Layered Video for Incentives in P2P Live Streaming Zhengye Liu Yanming Shen Shivendra Panwar Keith W. Ross Yao Wang Polytechnic University, Brooklyn, NY, USA

File Distribution: BitTorrent tracker obtain list of peers trading chunks peer

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

BitTorrent: Trading Alice measures rate she receives bits from each neighbor. Alice sends chunks to four best neighbors. Every 10 seconds, she recalculates rates & possibly modifies set of four peers. Every 30 seconds, she “optimistically unchokes” random peer.

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!

Basic idea P2P live streaming tracker obtain list of peers peer trade chunks Source of video

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.

Layered Video Single layer Video Layered video All peers receive the same video quality Layered video A video is encoded into several layers More layers introduce better video quality Nested dependence between layers Higher upload contribution results in better received video quality

Layered Video w/ Tit-for-Tat Generate multiple layers, each divided into layer chunks (LCs) Exchange LCs Measure download rates from neighbors Reciprocate to neighbors based on their contributions

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

Supplier Side Scheduler Goal: Supply neighbors in proportion to their contributions Measure the download rates, dk from neighbor k Maintain separate FIFO rqst queue for each neighbor Serve neighbor k next with probability:

Receiver Side State Request LCs at beginnings of rounds Can request in a window up to B chunks into future

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

Receiver Side Scheduler (2) Where to send the request for the LC? Estimate the current delay from each neighbor: where mk is # of outstanding requests, r is video rate, Δis chunk length Send request to neighbor that will send it first As long as it can come before deadline

Performance Study: Schemes Single layer video without incentives (Single-Layer) Layered video without incentives (Layered) MDC with incentives (MDC-Incent) Layered video with incentives (Layered-Incent)

System Setup Peers Video Overlay 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 Video Foreman video sequence (CIF, 30 frames/sec) SVC video codec 20 layers, with each layer having a rate of 50 kbps Overlay Each peer has 14 to 18 neighbors Randomly replace worst neighbor every 30 seconds

Performance Metrics Useful rate received (R) Discontinuity ratio (α) 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

Differentiated Service Peers with high upload contributions receive better video quality; Peers with low contributions receive relatively low but still acceptable video quality; Free-riders receive unacceptable video quality.

Free-Riding Received video quality does not degrade with free-riding

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