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FTP Internal Protocol Commands and Command Groups
(Page 1 of 2)
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.
FTP Command Groups
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”.
 Key Concept: FTP operation is controlled through the issuing of protocol commands from the FTP client to the FTP server. Each command has a three- or four-letter command code that indicates its function. The commands are organized into three groups: access control commands used for login and general session control; transfer parameter commands that control how transfers are performed; and FTP service commands, which are used to perform actual file operations.
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