larger VPN handoff

achieved. In the real world experiment, a larger VPN handoff time was measured and analysed. This thesis also gives some suggestion of how to reduce the handoff time in the real world which may open a range of new research opportunities. One journal paper and one conference paper related to this thesis have been published: C. Xu and P. Radcliffe, "Building Secure Tunnel from PPP Wireless Network", Wireless Personal Communications, DOI 10.1007/s11277-009-9894-x, 2009.  C. Xu and P. Radcliffe, "A novel mobility solution  based on L2TP/IPsec tunnel",  2009 IEEE Sarnoff Symposium, 2009. 1.3 LITERATURE SEARCH The most common solution to mobility problems in a VPN is to run tunnels, such as L2TP or IPsec, over Mobile IP (double tunneling) [21, 22]. However, that is inefficient due to the overhead of network traffic (see Chapter 8.4). The packet structures of these tunnels are shown below. Figure 1-2 Packet Structures of Double Tunneling Some solutions [19, 53] are to add mobility support only to IPsec. These solutions can only transfer IP packets (Layer 3 packet). If user wants to send Layer 2 packets (such as PPP packets [39]), an IPsec tunnel cannot provide a solution. Furthermore, these solutions lose packets as they do not provide a method for sending old packets to the new IP address, and have relatively lower performance when handling handoff. Synchronization problems may also occur when the IPsec server updates tunnel information and sends IPsec packets at the same time. Some solution [19] also has security problems. A detailed analysis of these solutions will be shown in Chapter 8.4.