binding Acknowledgement is not authenticated in other ways than
   including the right sequence number in the reply.


14.4.3. Comparison to Regular IPv6 Communications

   This section discusses the protection offered by the return
   routability method by comparing it to the security of regular IPv6
   communications.  We will divide vulnerabilities in three classes:
   (1) those related to attackers on the local network of the mobile
   node, home agent, or the correspondent node, (2) those related to
   attackers on the path between the MN/HA and the CN, and (3) off-path
   attackers, i.e.  the rest of the Internet.

   We will now discuss the vulnerabilities of regular IPv6
   communications.  The on-link vulnerabilities of IPv6 communications
   include Denial-of-Service, Masquerading, Man-in-the-Middle,
   Eavesdropping, and other attacks.  These attacks can be launched
   through spoofing Router Discovery, Neighbor Discovery and other IPv6
   mechanisms.  Some of these attacks can be prevented with the use of
   cryptographic protection in the packets.

   A similar situation exists with on-path attackers.  That is, without
   cryptographic protection the traffic is completely vulnerable.

   Assuming that attackers have not penetrated the security of the
   Internet routing protocols, attacks are much harder to launch
   from off-path locations.  Attacks that can be launched from these
   locations are mainly Denial-of-Service attacks, such as flooding
   and/or reflection attacks.  It is not possible for an off-path
   attacker to become a MitM. (Since IPv6 communications are relatively
   well protected against off-path attackers, it is important that
   Mobile IPv6 prevents off-path attacks as well.)

   Next, we will consider the vulnerabilities that exist when IPv6 is
   used together with Mobile IPv6 and the return routability procedure.
   On the local link the vulnerabilities are same as those as in IPv6,
   but Masquerade and MitM attacks can now be launched also against
   future communications, and not just against current communications.
   If a binding update was sent while the attacker was present on the
   link, its effects stay during the lifetime of the binding.  This



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   happens even if the attacker moves away from the link.  In regular
   IPv6, the attacker generally has to be stay on the link in order to
   continue the attack.  Note that in order to launch these new attacks,
   the IP address of the victim must be known.  This makes this attack
   feasible mainly in the context of well-known interface IDs, such as
   those already appearing in the traffic on the link or registered in
   the DNS.

   On-path attackers can exploit similar vulnerabilities as in regular
   IPv6.  There are some minor differences, however.  Masquerade, MitM,
   and DoS attacks can be launched with just the interception of a few
   packets, whereas in regular IPv6 it is necessary to intercept every
   packet.  The effect of the attacks is the same regardless of the
   method, however.  In any case, the most difficult task attacker faces
   in these attacks is getting to the right path.

   The vulnerabilities for off-path attackers are the same as in regular
   IPv6.  Those nodes that are not on the path between the home agent
   and the correspondent node will not be able to receive the probe
   messages.

   In conclusion, we can state the following main results from this
   comparison:

    -  Return routability procedure prevents any off-path attacks beyond
       those that are already possible in regular IPv6.  This is the
       most important result, and prevents attackers from the Internet
       from exploiting any vulnerabilities.

    -  Vulnerabilities to attackers on the home agent link, the
       correspondent node link, and the path between them are roughly
       the same as in regular IPv6.

    -  However, one difference is that in basic IPv6 an on-path attacker
       must be constantly present on the link or the path, whereas with
       Mobile IPv6 an attacker can leave a binding behind after moving
       away.

       For this reason, this specification limits the creation of
       bindings to at most MAX_COOKIE_LIFE seconds after the last
       routability check has been performed, and limits the duration of
       a binding to at most MAX_RR_BINDING_LIFE seconds.  With these
       limitation, attackers can not take practical advantages of this
       vulnerability.  This limited vulnerability can also be compared
       to similar vulnerabilities in IPv6 Neighbor Discovery, with
       Neighbour Cache entries having a limited lifetime.

    -  There are some other minor differences, such as an effect
       to the DoS vulnerabilities.  These can be considered to be
       insignificant.




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    -  The path between the home agent and a correspondent node is
       typically easiest to attack on the links at either end, in
       particular if these links are publicly accessible wireless LANs.
       Attacks against the routers or switches on the path are typically
       harder to accomplish.  The security on layer 2 of the links plays
       then a major role in the resulting overall network security.
       Similarly, security of IPv6 Neighbor and Router Discovery on
       these links has a large impact.  If these were secured using
       some new technology in the future, this could make the return
       routability procedure the easiest route for attackers.  For this
       reason, this specification should have a protection mechanism for
       selecting between return routability and potential other future
       mechanisms.

   For a more in-depth discussion of these issues, see [32].


14.4.4. Return Routability Replays

   The return routability procedure also protects the participants
   against replayed Binding Updates.  The attacker is unable replay
   the same message due to the sequence number which is a part of the
   Binding Update.  It is also unable to modify the Binding Update since
   the MAC would not verify after such modification.

   Care must be taken when removing bindings at the correspondent
   node, however.  If a binding is removed while the nonce used in its
   creation is still valid, an attacker could replay the old Binding
   Update.  Rules outlined in Section 5.2.8 ensure that this can not
   happen.


