Reference for all TCP header flags (SYN, ACK, FIN, RST, PSH, URG, ECE, CWR) with descriptions and a flag byte decoder.
| Flag | Bit | Name | Description |
|---|---|---|---|
| SYN | 1 | Synchronize | Initiates a connection. Used in the TCP 3-way handshake. |
| ACK | 4 | Acknowledgment | Confirms receipt of data. Set in all packets after initial SYN. |
| FIN | 0 | Finish | Graceful connection termination. Sender has no more data. |
| RST | 2 | Reset | Immediately terminates the connection. No graceful close. |
| PSH | 3 | Push | Push data to the application immediately, bypassing the buffer. |
| URG | 5 | Urgent | Urgent data present. Urgent pointer field is valid. |
| ECE | 6 | ECN-Echo | Explicit Congestion Notification echo. Network congestion signaling. |
| CWR | 7 | Congestion Window Reduced | Response to ECE — sender reduced congestion window. |
| Pattern | Description |
|---|---|
| SYN | Connection initiation (step 1 of 3-way handshake) |
| SYN+ACK | Connection accepted (step 2 of 3-way handshake) |
| ACK | Connection established (step 3) or data acknowledgment |
| PSH+ACK | Data segment with push (most common data transfer pattern) |
| FIN+ACK | Graceful connection close |
| RST | Abrupt connection termination or port closed |
| RST+ACK | Reset acknowledgment |
Enter a decimal or hex value to see which flags are set (e.g. 18 or 0x12 for SYN+ACK)
marduc812
2026