Slide 1Major issue in real P2P systemsChurn was just one issue!Consequences?Today look at two solutionsConnection to Game TheoryBitTorrentCommon ScenarioIP Multicast?Slide 10Client-ServerClient-ServerIP multicastEnd-host based multicastEnd-host based multicastEnd-host based multicastEnd-host multicast using single treeEnd-host multicast using single treeEnd-host multicast using single treeEnd-host multicast using single treeBitTorrentBitTorrent SwarmHow a node enters a swarm for file “popeye.mp4”How a node enters a swarm for file “popeye.mp4”How a node enters a swarm for file “popeye.mp4”How a node enters a swarm for file “popeye.mp4”Contents of .torrent fileTerminologyPeer-peer transactions: Choosing pieces to requestChoosing pieces to requestChoosing pieces to requestTit-for-tat as incentive to uploadAnti-snubbingWhy BitTorrent took offWhy BitTorrent took offPros and cons of BitTorrentPros and cons of BitTorrentPros and cons of BitTorrent“Trackerless” BitTorrentWhy is (studying) BitTorrent important?Why is (studying) BitTorrent important?Other file-sharing systemsFile-sharing systems…ReferencesGame-Based ApproachesKen BirmanCornell University. CS5410 Fall 2008.Major issue in real P2P systemsSuppose that John Smith signs up to use FileSnarfer and checks all the boxes off: he’s happy to upload files, has ample space, his machine is always turned on.… but over time his disk starts to fill up… and he enables power management, so his machine starts to turn itself off automatically sometimes… and his bandwidth changes when he moves to his room in the coop he’ll live in next yearFrom the perspective of FileSnarfer, John made a lot of promises and isn’t keeping them!Churn was just one issue!With churn, nodes come and go unexpectedly, but at least you can “detect” the failure in most casesWith John’s scenario, nodes exhibitHeterogenous capabilities: some are up for long periods, have lots of resources, and are well connectedOthers may be down a lot, low on stuff, behind firewalls, have slow linksAnd these attributes “churn” too: a single machine may behave very differently over the course of a day, or over its lifetime in the FileSnarfer overlayConsequences?When people try to download from John they may get very poor performanceAnd overlay routes via John could failIn fact this suggests a way to attack the FileSnarfer platform: just sign up lots of people just like John!Like a Sybil attack but even betterThese guys are just eager users and lousy “peers”!Topic this raises: how to do P2P stuff in a world of heterogeneous peers that churn in many dimensionsToday look at two solutionsBitTorrentA famous platform and in fact a very successful commercial product nowCore idea is to create “swarms” in which nodes that currently have more resource tend to bubble to the top and play more active roles and nodes with less resource bubble to the edges and are served last (and least)BAR GossipExtends the BitTorrent concept and also generalizes it. A model of Byzantine, Altruistic and Rational behaviorConnection to Game TheoryThe mathematical theory of games dates back centuries, but the recent version was pioneered by Nash at Princeton in 1940’s (“A beautiful mind”)He imagined games with two or more players, in which each makes moves and tries to win some payoffEach is selfish. Question Nash posed: what strategy works best if you are a game player?His deep idea: If we formalize notions of payoff some games settle into a Nash EquilibriumBitTorrent and BAR Gossip create scenarios in which doing what’s best for you is also best for the system!BitTorrentCS514Vivek Vishnumurthy, TACommon ScenarioMillions want to download the same popular huge files (for free)ISO’sMedia (the real example!)Client-server model failsSingle server failsCan’t afford to deploy enough serversIP Multicast?Recall: IP Multicast not a real option in general settingsNot scalableOnly used in private settingsAlternativesEnd-host based MulticastBitTorrentOther P2P file-sharing schemes (later in lecture)Router“Interested” End-hostSourceRouter“Interested” End-hostSourceClient-ServerRouter“Interested” End-hostSourceClient-ServerOverloaded!Router“Interested” End-hostSourceIP multicastRouter“Interested” End-hostSourceEnd-host based multicastEnd-host based multicast“Single-uploader”  “Multiple-uploaders”Lots of nodes want to downloadMake use of their uploading abilities as wellNode that has downloaded (part of) file will then upload it to other nodes.Uploading costs amortized across all nodesEnd-host based multicastAlso called “Application-level Multicast”Many protocols proposed early this decadeYoid (2000), Narada (2000), Overcast (2000), ALMI (2001)All use single treesProblem with single trees?End-host multicast using single treeSourceEnd-host multicast using single treeSourceEnd-host multicast using single treeSourceSlow data transferEnd-host multicast using single treeTree is “push-based” – node receives data, pushes data to childrenFailure of “interior”-node affects downloads in entire subtree rooted at nodeSlow interior node similarly affects entire subtreeAlso, leaf-nodes don’t do any sending!Though later multi-tree / multi-path protocols (Chunkyspread (2006), Chainsaw (2005), Bullet (2003)) mitigate some of these issuesBitTorrentWritten by Bram Cohen (in Python) in 2001“Pull-based” “swarming” approachEach file split into smaller piecesNodes request desired pieces from neighborsAs opposed to parents pushing data that they receivePieces not downloaded in sequential orderPrevious multicast schemes aimed to support “streaming”; BitTorrent does notEncourages contribution by all nodesBitTorrent SwarmSwarmSet of peers all downloading the same fileOrganized as a random meshEach node knows list of pieces downloaded by neighborsNode requests pieces it does not own from neighborsExact method explained laterHow a node enters a swarm for file “popeye.mp4”File popeye.mp4.torrent hosted at a (well-known) webserverThe .torrent has address of tracker for fileThe tracker, which runs on a webserver as well, keeps track of all peers downloading fileHow a node enters a swarm for file “popeye.mp4”File