Bandwidth-Delay Product Calculator

Frequently Asked Questions

What is a long fat network (LFN)?

A link with high bandwidth and high latency - such as satellite or transcontinental fiber - is called a long fat network. It has a large BDP and benefits most from window scaling and properly tuned buffers.

Why round the window up to a power of two?

TCP window scaling is expressed as a shift count (a power of two multiplier). Aligning to the nearest power of two ensures the OS can express the window exactly without truncation.

My 1 Gbps link only achieves 100 Mbps. Is BDP the cause?

Quite possibly. Calculate the BDP for your link's RTT. If the result exceeds your current TCP receive window, the window is the bottleneck. Check socket buffer sizes and window scaling settings on both endpoints.

Does BDP apply to UDP?

BDP is most relevant to TCP because TCP's flow control is tied directly to the window. UDP and application protocols with their own flow control have similar buffer sizing considerations.

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How It Works

The bandwidth-delay product (BDP) is the amount of data that can be in flight on a network link at any instant. It equals the link bandwidth multiplied by the round-trip time. Think of the link as a pipe: bandwidth is how wide the pipe is, latency is how long it is, and BDP is the volume the pipe holds.

This calculator multiplies bandwidth in megabits per second by round-trip time in milliseconds and converts to bytes (BDP = Mbps x ms x 125). It also recommends a TCP receive window by rounding the BDP up to the next power of two in kilobytes, which is convenient because TCP window scaling uses powers of two.

TCP can only have as much unacknowledged data outstanding as its window allows. If the window is smaller than the BDP, the sender stalls waiting for acknowledgments and the link sits underused, no matter how fast it is. Setting the TCP window equal to or greater than the BDP lets a single connection fill the pipe.

Use Cases

Network engineers use the BDP to:

Tune TCP socket buffer sizes on Linux servers to maximize throughput on high-bandwidth, high-latency links such as intercontinental fiber or satellite.

Diagnose why a 10 Gbps link between two data centers carries only 200 Mbps - the answer is almost always an undersized TCP window.

Size application-layer buffers for streaming protocols and file transfer tools that allow manual window configuration.

Justify WAN optimization appliances by quantifying exactly how much the latency tax is costing in wasted bandwidth.

Evaluate CDN placement by showing how much closer servers need to be to fill a given client connection.

Tips

Enable TCP window scaling (RFC 1323) - modern operating systems do this automatically, but confirm it is not disabled by a firewall or load balancer in your path.

Measure RTT under load, not just with an idle ping; queuing delay at intermediate routers raises the effective RTT and therefore the required window size.

Oversized buffers add latency - buffer bloat occurs when buffers far exceed the BDP; size them close to the BDP, not to the maximum allowed.

Use iperf3 with -P parallel streams if a single TCP connection cannot reach line rate even after window tuning; some operating systems cap per-socket buffer sizes.

Check /proc/sys/net/ipv4/tcp_rmem and tcp_wmem on Linux to confirm kernel receive and send buffer limits are at least 2x the calculated BDP.

FAQ

What is a long fat network (LFN)?

A link with high bandwidth and high latency - such as a satellite connection or a transcontinental fiber circuit - is called a long fat network. It has a large BDP and benefits most from window scaling and properly tuned buffers to reach its full throughput potential.

Why do I need to round the window up to a power of two?

TCP window scaling is expressed as a shift count (a power of two multiplier). Aligning to the nearest higher power of two ensures the OS can express the window exactly without truncation, and it makes the math straightforward when reading kernel tuning guides.

My 1 Gbps link only achieves 100 Mbps. Is BDP the cause?

Quite possibly. Calculate the BDP for your link's RTT. If the result is larger than your current TCP receive window, the window is the bottleneck. Check socket buffer sizes and window scaling settings on both endpoints.

Does BDP apply to UDP or other protocols?

BDP is most relevant to TCP because TCP's flow control is tied directly to the window. UDP and application protocols with their own flow control have similar buffer sizing considerations but are not limited by the TCP window mechanism.

Bandwidth-Delay Product Calculator

Frequently Asked Questions

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