There's a chance of a server receiving duplicate data from SYN packets, which some applications won't tolerate. https://datatracker.ietf.org/doc/html/rfc7413#s

There's a chance of a server receiving duplicate data from SYN packets, which some applications won't tolerate. https://datatracker.ietf.org/doc/html/rfc7413#section-2.1 https://datatracker.ietf.org/doc/html/rfc7413#section-2.1 There are some extra tcp_fastopen sysctl values that do enable it transparently.