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IETF Mobile IP Working Group                            David B. Johnson
INTERNET-DRAFT                                           Rice University
                                                         Charles Perkins
                                                                   Nokia
                                                        17 November 2000


                        Mobility Support in IPv6

                   <draft-ietf-mobileip-ipv6-13.txt>


Status of This Memo

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

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

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

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

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


Abstract

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








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                                Contents


Status of This Memo                                                    i

Abstract                                                               i

 1. Introduction                                                       1

 2. Comparison with Mobile IP for IPv4                                 3

 3. Terminology                                                        6
     3.1. General Terms . . . . . . . . . . . . . . . . . . . . . .    6
     3.2. Mobile IPv6 Terms . . . . . . . . . . . . . . . . . . . .    7
     3.3. Specification Language  . . . . . . . . . . . . . . . . .    8

 4. Overview of Mobile IPv6                                            9
     4.1. Basic Operation . . . . . . . . . . . . . . . . . . . . .    9
     4.2. New IPv6 Destination Options  . . . . . . . . . . . . . .   11
     4.3. Alignment Requirements for New Destination Options  . . .   13
     4.4. IPsec Requirements for New Destination Options  . . . . .   13
     4.5. New IPv6 ICMP Messages  . . . . . . . . . . . . . . . . .   14
     4.6. Conceptual Data Structures  . . . . . . . . . . . . . . .   14
     4.7. Binding Management  . . . . . . . . . . . . . . . . . . .   19

 5. New IPv6 Destination Options and Message Types                    21
     5.1. Binding Update Option . . . . . . . . . . . . . . . . . .   21
     5.2. Binding Acknowledgement Option  . . . . . . . . . . . . .   25
     5.3. Binding Request Option  . . . . . . . . . . . . . . . . .   29
     5.4. Home Address Option . . . . . . . . . . . . . . . . . . .   31
     5.5. Mobile IPv6 Destination Option Sub-Options  . . . . . . .   34
     5.6. ICMP Home Agent Address Discovery Request Message . . . .   37
     5.7. ICMP Home Agent Address Discovery Reply Message . . . . .   39

 6. Modifications to IPv6 Neighbor Discovery                          41
     6.1. Modified Router Advertisement Message Format  . . . . . .   41
     6.2. Modified Prefix Information Option Format . . . . . . . .   42
     6.3. New Advertisement Interval Option Format  . . . . . . . .   44
     6.4. New Home Agent Information Option Format  . . . . . . . .   45
     6.5. Changes to Sending Router Advertisements  . . . . . . . .   47
     6.6. Changes to Sending Router Solicitations . . . . . . . . .   48

 7. Requirements for Types of IPv6 Nodes                              50
     7.1. Requirements for All IPv6 Hosts and Routers . . . . . . .   50
     7.2. Requirements for All IPv6 Routers . . . . . . . . . . . .   50
     7.3. Requirements for IPv6 Home Agents . . . . . . . . . . . .   50
     7.4. Requirements for IPv6 Mobile Nodes  . . . . . . . . . . .   51





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 8. Correspondent Node Operation                                      53
     8.1. Receiving Packets from a Mobile Node  . . . . . . . . . .   53
     8.2. Receiving Binding Updates . . . . . . . . . . . . . . . .   53
     8.3. Requests to Cache a Binding . . . . . . . . . . . . . . .   54
     8.4. Requests to Delete a Binding  . . . . . . . . . . . . . .   55
     8.5. Sending Binding Acknowledgements  . . . . . . . . . . . .   55
     8.6. Sending Binding Requests  . . . . . . . . . . . . . . . .   55
     8.7. Cache Replacement Policy  . . . . . . . . . . . . . . . .   56
     8.8. Receiving ICMP Error Messages . . . . . . . . . . . . . .   57
     8.9. Sending Packets to a Mobile Node  . . . . . . . . . . . .   58

 9. Home Agent Operation                                              60
     9.1. Receiving Router Advertisement Messages . . . . . . . . .   60
     9.2. Dynamic Home Agent Address Discovery  . . . . . . . . . .   61
     9.3. Primary Care-of Address Registration  . . . . . . . . . .   63
     9.4. Primary Care-of Address De-registration . . . . . . . . .   65
     9.5. Intercepting Packets for a Mobile Node  . . . . . . . . .   66
     9.6. Tunneling Intercepted Packets to a Mobile Node  . . . . .   68
     9.7. Handling Reverse Tunneled Packets from a Mobile Node  . .   69
     9.8. Renumbering the Home Subnet . . . . . . . . . . . . . . .   70
           9.8.1. Building Aggregate List of Home Network Prefixes    70
           9.8.2. Sending Changed Prefix Information to the Mobile
                          Node . . . . . . . . . . . . . . . . . . .  71
           9.8.3. Tunneling Router Advertisements to the Mobile Node  73
           9.8.4. Lifetimes for Changed Prefixes  . . . . . . . . .   74

10. Mobile Node Operation                                             75
    10.1. Sending Packets While Away from Home  . . . . . . . . . .   75
    10.2. Interaction with Outbound IPsec Processing  . . . . . . .   76
    10.3. Receiving Packets While Away from Home  . . . . . . . . .   78
    10.4. Movement Detection  . . . . . . . . . . . . . . . . . . .   80
    10.5. Forming New Care-of Addresses . . . . . . . . . . . . . .   82
    10.6. Sending Binding Updates to the Home Agent . . . . . . . .   84
    10.7. Dynamic Home Agent Address Discovery  . . . . . . . . . .   86
    10.8. Sending Binding Updates to Correspondent Nodes  . . . . .   87
    10.9. Establishing Forwarding from a Previous Care-of Address .   89
   10.10. Retransmitting Binding Updates  . . . . . . . . . . . . .   90
   10.11. Rate Limiting for Sending Binding Updates . . . . . . . .   91
   10.12. Receiving Binding Acknowledgements  . . . . . . . . . . .   91
   10.13. Receiving Binding Requests  . . . . . . . . . . . . . . .   92
   10.14. Receiving ICMP Error Messages . . . . . . . . . . . . . .   93
   10.15. Receiving Local Router Advertisement Messages . . . . . .   94
   10.16. Sending Tunneled Router Solicitations . . . . . . . . . .   95
   10.17. Receiving Tunneled Router Advertisements  . . . . . . . .   96
   10.18. Using Multiple Care-of Addresses  . . . . . . . . . . . .   97
   10.19. Routing Multicast Packets . . . . . . . . . . . . . . . .   97
   10.20. Returning Home  . . . . . . . . . . . . . . . . . . . . .   98

11. Protocol Constants                                               100

12. IANA Considerations                                              101



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13. Security Considerations                                          102
    13.1. Binding Updates, Acknowledgements, and Requests . . . . .  102
    13.2. Security for the Home Address Option  . . . . . . . . . .  102
    13.3. General Mobile Computing Issues . . . . . . . . . . . . .  103


















































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Changes from Previous Version of the Draft                           104

Acknowledgements                                                     105

References                                                           106

 A. Remote Home Address Configuration                                108

Chair's Address                                                      109

Authors' Addresses                                                   110











































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1. Introduction

   This document specifies the operation of mobile computers using
   Internet Protocol Version 6 (IPv6) [6].  Without specific support
   for mobility in IPv6, packets destined to a mobile node (host or
   router) would not be able to reach it while the mobile node is away
   from its home link (the link on which its home IPv6 subnet prefix is
   in use), since routing is based on the subnet prefix in a packet's
   destination IP address.  In order to continue communication in spite
   of its movement, a mobile node could change its IP address each time
   it moves to a new link, but the mobile node would then not be able
   to maintain transport and higher-layer connections when it changes
   location.  Mobility support in IPv6 is particularly important, as
   mobile computers are likely to account for a majority or at least a
   substantial fraction of the population of the Internet during the
   lifetime of IPv6.

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

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

   One can think of the Mobile IPv6 protocol as solving the
   network-layer mobility management problem.  Some mobility management
   applications -- for example, handoff 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
   "handoff" 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 handoff 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, such as typical
       wireless networks.  Some aspects of this problem are addressed
       by the movement detection procedure described in Section 10.4,
       but no attempt has been made to fully solve this problem in its
       general form.  Most aspects of this problem can be solved by the
       workaround of restricting such networks to only one router per
       link, although there are still possible hidden terminal problems
       when two nodes on the same link (on opposite sides of the router)
       attempt to communicate directly.

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

































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2. Comparison with Mobile IP for IPv4

   The design of Mobile IP support in IPv6 (Mobile IPv6) represents a
   natural combination of the experiences gained from the development
   of Mobile IP support in IPv4 (Mobile IPv4) [19, 18, 20], 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" [21] 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 [19].  The Mobile IPv4
       "registration" functionality and the Mobile IPv4 Route
       Optimization functionality are performed by a single protocol
       rather than two separate (and different) protocols.

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

    -  The use of 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,



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       mobile nodes make use of IPv6 features, such as Neighbor
       Discovery [17] and Address Autoconfiguration [27], to operate in
       any location away from home without any special support required
       from its local router.

    -  Unlike Mobile IPv4, Mobile IPv6 utilizes IP Security
       (IPsec) [11, 12, 13] for all security requirements (sender
       authentication, data integrity protection, and replay protection)
       for Binding Updates (which serve the role of both registration
       and Route Optimization in Mobile IPv4).  Mobile IPv4 relies
       on its own security mechanisms for these functions, based on
       statically configured "mobility security associations".

    -  The movement detection mechanism in Mobile IPv6 provides
       bidirectional confirmation of a mobile node's ability to
       communicate with its default router in its current location
       (packets that the router sends are reaching the mobile node, and
       packets that the mobile node sends are reaching the router).
       This confirmation provides a detection of the "black hole"
       situation that may exist in some wireless environments where the
       link to the router does not work equally well in both directions,
       such as when the mobile node has moved out of good wireless
       transmission range from the router.  The mobile node may then
       attempt to find a new router and begin using a new care-of
       address if its link to its current router is not working well.
       In contrast, in Mobile IPv4, only the forward direction (packets
       from the router are reaching the mobile node) is confirmed,
       allowing the black hole condition to persist.

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

    -  While a mobile node is away from home, its home agent intercepts
       any packets for the mobile node that arrive at the home network,
       using IPv6 Neighbor Discovery [17] rather than ARP [23] as is
       used in Mobile IPv4.  The use of Neighbor Discovery improves
       the robustness of the protocol (e.g., due to the Neighbor
       Advertisement "override" bit) and simplifies implementation
       of Mobile IP due to the ability to not be concerned with any
       particular link layer as is required in ARP.

    -  The use of IPv6 encapsulation (and the Routing header) removes
       the need in Mobile IPv6 to manage "tunnel soft state", which was
       required in Mobile IPv4 due to limitations in ICMP for IPv4.  Due



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

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

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





























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3. Terminology

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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3.2. Mobile IPv6 Terms

      home address

         An IP address assigned to a mobile node within its home link.

      home subnet prefix

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

      home link

         The link on which a mobile node's home subnet prefix is
         defined.  Standard IP routing mechanisms will deliver packets
         destined for a mobile node's home address to its home link.

      mobile node

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

      movement

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

      correspondent node

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

      foreign subnet prefix

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

      foreign link

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

      home agent

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



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      care-of address

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

      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.


3.3. Specification Language

   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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4. Overview of Mobile IPv6

4.1. Basic Operation

   A mobile node is always addressable by its home address, whether it
   is currently attached to its home link or is away from home.  While
   a mobile node is at home, packets addressed to its home address are
   routed to it using conventional Internet routing mechanisms in the
   same way as if the node were never mobile.  Since the subnet prefix
   of a mobile node's home address is the subnet prefix (or one of the
   subnet prefixes) on the mobile node's home link (it is the mobile
   node's home subnet prefix), packets addressed to it will be routed to
   its home link.

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

   The association between a mobile node's home address and care-of
   address is known as a "binding" for the mobile node.  A mobile node
   typically acquires its care-of address through stateless [27] or
   stateful (e.g., DHCPv6 [2]) Address Autoconfiguration, according
   to the methods of IPv6 Neighbor Discovery [17].  Other methods
   of acquiring a care-of address are also possible, such as static
   pre-assignment by the owner or manager of a particular foreign link,
   but details of such other methods are beyond the scope of this
   document.

   While away from home, a mobile node registers one of its care-of
   addresses with a router on its home link, requesting this router
   to function as the "home agent" for the mobile node.  This binding
   registration is done by the mobile node sending to the home agent
   a packet containing a "Binding Update" destination option; the
   home agent then replies to the mobile node by returning a packet
   containing a "Binding Acknowledgement" destination option.  The
   care-of address in this binding registered with its home agent is
   known as the mobile node's "primary care-of address".  The mobile
   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.



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   When a mobile node moves from one care-of address to a new care-of
   address on a new link, it is desirable for packets arriving at the
   previous care-of address to be tunneled to the mobile node's care-of
   address.  Since the purpose of a Binding Update is to establish
   exactly this kind of tunneling, it is specified to be used (at
   least temporarily) for tunnels originating at the mobile node's
   previous care-of address, in exactly the same way that it is used
   for establishing tunnels from the mobile node's home address to the
   mobile node's current care-of address.  Section 10.9 describes the
   use of the Binding Update for this purpose.

   Section 10.18 discusses the reasons why it may be desirable for
   a mobile node to use more than one care-of address at the same
   time.  However, a mobile node's primary care-of address is distinct
   among these in that the home agent maintains only a single care-of
   address registered for each mobile node, and always tunnels a mobile
   node's packets intercepted from its home link to this mobile node's
   registered primary care-of address.  The home agent thus need not
   implement any policy to determine which of possibly many care-of
   addresses to which to tunnel each intercepted packet.  The mobile
   node alone controls the policy by which it selects the care-of
   addresses to register with its home agent.

