(Aug. 12, 1973)
RFC542 NIC 17759
Nancy J. Neigus See Also: RFCs 354, 454, 495
Bolt Beranek and Newman, Inc.
Cambridge, Mass.
File Transfer Protocol for the ARPA Network
File Transfer Protocol
(Aug. 12, 1973)
RFC542 NIC 17759
PREFACE
This document is the result of several months discussion via RFC
(relevant numbers are 430, 448, 454, 463, 468, 478, 480), followed by a
meeting of the FTP committee at BBN on March 16, followed by further
communication among committee members. There are a considerable number
of changes for the last "official" version, see RFCs 354, 385, but the
gross structure remains the same. The places to look for differences
are (1) in the definitions pf types and modes, (2) in the specification
of the data connection and data sockets, (3) in the command-reply
sequences, (4) in the functions dependent on the TELNET protocol (FTP
has been altered to correspond to the new TELNET spec). The model has
been clarified and enlarged to allow inter-server file transfer, and
several new commands have been added to accommodate more specialized (or
site-specific) functions. It is my belief that this new specificiation
reflects the views expressed by the committee at the above-mentioned
meeting and in subsequent conversations.
The large number of incompatibilities would complicate a phased
implementation schedule, such as is in effect for the TELNET protocol.
Therefore we have assigned a new socket, decimal 21, as a temporary
logger socket for the new version and a change-over date of 1 February
1974. Until that date the old (354, 385) version of FTP will be
available on Socket 3 and the new version (attached) should be
implemented on Socket 21. On 1 February the new version will shift to
Socket 3 and the old disappear from view.
The File Transfer protocol should be considered stable at least until
February, though one should feel free to propose further changes via
RFC. (Implementation of new commands on an experimental basis is
encouraged and should also be reported by RFC.) In addition, members of
the FTP committee may be contacted directly about changes. Based on
attendance at the March 16 meeting, they are:
Abhay Bhushan MIT-DMCG
Bob Braden UCLA-CCN
Bob Bressler BBN-NET
Bob Clements BBN-TENEX
John Day ILL-ANTS
Peter Deutsch PARC-MAXC
Wayne Hathaway AMES-67
Mike Kudlick SRI-ARC
Alex McKenzie BBN-NET
Bob Merryman UCSD-CC
Nancy Neigus BBN-NET
Mike Padlipsky MIT-Multics
Jim Pepin USC-44
Ken Pogran MIT-Multics
Jon Postel UCLA-NMC
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Milton Reese FNWC
Brad Reussow HARV-10
Marc Seriff MIT-DMCG
Ed Taft HARV-10
Bob Thomas BBN-TENEX
Ric Werme CMU-10
Jim White SRI-ARC
I would especially like to thank Bob Braden, Ken Pogran, Wayne Hathaway,
Jon Postel, Ed Taft and Alex McKenzie for their help in preparing this
document.
NJN/jm
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FILE TRANSFER PROTOCOL
INTRODUCTION
The File Transfer Protocol (FTP) is a protocol for file transfer
between Hosts (including Terminal Interface Message Processors
(TIPs)) 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) 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, TIPs, and the Datacomputer, with a
simple, and easily implemented protocol design.
This paper assumes knowledge of the following protocols described in
NIC #7104:
The Host-Host Protocol
The Initial Connection Protocol
The TELNET Protocol
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.
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 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.
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RFC542 NIC 17759
It is the prerogative of a server-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. The data connection
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.
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
path may be between a server-DTP and a user-DTP, or between two
server-DTPs.
data socket
The passive data transfer process "listens" on the data socket
for an RFCfrom the active transfer process (server) in order
to open the data connection. The server has fixed data
sockets; the passive process may or may not.
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.
FTP commands
A set of commands that comprise the control information flowing
from the user-FTP to the server-FTP process.
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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 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 NVFS
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 the 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 a file need not have record structure.
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 a text string. The codes
are for use by programs and the text is usually intended for
human users.
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server-DTP
The data transfer process, in its normal "active" state,
establishes the data connection by RFCto the "listening" data
socket, sets up parameters for transfer and storage, and
tranfers data on command from its PI. The DTP can be placed in
a "passive" state to listen for, rather than initiate, an RFC
on the data socket.
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 protocol interpreter "listens" on Socket 3 for an ICP from
a user-PI and establishes a TELNET communication connection.
It receives standard FTP commands from the user-PI, sends
replies, and governs the server-DTP.
TELNET connections
The full-duplex communication path between a user-PI and a
server-PI. 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 the Section on Establishing
Data Connections.
user
A human being or a process on behalf of a human being 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.
