What is tcp_info->tcpi_rtt field and how it is calculated?

Question

I am trying to write some maintenance code which may help me in identifying TCP connection quality over time. I am using getsockopt() to get tcp_info struct every second and the values I get for tcpi_rtt field in microseconds are in the pattern, 5340 9140 12007 14115 15764 17058 17990 18709 19298 19767 20063 20276 20543 20699 20804 20914 21004 21126 21131 21110 21135 21115 21199 21160 21162 21137 21204 21267 21251 21234 21268 21204... After sometimes, it stays in this 21K range.

With this, I am also trying to calculate application RTT, which is to identify HTTP req/res round trip time over the same TCP connection and the values I am able to calculate in microseconds are, 1811 869 1356 965 984 2312 841 822 680 1140 3140 789 859 844 1575 1883 964 2107 890 1034 856 1337 1182 969 855 821 1583 907 1427 705 813 1318 681 1181 804 834 915 846 832 2256 977 860 922 699 872 1577 ...

With this exercise, I was expecting tcpi_rtt values to be smaller than app_rtt values because the HTTP app is running over the same TCP connection.

I am not able to find any good documentation for tcpi_rtt field, first of all what it means and how it is calculated?

Any documentation link would be helpful. Thank you !