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The following diagram shows the communication flow between the media player application and the server during playback using the Windows Media HTTP Steaming Protocol (WMSP).
Figure 17: Unicast playback communication flow using WMSP
Content request: The trigger for the unicast playback is the media player application providing the content URL to the media player client. The media player client uses the content URL to initiate the discovery. For examples of the exact message content and format, see the individual member protocol TDs. After receiving the request to stream from the media player application, the media player client establishes a TCP connection to the server by using the IP address and port number obtained from the URL that is provided by the media player application. With the IP address known, the client can request to configure the server for streaming. How this is accomplished is protocol-specific. For examples of the exact message content and format, see [MS-WMSP] section 3.1.4.2.1.
As part of the Describe request for [MS-WMSP] section 3.1.4.2.1, the client can request packet-pair bandwidth estimation. See section 3.3 for more details on packet-pair bandwidth estimation.
Content response: The media player client obtains information on the stream itself. With WMSP, this data is included in the Describe response. The Describe response includes stream-specific information. For examples of the exact message content and format for WMSP, see [MS-WMSP] section 3.1.5.5. This Describe response is the same Describe response that is described in section 3.3 step 3.
$M and $H packets: After the content response, the media server sends $M and $H packets. These packets include stream-specific information. This information can be used by the higher layer to select a specific stream during the Play request in step 9. For examples of the exact message content and format, see [MS-WMSP] sections 3.1.5.7 and 3.1.5.8.
Play request: As part of the Play request, the media player client specifies which stream to play. For examples of the exact message content and format for WMSP, see [MS-WMSP] section 3.1.4.3.1.
Play response: The server responds to the Play request with a Play response. For examples of the exact message content and format for WMSP, see [MS-WMSP] section 3.1.5.11.
Playback packets: Following the Play request, the server begins sending the packets to the client. For examples of the exact message content and format for WMSP, see [MS-WMSP] section 2.2.3.
End of stream: When the server has completed playing the file, it notifies the media player client that playback is completed. For examples of the exact message content and format for WMSP, see [MS-WMSP] section 3.2.4.1. Following receiving the playback complete notification from the media server, the client sends a playback complete notification to the media player client application.