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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