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IETF Mobile IP Working Group                            David B. Johnson
INTERNET-DRAFT                                           Rice University
                                                         Charles Perkins
                                                   Nokia Research Center
                                                           22 March 2002


                        Mobility Support in IPv6
                  <draft-ietf-mobileip-ipv6-16.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.


Abstract

   This document specifies the operation of mobile computers using IPv6.
   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 four new IPv6 destination options,
   including one that MUST be supported in packets received by any node,
   whether mobile or stationary.









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                                Contents


Status of This Memo                                                    i

Abstract                                                               i

 1. Introduction                                                       1

 2. Comparison with Mobile IP for IPv4                                 3

 3. Terminology                                                        6
     3.1. General Terms . . . . . . . . . . . . . . . . . . . . . .    6
     3.2. Mobile IPv6 Terms . . . . . . . . . . . . . . . . . . . .    7

 4. Overview of Mobile IPv6                                            8
     4.1. New IPv6 Protocols  . . . . . . . . . . . . . . . . . . .    8
     4.2. Basic Operation . . . . . . . . . . . . . . . . . . . . .   10
     4.3. New IPv6 Destination Options  . . . . . . . . . . . . . .   13
     4.4. Alignment Requirements for New Destination Options  . . .   13
     4.5. Security Design . . . . . . . . . . . . . . . . . . . . .   14
           4.5.1. Security Threats  . . . . . . . . . . . . . . . .   14
           4.5.2. Security Features . . . . . . . . . . . . . . . .   15
           4.5.3. Securing Tunnels to and from the Home Agents  . .   17
           4.5.4. Securing Binding Updates to Home Agents . . . . .   17
           4.5.5. Securing Binding Updates to Correspondent Nodes .   18
           4.5.6. Preventing Denial-of-Service Attacks  . . . . . .   22
           4.5.7. Design Rationale  . . . . . . . . . . . . . . . .   23
     4.6. New IPv6 ICMP Messages  . . . . . . . . . . . . . . . . .   24
     4.7. Conceptual Data Structures  . . . . . . . . . . . . . . .   25
     4.8. Binding Management  . . . . . . . . . . . . . . . . . . .   30

 5. New IPv6 Destination Options and Message Types                    31
     5.1. Mobility Header . . . . . . . . . . . . . . . . . . . . .   31
           5.1.1. Format  . . . . . . . . . . . . . . . . . . . . .   32
           5.1.2. Binding Request (BR) Message  . . . . . . . . . .   33
           5.1.3. Home Test Init (HoTI) Message . . . . . . . . . .   34
           5.1.4. Care-of Test Init (CoTI) Message  . . . . . . . .   35
           5.1.5. Home Test (HoT) Message . . . . . . . . . . . . .   36
           5.1.6. Care-of Test (CoT) Message  . . . . . . . . . . .   38
           5.1.7. Binding Update (BU) Message . . . . . . . . . . .   40
           5.1.8. Binding Acknowledgement (BA) Message  . . . . . .   44
           5.1.9. Binding Missing (BM) Message  . . . . . . . . . .   49
     5.2. Mobility Header Parameters  . . . . . . . . . . . . . . .   51
           5.2.1. Format  . . . . . . . . . . . . . . . . . . . . .   51
           5.2.2. Pad1  . . . . . . . . . . . . . . . . . . . . . .   52
           5.2.3. PadN  . . . . . . . . . . . . . . . . . . . . . .   52
           5.2.4. Unique Identifier . . . . . . . . . . . . . . . .   53
           5.2.5. Alternate Care-of Address . . . . . . . . . . . .   53



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           5.2.6. Nonce Indices . . . . . . . . . . . . . . . . . .   54
           5.2.7. Authentication Data . . . . . . . . . . . . . . .   54
     5.3. Home Address Option . . . . . . . . . . . . . . . . . . .   55
     5.4. Routing Header type 2 . . . . . . . . . . . . . . . . . .   58
           5.4.1. Routing Header Packet format  . . . . . . . . . .   58
           5.4.2. Sending RH type 2 . . . . . . . . . . . . . . . .   59
           5.4.3. Verification by receiver  . . . . . . . . . . . .   60
           5.4.4. Extension header ordering . . . . . . . . . . . .   60
           5.4.5. Reversing type 2 routing headers  . . . . . . . .   60
     5.5. Mobile IPv6 Destination Option Sub-Options  . . . . . . .   61
           5.5.1. Pad1  . . . . . . . . . . . . . . . . . . . . . .   62
           5.5.2. PadN  . . . . . . . . . . . . . . . . . . . . . .   62
     5.6. ICMP Home Agent Address Discovery Request Message . . . .   63
     5.7. ICMP Home Agent Address Discovery Reply Message . . . . .   64
     5.8. ICMP Mobile Prefix Solicitation Message Format  . . . . .   66
     5.9. ICMP Mobile Prefix Advertisement Message Format . . . . .   68

 6. Modifications to IPv6 Neighbor Discovery                          70
     6.1. Modified Router Advertisement Message Format  . . . . . .   70
     6.2. Modified Prefix Information Option Format . . . . . . . .   71
     6.3. New Advertisement Interval Option Format  . . . . . . . .   73
     6.4. New Home Agent Information Option Format  . . . . . . . .   74
     6.5. Changes to Sending Router Advertisements  . . . . . . . .   76
     6.6. Changes to Sending Router Solicitations . . . . . . . . .   77

 7. Requirements for Types of IPv6 Nodes                              79
     7.1. Requirements for All IPv6 Routers . . . . . . . . . . . .   79
     7.2. Requirements for IPv6 Home Agents . . . . . . . . . . . .   79
     7.3. Requirements for IPv6 Mobile Nodes  . . . . . . . . . . .   80

























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 8. Correspondent Node Operation                                      82
     8.1. Receiving Packets from a Mobile Node  . . . . . . . . . .   82
     8.2. Receiving Binding Updates . . . . . . . . . . . . . . . .   82
     8.3. Requests to Cache a Binding . . . . . . . . . . . . . . .   83
     8.4. Requests to Delete a Binding  . . . . . . . . . . . . . .   84
     8.5. Sending Binding Acknowledgements  . . . . . . . . . . . .   84
     8.6. Sending Binding Requests  . . . . . . . . . . . . . . . .   85
     8.7. Cache Replacement Policy  . . . . . . . . . . . . . . . .   85
     8.8. Receiving ICMP Error Messages . . . . . . . . . . . . . .   86
     8.9. Sending Packets to a Mobile Node  . . . . . . . . . . . .   87

 9. Home Agent Operation                                              89
     9.1. Primary Care-of Address Registration  . . . . . . . . . .   89
     9.2. Primary Care-of Address De-Registration . . . . . . . . .   92
     9.3. Intercepting Packets for a Mobile Node  . . . . . . . . .   92
     9.4. Tunneling Intercepted Packets to a Mobile Node  . . . . .   94
     9.5. Handling Reverse Tunneled Packets from a Mobile Node  . .   96
     9.6. Protecting Return Routability Packets . . . . . . . . . .   96
     9.7. Home Prefix Propagation . . . . . . . . . . . . . . . . .   96
     9.8. Receiving Router Advertisement Messages . . . . . . . . .   97
     9.9. Dynamic Home Agent Address Discovery  . . . . . . . . . .   98
           9.9.1. Aggregate List of Home Network Prefixes . . . . .  100
           9.9.2. Scheduling Prefix Deliveries to the Mobile Node .  101
           9.9.3. Sending Advertisements to the Mobile Node . . . .  103
           9.9.4. Lifetimes for Changed Prefixes  . . . . . . . . .  105

10. Mobile Node Operation                                            105
    10.1. Sending Packets While Away from Home  . . . . . . . . . .  105
    10.2. Interaction with Outbound IPsec Processing  . . . . . . .  107
    10.3. Receiving Packets While Away from Home  . . . . . . . . .  108
    10.4. Movement Detection  . . . . . . . . . . . . . . . . . . .  109
    10.5. Receiving Local Router Advertisement Messages . . . . . .  112
    10.6. Forming New Care-of Addresses . . . . . . . . . . . . . .  114
    10.7. Sending Binding Updates to the Home Agent . . . . . . . .  115
    10.8. Dynamic Home Agent Address Discovery  . . . . . . . . . .  117
    10.9. Sending Binding Updates to Correspondent Nodes  . . . . .  118
   10.10. Receiving RR Messages . . . . . . . . . . . . . . . . . .  121
   10.11. Establishing Forwarding from a Previous Care-of Address .  122
   10.12. Retransmitting Binding Updates  . . . . . . . . . . . . .  123
   10.13. Rate Limiting for Sending Binding Updates . . . . . . . .  124
   10.14. Receiving Binding Acknowledgements  . . . . . . . . . . .  124
   10.15. Receiving Binding Requests  . . . . . . . . . . . . . . .  125
   10.16. Receiving ICMP Error Messages . . . . . . . . . . . . . .  126
   10.17. Receiving ICMP Error Messages . . . . . . . . . . . . . .  126
   10.18. Sending Mobile Prefix Solicitations . . . . . . . . . . .  126
   10.19. Receiving Mobile Prefix Advertisements  . . . . . . . . .  127
   10.20. Using Multiple Care-of Addresses  . . . . . . . . . . . .  128
   10.21. Routing Multicast Packets . . . . . . . . . . . . . . . .  128
   10.22. Returning Home  . . . . . . . . . . . . . . . . . . . . .  129

11. Protocol Constants                                               131



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12. Future Extensions                                                132
    12.1. Piggybacking  . . . . . . . . . . . . . . . . . . . . . .  132
    12.2. Triangular Routing and Unverified HAOs  . . . . . . . . .  132
    12.3. New Authorization Methods beyond RR . . . . . . . . . . .  132

13. IANA Considerations                                              133

14. Security Considerations                                          134
    14.1. Security for the Tunneling to and from the Home Agent . .  134
    14.2. Security for the Binding Updates to the Home Agent  . . .  135
    14.3. Security for the Binding Updates to the Correspondent
             Nodes  . . . . . . . . . . . . . . . . . . . . . . . .  135
    14.4. Security for the Home Address Option  . . . . . . . . . .  135
    14.5. Firewall considerations . . . . . . . . . . . . . . . . .  136

Acknowledgements                                                     137

References                                                           138

 A. Changes from Previous Version of the Draft                       140
     A.1. Changes from Draft Version ...-15 . . . . . . . . . . . .  140
     A.2. Changes from Previous Versions of the Draft . . . . . . .  142

 B. Remote Home Address Configuration                                144

Chairs' Addresses                                                    145

Authors' Addresses                                                   146


























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1. Introduction

   This document specifies the operation of mobile computers using
   Internet Protocol Version 6 (IPv6) [6].  Without specific support
   for mobility in IPv6, packets destined to a mobile node (host or
   router) would not be able to reach it while the mobile node is away
   from its home link (the link on which its home IPv6 subnet prefix is
   in use), since routing is based on the subnet prefix in a packet's
   destination IP address.  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 operation defined here, 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, and the mobile node may
   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.  Within
   the natural limitations imposed by link-management solutions, and as
   long as such handover occurs only within cells of the mobile node's
   home link, such link-layer mobility mechanisms MAY offer faster
   convergence and lower overhead than Mobile IPv6.  Extensions to the
   Mobile IPv6 protocol have been proposed to support a more local,
   hierarchical form of mobility management, but such extensions are
   beyond the scope of this document.



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   The protocol specified in this document solves the problem of
   transparently routing packets to and from mobile nodes while away
   from home.  However, it 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.  Some
       aspects of this problem are addressed by the movement detection
       procedure described in Section 10.4, but no attempt has been made
       to fully solve this problem in its general form.  Most aspects of
       this problem can be solved by the workaround of restricting such
       networks to only one router per link, although there are still
       possible hidden terminal problems when two nodes on the same
       link (on opposite sides of the router) attempt to communicate
       directly.

    -  Access control on a link being visited by a mobile node.  This
       is a general problem any time an unauthenticated node is allowed
       to connect to any link layer.  It is independent of whether the
       connecting node uses Mobile IP, DHCP [2], or just "borrows" an IP
       address on the link.
































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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) [25, 24, 26], together
   with the opportunities provided by the design and deployment of a new
   version of IP itself (IPv6) and the new protocol features offered
   by IPv6.  Mobile IPv6 thus shares many features with Mobile IPv4,
   but the protocol is now fully integrated into IP and provides many
   improvements over Mobile IPv4.  This section summarizes the major
   differences between Mobile IPv4 and Mobile IPv6:

    -  Support for what is known in Mobile IPv4 as "Route
       Optimization" [27] 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.  This integration of Route Optimization
       functionality allows direct routing from any correspondent
       node to any mobile node, without needing to pass through
       the mobile node's home network and be forwarded by its home
       agent, and thus eliminates the problem of "triangle routing"
       present in the base Mobile IPv4 protocol [25].  The Mobile IPv4
       "registration" functionality and the Mobile IPv4 Route
       Optimization functionality are performed by a single protocol
       rather than two separate (and different) protocols.

    -  Support is also integrated into Mobile IPv6 -- and into IPv6
       itself -- for allowing mobile nodes and Mobile IP to coexist
       efficiently with routers that perform "ingress filtering" [7].  A
       mobile node now uses its care-of address as the Source Address in
       the IP header of packets it sends, allowing the packets to pass
       normally through ingress filtering routers.  The home address
       of the mobile node is carried in the packet in a Home Address
       destination option, allowing the use of the care-of address in
       the packet to be transparent above the IP layer.  The ability
       to correctly process a Home Address option in a received packet
       is required in all IPv6 nodes, whether mobile nor stationary,
       whether host or router.

    -  The use of IPv6 destination options allows all Mobile IPv6
       control traffic to be piggybacked on any existing IPv6 packets,
       whereas in Mobile IPv4 and its Route Optimization extensions,
       separate UDP packets were required for each control message.

    -  The use of the care-of address as the Source Address in each
       packet's IP header also simplifies routing of multicast packets
       sent by a mobile node.  With Mobile IPv4, the mobile node
       had to tunnel multicast packets to its home agent in order to
       transparently use its home address as the source of the multicast
       packets.  With Mobile IPv6, the use of the Home Address option
       allows the home address to be used but still be compatible with



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       multicast routing that is based in part on the packet's Source
       Address.

    -  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, such as Neighbor
       Discovery [20] and Address Autoconfiguration [35], to operate in
       any location away from home without any special support required
       from its local router.

    -  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
       (packets that the router sends are reaching the mobile node, and
       packets that the mobile node sends are reaching the router).
       This confirmation provides a detection of the "black hole"
       situation that may exist in some wireless environments where the
       link to the router does not work equally well in both directions,
       such as when the mobile node has moved out of good wireless
       transmission range from the router.  The mobile node may then
       attempt to find a new router and begin using a new care-of
       address if its link to its current router is not working well.
       In contrast, in Mobile IPv4, only the forward direction (packets
       from the router are reaching the mobile node) is confirmed,
       allowing the black hole condition to persist.

    -  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, whereas Mobile IPv4 must use encapsulation for all
       packets.  The use of a Routing header requires less additional
       header bytes to be added to the packet, reducing the overhead
       of Mobile IP packet delivery.  To avoid modifying the packet in
       flight, however, packets intercepted and tunneled by a mobile
       node's home agent in Mobile IPv6 must still use encapsulation for
       delivery to the mobile node.

    -  While a mobile node is away from home, its home agent intercepts
       any packets for the mobile node that arrive at the home network,
       using IPv6 Neighbor Discovery [20] rather than ARP [29] as is
       used in Mobile IPv4.  The use of Neighbor Discovery improves
       the robustness of the protocol (e.g., due to the Neighbor
       Advertisement "override" bit) and simplifies implementation
       of Mobile IP due to the ability to not be concerned with any
       particular link layer as is required in ARP.

    -  The use of IPv6 encapsulation (and the Routing header) removes
       the need in Mobile IPv6 to manage "tunnel soft state", which was
       required in Mobile IPv4 due to limitations in ICMP for IPv4.  Due
       to the definition of ICMP for IPv6, the use of tunnel soft state
       is no longer required in IPv6 for correctly relaying ICMP error




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       messages from within the tunnel back to the original sender of
       the packet.

    -  The dynamic home agent address discovery mechanism in Mobile IPv6
       uses IPv6 anycast [11] and returns a single reply to the mobile
       node, rather than the corresponding Mobile IPv4 mechanism that
       used IPv4 directed broadcast and returned a separate reply from
       each home agent on the mobile node's home link.  The Mobile IPv6
       mechanism is more efficient and more reliable, since only one
       packet need be sent back to the mobile node.  The mobile node is
       less likely to lose one of the replies because no "implosion" of
       replies is required by the protocol.

    -  Mobile IPv6 defines an Advertisement Interval option on
       Router Advertisements (equivalent to Agent Advertisements in
       Mobile IPv4), allowing a mobile node to decide for itself how
       many Router Advertisements (Agent Advertisements) it is willing
       to miss before declaring its current router unreachable.




































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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 [3].


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.

      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, as defined above).

      Security Policy Database (SPD)
                    A database of security associations selectable by
                    rulesets (policies) that determine the packets for
                    which each security association is to be applied.



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3.2. Mobile IPv6 Terms

      home address  An IP address assigned to a mobile node within 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.  Standard IP routing mechanisms will
                    deliver packets destined for a mobile node's home
                    address to its home link.

      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
                    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.

      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.

      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 a time (e.g., with different subnet
                    prefixes), the one registered with the mobile node's
                    home agent is called its "primary" care-of address.



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      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 Key
                    a key used for authenticating Binding Update
                    messages.

      Binding Security Association (BSA)
                    a security association established specifically
                    for the purpose of producing and verifying
                    authentication data passed with a Binding Update
                    destination option.


