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

• A. It establishes the connection between the two hosts before data transfer.
• B. It immediately starts sending data without coordination.
• C. It converts data to a different protocol.
• D. It performs a DNS lookup before communication.

Answer: (A)

TCP is a connection-oriented protocol, so it must establish a session before any data moves. The three-way handshake is the process that sets up that reliable channel and negotiates the initial state for communication. It works like this: one side sends a synchronization packet to request a connection and pick an initial sequence number. The other side replies with a synchronization-acknowledgment packet, confirming it received the request and offering its own sequence number. The first side then sends a final acknowledgment to confirm, and only then does data transfer begin, with both sides ready to guarantee order, delivery, and error checking.

This is why starting to send data right away without coordination isn’t correct—the connection isn’t established yet, so reliability isn’t guaranteed. And DNS lookups or protocol conversion aren’t what the handshake does, so those explanations don’t fit the purpose of the three-way handshake.