14.4.5. Return Routability Denial-of-Service

   The return routability procedure has protection against resource
   exhaustion Denial-of-Service attacks.  The correspondent nodes do not
   retain any state about individual mobile nodes until an authentic
   Binding Update arrives.  This is achieved through the use of the
   nonces and node keys that are not specific to individual mobile
   nodes.  The cookies are specific, but they can be reconstructed
   based on the home and care-of address information that arrives
   with the Binding Update.  This means that the correspondent nodes
   are safe against memory exhaustion attacks except where on-path
   attackers are concerned.  Due to the use of symmetric cryptography,
   the correspondent nodes are relatively safe against CPU resource
   exhaustion attacks as well.

   Nevertheless, as [29] describes, there are situations in which it is
   impossible for the mobile and correspondent nodes to determine if
   they actually need a binding or whether they just have been fooled




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   into believing so by an attacker.  Therefore, it is necessary to
   consider situations where such attacks are being made.

   Even if route optimization is a very important optimization, it is
   still only an optimization.  A mobile node can communicate with a
   correspondent node even if the correspondent refuses to accept any
   Binding Updates.  However, performance will suffer because packets
   from the correspondent node to the mobile node will be routed via the
   mobile's home agent rather than a more direct route.  A correspondent
   node can protect itself against some of these resource exhaustion
   attacks as follows.  If the correspondent node is flooded with a
   large number of Binding Updates that fail the cryptographic integrity
   checks, it can stop processing Binding Updates.  If a correspondent
   node finds that it is spending more resources on checking bogus
   Binding Updates than it is likely to save by accepting genuine
   Binding Updates, then it may reject some or all Binding Updates
   without performing any cryptographic operations.

   Layers above IP can usually provide additional information to decide
   if there is a need to establish a binding with a specific peer.  For
   example, TCP knows if the node has a queue of data that it is trying
   to send to a peer.  An implementation of this specification is not
   required to make use of information from higher protocol layers, but
   some implementations are likely to be able to manage resources more
   effectively by making use of such information.

   We also require that all implementations MUST allow route
   optimization to be administratively enabled or disabled.  The default
   SHOULD be enabled.


14.5. Tunneling via the Home Agent

   Tunnels between the mobile node and the home agent can be
   protected by ensuring proper use of source addresses, and optional
   cryptographic protection.  These procedures are discussed in
   Section 5.3.

   Binding Updates to the home agents are secure.  When receiving
   tunneled traffic the home agent verifies the outer IP address
   corresponds to the current location of the mobile node.  This
   prevents attacks where the attacker is controlled by ingress
   filtering.  It also prevents attacks when the attacker does not know
   the current care-of address of the mobile node.  Attackers who know
   the care-of address and are not controlled by ingress filtering could
   still send traffic through the home agent.  This includes attackers
   on the same local link as the mobile node is currently on.  But such
   attackers could also send spoofed packets without using a tunnel.






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   Home agents and mobile nodes may use IPsec AH or ESP to protect
   payload packets tunneled between themselves.  This is useful to
   protect communications against attackers on the path of the tunnel.

   When site local home address are used, reverse tunneling can be used
   to send site local traffic from another location.  Administrators
   should be aware of this when allowing such home addresses.  In
   particular, the outer IP address check described above is not
   sufficient against all attackers.  The use of encrypted tunnels is
   particularly useful for this kind of home addresses.


14.6. Home Address Destination Option

   When the mobile node sends packets directly to the correspondent
   node, the Source Address field of the packet's IPv6 header is the
   care-of address.  Ingress filtering [24] works therefore in the usual
   manner even for mobile nodes, as the Source Address is topologically
   correct.  The Home Address destination option is used to inform the
   correspondent node of the mobile node's home address.

   However, the care-of address in the Source Address field does
   not survive in replies sent by the correspondent node unless
   it has a binding for this mobile node.  Also, not all attacker
   tracing mechanisms work when packets are being reflected through
   correspondent nodes using the Home Address option.  For these
   reasons, this specification restricts the use of the Home Address
   option.  It may only used when a binding has already been established
   with the participation of the node at the home address, as described
   in Sections 5.3 and 6.3.  This prevents reflection attacks through
   the use of the Home Address option.  It also ensures that the
   correspondent nodes reply to the same address as the mobile node
   sends traffic from.

   No special authentication of the Home Address option is required
   beyond the above, except that if the IPv6 header of a packet is
   covered by authentication, then that authentication MUST also cover
   the Home Address option; this coverage is achieved automatically by
   the definition of the Option Type code for the Home Address option
   (Section 6.3), since it indicates that the option is included in the
   authentication computation.  Thus, even when authentication is used
   in the IPv6 header, the security of the Source Address field in the
   IPv6 header is not compromised by the presence of a Home Address
   option.  Without authentication of the packet, then any field in
   the IPv6 header, including the Source Address field, and any other
   parts of the packet, including the Home Address option, can be forged
   or modified in transit.  In this case, the contents of the Home
   Address option is no more suspect than any other part of the packet.
   Additionally, if a security association has been used to protect
   the packet, we allow the Home Address option to be used even if the
   correspondent node does not have a binding for this mobile node.



