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Multimedia Streaming via TCP: An Analytic Performance Study

Bing Wang, Jim Kurose, Prashant Shenoy, Don Towsley

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Introduction

Under what circumstances can TCP streaming provide satisfactory performance?

Live video streaming is constrained streaming

Stored video streaming is unconstrained streaming

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Outline

Analytic model Simulation Experiments Effect of parameters on performance

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Contribution of Paper

develop discrete-time Markov models for live and stored video streaming

explore how parameters (i.e. loss rate, round trip time, timeout value & playback rate) affect TCP streaming performance

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Assumption

Average TCP throughput is no less than the video bitrate

Startup delay on the order of seconds Videos are of constant bit rate (CBR)

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Performance metrics

Fraction of late packets no known metric directly for viewing quality

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Notations

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Model for TCP

time unit is round (length of a round = a round trip time)

Xi is the state of model in the ith round Xi = (Wi, Ci, Li, Ei, Ri) Wi = window size Ci = delayed ACK behavior Li = # packets lost in (i-1)th round Ei = backoff exponent if in timeout state Ri = 0 for new packet; =1 for retransmission

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Model for streaming

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Model for constrained streaming Yi is the state of the model in the ith round

Yi = (Xi, Ni)

Xi = state of TCP (mentioned before)

Ni = # early packets

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Model for unconstrained streaming As length go to infinity, # early packets go to i

nfinity and fraction of late packets go to zero use transient analysis Yi is the state of the model in the ith round

Yi = Xi

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Model for unconstrained streaming use impulse reward to obtain transient distrib

ution of Ni

impulse reward = difference % # packets received and played back in transition

Ni’ = accumulation of the impulse reward up to ith round

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Model for unconstrained streaming

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Simulation

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Simulation

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Simulation

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Simulation

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Simulation

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Simulation

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Simulation

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Simulation

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Experiments

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Experiment

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Experiment

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Exploring parameter space

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Effect of video length

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Effect of T/μ

playback rate

T/μ

51 1.4

55 1.3

60 1.2

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Sensitivity to parameters

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Conditions for satisfactory performance

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Summary of results

fraction of late packets increases with length in live streaming, but decreases with length in stored streaming

performance increases with T/μ; beyond a certain point yields diminishing gain

performance is not solely determined by T/μ but also sensitive to parameters like R, p, T0

For large R, p and T0, either long startup delay or T/μ greater than 2 is needed for low fraction of late packets

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Implication

large fraction of streaming video clips are encoded at 300Kbps

most DSL and cable modem connection support 750Kbps – 1.5Mbps

TCP streaming is adequate for broadband users

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Conclusion

Discrete-time Markov models for live and stored video streaming

Simulation and experiments show models are accurate

Study effect of various parameters on performance with the models