Windows Media Multicast Support


Fabrikam already tested multicast using the closed LAN (see chapter 17). The technicians configured a broadcast publishing point for multicast streaming, and then used Windows Media Player on the client computer to connect to the stream. When the publishing point was started, the switch connected to the server flooded the stream to all ports. If the switch had supported CGMP or IGMP traffic snooping, the multicast stream would not have been sent to a port until the switch received a membership request from a client.

Windows Media Services—or any multicast source for that matter—does not need to maintain any sort of state because multicast is a connectionless protocol. The server merely sends the packets, which contain the digital media content and the multicast group address.

A client running Windows Media Player 9 Series does not maintain a connection state either. The Player sends a request for the stream, and then plays the packets as they arrive. The devices that maintain state and are involved with actively routing and moving multicast traffic are the routers and switches, as you have seen.

To enable the routers and switches to create states and route packets, the Player sends messages and the server sends IP multicast packets. From the Player and server points of view, you configure a multicast broadcast as follows:

  1. Configure a broadcast publishing point for multicast by enabling the WMS Multicast Data Writer plug-in. The data writer converts the source stream into a stream of multicast packets.

  2. Create a multicast information (.nsc) file and an announcement (.asx) file. When you finish the configuration, the data writer is configured with the multicast group address. The .nsc file contains stream format information, the IP address of the source, and the multicast group address. The .asx file contains the URL of the .nsc file.

    The live stream from Windows Media Encoder is a common source for a broadcast publishing point configured for multicast. For more information about configuring the encoder and server, see chapter 18.

  3. An end user enters the URL of the .asx file in the Player.

  4. The Player downloads the .asx file, and then downloads the .nsc file.

  5. The Player uses the following information in the .nsc file:

    • Stream format. The Player loads and configures codecs to decode and display audio and video.

    • Multicast IP address. The IP multicast group address is used to create an IGMPv2 Membership Report. There are other elements such as time-to-live (TTL), which is used by routers for distributing the stream, and the port number on which the multicast is streamed.

    • Log URL (optional). The URL of a multicast logging program, which runs on a Web server and receives client usage information when a user ends a multicast session.

    • Unicast rollover address (optional). The unicast URL of the publishing point.

  6. The Player sends the Membership Report, which is received by the leaf router.

  7. The leaf router uses the multicast group address to create a shared tree through an RP.

When an end user stops the Player, the following events occur:

  1. The Player sends an IGMP leave message to the leaf router.

  2. The router receives the message. If there are no more Player requests for the stream on that router, the router sends a prune message upstream.

  3. The prune message is used by upstream routers to stop routing the stream to the router.

  4. The Player sends logging information to the URL specified as the LogURL in the .nsc file.




Microsoft Windows Media Resource Kit
Microsoft Windows Media Resource Kit (Pro-Resource Kit)
ISBN: 0735618070
EAN: 2147483647
Year: 2005
Pages: 258

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