Opportunistic Overlays: Efficient Content Delivery in Mobile Ad Hoc Networks
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Current content-based publish/subscribe systems assume a fixed network environment in which nodes do not move and where the network topology remains relatively stable. For mobile environments, the resulting problem is a mismatch between static broker topologies and dynamic underlying network topologies. This mismatch will result in inefficiencies in event delivery, especially in mobile ad hoc networks where nodes frequently change their locations. This paper presents a novel middleware approach termed opportunistic overlays, and its dynamically reconfigurable support framework to address such inefficiencies introduced by node mobility in publish/subscribe systems. The opportunistic overlay approach dynamically adapts event dissemination structures (i.e., broker overlays) to changes in physical network topology, in nodes' physical locations, and in network node behaviors, with the goal of optimizing end-to-end delays in event delivery. Adaptation techniques include the dynamic construction of broker overlay networks and runtime changes of mobile clients' assignments to brokers. Opportunistic overlays and the adaptive methods they use are realized by a set of distributed protocols implemented in a Java-based publish/subscribe infrastructure. Performance evaluations use simulation, system emulation, and also involve running a representative application on an experimental testbed. Experimental results demonstrate that the opportunistic overlay approach is practically applicable and the performance advantages attained from the use of opportunistic overlays can be substantial.
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