   It is possible that while a mobile node is away from home, some nodes
   on its home link may be reconfigured, such that the router that was
   operating as the mobile node's home agent is replaced by a different
   router serving this role.  In this case, the mobile node may not
   know the IP address of its own home agent.  Mobile IPv6 provides a
   mechanism, known as "dynamic home agent address discovery", that
   allows a mobile node to dynamically discover the IP address of a home
   agent on its home link with which it may register its 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 [10] and thus reaches
   one of the (possibly many) routers on its home link currently
   operating as a home agent.  This home agent then returns an ICMP
   "Home Agent Address Discovery Reply" message to the mobile node,
   including a list of home agents on the home link.  This list of home
   agents is maintained by each home agent on the home link through use
   of the Home Agent (H) bit in each home agent's periodic unsolicited
   multicast Router Advertisements.

   The Binding Update and Binding Acknowledgement destination options,
   together with a "Binding Request" destination option, are also used
   to allow IPv6 nodes communicating with a mobile node, to dynamically
   learn and cache the mobile node's binding.  When sending a packet
   to any IPv6 destination, a node checks its cached bindings for an
   entry for the packet's destination address.  If a cached binding for
   this destination address is found, the node uses an IPv6 Routing
   header [6] (instead of IPv6 encapsulation) to route the packet to
   the mobile node by way of the care-of address indicated in this



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   binding.  If, instead, the sending node has no cached binding for
   this destination address, the node sends the packet normally (with
   no Routing header), and the packet is subsequently intercepted and
   tunneled by the mobile node's home agent as described above.  Any
   node communicating with a mobile node is referred to in this document
   as a "correspondent node" of the mobile node, and may itself be
   either a stationary node or a mobile node.

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

    -  the messages can be included within any IPv6 packet carrying any
       payload such as TCP [25] or UDP [24].

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

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


4.2. New IPv6 Destination Options

   As mentioned in Section 4.1, the following four new IPv6 destination
   options are defined for Mobile IPv6:






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      Binding Update

         A Binding Update option 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".  Any packet that includes a Binding
         Update option MUST be protected by IPsec [13], as defined in
         Section 4.4, to guard against malicious Binding Updates.  The
         Binding Update option and its specific IPsec requirements are
         described in detail in Section 5.1.

      Binding Acknowledgement

         A Binding Acknowledgement option is used to acknowledge receipt
         of a Binding Update, if an acknowledgement was requested
         in the Binding Update.  Any packet that includes a Binding
         Acknowledgement option MUST be protected by IPsec [13], as
         defined in Section 4.4, to guard against malicious Binding
         Acknowledgements.  The Binding Acknowledgement option and
         its specific IPsec requirements are described in detail in
         Section 5.2.

      Binding Request

         A Binding Request option is used to request a mobile node to
         send to the requesting node a Binding Update containing the
         mobile node's current binding.  This option is typically used
         by a correspondent node to refresh a cached binding for a
         mobile node, when the cached binding is in active use but the
         binding's lifetime is close to expiration.  No authentication
         is required for the Binding Request option.  The Binding
         Request option is described in detail in Section 5.3.

      Home Address

         A Home Address option is used in a packet sent by a mobile
         node to inform the recipient of that packet of the mobile
         node's home address.  For packets sent by a mobile node while
         away from home, the mobile node generally uses one of its
         care-of addresses as the Source Address in the packet's IPv6
         header.  By including a Home Address option in the packet, the
         correspondent node receiving the packet is able to substitute
         the mobile node's home address for this care-of address when
         processing the packet, thus making the use of the care-of
         address transparent to the correspondent node.  If the IP
         header of a packet carrying a Home Address option is covered
         by authentication, then the Home Address option MUST also be
         covered by this authentication, but no other authentication
         is required for the Home Address option.  See sections 10.2
         and 5.4 for additional details about requirements for the



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         calculation and verification of the authentication data.  The
         Home Address option is described in detail in Section 5.4.

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


4.3. Alignment Requirements for New Destination Options

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

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

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


4.4. IPsec Requirements for New Destination Options

   Any packet that includes a Binding Update or Binding Acknowledgement
   option MUST be protected by IPsec [13] to guard against malicious
   Binding Updates or Acknowledgements.  Specifically, any packet that
   includes a Binding Update or Binding Acknowledgement option MUST
   utilize IPsec sender authentication, data integrity protection, and
   replay protection.

   Mobile IPv6 requires that this protection covering a Binding Update
   or Binding Acknowledgement MUST be provided by use of AH [11].  If
   another Security Association applied to the packet for other reasons
   requires use of ESP [12], for example to encrypt the transport layer
   data carried in the packet, this use of ESP is not sufficient to
   satisfy the authentication requirements of Mobile IPv6; instead,



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   the packet MUST use both AH and ESP.  Use of ESP for protecting the
   Binding Update or Binding Acknowledgement is not currently defined in
   this document, since ESP does not protect the portion of the packet
   above the ESP header itself [12].


4.5. New IPv6 ICMP Messages

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

      Home Agent Address Discovery Request

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

      Home Agent Address Discovery Reply

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


4.6. Conceptual Data Structures

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

      Binding Cache

         A cache, maintained by each IPv6 node, of bindings for other
         nodes.  A separate Binding Cache SHOULD be maintained by each
         IPv6 node for each of its IPv6 addresses.  The Binding Cache
         MAY be implemented in any manner consistent with the external
         behavior described in this document, for example by being
         combined with the node's Destination Cache as maintained by
         Neighbor Discovery [17].  When sending a packet, the Binding
         Cache is searched before the Neighbor Discovery conceptual



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         Destination Cache [17] (i.e., any Binding Cache entry for this
         destination SHOULD take precedence over any Destination Cache
         entry for the same destination).  Each Binding Cache entry
         conceptually contains the following fields:

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

          -  The care-of address for the mobile node indicated by
             the home address field in this Binding Cache entry.  If
             the destination address of a packet being routed by a
             node matches the home address in this entry, the packet
             SHOULD be routed to this care-of address, as described in
             Section 8.9, for packets originated by this node, or in
             Section 9.6, 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 on this entry expires, the entry MUST be
             deleted from the Binding Cache.

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

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

          -  The value of the Prefix Length field received in the
             Binding Update that created or last modified this Binding
             Cache entry.  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




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             value B if ((short)((a) - (b)) > 0), if a "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 Request should be sent when the lifetime on this
             entry nears expiration.

          -  The time at which a Binding Request was last sent for this
             entry, as needed to implement the rate limiting restriction
             for sending Binding Requests.

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

      Binding Update List

         A list, maintained by each mobile node, recording information
         for each Binding Update sent by this mobile node, for which the
         Lifetime sent in that Binding Update has not yet expired.  The
         Binding Update List includes all bindings sent by the mobile
         node:  those to correspondent nodes, those to the mobile node's
         home agent, and those to a home agent on the link on which the
         mobile node's previous care-of address is located.  However,
         for multiple Binding Updates sent to the same destination
         address, the Binding Update List contains only the most recent
         Binding Update (i.e., with the greatest Sequence Number value)
         sent to that destination.  The Binding Update List MAY be
         implemented in any manner consistent with the external behavior
         described in this document.  Each Binding Update List entry
         conceptually contains the following fields:

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



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

              *  the mobile node's home addresses for typical Binding
                 Updates (Sections 10.6 and 10.8), or

              *  the mobile node's previous care-of address for Binding
                 Updates sent to establish forwarding from the mobile
                 node's previous care-of address by a home agent from
                 this previous care-of address (Section 10.9).

          -  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 ((short)((a) - (b)) > 0), if a
             "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 2,
             error message in response to a Binding Update sent to that
             destination, as described in Section 10.14.



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      Home Agents List

         A list, maintained by each home agent and each mobile node,
         recording information about each home agent from which this
         node has received a Router Advertisement in which the Home
         Agent (H) bit is set, for which the remaining lifetime for
         this list entry (defined below) has not yet expired.  The
         home agents list is thus similar to the Default Router
         List conceptual data structure maintained by each host for
         Neighbor Discovery [17], 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 [17].

          -  One or more global IP addresses for this home agent,
             learned through Prefix Information options with
             the Router Address (R) bit set, received in Router
             Advertisements from this link-local address.  Global
             addresses for the router in a Home Agents List entry MUST
             be deleted once the prefix associated with that address is
             no longer valid [17].

          -  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.



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          -  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.


4.7. 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 assumes 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
   thus returns a Binding Update to the correspondent node, allowing
   it to cache the mobile node's binding for routing future packets to
   it.  Although the mobile node may request an acknowledgement for
   this Binding Update, it need not, since subsequent packets from the
   correspondent node will continue to be intercepted and tunneled by
   the mobile node's home agent, effectively causing any needed Binding
   Update retransmission.

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

   A mobile node may use more than one care-of address at the same
   time, although only one care-of address may be registered for it at



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

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























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5. New IPv6 Destination Options and Message Types

5.1. Binding Update Option

   The Binding Update destination option is used by a mobile node
   to notify other nodes of a new care-of address for itself.  As a
   destination option, it MAY be included in any existing packet being
   sent to this same destination or MAY be sent in a packet by itself;
   a packet containing a Binding Update is sent in the same way as any
   packet sent by a mobile node (Section 10.1).

   The Binding Update 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 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |A|H|R|D|Reservd| Prefix Length |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Lifetime                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Sub-Options...
   +-+-+-+-+-+-+-+-+-+-+-+-

      Option Type

         198 = 0xC6

      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 8 plus the total length of all sub-options
         present, including their Sub-Option Type and Sub-Option Len
         fields.

      Acknowledge (A)

         The Acknowledge (A) bit is set by the sending mobile node to
         request a Binding Acknowledgement (Section 5.2) be returned
         upon receipt of the Binding Update.

      Home Registration (H)

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




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         of the mobile node in the binding (given by the Home Address
         field in the Home Address option in the packet).

      Router (R)

         The Router (R) bit, when set, indicates that the sending
         mobile node is a router.  This bit is only valid when the
         Home Registration (H) bit is also set, and MUST NOT be set
         otherwise.  This bit is saved in the home agent's "home
         registration" Binding Cache entry for the mobile node, and
         is copied into the corresponding bit in all proxy Neighbor
         Advertisement messages sent on behalf of this mobile node by
         the home agent using this Binding Cache entry.

      Duplicate Address Detection (D)

         The Duplicate Address Detection (D) bit is set by the sending
         mobile node to request the receiving node (the mobile node's
         home agent) to perform Duplicate Address Detection [27] 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).

      Reservd

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

      Prefix Length

         The Prefix Length field is valid only for a "home registration"
         Binding Update; this field MUST be zero if the Home
         Registration (H) bit is not set in the Binding Update.  The
         Prefix Length field is set by the sending mobile node to the
         (nonzero) length of its subnet prefix in its home address
         (given in the Home Address option in the packet) to request
         its home agent to use the interface identifier in the mobile
         node's home address (the remaining low-order bits after the
         indicated subnet prefix) to form all other home addresses for
         the mobile node on the home link.  The home agent becomes the
         home agent not only for the individual home address given in
         this binding, but also for all other home addresses for this
         mobile node formed from this interface identifier.  That is,
         for each on-link prefix on the home link, the home agent uses
         the interface identifier to form other valid addresses for
         the mobile node on the home link, and acts as a home agent
         also for those addresses.  In addition, the home agent forms



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         the link-local address and site-local address corresponding
         to this interface identifier, and defends each for purposes
         of Duplicate Address Detection.  The home agent also performs
         Duplicate Address Detection on at least one such address as
         part of the home registration processing (before returning
         the Binding Acknowledgement), if the Duplicate Address
         Detection (D) bit is set in the Binding Update; it is not
         necessary to perform Duplicate Address Detection individually
         on each of these addresses, since address uniqueness here is
         determined solely by the interface identifier [27].  Details of
         this operation are described in Section 9.3.

      Sequence 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 4.6).  There is no requirement, however, that the
         Sequence Number value strictly increase by 1 with each new
         Binding Update sent or received.

      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 is zero
         indicates that the Binding Cache entry for the mobile node MUST
         be deleted.

      Sub-Options

         Additional information, associated with this Binding Update
         option, that need not be present in all Binding Updates sent.
         This use of sub-options also allows for future extensions to
         the format of the Binding Update option to be defined.  The
         encoding and format of defined sub-options are described in
         Section 5.5.  The following sub-options are valid in a Binding
         Update option:

          -  Unique Identifier Sub-Option

          -  Alternate Care-of Address Sub-Option

   The alignment requirement [6] for the Binding Update option is 4n+2.

   Any packet that includes a Binding Update option MUST also include
   a Home Address option.  The home address of the mobile node in the
   binding given in the Binding Update option is that which was received



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   as the value of the Home Address field in the Home Address option in
   the packet.

   The care-of address for the binding given in the Binding Update
   option 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 option.  However, a care-of address different from the Source
   Address MAY be specified by including an Alternate Care-of Address
   sub-option in the Binding Update option.

   Any packet that includes a Binding Update option MUST be protected by
   IPsec [13] to guard against malicious Binding Updates.  The specific
   requirements for this protection are defined in Section 4.4.

   If the care-of address for the binding (specified either in an
   Alternate Care-of Address sub-option in the Binding Update option, if
   present, or in the Source Address field in the packet's IPv6 header)
   is equal to the home address of the mobile node, the Binding Update
   option indicates that any existing binding for the mobile node MUST
   be deleted.  Likewise, if the Lifetime field in the Binding Update
   option is equal to 0, the Binding Update option indicates that any
   existing binding for the mobile node MUST be deleted.  In each of
   these cases, a Binding Cache entry for the mobile node MUST NOT be
   created in response to receiving the Binding Update.

   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; the last Sequence Number value received from
   a mobile node in a Binding Update is stored by a correspondent node
   in its Binding Cache entry for that mobile node.  Thus, the mobile
   node's and the correspondent node's knowledge of the last sequence
   number expire at the same time.  If the sending mobile node has no
   Binding Update List entry, the Sequence Number may start at any
   value; if the receiving correspondent node has no Binding Cache entry
   for the sending mobile node, it MUST accept any Sequence Number value
   in a received Binding Update from this mobile node.

   The three highest-order bits of the Option Type are encoded to
   indicate specific processing of the option [6].  For the Binding
   Update option, these three bits are set to 110, indicating that any
   IPv6 node processing this option that does not recognize the Option
   Type must discard the packet and, only if the packet's Destination
   Address was not a multicast address, return an ICMP Parameter
   Problem, Code 2, message to the packet's Source Address; and that the
   data within the option cannot change en-route to the packet's final
   destination.








