Network Working Group J. Postel
Request for Comments: 959 J. Reynolds
ISI
Obsoletes RFC: 765 (IEN 149) October 1985
FILE TRANSFER PROTOCOL (FTP)
Status of this Memo
This memo is the official specification of the File Transfer
Protocol (FTP). Distribution of this memo is unlimited.
The following new optional commands are included in this edition of
the specification:
CDUP (Change to Parent Directory), SMNT (Structure Mount), STOU
(Store Unique), RMD (Remove Directory), MKD (Make Directory), PWD
(Print Directory), and SYST (System).
Note that this specification is compatible with the previous edition.
1. INTRODUCTION
The objectives of FTP are 1) to promote sharing of files (computer
programs and/or data), 2) to encourage indirect or implicit (via
programs) use of remote computers, 3) to shield a user from
variations in file storage systems among hosts, and 4) to transfer
data reliably and efficiently. FTP, though usable directly by a user
at a terminal, is designed mainly for use by programs.
The attempt in this specification is to satisfy the diverse needs of
users of maxi-hosts, mini-hosts, personal workstations, and TACs,
with a simple, and easily implemented protocol design.
This paper assumes knowledge of the Transmission Control Protocol
(TCP) [2] and the Telnet Protocol [3]. These documents are contained
in the ARPA-Internet protocol handbook [1].
2. OVERVIEW
In this section, the history, the terminology, and the FTP model are
discussed. The terms defined in this section are only those that
have special significance in FTP. Some of the terminology is very
specific to the FTP model; some readers may wish to turn to the
section on the FTP model while reviewing the terminology.
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File Transfer Protocol
2.1. HISTORY
FTP has had a long evolution over the years. Appendix III is a
chronological compilation of Request for Comments documents
relating to FTP. These include the first proposed file transfer
mechanisms in 1971 that were developed for implementation on hosts
at M.I.T. (RFC 114), plus comments and discussion in RFC 141.
RFC 172 provided a user-level oriented protocol for file transfer
between host computers (including terminal IMPs). A revision of
this as RFC 265, restated FTP for additional review, while RFC 281
suggested further changes. The use of a "Set Data Type"
transaction was proposed in RFC 294 in January 1982.
RFC 354 obsoleted RFCs 264 and 265. The File Transfer Protocol
was now defined as a protocol for file transfer between HOSTs on
the ARPANET, with the primary function of FTP defined as
transfering files efficiently and reliably among hosts and
allowing the convenient use of remote file storage capabilities.
RFC 385 further commented on errors, emphasis points, and
additions to the protocol, while RFC 414 provided a status report
on the working server and user FTPs. RFC 430, issued in 1973,
(among other RFCs too numerous to mention) presented further
comments on FTP. Finally, an "official" FTP document was
published as RFC 454.
By July 1973, considerable changes from the last versions of FTP
were made, but the general structure remained the same. RFC 542
was published as a new "official" specification to reflect these
changes. However, many implementations based on the older
specification were not updated.
In 1974, RFCs 607 and 614 continued comments on FTP. RFC 624
proposed further design changes and minor modifications. In 1975,
RFC 686 entitled, "Leaving Well Enough Alone", discussed the
differences between all of the early and later versions of FTP.
RFC 691 presented a minor revision of RFC 686, regarding the
subject of print files.
Motivated by the transition from the NCP to the TCP as the
underlying protocol, a phoenix was born out of all of the above
efforts in RFC 765 as the specification of FTP for use on TCP.
This current edition of the FTP specification is intended to
correct some minor documentation errors, to improve the
explanation of some protocol features, and to add some new
optional commands.
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File Transfer Protocol
In particular, the following new optional commands are included in
this edition of the specification:
CDUP - Change to Parent Directory
SMNT - Structure Mount
STOU - Store Unique
RMD - Remove Directory
MKD - Make Directory
PWD - Print Directory
SYST - System
This specification is compatible with the previous edition. A
program implemented in conformance to the previous specification
should automatically be in conformance to this specification.
2.2. TERMINOLOGY
ASCII
The ASCII character set is as defined in the ARPA-Internet
Protocol Handbook. In FTP, ASCII characters are defined to be
the lower half of an eight-bit code set (i.e., the most
significant bit is zero).
access controls
Access controls define users' access privileges to the use of a
system, and to the files in that system. Access controls are
necessary to prevent unauthorized or accidental use of files.
It is the prerogative of a server-FTP process to invoke access
controls.
byte size
There are two byte sizes of interest in FTP: the logical byte
size of the file, and the transfer byte size used for the
transmission of the data. The transfer byte size is always 8
bits. The transfer byte size is not necessarily the byte size
in which data is to be stored in a system, nor the logical byte
size for interpretation of the structure of the data.
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File Transfer Protocol
control connection
The communication path between the USER-PI and SERVER-PI for
the exchange of commands and replies. This connection follows
the Telnet Protocol.
data connection
A full duplex connection over which data is transferred, in a
specified mode and type. The data transferred may be a part of
a file, an entire file or a number of files. The path may be
between a server-DTP and a user-DTP, or between two
server-DTPs.
data port
The passive data transfer process "listens" on the data port
for a connection from the active transfer process in order to
open the data connection.
DTP
The data transfer process establishes and manages the data
connection. The DTP can be passive or active.
End-of-Line
The end-of-line sequence defines the separation of printing
lines. The sequence is Carriage Return, followed by Line Feed.
EOF
The end-of-file condition that defines the end of a file being
transferred.
EOR
The end-of-record condition that defines the end of a record
being transferred.
error recovery
A procedure that allows a user to recover from certain errors
such as failure of either host system or transfer process. In
FTP, error recovery may involve restarting a file transfer at a
given checkpoint.
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FTP commands
A set of commands that comprise the control information flowing
from the user-FTP to the server-FTP process.
file
An ordered set of computer data (including programs), of
arbitrary length, uniquely identified by a pathname.
mode
The mode in which data is to be transferred via the data
connection. The mode defines the data format during transfer
including EOR and EOF. The transfer modes defined in FTP are
described in the Section on Transmission Modes.
NVT
The Network Virtual Terminal as defined in the Telnet Protocol.
NVFS
The Network Virtual File System. A concept which defines a
standard network file system with standard commands and
pathname conventions.
page
A file may be structured as a set of independent parts called
pages. FTP supports the transmission of discontinuous files as
independent indexed pages.
pathname
Pathname is defined to be the character string which must be
input to a file system by a user in order to identify a file.
Pathname normally contains device and/or directory names, and
file name specification. FTP does not yet specify a standard
pathname convention. Each user must follow the file naming
conventions of the file systems involved in the transfer.
PI
The protocol interpreter. The user and server sides of the
protocol have distinct roles implemented in a user-PI and a
server-PI.
