Compare with what existed when NCP (RFC 33) was in use. An early FTP protocol (452, 454, 542) originally used a server listen port of 3 (and hence a server "re

Compare with what existed when NCP (RFC 33) was in use. An early FTP protocol (452, 454, 542) originally used a server listen port of 3 (and hence a server "return port" of 2*), but 542 allowed temporary use of port 21 as the protocol was changed. A different pair of ports (even or odd depending upon TX or RX end) were used for the data channel, and the port picked was related to the Telnet protocol port in use. RFC 640 then seems to have fixed the use of port 21 instead of 3. Then when TCP came in to use RFC 765 seems to have simplified things somewhat, sticking to port 21. Also see RFC 165 for the/a Telnet Initial Connection Protocol showing the use of the two even/odd ports for forming a bidirectional connection. * Or 4, but I believe the return channel was one port below, not above.