Data Delivery in Delay Tolerant Networks

Current Internet and mobile ad hoc protocols do not properly handle the delivery of data in networks with intermittent connectivity due to link instability. Such networks where nodes are characterized by opportunistic connectivity are referred to as Delay Tolerant Networks (DTNs). Delay tolerant networking is a networking paradigm that deals with the establishment of communication between two nodes when the connectivity is intermittent and/or subject to disruptions and thus a path connecting source and destination cannot be maintained over time. As a consequence, end-to-end latency cannot be guaranteed. A typical example of DTN are vehicular Ad Hoc networks, where a vehicle can send a message to the intended destination only when they are close, or by means of intermediate vehicles carrying a copy of the message. Other examples are military communications in the battlefield, deep space communications and wireless sensor networks. DTNs have been one of the growing topics of interest characterized by significant amount of research efforts invested in this area over the past decade. Routing is one of the major components significantly affecting the overall performance of DTNs in terms of resource consumption, delivery ratio and latency. Data delivery in DTNs mainly rely on the mobility of nodes, where data is transferred in a store-carry-forward fashion between a source and a destination using relays. Whenever two nodes are in proximity (within radio range of one another), they exchange information based on local knowledge to make forwarding decisions. In this context, several data delivery schemes have been proposed to choose the best relays that would bring data closer to the destination In this thesis, we discuss several aspects concerning routing and data delivery problems in DTNs. We start by proposing a reference architecture for DTNs routing protocols, which consider queue management, forwarding and replication as the main components for the routing process. According to this architecture we classify most of the routing protocols proposed in the literature. Moreover we present the first thorough quantitative evaluation and comparison of the protocols based on real traces and synthetic mobility models. We stress that in previous works either only qualitative comparisons have been presented or only a single category of protocols have been analyzed. Concerning real experimentation and implementation for data delivery in DTNs, we present two experiments. The first one is implementing our reference architecture for implementing DTNs routing protocols on resource constrained devices. The proposed architecture, has been employed and implemented on Openbeacon active RFID tags. In the second experimental activity we show how DTNs can be effectively used to complement wireless sensor networks (WSNs) in a data collection task. In particular, to increase the robustness of the WSN by using mobile nodes running DTNs protocols as data mules to connect two otherwise unconnected regions of the WSN. In the final part of the thesis we present a study on exploiting erasure coding technique for data delivery in DTNs as a means to generate a suitable amount of content redundancy. Here we present a mathematical model that captures the effect of redundancy on the delivery delay. Based on this study, we propose two data delivery schemes for biased and unbiased contact models that can reduce both storage overhead and delivery delay.