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IETF Mobile IP Working Group                            David B. Johnson
INTERNET-DRAFT                                Carnegie Mellon University
                                                         Charles Perkins
                                                        Sun Microsystems
                                                           4 August 1998


                        Mobility Support in IPv6

                   <draft-ietf-mobileip-ipv6-06.txt>


Status of This Memo

   This document is a submission by the Mobile IP Working Group of the
   Internet Engineering Task Force (IETF).  Comments should be submitted
   to the Working Group mailing list at "mobile-ip@SmallWorks.COM".
   Distribution of this memo is unlimited.

   This document is an Internet-Draft.  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."

   To view the entire list of current Internet-Drafts, please check
   the "1id-abstracts.txt" listing contained in the Internet-Drafts
   Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net
   (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au
   (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu
   (US West Coast).

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.








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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. Conceptual Data Structures  . . . . . . . . . . . . . . .   13
     4.4. Binding Management  . . . . . . . . . . . . . . . . . . .   16

 5. New IPv6 Destination Options                                      19
     5.1. Binding Update Option Format  . . . . . . . . . . . . . .   19
     5.2. Binding Acknowledgement Option Format . . . . . . . . . .   23
     5.3. Binding Request Option Format . . . . . . . . . . . . . .   27
     5.4. Home Address Option Format  . . . . . . . . . . . . . . .   28

 6. Modifications to IPv6 Neighbor Discovery                          30
     6.1. Modified Router Advertisement Message Format  . . . . . .   30
     6.2. Modified Prefix Information Option Format . . . . . . . .   31
     6.3. New Advertisement Interval Option Format  . . . . . . . .   33
     6.4. New Home Agent Information Option Format  . . . . . . . .   34
     6.5. Changes to Sending Router Advertisements  . . . . . . . .   36
     6.6. Changes to Sending Router Solicitations . . . . . . . . .   37

 7. Requirements for IPv6 Nodes                                       39
     7.1. Requirements for All IPv6 Hosts and Routers . . . . . . .   39
     7.2. Requirements for All IPv6 Routers . . . . . . . . . . . .   39
     7.3. Requirements for IPv6 Home Agents . . . . . . . . . . . .   39
     7.4. Requirements for IPv6 Mobile Nodes  . . . . . . . . . . .   40

 8. Correspondent Node Operation                                      42
     8.1. Receiving Packets from a Mobile Node  . . . . . . . . . .   42
     8.2. Receiving Binding Updates . . . . . . . . . . . . . . . .   42
     8.3. Requests to Cache a Binding . . . . . . . . . . . . . . .   43
     8.4. Requests to Delete a Binding  . . . . . . . . . . . . . .   44



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     8.5. Sending Binding Acknowledgements  . . . . . . . . . . . .   44
     8.6. Sending Binding Requests  . . . . . . . . . . . . . . . .   45
     8.7. Cache Replacement Policy  . . . . . . . . . . . . . . . .   45
     8.8. Receiving ICMP Error Messages . . . . . . . . . . . . . .   46
     8.9. Sending Packets to a Mobile Node  . . . . . . . . . . . .   47

 9. Home Agent Operation                                              49
     9.1. Receiving Router Advertisement Messages . . . . . . . . .   49
     9.2. Dynamic Home Agent Address Discovery  . . . . . . . . . .   50
     9.3. Primary Care-of Address Registration  . . . . . . . . . .   51
     9.4. Primary Care-of Address De-registration . . . . . . . . .   54
     9.5. Intercepting Packets for a Mobile Node  . . . . . . . . .   54
     9.6. Tunneling Intercepted Packets to a Mobile Node  . . . . .   56
     9.7. Renumbering the Home Subnet . . . . . . . . . . . . . . .   57

10. Mobile Node Operation                                             60
    10.1. Sending Packets While Away from Home  . . . . . . . . . .   60
    10.2. Receiving Packets While Away from Home  . . . . . . . . .   62
    10.3. Movement Detection  . . . . . . . . . . . . . . . . . . .   63
    10.4. Forming New Care-of Addresses . . . . . . . . . . . . . .   66
    10.5. Sending Binding Updates to the Home Agent . . . . . . . .   67
    10.6. Dynamic Home Agent Address Discovery  . . . . . . . . . .   68
    10.7. Sending Binding Updates to Correspondent Nodes  . . . . .   69
    10.8. Sending Binding Updates to the Previous Default Router  .   71
    10.9. Retransmitting Binding Updates  . . . . . . . . . . . . .   72
   10.10. Rate Limiting for Sending Binding Updates . . . . . . . .   72
   10.11. Receiving Binding Acknowledgements  . . . . . . . . . . .   72
   10.12. Receiving Binding Requests  . . . . . . . . . . . . . . .   73
   10.13. Receiving ICMP Error Messages . . . . . . . . . . . . . .   74
   10.14. Receiving Tunneled Router Advertisements  . . . . . . . .   74
   10.15. Using Multiple Care-of Addresses  . . . . . . . . . . . .   75
   10.16. Routing Multicast Packets . . . . . . . . . . . . . . . .   76
   10.17. Returning Home  . . . . . . . . . . . . . . . . . . . . .   76

11. Constants                                                         78

12. IANA Considerations                                               79

13. Security Considerations                                           80
    13.1. Binding Updates, Acknowledgements, and Requests . . . . .   80
    13.2. Home Address Option . . . . . . . . . . . . . . . . . . .   80
    13.3. General Mobile Computing Issues . . . . . . . . . . . . .   81











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Changes from Previous Draft                                           83

Acknowledgements                                                      85

References                                                            86

Chair's Address                                                       88

Authors' Addresses                                                    89












































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1. Introduction

   This document specifies the operation of mobile computers using
   Internet Protocol Version 6 (IPv6) [5].  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 "macro"
   mobility management problem.  More "micro" mobility management
   applications -- for example, handoff among wireless transceivers,
   each of which covers only a very small geographic area -- are
   possibly more suited to other solutions.  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.  As long
   as such handoff occurs only within cells of the mobile node's home
   link, such link-layer mobility mechanisms are likely to offer faster
   convergence and lower overhead than Mobile IPv6.  Extensions to the
   Mobile IPv6 protocol are also possible 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 10.3,
       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 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) [15, 14, 16], 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" [17] is now built in as a fundamental part
       of the protocol, rather than being added on as a 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 [15].  This integration also allows the
       Mobile IPv4 "registration" functionality and the Mobile IPv4
       Route Optimization functionality to be 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" [6].  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.





