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4.10 Stream Selection

The SelectStream request is sent by the client to notify the server which streams it requests. If a file contains only one stream, the SelectStream request will request that the server send the default streams for that file. A SelectStream request will always accompany a Play request to denote which audio and/or video streams the client wants to play based on preferences and bandwidth considerations. As with Log and KeepAlive requests, SelectStream request can be sent multiple times during a session, and they are always sent as a POST request on a separate TCP connection.

The following scenarios are examples of when a SelectStream request would be sent by the client:

  • When streaming a server-side playlist, the client will send a SelectStream request between each playlist entry.

  • When streaming a multi-language Windows Media Audio (.wma) file, the client will send a SelectStream request whenever the client changes from one language to another.

  • When streaming a multiple bit rate ASF file, the client will send a SelectStream request whenever it experiences a prolonged change in bandwidth conditions.

The sequencing applies to both the pipelined and the non-pipelined mode of the protocol:

  1. The client sends a Describe request to retrieve the ASF header.

  2. The server responds with a $H (Header) packet.

  3. The client sends a Play request for the file, selecting one or more streams.

  4. The server responds with a $H (Header) packet and $D (Data) packets.

  5. To change streams, the client passes the stream selection information using a SelectStream request with the stream-switch-entry token on a Pragma header.

  6. The server responds by acknowledging the client request and switching to the requested stream.

    Note  To avoid disruptions in a video stream, the server will switch the streams at a key frame.

  7. The client detects the stream change in the data stream and renders the new stream.

    Note  The client will not send a message to the server as a result of this stream change.

The following figure shows the previously described sequence.

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Figure 13: Using the SelectStream Request packet sequencing

 
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