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Internet Engineering Task Force                       C. Perkins, editor
INTERNET DRAFT                                                       IBM
                                                        21 December 1995


                          IP Mobility Support
                  draft-ietf-mobileip-protocol-14.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 mobile-ip@tadpole.com mailing list.

   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
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   To learn the current status of any Internet-Draft, please check the
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   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).


Abstract

   This document specifies protocol enhancements that allow transparent
   routing of IP datagrams to mobile nodes in the Internet.  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 its current point
   of attachment to the Internet.  The protocol provides for registering
   the care-of address with a home agent.  The home agent sends packets
   destined for the mobile node through a tunnel to the care-of address.
   After arriving at the end of the tunnel, the packets are then
   delivered to the mobile node.







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                                Contents



Status of This Memo                                                    i

Abstract                                                               i

 1. Introduction                                                       1
     1.1. Protocol Requirements . . . . . . . . . . . . . . . . . .    1
     1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . .    2
     1.3. Assumptions . . . . . . . . . . . . . . . . . . . . . . .    2
     1.4. Applicability . . . . . . . . . . . . . . . . . . . . . .    2
     1.5. New Architectural Entities  . . . . . . . . . . . . . . .    3
     1.6. Terminology . . . . . . . . . . . . . . . . . . . . . . .    3
     1.7. Protocol Overview . . . . . . . . . . . . . . . . . . . .    5
     1.8. Specification Language  . . . . . . . . . . . . . . . . .    9
     1.9. Message Format and Protocol Extensibility . . . . . . . .    9

 2. Agent Discovery                                                   12
     2.1. Agent Advertisement . . . . . . . . . . . . . . . . . . .   12
           2.1.1. Mobile Service Extension  . . . . . . . . . . . .   14
           2.1.2. Prefix-Lengths Extension  . . . . . . . . . . . .   15
           2.1.3. One-byte Padding Extension  . . . . . . . . . . .   16
     2.2. Agent Solicitation  . . . . . . . . . . . . . . . . . . .   17
     2.3. Foreign/Home Agent Considerations . . . . . . . . . . . .   17
           2.3.1. Advertised Router Addresses . . . . . . . . . . .   18
           2.3.2. Sequence Numbers, and Rollover Handling . . . . .   18
     2.4. Mobile Node Considerations  . . . . . . . . . . . . . . .   18
           2.4.1. Registration Required . . . . . . . . . . . . . .   19
           2.4.2. Move Detection  . . . . . . . . . . . . . . . . .   19
           2.4.3. Returning Home  . . . . . . . . . . . . . . . . .   20
           2.4.4. Sequence Numbers, and Rollover Handling . . . . .   20

 3. Registration                                                      21
     3.1. Registration Overview . . . . . . . . . . . . . . . . . .   21
     3.2. Authentication  . . . . . . . . . . . . . . . . . . . . .   22
     3.3. Registration Request  . . . . . . . . . . . . . . . . . .   22
     3.4. Registration Reply  . . . . . . . . . . . . . . . . . . .   24
     3.5. Registration Extensions . . . . . . . . . . . . . . . . .   27
           3.5.1. Mobile-Home Authentication Extension  . . . . . .   27
           3.5.2. Mobile-Foreign Authentication Extension . . . . .   28
           3.5.3. Foreign-Home Authentication Extension . . . . . .   28
     3.6. Mobile Node Considerations  . . . . . . . . . . . . . . .   29
           3.6.1. Sending Registration Requests . . . . . . . . . .   30
           3.6.2. Receiving Registration Replies  . . . . . . . . .   33



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           3.6.3. Registration Retransmission . . . . . . . . . . .   35
     3.7. Foreign Agent Considerations  . . . . . . . . . . . . . .   36
           3.7.1. Configuration and Registration Tables . . . . . .   36
           3.7.2. Receiving Registration Requests . . . . . . . . .   37
           3.7.3. Receiving Registration Replies  . . . . . . . . .   39
     3.8. Home Agent Considerations . . . . . . . . . . . . . . . .   40
           3.8.1. Configuration and Registration Tables . . . . . .   40
           3.8.2. Receiving Registration Requests . . . . . . . . .   41
           3.8.3. Sending Registration Replies  . . . . . . . . . .   44

 4. Routing Considerations                                            46
     4.1. Encapsulation Types . . . . . . . . . . . . . . . . . . .   46
     4.2. Unicast Packet Routing  . . . . . . . . . . . . . . . . .   46
           4.2.1. Mobile Node Considerations  . . . . . . . . . . .   46
           4.2.2. Foreign Agent Considerations  . . . . . . . . . .   47
           4.2.3. Home Agent Considerations . . . . . . . . . . . .   48
     4.3. Broadcast packets . . . . . . . . . . . . . . . . . . . .   48
     4.4. Multicast Packet Routing  . . . . . . . . . . . . . . . .   49
     4.5. Mobile Routers  . . . . . . . . . . . . . . . . . . . . .   50
     4.6. ARP, Proxy ARP, and Gratuitous ARP  . . . . . . . . . . .   51

 5. Security Considerations                                           55
     5.1. Message Authentication Codes  . . . . . . . . . . . . . .   55
     5.2. Areas of security concern in this protocol  . . . . . . .   55
     5.3. Key management  . . . . . . . . . . . . . . . . . . . . .   55
     5.4. Picking good random numbers . . . . . . . . . . . . . . .   56
     5.5. Privacy . . . . . . . . . . . . . . . . . . . . . . . . .   56
     5.6. Replay Protection for Registration Requests . . . . . . .   56
           5.6.1. Replay Protection using Timestamps  . . . . . . .   57
           5.6.2. Replay Protection using Nonces  . . . . . . . . .   58

 6. Acknowledgments                                                   58

 A. Patent Issues                                                     59
     A.1. IBM Patent #5,159,592 . . . . . . . . . . . . . . . . . .   60
     A.2. IBM Patent #5,148,479 . . . . . . . . . . . . . . . . . .   60

 B. Link-Layer considerations                                         61
     B.1. Point-to-Point Link-Layers  . . . . . . . . . . . . . . .   61
     B.2. Multi-Point Link-Layers . . . . . . . . . . . . . . . . .   61

 C. TCP Considerations                                                61
     C.1. TCP Timers  . . . . . . . . . . . . . . . . . . . . . . .   61
     C.2. TCP Congestion Management . . . . . . . . . . . . . . . .   62

 D. Example Scenarios                                                 63
     D.1. Registering with a Foreign Agent's Care-of Address  . . .   63
     D.2. Registering with a Dynamic Care-of Address  . . . . . . .   63



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     D.3. Deregistration  . . . . . . . . . . . . . . . . . . . . .   64

 E. Applicability of Prefix Lengths Extension                         65

Chair's Address                                                       68

Editor's Address                                                      68












































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1. Introduction

   IP version 4, like its predecessors, assumes that a node's IP address
   uniquely identifies the node's point of attachment to the Internet.
   Therefore, a node must be located on the network indicated by its
   IP address in order to receive packets destined to it; otherwise,
   packets destined to the node would be undeliverable.  For a node
   to change its point of attachment without losing its ability to
   communicate, currently one of the two following mechanisms must
   typically be employed:

      a)   the node must change its IP address whenever it changes its
           point of attachment, or

      b)   host-specific routes must be propagated throughout much of
           the Internet routing fabric.

   Both of these alternatives are often unacceptable.  The first makes
   it impossible for a node to maintain transport and higher-layer
   connections when the node changes location.  The second has obvious
   and severe scaling problems, especially relevant considering the
   explosive growth in sales of notebook computers.

   A new, scalable, mechanism is evidently required to accommodate node
   mobility within the Internet.  This document defines such a mechanism
   and enables nodes to change their point of attachment to the Internet
   without changing their IP address.


1.1. Protocol Requirements

   Implementation of the protocol described in this document shall allow
   a mobile node to communicate with other nodes after changing its
   point of physical attachment to the Internet, yet without changing
   its IP address.

   Implementation of the protocol described in this document shall allow
   a mobile node to communicate with other nodes that do not implement
   these mobility functions.  No protocol enhancements are required
   in hosts or routers that are not providing any of the mobility
   functions.

   Messages used by this protocol which exchange location information
   must be authenticated in order to protect against remote redirection
   attacks.






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1.2. Goals

   The link by which the mobile node is directly attached to the
   Internet is likely to be bandwidth limited, and experience a higher
   rate of errors than traditional wired networks.  Moreover, mobile
   nodes are more likely to be battery powered, and minimizing power
   consumption is important.  Therefore, only a few administrative
   messages should be sent between a mobile node and an agent, and the
   size of these messages should be kept as short as is reasonably
   possible.


1.3. Assumptions

   The protocols defined in this document place no additional
   constraints on assignment of IP addresses.  That is, a mobile node
   can be assigned an IP address by the organization that owns the
   machine, and will be able to use that IP address regardless of the
   current point of attachment.

   It is assumed that mobile nodes will not change their point of
   attachment to the Internet more frequently than once per second.

   It is assumed that IP unicast datagrams are routed based on the
   destination address in the datagram header (i.e., not by source
   address).


1.4. Applicability

   Mobile IP is intended to solve node mobility across changes in IP
   subnet.  It is just as suitable for mobility across homogeneous media
   as it is for mobility across heterogeneous media.  That is, Mobile
   IP facilitates node movement from one Ethernet segment to another as
   well as it accommodates node movement from an Ethernet segment to a
   wireless LAN, as long as the mobile node's IP address remains the
   same after such a movement.

   One can think of Mobile IP as solving the "macro" mobility management
   problem.  It is less well suited for more "micro" mobility management
   applications -- for example, handoff amongst wireless transceivers,
   each of which covers only a very small geographic area.  In this
   later situation, link-layer mechanisms for link maintenance (i.e.
   link-layer handoff) may offer faster convergence and far less
   overhead than Mobile IP.






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1.5. New Architectural Entities

   Mobile IP introduces these new functional entities:

      Mobile Node

         A host or router that changes its point of attachment from one
         network or subnetwork to another.  A mobile node may change its
         location without losing connectivity and without changing its
         IP address.

      Home Agent

         A router on a mobile node's home network which tunnels packets
         for delivery to the mobile node when it is away from home, and
         maintains current location information for the mobile node.

      Foreign Agent

         A router on a mobile node's visited network which provides
         routing services to the mobile node when it registers.  The
         foreign agent detunnels and delivers packets to the mobile node
         that were tunneled by the mobile node's home agent.  In the
         reverse direction, the foreign agent may serve as a default
         router for registered mobile node.

   A mobile node is given a long-term IP address on a home network.
   This home address is administered in much the same way as "permanent"
   IP addresses are provided to stationary hosts.  When away from its
   home network, the "care-of address" associated with the mobile node
   reflects the mobile node's current point of attachment.  The mobile
   node creates new network connections with existing Internet hosts
   using its home address.


1.6. Terminology

   This document frequently uses the following terms:

      Agent Advertisement

         A periodic advertisement constructed by attaching a special
         extension to a router advertisement [7] message.

      Care-of Address

         The care-of address is the termination point of a tunnel toward
         a mobile node that is away from its home network.  Depending



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         on the configuration, the care-of address can be either an
         address of a foreign agent or a temporary address acquired by
         the mobile node.

      Correspondent

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

      Foreign Network

         Any network other than the mobile node's Home Network.

      Home Address

         An IP address that is assigned for an extended period of time
         to a mobile node.  It remains unchanged regardless of where the
         node is attached to the Internet.

      Home Network

         A network, possibly virtual, having a network prefix matching
         that of a mobile node's home address.  Note that standard IP
         routing mechanisms will deliver packets destined to a mobile
         node's Home Address to the mobile node's Home Network.

      Link

         A facility or medium over which nodes can communicate at the
         link layer.  A link underlies the network layer.

      Link-Layer Address

         The address used to identify the endpoints of the communication
         over a physical link.  Typically, the Link-Layer address is an
         interface's Media Access Control (MAC) address.

      Mobility Agent

         Either a home agent or a foreign agent.

      Mobility Binding

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






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      Mobility Security Association

         The mobility security association between a pair of nodes
         is a collection of security contexts which may be applied
         to Mobile IP protocol messages exchanged by them.  Each
         context indicates an authentication algorithm and mode
         (subsection 5.1), a secret (a shared key, or appropriate
         public/private key pair), and a style of replay protection in
         use (subsection 5.6).

      Node

         A host or a router.

      Nonce

         A random value, different from previous choices, inserted in a
         packet to protect against replays.

      Security Parameter Index (SPI)

         The SPI indicates the security context between a pair of nodes
         among those available in the Mobility Security Association.  A
         value of zero is reserved and MUST NOT be used in any Mobility
         Security Association.

      Tunnel

         The path followed by a packet while it is encapsulated.  The
         model is that, while it is encapsulated, a packet is routed
         to a knowledgeable decapsulating agent, which decapsulates
         the packet and then correctly delivers it to its ultimate
         destination.

      Visited Network

         A network other than a mobile node's Home Network to which the
         mobile node is currently connected.

      Visitor List

         The list of mobile nodes visiting a foreign agent.


1.7. Protocol Overview

   The following support services are defined for Mobile IP:




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      Agent Discovery

         Home agents and foreign agents may advertise their availability
         on each link for which they provide service.  A newly arrived
         mobile node can send a solicitation on the link to learn if any
         prospective agents are present.

      Registration

         When the mobile node is away from home, it registers its
         care-of address with its home agent.  Depending on its method
         of attachment, the mobile node will register either directly
         with its home agent, or through a foreign agent which forwards
         the registration to the home agent.

   The following is a rough outline of the mobile-IP protocol:

    -  Foreign agents and home agents advertise their presence via Agent
       Advertisements (see section 2).

    -  A mobile node receives these advertisements and determines
       whether it is on its home network or a foreign network.

    -  When the mobile node detects that it is located on its home
       network, it operates without mobility services.

    -  When a mobile node detects that it has moved to a foreign
       network, it obtains a care-of address on the foreign network.
       The care-of address can either be determined from a foreign
       agent's advertisements, or by some assignment mechanism (for
       example, DHCP [8]).

    -  The mobile node then registers its new care-of address with its
       home agent, possibly via a foreign agent (see section 3).

    -  Packets sent to the mobile node's Home Address are received
       by the home agent, tunneled by the home agent to the care-of
       address, received at the tunnel endpoint (either at a foreign
       agent or at the mobile node itself), and finally delivered to the
       mobile node (see subsection 4.2.3).

