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


                        Mobility Support in IPv6
                   <draft-ietf-mobileip-ipv6-17.txt>


Status of This Memo

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

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

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

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

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

   This document specifies the operation 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 a new IPv6 protocol and a new
   destination option.  All IPv6 nodes, whether mobile or stationary,
   MUST support communications with mobile nodes.










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                                Contents


Status of This Memo                                                    i

Abstract                                                               i

 1. Introduction                                                       1

 2. Comparison with Mobile IP for IPv4                                 2

 3. Terminology                                                        4
     3.1. General Terms . . . . . . . . . . . . . . . . . . . . . .    5
     3.2. Mobile IPv6 Terms . . . . . . . . . . . . . . . . . . . .    6

 4. Overview of Mobile IPv6                                            9
     4.1. Basic Operation . . . . . . . . . . . . . . . . . . . . .    9
     4.2. New IPv6 Protocols  . . . . . . . . . . . . . . . . . . .   12
     4.3. New IPv6 Destination Options  . . . . . . . . . . . . . .   13
     4.4. New IPv6 ICMP Messages  . . . . . . . . . . . . . . . . .   14
     4.5. Conceptual Data Structures  . . . . . . . . . . . . . . .   15
     4.6. Binding Management  . . . . . . . . . . . . . . . . . . .   16

 5. Overview of Mobile IPv6 Security                                  17
     5.1. Threats . . . . . . . . . . . . . . . . . . . . . . . . .   17
     5.2. Features  . . . . . . . . . . . . . . . . . . . . . . . .   18
     5.3. Tunnels to and from the Home Agents . . . . . . . . . . .   20
     5.4. Binding Updates to Home Agents  . . . . . . . . . . . . .   20
     5.5. Binding Updates to Correspondent Nodes  . . . . . . . . .   21
           5.5.1. Node Keys . . . . . . . . . . . . . . . . . . . .   22
           5.5.2. Nonces  . . . . . . . . . . . . . . . . . . . . .   23
           5.5.3. Cookies . . . . . . . . . . . . . . . . . . . . .   23
           5.5.4. Cryptographic Functions . . . . . . . . . . . . .   24
           5.5.5. Return Routability Procedure  . . . . . . . . . .   24
           5.5.6. Applying Return Routability for Correspondent
                          Bindings . . . . . . . . . . . . . . . . .  28
           5.5.7. Updating Node Keys and Nonces . . . . . . . . . .   29
           5.5.8. Preventing Replay Attacks . . . . . . . . . . . .   30
           5.5.9. Preventing Denial-of-Service Attacks  . . . . . .   30
          5.5.10. Correspondent Binding Procedure Extensibility . .   31

 6. New IPv6 Protocols, Message Types, and Destination Option         31
     6.1. Mobility Header . . . . . . . . . . . . . . . . . . . . .   31
           6.1.1. Format  . . . . . . . . . . . . . . . . . . . . .   32
           6.1.2. Binding Refresh Request (BRR) Message . . . . . .   33
           6.1.3. Home Test Init (HoTI) Message . . . . . . . . . .   34
           6.1.4. Care-of Test Init (CoTI) Message  . . . . . . . .   36
           6.1.5. Home Test (HoT) Message . . . . . . . . . . . . .   37
           6.1.6. Care-of Test (CoT) Message  . . . . . . . . . . .   39



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           6.1.7. Binding Update (BU) Message . . . . . . . . . . .   41
           6.1.8. Binding Acknowledgement (BA) Message  . . . . . .   45
           6.1.9. Binding Error (BE) Message  . . . . . . . . . . .   49
     6.2. Mobility Options  . . . . . . . . . . . . . . . . . . . .   51
           6.2.1. Format  . . . . . . . . . . . . . . . . . . . . .   51
           6.2.2. Pad1  . . . . . . . . . . . . . . . . . . . . . .   52
           6.2.3. PadN  . . . . . . . . . . . . . . . . . . . . . .   52
           6.2.4. Unique Identifier . . . . . . . . . . . . . . . .   53
           6.2.5. Alternate Care-of Address . . . . . . . . . . . .   53
           6.2.6. Nonce Indices . . . . . . . . . . . . . . . . . .   54
           6.2.7. Binding Authorization Data  . . . . . . . . . . .   54
     6.3. Home Address Destination Option . . . . . . . . . . . . .   55
     6.4. Routing Header type 2 . . . . . . . . . . . . . . . . . .   58
           6.4.1. Routing Header Packet format  . . . . . . . . . .   58
     6.5. ICMP Home Agent Address Discovery Request Message . . . .   59
     6.6. ICMP Home Agent Address Discovery Reply Message . . . . .   61
     6.7. ICMP Mobile Prefix Solicitation Message Format  . . . . .   63
     6.8. ICMP Mobile Prefix Advertisement Message Format . . . . .   65

 7. Modifications to IPv6 Neighbor Discovery                          67
     7.1. Modified Router Advertisement Message Format  . . . . . .   67
     7.2. Modified Prefix Information Option Format . . . . . . . .   68
     7.3. New Advertisement Interval Option Format  . . . . . . . .   70
     7.4. New Home Agent Information Option Format  . . . . . . . .   71
     7.5. Changes to Sending Router Advertisements  . . . . . . . .   73
     7.6. Changes to Sending Router Solicitations . . . . . . . . .   74

 8. Requirements for Types of IPv6 Nodes                              75
     8.1. Requirements for All IPv6 Hosts and Routers . . . . . . .   75
     8.2. Requirements for All IPv6 Routers . . . . . . . . . . . .   75
     8.3. Requirements for IPv6 Home Agents . . . . . . . . . . . .   76
     8.4. Requirements for IPv6 Mobile Nodes  . . . . . . . . . . .   77

 9. Correspondent Node Operation                                      78
     9.1. Conceptual Data Structures  . . . . . . . . . . . . . . .   78
     9.2. Receiving Packets from a Mobile Node  . . . . . . . . . .   79
           9.2.1. Processing Mobility Header (MH) Messages  . . . .   79
           9.2.2. Receiving Packets with Home Address Destination
                          Option . . . . . . . . . . . . . . . . . .  80
     9.3. Return Routability Procedure  . . . . . . . . . . . . . .   80
           9.3.1. Receiving HoTI Messages . . . . . . . . . . . . .   81
           9.3.2. Receiving CoTI Messages . . . . . . . . . . . . .   81
           9.3.3. Sending HoT Messages  . . . . . . . . . . . . . .   82
           9.3.4. Sending CoT Messages  . . . . . . . . . . . . . .   82
     9.4. Processing Bindings . . . . . . . . . . . . . . . . . . .   82
           9.4.1. Receiving Binding Updates . . . . . . . . . . . .   82
           9.4.2. Requests to Cache a Binding . . . . . . . . . . .   84
           9.4.3. Requests to Delete a Binding  . . . . . . . . . .   84
           9.4.4. Sending Binding Acknowledgements  . . . . . . . .   85
           9.4.5. Sending Binding Refresh Requests  . . . . . . . .   86
           9.4.6. Sending Binding Error Messages  . . . . . . . . .   87



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     9.5. Cache Replacement Policy  . . . . . . . . . . . . . . . .   87
     9.6. Sending Packets to a Mobile Node  . . . . . . . . . . . .   88
     9.7. Receiving ICMP Error Messages . . . . . . . . . . . . . .   89

10. Home Agent Operation                                              90
    10.1. Conceptual Data Structures  . . . . . . . . . . . . . . .   90
    10.2. Primary Care-of Address Registration  . . . . . . . . . .   91
    10.3. Primary Care-of Address De-Registration . . . . . . . . .   94
    10.4. Intercepting Packets for a Mobile Node  . . . . . . . . .   95
    10.5. Tunneling Intercepted Packets to a Mobile Node  . . . . .   97
    10.6. Handling Reverse Tunneled Packets from a Mobile Node  . .   98
    10.7. Protecting Return Routability Packets . . . . . . . . . .   99
    10.8. Receiving Router Advertisement Messages . . . . . . . . .   99
    10.9. Dynamic Home Agent Address Discovery  . . . . . . . . . .  101
          10.9.1. Aggregate List of Home Network Prefixes . . . . .  102
          10.9.2. Scheduling Prefix Deliveries to the Mobile Node .  104
          10.9.3. Sending Advertisements to the Mobile Node . . . .  106
          10.9.4. Lifetimes for Changed Prefixes  . . . . . . . . .  107

11. Mobile Node Operation                                            107
    11.1. Conceptual Data Structures  . . . . . . . . . . . . . . .  107
    11.2. Packet Processing . . . . . . . . . . . . . . . . . . . .  110
          11.2.1. Sending Packets While Away from Home  . . . . . .  110
          11.2.2. Interaction with Outbound IPsec Processing  . . .  112
          11.2.3. Receiving Packets While Away from Home  . . . . .  114
          11.2.4. Routing Multicast Packets . . . . . . . . . . . .  116
    11.3. Home Agent and Prefix Management  . . . . . . . . . . . .  116
          11.3.1. Receiving Local Router Advertisement Messages . .  116
          11.3.2. Dynamic Home Agent Address Discovery  . . . . . .  118
          11.3.3. Sending Mobile Prefix Solicitations . . . . . . .  119
          11.3.4. Receiving Mobile Prefix Advertisements  . . . . .  120
    11.4. Movement  . . . . . . . . . . . . . . . . . . . . . . . .  121
          11.4.1. Movement Detection  . . . . . . . . . . . . . . .  121
          11.4.2. Forming New Care-of Addresses . . . . . . . . . .  124
          11.4.3. Using Multiple Care-of Addresses  . . . . . . . .  125
    11.5. Return Routability Procedure  . . . . . . . . . . . . . .  126
          11.5.1. Sending Home and Care-of Test Init Messages . . .  126
          11.5.2. Receiving Return Routability Messages . . . . . .  126
          11.5.3. Retransmitting in the Return Routability Procedure 128
          11.5.4. Rate Limiting for Return Routability Procedure  .  128
    11.6. Processing Bindings . . . . . . . . . . . . . . . . . . .  128
          11.6.1. Sending Binding Updates to the Home Agent . . . .  128
          11.6.2. Correspondent Binding Procedure . . . . . . . . .  130
          11.6.3. Receiving Binding Acknowledgements  . . . . . . .  133
          11.6.4. Receiving Binding Refresh Requests  . . . . . . .  134
          11.6.5. Receiving Binding Error Messages  . . . . . . . .  135
          11.6.6. Forwarding from a Previous Care-of Address  . . .  136
          11.6.7. Returning Home  . . . . . . . . . . . . . . . . .  137
          11.6.8. Retransmitting Binding Updates  . . . . . . . . .  139
          11.6.9. Rate Limiting Binding Updates . . . . . . . . . .  140
    11.7. Receiving ICMP Error Messages . . . . . . . . . . . . . .  140



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12. Protocol Constants                                               141

13. IANA Considerations                                              142

14. Security Considerations                                          143
    14.1. Security for the Tunneling to and from the Home Agent . .  143
    14.2. Security for the Binding Updates to the Home Agent  . . .  144
    14.3. Security for the Binding Updates to the Correspondent
             Nodes  . . . . . . . . . . . . . . . . . . . . . . . .  144
    14.4. Security for the Home Address Destination Option  . . . .  145
    14.5. Firewall considerations . . . . . . . . . . . . . . . . .  145

Acknowledgements                                                     146

References                                                           147

 A. State Machine for the Correspondent Binding Procedure            150

 B. Changes from Previous Version of the Draft                       159
     B.1. Changes from Draft Version 16 . . . . . . . . . . . . . .  159
     B.2. Changes from Draft Version 15 . . . . . . . . . . . . . .  161
     B.3. Changes from Earlier Versions of the Draft  . . . . . . .  162

 C. Remote Home Address Configuration                                164

 D. Future Extensions                                                165
     D.1. Piggybacking  . . . . . . . . . . . . . . . . . . . . . .  165
     D.2. Triangular Routing and Unverified Home Addresses  . . . .  166
     D.3. New Authorization Methods beyond Return Routability . . .  166

Chairs' Addresses                                                    167

Authors' Addresses                                                   167





















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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 defined in this document, known as Mobile IPv6, allows
   a mobile node to move from one link to another without changing the
   mobile node's IP address.  A mobile node is always addressable by
   its "home address", an IP address assigned to the mobile node within
   its home subnet prefix on its home link.  Packets may be routed to
   the mobile node using this address regardless of the mobile node's
   current point of attachment to the Internet, 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 (but
       see Section 11.4.1).

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

    -  Assistance for adaptive applications

    -  Mobile routers

    -  Service Discovery

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


2. Comparison with Mobile IP for IPv4

   The design of Mobile IP support in IPv6 (Mobile IPv6) represents a
   natural combination of the experiences gained from the development
   of Mobile IP support in IPv4 (Mobile IPv4) [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 Route Optimization to coexist efficiently
       with routers that perform "ingress filtering" [7].  A mobile



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       node 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 or stationary, whether
       host or router.

    -  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
       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 [33], to operate in
       any location away from home without any special support required
       from the 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




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       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 decouples Mobile IP from any
       particular link layer, unlike 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
       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
       uses IPv4 directed broadcast and returns 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 has to be sent back to the mobile node.

    -  Mobile IPv6 defines an Advertisement Interval option for
       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.

    -  The return routability procedure (see section 5.5) provides a
       way to verify the that a mobile node is reachable at its claimed
       home address and at its claimed care-of address.  This allows
       correspondent nodes to verify the authority of the Binding
       Updates sent to it.  Given that the return routability procedure
       is light-weight and does not require participation in a security
       infrastructure, it is expected that Route Optimization can
       be deployed on a global scale between all mobile nodes and
       correspondent nodes.


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].





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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.






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      security association

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

      security policy database

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

      destination option

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


3.2. Mobile IPv6 Terms

      home address

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

      home subnet prefix

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

      home link

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

      mobile node

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

      movement

         A change in a mobile node's point of attachment to the Internet
         such that it is no longer connected to the same link as it was
         previously.  If a mobile node is not currently attached to its
         home link, the mobile node is said to be "away from home".





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      correspondent node

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

      foreign subnet prefix

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

      foreign link

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

      care-of address

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

      home agent

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

      binding

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

      binding procedure

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

      binding authorization

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





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      return routability procedure

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

      correspondent binding procedure

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

      home binding procedure

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

      nonce

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

      cookie

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

      care-of cookie

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

      home cookie

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

      mobile cookie

         A cookie sent to the correspondent node from the mobile node,
         and later returned to the mobile node.  Mobile cookies are
         produced randomly.

      nonce index

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



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      binding key

         a key used for authenticating binding cache management
         messages.

      binding security association

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


4. Overview of Mobile IPv6

4.1. Basic Operation

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

   While a mobile node is attached to some foreign link away from home,
   it is also addressable at one or more care-of addresses, 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 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 [33] 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.  The operation of the mobile node is specified in
   Section 11.

   While away from home, a mobile node registers one of its care-of
   addresses with a router on its home link, requesting this router to
   function as the "home agent" for the mobile node.  The mobile node
   performs this binding registration by sending a "Binding Update"
   message to the home agent; the home agent then replies to the mobile



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   node by returning a "Binding Acknowledgement" message.  The care-of
   address associated with this binding registration is known as the
   mobile node's "primary care-of address".  The mobile node's home
   agent thereafter uses proxy Neighbor Discovery to intercept any
   IPv6 packets addressed to the mobile node's home address (or home
   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.  The operation of the home agent is
   specified in Section 10.

   The Binding Update and Binding Acknowledgement messages, together
   with a "Binding Refresh Request" message, are also used to allow IPv6
   nodes communicating with a mobile node are capable of dynamically
   learning and caching the mobile node's binding.  This happens
   through the correspondent binding procedure which involves a return
   routability test in order to authorize the establishment of the
   binding, as specified in Sections 5.5.5 and 5.5.6.  When sending a
   packet to any IPv6 destination, a node checks its cached bindings
   for an entry for the packet's destination address.  If a cached
   binding for this destination address is found, the node uses a new
   type of IPv6 Routing header [6] (see section 6.4) to route the packet
   to the mobile node by way of the care-of address indicated in this
   binding.  If, instead, the sending node has no cached binding for
   this destination address, the node sends the packet normally (with
   no Routing header), and the packet is subsequently intercepted and
   tunneled by the mobile node's home agent as described above.  Any
   node communicating with a mobile node is referred to in this document
   as a "correspondent node" of the mobile node, and may itself be
   either a stationary node or a mobile node.  The operation of the
   correspondent node is specified in Section 9.

   Mobile IPv6 also defines one additional IPv6 destination option.
   When a mobile node sends a packet while away from home, it could
   generally use a tunnel via the home agent to send this packet.
   However, if the correspondent node in question has a binding for this
   mobile node it can use deliver packets more directly.  In this case
   the mobile node can the Source Address in the packet's IPv6 header to
   one of its current care-of addresses, and include a "Home Address"
   destination option in the packet, giving the mobile node's home
   address.  Many routers implement security policies such as "ingress
   filtering" [7] that do not allow forwarding of packets having a
   Source Address that 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, and 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 destination 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



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   care-of address to be transparent above the Mobile IPv6 support level
   (e.g., at the transport layer).  The inclusion of a Home Address
   destination 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
   destination option in a packet.

   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 routers on its home link currently operating
   as a home agent.  This home agent then returns an ICMP "Home Agent
   Address Discovery Reply" message to the mobile node, including a list
   of home agents on the home link.  This procedure is specified in
   Sections 10.9 and 11.3.2.

   When a mobile node 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
   new care-of address.  Since the purpose of a Binding Update is
   to establish exactly this kind of tunneling, it can 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 11.6.6 describes the
   use of the Binding Update for this purpose.

