An apparatus comprising: a first device (105, 210) comprising: a port (142, 144, 146, 220), wherein the port (142, 144, 146, 220) is configured to enable a connection between the first device (105, 210) and a second device (160, 162, 164, 230), and the port (142, 144, 146, 220) is configured to: receive a latency tolerance reporting, LTR, message from the second device (160, 162, 164, 230), wherein the LTR message indicates a service latency requirement of the second device (160, 162, 164, 230); enter a first sub-state of a low power link state from an active link state, wherein the first sub-state is one of a set of sub-states of the low-power link state; de-assert a signal to cause a bidirectional open-drain sideband signal to indicate to the second device (160, 162, 164, 230) a request to enter a second one of the set of sub-states of the low-power link state from the first sub-state, wherein a reference clock is to be used in the first sub-state and is to be disabled in the second sub-state, and the signal is de-asserted when an LTR value in the LTR message is greater than or equal to a threshold; and enter the second sub-state, wherein the port (142, 144, 146, 220) is compliant with a Peripheral Component Interconnect Express, PCIe, based protocol. a port (142, 144, 146, 220), wherein the port (142, 144, 146, 220) is configured to enable a connection between the first device (105, 210) and a second device (160, 162, 164, 230), and the port (142, 144, 146, 220) is configured to: receive a latency tolerance reporting, LTR, message from the second device (160, 162, 164, 230), wherein the LTR message indicates a service latency requirement of the second device (160, 162, 164, 230); enter a first sub-state of a low power link state from an active link state, wherein the first sub-state is one of a set of sub-states of the low-power link state; de-assert a signal to cause a bidirectional open-drain sideband signal to indicate to the second device (160, 162, 164, 230) a request to enter a second one of the set of sub-states of the low-power link state from the first sub-state, wherein a reference clock is to be used in the first sub-state and is to be disabled in the second sub-state, and the signal is de-asserted when an LTR value in the LTR message is greater than or equal to a threshold; and enter the second sub-state, receive a latency tolerance reporting, LTR, message from the second device (160, 162, 164, 230), wherein the LTR message indicates a service latency requirement of the second device (160, 162, 164, 230); enter a first sub-state of a low power link state from an active link state, wherein the first sub-state is one of a set of sub-states of the low-power link state; de-assert a signal to cause a bidirectional open-drain sideband signal to indicate to the second device (160, 162, 164, 230) a request to enter a second one of the set of sub-states of the low-power link state from the first sub-state, wherein a reference clock is to be used in the first sub-state and is to be disabled in the second sub-state, and the signal is de-asserted when an LTR value in the LTR message is greater than or equal to a threshold; and enter the second sub-state,