Both SYN and ACK TCP flags play an important role in stateful network communications. SYN requests allow the establishment of new TCP sessions, while ACK responses are used to sustain a session until it is closed. A port that responds to one of these types of packets, but not the other, is most likely subject to filters that restrict traffic based on the session state. By identifying cases such as this, it is possible to infer that stateful filtering exists on the port in question.