he video delivered to some of participants will be of low quality (since packet losses and/or delays will be high on some branches of the tree). The second case is safest, but it results in disturbing the bulk of the participants. In practice, it is not clear how to choose an adequate value for this fraction. In IVS, we set the fraction to a few percent [2, 3]. Ideally, however, the source should be able to single out the parts of the multicast tree that experience congestion. In order not to disturb the bulk of participants in the tree, these branches should be treated separately. Two possible solutions include video gateways, and using some form of layered coding. Video gateways or layered coding schemes are not new ideas. Our contribution is to identify and discuss the issues associated with using these techniques in the Internet. We illustrate our points with the H.261[10] software coder of IVS. 2 Video gateways Video gateways take

This report describes a low-bandwidth videoconferencing application on the Internet using the IP multicast extensions and the User Datagram Protocol (UDP) transport protocol. The video coder-decoder is a software implementation of the CCITT recommendation H.261 originally developped for the Integrated Services Digital Network (ISDN). Until now, H.261 codecs have been implemented in hardware. We find that the mean output rate of the coder is less than 30 kb/s, thus making videoconferencing applications possible over low-speed networks such as the Internet. After a brief overview of the different data compression techniques and a description of the recommendation H.261, we describe in more details IVS, our videoconferencing application which is freely available in the public domain. Keywords codec, CCITT recommendation H.261, data compression, Internet, IP multicast extensions, UDP, videoconferencing. 1 Introduction Videoconference applications have often been described as requiring ...