    -  In the reverse direction, packets originated by the mobile node
       are typically delivered to their destination using standard IP
       routing mechanisms, not necessarily passing through the home
       agent.

   When away from home, Mobile IP uses protocol tunneling to hide a
   mobile node's home address from intervening routers between its home



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   network and its current location.  The tunnel terminates at the
   mobile node's care-of address -- an address to which packets can
   be delivered via conventional IP routing.  At the care-of address,
   the original packet is removed from the tunnel and delivered to the
   mobile node.

   Mobile IP provides two distinct modes for the acquisition of a
   care-of address:

      a)   A care-of address may be provided by a foreign agent.  This
           mode is preferred because it allows many mobile nodes to
           share the same care-of address and therefore does not place
           unnecessary demands on the already limited IPv4 address
           space.  In this mode, the foreign agent is the endpoint of
           the tunnel and, upon receiving tunneled packets from the
           mobile node's home agent, decapsulates them and delivers the
           inner packet to the mobile node.

      b)   A care-of address may be acquired by, and/or owned by, a
           mobile node when visiting a foreign network.  The method
           by which it obtains such an address is beyond the scope
           of this document (but see, for example, DHCP [8]).  With
           its own care-of address, the mobile node itself performs
           decapsulation of the packets tunneled by its home agent.

   The latter mode has the advantage that it allows a mobile node to
   function without a foreign agent, for example, in installations that
   have not yet deployed Mobile IP. It does, however, place additional
   burden on the IPv4 address space because it requires a pool of
   addresses to be made available to visiting mobile nodes.  It is
   difficult to efficiently maintain pools of addresses for each subnet
   which may permit mobile nodes to visit.

   It is important to understand the distinction between the care-of
   address and the foreign agent functions.  The care-of address is
   simply the endpoint of the tunnel.  It might indeed be an address
   of a foreign agent, but it also might be an address temporarily
   acquired by the mobile node.  On the other hand, a foreign agent
   is a mobility agent that provides services to mobile nodes.  See
   subsections 3.7 and 4.2.2 for additional details.

   A home agent MUST be able to attract and intercept packets that
   are destined to the home address of any of its registered mobile
   nodes.  Using the proxy and gratuitous ARP mechanisms described in
   section 4.6, this requirement can be satisfied if the home agent has
   a network interface on the link indicated by the mobile node's home
   address.  Other placements of the home agent relative to the mobile
   node's home location MAY also be possible using other mechanisms for



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   intercepting datagrams destined to the mobile node's home address.
   Such placements are beyond the scope of this document.

   Similarly, a mobile node and a prospective or current foreign agent
   MUST be able to exchange packets without relying on standard IP
   routing mechanisms; that is, those mechanisms which make forwarding
   decisions based upon the network-prefix of the destination address
   in the IP header.  This requirement can be satisfied if the foreign
   agent and the mobile node have an interface on the same link.  In
   this case, the mobile node and foreign agent simply bypass their
   normal IP routing mechanism when sending packets to each other,
   addressing the underlying link-layer frames to their respective
   link-layer addresses.  Other placements of the foreign agent relative
   to the mobile node MAY also be possible using other mechanisms to
   exchange packets between these nodes.  Such placements are beyond the
   scope of this document.

   If a mobile node owns its own care-of address (as described in (b)
   above), the mobile node MUST be located on the link identified by the
   network prefix of this care-of address.  Otherwise, packets destined
   to the care-of address will be undeliverable.

   .....................................................................
   :                                                                   :
   :                 2) packet is received   3) packet is              :
   :                    by home agent and       detunneled             :
   :                    is tunneled to the      and delivered          :
   :                    care-of address         to mobile node         :
   :                                                                   :
   :                 +-----+          +-------+         +------+       :
   :                 |home | =======> |foreign| ------> |mobile|       :
   :                 |agent|          | agent | <------ | node |       :
   :                 +-----+          +-------+         +------+       :
   : 1) packet to      /|\         /                                   :
   :    mobile node     |        /   4) In the opposite direction,     :
   :    arrives on      |      /        standard IP routing delivers   :
   :    home network    |    /          the packet to its destination. :
   :    via standard    |  |_           In this figure, the foreign    :
   :    IP routing.   +----+            agent is the mobile node's     :
   :                  |host|            default router.                :
   :                  +----+                                           :
   :                                                                   :
   :     Figure 1: Packet Delivery for Mobile Nodes Away from Home     :
   :...................................................................:

   Figure one illustrates packet routing to/from a mobile node away from
   home, once the mobile node has registered with its home agent.  Shown




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   is the mode in which the care-of address is provided by a foreign
   agent.


1.8. 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 this item,
                         but the full implications must be understood
                         and carefully weighed before choosing a
                         different course.  Unexpected results may
                         result otherwise.

      MAY                This word, or the adjective "optional", means
                         that this item is one of an allowed set of
                         alternatives.  An implementation which does
                         not include this 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.


1.9. Message Format and Protocol Extensibility

   Each message in Mobile IP begins with a short fixed part, followed
   by one or more extensions in Type-Length-Value format, with one
   exception.  That exception is the One-Byte Padding Extension which
   has only a Type but no Length and no Data fields.





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   There are two separately maintained sets of numbering spaces from
   which extension Type values are allocated.  The first set defines
   those extensions which may only appear in ICMP Router Advertisement
   messages.  The second set defines those extensions which may only
   appear in Mobile IP messages (those sent to and from UDP port number
   434).  Each individual extension is described in detail within the
   section of this document that is applicable.

   Due to the separation (orthogonality) of these sets, it is
   conceivable that two extensions that are defined at a later date
   could have identical Type values, so long as one of the extensions
   applies to ICMP Router Advertisement and the other applies to Mobile
   IP registration.

   When an extension numbered in either of these sets within the range
   0-127 is encountered but not recognized, the packet containing the
   extension must be dropped.  When an extension numbered in the range
   128-255 is encountered which is not recognized, that particular
   extension is ignored, but the rest of the extensions and packet data
   can still be processed.  The Length field of the extension is used to
   skip the Data field in searching for the next extension.

   Below is the list of extensions that are currently defined within
   each respective set.  Up-to-date values for extensions to ICMP Router
   Advertisement and to Mobile IP messages are specified in the most
   recent "Assigned Numbers" [21].

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



















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      Type

         The following extension Type values are allocated from the set
         of extensions to ICMP Router Advertisement:

          0   One-byte Padding (has no Length nor Data field)
         16   Mobile Service
         19   Prefix-Lengths

         The following extension Type values are allocated from the set
         of extensions to Mobile IP messages (e.g.  registration):

         32   Mobile-Home Authentication
         33   Mobile-Foreign Authentication
         34   Foreign-Home Authentication

         All other values in these respective sets are currently
         undefined (see [21] for the most up-to-date lists).

      Length

         Indicates the length (in bytes) of the data field.  The length
         does not include the Type and Length bytes.

      Data

         This field is zero or more bytes in length and contains the
         value(s) for this extension.  The format and length of the data
         field is determined by the type and length fields.

   Extensions allow variable amounts of information to be carried within
   each datagram.  The end of the list of extensions is indicated by the
   total length of the IP datagram.


















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2. Agent Discovery

   Agent Discovery is the method by which a mobile node determines
   whether it is currently connected to its home network or a foreign
   network.  When on a foreign network, the methods specified in this
   section allow the mobile node to determine the care-of address being
   offered by foreign agents on that network.

   Mobile IP extends ICMP Router Discovery [7] as its primary mechanism
   for Agent Discovery.  An Agent Advertisement is formed by appending
   one or more of the extensions defined in this section to an ICMP
   Router Advertisement.  An Agent Solicitation is identical to an ICMP
   Router Solicitation.  This section describes the message formats and
   procedures by which mobile nodes, foreign agents, and home agents
   cooperate to realize Agent Discovery.

   Agent Advertisement and Agent Solicitation may not be necessary
   for link layers which already provide this functionality.  The
   method by which mobile nodes establish link-layer connections
   with prospective agents is outside the scope of this document (but
   see Appendix B).  The procedures described below assume that such
   link-layer connectivity has already been established.

   No authentication is required for Agent Advertisement and Agent
   Solicitation messages.  They MAY be authenticated using the IP
   Authentication Header [2], which is external to the messages
   described here.  Further specification of the way that advertisement
   and solicitation messages are authenticated is outside of the scope
   of this document.


2.1. Agent Advertisement

   Agent Advertisements are periodic transmissions sent by a mobility
   agent.  Mobile nodes use these advertisements to determine their
   current point of attachment to the Internet.  Agent advertisements,
   as specified in this document, are ICMP Router Advertisements
   that have been modified to carry the Mobile Service Extension and,
   optionally, the Prefix-Lengths Extension or future extensions that
   might be defined.

   The following fields within the ICMP Router Advertisement portion of
   the Agent Advertisement are further refined as follows:

    -  Link Layer Fields






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          Destination Address

             The link-layer destination address of a unicast Agent
             Advertisement MUST be the same as the source link-layer
             address of the Agent Solicitation which prompted the
             Advertisement.

    -  IP Fields

          TTL

             The TTL for Agent Advertisements MUST be set to 1.

          Destination Address

             As per RFC1256 [7], the IP destination address of an
             Agent Advertisement MUST be either the "all systems
             on this link" multicast address (224.0.0.1) [6] or the
             "Limited broadcast" address (255.255.255.255).  This is
             because subnet-directed broadcast addresses of the form
             <prefix>.<-1> are generally useless to mobile nodes that
             are visiting foreign networks.  Such mobile nodes will have
             a different prefix than that of the advertising agents.

    -  ICMP Fields

          Code

             The Code field of the agent advertisement is interpreted as
             follows:

                0    The mobility agent handles common traffic -- that
                     is, IP data packets not necessarily related to
                     mobile nodes.

                16   The mobility agent does not route common traffic.
                     However, all foreign agents MUST (minimally)
                     forward to a default router those packets
                     received from a registered mobile node (see
                     subsection 4.2.2).

          Lifetime

             The maximum length of time that the Advertisement is
             considered valid in the absence of further Advertisements.






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          Router Addresses

             See subsection 2.3.1 for a discussion of the addresses that
             may appear in this portion of the Agent Advertisement.

   The nominal advertisement period as shown in the Lifetime field
   SHOULD be 1/3 of the advertisement Lifetime.  This allows a mobile
   node to miss three successive advertisements before deleting the
   agent from its list of valid agents.  The actual time of issuance of
   advertisements should be slightly randomized to avoid synchronization
   and subsequent collisions with other agent advertisements which
   may be present on the same medium.  Note that this field has no
   relation whatsoever to the "Lifetime" field within the Mobile Service
   Extension defined below.

   The ICMP fields are immediately followed by the Mobile Service
   Extension defined in subsection 2.1.1.  The Mobile Service
   Extension MAY optionally be followed by the Prefix-Lengths
   Extension (subsection 2.1.2), the One-byte Padding Extension
   (subsection 2.1.3), or future extensions which might be defined.


2.1.1. Mobile Service Extension

   The Mobile Service Extension immediately follows the ICMP Router
   Advertisement fields.  It is used to indicate that an ICMP Router
   Advertisement message is actually an Agent Advertisement being sent
   by a mobility agent.  The Mobile Service Extension is defined as
   follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Lifetime            |R|B|H|F|M|G|V|    reserved     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  zero or more Care-of Addresses               |
   |                              ...                              |

      Type              16

      Length            (6 + 4*N), where N is the number of
                        care-of addresses advertised.

      Sequence number   The count of advertisement messages sent since
                        the agent was initialized (see section 2.3.2).




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      Lifetime          The longest lifetime (measured in seconds)
                        that the agent is willing to accept in any
                        registration request.  A value of all ones
                        indicates infinity.  This field has nothing
                        whatsoever to do with the "Lifetime" field
                        within the ICMP Router Advertisement portion of
                        the Agent Advertisement described above.

      R                 Foreign agent registration required bit.

      B                 Busy bit.  The foreign agent will not accept
                        more registrations.  Only valid if F=1.

      H                 Agent offers service as a home agent.

      F                 Agent offers service as a foreign agent.

      M                 Agent offers minimal encapsulation (see [16]).

      G                 Agent offers GRE encapsulation (see [10]).

      V                 Agent supports Van Jacobson header compression
                        [11]

      reserved          Sent as zero; ignored on reception.

      Care of Address   A foreign agent's care-of address(es).  An Agent
                        Advertisement MUST include at least one care-of
                        address if F=1.

   Any home agent MUST always be prepared to serve its mobile nodes.
   Thus, it is an error to have the 'B' bit set without also having the
   'F' bit set, since only foreign agents are permitted to be too busy
   to service new requests.  An agent MUST NOT send Agent Advertisements
   with neither the 'F' bit nor the 'H' bit set to one.

   When a foreign agent wishes to require registration even from those
   mobile nodes which have acquired local, temporary care-of addresses,
   it sets the 'R' bit to one.  Because this applies only to foreign
   agents, an agent MUST NOT set the 'R' bit to one unless the 'F' bit
   is also set to one.


2.1.2. Prefix-Lengths Extension

   The Prefix-Lengths Extension MAY follow the Mobile Service Extension.
   It is used to indicate the number of bits of network prefix that
   applies to each address listed in the ICMP Router Advertisement



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   portion of the Agent Advertisement.  Note that the prefix lengths
   DO NOT apply to care-of address(es) listed in the Mobile Service
   Extension.  The Prefix-Lengths Extension is defined as follows:

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

      Type               19 (Prefix-Lengths Extension)

      Length             The length of this extension excluding the
                         Type field and the Length field.  This field
                         MUST equal N, where N is the value of the "Num
                         Addrs" field in the ICMP Router Advertisement
                         portion of the Agent Advertisement.

      Prefix length(s)   The number of leading bits which define the
                         network number of the corresponding router
                         address listed in the ICMP Router Advertisement
                         portion of the message.

   See subsection 2.4.2 for information about how to use the Prefix
   Lengths extension when determining whether movement has occurred.
   See appendix E for implementation details about the use of this
   extension.


2.1.3. One-byte Padding Extension

   Some kernel implementations insist upon padding ICMP packets to an
   even number of bytes.  If the ICMP length of an Agent Advertisement
   is odd, this extension MAY be included in order to make the ICMP
   length even.  Note that this extension is NOT intended to be
   a general-purpose extension to be included in order to word or
   long-align the various fields of the Agent Advertisement.  In fact,
   an Agent Advertisement SHOULD NOT include more than one One-byte
   Padding Extension and this extension SHOULD be the last extension
   present in an Agent Advertisement.  NOTE the absence of both a
   "Length" field and a "Data" field in the One-byte Pad Extension.










