Which statement best describes the SYN-ACK message?

• A. It is the initial packet sent to establish a connection.
• B. It is the client's ACK that follows the server's response.
• C. It acknowledges a received SYN and then completes the handshake by sending an ACK.
• D. It is the server's response to a received SYN that includes both a SYN and an ACK.

Answer: (D)

In TCP, a connection is established through a three-way handshake. The client starts by sending a segment with only the SYN flag set to initiate the connection and propose an initial sequence number. The server replies with a packet that has both the SYN and ACK flags set. This SYN-ACK serves two purposes: it acknowledges the client's SYN (the ACK part) and it also starts the server’s own sequence numbering by including its own SYN (the SYN part). The client then responds with a final ACK to complete the handshake.

So this statement is correct because a SYN-ACK is specifically the server’s response to a received SYN that includes both a SYN and an ACK, signaling both acknowledgment of the request and synchronization for the server side. The first packet is just a SYN, and the final step is the client’s ACK, not the server’s.