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File Transfer Protocol
record
A sequential file may be structured as a number of contiguous
parts called records. Record structures are supported by FTP
but a file need not have record structure.
reply
A reply is an acknowledgment (positive or negative) sent from
server to user via the control connection in response to FTP
commands. The general form of a reply is a completion code
(including error codes) followed by a text string. The codes
are for use by programs and the text is usually intended for
human users.
server-DTP
The data transfer process, in its normal "active" state,
establishes the data connection with the "listening" data port.
It sets up parameters for transfer and storage, and transfers
data on command from its PI. The DTP can be placed in a
"passive" state to listen for, rather than initiate a
connection on the data port.
server-FTP process
A process or set of processes which perform the function of
file transfer in cooperation with a user-FTP process and,
possibly, another server. The functions consist of a protocol
interpreter (PI) and a data transfer process (DTP).
server-PI
The server protocol interpreter "listens" on Port L for a
connection from a user-PI and establishes a control
communication connection. It receives standard FTP commands
from the user-PI, sends replies, and governs the server-DTP.
type
The data representation type used for data transfer and
storage. Type implies certain transformations between the time
of data storage and data transfer. The representation types
defined in FTP are described in the Section on Establishing
Data Connections.
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user
A person or a process on behalf of a person wishing to obtain
file transfer service. The human user may interact directly
with a server-FTP process, but use of a user-FTP process is
preferred since the protocol design is weighted towards
automata.
user-DTP
The data transfer process "listens" on the data port for a
connection from a server-FTP process. If two servers are
transferring data between them, the user-DTP is inactive.
user-FTP process
A set of functions including a protocol interpreter, a data
transfer process and a user interface which together perform
the function of file transfer in cooperation with one or more
server-FTP processes. The user interface allows a local
language to be used in the command-reply dialogue with the
user.
user-PI
The user protocol interpreter initiates the control connection
from its port U to the server-FTP process, initiates FTP
commands, and governs the user-DTP if that process is part of
the file transfer.
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2.3. THE FTP MODEL
With the above definitions in mind, the following model (shown in
Figure 1) may be diagrammed for an FTP service.
-------------
|/---------\|
|| User || --------
||Interface|<--->| User |
|\----^----/| --------
---------- | | |
|/------\| FTP Commands |/----V----\|
||Server|<---------------->| User ||
|| PI || FTP Replies || PI ||
|\--^---/| |\----^----/|
| | | | | |
-------- |/--V---\| Data |/----V----\| --------
| File |<--->|Server|<---------------->| User |<--->| File |
|System| || DTP || Connection || DTP || |System|
-------- |\------/| |\---------/| --------
---------- -------------
Server-FTP USER-FTP
NOTES: 1. The data connection may be used in either direction.
2. The data connection need not exist all of the time.
Figure 1 Model for FTP Use
In the model described in Figure 1, the user-protocol interpreter
initiates the control connection. The control connection follows
the Telnet protocol. At the initiation of the user, standard FTP
commands are generated by the user-PI and transmitted to the
server process via the control connection. (The user may
establish a direct control connection to the server-FTP, from a
TAC terminal for example, and generate standard FTP commands
independently, bypassing the user-FTP process.) Standard replies
are sent from the server-PI to the user-PI over the control
connection in response to the commands.
The FTP commands specify the parameters for the data connection
(data port, transfer mode, representation type, and structure) and
the nature of file system operation (store, retrieve, append,
delete, etc.). The user-DTP or its designate should "listen" on
the specified data port, and the server initiate the data
connection and data transfer in accordance with the specified
parameters. It should be noted that the data port need not be in
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the same host that initiates the FTP commands via the control
connection, but the user or the user-FTP process must ensure a
"listen" on the specified data port. It ought to also be noted
that the data connection may be used for simultaneous sending and
receiving.
In another situation a user might wish to transfer files between
two hosts, neither of which is a local host. The user sets up
control connections to the two servers and then arranges for a
data connection between them. In this manner, control information
is passed to the user-PI but data is transferred between the
server data transfer processes. Following is a model of this
server-server interaction.
Control ------------ Control
---------->| User-FTP |<-----------
| | User-PI | |
| | "C" | |
V ------------ V
-------------- --------------
| Server-FTP | Data Connection | Server-FTP |
| "A" |<---------------------->| "B" |
-------------- Port (A) Port (B) --------------
Figure 2
The protocol requires that the control connections be open while
data transfer is in progress. It is the responsibility of the
user to request the closing of the control connections when
finished using the FTP service, while it is the server who takes
the action. The server may abort data transfer if the control
connections are closed without command.
The Relationship between FTP and Telnet:
The FTP uses the Telnet protocol on the control connection.
This can be achieved in two ways: first, the user-PI or the
server-PI may implement the rules of the Telnet Protocol
directly in their own procedures; or, second, the user-PI or
the server-PI may make use of the existing Telnet module in the
system.
Ease of implementaion, sharing code, and modular programming
argue for the second approach. Efficiency and independence
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argue for the first approach. In practice, FTP relies on very
little of the Telnet Protocol, so the first approach does not
necessarily involve a large amount of code.
3. DATA TRANSFER FUNCTIONS
Files are transferred only via the data connection. The control
connection is used for the transfer of commands, which describe the
functions to be performed, and the replies to these commands (see the
Section on FTP Replies). Several commands are concerned with the
transfer of data between hosts. These data transfer commands include
the MODE command which specify how the bits of the data are to be
transmitted, and the STRUcture and TYPE commands, which are used to
define the way in which the data are to be represented. The
transmission and representation are basically independent but the
"Stream" transmission mode is dependent on the file structure
attribute and if "Compressed" transmission mode is used, the nature
of the filler byte depends on the representation type.
3.1. DATA REPRESENTATION AND STORAGE
Data is transferred from a storage device in the sending host to a
storage device in the receiving host. Often it is necessary to
perform certain transformations on the data because data storage
representations in the two systems are different. For example,
NVT-ASCII has different data storage representations in different
systems. DEC TOPS-20s's generally store NVT-ASCII as five 7-bit
ASCII characters, left-justified in a 36-bit word. IBM Mainframe's
store NVT-ASCII as 8-bit EBCDIC codes. Multics stores NVT-ASCII
as four 9-bit characters in a 36-bit word. It is desirable to
convert characters into the standard NVT-ASCII representation when
transmitting text between dissimilar systems. The sending and
receiving sites would have to perform the necessary
transformations between the standard representation and their
internal representations.
A different problem in representation arises when transmitting
binary data (not character codes) between host systems with
different word lengths. It is not always clear how the sender
should send data, and the receiver store it. For example, when
transmitting 32-bit bytes from a 32-bit word-length system to a
36-bit word-length system, it may be desirable (for reasons of
efficiency and usefulness) to store the 32-bit bytes
right-justified in a 36-bit word in the latter system. In any
case, the user should have the option of specifying data
representation and transformation functions. It should be noted
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File Transfer Protocol
that FTP provides for very limited data type representations.
Transformations desired beyond this limited capability should be
performed by the user directly.
