On the Snapshot Problem in Mobile Ad Hoc Network Systems

Computing consistent global states in a distributed system is a fundamental problem. A large class of distributed system problems can be cast as construction of consistent global states and evaluation of relevant properties over these global states. Examples of such problems include the monitoring and debugging of distributed systems, the detection of stable properties such as deadlock, termination, and loss of tokens, protocol specification and verification, garbage collection, checkpointing and failure recovery, and many others.;However, consistent global states are not freely available in distributed systems without shared memory and synchronized clocks. In the literature, the fundamental problem of constructing consistent global states in a distributed system was defined as the snapshot problem. Although many solutions to the snapshot problem have been developed for various types of distributed systems, most of them cannot be applied directly to a mobile ad hoc network system, which has no fixed network infrastructure for operating support and may experience dynamic topology changes due to node mobility. In this thesis, we present a systematic study on the snapshot problem in mobile ad hoc network systems.;Firstly, we will develop a system model for a mobile ad hoc network system. In addition to the internal, send and receive events that are considered in a traditional system model for static systems, two novel types of events called on and off are introduced to represent dynamic topology changes in our system model. Based on these on and off events, it is convenient to devise new distributed algorithms that can handle dynamic topology changes for the mobile ad hoc network environment. We will also propose a new relation called the extended-happened-before relation, which is generalized from the well-known happened-before relation defined by Lamport, to fully model the ordering of events in a mobile ad hoc network system. The event orders captured by the extended-happened-before relation playa critical role in solving the snapshot problem at hand.;Secondly, we will derive a new consistency criterion for constructing a global state in a mobile ad hoc network system. Without a common time base and shared memory, the development of the new consistency criterion sticks to the fundamental principle that if the events for recording the local states are ensured to be concurrent, then the recorded local states are equivalent to those that are recorded simultaneously in real time and the resulting global state is guaranteed to be consistent. Importantly, we will also show a consistency theorem, which gives a necessary and sufficient condition for computing consistent global states in a mobile ad hoc network system. That is, a global state of a mobile ad hoc network system is consistent if and only if its corresponding cut is not only causally consistent but also topologically consistent.;Thirdly, we will propose a simple method called timing-and-tailoring to design and analyze snapshot algorithms in a well structured approach. In this generic method, a snapshot algorithm is decomposed into two basic components. The first component called timing is used to record the ordering of events by using logical time algorithms. The second component called tailoring is used to find a consistent global state based on known event ordering. To demonstrate the proposed timing-and-tailoring method, we will also present several examples of using this method to design and analyze snapshot algorithms. These examples provide helpful insights in designing new snapshot algorithms for mobile ad hoc network systems.;Finally, we will present a suit of three new snapshot algorithms for mobile ad hoc network systems: cooperative, localized, and centralized. These new snapshot algorithms have different timing and tailoring components, Compared to existing snapshot algorithms, our snapshot algorithms can be proven to work in the presence of node mobility and dynamic topology changes, i.e., all of them can compute consistent global states in a mobile ad hoc network system. We will also evaluate the effectiveness and efficiency of these newly proposed snapshot algorithms by using extensive simulations.