TCP Fast Open (TFO) mechanism is described in https://tools.ietf.org/html/draft-ietf-tcpm-fastopen-10.
Other considerations can be found in this white paper.
To enable TFO on the server machine, the kernel parameters need to be configured to support TCP_FASTOPEN in server mode. To enable it, run the following command as root:
To act in pure server mode, set it to 2. If also running a TFO client on the machine, set it to 3.
More details from Documentation/networking/ip-sysctl.txt: