TCP Control Bits and Flow Records
Every TCP packet includes one or more control bits, which are on-off switches that tell the connection participants how to handle a particular packet. Flow records capture the control bits used by each TCP flow. These control bits are not terribly useful when a connection works correctly but are invaluable when identifying problems. If you're not familiar with TCP control bits, read a good TCP/IP primer such as Charles A. Kozierok's The TCP/IP Guide (No Starch Press, 2005).
Control bits are given this name because TCP has six bits set aside just for them. These bits are as follows:
The SYN (synchronize) bit indicates a connection synchronization request. It permits the sender and receiver to synchronize TCP sequence ...
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