Network Working Group Abhay Bhushan
Request for Comments: 354 MIT-MAC
NIC: 10596 July 8, 1972
Categories D.4, D.5, D.7
Obsoletes: RFC264 and 265
THE FILE TRANSFER PROTOCOL
I. INTRODUCTION
The File Transfer Protocol (FTP) is a protocol for file
transfer betweet HOSTs (including terminal IMPs), on the ARPA
Computer Network (ARPANET). The primary function of FTP is to
transfer files efficiently and reliably among HOSTs and to allow
the convenient use of remote file storage capabilities.
The objectives of FTP are 1) to promote sharing of files
(computer programs and/or data), 2) fo 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 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, TIPs, and the
Datacomputer, with a simple, elegant, and easily implemented
protocol design.
This paper assumes knowledge of the following protocols:
1) The HOST-HOST Protocol (NIC #8246)
2) The initial Connection Protocol (NIC #7101)
3) The TELNET Protocol (NWG/RFC#318, NIC #9348)
II. DISCUSSION
In this section, the terminology and the FTP model are
discussed. The terms defined in this section are only those that
have special significance in FTP.
11.A. Terminology
ASCII The USASCII character set as defined in NIC
#7104. In FTP, ASCII characters are defined
to be the lower half of an eight bit code set
(i.e., the most significant bit es 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 user-FTP process to provide
access controls.
byte size The byte size specified for the transfer of
data. The data connection is opened with
this byte size. Data connection byte size is
not necessarily the byte size in which data
is to be stored in a system, and may not be
related to the structure of data.
data connection A simplex connection over which data is
transferred, in a specified byte size, mode
and type. The data transferred may be a part
of a file, an entire file or a number of
files. The data connection may be in either
direction (server-to-user or user-to server).
data socket The socket on which a User-FTP process
"listens" for a data connection.
EOF The end-of-file conidition 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
form 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
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 including EOR and EOF. The
transfer modes defined in FTP are described
in Section III.A.
NVT The Network Virtual Terminal as defined in
the ARPANET TELNET Protocol.
NVFS The Network Virtual File System. A concept
which defines a standard network file system
with standard commands and pathname
conventions. FTP only partially embraces the
NFS concept at this time.
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 hte file systems he
wishes to use.
record A sequential file may be structured as a
number of contiguous parts called records.
Record structures are supported by FTP but
are not mandatory.
reply A reply is an acknowledgment (positive or
negative) sent from server to user via the
telnet connections in response to FTP
commands. The general form of a reply is a
completion code (including error codes)
followed by an ASCII text string. The codes
are for use by programs and the text is for
human users.
server-FTP process A process or set of processes which perform
the function of file transfer in cooperation
with a user-FTP process. The server-FTP
process must interpret and respond to user
commands and initiate the data connection.
server site A HOST site wich has a server-FTP process.
server-TELNET A TELNET process which listens on a specified
socket for an ICP initiated by a user-TELNET,
and perform in accordance with the ARPANET
TELNET Protocol.
TELNET connections The full-duplex communication path between a
user-TELNET and a server-TELNET. The TELNET
connections are established via the standard
ARPANET initial Connection Protocol (ICP).
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 Section III.B.
user A process on behalf of a human being or a
human being wishing to oBTain file transfer
service.
user site A HOST site satisfying any of the following
conditions: 1) The site where a user is
located, 2) a site where a user-FTP process
is located, 3) a site to which a data
connection is made by a server. In the
normal case, the sites defined by 1, 2, and 3
are the same site, but nothing in FTP
requires that this be so.
user-FTP process A process or set of precesses which perform
the function of file transfer in cooperation
with a server-FTP process. The user-FTP
process 1) initiates the ICP (via a
user-TELNET, 2) initiates FTP commands and
3) "listens" on the data socket for the data
connection. In some obvious cases (use from
TIPs and other mini-HOSTs) a user-FTP process
will be subsumed under the term "user".
user-TELNET A TELNET process which initiates an ICP to a
specified server-TELNET socket, and performs
in accordance with the ARPANET TELNET
protocol.
II.B. The FTP Model
With the above definitions in mind, the following model
(shown in Figure 1) may be diagrammed for an FTP service.