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    -  There is no longer any need to deploy special routers as
       "foreign agents" as are used in Mobile IPv4.  In Mobile IPv6,
       mobile nodes make use of the enhanced features of IPv6, such
       as Neighbor Discovery [13] and Address Autoconfiguration [22],
       to operate in any location away from home without any special
       support required from its local router.

    -  Unlike Mobile IPv4, Mobile IPv6 utilizes IPsec [8, 9, 10] 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 tunneled 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 tunneling.

    -  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 [13] rather than ARP [18] 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.




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

    -  The dynamic home agent address discovery mechanism in Mobile IPv6
       uses IPv6 anycast 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 and since 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




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         address, encapsulates them, and tunnels them to the mobile
         node's registered care-of address.

      care-of address

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

      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 [22] or
   stateful (e.g., DHCPv6 [2]) address autoconfiguration, according
   to the methods of IPv6 Neighbor Discovery [13].  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




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   packet using IPv6 encapsulation [4], with the outer IPv6 header
   addressed to the mobile node's primary care-of address.

   Section 10.15 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, leaving the
   mobile node entirely in control of this policy by which of its
   care-of addresses it registers 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 a Binding Update to the
   "Home-Agents anycast address" for its own home subnet prefix and thus
   reaches one of the (possibly many) routers on its home link currently
   operating as a home agent.  This home agent rejects the mobile
   node's Binding Update, but returns in the Binding Acknowledgement
   in response a list of all 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 [5] (instead of IPv6 encapsulation) to route the packet to
   the mobile node by way of the care-of address indicated in this
   binding.  If, instead, the sending node has no cached binding for
   this destination address, the node sends the packet normally (with
   no Routing header), and the packet is subsequently intercepted and
   tunneled by the mobile node's home agent as described above.  Any
   node communicating with a mobile node is referred to in this document
   as a "correspondent node" of the mobile node, and may itself be
   either a stationary node or a mobile node.




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   Since a Binding Update, Binding Acknowledgement, and Binding Request
   are each represented in a packet as an IPv6 destination option [5],
   they may be included in any IPv6 packet.  Any of these options can be
   sent in either of two ways:

    -  A Binding Update, Binding Acknowledgement, or Binding Request can
       be included within any IPv6 packet carrying any payload such as
       TCP [20] or UDP [19].

    -  A Binding Update, Binding Acknowledgement, or Binding Request 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" [5].

   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" [6] that do not allow forwarding of packets that
   appear to have a Source Address that is not topologically correct.
   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 discussed in general in Section 4.1, the following four new IPv6
   destination options are defined for Mobile IPv6:

      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 also include either an AH [8] or



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         ESP [9] header providing sender authentication, data integrity
         protection, and replay protection.  The Binding Update option
         is 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 also include either an AH [8] or
         ESP [9] header providing sender authentication, data integrity
         protection, and replay protection.  The Binding Acknowledgement
         option is 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.  The Home Address option
         is described in detail in Section 5.4.

   Extensions to the format of these options MAY be included after the
   fixed portion of the option data specified in this document.  The
   presence of such extensions 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 extensions.  Currently, no extensions have been
   defined.



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4.3. 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.  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 [13].
         When sending a packet, the Binding Cache is searched before the
         Neighbor Discovery conceptual Destination Cache [13] (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.

          -  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.




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          -  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.

          -  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.  Any node's Binding Cache 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.

      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, to the mobile
         node's home agent, and to a previous default router of the
         mobile node.  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).

          -  The home address for which that Binding Update was sent.
             This will be one of the mobile node's home addresses for



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             most Binding Updates (Sections 10.5 and 10.7), but will
             be the mobile node's previous care-of address for Binding
             Updates sent to the mobile node's previous default router
             (Section 10.8).

          -  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 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.

          -  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 10.13.

      Home Agents List

         A list, maintained by each home agent, recording information
         about each other home agent on a link on which this node
         is serving as a home agent; each home agent maintains a
         separate Home Agents List for each such link on which it is
         serving.  This list is used in the dynamic home agent address
         discovery mechanism.  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



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         Default Router List conceptual data structure maintained by
         each host for Neighbor Discovery [13].  The Home Agents List
         MAY be implemented in any manner consistent with the external
         behavior described in this document.  Each Home Agents List
         entry conceptually contains the following fields:

          -  The IP address of another router on the home link that this
             node currently believes is operating as a home agent for
             this 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 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 this home agent
             is initialized from the Home Agent Lifetime field in the
             option; otherwise, the lifetime is initialized from the
             Router Lifetime field in the received Router Advertisement.
             The Home Agents List entry lifetime is decremented until it
             reaches zero, at which time this entry MUST be deleted from
             the Home Agents List.

          -  The preference for this home agent, for use in ordering the
             Home Agents List returned in a Binding Acknowledgement;
             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.


4.4. 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



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   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
   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




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   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

5.1. Binding Update Option Format

   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|C| Reserved| Prefix Length |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Lifetime                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                        Care-of Address                        +
   |                  (only present if C bit set)                  |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Option Type

         195 ???

      Option Length

         8-bit unsigned integer.  Length of the option, in octets,
         excluding the Option Type and Option Length fields.  For the
         current definition of the Binding Update option, the minimum
         value for this field is 8; the length is 24 if the Care-of
         Address Present (C) bit is set.

      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.




