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gnutls_transport_set_fastopen(3) gnutls gnutls_transport_set_fastopen(3)


gnutls_transport_set_fastopen - API function


#include <gnutls/socket.h>

void gnutls_transport_set_fastopen(gnutls_session_t session, int fd, struct sockaddr * connect_addr, socklen_t connect_addrlen, unsigned int flags);


gnutls_session_t session
is a gnutls_session_t type.
int fd
is the session's socket descriptor
struct sockaddr * connect_addr
is the address we want to connect to
socklen_t connect_addrlen
is the length of connect_addr
unsigned int 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 connect_addr and connect_addrlen , and the socket specified by 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 connect() transparently during the first write.


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 gnutls_transport_set_ptr() or 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.




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