05 Fakultät Informatik, Elektrotechnik und Informationstechnik

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/6

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End date: 2001Author: search.filters.author.Rothermel, Kurt

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    A protocol for preserving the exactly-once property of mobile agents
    (1997) Rothermel, Kurt; Straßer, Markus
    Mobile agents are autonomous objects that can migrate from node to node of a computer network. Mobile agent technology has been proposed for various application areas, including electronic commerce, systems management and active messaging. Many of these applications - especially those for electronic commerce - require agents to be performed 'exactly once', independent of communication and node failures. In other words, once a mobile agent has been launched, it must never be lost before its execution is finished. Moreover, each 'portion' of the agent performed at the visited nodes is performed exactly once. Due to the autonomy of mobile agents, there is no 'natural' instance that monitors the progress of an agent's execution. As a result of that agents may be blocked due to node crashes or network partitioning even if there are other nodes available that could continue processing. In this paper, we will describe a protocol that ensures the exactly once property of agents and additionally reduces the blocking probability of agents by introducing so-called observer nodes for monitoring the progress of agents. This protocol is based on conventional transactional technology, such as defined by X/Open DTP or CORBA OTS. It is implemented in the Mole, a mobile agent system developed at Stuttgart University.
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    A context-aware hoarding mechanism for location-dependent information systems
    (2000) Kubach, Uwe; Rothermel, Kurt
    When used in an outdoor environment mobile information systems often suffer from the disadvantages of wireless WANs, especially low bandwidth, high delay, and frequent disconnections. Hoarding is an effective method to overcome these disadvantages by transferring information which is probably needed by the user in advance. In this paper we propose a generic, context-aware hoarding mechanism. When selecting the information to hoard, it considers the user's future location as well as the expected speed of movement. In contrast to existing hoarding mechanisms it is universally applicable for different types of location-dependent, mobile information systems. Its flexibility allows it to rely on different knowledge sources in order to get information about a user's context.
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    Application sharing in teaching context with wireless networks
    (2001) Burger, Cora; Papakosta, Stella; Rothermel, Kurt
    The success of teaching is depending on a couple of factors: on how far students are involved into lectures, on the material, its completeness and on co-learning of students. Involvement of students into lectures means, being able to follow the thoughts of the teacher, ask questions and make comments. The material must be presented in a suitable form and essential parts of it have to be available during the whole learning process, for preparing participation in lectures and exercises as well as for exams. For more effective learning and training of social abilities, working in groups of co-learners has to be encouraged. Mobile and ubiquitous computing offer new possibilities to achieve these goals by increasing the awareness in class and supporting an active participation of students. By promoting existing concepts and enabling new ways of application sharing, the project SASCIA (System architecture supporting cooperative and interactive applications) aims at developing a framework for multiple applications to support teaching in collocated, remote and hybrid scenarios. Its core is composed of components to capture and distribute context information about sessions, participants and those applications that are used during a lecture or encounter among students. A configurable floor control was designed to cope with a wide spectrum of applications and learning situations. For some cases, even a control for semantic consistency can be necessary. In combination with a suitable user and session management, a whiteboard for annotations and a recording facility to support latecomers as well as subsequent replay, these components are providing the required functionality. As a consequence, SASCIA offers remote control and viewing facilities to all participants during lectures and co-learning sessions.
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    ATOMAS : a transaction-oriented open multi agent system; final report
    (1998) Straßer, Markus; Baumann, Joachim; Hohl, Fritz; Schwehm, Markus; Rothermel, Kurt
    The electronic marketplace of the future will consist of a large number of services located on an open, distributed and heterogeneous platform, which will be used by an even larger number of clients. Mobile Agent Systems are considered to be a precondition for the evolution of such an electronic market. They can provide a flexible infrastructure for this market, i.e. for the installation of new services by service agents as well as for the utilization of these services by client agents. Mobile Agent Systems basically consist of a number of locations and agents. Locations are (logical) abstractions for (physical) hosts in a computer network. The network of locations serves as a unique and homogeneous platform, while the underlying network of hosts may be heterogeneous and widely distributed. Locations therefore have to guarantee independence from the underlying hard- and software. To make the Mobile Agent System an open platform, the system furthermore has to guarantee security of hosts against malicious attacks.
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    System mechanisms for partial rollback of mobile agent execution
    (1999) Straßer, Markus; Rothermel, Kurt
    Mobile agent technology has been proposed for various fault-sensitive application areas, including electronic commerce, systems management and active messaging. Recently proposed protocols providing the exactly-once execution of mobile agents allow the usage of mobile agents in these application areas. Based on these protocols, a mechanism for the application-initiated partial rollback of the agent execution is presented in this paper. The rollback mechanism uses compensating operations to roll back the effects of the agent execution on the resources and uses a mixture of physical logging and compensating operations to rollback the state of the agent. The introduction of different types of compensating operations and the integration of an itinerary concept with the rollback mechanism allows performance improvements during the agent rollback as well as during the normal agent execution.
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    Dynamic distance maps of the Internet
