Assessing DTN Architecture Reliability for Distributed Satellite Constellations: Preliminary Results from a Case Study Juan A. Fraire *1 and Pablo A. Ferreyra † 2 * Digital Communications Research Lab, FCEFyN, Universidad Nacional de C´ ordoba † FCEFyN, Universidad Nacional de C´ ordoba; and Posgrado Sistemas Embebidos, IUA Argentina 1 juanfraire@famaf.unc.edu.ar 2 ferreyra@famaf.unc.edu.ar Abstract—Networked small satellites constellations can yield, in general, not only higher revisit rates but new mission opportunities with important cost and risk saving by means of successive small launches and distributed functionalities such as payload, storage, processing, or data downlink. Nevertheless, as this networks operates in challenged environments, they usually face resources constraints; moreover, orbital dynamics might impose sporadic channel availability. As a result, these intermittent inter-satellite communications challenges existing networking protocols as they assume persistent connectivity. To this end, Delay Tolerant Networking (DTN) has emerged as an automated store-carry-and-forward communication architecture capable to cope with contact disruption. In order to assess DTN reliability, we generalize the communications disruptions to also include transient and permanent component faults so as to demonstrate that DTN architecture result inherently fault tolerant as failures no longer implies a service outage but an overall system capacity degradation. To this end, we developed a network model encompassing DTN communication protocols, routing algorithms, and satellite failure models to measure the system capacity degradation under specific constellation topologies. Resumen— Las constelaciones de sat´ elites, en general, pueden ofrecer mayores tasas de revisita y nuevas oportunidades de misi´ on con disminuciones de costos y riesgos por medio del uso de lanzadores de menor escala y distribuci´ on de funcionalidades como carga ´ util, almacenamiento, procesamiento, o bajada de datos a tierra. Sin embargo, al operar en entornos extremos, con limitaci´ on de recursos, y bajo cierta din´ amica orbital, la disponibilidad de los canales de comunicaci´ on puede resultar espor´ adica. Como los protocolos tradicionales de Internet asumen conectividad permanente, la arquitectura Delay Tolerant Networking (DTN) ha sido propuesta para este contexto. Con el fin de evaluar la confiabilidad de este tipo de redes, incluimos las fallas como parte de las disrupciones para demostrar que la arquitectura DTN resulta inherentemente tolerante a fallas, donde las mismas ya no implican la discontinuidad del servicio, si no que una degradaci´ on de la capacidad del sistema. Para poder demostrar esto ´ ultimo, proponemos medir dicha degradaci´ on en un caso de estudio espec´ ıfico de constelaci´ on de baja ´ orbita por medio del desarrollo de modelos de red DTN, incluyendo sus protocolos y ruteo, as´ ı como modelos de fallas. I. I NTRODUCTION A spacecraft complexity is product of a required capabil- ity with certain robustness towards technical or environmen- tal uncertainty, due to the significant amount of time and costs involved in the design, manufacture, and operations process. Traditionally, the approach to optimize reliability in spacecraft is to provide layers of redundancy, fault tolerance through detection and recovery circuitry, coupled with radiation hardening [1]. As a result, the inclusion of more components to the platform also increases complexity (more parts and more complex interconnection network) creating new fault modes to be counteracted. Moreover, as complexity grows, so does the system size, cost, and schedule. As a self-fulfilling consequence of the system’s increased cost, a higher degree of reliability tends to be imposed to insure against a catastrophic loss. Therefore more margins and redundancy tend to be imposed: which is exactly the same reason for which component redundancy was considered on the first place. The described phenomena has shaped the actual space industry and the term death spiral was coined by recent researches in the area [2]. Distributed spacecraft architec- tures had emerged as an alternative approach by means of exploding spatial distribution of complexity. This is, dis- tributing the functionality in wirelessly interconnected mod- ules that together behaves at least as good as it monolithic counterpart. Benefits of this paradigm are risk reduction, re- placeability and upgradeability, production learning, payload coverage, among others. However many technological chal- lenges, such as inter-networking, need to be addressed [3]. In particular, if the system modules are autonomous enough to allow for intermittent inter-satellite communications, Delay- Tolerant Networking (DTN) results appealing as it assumes no persistent connectivity as traditional Internet-based pro- tocols. DTNs have received much attention during the last years as they have been proposed for several environments where communications can be challenged by either latency, band- width, errors, or stability issues [4]. DTN communication architecture is also designed to provide automated data communication services in networks characterized by het- erogeneity in protocol support below the application layer. Even if originally studied to develop a network architecture for the Interplanetary Internet (IPN) [5], DTNs have been recently recognized as an alternative solution for building future satellite applications [6]; in particular, to cope with typical intermittent (but predictable) channels of LEO (Low Earth Orbit) constellation systems [7].