RFC 0765
File Transfer Protocol specification
Replicated Byte:
2 6 8
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
|1 0| n | | d |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
A string of n filler bytes can be compressed into a single
byte, where the filler byte varies with the representation
type. If the type is ASCII or EBCDIC the filler byte is
<SP> (Space, ASCII code 32., EBCDIC code 64). If the type
is Image or Local byte the filler is a zero byte.
Filler String:
2 6
+-+-+-+-+-+-+-+-+
|1 1| n |
+-+-+-+-+-+-+-+-+
The escape sequence is a double byte, the first of which is
the escape byte (all zeros) and the second of which contains
descriptor codes as defined in Block mode. The descriptor
codes have the same meaning as in Block mode and apply to
the succeeding string of bytes.
Compressed mode is useful for obtaining increased bandwidth
on very large network transmissions at a little extra CPU
cost. It can be most effectively used to reduce the size of
printer files such as those generated by RJE Hosts.
ERROR RECOVERY AND RESTART
There is no provision for detecting bits lost or scrambled in data
transfer; this level of error control is handled by the TCP.
However, a restart procedure is provided to protect users from
gross system failures (including failures of a Host, an
FTP-process, or the underlying network).
The restart procedure is defined only for the block and compressed
modes of data transfer. It requires the sender of data to insert
a special marker code in the data stream with some marker
information. The marker information has meaning only to the
sender, but must consist of printable characters in the default or
negotiated language of the TELNET connection (ASCII or EBCDIC).
The marker could represent a bit-count, a record-count, or any
other information by which a system may identify a data
checkpoint. The receiver of data, if it implements the restart
procedure, would then mark the corresponding position of this
marker in the receiving system, and return this information to the
user.
In the event of a system failure, the user can restart the data
transfer by identifying the marker point with the FTP restart
procedure. The following example illustrates the use of the
restart procedure.
The sender of the data inserts an appropriate marker block in the
data stream at a convenient point. The receiving Host marks the
corresponding data point in its file system and conveys the last
known sender and receiver marker information to the user, either
directly or over the TELNET connection in a 110 reply (depending
on who is the sender). In the event of a system failure, the user
or controller process restarts the server at the last server
marker by sending a restart command with server's marker code as
its argument. The restart command is transmitted over the TELNET
connection and is immediately followed by the command (such as
RETR, STOR or LIST) which was being executed when the system
failure occurred.
FILE TRANSFER FUNCTIONS
The communication channel from the user-PI to the server-PI is
established by a TCP connection from the user to a standard server
port. The user protocol interpreter is responsible for sending FTP
commands and interpreting the replies received; the server-PI
interprets commands, sends replies and directs its DTP to set up the
data connection and transfer the data. If the second party to the
data transfer (the passive transfer process) is the user-DTP then it
is governed through the internal protocol of the user-FTP Host; if it
is a second server-DTP then it is governed by its PI on command from
the user-PI. The FTP replies are discussed in the next section. In
the description of a few of the commands in this section it is
helpful to be explicit about the possible replies.
FTP COMMANDS
ACCESS CONTROL COMMANDS
The following commands specify access control identifiers
(command codes are shown in parentheses).
USER NAME (USER)
The argument field is a TELNET string identifying the user.
The user identification is that which is required by the
server for access to its file system. This command will
normally be the first command transmitted by the user after
the TELNET connections are made (some servers may require
this). Additional identification information in the form of
a password and/or an account command may also be required by
some servers. Servers may allow a new USER command to be
entered at any point in order to change the access control
and/or accounting information. This has the effect of
flushing any user, password, and account information already
supplied and beginning the login sequence again. All
transfer parameters are unchanged and any file transfer in
progress is completed under the old account.
PASSWORD (PASS)
The argument field is a TELNET string identifying the user's
password. This command must be immediately preceded by the
user name command, and, for some sites, completes the user's
identification for access control. Since password
information is quite sensitive, it is desirable in general
to "mask" it or suppress typeout. It appears that the
server has no foolproof way to achieve this. It is
therefore the responsibility of the user-FTP process to hide
the sensitive password information.