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14.7. Type 2 Routing Header

   The definition of the Type 2 Routing Header is described in
   Section 6.4.  This definition and the associated processing rules
   have been chosen so that the header can not be used for what is
   traditionally viewed as source routing.  In particular, the IPv6 the
   Home Address in the routing header will always have to be assigned to
   the home address of the receiving node.  Otherwise the packet will be
   dropped.

   Generally, source routing has a number of security concerns.  These
   include the automatic reversal of unauthenticated source routes
   (which is an issue for IPv4, but not for IPv6).  Another concern is
   the ability to use source routing to "jump" between nodes inside, as
   well as outside a firewall.  These security concerns are not issues
   in Mobile IPv6, due to the rules mentioned above.

   In essence the semantics of the type 2 routing header is the same as
   a special form of IP-in-IP tunneling where the inner and outer source
   addresses are the same.

   This implies that a device which implements filtering of packets
   should be able to distinguish between a Type 2 Routing header and
   other Routing headers, as required in section 8.3.  This is necessary
   in order to allow Mobile IPv6 traffic while still having the option
   to filter out other uses of Routing headers.


Acknowledgements

   We would like to thank the members of the Mobile IP and IPng Working
   Groups for their comments and suggestions on this work.  We would
   particularly like to thank (in alphabetical order) Fred Baker
   (Cisco), Josh Broch (Carnegie Mellon University), Robert Chalmers
   (University of California, Santa Barbara), Noel Chiappa (MIT),
   Vijay Devarapalli (Nokia Research Center), Rich Draves (Microsoft
   Research), Francis Dupont (ENST Bretagne), Thomas Eklund (Xelerated),
   Jun-Ichiro Itojun Hagino (IIJ Research Laboratory), Krishna Kumar
   (IBM Research), T.J. Kniveton (Nokia Research), Jiwoong Lee (KTF),
   Aime Lerouzic (Bull S.A.), Vesa-Matti Mantyla (Ericsson), Thomas
   Narten (IBM), Simon Nybroe (Ericsson Telebit), David Oran (Cisco),
   Lars Henrik Petander (HUT), Basavaraj Patil (Nokia), Ken Powell
   (Compaq), Phil Roberts (Motorola), Patrice Romand (Bull S.A.),
   Jeff Schiller (MIT) Tom Soderlund (Nokia Research), Hesham Soliman
   (Ericsson), Jim Solomon (RedBack Networks), Tapio Suihko (Technical
   Research Center of Finland), Benny Van Houdt (University of Antwerp),
   Jon-Olov Vatn (KTH), Alper Yegin (Sun Microsystems), and Xinhua Zhao
   (Stanford University) for their detailed reviews of earlier versions
   of this document.  Their suggestions have helped to improve both the
   design and presentation of the protocol.




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   We would also like to thank the participants in the Mobile IPv6
   testing event held at Nancy, France, September 15-17, 1999, for
   their valuable feedback as a result of interoperability testing
   of four Mobile IPv6 implementations coming from four different
   organizations:  Bull (AIX), Ericsson Telebit (FreeBSD), NEC
   (FreeBSD), and INRIA (FreeBSD). Further, we would like to thank the
   feedback from the implementors who participated in the Mobile IPv6
   interoperability testing at Connectathons 2000, 2001, and 2002
   in San Jose, California.  Similarly, we would like to thank the
   participants at the ETSI interoperability testing at ETSI, in Sophia
   Antipolis, France, during October 2-6, 2000, including teams from
   Compaq, Ericsson, INRIA, Nokia, and Technical University of Helsinki.

   We would also like to thank Tuomas Aura, Mike Roe, and Greg
   O'Shea (Microsoft), Pekka Nikander (Ericsson), Erik Nordmark (Sun
   Microsystems), and Michael Thomas (Cisco) for the work on the return
   routability protocols which eventually led to the procedures used in
   this protocol.  The procedures described in [30] were adopted in the
   protocol.

   Lastly, we must express our appreciation for the significant
   contributions made by members of the Mobile IPv6 Security Design
   Team, including (in alphabetical order) Gabriel Montenegro, Erik
   Nordmark, (Sun Microsystems) and Pekka Nikander (Ericsson), who have
   contributed volumes of text to this specification.





























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References

    [1] D. Eastlake, 3rd, S. Crocker, and J. Schiller.  Randomness
        Recommendations for Security.  Request for Comments
        (Informational) 1750, Internet Engineering Task Force, December
        1994.

    [2] S. Bradner.  Key words for use in RFCs to Indicate Requirement
        Levels.  Request for Comments (Best Current Practice) 2119,
        Internet Engineering Task Force, March 1997.

    [3] R. Hinden and S. Deering.  IP Version 6 Addressing Architecture.
        Request for Comments (Proposed Standard) 2373, Internet
        Engineering Task Force, July 1998.