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

      Care-of Address Present (C)

         The Care-of Address Present (C) bit indicates the presence of
         the Care-of Address field in the Binding Update.  The care-of
         address for this binding is either the address in the Care-of
         Address field in the Binding Update, if this bit is set, or the
         Source Address in the packet's IPv6 header, if this bit is not
         set.

      Reserved

         Sent as 0; ignored on reception.

      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 the
         link-local address and site-local address corresponding to
         this interface identifier, and defends each for purposes of
         Duplicate Address Detection.  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



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         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).

      Lifetime

         32-bit unsigned integer.  The number of seconds remaining
         before the binding must be considered expired.  A value of all
         one bits (0xffffffff) indicates infinity.  A value of zero
         indicates that the Binding Cache entry for the mobile node
         should be deleted.

      Care-of Address

         This field in the Binding Update is optional and is only
         present when the Care-of Address Present (C) bit is set.  If
         present, it gives the care-of address of the mobile node for
         this binding.  For most Binding Updates sent, it is expected
         that this field will not be present, and instead that the
         care-of address for the binding will be given by the Source
         Address field in the packet's IPv6 header.

   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 indicated by the Home
   Address field in the Home Address option in the packet.

   Any packet that includes a Binding Update option MUST also include
   either an AH [8] or ESP [9] header providing sender authentication,
   data integrity protection, and replay protection.

   If the care-of address in the binding (either the Care-of Address
   field in the Binding Update option or 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



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   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 [5].  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.

   Extensions to the Binding Update option format may be included after
   the fixed portion of the Binding Update option specified above.
   The presence of such extensions will be indicated by the Option
   Length field.  When the Option Length is greater than the length
   defined above, the remaining octets are interpreted as extensions.
   Currently, no extensions have been defined.
































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5.2. Binding Acknowledgement Option Format

   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 included it 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 (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                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   .                                                               .
   .                        Home Agents List                       .
   .                                                               .
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Option Type

         2 ???

      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, except when the Status field is equal to 135
         (dynamic home agent address discovery response), in which case
         this field MUST be set to 11 + 16 * N, where N is the number of
         IP addresses included in the Home Agents List field; the Home




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         Agents List field MUST NOT be included in the option if the
         Status field is not set to 135.

      Status

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

              0   Binding Update accepted

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

            128   Reason unspecified
            129   Poorly formed Binding Update
            130   Administratively prohibited
            131   Insufficient resources
            132   Home registration not supported
            133   Not home subnet
            134   Sequence Number field value too small
            135   Dynamic home agent address discovery response
            136   Incorrect interface identifier length
            137   Not home agent for this mobile node

         Up-to-date values of the Status field are to be specified in
         the most recent "Assigned Numbers" [21].

      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 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




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         if the Status field indicates that the Binding Update was
         rejected.

      Refresh

         The recommended interval at which the mobile node SHOULD send
         a new Binding Update to this node in order to "refresh" the
         mobile node's binding in this node's Binding Cache.  This
         refreshing of the binding is useful in case the node fails and
         loses its cache state.  The Refresh period is determined by
         the node sending the Binding Acknowledgement (the node caching
         the binding).  If this node is serving as the mobile node's
         home agent, the Refresh value may be set, for example, based on
         whether the node stores its Binding Cache in volatile storage
         or in nonvolatile storage.  If the node sending the Binding
         Acknowledgement is not serving as the mobile node's home agent,
         the Refresh period SHOULD be set equal to the Lifetime period
         in the Binding Acknowledgement; even if this node loses this
         cache entry due to a failure of the node, packets from it can
         still reach the mobile node through the mobile node's home
         agent, causing a new Binding Update to this node to allow it
         to recreate this cache entry.  The value of this field is
         undefined if the Status field indicates that the Binding Update
         was rejected.

      Home Agents List

         A list of home agents on the home link for the mobile node to
         which this Binding Acknowledgement is sent.  This field MUST
         NOT be present (zero addresses listed) unless the Binding
         Acknowledgement is sent in response to an anycast Binding
         Update sent by this mobile node attempting dynamic home agent
         address discovery.  In this case, the Status field MUST be
         set to 135 (dynamic home agent address discovery response).
         The construction of the Home Agents List field in a Binding
         Acknowledgement is defined in Section 9.2.

   Any packet that includes a Binding Acknowledgement option MUST
   also include either an AH [8] or ESP [9] header providing sender
   authentication, data integrity protection, and replay protection.

   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, and are described in Section 8.9.  The



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   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
   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" [5]).

   The three highest-order bits of the Option Type are encoded to
   indicate specific processing of the option [5].  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 Format

   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 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Option Type

         3 ???

      Option Length

         8-bit unsigned integer.  Length of the option, in octets,
         excluding the Option Type and Option Length fields.  For the
         current definition of the Binding Request option, this field
         MUST be set to 0.

   The three highest-order bits of the Option Type are encoded to
   indicate specific processing of the option [5].  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
   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.

   Extensions to the Binding Request option format may be included after
   the fixed portion of the Binding Request option specified above.
   The presence of such extensions will be indicated by the Option
   Length field.  When the Option Length is greater than 0 octets,
   the remaining octets are interpreted as extensions.  Currently, no
   extensions have been defined.








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5.4. Home Address Option Format

   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                         +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Option Type

         196 ???

      Option Length

         8-bit unsigned integer.  Length of the option, in octets,
         excluding the Option Type and Option Length fields.  For the
         current definition of the Home Address option, this field MUST
         be set to 16.

      Home Address

         The home address of the mobile node sending the packet.

   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



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   of the receiver's Binding Cache and MUST NOT cause any changes in the
   routing of subsequent packets sent by this receiving node.

   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.  If the packet carries no IP authentication, then the
   contents of the Home Address option, as well as the Source Address
   field or any other field in the IPv6 header, may have been forged or
   altered during transit.

   Upon receipt of a packet containing a Home Address option, the
   receiving node replaces the Source Address in the IPv6 header with
   the Home Address in the Home Address option.  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.

