Pilot : P robabilist i c L ightweight Gr o up Communication Sys t em for Ad Hoc Networks

1. Pilot: Probabilistic Lightweight Group Communication System for Ad Hoc Networks Authored by Luo, Eugster, and Hubaux Presented by Jin-Hee Cho

2. Group Communication System (GCS) in Ad Hoc Networks • GCS: A useful infrastructure on which various reliable distributed computing functions can be built. • Mobility management • Distributed management of cryptographic keys or certificates • Access control • Key management

3. Overview of Paper • Pilot provides probabilistic reliability for: • Multicast: RDG & RRDG(R2DG) • Use gossip mechanisms • Data Sharing: PAN • Use probabilistic quorum systems • Present analytical results & simulation results using ns-2 • Show a tradeoff between reliability (Rd) and efficiency (Nl)

4. Related Work: Gossip-Based Probabilistic Reliable Multicast • Probabilistic reliable multicast protocols • Reduce the protocol overhead by sacrificing safety guarantees by using gossip-based dissemination scheme. • Examples • Probabilistic Broadcast • Lightweight Probabilistic Broadcast • Anonymous Gossip (AG)

5. Related Work: Probabilistic Quorum Systems • State Machine approach • “write all- read one.” • Synchronizing a set of replica to handle all updates and queries in the same way. • Perfect guarantee in theory BUT too expensive in reality. • Original (Strict) Quorum System • “write many-read many.” • A subset of quorums, each consisting of a subset of server replica, i.e. read quorum and write quorum. • Ensure the intersection of read and write quorums. • Probabilistic Quorum System • Relax the intersection property. i.e. no intersection may happen. • First introduced by randomized database groups [Hass99].

6. Related Work: Data Management in Ad Hoc Networks • 7DS [Papadopouli01]vs. Pilot • Diffusion scheme under different network environment: low density vs. high density • Data dissemination: node mobility vs. a gossip-based protocol. • [Hara01] & [Wang02]vs. Pilot • Mobility Model: network partition/prediction vs. random way point model.

7. Model • An ad hoc network with a set N of nodes • Every node with unique id. • Node failure/recovery • Underlying unicast protocol: DSR • Address two fundamental problems: multicast and data sharing.

8. Metrics for Reliable Multicast • Reliable Multicast Protocol: • Disseminates packets within a multicast group, G  N • Reliability Degree of Single Packet Dissemination (Rds): the fraction of group members that receive the packet sent by a certain member. • Reliability Degree of Continuous Dissemination (Rdc): the fraction of all packets that are received by a certain member with the rate λ0 • Both metrics are described by respective cdf F(x): [0,1][1,0] the probability that Rds or Rds is at most x.

9. Metric for Reliable Data Sharing • Reliable Data Sharing Service • Storage set, STS  N where is a set of nodes. • ρ is a set of access protocols for STS. • STS holds shard data in a replicated fashion using the consistency model of data replication called “shared private.” • “shared private”: the object is owned by a particular node. Only this node can modify the object while others can read it. • Given access rates λu andλq for updates and queries, the data sharing service is probabilistically reliable in nature if a query access ρq (STS, λq) obtains, with a certain probability, the latest version of a data object resulting from an update access ρu (STS, λu). • Reliability Degree of Access (Rda): Probability that a query operation acquires the most recent update of the corresponding data object, considering both node and channel failures.

10. Metric for Overhead & Goal • Network load (Nl) • Average number of unicast packet*hop per multicast packet to achieve a certain Rds or per unit time to achieve a certain Rdc or Rda • Goal: design a set of protocols that achieve a high reliability degree Rd (Rds, Rdc, & Rda) even under high λ0 while incurring reasonable overhead, Nl. • Show an efficient tradeoff between Rd and Nl.

11. Route Driven Gossip (RDG): a gossip-based probabilistic multicast protocol Reliable RDG (R2DG): devised for continuous packet dissemination and detects packet loss Probabilistic quorum system in Ad hoc Network (PAN): Any node in STS is termed server while the rest of the nodes are termed clients of STS. Layered Architecture of Pilot • Data query and updates: arbitrary servers in STS • Message dissemination within STS: RDG.

12. Gossip-based multicasting in RDG

13. Message exchange for updating and querying the STS in PAN

14. RDG: Basic Pilot Multicast Protocol • Pure gossip scheme basis • Use available routing information • Random subview works well • Each packet with pid [gid, sid, seq] • Four data structures • Data management: pidList, Buffer • Membership management: gidList, Views • AView (active view), PView (passive view), RView (remove view) • Each node has four subrecords (pidListigid,Bufferigid, gidListigid, Viewigid)

15. RDG Operations: Join session • GROUPREQUEST message • Update AView by all other members • GROUPREPLY with probability Preply • Update AView by initiator • Maintain AView and Pview updated by recording the route of each incoming packet • Reinitiate if the size of AView drops under threshold.

16. RDG Operations: Gossip/Leave session • Three protocol parameters in GOSSIP task • F: fanout is the number of gossip destinations randomly selected from the AView for each gossip emission. • τa : the quiescence threshold means that each packet will be removed from Buffer after having been gossiped for τq rounds by individual nodes. Thus, it limits the number of gossip rounds. • τq: the age threshold limits the propagation range of each packet. Thus, it indicates how many times a packet is repeatedly relayed by a certain group member.

