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4.12 Windows Media Services HTTP 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 the 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 header contains directives about the content that indicates to the proxy server how it must handle 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 may include one or more directives and can be passed from the origin server to client through the intermediate proxy server. For more information about the Cache-Control header and the directives, see section 2.2.1.1.

When a client requests on-demand content from a caching proxy server, the proxy server is allowed to transmit the content to the client. If the cached copy of the content is not valid 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.

For broadcast content, a proxy server might provide the capability to proxy live content from an origin server through another server. When a client requests live content, the proxy server should check whether it is already proxying the content. If so, it could split the streams and delivers the content to all of the requesting clients. If the proxy server is not proxying the content, it could establish a connection to the origin server and forward the content from the origin server to the client. For broadcast content, it is conceivable that only one TCP connection would exist between the proxy server and the origin server.

A proxy server, whether acting as a server or as a client, is largely identical in state to an origin server (as illustrated in sections 4.1 and 4.2.) 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 diagram, much of the decision matrix described previously occurs as a result of the Describe request that causes the initial transition into the "Idle" state. One additional state — the "Waiting for GetContentInfo response" state — becomes available with the introduction of a caching proxy server. This state 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 will remain in the "Waiting for GetContentInfo response" state until it receives the GetContentInfo response from the origin server. The response to the GetContentInfo request will determine 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 "Idle" state.

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Figure 15: Caching proxy server states

Other request types have a negligible impact on state of a proxy server. As indicated in the following flow diagram, if a proxy server receives a rendering log from a client, it should forward it to the origin server regardless of the then current state of the proxy server. For example, if the proxy server is streaming content to a client and receives a rendering log, it should immediately transmit the log to the origin server. The proxy server does not have to wait for a response from the origin server.

On the contrary, if a proxy server receives a streaming log or a "legacy" log from a client, it should not forward it to the origin server. In the case of a "legacy" log, a proxy server implementation ought to synthesize a rendering log from the "legacy" log and transmit the synthesized rendering log to the origin server (assuming that the origin server has indicated that it will accept rendering logs).

Regardless of the type of log message received by the proxy server, there is no state change caused by the transmittal of a log from the proxy server to the origin server. More details on the different Windows Media Log types are specified in [MS-WMLOG].

In the case that the request received by the proxy server is not log-related, the proxy server will attempt to service the request locally. If it cannot, the request will be forwarded to the origin server. The proxy server will wait until it receives a response from the origin server before sending a response to the client that originated the request. It is important to note that this interaction does not trigger a state transition; the proxy server will remain in its then current state.

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Figure 16: Caching proxy server state flow diagram

 
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