MO B LEc。 M PUTIN C mobile nodes network interface. In this sit uation, the mobile node should begin to FA advertises service hunt for a new care-of address, or possibly se a care-of address known from advertise ments it is still receiving. The mobile node MH requests service may choose to wait for another advertise- FA relays request to HA ment if it has not received any recently adver tised care-of addresses, or it may send an nt Registering the Care-of Address Once a mobile node has a care-of address FA relays status to MH HA accepts or denies its home agent must find out about it. Fig- ure 1 shows the registration process defined by Mobile IP for this purpose. The process begins when the mobile node, possibly with the assistance of a foreign agent, sends a reg Figure 1. Registration operations in Mobile IP. FA is foreign agent, HA is istration request with the care-of address home address and MH is mobile host information. When the home agent receives this request, it(typically) adds the necessary information to its routing table, approves Home agents use agent advertisements to make them- the request, and sends a registration reply back to the mobile selves known, even if they do not offer any care-of address- node. Although the home agent is not required by the es. However, it is not possible to associate preferences to the Mobile IP protocol to handle registration requests by updat various care-of addresses in the router advertisement, as is ing entries in its routing table, doing so offers a natural the case with default routers. The IETF working group was implementation strategy, and all implementations I know concerned that dynamic preference values might destabilize of take this approach the operation of Mobile IP. Because no one could defend static preference assignments except for backup mobility Authentication. Registration requests contain parameters agents, which do not help distribute the routing load, the and flags that characterize the tunnel through which the group eventually decided not to use the preference assign- home agent will deliver packets to the care-of address. Tun ments with the care-of address list nels can be constructed in various ways, described briefly in Thus, an agent advertisement performs the following the next section. 0. I When a home agent accepts the functions. request, it begins to associate the home address of the mobile node with the care-of address and maintains this associa allows for the detection of mobility agents tion until the registration lifetime expires. The triplet that a lists one or more available care-of addresses contains the home address, care-of address, and registration forms the mobile node about special features provided lifetime is called a binding for the mobile node. a registra by foreign agents, for example, alternative encapsulation tion request can be considered a binding update sent by the a lets mobile nodes determine the network number and mobile node techniques a binding update is an example of a remote redirect, status of their link to the internet and because it is sent remotely to the home agent to affect the lets the mobile node know whether the agent is a home home agents routing table. This view of registration makes agent, a foreign agent, or both, and therefore whether it the need for authentication very clear. 2 The home agent is on its home network or a foreign network must be certain registration was originated by the mobile node and not by some other malicious node pretending to Mobile nodes use router solicitations as defined in rfc 1256 be the mobile node. a malicious node could cause the home to detect any change in the set of mobility agents available at agent to alter its routing table with erroneous care-of address the current point of attachment. (In Mobile IP this is then information, and the mobile node would be unreachable to termed agent solicitation. If advertisements are no longer all incoming communications from the Internet. detectable from a foreign agent that previously had offered a The need to authenticate registration information has care-of address to the mobile node, the mobile node should played a major role in determining the acceptable design presume that foreign agent is no longer within range of the parameters for Mobile IP. Each mobile node and home agent JanUarY.feBruAry1998http://computer.org/internet/ IEEE INTERNET COMPUTINGHome agents use agent advertisements to make them￾selves known, even if they do not offer any care-of address￾es. However, it is not possible to associate preferences to the various care-of addresses in the router advertisement, as is the case with default routers. The IETF working group was concerned that dynamic preference values might destabilize the operation of Mobile IP. Because no one could defend static preference assignments except for backup mobility agents, which do not help distribute the routing load, the group eventually decided not to use the preference assign￾ments with the care-of address list. Thus, an agent advertisement performs the following functions: ■ allows for the detection of mobility agents; ■ lists one or more available care-of addresses; ■ informs the mobile node about special features provided by foreign agents, for example, alternative encapsulation techniques; ■ lets mobile nodes determine the network number and status of their link to the Internet; and ■ lets the mobile node know whether the agent is a home agent, a foreign agent, or both, and therefore whether it is on its home network or a foreign network. Mobile nodes use router solicitations as defined in RFC 1256 to detect any change in the set of mobility agents available at the current point of attachment. (In Mobile IP this is then termed agent solicitation.) If advertisements are no longer detectable from a foreign agent that previously had offered a care-of address to the mobile node, the mobile node should presume that foreign agent is no longer within range of the mobile node’s network interface. In this sit￾uation, the mobile node should begin to hunt for a new care-of address, or possibly use a care-of address known from advertise￾ments it is still receiving. The mobile node may choose to wait for another advertise￾ment if it has not received any recently adver￾tised care-of addresses, or it may send an agent solicitation. Registering the Care-of Address Once a mobile node has a care-of address, its home agent must find out about it. Fig￾ure 1 shows the registration process defined by Mobile IP for this purpose. The process begins when the mobile node, possibly with the assistance of a foreign agent, sends a reg￾istration request with the care-of address information. When the home agent receives this request, it (typically) adds the necessary information to its routing table, approves the request, and sends a registration reply back to the mobile node. Although the home agent is not required by the Mobile IP protocol to handle registration requests by updat￾ing entries in its routing table, doing so offers a natural implementation strategy, and all implementations I know of take this approach. Authentication. Registration requests contain parameters and flags that characterize the tunnel through which the home agent will deliver packets to the care-of address. Tun￾nels can be constructed in various ways, described briefly in the next section.10,11 When a home agent accepts the request, it begins to associate the home address of the mobile node with the care-of address, and maintains this associa￾tion until the registration lifetime expires. The triplet that contains the home address, care-of address, and registration lifetime is called a binding for the mobile node. A registra￾tion request can be considered a binding update sent by the mobile node. A binding update is an example of a remote redirect, because it is sent remotely to the home agent to affect the home agent’s routing table. This view of registration makes the need for authentication very clear.12 The home agent must be certain registration was originated by the mobile node and not by some other malicious node pretending to be the mobile node. A malicious node could cause the home agent to alter its routing table with erroneous care-of address information, and the mobile node would be unreachable to all incoming communications from the Internet. The need to authenticate registration information has played a major role in determining the acceptable design parameters for Mobile IP. Each mobile node and home agent MOBILE COMPUTING 62 JANUARY • FEBRUARY 1998 http://computer.org/internet/ IEEE INTERNET COMPUTING . FA FA HA FA FA advertises service MH requests service FA relays request to HA FA relays status to MH HA accepts or denies Figure 1. Registration operations in Mobile IP. FA is foreign agent, HA is home address, and MH is mobile host