TELNET
connections
File Server Server<------------ User User File
Systems<-> FTP <->TELNET FTP Commands TELNET<->FTP <->System
Process ------------> Process
Data
<------------------------------>Socket
Data Connection(s)
USER
Notes: 1. The data connection may be in either direction.
2. The data connection need not exist all of the time.
3. The distinctions between user-FTP and user-TELNET,
and between server-FTP and server-TELNET may not
be as clear-cut as shown above. For example, a user-
TELNET may be directly driven by the user.
FIGURE 1 Model for FTP Use
In the model described in Figure 1, the user-TELNET
initiates the TELNET connection. Standard FTP commands are then
generated by the user and transmitted to the server site via the TELNET
connections. FTP commands are in ASCII, in accordance with NVT
conventions and the TELNET protocol. Note that commands may be initiated
by the user directly through the user-TELNET or via a user-FTP process.
Standard replies are sent from the server to the user in response to the
commands over the TELNET connections.
The FTP commands specify the parameters for teh data
connection (data socket, byte size, transfer mode, and representation
type), and the nature of file system operation (store, retrieve, append,
delete, etc.). The user-FTP process or its designate should "listen" on
the specified data socket, and it is the server's responsibility to
initiate the data connection and data transfer in accordance with the
specified data connection parameters. It should be noted that the data
socket need not be in the same HOST that initiates the FTP commands via
the TELNET connection, but the user or his user-FTP process must ensure
a "listen" on the specified data socket. A practical example of such
file transfer to third HOSTs is a maxi-HOST user (who may actually be a
TIP user) whishing to transmit a file to or from an I/O device attached
to a TIP. It should also be noted that two data connections, one for
send and the other for receive, may exist simultaneously.
The protocol requires that the TELNET connections be open
while data transfer is in progress. It is the responsibility of the user
to close the TELNET connections when finished using the FTP service. The
server may abort data transfer if the TELNET connections are closed.
III. DATA TRANSFER FUNCTIONS
Data and files are transferred only via the data connection.
The data transfer of data is governed by FTP data transfer commands
received on the TELNET connections. The data transfer functions include
establishing the data connection to the specified data socket in the
specified HOST (using the specified byte size), transmitting and
receiving data in the specified representation type and transfer mode,
handling EOR and EOF conditions and error recovery (where applicable).
III.A Establishing Data Connection
The user site shall "listen" on the specified data socket.
The FTP request command determines the direction of data transfer, and
the socket number (odd or even) which is to be used in establishing the
data connection. The server on receiving the appropriate store or
retrieve request shall initiate the data connection to the specified
user data socket in the specified byte size (default byte size is 8
bits) and send a reply indicating that file transfer may proceed. Prior
to this the server should send a reply indicating the server socket for
the data connection. The user may use this server socket information to
ensure the security of his data transfer. The server may send this reply
either before of after initiating the data connection.
The byte size for the data connection is specified by the
TYPE (ASCII is 8 bits), or TYPE and BYTE commands. It is not required by
the protocol that servers accept all possible byte size. The user of
various byte size is for efficiency in data transfer and servers may
implement only those byte size for which their data transfer is
efficient. It is however recommended that servers implement at least the
byte size of 8 bits.
After the data transfer is completed, it is the server's
responsibility to close the data connection except when the user is
sender of data. The data connection shall be closed under any of the
following conditions:
1) server receives an abort command form user.
2) EOF in stream mode indicated by closing data connection.
3) the socket or byte size specification is changed.
4) any of the TELNET connections are closed.
5) an irrecoverable error condition.
It should be noted that two simultaneous data connections
(for send and receive) may exist. It is a server option, however, to
close the data connection after each instance of file transfer.
III.B Data Representation and Storage
Data is transferred from a storage device in sending HOST to
a storage device in 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. PDP-10's generally
store NVT-ASCII as five 7-bit ASCII characters, left justified in a 36
bit Word. 360's store NVT-ASCII as 8-bit EBCDIC codes. Multics stores
NVT-ASCII as four 9-bit characters in a 36-bit word. It may desirable to
convert characters into the standard NVT-ASCII representation when
transmitting text between disimilar systems. The sending and receiving
site 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 length. 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) o 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 that FTP
provides for very limited data types reprentations. Transformations
desired beyond this limited capability should be performed by the user
directly or via the use of the Data Reconfiguration Service (DRS, RFC
#138, NIC #6715). Additional representation types may be defined later
if there is a demonstrable need.
