Network Working Group C. Perkins, Ed.
Request for Comments: 3344 Nokia Research Center
Obsoletes: 3220 August 2002
Category: Standards Track
 IP Mobility Support for IPv4
Status of this Memo
 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements. Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
 Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
 This document specifies protocol enhancements that allow transparent
 routing of IP datagrams to mobile nodes in the Internet. Each mobile
 node is always identified by its home address, regardless of its
 current point of attachment to the Internet. While situated away
 from its home, a mobile node is also associated with a care-of
 address, which provides information about its current point of
 attachment to the Internet. The protocol provides for registering
 the care-of address with a home agent. The home agent sends
 datagrams destined for the mobile node through a tunnel to the care-
 of address. After arriving at the end of the tunnel, each datagram
 is then delivered to the mobile node.
Contents
 1. Introduction 3
 1.1. Protocol Requirements . . . . . . . . . . . . . . . . . 4
 1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . 4
 1.3. Assumptions . . . . . . . . . . . . . . . . . . . . . . 5
 1.4. Applicability . . . . . . . . . . . . . . . . . . . . . 5
 1.5. New Architectural Entities . . . . . . . . . . . . . . 5
 1.6. Terminology . . . . . . . . . . . . . . . . . . . . . . 6
 1.7. Protocol Overview . . . . . . . . . . . . . . . . . . . 9
 1.8. Message Format and Protocol Extensibility . . . . . . . 13
 1.9. Type-Length-Value Extension Format for Mobile IP
 Extensions . . . . . . . . . . . . . . . . . . . . . 15
 1.10. Long Extension Format . . . . . . . . . . . . . . . . . 16
 1.11. Short Extension Format . . . . . . . . . . . . . . . . 16
 2. Agent Discovery 17
 2.1. Agent Advertisement . . . . . . . . . . . . . . . . . . 18
 2.1.1. Mobility Agent Advertisement Extension . . . . 20
 2.1.2. Prefix-Lengths Extension . . . . . . . . . . . 22
 2.1.3. One-byte Padding Extension . . . . . . . . . . 22
 2.2. Agent Solicitation . . . . . . . . . . . . . . . . . . 23
 2.3. Foreign Agent and Home Agent Considerations . . . . . . 23
 2.3.1. Advertised Router Addresses . . . . . . . . . . 24
 2.3.2. Sequence Numbers and Rollover Handling . . . . 24
 2.4. Mobile Node Considerations . . . . . . . . . . . . . . 25
 2.4.1. Registration Required . . . . . . . . . . . . . 26
 2.4.2. Move Detection . . . . . . . . . . . . . . . . 26
 2.4.3. Returning Home . . . . . . . . . . . . . . . . 27
 2.4.4. Sequence Numbers and Rollover Handling . . . . 28
 3. Registration 28
 3.1. Registration Overview . . . . . . . . . . . . . . . . . 29
 3.2. Authentication . . . . . . . . . . . . . . . . . . . . 30
 3.3. Registration Request . . . . . . . . . . . . . . . . . 30
 3.4. Registration Reply . . . . . . . . . . . . . . . . . . 33
 3.5. Registration Extensions . . . . . . . . . . . . . . . . 36
 3.5.1. Computing Authentication Extension Values . . . 36
 3.5.2. Mobile-Home Authentication Extension . . . . . 37
 3.5.3. Mobile-Foreign Authentication Extension . . . . 37
 3.5.4. Foreign-Home Authentication Extension . . . . . 38
 3.6. Mobile Node Considerations . . . . . . . . . . . . . . 38
 3.6.1. Sending Registration Requests . . . . . . . . . 40
 3.6.2. Receiving Registration Replies . . . . . . . . 44
 3.6.3. Registration Retransmission . . . . . . . . . . 47
 3.7. Foreign Agent Considerations . . . . . . . . . . . . . 47
 3.7.1. Configuration and Registration Tables . . . . . 48
 3.7.2. Receiving Registration Requests . . . . . . . . 49
 3.7.3. Receiving Registration Replies . . . . . . . . 52
 3.8. Home Agent Considerations . . . . . . . . . . . . . . . 54
 3.8.1. Configuration and Registration Tables . . . . . 55
 3.8.2. Receiving Registration Requests . . . . . . . . 56
 3.8.3. Sending Registration Replies . . . . . . . . . 59
 4. Routing Considerations 62
 4.1. Encapsulation Types . . . . . . . . . . . . . . . . . . 62
 4.2. Unicast Datagram Routing . . . . . . . . . . . . . . . 62
 4.2.1. Mobile Node Considerations . . . . . . . . . . 62
 4.2.2. Foreign Agent Considerations . . . . . . . . . 63
 4.2.3. Home Agent Considerations . . . . . . . . . . . 64
 4.3. Broadcast Datagrams . . . . . . . . . . . . . . . . . . 66
 4.4. Multicast Datagram Routing . . . . . . . . . . . . . . 66
 4.5. Mobile Routers . . . . . . . . . . . . . . . . . . . . 67
 4.6. ARP, Proxy ARP, and Gratuitous ARP . . . . . . . . . . 69
 5. Security Considerations 73
 5.1. Message Authentication Codes . . . . . . . . . . . . . 73
 5.2. Areas of Security Concern in this Protocol . . . . . . 73
 5.3. Key Management . . . . . . . . . . . . . . . . . . . . 74
 5.4. Picking Good Random Numbers . . . . . . . . . . . . . . 74
 5.5. Privacy . . . . . . . . . . . . . . . . . . . . . . . . 74
 5.6. Ingress Filtering . . . . . . . . . . . . . . . . . . . 75
 5.7. Replay Protection for Registration Requests . . . . . . 75
 5.7.1. Replay Protection using Timestamps . . . . . . 75
 5.7.2. Replay Protection using Nonces . . . . . . . . 77
 6. IANA Considerations 77
 6.1. Mobile IP Message Types . . . . . . . . . . . . . . . . 78
 6.2. Extensions to RFC 1256 Router Advertisement . . . . . . 78
 6.3. Extensions to Mobile IP Registration Messages . . . . . 79
 6.4. Code Values for Mobile IP Registration Reply
 Messages. . . . . . . . . . . . . . . . . . . . . . 79
 7. Acknowledgments 80
 A. Patent Issues 82
 B. Link-Layer Considerations 82
 C. TCP Considerations 83
 C.1. TCP Timers . . . . . . . . . . . . . . . . . . . . . . 83
 C.2. TCP Congestion Management . . . . . . . . . . . . . . . 83
 D. Example Scenarios 84
 D.1. Registering with a Foreign Agent Care-of Address . . . 84
 D.2. Registering with a Co-Located Care-of Address . . . . . 84
 D.3. Deregistration . . . . . . . . . . . . . . . . . . . . 85
 E. Applicability of Prefix-Lengths Extension 86
 F. Interoperability Considerations 86
 G. Changes since RFC 2002 87
 G.1. Major Changes . . . . . . . . . . . . . . . . . . . . . 87
 G.2. Minor Changes . . . . . . . . . . . . . . . . . . . . . 89
 G.3. Changes since revision 04 of RFC2002bis . . . . . . . . 91
 H. Example Messages 92
 H.1. Example ICMP Agent Advertisement Message Format . . . . 92
 H.2. Example Registration Request Message Format . . . . . . 93
 H.3. Example Registration Reply Message Format . . . . . . . 94
 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 98
 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 99
1. Introduction
 IP version 4 assumes that a node's IP address uniquely identifies the
 node's point of attachment to the Internet. Therefore, a node must
 be located on the network indicated by its IP address in order to
 receive datagrams destined to it; otherwise, datagrams destined to
 the node would be undeliverable. For a node to change its point of
 attachment without losing its ability to communicate, currently one
 of the two following mechanisms must typically be employed:
 a) the node must change its IP address whenever it changes its
 point of attachment, or
 b) host-specific routes must be propagated throughout much of the
 Internet routing fabric.
 Both of these alternatives are often unacceptable. The first makes
 it impossible for a node to maintain transport and higher-layer
 connections when the node changes location. The second has obvious
 and severe scaling problems, especially relevant considering the
 explosive growth in sales of notebook (mobile) computers.
 A new, scalable, mechanism is required for accommodating node
 mobility within the Internet. This document defines such a
 mechanism, which enables nodes to change their point of attachment to
 the Internet without changing their IP address.
 Changes between this revised specification for Mobile IP and the
 original specifications (see [33, 32, 34, 43, 8]) are detailed in the
 appendix section G.
1.1. Protocol Requirements
 A mobile node must be able to communicate with other nodes after
 changing its link-layer point of attachment to the Internet, yet
 without changing its IP address.
 A mobile node must be able to communicate with other nodes that do
 not implement these mobility functions. No protocol enhancements are
 required in hosts or routers that are not acting as any of the new
 architectural entities introduced in Section 1.5.
 All messages used to update another node as to the location of a
 mobile node must be authenticated in order to protect against remote
 redirection attacks.
1.2. Goals
 The link by which a mobile node is directly attached to the Internet
 may often be a wireless link. This link may thus have a
 substantially lower bandwidth and higher error rate than traditional
 wired networks. Moreover, mobile nodes are likely to be battery
 powered, and minimizing power consumption is important. Therefore,
 the number of administrative messages sent over the link by which a
 mobile node is directly attached to the Internet should be minimized,
 and the size of these messages should be kept as small as is
 reasonably possible.
1.3. Assumptions
 The protocols defined in this document place no additional
 constraints on the assignment of IP addresses. That is, a mobile
 node can be assigned an IP address by the organization that owns the
 machine.
 This protocol assumes that mobile nodes will generally not change
 their point of attachment to the Internet more frequently than once
 per second.
 This protocol assumes that IP unicast datagrams are routed based on
 the destination address in the datagram header (and not, for example,
 by source address).
1.4. Applicability
 Mobile IP is intended to enable nodes to move from one IP subnet to
 another. It is just as suitable for mobility across homogeneous
 media as it is for mobility across heterogeneous media. That is,
 Mobile IP facilitates node movement from one Ethernet segment to
 another as well as it accommodates node movement from an Ethernet
 segment to a wireless LAN, as long as the mobile node's IP address
 remains the same after such a movement.
 One can think of Mobile IP as solving the "macro" mobility management
 problem. It is less well suited for more "micro" mobility management
 applications -- for example, handoff amongst wireless transceivers,
 each of which covers only a very small geographic area. As long as
 node movement does not occur between points of attachment on
 different IP subnets, link-layer mechanisms for mobility (i.e.,
 link-layer handoff) may offer faster convergence and far less
 overhead than Mobile IP.
1.5. New Architectural Entities
 Mobile IP introduces the following new functional entities:
 Mobile Node
 A host or router that changes its point of attachment from one
 network or subnetwork to another. A mobile node may change its
 location without changing its IP address; it may continue to
 communicate with other Internet nodes at any location using its
 (constant) IP address, assuming link-layer connectivity to a
 point of attachment is available.
 Home Agent
 A router on a mobile node's home network which tunnels
 datagrams for delivery to the mobile node when it is away from
 home, and maintains current location information for the mobile
 node.
 Foreign Agent
 A router on a mobile node's visited network which provides
 routing services to the mobile node while registered. The
 foreign agent detunnels and delivers datagrams to the mobile
 node that were tunneled by the mobile node's home agent. For
 datagrams sent by a mobile node, the foreign agent may serve as
 a default router for registered mobile nodes.
 A mobile node is given a long-term IP address on a home network.
 This home address is administered in the same way as a "permanent" IP
 address is provided to a stationary host. When away from its home
 network, a "care-of address" is associated with the mobile node and
 reflects the mobile node's current point of attachment. The mobile
 node uses its home address as the source address of all IP datagrams
 that it sends, except where otherwise described in this document for
 datagrams sent for certain mobility management functions (e.g., as in
 Section 3.6.1.1).
1.6. Terminology
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [4].
 In addition, this document frequently uses the following terms:
 Authorization-enabling extension
 An authentication which makes a (registration) message
 acceptable to the ultimate recipient of the registration
 message. An authorization-enabling extension MUST contain
 an SPI.
 In this document, all uses of authorization-enabling
 extension refer to authentication extensions that enable the
 Registration Request message to be acceptable to the home
 agent. Using additional protocol structures specified
 outside of this document, it may be possible for the mobile
 node to provide authentication of its registration to the
 home agent, by way of another authenticating entity within
 the network that is acceptable to the home agent (for
 example, see RFC 2794 [6]).
 Agent Advertisement
 An advertisement message constructed by attaching a special
 Extension to a router advertisement [10] message.
 Authentication
 The process of verifying (using cryptographic techniques,
 for all applications in this specification) the identity of
 the originator of a message.
 Care-of Address
 The termination point of a tunnel toward a mobile node, for
 datagrams forwarded to the mobile node while it is away from
 home. The protocol can use two different types of care-of
 address: a "foreign agent care-of address" is an address of
 a foreign agent with which the mobile node is registered,
 and a "co-located care-of address" is an externally obtained
 local address which the mobile node has associated with one
 of its own network interfaces.
 Correspondent Node
 A peer with which a mobile node is communicating. A
 correspondent node may be either mobile or stationary.
 Foreign Network
 Any network other than the mobile node's Home Network.
 Gratuitous ARP
 An ARP packet sent by a node in order to spontaneously cause
 other nodes to update an entry in their ARP cache [45]. See
 section 4.6.
 Home Address
 An IP address that is assigned for an extended period of
 time to a mobile node. It remains unchanged regardless of
 where the node is attached to the Internet.
 Home Network
 A network, possibly virtual, having a network prefix
 matching that of a mobile node's home address. Note that
 standard IP routing mechanisms will deliver datagrams
 destined to a mobile node's Home Address to the mobile
 node's Home Network.
 Link
 A facility or medium over which nodes can communicate at the
 link layer. A link underlies the network layer.
 Link-Layer Address
 The address used to identify an endpoint of some
 communication over a physical link. Typically, the Link-
 Layer address is an interface's Media Access Control (MAC)
 address.
 Mobility Agent
 Either a home agent or a foreign agent.
 Mobility Binding
 The association of a home address with a care-of address,
 along with the remaining lifetime of that association.
 Mobility Security Association
 A collection of security contexts, between a pair of nodes,
 which may be applied to Mobile IP protocol messages
 exchanged between them. Each context indicates an
 authentication algorithm and mode (Section 5.1), a secret (a
 shared key, or appropriate public/private key pair), and a
 style of replay protection in use (Section 5.7).
 Node
 A host or a router.
 Nonce
 A randomly chosen value, different from previous choices,
 inserted in a message to protect against replays.
 Security Parameter Index (SPI)
 An index identifying a security context between a pair of
 nodes among the contexts available in the Mobility Security
 Association. SPI values 0 through 255 are reserved and MUST
 NOT be used in any Mobility Security Association.
 Tunnel
 The path followed by a datagram while it is encapsulated.
 The model is that, while it is encapsulated, a datagram is
 routed to a knowledgeable decapsulating agent, which
 decapsulates the datagram and then correctly delivers it to
 its ultimate destination.
 Virtual Network
 A network with no physical instantiation beyond a router
 (with a physical network interface on another network). The
 router (e.g., a home agent) generally advertises
 reachability to the virtual network using conventional
 routing protocols.
 Visited Network
 A network other than a mobile node's Home Network, to which
 the mobile node is currently connected.
 Visitor List
 The list of mobile nodes visiting a foreign agent.
1.7. Protocol Overview
 The following support services are defined for Mobile IP:
 Agent Discovery
 Home agents and foreign agents may advertise their
 availability on each link for which they provide service. A
 newly arrived mobile node can send a solicitation on the
 link to learn if any prospective agents are present.
 Registration
 When the mobile node is away from home, it registers its
 care-of address with its home agent. Depending on its
 method of attachment, the mobile node will register either
 directly with its home agent, or through a foreign agent
 which forwards the registration to the home agent.
 silently discard
 The implementation discards the datagram without further
 processing, and without indicating an error to the sender.
 The implementation SHOULD provide the capability of logging
 the error, including the contents of the discarded datagram,
 and SHOULD record the event in a statistics counter.
 The following steps provide a rough outline of operation of the
 Mobile IP protocol:
 - Mobility agents (i.e., foreign agents and home agents)
 advertise their presence via Agent Advertisement messages
 (Section 2). A mobile node may optionally solicit an Agent
 Advertisement message from any locally attached mobility agents
 through an Agent Solicitation message.
 - A mobile node receives these Agent Advertisements and
 determines whether it is on its home network or a foreign
 network.
 - When the mobile node detects that it is located on its home
 network, it operates without mobility services. If returning
 to its home network from being registered elsewhere, the mobile
 node deregisters with its home agent, through exchange of a
 Registration Request and Registration Reply message with it.
 - When a mobile node detects that it has moved to a foreign
 network, it obtains a care-of address on the foreign network.
 The care-of address can either be determined from a foreign
 agent's advertisements (a foreign agent care-of address), or by
 some external assignment mechanism such as DHCP [13] (a co-
 located care-of address).
 - The mobile node operating away from home then registers its new
 care-of address with its home agent through exchange of a
 Registration Request and Registration Reply message with it,
 possibly via a foreign agent (Section 3).
 - Datagrams sent to the mobile node's home address are
 intercepted by its home agent, tunneled by the home agent to
 the mobile node's care-of address, received at the tunnel
 endpoint (either at a foreign agent or at the mobile node
 itself), and finally delivered to the mobile node (Section
 4.2.3).
 - In the reverse direction, datagrams sent by the mobile node are
 generally delivered to their destination using standard IP
 routing mechanisms, not necessarily passing through the home
 agent.
 When away from home, Mobile IP uses protocol tunneling to hide a
 mobile node's home address from intervening routers between its home
 network and its current location. The tunnel terminates at the
 mobile node's care-of address. The care-of address must be an
 address to which datagrams can be delivered via conventional IP
 routing. At the care-of address, the original datagram is removed
 from the tunnel and delivered to the mobile node.
 Mobile IP provides two alternative modes for the acquisition of a
 care-of address:
 a) A "foreign agent care-of address" is a care-of address provided
 by a foreign agent through its Agent Advertisement messages.
 In this case, the care-of address is an IP address of the
 foreign agent. In this mode, the foreign agent is the endpoint
 of the tunnel and, upon receiving tunneled datagrams,
 decapsulates them and delivers the inner datagram to the mobile
 node. This mode of acquisition is preferred because it allows
 many mobile nodes to share the same care-of address and
 therefore does not place unnecessary demands on the already
 limited IPv4 address space.
 b) A "co-located care-of address" is a care-of address acquired by
 the mobile node as a local IP address through some external
 means, which the mobile node then associates with one of its
 own network interfaces. The address may be dynamically
 acquired as a temporary address by the mobile node such as
 through DHCP [13], or may be owned by the mobile node as a
 long-term address for its use only while visiting some foreign
 network. Specific external methods of acquiring a local IP
 address for use as a co-located care-of address are beyond the
 scope of this document. When using a co-located care-of
 address, the mobile node serves as the endpoint of the tunnel
 and itself performs decapsulation of the datagrams tunneled to
 it.
 The mode of using a co-located care-of address has the advantage that
 it allows a mobile node to function without a foreign agent, for
 example, in networks that have not yet deployed a foreign agent. It
 does, however, place additional burden on the IPv4 address space
 because it requires a pool of addresses within the foreign network to
 be made available to visiting mobile nodes. It is difficult to
 efficiently maintain pools of addresses for each subnet that may
 permit mobile nodes to visit.
 It is important to understand the distinction between the care-of
 address and the foreign agent functions. The care-of address is
 simply the endpoint of the tunnel. It might indeed be an address of
 a foreign agent (a foreign agent care-of address), but it might
 instead be an address temporarily acquired by the mobile node (a co-
 located care-of address). A foreign agent, on the other hand, is a
 mobility agent that provides services to mobile nodes. See Sections
 3.7 and 4.2.2 for additional details.
 For example, figure 1 illustrates the routing of datagrams to and
 from a mobile node away from home, once the mobile node has
 registered with its home agent. In figure 1, the mobile node is
 using a foreign agent care-of address, not a co-located care-of
 address.
 2) Datagram is intercepted 3) Datagram is
 by home agent and detunneled and
 is tunneled to the delivered to the
 care-of address. mobile node.
 +-----+ +-------+ +------+
 |home | =======> |foreign| ------> |mobile|
 |agent| | agent | <------ | node |
 +-----+ +-------+ +------+
 1) Datagram to /|\ /
 mobile node | / 4) For datagrams sent by the
 arrives on | / mobile node, standard IP
 home network | / routing delivers each to its
 via standard | |_ destination. In this figure,
 IP routing. +----+ the foreign agent is the
 |host| mobile node's default router.
