In the context of networks offering Differentiated Services (DiffServ), we investigate the effect of acknowledgment treatment on the throughput of TCP connections. We carry out experiments on a testbed offering three classes of service (Premium, Assured and Best-Effort), and different levels of congestion on the data and acknowledgment path. We apply a full factorial statistical design and deduce that treatment of TCP data packets is not sufficient and that acknowledgment treatment on the reverse path is a necessary condition to reach the targeted performance in DiffServ efficiently. We find that the optimal marking strategy depends on the level of congestion on the reverse path. In the practical case where Internet Service Providers cannot obtain such information in order to mark acknowledgment packets, we show that the strategy leading to optimal overall performance is to copy the mark from the respective data packet into returned acknowledgement packets, provided that the affected service class is appropriately provisioned
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