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

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 Leacher Torrent file Tracker 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. 4 4

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. 7

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! 8

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. 10

Layered Video Single layer Video Layered 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 Higher upload contribution results in better received video quality 11

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 Video quality MDC MDC Video content encoding decoding # of description 12

Layered Video w/ Tit-for-Tat Generate multiple layers, each layer divided into layer chunks (LCs) Assign higher priorities to the lower layers Exchange LCs Measure download rates from neighbors Reciprocate to neighbors based on their contributions 13

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 14

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: 15

Receiver Side State Peer request LCs at beginnings of rounds 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. 16

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 17

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 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 18

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 19

System Setup (1) Peers Video 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. 20

System Setup (2) Video Overlay MDC-Incent scheme Single-Layer 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

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 22

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.

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.

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.

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 free- riding. 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%.

Free-Riding (2) The Single-Layer scheme is seriously affected by free-riding.

Free-Riding (3) The video quality degradation is less severe than that with Single-Layer, the average PSNR Q decreases more than 2 dB. 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 29

References [6] A. Habib and J. Chuang, “Incentive mechanism for peer-to-peer media streaming,” Proceedings of International Workshop on Quality of Service (IWQoS) 2004, 2004. [7] X. Hei, , C. Liang, J. Liang, Y. Liu, and K. W. Ross, “A measurement study of a large-Scale P2P IPTV system,” IEEE Transactions on Multimedia, December 2007. [9] Z. Liu, Y. Shen, S. Panwar, K. Ross, and Y. Wang, “P2P video live streaming with MDC: Providing incentives for redistribution,” IEEE ICME 2007, Beijing, 2007. [17] Y. Wang, A. R. Reibman, and S. Lin, “Multiple description coding for video delivery,” Proceeding of the IEEE, pages 57-70, Jan. 2005. [18] X. Hei and Y. Liu and K.W. Ross, “Inferring network-wide quality in P2P live streaming systems,” IEEE JSAC, special issue on P2P Streaming, December 2007 . Z. Liu, Y. Shen, K.W. Ross, S. Panwar, K.W. Ross, Y. Wang, “Substream Trading: Towards an Open P2P Live Streaming System,” International Conference on Network Protocols (ICNP), Orlando, October 2008.