Ferry access points and sticky transfers: Improving communication in ferry-assisted DTNs

Message ferries (MF) have been proposed as special nodes to improve communication in sparse, intermittent networks, such as delay-and disruption-tolerant networks (DTNs) by providing non-random message transfer opportunities to participating nodes. However, considering the mobility of nodes and the speed of the ferry, merely ensuring non-random encounters with hosts and the ferry do not guarantee optimal performance as the contact duration may not be enough to ensure adequate message transfers. The end-to-end performance could suffer significantly if message sizes are large. To address this issue, we present two methods that improve the performance of ferry-assisted DTNs: ferry access points (FAPs) and sticky transfers. FAPs supplement a ferry-assisted DTN with rendezvous points that increase the contact opportunities between mobile nodes and the ferry. Sticky transfers, upon mutual agreement, allow two encountered nodes to remain within transmission range of each other until they are able to complete necessary message transfers. This improves the number of successfully forwarded messages by allowing more messages to be transferred during the contact duration. Sticky transfers inherently reduce message transfer aborts. Our simulation results demonstrate that particular FAP placements improve the delivery ratio by as much as 25% for sparse networks and reduce the delay by as much as 40% for relatively dense networks. Sticky transfers on top of FAP-based DTNs achieved 100% delivery ratio for most situations, except in the case of highly-loaded, dense networks. Additionally, using sticky transfers resulted in lower delays by as much as 65% compared to non-sticky transfer cases.