 +----+
 Figure 1: Operation of Mobile IPv4
 A home agent MUST be able to attract and intercept datagrams that are
 destined to the home address of any of its registered mobile nodes.
 Using the proxy and gratuitous ARP mechanisms described in Section
 4.6, this requirement can be satisfied if the home agent has a
 network interface on the link indicated by the mobile node's home
 address. Other placements of the home agent relative to the mobile
 node's home location MAY also be possible using other mechanisms for
 intercepting datagrams destined to the mobile node's home address.
 Such placements are beyond the scope of this document.
 Similarly, a mobile node and a prospective or current foreign agent
 MUST be able to exchange datagrams without relying on standard IP
 routing mechanisms; that is, those mechanisms which make forwarding
 decisions based upon the network-prefix of the destination address in
 the IP header. This requirement can be satisfied if the foreign
 agent and the visiting mobile node have an interface on the same
 link. In this case, the mobile node and foreign agent simply bypass
 their normal IP routing mechanism when sending datagrams to each
 other, addressing the underlying link-layer packets to their
 respective link-layer addresses. Other placements of the foreign
 agent relative to the mobile node MAY also be possible using other
 mechanisms to exchange datagrams between these nodes, but such
 placements are beyond the scope of this document.
 If a mobile node is using a co-located care-of address (as described
 in (b) above), the mobile node MUST be located on the link identified
 by the network prefix of this care-of address. Otherwise, datagrams
 destined to the care-of address would be undeliverable.
1.8. Message Format and Protocol Extensibility
 Mobile IP defines a set of new control messages, sent with UDP [37]
 using well-known port number 434. The following two message types
 are defined in this document:
 1 Registration Request
 3 Registration Reply
 Up-to-date values for the message types for Mobile IP control
 messages are specified in the most recent "Assigned Numbers" [40].
 In addition, for Agent Discovery, Mobile IP makes use of the
 existing Router Advertisement and Router Solicitation messages
 defined for ICMP Router Discovery [10].
 Mobile IP defines a general Extension mechanism to allow optional
 information to be carried by Mobile IP control messages or by ICMP
 Router Discovery messages. Some extensions have been specified to
 be encoded in the simple Type-Length-Value format described in
 Section 1.9.
 Extensions allow variable amounts of information to be carried
 within each datagram. The end of the list of Extensions is
 indicated by the total length of the IP datagram.
 Two separately maintained sets of numbering spaces, from which
 Extension Type values are allocated, are used in Mobile IP:
 - The first set consists of those Extensions which may appear
 only in Mobile IP control messages (those sent to and from UDP
 port number 434). In this document, the following Types are
 defined for Extensions appearing in Mobile IP control messages:
 32 Mobile-Home Authentication
 33 Mobile-Foreign Authentication
 34 Foreign-Home Authentication
 - The second set consists of those extensions which may appear
 only in ICMP Router Discovery messages [10]. In this document,
 the following Types are defined for Extensions appearing in
 ICMP Router Discovery messages:
 0 One-byte Padding (encoded with no Length nor Data field)
 16 Mobility Agent Advertisement
 19 Prefix-Lengths
 Each individual Extension is described in detail in a separate
 section later in this document. Up-to-date values for these
 Extension Type numbers are specified in the most recent "Assigned
 Numbers" [40].
 Due to the separation (orthogonality) of these sets, it is
 conceivable that two Extensions that are defined at a later date
 could have identical Type values, so long as one of the Extensions
 may be used only in Mobile IP control messages and the other may be
 used only in ICMP Router Discovery messages.
 The type field in the Mobile IP extension structure can support up to
 255 (skippable and not skippable) uniquely identifiable extensions.
 When an Extension numbered in either of these sets within the range 0
 through 127 is encountered but not recognized, the message containing
 that Extension MUST be silently discarded. When an Extension
 numbered in the range 128 through 255 is encountered which is not
 recognized, that particular Extension is ignored, but the rest of the
 Extensions and message data MUST still be processed. The Length
 field of the Extension is used to skip the Data field in searching
 for the next Extension.
 Unless additional structure is utilized for the extension types, new
 developments or additions to Mobile IP might require so many new
 extensions that the available space for extension types might run
 out. Two new extension structures are proposed to solve this
 problem. Certain types of extensions can be aggregated, using
 subtypes to identify the precise extension, for example as has been
 done with the Generic Authentication Keys extensions [35]. In many
 cases, this may reduce the rate of allocation for new values of the
 type field.
 Since the new extension structures will cause an efficient usage of
 the extension type space, it is recommended that new Mobile IP
 extensions follow one of the two new extension formats whenever there
 may be the possibility to group related extensions together.
 The following subsections provide details about three distinct
 structures for Mobile IP extensions:
 - The simple extension format
 - The long extension format
 - The short extension format
1.9. Type-Length-Value Extension Format for Mobile IP Extensions
 The Type-Length-Value format illustrated in figure 2 is used for
 extensions which are specified in this document. Since this simple
 extension structure does not encourage the most efficient usage of
 the extension type space, it is recommended that new Mobile IP
 extensions follow one of the two new extension formats specified in
 sections 1.10 or 1.11 whenever there may be the possibility to group
 related extensions together.
 0 1 2
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 | Type | Length | Data ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Figure 2: Type-Length-Value extension format for Mobile IPv4
 Type Indicates the particular type of Extension.
 Length Indicates the length (in bytes) of the data field within
 this Extension. The length does NOT include the Type and
 Length bytes.
 Data The particular data associated with this Extension. This
 field may be zero or more bytes in length. The format
 and length of the data field is determined by the type
 and length fields.
1.10. Long Extension Format
 This format is applicable for non-skippable extensions which carry
 information more than 256 bytes.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type | Sub-Type | Length |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Data .....
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The Long Extension format requires that the following fields be
 specified as the first fields of the extension.
 Type is the type, which describes a collection of extensions
 having a common data type.
 Sub-Type is a unique number given to each member in the aggregated
 type.
 Length indicates the length (in bytes) of the data field within
 this Extension. It does NOT include the Type, Length and
 Sub-Type bytes.
 Data is the data associated with the subtype of this
 extension. This specification does not place any
 additional structure on the subtype data.
 Since the length field is 16 bits wide, a the extension data can
 exceed 256 bytes in length.
1.11. Short Extension Format
 This format is compatible with the skippable extensions defined in
 section 1.9. It is not applicable for extensions which require more
 than 256 bytes of data; for such extensions, use the format described
 in section 1.10.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type | Length | Sub-Type | Data ....
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The Short Extension format requires that the following fields be
 specified as the first fields of the extension:
 Type is the type, which describes a collection of extensions
 having a common data type.
 Sub-Type is a unique number given to each member in the aggregated
 type.
 Length 8-bit unsigned integer. Length of the extension, in
 bytes, excluding the extension Type and the extension
 Length fields. This field MUST be set to 1 plus the
 total length of the data field.
 Data is the data associated with this extension. This
 specification does not place any additional structure on
 the subtype data.
2. Agent Discovery
 Agent Discovery is the method by which a mobile node determines
 whether it is currently connected to its home network or to a foreign
 network, and by which a mobile node can detect when it has moved from
 one network to another. When connected to a foreign network, the
 methods specified in this section also allow the mobile node to
 determine the foreign agent care-of address being offered by each
 foreign agent on that network.
 Mobile IP extends ICMP Router Discovery [10] as its primary mechanism
 for Agent Discovery. An Agent Advertisement is formed by including a
 Mobility Agent Advertisement Extension in an ICMP Router
 Advertisement message (Section 2.1). An Agent Solicitation message
 is identical to an ICMP Router Solicitation, except that its IP TTL
 MUST be set to 1 (Section 2.2). This section describes the message
 formats and procedures by which mobile nodes, foreign agents, and
 home agents cooperate to realize Agent Discovery.
 Agent Advertisement and Agent Solicitation may not be necessary for
 link layers that already provide this functionality. The method by
 which mobile nodes establish link-layer connections with prospective
 agents is outside the scope of this document (but see Appendix B).
 The procedures described below assume that such link-layer
 connectivity has already been established.
 No authentication is required for Agent Advertisement and Agent
 Solicitation messages. They MAY be authenticated using the IP
 Authentication Header [22], which is unrelated to the messages
 described in this document. Further specification of the way in
 which Advertisement and Solicitation messages may be authenticated is
 outside of the scope of this document.
2.1. Agent Advertisement
 Agent Advertisements are transmitted by a mobility agent to advertise
 its services on a link. Mobile nodes use these advertisements to
 determine their current point of attachment to the Internet. An
 Agent Advertisement is an ICMP Router Advertisement that has been
 extended to also carry an Mobility Agent Advertisement Extension
 (Section 2.1.1) and, optionally, a Prefix-Lengths Extension (Section
 2.1.2), One-byte Padding Extension (Section 2.1.3), or other
 Extensions that might be defined in the future.
 Within an Agent Advertisement message, ICMP Router Advertisement
 fields of the message are required to conform to the following
 additional specifications:
 - Link-Layer Fields
 Destination Address
 The link-layer destination address of a unicast Agent
 Advertisement MUST be the same as the source link-layer
 address of the Agent Solicitation which prompted the
 Advertisement.
 - IP Fields
 TTL The TTL for all Agent Advertisements MUST be set
 to 1.
 Destination Address
 As specified for ICMP Router Discovery [10], the IP
 destination address of an multicast Agent Advertisement
 MUST be either the "all systems on this link" multicast
 address (224.0.0.1) [11] or the "limited broadcast"
 address (255.255.255.255). The subnet-directed broadcast
 address of the form <prefix>.<-1> cannot be used since
 mobile nodes will not generally know the prefix of the
 foreign network. When the Agent Advertisement is unicast
 to a mobile node, the IP home address of the mobile node
 SHOULD be used as the Destination Address.
 - ICMP Fields
 Code The Code field of the agent advertisement is
 interpreted as follows:
 0 The mobility agent handles common traffic -- that
 is, it acts as a router for IP datagrams not
 necessarily related to mobile nodes.
 16 The mobility agent does not route common traffic.
 However, all foreign agents MUST (minimally)
 forward to a default router any datagrams received
 from a registered mobile node (Section 4.2.2).
 Lifetime
 The maximum length of time that the Advertisement is
 considered valid in the absence of further
 Advertisements.
 Router Address(es)
 See Section 2.3.1 for a discussion of the addresses that
 may appear in this portion of the Agent Advertisement.
 Num Addrs
 The number of Router Addresses advertised in this
 message. Note that in an Agent Advertisement message,
 the number of router addresses specified in the ICMP
 Router Advertisement portion of the message MAY be set to
 0. See Section 2.3.1 for details.
 If sent periodically, the nominal interval at which Agent
 Advertisements are sent SHOULD be no longer than 1/3 of the
 advertisement Lifetime given in the ICMP header. This interval MAY
 be shorter than 1/3 the advertised Lifetime. This allows a mobile
 node to miss three successive advertisements before deleting the
 agent from its list of valid agents. The actual transmission time
 for each advertisement SHOULD be slightly randomized [10] in order to
 avoid synchronization and subsequent collisions with other Agent
 Advertisements that may be sent by other agents (or with other Router
 Advertisements sent by other routers). Note that this field has no
 relation to the "Registration Lifetime" field within the Mobility
 Agent Advertisement Extension defined below.
2.1.1. Mobility Agent Advertisement Extension
 The Mobility Agent Advertisement Extension follows the ICMP Router
 Advertisement fields. It is used to indicate that an ICMP Router
 Advertisement message is also an Agent Advertisement being sent by a
 mobility agent. The Mobility Agent Advertisement Extension is
 defined as follows:
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type | Length | Sequence Number |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Registration Lifetime |R|B|H|F|M|G|r|T| reserved |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | zero or more Care-of Addresses |
 | ... |
 Type 16
 Length (6 + 4*N), where 6 accounts for the number of bytes in
 the Sequence Number, Registration Lifetime, flags, and
 reserved fields, and N is the number of care-of addresses
 advertised.
 Sequence Number
 The count of Agent Advertisement messages sent since the
 agent was initialized (Section 2.3.2).
 Registration Lifetime
 The longest lifetime (measured in seconds) that this
 agent is willing to accept in any Registration Request.
 A value of 0xffff indicates infinity. This field has no
 relation to the "Lifetime" field within the ICMP Router
 Advertisement portion of the Agent Advertisement.
 R Registration required. Registration with this foreign
 agent (or another foreign agent on this link) is required
 even when using a co-located care-of address.
 B Busy. The foreign agent will not accept registrations
 from additional mobile nodes.
 H Home agent. This agent offers service as a home agent on
 the link on which this Agent Advertisement message is
 sent.
 F Foreign agent. This agent offers service as a foreign
 agent on the link on which this Agent Advertisement
 message is sent.
 M Minimal encapsulation. This agent implements receiving
 tunneled datagrams that use minimal encapsulation [34].
 G GRE encapsulation. This agent implements receiving
 tunneled datagrams that use GRE encapsulation [16].
 r Sent as zero; ignored on reception. SHOULD NOT be
 allocated for any other uses.
 T Foreign agent supports reverse tunneling [27].
 reserved
 Sent as zero; ignored on reception.
 Care-of Address(es)
 The advertised foreign agent care-of address(es) provided
 by this foreign agent. An Agent Advertisement MUST
 include at least one care-of address if the 'F' bit is
 set. The number of care-of addresses present is
 determined by the Length field in the Extension.
 A home agent MUST always be prepared to serve the mobile nodes for
 which it is the home agent. A foreign agent may at times be too busy
 to serve additional mobile nodes; even so, it must continue to send
 Agent Advertisements, so that any mobile nodes already registered
 with it will know that they have not moved out of range of the
 foreign agent and that the foreign agent has not failed. A foreign
 agent may indicate that it is "too busy" to allow new mobile nodes to
 register with it, by setting the 'B' bit in its Agent Advertisements.
 An Agent Advertisement message MUST NOT have the 'B' bit set if the
 'F' bit is not also set. Furthermore, at least one of the 'F' bit
 and the 'H' bit MUST be set in any Agent Advertisement message sent.
 When a foreign agent wishes to require registration even from those
 mobile nodes which have acquired a co-located care-of address, it
 sets the 'R' bit to one. Because this bit applies only to foreign
 agents, an agent MUST NOT set the 'R' bit to one unless the 'F' bit
 is also set to one.
2.1.2. Prefix-Lengths Extension
 The Prefix-Lengths Extension MAY follow the Mobility Agent
 Advertisement Extension. It is used to indicate the number of bits
 of network prefix that applies to each Router Address listed in the
 ICMP Router Advertisement portion of the Agent Advertisement. Note
 that the prefix lengths given DO NOT apply to care-of address(es)
 listed in the Mobility Agent Advertisement Extension. The Prefix-
 Lengths Extension is defined as follows:
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type | Length | Prefix Length | ....
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type 19 (Prefix-Lengths Extension)
 Length N, where N is the value (possibly zero) of the Num Addrs
 field in the ICMP Router Advertisement portion of the
 Agent Advertisement.
 Prefix Length(s)
 The number of leading bits that define the network number
 of the corresponding Router Address listed in the ICMP
 Router Advertisement portion of the message. The prefix
 length for each Router Address is encoded as a separate
 byte, in the order that the Router Addresses are listed
 in the ICMP Router Advertisement portion of the message.
 See Section 2.4.2 for information about how the Prefix-Lengths
 Extension MAY be used by a mobile node when determining whether it
 has moved. See Appendix E for implementation details about the use
 of this Extension.
2.1.3. One-byte Padding Extension
 Some IP protocol implementations insist upon padding ICMP messages to
 an even number of bytes. If the ICMP length of an Agent
 Advertisement is odd, this Extension MAY be included in order to make
 the ICMP length even. Note that this Extension is NOT intended to be
 a general-purpose Extension to be included in order to word- or
 long-align the various fields of the Agent Advertisement. An Agent
 Advertisement SHOULD NOT include more than one One-byte Padding
 Extension and if present, this Extension SHOULD be the last Extension
 in the Agent Advertisement.
 Note that unlike other Extensions used in Mobile IP, the One-byte
 Padding Extension is encoded as a single byte, with no "Length" nor
 "Data" field present. The One-byte Padding Extension is defined as
 follows:
 0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+
 | Type |
 +-+-+-+-+-+-+-+-+
 Type 0 (One-byte Padding Extension)
2.2. Agent Solicitation
 An Agent Solicitation is identical to an ICMP Router Solicitation
 with the further restriction that the IP TTL Field MUST be set to 1.
2.3. Foreign Agent and Home Agent Considerations
 Any mobility agent which cannot be discovered by a link-layer
 protocol MUST send Agent Advertisements. An agent which can be
 discovered by a link-layer protocol SHOULD also implement Agent
 Advertisements. However, the Advertisements need not be sent, except
 when the site policy requires registration with the agent (i.e., when
 the 'R' bit is set), or as a response to a specific Agent
 Solicitation. All mobility agents MUST process packets that they
 receive addressed to the Mobile-Agents multicast group, at address
 224.0.0.11. A mobile node MAY send an Agent Solicitation to
 224.0.0.11. All mobility agents SHOULD respond to Agent
 Solicitations.
 The same procedures, defaults, and constants are used in Agent
 Advertisement messages and Agent Solicitation messages as specified
 for ICMP Router Discovery [10], except that:
 - a mobility agent MUST limit the rate at which it sends broadcast
 or multicast Agent Advertisements; the maximum rate SHOULD be
 chosen so that the Advertisements do not consume a significant
 amount of network bandwidth, AND
 - a mobility agent that receives a Router Solicitation MUST NOT
 require that the IP Source Address is the address of a neighbor
 (i.e., an address that matches one of the router's own addresses
 on the arrival interface, under the subnet mask associated with
 that address of the router).
 - a mobility agent MAY be configured to send Agent Advertisements
 only in response to an Agent Solicitation message.
 If the home network is not a virtual network, then the home agent for
 any mobile node SHOULD be located on the link identified by the
 mobile node's home address, and Agent Advertisement messages sent by
 the home agent on this link MUST have the 'H' bit set. In this way,
 mobile nodes on their own home network will be able to determine that
 they are indeed at home. Any Agent Advertisement messages sent by
 the home agent on another link to which it may be attached (if it is
 a mobility agent serving more than one link), MUST NOT have the 'H'
 bit set, unless the home agent also serves as a home agent (to other
 mobile nodes) on that other link. A mobility agent MAY use different
 settings for each of the 'R', 'H', and 'F' bits on different network
 interfaces.
 If the home network is a virtual network, the home network has no
 physical realization external to the home agent itself. In this
 case, there is no physical network link on which to send Agent
 Advertisement messages advertising the home agent. Mobile nodes for
 which this is the home network are always treated as being away from
 home.
 On a particular subnet, either all mobility agents MUST include the
 Prefix-Lengths Extension or all of them MUST NOT include this
 Extension. Equivalently, it is prohibited for some agents on a given
 subnet to include the Extension but for others not to include it.
 Otherwise, one of the move detection algorithms designed for mobile
 nodes will not function properly (Section 2.4.2).
2.3.1. Advertised Router Addresses
 The ICMP Router Advertisement portion of the Agent Advertisement MAY
 contain one or more router addresses. An agent SHOULD only put its
 own addresses, if any, in the advertisement. Whether or not its own
 address appears in the Router Addresses, a foreign agent MUST route
 datagrams it receives from registered mobile nodes (Section 4.2.2).
2.3.2. Sequence Numbers and Rollover Handling
 The sequence number in Agent Advertisements ranges from 0 to 0xffff.
 After booting, an agent MUST use the number 0 for its first
 advertisement. Each subsequent advertisement MUST use the sequence
 number one greater, with the exception that the sequence number
 0xffff MUST be followed by sequence number 256. In this way, mobile
 nodes can distinguish a reduction in the sequence number that occurs
 after a reboot from a reduction that results in rollover of the
 sequence number after it attains the value 0xffff.
2.4. Mobile Node Considerations
 Every mobile node MUST implement Agent Solicitation. Solicitations
 SHOULD only be sent in the absence of Agent Advertisements and when a
 care-of address has not been determined through a link-layer protocol
 or other means. The mobile node uses the same procedures, defaults,
 and constants for Agent Solicitation as specified for ICMP Router
 Solicitation messages [10], except that the mobile node MAY solicit
 more often than once every three seconds, and that a mobile node that
 is currently not connected to any foreign agent MAY solicit more
 times than MAX_SOLICITATIONS.
 The rate at which a mobile node sends Solicitations MUST be limited
 by the mobile node. The mobile node MAY send three initial
 Solicitations at a maximum rate of one per second while searching for
 an agent. After this, the rate at which Solicitations are sent MUST
 be reduced so as to limit the overhead on the local link. Subsequent
 Solicitations MUST be sent using a binary exponential backoff
 mechanism, doubling the interval between consecutive Solicitations,
 up to a maximum interval. The maximum interval SHOULD be chosen
 appropriately based upon the characteristics of the media over which
 the mobile node is soliciting. This maximum interval SHOULD be at
 least one minute between Solicitations.
