Network Working Group C. Perkins, Editor Request for Comments: 2002 IBM Category: Standards Track October 1996 IP Mobility Support 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. 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. Table of Contents 1. Introduction 3 1.1. Protocol Requirements . . . . . . . . . . . . . . . . . . 3 1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 4 1.4. Applicability . . . . . . . . . . . . . . . . . . . . . . 4 1.5. New Architectural Entities . . . . . . . . . . . . . . . 5 1.6. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 1.7. Protocol Overview . . . . . . . . . . . . . . . . . . . . 8 1.8. Specification Language . . . . . . . . . . . . . . . . . 11 1.9. Message Format and Protocol Extensibility . . . . . . . . 12 2. Agent Discovery 14 2.1. Agent Advertisement . . . . . . . . . . . . . . . . . . . 14 2.1.1. Mobility Agent Advertisement Extension . . . . . 16 2.1.2. Prefix-Lengths Extension . . . . . . . . . . . . 18 2.1.3. One-byte Padding Extension . . . . . . . . . . . 19 2.2. Agent Solicitation . . . . . . . . . . . . . . . . . . . 19 2.3. Foreign Agent and Home Agent Considerations . . . . . . . 19 2.3.1. Advertised Router Addresses . . . . . . . . . . . 20
2.3.2. Sequence Numbers and Rollover Handling . . . . . 21 2.4. Mobile Node Considerations . . . . . . . . . . . . . . . 21 2.4.1. Registration Required . . . . . . . . . . . . . . 22 2.4.2. Move Detection . . . . . . . . . . . . . . . . . 22 2.4.3. Returning Home . . . . . . . . . . . . . . . . . 24 2.4.4. Sequence Numbers and Rollover Handling . . . . . 24 3. Registration 24 3.1. Registration Overview . . . . . . . . . . . . . . . . . . 25 3.2. Authentication . . . . . . . . . . . . . . . . . . . . . 26 3.3. Registration Request . . . . . . . . . . . . . . . . . . 26 3.4. Registration Reply . . . . . . . . . . . . . . . . . . . 29 3.5. Registration Extensions . . . . . . . . . . . . . . . . . 32 3.5.1. Computing Authentication Extension Values . . . . 32 3.5.2. Mobile-Home Authentication Extension . . . . . . 33 3.5.3. Mobile-Foreign Authentication Extension . . . . . 33 3.5.4. Foreign-Home Authentication Extension . . . . . . 34 3.6. Mobile Node Considerations . . . . . . . . . . . . . . . 34 3.6.1. Sending Registration Requests . . . . . . . . . . 36 3.6.2. Receiving Registration Replies . . . . . . . . . 40 3.6.3. Registration Retransmission . . . . . . . . . . . 42 3.7. Foreign Agent Considerations . . . . . . . . . . . . . . 43 3.7.1. Configuration and Registration Tables . . . . . . 44 3.7.2. Receiving Registration Requests . . . . . . . . . 44 3.7.3. Receiving Registration Replies . . . . . . . . . 47 3.8. Home Agent Considerations . . . . . . . . . . . . . . . . 49 3.8.1. Configuration and Registration Tables . . . . . . 49 3.8.2. Receiving Registration Requests . . . . . . . . . 49 3.8.3. Sending Registration Replies . . . . . . . . . . 53 4. Routing Considerations 55 4.1. Encapsulation Types . . . . . . . . . . . . . . . . . . . 56 4.2. Unicast Datagram Routing . . . . . . . . . . . . . . . . 56 4.2.1. Mobile Node Considerations . . . . . . . . . . . 56 4.2.2. Foreign Agent Considerations . . . . . . . . . . 57 4.2.3. Home Agent Considerations . . . . . . . . . . . . 58 4.3. Broadcast Datagrams . . . . . . . . . . . . . . . . . . . 59 4.4. Multicast Datagram Routing . . . . . . . . . . . . . . . 60 4.5. Mobile Routers . . . . . . . . . . . . . . . . . . . . . 61 4.6. ARP, Proxy ARP, and Gratuitous ARP . . . . . . . . . . . 62 5. Security Considerations 66 5.1. Message Authentication Codes . . . . . . . . . . . . . . 66 5.2. Areas of Security Concern in this Protocol . . . . . . . 66 5.3. Key Management . . . . . . . . . . . . . . . . . . . . . 67 5.4. Picking Good Random Numbers . . . . . . . . . . . . . . . 67 5.5. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 67 5.6. Replay Protection for Registration Requests . . . . . . . 68 5.6.1. Replay Protection using Timestamps . . . . . . . 68 5.6.2. Replay Protection using Nonces . . . . . . . . . 69 6. Acknowledgments 71
A. Patent Issues 72 A.1. IBM Patent #5,159,592 . . . . . . . . . . . . . . . . . . 72 A.2. IBM Patent #5,148,479 . . . . . . . . . . . . . . . . . . 72 B. Link-Layer Considerations 73 C. TCP Considerations 73 C.1. TCP Timers . . . . . . . . . . . . . . . . . . . . . . . 73 C.2. TCP Congestion Management . . . . . . . . . . . . . . . . 73 D. Example Scenarios 74 D.1. Registering with a Foreign Agent Care-of Address . . . . 74 D.2. Registering with a Co-Located Care-of Address . . . . . . 75 D.3. Deregistration . . . . . . . . . . . . . . . . . . . . . 76 E. Applicability of Prefix Lengths Extension 76 Editor's Address 79 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. 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 188.8.131.52).
