Three-Way Handshake
The SYN, SYN-ACK, ACK exchange that establishes a reliable TCP connection.
The TCP three-way handshake establishes a reliable, connection-oriented session before data is exchanged. The initiating host sends a SYN carrying its initial sequence number (ISN); the receiver replies with a SYN-ACK acknowledging that ISN and carrying its own; the initiator finishes with a final ACK. Only then does the connection enter the ESTABLISHED state and data transfer begin. A common exam trap is confusing setup with teardown, which uses a four-step FIN/ACK/FIN/ACK exchange rather than three. The handshake is exclusive to TCP; UDP is connectionless and performs no handshake, making it faster but unreliable.
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