16287780
Distributed object invocation in OBIWAN
The need for sharing is well-known in a large number of distributed collaborative applications. These applications are difficult to develop for an environment in which network connections are slow and not reliable.For this purpose, we developed a platform called OBIWAN that: i) allows the application programmer to decide the mechanism by which objects should be invoked, remote method invocation or invocation on a local replica, and ii) provides hooks for the application programmer to implement a set of application specific properties such as transactional support, for example.This functionality allows the application programmer to deal with situations that frequently occur in a wide-area network, such as disconnections and slow links. As a matter of fact, as long as the objects needed by an application are locally accessible, there is no need to be connected to the network. In addition, it allows the programmer to easily replace, in run-time, remote by local invocations, thus improving the performance of his application and its adaptability.The prototype is developed in Java, is very small and simple to use, the preliminary performance results are very encouraging, and existing applications can be easily modified to take advantage of the OBIWAN functionality.
6931993
PerDiS: Design, Implementation, and Use of a PERsistent DIstributed Store
The PerDis (Persisent Distributed Store) project addresses the issue of providing support for distributed collaborative engineering applications. We describe the design and implementation of the PerDis platform, and its support for such applications. 
 
Collaborative engineering raises system issues related to the sharing of large volumes of fine-grain, complex objects across wide-area networks and administrative boundaries. PerDiS manages all these aspects in a well defined, integrated, and automatic way. Distributed application programming is simplified because it uses the same memory abstraction as in the centralized case. Porting an existing centralized program written in C or C++ is usually a matter of a few, well-isolated changes. 
 
We present some performance results from a proof-of-concept platform that runs a number of small, but real, distributed applications on Unix and Windows NT. These confirm that the PerDiS abstraction is well adapted to the targeted application area and that the overall performance is promising compared to alternative approaches.
1020859
Context Awareness: An Experiment with Hoarding
Computer mobility allows people to use computers in varied and changing environments. This variability forces applications to adapt thus requiring awareness of the computational and physical environment (e.g. information about power management, network connections, synchronization opportunities, storage, computation, location-based services, etc.). 
 
An important application for mobility is hoarding, i.e. automatic file replication between devices. To be accurate and not obstructive to the user, the hoarding mechanism requires both context awareness (e.g. amount of usable storage) and estimation of future environment conditions (e.g. network connection, tasks to be performed by the user in the near future, etc.). However, making applications context-aware is hindered by the complexity of dealing with the large variety of different modules, sensors and service platforms, i.e. there is no middleware supporting such applications and their development in a uniform and integrated way. 
 
