Equation-Based Congestion Control for Unicast Applications
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Transcript of Equation-Based Congestion Control for Unicast Applications
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Equation-Based Congestion Control for Unicast Applications
Sally Floyd, Mark Handley, Jitendra Padhye & Jorg Widmer
August 2000, ACM SIGCOMM Computer Communication Review, Proceedings of the conference on Applications, Technologies, Architectures, and Protocols for Computer Communication, Volume 30 Issue 4
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Motivation• Smooth adjustment of sending rate
− Respond to congestion slower and less severe
• TCP-friendly − Coexist
TCP-Friendly Rate Control (TFRC)
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Outline• Introduction of TFRC• TCP response function• Protocol features• Simulation and experiments• Conclusion
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TCP-Friendly Rate Control (TFRC)• Equation-based (c.f. window-based of TCP)
− Adjust sending rate according to control equation
− Calculate at sender side with the aid of receiver feedback
• Do not aggressively seek out available bandwidth; increase sending rate slowly in response to a decrease in loss event rate
• Do not halve sending rate upon single loss event; however, do halve in response to several successive loss event
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TFRC• Advantage:
− Smooth-changing sending rate• Disadvantage:
− Slower response to sudden bandwidth increase
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TCP response function • T: sending rating (calculated at sender)• s: packet size (known by sender)• R: round trip time (calculated at sender)• tRTO: timeout value, estimated from R
• p: loss event rate (calculated at receiver)
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TCP response function• SRTT: estimate round trip time
(calculated from receiver feedback)• RTTvar: variance of round trip time
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TCP response function• p is loss event rate instead of packet
loss rate• loss event can consist of several packet
lost within a round-trip time• loss interval is defined as the number
of packets between loss events• use Average Loss interval method
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Average Loss Interval method
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Average Loss Interval method
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Average Loss Interval method• s0 is the most recent loss interval
• when a loss event occurs, s0 becomes s1 and new s0 becomes zero
• ignore s0 unless s0 is large enough to increase the average
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History discounting• problem of average loss interval method:
− slow to respond to a sustained decrease in congestion
• when s0 > twice the average loss interval− reduce the weights of older loss intervals
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TCP response function• If Tactual < Tnew
increase sending rateelse
decrease sending rate
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Slowstart• Reno increase sending rate by 2 for each
round-trip time• rate-based protocol does not have such a
limitation; to prevent overshoot
• Treceived : rate of packets arrived at receiver
• slowstart terminates upon loss occurs
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Protocol features• loss fraction vs loss event fraction
− stable steady-state packet loss rate, difference at most 10%
− multiple packet drops is uncommon in RED, but relatively more common in droptail
− difference diminishes if congestion persists
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Protocol features• increasing transmission rate
− ~0.14 packet/RTT (without history discounting)*
− ~0.22 packet/RTT (with history discounting)*
− no need of explicit control of bursty traffic • response to persistent congestion
− require 4-8 RTT to halve sending rate• response to quiescent senders
*derivation skipped, interested readers may refer to the paper
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Simulation Results
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Simulation Results
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Simulation Results
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Simulation Results
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Long background traffic
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Short background traffic
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Experiment Results
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Experiment Results
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Conclusion• highly varying throughput not suitable
for streaming• TFRC is one of the protocols trying to
cope to it• smoothness and interflow fairness• loss event • do not halve sending rate upon a loss
event• do halve sending rate upon persistent
congestion and more gentle increase in sending rate