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Network Working Group                                    C. Perkins, Ed.
Request for Comments: 3344                         Nokia Research Center
Obsoletes: 3220                                              August 2002
Category: Standards Track


                      IP Mobility Support for IPv4

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

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
   datagrams destined for the mobile node through a tunnel to the care-
   of address.  After arriving at the end of the tunnel, each datagram
   is then delivered to the mobile node.

Contents

   1. Introduction                                                     3
       1.1. Protocol Requirements . . . . . . . . . . . . . . . . .    4
       1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . .    4
       1.3. Assumptions . . . . . . . . . . . . . . . . . . . . . .    5
       1.4. Applicability . . . . . . . . . . . . . . . . . . . . .    5
       1.5. New Architectural Entities  . . . . . . . . . . . . . .    5
       1.6. Terminology . . . . . . . . . . . . . . . . . . . . . .    6
       1.7. Protocol Overview . . . . . . . . . . . . . . . . . . .    9
       1.8. Message Format and Protocol Extensibility . . . . . . .   13
       1.9. Type-Length-Value Extension Format for Mobile IP
               Extensions . . . . . . . . . . . . . . . . . . . . .   15
      1.10. Long Extension Format . . . . . . . . . . . . . . . . .   16



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      1.11. Short Extension Format  . . . . . . . . . . . . . . . .   16
   2. Agent Discovery                                                 17
       2.1. Agent Advertisement . . . . . . . . . . . . . . . . . .   18
             2.1.1. Mobility Agent Advertisement Extension  . . . .   20
             2.1.2. Prefix-Lengths Extension  . . . . . . . . . . .   22
             2.1.3. One-byte Padding Extension  . . . . . . . . . .   22
       2.2. Agent Solicitation  . . . . . . . . . . . . . . . . . .   23
       2.3. Foreign Agent and Home Agent Considerations . . . . . .   23
             2.3.1. Advertised Router Addresses . . . . . . . . . .   24
             2.3.2. Sequence Numbers and Rollover Handling  . . . .   24
       2.4. Mobile Node Considerations  . . . . . . . . . . . . . .   25
             2.4.1. Registration Required . . . . . . . . . . . . .   26
             2.4.2. Move Detection  . . . . . . . . . . . . . . . .   26
             2.4.3. Returning Home  . . . . . . . . . . . . . . . .   27
             2.4.4. Sequence Numbers and Rollover Handling  . . . .   28
   3. Registration                                                    28
       3.1. Registration Overview . . . . . . . . . . . . . . . . .   29
       3.2. Authentication  . . . . . . . . . . . . . . . . . . . .   30
       3.3. Registration Request  . . . . . . . . . . . . . . . . .   30
       3.4. Registration Reply  . . . . . . . . . . . . . . . . . .   33
       3.5. Registration Extensions . . . . . . . . . . . . . . . .   36
             3.5.1. Computing Authentication Extension Values . . .   36
             3.5.2. Mobile-Home Authentication Extension  . . . . .   37
             3.5.3. Mobile-Foreign Authentication Extension . . . .   37
             3.5.4. Foreign-Home Authentication Extension . . . . .   38
       3.6. Mobile Node Considerations  . . . . . . . . . . . . . .   38
             3.6.1. Sending Registration Requests . . . . . . . . .   40
             3.6.2. Receiving Registration Replies  . . . . . . . .   44
             3.6.3. Registration Retransmission . . . . . . . . . .   47
       3.7. Foreign Agent Considerations  . . . . . . . . . . . . .   47
             3.7.1. Configuration and Registration Tables . . . . .   48
             3.7.2. Receiving Registration Requests . . . . . . . .   49
             3.7.3. Receiving Registration Replies  . . . . . . . .   52
       3.8. Home Agent Considerations . . . . . . . . . . . . . . .   54
             3.8.1. Configuration and Registration Tables . . . . .   55
             3.8.2. Receiving Registration Requests . . . . . . . .   56
             3.8.3. Sending Registration Replies  . . . . . . . . .   59
   4. Routing Considerations                                          62
       4.1. Encapsulation Types . . . . . . . . . . . . . . . . . .   62
       4.2. Unicast Datagram Routing  . . . . . . . . . . . . . . .   62
             4.2.1. Mobile Node Considerations  . . . . . . . . . .   62
             4.2.2. Foreign Agent Considerations  . . . . . . . . .   63
             4.2.3. Home Agent Considerations . . . . . . . . . . .   64
       4.3. Broadcast Datagrams . . . . . . . . . . . . . . . . . .   66
       4.4. Multicast Datagram Routing  . . . . . . . . . . . . . .   66
       4.5. Mobile Routers  . . . . . . . . . . . . . . . . . . . .   67
       4.6. ARP, Proxy ARP, and Gratuitous ARP  . . . . . . . . . .   69
   5. Security Considerations                                         73



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       5.1. Message Authentication Codes  . . . . . . . . . . . . .   73
       5.2. Areas of Security Concern in this Protocol  . . . . . .   73
       5.3. Key Management  . . . . . . . . . . . . . . . . . . . .   74
       5.4. Picking Good Random Numbers . . . . . . . . . . . . . .   74
       5.5. Privacy . . . . . . . . . . . . . . . . . . . . . . . .   74
       5.6. Ingress Filtering . . . . . . . . . . . . . . . . . . .   75
       5.7. Replay Protection for Registration Requests . . . . . .   75
             5.7.1. Replay Protection using Timestamps  . . . . . .   75
             5.7.2. Replay Protection using Nonces  . . . . . . . .   77
   6. IANA Considerations                                             77
       6.1. Mobile IP Message Types . . . . . . . . . . . . . . . .   78
       6.2. Extensions to RFC 1256 Router Advertisement . . . . . .   78
       6.3. Extensions to Mobile IP Registration Messages . . . . .   79
       6.4. Code Values for Mobile IP Registration Reply
                Messages. . . . . . . . . . . . . . . . . . . . . .   79
   7. Acknowledgments                                                 80
   A. Patent Issues                                                   82
   B. Link-Layer Considerations                                       82
   C. TCP Considerations                                              83
       C.1. TCP Timers  . . . . . . . . . . . . . . . . . . . . . .   83
       C.2. TCP Congestion Management . . . . . . . . . . . . . . .   83
   D. Example Scenarios                                               84
       D.1. Registering with a Foreign Agent Care-of Address  . . .   84
       D.2. Registering with a Co-Located Care-of Address . . . . .   84
       D.3. Deregistration  . . . . . . . . . . . . . . . . . . . .   85
   E. Applicability of Prefix-Lengths Extension                       86
   F. Interoperability Considerations                                 86
   G. Changes since RFC 2002                                          87
       G.1. Major Changes . . . . . . . . . . . . . . . . . . . . .   87
       G.2. Minor Changes . . . . . . . . . . . . . . . . . . . . .   89
       G.3. Changes since revision 04 of RFC2002bis . . . . . . . .   91
   H. Example Messages                                                92
       H.1. Example ICMP Agent Advertisement Message Format . . . .   92
       H.2. Example Registration Request Message Format . . . . . .   93
       H.3. Example Registration Reply Message Format . . . . . . .   94
   References  . . . . . . . . . . . . . . . . . . . . . . . . . . .  94
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .   98
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . .   99

1. Introduction

   IP version 4 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 datagrams destined to it; otherwise, datagrams 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:



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      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 (mobile) computers.

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

   Changes between this revised specification for Mobile IP and the
   original specifications (see [33, 32, 34, 43, 8]) are detailed in the
   appendix section G.

1.1. Protocol Requirements

   A mobile node must be able to communicate with other nodes after
   changing its link-layer point of attachment to the Internet, yet
   without changing its IP address.

   A mobile node must be able to communicate with other nodes that do
   not implement these mobility functions.  No protocol enhancements are
   required in hosts or routers that are not acting as any of the new
   architectural entities introduced in Section 1.5.

   All messages used to update another node as to the location of a
   mobile node must be authenticated in order to protect against remote
   redirection attacks.

1.2. Goals

   The link by which a mobile node is directly attached to the Internet
   may often be a wireless link.  This link may thus have a
   substantially lower bandwidth and higher error rate than traditional
   wired networks.  Moreover, mobile nodes are likely to be battery
   powered, and minimizing power consumption is important.  Therefore,
   the number of administrative messages sent over the link by which a
   mobile node is directly attached to the Internet should be minimized,
   and the size of these messages should be kept as small as is
   reasonably possible.




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1.3. Assumptions

   The protocols defined in this document place no additional
   constraints on the assignment of IP addresses.  That is, a mobile
   node can be assigned an IP address by the organization that owns the
   machine.

   This protocol assumes that mobile nodes will generally not change
   their point of attachment to the Internet more frequently than once
   per second.

   This protocol assumes that IP unicast datagrams are routed based on
   the destination address in the datagram header (and not, for example,
   by source address).

1.4. Applicability

   Mobile IP is intended to enable nodes to move from one IP subnet to
   another.  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.  As long as
   node movement does not occur between points of attachment on
   different IP subnets, link-layer mechanisms for mobility (i.e.,
   link-layer handoff) may offer faster convergence and far less
   overhead than Mobile IP.

1.5. New Architectural Entities

   Mobile IP introduces the following 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 changing its IP address; it may continue to
         communicate with other Internet nodes at any location using its
         (constant) IP address, assuming link-layer connectivity to a
         point of attachment is available.





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      Home Agent

         A router on a mobile node's home network which tunnels
         datagrams 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 while registered.  The
         foreign agent detunnels and delivers datagrams to the mobile
         node that were tunneled by the mobile node's home agent.  For
         datagrams sent by a mobile node, the foreign agent may serve as
         a default router for registered mobile nodes.

   A mobile node is given a long-term IP address on a home network.
   This home address is administered in the same way as a "permanent" IP
   address is provided to a stationary host.  When away from its home
   network, a "care-of address" is associated with the mobile node and
   reflects the mobile node's current point of attachment.  The mobile
   node uses its home address as the source address of all IP datagrams
   that it sends, except where otherwise described in this document for
   datagrams sent for certain mobility management functions (e.g., as in
   Section 3.6.1.1).

1.6. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [4].

   In addition, this document frequently uses the following terms:

      Authorization-enabling extension

            An authentication which makes a (registration) message
            acceptable to the ultimate recipient of the registration
            message.  An authorization-enabling extension MUST contain
            an SPI.

            In this document, all uses of authorization-enabling
            extension refer to authentication extensions that enable the
            Registration Request message to be acceptable to the home
            agent.  Using additional protocol structures specified
            outside of this document, it may be possible for the mobile
            node to provide authentication of its registration to the




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            home agent, by way of another authenticating entity within
            the network that is acceptable to the home agent (for
            example, see RFC 2794 [6]).

      Agent Advertisement

            An advertisement message constructed by attaching a special
            Extension to a router advertisement [10] message.

      Authentication

            The process of verifying (using cryptographic techniques,
            for all applications in this specification) the identity of
            the originator of a message.

      Care-of Address

            The termination point of a tunnel toward a mobile node, for
            datagrams forwarded to the mobile node while it is away from
            home.  The protocol can use two different types of care-of
            address:  a "foreign agent care-of address" is an address of
            a foreign agent with which the mobile node is registered,
            and a "co-located care-of address" is an externally obtained
            local address which the mobile node has associated with one
            of its own network interfaces.

      Correspondent Node

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

      Foreign Network

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

      Gratuitous ARP

            An ARP packet sent by a node in order to spontaneously cause
            other nodes to update an entry in their ARP cache [45].  See
            section 4.6.

      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.





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      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 datagrams
            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 an endpoint of some
            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.

      Mobility Security Association

            A collection of security contexts, between a pair of nodes,
            which may be applied to Mobile IP protocol messages
            exchanged between them.  Each context indicates an
            authentication algorithm and mode (Section 5.1), a secret (a
            shared key, or appropriate public/private key pair), and a
            style of replay protection in use (Section 5.7).

      Node

            A host or a router.

      Nonce

            A randomly chosen value, different from previous choices,
            inserted in a message to protect against replays.





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      Security Parameter Index (SPI)

            An index identifying a security context between a pair of
            nodes among the contexts available in the Mobility Security
            Association.  SPI values 0 through 255 are reserved and MUST
            NOT be used in any Mobility Security Association.

      Tunnel

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

      Virtual Network

            A network with no physical instantiation beyond a router
            (with a physical network interface on another network).  The
            router (e.g., a home agent) generally advertises
            reachability to the virtual network using conventional
            routing protocols.

      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:

      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



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            directly with its home agent, or through a foreign agent
            which forwards the registration to the home agent.

      silently discard

            The implementation discards the datagram 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 datagram,
            and SHOULD record the event in a statistics counter.

   The following steps provide a rough outline of operation of the
   Mobile IP protocol:

      -  Mobility agents (i.e., foreign agents and home agents)
         advertise their presence via Agent Advertisement messages
         (Section 2).  A mobile node may optionally solicit an Agent
         Advertisement message from any locally attached mobility agents
         through an Agent Solicitation message.

      -  A mobile node receives these Agent 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.  If returning
         to its home network from being registered elsewhere, the mobile
         node deregisters with its home agent, through exchange of a
         Registration Request and Registration Reply message with it.

      -  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 (a foreign agent care-of address), or by
         some external assignment mechanism such as DHCP [13] (a co-
         located care-of address).

      -  The mobile node operating away from home then registers its new
         care-of address with its home agent through exchange of a
         Registration Request and Registration Reply message with it,
         possibly via a foreign agent (Section 3).

      -  Datagrams sent to the mobile node's home address are
         intercepted by its home agent, tunneled by the home agent to
         the mobile node's 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 (Section
         4.2.3).



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      -  In the reverse direction, datagrams sent by the mobile node are
         generally 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
   network and its current location.  The tunnel terminates at the
   mobile node's care-of address.  The care-of address must be an
   address to which datagrams can be delivered via conventional IP
   routing.  At the care-of address, the original datagram is removed
   from the tunnel and delivered to the mobile node.

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

      a) A "foreign agent care-of address" is a care-of address provided
         by a foreign agent through its Agent Advertisement messages.
         In this case, the care-of address is an IP address of the
         foreign agent.  In this mode, the foreign agent is the endpoint
         of the tunnel and, upon receiving tunneled datagrams,
         decapsulates them and delivers the inner datagram to the mobile
         node.  This mode of acquisition 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.

      b) A "co-located care-of address" is a care-of address acquired by
         the mobile node as a local IP address through some external
         means, which the mobile node then associates with one of its
         own network interfaces.  The address may be dynamically
         acquired as a temporary address by the mobile node such as
         through DHCP [13], or may be owned by the mobile node as a
         long-term address for its use only while visiting some foreign
         network.  Specific external methods of acquiring a local IP
         address for use as a co-located care-of address are beyond the
         scope of this document.  When using a co-located care-of
         address, the mobile node serves as the endpoint of the tunnel
         and itself performs decapsulation of the datagrams tunneled to
         it.

   The mode of using a co-located care-of address has the advantage that
   it allows a mobile node to function without a foreign agent, for
   example, in networks that have not yet deployed a foreign agent.  It
   does, however, place additional burden on the IPv4 address space
   because it requires a pool of addresses within the foreign network to





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   be made available to visiting mobile nodes.  It is difficult to
   efficiently maintain pools of addresses for each subnet that 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 (a foreign agent care-of address), but it might
   instead be an address temporarily acquired by the mobile node (a co-
   located care-of address).  A foreign agent, on the other hand, is a
   mobility agent that provides services to mobile nodes.  See Sections
   3.7 and 4.2.2 for additional details.

   For example, figure 1 illustrates the routing of datagrams to and
   from a mobile node away from home, once the mobile node has
   registered with its home agent.  In figure 1, the mobile node is
   using a foreign agent care-of address, not a co-located care-of
   address.