3.1.1. DATA TYPES
Data representations are handled in FTP by a user specifying a
representation type. This type may implicitly (as in ASCII or
EBCDIC) or explicitly (as in Local byte) define a byte size for
interpretation which is referred to as the "logical byte size."
Note that this has nothing to do with the byte size used for
transmission over the data connection, called the "transfer
byte size", and the two should not be confused. For example,
NVT-ASCII has a logical byte size of 8 bits. If the type is
Local byte, then the TYPE command has an obligatory second
parameter specifying the logical byte size. The transfer byte
size is always 8 bits.
3.1.1.1. ASCII TYPE
This is the default type and must be accepted by all FTP
implementations. It is intended primarily for the transfer
of text files, except when both hosts would find the EBCDIC
type more convenient.
The sender converts the data from an internal character
representation to the standard 8-bit NVT-ASCII
representation (see the Telnet specification). The receiver
will convert the data from the standard form to his own
internal form.
In accordance with the NVT standard, the <CRLF> sequence
should be used where necessary to denote the end of a line
of text. (See the discussion of file structure at the end
of the Section on Data Representation and Storage.)
Using the standard NVT-ASCII representation means that data
must be interpreted as 8-bit bytes.
The Format parameter for ASCII and EBCDIC types is discussed
below.
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3.1.1.2. EBCDIC TYPE
This type is intended for efficient transfer between hosts
which use EBCDIC for their internal character
representation.
For transmission, the data are represented as 8-bit EBCDIC
characters. The character code is the only difference
between the functional specifications of EBCDIC and ASCII
types.
End-of-line (as opposed to end-of-record--see the discussion
of structure) will probably be rarely used with EBCDIC type
for purposes of denoting structure, but where it is
necessary the <NL> character should be used.
3.1.1.3. IMAGE TYPE
The data are sent as contiguous bits which, for transfer,
are packed into the 8-bit transfer bytes. The receiving
site must store the data as contiguous bits. The structure
of the storage system might necessitate the padding of the
file (or of each record, for a record-structured file) to
some convenient boundary (byte, word or block). This
padding, which must be all zeros, may occur only at the end
of the file (or at the end of each record) and there must be
a way of identifying the padding bits so that they may be
stripped off if the file is retrieved. The padding
transformation should be well publicized to enable a user to
process a file at the storage site.
Image type is intended for the efficient storage and
retrieval of files and for the transfer of binary data. It
is recommended that this type be accepted by all FTP
implementations.
3.1.1.4. LOCAL TYPE
The data is transferred in logical bytes of the size
specified by the obligatory second parameter, Byte size.
The value of Byte size must be a decimal integer; there is
no default value. The logical byte size is not necessarily
the same as the transfer byte size. If there is a
difference in byte sizes, then the logical bytes should be
packed contiguously, disregarding transfer byte boundaries
and with any necessary padding at the end.
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When the data reaches the receiving host, it will be
transformed in a manner dependent on the logical byte size
and the particular host. This transformation must be
invertible (i.e., an identical file can be retrieved if the
same parameters are used) and should be well publicized by
the FTP implementors.
For example, a user sending 36-bit floating-point numbers to
a host with a 32-bit word could send that data as Local byte
with a logical byte size of 36. The receiving host would
then be expected to store the logical bytes so that they
could be easily manipulated; in this example putting the
36-bit logical bytes into 64-bit double words should
suffice.
In another example, a pair of hosts with a 36-bit word size
may send data to one another in words by using TYPE L 36.
The data would be sent in the 8-bit transmission bytes
packed so that 9 transmission bytes carried two host words.
3.1.1.5. FORMAT CONTROL
The types ASCII and EBCDIC also take a second (optional)
parameter; this is to indicate what kind of vertical format
control, if any, is associated with a file. The following
data representation types are defined in FTP:
A character file may be transferred to a host for one of
three purposes: for printing, for storage and later
retrieval, or for processing. If a file is sent for
printing, the receiving host must know how the vertical
format control is represented. In the second case, it must
be possible to store a file at a host and then retrieve it
later in exactly the same form. Finally, it should be
possible to move a file from one host to another and process
the file at the second host without undue trouble. A single
ASCII or EBCDIC format does not satisfy all these
conditions. Therefore, these types have a second parameter
specifying one of the following three formats:
3.1.1.5.1. NON PRINT
This is the default format to be used if the second
(format) parameter is omitted. Non-print format must be
accepted by all FTP implementations.
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File Transfer Protocol
The file need contain no vertical format information. If
it is passed to a printer process, this process may
assume standard values for spacing and margins.
Normally, this format will be used with files destined
for processing or just storage.
3.1.1.5.2. TELNET FORMAT CONTROLS
The file contains ASCII/EBCDIC vertical format controls
(i.e., <CR>, <LF>, <NL>, <VT>, <FF>) which the printer
process will interpret appropriately. <CRLF>, in exactly
this sequence, also denotes end-of-line.
3.1.1.5.2. CARRIAGE CONTROL (ASA)
The file contains ASA (FORTRAN) vertical format control
characters. (See RFC 740 Appendix C; and Communications
of the ACM, Vol. 7, No. 10, p. 606, October 1964.) In a
line or a record formatted according to the ASA Standard,
the first character is not to be printed. Instead, it
should be used to determine the vertical movement of the
paper which should take place before the rest of the
record is printed.
The ASA Standard specifies the following control
characters:
Character Vertical Spacing
blank Move paper up one line
0 Move paper up two lines
1 Move paper to top of next page
+ No movement, i.e., overprint
Clearly there must be some way for a printer process to
distinguish the end of the structural entity. If a file
has record structure (see below) this is no problem;
records will be explicitly marked during transfer and
storage. If the file has no record structure, the <CRLF>
end-of-line sequence is used to separate printing lines,
but these format effectors are overridden by the ASA
controls.
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3.1.2. DATA STRUCTURES
In addition to different representation types, FTP allows the
structure of a file to be specified. Three file structures are
defined in FTP:
file-structure, where there is no internal structure and
the file is considered to be a
continuous sequence of data bytes,
record-structure, where the file is made up of sequential
records,
and page-structure, where the file is made up of independent
indexed pages.
File-structure is the default to be assumed if the STRUcture
command has not been used but both file and record structures
must be accepted for "text" files (i.e., files with TYPE ASCII
or EBCDIC) by all FTP implementations. The structure of a file
will affect both the transfer mode of a file (see the Section
on Transmission Modes) and the interpretation and storage of
the file.
The "natural" structure of a file will depend on which host
stores the file. A source-code file will usually be stored on
an IBM Mainframe in fixed length records but on a DEC TOPS-20
as a stream of characters partitioned into lines, for example
by <CRLF>. If the transfer of files between such disparate
sites is to be useful, there must be some way for one site to
recognize the other's assumptions about the file.
With some sites being naturally file-oriented and others
naturally record-oriented there may be problems if a file with
one structure is sent to a host oriented to the other. If a
text file is sent with record-structure to a host which is file
oriented, then that host should apply an internal
transformation to the file based on the record structure.
