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1. Introduction
IP version 4 assumes that a node’s IP address uniquely identifies the node’s point of attachment to the Internet.
Therefore, a node must be located on the network indicated by its IP address in order to receive datagrams destined to it;
otherwise, datagrams destined to the node would be undeliverable.
For a node to change its point of attachment without losing its ability to communicate, currently one of the two following mechanisms must typically be employed:
a) the node must change its IP address whenever it changes its point of attachment, or
b) host-specific routes must be propagated throughout much of the Internet routing fabric.
Both of these alternatives are often unacceptable.
The first makes it impossible for a node to maintain transport and higher-layer connections when the node changes location.
The second has obvious and severe scaling problems, especially relevant considering the explosive growth in sales of notebook (mobile) computers.
A new, scalable, mechanism is required for accommodating node mobility within the Internet.
This document defines such a mechanism, which enables nodes to change their point of attachment to the Internet without changing their IP address.
Changes between this revised specification for Mobile IP and the original specifications (see [33, 32, 34, 43, 8]) are detailed in the appendix section G.

1.1. Protocol Requirements
A mobile node must be able to communicate with other nodes after changing its link-layer point of attachment to the Internet, yet without changing its IP address.
A mobile node must be able to communicate with other nodes that do not implement these mobility functions.
No protocol enhancements are required in hosts or routers that are not acting as any of the new architectural entities introduced in Section 1.5.
All messages used to update another node as to the location of a mobile node must be authenticated in order to protect against remote redirection attacks.

1.2. Goals
The link by which a mobile node is directly attached to the Internet may often be a wireless link.
This link may thus have a substantially lower bandwidth and higher error rate than traditional wired networks.
Moreover, mobile nodes are likely to be battery powered, and minimizing power consumption is important.
Therefore, the number of administrative messages sent over the link by which a mobile node is directly attached to the Internet should be minimized, and the size of these messages should be kept as small as is reasonably possible.

1.3. Assumptions
The protocols defined in this document place no additional constraints on the assignment of IP addresses.
That is, a mobile node can be assigned an IP address by the organization that owns the machine.
This protocol assumes that mobile nodes will generally not change their point of attachment to the Internet more frequently than once per second.
This protocol assumes that IP unicast datagrams are routed based on the destination address in the datagram header (and not, for example, by source address).

1.4. Applicability
Mobile IP is intended to enable nodes to move from one IP subnet to another.
It is just as suitable for mobility across homogeneous media as it is for mobility across heterogeneous media.
That is, Mobile IP facilitates node movement from one Ethernet segment to another as well as it accommodates node movement from an Ethernet segment to a wireless LAN, as long as the mobile node’s IP address remains the same after such a movement.
One can think of Mobile IP as solving the "macro" mobility management problem.
It is less well suited for more "micro" mobility management applications -- for example, handoff amongst wireless transceivers, each of which covers only a very small geographic area.
As long as node movement does not occur between points of attachment on different IP subnets, link-layer mechanisms for mobility (i.e., link-layer handoff) may offer faster convergence and far less overhead than Mobile IP.

1.5. New Architectural Entities
Mobile IP introduces the following new functional entities:

Mobile Node

A host or router that changes its point of attachment from one network or subnetwork to another.
A mobile node may change its location without changing its IP address;
it may continue to communicate with other Internet nodes at any location using its (constant) IP address, assuming link-layer connectivity to a point of attachment is available.

Home Agent

A router on a mobile node’s home network which tunnels datagrams for delivery to the mobile node when it is away from home, and maintains current location information for the mobile node.

Foreign Agent
A router on a mobile node’s visited network which provides routing services to the mobile node while registered.
The foreign agent detunnels and delivers datagrams to the mobile node that were tunneled by the mobile node’s home agent.
For datagrams sent by a mobile node, the foreign agent may serve as a default router for registered mobile nodes.

A mobile node is given a long-term IP address on a home network.
This home address is administered in the same way as a "permanent" IP address is provided to a stationary host.
When away from its home network, a "care-of address" is associated with the mobile node and reflects the mobile node’s current point of attachment.
The mobile node uses its home address as the source address of all IP datagrams that it sends, except where otherwise described in this document for datagrams sent for certain mobility management functions (e.g., as in Section 3.6.1.1).

1.6. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [4].
In addition, this document frequently uses the following terms:

Authorization-enabling extension

An authentication which makes a (registration) message acceptable to the ultimate recipient of the registration message. An authorization-enabling extension MUST contain an SPI.

In this document, all uses of authorization-enabling extension refer to authentication extensions that enable the Registration Request message to be acceptable to the home agent.
Using additional protocol structures specified outside of this document, it may be possible for the mobile node to provide authentication of its registration to the home agent, by way of another authenticating entity within the network that is acceptable to the home agent (for example, see RFC 2794 [6]).

Agent Advertisement
An advertisement message constructed by attaching a special Extension to a router advertisement [10] message.

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

Care-of Address

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

Correspondent Node
A peer with which a mobile node is communicating.
A correspondent node may be either mobile or stationary.

Foreign Network
Any network other than the mobile node’s Home Network.

Gratuitous ARP
An ARP packet sent by a node in order to spontaneously cause other nodes to update an entry in their ARP cache [45].
See section 4.6.

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

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

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

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

Mobility Agent
Either a home agent or a foreign agent.

Mobility Binding
The association of a home address with a care-of address, along with the remaining lifetime of that association.

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

Node
A host or a router.

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

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

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

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

Visited Network
A network other than a mobile node’s Home Network, to which the mobile node is currently connected.

Visitor List
The list of mobile nodes visiting a foreign agent.

1.7. Protocol Overview

The following support services are defined for Mobile IP:

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

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

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

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

- Mobility agents (i.e., foreign agents and home agents) advertise their presence via Agent Advertisement messages (Section 2).
A mobile node may optionally solicit an Agent Advertisement message from any locally attached mobility agents through an Agent Solicitation message.
- A mobile node receives these Agent Advertisements and determines whether it is on its home network or a foreign network.
- When the mobile node detects that it is located on its home network, it operates without mobility services.
If returning to its home network from being registered elsewhere, the mobile node deregisters with its home agent, through exchange of a Registration Request and Registration Reply message with it.
- When a mobile node detects that it has moved to a foreign network, it obtains a care-of address on the foreign network.
The care-of address can either be determined from a foreign agent’s advertisements (a foreign agent care-of address), or by some external assignment mechanism such as DHCP [13] (a colocated 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 colocated 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.

Figure 1: Operation of Mobile IPv4
1) Datagram to mobile node arrives on home network via standard IP routing.
2) Datagram is intercepted by home agent and is tunneled to the care-of address.
3) Datagram is detunneled and delivered to the mobile node.
4) For datagrams sent by the mobile node, standard IP routing delivers each to its destination.
In this figure, the foreign agent is the mobile node’s default router.

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.