The Disruption Tolerant Networking (DTN) program establishes a long-term, readily accessible communications test-bed onboard the International Space Station (ISS). Two Commercial Generic Bioprocessing Apparatus (CGBA), CGBA-5 and CGBA-4, will serve as communications test computers that transmit messages between ISS and ground Mission Control Centers. All data will be monitored and controlled at the BioServe remote Payload Operations Control Center (POCC) located on the Engineering Center premises at the University of Colorado – Boulder.
The Office of Space Communications and Navigation (SCaN) at NASA Headquarters leads the Delay Tolerant Networking (DTN) investigation with the goal of advancing the maturity and heritage (space flight use) of the DTN communication protocols. Delay tolerant networks make use of store-and-forward techniques within the network in order to compensate for intermittent link connectivity. In the DTN the fundamental concept is an architecture based on Internet-independent middleware where protocols at all layers are used that best suit the operation within each environment, with a new overlay network protocol (bundle protocol) inserted between the applications and the locally optimized communications stacks. Many applications can benefit from the reliable delivery of messages in a disconnected network.?? The internet, in contrast, is a connected network where internet protocols, most notably transmission control protocol/internet protocol (TCP/IP), are dependent upon (low) latencies of approximately milliseconds. This low latency, coupled with low bit error rates (BER), allows TCP to reliably transmit and receive acknowledgements for messages traversing the terrestrial Internet. One of the best examples of high latency, high BER links, with intermittent connectivity is that of space communications. One-way trip times, at the speed of light, from the Earth to the moon incurs a delay of 1.7 seconds; while one-way trip times to Mars incur a minimum delay of 8 minutes. The problem of latency for interplanetary links is exasperated with increased BER due to solar radiation. In addition, the celestial bodies are in constant motion, which can block the required line-of-sight between transmit and receive antennas, resulting in links that at best are only intermittently connected. Intermittent link connectivity is commonplace terrestrially as well. One example is the plethora of battery-powered mobile communications devices that go in and out of communication range to wired service interface points and are turned on and off at the users discretion.
Military applications in the DTN arena are substantial, allowing the retrieval of critical information in mobile battlefield scenarios using only intermittently connected network communications. For these types of applications, the delay tolerant protocol should transmit data segments across multiple-hop networks that consist of differing regional networks based on environmental network parameters (latency, loss, BER). This essentially implies that data from low-latency networks for which TCP may be suitable must also be forward across the long-haul interplanetary link. DTN achieves message reliability via employing custody transfer. The concept of custody transfer, where responsibility of some data segment (bundle or bundle fragment), migrates with the data segment as it progresses across a series of network hops is a fundamental strategy such that reliable delivery is accomplished on a hop-by-hop basis instead of an end-to-end basis which is impractical over high latency links.?? DTN is a set of protocols that act together to enable a standardized method of performing store and forward communications. DTN operates in two basic environments: low-propagation delay and high-propagation delay. In a low-propagation environment such as may occur in near-planetary or planetary surface environments, DTN bundle agents can utilize underlying Internet protocols that negotiate connectivity in real-time. In high-propagation delay environments such as deep space, DTN bundle agents must use other methods, such as some form of scheduling, to enable connectivity between the two agents.?? The convergence layer protocols provide the standard methods for transferring the bundles over various communications paths. The bundle agent discovery protocols are the equivalent to dynamic routing protocols in IP networks. To date the location of bundle agents, DTN agents, has been managed, analogous to static routing in internet protocol (IP) networks.?? The security protocols for DTN are important for the bundle protocol. The stressed environment of the underlying networks over which the bundle protocol will operate makes it important that the DTN be protected from unauthorized use, and this stressed environment poses unique challenges on the mechanisms needed to secure the bundle protocol. DTNs are likely to be deployed in organizationally heterogeneous environments where one does not control the entire network infrastructure. Furthermore, DTNs may very likely be deployed in environments where a portion of the network might become compromised, posing the usual security challenges related to confidentiality, integrity and availability.
The DTN protocol suite is still under active development. In addition to network security, research goals for the DTN activity will focus on testing and evolving important network services including naming and addressing, time synchronization, routing, network management and class of service.?? The DTN experiments on ISS consist of software which is to be placed on both Commercial Generic Bioprocessing Apparatus, CGBA-4 and CGBA-5, and then tested from a ground operations center. This software is not in any critical path of the CGBA operations and may be turned off at anytime. This software does not preclude the use of the CGBA units for other purposes or research support.
As NASA extends its reach to the Moon and beyond, a networked architecture such as DTN will be required to successfully complete these missions. The experiments that will be performed are designed to test the DTN protocol suite in an actual space environment, and to determine how well the protocols perform and what improvements may need to be made. The impact of the results of the research will help to advance the technical maturity of the DTN communications technology so that it is available for NASA use in both human and robotic Exploration missions.