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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 Commands and Replies

FTP Commands and Replies 1 2 FTP Replies, Reply Code Format and Important Reply Codes

An FTP session begins with the establishment of a TCP connection from an FTP client (user device) to an FTP server. Once established, the control channel is logically in place between the Server-PI and the User-PI. All communication to manage the operation of the protocol takes place over this channel. The User-PI sends protocol commands to the Server-PI, which processes them and takes appropriate action. The Server-PI responds back with reply codes to tell the User-PI the result of the commands it issued and convey other important information.

Each command is identified by a short three-letter or four-letter command code for convenience, and performs a specific task in the overall functionality of FTP. There are several dozen of these protocol commands, and to help organize them, the FTP standard categorizes them into three groups, based on overall function type:

• Access Control Commands: Commands that are part of the user login and authentication process, are used for resource access, or are part of general session control.
  
  
• Transfer Parameter Commands: Commands that specify parameters for how data transfers should occur. For example, commands in this group specify the data type of a file to be sent, indicate whether passive or active data connections will be used, and so forth.
  
  
• FTP Service Commands: This is the largest group, containing all the commands that actually perform file operations, such as sending and receiving files. Commands to implement support functions, such as deleting or renaming files, are also here.

Interestingly, the actual transmission of FTP commands over the control channel is done using specifications based on the Telnet protocol. You may recall from the FTP overview that Telnet and FTP are two of the very oldest TCP/IP applications, the former being for “direct” network use and the latter for “indirect” resource access. They were developed at around the same time, and setting up the FTP control channel to act as a type of Telnet connection is a good example of how Internet standards try not to “reinvent the wheel”.

FTP Commands and Replies 1 2 FTP Replies, Reply Code Format and Important Reply Codes