1. CMPT 401 Summer 2007 Dr. Alexandra Fedorova Lecture XV: Real P2P Systems

2. 2 CMPT 401 Summer 2007 © A. Fedorova Outline Skype BitTorrent

3. 3 CMPT 401 Summer 2007 © A. Fedorova Skype IP telephony system Allows users to make phone calls –to Skype users –to regular phone users Calls are routed via Skype nodes Central login server Super-nodes –Nodes that know about location of other nodes

4. 4 CMPT 401 Summer 2007 © A. Fedorova Skype P2P Network Super-nodes are nodes with powerful CPU and network bandwidth One has to qualify to become a super-node Super-nodes know about locations of other nodes Ordinary nodes contact super-nodes to place calls

5. 5 CMPT 401 Summer 2007 © A. Fedorova Skype Components Skype client – the client program used to make phone calls Host cache – list of other known Skype users, maintained at each Skype client Skype login server – a centralized component. Processes account information, authentication

6. 6 CMPT 401 Summer 2007 © A. Fedorova Skype Bootstrap Procedure Skype client depends on host cache to find other users How does a new Skype client populate its host cache? There is a list of bootstrap supernodes hard-coded into the Skype client Skype client contacts them to –Find the login server –Find names of other Skype peers Skype client continues maintaining and refreshing its host cache throughput its lifetime –It’s a good idea to let Skype running for a while before making a phone call!

7. 7 CMPT 401 Summer 2007 © A. Fedorova Skype User Search Procedure A Skype client making a phone call needs to find other users It contacts super-nodes from its host cache, asking them to help find the user Super-nodes return a list of nodes to contact The client contacts those nodes If unsuccessful, the client asks for more nodes Guarantees to find any user that has logged in within the last 72 hours Not much specific information on Skype protocol is available…

8. 8 CMPT 401 Summer 2007 © A. Fedorova Outline Skype BitTorrent

9. 9 CMPT 401 Summer 2007 © A. Fedorova

10. 10 CMPT 401 Summer 2007 © A. Fedorova BitTorrent Overview Website allowing peers to share music, video and other media files Central server helps users find initial set of peers that have pieces of the file Tracker server keeps track of peers possessing content of individual files Users download the file by participating in exchange: –They exchange pieces that they have –for pieces that they don’t have Therefore, for the system to work, users must have incentive to give Users who just get, but do not give are called free riders Protocol must discourage free riding

11. 11 CMPT 401 Summer 2007 © A. Fedorova BitTorrent Download Mechanism Peers download pieces they need and upload pieces they have Keep track of download rates from all links Keep those with highest rate Keep uploading to those peers Choke the others (stop upload) fatter link = higher rate

12. 12 CMPT 401 Summer 2007 © A. Fedorova Selective Unchoking Once in a while selectively unchoke one of the links Allow some time (30 sec.) to see if the download rate will increase If the download rate becomes greater than the rate at one of the existing links, chop the slowest of the existing links

13. 13 CMPT 401 Summer 2007 © A. Fedorova Game Theory on Exchange Two traders A and B simultaneously exchange goods They have to decide whether to participate in the exchange without knowing the other person’s decision If a trader makes the exchange, he co-operates If a trader does not make the exchange, he defects Possible outcomes: –A co-operates, B co-operates – both benefit from receiving the other trader’s good –A co-operates, B defects – B benefits by receiving A’s good, by not giving away his own good –A defects, B defects – none receives the other trader’s good, but each gets to keep his own good

14. 14 CMPT 401 Summer 2007 © A. Fedorova Incentives P – payoff P(C) – payoff to co-operating player P(D) – payoff to defecting player CooperateDefect CooperateP(C) = 3, P(C) = 3P(C) = 0, P(D) = 5 DefectP(D) = 5, P(C) = 0P(D) = 1, P(D) = 1

15. 15 CMPT 401 Summer 2007 © A. Fedorova Strategies Encouraged By Incentives Best individual strategy is to defect But if both traders defect they are worse off than if they both co-operated So the best strategy is to co-operate, but only if the other player co-operates CooperateDefect CooperateP(C) = 3, P(C) = 3P(C) = 0, P(D) = 5 DefectP(D) = 5, P(C) = 0P(D) = 1, P(D) = 1

16. 16 CMPT 401 Summer 2007 © A. Fedorova Winning the Exchange Game: TIT-FOR-TAT The exchange game: –Repeat rounds of exchanges with other players –Remember what other players did in previous rounds –Goal: maximize personal payoff The winning strategy: TIT-FOR-TAT –Co-operate in the first round –In subsequent rounds do the same what the other player did Properties of the winning player: –Nice – he begins by co-operating –Retaliatory – he won’t co-operate with players that do not themselves co-operate –Forgiving – will co-operate again if bad players fix their behavior –Clear – clearly communicates his intentions

17. 17 CMPT 401 Summer 2007 © A. Fedorova Analogy With BitTorrent Goods exchange: exchange the piece of file that you have for the piece that you don’t have Co-operate: upload the piece you have in exchange for downloading the piece you need Defect: download the piece you need, but don’t upload (choke) Payoff: –The piece you download –minus the cost of uploading the file

18. 18 CMPT 401 Summer 2007 © A. Fedorova Is BitTorrent Using the Winning Strategy? Is BitTorrent player NICE? –No, it may choke an existing link upon finding a faster one Is BitTorrent player RETALIATORY? –No, it uses optimistic unchoking Is BitTorrent player FORGIVING? –Not necessarily – unchoked links are chosen arbitrarily Is BitTorrent player CLEAR? No, unchoked links are chosen arbitrarily

19. 19 CMPT 401 Summer 2007 © A. Fedorova Improved Incentive Mechanism For each link, maintain: –Download amount d –Upload amount u Upload to the peer as long as: u – d ≤ f * c –c – the fragment size –f – some constant If u – d is large, the other peer is not co-operating –So you stop co-operating in return u d

20. 20 CMPT 401 Summer 2007 © A. Fedorova Comparison of the Mechanisms Old mechanism: everyone experiences the same download time, regardless of the upload rate they provide New mechanism: nodes that upload more enjoy faster completion time

21. 21 CMPT 401 Summer 2007 © A. Fedorova Comparison of the Mechanisms (cont.) FO – free riders + old players FN – free riders + new players FNO – free riders + new players + old players

22. 22 CMPT 401 Summer 2007 © A. Fedorova Summary Skype –IP telephony system –Relies on super-nodes, recorded in host cache –Skype client learns about initial super-nodes from bootstrap super-nodes –Centralized login server BitTorrent –Media sharing network –File location is found in a centralized fashion (central server + tracker) –File download is P2P –System must encourage co-operation, discourage free riding –The right incentive mechanism is key