   Section 11.4.3 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 home address belonging to a mobile node, and
   always tunnels packets sent to a mobile node's home address and
   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.







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4.2. New IPv6 Protocols

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

      Home Test Init

         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 subsequent 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 6.1.3.

      Care-of Test Init

         The Care-of Test Init message is used to initiate the
         correspondent routability procedure, for a particular care-of
         address.  The Care-of Test Init message is described in detail
         in Section 6.1.4.

      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 sent in reply to the Home Test
         Init message, and is described in detail in Section 6.1.5.

      Care-of Test

         The Care-of Test message carries another cookie which the
         mobile node needs before it can properly authorize itself for
         sending a Binding Update.  This message is sent in reply to
         the Care-of Test Init message, and is described in detail in
         Section 6.1.6.

      Binding Update

         A Binding Update message is used by a mobile node to notify
         a correspondent node or the mobile node's home agent of its
         current binding.  The Binding Update sent to the mobile node's
         home agent to register its primary care-of address is marked
         as a "home registration".  The Binding Update message and its
         specific authentication requirements are described in detail in
         Section 6.1.7.

      Binding Acknowledgement

         A Binding Acknowledgement message is used to acknowledge
         receipt of a Binding Update, if an acknowledgement was



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         requested in the Binding Update.  The Binding Acknowledgement
         message and its specific authentication requirements are
         described in detail in Section 6.1.8.

      Binding Refresh Request

         A Binding Refresh Request message is used to request that
         a mobile node 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.  The
         Binding Refresh Request message is described in detail in
         Section 6.1.2.

         No authentication is required for the Binding Refresh Request
         message.

      Binding Error

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


4.3. New IPv6 Destination Options

   Mobile IPv6 defines a new IPv6 destination option, the Home Address
   destination option.  This option is 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 above the
   Mobile IPv6 support level.  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 6.3 and 11.2.2 for additional details about requirements
   for the calculation and verification of the authentication data.
   The Home Address destination option is described in detail in
   Section 6.3.







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4.4. New IPv6 ICMP Messages

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

      Home Agent Address Discovery Request

         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 6.5.

      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 6.6.

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

      Mobile Prefix Solicitation

         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
         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 6.7.

      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




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         or other prefix changes.  This message is specified in Section
         Section 10.9.3.


4.5. Conceptual Data Structures

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

      Binding Cache

         A cache, maintained by each IPv6 node, of bindings for other
         nodes.  A separate Binding Cache is maintained by each IPv6
         node for each of its IPv6 addresses.  When sending a packet,
         the Binding Cache is searched before the Neighbor Discovery
         conceptual Destination Cache [20].

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

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

      Binding Update List

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

      Home Agents List

         A list, maintained by each home agent and each mobile node,
         recording information about each home agent from which this
         node has received recent a Router Advertisement in which the
         Home Agent (H) bit is set.  The home agents list is thus
         similar to the Default Router List conceptual data structure
         maintained by each host for Neighbor Discovery [20].

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



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         maintains a Home Agents List, to enable it to notify a home
         agent on its previous link when it moves to a new link.


4.6. 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.  The
   mobile node SHOULD then start a correspondent binding procedure in
   order to establish a binding.  This would allow the correspondent
   node to cache the mobile node's binding for routing future packets to
   it.

   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 Refresh 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 Refresh Request, it MAY reply
   by initiating a correspondent binding procedure.

   A mobile node may use more than one care-of address at the same
   time.  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



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   node's care-of address, so that the home agent is rarely involved
   with packet transmission to the mobile node.  This is important for
   scalability and reliability, and for minimizing overall network load.
   By caching the care-of address of a mobile node, direct delivery 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. Overview of Mobile IPv6 Security

5.1. Threats

   Any mobility solution must protect itself against misuses of the
   mobility features.  In Mobile IPv6, most of the potential threats
   are concerned with denial of service.  Some of the threats also
   include potential for man-in-the-middle, hijacking, and impersonation
   attacks.  The main threats this protocol protects against are as
   follows:

    1. Threats against Binding Updates sent to home agents and
       correspondent nodes.  For instance, an attacker might claim that
       a certain mobile node is currently at a different location than
       it really is.  If 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 did not want.

       Similarly, a malicious mobile node might use the home address of
       a victim node in a forged Binding Update to a correspondent node.
       If such Binding Updates were accepted, the communications between
       the correspondent node and the victim would be then 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 also send 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 such Binding
       Updates were accepted, the malicious mobile node could force the
       correspondent node into sending data to the victim node or the
       victim network; the correspondent node's replies to messages sent
       by the malicious mobile node will be sent to the victim host
       or network.  This could be used to cause a distributed denial
       of service attack.  Variations of this threat are described
       elsewhere [1][31].



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       A malicious node might also send a large number of invalid
       Binding Updates to a victim node.  If each Binding Update takes a
       significant amount of resources (such as CPU) to process before
       it can be recognized either as valid or as invalid, then a denial
       of service attack can be caused by sending the correspondent node
       so many invalid Binding Updates that it has no resources left for
       other tasks.

       An attacker might also attempt to disrupt a mobile node's
       communications by replaying a Binding Update that the node had
       sent earlier.  If the old Binding Update was accepted, packets
       destined for the mobile node would be sent to its old location
       and not its current location.

    2. Reflection attack threats against third partied with the help
       of Mobile IPv6 correspondent nodes that do not use appropriate
       security precautions.  The Home Address destination option can be
       used to direct response traffic toward a node whose IP address
       appears in the option, without allowing ingress filtering to
       catch the forged "return address"  [32] [23].

    3. Threats where an attacker forges tunneled packets between the
       mobile node and the home agent, making it appear that the traffic
       is coming from the mobile node when it is not.

    4. Threats against IPv6 functionality used by Mobile IPv6, such as
       the Routing header.  The generality of the regular Routing Header
       would allow circumvention of IP-address based rules in firewalls
       or reflection of traffic to other nodes, even if the usage that
       Mobile IPv6 requires is safe.

    5. The security mechanisms of Mobile IPv6 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.


5.2. 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
       destination 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 Mobile IPv6 security
       mechanisms themselves.

   Protecting the Binding Updates to home agents and to arbitrary
   correspondent nodes require very different security solutions due
   to the different situations.  Mobile nodes and home agents are
   expected to be naturally subject to the network administration of
   the home domain, and thus to have a strong security association to
   reliably authenticate the exchanged messages.  With such a security
   arrangement, IPsec Encapsulating Security Payload (ESP) can be used
   to implement the necessary security features.  See Section 5.4.

   It is expected that Mobile IPv6 will be used on a global basis
   between nodes belonging to different administrative domains.
   Building an authentication infrastructure to authenticate mobile
   nodes and correspondent nodes would be a very demanding task in this
   scale.  Furthermore, 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.  Thus, an "infrastructureless" approach is necessary.

   The chosen infrastructureless 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 nodes
   in question, and by requiring that the eventual Binding Update is
   cryptographically bound to the exchanged cookies.  Some additional
   protection is provided by requiring the cookies be protected by
   ESP when exchanged between the mobile node and the correspondent
   node via the home agent.  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 destination option can be solved as
   follows:  We ensure that the mobile node is authorized to use a given
   home address before this option can be used.  Such authorization is
   already performed in the context of Route Optimization, and therefore
   this specification limits the use of the Home Address option 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 5.3.




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   Potential abuses of the Routing Header can be prevented by using a
   Mobile IPv6 specific type of a Routing Header.  This type provides
   the necessary functionality but does not open vulnerabilities.

   Denial-of-Service threats against Mobile IPv6 security mechanisms
   themselves concern mainly the Binding Update procedures with
   correspondent nodes.  The protocol has been designed to limit the
   effects of such attacks, as will be described in Section 5.5.9.


5.3. Tunnels to and from the Home Agents

   Mobile IPv6 tunneling -- as tunneling in general --  needs protection
   so that it isn't possible, e.g., for anyone to pose as the home agent
   and send traffic to the mobile node.  To protect the tunnels to the
   mobile node, the mobile node verifies that the outer IP address
   corresponds to its home agent, to prevent attacks against the tunnel
   from other IP addresses.

   Tunnels from the mobile node to 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.

   Binding Updates sent to the home agents are secure.  The home
   agent verifies that the outer IP address corresponds to the current
   location of the mobile node, to prevent attacks against the tunnel
   from other IP addresses.

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


5.4. 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 for integrity protection when a non-null
   authentication algorithm is applied.

   However, IPsec can easily provide replay protection only if dynamic
   security association establishment is used.  This may not always be
   possible, and manual keying would be preferred in some cases.  IPsec
   also does not guarantee correct ordering of packets, only that they
   have not been replayed.  Because of this, Mobile IPv6 provides its
   own mechanism inside the Binding Update and Acknowledgement messages.



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   A sequence number field is used to ensure correct ordering.  If the
   mobile node reboots and forgets its current sequence number, the home
   agent uses the status value 141 (Sequence number out of window, see
   Section 6.1.8) to inform the mobile node of the use of an improper
   sequence number.

   Note that the the sequence number mechanism provides also a weak form
   of replay protection.  However, if a home agent reboots and loses its
   state regarding the sequence numbers, replay attacks become possible.
   If the home agent is vulnerable to this, the use of a key management
   mechanism together with IPsec can be used to prevent replay attacks.

   A sliding window scheme is used for the sequence numbers.  The
   protection against replays and reordering attacks without a key
   management mechanism works when the attacker remembers up to a
   maximum of 2**15 Binding Updates.

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


5.5. Binding Updates to Correspondent Nodes

   Binding Updates to correspondent nodes are protected using the return
   routability procedure.  The motivation for designing the return
   routability procedure was to have sufficient support for Mobile IP,
   without creating major new security problems.  It was not our goal
   to protect against attacks that were already possible before the
   introduction of Mobile IP. 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 regardless of whether
   Mobile IP is in use.

   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 node
   to send the victim traffic that it does not expect.  For example,



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   suppose that the correspondent node 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
   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 node 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 rationale of the return
   routability procedure, see [1] [31] [22] [23].

   The return routability procedure method uses the following
   principles:

    -  A cookie exchange verifies that the mobile node is reachable at
       its addresses i.e.  is at least able to transmit and receive
       traffic at its addresses.

    -  The eventual Binding Update is protected cryptographically using
       the cookies.

    -  Requiring that the cookies be protected by ESP when forwarded by
       the home agent to the mobile node.

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

    -  Correspondent nodes operate in a stateless manner until they
       receive a Binding Update that can be authorized.

   The return routability procedure 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.


5.5.1. Node Keys

   Each correspondent node has a secret key, Kcn.  This key is used by
   the correspondent node to accept only the use of cookies which it has
   created itself.  This key does not need to be shared with any other
   entity, so no key distribution mechanism is needed for it.

   A correspondent node can generate a fresh Kcn each time that it boots
   to avoid the need for secure persistent storage for Kcn.  Kcn can be



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   either a fixed value or regularly updated.  Procedures for updating
   Kcn are discussed later in Section 5.5.7.

   Kcn consists of 20 octets.


5.5.2. Nonces

   Each correspondent node also generates a nonce at regular intervals,
   for example every few minutes.  A correspondent node uses the same
   Kcn and nonce with all the mobiles it is in communication with, so
   that it does not need to generate and store a new nonce when a new
   mobile contacts it.  Each nonce is identified by a nonce index.
   Nonce indices are 16-bit values that are e.g.  incremented each time
   a new nonce is created.  The index value is communicated in the
   protocol, so that if a nonce is replaced by new nonce during the run
   of a protocol, the correspondent node can distinguish messages that
   should be checked against the old nonce from messages that should be
   checked against the new nonce.  Correspondent nodes keep both the
   current nonce and a small set of old nonces.  Older values can be
   discarded, and messages using them will be rejected as replays.

   The specific nonce index values can not be used by mobile nodes to
   determine the validity of the nonce.  Expected validity times for
   the nonces values and the procedures for updating them are discussed
   later in Section 5.5.7.

   Nonce is an octet string of any length.  The recommended length is 16
   octets.


5.5.3. Cookies

   Three different types of cookies are used in the protocol:

    -  Mobile cookie is sent to the correspondent node from the mobile
       node, and later returned to the mobile node.  Mobile cookies are
       produced randomly, and used to verify that the response matches
       the request, and to ensure that parties who have not seen the
       request can not spoof responses.

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

    -  A care-of cookie sent directly to the mobile node from the
       correspondent node.  Home cookies are produced cryptographically
       from nonces.

   Mobile cookies are typically newly generated random values for each
   new request that needs them.  They could also be changed periodically



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   only.  The policy to use new or old mobile cookies is purely a local
   matter for the mobile node.

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


5.5.4. Cryptographic Functions

   MAC_K(m) denotes a Message Authentication Code computed on message
   m with key K. In this specification, HMAC SHA1 function [15][21] is
   used to compute these codes.

   H(m) denotes a hash of message m.  In this specification, SHA1
   function [21] is used to compute the hash.


5.5.5. Return Routability Procedure

   The return routability signaling happens as follows:


Mobile node                 Home agent           Correspondent node
     |                                                     |
     |  Home Test Init(HoTI)                               |
     |  Src = home address,                                |
     |  Dst = correspondent     |                          |
     |  Parameters:             |                          |
     |     - mobile cookie 1    |                          |
     |------------------------->|------------------------->|
     |                          |                          |
     |                                                     |
     |  Care-of Test Init(CoTI)                            |
     |  Src = care-of address                              |
     |  Dst = correspondent                                |
     |  Parameters:                                        |
     |     - mobile cookie 2                               |
     |---------------------------------------------------->|
     |                                                     |
     |                              Home Test (HoT)        |
     |                              Src = correspondent,   |
     |                              Dst = home address     |
     |                              Parameters:            |
     |                               - mobile cookie 1     |
     |                          |    - home cookie         |
     |                          |    - home nonce index    |
     |<-------------------------|<-------------------------|
     |                          |                          |



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     |                                                     |
     |                              Care-of Test(CoT)      |
     |                              Src = correspondent,   |
     |                              Dst = care-of address  |
     |                              Parameters:            |
     |                               - mobile cookie 2     |
     |                               - care-of cookie      |
     |                               - care-of nonce index |
     |<----------------------------------------------------|
     |                                                     |

   The HoTI and CoTI messages are sent at the same time.  The
   correspondent node returns the HoT and CoT messages as quickly as
   possible, and perhaps nearly simultaneously, requiring very little
   processing.  The four messages form the return routability procedure.
   (After the return routability procedure, a binding will be created
   with a single request with an optional response.)  Due to the
   simultaneous sending of messages, the return routability procedure
   completes in 1 roundtrip (and the whole process completes in 1.5
   roundtrips excluding the acknowledgement message).

   The four messages (HoTI, CoTI, HoT, and CoT) belonging to the return
   routability procedure are described in more detail below.  The use of
   the results of the return routability procedure for authenticating a
   correspondent binding procedure is described in Section 5.5.6.

      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.

           Src = home address
           Dst = correspondent
           Parameters:
            - mobile cookie 1

         This message conveys the mobile node's home address to the
         correspondent node.  The mobile node also sends along mobile
         cookie C0 that the correspondent node must return later,
         along with its own cookie that it generates based on the home
         address.  The HoTI message is reverse tunneled through the home
         agent.

      CoTI

         Care-of Test Init Message:




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         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.

           Src = care-of address
           Dst = correspondent
           Parameters:
            - mobile cookie 2

         The second message is sent in parallel with the first one.  It
         conveys the mobile node's care-of address to the correspondent
         node.  The mobile node also sends along mobile cookie C1 that
         the correspondent node must return later, along with its own
         cookie that it generates based on the care-of address.  The
         CoTI message is sent directly to the correspondent node.

      HoT

         Home Test Message:

         This message is sent in response to a HoTI message.

           Src = correspondent
           Dst = home address
           Parameters:
           - mobile cookie 1
           - home cookie
           - home nonce index

         When the correspondent node receives the HoTI message, it
         generates a 16 octet home cookie as follows:

             home cookie = MAC_Kcn(home address | nonce)

         The cookie is sent in the message to the mobile node via the
         Home Agent; it is an assumption of the protocol that the home
         agent - mobile node route is secure.  Home cookie also acts as
         a challenge to test that the mobile can receive messages sent
         to its home address.  Kcn is used in the production of home
         cookie in order to allow the correspondent node to verify that
         the cookies used later really came from itself, without forcing
         the correspondent node to remember a list of all cookies it has
         handed out.

         Mobile cookie 1 from the mobile node is returned as well in the
         HoT message, to ensure that the message comes from someone on
         the path to the correspondent node.






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         The home nonce index is carried along in the protocol to allow
         the correspondent node to later efficiently find the nonce
         value Ni that it used in creating this cookie.

      CoT

         Care-of Test Message:

         This message is sent in response to a CoTI message.

           Src = correspondent
           Dst = care-of address
           Parameters:
           - mobile cookie 2
           - care-of cookie
           - care-of nonce index

         The correspondent node also sends a challenge to the mobile's
         care-of address.  When the correspondent node receives the CoTI
         message, it generates a 16 octet care-of cookie as follows:

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

         The cookie is sent directly to the mobile node at its care-of
         address.  Mobile cookie 2 from the mobile node is returned as
         well, to ensure that the message comes from someone on the path
         to the correspondent node.

         Again, an index is sent along the cookie in order to identify
         the used nonce.  Note that home and care-of nonce indices are
         likely to be the same in HoT and CoT messages, except when
         the correspondent node changed its nonce value between the
         reception of HoTI and the CoTI messages.