    (1999) Theilmann, Wolfgang; Rothermel, Kurt
    There is an increasing number of Internet applications that attempt to optimize their network communication by considering the network distance across which data is transferred. Such applications range from replication management to mobile agent applications. One major problem of these applications is to efficiently acquire distance information for large computer networks. This paper presents an approach to create a global view on the Internet, a so-called network distance map, which realizes a hierarchical decomposition of the network into regions and which allows to estimate the network distance between any two hosts. This view is not only a single snapshot but is dynamically adapted to the continuously changing network conditions. The main idea is to use a certain set of hosts for performing distance measurements and to use the so gained information for estimating the distance between arbitrary hosts. A hierarchical clustering provides the notion of regions and allows to coordinate the measurements in such a way that the resulting network load is minimized. An experimental evaluation on the basis of 119 globally distributed measurement servers shows that already a small number of measurement servers allows to construct fairly accurate distance maps at low costs.
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    A delay analysis of tree-based reliable multicast protocols
    (2001) Maihöfer, Christian; Rothermel, Kurt
    We present a comparative delay analysis of tree-based reliable multicast protocols and show the influence of varying sending rates, group sizes, packet loss probabilities and branching factors of the control tree. Besides the average delivery delay we consider the delay to reliably deliver all packets and the round trip delay. The former two examines the delay between generation of a packet at the sender and correct reception at a randomly chosen receiver or all receivers, respectively. The latter is the delay between generation of a packet at the sender and reception of all acknowledgment packets at the sender. Our numerical results show that all tree-based protocols provide low delays and good scalability. From the four considered protocol classes, NAK-based protocols achieve the best scalability but ACK-based protocols achieve the lowest delays. An important aspect of our work is to be of practical relevance rather than being of only theoretical nature. Therefore, we have compared the analytical results with a RMTP and TMTP simulation. Both show similar results which confirms that our analysis can help to choose a suitable protocol and to tune them for improved performance.
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    Representing time in multimedia systems
    (1993) Wahl, Thomas; Rothermel, Kurt
    As multimedia systems deal with a variety of temporally interrelated media items, synchronization is an important issue in those systems. One part of synchronization is the representation of temporal information. In contrast to traditional computing tasks, multimedia imposes new requirements on the representation of time. Specifically, a fine-grained and a flexible temporal model is required. Therefore, a number of temporal models have been suggested by various authors. This paper evaluates and classifies a selection of the most common existing models applying fundamental statements of the time theory and temporal logic. Learning from the deficits of the existing models, a new temporal model based on interval operators is proposed for multimedia systems.
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    Verbundprojekt PoliFlow : Abschlußbericht
    (1999) Kindler, Thomas; Kulendik, Ottokar; Rothermel, Kurt; Schneider, Kerstin; Siebert, Reiner; Soyez, Tobias
    In den letzten Jahren fanden Groupware- und Workflow-Systeme ein große Beachtung bei Herstellern, Anwendern und Wissenschaftlern. Nach anfänglich unkritischer Euphorie wurden in den letzten Jahren jedoch auch einige Schwachstellen der noch jungen Technologien erkannt. Von der Behebung dieser Schwachstellen wird die weitere Entwicklung maßgeblich beeinflußt werden. In der Förderinitiative POLIKOM wurde untersucht, wie diese Technologien im Anwendungsbereich der öffentlichen Verwaltung effektiv und effizient eingesetzt werden können. Im Projekt PoliFlow wurden Mechanismen und Modelle entworfen, mit denen einige existentiellen Mängel in den Bereichen Sicherheit und Flexibilität behoben werden konnten. Hierbei wurden die Beschreibungsmodelle für Workflows um spezifische Aspekte und die Ausführungsmodelle um entsprechende Funktionalitäten erweitert. Um diese erweiterte Funktionalität in verschiedene bestehende Systeme integrieren zu können, wurden Referenzarchitekturen entworfen, die auf eine Vielzahl bestehender Modelle und Systeme übertragbar sind. Weitere erfolgreiche Konzepte wurden zur Integration von Workflow und synchroner Telekooperation sowie zur zuverlässigen Ausführung langlebiger Prozesse entwickelt. Eine weitere Schwachstelle der Technologie war die mangelnde Unterstützung heterogener System- und Anwendungsumgebungen. Um eine große Verbreitung dieser strategischen und hoch integrierten Informationssysteme zu erreichen, müssen die beteiligten Personen von unterschiedlichen Rechnern und Netzen eine entsprechende Zugangsmöglichkeit erhalten. Mit der Realisierung des Stuttgarter Workflow- und Telekooperationssystems (SWATS), bei dem neueste Intra-/Internet-Technologien (wie Java und CORBA) berücksichtigt wurden, konnten auch diese Anforderungen erfüllt werden. Darüber hinaus bildete das Grundsystem von SWATS die Basis zur Integration der Prototypen aus den oben genannten Arbeitsbereichen.
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    The shadow approach : an orphan detection protocol for mobile agents
    (1998) Baumann, Joachim; Rothermel, Kurt
    Orphan detection in distributed systems is a well researched field for which many solutions exist. These solutions exploit well defined parent-child relationships given in distributed systems. But they are not applicable in mobile agent systems, since no similar natural relationship between agents exist. Thus new protocols have to be developed. In this paper one such protocol for controlling mobile mobile agents and for orphan detection is presented. The shadow' approach presented in this paper uses the idea of a placeholder (shadow) which is assigned by the agent system to each new agent. This defines an artificial relationship between agents and shadow. The shadow records the location of all dependent agents. Removing the root shadow implies that all dependent agents are declared orphan and are eventually terminated. We introduce agent proxies that create a path from shadow to every agent. In an extension of the basic protocol we additionally allow the shadow to be mobile. The shadow approach can be used for termination of groups of agents even if the exact location of each single agent is not known.