speeding up short data transfers
DESCRIPTION
Speeding Up Short Data Transfers. Theory, Architectural Support, and Simulation Results. Yin Zhang, Lili Qiu Cornell University. Srinivasan Keshav Ensim Corporation. NOSSDAV’00, Chapel Hill, NC, June 2000. Outline. Motivation Related Work Theory Architectural Support Simulation Results - PowerPoint PPT PresentationTRANSCRIPT
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Speeding Up Short Data Transfers
Yin Zhang, Lili Qiu
Cornell University
Srinivasan Keshav
Ensim Corporation
NOSSDAV’00, Chapel Hill, NC, June 2000
Theory, Architectural Support, and Simulation Results
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Outline
MotivationRelated WorkTheoryArchitectural SupportSimulation ResultsConclusions and Future Work
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Motivation
Dominance of Web data transfersShort & bursty [Mah97]Small downloading time is important!
Dominance of TCPProblem: Short data transfers interact poorly with TCP !
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TCP/Reno Basics
Slow StartExponential growth in congestion window,Slow: log(n) round trips for n segments
Congestion Avoidance
Linear probing of BW
Fast RetransmissionTriggered by 3 Duplicated ACK’s
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Related Work
P-HTTP [PM94]Avoid repeated probing only for components within the SAME page.
T/TCP [Bra94]Cache connection count, RTT
TCP Control Block Interdependence [Tou97]:Cache cwnd, but large bursts cause losses
Rate Based Pacing [VH97]4K Initial Window [AFP98]Fast Start [PK98, Pad98]
Most similar to our work, but need router support to ensure TCP friendliness
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Our Approach
Directly enter Congestion AvoidanceChoose optimal initial congestion window
A Geometry Problem: Fitting a block to the service rate curve to minimize completion time
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Optimal Initial cwnd
Minimize completion time by having the transfer end at an Epoch boundary.
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Shift Optimization
Minimize initial cwnd while keeping the same integer number of RTT’s
Before optimization: cwnd = 9
After optimization: cwnd = 5
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Effect of Shift Optimization
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TCP/SPAND
Estimate network state by sharing performance information
SPAND: Shared PAssive Network Discovery [SSK97]
Directly enter Congestion Avoidance, starting with the optimal initial cwndAvoid large bursts by pacing
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Implementation Issues
Scope for sharing and aggregation24-bit heuristicnetwork-aware clustering [KW00]
Collecting performance informationNew TCP option, Windmill’s approach, …
Information aggregationSliding window average
Retrieving estimation of network stateExplicit query, active push, …
PacingLeaky bucket based pacing
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Opportunity for Sharing
MSNBC: 90% requests arrive within 5 minutes since the most recent request from the same client network (using 24-bit heuristic)
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Cost for Sharing
MSNBC: 15,000-25,000 different client networks in a 5-minute interval during peak hours (using 24-bit heuristic)
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Simulation Results
MethodologyDownload files in rounds
Performance MetricAverage completion time
TCP flavors consideredreno-ssr: Reno with slow start restartreno-nssr: Reno w/o slow start restartnewreno-ssr: NewReno with slow start restartnewreno-nssr: NewReno w/o slow start restart
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Simulation Topologies
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T1 Terrestrial WAN Link withSingle Bottleneck
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T1 Terrestrial WAN Link withMultiple Bottlenecks
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T1 Terrestrial WAN Link with Multiple Bottlenecks and Heavy
Congestion
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TCP Friendliness (I)Against reno-ssr with 50-ms
Timer
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TCP Friendliness (II)Against reno-ssr with 200-ms
Timer
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Conclusions
TCP/SPAND significantly reduces latency for short data transfers
35-65% compared to reno-ssr / newreno-ssr20-50% compared to reno-nssr / newreno-nssrEven higher for fatter pipes
TCP/SPAND is TCP-friendlyTCP/SPAND is incrementally deployable
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Future Work
Real implementation for TCP/SPANDBetter information aggregation
Exponential decay when there is not enough feedback
Understand pacing for short flows
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Acknowledgement
Brad KarpGeoffrey M. VoelkerVenkata N. Padmanabhan