Computer based community systems can provide powerful support in knowledge transfer, both in the direct exchange of information and in finding people to exchange information with. Several such information exchange and expert finding services have already been implemented. The problem of current approaches is that they are not able to exchange data with each other. User profile information and contributed information have to be entered separately for every community-based information system. So, what one system has learned is not available for other systems. We present our ideas for a community services infrastructure as an enabling technology for further integration of community related services.

We examine the impact of the loss recovery mechanism on the performance of a reliable multicast protocol. Approaches for loss recovery in reliable multicast can be divided into two major classes: centralized (source-based) recovery and distributed recovery. For both classes we consider the state of the art: for centralized recovery, an integrated transport layer scheme using parity multicast for error recovery (hybrid ARQ type 2) as well as timer-based feedback suppression, and for distributed recovery, a scheme with local data multicast retransmission and feedback processing in a local neighborhood. We also evaluate the benefits of combining the two approaches into distributed error recovery (DER) with local retransmissions using a type 2 hybrid ARQ scheme. The schemes are evaluated for up to 10/sup 6/ receivers under different loss scenarios with respect to network bandwidth usage and completion time of a reliable transfer. We show that using DER with type 2 hybrid ARQ gives best performance in terms of bandwidth and latency. For networks, where local retransmission is not possible, we show that a centralized protocol based on type 2 hybrid ARQ comes close to the performance of a protocol with local retransmissions.

We examine the impact of the loss recovery mechanisms on the performance of a reliable multicast protocol. Approaches to reliable multicast can be divided into two major classes: source-based recovery, and distributed recovery. For both classes we consider the state of the art: for source-based recovery, a type 2 hybrid ARQ scheme with parity retransmission; for distributed recovery, a scheme with local multicast retransmission and local feedback processing. We further show the benefits of combining the two approaches and consider a type 2 hybrid ARQ scheme with local retransmission. The schemes are compared for up to 10/sup 6/ receivers under different loss scenarios with respect to network bandwidth usage and completion time of a reliable transfer. We show that the protocol based on local retransmissions via type 2 hybrid ARQ performs best for bandwidth and latency. For networks, where local retransmission is not possible, we show that a protocol based on type 2 hybrid ARQ comes close to the performance of a protocol with local retransmissions.