Adaptive Reliable Multicast

An increasing number of applications, such as distributed interactive simulation, live auctions, distributed games and collaborative systems, require the network to provide a reliable multicast service. This service enables one sender to reliably transmit data to multiple receivers. Reliability is traditionally achieved by having receivers send negative acknowledgments (NACKs) to request from the sender the retransmission of lost (or missing) data packets. However, this Automatic Repeat reQuest (ARQ) approach results in the well-known NACK implosion problem at the sender. Many reliable multicast protocols have been recently proposed to reduce NACK implosion. But, the message overhead due to NACK requests remains significant. Another approach, based on Forward Error Correction (FEC), requires the sender to encode additional redundant information so that a receiver can independently recover from losses. However, due to the lack of feedback from receivers, it is impossible for the sender to determine how much redundancy is needed. In this paper, we propose a new reliable multicast protocol, called ARM for Adaptive Reliable Multicast. Our protocol integrates ARQ and FEC techniques. The objectives of ARM are (1) reduce the message overhead due to NACK requests, (2) reduce the amount of data transmission, and (3) reduce the time it takes for all receivers to receive the data intact (without loss). During data transmission, the sender periodically informs the receivers of the number of packets that are yet to be transmitted. Based on this information, each receiver predicts whether this amount is enough to recover its losses. Only if it is not enough, that the receiver requests the sender to encode additional redundant packets. Using ns simulations, we show the superiority of our hybrid ARQ-FEC protocol over the well-known Scalable Reliable Multicast (SRM) protocol.

Jam To-morrow and Jam Yesterday, but Never Jam To-day: The of Theology Libraries Planning the Twenty-first Century

The future of theology libraries is far from clear. Since the nineteenth century, theology libraries have evolved to support the work of theological education. This article briefly reviews the development of theology libraries in North America and examines the contextual changes impacting theology libraries today. Three significant factors that will shape theology libraries in the coming decade are collaborative models of pedagogy and scholarship, globalization and rapid changes in information technology, and changes in the nature of scholarly publishing including the digitization of information. A large body of research is available to assist those responsible for guiding the direction of theology libraries in the next decade, but there are significant gaps in what we know about the impact of technology on how people use information that must be filled in order to provide a solid foundation for planning.