TCP Connection details (no replies)

I am using the Nginx as a reverse proxy and I want to find out the TCP connection information on both east and west bound connections.
With the following params on the access log, I am able to get the info about the client TCP connection. Now, I want to find the RTT between Nginx and the backend webserver. Any idea how can I get this ?

$tcpinfo_rtt, $tcpinfo_rttvar, $tcpinfo_snd_cwnd, $tcpinfo_rcv_space
information about the client TCP connection; available on systems that support the TCP_INFO socket option