Sdr is a session directory designed for announcing and scheduling multimedia conferences on the mbone - the multicast backbone of the internet. sdr is loosely modelled on sd - lbl's mbone session directory. sdr extends the sd model in a number of ways, particularly in the degree of detail about the timing and resources required by a conference, and in the provision of a much more flexible interface for querying the existence of sessions or of any sessions that may potentially clash with a new session.