The server that has moved countless Petabytes is glFTPd that allows FXP ( clients without bandwidth can initiate to transfer files from server to server ).
Edit: Source: https://en.wikipedia.org/wiki/File_eXchange_Protocol#Technic...
1. You connect to servers A and B.
2. Tell B to receive a PASV transfer. It replies with the IP address and port it's receiving on.
3. Tell A to send to that address and port.
This is documented in RFC 959, starting with
"In another situation a user might wish to transfer files between two hosts, neither of which is a local host."