This paper deals with the TCP bandwidth guarantee problem in a Differentiated Services (Diffserv) network. Several researches proposed a RIO mechanism for the assured service. In RIO, IN packets experience lower drop probability than OUT packets to guarantee the promised bandwidth even under network congestion. However a token bucket marker fails to provide adequate performance to TCP connections because TCP generates bursty packets due to the unique TCP congestion control mechanism. We propose a new marker that uses a data buffer as well as a token buffer. The marker with a data buffer works well with RIO because it smooths TCP traffic. We show that the marker with a data buffer achieves the target throughput better than a marker with a token buffer only. We also show that the optimal data buffer size is proportional to reserved throughput and RTT.
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