





Internet Engineering Task Force (IETF)                   C. Perkins, Ed.
Request for Comments: 5944                                 WiChorus Inc.
Obsoletes: 3344                                            November 2010
Category: Standards Track
ISSN: 2070-1721


                 IP Mobility Support for IPv4, Revised

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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5944.















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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

























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Table of Contents

   1. Introduction ....................................................5
      1.1. Protocol Requirements ......................................5
      1.2. Goals ......................................................6
      1.3. Assumptions ................................................6
      1.4. Applicability ..............................................6
      1.5. New Architectural Entities .................................7
      1.6. Terminology ................................................7
      1.7. Protocol Overview .........................................11
      1.8. Message Format and Protocol Extensibility .................14
      1.9. Type-Length-Value Extension Format for Mobile IP
           Extensions ................................................16
      1.10. Long Extension Format ....................................17
      1.11. Short Extension Format ...................................18
   2. Agent Discovery ................................................18
      2.1. Agent Advertisement .......................................19
           2.1.1. Mobility Agent Advertisement Extension .............21
           2.1.2. Prefix-Lengths Extension ...........................23
           2.1.3. One-Byte Padding Extension .........................24
      2.2. Agent Solicitation ........................................24
      2.3. Foreign Agent and Home Agent Considerations ...............24
           2.3.1. Advertised Router Addresses ........................26
           2.3.2. Sequence Numbers and Rollover Handling .............26
      2.4. Mobile Node Considerations ................................26
           2.4.1. Registration Required ..............................28
           2.4.2. Move Detection .....................................28
           2.4.3. Returning Home .....................................29
           2.4.4. Sequence Numbers and Rollover Handling .............29
   3. Registration ...................................................29
      3.1. Registration Overview .....................................30
      3.2. Authentication ............................................31
      3.3. Registration Request ......................................32
      3.4. Registration Reply ........................................34
      3.5. Registration Extensions ...................................38
           3.5.1. Computing Authentication Extension Values ..........38
           3.5.2. Mobile-Home Authentication Extension ...............39
           3.5.3. Mobile-Foreign Authentication Extension ............40
           3.5.4. Foreign-Home Authentication Extension ..............40
      3.6. Mobile Node Considerations ................................41
           3.6.1. Sending Registration Requests ......................43
           3.6.2. Receiving Registration Replies .....................47
           3.6.3. Registration Retransmission ........................50
      3.7. Foreign Agent Considerations ..............................50
           3.7.1. Configuration and Registration Tables ..............51
           3.7.2. Receiving Registration Requests ....................52
           3.7.3. Receiving Registration Replies .....................56




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      3.8. Home Agent Considerations .................................58
           3.8.1. Configuration and Registration Tables ..............58
           3.8.2. Receiving Registration Requests ....................59
           3.8.3. Sending Registration Replies .......................64
   4. Routing Considerations .........................................66
      4.1. Encapsulation Types .......................................67
      4.2. Unicast Datagram Routing ..................................67
           4.2.1. Mobile Node Considerations .........................67
           4.2.2. Foreign Agent Considerations .......................68
           4.2.3. Home Agent Considerations ..........................69
      4.3. Broadcast Datagrams .......................................70
      4.4. Multicast Datagram Routing ................................71
      4.5. Mobile Routers ............................................72
      4.6. ARP, Proxy ARP, and Gratuitous ARP ........................74
   5. Security Considerations ........................................77
      5.1. Message Authentication Codes ..............................77
      5.2. Areas of Security Concern in This Protocol ................78
      5.3. Key Management ............................................78
      5.4. Picking Good Random Numbers ...............................78
      5.5. Privacy ...................................................79
      5.6. Ingress Filtering .........................................79
      5.7. Replay Protection for Registration Requests ...............79
           5.7.1. Replay Protection Using Timestamps .................80
           5.7.2. Replay Protection Using Nonces .....................81
   6. IANA Considerations ............................................82
      6.1. Mobile IP Message Types ...................................82
      6.2. Extensions to RFC 1256 Router Advertisement Messages ......83
      6.3. Extensions to Mobile IP Registration Messages .............83
      6.4. Code Values for Mobile IP Registration Reply Messages .....84
   7. Acknowledgments ................................................84
   8. References .....................................................86
      8.1. Normative References ......................................86
      8.2. Informative References ....................................87
   Appendix A. Link-Layer Considerations .............................90
   Appendix B. TCP Considerations ....................................90
      B.1. TCP Timers ................................................90
      B.2. TCP Congestion Management .................................91
   Appendix C.  Example Scenarios ....................................92
      C.1. Registering with a Foreign Agent Care-of Address ..........92
      C.2. Registering with a Co-Located Care-of Address .............93
      C.3. Deregistration ............................................94
   Appendix D. Applicability of Prefix-Lengths Extension .............94
   Appendix E. Interoperability Considerations .......................95
   Appendix F. Changes since RFC 3344 ................................96
   Appendix G. Example Messages ......................................98
      G.1. Example ICMP Agent Advertisement Message Format ...........98
      G.2. Example Registration Request Message Format ...............99
      G.3. Example Registration Reply Message Format ................100



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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:

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

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

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

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

   Changes between this revised specification for Mobile IP and the
   original specifications (see [44], [14], [15], [20], [4], and [50])
   are detailed in Appendix F.

1.1.  Protocol Requirements

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

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

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






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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 from an Ethernet segment to a wireless LAN, as
   long as the mobile node's IP address remains the same after such a
   movement.

   One can think of Mobile IP as solving the "macro" mobility management
   problem.  It is less well suited for more "micro" mobility management
   applications -- for example, handoff amongst wireless transceivers,
   each of which covers only a very small geographic area.  As long as
   node movement does not occur between points of attachment on
   different IP subnets, link-layer mechanisms for mobility (i.e., link-
   layer handoff) may offer faster convergence and far less overhead
   than Mobile IP.






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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 that 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 that provides routing
      services to the mobile node while registered.  The foreign agent
      detunnels and delivers to the mobile node datagrams 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 that a "permanent"
   IP address is provided to a stationary host.  When away from its home
   network, a "care-of address" is associated with the mobile node and
   reflects the mobile node's current point of attachment.  The mobile
   node uses its home address as the source address of all IP datagrams
   that it sends, except where otherwise described in this document for
   datagrams sent for certain mobility management functions (e.g., as in
   Section 3.6.1.1).

1.6.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].








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   In addition, this document frequently uses the following terms:

   Authorization-Enabling Extension

      An authentication that makes a (registration) message acceptable
      to the ultimate recipient of the registration message.  An
      authorization-enabling extension MUST contain a Security Parameter
      Index (SPI).

      In this document, all uses of authorization-enabling extension
      refer to authentication extensions that enable the Registration
      Request message to be acceptable to the home agent.  Using
      additional protocol structures specified outside of this document,
      it may be possible for the mobile node to provide authentication
      of its registration to the home agent, by way of another
      authenticating entity within the network that is acceptable to the
      home agent (for example, see RFC 2794 [2]).

   Agent Advertisement

      An advertisement message constructed by attaching a special
      Extension to a Router Advertisement [5] message.

   Authentication

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

   Care-of Address

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




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   Gratuitous ARP

      An Address Resolution Protocol (ARP) packet sent by a node in
      order to spontaneously cause other nodes to update an entry in
      their ARP cache [45].  See Section 4.6.

   Home Address

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

   Home Network

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

   Link

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

   Link-Layer Address

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

   Mobility Agent

      Either a home agent or a foreign agent.

   Mobility Binding

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

   Mobility Security Association

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




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   Node

      A host or a router.

   Nonce

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

   Security Parameter Index (SPI)

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

   Tunnel

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

   Virtual Network

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

   Visited Network

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

   Visitor List

      The list of mobile nodes visiting a foreign agent.













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1.7.  Protocol Overview

   The following support services are defined for Mobile IP:

   Agent Discovery

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

   Registration

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

   Silently Discard

      The implementation discards the datagram without further
      processing, and without indicating an error to the sender.  The
      implementation SHOULD provide the capability of logging the error,
      including the contents of the discarded datagram, and SHOULD
      record the event in a statistics counter.

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

   o  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.

   o  A mobile node receives these Agent Advertisements and determines
      whether it is on its home network or a foreign network.

   o  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.

   o  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



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      advertisements (a foreign agent care-of address), or by some
      external assignment mechanism such as DHCP [34] (a co-located
      care-of address).

   o  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 the home
      agent, possibly via a foreign agent (Section 3).

   o  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).

   o  In the reverse direction, datagrams sent by the mobile node are
      generally delivered to their destination using standard IP routing
      mechanisms, not necessarily passing through the home agent.

   When away from home, Mobile IP uses protocol tunneling to hide a
   mobile node's home address from intervening routers between its home
   network and its current location.  The tunnel terminates at the
   mobile node's care-of address.  The care-of address must be an
   address to which datagrams can be delivered via conventional IP
   routing.  At the care-of address, the original datagram is removed
   from the tunnel and delivered to the mobile node.

