Extremum Feedback for Very Large Multicast Groups

In multicast communication, it is often required that feedback is received from a potentially very large group of responders while at the same time a feedback implosion needs to be prevented. To this end, a number of feedback control mechanisms have been proposed, which rely either on tree-based feedback aggregation or timer-based feedback suppression. Usually, these mechanisms assume that it is not necessary to discriminate between feedback from different receivers. However, for many applications this is not the case and feedback from receivers with certain response values is preferred (e.g., highest loss or largest delay). In this paper, we present modifications to timer-based feedback suppression mechanisms that introduce such a preference scheme to differentiate between receivers. The modifications preserve the desirable characteristic of reliably preventing a feedback implosion

Extremum Feedback for Very Large Multicast Groups

Abstract. In multicast communication, it is often required that feedback is received from a poten-tially very large group of responders while at the same time a feedback implosion needs to be prevented. To this end, a number of feedback control mechanisms have been proposed, which rely either on tree-based feedback aggregation or timer-based feedback suppression. Usually, these mechanisms assume that it is not necessary to discriminate between feedback from different receivers. However, for many applications this is not the case and feedback from receivers with certain response values is preferred (e.g., highest loss or largest delay). In this paper, we present modifications to timerbased feedback suppression mechanisms that introduce such a preference scheme to differentiate between receivers. The modifications preserve the desirable characteristic of reliably preventing a feedback implosion

Equation-Based Congestion Control for Unicast and Multicast Data Streams

We believe that the emergence of congestion control mechanisms for relatively-smooth congestion control for unicast and multicast traffic can play a key role in preventing the degradation of end-to-end congestion control in the public Internet, by providing a viable alternative for multimedia flows that would otherwise be tempted to avoid end-to-end congestion control altogether. The design of good congestion control mechanisms is a hard problem, even more so for multicast environments where scalability issues are much more of a concern than for unicast. In this dissertation, equation-based congestion control is presented as an alternative form of congestion control to the well-known TCP protocol. We focus on areas of equation-based congestion control which were not yet well understood and for which no adequate solutions existed. Starting from a unicast congestion control mechanism which in contrast to TCP provides smooth rate changes, we extend equation-based congestion control in several ways. We investigate how it can work together with applications which can only operate in a very limited region of available bandwidth and whose rate can thus not be adapted to the network conditions in the usual way. Such a congestion control mechanism can also complement conventional equation-based congestion control in regimes where available bandwidth is too low for further rate reduction. When extending unicast congestion control to multicast, it is of paramount importance to ensure that changes in the network conditions anywhere in the multicast tree are reported back to the sender as quickly as possible to allow the sender to adjust the rate accordingly. A scalable feedback mechanism that allows timely congestion feedback in the face of potentially very large receiver sets is one of the contributions of this dissertation. But also other components of a congestion control protocol, such as the rate increase/decrease policy or the slow-start mechanism, need to be adjusted to be able to use them in a multicast environment. Our resulting multicast congestion control protocol was implemented in a simulation environment for extensive protocol testing and turned into a library for the use in real-world applications. In addition, a simple video transmission tool was built for test purposes that uses this congestion control library

Consistency Algorithms and Protocols for Distributed Interactive Applications

The Internet has a major impact not only on how people retrieve information but also on how they communicate. Distributed interactive applications support the communication and collaboration of people through the sharing and manipulation of rich multimedia content via the Internet. Aside from shared text editors, meeting support systems, and distributed virtual environments, shared whiteboards are a prominent example of distributed interactive applications. They allow the presentation and joint editing of documents in video conferencing scenarios. The design of such a shared whiteboard application, the multimedia lecture board (mlb), is a main contribution of this thesis. Like many other distributed interactive applications, the mlb has a replicated architecture where each user runs an instance of the application. This has the distinct advantage that the application can be deployed in a lightweight fashion, without relying on a supporting server infrastructure. But at the same time, this peer-to-peer architecture raises a number of challenging problems: First, application data needs to be distributed among all instances. For this purpose, we present the network protocol RTP/I for the standardized communication of distributed interactive applications, and a novel application-level multicast protocol that realizes efficient group communication while taking application-level knowledge into account. Second, consistency control mechanisms are required to keep the replicated application data synchronized. We present the consistency control algorithms “local lag”, “Timewarp”, and “state request”, show how they can be combined, and discuss how to provide visual feedback so that the session members are able to handle conflicting actions. Finally, late-joining participants need to be initialized with the current application state before they are able to participate in a collaborative session. We propose a novel late-join algorithm, which is both flexible and scalable. All algorithms and protocols presented in this dissertation solve the aforementioned problems in a generic way. We demonstrate how they can be employed for the mlb as well as for other distributed interactive applications