              2) Datagram is intercepted   3) Datagram is
                 by home agent and            detunneled and
                 is tunneled to the           delivered to the
                 care-of address.             mobile node.

                   +-----+          +-------+         +------+
                   |home | =======> |foreign| ------> |mobile|
                   |agent|          | agent | <------ | node |
                   +-----+          +-------+         +------+
   1) Datagram to    /|\         /
      mobile node     |        /   4) For datagrams sent by the
      arrives on      |      /        mobile node, standard IP
      home network    |    /          routing delivers each to its
      via standard    |  |_           destination.  In this figure,
      IP routing.   +----+            the foreign agent is the
                    |host|            mobile node's default router.
                    +----+

                 Figure 1: Operation of Mobile IPv4

   A home agent MUST be able to attract and intercept datagrams 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
   intercepting datagrams destined to the mobile node's home address.
   Such placements are beyond the scope of this document.



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   Similarly, a mobile node and a prospective or current foreign agent
   MUST be able to exchange datagrams 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 visiting 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 datagrams to each
   other, addressing the underlying link-layer packets 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 datagrams between these nodes, but such
   placements are beyond the scope of this document.

   If a mobile node is using a co-located 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, datagrams
   destined to the care-of address would be undeliverable.

1.8. Message Format and Protocol Extensibility

   Mobile IP defines a set of new control messages, sent with UDP [37]
   using well-known port number 434.  The following two message types
   are defined in this document:

      1  Registration Request
      3  Registration Reply

      Up-to-date values for the message types for Mobile IP control
      messages are specified in the most recent "Assigned Numbers" [40].

      In addition, for Agent Discovery, Mobile IP makes use of the
      existing Router Advertisement and Router Solicitation messages
      defined for ICMP Router Discovery [10].

      Mobile IP defines a general Extension mechanism to allow optional
      information to be carried by Mobile IP control messages or by ICMP
      Router Discovery messages.  Some extensions have been specified to
      be encoded in the simple Type-Length-Value format described in
      Section 1.9.

      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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      Two separately maintained sets of numbering spaces, from which
      Extension Type values are allocated, are used in Mobile IP:

      -  The first set consists of those Extensions which may appear
         only in Mobile IP control messages (those sent to and from UDP
         port number 434).  In this document, the following Types are
         defined for Extensions appearing in Mobile IP control messages:

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

      -  The second set consists of those extensions which may appear
         only in ICMP Router Discovery messages [10].  In this document,
         the following Types are defined for Extensions appearing in
         ICMP Router Discovery messages:

             0  One-byte Padding (encoded with no Length nor Data field)
            16  Mobility Agent Advertisement
            19  Prefix-Lengths

   Each individual Extension is described in detail in a separate
   section later in this document.  Up-to-date values for these
   Extension Type numbers are specified in the most recent "Assigned
   Numbers" [40].

   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
   may be used only in Mobile IP control messages and the other may be
   used only in ICMP Router Discovery messages.

   The type field in the Mobile IP extension structure can support up to
   255 (skippable and not skippable) uniquely identifiable extensions.
   When an Extension numbered in either of these sets within the range 0
   through 127 is encountered but not recognized, the message containing
   that Extension MUST be silently discarded.  When an Extension
   numbered in the range 128 through 255 is encountered which is not
   recognized, that particular Extension is ignored, but the rest of the
   Extensions and message data MUST still be processed.  The Length
   field of the Extension is used to skip the Data field in searching
   for the next Extension.

   Unless additional structure is utilized for the extension types, new
   developments or additions to Mobile IP might require so many new
   extensions that the available space for extension types might run
   out.  Two new extension structures are proposed to solve this
   problem.  Certain types of extensions can be aggregated, using



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   subtypes to identify the precise extension, for example as has been
   done with the Generic Authentication Keys extensions [35].  In many
   cases, this may reduce the rate of allocation for new values of the
   type field.

   Since the new extension structures will cause an efficient usage of
   the extension type space, it is recommended that new Mobile IP
   extensions follow one of the two new extension formats whenever there
   may be the possibility to group related extensions together.

   The following subsections provide details about three distinct
   structures for Mobile IP extensions:

      - The simple extension format
      - The long extension format
      - The short extension format

1.9. Type-Length-Value Extension Format for Mobile IP Extensions

   The Type-Length-Value format illustrated in figure 2 is used for
   extensions which are specified in this document.  Since this simple
   extension structure does not encourage the most efficient usage of
   the extension type space, it is recommended that new Mobile IP
   extensions follow one of the two new extension formats specified in
   sections 1.10 or 1.11 whenever there may be the possibility to group
   related extensions together.

    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 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

      Figure 2: Type-Length-Value extension format for Mobile IPv4

      Type     Indicates the particular type of Extension.

      Length   Indicates the length (in bytes) of the data field within
               this Extension.  The length does NOT include the Type and
               Length bytes.

      Data     The particular data associated with this Extension.  This
               field may be zero or more bytes in length.  The format
               and length of the data field is determined by the type
               and length fields.






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1.10.  Long Extension Format

   This format is applicable for non-skippable extensions which carry
   information more than 256 bytes.

     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      |  Sub-Type     |           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Data      .....
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Long Extension format requires that the following fields be
   specified as the first fields of the extension.

      Type     is the type, which describes a collection of extensions
               having a common data type.

      Sub-Type is a unique number given to each member in the aggregated
               type.

      Length   indicates the length (in bytes) of the data field within
               this Extension.  It does NOT include the Type, Length and
               Sub-Type bytes.

      Data     is the data associated with the subtype of this
               extension.  This specification does not place any
               additional structure on the subtype data.

   Since the length field is 16 bits wide, a the extension data can
   exceed 256 bytes in length.

1.11.  Short Extension Format

   This format is compatible with the skippable extensions defined in
   section 1.9.  It is not applicable for extensions which require more
   than 256 bytes of data; for such extensions, use the format described
   in section 1.10.

     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      |    Sub-Type   |    Data ....
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Short Extension format requires that the following fields be
   specified as the first fields of the extension:



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      Type     is the type, which describes a collection of extensions
               having a common data type.

      Sub-Type is a unique number given to each member in the aggregated
               type.

      Length   8-bit unsigned integer.  Length of the extension, in
               bytes, excluding the extension Type and the extension
               Length fields.  This field MUST be set to 1 plus the
               total length of the data field.

      Data     is the data associated with this extension.  This
               specification does not place any additional structure on
               the subtype data.

2. Agent Discovery

   Agent Discovery is the method by which a mobile node determines
   whether it is currently connected to its home network or to a foreign
   network, and by which a mobile node can detect when it has moved from
   one network to another.  When connected to a foreign network, the
   methods specified in this section also allow the mobile node to
   determine the foreign agent care-of address being offered by each
   foreign agent on that network.

   Mobile IP extends ICMP Router Discovery [10] as its primary mechanism
   for Agent Discovery.  An Agent Advertisement is formed by including a
   Mobility Agent Advertisement Extension in an ICMP Router
   Advertisement message (Section 2.1).  An Agent Solicitation message
   is identical to an ICMP Router Solicitation, except that its IP TTL
   MUST be set to 1 (Section 2.2).  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 that 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 [22], which is unrelated to the messages
   described in this document.  Further specification of the way in
   which Advertisement and Solicitation messages may be authenticated is
   outside of the scope of this document.




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2.1. Agent Advertisement

   Agent Advertisements are transmitted by a mobility agent to advertise
   its services on a link.  Mobile nodes use these advertisements to
   determine their current point of attachment to the Internet.  An
   Agent Advertisement is an ICMP Router Advertisement that has been
   extended to also carry an Mobility Agent Advertisement Extension
   (Section 2.1.1) and, optionally, a Prefix-Lengths Extension (Section
   2.1.2), One-byte Padding Extension (Section 2.1.3), or other
   Extensions that might be defined in the future.

   Within an Agent Advertisement message, ICMP Router Advertisement
   fields of the message are required to conform to the following
   additional specifications:

      -  Link-Layer Fields

         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 all Agent Advertisements MUST be set
                  to 1.

         Destination Address

               As specified for ICMP Router Discovery [10], the IP
               destination address of an multicast Agent Advertisement
               MUST be either the "all systems on this link" multicast
               address (224.0.0.1) [11] or the "limited broadcast"
               address (255.255.255.255).  The subnet-directed broadcast
               address of the form <prefix>.<-1> cannot be used since
               mobile nodes will not generally know the prefix of the
               foreign network.  When the Agent Advertisement is unicast
               to a mobile node, the IP home address of the mobile node
               SHOULD be used as the Destination Address.










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      -  ICMP Fields

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

                   0 The mobility agent handles common traffic -- that
                     is, it acts as a router for IP datagrams 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 any datagrams received
                     from a registered mobile node (Section 4.2.2).

         Lifetime

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

         Router Address(es)

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

         Num Addrs

               The number of Router Addresses advertised in this
               message.  Note that in an Agent Advertisement message,
               the number of router addresses specified in the ICMP
               Router Advertisement portion of the message MAY be set to
               0.  See Section 2.3.1 for details.

   If sent periodically, the nominal interval at which Agent
   Advertisements are sent SHOULD be no longer than 1/3 of the
   advertisement Lifetime given in the ICMP header.  This interval MAY
   be shorter than 1/3 the advertised Lifetime.  This allows a mobile
   node to miss three successive advertisements before deleting the
   agent from its list of valid agents.  The actual transmission time
   for each advertisement SHOULD be slightly randomized [10] in order to
   avoid synchronization and subsequent collisions with other Agent

   Advertisements that may be sent by other agents (or with other Router
   Advertisements sent by other routers).  Note that this field has no
   relation to the "Registration Lifetime" field within the Mobility
   Agent Advertisement Extension defined below.






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2.1.1. Mobility Agent Advertisement Extension

   The Mobility Agent Advertisement Extension follows the ICMP Router
   Advertisement fields.  It is used to indicate that an ICMP Router
   Advertisement message is also an Agent Advertisement being sent by a
   mobility agent.  The Mobility Agent Advertisement 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        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Registration Lifetime      |R|B|H|F|M|G|r|T|   reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  zero or more Care-of Addresses               |
   |                              ...                              |

      Type     16

      Length   (6 + 4*N), where 6 accounts for the number of bytes in
               the Sequence Number, Registration Lifetime, flags, and
               reserved fields, and N is the number of care-of addresses
               advertised.

      Sequence Number

               The count of Agent Advertisement messages sent since the
               agent was initialized (Section 2.3.2).

      Registration Lifetime

               The longest lifetime (measured in seconds) that this
               agent is willing to accept in any Registration Request.
               A value of 0xffff indicates infinity.  This field has no
               relation to the "Lifetime" field within the ICMP Router
               Advertisement portion of the Agent Advertisement.

      R        Registration required.  Registration with this foreign
               agent (or another foreign agent on this link) is required
               even when using a co-located care-of address.

      B        Busy.  The foreign agent will not accept registrations
               from additional mobile nodes.

      H        Home agent.  This agent offers service as a home agent on
               the link on which this Agent Advertisement message is
               sent.



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      F        Foreign agent.  This agent offers service as a foreign
               agent on the link on which this Agent Advertisement
               message is sent.

      M        Minimal encapsulation.  This agent implements receiving
               tunneled datagrams that use minimal encapsulation [34].

      G        GRE encapsulation.  This agent implements receiving
               tunneled datagrams that use GRE encapsulation [16].

      r        Sent as zero; ignored on reception.  SHOULD NOT be
               allocated for any other uses.

      T        Foreign agent supports reverse tunneling [27].

      reserved
               Sent as zero; ignored on reception.

      Care-of Address(es)

               The advertised foreign agent care-of address(es) provided
               by this foreign agent.  An Agent Advertisement MUST
               include at least one care-of address if the 'F' bit is
               set.  The number of care-of addresses present is
               determined by the Length field in the Extension.

   A home agent MUST always be prepared to serve the mobile nodes for
   which it is the home agent.  A foreign agent may at times be too busy
   to serve additional mobile nodes; even so, it must continue to send
   Agent Advertisements, so that any mobile nodes already registered
   with it will know that they have not moved out of range of the
   foreign agent and that the foreign agent has not failed.  A foreign
   agent may indicate that it is "too busy" to allow new mobile nodes to
   register with it, by setting the 'B' bit in its Agent Advertisements.
   An Agent Advertisement message MUST NOT have the 'B' bit set if the
   'F' bit is not also set.  Furthermore, at least one of the 'F' bit
   and the 'H' bit MUST be set in any Agent Advertisement message sent.

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








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2.1.2. Prefix-Lengths Extension

   The Prefix-Lengths Extension MAY follow the Mobility Agent
   Advertisement Extension.  It is used to indicate the number of bits
   of network prefix that applies to each Router Address listed in the
   ICMP Router Advertisement portion of the Agent Advertisement.  Note
   that the prefix lengths given DO NOT apply to care-of address(es)
   listed in the Mobility Agent Advertisement 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   N, where N is the value (possibly zero) of the Num Addrs
               field in the ICMP Router Advertisement portion of the
               Agent Advertisement.

      Prefix Length(s)

               The number of leading bits that define the network number
               of the corresponding Router Address listed in the ICMP
               Router Advertisement portion of the message.  The prefix
               length for each Router Address is encoded as a separate
               byte, in the order that the Router Addresses are listed
               in the ICMP Router Advertisement portion of the message.

   See Section 2.4.2 for information about how the Prefix-Lengths
   Extension MAY be used by a mobile node when determining whether it
   has moved.  See Appendix E for implementation details about the use
   of this Extension.

2.1.3. One-byte Padding Extension

   Some IP protocol implementations insist upon padding ICMP messages 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.  An Agent
   Advertisement SHOULD NOT include more than one One-byte Padding
   Extension and if present, this Extension SHOULD be the last Extension
   in the Agent Advertisement.




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   Note that unlike other Extensions used in Mobile IP, the One-byte
   Padding Extension is encoded as a single byte, with no "Length" nor
   "Data" field present.  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 Agent and Home Agent Considerations

   Any mobility agent which cannot be discovered by a link-layer
   protocol MUST send Agent Advertisements.  An agent which can be
   discovered 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 Agent
   Solicitation.  All mobility agents MUST process packets that they
   receive addressed to the Mobile-Agents multicast group, at address
   224.0.0.11.  A mobile node MAY send an Agent Solicitation to
   224.0.0.11.  All mobility agents SHOULD respond to Agent
   Solicitations.

   The same procedures, defaults, and constants are used in Agent
   Advertisement messages and Agent Solicitation messages as specified
   for ICMP Router Discovery [10], except that:

   -  a mobility agent MUST limit the rate at which it sends broadcast
      or multicast Agent Advertisements; the maximum rate SHOULD be
      chosen so that the Advertisements do not consume a significant
      amount of network bandwidth, AND

   -  a mobility agent that receives a Router Solicitation MUST 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 of the router).

   -  a mobility agent MAY be configured to send Agent Advertisements
      only in response to an Agent Solicitation message.



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   If the home network is not a virtual network, then the home agent for
   any mobile node SHOULD be located on the link identified by the
   mobile node's home address, and Agent Advertisement messages sent by
   the home agent on this link MUST have the 'H' bit set.  In this way,
   mobile nodes on their own home network will be able to determine that
   they are indeed at home.  Any Agent Advertisement messages sent by
   the home agent on another link to which it may be attached (if it is
   a mobility agent serving more than one link), MUST NOT have the 'H'
   bit set, unless the home agent also serves as a home agent (to other
   mobile nodes) on that other link.  A mobility agent MAY use different
   settings for each of the 'R', 'H', and 'F' bits on different network
   interfaces.