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

   a.  A "foreign agent care-of address" is a care-of address provided
       by a foreign agent through its Agent Advertisement messages.  In
       this case, the care-of address is an IP address of the foreign
       agent.  In this mode, the foreign agent is the endpoint of the
       tunnel and, upon receiving tunneled datagrams, decapsulates them
       and delivers the inner datagram to the mobile node.  This mode of
       acquisition is preferred because it allows many mobile nodes to
       share the same care-of address and therefore does not place
       unnecessary demands on the already limited IPv4 address space.

   b.  A "co-located care-of address" is a care-of address acquired by
       the mobile node as a local IP address through some external
       means, which the mobile node then associates with one of its own
       network interfaces.  The address may be dynamically acquired as a
       temporary address by the mobile node, such as through DHCP [34],
       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



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       care-of address are beyond the scope of this document.  When
       using a co-located care-of address, the mobile node serves as the
       endpoint of the tunnel and itself performs decapsulation of the
       datagrams tunneled to it.

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





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

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

                    Figure 1:  Operation of Mobile IPv4

   If a mobile node is using a co-located care-of address (as described
   in item (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, Figure 1 illustrates the routing of datagrams to and
   from a mobile node away from home, once the mobile node has
   registered with its home agent.  In Figure 1, the mobile node is
   using a foreign agent care-of address, not a co-located care-of
   address.

1.8.  Message Format and Protocol Extensibility

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

      1 Registration Request

      3 Registration Reply

   Up-to-date values for the message types for Mobile IP control
   messages are specified in the IANA online database [48].

   In addition, for Agent Discovery, Mobile IP makes use of the existing
   Router Advertisement and Router Solicitation messages defined for
   ICMP Router Discovery [5].




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

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

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

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

        0  One-byte Padding (encoded with neither Length nor Data field)
        32 Mobile-Home Authentication
        33 Mobile-Foreign Authentication
        34 Foreign-Home Authentication

   o  The second set consists of those Extensions that may appear in
      ICMP Router Discovery messages [5].  In this document, the
      following types are defined for Extensions appearing in ICMP
      Router Discovery messages:

        0  One-byte Padding (encoded with neither 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 IANA online database
   [48].

   Due to the separation (orthogonality) of these sets, it is
   conceivable that two Extensions that are defined at a later date
   could have identical Type values, so long as one of the Extensions
   may be used only in Mobile IP control messages and the other may be
   used only in ICMP Router Discovery messages.

   The Type field in the Mobile IP extension structure can support up to
   255 (skippable and non-skippable) uniquely identifiable extensions.
   When an Extension numbered in either of these sets within the range 0
   through 127 is encountered but not recognized, the message containing
   that Extension MUST be silently discarded.  When an Extension



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   numbered in the range 128 through 255 is encountered that is not
   recognized, that particular Extension is ignored, but the rest of the
   Extensions and message data MUST still be processed.  The Length
   field of the Extension is used to skip the Data field in searching
   for the next Extension.

   Unless additional structure is utilized for the extension types, new
   developments or additions to Mobile IP might require so many new
   extensions that the available space for extension types might run
   out.  Two new extension structures are proposed to solve this
   problem.  Certain types of extensions can be aggregated, using
   subtypes to identify the precise extension, for example as has been
   done with the Generic Authentication Keys extensions [46].  In many
   cases, this may reduce the rate of allocation for new values of the
   Type field.

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

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

   o  The simple extension format

   o  The long extension format

   o  The short extension format

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

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

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

       Figure 2:  Type-Length-Value Extension Format for Mobile IPv4




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   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.

1.10.  Long Extension Format

   This format is applicable for non-skippable extensions that carry
   information of more than 256 bytes.  Skippable extensions can never
   use the long format, because the receiver is not required to include
   parsing code and is likely to treat the 8 bits immediately following
   the Type as the Length field.

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

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

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

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

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

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

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





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1.11.  Short Extension Format

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

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

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

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

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

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

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

2.  Agent Discovery

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

   Mobile IP extends ICMP Router Discovery [5] 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 Time





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   to Live (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 A).
   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 [9], 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 a Mobility Agent Advertisement Extension
   (Section 2.1.1) and, optionally, a Prefix-Lengths Extension (Section
   2.1.2), One-byte Padding Extension (Section 2.1.3), or other
   Extensions that might be defined in the future.

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

      -  Link-Layer Fields

         Destination Address

                  The link-layer Destination Address of a unicast Agent
                  Advertisement MUST be the same as the source link-
                  layer address of the Agent Solicitation that prompted
                  the Advertisement.

      -  IP Fields

         TTL      The TTL for all Agent Advertisements MUST be set to 1.






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         Destination Address

                  As specified for ICMP Router Discovery [5], the IP
                  Destination Address of a multicast Agent Advertisement
                  MUST be either the "all systems on this link"
                  multicast address (224.0.0.1) [6] or the "limited
                  broadcast" address (255.255.255.255).  The subnet-
                  directed broadcast address of the form <prefix>.<-1>
                  cannot be used since mobile nodes will not generally
                  know the prefix of the foreign network.  When the
                  Agent Advertisement is unicast to a mobile node, the
                  IP home address of the mobile node SHOULD be used as
                  the Destination Address.

      -  ICMP Fields

         Code     The Code field of the Agent Advertisement is
                  interpreted as follows:

                  0  The mobility agent handles common traffic -- that
                     is, it acts as a router for IP datagrams not
                     necessarily related to mobile nodes.

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

         Lifetime

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

         Router Address(es)

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

         Num Addrs

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





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   If sent periodically, the nominal interval at which Agent
   Advertisements are sent SHOULD be no longer than 1/3 of the
   advertisement Lifetime given in the ICMP header.  This interval MAY
   be shorter than 1/3 the advertised Lifetime.  This allows a mobile
   node to miss three successive advertisements before deleting the
   agent from its list of valid agents.  The actual transmission time
   for each advertisement SHOULD be slightly randomized [5] 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|r|T|U|X|I|reserved |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  zero or more Care-of Addresses               |
    |                              ...                              |

      Type     16

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

      Sequence Number

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









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      Registration Lifetime

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

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

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

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

      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        Generic Routing Encapsulation (GRE) encapsulation.  This
               agent implements receiving tunneled datagrams that use
               GRE encapsulation [13].

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

      T        Foreign agent supports reverse tunneling as specified in
               [12].

      U        Mobility agent supports UDP Tunneling as specified in
               [27].

      X        Mobility agent supports Registration Revocation as
               specified in [28].

      I        Foreign agent supports Regional Registration as specified
               in [29].

      reserved
               Sent as zero; ignored on reception.




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      Care-of Address(es)

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

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

   When a foreign agent wishes to require registration even from those
   mobile nodes that 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 (possibly zero) of the Num Addrs
               field in the ICMP Router Advertisement portion of the
               Agent Advertisement.




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      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 D for implementation details about the use
   of this Extension.

2.1.3.  One-Byte Padding Extension

   Some IP protocol implementations insist upon padding ICMP messages to
   an even number of bytes.  If the ICMP length of an Agent
   Advertisement is odd, this Extension MAY be included in order to make
   the ICMP length even.  Note that this Extension is NOT intended to be
   a general-purpose Extension to be included in order to word- or long-
   align the various fields of the Agent Advertisement.  An Agent
   Advertisement SHOULD NOT include more than one One-byte Padding
   Extension and if present, this Extension SHOULD be the last Extension
   in the Agent Advertisement.

   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 that cannot be discovered by a link-layer protocol
   MUST send Agent Advertisements.  An agent that can be discovered by a
   link-layer protocol SHOULD also implement Agent Advertisements.



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   However, the Advertisements need not be sent, except when the site
   policy requires registration with the agent (i.e., when the 'R' bit
   is set), or as a response to a specific Agent Solicitation.  All
   mobility agents MUST process packets that they receive addressed to
   the Mobile-Agents multicast group, at address 224.0.0.11.  A mobile
   node MAY send an Agent Solicitation to 224.0.0.11.  All mobility
   agents SHOULD respond to Agent Solicitations.

   The same procedures, defaults, and constants are used in Agent
   Advertisement messages and Agent Solicitation messages as specified
   for ICMP Router Discovery [5], except that:

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

   o  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).

   o  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.  A mobility agent MAY use different
   settings for each of the 'R', 'H', and 'F' bits on different network
   interfaces.

   If the home network is a virtual network, the home network has no
   physical realization external to the home agent itself.  In this
   case, there is no physical network link on which to send Agent
   Advertisement messages advertising the home agent.  Mobile nodes for
   which this is the home network are always treated as being away from
   home.






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   On a particular subnet, either all mobility agents MUST include the
   Prefix-Lengths Extension or all of them MUST NOT include this
   Extension.  Equivalently, it is prohibited for some agents on a given
   subnet to include the Extension but for others not to include it.
   Otherwise, one of the move detection algorithms designed for mobile
   nodes will not function properly (Section 2.4.2).

2.3.1.  Advertised Router Addresses

   The ICMP Router Advertisement portion of the Agent Advertisement MAY
   contain one or more router addresses.  An agent SHOULD only put its
   own addresses, if any, in the advertisement.  Whether or not its own
   address appears in the router addresses, a foreign agent MUST route
   datagrams it receives from registered mobile nodes (Section 3.7).

