A normal TCP segment header has at least one flag control set. A TCP segment with no control flags set is an anomalous event. Because different operating systems respond differently to such anomalies, the response (or lack of response) from the targeted device can provide a clue as to the type of OS it is running. See Figure 33.
Figure 33: TCP Header with No Flags Set
When you enable the device to detect TCP segment headers with no flags set, the device drops all TCP packets with a missing or malformed flags field.