Which header fields participate in the TCP three-way handshake?

• A. Sequence number, Acknowledgement numbers, SYN/ACK flags
• B. Source IP address, Destination IP address, TTL
• C. Window size, Checksum, URG flag
• D. Options field

Answer: (A)

The key idea is that the TCP three-way handshake uses specific header information to establish and synchronize a connection. The essential elements are the sequence number, the acknowledgment number, and the flags that indicate SYN and ACK.

In the first segment, the client starts the connection by sending a request with its initial sequence number and the SYN flag set. The server replies with a segment that has both the SYN and ACK flags set, uses its own initial sequence number, and acknowledges the client’s SYN by setting the acknowledgment number to the client’s ISN plus one. Finally, the client sends a final segment with the ACK flag set, acknowledging the server’s ISN plus one, and the connection is established.

Other header fields, like the source and destination IP addresses or TTL, belong to other layers or are always present in all TCP segments but aren’t specific to performing the handshake. The window size and checksum are important for data transfer and integrity in all segments, not just the handshake, and the options field may carry optional parameters but is not essential to establishing the connection.