   The three highest-order bits of the Option Type are encoded to
   indicate specific processing of the option [5].  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.

   Extensions to the Home Address option format may be included after
   the fixed portion of the Home Address option specified above.
   The presence of such extensions will be indicated by the Option
   Length field.  When the Option Length is greater than 8 octets,
   the remaining octets are interpreted as extensions.  Currently, no
   extensions have been defined.













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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 [13] by the addition of a single flag bit for use in the
   dynamic home agent address discovery mechanism (Sections 9.2
   and 10.6).  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 [13]:

      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.

      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 proving 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 10.6).

    -  To allow a mobile node to send a Binding Update to its previous
       default router, after moving to a new subnet and acquiring a new
       care-of address (Section 10.8).

   However, Neighbor Discovery [13] 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 [13]:






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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 [13].  In this case, at least one of these
   multiple Advertisements begin 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

         6 ???

      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 [13], 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 10.3.

   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

         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.

      Home Agent Preference

         16-bit signed, twos-complement integer.  The preference
         for the home agent sending this Router Advertisement, for
         use in ordering the Home Agents List returned in a Binding
         Acknowledgement; 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.

      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



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         a Router Advertisement in which the Home Agent (H) bit is set,
         the lifetime for this home agent SHOULD 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 (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 [13] 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 home agent 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.





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    -  A mobile node that is currently configured with a care-of
       address SHOULD NOT send Router Solicitations, until its movement
       detection algorithm (Section 10.3) determines that it has moved
       and that its current care-of address might no longer be valid.

















































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7. Requirements for 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 be able to accept packets addressed to the
       Home-Agents anycast address for the subnet on which it is serving
       as a home agent, and MUST be able to participate in dynamic home
       agent address discovery (Section 9.2).


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 10.5, 10.7, and 10.8; and MUST
       be able to receive and process Binding Acknowledgement options,
       as specified in Section 10.11.

    -  Every IPv6 mobile node MUST support use of the dynamic home agent
       address discovery mechanism, as described in Section 10.6.

    -  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.




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    -  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.
















































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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 copying the Home Address field from the Home Address
   option into the IPv6 header, replacing the original value of the
   Source Address field there.

   Further processing of such a packet (e.g., at the transport layer)
   thus need not 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 contains a valid AH [8] or ESP [9] header that
       provides sender authentication, integrity protection, and replay
       protection.

    -  The packet MUST contain a valid Home Address option.  The home
       address for the binding is specified by the Home Address field of
       the 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.  The Sequence Number comparison is
       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.

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

    -  If the Destination Address in the packet's IPv6 header is the
       Home-Agents anycast address for a local prefix and this address
       is assigned to one of this node's network interfaces, then the
       mobile node sending this Binding Update is attempting dynamic
       home agent address discovery.  Processing for this type of
       received Binding Update is described in Section 9.2.  (If the
       Destination Address is not assigned to one of this node's network
       interfaces, then the packet would have been forwarded as a normal
       packet and the Binding Update, as a destination option, would not
       be processed in any way by this node.)

    -  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 (as given in the Home Address option in the
       packet), 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 home address of the mobile node is taken from the Home Address
   field in the packet's Home Address option.  The new Binding Cache



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   entry records the association between this home address and the
   care-of address for the binding, as specified in either the Care-of
   Address field of the Binding Update or in the Source Address field
   in the packet's IPv6 header.  Any Binding Cache entry created or
   updated in response to processing this Binding Update MUST be deleted
   after the expiration of the Lifetime period specified in the Binding
   Update.


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.  The home address of the
   mobile node is taken from the Home Address field in the packet's Home
   Address option.


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 SHOULD 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, the Status field in
   the Binding Acknowledgement MUST be set to a value less than 128;
   if the node rejects the Binding Update and does not create or
   update an entry for this binding, 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" [21].

   As described in Section 5.2, the packet in which the Binding
   Acknowledgement is returned MUST include either an AH [8] or ESP [9]
   header providing sender authentication, data integrity protection,
   and replay protection; 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.  The packet is routed first to the care-of
   address contained in the Binding Update being acknowledged, and
   then to the mobile node's home address.  This use of the Routing
   header ensures that the Binding Acknowledgement will be routed to the
   current location of the node sending the Binding Update, whether the
   Binding Update was accepted or rejected.



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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.

   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 (or
   can 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"



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   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
   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 returned 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 default router (e.g., the mobile node moved after the packet
   was sent), the router will encapsulate and tunnel the packet to the
   mobile node's new care-of address (if it has a Binding Cache entry
   for the mobile node).  As above, any ICMP error message caused by the
   packet while in this tunnel will be returned to the previous default
   router (the source of the tunnel), which MUST relay certain ICMP
   error messages back to the correspondent node [4].




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   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.


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 [5], 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 [5], 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



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       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 10.7,
   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 [13].

   On receipt of a valid Router Advertisement, as defined in the
   processing algorithm specified for Neighbor Discovery [13], 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 [13].
       Determine the global address of the router based on the
       Prefix Information option received from it in which the Router
       Address (R) bit is set (Section 6.2).

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



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

    -  If the address 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.

   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

   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 indicates that the mobile node sending it is
   attempting dynamic home agent address discovery.

   As described in Section 10.6, a mobile node attempts dynamic home
   agent address discovery by sending its "home registration" Binding
   Update to the Home-Agents anycast address for its home IP subnet
   prefix (the packet MUST also include a Home Address option).  A home
   agent receiving such a Binding Update that is serving this subnet
   (the home agent is configured with this anycast address on one of its
   network interfaces) MUST reject the Binding Update and SHOULD return
   a Binding Acknowledgement indicating this rejection, with the Source
   Address of the packet carrying the Binding Acknowledgement set to one
   of the global unicast addresses of the home agent.  The Status field
   in the Binding Acknowledgement MUST be set to 135 (dynamic home agent
   address discovery response).

