What latency should I use to calculate RWIN ?
"Maximum anticipated latency" is an important estimated value, used to calculate the TCP Receive Window (RWIN) using the bandwidth*delay product (BDP).
You should use the maximum anticipated latency that you may experience at peak/congestion times. In other words, it is an estimate of the maximum ping/rtt/latency/delay.
If you use a latency number that is too small, and your latency ever goes higher, your RWIN will get filled and throttle your transfer rate.
On the other hand, if you use a latency number that is too large, your RWIN will be unnecessarily large as well. A large RWIN, combined with congested nodes and packet loss can have negative impact on your transfers as well.
Generally, as a very rough estimate, you can use the following maximum latencies:
US commercial/fiber, or residential broadband land line: 200 - 300ms
Western Europe residential broadband land line: 300 - 400ms
East Asia - residential broadband: 300 - 500ms
Wifi Internet: 400 - 600ms
Satellite connections ~700ms+
Even though the numbers above may seem large, remember those are maximum anticipated, not average latencies.
For a bit more accurate estimate, you can use the "Latency" test in the TCP Optimizer: you can try pinging a larger number of hosts, and with larger packet sizes (default is 32-byte packets that tend to travel faster)... I'd use twice the average latency from the Optimizer as your anticipated max latency.
SG BDP Calculator