ACCOUNT (ACCT)
The argument field is a TELNET string identifying the user's
account. The command is not necessarily related to the USER
command, as some sites may require an account for login and
others only for specific access, such as storing files. In
the latter case the command may arrive at any time.
There are reply codes to differentiate these cases for the
automaton: when account information is required for login,
the response to a successful PASSword command is reply code
332. On the other hand, if account information is NOT
required for login, the reply to a successful PASSword
command is 230; and if the account information is needed for
a command issued later in the dialogue, the server should
return a 332 or 532 reply depending on whether he stores
(pending receipt of the ACCounT command) or discards the
command, respectively.
REINITIALIZE (REIN)
This command terminates a USER, flushing all I/O and account
information, except to allow any transfer in progress to be
completed. All parameters are reset to the default settings
and the TELNET connection is left open. This is identical
to the state in which a user finds himself immediately after
the TELNET connection is opened. A USER command may be
expected to follow.
LOGOUT (QUIT)
This command terminates a USER and if file transfer is not
in progress, the server closes the TELNET connection. If
file transfer is in progress, the connection will remain
open for result response and the server will then close it.
If the user-process is transferring files for several USERs
but does not wish to close and then reopen connections for
each, then the REIN command should be used instead of QUIT.
An unexpected close on the TELNET connection will cause the
server to take the effective action of an abort (ABOR) and a
logout (QUIT).
TRANSFER PARAMETER COMMANDS
All data transfer parameters have default values, and the
commands specifying data transfer parameters are required only
if the default parameter values are to be changed. The default
value is the last specified value, or if no value has been
specified, the standard default value as stated here. This
implies that the server must "remember" the applicable default
values. The commands may be in any order except that they must
precede the FTP service request. The following commands
specify data transfer parameters.
DATA PORT (PORT)
The argument is a HOST-PORT specification for the data port
to be used in data connection. There defaults for both the
user and server data ports, and under normal circumstances
this command and its reply are not needed. If this command
is used the argument is the concatenation of a 32-bit
internet host address and a 16-bit TCP port address. This
address information is broken into 8-bit fields and the
value of each field is transmitted as a decimal number (in
character string representation). The fields are separated
by commas. A port command would be:
PORT h1,h2,h3,h4,p1,p2
where, h1 is the high order 8 bits of the internet host
address.
PASSIVE (PASV)
This command requests the server-DTP to "listen" on a data
port (which is not its default data port) and to wait for a
connection rather than initiate one upon receipt of a
transfer command. The response to this command includes the
host and port address this server is listening on.
REPRESENTATION TYPE (TYPE)
The argument specifies the representation type as described
in the Section on Data Representation and Storage. Several
types take a second parameter. The first parameter is
denoted by a single TELNET character, as is the second
Format parameter for ASCII and EBCDIC; the second parameter
for local byte is a decimal integer to indicate Bytesize.
The parameters are separated by a <SP> (Space, ASCII code
32.).
The following codes are assigned for type:
\ /
A - ASCII | | N - Non-print
|-><-| T - TELNET format effectors
E - EBCDIC| | C - Carriage Control (ASA)
/ \
I - Image
L <byte size> - Local byte Byte size
The default representation type is ASCII Non-print. If the
Format parameter is changed, and later just the first
argument is changed, Format then returns to the Non-print
default.
FILE STRUCTURE (STRU)
The argument is a single TELNET character code specifying
file structure described in the Section on Data
Representation and Storage.
The following codes are assigned for structure:
F - File (no record structure)
R - Record structure
P - Page structure
The default structure is File.
TRANSFER MODE (MODE)
The argument is a single TELNET character code specifying
the data transfer modes described in the Section on
Transmission Modes.
The following codes are assigned for transfer modes:
S - Stream
B - Block
C - Compressed
The default transfer mode is Stream.
FTP SERVICE COMMANDS
The FTP service commands define the file transfer or the file
system function requested by the user. The argument of an FTP
service command will normally be a pathname. The syntax of
pathnames must conform to server site conventions (with
standard defaults applicable), and the language conventions of
the TELNET connection. The suggested default handling is to
use the last specified device, directory or file name, or the
standard default defined for local users. The commands may be
in any order except that a "rename from" command must be
followed by a "rename to" command and the restart command must
be followed by the interrupted service command. The data, when
transferred in response to FTP service commands, shall always
be sent over the data connection, except for certain
informative replies. The following commands specify FTP
service requests:
RETRIEVE (RETR)
This command causes the server-DTP to transfer a copy of the
file, specified in the pathname, to the server- or user-DTP
at the other end of the data connection. The status and
contents of the file at the server site shall be unaffected.
