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rfc:rfc2002

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

Perkins Standards Track [Page 1] RFC 2002 IP Mobility Support October 1996

         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

Perkins Standards Track [Page 2] RFC 2002 IP Mobility Support October 1996

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.

Perkins Standards Track [Page 3] RFC 2002 IP Mobility Support October 1996

 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

Perkins Standards Track [Page 4] RFC 2002 IP Mobility Support October 1996

 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).

Perkins Standards Track [Page 5] RFC 2002 IP Mobility Support October 1996

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.

Perkins Standards Track [Page 6] RFC 2002 IP Mobility Support October 1996

    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.

Perkins Standards Track [Page 7] RFC 2002 IP Mobility Support October 1996

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:
  1. 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.
  1. A mobile node receives these Agent Advertisements and determines

whether it is on its home network or a foreign network.

  1. 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.
  1. 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).
  1. 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).

Perkins Standards Track [Page 8] RFC 2002 IP Mobility Support October 1996

  1. 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).
  1. 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:
  1. 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.
  1. 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.

Perkins Standards Track [Page 9] RFC 2002 IP Mobility Support October 1996

 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:

Perkins Standards Track [Page 10] RFC 2002 IP Mobility Support October 1996

              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.

Perkins Standards Track [Page 11] RFC 2002 IP Mobility Support October 1996

    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.

Perkins Standards Track [Page 12] RFC 2002 IP Mobility Support October 1996

 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:
  1. 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
  1. 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.

Perkins Standards Track [Page 13] RFC 2002 IP Mobility Support October 1996

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:

Perkins Standards Track [Page 14] RFC 2002 IP Mobility Support October 1996

  1. 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.
  1. 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.
  1. 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.

Perkins Standards Track [Page 15] RFC 2002 IP Mobility Support October 1996

        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).

Perkins Standards Track [Page 16] RFC 2002 IP Mobility Support October 1996

    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

Perkins Standards Track [Page 17] RFC 2002 IP Mobility Support October 1996

 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.

Perkins Standards Track [Page 18] RFC 2002 IP Mobility Support October 1996

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:
  1. 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

Perkins Standards Track [Page 19] RFC 2002 IP Mobility Support October 1996

  1. 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).
  1. 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).

Perkins Standards Track [Page 20] RFC 2002 IP Mobility Support October 1996

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].

Perkins Standards Track [Page 21] RFC 2002 IP Mobility Support October 1996

 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.

Perkins Standards Track [Page 22] RFC 2002 IP Mobility Support October 1996

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.

Perkins Standards Track [Page 23] RFC 2002 IP Mobility Support October 1996

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).

3. Registration

 Mobile IP registration provides a flexible mechanism for mobile nodes
 to communicate their current reachability information to their home
 agent.  It is the method by which mobile nodes:
  1. request forwarding services when visiting a foreign network,
  1. inform their home agent of their current care-of address,
  1. renew a registration which is due to expire, and/or
  1. deregister when they return home.
 Registration messages exchange information between a mobile node,
 (optionally) a foreign agent, and the home agent.  Registration
 creates or modifies a mobility binding at the home agent, associating
 the mobile node's home address with its care-of address for the
 specified Lifetime.

Perkins Standards Track [Page 24] RFC 2002 IP Mobility Support October 1996

 Several other (optional) capabilities are available through the
 registration procedure, which enable a mobile node to:
  1. maintain multiple simultaneous registrations, so that a copy of

each datagram will be tunneled to each active care-of address

  1. deregister specific care-of addresses while retaining other

mobility bindings, and

  1. discover the address of a home agent if the mobile node is not

configured with this information.

3.1. Registration Overview

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

the mobile node MUST register via that foreign agent.

  1. If a mobile node is using a co-located care-of address, and

receives an Agent Advertisement from a foreign agent on the

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

the mobile node MUST register directly with its home agent.

  1. If a mobile node has returned to its home network and is

(de)registering with its home agent, the mobile node MUST

     register directly with its home agent.
 Both registration procedures involve the exchange of Registration
 Request and Registration Reply messages (Sections 3.3 and 3.4).  When
 registering via a foreign agent, the registration procedure requires
 the following four messages:
    a)   The mobile node sends a Registration Request to the
         prospective foreign agent to begin the registration process.
    b)   The foreign agent processes the Registration Request and then
         relays it to the home agent.

Perkins Standards Track [Page 25] RFC 2002 IP Mobility Support October 1996

    c)   The home agent sends a Registration Reply to the foreign
         agent to grant or deny the Request.
    d)   The foreign agent processes the Registration Reply and then
         relays it to the mobile node to inform it of the disposition
         of its Request.
 When the mobile node instead registers directly with its home agent,
 the registration procedure requires only the following two messages:
       a)   The mobile node sends a Registration Request to the home
            agent.
       b)   The home agent sends a Registration Reply to the mobile
            node, granting or denying the Request.
 The registration messages defined in Sections 3.3 and 3.4 use the
 User Datagram Protocol (UDP) [17].  A nonzero UDP checksum SHOULD be
 included in the header, and MUST be checked by the recipient.

3.2. Authentication

 Each mobile node, foreign agent, and home agent MUST be able to
 support a mobility security association for mobile entities, indexed
 by their SPI and IP address.  In the case of the mobile node, this
 must be its Home Address.  See Section 5.1 for requirements for
 support of authentication algorithms.  Registration messages between
 a mobile node and its home agent MUST be authenticated with the
 Mobile-Home Authentication Extension (Section 3.5.2).  This Extension
 immediately follows all non-authentication Extensions, except those
 foreign agent-specific Extensions which may be added to the message
 after the mobile node computes the authentication.

3.3. Registration Request

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

Perkins Standards Track [Page 26] RFC 2002 IP Mobility Support October 1996

    Destination Address Typically that of the foreign agent or the
             home agent.
 See Sections 3.6.1.1 and 3.7.2.2 for details.
 UDP fields:
    Source Port        variable
    Destination Port   434
 The UDP header is followed by the Mobile IP fields shown below:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |S|B|D|M|G|V|rsv|          Lifetime             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          Home Address                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           Home Agent                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Care-of Address                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                         Identification                        +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Extensions ...
 +-+-+-+-+-+-+-+-
    Type     1 (Registration Request)
    S        Simultaneous bindings.  If the 'S' bit is set, the mobile
             node is requesting that the home agent retain its prior
             mobility bindings, as described in Section 3.6.1.2.
    B        Broadcast datagrams.  If the 'B' bit is set, the mobile
             node requests that the home agent tunnel to it any
             broadcast datagrams that it receives on the home network,
             as described in Section 4.3.
    D        Decapsulation by mobile node.  If the 'D' bit is set, the
             mobile node will itself decapsulate datagrams which are
             sent to the care-of address.  That is, the mobile node is
             using a co-located care-of address.

Perkins Standards Track [Page 27] RFC 2002 IP Mobility Support October 1996

    M        Minimal encapsulation.  If the 'M' bit is set, the
             mobile node requests that its home agent use minimal
             encapsulation [15] for datagrams tunneled to the mobile
             node.
    G        GRE encapsulation.  If the 'G' bit is set, the
             mobile node requests that its home agent use GRE
             encapsulation [8] for datagrams tunneled to the mobile
             node.
    V        The mobile node requests that its mobility agent use Van
             Jacobson header compression [10] over its link with the
             mobile node.
    rsv      Reserved bits; sent as zero
    Lifetime
             The number of seconds remaining before the registration
             is considered expired.  A value of zero indicates a
             request for deregistration.  A value of 0xffff indicates
             infinity.
    Home Address
             The IP address of the mobile node.
    Home Agent
             The IP address of the mobile node's home agent.
    Care-of Address
             The IP address for the end of the tunnel.
    Identification
             A 64-bit number, constructed by the mobile node, used for
             matching Registration Requests with Registration Replies,
             and for protecting against replay attacks of registration
             messages.  See Sections 5.4 and 5.6.
    Extensions
             The fixed portion of the Registration Request is followed
             by one or more of the Extensions listed in Section 3.5.
             The Mobile-Home Authentication Extension MUST be included
             in all Registration Requests.  See Sections 3.6.1.3
             and  3.7.2.2 for information on the relative order in
             which different extensions, when present, MUST be placed
             in a Registration Request message.

Perkins Standards Track [Page 28] RFC 2002 IP Mobility Support October 1996

3.4. Registration Reply

 A mobility agent returns a Registration Reply message to a mobile
 node which has sent a Registration Request (Section 3.3) message.  If
 the mobile node is requesting service from a foreign agent, that
 foreign agent will receive the Reply from the home agent and
 subsequently relay it to the mobile node.  The Reply message contains
 the necessary codes to inform the mobile node about the status of its
 Request, along with the lifetime granted by the home agent, which MAY
 be smaller than the original Request.
 The foreign agent MUST NOT increase the Lifetime selected by the
 mobile node in the Registration Request, since the Lifetime is
 covered by the Mobile-Home Authentication Extension, which cannot be
 correctly (re)computed by the foreign agent.  The home agent MUST NOT
 increase the Lifetime selected by the mobile node in the Registration
 Request, since doing so could increase it beyond the maximum
 Registration Lifetime allowed by the foreign agent.  If the Lifetime
 received in the Registration Reply is greater than that in the
 Registration Request, the Lifetime in the Request MUST be used.  When
 the Lifetime received in the Registration Reply is less than that in
 the Registration Request, the Lifetime in the Reply MUST be used.
 IP fields:
    Source Address        Typically copied from the destination
                          address of the Registration Request to which
                          the agent is replying.  See Sections 3.7.2.3
                          and 3.8.3.1 for complete details.
    Destination Address   Copied from the source address of the
                          Registration Request to which the agent is
                          replying
 UDP fields:
    Source Port           <variable>
    Destination Port      Copied from the source port of the
                          corresponding Registration Request
                          (Section 3.7.1).

Perkins Standards Track [Page 29] RFC 2002 IP Mobility Support October 1996

 The UDP header is followed by the Mobile IP fields shown below:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |     Code      |           Lifetime            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          Home Address                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           Home Agent                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                         Identification                        +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Extensions ...
 +-+-+-+-+-+-+-+-
    Type     3 (Registration Reply)
    Code     A value indicating the result of the Registration
             Request.  See below for a list of currently defined Code
             values.
    Lifetime
             If the Code field indicates that the registration was
             accepted, the Lifetime field is set to the number of
             seconds remaining before the registration is considered
             expired.  A value of zero indicates that the mobile node
             has been deregistered.  A value of 0xffff indicates
             infinity.  If the Code field indicates that the
             registration was denied, the contents of the Lifetime
             field are unspecified and MUST be ignored on reception.
    Home Address
             The IP address of the mobile node.
    Home Agent
             The IP address of the mobile node's home agent.

Perkins Standards Track [Page 30] RFC 2002 IP Mobility Support October 1996

    Identification
             A 64-bit number used for matching Registration Requests
             with Registration Replies, and for protecting against
             replay attacks of registration messages.  The value is
             based on the Identification field from the Registration
             Request message from the mobile node, and on the style of
             replay protection used in the security context between
             the mobile node and its home agent (defined by the
             mobility security association between them, and SPI
             value in the Mobile-Home Authentication Extension).  See
             Sections 5.4 and 5.6.
    Extensions
             The fixed portion of the Registration Reply is followed
             by one or more of the Extensions listed in Section 3.5.
             The Mobile-Home Authentication Extension MUST be included
             in all Registration Replies returned by the home agent.
             See Sections 3.7.2.2 and 3.8.3.3 for rules on placement
             of extensions to Reply messages.
 The following values are defined for use within the Code field.
 Registration successful:
      0 registration accepted
      1 registration accepted, but simultaneous mobility
        bindings unsupported
 Registration denied by the foreign agent:
     64 reason unspecified
     65 administratively prohibited
     66 insufficient resources
     67 mobile node failed authentication
     68 home agent failed authentication
     69 requested Lifetime too long
     70 poorly formed Request
     71 poorly formed Reply
     72 requested encapsulation unavailable
     73 requested Van Jacobson compression unavailable
     80 home network unreachable (ICMP error received)
     81 home agent host unreachable (ICMP error received)
     82 home agent port unreachable (ICMP error received)
     88 home agent unreachable (other ICMP error received)

Perkins Standards Track [Page 31] RFC 2002 IP Mobility Support October 1996

 Registration denied by the home agent:
    128 reason unspecified
    129 administratively prohibited
    130 insufficient resources
    131 mobile node failed authentication
    132 foreign agent failed authentication
    133 registration Identification mismatch
    134 poorly formed Request
    135 too many simultaneous mobility bindings
    136 unknown home agent address
 Up-to-date values of the Code field are specified in the most recent
 "Assigned Numbers" [20].

