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

2. Outline • Skype • BitTorrent

3. 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. 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. 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. 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. 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. Outline • Skype • BitTorrent

10. 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. 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. 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. 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. Incentives • P – payoff • P(C) – payoff to co-operating player • P(D) – payoff to defecting player

15. 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

16. 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. 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. 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. 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 d u

20. 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. Comparison of the Mechanisms (cont.) • FO – free riders + old players • FN – free riders + new players • FNO – free riders + new players + old players

22. 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