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Length: 159078 (0x26d66)
Types: TextFile
Names: »RFC765«
└─⟦a0efdde77⟧ Bits:30001252 EUUGD11 Tape, 1987 Spring Conference Helsinki
└─ ⟦this⟧ »EUUGD11/euug-87hel/sec7/RFC/RFC765«
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IEN 149 J. Postel
RFC 765 ISI
June 1980
FILE TRANSFER PROTOCOL
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, and TIPs, with a simple, and easily
implemented protocol design.
This paper assumes knowledge of the following protocols described in
the ARPA Internet Protocol Handbook.
The Transmission Control 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. 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.
TERMINOLOGY
ASCII
The ASCII character set 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.
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File Transfer Protocol RFC 765
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.
data connection
A simplex 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.
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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RFC 765 File Transfer Protocol
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. FTP only partially implements the NVFS
concept at this time.
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.
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.
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File Transfer Protocol RFC 765
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.
server-DTP
The data transfer process, in its normal "active" state,
establishes the data connection with the "listening" data port,
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 protocol interpreter "listens" on Port L for a connection
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, operating according to the TELNET Protocol.
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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RFC 765 File Transfer Protocol
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.
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 protocol interpreter initiates the TELNET 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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File Transfer Protocol RFC 765
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 || FTP Replies || PI ||
|\--:---/| |\----:----/|
| V | | V |
-------- |/------\| Data |/---------\| --------
| 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 TELNET connection. At the initiation of the user,
standard FTP commands are generated by the user-PI and transmitted
to the server process via the TELNET connection. (The user may
establish a direct TELNET connection to the server-FTP, from a TIP
terminal for example, and generate standard FTP commands himself,
bypassing the user-FTP process.) Standard replies are sent from
the server-PI to the user-PI over the TELNET 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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RFC 765 File Transfer Protocol
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 port. It should 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 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 the 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" |
-------------- Port (A) Port (B) --------------
Figure 2
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. 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 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
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File Transfer Protocol RFC 765
"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 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 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
performed by the user directly.
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, 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.
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RFC 765 File Transfer Protocol
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.
The Format parameter for ASCII and EBCDIC types is discussed
below.
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.
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File Transfer Protocol RFC 765
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 RFC 740 Appendix C and Communications of
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.
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RFC 765 File Transfer Protocol
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 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.
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
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File Transfer Protocol RFC 765
difference in byte sizes, 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.
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.
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.
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. 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
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RFC 765 File Transfer Protocol
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 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, <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.
Page Structure
To transmit files that are discontinuous FTP defines a page
structure. Files of this type are sometimes know 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:
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File Transfer Protocol RFC 765
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.
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 Controled Page
This is type includes an additional header field
for paged files with page level access control
information. The header length must be 5.
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RFC 765 File Transfer Protocol
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.
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.
It is possible for the user to specify an alternate data port by
use of the PORT 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.
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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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 TELNET connection is 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.
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 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
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RFC 765 File Transfer Protocol
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.
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 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 file structure, the EOF is indicated by
the sending Host closing the data connection and all bytes
are data bytes.
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 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
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File Transfer Protocol RFC 765
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.
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 TELNET
connection (e.g., default--NVT-ASCII). <SP> (Space, in the
appropriate language) must not be used WITHIN a restart
marker.
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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 |
+--------+--------+--------+
COMPRESSED
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:
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Replicated Byte:
2 6 8
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
|1 0| n | | d |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
A string of n filler bytes can be compressed into a single
byte, where the filler byte varies with the representation
type. If the type is ASCII or EBCDIC the filler byte is
<SP> (Space, ASCII code 32., EBCDIC code 64). If the type
is Image or Local byte the filler is a zero byte.
Filler String:
2 6
+-+-+-+-+-+-+-+-+
|1 1| n |
+-+-+-+-+-+-+-+-+
The escape sequence is a double byte, the first of which is
the escape byte (all zeros) and the second of which contains
descriptor codes as defined in Block mode. The descriptor
codes have the same meaning as in Block mode and apply to
the succeeding string of bytes.
Compressed mode is useful for obtaining increased bandwidth
on very large network transmissions at a little extra CPU
cost. It can be most effectively used to reduce the size of
printer files such as those generated by RJE Hosts.
ERROR RECOVERY AND RESTART
There is no provision for detecting bits lost or scrambled in data
transfer; this level of error control is handled by the TCP.
However, a restart procedure is provided to protect users from
gross system failures (including failures of a Host, an
FTP-process, or the underlying network).
The restart procedure is defined only for the block and compressed
modes of data transfer. It requires the sender of data to insert
a special marker code in the data stream with some marker
information. The marker information has meaning only to the
sender, but must consist of printable characters in the default or
negotiated language of the TELNET connection (ASCII or EBCDIC).
The marker could represent a bit-count, a record-count, or any
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other information by which a system may identify a data
checkpoint. The receiver of data, if it implements the restart
procedure, would then mark the corresponding position of this
marker in the receiving system, and return this information to the
user.
In the event of a system failure, the user can restart the data
transfer by identifying the marker point with the FTP restart
procedure. The following example illustrates the use of the
restart procedure.
The sender of the data inserts an appropriate marker block in the
data stream at a convenient point. The receiving Host marks the
corresponding data point in its file system and conveys the last
known sender and receiver marker information to the user, either
directly or over the TELNET connection in a 110 reply (depending
on who is the sender). In the event of a system failure, the user
or controller process restarts the server at the last server
marker by sending a restart command with server's marker code as
its argument. The restart command is transmitted over the TELNET
connection and is immediately followed by the command (such as
RETR, STOR or LIST) which was being executed when the system
failure occurred.
FILE TRANSFER FUNCTIONS
The communication channel from the user-PI to the server-PI is
established by a TCP connection from the user to a standard server
port. The user protocol interpreter is responsible for sending FTP
commands and interpreting the replies received; the server-PI
interprets commands, sends replies and directs its DTP to set up the
data connection and transfer the data. If the second party to the
data transfer (the passive transfer process) is the user-DTP then it
is governed through the internal protocol of the user-FTP Host; if it
is a second server-DTP then it is governed by its PI on command from
the user-PI. The FTP replies are discussed in the next section. In
the description of a few of the commands in this section it is
helpful to be explicit about the possible replies.
FTP COMMANDS
ACCESS CONTROL COMMANDS
The following commands specify access control identifiers
(command codes are shown in parentheses).
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USER NAME (USER)
The argument field is a TELNET string identifying the user.
The user identification is that which is required by the
server for access to its file system. This command will
normally be the first command transmitted by the user after
the TELNET connections are made (some servers may require
this). Additional identification information in the form of
a password and/or an account command may also be required by
some servers. Servers may allow a new USER command to be
entered at any point in order to change the access control
and/or accounting information. This has the effect of
flushing any user, password, and account information already
supplied and beginning the login sequence again. All
transfer parameters are unchanged and any file transfer in
progress is completed under the old account.
PASSWORD (PASS)
The argument field is a TELNET string identifying the user's
password. This command must be immediately preceded by the
user name command, and, for some sites, completes the user's
identification for access control. Since password
information is quite sensitive, it is desirable in general
to "mask" it or suppress typeout. It appears that the
server has no foolproof way to achieve this. It is
therefore the responsibility of the user-FTP process to hide
the sensitive password information.
