1 Incentives Build Robustness in BitTorrent? Analysis on Bit Torrent Client performance By Jimmy Wong

2 Agenda Introduction BitTorrent Basics Bit Tyrant: A strategic client Bit Thief: A free-riding client Other Tricks of BT Client Real World Experiments Avoiding of Tricks Conclusion

3 Introduction BitTorrent core incentive –Upload More, Download More Problem Definition –I am a selfish user –Can I gain without pay / pay less?

4 BitTorrent Basics Step 0 – Publishing Content –Create Torrent file Tracker URL SHA-1 Hash Checksum –Assign a Tracker –Distribute Torrent to users through Web Site, Newsgroup or Forum

5 BitTorrent Basics Step 1 – Joining –Peers get the Torrent –Connect to Tracker Look for available peers DHT (Distributed Hash Table) –Create connection with other peers –Start with Others’ Optimistic Unchoking Look for better peers Let other peers join the swarm

6 BitTorrent Basics Step 2 to N-1 – Pipelining –Exchange bitmap about which chunks are available –Pay more, get more To build incentive for peers to upload Reciprocal –Rarest first To preserve File completeness among the swarm To build up peer’s reputation

7 BitTorrent Basics Choking –Send data to only few good peers Contribute data to you Have pieces that you wants A resourceful peers –It will be nice if I can pay minimal cost to be un-choke!!!!

8 BitTorrent Basics Optimistic Un-choking –Allow new peers to get some pieces for exchange –Allow discovery of better exchanging partners –Most peers are so friendly –I can gain without contribute!!!!

9 BitTorrent Basics Ending Phase –Toward the end of download –Request for missing pieces –Search for available peers –20-30 pieces left

10 Bit Tyrant: A strategic client Incentive –100KB/s  15KB/s 1% drop –15KB/s  10KB/s 40% drop – About 1.9Mbps World Wide Bandwidth 29-survey-average-broadband-speed-in-us- is-1-9mbps.html –Why do I still need to give u so much?

11 Bit Tyrant: A strategic client Resource Management –Bandwidth fixed –# Conn x Speed = Constant Current Official BT Protocol (<=5.0) –4 un-choking peers –1 optimistic un-choking A Optimization Problem!!

12 Bit Tyrant: A strategic client Bit Tyrant Un-chocking algorithm –Mark Dp be download rate of conn p –Mark Up be upload rate of conn p –For each round Calculate performance metric Dp / Up Un-choking the conn with the highest metric –At the end of each round If peer doesn’t un-choking us  Up  (1+delta)Up If peer un-choking us  Dp = downlod rate If peer has un-choked us for r round  Up  (1-lamda)Up

13 Bit Tyrant: A strategic client Very simple algorithm Net Effect –Select friendly peers –Filter out unfriendly peers –Gain from peers most of the time

14 Bit Tyrant: A strategic client Dynamic sizing of active set –Serving the best peers first –The 2nd best next and so on –Until upload rate is exhausted Local Optimal trading –Not global, because only concern your local peers group

15 BitThief : A free-riding client Download only –No copy right issue (Only Contributor is sued) –Conserve Resource –Use to spoil the community Basic Idea –Gain from optimistic un-choking –Gain from free sharing of Seeder

16 BitThief : A free-riding client Gain From Optimistic Un-choking –Request for as many clients as possible May be block by the Tracker SIMD like query to Tracker –Connect to all client available Higher Chance of being un-choking –Always pretend to be a new comer A bitmap with no available piece Try to download whatever available Most Clients are so nice

17 BitThief : A free-riding client Gain From Free Sharing of Seeder –Seeder select peers in two ways The Highest Bandwidth (from tracker) Round Robin –Bit Thief report high upload rate –# of Seeder > # of Leecher  Even better Gain from both methodology

18 Other Tricks of BT Client Client Discrimination –Higher priority for Similar Client –Delibrately choke other client Pretend to be Local Neighbor –Fake tracker with wrong upload rate –Retrieve from Seed Rapid Disconnect and Reconnect –Hope to gain from optimistic unchoking –Remove from black- list Multiple Connection to the same client –For high speed BB –Gain the most from the same client

19 Experiment with BT Client Testing Environment –Athlon –1GB Ram –120GB 7200 rpm HDD –Windows XP SP2 w/ TCP Conn Crack –BT Software Utorrent / Mainline Bit Comet Bit Tryant –iCable internet Connection

20 Experiment with BT Client Gentoo AMD 64 Minimal ISO –54.4MB –34 Seeders, 0 Leechers Ubuntu 7.10 i386 Desktop ISO –695MB –928 Seeders / 78 Leechers NBA Match –921MB –120 Seeders, 714 Leechers

21 Experiment with BT Client Gentoo UploadTime Avg Speed KB/s Conn Peer Seed/Leec h Bit Comet018: /0 uTorrent017: /0 Bit Tyrant015: /0

22 Experiment with BT Client Ubuntu UploadTime Avg Speed KB/s Conn Peer Seed/Leec h Bit Comet32MB15: /70 uTorrent112KB69: /1 Bit Tyrant100KB26: /1

23 Experiment with BT Client NBA UploadTime Avg Speed KB/s Conn Peer Seed/Leec h Bit Comet1070MB52: /53 uTorrent1035MB1:42:501527/29 Bit Tyrant540MB1:20: /22

24 Experiment with BT Client Bit Thief –Achieve a high download rate (~150KB/s) for Ubuntu and NBA –Suck at 25%

25 Avoiding Tricks Target –Stop Fake Information –Identify Free-riding Difficulties –Bit Torrent is so wide spread –Too many implementations Incentive driven works again!!! –Co-operative identify bad client –Block bad peers, Share with good peers

26 Avoiding Tricks Sharing with Peers –Search for local optimal best peers –Search for potential bad peers –Broadcast of bad peers (In a majority basics) –It will fail if more than half of clients are in-honest

27 Conclusion BT Robustness –Most peers are friendly Ways to Crack –Gain from cracking the protocol –Bit Tyrant: Download > Upload –Bit Thief: Free Riding Proposed Solution –Decentralized Client Solution

28 Reference Incentives Build Robustness in BitTorrent Do incentives build robustness in BitTorrent? /papers/BitTyrant.pdf /papers/BitTyrant.pdf Free Riding in BitTorrent is Cheap f f