2.3.2.  Sequence Numbers and Rollover Handling

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

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 [5], 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,






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

   While still searching for an agent, the mobile node MUST NOT increase
   the rate at which it sends solicitations unless it has received a
   positive indication that it has moved to a new link.  After
   successfully registering with an agent, the mobile node 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 [5].

   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.

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

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

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






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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) that require exchanges of
   authorization.

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

2.4.2.  Move Detection

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

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



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   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 the
   foreign agent sending the new Advertisement with the different
   network prefix.  The Agent Advertisement on which the new
   registration is based MUST NOT have expired according to its Lifetime
   field.

2.4.3.  Returning Home

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

   o  request forwarding services when visiting a foreign network,

   o  inform their home agent of their current care-of address,



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   o  renew a registration that is due to expire, and/or

   o  deregister when they return home.

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

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

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

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

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

   o  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:

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

   o  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.

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






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   o  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 (Section 3.3 and Section
   3.4).  When registering via a foreign agent, the registration
   procedure requires the following four messages:

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

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

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

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

   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.  A zero
   UDP checksum SHOULD be accepted by the recipient.  The behavior of
   the mobile node and the home agent with respect to their mutual
   acceptance of packets with zero UDP checksums SHOULD be defined as
   part of the Mobility Security Association that exists between them.

3.2.  Authentication

   Each mobile node, foreign agent, and home agent MUST be able to
   support a Mobility Security Association for mobile entities, indexed
   by their SPI and IP address.  In the case of the mobile node, this
   must be its home address.  See Section 5.1 for requirements for
   support of authentication algorithms.  Registration messages between
   a mobile node and its home agent MUST be authenticated with an
   authorization-enabling extension, e.g., the Mobile-Home
   Authentication Extension (Section 3.5.2).  This extension MUST be the



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   first authentication extension; other foreign-agent-specific
   extensions MAY be added to the message after the mobile node computes
   the authentication.

3.3.  Registration Request

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

   IP fields:

      Source Address

                     Typically the interface address from which the
                     message is sent.

      Destination Address

                     Typically that of the foreign agent or the home
                     agent.

      See Sections 3.6.1.1 and 3.7.2.2 for details.

   UDP fields:

      Source Port       variable

      Destination Port  434


















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

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

      Type     1 (Registration Request)

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

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

      D        Decapsulation by mobile node.  If the 'D' bit is set, the
               mobile node will itself decapsulate datagrams that are
               sent to the care-of address.  That is, the mobile node is
               using a co-located care-of address.

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

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




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      T        Reverse Tunneling requested; see [12].

      x        Sent as zero; ignored on reception.

      Lifetime

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

      Home Address

               The IP address of the mobile node.

      Home Agent

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

      Care-of Address

               The IP address for the end of the tunnel.

      Identification

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

      Extensions

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

3.4.  Registration Reply

   A mobility agent typically returns a Registration Reply message to a
   mobile node that has sent a Registration Request message.  If the
   mobile node is requesting service from a foreign agent, that foreign
   agent will typically receive the Reply from the home agent and





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   subsequently relay it to the mobile node.  Reply messages contain the
   necessary codes to inform the mobile node about the status of its
   Request, along with the lifetime granted by the home agent, which MAY
   be smaller than the original Request.

   The foreign agent MUST NOT increase the Lifetime selected by the
   mobile node in the Registration Request, since the Lifetime is
   covered by an authentication extension that enables authorization by
   the home agent.  Such an extension contains authentication data that
   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 3.8.3.2 for
                     complete details.

      Destination Address

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

   UDP fields:

      Source Port

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

      Destination Port

                     Copied from the source port of the corresponding
                     Registration Request (Section 3.7.1).







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

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

      Type     3 (Registration Reply)

      Code

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

      Lifetime

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

      Home Address

               The IP address of the mobile node.

      Home Agent

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







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      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 authorization-enabling extension).  See Sections
               5.4 and 5.7.

      Extensions

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

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

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

      Registration denied by the foreign agent:

         64  reason unspecified
         65  administratively prohibited
         66  insufficient resources
         67  mobile node failed authentication
         68  home agent failed authentication
         69  requested Lifetime too long
         70  poorly formed Request
         71  poorly formed Reply
         72  requested encapsulation unavailable
         73  reserved and unavailable
         77  invalid care-of address
         78  registration timeout
         80  home network unreachable (ICMP error received)
         81  home agent host unreachable (ICMP error received)
         82  home agent port unreachable (ICMP error received)
         88  home agent unreachable (other ICMP error received)
         194 Invalid Home Agent Address



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      Registration denied by the home agent:

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

      Up-to-date values of the Code field are specified in the IANA
      online database [48].

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:

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

   o  all prior Extensions in their entirety, and

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

   The default authentication algorithm uses HMAC-MD5 [10] to compute a
   128-bit "message digest" of the registration message.  The data over
   which the HMAC is computed is defined as:

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

   o  all prior Extensions in their entirety, and

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

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






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   The Security Parameter Index (SPI) within any of the authentication
   Extensions defines the security context that is used to compute the
   Authenticator value and that 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 that it implements.  In addition, all implementations of Mobile
   IP MUST implement the default authentication algorithm (HMAC-MD5)
   specified above.

3.5.2.  Mobile-Home Authentication Extension

   At least one authorization-enabling extension MUST be present in all
   Registration Requests, and also in all Registration Replies generated
   by the home agent.  The Mobile-Home Authentication Extension is
   always an authorization-enabling extension for registration messages
   specified in this document.  This requirement is intended to
   eliminate problems [30] that result from the uncontrolled propagation
   of remote redirects in the Internet.  The location of the
   authorization-enabling extension marks the end of the data to be
   authenticated by the authorizing agent interpreting that
   authorization-enabling extension.

     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.)







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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.

     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, as long as the Registration Request
   is not a deregistration (i.e., the mobile node requested a nonzero
   Lifetime and the home address is different than the care-of address).
   The Foreign-Home Authentication extension MUST NOT be applied to
   deregistration messages.  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.



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

      Authenticator

               (variable length) (See Section 3.5.1).

   In order to perform the authentication, the home agent and the
   foreign agent are configured with a Mobility Security Association
   that is indexed by the SPI (in the appended Foreign-Home
   Authentication Extension) and the IP Source Address of the
   Registration Request.  When the extension is used with a Registration
   Reply message, the foreign agent address MUST be used as the
   Destination IP Address in the IP header.

   When this extension is applied to a Registration Request message, the
   Mobility Security Association for verifying the correctness of the
   authentication data is selected by the home agent based on the value
   of the Source IP Address field of the Registration Request and the
   SPI of the Authentication extension.  The Source IP Address will be
   the same as the Care-of Address field of the Registration Request
   (see Section 3.7.2.2).

   When this extension is applied to a Registration Reply message, the
   Mobility Security Association for verifying the correctness of the
   authentication data is selected by the foreign agent based on the
   value of the home agent Address field of the Registration Reply.

   If the Care-of Address in the Registration Request is not in the
   Agent Advertisement, then the foreign agent MUST NOT append the
   Foreign-Home Authentication Extension when relaying the message to
   the home agent.  Moreover, for a deregistration message (i.e.,
   Lifetime = 0), the foreign agent MUST NOT append the Foreign-Home
   Authentication Extension when relaying the message to the home agent.
   Consequently, when the home agent (HA) receives a deregistration
   request that does not contain a Foreign-Home Authentication
   Extension, it MUST NOT for this reason discard the request as part of
   security association processing.

3.6.  Mobile Node Considerations

   A mobile node MUST be configured (statically or dynamically) with a
   netmask and a Mobility Security Association for each of its home
   agents.  In addition, a mobile node MAY be configured with its home
   address, and the IP 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.




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   If the mobile node is not configured with a home address, it MAY use
   the Mobile Node Network Access Identifier (NAI) extension [2] to
   identify itself, and set the Home Address field of the Registration
   Request to 0.0.0.0.  In this case, the mobile node MUST be able to
   assign its home address after extracting this information from the
   Registration Reply from the home agent.

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

   o  the link-layer address of the foreign agent to which the
      Registration Request was sent, if applicable,

   o  the IP Destination Address of the Registration Request,

   o  the care-of address used in the registration,

   o  the Identification value sent in the registration,

   o  the originally requested Lifetime, and

   o  the remaining Lifetime of the pending registration.

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

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

   In the absence of link-layer indications of changes in point of
   attachment, Agent Advertisements from new agents SHOULD NOT cause a
   mobile node to attempt a new registration, if its current
   registration has not expired and it is still also receiving Agent
   Advertisements from the foreign agent with which it is currently
   registered.  In the absence of link-layer indications, a mobile node
   MUST NOT attempt to register more often than once per second.