 While still searching for an agent, the mobile node MUST NOT increase
 the rate at which it sends Solicitations unless it has received a
 positive indication that it has moved to a new link. After
 successfully registering with an agent, the mobile node SHOULD also
 increase the rate at which it will send Solicitations when it next
 begins searching for a new agent with which to register. The
 increased solicitation rate MAY revert to the maximum rate, but then
 MUST be limited in the manner described above. In all cases, the
 recommended solicitation intervals are nominal values. Mobile nodes
 MUST randomize their solicitation times around these nominal values
 as specified for ICMP Router Discovery [10].
 Mobile nodes MUST process received Agent Advertisements. A mobile
 node can distinguish an Agent Advertisement message from other uses
 of the ICMP Router Advertisement message by examining the number of
 advertised addresses and the IP Total Length field. When the IP
 total length indicates that the ICMP message is longer than needed
 for the number of advertised addresses, the remaining data is
 interpreted as one or more Extensions. The presence of a Mobility
 Agent Advertisement Extension identifies the advertisement as an
 Agent Advertisement.
 If there is more than one advertised address, the mobile node SHOULD
 pick the first address for its initial registration attempt. If the
 registration attempt fails with a status Code indicating rejection by
 the foreign agent, the mobile node MAY retry the attempt with each
 subsequent advertised address in turn.
 When multiple methods of agent discovery are in use, the mobile node
 SHOULD first attempt registration with agents including Mobility
 Agent Advertisement Extensions in their advertisements, in preference
 to those discovered by other means. This preference maximizes the
 likelihood that the registration will be recognized, thereby
 minimizing the number of registration attempts.
 A mobile node MUST ignore reserved bits in Agent Advertisements, as
 opposed to discarding such advertisements. In this way, new bits can
 be defined later, without affecting the ability for mobile nodes to
 use the advertisements even when the newly defined bits are not
 understood.
2.4.1. Registration Required
 When the mobile node receives an Agent Advertisement with the 'R' bit
 set, the mobile node SHOULD register through the foreign agent, even
 when the mobile node might be able to acquire its own co-located
 care-of address. This feature is intended to allow sites to enforce
 visiting policies (such as accounting) which require exchanges of
 authorization.
 If formerly reserved bits require some kind of monitoring/enforcement
 at the foreign link, foreign agents implementing the new
 specification for the formerly reserved bits can set the 'R' bit.
 This has the effect of forcing the mobile node to register through
 the foreign agent, so the foreign agent could then monitor/enforce
 the policy.
2.4.2. Move Detection
 Two primary mechanisms are provided for mobile nodes to detect when
 they have moved from one subnet to another. Other mechanisms MAY
 also be used. When the mobile node detects that it has moved, it
 SHOULD register (Section 3) with a suitable care-of address on the
 new foreign network. However, the mobile node MUST NOT register more
 frequently than once per second on average, as specified in Section
 3.6.3.
2.4.2.1. Algorithm 1
 The first method of move detection is based upon the Lifetime field
 within the main body of the ICMP Router Advertisement portion of the
 Agent Advertisement. A mobile node SHOULD record the Lifetime
 received in any Agent Advertisements, until that Lifetime expires.
 If the mobile node fails to receive another advertisement from the
 same agent within the specified Lifetime, it SHOULD assume that it
 has lost contact with that agent. If the mobile node has previously
 received an Agent Advertisement from another agent for which the
 Lifetime field has not yet expired, the mobile node MAY immediately
 attempt registration with that other agent. Otherwise, the mobile
 node SHOULD attempt to discover a new agent with which to register.
2.4.2.2. Algorithm 2
 The second method uses network prefixes. The Prefix-Lengths
 Extension MAY be used in some cases by a mobile node to determine
 whether or not a newly received Agent Advertisement was received on
 the same subnet as the mobile node's current care-of address. If the
 prefixes differ, the mobile node MAY assume that it has moved. If a
 mobile node is currently using a foreign agent care-of address, the
 mobile node SHOULD NOT use this method of move detection unless both
 the current agent and the new agent include the Prefix-Lengths
 Extension in their respective Agent Advertisements; if this Extension
 is missing from one or both of the advertisements, this method of
 move detection SHOULD NOT be used. Similarly, if a mobile node is
 using a co-located care-of address, it SHOULD not use this method of
 move detection unless the new agent includes the Prefix-Lengths
 Extension in its Advertisement and the mobile node knows the network
 prefix of its current co-located care-of address. On the expiration
 of its current registration, if this method indicates that the mobile
 node has moved, rather than re-registering with its current care-of
 address, a mobile node MAY choose instead to register with a the
 foreign agent sending the new Advertisement with the different
 network prefix. The Agent Advertisement on which the new
 registration is based MUST NOT have expired according to its Lifetime
 field.
2.4.3. Returning Home
 A mobile node can detect that it has returned to its home network
 when it receives an Agent Advertisement from its own home agent. If
 so, it SHOULD deregister with its home agent (Section 3). Before
 attempting to deregister, the mobile node SHOULD configure its
 routing table appropriately for its home network (Section 4.2.1). In
 addition, if the home network is using ARP [36], the mobile node MUST
 follow the procedures described in Section 4.6 with regard to ARP,
 proxy ARP, and gratuitous ARP.
2.4.4. Sequence Numbers and Rollover Handling
 If a mobile node detects two successive values of the sequence number
 in the Agent Advertisements from the foreign agent with which it is
 registered, the second of which is less than the first and inside the
 range 0 to 255, the mobile node SHOULD register again. If the second
 value is less than the first but is greater than or equal to 256, the
 mobile node SHOULD assume that the sequence number has rolled over
 past its maximum value (0xffff), and that reregistration is not
 necessary (Section 2.3).
3. Registration
 Mobile IP registration provides a flexible mechanism for mobile nodes
 to communicate their current reachability information to their home
 agent. It is the method by which mobile nodes:
 - request forwarding services when visiting a foreign network,
 - inform their home agent of their current care-of address,
 - renew a registration which is due to expire, and/or
 - deregister when they return home.
 Registration messages exchange information between a mobile node,
 (optionally) a foreign agent, and the home agent. Registration
 creates or modifies a mobility binding at the home agent, associating
 the mobile node's home address with its care-of address for the
 specified Lifetime.
 Several other (optional) capabilities are available through the
 registration procedure, which enable a mobile node to:
 - discover its home address, if the mobile node is not configured
 with this information.
 - maintain multiple simultaneous registrations, so that a copy of
 each datagram will be tunneled to each active care-of address
 - deregister specific care-of addresses while retaining other
 mobility bindings, and
 - discover the address of a home agent if the mobile node is not
 configured with this information.
3.1. Registration Overview
 Mobile IP defines two different registration procedures, one via a
 foreign agent that relays the registration to the mobile node's home
 agent, and one directly with the mobile node's home agent. The
 following rules determine which of these two registration procedures
 to use in any particular circumstance:
 - If a mobile node is registering a foreign agent care-of
 address, the mobile node MUST register via that foreign agent.
 - If a mobile node is using a co-located care-of address, and
 receives an Agent Advertisement from a foreign agent on the
 link on which it is using this care-of address, the mobile node
 SHOULD register via that foreign agent (or via another foreign
 agent on this link) if the 'R' bit is set in the received Agent
 Advertisement message.
 - If a mobile node is otherwise using a co-located care-of
 address, the mobile node MUST register directly with its home
 agent.
 - If a mobile node has returned to its home network and is
 (de)registering with its home agent, the mobile node MUST
 register directly with its home agent.
 Both registration procedures involve the exchange of Registration
 Request and Registration Reply messages (Sections 3.3 and 3.4). When
 registering via a foreign agent, the registration procedure requires
 the following four messages:
 a) The mobile node sends a Registration Request to the prospective
 foreign agent to begin the registration process.
 b) The foreign agent processes the Registration Request and then
 relays it to the home agent.
 c) The home agent sends a Registration Reply to the foreign agent
 to grant or deny the Request.
 d) The foreign agent processes the Registration Reply and then
 relays it to the mobile node to inform it of the disposition of
 its Request.
 When the mobile node instead registers directly with its home agent,
 the registration procedure requires only the following two messages:
 a) The mobile node sends a Registration Request to the home agent.
 b) The home agent sends a Registration Reply to the mobile node,
 granting or denying the Request.
 The registration messages defined in Sections 3.3 and 3.4 use the
 User Datagram Protocol (UDP) [37]. A nonzero UDP checksum SHOULD be
 included in the header, and MUST be checked by the recipient. A zero
 UDP checksum SHOULD be accepted by the recipient. The behavior of
 the mobile node and the home agent with respect to their mutual
 acceptance of packets with zero UDP checksums SHOULD be defined as
 part of the mobility security association which exists between them.
3.2. Authentication
 Each mobile node, foreign agent, and home agent MUST be able to
 support a mobility security association for mobile entities, indexed
 by their SPI and IP address. In the case of the mobile node, this
 must be its Home Address. See Section 5.1 for requirements for
 support of authentication algorithms. Registration messages between
 a mobile node and its home agent MUST be authenticated with an
 authorization-enabling extension, e.g. the Mobile-Home Authentication
 Extension (Section 3.5.2). This extension MUST be the first
 authentication extension; other foreign agent-specific extensions MAY
 be added to the message after the mobile node computes the
 authentication.
3.3. Registration Request
 A mobile node registers with its home agent using a Registration
 Request message so that its home agent can create or modify a
 mobility binding for that mobile node (e.g., with a new lifetime).
 The Request may be relayed to the home agent by the foreign agent
 through which the mobile node is registering, or it may be sent
 directly to the home agent in the case in which the mobile node is
 registering a co-located care-of address.
 IP fields:
 Source Address Typically the interface address from which the
 message is sent.
 Destination Address Typically that of the foreign agent or the
 home agent.
 See Sections 3.6.1.1 and 3.7.2.2 for details. UDP fields:
 Source Port variable
 Destination Port 434
 The UDP header is followed by the Mobile IP fields shown below:
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type |S|B|D|M|G|r|T|x| Lifetime |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Home Address |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Home Agent |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Care-of Address |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 + Identification +
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Extensions ...
 +-+-+-+-+-+-+-+-
 Type 1 (Registration Request)
 S Simultaneous bindings. If the 'S' bit is set, the mobile
 node is requesting that the home agent retain its prior
 mobility bindings, as described in Section 3.6.1.2.
 B Broadcast datagrams. If the 'B' bit is set, the mobile
 node requests that the home agent tunnel to it any
 broadcast datagrams that it receives on the home network,
 as described in Section 4.3.
 D Decapsulation by mobile node. If the 'D' bit is set, the
 mobile node will itself decapsulate datagrams which are
 sent to the care-of address. That is, the mobile node is
 using a co-located care-of address.
 M Minimal encapsulation. If the 'M' bit is set, the mobile
 node requests that its home agent use minimal
 encapsulation [34] for datagrams tunneled to the mobile
 node.
 G GRE encapsulation. If the 'G' bit is set, the mobile
 node requests that its home agent use GRE encapsulation
 [16] for datagrams tunneled to the mobile node.
 r Sent as zero; ignored on reception. SHOULD NOT be
 allocated for any other uses.
 T Reverse Tunneling requested; see [27].
 x Sent as zero; ignored on reception.
 Lifetime
 The number of seconds remaining before the registration
 is considered expired. A value of zero indicates a
 request for deregistration. A value of 0xffff indicates
 infinity.
 Home Address
 The IP address of the mobile node.
 Home Agent
 The IP address of the mobile node's home agent.
 Care-of Address
 The IP address for the end of the tunnel.
 Identification
 A 64-bit number, constructed by the mobile node, used for
 matching Registration Requests with Registration Replies,
 and for protecting against replay attacks of registration
 messages. See Sections 5.4 and 5.7.
 Extensions
 The fixed portion of the Registration Request is followed
 by one or more of the Extensions listed in Section 3.5.
 An authorization-enabling extension MUST be included in
 all Registration Requests. See Sections 3.6.1.3 and
 3.7.2.2 for information on the relative order in which
 different extensions, when present, MUST be placed in a
 Registration Request message.
3.4. Registration Reply
 A mobility agent returns a Registration Reply message to a mobile
 node which has sent a Registration Request (Section 3.3) message. If
 the mobile node is requesting service from a foreign agent, that
 foreign agent will receive the Reply from the home agent and
 subsequently relay it to the mobile node. The Reply message contains
 the necessary codes to inform the mobile node about the status of its
 Request, along with the lifetime granted by the home agent, which MAY
 be smaller than the original Request.
 The foreign agent MUST NOT increase the Lifetime selected by the
 mobile node in the Registration Request, since the Lifetime is
 covered by an authentication extension which enables authorization by
 the home agent. Such an extension contains authentication data which
 cannot be correctly (re)computed by the foreign agent. The home
 agent MUST NOT increase the Lifetime selected by the mobile node in
 the Registration Request, since doing so could increase it beyond the
 maximum Registration Lifetime allowed by the foreign agent. If the
 Lifetime received in the Registration Reply is greater than that in
 the Registration Request, the Lifetime in the Request MUST be used.
 When the Lifetime received in the Registration Reply is less than
 that in the Registration Request, the Lifetime in the Reply MUST be
 used.
 IP fields:
 Source Address Typically copied from the destination address
 of the Registration Request to which the
 agent is replying. See Sections 3.7.2.3 and
 3.8.3.1 for complete details.
 Destination Address Copied from the source address of the
 Registration Request to which the agent is
 replying
 UDP fields:
 Source Port <variable>
 Destination Port Copied from the source port of the
 corresponding Registration Request (Section
 3.7.1).
 The UDP header is followed by the Mobile IP fields shown below:
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type | Code | Lifetime |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Home Address |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Home Agent |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 + Identification +
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Extensions ...
 +-+-+-+-+-+-+-+-
 Type 3 (Registration Reply)
 Code A value indicating the result of the Registration
 Request. See below for a list of currently defined Code
 values.
 Lifetime
 If the Code field indicates that the registration was
 accepted, the Lifetime field is set to the number of
 seconds remaining before the registration is considered
 expired. A value of zero indicates that the mobile node
 has been deregistered. A value of 0xffff indicates
 infinity. If the Code field indicates that the
 registration was denied, the contents of the Lifetime
 field are unspecified and MUST be ignored on reception.
 Home Address
 The IP address of the mobile node.
 Home Agent
 The IP address of the mobile node's home agent.
 Identification
 A 64-bit number used for matching Registration Requests
 with Registration Replies, and for protecting against
 replay attacks of registration messages. The value is
 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)
 Registration denied by the home agent:
 128 reason unspecified
 129 administratively prohibited
 130 insufficient resources
 131 mobile node failed authentication
 132 foreign agent failed authentication
 133 registration Identification mismatch
 134 poorly formed Request
 135 too many simultaneous mobility bindings
 136 unknown home agent address
 Up-to-date values of the Code field are specified in the most recent
 "Assigned Numbers" [40].
3.5. Registration Extensions
3.5.1. Computing Authentication Extension Values
 The Authenticator value computed for each authentication Extension
 MUST protect the following fields from the registration message:
 - the UDP payload (that is, the Registration Request or
 Registration Reply data),
 - all prior Extensions in their entirety, and
 - the Type, Length, and SPI of this Extension.
 The default authentication algorithm uses HMAC-MD5 [23] to compute a
 128-bit "message digest" of the registration message. The data over
 which the HMAC is computed is defined as:
 - the UDP payload (that is, the Registration Request or
 Registration Reply data),
 - all prior Extensions in their entirety, and
 - the Type, Length, and SPI of this Extension.
 Note that the Authenticator field itself and the UDP header are NOT
 included in the computation of the default Authenticator value. See
 Section 5.1 for information about support requirements for message
 authentication codes, which are to be used with the various
 authentication Extensions.
 The Security Parameter Index (SPI) within any of the authentication
 Extensions defines the security context which is used to compute the
 Authenticator value and which MUST be used by the receiver to check
 that value. In particular, the SPI selects the authentication
 algorithm and mode (Section 5.1) and secret (a shared key, or
 appropriate public/private key pair) used in computing the
 Authenticator. In order to ensure interoperability between different
 implementations of the Mobile IP protocol, an implementation MUST be
 able to associate any SPI value with any authentication algorithm and
 mode which it implements. In addition, all implementations of Mobile
 IP MUST implement the default authentication algorithm (HMAC-MD5)
 specified above.
3.5.2. Mobile-Home Authentication Extension
 Exactly one authorization-enabling extension MUST be present in all
 Registration Requests, and also in all Registration Replies generated
 by the Home Agent. The Mobile-Home Authentication Extension is
 always an authorization-enabling for registration messages specified
 in this document. This requirement is intended to eliminate problems
 [2] which result from the uncontrolled propagation of remote
 redirects in the Internet. The location of the extension marks the
 end of the authenticated data.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type | Length | SPI ....
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 ... SPI (cont.) | Authenticator ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type 32
 Length 4 plus the number of bytes in the Authenticator.
 SPI Security Parameter Index (4 bytes). An opaque
 identifier (see Section 1.6).
 Authenticator (variable length) (See Section 3.5.1.)
3.5.3. Mobile-Foreign Authentication Extension
 This Extension MAY be included in Registration Requests and Replies
 in cases in which a mobility security association exists between the
 mobile node and the foreign agent. See Section 5.1 for information
 about support requirements for message authentication codes.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type | Length | SPI ....
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 ... SPI (cont.) | Authenticator ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type 33
 Length 4 plus the number of bytes in the Authenticator.
 SPI Security Parameter Index (4 bytes). An opaque
 identifier (see Section 1.6).
 Authenticator (variable length) (See Section 3.5.1.)
3.5.4. Foreign-Home Authentication Extension
 This Extension MAY be included in Registration Requests and Replies
 in cases in which a mobility security association exists between the
 foreign agent and the home agent. See Section 5.1 for information
 about support requirements for message authentication codes.
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type | Length | SPI ....
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 ... SPI (cont.) | Authenticator ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type 34
 Length 4 plus the number of bytes in the Authenticator.
 SPI Security Parameter Index (4 bytes). An opaque
 identifier (see Section 1.6).
 Authenticator (variable length) (See Section 3.5.1.)
3.6. Mobile Node Considerations
 A mobile node MUST be configured with a netmask and a mobility
 security association for each of its home agents. In addition, a
 mobile node MAY be configured with its home address, and the IP
 address of one or more of its home agents; otherwise, the mobile node
 MAY discover a home agent using the procedures described in Section
 3.6.1.2.
 If the mobile node is not configured with a home address, it MAY use
 the Mobile Node NAI extension [6] to identify itself, and set the
 Home Address field of the Registration Request to 0.0.0.0. In this
 case, the mobile node MUST be able to assign its home address after
 extracting this information from the Registration Reply from the home
 agent.
 For each pending registration, the mobile node maintains the
 following information:
 - the link-layer address of the foreign agent to which the
 Registration Request was sent, if applicable,
 - the IP destination address of the Registration Request,
 - the care-of address used in the registration,
 - the Identification value sent in the registration,
 - the originally requested Lifetime, and
 - the remaining Lifetime of the pending registration.
 A mobile node SHOULD initiate a registration whenever it detects a
 change in its network connectivity. See Section 2.4.2 for methods by
 which mobile nodes MAY make such a determination. When it is away
 from home, the mobile node's Registration Request allows its home
 agent to create or modify a mobility binding for it. When it is at
 home, the mobile node's (de)Registration Request allows its home
 agent to delete any previous mobility binding(s) for it. A mobile
 node operates without the support of mobility functions when it is at
 home.
 There are other conditions under which the mobile node SHOULD
 (re)register with its foreign agent, such as when the mobile node
 detects that the foreign agent has rebooted (as specified in Section
 2.4.4) and when the current registration's Lifetime is near
 expiration.
 In the absence of link-layer indications of changes in point of
 attachment, Agent Advertisements from new agents SHOULD NOT cause a
 mobile node to attempt a new registration, if its current
 registration has not expired and it is still also receiving Agent
 Advertisements from the foreign agent with which it is currently
 registered. In the absence of link-layer indications, a mobile node
 MUST NOT attempt to register more often than once per second.
 A mobile node MAY register with a different agent when transport-
 layer protocols indicate excessive retransmissions. A mobile node
 MUST NOT consider reception of an ICMP Redirect from a foreign agent
 that is currently providing service to it as reason to register with
 a new foreign agent. Within these constraints, the mobile node MAY
 register again at any time.
 Appendix D shows some examples of how the fields in registration
 messages would be set up in some typical registration scenarios.
