file: /proc/sys/net/ipv4/tcp_fastopen
variable: net.ipv4.tcp_fastopen
Official reference

Enable TCP Fast Open (RFC7413) to send and accept data in the opening SYN packet.

The client support is enabled by flag 0x1 (on by default). The client then must use sendmsg() or sendto() with the MSG_FASTOPEN flag, rather than connect() to send data in SYN.

The server support is enabled by flag 0x2 (off by default). Then either enable for all listeners with another flag (0x400) or enable individual listeners via TCP_FASTOPEN socket option with the option value being the length of the syn-data backlog.

The values (bitmap) are 0x1: (client) enables sending data in the opening SYN on the client. 0x2: (server) enables the server support, i.e., allowing data in a SYN packet to be accepted and passed to the application before 3-way handshake finishes. 0x4: (client) send data in the opening SYN regardless of cookie availability and without a cookie option. 0x200: (server) accept data-in-SYN w/o any cookie option present. 0x400: (server) enable all listeners to support Fast Open by default without explicit TCP_FASTOPEN socket option.

Default: 0x1

Note that that additional client or server features are only effective if the basic support (0x1 and 0x2) are enabled respectively.