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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