4.6.1 Sequencing for Playlist Content Delivery
The following sequence occurs when a client is requesting playlist content. A proxy server handles requests and content delivery to and from the client, connecting with the playlist origin server if required.
The proxy server comprises a server role and a client role. In the following description, the client that connects to the proxy server is referred to as the media player client to keep it distinct from the client role of the proxy server.
The media player client sends a Describe (section 2.2.7.2) request.
The higher layer of the proxy server checks whether the requested content is in its local cache. If the content is not in the cache, the higher layer of the proxy server creates an instance of the client role and uses the client role to open a connection with the origin server. These steps are described in section 4.6.3.
The higher layer of the proxy server checks whether the content in the local cache is valid. If the content is not valid, the higher layer of the proxy server creates an instance of the client role and uses the client role to revalidate the content. If the content is still not valid, then the higher layer of the proxy server opens a connection with the origin server. These steps are described in section 4.6.3.
The server role of the proxy server sends a "200 OK" response that includes the SDP record and the Cache-Control (section 2.2.6.2) header, as specified in 3.2.5.4.
The media player client sends a SelectStream (2.2.7.10) request using the Setup method for each selected stream.
The server role of the proxy server sends a "200 OK" response to the media player client, as specified in 3.2.5.6.
The media player client submits a Play request to start receiving the RTP packets.
The server role of the proxy server responds with "200 OK" and begins sending the RTP packets for the requested streams to the client, as specified in 3.2.5.8.
The client role of the proxy server communicates any subscribed-to events to the origin server using the SendEvent (2.2.7.11) request, as specified in 3.1.4.7 and3.1.4.9.
The media player client submits play statistics using a LogPlay (section 2.2.7.7) request.
The server role of the proxy server responds with "200 OK", as specified in 3.2.5.12.
The following illustration shows the sequencing that occurs when the media player client requests a file from a media server that is configured as a proxy server.

Figure 21: Playlist content delivery