Which TCP flag is used to initiate a connection in the TCP three-way handshake?

Which TCP flag is used to initiate a connection in the TCP three-way handshake?

• A. Synchronize (SYN)
• B. Acknowledgement (ACK)
• C. Push (PSH)
• D. Port Scanning

Answer: (A)

Starting a TCP connection hinges on the SYN flag. When a client wants to begin communication, it sends a segment with the SYN bit set and an initial sequence number. This "synchronization" step gets the two hosts aligned for the data exchange. The other side replies with a segment that has both SYN and ACK set, acknowledging the request and establishing its own sequence space. The client then sends a final ACK to complete the three-way handshake and establish the connection. The ACK flag shown in subsequent messages is for acknowledging received data, not for starting the connection. The PSH flag is used to push data through the connection, not to initiate a handshake. Port scanning isn’t a TCP flag at all—it’s a technique used to probe for open ports.

Starting a TCP connection hinges on the SYN flag. When a client wants to begin communication, it sends a segment with the SYN bit set and an initial sequence number. This "synchronization" step gets the two hosts aligned for the data exchange. The other side replies with a segment that has both SYN and ACK set, acknowledging the request and establishing its own sequence space. The client then sends a final ACK to complete the three-way handshake and establish the connection.

The ACK flag shown in subsequent messages is for acknowledging received data, not for starting the connection. The PSH flag is used to push data through the connection, not to initiate a handshake. Port scanning isn’t a TCP flag at all—it’s a technique used to probe for open ports.