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5.2. Binding Acknowledgement Option

   The Binding Acknowledgement destination option is used to acknowledge
   receipt of a Binding Update option (Section 5.1).  When a node
   receives a packet containing a Binding Update option, with this
   node being the destination of the packet (only the destination node
   processes the option since it is a destination option), this node
   MUST return a Binding Acknowledgement to the source of the packet,
   if the Acknowledge (A) bit is set in the Binding Update.  As a
   destination option, this node MAY include the Binding Acknowledgement
   in any existing packet being sent to the mobile node or MAY send it
   in a packet by itself.  A packet containing a Binding Acknowledgement
   is sent in the same way as any packet to a mobile node, using a
   Routing header to route the packet to the mobile node by way of the
   care-of address in the binding (Section 8.9).

   The Binding Acknowledgement 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 |    Status     |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Lifetime                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Refresh                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Sub-Options...
   +-+-+-+-+-+-+-+-+-+-+-+-

      Option Type

         7

      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 11 plus the total length of all sub-options
         present, including their Sub-Option Type and Sub-Option Len
         fields.

      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:



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              0   Binding Update accepted

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

            128   Reason unspecified
            130   Administratively prohibited
            131   Insufficient resources
            132   Home registration not supported
            133   Not home subnet
            136   Incorrect interface identifier length
            137   Not home agent for this mobile node
            138   Duplicate Address Detection failed

         Up-to-date values of the Status field are to be specified in
         the most recent "Assigned Numbers" [26].

      Sequence Number

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

      Lifetime

         The granted lifetime, in seconds, for which this node will
         attempt to retain the entry for this mobile node in its Binding
         Cache.  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



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

      Sub-Options

         Additional information, associated with this Binding
         Acknowledgement option, that need not be present in all Binding
         Acknowledgements sent.  This use of sub-options also allows for
         future extensions to the format of the Binding Acknowledgement
         option to be defined.  The encoding and format of defined
         sub-options are described in Section 5.5.  Currently, no valid
         sub-options are defined for a Binding Acknowledgement option.

   The alignment requirement [6] for the Binding Acknowledgement option
   is 4n+3.

   Any packet that includes a Binding Acknowledgement option MUST
   be protected by IPsec [13] to guard against malicious Binding
   Acknowledgements.  The specific requirements for this protection are
   defined in Section 4.4.

   If the node returning the Binding Acknowledgement accepted the
   Binding Update for which the Acknowledgement is being returned (the
   value of the Status field in the Acknowledgement is less than 128),
   this node will have an entry for the mobile node in its Binding Cache
   and MUST use this entry (which includes the care-of address received
   in the Binding Update) in sending the packet containing the Binding
   Acknowledgement to the mobile node.  The details of sending this
   packet to the mobile node are the same as for sending any packet to
   a mobile node using a binding, as are described in Section 8.9.  The
   packet is sent using a Routing header, routing the packet to the
   mobile node by way of its care-of address recorded in the Binding
   Cache entry.

   If the node returning the Binding Acknowledgement instead
   rejected the Binding Update (the value of the Status field in the
   Acknowledgement is greater than or equal to 128), this node MUST
   similarly use a Routing header in sending the packet containing the
   Binding Acknowledgement, as described in Section 8.9, but MUST NOT
   use its Binding Cache in forming the IP header or Routing header
   in this packet.  Rather, the care-of address used by this node in
   sending the packet containing the Binding Acknowledgement MUST be
   copied from the care-of address received in the rejected Binding



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   Update; this node MUST NOT modify its Binding Cache in response
   to receiving this rejected Binding Update and MUST ignore its
   Binding Cache in sending the packet in which it returns this Binding
   Acknowledgement.  The packet is sent using a Routing header, routing
   the packet to the home address of the rejected Binding Update by
   way of the care-of address indicated in the packet containing the
   Binding Update.  When sending a Binding Acknowledgement to reject a
   Binding Update, the Binding Acknowledgement MUST be sent in an IPv6
   packet containing no payload (with the Next Header field in the last
   extension header in the packet set to indicate "No Next Header" [6]).

   The three highest-order bits of the Option Type are encoded to
   indicate specific processing of the option [6].  For the Binding
   Acknowledgement option, these three bits are set to 000, indicating
   that any IPv6 node processing this option that does not recognize the
   Option Type must skip over this option and continue processing the
   header, and that the data within the option cannot change en-route to
   the packet's final destination.




































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5.3. Binding Request Option

   The Binding Request destination option is used to request a mobile
   node's binding from the mobile node.  As a destination option, it
   MAY be included in any existing packet being sent to the mobile
   node or MAY be sent in a packet by itself; a packet containing a
   Binding Request option is sent in the same way as any packet to a
   mobile node (Section 8.9).  When a mobile node receives a packet
   containing a Binding Request option, it SHOULD return a Binding
   Update (Section 5.1) to the source of the Binding Request.

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

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |  Option Type  | Option Length |   Sub-Options...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

      Option Type

         8

      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 0 plus the total length of all sub-options
         present, including their Sub-Option Type and Sub-Option Len
         fields.

      Sub-Options

         Additional information, associated with this Binding Request
         option, that need not be present in all Binding Requests sent.
         This use of sub-options also allows for future extensions to
         the format of the Binding Request option to be defined.  The
         encoding and format of defined sub-options are described in
         Section 5.5.  The following sub-options are valid in a Binding
         Request option:

          -  Unique Identifier Sub-Option

   There is no requirement for alignment [6] of the Binding Request
   option.

   The three highest-order bits of the Option Type are encoded to
   indicate specific processing of the option [6].  For the Binding
   Request option, these three bits are set to 000, indicating that any
   IPv6 node processing this option that does not recognize the Option



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   Type must skip over this option and continue processing the header,
   and that the data within the option cannot change en-route to the
   packet's final destination.



















































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5.4. Home Address Option

   The Home Address destination option is used in a packet sent by a
   mobile node while away from home, to inform the recipient of that
   packet of the mobile node's home address.  For packets sent by a
   mobile node while away from home, the mobile node generally uses
   one of its care-of addresses as the Source Address in the packet's
   IPv6 header.  By including a Home Address option in the 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.

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

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

      Option Type

         201 = 0xC9

      Option Length

         8-bit unsigned integer.  Length of the option, in octets,
         excluding the Option Type and Option Length fields.  This field
         MUST be set to 16 plus the total length of all sub-options
         present, including their Sub-Option Type and Sub-Option Len
         fields.

      Home Address

         The home address of the mobile node sending the packet.







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      Sub-Options

         Additional information, associated with this Home Address
         option, that need not be present in all Home Address options
         sent.  This use of sub-options also allows for future
         extensions to the format of the Home Address option to be
         defined.  The encoding and format of defined sub-options are
         described in Section 5.5.  Currently, no valid sub-options are
         defined for use in a Home Address option.

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

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

   The Home Address option MUST be placed as follows:

    -  After the Routing Header, if that header is present

    -  Before the Fragment Header, if that header is present

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

   No 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 13.  When
   attempting to verify authentication data in a packet that contains
   a Home Address option, the receiving node MUST make the calculation
   as if the care-of address were present in the Home Address option,
   and the home address were present in the source IPv6 address field
   of the IPv6 header.  This conforms with the calculation specified in
   section 10.2.

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



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   The three highest-order bits of the Option Type are encoded to
   indicate specific processing of the option [6].  For the Home Address
   option, these three bits are set to 110, indicating that any IPv6
   node processing this option that does not recognize the Option Type
   must discard the packet and, only if the packet's Destination Address
   was not a multicast address, return an ICMP Parameter Problem,
   Code 2, message to the packet's Source Address; and that the data
   within the option cannot change en-route to the packet's final
   destination.













































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5.5. Mobile IPv6 Destination Option Sub-Options

   In order to allow optional fields that may not be needed in every
   use of any given Mobile IPv6 destination option, and to allow future
   extensions to the format of these destination options to be defined,
   any of the Mobile IPv6 destination options defined in this document
   MAY include one or more sub-options.

   Such sub-options are included in the data portion of the destination
   option itself, after the fixed portion of the option data specified
   for that particular destination option (Sections 5.1 through 5.4).
   The presence of such sub-options will be indicated by the Option
   Length field.  When the Option Length is greater than the standard
   length defined for that destination option, the remaining octets are
   interpreted as sub-options.

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

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

      Sub-Option Type

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

      Sub-Option Length

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

      Sub-Option Data

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

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



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   of each sub-option is specified as part of the definition of each
   sub-option below.

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

   Currently, the following sub-option types are defined for use in
   Mobile IPv6 destination options:

   Pad1 Sub-Option   (alignment requirement: none)

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

      NOTE! the format of the Pad1 sub-option is a special
      case -- it does not have Sub-Option Len and Sub-Option Data
      fields.

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

   PadN Sub-Option   (alignment requirement: none)

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

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




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   Unique Identifier Sub-Option   (alignment requirement: 2n)

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

      The Unique Identifier sub-option is valid only in Binding
      Request and Binding Update destination options.  The Unique
      Identifier field contains a 16-bit value that serves to
      uniquely identify a Binding Request among those sent by this
      Source Address, and to allow the Binding Update to identify
      the specific Binding Request to which it responds.  This
      matching of Binding Updates to Binding Requests is required
      in the procedure for renumbering the home subnet while a
      mobile node is away from home (Section 9.8).

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

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

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















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5.6. ICMP Home Agent Address Discovery Request Message

   The ICMP Home Agent Address Discovery Request message is used by a
   mobile node to initiate the dynamic home agent address discovery
   mechanism, as described in Sections 9.2 and 10.7.  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 [10], and one of the home agents there responds to the mobile
   node with a Home Agent Address Discovery Reply message giving a list
   of the routers on the mobile node's home link serving as home agents.

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

      Type

         <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.






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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 Address

         The home address of the mobile node sending the Home Agent
         Address Discovery Request message.

   The Source Address of the Home Agent Address Discovery Request
   message packet MUST be one of the mobile node's current care-of
   addresses, and the mobile node MUST NOT include a Home Address
   option in this packet; the home agent then MUST return the Home
   Agent Address Discovery Reply message directly to this care-of
   address.  These restrictions are necessary, since at the time of
   performing this dynamic home agent address discovery, the mobile node
   is generally not registered with its home agent; using the mobile
   node's care-of address simplifies the return of the Reply message to
   the mobile node.


































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5.7. ICMP Home Agent Address Discovery Reply Message

   The ICMP Home Agent Address Discovery Reply message is used by a
   home agent to respond to a mobile node using the dynamic home agent
   address discovery mechanism, as described in Sections 9.2 and 10.7.
   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 [10], and one of the home agents there responds to the
   mobile node with a Home Agent Address Discovery Reply message giving
   a list of the routers on the mobile node's home link serving as home
   agents.

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

      Type

         <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. Modifications to IPv6 Neighbor Discovery

6.1. Modified Router Advertisement Message Format

   Mobile IPv6 modifies the format of the Router Advertisement
   message [17] 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 [17]:

      Home Agent (H)

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

      Reserved

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

















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6.2. Modified Prefix Information Option Format

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

    -  To allow a home agent (a router) to learn the address of all
       other home agents on the link for which it is providing home
       agent service, for use in building its Home Agents List as
       part of the dynamic home agent address discovery mechanism
       (Sections 9.2 and 10.7).

    -  To allow a mobile node to send a Binding Update to a router on
       the link on which its previous care-of address is located, for
       purposes of establishing forwarding from this previous care-of
       address to its new care-of address (Section 10.9).

   However, Neighbor Discovery [17] 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 [17]:






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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 router MUST 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 [17].  In this case, at least one of these
   multiple Advertisements being sent instead of a single larger
   solicited Advertisement, MUST include a Prefix Information option
   with the Router Address (R) bit set.

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


















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6.3. New Advertisement Interval Option Format

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

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

      Type

         7

      Length

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

      Reserved

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

      Advertisement Interval

         32-bit unsigned integer.  The maximum time, in milliseconds,
         between successive unsolicited router Router Advertisement
         messages sent by this router on this network interface.  Using
         the conceptual router configuration variables defined by
         Neighbor Discovery [17], this field MUST be equal to the value
         MaxRtrAdvInterval, expressed in milliseconds.

   Routers MAY include this option in their Router Advertisements.  A
   mobile node receiving a Router Advertisement containing this option
   SHOULD utilize the specified Advertisement Interval for that router
   in its movement detection algorithm, as described in Section 10.4.

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








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6.4. New Home Agent Information Option Format

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

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

      Type

         8

      Length

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

      Reserved

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

      Home Agent Preference

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

         The manual configuration of the Home Agent Preference value
         is described in Section 7.3.  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 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.  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 specified in the main body of the
         Router Advertisement message.

   Home agents MAY include this option in their Router Advertisements.
   This option MUST NOT be included in a Router Advertisement in which
   the Home Agent (H) bit (see Section 6.1) is not set.

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

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
























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6.5. Changes to Sending Router Advertisements

   The Neighbor Discovery protocol specification [17] 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       0.5 seconds

    -  MaxRtrAdvInterval       1.5 seconds

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

   When sending unsolicited multicast Router Advertisements more
   frequently than the standard limit on unsolicited multicast
   Advertisement frequency, the sending router need not include all
   options in each of these Advertisements, but it SHOULD include at
   least one Prefix Information option with the Router Address (R) bit
   set (Section 6.2) in each.




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6.6. Changes to Sending Router Solicitations

   In addition to the limit on routers sending unsolicited multicast
   Router Advertisement messages (Section 6.5), Neighbor Discovery
   defines limits on nodes sending Router Solicitation messages, such
   that a node SHOULD send no more than 3 Router Solicitations, and that
   these 3 transmissions SHOULD be spaced at least 4 seconds apart.
   However, these limits prevent a mobile node from finding a new
   default router (and thus a new care-of address) quickly as it moves
   about.

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

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

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

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

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

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



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       on it current link, until its movement detection algorithm
       (Section 10.4) determines that it has moved and that its current
       care-of address might no longer be valid.



















































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7. Requirements for Types of IPv6 Nodes

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


7.1. Requirements for All IPv6 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 process a Binding Update option
       received in a packet, 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.