   In this Binding Acknowledgement rejecting the dynamic home agent
   address discovery Binding Update, this home agent SHOULD set the Home
   Agents List as follows:

    -  The Home Agents List in this Binding Acknowledgement SHOULD
       contain the IP address of all home agents currently listed in
       this home agent's own Home Agents List (Section 4.3).  However,
       if this home agent's own 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 list in
       the Binding Acknowledgement.  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.



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    -  The IP addresses in the Home Agents List should be placed in
       the Home Agents List in the Binding Acknowledgement 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, and the home agent with the lowest preference SHOULD be
       listed last.

    -  Among home agents with equal preference, their IP addresses in
       the Home Agents List SHOULD be listed in an order randomized with
       respect to other home agents with equal preference, each time
       a Binding Acknowledgement with a non-empty Home Agents List is
       returned by this home agent.

    -  The Option Length field in this Binding Acknowledgement
       MUST be set to 11 + 16 * N, where N is the number of IP
       addresses included in the Home Agents List field in the Binding
       Acknowledgement.

   The mobile node, upon receiving this Binding Acknowledgement, MAY
   then resend its Binding Update to the home agent address given as the
   IP Source Address of the packet carrying the Binding Acknowledgement
   or to any of the unicast IP addresses listed in the Home Agents List
   field in the Acknowledgement.  For example, the mobile node may
   re-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 Agents List in the Binding Acknowledgement.  If the home agent
   identified by the Source Address field in the IP header of the packet
   carrying the Binding Acknowledgement is not listed in the Home Agents
   List, it SHOULD be tried before the first address given in the list;
   otherwise, it SHOULD be tried in the in its listed order.


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:





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

   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 from the Home Address
   field in the packet's Home Address option.  The care-of address for
   this Binding Cache entry is taken from the Care-of Address field of
   the Binding Update (if the Care-of Address Present (C) bit is set in
   the Binding Update) or from the Source Address field in the packet's
   IPv6 header (otherwise).

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

   The lifetime for the Binding Cache entry MUST NOT be greater than the
   remaining valid lifetime for the subnet prefix in the mobile node's
   home address specified with the Binding Update.  The remaining valid



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   lifetime for this prefix is determined by the home agent based on
   its own Prefix List entry for this prefix [13].  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.

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

   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 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.




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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:

    -  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].




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   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 [13] 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,
       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 [13] on behalf of the mobile node, to
       advertise the home agent's own link-layer address for this IP
       address.  The Target Address in the Neighbor Advertisement
       message MUST be set to this IP address for the mobile node, and
       the Advertisement MUST include a Target Link-layer Address option
       specifying the home agent's link-layer address.  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 [13].





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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 [13]
   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.  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 to defend these addresses on the home link for
   Duplicate Address Detection [13].


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 [8] or ESP [9] header present in the packet.

   For forwarding each intercepted packet to the mobile node, the
   home agent MUST tunnel the packet to the mobile node using IPv6



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   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 [7],
   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) [7], 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. Renumbering the Home Subnet

   IPv6 provides mechanisms through Neighbor Discovery [13] and Address
   Autoconfiguration [22] 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.



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   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 [13].  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 AH [8] or
   ESP [9].

   Specifically, a home agent serving some mobile node SHOULD construct
   and tunnel to the mobile node a new Router Advertisement when any of
   the following conditions occur:

    -  The preferred or valid lifetime for an existing prefix on the
       home link is reduced.

    -  A new prefix is introduced on the home link.

    -  The state of the home agent's AdvManagedFlag flag [13] changes
       from FALSE to TRUE or from TRUE to FALSE.

   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 constructs a new
   Router Advertisement message containing no options other than the
   Prefix Information options describing the prefixes for which one of
   the conditions above has occurred since the last Router Advertisement
   tunneled to and acknowledged by the mobile node.  When multiple
   conditions occur at or near the same time, the home agent SHOULD
   attempt to combine them into a single Router Advertisement message to
   the mobile node.

   In tunneling each such 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 include either an AH [8] or ESP [9] header
       providing sender authentication, data integrity protection, and
       replay protection.

    -  The packet MUST include a Binding Request destination option.




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    -  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).  If
   while the mobile node 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.

   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 10.7, 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 a binding lifetimes ensure that no node uses a mobile node's home
   address beyond the time that 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 [11, 12].  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.

   If the mobile node uses one of its care-of addresses as the source
   of some packet while away from home, no special Mobile IP processing



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   is required for sending this packet.  The packet is simply addressed
   and transmitted in the same way as any normal IPv6 packet, setting
   the Source Address field in the packet's IPv6 header to this care-of
   address.

   On the other hand, if while away from home, the mobile node uses its
   home address as the source of a packet from the point of view of
   higher protocol layers or applications as described above, special
   Mobile IP processing of this packet is required for the insertion of
   the Home Address option.  Specifically:

    -  Since Mobile IP is transparent to higher protocol layers (e.g.,
       to TCP), the packet is initially constructed 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.

    -  If the mobile node is at home, no special Mobile IP processing
       for this packet is required.  The packet is sent normally and the
       following additional steps are not performed.

    -  Likewise, if the Source Address field in the packet's IPv6 header
       is not the mobile node's home address, no special Mobile IP
       processing for this packet is required.  The packet is sent
       normally and the following additional steps are not performed.

    -  Otherwise, 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.

   This addition of the Home Address option to a packet MUST be
   performed before outgoing IPsec processing, such as the addition of
   an AH [8] or ESP [9] header to the packet, is performed.  Likewise,
   IPsec processing for a received packet containing a Home Address
   option MUST be performed before the packet is possibly modified as
   part of processing the Home Address option.  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 [6].







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10.2. 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
       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 its previous default router when
       moving from this care-of address to another, and if the Binding
       Cache entry that was created from this Binding Update is still
       present in this router's Binding Cache, then such a packet
       will be intercepted by this router, encapsulated using IPv6
       encapsulation [4], and tunneled to the mobile node's primary
       care-of address (registered with this router, acting as a home
       agent for this out-of-date care-of address).