3.5. Registration Extensions

3.5.1. Computing Authentication Extension Values

 The Authenticator value computed for each authentication Extension
 MUST protect the following fields from the registration message:
  1. the UDP payload (that is, the Registration Request or

Registration Reply data),

  1. all prior Extensions in their entirety, and
  1. the Type and Length of this Extension.
 The default authentication algorithm uses keyed-MD5 [21] in
 "prefix+suffix" mode to compute a 128-bit "message digest" of the
 registration message.  The default authenticator is a 128-bit value
 computed as the MD5 checksum over the the following stream of bytes:
  1. the shared secret defined by the mobility security association

between the nodes and by SPI value specified in the

     authentication Extension, followed by
  1. the protected fields from the registration message, in the order

specified above, followed by

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

Perkins Standards Track [Page 32] RFC 2002 IP Mobility Support October 1996

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

3.5.2. Mobile-Home Authentication Extension

 Exactly one Mobile-Home Authentication Extension MUST be present in
 all Registration Requests and Registration Replies, and is intended
 to eliminate problems [2] which result from the uncontrolled
 propagation of remote redirects in the Internet.  The location of the
 extension marks the end of the authenticated data.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |     Length    |         SPI  ....
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ... SPI (cont.)          |       Authenticator ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type            32
    Length          4 plus the number of bytes in the Authenticator.
    SPI             Security Parameter Index (4 bytes).  An opaque
                    identifier (see Section 1.6).
    Authenticator   (variable length) (See Section 3.5.1.)

3.5.3. Mobile-Foreign Authentication Extension

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

Perkins Standards Track [Page 33] RFC 2002 IP Mobility Support October 1996

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |     Length    |         SPI  ....
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ... SPI (cont.)          |       Authenticator ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type            33
    Length          4 plus the number of bytes in the Authenticator.
    SPI             Security Parameter Index (4 bytes).  An opaque
                    identifier (see Section 1.6).
    Authenticator   (variable length) (See Section 3.5.1.)

3.5.4. Foreign-Home Authentication Extension

 This Extension MAY be included in Registration Requests and Replies
 in cases in which a mobility security association exists between the
 foreign agent and the home agent.  See Section 5.1 for information
 about support requirements for message authentication codes.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |     Length    |         SPI  ....
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ... SPI (cont.)          |       Authenticator ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type            34
    Length          4 plus the number of bytes in the Authenticator.
    SPI             Security Parameter Index (4 bytes).  An opaque
                    identifier (see Section 1.6).
    Authenticator   (variable length) (See Section 3.5.1.)

3.6. Mobile Node Considerations

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

Perkins Standards Track [Page 34] RFC 2002 IP Mobility Support October 1996

 For each pending registration, the mobile node maintains the
 following information:
  1. the link-layer address of the foreign agent to which the

Registration Request was sent, if applicable,

  1. the IP destination address of the Registration Request,
  2. the care-of address used in the registration,
  3. the Identification value sent in the registration,
  4. the originally requested Lifetime, and
  5. the remaining Lifetime of the pending registration.
 A mobile node SHOULD initiate a registration whenever it detects a
 change in its network connectivity.  See Section 2.4.2 for methods by
 which mobile nodes MAY make such a determination.  When it is away
 from home, the mobile node's Registration Request allows its home
 agent to create or modify a mobility binding for it.  When it is at
 home, the mobile node's (de)Registration Request allows its home
 agent to delete any previous mobility binding(s) for it.  A mobile
 node operates without the support of mobility functions when it is at
 home.
 There are other conditions under which the mobile node SHOULD
 (re)register with its foreign agent, such as when the mobile node
 detects that the foreign agent has rebooted (as specified in Section
 2.4.4) and when the current registration's Lifetime is near
 expiration.
 In the absence of link-layer indications of changes in point of
 attachment, Agent Advertisements from new agents SHOULD NOT cause a
 mobile node to attempt a new registration, if its current
 registration has not expired and it is still also receiving Agent
 Advertisements from the foreign agent with which it is currently
 registered.  In the absence of link-layer indications, a mobile node
 MUST NOT attempt to register more often than once per second.
 A mobile node MAY register with a different agent when transport-
 layer protocols indicate excessive retransmissions.  A mobile node
 MUST NOT consider reception of an ICMP Redirect from a foreign agent
 that is currently providing service to it as reason to register with
 a new foreign agent.  Within these constraints, the mobile node MAY
 register again at any time.
 Appendix D shows some examples of how the fields in registration
 messages would be set up in some typical registration scenarios.

Perkins Standards Track [Page 35] RFC 2002 IP Mobility Support October 1996

3.6.1. Sending Registration Requests

 The following sections specify details for the values the mobile node
 MUST supply in the fields of Registration Request messages.

3.6.1.1. IP Fields

 This section provides the specific rules by which mobile nodes pick
 values for the IP header fields of a Registration Request.
 IP Source Address:
  1. When registering on a foreign network with a co-located care-of

address, the IP source address MUST be the care-of address.

  1. In all other circumstances, the IP source address MUST be the

mobile node's home address.

 IP Destination Address:
  1. When the mobile node has discovered the agent with which it is

registering, through some means (e.g., link-layer) that does not

     provide the IP address of the agent (the IP address of the agent
     is unknown to the mobile node), then the "All Mobility Agents"
     multicast address (224.0.0.11) MUST be used.  In this case, the
     mobile node MUST use the agent's link-layer unicast address in
     order to deliver the datagram to the correct agent.
  1. When registering with a foreign agent, the address of the agent

as learned from the IP source address of the corresponding Agent

     Advertisement MUST be used.  In addition, when transmitting
     this Registration Request message, the mobile node MUST use a
     link-layer destination address copied from the link-layer source
     address of the Agent Advertisement message in which it learned
     this foreign agent's IP address.
  1. When the mobile node is registering directly with its home

agent and knows the (unicast) IP address of its home agent, the

     destination address MUST be set to this address.

Perkins Standards Track [Page 36] RFC 2002 IP Mobility Support October 1996

  1. If the mobile node is registering directly with its home

agent, but does not know the IP address of its home agent,

     the mobile node may use dynamic home agent address resolution
     to automatically determine the IP address of its home agent
     (Section 3.6.1.2).  In this case, the IP destination address is
     set to the subnet-directed broadcast address of the mobile node's
     home network.  This address MUST NOT be used as the destination
     IP address if the mobile node is registering via a foreign agent,
     although it MAY be used as the Home Agent address in the body of
     the Registration Request when registering via a foreign agent.
 IP Time to Live:
  1. The IP TTL field MUST be set to 1 if the IP destination address

is set to the "All Mobility Agents" multicast address as

     described above.  Otherwise a suitable value should be chosen in
     accordance with standard IP practice [19].

3.6.1.2. Registration Request Fields

 This section provides specific rules by which mobile nodes pick
 values for the fields within the fixed portion of a Registration
 Request.
 A mobile node MAY set the 'S' bit in order to request that the home
 agent maintain prior mobility binding(s).  Otherwise, the home agent
 deletes any previous binding(s) and replaces them with the new
 binding specified in the Registration Request.  Multiple simultaneous
 mobility bindings are likely to be useful when a mobile node using at
 least one wireless network interface moves within wireless
 transmission range of more than one foreign agent.  IP explicitly
 allows duplication of datagrams.  When the home agent allows
 simultaneous bindings, it will tunnel a separate copy of each
 arriving datagram to each care-of address, and the mobile node will
 receive multiple copies of datagrams destined to it.
 The mobile node SHOULD set the 'D' bit if it is registering with a
 co-located care-of address.  Otherwise, the 'D' bit MUST NOT be set.
 A mobile node MAY set the 'B' bit to request its home agent to
 forward to it, a copy of broadcast datagrams received by its home
 agent from the home network.  The method used by the home agent to
 forward broadcast datagrams depends on the type of care-of address
 registered by the mobile node, as determined by the 'D' bit in the
 mobile node's Registration Request:

Perkins Standards Track [Page 37] RFC 2002 IP Mobility Support October 1996

  1. If the 'D' bit is set, then the mobile node has indicated that it

will decapsulate any datagrams tunneled to this care-of address

     itself (the mobile node is using a co-located care-of address).
     In this case, to forward such a received broadcast datagram to
     the mobile node, the home agent MUST tunnel it to this care-of
     address.  The mobile node de-tunnels the received datagram in the
     same way as any other datagram tunneled directly to it.
  1. If the 'D' bit is NOT set, then the mobile node has indicated

that it is using a foreign agent care-of address, and that the

     foreign agent will thus decapsulate arriving datagrams before
     forwarding them to the mobile node.  In this case, to forward
     such a received broadcast datagram to the mobile node, the home
     agent MUST first encapsulate the broadcast datagram in a unicast
     datagram addressed to the mobile node's home address, and then
     MUST tunnel this resulting datagram to the mobile node's care-of
     address.
    When decapsulated by the foreign agent, the inner datagram will
    thus be a unicast IP datagram addressed to the mobile node,
    identifying to the foreign agent the intended destination of the
    encapsulated broadcast datagram, and will be delivered to the
    mobile node in the same way as any tunneled datagram arriving for
    the mobile node.  The foreign agent MUST NOT decapsulate the
    encapsulated broadcast datagram and MUST NOT use a local network
    broadcast to transmit it to the mobile node.  The mobile node thus
    MUST decapsulate the encapsulated broadcast datagram itself, and
    thus MUST NOT set the 'B' bit in its Registration Request in this
    case unless it is capable of decapsulating datagrams.
 The mobile node MAY request alternative forms of encapsulation by
 setting the 'M' bit and/or the 'G' bit, but only if the mobile node
 is decapsulating its own datagrams (the mobile node is using a co-
 located care-of address) or if its foreign agent has indicated
 support for these forms of encapsulation by setting the corresponding
 bits in the Mobility Agent Advertisement Extension of an Agent
 Advertisement received by the mobile node.  Otherwise, the mobile
 node MUST NOT set these bits.
 The Lifetime field is chosen as follows:
  1. If the mobile node is registering with a foreign agent, the

Lifetime SHOULD NOT exceed the value in the Registration Lifetime

     field of the Agent Advertisement message received from the
     foreign agent.  When the method by which the care-of address is
     learned does not include a Lifetime, the default ICMP Router
     Advertisement Lifetime (1800 seconds) MAY be used.

Perkins Standards Track [Page 38] RFC 2002 IP Mobility Support October 1996

  1. The mobile node MAY ask a home agent to delete a particular

mobility binding, by sending a Registration Request with the

     care-of address for this binding, with the Lifetime field set to
     zero (Section 3.8.2).
  1. Similarly, a Lifetime of zero is used when the mobile node

deregisters all care-of addresses, such as upon returning home.

 The Home Agent field MUST be set to the address of the mobile node's
 home agent, if the mobile node knows this address.  Otherwise, the
 mobile node MAY use dynamic home agent address resolution to learn
 the address of its home agent.  In this case, the mobile node MUST
 set the Home Agent field to the subnet-directed broadcast address of
 the mobile node's home network.  Each home agent receiving such a
 Registration Request with a broadcast destination address MUST reject
 the mobile node's registration and SHOULD return a rejection
 Registration Reply indicating its unicast IP address for use by the
 mobile node in a future registration attempt.
 The Care-of Address field MUST be set to the value of the particular
 care-of address that the mobile node wishes to (de)register.  In the
 special case in which a mobile node wishes to deregister all care-of
 addresses, it MUST set this field to its home address.
 The mobile node chooses the Identification field in accordance with
 the style of replay protection it uses with its home agent.  This is
 part of the mobility security association the mobile node shares with
 its home agent.  See Section 5.6 for the method by which the mobile
 node computes the Identification field.

