Network performance: delay, throughput and the bandwidth-delay product

A company in Madrid pays for a 1 Gb/s circuit to its backup site in New York. The first copy runs at under 9 Mb/s. The provider's tests show the link is clean: no errors, no loss, plenty of spare capacity. Nothing is broken.

The culprit is arithmetic. The old backup client does not use TCP window scaling, so it can never have more than 64 KiB in flight, and on a path that long 64 KiB is a few drops in a very wide pipe. In 1988 Van Jacobson and Bob Braden gave such paths a name, “long fat pipes”, in RFC 1072, and proposed the fix.