There are many issues that limit the performance of wireless mobile multi-hop ad hoc networks (MANETs). One of them is that TCP is not well adapted to networks where routes can change or disappear often. In this paper the behaviour of a standard TCP implementation is studied in situations typical for MANETs and compared to the behaviour of a partial implementation of a ATCP, a TCP modification that is intended to increase performance in MANETs. Simulations with simple scenarios show that TCP easily creates a full network load which causes send failures and decreased throughput performance. In some cases the partial ATCP implementation increases throughput but more often it causes an increased amount of duplicate retransmissions. In these scenarios it is unlikely that even a complete ATCP implementation would increase throughput performance. A few modifications to ATCP and TCP are analysed. Especially a limit of the congestion window size shows a large throughput increase. The results are inconclusive, the simulations are too simple to show if the results are applicable in more complex scenarios. It is not clear if ATCP actually is useful in a MANET. / Många faktorer begränsar prestandan i trådlösa mobila multi-hopp ad hoc-nätverk (MANET:er). En av dem är att TCP inte är anpassat till nätverk där rutter ofta kan förändras eller försvinna. I den här rapporten studeras hur en vanlig TCP-implementation uppför sig i typiska MANET-situationer. Detta beteende jämförs mot en partiell implementation av ATCP, en TCPmodifiering som är tänkt att öka prestanda i MANET:er. Simuleringar med enkla scenarier visar att TCP lätt genererar en full nätverkslast vilket orsakar misslyckade sändningar och en minskad genomströmningsprestanda. I vissa fall ökar den partiella ATCP-implementationen genomströmningen, men oftare ger den en ökad mängd onödiga omsändningar. I dessa scenarier är det inte troligt att ens en komplett ATCP-implementation skulle öka genomströmningsprestanda. Några mindre förändringar av ATCP och TCP analyseras. Särskilt ger en begränsning av stockningsfönstret en stor ökning av genomströmningen. Resultaten är ofullständiga. Simuleringarna är för enkla för att kunna visa om om resultaten är tillämpliga i mer komplexa scenarier. Det är inte klarlagt ifall ATCP verkligen är användbart i ett MANET.

Although smart mobile devices have only come into prominence recently, they have quickly become a necessity in the modern world. In 2012, more than 450 million new smartphones are expected to be purchased around the world, exceeding, for the first time, purchases of laptops and desktop PCs combined in a single year. That, in addition to the increasing processing power and low cost of these emerging mobile devices, creates an increasing demand for mobile applications that work in infrastructure-supported environments like WiFi and cellular networks as well as infrastructure-less environments like ad hoc networks. Therefore, the behavior of mobile devices in such scenarios should be a continued focus of research. Several factors contribute to the observed behavior of nodes in Mobile Ad-hoc Networks MANETs. For example, nodes may act selfishly to preserve their limited energy resources. This selfishness may be detrimental to network performance. Therefore, cooperation between peers is necessary to keep these MANETs operational. Beside the need for actively encouraging cooperation by providing incentives, passive encouragement is also needed to overcome the effect of factors that limit cooperation, including malicious behavior, environmental obstruction, and mobility. The contribution of this work is to provide a cooperation model in MANETs that is capable of surviving topology distortions caused by mobility, and is operable in practical distributed scenarios. Towards this goal, we first provide a study of the topology characteristics of MANETs based on real experiments. We study the node degree, link stability, and link symmetry of these networks, and, based on our observations, we suggest a two-state Markov model to model link state in such networks, demonstrating the superiority of this model over the widely-used disk model with mobility. We conclude from this study that both mobility and channel fluctuations have a significant influence on the network topology, which makes it important to study cooperation in scenarios where the topology is changing rapidly. Based on experimental observations of a real network, we propose a coalition game model for cooperation in MANETs that shows that stable, effective coalitions can be maintained, even in the face of a dynamic network topology. We provide an initial evaluation of the model using a centralized simulation approach. We use the notion of reachability to evaluate the proposed model, and we simulate the model under different speeds and node densities. Our simulations show that reachability can be sustained at stable levels despite the deterioration caused by mobility. In addition, we show that our cumulative coalition formation approach gives good results in terms of reachability level and computational complexity. We also show that our proposed model achieves a fair payoff distribution among participating nodes. Motivated by the promising results of our centralized simulation approach, we take a further step towards more practical evaluation. We integrate the cooperation model with an existing MANET routing protocol, OLSR, and evaluate it in this distributed environment. We modify and augment the OLSR messaging mechanism to enable the exchange of the coalition information required to keep the model operating. Beside ensuring that the reachability gain is still attained and the coalition structure is stable, we study the effect of the extra control traffic overhead incurred by the model. We compare deliverability over the network with and without the cooperation model. Although our results show that the cooperation model incurs an average overhead exceeding $100\%$ of that incurred by OLSR in high density scenarios, it shows better reliability in delivering traffic especially among selfish nodes in low and average density scenarios. Counter to what is commonly assumed in the literature, this study shows that cooperation can be be maintained in a distributed manner without causing significant traffic overhead to MANETs run by proactive routing protocols. Due to the simplicity, several extensions can be applied to enhance the performance of the proposed model and diversify its usage. We propose these extensions at the end of this dissertation. / Ph. D.