3.6.1. Sending Registration Requests
 The following sections specify details for the values the mobile node
 MUST supply in the fields of Registration Request messages.
3.6.1.1. IP Fields
 This section provides the specific rules by which mobile nodes pick
 values for the IP header fields of a Registration Request.
 IP Source Address:
 - When registering on a foreign network with a co-located care-of
 address, the IP source address MUST be the care-of address.
 - Otherwise, if the mobile node does not have a home address, the
 IP source address MUST be 0.0.0.0.
 - In all other circumstances, the IP source address MUST be the
 mobile node's home address.
 IP Destination Address:
 - When the mobile node has discovered the agent with which it is
 registering, through some means (e.g., link-layer) that does
 not provide the IP address of the agent (the IP address of the
 agent is unknown to the mobile node), then the "All Mobility
 Agents" multicast address (224.0.0.11) MUST be used. In this
 case, the mobile node MUST use the agent's link-layer unicast
 address in order to deliver the datagram to the correct agent.
 - When registering with a foreign agent, the address of the agent
 as learned from the IP source address of the corresponding
 Agent Advertisement MUST be used. This MAY be an address which
 does not appear as an advertised care-of address in the Agent
 Advertisement. In addition, when transmitting this
 Registration Request message, the mobile node MUST use a link-
 layer destination address copied from the link-layer source
 address of the Agent Advertisement message in which it learned
 this foreign agent's IP address.
 - When the mobile node is registering directly with its home
 agent and knows the (unicast) IP address of its home agent, the
 destination address MUST be set to this address.
 - If the mobile node is registering directly with its home agent,
 but does not know the IP address of its home agent, the mobile
 node may use dynamic home agent address resolution to
 automatically determine the IP address of its home agent
 (Section 3.6.1.2). In this case, the IP destination address is
 set to the subnet-directed broadcast address of the mobile
 node's home network. This address MUST NOT be used as the
 destination IP address if the mobile node is registering via a
 foreign agent, although it MAY be used as the Home Agent
 address in the body of the Registration Request when
 registering via a foreign agent.
 IP Time to Live:
 - The IP TTL field MUST be set to 1 if the IP destination address
 is set to the "All Mobility Agents" multicast address as
 described above. Otherwise a suitable value should be chosen
 in accordance with standard IP practice [38].
3.6.1.2. Registration Request Fields
 This section provides specific rules by which mobile nodes pick
 values for the fields within the fixed portion of a Registration
 Request.
 A mobile node MAY set the 'S' bit in order to request that the home
 agent maintain prior mobility binding(s). Otherwise, the home agent
 deletes any previous binding(s) and replaces them with the new
 binding specified in the Registration Request. Multiple simultaneous
 mobility bindings are likely to be useful when a mobile node using at
 least one wireless network interface moves within wireless
 transmission range of more than one foreign agent. IP explicitly
 allows duplication of datagrams. When the home agent allows
 simultaneous bindings, it will tunnel a separate copy of each
 arriving datagram to each care-of address, and the mobile node will
 receive multiple copies of datagrams destined to it.
 The mobile node SHOULD set the 'D' bit if it is registering with a
 co-located care-of address. Otherwise, the 'D' bit MUST NOT be set.
 A mobile node MAY set the 'B' bit to request its home agent to
 forward to it, a copy of broadcast datagrams received by its home
 agent from the home network. The method used by the home agent to
 forward broadcast datagrams depends on the type of care-of address
 registered by the mobile node, as determined by the 'D' bit in the
 mobile node's Registration Request:
 - If the 'D' bit is set, then the mobile node has indicated that
 it will decapsulate any datagrams tunneled to this care-of
 address itself (the mobile node is using a co-located care-of
 address). In this case, to forward such a received broadcast
 datagram to the mobile node, the home agent MUST tunnel it to
 this care-of address. The mobile node de-tunnels the received
 datagram in the same way as any other datagram tunneled
 directly to it.
 - If the 'D' bit is NOT set, then the mobile node has indicated
 that it is using a foreign agent care-of address, and that the
 foreign agent will thus decapsulate arriving datagrams before
 forwarding them to the mobile node. In this case, to forward
 such a received broadcast datagram to the mobile node, the home
 agent MUST first encapsulate the broadcast datagram in a
 unicast datagram addressed to the mobile node's home address,
 and then MUST tunnel this resulting datagram to the mobile
 node's care-of address.
 When decapsulated by the foreign agent, the inner datagram will
 thus be a unicast IP datagram addressed to the mobile node,
 identifying to the foreign agent the intended destination of
 the encapsulated broadcast datagram, and will be delivered to
 the mobile node in the same way as any tunneled datagram
 arriving for the mobile node. The foreign agent MUST NOT
 decapsulate the encapsulated broadcast datagram and MUST NOT
 use a local network broadcast to transmit it to the mobile
 node. The mobile node thus MUST decapsulate the encapsulated
 broadcast datagram itself, and thus MUST NOT set the 'B' bit in
 its Registration Request in this case unless it is capable of
 decapsulating datagrams.
 The mobile node MAY request alternative forms of encapsulation by
 setting the 'M' bit and/or the 'G' bit, but only if the mobile node
 is decapsulating its own datagrams (the mobile node is using a co-
 located care-of address) or if its foreign agent has indicated
 support for these forms of encapsulation by setting the corresponding
 bits in the Mobility Agent Advertisement Extension of an Agent
 Advertisement received by the mobile node. Otherwise, the mobile
 node MUST NOT set these bits.
 The Lifetime field is chosen as follows:
 - If the mobile node is registering with a foreign agent, the
 Lifetime SHOULD NOT exceed the value in the Registration
 Lifetime field of the Agent Advertisement message received from
 the foreign agent.
 When the method by which the care-of address is learned does
 not include a Lifetime, the default ICMP Router Advertisement
 Lifetime (1800 seconds) MAY be used.
 - The mobile node MAY ask a home agent to delete a particular
 mobility binding, by sending a Registration Request with the
 care-of address for this binding, with the Lifetime field set
 to zero (Section 3.8.2).
 - Similarly, a Lifetime of zero is used when the mobile node
 deregisters all care-of addresses, such as upon returning home.
 The Home Address field MUST be set to the mobile node's home address,
 if this information is known. Otherwise, the Home Address MUST be
 set to zeroes.
 The Home Agent field MUST be set to the address of the mobile node's
 home agent, if the mobile node knows this address. Otherwise, the
 mobile node MAY use dynamic home agent address resolution to learn
 the address of its home agent. In this case, the mobile node MUST
 set the Home Agent field to the subnet-directed broadcast address of
 the mobile node's home network. Each home agent receiving such a
 Registration Request with a broadcast destination address MUST reject
 the mobile node's registration and SHOULD return a rejection
 Registration Reply indicating its unicast IP address for use by the
 mobile node in a future registration attempt.
 The Care-of Address field MUST be set to the value of the particular
 care-of address that the mobile node wishes to (de)register. In the
 special case in which a mobile node wishes to deregister all care-of
 addresses, it MUST set this field to its home address.
 The mobile node chooses the Identification field in accordance with
 the style of replay protection it uses with its home agent. This is
 part of the mobility security association the mobile node shares with
 its home agent. See Section 5.7 for the method by which the mobile
 node computes the Identification field.
3.6.1.3. Extensions
 This section describes the ordering of any mandatory and any optional
 Extensions that a mobile node appends to a Registration Request.
 This following ordering MUST be followed:
 a) The IP header, followed by the UDP header, followed by the
 fixed-length portion of the Registration Request, followed by
 b) If present, any non-authentication Extensions expected to be
 used by the home agent (which may or may not also be useful to
 the foreign agent), followed by
 c) An authorization-enabling extension, followed by
 d) If present, any non-authentication Extensions used only by the
 foreign agent, followed by
 e) The Mobile-Foreign Authentication Extension, if present.
 Note that items (a) and (c) MUST appear in every Registration Request
 sent by the mobile node. Items (b), (d), and (e) are optional.
 However, item (e) MUST be included when the mobile node and the
 foreign agent share a mobility security association.
3.6.2. Receiving Registration Replies
 Registration Replies will be received by the mobile node in response
 to its Registration Requests. Registration Replies generally fall
 into three categories:
 - the registration was accepted,
 - the registration was denied by the foreign agent, or
 - the registration was denied by the home agent.
 The remainder of this section describes the Registration Reply
 handling by a mobile node in each of these three categories.
3.6.2.1. Validity Checks
 Registration Replies with an invalid, non-zero UDP checksum MUST be
 silently discarded.
 In addition, the low-order 32 bits of the Identification field in the
 Registration Reply MUST be compared to the low-order 32 bits of the
 Identification field in the most recent Registration Request sent to
 the replying agent. If they do not match, the Reply MUST be silently
 discarded.
 Also, the Registration Reply MUST be checked for presence of an
 authorization-enabling extension. For all Registration Reply
 messages containing a Status Code indicating status from the Home
 Agent, the mobile node MUST check for the presence of an
 authorization-enabling extension, acting in accordance with the Code
 field in the Reply. The rules are as follows:
 a) If the mobile node and the foreign agent share a mobility
 security association, exactly one Mobile-Foreign Authentication
 Extension MUST be present in the Registration Reply, and the
 mobile node MUST check the Authenticator value in the
 Extension. If no Mobile-Foreign Authentication Extension is
 found, or if more than one Mobile-Foreign Authentication
 Extension is found, or if the Authenticator is invalid, the
 mobile node MUST silently discard the Reply and SHOULD log the
 event as a security exception.
 b) If the Code field indicates that service is denied by the home
 agent, or if the Code field indicates that the registration was
 accepted by the home agent, exactly one Mobile-Home
 Authentication Extension MUST be present in the Registration
 Reply, and the mobile node MUST check the Authenticator value
 in the Extension. If the Registration Reply was generated by
 the home agent but no Mobile-Home Authentication Extension is
 found, or if more than one Mobile-Home Authentication Extension
 is found, or if the Authenticator is invalid, the mobile node
 MUST silently discard the Reply and SHOULD log the event as a
 security exception.
 If the Code field indicates an authentication failure, either at the
 foreign agent or the home agent, then it is quite possible that any
 authenticators in the Registration Reply will also be in error. This
 could happen, for example, if the shared secret between the mobile
 node and home agent was erroneously configured. The mobile node
 SHOULD log such errors as security exceptions.
3.6.2.2. Registration Request Accepted
 If the Code field indicates that the request has been accepted, the
 mobile node SHOULD configure its routing table appropriately for its
 current point of attachment (Section 4.2.1).
 If the mobile node is returning to its home network and that network
 is one which implements ARP, the mobile node MUST follow the
 procedures described in Section 4.6 with regard to ARP, proxy ARP,
 and gratuitous ARP.
 If the mobile node has registered on a foreign network, it SHOULD
 re-register before the expiration of the Lifetime of its
 registration. As described in Section 3.6, for each pending
 Registration Request, the mobile node MUST maintain the remaining
 lifetime of this pending registration, as well as the original
 Lifetime from the Registration Request. When the mobile node
 receives a valid Registration Reply, the mobile node MUST decrease
 its view of the remaining lifetime of the registration by the amount
 by which the home agent decreased the originally requested Lifetime.
 This procedure is equivalent to the mobile node starting a timer for
 the granted Lifetime at the time it sent the Registration Request,
 even though the granted Lifetime is not known to the mobile node
 until the Registration Reply is received. Since the Registration
 Request is certainly sent before the home agent begins timing the
 registration Lifetime (also based on the granted Lifetime), this
 procedure ensures that the mobile node will re-register before the
 home agent expires and deletes the registration, in spite of possibly
 non-negligible transmission delays for the original Registration
 Request and Reply that started the timing of the Lifetime at the
 mobile node and its home agent.
3.6.2.3. Registration Request Denied
 If the Code field indicates that service is being denied, the mobile
 node SHOULD log the error. In certain cases the mobile node may be
 able to "repair" the error. These include:
 Code 69: (Denied by foreign agent, Lifetime too long)
 In this case, the Lifetime field in the Registration Reply will
 contain the maximum Lifetime value which that foreign agent is
 willing to accept in any Registration Request. The mobile node
 MAY attempt to register with this same agent, using a Lifetime
 in the Registration Request that MUST be less than or equal to
 the value specified in the Reply.
 Code 133: (Denied by home agent, Identification mismatch)
 In this case, the Identification field in the Registration
 Reply will contain a value that allows the mobile node to
 synchronize with the home agent, based upon the style of replay
 protection in effect (Section 5.7). The mobile node MUST
 adjust the parameters it uses to compute the Identification
 field based upon the information in the Registration Reply,
 before issuing any future Registration Requests.
 Code 136: (Denied by home agent, Unknown home agent address)
 This code is returned by a home agent when the mobile node is
 performing dynamic home agent address resolution as described
 in Sections 3.6.1.1 and 3.6.1.2. In this case, the Home Agent
 field within the Reply will contain the unicast IP address of
 the home agent returning the Reply. The mobile node MAY then
 attempt to register with this home agent in future Registration
 Requests. In addition, the mobile node SHOULD adjust the
 parameters it uses to compute the Identification field based
 upon the corresponding field in the Registration Reply, before
 issuing any future Registration Requests.
3.6.3. Registration Retransmission
 When no Registration Reply has been received within a reasonable
 time, another Registration Request MAY be transmitted. When
 timestamps are used, a new registration Identification is chosen for
 each retransmission; thus it counts as a new registration. When
 nonces are used, the unanswered Request is retransmitted unchanged;
 thus the retransmission does not count as a new registration (Section
 5.7). In this way a retransmission will not require the home agent
 to resynchronize with the mobile node by issuing another nonce in the
 case in which the original Registration Request (rather than its
 Registration Reply) was lost by the network.
 The maximum time until a new Registration Request is sent SHOULD be
 no greater than the requested Lifetime of the Registration Request.
 The minimum value SHOULD be large enough to account for the size of
 the messages, twice the round trip time for transmission to the home
 agent, and at least an additional 100 milliseconds to allow for
 processing the messages before responding. The round trip time for
 transmission to the home agent will be at least as large as the time
 required to transmit the messages at the link speed of the mobile
 node's current point of attachment. Some circuits add another 200
 milliseconds of satellite delay in the total round trip time to the
 home agent. The minimum time between Registration Requests MUST NOT
 be less than 1 second. Each successive retransmission timeout period
 SHOULD be at least twice the previous period, as long as that is less
 than the maximum as specified above.
3.7. Foreign Agent Considerations
 The foreign agent plays a mostly passive role in Mobile IP
 registration. It relays Registration Requests between mobile nodes
 and home agents, and, when it provides the care-of address,
 decapsulates datagrams for delivery to the mobile node. It SHOULD
 also send periodic Agent Advertisement messages to advertise its
 presence as described in Section 2.3, if not detectable by link-layer
 means.
 A foreign agent MUST NOT transmit a Registration Request except when
 relaying a Registration Request received from a mobile node, to the
 mobile node's home agent. A foreign agent MUST NOT transmit a
 Registration Reply except when relaying a Registration Reply received
 from a mobile node's home agent, or when replying to a Registration
 Request received from a mobile node in the case in which the foreign
 agent is denying service to the mobile node. In particular, a
 foreign agent MUST NOT generate a Registration Request or Reply
 because a mobile node's registration Lifetime has expired. A foreign
 agent also MUST NOT originate a Registration Request message that
 asks for deregistration of a mobile node; however, it MUST relay
 valid (de)Registration Requests originated by a mobile node.
3.7.1. Configuration and Registration Tables
 Each foreign agent MUST be configured with a care-of address. In
 addition, for each pending or current registration the foreign agent
 MUST maintain a visitor list entry containing the following
 information obtained from the mobile node's Registration Request:
 - the link-layer source address of the mobile node
 - the IP Source Address (the mobile node's Home Address) or its
 co-located care-of address (see description of the 'R' bit in
 section 2.1.1)
 - the IP Destination Address (as specified in 3.6.1.1)
 - the UDP Source Port
 - the Home Agent address
 - the Identification field
 - the requested registration Lifetime, and
 - the remaining Lifetime of the pending or current registration.
 If the mobile node's Home Address is zero in the Registration Request
 message, then the foreign agent MUST follow the procedures specified
 in RFC 2794 [6]. In particular, if the foreign agent cannot manage
 pending registration request records with such a zero Home Address
 for the mobile node, the foreign agent MUST return a Registration
 Reply with Code indicating NONZERO_HOMEADDR_REQD (see [6]).
 The foreign agent MAY configure a maximum number of pending
 registrations that it is willing to maintain (typically 5).
 Additional registrations SHOULD then be rejected by the foreign agent
 with code 66. The foreign agent MAY delete any pending Registration
 Request after the request has been pending for more than 7 seconds;
 in this case, the foreign agent SHOULD reject the Request with code
 78 (registration timeout).
 As with any node on the Internet, a foreign agent MAY also share
 mobility security associations with any other nodes. When relaying a
 Registration Request from a mobile node to its home agent, if the
 foreign agent shares a mobility security association with the home
 agent, it MUST add a Foreign-Home Authentication Extension to the
 Request and MUST check the required Foreign-Home Authentication
 Extension in the Registration Reply from the home agent (Sections 3.3
 and 3.4). Similarly, when receiving a Registration Request from a
 mobile node, if the foreign agent shares a mobility security
 association with the mobile node, it MUST check the required Mobile-
 Foreign Authentication Extension in the Request and MUST add a
 Mobile-Foreign Authentication Extension to the Registration Reply to
 the mobile node.
3.7.2. Receiving Registration Requests
 If the foreign agent accepts a Registration Request from a mobile
 node, it checks to make sure that the indicated home agent address
 does not belong to any network interface of the foreign agent. If
 not, the foreign agent then MUST relay the Request to the indicated
 home agent. Otherwise, if the foreign agent denies the Request, it
 MUST send a Registration Reply to the mobile node with an appropriate
 denial Code, except in cases where the foreign agent would be
 required to send out more than one such denial per second to the same
 mobile node. The following sections describe this behavior in more
 detail.
 If the foreign agent has configured one of its network interfaces
 with the IP address specified by the mobile node as its home agent
 address, the foreign agent MUST NOT forward the request again. If
 the foreign agent serves the mobile node as a home agent, the foreign
 agent follows the procedures specified in section 3.8.2. Otherwise,
 if the foreign agent does not serve the mobile node as a home agent,
 the foreign agent rejects the Registration Request with code 136
 (unknown home agent address).
 If a foreign agent receives a Registration Request from a mobile node
 in its visitor list, the existing visitor list entry for the mobile
 node SHOULD NOT be deleted or modified until the foreign agent
 receives a valid Registration Reply from the home agent with a Code
 indicating success. The foreign agent MUST record the new pending
 Request as a separate part of the existing visitor list entry for the
 mobile node. If the Registration Request requests deregistration,
 the existing visitor list entry for the mobile node SHOULD NOT be
 deleted until the foreign agent has received a successful
 Registration Reply. If the Registration Reply indicates that the
 Request (for registration or deregistration) was denied by the home
 agent, the existing visitor list entry for the mobile node MUST NOT
 be modified as a result of receiving the Registration Reply.
3.7.2.1. Validity Checks
 Registration Requests with an invalid, non-zero UDP checksum MUST be
 silently discarded. Requests with non-zero bits in reserved fields
 MUST be rejected with code 70 (poorly formed request). Requests with
 the 'D' bit set to 0, and specifying a care-of address not offered by
 the foreign agent, MUST be rejected with code 77 (invalid care-of
 address).
 Also, the authentication in the Registration Request MUST be checked.
 If the foreign agent and the mobile node share a mobility security
 association, exactly one Mobile-Foreign Authentication Extension MUST
 be present in the Registration Request, and the foreign agent MUST
 check the Authenticator value in the Extension. If no Mobile-Foreign
 Authentication Extension is found, or if more than one Mobile-Foreign
 Authentication Extension is found, or if the Authenticator is
 invalid, the foreign agent MUST silently discard the Request and
 SHOULD log the event as a security exception. The foreign agent also
 SHOULD send a Registration Reply to the mobile node with Code 67.
3.7.2.2. Forwarding a Valid Request to the Home Agent
 If the foreign agent accepts the mobile node's Registration Request,
 it MUST relay the Request to the mobile node's home agent as
 specified in the Home Agent field of the Registration Request. The
 foreign agent MUST NOT modify any of the fields beginning with the
 fixed portion of the Registration Request up through and including
 the Mobile-Home Authentication Extension or other authentication
 extension supplied by the mobile node as an authorization-enabling
 extension for the home agent. Otherwise, an authentication failure
 is very likely to occur at the home agent. In addition, the foreign
 agent proceeds as follows:
 - It MUST process and remove any Extensions following the
 Mobile-Home Authentication Extension,
 - It MAY append any of its own non-authentication Extensions of
 relevance to the home agent, if applicable, and
 - It MUST append the Foreign-Home Authentication Extension, if
 the foreign agent shares a mobility security association with
 the home agent.