3.6.1.3. Extensions

 This section describes the ordering of any mandatory and any optional
 Extensions that a mobile node appends to a Registration Request.
 This following ordering MUST be followed:
    a)   The IP header, followed by the UDP header, followed by the
         fixed-length portion of the Registration Request, followed by
    b)   If present, any non-authentication Extensions expected to be
         used by the home agent (which may or may not also be used by
         the foreign agent), followed by
    c)   The Mobile-Home Authentication Extension, followed by
    d)   If present, any non-authentication Extensions used only by
         the foreign agent, followed by

Perkins Standards Track [Page 39] RFC 2002 IP Mobility Support October 1996

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

3.6.2. Receiving Registration Replies

 Registration Replies will be received by the mobile node in response
 to its Registration Requests.  Registration Replies generally fall
 into three categories:
  1. the registration was accepted,
  2. the registration was denied by the foreign agent, or
  3. the registration was denied by the home agent.
 The remainder of this section describes the Registration Reply
 handling by a mobile node in each of these three categories.

3.6.2.1. Validity Checks

 Registration Replies with an invalid, non-zero UDP checksum MUST be
 silently discarded.
 In addition, the low-order 32 bits of the Identification field in the
 Registration Reply MUST be compared to the low-order 32 bits of the
 Identification field in the most recent Registration Request sent to
 the replying agent.  If they do not match, the Reply MUST be silently
 discarded.
 Also, the authentication in the Registration Reply MUST be checked.
 That is, the mobile node MUST check for the presence of a valid
 authentication Extension, acting in accordance with the Code field in
 the Reply.  The rules are as follows:
    a)   If the mobile node and the foreign agent share a
         mobility security association, exactly one Mobile-Foreign
         Authentication Extension MUST be present in the Registration
         Reply, and the mobile node MUST check the Authenticator
         value in the Extension.  If no Mobile-Foreign Authentication
         Extension is found, or if more than one Mobile-Foreign
         Authentication Extension is found, or if the Authenticator is
         invalid, the mobile node MUST silently discard the Reply and
         SHOULD log the event as a security exception.

Perkins Standards Track [Page 40] RFC 2002 IP Mobility Support October 1996

    b)   If the Code field indicates that service is denied by
         the home agent, or if the Code field indicates that the
         registration was accepted by the home agent, exactly one
         Mobile-Home Authentication Extension MUST be present in
         the Registration Reply, and the mobile node MUST check the
         Authenticator value in the Extension.  If no Mobile-Home
         Authentication Extension is found, or if more than one
         Mobile-Home Authentication Extension is found, or if the
         Authenticator is invalid, the mobile node MUST silently
         discard the Reply and SHOULD log the event as a security
         exception.
 If the Code field indicates an authentication failure, either at the
 foreign agent or the home agent, then it is quite possible that any
 authenticators in the Registration Reply will also be in error.  This
 could happen, for example, if the shared secret between the mobile
 node and home agent was erroneously configured.  The mobile node
 SHOULD log such errors as security exceptions.

3.6.2.2. Registration Request Accepted

 If the Code field indicates that the request has been accepted, the
 mobile node SHOULD configure its routing table appropriately for its
 current point of attachment (Section 4.2.1).
 If the mobile node is returning to its home network and that network
 is one which implements ARP, the mobile node MUST follow the
 procedures described in Section 4.6 with regard to ARP, proxy ARP,
 and gratuitous ARP.
 If the mobile node has registered on a foreign network, it SHOULD
 re-register before the expiration of the Lifetime of its
 registration.  As described in Section 3.6, for each pending
 Registration Request, the mobile node MUST maintain the remaining
 lifetime of this pending registration, as well as the original
 Lifetime from the Registration Request.  When the mobile node
 receives a valid Registration Reply, the mobile node MUST decrease
 its view of the remaining lifetime of the registration by the amount
 by which the home agent decreased the originally requested Lifetime.
 This procedure is equivalent to the mobile node starting a timer for
 the granted Lifetime at the time it sent the Registration Request,
 even though the granted Lifetime is not known to the mobile node
 until the Registration Reply is received.  Since the Registration
 Request is certainly sent before the home agent begins timing the
 registration Lifetime (also based on the granted Lifetime), this
 procedure ensures that the mobile node will re-register before the
 home agent expires and deletes the registration, in spite of possibly
 non-negligible transmission delays for the original Registration

Perkins Standards Track [Page 41] RFC 2002 IP Mobility Support October 1996

 Request and Reply that started the timing of the Lifetime at the
 mobile node and its home agent.

3.6.2.3. Registration Request Denied

 If the Code field indicates that service is being denied, the mobile
 node SHOULD log the error.  In certain cases the mobile node may be
 able to "repair" the error.  These include:
    Code 69:  (Denied by foreign agent, Lifetime too long)
       In this case, the Lifetime field in the Registration Reply will
       contain the maximum Lifetime value which that foreign agent is
       willing to accept in any Registration Request.  The mobile node
       MAY attempt to register with this same agent, using a Lifetime
       in the Registration Request that MUST be less than or equal to
       the value specified in the Reply.
    Code 133:  (Denied by home agent, Identification mismatch)
       In this case, the Identification field in the Registration
       Reply will contain a value that allows the mobile node to
       synchronize with the home agent, based upon the style of replay
       protection in effect (Section 5.6).  The mobile node MUST
       adjust the parameters it uses to compute the Identification
       field based upon the information in the Registration Reply,
       before issuing any future Registration Requests.
    Code 136:  (Denied by home agent, Unknown home agent address)
       This code is returned by a home agent when the mobile node is
       performing dynamic home agent address resolution as described
       in Sections 3.6.1.1 and 3.6.1.2.  In this case, the Home Agent
       field within the Reply will contain the unicast IP address of
       the home agent returning the Reply.  The mobile node MAY then
       attempt to register with this home agent in future Registration
       Requests.  In addition, the mobile node SHOULD adjust the
       parameters it uses to compute the Identification field based
       upon the corresponding field in the Registration Reply, before
       issuing any future Registration Requests.

3.6.3. Registration Retransmission

 When no Registration Reply has been received within a reasonable
 time, another Registration Request MAY be transmitted.  When
 timestamps are used, a new registration Identification is chosen for
 each retransmission; thus it counts as a new registration.  When
 nonces are used, the unanswered Request is retransmitted unchanged;

Perkins Standards Track [Page 42] RFC 2002 IP Mobility Support October 1996

 thus the retransmission does not count as a new registration (Section
 5.6).  In this way a retransmission will not require the home agent
 to resynchronize with the mobile node by issuing another nonce in the
 case in which the original Registration Request (rather than its
 Registration Reply) was lost by the network.
 The maximum time until a new Registration Request is sent SHOULD be
 no greater than the requested Lifetime of the Registration Request.
 The minimum value SHOULD be large enough to account for the size of
 the messages, twice the round trip time for transmission to the home
 agent, and at least an additional 100 milliseconds to allow for
 processing the messages before responding.  The round trip time for
 transmission to the home agent will be at least as large as the time
 required to transmit the messages at the link speed of the mobile
 node's current point of attachment.  Some circuits add another 200
 milliseconds of satellite delay in the total round trip time to the
 home agent.  The minimum time between Registration Requests MUST NOT
 be less than 1 second.  Each successive retransmission timeout period
 SHOULD be at least twice the previous period, as long as that is less
 than the maximum as specified above.

3.7. Foreign Agent Considerations

 The foreign agent plays a mostly passive role in Mobile IP
 registration.  It relays Registration Requests between mobile nodes
 and home agents, and, when it provides the care-of address,
 decapsulates datagrams for delivery to the mobile node.  It SHOULD
 also send periodic Agent Advertisement messages to advertise its
 presence as described in Section 2.3, if not detectable by link-layer
 means.
 A foreign agent MUST NOT transmit a Registration Request except when
 relaying a Registration Request received from a mobile node, to the
 mobile node's home agent.  A foreign agent MUST NOT transmit a
 Registration Reply except when relaying a Registration Reply received
 from a mobile node's home agent, or when replying to a Registration
 Request received from a mobile node in the case in which the foreign
 agent is denying service to the mobile node.  In particular, a
 foreign agent MUST NOT generate a Registration Request or Reply
 because a mobile node's registration Lifetime has expired.  A foreign
 agent also MUST NOT originate a Registration Request message that
 asks for deregistration of a mobile node; however, it MUST relay
 valid (de)Registration Requests originated by a mobile node.

Perkins Standards Track [Page 43] RFC 2002 IP Mobility Support October 1996

3.7.1. Configuration and Registration Tables

 Each foreign agent MUST be configured with a care-of address.  In
 addition, for each pending or current registration, the foreign agent
 MUST maintain a visitor list entry containing the following
 information obtained from the mobile node's Registration Request:
  1. the link-layer source address of the mobile node
  2. the IP Source Address (the mobile node's Home Address)
  3. the IP Destination Address (as specified in 3.6.2.3)
  4. the UDP Source Port
  5. the Home Agent address
  6. the Identification field
  7. the requested registration Lifetime, and
  8. the remaining Lifetime of the pending or current registration.
 As with any node on the Internet, a foreign agent MAY also share
 mobility security associations with any other nodes.  When relaying a
 Registration Request from a mobile node to its home agent, if the
 foreign agent shares a mobility security association with the home
 agent, it MUST add a Foreign-Home Authentication Extension to the
 Request and MUST check the required Foreign-Home Authentication
 Extension in the Registration Reply from the home agent (Sections 3.3
 and 3.4).  Similarly, when receiving a Registration Request from a
 mobile node, if the foreign agent shares a mobility security
 association with the mobile node, it MUST check the required Mobile-
 Foreign Authentication Extension in the Request and MUST add a
 Mobile-Foreign Authentication Extension to the Registration Reply to
 the mobile node.

3.7.2. Receiving Registration Requests

 If the foreign agent accepts a Registration Request from a mobile
 node, it then MUST relay the Request to the indicated home agent.
 Otherwise, if the foreign agent denies the Request, it MUST send a
 Registration Reply to the mobile node with an appropriate denial
 Code, except in cases where the foreign agent would be required to
 send out more than one such denial per second to the same mobile
 node.  The following sections describe this behavior in more detail.
 If a foreign agent receives a Registration Request from a mobile node
 in its visitor list, the existing visitor list entry for the mobile
 node SHOULD NOT be deleted or modified until the foreign agent
 receives a valid Registration Reply from the home agent with a Code
 indicating success.  The foreign agent MUST record the new pending

Perkins Standards Track [Page 44] RFC 2002 IP Mobility Support October 1996

 Request separately from the existing visitor list entry for the
 mobile node.  If the Registration Request requests deregistration,
 the existing visitor list entry for the mobile node SHOULD NOT be
 deleted until the foreign agent has received a successful
 Registration Reply.  If the Registration Reply indicates that the
 Request (for registration or deregistration) was denied by the home
 agent, the existing visitor list entry for the mobile node MUST NOT
 be modified as a result of receiving the Registration Reply.

3.7.2.1. Validity Checks

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

3.7.2.2. Forwarding a Valid Request to the Home Agent

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

Mobile-Home Authentication Extension,

  1. It MAY append any of its own non-authentication Extensions of

relevance to the home agent, if applicable, and

  1. It MUST append the Foreign-Home Authentication Extension, if the

foreign agent shares a mobility security association with the home

    agent.