   If the home network is a virtual network, the home network has no
   physical realization external to the home agent itself.  In this
   case, there is no physical network link on which to send Agent
   Advertisement messages advertising the home agent.  Mobile nodes for
   which this is the home network are always treated as being away from
   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
   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 (Section 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.  An agent SHOULD only put its
   own addresses, if any, in the advertisement.  Whether or not its own
   address appears in the Router Addresses, a foreign agent MUST route
   datagrams it receives from registered mobile nodes (Section 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 MUST use the number 0 for its first
   advertisement.  Each subsequent advertisement MUST use the sequence
   number one greater, with the exception that the sequence number
   0xffff MUST be followed by sequence number 256.  In this way, mobile
   nodes can distinguish a reduction in the sequence number that occurs
   after a reboot from a reduction that results in rollover of the
   sequence number after it attains the value 0xffff.






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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 specified for ICMP Router
   Solicitation messages [10], except that the mobile node MAY solicit
   more often than once every three seconds, and that a mobile node that
   is currently not connected to any foreign agent MAY solicit more
   times than MAX_SOLICITATIONS.

   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,
   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 SHOULD also
   increase the rate at which it will send Solicitations when it next
   begins searching for a new agent with which to register.  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 specified for ICMP Router Discovery [10].

   Mobile nodes MUST process received Agent Advertisements.  A mobile
   node can distinguish an Agent Advertisement message from other uses
   of the ICMP Router Advertisement message 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 Mobility
   Agent Advertisement Extension identifies the advertisement as an
   Agent Advertisement.






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   If there is more than one advertised address, the mobile node SHOULD
   pick the first address for its initial registration attempt.  If the
   registration attempt fails with a status Code indicating rejection by
   the foreign agent, the mobile node MAY retry the attempt with each
   subsequent advertised address in turn.

   When multiple methods of agent discovery are in use, the mobile node
   SHOULD first attempt registration with agents including Mobility
   Agent Advertisement Extensions in their advertisements, in preference
   to those discovered by other means.  This preference maximizes the
   likelihood that the registration will be recognized, thereby
   minimizing the number of registration attempts.

   A mobile node MUST ignore reserved bits in Agent Advertisements, as
   opposed to discarding such advertisements.  In this way, new bits can
   be defined later, without affecting the ability for mobile nodes to
   use the advertisements even when the newly defined bits are not
   understood.

2.4.1. Registration Required

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

   If formerly reserved bits require some kind of monitoring/enforcement
   at the foreign link, foreign agents implementing the new
   specification for the formerly reserved bits can set the 'R' bit.
   This has the effect of forcing the mobile node to register through
   the foreign agent, so the foreign agent could then monitor/enforce
   the policy.

2.4.2. Move Detection

   Two primary mechanisms are provided for mobile nodes to detect when
   they have moved from one subnet to another.  Other mechanisms MAY
   also be used.  When the mobile node detects that it has moved, it
   SHOULD register (Section 3) with a suitable care-of address on the
   new foreign network.  However, the mobile node MUST NOT register more
   frequently than once per second on average, as specified in Section
   3.6.3.







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2.4.2.1. Algorithm 1

   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, until that Lifetime expires.
   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 that agent.  If the mobile node has previously
   received an Agent Advertisement from another agent for which the
   Lifetime field has not yet expired, the mobile node MAY immediately
   attempt registration with that other agent.  Otherwise, the mobile
   node SHOULD attempt to discover a new agent with which to register.

2.4.2.2. Algorithm 2

   The second method uses network prefixes.  The Prefix-Lengths
   Extension MAY be used in some cases by a mobile node to determine
   whether or not a newly received Agent Advertisement was received on
   the same subnet as the mobile node's current care-of address.  If the
   prefixes differ, the mobile node MAY assume that it has moved.  If a
   mobile node is currently using a foreign agent care-of address, the
   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.  Similarly, if a mobile node is
   using a co-located care-of address, it SHOULD not use this method of
   move detection unless the new agent includes the Prefix-Lengths
   Extension in its Advertisement and the mobile node knows the network
   prefix of its current co-located care-of address.  On the expiration
   of its current registration, if this method indicates that the mobile
   node has moved, rather than re-registering with its current care-of
   address, a mobile node MAY choose instead to register with a the
   foreign agent sending the new Advertisement with the different
   network prefix.  The Agent Advertisement on which the new
   registration is based MUST NOT have expired according to its Lifetime
   field.

2.4.3. Returning Home

   A mobile node can detect that it has returned to its home network
   when it receives an Agent Advertisement from its own home agent.  If
   so, it SHOULD deregister with its home agent (Section 3).  Before
   attempting to deregister, the mobile node SHOULD configure its
   routing table appropriately for its home network (Section 4.2.1).  In





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   addition, if the home network is using ARP [36], 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 Agent Advertisements from the foreign agent with which it is
   registered, the second of which is less than the first and inside the
   range 0 to 255, the mobile node SHOULD register again.  If the second
   value is less than the first but is greater than or equal to 256, the
   mobile node SHOULD assume that the sequence number has rolled over
   past its maximum value (0xffff), and that reregistration is not
   necessary (Section 2.3).

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:

      -  discover its home address, if the mobile node is not configured
         with this information.

      -  maintain multiple simultaneous registrations, so that 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





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

3.1. Registration Overview

   Mobile IP defines two different registration procedures, one via a
   foreign agent that relays the registration to the mobile node's home
   agent, and one directly with the mobile node's home agent.  The
   following rules determine which of these two registration procedures
   to use in any particular circumstance:

      -  If a mobile node is registering a foreign agent care-of
         address, the mobile node MUST register via that foreign agent.

      -  If a mobile node is using a co-located care-of address, and
         receives an Agent Advertisement from a foreign agent on the
         link on which it is using this care-of address, the mobile node
         SHOULD register via that foreign agent (or via another foreign
         agent on this link) if the 'R' bit is set in the received Agent
         Advertisement message.

      -  If a mobile node is otherwise using a co-located care-of
         address, the mobile node MUST register directly with its home
         agent.

      -  If a mobile node has returned to its home network and is
         (de)registering with its home agent, the mobile node MUST
         register directly with its home agent.

   Both registration procedures involve the exchange of Registration
   Request and Registration Reply messages (Sections 3.3 and 3.4).  When
   registering via a foreign agent, the registration procedure requires
   the following four messages:

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

      b) The foreign agent processes the Registration Request and then
         relays it to the home agent.

      c) The home agent sends a Registration Reply to the foreign agent
         to grant or deny the Request.

      d) The foreign agent processes the Registration Reply and then
         relays it to the mobile node to inform it of the disposition of
         its Request.





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   When the mobile node instead registers directly with its home agent,
   the registration procedure requires only the following two messages:

      a) The mobile node sends a Registration Request to the home agent.

      b) The home agent sends a Registration Reply to the mobile node,
         granting or denying the Request.

   The registration messages defined in Sections 3.3 and 3.4 use the
   User Datagram Protocol (UDP) [37].  A nonzero UDP checksum SHOULD be
   included in the header, and MUST be checked by the recipient.  A zero
   UDP checksum SHOULD be accepted by the recipient.  The behavior of
   the mobile node and the home agent with respect to their mutual
   acceptance of packets with zero UDP checksums SHOULD be defined as
   part of the mobility security association which exists between them.

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 SPI and IP address.  In the case of the mobile node, this
   must be its Home Address.  See Section 5.1 for requirements for
   support of authentication algorithms.  Registration messages between
   a mobile node and its home agent MUST be authenticated with an
   authorization-enabling extension, e.g. the Mobile-Home Authentication
   Extension (Section 3.5.2).  This extension MUST be the first
   authentication extension; other foreign agent-specific extensions MAY
   be added to the message after the mobile node computes the
   authentication.

3.3. Registration Request

   A mobile node registers with its home agent using a Registration
   Request message so that its home agent can create or modify a
   mobility binding for that mobile node (e.g., with a new lifetime).
   The Request may be relayed to the home agent by the foreign agent
   through which the mobile node is registering, or it may be sent
   directly to the home agent in the case in which the mobile node is
   registering a co-located care-of address.

   IP fields:

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

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




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   See Sections 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|r|T|x|          Lifetime             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Home Address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Home Agent                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Care-of Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Identification                        +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Extensions ...
   +-+-+-+-+-+-+-+-

      Type     1 (Registration Request)

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

      B        Broadcast datagrams.  If the 'B' bit is set, the mobile
               node requests that the home agent tunnel to it any
               broadcast datagrams that it receives on the home network,
               as described in Section 4.3.

      D        Decapsulation by mobile node.  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 is
               using a co-located care-of address.

      M        Minimal encapsulation.  If the 'M' bit is set, the mobile
               node requests that its home agent use minimal
               encapsulation [34] for datagrams tunneled to the mobile
               node.





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      G        GRE encapsulation.  If the 'G' bit is set, the mobile
               node requests that its home agent use GRE encapsulation
               [16] for datagrams tunneled to the mobile node.

      r        Sent as zero; ignored on reception.  SHOULD NOT be
               allocated for any other uses.

      T        Reverse Tunneling requested; see [27].

      x        Sent as zero; ignored on reception.

      Lifetime

               The number of seconds remaining before the registration
               is considered expired.  A value of zero indicates a
               request for deregistration.  A value of 0xffff indicates
               infinity.

      Home Address

               The IP address of the mobile node.

      Home Agent

               The IP address of the mobile node's home agent.

      Care-of Address

               The IP address for the end of the tunnel.

      Identification

               A 64-bit number, constructed by the mobile node, used for
               matching Registration Requests with Registration Replies,
               and for protecting against replay attacks of registration
               messages.  See Sections 5.4 and 5.7.

      Extensions

               The fixed portion of the Registration Request is followed
               by one or more of the Extensions listed in Section 3.5.
               An authorization-enabling extension MUST be included in
               all Registration Requests.  See Sections 3.6.1.3 and
               3.7.2.2 for information on the relative order in which
               different extensions, when present, MUST be placed in a
               Registration Request message.





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3.4. Registration Reply

   A mobility agent returns a Registration Reply message to a mobile
   node which has sent a Registration Request (Section 3.3) message.  If
   the mobile node is requesting 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.

   The foreign agent MUST NOT increase the Lifetime selected by the
   mobile node in the Registration Request, since the Lifetime is
   covered by an authentication extension which enables authorization by
   the home agent.  Such an extension contains authentication data which
   cannot be correctly (re)computed by the foreign agent.  The home
   agent MUST NOT increase the Lifetime selected by the mobile node in
   the Registration Request, since doing so could increase it beyond the
   maximum Registration Lifetime allowed by the foreign agent.  If the
   Lifetime received in the Registration Reply is greater than that in
   the Registration Request, the Lifetime in the Request MUST be used.
   When the Lifetime received in the Registration Reply is less than
   that in the Registration Request, the Lifetime in the Reply MUST be
   used.

   IP fields:

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

      Destination Address  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 (Section
                           3.7.1).








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   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     A value indicating the result of the Registration
               Request.  See below for a list of currently defined Code
               values.

      Lifetime

               If the Code field indicates that the registration was
               accepted, the Lifetime field is set to the number of
               seconds remaining before the registration is considered
               expired.  A value of zero indicates that the mobile node
               has been deregistered.  A value of 0xffff indicates
               infinity.  If the Code field indicates that the
               registration was denied, the contents of the Lifetime
               field are unspecified and MUST be ignored on reception.

      Home Address

               The IP address of the mobile node.

      Home Agent

               The IP address of the mobile node's home agent.

      Identification

               A 64-bit number used for matching Registration Requests
               with Registration Replies, and for protecting against
               replay attacks of registration messages.  The value is



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               based on the Identification field from the Registration
               Request message from the mobile node, and on the style of
               replay protection used in the security context between
               the mobile node and its home agent (defined by the
               mobility security association between them, and SPI value
               in the authorization-enabling extension).  See Sections
               5.4 and 5.7.

      Extensions

               The fixed portion of the Registration Reply is followed
               by one or more of the Extensions listed in Section 3.5.
               An authorization-enabling extension MUST be included in
               all Registration Replies returned by the home agent.  See
               Sections 3.7.2.2 and 3.8.3.3 for rules on placement of
               extensions to Reply messages.

   The following values are defined for use within the Code field.
   Registration successful:

      0 registration accepted
      1 registration accepted, but simultaneous mobility
       bindings unsupported

   Registration 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 reserved and unavailable
      77 invalid care-of address
      78 registration timeout
      80 home network unreachable (ICMP error received)
      81 home agent host unreachable (ICMP error received)
      82 home agent port unreachable (ICMP error received)
      88 home agent unreachable (other ICMP error received)









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   Registration 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 registration 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" [40].

3.5. Registration Extensions

3.5.1. Computing Authentication Extension Values

   The Authenticator value computed for each authentication Extension
   MUST protect the following fields from the registration message:

      -  the UDP payload (that is, the Registration Request or
         Registration Reply data),

      -  all prior Extensions in their entirety, and

      -  the Type, Length, and SPI of this Extension.

   The default authentication algorithm uses HMAC-MD5 [23] to compute a
   128-bit "message digest" of the registration message.  The data over
   which the HMAC is computed is defined as:

      -  the UDP payload (that is, the Registration Request or
         Registration Reply data),

      -  all prior Extensions in their entirety, and

      -  the Type, Length, and SPI of this Extension.

   Note that the Authenticator field itself and the UDP header are NOT
   included in the computation of the default Authenticator value.  See
   Section 5.1 for information about support requirements for message
   authentication codes, which are to be used with the various
   authentication Extensions.






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   The Security Parameter Index (SPI) within any of the authentication
   Extensions defines the security context which is used to compute the
   Authenticator value and which MUST be used by the receiver to check
   that value.  In particular, the SPI selects the authentication
   algorithm and mode (Section 5.1) and secret (a shared key, or
   appropriate public/private key pair) used in computing the
   Authenticator.  In order to ensure interoperability between different
   implementations of the Mobile IP protocol, an implementation MUST be
   able to associate any SPI value with any authentication algorithm and
   mode which it implements.  In addition, all implementations of Mobile
   IP MUST implement the default authentication algorithm (HMAC-MD5)
   specified above.

3.5.2. Mobile-Home Authentication Extension

   Exactly one authorization-enabling extension MUST be present in all
   Registration Requests, and also in all Registration Replies generated
   by the Home Agent.  The Mobile-Home Authentication Extension is
   always an authorization-enabling for registration messages specified
   in this document.  This requirement is intended to eliminate problems
   [2] which result from the uncontrolled propagation of remote
   redirects in the Internet.  The location of the 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         4 plus the number of bytes in the Authenticator.

      SPI            Security Parameter Index (4 bytes).  An opaque
                     identifier (see Section 1.6).

      Authenticator  (variable length) (See Section 3.5.1.)

3.5.3. Mobile-Foreign Authentication Extension

   This Extension MAY be included in Registration Requests and Replies
   in cases in which a mobility security association exists between the
   mobile node and the foreign agent.  See Section 5.1 for information
   about support requirements for message authentication codes.




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    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

      Length         4 plus the number of bytes in the Authenticator.

      SPI            Security Parameter Index (4 bytes).  An opaque
                     identifier (see Section 1.6).

      Authenticator  (variable length) (See Section 3.5.1.)

3.5.4. Foreign-Home Authentication Extension

   This Extension MAY be included in Registration Requests and Replies
   in cases in which a mobility security association exists between the
   foreign agent and the home agent.  See Section 5.1 for information
   about support requirements for message authentication codes.

    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

      Length         4 plus the number of bytes in the Authenticator.

      SPI            Security Parameter Index (4 bytes).  An opaque
                     identifier (see Section 1.6).

