Describe the TCP three-way handshake and what each step accomplishes.

• A. Client sends ACK to acknowledge a connection; server replies with FIN; client sends SYN.
• B. Client sends SYN to initiate; server replies with SYN-ACK to acknowledge and provide its own ISN; client sends ACK to establish the connection.
• C. Client sends SYN-ACK, server sends ACK, client sends FIN.
• D. Client sends FIN to terminate; server acknowledges.

Answer: (B)

The TCP three-way handshake establishes a reliable, synchronized connection by exchanging control messages that set up sequencing and readiness to transmit. The client first sends a segment with the SYN flag set, choosing an initial sequence number. This starts the connection request and tells the server where the byte stream will begin from the client’s side.

The server then replies with a segment that has both SYN and ACK flags set. This serves two purposes: it acknowledges the receipt of the client's SYN (acknowledging the client’s ISN) and it provides its own initial sequence number for the server’s side. This step synchronizes both ends’ expectations for the upcoming data.

Finally, the client responds with a segment that has the ACK flag set, acknowledging the server’s SYN (its ISN). With both sides’ sequence numbers established and acknowledged, the connection is fully open and data transfer can begin in both directions.

The key idea is that this three-step exchange sets up a reliable, ordered channel by agreeing on initial sequence numbers and confirming both sides are ready to communicate. Other sequences that involve terminating messages (like FIN) or omit the proper SYN/SYN-ACK/ACK order do not establish a connection.