STORE (STOR)
This command causes the server-DTP to accept the data
transferred via the data connection and to store the data as
a file at the server site. If the file specified in the
pathname exists at the server site then its contents shall
be replaced by the data being transferred. A new file is
created at the server site if the file specified in the
pathname does not already exist.
APPEND (with create) (APPE)
This command causes the server-DTP to accept the data
transferred via the data connection and to store the data in
a file at the server site. If the file specified in the
pathname exists at the server site, then the data shall be
appended to that file; otherwise the file specified in the
pathname shall be created at the server site.
MAIL FILE (MLFL)
The intent of this command is to enable a user at the user
site to mail data (in form of a file) to another user at the
server site. It should be noted that the files to be mailed
are transmitted via the data connection in ASCII or EBCDIC
type. (It is the user's responsibility to ensure that the
type is correct.) These files should be inserted into the
destination user's mailbox by the server in accordance with
serving Host mail conventions. The mail may be marked as
sent from the particular user HOST and the user specified by
the 'USER' command. The argument field may contain a Host
system ident, or it may be empty. If the argument field is
empty or blank (one or more spaces), then the mail is
destined for a printer or other designated place for general
delivery site mail.
MAIL (MAIL)
This command allows a user to send mail that is NOT in a
file over the TELNET connection. The argument field may
contain system ident, or it may be empty. The ident is
defined as above for the MLFL command. After the 'MAIL'
command is received, the server is to treat the following
lines as text of the mail sent by the user. The mail text
is to be terminated by a line containing only a single
period, that is, the character sequence "CRLF.CRLF". It is
suggested that a modest volume of mail service should be
free; i.e., it may be entered before a USER command.
MAIL SEND TO TERMINAL (MSND)
This command is like the MAIL command, except that the data
is displayed on the addressed user's terminal, if such
access is currently allowed, otherwise an error is returned.
MAIL SEND TO TERMINAL OR MAILBOX (MSOM)
This command is like the MAIL command, except that the data
is displayed on the addressed user's terminal, if such
access is currently allowed, otherwise the data is placed in
the user's mailbox.
MAIL SEND TO TERMINAL AND MAILBOX (MSAM)
This command is like the MAIL command, except that the data
is displayed on the addressed user's terminal, if such
access is currently allowed, and, in any case, the data is
placed in the user's mailbox.
MAIL RECIPIENT SCHEME QUESTION (MRSQ)
This FTP command is used to select a scheme for the
transmission of mail to several users at the same host. The
schemes are to list the recipients first, or to send the
mail first.
MAIL RECIPIENT (MRCP)
This command is used to identify the individual recipients
of the mail in the transmission of mail for multiple users
at one host.
ALLOCATE (ALLO)
This command may be required by some servers to reserve
sufficient storage to accommodate the new file to be
transferred. The argument shall be a decimal integer
representing the number of bytes (using the logical byte
size) of storage to be reserved for the file. For files
sent with record or page structure a maximum record or page
size (in logical bytes) might also be necessary; this is
indicated by a decimal integer in a second argument field of
the command. This second argument is optional, but when
present should be separated from the first by the three
TELNET characters <SP> R <SP>. This command shall be
followed by a STORe or APPEnd command. The ALLO command
should be treated as a NOOP (no operation) by those servers
which do not require that the maximum size of the file be
declared beforehand, and those servers interested in only
the maximum record or page size should accept a dummy value
in the first argument and ignore it.
RESTART (REST)
The argument field represents the server marker at which
file transfer is to be restarted. This command does not
cause file transfer but "spaces" over the file to the
specified data checkpoint. This command shall be
immediately followed by the appropriate FTP service command
which shall cause file transfer to resume.
RENAME FROM (RNFR)
This command specifies the file which is to be renamed.
This command must be immediately followed by a "rename to"
command specifying the new file pathname.