      Authenticator  (variable length) (See Section 3.5.1.)

3.6. Mobile Node Considerations

   A mobile node MUST be configured with a netmask and a mobility
   security association for each of its home agents.  In addition, a
   mobile node MAY be configured with its home address, and the IP





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   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.

   If the mobile node is not configured with a home address, it MAY use
   the Mobile Node NAI extension [6] to identify itself, and set the
   Home Address field of the Registration Request to 0.0.0.0.  In this
   case, the mobile node MUST be able to assign its home address after
   extracting this information from the Registration Reply from the home
   agent.

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

      -  the link-layer address of the foreign agent to which the
         Registration Request was sent, if applicable,
      -  the IP destination address of the Registration Request,
      -  the care-of address used in the registration,
      -  the Identification value sent in the registration,
      -  the originally requested Lifetime, and
      -  the remaining Lifetime of the pending registration.

   A mobile node SHOULD initiate a registration whenever it detects a
   change in its network connectivity.  See Section 2.4.2 for methods by
   which mobile nodes MAY 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 for it.  When it is at
   home, the mobile node's (de)Registration Request allows its home
   agent to delete any previous mobility binding(s) for it.  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 foreign 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 SHOULD NOT cause a
   mobile node to attempt a new registration, if its current
   registration has not expired and it is still also receiving Agent
   Advertisements from the foreign agent with which it is currently
   registered.  In the absence of link-layer indications, a mobile node
   MUST NOT attempt to register more often than once per second.






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   A mobile node MAY register with a different agent when transport-
   layer protocols indicate excessive retransmissions.  A mobile node
   MUST NOT consider reception of an ICMP Redirect from a foreign agent
   that is currently providing service to it as reason to register with
   a new foreign agent.  Within these constraints, the mobile node MAY
   register again at any time.

   Appendix D shows some examples of how the fields in registration
   messages would be set up in some typical registration scenarios.

3.6.1. Sending Registration Requests

   The following sections specify details 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 mobile nodes pick
   values for the IP header fields of a Registration Request.

   IP Source Address:

      -  When registering on a foreign network with a co-located care-of
         address, the IP source address MUST be the care-of address.

      -  Otherwise, if the mobile node does not have a home address, the
         IP source address MUST be 0.0.0.0.

      -  In all other circumstances, the IP source address MUST be the
         mobile node's home address.

   IP Destination Address:

      -  When the mobile node has discovered the agent with which it is
         registering, through some means (e.g., link-layer) that does
         not provide the IP address of the agent (the IP address of the
         agent is unknown to the mobile node), then the "All Mobility
         Agents" multicast address (224.0.0.11) MUST be used.  In this
         case, the mobile node MUST 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.  This MAY be an address which
         does not appear as an advertised care-of address in the Agent
         Advertisement.  In addition, when transmitting this
         Registration Request message, the mobile node MUST use a link-




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         layer destination address copied from the link-layer source
         address of the Agent Advertisement message in which it learned
         this foreign agent's IP address.

      -  When the mobile node is registering directly with its home
         agent and knows the (unicast) IP address of its home agent, the
         destination address MUST be set to this address.

      -  If the mobile node is registering directly with its home agent,
         but does not know the IP address of its home agent, the mobile
         node may use dynamic home agent address resolution to
         automatically determine the IP address of its home agent
         (Section 3.6.1.2).  In this case, the IP destination address is
         set to the subnet-directed broadcast address of the mobile
         node's home network.  This address MUST NOT be used as the
         destination IP address if the mobile node is registering via a
         foreign agent, although it MAY be used as the Home Agent
         address in the body of the Registration Request when
         registering via a foreign agent.

   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 [38].

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 using at
   least one wireless network interface moves within wireless
   transmission range of more than one foreign agent.  IP explicitly
   allows duplication of datagrams.  When the home agent allows
   simultaneous bindings, it will tunnel a separate copy of each
   arriving datagram to each care-of address, and the mobile node will
   receive multiple copies of datagrams destined to it.

   The mobile node SHOULD set the 'D' bit if it is registering with a
   co-located care-of address.  Otherwise, the 'D' bit MUST NOT be set.




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   A mobile node MAY set the 'B' bit to request its home agent to
   forward to it, a copy of broadcast datagrams received by its home
   agent from the home network.  The method used by the home agent to
   forward broadcast datagrams depends on the type of care-of address
   registered by the mobile node, as determined by the 'D' bit in the
   mobile node's Registration Request:

      -  If the 'D' bit is set, then the mobile node has indicated that
         it will decapsulate any datagrams tunneled to this care-of
         address itself (the mobile node is using a co-located care-of
         address).  In this case, to forward such a received broadcast
         datagram to the mobile node, the home agent MUST tunnel it to
         this care-of address.  The mobile node de-tunnels the received
         datagram in the same way as any other datagram tunneled
         directly to it.

      -  If the 'D' bit is NOT set, then the mobile node has indicated
         that it is using a foreign agent care-of address, and that the
         foreign agent will thus decapsulate arriving datagrams before
         forwarding them to the mobile node.  In this case, to forward
         such a received broadcast datagram to the mobile node, the home
         agent MUST first encapsulate the broadcast datagram in a
         unicast datagram addressed to the mobile node's home address,
         and then MUST tunnel this resulting datagram to the mobile
         node's care-of address.

         When decapsulated by the foreign agent, the inner datagram will
         thus be a unicast IP datagram addressed to the mobile node,
         identifying to the foreign agent the intended destination of
         the encapsulated broadcast datagram, and will be delivered to
         the mobile node in the same way as any tunneled datagram
         arriving for the mobile node.  The foreign agent MUST NOT
         decapsulate the encapsulated broadcast datagram and MUST NOT
         use a local network broadcast to transmit it to the mobile
         node.  The mobile node thus MUST decapsulate the encapsulated
         broadcast datagram itself, and thus MUST NOT set the 'B' bit in
         its Registration Request in this case unless it is capable of
         decapsulating datagrams.

   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 datagrams (the mobile node is using a co-
   located care-of address) or if its foreign agent has indicated
   support for these forms of encapsulation by setting the corresponding
   bits in the Mobility Agent Advertisement Extension of an Agent
   Advertisement received by the mobile node.  Otherwise, the mobile
   node MUST NOT set these bits.




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   The Lifetime field is chosen as follows:

      -  If the mobile node is registering with a foreign agent, the
         Lifetime SHOULD NOT exceed the value in the Registration
         Lifetime field of the Agent Advertisement message received from
         the foreign agent.
         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 delete a particular
         mobility binding, by sending a Registration Request with the
         care-of address for this binding, with the Lifetime field set
         to zero (Section 3.8.2).

      -  Similarly, a Lifetime of zero is used when the mobile node
         deregisters all care-of addresses, such as upon returning home.

   The Home Address field MUST be set to the mobile node's home address,
   if this information is known.  Otherwise, the Home Address MUST be
   set to zeroes.

   The Home Agent field MUST be set to the address of the mobile node's
   home agent, if the mobile node knows this address.  Otherwise, the
   mobile node MAY use dynamic home agent address resolution to learn
   the address of its home agent.  In this case, the mobile node MUST
   set the Home Agent field to the subnet-directed broadcast address of
   the mobile node's home network.  Each home agent receiving such a
   Registration Request with a broadcast destination address MUST reject
   the mobile node's registration and SHOULD return a rejection
   Registration Reply indicating its unicast IP address for use by the
   mobile node in a future registration attempt.

   The Care-of Address field MUST be set to the value of the particular
   care-of address that the mobile node wishes to (de)register.  In the
   special case in which a mobile node wishes to deregister all care-of
   addresses, it MUST set this field to its home address.

   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 5.7 for the method by which the mobile
   node computes the Identification field.








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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, followed by

      b) If present, any non-authentication Extensions expected to be
         used by the home agent (which may or may not also be useful to
         the foreign agent), followed by

      c) An authorization-enabling extension, followed by

      d) If present, any non-authentication Extensions used only by the
         foreign agent, followed by

      e) The Mobile-Foreign Authentication Extension, if present.

   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 mobility 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:

      - the registration was accepted,
      - the registration was denied by the foreign agent, or
      - the registration was denied by the home agent.

   The remainder of this section describes the Registration Reply
   handling by a mobile node in each of these three categories.

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 the
   Identification field in the most recent Registration Request sent to
   the replying agent.  If they do not match, the Reply MUST be silently
   discarded.



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   Also, the Registration Reply MUST be checked for presence of an
   authorization-enabling extension.  For all Registration Reply
   messages containing a Status Code indicating status from the Home
   Agent, the mobile node MUST check for the presence of an
   authorization-enabling extension, acting in accordance with the Code
   field in the Reply.  The rules are as follows:

      a) If the mobile node and the foreign agent share a mobility
         security association, exactly one Mobile-Foreign Authentication
         Extension MUST be present in the Registration Reply, and the
         mobile node MUST check the Authenticator value in the
         Extension.  If no Mobile-Foreign Authentication Extension is
         found, or if more than one Mobile-Foreign Authentication
         Extension is found, or if the Authenticator is invalid, the
         mobile node MUST silently discard the Reply and 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 registration was
         accepted by the home agent, exactly one Mobile-Home
         Authentication Extension MUST be present in the Registration
         Reply, and the mobile node MUST check the Authenticator value
         in the Extension.  If the Registration Reply was generated by
         the home agent but no Mobile-Home Authentication Extension is
         found, or if more than one Mobile-Home Authentication Extension
         is found, or if the Authenticator is invalid, the mobile node
         MUST silently discard the Reply and 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. Registration Request Accepted

   If the Code field indicates that the request has been accepted, the
   mobile node SHOULD configure its routing table appropriately for its
   current point of attachment (Section 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.





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   If the mobile node has registered on a foreign network, it SHOULD
   re-register before the expiration of the Lifetime of its
   registration.  As described in Section 3.6, for each pending
   Registration Request, the mobile node MUST maintain the remaining
   lifetime of this pending registration, as well as the original
   Lifetime from the Registration Request.  When the mobile node
   receives a valid Registration Reply, the mobile node MUST decrease
   its view of the remaining lifetime of the registration by the amount
   by which the home agent decreased the originally requested Lifetime.
   This procedure is equivalent to the mobile node starting a timer for
   the granted Lifetime at the time it sent the Registration Request,
   even though the granted Lifetime is not known to the mobile node
   until the Registration Reply is received.  Since the Registration
   Request is certainly sent before the home agent begins timing the
   registration Lifetime (also based on the granted Lifetime), this
   procedure ensures that the mobile node will re-register before the
   home agent expires and deletes the registration, in spite of possibly
   non-negligible transmission delays for the original Registration
   Request and Reply that started the timing of the Lifetime at the
   mobile node and its home agent.

3.6.2.3. Registration Request Denied

   If the Code field indicates that service is being denied, the mobile
   node SHOULD log the error.  In certain cases 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 Lifetime value which that foreign agent is
         willing to accept in any Registration Request.  The mobile node
         MAY 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 (Section 5.7).  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.






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

         This code is returned by a home agent when the mobile node is
         performing dynamic home agent address resolution as described
         in Sections 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 MAY be 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 (Section
   5.7).  In this way a retransmission will not require the home agent
   to resynchronize with the mobile node by issuing another nonce in the
   case in which the original Registration Request (rather than its
   Registration Reply) was lost by the network.

   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 messages, twice the round trip time for transmission to the home
   agent, and at least an additional 100 milliseconds to allow for
   processing the messages before responding.  The round trip time for
   transmission to the home agent will be at least as large as the time
   required to transmit the messages at the link speed of the mobile
   node's current point of attachment.  Some circuits add another 200
   milliseconds of satellite delay in the total round trip time to the
   home agent.  The minimum time between Registration Requests MUST NOT
   be less than 1 second.  Each successive retransmission timeout period
   SHOULD be at least twice the previous period, as long as that is less
   than the maximum as specified above.

3.7. Foreign Agent Considerations

   The foreign agent plays a mostly 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 SHOULD
   also send periodic Agent Advertisement messages to advertise its



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   presence as described in Section 2.3, if not detectable by link-layer
   means.

   A foreign agent MUST NOT transmit a Registration Request except when
   relaying a Registration Request received from a mobile node, to the
   mobile node's home agent.  A foreign agent MUST NOT transmit a
   Registration Reply except when relaying a Registration Reply received
   from a mobile node's home agent, or when replying to a Registration
   Request received from a mobile node in the case in which the foreign
   agent is denying service to the mobile node.  In particular, a
   foreign agent MUST NOT generate a Registration Request or Reply
   because a mobile node's registration Lifetime has expired.  A foreign
   agent also MUST NOT originate a Registration Request message that
   asks for deregistration of a mobile node; however, it MUST relay
   valid (de)Registration Requests originated by a mobile node.

3.7.1. Configuration and Registration Tables

   Each foreign agent MUST be configured with 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:

      -  the link-layer source address of the mobile node
      -  the IP Source Address (the mobile node's Home Address) or its
         co-located care-of address (see description of the 'R' bit in
         section 2.1.1)
      -  the IP Destination Address (as specified in 3.6.1.1)
      -  the UDP Source Port
      -  the Home Agent address
      -  the Identification field
      -  the requested registration Lifetime, and
      -  the remaining Lifetime of the pending or current registration.

   If the mobile node's Home Address is zero in the Registration Request
   message, then the foreign agent MUST follow the procedures specified
   in RFC 2794 [6].  In particular, if the foreign agent cannot manage
   pending registration request records with such a zero Home Address
   for the mobile node, the foreign agent MUST return a Registration
   Reply with Code indicating NONZERO_HOMEADDR_REQD (see [6]).

   The foreign agent MAY configure a maximum number of pending
   registrations that it is willing to maintain (typically 5).
   Additional registrations SHOULD then be rejected by the foreign agent
   with code 66.  The foreign agent MAY delete any pending Registration
   Request after the request has been pending for more than 7 seconds;
   in this case, the foreign agent SHOULD reject the Request with code
   78 (registration timeout).



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   As with any node on the Internet, a foreign agent MAY also share
   mobility security associations with any other nodes.  When relaying a
   Registration Request from a mobile node to its home agent, if the
   foreign agent shares a mobility security association with the home
   agent, it MUST add a Foreign-Home Authentication Extension to the
   Request and MUST check the required Foreign-Home Authentication
   Extension in the Registration Reply from the home agent (Sections 3.3
   and 3.4).  Similarly, when receiving a Registration Request from a
   mobile node, if the foreign agent shares a mobility security
   association with the mobile node, it MUST check the required Mobile-
   Foreign Authentication Extension in the Request and MUST add a
   Mobile-Foreign Authentication Extension to the Registration Reply to
   the mobile node.

3.7.2. Receiving Registration Requests

   If the foreign agent accepts a Registration Request from a mobile
   node, it checks to make sure that the indicated home agent address
   does not belong to any network interface of the foreign agent.  If
   not, the foreign agent then MUST relay the Request to the indicated
   home agent.  Otherwise, if the foreign agent denies the Request, it
   MUST send a Registration Reply to the mobile node with an appropriate
   denial Code, except in cases where the foreign agent would be
   required to send out more than one such denial per second to the same
   mobile node.  The following sections describe this behavior in more
   detail.

   If the foreign agent has configured one of its network interfaces
   with the IP address specified by the mobile node as its home agent
   address, the foreign agent MUST NOT forward the request again.  If
   the foreign agent serves the mobile node as a home agent, the foreign
   agent follows the procedures specified in section 3.8.2.  Otherwise,
   if the foreign agent does not serve the mobile node as a home agent,
   the foreign agent rejects the Registration Request with code 136
   (unknown home agent address).