ACCOUNT (ACCT)
The argument field is a TELNET string identifying the user's
account. The command is not necessarily related to the USER
command, as some sites may require an account for login and
others only for specific access, such as storing files. In
the latter case the command may arrive at any time.
There are reply codes to differentiate these cases for the
automaton: when account information is required for login,
the response to a successful PASSword command is reply code
332. On the other hand, if account information is NOT
required for login, the reply to a successful PASSword
command is 230; and if the account information is needed for
a command issued later in the dialogue, the server should
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return a 332 or 532 reply depending on whether he stores
(pending receipt of the ACCounT command) or discards the
command, respectively.
REINITIALIZE (REIN)
This command terminates a USER, flushing all I/O and account
information, except to allow any transfer in progress to be
completed. All parameters are reset to the default settings
and the TELNET connection is left open. This is identical
to the state in which a user finds himself immediately after
the TELNET connection is opened. A USER command may be
expected to follow.
LOGOUT (QUIT)
This command terminates a USER and if file transfer is not
in progress, the server closes the TELNET connection. If
file transfer is in progress, the connection will remain
open for result response and the server will then close it.
If the user-process is transferring files for several USERs
but does not wish to close and then reopen connections for
each, then the REIN command should be used instead of QUIT.
An unexpected close on the TELNET connection will cause the
server to take the effective action of an abort (ABOR) and a
logout (QUIT).
TRANSFER PARAMETER COMMANDS
All data transfer parameters have default values, and the
commands specifying data transfer parameters are required only
if the default parameter values are to be changed. The default
value is the last specified value, or if no value has been
specified, the standard default value as stated here. This
implies that the server must "remember" the applicable default
values. The commands may be in any order except that they must
precede the FTP service request. The following commands
specify data transfer parameters.
DATA PORT (PORT)
The argument is a HOST-PORT specification for the data port
to be used in data connection. There defaults for both the
user and server data ports, and under normal circumstances
this command and its reply are not needed. If this command
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is used the argument is the concatenation of a 32-bit
internet host address and a 16-bit TCP port address. This
address information is broken into 8-bit fields and the
value of each field is transmitted as a decimal number (in
character string representation). The fields are separated
by commas. A port command would be:
PORT h1,h2,h3,h4,p1,p2
where, h1 is the high order 8 bits of the internet host
address.
PASSIVE (PASV)
This command requests the server-DTP to "listen" on a data
port (which is not its default data port) and to wait for a
connection rather than initiate one upon receipt of a
transfer command. The response to this command includes the
host and port address this server is listening on.
REPRESENTATION TYPE (TYPE)
The argument specifies the representation type as described
in the Section on Data Representation and Storage. Several
types take a second parameter. The first parameter is
denoted by a single TELNET character, as is the second
Format parameter for ASCII and EBCDIC; the second parameter
for local byte is a decimal integer to indicate Bytesize.
The parameters are separated by a <SP> (Space, ASCII code
32.).
The following codes are assigned for type:
\ /
A - ASCII | | N - Non-print
|-><-| T - TELNET format effectors
E - EBCDIC| | C - Carriage Control (ASA)
/ \
I - Image
L <byte size> - Local byte Byte size
The default representation type is ASCII Non-print. If the
Format parameter is changed, and later just the first
argument is changed, Format then returns to the Non-print
default.
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FILE STRUCTURE (STRU)
The argument is a single TELNET character code specifying
file structure described in the Section on Data
Representation and Storage.
The following codes are assigned for structure:
F - File (no record structure)
R - Record structure
P - Page structure
The default structure is File.
TRANSFER MODE (MODE)
The argument is a single TELNET character code specifying
the data transfer modes described in the Section on
Transmission Modes.
The following codes are assigned for transfer modes:
S - Stream
B - Block
C - Compressed
The default transfer mode is Stream.
FTP SERVICE COMMANDS
The FTP service commands define the file transfer or the file
system function requested by the user. The argument of an FTP
service command will normally be a pathname. The syntax of
pathnames must conform to server site conventions (with
standard defaults applicable), and the language conventions of
the TELNET connection. The suggested default handling is to
use the last specified device, directory or file name, or the
standard default defined for local users. The commands may be
in any order except that a "rename from" command must be
followed by a "rename to" command and the restart command must
be followed by the interrupted service command. The data, when
transferred in response to FTP service commands, shall always
be sent over the data connection, except for certain
informative replies. The following commands specify FTP
service requests:
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RETRIEVE (RETR)
This command causes the server-DTP to transfer a copy of the
file, specified in the pathname, to the server- or user-DTP
at the other end of the data connection. The status and
contents of the file at the server site shall be unaffected.
STORE (STOR)
This command causes the server-DTP to accept the data
transferred via the data connection and to store the data as
a file at the server site. If the file specified in the
pathname exists at the server site then its contents shall
be replaced by the data being transferred. A new file is
created at the server site if the file specified in the
pathname does not already exist.
APPEND (with create) (APPE)
This command causes the server-DTP to accept the data
transferred via the data connection and to store the data in
a file at the server site. If the file specified in the
pathname exists at the server site, then the data shall be
appended to that file; otherwise the file specified in the
pathname shall be created at the server site.
MAIL FILE (MLFL)
The intent of this command is to enable a user at the user
site to mail data (in form of a file) to another user at the
server site. It should be noted that the files to be mailed
are transmitted via the data connection in ASCII or EBCDIC
type. (It is the user's responsibility to ensure that the
type is correct.) These files should be inserted into the
destination user's mailbox by the server in accordance with
serving Host mail conventions. The mail may be marked as
sent from the particular user HOST and the user specified by
the 'USER' command. The argument field may contain a Host
system ident, or it may be empty. If the argument field is
empty or blank (one or more spaces), then the mail is
destined for a printer or other designated place for general
delivery site mail.
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MAIL (MAIL)
This command allows a user to send mail that is NOT in a
file over the TELNET connection. The argument field may
contain system ident, or it may be empty. The ident is
defined as above for the MLFL command. After the 'MAIL'
command is received, the server is to treat the following
lines as text of the mail sent by the user. The mail text
is to be terminated by a line containing only a single
period, that is, the character sequence "CRLF.CRLF". It is
suggested that a modest volume of mail service should be
free; i.e., it may be entered before a USER command.
MAIL SEND TO TERMINAL (MSND)
This command is like the MAIL command, except that the data
is displayed on the addressed user's terminal, if such
access is currently allowed, otherwise an error is returned.
MAIL SEND TO TERMINAL OR MAILBOX (MSOM)
This command is like the MAIL command, except that the data
is displayed on the addressed user's terminal, if such
access is currently allowed, otherwise the data is placed in
the user's mailbox.
MAIL SEND TO TERMINAL AND MAILBOX (MSAM)
This command is like the MAIL command, except that the data
is displayed on the addressed user's terminal, if such
access is currently allowed, and, in any case, the data is
placed in the user's mailbox.
MAIL RECIPIENT SCHEME QUESTION (MRSQ)
This FTP command is used to select a scheme for the
transmission of mail to several users at the same host. The
schemes are to list the recipients first, or to send the
mail first.
MAIL RECIPIENT (MRCP)
This command is used to identify the individual recipients
of the mail in the transmission of mail for multiple users
at one host.
