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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP General File Transfer Protocols (FTP and TFTP)                      9  File Transfer Protocol (FTP)                           9  FTP Concepts and General Operation

FTP Control Connection Establishment, User Authentication and Anonymous FTP Access 1 2 3 4 FTP General Data Communication and Transmission Modes

Passive Data Connections

The second method is called a passive data connection. The client tells the server to be “passive”, that is, to accept an incoming data connection initiated by the client. The server replies back giving the client the server IP address and port number that it should use. The Server-DTP then listens on this port for an incoming TCP connection from the User-DTP. By default, the user machine uses the same port number it used for the control connection, as in the active case. However, here again, the client can choose to use a different port number for the data connection if necessary (typically an ephemeral port number.)

Let's consider our example again, with the control connection from port 1678 on the client to port 21 on the server, but this time consider data transfer using a passive connection, as illustrated in Figure 291. The client would issue the PASV command to tell the server it wanted to use passive data control. The Server-PI would reply back with a port number for the client to use, say port 2223. The Server-PI would then instruct the Server-DTP to listen on this port 2223. The User-PI would instruct the User-DTP to create a connection from client port 1742 to server port 2223. The server would acknowledge this and then data could be sent and received, again in either direction.

Figure 291: FTP Passive Data Connection

In a passive FTP data connection, the client uses the PASV command to tell the server to wait for the client to establish the data connection. The server responds, telling the client what port it should use on the server for the data transmission, in this case port 2223. The client then opens the data connection using that port number on the server and a client port number of its own choosing, in this case 1742. Contrast to Figure 291.

FTP Control Connection Establishment, User Authentication and Anonymous FTP Access 1 2 3 4 FTP General Data Communication and Transmission Modes

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