Roxen Community: RFC 3220 IP Mobility Support for IPv4 (Standards Track)

[Text version]

This RFC is obsoleted by: RFC 3344

Network Working Group
Request for Comments: 3220
Obsoletes: 2002
Category: Standards Track C. Perkins, Ed.
Nokia Research Center
January 2002
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 © 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 . . . . . . . . 43 3.6.3. Registration Retransmission . . . . . . . . . . 46 3.7. Foreign Agent Considerations . . . . . . . . . . . . . 46 3.7.1. Configuration and Registration Tables . . . . . 47 3.7.2. Receiving Registration Requests . . . . . . . . 48 3.7.3. Receiving Registration Replies . . . . . . . . 51 3.8. Home Agent Considerations . . . . . . . . . . . . . . . 53 3.8.1. Configuration and Registration Tables . . . . . 54 3.8.2. Receiving Registration Requests . . . . . . . . 55 3.8.3. Sending Registration Replies . . . . . . . . . 58 4. Routing Considerations 61 4.1. Encapsulation Types . . . . . . . . . . . . . . . . . . 61 4.2. Unicast Datagram Routing . . . . . . . . . . . . . . . 61 4.2.1. Mobile Node Considerations . . . . . . . . . . 61 4.2.2. Foreign Agent Considerations . . . . . . . . . 62 4.2.3. Home Agent Considerations . . . . . . . . . . . 63 4.3. Broadcast Datagrams . . . . . . . . . . . . . . . . . . 65 4.4. Multicast Datagram Routing . . . . . . . . . . . . . . 65 4.5. Mobile Routers . . . . . . . . . . . . . . . . . . . . 66 4.6. ARP, Proxy ARP, and Gratuitous ARP . . . . . . . . . . 68 5. Security Considerations 72
5.1. Message Authentication Codes . . . . . . . . . . . . . 72 5.2. Areas of Security Concern in this Protocol . . . . . . 72 5.3. Key Management . . . . . . . . . . . . . . . . . . . . 73 5.4. Picking Good Random Numbers . . . . . . . . . . . . . . 73 5.5. Privacy . . . . . . . . . . . . . . . . . . . . . . . . 73 5.6. Ingress Filtering . . . . . . . . . . . . . . . . . . . 74 5.7. Replay Protection for Registration Requests . . . . . . 74 5.7.1. Replay Protection using Timestamps . . . . . . 74 5.7.2. Replay Protection using Nonces . . . . . . . . 76 6. IANA Considerations 76 6.1. Mobile IP Message Types . . . . . . . . . . . . . . . . 77 6.2. Extensions to RFC 1256 Router Advertisement . . . . . . 77 6.3. Extensions to Mobile IP Registration Messages . . . . . 78 6.4. Code Values for Mobile IP Registration Reply Messages. . . . . . . . . . . . . . . . . . . . . . 78 7. Acknowledgments 79 A. Patent Issues 81 B. Link-Layer Considerations 81 C. TCP Considerations 82 C.1. TCP Timers . . . . . . . . . . . . . . . . . . . . . . 82 C.2. TCP Congestion Management . . . . . . . . . . . . . . . 82 D. Example Scenarios 83 D.1. Registering with a Foreign Agent Care-of Address . . . 83 D.2. Registering with a Co-Located Care-of Address . . . . . 83 D.3. Deregistration . . . . . . . . . . . . . . . . . . . . 84 E. Applicability of Prefix-Lengths Extension 85 F. Interoperability Considerations 85 G. Changes since RFC 2002 86 G.1. Major Changes . . . . . . . . . . . . . . . . . . . . . 86 G.2. Minor Changes . . . . . . . . . . . . . . . . . . . . . 88 G.3. Changes since revision 04 of RFC2002bis . . . . . . . . 90 H. Example Messages 91 H.1. Example ICMP Agent Advertisement Message Format . . . . 91 H.2. Example Registration Request Message Format . . . . . . 92 H.3. Example Registration Reply Message Format . . . . . . . 93 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 97 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 98
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.
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