Automatically diagnosing operation of a communication network, including collecting, during multiple time periods spanning multiple days, a plurality of Layer-4 statistics for Layer-4 network connections on the network; determining, for each time period in the multiple time periods, a first Layer-4 condition metric for the time period based on the Layer-4 protocol statistics collected during the time period; determining a network performance indicator, based on a number of the time periods that a first threshold condition is met by the first Layer-4 condition metric; determining whether the communication network has experienced a performance issue based on the network performance indicator meeting a second threshold condition; and causing a diagnostic action, a corrective action, or a maintenance action to be performed based on the determination whether the communication network has experienced a performance issue.