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ALLOCATE (ALLO)
This command may be required by some servers to reserve
sufficient storage to accommodate the new file to be
transferred. The argument shall be a decimal integer
representing the number of bytes (using the logical byte
size) of storage to be reserved for the file. For files
sent with record or page structure a maximum record or page
size (in logical bytes) might also be necessary; this is
indicated by a decimal integer in a second argument field of
the command. This second argument is optional, but when
present should be separated from the first by the three
TELNET characters <SP> R <SP>. This command shall be
followed by a STORe or APPEnd command. The ALLO command
should be treated as a NOOP (no operation) by those servers
which do not require that the maximum size of the file be
declared beforehand, and those servers interested in only
the maximum record or page size should accept a dummy value
in the first argument and ignore it.
RESTART (REST)
The argument field represents the server marker at which
file transfer is to be restarted. This command does not
cause file transfer but "spaces" over the file to the
specified data checkpoint. This command shall be
immediately followed by the appropriate FTP service command
which shall cause file transfer to resume.
RENAME FROM (RNFR)
This command specifies the file which is to be renamed.
This command must be immediately followed by a "rename to"
command specifying the new file pathname.
RENAME TO (RNTO)
This command specifies the new pathname of the file
specified in the immediately preceding "rename from"
command. Together the two commands cause a file to be
renamed.
ABORT (ABOR)
This command tells the server to abort the previous FTP
service command and any associated transfer of data. The
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abort command may require "special action", as discussed in
the Section on FTP Commands, to force recognition by the
server. No action is to be taken if the previous command
has been completed (including data transfer). The TELNET
connection is not to be closed by the server, but the data
connection must be closed.
There are two cases for the server upon receipt of this
command: (1) the FTP service command was already completed,
or (2) the FTP service command is still in progress.
In the first case, the server closes the data connection
(if it is open) and responds with a 226 reply, indicating
that the abort command was successfully processed.
In the second case, the server aborts the FTP service in
progress and closes the data connection, returning a 426
reply to indicate that the service request terminated in
abnormally. The server then sends a 226 reply,
indicating that the abort command was successfully
processed.
DELETE (DELE)
This command causes the file specified in the pathname to be
deleted at the server site. If an extra level of protection
is desired (such as the query, "DO you really wish to
delete?"), it should be provided by the user-FTP process.
CHANGE WORKING DIRECTORY (CWD)
This command allows the user to work with a different
directory or dataset for file storage or retrieval without
altering his login or accounting information. Transfer
parameters are similarly unchanged. The argument is a
pathname specifying a directory or other system dependent
file group designator.
LIST (LIST)
This command causes a list to be sent from the server to the
passive DTP. If the pathname specifies a directory, the
server should transfer a list of files in the specified
directory. If the pathname specifies a file then the server
should send current information on the file. A null
argument implies the user's current working or default
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directory. The data transfer is over the data connection in
type ASCII or type EBCDIC. (The user must ensure that the
TYPE is appropriately ASCII or EBCDIC).
NAME-LIST (NLST)
This command causes a directory listing to be sent from
server to user site. The pathname should specify a
directory or other system-specific file group descriptor; a
null argument implies the current directory. The server
will return a stream of names of files and no other
information. The data will be transferred in ASCII or
EBCDIC type over the data connection as valid pathname
strings separated by <CRLF> or <NL>. (Again the user must
ensure that the TYPE is correct.)
SITE PARAMETERS (SITE)
This command is used by the server to provide services
specific to his system that are essential to file transfer
but not sufficiently universal to be included as commands in
the protocol. The nature of these services and the
specification of their syntax can be stated in a reply to
the HELP SITE command.
STATUS (STAT)
This command shall cause a status response to be sent over
the TELNET connection in the form of a reply. The command
may be sent during a file transfer (along with the TELNET IP
and Synch signals--see the Section on FTP Commands) in which
case the server will respond with the status of the
operation in progress, or it may be sent between file
transfers. In the latter case the command may have an
argument field. If the argument is a pathname, the command
is analogous to the "list" command except that data shall be
transferred over the TELNET connection. If a partial
pathname is given, the server may respond with a list of
file names or attributes associated with that specification.
If no argument is given, the server should return general
status information about the server FTP process. This
should include current values of all transfer parameters and
the status of connections.
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HELP (HELP)
This command shall cause the server to send helpful
information regarding its implementation status over the
TELNET connection to the user. The command may take an
argument (e.g., any command name) and return more specific
information as a response. The reply is type 211 or 214.
It is suggested that HELP be allowed before entering a USER
command. The server may use this reply to specify
site-dependent parameters, e.g., in response to HELP SITE.
NOOP (NOOP)
This command does not affect any parameters or previously
entered commands. It specifies no action other than that the
server send an OK reply.
The File Transfer Protocol follows the specifications of the
TELNET protocol for all communications over the TELNET connection.
Since, the language used for TELNET communication may be a
negotiated option, all references in the next two sections will be
to the "TELNET language" and the corresponding "TELNET end of line
code". Currently one may take these to mean NVT-ASCII and <CRLF>.
No other specifications of the TELNET protocol will be cited.
FTP commands are "TELNET strings" terminated by the "TELNET end of
line code". The command codes themselves are alphabetic
characters terminated by the character <SP> (Space) if parameters
follow and TELNET-EOL otherwise. The command codes and the
semantics of commands are described in this section; the detailed
syntax of commands is specified in the Section on Commands, the
reply sequences are discussed in the Section on Sequencing of
Commands and Replies, and scenarios illustrating the use of
commands are provided in the Section on Typical FTP Scenarios.
FTP commands may be partitioned as those specifying access-control
identifiers, data transfer parameters, or FTP service requests.
Certain commands (such as ABOR, STAT, QUIT) may be sent over the
TELNET connection while a data transfer is in progress. Some
servers may not be able to monitor the TELNET and data connections
simultaneously, in which case some special action will be
necessary to get the server's attention. The exact form of the
"special action" is undefined; but the following ordered format is
tentatively recommended:
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1. User system inserts the TELNET "Interrupt Process" (IP)
signal in the TELNET stream.
2. User system sends the TELNET "Synch" signal
3. User system inserts the command (e.g., ABOR) in the TELNET
stream.
4. Server PI,, after receiving "IP", scans the TELNET stream
for EXACTLY ONE FTP command.
(For other servers this may not be necessary but the actions
listed above should have no unusual effect.)
FTP REPLIES
Replies to File Transfer Protocol commands are devised to ensure
the synchronization of requests and actions in the process of file
transfer, and to guarantee that the user process always knows the
state of the Server. Every command must generate at least one
reply, although there may be more than one; in the latter case,
the multiple replies must be easily distinguished. In addition,
some commands occur in sequential groups, such as USER, PASS and
ACCT, or RNFR and RNTO. The replies show the existence of an
intermediate state if all preceding commands have been successful.
A failure at any point in the sequence necessitates the repetition
of the entire sequence from the beginning.
The details of the command-reply sequence are made explicit in
a set of state diagrams below.
An FTP reply consists of a three digit number (transmitted as
three alphanumeric characters) followed by some text. The number
is intended for use by automata to determine what state to enter
next; the text is intended for the human user. It is intended
that the three digits contain enough encoded information that the
user-process (the User-PI) will not need to examine the text and
may either discard it or pass it on to the user, as appropriate.