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

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

3.6.1.  Sending Registration Requests

   The following sections specify details for the values that 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:

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

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

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

   IP Destination Address:

   o  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.

   o  When registering with a foreign agent, the address of the agent as
      learned from the IP source address of the corresponding Agent
      Advertisement MUST be used.  This MAY be an address that does not
      appear as an advertised care-of address in the Agent
      Advertisement.  In addition, when transmitting this Registration





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      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.

   o  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.

   o  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:

   o  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 [18].

3.6.1.2.  Registration Request Fields

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

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

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




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

   o  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 detunnels the received datagram in the
      same way as any other datagram tunneled directly to it.

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

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

   The mobile node MAY request alternative forms of encapsulation by
   setting the 'M' bit and/or the 'G' bit, but only if the mobile node
   is decapsulating its own datagrams (the mobile node is using a
   co-located care-of address) or if its foreign agent has indicated
   support for these forms of encapsulation by setting the corresponding
   bits in the Mobility Agent Advertisement Extension of an Agent
   Advertisement received by the mobile node.  Otherwise, the mobile
   node MUST NOT set these bits.







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   The Lifetime field is chosen as follows:

   o  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.

   o  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).

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

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

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

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

   The mobile node chooses the Identification field in accordance with
   the style of replay protection it uses with its home agent.  This is
   part of the Mobility Security Association the mobile node shares with
   its home agent.  See Section 5.7 for the method by which the mobile
   node computes the Identification field.

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 ordering is REQUIRED:



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   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 or other authorizing agent (which may or may
       not also be useful to the foreign agent), followed by

   c.  All authorization-enabling extensions (see Section 1.6), followed
       by

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

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

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

3.6.2.  Receiving Registration Replies

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

   o  the registration was accepted,

   o  the registration was denied by the foreign agent, or

   o  the registration was denied by the home agent.

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

3.6.2.1.  Validity Checks

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

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






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

   a.  If the mobile node and the foreign agent share a Mobility
       Security Association, exactly one Mobile-Foreign Authentication
       Extension MUST be present in the Registration Reply, and the
       mobile node MUST check the Authenticator value in the Extension.
       If no Mobile-Foreign Authentication Extension is found, or if
       more than one Mobile-Foreign Authentication Extension is found,
       or if the Authenticator is invalid, the mobile node MUST silently
       discard the Reply and SHOULD log the event as a security
       exception.

   b.  If the Code field indicates that service is denied by the home
       agent, or if the Code field indicates that the registration was
       accepted by the home agent, exactly one Mobile-Home
       Authentication Extension MUST be present in the Registration
       Reply, and the mobile node MUST check the Authenticator value in
       the Extension.  If the Registration Reply was generated by the
       home agent but no Mobile-Home Authentication Extension is found,
       or if more than one Mobile-Home Authentication Extension is
       found, or if the Authenticator is invalid, the mobile node MUST
       silently discard the Reply and SHOULD log the event as a security
       exception.

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

3.6.2.2.  Registration Request Accepted

   If the Code field indicates that the request has been accepted, the
   mobile node SHOULD configure its routing table appropriately for its
   current point of attachment (Section 4.2.1).

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





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

3.6.2.3.  Registration Request Denied

   If the Code field indicates that service is being denied, the mobile
   node SHOULD log the error.  In certain cases, the mobile node may be
   able to "repair" the error.  These include:

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

      In this case, the Lifetime field in the Registration Reply will
      contain the maximum Lifetime value that the 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, registration Identification
      mismatch)

      In this case, the Identification field in the Registration Reply
      will contain a value that allows the mobile node to synchronize
      with the home agent, based upon the style of replay protection in
      effect (Section 5.7).  The mobile node MUST adjust the parameters
      it uses to compute the Identification field based upon the
      information in the Registration Reply, before issuing any future
      Registration Requests.





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

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

3.6.3.  Registration Retransmission

   When no Registration Reply has been received within a reasonable
   time, another Registration Request MAY be transmitted.  When
   timestamps are used, a new registration Identification is chosen for
   each retransmission; thus, it counts as a new registration.  When
   nonces are used, the unanswered Request is retransmitted unchanged;
   thus, the retransmission does not count as a new registration
   (Section 5.7).  In this way, a retransmission will not require the
   home agent to resynchronize with the mobile node by issuing another
   nonce in the case in which the original Registration Request (rather
   than its Registration Reply) was lost by the network.

   The maximum time until a new Registration Request is sent SHOULD be
   no greater than the requested Lifetime of the Registration Request.
   The minimum value SHOULD be large enough to account for the size of
   the messages, twice the round-trip time for transmission to the home
   agent, and at least an additional 100 milliseconds to allow for
   processing the messages before responding.  The round-trip time for
   transmission to the home agent will be at least as large as the time
   required to transmit the messages at the link speed of the mobile
   node's current point of attachment.  Some circuits add another 200
   milliseconds of satellite delay in the total round-trip time to the
   home agent.  The minimum time between Registration Requests MUST NOT
   be less than 1 second.  Each successive retransmission timeout period
   SHOULD be at least twice the previous period, as long as that is less
   than the maximum as specified above.

3.7.  Foreign Agent Considerations

   The foreign agent plays a mostly passive role in Mobile IP
   registration.  It relays Registration Requests between mobile nodes
   and home agents, and, when it provides the care-of address,





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   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, unless the
   request is being relayed from a mobile node to that 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 well-formed
   (de)Registration Requests originated by a mobile node.

3.7.1.  Configuration and Registration Tables

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

   o  the link-layer source address of the mobile node

   o  the IP Source Address (the mobile node's home address) or its co-
      located care-of address (see description of the 'R' bit in Section
      2.1.1)

   o  the IP Destination Address (as specified in Section 3.6.1.1)

   o  the UDP Source Port

   o  the home agent address

   o  the Identification field

   o  the requested registration Lifetime, and

   o  the remaining Lifetime of the pending or current registration

   If there is an NAI extension in the Registration Request message
   (often, for example, when the mobile node's Home Address is zero),
   then the foreign agent MUST follow the procedures specified in RFC





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   2794 [2].  In particular, if the foreign agent cannot manage pending
   Registration Request records with such a zero Home Address for the
   mobile node, the foreign agent MUST return a Registration Reply with
   a code indicating NONZERO_HOMEADDR_REQD (see [2]).

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

   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.  In this case, when the Registration Reply has nonzero
   Lifetime, the foreign agent MUST check the required Foreign-Home
   Authentication Extension in the Registration Reply from the home
   agent (Sections 3.3 and 3.4).  Similarly, when receiving a
   Registration Request from a mobile node, if the foreign agent shares
   a Mobility Security Association with the mobile node, it MUST check
   the required Mobile-Foreign Authentication Extension in the Request
   and MUST add a Mobile-Foreign Authentication Extension to the
   Registration Reply to the mobile node.

3.7.2.  Receiving Registration Requests

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

   If the foreign agent has configured one of its network interfaces
   with the IP address specified by the mobile node as its home agent
   address, the foreign agent MUST NOT forward the Request again.  If
   the foreign agent serves the mobile node as a home agent, the foreign
   agent follows the procedures specified in Section 3.8.2.  Otherwise,





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   if the foreign agent does not serve the mobile node as a home agent,
   the foreign agent rejects the Registration Request with Code 194
   (Invalid Home Agent Address).

   If a foreign agent receives a Registration Request from a mobile node
   in its visitor list, the existing visitor list entry for the mobile
   node SHOULD NOT be deleted or modified until the foreign agent
   receives a valid Registration Reply from the home agent with a code
   indicating success.  The foreign agent MUST record the new pending
   Request as a separate part of the existing visitor list entry for the
   mobile node.  If the Registration Request asks for 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.  Requests with non-zero bits in reserved fields
   MUST be rejected with Code 70 (poorly formed Request).  Requests with
   the 'D' bit set to 0, nonzero Lifetime, and specifying a care-of
   address not offered by the foreign agent, MUST be rejected with Code
   77 (invalid care-of address).

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

3.7.2.2.  Forwarding a Valid Request to the Home Agent

   If the foreign agent accepts the mobile node's Registration Request,
   it MUST relay the Request to the mobile node's home agent as
   specified in the Home Agent field of the Registration Request.  The
   foreign agent MUST NOT modify any of the fields beginning with the
   fixed portion of the Registration Request up through and including
   the Mobile-Home Authentication Extension or other authentication
   extension supplied by the mobile node as an authorization-enabling




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   extension for the home agent.  Otherwise, an authentication failure
   is very likely to occur at the home agent.  In addition, the foreign
   agent proceeds as follows:

   o  It MUST process and remove any extensions that do not precede any
      authorization-enabling extension,

   o  It MAY append any of its own non-authentication Extensions of
      relevance to the home agent, if applicable, and

   o  If the foreign agent shares a Mobility Security Association with
      the home agent, and the Request has Lifetime != 0, then it MUST
      append the Foreign-Home Authentication Extension.

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

      IP Source Address

         The care-of address offered by the foreign agent for the mobile
         node sending the Registration Request.

      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.










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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 the Registration
         Request, unless the "All Agents Multicast" address was used.
         In this case, the foreign agent's address (on the interface
         from which the message will be sent) MUST be used.

