An analytical model of fast retransmission and recovery in TCP-SACK

Fast retransmission and recovery are the two most important mechanisms employed by TCP to timely recover lost packets and efficiently improve performance. This paper presents a mathematical model to systematically analyze the characteristics of fast retransmission and recovery in TCP-SACK, while multiple packet losses in a congestion window are possible. One of the significant observations revealed by this paper is that whether or not the lost packets can be fast retransmitted at the ( y + 1 ) -th RTT (Round-Trip Time) round simply depends on N r [ y ] , representing the number of packets following the first lost packet and those packets have successfully arrived at the receiver at the y -th RTT round. For clarification, the analytical model consists of two cases. The first case assumes that N r [ y ] is greater than or equal to a threshold k (the number of duplicate acknowledgments), while the second case investigates the behavior when N r [ y ] is smaller than the threshold k . In the proposed theorems, we show that there is an upper bound in the number of packet losses, above which TCP-SACK may not be able to recover the lost packets successfully. Additionally, we develop a model to study the throughput degradation resulting from multiple packet losses in TCP windows. The analytical results of throughput degradation are verified through OPNET simulation.