Spurious TCP Timeouts in 802.11 Networks

In this paper, we investigate spurious TCP timeouts in 802.11 wireless networks. Though timeouts can be a problem for uploads from an 802.11 network, these timeouts are not spurious but are caused by a bottleneck at the access point. Once this bottleneck is removed, we find that spurious timeouts are rare, even in the face of large changes in numbers of active stations or PHY rate

Investigations on making TCP robust against spurious retransmissions

Master'sMASTER OF SCIENC

A middleware protocol for time-critical wireless communication of large data samples

We present a middleware-based protocol that reliably synchronizes large samples consisting of multiple frames efficiently and within application level QoS requirements over a lossy wireless channel. The protocol uses a custom retransmission scheme, exploiting the latency requirements on sample level for frame level scheduling. It can be integrated into the popular DDS middleware. We investigate some technical limits of such a protocol and compare it to existing error protocols in the software stack and in the wireless protocol and combinations thereof. The comparison is based on an Omnet++ simulation using an established wireless channel error model. For evaluation, we take a use case from automated valet parking where infrastructure data provided via a wireless link augments in-vehicle sensor data. The use case respects the related safety requirements. Results show that the application awareness of the presented protocol, significantly improves service availability by transmitting data efficiently in time even under higher frame error rates

LINK ADAPTATION IN WIRELESS NETWORKS: A CROSS-LAYER APPROACH

Conventional Link Adaptation Techniques in wireless networks aim to overcome harsh link conditions caused by physical environmental properties, by adaptively regulating modulation, coding and other signal and protocol specific parameters. These techniques are essential for the overall performance of the networks, especially for environments where the ambient noise level is high or the noise level changes rapidly. Link adaptation techniques answer the questions of What to change? and When to change? in order to improve the present layer performance. Once these decisions are made, other layers are expected to function perfectly with the new communication channel conditions. In our work, we have shown that this assumption does not always hold; and provide two mechanisms that lessen the negative outcomes caused by these decisions. Our first solution, MORAL, is a MAC layer link adaptation technique which utilizes the physical transmission information in order to create differentiation between wireless users with different communication capabilities. MORAL passively collects information from its neighbors and re-aligns the MAC layer parameters according to the observed conditions. MORAL improves the fairness and total throughput of the system through distributing the mutually shared network assets to the wireless users in a fairer manner, according to their capabilities. Our second solution, Data Rate and Fragmentation Aware Ad-hoc Routing protocol, is a network layer link adaptation technique which utilizes the physical transmission information in order to differentiate the wireless links according to their communication capabilities. The proposed mechanism takes the physical transmission parameters into account during the path creation process and produces energy-efficient network paths. The research demonstrated in this dissertation contributes to our understanding of link adaptation techniques and broadens the scope of such techniques beyond simple, one-step physical parameter adjustments. We have designed and implemented two cross-layer mechanisms that utilize the physical layer information to better adapt to the varying channel conditions caused by physical link adaptation mechanisms. These mechanisms has shown that even though the Link Adaptation concept starts at the physical layer, its effects are by no means restricted to this layer; and the wireless networks can benefit considerably by expanding the scope of this concept throughout the entire network stack