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user-DTP
The data transfer process "listens" on the data socket for an
RFCfrom 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 protocol interpreter initiates the ICP to the server-FTP
process, initiates FTP commands, and governs the user-DTP if
that process is part of the file transfer.
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 !
!\----:----/! --------
---------- ! V !
!/------\! FTP Commands !/---------\!
!!Server!<-----------------! User !!
!! PI !----------------->! PI !!
!\--:---/! FTP Replies !\----:----/!
! V ! ! V !
-------- !/------\! Data !/---------\! --------
! File !<--->!Server!<---------------->! User !<--->! File !
!System! !! DTP !! Connections !! DTP !! !System!
-------- !\------/! !\---------/! --------
---------- -------------
Server-FTP User-FTP
NOTES: 1. The data connection may be in either direction.
2. The data connection need not exist all of the time.
Figure 1 Model for FTP Use
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In the model described in Figure 1, the user-protocol interpreter
initiates the TELNET connections. At the initiation of the user,
standard FTP commands are generated by the user-PI and transmitted to
the server process via the TELNET connections. (The user may
establish a direct TELNET connection to the server-FTP, from a TIP
terminal for example, and generate standard FTP commands himself,
by-passing the user-FTP process.) Standard replies are sent from the
server-PI to the user-PI over the TELNET connections in response to
the commands.
The FTP commands specify the parameters for the data connection (data
socket, byte size, 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 socket, and the server initiate the data connection
and data transfer in accordance with the specified parameters. It
should be noted that the data socket need not be in the same Host
that initiates the FTP commands via the TELNET connections, but the
user or his user-FTP process must ensure a "listen" on the specified
data socket. It should also be noted that two data connections, one
for send and the other for receive, may exist simultaneously.
In another situation a user might wish to transfer files between two
Hosts, neither of which is his local Host. He sets up TELNET
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 he server data
transfer processes. Following is a model of this server-server
interaction.
TELNET ------------ TELNET
-----------! User-FTP !------------
! -------->! User-PI !<--------- !
! ! ! "C" ! ! !
V ! ------------ ! V
-------------- --------------
! Server-FTP ! Data Connection ! Server-FTP !
! "A" !<-----------------------! "B" !
-------------- Socket(A) Socket(B) --------------
Figure 2
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The protocol requires that the TELNET connections be open while data
transfer is in progress. It is the responsibility of the user to
request the closing of the TELNET connections when finished using the
FTP service, while it is the server who takes the action. The server
may abort data transfer if the TELNET connections are closed without
command.
DATA TRANSFER FUNCTIONS
Files are transferred only via the data connection(s). The TELNET
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 BYTE, MODE, and SOCKet commands 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 "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.
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 diffeent
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 be 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
that FTP provides for very limited data type representations.
Transformations desired beyond this limited capability should be
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performed by the user directly or via the use of the Data
Reconfiguration Sevice (DRS, RFC#138, NIC #6715). Additonal
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. 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." This
has nothing to do with the byte size used for transmission over the
data connection(s) (called the "transfer byte size") and the two
should not be confused. For example, NVT-ASCII has a logical byte
size of 8 bits but an ASCII file might be transferred using a
transfer byte size of 32. If the type is Local byte, then the TYPE
command has an obligatory second parameter specifying the logical
byte size.
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:
ASCII Format
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 his 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. If the BYTE command (see
the Section on Transfer Parameter Commands) specifies a
transfer byte size different from 8 bits, the 8-bit ASCII
characters should be packed contiguously without regard for
transfer byte boundaries.
The Format parameter for ASCII and EBCDIC types is discussed
below.
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RFC542 NIC 17759
EBCDIC Format
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.
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 ought to 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 and so these types have a
second parameter specifying one of the following three formats:
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.
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.
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.
Carriage Control (ASA)
The file contains ASA (FORTRAN) vertical format control
characters. (See NWG/RFC#189 Appendix C and Communications of
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the ACM, Vol. 7, No. 10, 606 (Oct. 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.
Image
The data are sent as contiguous bits which, for transfer, are
packed into transfer bytes of the size specified in the BYTE
command. 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 zeroes, 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.
Local byte Byte size
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,
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then the logical bytes should be packed contiguously,
disregarding transfer byte boundaries and with any necessary
padding at the end.
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 (that
is an identical file can be retrieved if the same parameters
are used) and should be well publicized by the FTP
implementors.
This type is intended for the transfer of structured data. For
example, a user sending 36-bit floating-point numbers to a Host
with a 32-bit word could send his 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.
A note of caution about parameters: a file must be stored and
retrieved with the same parameters if the retrieved version is to 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.
