Pseudo-DHT: Distributed Search Algorithm for P2P Video Streaming

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Pseudo-DHT: Distributed Search Algorithm for P2P Video Streaming. December 15, 2008 Jeonghun Noh Stanford University Sachin Deshpande Sharp Laboratories of America. P2P Live Video Streaming. Relaying video using uplink bandwidth. Video source. - PowerPoint PPT Presentation

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  • Pseudo-DHT: Distributed Search Algorithm for P2P Video StreamingDecember 15, 2008

    Jeonghun NohStanford University

    Sachin DeshpandeSharp Laboratories of America

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    P2P Live Video StreamingRelaying video using uplink bandwidthLive video: no needs for locating video chunksVideo source

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    P2P Time-Shifted StreamingLocal storage to store fractions of the videoTo locate video at arbitrary point, a query server may be used3~6mSeeking video of position 5m7~11m0~4mA scalable distributed content search is desiredThe server can become a bottleneck as peer population increasesNo dedicated server may be available

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    OutlineP2TSS systemPseudo-DHT: Distributed SearchPerformance evaluationNumerical analysisSimulation study

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Introduction to P2TSSP2TSS (P2P Time-shifted Streaming System) Serves both live streaming and time-shifted streamingTime-shifted stream (TSS) is the same as the original stream except being delayed in timePeers store a fraction of videoCached video chunks are later served to other peers[Deshpande et al., 2008]

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Caching Live StreamTimeVideo positionPeers cache live stream with another video connectionLive streamCached portionPlayback trajectory

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Chord: Distributed LookupChord: a distributed lookup overlay [Stoica et al., 2001] Nodes (peers) are connected as a circleKeys are mapped to successor nodeFast lookup by a finger table. Lookup latency: O( log(Np) )

    An example key/node spaceNodes and keys sorted by IDsID length: 6bitsCan be normalized to [0,1)

    N: nodeK: key

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Building Lookup OverlayNode is entered to overlay as peer joinsNode ID: uniformly drawn between [0,1)Node is placed in a distributed manner(Key, Value) is entered as peer registers buffer statusKey (K): hashed video chunk ID ( 0 K < 1)Value (V): peer network address(K,V) is mapped to the successor nodeExample

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Pseudo-DHT: RegistrationRegister (K, V) may result in collision

    Chunk IDiRepeat Register (K, V) until there is no collision K = K + (n-1) (n=# attempts, =offset base)Unlike original DHT, single key-multiple values is discouragedLeads to better load balancing and low latency retrieval

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Pseudo-DHT: RetrievalSeeking Video Chunk IRetrieve(i) may return a missK = i - (n-1) (n=# attempts, =offset base)Repeat Retrieve(K) with different keys until a hit occursBest-effort search, different from original DHT

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    OutlineP2TSS systemPseudo-DHT: Distributed SearchPerformance evaluationNumerical analysisSimulation study

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Preliminaries for AnalysisSymbolsNp: Number of peers (or nodes)L: Video length (in secs)Q: Video chunk size (in secs): Ratio between Np and available slots M (=L/Q)

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Registration LatencyIndependence model for the number of collision, C

    More sophisticated model

    where A denotes the number of peer arrival in Q seconds

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Experimental SetupPseudo-DHT implemented in PlanetSim [Garcia et al, 2004]

    Simulation setupMaximum successive lookup: 4Video length L: 7200sBuffer size D: 240s Chunk size Q: 5, 10, 15, 30s

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Registration LatencyWhen is small, models match simulation resultsRegistration latency of both forward/backward key change is identicalVideo chunk size (Q): 5s

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Retrieval LatencyEmpirical statisticsXk: 1 if video chunk Bk is occupied. 0 otherwiseConditional hit probability Pr (Xi-j=1| Xi=0), ( j: offset base )With a larger offset base , the correlation between successive retrievals becomes weaker

    Modeling retrieval latency N:

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Offset Base and Retrieval LatencyRetrieval latency decreases as an offset base increasesVideo chunk size Q: 5s

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Video Chunk Size and Retrieval LatencyAs chunk size Q increases, retrieval latency decreases

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Overhead of Key StorageBalls and bins problemHow many nodes (bins) hold k keys (balls)?Bins are created by nodesRandom keys fall into [0,1) uniformlyStatistically, larger bins will receive more balls

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Overhead of Key StorageOne nodes probability of storing k keys

    ObservationsLow overhead: keys are spread out on the overlay50% of nodes store no keys when N=K

    N=300K=300

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Conclusion and Future WorkProposed Pseudo-DHTAllows peers to register / retrieve video chunk in a scalable waySlightly different from original DHT due to video continuitySpreads out (key, value) items over the overlayP2TSS and Pseudo-DHTApplication to a P2P systemThorough evaluation with analysis and simulations

    Future research topicsChanging Q dynamically according to peer population sizeConsidering heterogeneous peer uplink capacity for registration

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Thank you!

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Backup Slides

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Preliminaries for AnalysisVideo chunk ID space

    0X0X1X2XM-1M-1

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Sample Key/Node SpaceInterval between nodes is not constant

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Chord: Distributed LookupChord: a distributed lookup overlay [Stoica et al., 2001] Nodes (peers) and keys are mapped to a common spaceFast lookup by a finger table. Lookup time: O( log(Np) )

    An example key/node space

    0N: nodeK: key

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Caching VideoPeers locally determine which portion to cacheDistributed Stream Buffer (DSB)Peers local buffer to hold a fraction of videoA finite size of cache (e.g., size of 2 to 4 minutes of video)Independent of playback buffer

    Static contents in cacheNo content change once the cache is fullProvides a bottom line performance

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    A lookup on Chord OverlayFast search using a finger table. Each node has more detailed knowledge about nodes closer to them.

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    RegistrationApproach 2: Simplified dependence modelAi : Number of insertions for key i ( Number of arrivals during the slot i)

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Simultaneous Retrieval

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Retrieval AnalysisWith larger offsets(+) The number of lookups decreases slightly.(-) Peers have to switch earlier to another peer.The simulation results match the model

    ParametersNp = 500L = 7200D = 480sQ = 5sNumber of slots: 1441 (M = 1440)

    *J. Noh : Pseudo-DHT: Distributed Search for P2P Streaming Dec. 15, 2008

    Analysis: Overhead for Key StoragePoisson process property: Given N(t) = N, N arrival times are independently uniformly distributed. In Poisson process, interarrival time between events is exponentially distributed. The converse is also true. Finally,

    *Define leech clearly.***Carol: I think this slide could be eliminated**Peers locally determine which portion to cacheDistributed Stream Buffer (DSB)Peers local buffer to hold a fraction of videoA finite size of cache (e.g., size of 2 to 4 minutes of video)Independent of playback bufferStatic contents in cacheNo content change once the cache is fullProvides a bottom line performance

    *Each peer participates in the search processComputational resources contributed by peers*Another consequence : low penalty due to node disconnect. But this is low level (Chord functionality.. Or DHT)Increases lookup performance.

    **Carol: I think this slide could be eliminated****Load factor = Num of peers.As alpha increases, higher-order model (sophisticated) matches the result better.*(Np=500, L=7200s, Q=5s, =0.346)Each trial is assumed to be independent