We will consider three types of functionality that can be associated with a network entity in an environment that supports mobility. Functionality is defined in terms of services that can be provided or are used. The following definition are similar to those found in .
A mobile node, MN, is a host or router that can change its point of attachment from one network or subnetwork to another. A change of location does not necessitate a change of IP address. Every mobile node has a fixed IP address on a fixed home network.
A home agent, HA, is a router or host which is capable of establishing a tunnel from a home network, on which it is located, to a mobile node which shares the same home network but is not physically located at the home network. Tunnels are used for the delivery of datagrams which are addressed to the mobile node's home network IP address.
A foreign agent, FA, is a router which supports the attachment of mobile nodes. In the context of this paper, a foreign agent supports wireless connections and therefore beacon its availability as an agent. A foreign agent provides routing services to any connected, registered mobile node. These services consist of establishing and maintaining a tunnel from a home agent to a mobile node; and as a default router for datagrams sent by the mobile node or for tunneling from the mobile node back to the home agent. Tunnels are unidirectional.
Note, any node may provide more than one type of functionality given that it contains the appropriate hardware and software. Therefore, it is practical to consider entities which function both as a home agent and a foreign agent or both as a foreign agent and a mobile node, etc. Based on the above definitions, it is not reasonable to consider an entity as both a home agent and a mobile node to the same subnet, since one by definition is located in its home network and the other is not. In this context, a node that is connected via a wireless connection directly to its home network is considered to be local and not mobile. For details on proposed mobile standards see .