In addition to different representation types, FTP allows the
structure of a file to be specified. Currently two file structures
are recognized in FTP: file-structure, where there is no internal
structure, and record-structure, where the file is made up of
records. File-structure is the default, to be assumed if the
STRUcture command has not been used but both 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 360 in
fixed length records but on a PDP-10 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
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record structure. Obviously this transformation should be useful but
it must also be invertible so that an identical file may be
retreieved 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, <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.
ESTABLISHING DATA CONNECTIONS
The mechanics of transferring data consists of setting up the data
connection to the appropriate sockets and choosing the parameters for
transfer--byte size and mode. Both the user and the server-DTPs have
default data sockets; these are the two sockets (for send and
receive) immediately following the standard ICP TELNET socket ,i.e.,
(U+4) and (U+5) for the user-process and (S+2), (S+3) for the server.
The use of default sockets will ensure the security of the data
transfer, without requiring the socket information to be explicitly
exchanged.
The byte size for the data connection is specified by the BYTE
command, or, if left unspecified, defaults to 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 protocol does not require servers to accept all possible byte
sizes. Since the use of various byte sizes is intended for efficiency
of transfer, servers may implement only those sizes for which their
data transfer is efficient including the default byte size of 8 bits.
The passive data transfer process (this may be a user-DTP or a second
server-DTP) shall "listen" on the data socket prior to sending a
transfer request command. The FTP request command determines the
direction of the data transfer and thus which data socket (odd or
even) is to be used in establishing the connection. The server, upon
receiving the transfer request, will initiate the data connection by
RFCto the appropriate socket using the specified (or default) byte
size. When the connection is opened, the data transfer begins
between DTP's, and the server-PI sends a confirming reply to the
user-PI.
It is possible for the user to specify an alternate data socket by
use of the SOCK command. He might want a file dumped on a TIP line
printer or retrieved from a third party Host. In the latter case the
user-PI sets up TELNET connections with both server-PI's and sends
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each a SOCK command indicating the fixed data sockets of the other.
One server is then told (by an FTP command) to "listen" for an RFC
which the other will initiate and 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 the RFC's and the closes. The exception to
this 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 socket or byte size specification is changed by a command
from the user.
4. The TELNET connections are closed legally or otherwise.
5. An irrecoverable error condition occurs.
Otherwise the close is a server option, the exercise of which he must
indicate to the user-process by an appropriate reply.
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.
Note: In the rest of this section, byte means "transfer byte" except
where explicitly stated otherwise.
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The following transmission modes are defined in FTP:
Stream
The data is transmitted as a stream of bytes. There is no
restriction on the representation type used; record structures
are allowed, in which case the transfer byte size must be at
least 3 bits!
In a record structured file EOR and EOF will each be indicated
by a two-byte control code of whatever byte size is used for
the transfer. 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 zeroes 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 file does not have record structure, the EOF is
indicated by the sending Host closing the data connection and
all bytes are data bytes.
For the purpose of standardized transfer, the sending Host will
translate his 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 his internal denotation. An IBM 360 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.
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 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
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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. There is no
restriction on the transfer byte size.
The header consists of the smallest integral number of bytes
whose length is greater than or equal to 24 bits. Only the
LEAST significant 24 bits (right-justified) of header shall
have information; the remaining most significant bits are
"don't care" bits. 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 number of bytes greater than or equal to 24 bits
--------------------------------------------------------
! Don't care ! Descriptor ! Byte Count !
! 0 to 231 bits ! 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 TELNET
connection (e.g., default--NVT-ASCII). These marker bytes are
right-justified in the smallest integral number of transfer
bytes greater than or equal to 8 bits. For example, if the
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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 !
-------------------------
If the transfer byte size is 16 or more bits, the maximum
possible number of complete marker bytes should be packed,
right-justified, into each transfer byte. The restart marker
should begin in the first marker byte. If there are any unused
marker bytes, these should be filled with the character <SP>
(Space, in the appropriate language). <SP> must not be used
WITHIN a restart marker. For example, to transmit a
six-character marker with a 36-bit transfer byte size, the
following three 36-bit bytes would be sent:
------------------------------------------
! Don't care !Descriptor! Byte count = 2 !
! 12 bits ! code = 16! !
------------------------------------------
------------------------------------------
! ! Marker ! Marker ! Marker ! Marker !
! ! 8 bits ! 8 bits ! 8 bits ! 8 bits !
------------------------------------------
------------------------------------------
! ! Marker ! Marker ! Space ! Space !
! ! 8 bits ! 8 bits ! 8 bits ! 8 bits !
