On ACK Filtering on a Slow Reverse Channel

ACK filtering has been proposed as a technique to alleviate the congestion at the input of a slow channel located on the reverse path of a TCP connection. Old ACKs waiting at the input of the slow channel are erased when new ACKs are to be queued. In the literature the case of one-ACK per connection at a time has been studied. In this paper we show that this latter scheme is too aggressive for short transfers where ACKs arrive in bursts due to the slow start phase, and where the TCP source needs to receive the maximum possible number of ACKs in order to quickly increase its congestion window. We study first static filtering where a fixed ACK queue length is allowed. We show analytically how this length needs to be chosen. We present then some mechanisms that adapt the filtering of ACKs as a function of the slow channel utilization rather than the ACK queue length. These mechanisms provide a good compromise between reducing the ACK queueing delay and passing to TCP source a large number of ACKs that guarantee a fast window increase.