4.6 RTSP Proxy Server Interaction
A server that is configured to operate as a proxy server provides the service of routing client requests to one or more origin servers that publish the streaming media content. In this case, the proxy server behaves as a client to the origin server. A proxy server might support caching of content. When the client requests content from a caching proxy server, it can either transmit the content from its local cache or obtain the content from the origin server and then transmit it to the requesting client.
A Cache-Control (section 2.2.6.2) header contains directives about the content that indicates to the proxy server how it handles the content. For example, the x-wms-stream-type directive is used to determine whether the requested content is broadcast or on-demand. The header can include one or more directives and can be passed from the origin server to the client through the intermediate proxy server. When the proxy server is caching content, the Cache-Control header is saved and added to the relevant response messages. For information on the Cache-Control header and the directives, see section 2.2.6.2.
Figure 18: RTSP state diagram with caching proxy server (client perspective)
Figure 19: RTSP state diagram with caching proxy server (server perspective)
A proxy server, whether acting as a server or as a client, is largely identical in state to an origin server (as specified in diagrams in sections 3.1 and 3.2.) That is, when streaming to a client, the proxy server is acting as a regular (origin) server. When acting as a logical client, the proxy server is forwarding requests to the origin server. As indicated in the following figure that illustrates caching proxy server states, much of the decision matrix previously described occurs as a result of the Setup request that causes the initial transition into the "Ready" state. One additional state—the Waiting for GetContentInfo response state—becomes available with the introduction of a caching proxy server. This state, which is indicated by the dotted boxes in the RTSP state diagrams shown here, is applicable only if the caching proxy server is acting as a client to the origin server and only if the content on the cache has expired. The caching proxy server remains in the "Waiting for GetContentInfo response" state until it receives the GetContentInfo response from the origin server. The response to the GetContentInfo request determines whether the session is streamed from the cache or from the origin server. In either case, both the origin server and the proxy server transition back to the "Ready" state.
When a client requests on-demand content from a caching proxy server, the proxy server first checks whether the content exists locally and whether the content is valid. If both conditions are true, then the proxy server can transmit the content from its local cache to the client. If the content has expired, the proxy server establishes a connection to the origin server to determine whether the cached copy of the content is still valid. If the proxy server is able to determine that the cached content is still valid, then the proxy server is allowed to transmit the content to the client. If the cached copy of the content is invalid and caching of the content is allowed, then the proxy server might replace its cached copy by downloading the content from the origin server into the cache. The proxy server would then be able to transmit the content to the requesting client.
Figure 20: Caching proxy server states