Why does the USB 2.0 control endpoint become stalled when the set_configuration request is made?

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When the set_configuration request is made, the USB 2.0 control endpoint becomes stalled because the USB host controller is changing the configuration of the attached USB device. During this process, the USB device may need to reset some of its endpoints and reconfigure its internal registers. As a result, the control endpoint may become temporarily unavailable for communication until the USB device completes its configuration changes. This is why the control endpoint is stalled during the configuration process. Once the configuration process is complete, the control endpoint becomes available again for communication.