------------------------------------------
Compressed
The file is transmitted as series of bytes of the size
specified by the BYTE command. 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
the byte size is B bits and n>0 bytes 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 B-1 bits containing the number n.
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1 B-1 B B
------- ------ ------
Byte string: !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:
2 B-2 B
--------------- ------
Replicated Byte: ! 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 transfer byte
size is not 8, the expanded byte string should be filled with
8-bit <SP> characters in the manner described in the definition
of ASCII representation type (see the Section on Data
Representation and Storage). If the type is Image or Local
byte the filler is a zero byte.
2 B-2
---------------
Filler String: ! 1 1 ! n !
---------------
The escape sequence is a double byte, the first of which is the
escape byte (all zeroes) and the second of which contains
descriptor codes as defined in Block mode. This implies that
the byte size must be at least 8 bits, which is not much of a
restriction for efficiency in this 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 is most efficient when the byte size chosen is that of the
word size of the transmitting Host, and can be most effectively
used to reduce the size of printer files such as those
generated by RJE Hosts.
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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 users from gross system failures (including failures of a
Host, an FTP-process, or the IMP subnet).
The restart procedure is defined only for the block and compressed
modes of data transfer. It requires the sender of data to insert a
special marker code in the data stream with some marker information.
The marker information has meaning only to the sender, but must
consist of printable characters in the default or negotiated language
of the TELNET connection. 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 recieving system, and return this information to the
user.
In the event of a system failure, the user can restart the data
transfer by identifying the marker point with the FTP restart
procedure. The following example illustrates the use of the restart
procedure.
The sender of the data inserts an appropriate marker block in the
data stream at a convenient point. The receiving Host marks the
corresponding data point in its file system and conveys the last
known sender and receiver marker information to the user, either
directly or over the TELNET connection in a 251 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 restrart command is transmitted over the TELNET connection and is
immediately followed by the command (such as RETR, STOR or LIST)
which was being executed when the system failure occurred.
FILE TRANSFER FUNCTIONS
The communication channel from the user-PI to the server-PI is
established by ICP from the user to a standard server socket. 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.
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FTP COMMANDS
The File Transfer Protocol follows the specifications of the TELNET
protocol for all communications over the TELNET connection - see NIC
#7104. Since, in the future, 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, BYE) may be sent over the
TELNET connections while a data transfer is in progress. Some
servers may not be able to monitor the TELNET and data connections
simultaneously, in which case some special action will be necessary
to get the server's attention. The exact form of the "special
action" is related to decisions currently under review by the TELNET
committee; but the following ordered format is tentatively
recommended:
1. User system inserts the TELNET "Interrupt Process" (IP) signal
in the TELNET stream.
2. User system sends the TELNET "Synch" signal
3. User system inserts the command (e.g., ABOR) in the TELNET
stream.
4. Server PI,, after receiving "IP", scans the TELNET stream for
EXACTLY ONE FTP command.
(For other servers this may not be necessary but the actions listed
above should have no unusual effect.)
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ACCESS CONTROL COMMANDS
The following commands specify access control identifiers (command
codes are shown in parentheses).
USER NAME (USER)
The argument field is a TELNET string identifying the user.
The user identification is that which is required by the server
for access to its file system. This command will normally be
the first command transmitted by the user after the TELNET
connections are made (some servers may require this).
Additional identification information in the form of a password
and/or an account command may also be required by some servers.
Servers may allow a new USER command to be entered at any point
in order to change the access control and/or accounting
information. This has the effect of flushing any user,
password, and account information already supplied and
beginning the login sequence again. All transfer parameters
are unchanged and any file transfer in progress is completed
under the old acccount.
PASSWORD (PASS)
The argument field is a TELNET string identifying the user's
password. This command must be immediately preceded by the
user name command, and, for some sites, completes the user's
identification for access control. Since password information
is quite sensitive, it is desirable in general to "mask" it or
suppress typeout. It appears that the server has no foolproof
way to achieve this. It is therefore the responsibility of the
user-FTP process to hide the sensitive password information.
ACCOUNT (ACCT)
The argument field is a TELNET string identifying the user's
account. The command is not necessarily related to the USER
command, as some sites may require an account for login and
others only for specific access, such as storing files. In the
latter case the command may arrive at any time. There are two
reply codes to differentiate these cases for the automaton:
when account information is required for login, the response to
a successful PASSword command is reply code 331; then if a
command other than ACCounT is sent, the server may remember it
and return a 331 reply, prepared to act on the command after
the account information is received; or he may flush the
command and return a 433 reply asking for the account. On the
other hand, if account information is NOT required for login,
the reply to a successful PASSword command is 230; and if the
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information is needed for a command issued later in the