Explain the TCP three-way handshake.

TCP connection establishment uses three-way handshake: 1) Client sends SYN (synchronize) with initial sequence number. 2) Server responds with SYN-ACK (synchronize-acknowledge) with its sequence number and acknowledgment of client's. 3) Client sends ACK acknowledging server's sequence. Connection is now established for bidirectional communication. Termination uses four-way handshake (FIN, ACK, FIN, ACK). SYN flood attacks exploit this by never completing handshake.