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 Commands and Replies

FTP Replies, Reply Code Format and Important Reply Codes 1 2 3 FTP Sample User and Internal Command Dialog

The FTP protocol command set provides a rich, complete set of instructions for implementing the File Transfer Protocol. A human user could employ those commands to perform file transfer functions directly with an FTP server. But to do this requires that the user have an intimate knowledge of how FTP works. The user must know exactly which commands to send at which time, and in what order.

Memorizing internal FTP commands might be a reasonable assignment for an internetworking expert, but not for a typical TCP/IP application user. For this reason, the FTP protocol defines an additional protocol component as part of the User-FTP Process: the FTP user interface. It provides three main benefits to the FTP user:

• User Friendliness: The FTP user interface presents FTP to the human user in a way that is easier and simpler to use than issuing protocol commands. Instead of requiring the knowledge of all those four-letter codes, the user interface can allow functions to be performed with more intuitive human-language commands. For example, we can say “get” a file instead of knowing to use the command RETR.
  
  
• Customization: The command used to perform a particular function can be customized based on common parlance in the networking industry, without requiring changes to be made to the FTP protocol itself. For example, the “image” transfer mode is now also commonly called “binary” mode, so a user command called “binary” has been created to set this mode.
  
  
• Detail Abstraction and Command Sequence Simplification: A single user command can be made to issue multiple FTP protocol commands, hiding internal FTP details and making the protocol easier to use. In particular, commands that are related to the maintenance of the connection and other “overhead” issues that users don't want to deal with can be automated. For example, an FTP client normally issues a PASV or PORT command prior to each data transfer. The user interface can take care of issuing this command automatically prior to a RETR or STOR command when a user tells FTP to get or send a file.

FTP Replies, Reply Code Format and Important Reply Codes 1 2 3 FTP Sample User and Internal Command Dialog