RENAME TO (RNTO)
This command specifies the new pathname of the file
specified in the immediately preceding "rename from"
command. Together the two commands cause a file to be
renamed.
ABORT (ABOR)
This command tells the server to abort the previous FTP
service command and any associated transfer of data. The
abort command may require "special action", as discussed in
the Section on FTP Commands, to force recognition by the
server. No action is to be taken if the previous command
has been completed (including data transfer). The TELNET
connection is not to be closed by the server, but the data
connection must be closed.
There are two cases for the server upon receipt of this
command: (1) the FTP service command was already completed,
or (2) the FTP service command is still in progress.
In the first case, the server closes the data connection
(if it is open) and responds with a 226 reply, indicating
that the abort command was successfully processed.
In the second case, the server aborts the FTP service in
progress and closes the data connection, returning a 426
reply to indicate that the service request terminated in
abnormally. The server then sends a 226 reply,
indicating that the abort command was successfully
processed.
DELETE (DELE)
This command causes the file specified in the pathname to be
deleted at the server site. If an extra level of protection
is desired (such as the query, "DO you really wish to
delete?"), it should be provided by the user-FTP process.
CHANGE WORKING DIRECTORY (CWD)
This command allows the user to work with a different
directory or dataset for file storage or retrieval without
altering his login or accounting information. Transfer
parameters are similarly unchanged. The argument is a
pathname specifying a directory or other system dependent
file group designator.
LIST (LIST)
This command causes a list to be sent from the server to the
passive DTP. If the pathname specifies a directory, the
server should transfer a list of files in the specified
directory. If the pathname specifies a file then the server
should send current information on the file. A null
argument implies the user's current working or default
directory. The data transfer is over the data connection in
type ASCII or type EBCDIC. (The user must ensure that the
TYPE is appropriately ASCII or EBCDIC).
NAME-LIST (NLST)
This command causes a directory listing to be sent from
server to user site. The pathname should specify a
directory or other system-specific file group descriptor; a
null argument implies the current directory. The server
will return a stream of names of files and no other
information. The data will be transferred in ASCII or
EBCDIC type over the data connection as valid pathname
strings separated by <CRLF> or <NL>. (Again the user must
ensure that the TYPE is correct.)
SITE PARAMETERS (SITE)
This command is used by the server to provide services
specific to his system that are essential to file transfer
but not sufficiently universal to be included as commands in
the protocol. The nature of these services and the
specification of their syntax can be stated in a reply to
the HELP SITE command.
STATUS (STAT)
This command shall cause a status response to be sent over
the TELNET connection in the form of a reply. The command
may be sent during a file transfer (along with the TELNET IP
and Synch signals--see the Section on FTP Commands) in which
case the server will respond with the status of the
operation in progress, or it may be sent between file
transfers. In the latter case the command may have an
argument field. If the argument is a pathname, the command
is analogous to the "list" command except that data shall be
transferred over the TELNET 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
TELNET 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 TELNET 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 <CRLF>.
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 <SP> (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
TELNET connection while a data transfer is in progress. Some
servers may not be able to monitor the TELNET and data connections
simultaneously, in which case some special action will be
necessary to get the server's attention. The exact form of the
"special action" is undefined; but 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.)
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.
Formally, a reply is defined to contain the 3-digit code, followed
by Space <SP>, followed by one line of text (where some maximum
line length has been specified), and terminated by the TELNET
end-of-line code. There will be cases, however, where the text is
longer than a single line. In these cases the complete text must
be bracketed so the User-process knows when it may stop reading
the reply (i.e. stop processing input on the TELNET connection)
and go do other things. This requires a special format on the
first line to indicate that more than one line is coming, and
another on the last line to designate it as the last. At least
one of these must contain the appropriate reply code to indicate
the state of the transaction. To satisfy all factions it was
decided that both the first and last line codes should be the
same.
Thus the format for multi-line replies is that the first line
will begin with the exact required reply code, followed
immediately by a Hyphen, "-" (also known as Minus), followed by
text. The last line will begin with the same code, followed
immediately by Space <SP>, optionally some text, and the TELNET
end-of-line code.