Perkins Standards Track [Page 45] RFC 2002 IP Mobility Support October 1996

 Specific fields within the IP header and the UDP header of the
 relayed Registration Request MUST be set as follows:
    IP Source Address
             The foreign agent's address on the interface from which
             the message will be sent.
    IP Destination Address
             Copied from the Home Agent field within the Registration
             Request.
    UDP Source Port
             <variable>
    UDP Destination Port
             434
 After forwarding a valid Registration Request to the home agent, the
 foreign agent MUST begin timing the remaining lifetime of the pending
 registration based on the Lifetime in the Registration Request.  If
 this lifetime expires before receiving a valid Registration Reply,
 the foreign agent MUST delete its visitor list entry for this pending
 registration.

3.7.2.3. Denying Invalid Requests

 If the foreign agent denies the mobile node's Registration Request
 for any reason, it SHOULD send the mobile node a Registration Reply
 with a suitable denial Code.  In such a case, the Home Address, Home
 Agent, and Identification fields within the Registration Reply are
 copied from the corresponding fields of the Registration Request.
 If the Reserved field is nonzero, the foreign agent MUST deny the
 Request and SHOULD return a Registration Reply with status code 70 to
 the mobile node.  If the Request is being denied because the
 requested Lifetime is too long, the foreign agent sets the Lifetime
 in the Reply to the maximum Lifetime value it is willing to accept in
 any Registration Request, and sets the Code field to 69.  Otherwise,
 the Lifetime SHOULD be copied from the Lifetime field in the Request.
 Specific fields within the IP header and the UDP header of the
 Registration Reply MUST be set as follows:
    IP Source Address
             Copied from the IP Destination Address of Registration
             Request, unless the "All Agents Multicast" address was
             used.  In this case, the foreign agent's address (on the
             interface from which the message will be sent) MUST be

Perkins Standards Track [Page 46] RFC 2002 IP Mobility Support October 1996

             used.
    IP Destination Address
             Copied from the IP Source Address of the Registration
             Request.
    UDP Source Port
             434
    UDP Destination Port
             Copied from the UDP Source Port of the Registration
             Request.

3.7.3. Receiving Registration Replies

 The foreign agent updates its visitor list when it receives a valid
 Registration Reply from a home agent.  It then relays the
 Registration Reply to the mobile node.  The following sections
 describe this behavior in more detail.
 If upon relaying a Registration Request to a home agent, the foreign
 agent receives an ICMP error message instead of a Registration Reply,
 then the foreign agent SHOULD send to the mobile node a Registration
 Reply with an appropriate "Home Agent Unreachable" failure Code
 (within the range 80-95, inclusive).  See Section 3.7.2.3 for details
 on building the Registration Reply.

3.7.3.1. Validity Checks

 Registration Replies with an invalid, non-zero UDP checksum MUST be
 silently discarded.
 When a foreign agent receives a Registration Reply message, it MUST
 search its visitor list for a pending Registration Request with the
 same mobile node home address as indicated in the Reply.  If no
 pending Request is found, the foreign agent MUST silently discard the
 Reply.  The foreign agent MUST also silently discard the Reply if the
 low-order 32 bits of the Identification field in the Reply do not
 match those in the Request.
 Also, the authentication in the Registration Reply MUST be checked.
 If the foreign agent and the home agent share a mobility security
 association, exactly one Foreign-Home Authentication Extension MUST
 be present in the Registration Reply, and the foreign agent MUST
 check the Authenticator value in the Extension.  If no Foreign-Home
 Authentication Extension is found, or if more than one Foreign-Home
 Authentication Extension is found, or if the Authenticator is
 invalid, the foreign agent MUST silently discard the Reply and SHOULD

Perkins Standards Track [Page 47] RFC 2002 IP Mobility Support October 1996

 log the event as a security exception.  The foreign agent also MUST
 reject the mobile node's registration and SHOULD send a Registration
 Reply to the mobile node with Code 68.

3.7.3.2. Forwarding Replies to the Mobile Node

 A Registration Reply which satisfies the validity checks of Section
 3.8.2.1 is relayed to the mobile node.  The foreign agent MUST also
 update its visitor list entry for the mobile node to reflect the
 results of the Registration Request, as indicated by the Code field
 in the Reply.  If the Code indicates that the mobile node has
 accepted the registration and the Lifetime field is nonzero, the
 foreign agent MUST set the Lifetime in the visitor list entry to the
 value specified in the Lifetime field of the Registration Reply.  If,
 instead, the Code indicates that the Lifetime field is zero, the
 foreign agent MUST delete its visitor list entry for the mobile node.
 Finally, if the Code indicates that the registration was denied by
 the home agent, the foreign agent MUST delete its pending
 registration list entry, but not its visitor list entry, for the
 mobile node.
 The foreign agent MUST NOT modify any of the fields beginning with
 the fixed portion of the Registration Reply up through and including
 the Mobile-Home Authentication Extension.  Otherwise, an
 authentication failure is very likely to occur at the mobile node.
 In addition, the foreign agent SHOULD perform the following
 additional procedures:
  1. It MUST process and remove any Extensions following the

Mobile-Home Authentication Extension,

  1. It MAY append its own non-authentication Extensions of relevance

to the mobile node, if applicable, and

  1. It MUST append the Mobile-Foreign Authentication Extension, if

the foreign agent shares a mobility security association with the

    mobile node.
 Specific fields within the IP header and the UDP header of the
 relayed Registration Reply are set according to the same rules
 specified in Section 3.7.2.3.
 After forwarding a valid Registration Reply to the mobile node, the
 foreign agent MUST update its visitor list entry for this
 registration as follows.  If the Registration Reply indicates that
 the registration was accepted by the home agent, the foreign agent
 resets its timer of the lifetime of the registration to the Lifetime
 granted in the Registration Reply; unlike the mobile node's timing of
 the registration lifetime as described in Section 3.6.2.2, the
 foreign agent considers this lifetime to begin when it forwards the

Perkins Standards Track [Page 48] RFC 2002 IP Mobility Support October 1996

 Registration Reply message, ensuring that the foreign agent will not
 expire the registration before the mobile node does.  On the other
 hand, if the Registration Reply indicates that the registration was
 rejected by the home agent, the foreign agent deletes its visitor
 list entry for this attempted registration.

3.8. Home Agent Considerations

 Home agents play a reactive role in the registration process.  The
 home agent receives Registration Requests from the mobile node
 (perhaps relayed by a foreign agent), updates its record of the
 mobility bindings for this mobile node, and issues a suitable
 Registration Reply in response to each.
 A home agent MUST NOT transmit a Registration Reply except when
 replying to a Registration Request received from a mobile node.  In
 particular, the home agent MUST NOT generate a Registration Reply to
 indicate that the Lifetime has expired.

3.8.1. Configuration and Registration Tables

 Each home agent MUST be configured with an IP address and with the
 prefix size for the home network.  The home agent MUST be configured
 with the home address and mobility security association of each
 authorized mobile node that it is serving as a home agent.  When the
 home agent accepts a valid Registration Request from a mobile node
 that it serves as a home agent, the home agent MUST create or modify
 the entry for this mobile node in its mobility binding list
 containing:
  1. the mobile node's care-of address
  2. the Identification field from the Registration Reply
  3. the remaining Lifetime of the registration
 The home agent MAY also maintain mobility security associations with
 various foreign agents.  When receiving a Registration Request from a
 foreign agent, if the home agent shares a mobility security
 association with the foreign agent, the home agent MUST check the
 Authenticator in the required Foreign-Home Authentication Extension
 in the message, based on this mobility security association.
 Similarly, when sending a Registration Reply to a foreign agent, if
 the home agent shares a mobility security association with the
 foreign agent, the home agent MUST include a Foreign-Home
 Authentication Extension in the message, based on this mobility
 security association.

3.8.2. Receiving Registration Requests

Perkins Standards Track [Page 49] RFC 2002 IP Mobility Support October 1996

 If the home agent accepts an incoming Registration Request, it MUST
 update its record of the the mobile node's mobility binding(s) and
 SHOULD send a Registration Reply with a suitable Code.  Otherwise
 (the home agent denies the Request), it SHOULD send a Registration
 Reply with an appropriate Code specifying the reason the Request was
 denied.  The following sections describe this behavior in more
 detail.

3.8.2.1. Validity Checks

 Registration Requests with an invalid, non-zero UDP checksum MUST be
 silently discarded by the home agent.
 The authentication in the Registration Request MUST be checked.  This
 involves the following operations:
    a)   The home agent MUST check for the presence of a valid
         Mobile-Home Authentication Extension, and perform the
         indicated authentication.  Exactly one Mobile-Home
         Authentication Extension MUST be present in the Registration
         Request, and the home agent MUST check the Authenticator
         value in the Extension.  If no Mobile-Home Authentication
         Extension is found, or if more than one Mobile-Home
         Authentication Extension is found, or if the Authenticator
         is invalid, the home agent MUST reject the mobile node's
         registration and SHOULD send a Registration Reply to the
         mobile node with Code 131.  The home agent MUST then discard
         the Request and SHOULD log the error as a security exception.
    b)   The home agent MUST check that the registration
         Identification field is correct using the context selected by
         the SPI within the Mobile-Home Authentication Extension.  See
         Section 5.6 for a description of how this is performed.  If
         incorrect, the home agent MUST reject the Request and SHOULD
         send a Registration Reply to the mobile node with Code 133,
         including an Identification field computed in accordance with
         the rules specified in Section 5.6.  The home agent MUST do
         no further processing with such a Request, though it SHOULD
         log the error as a security exception.
    c)   If the home agent shares a mobility security association with
         the foreign agent, the home agent MUST check for the presence
         of a valid Foreign-Home Authentication Extension.  Exactly
         one Foreign-Home Authentication Extension MUST be present in
         the Registration Request in this case, and the home agent
         MUST check the Authenticator value in the Extension.  If no
         Foreign-Home Authentication Extension is found, or if more
         than one Foreign-Home Authentication Extension is found, or

Perkins Standards Track [Page 50] RFC 2002 IP Mobility Support October 1996

         if the Authenticator is invalid, the home agent MUST reject
         the mobile node's registration and SHOULD send a Registration
         Reply to the mobile node with Code 132.  The home agent
         MUST then discard the Request and SHOULD log the error as a
         security exception.
 In addition to checking the authentication in the Registration
 Request, home agents MUST deny Registration Requests that are sent to
 the subnet-directed broadcast address of the home network (as opposed
 to being unicast to the home agent).  The home agent MUST discard the
 Request and SHOULD returning a Registration Reply with a Code of 136.
 In this case, the Registration Reply will contain the home agent's
 unicast address, so that the mobile node can re-issue the
 Registration Request with the correct home agent address.

3.8.2.2. Accepting a Valid Request

 If the Registration Request satisfies the validity checks in Section
 3.8.2.1, and the home agent is able to accommodate the Request, the
 home agent MUST update its mobility binding list for the requesting
 mobile node and MUST return a Registration Reply to the mobile node.
 In this case, the Reply Code will be either 0 if the home agent
 supports simultaneous mobility bindings, or 1 if it does not.  See
 Section 3.8.3 for details on building the Registration Reply message.
 The home agent updates its record of the mobile node's mobility
 bindings as follows, based on the fields in the Registration Request:
  1. If the Lifetime is zero and the Care-of Address equals the mobile

node's home address, the home agent deletes all of the entries in

     the mobility binding list for the requesting mobile node.  This
     is how a mobile node requests that its home agent cease providing
     mobility services.
  1. If the Lifetime is zero and the Care-of Address does not equal

the mobile node's home address, the home agent deletes only the

     entry containing the specified Care-of Address from the mobility
     binding list for the requesting mobile node.  Any other active
     entries containing other care-of addresses will remain active.
  1. If the Lifetime is nonzero, the home agent adds an entry

containing the requested Care-of Address to the mobility binding

     list for the mobile node.  If the 'S' bit is set and the home
     agent supports simultaneous mobility bindings, the previous
     mobility binding entries are retained.  Otherwise, the home agent
     removes all previous entries in the mobility binding list for the
     mobile node.