   If a foreign agent receives a Registration Request from a mobile node
   in its visitor list, the existing visitor list entry for the mobile
   node SHOULD NOT be deleted or modified until the foreign agent
   receives a valid Registration Reply from the home agent with a Code
   indicating success.  The foreign agent MUST record the new pending
   Request as a separate part of the existing visitor list entry for the
   mobile node.  If the Registration Request requests deregistration,
   the existing visitor list entry for the mobile node SHOULD NOT be
   deleted until the foreign agent has received a successful
   Registration Reply.  If the Registration Reply indicates that the





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   Request (for registration or deregistration) was denied by the home
   agent, the existing visitor list entry for the mobile node MUST NOT
   be modified as a result of receiving the Registration Reply.

3.7.2.1. Validity Checks

   Registration Requests with an invalid, non-zero UDP checksum MUST be
   silently discarded.  Requests with non-zero bits in reserved fields
   MUST be rejected with code 70 (poorly formed request).  Requests with
   the 'D' bit set to 0, and specifying a care-of address not offered by
   the foreign agent, MUST be rejected with code 77 (invalid care-of
   address).

   Also, the authentication in the Registration Request MUST be checked.
   If the foreign agent and the mobile node share a mobility security
   association, exactly one Mobile-Foreign Authentication Extension MUST
   be present in the Registration Request, and the foreign agent MUST
   check the Authenticator value in the Extension.  If no Mobile-Foreign
   Authentication Extension is found, or if more than one Mobile-Foreign
   Authentication Extension is found, or if the Authenticator is
   invalid, the foreign agent MUST silently discard the Request and
   SHOULD log the event as a security exception.  The foreign agent also
   SHOULD send a Registration Reply to the mobile node with Code 67.

3.7.2.2. Forwarding a Valid Request to the Home Agent

   If the foreign agent accepts the mobile node's Registration Request,
   it MUST relay the Request to the mobile node's home agent as
   specified in the Home Agent field of the Registration Request.  The
   foreign agent MUST NOT modify any of the fields beginning with the
   fixed portion of the Registration Request up through and including
   the Mobile-Home Authentication Extension or other authentication
   extension supplied by the mobile node as an authorization-enabling
   extension for the home agent.  Otherwise, an authentication failure
   is very likely to occur at the home agent.  In addition, the foreign
   agent proceeds as follows:

      -  It MUST process and remove any Extensions following the
         Mobile-Home Authentication Extension,
      -  It MAY 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 mobility security association with
         the home agent.







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

      IP Source Address

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

      IP Destination Address

               Copied from the Home Agent field within the Registration
               Request.

      UDP Source Port

               <variable>

      UDP Destination Port

               434

   After forwarding a valid Registration Request to the home agent, the
   foreign agent MUST begin timing the remaining lifetime of the pending
   registration based on the Lifetime in the Registration Request.  If
   this lifetime expires before receiving a valid Registration Reply,
   the foreign agent MUST delete its visitor list entry for this pending
   registration.

3.7.2.3. Denying Invalid Requests

   If the foreign agent denies the mobile node's Registration Request
   for any reason, it SHOULD send the mobile node a Registration Reply
   with a suitable denial 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 the Reserved field is nonzero, the foreign agent MUST deny the
   Request and SHOULD return a Registration Reply with status code 70 to
   the mobile node.  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 Lifetime value it is willing to accept in
   any Registration Request, and sets the Code field to 69.  Otherwise,
   the Lifetime SHOULD be copied from the Lifetime field in the Request.








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   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 message will be sent) MUST be
               used.

      IP Destination Address

               If the Registration Reply is generated by the Foreign
               Agent in order to reject a mobile node's Registration
               Request, and the Registration Request contains a Home
               Address which is not 0.0.0.0, then the IP Destination
               Address is copied from the Home Address field of the
               Registration Request.  Otherwise, if the Registration
               Reply is received from the Home Agent, and contains a
               Home Address which is not 0.0.0.0, then the IP
               Destination Address is copied from the Home Address field
               of the Registration Reply.  Otherwise, the IP Destination
               Address of the Registration Reply is set to be
               255.255.255.255.

      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 SHOULD send 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
   on building the Registration Reply.



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3.7.3.1. Validity Checks

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

   When a foreign agent receives a Registration Reply message, it MUST
   search its visitor list for a pending Registration Request with the
   same mobile node home address as indicated in the Reply.  If no such
   pending Request is found, and if the Registration Reply does not
   correspond with any pending Registration Request with a zero mobile
   node home address (see section 3.7.1), the foreign agent MUST
   silently discard the Reply.  The foreign agent MUST also silently
   discard the Reply if the low-order 32 bits of the Identification
   field in the Reply do not match those in the Request.

   Also, the authentication in the Registration Reply MUST be checked.
   If the foreign agent and the home agent share a mobility security
   association, exactly one Foreign-Home Authentication Extension MUST
   be present in the Registration Reply, and the foreign agent MUST
   check the Authenticator value in the Extension.  If no Foreign-Home
   Authentication Extension is found, or if more than one Foreign-Home
   Authentication Extension is found, or if the Authenticator is
   invalid, the foreign agent MUST silently discard the Reply and SHOULD
   log the event as a security exception.  The foreign agent also MUST
   reject the mobile node's registration and SHOULD send a Registration
   Reply to the mobile node with Code 68.

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.  The foreign agent MUST also
   update its visitor list entry for the mobile node to reflect the
   results of the Registration Request, as indicated by the Code field
   in the Reply.  If the Code indicates that the home agent has accepted
   the registration and the Lifetime field is nonzero, the foreign agent
   SHOULD set the Lifetime in the visitor list entry to the minimum of
   the following two values:

      -  the value specified in the Lifetime field of the Registration
         Reply, and

      -  the foreign agent's own maximum value for allowable
         registration lifetime.

   If, instead, the Code indicates that the Lifetime field is zero, the
   foreign agent MUST delete its visitor list entry for the mobile node.
   Finally, if the Code indicates that the registration was denied by




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   the home agent, the foreign agent MUST delete its pending
   registration list entry, but not its visitor list entry, for the
   mobile node.

   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 process and remove any Extensions following the
         Mobile-Home Authentication Extension,
      -  It MAY 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 mobility 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
   specified in Section 3.7.2.3.

   After forwarding a valid Registration Reply to the mobile node, the
   foreign agent MUST update its visitor list entry for this
   registration as follows.  If the Registration Reply indicates that
   the registration was accepted by the home agent, the foreign agent
   resets its timer of the lifetime of the registration to the Lifetime
   granted in the Registration Reply; unlike the mobile node's timing of
   the registration lifetime as described in Section 3.6.2.2, the
   foreign agent considers this lifetime to begin when it forwards the
   Registration Reply message, ensuring that the foreign agent will not
   expire the registration before the mobile node does.  On the other
   hand, if the Registration Reply indicates that the registration was
   rejected by the home agent, the foreign agent deletes its visitor
   list entry for this attempted registration.

3.8. Home Agent Considerations

   Home agents play a reactive role in the registration process.  The
   home agent receives Registration Requests from the mobile node
   (perhaps relayed by a foreign agent), updates its record of the
   mobility bindings for this mobile node, and issues a suitable
   Registration Reply in response to each.






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   A home agent MUST NOT transmit a Registration Reply except when
   replying to a Registration Request received from a mobile node.  In
   particular, the home agent MUST NOT generate a Registration Reply to
   indicate that the Lifetime has expired.

3.8.1. Configuration and Registration Tables

   Each home agent MUST be configured with an IP address and with the
   prefix size for the home network.  The home agent MUST be configured
   with the mobility security association of each authorized mobile node
   that it is serving as a home agent.

   When the home agent accepts a valid Registration Request from a
   mobile node that it serves as a home agent, the home agent MUST
   create or modify the entry for this mobile node in its mobility
   binding list containing:

      -  the mobile node's home address
      -  the mobile node's care-of address
      -  the Identification field from the Registration Reply
      -  the remaining Lifetime of the registration

   The home agent MAY optionally offer the capability to dynamically
   associate a home address to a mobile node upon receiving a
   Registration Request from that mobile node.  The method by which a
   home address is allocated to the mobile node is beyond the scope of
   this document, but see [6].  After the home agent makes the
   association of the home address to the mobile node, the home agent
   MUST put that home address into the Home Address field of the
   Registration Reply.

   The home agent MAY also maintain mobility security associations with
   various foreign agents.  When receiving a Registration Request from a
   foreign agent, if the home agent shares a mobility security
   association with the foreign agent, the home agent MUST check the
   Authenticator in the required Foreign-Home Authentication Extension
   in the message, based on this mobility security association.
   Similarly, when sending a Registration Reply to a foreign agent, if
   the home agent shares a mobility security association with the
   foreign agent, the home agent MUST include a Foreign-Home
   Authentication Extension in the message, based on this mobility
   security association.









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3.8.2. Receiving Registration Requests

   If the home agent accepts an incoming Registration Request, it MUST
   update its record of the the mobile node's mobility binding(s) and
   SHOULD send a Registration Reply with a suitable Code.  Otherwise
   (the home agent denies the Request), it SHOULD send a Registration
   Reply with an appropriate Code specifying the reason the Request was
   denied.  The following sections describe this behavior in more
   detail.  If the home agent does not support broadcasts (see section
   4.3), it MUST ignore the 'B' bit (as opposed to rejecting the
   Registration Request).

3.8.2.1. Validity Checks

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

   The authentication in the Registration Request MUST be checked.  This
   involves the following operations:

      a) The home agent MUST check for the presence of an
         authorization-enabling extension, and perform the indicated
         authentication.  Exactly one authorization-enabling extension
         MUST be present in the Registration Request; and the home agent
         MUST either check the Authenticator value in the extension or
         verify that the authenticator value has been checked by another
         agent with which it has a security association.  If no
         authorization-enabling extension is found, or if more than one
         authorization-enabling extension is found, or if the
         Authenticator is invalid, the home agent MUST reject the mobile
         node's registration and SHOULD send a Registration Reply to the
         mobile node with Code 131.  The home agent MUST then discard
         the Request and SHOULD log the error as a security exception.

      b) The home agent MUST check that the registration Identification
         field is correct using the context selected by the SPI within
         the authorization-enabling extension.  See Section 5.7 for a
         description of how this is performed.  If incorrect, the home
         agent MUST reject the Request and SHOULD send a Registration
         Reply to the mobile node with Code 133, including an
         Identification field computed in accordance with the rules
         specified in Section 5.7.  The home agent MUST do no further
         processing with such a Request, though it SHOULD log the error
         as a security exception.

      c) If the home agent shares a mobility security association with
         the foreign agent, the home agent MUST check for the presence
         of a valid Foreign-Home Authentication Extension.  Exactly one



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         Foreign-Home Authentication Extension MUST be present in the
         Registration Request in this case, and the home agent MUST
         check the Authenticator value in the Extension.  If no
         Foreign-Home Authentication Extension is found, or if more than
         one Foreign-Home Authentication Extension is found, or if the
         Authenticator is invalid, the home agent MUST reject the mobile
         node's registration and SHOULD send a Registration Reply to the
         mobile node with Code 132.  The home agent MUST then discard
         the Request and SHOULD log the error as a security exception.

   In addition to checking the authentication in the Registration
   Request, home agents MUST deny 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 MUST discard the
   Request and SHOULD returning a Registration Reply with a Code of 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.

   Note that some routers change the IP destination address of a
   datagram from a subnet-directed broadcast address to 255.255.255.255
   before injecting it into the destination subnet.  In this case, home
   agents that attempt to pick up dynamic home agent discovery requests
   by binding a socket explicitly to the subnet-directed broadcast
   address will not see such packets.  Home agent implementors should be
   prepared for both the subnet-directed broadcast address and
   255.255.255.255 if they wish to support dynamic home agent discovery.

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 MUST update its mobility binding list for the requesting
   mobile node and MUST return 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 on building the Registration Reply message.

   The home agent updates its record of the mobile node's mobility
   bindings as follows, based on the fields in the Registration Request:

      -  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.




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      -  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 and
         the home agent supports simultaneous mobility bindings, the
         previous mobility binding entries are retained.  Otherwise, the
         home agent removes all previous entries in the mobility binding
         list for the mobile node.

   In all cases, the home agent MUST send 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 mobility security
   association with the foreign agent whose care-of address is being
   deregistered, and that 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 mobility 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.

   The home agent MUST NOT increase the Lifetime above that specified by
   the mobile node in the Registration Request.  However, it is not an
   error for the mobile node to request a Lifetime longer than the home
   agent is willing to accept.  In this case, the home agent simply
   reduces the Lifetime to a permissible value and returns this value in
   the Registration Reply.  The Lifetime value in the Registration Reply
   informs the mobile node of the granted lifetime of the registration,
   indicating when it SHOULD re-register in order to maintain continued
   service.  After the expiration of this registration lifetime, the
   home agent MUST delete its entry for this registration in its
   mobility binding list.

   If the Registration Request duplicates an accepted current
   Registration Request, the new Lifetime MUST NOT extend beyond the
   Lifetime originally granted.  A Registration Request is a duplicate
   if the home address, care-of address, and Identification fields all
   equal those of an accepted current registration.






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   In addition, if the home network implements ARP [36], 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 Section 3.8.2.1, or the home agent is unable to accommodate the
   Request, the home agent SHOULD return 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.
   See Section 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 a Code of 129.

   Requests with non-zero bits in reserved fields MUST be rejected with
   code 134 (poorly formed request).

3.8.3. Sending Registration Replies

   If the home agent accepts a Registration Request, it then MUST update
   its record of the mobile node's mobility binding(s) and SHOULD send a
   Registration Reply with a suitable Code.  Otherwise (the home agent
   has denied the Request), it SHOULD send a Registration Reply with an
   appropriate Code specifying the reason the Request was denied.  The
   following sections provide additional detail for the values the home
   agent MUST supply in the fields of Registration Reply messages.





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3.8.3.1. IP/UDP Fields

   This section provides the specific rules by which home agents 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.  If the IP Destination Address of the Registration
               Request was a broadcast or multicast address, the IP
               Source Address of the Registration Reply MUST be set to
               the home agent's (unicast) IP address.

      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.

   When sending a Registration Reply in response to a Registration
   Request that requested deregistration of the mobile node (the
   Lifetime is zero and the Care-of Address equals the mobile node's
   home address) and in which the IP Source Address was also set to the
   mobile node's home address (this is the normal method used by a
   mobile node to deregister when it returns to its home network), the
   IP Destination Address in the Registration Reply will be set to the
   mobile node's home address, as copied from the IP Source Address of
   the Request.

   In this case, when transmitting the Registration Reply, the home
   agent MUST transmit the Reply directly onto the home network as if
   the mobile node were at home, bypassing any mobility binding list
   entry that may still exist at the home agent for the destination
   mobile node.  In particular, for a mobile node returning home after
   being registered with a care-of address, if the mobile node's new
   Registration Request is not accepted by the home agent, the mobility
   binding list entry for the mobile node will still indicate that
   datagrams addressed to the mobile node should be tunneled to the
   mobile node's registered care-of address; when sending the
   Registration Reply indicating the rejection of this Request, this
   existing binding list entry MUST be ignored, and the home agent MUST
   transmit this Reply as if the mobile node were at home.



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3.8.3.2. Registration Reply Fields

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

   The Code field of the Registration Reply is chosen in accordance with
   the rules specified in the previous sections.  When replying to an
   accepted registration, a home agent SHOULD respond with Code 1 if it
   does not support simultaneous registrations.

   The Lifetime field MUST be 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.  This reduced Lifetime SHOULD be the maximum Lifetime allowed
   by the home agent (for this mobile node and care-of address).