   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 10.7, subject to the
   rate limiting defined in Section 10.10.  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 [5], which
   will result in the packet being processed normally by upper-layer




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   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 [5].  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.


10.3. 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.  The description here is based on
   the conceptual model of the organization and data structures defined
   by Neighbor Discovery [13].

   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 [13].
   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) 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 10.4.  The mobile node registers
   its primary care-of address with its home agent, as described in
   Section 10.5.




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   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 to 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 [13], while the mobile node is actively sending
   packets to (or through) its default router, the mobile node can
   detect that the router (as its neighbor) is still reachable either
   through indications from upper layer protocols on the mobile node
   that a connection is making "forward progress" (e.g., receipt of TCP
   acknowledgements for new data transmitted), or through receipt of a
   Neighbor Advertisement message from its default router in response
   to an explicit Neighbor Solicitation messages to it.  Note that
   although this mechanism detects that the mobile node's default router
   has become unreachable to the mobile node only while the mobile node
   is actively sending packets to it, this is the only time that this
   direction of reachability confirmation is needed.  Confirmation
   that the mobile node is still reachable from the router is handled
   separately, as described below.

   For a mobile node to detect when it has become unreachable from its
   default router, the mobile node cannot efficiently rely on Neighbor
   Unreachability Detection alone, since the network overhead would be
   prohibitively high in many cases for a mobile node to continually
   probe its default router with Neighbor Solicitation messages even
   when it is not otherwise actively sending packets to it.  Instead,
   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



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   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 to detect reachability from the router.  This
   use of promiscuous mode may be useful on very low bandwidth (e.g.,
   wireless) links, but its use MUST be configurable on the mobile node.

   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



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   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.


10.4. Forming New Care-of Addresses

   After detecting that it has moved from one link to another (i.e., its
   current default router has become unreachable and it has discovered a
   new default router), a mobile node SHOULD form a new primary care-of
   address using one of the on-link subnet prefixes advertised by the
   new router.  A mobile node MAY form a new primary care-of address
   at any time, except that it MUST NOT do so too frequently (not 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 10.15.

   As described in Section 4, in order to form a new care-of address,
   a mobile node MAY use either stateless [22] 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



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   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.


10.5. Sending Binding Updates to the Home Agent

   After deciding to change its primary care-of address as described
   in Sections 10.3 and 10.4, 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, or the Care-of Address
       Present (C) bit MUST be set in the Binding Update and the care-of
       address for the binding MUST be specified in the Care-of Address
       field in the Binding Update.

    -  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 Acknowledge (A) bit in the Binding Update requests the home
   agent to return a Binding Acknowledgement in response to this
   Binding Update.  As described in Section 5.2, 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



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   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.

   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 contain an AH [8] or ESP [9] header to authenticate the Binding
   Update as coming from the home address being bound.


10.6. 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 10.5, 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 SHOULD use the dynamic home agent
   address discovery procedure to find the address of a suitable home
   agent on its home link.  To do so, the mobile node sends the packet,
   as described above, with the Destination Address in the packet's IPv6
   header set to the Home-Agents anycast address for its home subnet
   prefix.  As described in Section 9.2, the home agent on its home link
   that receives this Binding Update will reject the Update, returning
   to the mobile node the home agent's own global unicast IP address
   along with a list of the global unicast IP addresses of each other
   home agent operating on the home link.  The mobile node SHOULD then
   retransmit its Binding Update to one of these homes agent using the
   provided global unicast address; the mobile node MAY re-attempt
   this 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.

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



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10.7. Sending Binding Updates to Correspondent Nodes

   A mobile node MAY send a Binding Update to any correspondent node at
   any time to allow it to cache its current care-of address (subject to
   the rate limiting defined in Section 10.10).  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
   Lifetime value sent in the Binding Update MUST be no greater than
   the remaining lifetime of the mobile node's home registration of its
   primary care-of address at its home agent.

   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 remaining lifetime of the binding, initialized from the
       Lifetime field sent in the Binding Update.

   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 10.5), 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




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   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 10.10).  In particular, the mobile node SHOULD return a
   Binding Update in response to receiving a packet that meets all of
   the following tests:

    -  The packet was tunneled using IPv6 encapsulation.

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

    -  The Destination Address in the original (inner) IPv6 header
       is equal to one of the mobile node's home addresses; or this
       Destination Address is equal to one of the mobile node's previous
       care-of addresses, if the mobile node has an entry in its Binding
       Update List representing an unexpired Binding Update sent to
       a previous default router for this previous care-of address
       (Section 10.8).

    -  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.




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   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.


10.8. Sending Binding Updates to the Previous Default Router

   After switching to a new default router (and thus also changing its
   primary care-of address), a mobile node MAY send a Binding Update to
   its previous default router, giving its new care-of address.  The
   packet carrying the Binding Update MUST be addressed to the mobile
   node's previous default router's global unicast address, learned
   by the mobile node based on Prefix Information options received in
   Router Advertisements from it in which the Router Address (R) bit is
   set.

   If the mobile node sends such a Binding Update, the home address
   for the binding, specified in the Home Address option included in
   the packet carrying this Binding Update, MUST be set the mobile
   node's old primary care-of address (that it used while using this
   default router), and the care-of address for the binding (either the
   Source Address in the packet's IPv6 header or the Care-of Address
   field in the Binding Update) MUST be set to the mobile node's new
   primary care-of address.  In addition, the Home Registration (H)
   bit MUST also be set in this Binding Update, to request the mobile
   node's previous default router to temporarily act as a home agent
   for the mobile node's old primary care-of address.  The previous
   default router will thus tunnel packets for the mobile node to its
   new care-of address.  All of the procedures defined for home agent
   operation must be followed by this previous default router for this
   registration.  Note that the previous router does not necessarily
   know the mobile node's (permanent) home address as part of this
   registration.







