Rosario Firrincieli

Application of Contact Graph Routing to LEO satellite DTN communications

Delay-/Disruption-Tolerant Networking, which originated from research on deep space communications, has enlarged its scope to encompass all challenged networks, including LEO satellite communications. Focusing on single satellite or incomplete constellation cases, the advantages of DTN mainly relate to its ability to cope with disruption and intermittent connectivity, typical of LEOs. This, however, requires the adoption of routing solutions specifically designed for DTNs. Among the many proposals, Contact Graph Routing, designed by NASA for deep space, seems particularly appealing, as it takes advantage of the a priori knowledge of “contacts” between DTN nodes, a characteristic peculiar to both deep space and LEO environments. This paper aims to investigate the suitability of CGR in LEO satellite DTN communications, by focusing on two practical application scenarios: Earth observation and data mule. Results, obtained through a Linux testbed running ION, the DTN Bundle protocol and CGR implementation developed by NASA, highlight the advantages of CGR when applied to LEO satellite communications.

Analysis of TCP and DTN Retransmission Algorithms in Presence of Channel Disruptions

“Challenged Networks” often violate the TCP design key assumption of continuous path availability from the source to the sink node. These networks are the preferred target of Delay/Disruption Tolerant Networking (DTN), which aims at providing a more robust network architecture. Against disruptions, however, even standard TCP offers a certain level of robustness, through its many retransmission algorithms. The aim of the paper is to present a comparative analysis of the TCP and DTN Bundle protocol retransmission algorithms triggered by channel disruptions. This analysis provides the reader with a comprehensive view of the many mechanisms involved and a first assessment of their impact on some performance metrics. Although DTN put on top of TCP clearly offers higher resilience against long disruptions, TCP itself can provide a certain level of robustness, provided that the many retransmission parameters are properly set. The analysis is completed by some numerical evaluations on a GEO satellite channel obtained by means of a Linux testbed.

DTN Bundle Layer over TCP: Retransmission Algorithms in the Presence of Channel Disruptions

“Challenged n etworks” often violate the TCP design key assumption of continuous path availability from the source to the sink node. These networks are the preferred target of Delay/Disruption Tolerant Networking (DTN), which aims at providing a more robust network architecture. Against disruptions, however, even standard TCP offers a certain level of robustness, through its many retransmission algorithms. This intrinsic TCP resilience against disruptions is retained and enhanced if TCP is inserted in a DTN architecture. Focusing on this, the aim of the paper is to present an in-depth analysis of TCP and DTN bundle protocol retransmission algorithms that are triggered by channel disruptions, thus providing the reader with a comprehensive view of the many mechanisms involved and their complex interactions. The first sections, devoted to the description of TCP and DTN algorithms, are completed by the presentation of some numerical evaluations obtained by means of a Linux testbed. They refer to a GEO satellite environment and clarify the previous algorithm descriptions. Moreover, they also offer the reader some useful insights on both TCP and DTN resilience to disruptions, considering some performance metrics, like the “ maximum tolerable disrupti on length” and the “restart delay”, introduced and explained in the paper.

Analysis of TCP live experiments on a real GEO satellite testbed

The paper describes a measurement campaign carried out by the University of Bologna (UoB), the National Inter-University Consortium for Telecommunications (CNIT) and the University of California Los Angeles (UCLA). The aim of the experiments was the performance assessment of a wide range of TCP enhancements on network environments that include a real GEO satellite link. To this end, UoB and CNIT integrated their network tools and set up a testbed composed of a cluster of UoB Linux PCs connected to the CNIT GEO Skyplex satellite platform. Tests were conducted considering both end-to-end TCP enhancements and a TCP-splitting Performance Enhancing Proxy (PEP) developed by UoB. The analysis was not limited to isolated satellite links, but embraced more complex heterogeneous networks, where satellite connections have to compete with wired cross traffic for the network resources. The analysis of the large set of experimental data presented in the paper confirms the challenges posed by GEO satellite channels, which, in this case, were worsened also by the presence of Bandwidth on Demand technique, which impacts TCP performance. Among the end-to-end TCP variants, best results are generally shown by TCP Hybla, especially when heterogeneous environments are considered. On the other hand, the splitting PEP solution offers the best performance with respect to all TCP enhancements, due to its ability to isolate satellite channel impairments. The possible drawback of this approach is in that it violates the end-to-end semantics and is incompatible with IPsec protocol implementation.

TCP, PEP and DTN performance on disruptive satellite channels

Fixed GEO satellite communications are impaired by long RTTs and the possible presence of packet losses on the satellite radio channel. Moreover, when the satellite receiver is mobile, short and long disruptions due to line of sight obstructions can cause further performance deterioration. In this paper, we present a preliminary assessment of disruption impact on performance comparing different approaches, such as end-to-end TCP, PEP based on TCP splitting and DTN. Performance is evaluated on a Linux testbed, by emulating the satellite link interruptions caused by tunnels on a real railway line. Results show that good performance can be achieved by all of the approaches considered provided that on the satellite channel an efficient TCP variant, like Hybla, is adopted; however, DTN offers best performance. DTN superiority emerges when the disrupted satellite channel is the last leg of a congested path, thanks to the DTN custody transfer mechanism and store and forward capability.

