This paper proposes a feedback based rate control mechanism to achieve both smooth traffic and the high presentation quality of encoded video. We observed that the aggregated required bandwidth of uctuating consumption rate may be zero in time scale longer than negotiated at session establishment. We use such a notion in controlling rate. In our mechanism, optimism is found in that we do not make immediate response to high consumption rate. Traffic would not need to be changed by feedback if total aggregated bandwidth until next service round is smaller than the amount of resources negotiated at channel establishment. We show that rate control based on such an optimism guarantees acceptable QoS compared to previous mechanisms.