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10.9. Retransmitting Binding Updates

   If, after sending a Binding Update in which the Acknowledge (A) bit
   is set, a mobile node fails to receive a Binding Acknowledgement
   within INITIAL_BINDACK_TIMEOUT seconds, the mobile node SHOULD
   retransmit the Binding Update until a Binding Acknowledgement
   is received.  Such a retransmitted Binding Update MUST use the
   same Sequence Number value as the original transmission.  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.


10.10. Rate Limiting for Sending Binding Updates

   A mobile node MUST NOT send Binding Updates more often than once per
   MAX_UPDATE_RATE seconds to any node.  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.


10.11. 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 contains a valid AH [8] or ESP [9] header providing
       sender authentication, data integrity protection, and replay
       protection.

    -  The Option Length field in the option is greater than or equal to
       11 octets.

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

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





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   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.

    -  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 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 10.10.  In particular, if the Status field
       is equal to 135 (dynamic home agent address discovery response),
       then the mobile node MAY reattempt its home registration with
       the home agent address given in the Source Address field of the
       packet carrying the Binding Acknowledgement or with any of the
       home agent IP addresses listed in the Home Agents List field in
       the Binding Acknowledgement.  If any of these addresses is not a
       global unicast address or does not have a subnet prefix equal to
       the mobile node's own subnet prefix, then that particular address
       MUST be ignored and the mobile node MUST NOT reattempt its home
       registration with that home agent.


10.12. Receiving Binding Requests

   When a mobile node receives a packet containing a Binding Request,
   it SHOULD return to the sender a packet containing a Binding Update.
   The Lifetime field in this Binding Update SHOULD be set to a new
   lifetime, extending any current lifetime remaining from a previous
   Binding Update sent to this node (as indicated in any existing
   Binding Update List entry for this node), except that this lifetime
   MUST NOT exceed the remaining lifetime for the mobile node's primary
   care-of address registration at its home agent.  When sending this
   Binding Update, the mobile node MUST update its Binding Update List
   in the same way as for any other Binding Update sent by the mobile
   node.

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




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10.13. Receiving ICMP Error Messages

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

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


10.14. Receiving Tunneled Router Advertisements

   Section 9.7 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 contains either an AH [8] or ESP [9] header providing
       sender authentication, data integrity protection, and replay
       protection.

    -  The packet contains a Binding Request destination option.



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   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 [13].
   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 addition, 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.


10.15. Using Multiple Care-of Addresses

   As described in Section 10.4, 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 10.3.  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 10.5.

   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.







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10.16. 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 its care-of
   address sharing a subnet prefix with the multicast router, as
   the source IPv6 address of its multicast group membership control
   messages.

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

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


10.17. Returning Home

   A mobile node detects that it has returned to its home link through
   the movement detection algorithm in use (Section 10.3), 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.

   In addition, the mobile node MUST multicast onto the home link
   (to the all-nodes multicast address) a Neighbor Advertisement
   message [13], to advertise the mobile node's own link-layer address
   for its own home address.  The Target Address in this Neighbor



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   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 [13].


































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11. Constants

      INITIAL_BINDACK_TIMEOUT   1 second

      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








































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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

    -  The Home Address option, described in Section 5.4

   In addition, this document defines two new Neighbor Discovery [13]
   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.

    -  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 Home-Agents anycast address, used in the dynamic home agent
       address discovery procedure described in Sections 9.2 and 10.6.


























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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
   worry about replay protection such as through the sequence number
   wrapping around.


13.2. 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



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   IPv6 header, including the Source Address field, and any other parts
   of the packet, including the Home Address option, can be forged or
   modified in transit.  In this case, the contents of the Home Address
   option is no more suspect than any other part of the packet.

   The use of the Home Address option allows packets sent by a
   mobile node to pass normally through routers implementing ingress
   filtering [6].  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.

   However, if a node receiving a packet that includes a Home Address
   option implements the processing of this option by physically
   copying the Home Address field from the option into the IPv6 header,
   replacing the Source Address field there, then the ability to
   trace the true location of the sender is removed once this step
   in the processing is performed.  This diminishing of the power of
   ingress filtering only occurs once the packet has been received at
   its ultimate destination, and does not affect the capability of
   ingress filtering while the packet is in transit.  Furthermore, this
   diminishing can be entirely eliminated by appropriate implementation
   techniques in the receiving node.  For example, the original contents
   of the Source Address field (the sending care-of address) could be
   saved elsewhere in memory with the packet, until all processing of
   the packet is completed.


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




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   pinpoint the location of the mobile node.  Such mechanisms are all
   beyond the scope of this document.



















































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Changes from Previous 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-ipv6-05.txt:

    -  Clarified that the Advertisement Interval option in Section 6.3
       MAY be included in Router Advertisements by any router, not just
       by home agents.

    -  Modified Section 6.5 to document a required change to the
       MaxRtrAdvInterval limit, in addition to the change to the
       MinRtrAdvInterval limit, and clarified that these new limits MAY
       be used by any router, not just by home agents.

    -  Added Section 6.6 to document new limits on sending Router
       Solicitations by a mobile node while away from home.  These
       changes are related to the MAX_RTR_SOLICITATIONS and
       RTR_SOLICITATION_INTERVAL Neighbor Discovery constants.

    -  Added Section 6.2 documenting a modification to the format of
       a Prefix Information option for use in Router Advertisement
       messages.  This modification allows a router to easily and
       efficiently advertise its own global unicast address.

    -  Defined a new Home Agent Information Option for Router
       Advertisements (Section 6.4).  This option allows those routers
       functioning as a home agent to optionally specify a preference
       (relative to other home agents on this link) and a lifetime for
       this advertisement for providing home agent service.  Use of this
       option by home agents is optional.

    -  Added Section 10.2, defining the general procedures to be used
       by a mobile node in receiving packets while away from home.  In
       particular, for packets received with a Routing header, this
       section defines an exception for any use of a Routing header
       automatically derived by "reversing" the received Routing header,
       for any response packets sent by upper-layer protocols.