   When the mobile node has received both the HoT and CoT messages, the
   return routability procedure is complete.  As a result, the mobile
   node has the means to prove its authority to send a Binding Update
   to the correspondent node.  The mobile node hashes together the
   challenges to form a 20 octet session key (Kbu):

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

   Note that the correspondent node has not created any state at this
   point.  It is unaware of the session key Kbu, though it can recreate
   Kbu if it is presented the right addresses and nonce indices.









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5.5.6. Applying Return Routability for Correspondent Bindings

   After the return routability procedure, the mobile node can proceed
   to perform a binding procedure with the correspondent node.  An
   overview of the binding procedure is shown below.

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

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

      1.

         Binding Update (BU) Message:

         The mobile node uses the created session key Kbu to authorize
         the Binding Update.

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





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

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

      2.

         Binding Acknowledgement (BA) Message:

         The Binding Update is optionally acknowledged by the
         correspondent node.

           Src = correspondent
           Dst = care-of address
           Parameters:
           - sequence number
           - ...

         The Binding Acknowledgement is not authenticated in other ways
         than including the right sequence number in the reply.  The
         three dots represent all the remaining (not security related)
         information in the message.


5.5.7. Updating Node Keys and Nonces

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

   Before sending a Binding Update in Step 3, the mobile node has
   to wait for both the Home and Care-of Cookies to arrive.  Due
   to resource limitations, rapid deletion of bindings, or reboots
   it can not be guaranteed that the cookies are still fresh and
   acceptable when the correspondent node uses them in the processing
   of the Binding Update.  If the cookies have become too old, the
   correspondent node replies with an an error code in the Binding
   Acknowledgement.  The mobile node can then retry the return
   routability procedure.  However, it is recommended that correspondent



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   nodes try to keep these cookies acceptable as long as possible and
   SHOULD NOT accept them beyond MAX_COOKIE_LIFE seconds.

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


5.5.8. Preventing Replay Attacks

   The return routability procedure 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
   Binding Update, 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 node can batch them
   together to avoid having to increment the nonce index too often.


5.5.9. Preventing Denial-of-Service Attacks

   The return routability procedure 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.  The cookies are specific, but
   they can be reconstructed based on the home and care-of address
   information that arrives with the Binding Update.  This means that
   the correspondent nodes are safe against memory exhaustion attacks
   except where on-path attackers are concerned.  Due to the use of




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   symmetric cryptography, the correspondent nodes are relatively safe
   against CPU resource exhaustion attacks as well.

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

   The binding updates that are used in Mobile IPv6 are only an
   optimization, albeit a very important 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 node to the mobile
   node will be routed via the mobile's home agent rather than a more
   direct route.  A correspondent node can protect itself against some
   of the resource exhaustion attacks by not processing binding updates
   when it is flooded with a large number of binding updates that fail
   the cryptographic integrity checks.  If a correspondent node finds
   that it is spending more resources on checking bogus binding updates
   than it is likely to save by accepting genuine binding updates, then
   it MAY reject some or all Binding Updates without performing any
   cryptographic operations.

   Additional information needed to make this decision 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.  A conformant implementation of the protocols in this
   specification is not required to make use of information from higher
   protocol layers, but implementations are likely to be able to manage
   resources more effectively by making use of such information.


5.5.10. Correspondent Binding Procedure Extensibility

   As discussed in Appendix D.3, in the future there may be other
   mechanisms beyond the return routability procedure for authorizing
   mobile nodes to correspondent nodes.  The nodes can use other methods
   based on future definition of flag values in the Reserved fields of
   HoTI, HoT, CoTI, CoT, and BU messages.  Nodes need assurance against
   bidding down attacks in this selection by following the procedure
   described in Section 14.3.


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

6.1. Mobility Header

   The Mobility Header is used by mobile nodes, correspondent nodes, and
   home agents in all messaging related to the creation and management
   of bindings.  The Mobility Header is an IPv6 protocol.  Rules



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   regarding how it is sent and what addresses are used in the IPv6
   header are given separately in Sections 6.1.2 through 6.1.9, which
   describe the message types used in this protocol.


6.1.1. 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 D.1).

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

      Header Len

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

      MH Type

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







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      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 an octet string
         consisting of a "pseudo-header" followed by the entire
         Mobility Header starting with the Payload Proto field.  The
         pseudo-header contains IPv6 header fields, as specified
         in Section 8.1 of [6].  The Next Header value used in the
         pseudo-header 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.

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

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


6.1.2. Binding Refresh Request (BRR) Message

   The Binding Refresh Request (BRR) message is used to request a
   mobile node's binding from the mobile node.  A packet containing
   a Binding Refresh Request message is sent in the same way as any
   packet to a mobile node (Section 9.6).  When a mobile node receives
   a packet containing a Binding Refresh Request message and there
   already exists a Binding Update List entry for the source of the
   Binding Refresh Request, it MAY start a return routability procedure
   (see Section 5.5) if it believes the amount of traffic with the
   correspondent justifies the use of Route Optimization.  Note that
   the mobile node SHOULD NOT respond to Binding Refresh Requests from
   previously unknown correspondent nodes due to Denial-of-Service
   concerns.








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

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

      Reserved

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

      Mobility options

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

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

          -  Unique Identifier Option

          -  Binding Authorization option

   The Header Length field in the Mobility Header for this message
   MUST be set to 1 (since unit is 8 octets) plus the total length of
   all mobility options present (also in 8 octet units).  If no actual
   options are present in this message, no padding is necessary.


6.1.3. 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 11.6.2).  The purpose of this message is to test the
   reachability of the home address.  This message is always sent with



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   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.  Such tunneling SHOULD employ IPsec ESP in tunnel mode between
   the home agent and the mobile node.  This protection is guided by the
   IPsec Policy Data Base.  (Note the protection of HoTI messages is
   different from the requirement to protect regular payload traffic,
   which MAY use such tunnels as well.)

   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:

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

      Reserved

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

      Mobile cookie

         32-bit field which contains a random value, mobile cookie 1,
         selected by the mobile node.

      Mobility options

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

         There MAY be additional information, associated with this
         message that need not be present in all HoTI messages.  This
         use of mobility options also allows for future extensions to
         the format of the HoTI message to be defined.  The encoding and
         format of defined options are described in Section 6.2.  The
         following options are valid in a HoTI message:

          -  Unique Identifier Option



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   The Header Length field in the Mobility Header for this message
   MUST be set to 2 (since unit is 8 octets) plus the total length of
   all mobility options present (also in 8 octet units).  If no actual
   options are present in this message, 4 bytes of padding is necessary.

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


6.1.4. 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
   (see Section 11.6.2).  The purpose of this message is to test the
   reachability of the care-of address.  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             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Mobile cookie                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Mobility Options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Reserved

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

      Mobile cookie

         32-bit field which contains a random value, mobile cookie 2,
         selected by the mobile node.

      Mobility options

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



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         There MAY be additional information, associated with this
         message that need not be present in all CoTI messages.  This
         use of mobility options also allows for future extensions to
         the format of the CoTI message to be defined.  The encoding and
         format of defined options are described in Section 6.2.  The
         following options are valid in a CoTI message:

          -  Unique Identifier Option

   The Header Length field in the Mobility Header for this message
   MUST be set to 2 (since unit is 8 octets) plus the total length of
   all mobility options present (also in 8 octet units).  If no actual
   options are present in this message, 4 bytes of padding is necessary.

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


6.1.5. Home Test (HoT) Message

   The Home Test (HoT) message is a response to the HoTI message, and
   is sent from the correspondent node to the mobile node (see Section
   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.  Such tunneling SHOULD
   employ IPsec ESP in tunnel mode between the home agent and the mobile
   node.  This protection is guided by the IPsec Policy Data Base.


























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   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       |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Mobile cookie                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                      Home Cookie (128 bits)                   |
    +                                                               +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                         Mobility options                      .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Reserved

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

      Home Nonce Index

         This field will be echoed back by the mobile node to the
         correspondent node in a subsequent binding update.  Strictly
         speaking, this value is not necessary in the authentication,
         but allows the correspondent node to efficiently find the nonce
         value Ni that it used in creating the Home Cookie.  Without
         this field, the correspondent node would have to search through
         all currently acceptable nonce values when testing for the
         correctness of the authenticator sent in a Binding Update.

      Mobile cookie

         32-bit field which contains mobile cookie 1, returned by the
         correspondent node.







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      Home Cookie

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

      Mobility options

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

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

   The Header Length field in the Mobility Header for this message
   MUST be set to 4 (since unit is 8 octets) plus the total length of
   all mobility options present (also in 8 octet units).  If no actual
   options are present in this message, no padding is necessary.


6.1.6. Care-of Test (CoT) Message

   The Care-of Test (CoT) message is a response to the CoTI message, and
   is sent from the correspondent node to the mobile node (see Section
   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      |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Mobile cookie                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                     Care-of Cookie (128 bits)                 |
    +                                                               +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Mobility Options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Reserved

         The two 16-bit fields and the one 32-bit field are reserved for
         future use.  These values MUST be initialized to zero by the
         sender, and MUST be ignored by the receiver.

      Care-of Nonce Index

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

      Mobile cookie

         32-bit field which contains the mobile cookie 2, returned by
         the correspondent node.

      Care-of Cookie

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




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      Mobility options

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

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

   The Header Length field in the Mobility Header for this message
   MUST be set to 4 (since unit is 8 octets) plus the total length of
   all mobility options present (also in 8 octet units).  If no actual
   options are present in this message, no padding is necessary.


6.1.7. Binding Update (BU) Message

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

























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

      Acknowledge (A)

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

      Home Registration (H)

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

      Single Address Only (S)

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






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      Duplicate Address Detection (D)

         The Duplicate Address Detection (D) bit is set by the sending
         mobile node to request that the receiving node (the mobile
         node's home agent) perform Duplicate Address Detection [33]
         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 4.5).  There is no requirement, however,
         that the Sequence Number value strictly increase by 1 with each
         new Binding Update sent or received, as long as the value stays
         within the window.  A Binding Acknowledgement with Status field
         set to 141 (Sequence number out of window) will be returned
         if the value is outside the window.  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.

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

      Home Address

         The home address of the mobile node associated with this
         Binding Update.



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      Mobility options

         Variable-length field of such length that the complete Mobility
         Header is an integer multiple of 8 octets long.  Contains one
         or more TLV-encoded mobility options.  The encoding and format
         of defined options are described in Section 6.2.  The receiver
         MUST ignore and skip any options which it does not understand.
         A Binding Update sent to a correspondent node MUST include the
         following options when the return routability procedure is used
         as the authorization method:

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

          -  Binding Authorization Data option.  This option 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 96 bits and computed over a string of octets
             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 Binding Authorization
                 Data mobility option) which is not included for the
                 purposes of calculating the Authenticator.  Options of
                 the Mobility Header are included in the calculation.

             The actual authenticator calculation over a sequence of
             bits is described in Section 5.5.

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

          -  Unique Identifier option

          -  Binding Authorization Data option

          -  Alternate Care-of Address option




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   The Header Length field in the Mobility Header for this message
   MUST be set to 4 (since unit is 8 octets) plus the total length of
   all mobility options present (also in 8 octet units).  If no actual
   options are present in this message, no padding is necessary.

   A Binding Update to the home agent MUST include the Home Address
   destination option in order to allow for the use of manually keyed
   IPsec in the protection of these messages.  Note also that as
   described in Section 6.3, the Home Address destination option is not
   accepted by correspondent nodes that do not have an existing binding
   with the sender.

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

   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 mobility option in the Binding Update message.  When such
   message is sent to the correspondent node and the return routability
   procedure is used as the authorization method, the Care-of Test Init
   and Care-of Test messages MUST have been performed for the address in
   the Alternate Care-of Address option (not the Source Address).  The
   contents of the Nonce Indices and the Authenticator mobility options
   MUST be based on information gained in this test.

   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.

   The deletion of a binding can be indicated by setting the Lifetime
   field to 0 or by setting the care-of address as equal to the home
   address (the care-of address can be specified either in an Alternate
   Care-of Address mobility option in the Binding Update message, if
   present, or in the Source Address field in the packet's IPv6 header).


6.1.8. Binding Acknowledgement (BA) Message

   The Binding Acknowledgement message is used to acknowledge receipt
   of a Binding Update message (Section 6.1.7).  When a node receives
   a packet containing a Binding Update message, with this node being
   the destination of the packet, this node MUST return a Binding
   Acknowledgement to the mobile node, if the Acknowledge (A) bit
   is set in the the Binding Update.  The Binding Acknowledgement



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   message is sent to the Source Address of the Binding Update message
   which is being acknowledged.  The Source Address of the Binding
   Acknowledgement is the Destination Address from the Binding Update.

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

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

      Reserved

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

      Status

         8-bit unsigned integer indicating the disposition of the
         Binding Update.  Values of the Status field less than 128
         indicate that the Binding Update was accepted by the receiving
         node.  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



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      131

         Insufficient resources

      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

         Expired Home Nonce Index

      145

         Expired Care-of Nonce Index

         Up-to-date values of the Status field are to be specified in
         the most recent "Assigned Numbers" [30].

      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.





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      Lifetime

         The granted lifetime, in seconds, for which this node SHOULD
         retain the entry for this mobile node in its Binding Cache.
         Correspondent nodes should make an effort to honor 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 destination option.
         While this situation is recoverable since an error message is
         sent to the mobile node, it causes an unnecessary break in the
         communications.

         Mobile nodes SHOULD send a new Binding Update well before the
         expiration of this period in order to extend the lifetime and
         not cause a disruption in communications.  This is particularly
         necessary in order to prevent packets from being dropped due
         to the use of the Home Address destination option without an
         existing Binding Cache Entry, and the possibility of clock
         drift.

         If the 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.

         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



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

      Mobility options

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

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

          -  Binding Authorization Data option

   The Header Length field in the Mobility Header for this message
   MUST be set to 3 (since unit is 8 octets) plus the total length of
   all mobility options present (also in 8 octet units).  If no actual
   options are present in this message, 4 bytes of Pad1 or PadN mobility
   options are needed to make the length of the message a multiple of 8.
   The Header Length field does include this padding.

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

   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
   accepted sequence number in the Sequence Number field of the Binding
   Acknowledgement message.


6.1.9. Binding Error (BE) Message

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




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

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

      Status

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

        1

         Home Address destination option used without a binding

        2

         Received message had an unknown value for the MH Type field

      Reserved

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

      Home Address

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

      Mobility options

         Variable-length field of such length that the complete Mobility
         Header is an integer multiple of 8 octets long.  Contains one



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         or more TLV-encoded mobility options.  The receiver MUST ignore
         and skip any options which it does not understand.

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

   The Header Length field in the Mobility Header for this message
   MUST be set to 3 (since unit is 8 octets) plus the total length of
   all mobility options present (also in 8 octet units).  If no actual
   options are present in this message, no padding is necessary.


6.2. Mobility Options

6.2.1. Format

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

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

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

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

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

      Option Type

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




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      Option Length

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

      Option Data

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

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

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


6.2.2. Pad1

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

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

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

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


6.2.3. PadN

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

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

   The PadN option is used to insert two or more octets of padding in
   the Mobility Options area of a Mobility Header message.  For N octets



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   of padding, the Option Len field contains the value N, and the Option
   Data consists of N-2 zero-valued octets.  Option data MUST be ignored
   by the receiver.


6.2.4. Unique Identifier

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

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

   The Unique Identifier option is valid only in Binding Refresh
   Request, HoTI, CoTI, 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 HoTI, CoTI, and Binding Update to identify the
   specific Binding Refresh Request to which it responds.  This matching
   of Binding Updates to Binding Refresh Requests is required in the
   procedure for renumbering the home subnet while a mobile node is away
   from home (Section 10.9.1).


6.2.5. Alternate Care-of Address

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

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |       3       |      18       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                                                               |
    +                   Alternate Care-of Address                   +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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




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6.2.6. Nonce Indices

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

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

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

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

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


6.2.7. Binding Authorization Data

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

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

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

   The Option Len field contains the value 2  +   Len, where Len is the
   length of the authenticator in octets.

   The Authenticator field contains a cryptographic value which can be
   used to determine that the message in question comes from the right



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   authority.  Rules for calculating this value depend on the used
   authorization procedure.  This specification gives the rules only for
   the return routability procedure.  For this procedure, this option
   can only appear in a Binding Update message and rules for calculating
   the Authenticator value are described in Section 6.1.7.


6.3. Home Address Destination Option

   The Home Address destination option is used in a packet sent by a
   mobile node while away from home, to inform the recipient of 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
   above the Mobile IPv6 support level.  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 MUST not appear more than once in
   any given packet, except inside the payload part of the packet if
   tunneling is involved.

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

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

      Option Type

         201 = 0xC9








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      Option Length

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

      Home Address

         The home address of the mobile node sending the packet.

   IPv6 requires that options appearing in a Hop-by-Hop Options
   header or Destination Options header be aligned in a packet so that
   multi-octet values within the Option Data field of each option fall
   on natural boundaries (i.e., fields of width n octets are placed at
   an integer multiple of n octets from the start of the header, for
   n = 1, 2, 4, or 8) [6].  The alignment requirement [6] for the Home
   Address option is 8n+6.

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

   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 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
   Error message to the source address of the packet that contained the
   Home Address option (see Section 6.1.9).  The Status field in this
   message should be set to 1.  These messages SHOULD be rate-limited.




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

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



























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6.4. Routing Header type 2

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

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


6.4.1. Routing Header Packet format

   The Type 2 Routing header has the following format:

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

      Next Header

         8-bit selector.  Identifies the type of header immediately
         following the Routing header.  Uses the same values as the 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

         8-bit unsigned integer that contains the value 2.






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      Segments Left

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

      Reserved

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

      Home Address

         The Home Address of the destination Mobile Node.

