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The Design of a Robust Peer-to-Peer System

The Design of a Robust Peer-to-Peer System. Reference: SIGOPS European Workshop 25/09/2002 Rodrigo Rodrigues(MIT). Gisik Kwon Dept. of Computer Science and Engineering Arizona State University. Motivation. What if we wanted to store the state of the application in a P2P system?

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The Design of a Robust Peer-to-Peer System

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  1. The Design of a RobustPeer-to-Peer System Reference: SIGOPS European Workshop 25/09/2002 Rodrigo Rodrigues(MIT) Gisik Kwon Dept. of Computer Science and Engineering Arizona State University

  2. Motivation • What if we wanted to store the state of the application in a P2P system? • E.g., mail service, archiving • Need a robust P2P storage system • Current algorithms do not provide reliable service: • No admission control • Trust configuration information • Vulnerable to malicious routing • Tolerance to Byzantine faults

  3. Configuration Service P2P Nodes Architecture • Configuration Services(CS) • Selected statically(a subset of P2P nodes) or dynamically • P2P nodes • set of servers, not clients • Common to equip with • Co-processor, read-only disk, watch-dog timer, Cryptographic co-processor

  4. Configuration Service • Four main functions: • Admission control • Node monitoring • Deciding on a new configuration • Propagating configuration information • CS nodes carry BFT protocol [CL99]

  5. Admission Control • Prevents single user controlling large fraction of the ID space • Maintains the list of node • <ID, IP, public key> • Hard to do in volunteer-based system • We assume servers • Nodes have public / private keys

  6. Node Monitoring • Detection fault node • Fail-stop failure: ping protocol • Byzantine failure: hard! • proactively recovered frequently -> restart from the correct code in read-only disk -> copy state from other nodes • All CS do its own monitoring for all P2P nodes

  7. Configuration Information • What to propagate? • Incomplete config is a limitation of p2p • Disseminate entire config • Transmit using diffs • When to propagate? • Server machines: not very often (e.g., every hour) • Configs include start and expiration times • Periodically deciding the new config • Using non-deterministic choices validation • Send eviction request to other CS -> validation

  8. Application Storage Storage Storage Lookup Lookup Lookup Client Server Server Architecture – P2P Nodes • Lookup layer • Periodically receive the latest config from CS • Notify to storage layer • Storage layer • Uses Byzantine quorums to store data • Data placement algorithm assigns 3 f + 1 replicas to each data item

  9. Correctness Condition • “At any moment, any group of 3 f +1 replicas of a data item contains no more than f faulty replicas” • Reconfiguration must be frequent enough to preserve this invariant

  10. State Transfer • During reconfiguration, set of replicas may change • Replica finds out of new configuration: • Know old configuration • Pulls data items from old replicas (as seen before)

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