Faster processors are quickly enabling a new class of computationally intensive applications that actively transform information flows. Performing such computation at the terminal end is contrary to current trends toward low-power terminal devices. Moreover, scalability and efficiency concerns are also encouraging service providers to outsource computation when operating under loaded conditions. To address the problem of deploying such applications, we introduce the JOURNEY network model, which provides computation as an integrated network service. Contrary to other distributed computing models, JOURNEY does not attempt to guarantee that a given computational job will indeed be completed. Instead, the JOURNEY model trades off such hard guarantees in favor of architectural simplicity, and hence scalability. In order for the JOURNEY model to be applicable to real-time multimedia flows, we introduce the notion of soft quality-of-service (QoS) that provides a probabilistic bound on the unprocessed packet rate (UPR). Based on this notion, we describe a packet-processing admission control algorithm that additionally takes into consideration a flow's real-time constraints. We also propose a computing router architecture that is based on cluster technology. This architecture can track technology advances in both routing and computing independently. We further present a motivating multimedia application that employs a semantically driven video transcoding service within the JOURNEY framework we implemented, and describe our experience along with performance measurements.

In distributed object systems, remote method invocation attempts to abstract communication between different machines to the level of object method invocation. Link failure in such systems often appears as a failed method invocation, which must be explicitly handled outside the remote object infrastructure. This usually implies that techniques to handle the effects of link failure are embedded within application functionality. The resulting application can become unnecessarily specialized to a particular environment, restricting the situations where it could be used. We explore the possibility of cleanly decoupling policies for handling a link failure from the application. With this goal in mind, we present extensions to the Java RMI system in the context of a distributed system with mobile hosts. An intermittent communication link is typical in such systems, and applications use techniques such as data caching or queuing of method calls to alleviate the effects. We show how such techniques and policies can be implemented generically so that not only does the application become oblivious of them, but also policies can be chosen dynamically depending on a particular situation.

In mobile computing, factors such as add-on hardware components and heterogeneous networks result in an environment of changing resource constraints. An application in such a constrained environment must adapt to these changes so that available resources are properly utilized. We propose an architecture for exporting awareness of the mobile computing environment to an application. In this architecture, a change in the environment is modeled as an asynchronous event that includes information related to the change. Events are typed and are organized as an extensible class hierarchy so that they can be handled at different levels of abstraction according to the requirement of each application. We also compare two approaches to structure an adaptive application. One addresses the problem of incorporating adaptiveness into legacy applications, while the other considers the design of an application with adaptiveness in mind.

In mobile computing, factors such as add-on hardware components and heterogeneous networks result in an environment of changing resource constraints. An application in such a constrained environment must react to these changes so that available resources are properly utilized. In this paper, we propose a framework to build environment aware applications. The architecture is based on an event delivery mechanism that decouples event detection from delivery, giving the flexibility and extensibility that is necessary in a mobile computing environment. Information associated with the event is delivered as part of the event notification, while delivery latency is reduced by clever thread scheduling. We demonstrate the utility of this framework by structuring an environment aware networking subsystem around a prototype implementation. The performance of this implementation is competitive with current event delivery mechanisms such as the Unix signal.

This paper accompanies a demonstration of a variable bit-rate (VBR) video server with scalable quality-of-service (QoS) control. Multimedia applications dynamically demand different QoS grades from servers and networks during a session. Since video is a critical component of distributed multimedia applications, video servers have to be responsive to the applications' requirements and to the availability of resources on heterogeneous terminals and networks. For this purpose a video server has to dynamically scale the source's quality in the control space formed by frame resolution, quantization and display rate. Experiences with a proof-of-concept implementation of a server with scalable QoS control show that it is able to jointly match the target quality of applications, the processing limitations of terminals and the capacity limitations of networks.

The client-agent-server architecture, which serves as a flexible mechanism for many-to-many remote procedure calls, is discussed. The architecture provides a transparent view of the server to a client. The client accesses the server group through the agent while retaining only an abstract view of the server. The call primitives for the client, agent groups, hook construction and primitives (hooks are basic building block which a server group provides to the agent group), and server groups used in the architecture are described. Implementation issues are reviewed.>