CURLOPT_TCP_FASTOPEN man page
CURLOPT_TCP_FASTOPEN — enable TCP Fast Open
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_TCP_FASTOPEN, long enable);
Pass a long as parameter set to 1 to enable or 0 to disable.
TCP Fast Open (RFC7413) is a mechanism that allows data to be carried in the SYN and SYN-ACK packets and consumed by the receiving end during the initial connection handshake, saving up to one full round-trip time (RTT).
Added in 7.49.0. This option is currently only supported on Linux and OS X El Capitan.
Returns CURLE_OK if fast open is supported by the operating system, otherwise returns CURLE_NOT_BUILT_IN.