Data representations are handled in FTP by a user specifying
a representation type. The type may also specify a fixed transfer byte
size. For example in ASCII and Print File representations, the transfer
byte size must be 8 bits. Only in the Image and Local Byte
representations the byte size specified by the BYTE command is to be
used. The following data representation types are currently defined in
FTP:
1. ASCII The sender converts data form its internal
character representation to the standard
ARPANET ASCII form. The receiver converts
the data from the standard form to its own
internal form. The data is transferred in
the standard form. The transfer byte size
must be 8 bits. This type would be used for
transfer of text files. This is be default
type, and it is recommended that this type be
implemented by all.
2. Image The sender transforms data from contiguous
bits to bytes for transfer. The receiver
transforms the bytes into bits, storing them
contiguously independent of the byte size
chosen for data transfer. Typical uses for
the Image type are transfer of executable
programs between like machines, and transfer
of binary (non-text) data. It is recommended
that this type be implemented by all for some
byte size preferably including the 8 bit byte
size.
3. Local Byte This representation allows for efficient
storage, use, and retrieval of data. The
mann in which data is to be transformed
depends on the byte size for data transfer,
and the particular HOST being used. The
transformation scheme for different byte size
is to be well publicized b all server sites.
This transformation shall be invertible
(i.e., if a file is stored using a certain
transfer byte size, an identical file must be
retrievable using the same byte size and
representation type). It is the user's
responsibility to keep track of the
representation type and byte size used for
his transfer. Typical uses of the Local Byte
type are in efficient storage and retrieval
of files, and transfer of structured binary
data. This type may be identical to the
image type for byte size which are integral
multiples of or factors of the computer word
length-
4. Print File- The server site will transform the ASCII
ASCII file in a form suitable for printing at the
server site. The byte size must be 8 bits.
The transformation may not be invertible.
This type is different from ASCII in that
TABs, FFs and other ASCII format effector
characters may be replaced by SPs, LFs, and
other substitute characters. The print file
conversions are to be well publicized by all
server sites. This type would be used when
the file is destined for an ASCII printer.
This type in some systems may be identical to
the ASCII type. It is recommended that this
type be implemented by all.
5. EBCDIC Print The server site will transform the EBCDIC
File file into a form suitable for printing at the
server site. The byte size must be 8 bits.
the transformation may not be invertible.
This type would be used when the file is
destined for an EBCDIC printer. Only systems
which use EBCDIC for their internal character
representation need accept this type.
It should be noted that a serving HOST need not accept all
representation types and/or byte size, but it must inform the user of
the fact by sending an appropriate reply.
III.C File Structure and Transfer Modes
The only file structures supported directly in FTP at the
present time are record structures. However, the use of record
structures is not mandatory. A user with no record structure in his file
should be able to store and retrieve his file at any HOST. A user
wishing to transmit a record structured file must send the appropriate
FTP 'STRU' command (the default assumption is no record structure). A
serving HOST need not accept record structures, but it must inform the
user of this fact by sending an appropriate reply. Any record structure
information in the data stream may subsequently be discarded by the
receiver.
All data transfer must end with an EOF. The EOF is defined
by the data transfer mode. For files that have record structures, an EOR
is also defined by the transfer mode. Only the transfer modes and
representation type combinations that have EOR defined may be used for
transfer or files with record structures. Records may be of zero length
but they must be contained in file boundaries. The relationship between
files and records is heirarchical and an EOF implies an EOR.
The following data transfer modes are defined in FTP:
1. Stream The file is transmitted as a stream of bytes of the
specified byte size. The EOF is signalled by
closing the data connection. Any representation
type and byte size may be used in the stream mode
but record structures are possible only with the
ASCII representation type. The convention is that
the ASCII character CR (Carriage Return, Code 13.)
followed by LF (Line Feed, Code 10.) Indicates an
EOR in stream mode and ASCII representation type.
This is the default mode, and it is recommended
that this mode be implemented by all.
2. Text The file is ASCII text transmitted as sequence of
8-bit bytes in the ASCII representation type.
Record structures are allowed in this mode. The
EOR and EOF are defined by the presence of special
"TELNET-control" codes (most significant bit set
of one) in the data stream. The EOR code is 192
(octal 300, hex C0). The EOF code os 193 (octal
301, hex C1). The byte size for transfer is 8
bits.
