A mobile node uses a method that is known as agent discovery to determine the following information:
When the node has moved from one network to another
Whether the network is the home network or a foreign network
The foreign agent care-of address that is offered by each foreign agent on that network
Mobility services that are provided by the mobility agent, advertised as flags, and additional extensions in the agent advertisement
Mobility agents transmit agent advertisements to advertise services on a network. In the absence of agent advertisements, a mobile node can solicit advertisements. This capability is known as agent solicitation. If a mobile node is capable of supporting its own co-located care-of address, the mobile node can use regular router advertisements for the same purposes. See the next section.
Mobile nodes use agent advertisements to determine the current point of attachment to the Internet or to an organization's network. An agent advertisement is an Internet Control Message Protocol (ICMP) router advertisement that has been extended to carry also a mobility agent advertisement extension.
A foreign agent (FA) can be too busy to serve additional mobile nodes. However, a foreign agent must continue to send agent advertisements. Then the mobile node, which is already registered with a foreign agent, knows that the mobile node has not moved out of range of the foreign agent. The mobile node also knows that the foreign agent has not failed. A mobile node that is registered with a foreign agent from which it no longer receives agent advertisements, probably knows that the mobile node can no longer contact that foreign agent.
You can configure the implementation of the foreign agent to send advertisements over dynamically created interfaces. You have options to enable or disable limited unsolicited advertisements over the advertising interfaces. Dynamically created interfaces are defined as only those interfaces that are configured after the mipagent starts. Advertisement over dynamic interfaces is useful for applications that are to support transient mobility interfaces. Moreover, by limiting unsolicited advertisement, network bandwidth might be saved.
Every mobile node should implement agent solicitation. The mobile node uses the same procedures, defaults, and constants for agent solicitation that are specified for solicitation messages of ICMP routers.
The rate that a mobile node sends solicitations is limited by the mobile node. The mobile node can send three initial solicitations at a maximum rate of one per second while the mobile node searches for an agent. After the mobile node registers with an agent, the rate that solicitations are sent is reduced to limit the overhead on the local network.