7.2. 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 its Router Advertisements, to aid movement
       detection by mobile nodes.  The use of this option in Router
       Advertisements MUST be configurable.

    -  Every IPv6 router SHOULD be able to support sending unsolicited
       multicast Router Advertisements at the faster rate described in
       Section 6.5.  The use of this faster rate MUST be configurable.


7.3. 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:




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

    -  Every home agent MUST maintain a separate Home Agents List for
       each link on which it is serving as a home agent, as described in
       Section 4.6.

    -  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 [10], and MUST be
       able to participate in dynamic home agent address discovery
       (Section 9.2).

    -  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.


7.4. 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
       decapsulation [4].

    -  Every IPv6 mobile node MUST support sending Binding Update
       options, as specified in Sections 10.6, 10.8, and 10.9; and MUST
       be able to receive and process Binding Acknowledgement options,
       as specified in Section 10.12.

    -  Every IPv6 mobile node MUST support use of the dynamic home agent
       address discovery mechanism, as described in Section 10.7.



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    -  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 Request
       option, by responding with a Binding Update option.

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

    -  Every IPv6 mobile node MUST maintain a Home Agents List, as
       described in Section 4.6.






































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8. Correspondent Node Operation

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


8.1. Receiving Packets from a Mobile Node

   Packets sent by a mobile node while away from home generally include
   a Home Address option.  When any node receives a packet containing
   a Home Address option, it MUST process the option in a manner
   consistent with exchanging the Home Address field from the Home
   Address option into the IPv6 header, replacing the original value of
   the Source Address field there.  However, any actual modifications
   to the Source Address field in the packet's IPv6 header MUST not
   be performed until after all processing of other options contained
   in the same Destination Options extension header is completed.
   Currently, no other such options are defined.

   Further processing of such a packet after all IPv6 options processing
   (e.g., at the transport layer) thus does not need to know that the
   original Source Address was a care-of address, or that the Home
   Address option was used in the packet.  Since the sending mobile
   node uses its home address at the transport layer when sending such
   a packet, the use of the care-of address and Home Address option is
   transparent to both the mobile node and the correspondent node above
   the level of the Home Address option generation and processing.


8.2. Receiving Binding Updates

   Upon receiving a Binding Update option in some packet, the receiving
   node MUST validate the Binding Update according to the following
   tests:

    -  The packet meets the specific IPsec requirements for Binding
       Updates, defined in Section 4.4.

    -  The packet MUST contain a Home Address option.

    -  The Option Length field in the Binding Update option is greater
       than or equal to the length specified in Section 5.1.

    -  The Sequence Number field in the Binding Update option is greater
       than the Sequence Number received in the previous Binding Update
       for this home address, if any.  As noted in Section 4.6, this
       Sequence Number comparison MUST be performed modulo 2**16.





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   Any Binding Update not satisfying all of these tests MUST be
   silently ignored, and the packet carrying the Binding Update MUST be
   discarded.

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

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

    -  If the Lifetime specified in the Binding Update is nonzero and
       the specified Care-of Address is not equal to the home address
       for the binding, then this is a request to cache a binding for
       the mobile node.  If the Home Registration (H) bit is set in the
       Binding Update, the Binding Update is processed according to the
       procedure specified in Section 9.3; otherwise, it is processed
       according to the procedure specified in Section 8.3.

    -  If the Lifetime specified in the Binding Update is zero or the
       specified Care-of Address matches the home address for the
       binding, then this is a request to delete the mobile node's
       cached binding.  If the Home Registration (H) bit is set in the
       Binding Update, the Binding Update is processed according to the
       procedure specified in Section 9.4; otherwise, it is processed
       according to the procedure specified in Section 8.4.


8.3. Requests to Cache a Binding

   When a node receives a Binding Update, it MUST validate it and
   determine the type of Binding Update according to the steps described
   in Section 8.2.  This section describes the processing of a valid
   Binding Update that requests a node to cache a mobile node's binding,
   for which the Home Registration (H) bit is not set in the Binding
   Update.

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






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8.4. Requests to Delete a Binding

   When a node receives a Binding Update, it MUST validate it and
   determine the type of Binding Update according to the steps described
   in Section 8.2.  This section describes the processing of a valid
   Binding Update that requests a node to delete a mobile node's binding
   from its Binding Cache, for which the Home Registration (H) bit is
   not set in the Binding Update.  In this case, the receiving node MUST
   delete any existing entry in its Binding Cache for this mobile node.


8.5. Sending Binding Acknowledgements

   When any node receives a packet containing a Binding Update option
   in which the Acknowledge (A) bit is set, it MUST return a Binding
   Acknowledgement option acknowledging receipt of the Binding Update.
   If the node accepts the Binding Update and creates or updates
   an entry in its Binding Cache for this binding, and the `A' bit
   was set in the Binding Update, the Status field in the Binding
   Acknowledgement MUST be set to a value less than 128; if, on the
   other hand the Binding Update is accepted and the `A' bit is not set,
   the node SHOULD NOT send a Binding Acknowledgement.  If the node
   rejects the Binding Update and does not create or update an entry for
   this binding, a Binding Acknowledgement MUST be sent even if the `A'
   bit was not sent, and the Status field in the Binding Acknowledgement
   MUST be set to a value greater than or equal to 128.  Specific values
   for the Status field are described in Section 5.2 and in the most
   recent "Assigned Numbers" [26].

   The packet in which the Binding Acknowledgement is returned MUST meet
   the specific IPsec requirements for Binding Acknowledgements, defined
   in Section 4.4; and the packet MUST be sent using a Routing header
   in the same way as any other packet sent to a mobile node using a
   care-of address (even if the binding was rejected), as described in
   Section 8.9.


8.6. Sending Binding Requests

   Entries in a node's Binding Cache MUST be deleted when their lifetime
   expires.  If such an entry is still in active use in sending packets
   to a mobile node, the next packet sent to the mobile node will be
   routed normally to the mobile node's home link, where it will be
   intercepted and tunneled to the mobile node.  The mobile node will
   then return a Binding Update to the sender, allowing it to create
   a new Binding Cache entry for sending future packets to the mobile
   node.  Communication with the mobile node continues uninterrupted,
   but the forwarding of this packet through the mobile node's home
   agent creates additional overhead and latency in delivering packets
   to the mobile node.




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


8.7. Cache Replacement Policy

   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.  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.  When a Binding Cache entry's timer
   expires, the node deletes the entry.

   Each node's Binding Cache will, by necessity, have a finite size.
   A node MAY use any reasonable local policy for managing the space
   within its Binding Cache, except that any entry marked as a "home
   registration" (Section 9.3) MUST NOT be deleted from the cache until
   the expiration of its lifetime period.  When attempting to add a new
   "home registration" entry in response to a Binding Update with the
   Home Registration (H) bit set, if insufficient space exists (and
   sufficient space cannot be reclaimed) in the node's Binding Cache,
   the node MUST reject the Binding Update and SHOULD return a Binding
   Acknowledgement to the sending mobile node, in which the Status field
   is set to 131 (insufficient resources).  When otherwise attempting to
   add a new entry to its Binding Cache, a node MAY, if needed, choose
   to drop any entry already in its Binding Cache, other than a "home
   registration" entry, in order to make space for the new entry.  For
   example, a "least-recently used" (LRU) strategy for cache entry
   replacement among entries not marked as a "home registration" is
   likely to work well.

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



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   a Binding Update to this sending node when it receives the tunneled
   packet, allowing it to add an entry again for this destination mobile
   node to its Binding Cache.


8.8. Receiving ICMP Error Messages

   When a correspondent node sends a packet to a mobile node, if the
   correspondent node has a Binding Cache entry for the destination
   address of the packet, then the correspondent node uses a Routing
   header to deliver the packet to the mobile node through the care-of
   address in the binding recorded in the Binding Cache entry.  Any ICMP
   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], this encapsulating node MUST
   relay certain ICMP error messages back to the original sender of the
   packet, which in this case is the correspondent node.

   Likewise, if a packet for a mobile node arrives at the mobile node's
   previous link and is intercepted there by a home agent for the mobile
   node's previous care-of address as described in Section 10.9 (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].

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





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8.9. Sending Packets to a Mobile Node

   Before sending any packet, the sending node SHOULD examine its
   Binding Cache for an entry for the destination address to which the
   packet is being sent.  If the sending node has a Binding Cache entry
   for this address, the sending node SHOULD use a Routing header to
   route the packet to this mobile node (the destination node) by way
   of the care-of address in the binding recorded in that Binding Cache
   entry.  For example, assuming use of a Type 0 Routing header [6], if
   no other use of a Routing header is involved in the routing of this
   packet, the mobile node sets the fields in the packet's IPv6 header
   and Routing header as follows:

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

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

   Following the definition of a Type 0 Routing header [6], this packet
   will be routed to the mobile node's care-of address, where it will
   be delivered to the mobile node (the mobile node has associated the
   care-of address with its network interface).  Normal processing of
   the Routing header by the mobile node will then proceed as follows:

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

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

   If, instead, the sending node has no Binding Cache entry for the
   destination address to which the packet is being sent, the sending
   node simply sends the packet normally, with no Routing header.  If
   the destination node is not a mobile node (or is a mobile node that
   is currently at home), the packet will be delivered directly to this
   node and processed normally by it.  If, however, the destination node
   is a mobile node that is currently away from home, the packet will
   be intercepted by the mobile node's home agent and tunneled (using
   IPv6 encapsulation [4]) to the mobile node's current primary care-of
   address, as described in Section 9.6.  The mobile node will then send
   a Binding Update to the sending node, as described in Section 10.8,




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   allowing the sending node to create a Binding Cache entry for its use
   in sending subsequent packets to this mobile node.




















































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9. Home Agent Operation

9.1. Receiving Router Advertisement Messages

   For each link on which a router provides service as a home agent, the
   router maintains a Home Agents List recording information about all
   other home agents on that link.  This list is used in the dynamic
   home agent address discovery mechanism, described in Section 9.2.
   The information for the list is learned through receipt of the
   periodic unsolicited multicast Router Advertisements from each other
   home agent on the link, in which the Home Agent (H) bit is set, in a
   manner similar to the Default Router List conceptual data structure
   maintained by each host for Neighbor Discovery [17].

   On receipt of a valid Router Advertisement, as defined in the
   processing algorithm specified for Neighbor Discovery [17], the home
   agent performs the following steps, in addition to any steps already
   required of it by Neighbor Discovery:

    -  If the Home Agent (H) bit in the Router Advertisement is not set,
       skip all of the following steps.  There are no special processing
       steps required by Mobile IP for this Router Advertisement, since
       the Advertisement was not sent by a home agent.

    -  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 [17].

    -  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.




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

    -  If the Home Agents List entry for the link-local address of
       the home agent sending this Advertisement was not deleted as
       described above, determine any global address(es) of the home
       agent based on each Prefix Information option received in
       this Advertisement in which the Router Address (R) bit is set
       (Section 6.2).  For each such global address determined from this
       Advertisement, add this global address to the list of global
       addresses for this home agent in this Home Agents List entry.

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


9.2. Dynamic Home Agent Address Discovery

   A mobile node, while away from home, MAY use the dynamic home agent
   address discovery mechanism to attempt to discover the address of
   one or more routers serving as home agents on its home link.  This
   discovery may be 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 10.7, 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 [10] 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 4.6).  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



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       Addresses field in the Reply message.  Not placing this home
       agent's own IP address in the list will cause the receiving
       mobile node to consider this home agent as the most preferred
       home agent; otherwise, this home agent will be considered to be
       preferred in its order given by its place in the list returned.

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

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

    -  For each entry in this home agent's Home Agents List, if more
       than one global IP address is associated with this list entry,
       then one of these global IP addresses SHOULD be selected to
       include in the Home Agent Addresses field in the Reply message.
       As described in Section 4.6, 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 mobile node's home address
       as indicated in the Home Address option in 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.



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9.3. Primary Care-of Address Registration

   When a node receives a Binding Update, it MUST validate it and
   determine the type of Binding Update according to the steps described
   in Section 8.2.  This section describes the processing of a valid
   Binding Update that requests the receiving node to serve as its home
   agent, registering its primary care-of address.

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

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

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

    -  Else, if the Prefix Length field is nonzero in the Binding Update
       and this length differs from the length of the home agent's own
       knowledge of the corresponding subnet prefix on the home link,
       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 136 (incorrect subnet prefix length).

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

    -  Finally, if the Duplicate Address Detection (D) bit is set in
       the Binding Update, this home agent MUST perform Duplicate
       Address Detection [27] on the mobile node's home link for the
       home address in this binding (before returning the Binding
       Acknowledgement); if the Prefix Length field is nonzero in the
       Binding Update, the home agent MAY choose to perform Duplicate
       Address Detection for only one of the addresses formed from the
       interface identifier for this binding, and if so, 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 Solication message
       of Duplicate Address Detection by a random delay between 0 and



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       MAX_RTR_SOLICITATION_DELAY [17, 27]; however, in this case, the
       home agent SHOULD NOT perform such a delay.  If this Duplicate
       Address Detection fails, then the home agent MUST reject the
       Binding Update and SHOULD return a Binding Acknowledgement to the
       mobile node, in which the Status field is set to 138 (Duplicate
       Address Detection failed).

   If the home agent does not reject the Binding Update as described
   above, then it becomes the home agent for the mobile node.  The new
   home agent (the receiving node) MUST then create a new entry in its
   Binding Cache for this mobile node (or update its existing Binding
   Cache entry for this mobile node, if such an entry already exists)
   The home address of the mobile node is taken to be the value which,
   when the packet was originally received, was located in the Home
   Address field in the packet's Home Address option.  The care-of
   address for this Binding Cache entry is taken to be the value which,
   when the packet was originally received, was located either in the
   Alternate Care-of Address sub-option in the Binding Update option,
   if present, or from the Source Address field in the packet's IPv6
   header, otherwise.

   The home agent MUST mark this Binding Cache entry as a "home
   registration" to indicate that the node is serving as a home
   agent for this binding.  Binding Cache entries marked as a "home
   registration" MUST be excluded from the normal cache replacement
   policy used for the Binding Cache (Section 8.7) and MUST NOT be
   removed from the Binding Cache until the expiration of the Lifetime
   period.