Obviously, this transformation should be useful, but it must
also be invertible so that an identical file may be retrieved
using record structure.
In the case of a file being sent with file-structure to a
record-oriented host, there exists the question of what
criteria the host should use to divide the file into records
which can be processed locally. If this division is necessary,
the FTP implementation should use the end-of-line sequence,
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File Transfer Protocol
<CRLF> for ASCII, or <NL> for EBCDIC text files, as the
delimiter. If an FTP implementation adopts this technique, it
must be prepared to reverse the transformation if the file is
retrieved with file-structure.
3.1.2.1. FILE STRUCTURE
File structure is the default to be assumed if the STRUcture
command has not been used.
In file-structure there is no internal structure and the
file is considered to be a continuous sequence of data
bytes.
3.1.2.2. RECORD STRUCTURE
Record structures must be accepted for "text" files (i.e.,
files with TYPE ASCII or EBCDIC) by all FTP implementations.
In record-structure the file is made up of sequential
records.
3.1.2.3. PAGE STRUCTURE
To transmit files that are discontinuous, FTP defines a page
structure. Files of this type are sometimes known as
"random access files" or even as "holey files". In these
files there is sometimes other information associated with
the file as a whole (e.g., a file descriptor), or with a
section of the file (e.g., page access controls), or both.
In FTP, the sections of the file are called pages.
To provide for various page sizes and associated
information, each page is sent with a page header. The page
header has the following defined fields:
Header Length
The number of logical bytes in the page header
including this byte. The minimum header length is 4.
Page Index
The logical page number of this section of the file.
This is not the transmission sequence number of this
page, but the index used to identify this page of the
file.
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Data Length
The number of logical bytes in the page data. The
minimum data length is 0.
Page Type
The type of page this is. The following page types
are defined:
0 = Last Page
This is used to indicate the end of a paged
structured transmission. The header length must
be 4, and the data length must be 0.
1 = Simple Page
This is the normal type for simple paged files
with no page level associated control
information. The header length must be 4.
2 = Descriptor Page
This type is used to transmit the descriptive
information for the file as a whole.
3 = Access Controlled Page
This type includes an additional header field
for paged files with page level access control
information. The header length must be 5.
Optional Fields
Further header fields may be used to supply per page
control information, for example, per page access
control.
All fields are one logical byte in length. The logical byte
size is specified by the TYPE command. See Appendix I for
further details and a specific case at the page structure.
A note of caution about parameters: a file must be stored and
retrieved with the same parameters if the retrieved version is to
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be identical to the version originally transmitted. Conversely,
FTP implementations must return a file identical to the original
if the parameters used to store and retrieve a file are the same.
3.2. ESTABLISHING DATA CONNECTIONS
The mechanics of transferring data consists of setting up the data
connection to the appropriate ports and choosing the parameters
for transfer. Both the user and the server-DTPs have a default
data port. The user-process default data port is the same as the
control connection port (i.e., U). The server-process default
data port is the port adjacent to the control connection port
(i.e., L-1).
The transfer byte size is 8-bit bytes. This byte size is relevant
only for the actual transfer of the data; it has no bearing on
representation of the data within a host's file system.
The passive data transfer process (this may be a user-DTP or a
second server-DTP) shall "listen" on the data port prior to
sending a transfer request command. The FTP request command
determines the direction of the data transfer. The server, upon
receiving the transfer request, will initiate the data connection
to the port. When the connection is established, the data
transfer begins between DTP's, and the server-PI sends a
confirming reply to the user-PI.
Every FTP implementation must support the use of the default data
ports, and only the USER-PI can initiate a change to non-default
ports.
It is possible for the user to specify an alternate data port by
use of the PORT command. The user may want a file dumped on a TAC
line printer or retrieved from a third party host. In the latter
case, the user-PI sets up control connections with both
server-PI's. One server is then told (by an FTP command) to
"listen" for a connection which the other will initiate. The
user-PI sends one server-PI a PORT command indicating the data
port of the other. Finally, both are sent the appropriate
transfer commands. The exact sequence of commands and replies
sent between the user-controller and the servers is defined in the
Section on FTP Replies.
In general, it is the server's responsibility to maintain the data
connection--to initiate it and to close it. The exception to this
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File Transfer Protocol
is when the user-DTP is sending the data in a transfer mode that
requires the connection to be closed to indicate EOF. The server
MUST close the data connection under the following conditions:
1. The server has completed sending data in a transfer mode
that requires a close to indicate EOF.
2. The server receives an ABORT command from the user.
3. The port specification is changed by a command from the
user.
4. The control connection is closed legally or otherwise.
5. An irrecoverable error condition occurs.
Otherwise the close is a server option, the exercise of which the
server must indicate to the user-process by either a 250 or 226
reply only.
3.3. DATA CONNECTION MANAGEMENT
Default Data Connection Ports: All FTP implementations must
support use of the default data connection ports, and only the
User-PI may initiate the use of non-default ports.
Negotiating Non-Default Data Ports: The User-PI may specify a
non-default user side data port with the PORT command. The
User-PI may request the server side to identify a non-default
server side data port with the PASV command. Since a connection
is defined by the pair of addresses, either of these actions is
enough to get a different data connection, still it is permitted
to do both commands to use new ports on both ends of the data
connection.
Reuse of the Data Connection: When using the stream mode of data
transfer the end of the file must be indicated by closing the
connection. This causes a problem if multiple files are to be
transfered in the session, due to need for TCP to hold the
connection record for a time out period to guarantee the reliable
communication. Thus the connection can not be reopened at once.
There are two solutions to this problem. The first is to
negotiate a non-default port. The second is to use another
transfer mode.
A comment on transfer modes. The stream transfer mode is
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File Transfer Protocol
inherently unreliable, since one can not determine if the
connection closed prematurely or not. The other transfer modes
(Block, Compressed) do not close the connection to indicate the
end of file. They have enough FTP encoding that the data
connection can be parsed to determine the end of the file.
Thus using these modes one can leave the data connection open
for multiple file transfers.
3.4. TRANSMISSION MODES
The next consideration in transferring data is choosing the
appropriate transmission mode. There are three modes: one which
formats the data and allows for restart procedures; one which also
compresses the data for efficient transfer; and one which passes
the data with little or no processing. In this last case the mode
interacts with the structure attribute to determine the type of
processing. In the compressed mode, the representation type
determines the filler byte.
All data transfers must be completed with an end-of-file (EOF)
which may be explicitly stated or implied by the closing of the
data connection. For files with record structure, all the
end-of-record markers (EOR) are explicit, including the final one.
For files transmitted in page structure a "last-page" page type is
used.
NOTE: In the rest of this section, byte means "transfer byte"
except where explicitly stated otherwise.
