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RFC 2002


Pages: 79
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IP Mobility Support

Part 1 of 3, p. 1 to 24
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Obsoleted by:    3220
Updated by:    2290


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Network Working Group                                 C. Perkins, Editor
Request for Comments: 2002                                           IBM
Category: Standards Track                                   October 1996

                          IP Mobility Support

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.

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.

Table of Contents

 1. Introduction                                                       3
     1.1. Protocol Requirements . . . . . . . . . . . . . . . . . .    3
     1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . .    4
     1.3. Assumptions . . . . . . . . . . . . . . . . . . . . . . .    4
     1.4. Applicability . . . . . . . . . . . . . . . . . . . . . .    4
     1.5. New Architectural Entities  . . . . . . . . . . . . . . .    5
     1.6. Terminology . . . . . . . . . . . . . . . . . . . . . . .    6
     1.7. Protocol Overview . . . . . . . . . . . . . . . . . . . .    8
     1.8. Specification Language  . . . . . . . . . . . . . . . . .   11
     1.9. Message Format and Protocol Extensibility . . . . . . . .   12
 2. Agent Discovery                                                   14
     2.1. Agent Advertisement . . . . . . . . . . . . . . . . . . .   14
           2.1.1. Mobility Agent Advertisement Extension  . . . . .   16
           2.1.2. Prefix-Lengths Extension  . . . . . . . . . . . .   18
           2.1.3. One-byte Padding Extension  . . . . . . . . . . .   19
     2.2. Agent Solicitation  . . . . . . . . . . . . . . . . . . .   19
     2.3. Foreign Agent and Home Agent Considerations . . . . . . .   19
           2.3.1. Advertised Router Addresses . . . . . . . . . . .   20

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           2.3.2. Sequence Numbers and Rollover Handling  . . . . .   21
     2.4. Mobile Node Considerations  . . . . . . . . . . . . . . .   21
           2.4.1. Registration Required . . . . . . . . . . . . . .   22
           2.4.2. Move Detection  . . . . . . . . . . . . . . . . .   22
           2.4.3. Returning Home  . . . . . . . . . . . . . . . . .   24
           2.4.4. Sequence Numbers and Rollover Handling  . . . . .   24
 3. Registration                                                      24
     3.1. Registration Overview . . . . . . . . . . . . . . . . . .   25
     3.2. Authentication  . . . . . . . . . . . . . . . . . . . . .   26
     3.3. Registration Request  . . . . . . . . . . . . . . . . . .   26
     3.4. Registration Reply  . . . . . . . . . . . . . . . . . . .   29
     3.5. Registration Extensions . . . . . . . . . . . . . . . . .   32
           3.5.1. Computing Authentication Extension Values . . . .   32
           3.5.2. Mobile-Home Authentication Extension  . . . . . .   33
           3.5.3. Mobile-Foreign Authentication Extension . . . . .   33
           3.5.4. Foreign-Home Authentication Extension . . . . . .   34
     3.6. Mobile Node Considerations  . . . . . . . . . . . . . . .   34
           3.6.1. Sending Registration Requests . . . . . . . . . .   36
           3.6.2. Receiving Registration Replies  . . . . . . . . .   40
           3.6.3. Registration Retransmission . . . . . . . . . . .   42
     3.7. Foreign Agent Considerations  . . . . . . . . . . . . . .   43
           3.7.1. Configuration and Registration Tables . . . . . .   44
           3.7.2. Receiving Registration Requests . . . . . . . . .   44
           3.7.3. Receiving Registration Replies  . . . . . . . . .   47
     3.8. Home Agent Considerations . . . . . . . . . . . . . . . .   49
           3.8.1. Configuration and Registration Tables . . . . . .   49
           3.8.2. Receiving Registration Requests . . . . . . . . .   49
           3.8.3. Sending Registration Replies  . . . . . . . . . .   53
 4. Routing Considerations                                            55
     4.1. Encapsulation Types . . . . . . . . . . . . . . . . . . .   56
     4.2. Unicast Datagram Routing  . . . . . . . . . . . . . . . .   56
           4.2.1. Mobile Node Considerations  . . . . . . . . . . .   56
           4.2.2. Foreign Agent Considerations  . . . . . . . . . .   57
           4.2.3. Home Agent Considerations . . . . . . . . . . . .   58
     4.3. Broadcast Datagrams . . . . . . . . . . . . . . . . . . .   59
     4.4. Multicast Datagram Routing  . . . . . . . . . . . . . . .   60
     4.5. Mobile Routers  . . . . . . . . . . . . . . . . . . . . .   61
     4.6. ARP, Proxy ARP, and Gratuitous ARP  . . . . . . . . . . .   62
 5. Security Considerations                                           66
     5.1. Message Authentication Codes  . . . . . . . . . . . . . .   66
     5.2. Areas of Security Concern in this Protocol  . . . . . . .   66
     5.3. Key Management  . . . . . . . . . . . . . . . . . . . . .   67
     5.4. Picking Good Random Numbers . . . . . . . . . . . . . . .   67
     5.5. Privacy . . . . . . . . . . . . . . . . . . . . . . . . .   67
     5.6. Replay Protection for Registration Requests . . . . . . .   68
           5.6.1. Replay Protection using Timestamps  . . . . . . .   68
           5.6.2. Replay Protection using Nonces  . . . . . . . . .   69
 6. Acknowledgments                                                   71

