Describe the TCP three-way handshake and why it establishes a reliable connection.

• A. Client sends ACK, server responds ACK, data transfer begins.
• B. Client sends SYN, data is sent, connection closed.
• C. Client sends FIN, server responds FIN-ACK, connection reset.
• D. Client sends SYN, server replies SYN-ACK, client sends ACK; establishes connection and sequence numbers for reliability.

Answer: (D)

Opening a TCP connection uses a three-way handshake that sets up a synchronized, reliable channel. The client starts by sending a segment with the SYN flag set, indicating a request to open a connection and carrying the client’s initial sequence number. The server replies with a segment that has both SYN and ACK flags, acknowledging the client’s ISN and providing its own initial sequence number. Finally, the client sends an ACK back to acknowledge the server’s ISN, completing the handshake. At this point both sides know the other is reachable and have agreed on starting sequence numbers, establishing a stateful, connection-oriented path for data transfer.

That handshake is what makes the connection reliable: the two ends coordinate their sequence spaces, and every subsequent data segment is acknowledged. If something is lost, retransmission is triggered based on those ACKs and timers, ensuring data arrives in order and without gaps. The other options describe scenarios that skip the handshake, try to send data before a connection is established, or depict closing rather than opening, which does not explain how reliability and synchronization are achieved.