Congestion control on spurious TCP retransmission timeouts

Spurious TCP retransmission timeouts are a considerable problem degrading the TCP efficiency in communication over mobile 2.5G and 3G networks. They cause several unnecessary retransmissions and diminish TCP throughput. F-RTO algorithm can be used to detect spurious TCP RTOs, thus making it possible to avoid unnecessary retransmissions in such occasions and improve TCP performance. In this paper we present a SACK-based enhancement to the F-RTO algorithm that further improves the efficiency of F-RTO in detecting spurious RTOs. Additionally, we evaluate three congestion control alternatives to be applied when spurious TCP RTO is detected under different levels of congestion. The results show that choosing the most efficient congestion control alternative depends on the available link bandwidth, amount of buffering space in the bottleneck router, and the number of parallel TCP connections. If the wireless link has a low bandwidth, sufficient conservativeness at the sender not only prevents congestion losses, but also improves TCP throughput. On a faster link more aggressive congestion control yields better throughput despite of increased number of congestion losses.