    [4] S. Kent and R. Atkinson.  Security Architecture for the Internet
        Protocol.  Request for Comments (Proposed Standard) 2401,
        Internet Engineering Task Force, November 1998.

    [5] S. Kent and R. Atkinson.  IP Authentication Header.  Request for
        Comments (Proposed Standard) 2402, Internet Engineering Task
        Force, November 1998.

    [6] S. Kent and R. Atkinson.  IP Encapsulating Security Payload
        (ESP).  Request for Comments (Proposed Standard) 2406, Internet
        Engineering Task Force, November 1998.

    [7] D. Piper.  The Internet IP Security Domain of Interpretation for
        ISAKMP.  Request for Comments (Proposed Standard) 2407, Internet
        Engineering Task Force, November 1998.

    [8] D. Maughan, M. Schertler, M. Schneider, and J. Turner.  Internet
        Security Association and Key Management Protocol (ISAKMP).
        Request for Comments (Proposed Standard) 2408, Internet
        Engineering Task Force, November 1998.

    [9] D. Harkins and D. Carrel.  The Internet Key Exchange (IKE).
        Request for Comments (Proposed Standard) 2409, Internet
        Engineering Task Force, November 1998.

   [10] T. Narten and H. Alvestrand.  Guidelines for Writing an IANA
        Considerations Section in RFCs.  Request for Comments (Best
        Current Practice) 2434, Internet Engineering Task Force, October
        1998.

   [11] S. Deering and R. Hinden.  Internet Protocol, Version 6 (ipv6)
        Specification.  Request for Comments (Draft Standard) 2460,
        Internet Engineering Task Force, December 1998.






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   [12] T. Narten, E. Nordmark, and W. Simpson.  Neighbor Discovery for
        IP Version 6 (ipv6).  Request for Comments (Draft Standard)
        2461, Internet Engineering Task Force, December 1998.

   [13] S. Thomson and T. Narten.  IPv6 Stateless Address
        Autoconfiguration.  Request for Comments (Draft Standard) 2462,
        Internet Engineering Task Force, December 1998.

   [14] A. Conta and S. Deering.  Internet Control Message Protocol
        (ICMPv6) for the Internet protocol version 6 (ipv6)
        specification.  Request for Comments (Draft Standard) 2463,
        Internet Engineering Task Force, December 1998.

   [15] A. Conta and S. Deering.  Generic Packet Tunneling in IPv6
        Specification.  Request for Comments (Proposed Standard) 2473,
        Internet Engineering Task Force, December 1998.

   [16] D. Johnson and S. Deering.  Reserved IPv6 Subnet Anycast
        Addresses.  Request for Comments (Proposed Standard) 2526,
        Internet Engineering Task Force, March 1999.

   [17] T. Narten and R. Draves.  Privacy Extensions for Stateless
        Address Autoconfiguration in IPv6, January 2001.

   [18] Editor J. Reynolds.  Assigned Numbers:  RFC 1700 is Replaced by
        an On-line Database.  Request for Comments (Informational) 3232,
        Internet Engineering Task Force, January 2002.

   [19] NIST.  Secure hash standard.  FIPS PUB 180-1, April 1995.

   [20] C. Perkins.  IP Mobility Support.  Request for Comments
        (Proposed Standard) 2002, Internet Engineering Task Force,
        October 1996.

   [21] C. Perkins.  IP Encapsulation within IP.  Request for Comments
        (Proposed Standard) 2003, Internet Engineering Task Force,
        October 1996.

   [22] C. Perkins.  Minimal Encapsulation within IP.  Request for
        Comments (Proposed Standard) 2004, Internet Engineering Task
        Force, October 1996.

   [23] C. Perkins and D. Johnson.  Route optimization in mobile IP
        (work in progress).  Internet Draft, Internet Engineering Task
        Force, September 2001.

   [24] P. Ferguson and D. Senie.  Network Ingress Filtering:  Defeating
        Denial of Service Attacks which employ IP source address
        spoofing.  Request for Comments (Informational) 2267, Internet
        Engineering Task Force, January 1998.




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   [25] J. Bound, C. Perkins, M. Carney, and R. Droms.  Dynamic host
        configuration protocol for IPv6 (DHCPv6) (work in progress).
        Internet Draft, Internet Engineering Task Force, January 2001.

   [26] H. Krawczyk, M. Bellare, and R. Canetti.  HMAC: Keyed-Hashing
        for Message Authentication.  Request for Comments
        (Informational) 2104, Internet Engineering Task Force,
        February 1997.

   [27] P. V. Mockapetris.  Domain names - concepts and facilities.
        Request for Comments (Standard) 1034, Internet Engineering Task
        Force, November 1987.

   [28] P. V. Mockapetris.  Domain names - implementation and
        specification.  Request for Comments (Standard) 1035, Internet
        Engineering Task Force, November 1987.

   [29] Tuomas Aura and Jari Arkko.  MIPv6 BU attacks and defenses.
        Internet Draft draft-aura-mipv6-bu-attacks-01.txt (Work In
        Progress), IETF, February 2002.