      IP Destination Address

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

      UDP Source Port

         434

      UDP Destination Port

         Copied from the UDP Source Port of the Registration Request.





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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 there
   are multiple entries with the same home address, and if the
   Registration Reply has the Mobile Node NAI extension [2], the foreign
   agent MUST use the NAI to disambiguate the pending Registration
   Requests with the same home address.  If no matching pending Request
   is found, and if the Registration Reply does not correspond with any
   pending Registration Request with a zero mobile node home address
   (see Section 3.7.1), the foreign agent MUST silently discard the
   Reply.  The foreign agent MUST also silently discard the Reply if the
   low-order 32 bits of the Identification field in the Reply do not
   match those in the Request.

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







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3.7.3.2.  Forwarding Replies to the Mobile Node

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

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

   o  the foreign agent's own maximum value for allowable registration
      Lifetime.

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

   o  It MUST process and remove any Extensions that are not covered by
      any authorization-enabling extension,

   o  It MAY append its own non-authentication Extensions that supply
      information to the mobile node, if applicable, and

   o  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



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

3.8.  Home Agent Considerations

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

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

3.8.1.  Configuration and Registration Tables

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

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

   o  the mobile node's home address

   o  the mobile node's care-of address

   o  the Identification field from the Registration Reply

   o  the remaining Lifetime of the registration

   The home agent MAY optionally offer the capability to dynamically
   associate a home address to a mobile node upon receiving a
   Registration Request from that mobile node.  The method by which a
   home address is allocated to the mobile node is beyond the scope of



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   this document, but see [2].  After the home agent makes the
   association of the home address to the mobile node, the home agent
   MUST put that home address into the Home Address field of the
   Registration Reply.

   The home agent MAY also maintain Mobility Security Associations with
   various foreign agents.  When receiving a Registration Request from a
   foreign agent, if the home agent shares a Mobility Security
   Association with the foreign agent, the home agent MUST check the
   Authenticator in the required Foreign-Home Authentication Extension
   in the message, based on this Mobility Security Association, unless
   the Lifetime field equals 0.  When processing a Registration Request
   with Lifetime = 0, the HA MAY skip checking for the presence and
   validity of a Foreign-Home Authentication Extension.  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

   If the home agent accepts an incoming Registration Request, it 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 in most cases send a
   Registration Reply with an appropriate code specifying the reason the
   Request was denied.  The following sections describe this behavior in
   more detail.  If the home agent does not support broadcasts (see
   Section 4.3), it MUST ignore the 'B' bit (as opposed to rejecting the
   Registration Request).

3.8.2.1.  Validity Checks

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

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

   a.  The home agent MUST check for the presence of at least one
       authorization-enabling extension, and ensure that all indicated
       authentications are carried out.  At least one authorization-
       enabling extension MUST be present in the Registration Request,
       and the home agent MUST either check the Authenticator value in
       the extension or verify that the Authenticator Value has been
       checked by another agent with which it has a security
       association.




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       If the home agent receives a Registration Request from a mobile
       node with which it does not have any security association, the
       home agent MUST silently discard the Registration Request.

       If the home agent receives a Registration Request without any
       authorization-enabling extension, the home agent MUST silently
       discard the Registration Request.

       If the Authenticator is invalid, the home agent MUST reject the
       mobile node's registration.  Further action to be taken in this
       case depends upon whether the Request has a valid Foreign-Home
       authentication extension (as follows):

       *  If there is a valid Foreign-Home authentication extension, the
          home agent MUST send a Registration Reply with Code 131.

       *  Otherwise, if there is no Foreign-Home Security Association,
          the home agent MAY send a Registration Reply with Code 131.
          If the home agent sends a Registration Reply, it MUST contain
          a valid Mobile-Home Authentication Extension.  In constructing
          the Reply, the home agent SHOULD choose a security association
          that is likely to exist in the mobile node; for example, this
          may be an older security association or one with a longer
          lifetime than the one that the mobile node attempted to use in
          its Request.  Deployments should take care when updating
          security associations to ensure that there is at least one
          common security association shared between the mobile node and
          home agent.  In any case of a failed Authenticator, the home
          agent MUST then discard the Request without further processing
          and SHOULD log the error as a security exception.

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

   c.  If the home agent shares a Mobility Security Association with the
       foreign agent, and this is a Registration Request (has non-zero
       Lifetime), 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



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

   d.  If the home agent and the foreign agent do not share a Mobility
       Security Association, and the Registration contains a Foreign-
       Home Authentication Extension, the home agent MUST 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 return a Registration Reply with a Code of 136.
   In this case, the Registration Reply will contain the home agent's
   unicast address, so that the mobile node can re-issue the
   Registration Request with the correct home agent address.

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

3.8.2.2.  Accepting a Valid Request

   If the Registration Request satisfies the validity checks in Section
   3.8.2.1, and the home agent is able to accommodate the Request, the
   home agent MUST update its mobility binding list for the requesting
   mobile node and MUST return a Registration Reply to the mobile node.
   In this case, the code in the Registration Reply 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:






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   o  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.

   o  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.

   o  If the Lifetime is nonzero, the home agent adds an entry
      containing the requested Care-of Address to the mobility binding
      list for the mobile node.  If the 'S' bit is set and the home
      agent supports simultaneous mobility bindings, the previous
      mobility binding entries are retained.  Otherwise, the home agent
      removes all previous entries in the mobility binding list for the
      mobile node.

   In all cases, the home agent MUST send a Registration Reply to the
   source of the Registration Request, which might indeed be a different
   foreign agent than that whose care-of address is being
   (de)registered.  If the home agent shares a Mobility Security
   Association with the foreign agent whose care-of address is being
   deregistered, and that foreign agent is different from the one that
   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.

   When a foreign agent relays a deregistration message containing a
   care-of address that it does not own, it MUST NOT add a Foreign-Home
   Authentication Extension to that deregistration.  See Section 3.5.4
   for more details.

   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





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

   This section lists a number of reasons the home agent might reject a
   Request, and provides the code value it should use in each instance.
   See Section 3.8.3 for additional details on building the Registration
   Reply message.

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

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





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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 in most cases 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 home agents pick
   values for the IP and UDP header fields of a Registration Reply.

      IP Source Address

         Copied from the IP Destination Address of the Registration
         Request, unless a multicast or broadcast address was used.  If
         the IP Destination Address of the Registration Request was a
         broadcast or multicast address, the IP Source Address of the
         Registration Reply MUST be set to the home agent's (unicast) IP
         address.

      IP Destination Address

         Copied from the IP Source Address of the Registration Request.

      UDP Source Port

         Copied from the UDP Destination Port of the Registration
         Request.

      UDP Destination Port

         Copied from the UDP Source Port of the Registration Request.

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




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

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

   The Lifetime field MUST be copied from the corresponding field in the
   Registration Request, unless the requested value is greater than the
   maximum length of time the home agent is willing to provide the
   requested service.  In such a case, the Lifetime MUST be set to the
   length of time that service will actually be provided by the home
   agent.  This reduced Lifetime SHOULD be the maximum Lifetime allowed
   by the home agent (for this mobile node and care-of address).

   If the Home Address field of the Registration Request is non-zero, it
   MUST be copied into the Home Address field of the Registration Reply
   message.  If the home agent cannot support the specified nonzero
   unicast address in the Home Address field of the Registration
   Request, then the home agent MUST reject the Registration Request
   with a Code of 129.

   Otherwise, if the Home Address field of the Registration Request is
   zero as specified in Section 3.6, the home agent SHOULD arrange for
   the selection of a home address for the mobile node, and insert the
   selected address into the Home Address field of the Registration
   Reply message.  See [2] for further relevant details in the case
   where mobile nodes identify themselves using an NAI instead of their
   IP home address.




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   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.








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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
   [13] are alternate encapsulation methods that 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 [5] is one such method.

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

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

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




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

4.2.2.  Foreign Agent Considerations

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

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

   The foreign agent MUST route datagrams it receives from registered
   mobile nodes.  At a minimum, this means that the foreign agent must
   verify the IP Header Checksum, decrement the IP Time To Live,
   recompute the IP Header Checksum, and forward such datagrams to a
   default router.

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

   Each foreign agent SHOULD support the mandatory features for reverse
   tunneling [12].






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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.

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

   See Section 4.1 regarding methods of encapsulation that may be used
   for tunneling.  Nodes implementing tunneling SHOULD also implement
   the "tunnel soft state" mechanism [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 MUST decapsulate packets addressed to themselves, sent by
   a mobile node for the purpose of maintaining location privacy, as
   described in Section 5.5.  This feature is also required for support
   of reverse tunneling [12].

   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



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   agent MUST delete the binding, but it MUST retain any other (non-
   expired) simultaneous mobility bindings that it holds for the mobile
   node.

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

   o  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.

