How long does it take to transfer 1 TB over a WAN with 100 ms latency? The TCP window trap

How this is calculated

The naive math (1 TB divided by 1000 Mbps) gives 2 hours 13 minutes, and that is what every size-over-speed calculator on the web returns. It is correct for a USB cable and wrong for the internet. On a long-distance link the throughput ceiling is the bandwidth-delay product: the receive window divided by the round-trip time. With an unscaled 64 KB window and 100 ms RTT that ceiling is 65536 bytes times 8 bits divided by 0.1 seconds, which works out to 5.24 Mbps. The link fills to 0.52% of its rated speed because TCP is not allowed to put more data in flight than the window allows, and the acknowledgments take too long to return.

Verdict

1 TB over a 1 Gbps WAN with 100 ms round-trip time takes about 17 days 16 hours when the default unscaled 64 KB TCP receive window is in play, which is the gap between the 2 hours 13 minutes a naive size-divided-by-speed calculator returns and what actually happens on the wire. The mechanism is the bandwidth-delay product: TCP is only allowed to have one receive window worth of data in flight at a time, so the throughput ceiling is the window size divided by the round-trip time. With 65536 bytes in the window and a 0.1 second round trip, that ceiling is 5.24 Mbps, about 0.52% of the 1000 Mbps link. The fix is RFC 1323 window scaling, which lets the window grow up to 1 GiB and pushes the ceiling well past the link rate so the transfer drops back to the 2 hours 13 minutes the link is actually capable of.