This paper presents the architecture for an environment awareness system (EAS) and how it applies to hoarding. EAS is a middleware component that acts as an intermediary between applications and all mechanisms that assess the surrounding environment. It lets applications query and combine environment properties in a standardized way. Crucial for the success of automatic file hoarding is the EAS's capability of supporting environment prediction based on simple reasoning and pattern detection. Thus, applications may advise users accordingly or even make decisions on their behalf.
15278123
Taxonomy and survey of retransmission-based partially reliable transport protocols
The mismatch between the services offered by the two standard transport protocols in the Internet, TCP and UDP, and the services required by distributed multimedia applications has led to the development of a large number of partially reliable transport protocols: protocols that in terms of reliability place themselves between TCP and UDP. This paper presents taxonomy for retransmission-based, partially reliable transport protocols, i.e. the subclass of partially reliable transport protocols that makes error recovery through retransmissions. The taxonomy comprises two classification schemes: one that classifies protocols with respect to the reliability service they offer and one that classifies them with respect to their error control scheme. The objective of our taxonomy is fourfold: to introduce a unified terminology; to provide a framework in which this class of protocols can be examined, compared and contrasted; to make explicit the error control schemes used by these protocols; and, finally, to gain new insight into these protocols and thereby suggest avenues for future research. Based on our taxonomy, a survey is made of existing retransmission-based, partially reliable transport protocols. The survey shows how protocols are categorized according to our taxonomy and exemplifies the majority of reliability services and error control schemes detailed in our taxonomy.
16411220
The Impact of Loss Generation on Emulation-based Protocol Evaluation
Network emulation has for a long time been an important tool for evaluating the performance of communication protocols. By emulating network characteristics, such as restricted bandwidth, delay and losses, knowledge about the behavior and performance of actual protocol implementations can be obtained. This paper focuses on the generation of losses in network emulators and shows the beneficial effects of being able to control the generation of losses in a precise way. Both the possibility to get additional knowledge about a protocol implementations behavior, as well as statistical benefits such as paired experiments are discussed. By extending the loss generation to also include bit-error generation, in addition to packet losses, a finer level of abstraction is provided. Deterministic bit-error generation allows detailed and repeatable studies of bit-error sensitive protocol behavior. TCP and a loss differentiating variant of TCP is used to illustrate the utility of improved loss generation.
7238873
A Robust JPEG Coder for a Partially Reliable Transport Service
The usage of multimedia applications on the Internet has seen phenomenal growth in recent years. Transport protocols that provide partially reliable service have been suggested as one approach to better handle the requirements of these applications. A partially reliable service provides applications with the possibility of a flexible tradeoff between reliability and delay/throughput. Appropriately designed coders are, however, required to fully utilize a partially reliable service. In this paper we present a JPEG image coder tailored to suit the behavior of a partially reliable byte stream service. With regular JPEG, data loss typically results in severely distorted images. The robust recoder employs three major modifications to standard JPEG in order to adapt to the partially reliable transport: (1) extended resynchronization markers in order to be able to resynchronize effectively, (2) block interleaving in order to spread out the loss of a packet across the image and (3) error concealment in order to minimize the perceived quality loss. The modifications incorporate both new inventions, such as random window interleaving, as well as variations of previously known techniques.
4596877
An Experimental Study on the Performance and Fairness of Loss Differentiation for TCP
The evolution of computer communications and the Internet has led to the emergence of a large number of communication technologies with widely different capabilities and characteristics. While this multitude of technologies provides a wide array of possibilities it also creates a complex and heterogeneous environment for higher-layer communication protocols. Specific link technologies, as well as overall network heterogeneity, can hamper user-perceived performance or impede end-to-end throughput. In this thesis we examine two transport layer centered approaches to improve performance. The first approach addresses the decrease in user satisfaction that occurs when web waiting times become too long. Increased transport layer flexibility with regards to reliability, together with error-resilient image coding, is used to enable a new trade-off. The user is given the possibility to reduce waiting times, at the expense of image fidelity. An experimental examination of this new functionality is provided, with a focus on image-coding aspects. The results show that reduced waiting times can be achieved, and user studies indicate the usefulness of this new trade-off. The second approach concerns the throughput degradations that can occur as a consequence of link and transport layer interactions. An experimental evaluation of the GSM environment shows that when negative interactions do occur, they are coupled to large variability in link layer round-trip times rather than simply to poor radio conditions. Another type of interaction can occur for link layers which expose higher layers to residual bit errors. Residual bit errors create an ambiguity problem for congestion controlled transport layer protocols which cannot correctly determine the cause for a loss. This ambiguity leads to an unnecessary throughput degradation. To mitigate this degradation, loss differentiation and notification mechanisms are proposed and experimentally evaluated from both performance and fairness perspectives. The results show that considerable performance improvements can be realized. However, there are also fairness implications that need to be taken into account since the same mechanisms that improve performance may also lead to unfairness towards flows that do not employ loss differentiation.
42455029
A GSM/GPRS Testbed for TCP Performance Evaluation
Investigating the wireless Internet is a nontrivial task dueto the complexity of wireless systems and the many protocolinteractions that appear. In this paper we present a measurementtestbed that a ...