Modern software must evolve in response to changing conditions. In the most widely used programming environments, code is static and cannot change at runtime. This poses problems for applications, that have limited down-time. More support is needed for dynamic evolution. In this paper we present an approach for supporting dynamic evolution of Java programs. In this approach, Java programs can evolve by changing their components, namely classes, during their execution. Changes in a class lead to changes in the its instances, thereby allowing evolution of both code and state. The approach promotes compatibility with existing Java applications, and maintains the security and type safety controls imposed by Java's dynamic linking mechanism. Experimental analyses of our implementation indicate that the implementation imposes a moderate performance penalty relative to the unmodified virtual machine. This work is supported by the Defense Advanced Research Project Agency (DARPA) and Rome Laboratory, Air Force Materiel Command, USAF, under agreement number F30602-97-1-0221. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright annotation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Defense Advanced Research Project Agency (DARPA), Rome Laboratory, or the U.S. Government. Report Documentation PageForm Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. Modern software must evolve in response to changing conditions. In the most widely used programming environments, code is static and cannot change at runtime. This poses problems for applications, that have limited down-time. More support is needed for dynamic evolution. In this paper we present an approach for supporting dynamic evolution of Java programs. In this approach, Java programs can evolve by changing their components, namely classes, during their execution. Changes in a class lead to changes in the its instances, thereby allowing evolution of both code and state. The approach promotes compatibility with existing Java applications, and maintains the security and typ...

Most implementationsof HTTP servers do not distinguish among requests to differeut pages. This has the implicatiou that requests for popular pages have the tendency to overwhelm the requests for other pages. In addition, HTTP servers do not allow a site to specify policies for server resource allocation.This paper presents a notion of wality of service that enables a site to customize how an HTTP server should respond to external requests by setting priorities among page requests and allocating server resources.It also describes a design and an implementation of a distributed HTTP server, QoS Web Server, that enforces the quality of service constraints.The performance aualysis of the prototype server indicates that the server provides the desired quality of service with minimal overhead.

Abstract. The continual improvement in computer performance together with the prevalence of high-speed network connections having high throughput and moderate latencies enables the deployment of multimedia applications, such as collaborative virtual environments, over wide area networks. These applications can serve as simulated environments in scenarios such as emergency response training to catastrophic disasters, military training, and entertainment. Many of these systems use 3D graphics for display and may be required to distribute geometric models on demand between participants. Progressive meshes provide an attractive mechanism for such distribution. Previous uses of progressive meshes have sent data using reliable protocols (TCP). However, such protocols have disadvantages in on-demand settings, in that they: (1) use flow control, which limits performance in wide area networks; (2) add additional bandwidth when there is loss; (3) treat all loss as an indication of congestion; and (4) require feature-rich multicast support, which is not always available. In this paper, we modify progressive mesh models to allow reconstruction even in the event of packet loss. We use these modifications in two transmission schemes, a hybrid transmission that uses TCP and UDP to send packets and a forward error-correcting transmission scheme that uses redundancy to decode the information sent. We assess the performance of these transmission schemes when deployed on network testbeds that simulate wide-area and wireless characteristics.

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