1. MULTI-TORRENT: A PERFORMANCE STUDY Yan Yang, Alix L.H. Chow, Leana Golubchik Internet Multimedia Lab University of Southern California

2. Internet Multimedia Lab OUTLINE Background and Introduction Proposed Approach Simulation Study Related Work and Conclusion 2

3. Internet Multimedia Lab BIT-TORRENT (BT) OVERVIEW An extremely popular P2P file sharing application. Nodes downloading same file (torrent) form an overlay network. File is divided into pieces (chunks) with fixed size. Default size is 256KB. Upload while downloading Nodes download chunks from their neighbors. Each node upload file chunks to a number of neighbors. Local scheduling file exchange. 3

4. Internet Multimedia Lab HOW BT WORKS? Node Seed Leecher Unchoking Tit-For-Tat (TFT) Leecher: 4TFT + 1 optimistic Seeds: 5 random 4 seed leeche r 4TFT + 1 optimistic 5 random seed

5. Internet Multimedia Lab TORRENT IN REAL LIFE Downloading multiple torrents simultaneously is common Previous study shows ~85% peers participate in multiple torrents. [Guo et al. IMC 2005] Overlap in downloading torrents: common interests do exist in real life! Many torrents are related to each other. Different episodes of a TV show. Movies/music form the same genre, etc. Start-up information are obtained from a similar source. Similar web site, online forum, etc. 5

6. Internet Multimedia Lab MULTI-TORRENT APPROACH Current BT: single-torrent approach Torrents are independent Doesn’t take advantages of comment interest How about multi-torrent approach ? Relating torrents together. A natural idea but is significantly overlooked. Encourages seeding. (Key to system performance) Advantages of multi-torrent Help newly joined nodes ramp up faster. Help nodes nearing end of their downloads find the last few file chunks faster. (end-game behavior) Keep a torrent “alive”. 6

7. Internet Multimedia Lab CONTRIBUTIONS A multi-torrent BT system Small modification to the current BT protocol. Completely decentralized, all local decisions. A “cross-torrent” TFT (CTFT) Simple modification to current BT TFT. Extensive simulation-based study shows: Multi-torrent approach improves system performance. CTFT provides incentives for nodes to stay as seeds. 7

8. Internet Multimedia Lab OUTLINE Background and Introduction Proposed Approach Simulation Study Related Work and Conclusion 8

9. Internet Multimedia Lab MOTIVATIONAL EXAMPLE 9 Slow: +21% Fast: +17% Staying nodes are better! SettingValue Node classFast nodes (5000/512kbps) (0.6), Slow nodes (1500/128 kbps) (0.4). File selectionFixed selection 2 torrents out of 10 (200MB each). No StayNode leaves torrent as soon as it finishes. StayNode leaves torrent until all participating torrents finishes.

10. Internet Multimedia Lab PROPOSED APPROACH Current BT uses download rate for nodes to decide unchoke. Such unchoking for each torrent is independent. Cross-Torrent TFT (CTFT) Aggregate download rate on each torrent in peer’s all participating torrents. Aggregation is done in a weighted manner. Higher weight (> 1) given to the downloading rate from seeding torrents. Normal weight (=1) given to the downloading rate from leecher torrents. Total contribution of a peer N y to node N x is: 10 Download rate on torrent i of node N x from node N y

11. Internet Multimedia Lab OUTLINE Background and Introduction Motivation and Proposed Approach Simulation Study Related Work and Conclusion 11

12. Internet Multimedia Lab SIMULATION SETUP Parameters Value File size200MB Simulation time12 hours (+3 hours warm-up) Average node inter- arrival time 45 sec Peer set size40 #Leccher unchoking4 TFT + 1 optimistic #Seed unchoking5 Original seed1 per torrent CTFT weight4 12 Simulator Event based in [Bharambe et al. INFOCOM 2006] Extension for multi-torrent and latest BT protocol.

13. Internet Multimedia Lab STAY IMPROVEMENT: # OF TORRENT 13 Have nodes staying helps both metrics. SettingValue Node ClassFast nodes (5000kbps / 512kbps) File selection1.Fixed selection out of 10. 2.Random selection out of 10.

14. Internet Multimedia Lab STAY IMPROVEMENT: INTER- ARRIVAL TIME 14 More improvement for longer inter- arrival time. More obvious # nodes change for longer inter- arrival time. SettingValue Node classFast nodes (5000kbps / 512kbps) File selectionFixed selection 3 torrents out of 10.

15. Internet Multimedia Lab APPLICATION: GAME PATCH SYSTEM 15 492MB: -77% 256MB: +84% 80MB:+92% 492MB: -77% 256MB: +84% 80MB:+92% SettingValue Node classFast nodes (5000kbps / 512kbps) File size492MB, 256MB, 80MB (WoW major patch 2.0, 2.1, 2.2) File selectionFixed selection of all 3 torrents

16. Internet Multimedia Lab APPLICATION: GAME PATCH SYSTEM Dynamic seeding (DS) Each node does a local estimation of the ratio of seeds to leechers. Only participate as seed in those torrents where seed to leecher ratio is below R. This local estimation re-evaluated every S time units. 16 492MB: +2% 256MB: +47% 80MB: +69% 492MB: +2% 256MB: +47% 80MB: +69%

17. Internet Multimedia Lab OUTLINE Background and Introduction Motivation and Proposed Approach Simulation Study Related Work and Conclusion 17

18. Internet Multimedia Lab RELATED WORK Explore multi-torrent Multi-torrent system using tracker overlay. [Guo et al. IMC 2005] Our focus is on performance. Our CTFT provides incentives to stay as seeds. Our design requires only local client modification. Explore BT incentives in single torrent A market based incentives. [Freedman et al. IPTPS 2008] Reputation based incentives. [Lian et al. IPTPS 2006] Our CTFT-DS matches seeding capacity demand and supply with local knowledge. Investigating other incentive approaches are part of our future work. 18

19. Internet Multimedia Lab CONCLUSION Performance gains are possible through multi- torrent. Nodes staying as seeds improve performance. CTFT provides incentive for nodes to stay as seed. CTFT-DS makes seeding efficient. Multi-torrent is a promising research area with remaining future directions. Malicious users. More exploration on system parameters. R, S, CTFT weight, etc. Real word implementations. 19

20. Internet Multimedia Lab Q & A 20 Thank You!

21. Internet Multimedia Lab STAY IMPROVEMENT: CTFT WEIGHT 21 Staying nodes perform better in both class. SettingValue Node classFast nodes (0.6), Slow nodes (0.4) File selectionRandom selection of 3 out of 10 torrents Stay probability0.5

22. Internet Multimedia Lab STAY IMPROVEMENT: FRACTION OF STAY 22 No Incentives Incentives! SettingValue Node classFast nodes (0.6), Slow nodes (0.4) File selectionRandom selection of 3 out of 10 torrents

23. Internet Multimedia Lab DISCUSSION (BACKUP) Performance in the real world We expect even bigger improvement. System parameters Exploration of various system and proposed schemes’ parameters is our on-going effort. Malicious behavior A malicious node can take advantage of the weight of CTFT, how to detect malicious node is one of our future works. How malicious behavior affects the system performance is one of our future works. 23