What it measures
The bandwidth delay product, or BDP, is the amount of data that can be in transit on a link at one moment. You compute it by multiplying the link bandwidth by the round trip time. It tells you how many unacknowledged bytes a sender must keep on the wire to fully use the path, much like the volume of water filling a pipe of given width and length.
Why it matters for throughput
- A sender limited by a small window cannot fill a high BDP path and wastes capacity.
- The receive window and send buffer must be at least the BDP to reach full speed.
- Long fat networks with high bandwidth and high latency have a large BDP and demand large windows.
- Underestimating BDP causes the link to sit idle while waiting for acknowledgments.
Suppose a path has high bandwidth but also a long round trip across continents. If the sender stops after a few packets to wait for acknowledgments, the pipe drains and throughput collapses far below the link rate. To avoid this, TCP scales its window so the bytes in flight match the BDP. Tuning buffers to the BDP is the key to high throughput on long fat networks.
Key idea
The bandwidth delay product is bandwidth times round trip time, giving the bytes that must be in flight to keep a path busy, so windows and buffers must match it to achieve full throughput on long fast links.