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   The One-byte Padding Extension is defined as follows:

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

      Type 0 (One-byte Padding Extension)


2.2. Agent Solicitation

   An Agent Solicitation is identical to an ICMP Router Solicitation
   with the further restriction that the IP TTL Field MUST be set to 1.


2.3. Foreign/Home Agent Considerations

   Any mobility agent which is not indicated by a link-layer protocol
   MUST send Agent Advertisements.  An agent which is indicated by a
   link-layer protocol SHOULD also implement Agent Advertisements.
   However, the advertisements need not be sent, except when the site
   policy requires registration with the agent (i.e.  when the 'R' bit
   is set), or as a response to a specific solicitation.  All mobility
   agents SHOULD respond to Agent Solicitations.

   The same procedures, defaults, and constants are used in agent
   advertisements as described in RFC 1256 [7], except that:

    -  a mobility agent MUST limit the rate at which it sends broadcast
       or multicast agent advertisements; a recommended maximum rate is
       once per second, AND

    -  a mobility agent that receives a Router Solicitation does not
       require that the IP Source Address is the address of a neighbor
       (i.e., an address that matches one of the router's own addresses
       on the arrival interface, under the subnet mask associated with
       that address.)

   The home agent for a given mobile node SHOULD be located on the link
   identified by the home address, if the home network is not merely a
   virtual network.  In this case, the home agent MUST send out agent
   advertisements with the 'H' bit set, so that mobile nodes on their
   home network will be able to determine that they are indeed at home.

   On a particular subnet, either all mobility agents MUST include
   the Prefix-Lengths Extension or all of them MUST NOT include this
   extension.  Equivalently, it is prohibited for some agents on a given



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   subnet to include the extension but for others not to include it.
   Otherwise, one of the move detection algorithms designed for mobile
   nodes will not function properly (see subsection 2.4.2).


2.3.1. Advertised Router Addresses

   The ICMP Router Advertisement portion of the Agent Advertisement MAY
   contain one or more router addresses.  Thus, an agent MAY include
   one of its own addresses in the advertisement.  A foreign agent
   MAY discourage use of this address as a default router by setting
   the preference to a low value and by including the address of
   another router in the advertisement (with a correspondingly higher
   preference).  A foreign agent MUST route packets it receives from
   registered mobile nodes (see subsection 4.2.2).


2.3.2. Sequence Numbers, and Rollover Handling

   The sequence number in Agent Advertisements ranges from 0 to
   0xffff.  After booting, an agent shall use the number 0 for its first
   advertisement.  Each subsequent advertisement shall use the sequence
   number one greater, with the exception that the sequence number
   0xffff shall be followed by sequence number 256.  In this way, mobile
   nodes can distinguish reductions in sequence numbers that result from
   reboots, from reductions that result in rollover of the sequence
   number after it attains the value 0xffff.


2.4. Mobile Node Considerations

   Every mobile node MUST implement Agent Solicitation.  Solicitations
   SHOULD only be sent in the absence of Agent Advertisements and when a
   care-of address has not been determined through a link-layer protocol
   or other means.  The mobile node uses the same procedures, defaults,
   and constants for Agent Solicitation as described in RFC 1256 for
   router solicitation, except that the mobile node may solicit more
   often than once every three seconds and MAX_SOLICITATIONS does not
   apply for mobile nodes that are currently unconnected to any foreign
   agent.

   The rate at which a mobile node sends solicitations MUST be
   limited by the mobile node.  The mobile node MAY send three initial
   solicitations at a maximum rate of one per second while searching for
   an agent.  After this, the rate at which solicitations are sent MUST
   be reduced so as to limit the overhead on the local link.  Subsequent
   solicitations MUST be sent using a binary exponential backoff
   mechanism, doubling the interval between consecutive solicitations,



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   up to a maximum interval.  The maximum interval SHOULD be chosen
   appropriately based upon the characteristics of the media over which
   the mobile node is soliciting.  This maximum interval SHOULD be at
   least one minute between solicitations.

   While still searching for an agent, the mobile node MUST NOT increase
   the rate at which it sends solicitations unless it has received
   a positive indication that it has moved to a new link.  After
   successfully registering with an agent, the mobile node MAY also
   increase the rate at which it will send solicitations when it next
   begins searching for a new agent with which to register.  In both
   cases, the increased solicitation rate MAY revert to the maximum
   rate, but then MUST be limited in the manner described above.  In all
   cases, the recommended solicitation intervals are nominal values.
   Mobile nodes MUST randomize their solicitation times around these
   nominal values as mandated by RFC 1256 [7].

   Mobile nodes must process Agent Advertisements.  Mobile nodes
   can distinguish Agent Advertisements from "standard" ICMP Router
   Advertisements by examining the number of advertised addresses and
   the IP Total Length field.  When the IP total length indicates
   that the ICMP message is longer than needed for the number of
   advertised addresses, the remaining data is interpreted as one
   or more extensions.  The presence of a Mobile Service Extension
   identifies the advertisement as an Agent Advertisement.

   When multiple methods of agent identification are in use, the
   mobile node SHOULD first attempt registration with agents including
   Mobile Service Extensions in their advertisements in preference
   to those sending link-layer advertisements.  This order maximizes
   the likelihood that the registration will be recognized, thereby
   minimizing the number of registration attempts.


2.4.1. Registration Required

   When the mobile node receives an Agent Advertisement with the 'R'
   bit set to 1, the mobile node SHOULD register through the foreign
   agent, even when the mobile node might be able to acquire its own
   temporary care-of address.  This feature allows sites to enforce
   visiting policies (such as accounting) which require exchanges of
   authorization.


2.4.2. Move Detection

   Two primary mechanisms are provided for mobile nodes to detect
   movement from one subnet to another.  Other mechanisms MAY be used.



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   When the mobile node detects that it has moved, it SHOULD register
   (see section 3) with a suitable care-of address on the new foreign
   network (but not too often -- see subsection 3.6.3).

   The first method of move detection is based upon the Lifetime field
   within the main body of the ICMP Router Advertisement portion of
   the Agent Advertisement.  A mobile node SHOULD record the Lifetime
   received in any Agent Advertisements.  If the mobile node fails
   to receive another advertisement from the same agent within the
   specified Lifetime, it SHOULD assume that it has lost contact with
   the agent and therefore it SHOULD attempt to discover and then
   register with other agents.

   The second method uses network prefixes.  The Prefix-Lengths
   Extension MAY be used by mobile nodes to determine whether or not a
   newly received Agent Advertisement was received on the same subnet
   as the mobile node's current agent.  If the prefixes differ, the
   mobile node assumes that it has moved.  A mobile node SHOULD NOT use
   this method of move detection unless both the current agent and the
   new agent include the Prefix-Lengths Extension in their respective
   Agent Advertisements.  If this extension is missing from one or both
   of the advertisements, this method of move detection SHOULD NOT be
   used.  On the expiration of registration, a mobile node may choose to
   register with the care-of addresses received in the most recent agent
   advertisement.


2.4.3. Returning Home

   If the mobile node detects that it has moved to its home network,
   it MUST deregister with its home agent (see section 3).  Before
   attempting to deregister, the mobile node SHOULD configure
   its routing table appropriately for its home network (see
   subsection 4.2.1).  In addition, if the home network is using
   ARP [17], the mobile node MUST follow the procedures described in
   section 4.6 with regard to ARP, proxy ARP, and gratuitous ARP.


2.4.4. Sequence Numbers, and Rollover Handling

   If a mobile node detects two successive values of the sequence number
   in the advertisements from its foreign agent, the second of which is
   less than the first and inside the range 0 to 255, the mobile node
   MUST register again.  If the second value is less than the first, but
   greater than or equal to 256, the mobile node may assume that the
   sequence number has rolled over past its maximum value (0xffff), and
   that there is no need to re-register (see subsection 2.3).




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3. Registration

   Mobile IP registration provides a flexible mechanism for mobile nodes
   to communicate their current reachability information to their home
   agent.  It is the method by which mobile nodes:

    - request forwarding services when visiting a foreign network,
    - inform their home agent of their current care-of address,
    - renew a registration which is due to expire, and/or
    - deregister when they return home.

   Registration messages exchange information between a mobile node,
   (optionally) a foreign agent, and the home agent.  Registration
   creates or modifies a mobility binding at the home agent, associating
   the mobile node's home address with its care-of address for the
   specified Lifetime.

   Several other (optional) capabilities are available through the
   registration procedure, which enable a mobile node to:

    -  maintain multiple simultaneous registrations, wherein a copy of
       each datagram will be tunneled to each active care-of address

    -  deregister specific care-of addresses while retaining other
       mobility bindings, and

    -  discover the address of a home agent if the mobile node is not
       configured with this information.


3.1. Registration Overview

   If a mobile node has associated a care-of address to an interface on
   a link over which it has not received Agent Advertisements with the
   'R' bit set, or if the mobile node is returning home, the mobile node
   registers or deregisters directly with a home agent by the exchange
   of only 2 messages:

      a)   The mobile node sends a registration request to a home agent,
           asking it to provide service.

      b)   The home agent sends a registration reply to the mobile node,
           granting or denying service.

   If a mobile node is using a care-of address offered by a foreign
   agent, the mobile node MUST register via that foreign agent.  Even
   if a mobile node has associated a care-of address with one of its
   own interfaces on a link over which it receives Agent Advertisements



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   with the 'R' bit set, the mobile node SHOULD register via one of the
   foreign agents on that link.  In these cases, the foreign agent acts
   as a relay between the mobile node and the home agent.  This extended
   registration process requires 4 messages:

      a)   The mobile node sends a registration request to the
           prospective foreign agent to begin the registration process.

      b)   The foreign agent relays the request to the home agent,
           asking the home agent to register the mobile node at the
           foreign agent's care-of address.

      c)   The home agent sends a registration reply to the foreign
           agent to grant or deny service.

      d)   The foreign agent relays the registration reply to the mobile
           node to inform it of the disposition of its request.

   The registration messages defined in subsections 3.3 and 3.4 use the
   User Datagram Protocol (UDP) header [18].  A nonzero UDP checksum
   SHOULD be included in the header, and checked by each recipient.


3.2. Authentication

   Each mobile node, foreign agent, and home agent MUST be able to
   support a mobility security association for mobile entities, indexed
   by their IP address.  See section 5.1 for requirements for support
   of authentication algorithms.  Registration messages between mobile
   node and home agent MUST be authenticated with the Mobile-Home
   Authentication Extension (subsection 3.5.1).  This extension
   immediately follows all non-authentication extensions, except those
   foreign agent-specific extensions which may be added to the packet
   after the mobile node computes the authentication.


3.3. Registration Request

   A mobile node sends a registration request message so that its home
   agent can create or modify a mobility binding for that mobile node
   (with a new lifetime).  The request may be relayed to the home agent
   by the foreign agent from which the mobile node is accepting service,
   or it may be sent directly in case the mobile node has received a
   care-of address by some other means (e.g, DHCP [8]).

   IP fields:





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      Source Address        Typically the interface address from which
                            the packet is sent.

      Destination Address   Typically that of the foreign agent or the
                            home agent.

   See subsections  3.6.1.1 and 3.7.2.2 for details.

   UDP fields:

      Source Port        variable

      Destination Port   434

   The UDP header is followed by the Mobile-IP fields shown below:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |S|B|D|M|G|rsvd |          Lifetime             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Home Address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Home Agent                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Care-of Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Identification                        |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Extensions ...
   +-+-+-+-+-+-+-+-

      Type              1 (Registration Request)

      S                 If the 'S' bit is set, the mobile node is
                        requesting that the home agent retain its
                        prior mobility bindings, as described in
                        subsection 3.6.1.2.

      B                 If the 'B' bit is set, the mobile node requests
                        that the home agent send to it broadcasts on the
                        home network, as described in subsection 4.3.

      D                 If the 'D' bit is set, the mobile node will
                        itself decapsulate datagrams which are sent to
                        the care-of address.  That is, the mobile node




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                        has assocated a care-of address with one of its
                        own interfaces.

      M                 If the 'M' bit is set, the mobile node asks its
                        home agent to use minimal encapsulation [16].

      G                 If the 'G' bit is set, the mobile node asks its
                        home agent to use GRE encapsulation [10].

      rsvd              Reserved bits; sent as zero

      Lifetime          The number of seconds remaining before the
                        registration is considered expired.  A value of
                        zero indicates a request for deregistration.  A
                        value of all ones indicates infinity.

      Home Address      The IP address of the mobile node.

      Home Agent        The IP address of a home agent.

      Care-of Address   The IP address for the end of a tunnel.

      Identification    A 64-bit number, constructed by the mobile node,
                        useful for matching requests with replies, and
                        for protecting against replay attacks (see
                        subsections 5.4, 5.6).

      Extensions        The fixed portion of the Registration Request
                        is followed by one or more of the extensions
                        listed in subsection 3.5.  The Mobile-Home
                        Authentication Extension MUST be included in
                        all Registration Requests.  See the sections on
                        mobile node, and foreign agent considerations
                        (3.6.1.3 and  3.7.2.2) for ordering rules on
                        extensions.


3.4. Registration Reply

   A mobility agent returns a registration reply message to a mobile
   node which has sent a registration request (subsection 3.3) message.
   If the mobile node is accepting service from a foreign agent,
   that foreign agent will receive the reply from the home agent and
   subsequently relay it to the mobile node.  The reply message contains
   the necessary codes to inform the mobile node about the status of its
   request, along with the lifetime granted by the home agent, which MAY
   be smaller than the original request.




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   Mobility agents MUST NOT increase the lifetime selected by the mobile
   node in the registration request.  If the lifetime of the reply is
   greater than the original request, the excess time MUST be ignored.
   When the lifetime of the reply is smaller than the original request,
   another registration SHOULD occur before the smaller lifetime
   expires.

   IP fields:

      Source        Typically copied from the destination address of the
                    Registration Request to which the agent is replying.
                    See subsections 3.7.2.3 and 3.8.3.1 for complete
                    details.

      Destination   Copied from the source address of the Registration
                    Request to which the agent is replying

   UDP fields:

      Source Port        <variable>

      Destination Port   Copied from the source port of the
                         corresponding Registration Request
                         (subsection 3.7.1).