4. Overview of Mobile IPv6

4.1. New IPv6 Protocols

   As mentioned in Section 4.2, Mobile IPv6 defines a new IPv6 protocol,
   the Mobility Header.  This protocol is used to carry the following
   messages:

      Home Test Init

         The Home Test Init message is used to initiate the Return
         Routability procedure from the mobile node to a correspondent
         node.  This procedure ensures that subsequence Binding Updates
         are properly authorized to redirect the traffic of a particular
         home address.  The Home Test Init message is described in
         detail in Section 5.1.3.

      Care-of Test Init

         The Care-of Test Init message is also used to initiate the
         Return Routability procedure, for a particular care-of address.
         The Care-of Test Init message is described in detail in
         Section 5.1.4.

      Home Test

         The Home Test message carries a cookie which the mobile node
         needs before it can properly authorize itself for sending a
         Binding Update.  This message is an answer to the Home Test
         Init message, and is described in detail in Section 5.1.5.

      Care-of Test

         The Care-of Test message carries a cookie which the mobile node
         needs before it can properly authorize itself for sending a




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         Binding Update.  This message is an answer to the Care-of Test
         Init message, and is described in detail in Section 5.1.6.

      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
         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".  A home registration MUST be protected
         with IPsec, while other Binding Updates MUST be protected with
         an authenticator as described in Section 4.5.  The Binding
         Update message and its specific authentication requirements are
         described in detail in Section 5.1.7.

      Binding Acknowledgement

         A Binding Acknowledgement message is used to acknowledge
         receipt of a Binding Update, if an acknowledgement was
         requested in the Binding Update.  An acknowledgement of a home
         registration MUST be protected with IPsec, while other Binding
         Update acknowledgements MUST be protected with an authenticator
         as described in Section 4.5.  The Binding Acknowledgement
         message and its specific authentication requirements are
         described in detail in Section 5.1.8.

      Binding Request

         A Binding Request message is used to request a mobile node
         to send to the requesting node a Binding Update containing
         the mobile node's current binding.  This message is typically
         used by a correspondent node to refresh a cached binding for a
         mobile node, when the cached binding is in active use but the
         binding's lifetime is close to expiration.  No authentication
         is required for the Binding Request message.  The Binding
         Request message is described in detail in Section 5.1.2.

      Binding Missing

         The Binding Missing message is used by the correspondent node
         to signal an inappropriate attempt to use the Home Address
         Option without an existing binding.  This message is described
         in detail in Section 5.1.9.

   Mobile IPv6 also defines a number of "parameters" for use within
   these messages; if included, any parameters MUST appear after the
   fixed portion of the option data specified in this document.  The
   presence of such parameters 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




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   are interpreted as parameters.  The encoding and format of defined
   parameters are described in Section 5.2.

   Alignment requirements for the Mobility Header are same as for any
   IPv6 protocol, i.e.  they MUST be aligned on an 8-octet boundary.
   We also require that the Mobility Header length is a multiple of 8
   octets.


4.2. Basic Operation

   A mobile node is always addressable by 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 never mobile.  Since the subnet prefix
   of a mobile node's home address is the subnet prefix (or one of the
   subnet prefixes) on the mobile node's home link (it is the mobile
   node's home subnet prefix), packets addressed to it will be routed to
   its home link.

   While a mobile node is attached to some foreign link away from home,
   it is also addressable by one or more care-of addresses, in addition
   to its home address.  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 the subnet prefix
   (or one of the subnet prefixes) on the foreign link being visited by
   the mobile node; if the mobile node is connected to this foreign link
   while using that care-of address, packets addressed to this care-of
   address will be routed to the mobile node in its location away from
   home.

   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 [35] or
   stateful (e.g., DHCPv6 [2]) Address Autoconfiguration, according
   to the methods of IPv6 Neighbor Discovery [20].  Other methods
   of acquiring a care-of address are also possible, such as static
   pre-assignment by the owner or manager of a particular foreign link,
   but details of such other methods are beyond the scope of this
   document.

   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.  This binding
   registration is done by the mobile node sending to the home agent
   a packet containing a "Binding Update" destination option; the
   home agent then replies to the mobile node by returning a packet
   containing a "Binding Acknowledgement" destination option.  The
   care-of address in this binding registered with its home agent is
   known as the mobile node's "primary care-of address".  The mobile



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   node's home agent thereafter uses proxy Neighbor Discovery to
   intercept any IPv6 packets addressed to the mobile node's home
   address (or home addresses) on the home link, and tunnels each
   intercepted packet to the mobile node's primary care-of address.
   To tunnel each intercepted packet, the home agent encapsulates the
   packet using IPv6 encapsulation [4], with the outer IPv6 header
   addressed to the mobile node's primary care-of address.

   When a mobile node moves from one care-of address to a new care-of
   address on a new link, it is desirable for packets arriving at the
   previous care-of address to be tunneled to the mobile node's care-of
   address.  Since the purpose of a Binding Update is to establish
   exactly this kind of tunneling, it is specified to be used (at
   least temporarily) for tunnels originating at the mobile node's
   previous care-of address, in exactly the same way that it is used
   for establishing tunnels from the mobile node's home address to the
   mobile node's current care-of address.  Section 10.11 describes the
   use of the Binding Update for this purpose.

   Section 10.20 discusses the reasons why it may be desirable for
   a mobile node to use more than one care-of address at the same
   time.  However, a mobile node's primary care-of address is distinct
   among these in that the home agent maintains only a single care-of
   address registered for each mobile node, and always tunnels a mobile
   node's packets intercepted from its home link to this mobile node's
   registered primary care-of address.  The home agent thus need not
   implement any policy to determine the particular care-of address to
   which it will tunnel each intercepted packet.  The mobile node alone
   controls the policy by which it selects the care-of addresses to
   register with its home agent.

   It is possible that while a mobile node is away from home, some nodes
   on its home link may be reconfigured, such that the router that was
   operating as the mobile node's home agent is replaced by a different
   router serving this 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 [11] and
   thus reaches one of the (possibly many) 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 list of home
   agents is maintained by each home agent on the home link through use
   of the Home Agent (H) bit in each home agent's periodic unsolicited
   multicast Router Advertisements.





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   The Binding Update and Binding Acknowledgement destination options,
   together with a "Binding Request" destination option, are also used
   to allow IPv6 nodes communicating with a mobile node, to dynamically
   learn and cache the mobile node's binding.  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 an IPv6 Routing
   header [6] (instead of IPv6 encapsulation) 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.  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.

   Since a Binding Update, Binding Acknowledgement, and Binding Request
   are each represented in a packet as an IPv6 destination option [6],
   they may be included in any IPv6 packet.  Any of these options can be
   sent in either of two ways:

    -  the messages can be included within any IPv6 packet carrying any
       payload such as TCP [31] or UDP [30].

    -  the messages can be sent as a separate IPv6 packet containing
       no payload.  In this case, the Next Header field in the last
       extension header in the packet is set to the value 59, to
       indicate "No Next Header" [6].

   Mobile IPv6 also defines one additional IPv6 destination option.
   When a mobile node sends a packet while away from home, it will
   generally set the Source Address in the packet's IPv6 header to one
   of its current care-of addresses, and will also include a "Home
   Address" destination option in the packet, giving the mobile node's
   home address.  Many routers implement security policies such as
   "ingress filtering" [7] that do not allow forwarding of packets
   that have a Source Address which appears topologically incorrect.
   By using the care-of address as the IPv6 header Source Address,
   the packet will be able to pass normally through such routers,
   yet ingress filtering rules will still be able to locate the true
   topological source of the packet in the same way as packets from
   non-mobile nodes.  By also including the Home Address option in each
   packet, the sending mobile node can communicate its home address to
   the correspondent node receiving this packet, allowing the use of
   the care-of address to be transparent above the Mobile IPv6 support
   level (e.g., at the transport layer).  The inclusion of a Home
   Address option in a packet affects only the correspondent node's
   receipt of this single packet; no state is created or modified in the
   correspondent node as a result of receiving a Home Address option in
   a packet.



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4.3. New IPv6 Destination Options

   As mentioned in Section 4.2, the following new IPv6 destination
   option is defined for Mobile IPv6:

      Home Address

         A Home Address option is used in a packet sent by a mobile
         node to inform the recipient of that packet of the mobile
         node's home address.  For packets sent by a mobile node while
         away from home, the mobile node generally uses one of its
         care-of addresses as the Source Address in the packet's IPv6
         header.  By including a Home Address option in the packet, the
         correspondent node receiving the packet is able to substitute
         the mobile node's home address for this care-of address when
         processing the packet, thus making the use of the care-of
         address transparent to the correspondent node.  If the IP
         header of a packet carrying a Home Address option is covered
         by authentication, then the Home Address option MUST also be
         covered by this authentication, but no other authentication
         is required for the Home Address option.  See sections 10.2
         and 5.3 for additional details about requirements for the
         calculation and verification of the authentication data.  The
         Home Address option is described in detail in Section 5.3.

   Mobile IPv6 also defines a number of "sub-options" for use within
   destination options.  If included, any sub-options MUST appear after
   the fixed portion of the option data specified in this document.  The
   presence of such sub-options will be indicated by the Option Length
   field within the option.  When the Option Length is greater than the
   length required for the option specified here, the remaining octets
   are interpreted as sub-options.  The encoding and format of defined
   sub-options are described in Section 5.5.


4.4. Alignment Requirements for New Destination Options

   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 an integer multiple of n octets from the start of the header,
   for n = 1, 2, 4, or 8) [6].  Mobile IPv6 sub-options have similar
   alignment requirements, so that multi-octet values within the
   Sub-Option Data field of each sub-option fall on natural boundaries.
   The alignment requirement of an option or sub-option is specified in
   this document using the standard notation used elsewhere for IPv6
   alignment requirements [6].  Specifically, the notation xn+y means
   that the Option Type or Sub-Option Type field must fall at an integer
   multiple of x octets from the start of the header, plus y octets.
   For example:



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      2n    means any 2-octet offset from the start of the header.

      8n+2  means any 8-octet offset from the start of the header,
            plus 2 octets.


4.5. Security Design

4.5.1. Security Threats

   The MIPv6 protocol needs to protect itself against the following main
   threats:

    1. Threats against Binding Updates sent to home agents:  a attacker
       might claim that a certain mobile node is currently at a
       different location than it really is.  If the home agent accepts
       the information sent to it as is, the mobile node might not get
       traffic destined to it, and other nodes might get traffic they
       didn't want.

    2. Threats against route optimization with correspondent nodes:
       A malicious mobile node might lie about its home address.  A
       malicious mobile node might send a correspondent node binding
       updates in which the home address is set to the address of
       another node, the victim.  If the correspondent node accepted
       this forged binding update, then communications between the
       correspondent node and the victim would be disrupted, because
       packets that the correspondent node intended to send to the
       victim would be sent to the wrong care-of address.  This is a
       threat to confidentiality as well as availability, because an
       attacker might redirect packets meant for another node to itself
       in order to learn the content of those packets.

       A malicious mobile node might lie about its care-of address.  A
       malicious mobile node might send a correspondent node binding
       updates in which the care-of address is set to the address of
       a victim node or an address within a victim network.  If the
       correspondent node accepted this forged binding update, then the
       malicious mobile could trick the correspondent into sending data
       to the victim node or the victim network; the correspondent's
       replies to messages sent by the malicious mobile will be sent
       to the victim host or network.  This could be used to cause a
       distributed denial of service attack; the malicious mobile could
       trick a large number of servers so that they all send a large
       amount of data to the same victim node or network.  Several
       variations of this threat are described elsewhere [1][33].

       A malicious node might also send a large number of invalid
       binding updates to a victim correspondent node.  If each invalid
       binding update took a significant amount of resources (such as
       CPU) to process before it could be recognized as invalid, then it



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       might be possible to cause a denial of service attack by sending
       the correspondent so may invalid binding updates that it has no
       resources left for other tasks.

       An attacker might also replay an old binding update.  An attacker
       might 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.

    3. Threats where MIPv6 correspondent node functionality is used
       to launch reflection attacks against other parties [34] [23].
       The Home Address Option can be used to direct response traffic
       against a node whose IP address appears in the option, without
       giving a possibility for ingress filtering to catch the forged
       "return address".

    4. Threats where the tunnels between the mobile node and the home
       agent are attacked to make it appear like the mobile node is
       sending traffic while it is not.

    5. Threats where IPv6 Routing Header -- which is employed in
       MIPv6 -- is used to circumvent IP-address based rules in
       firewalls or to reflect traffic from other nodes.  The generality
       of the Routing Header allows the kind of usage that opens
       vulnerabilities, even if the usage that MIPv6 needs is safe.

    6. The security mechanisms of MIPv6 may also be attacked themselves,
       e.g.  in order to force the participants to execute expensive
       cryptographic operations or allocate memory for the purpose of
       keeping state.

   Most of the above threats are concerned with denial of service.  Some
   of the threats also open up possibilities for man-in-the-middle,
   hijacking, and impersonation attacks.


4.5.2. Security Features

   This specification provides a number of security features.  The main
   features are:

    -  Protection of Binding Updates to home agents.

    -  Protection of Binding Updates to correspondent nodes.

    -  Protection against reflection attacks through the Home Address
       Option.

    -  Protection of tunnels between the mobile node and the home agent.



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    -  Preventing Routing Header vulnerabilities.

    -  Preventing Denial-of-Service attacks to the MIPv6 security
       mechanisms themselves.

   Protecting the Binding Updates to home agents and to correspondent
   nodes require very different security solutions due to the different
   situations.  Mobile nodes and home agents know each other, and can
   thus have a strong security association to reliably authenticate
   the exchanged messages.  In this environment, IPsec Encapsulating
   Security Payload (ESP) can be used to implement the necessary
   security features.  See Section 4.5.5.

   The protection of Binding Updates to correspondents is a much harder
   problem for the traditional strong authentication approach.  It is
   expected that MIPv6 will be used on a global basis between nodes
   belonging under different administrative domains, hence building
   an authentication infrastructure to authenticate mobile nodes
   and correspondent nodes would be a very demanding task.  Thus, an
   infrastructureless approach is necessary.  Furthermore, making a
   traditional authentication infrastructure keep track of correct
   IP addresses for all hosts is either impossible or at least very
   hard.  That is, it isn't sufficient to authenticate mobile nodes,
   authorization to claim right to use an address is needed.

   A different approach is therefore necessary.  The chosen method
   verifies that the mobile node is ``live'' (that is, it responds to
   probes) at its home and care-of addresses by performing a cookie
   exchange with the addresses, and by requiring that the eventual
   binding update is cryptographically bound to the sent cookies.
   Some additional protection is provided by requiring the cookies be
   protected by ESP when forwarded by the Home Agent to the mobile node.
   This method limits the vulnerabilities to those attackers who are
   on the path between the Home Agent and the correspondent node.  As
   adversaries on this path would be able to cause also other types of
   attacks, this is seen as sufficient base security between mobile and
   correspondent nodes.

   Vulnerabilities relating to the use of correspondent nodes as
   reflectors via the Home Address Option can be solved as follows.  We
   ensure that the mobile node is authorized to use a given home address
   before HAO can be used.  Such authorization is already performed in
   the context of Route Optimization, and therefore this specification
   limits the use of the HAO to the situation where the correspondent
   node already has a binding cache entry for the given home address.

   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 4.5.3.




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   Vulnerabilities related to the Routing Header can be prevented by
   using a MIPv6 specific type of a Routing Header.  This type provides
   the necessary functionality but does not open vulnerabilities.

   Denial-of-Service threats against MIPv6 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 4.5.6.


4.5.3. Securing Tunnels to and from the Home Agents

   Tunnels between the mobile node and the home agent need protection
   so that it isn't possible for anyone to send traffic through the
   home agent, pose as the mobile node, and escape detection through
   traditional tracing mechanisms.

   If Binding Updates sent to the home agents are secure, and the home
   agent verifies the outer IP address corresponds to the current
   location of the mobile node, this prevents attacks against the tunnel
   from other IP addresses.  This prevents attacks where the attacker
   is controlled by ingress filtering, as well as attacks where the
   attacker does not know the current care-of address of the mobile
   node.  Attackers who know the care-of address are not controlled by
   ingress filtering could still send traffic through the home agent,
   but they could also send spoofed packets without using a tunnel.

   Encapsulating the tunneled traffic inside IPsec ESP offers an
   optional mechanism to protect the confidentiality and integrity of
   the traffic against on-path attackers.


4.5.4. Securing Binding Updates to Home Agents

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

   In order to have this protection, the mobile node and the home agent
   must have a security association.  IPsec Encapsulating Security
   Payload (ESP) can be used to for integrity protection when a non-null
   authentication algorithm is applied.

   However, IPsec can provide replay protection only when 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, Mobile IPv6 does not rely
   on IPsec replay protection and provides its own mechanism inside the
   Binding Update and Acknowledgement messages.  A sequence number field
   is used to ensure correct ordering and to prevent replay protection.
   Both the mobile node and the home agent MUST use non-volatile memory



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   to store the sequence number so that a reboot does not prevent the
   acceptance of new Binding Updates.

   A sliding window scheme is used for the sequence numbers.  Therefore
   the protection against replays and reordering attacks works when
   the attacker remembers up to a maximum of 2^31 Binding Updates.
   The mobile node and the home agent MAY agree on a new key for the
   security association before this many Binding Updates have been sent
   if this is an issue.

   Note that while the required non-volatile memory is an additional
   burden in particular for the mobile node, the use of sequence numbers
   reduces the number of roundtrips necessary for the update procedure
   compared to other schemes that would not have required non-volatile
   memory.  Note also that implementations do not necessarily have to
   write the non-volatile memory every time they send a Binding Update,
   if they always write a somewhat larger sequence number to the memory
   and only update the memory again once the used sequence numbers go
   beyond this larger number.  For instance, if the sequence number
   starts at 0, the value 100 can be written to the memory so that
   the next write can be done when the sequence numbers from 0 to 99
   have been used.  This reduces the need for frequent updates to the
   non-volatile memory, which improves performance and may be necessary
   in some cases to lengthen the lifetime of the used memory components.