   The ordering rules for extension headers in an IPv6 packet are
   described in Section 4.1 of [6].  The new Routing header (Type 2)
   defined for Mobile IPv6 follows the same ordering as other routing
   headers.  If more than one Routing header (e.g., both a Type 0 and a
   Type 2 Routing header are present), the Type 2 Routing header should
   follow all other Routing headers.  Otherwise the order of routing
   headers is independent of their type and follows [6].

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


6.5. ICMP Home Agent Address Discovery Request Message

   The ICMP Home Agent Address Discovery Request message is used by a
   mobile node to initiate the dynamic home agent address discovery
   mechanism, as described in Sections 10.9 and 11.3.2.  The mobile
   node sends a Home Agent Address Discovery Request message to the
   "Mobile IPv6 Home-Agents" anycast address for its own home subnet
   prefix [11], and one of the home agents responds to the mobile node
   with a Home Agent Address Discovery Reply message, providing a list
   of the routers on the mobile node's home link serving as home agents.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |            Checksum           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Identifier           |            Reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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      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 MUST then return the Home Agent Address
   Discovery Reply message directly to the Source Address chosen by the
   mobile node.  Note that, at the time of performing this dynamic home
   agent address discovery, it is likely that the mobile node is not
   registered with any home agent within the specified anycast group.
























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6.6. ICMP Home Agent Address Discovery Reply Message

   The ICMP Home Agent Address Discovery Reply message is used by
   a home agent to respond to a mobile node using the dynamic home
   agent address discovery mechanism, as described in Sections 10.9
   and 11.3.2.  The mobile node sends a Home Agent Address Discovery
   Request message to the "Mobile IPv6 Home-Agents" anycast address
   for its own home subnet prefix [11], and one of the home agents
   responds to the mobile node with a Home Agent Address Discovery Reply
   message, providing a list of the routers on the mobile node's home
   link serving as home agents.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |            Checksum           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Identifier          |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   +                            Reserved                           +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   .                                                               .
   .                      Home Agent Addresses                     .
   .                                                               .
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type

         151 <To Be Assigned by IANA>

      Code

         0

      Checksum

         The ICMP checksum [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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6.7. ICMP Mobile Prefix Solicitation Message Format

   The ICMP Mobile Prefix Solicitation Message is sent by a mobile node
   to its home agent while it is away from home.  The purpose of the
   message is to solicit a Mobile Prefix Advertisement from the home
   agent, which will allow the mobile node to gather prefix information
   about its home network.  This information can be used to configure
   home address(es) by stateless address autoconfiguration [33],
   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 [30].

      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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6.8. ICMP Mobile Prefix Advertisement Message Format

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Options ...
   +-+-+-+-+-+-+-+-+-+-+-+-

   IP Fields:

      Source Address


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

      Destination Address


         If this message is a response to a Mobile Prefix Solicitation,
         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


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

   ICMP Fields:

      Type

         153 <To Be Assigned by IANA>

      Code

         0





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      Checksum

         The ICMP checksum [5].

   Options:

      Prefix Information

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

         The Prefix Information option is defined in Section 4.6.2
         of [20], with modifications defined in Section 7.2 of this
         specification.  The home agent MUST use this modified Prefix
         Information option to send the aggregate list of home network
         prefixes as defined in Section 10.9.1.

   The Mobile Prefix Advertisement sent by the home agent MAY include
   the Source Link-layer Address option defined in RFC 2461 [20], or the
   Advertisement Interval option specified in Section 7.3.

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



























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7. Modifications to IPv6 Neighbor Discovery

7.1. Modified Router Advertisement Message Format

   Mobile IPv6 modifies the format of the Router Advertisement
   message [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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7.2. Modified Prefix Information Option Format

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

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

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

   However, Neighbor Discovery [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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7.3. New Advertisement Interval Option Format

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

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

      Type

         7

      Length

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

      Reserved

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

      Advertisement Interval

         32-bit unsigned integer.  The maximum time, in milliseconds,
         between successive unsolicited router Router Advertisement
         messages sent by this router on this network interface.  Using
         the conceptual router configuration variables defined by
         Neighbor Discovery [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 11.4.1.

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








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7.4. New Home Agent Information Option Format

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

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

      Type

         8

      Length

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

      Reserved

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

      Home Agent Preference

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

         The manual configuration of the Home Agent Preference value
         is described in Section 8.3.  In addition, the sending home
         agent MAY dynamically set the Home Agent Preference value, for
         example basing it on the number of mobile nodes it is currently
         serving or on its remaining resources for serving additional
         mobile nodes; such dynamic settings are beyond the scope of
         this document.  Any such dynamic setting of the Home Agent
         Preference, however, MUST set the preference appropriately,



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

      Home Agent Lifetime

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

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

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

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


















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7.5. Changes to Sending Router Advertisements

   The Neighbor Discovery protocol specification [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 7.2) in each.




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7.6. Changes to Sending Router Solicitations

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

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

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

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

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

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

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



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


8. Requirements for Types of IPv6 Nodes

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


8.1. Requirements for All IPv6 Hosts and Routers

   Since any IPv6 node may at any time be a correspondent node of a
   mobile node, either sending a packet to a mobile node or receiving a
   packet from a mobile node, the following requirements apply to ALL
   IPv6 nodes (whether host or router, whether mobile or stationary):

    -  Every IPv6 node MUST be able to process a Home Address option
       received in any IPv6 packet.

    -  Every IPv6 node SHOULD be able to participate in a return
       routability procedure, process Binding Update messages, and to
       return a Binding Acknowledgement option if the Acknowledge (A)
       bit is set in the received Binding Update.

    -  Every IPv6 node SHOULD be able to maintain a Binding Cache of the
       bindings received in accepted Binding Updates.


8.2. 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 7.5.  The use of this faster rate MUST be configurable.

    -  Each router SHOULD include at least one prefix with the 'R' bit
       set and with its full IP address in its router advertisements.





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


8.3. 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.

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

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

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

    -  Every home agent MUST be able to accept packets addressed to
       the "Mobile IPv6 Home-Agents" anycast address for the subnet
       on which it is serving as a home agent [11], and MUST be
       able to participate in dynamic home agent address discovery
       (Section 10.9).





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    -  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.


8.4. Requirements for IPv6 Mobile Nodes

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

    -  Every IPv6 mobile node MUST be able to perform IPv6 encapsulation
       and decapsulation [4].

    -  Every IPv6 mobile node MUST support the return routability
       procedure and sending Binding Update messages, as specified in
       Sections 11.6.1, 11.6.2, and 11.6.6; and MUST be able to receive
       and process Binding Acknowledgement messages, as specified in
       Section 11.6.3.

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

    -  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.

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

    -  Every IPv6 mobile node MUST support sending packets containing a
       Home Address option.  This option MUST be included in all packets
       sent to a correspondent node when the following three conditions
       apply:  The correspondent node has a binding with this mobile
       node.  The mobile node is away from home.  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 4.5.

    -  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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9. Correspondent Node Operation

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


9.1. Conceptual Data Structures

   Each IPv6 node maintains a Binding Cache of bindings for other nodes.
   A separate Binding Cache SHOULD be maintained by each IPv6 node for
   each of its IPv6 addresses.  The Binding Cache MAY be implemented in
   any manner consistent with the external behavior described in this
   document, for example by being combined with the node's Destination
   Cache as maintained by Neighbor Discovery [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 9.6, for packets
       originated by this node, or in Section 10.5, if this node is the
       mobile node's home agent and the packet was intercepted by it on
       the home link.

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

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

    -  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.




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       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 16 bits long, and all comparisons
       between Sequence Number values MUST be performed modulo 2**16.
       For example, using an implementation in the C programming
       language, a Sequence Number value A is greater than another
       Sequence Number value B if ((short)((a) - (b)) > 0), if the
       "short" data type is a 16-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 Refresh
       Request should be sent when the lifetime of this entry nears
       expiration.

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


9.2. Receiving Packets from a Mobile Node

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


9.2.1. Processing Mobility Header (MH) Messages

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

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





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

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

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

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


9.2.2. Receiving Packets with Home Address Destination Option

   Packets sent by a mobile node while away from home MAY include a Home
   Address destination option, if the correspondent node has a Binding
   Cache Entry for that home address.  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 depend on that information 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.

   Packets containing Home Address Option MUST be dropped if there is
   no corresponding Binding Cache Entry for that home address.  In this
   case, the correspondent nodes SHOULD send the Binding Error message
   to the source address of the packet that contained the Home Address
   Option (see Section 6.1.9).


9.3. Return Routability Procedure

   A correspondent node engages in the return routability procedure in
   order to secure a subsequent Binding Update.  This is a requirement
   in order to authorize the creation of new bindings as well as to
   refresh existing ones.  In particular, these messages are used to
   establish the mobile node's liveness (responsiveness to packets) at
   both its care-of address as well as its home address.



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9.3.1. Receiving HoTI Messages

   The HoTI message initiates the return routability procedure from the
   mobile node's home address to the correspondent node.

   The correspondent node verifies the following:

    -  MH Type field for this message is 1.

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

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

   In preparation for sending the corresponding HoT Message, the
   correspondent node checks that it has the necessary material
   to engage in a return routability procedure, as specified in
   Section 5.5.  For that procedure, the correspondent node MUST have a
   secret Kcn and a nonce Nj.  If it does not have this material yet,
   it MUST produce it before continuing with the return routability
   procedure.

   Section 9.3.3 specifies further processing.


9.3.2. Receiving CoTI Messages

   The CoTI message initiates the return routability procedure from the
   mobile node's care-of address location to the correspondent node.

   The correspondent node verifies the following:

    -  MH Type field for this message is 2.

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

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

   In preparation for sending the corresponding CoT Message, the
   correspondent node checks that it has the necessary material
   to engage in a return routability procedure, as specified in
   Section 5.5.  For that procedure, the correspondent node MUST have a
   secret Kcn and a nonce Nl.  If it does not have this material yet,
   it MUST produce it before continuing with the return routability
   procedure.

   Section 9.3.4 specifies further processing.






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9.3.3. Sending HoT Messages

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


9.3.4. Sending CoT Messages

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


9.4. Processing Bindings

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

    -  Binding Update

    -  Binding Refresh Request

    -  Binding Acknowledgement

    -  Binding Error


9.4.1. Receiving Binding Updates

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

    -  The packet MUST NOT contain a Home Address option.

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

    -  The Sequence Number field in the Binding Update message is
       greater than the Sequence Number received in the previous Binding
       Update for this home address, if any.  As noted in Section 5.5,
       this Sequence Number comparison MUST be performed modulo 2**16.

    -  The packet meets the specific authentication requirements for
       Binding Updates, defined in Section 5.5.

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




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

    -  The Home and Care-of Nonce Index values in the Nonce Indices
       mobility option are recognized by the correspondent node.  As
       described in Section 5.5, the correspondent node discards Nonce
       values that are too old.

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

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

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

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

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

    -  If the Lifetime specified in the Binding Update is nonzero and
       the specified Care-of Address is not equal to the home address
       for the binding, then this is a request to cache a binding for
       the mobile node.  If the Home Registration (H) bit is set in the
       Binding Update, the Binding Update is processed according to the
       procedure specified in Section 10.2; otherwise, it is processed
       according to the procedure specified in Section 9.4.2.

    -  If the Lifetime specified in the Binding Update is zero or the
       specified Care-of Address matches the home address for the
       binding, then this is a request to delete the mobile node's



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       cached binding.  If the Home Registration (H) bit is set in the
       Binding Update, the Binding Update is processed according to the
       procedure specified in Section 10.3; otherwise, it is processed
       according to the procedure specified in Section 9.4.3.


9.4.2. Requests to Cache a Binding

   When a node receives a Binding Update, it MUST validate it and
   determine the type of Binding Update according to the steps described
   in Section 9.4.1.  This section describes the processing of a valid
   Binding Update that requests 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 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
   the Binding Update.  Any Binding Cache entry MUST be deleted after
   the expiration its lifetime.

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


9.4.3. Requests to Delete a Binding

   When a node receives a Binding Update, it MUST validate it and
   determine the type of Binding Update according to the steps described
   in Section 9.4.1.  This section describes the processing of a valid
   Binding Update that requests a node to delete a mobile node's binding
   from its Binding Cache, for which the Home Registration (H) bit is
   not set in the Binding Update.

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

   In order to prevent replayed binding updates after a binding cache
   entry has been deleted the correspondent node needs to make sure that
   the nonce indices used to create the binding are no longer valid.



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   This applies whether the binding is deleted due to it timing out
   (lifetime expiry) or being deleted explicitly by the mobile node.

   If a binding cache entry is logically deleted and either the home
   nonce index or the care-of nonce index used to create (or last
   update) the binding are still valid, the correspondent node must
   behave as if it retains the state about the binding (including the
   sequence number) until at least one of the cookies has become too
   old.

   A possible way to implement this is to mark the binding cache entry
   so that it does not effect sending and receiving of packets, but
   so that it is found when a binding update is received.  Another
   way is to mark the used nonces immediately too old.  However, this
   method may cause some unnecessary failures and retries with ongoing
   return routability procedures with other mobile nodes.  Furthermore,
   unless the mobile node has requested a Binding Acknowledgement,
   it is possible that this method may even cause an error in the
   return routability procedure procedure to go unnoticed, and data
   packets to be dropped through the use of the Home Address destination
   option without an existing binding.  The effect is similar to packet
   loss during the return routability procedure, but may in certain
   circumstances significantly increase the problems.


9.4.4. Sending Binding Acknowledgements

   When any node receives a packet containing a Binding Update message
   in which the Acknowledge (A) bit is set, it MUST return a Binding
   Acknowledgement message acknowledging receipt of the Binding Update.
   If the node accepts the Binding Update and creates or updates an
   entry in its Binding Cache for this binding, the Status field in the
   Binding Acknowledgement MUST be set to a value less than 128; if, on
   the other hand the Binding Update is accepted and the `A' bit is not
   set, the node SHOULD NOT send a Binding Acknowledgement.  If the node
   rejects the Binding Update and does not create or update an entry for
   this binding, a Binding Acknowledgement MUST be sent even if the `A'
   bit was not set, and the Status field in the Binding Acknowledgement
   MUST be set to a value greater than or equal to 128.  Specific values
   for the Status field are described in Section 6.1.8 and in the 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 5.5.  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:




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

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

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


9.4.5. Sending Binding Refresh 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 Refresh Request message to the mobile node
   in an attempt to avoid this overhead and latency due to deleting and
   recreating the Binding Cache entry.  When the mobile node receives a
   packet from some sender containing a Binding Refresh Request option,
   it MAY start a return routability procedure, if necessary, before
   sending its current binding and a new lifetime in a new Binding
   Update.

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





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9.4.6. Sending Binding Error Messages

   If the correspondent node receives a packet with a Home Address
   destination option it MUST verify that it has a binding for that
   mobile node.  Specifically, it MUST have a binding entry for the
   mobile node's home address (as obtained from the Home Address option)
   at the mobile node's care-of address (from the IP source address of
   the packet).  If the correspondent node does not find such a binding
   entry, it MUST discard the packet and return a Binding Error message
   (Section 6.1.9).


9.5. Cache Replacement Policy

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

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

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

   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,



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   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.  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 again add an entry for this
   destination mobile node to its Binding Cache.


9.6. Sending Packets to a Mobile Node

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

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

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

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

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

   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 10.5.  The mobile node MAY then send



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   a Binding Update to the sending node, as described in Section 11.6.2,
   allowing the sending node to create a Binding Cache entry for its use
   in sending subsequent packets to this mobile node.


9.7. Receiving ICMP Error Messages

   When 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
   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 11.6.6 (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 SHOULD 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




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   the correspondent node, allowing it to recreate a (correct) Binding
   Cache entry for the mobile node.


10. Home Agent Operation

10.1. Conceptual Data Structures

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

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

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




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       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 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?


10.2. 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 9.4.1.  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 MUST return a Binding Acknowledgement to the mobile node,
       in which the Status field is set to 132 (home registration not
       supported).

    -  Else, if the home address for the binding (the Home Address field
       in the packet's Home Address option) is not an on-link IPv6
       address with respect to the home agent's current Prefix List,



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       then the home agent MUST reject the Binding Update and SHOULD
       return a Binding Acknowledgement to the mobile node, in which the
       Status field is set to 133 (not home subnet).

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

    -  A Home Address destination option MUST be present in the message,
       and the value of the Home Address field in this option MUST
       correspond to the Home Address field in the Binding Update.

    -  Finally, if the Duplicate Address Detection (D) bit is set in
       the Binding Update, this home agent MUST perform Duplicate
       Address Detection [33] on the mobile node's home link for the
       link-local address associated with the home address in this
       binding, before returning the Binding Acknowledgement.  This
       ensures that no other node on the home link can possibly use
       the mobile node's home address.  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, 33];
       however, in this case, the home agent SHOULD NOT perform such a
       delay.  If this Duplicate Address Detection fails, then the home
       agent MUST reject the Binding Update and MUST return a Binding
       Acknowledgement to the mobile node, in which the Status field is
       set to 138 (Duplicate Address Detection failed).  When the home
       agent sends a successful Binding Acknowledgement to the mobile
       node, in response to a Binding Update with the `D' bit set, the
       home agent assures to the mobile node that its home address will
       continue to be kept unique by the home agent at least as long
       as the 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, 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 option in the Binding Update option,
   if present, or from the Source Address field in the packet's IPv6
   header, otherwise.



