The device is authenticated by the home agent, a temporary data tunnel is set up between the home agent and COA, and packets are forwarded to the mobile device via the tunnel. GRE is specified as an Internet standard in IETF RFC 1701. Mobile IP uses tunneling from the home agent to the mobile node's COA, but rarely in the reverse direction, notes Michael DeLeo, Cisco Consulting Engineer. Usually, a mobile node sends its packets through a router on the foreign network and assumes that routing is independent of source address. Backward-compatible extensions to Mobile IP have been defined by the IETF to support topologically correct reverse tunnels and are supported in Cisco's Mobile IP implementation.
Routers acting as home or foreign agents regularly advertise their existence.
If a mobile node picks up its own home agent's advertisement, it knows it is on its home network and does not need to do anything special to receive its datagrams. If a mobile node receives an IRDP advertisement from another mobility agent, it will register its location via a foreign agent or directly with its home agent, which, in turn, authenticates the device. The COA identifies the mobile node's current, topological point of attachment to the Internet and routes packets to the mobile node when it is attached to a different network. In this case, the COA is the IP address of the foreign agent. 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.