   In order to protect messages exchanged between the mobile node and
   the home agent with IPsec, appropriate Security Policy Database (SPD)
   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 SPD 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 Option in
   Binding Updates sent to the home agent.  The home address in the Home
   Address Option and the Binding Update message MUST be equal and MUST
   be checked by the home agent.


4.5.5. Securing Binding Updates to Correspondent Nodes

   Binding Updates to correspondent nodes are protected using the Return
   Routability (RR) method.  This method uses the following principles:

    -  A cookie exchange verifies that the mobile node is ``live'' at
       its addresses, or is at least able to receive traffic on them.

    -  Protecting the eventual binding update cryptographically using
       the cookies.





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    -  Requiring that the cookies be protected by ESP when forwarded by
       the Home Agent to the mobile node.

    -  The use of symmetric exchanges were responses are sent to the
       same address as the request was sent from, to avoid the use of
       this protocol in reflection attacks.

    -  Stateless operation at correspondent nodes until they receive the
       a binding update that can be authorized.

   The RR protocol can be broken by an attacker on the route between the
   home agent and the correspondent node, but not by attackers on the
   network the mobile node is currently at and not from elsewhere on the
   Internet.

   Each correspondent node has a secret key, Kcn.  This key does not
   need to be shared with any other entity, so no key distribution
   mechanism is needed for it.  Each correspondent node also generates
   a nonce, Nj, at regular intervals, for example every few minutes.  A
   correspondent node uses the same Kcn and Nj with all the mobiles it
   is in communication with, so that it does not need to generate and
   store a new Nj when a new mobile contacts it.  Each value of Nj is
   identified by the subscript j.  This subscript is communicated in the
   protocol, so that if Nj is replaced by N(j+1) part way through a run
   of a protocol, the correspondent can distinguish messages that should
   be checked against the old nonce from messages that should be checked
   against the new nonce.  Correspondent nodes keep both the current
   value of Nj and a small set of previous values N(j-1), N(j-2), ...
   Older values can be discarded, and messages using them will can be
   rejected as replays.

   Kcn can be either a fixed value or regularly updated.  An update
   of Kcn can be done at the same time as an update of Nj, so that j
   identifies both the nonce and the key.  A correspondent node can
   generate a fresh Kcn each time that it boots to avoid the need for
   secure persistent storage for Kcn.

   The RR signaling happens as follows:

    1. MN(HoA) -> CN: HoTI(HoA)
    2. MN(CoA) -> CN: CoTI(CoA)
    3. CN -> MN(HoA): HoT(K0, j)
    4. CN -> MN(CoA): CoT(K1, l)
    5. MN(CoA) -> CN: BU(HoA, CoA, MAC, j, l)
    6. CN -> MN(CoA): BA(MAC)
    7. CN -> MN(HoA): BR(MAC)

   Messages 1 and 2 are sent simultaneously, as are messages 3 and
   4.  Message 5 actually creates a binding, and messages 6 and 7 are
   optional.  The messages are described in more detail below:




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    1. HoTI (Home Test Init) Message:

       When a mobile nodes wants to perform route optimization it sends
       a HoTI message to the correspondent node in order to initiate the
       return routability verification for the Home Address.

           MN(HoA) -> CN: HoA

       This message tells the mobile node's home address to the
       correspondent node.  The address is used later to create a
       cookie.  This message is reverse tunneled through the Home Agent.

    2. CoTI (Care-of Test Init) Message:

       When a mobile nodes wants to perform route optimization it sends
       a CoTI message to the correspondent node in order to initiate the
       return routability verification for the care-of Address.

           MN(CoA) -> CN: CoA

       The second message is sent in parallel with the first one.  It
       tells the correspondent node the mobile node's care-of address,
       which is later used to create a cookie.  This message is sent
       directly to the correspondent node.

    3. HoT (Home Test) Message:

       This message is sent in response to a HoTI message.

           CN -> MN(HoA): K0, j

       When the correspondent node receives the HoTI message, it
       generates a cookie K0 as follows:

           K0 = MAC_Kcn(HoA | Nj | 0)

       The cookie and the value j are sent to the mobile node via the
       Home Agent; it is an assumption of the protocol that the home
       agent - mobile node route is secure.  K0 also acts as a challenge
       to test that the mobile can receive messages sent to its home
       address.

       The index j is carried along in the protocol to allow the CN
       to later efficiently find the nonce value Nj that it used in
       creating this cookie.

       The notation used here is as follows.  MAC_K(m) denotes a
       message authentication code computed on message m with key K.
       H(m) denotes a hash of message m.  HMAC SHA1 function [15][21]
       is used to compute the message authentication code, and SHA1
       function [21] is used to compute the hash.  The final ``0''



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       inside the MAC function is a 32-bit integer used to distinguish
       home and care-of cookies from each other.

    4. CoT (Care-of Test) Message:

       This message is sent in response to a CoTI message.

           CN -> MN(CoA): K1, i

       The correspondent also sends a challenge to the mobile's care-of
       address.  When the correspondent node receives the CoTI message,
       it generates a cookie K1 as follows:

           K1 = MAC_Kcn(CoA | Ni | 1)

       The cookie and the value i are sent directly the mobile node.
       The final 1 inside the MAC function is a 32-bit integer, again
       used for distinguishing home and care-of cookies from each other.

       Again, an index is sent along the cookie in order to identify
       the used nonce Ni.  Note that i and j are likely to be the same
       in HoT and CoT messages, except when nonce values happen to have
       changed between the reception of HoT and the CoT messages.

    5. BU (Binding Update) Message:

       When the MN has received both the HoT and CoT is has the cookies
       necessary to send the Binding Update.

           MN(CoA) -> CN: BU, HoA, CoA,
                          MAC_Kbu(CoA | HoA | BU | 0),
                          j, i

       The mobile node hashes together the challenges and to form a
       session key (Kbu), and then uses this session key to authenticate
       a binding update:

           Kbu = H(K0 | K1)

       The message contains j and i, so that the correspondent knows
       which value of Nj and Ni to use to recompute the session key.
       "BU" is the content of the BU message.  Once the correspondent
       node has verified the MAC, it can create a binding cache entry
       for the mobile.

    6. BA (Binding Acknowledgement) Message:

       The Binding Update is optionally acknowledged by the
       correspondent node.

           CN -> MN(CoA): BA,



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                          MAC_Kbu(CoA | HoA | BA | 0),

       The correspondent node uses the same key (Kbu) to authenticate a
       binding acknowledgement.  "BA" is the content of the BA message.

    7. BR (Binding Request) Message:

       The correspondent node can optionally request a binding to be
       refreshed using the Binding Request message.  This message can be
       authenticated using the same Kbu that was created earlier.

           CN -> MN(HoA): BR,
                          MAC_Kbu(HoA | BR | 0),

       "BR" is the content of the BR message.

   This protocol also protects the participants against replayed binding
   updates.  The attacker can't replay the same message due to the
   sequence number which is a part of the MIPv6 binding update itself,
   and the attacker can't modify the binding update since the MAC would
   not verify after that.  Care must be taken when removing bindings
   at the correspondent node, however.  If a binding is removed either
   due to garbage collection, request, or expiration and the nonce
   used in its creation is still valid, an attacker can replay the old
   binding update.  This can be prevented by having the correspondent
   node 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 can batch them together to avoid
   having to increment the nonce index too often.


4.5.6. Preventing Denial-of-Service Attacks

   The RR protocol has been designed with protection against resource
   exhaustion Denial-of-Service attacks.  In these attacks the victim
   has only a limited amount of some resource (such as network bandwidth
   or CPU cycles), and the attack consumes some of this resource.  This
   leaves the victim without enough resources to carry out other work.

   The correspondent nodes do not have to retain any state about
   individual mobile nodes until an authentic binding update arrives.
   This is achieved through the use of the nonces and Kcn that are
   not specific to individual mobile nodes.  Yet the cookies are
   specific, but they can be reconstructed based on the CoA and HoA
   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.





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   Nevertheless, as [1] describes, there are situations it is impossible
   for the mobile and correspondent nodes to determine if they actually
   need a binding or whether they just have been fooled into believing
   so by an attacker.  Therefore, it is necessary to treat situations
   where such attacks are being made.

   The binding updates that are used in mobile IPv6 are only an
   optimization.  A mobile node can communicate with a correspondent
   node even if the correspondent refuses to accept any of its binding
   updates.  However, performance will suffer because packets from the
   correspondent to the mobile will be routed via the mobile's home
   agent rather than a more direct route.  A correspondent can protect
   itself against some of the resource exhaustion attacks by stopping
   processing binding updates when it is flooded with a large number of
   binding updates that fail the cryptographic integrity checks.  If a
   correspondent 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 can decide to reject all binding updates
   without performing any cryptographic operations.

   Additional information needed to make this decisions about responding
   to requests will usually originate in layers above IP. For example,
   TCP knows if the node has a queue of data that it is trying to send
   to a peer.  It is possible to produce a conforming implementation of
   the protocols in this memo without making use of information from
   higher protocol layers, but implementations MAY be able to manage
   resources more effectively by making use of such information.


4.5.7. Design Rationale

   The motivation for designing the RR protocol was to have sufficient
   support for mobile IP, without creating major new security problems.
   A protocol was needed against the new vulnerabilities introduced by
   IP mobility.  It was not our goal to protect against attacks that
   were already possible before the introduction of IP mobility.  This
   protocol does not defend against an attacker who can monitor the home
   agent to correspondent node path, as such attackers would in any case
   be 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
   impediment to the deployment of mobile IP, because these attacks are
   possible irrespective of whether mobile IP is in use or not.

   This protocol 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, and so causes the correspondent
   to send the victim traffic that it does not want.  For example,
   suppose that the correspondent is a news 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



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   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 then redirect the stream
   to the victim's address.  This protocol defends against these attacks
   by only completing if packets sent by the correspondent to the care
   of address are received and processed by an entity that is willing to
   participate in the protocol.  Normally, this will be the mobile node.

   For further information about the design of RR and other protocols,
   see [1] [33] [22] [23].


4.6. 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 4.2, the following two new ICMP message types are
   used for home agent address discovery:

      Home Agent Address Discovery Request

         The ICMP Home Agent Address Discovery Request message is used
         by a mobile node to initiate the dynamic home agent address
         discovery mechanism.  When attempting a home registration, the
         mobile node may use this mechanism to discover the address of
         one or more routers currently operating as home agents on its
         home link, with which it may register while away from home.
         The Home Agent Address Discovery Request message is described
         in detail in Section 5.6.

      Home Agent Address Discovery Reply

         The ICMP Home Agent Address Discovery Reply message is used by
         a home agent to respond to a mobile node using the dynamic home
         agent address discovery mechanism.  When a home agent receives
         a Home Agent Address Discovery Request message, it replies with
         a Home Agent Address Discovery Reply message, giving a list
         of the routers on the mobile node's home link serving as home
         agents.  The Home Agent Address Discovery Reply message is
         described in detail in Section 5.7.

   The next two message types are used for network renumbering
   and address configuration on the mobile node, as described in
   Section 9.7:

      Mobile Prefix Solicitation

         The ICMP Mobile Prefix Solicitation message is used by a mobile
         node to request prefix information about the home subnet, in
         order to retrieve prefixes that are served by home agents and



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         can be used to configure one or more home addresses, or to
         refresh home addresses before the expiration of their validity.
         This message is specified in Section 5.8.

      Mobile Prefix Advertisement

         The ICMP Mobile Prefix Advertisement is used by a home agent to
         distribute information to a mobile node about prefixes on the
         home link which are available for use by the mobile node while
         away from home.  This message may be sent as a response to a
         Mobile Prefix Solicitation, or due to network renumbering or
         other prefix changes as specified in Section 9.9.3


4.7. 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 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 [20].  When sending a packet, the Binding
         Cache is searched before the Neighbor Discovery conceptual
         Destination Cache [20] (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, as described in
             Section 8.9, for packets originated by this node, or in
             Section 9.4, if this node is the mobile node's home agent
             and the packet was intercepted by it on the home link.





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          -  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 on 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.

          -  A flag indicating whether or not this Binding Cache entry
             represents a mobile node that should be advertised as a
             router in proxy Neighbor Advertisements sent by this node
             on its behalf.  This flag is only valid if the Binding
             Cache entry indicates that this is a "home registration"
             entry.

          -  The length of the routing prefix for the home address.
             This field is only valid if the "home registration" flag is
             set on this Binding Cache 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 8 bits long,
             and all comparisons between Sequence Number values
             MUST be performed modulo 2**8.  For example, using an
             implementation in the C programming language, a Sequence
             Number value A is greater than another Sequence Number
             value B if ((char)((a) - (b)) > 0), if the "int" data type
             is a 8-bit signed integer.

          -  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 Request should be sent when the lifetime on this
             entry nears expiration.

          -  The Binding Security Association (BSA) to be be used when
             authenticating Binding Updates that are received for this
             Binding Cache entry.

          -  The Binding Security Association (BSA) to be be used when
             calculating authentication data for inclusion in Binding
             Acknowledgements in response to Binding Updates that are
             received for this Binding Cache entry.

         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 and SHOULD NOT be deleted by the home agent until the
         expiration of its binding lifetime.  Other Binding Cache
         entries MAY be replaced at any time by any reasonable local



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         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.

      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.  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.  That node might still have a Binding Cache entry
             created or updated from this Binding Update, if the Binding
             Update was successfully received by that node (e.g., not
             lost by the network) and 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:

              *  the mobile node's home addresses for typical Binding
                 Updates (Sections 10.7 and 10.9), or

              *  the mobile node's previous care-of address for Binding
                 Updates sent to establish forwarding from the mobile
                 node's previous care-of address by a home agent from
                 this previous care-of address (Section 10.11).

          -  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.





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          -  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 8 bits long, and all comparisons between
             Sequence Number values MUST be performed modulo 2**8.  For
             example, using an implementation in the C programming
             language, a Sequence Number value A is greater than another
             Sequence Number value B if ((char)((a) - (b)) > 0), if the
             "char" data type is a 8-bit signed integer.

          -  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 2,
             error message in response to a Binding Update sent to that
             destination, as described in Section 10.17.

      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 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 [20], 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



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         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 to which it is (or has recently) connected, or it MAY
         maintain a single list for all links.  Each Home Agents List
         entry conceptually contains the following fields:

          -  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 [20].

          -  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 [20].

                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



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             of the home agents on its previous link to notify when it
             moves to a new link.

                Can we delete the preference stuff?  Is anyone using
                it?


4.8. Binding Management

   When a mobile node configures a new care-of address and decides to
   use this new address as its primary care-of address, the mobile
   node registers this new binding with its home agent by sending
   the home agent a Binding Update.  The mobile node indicates
   that an acknowledgement is needed for this Binding Update and
   continues to periodically retransmit it until acknowledged.  The
   home agent acknowledges the Binding Update by returning a Binding
   Acknowledgement to the mobile node.

   When a mobile node receives a packet tunneled to it from its home
   agent, the mobile node uses that as an indication that the original
   sending correspondent node has no Binding Cache entry for the mobile
   node, since the correspondent node would otherwise have sent the
   packet directly to the mobile node using a Routing header.  If the
   mobile node has a Binding Security Association (BSA) with that
   correspondent node, the mobile node thus returns a Binding Update
   to the correspondent node, allowing it to cache the mobile node's
   binding for routing future packets to it.  Although the mobile node
   may request an acknowledgement for this Binding Update, it need not,
   since subsequent packets from the correspondent node will continue
   to be intercepted and tunneled by the mobile node's home agent,
   effectively causing any needed Binding Update retransmission.

   If the mobile node receives such a tunneled packet but does not have
   a BSA with the correspondent node, the mobile node SHOULD initiate
   the process of establishing the necessary security association by
   following the procedures outlined in Section 4.5

   A correspondent node with a Binding Cache entry for a mobile node
   may refresh this binding, for example if the binding's lifetime
   is near expiration, by sending a Binding Request to the mobile
   node.  Normally, a correspondent node will only refresh a Binding
   Cache entry in this way if it is actively communicating with the
   mobile node and has indications, such as an open TCP connection to
   the mobile node, that it will continue this communication in the
   future.  When a mobile node receives a Binding Request, it replies by
   returning a Binding Update to the node sending the Binding Request.

   A mobile node may use more than one care-of address at the same time,
   although only one care-of address may be registered for it at its
   home agent as its primary care-of address.  The mobile node's home
   agent will tunnel all intercepted packets for the mobile node to its



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   (single) registered primary care-of address, but the mobile node
   will accept packets that it receives at any of its current care-of
   addresses.  Use of more than one care-of address by a mobile node
   may be useful, for example, to improve smooth handover when the
   mobile node moves from one wireless link to another.  If each of
   these wireless links is connected to the Internet through a separate
   base station, such that the wireless transmission range from the
   two base stations overlap, the mobile node may be able to remain
   connected to both links while in the area of overlap.  In this case,
   the mobile node could acquire a new care-of address on the new link
   before moving out of transmission range and disconnecting from the
   old link.  The mobile node may thus still accept packets at its
   old care-of address while it works to update its home agent and
   correspondent nodes, notifying them of its new care-of address on the
   new link.

   Since correspondent nodes cache bindings, 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 essential for
   scalability and reliability, and for minimizing overall network load.
   By caching the care-of address of a mobile node, optimal routing of
   packets can be achieved from the correspondent node to the mobile
   node.  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.


5. New IPv6 Destination Options and Message Types

5.1. 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.  The Mobility Header is an IPv6 protocol.  Rules
   regarding how it is sent and what addresses are used in the IPv6
   header are given separately in Sections 5.1.2 to 5.1.9.  Mobile nodes
   MUST use reverse tunneling to send Mobility Header messages when the
   source address is set to the home address of the mobile node.












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5.1.1. Format

   The Mobility Header is identified by a Next Header value of 62 (XXX)
   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
         IPv4 Protocol field [10].

         This field is intended to be used by a future specification
         of piggybacking binding messages on payload packets (see
         Section 12.1).