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

   If the `S' bit field in the Binding Update is zero, The home agent
   creates or updates Binding Cache entries for each of possibly
   several home addresses.  The set of such home addresses is formed
   by replacing the routing prefix for the given home address with
   all other routing prefixes 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 of the Binding Cache entry depends on a number of
   factors:

    -  The lifetime for the Binding Cache entry MUST NOT be greater
       than the remaining valid lifetime for the subnet prefix in the
       mobile node's home address specified with the Binding Update,
       and MUST NOT be greater than the Lifetime value specified in the
       Binding Update.  The remaining valid lifetime for this prefix is
       determined by the home agent based on its own Prefix List entry
       for this prefix [20].

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

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

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

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





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


10.3. Primary Care-of Address De-Registration

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

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

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

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




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

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

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

    -  The Lifetime field MUST be set to zero.

    -  The Refresh field MUST be set to zero.

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

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


10.4. 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 that are addressed to the
   mobile node, and MUST tunnel each intercepted packet to the mobile
   node 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).





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    -  For each specific IP address for the mobile node determined
       in the first step above, the home agent sends a Neighbor
       Advertisement message [20] to the all-nodes multicast address
       on the home link, to advertise the home agent's own link-layer
       address for this IP address on behalf of the mobile node.

       All fields in each such Neighbor Advertisement message SHOULD be
       set in the same way they would be set by the mobile node itself
       if sending this Neighbor Advertisement while at home [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 set to zero.

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

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

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



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   registration".  (Note that Binding Update messages with the `S' bit
   set to zero will result in multiple Binding Cache entries, so checks
   on all these entries necessarily include all possible home addresses
   for the mobile node).

   If such an entry exists in the home agent's Binding Cache, the home
   agent MUST reply to the Neighbor Solicitation message with a Neighbor
   Advertisement message, giving the home agent's own link-layer address
   as the link-layer address for the specified Target Address.  In
   addition, the Router (R) bit in the Advertisement MUST be 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].


10.5. Tunneling Intercepted Packets to a Mobile Node

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

   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 10.4.  The home agent cannot use a Routing header to
   forward these intercepted packets to the mobile node, since it cannot
   modify the packet in flight without invalidating any existing IPv6
   AH [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 new tunnel IP header to the
   home agent's own IP address, and sets the Destination Address
   in the tunnel IP header to the mobile node's primary care-of
   address.  When received by the mobile node (using its primary care-of
   address), normal processing of the tunnel header [4] will result in
   decapsulation and processing of the original packet by the mobile
   node.



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   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
   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.


10.6. Handling Reverse Tunneled Packets from a Mobile Node

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

    -  The tunneled traffic arrives to the home agent using IPv6
       encapsulation [4].

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

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

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



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   Internet could send traffic through the home agent and escape ingress
   filtering limitations.

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


10.7. Protecting Return Routability Packets

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

   The use of ESP for protection of the return routability procedure is
   optional and controlled by configuration of the IPsec security policy
   database both at the mobile node and at the home agent.

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


10.8. Receiving Router Advertisement Messages

   For each link on which a router provides service as a home agent,
   the router maintains a Home Agents List recording information
   about all other home agents on that link.  This list is used in
   the dynamic home agent address discovery mechanism, described in
   Section 10.9.  The information for the list is learned through
   receipt of the periodic unsolicited multicast Router Advertisements,
   in a manner similar to the Default Router List conceptual data
   structure maintained by each host for Neighbor Discovery [20].  In
   the construction of the Home Agents List, the Router Advertisements
   are from each other home agent on the link, and the Home Agent (H)
   bit is set in them.

   On receipt of a valid Router Advertisement, as defined in the
   processing algorithm specified for Neighbor Discovery [20], the home



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   agent performs the following steps, in addition to any steps already
   required of it by Neighbor Discovery:

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

    -  Otherwise, extract the Source Address from the IP header of the
       Router Advertisement.  This is the link-local IP address on this
       link of the home agent sending this Advertisement [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 home agent's Home
       Agents List and the received home agent lifetime value is zero,
       immediately delete this entry in the Home Agents List.

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

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

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




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


10.9. Dynamic Home Agent Address Discovery

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

   As described in Section 11.3.2, a mobile node attempts dynamic
   home agent address discovery by sending an ICMP Home Agent Address
   Discovery Request message to the "Mobile IPv6 Home-Agents" anycast
   address [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 4.5).  However, if this
       home agent's own global IP address would be placed in the list
       (as described below) as the first entry in the list, then this
       home agent SHOULD NOT include its own address in the Home Agent
       Addresses field in the Reply message.  Not placing this home
       agent's own IP address in the list will cause the receiving
       mobile node to consider this home agent as the most preferred
       home agent; otherwise, this home agent will be considered to be
       preferred in its order given by its place in the list returned.

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



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

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

       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.


10.9.1. Aggregate List of Home Network Prefixes

   IPv6 provides mechanisms for node configuration when it turns on,
   and in renumbering a subnet, such as when a site switches to a new
   network service provider.  These mechanisms are a part of Neighbor
   Discovery [20] and Address Autoconfiguration [33].

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



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

   To avoid possible security attacks from forged Mobile Prefix
   Advertisements all such Advertisements must be authenticated to the
   mobile node by its home agent using IPsec [14, 12, 13] if a security
   associate exists (i.e.  unless the mobile node does not yet have a
   home address configured).

   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
       (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




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       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 [33]).

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


10.9.2. Scheduling Prefix Deliveries to the Mobile Node

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

   MUST:

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

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

   MAY:

    -  A new prefix is added to the aggregate list.

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

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




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

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

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

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

   Assume that the home agent already has scheduled the transmission of
   a Router Advertisement to the mobile node.  New information should
   be added to the existing scheduled transmission, if the freshly
   calculated RAND_ADV_DELAY would cause another transmission before
   the expiration of the Preferred Lifetime of the mobile node's home
   address derived from the prefix whose advertisement information has
   changed.  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
   transmission, add the data from the existing scheduled transmission
   to the newly scheduled transmission, deleting the previously
   scheduled transmission event.

   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.

   Calculate the newly advertised Preferred Lifetime as follows.
       MAX_SCHEDULE_DELAY == min (MAX_PFX_ADV_DELAY, Preferred Lifetime)

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

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



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   matching Unique Identifier mobility option).  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 Refresh Request if it specifies
   a binding for the mobile node to which the Binding Refresh Request
   was sent and contains a Unique Identifier mobility option matching
   the unique identifier sent in the Unique Identifier option in the
   Binding Refresh 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 10.9.2.  The home agent MUST generate a new unique identifier
   for use in the Unique Identifier Option in the Binding Refresh
   Request tunneled with the new Mobile Prefix Advertisement.


10.9.3. Sending Advertisements to the Mobile Node

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

    -  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.

    -  A separate Binding Refresh Request message MUST be sent in
       addition to the advertisement, if this is the first advertisement



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       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 Refresh
       Request message MUST include a Unique Identifier mobility option
       (Section 6.2.4), with the unique identifier in the option data
       set to a value different than that in any other Binding Refresh
       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
       Refresh Requests that contain a Unique Identifier option,
       incremented each time a Binding Refresh Request containing a
       Unique Identifier 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).


10.9.4. Lifetimes for Changed Prefixes

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


11. Mobile Node Operation

11.1. Conceptual Data Structures

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

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

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



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

   Each Binding Update List entry conceptually contains the following
   fields:

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

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

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

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

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

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

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

    -  The maximum value of the Sequence Number field sent in previous
       Binding Updates to this destination.  The Sequence Number field
       is 16 bits long, and all comparisons between Sequence Number
       values MUST be performed modulo 2**16.  For example, using an
       implementation in the C programming language, a Sequence Number
       value A is greater than another Sequence Number value B if



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       ((short)((a) - (b)) > 0), if the "short" data type is a 16-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 1, error message in response to
       a return routability message or Binding Update sent to that
       destination, as described in Section 11.7.

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

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

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

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

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

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

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










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11.2. Packet Processing

11.2.1. Sending Packets While Away from Home

   While a mobile node is away from home, it continues to use its home
   address, as well as also using one or more care-of addresses.  When
   sending a packet while away from home, a mobile node MAY choose among
   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.
       For packets sent that are part of transport-level connections
       established while the mobile node was at home, the mobile node
       MUST use its home address.  Likewise, for packets sent that are
       part of transport-level connections that the mobile node may
       still be using after moving to a new location, the mobile node
       SHOULD use its home address in this way.  When sending such
       packets, the delivery method depends on whether a binding exists
       with the correspondent node.  If a binding exists, the mobile
       node SHOULD send the packets directly to the correspondent node.
       Otherwise, if a binding does not exist, the mobile node MUST use
       reverse tunneling.  Detailed operation for both of these cases is
       described later in this section.

    -  For short-term communication, particularly for communication that
       may easily be retried if it fails, the mobile node MAY choose
       to directly use one of its care-of addresses as the source of
       the packet, thus not requiring the use of a Home Address option
       in the packet.  An example of this type of communication might
       be DNS queries sent by the mobile node [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.




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   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.  This can be done in two ways, as described above.  These
   ways are:

      direct delivery

         This is manner of delivering packets does not require going
         through the home network, and typically will enable faster and
         more reliable transmission.  A mobile node SHOULD arrange to
         supply the home address in a Home Address option, and allowing
         the IPv6 header's Source Address field to be set to one of the
         mobile node's care-of addresses; the correspondent node will
         then use the address supplied in the Home Address option to
         serve the function traditionally done by the Source IP address
         in the IPv6 header.  the mobile node's home address is then
         supplied to higher protocol layers and applications.

         Specifically:

          -  Construct the packet using the mobile node's home address
             as the packet's Source Address, in the same way as if the
             mobile node were at home.  This preserves the transparency
             of Mobile IP to higher protocol layers (e.g., 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.

         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].

      reverse tunneling

         This is the mechanism which tunnels the packets via the home
         agent.  It isn't as efficient as the above mechanism, but is




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         needed if there is no binding yet with the correspondent node.
         Specifically:

          -  The packet is sent to the home agent using IPv6
             encapsulation [4].

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

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

         Reverse tunneled packets MAY be protected using a AH or ESP
         header, depending on the security policies used by the home
         agent.  The support for encrypted reverse tunneling allows
         mobile nodes to defeat certain kinds of traffic analysis, and
         provides a mechanism by which routers on the home network can
         distinguish authorized traffic from other possibly malicious
         traffic.


11.2.2. Interaction with Outbound IPsec Processing

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

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






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

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

       If route optimization is in use, the mobile node inserts a Home
       Address destination option into the packet, replacing the Source
       Address in the packet's IP header with a care-of address suitable
       for the link on which the packet is being sent, as described in
       Section 11.2.1.  The Destination Options header in which the Home
       Address destination option is inserted MUST appear in the packet
       after the Routing Header, if present, and before the AH [12] (or
       ESP [13]) header, so that the Home Address destination option is
       processed by the destination node before the AH or ESP header is
       processed.

       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 6.3) contains the new care-of address.

    -  This allows, but does not require, the receiver of the packet
       containing a Home Address destination option to exchange the
       two fields of the incoming packet, simplifying processing for
       all subsequent packet headers.  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 remains the same.

   In addition, when using any automated key management protocol [14]
   (such as IKE [8]) to create a new SA (or SA bundle) while away from
   home, a mobile node MUST take special care in its processing of the
   key management protocol.  Otherwise, other nodes with which the
   mobile node must communicate as part of the automated key management
   protocol processing may be unable to correctly deliver packets to
   the mobile node if they and/or the mobile node's home agent do
   not then have a current Binding Cache entry for the mobile node.
   For the default case of using IKE as the automated key management
   protocol [8][14], such problems can be avoided by the following
   requirements on the use of IKE by a mobile node while away from home:





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

    -  In addition, for all security associations bound to the mobile
       node's home address established by way of IKE, the mobile node
       MUST include an ISAKMP Identification Payload [16] in the IKE
       exchange, giving the mobile node's home address as the initiator
       of the Security Association [28].


11.2.3. Receiving Packets While Away from Home

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

    -  Packets sent by a correspondent node that does not have a
       Binding Cache entry for the mobile node, will be sent by the
       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 type 2 Routing header.  The packet will be addressed to the
       mobile node's care-of address, with the final hop in the Routing
       header directing the packet to the mobile node's home address;
       the processing of this last hop of the Routing header is entirely
       internal to the mobile node, since the care-of address and home
       address are both addresses within the mobile node.

    -  Packets sent by a correspondent node that has a Binding
       Cache entry for the mobile node that contains an out-of-date
       care-of address for the mobile node, will also be sent by the
       correspondent node using a type 2 Routing header, as described
       above.  If the mobile node sent a Binding Update to a home agent
       on the link on which its previous care-of address is located
       (Section 11.6.6), and if this home agent is still serving as
       a home agent for the mobile node's previous care-of address,
       then such a packet will be 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 the first of these methods, the mobile node
   MUST check that the IPv6 source address of the tunnel packet is the
   IP address of its home agent.




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   For packets received by either the first or last of these three
   methods, the mobile node SHOULD send a Binding Update to the original
   sender of the packet, as described in Section 11.6.2, subject to
   the rate limiting defined in Section 11.6.9.  The mobile node MUST
   also process the received packet in the manner defined for IPv6
   encapsulation [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 Type 2
   Routing header), the following rules will result in the packet being
   processed normally by upper-layer protocols within the mobile node,
   as if it had been addressed to the mobile node's home address.

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

    -  The length field in the RH is exactly 2.

    -  The segments left field in the RH is either 0 or 1.

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

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

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

   /








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11.2.4. Routing Multicast Packets

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

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

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

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


11.3. Home Agent and Prefix Management

11.3.1. Receiving Local Router Advertisement Messages

   Each mobile node maintains a Home Agents List recording information
   about all home agents from which it receives a Router Advertisement,
   for which the home agent lifetime indicated in that Router
   Advertisement has not yet expired.  This list is used by the mobile
   node to enable it to send a Binding Update to the global unicast
   address of a home agent on its previous link when it moves to a new
   link, as described in Section 11.6.6.  On receipt of a valid Router
   Advertisement, as defined in the processing algorithm specified for
   Neighbor Discovery [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 7.2).  For each such global address determined from this
       Advertisement, add this global address to the list of global
       addresses for this home agent in this Home Agents List entry.

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



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11.3.2. Dynamic Home Agent Address Discovery

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

   In this case, the mobile node MAY attempt to discover the address of
   a suitable home agent on its home link.  To do so, the mobile node
   sends an ICMP Home Agent Address Discovery Request message to the
   "Mobile IPv6 Home-Agents" anycast address [11] for its home subnet
   prefix.  As described in Section 10.9, the home agent on its home
   link that receives this Request message will return an ICMP Home
   Agent Address Discovery Reply message, giving this home agent's own
   global unicast IP address along with a list of the global unicast IP
   address of each other home agent operating on the home link.

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

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

   If, after a mobile node transmits a Home Agent Address Discovery
   Request message to the Home Agents Anycast address, it does not



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   receive a corresponding Home Agent Address Discovery Reply message
   within INITIAL_DHAAD_TIMEOUT seconds, the mobile node MAY retransmit
   the same Request message to the same anycast address.  This
   retransmission MAY be repeated up to a maximum of DHAAD_RETRIES
   attempts.  Each retransmission MUST be delayed by twice the time
   interval of the previous retransmission.


11.3.3. Sending Mobile Prefix Solicitations

   When a mobile node has a home address that is about to become
   invalid, it sends a Mobile Prefix Solicitation to its home agent
   in an attempt to acquire fresh routing prefix information.  The
   new information also enables the mobile node to participate in
   renumbering operations affecting the home network, as described in
   section 10.9.1.

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

   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 12).

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

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

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






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11.3.4. Receiving Mobile Prefix Advertisements

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

   When a mobile node receives a Mobile Prefix Advertisement, it MUST
   validate it according to the following 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, if a security
       association exists (i.e.  unless the mobile node does not yet
       have a home address configured).  The IPsec protection MUST
       provide sender authentication, data integrity protection, and
       replay protection, covering the advertisement.

   Any received Mobile Prefix Advertisement not meeting 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 Refresh Request option, the
   mobile node SHOULD return a Binding Update, which will be viewed by
   the home agent as an acknowledgement of the corresponding Mobile
   Prefix Advertisement, which it can cease transmitting.

   In addition, if 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 [33] 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



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   its home agent, to request the home agent to perform this Duplicate
   Address Detection on behalf of the mobile node.


11.4. Movement

11.4.1. Movement Detection

   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
   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 11.4.2.  The mobile node registers
   its primary care-of address with its home agent, as described in
   Section 11.6.1.

   While a mobile node is away from home 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




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   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
   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



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

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

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

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





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11.4.2. Forming New Care-of Addresses

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

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

   As described in Section 4, in order to form a new care-of address,
   a mobile node MAY use either stateless [33] 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 [33] on that new address to confirm its
   uniqueness.  However, doing so represents a trade-off between safety
   (ensuring that the new address is not used if it is a duplicate
   address) and overhead (performing Duplicate Address Detection
   requires the sending of one or more additional packets over what
   may be, for example, a slow wireless link through which the mobile
   node is connected).  Performing Duplicate Address Detection also in



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

   In addition, normal processing for Duplicate Address Detection
   specifies that, in certain cases, the node SHOULD delay sending the
   initial Neighbor Solicitation message of Duplicate Address Detection
   by a random delay between 0 and MAX_RTR_SOLICITATION_DELAY [20, 33];
   however, in this case, the mobile node SHOULD NOT perform such a
   delay in its use of Duplicate Address Detection, unless the mobile
   node is initializing after rebooting.


11.4.3. Using Multiple Care-of Addresses

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

   To assist with smooth handovers, a mobile node SHOULD retain
   its previous primary care-of address as a (non-primary) care-of
   address, and SHOULD still accept packets at this address, even after
   registering its new primary care-of address with its home agent.
   This is reasonable, since the mobile node could only receive packets
   at its previous primary care-of address if it were indeed still
   connected to that link.  If the previous primary care-of address was
   allocated using stateful Address Autoconfiguration [2], the mobile
   node may not wish to release the address immediately upon switching
   to a new primary care-of address.






