Browsing by Author "Rothermel, Kurt (Prof. Dr. rer. nat. Dr. h.c.)"

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Open Access
Mobility-awareness in complex event processing systems
(2016) Ottenwälder, Beate; Rothermel, Kurt (Prof. Dr. rer. nat. Dr. h.c.)
The proliferation and vast deployment of mobile devices and sensors over the last couple of years enables a huge number of Mobile Situation Awareness (MSA) applications. These applications need to react in near real-time to situations in the environment of mobile objects like vehicles, pedestrians, or cargo. To this end, Complex Event Processing (CEP) is becoming increasingly important as it allows to scalably detect situations “on-the-fly” by continously processing distributed sensor data streams. Furthermore, recent trends in communication networks promise high real-time conformance to CEP systems by processing sensor data streams on distributed computing resources at the edge of the network, where low network latencies can be achieved. Yet, supporting MSA applications with a CEP middleware that utilizes distributed computing resources proves to be challenging due to the dynamics of mobile devices and sensors. In particular, situations need to be efficiently, scalably, and consistently detected with respect to ever-changing sensors in the environment of a mobile object. Moreover, the computing resources that provide low latencies change with the access points of mobile devices and sensors. The goal of this thesis is to provide concepts and algorithms to i) continuously detect situations that recently occurred close to a mobile object, ii) support bandwidth and computational efficient detections of such situations on distributed computing resources, and iii) support consistent, low latency, and high quality detections of such situations. To this end, we introduce the distributed Mobile CEP (MCEP) system which automatically adapts the processing of sensor data streams according to a mobile object’s location. MCEP provides an expressive, location-aware query model for situations that recently occurred at a location close to a mobile object. MCEP significantly reduces latency, bandwidth, and processing overhead by providing on-demand and opportunistic adaptation algorithms to dynamically assign event streams to queries of the MCEP system. Moreover, MCEP incorporates algorithms to adapt the deployment of MCEP queries in a network of computing resources. This way, MCEP supports latency-sensitive, large-scale deployments of MSA applications and ensures a low network utilization while mobile objects change their access points to the system. MCEP also provides methods to increase the scalability in terms of deployed MCEP queries by reusing event streams and computations for detecting common situations for several mobile objects.
Open Access
Position sharing for location privacy in non-trusted systems
(2015) Skvortsov, Pavel; Rothermel, Kurt (Prof. Dr. rer. nat. Dr. h.c.)
Currently, many location-aware applications are available for mobile users of location-based services. Applications such as Google Now, Trace4You or FourSquare are being widely used in various environments where privacy is a critical issue for users. A general solution for preserving location privacy for a user is to degrade the quality of his or her position information. In this work, we propose an approach that uses spatial obfuscation to secure the users’ position information. By revealing the user’s position with a certain degree of obfuscation, the first crucial issue is the tradeoff between privacy and precision. This tradeoff problem is caused by limited trust in the location service providers: higher obfuscation increases privacy but leads to lower quality of service. We overcome this problem by introducing the position sharing approach. Our main idea is that position information is distributed amongst multiple providers in the form of separate data pieces called position shares. Our approach allows for the usage of non-trusted providers and flexibly manages the user’s location privacy level based on probabilistic privacy metrics. In this work, we present the multi-provider based position sharing approach, which includes algorithms for the generation of position shares and share fusion algorithms. The second challenge that must be addressed is that the user’s environmental context can significantly decrease the level of obfuscation. For example, a plane, a boat and a car create different requirements for the obfuscated region. Therefore, it is very important to consider map-awareness in selecting the obfuscated areas. We assume that a static map is known to an adversary, which may help in deriving the user’s true position. We analyze both how map-awareness affects the generation and fusion of position shares and the difference between the map-aware position sharing approach and its open space based version. Our security analysis shows that the proposed position sharing approach provides good security guarantees for both open space and constrained space based models. The third challenge is that multiple location servers and/or their providers may have different trustworthiness from the user’s point of view. In this case, the user would prefer not to reveal an equal level (precision) of position information to every server. We propose a placement optimization approach that ensures that risk is balanced among the location servers according to their individual trust levels. Our evaluation shows significant improvement of privacy guarantees after applying the optimized share distribution, in comparison with the equal share distribution. The fourth related problem is the location update algorithm. A high number of different location servers n (corresponding to n privacy levels) may lead to significant communication overhead. Each update would require n messages from the mobile user to the location servers, especially in cases of high update rate. Therefore, we propose an optimized location update algorithm to decrease the number of messages sent without reducing the number of privacy levels and the user’s privacy.