         Thus implementations conforming to this specification MUST 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.

      MH Type

         16-bit selector.  Identifies the particular mobility message in
         question.  Legal values are defined in Sections 5.1.2 to 5.1.8.
         An unrecognized MH Type field SHOULD cause 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 the 16-bit one's
         complement of the one's complement sum of the entire Mobility
         Header starting with the Payload Proto field, prepended with a
         "pseudo-header" of IPv6 header fields, as specified in [IPv6,
         section 8.1].  The Next Header value used in the pseudo-header



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         is 62 (XXX). 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.


5.1.2. Binding Request (BR) Message

   The Binding Request (BR) message is used to request a mobile node's
   binding from the mobile node.  A packet containing a Binding Request
   message is sent in the same way as any packet to a mobile node
   (Section 8.9).  When a mobile node receives a packet containing a
   Binding Request message and there already exists a Binding Update
   List entry for the source of the Binding Request, it MAY start
   a Return Routability Procedure (see Section 4.5) 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.

   The Binding 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             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                          Parameters                           .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Reserved

         16-bit field reserved for future flags.  These flag bits are
         reserved for future use, and MUST be initialized to zero by the
         sender, and MUST be ignored by the receiver.

      Parameters

         Variable-length field, of length such that the complete
         Mobility Header is an integer multiple of 8 octets long.
         Contains one or more TLV-encoded parameters.  The receiver MUST
         ignore and skip any parameters which it does not understand.  A
         Binding Request MUST include the following parameter:




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          -  Authentication Data parameter.  This parameter contains a
             cryptographic hash value which is used to ensure that it
             has been sent by the correspondent node.  The authenticator
             covering a Binding Acknowledgement MUST be computed over
             a bitstring containing the following fields of the IPv6
             header and the Mobility Header, in order:

              *  The Home Address of the mobile node, from the
                 Destination Address field of the IPv6 header.

              *  The address of the correspondent node, from the Source
                 Address field of the IPv6 header.

              *  The contents of the Mobility Header, excluding the
                 Authenticator field (inside the Authentication Data
                 parameter) which is included as zeroes for the purposes
                 of calculating the Authenticator.

              *  Four bytes of zero.  This is included for a potential
                 future extension.

             The actual authenticator calculation over sequence of bits
             is described in Section 4.5.

         There MAY be additional information, associated with this
         Binding Request message, that need not be present in all
         Binding Requests sent.  This use of MH parameters also allows
         for future extensions to the format of the Binding Request
         message to be defined.  The encoding and format of defined
         parameters are described in Section 5.2.  The following
         parameters are valid in a Binding Request message:

          -  Unique Identifier Parameter

   If no actual parameters are present in this message, no padding is
   necessary.


5.1.3. Home Test Init (HoTI) Message

   The Home Test Init (HoTI) message is used to initiate the Return
   Routability procedure from the mobile node to a correspondent node
   (see Section 10.9).  This message is always sent with the Source
   Address set to the home address of the mobile node, Destination
   Address set to the correspondent node's address, and is tunneled
   through the home agent when the mobile node is away from home.








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   The HoTI message uses the MH Type value 1.  When this value is
   indicated in the MH Type field, the format of the Message Data field
   in the Mobility Header is as follows:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |           Flags               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                          Parameters                           .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Flags

         16-bit field reserved for future flags.  These flag bits are
         reserved for future use, and MUST be initialized to zero by the
         sender, and MUST be ignored by the receiver.

      Parameters

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

         There MAY be additional information, associated with this
         message that need not be present in all HoTI messages.  This
         use of MH parameters also allows for future extensions to the
         format of the HoTI message to be defined.  The encoding and
         format of defined parameters are described in Section 5.2.  The
         following parameters are valid in a HoTI message:

          -  Unique Identifier Parameter

   If no actual parameters are present in this message, 4 bytes of Pad1
   or PadN parameters are needed to make the length of the message a
   multiple of 8.

   The HoTI message SHOULD be protected by an IPsec policy that employs
   ESP between the home agent and the mobile node.

   A packet that includes a HoTI message MUST NOT include a Home Address
   option.


5.1.4. Care-of Test Init (CoTI) Message

   The Care-of Test Init (CoTI) message is used to initiate the Return
   Routability procedure from the mobile node to a correspondent node



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   (see Section 10.9).  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.

   The CoTI 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             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                          Parameters                           .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Reserved

         16-bit field reserved for future flags.  These flag bits are
         reserved for future use, and MUST be initialized to zero by the
         sender, and MUST be ignored by the receiver.

      Parameters

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

         There MAY be additional information, associated with this
         message that need not be present in all CoTI messages.  This
         use of MH parameters also allows for future extensions to the
         format of the CoTI message to be defined.  The encoding and
         format of defined parameters are described in Section 5.2.  The
         following parameters are valid in a CoTI message:

          -  Unique Identifier Parameter

   If no actual parameters are present in this message, no padding is
   necessary.

   A packet that includes a CoTI message MUST NOT include a Home Address
   option.


5.1.5. Home Test (HoT) Message

   The Home Test (HoT) message is an answer to the HoTI message, and
   is sent from the correspondent node to the mobile node (see Section



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   8.2).  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.

   The HoT 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:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    |        Home Nonce Index       |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                      Home Cookie (128 bits)                   |
    +                                                               +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                           Parameters                          .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Reserved

         32-bit field reserved for future flags.  These flag bits are
         reserved for future use, and MUST be set to zero, otherwise an
         error MUST be returned to the sender.

      Home Nonce Index

         This field will be echoed back by the mobile node to the
         correspondent node in a subsequent binding update.  It
         will allow the correspondent node to select the appropriate
         challenge values to authenticate the binding update.

      Home Cookie

         This field contains the cookie K0 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.






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      Parameters

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

         There MAY be additional information, associated with this
         message that need not be present in all HoT messages.  MH
         parameters are used to carry that information.  The encoding
         and format of defined parameters are described in Section 5.2.
         This use of MH parameters also allows for future extensions
         to the format of the HoT message to be defined.  This
         specification does not define any optional parameters for the
         HoT message.

   If no actual parameters are present in this message, 4 bytes of Pad1
   or PadN parameters are needed to make the length of the message a
   multiple of 8.

   The HoT message SHOULD be protected by an IPsec policy that employs
   ESP between the home agent and the mobile node, in order to prevent
   attackets e.g.  on the same link as the MN to receive the Home
   Cookie.


5.1.6. Care-of Test (CoT) Message

   The Care-of Test (CoT) message is an answer to the CoTI message, and
   is sent from the correspondent node to the mobile node (see Section
   8.2).  This message is always sent with the Source Address set to the
   address of the correspondent node, the Destination Address set to
   the care-of address of the mobile node, and is sent directly to the
   mobile node.




















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   The CoT 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:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    |      Care-of Nonce Index      |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                      Care-of Cookie (128 bits)                |
    +                                                               +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                          Parameters                           .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Reserved

         32-bit field reserved for future flags.  These flag bits are
         reserved for future use, and MUST be set to zero, otherwise an
         error MUST be returned to the sender.

      Care-of Nonce Index

         This field will be echoed back by the mobile node to the
         correspondent node in a subsequent binding update.  It
         will allow the correspondent node to select the appropriate
         challenge values to authenticate the binding update.

      Care-of Cookie

         This field contains the cookie K1 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.

      Parameters

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




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         There MAY be additional information, associated with this
         message that need not be present in all CoT messages.  MH
         parameters are used to carry that information.  The encoding
         and format of defined parameters are described in Section 5.2.
         This use of MH parameters also allows for future extensions
         to the format of the CoT message to be defined.  This
         specification does not define any optional parameters for the
         CoT message.

   If no actual parameters are present in this message, 4 bytes of Pad1
   or PadN parameters are needed to make the length of the message a
   multiple of 8.


5.1.7. 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:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |A|H|S|D|      Reserved         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Sequence #           |          Reserved             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            Lifetime                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                                                               |
    +                           Home Address                        +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                          Parameters                           .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+







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      Acknowledge (A)

         The Acknowledge (A) bit is set by the sending mobile node to
         request a Binding Acknowledgement (Section 5.1.8) 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 the receiving node to 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 (given by the Home
         Address field in the Home Address option in the packet).

      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 option.  This disables home agent processing
         for other related addresses, as is described in Section 9.1.

      Duplicate Address Detection (D)

         The Duplicate Address Detection (D) bit is set by the sending
         mobile node to request the receiving node (the mobile node's
         home agent) to perform Duplicate Address Detection [35] 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.  If the Duplicate Address Detection performed by
         the home agent fails, the Status field in the returned Binding
         Acknowledgement will be set to 138 (Duplicate Address Detection
         failed).

      Reserved

         This field is unused.  It 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.  Each Binding Update
         sent by a mobile node MUST use 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
         defined in Section 4.7).  There is no requirement, however,
         that the Sequence Number value strictly increase by 1 with each



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         new Binding Update sent or received.  Both home agents and
         correspondent nodes use the sequence number also to prevent
         replay attacks.

      Lifetime

         32-bit unsigned integer.  The number of seconds 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.

      Home Address

         This field tells the correspondent node the home address of the
         mobile node.

      Parameters

         Variable-length field, of length such that the complete
         Mobility Header is an integer multiple of 8 octets long.
         Contains one or more TLV-encoded parameters.  The receiver MUST
         ignore and skip any parameters which it does not understand.  A
         Binding Update sent to a correspondent node MUST include the
         following parameters:

          -  Nonce Indices parameter.  This parameter contains
             information the correspondent node needs in order to find
             the challenge values Nj and Ni.

          -  Authentication Data parameter.  This parameter contains a
             cryptographic hash value which is used to ensure that it
             has been sent by the same party who received the HoT and
             CoT messages.  The authenticator covering a Binding Update
             MUST be computed over a bitstring containing the following
             fields of the IPv6 header and the Mobility Header, in
             order:

              *  Care-of Address, in the Source Address field of the
                 IPv6 header

              *  The address of the correspondent node, in the
                 Destination Address field of the IPv6 header.

              *  The contents of the Mobility Header, excluding the
                 Authenticator field (within the Authentication Data
                 parameter) which is included as zeroes for the purposes
                 of calculating the Authenticator.  Parameters of the
                 Mobility Header are included in the calculation.

              *  Four bytes of zero.



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             The actual authenticator calculation over sequence of bits
             is described in Section 4.5.

         There MAY be additional information, associated with this
         Binding Update message, that need not be present in all Binding
         Updates sent.  This use of MH parameters also allows for future
         extensions to the format of the Binding Update message to be
         defined.  The encoding and format of defined parameters are
         described in Section 5.2.  The following parameters are valid
         in a Binding Update message:

          -  Unique Identifier Parameter

          -  Alternate Care-of Address Parameter

   If no actual parameters are present in this message, 4 bytes of Pad1
   or PadN parameters are needed to make the length of the message a
   multiple of 8.

   A Binding Update message to the correspondent node MUST NOT include
   the Home Address option in order to avoid reflection attacks
   described in Section 4.5.  A Binding Update to the home agent MUST
   include the Home Address option in order to allow for the use of
   manually keyed IPsec in the protection of these messages.

   When a packet contains both a Home Address Option and a Binding
   Update message, the sender MUST use the same address in both.  The
   receiver MUST check for the equal values and MUST silently discard a
   packet that does not pass this test.

   The home address of the mobile node in the binding given in the
   Binding Update message is that which was received as the value of the
   Home Address field in the Home Address option in the packet.

   The care-of address for the binding given in the Binding Update
   message is normally that which was received as the value in the
   Source Address field in the IPv6 header of the packet carrying the
   Binding Update message.  However, a care-of address different from
   the Source Address MAY be specified by including an Alternate Care-of
   Address parameter in the Binding Update message.  In any case, 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.

   If the care-of address for the binding (specified either in an
   Alternate Care-of Address parameter in the Binding Update message, if
   present, or in the Source Address field in the packet's IPv6 header)
   is equal to the home address of the mobile node, the Binding Update
   message indicates that any existing binding for the mobile node MUST



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   be deleted.  Likewise, if the Lifetime field in the Binding parameter
   is equal to 0, the Binding Update message indicates that any existing
   binding for the mobile node MUST be deleted.  In each of these cases,
   a Binding Cache entry for the mobile node MUST NOT be created in
   response to receiving the Binding Update.

      When the care-of address is NOT equal to the home address,
      what if we just delete that particular care-of address?

   The last Sequence Number value sent to a destination in a Binding
   parameter is stored by the mobile node in its Binding Update List
   entry for that destination; the last 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.  Thus, the
   mobile node's and the correspondent node's knowledge of the last
   sequence number expire at the same time.  If the sending mobile node
   has no Binding Update List entry, the Sequence Number may start at
   any value; 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.  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.


5.1.8. Binding Acknowledgement (BA) Message

   The Binding Acknowledgement message is used to acknowledge receipt
   of a Binding Update message (Section 5.1.7).  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 Binding Parameter carried in the Binding Update.  The
   Binding Acknowledgement message is sent to the Source Address of the
   Binding Update message it is an answer to, with the source being the
   Destination Address from the Binding Update.


















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   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:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Status     |   Reserved      |           Sequence #          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            Lifetime                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            Refresh                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                           Parameters                          .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      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.  The following such Status values are currently defined:

        0

         Binding Update accepted

         Values of the Status field greater than or equal to 128
         indicate that the Binding Update was rejected by the receiving
         node.  The following such Status values are currently defined:

      128

         Reason unspecified

      130

         Administratively prohibited

      131

         Insufficient resources



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      132

         Home registration not supported

      133

         Not home subnet

      137

         Not home agent for this mobile node

      138

         Duplicate Address Detection failed

      141

         Sequence number out of window

      142

         Route optimization unnecessary due to low traffic

      143

         Invalid authenticator

      144

         Too old Home Nonce Index

      145

         Too old Care-of Nonce Index

         Up-to-date values of the Status field are to be specified in
         the most recent "Assigned Numbers" [32].

      Sequence #

         The Sequence Number in the Binding Acknowledgement is copied
         from the Sequence Number field in the Binding Update being
         acknowledged, for use by the mobile node in matching this
         Acknowledgement with an outstanding Binding Update.

      Lifetime

         The granted lifetime, in seconds, for which this node will
         SHOULD retain the entry for this mobile node in its Binding
         Cache.  Correspondent nodes should make an effort to honor



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         the lifetimes, since an entry that was garbage collected too
         early might cause subsequent packets from the mobile node to
         be dropped, if they contained the Home Address Option.  While
         this situation is recoverable since an error message is sent
         to the mobile node, it causes an unnecessary break in the
         communications.

         Correspondent nodes SHOULD also retain a BCE entry for the
         purposes of accepting Home Address Options somewhat longer than
         they keep on using the entry for Route Optimization of outgoing
         packets.  This helps to avoid dropped packets, particularly
         when clock drift can be a problem.

         If the node sending the Binding Acknowledgement is serving
         as the mobile node's home agent, the Lifetime period also
         indicates the period for which this node will continue this
         service; if the mobile node requires home agent service from
         this node beyond this period, the mobile node MUST send a new
         Binding Update to it before the expiration of this period (even
         if it is not changing its primary care-of address), in order
         to extend the lifetime.  The value of this field is undefined
         if the Status field indicates that the Binding Update was
         rejected.

      Refresh

         The recommended interval, in seconds, at which the mobile
         node SHOULD send a new Binding Update to this node in order
         to "refresh" the mobile node's binding in this node's Binding
         Cache.  This refreshing of the binding is useful in case the
         node fails and loses its cache state.  The Refresh period is
         determined by the node sending the Binding Acknowledgement (the
         node caching the binding).  If this node is serving as the
         mobile node's home agent, the Refresh value may be set, for
         example, based on whether the node stores its Binding Cache
         in volatile storage or in nonvolatile storage.  Note that as
         discussed in Section 4.5.4, home agents need to keep at least
         some information about sequence numbers in non-volatile memory.

         If the node sending the Binding Acknowledgement is not
         serving as the mobile node's home agent, the Refresh period
         SHOULD be set equal to the Lifetime period in the Binding
         Acknowledgement; even if this node loses this cache entry due
         to a failure of the node, packets from it can still reach the
         mobile node through the mobile node's home agent, causing a new
         Binding Update to this node to allow it to recreate this cache
         entry.  The value of this field is undefined if the Status
         field indicates that the Binding Update was rejected.






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      Parameters

         Variable-length field, of length such that the complete
         Mobility Header is an integer multiple of 8 octets long.
         Contains one or more TLV-encoded parameters.  The receiver MUST
         ignore and skip any parameters which it does not understand.
         A Binding Acknowledgement sent by a correspondent node MUST
         include the following parameter:

          -  Authentication Data parameter.  This parameter contains a
             cryptographic hash value which is used to ensure that it
             has been sent by the correspondent node.  The authenticator
             covering a Binding Acknowledgement MUST be computed over
             a bitstring containing the following fields of the IPv6
             header and the Mobility Header, in order:

              *  Care-of Address, in the Destination Address field of
                 the IPv6 header

              *  The address of the correspondent node, in the Source
                 Address field of the IPv6 header.

              *  The contents of the Mobility Header, excluding the
                 Authenticator field (inside the Authentication Data
                 parameter) which is included as zeroes for the purposes
                 of calculating the Authenticator.

              *  Four bytes of zero.

             The actual authenticator calculation over sequence of bits
             is described in Section 4.5.

         There MAY be additional information, associated with this
         Binding Acknowledgement message, that need not be present in
         all Binding Acknowledgements sent.  This use of MH parameters
         also allows for future extensions to the format of the Binding
         Acknowledgement message to be defined.  The encoding and format
         of defined parameters are described in Section  5.2.  This
         specification does not define any parameters valid for the
         Binding Acknowledgement message.

   If no actual parameters are present in this message, no padding is
   needed.

   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 inserted to the message.

