@deftypefun {void} {gnutls_transport_set_fastopen} (gnutls_session_t @var{session}, int @var{fd}, struct sockaddr * @var{connect_addr}, socklen_t @var{connect_addrlen}, unsigned int @var{flags})
@var{session}: is a @code{gnutls_session_t}  type.

@var{fd}: is the session's socket descriptor

@var{connect_addr}: is the address we want to connect to

@var{connect_addrlen}: is the length of  @code{connect_addr} 

@var{flags}: must be zero

Enables TCP Fast Open (TFO) for the specified TLS client session.
That means that TCP connection establishment and the transmission
of the first TLS client hello packet are combined. The
peer's address must be  specified in  @code{connect_addr} and  @code{connect_addrlen} ,
and the socket specified by  @code{fd} should not be connected.

TFO only works for TCP sockets of type AF_INET and AF_INET6.
If the OS doesn't support TCP fast open this function will result
to gnutls using @code{connect()}  transparently during the first write.

@strong{Note:} This function overrides all the transport callback functions.
If this is undesirable, TCP Fast Open must be implemented on the user
callback functions without calling this function. When using
this function, transport callbacks must not be set, and 
@code{gnutls_transport_set_ptr()}  or @code{gnutls_transport_set_int()} 
must not be called.

On GNU/Linux TFO has to be enabled at the system layer, that is
in /proc/sys/net/ipv4/tcp_fastopen, bit 0 has to be set.

This function has no effect on server sessions.

@strong{Since:} 3.5.3
@end deftypefun