This paper tries to tackle the problem of providing retrofitting network QoS in clustered configurations. For this purpose, we designed a QoS manager which runs on each of the internal cluster nodes and controls network I/O of local interface cooperating with peer managers on other nodes towards a certain QoS policy. First, we show the design of control framework, contending that an end-host manager-based mechanism is a desirable approach, which utilizes an end-host oriented network control primitive, Netnice. Second, for flexibility of configuration, we propose object-oriented modeling of the QoS manager with event-handler based configuration mechanism, and show the design of an object-oriented configuration language that allow simple and flexible definition of QoS policies. Lastly, results from two simple experiments with a Web server cluster are analyzed.