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Many distributed applications running on pervasive computing devices require a more general resolution of application-specific keys to application-specific values than just service discovery, a functionality provided by a lookup service. Such distributed application include peer-to-peer (P2P) file sharing, instant messaging, search in information portals and search in the Web. In mobile ad hoc networks (MANET), the lack of a fixed infrastructure, as well as interrupted connectivity due to device mobility hampers the employment of a centralized index for service location and lookup services.
In this talk, we first introduce a general-purpose distributed lookup service, denoted Passive Distributed Indexing (PDI). PDI stores index entries in form of (key, value) pairs in index caches located in each mobile device. Index caches are filled by epidemic dissemination of popular index entries. Thus, by exploiting node mobility PDI can resolve most queries locally without broadcasting messages outside the radio coverage of the inquiring node. For keeping index caches coherent, we introduce two novel consistency mechanisms for PDI index caches: configurable value timeouts implementing implicit invalidation and lazy invalidation caches implementing explicit invalidation. Subsequently, we show how to customize PDI for the mobile applications P2P file sharing, disconnected search in information portals and disconnected Web search.
In the second part of the talk, we outline a special-purpose approach for P2P file sharing tailored to MANET denoted Optimized Routing Independent Overlay Network (ORION). ORION comprises of an algorithm for construction and maintenance of an application-layer overlay network that enables routing of all types of messages required to operate a P2P file sharing system, i.e., queries, responses, and file transmissions. Overlay connections are set up on demand and maintained only as long as necessary, closely matching the current topology of the underlying network. In a detailed ns-2 simulation study, we show that the search algorithm outperform the off-the-shelf approach based on a state-of-the-art techniques for P2P file sharing system for the wireline Internet, TCP, and a MANET routing protocol.
Open source simulation software for the simulator ns-2 and related publications are available at the Web site http://www4.cs.uni-dortmund.de/~Lindemann/projects/MobileP2P/
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