17. RDG Operations: Gossip/Leave session • Multicast • Node Leave

18. RDG Operations: Gossip/Leave session Update views   gossiping gossiping • Packet Emission

19. RDG Operations: Gossip/Leave session • Packet Reception

20. R2DG: Continuous Packet Multicasting Service • Same data structure as RDG except larger Buffer size to detect missing packets • Detects missing packets by examining the pid of sequence of received packets • Pull packets or piggybacks the pull information. • Multicast & Pull task

21. R2DG: Continuous Packet Multicasting Service • Packet reception and the response to pull

22. PAN: Reliable Data Sharing Service • Client protocol: requests to an arbitrary server in the STS, which is terms an agent. • One-to-one connection. • Each message: mid [sid, oid, ver] • Server protocol: maintains a quorum system building upon the STS with the support from the underlying RDG protocol. • Nominal quorum size: • Real quorum size: • Read quorum: R • Write quorum: W

23. PAN: Server Update Protocol • UPDATE emission • UPDATE reception

24. PAN: Server Query Protocol • QUERY emission (Agent Servers)

25. PAN: Server Query Protocol • QUERY reception (Servers Agent)

26. PAN: Server Query Protocol • REPLY reception at an agent

27. Examples of Protocol Operations All members receive the message. 9 3, 5 51, 19 101, 13 1310, 15 21, 8 15 5, 3 910, 13 10 2, 8 15 9, 10 F = 2 τa= 2 |G| = 10 N = 20

28. Examples of Update/Query in PAN |STS| = 25 N = 50 M25: Update M27: Query M12: the intersection of write and read quorum

29. Analysis Model • |G| = n members/servers • Gossiping operations: a uniform random selection of F members out of n. • Infected member: A member that has received a certain packet. • Susceptible member: A member that has NOT received a certain packet. • Infectious member: A infected member who keeps gossiping the packet. • A node gossips in synchronous rounds (Tms) • pf: failure probability for each hop along a routing path (pfc « pfmo ≈pf) failure due to network condition. • H: the number of hops • pe: server unavailability due to failure at any instant time.

30. Analysis Model • No consideration for R2DG and client protocol in PAN. • Overall access rate λ0 = λq + λu • The dissemination process of the server update by RDG • Consider only the second query to a data object that was modified by the most recent update, while considering the first query as happening before the update.

31. Stochastic Behavior of RDG • Sr: the number of members infected with the packet after round r.

32. Stochastic Behavior of RDG: Recurrence Relation (1) • The probability of having successfully infected members in round r+1 among susceptible members in round r by infectious members of round r. Group r+1 round n k = j-i i r round

33. Stochastic Behavior of RDG: Recurrence Relation (2) & (3) • The probability of having i infected members in round r.

34. Stochastic Behavior of RDG: Computation of p • The probability of infection p can be estimated under two conditions: (i) the considered node is chosen as the gossip destination and (ii) the gossip message is successfully received.

35. Stochastic Behavior of RDG: Reliability Degree Rds and Rdc p1 is the probability that a certain member is infected by receiving a single packet in round r. M: the number of packets in a stream.

36. Stochastic Behavior of RDG: Network Load (Nl) • Network Load for a single packet dissemination • Network Load for continuous packet dissemination

37. Stochastic Behavior of PAN: Reliability Degree Rda The probability for read quorums to intersect with write quorums in each round r pdf of real read quorum pdf of real write quorum probability that a query happens within a certain period (between round r and r+1)

38. Stochastic Behavior of PAN: Network Load (Nl)

39. Simulations: Model and Parameters • ns-2 with the Monarch Project wireless and mobile extension. • Simulation area: 1km2 • Random waypoint model • Gossip period: 200ms • λ0 = 5 pkt/s • λ0 = 8 λu • Vary F and Ξr • pe = 0.01 • Simulated time: 400 seconds • N = 100, |G| = n = 50 • Paired speed/pause time: 2m/s, 5m/s, 10m/s and 20m/s, and 10s, 20s, 40s, and 80s.

40. Single Packet Dissemination ReliabilityRds • Speedmax = 2m/s Timepause = 40s • (a) vary F with τa= 1 (2) vary τa with F = 3

41. Continuous Packet Dissemination Reliability (Rds) and Network Load (Nl) • |G| = 50. mobility is varied. R2DG-pull mechanism.

42. Continuous Packet Dissemination Reliability (Rds) and Network Load (Nl) • Speedmax = 1m/s, Timepause = 10s • Scalability Effect

43. Impact of λ0 on PAN Performance • F = 2, ξR = 4, relatively stableduring 1.5/s< λ0 <3/s

44. Access Reliability (Rda) and Network Load (Nl) • Normal density network, F = 2, ξR = 4

45. Conclusion • To realize probabilistic reliable group communication in mobile ad hoc networks, two fundamental problems are studied, namely multicast and data sharing. • Pilot uses gossip mechanism and probalistic quorum systems. • Analytical results and simulation results.

46. Questions?