In particular, the text may be server-dependent, so there are
likely to be varying texts for each reply code.
Formally, a reply is defined to contain the 3-digit code, followed
by Space <SP>, followed by one line of text (where some maximum
line length has been specified), and terminated by the TELNET
end-of-line code. There will be cases, however, where the text is
longer than a single line. In these cases the complete text must
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be bracketed so the User-process knows when it may stop reading
the reply (i.e. stop processing input on the TELNET connection)
and go do other things. This requires a special format on the
first line to indicate that more than one line is coming, and
another on the last line to designate it as the last. At least
one of these must contain the appropriate reply code to indicate
the state of the transaction. To satisfy all factions it was
decided that both the first and last line codes should be the
same.
Thus the format for multi-line replies is that the first line
will begin with the exact required reply code, followed
immediately by a Hyphen, "-" (also known as Minus), followed by
text. The last line will begin with the same code, followed
immediately by Space <SP>, optionally some text, and the TELNET
end-of-line code.
For example:
123-First line
Second line
234 A line beginning with numbers
123 The last line
The user-process then simply needs to search for the second
occurrence of the same reply code, followed by <SP> (Space), at
the beginning of a line, and ignore all intermediary lines. If
an intermediary line begins with a 3-digit number, the Server
must pad the front to avoid confusion.
This scheme allows standard system routines to be used for
reply information (such as for the STAT reply), with
"artificial" first and last lines tacked on. In the rare
cases where these routines are able to generate three digits
and a Space at the beginning of any line, the beginning of
each text line should be offset by some neutral text, like
Space.
This scheme assumes that multi-line replies may not be nested.
We have found that, in general, nesting of replies will not
occur, except for random system messages (also called
spontaneous replies) which may interrupt another reply. System
messages (i.e. those not processed by the FTP server) will NOT
carry reply codes and may occur anywhere in the command-reply
sequence. They may be ignored by the User-process as they are
only information for the human user.
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The three digits of the reply each have a special significance.
This is intended to allow a range of very simple to very
sophisticated response by the user-process. The first digit
denotes whether the response is good, bad or incomplete.
(Referring to the state diagram) an unsophisticated user-process
will be able to determine its next action (proceed as planned,
redo, retrench, etc.) by simply examining this first digit. A
user-process that wants to know approximately what kind of error
occurred (e.g. file system error, command syntax error) may
examine the second digit, reserving the third digit for the finest
gradation of information (e.g. RNTO command without a preceding
RNFR.)
There are five values for the first digit of the reply code:
1yz Positive Preliminary reply
The requested action is being initiated; expect another
reply before proceeding with a new command. (The
user-process sending another command before the
completion reply would be in violation of protocol; but
server-FTP processes should queue any commands that
arrive while a preceding command is in progress.) This
type of reply can be used to indicate that the command
was accepted and the user-process may now pay attention
to the data connections, for implementations where
simultaneous monitoring is difficult.
2yz Positive Completion reply
The requested action has been successfully completed. A
new request may be initiated.
3yz Positive Intermediate reply
The command has been accepted, but the requested action
is being held in abeyance, pending receipt of further
information. The user should send another command
specifying this information. This reply is used in
command sequence groups.
4yz Transient Negative Completion reply
The command was not accepted and the requested action did
not take place, but the error condition is temporary and
the action may be requested again. The user should
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return to the beginning of the command sequence, if any.
It is difficult to assign a meaning to "transient",
particularly when two distinct sites (Server and
User-processes) have to agree on the interpretation.
Each reply in the 4yz category might have a slightly
different time value, but the intent is that the
user-process is encouraged to try again. A rule of thumb
in determining if a reply fits into the 4yz or the 5yz
(Permanent Negative) category is that replies are 4yz if
the commands can be repeated without any change in
command form or in properties of the User or Server (e.g.
the command is spelled the same with the same arguments
used; the user does not change his file access or user
name; the server does not put up a new implementation.)
5yz Permanent Negative Completion reply
The command was not accepted and the requested action did
not take place. The User-process is discouraged from
repeating the exact request (in the same sequence). Even
some "permanent" error conditions can be corrected, so
the human user may want to direct his User-process to
reinitiate the command sequence by direct action at some
point in the future (e.g. after the spelling has been
changed, or the user has altered his directory status.)
The following function groupings are encoded in the second
digit:
x0z Syntax - These replies refer to syntax errors,
syntactically correct commands that don't fit any
functional category, unimplemented or superfluous
commands.
x1z Information - These are replies to requests for
information, such as status or help.
x2z Connections - Replies referring to the TELNET and data
connections.
x3z Authentication and accounting - Replies for the login
process and accounting procedures.
x4z Unspecified as yet
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x5z File system - These replies indicate the status of the
Server file system vis-a-vis the requested transfer or
other file system action.
The third digit gives a finer gradation of meaning in each of
the function categories, specified by the second digit. The
list of replies below will illustrate this. Note that the text
associated with each reply is recommended, rather than
mandatory, and may even change according to the command with
which it is associated. The reply codes, on the other hand,
must strictly follow the specifications in the last section;
that is, Server implementations should not invent new codes for
situations that are only slightly different from the ones
described here, but rather should adapt codes already defined.
A command such as TYPE or ALLO whose successful execution
does not offer the user-process any new information will
cause a 200 reply to be returned. If the command is not
implemented by a particular Server-FTP process because it
has no relevance to that computer system, for example ALLO
at a TOPS20 site, a Positive Completion reply is still
desired so that the simple User-process knows it can proceed
with its course of action. A 202 reply is used in this case
with, for example, the reply text: "No storage allocation
necessary." If, on the other hand, the command requests a
non-site-specific action and is unimplemented, the response
is 502. A refinement of that is the 504 reply for a command
that IS implemented, but that requests an unimplemented
parameter.
Reply Codes by Function Groups
200 Command okay
500 Syntax error, command unrecognized
[This may include errors such as command line too long.]
501 Syntax error in parameters or arguments
202 Command not implemented, superfluous at this site.
502 Command not implemented
503 Bad sequence of commands
504 Command not implemented for that parameter
110 Restart marker reply.
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In this case the text is exact and not left to the
particular implementation; it must read:
MARK yyyy = mmmm
where yyyy is User-process data stream marker, and mmmm
server's equivalent marker. (note the spaces between
markers and "=".)
119 Terminal not available, will try mailbox.
211 System status, or system help reply
212 Directory status
213 File status
214 Help message
(on how to use the server or the meaning of a particular
non-standard command. This reply is useful only to the
human user.)
215 <scheme> is the preferred scheme.
120 Service ready in nnn minutes
220 Service ready for new user
221 Service closing TELNET connection
(logged out if appropriate)
421 Service not available, closing TELNET connection.
This may be a reply to any command if the service knows it
must shut down.]
125 Data connection already open; transfer starting
225 Data connection open; no transfer in progress
425 Can't open data connection
226 Closing data connection;
requested file action successful (for example, file transfer
or file abort.)