   If the mobile node sends a sequence number which is not within the
   window of acceptable sequence numbers, then the home agent MUST send
   back a Binding Acknowledgement with status code 141, and the last



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   accepted sequence number in the Sequence Number field of the Binding
   Ack Parameter.


5.1.9. Binding Missing (BM) Message

   The Binding Missing (BM) message is used by the correspondent node
   to signal an inappropriate attempt to use the Home Address Option
   without an existing binding.  A packet containing a Binding Missing
   message is sent to the source address of the packet that contained
   the Home Address Option i.e.  to the care-of address of the mobile
   node.  The source address of the Binding Missing message is the
   correspondent node's address.

   When a mobile node receives a packet containing a Binding Missing
   message, it should perform one of the following three actions:

    -  If the mobile node does not have a Binding Update List entry for
       the source of the Binding Missing message, it MUST ignore the
       message.  This is necessary to prevent loss of resources spent on
       the Route Optimization signaling due to spoofed Binding Missing
       messages.

    -  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 Missing 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 4.5).



















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   The Binding Missing 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:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |          Reserved             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                                                               |
    +                          Home Address                         +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                          Parameters                           .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Reserved

         16-bit field reserved for future flags.  These flag bits are
         reserved for future use, and 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 Option.
         The mobile node uses this information to determine which
         binding does not exist, if there mobile node has several home
         addresses.

      Parameters

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

         There MAY be additional information, associated with this
         Binding Missing message, that need not be present in all
         Binding Missings sent.  This use of MH parameters also allows
         for future extensions to the format of the Binding Missing
         message to be defined.  The encoding and format of defined
         parameters are described in Section 5.2.  This specification
         does not define any parameters for the Binding Missing message.





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   If no actual parameters are present in this message, no padding is
   needed.


5.2. Mobility Header Parameters

5.2.1. 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, any of the Mobility Header
   messages defined in this document MAY include one or more parameters.

   Such parameters are included in the data portion of the message
   itself, after the fixed portion of the message data specified in
   section 5.1.

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

   These parameters 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Parameter Type | Parameter Len |   Parameter Data...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Parameter Type

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

      Parameter Length

         8-bit unsigned integer.  Length of the Parameter Data field
         of this parameter, in octets.  The Parameter Len includes the
         length of the Parameter Type and Parameter Len fields.

      Parameter Data

         Variable-length field.  Parameter -Type-specific data.

   Parameters MUST be aligned on an 8-byte boundary, and have a length
   which is a multiple of 8.




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

   Implementations MUST silently ignore any parameters that they do not
   understand.


5.2.2. Pad1

   Pad1 Parameter   (alignment requirement: none)

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

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

   The Pad1 parameter is used to insert one octet of padding into the
   Parameters area of a Mobility Header.  If more than one octet of
   padding is required, the PadN parameter, described next, should be
   used, rather than multiple Pad1 parameters.


5.2.3. PadN

   PadN Parameter   (alignment requirement: none)

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

   The PadN parameter is used to insert two or more octets of padding
   into the Parameters area of some Mobility Header message.  For N
   octets of padding, the Parameter Len field contains the value N, and
   the Parameter Data consists of N-2 zero-valued octets.  Parameter
   data MUST be ignored by the receiver.













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5.2.4. Unique Identifier

   Unique Identifier parameter   (alignment requirement: 2n)

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       2       |       4       |       Unique Identifier       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Unique Identifier parameter is valid only in Binding 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 Request to which it responds.  This
   matching of Binding Updates to Binding Requests is required in the
   procedure for renumbering the home subnet while a mobile node is away
   from home (Section 9.7).


5.2.5. Alternate Care-of Address

   Alternate Care-of Address parameter   (alignment requirement: 8n+6)

     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
                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |       3       |      18       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                                                               |
    +                   Alternate Care-of Address                   +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Alternate Care-of Address parameter 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.












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5.2.6. Nonce Indices

   Nonce Indices parameter   (alignment requirement: 8n+6)

     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
                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |       4       |       6       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Home Nonce Index      |     Care-of Nonce Index       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   The Home 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.

   The Care-of 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.


5.2.7. Authentication Data

   Authentication Data parameter   (alignment requirement: 8n+6)

     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
                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |       5       |      18       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                             SPI                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                         Authenticator                         |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Authentication Data parameter is valid only in the Binding
   Request, Binding Update, and Binding Acknowledgment messages.

   The Security Parameters Index (SPI) field contains an arbitrary
   32-bit value that uniquely identifies the used security association.
   This document specifies only one legal value for the SPI field.  This
   value, 0, signifies that no security association is present and the
   cryptographic context MUST be established temporarily only for the



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   duration of processing this message.  Messages that contain other
   values of the SPI field SHOULD be silently discarded.

   The Authenticator field contains a 96-bit cryptographic hash
   value.  Rules for calculating this value are different for different
   messages, and are described in Sections 5.1.2,  5.1.7 and 5.1.8.


5.3. Home Address 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 that
   packet of the mobile node's home address.  For packets sent by a
   mobile node while away from home, the mobile node generally uses one
   of its care-of addresses as the Source Address in the packet's IPv6
   header.  By including a Home Address option in the IPv6 Destination
   Options header of the packet, the correspondent node receiving the
   packet is able to substitute the mobile node's home address for this
   care-of address when processing the packet.  This makes the use of
   the care-of address transparent to the correspondent node.  Note
   that 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 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                         +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Sub-Options...
   +-+-+-+-+-+-+-+-+-+-+-+-

      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



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         MUST be set to 16 plus the total length of all sub-options
         present, including their Sub-Option Type and Sub-Option Len
         fields.

      Home Address

         The home address of the mobile node sending the packet.

      Sub-Options

         Additional information, associated with this Home Address
         option, that need not be present in all Home Address options
         sent.  This use of sub-options also allows for future
         extensions to the format of the Home Address option to be
         defined.  Currently, no valid sub-options are defined for use
         in a Home Address option.

   The alignment requirement [6] for the Home Address option is 8n+6.

   The inclusion of a Home Address 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.

   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

   Due to the threat of reflection attacks through the use of this
   option, this specification requires that packets containing Home
   Address Option MUST be dropped if there is no corresponding Binding
   Cache Entry for that home address with the currently registered
   care-of address matching the source address of the packet.  If the
   packet is dropped, the correspondent nodes SHOULD send the Binding
   Missing message to the source address of the packet that contained
   the Home Address Option (see Section 5.1.9).  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



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   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 4.5.  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 10.2.

   A packet MUST NOT contain more than one Home Address option, except
   that an encapsulated packet [4] MAY contain a separate Home Address
   option associated with each encapsulating IP header.

   The three highest-order bits of the Option Type are encoded to
   indicate specific processing of the option [6].  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.



























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5.4. 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, which the
   Care of Address of the MN is carried in the IPv6 destination field.

   This uses a different routing header type than what is defined
   for ``regular'' IPv6 source routing to make it possible for e.g.,
   firewalls to have different rules for source routing versus MIPv6.
   This routing header type (type 2) is restricted to only carry one
   IPv6 address and all IPv6 nodes which process it MUST verify that the
   address contained in the routing header is the home address of the
   node in order to prevent packets with this routing header type to be
   forwarded after decrementing the segments left field.


5.4.1. 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 IPv4
         Protocol field [10].

      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

         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.  For the type
         2 routing header, Segments left is always 1.

      Reserved

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

      Home Address

         The Home Address of the destination Mobile Node.


5.4.2. Sending RH type 2

   A correspondent node sends packets with a routing header based on the
   content of the binding cache.  Conceptually this is done by the IP
   layer inspecting the binding cache and if there is an entry for the
   destination address (the Home Address) then the IP layer inserts a
   routing header of type 2 based on the ordering rules below and moves
   the Home Address to the Home Address field in the RH and places the
   Care of Address in the IPv6 destination field.

   Note that following the above conceptually model in an implementation
   creates some additional requirements for path MTU discovery since the
   packetization layer (e.g., TCP and applications using UDP) need 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.















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5.4.3. Verification by receiver

   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
   routing header of type 2 during this processing it performs the
   following checks.  If any of these checks fail the node MUST silently
   discard the packet.

    -  The node is a mobile node.

    -  The length field in the RH is exactly 2

    -  The segments left field in the RH is exactly 1

    -  The Home Address field in the RH is (one of) the node's Home
       Address(es)

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

   Since IPsec headers follow the Routing Header any IPsec processing
   will operate on the packet with the HoA in the IP destination field
   and segments left being zero.  Thus AH will see the packet in the
   same "shape" as the AH calculation on the sender.


5.4.4. Extension header ordering

   Section 4.1 in RFC 2460 lists the extension header ordering.  The
   introduction of Routing Header type 2 potentially allows there to be
   multiple routing headers in a single packet.  If this is the case
   the Routing Header type 2 should follow any Routing header of other
   type but otherwise the order constraints for routing headers is
   independent of their type and follows RFC 2460.


5.4.5. Reversing type 2 routing headers

   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 to type 2 routing
   headers.




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5.5. Mobile IPv6 Destination Option Sub-Options

   In order to allow future extensions to the format of MIPv6
   destination options, any of the Mobile IPv6 destination options
   defined in this document MAY include one or more sub-options.

   Such sub-options are included in the data portion of the destination
   option itself, after the fixed portion of the option data specified
   for that particular destination option (Section 5.3).  The presence
   of such sub-options will be indicated by the Option Length field.
   When the Option Length is greater than the standard length defined
   for that destination option, the remaining octets are interpreted as
   sub-options.

   These sub-options are encoded within the remaining space of the
   option data for that option, 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Sub-Option Type| Sub-Option Len|   Sub-Option Data...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Sub-Option Type

         8-bit identifier of the type of sub-option.  When processing
         a Mobile IPv6 destination option containing a sub-option for
         which the Sub-Option Type value is not recognized by the
         receiver, the receiver SHOULD quietly ignore and skip over the
         sub-option, correctly handling any remaining sub-options in the
         option.

      Sub-Option Length

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

      Sub-Option Data

         Variable-length field.  Sub-Option-Type-specific data.

   As with IPv6 options appearing in a Hop-by-Hop Options header
   or Destination Options header [6], individual sub-options within
   a Mobile IPv6 destination option may have specific alignment
   requirements, to ensure that multi-octet values within Sub-Option
   Data fields fall on natural boundaries.  The alignment requirement
   of each sub-option is specified as part of the definition of each
   sub-option below.



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   Each section above defining the Mobile IPv6 destination options
   specifies which of the defined sub-options is valid for that
   destination option.  In addition, there are two padding sub-options,
   Pad1 and PadN (defined below), which are used when necessary to align
   subsequent sub-options.  The Pad1 and PadN sub-options are valid for
   all Mobile IPv6 destination options.  Unlike the padding options
   used in Hop-by-Hop Options header or Destination Options header [6],
   there is no requirement for padding the total size of any Mobile IPv6
   destination option to a multiple of 8 octets in length, and the
   Pad1 and PadN sub-options SHOULD NOT be used for this purpose.  All
   Mobile IPv6 sub-options defined in this document MUST be recognized
   by all Mobile IPv6 implementations.


5.5.1. Pad1

   Pad1 Sub-Option   (alignment requirement: none)

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

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

      The Pad1 sub-option is used to insert one octet of padding
      into the Sub-Options area of a Mobile IPv6 option.  If more
      than one octet of padding is required, the PadN sub-option,
      described next, should be used, rather than multiple Pad1
      sub-options.


5.5.2. PadN

   PadN Sub-Option   (alignment requirement: none)

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

      The PadN sub-option is used to insert two or more octets of
      padding into the Sub-Options area of a Mobile IPv6 option.
      For N octets of padding, the Sub-Option Len field contains
      the value N-2, and the Sub-Option Data consists of N-2
      zero-valued octets.




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5.6. 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 Sections 9.9 and 10.8.  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 [11], and one of the home agents there responds to the mobile
   node with a Home Agent Address Discovery Reply message giving 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           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         150 <To Be Assigned by IANA>

      Code

         0

      Checksum

         The ICMP checksum [5].

      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 then MUST 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 not
   registered with any home agent within the specified anycast group.





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5.7. 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 using the dynamic home agent
   address discovery mechanism, as described in Sections 9.9 and 10.8.
   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 [11], and one of the home agents there responds to the
   mobile node with a Home Agent Address Discovery Reply message giving
   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 [5].

      Identifier

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






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      Reserved

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

      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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5.8. 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 [35],
   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 [20], 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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            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, and this message is routed
         according to the rules of a typical unicast packet.  A hop
         limit of 64 is currently suggested [32].

      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 [5].

      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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5.9. 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 5.9.

    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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Options ...
   +-+-+-+-+-+-+-+-+-+-+-+-

   IP Fields:

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

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

      Authentication Header
                    This header MUST be sent, unless the mobile node
                    has not yet configured, and is using its care-of
                    address.  [subject to change]

   ICMP Fields:

      Type

         153 <To Be Assigned by IANA>

      Code

         0

      Checksum

         The ICMP checksum [5].

   Options:



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      Prefix Information

         Each message contains one or more Prefix Information options,
         which contain the prefix(es) the mobile node should configure
         its home address(es) with.  Section 9.7 describes which
         prefixes should be advertised to the mobile node.

         The Prefix Information option is defined in Section 4.6.2
         of [20], with modifications defined in Section 6.2 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 9.9.1.

   The Mobile Prefix Advertisement sent by the home agent MAY include
   the Source Link-layer Address option defined in RFC 2461 [20], or the
   Advertisement Interval option specified in Section 6.3.

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


































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6. Modifications to IPv6 Neighbor Discovery

6.1. Modified Router Advertisement Message Format

   Mobile IPv6 modifies the format of the Router Advertisement
   message [20] 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 [20]:

      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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6.2. 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 9.9 and 10.8).

    -  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 10.11).

   However, Neighbor Discovery [20] 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 [20]:






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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 [20].  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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6.3. 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 [20], 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 10.4.

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








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6.4. 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 7.2.  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 6.1) 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.

   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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6.5. Changes to Sending Router Advertisements

   The Neighbor Discovery protocol specification [20] 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 6.2) in each.




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6.6. Changes to Sending Router Solicitations

   In addition to the limit on routers sending unsolicited multicast
   Router Advertisement messages (Section 6.5), 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 it current link, until its movement detection algorithm
       (Section 10.4) determines that it has moved and that its current
       care-of address might no longer be valid.



















































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7. 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.  Further details on this functionality is
   provided in the following sections.


7.1. Requirements for All IPv6 Routers

   The following requirements apply to all IPv6 routers, even those not
   serving as a home agent for Mobile IPv6:

    -  Every IPv6 router SHOULD be able to send an Advertisement
       Interval option in each of its Router Advertisements, to aid
       movement detection by mobile nodes.  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 6.5.  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.

    -  Filtering routers SHOULD be able to have different rules for
       routing header type 2 than for other routing headers so that
       type 2 can be allowed in order to allow Mobile IPv6 traffic
       while still having the option to filter out other use of routing
       headers.


7.2. 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.  Each such Binding Cache entry records the mobile node's
       binding with its primary care-of address and is marked as a "home
       registration".

    -  Every home agent MUST be able to intercept packets (using proxy
       Neighbor Discovery) 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.



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    -  Every home agent MUST be able to encapsulate 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.

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

    -  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 4.7.

    -  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 [11], and MUST be
       able to participate in dynamic home agent address discovery
       (Section 9.9).

    -  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.

    -  Every home agent SHOULD support sending ICMP Mobile
       Prefix Advertisements, and SHOULD respond to Mobile Prefix
       Solicitations.


7.3. 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
       decapsulation [4].

    -  Every IPv6 mobile node MUST support sending Binding Update
       options, as specified in Sections 10.7, 10.9, and 10.11; and MUST
       be able to receive and process Binding Acknowledgement options,
       as specified in Section 10.14.

    -  Every IPv6 mobile node MUST support use of the dynamic home agent
       address discovery mechanism, as described in Section 10.8.

    -  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.





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    -  Every IPv6 mobile node MUST support receiving a Binding Request
       option, by responding with a Binding Update option.

    -  Every IPv6 mobile node MUST support sending packets containing a
       Home Address option; this option MUST be included in all packets
       sent while away from home, if the packet would otherwise have
       been sent with the mobile node's home address as the IP Source
       Address.

    -  Every IPv6 mobile node MUST maintain a Home Agents List, as
       described in Section 4.7.

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







































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8. Correspondent Node Operation

   A correspondent node is any node communicating with a mobile node.
   The correspondent node, itself, may be stationary or mobile, and may
   possibly also be functioning as a home agent for Mobile IPv6.  The
   procedures in this section thus apply to all IPv6 nodes.


8.1. Receiving Packets from a Mobile Node

   Packets sent by a mobile node while away from home generally include
   a Home Address option.  When any node receives a packet containing
   a Home Address option, it MUST process the option in a manner
   consistent with exchanging the Home Address field from the Home
   Address option into the IPv6 header, replacing the original value of
   the Source Address field there.  However, any actual modifications to
   the Source Address field in the packet's IPv6 header MUST be carried
   out in such a fashion that further processing of such a packet after
   all IPv6 options processing (e.g., at the transport layer) does not
   need to know that the original Source Address was a care-of address,
   or that the Home Address option was used in the packet.

   Since the sending mobile node uses its home address at the transport
   layer when sending such a packet, the use of the care-of address
   and Home Address option is transparent to both the mobile node and
   the correspondent node above the level of the Home Address option
   generation and processing.


8.2. Receiving Binding Updates

   Before accepting a Binding Update option received in any packet, the
   receiving node MUST validate the Binding Update according to the
   following tests:

    -  The packet meets the specific authentication requirements for
       Binding Updates, defined in Section 4.5.

    -  The packet MUST contain a Home Address option.

    -  The Option Length field in the Binding Update option is greater
       than or equal to the length specified in Section 5.1.7.

    -  The Sequence Number field in the Binding Update option is greater
       than the Sequence Number received in the previous Binding Update
       for this home address, if any.  As noted in Section 4.7, this
       Sequence Number comparison MUST be performed modulo 2**8.