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11.5. Return Routability Procedure

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


11.5.1. Sending Home and Care-of Test Init Messages

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

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

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

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

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

    -  The mobile cookie used in the messages.


11.5.2. Receiving Return Routability Messages

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

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

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

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

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



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

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

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

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

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

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

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

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

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

   If after receiving either the Home Test or the Care-of Test message
   and performing the above actions, the Binding Update List entry
   has both the Home and the Care-of Cookies, the return routability
   procedure is complete.  The mobile node SHOULD then proceed with
   sending a Binding Update message as described in Section 11.6.2.

   Correspondent nodes from the time before this specification was
   published may not not support the Mobility Header protocol.  These
   nodes will respond to Home Test Init and Care-of Test Init messages
   with an ICMP Parameter Problem code 1.  The mobile node SHOULD
   take such messages as an indication that the correspondent node
   can not provide route optimization, and revert back to the use of
   bidirectional routing.






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11.5.3. Retransmitting in the Return Routability Procedure

   The mobile node is responsible for retransmissions in the return
   routability procedure.

   When the mobile node sends a Home Test Init or Care-of Test Init
   message, it has to determine a value for the initial retransmission
   timer.  It should use the specified value of INITIAL_BINDACK_TIMEOUT
   for this initial retransmission timer.

   If, after sending either a Home Test Init or Care-of Test Init
   message and the mobile node fails to receive a valid, matching
   Home Test or Care-of Test message within the selected initial
   retransmission interval, the mobile node SHOULD retransmit
   the original message, until a valid answer is received.  The
   retransmissions by the mobile node MUST use an exponential
   back-off process, in which the timeout period is doubled upon each
   retransmission until either the node receives a valid response or the
   timeout period reaches the value MAX_BINDACK_TIMEOUT.


11.5.4. Rate Limiting for Return Routability Procedure

   A mobile node MUST NOT send Home Test Init or Care-of Test
   Init messages to any individual node more often than once per
   MAX_UPDATE_RATE seconds.  After sending MAX_FAST_UPDATES consecutive
   messages to a particular node with the same care-of address, the
   mobile node SHOULD reduce its rate of sending these messages to that
   node, to the rate of SLOW_UPDATE_RATE per second.  The mobile node
   MAY continue to send these messages at this slower rate indefinitely,
   in hopes that the node will eventually be able to complete the return
   routability procedure.


11.6. Processing Bindings

11.6.1. Sending Binding Updates to the Home Agent

   After deciding to change its primary care-of address as described in
   Sections 11.4.1 and 11.4.2, a mobile node MUST register this care-of
   address with its home agent in order to make this its primary care-of
   address.  To do so, the mobile node sends a packet to its home agent
   containing a Binding Update message, with the packet constructed as
   follows:

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

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

    -  The packet MUST contain a Home Address destination option, giving
       the mobile node's home address for the binding.



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    -  The care-of address for the binding MUST be used as the Source
       Address in the packet's IPv6 header, unless an Alternate Care-of
       Address mobility option is included in the Binding Update
       message.

    -  The `S' bit is set to the zero to request the mobile node's home
       agent to serve as a home agent for all home addresses for the
       mobile node based on all on-link subnet prefixes on the home
       link; this is the default behavior.  If the mobile node desires
       that only a single home address should be affected by this
       Binding Update, the `S' bit can be set to 1.

    -  The value specified in the Lifetime field SHOULD be less than
       or equal to the remaining lifetime of the home address and the
       care-of address specified for the binding.

   The Acknowledge (A) bit in the Binding Update requests the home agent
   to return a Binding Acknowledgement in response to this Binding
   Update.  As described in Section 6.1.8, the mobile node SHOULD
   retransmit this Binding Update to its home agent until it receives
   a matching Binding Acknowledgement.  Once reaching a retransmission
   timeout period of MAX_BINDACK_TIMEOUT, the mobile node SHOULD restart
   the process of delivering the Binding Update, but trying instead the
   next home agent from its Home Agents List (see Section 11.3.2).  If
   there is only one home agent in the Home Agents List, the mobile node
   instead SHOULD continue to periodically retransmit the Binding Update
   at this rate until acknowledged (or until it begins attempting to
   register a different primary care-of address).  See Section 11.6.8
   for information about retransmitting Binding Updates.

   Depending on the value of the Single Address Only (S) bit in the
   Binding Update, the home agent is requested to serve either a single
   home address or all home home addresses for the mobile node.  Until
   the lifetime of this registration expires, the home agent considers
   itself the home agent for each such home address of the mobile node.
   As the set of on-link subnet prefixes on the home link changes over
   time, the home agent changes the set of home addresses for this
   mobile node for which it is serving as the home agent.

   Each Binding Update MUST be authenticated as coming from the right
   mobile node, as defined in Section 5.4.  The mobile node MUST use a
   Home Address destination option in Binding Updates sent to the home
   agent in order to allow the IPsec policies to be matched with the
   right home address.  The home address in the Home Address destination
   option and the Binding Update message MUST be equal (and this will be
   checked by 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 11.6.2.




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   The last Sequence Number value sent to the home agent in a Binding
   Update is stored by the mobile node.  If the sending mobile node has
   no knowledge of the right Sequence Number value, it may start at any
   value.  If the home agent rejects the value, it sends back a Binding
   Acknowledgement with status code 141, and the last accepted sequence
   number in the Sequence Number field of the Binding Acknowledgement.
   The mobile node MUST store this information and use the next Sequence
   Number value for the next Binding Update it sends.

   If the mobile node has additional home addresses using a different
   interface identifier, then the mobile node SHOULD send an additional
   packet containing a Binding Update to its home agent to register the
   care-of address for each such other home address (or set of home
   addresses sharing an interface identifier).

   While the mobile node is away from home, it relies on the home agent
   to participate in Duplicate Address Detection (DAD) to defend its
   home address against stateless autoconfiguration performed by another
   node.  Therefore, the mobile node SHOULD set the Duplicate Address
   Detection (D) bit based on any requirements for DAD that would apply
   to the mobile node if it were at home [20][33].  If the mobile
   node's recent Binding Update was accepted by the home agent, and the
   lifetime for that Binding Update has not yet expired, the mobile node
   SHOULD NOT set the `D' bit in the new Binding Update; the home agent
   will already be defending the home address(es) of the mobile node and
   does not need to perform DAD again.

   The home agent will only perform DAD for the mobile node's home
   address when the mobile node has supplied a valid binding between
   its home address and a care-of address.  If some time elapses during
   which the mobile node has no binding at the home agent, it might
   be possible for another node to autoconfigure the mobile node's
   home address.  Therefore, the mobile node MUST treat creation of
   a new binding with the home agent using an existing home address
   the same as creation of a new home address.  In the unlikely event
   that the mobile node's home address is autoconfigured as the IPv6
   address of another network node on the home network, the home agent
   will reply to the mobile node's subsequent Binding Update with a
   Binding Acknowledgement containing a Status of 138, Duplicate Address
   Detection failed.  In this case, the mobile node MUST NOT attempt to
   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.


11.6.2. Correspondent Binding Procedure

   When the mobile node is assured that its home address is valid, it
   MAY at any time initiate a correspondent binding procedure with



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   the purpose of allowing the correspondent node to cache the mobile
   node's current care-of address.  The mobile node is responsible for
   the initiation and completion of this procedure, as well as any
   retransmissions that may be needed (subject to the rate limiting
   defined in Section 11.6.9).

   This section defines the rules that the mobile node must follow
   when performing the correspondent binding procedure.  Appendix A
   specifies also a (non-normative) state-machine that describes the
   same procedure.

   The mobile node can be assured that its home address is still valid,
   for example, by the home agent's use the 'D' bit of Binding Updates
   (see Section 10.2).  In any Binding Update sent by a mobile node,
   the care-of address (either the Source Address in the packet's IPv6
   header or the Care-of Address in the Alternate Care-of Address
   mobility option of the Binding Update) MUST be set to one of the
   care-of addresses currently in use by the mobile node or to the
   mobile node's home address.  A mobile node MAY set the care-of
   address differently for sending Binding Updates to different
   correspondent nodes.

   A mobile node MAY choose to keep its location private from
   certain correspondent nodes, and thus need not initiate the
   return routability procedure, or send new Binding Updates to those
   correspondents.  A mobile node MAY also send a Binding Update to
   such a correspondent node to instruct it to delete any existing
   binding for the mobile node from its Binding Cache, as described in
   Section 6.1.7.  However, all Binding Updates to the correspondent
   node require the successful completion of the return routability
   procedure first, as no other IPv6 nodes are authorized to send
   Binding Updates on behalf of a mobile node.

   If set to one of the mobile node's current care-of addresses (the
   care-of address given MAY differ from the mobile node's primary
   care-of address), the Binding Update requests the correspondent node
   to create or update an entry for the mobile node in the correspondent
   node's Binding Cache in order to record this care-of address for use
   in sending future packets to the mobile node.  In this case, the
   value specified in the Lifetime field sent in the Binding Update
   SHOULD be less than or equal to the remaining lifetime of the home
   address and the care-of address specified for the binding.

   If, instead, the care-of address is set to the mobile node's home
   address, the Binding Update requests the correspondent node to delete
   any existing Binding Cache entry that it has for the mobile node.

   When a mobile node sends a Binding Update to its home agent
   to register a new primary care-of address (as described in
   Section 11.6.1), the mobile node SHOULD also start a return
   routability procedure to each other node for which an entry exists



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   in the mobile node's Binding Update List, as detailed below.  Upon
   successful return routability procedure, a Binding Update message is
   sent.  Thus, other relevant nodes are generally kept updated about
   the mobile node's binding and can send packets directly to the mobile
   node using the mobile node's current care-of address.

   The mobile node, however, need not initiate these actions immediately
   after configuring a new care-of address.  For example, the mobile
   node MAY delay initiating the return routability procedure to any
   correspondent node for a short period of time, if it isn't certain
   that there's traffic to the correspondent node.  This is particularly
   useful if the mobile node anticipates that it is not going to stay
   long in this location.

   In addition, when a mobile node receives a packet for which the
   mobile node can deduce that the original sender of the packet either
   has no Binding Cache entry for the mobile node, or a stale entry
   for the mobile node in its Binding Cache, the mobile node SHOULD
   initiate a return routability procedure with the sender, in order to
   finally update the sender's Binding Cache with the current care-of
   address (subject to the rate limiting defined in Section 11.6.9).
   In particular, the mobile node SHOULD initiate a return routability
   procedure in response to receiving a packet that meets all of the
   following tests:

    -  The packet was tunneled using IPv6 encapsulation.

    -  The Destination Address in the tunnel (outer) IPv6 header is
       equal to any of the mobile node's care-of addresses.

    -  The Destination Address in the original (inner) IPv6 header
       is equal to one of the mobile node's home addresses; or this
       Destination Address is equal to one of the mobile node's previous
       care-of addresses for which the mobile node has an entry in its
       Binding Update List, representing an unexpired Binding Update
       sent to a home agent on the link on which its previous care-of
       address is located (Section 11.6.6).

    -  The Source Address in the tunnel (outer) IPv6 header differs from
       the Source Address in the original (inner) IPv6 header.

   The destination address to which the procedure should be initiated to
   in response to receiving a packet meeting all of the above tests is
   the Source Address in the original (inner) IPv6 header of the packet.
   The home address for which this Binding Update is sent should be the
   Destination Address of the original (inner) packet.

   Binding Updates sent to correspondent nodes are not generally
   required to be acknowledged.  However, if the mobile node wants
   to be sure that its new care-of address has been entered into a
   correspondent node's Binding Cache, the mobile node MAY request an



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   acknowledgement by setting the Acknowledge (A) bit in the Binding
   Update.  In this case, however, the mobile node SHOULD NOT continue
   to retransmit the Binding Update once the retransmission timeout
   period has reached MAX_BINDACK_TIMEOUT.

   The mobile node SHOULD create a Binding Update message as follows:

    -  The Source Address of the IPv6 header MUST contain the current
       care-of address of the mobile node.

    -  The Destination Address of the IPv6 header MUST contain the
       address of the correspondent node.

    -  The Mobility Header is constructed according to rules in
       Section 6.1.7, including the authenticator field which is
       calculated based on the received Home and Care-of Cookies.

   The last Sequence Number value sent to a destination in a Binding
   Update is stored by the mobile node in its Binding Update List entry
   for that destination.  If the sending mobile node has no Binding
   Update List entry, the Sequence Number SHOULD start at a random
   value.  The mobile node MUST NOT use the same Sequence Number in two
   different Binding Updates to the same correspondent node, even if the
   Binding Updates provide different care-of addresses.


11.6.3. Receiving Binding Acknowledgements

   Upon receiving a packet carrying a Binding Acknowledgement, a mobile
   node MUST validate the packet according to the following tests:

    -  The packet meets the authentication requirements for Binding
       Acknowledgements, defined in Sections 6.1.8 and 5.  That is,
       if the Binding Update was sent to the home agent, underlying
       IPsec protection is used.  If the Binding Update was sent to the
       correspondent node, the authenticator field MUST be present and
       have a valid value.

    -  The Header Len field in the Binding Acknowledgement message is
       greater than or equal to the length specified in Section 6.1.8.

    -  The Sequence Number field matches the Sequence Number sent by the
       mobile node to this destination address in an outstanding Binding
       Update.

   Any Binding Acknowledgement not satisfying all of these tests MUST be
   silently ignored.

   When a mobile node receives a packet carrying a valid Binding
   Acknowledgement, the mobile node MUST examine the Status field as
   follows:



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    -  If the Status field indicates that the Binding Update was
       accepted (the Status field is less than 128), then the mobile
       node MUST update the corresponding entry in its Binding Update
       List to indicate that the Binding Update has been acknowledged;
       the mobile node MUST then stop retransmitting the Binding Update.
       In addition, if the value specified in the Lifetime field in the
       Binding Acknowledgement is less than the Lifetime value sent
       in the Binding Update being acknowledged, then the mobile node
       MUST subtract the difference between these two Lifetime values
       from the remaining lifetime for the binding as maintained in the
       corresponding Binding Update List entry (with a minimum value
       for the Binding Update List entry lifetime of 0).  That is, if
       the Lifetime value sent in the Binding Update was L_update, the
       Lifetime value received in the Binding Acknowledgement was L_ack,
       and the current remaining lifetime of the Binding Update List
       entry is L_remain, then the new value for the remaining lifetime
       of the Binding Update List entry should be

          max((L_remain - (L_update - L_ack)), 0)

       where max(X, Y) is the maximum of X and Y. The effect of this
       step is to correctly manage the mobile node's view of the
       binding's remaining lifetime (as maintained in the corresponding
       Binding Update List entry) so that it correctly counts down from
       the Lifetime value given in the Binding Acknowledgement, but with
       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 11.6.9.


11.6.4. Receiving Binding Refresh Requests

   When a mobile node receives a packet containing a Binding Refresh
   Request message and there already exists a Binding Update List
   entry for the source of the Binding Refresh Request, it MAY start
   a return routability procedure (see Section 5) if it believes
   the amount of traffic with the correspondent justifies the use of
   Route Optimization.  Note that the mobile node SHOULD NOT respond
   Binding Requests from previously unknown correspondent nodes due to
   Denial-of-Service concerns.

   If the return routability procedure completes successfully, a
   Binding Update message SHOULD be sent as described in Section 11.6.2.



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   The Lifetime field in this Binding Update SHOULD be set to a new
   lifetime, extending any current lifetime remaining from a previous
   Binding Update sent to this node (as indicated in any existing
   Binding Update List entry for this node), and lifetime SHOULD
   again be less than or equal to the remaining lifetime of the home
   registration and the care-of address specified for the binding.  When
   sending this Binding Update, the mobile node MUST update its Binding
   Update List in the same way as for any other Binding Update sent by
   the mobile node.

   Note, however, that the mobile node MAY choose to delete its binding
   from the sender of the Binding Refresh Request.  In this case, the
   mobile node instead SHOULD return a Binding Update to the sender, in
   which the Lifetime field is set to zero and the care-of address is
   set to the mobile node's home address.

   If the Binding Refresh Request for which the Binding Update is being
   returned contains a Unique Identifier mobility option, the resulting
   Home Test Init, Care-of Test Init, and Update messages MUST also
   include a Unique Identifier mobility option.  The unique identifier
   in the Option Data field of the Unique Identifier mobility option
   MUST be copied from the unique identifier carried in the Binding
   Refresh Request.


11.6.5. Receiving Binding Error Messages

   When a mobile node receives a packet containing a Binding Error
   message, it should first check if the mobile node has a Binding
   Update List entry for the the source of the Binding Error message.
   If the mobile node does not have such entry, it MUST ignore the
   message.  This is necessary to prevent a waste of resources on e.g.
   return routability procedure due to spoofed Binding Error messages.

   Otherwise, if the message Status field was 1 (Home Address
   destination option used without a binding), the mobile node should
   perform one of the following two actions:

    -  If the mobile node does have a Binding Update List entry but
       has recent upper layer progress information that indicates
       communications with the correspondent node are progressing, it
       MAY ignore the message.  This can be done in order to limit the
       damage that spoofed Binding Error messages can cause to ongoing
       communications.

    -  If the mobile node does have a Binding Update List entry but
       no upper layer progress information, it MUST remove the entry
       and route further communications through the home agent.  It
       MAY also optionally start a return routability procedure (see
       Section 5.5).