DTNperf_2: A performance evaluation tool for Delay/Disruption Tolerant Networking

Delay/Disruption Tolerant Networking (DTN) has recently been proposed as a very effective solution to cope with the problems posed by challenged networks (long delays, disruptions, absence of end-to-end connectivity, etc.). In order to assess DTN performance in such scenarios, it is essential to be able to rely on specific evaluation tools. In this paper we present DTNperf_2, which is a client-server evaluation tool designed to assess goodput and to provide logs in DTN bundle layer architectures. The aim is to facilitate the adoption of this tool by the DTN research community. DTNperf_2 features, design and usage are discussed in detail. Moreover, an example of use is given, to demonstrate its capabilities. DTNperf_2 is included in the official DTN suite available at DTNRG web site.

End-to-End TCP Enhancements Performance on Satellite Links

Although TCP has proved very effective and robust for many years, its performance on emerging heterogeneous networks is challenged by the impairments originated by the presence of radio links. In particular, satellite communications are affected by long RTTs and possibly also by random segment losses, which can severely affect end-to-end performance. To cope with these problems, several TCP enhancements have been presented in the literature. The paper aim is to investigate the effectiveness of such modifications, when applied to TCP satellite connections. In particular, the analysis focuses on some emerging proposals, namely TCP Hybla, developed by the authors, and TCP Westwood, examined here in three variants. They are compared with three well established TCP variants, such as NewReno, SACK and Vegas, taking into account both a pure satellite environment and more challenging, but also perhaps closer to reality, heterogeneous network. Performance is assessed by means of ns-2 simulations considering goodput, fairness and friendliness as performance metrics. Results show that large performance improvements may be achieved by some of the considered TCP enhancements, without infringing the end-to-end semantics of this protocol.

The TATPA Testbed; A Testbed for Advanced Transport Protocols and Architecture performance evaluation on wireless channels

The paper describes the Testbed for Advanced Transport Protocols and Architecture (TATPA) developed by the University of Bologna, inside the SatNEx (Satellite Network of Excellence) framework. The aim is to provide partners, and in general the scientific community, with a powerful and flexible emulation tool to evaluate new proposal at transport layer (both protocols and architectures). In accordance with our research interests, the testbed is focused on performance evaluation on heterogeneous networks that include satellite links. The testbed architecture is based on a cluster of Linux PCs and, in addition to some widely adopted networking tools (NistNet, Ipcrf), it also exploits software packages specifically developed by the authors (Multi TCP, PEPsal, released under GNU license). The testbed is controlled by remote through a web interface, which provides both users and administrators with a ubiquitous easy access to the testbed features. A dedicated PC, acting as testbed controller, hosts the web server and the control engine, developed in PHP and bash and based on a MySQL database. In the paper, a particular attention is devoted to the description of the innovative design and the many advanced features of this controller. An example of use, seen from the final user point of view, completes the description of the TATPA testbed.

On The Application of MPE-FEC to Mobile DVB-S2: Performance Evaluation in Deep Fading Conditions

Digital video broadcasting through satellite transmission will provide new services to aeronautical, railway and maritime mobile terminals. However, mobile channel impairments request effective countermeasures to improve the C/N performance and counteract Doppler spread. The paper investigates the application of MPE-FEC (multi protocol encapsulation -forward error correction) to DVB-S2 standard chain for railway applications. In this scenario, trellis power arches may introduce deep fading events which cause bursts of data losses. An analytical model describing MPE-FEC decoder performance has been derived and validated through numerical simulation considering different train speed values, MOD-COD (modulation order and physical layer coding rate) configurations and number of rows in the MPE-FEC frame. Results show that, depending on the train speed, enlarging the number of rows or adopting a sliding encoding mechanism are effective in counteracting power arches impairments.

An integrated testbed for wireless advanced transport protocols and architectures

The challenges posed by data communications over both satellite and terrestrial wireless systems make the adoption of adequate countermeasures advisable. In particular, the poor performance experienced by TCP transport protocol in such environments has fed the scientific community to propose novel solutions, from both the protocols and architecture points of view. In order to perform a thorough assessment of such proposals, University of Bologna (UoB) and National Inter-University Consortium for Telecommunications (CNIT) have conceived and set-up an integrated testbed. Its characteristics and capabilities are presented and discussed in this work, focusing in particular on the advantages offered by the ubiquitous remote control of the testbed, provided by a powerful web interface specifically designed for this purpose