   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



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   code 141, and the last accepted sequence number in the Sequence
   Number field of the Binding Acknowledgement.

   Any Binding Update which fails to satisfy all of these tests for any
   other reason (than insufficiency of the Sequence Number) 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
       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 9.1; otherwise, it is processed
       according to the procedure specified in Section 8.3.

    -  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 9.2; otherwise, it is processed
       according to the procedure specified in Section 8.4.


8.3. Requests to Cache a Binding

   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 8.2.  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 new Binding Cache entry records the association between this
   home address and the care-of address for the binding.  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




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   the Binding Update.  Any Binding Cache entry MUST be deleted after
   the expiration of this lifetime in the Binding Cache entry.


8.4. Requests to Delete a Binding

   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 8.2.  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.  In this case, the receiving node MUST
   delete any existing entry in its Binding Cache for this mobile node.


8.5. Sending Binding Acknowledgements

   When any node receives a packet containing a Binding Update option
   in which the Acknowledge (A) bit is set, it MUST return a Binding
   Acknowledgement option 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, and the `A' bit
   was set in the Binding Update, 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 sent, 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 5.1.8 and in the most
   recent "Assigned Numbers" [10].

   The packet in which the Binding Acknowledgement is returned
   MUST meet the specific authentication requirements for Binding
   Acknowledgements, defined in Section 4.5.  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 8.9.

    -  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,



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       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.

   In response to a Binding Update, a node MAY send a Binding
   Acknowledgement even when the 'A' bit is not set in the Binding
   Update.  This would happen, for instance, if a mobile node attempted
   to send a Binding Update with the 'H' bit set to a correspondent
   node.


8.6. Sending Binding Requests

   Entries in a node's Binding Cache MUST be deleted when their lifetime
   expires.  If such an entry 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, where it will be
   intercepted and tunneled to the mobile node.  The mobile node will
   then return a Binding Update to the sender, allowing it to create
   a new Binding Cache entry for sending future packets to the mobile
   node.  Communication with the mobile node continues uninterrupted,
   but the forwarding of this packet through the mobile node's home
   agent creates additional overhead and latency in delivering packets
   to the mobile node.  Such routing paths could, for instance,
   temporarily or permanently disrupt any negotiated Quality of Service
   reservations which had been made by the mobile node on its home
   network.

   If the sender knows that the Binding Cache entry is still in active
   use, it MAY send a Binding Request option to the mobile node in
   an attempt to avoid this overhead and latency due to deleting and
   recreating the Binding Cache entry.  Since a Binding Request is a
   destination option, it may, for example, be included in any packet
   already being sent to the mobile node, such as a packet that is part
   of ongoing TCP communication with the mobile node.  When the mobile
   node receives a packet from some sender containing a Binding Request
   option, it returns a Binding Update option to that sender, giving its
   current binding and a new lifetime.


8.7. 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



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   Lifetime specified in the Binding Update from which the entry was
   created or last updated.

   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 9.1) MUST NOT be deleted from the cache
   until the expiration of its lifetime period.  When such a "home
   registration" entry is deleted, in addition the home agent MUST also
   cease intercepting packets on the mobile node's home link addressed
   to the mobile node (Section 9.3), just as if the mobile node had
   deregistered its primary care-of address (see Section 9.2).

   When attempting to add a new "home registration" entry in response
   to a Binding Update with the Home Registration (H) bit set, if
   insufficient space exists (and sufficient space cannot be reclaimed)
   in the node's Binding Cache, the node MUST reject the Binding
   Update and SHOULD 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 a "home registration"
   entry, 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 a "home registration" is likely to
   work well unless the size of the Binding Cache is substantially
   insufficient.

   Any binding dropped from a node's Binding Cache due to lack of cache
   space will be rediscovered and a new cache entry created, if the
   binding is still in active use by the node for sending packets.  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 normally,
   leading to the mobile node's home link.  There, the packet will be
   intercepted by the mobile node's home agent and tunneled to the
   mobile node's current primary care-of address.  As when a Binding
   Cache entry is initially created, this indirect routing to the mobile
   node through its home agent will result in the mobile node sending
   a Binding Update to this sending node when it receives the tunneled
   packet, allowing it to add an entry again for this destination mobile
   node to its Binding Cache.


8.8. Receiving ICMP Error Messages

   When a correspondent node sends a packet to a mobile node, if the
   correspondent node has a Binding Cache entry for the destination
   address of the packet, then the correspondent node uses a Routing
   header to deliver the packet to the mobile node through the care-of
   address in the binding recorded in the Binding Cache entry.  Any ICMP




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   error message caused by the packet on its way to the mobile node will
   be returned normally 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 to the mobile
   node's home link.  There, it will be intercepted by the mobile node's
   home agent, encapsulated, and tunneled to the mobile node's primary
   care-of address.  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 (the source of the tunnel).  By
   the definition of IPv6 encapsulation [4], the home agent (as the
   encapsulating node) MUST relay certain ICMP error messages back
   to the original sender of the packet, which in this case is the
   correspondent node.

   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 as described in Section 10.11 (e.g.,
   the mobile node moved after the packet was sent), that home agent
   will encapsulate and tunnel the packet 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 (the
   source of the tunnel), which MUST relay certain ICMP error messages
   back to the correspondent node [4].  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 MUST delete this Binding Cache
   entry.  If the correspondent node subsequently transmits another
   packet to the mobile node, the packet will be routed to the mobile
   node's home link, intercepted by the mobile node's home agent, and
   tunneled to the mobile node's primary care-of address using IPv6
   encapsulation.  The mobile node will then return a Binding Update to
   the correspondent node, allowing it to recreate a (correct) Binding
   Cache entry for the mobile node.


8.9. 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 0 Routing header [6], if



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   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).

   Following the definition of a Type 0 Routing header [6], 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).  Normal processing of
   the Routing header by the mobile node will then proceed as follows:

    -  The mobile node swaps the Destination Address in the packet's
       IPv6 header and the Address specified in the Routing header.
       This results in the packet's IP Destination Address being set to
       the mobile node's home address.

    -  The mobile node then resubmits the packet to its IPv6 module for
       further processing, "looping back" the packet inside the mobile
       node.  Since the mobile node recognizes its own home address as
       one of its current IP addresses, the packet is processed further
       within the mobile node, in the same way then as if the mobile
       node was at home.

   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 [4]) to the mobile node's current primary care-of
   address, as described in Section 9.4.  The mobile node will then send
   a Binding Update to the sending node, as described in Section 10.9,
   allowing the sending node to create a Binding Cache entry for its use
   in sending subsequent packets to this mobile node.

   It is possible that a correspondent node, having no knowledge
   of the mobile node's care-of address, would still (for reasons
   unspecified here but not necessarily related to mobility) attempt
   to deliver a packet, either to or by way of the mobile node's home
   address, by using a routing header.  If the correspondent node
   subsequently accepts a Binding Update and creates a Binding Cache
   entry for the mobile node, then afterwards, the routing header used



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   by the corresponding node which includes the mobile node's home
   address SHOULD also include the mobile node's care-of address.  The
   correspondent node SHOULD put the mobile node's care-of address as
   the intermediate node address immediately preceding the mobile node's
   home address.  When the care-of address is the first intermediate
   node address, this implies that the care-of address is to be placed
   in the Destination Address of the IPv6 header, and the mobile
   node's home address is the first entry in the type 0 routing header.
   Otherwise, the correspondent node MUST insert the mobile node's
   care-of address immediately before the home address entry in the
   routing header.


9. Home Agent Operation

9.1. Primary Care-of Address 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 8.2.  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:

    -  If the node is not a router that implements home agent
       functionality, then the node MUST reject the Binding Update and
       SHOULD 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.

    -  Finally, if the Duplicate Address Detection (D) bit is set in
       the Binding Update, this home agent MUST ensure that Duplicate
       Address Detection [35] has been performed on the mobile node's
       home link for the link-local address associated with the home
       address in this binding, and thus to ensure that no other node



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       on the home link can possibly use the mobile node's home address
       (before returning the Binding Acknowledgement).  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 [20, 35]; 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 SHOULD 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 valid at least as long as the mobile node transmits Binding
       Updates with new care-of addresses for that home address.

   If the home agent does not reject the Binding Update, then it becomes
   or remains the home agent for the mobile node.  The home agent MUST
   then 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 home address of the mobile node is
   taken to be the value which, when the packet was originally received,
   was located in the Home Address field in the packet's Home Address
   option.  The care-of address for this Binding Cache entry is taken
   to be the value which, when the packet was originally received, was
   located either in the Alternate Care-of Address sub-option in the
   Binding Update option, if present, or from the Source Address field
   in the packet's IPv6 header, otherwise.

   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 8.7) and MUST NOT be
   removed from the Binding Cache until the expiration of the Lifetime
   period.

   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 determined by the home
   agent based on its own Prefix List entry for this prefix [20].

   If the `S' bit field in the Binding Update is zero, The Home Agent
   creates or updates Binding Cache entries for each of possible
   several home addresses.  The set of such home addresses is formed
   by replacing the routing prefix for the given home address with



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   all other routing prefixes 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 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 indicating success; this
       value MUST be less than 128.  The only currently defined success
       Status value is 0, indicating simply that the Binding Update was
       accepted.

    -  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).

   In addition, the home agent MUST follow the procedure defined in
   Section 9.3 to intercept packets on the mobile node's home link
   addressed to the mobile node, while the home agent is serving as the
   home agent for this mobile node.




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9.2. 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 8.2.  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 in its Binding Cache for this
       mobile node that is marked as a "home registration", 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 indicating success (the
       value MUST be less than 128).  The only currently defined success
       Status value is 0, indicating simply that the Binding Update was
       accepted.

    -  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 addressed to the mobile node (Section 9.3).

   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 8.5.


9.3. Intercepting Packets for a Mobile Node

   While a node is serving as the home agent for mobile node (while the
   node has an entry in its Binding Cache for this mobile node that is
   marked as a "home registration"), this node MUST attempt to intercept
   packets on the mobile node's home link addressed to the mobile node,




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   and MUST tunnel each intercepted packet to the mobile node using
   using IPv6 encapsulation [4].

   In order to intercept such packets on the home link, when a node
   begins serving as the home agent for some mobile node (it did not
   already have a Binding Cache entry for this mobile node marked as a
   "home registration"), then the home agent MUST multicast onto the
   home link a "gratuitous" Neighbor Advertisement message [20] 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 new "home
       registration" Binding Cache entry.  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 multicasts onto the
       home link (to the all-nodes multicast address) a Neighbor
       Advertisement message [20] on behalf of the mobile node, to
       advertise the home agent's own link-layer address for this IP
       address.

       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 [20], 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 copied from
           the corresponding bit in the home agent's Binding Cache entry
           for the mobile node.

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






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        *  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.

   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 multicasts on the local link (such as
   Ethernet) are 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 [20].

   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 [20] 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".  This check MUST include all possible
   home addresses for the mobile node, based on the subnet prefixes
   currently considered to be on-link by the home agent (including the
   corresponding link-local address and site-local address), if the
   Prefix Length in the Binding Cache entry for this mobile node (from
   the Binding Update that created this Cache entry) is nonzero.

   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 copied from
   the corresponding bit in the home agent's Binding Cache entry for the
   mobile node.  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 [20].


9.4. 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



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   packet), the home agent is operating as a correspondent node of
   the mobile node for this packet and the procedures described in
   Section 8.9 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).

   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
   Prefix Length field was nonzero in the Binding Update), as described
   in Section 9.3.  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 [12] or ESP [13] 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 [4]; 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 prepended 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 [4] 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 [9],
   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



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   organization-local) [9], 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.


9.5. Handling Reverse Tunneled Packets from a Mobile Node

   A home agent MUST support decapsulating reverse tunneled packets
   sent to it from a mobile node's home address.  Such reverse tunneled
   packets MAY be discarded unless accompanied by a valid AH or ESP
   header.  This support for reverse tunneling allows mobile nodes to
   defeat certain kinds of traffic analysis.  Requiring IPsec headers
   on reverse tunneled packets allows the home agent to protect the
   home network against unwarranted intrusions by malicious nodes
   masquerading as a mobile node with a home address on the network
   served by the home agent.


9.6. Protecting Return Routability Packets

   The Return Routability procedure described in Section 4.5 assumes
   that the confidentiality of the HoT message is protected as it is
   tunneled from the home agent to the mobile node.  Therefore, the home
   agent MUST protect these packets using IPsec ESP with a non-null
   encryption transform.  It is also useful, but not required to protect
   other RR related messages.

   As described earlier, the Binding Update and Binding Acknowledgement
   messages already need protection even if they are not tunneled.  As
   these messages employ the Mobility Header in a similar manner to
   the RR messages, it is sufficient to set up IPsec Security Policy
   Database in a manner that protects all traffic to the mobile node
   and back with IPsec ESP if the protocol in the packet is Mobility
   Header.


9.7. Home Prefix Propagation

   IPv6 provides mechanisms as part of Neighbor Discovery [20] and
   Address Autoconfiguration [35] to aid in mobile node configuration
   when a mobile node turns on, and in renumbering a subnet, such as
   when a site switches to a new network service provider.

   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 for the case in which one or more mobile




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   nodes using the old prefixes are away from home and registered at the
   home agent while the renumbering takes place.

   In the IPv6 renumbering mechanism, nodes on the visited link receive
   Mobile Prefix Advertisements messages with Prefix Information
   Options, which give the valid lifetime and preferred lifetime for
   available prefixes on the link [20].

   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.  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 [14, 12, 13] if a
   security associate exists (i.e.  unless the mobile node does not yet
   have a home address configured).


9.8. 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 9.9.
   The information for the list is learned through receipt of the
   periodic unsolicited multicast Router Advertisements from each other
   home agent on the link, in which the Home Agent (H) bit is set, in a
   manner similar to the Default Router List conceptual data structure
   maintained by each host for Neighbor Discovery [20].

   On receipt of a valid Router Advertisement, as defined in the
   processing algorithm specified for Neighbor Discovery [20], 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,
       skip all of the following steps.

    -  If the Home Agent (H) bit in the Router Advertisement is not set,
       and the sending node has an entry in the current 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 [20].

    -  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




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       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
       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 6.2).  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.


9.9. Dynamic Home Agent Address Discovery

   A mobile node, while away from home, MAY use the dynamic home agent
   address discovery mechanism in section 10.8 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.



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   As described in Section 10.8, 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 [11] for its home IP subnet prefix, using its care-of address
   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 4.7).  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 4.7, 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



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       Router Address (R) bit is set, received in Router Advertisements
       from this link-local address.

       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 [5]), 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 [6], 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.


9.9.1. Aggregate List of Home Network Prefixes

   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
   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




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       (Neighbor Discovery [20]).  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 [20].

    -  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 [20]) and section
       5.5.3 of RFC 2462 (Stateless Address Autoconfiguration [35]).

    -  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.

   The home agent uses the information in the aggregate list to
   construct Mobile Prefix Advertisements, possibly including Binding
   Acknowledgement or Binding Request destination options, for delivery
   to a mobile node for which it is maintaining a current binding.
   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 10.8).


9.9.2. 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
       changes



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    -  The mobile node requests the information with a Router
       Solicitation (see section 10.18).

   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, and
       calculate RAND_ADV_DELAY in order to randomize the time at which
       the transmission is scheduled.

    -  If there are any unacknowledged changes to prefix information
       when a Binding Update arrives for the home registration, send
       a Mobile Prefix Advertisement to the mobile node immediately.
       The Mobile Prefix Advertisement SHOULD have the Binding
       Acknowledgement as a Destination Option.  If an advertisement
       was previously scheduled for the mobile node, cancel that
       advertisement.

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

   If the home agent already has scheduled the transmission of a Router
   Advertisement to the mobile node, and if the freshly calculated
   RAND_ADV_DELAY would cause another transmission BEFORE the Preferred
   Lifetime of the mobile node's home address derived from the prefix
   whose advertisement information has changed, then add the new
   information to be transmitted to the existing scheduled transmission.
   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.  If there is already a scheduled



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   transmission, add the data from the existing scheduled transmission
   to the newly scheduled transmission, deleting the previously
   scheduled transmission event.

   If the mobile node's binding expires before the Preferred Lifetime,
   then return.  The mobile node will get the revised information with
   its next Binding Acknowledgement.  Otherwise, continue with the
   following computation.
       MAX_SCHEDULE_DELAY == min (MAX_PFX_ADV_DELAY, Preferred Lifetime)
   for the newly advertised Preferred Lifetime.

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

   RAND_ADV_DELAY is the offset from the current time to be used
   to schedule the necessary advertisement to the mobile node.  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.


9.9.3. 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:

    -  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 [14, 12, 13] to guard
       against malicious Mobile Prefix Advertisements.  The IPsec
       protection MUST provide sender authentication, data integrity
       protection, and replay protection, covering the Mobile Prefix
       Advertisement.

    -  The advertisement MUST include a Binding Request destination
       option if this is the first advertisement for a home
       registration, or if there was a change in prefix information
       since the last acknowledged advertisement was sent to the mobile
       node for the home registration.  The Binding Request destination
       option MUST include a Unique Identifier Sub-Option (Section 5.5),
       with the unique identifier in the sub-option data set to a value
       different than that in any other Binding Request sent recently by
       this home agent.  It is assumed that this requirement can be met
       by maintaining a simple 16-bit "wrap-around" counter to generate
       unique identifiers for Binding Requests that contain a Unique



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       Identifier Sub-Option, incremented each time a Binding Request
       containing a Unique Identifier Sub-Option is sent.

    -  If the advertisement was solicited, it should be destined
       (and authenticated, if possible) 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).

   The home agent SHOULD periodically continue to retransmit an
   unsolicited Advertisement to the mobile node, until it is
   acknowledged by the receipt from the mobile node of a Binding Update
   matching the Binding Request in the packet (i.e., with matching
   Sequence Number).  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.

