To measure RTT you need to perform a round trip. Hence the name. But the _whole point_ of this feature is to avoid incurring the cost of an extra round trip if possible.
There is no need to send extra data to measure RTT. On the TCP handshake SYN/ACK you already know RTT. Linux kernel provides this info in tcp_info data structure.
To measure RTT you need to perform a round trip. Hence the name. But the _whole point_ of this feature is to avoid incurring the cost of an extra round trip if possible.