   The UDP header is followed by the Mobile-IP fields shown below:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |           Lifetime            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Home Address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Home Agent                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Identification                        |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Extensions ...
   +-+-+-+-+-+-+-+-

      Type             3 (Registration Reply)

      Code             One of the following codes:

                        0 registration accepted




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                        1 registration accepted; simultaneous mobility
                          bindings unsupported

                       Service denied by the foreign agent:

                       64 reason unspecified
                       65 administratively prohibited
                       66 insufficient resources
                       67 mobile node failed authentication
                       68 home agent failed authentication
                       69 requested lifetime too long
                       70 poorly formed request
                       71 poorly formed reply
                       72 requested encapsulation unavailable
                       73 requested VJ compression unavailable
                       80 home network unreachable (ICMP error)
                       81 home agent host unreachable (ICMP error)
                       82 home agent port unreachable (ICMP error)
                       88 home agent unreachable (other ICMP error)

                       Service denied by the home agent:

                       128 reason unspecified
                       129 administratively prohibited
                       130 insufficient resources
                       131 mobile node failed authentication
                       132 foreign agent failed authentication
                       133 identification mismatch
                       134 poorly formed request
                       135 too many simultaneous mobility bindings
                       136 unknown home agent address

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

      Lifetime         The seconds remaining before the registration is
                       considered expired.  A value of zero confirms a
                       request for deregistration.  A value of all ones
                       indicates infinity.

      Home Address     The IP address of the mobile node.

      Home Agent       The IP address of a home agent.

      Identification   The registration identification is copied from
                       the request message, for use by the mobile
                       node in matching its reply with an outstanding
                       request.



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      Extensions       The fixed portion of the Registration Reply
                       is followed by one or more of the extensions
                       listed in subsection 3.5.  The Mobile-Home
                       Authentication Extension MUST be included in all
                       Registration Replies returned by the home agent.
                       See the sections on foreign agent and home agent
                       considerations (3.7.2.2 and 3.8.3.3) for the
                       ordering rules on extensions.


3.5. Registration Extensions

   Each authenticator required in the authentication extensions which
   follow is defined to be a value computed from a stream of bytes
   including:

    - the shared secret,
    - the UDP payload (that is, the registration request or reply data),
    - all prior extensions in their entirety, and
    - the type and length of this extension,

   but not including the Authenticator field itself nor the UDP header.
   See subsection 5.1 for information about support requirements for
   message authentication codes, etc.  which are to be used with the
   various authentication extensions.


3.5.1. Mobile-Home Authentication Extension

   This extension must be present in all registration requests and
   replies, and is intended to eliminate problems [3] which result from
   the uncontrolled propagation of remote redirects in the Internet.
   The location of the authentication extension marks the end of the
   authenticated data.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |         SPI  ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          ... SPI (cont.)          |       Authenticator ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type            32

      Length          The number of data bytes in the extension.





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      SPI             Security Parameter Index.  (4 bytes) An opaque
                      identifier other than zero (reserved).

      Authenticator   (variable length) (see 3.5)


3.5.2. Mobile-Foreign Authentication Extension

   This extension may be found in registration requests and replies
   where a security association exists between the mobile node and a
   foreign agent.  See subsection 5.1 for information about support
   requirements for message authentication codes, etc.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |         SPI  ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          ... SPI (cont.)          |       Authenticator ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type            33

      SPI             Security Parameter Index.  (4 bytes) An opaque
                      identifier other than zero (reserved).

      Length          The number of data bytes in the extension.

      Authenticator   (variable length) (see 3.5)


3.5.3. Foreign-Home Authentication Extension

   This extension may be found in registration requests and replies
   where a security association exists between the foreign agent and
   a home agent.  See subsection 5.1 for information about support
   requirements for message authentication codes, etc.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |         SPI  ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          ... SPI (cont.)          |       Authenticator ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type            34




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      SPI             Security Parameter Index.  (4 bytes) An opaque
                      identifier other than zero (reserved).

      Length          The number of data bytes in the extension.

      Authenticator   (variable length) (see 3.5)


3.6. Mobile Node Considerations

   A mobile node must be configured with its home address, a netmask,
   and a mobility security association for each home agent.  In
   addition, a mobile node MAY be configured with the IP address of one
   or more of its home agents; otherwise, the mobile node MAY discover a
   home agent using the procedures described in section 3.6.1.2.

   For each pending registration, the mobile node maintains the
   following:

    - link-layer address of foreign agent, if applicable
    - Care-of Address
    - registration Identification
    - registration Lifetime

   A mobile node initiates registration whenever it detects a change in
   its network connectivity.  See section 2.4.2 for methods by which
   mobile nodes can make such a determination.  When it is away from
   home, the mobile node's Registration Request allows its home agent to
   create or modify a mobility binding.  When it is at home, the mobile
   node's (de)Registration Request allows its home agent to erase any
   previous mobility binding(s).  A mobile node operates without the
   support of mobility functions when it is at home.

   There are other conditions under which the mobile node SHOULD
   (re)register with its foreign agent, such as when the mobile node
   detects that the agent has rebooted (as specified in section 2.4.4)
   and when the current registration's Lifetime is near expiration.

   In the absence of link-layer indications of changes in point of
   attachment, Agent Advertisements from new agents do not necessarily
   affect a current registration.  In the absence of link-layer
   indications, a mobile node MUST NOT attempt to register more often
   than once per second.

   A mobile node MAY register with a different agent when
   transport-layer protocols indicate excessive retransmissions.
   A mobile node MUST NOT register with a new foreign agent because
   it has received an ICMP Redirect from the foreign agent that is



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   currently providing service to it.  Within these constraints, the
   mobile node MAY register again at any time.

   Please refer to the examples in appendix D to see how the fields in
   registration messages would be set up in some typical registration
   scenarios.


3.6.1. Sending Registration Requests

   The following sections provide additional detail for the values
   the mobile node MUST supply in the fields of Registration Request
   messages.


3.6.1.1. IP Fields

   This section provides the specific rules by which mobility agents
   pick values for the IP header fields of a Registration Request.

   IP Source Address:

    - When registering on a foreign network with a care-of address
      associated to one of its own interfaces, the IP source address
      MUST be the care-of address.
    - In all other circumstances, the IP source address MUST be the
      mobile node's home address.

   IP Destination Address:

    - When the IP address is unknown (e.g., the agent was discovered
      via a link-layer protocol), the "All Mobility Agents" multicast
      address (224.0.0.11) MUST be used.  In such a case, the mobile
      node SHOULD use the agent's link-layer unicast address in order to
      deliver the datagram to the correct agent.
    - When registering with a foreign agent, the address of the agent
      as learned from the IP source address of the corresponding Agent
      Advertisement MUST be used.
    - When registering directly with the home agent, the destination
      address is set to the (unicast) address that the mobile node uses
      for its home agent.
    - If the mobile node is connected to its home network or if the
      mobile node is registering without a foreign agent, and the mobile
      node is attempting to perform home agent discovery, then the
      IP destination address is set to the subnet-directed broadcast
      address of the home network.  This address MUST NOT be used as the
      destination IP address if the mobile node is registering via a
      foreign agent.



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   IP Time to Live:

    - The IP TTL field MUST be set to 1 if the IP destination address is
      set to the "All Mobility Agents" multicast address as described
      above.  Otherwise a suitable value should be chosen in accordance
      with standard IP practice [19].


3.6.1.2. Registration Request Fields

   This section provides specific rules by which mobile nodes pick
   values for the fields within the fixed portion of a Registration
   Request.

   A mobile node MAY set the 'S' bit in order to request that the home
   agent maintain prior mobility binding(s).  Otherwise, the home agent
   deletes any previous binding(s) and replaces them with the new
   binding specified in the Registration Request.  Multiple simultaneous
   mobility bindings are likely to be useful when a mobile node moves
   within range of multiple cellular systems.  IP explicitly allows
   duplication of datagrams.  When the home agent allows simultaneous
   bindings, it will encapsulate a separate copy of each arriving
   datagram to each care-of address, and the mobile node will receive
   multiple copies of its datagrams.

   The mobile node SHOULD set the 'D' bit if it is registering with
   a care-of address associated with one of its own interfaces.
   Otherwise, this bit MUST NOT be set.

   A mobile node MAY set the 'B' bit if the mobile node would like to
   receive a copy of IP broadcasts on its home network.  Note that in
   order to "shield" local broadcast packets from nodes on the foreign
   network (particularly the foreign agent), the home agent is required
   to tunnel broadcasts either directly to a care-of address associated
   with one of the mobile node's interfaces (hence the 'D' bit) -or- it
   must recursively tunnel such packets first to the mobile node's home
   address and then to the (foreign agent-provided) care-of address.
   The mobile node must be capable of de-tunneling packets in order to
   obtain the original broadcast datagram.  For this reason, the 'B' bit
   MUST NOT be set unless the mobile node is capable of de-tunneling
   packets.

   The mobile node MAY request alternative forms of encapsulation by
   setting the 'M' bit and/or the 'G' bit, but only if the mobile node
   is decapsulating its own packets (with a care-of address associated
   with one of its own interfaces) or if its foreign agent has indicated
   support for these forms of encapsulation by setting the corresponding




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   bits in the Mobile Service Extension of an Agent Advertisement.
   Otherwise, the mobile node MUST NOT set these bits.

   The Lifetime field is chosen as follows:

    - If the mobile node is registering with a foreign agent, the
      Lifetime SHOULD NOT exceed the value learned in the Agent
      Advertisement.  When the method by which the care-of address is
      learned does not include a Lifetime, the default ICMP Router
      Advertisement Lifetime (1800 seconds) MAY be used.
    - The mobile node MAY ask a home agent to terminate forwarding
      service to a particular care-of address, by sending a registration
      with a Lifetime of zero (see section 3.8.2).
    - Similarly, a Lifetime of zero is used when the mobile node
      deregisters all care-of addresses upon returning home.

   The Home Agent field is set to the address of one of the mobile
   node's home agents, if the mobile node possesses this information.
   Otherwise, the mobile node MAY discover a home agent by setting this
   field to the subnet-directed broadcast address of the home network.
   (Each home agent will reject the mobile node's registration, but in
   the reply they will provide their unicast address for use by the
   mobile node in a future registration attempt).

   The Care-of Address field is set to the value of the particular
   care-of address that the mobile node wishes to (de)register.  In the
   special case when a mobile node wishes to deregister all care-of
   addresses, it sets this field to the value of the its home address.
   A mobile node on its home network need not register again with a home
   agent when a change of sequence number occurs, or the advertisement
   lifetime expires, or even when the home agent crashes, since it is
   not seeking service from the home agent.

   The mobile node chooses the Identification field in accordance with
   the style of replay protection it uses with its home agent.  This is
   part of the mobility security association the mobile node shares with
   its home agent.  See section 5.6 on replay protection for the method
   by which the mobile node computes the Identification field.


3.6.1.3. Extensions

   This section describes the ordering of any mandatory and any optional
   extensions that a mobile node appends to a Registration Request.
   This following ordering MUST be followed:

      a)   The IP header, followed by the UDP header, followed by the
           fixed-length portion of the Registration Request



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      b)   Any non-authentication extensions relevant to the home agent
           (which may or may not also be relevant to the foreign agent)

      c)   The Mobile-Home Authentication Extension

      d)   Any non-authentication extensions relevant only to the
           foreign agent.

      e)   The Mobile-Foreign Authentication Extension

   Note that items (a) and (c) MUST appear in every Registration Request
   sent by the mobile node.  Items (b), (d), and (e) are optional.
   However, item (e) MUST be included when the mobile node and the
   foreign agent share a security association.


3.6.2. Receiving Registration Replies

   Registration Replies will be received by the mobile node in response
   to its Registration Requests.  Registration Replies generally fall
   into three categories:

    - service request was accepted,
    - service request was denied by foreign agent, and
    - service request was denied by home agent.

   The remainder of this section describes handling by mobile nodes
   under these various categories, based upon the contents of the
   Registration Reply.


3.6.2.1. Validity Checks

   Registration Replies with an invalid, non-zero UDP checksum MUST be
   silently discarded.

   In addition, the low-order 32 bits of the Identification field in
   the Registration Reply MUST be compared to the low-order 32 bits of
   Identification field in the most recent Registration Request sent to
   the replying agent.  If they do not match, the Reply MUST be silently
   discarded.

   Also, the Registration Reply MUST be checked for authenticity.
   That is, the mobile node MUST check for the presence of a valid
   authentication extension, based upon the Code field in the Reply.
   The rules are as follows:





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      a)   If the mobile node and foreign agent share a security
           association, the mobile node MUST check for a valid
           Mobile-Foreign Authentication Extension.  If no such
           extension is found, the mobile node MUST silently discard the
           Reply and the mobile node SHOULD log the event as a security
           exception.

      b)   If the Code field indicates that service is denied by the
           home agent, or if the Code field indicates that the service
           request was accepted, the mobile node MUST check for a valid
           Mobile-Home Authentication Extension.  If no such extension
           is found, the mobile node MUST silently discard the Reply and
           the mobile node SHOULD log the event as a security exception.

   If the Code field indicates an authentication failure, either at the
   foreign agent or the home agent, then it is quite possible that any
   authenticators in the Registration Reply will also be in error.  This
   could happen, for example, if the shared secret between the mobile
   node and home agent was erroneously configured.  The mobile node
   SHOULD log such errors as security exceptions.


3.6.2.2. Service Request Accepted

   If the Code field indicates that service will be provided, the mobile
   node SHOULD configure its routing table appropriately for its current
   point of attachment (see subsection 4.2.1).

   If the mobile node is returning to its home network and that
   network is one which implements ARP, the mobile node MUST follow the
   procedures described in section 4.6 with regard to ARP, proxy ARP,
   and gratuitous ARP.

   If the mobile node has registered on a foreign network, it SHOULD
   re-register before the granted Lifetime expires.


3.6.2.3. Service Request Denied

   If the Code field indicates that service is being denied, the mobile
   node SHOULD log the error.  There are several scenarios under which
   the mobile node may be able to "repair" the error.  These include:

      Code 69:  (Denied by foreign agent, Lifetime too long)

         In this case, the Lifetime field in the Registration Reply
         will contain the maximum amount of time for which that foreign
         agent is willing to accept registrations.  The mobile node MAY



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         attempt to register with this same agent, using a Lifetime in
         the Registration Request that MUST be less than or equal to the
         value specified in the Reply.