   A Binding Update matches a Binding Request if it specifies a
   binding for the mobile node to which the Binding Request was sent
   and contains a Unique Identifier Sub-Option matching the unique
   identifier sent in the Unique Identifier Sub-Option in the Binding
   Request.  In the solicited case, the mobile node will retransmit
   solicitations until one is received; thus, the home agent SHOULD NOT
   retransmit the responding 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 algorithm in
   section 9.9.2.  The home agent MUST generate a new unique identifier
   for use in the Unique Identifier Sub-Option in the Binding Request
   tunneled with the new Mobile Prefix Advertisement.



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9.9.4. Lifetimes for Changed Prefixes

   As described in Section 9.1, 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.  Furthermore, as described in Section 10.9, 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.  These limits on the binding lifetime serve
   to prohibit use of a mobile node's home address after it becomes
   invalid.  The mobile node SHOULD further limit the lifetimes that it
   sends on any Binding Updates to be within the remaining preferred
   lifetime 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.


10. Mobile Node Operation

10.1. 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
   these in selecting the address that it will use as the source of the
   packet, as follows:

    -  From the point of view of protocol layers and applications above
       Mobile IP (e.g., transport protocols), the mobile node will
       generally use its home address as the source of the packet for
       most packets, even while away from home, since Mobile IP is
       designed to make mobility transparent to such software.  Doing
       so also makes the node's mobility---and the fact that it is
       currently away from home---transparent to the correspondent nodes
       with which it communicates.  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 in this
       way.  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, Mobile IP will modify
       the packet to move the home address into a Home Address option
       and will set the IPv6 header's Source Address field to one of
       the mobile node's care-of addresses; these modifications to
       the packet are then reversed in the node receiving the packet,
       restoring the mobile node's home address to be the packet's




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       Source Address before processing by higher protocol layers and
       applications.

    -  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 [17, 18].  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 its home address as
   the source of a packet sent while away from home (from the point of
   view of higher protocol layers or applications, as described above),
   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 each other packet sent by the mobile node (i.e., packets sent
   while away from home, using the mobile node's home address as
   the source, from the point of view of higher protocol layers and
   applications), special Mobile IP processing of the packet is required
   for the insertion of the Home Address option.  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.  This preserves the transparency of Mobile IP to
       higher protocol layers (e.g., to TCP).

    -  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.





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   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 [7].


10.2. 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 [14] 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 10.1):

    -  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.  Mobile IP is transparent to such higher
       layers.

    -  As part of outbound packet processing in IP, the packet is
       compared against the IPsec Security Policy Database (SPD) to
       determine what processing is required for the packet [14].

    -  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 through the home agent.

       If route optimization is in use, the mobile node inserts a Home
       Address 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 10.1.  The Destination Options header in which the Home
       Address option is inserted MUST appear in the packet after the
       Routing Header, if present, and before the AH [12] (or ESP [13])
       header, so that the Home Address option is processed by the
       destination node before the AH or ESP 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 AH or ESP 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 5.3) contains the new care-of address.

    -  This allows, but does not require, the receiver of the
       packet containing a Home Address Option to exchange the two
       fields of the incoming packet, simplifying processing for all
       subsequent packet headers.  The mechanics of implementation
       do not absolutely require such an exchange to occur; other
       implementation strategies may be more appropriate, as long as the
       result of the authentication calculation remain the same.

   In addition, when using any automated key management protocol [14]
   (such as IKE [8]) to create any 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 [8][14], 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 10.1).

    -  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 [16] in the IKE
       exchange, giving the mobile node's home address as the initiator
       of the Security Association [28].


10.3. 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 sent by the



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       correspondent node in the same way as any normal IP packet.  Such
       packets will then be intercepted by the mobile node's home agent,
       encapsulated using IPv6 encapsulation [4], and tunneled to the
       mobile node's primary care-of address.

    -  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 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 be sent by the correspondent
       node using a 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 10.11), 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 intercepted by this home agent, encapsulated using IPv6
       encapsulation [4], and tunneled to the mobile node's new care-of
       address (registered with this 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 10.9, subject to
   the rate limiting defined in Section 10.13.  The mobile node MUST
   also process the received packet in the manner defined for IPv6
   encapsulation [4], 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 Routing
   header), the mobile node MUST process the received packet in
   the manner defined for the type of IPv6 Routing header used (see
   Section 5.4), which will result in the 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.


10.4. Movement Detection

   A mobile node MAY use any combination of mechanisms available to it
   to detect when it has moved from one link to another.  The primary
   movement detection mechanism for Mobile IPv6 defined here uses the
   facilities of IPv6 Neighbor Discovery, including Router Discovery and



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   Neighbor Unreachability Detection, although 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 [20].

   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 [20].
   Based on received Router Advertisement messages, a mobile node (in
   the same way as any other node) maintains an entry in its Default
   Router List for each router, and 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 (extracted
   from the Router Advertisement and Prefix Information options) used to
   expire the entry when it becomes invalid.

   While away from home, a mobile node typically selects one router
   from its Default Router List to use as its default router, and one
   subnet prefix advertised by that router from its Prefix List 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 10.6.  The mobile node registers
   its primary care-of address with its home agent, as described in
   Section 10.7.

   While a mobile node is away from home and using some router as its
   default router, it is important for the mobile node to be able to
   quickly detect when that router becomes unreachable, so that it
   can switch to a new default router and (if needed, according to
   prefix advertisement) to a new primary care-of address.  Since some
   links (notably wireless) do not necessarily work equally well in
   both directions, it is likewise important for the mobile node to
   detect when it becomes unreachable for packets sent from its default
   router, so that the mobile node can take steps to ensure that any
   correspondent nodes attempting to communicate with it can still reach
   it through some other route.

   To detect when its default router becomes unreachable, a mobile
   node SHOULD use Neighbor Unreachability Detection.  As specified in
   Neighbor Discovery [20], while the mobile node is actively sending
   packets to (or through) its default router, the mobile node can
   detect that the router (as its neighbor) is still reachable either
   through indications from upper layer protocols on the mobile node
   that a connection is making "forward progress" (e.g., receipt of TCP
   acknowledgements for new data transmitted), or through receipt of a
   Neighbor Advertisement message from its default router in response
   to an explicit Neighbor Solicitation messages to it.  Note that



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   although this mechanism detects that the mobile node's default router
   has become unreachable to the mobile node only while the mobile node
   is actively sending packets to it, this is the only time that this
   direction of reachability confirmation is needed.  Confirmation
   that the mobile node is still reachable from the router is handled
   separately, as described below.

   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 for a mobile node to continually
   probe its default router with Neighbor Solicitation messages even
   when it is not otherwise actively sending packets to it.  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
   to receive all packets on the link, including those not link-level
   addressed to it (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



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   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.


10.5. 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 10.11.  On receipt of a valid Router
   Advertisement, as defined in the processing algorithm specified for
   Neighbor Discovery [20], the mobile node performs the following
   steps, in addition to any steps already required of it by Neighbor
   Discovery.



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    -  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 [20].

    -  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
       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 6.2).  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.



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10.6. 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 10.20.

   As described in Section 4, in order to form a new care-of address,
   a mobile node MAY use either stateless [35] or stateful (e.g.,
   DHCPv6 [2]) 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 [35] on that new address to confirm its
   uniqueness.  However, doing so represents a tradeoff 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 [20, 35];
   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.


10.7. Sending Binding Updates to the Home Agent

   After deciding to change its primary care-of address as described
   in Sections 10.4 and 10.6, a mobile node MUST register this care-of
   address with its home agent in order to make this its primary care-of
   address.  To do so, the mobile node sends a packet to its home agent
   containing a Binding Update option, with the packet constructed as
   follows:

    -  The Home Registration (H) bit MUST be set in the Binding Update.

    -  The Acknowledge (A) bit MUST be set in the Binding Update.

    -  The packet MUST contain a Home Address 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 sub-option is included in the Binding Update option.

    -  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.





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    -  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 5.1.8, 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 Agent list (see section 10.8).  If
   there is only one home agent in the Home Agent 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 10.12 for
   information about retransmitting Binding Updates.

   The Prefix Length field in the Binding Update allows the mobile node
   to request its home agent to serve all home addresses for the mobile
   node, as indicated by the interface identifier in the mobile node's
   home address (the remaining low-order bits after the indicated subnet
   prefix), together with each on-link subnet prefix on the home link.
   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.

   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 10.9.

   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).  These additional Binding
   Updates MUST each be sent as a separate packet.  Each care-of address
   MUST be authenticated within the Binding Update as coming from the
   home address being associated with the care-of address, as defined in
   Section 4.5.

   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 [20, 35].  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



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   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 showing Status 138, Duplicate Address Detection
   failed.  In this case, the mobile node MUST NOT attempt to 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 B.


10.8. 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 10.7, 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 discovery 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 [11] for its home subnet
   prefix.  This packet MUST NOT contain a Home Address option and must
   be sent using the mobile node's care-of address as the Source Address
   in the packet's IP header (the packet is sent from the care-of
   address, not using Mobile IP). As described in Section 9.9, 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



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   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.


10.9. Sending Binding Updates to Correspondent Nodes

   When the mobile node is assured that its home address is valid,
   it MAY send a Binding Update to any correspondent node at any
   time to allow the correspondent node to cache the mobile node's
   current care-of address (subject to the rate limiting defined in
   Section 10.13).  See for example the home agent's use the 'D' bit
   of Binding Updates (in section 9.1) for how the mobile node can be
   assured that its home address is still valid.  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 Sub-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.




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   A mobile node MAY choose to keep its location private from certain
   correspondent nodes, and thus need not 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 5.1.7.  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 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 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.
   A mobile node MAY set the care-of address differently for sending
   Binding Updates to different correspondent nodes.

   When sending any Binding Update, the mobile node MUST record in its
   Binding Update List the following fields from the Binding Update:

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

    -  The home address for which the Binding Update was sent (the value
       in the Home Address option in the packet carrying the Binding
       Update).

    -  The initial lifetime of the binding, initialized from the
       Lifetime field sent in the Binding Update.

    -  The remaining lifetime of the binding, also initialized from
       the Lifetime field sent in the Binding Update.  This remaining
       lifetime value counts down and may also be reduced when the
       matching Binding Acknowledgement is received, based on the
       Lifetime value specified in that Binding Acknowledgement, as
       described in Section 10.14.  When the remaining lifetime reaches
       zero, the Binding Update List entry MUST be deleted.

   The mobile node MUST retain in its Binding Update List information
   about all Binding Updates sent, for which the lifetime of the binding
   has not yet expired.  However, when sending a Binding Update, if an
   entry already exists in the mobile node's Binding Update List for
   an earlier Binding Update sent to that same destination node, the
   existing Binding Update List entry is updated to reflect the new
   Binding Update rather than creating a new Binding Update List entry.



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   When a mobile node sends a Binding Update to its home agent
   to register a new primary care-of address (as described in
   Section 10.7), the mobile node SHOULD also send a Binding Update
   to each other node for which an entry exists in the mobile node's
   Binding Update List, as detailed below.  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 send these Binding Updates
   immediately after configuring a new care-of address.  For example,
   since the Binding Update is a destination option and can be included
   in any packet sent by a mobile node, the mobile node MAY delay
   sending a new Binding Update to any correspondent node for a
   short period of time, in hopes that the needed Binding Update
   can be included in some packet that the mobile node sends to that
   correspondent node for some other reason (for example, as part of
   some TCP connection in use).  In this case, when sending a packet
   to some correspondent node, the mobile node SHOULD check in its
   Binding Update List to determine if a new Binding Update to this
   correspondent node is needed, and SHOULD include the new Binding
   Update in this packet as necessary.

   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
   return a Binding Update to the sender giving its current care-of
   address (subject to the rate limiting defined in Section 10.13).
   In particular, the mobile node SHOULD return a Binding Update 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 10.11).

    -  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 Binding Update should be sent
   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.



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   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.


10.10. Receiving RR Messages

   Upon receiving a packet carrying a HoT 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 5.1.5.

    -  The Source Address of the packet belongs to a correspondent node
       that the mobile node wishes to use Route Optimization with.

    -  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 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 agenet.

   Any HoT 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 CoT 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 5.1.5.

    -  The Source Address of the packet belongs to a correspondent node
       that the mobile node wishes to use Route Optimization with.

    -  The Source Address of the packet belongs to a correspondent
       node for which the mobile node has a Binding Update List entry




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       with a state indicating that Return Routability procedure is in
       progress.

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

   Any CoT 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 HoT or the CoT message and performing
   the above actions, the Binding Update List entry has both the Home
   and the Care-of Cookies, the mobile node SHOULD send a Binding Update
   message as described in Section 10.9.


10.11. Establishing 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.

   In constructing this Binding Update, the mobile node utilizes the
   following specific steps:

    -  The Home Address field in the Home Address 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
       sub-option in the Binding Update option, 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.



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   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
   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 6.2) 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 4.7).  Alternatively,
   the mobile node MAY use dynamic home agent address discovery
   (Section 10.8) 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 5.1.7),
   the Binding Update packet to this home agent MUST meet the
   authentication requirements for Binding Updates, defined in
   Section 4.5.


10.12. Retransmitting Binding Updates

   When the mobile node sends a Binding Update, 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 10.7.

   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



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   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.


10.13. Rate Limiting for Sending Binding Updates

   A mobile node MUST NOT send Binding Updates about the same binding to
   any individual node more often than once per MAX_UPDATE_RATE seconds.
   After sending MAX_FAST_UPDATES consecutive Binding Updates to a
   particular node with the same care-of address, the mobile node SHOULD
   reduce its rate of sending Binding Updates to that node, to the rate
   of SLOW_UPDATE_RATE per second.  The mobile node MAY continue to send
   Binding Updates 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.


10.14. 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 Section 4.5.

    -  The Option Length field in the Binding Acknowledgement option is
       greater than or equal to the length specified in Section 5.1.8.

    -  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, although the remainder of the packet (i.e., other
   options, extension headers, or payload) SHOULD be processed normally
   according to any procedure defined for that part of the packet.

   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



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       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
       the timer countdown beginning at the time that the Binding Update
       was sent.

    -  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 10.13.


10.15. Receiving Binding Requests

   When a mobile node receives a packet containing a Binding Request,
   it SHOULD return to the sender a packet containing a Binding Update.
   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), except that this lifetime
   MUST NOT exceed the remaining lifetime for the mobile node's primary
   care-of address registration at its home agent.  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 keep its current
   binding private from the sender of the Binding 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 is set to the mobile node's home address.




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   If the Binding Request for which the Binding Update is being returned
   contains a Unique Identifier Sub-Option, the Binding Update MUST also
   include a Unique Identifier Sub-Option.  The unique identifier in the
   Sub-Option Data field of the Unique Identifier Sub-Option MUST be
   copied from the unique identifier carried in the Binding Request.


10.16. Receiving ICMP Error Messages

10.17. Receiving ICMP Error Messages

   The Option Type value for a Binding Update option specifies that
   any node receiving this option that does not recognize the Option
   Type SHOULD return an ICMP Parameter Problem, Code 2, message to
   the sender of the packet containing the Binding Update option.  If
   a node 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.

   Likewise, although ALL IPv6 nodes (whether host or router, whether
   mobile or stationary) MUST implement the ability to correctly process
   received packets containing a Home Address option, all Option Type
   values in IPv6 include a specification of the behavior that a node
   receiving a packet containing this option performs if it does not
   implement receipt of that type of option.  For the Home Address
   option, the Option Type value specifies that any node receiving
   this option that does not recognize the Option Type SHOULD return
   an ICMP Parameter Problem, Code 2, message to the sender of the
   packet containing the Home Address option.  If a mobile node receives
   such an ICMP error message from some node indicating that it does
   not recognize the mobile node's Home Address option, the mobile
   node SHOULD log the error and then discard the ICMP message; this
   error message indicates that the node to which the original packet
   was addressed (the node returning the ICMP error message) does not
   correctly implement this required part of the IPv6 protocol.


10.18. 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
   renumbering operations affecting the home network, as described in
   section 9.7.

   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 11).



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   If the mobile node does not have a valid home address available for
   use as the IP source address, it MAY use its care-of address (but
   there will not be a security association between the Home Agent
   and the care-of address for the corresponding Advertisement to be
   authenticated).

   This solicitation follows the same retransmission rules specified for
   Router Solicitations [20], except that the initial retransmission
   interval is specified to be INITIAL_SOLICIT_TIMER (see section 11).


10.19. Receiving Mobile Prefix Advertisements

   Section 9.7 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 tests:

    -  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.

    -  The packet MUST be protected by IPsec [14, 12, 13] to guard
       against malicious prefix advertisements.  The IPsec protection
       MUST provide sender authentication, data integrity protection,
       and replay protection, covering the advertisement.

   Any received Mobile Prefix Advertisement not meeting all of these
   tests MUST be silently discarded.

   If a received Mobile Prefix Advertisement is not discarded according
   to the tests listed above, the mobile node MUST process the Prefix
   Information Options as if they arrived in a Router Advertisement
   on the mobile node's home link [20].  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.

   If the advertisement contains a Binding Request option, the mobile
   node SHOULD return a Binding Update, which will be viewed by the



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   home agent as an acknowledgement of the corresponding Mobile Prefix
   Advertisement, which it can cease transmitting.

   In addition, if processing of this Advertisement resulted in the
   mobile node configuring a new home address, and if the method used
   for this new home address configuration would require the mobile node
   to perform Duplicate Address Detection [35] 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.


10.20. Using Multiple Care-of Addresses

   As described in Section 10.6, 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 10.4.  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 10.7.

   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 [2], the mobile
   node may not wish to release the address immediately upon switching
   to a new primary care-of address.


10.21. 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



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   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.  The mobile node MUST insert a Home Address
   destination option in such outgoing multicast packets, so that any
   multicast applications that depend on the address of the sending node
   will correctly use the mobile node's home address for that value.

   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.