For the purpose of standardized transfer, the sending host will
translate its internal end of line or end of record denotation
into the representation prescribed by the transfer mode and file
structure, and the receiving host will perform the inverse
translation to its internal denotation. An IBM Mainframe record
count field may not be recognized at another host, so the
end-of-record information may be transferred as a two byte control
code in Stream mode or as a flagged bit in a Block or Compressed
mode descriptor. End-of-line in an ASCII or EBCDIC file with no
record structure should be indicated by <CRLF> or <NL>,
respectively. Since these transformations imply extra work for
some systems, identical systems transferring non-record structured
text files might wish to use a binary representation and stream
mode for the transfer.
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File Transfer Protocol
The following transmission modes are defined in FTP:
3.4.1. STREAM MODE
The data is transmitted as a stream of bytes. There is no
restriction on the representation type used; record structures
are allowed.
In a record structured file EOR and EOF will each be indicated
by a two-byte control code. The first byte of the control code
will be all ones, the escape character. The second byte will
have the low order bit on and zeros elsewhere for EOR and the
second low order bit on for EOF; that is, the byte will have
value 1 for EOR and value 2 for EOF. EOR and EOF may be
indicated together on the last byte transmitted by turning both
low order bits on (i.e., the value 3). If a byte of all ones
was intended to be sent as data, it should be repeated in the
second byte of the control code.
If the structure is a file structure, the EOF is indicated by
the sending host closing the data connection and all bytes are
data bytes.
3.4.2. BLOCK MODE
The file is transmitted as a series of data blocks preceded by
one or more header bytes. The header bytes contain a count
field, and descriptor code. The count field indicates the
total length of the data block in bytes, thus marking the
beginning of the next data block (there are no filler bits).
The descriptor code defines: last block in the file (EOF) last
block in the record (EOR), restart marker (see the Section on
Error Recovery and Restart) or suspect data (i.e., the data
being transferred is suspected of errors and is not reliable).
This last code is NOT intended for error control within FTP.
It is motivated by the desire of sites exchanging certain types
of data (e.g., seismic or weather data) to send and receive all
the data despite local errors (such as "magnetic tape read
errors"), but to indicate in the transmission that certain
portions are suspect). Record structures are allowed in this
mode, and any representation type may be used.
The header consists of the three bytes. Of the 24 bits of
header information, the 16 low order bits shall represent byte
count, and the 8 high order bits shall represent descriptor
codes as shown below.
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File Transfer Protocol
Block Header
+----------------+----------------+----------------+
| Descriptor | Byte Count |
| 8 bits | 16 bits |
+----------------+----------------+----------------+
The descriptor codes are indicated by bit flags in the
descriptor byte. Four codes have been assigned, where each
code number is the decimal value of the corresponding bit in
the byte.
Code Meaning
128 End of data block is EOR
64 End of data block is EOF
32 Suspected errors in data block
16 Data block is a restart marker
With this encoding, more than one descriptor coded condition
may exist for a particular block. As many bits as necessary
may be flagged.
The restart marker is embedded in the data stream as an
integral number of 8-bit bytes representing printable
characters in the language being used over the control
connection (e.g., default--NVT-ASCII). <SP> (Space, in the
appropriate language) must not be used WITHIN a restart marker.
For example, to transmit a six-character marker, the following
would be sent:
+--------+--------+--------+
|Descrptr| Byte count |
|code= 16| = 6 |
+--------+--------+--------+
+--------+--------+--------+
| Marker | Marker | Marker |
| 8 bits | 8 bits | 8 bits |
+--------+--------+--------+
+--------+--------+--------+
| Marker | Marker | Marker |
| 8 bits | 8 bits | 8 bits |
+--------+--------+--------+
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File Transfer Protocol
3.4.3. COMPRESSED MODE
There are three kinds of information to be sent: regular data,
sent in a byte string; compressed data, consisting of
replications or filler; and control information, sent in a
two-byte escape sequence. If n>0 bytes (up to 127) of regular
data are sent, these n bytes are preceded by a byte with the
left-most bit set to 0 and the right-most 7 bits containing the
number n.
Byte string:
1 7 8 8
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
|0| n | | d(1) | ... | d(n) |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
^ ^
|---n bytes---|
of data
String of n data bytes d(1),..., d(n)
Count n must be positive.
To compress a string of n replications of the data byte d, the
following 2 bytes are sent:
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
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File Transfer Protocol
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.
3.5. 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 control 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 control 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 control
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File Transfer Protocol
connection and is immediately followed by the command (such as
RETR, STOR or LIST) which was being executed when the system
failure occurred.
4. FILE TRANSFER FUNCTIONS
The communication channel from the user-PI to the server-PI is
established as a TCP connection from the user to the 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.
4.1. FTP COMMANDS
4.1.1. 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 control 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 access control
parameters.
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File Transfer Protocol
PASSWORD (PASS)
The argument field is a Telnet string specifying 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
automation: 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 it stores
(pending receipt of the ACCounT command) or discards the
command, respectively.
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.
CHANGE TO PARENT DIRECTORY (CDUP)
This command is a special case of CWD, and is included to
simplify the implementation of programs for transferring
directory trees between operating systems having different
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File Transfer Protocol
syntaxes for naming the parent directory. The reply codes
shall be identical to the reply codes of CWD. See
Appendix II for further details.
STRUCTURE MOUNT (SMNT)
This command allows the user to mount a different file
system data structure 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.
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 control connection is left open. This is identical
to the state in which a user finds himself immediately after
the control 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 control 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 control connection will cause the
server to take the effective action of an abort (ABOR) and a
logout (QUIT).
4.1.2. 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 is 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:
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File Transfer Protocol
DATA PORT (PORT)
The argument is a HOST-PORT specification for the data port
to be used in data connection. There are 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
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File Transfer Protocol
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.
4.1.3. 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 control 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 (e.g., STOR or
RETR). The data, when transferred in response to FTP service
Postel & Reynolds [Page 29]RFC 959 October 1985
File Transfer Protocol
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.
STORE UNIQUE (STOU)
This command behaves like STOR except that the resultant
file is to be created in the current directory under a name
unique to that directory. The 250 Transfer Started response
must include the name generated.
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.
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
Postel & Reynolds [Page 30]RFC 959 October 1985
File Transfer Protocol
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 skips 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 old pathname of 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 control
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.
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File Transfer Protocol
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
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.
REMOVE DIRECTORY (RMD)
This command causes the directory specified in the pathname
to be removed as a directory (if the pathname is absolute)
or as a subdirectory of the current working directory (if
the pathname is relative). See Appendix II.
MAKE DIRECTORY (MKD)
This command causes the directory specified in the pathname
to be created as a directory (if the pathname is absolute)
or as a subdirectory of the current working directory (if
the pathname is relative). See Appendix II.
PRINT WORKING DIRECTORY (PWD)
This command causes the name of the current working
directory to be returned in the reply. See Appendix II.
