Nikolaos Bezirgiannidis

Contact graph routing in DTN space networks: overview, enhancements and performance

Delay- and Disruption Tolerant Networks (DTNs) are based on an overlay protocol and on the store-carry-forward paradigm. In practice, each DTN node can store information for a long time before forwarding it. DTNs are particularly suited to cope with the challenges imposed by the space environment. This paper is focused on routing in space DTNs, and in particular on contact graph routing (CGR) and its most representative enhancements, available in the literature, which are briefly surveyed in this work. Moreover, the applicability and the obtained performance of the DTN protocol stack and of the CGR have been evaluated by presenting results from real experimental experiences such as the Deep Impact Network experiment (employing the EPOXI space cruise), the JAXA jointly performed space link demonstrations with NASA (where the JAXAs GEO relay satellite called Data Relay Test Satellite has been used), the Space Data Routers European Project, and the pilot operation of a DTN implementation on the International Space Station (ISS).

Packet size and DTN transport service: Evaluation on a DTN Testbed

We present a study of DTN transport layer service, evaluating its performance during a file transmission from a Mars relay satellite towards Earth. The experiments are being held in a DTN Testbed designed and implemented for emulating and evaluating deep-space communications. Bundle Protocol truncates application data into bundles of different size and Licklider Transmission Protocol is used as the convergence layer protocol, encapsulating bundles into blocks and fragmenting blocks into segments. Bundle size, LTP block size and segment size are under consideration to achieve better transmission in terms of delivery time, overhead and memory occupancy at the sending node. Our most interesting conclusion is that memory is released faster when using small bundles and LTP blocks. We also conclude that segments should be close to the maximum boundary of underlying layers MTU, and that ignoring these dependencies, file delivery time in a channel of steady bit error rate has little dependence from packet sizes.

Delivery Time Estimation for Space Bundles

We present a method for predicting delivery time of bundles in space internetworks. The bundle delivery time estimation (BDTE) tool exploits contact graph routing (CGR), predicts bundle route, and calculates plausible arrival times along with the corresponding probabilities. Latency forecasts are performed in an administrative node with access to an instrumentation database (DB) appropriate for statistical processing. Through both analysis and experimentation, we demonstrate that estimates of bundle earliest plausible delivery time and destination arrival probabilities can be provided.

Contact Graph Routing enhancements for delay tolerant space communications

In this paper we present two enhancements to Contact Graph Routing (CGR), a Delay-/ Disruption- Tolerant Networking routing algorithm developed by NASA JPL for space environments with predetermined connectivity schedules, such as Interplanetary communications and LEO satellite systems. The first enhancement, CGR-ETO, aims to improve the accuracy of predicted bundle delivery time by considering the available information on queueing delay. The second, the Overbooking Management, aims to proactively handle contact oversubscription, which occurs when high priority bundles are forwarded for transmission on a contact that is already fully subscribed by lower priority bundles. Both enhancements have been inserted as optional features in the Interplanetary Overlay Network CGR implementation in order to comparatively evaluate their performance on a GNU/Linux testbed running the full protocol stack. The results show that the two enhancements are complementary and can significantly improve routing decisions compared to standard CGR.

Towards flexibility and accuracy in space DTN communications

Although Interplanetary Telecommunications rely on preconfigured contact schedules to make routing decisions, there is a lack of appropriate mechanisms to notify the network about contact plan changes. In order to fill this gap, we propose and evaluate a framework for disseminating information about queueing delays and link disruptions. In this context, we present such a mechanism, focusing not only on its functional properties, but rather on its impact objectives: to improve accuracy and routing performance. Supportively, we couple this mechanism with a DTN-compatible protocol, namely Contact Plan Update Protocol (CPUP), which implements our dissemination policy. Through simulation of space scenarios we show that accuracy can be significantly improved in all cases while routing performance can achieve a wide range, from minor through to significant gains, conditionally.