For example:
123-First line
Second line
234 A line beginning with numbers
123 The last line
The user-process then simply needs to search for the second
occurrence of the same reply code, followed by <SP> (Space), at
the beginning of a line, and ignore all intermediary lines. If
an intermediary line begins with a 3-digit number, the Server
must pad the front to avoid confusion.
This scheme allows standard system routines to be used for
reply information (such as for the STAT reply), with
"artificial" first and last lines tacked on. In the rare
cases where these routines are able to generate three digits
and a Space at the beginning of any line, the beginning of
each text line should be offset by some neutral text, like
Space.
This scheme assumes that multi-line replies may not be nested.
We have found that, in general, nesting of replies will not
occur, except for random system messages (also called
spontaneous replies) which may interrupt another reply. System
messages (i.e. those not processed by the FTP server) will NOT
carry reply codes and may occur anywhere in the command-reply
sequence. They may be ignored by the User-process as they are
only information for the human user.
The three digits of the reply each have a special significance.
This is intended to allow a range of very simple to very
sophisticated response by the user-process. The first digit
denotes whether the response is good, bad or incomplete.
(Referring to the state diagram) an unsophisticated user-process
will be able to determine its next action (proceed as planned,
redo, retrench, etc.) by simply examining this first digit. A
user-process that wants to know approximately what kind of error
occurred (e.g. file system error, command syntax error) may
examine the second digit, reserving the third digit for the finest
gradation of information (e.g. RNTO command without a preceding
RNFR.)
There are five values for the first digit of the reply code:
1yz Positive Preliminary reply
The requested action is being initiated; expect another
reply before proceeding with a new command. (The
user-process sending another command before the
completion reply would be in violation of protocol; but
server-FTP processes should queue any commands that
arrive while a preceding command is in progress.) This
type of reply can be used to indicate that the command
was accepted and the user-process may now pay attention
to the data connections, for implementations where
simultaneous monitoring is difficult.
2yz Positive Completion reply
The requested action has been successfully completed. A
new request may be initiated.
3yz Positive Intermediate reply
The command has been accepted, but the requested action
is being held in abeyance, pending receipt of further
information. The user should send another command
specifying this information. This reply is used in
command sequence groups.
4yz Transient Negative Completion reply
The command was not accepted and the requested action did
not take place, but the error condition is temporary and
the action may be requested again. The user should
return to the beginning of the command sequence, if any.
It is difficult to assign a meaning to "transient",
particularly when two distinct sites (Server and
User-processes) have to agree on the interpretation.
Each reply in the 4yz category might have a slightly
different time value, but the intent is that the
user-process is encouraged to try again. A rule of thumb
in determining if a reply fits into the 4yz or the 5yz
(Permanent Negative) category is that replies are 4yz if
the commands can be repeated without any change in
command form or in properties of the User or Server (e.g.
the command is spelled the same with the same arguments
used; the user does not change his file access or user
name; the server does not put up a new implementation.)
5yz Permanent Negative Completion reply
The command was not accepted and the requested action did
not take place. The User-process is discouraged from
repeating the exact request (in the same sequence). Even
some "permanent" error conditions can be corrected, so
the human user may want to direct his User-process to
reinitiate the command sequence by direct action at some
point in the future (e.g. after the spelling has been
changed, or the user has altered his directory status.)
The following function groupings are encoded in the second
digit:
x0z Syntax - These replies refer to syntax errors,
syntactically correct commands that don't fit any
functional category, unimplemented or superfluous
commands.
x1z Information - These are replies to requests for
information, such as status or help.
x2z Connections - Replies referring to the TELNET and data
connections.
x3z Authentication and accounting - Replies for the login
process and accounting procedures.
x4z Unspecified as yet
x5z File system - These replies indicate the status of the
Server file system vis-a-vis the requested transfer or
other file system action.
The third digit gives a finer gradation of meaning in each of
the function categories, specified by the second digit. The
list of replies below will illustrate this. Note that the text
associated with each reply is recommended, rather than
mandatory, and may even change according to the command with
which it is associated. The reply codes, on the other hand,
must strictly follow the specifications in the last section;
that is, Server implementations should not invent new codes for
situations that are only slightly different from the ones
described here, but rather should adapt codes already defined.
