TCP three-way handshake
The classic SYN → SYN-ACK → ACK sequence that establishes a TCP connection. Ensures both sides are synchronised before data flows.
The client sends SYN (with its initial sequence number), the server responds with SYN-ACK (acks the client's + sends its own), the client responds with ACK. Takes 1 RTT — which is the entire reason HTTP/2/3 over keep-alive shines compared to HTTP/1.0.
SYN flood attacks abused the three-way handshake by sending many SYNs without ACK → the server held half-open connections. The mitigation is SYN cookies. TCP Fast Open (RFC 7413) allows data in the SYN to skip an RTT.