 Specific fields within the IP header and the UDP header of the
 relayed Registration Request MUST be set as follows:
 IP Source Address
 The foreign agent's address on the interface from which
 the message will be sent.
 IP Destination Address
 Copied from the Home Agent field within the Registration
 Request.
 UDP Source Port
 <variable>
 UDP Destination Port
 434
 After forwarding a valid Registration Request to the home agent, the
 foreign agent MUST begin timing the remaining lifetime of the pending
 registration based on the Lifetime in the Registration Request. If
 this lifetime expires before receiving a valid Registration Reply,
 the foreign agent MUST delete its visitor list entry for this pending
 registration.
3.7.2.3. Denying Invalid Requests
 If the foreign agent denies the mobile node's Registration Request
 for any reason, it SHOULD send the mobile node a Registration Reply
 with a suitable denial Code. In such a case, the Home Address, Home
 Agent, and Identification fields within the Registration Reply are
 copied from the corresponding fields of the Registration Request.
 If the Reserved field is nonzero, the foreign agent MUST deny the
 Request and SHOULD return a Registration Reply with status code 70 to
 the mobile node. If the Request is being denied because the
 requested Lifetime is too long, the foreign agent sets the Lifetime
 in the Reply to the maximum Lifetime value it is willing to accept in
 any Registration Request, and sets the Code field to 69. Otherwise,
 the Lifetime SHOULD be copied from the Lifetime field in the Request.
 Specific fields within the IP header and the UDP header of the
 Registration Reply MUST be set as follows:
 IP Source Address
 Copied from the IP Destination Address of Registration
 Request, unless the "All Agents Multicast" address was
 used. In this case, the foreign agent's address (on the
 interface from which the message will be sent) MUST be
 used.
 IP Destination Address
 If the Registration Reply is generated by the Foreign
 Agent in order to reject a mobile node's Registration
 Request, and the Registration Request contains a Home
 Address which is not 0.0.0.0, then the IP Destination
 Address is copied from the Home Address field of the
 Registration Request. Otherwise, if the Registration
 Reply is received from the Home Agent, and contains a
 Home Address which is not 0.0.0.0, then the IP
 Destination Address is copied from the Home Address field
 of the Registration Reply. Otherwise, the IP Destination
 Address of the Registration Reply is set to be
 255.255.255.255.
 UDP Source Port
 434
 UDP Destination Port
 Copied from the UDP Source Port of the Registration
 Request.
3.7.3. Receiving Registration Replies
 The foreign agent updates its visitor list when it receives a valid
 Registration Reply from a home agent. It then relays the
 Registration Reply to the mobile node. The following sections
 describe this behavior in more detail.
 If upon relaying a Registration Request to a home agent, the foreign
 agent receives an ICMP error message instead of a Registration Reply,
 then the foreign agent SHOULD send to the mobile node a Registration
 Reply with an appropriate "Home Agent Unreachable" failure Code
 (within the range 80-95, inclusive). See Section 3.7.2.3 for details
 on building the Registration Reply.
3.7.3.1. Validity Checks
 Registration Replies with an invalid, non-zero UDP checksum MUST be
 silently discarded.
 When a foreign agent receives a Registration Reply message, it MUST
 search its visitor list for a pending Registration Request with the
 same mobile node home address as indicated in the Reply. If no such
 pending Request is found, and if the Registration Reply does not
 correspond with any pending Registration Request with a zero mobile
 node home address (see section 3.7.1), the foreign agent MUST
 silently discard the Reply. The foreign agent MUST also silently
 discard the Reply if the low-order 32 bits of the Identification
 field in the Reply do not match those in the Request.
 Also, the authentication in the Registration Reply MUST be checked.
 If the foreign agent and the home agent share a mobility security
 association, exactly one Foreign-Home Authentication Extension MUST
 be present in the Registration Reply, and the foreign agent MUST
 check the Authenticator value in the Extension. If no Foreign-Home
 Authentication Extension is found, or if more than one Foreign-Home
 Authentication Extension is found, or if the Authenticator is
 invalid, the foreign agent MUST silently discard the Reply and SHOULD
 log the event as a security exception. The foreign agent also MUST
 reject the mobile node's registration and SHOULD send a Registration
 Reply to the mobile node with Code 68.
3.7.3.2. Forwarding Replies to the Mobile Node
 A Registration Reply which satisfies the validity checks of Section
 3.8.2.1 is relayed to the mobile node. The foreign agent MUST also
 update its visitor list entry for the mobile node to reflect the
 results of the Registration Request, as indicated by the Code field
 in the Reply. If the Code indicates that the home agent has accepted
 the registration and the Lifetime field is nonzero, the foreign agent
 SHOULD set the Lifetime in the visitor list entry to the minimum of
 the following two values:
 - the value specified in the Lifetime field of the Registration
 Reply, and
 - the foreign agent's own maximum value for allowable
 registration lifetime.
 If, instead, the Code indicates that the Lifetime field is zero, the
 foreign agent MUST delete its visitor list entry for the mobile node.
 Finally, if the Code indicates that the registration was denied by
 the home agent, the foreign agent MUST delete its pending
 registration list entry, but not its visitor list entry, for the
 mobile node.
 The foreign agent MUST NOT modify any of the fields beginning with
 the fixed portion of the Registration Reply up through and including
 the Mobile-Home Authentication Extension. Otherwise, an
 authentication failure is very likely to occur at the mobile node.
 In addition, the foreign agent SHOULD perform the following
 additional procedures:
 - It MUST process and remove any Extensions following the
 Mobile-Home Authentication Extension,
 - It MAY append its own non-authentication Extensions of
 relevance to the mobile node, if applicable, and
 - It MUST append the Mobile-Foreign Authentication Extension, if
 the foreign agent shares a mobility security association with
 the mobile node.
 Specific fields within the IP header and the UDP header of the
 relayed Registration Reply are set according to the same rules
 specified in Section 3.7.2.3.
 After forwarding a valid Registration Reply to the mobile node, the
 foreign agent MUST update its visitor list entry for this
 registration as follows. If the Registration Reply indicates that
 the registration was accepted by the home agent, the foreign agent
 resets its timer of the lifetime of the registration to the Lifetime
 granted in the Registration Reply; unlike the mobile node's timing of
 the registration lifetime as described in Section 3.6.2.2, the
 foreign agent considers this lifetime to begin when it forwards the
 Registration Reply message, ensuring that the foreign agent will not
 expire the registration before the mobile node does. On the other
 hand, if the Registration Reply indicates that the registration was
 rejected by the home agent, the foreign agent deletes its visitor
 list entry for this attempted registration.
3.8. Home Agent Considerations
 Home agents play a reactive role in the registration process. The
 home agent receives Registration Requests from the mobile node
 (perhaps relayed by a foreign agent), updates its record of the
 mobility bindings for this mobile node, and issues a suitable
 Registration Reply in response to each.
 A home agent MUST NOT transmit a Registration Reply except when
 replying to a Registration Request received from a mobile node. In
 particular, the home agent MUST NOT generate a Registration Reply to
 indicate that the Lifetime has expired.
3.8.1. Configuration and Registration Tables
 Each home agent MUST be configured with an IP address and with the
 prefix size for the home network. The home agent MUST be configured
 with the mobility security association of each authorized mobile node
 that it is serving as a home agent.
 When the home agent accepts a valid Registration Request from a
 mobile node that it serves as a home agent, the home agent MUST
 create or modify the entry for this mobile node in its mobility
 binding list containing:
 - the mobile node's home address
 - the mobile node's care-of address
 - the Identification field from the Registration Reply
 - the remaining Lifetime of the registration
 The home agent MAY optionally offer the capability to dynamically
 associate a home address to a mobile node upon receiving a
 Registration Request from that mobile node. The method by which a
 home address is allocated to the mobile node is beyond the scope of
 this document, but see [6]. After the home agent makes the
 association of the home address to the mobile node, the home agent
 MUST put that home address into the Home Address field of the
 Registration Reply.
 The home agent MAY also maintain mobility security associations with
 various foreign agents. When receiving a Registration Request from a
 foreign agent, if the home agent shares a mobility security
 association with the foreign agent, the home agent MUST check the
 Authenticator in the required Foreign-Home Authentication Extension
 in the message, based on this mobility security association.
 Similarly, when sending a Registration Reply to a foreign agent, if
 the home agent shares a mobility security association with the
 foreign agent, the home agent MUST include a Foreign-Home
 Authentication Extension in the message, based on this mobility
 security association.
3.8.2. Receiving Registration Requests
 If the home agent accepts an incoming Registration Request, it MUST
 update its record of the the mobile node's mobility binding(s) and
 SHOULD send a Registration Reply with a suitable Code. Otherwise
 (the home agent denies the Request), it SHOULD send a Registration
 Reply with an appropriate Code specifying the reason the Request was
 denied. The following sections describe this behavior in more
 detail. If the home agent does not support broadcasts (see section
 4.3), it MUST ignore the 'B' bit (as opposed to rejecting the
 Registration Request).
3.8.2.1. Validity Checks
 Registration Requests with an invalid, non-zero UDP checksum MUST be
 silently discarded by the home agent.
 The authentication in the Registration Request MUST be checked. This
 involves the following operations:
 a) The home agent MUST check for the presence of an
 authorization-enabling extension, and perform the indicated
 authentication. Exactly one authorization-enabling extension
 MUST be present in the Registration Request; and the home agent
 MUST either check the Authenticator value in the extension or
 verify that the authenticator value has been checked by another
 agent with which it has a security association. If no
 authorization-enabling extension is found, or if more than one
 authorization-enabling extension is found, or if the
 Authenticator is invalid, the home agent MUST reject the mobile
 node's registration and SHOULD send a Registration Reply to the
 mobile node with Code 131. The home agent MUST then discard
 the Request and SHOULD log the error as a security exception.
 b) The home agent MUST check that the registration Identification
 field is correct using the context selected by the SPI within
 the authorization-enabling extension. See Section 5.7 for a
 description of how this is performed. If incorrect, the home
 agent MUST reject the Request and SHOULD send a Registration
 Reply to the mobile node with Code 133, including an
 Identification field computed in accordance with the rules
 specified in Section 5.7. The home agent MUST do no further
 processing with such a Request, though it SHOULD log the error
 as a security exception.
 c) If the home agent shares a mobility security association with
 the foreign agent, the home agent MUST check for the presence
 of a valid Foreign-Home Authentication Extension. Exactly one
 Foreign-Home Authentication Extension MUST be present in the
 Registration Request in this case, and the home agent MUST
 check the Authenticator value in the Extension. If no
 Foreign-Home Authentication Extension is found, or if more than
 one Foreign-Home Authentication Extension is found, or if the
 Authenticator is invalid, the home agent MUST reject the mobile
 node's registration and SHOULD send a Registration Reply to the
 mobile node with Code 132. The home agent MUST then discard
 the Request and SHOULD log the error as a security exception.
 In addition to checking the authentication in the Registration
 Request, home agents MUST deny Registration Requests that are sent to
 the subnet-directed broadcast address of the home network (as opposed
 to being unicast to the home agent). The home agent MUST discard the
 Request and SHOULD returning a Registration Reply with a Code of 136.
 In this case, the Registration Reply will contain the home agent's
 unicast address, so that the mobile node can re-issue the
 Registration Request with the correct home agent address.
 Note that some routers change the IP destination address of a
 datagram from a subnet-directed broadcast address to 255.255.255.255
 before injecting it into the destination subnet. In this case, home
 agents that attempt to pick up dynamic home agent discovery requests
 by binding a socket explicitly to the subnet-directed broadcast
 address will not see such packets. Home agent implementors should be
 prepared for both the subnet-directed broadcast address and
 255.255.255.255 if they wish to support dynamic home agent discovery.
3.8.2.2. Accepting a Valid Request
 If the Registration Request satisfies the validity checks in Section
 3.8.2.1, and the home agent is able to accommodate the Request, the
 home agent MUST update its mobility binding list for the requesting
 mobile node and MUST return a Registration Reply to the mobile node.
 In this case, the Reply Code will be either 0 if the home agent
 supports simultaneous mobility bindings, or 1 if it does not. See
 Section 3.8.3 for details on building the Registration Reply message.
 The home agent updates its record of the mobile node's mobility
 bindings as follows, based on the fields in the Registration Request:
 - If the Lifetime is zero and the Care-of Address equals the
 mobile node's home address, the home agent deletes all of the
 entries in the mobility binding list for the requesting mobile
 node. This is how a mobile node requests that its home agent
 cease providing mobility services.
 - If the Lifetime is zero and the Care-of Address does not equal
 the mobile node's home address, the home agent deletes only the
 entry containing the specified Care-of Address from the
 mobility binding list for the requesting mobile node. Any
 other active entries containing other care-of addresses will
 remain active.
 - If the Lifetime is nonzero, the home agent adds an entry
 containing the requested Care-of Address to the mobility
 binding list for the mobile node. If the 'S' bit is set and
 the home agent supports simultaneous mobility bindings, the
 previous mobility binding entries are retained. Otherwise, the
 home agent removes all previous entries in the mobility binding
 list for the mobile node.
 In all cases, the home agent MUST send a Registration Reply to the
 source of the Registration Request, which might indeed be a different
 foreign agent than that whose care-of address is being
 (de)registered. If the home agent shares a mobility security
 association with the foreign agent whose care-of address is being
 deregistered, and that foreign agent is different from the one which
 relayed the Registration Request, the home agent MAY additionally
 send a Registration Reply to the foreign agent whose care-of address
 is being deregistered. The home agent MUST NOT send such a Reply if
 it does not share a mobility security association with the foreign
 agent. If no Reply is sent, the foreign agent's visitor list will
 expire naturally when the original Lifetime expires.
 The home agent MUST NOT increase the Lifetime above that specified by
 the mobile node in the Registration Request. However, it is not an
 error for the mobile node to request a Lifetime longer than the home
 agent is willing to accept. In this case, the home agent simply
 reduces the Lifetime to a permissible value and returns this value in
 the Registration Reply. The Lifetime value in the Registration Reply
 informs the mobile node of the granted lifetime of the registration,
 indicating when it SHOULD re-register in order to maintain continued
 service. After the expiration of this registration lifetime, the
 home agent MUST delete its entry for this registration in its
 mobility binding list.
 If the Registration Request duplicates an accepted current
 Registration Request, the new Lifetime MUST NOT extend beyond the
 Lifetime originally granted. A Registration Request is a duplicate
 if the home address, care-of address, and Identification fields all
 equal those of an accepted current registration.
 In addition, if the home network implements ARP [36], and the
 Registration Request asks the home agent to create a mobility binding
 for a mobile node which previously had no binding (the mobile node
 was previously assumed to be at home), then the home agent MUST
 follow the procedures described in Section 4.6 with regard to ARP,
 proxy ARP, and gratuitous ARP. If the mobile node already had a
 previous mobility binding, the home agent MUST continue to follow the
 rules for proxy ARP described in Section 4.6.
3.8.2.3. Denying an Invalid Request
 If the Registration Reply does not satisfy all of the validity checks
 in Section 3.8.2.1, or the home agent is unable to accommodate the
 Request, the home agent SHOULD return a Registration Reply to the
 mobile node with a Code that indicates the reason for the error. If
 a foreign agent was involved in relaying the Request, this allows the
 foreign agent to delete its pending visitor list entry. Also, this
 informs the mobile node of the reason for the error such that it may
 attempt to fix the error and issue another Request.
 This section lists a number of reasons the home agent might reject a
 Request, and provides the Code value it should use in each instance.
 See Section 3.8.3 for additional details on building the Registration
 Reply message.
 Many reasons for rejecting a registration are administrative in
 nature. For example, a home agent can limit the number of
 simultaneous registrations for a mobile node, by rejecting any
 registrations that would cause its limit to be exceeded, and
 returning a Registration Reply with error code 135. Similarly, a
 home agent may refuse to grant service to mobile nodes which have
 entered unauthorized service areas by returning a Registration Reply
 with a Code of 129.
 Requests with non-zero bits in reserved fields MUST be rejected with
 code 134 (poorly formed request).
3.8.3. Sending Registration Replies
 If the home agent accepts a Registration Request, it then MUST update
 its record of the mobile node's mobility binding(s) and SHOULD send a
 Registration Reply with a suitable Code. Otherwise (the home agent
 has denied the Request), it SHOULD send a Registration Reply with an
 appropriate Code specifying the reason the Request was denied. The
 following sections provide additional detail for the values the home
 agent MUST supply in the fields of Registration Reply messages.
3.8.3.1. IP/UDP Fields
 This section provides the specific rules by which home agents pick
 values for the IP and UDP header fields of a Registration Reply.
 IP Source Address
 Copied from the IP Destination Address of Registration
 Request, unless a multicast or broadcast address was
 used. If the IP Destination Address of the Registration
 Request was a broadcast or multicast address, the IP
 Source Address of the Registration Reply MUST be set to
 the home agent's (unicast) IP address.
 IP Destination Address
 Copied from the IP Source Address of the Registration
 Request.
 UDP Source Port
 Copied from the UDP Destination Port of the Registration
 Request.
 UDP Destination Port
 Copied from the UDP Source Port of the Registration
 Request.
 When sending a Registration Reply in response to a Registration
 Request that requested deregistration of the mobile node (the
 Lifetime is zero and the Care-of Address equals the mobile node's
 home address) and in which the IP Source Address was also set to the
 mobile node's home address (this is the normal method used by a
 mobile node to deregister when it returns to its home network), the
 IP Destination Address in the Registration Reply will be set to the
 mobile node's home address, as copied from the IP Source Address of
 the Request.
 In this case, when transmitting the Registration Reply, the home
 agent MUST transmit the Reply directly onto the home network as if
 the mobile node were at home, bypassing any mobility binding list
 entry that may still exist at the home agent for the destination
 mobile node. In particular, for a mobile node returning home after
 being registered with a care-of address, if the mobile node's new
 Registration Request is not accepted by the home agent, the mobility
 binding list entry for the mobile node will still indicate that
 datagrams addressed to the mobile node should be tunneled to the
 mobile node's registered care-of address; when sending the
 Registration Reply indicating the rejection of this Request, this
 existing binding list entry MUST be ignored, and the home agent MUST
 transmit this Reply as if the mobile node were at home.
3.8.3.2. Registration Reply Fields
 This section provides the specific rules by which home agents pick
 values for the fields within the fixed portion of a Registration
 Reply.
 The Code field of the Registration Reply is chosen in accordance with
 the rules specified in the previous sections. When replying to an
 accepted registration, a home agent SHOULD respond with Code 1 if it
 does not support simultaneous registrations.
 The Lifetime field MUST be copied from the corresponding field in the
 Registration Request, unless the requested value is greater than the
 maximum length of time the home agent is willing to provide the
 requested service. In such a case, the Lifetime MUST be set to the
 length of time that service will actually be provided by the home
 agent. This reduced Lifetime SHOULD be the maximum Lifetime allowed
 by the home agent (for this mobile node and care-of address).
 If the Home Address field of the Registration Request is nonzero, it
 MUST be copied into the Home Address field of the Registration Reply
 message. Otherwise, if the Home Address field of the Registration
 Request is zero as specified in section 3.6, the home agent SHOULD
 arrange for the selection of a home address for the mobile node, and
 insert the selected address into the Home Address field of the
 Registration Reply message. See [6] for further relevant details in
 the case where mobile nodes identify themselves using an NAI instead
 of their IP home address.
 If the Home Agent field in the Registration Request contains a
 unicast address of this home agent, then that field MUST be copied
 into the Home Agent field of the Registration Reply. Otherwise, the
 home agent MUST set the Home Agent field in the Registration Reply to
 its unicast address. In this latter case, the home agent MUST reject
 the registration with a suitable code (e.g., Code 136) to prevent the
 mobile node from possibly being simultaneously registered with two or
 more home agents.
3.8.3.3. Extensions
 This section describes the ordering of any required and any optional
 Mobile IP Extensions that a home agent appends to a Registration
 Reply. The following ordering MUST be followed:
 a) The IP header, followed by the UDP header, followed by the
 fixed-length portion of the Registration Reply,
 b) If present, any non-authentication Extensions used by the
 mobile node (which may or may not also be used by the foreign
 agent),
 c) The Mobile-Home Authentication Extension,
 d) If present, any non-authentication Extensions used only by the
 foreign agent, and
 e) The Foreign-Home Authentication Extension, if present.
 Note that items (a) and (c) MUST appear in every Registration Reply
 sent by the home agent. Items (b), (d), and (e) are optional.
