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Mobile IP Working Group                                 David B. Johnson
INTERNET-DRAFT                                Carnegie Mellon University
                                                         Charles Perkins
                                                        Sun Microsystems
                                                            30 July 1997


                        Mobility Support in IPv6

                   <draft-ietf-mobileip-ipv6-03.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 (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (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 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. Terminology                                                        2
     2.1. General Terms . . . . . . . . . . . . . . . . . . . . . .    2
     2.2. Mobile IPv6 Terms . . . . . . . . . . . . . . . . . . . .    3
     2.3. Specification Language  . . . . . . . . . . . . . . . . .    4

 3. Overview of Mobile IPv6 Operation                                  6
     3.1. Protocol Summary  . . . . . . . . . . . . . . . . . . . .    6
     3.2. Comparison with Mobile IP for IPv4  . . . . . . . . . . .   11

 4. New IPv6 Destination Options                                      12
     4.1. Binding Update Option . . . . . . . . . . . . . . . . . .   12
     4.2. Binding Acknowledgement Option  . . . . . . . . . . . . .   16
     4.3. Binding Request Option  . . . . . . . . . . . . . . . . .   20
     4.4. Home Address Option . . . . . . . . . . . . . . . . . . .   21

 5. Requirements for IPv6 Nodes                                       23

 6. Correspondent Node Operation                                      25
     6.1. Receiving Packets from a Mobile Node  . . . . . . . . . .   25
     6.2. Receiving Binding Updates . . . . . . . . . . . . . . . .   25
     6.3. Requests to Cache a Binding . . . . . . . . . . . . . . .   26
     6.4. Requests to Delete a Binding  . . . . . . . . . . . . . .   27
     6.5. Sending Binding Acknowledgements  . . . . . . . . . . . .   27
     6.6. Cache Replacement Policy  . . . . . . . . . . . . . . . .   28
     6.7. Receiving ICMP Error Messages . . . . . . . . . . . . . .   28
     6.8. Sending Packets to a Mobile Node  . . . . . . . . . . . .   29

 7. Home Agent Operation                                              31
     7.1. Dynamic Home Agent Address Discovery  . . . . . . . . . .   31
     7.2. Primary Care-of Address Registration  . . . . . . . . . .   31
     7.3. Primary Care-of Address De-registration . . . . . . . . .   33
     7.4. Tunneling Intercepted Packets to a Mobile Node  . . . . .   34
     7.5. Renumbering the Home Subnet . . . . . . . . . . . . . . .   35

 8. Mobile Node Operation                                             37
     8.1. Sending Packets While Away from Home  . . . . . . . . . .   37



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     8.2. Movement Detection  . . . . . . . . . . . . . . . . . . .   38
     8.3. Forming New Care-of Addresses . . . . . . . . . . . . . .   40
     8.4. Sending Binding Updates to the Home Agent . . . . . . . .   41
     8.5. Sending Binding Updates to Correspondent Nodes  . . . . .   42
     8.6. Sending Binding Updates to the Previous Default Router  .   45
     8.7. Retransmitting Binding Updates  . . . . . . . . . . . . .   45
     8.8. Rate Limiting for Sending Binding Updates . . . . . . . .   46
     8.9. Receiving Binding Acknowledgements  . . . . . . . . . . .   46
    8.10. Using Multiple Care-of Addresses  . . . . . . . . . . . .   47
    8.11. Returning Home  . . . . . . . . . . . . . . . . . . . . .   47

 9. Routing Multicast Packets                                         49

10. Constants                                                         50

11. Security Considerations                                           51
    11.1. Binding Updates, Acknowledgements, and Requests . . . . .   51
    11.2. Home Address Options  . . . . . . . . . . . . . . . . . .   51
    11.3. General Mobile Computing Issues . . . . . . . . . . . . .   52

Appendix A. Changes from Previous Draft                               53

Acknowledgements                                                      54

References                                                            55

Chair's Address                                                       57

Authors' Addresses                                                    58






















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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 IPv6 subnet, since routing is based on the network prefix in a
   packet's destination IP address.  In order continue communication
   in spite of its movement, a mobile node could change its IP address
   each time it moves to a new IPv6 subnet, 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 IPv6 subnet to another without changing
   the mobile node's IP address.  A mobile node is always addressable
   by its "home address", the IP address assigned to the mobile node
   within its home IPv6 subnet.  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 subnet.  The movement of a mobile node away from its home
   subnet 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 amongst wireless transceivers,
   each of which covers only a very small geographic area -- are
   possibly more suited to other solutions.  For example, as long as
   node movement does not occur between link-level points of attachment
   on different IPv6 subnets, link-layer mobility support offered by a
   number of current wireless LAN products is 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 handoff, but such extensions are beyond the sope
   of this document.




