Understanding the interaction of TCP retransmissions and link layer ARQ mechanisms in a wireless environment

Abstract

TCP, the widely used transport protocol in the Internet, assumes that packet losses are because of congestion. Wireless networks have higher error rates than wired networks; TCP misinterprets the error losses as congestion and wrongly invokes congestion control algorithms. Link layer ARQ mechanisms can be used to hide the losses from the TCP sender, thereby improving the throughput performance of TCP. Such link layer mechanisms have been criticized in the past citing adverse interaction between TCP retransmissions and link layer retransmissions. The adverse interaction is due to TCP timing out when the link layer is trying to recover and reordered packet delivery at the link layer leading to fast retransmission by the TCP sender. The latter problem can be solved by employing packet reordering. This leaves us with avoiding TCP timeouts when link layer recovery is attempted. It is found that TCP timeout behaviour is affected by TCP timer granularity, queueing delay, congestion window size and inaccurate retransmission timer implementation. A systematic study of the performance of TCP with link level retransmissions is made considering these factors. Finally, TCP throughput achieved with link layer ARQ mechanism is compared with SNOOP.