Using Layered Video to Provide
Incentives in P2P Live Streaming
Zhengye Liu
Yanming Shen
Shivendra Panwar
Keith W. Ross
Yao Wang
Polytechnic University, Brooklyn, NY, USA
ACM SIGCOMM P2P-TV Workshop, Kyoto, 2007
1
Outlines
• Introduction
• P2P live streaming
• System architecture and design
– Layered Video / MDC
• Simulation results
• Conclusion
2
BT (Basic idea)
• Chop file into many pieces.
– piece : typically 256KB in size.
• Replicate different pieces on different
peers as soon as possible.
• As soon as a peer has a complete piece,
it can trade it with other peers.
• Hopefully, we will be able to assemble
the entire file at the end.
3
http://vc.cs.nthu.edu.tw/home/paper/codfiles/jwhuang/200803251226/BiToS970325.ppt 3
BT (Basic components)
• Seed
– Peer that has the entire file.
• Leacher
– Peer that has an incomplete copy of the file.
• Torrent file
– Files are typically fragmented into 256KB pieces.
– The torrent file lists the hashes of all the pieces to
allow peers to verify integrity.
– Typically hosted on a web server.
• Tracker
– The tracker is responsible to help the peers find each
other and to keep the download/upload statistics of
each peer.
– Returns a random list of peers.
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File Distribution: BitTorrent
tracker
obtain list
of peers
trading
chunks
peer
5
BitTorrent: Incentive
• Question: What is the incentive to
provide higher upload rate?
• Answer: To get file faster
• Implementation: Tit-for-tat mechanism.
Search for trading partners that upload
to you at higher rates
6
BitTorrent: Trading
• Alice measures rate she receives bits
from each neighbor.
• Alice sends chunks to 4 best neighbors.
• Every 10 seconds, she recalculates rates
& possibly modifies set of 4 peers.
• Every 30 seconds, she “optimistically
unchokes” random peer.
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BitTorrent: Trading
(1) Alice “optimistically unchokes” Bob
(2) Alice becomes one of Bob’s top-four providers; Bob reciprocates
(3) Bob becomes one of Alice’s top-four providers
With higher upload rate,
can find better trading
partners & get file faster!
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Basic idea P2P live streaming
tracker
obtain list
of peers
peer
trade
chunks
Source
of video
9
Incentives for Live Streaming
• Why upload at all?
– Currently no tit-for-tat mechanism in
existing deployments
• Is tit-for-tat a sufficient incentive?
– No! Why provide more upload bandwidth if
you’re receiving the video at the full rate?
• Our main idea:
– If you upload more, you get better quality.
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Layered Video
• Single layer Video
– All peers receive the same video quality
– PPStream, PPLive, CoolStreaming
• Layered video
–
–
–
–
A video is encoded into several layers
More layers introduce better video quality
Nested dependence between layers
A higher layer can be decoded only if all the lower layers are
available
L4
L3
L2
L1
L2
L1
• Higher upload contribution results in better received video
quality
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Multiple Description Coding
(MDC)
• Video encoding/decoding technology
– Video content is encoded into several descriptions
– Each description can be decoded independently
– Even receiver only receives one description, the video is
displayed with low quality
– The more descriptions received, the better video quality
– Compatible for transmitting video streaming in the Internet
MDC
encoding
MDC
decoding
Video
quality
Video
content
# of
description
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Layered Video w/ Tit-for-Tat
• Generate multiple layers, each layer divided
into layer chunks (LCs)
– Assign higher priorities to the lower layers
Layer 3
LC31
LC32
LC33
LC34
Layer 2
LC21
LC22
LC23
LC24
Layer 1
LC11
LC12
LC13
LC14
• Exchange LCs
• Measure download rates from neighbors
• Reciprocate to neighbors based on their
contributions
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Supplier & Receiver Side
Schedulers
• Supplier: How to allocate uplink
bandwidth to neighbors?
– BitTorrent roughly gives each unchoked
neighbor an equal share.
• Receiver: How to maximize the received
video quality
– Multiple LCs are to be requested
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Supplier Side Scheduler
• Goal: Supply neighbors in proportion to their
contributions
• Measure the download rates, dk from neighbor k
Receiver 1
Receiver 2
• Maintain separate FIFO rqst
queue for each neighbor
Receiver K
...
