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1.9 Applications

The most widely used application on the old MBone was SDR. By launching SDR, a host listens to the well-known SAP group, Any source host that wants to advertise a session (usually audio and/or video) describes its session in SDP messages. These SDP messages contain the address of the source, type of session, contact information for the source, and so on and are transmitted on the SAP multicast group.

Thus every host running SDR learns about every session on the Internet by receiving these SDP messages on the SAP group. By clicking one of the sessions listed in the SDR window, applications such as VIC (video conferencing tool) or VAT (visual audio tool) are launched to display the video or audio. An interesting feature of many of the applications launched by SDR is that by joining a session, you also become a source for that session. Because every receiver is also a source, each participant can see the others, which makes these applications ideal for collaboration and videoconferencing (and unscalable for sessions with lots of participants!).

Most agree that SDR is a "neat little toy" but not really a commercially viable application. Most SDR sessions are "cube-cams" or video camera shots of ISP parking lots. Because SDR acts as a global directory service for all multicast content on the Internet, it is not expected to scale to support large numbers of sessions.

Windows Media Player (WMP) is currently a popular application for accessing multicast audio and video content. WMP has excellent scaling potential for the Internet because, unlike many SDR-launched applications, receivers do not become sources to the group they join. Also, WMP has the capability to attempt to join a multicast session first, failing over to a unicast session if unsuccessful, which is ideal for content providers seeking the efficiency of multicast and the availability of unicast. Cisco System's IP/TV is another promising application for delivering multicast multimedia content. IP/TV supports multicast content only.

Juniper Networks and Cisco System routers can be configured to listen to the SAP group and keep a cache of SDR sessions. Joining the SAP group is useful in troubleshooting. It is a quick and easy way to determine whether the router has multicast connectivity with the rest of the Internet.

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