   If the Home Address field of the Registration Request is nonzero, it
   MUST be copied into the Home Address field of the Registration Reply
   message.  Otherwise, if the Home Address field of the Registration
   Request is zero as specified in section 3.6, the home agent SHOULD
   arrange for the selection of a home address for the mobile node, and
   insert the selected address into the Home Address field of the
   Registration Reply message.  See [6] for further relevant details in
   the case where mobile nodes identify themselves using an NAI instead
   of their IP home address.

   If the Home Agent field in the Registration Request contains a
   unicast address of this home agent, then that field MUST be copied
   into the Home Agent field of the Registration Reply.  Otherwise, the
   home agent MUST set the Home Agent field in the Registration Reply to
   its unicast address.  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 possibly being simultaneously registered with two or
   more home agents.

3.8.3.3. Extensions

   This section describes the ordering of any required and any optional
   Mobile IP 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,





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      b) If present, any non-authentication Extensions used by the
         mobile node (which may or may not also be used by the foreign
         agent),

      c) The Mobile-Home Authentication Extension,

      d) If present, any non-authentication Extensions used only by the
         foreign agent, and

      e) The Foreign-Home Authentication Extension, if present.

   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 mobility security association.

4. Routing Considerations

   This section describes how mobile nodes, home agents, and (possibly)
   foreign agents cooperate to route datagrams 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

   Home agents and foreign agents MUST support tunneling datagrams using
   IP in IP encapsulation [32].  Any mobile node that uses a co-located
   care-of address MUST support receiving datagrams tunneled using IP in
   IP encapsulation.  Minimal encapsulation [34] and GRE encapsulation
   [16] are alternate encapsulation methods which MAY optionally be
   supported by mobility agents and mobile nodes.  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 Datagram 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 in the same
   way as any other (fixed) host or router.  The method by which a
   mobile node selects a default router when connected to its home





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   network, or when away from home and using a co-located care-of
   address, is outside the scope of this document.  ICMP Router
   Advertisement [10] is one such method.

   When registered on a foreign network, the mobile node chooses a
   default router by the following rules:

      -  If the mobile node is registered using a foreign agent care-of
         address, it MAY use its foreign agent as a first-hop router.
         The foreign agent's MAC address can be learned from Agent
         Advertisement.  Otherwise, the mobile node MUST choose its
         default router from among the Router Addresses advertised in
         the ICMP Router Advertisement portion of that Agent
         Advertisement message.

      -  If the mobile node is registered directly with its home agent
         using a co-located care-of address, then the mobile node SHOULD
         choose its default router from among those advertised in any
         ICMP Router Advertisement message that it receives for which
         its externally obtained care-of address and the Router Address
         match under the network prefix.  If the mobile node's
         externally obtained care-of address matches the IP source
         address of the Agent Advertisement under the network prefix,
         the mobile node MAY also consider that IP source address as
         another possible choice for the IP address of a default router.
         The network prefix MAY be obtained from the Prefix-Lengths
         Extension in the Router Advertisement, if present.  The prefix
         MAY also be obtained through other mechanisms beyond the scope
         of this document.

   While they are away from the home network, mobile nodes MUST NOT
   broadcast ARP packets to find the MAC address of another Internet
   node.  Thus, the (possibly empty) list of Router Addresses from the
   ICMP Router Advertisement portion of the message is not useful for
   selecting a default router, unless the mobile node has some means not
   involving broadcast ARP and not specified within this document for
   obtaining the MAC address of one of the routers in the list.
   Similarly, in the absence of unspecified mechanisms for obtaining MAC
   addresses on foreign networks, the mobile node MUST ignore redirects
   to other routers on foreign networks.

4.2.2. Foreign Agent Considerations

   Upon receipt of an encapsulated datagram 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 the address of any mobile node currently in the
   visitor list, the foreign agent MUST NOT forward the datagram without



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   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 forwards the decapsulated datagram 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
   (Section 4.5) or mobile node in its visitor list.

   The foreign agent MUST route datagrams 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 datagrams to a
   default router.

   A foreign agent MUST NOT use broadcast ARP for a mobile node's MAC
   address on a foreign network.  It may obtain the MAC address by
   copying the information from an Agent Solicitation or a Registration
   Request transmitted from a mobile node.  A foreign agent's ARP cache
   for the mobile node's IP address MUST NOT be allowed to expire before
   the mobile node's visitor list entry expires, unless the foreign
   agent has some way other than broadcast ARP to refresh its MAC
   address associated with the mobile node's IP address.

   Each foreign agent SHOULD support the mandatory features for reverse
   tunneling [27].

4.2.3. Home Agent Considerations

   The home agent MUST be able to intercept any datagrams on the home
   network addressed to the mobile node while the mobile node is
   registered away from home.  Proxy and gratuitous ARP MAY be used in
   enabling this interception, as specified in Section 4.6.

   The home agent must examine the IP Destination Address of all
   arriving datagrams to see if it is equal to the home address of any
   of its mobile nodes registered away from home.  If so, the home agent
   tunnels the datagram to the mobile node's currently registered care-
   of address or addresses.  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 MUST NOT attempt to intercept datagrams destined for the mobile
   node, and thus will not in general receive such datagrams.  However,
   if the home agent is also a router handling common IP traffic, it is
   possible that it will receive such datagrams for forwarding onto the




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   home network.  In this case, the home agent MUST assume the mobile
   node is at home and simply forward the datagram directly onto the
   home network.

   For multihomed home agents, the source address in the outer IP header
   of the encapsulated datagram MUST be the address sent to the mobile
   node in the home agent field of the registration reply.  That is, the
   home agent cannot use the the address of some other network interface
   as the source address.

   See Section 4.1 regarding methods of encapsulation that may be used
   for tunneling.  Nodes implementing tunneling SHOULD also implement
   the "tunnel soft state" mechanism [32], which allows ICMP error
   messages returned from the tunnel to correctly be reflected back to
   the original senders of the tunneled datagrams.

   Home agents MUST decapsulate packets addressed to themselves, sent by
   a mobile node for the purpose of maintaining location privacy, as
   described in Section 5.5.  This feature is also required for support
   of reverse tunneling [27].

   If the Lifetime for a given mobility binding expires before the home
   agent has received another valid Registration Request for that mobile
   node, then that binding is deleted from the mobility binding list.
   The home agent MUST NOT send any Registration Reply message simply
   because the mobile node's binding has expired.  The entry in the
   visitor list of the mobile node's current foreign agent will expire
   naturally, probably at the same time as the binding expired at the
   home agent.  When a mobility binding's lifetime expires, the home
   agent MUST delete the binding, but it MUST retain any other (non-
   expired) simultaneous mobility bindings that it holds for the mobile
   node.

   When a home agent receives a datagram, intercepted for one of its
   mobile nodes registered away from home, the home agent MUST examine
   the datagram to check if it is already encapsulated.  If so, special
   rules apply in the forwarding of that datagram to the mobile node:

      -  If the inner (encapsulated) Destination Address is the same as
         the outer Destination Address (the mobile node), then the home
         agent MUST also examine the outer Source Address of the
         encapsulated datagram (the source address of the tunnel).  If
         this outer Source Address is the same as the mobile node's
         current care-of address, the home agent MUST silently discard
         that datagram in order to prevent a likely routing loop.  If,
         instead, the outer Source Address is NOT the same as the mobile
         node's current care-of address, then the home agent SHOULD
         forward the datagram to the mobile node.  In order to forward



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         the datagram in this case, the home agent MAY simply alter the
         outer Destination Address to the care-of address, rather than
         re-encapsulating the datagram.

      -  Otherwise (the inner Destination Address is NOT the same as the
         outer Destination Address), the home agent SHOULD encapsulate
         the datagram again (nested encapsulation), with the new outer
         Destination Address set equal to the mobile node's care-of
         address.  That is, the home agent forwards the entire datagram
         to the mobile node in the same way as any other datagram
         (encapsulated already or not).

4.3. Broadcast Datagrams

   When a home agent receives a broadcast datagram, it MUST NOT forward
   the datagram to any mobile nodes in its mobility binding list other
   than those that have requested forwarding of broadcast datagrams.  A
   mobile node MAY request forwarding of broadcast datagrams by setting
   the 'B' bit in its Registration Request message (Section 3.3).  For
   each such registered mobile node, the home agent SHOULD forward
   received broadcast datagrams to the mobile node, although it is a
   matter of configuration at the home agent as to which specific
   categories of broadcast datagrams will be forwarded to such mobile
   nodes.

   If the 'D' bit was set in the mobile node's Registration Request
   message, indicating that the mobile node is using a co-located care-
   of address, the home agent simply tunnels appropriate broadcast IP
   datagrams to the mobile node's care-of address.  Otherwise (the 'D'
   bit was NOT set), 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 the
   foreign agent can determine which mobile node should receive the
   datagram 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 Datagram Routing

   As mentioned previously, a mobile node that is connected to its home
   network functions in the same way as any other (fixed) 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.



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   In order to receive multicasts, a mobile node MUST join the multicast
   group in one of two ways.  First, a mobile node 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.  If the mobile node is using a co-located care-of address, it
   SHOULD use this address as the source IP address of its IGMP [11]
   messages.  Otherwise, it MAY use its home address.

   Alternatively, a mobile node which wishes to receive multicasts MAY
   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 messages to its home agent and the home agent forwards multicast
   datagrams down the tunnel to the mobile node.  For packets tunneled
   to the home agent, the source address in the IP header SHOULD be the
   mobile node's home address.

   The rules for multicast datagram delivery to mobile nodes in this
   case are identical to those for broadcast datagrams (Section 4.3).
   Namely, if the mobile node is using a co-located care-of address (the
   'D' bit was set in the mobile node's Registration Request), then the
   home agent SHOULD tunnel the datagram to this care-of address;
   otherwise, the home agent MUST first encapsulate the datagram in a
   unicast datagram addressed to the mobile node's home address and then
   MUST tunnel the resulting datagram (nested tunneling) to the mobile
   node's care-of address.  For this reason, the mobile node MUST be
   capable of decapsulating packets sent to its home address in order to
   receive multicast datagrams using this method.

   A mobile node that wishes to send datagrams 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 datagrams directly on the visited network MUST use a co-
   located care-of address as the IP source address.  Similarly, a
   mobile node which tunnels a multicast datagram to its home agent MUST
   use its home address as the IP source address of both the (inner)
   multicast datagram and the (outer) encapsulating datagram.  This
   second option assumes that the home agent is a multicast router.

4.5. Mobile Routers

   A mobile node can be a router that 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 document, such
   networks are called "mobile networks".




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   A mobile router MAY act as a foreign agent and provide a foreign
   agent care-of address to mobile nodes connected to the mobile
   network.  Typical routing to a mobile node via a mobile router in
   this case is illustrated by the following example:

      a) A laptop computer is disconnected from its home network and
         later attached to a network port in the seat back of an
         aircraft.  The laptop computer uses Mobile IP to register on
         this foreign network, using a foreign agent care-of address
         discovered through an Agent Advertisement from the aircraft's
         foreign agent.

      b) The aircraft network is itself mobile.  Suppose the node
         serving as the foreign agent on the aircraft also serves as the
         default router that connects the aircraft network to the rest
         of the Internet.  When the aircraft is at home, this router is
         attached to some fixed network at the airline's headquarters,
         which is the router's home network.  While the aircraft is in
         flight, this router registers from time to time over its radio
         link with a series of foreign agents below it on the ground.
         This router's home agent is a node on the fixed network at the
         airline's headquarters.

      c) Some correspondent node sends a datagram to the laptop
         computer, addressing the datagram to the laptop's home address.
         This datagram is initially routed to the laptop's home network.

      d) The laptop's home agent intercepts the datagram on the home
         network and tunnels it to the laptop's care-of address, which
         in this example is an address of the node serving as router and
         foreign agent on the aircraft.  Normal IP routing will route
         the datagram to the fixed network at the airline's
         headquarters.

      e) The aircraft router and foreign agent's home agent there
         intercepts the datagram and tunnels it to its current care-of
         address, which in this example is some foreign agent on the
         ground below the aircraft.  The original datagram from the
         correspondent node has now been encapsulated twice:  once by
         the laptop's home agent and again by the aircraft's home agent.

      f) The foreign agent on the ground decapsulates the datagram,
         yielding a datagram still encapsulated by the laptop's home
         agent, with a destination address of the laptop's care-of
         address.  The ground foreign agent sends the resulting datagram
         over its radio link to the aircraft.





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      g) The foreign agent on the aircraft decapsulates the datagram,
         yielding the original datagram from the correspondent node,
         with a destination address of the laptop's home address.  The
         aircraft foreign agent delivers the datagram over the aircraft
         network to the laptop's link-layer address.

   This example illustrated the case in which a mobile node is attached
   to a mobile network.  That is, the mobile node is mobile with respect
   to the network, which itself is also mobile (here with respect to the
   ground).  If, instead, the node is fixed with respect to the mobile
   network (the mobile network is the fixed node's home network), then
   either of two methods may be used to cause datagrams from
   correspondent nodes to be routed to the fixed node.

   A home agent MAY be configured to have 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 datagrams sent to the fixed node will thus use
   nested 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 nested tunneling of datagrams.

4.6. ARP, Proxy ARP, and Gratuitous ARP

   The use of ARP [36] 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 [39] 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 [36],
         but supplies some configured link-layer address (generally, its
         own) in the Sender Hardware Address field.  The node receiving
         the Reply will then associate this link-layer address with the
         IP address of the original target node, causing it to transmit
         future datagrams for this target node to the node with that
         link-layer address.



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      -  A Gratuitous ARP [45] is an ARP packet sent by a node in order
         to spontaneously cause other nodes to update an entry in their
         ARP cache.  A gratuitous ARP MAY use either an ARP Request or
         an ARP Reply packet.  In either case, the ARP Sender Protocol
         Address and ARP Target Protocol Address are both set to the IP
         address of the cache entry to be updated, and the ARP Sender
         Hardware Address is set to the link-layer address to which this
         cache entry should be updated.  When using an ARP Reply packet,
         the Target Hardware Address is also set to the link-layer
         address to which this cache entry should be updated (this field
         is not used in an ARP Request packet).

         In either case, for a gratuitous ARP, the ARP packet MUST be
         transmitted as a local broadcast packet on the local link.  As
         specified in [36], any node receiving any ARP packet (Request
         or Reply) MUST update its local ARP cache with the Sender
         Protocol and Hardware Addresses in the ARP packet, if the
         receiving node has an entry for that IP address already in its
         ARP cache.  This requirement in the ARP protocol applies even
         for ARP Request packets, and for ARP Reply packets that do not
         match any ARP Request transmitted by the receiving node [36].

   While a mobile node is registered on a foreign network, its home
   agent uses proxy ARP [39] to reply to ARP Requests it receives that
   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 registered 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 [39], 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.  This gratuitous ARP
   packet MUST be transmitted as a broadcast packet on the link on which
   the mobile node's home address is located.  Since broadcasts on the
   local link (such as Ethernet) are typically not guaranteed to be
   reliable, the gratuitous ARP packet SHOULD be retransmitted a small
   number of times to increase its reliability.




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   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 network 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 this
   gratuitous ARP on its home network as a local broadcast on this link.
   The gratuitous ARP packet SHOULD be retransmitted a small number of
   times to increase its reliability, but these retransmissions SHOULD
   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 also transmit a
   gratuitous ARP on the mobile node's home network.  This gratuitous
   ARP also is used to associate the mobile node's home address with the
   mobile node's own link-layer address.  A gratuitous ARP is
   transmitted by both the mobile node and its home agent, since in the
   case of wireless network interfaces, the area within transmission
   range of the mobile node will likely differ from that within range of
   its home agent.  The ARP packet from the home agent MUST be
   transmitted as a local broadcast on the mobile node's home link, and
   SHOULD be retransmitted a small number of times to increase its
   reliability; these retransmissions, however, SHOULD proceed in
   parallel with the transmission and processing of its (de)Registration
   Reply.

