TCP Flags Reference
Reference guide for TCP flags: SYN, ACK, FIN, RST, PSH, URG.
TCP flags are control bits in the TCP header that manage the state of connections.
Synchronize β initiates a connection
Connection establishment (3-way handshake step 1&2)
Acknowledgment β acknowledges received data
Acknowledges all segments after initial SYN
Finish β gracefully closes a connection
Connection termination (4-way close)
Reset β abruptly closes a connection
Error recovery, refuse connections, close immediately
Push β push data to application immediately
Real-time applications like SSH, Telnet
Urgent β urgent pointer field is valid
Out-of-band data signaling (rarely used)
ECN-Echo β congestion notification
Explicit Congestion Notification support
Congestion Window Reduced
Tells peer congestion window was reduced
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Reference guide for TCP flags: SYN, ACK, FIN, RST, PSH, URG.