426 Connection closed; transfer aborted.
227 Entering Passive Mode. h1,h2,h3,h4,p1,p2
230 User logged in, proceed
530 Not logged in
331 User name okay, need password
332 Need account for login
532 Need account for storing files
150 File status okay; about to open data connection.
151 User not local; Will forward to <user>@<host>.
152 User Unknown; Mail will be forwarded by the operator.
250 Requested file action okay, completed.
350 Requested file action pending further information
450 Requested file action not taken:
file unavailable (e.g. file busy)
550 Requested action not taken:
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file unavailable (e.g. file not found, no access)
451 Requested action aborted: local error in processing
551 Requested action aborted: page type unknown
452 Requested action not taken:
insufficient storage space in system
552 Requested file action aborted:
exceeded storage allocation (for current directory or
dataset)
553 Requested action not taken:
file name not allowed
354 Start mail input; end with <CR><LF>.<CR><LF>
Numeric Order List of Reply Codes
110 Restart marker reply.
In this case the text is exact and not left to the
particular implementation; it must read:
MARK yyyy = mmmm
where yyyy is User-process data stream marker, and mmmm
server's equivalent marker. (note the spaces between
markers and "=".)
119 Terminal not available, will try mailbox.
120 Service ready in nnn minutes
125 Data connection already open; transfer starting
150 File status okay; about to open data connection.
151 User not local; Will forward to <user>@<host>.
152 User Unknown; Mail will be forwarded by the operator.
200 Command okay
202 Command not implemented, superfluous at this site.
211 System status, or system help reply
212 Directory status
213 File status
214 Help message
(on how to use the server or the meaning of a particular
non-standard command. This reply is useful only to the
human user.)
215 <scheme> is the preferred scheme.
220 Service ready for new user
221 Service closing TELNET connection
(logged out if appropriate)
225 Data connection open; no transfer in progress
226 Closing data connection;
requested file action successful (for example, file transfer
or file abort.)
227 Entering Passive Mode. h1,h2,h3,h4,p1,p2
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230 User logged in, proceed
250 Requested file action okay, completed.
331 User name okay, need password
332 Need account for login
350 Requested file action pending further information
354 Start mail input; end with <CR><LF>.<CR><LF>
421 Service not available, closing TELNET connection.
This may be a reply to any command if the service knows it
must shut down.]
425 Can't open data connection
426 Connection closed; transfer aborted.
450 Requested file action not taken:
file unavailable (e.g. file busy)
451 Requested action aborted: local error in processing
452 Requested action not taken:
insufficient storage space in system
500 Syntax error, command unrecognized
[This may include errors such as command line too long.]
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command not implemented for that parameter
530 Not logged in
532 Need account for storing files
550 Requested action not taken:
file unavailable (e.g. file not found, no access)
551 Requested action aborted: page type unknown
552 Requested file action aborted:
exceeded storage allocation (for current directory or
dataset)
553 Requested action not taken:
file name not allowed
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DECLARATIVE SPECIFICATIONS
MINIMUM IMPLEMENTATION
In order to make FTP workable without needless error messages, the
following minimum implementation is required for all servers:
TYPE - ASCII Non-print
MODE - Stream
STRUCTURE - File, Record
COMMANDS - USER, QUIT, PORT,
TYPE, MODE, STRU,
for the default values
RETR, STOR,
NOOP.
The default values for transfer parameters are:
TYPE - ASCII Non-print
MODE - Stream
STRU - File
All Hosts must accept the above as the standard defaults.
CONNECTIONS
The server protocol interpreter shall "listen" on Port L. The
user or user protocol interpreter shall initiate the full-duplex
TELNET connection. Server- and user- processes should follow the
conventions of the TELNET protocol as specified in the ARPA
Internet Protocol Handbook. Servers are under no obligation to
provide for editing of command lines and may specify that it be
done in the user Host. The TELNET connection shall be closed by
the server at the user's request after all transfers and replies
are completed.
The user-DTP must "listen" on the specified data port; this may be
the default user port (U) or a port specified in the PORT command.
The server shall initiate the data connection from his own default
data port (L-1) using the specified user data port. The direction
of the transfer and the port used will be determined by the FTP
service command.
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When data is to be transferred between two servers, A and B (refer
to Figure 2), the user-PI, C, sets up TELNET connections with both
server-PI's. One of the servers, say A, is then sent a PASV
command telling him to "listen" on his data port rather than
initiate a connection when he receives a transfer service command.
When the user-PI receives an acknowledgment to the PASV command,
which includes the identity of the host and port being listened
on, the user-PI then sends A's port, a, to B in a PORT command; a
reply is returned. The user-PI may then send the corresponding
service commands to A and B. Server B initiates the connection
and the transfer proceeds. The command-reply sequence is listed
below where the messages are vertically synchronous but
horizontally asynchronous:
User-PI - Server A User-PI - Server B
------------------ ------------------
C->A : Connect C->B : Connect
C->A : PASV
A->C : 227 Entering Passive Mode. A1,A2,A3,A4,a1,a2
C->B : PORT A1,A2,A3,A4,a1,a2
B->C : 200 Okay
C->A : STOR C->B : RETR
B->A : Connect to HOST-A, PORT-a
The data connection shall be closed by the server under the
conditions described in the Section on Establishing Data
Connections. If the server wishes to close the connection after a
transfer where it is not required, he should do so immediately
after the file transfer is completed. He should not wait until
after a new transfer command is received because the user-process
will have already tested the data connection to see if it needs to
do a "listen"; (recall that the user must "listen" on a closed
data port BEFORE sending the transfer request). To prevent a race
condition here, the server sends a reply (226) after closing the
data connection (or if the connection is left open, a "file
transfer completed" reply (250) and the user-PI should wait for
one of these replies before issuing a new transfer command.
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COMMANDS
The commands are TELNET character string transmitted over the
TELNET connections as described in the Section on FTP Commands.
The command functions and semantics are described in the Section
on Access Control Commands, Transfer Parameter Commands, FTP
Service Commands, and Miscellaneous Commands. The command syntax
is specified here.
The commands begin with a command code followed by an argument
field. The command codes are four or fewer alphabetic characters.
Upper and lower case alphabetic characters are to be treated
identically. Thus any of the following may represent the retrieve
command:
RETR Retr retr ReTr rETr
This also applies to any symbols representing parameter values,
such as A or a for ASCII TYPE. The command codes and the argument
fields are separated by one or more spaces.
The argument field consists of a variable length character string
ending with the character sequence <CRLF> (Carriage Return,
Linefeed) for NVT-ASCII representation; for other negotiated
languages a different end of line character might be used. It
should be noted that the server is to take NO action until the end
of line code is received.
The syntax is specified below in NVT-ASCII. All characters in the
argument field are ASCII characters including any ASCII
represented decimal integers. Square brackets denote an optional
argument field. If the option is not taken, the appropriate
default is implied.