LIST (LIST)
This command causes a list to be sent from the server to the
passive DTP. If the pathname specifies a directory or other
group of files, 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
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File Transfer Protocol
ensure that the TYPE is appropriately ASCII or EBCDIC).
Since the information on a file may vary widely from system
to system, this information may be hard to use automatically
in a program, but may be quite useful to a human user.
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.) This command is intended
to return information that can be used by a program to
further process the files automatically. For example, in
the implementation of a "multiple get" function.
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.
SYSTEM (SYST)
This command is used to find out the type of operating
system at the server. The reply shall have as its first
word one of the system names listed in the current version
of the Assigned Numbers document [4].
STATUS (STAT)
This command shall cause a status response to be sent over
the control 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
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File Transfer Protocol
transferred over the control 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
control 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 control 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
control connection while a data transfer is in progress. Some
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File Transfer Protocol
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.
A reply is defined to contain the 3-digit code, followed by Space
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File Transfer Protocol
<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 control 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 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.
The three digits of the reply each have a special significance.
This is intended to allow a range of very simple to very
sophisticated responses 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,
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File Transfer Protocol
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. The server-FTP
process may send at most, one 1yz reply per command.
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
Postel & Reynolds [Page 37]RFC 959 October 1985
File Transfer Protocol
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 control 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
Postel & Reynolds [Page 38]RFC 959 October 1985
File Transfer Protocol
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.
4.2.1 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.
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File Transfer Protocol
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.
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File Transfer Protocol
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.
4.2.2 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 "=").
120 Service ready in nnn minutes.
125 Data connection already open; transfer starting.
150 File status okay; about to open data connection.
Postel & Reynolds [Page 41]RFC 959 October 1985
File Transfer Protocol
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 NAME system type.
Where NAME is an official system name from the list in the
Assigned Numbers document.
220 Service ready for new user.
221 Service closing control 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.
257 "PATHNAME" created.
331 User name okay, need password.
332 Need account for login.
350 Requested file action pending further information.
421 Service not available, closing control 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.
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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.
5. DECLARATIVE SPECIFICATIONS
5.1. 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.
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5.2. CONNECTIONS
The server protocol interpreter shall "listen" on Port L. The
user or user protocol interpreter shall initiate the full-duplex
control connection. Server- and user- processes should follow the
conventions of the Telnet protocol as specified in the
ARPA-Internet Protocol Handbook [1]. Servers are under no
obligation to provide for editing of command lines and may require
that it be done in the user host. The control 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.
Note that all FTP implementation must support data transfer using
the default port, and that only the USER-PI may initiate the use
of non-default ports.
When data is to be transferred between two servers, A and B (refer
to Figure 2), the user-PI, C, sets up control 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:
Postel & Reynolds [Page 44]RFC 959 October 1985
File Transfer Protocol
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
Figure 3
The data connection shall be closed by the server under the
conditions described in the Section on Establishing Data
Connections. If the data connection is to be closed following a
data transfer where closing the connection is not required to
indicate the end-of-file, the server must do so immediately.
Waiting until after a new transfer command is not permitted
because the user-process will have already tested the data
connection to see if it needs to do a "listen"; (remember 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).
Any time either the user or server see that the connection is
being closed by the other side, it should promptly read any
remaining data queued on the connection and issue the close on its
own side.
5.3. COMMANDS
The commands are Telnet character strings transmitted over the
control 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:
Postel & Reynolds [Page 45]RFC 959 October 1985
File Transfer Protocol
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, Line
Feed) 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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File Transfer Protocol
5.3.1. FTP COMMANDS
The following are the FTP commands:
USER <SP> <username> <CRLF>
PASS <SP> <password> <CRLF>
ACCT <SP> <account-information> <CRLF>
CWD <SP> <pathname> <CRLF>
CDUP <CRLF>
SMNT <SP> <pathname> <CRLF>
QUIT <CRLF>
REIN <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>
STOU <CRLF>
APPE <SP> <pathname> <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>
RMD <SP> <pathname> <CRLF>
MKD <SP> <pathname> <CRLF>
PWD <CRLF>
LIST [<SP> <pathname>] <CRLF>
NLST [<SP> <pathname>] <CRLF>
SITE <SP> <string> <CRLF>
SYST <CRLF>
STAT [<SP> <pathname>] <CRLF>
HELP [<SP> <string>] <CRLF>
NOOP <CRLF>
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5.3.2. FTP COMMAND ARGUMENTS
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> ::= <number>
<host-port> ::= <host-number>,<port-number>
<host-number> ::= <number>,<number>,<number>,<number>
<port-number> ::= <number>,<number>
<number> ::= any decimal integer 1 through 255
<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>
<decimal-integer> ::= any decimal integer
Postel & Reynolds [Page 48]RFC 959 October 1985
File Transfer Protocol
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
Postel & Reynolds [Page 49]RFC 959 October 1985
File Transfer Protocol
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
Postel & Reynolds [Page 50]RFC 959 October 1985
File Transfer Protocol
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
Postel & Reynolds [Page 51]RFC 959 October 1985
File Transfer Protocol
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
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File Transfer Protocol
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
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File Transfer Protocol
6. 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, CDUP, SMNT, HELP, MODE, NOOP, PASV,
QUIT, SITE, PORT, SYST, STAT, RMD, MKD, PWD, STRU, and TYPE.
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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, NLST, REIN, RETR, STOR, and STOU.
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.
Remember that at most, one 100 series reply is allowed per command.
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 |
+---+ +---+ +---+
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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, or RETR.
We note that the above three models are similar. The Restart differs
from the Rename two only in the treatment of 100 series replies at
the second stage, while the second group expects (some may require)
100 series replies. Remember that at most, one 100 series reply is
allowed per command.
Postel & Reynolds [Page 56]RFC 959 October 1985
File Transfer Protocol
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 |
+---+ +---+------------->+---+
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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
Postel & Reynolds [Page 58]RFC 959 October 1985
File Transfer Protocol
7. 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 control 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<CRLF>.
Server makes data connection
to port U.
<---- 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> ---->
<---- 550 Access denied<CRLF>
terminate QUIT <CRLF> ---->
Server closes all
connections.
8. 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.
Postel & Reynolds [Page 59]RFC 959 October 1985
File Transfer Protocol
APPENDIX I - PAGE STRUCTURE
The need for FTP to support page structure derives principally from
the need to support efficient transmission of files between TOPS-20
systems, particularly the files used by NLS.
The file system of TOPS-20 is based on the concept of pages. The
operating system is most efficient at manipulating files as pages.
The operating system provides an interface to the file system so that
many applications view files as sequential streams of characters.
However, a few applications use the underlying page structures
directly, and some of these create holey files.
A TOPS-20 disk file consists of four things: a pathname, a page
table, a (possibly empty) set of pages, and a set of attributes.
The pathname is specified in the RETR or STOR command. It includes
the directory name, file name, file name extension, and generation
number.