3. Block 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 file block (EOF),
last record block (EOR), restart marker (see
section III.D), or suspect data (i.e. the data
being transferred is suspected of errors and is
not reliable). Record structures are allowed in
this mode, and any representation type or byte
size may be used. The header consists of integral
number of bytes whose length is greater than or
equal to 24 bits. Only the least significant 24
bits (right-jusified) of header shall have
information, other must significant bits must be
zero. 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.
Integral data bytes > 24
Must be Zero Descriptor Byte Count
0 to 231 bits 8 bits 16 bits
The following descriptor codes are assigned:
Code Meaning
0 An ordinary block of data.
1 End of data block is EOR.
2 End of data block is EOF.
3 Suspected errors in data block.
4 Data block is a restart marker.
The restart marker is imbedded in the data stream
as integral number of 8-bit bytes (representing
printable ASCII characters) right-justified in
integral number of data bytes greater than 8 bits.
For example if the byte size is 7 bits, the
restart marker byte would be one byte
right-justified per two 7-bit bytes as shown
below:
Two 7-bit bytes
Marker Char
8 bits
For byte size of 16 bits or more, two more
marker bytes shall be packed right-justified. The
end of the marker may be delimited by the
character SP (code 32.). If marker characters do
no exactly fit an integral byte, the unused
character slots should contain the ASCII character
SP (code 32.). For example, to transmit a six
character marker in a 36-bit byte size, the
following three 36.bit bytes would be sent:
Zero Descriptor
12 bits code=4 Byte count=2
Marker Marker Marker Marker
8 bits 8 bits 8 bits 8 bits
Marker Marker SP SP
8 bits 8 bits 8 bits 8 bits
4 HASP The file is transmitted as a sequence of 8-bit bytes
in the standard Hasp-compressed data format
(document to be issued by Bob Braden, UCLA). This
mode achieves considerable compression of data for
print files. Record structures are allowed in the
Hasp mode.
III.D Error Recovery and Restart
There is no provision for detecting bits lost or scrambled
in data transfer. This issue is perhaps handled best at the NCP level
where it benefits most users. However, a restart procedure is provided
to protect user from system failures (such as failure of either HOST,
FTP-process, or the IMP subnet).
The restart procedure is defined only for the block mode of
data transfer. It requires the sender of data to insert a special marker
code in teh data stream with some marker information. The marker
information has meaning only to the sender, but must consist of
printable ASCII characters. The printable ASCII characters are defined
to be codes 33. through 126. (i.e., not including codes 0. through 31.
and the characters SP and DEL). The marker could represent a bit-count,a
record-count, or any other information by wich 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 examples Illustrate the use of the restart
procedure.
1. When server is the sender of data, the server-FTP process
inserts an appropriate marker block in the data stream at a convenient
data point. The user-FTP process receiving the data, marks the
coressponding data point in its file system and conveys the last known
sender and receiver marker information to the user. In the event of
system failure, the user or user-FTP process restarts the server at the
last server marker by sending a restart command with the server's marker
code at its argument. The restart command is transmitted over the
TELNET connection and is immediately followed by the command (such as
store or retrieve) which was being executed when the system failure
occured.
2. When user is the sender of data, the user-FTP process
inserts the appropriate marker block in the data stream. The server-FTP
process receiving the data, marks the corresponding data point in its
file system. The server does not store this marker but conveys the last
known sender and receiver marker information to the user over the TELNET
connections by appropriate reply codes. The user or the user-FTP process
then restarts transfer in a manner identical to that described in the
first example.
IV. FILE TRANSFER FUNCTIONS
The TELNET connections on which FTP commands and replies are
transmitted, are initiated by the user-FTP process via an ICP to a
standard server socket. FTP commands are then transmitted from user to
server, and replies are transmitted from server to user. The user file
transfer functions invoive sending the FTP commands, interpreting the
replies received and transferring data over the data connection in the
specified manner. The server file transfer functions involve accepting
and interpreting FTP commands, sending replies, setting up the data
connection, and transferring data.
IV.A FTP Commands
FTP commands are ASCII terminated by the ASCII
character sequence CRLF (Carriage Return follow by Line Feed). The
command codes themselves are ASCII alpabetic characters terminated by
the ASCII character 'space' (code = 32.). For convenience, the command
codes are defined to be four (or less) ASCII alphanumeric characters
(including both upper and lower case alphabetic characters). The command
codes and the semantics of commands are described in this section, but
the detailed syntax of commands is specified in Section V.b, the reply
sequence are discussed in Section V.C, and scenarios illustrating the
use of commands are provided in Section V.D.