Perkins Standards Track [Page 51] RFC 2002 IP Mobility Support October 1996

 In all cases, the home agent MUST send a Registration Reply to the
 source of the Registration Request, which might indeed be a different
 foreign agent than that whose care-of address is being
 (de)registered.  If the home agent shares a mobility security
 association with the foreign agent whose care-of address is being
 deregistered, and that foreign agent is different from the one which
 relayed the Registration Request, the home agent MAY additionally
 send a Registration Reply to the foreign agent whose care-of address
 is being deregistered.  The home agent MUST NOT send such a Reply if
 it does not share a mobility security association with the foreign
 agent.  If no Reply is sent, the foreign agent's visitor list will
 expire naturally when the original Lifetime expires.
 The home agent MUST NOT increase the Lifetime above that specified by
 the mobile node in the Registration Request.  However, it is not an
 error for the mobile node to request a Lifetime longer than the home
 agent is willing to accept.  In this case, the home agent simply
 reduces the Lifetime to a permissible value and returns this value in
 the Registration Reply.  The Lifetime value in the Registration Reply
 informs the mobile node of the granted lifetime of the registration,
 indicating when it SHOULD re-register in order to maintain continued
 service.  After the expiration of this registration lifetime, the
 home agent MUST delete its entry for this registration in its
 mobility binding list.
 If the Registration Request duplicates an accepted current
 Registration Request, the new Lifetime MUST NOT extend beyond the
 Lifetime originally granted.  A Registration Request is a duplicate
 if the home address, care-of address, and Identification fields all
 equal those of an accepted current registration.
 In addition, if the home network implements ARP [16], and the
 Registration Request asks the home agent to create a mobility binding
 for a mobile node which previously had no binding (the mobile node
 was previously assumed to be at home), then the home agent MUST
 follow the procedures described in Section 4.6 with regard to ARP,
 proxy ARP, and gratuitous ARP.  If the mobile node already had a
 previous mobility binding, the home agent MUST continue to follow the
 rules for proxy ARP described in Section 4.6.

3.8.2.3. Denying an Invalid Request

 If the Registration Reply does not satisfy all of the validity checks
 in Section 3.8.2.1, or the home agent is unable to accommodate the
 Request, the home agent SHOULD return a Registration Reply to the
 mobile node with a Code that indicates the reason for the error.  If
 a foreign agent was involved in relaying the Request, this allows the
 foreign agent to delete its pending visitor list entry.  Also, this

Perkins Standards Track [Page 52] RFC 2002 IP Mobility Support October 1996

 informs the mobile node of the reason for the error such that it may
 attempt to fix the error and issue another Request.
 This section lists a number of reasons the home agent might reject a
 Request, and provides the Code value it should use in each instance.
 See Section 3.8.3 for additional details on building the Registration
 Reply message.
 Many reasons for rejecting a registration are administrative in
 nature.  For example, a home agent can limit the number of
 simultaneous registrations for a mobile node, by rejecting any
 registrations that would cause its limit to be exceeded, and
 returning a Registration Reply with error code 135.  Similarly, a
 home agent may refuse to grant service to mobile nodes which have
 entered unauthorized service areas by returning a Registration Reply
 with a Code of 129.
 If the Reserved field is nonzero, it MUST deny the Request with a
 Code of 134.

3.8.3. Sending Registration Replies

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

3.8.3.1. IP/UDP Fields

 This section provides the specific rules by which mobile nodes pick
 values for the IP and UDP header fields of a Registration Reply.
    IP Source Address
             Copied from the IP Destination Address of Registration
             Request, unless a multicast or broadcast address was
             used.  If the IP Destination Address of the Registration
             Request was a broadcast or multicast address, the IP
             Source Address of the Registration Reply MUST be set to
             the home agent's (unicast) IP address.
    IP Destination Address
             Copied from the IP Source Address of the Registration
             Request.

Perkins Standards Track [Page 53] RFC 2002 IP Mobility Support October 1996

    UDP Source Port
             Copied from the UDP Destination Port of the Registration
             Request.
    UDP Destination Port
             Copied from the UDP Source Port of the Registration
             Request.
 When sending a Registration Reply in response to a Registration
 Request that requested deregistration of the mobile node (the
 Lifetime is zero and the Care-of Address equals the mobile node's
 home address) and in which the IP Source Address was also set to the
 mobile node's home address (this is the normal method used by a
 mobile node to deregister when it returns to its home network), the
 IP Destination Address in the Registration Reply will be set to the
 mobile node's home address, as copied from the IP Source Address of
 the Request.
 In this case, when transmitting the Registration Reply, the home
 agent MUST transmit the Reply directly onto the home network as if
 the mobile node were at home, bypassing any mobility binding list
 entry that may still exist at the home agent for the destination
 mobile node.  In particular, for a mobile node returning home after
 being registered with a care-of address, if the mobile node's new
 Registration Request is not accepted by the home agent, the mobility
 binding list entry for the mobile node will still indicate that
 datagrams addressed to the mobile node should be tunneled to the
 mobile node's registered care-of address; when sending the
 Registration Reply indicating the rejection of this Request, this
 existing binding list entry MUST be ignored, and the home agent MUST
 transmit this Reply as if the mobile node were at home.

3.8.3.2. Registration Reply Fields

 This section provides specific rules by which home agents pick values
 for the fields within the fixed portion of a Registration Reply.  The
 Code field of the Registration Reply is chosen in accordance with the
 rules specified in the previous sections.  When replying to an
 accepted registration, a home agent SHOULD respond with Code 1 if it
 does not support simultaneous registrations.
 The Lifetime field MUST be copied from the corresponding field in the
 Registration Request, unless the requested value is greater than the
 maximum length of time the home agent is willing to provide the
 requested service.  In such a case, the Lifetime MUST be set to the
 length of time that service will actually be provided by the home
 agent.  This reduced Lifetime SHOULD be the maximum Lifetime allowed
 by the home agent (for this mobile node and care-of address).

Perkins Standards Track [Page 54] RFC 2002 IP Mobility Support October 1996

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

3.8.3.3. Extensions

 This section describes the ordering of any required and any optional
 Mobile IP Extensions that a home agent appends to a Registration
 Reply.  The following ordering MUST be followed:
    a)   The IP header, followed by the UDP header, followed by the
         fixed-length portion of the Registration Reply,
    b)   If present, any non-authentication Extensions used by the
         mobile node (which may or may not also be used by the foreign
         agent),
    c)   The Mobile-Home Authentication Extension,
    d)   If present, any non-authentication Extensions used only by
         the foreign agent, and
    e)   The Foreign-Home Authentication Extension, if present.
 Note that items (a) and (c) MUST appear in every Registration Reply
 sent by the home agent.  Items (b), (d), and (e) are optional.
 However, item (e) MUST be included when the home agent and the
 foreign agent share a mobility security association.

4. Routing Considerations

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

Perkins Standards Track [Page 55] RFC 2002 IP Mobility Support October 1996

4.1. Encapsulation Types

 Home agents and foreign agents MUST support tunneling datagrams using
 IP in IP encapsulation [14].  Any mobile node that uses a co-located
 care-of address MUST support receiving datagrams tunneled using IP in
 IP encapsulation.  Minimal encapsulation [15] and GRE encapsulation
 [8] are alternate encapsulation methods which MAY optionally be
 supported by mobility agents and mobile nodes.  The use of these
 alternative forms of encapsulation, when requested by the mobile
 node, is otherwise at the discretion of the home agent.

4.2. Unicast Datagram Routing

4.2.1. Mobile Node Considerations

 When connected to its home network, a mobile node operates without
 the support of mobility services.  That is, it operates in the same
 way as any other (fixed) host or router.  The method by which a
 mobile node selects a default router when connected to its home
 network, or when away from home and using a co-located care-of
 address, is outside the scope of this document.  ICMP Router
 Advertisement [4] is one such method.
 When registered on a foreign network, the mobile node chooses a
 default router by the following rules:
  1. If the mobile node is registered using a foreign agent care-of

address, then the mobile node MUST choose its default router

     from among the Router Addresses advertised in the ICMP Router
     Advertisement portion of that Agent Advertisement message.  The
     mobile node MAY also consider the IP source address of the Agent
     Advertisement as another possible choice for the IP address of a
     default router, along with the (possibly empty) list of Router
     Addresses from the ICMP Router Advertisement portion of the
     message.  In such cases, the IP source address MUST be considered
     to be the worst choice (lowest preference) for a default router.
  1. If the mobile node is registered directly with its home agent

using a co-located care-of address, then the mobile node SHOULD

     choose its default router from among those advertised in any
     ICMP Router Advertisement message that it receives for which
     its externally obtained care-of address and the Router Address
     match under the network prefix.  If the mobile node's externally
     obtained care-of address matches the IP source address of the
     Agent Advertisement under the network prefix, the mobile node
     MAY also consider that IP source address as another possible
     choice for the IP address of a default router, along with the
     (possibly empty) list of Router Addresses from the ICMP Router

Perkins Standards Track [Page 56] RFC 2002 IP Mobility Support October 1996

     Advertisement portion of the message.  If so, the IP source
     address MUST be considered to be the worst choice (lowest
     preference) for a default router.  The network prefix MAY
     be obtained from the Prefix-Lengths Extension in the Router
     Advertisement, if present.  The prefix MAY also be obtained
     through other mechanisms beyond the scope of this document.
 Beyond these rules, the actual selection of the default router is
 made by the selection method specified for ICMP Router Discovery [4],
 among the Router Addresses specified above.  In any case, a mobile
 node registered via a foreign agent MAY choose its foreign agent as a
 default router.
 Note that Van Jacobson header compression [10] will not function
 properly unless all TCP IP datagrams to and from the mobile node
 pass, respectively, through the same first and last-hop router.  The
 mobile node, therefore, MUST select its foreign agent as its default
 router if it performs Van Jacobson header compression with its
 foreign agent.

4.2.2. Foreign Agent Considerations

 Upon receipt of an encapsulated datagram sent to its advertised
 care-of address, a foreign agent MUST compare the inner destination
 address to those entries in its visitor list.  When the destination
 does not match the address of any mobile node currently in the
 visitor list, the foreign agent MUST NOT forward the datagram without
 modifications to the original IP header, because otherwise a routing
 loop is likely to result.  The datagram SHOULD be silently discarded.
 ICMP Destination Unreachable MUST NOT be sent when a foreign agent is
 unable to forward an incoming tunneled datagram.  Otherwise, the
 foreign agent forwards the decapsulated datagram to the mobile node.
 The foreign agent MUST NOT advertise to other routers in its routing
 domain, nor to any other mobile node, the presence of a mobile router
 (Section 4.5).
 The foreign agent MUST route datagrams it receives from registered
 mobile nodes.  At a minimum, this means that the foreign agent must
 verify the IP Header Checksum, decrement the IP Time To Live,
 recompute the IP Header Checksum, and forward such datagrams to a
 default router.  In addition, the foreign agent SHOULD send an
 appropriate ICMP Redirect message to the mobile node.