The page table contains up to 2**18 entries. Each entry may be
EMPTY, or may point to a page. If it is not empty, there are also
some page-specific access bits; not all pages of a file need have the
same access protection.
A page is a contiguous set of 512 words of 36 bits each.
The attributes of the file, in the File Descriptor Block (FDB),
contain such things as creation time, write time, read time, writer's
byte-size, end-of-file pointer, count of reads and writes, backup
system tape numbers, etc.
Note that there is NO requirement that entries in the page table be
contiguous. There may be empty page table slots between occupied
ones. Also, the end of file pointer is simply a number. There is no
requirement that it in fact point at the "last" datum in the file.
Ordinary sequential I/O calls in TOPS-20 will cause the end of file
pointer to be left after the last datum written, but other operations
may cause it not to be so, if a particular programming system so
requires.
In fact, in both of these special cases, "holey" files and
end-of-file pointers NOT at the end of the file, occur with NLS data
files.
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The TOPS-20 paged files can be sent with the FTP transfer parameters:
TYPE L 36, STRU P, and MODE S (in fact, any mode could be used).
Each page of information has a header. Each header field, which is a
logical byte, is a TOPS-20 word, since the TYPE is L 36.
The header fields are:
Word 0: Header Length.
The header length is 5.
Word 1: Page Index.
If the data is a disk file page, this is the number of that
page in the file's page map. Empty pages (holes) in the file
are simply not sent. Note that a hole is NOT the same as a
page of zeros.
Word 2: Data Length.
The number of data words in this page, following the header.
Thus, the total length of the transmission unit is the Header
Length plus the Data Length.
Word 3: Page Type.
A code for what type of chunk this is. A data page is type 3,
the FDB page is type 2.
Word 4: Page Access Control.
The access bits associated with the page in the file's page
map. (This full word quantity is put into AC2 of an SPACS by
the program reading from net to disk.)
After the header are Data Length data words. Data Length is
currently either 512 for a data page or 31 for an FDB. Trailing
zeros in a disk file page may be discarded, making Data Length less
than 512 in that case.
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APPENDIX II - DIRECTORY COMMANDS
Since UNIX has a tree-like directory structure in which directories
are as easy to manipulate as ordinary files, it is useful to expand
the FTP servers on these machines to include commands which deal with
the creation of directories. Since there are other hosts on the
ARPA-Internet which have tree-like directories (including TOPS-20 and
Multics), these commands are as general as possible.
Four directory commands have been added to FTP:
MKD pathname
Make a directory with the name "pathname".
RMD pathname
Remove the directory with the name "pathname".
PWD
Print the current working directory name.
CDUP
Change to the parent of the current working directory.
The "pathname" argument should be created (removed) as a
subdirectory of the current working directory, unless the "pathname"
string contains sufficient information to specify otherwise to the
server, e.g., "pathname" is an absolute pathname (in UNIX and
Multics), or pathname is something like "<abso.lute.path>" to
TOPS-20.
REPLY CODES
The CDUP command is a special case of CWD, and is included to
simplify the implementation of programs for transferring directory
trees between operating systems having different syntaxes for
naming the parent directory. The reply codes for CDUP be
identical to the reply codes of CWD.
The reply codes for RMD be identical to the reply codes for its
file analogue, DELE.
The reply codes for MKD, however, are a bit more complicated. A
freshly created directory will probably be the object of a future
Postel & Reynolds [Page 62]RFC 959 October 1985
File Transfer Protocol
CWD command. Unfortunately, the argument to MKD may not always be
a suitable argument for CWD. This is the case, for example, when
a TOPS-20 subdirectory is created by giving just the subdirectory
name. That is, with a TOPS-20 server FTP, the command sequence
MKD MYDIR
CWD MYDIR
will fail. The new directory may only be referred to by its
"absolute" name; e.g., if the MKD command above were issued while
connected to the directory <DFRANKLIN>, the new subdirectory
could only be referred to by the name <DFRANKLIN.MYDIR>.
Even on UNIX and Multics, however, the argument given to MKD may
not be suitable. If it is a "relative" pathname (i.e., a pathname
which is interpreted relative to the current directory), the user
would need to be in the same current directory in order to reach
the subdirectory. Depending on the application, this may be
inconvenient. It is not very robust in any case.
To solve these problems, upon successful completion of an MKD
command, the server should return a line of the form:
257<space>"<directory-name>"<space><commentary>
That is, the server will tell the user what string to use when
referring to the created directory. The directory name can
contain any character; embedded double-quotes should be escaped by
double-quotes (the "quote-doubling" convention).
For example, a user connects to the directory /usr/dm, and creates
a subdirectory, named pathname:
CWD /usr/dm
200 directory changed to /usr/dm
MKD pathname
257 "/usr/dm/pathname" directory created
An example with an embedded double quote:
MKD foo"bar
257 "/usr/dm/foo""bar" directory created
CWD /usr/dm/foo"bar
200 directory changed to /usr/dm/foo"bar
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The prior existence of a subdirectory with the same name is an
error, and the server must return an "access denied" error reply
in that case.
CWD /usr/dm
200 directory changed to /usr/dm
MKD pathname
521-"/usr/dm/pathname" directory already exists;
521 taking no action.
The failure replies for MKD are analogous to its file creating
cousin, STOR. Also, an "access denied" return is given if a file
name with the same name as the subdirectory will conflict with the
creation of the subdirectory (this is a problem on UNIX, but
shouldn't be one on TOPS-20).
Essentially because the PWD command returns the same type of
information as the successful MKD command, the successful PWD
command uses the 257 reply code as well.
SUBTLETIES
Because these commands will be most useful in transferring
subtrees from one machine to another, carefully observe that the
argument to MKD is to be interpreted as a sub-directory of the
current working directory, unless it contains enough information
for the destination host to tell otherwise. A hypothetical
example of its use in the TOPS-20 world:
CWD <some.where>
200 Working directory changed
MKD overrainbow
257 "<some.where.overrainbow>" directory created
CWD overrainbow
431 No such directory
CWD <some.where.overrainbow>
200 Working directory changed
CWD <some.where>
200 Working directory changed to <some.where>
MKD <unambiguous>
257 "<unambiguous>" directory created
CWD <unambiguous>
Note that the first example results in a subdirectory of the
connected directory. In contrast, the argument in the second
example contains enough information for TOPS-20 to tell that the
Postel & Reynolds [Page 64]RFC 959 October 1985
File Transfer Protocol
<unambiguous> directory is a top-level directory. Note also that
in the first example the user "violated" the protocol by
attempting to access the freshly created directory with a name
other than the one returned by TOPS-20. Problems could have
resulted in this case had there been an <overrainbow> directory;
this is an ambiguity inherent in some TOPS-20 implementations.
Similar considerations apply to the RMD command. The point is
this: except where to do so would violate a host's conventions for
denoting relative versus absolute pathnames, the host should treat
the operands of the MKD and RMD commands as subdirectories. The
257 reply to the MKD command must always contain the absolute
pathname of the created directory.
