Round Trip Time
Simply put, round trip time is calculated as the number of milliseconds required to receive a response to an nmap request. Nmap refers to these as "probe responses," and it keeps a running total of their round trip time and variability. As the scan progresses, nmap will dynamically adjust the round trip time to use for comparison with the minimum and maximum values.

Initial Round Trip Time Timeout (--initial_rtt_timeout <milliseconds>)
The --initial_rtt_timeout is the starting point for an nmap scan. Nmap can usually determine an accurate round trip time during the nmap ping. If nmap doesn't ping the remote device (option –P0), the --initial_rtt_timeout option can provide nmap with a timing foundation on which the rest of the scan will build.

The default value for the --initial_rtt_timeout is 1,000ms (one second). If this value will be configured manually, it must be greater than zero.

Since this value changes throughout the scan, nmap only uses this option as a starting point. A starting value that is relatively close to the actual response time will increase the efficiency of the nmap scan.

Minimum Round Trip Time Timeout (--min_rtt_timeout <milliseconds>)
As the nmap scan progresses, the minimum round trip time is automatically adjusted based on the actual response times. If the response times at the beginning of the scan are very small, the response time timeout will decrease to match this efficient response time.

If a network is experiencing throughput issues such as dropped packets, it may be necessary to force nmap to maintain a larger timeout value. Setting the --min_rtt_timeout value manually will keep the timeout value at a more reasonable setting for these circumstances. This could result in a longer total scan time, so this option should be configured carefully.

The default minimum round trip timeout value is 100 milliseconds. If the minimum is configured to be more than 50,000 milliseconds (50 seconds!), nmap will provide this warning:
Warning:  min_rtt_timeout is given in milliseconds, your value seems pretty large.

Maximum Round Trip Time Timeout (--max_rtt_timeout <milliseconds>)
The --max_rtt_timeout is used to determine when a response is taking too long. If a response takes longer than the configured value, nmap will categorize the request as lost and retransmit or time out the request.

The default maximum round trip time timeout is 10,000 milliseconds (10 seconds). If the nmap scan is operating across slow networks or networks with communication problems, this value may need to be increased to ensure an accurate scan.

The minimum possible --max_rtt_timeout value is 5 milliseconds. Although it's possible to set the value this low, it's impractical to run an nmap scan with such a low setting. If a --max_rtt_timeout value of less than 20 milliseconds is specified, nmap will provide a warning:
WARNING: You specified a round-trip time timeout (10 ms) that
is EXTRAORDINARILY SMALL.  Accuracy may suffer.