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Table Of Contents  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

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FTP Concepts and General Operation
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FTP Control Connection Establishment, User Authentication and Anonymous FTP Access
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FTP Operational Model, Protocol Components and Key Terminology
(Page 1 of 4)

The standards that define the File Transfer Protocol (FTP) describe its overall operation using a simple conceptual tool called the FTP model. This model defines the roles of the devices that participate in a file transfer, and the two communication channels that are established between them. It also describes the components of FTP that manage these channels, and defines the terminology used for the components. This makes it an ideal place for us to see how FTP works in broad terms.

The Server-FTP Process and User-FTP Process

FTP is a classical client/server protocol, as I mentioned in the overview. However, the client is not called by that name but rather is called the user. The name comes from the fact that the human user that issues FTP commands works on the client machine. The full set of FTP software operating on a device is called a process. The FTP software on the server is called the Server-FTP Process, while the software on the client is the User-FTP Process.

Key Concept: The FTP client is sometimes called the user device, since the human user interacts with the client directly. The FTP client software is called the User-FTP Process; the FTP server software, the Server-FTP Process.

FTP Control Connection and Data Connection

A critical concept in understanding FTP is that while it uses TCP like many other applications, it does not use just one TCP connection for all communication the way most protocols do. The FTP model is designed around two logical channels of communication between the server and user FTP processes:

  • Control Connection: This is the main logical TCP connection that is created when an FTP session is established. It is maintained throughout the FTP session and is used only for passing control information, such as FTP commands and replies. It is not used to send files.

  • Data Connection: Each time that data is sent from the server to the client or vice-versa, a distinct TCP data connection is established between them. Data is transferred over this connection. When the file transfer is complete, the connection is terminated.

The reason for having these separate channels is that it provides flexibility in how the protocol is used, as we will see later in this section. It does, however, add complexity to FTP.

Key Concept: Unlike most protocols, FTP does not use a single TCP connection. When a session is set up, a permanent control connection is established using TCP, for passing commands and replies. When files or other data are to be sent, they are passed over separate TCP data connections that are created and then dismantled as needed.

Previous Topic/Section
FTP Concepts and General Operation
Previous Page
Pages in Current Topic/Section
Next Page
FTP Control Connection Establishment, User Authentication and Anonymous FTP Access
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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