festive - dash
Post on 05-Apr-2017
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FESTIVEFairness, Efficiency and Stability in DASHYigit UNALLAR
• There are 3 goals to achieve:✓ Fairness,✓ Efficiency (Highest feasible set of bitrates), ✓ Stability (Avoid frequent bitrate switches),
• Perspective:✓ Scheduling a specific video chunk to be downloaded,✓ Bitrate selection for each chunk, ✓ BW estimation,
Introduction
• Problems:✓ False reflection of network as a feedback,✓ Periodic chunk selection, ✓ Stateless bitrate selection, ✓ Unnecessary bitrate switches,
Introduction
• Recommendations:✓ Randomized chunk scheduling,✓ Stateful bitrate selection,✓ Delayed update approach, ✓ BW estimator via harmonic mean,
Introduction
Compared to alternatives,
• Fairness by 40%,• Stability by 50%,• Efficiency by 10%
improved.
• Inefficiency,
• Unfairness,
• Instability,
w(d) = k − d, linear penalty to more recent switch!
Background& Motivation
Performances
Quite unfair, although it is the best player per the above results!
Performances
• Application-layer,
• Receiver driven,
• Decentralized adaptation.
Design Space
• Chunk Scheduling:✓ Immediate Download(Ramp-up state, Steady State)
➢ Greedy download @ high bitrates increase BW costs, ➢ Greedy download @ low bitrates preclude switching to
higher rate,✓ Periodic Download,✓ Random Scheduling,
Design
• Chunk Scheduling:✓ Immediate Download,✓ Periodic Download,
➢ Constant playback buffer to minimize rebuffering,
➢ Biased view of a network state!! ✓ Random Scheduling,
Design
Design
• Chunk Scheduling:✓ Immediate Download,✓ Periodic Download,✓ Random Scheduling,
➢ Randbuf at the range of (targetbuf − δ, targetbuf + δ],
Thus, each player is not biased by its start time!
Design
• Bitrate Selection:➢ Stateless Selection ( Highest bitrate lower than the
estimated BW),❖ Is current bitrate ramping up or down?
❖ Players using a higher bitrate observe a higher bandwidth!?
❖ Converging to an equilibrium state that is inherently unfair!!
BW=2Mbps,Bitrates= 600,1200,1500kbps
Design
• Bitrate Selection:➢ Stateful Selection,
❖ Players with lower bitrate to ramp up aggressively,❖ Players with higher bitrate to ramp down aggressively,
In our design, ➢ We keep the rate of decrease a constant function, ➢ Gradual switching strategy,
❖ Only to next highest level,❖ Lower rate of upward switches at higher bitrates,
Design
• Delayed Update:➢ Balancing efficiency and fairness on one hand vs.
stability on the other,➢ Computing how close the efficient or stable allocation the
bcur and the reference bitrate bref are.
Design
w, estimated BW
n, the number of switches in the last k=20 secsthe more switches, the more penalty of adding a new switch!
• The combined score is simply the weighted average:
Design
▪ Computed for both the current and reference bitrates and picked the one with lower combined score,
▪ α here provides a tunable knob to control the tradeoff,
• Bandwidth Estimation:➢ Smoothed value rather than instantaneous throughput,➢ Be robust to outliers,
✓ Harmonic mean of the last 20 samples to minimize the impact of outliers,
Design
The Fair, Efficient, Stable, adapTIVE Algorithm
• Harmonic Bandwidth Estimator,➢ Harmonic mean of the last k=20 throughput estimates,
• Increase the reference bitrate at bitrate level k only after k chunks,
• Decrease(bcurrent=0.85 x bestimated) the bitrate level after every chunk if a decrease is necessary, thus bitrates eventually converge to a fair allocation,
• Delayed update calculation α = 12,• Randomized scheduler,
➢ playback buffer < target buffer, download immediately,➢ Otherwise, download with delay, thus, there is no start
time biases.
Festive
• Implemented on OSMF by Adobe in AS,
Evaluation
Inefficiency, as the parameters are customized for chunk size and bitrate levels,
• Comparison w/ other players,➢ In order to do a head-to-head comparison, players emulated,➢ The chunk lengths and bitrate levels vary across commercial players,
Evaluation
3 Players sharing 3Mbps of bottleneck
• Component-wise Validation,➢ Bandwidth Estimator,
➢ Chunk Scheduling,➢ Stateful Bitrate Selection,➢ Delayed Update,
Evaluation
Using the update function bwnext = 0.9bwprev + 0.1bwcur
• Component-wise Validation,➢ Bandwidth Estimator,➢ Chunk Scheduling,
➢ Stateful Bitrate Selection,➢ Delayed Update,
Evaluation
Periodic S. leads to large bias in the estimated bw, and unfairnessin bitrate selection!
• Component-wise Validation,➢ Bandwidth Estimator,➢ Chunk Scheduling,➢ Stateful Bitrate Selection,
➢ Delayed Update,
Evaluation
Stateful bitrate selectionImproves fairness with Minimal impact on Efficiency!
• Component-wise Validation,➢ Bandwidth Estimator,➢ Chunk Scheduling,➢ Stateful Bitrate Selection,➢ Delayed Update,
Evaluation
Knee pts using α = 12!Larger α provides higherEfficiency at the cost of Stability!
Performance of Festive& Baseline Player
2-30 concurrent players sharing a 10Mbps bottleneck link
Any Questions?
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