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

4.3.  Broadcast Datagrams

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

   If the 'D' bit was set in the mobile node's Registration Request
   message, indicating that the mobile node is using a co-located care-
   of address, the home agent simply tunnels appropriate broadcast IP
   datagrams to the mobile node's care-of address.  Otherwise (the 'D'
   bit was NOT set), the home agent first encapsulates the broadcast



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   datagram in a unicast datagram addressed to the mobile node's home
   address, and then tunnels this encapsulated datagram to the foreign
   agent.  This extra level of encapsulation is required so that the
   foreign agent can determine which mobile node should receive the
   datagram after it is decapsulated.  When received by the foreign
   agent, the unicast encapsulated datagram is detunneled and delivered
   to the mobile node in the same way as any other datagram.  In either
   case, the mobile node must decapsulate the datagram it receives in
   order to recover the original broadcast datagram.

4.4.  Multicast Datagram Routing

   As mentioned previously, a mobile node that is connected to its home
   network functions in the same way as any other (fixed) host or
   router.  Thus, when it is at home, a mobile node functions
   identically to other multicast senders and receivers.  This section
   therefore describes the behavior of a mobile node that is visiting a
   foreign network.

   In order to receive multicasts, a mobile node MUST join the multicast
   group in one of two ways.  First, a mobile node MAY join the group
   via a (local) multicast router on the visited subnet.  This option
   assumes that there is a multicast router present on the visited
   subnet.  If the mobile node is using a co-located care-of address, it
   SHOULD use this address as the source IP address of its IGMP [6]
   messages.  Otherwise, it MAY use its home address.

   Alternatively, a mobile node that wishes to receive multicasts MAY
   join groups via a bidirectional tunnel to its home agent, assuming
   that its home agent is a multicast router.  The mobile node tunnels
   IGMP messages to its home agent, and the home agent forwards
   multicast datagrams down the tunnel to the mobile node.  For packets
   tunneled to the home agent, the source address in the IP header
   SHOULD be the mobile node's home address.

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





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   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 that 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 that tunnels a multicast datagram to its home agent MUST
   use its home address as the IP source address of both the (inner)
   multicast datagram and the (outer) encapsulating datagram.  This
   second option assumes that the home agent is a multicast router.

4.5.  Mobile Routers

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

   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.






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   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 the router and
       foreign agent on the aircraft.  Normal IP routing will route the
       datagram to the fixed network at the airline's headquarters.

   e.  The aircraft router and foreign agent's home agent there
       intercept the datagram and tunnel 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 illustrates 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.

   For the fixed node, a home agent MAY be configured to have a
   permanent registration that indicates the mobile router's address as
   the fixed host's care-of address.  The mobile router's home agent
   will normally be used for this purpose.  The home agent is then
   responsible for advertising connectivity using normal routing
   protocols to the fixed node.  Any datagrams sent to the fixed node
   will thus use nested tunneling as described above.

   Alternatively, the mobile router MAY advertise connectivity to the
   entire mobile network using normal IP routing protocols through a
   bidirectional tunnel to its own home agent.  This method avoids the
   need for nested tunneling of datagrams.






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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.

   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:

   o  A Proxy ARP [49] is an ARP Reply sent by one node on behalf of
      another node that 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.

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

      In either case, for a gratuitous ARP, the ARP packet MUST be
      transmitted as a local broadcast packet on the local link.  As
      specified in [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 [49] 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



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   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 [49], the
   home agent MUST set the sender link-layer address in the packet to
   the link-layer address of its own interface over which the Reply will
   be sent.

   When a mobile node leaves its home network and registers a binding on
   a foreign network, its home agent uses gratuitous ARP to update the
   ARP caches of nodes on the home network.  This causes such nodes to
   associate the link-layer address of the home agent with the mobile
   node's home (IP) address.  When registering a binding for a mobile
   node for which the home agent previously had no binding (the mobile
   node was assumed to be at home), the home agent MUST transmit a
   gratuitous ARP on behalf of the mobile node.  This gratuitous ARP
   packet MUST be transmitted as a broadcast packet on the link on which
   the mobile node's home address is located.  Since broadcasts on the
   local link (such as Ethernet) are typically not guaranteed to be
   reliable, the gratuitous ARP packet SHOULD be retransmitted a small
   number of times to increase its reliability.

   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 the
   mobile node's (de)Registration Request.

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




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

   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:

   o  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.

   o  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.

   o  The mobile node transmits its Registration Request.

   o  When the mobile node's home agent receives and accepts the
      Registration Request, it performs a gratuitous ARP on behalf of
      the mobile node, and begins using proxy ARP to reply to ARP
      Requests that it receives requesting the mobile node's link-layer
      address.  In the gratuitous ARP, the ARP Sender Hardware Address
      is set to the link-layer address of the home agent.  If, instead,
      the home agent rejects the Registration Request, no ARP processing
      (neither gratuitous nor proxy) is performed by the home agent.




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

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

   o  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.

   o  The mobile node performs a gratuitous ARP for itself.  In this
      gratuitous ARP, the ARP Sender Hardware Address is set to the
      link-layer address of the mobile node.

   o  The mobile node transmits its Registration Request.

   o  When the mobile node's home agent receives and accepts the
      Registration Request, it stops using proxy ARP to reply to ARP
      Requests that it receives requesting the mobile node's link-layer
      address, and then performs a gratuitous ARP on behalf of the
      mobile node.  In this gratuitous ARP, the ARP Sender Hardware
      Address is set to the link-layer address of the mobile node.  If,
      instead, the home agent rejects the Registration Request, the home
      agent MUST NOT make any change to the way it performs ARP
      processing (neither gratuitous nor proxy) for the mobile node.  In
      this latter case, the home agent should operate as if the mobile
      node has not returned home, and continue to perform proxy ARP on
      behalf of the mobile node.

5.  Security Considerations

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

5.1.  Message Authentication Codes

   Home agents and mobile nodes MUST be able to perform authentication.
   The default algorithm is HMAC-MD5 [10], with a key size of 128 bits.
   The foreign agent MUST also support authentication using HMAC-MD5 and
   key sizes of 128 bits or greater, with manual key distribution.  Keys
   with arbitrary binary values MUST be supported.

   The "prefix+suffix" use of MD5 to protect data and a shared secret is
   considered vulnerable to attack by the cryptographic community.
   Where backward compatibility with existing Mobile IP implementations



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   that use this mode is needed, new implementations SHOULD include
   keyed MD5 [19] as one of the additional authentication algorithms for
   use when producing and verifying the authentication data that is
   supplied with Mobile IP registration messages, for instance, in the
   extensions specified in Sections 3.5.2, 3.5.3, and 3.5.4.

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

5.2.  Areas of Security Concern in This Protocol

   The registration protocol described in this document will result in a
   mobile node's traffic being tunneled to its care-of address.  This
   tunneling feature could be a significant vulnerability if the
   registration were not authenticated.  Such remote redirection, for
   instance, as performed by the mobile registration protocol, is widely
   understood to be a security problem in the current Internet if not
   authenticated [30].  Moreover, the Address Resolution Protocol (ARP)
   is not authenticated, and can potentially be used to steal another
   host's traffic.  The use of gratuitous ARP (Section 4.6) brings with
   it all of the risks associated with the use of ARP.

5.3.  Key Management

   This specification requires a strong authentication mechanism (keyed
   MD5) that 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.





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   If nonces are used in connection with replay protection, they must
   also be selected carefully.  RFC 4086 [8] written by Eastlake, et al.
   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.

5.6.  Ingress Filtering

   Many routers implement security policies such as "ingress filtering"
   [35] that do not allow forwarding of packets that have a Source
   Address that appears topologically incorrect.  In environments where
   this is a problem, mobile nodes may use reverse tunneling [12] with
   the foreign agent supplied care-of address as the Source Address.
   Reverse-tunneled packets will be able to pass normally through such
   routers, while ingress filtering rules will still be able to locate
   the true topological source of the packet in the same way as packets
   from non-mobile nodes.

5.7.  Replay Protection for Registration Requests

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

   The style of replay protection in effect between a mobile node and
   its home agent is part of the Mobility 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
   field MUST be copied unchanged from the Registration Request to the
   Reply.  The foreign agent uses those bits (and the mobile node's home



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   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 field in a new Registration Request MUST NOT be
   the same as in an immediately preceding Request, and SHOULD NOT
   repeat while the same security context is being used between the
   mobile node and the home agent.  Retransmission as in Section 3.6.3
   is allowed.

5.7.1.  Replay Protection Using Timestamps

   The basic principle of timestamp replay protection is that the node
   generating a message inserts the current time of day, and the node
   receiving the message checks that this timestamp is sufficiently
   close to its own time of day.  Unless specified differently in the
   security association between the nodes, a default value of 7 seconds
   MAY be used to limit the time difference.  This value SHOULD be
   greater than 3 seconds.  Obviously the two nodes must have adequately
   synchronized time-of-day clocks.  As with any messages, time
   synchronization messages may be protected against tampering by an
   authentication mechanism determined by the security context between
   the two nodes.