10.22. Returning Home

   A mobile node detects that it has returned to its home link through
   the movement detection algorithm in use (Section 10.4), 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 Binding Update, the mobile node MUST set the care-of
   address for the binding (the Source Address field in the packet's
   IPv6 header) to the mobile node's own home address.  As with other
   Binding Updates sent to register with its home agent, the mobile
   node MUST set the Acknowledge (A) and Home Registration (H) bits,
   and SHOULD retransmit the Binding Update until a matching Binding
   Acknowledgement is received.

   When sending this Binding Update to its home agent, the mobile
   node must be careful in how it uses Neighbor Solicitation [20] (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



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   does Neighbor Solicitation to learn the home agent's link-layer
   address, in this special case of the mobile node returning home, the
   mobile node MUST unicast 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).  Since the solicitation is unicast, the home
   agent will be able to distinguish from a similar packet that would
   only be used 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.

   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 [20], 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
   home addresses, as defined by the current on-link prefixes, including
   its link-local address and site-local address.  The Solicited



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   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 multicasts on the local link (such as Ethernet) are 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 [20].


11. 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_BINDACK_TIMEOUT      256 seconds
      MAX_UPDATE_RATE          once per second
      MAX_FAST_UPDATES         5 transmissions
      MAX_ADVERT_REXMIT        3 transmissions
      MAX_PFX_ADV_DELAY        1,000 seconds
      PREFIX_ADV_RETRIES       3 retransmissions
      PREFIX_ADV_TIMEOUT       5 seconds
      SLOW_UPDATE_RATE         once per 10 second interval
























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12. Future Extensions

12.1. Piggybacking

   This document does not specify how to piggyback payload on the
   binding related messages.  However, it is envision 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 12.3).

   The idea is to use the Flag field in the HoTI message so that the MN
   can indicate that it supports the receipt of piggybacked messages,
   use the Flag field in the HoT message for the CN to indicate that
   it can support the receipt of piggybacked messages, and then carry
   the piggybacked payload after the MH header by specifying a payload
   protocol type other than NO_NXTHDR (59).

   Until such a separate specification exists implementations conforming
   to this specification MUST set the payload protocol type to NO_NXTHDR
   (59 decimal).


12.2. Triangular Routing and Unverified HAOs

   Due to the concerns about opening up reflection attacks with the home
   address 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 options in ways that do not introduce security issues.


12.3. New Authorization Methods beyond RR

   While the RR method provides a good level of security, there exists
   methods that have even higher level of security.  Secondly, as
   discussed in Section 14.3, future enhancements of IPv6 security may
   cause a need to improve also the security of RR. The question is then
   what is the method to securely agree on the use of another method,
   while still allowing RR for some hosts during a transition period.
   In some cases, a third party can help to make this selection.  But
   in general infrastructureless methods have little information beyond
   the exchanged messages and their contents.  For these reasons, this
   specification requires a protection mechanism for selecting between
   RR and potential other future mechanisms (see Section ??).  This
   isn't

   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



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   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.


13. IANA Considerations

   This document defines a new IPv6 protocol, the Mobility Header,
   described in Section 5.1.  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 5.1.2 to 5.1.9, and include the following:

     0 Binding Request

     1 HoA Test Init

     2 CoA Test Init

     3 HoA Test

     4 CoA Test

     5 Binding Update

     6 Binding Acknowledgement

     7 Binding Missing

   Future values of the MH Type can be allocated using standards
   action [19].

   Furthermore, each Mobility Header message may contain parameters as
   described in Section 5.2.  The current parameters are defined in
   Sections 5.2.2 to 5.2.5, and include the following:

     0 Pad1

     1 PadN

     2 Unique Identifier

     3 Alternate Care-of Address

   Future values of the Parameter Type can be allocated using standards
   action [19].

   This document also defines a new IPv6 destination option, the
   Home Address option, described in Section 5.3.  This option must



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   be assigned an Option Type value.  This destination option may
   also contain sub-options as described in Section 5.5.  The current
   sub-options are specified in Sections 5.5.1 to 5.5.2, and include the
   following:

     0 Pad1 sub-option

     1 PadN sub-option

   This document also defines a new IPv6 Routing Header type, 2,
   described in Section 5.4.  The value 2 is 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 5.6;

    -  The Home Agent Address Discovery Reply message, described in
       Section 5.7;

    -  The Mobile Prefix Solicitation message, described in Section 5.8;
       and

    -  The Mobile Prefix Advertisement message, described in
       Section 5.9.

   This document also defines two new Neighbor Discovery [20] options,
   which must be assigned Option Type values within the option numbering
   space for Neighbor Discovery messages:

    -  The Advertisement Interval option, described in Section 6.3; and

    -  The Home Agent Information option, described in Section 6.4.


14. Security Considerations

14.1. Security for the Tunneling to and from the Home Agent

   The use of IPsec ESP to protects payload packets tunneled to the
   mobile node from the home agent and reverse tunneled back to the
   home agent.  While this specification does not mandate the use of
   ESP, it is highly recommended due to unauthenticated tunnels opening
   a flexible reflection device to attackers, even if Mobile IPv6
   signaling such as Binding Updates are not affected.





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14.2. Security for the Binding Updates to the Home Agent

   The use of IPsec ESP to protect Mobility Header messages between the
   mobile node and the home agent protects the integrity of the Binding
   Updates and Binding Acknowledgements.  If manually keyed IPsec is
   used, it does not, however, protect against Binding Update replays.
   Replay protection is provided by the application layer sequence
   number described in Section 4.5.4.


14.3. Security for the Binding Updates to the Correspondent Nodes

   The introduction of home address and care-of-address based return
   routability (RR) tests prevent any off-path attacks beyond those that
   are already possible in basic IPv6 [23].

   Protection against attackers on the home agent link and the
   correspondent node link, as well as on the path between, are
   roughly similar to the situation in existing IPv6 as well.  However,
   one difference is that in basic IPv6 an on-path attacker must be
   constantly present on the link or the path (in order to perform e.g.
   a man-in-the-middle attack), whereas with Mobile IPv6 an attacker can
   leave an existing binding behind, even after it is no longer on the
   link or on the path [23].  For this reason, this specification limits
   the validity of RR authorized bindings to a maximum of five minutes
   after the last RR check has been performed.

   In practice the easiest attacks on the path between the Home Agent
   and a correspondent node is when multi-access links exist at either
   node; a correspondent node on a public access wireless LAN would be
   a good example.  There are attacks that can be launched on routers
   or switches on the path as well but those seem to be harder in many
   cases.  Thus, the weakest places are typically in layer 2 security as
   well in IPv6 Neighbour and Router discovery on multi-access links.
   If these were secured using some new technology in the future, this
   would make Mobile IPv6 an easier route for attackers to use.  For
   this reason, this specification requires a protection mechanism for
   selecting between RR and potential other future mechanisms (see
   Section ??).


14.4. Security for the Home Address Option

   The use of the Home Address option allows packets sent by a mobile
   node to pass normally through routers implementing ingress filtering
   [7].  Since the care-of address used in the Source Address field of
   the packet's IPv6 header is topologically correct for the sending
   location of the mobile node, ingress filtering can trace the
   location of the mobile node in the same way as can be done with any
   sender when ingress filtering is in use.  As this location does not
   survive in the answers sent by the correspondent node, this document



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   restricts the use of the Home Address Option to those situations
   where a binding has been established, and the home address has agreed
   to the participation in the binding.  This prevents reflection
   attacks through the use of the Home Address Option.

   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 5.3), 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.

   A node receiving a packet that includes a Home Address option MAY
   implement the processing of this option by physically exchanging the
   Home Address option field with the source IPv6 address in the IPv6
   header.


14.5. Firewall considerations

   The definition of Routing Header 2 in Section 5.4 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 destination and the Home Address in the routing header will
   always have to be assigned to the same node otherwise the packet will
   be dropped.

   This means that the typical security concerns for source routing
   including the automatic reversal of unauthenticated source routes
   (which is an issue for IPv4 but not for IPv6 source routing) and the
   ability to use source routing to "jump" between nodes inside as well
   as outside a firewall, are not at play.

   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 device which implement filtering of packets should
   be able to distinguish routing header type 2 from other routing
   headers, as required in section 7.1.  This is necessary in order to
   allow Mobile IPv6 traffic while still having the option to filter out
   other use of routing headers.




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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 at 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.), Thomas Narten (IBM), Erik Nordmark (Sun
   Microsystems), 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.

   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 Connectathon 2000 in San Jose,
   California, March 6-9, 2000.  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.

   Lastly, we must express our appreciation for the significant
   contributions made by the Mobile IPv6 Security Design Team, including
   (in alphabetical order) Jari Arkko, Gabriel Montenegro, Erik
   Nordmark, and Pekka Nikander, who have contributed volumes of text to
   this specification.













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   [22] Erik Nordmark.  Securing MIPv6 BUs using return routability
        (BU3WAY).  Internet Draft draft-nordmark-mobileip-bu3way-00.txt
        (Work In Progress), IETF, November 2001.

   [23] Erik Nordmark, Gabriel Montenegro, Pekka Nikander, and Jari
        Arkko.  Mobile IPv6 Security Design Rationale.  To appear, 2002.

   [24] C. Perkins.  IP Encapsulation within IP.  Request for Comments
        (Proposed Standard) 2003, Internet Engineering Task Force,
        October 1996.

   [25] C. Perkins.  IP Mobility Support.  Request for Comments
        (Proposed Standard) 2002, Internet Engineering Task Force,
        October 1996.



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   [26] C. Perkins.  Minimal Encapsulation within IP.  Request for
        Comments (Proposed Standard) 2004, Internet Engineering Task
        Force, October 1996.

   [27] Charles Perkins and David B. Johnson.  Route Optimization in
        Mobile IP, February 1999.  Work in progress.

   [28] D. Piper.  The Internet IP Security Domain of Interpretation for
        ISAKMP.  Request for Comments (Proposed Standard) 2407, Internet
        Engineering Task Force, November 1998.

   [29] D. C. Plummer.  Ethernet Address Resolution Protocol:  Or
        converting network protocol addresses to 48.bit Ethernet address
        for transmission on Ethernet hardware.  Request for Comments
        (Standard) 826, Internet Engineering Task Force, November 1982.

   [30] J. Postel.  User Datagram Protocol.  Request for Comments
        (Standard) 768, Internet Engineering Task Force, August 1980.

   [31] J. Postel.  Transmission Control Protocol.  Request for Comments
        (Standard) 793, Internet Engineering Task Force, September 1981.

   [32] J. Reynolds and J. Postel.  Assigned Numbers.  Request for
        Comments (Standard) 1700, Internet Engineering Task Force,
        October 1994.

   [33] 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.

   [34] 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.

   [35] S. Thomson and T. Narten.  IPv6 Stateless Address
        Autoconfiguration.  Request for Comments (Draft Standard) 2462,
        Internet Engineering Task Force, December 1998.


A. 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-ipv6-15.txt:


A.1. Changes from Draft Version ...-15

    -  A binding update authorization mechanism suitable for use
       between previously unknown peers in the global Internet has been



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       incorporated to the specification.  As a result, Sections 4.5,
       5.1, 14 and others have been substantially revised.

    -  A new IPv6 protocol has replaced IPv6 Destination Options for
       some of the MIPv6 signaling.  This was done in order to enable
       the use of standard IPsec for the protection of binding updates
       between the mobile node and the home agent, the protection of
       RR packets as they are forwarded to the mobile node from the
       home agent, and possibly in the future the protection of binding
       updates themselves to the correspondent nodes.  This has resulted
       in substantial modifications in Section 5.

    -  The use of the Home Address Option has been restricted to the
       situation where a binding already exists.  This has been done
       in order to limit distributed Denial-of-Service attacks through
       reflections attacks that employ the Home Address Option.

    -  A new Binding Missing message has been added to signal the
       mobile node that it has used the Home Address Option when the
       correspondent node has no existing binding to the node.

    -  The Authentication Data suboption has been made a part of
       the Binding Update and Acknowledgement messages, and is now
       calculated in the specific manner required by the authorization
       mechanism (RR).

    -  Sequence number length for Binding Update messages has been
       increased to 32 bits to protect home registrations against replay
       attacks.

    -  Mobile IPv6 uses now Routing Header type 2 instead of the
       general type 0, in order to limit potential dangers that
       general capabilities offers type 0 and to ensure that firewall
       administrators want to allow the type of Routing Header that
       Mobile IPv6 uses through.

    -  Requirements for all IPv6 routers have also been updated in order
       to describe the considerations relating to the new Routing Header
       type.

    -  Processing rules for mobile nodes, correspondent nodes, and to
       some extent home agents have been substantially modified in order
       to explain the new authorization scheme.

    -  Piggybacking is no longer possible due to the use of a new IPv6
       protocol and not a Destination Option.  (However, a separate
       extension to this specification will allow piggybacking and takes
       in account the necessary IPsec policy considerations to avoid
       problems.)





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    -  The security considerations in Section 14 have been revised to
       describe the threats that this specification protects against as
       well as any residual threats.


A.2. Changes from Previous Versions of the Draft

    -  Strengthened mandates for mobile nodes so that now a mobile node
       MUST support decapsulation and processing for routing headers
       (section 10.3).

    -  Enabled ESP to be a valid way to secure reverse tunneled packets
       (section 9.5).

    -  Removed mandate that mobile node select a default router,
       and instead described it as typical behavior (section 10.4).
       Also made it clear that picking a new default router does not
       automatically mean picking a new primary care-of address.

    -  Modified mandated behavior from Home Agent upon reception of a
       `D' bit in a Binding Update.  The home agent only has to make
       sure that DAD has been run, and that no other node on the home
       network could be using the mobile node's link-local address.

    -  Added provisional ICMP numbers for the new message types, which
       may be reassigned by IANA, but which will be useful for testing
       purposes.

    -  Removed the Mobile Router Prefix Length Sub-Option

    -  Removed the Prefix Length field from the Binding Update, and
       references to error number 136.

    -  Added the `S' bit so that the home agent can be instructed to
       *override* its default behavior.  That is, with the `S' bit
       set, the home agent will not attempt to be helpful by changing
       multiple Binding Cache entries, for multiple routing prefixes,
       after receiving only one Binding Update.

    -  Reworded the specification so that the Home Agent now has to
       perform Duplicate Address Detection for the mobile node's address
       on all the prefixes for which the router is performing home agent
       service.

    -  Removed the section about Mobile Routers

    -  Added the Authentication Data Sub-option; reorganized the section
       about computing authentication data.

    -  Specified that the Home Agent lifetime is by default the same as
       the Router lifetime, in a Router Advertisement.



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    -  Specified that Binding Updates with zero lifetime and the 'A' bit
       set should cause a Binding Acknowledgement to be sent back to the
       Source IP address of the Binding Update.

    -  Qualified the allowable times when a mobile node can send a
       Binding Update to a correspondent node

    -  Added text allowing the correspondent node to extend an existing
       Routing Header by also including the care-of address as the entry
       of a routing header to be visited immediately before the home
       address.  In this way, for instance, the mobile node can be an
       intermediate node of a path along the way to some other node.

    -  Removed the Home Address field from the Home Agent Address
       Discovery Request Message.

    -  Noted that ICMP Unreachable forms a potential mechanism by which
       a malicious node can cause a correspondent node to delete a valid
       entry from its Binding Cache.

    -  Specified that, when a router stops offering home agent services
       by turning off the 'H' flag, the mobile node has to delete the
       corresponding entry from its Home Agent list.

    -  Clarified language about how the aggregate list of prefixes is
       built by the home agent, to include only prefixes with the 'H'
       bit set.

    -  Specified a new error status (141) to handle cases for sequence
       number mismatches (e.g., when a mobile node reboots).

    -  Moved this section to the appendix, and reorganized other
       appendix sections.

    -  Reorganized some related sections to be adjacent to each other.

    -  Changed the Prefix Length of the Binding Update to be 7-bit only,
       in order to reserve more flag bits for the future.

    -  Changed the Sequence Number of the Binding Update and Binding
       Acknowledgement to be 8-bit only.

    -  Inserted specification that, after returning home and sending a
       Neighbor Solicitation to the home agent, a mobile node should
       accept any Neighbor Advertisement from the home agent as an
       indication that the home agent is REACHABLE.

    -  Inserted new terminology for Binding Key and Binding Security
       Association (BSA) in anticipation of eliminating the use of AH





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    -  Eliminated use of AH for authenticating Binding Update, and for
       authenticating Binding Acknowledgement

    -  Specified that all correspondent nodes MUST implement a base
       protocol for establishing a Binding Key; this has become the
       Return Routability protocol in this document.

    -  Added the following protocol constants:

          INITIAL_SOLICIT_TIMER:

    -  Created new ICMP messages for Mobile Prefix Solicitations and
       Advertisements (see sections 5.8 and 5.9).

    -  Changed Network Renumbering (Section 9.7) to encompass mobile
       node configuration issues, remove unspecified address usage,
       simplify rules for prefix maintenance and sending, and use new
       ICMP message types noted above.

    -  Added a paragraph to Returning Home (section 10.22) to describe
       how the Home Agent discovers the mobile node's link-layer address

    -  Reworded parts of appendix B as needed.

    -  Added the Mobile Router Prefix Length Sub-Option (section 5.5),
       along with text describing what a Mobile Router should do with
       it.


B. 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 using stateless or stateful
       autoconfiguration.

    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.




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    4. Receive Home Agent Address Discovery Reply message.

    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.


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
















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Authors' Addresses

   QuestionsDaboutathisvdocumenticandalsoBbe.directedJtoothehauthors:nson

     Rice University                   Charles Perkins
     Department of Computer Science,   Nokia
     MS 132                            313 Fairchild Drive
     6100 Main Street                  Mountain View, CA 94043
     Houston, TX 77005-1892            USA
     USA                               Phone:  +1 650 625-2986

     Phone:  +1 713 348-3063           Fax:  +1 650 625-2502
     Fax:  +1 713 348-5930             E-mail:  charliep@iprg.nokia.com
     E-mail:  dbj@cs.rice.edu








































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