Data migration calculator: how long will your transfer really take over the WAN?

How this is calculated

The gap between 11 hours and 35 days comes from the TCP receive window. TCP only sends one window of data before stopping to wait for an acknowledgment, and on a 40 ms same-continent link that acknowledgment takes 0.04 seconds to come back. With the default unscaled 64 KB window the throughput ceiling is 64 KB / 0.04 s = 13.1 Mbps, which is 1.3% of the 1 Gbps link. The bandwidth-delay product for 1 Gbps at 40 ms is about 5 MB, and the 64 KB window is 75 times too small to fill it. Enable RFC 1323 window scaling and the window grows past 5 MB, pushing the ceiling past the link rate so the 11-hour figure becomes the real one.

Verdict

A 5 TB data migration over a 1 Gbps WAN at 40 ms round-trip time takes about 35 days with the default unscaled 64 KB TCP receive window, which is the gap between the 11 hours a naive size-divided-by-speed calculator returns and what the migration actually delivers on the wire. The structural reason is the same bandwidth-delay product that hits every long-distance TCP transfer: the receive window is 75 times smaller than the 5 MB the link needs to stay full at 40 ms RTT, so the throughput collapses to 13.1 Mbps, which is 1.3% of the 1000 Mbps link. The planning move is to either validate that RFC 1323 window scaling is negotiated end to end (which pushes the window past 5 MB and drops the migration to the 11 hours the link is actually capable of), open parallel streams with rsync or SMB Multichannel to multiply the effective window, or budget for the 35-day figure if neither is possible. A data migration calculator that ignores the bandwidth-delay product is not a planning tool, it is a guess.