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RFC3344 - IP Mobility Support for IPv4

王朝other·作者佚名  2008-05-31
窄屏简体版  字體: |||超大  

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 RFC1256 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 RFC2002 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 RFC2119 [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 RFC2794 [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 RBHFMGrT 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 SBDMGrTx 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 RFC2794 [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 RFC1256 Router Advertisement

RFC1256 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 RFC2002 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 RFC2290 [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 RFC2988 [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 RFC2002 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 RFC2002 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 RFC2002.

- 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 RFC2002

This section details differences between the original Mobile IP base

specification (RFC2002 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 RFC2002.

- 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. RFC2002 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 RFC2290 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 RFCeditor.

- 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 RFC2290 within appendix B, which deals

with PPP operation.

- Revamped IANA Considerations section

- Revamped Changes section

- Replaced Patents section with wording mandated from RFC2026.

- 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 RBHFMGrT 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 SBDMGrTx 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", RFC3135, June 2001.

[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement

Levels", BCP 14, RFC2119, 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", RFC2794, January 2000.

[7] Calhoun, P. and C. Perkins, "Mobile IP Foreign Agent

Challenge/Response Extension", RFC3012, December 2000.

[8] Cong, D., Hamlen, M. and C. Perkins, "The Definitions of

Managed Objects for IP Mobility Support using SMIv2", RFC2006,

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, RFC3155, August 2001.

[10] Deering, S., "ICMP Router Discovery Messages", RFC1256,

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", RFC3115, April 2001.

[13] Droms, R., "Dynamic Host Configuration Protocol", RFC2131,

March 1997.

[14] Eastlake, D., Crocker, S. and J. Schiller, "Randomness

Recommendations for Security", RFC1750, December 1994.

[15] Ferguson P. and D. Senie, "Network Ingress Filtering: Defeating

Denial of Service Attacks which employ IP Source Address

Spoofing", BCP 38, RFC2827, May 2000.

[16] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic

Routing Encapsulation (GRE)", RFC1701, 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", RFC1144, 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", RFC2402,

November 1998.

[23] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing

for Message Authentication", RFC2104, February 1997.

[24] McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB",

RFC2863, June 2000.

[25] McGregor, G., "The PPP Internet Protocol Control Protocol

(IPCP)", RFC1332, May 1992.

[26] Mills, D., "Network Time Protocol (Version 3) Specification,

Implementation", RFC1305, March 1992.

[27] Montenegro, G., "Reverse Tunneling for Mobile IP (revised)",

RFC3024, January 2001.

[28] Montenegro, G., Dawkins, S., Kojo, M., Magret, V. and N.

Vaidya, "Long Thin Networks", RFC2757, January 2000.

[29] Montenegro, G. and V. Gupta, "Sun's SKIP Firewall Traversal for

Mobile IP", RFC2356, June 1998.

[30] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA

Considerations Section in RFCs", RFC2434, October 1998.

[31] Paxson, V. and M. Allman, "Computing TCP's Retransmission

Timer", RFC2988, November 2000.

[32] Perkins, C., "IP Encapsulation within IP", RFC2003, October

1996.

[33] Perkins, C., "IP Mobility Support", RFC2002, October 1996.

[34] Perkins, C., "Minimal Encapsulation within IP", RFC2004,

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, RFC768, August

1980.

[38] Postel, J., "Internet Protocol", STD 5, RFC791, September

1981.

[39] Postel, J., "Multi-LAN Address Resolution", RFC925, October

1984.

[40] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC

1700, October 1994.

[41] Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321, 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"

RFC2005, October 1996.

[44] Solomon J. and S. Glass, "Mobile-IPv4 Configuration Option for

PPP IPCP", RFC2290, 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.

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"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

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Acknowledgement

Funding for the RFCEditor function is currently provided by the

Internet Society.

 
 
 
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