PhD thesis abstracts

PhD thesis abstracts

• Apostolos Papageorgiou
• Kristian Evensen

Apostolos Papageorgiou

Service-Oriented Computing with Wireless Participants - Adaptation Mechanisms and Decision Support for Lightweight Web Service Consumption

At the crossroads of two hot trends of modern computer science, namely service-orientation and mobile computing, great potentials arise together with tough challenges. The list of advantages of combining these technologies is long and compelling: Outsourcing of data- and processing-intensive software tasks from mobile devices to more capable systems, quick mobile application development through the use of existing software services that are otherwise difficult to implement on mobile devices from scratch, and the list goes on. However, service-oriented messaging and description technologies are characterized by a verbose, self-descriptive nature, in order to achieve interoperability and platform-independence. This renders them often heavyweight and thus not always a good match for the resource-constrained nature of mobile, wireless devices.

In the face of the fact that very different devices will coexist in future service-oriented systems, researchers are diligently trying to tackle the aforementioned challenges by developing adaptation mechanisms. These are meant to allow the systems to dynamically and seamlessly switch to configurations that suit each particular participant. When it comes to one of the most important service-oriented technologies, namely Web services, adaptation for resource-constrained devices usually translates to an attempt to use lightweight ways of communication that are dictated by the system context.

The striving for enhancements in the field of Web service adaptation mechanisms for wireless participants starts with three simple questions: Which specific adaptation mechanisms can be used and what are their characteristics? Can the development of new adaptation mechanisms contribute to the currently existing possibilities? Which adaptation mechanisms should be used under which wireless system contexts? The systematic and scientific examination of these questions leads to the three corresponding main contributions of this thesis.

First, the conducted survey and comparison of Web service adaptation mechanisms is the first work examining the conditions of the wireless system context under which the adaptation mechanisms achieve significant benefits. Thus, the survey results can be used as a basis for examining the two further issues. Second, a new, caching-based Web service adaptation mechanism is presented. The developed mechanism is the first to enable the use of cached responses of external, i.e., third-party, Web services with guaranteed 100% freshness in an automated and generic manner. The freshness of cached objects refers to their probability of being up-to-date. The evaluation proves that the approach can lead to performance enhancements of mobile Web service invocations compared to other approaches that achieve absolute freshness. Third, provided that no single adaptation mechanism is the best-performing under all possible system contexts, the thesis offers insights with regard to the issue of corresponding decision support. Decision support algorithms that are based on the obtained survey results are developed and evaluated with focus on an important, yet uninvestigated aspect, namely the existence of missing system context data. The respective work reveals which data imputation approaches are best suited to the examined scenario.

By discussing the above issues within a well-defined future scenario of mediated Web service usage, the thesis provides further contributions, such as architectural solutions, engineering approaches, and problem formulations. All in all, its findings should be interesting for any work in the research area of mobile services.

Kristian Evensen

Aggregating the Bandwidth of Multiple Network Interfaces to Increase the Performance of Networked Applications

One the few constants in the world of computer networking, is that users always want more bandwidth. The usage of bandwidth-intensive services, like video streaming, video conferencing and games, have exploded over the last couple of years. At the same time, devices capable of connecting to two or more different networks simultaneously, known as host multihoming, have become the norm. For example, most laptops are equipped with a least a Local Area Network (LAN) and Wireless LAN (WLAN) interface, and smartphones can connect to both WLANs and HSDPA-networks. Being connected to multiple networks simultaneously allows for desirable features like bandwidth aggregation and redundancy.

Enabling and making efficient use of multiple links requires solving several challenges related to deployment, link heterogeneity and dynamic link behavior. Even though multihoming has existed for a long time, for example routers require being able to connect to different networks, most existing operating systems, network protocols and applications do not take host multihoming into consideration. The default behavior is still to use a single link for all traffic. Using a single link is, for example, often insufficient to meet the requirements of popular, bandwidth intensive services like video streaming.

In this thesis, we have focused on bandwidth aggregation on host multihomed devices. Bandwidth aggregation is to merge physical links into one logical link that offers a higher bandwidth. Even though bandwidth aggregation has been a research field for several years, the related work has failed to consider the challenges present in real world networks properly, or does not apply to scenarios where a device is connected to different networks.

In order to solve the deployment challenges and enable the use of multiple interfaces in a way that works in a real-world network environment, we have created a platform-independent framework, MULTI. MULTI works in the presence of NAT, automatically detects and configures the device based on changes in link state, and notifies the application(s) of any changes.

MULTI was used as the foundation for designing transparent and application-specific bandwidth aggregation techniques. A transparent bandwidth aggregation technique is a technique that requires no modifications to the application, nor the operating system at either the sender or receiver. An application-specific technique, on the other hand, is an extension to an existing application or application type.

The application-specific bandwidth aggregation technique presented in this thesis, improve the performance of quality adaptive video streaming. The technique was evaluated with different types of streaming in both a controlled network environment and real-world networks. Adding a second link gave a significant increase in both video and playback quality. The technique is not limited to video streaming or the system/protocols we have worked with. As long as two common requirements are met, the technique can be applied.

In many cases, it is not possible to extended the application directly with multilink support. Working on the network-layer allows for the creation of transparent bandwidth aggregation solutions. A transparent, network-layer bandwidth aggregation solution must cater to the behavior of the different transport protocol in order to achieve efficient bandwidth aggregation. The transparent bandwidth aggregation techniques introduced in this thesis are targeted at Universal Datagram Protocol (UDP) and Transmission Control Protocol (TCP), the two most popular transport protocols in the internet today.