P48/1 Minimizing Mobile IP Handoff Latency Ali Diab, Andreas Mitschele-Thiel {ali.diab|mitsch}@tu-ilmenau.de Ilmenau University of Technology, Chair for Integrated HW/SW-Systems Abstract Handoff latencies affect the service quality of real-time applications of mobile users. With Mobile IP (MIP), the handoff latency highly depends on the distance, i.e. delay, between Home Agent (HA) and Foreign Agent (FA). Hierarchical MIP (HMIP) minimizes handoff latencies but depends on additional network elements introduced on the path between HA and FA. The need for additional network elements contradicts with the principles of the Internet and impedes the wide use of HMIP in a world-wide network. In the paper, we propose an alternative to HMIP to reduce handoff latencies, which does not need additional network elements beyond the FA and HA already known from MIP. The proposal is based on a new technique that uses local authentication with the new FA independent of the re-authentication with the HA. With our approach some functions previously implemented by the HA are additional supported by the FA. Thus, the MN can quickly resume the transmission in up- and downlink after an handoff. We describe the modified MIP protocol and provide a simple performance analysis comparing the handoff delay of MIP, HMIP and our approach. The comparison shows that the performance of our proposal is comparable to the performance of HMIP. Keywords : Mobility Management, Mobile IP, Wireless LAN. I Introduction Public Wireless LANs (WLANs) show a steady growth in the Internet. IEEE 802.11 (the de- facto standard for WLANs) provides services at low cost. However, as the user mobility increases, the small cell size of WLANs will induce frequent handoffs causing inevitable delays. When the Mobile Node (MN) notices that the current Access Point (AP) is no longer reachable, it performs a handoff as follows: 1- When the MN moves to a new AP belonging to the same subnet, it performs a Layer2-Handoff (L2-HO) following these procedures: a. Discovery of the available AP and Layer2 authentication. b. Reassociation, and 802.11i authentication. 2- When the MN moves to a new AP belonging to another subnet, it performs a Layer3- Handoff (L3-HO) in addition to the L2-HO following the procedures: a. Discovery of the new Foreign Agent (FA). b. Registration and authentication with the Home Agent (HA) / Gateway Foreign Agent (GFA). In order to implement Layer3-Handoffs, several protocols ha ve been proposed. With Mobile IP Version 4 (MIPv4) [Per98], [Per02], [Gon98] the MN must be registered and authenticated by the HA every time it moves from one subnet to another. This introduces extra latency to the communication, especially when the HA is far away from the FA. The generation of secret keys for the security association between HA and FA, and/or between FA and MN is another reason for latency. This is optional with MIP and depends on the configuration. However, these keys are mandatory for some extensions of MIP (e.g. hierarchical MIP, route optimisation for MIP) [BA00], [PR01]. In order to avoid these sources for extra latency, an approach to use an Anchor FA (AFA) has been proposed [DY00]. If the MN is away from the home network, it will be initially registered by the HA. During this registration a shared secret between MN and FA (K MN-FA ) is established. The FA then acts as AFA. Thus, in subsequent registrations, the MN is registered at this AFA instead of the HA as long as it remains in the same domain, which the AFA belongs to. In this approach there is no need to generate more secret keys to authenticate the MN, and no need to establish a tunnel between HA and FA. Instead, an additional tunnel from the AFA to the current FA is established. However, the forwarding delay on the downlink as well as the uplink, i.e. the path from HA via AFA and current FA to MN, increases compared to MIP. Additional tunnels are needed if smooth