MobileCopy: Resisting correlated node failures to enhance data availability in DTNs

Motivated by the growing popularity of mobile devices and their increasing capacities, file sharing in disruption tolerant networks (DTNs) has attracted significant attention recently. Since nodes are sparsely distributed in separated areas and are intermittently disconnected in DTNs, it is difficult to achieve high data availability in file sharing. Many previous methods enhance file availability in DTNs through file replication. However, there has been no file replication method that tries to reduce data loss in correlated node failures, which however are common in wireless networks. In this paper, we propose a distributed file replication method (called MobileCopy) in DTNs, which aims to achieve low probability of totally losing a file at the expense of having a high number of impacted files in an individual large-scale correlated node failure. MobileCopy is designed for community-based file sharing systems. It has two main components: i) data loss resistant and popularity aware file replication, and ii) distributed hash table (DHT)-based file replica indexing. MobileCopy considers file popularity to determine the number of replicas of a file in each community. Through limiting the possible combination of candidate replica holders, MobileCopy greatly reduces the probability of node failures that will lead to data loss, i.e., losing all replicas of a file. Moreover, MobileCopy enables nodes to efficiently store and fetch the placement information of file replicas for efficient file searching. Extensive trace-driven experiments show that MobileCopy is robust against correlated node failures and efficient in file sharing in comparison with previous methods.