(ISC)2 Certified in Cybersecurity Practice Exam

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Which TCP Flag indicates a connection needs to be opened?

  1. A) SYN

  2. B) FIN

  3. C) ACK

  4. D) RST

The correct answer is: A) SYN

The TCP SYN flag is used to initiate a TCP connection. In the context of the TCP three-way handshake, it plays a crucial role in establishing a connection between a client and a server. When a client wants to establish a new connection, it sends a segment with the SYN flag set to '1' to the server. This signals the server that the client wants to start a session. Upon receiving the SYN packet, the server responds with a SYN-ACK packet, which indicates that it acknowledges the receipt of the SYN request and also wants to establish a connection. Finally, the client responds with an ACK packet to confirm the establishment of the connection. This three-step process ensures a reliable connection setup before any data transmission occurs. The other flags play different roles in the TCP protocol. The FIN flag is used to indicate that a sender has finished sending data and wants to close the connection. The ACK flag is utilized to acknowledge the receipt of data segments but does not, by itself, initiate a connection. The RST flag is employed to reset a connection, typically used when a connection needs to be abruptly terminated. Understanding the role of the SYN flag is essential for comprehending the foundational concepts of TCP connections.