Nodes taking part in mobile ad hoc networks (MANET) are expected to adhere to the rules dictated by the routing protocol employed in the subnet. Secure routing protocols attempt to reduce the ill-effect of nodes under the control of malicious entities who deliberately violate the protocol. Most secure routing protocols are reactive strategies which include elements like redundancies and cryptographic authentication to detect inconsistencies in routing data advertised by nodes, and perhaps explicit measures to react to detected inconsistencies. The approach presented in this dissertation is a proactive approach motivated by the question “what is a minimal trusted computing base for a MANET node?” Specifically, the goal of the research was to identify a small set of well-defined low-complexity functions, simple enough to be executed inside highly resource limited trusted boundaries, which can ensure that nodes will not be able to violate the protocol. In the proposed approach every node is assumed to possess a low complexity trusted MANET module (TMM). Only the TMM in a node is trusted - all other hardware and software are assumed to be untrusted or even hostile. TMMs offer a set of interfaces to the untrusted node housing the TMM, using which the node can submit data to the TMM for cryptographic verification and authentication. As other nodes will not accept packets that are not authenticated by TMMs, the untrusted node is forced to submit any data that it desires to advertise, to its TMM. TMMs will authenticate data only if the untrusted node can convince the TMM of the validity of the data. The operations performed by TMMs are to accept, verify, validate data submitted by the untrusted node, and authenticate such data to TMMs housed in other nodes. We enumerate various TMM interfaces and provide a concrete description of the functionality behind the interfaces for popular ad hoc routing protocols.

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We have witnessed two main trends in recent years that have shaped the current state of communication networks. First, the Internet was designed with the initial idea to provide remote access to resources in the network; today it is overwhelmingly being used for content distribution. In addition, the community of content creators has evolved from a small group of professionals into a global community where every user can generate his contents and share it with other users. Second, the proliferation of personal mobile devices, such as smartphones and media tablets, has altered the way people access, create and share information, leading to a significant migration from wired to wireless networks and raising user expectations for ubiquitous connectivity. These trends have incited research on new communication modes and in this thesis we consider a specific mode, namely opportunistic networking. Opportunistic networking is a communication paradigm that utilizes intermittent connectivity between mobile devices to enable communication in infrastructure-less environments, and to provide complementary transport mechanisms in wireless networks where infrastructure is present. The thesis focuses on two main topics: understanding and modeling human mobility, and opportunistic content distribution. Mobility modeling is one of the key issues in opportunistic networking research. First, we discuss the structure of human mobility and introduce a framework to study mobility at different behavioural levels. We propose a queuing model, denoted by meeting-point model, for pedestrian mobility in smaller urban areas, such as city squares, parks, shops or at bus stops. The model is also a contribution to the second topic we address in the thesis, since we will use it to study characteristics of content distribution in smaller areas. We envision this model as a building block in a library of analytical models that would be used to study the performance of pedestrian content distribution in common scenarios of urban mobility. Furthermore, we show how the proposed model can be used to build larger, more complex models. In the area of opportunistic content distribution, we apply both analytical and simulation-based evaluation. We empirically study the performance of epidemic content distribution by using real-life mobility traces and investigate the fitness of a homogeneous stochastic model to capture the epidemic process. In addition, we present the design, implementation and evaluation of a mobile peer-to-peer system for opportunistic networking and discuss some promising application scenarios. / <p>QC 20131115</p>

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