What is the purpose of the TCP three-way handshake?

• A. Request a DNS resolution
• B. Encrypt payload with TLS
• C. Establish a reliable connection by synchronizing sequence numbers: SYN, SYN-ACK, ACK
• D. Terminate a connection with FIN and ACK

Answer: (C)

A TCP three-way handshake is all about setting up a reliable connection between two hosts before any data is exchanged. It starts with the client sending a segment marked SYN that includes its initial sequence number. The server responds with a segment marked SYN and ACK, acknowledging the client’s ISN and providing its own initial sequence number. The client then sends a final ACK, acknowledging the server’s ISN. At this point, both sides know the initial sequence numbers and are ready to begin sending data in order, with reliable delivery and proper flow control.

This process ensures both ends are ready to communicate and agree on where the conversation starts, which is essential for guaranteeing that data arrives intact and in order. It’s not about resolving domain names, encrypting payloads with TLS, or simply closing a connection; those are separate tasks that happen later (encryption sits atop a established TCP connection, and termination uses FIN/ACK).