1. Distributed Token Circulation in Mobile Ad Hoc Networks Navneet Malpani, Intel Corp. Nitin Vaidya, Univ. Illinois Urbana-Champaign Jennifer Welch, Texas A&M Univ. Presented at Int’l Conf. on Network Protocols, Nov 2001 http://faculty.cs.tamu.edu/welch/papers/icnp01.ps or pdf

2. Introduction Mobile Ad Hoc Networks (MANETs) –Formed by a collection of wireless mobile hosts, without making use of any existing infrastructure (such as base stations or telephone lines). –Pair of nodes communicate with each other either over a wireless link between the two nodes, or by traversing a sequence of wireless links over several other intermediate nodes.

3. Example Mobile Ad Hoc Network AB C DE C DE BA

4. Introduction continued Usefulness –Disaster recovery –Search and rescue in remote areas –Military operations Characteristics of Mobile Ad Hoc Networks –Highly dynamic topology –Highly variable message delays –Variable transmission error rates –Constraints on energy consumption –Constraints imposed by wireless interfaces

5. Token Circulation Definition Ensure that a token circulates throughout the network, visiting every node infinitely often. Round: Minimal length sequence of nodes visited to ensure that every node is visited at least once.

6. Token Circulation Example AB C DE T T T T TT A B C E D C A B C E D C A... Length of round 1: 5 Length of round 2: 6 Length of round 3: 6

7. Token Circulation Application Total order of message delivery in a group communication service Key features of a group communication service: –Maintaining information regarding group membership –Communication among nodes in the group in an ordered manner

8. Token Circulation Application Token carries a sequence number, which is always incremented. Sender multicasts message with sequence number; receiver delivers in order. OR Messages are stored in the token itself (large token). Additional mechanisms are needed to obtain desired level of reliability.

9. Token Circulation Algorithms Local Least Recently Visited (LR): forward token to neighbor visited least recently Local Least Frequently Visited (LF): forward to neighbor visited least frequently AB C DE LR: ACBCDE CACBCDE CACBCD E... LF: ACBCDE DECACB CDEDEDECACB C...

10. More TC Algorithms Choose next destination among all nodes. Global Least Recently (GR): forward to any node in network visited least recently Least Frequently (GF): forward to any node in network visited least frequently

11. Yet More TC Algorithms GRN: Global Least Recently + visit intermediate nodes on the path GFN: Global Least Frequently + visit intermediate nodes on the path (not studied) Iterative Search: try to find Hamiltonian Path using more history information (see paper for more details)

12. Performance Measures Round length: number of nodes visited by the token in a round Message overhead: number of bytes sent per round Time overhead: time required to complete a to complete a round

13. Simulation Results ns-2 simulator with CMU extensions System contains 20 nodes initially placed randomly in a 1000m x 300m box Random Waypoint mobility model Each algorithm runs as an application on top of TCP and DSR protocol Results for Static and Dynamic topologies

14. Static Topologies Plots of –number of nodes visited –number of bytes sent –amount of time taken during each round, averaged over 50 different scenarios

18. Discussion of Static Results LF diverges GR and GF trivially have best round length, but not so good on messages & time LR is quite good Iterative Search is best overall

19. Dynamic Topologies Varying speed (6, 12, 18 and 24 m/sec) with constant hello interval of 0.5 sec Varying hello interval (0.1, 0.3, 0.5 and 0.7 sec) with constant speed of 12 m/sec Hello Threshold: 3 Number of scenarios: 30 Duration of simulation was varied inversely with the speed

26. Discussion of Dynamic Results Random Nature of Results –Effect of uncertainty in the topology knowledge due to the hello protocol –Effect of the TCP timeout intervals when partitions occur –Chaotic nature of the algorithms themselves LR is the best! Close to optimal round length.

27. Conclusion Identified new problem for MANETs -- token circulation Proposed several distributed algorithms Compared them by simulation Overall best algorithm : –Iterative Search in the static case –LR algorithm in the dynamic case

28. Future Work Identify characteristics of graphs on which LR has good performance -- there are graphs on which it has exponential round length (cf. recent work by Yu Chen) Integrate token circulation with mechanisms for complete group communication service Make tolerant of token loss / partitions Find lower bounds on possible performance and find optimal algorithms