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 node Features Not self-organized Not scalable Single point of failure/attack Most popular network - mp3 sharing Applications: Napster Central Server Peer 5 Peer 2 Peer 3 Peer 4

Unstructured P2P networks - Gnutella Random overlay networks No central index Start with nodes that know about peer servers and flood along the network Peers find neighbors Features: Scalability – Flooding limited by TTL Keyword search Cannot guarantee search Applications: Gnutella Freenet Peer 1 Peer 6 Peer 7 Peer 4 Peer 3 Peer 5 Peer 2

Structured P2P networks – Chord, Pastry, CAN Based on ‘Distributed Hash Tables’ Self-organized overlay networks Insertion and lookup in a bounded number of hops Features: Load balancing Fault-tolerance Decentralization Scalability Availability Flexible naming Applications: Chord Pastry Tapestry CAN N51 N8 N14 N21 N1 N38 N42 N48 N58 K54 Lookup (K54) N32

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

Central Server 0 and 640 Super Peer Super Peer Super Peer Super Peer Super Peer Chord-over-Chord Overlay Chord-over-Chord Overlay(CoCO) Chord used in local clusters – Super-peer as manager Super-peer redundancy - by assigning super-peers at the edge Super-peers form a Chord overlay network Super-peers maintain finger tables for cluster as well as the super-peer overlay Central Server consulted only if all Chord searches fail on the overlay

Central Server Super Peer Chord-over-Centralized Server Overlay Chord-over-Centralized server Overlay (CoCO) Super-peers maintain a direct link to the Central Server Central Server consulted in case of failed searches in local clusters Central Server may be single point of failure

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

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