a simulation based comparison between highspeed tcp and xcp

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

1. Motivation2. Overview of HSTCP and XCP3. Our Results

Utilization, Fairness, TCP-friendliness, Dynamics, Buffering, Deployment

4. Conclusion5. 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:

5.0

loglog

loglog5.0

rate) droppacket :)(( 2

2)(2

LowHigh

Low

WW

WwseHighDecreawb

wpwb

wbwpwwa

TCP HSTCP

a(w) 1 72

b(w) 0.5 0.1

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

SF0

SF1

SFn-1

R0 R1

SR0

SR1

SRm-1

Dumbbell TopologynFlows = variableRTT=100msBW=100Mbps (variable)qType=RED/DropTailqSize=BWDelay

7

Utilization Do HSTCP and XCP achieve high utilization in

high bandwidth-delay networks?

Bottleneck util. vs. variable BW Bottleneck util. vs. variable RTTSETUP RTT=(100ms)

Bottleneck BW=(200Mbps)Qsize=BW*RTT

8

Fairness (Jain’s Index) A fairness index proposed by Jain:

ii

ii

xn

x

xF2

2

)(

• For n flows:- Best Fairness F(x) = 1 - Worst Fairness F(x) = 1/n

Fairness Index vs. Number of FlowsSETUP RTT=100ms

Bottleneck BW=200MbpsQsize=BW*RTT

9

TCP-friendliness HighSpeed TCP flow starves a TCP flow even in

relatively low/moderate bandwidth (e. g. 50Mbps)

Starvation of TCP flow(>10x)

SETUP RTT=100msBottleneck BW=50Mbps

Qsize=BW*RTTQtype=DropTail

Standard TCP flows fair to each

other

1 HSTCP and 1 TCP flow 2 TCP flows

10

Dynamics (Convergence to Fairness)

XCPHSTCP

1. In general, HSTCP takes longer time to converge to fairness than XCP2. HSTCP may take longer time to converge than TCP, too.

Long convergence time to fairness (~4000 RTTs)

Very short convergence time to fairness



A new flow joins at t=40s

TCP

Long convergence time to fairness (~1700 RTTs)

11

Dynamics (Fetching BW)

XCPHSTCP

Takes ~310 RTTs to grab suddenly available bandwidth

HSTCP may rely on additive increase to fetch suddenly available bandwidth like TCP

Immediately grabs suddenly available bandwidth



TCP

Takes ~2550 RTTs to grab suddenly available bandwidth

12

Buffering (Utilization) What is the impact of buffering on

utilization?


NumFlows=10

*Buffer size is normalized to bandwidth-delay product

13

Buffering (Fairness) What is the impact of buffering on fairness?

qSize = 0.1 BW Delay

XCPHSTCP SETUP RTT=100msBottleneck BW=200Mbps

Qsize=0.1*BW*RTTQtype=DropTail/RED

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

Future Work

Evaluate QuickStart Obtain results for TCP friendliness of

XCP Simulate Web traffic Explain specific convergence issues with

HSTCP Analyze and in XCP

Appendix

19

Fairness (Dynamics) Are protocols scalable in terms of

fairness in a homogeneous environment of the future?

XCPHSTCP

Appendix

20

Dynamics (HSTCP and TCP)

Jain’s analysis for HSTCP

“Convergence to fairness” condition holds in HighSpeed TCP

Theta() analysis

For =the angle between a(w) & wb(w)

small quickly fetch available bw but, could cause slow conv. to

fairness

There is a tradeoff between convergence to fairness and efficiency in achieving full utilization.

1)(2

)(2 ),(),(

1

2

2.0

1

22121

wb

wb

w

wwwww DI

Efficiency line

Fairnessline

cwnd1

cwnd2

(w1,w2)

DI

HSTCP

TCP

Appendix

21

Theta analysis (Details)

Appendix

In order to converge to fairness fast:

Efficiency line

Fairnessline

cwnd1

cwnd2

(w1,w2)

DI

1. The larger D, the better

2. The smaller I, the better (I>45 always)

3. Combining 1 & 2, maximize tanD / tanI.

11

22tanwbw

wbwD

1))(2()(

))(2()(

))(2()(2078.0

))(2()(2078.0

)(

)(tan

218.0

1

128.0

2

212.1

221

122.1

122

1

2

wbwbw

wbwbw

wbwbww

wbwbww

wa

waI

1)(2

)(2

tan

tan

1

2

2.0

1

2

wb

wb

w

w

I

D

a simulation based comparison between highspeed tcp and xcp

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