Chord-over-Chord Overlay Sudhindra Rao Ph.D Qualifier Exam Department of ECECS Outline Peer-to-Peer systems Centralized P2P systems (hybrid) Unstructured P2P systems (pure) Structured P2P systems Super-peer networks CoCO Analysis and Conclusion Peer-To-Peer Systems Decentralized data and resource sharing All computers have equal capabilities The resources can include: Processing power Data Network bandwidth Applications Redundant storage Permanence Selection of nearby servers Anonymity Search Authentication Hierarchical naming Centralized Server P2P systems - Napster Used in large scale sharing of files Single server maintains a table of data Vs Central Server node Features Peer 5 Not self-organized Not scalable Single point of failure/attack Peer 2 Most popular network - mp3 sharing Applications: Peer 4 Napster Peer 3 Unstructured P2P networks - Gnutella Random overlay networks Peer 1 No central index Start with nodes that know about Peer 5 peer servers and flood along the network Peers find neighbors Peer 3 Features: Scalability – Flooding limited by TTL Peer 4 Peer 2 Keyword search Cannot guarantee search Applications: Peer 6 Peer 7 Gnutella Freenet Structured P2P networks – Chord, Pastry, CAN Based on ‘Distributed Hash Tables’ Self-organized overlay networks Insertion and lookup in a bounded Lookup (K54) number of hops N1 K54 N8 Features: N58 Load balancing N14 Fault-tolerance Decentralization N51 Scalability N48 N21 Availability Flexible naming N42 Applications: N38 N32 Chord Pastry Tapestry CAN Design and Analysis Chord provides fast distributed computation of a hash function, mapping keys to nodes responsible for them Assigns keys to nodes with consistent hashing A chord node needs only a small amount of routing information about other nodes A node resolves the hash function by communicating with other nodes With high probability, the number of nodes that must be contacted to find a successor is an N-node network is O(log N) Only O(log N) fingers need be stored When an Nth node joins or leaves the network, only an O(1/N) fraction of the keys are moved Super-Peer Networks Hierarchy introduces manageability Super-Peer networks combine features of distributed search and centralized search Super-Peer node acts as server for subset of peers Inherent heterogeneity in the capability of peers on the network Super-Peers are assigned based on processing power, network bandwidth, degree etc. Super-peers communicate by flooding to other super-peers Super-peer to peer communication – centralized server system Super-peer network Thumb rules for design Increasing cluster size reduces aggregate load Super-peer redundancy makes system resilient Super-peers should have higher out-degree Minimize TTL on floods Drawbacks Flooding does not guarantee search success Super-peers can be burdened Flooding traffic and duplicates Self-similarities in Mandelbrot Set Chord-over-Chord Overlay(CoCO) Chord used in local clusters – Central Server 0 and 640 Super-peer as manager Super-peer redundancy - by Super Peer Super Peer assigning super-peers at the edge 1 - 127 512 - 639 Super-peers form a Chord overlay network Super Peer Super Peer Super-peers maintain finger tables 128 - 255 384 - 511 for cluster as well as the super-peer Super Peer overlay 256 - 383 Central Server consulted only if all Chord searches fail on the overlay Chord-over-Chord Overlay Chord-over-Centralized server Overlay (CoCO) Super-peers maintain a direct link to the Central Server Central Server consulted in case of Super Peer Super Peer failed searches in local clusters Central Server may be single point of Central Server failure Super Peer Super Peer Super Peer Chord-over-Centralized Server Overlay CoCO Analysis Number of nodes to be contacted in the local cluster of size N/m - O(log N/m) Cost of searching on Super-peer overlay - O(log m) Only O(log N/m) fingers need to be stored in peers and O(log m) additional fingers on super-peers When an node joins or leaves the network, only an O(m/N) fraction of the keys are moved and when Super peer leaves a network chord flip reassigns O(log N/m) + O(log m) fingers. Discussion CoCO Uses DHT on all layers – hence resilient to failures, attacks. Increasing hierarchy improves manageability like Internet Efficient and guaranteed search results Joins/Leaves handled efficiently Super-Peer reassignment is integral part of the protocol Super-Peer networks using Gnutella Flooding can reduce efficiency Techniques to reduce flooding directly affect the network efficiency Super-peer failures are not accounted for Flooding on super-peers does not guarantee search results Conclusion Possible applications of CoCO University wide P2P networks Each department has its own super-peer Company wide P2P networks Geographically distant networks controlled by administrators – super-peer assignment ISP controlled Napster like central server Strategically placed Super-peers – like Akamai caches Better control over the network dynamics and easy to implement Structured network is key to simpler administration Thank you!
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