Open Access
Privacy-aware sharing of location information
(2015) Wernke, Marius; Rothermel, Kurt (Prof. Dr. rer. nat. Dr. h.c.)
Location-based applications such as Foursquare, Glympse, or Waze attract millions of users by implementing points of interest finders, geosocial networking, trajectory sharing, or real-time traffic monitoring. An essential requirement for these applications is the knowledge of user location information, i.e., the user's position or his movement trajectory. Location-based applications typically act as clients to a location service, which manages mobile object location information in a scalable fashion and provides various clients with this information. However, sharing location information raises user privacy concerns, especially if location service providers are not fully trustworthy and user location information can be exposed. For instance, an attacker successfully compromising a location service may misuse the revealed location information for stalking, mugging, or to derive personal user information like habits, preferences, or interests of the user. Driven by the increasing number of reported incidents where service providers did not succeed in protecting private user information adequately, user privacy concerns are further intensified. Therefore, we present novel approaches protecting user location privacy when sharing location information without assuming location service providers to be fully trustworthy. To protect user position information, we present our position sharing concept. Position sharing allows to reveal only positions of decreased precision to different location services, while clients can query position shares from different location services to increase precision. To protect movement trajectories, we introduce our trajectory fragmentation approach and an approach protecting the speed information of movement trajectories.
Open Access
Protocols for the efficient dissemination of context-aware messages
(2016) Geiger, Lars; Rothermel, Kurt (Prof. Dr. rer. nat. Dr. h.c.)
Context-aware applications are able to react and adapt to the context of their users. This context includes, for instance, location, properties of the user or their surroundings, nearby devices, etc. Over the last years, powerful mobile devices, i.e., smartphones or tablet computers, have become an important part in many people's computing life. Most of these devices maintain a continuous high-speed network connection, allowing to provide distributed applications with an uninterrupted stream of data. Additionally, a huge number of sensors, both in these mobile devices and deployed in our surroundings, enable the creation of comprehensive context models. Such large-scale context models open up new possibilities for the development of context-aware applications by providing access to relevant context information from providers all over the world. However, until now, applications need to query the context model for relevant information or register for events or messages; it is not possible to "push" information to the mobile devices, neither from the infrastructure nor from other mobile devices. To support application developers, we propose Contextcast, a novel communication paradigm that allows for the dissemination of context-aware (or contextual) messages in a system of context-aware routers. This includes the fundamental semantics to address clients using context constraints and a reference dissemination scheme for such messages. To enable Contextcast to grow to scales similar to the context-aware systems that it is intended to be used with, we also propose a couple of optimized routing approaches. They are designed to reduce the number of maintenance messages that are necessary for the dissemination of contextual messages. One optimized routing algorithm uses coarse context information to reduce the amount of context updates propagated to routers. To this end, routers use the similarity of contexts to automatically find groups of similar clients, whose information can then be propagated as a single, coarse context. While this reduces the amount of context information to be propagated, the resulting information loss causes more messages to be forwarded, since routers no longer possess exact information to match against the constraints in contextual messages. A configurable similarity threshold allows for various trade-offs between the coarseness of the context information and the resulting additional message load. The second orthogonal routing approach relies on statistics to determine the characteristics of contexts and messages in the system. Without context knowledge, routers must assume the presence of a matching recipient and forward a message speculatively to disseminate it to all recipients. Using statistics, routers can determine how often certain messages occur and then calculate the benefit of propagating contexts corresponding to these messages. Several parameters enable an administrator to adjust how fast the system reacts to changes, depending on the observed messages and context updates. Additionally, temporal support extends Contextcast with a powerful mechanism that allows application developers and clients to address messages to certain contexts in the past or future. This includes an additional context attribute \cattr{time} and a constraint with various, easy to use temporal operators. We also propose efficient routing approaches for historical and future messages. Routing historical messages focuses on efficient routing while effectively protecting the clients' privacy, i.e., their respective context history. The routing approach for future messages delays forwarding messages until a matching context is registered, thus preventing needlessly forwarded messages.
Open Access
Skalierbare und zuverlässige Gruppenkommunikation im Internet
(2002) Maihöfer, Christian; Rothermel, Kurt (Prof. Dr. rer. nat. Dr. h.c.)