A command such as TYPE or ALLO whose successful execution
does not offer the user-process any new information will
cause a 200 reply to be returned. If the command is not
implemented by a particular Server-FTP process because it
has no relevance to that computer system, for example ALLO
at a TOPS20 site, a Positive Completion reply is still
desired so that the simple User-process knows it can proceed
with its course of action. A 202 reply is used in this case
with, for example, the reply text: "No storage allocation
necessary." If, on the other hand, the command requests a
non-site-specific action and is unimplemented, the response
is 502. A refinement of that is the 504 reply for a command
that IS implemented, but that requests an unimplemented
parameter.
Reply Codes by Function Groups
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 "=".)
119 Terminal not available, will try mailbox.
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 <scheme> is the preferred scheme.
120 Service ready in nnn minutes
220 Service ready for new user
221 Service closing TELNET connection
(logged out if appropriate)
421 Service not available, closing TELNET 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.
151 User not local; Will forward to <user>@<host>.
152 User Unknown; Mail will be forwarded by the operator.
250 Requested file action okay, completed.
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
354 Start mail input; end with <CR><LF>.<CR><LF>
Numeric Order List of Reply Codes
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 "=".)
119 Terminal not available, will try mailbox.
120 Service ready in nnn minutes
125 Data connection already open; transfer starting
150 File status okay; about to open data connection.
151 User not local; Will forward to <user>@<host>.
152 User Unknown; Mail will be forwarded by the operator.
200 Command okay
202 Command not implemented, superfluous at this site.
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 <scheme> is the preferred scheme.
220 Service ready for new user
221 Service closing TELNET connection
(logged out if appropriate)
225 Data connection open; no transfer in progress
226 Closing data connection;
requested file action successful (for example, file transfer
or file abort.)
227 Entering Passive Mode. h1,h2,h3,h4,p1,p2
230 User logged in, proceed
250 Requested file action okay, completed.
331 User name okay, need password
332 Need account for login
350 Requested file action pending further information
354 Start mail input; end with <CR><LF>.<CR><LF>
421 Service not available, closing TELNET connection.
This may be a reply to any command if the service knows it
must shut down.]
425 Can't open data connection
426 Connection closed; transfer aborted.
450 Requested file action not taken:
file unavailable (e.g. file busy)
451 Requested action aborted: local error in processing
452 Requested action not taken:
insufficient storage space in system
500 Syntax error, command unrecognized
[This may include errors such as command line too long.]
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command not implemented for that parameter
530 Not logged in
532 Need account for storing files
550 Requested action not taken:
file unavailable (e.g. file not found, no access)
551 Requested action aborted: page type unknown
552 Requested file action aborted:
exceeded storage allocation (for current directory or
dataset)
553 Requested action not taken:
file name not allowed
DECLARATIVE SPECIFICATIONS
MINIMUM IMPLEMENTATION
In order to make FTP workable without needless error messages, the
following minimum implementation is required for all servers:
TYPE - ASCII Non-print
MODE - Stream
STRUCTURE - File, Record
COMMANDS - USER, QUIT, PORT,
TYPE, MODE, STRU,
for the default values
RETR, STOR,
NOOP.
The default values for transfer parameters are:
TYPE - ASCII Non-print
MODE - Stream
STRU - File
All Hosts must accept the above as the standard defaults.
CONNECTIONS
The server protocol interpreter shall "listen" on Port L. The
user or user protocol interpreter shall initiate the full-duplex
TELNET connection. Server- and user- processes should follow the
conventions of the TELNET protocol as specified in the ARPA
Internet Protocol Handbook. Servers are under no obligation to
provide for editing of command lines and may specify that it be
done in the user Host. The TELNET connection shall be closed by
the server at the user's request after all transfers and replies
are completed.
The user-DTP must "listen" on the specified data port; this may be
the default user port (U) or a port specified in the PORT command.
The server shall initiate the data connection from his own default
data port (L-1) using the specified user data port. The direction
of the transfer and the port used will be determined by the FTP
service command.
When data is to be transferred between two servers, A and B (refer
to Figure 2), the user-PI, C, sets up TELNET connections with both
server-PI's. One of the servers, say A, is then sent a PASV
command telling him to "listen" on his data port rather than
initiate a connection when he receives a transfer service command.
