A method for detecting a suspicious packet flow in a packet-switched network comprises the computer-implemented step of receiving a first packet in which the SYN bit but not the ACK or RST bit of the packet's TCP header is set. If a specified first time has elapsed, a packet counter associated with the destination address of the flow is incremented. A determination as to whether the packet counter is greater than a specified threshold values is made. If the packet counter is greater than the threshold value, a notification message is generated. In one embodiment, information identifying a packet flow is aggregated to an aggregation cache based on the destination address of the flow.