Mobile IP is both part of IPv4 and IPv6 (RFC 3344, RFC 2004) and enables a host device to roam different networks regardless of the access technology identified by a single fixed IP address, without the need for user intervention. Mobile IP uses protocol number 55, which is supported by the gre pseudo-device on OpenBSD. The implications are vast and the applications obvious?from cell phones over PDAs or handhelds to notebooks accessing the Internet via 802.11 Wi-Fi, GPRS, UMTS, Bluetooth, or other emerging access technologies. The challenge essentially is to provide routing reachability for roaming users/devices in combination with adequate security measures.
RFC 2002 introduces the concept of "mobile nodes" identified by care-of address, home agents, and foreign agents. The connection between a foreign and a home agent occurs via a virtual secure point-to-point tunnel of some sort (IP-IP or GRE). The mobile IP connection setup includes agent discovery (foreign and home) by the mobile user device, registration (with the foreign and the home agent), and the actual reverse tunneling from the home agent to the foreign agent. Both agents advertise their service via an extended version of the Internet Router Discovery Protocol (IRDP) to potential roaming customers. As in plain IRDP operation, the mobile node can send out agent solicitations and trigger agent advertisements. RFC 3519, "Mobile IP Traversal of Network Address Translation (NAT) Devices," covers this subject in detail.