Responding to spurious timeouts in TCP

Delays on Internet paths, especially including wireless links, can be highly variable. On the other hand, a current trend for modern TCPs is to deploy a fine-grain retransmission timer with a lower minimum timeout value than 1 s suggested by RFC2988. Spurious TCP timeouts cause unnecessary retransmissions and congestion control back-off. The Eifel algorithm detects spurious TCP timeouts and recovers by restoring the connection state saved before the timeout. This paper presents an enhanced version of the Eifel response to spurious timeouts and illustrates its performance benefits on paths with a high delay-bandwidth product. The refinements concern the following issues (1) an efficient operation in presence of packet losses (2) appropriate restoration of congestion control, and (3) adapting the retransmit timer to avoid further spurious timeouts. In our simulations the Eifel algorithm on paths with a high delay-bandwidth product can increase throughput by up to 250% and at the same decrease the load on the network by 3%. The proposed response also shows adequate performance on heavily congested paths.