Requests queue
• Serve neighbor k next with probability:
pk 
dk

i
di
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Receiver Side State
Figure 1: Buffer state at a given time.
• Peer request LCs at beginnings of rounds
• Peer may request LCs from the current time
up until a window of B chunks times into the
future, B=8 here.
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Receiver Side Scheduler (1)
• Goal: Maximize the received video quality
• Which LC should be requested first?
• Assign heuristic “importance” to each LC,
taking into account:
– Layer index
– Playback deadline
– Rareness
• Request LCs from the highest importance to
the lowest importance
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Receiver Side Scheduler (2)
• Where to send the request for the LC?
• Estimate the current delay from each
r
neighbor:
mk
dk
where mk is # of outstanding requests, r is the
video bit rate of one layer, Δis chunk length
• Send request to neighbor that will send it
first
– As long as it can come before deadline
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Performance Study: Schemes
• Single layer video without incentives (Single-Layer)
– A video is encoded by single layer video coding
• Layered video without incentives (Layered)
– A video is encoded into multiple layers; however, the upload
contribution is ignored and each peer is treated equally
• MDC with incentives (MDC-Incent)
– A video is encoded into multiple descriptions, and the
received video quality of a peer depends on its upload
contribution
• Layered video with incentives (Layered-Incent)
– our proposed scheme
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• Peers
System Setup (1)
– Uplink bandwidth: Ethernet peer: 1000 kbps; cable
peer: 300 kbps; free-rider: 0 kbps
– Fix ratio of Ethernet peers to cable peers: 3:7;
change percentage of free-riders (5%~30%)
• Video
– Foreman video sequence (CIF, 30 frames/sec)
– SVC (Scalable Video Coder) video codec
– 20 layers, with each layer having a bit rate of 50
kbps
– Each layer is further divided into 1 second LCs.
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System Setup (2)
• Video
– MDC-Incent scheme
• Apply multiple description FEC (MD-FEC) scheme on the
FGS encoded bit stream to generate 20 descriptions
• The bit rate of each description is 50 kbps
– Single-Layer scheme
• Code “Foreman“ sequence into a single layer bit stream
with bit rate 400 kbps using the H.264 codec JM9.6
• The GOP length is 1 second
• We divide each GOP into 8 chunks
• Overlay
– Each peer, total 1000 peers, has 14 to 18 neighbors
– Randomly replace worst neighbor every 30 seconds
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Performance Metrics
• Useful rate received (R)
– The bits that are useful for video
decoding
• Discontinuity ratio (α)
– The percentage of time that a video is
undecodable and unplayable
• Average PSNR (Q)
– Received video quality
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Differentiated Service (1)
(a) Ethernet peer
•Ethernet peers with
Layered-Incent scheme
have much higher
received useful rates.
•Peers with high upload
contributions receive
better video quality.
Figure 3: Cumulative distribution of received useful rate for all peers
during the simulation period under different schemes. The percentage
of free-rider is 5%. Average (Q, α) for each class of peers are
indicated.
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Differentiated Service (2)
(b) Cable peer
•Ethernet peer can receive more descriptions than a cable peer
•Peers with low contributions receive relatively low but still
acceptable video quality.
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Differentiated Service (3)
(c) Free rider
•Free-riders receive unacceptable video quality.
•The discontinuity ratio for free-rider is high (Layered-Incent: 13%
and 20%, MDC-Incent: 76%), which is unacceptable for video service.
•For the Single-Layer case, just about every peer receives the video at
the encoded rate of 400kbps.
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Free-Riding (1)
•R : the average received
useful rate for a particular
class of peers.
•With Layered-Incent, the
Ethernet peers receive
much higher rates.
•Received video quality does
not degrade with freeriding.
Figure 4: Robustness of schemes to free-riding for different users
under different performance metrics. (a)-(c) Ethernet peers. Vary the
percentage of free-riders in the system from 5% to 30%.
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Free-Riding (2)
•The Single-Layer scheme is seriously affected by free-riding.
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Free-Riding (3)
•The video quality degradation is less severe than that with SingleLayer, the average PSNR Q decreases more than 2 dB.
•Received video quality does not degrade with free- riding.
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Conclusion
• A decentralized incentive mechanism
for video streaming
• Performance studies show that the
scheme can
– Provide differentiated video quality
commensurate with a peer’s contribution
– Largely prevents free-riders
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References
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