      Code 133:  (Denied by home agent, Identification mismatch)

         In this case, the Identification field in the Registration
         Reply will contain a value that allows the mobile node to
         synchronize with the home agent, based upon the style of replay
         protection in effect.  (See section 5.6 for details).  The
         mobile node MUST adjust the parameters it uses to compute
         the Identification field based upon the information in the
         Registration Reply, before issuing any future Registration
         Requests.

      Code 136:  (Denied by home agent, Unknown home agent address)

         This code is returned by a home agent when the mobile
         node is performing home agent discovery as described in
         subsections 3.6.1.1 and 3.6.1.2.  In this case, the Home Agent
         field within the Reply will contain the unicast IP address of
         the home agent returning the reply.  The mobile node MAY then
         attempt to register with this home agent in future Registration
         Requests.  In addition, the mobile node SHOULD adjust the
         parameters it uses to compute the Identification field based
         upon the corresponding field in the Registration Reply, before
         issuing any future Registration Requests.


3.6.3. Registration Retransmission

   When no Registration Reply has been received within a reasonable
   time, another Registration Request is transmitted.  When timestamps
   are used, a new registration Identification is chosen for each
   retransmission; thus it counts as a new registration.  When nonces
   are used, the unanswered request is retransmitted unchanged;
   thus the retransmission does not count as a new registration (see
   subsection 5.6).  In this way a retransmission will not require the
   home agent to resynchronize with the mobile node by issuing another
   nonce.

   The maximum time until a new Registration Request is sent SHOULD be
   no greater than the requested Lifetime of the Registration Request.
   The minimum value SHOULD be large enough to account for the size
   of the packets, twice the round trip time for transmission at the
   link speed, and at least an additional 100 milliseconds to allow
   for processing the packets before responding.  Some circuits add
   another 200 milliseconds of satellite delay.  The minimum time



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   between Registration Requests MUST NOT be less than 1 second.  Each
   successive wait SHOULD be at least twice the previous wait, as long
   as that is less than the maximum.


3.7. Foreign Agent Considerations

   The foreign agent plays a passive role in Mobile IP registration.  It
   relays Registration Requests between mobile nodes and home agents,
   and, when it provides the care-of address, decapsulates datagrams for
   delivery to the mobile node.  It MAY also advertise its presence as
   described in subsection 2.3.

   The foreign agent MUST NOT originate a Registration Request or Reply
   that has not been prompted by the mobile node.  The foreign agent
   MUST NOT generate a Registration Request or Reply to indicate that
   the service Lifetime has expired.  A foreign agent MUST NOT originate
   a message that asks for deregistration of a mobile node; however, it
   MUST relay valid deregistration requests originated by a mobile node.


3.7.1. Configuration and Registration Tables

   Each foreign agent must offer a care-of address.  In addition, for
   each pending or current registration, the foreign agent must maintain
   a visitor list entry containing the following information obtained
   from the mobile node's Registration Request:

    - link-layer source address
    - IP Source Address (mobile node's Home Address)
    - UDP Source Port
    - Home Agent
    - Lifetime
    - Identification

   As with any host on the Internet, a foreign agent may also maintain
   a security association for each pending or current registrant,
   and use it to authenticate the Registration Requests and Replies
   of the mobile node or its home agent (subsections 3.3, 3.4).
   The foreign agent may use an available security association
   with the home agent to compute the authentication data for the
   Foreign-Home Authentication Extension.  Even if a foreign agent
   implements authentication, it might not use authentication with each
   registration, because of the key management difficulties.







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3.7.2. Receiving Registration Requests

   If the foreign agent is able to satisfy an incoming Registration
   Request, it then relays the Request to the home agent.  Otherwise, it
   denies the request by sending a Registration Reply to the mobile node
   with an appropriate rejection Code.  The following sections describe
   this behavior in more detail.

   If a foreign agent receives a deregistration request from a mobile
   node in its visitor list, the visitor list entry SHOULD NOT be purged
   until the home agent sends back a Registration Reply with a Code
   indicating success.


3.7.2.1. Validity Checks

   Registration Requests with an invalid, non-zero UDP checksum MUST be
   silently discarded.

   Also, the Registration Request MUST be checked for authenticity.
   That is, the foreign agent MUST check for the presence of a valid
   Mobile-Foreign Authentication Extension if it shares a security
   association with the mobile node.  If, under these circumstances, no
   such extension is found, the foreign agent MAY reject the Request by
   sending a Registration Reply to the mobile node with Code 67.  The
   foreign agent SHOULD do no further processing with such a Request.


3.7.2.2. Forwarding a Valid Request to the Home Agent

   If the foreign agent is able to satisfy the mobile node's
   Registration Request, it relays the Request to the mobile node's home
   agent.  The foreign agent MUST NOT modify any of the fields beginning
   within the fixed portion of the Registration Request up through and
   including the Mobile-Home Authentication Extension.  Otherwise, an
   authentication failure is very likely to occur at the home agent.  In
   addition, the foreign agent MUST perform the following additional
   procedures:

    - It MUST consume any extensions following the Mobile-Home
      Authentication Extension,
    - It SHOULD append any of its own non-authentication extensions of
      relevance to the home agent, if applicable, and
    - It MUST append the Foreign-Home Authentication Extension, if the
      foreign agent shares a security association with the home agent.

   Specific fields within the IP header and the UDP header of the
   relayed Registration Request MUST be set as follows:



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      IP Source Address

         The foreign agent's address on the interface from which the
         packet will be sent.

      IP Destination Address

         Copied from the Home Agent field within the Registration
         Request.

      UDP Source Port

         <variable>

      UDP Destination Port

         434


3.7.2.3. Denying Invalid Requests

   If the foreign agent is unable to satisfy the mobile node's
   Registration Request, it SHOULD send the mobile node a Registration
   Reply with a suitable rejection Code.  In such a case, the
   Home Address, Home Agent, and Identification fields within the
   Registration Reply are copied from the corresponding fields of the
   Registration Request.

   If a foreign agent detects unknown bits set in the Reserved Bits
   field of the registration request, it MUST deny the request with
   status code 134.  If the request is being denied because the
   requested Lifetime is too long, the foreign agent sets the Lifetime
   in the Reply to the maximum length of time for which it is willing to
   accept a registration, and sets the Code field to 69.  Otherwise, the
   Lifetime SHOULD be copied from the Lifetime field in the Request.

   Specific fields within the IP header and the UDP header of the
   Registration Reply MUST be set as follows:

      IP Source Address

         Copied from the IP Destination Address of Registration Request,
         unless the "All Agents Multicast" address was used.  In this
         case, the foreign agent's address (on the interface from which
         the packet will be sent) is used.






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      IP Destination Address

         Copied from the IP Source Address of the Registration Request.

      UDP Source Port

         434

      UDP Destination Port

         Copied from the UDP Source Port of the Registration Request.


3.7.3. Receiving Registration Replies

   The foreign agent updates its visitor list when it receives a
   valid Registration Reply from a home agent.  It then relays the
   Registration Reply to the mobile node.  The following sections
   describe this behavior in more detail.

   If upon relaying a Registration Request to a home agent, the foreign
   agent receives an ICMP error message instead of a Registration Reply,
   then the foreign agent sends to the mobile node a Registration Reply
   with an appropriate "Home Agent Unreachable" failure Code (within the
   range 80-95, inclusive).  See section 3.7.2.3 for details of building
   the Registration Reply.


3.7.3.1. Validity Checks

   Registration Replies with an invalid, non-zero UDP checksum MUST be
   silently discarded.

   A Registration Reply MUST be silently discarded if the low-order
   32 bits of the Identification field do not match that of a pending
   Registration Request.

   Also, the Registration Reply MUST be checked for authenticity.
   That is, the foreign agent MUST check for the presence of a valid
   Foreign-Home Authentication Extension if it shares a security
   association with the home agent.  If, under these circumstances, no
   such extension is found, the foreign agent MAY reject the Request
   by sending a Registration Reply to the mobile node with Code
   68.  The foreign agent MUST NOT perform further processing on the
   Reply, though the foreign agent SHOULD log the error as a security
   exception.





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3.7.3.2. Forwarding Replies to the Mobile Node

   A Registration Reply which satisfies the validity checks of
   section 3.8.2.1 is relayed to the mobile node.  If the reply contains
   a status code indicating that service will be provided, then the
   foreign agent updates its visitor list accordingly.

   The foreign agent MUST NOT modify any of the fields beginning
   with the fixed portion of the Registration Reply up through and
   including the Mobile-Home Authentication Extension.  Otherwise,
   an authentication failure is very likely to occur at the mobile
   node.  In addition, the foreign agent SHOULD perform the following
   additional procedures:

    - It MUST consume any extensions following the Mobile-Home
      Authentication Extension,
    - It SHOULD append its own non-authentication extensions of
      relevance to the mobile node, if applicable, and
    - It MUST append the Mobile-Foreign Authentication Extension, if the
      foreign agent shares a security association with the mobile node.

   Specific fields within the IP header and the UDP header of the
   relayed Registration Reply are set according to the same rules set
   forth in section 3.7.2.3.


3.8. Home Agent Considerations

   Home agents also play a reactive role in the registration process.
   They receive Registration Requests from mobile nodes (perhaps relayed
   by a foreign agent), update their mobility bindings appropriately,
   and issue suitable Registration Replies in response.

   A home agent MUST NOT originate a Registration Reply that has not
   been prompted by the mobile node.  The home agent MUST NOT generate a
   Registration Reply to indicate that the service Lifetime has expired.


3.8.1. Configuration and Registration Tables

   Each home agent must have an IP address, and, if the home network is
   not a virtual network, the prefix size for the home network.  The
   home agent must keep track of the home address and mobility security
   association of each authorized mobile node.  When an authorized
   mobile node becomes registered, the home agent will create or modify
   its mobility binding list entry containing:

    - care-of address



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    - registration Identification
    - registration Lifetime

   The home agent MAY also maintain security associations with
   various foreign agents.  The home agent may use these security
   associations to compute the authentication data for the Foreign-Home
   Authentication Extension.


3.8.2. Receiving Registration Requests

   If the home agent is able to satisfy an incoming Registration
   Request, it then updates the mobile node's mobility binding(s)
   and issues a Registration Reply with a suitable Code.  Otherwise,
   it denies the request by sending a Registration Reply with an
   appropriate Code specifying the reason the request was rejected.  The
   following sections describe this behavior in more detail.


3.8.2.1. Validity Checks

   Registration Requests with an invalid, non-zero UDP checksum MUST be
   silently discarded by the home agent.

   Also, the Registration Request MUST be checked for authenticity.
   This minimally involves the following operations:

      a)   The home agent MUST check for the presence of a valid
           Mobile-Home Authentication Extension.  If no such extension
           is found, the home agent MAY reject the Request by sending
           a Registration Reply to the mobile node with Code 131.  The
           home agent MUST do no further processing with such a Request,
           though it SHOULD log the error as a security exception.

      b)   The home agent MUST check that the registration
           Identification field is correct under the context selected
           by the security parameter index within the Mobile-Home
           Authentication Extension.  See section 5.6 for a description
           of how this is performed.  If incorrect, the home agent MAY
           reject the Request by sending a Registration Reply to the
           mobile node with Code 133, and including an Identification
           field computed in accordance with the rules set forth in 5.6.
           The home agent MUST do no further processing with such
           a Request, though it SHOULD log the error as a security
           exception.

      c)   In addition, the home agent MUST check for the presence of a
           valid Foreign-Home Authentication Extension if it shares a



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           security association with the foreign agent.  If, under these
           circumstances, no such extension is found, the home agent MAY
           reject the Request by sending a Registration Reply to the
           mobile node with Code 132.  The home agent MUST do no further
           processing with such a Request, but is SHOULD log the error
           as a security exception.

   In addition to validating the authenticity of a Registration Request,
   home agents MUST NOT grant service for Registration Requests that are
   sent to the subnet-directed broadcast address of the home network
   (as opposed to being unicast to the home agent).  The home agent MAY
   reject such a request by returning status code 136.  In this case,
   the Registration Reply will contain the home agent's unicast address,
   so that the mobile node can re-issue the Registration Request with
   the correct home agent address.


3.8.2.2. Accepting a Valid Request

   If the Registration Request satisfies the validity checks in
   section 3.8.2.1, and the home agent is able to accommodate the
   request, the home agent updates its mobility binding list for the
   requesting mobile node and returns a Registration Reply to the mobile
   node.  In this case, the reply Code will be either 0 if the home
   agent supports simultaneous mobility bindings or 1 if it does not.
   See section 3.8.3 for details of building the Registration Reply
   message.

   The home agent updates its mobility bindings as follows:

    - If the Lifetime is zero and the Care-of Address equals the mobile
      node's home address, the home agent deletes all of the entries in
      the mobility binding list for the requesting mobile node.  This
      is how a mobile node requests that its home agent cease providing
      mobility services.
    - If the Lifetime is zero and the Care-of Address does not equal the
      mobile node's home address, the home agent deletes only the entry
      containing the specified Care-of Address from the mobility binding
      list for the requesting mobile node.  Any other active entries
      containing other care-of addresses will remain active.
    - If the Lifetime is nonzero, the home agent adds an entry
      containing the requested Care-of Address to the mobility binding
      list for the mobile node.  If the 'S' bit is set to one, and the
      home agent supports simultaneous mobility bindings, the previous
      mobility binding entries remain active.  Otherwise, the home agent
      removes all previous entries in the mobility binding list for the
      mobile node.




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   In all cases, the home agent sends a Registration Reply to the
   source of the Registration Request, which might indeed be a
   different foreign agent than that whose care-of address is being
   (de)registered.  If the home agent shares a security association with
   the foreign agent whose care-of address is being deregistered --
   wherein said foreign agent is different from the one which relayed
   the Registration Request -- the home agent MAY additionally send a
   Registration Reply to the foreign agent whose care-of address is
   being deregistered.  The home agent MUST NOT send such a Reply if it
   does not share a security association with the foreign agent.  If no
   Reply is sent, the foreign agent's visitor list will expire naturally
   when the original Lifetime expires.

   It is not an error for the mobile node to request a Lifetime longer
   than the home agent is willing to accept.  In such a case, the home
   agent simply reduces the Lifetime to a permissible amount and returns
   this amount in the Registration Reply.  This informs the mobile node
   when it should reregister.  The home agent MUST NOT increase the
   Lifetime above that specified by the mobile node in the Registration
   Request.

