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 * RTT min ) 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*RTT min CWIN = amount of outstanding packets in the path BWE*RTT min = amount of packets in the virtual pipe

10 Early Window Reduction (EWR) The virtual queue length exceeds a threshold CWIN = BWE*RTT min – BWE*D a D a – the average queuing delay BWE*D a – 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, Q th = M(1-r) M = 3 (upper bound on backlogged packets) FTR, r = D m /(D a +δ) D m = αD m + (1-α) D min D a = αD a + (1-α) D avg α= 3/4 δ= 3x10 -6 /(RTT-RTT min ), 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