RFC765 (FTP) [part 2 of 3]
Brian Kantor
brian at sdcc3.UUCP
Sun Jan 6 10:25:49 AEST 1985
June 1980 IEN 149
File Transfer Protocol RFC 765
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
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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
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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.
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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:
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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
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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.
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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
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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
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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.
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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:
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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
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RFC 765 File Transfer Protocol
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
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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.
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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.
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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.
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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>
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The syntax of the above argument fields (using BNF notation where
applicable ) is:
<username> ::= <string>
<password> ::= <string>
<account information> ::= <string>
<string> ::= <char> | <char><string>
<char> ::= any of the 128 ASCII characters except <CR> and <LF>
<marker> ::= <pr string>
<pr string> ::= <pr char> | <pr char><pr string>
<pr char> ::= printable characters, any
ASCII code 33 through 126
<byte size> ::= any decimal integer 1 through 255
<Host-port> ::= <Host-number>,<Port-number>
<Host-number> ::= <number>,<number>,<number>,<number>
<Port-number> ::= <number>,<number>
<number> ::= any decimal integer 0 through 255
<ident> ::= <string>
<scheme> ::= R | T | ?
<form code> ::= N | T | C
<type code> ::= A [<SP> <form code>]
| E [<SP> <form code>]
| I
| L <SP> <byte size>
<structure code> ::= F | R | P
<mode code> ::= S | B | C
<pathname> ::= <string>
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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, he
should send a 120 "expected delay" reply immediately and a 220
reply when ready. The user will then know not to hang up if there
is a delay.
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
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June 1980 IEN 149
File Transfer Protocol RFC 765
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
Logout
QUIT
221
500
REIN
120
220
220
421
500, 502
Transfer parameters
PORT
200
500, 501, 421, 530
PASV
227
500, 501, 502, 421, 530
MODE, TYPE, 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
46
IEN 149 June 1980
RFC 765 File Transfer Protocol
STOR
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, 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
MLFL
125, 150, 151, 152
226, 250
425, 426, 451, 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, CWD
250
450, 550
500, 501, 502, 421, 530
47
June 1980 IEN 149
File Transfer Protocol RFC 765
ABOR
225, 226
500, 501, 502, 421
MAIL, MSND
151, 152
354
250
451, 552
354
250
451, 552
450, 550, 452, 553
500, 501, 502, 421, 530
MSOM, MSAM
119, 151, 152
354
250
451, 552
354
250
451, 552
450, 550, 452, 553
500, 501, 502, 421, 530
MRSQ
200, 215
500, 501, 502, 421, 530
MRCP
151, 152
200
200
450, 550, 452, 553
500, 501, 502, 503, 421
Informational commands
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
48
IEN 149 June 1980
RFC 765 File Transfer Protocol
NOOP
200
500 421
49
June 1980 IEN 149
File Transfer Protocol RFC 765
STATE DIAGRAMS
Here we present state diagrams for a very simple minded FTP
implementation. Only the first digit of the reply codes is used.
There is one state diagram for each group of FTP commands or command
sequences.
The command groupings were determined by constructing a model for
each command then collecting together the commands with structurally
identical models.
For each command or command sequence there are three possible
outcomes: success (S), failure (F), and error (E). In the state
diagrams below we use the symbol B for "begin", and the symbol W for
"wait for reply".
We first present the diagram that represents the largest group of FTP
commands:
1,3 +---+
----------->| E |
| +---+
|
+---+ cmd +---+ 2 +---+
| B |---------->| W |---------->| S |
+---+ +---+ +---+
|
| 4,5 +---+
----------->| F |
+---+
This diagram models the commands:
ABOR, ALLO, DELE, CWD, HELP, MODE, MRCP, MRSQ, NOOP, PASV,
QUIT, SITE, PORT, STAT, STRU, TYPE.
50
IEN 149 June 1980
RFC 765 File Transfer Protocol
The other large group of commands is represented by a very similar
diagram:
3 +---+
----------->| E |
| +---+
|
+---+ cmd +---+ 2 +---+
| B |---------->| W |---------->| S |
+---+ --->+---+ +---+
| | |
| | | 4,5 +---+
| 1 | ----------->| F |
----- +---+
This diagram models the commands:
APPE, LIST, MLFL, NLST, REIN, RETR, STOR.
Note that this second model could also be used to represent the first
group of commands, the only difference being that in the first group
the 100 series replies are unexpected and therefore treated as error,
while the second group expects (some may require) 100 series replies.
