Jürgen Vogel

Local-lag and timewarp: providing consistency for replicated continuous applications

In this paper, we investigate how consistency can be established for replicated applications changing their state in reaction to user-initiated operations as well as the passing of time. Typical examples of these applications are networked computer games and distributed virtual environments. We give a formal definition of the terms consistency and correctness for this application class. Based on these definitions, it is shown that an important tradeoff relationship exists between the responsiveness of the application and the appearance of short-term inconsistencies. We propose to exploit the knowledge of this tradeoff by voluntarily decreasing the responsiveness of the application in order to eliminate short-term inconsistencies. This concept is called local-lag. Furthermore, a timewarp scheme is presented that complements local-lag by guaranteeing consistency and correctness for replicated continuous applications. The computational complexity of the timewarp algorithm is determined in theory and practice by examining a simple networked computer game. The timewarp scheme is then compared to the well-known dead-reckoning approach. It is shown that the choice between both schemes is application-dependent.

Consistency control for distributed interactive media

In this paper we present a generic consistency control service for distributed interactive media, i.e. media which allow a distributed group of users to interact with the medium itself. Consistency control is vital to these media since they typically require that a local copy of the mediums state be maintained by each users application. Our service helps the applications to keep the local state copies consistent. The main characteristics of this service are as follows: a significant number of inconsistencies are prevented by using a mechanism called local lag. Inconsistencies that cannot be prevented are repaired by an improved timewarp algorithm that can be executed locally without burdening the network or the applications of other users. Exceptional situations and consistency during late-join situations are supported by a consistent state request mechanism. Moreover, the service also supports the application in detecting intention conflicts between the actions of distinct users. The major part of this functionality is based on a media model and the application level protocol for distributed interactive media (RTP/I) and can thus be reused by arbitrary RTP/I-based applications. In order to demonstrate the feasibility of our approach and to evaluate its performance we have integrated the generic consistency service into a shared whiteboard system.

Consistency Control for Synchronous and Asynchronous Collaboration Based on Shared Objects and Activities

We describe a new collaborative technology that bridges the gap between ad hoc collaboration in email and more formal collaboration in structured shared workspaces. Our approach is based on the notion of object-centric sharing, where users collaborate in a lightweight manner but aggregate and organize different types of shared artifacts into semi-structured activities with dynamic membership, hierarchical object relationships, as well as real-time and asynchronous collaboration. We present a working prototype that implements object-centric sharing on the basis of a replicated peer-to-peer architecture. In order to keep replicated data consistent in such a dynamic environment with blended synchronous and asynchronous collaboration, we designed appropriate consistency control algorithms, which we describe in detail. The performance of our approach is demonstrated by means of simulation results.

Priority-based distribution trees for application-level multicast

In this paper, we propose a novel multicast routing algorithm that is based on application-level priorities and network characteristics: The application may specify an individual priority for each packet-receiver pair. The multicast distribution tree is then constructed such that the higher the priority, the more direct the path from the sender to the packets destination and the lower the resulting end-to-end delay. This algorithm can be used to realize application-level multicast for delay-sensitive applications such as networked computer games. However, optimizing the multicast tree with respect to the end-to-end delay comes at the cost of an increase in link stress - the more direct a path, the less likely it is that it can be integrated efficiently into an overlay distribution tree. Our algorithm takes this tradeoff into account and constructs efficient priority-based multicast trees. We demonstrate the performance and characteristics of the algorithm through simulation.

