Abstract

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.

Keywords: Fast recovery; Fast retransmission; TCP-SACK; Duplicate ACK; Analytical modeling

Article Outline

1. Introduction

2. Assumptions and definitions

3. Multiple packet losses

3.1. Fast retransmission when N_r[j−1]≥k

3.2. Fast retransmission when N_r[j−1]<k

3.3. Validation of analytic results

4. Throughput degradation model

4.1. No packet loss indication

4.2. Multiple packet losses with a single-loss indication

4.3. Multiple packet losses with multiple loss indications

4.4. Calculations of average throughput

4.5. Numerical results versus simulation

5. Conclusions

Appendix A. Appendix

Appendix B. Appendix

Appendix C. Appendix

Appendix D. Appendix

Appendix E. Appendix

Appendix F. Appendix

Appendix G. Appendix

References

Vitae