The remaining diagrams model command sequences, perhaps the simplest
of these is the rename sequence:
+---+ RNFR +---+ 1,2 +---+
| B |---------->| W |---------->| E |
+---+ +---+ -->+---+
| | |
3 | | 4,5 |
-------------- ------ |
| | | +---+
| ------------->| S |
| | 1,3 | | +---+
| 2| --------
| | | |
V | | |
+---+ RNTO +---+ 4,5 ----->+---+
| |---------->| W |---------->| F |
+---+ +---+ +---+
51
June 1980 IEN 149
File Transfer Protocol RFC 765
A very similar diagram models the Mail and Send commands:
---- 1
| |
+---+ cmd -->+---+ 2 +---+
| B |---------->| W |---------->| E |
+---+ +---+ -->+---+
| | |
3 | | 4,5 |
-------------- ------ |
| | | +---+
| ------------->| S |
| | 1,3 | | +---+
| 2| --------
| | | |
V | | |
+---+ text +---+ 4,5 ----->+---+
| |---------->| W |---------->| F |
+---+ +---+ +---+
This diagram models the commands:
MAIL, MSND, MSOM, MSAM.
Note that the "text" here is a series of lines sent from the user
to the server with no response expected until the last line is
sent, recall that the last line must consist only of a single
period.
52
IEN 149 June 1980
RFC 765 File Transfer Protocol
The next diagram is a simple model of the Restart command:
+---+ REST +---+ 1,2 +---+
| B |---------->| W |---------->| E |
+---+ +---+ -->+---+
| | |
3 | | 4,5 |
-------------- ------ |
| | | +---+
| ------------->| S |
| | 3 | | +---+
| 2| --------
| | | |
V | | |
+---+ cmd +---+ 4,5 ----->+---+
| |---------->| W |---------->| F |
+---+ -->+---+ +---+
| |
| 1 |
------
Where "cmd" is APPE, STOR, RETR, or MLFL.
We note that the above three models are similar, in fact the Mail
diagram and the Rename diagram are structurally identical. The
Restart differs from the other two only in the treatment of 100
series replies at the second stage.
53
June 1980 IEN 149
File Transfer Protocol RFC 765
The most complicated diagram is for the Login sequence:
1
+---+ USER +---+------------->+---+
| B |---------->| W | 2 ---->| E |
+---+ +---+------ | -->+---+
| | | | |
3 | | 4,5 | | |
-------------- ----- | | |
| | | | |
| | | | |
| --------- |
| 1| | | |
V | | | |
+---+ PASS +---+ 2 | ------>+---+
| |---------->| W |------------->| S |
+---+ +---+ ---------->+---+
| | | | |
3 | |4,5| | |
-------------- -------- |
| | | | |
| | | | |
| -----------
| 1,3| | | |
V | 2| | |
+---+ ACCT +---+-- | ----->+---+
| |---------->| W | 4,5 -------->| F |
+---+ +---+------------->+---+
54
IEN 149 June 1980
RFC 765 File Transfer Protocol
Finally we present a generalized diagram that could be used to model
the command and reply interchange:
------------------------------------
| |
Begin | |
| V |
| +---+ cmd +---+ 2 +---+ |
-->| |------->| |---------->| | |
| | | W | | S |-----|
-->| | -->| |----- | | |
| +---+ | +---+ 4,5 | +---+ |
| | | | | | |
| | | 1| |3 | +---+ |
| | | | | | | | |
| | ---- | ---->| F |-----
| | | | |
| | | +---+
-------------------
|
|
V
End
55
June 1980 IEN 149
File Transfer Protocol RFC 765
TYPICAL FTP SCENARIO
User at Host U wanting to transfer files to/from Host S:
In general the user will communicate to the server via a mediating
user-FTP process. The following may be a typical scenario. The
user-FTP prompts are shown in parentheses, '---->' represents
commands from Host U to Host S, and '<----' represents replies from
Host S to Host U.
LOCAL COMMANDS BY USER ACTION INVOLVED
ftp (host) multics<CR> Connect to Host S, port L,
establishing TELNET connections
<---- 220 Service ready <CRLF>
username Doe <CR> USER Doe<CRLF>---->
<---- 331 User name ok,
need password<CRLF>
password mumble <CR> PASS mumble<CRLF>---->
<---- 230 User logged in.<CRLF>
retrieve (local type) ASCII<CR>
(local pathname) test 1 <CR> User-FTP opens local file in ASCII.
(for.pathname) test.pl1<CR> RETR test.pl1<CRLF> ---->
<---- 150 File status okay;
about to open data connection
Server makes data connection
to port U
<CRLF>
<---- 226 Closing data connection,
file transfer successful<CRLF>
type Image<CR> TYPE I<CRLF> ---->
<---- 200 Command OK<CRLF>
store (local type) image<CR>
(local pathname) file dump<CR> User-FTP opens local file in Image.
(for.pathname) >udd>cn>fd<CR> STOR >udd>cn>fd<CRLF> ---->
<---- 450 Access denied<CRLF>
terminate QUIT <CRLF> ---->
Server closes all
connections.
56
IEN 149 June 1980
RFC 765 File Transfer Protocol
CONNECTION ESTABLISHMENT
The FTP control connection is established via TCP between the user
process port U and the server process port L. This protocol is
assigned the service port 21 (25 octal), that is L=21.
57
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