Window-Based Error Recovery and Flow Control with a Slow Acknowledgement Channel: A Study of TCP/IP Performance

T. V. Lakshman, Upamanyu Madhow, Bernhard Suter

Abstract: With the envisaged growth in Internet access services over networks with asymmetric links such as asymmetric digital subscriber line (ADSL) and hybrid fiber coax (HFC), it becomes crucial to evaluate the performance of window-based protocols over systems in which the reverse link is considerably slower than the forward link. Even if the actual bandwidth asymmetry is moderate, high effective asymmetries can result because of bidirectional traffic. Our objective is to determine, whether TCP/IP performs reasonably in a setting in which the reverse link is the primary bottleneck. Our main results are as follows. (1) For both the prevalent Tahoe version with Fast Retransmit and the Reno version of TCP we determine the throughput as a function of buffering, round-trip times and normalized asymmetry (taken to be the ratio of the transmission time of ACKs in the reverse path to that of data packets in the forward path). We identify three modes of operation which are dependent on the forward buffer sizes and the normalized asymmetry. (2) Asymmetry increases the TCP's already high sensitivity to random packet losses that might be caused by transient bursts in real-time traffic. Specifically, random loss leads to significant throughput deterioration when the product of the loss probability, the asymmetry and the square of the bandwidth delay product is large. (3) Congestion in the reverse path adds considerably to the TCP's unfairness when multiple connections share the reverse link. Link bandwidth sharing is unfair even for connections with identical round-trip times and hence use of per connection buffer allocation on the reverse path appears essential.