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 A. Patent Issues                                                     72
     A.1. IBM Patent #5,159,592 . . . . . . . . . . . . . . . . . .   72
     A.2. IBM Patent #5,148,479 . . . . . . . . . . . . . . . . . .   72
 B. Link-Layer Considerations                                         73
 C. TCP Considerations                                                73
     C.1. TCP Timers  . . . . . . . . . . . . . . . . . . . . . . .   73
     C.2. TCP Congestion Management . . . . . . . . . . . . . . . .   73
 D. Example Scenarios                                                 74
     D.1. Registering with a Foreign Agent Care-of Address  . . . .   74
     D.2. Registering with a Co-Located Care-of Address . . . . . .   75
     D.3. Deregistration  . . . . . . . . . . . . . . . . . . . . .   76
 E. Applicability of Prefix Lengths Extension                         76
Editor's Address                                                      79

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:

      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.

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.

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   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.

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

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   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.

      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).

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1.6. Terminology

   This document frequently uses the following terms:

      Agent Advertisement
               An advertisement message constructed by attaching a
               special Extension to a router advertisement [4] 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.

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

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

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      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 5.6).

      Node     A host or a router.

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

      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.

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

   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 [6] (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).

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    -  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).

    -  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 "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.

    -  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 [6],
       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.

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   It does, however, place additional burden on the IPv4 address space
   because it requires a pool of addresses within the foreign network to
   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.

   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.

   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.

   For example, the figure below 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 the figure below, the mobile node
   is using a foreign agent care-of address:

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                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.
                      +----+

1.8. Specification Language

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

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

      MUST NOT   This phrase means that the definition is an absolute
                 prohibition of the specification.

      SHOULD     This word, or the adjective "recommended", means
                 that, in some circumstances, valid reasons may exist
                 to ignore this item, but the full implications must
                 be understood and carefully weighed before choosing
                 a different course.  Unexpected results may result
                 otherwise.

      MAY        This word, or the adjective "optional", means that this
                 item is one of an allowed set of alternatives.  An
                 implementation which does not include this option MUST
                 be prepared to interoperate with another implementation
                 which does include the option.

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      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.

1.9. Message Format and Protocol Extensibility

   Mobile IP defines a set of new control messages, sent with UDP [17]
   using well-known port number 434.  Currently, the following two
   message types are defined:

      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" [20].

   In addition, for Agent Discovery, Mobile IP makes use of the existing
   Router Advertisement and Router Solicitation messages defined for
   ICMP Router Discovery [4].

   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.  Each of these Extensions (with one
   exception) is encoded in the following Type-Length-Value format:

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

      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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   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.

   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).  Currently, 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 [4].  Currently, Mobile IP
       defines the following Types 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" [20].

   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.

   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.

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2. Agent Discovery

   Agent Discovery is the method by which a mobile node determines
   whether it is currently connected to its home network or 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 [4] 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 [1], 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.

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:

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    -  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 [4], the IP
                   destination address of an Agent Advertisement MUST
                   be either the "all systems on this link" multicast
                   address (224.0.0.1) [5] 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.

    -  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.

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          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 1/3 of the advertisement Lifetime
   given in the ICMP header.  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 [4] 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.

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|V|    reserved     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  zero or more Care-of Addresses               |
   |                              ...                              |

      Type     16

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

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

Top      ToC       Page 17 
      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
               rather than 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.

      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 [15].

      G        GRE encapsulation.  This agent implements receiving
               tunneled datagrams that use GRE encapsulation [8].

      V        Van Jacobson header compression.  This agent supports use
               of Van Jacobson header compression [10] over the link
               with any registered mobile node.

      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

Top      ToC       Page 18 
   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, and 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.

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 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.

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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.

   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 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 [4], except that:

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

Top      ToC       Page 20 
    -  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.

   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.

   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.  Thus, an agent MAY include one
   of its own addresses in the advertisement.  A foreign agent MAY
   discourage use of this address as a default router by setting the
   preference to a low value and by including the address of another
   router in the advertisement (with a correspondingly higher
   preference).  Nevertheless, a foreign agent MUST route datagrams it
   receives from registered mobile nodes (Section 4.2.2).

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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 reductions in sequence numbers that result from
   reboots, from reductions that result in rollover of the sequence
   number after it attains the value 0xffff.

2.4. Mobile Node Considerations

   Every mobile node MUST implement Agent Solicitation.  Solicitations
   SHOULD only be sent in the absence of Agent Advertisements and when a
   care-of address has not been determined through a link-layer protocol
   or other means.  The mobile node uses the same procedures, defaults,
   and constants for Agent Solicitation as specified for ICMP Router
   Solicitation messages [4], 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 [4].

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   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.

   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.

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.

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.

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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
   addition, if the home network is using ARP [16], 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).



(page 24 continued on part 2)

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