[bscphil]

> with a large enough receive window

Yeah, that's the issue. I didn't have root permissions on either side. Moreover, a transfer tool should just work without requiring its users to have expert knowledge like this.

In this case, I checked the roundtrip ping time and multiplied it by the buffer size, and it agreed with the speeds I was seeing within ~5%, so it was not an issue with throttling. Actually, if I were a network provider interested in doing this, I would throttle on the 2-tuple as well.