Question: In a PSS scenario, if a 'Switch-Stream' header field in a PLAY request from a client to a server only specifies the `new-stream` URL without mentioning the `old-stream`, how should the PSS server interpret this request?
Option 1: As a request to duplicate an existing media stream with the new URL.
Option 2: As a request to switch to the new media stream, with the server deciding the mapping and indicating the SSRC in the P-Info of the reply.
Option 3: As a request for the server to pause all current streams before initiating the new one.
Option 4: As an invalid request due to missing `old-stream` information.
Explanation: According to specifications, if only the new media stream URL is indicated in the 'Switch-Stream' header field of a PLAY request, the PSS server interprets this as a request to switch to the new media stream. The server decides how to map this new stream and must provide the SSRC of the new media stream in the response's P-Info, enabling seamless switching.