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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 CorporationUCLA Computer Science Department

IEEE Symp on Computers & Communications (ISCC), 2004

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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

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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

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Introduction

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

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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

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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

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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

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TCPW-LP

Early Window Reduction (EWR) Congestion window reduction scheme

Dynamic Threshold Adjustment Foreground Traffic Ratio, r

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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

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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

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Dynamic Threshold Adjustment

Foreground Traffic Ratio (FTR), r Ratio of Temporal Minimum Queuing Delay to

Average Queuing Delay When all queued packets belong to foregroun

d traffic r approaches 1

only background flows minimum queuing delay is small due to EWR average queuing delay grows according to the b

acklog threshold

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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

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Performance Evaluation

Simulation Topology

End-to-end round trip propagation delay = 74ms

FIFO queuing with drop tail discipline

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Coexistence with foreground traffic

Throughput

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Coexistence with foreground traffic

Congestion Window Behavior

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Coexistence with foreground traffic

Completion time evaluation using FTP traffic

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Coexistence with foreground traffic

Effect of packet losses

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Comparison of TCPW-LP and TCP-LP Throughput

20 identical flows TCP-LP flows utilize only 68% of the link

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Comparison of TCPW-LP and TCP-LP Effect of packet losses

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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

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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 an

d TCP-LP Protocol between ordinary TCP and TCPW-LP

/TCP-LP Receiver-side only modification scheme

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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