   If timestamps are used, the mobile node MUST set the Identification
   field to a 64-bit value formatted as specified by the Network Time
   Protocol [11].  The low-order 32 bits of the NTP format represent
   fractional seconds, and those bits that are not available from a time
   source SHOULD be generated from a good source of randomness.  Note,
   however, that when using timestamps, the 64-bit Identification used
   in a Registration Request from the mobile node MUST be greater than
   that used in any previous Registration Request, as the home agent
   uses this value as a sequence number.  Without such a sequence
   number, it would be possible for a delayed duplicate of an earlier
   Registration Request to arrive at the home agent (within the clock
   synchronization required by the home agent), and thus be applied out
   of order, mistakenly altering the mobile node's current registered
   care-of address.

   Upon receipt of a Registration Request with an authorization-enabling
   extension, the home agent MUST check the Identification field for
   validity.  In order to be valid, the timestamp contained in the
   Identification field MUST be close enough to the home agent's time-
   of-day clock, and the timestamp MUST be greater than all previously
   accepted timestamps for the requesting mobile node.  Time tolerances
   and resynchronization details are specific to a particular Mobility
   Security Association.



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   If the timestamp is valid, the home agent copies the entire
   Identification field into the Registration Reply it returns to the
   mobile node.  If the timestamp is not valid, the home agent copies
   only the low-order 32 bits into the Registration Reply, and supplies
   the high-order 32 bits from its own time of day.  In this latter
   case, the home agent MUST reject the registration by returning Code
   133 (registration 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 field in the Registration
   Reply are identical to those in the rejected registration attempt,
   before using the high-order bits for clock resynchronization.

5.7.2.  Replay Protection Using Nonces

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

   The home agent may be expected to have resources for computing
   pseudo-random numbers useful as nonces [8].  It inserts a new nonce
   as the high-order 32 bits of the Identification field of every
   Registration Reply.  The home agent copies the low-order 32 bits of
   the Identification field from the Registration Request message into
   the low-order 32 bits of the Identification field 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 field 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 field 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 bit values 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).




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

6.  IANA Considerations

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

   o  Mobile IP message types sent to UDP port 434, as defined in
      Section 1.8.

   o  types of extensions to Registration Request and Registration Reply
      messages (see Sections 3.3 and 3.4, and also consult [12], [43],
      [2], [3], and [7]).

   o  values for the code in the Registration Reply message (see Section
      3.4, and also consult [12], [43], [2], [3], and [7]).

   o  Mobile IP defines so-called Agent Solicitation and Agent
      Advertisement messages.  These messages are in fact Router
      Discovery messages [5] augmented with Mobile-IP-specific
      extensions.  Thus, they do not define a new name space, but do
      define additional Router Discovery extensions as described below
      in Section 6.2.  Also see Section 2.1, and consult [3] and [7].

   There are additional Mobile IP numbering spaces specified in [3].

   Information about assignment of Mobile IP numbers derived from
   specifications external to this document is given by IANA at
   http://www.iana.org/protocols.  From that URL, see the "Mobile
   Internet Protocol (IP) Numbers" section.

   In this revised specification, a new code value (for the field in the
   Registration Reply message) is needed within the range typically used
   for foreign agent messages.  This error code is needed to indicate
   the status "Invalid Home Agent Address".  See Section 3.7.2 for
   details.

6.1.  Mobile IP Message Types

   Mobile IP messages are defined to be those that are sent to a message
   recipient at port 434 (UDP or TCP).  The number space for Mobile IP
   messages is specified in Section 1.8.  Approval of new extension





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   numbers is subject to Expert Review, and a specification is required
   [22].  The currently standardized message types have the following
   numbers, and are specified in the indicated sections.

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

6.2.  Extensions to RFC 1256 Router Advertisement

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

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

   Approval of new extension numbers for use with Mobile IP is subject
   to Expert Review, and a specification is required [22].

6.3.  Extensions to Mobile IP Registration Messages

   The Mobile IP messages specified within this document and listed in
   Sections 1.8 and 6.1 may have extensions.  Mobile IP message
   extensions all share the same number space, even if they are to be
   applied to different Mobile IP messages.  The number space for Mobile
   IP message extensions is specified within this document.  Approval of
   new extension numbers is subject to Expert Review, and a
   specification is required [22].

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









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6.4.  Code Values for Mobile IP Registration Reply Messages

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

   +---------+------------------------------------------------------+
   | Code #s |                       Guideline                      |
   +---------+------------------------------------------------------+
   |   0-8   |                     Success Codes                    |
   |         |                                                      |
   |   9-63  | Allocation guidelines not specified in this document |
   |         |                                                      |
   |  64-127 |          Error Codes from the Foreign Agent          |
   |         |                                                      |
   | 128-192 |            Error Codes from the Home Agent           |
   |         |                                                      |
   | 193-200 |    Error Codes from the Gateway Foreign Agent [29]   |
   |         |                                                      |
   | 201-255 | Allocation guidelines not specified in this document |
   +---------+------------------------------------------------------+

         Approval of new code values requires Expert Review [22].

            Table 1:  Guidelines for Allocation of Code Values

7.  Acknowledgments

   Special thanks to Steve Deering (Xerox PARC), along with Dan Duchamp
   and John Ioannidis (JI) (Columbia University), for forming the
   working group, chairing it, and putting so much effort into its early
   development.  Columbia's early Mobile IP work can be found in [37],
   [38], [39].

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

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






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

   Thanks to Charlie Kunzinger and to Bill Simpson, the editors who
   produced the first drafts of this document, reflecting the
   discussions of the working group.  Much of the new text in the later
   revisions preceding RFC 2002 is due to Jim Solomon and Dave Johnson.

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

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

      Mohamed Khalil (Nortel Networks)
      Raja Narayanan (nVisible Networks)
      Haseeb Akhtar (Nortel Networks)
      Emad Qaddoura (Nortel Networks)

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

   Thanks to Vijay Devarapalli, who put in many hours to convert the
   source for this text document into XML format.











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8.  References

8.1.  Normative References

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

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

   [3]   Perkins, C., Calhoun, P., and J. Bharatia, "Mobile IPv4
         Challenge/Response Extensions (Revised)", RFC 4721, January
         2007.

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

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

   [6]   Deering, S., "Host extensions for IP multicasting", STD 5, RFC
         1112, August 1989.

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

   [8]   Eastlake 3rd, D., Schiller, J., and S. Crocker, "Randomness
         Requirements for Security", BCP 106, RFC 4086, June 2005.

   [9]   Kent, S., "IP Authentication Header", RFC 4302, December 2005.

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

   [11]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, "Network
         Time Protocol Version 4: Protocol and Algorithms
         Specification", RFC 5905, June 2010.

   [12]  Montenegro, G., Ed., "Reverse Tunneling for Mobile IP,
         revised", RFC 3024, January 2001.

   [13]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina,
         "Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.

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




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

   [16]  Plummer, D., "Ethernet Address Resolution Protocol: Or
         Converting Network Protocol Addresses to 48.bit Ethernet
         Address for Transmission on Ethernet Hardware", STD 37, RFC
         826, November 1982.

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

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

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

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

   [21]  Perkins, C., Ed., "IP Mobility Support for IPv4", RFC 3344,
         August 2002.

   [22]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.

8.2.  Informative References

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

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

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

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

   [27]  Levkowetz, H. and S. Vaarala, "Mobile IP Traversal of Network
         Address Translation (NAT) Devices", RFC 3519, April 2003.

   [28]  Glass, S. and M. Chandra, "Registration Revocation in Mobile
         IPv4", RFC 3543, August 2003.





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   [29]  Fogelstroem, E., Jonsson, A., and C. Perkins, "Mobile IPv4
         Regional Registration", RFC 4857, June 2007.

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

   [31]  Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
         Shelby, "Performance Enhancing Proxies Intended to Mitigate
         Link-Related Degradations", RFC 3135, June 2001.

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

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

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

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

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

   [37]  Ioannidis, J., Duchamp, D., and G. Maguire, "IP-Based Protocols
         for Mobile Internetworking", In Proceedings of the SIGCOMM '01
         Conference: Communications Architectures and Protocols, pages
         235-245, September 1991.

   [38]  Ioannidis, J. and G. Maguire, "The Design and Implementation of
         a Mobile Internetworking Architecture", In Proceedings of the
         Winter USENIX Technical Conference, pages 489-500, January
         1993.

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

   [40]  Jacobson, V., "Congestion Avoidance and Control", In
         Proceedings of the SIGCOMM '88 Workshop, ACM SIGCOMM, ACM
         Press, pages 314-329, August 1998.





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

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

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

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

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

   [46]  Perkins, C. and P. Calhoun, "Authentication, Authorization, and
         Accounting (AAA) Registration Keys for Mobile IPv4", RFC 3957,
         March 2005.

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

   [48]  IANA, "Mobile IPv4 Numbers", http://www.iana.org.

   [49]  Postel, J., "Multi-LAN address resolution", RFC 925, October
         1984.

   [50]  Perkins, C., Ed., "IP Mobility Support for IPv4", RFC 3220,
         January 2002.






















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Appendix A.  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 [41], 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) [47] and its Internet Protocol
   Control Protocol (IPCP) [42] negotiate 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.
   PPP extensions for Mobile IP have been specified in RFC 2290 [23].
   Please consult that document for additional details for how to handle
   care-of address assignment from PPP in a more efficient manner.