Postel & Reynolds [Page 65]RFC 959 October 1985
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APPENDIX III - RFCs on FTP
Bhushan, Abhay, "A File Transfer Protocol", RFC 114 (NIC 5823),
MIT-Project MAC, 16 April 1971.
Harslem, Eric, and John Heafner, "Comments on RFC 114 (A File
Transfer Protocol)", RFC 141 (NIC 6726), RAND, 29 April 1971.
Bhushan, Abhay, et al, "The File Transfer Protocol", RFC 172
(NIC 6794), MIT-Project MAC, 23 June 1971.
Braden, Bob, "Comments on DTP and FTP Proposals", RFC 238 (NIC 7663),
UCLA/CCN, 29 September 1971.
Bhushan, Abhay, et al, "The File Transfer Protocol", RFC 265
(NIC 7813), MIT-Project MAC, 17 November 1971.
McKenzie, Alex, "A Suggested Addition to File Transfer Protocol",
RFC 281 (NIC 8163), BBN, 8 December 1971.
Bhushan, Abhay, "The Use of "Set Data Type" Transaction in File
Transfer Protocol", RFC 294 (NIC 8304), MIT-Project MAC,
25 January 1972.
Bhushan, Abhay, "The File Transfer Protocol", RFC 354 (NIC 10596),
MIT-Project MAC, 8 July 1972.
Bhushan, Abhay, "Comments on the File Transfer Protocol (RFC 354)",
RFC 385 (NIC 11357), MIT-Project MAC, 18 August 1972.
Hicks, Greg, "User FTP Documentation", RFC 412 (NIC 12404), Utah,
27 November 1972.
Bhushan, Abhay, "File Transfer Protocol (FTP) Status and Further
Comments", RFC 414 (NIC 12406), MIT-Project MAC, 20 November 1972.
Braden, Bob, "Comments on File Transfer Protocol", RFC 430
(NIC 13299), UCLA/CCN, 7 February 1973.
Thomas, Bob, and Bob Clements, "FTP Server-Server Interaction",
RFC 438 (NIC 13770), BBN, 15 January 1973.
Braden, Bob, "Print Files in FTP", RFC 448 (NIC 13299), UCLA/CCN,
27 February 1973.
McKenzie, Alex, "File Transfer Protocol", RFC 454 (NIC 14333), BBN,
16 February 1973.
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RFC 959 October 1985
File Transfer Protocol
Bressler, Bob, and Bob Thomas, "Mail Retrieval via FTP", RFC 458
(NIC 14378), BBN-NET and BBN-TENEX, 20 February 1973.
Neigus, Nancy, "File Transfer Protocol", RFC 542 (NIC 17759), BBN,
12 July 1973.
Krilanovich, Mark, and George Gregg, "Comments on the File Transfer
Protocol", RFC 607 (NIC 21255), UCSB, 7 January 1974.
Pogran, Ken, and Nancy Neigus, "Response to RFC 607 - Comments on the
File Transfer Protocol", RFC 614 (NIC 21530), BBN, 28 January 1974.
Krilanovich, Mark, George Gregg, Wayne Hathaway, and Jim White,
"Comments on the File Transfer Protocol", RFC 624 (NIC 22054), UCSB,
Ames Research Center, SRI-ARC, 28 February 1974.
Bhushan, Abhay, "FTP Comments and Response to RFC 430", RFC 463
(NIC 14573), MIT-DMCG, 21 February 1973.
Braden, Bob, "FTP Data Compression", RFC 468 (NIC 14742), UCLA/CCN,
8 March 1973.
Bhushan, Abhay, "FTP and Network Mail System", RFC 475 (NIC 14919),
MIT-DMCG, 6 March 1973.
Bressler, Bob, and Bob Thomas "FTP Server-Server Interaction - II",
RFC 478 (NIC 14947), BBN-NET and BBN-TENEX, 26 March 1973.
White, Jim, "Use of FTP by the NIC Journal", RFC 479 (NIC 14948),
SRI-ARC, 8 March 1973.
White, Jim, "Host-Dependent FTP Parameters", RFC 480 (NIC 14949),
SRI-ARC, 8 March 1973.
Padlipsky, Mike, "An FTP Command-Naming Problem", RFC 506
(NIC 16157), MIT-Multics, 26 June 1973.
Day, John, "Memo to FTP Group (Proposal for File Access Protocol)",
RFC 520 (NIC 16819), Illinois, 25 June 1973.
Merryman, Robert, "The UCSD-CC Server-FTP Facility", RFC 532
(NIC 17451), UCSD-CC, 22 June 1973.
Braden, Bob, "TENEX FTP Problem", RFC 571 (NIC 18974), UCLA/CCN,
15 November 1973.
Postel & Reynolds [Page 67]
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File Transfer Protocol
McKenzie, Alex, and Jon Postel, "Telnet and FTP Implementation -
Schedule Change", RFC 593 (NIC 20615), BBN and MITRE,
29 November 1973.
Sussman, Julie, "FTP Error Code Usage for More Reliable Mail
Service", RFC 630 (NIC 30237), BBN, 10 April 1974.
Postel, Jon, "Revised FTP Reply Codes", RFC 640 (NIC 30843),
UCLA/NMC, 5 June 1974.
Harvey, Brian, "Leaving Well Enough Alone", RFC 686 (NIC 32481),
SU-AI, 10 May 1975.
Harvey, Brian, "One More Try on the FTP", RFC 691 (NIC 32700), SU-AI,
28 May 1975.
Lieb, J., "CWD Command of FTP", RFC 697 (NIC 32963), 14 July 1975.
Harrenstien, Ken, "FTP Extension: XSEN", RFC 737 (NIC 42217), SRI-KL,
31 October 1977.
Harrenstien, Ken, "FTP Extension: XRSQ/XRCP", RFC 743 (NIC 42758),
SRI-KL, 30 December 1977.
Lebling, P. David, "Survey of FTP Mail and MLFL", RFC 751, MIT,
10 December 1978.
Postel, Jon, "File Transfer Protocol Specification", RFC 765, ISI,
June 1980.
Mankins, David, Dan Franklin, and Buzz Owen, "Directory Oriented FTP
Commands", RFC 776, BBN, December 1980.
Padlipsky, Michael, "FTP Unique-Named Store Command", RFC 949, MITRE,
July 1985.
Postel & Reynolds [Page 68]
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File Transfer Protocol
REFERENCES
[1] Feinler, Elizabeth, "Internet Protocol Transition Workbook",
Network Information Center, SRI International, March 1982.
[2] Postel, Jon, "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC 793, DARPA, September 1981.
[3] Postel, Jon, and Joyce Reynolds, "Telnet Protocol
Specification", RFC 854, ISI, May 1983.
[4] Reynolds, Joyce, and Jon Postel, "Assigned Numbers", RFC 943,
ISI, April 1985.
Postel & Reynolds [Page 69]
