The communication between the user and server is intended to be an alternating dialogue. As such, the user issues an FTP command and the server responds with a prompt primary reply. The user should wait for this initial primary success or failure response before sending further commands.
Certain commands require a second reply for which the user should also wait. These replies may, for example, report on the progress or completion of file transfer or the closing of the data connection. They are secondary replies to file transfer commands.
One important group of informational replies is the connection greetings. Under normal circumstances, a server will send a 220 reply, "awaiting input", when the connection is completed. The user should wait for this greeting message before sending any commands. If the server is unable to accept input right away, a 120 "expected delay" reply should be sent immediately and a 220 reply when ready. The user will then know not to hang up if there is a delay.
Spontaneous Replies
Sometimes "the system" spontaneously has a message to be sent to a user (usually all users). For example, "System going down in 15 minutes". There is no provision in FTP for such spontaneous information to be sent from the server to the user. It is recommended that such information be queued in the server-PI and delivered to the user-PI in the next reply (possibly making it a multi-line reply).
The table below lists alternative success and failure replies for each command. These must be strictly adhered to; a server may substitute text in the replies, but the meaning and action implied by the code numbers and by the specific command reply sequence cannot be altered.
Command-Reply Sequences
In this section, the command-reply sequence is presented. Each command is listed with its possible replies; command groups are listed together. Preliminary replies are listed first (with their succeeding replies indented and under them), then positive and negative completion, and finally intermediary replies with the remaining commands from the sequence following. This listing forms the basis for the state diagrams, which will be presented separately.
Connection Establishment 120 220 220 421 Login USER 230 530 500, 501, 421 331, 332 PASS 230 202 530 500, 501, 503, 421 332 ACCT 230 202 530 500, 501, 503, 421 CWD 250 500, 501, 502, 421, 530, 550 CDUP 200 500, 501, 502, 421, 530, 550 SMNT 202, 250 500, 501, 502, 421, 530, 550 Logout REIN 120 220 220 421 500, 502 QUIT 221 500 Transfer parameters PORT 200 500, 501, 421, 530 PASV 227 500, 501, 502, 421, 530 MODE 200 500, 501, 504, 421, 530 TYPE 200 500, 501, 504, 421, 530 STRU 200 500, 501, 504, 421, 530 File action commands ALLO 200 202 500, 501, 504, 421, 530 REST 500, 501, 502, 421, 530 350 STOR 125, 150 (110) 226, 250 425, 426, 451, 551, 552 532, 450, 452, 553 500, 501, 421, 530 STOU 125, 150 (110) 226, 250 425, 426, 451, 551, 552 532, 450, 452, 553 500, 501, 421, 530 RETR 125, 150 (110) 226, 250 425, 426, 451 450, 550 500, 501, 421, 530 LIST 125, 150 226, 250 425, 426, 451 450 500, 501, 502, 421, 530 NLST 125, 150 226, 250 425, 426, 451 450 500, 501, 502, 421, 530 APPE 125, 150 (110) 226, 250 425, 426, 451, 551, 552 532, 450, 550, 452, 553 500, 501, 502, 421, 530 RNFR 450, 550 500, 501, 502, 421, 530 350 RNTO 250 532, 553 500, 501, 502, 503, 421, 530 DELE 250 450, 550 500, 501, 502, 421, 530 RMD 250 500, 501, 502, 421, 530, 550 MKD 257 500, 501, 502, 421, 530, 550 PWD 257 500, 501, 502, 421, 550 ABOR 225, 226 500, 501, 502, 421 Informational commands SYST 215 500, 501, 502, 421 STAT 211, 212, 213 450 500, 501, 502, 421, 530 HELP 211, 214 500, 501, 502, 421 Miscellaneous commands SITE 200 202 500, 501, 530 NOOP 200 500 421