Advances in network technology and research in real-time packet-switched networks have prompted the emergence of internet conferencing. While audio and video conferencing tools are readily available, shared workspaces and drawing tools have been slower to emerge. We present a design for a network conferencing whiteboard that differs in notable respects from other shared workspace prototypes. We describe its transport layer, based on IP multicasting and application level framing, and its object oriented imaging model, based on persistent PostScript graphics operations. We then propose a user interface, and finally, describe a C++ implementation based on the InterViews structured graphics library. 1 Introduction In the early days of the Internet, experiments with packet voice proved that computer networks could be used for integrated services [5]. However, the technological limitations of the time made such use impractical. This is now changing. Dramatic advances in network bandwidth an...

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We are investigating alternative transport protocol strategies for realizing large scale dissemination services across a wide area network. Communication requirements of such applications are distinct from those based on conventional client-server interactions. Conventional flow and error control methods based on the retransmissions-with-timeout paradigm are not appropriate for such applications. Instead, we are interested in using optimistic flow and error control strategies that take into account application-specific error tolerance and media rates of multimedia applications. This paper describes transport level policies that use a combination of redundant transmissions, rate-based flow control, and selective feedback from receivers. A simulation-based performance evaluation demonstrates that relatively simple techniques succeed well in meeting the QOS requirements of a multimedia multicast and in scaling to hundreds of recipients. 1 Introduction Our research is motivated by the co...

Distributed multipoint applications for group interaction across wide-area networks, such as for simulation and telecollaboration, are becoming increasingly popular. While reliable multicasting has made significant advances in recent years, effective mechanisms to synchronize and coordinate work within large multicast groups and across long distances are still lacking. Synchronous sharing of resources, whose operational semantics prohibits parallel usage, typically creates race conditions among users, which can be resolved through an access discipline called floor control. Existing solutions on floor control, implemented either at the session or application layer, are mostly proprietary, limited in scope and not scalable. Furthermore, no performance comparison of floor control protocols has been attempted to date. We present a novel taxonomy and comparative performance analysis of known classes of floor control protocols, ranging from socially mediated control to protocols operating on ring and tree topologies. We find that aggregation and selective transmission of control information in a tree structure is the most promising solution with regard to scalability, efficacy, and robustness. The principal operation of such a tree protocol is outlined, which dynamically organizes participants in a multi-level control tree and aggregates resource sharing directives on the paths between interacting stations.

The Development of distributed, multimedia, collaborative applications requires resolution of communication issues such as concurrency control, and temporal and causal synchronization of traffic over related data streams. Existing transport and/or session layer protocols do not include desired support for multi-stream, multipoint communication. In this paper, we propose new communication abstractions and mechanisms that facilitate implementation of the necessary coordination and concurrency control semantics in a collaborative application. We also propose a protocol suite called MCP (Multi-Flow Conversation Protocol) for realization of these abstractions and describe its prototype implementation in an internetwork of workstations. The paper also describes our experience with the prototype and results of a performance evaluation. KEYWORDS: multimedia communication, distributed collaboration, temporal and causal synchronization, Unix networking. This research was supported in p...

A causal ordering protocol ensures that if two messages are causally related and have the same destination, they are delivered to the application in their sending order. Causal order strongly simplifies the development of distributed object-oriented systems. To prevent causal order violation, either messages may be forced to wait for messages in their past, or late messages may have to be discarded. For a real-time setting, the first approach is not suitable since when a message misses a deadline, all the messages that causally depend on it may also be forced to miss their deadlines. We propose a novel causal ordering abstraction that takes messages deadlines into consideration. Two implementations are proposed in the context of multicast and broadcast communication that delivers as many messages as possible to the application. Examples of distributed soft real-time applications that benefit from the use of a deadline-constrained causal ordering primitive are given. 1

Receiver synchronization of continuous media streams is required to deal with delay differences and variations resulting from delivery over packet networks such as the Internet. This function is commonly provided using per-stream playout buffers which introduce additional delay in order to produce a playout schedule which meets the synchronization requirements. Packets which arrive after their scheduled playout time are considered late and are discarded. In this paper, we present the Concord algorithm, which provides a delay-sensitive solution for playout buffering. It records historical information and uses it to make short-term predictions about network delay with the aim of not reacting too quickly to short-lived delay variations. This allows an application-controlled tradeoff of packet lateness against buffering delay, suitable for applications which demand low delay but can tolerate or conceal a small amount of late packets. We present a selection of results from an extensive evaluation of Concord using Internet traffic traces. We explore the use of aging techniques to improve the effectiveness of the historical information and hence the delay predictions. The results show that Concord can produce significant reductions in buffering delay and delay variations at the expense of packet lateness values of less than 1%. Keywords--- multimedia stream synchronization, playout buffering. I.

Synchronizing different data streams from multiple sources simultaneously at a receiver is one of the basic problems involved in multimedia distributed systems. This requirement stems from the nature of packet based networks which can introduce end-to-end delays that vary both within and across streams. In this paper, we present a new algorithm called Concord, which provides an integrated solution for these single and multiple stream synchronization problems. It is notable because it defines a single framework to deal with both problems, and operates under the influence of parameters which can be supplied by the application involved. In particular, these parameters are used to allow a trade-off between the packet loss rates, total end-to-end delay and skew for each of the streams. For applications like conferencing this is used to reduce delay by determining the minimum buffer delay/size required. 1 Introduction The use of packet based communication networks to transport digitized str...

A specific class of multimedia applications is expected to be of importance for future communication networks: Multi-Party Interactive Multimedia (MIM). Based on the isolation and characterization of MIM applications, concrete network support requirements are derived in this paper. The varying degree of connectivity, the vastly different sizes in terms of participants and the reliance on a guaranteed Quality of Service make MIM support a difficult problem. Starting with the definition of multimedia communication abstractions, principles of solutions are sketched. For an important subclass of applications a particularly efficient and practicable alternative implementation based on half-duplex channels is introduced. Finally, interfaces at both the transport and network layers are considered. The Tenet Group, Computer Science Division, Department of EECS, University of California, Berkeley and International Computer Science Institute, Berkeley. E-mail: fszyperski,ventreg@icsi.berkeley...

\Delta-causal ordering is a communication abstraction designed for distributed applications whose messages (i) have to be delivered according to causal ordering and (ii) have a limited lifetime after which their data can no longer be used by the application. Example of such applications are: multimedia real-time collaborative applications and groupware real-time applications. For such applications, the broadcasting of information is of primary importance. In this paper, we propose a simple and efficient \Delta-causal ordering protocol in the context of broadcast communication. By taking into account transitive dependencies on message sends, this algorithm gets a significant reduction in the control information piggybacked on application messages, compared to previous algorithms. Keywords: distributed computing, causal ordering, multimedia, real-time communication. 1 Introduction The notion of \Delta-causal ordering was introduced in [Yav92], and later refined and formalized ...