   In addition, the home agent MUST copy the Router (R) bit from the
   Binding Update into the corresponding bit in this Binding Cache entry
   for this mobile node.

   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 [17].
   Furthermore, if the Prefix Length field in the Binding Update is
   nonzero, then the lifetime for the 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 implemented by the home
   agent.  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.



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   The Prefix Length in the Binding Update MUST also be saved in the
   Binding Cache entry.

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

    -  The Status field MUST be set to a value indicating success (the
       value MUST be less than 128).  The only currently defined success
       Status value is 0, indicating simply that the Binding Update was
       accepted.

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

    -  The Lifetime field MUST be set to 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,
       this lifetime MUST NOT be greater than the remaining valid
       lifetime for the subnet prefix in the mobile node's home address.

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

   In addition, the home agent MUST follow the procedure defined in
   Section 9.5 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.


9.4. Primary Care-of Address De-registration

   When a node receives a Binding Update, it MUST validate it and
   determine the type of Binding Update according to the steps described
   in Section 8.2.  This section describes the processing of a valid
   Binding Update that requests the receiving node to no longer serve as
   its home agent, de-registering its primary care-of address.

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





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

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

   If the Acknowledge (A) bit is set in the Binding Update (it SHOULD
   be), then the home agent MUST return a Binding Acknowledgement to the
   mobile node, constructed as follows:

    -  The Status field MUST be set to a value indicating success (the
       value MUST be less than 128).  The only currently defined success
       Status value is 0, indicating simply that the Binding Update was
       accepted.

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

    -  The Lifetime field MUST be set to zero.

    -  The Refresh field MUST be set to zero.

   In addition, the home agent MUST stop intercepting packets on the
   mobile node's home link addressed to the mobile node (Section 9.5).


9.5. Intercepting Packets for a Mobile Node

   While a node is serving as the home agent for mobile node (while the
   node has an entry in its Binding Cache for this mobile node that is
   marked as a "home registration"), this node MUST attempt to intercept
   packets on the mobile node's home link addressed to the mobile node,
   and MUST tunnel each intercepted packet to the mobile node using
   using IPv6 encapsulation [4].

   In order to intercept such packets on the home link, when a node
   becomes 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 [17] on behalf of the
   mobile node.  Specifically, the home agent performs the following
   steps:

    -  The home agent examines the value of the Prefix Length field
       in the new "home registration" Binding Cache entry.  If this
       value is zero, the following step is carried out only for the
       individual home address specified for this binding.  If, instead,



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       this field is nonzero, 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 indicated
       subnet prefix), together with each one of the subnet prefixes
       currently considered by the home agent to be on-link (including
       both the link-local and site-local prefix).

    -  For each specific IP address for the mobile node determined
       in the first step above, the home agent multicasts onto the
       home link (to the all-nodes multicast address) a Neighbor
       Advertisement message [17] on behalf of the mobile node, to
       advertise the home agent's own link-layer address for this IP
       address.

       All fields in each such Neighbor Advertisement message SHOULD be
       set in the same way they would be set by the mobile node itself
       if sending this Neighbor Advertisement while at home [17], with
       the following exceptions:

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

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

        *  The Router (R) bit in the Advertisement MUST be copied from
           the corresponding bit in the home agent's Binding Cache entry
           for the mobile node.

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

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

   Any node on the home link receiving one of the Neighbor Advertisement
   messages described above will thus update its Neighbor Cache to
   associate the mobile node's address with the home agent's link
   layer address, causing it to transmit any future packets for the
   mobile node normally destined to this address instead to the mobile
   node's home agent.  Since multicasts on the local link (such as
   Ethernet) are typically not guaranteed to be reliable, the home
   agent MAY retransmit this Neighbor Advertisement message up to
   MAX_ADVERT_REXMIT times to increase its reliability.  It is still
   possible that some nodes on the home link will not receive any of
   these Neighbor Advertisements, but these nodes will eventually be
   able to detect the link-layer address change for the mobile node's
   home address, through use of Neighbor Unreachability Detection [17].



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   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 [17]
   to intercept unicast packets on the home link addressed 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 to reply to
   any received Neighbor Solicitation messages for it.  When a home
   agent receives a Neighbor Solicitation message, it MUST check if the
   Target Address specified in the message matches the home address
   of any mobile node for which it has a Binding Cache entry marked
   as a "home registration".  This check MUST include all possible
   home addresses for the mobile node, based on the subnet prefixes
   currently considered to be on-link by the home agent (including the
   corresponding link-local address and site-local address), if the
   Prefix Length in the Binding Cache entry for this mobile node (from
   the Binding Update that created this Cache entry) is nonzero.

   If such an entry exists in the home agent's Binding Cache, the home
   agent MUST reply to the Neighbor Solicitation message with a Neighbor
   Advertisement message, giving the home agent's own link-layer address
   as the link-layer address for the specified Target Address.  In
   addition, the Router (R) bit in the Advertisement MUST be copied from
   the corresponding bit in the home agent's Binding Cache entry for the
   mobile node.  Acting as a proxy in this way allows other nodes on
   the mobile node's home link to resolve the mobile node's IPv6 home
   address, and allows the home agent to defend these addresses on the
   home link for Duplicate Address Detection [17].


9.6. 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 8.9 apply.  The home agent (as a correspondent node) uses a
   Routing header to route the packet to the mobile node by way of the
   care-of address in the home agent's Binding Cache (the mobile node's
   primary care-of address, in this case).

   While the mobile node is away from home and this node is acting
   as the mobile node's home agent, the home agent intercepts any
   packets on the home link addressed to the mobile node's home address
   (including addresses formed from other on-link prefixes, if the
   Prefix Length field was nonzero in the Binding Update), as described
   in Section 9.5.  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 [11] or ESP [12] header present in the packet.





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   For forwarding 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 (which is an address of the mobile
   node itself).  When a home agent encapsulates an intercepted packet
   for forwarding to the mobile node, the home agent sets the Source
   Address in the prepended tunnel IP header to the home agent's own IP
   address, and sets the Destination Address in the tunnel IP header
   to the mobile node's primary care-of address.  When received by the
   mobile node (using its primary care-of address), normal processing of
   the tunnel header [4] will result in decapsulation and processing of
   the original packet by the mobile node.

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


9.7. Handling Reverse Tunneled Packets from a Mobile Node

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



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9.8. Renumbering the Home Subnet

   IPv6 provides mechanisms through Neighbor Discovery [17] and Address
   Autoconfiguration [27] to aid in renumbering a subnet, such as when a
   site switches to a new network service provider.  In renumbering, new
   prefixes and addresses can be introduced for the subnet and old ones
   can be deprecated and removed.  These mechanisms are defined to work
   while all nodes using the old prefixes are at home, connected to the
   link using these prefixes.  Mobile IPv6 extends these mechanisms for
   the case in which one or more mobile nodes using the old prefixes are
   away from home while the renumbering takes place.

   The IPv6 renumbering mechanisms are based on nodes on the link
   receiving Prefix Information options in Router Advertisement
   messages giving the valid lifetime and preferred lifetime for
   different prefixes on the link [17].  Mobile IPv6 arranges to
   tunnel certain Router Advertisements giving "important" Prefix
   Information options to mobile nodes while away from home.  To avoid
   the need to tunnel all Router Advertisements from the home link to
   a mobile node away from home, those Router Advertisements that are
   tunneled to the mobile node are retransmitted until acknowledged.  To
   avoid possible security attacks from forged Router Advertisements
   tunneled to the mobile node, all such tunneled Router Advertisements
   must be authenticated to the mobile node by its home agent using
   IPsec [13, 11, 12].


9.8.1. Building Aggregate List of Home Network Prefixes

   A mobile node on a remote network SHOULD autoconfigure the same
   set of home addresses it would autoconfigure if it were attached
   to the home network.  To support this, the home agent monitors
   prefixes advertised by other routers on the home subnet and passes
   the aggregate list of home subnet prefixes on to the mobile node in
   Router 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 [17]).  Do not update the aggregate list with
       any information from received prefixes that fail this check.





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    -  Add valid prefixes received in Router Advertisements from the
       home network that are not yet in the aggregate list to the
       aggregate list along with the value of their L and A flags.
       Clear the R flag and zero the interface-id portion of the prefix
       field to prevent mobile nodes from treating another router's
       interface address as belonging to the home agent.  Treat the
       lifetimes of these prefixes as "deprecating".

    -  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 [17]) and section
       5.5.3 of RFC 2462 (Stateless Address Autoconfiguration [27]).

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

    -  Ignore the R flag and interface id portion of any prefix received
       in a Router Advertisement.

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

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


9.8.2. Sending Changed Prefix Information to the Mobile Node

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

    -  A valid or preferred lifetime of a prefix in the aggregate list
       of prefixes changes.

    -  The state of the flags for a prefix in the aggregate list
       changes.

    -  A new prefix is introduced on the home link.

    -  The mobile node requests the information with a Router
       Solicitation (see section 10.16).





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   The home agent determines these conditions based on its own
   configuration as a router and based on the Router Advertisements that
   it receives on the home link.

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

    -  If a mobile node sends a solicitation, answer with everything.

    -  If a prefix changes state in a way that causes a mobile node's
       address to go deprecated, send an advertisement right away.

    -  For any existing previx, if the mobile node's binding expires
       before the advertised Preferred Lifetime, do not schedule the
       advertisement.  The mobile node will get the revised information
       in its next Binding Acknowledgement.

    -  If a prefix is added, or if it changes in any way that does not
       cause the mobile node's address to go deprecated, ensure that a
       transmission is scheduled at time RAND_ADV_DELAY in the future.

    -  If a prefix advertisement is scheduled, and a Binding Update
       arrives, perform that advertisement and include the information
       in a Router Advertisement that has the Binding Acknowledgement as
       a Destination Option.  Remove the future scheduled advertisement.

   The home agent uses the following algorithm to compute
   RAND_ADV_DELAY, the offset from the current time for the
   scheduled transmission.

   If there is a transmission already scheduled, then

      if the current RAND_ADV_DELAY would cause another
      transmission BEFORE the Preferred Lifetime of the
      mobile node's home address derived from the prefix whose
      advertisement information has changed, then

         add the new information to be transmitted to the
         existing scheduled transmission -- return.

      otherwise,

         continue with the following computation, and add the
         data from the existing scheduled transmission to the
         newly scheduled transmission, deleting the previously
         scheduled transmission event.

   If the mobile node's binding expires after the Preferred Lifetime,
   then compute





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      MAX_SCHEDULE_DELAY ==
      min (MAX_PFX_ADV_DELAY, Preferred Lifetime)

   for the newly advertised Preferred Lifetime.
   Then compute RAND_ADV_DELAY =
   MinRtrAdvInt + rand()*(MAX_SCHEDULE_DELAY - MinRtrAdvInt)


9.8.3. Tunneling Router Advertisements to the Mobile Node

   When tunneling a Router 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.

    -  The packet MUST be protected by IPsec [13, 11, 12] to guard
       against malicious Router Advertisements.  The IPsec protection
       MUST provide sender authentication, data integrity protection,
       and replay protection, covering the Router Advertisement.

    -  The packet MUST include a Binding Request destination option.

    -  The Binding Request destination option MUST include a Unique
       Identifier Sub-Option (Section 5.5), with the unique identifier
       in the sub-option data set to a value different than that in
       any other Binding Request sent recently by this node.  The word
       "recently" here means within the maximum likely lifetime of a
       packet, including transit time from source to destination and
       time spent awaiting reassembly with other fragments of the same
       packet, if fragmented.  However, it is not required that a source
       node know the maximum packet lifetime.  Rather, it is assumed
       that the requirement can be met by maintaining a simple 16-bit
       "wrap-around" counter to generate unique identifiers for Binding
       Requests that contain a Unique Identifier Sub-Option, incremented
       each time a Binding Request containing a Unique Identifier
       Sub-Option is sent.

    -  The packet MUST be tunneled to the mobile node's primary care-of
       address using a Routing header, in the same way as any packet
       sent to the mobile node originated by the home agent (rather than
       using IPv6 encapsulation, as would be used by the home agent for
       intercepted packets).

   The home agent SHOULD periodically continue to retransmit this
   tunneled packet to the mobile node, until it is acknowledged by
   the receipt from the mobile node of a Binding Update matching
   the Binding Request in the packet (i.e., with matching Sequence
   Number).  A Binding Update matches a Binding Request if it specifies
   a binding for the mobile node to which the Binding Request was sent



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   and contains a Unique Identifier Sub-Option matching the unique
   identifier sent in the Unique Identifier Sub-Option in the Binding
   Request.

   If while the home agent is still retransmitting a Router
   Advertisement to the mobile node, another condition as described
   above occurs on the home link causing another Router Advertisement
   to be tunneled to the mobile node, the home agent SHOULD combine any
   Prefix Information options in the unacknowledged Router Advertisement
   into the new Router Advertisement and then begin retransmitting the
   new Router Advertisement rather than the old one.  When tunneling
   a new Router Advertisement, even if it contains Prefix Information
   options sent previously in an unacknowledged tunneled Router
   Advertisement, the home agent MUST generate a new unique identifer
   for use in the Unique Identifier Sub-Option in the Binding Request
   tunneled with the new Router Advertisement.

   Whenever a mobile node has a valid binding on a network other than
   its home network, the home agent MUST tunnel a router advertisement
   with all prefixes in the aggregate list to the mobile node at least
   once per HomeRtrAdvInterval seconds, and upon receipt of a valid
   Router Solicitation from the mobile node.


9.8.4. Lifetimes for Changed Prefixes

   In addition, as described in Section 9.3, the lifetime returned by a
   mobile node's home agent in its Binding Acknowledgement in response
   to registration of a new primary care-of address by the mobile node
   MUST be no greater than the remaining valid lifetime for the subnet
   prefix in the mobile node's home address.  Furthermore, as described
   in Section 10.8, Binding Updates sent by the mobile node to other
   nodes MUST use a lifetime no greater than the remaining lifetime of
   its home registration of its primary care-of address.  These limits
   on the binding lifetime serve to prohibit use of a mobile node's home
   address after it becomes invalid.  The mobile node SHOULD further
   limit the lifetimes that it sends on any Binding Updates to be within
   the remaining preferred lifetime for the prefix in its home address.
















