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 network
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
colocated care-of address, the mobile node can use regular router advertisements for the
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 also carry
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.
Agent Advertisement Over Dynamic Interfaces
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 daemon starts. Advertisement over dynamic interfaces
is useful for applications that 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 solicitation 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.