TCP handshake
The three-step process that opens a TCP connection: SYN → SYN-ACK → ACK.
Client: "want to talk?" (SYN). Server: "yep, you first" (SYN-ACK). Client: "go" (ACK). Only then does data transfer start. Costs a round trip. TLS adds 2-3 more. QUIC bundles everything into one.