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2. Terminology

2.1. 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.

      network prefix

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

      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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2.2. Mobile IPv6 Terms

      home address

         An IP address assigned to a mobile node within its home subnet.
         The network prefix in a mobile node's home address is equal to
         the network prefix of the home subnet.

      home subnet

         The IP subnet indicated by a mobile node's home address.
         Standard IP routing mechanisms will deliver packets destined
         for a mobile node's home address to its home subnet.

      mobile node

         A node that can change its link-level point of attachment from
         one IP subnet 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 link-level connected to the same IP
         subnet as it was previously.  If a mobile node is not currently
         link-level connected to its home subnet, 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

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

      home agent

         A router on a mobile node's home subnet 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 subnet destined to the mobile node's home
         address, encapsulates them, and tunnels them to the mobile
         node's registered care-of address.






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      care-of address

         An IP address associated with a mobile node while visiting
         a foreign subnet, which uses the network prefix of that
         foreign subnet.  Among the multiple care-of addresses that a
         mobile node may have at a time (e.g., with different network
         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.


2.3. Specification Language

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.

      MUST

         This word, or the adjective "REQUIRED", means that the
         definition is an absolute requirement of the specification.

      MUST NOT

         This phrase means that the definition is an absolute
         prohibition of the specification.

      SHOULD

         This word, or the adjective "RECOMMENDED", means that there may
         exist valid reasons in particular circumstances to ignore a
         particular item, but the full implications must be understood
         and carefully weighed before choosing a different course.

      SHOULD NOT

         This phrase means that there may exist valid reasons in
         particular circumstances when the particular behavior is
         acceptable or even useful, but the full implications should be
         understood and the case carefully weighed before implementing
         any behavior described with this label.






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      MAY

         This word, or the adjective "OPTIONAL", means that an item
         is truly optional.  For example, one vendor may choose to
         include the item because a particular marketplace requires
         it or because the vendor feels that it enhances the product,
         while another vendor may omit the same item.  An implementation
         which does not include a particular option MUST be prepared to
         interoperate with another implementation which does include the
         option.

      silently discard

         The implementation discards the packet without further
         processing, and without indicating an error to the sender.  The
         implementation SHOULD provide the capability of logging the
         error, including the contents of the discarded packet, and
         SHOULD record the event in a statistics counter.

































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3. Overview of Mobile IPv6 Operation

3.1. Protocol Summary

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

   While a mobile node is attached to some foreign subnet 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
   subnet.  The network prefix of a mobile node's care-of address is
   equal to the network prefix of the foreign subnet being visited
   by the mobile node; if the mobile node is link-level connected
   to this foreign subnet 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 [16] or stateful (e.g., DHCPv6 [3])
   address autoconfiguration, according to the methods of IPv6 Neighbor
   Discovery [9], although other methods of acquiring a care-of address
   are also possible.

   While away from home, the mobile node registers one of its bindings
   with a router in its home subnet, requesting this router to function
   as the "home agent" for the mobile node.  This binding registration
   is done by the mobile node sending a packet with a "Binding Update"
   destination option to the home agent, which replies by returning a
   packet containing a "Binding Acknowledgement" destination option to
   the mobile node.  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 on the home subnet, and tunnels each
   intercepted packet to the mobile node's primary care-of address.
   To tunnel each intercepted packet, the home agent encapsulates the
   packet using IPv6 encapsulation [4], addressed to the mobile node's
   primary care-of address.

   The Binding Update and Binding Acknowledgement destination options,
   together with a "Binding Request" destination option, are also used



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   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.  A node
   communicating with a mobile node is referred to in this document as a
   "correspondent node" of the mobile node.

   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 [14] or UDP [13].

    -  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 Destination Options header 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 physical source of the packet in the same way as packets
   from non-mobile nodes.  By also including the Home Address option,
   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




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   correspondent node as a result of receiving a Home Address option in
   a packet.

   In summary, 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 an IPv6 Authentication
         header [1], providing sender authentication, data integrity
         protection, and replay protection.  The Binding Update option
         is described in detail in Section 4.1.

      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 an IPv6 Authentication
         header [1], providing sender authentication, data integrity
         protection, and replay protection.  The Binding Acknowledgement
         option is described in detail in Section 4.2.

      Binding Request

         A Binding Request option is used to request a mobile node
         to send a Binding Update to the requesting node, 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 special
         authentication is required for the Binding Request option.  The
         Binding Request option is described in detail in Section 4.3.

      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



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         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 described in detail in Section 4.4.

   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.

   This document describes the Mobile IPv6 protocol in terms of the
   following two conceptual data structures used in the maintenance of
   cached bindings:

      Binding Cache

         A cache, maintained by each IPv6 node, of bindings for other
         nodes.  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, keyed by the mobile node's home
         address.  The contents of a node's Binding Cache MUST NOT be
         changed in response to a Home Address option in a received
         packet.  The 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 through Neighbor Discovery [9].

      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 of the binding 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.  Each Binding Update List entry records
         the IP address of the node to which the Update was sent, the
         home address for which one Binding Update was sent, and the
         remaining lifetime of that binding.  The Binding Update List




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         MAY be implemented in any manner consistent with the external
         behavior described in this document.