Perkins Standards Track [Page 57] RFC 2002 IP Mobility Support October 1996

4.2.3. Home Agent Considerations

 The home agent MUST be able to intercept any datagrams on the home
 network addressed to the mobile node while the mobile node is
 registered away from home.  Proxy and gratuitous ARP MAY be used in
 enabling this interception, as specified in Section 4.6.
 The home agent must examine the IP Destination Address of all
 arriving datagrams to see if it is equal to the home address of any
 of its mobile nodes registered away from home.  If so, the home agent
 tunnels the datagram to the mobile node's currently registered care-
 of address or addresses.  If the home agent supports the optional
 capability of multiple simultaneous mobility bindings, it tunnels a
 copy to each care-of address in the mobile node's mobility binding
 list.  If the mobile node has no current mobility bindings, the home
 agent MUST NOT attempt to intercept datagrams destined for the mobile
 node, and thus will not in general receive such datagrams.  However,
 if the home agent is also a router handling common IP traffic, it is
 possible that it will receive such datagrams for forwarding onto the
 home network.  In this case, the home agent MUST assume the mobile
 node is at home and simply forward the datagram directly onto the
 home network.
 See Section 4.1 regarding methods of encapsulation that may be used
 for tunneling.  Nodes implementing tunneling SHOULD also implement
 the "tunnel soft state" mechanism [14], which allows ICMP error
 messages returned from the tunnel to correctly be reflected back to
 the original senders of the tunneled datagrams.
 Home agents SHOULD be able to decapsulate and further deliver packets
 addressed to themselves, sent by a mobile node for the purpose of
 maintaining location privacy, as described in Section 5.5.
 If the Lifetime for a given mobility binding expires before the home
 agent has received another valid Registration Request for that mobile
 node, then that binding is deleted from the mobility binding list.
 The home agent MUST NOT send any Registration Reply message simply
 because the mobile node's binding has expired.  The entry in the
 visitor list of the mobile node's current foreign agent will expire
 naturally, probably at the same time as the binding expired at the
 home agent.  When a mobility binding's lifetime expires, the home
 agent MUST delete the binding, but it MUST retain any other (non-
 expired) simultaneous mobility bindings that it holds for the mobile
 node.
 When a home agent receives a datagram, intercepted for one of its
 mobile nodes registered away from home, the home agent MUST examine
 the datagram to check if it is already encapsulated.  If so, special

Perkins Standards Track [Page 58] RFC 2002 IP Mobility Support October 1996

 rules apply in the forwarding of that datagram to the mobile node:
  1. If the inner (encapsulated) Destination Address is the same

as the outer Destination Address (the mobile node), then the

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

outer Destination Address), the home agent SHOULD encapsulate

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

4.3. Broadcast Datagrams

 When a home agent receives a broadcast datagram, it MUST NOT forward
 the datagram to any mobile nodes in its mobility binding list other
 than those that have requested forwarding of broadcast datagrams.  A
 mobile node MAY request forwarding of broadcast datagrams by setting
 the 'B' bit in its Registration Request message (Section 3.3).  For
 each such registered mobile node, the home agent SHOULD forward
 received broadcast datagrams to the mobile node, although it is a
 matter of configuration at the home agent as to which specific
 categories of broadcast datagrams will be forwarded to such mobile
 nodes.
 If the 'D' bit was set in the mobile node's Registration Request
 message, indicating that the mobile node is using a co-located care-
 of address, the home agent simply tunnels appropriate broadcast IP
 datagrams to the mobile node's care-of address.  Otherwise (the 'D'
 bit was NOT set), the home agent first encapsulates the broadcast
 datagram in a unicast datagram addressed to the mobile node's home
 address, and then tunnels this encapsulated datagram to the foreign
 agent.  This extra level of encapsulation is required so that the
 foreign agent can determine which mobile node should receive the
 datagram after it is decapsulated.  When received by the foreign
 agent, the unicast encapsulated datagram is detunneled and delivered

Perkins Standards Track [Page 59] RFC 2002 IP Mobility Support October 1996

 to the mobile node in the same way as any other datagram.  In either
 case, the mobile node must decapsulate the datagram it receives in
 order to recover the original broadcast datagram.

4.4. Multicast Datagram Routing

 As mentioned previously, a mobile node that is connected to its home
 network functions in the same way as any other (fixed) host or
 router.  Thus, when it is at home, a mobile node functions
 identically to other multicast senders and receivers.  This section
 therefore describes the behavior of a mobile node that is visiting a
 foreign network.
 In order receive multicasts, a mobile node MUST join the multicast
 group in one of two ways.  First, a mobile node MAY join the group
 via a (local) multicast router on the visited subnet.  This option
 assumes that there is a multicast router present on the visited
 subnet.  If the mobile node is using a co-located care-of address, it
 SHOULD use this address as the source IP address of its IGMP [5]
 messages.  Otherwise, it MUST use its home address.
 Alternatively, a mobile node which wishes to receive multicasts MAY
 join groups via a bi-directional tunnel to its home agent, assuming
 that its home agent is a multicast router.  The mobile node tunnels
 IGMP messages to its home agent and the home agent forwards multicast
 datagrams down the tunnel to the mobile node.  The rules for
 multicast datagram delivery to mobile nodes in this case are
 identical to those for broadcast datagrams (Section 4.3).  Namely, if
 the mobile node is using a co-located care-of address (the 'D' bit
 was set in the mobile node's Registration Request), then the home
 agent SHOULD tunnel the datagram to this care-of address; otherwise,
 the home agent MUST first encapsulate the datagram in a unicast
 datagram addressed to the mobile node's home address and then MUST
 tunnel the resulting datagram (recursive tunneling) to the mobile
 node's care-of address.
 A mobile node that wishes to send datagrams to a multicast group also
 has two options:  (1) send directly on the visited network; or (2)
 send via a tunnel to its home agent.  Because multicast routing in
 general depends upon the IP source address, a mobile node which sends
 multicast datagrams directly on the visited network MUST use a co-
 located care-of address as the IP source address.  Similarly, a
 mobile node which tunnels a multicast datagram to its home agent MUST
 use its home address as the IP source address of both the (inner)
 multicast datagram and the (outer) encapsulating datagram.  This
 second option assumes that the home agent is a multicast router.

Perkins Standards Track [Page 60] RFC 2002 IP Mobility Support October 1996

4.5. Mobile Routers

 A mobile node can be a router, which is responsible for the mobility
 of one or more entire networks moving together, perhaps on an
 airplane, a ship, a train, an automobile, a bicycle, or a kayak.  The
 nodes connected to a network served by the mobile router may
 themselves be fixed nodes or mobile nodes or routers.  In this
 document, such networks are called "mobile networks".
 A mobile router MAY act as a foreign agent and provide a foreign
 agent care-of address to mobile nodes connected to the mobile
 network.  Typical routing to a mobile node via a mobile router in
 this case is illustrated by the following example:
    a)   A laptop computer is disconnected from its home network and
         later attached to a network port in the seat back of an
         aircraft.  The laptop computer uses Mobile IP to register on
         this foreign network, using a foreign agent care-of address
         discovered through an Agent Advertisement from the aircraft's
         foreign agent.
    b)   The aircraft network is itself mobile.  Suppose the node
         serving as the foreign agent on the aircraft also serves as
         the default router that connects the aircraft network to the
         rest of the Internet.  When the aircraft is at home, this
         router is attached to some fixed network at the airline's
         headquarters, which is the router's home network.  While
         the aircraft is in flight, this router registers from time
         to time over its radio link with a series of foreign agents
         below it on the ground.  This router's home agent is a node
         on the fixed network at the airline's headquarters.
    c)   Some correspondent node sends a datagram to the laptop
         computer, addressing the datagram to the laptop's home
         address.  This datagram is initially routed to the laptop's
         home network.
    d)   The laptop's home agent intercepts the datagram on the home
         network and tunnels it to the laptop's care-of address, which
         in this example is an address of the node serving as router
         and foreign agent on the aircraft.  Normal IP routing will
         route the datagram to the fixed network at the airline's
         headquarters.

Perkins Standards Track [Page 61] RFC 2002 IP Mobility Support October 1996

    e)   The aircraft router and foreign agent's home agent there
         intercepts the datagram and tunnels it to its current care-of
         address, which in this example is some foreign agent on the
         ground below the aircraft.  The original datagram from the
         correspondent node has now been encapsulated twice:  once
         by the laptop's home agent and again by the aircraft's home
         agent.
    f)   The foreign agent on the ground decapsulates the datagram,
         yielding a datagram still encapsulated by the laptop's home
         agent, with a destination address of the laptop's care-of
         address.  The ground foreign agent sends the resulting
         datagram over its radio link to the aircraft.
    g)   The foreign agent on the aircraft decapsulates the datagram,
         yielding the original datagram from the correspondent node,
         with a destination address of the laptop's home address.
         The aircraft foreign agent delivers the datagram over the
         aircraft network to the laptop's link-layer address.
 This example illustrated the case in which a mobile node is attached
 to a mobile network.  That is, the mobile node is mobile with respect
 to the network, which itself is also mobile (here with respect to the
 ground).  If, instead, the node is fixed with respect to the mobile
 network (the mobile network is the fixed node's home network), then
 either of two methods may be used to cause datagrams from
 correspondent nodes to be routed to the fixed node.
 A home agent MAY be configured to have a permanent registration for
 the fixed node, that indicates the mobile router's address as the
 fixed host's care-of address.  The mobile router's home agent will
 usually be used for this purpose.  The home agent is then responsible
 for advertising connectivity using normal routing protocols to the
 fixed node.  Any datagrams sent to the fixed node will thus use
 recursive tunneling as described above.
 Alternatively, the mobile router MAY advertise connectivity to the
 entire mobile network using normal IP routing protocols through a
 bi-directional tunnel to its own home agent.  This method avoids the
 need for recursive tunneling of datagrams.

4.6. ARP, Proxy ARP, and Gratuitous ARP

 The use of ARP [16] requires special rules for correct operation when
 wireless or mobile nodes are involved.  The requirements specified in
 this section apply to all home networks in which ARP is used for
 address resolution.

Perkins Standards Track [Page 62] RFC 2002 IP Mobility Support October 1996

 In addition to the normal use of ARP for resolving a target node's
 link-layer address from its IP address, this document distinguishes
 two special uses of ARP:
  1. A Proxy ARP [18] is an ARP Reply sent by one node on behalf

of another node which is either unable or unwilling to answer

        its own ARP Requests.  The sender of a Proxy ARP reverses the
        Sender and Target Protocol Address fields as described in [16],
        but supplies some configured link-layer address (generally, its
        own) in the Sender Hardware Address field.  The node receiving
        the Reply will then associate this link-layer address with the
        IP address of the original target node, causing it to transmit
        future datagrams for this target node to the node with that
        link-layer address.
  1. A Gratuitous ARP [23] is an ARP packet sent by a node in order to

spontaneously cause other nodes to update an entry in their ARP

        cache.  A gratuitous ARP MAY use either an ARP Request or an ARP
        Reply packet.  In either case, the ARP Sender Protocol Address
        and ARP Target Protocol Address are both set to the IP address
        of the cache entry to be updated, and the ARP Sender Hardware
        Address is set to the link-layer address to which this cache
        entry should be updated.  When using an ARP Reply packet, the
        Target Hardware Address is also set to the link-layer address to
        which this cache entry should be updated (this field is not used
        in an ARP Request packet).
        In either case, for a gratuitous ARP, the ARP packet MUST be
        transmitted as a local broadcast packet on the local link.  As
        specified in [16], any node receiving any ARP packet (Request or
        Reply) MUST update its local ARP cache with the Sender Protocol
        and Hardware Addresses in the ARP packet, if the receiving node
        has an entry for that IP address already in its ARP cache.  This
        requirement in the ARP protocol applies even for ARP Request
        packets, and for ARP Reply packets that do not match any ARP
        Request transmitted by the receiving node [16].
 While a mobile node is registered on a foreign network, its home
 agent uses proxy ARP [18] to reply to ARP Requests it receives that
 seek the mobile node's link-layer address.  When receiving an ARP
 Request, the home agent MUST examine the target IP address of the
 Request, and if this IP address matches the home address of any
 mobile node for which it has a registered mobility binding, the home
 agent MUST transmit an ARP Reply on behalf of the mobile node.  After
 exchanging the sender and target addresses in the packet [18], the
 home agent MUST set the sender link-layer address in the packet to
 the link-layer address of its own interface over which the Reply will
 be sent.