   If the Registration Request duplicates an accepted current
   Registration Request, the new Lifetime MUST NOT extend beyond the
   Lifetime originally granted.

   In addition, if the home network implements ARP [17], and the
   Registration Request asks the home agent to create a mobility binding
   for a mobile node which previously had no binding (the mobile node
   was previously assumed to be at home), then the home agent MUST
   follow the procedures described in section 4.6 with regard to ARP,
   proxy ARP, and gratuitous ARP. If the mobile node already had a
   previous mobility binding, the home agent MUST continue to follow the
   rules for proxy ARP described in section 4.6.


3.8.2.3. Denying an Invalid Request

   If the Registration Reply does not satisfy all of the validity checks
   in subsection 3.8.2.1, or the home agent is unable to accommodate the
   request, the home agent returns a Registration Reply to the mobile
   node with a Code that indicates the reason for the error.  If a
   foreign agent was involved in relaying the request, this allows the
   foreign agent to delete its pending visitor list entry.  Also, this
   informs the mobile node of the reason for the error such that it may
   attempt to fix the error and issue another request.

   This section lists a number of reasons the home agent might reject a
   request and provides the Code value it should use in each instance.



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   See subsection 3.8.3 for additional details on building the
   Registration Reply message.

   Many reasons for rejecting a registration are administrative
   in nature.  For example, a home agent can limit the number of
   simultaneous registrations for a mobile node, by rejecting any
   registrations that would cause its limit to be exceeded, and
   returning a Registration Reply with error code 135.  Similarly, a
   home agent may refuse to grant service to mobile nodes which have
   entered unauthorized service areas by returning a Registration Reply
   with error code 129.

   If a home agent detects unknown bits set in the Reserved Bits field
   of the registration request, it MUST deny the request with status
   code 134.


3.8.3. Sending Registration Replies

   If the home agent is able to satisfy an incoming Registration
   Request, it then updates the mobile node's mobility binding(s)
   and issues a Registration Reply with a suitable Code.  Otherwise,
   it denies the request by sending a Registration Reply with an
   appropriate Code specifying the reason the request was rejected.  The
   following sections provide additional detail for the values the home
   agent MUST supply in the fields of Registration Reply messages.


3.8.3.1. IP/UDP Fields

   This section provides the specific rules by which mobile nodes pick
   values for the IP and UDP header fields of a Registration Reply.

      IP Source Address        Copied from the IP Destination Address of
                               Registration Request, unless a multicast
                               or broadcast address was used.  In such a
                               case, the home agent's address by which
                               it is known to the requesting mobile node
                               is used.

      IP Destination Address   Copied from the IP Source Address of the
                               Registration Request.

      UDP Source Port          Copied from the UDP Destination Port of
                               the Registration Request.

      UDP Destination Port     Copied from the UDP Source Port of the
                               Registration Request.



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3.8.3.2. Registration Reply Fields

   This section provides specific rules by which home agents pick values
   for the fields within the fixed portion of a Registration Reply.

   The Type field in a Registration Reply is always set to 3.  The Code
   field is chosen according to the rules set forth in the previous
   sections.  When responding to accepted registrations, a home agent
   SHOULD respond with Code 1 if it does not support simultaneous
   registrations.

   The Lifetime field is copied from the corresponding field in the
   Registration Request, unless the requested value is greater than
   the maximum length of time the home agent is willing to provide the
   requested service.  In such a case, the Lifetime MUST be set to the
   length of time that service will actually be provided by the home
   agent.

   The Home Address field is copied from the corresponding field in the
   Registration Request.

   If the Home Agent field in the Registration Request contains a
   unicast address of this home agent, then that field is copied into
   the Home Agent field of the Registration Reply.  Otherwise, the home
   agent supplies its unicast address in the Home Agent field of the
   Registration Reply.  In this latter case, the home agent MUST reject
   the registration with a suitable code (e.g.  Code 136) to prevent the
   mobile node from being simultaneously registered with two or more
   home agents.


3.8.3.3. Extensions

   This section describes the ordering of any mandatory and any optional
   extensions that a home agent appends to a Registration Reply.  The
   following ordering MUST be followed:

      a)   The IP header, followed by the UDP header, followed by the
           fixed-length portion of the Registration Reply,

      b)   Any non-authentication extensions relevant to the mobile node
           (which may or may not also be relevant to the foreign agent),
           and

      c)   The Mobile-Home Authentication Extension

      d)   Any non-authentication extensions relevant only to the
           foreign agent.



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      e)   The Foreign-Home Authentication Extension

   Note that items (a) and (c) MUST appear in every Registration Reply
   sent by the home agent.  Items (b), (d), and (e) are optional.
   However, item (e) MUST be included when the home agent and the
   foreign agent share a security association.


4. Routing Considerations

   This section describes how mobile nodes, home agents, and (possibly)
   foreign agents cooperate to route packets to/from mobile nodes that
   are connected to a foreign network.  The mobile node informs its
   home agent of its current location using the registration procedure
   described in section 3.  See the protocol overview in section 1.7 for
   the relative locations of the mobile node's home address with respect
   to its home agent, and the mobile node itself with respect to any
   foreign agent with which it might attempt to register.


4.1. Encapsulation Types

   Support for IP in IP encapsulation [15] is required in home agents
   and foreign agents, and any mobile node which can associate a care-of
   address to one of its own interfaces.  Minimal encapsulation [16]
   and GRE encapsulation [10] are alternate encapsulation methods which
   MAY optionally be supported by mobility agents and mobile nodes.
   Minimal encapsulation MUST NOT be used when the original datagram is
   a fragment.  The use of these alternative forms of encapsulation,
   when requested by the mobile node, is otherwise at the discretion of
   the home agent.


4.2. Unicast Packet Routing

4.2.1. Mobile Node Considerations

   When connected to its home network, a mobile node operates without
   the support of mobility services.  That is, it operates just like
   any other (fixed) host or router.  The method by which a mobile node
   selects a default router when connected to its home network, or when
   away from home and using a care-of address associated to one of its
   own interfaces, is outside the scope of this document.  ICMP Router
   Advertisement [7] is one such method.

   When registered on a foreign network, the mobile node chooses a
   default router by examining the Agent Advertisements sent by its
   foreign agent.  The default router is selected by the procedure



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   described in RFC1256; that is, the mobile node SHOULD choose as its
   default router the highest preference router address listed in the
   ICMP Router Advertisement portion of the Agent Advertisement.  The
   mobile node MAY add the IP source address of the Agent Advertisement
   to this (possibly empty) list of router addresses wherein the
   IP source address is considered to be the worst choice (lowest
   preference) for a default router.  In any case, the mobile node MAY
   choose its foreign agent as its default router.

   Note that Van Jacobson header compression [11] will not function
   properly unless all TCP packets to and from the mobile node pass,
   respectively, through the same first and last-hop router.  The mobile
   node, therefore, SHOULD select its foreign agent as its default
   router if it performs Van Jacobson header compression with its
   foreign agent.

   In order for a mobile node to associate a care-of address to one of
   its own interfaces, it MUST be able to detunnel packets sent to this
   address.  The method by which the mobile node obtained its local
   care-of address SHOULD also be capable of supplying the mobile node
   with the address of a default router.


4.2.2. Foreign Agent Considerations

   Upon receipt of an encapsulated packet sent to its advertised care-of
   address, a foreign agent MUST compare the inner destination address
   to those entries in its visitor list.  When the destination does not
   match any node currently in the visitor list, the foreign agent MUST
   NOT forward the datagram without modifications to the original IP
   header, because otherwise a routing loop is likely to result.  The
   datagram SHOULD be silently discarded.  ICMP Destination Unreachable
   MUST NOT be sent when a foreign agent is unable to forward an
   incoming tunneled datagram.  Otherwise, the foreign agent naturally
   forwards the decapsulated packet to the mobile node.

   The foreign agent MUST NOT advertise to other routers in its routing
   domain, nor to any other mobile node, the presence of a mobile router
   (see subsection 4.5).

   The foreign agent MUST route packets it receives from registered
   mobile nodes.  At a minimum, this means that the foreign agent
   must verify the IP Header Checksum, decrement the IP Time To Live,
   recompute the IP Header Checksum, and forward such packets to
   a default router.  In addition, the foreign agent should send
   appropriate ICMP error messages to the mobile node.





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4.2.3. Home Agent Considerations

   Packets destined for a mobile node will arrive at a home agent that
   advertises connectivity to the home network indicated by the address
   of the mobile nodes.  The home agent must examine the IP header of
   all arriving traffic to see if it contains a destination address
   equal to the home address of any of its mobile nodes.

   If so, the home agent tunnels the datagram to the mobile node's most
   recently registered care-of address.  If the home agent supports the
   optional capability of multiple simultaneous mobility bindings, it
   tunnels a copy to each care-of address in the mobile node's mobility
   binding list.  If the mobile node has no current mobility bindings,
   the home agent assumes the mobile node is at home and simply forwards
   the datagram directly to it.  In this case, however, it is likely
   that the datagram will never be received by the home agent.

   See section 4.1 about methods of encapsulation that may be used for
   tunneling.  Maintenance of "soft tunnel state" (described in [15])
   effectively reduces transmission errors in the tunnel.

   If the lifetime for a given mobility binding expires before the home
   agent has received another Registration Request, then that binding is
   erased from the mobility binding list.  No special Registration Reply
   is sent to the foreign agent.  The entry in its visitor list will
   expire naturally, and probably at the same time.  When a mobility
   binding's lifetime expires, the home agent drops it regardless of
   whether or not simultaneous bindings are supported.

   Suppose an encapsulated datagram arrives at the home agent, that
   is to be delivered to one of its mobile nodes.  If the destination
   of the inner header is not that same mobile node, the home agent
   may recursively encapsulate it for delivery to the mobile node's
   care-of address.  Otherwise, the home agent may simply alter the
   outer destination to the care-of address, unless the care-of address
   is the same as the origination point of the encapsulated datagram.
   In the latter case, if the home agent receives a datagram for one of
   its mobile nodes, and the packet's IP source address is identical to
   the care-of address contained in the mobility binding list, the home
   agent MUST silently discard that packet.  Otherwise, a routing loop
   is likely to result.


4.3. Broadcast packets

   When a home agent receives a broadcast packet, it may transmit the
   packet to only those mobile nodes on its mobility binding list that
   have requested broadcast service.  Mobile nodes request encapsulated



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   delivery of broadcast packets by setting the 'B' bit in their
   Registration Request packets (subsection 3.3).  It is a matter of
   configuration at the home agent as to which specific categories of
   broadcast packets will be sent to such mobile nodes.

   If the mobile node is using a care-of address associated with one of
   its own interfaces, as indicated by the 'D' bit in its Registration
   Request packet, the home agent simply tunnels appropriate broadcast
   IP datagrams to the mobile node's care-of address.  Otherwise, when
   the mobile node registered through a foreign agent, the home agent
   first encapsulates the broadcast datagram in a unicast datagram
   addressed to the mobile node's home address, and then tunnels this
   encapsulated datagram to the foreign agent.  This extra level of
   encapsulation is required so that foreign agent can determine which
   mobile node should receive the packet after it is decapsulated.  When
   received by the foreign agent, the unicast encapsulated datagram is
   detunneled and delivered to the mobile node in the same way as any
   other datagram.  In either case, the mobile node must decapsulate
   the datagram it receives in order to recover the original broadcast
   datagram.


4.4. Multicast Packet Routing

   As mentioned previously, a mobile node that is connected to its home
   network functions just like any other (stationary) host or router.
   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 visiting a foreign
   network.

   In order receive multicasts, a mobile node must join the multicast
   group.  Mobile nodes MAY join multicast groups in order to receive
   transmissions in one of two ways.  First, they MAY join the group
   via a (local) multicast router on the visited subnet.  This option
   assumes that there is a multicast router present on the visited
   subnet.  The mobile node SHOULD use a care-of address associated with
   one of its own interfaces (if it has one) as the source IP address of
   its IGMP [6] packets.  Otherwise, it MAY use its home address.

   Alternatively, a mobile node which wishes to receive multicasts can
   join groups via a bi-directional tunnel to its home agent, assuming
   that its home agent is a multicast router.  The mobile node tunnels
   IGMP packets to its home agent and the home agent forwards multicast
   packets down the tunnel to the mobile node.  The rules for multicast
   packet delivery to mobile nodes in this case are identical to those
   for broadcast packets (see section 4.3).  Namely, the home agent
   must tunnel the packet directly to the care-of address associated to



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   one of the mobile node's network interfaces, or, the packet must be
   tunneled first to the mobile node's home address and then recursively
   tunneled to the foreign agent-provided care-of address.

   A mobile node which wishes to send packets to a multicast group
   also has two options:  (1) send directly on the visited network; or
   (2) send via a tunnel to its home agent.  Because multicast routing
   in general depends upon the IP source address, a mobile node which
   sends multicast packets directly on the visited network MUST use
   a care-of address which is assigned to one of its own interfaces
   as the IP source address.  Similarly, a mobile node which tunnels
   a multicast packet to its home agent MUST use its home address as
   the IP source address of both the (inner) multicast packet and the
   (outer) encapsulating packet.  This second option assumes that the
   home agent is a multicast router.


4.5. Mobile Routers

   A mobile node can be a router, which is responsible for the mobility
   of one or more entire networks moving together, perhaps on an
   airplane, a ship, a train, an automobile, a bicycle, or a kayak.
   The nodes connected to a network served by the mobile router may
   themselves be fixed nodes or mobile nodes or routers.  In this
   subsection, such networks are called "mobile networks".

   A mobile router may provide a care-of address to mobile nodes
   connected to the mobile network.  In this case, when a correspondent
   host sends a packet to the mobile node, the actions described in the
   next paragraph should occur.

   Normal IP procedures will route the packet addressed to the mobile
   node from the correspondent host to the mobile node's home agent.
   This home agent's binding for the mobile node causes it to tunnel the
   packet to the mobile router.  Normal IP procedures will then route
   the packet from this home agent to the mobile router's home agent.
   That home agent's binding for the mobile router causes the packet
   to be doubly tunneled to the mobile router's care-of address.  For
   the sake of discussion, assume there is a foreign agent available at
   that care-of address.  The mobile router's foreign agent will then
   detunnel the packet and use its visitor list entry to deliver the
   packet to the mobile router.  The mobile router will then detunnel
   the packet and use its visitor list entry to deliver the packet
   finally to the mobile node.