 However, item (e) MUST be included when the home agent and the
 foreign agent share a mobility security association.
4. Routing Considerations
 This section describes how mobile nodes, home agents, and (possibly)
 foreign agents cooperate to route datagrams to/from mobile nodes that
 are connected to a foreign network. The mobile node informs its home
 agent of its current location using the registration procedure
 described in Section 3. See the protocol overview in Section 1.7 for
 the relative locations of the mobile node's home address with respect
 to its home agent, and the mobile node itself with respect to any
 foreign agent with which it might attempt to register.
4.1. Encapsulation Types
 Home agents and foreign agents MUST support tunneling datagrams using
 IP in IP encapsulation [32]. Any mobile node that uses a co-located
 care-of address MUST support receiving datagrams tunneled using IP in
 IP encapsulation. Minimal encapsulation [34] and GRE encapsulation
 [16] are alternate encapsulation methods which MAY optionally be
 supported by mobility agents and mobile nodes. The use of these
 alternative forms of encapsulation, when requested by the mobile
 node, is otherwise at the discretion of the home agent.
4.2. Unicast Datagram Routing
4.2.1. Mobile Node Considerations
 When connected to its home network, a mobile node operates without
 the support of mobility services. That is, it operates in the same
 way as any other (fixed) host or router. The method by which a
 mobile node selects a default router when connected to its home
 network, or when away from home and using a co-located care-of
 address, is outside the scope of this document. ICMP Router
 Advertisement [10] is one such method.
 When registered on a foreign network, the mobile node chooses a
 default router by the following rules:
 - If the mobile node is registered using a foreign agent care-of
 address, it MAY use its foreign agent as a first-hop router.
 The foreign agent's MAC address can be learned from Agent
 Advertisement. Otherwise, the mobile node MUST choose its
 default router from among the Router Addresses advertised in
 the ICMP Router Advertisement portion of that Agent
 Advertisement message.
 - If the mobile node is registered directly with its home agent
 using a co-located care-of address, then the mobile node SHOULD
 choose its default router from among those advertised in any
 ICMP Router Advertisement message that it receives for which
 its externally obtained care-of address and the Router Address
 match under the network prefix. If the mobile node's
 externally obtained care-of address matches the IP source
 address of the Agent Advertisement under the network prefix,
 the mobile node MAY also consider that IP source address as
 another possible choice for the IP address of a default router.
 The network prefix MAY be obtained from the Prefix-Lengths
 Extension in the Router Advertisement, if present. The prefix
 MAY also be obtained through other mechanisms beyond the scope
 of this document.
 While they are away from the home network, mobile nodes MUST NOT
 broadcast ARP packets to find the MAC address of another Internet
 node. Thus, the (possibly empty) list of Router Addresses from the
 ICMP Router Advertisement portion of the message is not useful for
 selecting a default router, unless the mobile node has some means not
 involving broadcast ARP and not specified within this document for
 obtaining the MAC address of one of the routers in the list.
 Similarly, in the absence of unspecified mechanisms for obtaining MAC
 addresses on foreign networks, the mobile node MUST ignore redirects
 to other routers on foreign networks.
4.2.2. Foreign Agent Considerations
 Upon receipt of an encapsulated datagram sent to its advertised
 care-of address, a foreign agent MUST compare the inner destination
 address to those entries in its visitor list. When the destination
 does not match the address of any mobile node currently in the
 visitor list, the foreign agent MUST NOT forward the datagram without
 modifications to the original IP header, because otherwise a routing
 loop is likely to result. The datagram SHOULD be silently discarded.
 ICMP Destination Unreachable MUST NOT be sent when a foreign agent is
 unable to forward an incoming tunneled datagram. Otherwise, the
 foreign agent forwards the decapsulated datagram to the mobile node.
 The foreign agent MUST NOT advertise to other routers in its routing
 domain, nor to any other mobile node, the presence of a mobile router
 (Section 4.5) or mobile node in its visitor list.
 The foreign agent MUST route datagrams it receives from registered
 mobile nodes. At a minimum, this means that the foreign agent must
 verify the IP Header Checksum, decrement the IP Time To Live,
 recompute the IP Header Checksum, and forward such datagrams to a
 default router.
 A foreign agent MUST NOT use broadcast ARP for a mobile node's MAC
 address on a foreign network. It may obtain the MAC address by
 copying the information from an Agent Solicitation or a Registration
 Request transmitted from a mobile node. A foreign agent's ARP cache
 for the mobile node's IP address MUST NOT be allowed to expire before
 the mobile node's visitor list entry expires, unless the foreign
 agent has some way other than broadcast ARP to refresh its MAC
 address associated with the mobile node's IP address.
 Each foreign agent SHOULD support the mandatory features for reverse
 tunneling [27].
4.2.3. Home Agent Considerations
 The home agent MUST be able to intercept any datagrams on the home
 network addressed to the mobile node while the mobile node is
 registered away from home. Proxy and gratuitous ARP MAY be used in
 enabling this interception, as specified in Section 4.6.
 The home agent must examine the IP Destination Address of all
 arriving datagrams to see if it is equal to the home address of any
 of its mobile nodes registered away from home. If so, the home agent
 tunnels the datagram to the mobile node's currently registered care-
 of address or addresses. If the home agent supports the optional
 capability of multiple simultaneous mobility bindings, it tunnels a
 copy to each care-of address in the mobile node's mobility binding
 list. If the mobile node has no current mobility bindings, the home
 agent MUST NOT attempt to intercept datagrams destined for the mobile
 node, and thus will not in general receive such datagrams. However,
 if the home agent is also a router handling common IP traffic, it is
 possible that it will receive such datagrams for forwarding onto the
 home network. In this case, the home agent MUST assume the mobile
 node is at home and simply forward the datagram directly onto the
 home network.
 For multihomed home agents, the source address in the outer IP header
 of the encapsulated datagram MUST be the address sent to the mobile
 node in the home agent field of the registration reply. That is, the
 home agent cannot use the the address of some other network interface
 as the source address.
 See Section 4.1 regarding methods of encapsulation that may be used
 for tunneling. Nodes implementing tunneling SHOULD also implement
 the "tunnel soft state" mechanism [32], which allows ICMP error
 messages returned from the tunnel to correctly be reflected back to
 the original senders of the tunneled datagrams.
 Home agents MUST decapsulate packets addressed to themselves, sent by
 a mobile node for the purpose of maintaining location privacy, as
 described in Section 5.5. This feature is also required for support
 of reverse tunneling [27].
 If the Lifetime for a given mobility binding expires before the home
 agent has received another valid Registration Request for that mobile
 node, then that binding is deleted from the mobility binding list.
 The home agent MUST NOT send any Registration Reply message simply
 because the mobile node's binding has expired. The entry in the
 visitor list of the mobile node's current foreign agent will expire
 naturally, probably at the same time as the binding expired at the
 home agent. When a mobility binding's lifetime expires, the home
 agent MUST delete the binding, but it MUST retain any other (non-
 expired) simultaneous mobility bindings that it holds for the mobile
 node.
 When a home agent receives a datagram, intercepted for one of its
 mobile nodes registered away from home, the home agent MUST examine
 the datagram to check if it is already encapsulated. If so, special
 rules apply in the forwarding of that datagram to the mobile node:
 - If the inner (encapsulated) Destination Address is the same as
 the outer Destination Address (the mobile node), then the home
 agent MUST also examine the outer Source Address of the
 encapsulated datagram (the source address of the tunnel). If
 this outer Source Address is the same as the mobile node's
 current care-of address, the home agent MUST silently discard
 that datagram in order to prevent a likely routing loop. If,
 instead, the outer Source Address is NOT the same as the mobile
 node's current care-of address, then the home agent SHOULD
 forward the datagram to the mobile node. In order to forward
 the datagram in this case, the home agent MAY simply alter the
 outer Destination Address to the care-of address, rather than
 re-encapsulating the datagram.
 - Otherwise (the inner Destination Address is NOT the same as the
 outer Destination Address), the home agent SHOULD encapsulate
 the datagram again (nested encapsulation), with the new outer
 Destination Address set equal to the mobile node's care-of
 address. That is, the home agent forwards the entire datagram
 to the mobile node in the same way as any other datagram
 (encapsulated already or not).
4.3. Broadcast Datagrams
 When a home agent receives a broadcast datagram, it MUST NOT forward
 the datagram to any mobile nodes in its mobility binding list other
 than those that have requested forwarding of broadcast datagrams. A
 mobile node MAY request forwarding of broadcast datagrams by setting
 the 'B' bit in its Registration Request message (Section 3.3). For
 each such registered mobile node, the home agent SHOULD forward
 received broadcast datagrams to the mobile node, although it is a
 matter of configuration at the home agent as to which specific
 categories of broadcast datagrams will be forwarded to such mobile
 nodes.
 If the 'D' bit was set in the mobile node's Registration Request
 message, indicating that the mobile node is using a co-located care-
 of address, the home agent simply tunnels appropriate broadcast IP
 datagrams to the mobile node's care-of address. Otherwise (the 'D'
 bit was NOT set), the home agent first encapsulates the broadcast
 datagram in a unicast datagram addressed to the mobile node's home
 address, and then tunnels this encapsulated datagram to the foreign
 agent. This extra level of encapsulation is required so that the
 foreign agent can determine which mobile node should receive the
 datagram after it is decapsulated. When received by the foreign
 agent, the unicast encapsulated datagram is detunneled and delivered
 to the mobile node in the same way as any other datagram. In either
 case, the mobile node must decapsulate the datagram it receives in
 order to recover the original broadcast datagram.
4.4. Multicast Datagram Routing
 As mentioned previously, a mobile node that is connected to its home
 network functions in the same way as any other (fixed) host or
 router. Thus, when it is at home, a mobile node functions
 identically to other multicast senders and receivers. This section
 therefore describes the behavior of a mobile node that is visiting a
 foreign network.
 In order to receive multicasts, a mobile node MUST join the multicast
 group in one of two ways. First, a mobile node MAY join the group
 via a (local) multicast router on the visited subnet. This option
 assumes that there is a multicast router present on the visited
 subnet. If the mobile node is using a co-located care-of address, it
 SHOULD use this address as the source IP address of its IGMP [11]
 messages. Otherwise, it MAY use its home address.
 Alternatively, a mobile node which wishes to receive multicasts MAY
 join groups via a bi-directional tunnel to its home agent, assuming
 that its home agent is a multicast router. The mobile node tunnels
 IGMP messages to its home agent and the home agent forwards multicast
 datagrams down the tunnel to the mobile node. For packets tunneled
 to the home agent, the source address in the IP header SHOULD be the
 mobile node's home address.
 The rules for multicast datagram delivery to mobile nodes in this
 case are identical to those for broadcast datagrams (Section 4.3).
 Namely, if the mobile node is using a co-located care-of address (the
 'D' bit was set in the mobile node's Registration Request), then the
 home agent SHOULD tunnel the datagram to this care-of address;
 otherwise, the home agent MUST first encapsulate the datagram in a
 unicast datagram addressed to the mobile node's home address and then
 MUST tunnel the resulting datagram (nested tunneling) to the mobile
 node's care-of address. For this reason, the mobile node MUST be
 capable of decapsulating packets sent to its home address in order to
 receive multicast datagrams using this method.
 A mobile node that wishes to send datagrams to a multicast group also
 has two options: (1) send directly on the visited network; or (2)
 send via a tunnel to its home agent. Because multicast routing in
 general depends upon the IP source address, a mobile node which sends
 multicast datagrams directly on the visited network MUST use a co-
 located care-of address as the IP source address. Similarly, a
 mobile node which tunnels a multicast datagram to its home agent MUST
 use its home address as the IP source address of both the (inner)
 multicast datagram and the (outer) encapsulating datagram. This
 second option assumes that the home agent is a multicast router.
4.5. Mobile Routers
 A mobile node can be a router that is responsible for the mobility of
 one or more entire networks moving together, perhaps on an airplane,
 a ship, a train, an automobile, a bicycle, or a kayak. The nodes
 connected to a network served by the mobile router may themselves be
 fixed nodes or mobile nodes or routers. In this document, such
 networks are called "mobile networks".
 A mobile router MAY act as a foreign agent and provide a foreign
 agent care-of address to mobile nodes connected to the mobile
 network. Typical routing to a mobile node via a mobile router in
 this case is illustrated by the following example:
 a) A laptop computer is disconnected from its home network and
 later attached to a network port in the seat back of an
 aircraft. The laptop computer uses Mobile IP to register on
 this foreign network, using a foreign agent care-of address
 discovered through an Agent Advertisement from the aircraft's
 foreign agent.
 b) The aircraft network is itself mobile. Suppose the node
 serving as the foreign agent on the aircraft also serves as the
 default router that connects the aircraft network to the rest
 of the Internet. When the aircraft is at home, this router is
 attached to some fixed network at the airline's headquarters,
 which is the router's home network. While the aircraft is in
 flight, this router registers from time to time over its radio
 link with a series of foreign agents below it on the ground.
 This router's home agent is a node on the fixed network at the
 airline's headquarters.
 c) Some correspondent node sends a datagram to the laptop
 computer, addressing the datagram to the laptop's home address.
 This datagram is initially routed to the laptop's home network.
 d) The laptop's home agent intercepts the datagram on the home
 network and tunnels it to the laptop's care-of address, which
 in this example is an address of the node serving as router and
 foreign agent on the aircraft. Normal IP routing will route
 the datagram to the fixed network at the airline's
 headquarters.
 e) The aircraft router and foreign agent's home agent there
 intercepts the datagram and tunnels it to its current care-of
 address, which in this example is some foreign agent on the
 ground below the aircraft. The original datagram from the
 correspondent node has now been encapsulated twice: once by
 the laptop's home agent and again by the aircraft's home agent.
 f) The foreign agent on the ground decapsulates the datagram,
 yielding a datagram still encapsulated by the laptop's home
 agent, with a destination address of the laptop's care-of
 address. The ground foreign agent sends the resulting datagram
 over its radio link to the aircraft.
 g) The foreign agent on the aircraft decapsulates the datagram,
 yielding the original datagram from the correspondent node,
 with a destination address of the laptop's home address. The
 aircraft foreign agent delivers the datagram over the aircraft
 network to the laptop's link-layer address.
 This example illustrated the case in which a mobile node is attached
 to a mobile network. That is, the mobile node is mobile with respect
 to the network, which itself is also mobile (here with respect to the
 ground). If, instead, the node is fixed with respect to the mobile
 network (the mobile network is the fixed node's home network), then
 either of two methods may be used to cause datagrams from
 correspondent nodes to be routed to the fixed node.
 A home agent MAY be configured to have a permanent registration for
 the fixed node, that indicates the mobile router's address as the
 fixed host's care-of address. The mobile router's home agent will
 usually be used for this purpose. The home agent is then responsible
 for advertising connectivity using normal routing protocols to the
 fixed node. Any datagrams sent to the fixed node will thus use
 nested tunneling as described above.
 Alternatively, the mobile router MAY advertise connectivity to the
 entire mobile network using normal IP routing protocols through a
 bi-directional tunnel to its own home agent. This method avoids the
 need for nested tunneling of datagrams.
4.6. ARP, Proxy ARP, and Gratuitous ARP
 The use of ARP [36] requires special rules for correct operation when
 wireless or mobile nodes are involved. The requirements specified in
 this section apply to all home networks in which ARP is used for
 address resolution.
 In addition to the normal use of ARP for resolving a target node's
 link-layer address from its IP address, this document distinguishes
 two special uses of ARP:
 - A Proxy ARP [39] is an ARP Reply sent by one node on behalf of
 another node which is either unable or unwilling to answer its
 own ARP Requests. The sender of a Proxy ARP reverses the
 Sender and Target Protocol Address fields as described in [36],
 but supplies some configured link-layer address (generally, its
 own) in the Sender Hardware Address field. The node receiving
 the Reply will then associate this link-layer address with the
 IP address of the original target node, causing it to transmit
 future datagrams for this target node to the node with that
 link-layer address.
 - A Gratuitous ARP [45] is an ARP packet sent by a node in order
 to spontaneously cause other nodes to update an entry in their
 ARP cache. A gratuitous ARP MAY use either an ARP Request or
 an ARP Reply packet. In either case, the ARP Sender Protocol
 Address and ARP Target Protocol Address are both set to the IP
 address of the cache entry to be updated, and the ARP Sender
 Hardware Address is set to the link-layer address to which this
 cache entry should be updated. When using an ARP Reply packet,
 the Target Hardware Address is also set to the link-layer
 address to which this cache entry should be updated (this field
 is not used in an ARP Request packet).
 In either case, for a gratuitous ARP, the ARP packet MUST be
 transmitted as a local broadcast packet on the local link. As
 specified in [36], any node receiving any ARP packet (Request
 or Reply) MUST update its local ARP cache with the Sender
 Protocol and Hardware Addresses in the ARP packet, if the
 receiving node has an entry for that IP address already in its
 ARP cache. This requirement in the ARP protocol applies even
 for ARP Request packets, and for ARP Reply packets that do not
 match any ARP Request transmitted by the receiving node [36].
 While a mobile node is registered on a foreign network, its home
 agent uses proxy ARP [39] to reply to ARP Requests it receives that
 seek the mobile node's link-layer address. When receiving an ARP
 Request, the home agent MUST examine the target IP address of the
 Request, and if this IP address matches the home address of any
 mobile node for which it has a registered mobility binding, the home
 agent MUST transmit an ARP Reply on behalf of the mobile node. After
 exchanging the sender and target addresses in the packet [39], the
 home agent MUST set the sender link-layer address in the packet to
 the link-layer address of its own interface over which the Reply will
 be sent.
 When a mobile node leaves its home network and registers a binding on
 a foreign network, its home agent uses gratuitous ARP to update the
 ARP caches of nodes on the home network. This causes such nodes to
 associate the link-layer address of the home agent with the mobile
 node's home (IP) address. When registering a binding for a mobile
 node for which the home agent previously had no binding (the mobile
 node was assumed to be at home), the home agent MUST transmit a
 gratuitous ARP on behalf of the mobile node. This gratuitous ARP
 packet MUST be transmitted as a broadcast packet on the link on which
 the mobile node's home address is located. Since broadcasts on the
 local link (such as Ethernet) are typically not guaranteed to be
 reliable, the gratuitous ARP packet SHOULD be retransmitted a small
 number of times to increase its reliability.
 When a mobile node returns to its home network, the mobile node and
 its home agent use gratuitous ARP to cause all nodes on the mobile
 node's home network to update their ARP caches to once again
 associate the mobile node's own link-layer address with the mobile
 node's home (IP) address. Before transmitting the (de)Registration
 Request message to its home agent, the mobile node MUST transmit this
 gratuitous ARP on its home network as a local broadcast on this link.
 The gratuitous ARP packet SHOULD be retransmitted a small number of
 times to increase its reliability, but these retransmissions SHOULD
 proceed in parallel with the transmission and processing of its
 (de)Registration Request.
 When the mobile node's home agent receives and accepts this
 (de)Registration Request, the home agent MUST also transmit a
 gratuitous ARP on the mobile node's home network. This gratuitous
 ARP also is used to associate the mobile node's home address with the
 mobile node's own link-layer address. A gratuitous ARP is
 transmitted by both the mobile node and its home agent, since in the
 case of wireless network interfaces, the area within transmission
 range of the mobile node will likely differ from that within range of
 its home agent. The ARP packet from the home agent MUST be
 transmitted as a local broadcast on the mobile node's home link, and
 SHOULD be retransmitted a small number of times to increase its
 reliability; these retransmissions, however, SHOULD proceed in
 parallel with the transmission and processing of its (de)Registration
 Reply.
 While the mobile node is away from home, it MUST NOT transmit any
 broadcast ARP Request or ARP Reply messages. Finally, while the
 mobile node is away from home, it MUST NOT reply to ARP Requests in
 which the target IP address is its own home address, unless the ARP
 Request is unicast by a foreign agent with which the mobile node is
 attempting to register or a foreign agent with which the mobile node
 has an unexpired registration. In the latter case, the mobile node
 MUST use a unicast ARP Reply to respond to the foreign agent. Note
 that if the mobile node is using a co-located care-of address and
 receives an ARP Request in which the target IP address is this care-
 of address, then the mobile node SHOULD reply to this ARP Request.
 Note also that, when transmitting a Registration Request on a foreign
 network, a mobile node may discover the link-layer address of a
 foreign agent by storing the address as it is received from the Agent
 Advertisement from that foreign agent, but not by transmitting a
 broadcast ARP Request message.
 The specific order in which each of the above requirements for the
 use of ARP, proxy ARP, and gratuitous ARP are applied, relative to
 the transmission and processing of the mobile node's Registration
 Request and Registration Reply messages when leaving home or
 returning home, are important to the correct operation of the
 protocol.
