Transport layer protocols and architectures for satellite networks

Designing efficient transmission mechanisms for advanced satellite networks is a demanding task, requiring the definition and the implementation of protocols and architectures well suited to this challenging environment. In particular, transport protocols performance over satellite networks is impaired by the characteristics of the satellite radio link, specifically by the long propagation delay and the possible presence of segment losses due to physical channel errors. The level of impact on performance depends upon the link design (type of constellation, link margin, coding and modulation) and operational conditions (link obstructions, terminal mobility, weather conditions, etc.). To address these critical aspects a number of possible solutions have been presented in the literature, ranging from limited modifications of standard protocols (e.g. TCP, transmission control protocol) to completely alternative protocol and network architectures. However, despite the great number of different proposals (or perhaps also because of it), the general framework appears quite fragmented and there is a compelling need of an integration of the research competences and efforts. This is actually the intent of the transport protocols research line within the European SatNEx (Satellite Network of Excellence) project. Stemming from the authors' work on this project, this paper aims to provide the reader with an updated overview of all the possible approaches that can be pursued to overcome the limitations of current transport protocols and architectures, when applied to satellite communications. In the paper the possible solutions are classified in the following categories: optimization of TCP interactions with lower layers, TCP enhancements, performance enhancement proxies (PEP) and delay tolerant networks (DTN). Advantages and disadvantages of the different approaches, as well as their interactions, are investigated and discussed, taking into account performance improvement, complexity, and compliance to the standard semantics. From this analysis, it emerges that DTN architectures could integrate some of the most efficient solutions from the other categories, by inserting them in a new rigorous framework. These innovative architectures therefore may represent a promising solution for solving some of the important problems posed at the transport layer by satellite networks, at least in a medium-to-long-term perspective. Copyright (c) 2006 John Wiley & Sons, Ltd

Geomicrobiology of a seawater-influenced active sulfuric acid cave.

Fetida Cave is an active sulfuric acid cave influenced by seawater, showing abundant microbial communities that organize themselves under three main different morphologies: water filaments, vermiculations and moonmilk deposits. These biofilms/deposits have different cave distribution, pH, macro- and microelement and mineralogical composition, carbon and nitrogen content. In particular, water filaments and vermiculations had circumneutral and slightly acidic pH, respectively, both had abundant organic carbon and high microbial diversity. They were rich in macro- and microelements, deriving from mineral dissolution, and, in the case of water filaments, from seawater composition. Vermiculations had different color, partly associated with their mineralogy, and unusual minerals probably due to trapping capacities. Moonmilk was composed of gypsum, poor in organic matter, had an extremely low pH (0\u20131) and low microbial diversity. Based on 16S rRNA gene analysis, the microbial composition of the biofilms/deposits included autotrophic taxa associated with sulfur and nitrogen cycles and biomineralization processes. In particular, water filaments communities were characterized by bacterial taxa involved in sulfur oxidation and reduction in aquatic, aphotic, microaerophilic/anoxic environments (Campylobacterales, Thiotrichales, Arenicellales, Desulfobacterales, Desulforomonadales) and in chemolithotrophy in marine habitats (Oceanospirillales, Chromatiales). Their biodiversity was linked to the morphology of the water filaments and their collection site. Microbial communities within vermiculations were partly related to their color and showed high abundance of unclassified Betaproteobacteria and sulfur-oxidizing Hydrogenophilales (including Sulfuriferula), and Acidiferrobacterales (including Sulfurifustis), sulfur-reducing Desulfurellales, and ammonia-oxidizing Planctomycetes and Nitrospirae. The microbial community associated with gypsum moonmilk showed the strong dominance (>60%) of the archaeal genus Thermoplasma and lower abundance of chemolithotrophic Acidithiobacillus, metal-oxidizing Metallibacterium, Sulfobacillus, and Acidibacillus. This study describes the geomicrobiology of water filaments, vermiculations and gypsum moonmilk from Fetida Cave, providing insights into the microbial taxa that characterize each morphology and contribute to biogeochemical cycles and speleogenesis of this peculiar seawater-influenced sulfuric acid cave

