Delay performance of a Publish Subscribe system deployed over a memory-constrained, Delay Tolerant Network

In this work we assess the delay performance of a Publish Subscribe system built on top of a Delay Tolerant Network (DTN) composed of nodes with limited storage capacities. Many DTN routing protocols replicate the same data over several nodes, in order to deliver data to destination in a faster or in a more reliable way. Of course, increasing the number of replicas has the effect of decreasing the delivery delay perceived by the users, but increases the use of the system memory. Our goal is to investigate the trade-off between reduction of delay and storage requirements when nodes are memory constrained, in a Topic-based, Publish Subscribe system where we have different topics with different popularity. We provide some insights in this trade-off, which implies some unanticipated issues, and propose simple rules to dimension the number of replicas per topic. To this end, we derive analytical models and we validate them with simulations.