 To summarize the above requirements, when a mobile node leaves its
 home network, the following steps, in this order, MUST be performed:
 - The mobile node decides to register away from home, perhaps
 because it has received an Agent Advertisement from a foreign
 agent and has not recently received one from its home agent.
 - Before transmitting the Registration Request, the mobile node
 disables its own future processing of any ARP Requests it may
 subsequently receive requesting the link-layer address
 corresponding to its home address, except insofar as necessary
 to communicate with foreign agents on visited networks.
 - The mobile node transmits its Registration Request.
 - When the mobile node's home agent receives and accepts the
 Registration Request, it performs a gratuitous ARP on behalf of
 the mobile node, and begins using proxy ARP to reply to ARP
 Requests that it receives requesting the mobile node's link-
 layer address. In the gratuitous ARP, the ARP Sender Hardware
 Address is set to the link-layer address of the home agent.
 If, instead, the home agent rejects the Registration Request,
 no ARP processing (gratuitous nor proxy) is performed by the
 home agent.
 When a mobile node later returns to its home network, the following
 steps, in this order, MUST be performed:
 - The mobile node decides to register at home, perhaps because it
 has received an Agent Advertisement from its home agent.
 - Before transmitting the Registration Request, the mobile node
 re-enables its own future processing of any ARP Requests it may
 subsequently receive requesting its link-layer address.
 - The mobile node performs a gratuitous ARP for itself. In this
 gratuitous ARP, the ARP Sender Hardware Address is set to the
 link-layer address of the mobile node.
 - The mobile node transmits its Registration Request.
 - When the mobile node's home agent receives and accepts the
 Registration Request, it stops using proxy ARP to reply to ARP
 Requests that it receives requesting the mobile node's link-
 layer address, and then performs a gratuitous ARP on behalf of
 the mobile node. In this gratuitous ARP, the ARP Sender
 Hardware Address is set to the link-layer address of the mobile
 node. If, instead, the home agent rejects the Registration
 Request, the home agent MUST NOT make any change to the way it
 performs ARP processing (gratuitous nor proxy) for the mobile
 node. In this latter case, the home agent should operate as if
 the mobile node has not returned home, and continue to perform
 proxy ARP on behalf of the mobile node.
5. Security Considerations
 The mobile computing environment is potentially very different from
 the ordinary computing environment. In many cases, mobile computers
 will be connected to the network via wireless links. Such links are
 particularly vulnerable to passive eavesdropping, active replay
 attacks, and other active attacks.
5.1. Message Authentication Codes
 Home agents and mobile nodes MUST be able to perform authentication.
 The default algorithm is HMAC-MD5 [23], with a key size of 128 bits.
 The foreign agent MUST also support authentication using HMAC-MD5 and
 key sizes of 128 bits or greater, with manual key distribution. Keys
 with arbitrary binary values MUST be supported.
 The "prefix+suffix" use of MD5 to protect data and a shared secret is
 considered vulnerable to attack by the cryptographic community.
 Where backward compatibility with existing Mobile IP implementations
 that use this mode is needed, new implementations SHOULD include
 keyed MD5 [41] as one of the additional authentication algorithms for
 use when producing and verifying the authentication data that is
 supplied with Mobile IP registration messages, for instance in the
 extensions specified in sections 3.5.2, 3.5.3, and 3.5.4.
 More authentication algorithms, algorithm modes, key distribution
 methods, and key sizes MAY also be supported for all of these
 extensions.
5.2. Areas of Security Concern in this Protocol
 The registration protocol described in this document will result in a
 mobile node's traffic being tunneled to its care-of address. This
 tunneling feature could be a significant vulnerability if the
 registration were not authenticated. Such remote redirection, for
 instance as performed by the mobile registration protocol, is widely
 understood to be a security problem in the current Internet if not
 authenticated [2]. Moreover, the Address Resolution Protocol (ARP)
 is not authenticated, and can potentially be used to steal another
 host's traffic. The use of "Gratuitous ARP" (Section 4.6) brings
 with it all of the risks associated with the use of ARP.
5.3. Key Management
 This specification requires a strong authentication mechanism (keyed
 MD5) which precludes many potential attacks based on the Mobile IP
 registration protocol. However, because key distribution is
 difficult in the absence of a network key management protocol,
 messages with the foreign agent are not all required to be
 authenticated. In a commercial environment it might be important to
 authenticate all messages between the foreign agent and the home
 agent, so that billing is possible, and service providers do not
 provide service to users that are not legitimate customers of that
 service provider.
5.4. Picking Good Random Numbers
 The strength of any authentication mechanism depends on several
 factors, including the innate strength of the authentication
 algorithm, the secrecy of the key used, the strength of the key used,
 and the quality of the particular implementation. This specification
 requires implementation of keyed MD5 for authentication, but does not
 preclude the use of other authentication algorithms and modes. For
 keyed MD5 authentication to be useful, the 128-bit key must be both
 secret (that is, known only to authorized parties) and pseudo-random.
 If nonces are used in connection with replay protection, they must
 also be selected carefully. Eastlake, et al. [14] provides more
 information on generating pseudo-random numbers.
5.5. Privacy
 Users who have sensitive data that they do not wish others to see
 should use mechanisms outside the scope of this document (such as
 encryption) to provide appropriate protection. Users concerned about
 traffic analysis should consider appropriate use of link encryption.
 If absolute location privacy is desired, the mobile node can create a
 tunnel to its home agent. Then, datagrams destined for correspondent
 nodes will appear to emanate from the home network, and it may be
 more difficult to pinpoint the location of the mobile node. Such
 mechanisms are all beyond the scope of this document.
5.6. Ingress Filtering
 Many routers implement security policies such as "ingress filtering"
 [15] that do not allow forwarding of packets that have a Source
 Address which appears topologically incorrect. In environments where
 this is a problem, mobile nodes may use reverse tunneling [27] with
 the foreign agent supplied care-of address as the Source Address.
 Reverse tunneled packets will be able to pass normally through such
 routers, while ingress filtering rules will still be able to locate
 the true topological source of the packet in the same way as packets
 from non-mobile nodes.
5.7. Replay Protection for Registration Requests
 The Identification field is used to let the home agent verify that a
 registration message has been freshly generated by the mobile node,
 not replayed by an attacker from some previous registration. Two
 methods are described in this section: timestamps (mandatory) and
 "nonces" (optional). All mobile nodes and home agents MUST implement
 timestamp-based replay protection. These nodes MAY also implement
 nonce-based replay protection (but see Appendix A).
 The style of replay protection in effect between a mobile node and
 its home agent is part of the mobile security association. A mobile
 node and its home agent MUST agree on which method of replay
 protection will be used. The interpretation of the Identification
 field depends on the method of replay protection as described in the
 subsequent subsections.
 Whatever method is used, the low-order 32 bits of the Identification
 MUST be copied unchanged from the Registration Request to the Reply.
 The foreign agent uses those bits (and the mobile node's home
 address) to match Registration Requests with corresponding replies.
 The mobile node MUST verify that the low-order 32 bits of any
 Registration Reply are identical to the bits it sent in the
 Registration Request.
 The Identification in a new Registration Request MUST NOT be the same
 as in an immediately preceding Request, and SHOULD NOT repeat while
 the same security context is being used between the mobile node and
 the home agent. Retransmission as in Section 3.6.3 is allowed.
5.7.1. Replay Protection using Timestamps
 The basic principle of timestamp replay protection is that the node
 generating a message inserts the current time of day, and the node
 receiving the message checks that this timestamp is sufficiently
 close to its own time of day. Unless specified differently in the
 security association between the nodes, a default value of 7 seconds
 MAY be used to limit the time difference. This value SHOULD be
 greater than 3 seconds. Obviously the two nodes must have adequately
 synchronized time-of-day clocks. As with any messages, time
 synchronization messages may be protected against tampering by an
 authentication mechanism determined by the security context between
 the two nodes.
 If timestamps are used, the mobile node MUST set the Identification
 field to a 64-bit value formatted as specified by the Network Time
 Protocol [26]. The low-order 32 bits of the NTP format represent
 fractional seconds, and those bits which are not available from a
 time source SHOULD be generated from a good source of randomness.
 Note, however, that when using timestamps, the 64-bit Identification
 used in a Registration Request from the mobile node MUST be greater
 than that used in any previous Registration Request, as the home
 agent uses this field also as a sequence number. Without such a
 sequence number, it would be possible for a delayed duplicate of an
 earlier Registration Request to arrive at the home agent (within the
 clock synchronization required by the home agent), and thus be
 applied out of order, mistakenly altering the mobile node's current
 registered care-of address.
 Upon receipt of a Registration Request with an authorization-enabling
 extension, the home agent MUST check the Identification field for
 validity. In order to be valid, the timestamp contained in the
 Identification field MUST be close enough to the home agent's time of
 day clock and the timestamp MUST be greater than all previously
 accepted timestamps for the requesting mobile node. Time tolerances
 and resynchronization details are specific to a particular mobility
 security association.
 If the timestamp is valid, the home agent copies the entire
 Identification field into the Registration Reply it returns the Reply
 to the mobile node. If the timestamp is not valid, the home agent
 copies only the low-order 32 bits into the Registration Reply, and
 supplies the high-order 32 bits from its own time of day. In this
 latter case, the home agent MUST reject the registration by returning
 Code 133 (identification mismatch) in the Registration Reply.
 As described in Section 3.6.2.1, the mobile node MUST verify that the
 low-order 32 bits of the Identification in the Registration Reply are
 identical to those in the rejected registration attempt, before using
 the high-order bits for clock resynchronization.
5.7.2. Replay Protection using Nonces
 The basic principle of nonce replay protection is that node A
 includes a new random number in every message to node B, and checks
 that node B returns that same number in its next message to node A.
 Both messages use an authentication code to protect against
 alteration by an attacker. At the same time node B can send its own
 nonces in all messages to node A (to be echoed by node A), so that it
 too can verify that it is receiving fresh messages.
 The home agent may be expected to have resources for computing
 pseudo-random numbers useful as nonces [14]. It inserts a new nonce
 as the high-order 32 bits of the identification field of every
 Registration Reply. The home agent copies the low-order 32 bits of
 the Identification from the Registration Request message into the
 low-order 32 bits of the Identification in the Registration Reply.
 When the mobile node receives an authenticated Registration Reply
 from the home agent, it saves the high-order 32 bits of the
 identification for use as the high-order 32 bits of its next
 Registration Request.
 The mobile node is responsible for generating the low-order 32 bits
 of the Identification in each Registration Request. Ideally it
 should generate its own random nonces. However it may use any
 expedient method, including duplication of the random value sent by
 the home agent. The method chosen is of concern only to the mobile
 node, because it is the node that checks for valid values in the
 Registration Reply. The high-order and low-order 32 bits of the
 identification chosen SHOULD both differ from their previous values.
 The home agent uses a new high-order value and the mobile node uses a
 new low-order value for each registration message. The foreign agent
 uses the low-order value (and the mobile host's home address) to
 correctly match registration replies with pending Requests (Section
 3.7.1).
 If a registration message is rejected because of an invalid nonce,
 the Reply always provides the mobile node with a new nonce to be used
 in the next registration. Thus the nonce protocol is self-
 synchronizing.
6. IANA Considerations
 Mobile IP specifies several new number spaces for values to be used
 in various message fields. These number spaces include the
 following:
 - Mobile IP message types sent to UDP port 434, as defined in
 section 1.8.
 - types of extensions to Registration Request and Registration
 Reply messages (see sections 3.3 and 3.4, and also consult [27,
 29, 6, 7, 12])
 - values for the Code in the Registration Reply message (see
 section 3.4, and also consult [27, 29, 6, 7, 12])
 - Mobile IP defines so-called Agent Solicitation and Agent
 Advertisement messages. These messages are in fact Router
 Discovery messages [10] augmented with mobile-IP specific
 extensions. Thus, they do not define a new name space, but do
 define additional Router Discovery extensions as described
 below in Section 6.2. Also see Section 2.1 and consult [7,
 12].
 There are additional Mobile IP numbering spaces specified in [7].
 Information about assignment of mobile-ip numbers derived from
 specifications external to this document is given by IANA at
 http://www.iana.org/numbers.html. From that URL, follow the
 hyperlinks to [M] within the "Directory of General Assigned Numbers",
 and subsequently to the specific section for "Mobile IP Numbers".
6.1. Mobile IP Message Types
 Mobile IP messages are defined to be those that are sent to a message
 recipient at port 434 (UDP or TCP). The number space for Mobile IP
 messages is specified in Section 1.8. Approval of new extension
 numbers is subject to Expert Review, and a specification is required
 [30]. The currently standardized message types have the following
 numbers, and are specified in the indicated sections.
 Type Name Section
 ---- -------------------------------------------- ---------
 1 Registration Request 3.3
 3 Registration Reply 3.4
6.2. Extensions to RFC 1256 Router Advertisement
 RFC 1256 defines two ICMP message types, Router Advertisement and
 Router Solicitation. Mobile IP defines a number space for extensions
 to Router Advertisement, which could be used by protocols other than
 Mobile IP. The extension types currently standardized for use with
 Mobile IP have the following numbers.
 Type Name Reference
 ---- -------------------------------------------- ---------
 0 One-byte Padding 2.1.3
 16 Mobility Agent Advertisement 2.1.1
 19 Prefix-Lengths 2.1.2
 Approval of new extension numbers for use with Mobile IP is subject
 to Expert Review, and a specification is required [30].
6.3. Extensions to Mobile IP Registration Messages
 The Mobile IP messages, specified within this document, and listed in
 sections 1.8 and 6.1, may have extensions. Mobile IP message
 extensions all share the same number space, even if they are to be
 applied to different Mobile IP messages. The number space for Mobile
 IP message extensions is specified within this document. Approval of
 new extension numbers is subject to Expert Review, and a
 specification is required [30].
 Type Name Reference
 ---- -------------------------------------------- ---------
 0 One-byte Padding
 32 Mobile-Home Authentication 3.5.2
 33 Mobile-Foreign Authentication 3.5.3
 34 Foreign-Home Authentication 3.5.4
6.4. Code Values for Mobile IP Registration Reply Messages
 The Mobile IP Registration Reply message, specified in section 3.4,
 has a Code field. The number space for the Code field values is also
 specified in Section 3.4. The Code number space is structured
 according to whether the registration was successful, or whether the
 foreign agent denied the registration request, or lastly whether the
 home agent denied the registration request, as follows:
 0-8 Success Codes
 9-63 No allocation guidelines currently exist
 64-127 Error Codes from the Foreign Agent
 128-192 Error Codes from the Home Agent
 193-255 No allocation guidelines currently exist
 Approval of new Code values requires Expert Review [30].
7. Acknowledgments
 Special thanks to Steve Deering (Xerox PARC), along with Dan Duchamp
 and John Ioannidis (JI) (Columbia University), for forming the
 working group, chairing it, and putting so much effort into its early
 development. Columbia's early Mobile IP work can be found in [18,
 19, 17].
 Thanks also to Kannan Alaggapan, Greg Minshall, Tony Li, Jim Solomon,
 Erik Nordmark, Basavaraj Patil, and Phil Roberts for their
 contributions to the group while performing the duties of
 chairperson, as well as for their many useful comments.
 Thanks to the active members of the Mobile IP Working Group,
 particularly those who contributed text, including (in alphabetical
 order)
 - Ran Atkinson (Naval Research Lab),
 - Samita Chakrabarti (Sun Microsystems)
 - Ken Imboden (Candlestick Networks, Inc.)
 - Dave Johnson (Carnegie Mellon University),
 - Frank Kastenholz (FTP Software),
 - Anders Klemets (KTH),
 - Chip Maguire (KTH),
 - Alison Mankin (ISI)
 - Andrew Myles (Macquarie University),
 - Thomas Narten (IBM)
 - Al Quirt (Bell Northern Research),
 - Yakov Rekhter (IBM), and
 - Fumio Teraoka (Sony).
 - Alper Yegin (NTT DoCoMo)
 Thanks to Charlie Kunzinger and to Bill Simpson, the editors who
 produced the first drafts for of this document, reflecting the
 discussions of the Working Group. Much of the new text in the later
 revisions preceding RFC 2002 is due to Jim Solomon and Dave Johnson.
 Thanks to Greg Minshall (Novell), Phil Karn (Qualcomm), Frank
 Kastenholz (FTP Software), and Pat Calhoun (Sun Microsystems) for
 their generous support in hosting interim Working Group meetings.
 Sections 1.10 and 1.11, which specify new extension formats to be
 used with aggregatable extension types, were included from a
 specification document (entitled "Mobile IP Extensions
 Rationalization (MIER)", which was written by
 - Mohamed M.Khalil, Nortel Networks
 - Raja Narayanan, nVisible Networks
 - Haseeb Akhtar, Nortel Networks
 - Emad Qaddoura, Nortel Networks
 Thanks to these authors, and also for the additional work on
 MIER, which was contributed by Basavaraj Patil, Pat Calhoun, Neil
 Justusson, N. Asokan, and Jouni Malinen.
A. Patent Issues
 The IETF has been notified of intellectual property rights claimed
 in regard to some or all of the specification contained in this
 document. For more information consult the online list of claimed
 rights.
 The IETF takes no position regarding the validity or scope of any
 intellectual property or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; neither does it represent that it
 has made any effort to identify any such rights. Information on
 the IETF's procedures with respect to rights in standards-track and
 standards-related documentation can be found in BCP-11. Copies of
 claims of rights made available for publication and any assurances
 of licenses to be made available, or the result of an attempt
 made to obtain a general license or permission for the use of such
 proprietary rights by implementors or users of this specification can
 be obtained from the IETF Secretariat.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights which may cover technology that may be required to practice
 this standard. Please address the information to the IETF Executive
 Director.
B. Link-Layer Considerations
 The mobile node MAY use link-layer mechanisms to decide that its
 point of attachment has changed. Such indications include the
 Down/Testing/Up interface status [24], and changes in cell or
 administration. The mechanisms will be specific to the particular
 link-layer technology, and are outside the scope of this document.
 The Point-to-Point-Protocol (PPP) [42] and its Internet Protocol
 Control Protocol (IPCP) [25], negotiates the use of IP addresses.
 The mobile node SHOULD first attempt to specify its home address,
 so that if the mobile node is attaching to its home network, the
 unrouted link will function correctly. When the home address is
 not accepted by the peer, but a transient IP address is dynamically
 assigned to the mobile node, and the mobile node is capable of
 supporting a co-located care-of address, the mobile node MAY
 register that address as a co-located care-of address. When the peer
 specifies its own IP address, that address MUST NOT be assumed to be
 a foreign agent care-of address or the IP address of a home agent.
 PPP extensions for Mobile IP have been specified in RFC 2290 [44].
 Please consult that document for additional details for how to handle
 care-of address assignment from PPP in a more efficient manner.
C. TCP Considerations
C.1. TCP Timers
 When high-delay (e.g. SATCOM) or low-bandwidth (e.g. High-Frequency
 Radio) links are in use, some TCP stacks may have insufficiently
 adaptive (non-standard) retransmission timeouts. There may be
 spurious retransmission timeouts, even when the link and network
 are actually operating properly, but just with a high delay because
 of the medium in use. This can cause an inability to create or
 maintain TCP connections over such links, and can also cause unneeded
 retransmissions which consume already scarce bandwidth. Vendors
 are encouraged to follow the algorithms in RFC 2988 [31] when
 implementing TCP retransmission timers. Vendors of systems designed
 for low-bandwidth, high-delay links should consult RFCs 2757 and
 2488 [28, 1]. Designers of applications targeted to operate on
 mobile nodes should be sensitive to the possibility of timer-related
 difficulties.
C.2. TCP Congestion Management
 Mobile nodes often use media which are more likely to introduce
 errors, effectively causing more packets to be dropped. This
 introduces a conflict with the mechanisms for congestion management
 found in modern versions of TCP [21]. Now, when a packet is dropped,
 the correspondent node's TCP implementation is likely to react as
 if there were a source of network congestion, and initiate the
 slow-start mechanisms [21] designed for controlling that problem.
 However, those mechanisms are inappropriate for overcoming errors
 introduced by the links themselves, and have the effect of magnifying
 the discontinuity introduced by the dropped packet. This problem has
 been analyzed by Caceres, et al. [5]. TCP approaches to the problem
 of handling errors that might interfere with congestion management
 are discussed in documents from the [pilc] working group [3, 9].
 While such approaches are beyond the scope of this document,
 they illustrate that providing performance transparency to mobile
 nodes involves understanding mechanisms outside the network layer.
 Problems introduced by higher media error rates also indicate the
 need to avoid designs which systematically drop packets; such designs
 might otherwise be considered favorably when making engineering
 tradeoffs.
