Error handling engine.
The intercomputer connection is non-homogenic and therefore a reason for
many errors of different type - network failures, access failures, syntax
errors. For all those types of errors that can happen during network
operations an error handling engine was implemented.
The engine is based on the RFC959 which proposes following numerical
protocol codes for FTP server:
Note that only part of those codes are needed for our mechanism since
we use only limited set of commands. The relevant codes are marked with
"!!" on the right side.
200 Command okay. !!
500 Syntax error, command unrecognized. !!
This may include errors such as command line too long.
501 Syntax error in parameters or arguments. !!
202 Command not implemented, superfluous at this site.
502 Command not implemented. !!
503 Bad sequence of commands. !!
504 Command not implemented for that parameter. !!
110 Restart marker reply.
In this case, the text is exact and not left to the
particular implementation; it must read:
MARK yyyy = mmmm
Where yyyy is User-process data stream marker, and mmmm
server's equivalent marker (note the spaces between markers
and "=").
211 System status, or system help reply.
212 Directory status.
213 File status.
214 Help message.
On how to use the server or the meaning of a particular
non-standard command. This reply is useful only to the
human user.
215 NAME system type.
Where NAME is an official system name from the list in the
Assigned Numbers document.
120 Service ready in nnn minutes.
220 Service ready for new user. !!
221 Service closing control connection. !!
Logged out if appropriate.
421 Service not available, closing control connection. !!
This may be a reply to any command if the service knows it
must shut down.
125 Data connection already open; transfer starting.
225 Data connection open; no transfer in progress.
425 Can't open data connection. !!
226 Closing data connection. !!
Requested file action successful (for example, file
transfer or file abort).
426 Connection closed; transfer aborted. !!
227 Entering Passive Mode (h1,h2,h3,h4,p1,p2).
230 User logged in, proceed. !!
530 Not logged in. !!
331 User name okay, need password. !!
332 Need account for login.
532 Need account for storing files.
150 File status okay; about to open data connection. !!
250 Requested file action okay, completed. !!
257 "PATHNAME" created.
350 Requested file action pending further information.
450 Requested file action not taken. !!
File unavailable (e.g., file busy).
550 Requested action not taken. !!
File unavailable (e.g., file not found, no access).
451 Requested action aborted. Local error in processing.
551 Requested action aborted. Page type unknown.
452 Requested action not taken. !!
Insufficient storage space in system.
552 Requested file action aborted. !!
Exceeded storage allocation (for current directory or
dataset).
553 Requested action not taken.
File name not allowed.
5.4. SEQUENCING OF COMMANDS AND REPLIES
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
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
Error handling routine:
The function that handles errors is the error_proceed() function in
module connect.c. It receives the wrong protocol number as the argument
and is in fact one big switch of all possible error cases we may run into.
According to the error number a corresponding error message is being printed on
the screen. Then the program returns to the place the error was encountered and
the fail code of -1 or 0 is being returned back to the routine hierarchy.
romm@empire.tau.ac.il
Last modified: Tue Oct 22 14:04:34 1996