FTP commands may be partitioned as those specifying
access-control identifiers, data transfer parameters, or FTP service
requests.
IV.A.1 Access Control Commands
The following commands specify access control identifiers
(command codes are shown in parentheses).
User name (USER) - The argument field is an ASCII string
identifying the user. The user identification is that wich is
required by the server for access to its file system. This
command will normally be the first command transmitted by the
user after the TELNET connections are made (some servers may
require this). Additional identification information in the form
of pasword command may also be required by some servers.
Password (PASS) - The argument field is an ASCII string
identifying the user's password. This command must be immediately
preceded by the user name command, and together it completes the
user's identifecation for access control.
IV.A.2 Data Transfer 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 teh last specified value, or if no value has been
specified, the standard default value specified 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.
Byte size (BYTE) - The argument is an ASCII-represented decimal
integer (1 through 255), specifying the byte size for the data
connection for local byte and image representation types. The
default byte size is 8 bits. The byte size is always 8 bits in
the ASCII and Print file representation types. A server may
reject specific byte size/type combinations by sending an
appropriate reply.
Data socket (SOCK) - The argument is a HOST-socket specification
for the data socket to be used in data connection. There may be
two data sockets, one from server to user and the other for user
to server data transfer. An odd socket number defines a send
socket and an even socket number defines a receive socket. The
default HOST is the user HOST to which TELNET connections are
made. The default data sockets are (U+4) and (U+5) where U is the
socket number used in the TELNET ICP and the TELNET connections
are on sockets (U+2) and (U+3).
Representation Type (TYPE) - The argument is a single ASCII
character code specifying the representation types described in
section III.B. The following codes are assigned for type:
A - ASCII
I - Image
L - Local Byte
P - Print file in ASCII
E - EBCDIC print file
The default representation type is ASCII
File Structure (STRU) - The argument is a single ASCII character
code specifying file structure described in section III.C. The
following codes are assigned for structure:
F - File (no record structure)
R - Record structure
The default structure is File (i.e., no records).
Transfer Mode (MODE) - The argument is a single ASCII character
code specifying the data transfer modes described in Section
III.C. The following codes are assigned for transfer modes:
S - Stream (bytes, close is EOF)
B - Block (Header with descriptor and count)
T - Text (TELNET control mode for EOR, EOF)
H - Hasp (specially formatted compressed data)
The default transfer mode is Stream.
IV.A.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), except that ASCII characters must be used
(in conformance with the TELNET Protool). 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 some servers may
require an "allocate" command before a "store" command. The data
when transferred in response to FTP service commands shall always
be over the data connection. The following commands specify FTP
service requests:
Retrieve (RETR) - This command achieves the transfer of a copy of
file specified in pathname, from server to user site. The status
and contents of a file at server site shall be unaffected.
Store (STOR) - This command achieves the transfer of a copy of
file from user to server site. If file specified in pathname
exists at the server site, then its contents shall be replaced by
the contents of the file being transferred. A new file is created
at the server site if the file specified in pathname does not
already exist.
Append (with create) (APPE) - This command achieves the transfer
of data from using to serving site. If file specified in pathname
exists at the server site, then the data transferred shall be
appended to that file, otherwise the file specified in pathname
shall be created at the server site.
Rename from (RNFR) - This command specifies the file which is to
be renamed. This command must be immediately followed by a
"rename to" command specifying the new file pathname.
Delete (DELE) - This command causes teh file specified in
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.
List (LIST) - This command causes a list to be sent from server
to user site. If pathname specifies a directory, the server
should transfer a list of files in the specified directory. If
pathname specifies a file then server should send current
information on the file. This command may be used to obtain the
contents of a file directory (the response should be sent in
ASCII type) or test the existence of a file and its current
status.
Allocate (ALLO) - This command my be required by some servers to
reserve sufficient storage to accomodate the new file to be
transferred. The command field shall be a decimal integer
representing the number of bytes (of size specified by the byte
size command) of storage to be reserved for the file. This
command shall be followed by a store or append command. The ALLO
command should be treated as a NO-OP (no operation) by thuse
servers which do not require that the maximum size of the file be
declared beforehand.