   If a fixed node is connected to a mobile network then either of two
   methods may be used to cause packets from correspondent hosts to be
   routed to the fixed node.



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   A home agent may be configured that has a permanent registration for
   the fixed node that indicates the mobile router's address as the
   fixed host's care-of address.  The mobile router's home agent will
   usually be used for this purpose.  The home agent is then responsible
   for advertising connectivity using normal routing protocols to
   the fixed node.  Any packets sent to the fixed node will thus use
   recursive tunneling as described above.

   Alternatively, the mobile router may advertise connectivity to the
   entire mobile network using normal IP routing protocols through a
   bi-directional tunnel to its own home agent.  This method avoids the
   need for recursive tunneling of packets.


4.6. ARP, Proxy ARP, and Gratuitous ARP

   The use of ARP [17] requires special rules for correct operation when
   wireless or mobile nodes are involved.  The requirements specified
   in this section apply to all home networks in which ARP is used for
   address resolution.

   In addition to the normal use of ARP for resolving a target node's
   link-layer address from its IP address, this document distinguishes
   two special uses of ARP:

    -  A Proxy ARP [20] is an ARP Reply sent by one node on behalf
       of another node which is either unable or unwilling to answer
       its own ARP Requests.  The sender of a Proxy ARP reverses the
       Sender and Target Protocol Address fields as described in [17],
       but supplies its own link-layer address in the Sender Hardware
       Address field.  The node receiving the Reply will then associate
       the link-layer address of the replying node with the IP address
       of the original target node, causing it to transmit future
       datagrams for this target node to the node that sent the Proxy
       ARP Reply.

    -  A Gratuitous ARP is an ARP Reply that is sent without having been
       prompted by the receipt of any ARP Request.  The ARP Reply is
       transmitted as a local broadcast packet on the local link.  Any
       node receiving any ARP packet (Request or Reply) MUST update its
       local ARP cache with the sender IP and link-layer address in
       the ARP packet, if the receiving node has an entry for that IP
       address already in its ARP cache [17].  This requirement in the
       ARP protocol applies even if the ARP packet is an ARP Reply that
       does not match any ARP Request transmitted by the receiving node.

   While a mobile node is registered on a foreign network, its home
   agent uses proxy ARP [20] to reply to ARP Requests it receives that



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   seek the mobile node's link-layer address.  When receiving an ARP
   Request, the home agent MUST examine the target IP address of the
   Request, and if this IP address matches the home address of any
   mobile node for which it has a mobility binding, the home agent
   MUST transmit an ARP Reply on behalf of the mobile node.  After
   exchanging the sender and target addresses in the packet [20], the
   home agent MUST set the sender link-layer address in the packet to
   the link-layer address of its own interface over which the Reply will
   be sent.

   When a mobile node leaves its home network and registers a binding on
   a foreign network, its home agent uses gratuitous ARP to update the
   ARP caches of nodes on the home network.  This causes such nodes to
   associate the link-layer address of the home agent with the mobile
   node's home (IP) address.  When registering a binding for a mobile
   node for which the home agent previously had no binding (the mobile
   node was assumed to be at home), the home agent MUST transmit a
   gratuitous ARP on behalf of the mobile node.  The gratuitous ARP
   Reply packet is sent on the link on which the mobile node's home
   address is located.  In this ARP Reply, both the sender and target
   IP addresses MUST be set to the home address of the mobile node, and
   both the sender and target link-layer addresses MUST be set to the
   home agent's link-layer address on its interface to the link on which
   the mobile node's home address is located.  This ARP Reply packet
   SHOULD be retransmitted a small number of times to increase its
   reliability.

   When a mobile node returns to its home network, the mobile node and
   its home agent use gratuitous ARP to cause all nodes on the mobile
   node's home link to update their ARP caches to once again associate
   the mobile node's own link-layer address with the mobile node's home
   (IP) address.  Before transmitting the (de)Registration Request
   message to its home agent, the mobile node MUST transmit a gratuitous
   ARP Reply packet on its home network.  This ARP Reply packet MUST be
   transmitted as a local broadcast on this link.  Both the sender and
   target IP addresses MUST be set to the home address of the mobile
   node, and both the sender and target link-layer addresses MUST be
   set to the mobile node's link-layer address on its interface to
   the home network.  This ARP Reply packet SHOULD be retransmitted
   a small number of times to increase its reliability, but these
   retransmissions MAY proceed in parallel with the transmission and
   processing of its (de)Registration Request.

   When the mobile node's home agent receives and accepts this
   (de)Registration Request, the home agent MUST transmit a gratuitous
   ARP Reply packet on the mobile node's home network.  This ARP Reply
   packet MUST be transmitted as a local broadcast on this link.  As
   with the gratuitous ARP Reply sent by the mobile node above, both



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   the sender and target IP addresses in the home agent's gratuitous
   ARP Reply MUST be set to the home address of the mobile node, and
   both the sender and target link-layer addresses MUST be set to the
   mobile node's link-layer address on its interface to this link.  This
   ARP Reply packet SHOULD be retransmitted a small number of times to
   increase its reliability, but these retransmissions MAY proceed in
   parallel with the transmission and processing of its (de)Registration
   Reply.

   Finally, while the mobile node is away from home, it MUST NOT reply
   to ARP Requests that seek its own link-layer address, unless the ARP
   Request is sent by a foreign agent with which the mobile node is
   attempting to register or a foreign agent with which the mobile node
   has already successfully registered.

   The specific order in which each of the above requirements for the
   use of ARP, proxy ARP, and gratuitous ARP are applied, relative to
   the transmission and processing of the mobile node's Registration
   Request and Registration Reply messages when leaving home or
   returning home, are important to the correct operation of the
   protocol.

   To summarize the above requirements, when a mobile node leaves its
   home network, the following steps, in this order, MUST be performed:

    -  The mobile node decides to register away from home, perhaps
       because it has received an Agent Advertisement from a foreign
       agent and has not recently received one from its home agent.

    -  Before transmitting the Registration Request, the mobile node
       disables its own future processing of any ARP Requests it may
       subsequently receive requesting its link-layer address.

    -  The mobile node transmits its Registration Request.

    -  When the mobile node's home agent receives and accepts the
       Registration Request, it performs a gratuitous ARP on behalf
       of the mobile node, and begins using proxy ARP to reply to ARP
       Requests that it receives requesting the mobile node's link-layer
       address.  If, instead, the home agent rejects the Registration
       Request, no ARP processing (gratuitous nor proxy) is performed by
       the home agent.

   When a mobile node later returns to its home network, the following
   steps, in this order, MUST be performed:

    -  The mobile node decides to register at home, perhaps because it
       has received an Agent Advertisement from its home agent.



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    -  Before transmitting the Registration Request, the mobile node
       re-enables its own future processing of any ARP Requests it may
       subsequently receive requesting its link-layer address.

    -  The mobile node performs a gratuitous ARP for itself.

    -  The mobile node transmits its Registration Request.

    -  When the mobile node's home agent receives and accepts the
       Registration Request, it stops using proxy ARP to reply to
       ARP Requests that it receives requesting the mobile node's
       link-layer address, and then performs a gratuitous ARP on behalf
       of the mobile node.  If, instead, the home agent rejects the
       Registration Request, no ARP processing (gratuitous nor proxy) is
       performed by the home agent.




































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5. Security Considerations

   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.


5.1. Message Authentication Codes

   Home agents and mobile nodes MUST be able to perform authentication.
   The default algorithm is keyed MD5 [22], with a key size of 128
   bits.  The default mode of operation is to both precede and follow
   the data to be hashed, by the 128-bit key; that is, MD5 is to be
   used in suffix+prefix mode.  The foreign agent SHOULD also support
   authentication using keyed MD5 and key sizes of 128 bits or greater,
   with manual key distribution.  More authentication algorithms,
   algorithm modes, key distribution methods, and key sizes MAY also be
   supported.


5.2. Areas of security concern in this protocol

   The registration protocol described in this document will result
   in a mobile node's traffic being tunneled to its care-of address.
   This tunneling feature could be a significant vulnerability if the
   registration were not authentic.  Such remote redirection, for
   instance as performed by the mobile registration protocol, is widely
   understood to be a security problem in the current Internet [3].
   Moreover, the Address Resolution Protocol (ARP) is not authenticated,
   and can potentially be used to steal another host's traffic.  The use
   of "Gratuitous ARP" (see subsection 4.6) brings with it all of the
   risks associated with the use of ARP.


5.3. Key management

   This specification requires a strong authentication mechanism
   (keyed MD5) which precludes many potential attacks based on the
   Mobile IP registration protocol.  However, because key distribution
   is difficult in the absence of a network key management protocol,
   messages with the foreign agent are not all required to be
   authenticated.  In a commercial environment it might be important
   to authenticate all messages between the foreign agent and the home
   agent, so that billing is possible, and service providers don't
   provide service to users that are not legitimate customers of that
   service provider.



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5.4. Picking good random numbers

   The strength of any authentication mechanism is dependent on
   several factors, including the innate strength of the authentication
   algorithm, the secrecy of the key used, the strength of the key used,
   and the quality of the particular implementation.  This specification
   requires implementation of keyed MD5 for authentication, but does not
   preclude the use of other authentication algorithms and modes.  For
   keyed MD5 authentication to be useful, the 128-bit key must be both
   secret (that is, known only to authorized parties) and pseudo-random.
   If nonces are used in connection with replay protection, they must
   also be selected carefully.  Eastlake, et.al. [9] provides more
   information on generating pseudo-random numbers.


5.5. Privacy

   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 absolute location privacy is desired, the Mobile Node can create a
   tunnel to its Home Agent.  Then, packets destined for correspondent
   hosts will appear to emanate from the Home Network, and it may be
   more difficult to pinpoint the location of the mobile node.


5.6. Replay Protection for Registration Requests

   The Identification field is used to let the home agent verify that a
   registration message has been freshly generated by the mobile node,
   not replayed by an attacker from some previous registration.  Two
   methods are described here:  timestamps (mandatory) and "nonces"
   (optional).  All mobile nodes and home agents MUST implement
   timestamp-based replay protection.  These nodes MAY implement
   nonce-based replay protection (but see appendix A.2 for a patent that
   may apply to nonce-based replay protection).

   The style of replay protection in effect between a mobile node
   and its home agent is part of the mobile security association.  A
   mobile node and its home agent MUST agree on which method of replay
   protection will be used.  The interpretation of the Identification
   field depends on the method of replay protection as described in the
   subsequent subsections.

   Whatever method is used, the low-order 32 bits of the Identification
   MUST be copied unchanged from the Registration Request to the reply.
   The foreign agent uses those bits to match Registration Requests



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   with corresponding replies.  The mobile node MUST verify that the
   low-order 32 bits of any Registration Reply are identical to the bits
   it sent in the Registration Request.

   The Identification in a new registration request MUST NOT be the same
   as in an immediately preceding request, and SHOULD NOT repeat during
   the lifetime of the selected security context between the mobile
   node and the home agent.  Retransmission as in subsection 3.6.3 is
   allowed.


5.6.1. Replay Protection using Timestamps

   The basic principle of timestamp replay protection is that the node
   generating a message inserts the current time of day, and the node
   receiving the message checks that this timestamp is sufficiently
   close to its own time of day.  Obviously the two nodes must have
   adequately synchronized time of day clocks.  As usual all messages
   are protected against tampering by a cryptographic checksum.

   If timestamps are used, the mobile node sets the Identification
   field to a 64-bit value formatted as specified by the Network Time
   Protocol [14].  The low-order 32 bits of the NTP format represent
   fractional seconds, and those bits which are not available from a
   time source SHOULD be generated from a good source of randomness.

   Upon receipt of a Registration Request with a valid Mobile-Home
   Authentication Extension, the home agent MUST check the
   Identification field for validity.  In order to be valid, the
   timestamp contained in the Identification field MUST be close enough
   to the home agent's time of day clock and the timestamp MUST be
   greater than all previously accepted timestamps for the requesting
   mobile node.  Time tolerances and resynchronization details are
   specific to a particular mobile security association.

   If the timestamp is valid, the home agent copies the entire
   Identification field into the Registration Reply it returns to the
   mobile node.  If the timestamp is not valid, the home agent copies
   only the low-order 32 bits into the Registration Reply, and supplies
   the high-order 32 bits from its own time of day.  In this latter
   case, the home agent MUST reject the registration by returning Code
   133 (identification mismatch) in the Registration Reply.

   As described in section 3.6.2.1, the mobile node MUST verify that the
   low-order 32 bits of the Identification in the Registration Reply are
   identical to those in the rejected registration attempt, before using
   the high-order bits for clock resynchronization.




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5.6.2. Replay Protection using Nonces

   Implementors of this optional mechanism should examine appendix A.2
   for a patent that may be applicable to nonce-based replay protection.

   The basic principle of nonce replay protection is that Node A
   includes a new random number in every message to node B, and checks
   that Node B returns that same number in its next message to node
   A. Both messages use a cryptographic checksum to protect against
   alteration by an attacker.  At the same time Node B can send its own
   nonces in all messages to Node A (to be echoed by node A), so that it
   too can verify that it is receiving fresh messages.

   The home agent may be expected to have resources for computing
   pseudo-random numbers useful as nonces [9].  It inserts a new nonce
   as the high-order 32 bits of the identification field of every
   registration reply.  The home agent copies the low-order 32 bits of
   the Identification from the registration request message.  When the
   mobile node receives an authenticated registration reply from the
   home agent, it saves the high-order 32 bits of the identification for
   use as the high-order 32 bits of its next registration request.

   The mobile node is responsible for generating the low-order 32 bits
   of the Identification in each registration request.  Ideally it
   should generate its own random nonces.  However it may use any
   expedient method, including duplication of the random value sent by
   the home agent.  The method chosen is of concern only to the mobile
   node, because it is the node that checks for valid values in the
   registration reply.  The high-order and low-order 32 bits of the
   identification chosen SHOULD both differ from their previous values.
   The home agent uses a new high-order value and the mobile node uses
   a new low-order value for replay protection.  The foreign agent uses
   the low-order value to correctly match registration replies with
   pending requests (see subsection 3.7.1).

   If a registration message is rejected because of an invalid nonce,
   the reply always provides the mobile node with a new nonce to
   be used in the next registration.  Thus the nonce protocol is
   self-synchronizing.