Multicast erlaubt die bandbreitenschonende Auslieferung einer Nachricht an eine Gruppe von Empfängern. Anwendungen wie Informations- und Softwareverteilung an große Empfängergruppen, Audio- und Videokonferenzen, verteiltes Rechnen und verteilte Spiele lassen sich mit einem Multicast-Dienst effizient realisieren. Eine Reihe von Anwendungen erfordert dabei eine zuverlässige Nachrichtenauslieferung, die über den im Internet verfügbaren unzuverlässigen IP-Multicast-Dienst hinausgeht. Aufbauend auf IP-Multicast wird eine zuverlässige Nachrichtenauslieferung mittels Empfangsbestätigungen und Übertragungswiederholungen durch nicht-hierarchische oder hierarchische Transportprotokolle erreicht. Wie sich im Verlauf der Arbeit zeigen wird, erlauben lediglich hierarchische Transportprotokolle die skalierbare Realisierung eines zuverlässigen Multicast-Diensts zur Gruppenkommunikation. Diese ordnen alle Gruppenmitglieder in eine Hierarchie ein, die Kontrollbaum genannt wird. Durch den Kontrollbaum können notwendige Übertragungswiederholungen lokal begrenzt werden und müssen nicht mehr notwendigerweise vom ursprünglichen Sender durchgeführt werden. Die Verteilung dieser Last auf andere Gruppenmitglieder ermöglicht einen mit der Mitgliederzahl skalierbaren Dienst. Die Voraussetzung für den praktischen Einsatz ist ein skalierbares Verfahren zum Aufbau der Kontrollbäume. Mit dem Token-Repository-Service (TRS) wird in dieser Arbeit ein effizientes Verfahren zum Aufbau der Kontrollbäume vorgestellt. Drei Realisierungsvarianten des TRS-Diensts, TRS-R, TRS-K und TRS-M, erlauben unterschiedlichste Anforderungen zu berücksichtigen. Das wesentliche Entwurfskriterium aller Varianten ist eine skalierbare und effiziente Realisierung. Vergleichende Berechnungen und Simulationen mit alternativen Ansätzen unterstreichen nicht nur die Tragfähigkeit des TRS-Diensts. Dieser stellt momentan die einzige skalierbare Lösung zum Aufbau von Kontrollbäumen dar. Aufgrund der Unabhängigkeit des TRS-Diensts von einem Multicast-Routing-Protokoll ist es zudem die einzige Lösung, die durchgängig im gesamten Internet eingesetzt werden kann. Zur Erleichterung der Auswahl eines geeigneten zuverlässigen Multicast-Transportprotokolls und zur optimalen Konfiguration eines ausgewählten Transportprotokolls mit dem TRS-Dienst wird eine ausführliche probabilistische Analyse durchgeführt. Ausgangspunkt der Analyse ist nicht die Untersuchung konkreter Protokolle, sondern die Identifizierung wichtiger generischer Protokollklassen und deren anschließende Beurteilung. Dabei ergeben sich bedeutende Unterschiede bezogen auf den Bandbreitenbedarf, den Durchsatz und die Nachrichtenverzögerungen. Die Evaluation zeigt vielfache Vorteile für hierarchische Verfahren. Diese bieten nicht nur Skalierbarkeit, auch bezüglich des verursachten Nachrichtenaufwands, des Durchsatzes und der erzielbaren Nachrichtenverzögerung sind sie durch eine Verteilung der Last und kürzere Kommunikationswege gegenüber nicht-hierarchischen Protokollen im Vorteil. Als ein wesentlicher Parameter zur Konfiguration hierarchischer Transportprotokolle wird der Verzweigungsgrad des Kontrollbaums identifiziert werden. Im Gegensatz zu bisherigen Verfahren ermöglicht es der TRS-Dienst den Verzweigungsgrad flexibel zu konfigurieren. Die Analysen und Simulationen zeigen, dass bei einer geeigneten Wahl des Verzweigungsgrads der Durchsatz und die Verzögerung des zuverlässigen Transportprotokolls beträchtlich verbessert werden kann. Zusammenfassend erlaubt es der vorgestellte Token-Repository-Service durch seinen skalierbaren Kontrollbaumaufbau erstmals, zuverlässige Multicast-Kommunikation auch für große Gruppen zu etablieren. Durch die flexible Wahl des Verzweigungsgrads der entstehenden Kontrollbäume ist zudem eine beträchtliche Leistungsoptimierung des zuverlässigen Transportprotokolls sowie eine entscheidende Verringerung der Netzbelastung mit dem TRS-Dienst möglich.