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   If the message Status field was 2 (received message had an unknown
   value for the MH Type field), the mobile node should perform one of
   the following two actions:

    -  If the mobile node is not expecting an acknowledgement or
       response from the correspondent node, the mobile node SHOULD
       ignore this message.

    -  Otherwise, the mobile node SHOULD cease the use of any extensions
       to this specification.  If no extensions had been used, the
       mobile node should cease the attempt to use Route Optimization.


11.6.6. Forwarding from a Previous Care-of Address

   When a mobile node connects to a new link and forms a new care-of
   address, it MAY establish forwarding of packets from a previous
   care-of address to this new care-of address.  To do so, the mobile
   node sends a Binding Update to any home agent on the link on which
   the previous care-of address is located, indicating this previous
   care-of address as the home address for the binding, and giving its
   new care-of address as the binding's care-of address.  Such packet
   forwarding allows packets destined to the mobile node from nodes that
   have not yet learned the mobile node's new care-of address, to be
   forwarded to the mobile node rather than being lost once the mobile
   node is no longer reachable at this previous care-of address.

   This Binding Update is sent to a home agent, albeit a temporary
   one.  Nevertheless, the authentication requirements for Binding
   Updates from a mobile node to its home agent apply, as specified in
   Section 11.6.1.  This means that the mobile node MUST employ IPsec
   ESP as specified further below.

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

    -  The Home Address field in the Home Address destination option
       in the packet carrying the Binding Update MUST be set to the
       previous care-of address for which packet forwarding is being
       established.

    -  The care-of address for the new binding MUST be set to the new
       care-of address to which packets destined to the previous care-of
       address are to be forwarded.  Normally, this care-of address for
       the binding is specified by setting the Source Address of the
       packet carrying the Binding Update, to this address.  However,
       the mobile node MAY instead include an Alternate Care-of Address
       mobility option in the Binding Update message, with its Alternate
       Care-of Address field set to the care-of address for the binding.





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    -  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.

   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 7.2) while
   attached to the link on which this previous care-of address and this
   home agent are located; the mobile node obtains this home agent
   address from its Home Agents List (Section 4.4).  Alternatively,
   the mobile node MAY use dynamic home agent address discovery
   (Section 10.9) to discover the global unicast address of a home agent
   on this previous link, but it SHOULD use an address from its Home
   Agents List if available for the prefix it used to form this previous
   care-of address.

   As with any packet containing a Binding Update (see Section 6.1.7),
   the Binding Update packet to this home agent MUST meet the
   authentication requirements for Binding Updates, defined in
   Section 5.4.  Each Binding Update MUST be authenticated as coming
   from the right mobile node.  This means that the mobile node and the
   home agent MUST have a security association that employs IPsec ESP
   for protecting the Mobility Header with a non-null authentication
   algorithm.  The mobile node MUST use a Home Address destination
   option in Binding Updates sent to the home agent in order to allow
   the IPsec policies to be matched with the right home address.  The
   home address in the Home Address destination option and the Binding
   Update message MUST be equal (and this will be checked by the home
   agent), that is, it MUST be the mobile node's previous care-of
   address for which forwarding is being established.


11.6.7. Returning Home

   A mobile node detects that it has returned to its home link through
   the movement detection algorithm in use (Section 11.4.1), when the
   mobile node detects that its home subnet prefix is again on-link.
   The mobile node SHOULD then send a Binding Update to its home agent,
   to instruct its home agent to no longer intercept or tunnel packets
   for it.  In this 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



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   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
   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



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   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
   Flag (S) in these Advertisements MUST NOT be set, since they were
   not solicited by any Neighbor Solicitation message.  The Override
   Flag (O) in these Advertisements MUST be set, indicating that the
   Advertisements SHOULD override any existing Neighbor Cache entries at
   any node receiving them.

   Since multicasting on the local link (such as Ethernet) is typically
   not guaranteed to be reliable, the mobile node MAY retransmit these
   Neighbor Advertisement messages up to MAX_ADVERT_REXMIT times to
   increase their reliability.  It is still possible that some nodes on
   the home link will not receive any of these Neighbor Advertisements,
   but these nodes will eventually be able to recover through use of
   Neighbor Unreachability Detection [20].


11.6.8. Retransmitting Binding Updates

   The mobile node is responsible for retransmissions in the binding
   procedure.

   When the mobile node sends a Binding Update message, it has to
   determine a value for the initial retransmission timer.  If the
   mobile node is changing or updating an existing binding at the home
   agent, it should use the specified value of INITIAL_BINDACK_TIMEOUT
   for this initial retransmission timer.  If on the other hand the
   mobile node does not have an existing binding at the home agent, it
   SHOULD use a value for the initial retransmission timer that is at
   least 1.5 times longer than (RetransTimer * DupAddrDetectTransmits).
   This value is likely to be substantially longer than the otherwise
   specified value of INITIAL_BINDACK_TIMEOUT that would be used by the
   mobile node.  This longer retransmission interval will allow the the
   home agent to complete the DAD procedure which is mandated in this
   case, as detailed in Section 11.6.1.



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   If, after sending a Binding Update in which the care-of address has
   changed and the Acknowledge (A) bit is set, a mobile node fails
   to receive a valid, matching Binding Acknowledgement within the
   selected initial retransmission interval, the mobile node SHOULD
   retransmit the Binding Update, until a Binding Acknowledgement is
   received.  Such a retransmitted Binding Update MUST use a Sequence
   Number value greater than that used for the previous transmission of
   this Binding Update.  The retransmissions by the mobile node MUST
   use an exponential back-off process, in which the timeout period
   is doubled upon each retransmission until either the node receives
   a Binding Acknowledgement or the timeout period reaches the value
   MAX_BINDACK_TIMEOUT.


11.6.9. Rate Limiting Binding Updates

   A mobile node MUST NOT send Binding Update messages for the
   same binding to any individual node more often than once per
   MAX_UPDATE_RATE seconds.  After sending MAX_FAST_UPDATES consecutive
   messages to a particular node with the same care-of address, the
   mobile node SHOULD reduce its rate of sending these messages to that
   node, to the rate of SLOW_UPDATE_RATE per second.  The mobile node
   MAY continue to send these messages at this slower rate indefinitely,
   in hopes that the node will eventually be able to process a Binding
   Update, and begin to route its packets directly to the mobile node at
   its new care-of address.


11.7. Receiving ICMP Error Messages

   Any node receiving a Mobility header that does not recognize the
   protocol SHOULD return an ICMP Parameter Problem, Code 1, message
   to the sender of the packet.  If a node performing the return
   routability procedure or sending a Binding Update receives such an
   ICMP error message in response, it SHOULD record in its Binding
   Update List that future Binding Updates SHOULD NOT be sent to this
   destination.

   Correspondent nodes who have participated in the return routability
   procedure MUST implement the ability to correctly process received
   packets containing a Home Address option.  Therefore, correctly
   implemented correspondent nodes should always be able to recognize
   Home Address options.  If a mobile node receives an ICMP Parameter
   Problem, Code 2, message from some node indicating the that the Home
   Address option, the mobile node SHOULD log the error and then discard
   the ICMP message.








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12. Protocol Constants

      HomeRtrAdvInterval       3,600 seconds
      DHAAD_RETRIES            3 retransmissions
      INITIAL_BINDACK_TIMEOUT  1 second
      INITIAL_DHAAD_TIMEOUT    2 seconds
      INITIAL_SOLICIT_TIMER    2 seconds
      MAX_ADVERT_REXMIT        3 transmissions
      MAX_BINDACK_TIMEOUT      256 seconds
      MAX_COOKIE_LIFE          240 seconds
      MAX_FAST_UPDATES         5 transmissions
      MAX_PFX_ADV_DELAY        1,000 seconds
      MAX_RR_BINDING_LIFE      300 seconds
      MAX_UPDATE_RATE          once per second
      PREFIX_ADV_RETRIES       3 retransmissions
      PREFIX_ADV_TIMEOUT       5 seconds
      SLOW_UPDATE_RATE         once per 10 second interval





































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13. IANA Considerations

   This document defines a new IPv6 protocol, the Mobility Header,
   described in Section 6.1.  This protocol must be assigned a protocol
   number.  The MH Type field in the Mobility Header is used to indicate
   a particular type of a message.  The current message types are
   described in Sections 6.1.2 through 6.1.9, and include the following:

     0 Binding Refresh Request

     1 Home Test Init

     2 Care-of Test Init

     3 Home Test

     4 Care-of Test

     5 Binding Update

     6 Binding Acknowledgement

     7 Binding Error

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

   Furthermore, each Mobility Header message may contain mobility
   options as described in Section 6.2.  The current mobility options
   are defined in Sections 6.2.2 through 6.2.5, and include the
   following:

     0 Pad1

     1 PadN

     2 Unique Identifier

     3 Alternate Care-of Address

     4 Nonce Indices

     5 Authorization Data

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

   This document also defines a new IPv6 destination option, the Home
   Address option, described in Section 6.3.  This option must be
   assigned an Option Type value.




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   This document also defines a new IPv6 Type 2 Routing Header,
   described in Section 6.4.  The value 2 must be allocated by IANA when
   this specification becomes an RFC.

   In addition, this document defines four ICMP message types, two used
   as part of the dynamic home agent address discovery mechanism and
   two used in lieu of router solicitations and advertisements when the
   mobile node is away from the home link:

    -  The Home Agent Address Discovery Request message, described in
       Section 6.5;

    -  The Home Agent Address Discovery Reply message, described in
       Section 6.6;

    -  The Mobile Prefix Solicitation message, described in Section 6.7;
       and

    -  The Mobile Prefix Advertisement message, described in
       Section 6.8.

   This document also defines two new Neighbor Discovery [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 7.3; and

    -  The Home Agent Information option, described in Section 7.4.


14. Security Considerations

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

   Binding updates to the home agents are secure.  When receiving
   tunneled traffic the home agent verifies the outer IP address
   corresponds to the current location of the mobile node.  This
   prevents attacks where the attacker is controlled by ingress
   filtering, 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 and are not controlled by ingress filtering could
   still send traffic through the home agent.  This includes attackers
   on the same local link as the mobile node is currently on.  But such
   attackers could also send spoofed packets without using a tunnel.

   It is possible to use IPsec ESP to protect payload packets tunneled
   to the mobile node and back.  While this specification does not
   mandate the use of ESP, its use is recommended to protect the payload
   communications against attackers on the path between the home agent
   and the current location of the mobile node.




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   When site local home address are used, reverse tunneling can be used
   to send site local traffic from another location.  Administrators
   should be aware of this when allowing such home addresses.  In
   particular, the outer IP address check described above is not
   sufficient against all attackers and the use of encrypted tunnels is
   particularly useful for this kind of home addresses.


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.  Sequence numbers with
   the Mobile IPv6 messages ensure correct ordering (see Section 5.4).
   However, if a home agent reboots and loses its state regarding the
   sequence numbers, replay attacks become possible.  If the home agent
   is vulnerable to this, the use of a key management mechanism together
   with IPsec can be used to prevent replay attacks.


14.3. Security for the Binding Updates to the Correspondent Nodes

   The use of home address and care-of-address based return routability
   tests prevents 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 (e.g., in order to perform
   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 bindings authorized by return routability to
   a maximum of MAX_COOKIE_LIFE + MAX_RR_BINDING_LIFE seconds after the
   last routability check has been performed.

   The path between the home agent and a correspondent node is typically
   easiest to attack on the links at either end, in particular if these
   links are publicly accessible wireless LANs.  Attacks against the
   routers or switches on the path are typically harder to accomplish.
   Thus, the weakest points are typically on the links at either end,
   and their mechanisms for layer 2 security or IPv6 Neighbour and
   Router Discovery.  If these were secured using some new technology in
   the future, this could make the key establishment mechanism specified
   in this document to be an easier route for attackers to use.  For
   this reason, this specification should have a protection mechanism
   for selecting between return routability and potential other future
   mechanisms.




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14.4. Security for the Home Address Destination Option

   The use of the Home Address destination 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 replies sent by the correspondent node, this document
   restricts the use of the Home Address option to those situations
   where a binding has been established with the participation of the
   node at the home address.  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 6.3), since it indicates that the option is included in the
   authentication computation.  Thus, even when authentication is used
   in the IPv6 header, the security of the Source Address field in the
   IPv6 header is not compromised by the presence of a Home Address
   option.  Without authentication of the packet, then any field in the
   IPv6 header, including the Source Address field, and any other parts
   of the packet, including the Home Address option, can be forged or
   modified in transit.  In this case, the contents of the Home Address
   option is no more suspect than any other part of the packet.


14.5. Firewall considerations

   The definition of Routing Header 2 in Section 6.4 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.





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   This implies that a device which implements filtering of packets
   should be able to distinguish between a Type 2 Routing header and
   other Routing headers, as required in section 8.2.  This is necessary
   in order to allow Mobile IPv6 traffic while still having the option
   to filter out other uses of Routing headers.


Acknowledgements

   We would like to thank the members of the Mobile IP and IPng Working
   Groups for their comments and suggestions on this work.  We would
   particularly like to thank (in alphabetical order) Fred Baker
   (Cisco), Josh Broch (Carnegie Mellon University), Robert Chalmers
   (University of California, Santa Barbara), Noel Chiappa (MIT),
   Vijay Devarapalli (Nokia Research Center), Rich Draves (Microsoft
   Research), Francis Dupont (ENST Bretagne), Thomas Eklund (Xelerated),
   Jun-Ichiro Itojun Hagino (IIJ Research Laboratory), Krishna Kumar
   (IBM Research), T.J. Kniveton (Nokia Research), Jiwoong Lee (KTF),
   Aime Lerouzic (Bull S.A.), 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 Connectathons 2000, 2001, and 2002
   in San Jose, California.  Similarly, we would like to thank the
   participants at the ETSI interoperability testing at ETSI, in Sophia
   Antipolis, France, during October 2-6, 2000, including teams from
   Compaq, Ericsson, INRIA, Nokia, and Technical University of Helsinki.

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






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References

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    [2] J. Bound, C. Perkins, M. Carney, and R. Droms.  Dynamic Host
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    [9] R. Hinden and S. Deering.  IP Version 6 Addressing Architecture.
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   [10] Editor J. Reynolds.  Assigned Numbers:  RFC 1700 is Replaced by
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   [12] S. Kent and R. Atkinson.  IP Authentication Header.  Request for
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   [13] S. Kent and R. Atkinson.  IP Encapsulating Security Payload
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   [14] S. Kent and R. Atkinson.  Security Architecture for the Internet
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   [15] H. Krawczyk, M. Bellare, and R. Canetti.  HMAC: Keyed-Hashing
        for Message Authentication.  Request for Comments
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   [16] D. Maughan, M. Schertler, M. Schneider, and J. Turner.  Internet
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   [17] P. V. Mockapetris.  Domain names - concepts and facilities.
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   [19] T. Narten and H. Alvestrand.  Guidelines for Writing an IANA
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   [20] T. Narten, E. Nordmark, and W. Simpson.  Neighbor Discovery for
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   [21] NIST.  Secure Hash Standard.  FIPS PUB 180-1, April 1995.

   [22] Erik Nordmark.  Securing MIPv6 BUs using return routability
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A. State Machine for the Correspondent Binding Procedure

   Home agents and correspondent nodes are stateless until a binding is
   actually established.

   The mobile node, however, is responsible for initiating the
   correspondent binding procedure, keeping track of its state, handle
   retransmissions and failures, and completing the procedure.

   Section 11.6.2 defines the normative rules that the mobile node
   must follow when performing the correspondent procedure.  This
   appendix specifies an additional, non-normative, state-machine that
   illustrates the behaviour of the mobile node.

   The mobile node will keep the following states in its Binding List:

    -  Idle:  This is an abstract state that refers to the situation
       that the correspondent node in question does not appear in
       the Binding List.  In this state, all RR and binding related
       messaging is silently ignored.

    -  WaitHC: In this state, the mobile node has sent the Home Test
       Init and CoT Init messages, and is waiting for the Home Test and
       CoT messages to come back.  It will also be necessary to keep
       state of retransmissions for both.

    -  WaitH: In this state, the mobile node has a recent Care-of Cookie
       and is only waiting for the Home Test message to arrive.

    -  WaitC: In this state, the mobile node has a recent Home Cookie
       and is only waiting for the CoT message to arrive.

    -  WaitA: In this state, the mobile node has sent a Binding Update,
       and is only waiting for the Binding Acknowledgement message to
       arrive.

    -  WaitD: In this state, the mobile node has sent a de-registration
       Binding Update, and is only waiting for the Binding
       Acknowledgement message to arrive.

    -  WaitDH: In this state, the mobile node intends to send a
       de-registration Binding Update later but is first waiting for a
       home cookie before this can be done.  Note that if the mobile
       node is at home, it can use a home cookie also as care-of cookie.

    -  Bound:  In this state, the mobile node has established a binding
       with the correspondent node.

   The following events are possible:





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    -  Route Optimization desired.  This is a decision taken by
       the mobile node based on observing traffic to and from the
       correspondent node.

    -  Route Optimization not needed.  This is another decision taken by
       the mobile node, perhaps due to running out of resources or lack
       of sufficient traffic to justify route optimization with this
       particular correspondent node.  Another reason for not needing
       Route Optimization any more is that the mobile node has returned
       home.

    -  Movement.

    -  Valid BRR received.  A valid Binding Refresh Request message has
       been received.

    -  Valid HoT received.  A valid Home Test message has been received.

    -  Valid CoT received.  A valid Care-of Test message has been
       received.

    -  Valid BA received.  A valid Binding Acknowledgement message has
       been received.

    -  Valid BE received.  A valid Binding Error message has been
       received.

    -  ICMP Parameter Problem Code 1 received.  This can happen if the
       peer does not support this specification.

