When a mobile host moves between fixed and roaming state, it is required that it does not change IP address, as this is part of the unique TCP connection identifier. To retain TCP connections across state transfers, it must therefore be possible to change the route of IP packets. We have currently implemented such an IP packet redirection mechanism based on the ARP protocol [Plu82], but this mechanism only supports redirection to other hosts connected to the same LAN. Currently, support for mobile IP is developed by the Internet Engineering Task Force [Per95], and future plans include use of the IETF method of IP packet redirection.
When the mobile host goes from fixed to roaming state, the proxy endpoints of the center are inserted by the following disconnect procedure. The mobile host freezes it's TCP connections and transfers the states of these to the MSG. These states are used to initialize the proxy endpoints at the MSG. The IP packets of the mobile host are then redirected to the MSG, and the MSG restarts the TCP connections of the mobile host. At this point, all responsibility of the mobile host's TCP connections on the wired network are taken care of by the MSG.
When the mobile host goes from roaming to fixed state, it connects directly to the home network, and informs the MSG of it's presence. The mobile host and the MSG need to synchronize the states of the TCP connections. If the MSG has buffered data, these must be delivered and acknowledged by their destinations, before the center point of the connections may be removed. The MSG enters a synchronization mode, where data are sent from the proxy endpoints, but only acknowledgments are accepted. All new data packets are discarded. When the buffers at the MSG are empty, the real endpoints of the TCP connections are in a consistent state, as all data which one endpoint believes acknowledged actually has been acknowledged by the peer. Finally, the IP packets are redirected to the mobile host. At this point the mobile host may again function as a regular workstation.
During the connect and disconnect phase, the mobile host uses a temporary IP address. This allows the mobile host to use TCP/IP protocols for the communication described above without interference from the redirection of IP packets and the freezing of the TCP connections.