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10. Mobile Node Operation

10.1. Sending Packets While Away from Home

   While a mobile node is away from home, it continues to use its home
   address as well as also using one or more care-of addresses.  When
   sending a packet while away from home, a mobile node MAY choose among
   these in selecting the address that it will use as the source of the
   packet, as follows:

    -  From the point of view of protocol layers and applications above
       Mobile IP (e.g., transport protocols), the mobile node will
       generally use its home address as the source of the packet for
       most packets, even while away from home, since Mobile IP is
       designed to make mobility transparent to such software.  Doing
       so also makes the node's mobility---and the fact that it is
       currently away from home---transparent to the correspondent nodes
       with which it communicates.  For packets sent that are part of
       transport-level connections established while the mobile node
       was at home, the mobile node MUST use its home address in this
       way.  Likewise, for packets sent that are part of transport-level
       connections that the mobile node may still be using after moving
       to a new location, the mobile node SHOULD use its home address
       in this way.  When sending such packets, Mobile IP will modify
       the packet to move the home address into a Home Address option
       and will set the IPv6 header's Source Address field to one of
       the mobile node's care-of addresses; these modifications to
       the packet are then reversed in the node receiving the packet,
       restoring the mobile node's home address to be the packet's
       Source Address before processing by higher protocol layers and
       applications.

    -  For short-term communication, particularly for communication that
       may easily be retried if it fails, the mobile node MAY choose
       to directly use one of its care-of addresses as the source of
       the packet, thus not requiring the use of a Home Address option
       in the packet.  An example of this type of communication might
       be DNS queries sent by the mobile node [15, 16].  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



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   the source of a packet sent while away from home (from the point of
   view of higher protocol layers or applications, as described above),
   no special Mobile IP processing is required for sending that packet.
   In each case, the packet is simply addressed and transmitted in the
   same way as any normal IPv6 packet.

   For each other packet sent by the mobile node (i.e., packets sent
   while away from home, using the mobile node's home address as
   the source, from the point of view of higher protocol layers and
   applications), special Mobile IP processing of the packet is required
   for the insertion of the Home Address option.  Specifically:

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

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

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

   By using the care-of address as the Source Address in the IPv6
   header, with the mobile node's home address instead in the Home
   Address option, the packet will be able to safely pass through any
   router implementing ingress filtering [7].


10.2. Interaction with Outbound IPsec Processing

   This section sketches the interaction between outbound Mobile IP
   processing and outbound IP Security (IPsec) processing for
   packets sent by a mobile node while away from home.  Any specific
   implementation MAY use algorithms and data structures other than
   those suggested here, but its processing MUST be consistent with the
   effect of the operation described here and with the relevant IPsec
   specifications.  In the steps described below, it is assumed that
   IPsec is being used in transport mode [13] and that the mobile node
   is using its home address as the source for the packet (from the
   point of view of higher protocol layers or applications, as described
   in Section 10.1):

    -  The packet is created by higher layer protocols and applications
       (e.g., by TCP) as if the mobile node were at home and Mobile IP




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       were not being used.  Mobile IP is transparent to such higher
       layers.

    -  As part of outbound packet processing in IP, the packet is
       compared against the IPsec Security Policy Database (SPD) to
       determine what processing is required for the packet [13].

    -  As a special case for Mobile IP, if a Binding Update or
       Binding Acknowledgement is being included in the packet, IPsec
       authentication, integrity protection, and replay protection MUST
       be applied to the packet [13, 11, 12], as defined in Section 4.4.
       If the SPD check above has already indicated that authentication
       and replay protection are required, this processing is sufficient
       for the Mobile IP requirement that all packets containing Binding
       Updates or Binding Acknowledgements be authenticated and covered
       by replay protection.  Otherwise, an implementation can force
       the required IPsec processing on this individual packet by, for
       example, creating a temporary SPD entry for the handling of this
       packet.

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

    -  Since the mobile node is away from home, the mobile node inserts
       a Home Address option into the packet, replacing the Source
       Address in the packet's IP header with a care-of address suitable
       for the link on which the packet is being sent, as described in
       Section 10.1.  The Destination Options header in which the Home
       Address option is inserted MUST appear in the packet before the
       AH [11] (or ESP [12]) header, so that the Home Address option is
       processed by the destination node (and, possibly, intermediate
       routing nodes) before the AH or ESP header is processed.

    -  If a Binding Update is being included in the packet, it is
       also added to a Destination Options header in the packet.  The
       Destination Options header in which the Binding Update option is
       inserted MUST appear after the AH or ESP header.

    -  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 authentication data MUST be calculated
       as if the following were true:

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

        *  the Home Address field of the Home Address destination option
           (section 5.4) contains the new care-of address.



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

   In addition, when using any automated key management protocol [13]
   (such as IKE [8]) to create any new SA (or SA bundle) while away from
   home (whether due to the inclusion of a Binding Update or Binding
   Acknowledgement in an outgoing packet, or otherwise), 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, 13],
   such problems can be avoided by the following requirements on the use
   of IKE by a mobile node while away from home:

    -  The mobile node MUST use its care-of address as the Source
       Address of all packets it sends as part of the key management
       protocol (without use of Mobile IP for these packets, as
       suggested in Section 10.1).

    -  In addition, for all security associations bound to the mobile
       node's home address, the mobile node MUST include an ISAKMP
       Identification Payload [14] in the IKE exchange, giving the
       mobile node's home address as the initiator of the Security
       Association [22].


10.3. Receiving Packets While Away from Home

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

    -  Packets sent by a correspondent node that does not have a
       Binding Cache entry for the mobile node, will be sent by the
       correspondent node in the same way as any normal IP packet.  Such
       packets will then be intercepted by the mobile node's home agent,
       encapsulated using IPv6 encapsulation [4], and tunneled to the
       mobile node's primary care-of address.

    -  Packets sent by a correspondent node that has a Binding Cache
       entry for the mobile node that contains the mobile node's current
       care-of address, will be sent by the correspondent node using
       a Routing header.  The packet will be addressed to the mobile
       node's care-of address, with the final hop in the Routing header



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       directing the packet to the mobile node's home address; the
       processing of this last hop of the Routing header is entirely
       internal to the mobile node, since the care-of address and home
       address are both addresses within the mobile node.

    -  Packets sent by a correspondent node that has a Binding Cache
       entry for the mobile node that contains an out-of-date care-of
       address for the mobile node, will be sent by the correspondent
       node using a Routing header, as described above.  If the mobile
       node sent a Binding Update to a home agent on the link on which
       its previous care-of address is located (Section 10.9), and
       if this home agent is still serving as a home agent for the
       mobile node's previous care-of address, then such a packet will
       be intercepted by this home agent, encapsulated using IPv6
       encapsulation [4], and tunneled to the mobile node's new care-of
       address (registered with this home agent).

   For packets received by either the first or last of these three
   methods, the mobile node SHOULD send a Binding Update to the original
   sender of the packet, as described in Section 10.8, subject to the
   rate limiting defined in Section 10.11.  The mobile node SHOULD
   also process the received packet in the manner defined for IPv6
   encapsulation [4], which will result in the encapsulated (inner)
   packet being processed normally by upper-layer protocols within the
   mobile node, as if it had been addressed (only) to the mobile node's
   home address.

   For packets received by the second method above (using a Routing
   header), the mobile node SHOULD process the received packet in the
   manner defined for the type of IPv6 Routing header used [6], which
   will result in the packet being processed normally by upper-layer
   protocols within the mobile node, as if it had been addressed (only)
   to the mobile node's home address.

   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].  If this is done for upper-layer protocol response
   packets sent by a mobile node while away from home, the mobile
   node SHOULD NOT include its own care-of address, which appears in
   the Routing header of the received packet, in the reversed route
   for the response packet.  If the received Routing header contained
   no additional hops (other than the mobile node's home address and
   care-of address), then any upper-layer protocol response packet
   SHOULD NOT include a Routing header.








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10.4. Movement Detection

   A mobile node MAY use any combination of mechanisms available to it
   to detect when it has moved from one link to another.  The primary
   movement detection mechanism for Mobile IPv6 defined here uses the
   facilities of IPv6 Neighbor Discovery, including Router Discovery
   and Neighbor Unreachability Detection, although the mobile node MAY
   supplement this mechanism with other information 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 [17].

   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 [17].
   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 SHOULD select one router from
   its Default Router List to use as its default router, and one subnet
   prefix advertised by that router from its Prefix List to use as
   the subnet prefix in its primary care-of address.  A mobile node
   MAY also have associated additional care-of addresses, using other
   subnet prefixes from its Prefix List.  The method by which a mobile
   node selects and forms a care-of address from the available subnet
   prefixes is described in Section 10.5.  The mobile node registers
   its primary care-of address with its home agent, as described in
   Section 10.6.

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

   To detect when its default router becomes unreachable, a mobile
   node SHOULD use Neighbor Unreachability Detection.  As specified in
   Neighbor Discovery [17], 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



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   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,
   a mobile node SHOULD consider receipt of any IPv6 packets from its
   current default router as an indication that it is still reachable
   from the router.  Both packets from the router's IP address and
   (IPv6) packets from its link-layer address (e.g., those forwarded but
   not originated by the router) SHOULD be considered.

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

   On some types of network interfaces, the mobile node MAY also
   supplement this monitoring of Router Advertisements, by setting its
   network interface into "promiscuous" receive mode, so that it is able
   to receive all packets on the link, including those not link-level
   addressed to it (i.e., disabling link-level address filtering).  The
   mobile node will then be able to detect any packets sent by the
   router, in order to detect reachability from the router.  This use of




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   promiscuous mode may be useful on very low bandwidth (e.g., wireless)
   links, but its use MUST be configurable on the mobile node.

   If the above means do not provide indication that the mobile node is
   still reachable from its current default router (i.e., 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, it is possible that additional
   indications about link-layer mobility can be obtained from
   lower-layer protocol or device driver software within the mobile
   node.  However, a mobile node MUST NOT assume that all link-layer
   mobility indications from lower layers 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
   new routers (and new on-link subnet prefixes) are present on its new
   link.

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


10.5. 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



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

   In addition, after discovering a new on-link subnet prefix, a mobile
   node MAY form a new (non-primary) care-of address using that subnet
   prefix, even when it has not switched to a new default router.  A
   mobile node can have only one primary care-of address at a time
   (which is registered with its home agent), but it MAY have an
   additional care-of address for any or all of the prefixes on its
   current link.  Furthermore, since a wireless network interface may
   actually allow a mobile node to be reachable on more than one link at
   a time (i.e., within wireless transmitter range of routers on more
   than one separate link), a mobile node MAY have care-of addresses
   on more than one link at a time.  The use of more than one care-of
   address at a time is described in Section 10.18.

   As described in Section 4, in order to form a new care-of address,
   a mobile node MAY use either stateless [27] 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 [27] on that new address to confirm its
   uniqueness.  However, doing so represents a tradeoff between safety
   (ensuring that the new address is not used if it is a duplicate
   address) and overhead (performing Duplicate Address Detection
   requires the sending of one or more additional packets over what
   may be, for example, a slow wireless link through which the mobile
   node is connected).  Performing Duplicate Address Detection also in
   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



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   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 Solication message of Duplicate Address Detection
   by a random delay between 0 and MAX_RTR_SOLICITATION_DELAY [17, 27];
   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 intializing after rebooting.


10.6. Sending Binding Updates to the Home Agent

   After deciding to change its primary care-of address as described
   in Sections 10.4 and 10.5, a mobile node MUST register this care-of
   address with its home agent in order to make this its primary care-of
   address.  To do so, the mobile node sends a packet to its home agent
   containing a Binding Update option, with the packet constructed as
   follows:

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

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

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

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

    -  The Prefix Length field SHOULD be set to the length of the mobile
       node's subnet prefix in its home address, 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.  Otherwise, this field MUST be set to zero.

    -  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



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   Binding Update.  As described in Section 5.2, 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
   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).

   The Prefix Length field in the Binding Update allows the mobile node
   to request its home agent to serve all home addresses for the mobile
   node, as indicated by the interface identifier in the mobile node's
   home address (the remaining low-order bits after the indicated subnet
   prefix), together with each on-link subnet prefix on the home link.
   Until the lifetime of this registration expires, the home agent
   considers itself the home agent for each such home address of the
   mobile node.  As the set of on-link subnet prefixes on the home link
   changes over time, the home agent changes the set of home addresses
   for this mobile node for which it is serving as the home agent.

   When sending a Binding Update to its home agent, the mobile node MUST
   also create or update the corresponding Binding Update List entry, as
   specified in Section 10.8.

   If the mobile node has additional home addresses using a different
   interface identifier, then the mobile node SHOULD send an additional
   packet containing a Binding Update to its home agent to register the
   care-of address for each such other home address (or set of home
   addresses sharing an interface identifier).  These additional Binding
   Updates MUST each be sent as a separate packet, since each MUST be
   protected by IPsec [13, 11, 12] to authenticate the Binding Update as
   coming from the home address being bound, as defined in Section 4.4.

   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 Detection that
   would apply to the mobile node if it were at home [17, 27].

   The home agent will only perform DAD for the mobile node's home
   address when the mobile node has supplied a valid binding between
   its home address and a care-of address.  If some time elapses during
   which the mobile node has no binding at the home agent, it might be
   possible for another node to autoconfigure the mobile node's home
   address.  Therefore, the mobile node MUST treat creation of a new
   binding with the home agent using an existing home address the same
   as creation of a new home address.  In the unlikely event that the
   mobile node's home address is autoconfigured as the IPv6 address
   of another network node on the home network, the home agent will
   reply to the mobile node's subsequent Binding Update with a Binding
   Acknowledgement showing Status 138, Duplicate Address Detection



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   failed.  See section 10.10 for information about retransmitting
   Binding Updates.


