Service Differentiation at Transport Layer via TCP Westwood Low-Priority (TCPW-LP)

1. Service Differentiation at Transport Layer via TCP Westwood Low-Priority (TCPW-LP) H. Shimonishi, M.Y. Sanadidi and M. Geria System Platforms Research Laboratories, NEC Corporation UCLA Computer Science Department IEEE Symp on Computers & Communications (ISCC), 2004

2. Outline • Introduction • TCP Westwood (TCPW) • TCP Westwood Low Priority (TCPW-LP) • Performance Evaluation • Coexistence with foreground traffic • Comparison of TCPW-LP and TCP-LP • Conclusion

3. Introduction • TCP Westwood Low-Priority (TCPW-LP) • An end-to-end “foreground/background” priority scheme • Objectives • Non-intrusive to coexisting foreground traffic • Capable of fully utilizing the unused bandwidth • Capable of fairly sharing with other low-priority flows

4. Introduction • Application • Web objects pre-fetching (cache) • Large bulk transfers, e.g. FTP

5. Introduction • Related Works • DiffServ (proposed by IETF) • Support from the network router is required • End-to-end schemes (TCP-LP and TCP-Nice) • Unused bandwidth cannot be fully utilized • Pre-set queuing threshold is required

6. Background - TCPW • TCPW – a sender-side only modification • Reaction to packet losses • Duplicate ACKs • Reno • CWIN = CWIN/2 • Westwood • CWIN = (BWE * RTTmin) • Timeout expiration • Reno and Westwood • CWIN = 1

7. Background - TCPW • BWE – Bandwidth Estimation • Estimated from the rate of ACK • b = segment size / (ACKtime - lastACKtime) • segment size = average of last n received segment • BWE = αBWE + (1- α)*b • smoothing operator α=0.8

8. TCPW-LP • Early Window Reduction (EWR) • Congestion window reduction scheme • Dynamic Threshold Adjustment • Foreground Traffic Ratio, r

9. Early Window Reduction (EWR) • Limit the backlog over the path • Virtual queue length = CWIN – BWE*RTTmin • CWIN = amount of outstanding packets in the path • BWE*RTTmin = amount of packets in the virtual pipe

10. Early Window Reduction (EWR) • The virtual queue length exceeds a threshold • CWIN = BWE*RTTmin – BWE*Da • Da – the average queuing delay • BWE*Da – the packets backlogged at the bottleneck

11. Dynamic Threshold Adjustment • Foreground Traffic Ratio (FTR), r • Ratio of Temporal Minimum Queuing Delay to Average Queuing Delay • When all queued packets belong to foreground traffic • r approaches 1 • only background flows • minimum queuing delay is small due to EWR • average queuing delay grows according to the backlog threshold

12. Dynamic Threshold Adjustment • Dynamic Threshold, Qth = M(1-r) • M = 3 (upper bound on backlogged packets) • FTR, r = Dm /(Da+δ) • Dm = αDm + (1-α) Dmin • Da = αDa + (1-α) Davg • α= 3/4 • δ= 3x10-6/(RTT-RTTmin), ensuring non-zero delay in the calculation of r

13. Performance Evaluation • Simulation Topology • End-to-end round trip propagation delay = 74ms • FIFO queuing with drop tail discipline

14. Coexistence with foreground traffic • Throughput

15. Coexistence with foreground traffic • Congestion Window Behavior

16. Coexistence with foreground traffic • Completion time evaluation using FTP traffic

17. Coexistence with foreground traffic • Effect of packet losses

18. Comparison of TCPW-LP and TCP-LP • Throughput • 20 identical flows • TCP-LP flows utilize only 68% of the link

19. Comparison of TCPW-LP and TCP-LP • Effect of packet losses

20. Comparison of TCPW-LP and TCP-LP • Coexistence with UDP traffic • On-off UDP traffic • Available Bandwidth = 3.3Mbps(On), 10Mbps(Off) • Average available bandwidth = 6.7Mbps

21. Comments • Some Questions • TCP-LP, one-way delay? • Analytical study of Foreground Traffic Ratio? • Packet loss improvement? TCP Westwood? • Insight • No bandwidth guarantee in both TCPW-LP and TCP-LP • Protocol between ordinary TCP and TCPW-LP/TCP-LP • Receiver-side only modification scheme

22. Conclusion • TCPW-LP – an end-to-end scheme to realize two-class service prioritization • Dynamically adjusting the queuing threshold • Evaluation of its performance by simulation • Comparison of TCPW-LP and TCP-LP