We all know of the problems caused by the FTP data connection with NAT
routers, firewalls, etc, so it's interesting someone has come up with an
RFC proposal that allows the data connection to also run on port 21.  


Essentially, a second connection is opened to port 21, with a simple
logon process to tie it to the initial 21 connection, then the real data
sent after a 200 DATA response.  

   - The control connection ("--1-->") is established.
     S--1-->C   220 FTP server ready<CRLF>
     C--1-->S   USER u001<CRLF>
     S--1-->C   331 Enter password<CRLF>
     C--1-->S   PASS xyz<CRLF>
     S--1-->C   230 You are logged in<CRLF>
     C--1-->S   SPSV<CRLF>
     S--1-->C   227 Entering single-port mode (xYab1234)<CRLF>
   - The data connection ("---2->") is established
     S---2->C   220 FTP server ready<CRLF>
     C---2->S   SPDT xYab1234<CRLF>
     S---2->C   200 DATA<CRLF>
     C--1-->S   RETR contents.txt<CRLF>
     S--1-->C   150 Transmitting data<CRLF>
     S---2->C   (Contents of contents.txt)
   - The server closes the data connection ("---2->")
     S--1-->C   226 Data transferred<CRLF>


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