What is the rationale behind bandwidth delay product
My understanding is that Bandwidth delay product refers to the maximum amount of data "in-transit" at any point in time, between two endpoints.
The thing that I don't get is, why multiply bandwidth by RTT. Bandwidth is a function of underlying medium, such as copper wire, fire optics etc and RTT is function of how busy intermediate nodes are, any scheduling applied at the intermediate nodes, distance etc. RTT can change, but bandwidth for practical purposes can be considered as fixed. So how does multiplying a constant value (capacity aka bandwidth) by fluctuating value (RTT) represents total amount of data in transit?
Based on this, will a really really slow have very large capacity? Chances are the "Causes" of RTT will start dropping.
Look at the units:
- [bandwidth] = bytes / second
- [round trip time] = seconds
- [data volume] = bytes
[data volume] = [bandwidth] * [round trip time].
Unit-wise, it is correct. Semantically,
What is bandwidth * round trip time? It's the amount of data that left the sender before the first acknowledgement was received by the sender. That is, bandwidth * round trip time = the desired window size under perfect conditions.
If the round trip time is measured from the last packet and the sender's outbound bandwidth is perfectly stable and fully used, then the measured window size exactly calculates the number of packets (data and ACKs together) in transit. If you want only one direction, divide the quantity by two.
Since the round trip time is a measured quantity, it naturally fluctuates (and gets smoothed out). The measured bandwidth could fluctuate as well, and thus the estimated total volume of data in transit fluctuates as well.
Note that the amount of data in transit can vary with the data transfer rate. If the bottleneck is wire delay, then RTT can be considered constant, and the amount of data in transit will be proportional to the speed with which it's sent to the network.
Of course, if a round trip time suddenly rises dramatically, the estimated max. amount of data in transit rises as well, but that is correct. If there is no accompanying packet loss, the sliding window needs to expand. If there is packet loss, you need to reconsider the bandwidth estimate (and the bandwidth delay product drops accordingly).
To add to Jan Dvorak's answer, you can think of the 'big fat pipe' as a garden hose. We are interested in how much water is in the pipe. So, we take its 'bandwidth' i.e. how fast it can deliver water, which for a hose is determined by its cross-sectional area, and multiply by its length, which corresponds to the RTT, i.e. how 'long' a drop of water takes to get from one end to the other. The result is the volume of the hose, the volume of the pipe, the amount of data 'in the pipe'.
First, BDP is a calculated value used in performance tuning to determine the upper bounds of data which could be outstanding/unacknowledged. This, almost always, does not represent the quantity of "in-transit" data, but a target which tuning parameters are applied. If it represented "in-transit" data, always, there would be no room for performance tuning.
RTT does in fact fluctuate. This is why the expected worse case RTT is used in calculations. By tuning to the worse case, throughput efficiency will be at maximum when RTT is poorest. If RTT improves, we get outstanding Acks sooner, the pipe remains full and maximum throughput (efficiency) is maintained.
"Full pipe" is a misnomer. The goal is to keep the Tx side full, as the Rx contains Ack packets which are typically smaller than the transmitted packets.
RTT also aggregated asymmetrical upstream and downstream bandwidths (ADSL, satellite modem, cable modem, etc.).