Handoff management for infotainment services over vehicular networks

Intelligent Transportation Systems (ITS) has impulsed the vehicular communications at the present time. The vehicular communications field is a hot research topic and is attracting a great interest in the automotive industry and telecommunications. There are essentially two main lines of work: (1) communication services related to road safety and traffic information; and (2) information and entertainment services, also named infotainment services. These latter services include both transmitting multimedia (voice over IP, streaming, on-line gaming, etc.) and classic data services (e-mail, access to private networks, web browsing, file sharing, etc.). In this thesis we will focus on these infotainment services because further research in this immature research field is necessary and, until nowadays, the main effort of the research community regarding vehicular communication has been focused on road safety and traffic information. Vehicular nodes need to be reached from the Internet and vice versa to be able to access to infotainment services. While vehicles move along the road infrastructure, they change their wireless point of attachment to the network. During this process, connectivity breaks down until the vehicle is connected again to a new road side unit in its area. This disconnection causes a disruption in the communications. Fast handoffs are a crucial requirement for vehicular networks to avoid long disruption times, since the high speed of vehicular nodes involves suffering a lot of handoffs during an Internet connection. This thesis is focused on Vehicular-to-Infrastructure (V2I) real-time infotainment services. The main contributions of this thesis are: i) a new testing framework for V2I communications to be able to test infotainment services in an easy way; ii) the analysis of the deployability of infotainment video services in vehicular networks using mobility protocols; and iii) the development of a new TCP architecture that will provide a better performance for all TCP-based infotainment services in a vehicular scenario with handoffs. In this thesis, firstly, we propose a new testing framework for vehicular infotainment applications. This framework is a vehicular emulation platform that allows testing real applications installed on Linux virtual machines. Using emulation, we are able to evaluate the performance of real applications with real-time requirements, so we can test multimedia applications used to offer infotainment services in vehicular scenarios in a straightforward way. Secondly, using the testing framework implemented in the first part of the thesis, we have done a performance evaluation of an infotainment service. Among these services, we think that video on demand services on highways will be interesting for users, and generate revenue to network operators. So we evaluated how network-layer handoffs can limit the deployment of a video streaming service. According to the results obtained, driving at high speeds will be an issue for a correct playback of video content, even using fast handoffs techniques. Finally, we developed a new TCP architecture to enhance performance during handoffs. Most of the non-safety services on ITS rely on the Transport Control Protocol (TCP), one of the core protocols of the Internet Protocol Suite. However there exists several issues related to TCP and mobility that can affect to TCP performance, and these issues are particularly important in vehicular networks due to its high mobility. Using new IEEE 802.21 MIH services, we propose a new TCP architecture that is able to anticipate handoffs, permitting to resume the communication after a handoff, avoiding long delays caused by TCP issues and adapting the TCP parameters to the new characteristics of the network. Using the architecture proposed, the performance of TCP is enhanced, getting a higher overall throughput and avoiding TCP fairness issues between users.Els Sistemes de Transport Intel·ligents (ITS) han impulsat les comunicacions vehiculars en l'actualitat. Les comunicacions vehiculars és un camp d'investigació de moda, i està atraient un gran interès en la indústria automobilística i de les telecomunicacions. En el camp de les comunicacions vehiculars, hi ha principalment dues línies de treball: (1) serveis de comunicacions relacionats amb la seguretat viària i la informació del trànsit; i (2) serveis d'informació i entreteniment, també anomenats serveis d'infotainment. Aquests últims inclouen tant serveis multimèdia (veu sobre IP, streaming, jocs on-line, etc.), com serveis clàssics de dades (correu electrònic, accés a xarxes privades, navegació web, compartir arxius, etc.). En aquesta tesi ens centrarem en aquests serveis d'infotainment ja que és necessari aprofundir en la investigació per aquests tipus de serveis, ja que, fins avui, els esforços de la comunitat científica en el camp de les comunicacions vehiculars s'ha centrat en els serveis relacionats amb la seguretat viària i la informació del trànsit. Els nodes vehiculars necessiten tenir connexió a Internet per a poder tenir accés als serveis d'infotainment. Mentre els vehicles estan en moviment a través de la xarxa viària, els vehicles han d'anar canviant el punt de connexió sense fils amb la xarxa. Durant aquest procés de canvi de punt de connexió, anomenat handoff, es perd la connectivitat fins que el vehicle es reconnecta a un altre punt de connexió viària prop de la seva àrea. Aquesta desconnexió causa interrupcions en les comunicacions. Uns handoffs ràpids són bàsics a les xarxes vehiculars per a evitar llargs períodes d'interrupció durant les comunicacions, ja que la gran velocitat a la que es mouen els nodes vehiculars significa un gran nombre de handoffs durant una connexió a Internet. Aquesta tesi es centra en serveis d'infotaiment en temps real per a comunicacions Vehicle-a-Infraestructura (V2I). Les principals contribucions d'aquesta tesi son: i) un nou marc de proves per a les comunicacions (V2I) per a poder provar serveis d'infotainment d'una manera fàcil; ii) l'anàlisi de la viabilitat del desplegament de serveis d'infotainment de vídeo en xarxes vehiculars utilitzant protocols de mobilitat IP; i iii) el desenvolupament d'una nova arquitectura TCP que proporciona un millor funcionament per a tots aquells serveis d'infotainment basats en el protocol TCP en un escenari vehicular amb handoffs. En aquesta tesi, primer proposem un nou marc de proves per a aplicacions vehiculars d'infotainment. Aquest marc és una plataforma d'emulació vehicular que permet provar aplicacions reals instal·lades en màquines virtuals Linux. Utilitzant l'emulació, som capaços d'avaluar el rendiment d'aplicacions reals amb característiques de temps real. D'aquesta manera es poden avaluar aplicacions multimèdia utilitzades per oferir serveis d'infotainment d'una forma senzilla en escenaris vehiculars. Segon, utilitzant el marc de prova implementat en la primera part de la tesi, hem avaluat el rendiment d'un servei d'infotainment. Entre aquest tipus de servei, creem que els serveis de vídeo sota demanda en autopistes/autovies serà interessant pels usuaris i generarà beneficis per als operadors de la xarxa. Per tant, hem avaluat com els handoffs a nivell de la capa de xarxa poden limitar el desplegament d'un servei de streaming de vídeo sota demanda. D'acord amb els resultats obtinguts, conduir a grans velocitats podria ser un problema per a poder reproduir un vídeo correctament, tot i utilitzar tècniques de handoffs ràpids. Finalment, hem desenvolupat una nova arquitectura TCP per a millorar el rendiment del protocol durant els handoffs. La majoria dels serveis d'infotainment utilitzen el Protocol de Control de Transport (TCP), un dels principals protocols de la pila de protocols d'Internet. Però existeixen forces problemes relacionats amb l'ús de TCP i la mobilitat que n'afecta el rendiment, i aquests problemes són particular