Restart (REST) - The argument field represents the server marker
at which file transfer is to be restarted. This command does not
cause file transfer but "spaces" over the file to the specified
data checkpoint. This command shall be immediately followed by
the appropriate FTP service command which shall cause file
transfer to resume.
Status (STAT) - This command shall cause a status response to be
sent over the TELNET connection in form of a reply. The command
may have an argument field such as a pathname. if the argument
is a pathname, the command is analogous to the "list" command
except that data shall be transferred in ASCII on the TELNET
connection. If no argument is specified, the server should return
general status information about the server FTP process. This may
include service availability, the current settings for the
relevant FTP parameters (including default settings), and the
status of command execution and connections.
Abort (ABOR) - This command indicates to the server to abort the
previous FTP service command and any associated transfer of data.
The abort command should be preceded by the TELNET SYNCH
condition (indicated by the combination of the DATA MARK and the
INS). No action is to be taken if the previous command has been
completed (including data transfer). The TELNET connections is
not to be closed by the server, but the data connection may be
closed. An appropriate reply should be sent by the server.
Logout (BYE) - This command terminates a USER and if file
transfer is not in progress, closes the TELNET connection. If
file transfer is in progress, the connection will remain open for
result response and will then close. During the interim a new
USER command (and no other command) is acceptable.
An uneXPected close on TELNET connection will cause the server to
take the effective action of an abort (ABOR) and a logout (BYE).
IV.B FTP Replies
The server sends FTP replies to user over the TELNET
connections in response to FTP commands. The FTP replies constitute the
acknowledgement or completion code (including errors). The FTP-server
replies are formatted for human or program interpretation. The replies
consist of a leading three digit numeric code followed by a space
followed by a text explanation of the code. The numeric codes are
assigned by groups and for ease of interpretation by programs in a
manner consistent with other protocols such as the RJE protocol. The
three digits of the code are to be interpredet as follows:
a) The first digit specifies type of response as indicated below:
000 These replies are purely informative and constitute
neither a positive nor a negative acknowledgement.
1xx informative replies to status inrequiries. These constitute
a positive acknowledgment to the status command.
2xx Positive acknowledgment of previous command or other
successful action.
3xx Incomplete information. Activity cannot proceed without
further specification and input.
4xx Unsuccessful reply. The request is correctly specified
but the server is unsuccessful in corretly fulfilling
it.
5xx Incorrect or illegal command. The command or its
parameters were invalid or incomplete from a syntactic
viewpoint, or the command its inconsistent with a previous
command. The command in question has been completely
ignored.
6xx - 9xx Reserved for future expansion.
b) The second digit specifies the general category to which
the response refers:
x00-x29 General purpose replies, not assignable to other
categories.
x30 Primary access. Informative replies to the "log-on"
attempt.
x40 Secondary access. The primary server is commenting on its
ability to access a secondary service.
x5x FTP results.
x6x RJE resutls.
x7x-x9x Reserved for future expansion.
c) the final digit specifies a particular message type. Since the code
is designed for an automaton process to interpret, it is not necessary
for every variation of a reply to have a unique number. Only the basic
meaning of replies need have unique numbers. The text of a reply can
explain the specific reason for that reply to a human user.
Each TELNET line (ended by CRLF) from the server is intended
to be a complete reply message. if it is necessary to continue the text
of a reply onto following lines, then those continuation replies contain
the special reply code of three spaces. It should be noted that text of
replies are intended for a human user. Only the reply codes and in some
instances the first line of text are intended for programs.
The assigned reply codes relating to FTP are:
000 General information message (site, time of day, etc.)
030 Server availibility information.
050 FTP commentary or user information.
100 System status reply.
150 File status reply.
151 Directory listing reply.
200 Last command received correctly.
201 An ABORT has terminated activity, as requested.
202 Abort request ignored, no activity in progress.
230 User is "logged in". may proceed.
231 User is "logged out". Service terminated.
232 Logout command noted, will complete when transfer done.
250 FTP file transfer started correctly.
251 FTP Restart-marker reply
Text is: MARK yyyy = mmmm
where yyyy is user's data stream marker (yours)
and mmmm is server's equivalent marker (mine)
(Note the spaces between the markers and '=').
