What is TCP Fast Open?
The TCP Handshake currently consists of a connect() from the client, an accept() from the server, with an ack from the server and the client, costing three trips.
TCP fast open eliminates a round trip by using TFO (TCP Fast open cookies) to make sure that the client is not malicious.
To eliminate a round trip, servers can generate a cookie and ack in response, allowing clients to save a TFO cookie for later requests:
And on subsequent requests, the client can provide this cookie, saving a round trip:
