RFC 959 October 1985File Transfer Protocoltransferred over the control connection...

If a partial
pathname is given, the server may respond with a list of
file names or attributes associated with that specification.
If no argument is given, the server should return general
status information about the server FTP process. This
should include current values of all transfer parameters and
the status of connections.
HELP (HELP)
This command shall cause the server to send helpful
information regarding its implementation status over the
control connection to the user. The command may take an
argument (e.g., any command name) and return more specific
information as a response. The reply is type 211 or 214.
It is suggested that HELP be allowed before entering a USER
command. The server may use this reply to specify
site-dependent parameters, e.g., in response to HELP SITE.
NOOP (NOOP)
This command does not affect any parameters or previously
entered commands. It specifies no action other than that the
server send an OK reply.
The File Transfer Protocol follows the specifications of the Telnet
protocol for all communications over the control connection. Since
the language used for Telnet communication may be a negotiated
option, all references in the next two sections will be to the
"Telnet language" and the corresponding "Telnet end-of-line code".
Currently, one may take these to mean NVT-ASCII and . No other
specifications of the Telnet protocol will be cited.
FTP commands are "Telnet strings" terminated by the "Telnet end of
line code". The command codes themselves are alphabetic characters
terminated by the character (Space) if parameters follow and
Telnet-EOL otherwise. The command codes and the semantics of
commands are described in this section; the detailed syntax of
commands is specified in the Section on Commands, the reply sequences
are discussed in the Section on Sequencing of Commands and Replies,
and scenarios illustrating the use of commands are provided in the
Section on Typical FTP Scenarios.
FTP commands may be partitioned as those specifying access-control
identifiers, data transfer parameters, or FTP service requests.
Certain commands (such as ABOR, STAT, QUIT) may be sent over the
control connection while a data transfer is in progress. Some
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RFC 959 October 1985
File Transfer Protocol
servers may not be able to monitor the control and data connections
simultaneously, in which case some special action will be necessary
to get the server's attention. The following ordered format is
tentatively recommended:
1. User system inserts the Telnet "Interrupt Process" (IP) signal
in the Telnet stream.
2. User system sends the Telnet "Synch" signal.
3. User system inserts the command (e.g., ABOR) in the Telnet
stream.
4. Server PI, after receiving "IP", scans the Telnet stream for
EXACTLY ONE FTP command.
(For other servers this may not be necessary but the actions listed
above should have no unusual effect.)
4.2. FTP REPLIES
Replies to File Transfer Protocol commands are devised to ensure
the synchronization of requests and actions in the process of file
transfer, and to guarantee that the user process always knows the
state of the Server. Every command must generate at least one
reply, although there may be more than one; in the latter case,
the multiple replies must be easily distinguished. In addition,
some commands occur in sequential groups, such as USER, PASS and
ACCT, or RNFR and RNTO. The replies show the existence of an
intermediate state if all preceding commands have been successful.
A failure at any point in the sequence necessitates the repetition
of the entire sequence from the beginning.
The details of the command-reply sequence are made explicit in
a set of state diagrams below.
An FTP reply consists of a three digit number (transmitted as
three alphanumeric characters) followed by some text. The number
is intended for use by automata to determine what state to enter
next; the text is intended for the human user. It is intended