10.7. Dynamic Home Agent Address Discovery

   It is possible that when the mobile node needs to send a Binding
   Update to its home agent to register its new primary care-of address,
   as described in Section 10.6, 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 use the dynamic home agent address
   discovery mechanism to find 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 [10] for its home subnet prefix.  This packet MUST
   NOT contain a Home Address option and must be sent using the mobile
   node's care-of address as the Source Address in the packet's IP
   header (the packet is sent from the care-of address, not using
   Mobile IP). As described in Section 9.2, 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



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   (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.


10.8. Sending Binding Updates to Correspondent Nodes

   A mobile node MAY send a Binding Update to any correspondent node at
   any time to allow the correspondent node to cache the mobile node's
   current care-of address (subject to the rate limiting defined in
   Section 10.11).  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 field in 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.

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

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

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

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

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

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

    -  The remaining lifetime of the binding, also initialized from
       the Lifetime field sent in the Binding Update.  This remaining
       lifetime value counts down and may also be reduced when the
       matching Binding Acknowledgement is received, based on the
       Lifetime value specified in that Binding Acknowledgement, as



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       described in Section 10.12.  When the remaining lifetime reaches
       zero, the Binding Update List entry MUST be deleted.

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

   In general, when a mobile node sends a Binding Update to its home
   agent to register a new primary care-of address (as described in
   Section 10.6), the mobile node will also send a Binding Update to
   each other node for which an entry exists in the mobile node's
   Binding Update List.  Thus, other relevant nodes are generally kept
   updated about the mobile node's binding and can send packets directly
   to the mobile node using the mobile node's current care-of address.

   The mobile node, however, need not send these Binding Updates
   immediately after configuring a new care-of address.  For example,
   since the Binding Update is a destination option and can be included
   in any packet sent by a mobile node, the mobile node MAY delay
   sending a new Binding Update to any correspondent node for a
   short period of time, in hopes that the needed Binding Update
   can be included in some packet that the mobile node sends to that
   correspondent node for some other reason (for example, as part of
   some TCP connection in use).  In this case, when sending a packet
   to some correspondent node, the mobile node SHOULD check in its
   Binding Update List to determine if a new Binding Update to this
   correspondent node is needed, and SHOULD include the new Binding
   Update in this packet as necessary.

   In addition, when a mobile node receives a packet for which the
   mobile node can deduce that the original sender of the packet has
   no Binding Cache entry for the mobile node, or for which the mobile
   node can deduce that the original sender of the packet has an
   out-of-date care-of address for the mobile node in its Binding Cache,
   the mobile node SHOULD return a Binding Update to the sender giving
   its current care-of address (subject to the rate limiting defined
   in Section 10.11).  In particular, the mobile node SHOULD return a
   Binding Update in response to receiving a packet that meets all of
   the following tests:

    -  The packet was tunneled using IPv6 encapsulation.

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

    -  The Destination Address in the original (inner) IPv6 header
       is equal to one of the mobile node's home addresses; or this



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       Destination Address is equal to one of the mobile node's previous
       care-of addresses for which the mobile node has an entry in its
       Binding Update List, representing an unexpired Binding Update
       sent to a home agent on the link on which its previous care-of
       address is located (Section 10.9).

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

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

   Binding Updates sent to correspondent nodes are not generally
   required to be acknowledged.  However, if the mobile node wants
   to be sure that its new care-of address has been entered into a
   correspondent node's Binding Cache, the mobile node MAY request an
   acknowledgement by setting the Acknowledge (A) bit in the Binding
   Update.  In this case, however, the mobile node SHOULD NOT continue
   to retransmit the Binding Update once the retransmission timeout
   period has reached MAX_BINDACK_TIMEOUT.

   A mobile node MAY choose to keep its location private from certain
   correspondent nodes, and thus need not send new Binding Updates to
   those correspondents.  A mobile node MAY also send a Binding Update
   to such a correspondent node to instruct it to delete any existing
   binding for the mobile node from its Binding Cache, as described in
   Section 5.1.  No other IPv6 nodes are authorized to send Binding
   Updates on behalf of a mobile node.


10.9. Establishing Forwarding from a Previous Care-of Address

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

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





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

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

    -  The Home Registration (H) bit MUST also be set in this Binding
       Update, to request this home agent to temporarily act as a home
       agent for this previous care-of address.

   This home agent will thus tunnel packets for the mobile node (packets
   destined to its specified previous care-of address) to its new
   care-of address.  All of the procedures defined for home agent
   operation MUST be followed by this home agent for this registration.
   Note that this home agent does not necessarily know (and need not
   know) the mobile node's (permanent) home address as part of this
   registration.

   The packet carrying the Binding Update MUST be addressed to
   this home agent's global unicast address.  Normally, this global
   unicast address is learned by the mobile node based on the Router
   Advertisements received by the mobile node (Section 6.2) while
   attached to the link on which this previous care-of address and this
   home agent are located; the mobile node obtains this home agent
   address from its Home Agents List (Section 4.6).  Alternatively,
   the mobile node MAY use dynamic home agent address discovery
   (Section 10.7) to discover the global unicast address of a home agent
   on this previous link, but it SHOULD use an address from its Home
   Agents List if available for the prefix it used to form this previous
   care-of address.

   As with any packet containing a Binding Update (see section 5.1),
   the Binding Update packet to this home agent MUST meet the IPsec
   requirements for Binding Updates, defined in Section 4.4.


10.10. Retransmitting Binding Updates

   When the mobile node sends a Binding Update, it has to determine
   a value for the initial retransmission timer.  If the mobile node
   is changing or updating an existing binding at the home agent, it
   should use the specified value of INITIAL_BINDACK_TIMEOUT for this
   initial retransmission timer.  If on the other hand the mobile node
   does not have an existing binding at the home agent, it SHOULD use a



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   value for the initial retransmission timer that is at least 1.5 times
   longer than (RetransTimer * DupAddrDetectTransmits).  This value is
   likely to be substantially longer than the otherwise specified value
   of INITIAL_BINDACK_TIMEOUT that would be used by the mobile node.
   This longer retransmission interval will allow the the home agent
   to complete the DAD procedure which is mandated in this case, as
   detailed in section 10.6.

   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.


10.11. Rate Limiting for Sending Binding Updates

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


10.12. 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 specific IPsec requirements for Binding
       Acknowledgements, defined in Section 4.4.

    -  The Option Length field in the Binding Acknowledgement option is
       greater than or equal to the length specified in Section 5.2.

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




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   Any Binding Acknowledgement not satisfying all of these tests MUST be
   silently ignored, although the remainder of the packet (i.e., other
   options, extension headers, or payload) SHOULD be processed normally
   according to any procedure defined for that part of the packet.

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

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

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

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

    -  If the Status field indicates that the Binding Update was
       rejected (the Status field is greater than or equal to 128), then
       the mobile node MUST delete the corresponding Binding Update List
       entry, and it MUST also stop retransmitting the Binding Update.
       Optionally, the mobile node MAY then take steps to correct the
       cause of the error and retransmit the Binding Update (with a new
       Sequence Number value), subject to the rate limiting restriction
       specified in Section 10.11.


10.13. Receiving Binding Requests

   When a mobile node receives a packet containing a Binding Request,
   it SHOULD return to the sender a packet containing a Binding Update.



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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), except that this lifetime
   MUST NOT exceed the remaining lifetime for the mobile node's primary
   care-of address registration at its home agent.  When sending this
   Binding Update, the mobile node MUST update its Binding Update List
   in the same way as for any other Binding Update sent by the mobile
   node.

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

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


10.14. Receiving ICMP Error Messages

   The Option Type value for a Binding Update option specifies that
   any node receiving this option that does not recognize the Option
   Type SHOULD return an ICMP Parameter Problem, Code 2, message to
   the sender of the packet containing the Binding Update option.  If
   a node sending a Binding Update receives such an ICMP error message
   in response, it SHOULD record in its Binding Update List that future
   Binding Updates SHOULD NOT be sent to this destination.

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





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10.15. Receiving Local Router Advertisement Messages

   Each mobile node maintains a Home Agents List recording information
   about all home agents from which it receives a Router Advertisement,
   for which the home agent lifetime indicated in that Router
   Advertisement has not yet expired.  This list is used by the mobile
   node to enable it to send a Binding Update to the global unicast
   address of a home agent on its previous link when it moves to a new
   link, as described in Section 10.9.  On receipt of a valid Router
   Advertisement, as defined in the processing algorithm specified for
   Neighbor Discovery [17], the mobile node performs the following
   steps, in addition to any steps already required of it by Neighbor
   Discovery and by other procedures described in this document:

    -  If the Home Agent (H) bit in the Router Advertisement is not
       set, skip all of the following steps.  There are no special
       processing steps required by this aspect of Mobile IP for this
       Router Advertisement, since the Advertisement was not sent by a
       home agent.

    -  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 [17].

    -  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,



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       is non-zero, create a new entry in the list, and initialize its
       lifetime and preference to the values determined above.

    -  If the Home Agents List entry for the link-local address of
       the home agent sending this Advertisement was not deleted as
       described above, determine any global address(es) of the home
       agent based on each Prefix Information option received in
       this Advertisement in which the Router Address (R) bit is set
       (Section 6.2).  For each such global address determined from this
       Advertisement, add this global address to the list of global
       addresses for this home agent in this Home Agents List entry.

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


10.16. Sending Tunneled Router Solicitations

   When a mobile node has a home address that is about to become
   invalid, it tunnels a Router Solicitation to its home agent in
   an attempt to acquire fresh routing prefix information.  The new
   information enables the mobile node to participate in renumbering
   operations affecting the home network, as described in section 9.8.

   The mobile node SHOULD tunnel a Router Solicitation to the home agent
   when its home address will become invalid within MaxRtrAdvInterval
   seconds, where this value is acquired in a previous Router
   Advertisement from the home agent.  If no such value is known, the
   value MAX_PFX_ADV_DELAY seconds is used instead.

   The mobile node tunnels (using IPv6 encapsulation [4]) the
   solicitation, including the following IPv6 header fields:

      Outer src = care-of address
      Outer dst = Home Agent's global address
      Inner src = home address
      Inner dst = Home Agent's global address


   If the mobile node does not have a valid home address available for
   use as the Inner src address, it MAY use the unspecified IPv6 address
   (0:0:0:0:0:0:0:0).

   This solicitation follows the same retransmission rules as already
   specified for Router Solicitations [17], except that the initial
   retransmission interval is specified to be INITIAL_SOLICIT_TIMER.







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10.17. Receiving Tunneled Router Advertisements

   Section 9.8 describes the operation of a home agent to support
   renumbering a mobile node's home subnet while the mobile node is
   away from home.  The home agent tunnels certain Router 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 tunneled Router Advertisement, it MUST
   validate it according to the following tests:

    -  The Source Address of the IP packet carrying the Router
       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.

    -  The packet MUST be protected by IPsec [13, 11, 12] to guard
       against malicious Router Advertisements.  The IPsec protection
       MUST provide sender authentication, data integrity protection,
       and replay protection, covering the Router Advertisement.

    -  The packet contains a Binding Request destination option.

    -  The Binding Request option contains a Unique Identifier
       Sub-Option.

   Any received tunneled Router Advertisement not meeting all of these
   tests MUST be silently discarded.

   If a received tunneled Router Advertisement is not discarded
   according to the tests listed above, the mobile node MUST process the
   Router Advertisement as if it were connected to its home link [17].
   Such processing may result in the mobile node configuring a new home
   address, although due to separation between preferred lifetime and
   valid lifetime, such changes should not affect most communication by
   the mobile node, in the same way as for nodes that are at home.

   In processing the packet containing this Router Advertisement, the
   mobile node SHOULD return to the home agent a Binding Update in
   response to the Binding Request carried in the packet.  The correct
   formation of this Binding Update by the mobile node and processing
   of it by the home agent will be viewed by the home agent as an
   acknowledgement of this Router Advertisement, confirming to it that
   this Router Advertisement was received by the mobile node.

   In addition, if processing of this Router 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 [27] for the new address
   if the mobile node were located at home, then the mobile node MUST



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   set the Duplicate Address Detection (D) bit in this Binding Update to
   its home agent, to request the home agent to perform this Duplicate
   Address Detection on behalf of the mobile node.


10.18. Using Multiple Care-of Addresses

   As described in Section 10.5, a mobile node MAY use more than one
   care-of address at a time.  Particularly in the case of many wireless
   networks, a mobile node effectively might be reachable through
   multiple links at the same time (e.g., with overlapping wireless
   cells), on which different on-link subnet prefixes may exist.  A
   mobile node SHOULD select a primary care-of address from among those
   care-of addresses it has formed using any of these subnet prefixes,
   based on the movement detection mechanism in use, as described in
   Section 10.4.  When the mobile node selects a new primary care-of
   address, it MUST register it with its home agent by sending it a
   Binding Update with the Home Registration (H) and Acknowledge (A)
   bits set, as described in Section 10.6.

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


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




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

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


10.20. Returning Home

   A mobile node detects that it has returned to its home link through
   the movement detection algorithm in use (Section 10.4), when the
   mobile node detects that its home subnet prefix is again on-link.
   The mobile node SHOULD then send a Binding Update to its home agent,
   to instruct its home agent to no longer intercept or tunnel packets
   for it.  In this Binding Update, the mobile node MUST set the care-of
   address for the binding (the Source Address field in the packet's
   IPv6 header) to the mobile node's own home address.  As with other
   Binding Updates sent to register with its home agent, the mobile
   node MUST set the Acknowledge (A) and Home Registration (H) bits,
   and SHOULD retransmit the Binding Update until a matching Binding
   Acknowledgement is received.

   When sending this Binding Update to its home agent, the mobile
   node must be careful in how it uses Neighbor Solicitation [17] (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.




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   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 using 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 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 [17], 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 multicasts on the local link (such as Ethernet) are typically
   not guaranteed to be reliable, the mobile node MAY retransmit these
   Neighbor Advertisement messages up to MAX_ADVERT_REXMIT times to
   increase their reliability.  It is still possible that some nodes on
   the home link will not receive any of these Neighbor Advertisements,
   but these nodes will eventually be able to recover through use of
   Neighbor Unreachability Detection [17].















