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SCRIBE

SCRIBE. A large-scale and decentralized application-level multicast infrastructure. Overview. Pastry PAST distributed file system layered on top of Pastry SCRIBE decentralized publish/subscribe system. Pastry – Quick Review. Chord like routing Consistent hashing Prefix routing Leaf set.

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SCRIBE

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  1. SCRIBE A large-scale and decentralized application-level multicast infrastructure

  2. Overview • Pastry • PAST • distributed file system layered on top of Pastry • SCRIBE • decentralized publish/subscribe system

  3. Pastry – Quick Review • Chord like routing • Consistent hashing • Prefix routing • Leaf set

  4. Pastry – locality properties • Short routes • Total distance traveled • Average dist 1.59 to 2.2 times actual dist • Route convergence • Dist Traveled by 2 messages sent to same key • Equal to dist between to nodes before routes converge

  5. Pastry API • nodeID = pastryInit(Credentials) • Causes node to join pastry network • route(msg,key) • send(msg,IP-addr) • Applications must export: • deliver(msg,key) • forward(msg,key,nextID) • newLeafs(leafSet)

  6. SCRIBE • Built on top of Pastry • Support large number of groups • Handle a high rate of membership turnover • SCRIBE nodes can: • Create groups • Join groups • Multicast messages to groups

  7. SCRIBE API • create(credentials, groupID) • join(credentials, groupID) • leave(credentials, groupID) • multicast(credentials, groupID, message)

  8. SCRIBE – Creating a Group • Pastry route(msg, key) • SCRIBE route(CREATE, groupID) • groupID => hash textual name cat creator name • Message delivered to closest key which become rendez-vous point for the group (root of multicast tree for group) • Adds to local list of groups • Stores credentials • Alternative use itself as root • good choice if creator sends to group often

  9. SCRIBE – Joining a Group • Pastry route(msg, key) • SCRIBE route(JOIN, groupID) • routed to rendez-vous point • along the way multicast tree formed

  10. SCRIBE – Leaving a Group • Remove from local group children list • If list becomes empty forward to parent • Part of the multicast tree may be removed

  11. SCRIBE – Sending a multicast message • route(MULTICAST, groupID) • ask for rendez-vous IP address • If rendez-vous fails re-request rendez-vous point • Pastry handles node duplication • All messages are sent through the rendez-vous point

  12. SCRIBE – Repairing the Multicast Tree • Messages are delivered only in best-effort • may be out of order delivery • Periodic heartbeat message sent to all children • Child rejoins the tree through sending a new JOIN message if suspects parent has failed • Can repair rendez-vous point • Pastry handles node duplication in leaf nodes • Children nodes JOIN new root when missing heartbeat is detected

  13. SCRIBE – Forming a Multicast Tree • Rendez-vous point (root) • Forwarders • may or may not be members of the group • maintain a children table (IP and nodeID) for group

  14. SCRIBE - Strengths • Pastry handles root duplication • Rendez-vous point does not handle all join requests • Locality properties of Pastry • short routes • delay from rendez-vous point to member is short • route convergence • load imposed on physical network is small

  15. SCRIBE – Experimental Evalutation • Simulation experimental results • Focus on three metrics: • delay to deliver events to group members • stress on each node • stress on each physical network link

  16. SCRIBE – Simulator Evaluation • 5050 routers and 100,000 end nodes • 1,500 groups of different sizes • 10 different runs using same parameters but different random seeds • Averaged all results • Compared results with IP multicast

  17. SCRIBE – Delay Penalty • RMD – ratio between max delay using SCRIBE & max delay using IP multicast • RAD – ratio between average delay using SCRIBE • & average delay using IP multicast

  18. SCRIBE – Node Stress • Average node is responsible for forwarding a small number of multicast messages

  19. SCRIBE – Link Stress • Total num links = 1,035,295 • SCRIBE = 2,489,824 messages (mean 2.4) • IP multicast = 758,853 messages (mean 0.7)

  20. SCRIBE – Bottleneck remover • Bottlenecks • Low capacity nodes • High capacity nodes with extremely high children entries • Drop children if over capacity • Select child to drop and send message with children table • Child chooses new parent node and sends JOIN message • Result • Removes long tail in node stress graph • Increases average link stress

  21. SCRIBE – Scalability Small Groups • 50,000 nodes • 30,000 groups 11 members each group • SCRIBE performs poorly for large number small groups • SCRIBE collapse • Removes long paths • removing nodes that are not members of a group & have only one entry in their children table

  22. SCRIBE – Scalability Small Groups • Average link stress 6.1 to 3.3 • Average number of children 21.2 to 8.5

  23. SCRIBE - Conclusion • Fully decentralized • Support large number of groups • Support large group size • Multiple multicast sources per group • QUESTIONS

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