1. Formal Verification of AODV Protocol using cadence SMV Jun Wang and Xin Liu jwang@cs.ubc.cajwang@cs.ubc.ca, liu@cs.ubc.caliu@cs.ubc.ca (CPSC513 Course Project)

2. Outline  Motivation  AODV Introduction  cadence SMV Introduction  Building Model  Conclusion

3. Motivation  Find an appropriate approach to use cadence SMV verifying real-life software communication protocols, such as AODV. The emphasis is how to build the model.  Find some bugs in current AODV protocol (RFC3561)? Hope we can …RFC3561

4. AODV Introduction  Background A mobile ad hoc network is a collection of nodes which communicate over wireless channels. “ ad hoc ” means that the nodes are free to move randomly. need routing protocol  Proactive: table driven  Reactive: on demand  AODV (Ad hoc On-demand Distance Vector) Protocol a reactive routing protocol for ad hoc mobile networks. 3 control messages: RREQ, RREP, REER Sequence number Important property: Loop free

5. cadence SMV Introduction  Find an appropriate approach to use cadence SMV verifying real-life software communication protocol, such as AODV. The emphasis is how to build the model.  Find some bugs in current AODV protocol (RFC3561)? Hope we can …RFC3561

6. Distributed Architecture Distributed vs. Centralized  Distributed Robustness: failure of any participant has no effect on other participants. Scalability: while in centralized architecture, server is the bottleneck. Minimum delay: information crosses the network only once to reach its final destination, while in centralized architecture, information reaches its destination through the server.  Centralized Consistency: Global consistency is guaranteed. Cheating difficult: the presence of server makes cheating difficult. Easily charge : allow game companies to easily charge players.

7. Distributed Architecture (cont.)  MiMaze Architecture: Note: the game server is only used when a new entity joins a session, to learn the session group and to download the game.

8. Distributed Architecture (cont.)  Fully distributed, Serverless architecture.  Based on RTP/UDP/IP multicast.  Only one multicast group in MiMaze.  Game server is only used when new participant joining the game session.  Once the game has started, participants exchange their own states (including position, direction, speed, … ) to compute the game state.  In MiMaze, no central entity (or server) exists to compute the game state, each participant computes its own view of the game state (may be inconsistent).

9. Bucket Synchronization Mechanism  Distributed synchronization All ADUs issued at the same time are computed together to evaluate the state of the game. All the session entities display the same game state simultaneously.  MiMaze bucket synchromization Time is divided into fixed length periods and a bucket is associated with each period. All ADUs received by a player are stored in the bucket corresponding to the time interval in which senders issue the ADUs. At the end of every bucket interval, all ADUs in that bucket are used by the entity to compute its local view of the global game state. Bucket will be computed 100ms after the end of the sampling period during which ADUs have been issued, which is the playout delay.

10. Bucket Synchronization Mechanism  Bucket frequency The bucket frequency defines the rate at which a new game state is computed and displayed. In MiMaze, the bucket frequency is 25 buckets per second. The ADU transmission frequency is equal to the bucket frequency, thus, there should be one new ADU per entity at the time a bucket is processed.

11. Bucket Synchronization Mechanism  Example:

12. Bucket Synchronization Mechanism  Global clock mechanism The bucket synchronization mechanism uses NTP to evaluate the delay between participating entities. If NTP is not available, use a NTP-like algorithm based on the evaluation of the round trip time. There are 3 levels (strata) of NTP servers and it is very difficult to maintain good synchronization when level 3 servers are involved. In order to increase the precision of NTP under stratum 2 and 3, use both NTP and NTP-like algorithm.

13. Dead reckoning  What is dead reckoning? Dead reckoning is an extrapolation techniques used in the aviation systems to compute an estimate of the current position of a plane based on the knowledge of its position in the past and on its trajectory.  An important property of game objects ’ behavior: “ Continuous ”, which means that the behavior of avatar X at time t+1 can be extrapolated from its behavior at time t with high possibility.

14. Dead reckoning (cont.)  Why need dead reckoning? To deliver a complete view of the game, the bucket algorithm requires at least one ADU per participant to be available in each bucket. However, a ADU can be missing or late (received after 100ms).  MiMaze uses the simplest dead reckoning algorithm When an avatar position (ADU) is missing at the time we compute a bucket, simply replay the last known position of this avatar. The late ADUs are still stored in their destination bucket, since they are useful to replace a missing ADU when computing future buckets.

15. Dead reckoning (cont.)  Error Control: replace lost ADUs.  Synchronization: extrapolate late ADUs.  Trajectory smoothing: interpolation between received states when sender frequency is lower than display frequency.  Anticipating collisions: dead reckoning can be used by the sending entities to anticipate potential future collisions.  Congestion control: adapt ADU transmission frequency according to network load.

