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Peer-to-Peer Networks

This survey outlines the contributions of Peter Druschel to self-organizing overlay networks, exploring the background, distribution, and decentralized control. It covers examples like Napster, Gnutella, FreeNet, academic prototypes like Pastry, Chord, CAN, and common issues like organizing the overlay network, node arrivals, failures, resource allocation, and locality. The architecture involves a generic P2P substrate with TCP/IP Internet protocols, object distribution using consistent hashing, and routing properties for efficient communication and fault detection. The PAST system for file storage and distribution is layered on top of Pastry, providing strong persistence, high availability, scalability, and reduced costs.

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Peer-to-Peer Networks

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  1. Peer-to-Peer Networks Outline Survey Self-organizing overlay network File system on top of P2P network Contributions from Peter Druschel

  2. Background • Distribution • Decentralized control • Self-organization • Symmetric communication

  3. Examples • Pioneers • Napster, Gnutella, FreeNet • Academic Prototypes • Pastry, Chord, CAN,…

  4. Common Issues • Organize, maintain overlay network • node arrivals • node failures • Resource allocation/load balancing • Resource location • Locality (network proximity) Idea: generic p2p substrate

  5. Architecture Event notification Network storage ? P2p application layer self-organizing overlay network P2P Substrate TCP/IP Internet

  6. 128 - 0 2 1 objid nodeids Object Distribution • Consistent hashing[Karger et al. ‘97] • 128 bit circular id space • nodeIds(uniform random) • objIds (uniform random) • Invariant: node with numerically closest nodeId maintains object

  7. Object Insertion/Lookup 2128 - 1 O Msg with key X is routed to live node with nodeId closest to X Problem: complete routing table not feasible X Route(X)

  8. Routing Properties • log16 N steps • O(log N) state

  9. Leaf Sets • Each node maintains IP addresses of the nodes with the L numerically closest larger and smaller nodeIds, respectively. • routing efficiency/robustness • fault detection (keep-alive) • application-specific local coordination

  10. Routing Procedure if (destination is within range of our leaf set) forward to numerically closest member else let l = length of shared prefix let d = value of l-th digit in D’s address if (Rld exists) forward to Rld else forward to a known node that (a) shares at least as long a prefix (b) is numerically closer than this node

  11. Routing Integrity of overlay: • guaranteed unless L/2 simultaneous failures of nodes with adjacent nodeIds Number of routing hops: • No failures: < log16N expected, 128/b + 1 max • During failure recovery: • O(N) worst case, average case much better

  12. Node Addition

  13. Node Departure (Failure) Leaf set members exchange keep-alive messages • Leaf set repair (eager): request set from farthest live node in set • Routing table repair (lazy): get table from peers in the same row, then higher rows

  14. API • route(M, X): route message M to node with nodeId numerically closest to X • deliver(M): deliver message M to application • forwarding(M, X):message M is being forwarded towards key X • newLeaf(L): report change in leaf set L to application

  15. PAST: Cooperative, archival file storage and distribution • Layered on top of Pastry • Strong persistence • High availability • Scalability • Reduced cost (no backup) • Efficient use of pooled resources

  16. PAST API • Insert - store replica of a file at k diverse storage nodes • Lookup - retrieve file from a nearby live storage node that holds a copy • Reclaim - free storage associated with a file Files are immutable

  17. fileId Insert fileId PAST: File storage

  18. k=4 fileId Insert fileId PAST: File storage Storage Invariant: File “replicas” are stored on k nodes with nodeIds closest to fileId (k is bounded by the leaf set size)

  19. PAST: File Retrieval C k replicas Lookup file located in log16 N steps (expected) usually locates replica nearest client C fileId

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