A cluster-based server system is a developing technology that could achieve high scalability by using several dispatchers, such as layer-4 or layer-7 switches, to appropriately distribute requests from clients. Many recent Web server systems have been developed as cluster systems, but such systems are so complicated that important information for appropriate distribution decisions is in higher layer (i.e., application layer or layer-7). Although the L7 switches are appliances that can redirect requests by examining the application-layer information, it is difficult to update or modify their distribution algorithms. This paper proposes a novel architecture based on TCP-migration mechanism that provides complete redirection (displacement) of a TCP session from a dispatcher to Web servers. The key idea is physical separation of L7 switch functionality: packet forwarding and request dispatching mechanisms. With NAT mechanism on the L3 switch and sophisticated management of virtual private IP addresses on the cluster servers, the dispatcher is released from relaying or translating both in-bound and out-bound TCP packets after the request has been redirected. This architecture can achieve greater flexibility because the forwarding is performed fast by hardware (i.e., the switch) and the dispatching is managed by software (i.e., application servers). We have designed and implemented this mechanism on Linux 2.4 kernel and evaluated its performance. The experimental results show that the overhead for handling multiple virtual IP addresses is almost negligible. Furthermore, the overhead with TCP-migration by using mini httpd server and wget client is approximately 1 ms, regardless of the reply size, on 3.
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