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

   When a mobile node receives a packet tunneled to it from its
   home agent, the mobile node assumes that the original sending
   correspondent node has no binding cache entry for the mobile node,
   since the correspondent node would otherwise have sent the packet
   directly to the mobile node using a Routing header.  The mobile node
   thus returns a Binding Update to the correspondent node, allowing
   it to cache the mobile node's binding for routing future packets.
   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 IP subnet to another.  If each wireless
   subnet 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 link-level connected
   within both subnets while in the area of overlap.  In this case, the



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   mobile node could acquire a new care-of address in the new subnet
   before moving out of transmission range and link-level disconnecting
   from the old subnet.  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 in
   the new subnet.

   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
   subnet.  In addition, the impact of of any possible failure of the
   home agent, the home subnet, or intervening networks leading to or
   from the home subnet is reduced, since these nodes and links are not
   involved in the delivery of most packets to the mobile node.


3.2. Comparison with Mobile IP for IPv4

   [This section will include a comparison between the Mobile IPv6
   protocol and the Mobile IPv4 protocol [11, 10, 12].  However, this
   comparison has not yet been written.  It will be filled in with the
   next revsion to this draft.]























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4. New IPv6 Destination Options

4.1. Binding Update Option

   The Binding Update destination option is used by a mobile node to
   notify a correspondent node or the mobile node's home agent of a new
   care-of address.

   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|L|      Reserved         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Lifetime           |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                        Care-of Address                        +
   |                  (only present if C bit set)                  |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                    Home Link-Local Address                    +
   |                  (only present if L bit set)                  |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Option Type

         192 ???

      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 6, for the case in which neither the
         Care-of Address Present (C) bit nor the Home Link-Local Address
         Present (L) bit are set, and the maximum value is 38, for the
         case in which both of these bits are set.



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      Acknowledge (A)

         The Acknowledge (A) bit is set by the sending node to request a
         Binding Acknowledgement (Section 4.2) be returned upon receipt
         of the Binding Update option.

      Home Registration (H)

         The Home Registration (H) bit is set by the sending node to
         request the receiving node to act as this node's home agent.
         The Destination Address in the IP header of the packet carrying
         this option MUST be that of a router sharing the same network
         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.

      Home Link-Local Address Present (L)

         The Home Link-Local Address Present (L) bit indicates the
         presence of the Home Link-Local Address field in the Binding
         Update.  This bit is set by the sending node to request
         the receiving node to act as a proxy (for participating in
         the Neighbor Discovery Protocol) for the node while it is
         away from home.  This bit MUST NOT be set unless the Home
         Registration (H) bit is also set in the Binding Update.

      Reserved

         Sent as 0; ignored on reception.

      Lifetime

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





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      Sequence Number

         Used by the receiving node to sequence Binding Updates and by
         the sending node to match a returned Binding Acknowledgement
         with this Binding Update.  Each Binding Update sent by a mobile
         node MUST use a Sequence Number greater than the Sequence
         Number value sent in the previous Binding Update (if any) to
         the same destination address (modulo 2**16).  There is no
         requirement, however, that the Sequence Number value strictly
         increase by 1 with each new Binding Update sent.

      Care-of Address

         This field in the Binding Update is optional and is only
         present when the Care-of Address Present (L) 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.

      Home Link-Local Address

         This field in the Binding Update is optional and is only
         present when the Home Link-Local Address Present (L) bit is
         set.  If present, it gives the link-local address of the mobile
         node used by the mobile node when it was last attached to its
         home subnet.

   Any packet including 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 include an IPv6
   Authentication header [1] in order to protect against forged Binding
   Updates.  The authentication MUST provide 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 should 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 should
   be deleted.  In each of these cases, no Binding Cache entry for the




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   mobile node should be created in response to receiving the Binding
   Update.

   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 the
   data within the option cannot change en-route to the packet's final
   destination, and 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.

   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, depending on the state of the Care-of Address Present (C)
   and Home Link-Local Address Present (L) bits, the remaining octets
   are interpreted as extensions.  Currently, no extensions have been
   defined.






























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4.2. Binding Acknowledgement Option

   The Binding Acknowledgement destination option is used to acknowledge
   receipt of a Binding Update option (Section 4.1).  When a node
   receives a packet containing a Binding Update option, with this
   node being the destination node of the packet, this node MUST
   return a Binding Acknowledgement to the source of the packet, if the
   Acknowledge (A) bit is set in the Binding Update.

   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     |            Lifetime           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Refresh            |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Option Type

         193 ???

      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 Acknowledgement option, this
         field MUST be set to 9.

      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



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            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

         Up-to-date values of the Status field are to be specified in
         the most recent "Assigned Numbers" [15].

      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, in order to
         extend the lifetime.

      Refresh

         The recommended period 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 the mobile node's binding 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.

      Sequence Number

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



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   Any packet that includes a Binding Acknowledgement option MUST
   include an IPv6 Authentication header [1] in order to protect
   against forged Binding Acknowledgements.  The authentication MUST
   provide 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 6.8.
   The packet is sent using a Routing header, routing the packet to the
   mobile node by way of its care-of address recorded in the Binding
   Cache entry.

