CS 640: Introduction to Computer NetworksThe Road AheadP2p file-sharingWhat’s out there?Publishing/SearchingSearchingFrameworkNapster: OverviewNapster: PublishNapster: SearchNapster: DiscussionGnutella: OverviewGnutella: SearchGnutella: DiscussionKaZaA: OverviewKaZaA: Network DesignKaZaA: File InsertKaZaA: File SearchKaZaA: FetchingStability and Superpeers (Supernodes)KaZaA: DiscussionBitTorrent: HistoryBitTorrent: OverviewBitTorrent: Publish/JoinBitTorrent: FetchBitTorrent: Sharing StrategyBitTorrent: DiscussionDistrubuted Hash Table: OverviewLocate An ObjectTable in A NodeAdd A New NodeDHT: DiscussionP2P: SummaryCS 640: Introduction to Computer NetworksYu-Chi LaiLecture 18 - Peer-to-PeerThe Road Ahead•p2p file sharing techniques–Downloading: Whole-file vs. chunks–Searching•Centralized index (Napster, etc.)•Flooding (Gnutella, etc.)•Smarter flooding (KaZaA, …)•Routing (Freenet, etc.)•Challenges–Fairness, freeloading, security, …P2p file-sharing•Quickly grown in popularity–Dozens or hundreds of file sharing applications–35 million American adults use P2P networks -- 29% of all Internet users in US!–Audio/Video transfer now dominates traffic on the InternetWhat’s out there?Central Flood Super-node floodRouteWholeFileNapster Gnutella FreenetChunkBasedBitTorrent KaZaA (bytes, not chunks)DHTseDonkey2000Publishing/SearchingInternetN1N2N3N6N5N4PublisherKey=“title”Value=MP3 data…ClientLookup(“title”)?Searching•Needles vs. Haystacks–Searching for top 40, or an obscure punk track from 1981 that nobody’s heard of?•Search expressiveness–Whole word? Regular expressions? File names? Attributes? Whole-text search?•(e.g., p2p gnutella or p2p google?)Framework•Common Primitives:–Join: how to I begin participating?–Publish: how do I advertise my file?–Search: how to I find a file?–Fetch: how to I retrieve a file?Napster: Overview•History–1999: Sean Fanning launches Napster–Peaked at 1.5 million simultaneous users–Jul 2001: Napster shuts down•Centralized Database:–Join: on startup, client contacts central server–Publish: reports list of files to central server–Search: query the server => return someone that stores the requested file–Fetch: get the file directly from peerNapster: PublishI have X, Y, and Z!Publishinsert(X, 123.2.21.23)...123.2.21.23Napster: SearchWhere is file A?Query Replysearch(A)-->123.2.0.18Fetch123.2.0.18Napster: Discussion•Pros:–Simple–Search scope is O(1)–Controllable (pro or con?)•Cons:–Server maintains lot of state–Server does all processing–Single point of failureGnutella: Overview•History:–In 2000, J. Frankel and T. Pepper from Nullsoft released Gnutella–Soon many other clients: Bearshare, Morpheus, LimeWire…–In 2001, many protocol enhancements including “ultrapeers”•Query Flooding:–Join: on startup, client contacts a few other nodes; these become its “neighbors”•Ping-Pong protocol–Pub li sh: no need–Search: ask neighbors, who ask their neighbors, and so on... when/if found, reply to sender.•TTL limits propagation–Fetch : get the file directly from peerI have file A.I have file A.Gnutella: SearchWhere is file A?QueryReplyGnutella: Discussion•Pros:–Fully de-centralized–Search cost distributed–Processing @ each node permits powerful search semantics•Cons:–Search scope is O(N)–Search time is O(???)–Nodes leave often, network unstable•TTL-limited search works well for haystacks.–For scalability, does NOT search every node. May have to re-issue query laterKaZaA: Overview•Gnutella X Napster–No didicated server–But.. not all peers are equal!•“Smart” Query Flooding:–Join: on startup, client contacts a “supernode” ... may at some point become one itself–Publish: send list of files to supernode–Search: send query to supernode, supernodes flood query amongst themselves.–Fetch: get the file directly from peer(s); can fetch simultaneously from multiple peersKaZaA: Network Design“Super Nodes”KaZaA: File InsertI have X!Publishinsert(X, 123.2.21.23)...123.2.21.23KaZaA: File SearchWhere is file A?Querysearch(A)-->123.2.0.18search(A)-->123.2.22.50Replies123.2.0.18123.2.22.50KaZaA: Fetching•More than one node may have requested file...•How to tell?–Must be able to identify similar files with not necessarily same filename–Same filename not necessarily same file...•Use Hash of file–KaZaA uses UUHash: fast, but not secure–Alternatives: MD5, SHA-1•How to fetch?–Get bytes [0..1000] from A, [1001...2000] from B–Alternative: Erasure CodesStability and Superpeers (Supernodes)•Why superpeers?–Query consolidation•Many connected nodes may have only a few files•Propagating a query to a sub-node would take more b/w than answering it yourself•Superpeer selection is time-based–How long you’ve been on is a good predictor of how long you’ll be around.KaZaA: Discussion•Pros:–Tries to take into account node heterogeneity:•Bandwidth•Host Computational Resources•Host Availability (?)•Cons:–Mechanisms easy to circumvent•Can freeload easily–Still no real guarantees on search scope or search time•Similar behavior to Gnutella, but better.BitTorrent: History•Key Motivation:–Popularity exhibits temporal locality (Flash Crowds)–E.g., Slashdot effect, CNN on 9/11, new movie/game release•Focused on Efficient Fetching, not Searching:–Distribute the same file to all peers–Single publisher, multiple downloaders•Has some “real” publishersBitTorrent: Overview•Swarming:–Publish: Run a tracker server.–Search: Out-of-band. E.g., use Google to find a tracker for the file you want.–Join: contact centralized “tracker” server, get a list of peers.–Fetch: Download chunks of the file from your peers. Upload chunks you have to them.•Big differences from Napster:–Chunk based downloading–“few large files” focus–Anti-freeloading mechanismsBitTorrent: Publish/JoinTrackerBitTorrent: FetchBitTorrent: Sharing Strategy•Employ “Tit-for-tat” sharing strategy–A is downloading from some other people•A will let the fastest N of those download from him–Be optimistic: occasionally let freeloaders download•Otherwise no one would ever start!•Also allows you to discover better peers to download from when they reciprocateBitTorrent: Discussion•Pros:–Works reasonably well in practice–Gives peers incentive to share resources; avoids
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