Performance evaluations of data-centric information retrieval schemes for DTNs

Mobile nodes in some challenging network scenarios, e.g. battlefield and disaster recovery scenarios, suffer from intermittent connectivity and frequent partitions. Disruption Tolerant Network (DTN) technologies are designed to enable communications in such environments. Several DTN routing schemes have been proposed. However, not much work has been done on designing schemes that provide efficient information access in such challenging network scenarios. In this paper, we explore how a content-based information retrieval system can be designed for DTNs. There are three important design issues, namely (a) how data should be replicated and stored at multiple nodes, (b) how a query is disseminated in sparsely connected networks, and (c) how a query response is routed back to the issuing node. We first describe how to select nodes for storing the replicated copies of data items. We consider the random and the intelligent caching schemes. In the random caching scheme, nodes that are encountered first by a data-generating node are selected to cache the extra copies while in the intelligent caching scheme, nodes that can potentially meet more nodes, e.g. faster nodes, are selected to cache the extra data copies. The number of replicated data copies K can be the same for all data items or varied depending on the access frequencies of the data items. In this work, we consider fixed, proportional and square-root replication schemes. Then, we describe two query dissemination schemes: (a) W-copy Selective Query Spraying (WSS) scheme and (b) L-hop Neighborhood Spraying (LNS) scheme. In the WSS scheme, nodes that can move faster are selected to cache the queries while in the LNS scheme, nodes that are within L-hops of a querying node will cache the queries. For message routing, we use an enhanced Prophet scheme where a next-hop node is selected only if its predicted delivery probability to the destination is higher than a certain threshold. We conduct extensive simulation studies to evaluate different combinations of the replication and query dissemination algorithms. Our results reveal that the scheme that performs the best is the one that uses the WSS scheme combined with binary spread of replicated data copies. The WSS scheme can achieve a higher query success ratio when compared to a scheme that does not use any data and query replication. Furthermore, the square-root and proportional replication schemes provide higher query success ratio than the fixed copy approach with varying node density. In addition, the intelligent caching approach can further improve the query success ratio by 5.3-15.8% with varying node density. Our results using different mobility models reveal that the query success ratio degrades at most 7.3% when the Community-Based model is used compared to the Random Waypoint (RWP) model [J. Broch et al., A Performance Comparison of Multihop wireless Ad hoc Network Routing Protocols, ACM Mobicom, 1998, pp. 85-97]. Compared to the RWP and the Community-Based mobility models, the UmassBusNet model from the DieselNet project [X. Zhang et al., Modeling of a Bus-based Disruption Tolerant Network Trace, Proceedings of ACM Mobihoc, 2007.] achieves much lower query success ratio because of the longer inter-node encounter time.