When optimizing throughput, you would use the formula BW x RTT = BDP to calculate bandwidth delay product (BDP), where BW is the bandwidth and RTT is the round-trip time, also known as latency. BDP is the maximum amount of data that can exist on a network path at any given time. This value is often used to establish a maximum goal when optimizing the bandwidth used by Transmission Control Protocol (TCP) traffic flows.
When calculating BDP, you should ensure that the unit measurements match for each variable. For example, BW is typically measured in bits per second, where as RTT is typically measured in milliseconds. Therefore, you should convert the RTT value to seconds before multiplying the values. For example, a Fast Ethernet link with a latency of 40 milliseconds (ms) would have a BDP of 4 Mb:
100 Mbps x 0.040 seconds = 4 Mb
The BDP measurement will match the measurement that is used for the BW variable. In this example, the BDP is expressed in megabits. If you need to convert from bits (lowercase b) to bytes (uppercase B), you should divide by 8. Conversely, if the bandwidth is expressed in bytes and you need to convert to bits, you should multiply by 8.
The rwin variable is not used when calculating BDP. However, rwin is related to BDP. The TCP receive window size, sometimes expressed as rwin, is often a limiting factor in optimizing throughput. In order to achieve maximum throughput, you should set the TCP receive window size to a value equal to or greater than BDP, thereby ensuring that either bandwidth or latency is the limiting factor.