MANETs: Internet Connectivity and Transport Protocols

A Mobile Ad hoc Network (MANET) is a collection of mobile nodes connected together over a wireless medium, which self-organize into an autonomous multi-hop wireless network. This kind of networks allows people and devices to seamlessly internetwork in areas with no pre-existing communication infrastructure, e.g., disaster recovery environments. Ad hoc networking is not a new concept, having been around in various forms for over 20 years. However, in the past only tactical networks followed the ad hoc networking paradigm. Recently, the introduction of new technologies such as IEEE 802.11, are moved the application field of MANETs to a more commercial field. These evolutions have been generating a renewed and growing interest in the research and development of MANETs. It is widely recognized that a prerequisite for the commercial penetration of the ad hoc networking technologies is the integration with existing wired/wireless infrastructure-based networks to provide an easy and transparent access to the Internet and its services. However, most of the existing solutions for enabling the interconnection between MANETs and the Internet are based on complex and inefficient mechanisms, as Mobile-IP and IP tunnelling. This thesis describes an alternative approach to build multi-hop and heterogeneous proactive ad hoc networks, which can be used as flexible and low-cost extensions of traditional wired LANs. The proposed architecture provides transparent global Internet connectivity and address autocofiguration capabilities to mobile nodes without requiring configuration changes in the pre-existing wired LAN, and relying on basic layer-2 functionalities. This thesis also includes an experimental evaluation of the proposed architecture and a comparison between this architecture with a well-known alternative NAT-based solution. The experimental outcomes confirm that the proposed technique ensures higher per-connection throughputs than the NAT-based solution. This thesis also examines the problems encountered by TCP over multi-hop ad hoc networks. Research on efficient transport protocols for ad hoc networks is one of the most active topics in the MANET community. Such a great interest is basically motivated by numerous observations showing that, in general, TCP is not able to efficiently deal with the unstable and very dynamic environment provided by multi-hop ad hoc networks. This is because some assumptions, in TCP design, are clearly inspired by the characteristics of wired networks dominant at the time when it was conceived. More specifically, TCP implicitly assumes that packet loss is almost always due to congestion phenomena causing buffer overflows at intermediate routers. Furthermore, it also assumes that nodes are static (i.e., they do not change their position over time). Unfortunately, these assumptions do not hold in MANETs, since in this kind of networks packet losses due to interference and link-layer contentions are largely predominant, and nodes may be mobile. The typical approach to solve these problems is patching TCP to fix its inefficiencies while preserving compatibility with the original protocol. This thesis explores a different approach. Specifically, this thesis presents a new transport protocol (TPA) designed from scratch, and address TCP interoperability at a late design stage. In this way, TPA can include all desired features in a neat and coherent way. This thesis also includes an experimental, as well as, a simulative evaluation of TPA, and a comparison between TCP and TPA performance (in terms of throughput, number of unnecessary transmissions and fairness). The presented analysis considers several of possible configurations of the protocols parameters, different routing protocols, and various networking scenarios. In all the cases taken into consideration TPA significantly outperforms TCP

Moving toward the intra-protocol de-ossification of TCP in mobile networks: Start-up and mobility

