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 Concepts and General Operation 1 2 3 4 FTP Control Connection Establishment, User Authentication and Anonymous FTP Access

FTP Process Components and Terminology

Since the control and data functions are communicated using distinct channels, the FTP model divides the software on each device into two logical protocol components that are responsible for each channel. The protocol interpreter (PI) is a piece of software that is charged with managing the control connection, issuing and receiving commands and replies. The data transfer process (DTP) is responsible for actually sending and receiving data between the client and server. In addition to these two elements, the user FTP process includes a third component, a user interface, that interacts with the human FTP user; it is not present on the server side.

Thus, there are two server process components and three client (user) process components in FTP. These components are referred to in the FTP model by specific names, which are used in the standard to describe the detailed operation of the protocol. I plan to do the same in this section, so I will now describe more fully the components in each device of this model, which are illustrated in Figure 288.

Figure 288: File Transfer Protocol (FTP) Operational Model

FTP is a client/server protocol, with communication taking place between the User-FTP Process on the client and the Server-FTP Process on the server. Commands, replies and status information are passed between the User-PI and Server-PI over the control connection, which is established once and maintained for the session. Data is moved between devices over data connections that are set up for each transfer.

FTP Concepts and General Operation 1 2 3 4 FTP Control Connection Establishment, User Authentication and Anonymous FTP Access

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