Appendix B.  TCP Considerations

B.1.  TCP Timers

   When high-delay (e.g., SATCOM) or low-bandwidth (e.g., High-Frequency
   Radio) links are in use, some TCP stacks may have insufficiently
   adaptive (non-standard) retransmission timeouts.  There may be
   spurious retransmission timeouts, even when the link and network are
   actually operating properly, but just with a high delay because of
   the medium in use.  This can cause an inability to create or maintain
   TCP connections over such links, and can also cause unneeded
   retransmissions that consume already scarce bandwidth.  Vendors are
   encouraged to follow the algorithms in RFC 2988 [26] when
   implementing TCP retransmission timers.  Vendors of systems designed
   for low-bandwidth, high-delay links should consult RFCs 2757 and 2488
   [24], [25].  Designers of applications targeted to operate on mobile
   nodes should be sensitive to the possibility of timer-related
   difficulties.









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B.2.  TCP Congestion Management

   Mobile nodes often use media that 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 [40].  Now, when a packet is dropped,
   the correspondent node's TCP implementation is likely to react as if
   there were a source of network congestion, and initiate the slow-
   start mechanisms [40] 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. [32].  TCP approaches to the problem
   of handling errors that might interfere with congestion management
   are discussed in documents from the PILC working group [31] [33].
   While such approaches are beyond the scope of this document, they
   illustrate that providing performance transparency to mobile nodes
   involves understanding mechanisms outside the network layer.
   Problems introduced by higher media error rates also indicate the
   need to avoid designs that systematically drop packets; such designs
   might otherwise be considered favorably when making engineering
   tradeoffs.





























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Appendix C.  Example Scenarios

   This section shows example Registration Requests for several common
   scenarios.

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

   The mobile node receives an Agent Advertisement from a foreign agent
   and wishes to register with that agent using the advertised foreign
   agent care-of address.  The mobile node wishes only IP-in-IP
   encapsulation, does not want broadcasts, and does not want
   simultaneous mobility bindings:

        IP fields:
          Source Address = mobile node's home address
          Destination Address = copied from the IP source address of the
            Agent Advertisement
          Time to Live = 1
        UDP fields:
          Source Port = <any>
          Destination Port = 434
        Registration Request fields:
          Type = 1
          S=0,B=0,D=0,M=0,G=0
          Lifetime = the Registration Lifetime copied from the
            Mobility Agent Advertisement Extension of the
            Router Advertisement message
          Home Address = the mobile node's home address
          Home Agent = IP address of mobile node's home agent
          Care-of Address = the Care-of Address copied from the
            Mobility Agent Advertisement Extension of the
            Router Advertisement message
          Identification = Network Time Protocol timestamp or Nonce
        Extensions:
          An authorization-enabling extension (e.g., the Mobile-Home
            Authentication Extension)















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C.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 [34]
   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

























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C.3.  Deregistration

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

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

Appendix D.  Applicability of Prefix-Lengths Extension

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

   Foreign agents using different wireless interfaces would have to
   cooperate using special protocols to provide identical coverage in
   space, and thus be able to claim to have wireless interfaces situated
   on the same subnetwork.  In the case of wired interfaces, a mobile
   node disconnecting and subsequently connecting to a new point of
   attachment may well send in a new Registration Request no matter
   whether the new advertisement is on the same medium as the last
   recorded advertisement.  And, finally, in areas with dense
   populations of foreign agents it would seem unwise to require the
   propagation via routing protocols of the subnet prefixes associated
   with each individual wireless foreign agent; such a strategy could
   lead to quick depletion of available space for routing tables,





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   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".

Appendix E.  Interoperability Considerations

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

   However, this specification does have new features that, if used,
   would cause interoperability problems with older implementations.
   All features specified in RFC 2002 will work with the new
   implementations, except for V-J compression [36].  The following list
   details some of the possible areas of compatibility problems that may
   be experienced by nodes conforming to this revised specification,
   when attempting to interoperate with nodes obeying RFC 2002.

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

   o  Mobile nodes (MNs) that use the NAI extension to identify
      themselves would not work with old mobility agents.

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

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

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




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Appendix F.  Changes since RFC 3344

   The following revisions to details of the specification in this
   document were made after RFC 3344 was published.  A list of changes
   from RFC 2002 made during the development of RFC 3344 [21] may be
   found in the latter document.  For items marked with issue numbers,
   more information is available by consulting the MIP4 mailing list
   archives.

   o  Showed more bit definitions in the Agent Advertisement message
      structure (see Section 2.1.1).  New advertisement bits have been
      defined by other specification documents, but not reflected in
      previous publications of this specification; this has led to
      confusion.  Citations for the other specification documents have
      also been included.

   o  (Issue 6) The behavior of the home agent was changed to avoid
      mandating error replies to Registration Requests that were
      invalidated because the foreign agent failed authentication.  The
      intention is to make the home agent more robust against Denial of
      Service attacks in which the malicious device has no intention of
      providing a valid Registration Request but only wants to congest
      traffic on the home network.  See Section 3.8.2.1.

   o  Due to non-unique assignment of IPv4 addresses in many domains, it
      is possible for different mobile nodes to have the same home
      address.  If they use the NAI, the foreign agent can still
      distinguish them.  Language was added to Section 3.7.1 and Section
      3.7.3.1 to specify that the foreign agent MUST use the NAI to
      distinguish mobile nodes with the same home address.

   o  (Issue 45) Specified that a foreign agent MUST NOT apply a
      Foreign-Home Authentication extension to a mobile node's
      deregistration request.  Also, the foreign agent MUST NOT apply a
      Foreign-Home Authentication extension unless the Care-of Address
      in the Registration Request matches an address advertised by the
      foreign agent.

   o  Specified that the Mobility Security Association to be used by the
      foreign agent and home agent depends upon values contained in the
      message data, not the IP headers.

   o  (Issues 9, 18) Created a new error code for use by the foreign
      agent, for the case when the foreign agent does not serve the
      mobile node as a home agent.  Formerly, the foreign agent could
      use an error Code of 136 for this case.





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   o  (Issue 17) Specified that, if the home agent cannot support the
      requested nonzero unicast address in the Home Address field of the
      Registration Request, then it MUST reject the registration with an
      error Code of 129.  See Section 3.8.3.2.

   o  (Issue 19) Specified that multiple authorization-enabling
      extensions may be present in the Registration Request message, but
      that the home agent has to (somehow) ensure that all have been
      checked (see Section 3.8.3.1).

   o  (Issue 20) Specified that the foreign agent SHOULD NOT modify any
      of the fields of the Registration Reply message that are covered
      by the Mobile-Home Authentication Extension, when it relays the
      packet to the mobile node.

   o  (Issue 21) Clarified that the foreign agent removes extensions
      that do not precede any authorization-enabling extension, not just
      the Mobile-Home Authentication extension (Section 3.7.3.2).

   o  (Issue 44) Specified that the address advertised by the foreign
      agent in Agent Advertisements is the care-of address offered on
      that network interface, not necessarily the address of the network
      interface (Section 3.7.2.2).

   o  (Issue 45) Clarification in Section 3.7.2.1 that Code 77 can only
      apply to a Registration Request with nonzero Lifetime.

   o  Created a new error code for use when a foreign agent can detect
      that the Home Agent address field is incorrect.

   o  Prohibited the use of the Foreign-Home Authorization Extension on
      deregistration messages.

   o  Cleaned up some more wording having to do with authorization-
      enabling extensions.

   o  For consistency, changed some wording about copying UDP ports.

   o  Added wording to clearly not disallow dynamically configuring
      netmask and security information at the mobile node.

   o  Revamped Changes section.

   o  Updated citations.







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Appendix G.  Example Messages

G.1.  Example ICMP Agent Advertisement Message Format

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

















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G.2.  Example Registration Request Message Format

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

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type = 1  |S|B|D|M|G|r|T|x|          Lifetime             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Home Address                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Home Agent                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Care-of Address                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                         Identification                        +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Optional Non-Auth Extensions for HA ...        |
    |                     ( variable length )                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Type = 32   |      Length   |           SPI                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          SPI (cont.)          |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
    :         MN-HA Authenticator ( variable length )               :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :           Optional  Non-Auth Extensions for FA .........
    :           Optional  MN-FA  Authentication Extension...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




















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RFC 5944                   IP Mobility Support             November 2010


G.3.  Example Registration Reply Message Format

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

     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Type = 3    |     Code      |           Lifetime            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Home Address                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Home Agent                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                         Identification                        +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Optional  HA  Non-Auth Extensions ...         |
    |                     ( variable length )                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Type = 32   |      Length   |           SPI                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          SPI (cont.)          |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
    :         MN-HA Authenticator ( variable length )               :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :           Optional  Extensions used by FA.........
    :           Optional  MN-FA Authentication Extension...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Author's Address

   Charles E. Perkins (editor)
   WiChorus Inc.
   3590 N. 1st Street, Suite 300
   San Jose, CA  95134
   USA

   EMail: charliep@computer.org













Perkins                      Standards Track                  [Page 100]