6. Acknowledgments

   Special thanks to Steve Deering (Xerox PARC), along with Dan Duchamp
   and John Ioannidis (JI) (Columbia), for forming the working group,
   chairing it, and putting so much effort into its early development.





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   Thanks also to Kannan Alaggapan and Greg Minshall for their
   contributions to the group while performing the duties of
   chairperson.

   Thanks to the active members of the Mobile-IP working group,
   particularly those who contributed text, including (in alphabetical
   order)

    - Ran Atkinson (Naval Research Lab),
    - Dave Johnson (Carnegie Mellon University),
    - Frank Kastenholz (FTP Software)
    - Anders Klemets (KTH)
    - Chip Maguire (KTH - also, JI's advisor and early contributor)
    - Andrew Myles (Macquarie University),
    - Al Quirt (Bell Northern Research),
    - Yakov Rekhter (IBM), and
    - Fumio Teraoka (Sony).

   Thanks to Charlie Kunzinger and to Bill Simpson, the editors who
   produced the first drafts for of this document, reflecting the
   discussions of the Working Group.  Much of the new text in the latest
   drafts is due to Jim Solomon.

   Thanks to Greg Minshall (Novell), Phil Karn (Qualcomm), and Frank
   Kastenholz (FTP Software) for their generous support in hosting
   interim Working Group meetings.

   Implementors may note that Anders Klemets has an implementation
   of the protocol specified here for mobile nodes, foreign agents,
   and home agents running under SunOS v4.1.3.  He is willing to
   provide it to people wishing to perform beta testing.  Contact
   him at <klemets@sics.se> if you would like a copy.  There
   is also a version of mobile-IP which was developed by Vipul
   Gupta <vgupta@cs.binghamton.edu> at the State University of
   New York Binghamton.  The software (along with supporting
   documentation) is available from the Linux Mobile-IP home page at
   http://anchor.cs.binghamton.edu/~mobileip.


A. Patent Issues

   As of the time of publication, the IETF had been made aware of
   two patents that may be relevant to implementors of the protocol
   described in this technical specification.







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A.1. IBM Patent #5,159,592

   Charles Perkins, editor of this draft, is sole inventor of
   U.S. Patent No. 5,159,592, assigned to IBM. In a letter dated
   May 30, 1995, IBM brought this patent to the attention of the IETF,
   stating that this patent "relates to the Mobile IP." We understand
   that IBM did not intend to assert that any particular implementation
   of Mobile IP would or would not infringe the patent, but rather that
   IBM was meeting what it viewed as a duty to disclose information that
   could be relevant to the process of adopting a standard.

   Based on a review of the claims of the patent, IETF believes that
   a system of registering an address obtained from a foreign agent,
   as described in the draft, would not necessarily infringe any of
   the claims of the patent; and that a system in which an address is
   obtained elsewhere and then registered can be implemented without
   necessarily infringing any claims of the patent.  Accordingly,
   our view is that the proposed protocol can be implemented without
   necessarily infringing the Perkins Patent.

   Parties considering adopting this protocol must be aware that
   some specific implementations, or features added to otherwise
   non-infringing implementations, may raise an issue of infringement
   with respect to this patent or to some other patent.

   This statement is for the IETF's assistance in its standard-setting
   procedure, and should not be relied upon by any party as an opinion
   or guarantee that any implementation it might make or use would not
   be covered, or would not be asserted by IBM to be covered, by the
   Perkins Patent or any other patent.

   As the sole inventor of the patent, Charles Perkins also feels
   honor-bound to submit his opinion (not informed by legal counsel on
   the matter, nor by IBM corporate opinion) that the aforementioned
   patent is relevant to a wide range of technology involving the action
   of home agents assisting the routing of packets for mobile nodes
   addressed via a home network.


A.2. IBM Patent #5,148,479

   This patent, also assigned to IBM, may be relevant to those
   who implement nonce-based replay protection as described in
   section 5.6.2.  Note that nonce-based replay protection is an
   optional feature of this specification.  Conversely, timestamp-based
   replay protection (section 5.6.1) is a requirement of this
   specification.




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B. Link-Layer considerations

   The mobile node primarily uses link-layer mechanisms to decide that
   its point of attachment has changed.  Such indications include
   the Down/Testing/Up interface status [12], and changes in cell or
   administration.  The mechanisms will be specific to the particular
   link-layer technology, and are outside the scope of this document.


B.1. Point-to-Point Link-Layers

   The Point-to-Point-Protocol (PPP) [23] and its Internet Protocol
   Control Protocol (IPCP) [13], negotiates the use of IP addresses.

   The mobile node SHOULD first attempt to specify its home address.
   This allows an unrouted link to function correctly.

   When the home address is not accepted by the peer, but a transient
   IP address is dynamically assigned, that address MAY be used as the
   care-of address for registration.  When the peer specifies its own IP
   address, that address MUST NOT be assumed to be the care-of address
   of a foreign agent or the IP address of a home agent.

   When router advertisements are received which contain the Mobile
   Service Extension, registration with the agent SHOULD take place as
   usual.  If the link is bandwidth limited, this method is preferred
   over use of the transient care-of address.  The encapsulation will
   be removed by the peer, allowing header compression techniques to
   function correctly [11].


B.2. Multi-Point Link-Layers

   Another link establishment protocol, IEEE 802.11 [1], might yield the
   link address of an agent.  This link-layer address SHOULD be used to
   attempt registration.

   The receipt of an agent's address via a router advertisement
   supersedes that obtained via IEEE 802.11.


C. TCP Considerations

C.1. TCP Timers

   Most hosts and routers which implement TCP/IP do not permit easy
   configuration of the TCP timer values.  When high-delay (e.g.
   SATCOM) or low-bandwidth (e.g.  High-Frequency Radio) links are



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   in use, the default TCP timer values in many systems may cause
   retransmissions or timeouts, even when the link and network is
   actually operating properly with greater than usual delays because
   of the medium in use.  This can cause an inability to create or
   maintain connections over such links, and can also cause unneeded
   retransmissions which consume already scarce bandwidth.  Vendors are
   encouraged to make TCP timers more configurable.  Vendors of systems
   designed for the mobile computing markets should pick default timer
   values more suited to low-bandwidth, high-delay links.  Users of
   mobile nodes should be sensitive to the possibility of timer-related
   difficulties.


C.2. TCP Congestion Management

   Mobility nodes are likely to use media which have low bandwidth and
   are more likely to introduce errors, effectively causing more packets
   to be dropped.  This introduces a conflict with the mechanisms for
   congestion management found in modern versions of TCP. Now, when
   a packet is dropped, the correspondent's TCP implementation is
   likely to react as if there were a source of network congestion,
   and initiate the slow-start mechanisms [5] designed for controlling
   that problem.  However, those mechanisms are inappropriate for
   overcoming errors introduced by the links themselves, and have the
   effect of magnifying the discontinuity introduced by the dropped
   packet.  This problem has been analyzed by Caceres, et. al.   [4];
   there is no easy solution available, and certainly no solution likely
   to be installed soon on all correspondents.  While this problem has
   nothing to do with any of the specifications in this document, it
   does illustrate that providing performance transparency to mobile
   nodes involves understanding mechanisms outside the network layer.
   It also indicates the need to avoid designs which systematically drop
   packets; such designs might otherwise be considered favorably when
   making engineering tradeoffs.

















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D. Example Scenarios

   This section shows example Registration Requests for several common
   scenarios.


D.1. Registering with a Foreign Agent's Care-of Address

   The mobile node receives an Agent Advertisement from a foreign agent
   and wishes to register with that agent.  The mobile node wishes only
   standard encapsulation, does not want broadcasts, and does not want
   simultaneous mobility bindings:

       IP fields:
         Source Address = mobile node's home address
         Destination Address = copied from the IP source address of the
           Agent Advertisement
         Time to Live = 1
       UDP fields:
         Source Port = <any>
         Destination Port = 434
       Registration Request fields:
         Type = 1
         S=0,B=0,D=0,M=0,G=0
         Lifetime = the Lifetime copied from the Mobile Service
           Extension of the Agent Advertisement
         Home Address = the mobile node's home address
         Home Agent = IP address of mobile node's home agent
         Care-of Address = the Care-of Address copied from the Mobile
           Service Extension of the Agent Advertisement
         Identification = Network Time Protocol timestamp or Nonce
       Extensions:
         The Mobile-Home Authentication Extension


D.2. Registering with a Dynamic Care-of Address

   The mobile node enters a foreign network that contains no foreign
   agents.  The mobile node obtains an address from a DHCP server for
   use as its care-of address.  The mobile node supports all forms of











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   encapsulation, desires a copy of all broadcasts on the home network,
   and does not want simultaneous mobility bindings:

       IP fields:
         Source Address = care-of address obtained from DHCP server
         Destination Address = IP address of home agent
         Time to Live = 64
       UDP fields:
         Source Port = <any>
         Destination Port = 434
       Registration Request fields:
         Type = 1
         S=0,B=1,D=1,M=1,G=1
         Lifetime = 1800 (seconds)
         Home Address = the mobile node's home address
         Home Agent = IP address of mobile node's home agent
         Care-of Address = care-of address obtained from DHCP server
         Identification = Network Time Protocol timestamp or Nonce
       Extensions:
         The Mobile-Home Authentication Extension


D.3. Deregistration

   The mobile node returns home and wishes to deregister all care-of
   addresses with its home agent.

       IP fields:
         Source Address = mobile node's home address
         Destination Address = IP address of home agent
         Time to Live = 1
       UDP fields:
         Source Port = <any>
         Destination Port = 434
       Registration Request fields:
         Type = 1
         S=0,B=0,D=0,M=0,G=0
         Lifetime = 0
         Home Address = the mobile node's home address
         Home Agent = IP address of mobile node's home agent
         Care-of Address = the mobile node's home address
         Identification = Network Time Protocol timestamp or Nonce
       Extensions:
         The Mobile-Home Authentication Extension







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E. Applicability of Prefix Lengths Extension

   Caution is indicated with the use of the Prefix Lengths extension
   over wireless links, due to the irregular coverage areas provided
   by many wireless transmitters.  As a result, it is possible
   that two foreign agents advertising the same prefix might indeed
   provide different connectivity to prospective mobile nodes.  The
   Prefix-Lengths Extension SHOULD NOT be included in the advertisements
   sent by agents in such a configuration.

   Foreign agents using different wireless interfaces would have to
   cooperate using special protocols to provide identical coverage
   in space, and thus be able to claim to have wireless interfaces
   situated on the same subnetwork.  In the case of wired interfaces,
   a mobile node disconnecting and subsequently connecting to a new
   point of attachment to another may well send in a new registration
   request no matter whether the new advertisement is on the same
   medium as the last recorded advertisement.  And, finally, in areas
   with dense populations of foreign agents it would seem unwise to
   require the propagation via routing protocols of the subnet prefixes
   associated with each individual wireless foreign agent; such a
   strategy could lead to quick depletion of available space for routing
   tables, unwarranted increases in the time required for processing
   routing updates, and longer decision times for route selection if
   routes (which are almost always unnecessary) are stored for wireless
   "subnets".

























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References

    [1] Draft Standard, Wireless LAN MAC and PHY Specifications, Rev.
        D1.  IEEE Document P802.11/D1-94/12, Dec 1994.

    [2] R. Atkinson.  IP Authentication Header.  RFC 1826, August 1995.

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

    [4] Ramon Caceres and Liviu Iftode.  The Effects of Mobility on
        Reliable Transport Protocols.  In Proceedings of the 14th
        International Conference on Distributed Computing Systems, June
        1994.

    [5] Douglas E. Comer.  Principles, Protocols, and Architecture,
        volume 1 of Internetworking with TCP/IP.  Prentice Hall,
        Englewood Cliffs, N.J., second edition, 1991.

    [6] S. Deering.  Host Extensions for IP Multicasting.  RFC 1112,
        August 1989.

    [7] S. Deering.  Router Discovery.  RFC 1256, September 1991.

    [8] R. Droms.  Dynamic Host Configuration Protocol.  RFC 1541,
        October 1993.

    [9] D.E. Eastlake, S.D. Crocker, and J.I. Schiller.  Randomness
        Requirements for Security.  RFC 1750, December 1994.

   [10] S. Hanks, T. Li, D. Farinacci, and P. Traina.  Generic Routing
        Encapsulation (GRE).  RFC 1701, October 1994.

   [11] V. Jacobson.  Compressing TCP/IP Headers for Low-Speed Serial
        Links.  RFC 1144, February 1990.

   [12] K. McCloghrie and F. Kastenholz.  Evolution of the Interfaces
        Group MIP-II.  RFC 1573, January 1994.

   [13] G. McGregor.  The PPP Internet Procotol Control Protocol (IPCP).
        RFC 1332, May 1992.

   [14] D. Mills.  Network Time Protocol (Version 3).  RFC 1305, March
        1992.

   [15] C. Perkins.  IP Encapsulation within IP.  Internet Draft -- work
        in progress, October 1995.




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   [16] C. Perkins.  Minimal Encapsulation within IP.  Internet Draft --
        work in progress, July 1995.

   [17] D. Plummer.  An Ethernet Address Resolution Protocol.  RFC 826,
        November 1982.

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

   [19] J. Postel.  Internet Protocol.  RFC 791, September 1981.

   [20] J. Postel.  Multi-LAN Address Resolution.  RFC 925, October
        1984.

   [21] J. Reynolds and J. Postel.  Assigned Numbers.  RFC 1700, October
        1994.

   [22] R. Rivest.  The MD5 Message-Digest Algorithm.  RFC 1321, April
        1992.

   [23] W. Simpson (Editor).  The Point-to-Point Protocol (PPP).  RFC
        1661, July 1994.






























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Chair's Addresses

   The working group can be contacted via the current chairs:


        Jim Solomon                       Tony Li
        Motorola, Inc.                    cisco systems
        1301 E. Algonquin Rd.             170 W. Tasman Dr.
        Schaumburg, IL  60196             San Jose, CA  95134

        Work:   +1-708-576-2753           Work:   +1-408-526-8186
        E-mail: solomon@comm.mot.com      E-mail: tli@cisco.com



Editor's Address

   Questions about this memo can also be directed to:

          Charles Perkins
          Room J1-A25
          T. J. Watson Research Center
          IBM Corporation
          30 Saw Mill River Rd.
          Hawthorne, NY  10532

          Work:   +1-914-784-7350
          Fax:    +1-914-784-7007
          E-mail: perk@watson.ibm.com






















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