252 FTP transfer completed correctly.
253 Rename completed.
254 Delete completed.
255 FTP server data socket reply
Text is: SOCK nnnn
where nnnn is decimal integer representing
the server socket for data connection.
300 Connection greeting message, awaiting input.
301 Current command incomplete (no CRLF for long time).
330 Enter password (may be sent with hide-your-input).
400 This service not implemented.
401 This service not accepting users now, goodbye.
430 Log-on time or tries exceeded, goodbye.
431 Log-on unsuccessful. User and/or password invalid.
432 User not valid for this service.
434 Log-out forced by operator action. Phone site.
435 Log-out forced by system problem.
436 Service shutting down, goodbye.
450 FTP: File not found.
451 FTP: File access denied to you.
452 FTP: File transfer incomplete, data connection closed.
453 FTP: File transfer incomplete, insufficient storage space.
500 Last command line completely unrecognized.
501 Syntax of last command in incorrect.
502 Last command incomplete, parameters missing.
503 Last command invalid (ignored), illegal parameter combination.
504 Last command invalid, action not possible at this time.
505 Last command conflicts illegally with previous command(s).
506 Requested action not implemented by the server.
V. DECLARATIVE SPECIFICATIONS
V.A. Connections
The server-FTP process at the server site shall "listen" on
Socket 3, via its server-TELNET. The user or user-FTP process at the
user site shall initiate the full-duplex TELNET connections via its
user-TELNET performing the ARPANET standard initial connection protocol
(ICP) to server socket 3. The TELNET connections shall be closed by the
user site upon completion of use.
The user site shall "listen" on the specified data socket or
sockets (a send and/or a receive socket). The server site shall initiate
the data connection using the specified data socket and byte size. The
direction of data connection and the data socket used shall be
determined by the FTP service command. The server shall send a reply to
the user indicating the server data socket so that the user may ensure
the security of data transfer. This can be done at any time prior to the
first transfer of data over a data connection.
The data connection shall be closed by the server site under
the conditions described is Section III.A. The server should in general
send a reply before closing the data connection to avoid problems at the
user end.
V.B. Commands
The commands are ASCII character strings transmitted over
the TELNET connections as described in section IV.A. The command
functions and semantics are described in sections IV.A.1, IV.A.2,
IV.A,3, and IV.A.4. The command syntax is specified here.
The commands begin with a command code followed by an
argument field. The command codes are four of less ASCII alphabetic
characters. Upper and lower case alphabetic characters are to be treated
identically. Thus any of the following may represent the retrieve
command:
RETR Retr retr ReTr rETr
The command codes and the argument fields are separated by one or more
spaces.
The argument field consists of a variable length ASCII
character string ending with the character sequence CRLF (Carriage
Return immediately followed by Line Feed). In the following section on
syntax it should be stressed that all characters in the argument field
are ASCII characters. Thus a decimal integer shall mean an ASCII
represented decimal integer.
The following are all the currently defined FTP commands:
USER <user name> CRLF
PASS <password> CRLF
BYTE <byte size> CRLF
SOCK <HOST-socket> CRLF
TYPE <type code> CRLF
STRU <structure code> CRLF
MODE <mode code> CRLF
RETR <pathname> CRLF
STOR <pathname> CRLF
APPE <pathname> CRLF
RNFR <pathname> CRLF
RNTO <pathname> CRLF
DELE <pathname> CRLF
LIST <pathname> CRLF
ALLO <decimal integer> CRLF
REST <marker> CRLF
STAT <pathname> CRLF
ABOR <empty> CRLF
Bye <empty> CRLF
The syntax of the above argument fields (using BNF notation
where aplicable) is:
<username> ::= <string>
<password> ::= <string>
<string> ::= <empty> <char> <char><string>
<char> ::= any of the 128 ASCII characters except CR and LF.
<marker> ::= <pr string>
<pr string> ::= <empty> <pr char> <pr char><pr string>
<pr char> ::= any ASCII code 33 through 126.
<byte size> ::= any decimal integer 1 through 255.
<HOST-socket> ::= <socket> <HOST number>,<socket>
<HOST number> ::= a decimal integer specifying an ARPANET HOST.
<socket> ::= decimal integer between o and (2**32)-1
<type code> ::= AILPE
<structure code> ::= FR
<mode code> ::= SBTH
<pathname> ::= <string>
<decimal integer> ::= <digit> <digit><decimal integer>
<digit> ::= 0123456789
<empty> ::= the null string (specifies use of default).