   While the mobile node is away from home, it MUST NOT transmit any
   broadcast ARP Request or ARP Reply messages.  Finally, while the
   mobile node is away from home, it MUST NOT reply to ARP Requests in
   which the target IP address is its own home address, unless the ARP
   Request is unicast by a foreign agent with which the mobile node is
   attempting to register or a foreign agent with which the mobile node
   has an unexpired registration.  In the latter case, the mobile node
   MUST use a unicast ARP Reply to respond to the foreign agent.  Note
   that if the mobile node is using a co-located care-of address and
   receives an ARP Request in which the target IP address is this care-
   of address, then the mobile node SHOULD reply to this ARP Request.
   Note also that, when transmitting a Registration Request on a foreign
   network, a mobile node may discover the link-layer address of a
   foreign agent by storing the address as it is received from the Agent
   Advertisement from that foreign agent, but not by transmitting a
   broadcast ARP Request message.








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   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 the link-layer address
         corresponding to its home address, except insofar as necessary
         to communicate with foreign agents on visited networks.

      -  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.  In the gratuitous ARP, the ARP Sender Hardware
         Address is set to the link-layer address of the home agent.
         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.

      -  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.  In this
         gratuitous ARP, the ARP Sender Hardware Address is set to the
         link-layer address of the mobile node.

      -  The mobile node transmits its Registration Request.




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      -  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.  In this gratuitous ARP, the ARP Sender
         Hardware Address is set to the link-layer address of the mobile
         node.  If, instead, the home agent rejects the Registration
         Request, the home agent MUST NOT make any change to the way it
         performs ARP processing (gratuitous nor proxy) for the mobile
         node.  In this latter case, the home agent should operate as if
         the mobile node has not returned home, and continue to perform
         proxy ARP on behalf of the mobile node.

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 HMAC-MD5 [23], with a key size of 128 bits.
   The foreign agent MUST also support authentication using HMAC-MD5 and
   key sizes of 128 bits or greater, with manual key distribution.  Keys
   with arbitrary binary values MUST be supported.

   The "prefix+suffix" use of MD5 to protect data and a shared secret is
   considered vulnerable to attack by the cryptographic community.
   Where backward compatibility with existing Mobile IP implementations
   that use this mode is needed, new implementations SHOULD include
   keyed MD5 [41] as one of the additional authentication algorithms for
   use when producing and verifying the authentication data that is
   supplied with Mobile IP registration messages, for instance in the
   extensions specified in sections 3.5.2,  3.5.3, and 3.5.4.

   More authentication algorithms, algorithm modes, key distribution
   methods, and key sizes MAY also be supported for all of these
   extensions.

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 authenticated.  Such remote redirection, for



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   instance as performed by the mobile registration protocol, is widely
   understood to be a security problem in the current Internet if not
   authenticated [2].  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" (Section 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 do not
   provide service to users that are not legitimate customers of that
   service provider.

5.4. Picking Good Random Numbers

   The strength of any authentication mechanism depends 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. [14] 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, datagrams destined for correspondent
   nodes will appear to emanate from the home network, and it may be
   more difficult to pinpoint the location of the mobile node.  Such
   mechanisms are all beyond the scope of this document.






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5.6. Ingress Filtering

   Many routers implement security policies such as "ingress filtering"
   [15] that do not allow forwarding of packets that have a Source
   Address which appears topologically incorrect.  In environments where
   this is a problem, mobile nodes may use reverse tunneling [27] with
   the foreign agent supplied care-of address as the Source Address.
   Reverse tunneled packets will be able to pass normally through such
   routers, while ingress filtering rules will still be able to locate
   the true topological source of the packet in the same way as packets
   from non-mobile nodes.

5.7. 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 in this section:  timestamps (mandatory) and
   "nonces" (optional).  All mobile nodes and home agents MUST implement
   timestamp-based replay protection.  These nodes MAY also implement
   nonce-based replay protection (but see Appendix A).

   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 (and the mobile node's home
   address) to match Registration Requests 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 while
   the same security context is being used between the mobile node and
   the home agent.  Retransmission as in Section 3.6.3 is allowed.

5.7.1. 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.  Unless specified differently in the



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   security association between the nodes, a default value of 7 seconds
   MAY be used to limit the time difference.  This value SHOULD be
   greater than 3 seconds.  Obviously the two nodes must have adequately
   synchronized time-of-day clocks.  As with any messages, time
   synchronization messages may be protected against tampering by an
   authentication mechanism determined by the security context between
   the two nodes.

   If timestamps are used, the mobile node MUST set the Identification
   field to a 64-bit value formatted as specified by the Network Time
   Protocol [26].  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.
   Note, however, that when using timestamps, the 64-bit Identification
   used in a Registration Request from the mobile node MUST be greater
   than that used in any previous Registration Request, as the home
   agent uses this field also as a sequence number.  Without such a
   sequence number, it would be possible for a delayed duplicate of an
   earlier Registration Request to arrive at the home agent (within the
   clock synchronization required by the home agent), and thus be
   applied out of order, mistakenly altering the mobile node's current
   registered care-of address.

   Upon receipt of a Registration Request with an authorization-enabling
   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 mobility
   security association.

   If the timestamp is valid, the home agent copies the entire
   Identification field into the Registration Reply it returns the Reply
   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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5.7.2. Replay Protection using Nonces

   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 an authentication code 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 [14].  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 into the
   low-order 32 bits of the Identification in the Registration Reply.
   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 each registration message.  The foreign agent
   uses the low-order value (and the mobile host's home address) to
   correctly match registration replies with pending Requests (Section
   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. IANA Considerations

   Mobile IP specifies several new number spaces for values to be used
   in various message fields.  These number spaces include the
   following:

      -  Mobile IP message types sent to UDP port 434, as defined in
         section 1.8.



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      -  types of extensions to Registration Request and Registration
         Reply messages (see sections 3.3 and 3.4, and also consult [27,
         29, 6, 7, 12])

      -  values for the Code in the Registration Reply message (see
         section 3.4, and also consult [27, 29, 6, 7, 12])

      -  Mobile IP defines so-called Agent Solicitation and Agent
         Advertisement messages.  These messages are in fact Router
         Discovery messages [10] augmented with mobile-IP specific
         extensions.  Thus, they do not define a new name space, but do
         define additional Router Discovery extensions as described
         below in Section 6.2.  Also see Section 2.1 and consult [7,
         12].

   There are additional Mobile IP numbering spaces specified in [7].

   Information about assignment of mobile-ip numbers derived from
   specifications external to this document is given by IANA at
   http://www.iana.org/numbers.html.  From that URL, follow the
   hyperlinks to [M] within the "Directory of General Assigned Numbers",
   and subsequently to the specific section for "Mobile IP Numbers".

6.1. Mobile IP Message Types

   Mobile IP messages are defined to be those that are sent to a message
   recipient at port 434 (UDP or TCP).  The number space for Mobile IP
   messages is specified in Section 1.8.  Approval of new extension
   numbers is subject to Expert Review, and a specification is required
   [30].  The currently standardized message types have the following
   numbers, and are specified in the indicated sections.

   Type  Name                                             Section
   ----  --------------------------------------------     ---------
   1     Registration Request                             3.3
   3     Registration Reply                               3.4

6.2. Extensions to RFC 1256 Router Advertisement

   RFC 1256 defines two ICMP message types, Router Advertisement and
   Router Solicitation.  Mobile IP defines a number space for extensions
   to Router Advertisement, which could be used by protocols other than
   Mobile IP.  The extension types currently standardized for use with
   Mobile IP have the following numbers.







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   Type  Name                                             Reference
   ----  --------------------------------------------     ---------
   0     One-byte Padding                                 2.1.3
   16    Mobility Agent Advertisement                     2.1.1
   19    Prefix-Lengths                                   2.1.2

   Approval of new extension numbers for use with Mobile IP is subject
   to Expert Review, and a specification is required [30].

6.3. Extensions to Mobile IP Registration Messages

   The Mobile IP messages, specified within this document, and listed in
   sections 1.8 and 6.1, may have extensions.  Mobile IP message
   extensions all share the same number space, even if they are to be
   applied to different Mobile IP messages.  The number space for Mobile
   IP message extensions is specified within this document.  Approval of
   new extension numbers is subject to Expert Review, and a
   specification is required [30].

   Type  Name                                             Reference
   ----  --------------------------------------------     ---------
   0     One-byte Padding
   32    Mobile-Home Authentication                       3.5.2
   33    Mobile-Foreign Authentication                    3.5.3
   34    Foreign-Home Authentication                      3.5.4

6.4. Code Values for Mobile IP Registration Reply Messages

   The Mobile IP Registration Reply message, specified in section 3.4,
   has a Code field.  The number space for the Code field values is also
   specified in Section 3.4.  The Code number space is structured
   according to whether the registration was successful, or whether the
   foreign agent denied the registration request, or lastly whether the
   home agent denied the registration request, as follows:

      0-8        Success Codes
      9-63       No allocation guidelines currently exist
      64-127     Error Codes from the Foreign Agent
      128-192    Error Codes from the Home Agent
      193-255    No allocation guidelines currently exist

   Approval of new Code values requires Expert Review [30].









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7. Acknowledgments

   Special thanks to Steve Deering (Xerox PARC), along with Dan Duchamp
   and John Ioannidis (JI) (Columbia University), for forming the
   working group, chairing it, and putting so much effort into its early
   development.  Columbia's early Mobile IP work can be found in [18,
   19, 17].

   Thanks also to Kannan Alaggapan, Greg Minshall, Tony Li, Jim Solomon,
   Erik Nordmark, Basavaraj Patil, and Phil Roberts for their
   contributions to the group while performing the duties of
   chairperson, as well as for their many useful comments.

   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),
      - Samita Chakrabarti (Sun Microsystems)
      - Ken Imboden (Candlestick Networks, Inc.)
      - Dave Johnson (Carnegie Mellon University),
      - Frank Kastenholz (FTP Software),
      - Anders Klemets (KTH),
      - Chip Maguire (KTH),
      - Alison Mankin (ISI)
      - Andrew Myles (Macquarie University),
      - Thomas Narten (IBM)
      - Al Quirt (Bell Northern Research),
      - Yakov Rekhter (IBM), and
      - Fumio Teraoka (Sony).
      - Alper Yegin (NTT DoCoMo)

   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 later
   revisions preceding RFC 2002 is due to Jim Solomon and Dave Johnson.

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

   Sections 1.10 and 1.11, which specify new extension formats to be
   used with aggregatable extension types, were included from a
   specification document (entitled "Mobile IP Extensions
   Rationalization (MIER)", which was written by






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      -  Mohamed M.Khalil, Nortel Networks
      -  Raja Narayanan, nVisible Networks
      -  Haseeb Akhtar, Nortel Networks
      -  Emad Qaddoura, Nortel Networks

   Thanks to these authors, and also for the additional work on
   MIER, which was contributed by Basavaraj Patil, Pat Calhoun, Neil
   Justusson, N. Asokan, and Jouni Malinen.











































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A. Patent Issues

   The IETF has been notified of intellectual property rights claimed
   in regard to some or all of the specification contained in this
   document.  For more information consult the online list of claimed
   rights.

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on
   the IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances
   of licenses to be made available, or the result of an attempt
   made to obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.

B. Link-Layer Considerations

   The mobile node MAY use link-layer mechanisms to decide that its
   point of attachment has changed.  Such indications include the
   Down/Testing/Up interface status [24], 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.

   The Point-to-Point-Protocol (PPP) [42] and its Internet Protocol
   Control Protocol (IPCP) [25], negotiates the use of IP addresses.
   The mobile node SHOULD first attempt to specify its home address,
   so that if the mobile node is attaching to its home network, the
   unrouted link will function correctly.  When the home address is
   not accepted by the peer, but a transient IP address is dynamically
   assigned to the mobile node, and the mobile node is capable of
   supporting a co-located care-of address, the mobile node MAY
   register that address as a co-located care-of address.  When the peer
   specifies its own IP address, that address MUST NOT be assumed to be
   a foreign agent care-of address or the IP address of a home agent.





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   PPP extensions for Mobile IP have been specified in RFC 2290 [44].
   Please consult that document for additional details for how to handle
   care-of address assignment from PPP in a more efficient manner.

C. TCP Considerations

C.1. TCP Timers

   When high-delay (e.g. SATCOM) or low-bandwidth (e.g. High-Frequency
   Radio) links are in use, some TCP stacks may have insufficiently
   adaptive (non-standard) retransmission timeouts.  There may be
   spurious retransmission timeouts, even when the link and network
   are actually operating properly, but just with a high delay because
   of the medium in use.  This can cause an inability to create or
   maintain TCP connections over such links, and can also cause unneeded
   retransmissions which consume already scarce bandwidth.  Vendors
   are encouraged to follow the algorithms in RFC 2988 [31] when
   implementing TCP retransmission timers.  Vendors of systems designed
   for low-bandwidth, high-delay links should consult RFCs 2757 and
   2488 [28, 1].  Designers of applications targeted to operate on
   mobile nodes should be sensitive to the possibility of timer-related
   difficulties.

C.2. TCP Congestion Management

   Mobile nodes often use media which 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 [21].  Now, when a packet is dropped,
   the correspondent node's TCP implementation is likely to react as
   if there were a source of network congestion, and initiate the
   slow-start mechanisms [21] 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. [5].  TCP approaches to the problem
   of handling errors that might interfere with congestion management
   are discussed in documents from the [pilc] working group [3, 9].
   While such approaches are beyond the scope of this document,
   they illustrate that providing performance transparency to mobile
   nodes involves understanding mechanisms outside the network layer.
   Problems introduced by higher media error rates also indicate 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 Care-of Address

   The mobile node receives an Agent Advertisement from a foreign
   agent and wishes to register with that agent using the advertised
   foreign agent care-of address.  The mobile node wishes only
   IP-in-IP 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 Registration Lifetime copied from the
          Mobility Agent Advertisement Extension of the
          Router Advertisement message
        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
          Mobility Agent Advertisement Extension of the
          Router Advertisement message
        Identification = Network Time Protocol timestamp or Nonce
      Extensions:
        An authorization-enabling extension (e.g., the
     Mobile-Home Authentication Extension)

D.2. Registering with a Co-Located 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 [13]
   for use as a co-located care-of address.  The mobile node supports
   all forms of encapsulation (IP-in-IP, minimal encapsulation, and
   GRE), desires a copy of broadcast datagrams on the home network, and
   does not want simultaneous mobility bindings:






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      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:
        An authorization-enabling extension (e.g., 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
   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, 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".

F. Interoperability Considerations

   This document specifies revisions to RFC 2002 that are intended to
   improve interoperability by resolving ambiguities contained in the
   earlier text.  Implementations that perform authentication according
   to the new more precisely specified algorithm would be interoperable
   with earlier implementations that did what was originally expected
   for producing authentication data.  That was a major source of non-
   interoperability before.

   However, this specification does have new features that, if used,
   would cause interoperability problems with older implementations.
   All features specified in RFC 2002 will work with the new
   implementations, except for V-J compression [20].  The following list
   details some of the possible areas of compatibility problems that may
   be experienced by nodes conforming to this revised specification,
   when attempting to interoperate with nodes obeying RFC 2002.

      -  A client that expects some of the newly mandatory features
         (like reverse tunneling) from a foreign agent would still be
         interoperable as long as it pays attention to the `T' bit.



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      -  Mobile nodes that use the NAI extension to identify themselves
         would not work with old mobility agents.