   If the node returning the Binding Acknowledgement instead
   rejected the Binding Update (the value of the Status field in the
   Acknowledgement is greater than or equal to 128), this node MUST
   similarly use a Routing header in sending the packet containing the
   Binding Acknowledgement, as described in Section 6.8, 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.

   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 110, indicating
   that the data within the option cannot change en-route to the
   packet's final destination, and 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.

   Extensions to the Binding Acknowledgement option format may be
   included after the fixed portion of the Binding Acknowledgement



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   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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4.3. Binding Request Option

   The Binding Request destination option is used to request a mobile
   node's binding from the mobile node.  When a mobile node receives
   a packet containing a Binding Request option, it SHOULD return a
   Binding Update (Section 4.1) 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

         194 ???

      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 110, indicating that the
   data within the option cannot change en-route to the packet's final
   destination, and 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.

   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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4.4. Home Address Option

   The Home Address destination 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.

   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

         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 Home Address option, this field MUST
         be set to 8.

      Home Address

         The home address of the mobile node sending the packet.

   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



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   receiving a Home Address option in a packet.  In particular, the
   receipt of a packet containing a Home Address option MUST NOT alter
   the contents of the receiver's Binding Cache due to the presence of
   the Home Address option, and the mapping between the home address
   and care-of address indicated by the Home Address option MUST NOT be
   used as a basis for routing subsequent packets sent by this receiving
   node.

   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.  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 11.

   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 the data
   within the option cannot change en-route to the packet's final
   destination, and 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.

   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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5. Requirements for IPv6 Nodes

   Mobile IPv6 places some special requirements on the functions
   provided by different 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.

   Since any IPv6 node may at any time be a correspondent node of a
   mobile node, the following requirements pertain to all IPv6 nodes:

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

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

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

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

    -  Every home agent MUST be able to intercept packets (using proxy
       Neighbor Discovery) on the local subnet addressed to a mobile
       node for which it is currently serving as the home agent while
       that 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.

    -  Every home agent MUST be able to return a Binding Acknowledgement
       in response to a Binding Update 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



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       as a home agent, and MUST be able to participate in dynamic home
       agent address discovery.

   Finally, the following requirements pertain 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 Updates, as
       specified in Sections 8.4, 8.5, and 8.6; and MUST be able to
       receive and process Binding Acknowledgements, as specified in
       Section 8.9.

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

































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6. Correspondent Node Operation

   A correspondent node is any node communicating with a mobile node.
   The correspondent node, itself, may be fixed 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.


6.1. Receiving Packets from a Mobile Node

   Packets sent by a mobile node while away from home generally include
   a Home Address option.  When a 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 the 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 thus transparent to
   both the mobile node and the correspondent node above the level of
   the Home Address option generation and processing.


6.2. 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 an IP Authentication header and the
       authentication is valid [1].  The Authentication header MUST
       provide sender authentication, integrity protection, and replay
       protection.

    -  The Option Length field in the Binding Update option is greater
       than or equal to the length specified in Section 4.1.

    -  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.

    -  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.




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   Any Binding Update 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.

   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 subnet 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 7.1.  (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, then this is a request to cache a binding for
       the mobile node.  Processing for this type of received Binding
       Update is described in Section 6.3.

    -  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.  Processing for this type of received Binding
       Update is described in Section 6.4.


6.3. Requests to Cache a Binding

   If a node receives a valid Binding Update requesting it to cache a
   binding for a mobile node, as specified in Section 6.2, then the node
   MUST examine the Home Registration (H) bit in the Binding Update
   to determine how to further process the Binding Update.  If the
   Home Registration (H) bit is set, the Binding Update is processed
   according to the procedure specified in Section 7.2.

   If the Home Registration (H) bit is not set, then 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 entry records the association between
   this address and the care-of address for the binding, as specified



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   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.


6.4. Requests to Delete a Binding

   If a node receives a valid Binding Update requesting it to delete a
   cached binding for a mobile node, as specified in Section 6.2, then
   the node MUST examine the Home Registration (H) bit in the Binding
   Update to determine how to further process the Binding Update.  If
   the Home Registration (H) bit is set, the Binding Update is processed
   according to the procedure specified in Section 7.3.

   If the Home Registration (H) bit is not set, then 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.


6.5. 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 4.2 and
   in the most recent "Assigned Numbers" [15].

   As described in Section 4.2, the packet in which the Binding
   Acknowledgement is returned MUST include an IPv6 Authentication
   header [1] in order to protect against forged Binding
   Acknowledgements, 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 not accepted), as
   described in Section 6.8.  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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6.6. 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 was 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 7.2) 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"
   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 subnet.  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 to its
   Binding Cache.


6.7. Receiving ICMP Error Messages

   When a correspondent node sends a packet to a mobile node, if the
   correspondent node has a Binding Cache entry for the destination



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   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 subnet, where 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].

   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 Host Unreachable or Network Unreachable error
   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 subnet, 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.


6.8. 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



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   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.  Since the mobile node recognizes its own
       home address as one if 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 7.4.  The mobile node will then send
   a Binding Update to the sending node, as described in Section 8.5,
   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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7. Home Agent Operation

7.1. Dynamic Home Agent Address Discovery

   If a received Binding Update indicates that the mobile node sending
   it is attempting dynamic home agent address discovery, as described
   in Section 6.2, then the receiving node MUST process the Binding
   Update as specified in this section.

