Path: utzoo!utgpu!water!watmath!clyde!att!ucbvax!VENERA.ISI.EDU!deschon
From: deschon@VENERA.ISI.EDU (Annette DeSchon)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: simultaneous connections
Message-ID: <8810181854.AA09410@venera.isi.edu>
Date: 18 Oct 88 18:53:36 GMT
References: <8810181218.AA11951@gateway.mitre.org>
Sender: usenet@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 55

Bill,

You are correct.  BFTP makes use of the server-server, or third party,
model of FTP described in RFC 765.  It does not depend on simultaneous
TCP connections.

To illustrate this:  Using third party file transfer, or more specifically
BFTP, there are three TCP connections.  There is an FTP control connection 
between BFTP and each of the FTP servers, and a data connection between 
the two FTP servers. (P) indicates a passive open, and (A) indicates an 
active open of the TCP connection.

In the case where BFTP sends the "PASV" command to the Source Host, the
Destination Host opens the data connection:

		         ------
		        | BFTP |
		        |Daemon|
		         ------
		     (A)        (A)
		     ^		  ^
      control conn. /		   \ control conn.
		   v		    v
	         (P)		    (P)
	      ------                -----------
	     |Source|		   |Destination|
	     | Host |		   |   Host    |
	     | FTP  |(P)<------>(A)|   FTP     |
	     |Server|	   	   |  Server   |
	      ------   data conn.   -----------


Where BFTP sends the "PASV" command to the Destination Host, the Source
Host opens the data connection:

		         ------
		        | BFTP |
		        |Daemon|
		         ------
		     (A)        (A)
		     ^		  ^
      control conn. /		   \ control conn.
		   v		    v
	         (P)		    (P)
	      ------                -----------
	     |Source|		   |Destination|
	     | Host |		   |   Host    |
             | FTP  |(A)<------>(P)|   FTP     |
	     |Server|	   	   |  Server   |
	      ------   data conn.   -----------


The BFTP Daemon always initiates both control connections.

--Annette