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RFC 3344


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IP Mobility Support for IPv4

Part 1 of 4, p. 1 to 17
None       Next RFC Part

Obsoleted by:    5944
Obsoletes:    3220
Updated by:    4636    4721


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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.



(page 17 continued on part 2)

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