 commercial environment. Content providers want to be compensated for their Standards for Conferencing over IP Networks efforts, and RTSP facilitates delivery control and efficiency, quality of service, security, usage measurement, and rights management. RTSP provides a mechanism for choosing IP-based delivery channels (UDP, IP multicast and TCP) and supports delivery methods based upon the RTP standard. RealNetworks, one of the two primary RTSP sponsors, offers software developments kits for creating RTSP-compliant applications. In only four years, from 1995 to 1999, RealNetworks software systems became the most pervasive method of streaming media on the Internet. By 2000, the company claimed that more than 80 million unique users had been registered, its RealPlayer download rate had exceeded 175,000 per day, more than 145,000 hours of live entertainment had been broadcast over the Internet, and hundreds of thousands of hours of content were available on-demand. Of course, it must now compete with Microsoft's Windows Media product. IP MULTICASTING AND ITS PROTOCOLS IP multicasting (for that matter, multicasting in general) calls for the simultaneous transmission of data to a designated subset of network users. It exists somewhere between unicasting (sending a stream of data to a single user) and broadcasting (sending a data stream to everyone on a network.) Most frame-based networks (e.g., Ethernet) can communicate in all three modes. Most applications incorporate unicast. Examples of a unicast might include a client contacting a server or two users exchanging messages and files across the network. Broadcasts allow a single station to communicate simultaneously with all other devices on the network. Broadcasts must reach across subnets to reach all machines on an IP network. Broadcasts have their place in the corporate intranet, where they are useful for company meetings, product announcements and addresses by the chair of the board. Broadcasts are not as useful when interactivity is facilitative. IP multicastin