1.6. Terminology This document frequently uses the following terms: Agent Advertisement An advertisement message constructed by attaching a special Extension to a router advertisement  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. 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 5.6). 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. 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  (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 "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. - 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 , 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. 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. For example, the figure below 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 the figure below, the mobile node is using a foreign agent 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. +----+ 1.8. Specification Language In this document, several words are used to signify the requirements of the specification. These words are often capitalized. MUST This word, or the adjective "required", means that the definition is an absolute requirement of the specification. MUST NOT This phrase means that the definition is an absolute prohibition of the specification. SHOULD This word, or the adjective "recommended", means that, in some circumstances, valid reasons may exist to ignore this item, but the full implications must be understood and carefully weighed before choosing a different course. Unexpected results may result otherwise. MAY This word, or the adjective "optional", means that this item is one of an allowed set of alternatives. An implementation which does not include this option MUST be prepared to interoperate with another implementation which does include the option.
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. 1.9. Message Format and Protocol Extensibility Mobile IP defines a set of new control messages, sent with UDP  using well-known port number 434. Currently, the following two message types are defined: 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" . In addition, for Agent Discovery, Mobile IP makes use of the existing Router Advertisement and Router Solicitation messages defined for ICMP Router Discovery . 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. Each of these Extensions (with one exception) is encoded in the following Type-Length-Value format: 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 ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 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.
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). Currently, 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 . Currently, Mobile IP defines the following Types 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" . 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. 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.
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  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 , 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 , the IP destination address of an Agent Advertisement MUST be either the "all systems on this link" multicast address (184.108.40.206)  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. - 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 1/3 of the advertisement Lifetime given in the ICMP header. 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  in order to avoid synchronization and subsequent collisions with other Agent Advertisements that may be sent by other agents (or with other Router Advertisements sent by other routers). Note that this field has no relation to the "Registration Lifetime" field within the Mobility Agent Advertisement Extension defined below. 2.1.1. Mobility Agent Advertisement Extension The Mobility Agent Advertisement Extension follows the ICMP Router Advertisement fields. It is used to indicate that an ICMP Router Advertisement message is also an Agent Advertisement being sent by a mobility agent. The Mobility Agent Advertisement Extension is defined as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Registration Lifetime |R|B|H|F|M|G|V| reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | zero or more Care-of Addresses | | ... | Type 16 Length (6 + 4*N), where 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 rather than 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 . G GRE encapsulation. This agent implements receiving tunneled datagrams that use GRE encapsulation . V Van Jacobson header compression. This agent supports use of Van Jacobson header compression  over the link with any registered mobile node. 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, and 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 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 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 , except that: - a mobility agent MUST limit the rate at which it sends broadcast or multicast Agent Advertisements; a recommended maximum rate is once per second, 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. 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. Thus, an agent MAY include one of its own addresses in the advertisement. A foreign agent MAY discourage use of this address as a default router by setting the preference to a low value and by including the address of another router in the advertisement (with a correspondingly higher preference). Nevertheless, 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 reductions in sequence numbers that result from reboots, from reductions that result 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 , 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 .
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. 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. 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. 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.
220.127.116.11. 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. 18.104.22.168. 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 , 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).