Our testbed consists of a number of client machines connected to a cluster server. The configuration is shown in Figure 12. Traffic from the clients flows to the front-end (1) and is forwarded to the back-ends (2). Data packets transmitted from the back-ends to the clients bypass the front-end (3).
The front-end of the server cluster is a 300MHz Intel Pentium II based PC with 128MB of memory. The cluster back-end consists of six PCs of the same type and configuration as the front-end. All machines run FreeBSD 2.2.6. A loadable kernel module was added to the OS of the front-end and back-end nodes that implements the TCP single handoff protocol, and, in the case of the front-end, the forwarding module. The clients are seven 166MHz Intel Pentium Pro PCs, each with 64MB of memory.
The clients and back-end nodes in the cluster are connected using switched Fast Ethernet (100Mbps). The front-end and the back-end nodes are equipped with two network interfaces, one for communication with the clients, one for internal communication. Clients, front-end, and back-ends are connected through a single 24-port switch. All network interfaces are Intel EtherExpress Pro/100B running in full-duplex mode.
The Apache-1.3.3 [2] server was used on the back-end nodes. Our client software is an event-driven program that simulates multiple HTTP clients. Each simulated HTTP client makes HTTP requests as fast as the server cluster can handle them.
