The RFC talks about how to calculate RTT0 during handshake. But it's impossible for the sender to associate a handshake packet with its response, as on long latency links there is a retransmission every 0.25s. So it doesn't know if the handshake response is due to a lost packet and subsequent repetition or a long latency link.

libsrt github says "it should be estimated at the handshaking stage, but this is not yet implemented. "