Deep-Space Transport Protocol: A novel transport scheme for Space DTNs Giorgos Papastergiou, Ioannis Psaras * , Vassilis Tsaoussidis Dept. of Electrical and Computer Engineering, Democritus University of Thrace, 12 Vas. Sofias Str., 67100 Xanthi, Greece article info Article history: Available online 23 February 2009 Keywords: Deep-space communications Double Automatic Retransmission Deep-Space Transport Protocol High Packet Error Rate abstract The Delay-/Disruption-Tolerant Networking Architecture calls for new design principles that will govern data transmission and retransmission scheduling over challenged environments. In that context, novel routing, transport and application layer algorithms have to be established in order to achieve efficient and reliable communication between DTN-nodes. In this study, we focus on the evolution of the terrestrial Internet into the Interplanetary or Space Inter- net and propose adoption of the Deep-Space Transport Protocol (DS-TP) as the transport layer scheme of choice for the space networking protocol stack. We present DS-TP’s basic design principles and we eval- uate its performance both theoretically and experimentally. We verify that practice conforms with theory and observe great performance boost, in terms of file delivery time between DTN-nodes, in case of DS-TP. In particular, the gain of DS-TP against conventional proposals for deep-space communications increases with the link error rate; under conditions DS-TP can improve the performance of the transport layer pro- tocol by a factor of two (i.e., DS-TP can become two times faster than conventional protocols). Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The increasing interest for Space Exploration by the space agen- cies world-wide has forced the deployment and evolution of data communication networks into the outer space environment. There is common consent among space agencies that telecommunication technologies, such as circuit switched networks, do not perform efficiently in the space environment. Therefore, the Extension of the Internet into Space has become a common goal for scientists and space agencies world-wide. In this context, the Delay-/Disrup- tion-Tolerant Networking (DTN) Architecture [6,18] has been pro- posed to provide an overlay for the Interplanetary or the Space Internet [2,13]. Later, DTN became an interesting idea for challeng- ing environments within the terrestrial Internet as well. For exam- ple, DTNs are expected to provide connectivity to the edges of the current Internet infrastructure. In that sense, the DTN Architecture is a potential candidate as an overlay for ad hoc sensor networks, for instance. However, it is still not clear at all whether common rules can apply for the whole spectrum of Delay-/Disruption-Tolerant Net- works. For example, reliability guarantees are different for a sensor that gathers temperature samples on the surface of the Earth and a rover/sensor that collects scientific data on the surface of Mars. That said, routing, buffering and congestion avoidance and control issues may exhibit different properties for a terrestrial and a Space DTN. For example, although connectivity may be intermittent in both environments, in a terrestrial DTN once connectivity exists the propagation delay between any two nodes of the DTN will probably be in the order of tens or at most hundreds of millisec- onds. In contrast, in a Space or Interplanetary DTN, even when con- nectivity exists the protocol has to be delay-tolerant, since propagation delays are in the order of tens of minutes. In that con- text, the goals of a transport layer protocol for Space DTNs are dif- ferent from the goals of a transport scheme for terrestrial DTNs [8,9,1]. In this paper, we focus on (Deep-) Space or Interplanetary DTNs and evaluate the performance of a novel transport layer scheme, namely the Deep-Space Transport Protocol (DS-TP), which was ini- tially introduced in [15]. DS-TP introduces proactive transmission and retransmission scheduling rules in order to deal with the unique characteristics of the deep-space networking environment (e.g., huge propagation delays, high bit error rates, intermittent connec- tivity, etc.). In particular, DS-TP’s basic design principles are based on the fact that deep-space communications are handled, at least presently, by human-operated management procedures that take place long before the mission execution itself. Moreover, DS-TP uti- lizes the hop-by-hop, store and forward message switching principle that governs today’s space communications and mitigates the need for congestion avoidance and control. Based on the above, DS-TP transmits data at the a priori-known and predetermined line rate. This way, DS-TP achieves high link utilization from the beginning 0140-3664/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.comcom.2009.02.012 * Corresponding author. Tel.: +30 25410 79554; fax: +30 25410 79576. E-mail addresses: gpapaste@ee.duth.gr (G. Papastergiou), ipsaras@ee.duth.gr, ipsaras@gmail.com (I. Psaras), vtsaousi@ee.duth.gr (V. Tsaoussidis). Computer Communications 32 (2009) 1757–1767 Contents lists available at ScienceDirect Computer Communications journal homepage: www.elsevier.com/locate/comcom