A Simulation Based Comparison Between HighSpeed TCP and XCP

1. A Simulation Based Comparison Between HighSpeed TCP and XCP Jae Wook Lee and Gleb Chuvpilo 6.829 Final Project December 6, 2002

2. Talk at a Glance • Motivation • Overview of HSTCP and XCP • Our Results • Utilization, Fairness, TCP-friendliness, Dynamics, Buffering, Deployment • Conclusion • Future Work

3. Motivation • For a TCP throughput of 10 Gbps: • Need cwnd=80,000 packets • Increase cwnd by 1 packet/RTT  Takes thousands of RTT to ramp up to full utilization! • HighSpeed TCP and XCP are two proposed replacements  Which one is better and when?  What are the tradeoffs?

4. HighSpeed TCP (HSTCP) • Modify TCP response function when cwnd is high to: • The whole point is that a(w) increases and b(w) decreases as cwnd becomes larger. • Example: behavior when cwnd = 80,000 packets:

5. eXplicit Control Protocol (XCP) • Routers provide explicit feedback to senders • No per-flow state is maintained in routers • Decoupled fairness and efficiency controllers • Efficiency controller increases aggregate feedback proportionally to spare bandwidth and decreases proportionally to the persistent queue size • Fairness controller uses AIMD

6. Simulation Topology Dumbbell TopologynFlows = variableRTT=100msBW=100Mbps (variable)qType=RED/DropTailqSize=BWDelay SF0 SR0 SF1 SR1 R0 R1   SFn-1 SRm-1

7. Utilization • Do HSTCP and XCP achieve high utilization in high bandwidth-delay networks? SETUP RTT=(100ms) Bottleneck BW=(200Mbps) Qsize=BW*RTT Bottleneck util. vs. variable BW Bottleneck util. vs. variable RTT

8. Fairness (Jain’s Index) • A fairness index proposed by Jain: • For n flows: • - Best Fairness  F(x) = 1 • - Worst Fairness  F(x) = 1/n SETUP RTT=100ms Bottleneck BW=200Mbps Qsize=BW*RTT Fairness Index vs. Number of Flows

9. Starvation of TCP flow (>10x) TCP-friendliness • HighSpeed TCP flow starves a TCP flow even in relatively low/moderate bandwidth (e. g. 50Mbps) Standard TCP flows fair to each other SETUP RTT=100ms Bottleneck BW=50Mbps Qsize=BW*RTT Qtype=DropTail 2 TCP flows 1 HSTCP and 1 TCP flow

10. Dynamics (Convergence to Fairness) TCP XCP HSTCP Very short convergence time to fairness Long convergence time to fairness (~4000 RTTs) Long convergence time to fairness (~1700 RTTs) SETUP RTT=100ms Bottleneck BW=50Mbps Qsize=BW*RTT Qtype=DropTail A new flow joins at t=40s • In general, HSTCP takes longer time to converge to fairness than XCP • HSTCP may take longer time to converge than TCP, too.

11. Takes ~310 RTTs to grab suddenly available bandwidth Dynamics (Fetching BW) XCP HSTCP TCP Takes ~2550 RTTs to grab suddenly available bandwidth Immediately grabs suddenly available bandwidth SETUP RTT=100ms Bottleneck BW=50Mbps Qsize=BW*RTT Qtype=DropTail HSTCP may rely on additive increase to fetch suddenly available bandwidth like TCP

12. Buffering (Utilization) • What is the impact of buffering on utilization? SETUP RTT=100ms Bottleneck BW=200Mbps NumFlows=10 *Buffer size is normalized to bandwidth-delay product

13. Buffering (Fairness) • What is the impact of buffering on fairness? HSTCP XCP SETUP RTT=100ms Bottleneck BW=200Mbps Qsize=0.1*BW*RTT Qtype=DropTail/RED qSize = 0.1 BW  Delay

14. Effects of Reverse Flows • Two new phenomena present [Zhang, et al] • ACK-compression • Out-of-phase queue-synchronization • Reverse flows degrades min-max fairness (by factor of ~2) of HSTCP flows as well as utilization.

15. Deployment • What are the costs and side effects of deployment? • HSTCP • Easier to deploy – end-to-end protocol, no router support needed. • HighSpeed TCP flows starve TCP flows • XCP: • Harder to deploy – requires router support on the path. • Next generation of IP routers may accommodate XCP with a benefit of smaller buffering.

16. Conclusion • Both HSTCP and XCP demonstrate good utilization for high bandwidth-delay environments. • Fairness index: XCP > HSTCP > TCP, but all close to 1. • HSTCP starves TCP flows even in low-bandwidth networks. • Dynamics tradeoff: convergence to fairness vs. convergence to full utilization. • With reasonably small buffer size of 0.1×BW×Delay, both HSTCP and XCP work well in terms of utilization and fairness. But, XCP outperforms HSTCP for both criteria. • Both HSTCP and XCP face deployment problems: • HSTCP is TCP-unfriendly, • XCP needs router support on the path.

17. Questions?

18. Appendix Future Work • Evaluate QuickStart • Obtain results for TCP friendliness of XCP • Simulate Web traffic • Explain specific convergence issues with HSTCP • Analyze  and  in XCP

19. Appendix Fairness (Dynamics) • Are protocols scalable in terms of fairness in a homogeneous environment of the future? XCP HSTCP

20. Jain’s analysis for HSTCP “Convergence to fairness” condition holds in HighSpeed TCP Theta() analysis For =the angle between a(w) & wb(w) small   quickly fetch available bw but, could cause slow conv. to fairness Appendix cwnd2 Fairness line Efficiency line (w1,w2) I D cwnd1 Dynamics (HSTCP and TCP) There is a tradeoff between convergence to fairness and efficiency in achieving full utilization. TCP HSTCP

21. Appendix cwnd2 Fairness line Efficiency line (w1,w2) I D cwnd1 Theta analysis (Details) • In order to converge to fairness fast: • The larger D, the better • The smaller I, the better (I>45 always) • Combining 1 & 2, maximize tanD / tanI.