    -  Invalid BRR received.

    -  Invalid HoT received.

    -  Invalid CoT received.

    -  Invalid BA received.

    -  Invalid BE received.

    -  Retransmission needed.  A timer is set to expire when a
       retransmission of a packet needs to be made.

    -  Retransmission failed.  A timer is set to expire when all
       retransmissions have failed.

   The following additional conditions are also used:

    -  Acknowledgements are required.  This is a local configuration on
       the mobile node side, and indicates whether acknowledgements are
       required to binding updates.



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    -  Home cookie too old.  A cookie is too old if it has been received
       MIN_COOKIE_LIFE or over seconds ago.

    -  Care-of cookie too old.

    -  Reason to believe forward progress is being made.  Upper layer
       protocols such as TCP may provide hints to the IP layer regarding
       the successfullness of the recent communications.

    -  Tests of the Status values received in a BE or BA message.

    -  Binding lifetime left.  The remaining lifetime field of a Binding
       Update List entry tells whether the binding currently registered
       at the correspondent node still has some lifetime left, even if
       we are trying to create a new one.  This has relevance when an
       attempt at re-binding is aborted for some reason.

   The state machine for the mobile node is as follows:

    State   Event                         Action         New State
    --------------------------------------------------------------

    Idle    Route Optimization desired    Send HoTI,     WaitHC
                                          Send CoTI,
                                          Start retrans-
                                          mission and
                                          failure timers

    Idle    Valid HoT received            (None)         Idle

    Idle    Valid CoT received            (None)         Idle

    Idle    Valid BA received             (None)         Idle

    Idle    Valid BRR received            (None)         Idle

    Idle    ICMP Parameter Problem Code 1 (None)         Idle
            received

    Idle    Valid BE received and         (None)         Idle
            status = 1

    Idle    Valid BE received and         (None)         Idle
            status = 2

    Idle    Movement                      (None)         Idle


    State   Event                         Action         New State
    --------------------------------------------------------------




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    WaitHC  Valid HoT received            Store cookie   WaitC
                                          and nonce
                                          index

    WaitHC  Valid CoT received            Store cookie   WaitH
                                          and nonce
                                          index

    WaitHC  Valid BA received             (None)         WaitHC

    WaitHC  Valid BRR received            (None)         WaitHC

    WaitHC  Retransmission needed         Send HoTI,     WaitHC
                                          Send CoTI,
                                          Start timer
                                          TRetr

    WaitHC  Valid BE received and         (None)         WaitHC
            status = 1

    WaitHC  Valid BE received and         Stop timers    Idle
            status = 2

    WaitHC  Movement                      Send CoTI,     WaitHC
                                          Restart
                                          retransmission
                                          and failure
                                          timers

    WaitHC  Route Optimization not needed (None)         WaitHC

    WaitHC  ICMP Parameter Problem Code 1 Stop timers    Idle
            received

    State   Event                         Action         New State
    --------------------------------------------------------------

    WaitH   Valid HoT received and        Store cookie   WaitA
            acknowledgements required     and nonce
                                          index,
                                          Send BU,
                                          Start retrans-
                                          mission timer

    WaitH   Valid HoT received and        Store cookie   Bound
            acknowledgements not required and nonce
                                          index,
                                          Send BU,
                                          Stop timers

    WaitH   Valid CoT received            (None)         WaitH



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    WaitH   Valid BA received             (None)         WaitH

    WaitH   Valid BRR received            (None)         WaitH

    WaitH   Retransmission needed         Send HoTI,     WaitH
                                          Start retrans-
                                          mission timer

    WaitH   Valid BE received and         (None)         WaitH
            status = 1

    WaitH   Valid BE received and         Stop timers    Idle
            status = 2

    WaitH   Movement                      Send CoTI,     WaitH
                                          Restart
                                          retransmission
                                          and failure
                                          timers

    WaitH   Route Optimization not needed (None)         WaitH

    WaitH   ICMP Parameter Problem Code 1 (None)         WaitH
            received

    State   Event                         Action         New State
    --------------------------------------------------------------

    WaitC   Valid CoT received and        Store cookie   WaitA
            acknowledgements required     and nonce
                                          index,
                                          Send BU,
                                          Start retrans-
                                          mission timers

    WaitC   Valid CoT received            Store cookie   Bound
            and acknowledgements not      and nonce
            required                      index,
                                          Send BU,
                                          Stop timers

    WaitC   Valid HoT received            (None)         WaitC

    WaitC   Valid BA received             (None)         WaitC

    WaitC   Valid BRR received            (None)         WaitC

    WaitC   Valid BE received and         (None)         WaitC
            status = 1




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    WaitC   Valid BE received and         Stop timers    Idle
            status = 2

    WaitC   Retransmission needed         Send CoTI,     WaitC
                                          Start retrans-
                                          mission timer

    WaitC   Movement                      Send CoTI,     WaitC
                                          Restart
                                          retransmission
                                          and failure
                                          timers

    WaitC   Route Optimization not needed (None)         WaitC

    WaitC   ICMP Parameter Problem Code 1 (None)         WaitC
            received

    State   Event                         Action         New State
    --------------------------------------------------------------

    WaitA   Valid BA received and         Stop timers    Bound
            status < 128

    WaitA   Valid BA received and         Set sequence#, WaitA
            status = 141                  Send BU,
                                          Restart
                                          retransmission
                                          and failure
                                          timers

    WaitA   Valid BA received and         Send HoTI,     WaitHC
            status = 144 or 145           Send CoTI,
                                          Restart
                                          retransmission
                                          and failure
                                          timers

    WaitA   Valid BA received and         Stop timers    Idle
            status anything else

    WaitA   Valid HoT received            (None)         WaitA

    WaitA   Valid CoT received            (None)         WaitA

    WaitA   Valid BRR received            (None)         WaitA

    WaitA   Retransmission needed         Send BU,       WaitA
                                          Start retrans-
                                          mission timer




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    WaitA   Valid BE received and         (None)         WaitA
            status = 1

    WaitA   Valid BE received and         Stop timers    Idle
            status = 2

    WaitA   Movement                      Send CoTI,     WaitC
                                          Restart
                                          retransmission
                                          and failure
                                          timers

    WaitA   Route Optimization not needed (None)         WaitA

    WaitA   ICMP Parameter Problem Code 1 (None)         WaitA
            received

    State   Event                         Action         New State
    --------------------------------------------------------------

    WaitD   Valid BA received and         Stop timers    Idle
            status < 128

    WaitD   Valid BA received and         Set sequence#, WaitD
            status = 141                  Send BU,
                                          Restart
                                          retransmission
                                          and failure
                                          timers

    WaitD   Valid BA received and         Send HoTI,     WaitDH
            status = 144 or 145           Restart
                                          retransmission
                                          and failure
                                          timers

    WaitD   Valid BA received and         Stop timers    Idle
            status anything else

    WaitD   Valid HoT received            (None)         WaitD

    WaitD   Valid CoT received            (None)         WaitD

    WaitD   Valid BRR received            (None)         WaitD

    WaitD   Retransmission needed         Send BU,       WaitD
                                          Start retrans-
                                          mission timer

    WaitD   Valid BE received             Stop timers    Idle




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    WaitD   Movement                      (None)         WaitD

    WaitD   Route Optimization Desired    Send HoTI,     WaitHC
                                          Send CoTI,
                                          Restart
                                          retransmission
                                          and failure
                                          timers

    WaitD   ICMP Parameter Problem Code 1 (None)         WaitD
            received

    State   Event                         Action         New State
    --------------------------------------------------------------

    WaitDH  Valid HoT received and        Send BU,       WaitD
            acknowledgements required     Restart
                                          retransmission
                                          and failure
                                          timers

    WaitDH  Valid HoT received and        Send BU,       Idle
            acknowledgements not          Stop timers
            required

    WaitDH  Valid CoT received            (None)         WaitDH

    WaitDH  Valid BA received             (None)         WaitDH

    WaitDH  Valid BRR received            (None)         WaitDH

    WaitDH  Retransmission needed         Send HoTI,     WaitDH
                                          Start retrans-
                                          mission timer

    WaitDH  Valid BE received             Stop timers    Idle

    WaitDH  Movement                      (none)         WaitDH

    WaitDH  Route Optimization Desired    Send HoTI,     WaitHC
                                          Send CoTI,
                                          Restart
                                          retransmission
                                          and failure
                                          timers

    WaitDH  ICMP Parameter Problem Code 1 (None)         WaitDH
            received

    State   Event                         Action         New State
    --------------------------------------------------------------



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    Bound   Valid BRR received            Send HoTI,     WaitHC
                                          Send CoTI,
                                          Start retrans-
                                          mission timers

    Bound   Valid HoT received            (None)         Bound

    Bound   Valid CoT received            (None)         Bound

    Bound   Valid BA received             (None)         Bound

    Bound   Route Optimization not        Send BU        Idle
            needed and home cookie not
            too old and acknowledgements
            not required

    Bound   Route Optimization not        Send BU,       WaitD
            needed and home cookie not    Start retrans-
            too old and acknowledgements  mission and
            required                      failure timers

    Bound   Route Optimization not        Send HoTI,     WaitDH
            needed and home cookie too    Start retrans-
            old                           mission and
                                          failure timers

    Bound   ICMP Parameter Problem Code 1 (None)         Bound
            received

    Bound   Movement and home cookie      Send CoTI,     WaitC
            not too old                   Start retrans-
                                          mission and
                                          failure timers

    Bound   Movement and home cookie      Send HoTI,     WaitHC
            too old                       Send CoTI,
                                          Start retrans-
                                          mission and
                                          failure timers

    Bound   Valid BE received and         (None)         Bound
            status = 1 and reason to
            believe forward progress
            is being made

    Bound   Valid BE received and         Send HoTI,     WaitHC
            status = 1 and no reason to   Send CoTI,
            believe forward progress      Start retrans-
            is being made                 mission and
                                          failure timers



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    Bound   Valid BE received and         (None)         Bound
            status = 2

    Bound   ICMP Parameter Problem Code 1 (None)         Bound
            received

    State   Event                         Action         New State
    --------------------------------------------------------------

    (Any)   Retransmission failed         Stop retrans-  Idle
                                          mission timer

    (Any)   Invalid BRR received                         (No change)

    (Any)   Invalid HoT received                         (No change)

    (Any)   Invalid CoT received                         (No change)

    (Any)   Invalid BA received                          (No change)

    (Any)   Invalid BE received                          (No change)

    (Any)   Invalid MH Type received      Send BE with   (No change)
                                          status 2


B. Changes from Previous Version of the Draft

   This appendix briefly lists some of the major changes in this
   draft relative to the previous version of this same draft,
   draft-ietf-mobileip-ipv6-15.txt:


B.1. Changes from Draft Version 16

    -  The "rest" of the document has been updated to correspond to the
       new packet formats and messages.

    -  Correspondent node operation has been updated to include the new
       security mechanisms.

    -  Procedures for reverse tunneling have been described for both
       home agents and mobile nodes, and these requirements have been
       take in account in Section 8.

    -  Terminology has been aligned throughout the document.  Parameters
       are now mobility options.  Binding Request is Binding Refresh
       Request.  Capitalization of the terms has been aligned throughout
       the document.




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    -  Overview section is now shorter, security issues are discussed
       elsewhere and data structures are fully described later.

    -  Parts of the mobile node requirements under Section 10.9 were
       moved to Section 11.3.3.

    -  A mechanism for Binding Acknowledgement authorization has been
       clarified.

    -  Alignment rules, minimum lengths, and packet formats of Mobility
       Header message have been updated.

    -  Discussion on the use of Type 0 Routing header in addition to
       Type 2 Routing header has been removed from the correspondent
       node operation section, and we now rely only on the ordering
       requirements specified by the Routing Header Type 2 description.

    -  Type 2 Routing header rules have been rewritten to allow for
       Segments Left to be 0.  Explanation on how AH works with Routing
       header has been clarified.  Much of the text has been moved
       to the Mobile Node Operation and Correspondent Node Operation
       sections.

    -  The concept of "persistent" ICMP messages is no longer referred
       to by a MUST keyword in Section 9.7.

    -  References to the "Router (R)" bit have been changed to "Router
       Address (R)" bit.

    -  The Home Agent Information option now has to appear on all Prefix
       Advertisements, or on none of them.

    -  Sub-options have been removed.

    -  The Dynamic Home Agent Address Discovery procedures have been
       updated to not use piggybacking.  Binding Refresh Requests
       are still sent during these procedures in certain cases,
       however.  the Unique Identifier mobility option has been used
       to synchronize BRR and BU instead of the sequence number.  The
       scheduling of the prefix deliveries has been changed to send new
       information even when the current binding is close to expiring.

    -  Section 11.7 now uses ICMP Parameter Problem Code 1 instead of 2.

    -  Sections 11.3.4 and  10.9 now agree that IPsec need not be used
       for the first advertisement.

    -  The rules regarding addresses for receiving and sending multicast
       traffic and control messages have been clarified for mobile
       nodes.




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    -  The Binding Missing message has been renamed to Binding Error.

    -  Eliminated the use of symbols in the description of the return
       routability procedure.

    -  Wrote a new description of the return routability procedure.


B.2. Changes from Draft Version 15

    -  A binding update authorization mechanism suitable for use
       between previously unknown peers in the global Internet has been
       incorporated to the specification.  As a result, Sections 5, 6.1,
       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 return routability 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 6.

    -  The use of the Home Address destination 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 destination option when
       the correspondent node has no existing binding to the node.

    -  The Authorization Data mobility option 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 (return routability).

    -  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.





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    -  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.)

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


B.3. Changes from Earlier Versions of the Draft

    -  Strengthened mandates for mobile nodes so that now a mobile node
       MUST support decapsulation and processing for routing headers
       (section 11.2.3).

    -  Enabled ESP to be a valid way to secure reverse tunneled packets
       (section 10.6).

    -  Removed mandate that mobile node select a default router, and
       instead described it as typical behavior (section 11.4.1).
       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




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       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.

    -  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.



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    -  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 in anticipation of eliminating the use of AH

    -  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 procedure in this document.

    -  Added the following protocol constants:

       INITIAL_SOLICIT_TIMER:  XXX

    -  Created new ICMP messages for Mobile Prefix Solicitations and
       Advertisements (see sections 6.7 and 6.8).

    -  Changed Network Renumbering (Section 10.9.1) 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 11.6.7) to describe
       how the Home Agent discovers the mobile node's link-layer address

    -  Reworded parts of Appendix C as needed.

    -  Added the Mobile Router Prefix Length Sub-Option along with text
       describing what a Mobile Router should do with it.


C. 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



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   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.

    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.


D. Future Extensions

D.1. Piggybacking

   This document does not specify how to piggyback payload packets on
   the binding related messages.  However, it is envisioned that this
   can be specified in a separate document when currently discussed
   issues such as the interaction between piggybacking and IPsec are
   fully resolved (see also Section D.3).

   The idea is to use the Flag field in the HoTI message so that the
   mobile node can indicate that it supports the receipt of piggybacked
   messages, use the Flag field in the HoT message for the correspondent
   node to indicate that it can support the receipt of piggybacked



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   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).


D.2. Triangular Routing and Unverified Home Addresses

   Due to the concerns about opening reflection attacks with the Home
   Address destination option, this specification requires that this
   option must be verified against the binding cache, i.e., there must
   be a binding cache entry for the Home Address and Care-of Address.

   Future extensions may be specified that allow the use of unverified
   Home Address destination options in ways that do not introduce
   security issues.


D.3. New Authorization Methods beyond Return Routability

   While the return routability procedure provides a good level
   of security, there exists methods that have even higher levels
   of security.  Secondly, as discussed in Section 14.3, future
   enhancements of IPv6 security may cause a need to improve also the
   security of the return routability procedure.  The question is then
   what is the method to securely agree on the use of another method,
   while still allowing return routability procedure 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, the final version of this specification requires
   a protection mechanism for selecting between the return routability
   procedure and potential other future mechanisms (see Section 14.3)
   but this isn't ready yet.

   Using IPsec as the sole method for authorizing Binding Updates
   to correspondent nodes is also possible.  The protection of the
   Mobility Header for this purpose is easy, though one must ensure
   that the IPsec SA was created with appropriate authorization to use
   the home address referenced in the Binding Update.  For instance,
   a certificate used by IKE to create the security association might
   contain the home address.  A future specification may specify how
   this is done.









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Chairs' Addresses

   The Working Group can be contacted via its current chairs:


     Basavaraj Patil                   Phil Roberts
     Nokia Corporation                 Megisto Corp.
     6000 Connection Drive             Suite 120
     M/S M8-540                        20251 Century Blvd
     Irving, TX 75039                  Germantown MD 20874
     USA                               USA
     Phone:  +1 972-894-6709           Phone:  +1 847-202-9314
     Fax :  +1 972-894-5349            Email:  PRoberts@MEGISTO.com
     EMail:  Raj.Patil@nokia.com



Authors' Addresses

   Questions about this document can also be directed to the authors:


     David B. Johnson                  Charles Perkins
     Rice University                   Nokia Research Center
     Dept. of Computer Science, MS 132
     6100 Main Street                  313 Fairchild Drive
     Houston, TX 77005-1892            Mountain View, CA 94043
     USA                               USA

     Phone:  +1 713 348-3063           Phone:  +1 650 625-2986
     Fax:  +1 713 348-5930             Fax:  +1 650 625-2502
     E-mail:  dbj@cs.rice.edu          E-mail:  charliep@iprg.nokia.com


     Jari Arkko
     Ericsson
     Jorvas 02420
     Finland

     Phone:  +358 40 5079256
     E-mail:  jari.arkko@ericsson.com













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