Janey C. HoeLaboratory for Computer Science at MIT

노상훈 , 권민혁 @ Pllab

Abstract

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Introduction

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Related Work

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The Congestion Control Scheme

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Slow-start problem

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Fast retransmit problem #1: TimeoutCWND is full

51 is recoveredBy fast retransmit

53 couldn’t recoveredNetwork is idleRetransmission time0ut

Ignored cueMax seq is 54 and Last ack is 53Obviously 53 was lost

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Fast retransmit problem #2: False Fast Retransmit

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Modification: Slow-startUse better initial value of ssthresh

Estimated value is better than arbitrary default value

Estimation methodAt first, initialize as previous default value (=64

seg)During 3 ack (= establishing connection)

Least square estimation of bandwidth-delay product Even if default value is high, CWND not ramp up

riskily9

Modification: Slow-startPrevent multiple packet losses during start-

up period on severe environment

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Modification: Fast retransmit & Fast Recovery #1

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Modification: Fast retransmit & Fast Recovery #2

Fast retransmit phaseSetup(1)set ssthresh = ½ cwndcwnd = 1 segmentsnd_next = snd_una(2) set snd_high = snd_maxsave_cwnd = ssthresh + 1 segment

Fast retransmit phaseSetup(1)set ssthresh = ½ cwndcwnd = 1 segmentsnd_next = snd_una(2) set snd_high = snd_maxsave_cwnd = ssthresh + 1 segment

Upon receiving 3 duplicateACK’s begin fast restransmit Phase

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cwnd=save_cwnd

Modification: Fast retransmit & Fast Recovery #3Modified algorithm can handle multiple

packet losses

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Simulation EnvironmentBottle-neck link

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HOST 1 HOST 2 Switc

h1Switc

h2

10Mbps 10Mbps1600Kbps

50msdelay

Multiple packet losses occurred during start-up period

Simulation result - original

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Graph show new algorithm is better than original algorithm

Simulation result – improved

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ConclusionProposes several possible changes to improve

the start-up behavior of the congestion control scheme.Estimated ssthresh value.Modification Fast retransmit algorithm.

Tested in simulations and over real network.These changes can significantly improve TCP’s

performance during start-up periodBy eliminating the wait for unnecessary timeout

These changes are noticeable in short TCP data transfers

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Reference[1] Janey C. Hoe, Improving the Start-up

Behavior of a Congestion Control Scheme for TCP, Proceedings of the ACM SIGCOMM Conference, 1996

[2] Kevin Fall. Kannan Varadhan, ns Notes and Documentation, UC Berkeley, LBL, USC/ISI, and Xerox PARC

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Supplement 1:About Implementation.

[2] ns Notes and Documentation

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Supplement 2:About Implementation

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Supplement 3:About graphThere is no TCL variable which is contain

current sequence number.

We added new variable “just_seq_” and compiled.

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