Interplay of Contact Times, Fragmentation and Coding in DTNs

Models describing DTNs proposed in the last few years have been focusing on message replication policies able to achieve high delivery probability at the cost of network resources, e.g., message copies. To this respect, the duration of contact events is the physical constraint that dictates how fine a large message should be fragmented into packets in order to match finite contacts duration. The price to pay, indeed, is that the source has to deliver a larger number of packets per message. In this paper we model the combined effect of message fragmentation and buffering and describe the structure of the forwarding process in closed form when the message is split into K packets and delivered to the destination. We consider the specific case of sequential forwarding, where the source delivers fragmented message packets in order to relays. In this case the interplay of forwarding and message fragmentation can be expressed in closed form by coupling the combinatorial structure for packet forwarding and fluid models for the replication of packets in the network. By deriving the closed form expression for the delivery probability, we are able to derive the optimal fragmentation K* as a function of the contact time distribution. Finally, results on sequential forwarding are applied to derive performance figures for the case when fountain coding is applied: redundancy is added to the original information with the aim to increase the message delivery probability. The paper is completed by numerical results.