Hierarchical Home Agent Architecture for Mobile IP Communications Imed Romdhani (1) , Jose Munoz (1) , Hatem Bettahar (2) , Abdelmadjid Bouabdallah (2) Napier University (1) School of Computing 10 Colinton Road Edinburgh, EH10 5DT UK (+44) 0131 455 2726 {I.Romdhani, J.Munoz}@napier.ac.uk Université de Technologie de Compiègne (2) HeuDiaSyc, CNRS UMR 6599 BP 20529 - 60205 Compiègne Cedex. France. (+33) 03 44 23 44 23 {bettahar, bouabdal}@utc.fr Abstract While the Mobile IP protocol does not exclude the use of multiple Home Agents (HAs), it does not impose any particular model either. Recent solutions propose that a mobile node uses multiple HAs located in different physical links in his home network. However, no architectural scheme is proposed either for unicast or multicast communications. In this paper, we propose a Hierarchical Home Agent architecture in which each Home Agent (HA) is assigned a specific topological level in its domain. The mobile node is notified about such hierarchy and chooses the closest HA on a per-connection basis. Our primary goal is to optimise both unicast and multicast routing for mobile nodes. We prove that our solution avoids redundant multicast traffic in the home domain and it is easy to implement using the current Mobile IP specification without extra cost. 1. Introduction IP mobility allows mobile hosts to continue receiving services while moving amongst different IP networks. While moving, IP address changes are kept transparent to the higher layers in order to avoid their possible temporal disconnection. Thus, to guarantee the transparency of the handover, the IP routes between a mobile node (MN) and its correspondent node (CN), which is the mobile host’s communication endpoint, should be updated to reflect the current location of the MN. This requires special routes between MN and its CN as the MN’s Home Address (HoA) can no longer be used with the conventional IP routing protocols to deliver packets to the MN’s current location. This is because the HoA is topologically incorrect and cannot be used in foreign networks due to ingress filtering restrictions. For a highly-dynamic MN, the special route should be updated regularly, such by the MN itself, or by a third party. Depending on the location update and notification mechanisms, different mobility management schemes were proposed by the Internet community to support unicast communication. Some schemes were designated to handle IP mobility of IPv4-based mobile host [1], where others were developed for IPv6 [2]. While the Mobile IP protocols do not exclude the use of multiple Home Agents (HA), they do not impose any particular model either. Recent solutions have proposed that a mobile node uses multiple HAs located in different physical links. These solutions have been designed to solve different problems, such as: binding update load balancing between HAs [9] [14] [18]; fault