QuickTime Streaming

Multicast Streaming

In a multicast, one copy of each stream is sent over each branch of a network. This reduces the amount of network traffic required to send the streams to large numbers of clients. A client receives the streams by "joining" the multicast.

The client finds out how to join the multicast by opening an SDP (Session Description Protocol) file. The SDP file contains the information needed to join the multicast, such as group address and port number, as well as the stream description information that would come over RTSP for a unicast. SDP files are commonly posted on web servers to announce upcoming multicasts.

Figure 3 QuickTime 4 allows your computer to join a multicast by opening an SDP file.

Not all routers support multicasting. QuickTime clients behind routers that don't implement multicasting can still receive a multicast by requesting the streams from a reflector . A reflector is an RTSP server that joins a multicast, then converts the multicast into a series of unicasts, passing the streams to clients who request them (see diagram below). The original server may be sending live content, such as a concert or a news broadcast, or a previously-recorded movie. The reflector is always sending "live" data, passing the streams in real time.

Figure 4 You can also receive a multicast through a reflector

When a QuickTime client is viewing a multicast or a live unicast, the user's movie controller has no "thumb" control. The user can stop or resume the display, and may have audio controls if the movie includes a sound stream, but there is no way to skip forward or back in a live transmission or a multicast.

Figure 5 Movie controller without "thumb"