      -  Mobile nodes that use a zero home address and expect to receive
         their home address in the Registration Reply would not work
         with old mobility agents.

      -  Mobile nodes that attempt to authenticate themselves without
         using the Mobile-Home authentication extension will be unable
         to successful register with their home agent.

   In all of these cases, a robust and well-configured mobile node is
   very likely to be able to recover if it takes reasonable actions upon
   receipt of a Registration Reply with an error code indicating the
   cause for rejection.  For instance, if a mobile node sends a
   registration request that is rejected because it contains the wrong
   kind of authentication extension, then the mobile node could retry
   the registration with a mobile-home authentication extension, since
   the foreign agent and/or home agent in this case will not be
   configured to demand the alternative authentication data.

G. Changes since RFC 2002

   This section details differences between the original Mobile IP base
   specification (RFC 2002 and ff.)  that have been made as part of this
   revised protocol specification for Mobile IP.

G.1. Major Changes

      -  Specification for Destination IP address of Registration Reply
         transmitted from Foreign Agent, to avoid any possible
         transmission to IP address 0.0.0.0.

      -  Specification of two new formats for Mobile IP extensions,
         according to the ideas contained in MIER.

      -  Specification that the SPI of the MN-HA authentication
         extension is to be used as part of the data over which the
         authentication algorithm must be computed.

      -  Eliminated Van-Jacobson Compression feature

      -  Specification that foreign agents MAY send advertisements at a
         rate faster than once per second, but chosen so that the
         advertisements do not burden the capacity of the local link.
         For simplicity, the foreign agent now MAY send advertisements
         at an interval less than 1/3 the advertised ICMP Lifetime.




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      -  Specification that foreign agents SHOULD support reverse
         tunneling, and home agents MUST support decapsulation of
         reverse tunnels.

      -  Changed the preconfiguration requirements in section 3.6 for
         the mobile node to reflect the capability, specified in RFC
         2794 [6], for the mobile node to identify itself by using its
         NAI, and then getting a home address from the Registration
         Reply.

      -  Changed section 3.7.3.1 so that a foreign agent is not required
         to discard Registration Replies that have a Home Address field
         that does not match any pending Registration Request.

      -  Allowed registrations to be authenticated by use of a security
         association between the mobile node and a suitable
         authentication entity acceptable to the home agent.  Defined
         "Authorization-enabling Extension" to be an authentication
         extension that makes a registration message acceptable to the
         recipient.  This is needed according to specification in [6].

      -  Mandated that HMAC-MD5 be used instead of the "prefix+suffix"
         mode of MD5 as originally mandated in RFC 2002.

      -  Specified that the mobile node SHOULD take the first care-of
         address in a list offered by a foreign agent, and MAY try each
         subsequent advertised address in turn if the attempted
         registrations are rejected by the foreign agent

      -  Clarification that a mobility agent SHOULD only put its own
         addresses into the initial (i.e., not mobility-related) list of
         routers in the mobility advertisement.  RFC 2002 suggests that
         a mobility agent might advertise other default routers.

      -  Specification that a mobile node MUST ignore reserved bits in
         Agent Advertisements, as opposed to discarding such
         advertisements.  In this way, new bits can be defined later,
         without affecting the ability for mobile nodes to use the
         advertisements even when the newly defined bits are not
         understood.  Furthermore, foreign agents can set the `R' bit to
         make sure that new bits are handled by themselves instead of
         some legacy mobility agent.

      -  Specification that the foreign agent checks to make sure that
         the indicated home agent address does not belong to any of its
         network interfaces before relaying a Registration Request.  If





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         the check fails, and the foreign agent is not the mobile node's
         home agent, then the foreign agent rejects the request with
         code 136 (unknown home agent address).

      -  Specification that, while they are away from the home network,
         mobile nodes MUST NOT broadcast ARP packets to find the MAC
         address of another Internet node.  Thus, the (possibly empty)
         list of Router Addresses from the ICMP Router Advertisement
         portion of the message is not useful for selecting a default
         router, unless the mobile node has some means not involving
         broadcast ARP and not specified within this document for
         obtaining the MAC address of one of the routers in the list.
         Similarly, in the absence of unspecified mechanisms for
         obtaining MAC addresses on foreign networks, the mobile node
         MUST ignore redirects to other routers on foreign networks.

      -  Specification that a foreign agent MUST NOT use broadcast ARP
         for a mobile node's MAC address on a foreign network.  It may
         obtain the MAC address by copying the information from an Agent
         Solicitation or a Registration Request transmitted from a
         mobile node.

      -  Specification that a foreign agent's ARP cache for the mobile
         node's IP address MUST NOT be allowed to expire before the
         mobile node's visitor list entry expires, unless the foreign
         agent has some way other than broadcast ARP to refresh its MAC
         address associated to the mobile node's IP address.

      -  At the end of section 4.6, clarified that a home agent MUST NOT
         make any changes to the way it performs proxy ARP after it
         rejects an invalid deregistration request.

      -  In section 4.2.3, specification that multihomed home agents
         MUST use the the address sent to the mobile node in the home
         agent field of the registration reply as the source address in
         the outer IP header of the encapsulated datagram.

      -  Inserted 'T' bit into its proper place in the Registration
         Request message format (section 3.3).

G.2. Minor Changes

      -  Allowed registration replies to be processed by the mobile
         node, even in the absence of any Mobile-Home Authentication
         extension, when containing rejection code by the foreign agent.






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      -  Specification that the foreign agent MAY configure a maximum
         number of pending registrations that it is willing to maintain
         (typically 5).  Additional registrations SHOULD then be
         rejected by the foreign agent with code 66.  The foreign agent
         MAY delete any pending Registration Request after the request
         has been pending for more than 7 seconds; in this case, the
         foreign agent SHOULD reject the Request with code 78
         (registration timeout).

      -  Relaxation of the requirement that, when a mobile node has
         joined a multicast group at the router on the foreign network,
         the mobile node MUST use its home address as the source IP
         address for multicast packets,

      -  Clarification that a mobility agent MAY use different settings
         for each of the 'R', 'H', and 'F' bits on different network
         interfaces.

      -  Replacement of the terminology "recursive tunneling" by the
         terminology "nested tunneling".

      -  Specification that the mobile node MAY use the IP source
         address of an agent advertisement as its default router
         address.

      -  Clarification that keys with arbitrary binary values MUST be
         supported as part of mobility security associations.

      -  Specification that the default value may be chosen as 7
         seconds, for allowable time skews between a home agent and
         mobile node using timestamps for replay protection.  Further
         specification that this value SHOULD be greater than 3 seconds.

      -  Specification that Registration Requests with the 'D' bit set
         to 0, and specifying a care-of address not offered by the
         foreign agent, MUST be rejected with code 77 (invalid care-of
         address).

      -  Clarification that the foreign agent SHOULD consider its own
         maximum value when handling the Lifetime field of the
         Registration Reply.

      -  Clarification that the home agent MUST ignore the 'B' bit (as
         opposed to rejecting the Registration Request) if it does not
         support broadcasts.






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      -  Advice about the impossibility of using dynamic home agent
         discovery in the case when routers change the IP destination
         address of a datagram from a subnet-directed broadcast address
         to 255.255.255.255 before injecting it into the destination
         subnet.

      -  Clarified that when an Agent Advertisement is unicast to a
         mobile node, the specific IP home address of a mobile node MAY
         be used as the destination IP address.

      -  Included a reference to RFC 2290 within appendix B, which deals
         with PPP operation.

      -  Created IANA Considerations section

      -  In section 3.8.3, clarified that a home agent SHOULD arrange
         the selection of a home address for a mobile node when the
         Registration Reply contains a zero Home Address.

G.3. Changes since revision 04 of RFC2002bis

   This section lists the changes between this version (...-06.txt) and
   the previous version (...-04.txt) of the document.  This section can
   be deleted by the RFC editor.

      -  Noted that HMAC-MD5 should be considered for use in place of
         the "prefix+suffix" mode of MD5 as originally mandated in RFC
         2002.

      -  Included a reference to RFC 2290 within appendix B, which deals
         with PPP operation.

      -  Revamped IANA Considerations section

      -  Revamped Changes section

      -  Replaced Patents section with wording mandated from RFC 2026.

      -  Updated citations.












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H. Example Messages

H.1. Example ICMP Agent Advertisement Message Format

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Num Addrs   |Addr Entry Size|           Lifetime            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Router Address[1]                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Preference Level[1]                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Router Address[2]                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Preference Level[2]                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        ....                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type = 16   |     Length    |      Sequence Number          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Registration Lifetime         |R|B|H|F|M|G|r|T|  reserved     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Care-of Address[1]                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Care-of Address[2]                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         ....                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                     Optional  Extensions                      :
   :   ....                ......                      ......      :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

















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H.2. Example Registration Request Message Format

   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 = 1  |S|B|D|M|G|r|T|x|          Lifetime             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Home Address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Home Agent                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Care-of Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Identification                        +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Optional Non-Auth Extensions for HA ...        |
   |                     ( variable length )                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Type =32   |      Length   |           SPI                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          SPI (cont..)         |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   :         MN-HA Authenticator ( variable length )               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :           Optional  Non-Auth Extensions for FA .........
   :           Optional  MN-FA  Authentication Extension...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




















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H.3. Example Registration Reply Message Format

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

    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 = 3    |     Code      |           Lifetime            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Home Address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Home Agent                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Identification                        +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Optional  HA  Non-Auth Extensions ...         |
   |                     ( variable length )                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Type =32   |      Length   |           SPI                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          SPI (cont...)        |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   :         MN-HA Authenticator ( variable length )               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :           Optional  Extensions  used by FA.........
   :           Optional  MN-FA Authentication Extension...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

References

   [1]   Allman, M., Glover, D. and L. Sanchez, "Enhancing TCP Over
         Satellite Channels using Standard Mechanisms", BCP 28, RFC
         2488, January 1999.

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

   [3]   Border, J., Kojo, M., Griner, J., Montenegro, G. and Z. Shelby,
         "Performance Enhancing Proxies", RFC 3135, June 2001.

   [4]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.








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   [5]   Ramon Caceres and Liviu Iftode.  Improving the Performance of
         Reliable Transport Protocols in Mobile Computing Environments.
         IEEE Journal on Selected Areas in Communications, 13(5):850--
         857, June 1995.

   [6]   Calhoun P. and C. Perkins, "Mobile IP Network Access Identifier
         Extension for IPv4", RFC 2794, January 2000.

   [7]   Calhoun, P. and C. Perkins, "Mobile IP Foreign Agent
         Challenge/Response Extension", RFC 3012, December 2000.

   [8]   Cong, D., Hamlen, M. and C. Perkins, "The Definitions of
         Managed Objects for IP Mobility Support using SMIv2", RFC 2006,
         October 1996.

   [9]   Dawkins, S., Montenegro, G., Kojo, M., Magret, V. and N.
         Vaidya, "End-to-end Performance Implications of Links with
         Errors", BCP 50, RFC 3155, August 2001.

   [10]  Deering, S., "ICMP Router Discovery Messages", RFC 1256,
         September 1991.

   [11]  Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
         1112, August 1989.

   [12]  Dommety, G. and K. Leung, "Mobile IP Vendor/Organization-
         Specific Extensions", RFC 3115, April 2001.

   [13]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
         March 1997.

   [14]  Eastlake, D., Crocker, S. and J. Schiller, "Randomness
         Recommendations for Security", RFC 1750, December 1994.

   [15]  Ferguson P. and D. Senie, "Network Ingress Filtering: Defeating
         Denial of Service Attacks which employ IP Source Address
         Spoofing", BCP 38, RFC 2827, May 2000.

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

   [17]  J. Ioannidis.  Protocols for Mobile Internetworking.  PhD
         Dissertation - Columbia University in the City of New York,
         July 1993.







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RFC 3344              IP Mobility Support for IPv4           August 2002


   [18]  John Ioannidis, Dan Duchamp, and Gerald Q. Maguire Jr.  IP-
         Based Protocols for Mobile Internetworking.  In Proceedings of
         the SIGCOMM '91 Conference:  Communications Architectures &
         Protocols, pages 235--245, September 1991.

   [19]  John Ioannidis and Gerald Q. Maguire Jr.  The Design and
         Implementation of a Mobile Internetworking Architecture.  In
         Proceedings of the Winter USENIX Technical Conference, pages
         489--500, January 1993.

   [20]  Jacobson, V., "Compressing TCP/IP headers for low-speed serial
         links", RFC 1144, February 1990.

   [21]  Jacobson, V., "Congestion Avoidance and Control.  In
         Proceedings, SIGCOMM '88 Workshop, pages 314--329.  ACM Press,
         August 1988.  Stanford, CA.

   [22]  Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
         November 1998.

   [23]  Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
         for Message Authentication", RFC 2104, February 1997.

   [24]  McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB",
         RFC 2863, June 2000.

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

   [26]  Mills, D., "Network Time Protocol (Version 3) Specification,
         Implementation", RFC 1305, March 1992.

   [27]  Montenegro, G., "Reverse Tunneling for Mobile IP (revised)",
         RFC 3024, January 2001.

   [28]  Montenegro, G., Dawkins, S., Kojo, M., Magret, V. and N.
         Vaidya, "Long Thin Networks", RFC 2757, January 2000.

   [29]  Montenegro, G. and V. Gupta, "Sun's SKIP Firewall Traversal for
         Mobile IP", RFC 2356, June 1998.

   [30]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", RFC 2434, October 1998.

   [31]  Paxson, V. and M. Allman, "Computing TCP's Retransmission
         Timer", RFC 2988, November 2000.





Perkins                     Standards Track                    [Page 96]

RFC 3344              IP Mobility Support for IPv4           August 2002


   [32]  Perkins, C., "IP Encapsulation within IP", RFC 2003, October
         1996.

   [33]  Perkins, C., "IP Mobility Support", RFC 2002, October 1996.

   [34]  Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
         October 1996.

   [35]  Perkins, C. and P. Calhoun, "AAA Registration Keys for Mobile
         IP", Work in Progress, July 2001.

   [36]  Plummer, D., "Ethernet Address Resolution Protocol: Or
         converting network protocol addresses to 48.bit Ethernet
         address for transmission on Ethernet hardware", STD 37, RFC
         826, November 1982.

   [37]  Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
         1980.

   [38]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
         1981.

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

   [40]  Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
         1700, October 1994.

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

   [42]  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
         1661, July 1994.

   [43]  Solomon, J., "Applicability Statement for IP Mobility Support"
         RFC 2005, October 1996.

   [44]  Solomon J. and S. Glass, "Mobile-IPv4 Configuration Option for
         PPP IPCP", RFC 2290, February 1998.

   [45]  Stevens, W., "TCP/IP Illustrated, Volume 1: The Protocols"
         Addison-Wesley, Reading, Massachusetts, 1994.









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RFC 3344              IP Mobility Support for IPv4           August 2002


Authors' Addresses

   The working group can be contacted via the current chairs:

   Basavaraj Patil
   Nokia
   6000 Connection Dr.
   Irving, TX. 75039
   USA

   Phone:  +1 972-894-6709
   EMail:  Basavaraj.Patil@nokia.com

   Phil Roberts
   Megisto Corp. Suite 120
   20251 Century Blvd
   Germantown MD 20874
   USA

   Phone:  +1 847-202-9314
   EMail:  PRoberts@MEGISTO.com

   Questions about this memo can also be directed to the editor:

   Charles E. Perkins
   Communications Systems Lab
   Nokia Research Center
   313 Fairchild Drive
   Mountain View, California 94043
   USA

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

















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RFC 3344              IP Mobility Support for IPv4           August 2002


Full Copyright Statement

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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