D. Example Scenarios
 This section shows example Registration Requests for several common
 scenarios.
D.1. Registering with a Foreign Agent Care-of Address
 The mobile node receives an Agent Advertisement from a foreign
 agent and wishes to register with that agent using the advertised
 foreign agent care-of address. The mobile node wishes only
 IP-in-IP encapsulation, does not want broadcasts, and does not want
 simultaneous mobility bindings:
 IP fields:
 Source Address = mobile node's home address
 Destination Address = copied from the IP source address of the
 Agent Advertisement
 Time to Live = 1
 UDP fields:
 Source Port = <any>
 Destination Port = 434
 Registration Request fields:
 Type = 1
 S=0,B=0,D=0,M=0,G=0
 Lifetime = the Registration Lifetime copied from the
 Mobility Agent Advertisement Extension of the
 Router Advertisement message
 Home Address = the mobile node's home address
 Home Agent = IP address of mobile node's home agent
 Care-of Address = the Care-of Address copied from the
 Mobility Agent Advertisement Extension of the
 Router Advertisement message
 Identification = Network Time Protocol timestamp or Nonce
 Extensions:
 An authorization-enabling extension (e.g., the
 Mobile-Home Authentication Extension)
D.2. Registering with a Co-Located Care-of Address
 The mobile node enters a foreign network that contains no foreign
 agents. The mobile node obtains an address from a DHCP server [13]
 for use as a co-located care-of address. The mobile node supports
 all forms of encapsulation (IP-in-IP, minimal encapsulation, and
 GRE), desires a copy of broadcast datagrams on the home network, and
 does not want simultaneous mobility bindings:
 IP fields:
 Source Address = care-of address obtained from DHCP server
 Destination Address = IP address of home agent
 Time to Live = 64
 UDP fields:
 Source Port = <any>
 Destination Port = 434
 Registration Request fields:
 Type = 1
 S=0,B=1,D=1,M=1,G=1
 Lifetime = 1800 (seconds)
 Home Address = the mobile node's home address
 Home Agent = IP address of mobile node's home agent
 Care-of Address = care-of address obtained from DHCP server
 Identification = Network Time Protocol timestamp or Nonce
 Extensions:
 The Mobile-Home Authentication Extension
D.3. Deregistration
 The mobile node returns home and wishes to deregister all care-of
 addresses with its home agent.
 IP fields:
 Source Address = mobile node's home address
 Destination Address = IP address of home agent
 Time to Live = 1
 UDP fields:
 Source Port = <any>
 Destination Port = 434
 Registration Request fields:
 Type = 1
 S=0,B=0,D=0,M=0,G=0
 Lifetime = 0
 Home Address = the mobile node's home address
 Home Agent = IP address of mobile node's home agent
 Care-of Address = the mobile node's home address
 Identification = Network Time Protocol timestamp or Nonce
 Extensions:
 An authorization-enabling extension (e.g., the
 Mobile-Home Authentication Extension)
E. Applicability of Prefix-Lengths Extension
 Caution is indicated with the use of the Prefix-Lengths Extension
 over wireless links, due to the irregular coverage areas provided by
 wireless transmitters. As a result, it is possible that two foreign
 agents advertising the same prefix might indeed provide different
 connectivity to prospective mobile nodes. The Prefix-Lengths
 Extension SHOULD NOT be included in the advertisements sent by agents
 in such a configuration.
 Foreign agents using different wireless interfaces would have to
 cooperate using special protocols to provide identical coverage in
 space, and thus be able to claim to have wireless interfaces situated
 on the same subnetwork. In the case of wired interfaces, a mobile
 node disconnecting and subsequently connecting to a new point of
 attachment, may well send in a new Registration Request no matter
 whether the new advertisement is on the same medium as the last
 recorded advertisement. And, finally, in areas with dense
 populations of foreign agents it would seem unwise to require the
 propagation via routing protocols of the subnet prefixes associated
 with each individual wireless foreign agent; such a strategy could
 lead to quick depletion of available space for routing tables,
 unwarranted increases in the time required for processing routing
 updates, and longer decision times for route selection if routes
 (which are almost always unnecessary) are stored for wireless
 "subnets".
F. Interoperability Considerations
 This document specifies revisions to RFC 2002 that are intended to
 improve interoperability by resolving ambiguities contained in the
 earlier text. Implementations that perform authentication according
 to the new more precisely specified algorithm would be interoperable
 with earlier implementations that did what was originally expected
 for producing authentication data. That was a major source of non-
 interoperability before.
 However, this specification does have new features that, if used,
 would cause interoperability problems with older implementations.
 All features specified in RFC 2002 will work with the new
 implementations, except for V-J compression [20]. The following list
 details some of the possible areas of compatibility problems that may
 be experienced by nodes conforming to this revised specification,
 when attempting to interoperate with nodes obeying RFC 2002.
 - A client that expects some of the newly mandatory features
 (like reverse tunneling) from a foreign agent would still be
 interoperable as long as it pays attention to the `T' bit.
 - Mobile nodes that use the NAI extension to identify themselves
 would not work with old mobility agents.
 - Mobile nodes that use a zero home address and expect to receive
 their home address in the Registration Reply would not work
 with old mobility agents.
 - Mobile nodes that attempt to authenticate themselves without
 using the Mobile-Home authentication extension will be unable
 to successful register with their home agent.
 In all of these cases, a robust and well-configured mobile node is
 very likely to be able to recover if it takes reasonable actions upon
 receipt of a Registration Reply with an error code indicating the
 cause for rejection. For instance, if a mobile node sends a
 registration request that is rejected because it contains the wrong
 kind of authentication extension, then the mobile node could retry
 the registration with a mobile-home authentication extension, since
 the foreign agent and/or home agent in this case will not be
 configured to demand the alternative authentication data.
G. Changes since RFC 2002
 This section details differences between the original Mobile IP base
 specification (RFC 2002 and ff.) that have been made as part of this
 revised protocol specification for Mobile IP.
G.1. Major Changes
 - Specification for Destination IP address of Registration Reply
 transmitted from Foreign Agent, to avoid any possible
 transmission to IP address 0.0.0.0.
 - Specification of two new formats for Mobile IP extensions,
 according to the ideas contained in MIER.
 - Specification that the SPI of the MN-HA authentication
 extension is to be used as part of the data over which the
 authentication algorithm must be computed.
 - Eliminated Van-Jacobson Compression feature
 - Specification that foreign agents MAY send advertisements at a
 rate faster than once per second, but chosen so that the
 advertisements do not burden the capacity of the local link.
 For simplicity, the foreign agent now MAY send advertisements
 at an interval less than 1/3 the advertised ICMP Lifetime.
 - Specification that foreign agents SHOULD support reverse
 tunneling, and home agents MUST support decapsulation of
 reverse tunnels.
 - Changed the preconfiguration requirements in section 3.6 for
 the mobile node to reflect the capability, specified in RFC
 2794 [6], for the mobile node to identify itself by using its
 NAI, and then getting a home address from the Registration
 Reply.
 - Changed section 3.7.3.1 so that a foreign agent is not required
 to discard Registration Replies that have a Home Address field
 that does not match any pending Registration Request.
 - Allowed registrations to be authenticated by use of a security
 association between the mobile node and a suitable
 authentication entity acceptable to the home agent. Defined
 "Authorization-enabling Extension" to be an authentication
 extension that makes a registration message acceptable to the
 recipient. This is needed according to specification in [6].
 - Mandated that HMAC-MD5 be used instead of the "prefix+suffix"
 mode of MD5 as originally mandated in RFC 2002.
 - Specified that the mobile node SHOULD take the first care-of
 address in a list offered by a foreign agent, and MAY try each
 subsequent advertised address in turn if the attempted
 registrations are rejected by the foreign agent
 - Clarification that a mobility agent SHOULD only put its own
 addresses into the initial (i.e., not mobility-related) list of
 routers in the mobility advertisement. RFC 2002 suggests that
 a mobility agent might advertise other default routers.
 - Specification that a mobile node MUST ignore reserved bits in
 Agent Advertisements, as opposed to discarding such
 advertisements. In this way, new bits can be defined later,
 without affecting the ability for mobile nodes to use the
 advertisements even when the newly defined bits are not
 understood. Furthermore, foreign agents can set the `R' bit to
 make sure that new bits are handled by themselves instead of
 some legacy mobility agent.
 - Specification that the foreign agent checks to make sure that
 the indicated home agent address does not belong to any of its
 network interfaces before relaying a Registration Request. If
 the check fails, and the foreign agent is not the mobile node's
 home agent, then the foreign agent rejects the request with
 code 136 (unknown home agent address).
 - Specification that, while they are away from the home network,
 mobile nodes MUST NOT broadcast ARP packets to find the MAC
 address of another Internet node. Thus, the (possibly empty)
 list of Router Addresses from the ICMP Router Advertisement
 portion of the message is not useful for selecting a default
 router, unless the mobile node has some means not involving
 broadcast ARP and not specified within this document for
 obtaining the MAC address of one of the routers in the list.
 Similarly, in the absence of unspecified mechanisms for
 obtaining MAC addresses on foreign networks, the mobile node
 MUST ignore redirects to other routers on foreign networks.
 - Specification that a foreign agent MUST NOT use broadcast ARP
 for a mobile node's MAC address on a foreign network. It may
 obtain the MAC address by copying the information from an Agent
 Solicitation or a Registration Request transmitted from a
 mobile node.
 - Specification that a foreign agent's ARP cache for the mobile
 node's IP address MUST NOT be allowed to expire before the
 mobile node's visitor list entry expires, unless the foreign
 agent has some way other than broadcast ARP to refresh its MAC
 address associated to the mobile node's IP address.
 - At the end of section 4.6, clarified that a home agent MUST NOT
 make any changes to the way it performs proxy ARP after it
 rejects an invalid deregistration request.
 - In section 4.2.3, specification that multihomed home agents
 MUST use the the address sent to the mobile node in the home
 agent field of the registration reply as the source address in
 the outer IP header of the encapsulated datagram.
 - Inserted 'T' bit into its proper place in the Registration
 Request message format (section 3.3).
G.2. Minor Changes
 - Allowed registration replies to be processed by the mobile
 node, even in the absence of any Mobile-Home Authentication
 extension, when containing rejection code by the foreign agent.
 - Specification that the foreign agent MAY configure a maximum
 number of pending registrations that it is willing to maintain
 (typically 5). Additional registrations SHOULD then be
 rejected by the foreign agent with code 66. The foreign agent
 MAY delete any pending Registration Request after the request
 has been pending for more than 7 seconds; in this case, the
 foreign agent SHOULD reject the Request with code 78
 (registration timeout).
 - Relaxation of the requirement that, when a mobile node has
 joined a multicast group at the router on the foreign network,
 the mobile node MUST use its home address as the source IP
 address for multicast packets,
 - Clarification that a mobility agent MAY use different settings
 for each of the 'R', 'H', and 'F' bits on different network
 interfaces.
 - Replacement of the terminology "recursive tunneling" by the
 terminology "nested tunneling".
 - Specification that the mobile node MAY use the IP source
 address of an agent advertisement as its default router
 address.
 - Clarification that keys with arbitrary binary values MUST be
 supported as part of mobility security associations.
 - Specification that the default value may be chosen as 7
 seconds, for allowable time skews between a home agent and
 mobile node using timestamps for replay protection. Further
 specification that this value SHOULD be greater than 3 seconds.
 - Specification that Registration Requests with the 'D' bit set
 to 0, and specifying a care-of address not offered by the
 foreign agent, MUST be rejected with code 77 (invalid care-of
 address).
 - Clarification that the foreign agent SHOULD consider its own
 maximum value when handling the Lifetime field of the
 Registration Reply.
 - Clarification that the home agent MUST ignore the 'B' bit (as
 opposed to rejecting the Registration Request) if it does not
 support broadcasts.
 - Advice about the impossibility of using dynamic home agent
 discovery in the case when routers change the IP destination
 address of a datagram from a subnet-directed broadcast address
 to 255.255.255.255 before injecting it into the destination
 subnet.
 - Clarified that when an Agent Advertisement is unicast to a
 mobile node, the specific IP home address of a mobile node MAY
 be used as the destination IP address.
 - Included a reference to RFC 2290 within appendix B, which deals
 with PPP operation.
 - Created IANA Considerations section
 - In section 3.8.3, clarified that a home agent SHOULD arrange
 the selection of a home address for a mobile node when the
 Registration Reply contains a zero Home Address.
G.3. Changes since revision 04 of RFC2002bis
 This section lists the changes between this version (...-06.txt) and
 the previous version (...-04.txt) of the document. This section can
 be deleted by the RFC editor.
 - Noted that HMAC-MD5 should be considered for use in place of
 the "prefix+suffix" mode of MD5 as originally mandated in RFC
 2002.
 - Included a reference to RFC 2290 within appendix B, which deals
 with PPP operation.
 - Revamped IANA Considerations section
 - Revamped Changes section
 - Replaced Patents section with wording mandated from RFC 2026.
 - Updated citations.
H. Example Messages
H.1. Example ICMP Agent Advertisement Message Format
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type | Code | Checksum |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Num Addrs |Addr Entry Size| Lifetime |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Router Address[1] |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Preference Level[1] |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Router Address[2] |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Preference Level[2] |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | .... |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type = 16 | Length | Sequence Number |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Registration Lifetime |R|B|H|F|M|G|r|T| reserved |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Care-of Address[1] |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Care-of Address[2] |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | .... |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 : Optional Extensions :
 : .... ...... ...... :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
H.2. Example Registration Request Message Format
 The UDP header is followed by the Mobile IP fields shown below:
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type = 1 |S|B|D|M|G|r|T|x| Lifetime |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Home Address |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Home Agent |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Care-of Address |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 + Identification +
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Optional Non-Auth Extensions for HA ... |
 | ( variable length ) |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type =32 | Length | SPI |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | SPI (cont..) | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
 : MN-HA Authenticator ( variable length ) :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 : Optional Non-Auth Extensions for FA .........
 : Optional MN-FA Authentication Extension...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
H.3. Example Registration Reply Message Format
 The UDP header is followed by the Mobile IP fields shown below:
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type = 3 | Code | Lifetime |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Home Address |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Home Agent |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 + Identification +
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Optional HA Non-Auth Extensions ... |
 | ( variable length ) |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type =32 | Length | SPI |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | SPI (cont...) | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
 : MN-HA Authenticator ( variable length ) :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 : Optional Extensions used by FA.........
 : Optional MN-FA Authentication Extension...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
References
 [1] Allman, M., Glover, D. and L. Sanchez, "Enhancing TCP Over
 Satellite Channels using Standard Mechanisms", BCP 28, RFC
 2488, January 1999.
 [2] S. M. Bellovin. Security Problems in the TCP/IP Protocol
 Suite. ACM Computer Communications Review, 19(2), March 1989.
 [3] Border, J., Kojo, M., Griner, J., Montenegro, G. and Z. Shelby,
 "Performance Enhancing Proxies", RFC 3135, June 2001.
 [4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
 Levels", BCP 14, RFC 2119, March 1997.
 [5] Ramon Caceres and Liviu Iftode. Improving the Performance of
 Reliable Transport Protocols in Mobile Computing Environments.
 IEEE Journal on Selected Areas in Communications, 13(5):850--
 857, June 1995.
 [6] Calhoun P. and C. Perkins, "Mobile IP Network Access Identifier
 Extension for IPv4", RFC 2794, January 2000.
 [7] Calhoun, P. and C. Perkins, "Mobile IP Foreign Agent
 Challenge/Response Extension", RFC 3012, December 2000.
 [8] Cong, D., Hamlen, M. and C. Perkins, "The Definitions of
 Managed Objects for IP Mobility Support using SMIv2", RFC 2006,
 October 1996.
 [9] Dawkins, S., Montenegro, G., Kojo, M., Magret, V. and N.
 Vaidya, "End-to-end Performance Implications of Links with
 Errors", BCP 50, RFC 3155, August 2001.
 [10] Deering, S., "ICMP Router Discovery Messages", RFC 1256,
 September 1991.
 [11] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
 1112, August 1989.
 [12] Dommety, G. and K. Leung, "Mobile IP Vendor/Organization-
 Specific Extensions", RFC 3115, April 2001.
 [13] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
 March 1997.
 [14] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
 Recommendations for Security", RFC 1750, December 1994.
 [15] Ferguson P. and D. Senie, "Network Ingress Filtering: Defeating
 Denial of Service Attacks which employ IP Source Address
 Spoofing", BCP 38, RFC 2827, May 2000.
 [16] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic
 Routing Encapsulation (GRE)", RFC 1701, October 1994.
 [17] J. Ioannidis. Protocols for Mobile Internetworking. PhD
 Dissertation - Columbia University in the City of New York,
 July 1993.
 [18] John Ioannidis, Dan Duchamp, and Gerald Q. Maguire Jr. IP-
 Based Protocols for Mobile Internetworking. In Proceedings of
 the SIGCOMM '91 Conference: Communications Architectures &
 Protocols, pages 235--245, September 1991.
 [19] John Ioannidis and Gerald Q. Maguire Jr. The Design and
 Implementation of a Mobile Internetworking Architecture. In
 Proceedings of the Winter USENIX Technical Conference, pages
 489--500, January 1993.
 [20] Jacobson, V., "Compressing TCP/IP headers for low-speed serial
 links", RFC 1144, February 1990.
 [21] Jacobson, V., "Congestion Avoidance and Control. In
 Proceedings, SIGCOMM '88 Workshop, pages 314--329. ACM Press,
 August 1988. Stanford, CA.
 [22] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
 November 1998.
 [23] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
 for Message Authentication", RFC 2104, February 1997.
 [24] McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB",
 RFC 2863, June 2000.
 [25] McGregor, G., "The PPP Internet Protocol Control Protocol
 (IPCP)", RFC 1332, May 1992.
 [26] Mills, D., "Network Time Protocol (Version 3) Specification,
 Implementation", RFC 1305, March 1992.
 [27] Montenegro, G., "Reverse Tunneling for Mobile IP (revised)",
 RFC 3024, January 2001.
 [28] Montenegro, G., Dawkins, S., Kojo, M., Magret, V. and N.
 Vaidya, "Long Thin Networks", RFC 2757, January 2000.
 [29] Montenegro, G. and V. Gupta, "Sun's SKIP Firewall Traversal for
 Mobile IP", RFC 2356, June 1998.
 [30] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
 Considerations Section in RFCs", RFC 2434, October 1998.
 [31] Paxson, V. and M. Allman, "Computing TCP's Retransmission
 Timer", RFC 2988, November 2000.
 [32] Perkins, C., "IP Encapsulation within IP", RFC 2003, October
 1996.
 [33] Perkins, C., "IP Mobility Support", RFC 2002, October 1996.
 [34] Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
 October 1996.
 [35] Perkins, C. and P. Calhoun, "AAA Registration Keys for Mobile
 IP", Work in Progress, July 2001.
 [36] Plummer, D., "Ethernet Address Resolution Protocol: Or
 converting network protocol addresses to 48.bit Ethernet
 address for transmission on Ethernet hardware", STD 37, RFC
 826, November 1982.
 [37] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
 1980.
 [38] Postel, J., "Internet Protocol", STD 5, RFC 791, September
 1981.
 [39] Postel, J., "Multi-LAN Address Resolution", RFC 925, October
 1984.
 [40] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
 1700, October 1994.
 [41] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
 1992.
 [42] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
 1661, July 1994.
 [43] Solomon, J., "Applicability Statement for IP Mobility Support"
 RFC 2005, October 1996.
 [44] Solomon J. and S. Glass, "Mobile-IPv4 Configuration Option for
 PPP IPCP", RFC 2290, February 1998.
 [45] Stevens, W., "TCP/IP Illustrated, Volume 1: The Protocols"
 Addison-Wesley, Reading, Massachusetts, 1994.
Authors' Addresses
 The working group can be contacted via the current chairs:
 Basavaraj Patil
 Nokia
 6000 Connection Dr.
 Irving, TX. 75039
 USA
 Phone: +1 972-894-6709
 EMail: Basavaraj.Patil@nokia.com
 Phil Roberts
 Megisto Corp. Suite 120
 20251 Century Blvd
 Germantown MD 20874
 USA
 Phone: +1 847-202-9314
 EMail: PRoberts@MEGISTO.com
 Questions about this memo can also be directed to the editor:
 Charles E. Perkins
 Communications Systems Lab
 Nokia Research Center
 313 Fairchild Drive
 Mountain View, California 94043
 USA
 Phone: +1-650 625-2986
 EMail: charliep@iprg.nokia.com
 Fax: +1 650 625-2502
Full Copyright Statement
 Copyright (C) The Internet Society (2002). All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works. However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
 Funding for the RFC Editor function is currently provided by the
 Internet Society.