Why 64 KiB and not more

A college transfers a nightly dataset to a partner institution across the country. The link is 100 Mbps end to end and measured round trip time is 80 ms. A single TCP transfer sustains only 6.5 Mbps.

Running eight transfers in parallel gets close to 100 Mbps in total, which rules out a hardware bottleneck: the path can carry the traffic.

Packet capture shows almost no retransmission, and the receiver advertises a window of 64 KiB and never more.

For reference, 64 KiB delivered once per 80 ms round trip is 65,536 × 8 ÷ 0.08 = 6.55 Mbps, which is what the transfer achieves.