   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 (the packet MUST also include
   a Home Address option, as described in Section 8.4).  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 and giving its
   unicast address.  The Status field in the Binding Acknowledgement
   MUST be set to 135 (dynamic home agent address discovery response).
   The mobile node, upon receiving this Binding Acknowledgement, MAY
   then resend its Binding Update to the unicast home agent address
   given in the Acknowledgement.


7.2. Primary Care-of Address Registration

   General processing of a received Binding Update that requests a
   binding to be cached, is described in Section 6.3.  However, if the
   Home Registration (H) bit is set in the Binding Update, then the
   receiving node MUST process the Binding Update as specified in this
   section, rather than following the general procedure specified in
   Section 6.3.

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

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

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



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    -  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 or
   update the existing entry in its Binding Cache for this mobile node's
   home address, as described in Section 6.3.  In addition, 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 6.6) and MUST NOT be removed from the Binding
   Cache until the expiration of the Lifetime period.

   If the home agent was not already serving as a home agent for this
   mobile node (the home agent did not already have a Binding Cache
   entry for this home address marked as a "home registration"), then
   the home agent MUST multicast onto the home subnet (to the all-nodes
   multicast address) a Neighbor Advertisement message [9] on behalf
   of the mobile node, to advertise the home agent's own link-layer
   address for the mobile node's home IP address.  The Target Address in
   the Neighbor Advertisement message MUST be set to the mobile node's
   home address, and the Advertisement MUST include a Target Link-layer
   Address option specifying the 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 subnet receiving this Neighbor Advertisement
   message will thus update its Neighbor Cache to associate the mobile
   node's home address with the home agent's link layer address, causing
   it to transmit any future packets for the mobile node 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 subnet 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 [9].




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   In addition, while this node is serving as a home agent for this
   mobile node (it still has a "home registration" entry for this mobile
   node in its Binding Cache), it 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".  If such an entry exists
   in its 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.  Likewise, if the mobile node included
   its home link-local address and set the Home Link-Local Address
   Present (L) bit in its Binding Update with which it established
   this "home registration" with its home agent, its home agent MUST
   also similarly act as a proxy for the mobile node's home link-local
   address while it has this "home registration" entry in its Binding
   Cache.  Acting as a proxy in this way allows other nodes on the
   mobile node's home subnet to resolve the mobile node's IPv6 home
   address and IPv6 link-local address, and allows the home agent to
   to defend these addresses on the home subnet for Duplicate Address
   Detection [9].


7.3. Primary Care-of Address De-registration

   General processing of a received Binding Update that requests a
   binding to be deleted, is described in Section 6.4.  However, if the
   Home Registration (H) bit is set in the Binding Update, then the
   receiving node MUST process the Binding Update as specified in this
   section, rather than following the general procedure specified in
   Section 6.4.

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

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

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



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   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.

   In addition, in this case, the home agent MUST multicast a Neighbor
   Advertisement message (to the all-nodes multicast address), giving
   the mobile node's home address as the Target Address, and specifying
   the mobile node's link-layer address in a Target Link-layer
   Address option in the Neighbor Advertisement message.  The home
   agent MAY retransmit this Neighbor Advertisement message up to
   MAX_ADVERT_REXMIT times to increase its reliability; any nodes on the
   home subnet that miss all of these Neighbor Advertisements can also
   eventually detect the link-layer address change for the mobile node's
   home address, through use of Neighbor Unreachability Detection [9].


7.4. Tunneling Intercepted Packets to a Mobile Node

   For any packet sent to a mobile node from the mobile node's home
   agent (for which the home agent is the original sender of the
   packet), the home agent is operating as a correspondent node of
   the mobile node for this packet and the procedures described in
   Section 6.8 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).

   In addition, 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 subnet addressed to the mobile node's
   home address, as described in Section 7.2.  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 Authentication header present in the
   packet [1].

   For forwarding each intercepted packet to the mobile node, the
   home agent MUST tunnel the packet to the mobile node using IPv6
   encapsulation [4]; the tunnel entry point node is the home agent,
   and the tunnel exit point node is the mobile node itself (using its
   primary care-of address as registered with the home agent).  When a
   home agent encapsulates an intercepted packet for forwarding to the
   mobile node, the home agent sets the Source Address in the prepended
   tunnel IP header to the home agent's own IP address, and sets the
   Destination Address in the tunnel IP header to the mobile node's
   primary care-of address.  When received by the mobile node (using its
   primary care-of address), normal processing of the tunnel header [4]




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   will result in decapsulation and processing of the original packet by
   the mobile node.


7.5. Renumbering the Home Subnet

   Neighbor Discovery [9] specifies a mechanism by which all nodes on a
   subnet can gracefully autoconfigure new addresses, say by each node
   combining a new routing prefix with its existing link-layer address.
   As currently specified, this mechanism works when the nodes are on
   the same link as the router issuing the necessary multicast packets
   to advertise the new routing prefix(es) appropriate for the link.

