37  * Kernel only stores RTT and RTTVAR in usec resolution
347 	unsigned long rtt;  in tcp_update_metrics()  local
357 		/* This session failed to estimate rtt. Why?  in tcp_update_metrics()
371 	rtt = tcp_metric_get(tm, TCP_METRIC_RTT);  in tcp_update_metrics()
372 	m = rtt - tp->srtt_us;  in tcp_update_metrics()
374 	/* If newly calculated rtt larger than stored one, store new  in tcp_update_metrics()
375 	 * one. Otherwise, use EWMA. Remember, rtt overestimation is  in tcp_update_metrics()
380 			rtt = tp->srtt_us;  in tcp_update_metrics()
382 			rtt -= (m >> 3);  in tcp_update_metrics()
383 		tcp_metric_set(tm, TCP_METRIC_RTT, rtt);  in tcp_update_metrics()
470 	u32 val, crtt = 0; /* cached RTT scaled by 8 */  in tcp_init_metrics()
504 	/* The initial RTT measurement from the SYN/SYN-ACK is not ideal  in tcp_init_metrics()
507 	 * the RTT estimator variables intact (e.g., srtt, mdev, rttvar).  in tcp_init_metrics()
509 	 * data RTT sample (tcp_rtt_estimator()). Hence the cached RTT only  in tcp_init_metrics()
510 	 * influences the first RTO but not later RTT estimation.  in tcp_init_metrics()
512 	 * But if RTT is not available from the SYN (due to retransmits or  in tcp_init_metrics()
515 	 * A bit of theory. RTT is time passed after "normal" sized packet  in tcp_init_metrics()
519 	 * NEVER underestimate RTT. BUT! If peer tries to make some clever  in tcp_init_metrics()
520 	 * tricks sort of "quick acks" for time long enough to decrease RTT  in tcp_init_metrics()
525 		/* Set RTO like tcp_rtt_estimator(), but from cached RTT. */  in tcp_init_metrics()
529 		/* RFC6298: 5.7 We've failed to get a valid RTT sample from  in tcp_init_metrics()