During operation, a radio node may provide, at a transmit time during a first time interval, a first instance of a keep-alive message to a computer. Then, the radio node may receive, at a receive time prior to a first instance of a transmit expire time, an instance of a keep-alive response from the computer, where the instance of the keep-alive response authorizes the radio node to transmit in a granted portion of the spectrum until a second instance of the transmit expire time has elapsed. Moreover, the radio node may determine, based at least in part on the receive time and the first instance of the transmit expire time, an updated transmit time. Next, the radio node may provide, at the updated transmit time, which is prior to the second instance of the transmit expire time, a second instance of the keep-alive message addressed to the computer.