Tangentially, back in the early-2000s, we used something called FlashFXP which was just a fine GUI over FTP's "feature" that the control and data channels were

Tangentially, back in the early-2000s, we used something called FlashFXP which was just a fine GUI over FTP's "feature" that the control and data channels were separate. With that, you could, from a thin pipe, connect to 2 FTP servers that had a fat pipe between them, and transfer files between those two. This was popular for file-sharing, especially since you'd never actually end up with the files on your computer, so, plausible deniability.