16. Performance evaluation  Experimental environment: The evaluation of MiMaze is performed on the MBone with up to 25 players located in various places in France. The number of participants are varied in order to vary the loss rate. The architecture of the experimental multicast tree:

17. Performance evaluation (cont.)  Monitoring During the experiment, participants play MiMaze and collect traces which is composed of: Network level data: for each received ADU, collect the senders ’ identity, the transmission and reception time-stamp, etc. Application level data: information contained in all ADUs sent and received is time-stamped and collected in the trace file, which is used to reconstruct the state of game computed by each participant. Synchronization data: network delays and clock offsets with respect to each participant are collected. These data is used to resynchronize the traces.

18. Performance evaluation (cont.)  Resynchronize traces (main difficulty in analysis) Since the system is distributed, there is no absolute clock in a game session, each participant timestamps its local traces. Global clock mechanism is imperfect, use an algorithm based on least-squares to resynchronize traces. Omit time intervals where relative clock information is not interpretable.

19. Performance evaluation (cont.)  Distributed game metrics Need a meaningful performance metric to analyze distributed multiplayer games. This metric should reflect how far the distributed game behavior is from the perfect behavior if without the network delay and lost. Use “ drift distance ”. The drift distance represents the distance units between the real position of an avatar in local entity, and the position of the same avatar computed by a remote entity. The game is consistent if the drift distance is zero, however, a small drift does not means that the game is inconsistent.

20. Performance evaluation (cont.)  In order to keep figures clear, the paper present performance observed between only two participants (droopy and hugo).  Delay distribution: (between hosts hugo and droopy)

21. Performance evaluation (cont.)  Distribution of late ADUs:  Percentage of late ADUs: Note: between hosts hugo and droopy.

22. Performance evaluation (cont.)  Percentage of lost ADUs:  Distribution of lost ADUs: Note: between hosts hugo and droopy.

23. Performance evaluation (cont.)  Drift distance:  Distribution of drift distance: Note: between hosts hugo and droopy.

24. Performance evaluation (cont.)  Consistency improvement (gain) with distributed synchronization: Note: a value 1 on the vertical axis corresponds to no gain and 2 corresponds to a 100% gain.

25. Summary  MiMaze is implemented on a completely distributed communication architecture based on the IP multicast protocol suit (RTP/UDP/IP multicast).  MiMaze use a simple distributed synchronization mechanism - the bucket mechanism to provide an acceptable level of game state consistency.  MiMaze shows that real-time interactive can be maintained in the fully distributed applications, provided that some level of inconsistency can be tolerated by the applications.

26. Discuss  Main Contribution: This work shows that with a multicast communication architecture and with a simple synchronization mechanism (the bucket mechanism), a fully distributed interactive application can provide an acceptable level of consistency. It also identified the problem of defining a proper metric for the evaluation of the application consistency. This work is a proof of feasibility of distributed multiplayer architectures on heterogeneous best-effort networks (Internet).

27. Discuss (cont.)  Related Work: Amaze: can be considered as MiMaze ’ s ancestor. It is played on a LAN, using point-to-point communication. Amaze transmits the game state on the network, and maintains replicated copies of the game state. Spline: a virtual distributed interactive world. It has a distributed architecture based on the DIS standard. Most of the efforts has been done on local flow synchronization mechanism. PARADISE: aims to architect and build a large-scale internet worked simulation environment that supports multiplayer interactive applications. It aims primarily at aggregation and specific dead reckoning algorithms.

28. Discuss (cont.)  Future Work: Extensions of MiMaze (has done)  Using Synchronized 3D Virtual Reality Modeling Language (VRML) to get a 3D game.  Mapping MPEG video on the maze wall.  Supports 3D spatial audio. Congestion control Avatar collision detection and anticipation Session management in subgroups of participants.

29. References  MiMaze http://www-sop.inria.fr/rodeo/MiMaze/http://www-sop.inria.fr/rodeo/MiMaze/  Design and Evaluation of MiMaze, a Multi-player Game on the Internet, Laurent Gautier, Christophe Diot, IEEE Multimedia Systems Conference, Austin, June 28 - July 1, 1998.  Distributed Synchronization for Multiplayer Interactive Applications on the Internet, Laurent Gautier, Christophe Diot, IEEE ICNP Conference, Austin, October 1998.  MiMaze, a 3D Multi-Player Game on the Internet, Emmanuel Lety, Laurent Gautier, Christophe Diot, In the Proceedings of the 4th International Conference on Virtual System and MultiMedia, Nov 98, Gifu, Japan.  IEEE Standard for Distributed Interactive Simulation -- Application Protocols (IEEE Std 1278.1-1995). IEEE Computer Society. 1995.  S. Deering. "Host Extensions for IP Multicasting". RFC 1112. 17. August 1989.

30. A Distributed Architecture for Multiplayer Interactive Applications on the Internet Thanks! Thanks!