Perkins Standards Track [Page 63] RFC 2002 IP Mobility Support October 1996

 When a mobile node leaves its home network and registers a binding on
 a foreign network, its home agent uses gratuitous ARP to update the
 ARP caches of nodes on the home network.  This causes such nodes to
 associate the link-layer address of the home agent with the mobile
 node's home (IP) address.  When registering a binding for a mobile
 node for which the home agent previously had no binding (the mobile
 node was assumed to be at home), the home agent MUST transmit a
 gratuitous ARP on behalf of the mobile node.  This gratuitous ARP
 packet MUST be transmitted as a broadcast packet on the link on which
 the mobile node's home address is located.  Since broadcasts on the
 local link (such as Ethernet) are typically not guaranteed to be
 reliable, the gratuitous ARP packet SHOULD be retransmitted a small
 number of times to increase its reliability.
 When a mobile node returns to its home network, the mobile node
 and its home agent use gratuitous ARP to cause all nodes on the
 mobile node's home network to update their ARP caches to once again
 associate the mobile node's own link-layer address with the mobile
 node's home (IP) address.  Before transmitting the (de)Registration
 Request message to its home agent, the mobile node MUST transmit this
 gratuitous ARP on its home network as a local broadcast on this link.
 The gratuitous ARP packet SHOULD be retransmitted a small number of
 times to increase its reliability, but these retransmissions SHOULD
 proceed in parallel with the transmission and processing of its
 (de)Registration Request.
 When the mobile node's home agent receives and accepts this
 (de)Registration Request, the home agent MUST also transmit a
 gratuitous ARP on the mobile node's home network.  This gratuitous
 ARP also is used to associate the mobile node's home address with
 the mobile node's own link-layer address.  A gratuitous ARP is
 transmitted by both the mobile node and its home agent, since in the
 case of wireless network interfaces, the area within transmission
 range of the mobile node will likely differ from that within range
 of its its home agent.  Th ARP packet from the home agent MUST be
 transmitted as a local broadcast on the mobile node's home link,
 and SHOULD be retransmitted a small number of times to increase
 its reliability; these retransmissions, however, SHOULD proceed in
 parallel with the transmission and processing of its (de)Registration
 Reply.
 While the mobile node is away from home, it MUST NOT transmit any
 broadcast ARP Request or ARP Reply messages.  Finally, while the
 mobile node is away from home, it MUST NOT reply to ARP Requests
 in which the target IP address is its own home address, unless the
 ARP Request is sent by a foreign agent with which the mobile node
 is attempting to register or a foreign agent with which the mobile
 node has an unexpired registration.  In the latter case, the mobile

Perkins Standards Track [Page 64] RFC 2002 IP Mobility Support October 1996

 node MUST use a unicast ARP Reply to respond to the foreign agent.
 Note that if the mobile node is using a co-located care-of address
 and receives an ARP Request in which the target IP address is this
 care-of address, then the mobile node SHOULD reply to this ARP
 Request.  Note also that, when transmitting a Registration Request on
 a foreign network, a mobile node may discover the link-layer address
 of a foreign agent by storing the address as it is received from the
 Agent Advertisement from that foreign agent, but not by transmitting
 a broadcast ARP Request message.
 The specific order in which each of the above requirements for the
 use of ARP, proxy ARP, and gratuitous ARP are applied, relative to
 the transmission and processing of the mobile node's Registration
 Request and Registration Reply messages when leaving home or
 returning home, are important to the correct operation of the
 protocol.
 To summarize the above requirements, when a mobile node leaves its
 home network, the following steps, in this order, MUST be performed:
  1. The mobile node decides to register away from home, perhaps

because it has received an Agent Advertisement from a foreign

     agent and has not recently received one from its home agent.
  1. Before transmitting the Registration Request, the mobile node

disables its own future processing of any ARP Requests it

     may subsequently receive requesting the link-layer address
     corresponding to its home address, except insofar as necessary to
     communicate with foreign agents on visited networks.
  1. The mobile node transmits its Registration Request.
  1. When the mobile node's home agent receives and accepts the

Registration Request, it performs a gratuitous ARP on behalf

     of the mobile node, and begins using proxy ARP to reply to ARP
     Requests that it receives requesting the mobile node's link-layer
     address.  If, instead, the home agent rejects the Registration
     Request, no ARP processing (gratuitous nor proxy) is performed by
     the home agent.
 When a mobile node later returns to its home network, the following
 steps, in this order, MUST be performed:
  1. The mobile node decides to register at home, perhaps because it

has received an Agent Advertisement from its home agent.

Perkins Standards Track [Page 65] RFC 2002 IP Mobility Support October 1996

  1. Before transmitting the Registration Request, the mobile node

re-enables its own future processing of any ARP Requests it may

     subsequently receive requesting its link-layer address.
  1. The mobile node performs a gratuitous ARP for itself.
  1. The mobile node transmits its Registration Request.
  1. When the mobile node's home agent receives and accepts the

Registration Request, it stops using proxy ARP to reply to

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

5. Security Considerations

 The mobile computing environment is potentially very different from
 the ordinary computing environment.  In many cases, mobile computers
 will be connected to the network via wireless links.  Such links are
 particularly vulnerable to passive eavesdropping, active replay
 attacks, and other active attacks.

5.1. Message Authentication Codes

 Home agents and mobile nodes MUST be able to perform authentication.
 The default algorithm is keyed MD5 [21], with a key size of 128 bits.
 The default mode of operation is to both precede and follow the data
 to be hashed, by the 128-bit key; that is, MD5 is to be used in
 "prefix+suffix" mode.  The foreign agent MUST also support
 authentication using keyed MD5 and key sizes of 128 bits or greater,
 with manual key distribution.  More authentication algorithms,
 algorithm modes, key distribution methods, and key sizes MAY also be
 supported.

5.2. Areas of Security Concern in this Protocol

 The registration protocol described in this document will result in a
 mobile node's traffic being tunneled to its care-of address.  This
 tunneling feature could be a significant vulnerability if the
 registration were not authenticated.  Such remote redirection, for
 instance as performed by the mobile registration protocol, is widely
 understood to be a security problem in the current Internet if not
 authenticated [2].  Moreover, the Address Resolution Protocol (ARP)
 is not authenticated, and can potentially be used to steal another
 host's traffic.  The use of "Gratuitous ARP" (Section 4.6) brings
 with it all of the risks associated with the use of ARP.

Perkins Standards Track [Page 66] RFC 2002 IP Mobility Support October 1996

5.3. Key Management

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

5.4. Picking Good Random Numbers

 The strength of any authentication mechanism depends on several
 factors, including the innate strength of the authentication
 algorithm, the secrecy of the key used, the strength of the key used,
 and the quality of the particular implementation.  This specification
 requires implementation of keyed MD5 for authentication, but does not
 preclude the use of other authentication algorithms and modes.  For
 keyed MD5 authentication to be useful, the 128-bit key must be both
 secret (that is, known only to authorized parties) and pseudo-random.
 If nonces are used in connection with replay protection, they must
 also be selected carefully.  Eastlake, et al. [7] provides more
 information on generating pseudo-random numbers.

5.5. Privacy

 Users who have sensitive data that they do not wish others to see
 should use mechanisms outside the scope of this document (such as
 encryption) to provide appropriate protection.  Users concerned about
 traffic analysis should consider appropriate use of link encryption.
 If absolute location privacy is desired, the mobile node can create a
 tunnel to its home agent.  Then, datagrams destined for correspondent
 nodes will appear to emanate from the home network, and it may be
 more difficult to pinpoint the location of the mobile node.  Such
 mechanisms are all beyond the scope of this document.

Perkins Standards Track [Page 67] RFC 2002 IP Mobility Support October 1996

5.6. Replay Protection for Registration Requests

 The Identification field is used to let the home agent verify that a
 registration message has been freshly generated by the mobile node,
 not replayed by an attacker from some previous registration.  Two
 methods are described in this section:  timestamps (mandatory) and
 "nonces" (optional).  All mobile nodes and home agents MUST implement
 timestamp-based replay protection.  These nodes MAY also implement
 nonce-based replay protection (but see Appendix A.2 for a patent that
 may apply to nonce-based replay protection).
 The style of replay protection in effect between a mobile node and
 its home agent is part of the mobile security association.  A mobile
 node and its home agent MUST agree on which method of replay
 protection will be used.  The interpretation of the Identification
 field depends on the method of replay protection as described in the
 subsequent subsections.
 Whatever method is used, the low-order 32 bits of the Identification
 MUST be copied unchanged from the Registration Request to the Reply.
 The foreign agent uses those bits (and the mobile node's home
 address) to match Registration Requests with corresponding replies.
 The mobile node MUST verify that the low-order 32 bits of any
 Registration Reply are identical to the bits it sent in the
 Registration Request.
 The Identification in a new Registration Request MUST NOT be the same
 as in an immediately preceding Request, and SHOULD NOT repeat while
 the same security context is being used between the mobile node and
 the home agent.  Retransmission as in Section 3.6.3 is allowed.

5.6.1. Replay Protection using Timestamps

 The basic principle of timestamp replay protection is that the node
 generating a message inserts the current time of day, and the node
 receiving the message checks that this timestamp is sufficiently
 close to its own time of day.  Obviously the two nodes must have
 adequately synchronized time-of-day clocks.  As with any messages,
 time synchronization messages may be protected against tampering by
 an authentication mechanism determined by the security context
 between the two nodes.
 If timestamps are used, the mobile node MUST set the Identification
 field to a 64-bit value formatted as specified by the Network Time
 Protocol [13].  The low-order 32 bits of the NTP format represent
 fractional seconds, and those bits which are not available from a
 time source SHOULD be generated from a good source of randomness.
 Note, however, that when using timestamps, the 64-bit Identification

Perkins Standards Track [Page 68] RFC 2002 IP Mobility Support October 1996

 used in a Registration Request from the mobile node MUST be greater
 than that used in any previous Registration Request, as the home
 agent uses this field also as a sequence number.  Without such a
 sequence number, it would be possible for a delayed duplicate of an
 earlier Registration Request to arrive at the home agent (within the
 clock synchronization required by the home agent), and thus be
 applied out of order, mistakenly altering the mobile node's current
 registered care-of address.
 Upon receipt of a Registration Request with a valid Mobile-Home
 Authentication Extension, the home agent MUST check the
 Identification field for validity.  In order to be valid, the
 timestamp contained in the Identification field MUST be close enough
 to the home agent's time of day clock and the timestamp MUST be
 greater than all previously accepted timestamps for the requesting
 mobile node.  Time tolerances and resynchronization details are
 specific to a particular mobility security association.
 If the timestamp is valid, the home agent copies the entire
 Identification field into the Registration Reply it returns the Reply
 to the mobile node.  If the timestamp is not valid, the home agent
 copies only the low-order 32 bits into the Registration Reply, and
 supplies the high-order 32 bits from its own time of day.  In this
 latter case, the home agent MUST reject the registration by returning
 Code 133 (identification mismatch) in the Registration Reply.
 As described in Section 3.6.2.1, the mobile node MUST verify that the
 low-order 32 bits of the Identification in the Registration Reply are
 identical to those in the rejected registration attempt, before using
 the high-order bits for clock resynchronization.

5.6.2. Replay Protection using Nonces

 Implementors of this optional mechanism should examine Appendix A.2
 for a patent that may be applicable to nonce-based replay protection.
 The basic principle of nonce replay protection is that node A
 includes a new random number in every message to node B, and checks
 that node B returns that same number in its next message to node A.
 Both messages use an authentication code to protect against
 alteration by an attacker.  At the same time node B can send its own
 nonces in all messages to node A (to be echoed by node A), so that it
 too can verify that it is receiving fresh messages.
 The home agent may be expected to have resources for computing
 pseudo-random numbers useful as nonces [7].  It inserts a new nonce
 as the high-order 32 bits of the identification field of every
 Registration Reply.  The home agent copies the low-order 32 bits of

Perkins Standards Track [Page 69] RFC 2002 IP Mobility Support October 1996

 the Identification from the Registration Request message into the
 low-order 32 bits of the Identification in the Registration Reply.
 When the mobile node receives an authenticated Registration Reply
 from the home agent, it saves the high-order 32 bits of the
 identification for use as the high-order 32 bits of its next
 Registration Request.
 The mobile node is responsible for generating the low-order 32 bits
 of the Identification in each Registration Request.  Ideally it
 should generate its own random nonces.  However it may use any
 expedient method, including duplication of the random value sent by
 the home agent.  The method chosen is of concern only to the mobile
 node, because it is the node that checks for valid values in the
 Registration Reply.  The high-order and low-order 32 bits of the
 identification chosen SHOULD both differ from their previous values.
 The home agent uses a new high-order value and the mobile node uses a
 new low-order value for each registration message.  The foreign agent
 uses the low-order value (and the mobile host's home address) to
 correctly match registration replies with pending Requests (Section
 3.7.1).
 If a registration message is rejected because of an invalid nonce,
 the Reply always provides the mobile node with a new nonce to be used
 in the next registration.  Thus the nonce protocol is self-
 synchronizing.