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The following are the FTP commands:
USER <SP> <username> <CRLF>
PASS <SP> <password> <CRLF>
ACCT <SP> <account information> <CRLF>
REIN <CRLF>
QUIT <CRLF>
PORT <SP> <Host-port> <CRLF>
PASV <CRLF>
TYPE <SP> <type code> <CRLF>
STRU <SP> <structure code> <CRLF>
MODE <SP> <mode code> <CRLF>
RETR <SP> <pathname> <CRLF>
STOR <SP> <pathname> <CRLF>
APPE <SP> <pathname> <CRLF>
MLFL [<SP> <ident>] <CRLF>
MAIL [<SP> <ident>] <CRLF>
MSND [<SP> <ident>] <CRLF>
MSOM [<SP> <ident>] <CRLF>
MSAM [<SP> <ident>] <CRLF>
MRSQ [<SP> <scheme>] <CRLF>
MRCP <SP> <ident> <CRLF>
ALLO <SP> <decimal integer>
[<SP> R <SP> <decimal integer>] <CRLF>
REST <SP> <marker> <CRLF>
RNFR <SP> <pathname> <CRLF>
RNTO <SP> <pathname> <CRLF>
ABOR <CRLF>
DELE <SP> <pathname> <CRLF>
CWD <SP> <pathname> <CRLF>
LIST [<SP> <pathname>] <CRLF>
NLST [<SP> <pathname>] <CRLF>
SITE <SP> <string> <CRLF>
STAT [<SP> <pathname>] <CRLF>
HELP [<SP> <string>] <CRLF>
NOOP <CRLF>
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The syntax of the above argument fields (using BNF notation where
applicable ) is:
<username> ::= <string>
<password> ::= <string>
<account information> ::= <string>
<string> ::= <char> | <char><string>
<char> ::= any of the 128 ASCII characters except <CR> and <LF>
<marker> ::= <pr string>
<pr string> ::= <pr char> | <pr char><pr string>
<pr char> ::= printable characters, any
ASCII code 33 through 126
<byte size> ::= any decimal integer 1 through 255
<Host-port> ::= <Host-number>,<Port-number>
<Host-number> ::= <number>,<number>,<number>,<number>
<Port-number> ::= <number>,<number>
<number> ::= any decimal integer 0 through 255
<ident> ::= <string>
<scheme> ::= R | T | ?
<form code> ::= N | T | C
<type code> ::= A [<SP> <form code>]
| E [<SP> <form code>]
| I
| L <SP> <byte size>
<structure code> ::= F | R | P
<mode code> ::= S | B | C
<pathname> ::= <string>
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SEQUENCING OF COMMANDS AND REPLIES
The communication between the user and server is intended to be an
alternating dialogue. As such, the user issues an FTP command and
the server responds with a prompt primary reply. The user should
wait for this initial primary success or failure response before
sending further commands.
Certain commands require a second reply for which the user should
also wait. These replies may, for example, report on the progress
or completion of file transfer or the closing of the data
connection. They are secondary replies to file transfer commands.
One important group of informational replies is the connection
greetings. Under normal circumstances, a server will send a 220
reply, "awaiting input", when the connection is completed. The
user should wait for this greeting message before sending any
commands. If the server is unable to accept input right away, he
should send a 120 "expected delay" reply immediately and a 220
reply when ready. The user will then know not to hang up if there
is a delay.
The table below lists alternative success and failure replies for
each command. These must be strictly adhered to; a server may
substitute text in the replies, but the meaning and action implied
by the code numbers and by the specific command reply sequence
cannot be altered.
Command-Reply Sequences
In this section, the command-reply sequence is presented. Each
command is listed with its possible replies; command groups are
listed together. Preliminary replies are listed first (with
their succeeding replies indented and under them), then
positive and negative completion, and finally intermediary
replies with the remaining commands from the sequence
following. This listing forms the basis for the state
diagrams, which will be presented separately.
Connection Establishment
120
220
220
421
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Login
USER
230
530
500, 501, 421
331, 332
PASS
230
202
530
500, 501, 503, 421
332
ACCT
230
202
530
500, 501, 503, 421
Logout
QUIT
221
500
REIN
120
220
220
421
500, 502
Transfer parameters
PORT
200
500, 501, 421, 530
PASV
227
500, 501, 502, 421, 530
MODE, TYPE, STRU
200
500, 501, 504, 421, 530
File action commands
ALLO
200
202
500, 501, 504, 421, 530
REST
500, 501, 502, 421, 530
350
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STOR
125, 150
(110)
226, 250
425, 426, 451, 551, 552
532, 450, 452, 553
500, 501, 421, 530
RETR
125, 150
(110)
226, 250
425, 426, 451
450, 550
500, 501, 421, 530
LIST, NLST
125, 150
226, 250
425, 426, 451
450
500, 501, 502, 421, 530
APPE
125, 150
(110)
226, 250
425, 426, 451, 551, 552
532, 450, 550, 452, 553
500, 501, 502, 421, 530
MLFL
125, 150, 151, 152
226, 250
425, 426, 451, 552
532, 450, 550, 452, 553
500, 501, 502, 421, 530
RNFR
450, 550
500, 501, 502, 421, 530
350
RNTO
250
532, 553
500, 501, 502, 503, 421, 530
DELE, CWD
250
450, 550
500, 501, 502, 421, 530
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ABOR
225, 226
500, 501, 502, 421
MAIL, MSND
151, 152
354
250
451, 552
354
250
451, 552
450, 550, 452, 553
500, 501, 502, 421, 530
MSOM, MSAM
119, 151, 152
354
250
451, 552
354
250
451, 552
450, 550, 452, 553
500, 501, 502, 421, 530
MRSQ
200, 215
500, 501, 502, 421, 530
MRCP
151, 152
200
200
450, 550, 452, 553
500, 501, 502, 503, 421
Informational commands
STAT
211, 212, 213
450
500, 501, 502, 421, 530
HELP
211, 214
500, 501, 502, 421
Miscellaneous commands
SITE
200
202
500, 501, 530
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NOOP
200
500 421
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STATE DIAGRAMS
Here we present state diagrams for a very simple minded FTP
implementation. Only the first digit of the reply codes is used.
There is one state diagram for each group of FTP commands or command
sequences.
The command groupings were determined by constructing a model for
each command then collecting together the commands with structurally
identical models.
For each command or command sequence there are three possible
outcomes: success (S), failure (F), and error (E). In the state
diagrams below we use the symbol B for "begin", and the symbol W for
"wait for reply".
We first present the diagram that represents the largest group of FTP
commands:
1,3 +---+
----------->| E |
| +---+
|
+---+ cmd +---+ 2 +---+
| B |---------->| W |---------->| S |
+---+ +---+ +---+
|
| 4,5 +---+
----------->| F |
+---+
This diagram models the commands:
ABOR, ALLO, DELE, CWD, HELP, MODE, MRCP, MRSQ, NOOP, PASV,
QUIT, SITE, PORT, STAT, STRU, TYPE.
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The other large group of commands is represented by a very similar
diagram:
3 +---+
----------->| E |
| +---+
|
+---+ cmd +---+ 2 +---+
| B |---------->| W |---------->| S |
+---+ --->+---+ +---+
| | |
| | | 4,5 +---+
| 1 | ----------->| F |
----- +---+
This diagram models the commands:
APPE, LIST, MLFL, NLST, REIN, RETR, STOR.
Note that this second model could also be used to represent the first
group of commands, the only difference being that in the first group
the 100 series replies are unexpected and therefore treated as error,
while the second group expects (some may require) 100 series replies.
The remaining diagrams model command sequences, perhaps the simplest
of these is the rename sequence:
+---+ RNFR +---+ 1,2 +---+
| B |---------->| W |---------->| E |
+---+ +---+ -->+---+
| | |
3 | | 4,5 |
-------------- ------ |
| | | +---+
| ------------->| S |
| | 1,3 | | +---+
| 2| --------
| | | |
V | | |
+---+ RNTO +---+ 4,5 ----->+---+
| |---------->| W |---------->| F |
+---+ +---+ +---+
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A very similar diagram models the Mail and Send commands:
---- 1
| |
+---+ cmd -->+---+ 2 +---+
| B |---------->| W |---------->| E |
+---+ +---+ -->+---+
| | |
3 | | 4,5 |
-------------- ------ |
| | | +---+
| ------------->| S |
| | 1,3 | | +---+
| 2| --------
| | | |
V | | |
+---+ text +---+ 4,5 ----->+---+
| |---------->| W |---------->| F |
+---+ +---+ +---+
This diagram models the commands:
MAIL, MSND, MSOM, MSAM.
