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The present paper proposes and evaluates some extensions to the CORBA Event Service, based on the use of the Light-weight Reliable Multicast Protocol (LRMP). These extensions address some limitations of the CORBA Event Service, namely multicasting, reliability and total ordering. The extensions were developed and tested in a prototype environment created for this purpose, and are transparent and fully compatible with the standard OMG Event Service, enabling the user to decide for the use of standard IIOP or IP multicasting.

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The present paper addresses the main features and performance test results of the CONCHA -CONference system based on java and corba event service CHAnnels, developed at the Communications and Telematic Services Group of the University of Coimbra. The conference framework was conceived using the Java platform, based on the ITU T. 120 recommendations, with reliable multicasting communication for both data and control information. The conference control service extends the CORBA Event Service architecture with mechanisms for native H? multicasting mapping, optimization of reliability mechanisms and processes, filtering, total ordering of events/messages and fragmenting/reassembling of large messages. STATE OF THE ART AND RELATED WORKThere are several proprietary architectures for this type of collaborative applications, some of which are referenced below. However, the proposal under consideration in the present paper is based on solutions recommended by international standardisation institutions (basically, ITU-T and LETF) and, thus, we will focus our attention on systems that somehow relate to these standardised solutions. There are two basic types of conference control: tightly-coupled/formal and loosely-coupled/informal, which, some times, can be combined into hybrid control schemes. The tightly coupled approach is based on a central and authoritative control mechanism, managing the members and the conference access. On the other hand, the loosely coupled approach uses a distributed control that does not require an explicit control mechanism for members and applications. These two control models are so different that interaction between conference applications using different control models is, in practice, ruled out. These two approaches are proposed by two standardisation institutions: International Telecommunication Union (ITU) and Internet Engineering Task Force (IETF). Typically, a loosely coupled conference consists of multipoint sessions with audio and video streams, supported by RTP and RTCP protocols, using IP Multicasting, as it is the case of Mbone [1] in the Internet community. ITU defined two standards for tightly coupled conferences: T.120 [2-10] recommendations for data conferences and 11.323 recommendations [1 1] for multimedia conferences. The increasing interest in more formal conferences lead IETF to the development (still as work in progress) of a tightly coupled model -"SCCP, Simple Conference Control Protocol" [12] -with functionalities that are similar to ITU T.120 and H.323 functionalities. Generally, typical conference systems (whiteboard, audio e video) are characterised by the following functionality, in addition to security, streaming and application-specific functionalities [13]: Data transmission unicastimulticast, data seriaJisation-marshalling, priorities, addressing and transparent localisation, internetworking Conference management conference set-up and termination, database administration, conference merging and splitting, resource management, etc. Distributed applications suppo...