182 p.El uso de las redes móviles de banda ancha ha aumentado significativamente los últimos años y se espera un crecimiento aún mayor con la inclusión de las futuras capacidades 5G. 5G proporcionará unas velocidades de transmisión y reducidos retardos nunca antes vistos. Sin embargo, la posibilidad de alcanzar las mencionadas cuotas está limitada por la gestión y rendimiento de los protocolos de transporte. A este respecto, TCP sigue siendo el protocolo de transporte imperante y sus diferentes algoritmos de control de congestión (CCA) los responsables finales del rendimiento obtenido. Mientras que originalmente los distintos CCAs han sido implementados para hacer frente a diferentes casos de uso en redes fijas, ninguno de los CCAs ha sido diseñado para poder gestionar la variabilidad de throughput y retardos de diferentes condiciones de red redes móviles de una manera fácilmente implantable. Dado que el análisis de TCP sobre redes móviles es complejo debido a los múltiples factores de impacto, nuestro trabajo se centra en dos casos de uso generalizados que resultan significativos en cuanto a afección del rendimiento: movimiento de los usuarios como representación de la característica principal de las redes móviles frente a las redes fijas y el rendimiento de la fase de Start-up de TCP debido a la presencia mayoritaria de flujos cortos en Internet. Diferentes trabajos han sugerido la importancia de una mayor flexibilidad en la capa de transporte, creando servicios de transporte sobre TCP o UDP. Sin embargo, estas propuestas han encontrado limitaciones relativas a las dependencias arquitecturales de los protocolos utilizados como sustrato (p.ej. imposibilidad de cambiar la configuración de la capa de transporte una vez la transmisión a comenzado), experimentando una capa de transporte "osificada". Esta tesis surge como respuesta a fin de abordar la citada limitación y demostrando que existen posibilidades de mejora dentro de la familia de TCP (intra-protocolar), proponiendo un marco para solventar parcialmente la restricción a través de la selección dinámica del CCA más apropiado. Para ello, se evalúan y seleccionan los mayores puntos de impacto en el rendimiento de los casos de uso seleccionados en despliegues de red 4G y en despliegues de baja latencia que emulan las potenciales latencias en las futuras capacidades 5G. Estos puntos de impacto sirven como heurísticas para decidir el CCA más apropiado en el propuesto marco. Por último, se valida la propuesta en entornos de movilidad con dos posibilidades de selección: al comienzo de la transmisión (limitada flexibilidad de la capa de transporte) y dinámicamente durante la transmisión (con una capa de transporte flexible). Se concluye que la propuesta puede acarrear importantes mejoras de rendimiento al seleccionar el CCA más apropiado teniendo en cuenta la situación de red y los requerimientos de la capa de aplicación