   [30] Michael Roe, Greg O'Shea, Tuomas Aura, and Jari Arkko.
        Authentication of Mobile IPv6 binding updates and
        acknowledgments.  Internet Draft draft-roe-mobileip-updateauth-02.txt
        (Work In Progress), IETF, February 2002.

   [31] Pekka Savola.  Security of IPv6 routing header and home address
        options.  Internet Draft draft-savola-ipv6-rh-ha-security-01.txt
        (Work In Progress), IETF, November 2001.

   [32] Erik Nordmark, Gabriel Montenegro, Pekka Nikander, and Jari
        Arkko.  Mobile ipv6 security design rationale.  To appear, 2002.

   [33] Erik Nordmark.  Securing MIPv6 BUs using return routability
        (BU3WAY).  Internet Draft draft-nordmark-mobileip-bu3way-00.txt
        (Work In Progress), IETF, November 2001.

   References [1] through [19] are normative and others are informative.


A. State Machine for the Correspondent Binding Procedure

   Home agents and correspondent nodes are stateless until a binding
   is actually established.  The mobile node, however, is responsible
   for initiating the correspondent binding procedure, keeping track of
   its state, handle retransmissions and failures, and completing the
   procedure.

   Section 11.6.2 defines the normative rules that the mobile node must
   follow when performing the correspondent binding procedure.  This
   appendix specifies two additional, non-normative, state-machines



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   that illustrates the behaviour of the mobile node.  The first
   state machine describes the full correspondent binding, and the
   second state machine describes the details relating to the return
   routability procedure.

   The state machines in this appendix do not attempt to define
   how recently received cookies can be used when moving fast or
   when performing deregistrations.  They also do not attempt to
   define how to behave when the mobile node may be simultaneously
   attached to several locations.  The state machines also assume that
   acknowledgements are always required.


A.1. Main State Machine

   The mobile node will keep the following states in its Binding List:

      Idle

         This is an imaginary state that refers to the situation that
         the correspondent node in question does not appear in the
         Binding List.  In this state, all RR and binding related
         messaging is silently ignored.

      RRInit

         This is a composite state.  While in this state, the mobile
         node has initiated the return routability procedure but has not
         yet completed it, and has no existing binding.  The internal
         details of this state are described in the second state
         machine, later in this appendix.

      RRRedo

         This is another composite state.  While in this state, the
         mobile node has an existing binding but has initiated the
         return routability procedure in order to refresh it.  The
         internal details of this state are the same as above.  In other
         words, the same second state machine is again used.

      RRDel

         In this state, the mobile node intends to send a
         de-registration Binding Update later but is first
         waiting for a home cookie before this can be done.  (Note that
         if the mobile node is at home, it can use a home cookie also as
         care-of cookie.)







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      WaitA

         In this state, the mobile node has sent a Binding Update, and
         is only waiting for the Binding Acknowledgement message to
         arrive.  There is no existing binding.

      WaitAR

         In this state, the mobile node has an existing binding, which
         is being refreshed with a Binding Update.  The mobile node is
         only waiting for the Binding Acknowledgement message to arrive.

      WaitD

         In this state, the mobile node has sent a de-registration
         Binding Update, and is only waiting for the Binding
         Acknowledgement message to arrive.

      Bound

         In this state, the mobile node has established a binding with
         the correspondent node.  In this state, the mobile node can
         send packets directly to the correspondent node.

   The following events are possible:

      Movement

         The mobile node moves off the home-link, or to a new location.
         (Note that in some cases the mobile node might not take
         movements immediately in account for the purposes of route
         optimization.)

      Returning home

         The mobile node moves back to its home link.  (Note that
         in some cases the mobile node may wish to give up route
         optimization by telling the correspondent nodes that it has
         returned home, when it has not.)

      Valid BRR received

         A valid Binding Refresh Request message has been received.

      Valid BA received

         A valid Binding Acknowledgement message has been received.

      Valid BE received

         A valid Binding Error message has been received.



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      Invalid MH Type received

         A Mobility Header message with an unrecognized MH Type field
         has been received.

      ICMP Problem 1 received

         An ICMP Parameter Problem Code 1 message has been
         received.  This can happen if the peer does not support this
         specification.

      Retransmission timer

         A timer is set to expire when a retransmission of a packet
         needs to be made.

      Failure timer

         A timer is set to expire when all retransmissions have failed.

   The following additional conditions are also used:

      Forward progress

         There is reason to believe forward progress is being made.
         Upper layer protocols such as TCP may provide hints to the IP
         layer regarding recent successful communications.

      Specific Status

         Tests of the Status values received in a BE or BA message.

   The following actions are possible:

      Start RR

         This is an abstract action that initiates the second state
         machine.

      Stop RR

         This is an abstract action that stops the second state machine.

      Send BU

         Send a Binding Update.

      Send BE

         Send a Binding Error with status 2.




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      Set sequence number

         Set the sequence number we use towards the correspondent node.

