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FTP Data Connection Management, Normal (Active) and Passive Data Connections and Port Usage
(Page 1 of 4)
The control channel created between
the Server-PI and the User-PI using the FTP
connection establishment and authentication process
is maintained throughout the FTP session. Commands and replies are exchanged
between the protocol interpreters over this channel, but not data.
Each time files or other data need
to be sent between the server and user FTP processes, a data
connection must be created. The data connection
links the User-DTP with the Server-DTP. This connection is required
both for explicit file transfer actions (getting or receiving a file)
and also for implicit data transfers, such as requesting a list of files
from a directory on the server.
The FTP standard specifies two different
ways of creating a data connection, though it doesn't really explain
them in a way that is very easy to understand. That's my job. The two
methods differ primarily in which device, the client or the server,
initiates the connection. This may at first seem like a trivial matter,
but as we'll see shortly, it is actually quite important.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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