1. Incentives Build Robustness in BitTorrent Author: Bram Cohen Presenter: Brian Liao

2. About Bram Cohen  Live in Berkeley  Only publish one paper: Noise Strategies for Local Search  Interest : Complexity theory Multi-agent systems Usability

3. What is BitTorrent? New file-sharing strategy:  Minimize the file hosting cost  Increase scalability

4. Traditional File Hosting

5. BitTorrent Strategy

6. Technical Framework  Publishing Content  Peer Distribution  Pipelining  Piece Selection

7. Technical Framework  Publishing Content  Peer Distribution  Pipelining  Piece Selection

8. Publishing Content  Distribute.torrent file over the web. Contains file info, name, hash info, url for a tracker

9. Technical Framework  Publishing Content  Peer Distribution  Pipelining  Piece Selection

10. Peer Distribution  Standard tracker algorithm will return a random list of peers.  cut files into pieces of fixed size for easy bookkeeping

11. Technical Framework  Publishing Content  Peer Distribution  Pipelining  Piece Selection

12. Pipelining  Avoid delay between pieces so a pipeline is required.  Further breaking pieces into sub- pieces (usually 16 kilobytes) to put in the pipeline.  Usually 5 sub-pieces pipelined at once.

13. Technical Framework  Publishing Content  Peer Distribution  Pipelining  Piece Selection

14. Piece Selection  Strict Priority  Rarest First  Random First Piece  Endgame Mode

15. Strict Priority  Finish complete piece as quickly as possible

16. Rarest First  Download pieces which all others want  Better performance  Prevent rarest piece missing

17. Randnom First Piece & Endgame Mode  Random First Piece For the initial stage of download  Endgame Mode Multiple requests for sub-pieces to prevent delay.

18. Choking Algorithms  Punishment to selfish users  Utilize available resources & provide consistent service

19. Pareto Efficiency  Def: no two parties can exchange resource and make both happier.  Local optimization –> global optima

20. Choke & Unchoke  Purely based on download rate  Every 10 seconds calculate who to choke  Unchoke limit is 4

21. Optimistic Unchoking  Discover unused connection  Rotate every 30 seconds

22. Upload only  After finish download, only upload to high upload rate users.

23. My thought on implementing a price scheme for BT  DC: download charge Static price base on content or fixed per download fee  SC: service charge Negatively proportion to the contribution to the network.  Goal: encourage more seeds

24.  Price Paid = DC + SC DC Price paid Finish download

25. Price Measurement  Local measurement  Periodically report to host