      Start retransmission timer

         Start the retransmission timer that controls the sending of
         additional messages after the first message.

      Start failure timer

         Start the failure timer that controls how long we keep on
         trying to retransmit.

      Stop retransmission timer

         Stop the retransmission timer.

      Stop timers

         Stop all timers.

   The state machine for performing the correspondent binding procedure
   is described below.  We have omitted events that have no actions and
   do not change the current state.

    State   Event                         Action         New State
    --------------------------------------------------------------

    Idle    Movement                      Start RR       RRInit

    State   Event                         Action         New State
    --------------------------------------------------------------

    RRInit  RR Done                       Send BU,       WaitA
                                          Start retrans-
                                          mission timer

    RRInit  Valid BE received and         Stop RR        Idle
            status = 2

    RRInit  Movement                      Stop RR        RRInit
                                          Start RR

    RRInit  Returning home                Stop RR        Idle

    RRInit  ICMP Problem 1 received       Stop timers    Idle

    State   Event                         Action         New State
    --------------------------------------------------------------




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    RRRedo  RR Done                       Send BU,       WaitAR
                                          Start retrans-
                                          mission timer

    RRRedo  Valid BE received and         Stop RR        Idle
            status = 2

    RRRedo  Movement                      Stop RR        RRInit
                                          Start RR

    RRRedo  Returning home                Stop RR        RRDel

    RRRedo  ICMP Problem 1 received       Stop timers    Idle

    State   Event                         Action         New State
    --------------------------------------------------------------

    WaitA   Valid BA received and         Stop timers    Bound
            status < 128

    WaitA   Valid BA received and         Set sequence   WaitA
            status = 141                  number,
                                          Send BU,
                                          Restart
                                          retransmission
                                          timer,
                                          Start failure
                                          timer

    WaitA   Valid BA received and         Start RR       RRInit
            status = 144 or 145

    WaitA   Valid BA received and         Stop timers    Idle
            status anything else

    WaitA   Retransmission timer          Send BU,       WaitA
                                          Start retrans-
                                          mission timer

    WaitA   Valid BE received and         Stop timers    Idle
            status = 2

    WaitA   Movement                      Start RR       RRInit

    WaitA   Returning home                Start RR       RRDel

    State   Event                         Action         New State
    --------------------------------------------------------------

    WaitAR  Valid BA received and         Stop timers    Bound
            status < 128



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    WaitAR  Valid BA received and         Set sequence   WaitAR
            status = 141                  number,
                                          Send BU,
                                          Restart
                                          retransmission
                                          timer,
                                          Start failure
                                          timer

    WaitAR  Valid BA received and         Start RR       RRInit
            status = 144 or 145

    WaitAR  Valid BA received and         Stop timers    Idle
            status anything else

    WaitAR  Retransmission timer          Send BU,       WaitAR
                                          Start retrans-
                                          mission timer

    WaitAR  Valid BE received and         Stop timers    Idle
            status = 2

    WaitAR  Movement                      Start RR       RRInit

    WaitAR  Returning home                Start RR       RRDel

    State   Event                         Action         New State
    --------------------------------------------------------------

    WaitD   Valid BA received and         Stop timers    Idle
            status < 128

    WaitD   Valid BA received and         Set sequence   WaitD
            status = 141                  number,
                                          Send BU,
                                          Stop timers,
                                          Start retrans-
                                          mission timer,
                                          Start failure
                                          timer

    WaitD   Valid BA received and         Start RR       RRDel
            status = 144 or 145

    WaitD   Valid BA received and         Stop timers    Idle
            status anything else

    WaitD   Retransmission timer          Send BU,       WaitD
                                          Start retrans-
                                          mission timer



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    WaitD   Valid BE received             Stop timers    Idle

    WaitD   Movement                      Start RR       RRInit

    State   Event                         Action         New State
    --------------------------------------------------------------

    RRDel   RR Done                       Send BU,       WaitD
                                          Stop timers,
                                          Start retrans-
                                          mission timer,
                                          Start failure
                                          timer

    RRDel   Valid BE received             Stop RR        Idle

    RRDel   Movement                      Stop RR,       RRInit
                                          Start RR

    RRDel   ICMP Problem 1 received       (None)         RRDel

    State   Event                         Action         New State
    --------------------------------------------------------------

    Bound   Valid BRR received            Start RR       RRRedo

    Bound   Returning home                Start RR       RRDel

    Bound   Movement                      Start RR       RRInit

    Bound   Valid BE received and         Start RR       RRInit
            status = 1 and no reason to
            believe forward progress
            is being made

    State   Event                         Action         New State
    --------------------------------------------------------------

    (Any)   RR Failed                     (None)         Idle

    (Any)   Failure timer                 Stop retrans-  Idle
                                          mission timer

    (Any)   Invalid MH Type received      Send BE        (No change)


A.2. Return Routability Procedure

   The second state machine describes how the return routability
   procedure is performed.  This state machine is used in three



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   situations in the first state machine, when creating a binding
   for the first time, when refreshing an existing binding, and when
   performing a deregistration.