   However, for mobile nodes away from home, special care must be taken
   to allow the mobile nodes to renumber gracefully.  The most direct
   method of ensuring this is for the home agent to encapsulate and
   tunnel the multicast packets to the primary care-of address of each
   mobile node for which it is serving as the home agent.  The rules for
   this are as follows:

    -  A mobile node assumes that its routing prefix has not changed
       unless it receives an authenticated Router Advertisement message
       from its home agent that the prefix has changed.

    -  When the mobile node is at home, the home agent does not tunnel
       Router Advertisements to it.

    -  The mobile node's home agent serves as a proxy for the mobile
       node's home address and link-local address, including defending
       these addresses for Duplicate Address Detection, while the mobile
       node is registered with the home agent away from home.

    -  When a home subnet prefix changes, the home agent tunnels Router
       Advertisement packets to each mobile node registered with it that
       is currently away from home and using a home address with the
       affected routing prefix.  Such tunneled Router Advertisements
       MUST be authenticated [1].

    -  When a mobile node receives a tunneled Router Advertisement
       containing a new routing prefix, it MUST perform the standard
       autoconfiguration operation to create its new address.

    -  When a mobile node returns to its home subnet, it must again
       perform Duplicate Address Detection at the earliest possible
       moment after it has deleted its "home registration" binding with
       its home agent.





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    -  A mobile node MAY send a Router Solicitation to its home agent at
       any time, within the constraints imposed by rate control defined
       by Neighbor Discovery [9].
















































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8. Mobile Node Operation

8.1. 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:

    -  For most packets, the mobile node will generally use its home
       address as the source of the packet.  Doing so makes its 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.  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.

    -  For short-term communication, particularly for communication
       that may easily be retried if it fails, the mobile node MAY
       choose to use one of its care-of addresses as the source of the
       packet.  An example of this type of communication might be DNS
       queries sent by the mobile node [7, 8].  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 type of communication, however, the
       mobile node SHOULD use its home address.

   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
   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 the mobile node uses its home address as the
   source of a packet while away from home, the mobile node SHOULD
   construct the packet as follows:

    -  The Source Address field in the packet's IPv6 header is set to
       one of the mobile node's care-of addresses.



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    -  A Home Address option is included in the packet, with the Home
       Address field set to the mobile node's home address.

   Without this use of the care-of address in the IPv6 header, with the
   mobile node's home address instead in the Home Address option, the
   packet will likely be discarded by any router implementing ingress
   filtering [6].


8.2. Movement Detection

   A mobile node MAY use any combination of mechanisms available to
   it to detect when its link-level point of attachment has moved
   from one IP subnet 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 [9].

   Mobile nodes SHOULD use Router Discovery to discover new routers and
   on-link network prefixes; a mobile node MAY send Router Solicitation
   messages, or MAY wait for unsolicited (periodic) Router Advertisement
   messages, as specified for Router Discovery [9].  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 network 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 network
   prefix advertised by that router from its Prefix List to use as
   the network prefix in its primary care-of address.  A mobile node
   MAY also have associated additional care-of addresses, using other
   network prefixes from its Prefix List.  The method by which a mobile
   node selects and forms a care-of address from the available network
   prefixes is described in Section 8.3.  The mobile node registers
   its primary care-of address with its home agent, as described in
   Section 8.4.

   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



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   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 the 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 [9], while the mobile node is actively sending
   packets to (or through) its default router, the mobile node can
   detect that the router 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 form its default router in response to an explicit Neighbor
   Solicitation messages to it.  Note that although this mechanism only
   detects that the mobile node's default router has become unreachable
   to the mobile node 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 to its
   default router, however, 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 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.
   On some types of network interfaces, the mobile node MAY also
   supplement this 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.  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 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



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   mobile node receives no packets from the router for a period of
   time), then the mobile node SHOULD 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's link-layer connection to a new IP subnet, such that the
   mobile node would need to switch to a new default router and primary
   care-of address.  Upon lower-layer indication of link-layer mobility,
   the mobile node SHOULD send Router Solicitation messages to determine
   if new routers (and new on-link network prefixes) are present on its
   new link.

   Such lower-layer information might also be useful to a mobile node in
   deciding to switch its primary care-of address to one of the other
   care-of addresses it has formed from the on-link network prefixes
   currently available through different default 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 default routers to decide when to
   switch to a new primary care-of address using that default 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.


8.3. Forming New Care-of Addresses

   After detecting that its link-layer point of attachment has moved
   from one IPv6 subnet 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 network 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).



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   In addition, after discovering a new on-link network prefix, a
   mobile node MAY form a new (non-primary) care-of address using that
   network 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 network 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 8.10.

   As described in Section 3.1, in order to form a new care-of address,
   a mobile node MAY use either stateless [16] or stateful (e.g.,
   DHCPv6 [3]) 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 subnet.  For example, a mobile node may be
   statically configured with an IPv6 address assigned by the system
   administrator of some foreign subnet, for its use while visiting that
   subnet.  If so, rather than using address autoconfiguration to form
   a new care-of address using this network prefix, the mobile node
   MAY use its own pre-assigned address as its care-of address on this
   subnet.