    -  Changed the treatment of packets addressed to a mobile node's
       site-local address while the mobile node is away from home.  The
       current consensus of the Mobile IP Working Group is that such
       packets 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.

    -  Added a definition of the treatment of multicast packets
       addressed to a multicast group to which a mobile node is



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       subscribed, for which the multicast scope is link-local,
       site-local, organization-local, etc.  As with packets sent to a
       mobile node's link-local and site-local addresses, link-local
       multicast packets MUST NOT be tunneled to the mobile node, and
       multicast packets addressed to a multicast address with scope
       larger than link-local but smaller than global (e.g., site-local
       and organization-local) SHOULD be tunneled to the mobile node by
       default, but this behavior MUST be configurable to disable it.

    -  Added Section 7.2, detailing Mobile IP requirements on all IPv6
       routers.  They SHOULD be able to send an Advertisement Interval
       option in their Router Advertisements, and SHOULD be able to
       support sending unsolicited multicast Router Advertisements at
       the faster rate described in Section 6.5.

    -  Added Section 10.14 describing the mobile node side of
       renumbering the home network, matching the home agent processing
       described in Section 9.7.

    -  Simplified the sequence of tests in Section 9.4 performed by a
       home agent being requested to no longer serve as the sending
       mobile node's home agent.

    -  Clarified in Section 10.5 that if a mobile node has multiple home
       addresses using different interface identifiers, then it SHOULD
       send a separate Binding Update to its home agent for each.

    -  Finally filled in Section 2, giving a comparison of Mobile IPv6
       with Mobile IP for IPv4.
























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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 Josh Broch, Thomas Narten, Erik Nordmark,
   and Jim Solomon for their detailed reviews of earlier versions of
   this draft.  Their suggestions have helped to improve both the design
   and presentation of the protocol.













































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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).  Internet-Draft,
        draft-ietf-dhc-dhcpv6-10.txt, May 1997.  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.  Internet-Draft,
        draft-ietf-ipngwg-ipv6-tunnel-07.txt, December 1996.
        Work in progress.

    [5] Stephen E. Deering and Robert M. Hinden.  Internet
        Protocol version 6 (IPv6) specification.  Internet-Draft,
        draft-ietf-ipngwg-ipv6-spec-v2-00.txt, July 1997.  Work in
        progress.

    [6] Paul Ferguson and Daniel Senie.  Network ingress filtering:
        Defeating denial of service attacks which employ IP source
        address spoofing.  RFC 2267, January 1998.

    [7] Robert M. Hinden and Stephen E. Deering.  IP Version 6
        addressing architecture.  Internet-Draft,
        draft-ietf-ipngwg-addr-arch-v2-06.txt, January 1998.  Work in
        progress.

    [8] Stephen Kent and Randall Atkinson.  IP Authentication header.
        Internet-Draft, draft-ietf-ipsec-auth-header-02.txt, October
        1997.  Work in progress.

    [9] Stephen Kent and Randall Atkinson.  IP Encapsulating Security
        Payload (ESP).  Internet-Draft, draft-ietf-ipsec-esp-v2-01.txt,
        October 1997.  Work in progress.

   [10] Stephen Kent and Randall Atkinson.  Security architecture for
        the Internet Protocol.  Internet-Draft,
        draft-ietf-ipsec-arch-sec-02.txt, November 1997.  Work in
        progress.

   [11] P. Mockapetris.  Domain Names -- concepts and facilities.
        RFC 1034, November 1987.

   [12] P. Mockapetris.  Domain Names -- implementation and
        specification.  RFC 1035, November 1987.




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   [13] Thomas Narten, Erik Nordmark, and William Allen Simpson.
        Neighbor Discovery for IP version 6 (IPv6).  Internet-Draft,
        draft-ietf-ipngwg-discovery-v2-00.txt, July 1997.  Work in
        progress.

   [14] Charles Perkins.  IP encapsulation within IP.  RFC 2003, October
        1996.

   [15] Charles Perkins, editor.  IP mobility support.  RFC 2002,
        October 1996.

   [16] Charles Perkins.  Minimal encapsulation within IP.  RFC 2004,
        October 1996.

   [17] Charles Perkins and David B. Johnson.  Route optimization in
        Mobile IP.  Internet-Draft, draft-ietf-mobileip-optim-07.txt,
        November 1997.  Work in progress.

   [18] 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.

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

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

   [21] Joyce K. Reynolds and Jon Postel.  Assigned numbers.  RFC 1700,
        October 1994.

   [22] Susan Thomson and Thomas Narten.  IPv6 stateless address
        autoconfiguration.  Internet-Draft,
        draft-ietf-ipngwg-addrconf-v2-00.txt, July 1997.



















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Chair's Address

   The Working Group can be contacted via its current chairs:

        Jim Solomon
        RedBack Networks
        1389 Moffett Park Drive
        Sunnyvale, CA  94089-1134
        USA

        Phone:  +1 408 548-3583
        Fax:    +1 408 548-3599
        E-mail: solomon@rback.com


        Erik Nordmark
        Sun Microsystems, Inc.
        2550 Garcia Avenue
        Mt. View, CA  94041
        USA

        Phone:  +1 415 786-5166
        Fax:    +1 415 786-5896
        E-mail: nordmark@sun.com





























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Authors' Addresses

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

        David B. Johnson
        Carnegie Mellon University
        Computer Science Department
        5000 Forbes Avenue
        Pittsburgh, PA  15213-3891
        USA

        Phone:  +1 412 268-7399
        Fax:    +1 412 268-5576
        E-mail: dbj@cs.cmu.edu


        Charles Perkins
        Sun Microsystems, Inc.
        Technology Development Group
        Mail Stop MPK15-214
        Room 2682
        901 San Antonio Road
        Palo Alto, CA  94303
        USA

        Phone:  +1 415 786-6464
        Fax:    +1 415 786-6445
        E-mail: cperkins@eng.sun.com

























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