Note that the "text" here is a series of lines sent from the user
to the server with no response expected until the last line is
sent, recall that the last line must consist only of a single
period.
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The next diagram is a simple model of the Restart command:
+---+ REST +---+ 1,2 +---+
| B |---------->| W |---------->| E |
+---+ +---+ -->+---+
| | |
3 | | 4,5 |
-------------- ------ |
| | | +---+
| ------------->| S |
| | 3 | | +---+
| 2| --------
| | | |
V | | |
+---+ cmd +---+ 4,5 ----->+---+
| |---------->| W |---------->| F |
+---+ -->+---+ +---+
| |
| 1 |
------
Where "cmd" is APPE, STOR, RETR, or MLFL.
We note that the above three models are similar, in fact the Mail
diagram and the Rename diagram are structurally identical. The
Restart differs from the other two only in the treatment of 100
series replies at the second stage.
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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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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
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TYPICAL FTP SCENARIO
User at Host U wanting to transfer files to/from Host S:
In general the user will communicate to the server via a mediating
user-FTP process. The following may be a typical scenario. The
user-FTP prompts are shown in parentheses, '---->' represents
commands from Host U to Host S, and '<----' represents replies from
Host S to Host U.
LOCAL COMMANDS BY USER ACTION INVOLVED
ftp (host) multics<CR> Connect to Host S, port L,
establishing TELNET connections
<---- 220 Service ready <CRLF>
username Doe <CR> USER Doe<CRLF>---->
<---- 331 User name ok,
need password<CRLF>
password mumble <CR> PASS mumble<CRLF>---->
<---- 230 User logged in.<CRLF>
retrieve (local type) ASCII<CR>
(local pathname) test 1 <CR> User-FTP opens local file in ASCII.
(for.pathname) test.pl1<CR> RETR test.pl1<CRLF> ---->
<---- 150 File status okay;
about to open data connection
Server makes data connection
to port U
<CRLF>
<---- 226 Closing data connection,
file transfer successful<CRLF>
type Image<CR> TYPE I<CRLF> ---->
<---- 200 Command OK<CRLF>
store (local type) image<CR>
(local pathname) file dump<CR> User-FTP opens local file in Image.
(for.pathname) >udd>cn>fd<CR> STOR >udd>cn>fd<CRLF> ---->
<---- 450 Access denied<CRLF>
terminate QUIT <CRLF> ---->
Server closes all
connections.
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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.
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APPENDIX ON MAIL
The basic commands transmitting mail are the MAIL and the MLFL
commands. These commands cause the transmitted data to be entered
into the recipients mailbox.
MAIL <SP> <recipient name> <CRLF>
If accepted, returns 354 reply and considers all succeeding
lines to be the message text, terminated by a line containing
only a period, upon which a 250 completion reply is returned.
Various errors are possible.
MLFL <SP> <recipient name> <CRLF>
If accepted, acts like a STOR command, except that the data is
considered to be the message text. Various errors are
possible.
There are two possible preliminary replies that a server may use to
indicate that it is accepting mail for a user whose mailbox is not at
that server.
151 User not local; Will forward to <user>@<host>.
This reply indicates that the server knows the user's mailbox
is on another host and will take responsibility for forwarding
the mail to that host. For example, at BBN (or ISI) there are
several host which each have a list of many of the users on
several of the host. These hosts then can accept mail for any
user on their list and forward it to the correct host.
152 User Unknown; Mail will be forwarded by the operator.
This reply indicates that the host does not recognize the user
name, but that it will accept the mail and have the operator
attempt to deliver it. This is useful if the user name is
misspelled, but may be a disservice if the mail is really
undeliverable.
Three FTP commands provide for "sending" a message to a logged-in
user's terminal, as well as variants for mailing it normally whether
the user is logged in or not.
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MSND -- SeND to terminal.
Returns 450 failure reply if the addressee is refusing or not
logged in.
MSOM -- Send to terminal Or Mailbox.
Returns 119 notification reply if terminal is not accessible.
MSAM -- Send to terminal And Mailbox.
Returns 119 notification reply if terminal is not accessible.
Note that for MSOM and MSAM, it is the mailing which determines
success, not the sending, although MSOM as implemented uses a 119
reply (in addition to the normal success/failure code) to indicate
that because the SEND failed, an attempt is being made to mail the
message instead. There are no corresponding variants for MLFL, since
messages transmitted in this way are generally short.
There are two FTP commands which allow one to mail the text of a
message to several recipients simultaneously; such message
transmission is far more efficient than the practice of sending the
text again and again for each additional recipient at a site.
There are two basic ways of sending a single text to several
recipients. In one, all recipients are specified first, and then the
text is sent; in the other, the order is reversed and the text is
sent first, followed by the recipients. Both schemes are necessary
because neither by itself is optimal for all systems, as will be
explained later. To select a particular scheme, the MRSQ command is
used; to specify recipients after a scheme is chosen, MRCP commands
are given; and to furnish text, the MAIL or MLFL commands are used.
Scheme Selection: MRSQ
MRSQ is the means by which a user program can test for
implementation of MRSQ/MRCP, select a particular scheme, reset its
state thereof, and even do some rudimentary negotiation. Its
format is like that of the TYPE command, as follows:
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MRSQ [<SP> <scheme>] <CRLF>
<scheme> = a single character. The following are defined:
R Recipients first. If not implemented, T must be.
T Text first. If this is not implemented, R must be.
? Request for preference. Must always be implemented.
No argument means a "selection" of none of the schemes (the
default).
Replies:
200 OK, we'll use specified scheme.
215 <scheme> This is the scheme I prefer.
501 I understand MRSQ but can't use that scheme.
5xx Command unrecognized or unimplemented.
Three aspects of MRSQ need to be pointed out here. The first is
that an MRSQ with no argument must always return a 200 reply and
restore the default state of having no scheme selected. Any other
reply implies that MRSQ and hence MRCP are not understood or
cannot be performed correctly.
The second is that the use of "?" as a <scheme> asks the FTP
server to return a 215 reply in which the server specifies a
"preferred" scheme. The format of this reply is simple:
215 <SP> <scheme> [<SP> <arbitrary text>] <CRLF>
Any other reply (e.g. 4xx or 5xx) implies that MRSQ and MRCP
are not implemented, because "?" must always be implemented if
MRSQ is.
The third important thing about MRSQ is that it always has the
side effect of resetting all schemes to their initial state. This
reset must be done no matter what the reply will be - 200, 215, or
501. The actions necessary for a reset will be explained when
discussing how each scheme actually works.
Message Text Specification: MAIL/MLFL
Regardless of which scheme (if any) has been selected, a MAIL or
MLFL with a non-null argument will behave exactly as before; the
MRSQ/MRCP commands have no effect on them. However, such normal
MAIL/MLFL commands do have the same side effect as MRSQ; they
"reset" the current scheme to its initial state.
