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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 that the three digits contain enough encoded information that the
user-process (the User-PI) will not need to examine the text and may
either discard it or pass it on to the user, as appropriate.
In particular, the text may be
server-dependent, so there are likely to be varying texts for each reply
code.
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