TFO (TCP Fast Open)

RU: TFO (TCP Fast Open)

TCP Fast Open (RFC 7413): an extension that allows a client to include application data in the SYN packet of a subsequent connection, saving one RTT compared to a standard three-way handshake. On the first connection the server issues a cryptographic cookie; the client presents that cookie in future SYNs to authenticate the early data. TFO is vulnerable to replay of the in-SYN data on idempotent endpoints, so server-side acceptance is limited to safe/idempotent requests.