8.4. Sending Binding Updates to the Home Agent

   After deciding to change its primary care-of address as described
   in Sections 8.2 and 8.3, 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.




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    -  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 binding MUST be specified in the Care-of Address
       field in the Binding Update.

   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 4.2, 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).

   It is possible that when the mobile node needs to send such a Binding
   Update to its home agent, that the mobile node does not know the
   address of any router in its home subnet that can serve as a home
   agent for it.  In this case, the mobile node SHOULD use the dynamic
   home agent address resolution procedure to find the address of a
   suitable home agent in its home subnet.  To do so, the mobile node
   sends the packet, as described above, with the Destination Address in
   the packet's IPv6 header set the Home-Agents anycast address for its
   home subnet.  The home agent in its home subnet that receives this
   Binding Update will reject the Update, returning to the mobile node
   the home agent's unicast IP address.  The mobile node SHOULD then
   retransmit its Binding Update to this home agent using the provided
   unicast address.

   If the mobile node has a current registration with some home agent
   in its home subnet (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 in its home
   subnet.  If the mobile node knows of no other suitable home agent,
   then it MAY attempt the dynamic home agent address resolution
   procedure described above.


8.5. 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 8.8).  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



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   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.  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.  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 8.4), the mobile node will also send a Binding Update to each
   correspondent node for which an entry exists in the mobile node's
   Binding Update List.  Thus, correspondent nodes are generally kept
   updated about the mobile node's binding and can send packets directly
   to the mobile node using the mobile node's current care-of address.

   The mobile node, however, need not send these Binding Updates
   immediately after configuring a new care-of address.  For example,
   since the Binding Update is a destination option and can be included
   in any packet sent by a mobile node, the mobile node MAY delay
   sending a new Binding Update to any correspondent node for a
   short period of time, in hopes that the needed Binding Update
   can be included in some packet that the mobile node sends to that
   correspondent node for some other reason (for example, as part of



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   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.  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 the mobile node's home address.  If the original packet
       contains a Routing header, the final Address indicated in the
       Routing header should be used in this comparison rather than the
       Destination Address in the original IPv6 header.

    -  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 tests above, is the
   Source Address in the original (inner) IPv6 header of the packet.

   Binding Updates sent to correspondent nodes are not generally
   required to be acknowledged.  However, if the mobile node wants to be
   sure that its new care-of address has been added to 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



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   Section 4.1.  No other IPv6 nodes are authorized to send Binding
   Updates on behalf of a mobile node.


8.6. 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 SHOULD send a Binding Update
   to its previous default router, giving its new care-of address.  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.  Note that the previous
   router does not necessarily know the mobile node's (permanent) home
   address as part of this registration.

   If any subsequent packets arrive at this previous router for
   forwarding to the mobile node's old primary care-of address,
   the router SHOULD encapsulate each such packet (using IPv6
   encapsulation [4]) and tunnel it to the mobile node at its new
   primary care-of address.  Moreover, for the lifetime of the "home
   registration" Binding Cache entry for the mobile node at this
   router, this router MUST act as a proxy for the mobile node's
   previous care-of address, for purposes of participation in Neighbor
   Discovery [9], in the same way as any home agent does for a mobile
   node's home address (Section 7.2).  This allows the router to
   intercept packets addressed to the mobile node's previous care-of
   address, and to encapsulate and tunnel them to the mobile node's new
   care-of address, as described in Section 7.4.


8.7. 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 he
   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



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   upon each retransmission until either the node receives a
   Binding Acknowledgement or the timeout period reaches the value
   MAX_BINDACK_TIMEOUT.


8.8. 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 the
   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.


8.9. 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 an IP Authentication header and the
       authentication is valid [1].  The Authentication header MUST
       provide both sender authentication, integrity protection, and
       replay protection.

    -  The Option Length field in the option is greater than or equal to
       9 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.

   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




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       List to indicate that the Binding Update has been acknowledged.
       The mobile node MUST thus stop retransmitting the Binding Update.

    -  If the Status field indicates that the Binding Update was not
       accepted (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 8.8.


8.10. Using Multiple Care-of Addresses

   As described in Section 8.3, a mobile node MAY have 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 link-level points of attachment at the same time (e.g.,
   with overlapping wireless cells), on which different on-link network
   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 network prefixes, based on the movement detection mechanism
   in use, as described in Section 8.2.  When the mobile node selects
   a new primary care-of address, it MUST register it with its home
   agent through a Binding Update with the Home Registration (H) and
   Acknowledge (A) bits set, as described in Section 8.4.

   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 [3], the
   mobile node may not wish to release the address immediately upon
   switching to a new primary care-of address.  The stateful address
   autoconfiguration server will allow mobile nodes to acquire new
   addresses while still using previously allocated addresses.