   The mobile node will keep the following states in its Binding List
   for the return routability procedure:

      Start

         In this state, the return routability procedure is not active.

      WaitHC

         In this state, the mobile node has sent the Home Test Init
         and Care-of Test Init messages, and is waiting for the Home
         Test and Care-of Test messages to come back.  It will also be
         necessary to keep state of retransmissions for both.

      WaitH

         In this state, the mobile node has a recent Care-of Cookie and
         is only waiting for the Home Test message to arrive.

      WaitC

         In this state, the mobile node has a recent Home Cookie and is
         only waiting for the CoT message to arrive.

   The following events are possible:

      Start RR

         This is an abstract message from the first state machine.  It
         indicates that the return routability procedure needs to be
         run.

      Start home RR

         This a second abstract message from the first state machine.
         It indicates that the return routability procedure needs to be
         run, but only for the home address.

      Stop RR

         This is a third abstract message.  It signifies that the return
         routability procedure is no longer needed.

      Valid HoT received

         A valid Home Test message has been received.




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      Valid CoT received

         A valid Care-of Test message has been received.

   In addition, this state machine uses the Retransmission timer and
   Failure timer events from the previous state machine.

   The following actions are possible:

      Send HoTI

         Send the Home Test Init message.

      Send CoTI

         Send the Care-of Test message.

      Store cookie and nonce index

         Store the received cookie and nonce index in the appropriate
         place in the Binding Update List.

      RR Done

         This is an abstract event and signals that the return
         routability procedure has been completed.

      RR Failed

         This is an abstract event and signals that the return
         routability procedure has failed.

   In addition, this state machine uses the Start retransmission timer,
   Start failure timer, Stop retransmission timer, and Stop timers
   actions from the previous state machine.

   The state machine is as follows:

    State   Event                         Action         New State
    --------------------------------------------------------------
    Start   Start RR                      Send HoTI,     WaitHC
                                          Send CoTI,
                                          Start retrans-
                                          mission timer,
                                          Start failure
                                          timer

    Start   Start home RR                 Send HoTI,     WaitH
                                          Start retrans-
                                          mission timer,
                                          Start failure



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                                          timer

    State   Event                         Action         New State
    --------------------------------------------------------------

    WaitHC  Valid HoT received            Store cookie   WaitC
                                          and nonce
                                          index

    WaitHC  Valid CoT received            Store cookie   WaitH
                                          and nonce
                                          index

    WaitHC  Retransmission timer          Send HoTI,     WaitHC
                                          Send CoTI,
                                          Start retrans-
                                          mission timer

    State   Event                         Action         New State
    --------------------------------------------------------------

    WaitH   Valid HoT received            Store cookie   Start
                                          and nonce
                                          index,
                                          Stop timers,
                                          RR Done

    WaitH   Retransmission timer          Send HoTI,     WaitH
                                          Start retrans-
                                          mission timer

    State   Event                         Action         New State
    --------------------------------------------------------------

    WaitC   Valid CoT received            Store cookie   Start
                                          and nonce
                                          index,
                                          Stop timers,
                                          RR Done

    WaitC   Retransmission timer          Send CoTI,     WaitC
                                          Start retrans-
                                          mission timer

    State   Event                         Action         New State
    --------------------------------------------------------------

    (Any)   Stop RR                       Stop timers    Start

    (Any)   Failure timer                 Stop retrans-  Start
                                          mission timer,



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                                          RR Failed


B. Changes from Previous Version of the Draft

   This appendix briefly lists some of the major changes in this
   draft relative to the previous version of this same draft,
   draft-ietf-mobileip-ipv6-17.txt:

    -  Substantial editorial modifications have taken place,
       including the reduction of repetitive text at many places, the
       reorganization of security threat and remaining vulnerability
       discussion to the Security Considerations section, and so on.

    -  Mobility Header messages have been reformatted in various ways,
       including the reduction of the size of the cookies.

    -  Recommendations for the RtrAdvInterval, MAX_RR_BINDING_LIFE, and
       MAX_COOKIE_LIFE constants have been updated.

    -  Explanation on how to find the home agent's link-layer address
       has been updated.

    -  Status code values for Binding Acknowledgment have been
       renumbered.

    -  It is required that the (D) and the (L) bits have to be set when
       requesting forwarding from a previous care-of address.

    -  Requirements for IPv6 nodes, routers, and so on have been written
       on a new style and without giving a keyword for the support
       of route optimization.  It is expected that other documents
       currently being prepared will have these keywords.

    -  Binding Error messages are now rate-limited.

    -  A new flag, the L bit has been introduced to Binding Update
       message.

    -  The refresh field has been moved to a mobility option.

    -  The Home Address destination option is now used even with Binding
       Updates to correspondent nodes.  The option MAY used without an
       existing binding if the packet has been protected with IPsec.

    -  The key Kbu is now used also to protect the Binding
       Acknowledgement message.

    -  Both IPsec AH and ESP can be used to protect Binding Updates to
       home agents.




