What is TCP Fast Open (TFO) and how does it reduce latency?

TCP Fast Open allows data transmission during the handshake, reducing round trips. On first connection, server provides TFO cookie. On subsequent connections, client sends SYN + cookie + data; server can process data immediately while completing handshake, saving 1 RTT. Security: cookie prevents amplification attacks. Limitations: requires client/server support, not all middleboxes compatible, cookies can be invalidated. TLS 1.3 0-RTT is similar concept for encryption. Benefits for short-lived connections.