When the user-PI receives an acknowledgment to the PASV command,
which includes the identity of the host and port being listened
on, the user-PI then sends A's port, a, to B in a PORT command; a
reply is returned. The user-PI may then send the corresponding
service commands to A and B. Server B initiates the connection
and the transfer proceeds. The command-reply sequence is listed
below where the messages are vertically synchronous but
horizontally asynchronous:
User-PI - Server A User-PI - Server B
------------------ ------------------
C->A : Connect C->B : Connect
C->A : PASV
A->C : 227 Entering Passive Mode. A1,A2,A3,A4,a1,a2
C->B : PORT A1,A2,A3,A4,a1,a2
B->C : 200 Okay
C->A : STOR C->B : RETR
B->A : Connect to HOST-A, PORT-a
The data connection shall be closed by the server under the
conditions described in the Section on Establishing Data
Connections. If the server wishes to close the connection after a
transfer where it is not required, he should do so immediately
after the file transfer is completed. He should not wait until
after a new transfer command is received because the user-process
will have already tested the data connection to see if it needs to
do a "listen"; (recall that the user must "listen" on a closed
data port BEFORE sending the transfer request). To prevent a race
condition here, the server sends a reply (226) after closing the
data connection (or if the connection is left open, a "file
transfer completed" reply (250) and the user-PI should wait for
one of these replies before issuing a new transfer command.
COMMANDS
The commands are TELNET character string transmitted over the
TELNET connections as described in the Section on FTP Commands.
The command functions and semantics are described in the Section
on Access Control Commands, Transfer Parameter Commands, FTP
Service Commands, and Miscellaneous Commands. The command syntax
is specified here.
The commands begin with a command code followed by an argument
field. The command codes are four or fewer alphabetic characters.
Upper and lower case alphabetic characters are to be treated
identically. Thus any of the following may represent the retrieve
command:
RETR Retr retr ReTr rETr
This also applies to any symbols representing parameter values,
such as A or a for ASCII TYPE. The command codes and the argument
fields are separated by one or more spaces.
The argument field consists of a variable length character string
ending with the character sequence <CRLF> (Carriage Return,
Linefeed) for NVT-ASCII representation; for other negotiated
languages a different end of line character might be used. It
should be noted that the server is to take NO action until the end
of line code is received.
The syntax is specified below in NVT-ASCII. All characters in the
argument field are ASCII characters including any ASCII
represented decimal integers. Square brackets denote an optional
argument field. If the option is not taken, the appropriate
default is implied.
The following are the FTP commands:
USER <SP> <username> <CRLF>
PASS <SP> <password> <CRLF>
ACCT <SP> <account information> <CRLF>
REIN <CRLF>
QUIT <CRLF>
PORT <SP> <Host-port> <CRLF>
PASV <CRLF>
TYPE <SP> <type code> <CRLF>
STRU <SP> <structure code> <CRLF>
MODE <SP> <mode code> <CRLF>
RETR <SP> <pathname> <CRLF>
STOR <SP> <pathname> <CRLF>
APPE <SP> <pathname> <CRLF>
MLFL [<SP> <ident>] <CRLF>
MAIL [<SP> <ident>] <CRLF>
MSND [<SP> <ident>] <CRLF>
MSOM [<SP> <ident>] <CRLF>
MSAM [<SP> <ident>] <CRLF>
MRSQ [<SP> <scheme>] <CRLF>
MRCP <SP> <ident> <CRLF>
ALLO <SP> <decimal integer>
[<SP> R <SP> <decimal integer>] <CRLF>
REST <SP> <marker> <CRLF>
RNFR <SP> <pathname> <CRLF>
RNTO <SP> <pathname> <CRLF>
ABOR <CRLF>
DELE <SP> <pathname> <CRLF>
CWD <SP> <pathname> <CRLF>
LIST [<SP> <pathname>] <CRLF>
NLST [<SP> <pathname>] <CRLF>
SITE <SP> <string> <CRLF>
STAT [<SP> <pathname>] <CRLF>
HELP [<SP> <string>] <CRLF>
NOOP <CRLF>
(next page on part 3)
