TCP flags are used in the header of TCP segments to control the flow of data and to indicate the status of a connection. Valid TCP flags include:
FIN: Finish, used to terminate the connection.
PSH: Push, instructs the receiver to pass the data to the application immediately.
URG: Urgent, indicates that the data contained in the segment should be processed urgently.
RST: Reset, abruptly terminates the connection upon error or other conditions.
SYN: Synchronize, used during the initial handshake to establish a connection.
These flags are integral to managing the state and flow of TCP connections.
Reference:
Douglas E. Comer, "Internetworking with TCP/IP Vol.1: Principles, Protocols, and Architecture".