Recording and playing back interactive media streams

Recording systems and media servers for networked audio and video streams have become an important part of todays Internet. In contrast to this, only a few recording and playback solutions currently exist for the data streams of interactive media applications (e.g., shared whiteboards and distributed virtual environments). So far these solutions are application-specific: individual algorithms and implementations are required for each application that is to be recorded. In this paper, we are proposing generic algorithms for the recording and playback of interactive media streams. These algorithms are based on a common model for the class of interactive media. They enable full random access to recordings by initializing the replaying applications with the required state information (e.g., the current slide in a recorded presentation). We have implemented these algorithms in the Interactive Media on Demand (IMoD) system. In order to interpret the semantics of an interactive media stream, the system requires that the Real-Time Application-Level Protocol for Distributed Interactive Media (RTP/I) protocol is used for the framing of the transmitted data. Any application using RTP/I can be recorded directly using the system without any modification. Interactive media streams not using RTP/I can be recorded using the generic recording algorithms. However, they require an adaptation of the system so that it is able to extract a minimal set of information from the application-level protocol of these streams. In addition to the generic recording algorithms, we present the architecture and major design considerations of the system and discuss the experiences gained from recording different interactive media applications.

Late join algorithms for distributed interactive applications

Abstract.Distributed interactive applications such as shared whiteboards and multiplayer games often support dynamic groups where users may join and leave at any time. A participant joining an ongoing session has missed the data that have previously been exchanged by the other session members. It is therefore necessary to initialize the application instance of the latecomer with the current state. In this paper, we propose a late join algorithm for distributed interactive applications that provides such an initialization of applications. The algorithm is scalable and robust and can be easily adapted to the needs of different applications by means of late join policies. The behavior of the late join algorithm and the impact of design alternatives are investigated in detail by means of an extensive simulation study. This study also shows that an improper handling of the late join problem can cause very high application and network load.

Interactive media on demand: generic recording and replay of interactive media streams

In this demonstration we present a generic recording service for distributed interactive media, i.e., media which allow a distributed group of users to interact with the medium itself. Typical examples of this media class are shared whiteboard systems, distributed virtual environments and networked computer games. By defining an abstract model and an application level protocol we were able to develop the first generic recording service for arbitrary distributed interactive media. This service is called Interactive Media on Demand (IMoD), and it is able to record and replay all applications that are based on the RTP/I protocol. In particular it allows random access to the recorded media streams. We demonstrate the IMoD system by recording and replaying a shared whiteboard, a networked computer game and a 3D telecooperation application.

How to Make a Digital Whiteboard Secure - Using Java-Cards for Multimedia Applications

In this paper we propose a light-weight, provable secure smart card integration for the OpenPGP secure message format. The basic idea is that the secret keys are stored on a smart card and never leave it. We have integrated this new security approach into an enhanced whiteboard, the digital lecture board (dlb). Existing whiteboards neglect security mechanisms almost completely, even though these mechanisms are extremely important to allow confidential private sessions and billing. The primary application field of our concept are small and closed groups, whereas the smart card serves to testify group membership. Our first implementation supports the JAVA i-Button which provides an additional hardware security.

Robustness in Network Protocols and Distributed Applications of the Internet

The Internet connects computers from all over the world for a fast and reliable exchange of data, e.g., from e-mail or Web applications. Considering its sheer size and heterogeneity, the Internet is the most complex computer system ever built. It has coped with a tremendous growth, has handled many critical situations, and, overall, does work quite well. This is because robustness was a major goal from the very beginning, which led to a network design that is self-regulatory, redundant, scalable, and updatable. In this chapter, we will discuss these design principles and the Internet’s architecture, the core protocols IP and TCP, protocols for wireless communication, and applications such as the popular BitTorrent file exchange.

Erzeugung und Verwendung multimedialer Teachware im synchronen und asynchronen Teleteaching

Authoring multimedia teachware is a costly process. In traditional teaching environments a number of documents are created to meet the requirements of different teaching scenarios (e. g. lectures, student homework) but deal with the same content. Furthermore, explanations of a teacher during a lecture implicitly form a document that may be useful beyond the boundaries of the lecture. In this paper we describe how documents created during a lecture can be recorded and how these recordings can be made available on a server in the Internet. The presented scheme allows them to be linked to additional multimedia documents, making a Computer Based Training unit. Thus, our scheme enables the efficient use of teachware in synchronous as well as in asynchronous teaching scenarios.