From owner-freebsd-hackers  Wed Aug 27 19:23:02 1997
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To: Nate Williams <nate@mt.sri.com>
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From: "Louis A. Mamakos" <louie@TransSys.COM>
Subject: Re: FTP protocol questions (non-passive mode) 
References: <199708272234.QAA02520@rocky.mt.sri.com> 
In-reply-to: Your message of "Wed, 27 Aug 1997 16:34:07 MDT."
             <199708272234.QAA02520@rocky.mt.sri.com> 
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Date: Wed, 27 Aug 1997 22:22:49 -0400
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Ok, so the client opens a TCP connection to an FTP server:

	(Client-IP, Client-Port) <-> (Server-IP, 21)

in the absense of any other information, the data connection
is made (per transfer) as:

	(Client-IP, Client-Port) <-> (Server-IP, 20)

that is, the client reuses the same local port number, but uses
the FTP data port on the server.

BSD ftp clients use the PORT command to override this default port
number selection.  This allows successive data transfers to proceed
more rapidly since you don't have to wait for the previous connection
state to time out (it might be in TIMEWAIT state) before you can re-bind
the same port number.

louie