Bandwidth Delay Product Calculator
Calculate the bandwidth delay product and the TCP window size a link needs to reach full throughput, and see why a fast high latency link delivers a fraction of its capacity by default.
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A fast link with high latency is slow by default
TCP sends a window of data and waits for acknowledgement before sending more. If the window is smaller than the amount of data in flight on the link, the sender spends most of its time waiting and throughput is capped regardless of the available bandwidth. On a one gigabit link with 100 milliseconds of round trip time, a 64 kilobyte window delivers about 5 megabits per second: half a percent of the link. This is why a transatlantic transfer over a fast circuit can be slower than the same transfer over a slow local one.
The product is the amount of data in flight
Bandwidth multiplied by round trip time gives the number of bytes that fit on the wire between sender and receiver at any instant. That figure is the minimum window needed to keep the link busy, and the sender's socket buffer must be at least that size on both ends. The classic 64 kilobyte limit comes from the 16 bit window field in the TCP header, which is why the window scaling option exists and why it must be enabled for any link where the product exceeds that.
Loss matters more as the product grows
The Mathis formula bounds throughput at roughly the maximum segment size divided by the round trip time, multiplied by one over the square root of the loss rate. On a long fat network, a loss rate of one in ten thousand, which is invisible on a local link, caps throughput well below the line rate. That is why tuning window sizes on a lossy long distance link produces disappointing results, and why loss should be measured before buffers are enlarged.
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Frequently Asked Questions
What is the bandwidth delay product?
Bandwidth multiplied by round trip time, which gives the amount of data in flight on the link at any moment. It is the minimum TCP window needed to keep the link fully busy, and therefore the minimum socket buffer size on both ends.
Why is my gigabit link only delivering a few megabits?
Almost certainly the TCP window. Without window scaling the maximum is 64 kilobytes, which at 100 ms of round trip time caps throughput at about 5 Mbit/s regardless of the link speed. Enable window scaling and raise the socket buffer limits.
What is TCP window scaling?
An option that multiplies the 16 bit window field by a power of two, allowing windows far above the 64 kilobyte limit. It is negotiated in the SYN, so both ends must support it, and some old middleboxes strip the option and silently cap the connection.
Does packet loss matter more on long links?
Considerably. The Mathis formula bounds throughput at roughly the segment size over the round trip time divided by the square root of the loss rate, so a loss rate invisible on a LAN caps a long distance link well below its line rate. Measure loss before enlarging buffers.
Should I just set very large buffers?
No. Oversized buffers cause bufferbloat, where queues fill and latency rises sharply without improving throughput. Size the buffer to the bandwidth delay product with modest headroom, and use a modern congestion control algorithm such as BBR that responds to latency as well as loss.
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How to Use
Enter link bandwidth and round trip time to get the required window size and achievable throughput.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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