On the performance of erasure coding over space DTNs

Erasure coding has attracted the attention of space research community due to its potential to present an alternative or complementary solution to ARQ schemes. Typically, erasure coding can enhance reliability and decrease delivery latency when long delays render ARQ-based solutions inefficient. In this paper, we explore the benefits of erasure coding for file transfers over space Delay Tolerant Networks, using a generic end-to-end mechanism built on top of the Bundle Protocol that incorporates LDPC codes along with an ARQ scheme. The results reveal significant insights on the tradeoff among efficient bandwidth exploitation and delivery latency. We quantify the performance gains when optimal erasure coding is applied and investigate in what extent theoretically optimal performance is affected when suboptimal code rates are used. Beyond that, we highlight the ability of erasure coding to provide different QoS to applications, in terms of file delivery latency, by properly tuning the code rate.

Analysis of contact graph routing enhancements for DTN space communications

Summary In this paper, we analyze the performance of two contact graph routing (CGR) enhancements, namely, CGR with earliest transmission opportunity (CGR-ETO) and overbooking management. CGR-ETO aims to improve the accuracy of predicted bundle delivery time by exploiting existing information on queueing delay, in routing decisions. Overbooking management aims to proactively handle contact oversubscription, which occurs when high-priority bundles are forwarded to a contact that is already fully subscribed by lower-priority bundles. These two enhancements have been recently included in the official CGR version as part of the Interplanetary Overlay Network delay-tolerant/disruption-tolerant networking implementation maintained by the National Aeronautics and Space Administration. In parallel to the comparative evaluation of the enhancements against the original CGR, we introduce an experimental version of CGR-ETO that exploits information on locally routed data to calculate queueing delays in all hops through the path to destination, rather than in the first hop only. We evaluate the aforementioned enhancements in a set of emulation experiments conducted on a GNU/Linux testbed and compare official and experimental versions of CGR. Results show that the two enhancements are complementary and can significantly improve routing decisions compared with older versions of CGR, particularly in the presence of parallel routes and traffic of different priorities. These advantages are further extended when the experimental version of ETO is considered. Copyright

Predicting Queueing Delays in Delay Tolerant Networks with Application in Space

In this paper we present a study of queueing delays experienced in Delay Tolerant Networks with topology based on deterministic contact plan schedules. We examine a generic scenario and propose a sampling procedure that extracts measurements of queueing rates and queue lengths. Sampling queueing information is transmitted to network nodes, which then form time series and can be used to forecast future queueing rates. Through simulations we show that the introduced method can be useful for DTNs with predetermined contact schedules, such as the Interplanetary Internet, providing accurate end-to-end delivery delay predictions.

SPICE Testbed: A DTN Testbed for Satellite and Space Communications

This paper presents SPICE testbed, a state-of-the-art Delay Tolerant Networking testbed for satellite and space communications deployed at the Space Internetworking Center, Greece. The core of the testbed relies on the Bundle Protocol and its architecture has been designed to support multiple DTN implementations and a variety of underlying and overlying protocols. SPICE testbed is equipped with specialised hardware components for the accurate emulation of space links and ground stations, such as Portable Satellite Simulator (PSS) and CORTEX CRT system, as well as protocols and mechanisms specifically designed for space DTNs. Performance and functionality evaluations on SPICE testbed show that it is an ideal platform to evaluate new mechanisms in a variety of space communication scenarios.

An adaptive end-to-end RTO calculation framework for DTNs with scheduled connectivity

Summary In this work, we propose an end-to-end retransmission framework for dynamically calculating efficient retransmission time-out intervals in delay-tolerant networks (DTNs) with scheduled connectivity. The proposed framework combines deterministic and statistical information about the network state to calculate worst-case estimates about the expected round trip times. Such information includes connectivity schedules, convergence layer protocols specifics, communication link characteristics, and network statistics about the maximum expected packet error rates and storage congestion. We detail the implementation of the proposed framework within the end-to-end application data conditioning layer proposed for the DTN architecture, realized by the Delay-Tolerant Payload Conditioning protocol, as part of the Interplanetary Overlay Network–DTN reference implementation, and evaluate its performance in a complex deep-space emulation scenario in our DTN testbed. Our results show that our approach achieves great accuracy in round-trip time estimations and, therefore, faster retransmissions of lost data, in comparison to the statically configured retransmission mechanism of the original Delay-Tolerant Payload Conditioning protocol. As a result, in-order data reception rate and storage requirements on the receiver side are significantly improved, at minimum or even zero extra cost in transmission overhead due to duplicate transmissions.