TCP Fast Open TFO
TCP extension that lets the client send data directly in the SYN packet — saves 1 RTT on repeat connections. RFC 7413.
Server gives client a "TFO cookie" on first connection. On second connection client sends SYN + cookie + data. Server replies SYN-ACK + already-processed response. Google project 2011-2014. Adoption: Linux + Android since ~2014, macOS/iOS since macOS 10.11. Some middlebox issues stop deployment sometimes. Mostly overshadowed by QUIC (0-RTT handshake in HTTP/3).