V.C 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 of failure
response before sending further commands.
A second type of reply is sent asynchronously with respect
to user commands. These replies may for example report on the
progress or completion of file transfer and as such are secondary
replies to file transfer commands.
The third class of replies are informational and spontaneous
replies which may arrive at any time. These replies are listed
below as spontaneous.
COMMAND-REPLY CORRESPONDENCE TABLE
COMMAND SUCCESS FAIL
USER 230,330 430-432,500-505
PASS 230 430-432,500-505
BYE 231,232 430-432,500-505
BYTE 200 500-506
SOCK 200 500-506
TYPE 200 500-506
MODE 200 500-506
RETR 250 450,451,500-506
Secondary Reply 252 452
STOR 250 451,451,500-506
Secondary Reply 252 452,453
APPE 250 451,451,500-506
Secondary Reply 252 452,453
RNFR 200 450,451,500-506
RNTO 253 450,451,500-505
DELE 254 450,451,500-506
LIST 250 450,453,500-506
Secondary Reply 252 452
ALLO 200 500-506
STAT 100,150,151 450,451,500-506
REST 200 500-506
ABOR 201,202 500-505
Spontaneous 0xx,300,301 400,401,434-436
Replies 251,255
V.D. Tyical FTP Scenarious
1. TIP User wanting o transfer file from FOST X to local printer:
a) TIP user opens TELNET connections by ICP to HOST X, socket 3.
b) The following commands and replies are exchanged:
TIP HOST X
USER username CRLF -------->
<------- 330 Enter Password CRLF
PASS password CRLF -------->
<------- 230 User logged in CRLF
SOCK 65538 CRLF -------->
<------- 200 Command received OK CRLF
RETR this.file CRLF -------->
<------- 255 SOCK 5533 CRLF
(HOST X initiates data connection to
TIP socket 65538, i.e., PORT 1 receive)
<------- 250 File transfer started
BYE CRLF ------_->
<------- 252 File transfer completed
c) HOST X closes the TELNET and data connections.
Note: The TIP user should be in line mode and can thus use
local TIP editing such as character delete.
2. User at Host U wanting to transfer files to/from HOST S:
In general the user would communicate to the server via a
mediating user-FTP process. The following may be a typical
scenario. The user-FTP prompts are shown in parenthesis,
'---->' 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 ICP to HOST S, socket 3,
establishing TELNET connections.
username Doe CR USER DoeCRLF ---->
<---- 330 passwordCRLF
password mumble CR PASS mumbleCRLF ---->
<---- 230 Doe logged in.CRLF
retrieve (local type ASCIICR
(local pathname) test 1 CR USER-FTP open local file in ASCII.
(for. pathname) test.pl1CR RETR test.pl1 CRLF ---->
<---- 255 SOCK 1233CRLF
Server makes data connection to (U+4).
<---- 250 File transfer startsCRLF
<---- 252 File transfer completeCRLF
type imageCR TYPE CRLF ---->
<---- 200 Command OKCRLF
byte 36CR BYTE 36CRLF ---->
<---- 200 Command OKCRLF
store (local type) ImageCR
(local pathname) file dumpCR User-FTP opens local file in Image.
(for. pathname) >udd>cn>fdCR STOR >udd>cn>fdCRLF ---->
<---- 451 Access deniedCRLF
terminate BYECRLF
<---- 231 Doe logged outCRLF
Server closes all connections.
ACKNOWLEDGEMENTS
The work on file transfer protocol has involved many people.
This document reports the work of a group rather than the author
alone. The author gratefully acknowledges the conributions of
the following:
Bob Braden UCLA-CCCN
Arvola Chan MIT-MAC
Bill Crowther BBN-TIP
Eric Harslem RAND
John Heafner RAND
Chuck Holland UCSD
Alex McKenzie BBN (NET)
Bob Metcalfe XPARC
Jon Postel UCLA
Neal Ryan MIT-MAC
Bob Sundberg HARVARD
Ray Tomlinson BBN (TENEX)
Dick Watson SRI-ARC
Jim White SRI-ARC
Richard Winter CCA
[ This RFCwas put into machine readable form for entry ]
[ into the online RFCarchives by Gottfried Janik 9/97 ]
