# Mars to Earth Data Downloading: A Directory Synchronization Approach
## 1 Introduction  
Delay/Disruption-Tolerant Networking (DTN) architecture originates from research on Inter-Planetary Networking (IPN). In the early 1990's [PERSON] proposed broadening the scope of IPN to embrace all \"challenged networks\" such as satellite and interplanetary networks, wireless sensor networks, tactical networks, opportunistic communications, emergency communications, underwater communications, etc. [1; 2]. These networks are dissimilar, but have in common the fact that the TCP/IP architecture is challenged by one or more of the following impairments: long delays, frequent random losses (due to the channel), disruption, opportunistic contacts and network partitioning [3]. To cope with these problems, the DTN architecture introduces a new \"Bundle Layer\", to be inserted between the Application and Transport layers of the ISO-OSI model. The new architecture was defined in RFC4838 [4] and its pillar, the Bundle Protocol (BP), in RFC5050 [5]. At present, standardization is carried out by IETF (Internet Engineering Task Force) [6] and by CCSDS (Consultative Committee for Space Data Systems) for space environments [7]. DTNbox is a DTN application developed by the authors, which aims to synchronize directories between DTN nodes [8]. It was designed to be peer-to-peer and robust against long delays and link disruption, not an easy task, as synchronization in general would clearly benefit from short latency and continuous links, exactly what is missing in challenged networks. As other applications of DTNsuite [9], DTNbox is fully compatible with all the major open source implementations of the BP: DTN2 [10], the original \"reference\" implementation of the IRTF DTN group; ION [11] by NASA-JPL; IBR-DTN, by the University of Braunshweig [12]. Although the rationale and features of DTNbox are fully described in [8], the advantages that can derive from its application to actual scenarios have not yet been investigated. The idea explored in this paper is to use DTNbox as an alternative to direct data transfer for bulk data, which would have the advantage of decoupling data generation from transmission. If the source already generates bulk data as files (e.g., images or results of scientific experiments), these files could be put into a synchronized directory and then automatically transferred to destination by means of DTNbox. This way there would be no need to modify the data generation program to interface it with the API of one of the BP implementations, or to write custom scripts for file transfers, which would be feasible but peculiar to each application and probably not as robust as DTNbox. The disadvantage is that indirect file transfer necessarily adds some delay to delivery, which is the reason why this solution is not appealing in ordinary networks where connectivity is continuous and delays are of the order of only very few milliseconds. By contrast, in a challenged network, propagation delays can be huge (minutes or hours) and disruption can force bundles to be stored for long periods at intermediate nodes, waiting for a new contact to the next node. For both reasons, the additional delay due to indirect transmission can become negligible, which makes the directory synchronization solution appealing, at least for bulk data, for which fastest delivery is not the primary concern. To assess validity of the proposed DTNbox-based approach, we have preferred to rely on a scenario already investigated instead of building an \"ad hoc\" scenario from scratch, thus avoiding by root any possible bias. To this end, we have considered a simplified variant of the Earth-to-Mars scenario already studied in the context of the \"Delay tolerant network for flexible Communication with Earth Observation satellites\" project funded by ESA/ESOC [13], whose results were preliminary presented in [14] and then fully analyzed in [15].