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

DTN and Satellite Communications

Satellite communications are an interesting and promising application field for Delay/Disruption Tolerant Networking (DTN). Although primarily conceived for deep space communications and sensor networks, DTN was immediately recognized as applicable to satellite environments, in particular to cope with the intermittent channels typical of LEO (Low Earth Orbit) constellation satellite systems. However, DTN applicability to the satellite field is not limited to LEOs, but extends to other systems, like GEOs (Geostationary Earth Orbit), where performance of reliable transport protocols, like TCP, is impaired by the characteristic of the satellite channel itself. Even in the most favorable case of continuous end-to-end connectivity, long delays inherent in GEO systems, together with the possible presence of a high Packet Error Rate (PER) due to the wireless channel, severely affect performance. The most common countermeasure is the insertion of intermediate agents, called PEPs (Performance Enhancing Proxies), which generally offer good performance but violate the end-to-end semantics of transport protocols. In this case too DTNs can have an important role. The possible applications of the DTN concept in satellite communications are various and can be conveniently classified on the basis of the presence or not of end-to-end connectivity: \u2022 continuous end-to-end connectivity; end-to-end connectivity is usually present; this may be the case with GEO satellite systems with terrestrial fixed terminals; \u2022 random intermittent end-to-end connectivity; end-to-end connectivity may be present but channel disruptions are frequent, and difficult, or impossible, to predict; for example, a GEO satellite connecting mobile terminals on means of transport, where the satellite link is disrupted by tunnels or other obstructions; \u2022 scheduled intermittent end-to-end connectivity; end-to-end connectivity is assured at regular intervals; for example, a LEO satellite for Earth observation, which can connect to its gateway stations only at intermittent but predictable intervals due to its orbital motion; \u2022 no end-to-end connectivity; there is never end-to-end connectivity between end-points; for example, a single LEO satellite acting as a \u201cmule\u201d between a terrestrial sensor network and a remote satellite gateway station, which are never in satellite visibility at the same time. These scenarios will be investigated in specific sections. However, let us briefly anticipate here the reasons why DTN can be more advantageous with respect to other approaches, like end-to-end transport protocols or PEPs. To this end, let us re-consider the four above scenarios in reverse order, as the last is the most suited to DTN. In the last scenario the absence of end-to-end connectivity prevents the establishment of TCP, or TCP-like, connections. Even unreliable UDP transfers are impossible, due the lack of a continuous path between end nodes, and the only possible approach is the application of DTN \u201cstore-and forward\u201d techniques. Data must first be transferred on intermediate nodes, then, when possible, transferred to the receiver. This task can easily be accomplished by DTN, unlike PEPs or end-to-end transport protocols. The third scenario (intermittent end-to-end connectivity) is the next most suited, as end-to-end transfers are now possible, but only at scheduled times and for a limited amount of time. This latter restriction poses a strict limit on the total data that can be transferred at each availability interval. For example, files exceeding the maximum data constraint must be divided into multiple parts. In this case DTN is preferable. If the DTN concept is implemented through the \u201cbundle layer\u201d, long files can be transferred by segmenting them in multiple bundles of the right dimension or, alternatively, by inserting the whole file in a single bundle, by relying on pro-active bundle segmentation. In the second scenario there is end-to-end connectivity but..

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 \u201ccontacts\u201d 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

The TCP \u201cAdaptive-Selection\u201d Concept

The rapidly increasing importance of wireless communications (including satellite) together with the rapid growth of high speed networks pose new challenges to TCP. To overcome them, a wide variety of TCP enhancements has been presented in literature. However, because most proposals aim to address different impairments, the choice of \u201cthe best\u201d TCP enhancement becomes arduous, given the increasing level of heterogeneity of present and future networks. The TCP adaptive-selection concept, introduced by the authors in this paper, aims at providing an alternative approach, by challenging the idea that only one TCP enhancement must be adopted. In fact, by extending the concept that underlies ACM (Adaptive Coding and Modulation) to Transport layer, it envisages concurrent adoption of different TCP versions on the same server, the better to match the different impairments present on different connections. Preliminary results obtained by considering a simple application example are encouraging, and justify the following remarks on feasibility and the discussion of some implementation proposals

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

\u201cChallenged networks\u201d 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 \u201cmaximum tolerable disruption length\u201d and the \u201crestart delay\u201d, introduced and explained in the paper

PEPsal Performance Analysis on Disruptive Radio Channels

Fixed GEO satellite communications are impaired by long RTTs (especially GEO) and the possible presence of packet losses on the satellite radio channels. Moreover, when the satellite receiver is mobile, short and long disruptions due to line of sight obstructions associated with the presence of shadowing can cause further performance deterioration. In this paper, we evaluate the impact of a disruptive channel on PEPsal, a TCP-splitting PEP previously developed by the authors. Results, obtained by emulating the satellite link interruptions caused by tunnels of a real railway line, highlight the advantages of the TCP-splitting architecture. By enabling the adoption of optimized version of TCP on the satellite connection and a satellite-specific tuning of TCP parameters, PEPsal can offer a significant resilience against all kind of satellite impairments