Perkins Standards Track [Page 70] RFC 2002 IP Mobility Support October 1996

6. Acknowledgments

 Special thanks to Steve Deering (Xerox PARC), along with Dan Duchamp
 and John Ioannidis (JI) (Columbia), for forming the working group,
 chairing it, and putting so much effort into its early development.
 Thanks also to Kannan Alaggapan, Greg Minshall, and Tony Li for their
 contributions to the group while performing the duties of
 chairperson, as well as for their many useful comments.
 Thanks to the active members of the Mobile IP Working Group,
 particularly those who contributed text, including (in alphabetical
 order)
  1. Ran Atkinson (Naval Research Lab),
  2. Dave Johnson (Carnegie Mellon University),
  3. Frank Kastenholz (FTP Software),
  4. Anders Klemets (KTH),
  5. Chip Maguire (KTH),
  6. Andrew Myles (Macquarie University),
  7. Al Quirt (Bell Northern Research),
  8. Yakov Rekhter (IBM), and
  9. Fumio Teraoka (Sony).
 Thanks to Charlie Kunzinger and to Bill Simpson, the editors who
 produced the first drafts for of this document, reflecting the
 discussions of the Working Group.  Much of the new text of this memo
 is due to Jim Solomon and Dave Johnson.
 Thanks to Greg Minshall (Novell), Phil Karn (Qualcomm), and Frank
 Kastenholz (FTP Software) for their generous support in hosting
 interim Working Group meetings.

Perkins Standards Track [Page 71] RFC 2002 IP Mobility Support October 1996

A. Patent Issues

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

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

 Charles Perkins, editor of this memo, is sole inventor of U.S. Patent
 No. 5,159,592, assigned to IBM.  In a letter dated May 30, 1995, IBM
 brought this patent to the attention of the IETF, stating that this
 patent "relates to the Mobile IP." We understand that IBM did not
 intend to assert that any particular implementation of Mobile IP
 would or would not infringe the patent, but rather that IBM was
 meeting what it viewed as a duty to disclose information that could
 be relevant to the process of adopting a standard.
 Based on a review of the claims of the patent, IETF believes that a
 system of registering an address obtained from a foreign agent, as
 described in the document, would not necessarily infringe any of the
 claims of the patent; and that a system in which an address is
 obtained elsewhere and then registered can be implemented without
 necessarily infringing any claims of the patent.  Accordingly, our
 view is that the proposed protocol can be implemented without
 necessarily infringing the Perkins Patent.
 Parties considering adopting this protocol must be aware that some
 specific implementations, or features added to otherwise non-
 infringing implementations, may raise an issue of infringement with
 respect to this patent or to some other patent.
 This statement is for the IETF's assistance in its standard-setting
 procedure, and should not be relied upon by any party as an opinion
 or guarantee that any implementation it might make or use would not
 be covered by the Perkins Patent and any other patents.  In
 particular, IBM might disagree with the interpretation of this patent
 described herein.

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

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

Perkins Standards Track [Page 72] RFC 2002 IP Mobility Support October 1996

B. Link-Layer Considerations

 The mobile node MAY use link-layer mechanisms to decide that its
 point of attachment has changed.  Such indications include the
 Down/Testing/Up interface status [11], and changes in cell or
 administration.  The mechanisms will be specific to the particular
 link-layer technology, and are outside the scope of this document.
 The Point-to-Point-Protocol (PPP) [22] and its Internet Protocol
 Control Protocol (IPCP) [12], negotiates the use of IP addresses.
 The mobile node SHOULD first attempt to specify its home address, so
 that if the mobile node is attaching to its home network, the
 unrouted link will function correctly.  When the home address is not
 accepted by the peer, but a transient IP address is dynamically
 assigned to the mobile node, and the mobile node is capable of
 supporting a co-located care-of address, the mobile node MAY register
 that address as a co-located care-of address.  When the peer
 specifies its own IP address, that address MUST NOT be assumed to be
 a foreign agent care-of address or the IP address of a home agent.

C. TCP Considerations

C.1. TCP Timers

 Most hosts and routers which implement TCP/IP do not permit easy
 configuration of the TCP timer values.  When high-delay (e.g.,
 SATCOM) or low-bandwidth (e.g., High-Frequency Radio) links are in
 use, the default TCP timer values in many systems may cause
 retransmissions or timeouts, even when the link and network are
 actually operating properly with greater than usual delays because of
 the medium in use.  This can cause an inability to create or maintain
 TCP connections over such links, and can also cause unneeded
 retransmissions which consume already scarce bandwidth.  Vendors are
 encouraged to make TCP timers more configurable.  Vendors of systems
 designed for the mobile computing markets should pick default timer
 values more suited to low-bandwidth, high-delay links.  Users of
 mobile nodes should be sensitive to the possibility of timer-related
 difficulties.

C.2. TCP Congestion Management

 Mobile nodes often use media which are more likely to introduce
 errors, effectively causing more packets to be dropped.  This
 introduces a conflict with the mechanisms for congestion management
 found in modern versions of TCP [9].  Now, when a packet is dropped,
 the correspondent node's TCP implementation is likely to react as if
 there were a source of network congestion, and initiate the slow-

Perkins Standards Track [Page 73] RFC 2002 IP Mobility Support October 1996

 start mechanisms [9] designed for controlling that problem.  However,
 those mechanisms are inappropriate for overcoming errors introduced
 by the links themselves, and have the effect of magnifying the
 discontinuity introduced by the dropped packet.  This problem has
 been analyzed by Caceres, et al. [3]; there is no easy solution
 available, and certainly no solution likely to be installed soon on
 all correspondent nodes.  While this problem is beyond the scope of
 this document, it does illustrate that providing performance
 transparency to mobile nodes involves understanding mechanisms
 outside the network layer.  It also indicates the need to avoid
 designs which systematically drop packets; such designs might
 otherwise be considered favorably when making engineering tradeoffs.

D. Example Scenarios

 This section shows example Registration Requests for several common
 scenarios.

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

 The mobile node receives an Agent Advertisement from a foreign agent
 and wishes to register with that agent using the advertised foreign
 agent care-of address.  The mobile node wishes only IP-in-IP
 encapsulation, does not want broadcasts, and does not want
 simultaneous mobility bindings:
     IP fields:
       Source Address = mobile node's home address
       Destination Address = copied from the IP source address of the
         Agent Advertisement
       Time to Live = 1
     UDP fields:
       Source Port = <any>
       Destination Port = 434
     Registration Request fields:
       Type = 1
       S=0,B=0,D=0,M=0,G=0
       Lifetime = the Registration Lifetime copied from the
         Mobility Agent Advertisement Extension of the
         Router Advertisement message
       Home Address = the mobile node's home address
       Home Agent = IP address of mobile node's home agent
       Care-of Address = the Care-of Address copied from the
         Mobility Agent Advertisement Extension of the
         Router Advertisement message
       Identification = Network Time Protocol timestamp or Nonce
     Extensions:
       The Mobile-Home Authentication Extension

Perkins Standards Track [Page 74] RFC 2002 IP Mobility Support October 1996

D.2. Registering with a Co-Located Care-of Address

 The mobile node enters a foreign network that contains no foreign
 agents.  The mobile node obtains an address from a DHCP server [6]
 for use as a co-located care-of address.  The mobile node supports
 all forms of encapsulation (IP-in-IP, minimal encapsulation, and
 GRE), desires a copy of broadcast datagrams on the home network, and
 does not want simultaneous mobility bindings:
     IP fields:
       Source Address = care-of address obtained from DHCP server
       Destination Address = IP address of home agent
       Time to Live = 64
     UDP fields:
       Source Port = <any>
       Destination Port = 434
     Registration Request fields:
       Type = 1
       S=0,B=1,D=1,M=1,G=1
       Lifetime = 1800 (seconds)
       Home Address = the mobile node's home address
       Home Agent = IP address of mobile node's home agent
       Care-of Address = care-of address obtained from DHCP server
       Identification = Network Time Protocol timestamp or Nonce
     Extensions:
       The Mobile-Home Authentication Extension

Perkins Standards Track [Page 75] RFC 2002 IP Mobility Support October 1996

D.3. Deregistration

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

E. Applicability of Prefix Lengths Extension

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

Perkins Standards Track [Page 76] RFC 2002 IP Mobility Support October 1996

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

References

 [1] Atkinson, R., "IP Authentication Header", RFC 1826, August 1995.
 [2] S. M. Bellovin.  Security Problems in the TCP/IP Protocol Suite.
     ACM Computer Communications Review, 19(2), March 1989.
 [3] Ramon Caceres and Liviu Iftode.  Improving the Performance
     of Reliable Transport Protocols in Mobile Computing
     Environments.  IEEE Journal on Selected Areas in Communications,
     13(5):850--857, June 1995.
 [4] Deering, S., Editor, "ICMP Router Discovery Messages",
     RFC 1256, September 1991.
 [5] Deering, S., "Host Extensions for IP Multicasting", STD 5,
     RFC 1112, August 1989.
 [6] Droms, R., "Dynamic Host Configuration Protocol", RFC 1541,
     October 1993.
 [7] Eastlake, D., Crocker, S., and J. Schiller, "Randomness
     Requirements for Security", RFC 1750, December 1994.
 [8] Hanks, S., Li, R., Farinacci, D., and P. Traina, "Generic
     Routing Encapsulation (GRE)", RFC 1701, October 1994.
 [9] Van Jacobson.  Congestion Avoidance and Control.  In Proceedings
     of the SIGCOMM '88 Symposium:  Communications Architectures &
     Protocols, pages 314--329, August 1988.

Perkins Standards Track [Page 77] RFC 2002 IP Mobility Support October 1996

 [10] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed Serial
      Links", RFC 1144, February 1990.
 [11] McCloghrie, K., and F. Kastenholz, "Evolution of the
      Interfaces Group of MIB-II", RFC 1573, January 1994.
 [12] McGregor, G., "The PPP Internet Protocol Control Protocol
      (IPCP)", RFC 1332, May 1992.
 [13] Mills, D., "Network Time Protocol (Version 3):
      Specification, Implementation and Analysis", RFC 1305, March
      1992.
 [14] Perkins, C., "IP Encapsulation within IP", RFC 2003,
      October 1996.
 [15] Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
      October 1996.
 [16] Plummer, D., "An Ethernet Address Resolution Protocol:
      Or Converting Network Protocol Addresses to 48.bit Ethernet
      Addresses for Transmission on Ethernet Hardware", STD 37,
      RFC 826, November 1982.
 [17] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
      1980.
 [18] Postel, J., "Multi-LAN Address Resolution", RFC 925, October
      1984.
 [19] Postel, J., Editor, "Internet Protocol", STD 5, RFC 791,
      September 1981.
 [20] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
      RFC 1700, October 1994.
 [21] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
      April 1992.
 [22] Simpson, W., Editor, "The Point-to-Point Protocol
      (PPP)", STD 51, RFC 1661, July 1994.
 [23] W. Richard Stevens.  TCP/IP Illustrated, Volume 1:  The
      Protocols.  Addison-Wesley, Reading, Massachusetts, 1994.

Perkins Standards Track [Page 78] RFC 2002 IP Mobility Support October 1996

Editor's Address

 Questions about this memo can also be directed to the editor:
 Charles Perkins
 Room H3-D34
 T. J. Watson Research Center
 IBM Corporation
 30 Saw Mill River Rd.
 Hawthorne, NY  10532
 Work:   +1-914-784-7350
 Fax:    +1-914-784-6205
 EMail: perk@watson.ibm.com
 The working group can be contacted via the current chair:
    Jim Solomon
    Motorola, Inc.
    1301 E. Algonquin Rd.
    Schaumburg, IL  60196
    Work:   +1-847-576-2753
    EMail: solomon@comm.mot.com

Perkins Standards Track [Page 79]

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