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    -  Unsolicited prefix advertisement now stops after a solicitation
       arrives to the home agent.

    -  Security is now required for all prefix discovery messages.

    -  Limitations regarding the generation of new home addresses for
       securing Binding Updates to home agents have been described.

    -  The state machine for the mobile node has been reorganized.

    -  Padding and header length texts for the Mobility header have been
       corrected.

    -  Option number and length fields have been corrected for many
       mobility options.

    -  References to mobile router functionality have been removed.

    -  Acknowledgements section has been updated.

    -  The mobile cookies have been renamed to be the HoT cookie and the
       CoT cookie.

    -  The Unique Identifier option is no longer specified for use with
       the HoTI, CoTI, HoT, or CoT messages.


C. Future Extensions

C.1. Piggybacking

   This document does not specify how to piggyback payload packets on
   the binding related messages.  However, it is envisioned that this
   can be specified in a separate document when currently discussed
   issues such as the interaction between piggybacking and IPsec are
   fully resolved (see also Section C.3).  The return routability
   messages can indicate support for piggybacking with a new mobility
   option.


C.2. Triangular Routing and Unverified Home Addresses

   Due to the concerns about opening reflection attacks with the Home
   Address destination option, this specification requires that this
   option must be verified against the binding cache, i.e., there must
   be a binding cache entry for the Home Address and Care-of Address.

   Future extensions may be specified that allow the use of unverified
   Home Address destination options in ways that do not introduce
   security issues.




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C.3. New Authorization Methods beyond Return Routability

   While the return routability procedure provides a good level
   of security, there exists methods that have even higher levels
   of security.  Secondly, as discussed in Section 14.4, future
   enhancements of IPv6 security may cause a need to improve also the
   security of the return routability procedure.

   Using IPsec as the sole method for authorizing Binding Updates
   to correspondent nodes is also possible.  The protection of the
   Mobility Header for this purpose is easy, though one must ensure
   that the IPsec SA was created with appropriate authorization to use
   the home address referenced in the Binding Update.  For instance,
   a certificate used by IKE to create the security association might
   contain the home address.  A future specification may specify how
   this is done.


C.4. Security and Dynamically Generated Home Addresses

   A future version of this specification may include functionality
   that allows the generation of new home addresses without requiring
   pre-arranged security associations or certificates even for the new
   addresses.


C.5. Remote Home Address Configuration

   The method for initializing a mobile node's home addresses on
   power-up or after an extended period of being disconnected from
   the network is beyond the scope of this specification.  Whatever
   procedure is used should result in the mobile node having the same
   stateless or stateful (e.g., DHCPv6) home address autoconfiguration
   information it would have if it were attached to the home network.
   Due to the possibility that the home network could be renumbered
   while the mobile node is disconnected, a robust mobile node would not
   rely solely on storing these addresses locally.

   Such a mobile node could initialize by using the following procedure:

    1. Generate a care-of address.

    2. Query DNS for the home network's mobile agent anycast address.

    3. Send a Home Agent Address Discovery Request message to the home
       network.

    4. Receive Home Agent Address Discovery Reply message.






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    5. Select the most preferred home agent and establish a security
       association between the mobile node's current care-of address and
       the home agent for temporary use during initialization only.

    6. Send a Home Prefix Solicitation message with the Request All
       Prefixes flag set to the home agent from the mobile node's
       care-of address.

    7. Receive a Home Prefix Advertisement message from the home agent,
       follow stateless address autoconfiguration rules to configure
       home addresses for prefixes received.

    8. Create a security association between the mobile node's home
       address and the home agent.

    9. Send a binding update(s) to the home agent to register the mobile
       node's home addresses.

   10. Receive binding acknowledgement(s) then begin normal
       communications.


































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Chairs' Addresses

   The Working Group can be contacted via its current chairs:


     Basavaraj Patil                   Phil Roberts
     Nokia Corporation                 Megisto Corp.
     6000 Connection Drive             Suite 120
     M/S M8-540                        20251 Century Blvd
     Irving, TX 75039                  Germantown MD 20874
     USA                               USA
     Phone:  +1 972-894-6709           Phone:  +1 847-202-9314
     Fax :  +1 972-894-5349            Email:  PRoberts@MEGISTO.com
     EMail:  Raj.Patil@nokia.com



Authors' Addresses

   Questions about this document can also be directed to the authors:


     David B. Johnson                  Charles E. Perkins
     Rice University                   Nokia Research Center
     Dept. of Computer Science, MS 132
     6100 Main Street                  313 Fairchild Drive
     Houston, TX 77005-1892            Mountain View, CA 94043
     USA                               USA

     Phone:  +1 713 348-3063           Phone:  +1 650 625-2986
     Fax:  +1 713 348-5930             Fax:  +1 650 625-2502
     E-mail:  dbj@cs.rice.edu          E-mail:  charliep@iprg.nokia.com


     Jari Arkko
     Ericsson
     Jorvas 02420
     Finland

     Phone:  +358 40 5079256
     E-mail:  jari.arkko@ericsson.com













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