A TWS3CDM based Marking Policy in DiffServ Networks

Previous marking policy for the AF service of TCP traffic in the Diffserv network has no sufficient consideration on the effect of RTT and target rate of TCP connections. In this paper, in order to improve fairness index of TCP flows, we propose the TSW3CDM_FS(Time Sliding Window Three Color Dynamic Marker with Flow Status) based on average transfer rate estimation. The proposed algorithm is based on dynamic marking policy that allocates band-width in proportion to transmission rate of flows. We implement the proposed marking policy and evaluate the performance of the proposed marking policy by a computer simulation using NS2. From simulation results, the TSW3CDM_FS algorithm improves fairness index by comparison with TSW3CM. DiffServ provides a classification of service that based on required quality of service and differential service to flows in each class based on SLA(Service Level Agreement). Nevertheless, differences occur in the occupied bandwidths due to influences of UPN properties, differences of targeted transmission and RTT(Round Trip Time) between TCP flow in actual network environments 1 . Moreover, it has problems of operating like the best effort services during UPN(Under-Provisioned Network) occurrences, where the bandwidth of bottleneck link in the Diffserv domain is smaller than the sum of target transmission rates, which disables differentiated marking. Up until now, great efforts have been placed to improve such issues 2, 3 . In this paper, we propose a mechanism that determines the status of each flow by using the achieved rate and fairness of TCP flow based on average transfer rate estimation. The status of each flow can be classified to High, Steady or Low. "High" status indicates that a flow is served more than other flows in the same class, and "Low" status indicates that a flow get lower service than other flows in the same class. Therefore, different dynamic marking policies can be conducted in accordance with the status of each flow. Since these estimated values are enabled to calculate the bandwidth of the bottleneck link in the Diffserv network, it is possible to detect the UPN conditions without receiving feedback information from the network.