8.11. Returning Home

   A mobile node detects that it has returned to its home subnet through
   the movement detection algorithm in use (Section 8.2), when the
   mobile node detects that the network prefix of its home subnet is
   again on-link.  The mobile node SHOULD then send a Binding Update to



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   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 (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 subnet
   (to the all-nodes multicast address) a Neighbor Advertisement
   message [9], to advertise the mobile node's own link-layer address
   for its own home address.  The Target Address in this Neighbor
   Advertisement message MUST be set to the mobile node's home address,
   and the Advertisement MUST include a Target Link-layer Address
   option specifying the mobile node's link-layer address.  Similarly,
   the mobile node MUST multicast a Neighbor Advertisement message to
   advertise its link-layer address for its IPv6 link-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 subnet will not receive any of these Neighbor
   Advertisements, but these nodes will eventually be able to recover
   through use of Neighbor Unreachability Detection [9].




















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9. Routing Multicast Packets

   A mobile node that is connected to its home subnet 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 subnet.

   In order receive packets sent to some multicast group, a mobile node
   must join the that multicast group.  One method by which a mobile
   node MAY join the group is via a (local) multicast router on the
   foreign subnet being visited.  This option assumes that there is a
   multicast router present on the foreign subnet.  The mobile node
   SHOULD use its care-of address sharing a network 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, assuming that its home agent
   is a multicast router.  The mobile node tunnels the appropriate
   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 subnet
   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
   sends multicast packets directly on the foreign subnet MUST use its
   care-of address as the IPv6 Source Address of each multicast packet.
   Similarly, 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.  This second option assumes that the home
   agent is a multicast router.
















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10. 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

      MAX_ADVERT_REXMIT         3






































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11. Security Considerations

11.1. 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 [2].

   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.  This 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.


11.2. Home Address Options

   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.  Thus, even when authentication is used in the IPv6
   header, the security of the Source Address field in the IPv6 header
   is not compromised by the presence of a Home Address option.  Without
   authentication of the packet, then any field in the IPv6 header,
   including the Source Address field, and any other parts of the
   packet, including the Home Address option, can be forged or modified



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   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 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.


11.3. 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 see
   should use mechanisms outside the scope of this document (such as
   encryption) to provide appropriate protection.  Users concerned about
   traffic analysis should consider appropriate use of link encryption.
   If stronger location privacy is desired, the mobile node can create a
   tunnel to its home agent.  Then, packets destined for correspondent
   nodes will appear to emanate from the home subnet, and it may be
   more difficult to pinpoint the location of the mobile node.  Such
   mechanisms are all beyond the scope of this document.



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Appendix A. 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-ipv6-02.txt:

    -  Added a comparison to Mobile IP for IPv4 and added this section
       listing changes from the previous version of this draft.

    -  Introduced the Home Address destination option, to allow packets
       sent by a mobile node while away from home to pass normally
       through routers implementing ingress filtering.

    -  Added the requirement that all IPv6 nodes MUST be able to
       correctly process a Home Address destination option in a received
       packet.

    -  Changed the interpretation of the Binding Update option such
       that the home address in the binding is the address in the Home
       Address option, not the Source Address in the IPv6 header.

    -  Made the Care-of Address field in the Binding Update optional,
       controlled by whether or not the new Care-of Address Present (C)
       bit is set in the option.  With the new use of the Home Address
       option, the care-of address for a binding will usually be
       specified by the Source Address field in the packet's IPv6
       header, but by retaining this field (and making it optional),
       it is possible to send a binding update using a Source Address
       different from the care-of address for the binding.

    -  Changed the 32-bit Identification field in the Binding Update and
       Binding Acknowledgement to a 16-bit Sequence Number field, and
       clarified the use of this field.  Replay protection for Binding
       Updates and Binding Acknowledgements is provided by the IPsec
       authentication in the packet, but this replay protection does
       not provide sequencing due to the use of the replay protection
       window.  This field satisfies that the additional sequencing
       requirement.

    -  Added a description of the dynamic home agent address discovery
       procedure and the use of the new Home-Agents anycast address.










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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 Thomas Narten and Erik Nordmark 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] Randall Atkinson.  IP Authentication header.  RFC 1826, August
        1995.

    [2] S. M. Bellovin.  Security problems in the TCP/IP protocol suite.
        ACM Computer Communications Review, 19(2), March 1989.

    [3] 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.

    [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.  RFC 1883, December 1995.

    [6] Paul Ferguson, editor.  Network ingress filtering:  Defeating
        IP source address spoofing denial of service attacks.
        Internet-Draft, draft-ferguson-ingress-filtering-02.txt, July
        1997.  Work in progress.

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

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

    [9] Thomas Narten, Erik Nordmark, and William Allen Simpson.
        Neighbor Discovery for IP version 6 (IPv6).  RFC 1970, August
        1996.

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

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

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

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

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



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

   [16] Susan Thomson and Thomas Narten.  IPv6 stateless address
        autoconfiguration.  RFC 1971, August 1996.














































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Chair's Address

   The Working Group can be contacted via its current chairs:

        Jim Solomon
        Motorola, Inc.
        1301 E. Algonquin Rd.
        Schaumburg, IL  60196
        USA

        Phone:  +1 847 576-2753
        E-mail: solomon@comm.mot.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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