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11. Protocol Constants

      INITIAL_BINDACK_TIMEOUT   1 second

      INITIAL_SOLICIT_TIMER 2 seconds

      MAX_BINDACK_TIMEOUT       256 seconds

      MAX_UPDATE_RATE           once per second

      SLOW_UPDATE_RATE          once per 10 seconds

      MAX_FAST_UPDATES          5 transmissions

      MAX_ADVERT_REXMIT         3 transmissions

      MAX_PFX_ADV_DELAY         1,000 seconds

      HomeRtrAdvInterval        1,000 seconds



































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12. IANA Considerations

   This document defines four new types of IPv6 destination options,
   each of which must be assigned an Option Type value:

    -  The Binding Update option, described in Section 5.1;

    -  The Binding Acknowledgement option, described in Section 5.2;

    -  The Binding Request option, described in Section 5.3; and

    -  The Home Address option, described in Section 5.4.

   In addition, this document defines two ICMP message types, used as
   part of the dynamic home agent address discovery mechanism:

    -  The Home Agent Address Discovery Request message, described in
       Section 5.6; and

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

   This document also defines two new Neighbor Discovery [17] options,
   which must be assigned Option Type values within the option numbering
   space for Neighbor Discovery messages:

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

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

   Finally, this document defines a new type of anycast address, which
   must be assigned a reserved value for use with any subnet prefix to
   define this anycast address on each subnet:

    -  The "Mobile IPv6 Home-Agents" anycast address [10], used in the
       dynamic home agent address discovery mechanism described in
       Sections 9.2 and 10.7.

















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13. Security Considerations

13.1. Binding Updates, Acknowledgements, and Requests

   The Binding Update option described in this document will result
   in packets addressed to a mobile node being delivered instead to
   its care-of address.  This ability to change the routing of these
   packets could be a significant vulnerability if any packet containing
   a Binding Update option was not authenticated.  Such use of "remote
   redirection", for instance as performed by the Binding Update option,
   is widely understood to be a security problem in the current Internet
   if not authenticated [1].

   The Binding Acknowledgement option also requires authentication,
   since, for example, an attacker could otherwise trick a mobile node
   into believing a different outcome from a registration attempt with
   its home agent.

   No authentication is required for the Binding Request option, since
   the use of this option does not modify or create any state in either
   the sender or the receiver.  The Binding Request option does open
   some issues with binding privacy, but those issues can be dealt with
   either through existing IPsec encryption mechanisms or through use of
   firewalls.

   The existing IPsec replay protection mechanisms allow a "replay
   protection window" to support receiving packets out of order.
   Although appropriate for many forms of communication, Binding Updates
   MUST be applied only in the order sent.  The Binding Update option
   thus includes a Sequence Number field to provide this necessary
   sequencing.  The use of this Sequence Number together with IPsec
   replay protection is similar in many ways, for example, to the the
   sequence number in TCP.  IPsec provides strong replay protection but
   no ordering, and the sequence number provides ordering but need not
   protect against replays such as may occur when the sequence number
   wraps around.


13.2. Security for the Home Address Option

   No special 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
   (Section 5.4), 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



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   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.

   The use of the Home Address option allows packets sent by a
   mobile node to pass normally through routers implementing ingress
   filtering [7].  Since the care-of address used in the Source Address
   field of the packet's IPv6 header is topologically correct for the
   sending location of the mobile node, ingress filtering can trace the
   location of the mobile node in the same way as can be done with any
   sender when ingress filtering is in use.  A node receiving a packet
   that includes a Home Address option MAY implement the processing of
   this option by physically exchanging the Home Address option field
   with the source IPv6 address in the IPv6 header.


13.3. General Mobile Computing Issues

   The mobile computing environment is potentially very different from
   the ordinary computing environment.  In many cases, mobile computers
   will be connected to the network via wireless links.  Such links
   are particularly vulnerable to passive eavesdropping, active replay
   attacks, and other active attacks.  Furthermore, mobile computers
   are more susceptible to loss or theft than stationary computers.
   Any secrets such as authentication or encryption keys stored on the
   mobile computer are thus subject to compromise in ways generally not
   common in the non-mobile environment.

   Users who have sensitive data that they do not wish others to have
   access to SHOULD use additional mechanisms (such as encryption) to
   provide privacy protection, but such mechanisms are beyond the scope
   of this document.  Users concerned about traffic analysis SHOULD
   consider appropriate use of link encryption.  If stronger location
   privacy is desired, the mobile node can create a tunnel to its home
   agent.  Then, packets destined for correspondent nodes will appear
   to emanate from the home subnet, and it may be more difficult to
   pinpoint the location of the mobile node.  Such mechanisms are all
   beyond the scope of this document.

   Whether or not the mobile node is away from home is likely to
   influence the choice of security policy from the SPD. For instance,
   if a mobile node is connected to its home network and it communicates
   with a correspondent node on its home network, no security may be
   needed.  If, on the other hand, the mobile node is attached to
   foreign network and has sent a Binding Update to its home agent, then
   the mobile node may need to make use of security features in order to
   communicate with that same correspondent node.







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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-ipv6-12.txt:

    -  Specified that the Home Address destination option MUST be
       inserted between Routing Header and Fragment Header

    -  Specified that the Binding Update MUST be located after the IPsec
       header(s).

    -  Specified that the AH is to be calculated as if the home address
       were in the IPv6 header, and the care-of address were in the Home
       Address destination option.

    -  Changed SHOULD to MUST for treating unspecified home agent
       preferences as 0.

    -  Introduced the notion of scheduling Router Advertisements to
       be sent to the mobile node whenever a prefix advertisement or
       internal reconfiguration causes a mobile node's home address to
       be in danger of becoming deprecated.

    -  Specified that the mobile node MUST set the `D' bit whenever it
       sends a Binding Update that is new, instead of simply updating an
       existing binding to a new care-of address or binding lifetime.

    -  Added new section on mobile node tunneling Router Solicitations

    -  Added appendix about remote autoconfiguration for home addresses.

    -  Added specification about picking a longer initial retransmission
       interval for initial Binding Updates sent to the home agent,
       because the home agent will take longer since it has to perform
       DAD.

    -  Added the following protocol constants:

          INITIAL_SOLICIT_TIMER:     2 seconds

          MAX_PFX_ADV_DELAY:         1,000 seconds

          HomeRtrAdvInterval:        1,000 seconds










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Acknowledgements

   We would like to thank the members of the Mobile IP and IPng Working
   Groups for their comments and suggestions on this work.  We would
   particularly like to thank (in alphabetical order) Fred Baker
   (Cisco), Josh Broch (Carnegie Mellon University), Robert Chalmers
   (University of California at Santa Barbara), Rich Draves (Microsoft
   Research), Francis Dupont (ENST Bretagne), Thomas Eklund (SwithCore),
   Jun-Ichiro Itojun Hagino (IIJ Research Laboratory), Aime Lerouzic
   (Bull S.A.), Thomas Narten (IBM), Erik Nordmark (Sun Microsystems),
   Simon Nybroe (Ericsson Telebit), David Oran (Cisco), Basavaraj Patil
   (Nokia), Ken Powell (Compaq), Phil Roberts (Motorola), Patrice
   Romand (Bull S.A.), 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), and Xinhua Zhao (Stanford University) for
   their detailed reviews of earlier versions of this document.  Their
   suggestions have helped to improve both the design and presentation
   of the protocol.

   We would also like to thank the participants in the Mobile IPv6
   testing event held at Nancy, France, September 15-17, 1999, for
   their valuable feedback as a result of interoperability testing
   of four Mobile IPv6 implementations coming from four different
   organizations:  Bull (AIX), Ericsson Telebit (FreeBSD), NEC
   (FreeBSD), and INRIA (FreeBSD). Further, we would like to thank the
   feedback from the implementors who participated in the Mobile IPv6
   interoperability testing at Connectathon 2000 in San Jose,
   California, March 6-9, 2000.  Finally, 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.






















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References

    [1] S. M. Bellovin.  Security Problems in the TCP/IP Protocol Suite.
        ACM Computer Communications Review, 19(2), March 1989.

    [2] Jim Bound and Charles Perkins.  Dynamic Host Configuration
        Protocol for IPv6 (DHCPv6), February 1999.  Work in progress.

    [3] Scott Bradner.  Key words for use in RFCs to Indicate
        Requirement Levels.  RFC 2119, March 1997.

    [4] Alex Conta and Stephen Deering.  Generic Packet Tunneling in
        IPv6 Specification.  RFC 2473, December 1998.

    [5] Alex Conta and Stephen Deering.  Internet Control Message
        Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6)
        Specification.  RFC 2463, December 1998.

    [6] Stephen E. Deering and Robert M. Hinden.  Internet Protocol
        Version 6 (IPv6) Specification.  RFC 2460, December 1998.

    [7] Paul Ferguson and Daniel Senie.  Network Ingress Filtering:
        Defeating Denial of Service Attacks which employ IP Source
        Address Spoofing.  RFC 2267, January 1998.

    [8] Dan Harkins and Dave Carrel.  The Internet Key Exchange (IKE).
        RFC 2409, November 1998.

    [9] Robert M. Hinden and Stephen E. Deering.  IP Version 6
        Addressing Architecture.  RFC 2373, July 1998.

   [10] David B. Johnson and Stephen E. Deering.  Reserved IPv6 Subnet
        Anycast Addresses.  RFC 2526, March 1999.

   [11] Stephen Kent and Randall Atkinson.  IP Authentication Header.
        RFC 2402, November 1998.

   [12] Stephen Kent and Randall Atkinson.  IP Encapsulating Security
        Payload (ESP).  RFC 2406, November 1998.

   [13] Stephen Kent and Randall Atkinson.  Security Architecture for
        the Internet Protocol.  RFC 2401, November 1998.

   [14] Douglas Maughan, Mark Schneider, Mark Schertler, and Jeff
        Turner.  Internet Security Association and Key Management
        Protocol (ISAKMP).  RFC 2408, November 1998.

   [15] P. Mockapetris.  Domain Names -- Concepts and Facilities.
        RFC 1034, November 1987.





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   [16] P. Mockapetris.  Domain Names -- Implementation and
        Specification.  RFC 1035, November 1987.

   [17] Thomas Narten, Erik Nordmark, and William Allen Simpson.
        Neighbor Discovery for IP Version 6 (IPv6).  RFC 2461, December
        1998.

   [18] Charles Perkins.  IP Encapsulation within IP.  RFC 2003, October
        1996.

   [19] Charles Perkins, editor.  IP Mobility Support.  RFC 2002,
        October 1996.

   [20] Charles Perkins.  Minimal Encapsulation within IP.  RFC 2004,
        October 1996.

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

   [22] Derrell Piper.  The Internet IP Security Domain of
        Interpretation for ISAKMP.  RFC 2407, November 1998.

   [23] David C. Plummer.  An Ethernet Address Resolution Protocol:
        Or Converting Network Protocol Addresses to 48.bit Ethernet
        Addresses for Transmission on Ethernet Hardware.  RFC 826,
        November 1982.

   [24] J. B. Postel.  User Datagram Protocol.  RFC 768, August 1980.

   [25] J. B. Postel, editor.  Transmission Control Protocol.  RFC 793,
        September 1981.

   [26] Joyce K. Reynolds and Jon Postel.  Assigned Numbers.  RFC 1700,
        October 1994.  See also http://www.iana.org/numbers.html.

   [27] Susan Thomson and Thomas Narten.  IPv6 Stateless Address
        Autoconfiguration.  RFC 2462, December 1998.

















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A. Remote Home Address Configuration

   The method for initializing a mobile node's home addresses on
   power-up or after an extended period of being disconnected from
   the network is beyond the scope of this specification.  Whatever
   procedure is used should result in the mobile node having the same
   stateless or stateful (e.g., DHCPv6) home address autoconfiguration
   information it would have if it were attached to the home network.
   Due to the possibility that the home network could be renumbered
   while the mobile node is disconnected, a robust mobile node would not
   rely solely on storing these addresses locally.

   A mobile node MAY generate a temporary home address using the
   following information:

    -  the subnet prefix from the home network's mobile agent anycast
       address, and

    -  the globally unique interface identifier that would have been
       used to generate the link local address if the mobile node were
       attached directly to the home network.

   Such a temporary address could be used to establish a binding with
   a home agent in the absence of any other known home addresses.  It
   could be created with short valid lifetime and a preferred lifetime
   of zero to ensure a quick transition to other addresses generated
   when stateless or stateful (DHCPv6) address autoconfiguration runs.

   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 address and use it to
       generate a temporary home address for the mobile node using the
       rules defined above.

    6. Send a binding update option with a Router Solicitation to the
       home agent.  This registers the mobile node's temporary home
       address and requests a router advertisement to initiate stateless
       address autoconfiguration at the same time.

    7. Receive binding acknowledgement and binding request options with
       a router advertisement from the home agent.



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    8. Parse the Router Advertisement and configure all prefixes and
       addresses according to the method stated there.  If the M or O
       flags are set in the router advertisement, follow the stateful
       (DHCPv6) configuration procedures.  These procedures could make
       the temporary home address permanent by increasing its valid and
       preferred lifetimes.

    9. Send binding update option(s) to update the binding for the
       temporary home address and to establish bindings for any new home
       addresses.


Chair's Address

   The Working Group can be contacted via its current chairs:

        Phil Roberts
        Motorola
        1501 West Shure Drive
        Arlington Heights, IL 60004

        Phone:  +1 847 632-3148
        E-mail: qa3445@email.mot.com


        Basavaraj Patil
        Nokia
        6000 Connection Drive
        M/S M8-540
        Irving, TX 75039
        USA

        Phone:  +1 972 894-6709
        Fax:    +1 972 894-5349
        E-mail: raj.patil@nokia.com



















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Authors' Addresses

   Questions about this document can also be directed to the authors:

        David B. Johnson
        Rice University
        Department of Computer Science, MS 132
        6100 Main Street
        Houston, TX 77005-1892
        USA

        Phone:  +1 713 348-3063
        Fax:    +1 713 348-5930
        E-mail: dbj@cs.rice.edu


        Charles Perkins
        Nokia
        313 Fairchild Drive
        Mountain View, CA 94043
        USA

        Phone:  +1 650 625-2986
        Fax:    +1 650 625-2502
        E-mail: charliep@iprg.nokia.com





























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