Investigating TCP performance in mobile ad hoc networks

Mobile ad hoc networks (MANETs) have become increasingly important in view of their promise of ubiquitous connectivity beyond traditional fixed infrastructure networks. Such networks, consisting of potentially highly mobile nodes, have provided new challenges by introducing special consideration stemming from the unique characteristics of the wireless medium and the dynamic nature of the network topology. The TCP protocol, which has been widely deployed on a multitude of internetworks including the Internet, is naturally viewed as the de facto reliable transport protocol for use in MANETs. However, assumptions made at TCP’s inception reflected characteristics of the prevalent wired infrastructure of networks at the time and could subsequently lead to sub-optimal performance when used in wireless ad hoc environments. The basic presupposition underlying TCP congestion control is that packet losses are predominantly an indication of congestion in the network. The detrimental effect of such an assumption on TCP’s performance in MANET environments has been a long-standing research problem. Hence, previous work has focused on addressing the ambiguity behind the cause of packet loss as perceived by TCP by proposing changes at various levels across the network protocol stack, such as at the MAC mechanism of the transceiver or via coupling with the routing protocol at the network layer. The main challenge addressed by the current work is to propose new methods to ameliorate the illness-effects of TCP’s misinterpretation of the causes of packet loss in MANETs. An assumed restriction on any proposed modifications is that resulting performance increases should be achievable by introducing limited changes confined to the transport layer. Such a restriction aids incremental adoption and ease of deployment by requiring minimal implementation effort. Further, the issue of packet loss ambiguity, from a transport layer perspective, has, by definition, to be dealt with in an end-to-end fashion. As such, a proposed solution may involve implementation at the sender, the receiver or both to address TCP shortcomings. Some attempts at describing TCP behaviour in MANETs have been previously reported in the literature. However, a thorough enquiry into the performance of those TCP agents popular in terms of research and adoption has been lacking. Specifically, very little work has been performed on an exhaustive analysis of TCP variants across different MANET routing protocols and under various mobility conditions. The first part of the dissertation addresses this shortcoming through extensive simulation evaluation in order to ascertain the relative performance merits of each TCP variant in terms of achieved goodput over dynamic topologies. Careful examination reveals sub-par performance of TCP Reno, the largely equivalent performance of NewReno and SACK, whilst the effectiveness of a proactive TCP variant (Vegas) is explicitly stated and justified for the first time in a dynamic MANET environment. Examination of the literature reveals that in addition to losses caused by route breakages, the hidden terminal effect contributes significantly to non-congestion induced packet losses in MANETs, which in turn has noticeably negative impact on TCP goodput. By adapting the conservative slow start mechanism of TCP Vegas into a form suitable for reactive TCP agents, like Reno, NewReno and SACK, the second part of the dissertation proposes a new Reno-based congestion avoidance mechanism which increases TCP goodput considerably across long paths by mitigating the negative effects of hidden terminals and alleviating some of the ambiguity of non-congestion related packet loss in MANETs. The proposed changes maintain intact the end-to-end semantics of TCP and are solely applicable to the sender. The new mechanism is further contrasted with an existing transport layer-focused solution and is shown to perform significantly better in a range of dynamic scenarios. As solution from an end-to-end perspective may be applicable to either or both communicating ends, the idea of implementing receiver-side alterations is also explored. Previous work has been primarily concerned with reducing receiver-generated cumulative ACK responses by “bundling” them into as few packets as possible thereby reducing misinterpretations of packet loss due to hidden terminals. However, a thorough evaluation of such receiver-side solutions reveals limitations in common evaluation practices and the solutions themselves. In an effort to address this shortcoming, the third part of this research work first specifies a tighter problem domain, identifying the circumstances under which the problem may be tackled by an end-to-end solution. Subsequent original analysis reveals that by taking into account optimisations possible in wireless communications, namely the partial or complete omission of the RTS/CTS handshake, noticeable improvements in TCP goodput are achievable especially over long paths. This novel modification is activated in a variety of topologies and is assessed using new metrics to more accurately gauge its effectiveness in a wireless multihop environment

Improving TCP Performance in the Mobile, High Speed, Heterogeneous and Evolving Internet

Ph.DDOCTOR OF PHILOSOPH

Optimising the data-collection time of a large-scale data-acquisition system

Data-acquisition systems are a fundamental component of modern scientific experiments. Large-scale experiments, particularly in the field of particle physics, comprise millions of sensors and produce petabytes of data per day. Their data-acquisition systems digitise, collect, filter, and store experimental signals for later analysis. The performance and reliability of these systems are critical to the operation of the experiment: insufficient performance and failures result in the loss of valuable scientific data. By its very nature, data acquisition is a synchronous many-to-one operation: every time a phenomenon is observed by the experiment, data from its various sensors must be assembled into a single coherent dataset. This characteristic yields a particularly challenging traffic pattern for computer networks dedicated to data acquisition. If no corrective measures are taken, this pattern, known as incast, results in a significant underutilisation of the network resources, with a direct impact on a data-acquisition systems' throughput. This thesis presents effective and feasible approaches to maximising network utilisation in data-acquisition systems, avoiding the incast problem without sacrificing throughput. Rather than using abstract models, it focuses on an existing large-scale experiment, used as a case-study: the ATLAS detector at the Large Hadron Collider. First, the impact of incast on data-acquisition performance is characterised through a series of measurements performed on the actual data-acquisition system of the ATLAS experiment. As the size of the data sent synchronously by multiple sources to the same destination grows past the size of the network buffers, the throughput falls. A simple but effective mitigation is proposed and tested: at the application-layer, the data-collection receivers can limit the number of senders they simultaneously collect data from. This solution recovers a large part of the throughput lost to incast, but introduces some performance losses of its own. Further investigations are enabled by the development of a complete packet-level model of the ATLAS data-acquisition network in an event-based simulation framework. Comparing real-world measurements and simulation results, the model is shown to be accurate enough to be used for studying the incast phenomenon in a data-acquisition system. Leveraging the simulation model, various optimisations are analysed. The focus is kept on practical software changes, that can realistically be deployed on otherwise unmodified existing systems. Receiver-side traffic-shaping, incast- and traffic-shaping-aware work scheduling policies, tuning of TCP's timeouts, and centralised network packet injection scheduling are evaluated alone and in combination. Used together, the first three techniques result in a very significant increase of the system's throughput, which gets within 10% of the ideal maximum performance, even with a high network traffic load