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It is only when the argument is null (e.g. MAIL<CRLF> or
MLFL<CRLF>) that the particular scheme being used is important,
because rather than producing an error (as most servers currently
do), the server will accept message text for this "null"
specification; what it does with it depends on which scheme is in
effect, and will be described in "Scheme Mechanics".
Recipient specification: MRCP
In order to specify recipient names (i.e., idents) and receive
some acknowledgment (or refusal) for each name, the following
command is used:
MRCP <SP> <ident> <CRLF>
Reply for no scheme:
503 No scheme specified yet; use MRSQ.
Replies for scheme T are identical to those for MAIL/MLFL.
Replies for scheme R (recipients first):
200 OK, name stored.
452 Recipient table full, this name not stored.
553 Recipient name rejected.
4xx Temporary error, try this name again later.
5xx Permanent error, report to sender.
Note that use of this command is an error if no scheme has been
selected yet; an MRSQ <scheme> must have been given if MRCP is to
be used.
Scheme mechanics: MRSQ R (Recipients first)
In the recipients-first scheme, MRCP is used to specify names
which the FTP server stores in a list or table. Normally the
reply for each MRCP will be either a 200 for acceptance, or a
4xx/5xx code for rejection; all 5xx codes are permanent rejections
(e.g. user not known) which should be reported to the human
sender, whereas 4xx codes in general connote some temporary error
that may be rectified later. None of the 4xx/5xx replies impinge
on previous or succeeding MRCP commands, except for 452 which
indicates that no further MRCP's will succeed unless a message is
sent to the already stored recipients or a reset is done.
Sending message text to stored recipients is done by giving a MAIL
or MLFL command with no argument; that is, just MAIL<CRLF> or
MLFL<CRLF>. Transmission of the message text is exactly the same
as for normal MAIL/MLFL; however, a positive acknowledgment at the
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end of transmission means that the message has been sent to ALL
recipients that were remembered with MRCP, and a failure code
means that it should be considered to have failed for ALL of these
specified recipients. This applies regardless of the actual error
code; and whether the reply signifies success or failure, all
stored recipient names are flushed and forgotten - in other words,
things are reset to their initial state. This purging of the
recipient name list must also be done as the "reset" side effect
of any use of MRSQ.
A 452 reply to an MRCP can thus be handled by using a MAIL/MLFL to
specify the message for currently stored recipients, and then
sending more MRCP's and another MAIL/MLFL, as many times as
necessary; for example, if a server only had room for 10 names
this would result in a 50-recipient message being sent 5 times, to
10 different recipients each time.
If a user attempts to specify message text (MAIL/MLFL with no
argument) before any successful MRCP's have been given, this
should be treated exactly as a "normal" MAIL/MLFL with a null
recipient would be; some servers will return an error of some
type, such as "550 Null recipient".
See Example 1 for an example using MRSQ R.
Scheme mechanics: MRSQ T (Text first)
In the text-first scheme, MAIL/MLFL with no argument is used to
specify message text, which the server stores away. Succeeding
MRCP's are then treated as if they were MAIL/MLFL commands, except
that none of the text transfer manipulations are done; the stored
message text is sent to the specified recipient, and a reply code
is returned identical to that which an actual MAIL/MLFL would
invoke. (Note ANY 2xx code indicates success.)
The stored message text is not forgotten until the next MAIL/MLFL
or MRSQ, which will either replace it with new text or flush it
entirely. Any use of MRSQ will reset this scheme by flushing
stored text, as will any use of MAIL/MLFL with a non-null
argument.
If an MRCP is seen before any message text has been stored, the
user in effect is trying to send a null message; some servers
might allow this, others would return an error code.
See Example 2 for an example using MRSQ T.
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Why two schemes anyway?
Because neither by itself is optimal for all systems. MRSQ R
allows more of a "bulk" mailing, because everything is saved up
and then mailed simultaneously; this is very useful for systems
such as ITS where the FTP server does not itself write mail
directly, but hands it on to a central mailer demon of great
power; the more information (e.g. recipients) associated with a
single "hand-off", the more efficiently mail can be delivered.
By contrast, MRSQ T is geared to FTP servers which want to deliver
mail directly, in one-by-one incremental fashion. This way they
can return an individual success/failure reply code for each
recipient given which may depend on variable file system factors
such as exceeding disk allocation, mailbox access conflicts, and
so forth; if they tried to emulate MRSQ R's bulk mailing, they
would have to ensure that a success reply to the MAIL/MLFL indeed
meant that it had been delivered to ALL recipients specified - not
just some.
Notes:
* Because these commands are not required in the minimum
implementation of FTP, one must be prepared to deal with sites
which don't recognize either MRSQ or MRCP. "MRSQ" and "MRSQ ?"
are explicitly designed as tests to see whether either scheme is
implemented; MRCP is not, and a failure return of the
"unimplemented" variety could be confused with "No scheme
selected yet", or even with "Recipient unknown". Be safe, be
sure, use MRSQ!
* There is no way to indicate in a positive response to "MRSQ ?"
that the preferred "scheme" for a server is that of the default
state; i.e. none of the multi-recipient schemes. The rationale
is that in this case, it would be pointless to implement
MRSQ/MRCP at all, and the response would therefore be negative.
* One reason that the use of MAIL/MLFL is restricted to null
arguments with this multi-recipient extension is the ambiguity
that would result if a non-null argument were allowed; for
example, if MRSQ R was in effect and some MRCP's had been given,
and a MAIL FOO<CRLF> was done, there would be no way to
distinguish a failure reply for mailbox "FOO" from a global
failure for all recipients specified. A similar situation
exists for MRSQ T; it would not be clear whether the text was
stored and the mailbox failed, or vice versa, or both.
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* "Resets" are done by all MRSQ's and "normal" MAIL/MLFL's to
avoid confusion and overly complicated implementation. The MRSQ
command implies a change or uncertainty of status, and the
latter commands would otherwise have to use some independent
mechanisms to avoid clobbering the data bases (e.g., message
text storage area) used by the T/R schemes. However, once a
scheme is selected, it remains "in effect" just as a "TYPE A"
remains selected. The recommended way for doing a reset,
without changing the current selection, is with "MRSQ ?".
Remember that "MRSQ" alone reverts to the no-scheme state.
* It is permissible to intersperse other FTP commands among the
MRSQ/MRCP/MAIL sequences.
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Example 1
Example of MRSQ R (Recipients first)
This is an example of how MRSQ R is used; first the user must
establish that the server in fact implements MRSQ:
U: MRSQ
S: 200 OK, no scheme selected.
An MRSQ with a null argument always returns a 200 if implemented,
selecting the "scheme" of null, i.e. none of them. If MRSQ were
not implemented, a code of 4xx or 5xx would be returned.
U: MRSQ R
S: 200 OK, using that scheme
All's well; now the recipients can be specified.
U: MRCP Foo
S: 200 OK
U: MRCP Raboof
S: 553 Who's that? No such user here.
U: MRCP bar
S: 200 OK
Well, two out of three ain't bad. Note that the demise of
"Raboof" has no effect on the storage of "Foo" or "bar". Now to
furnish the message text, by giving a MAIL or MLFL with no
argument:
U: MAIL
S: 354 Type mail, ended by <CRLF>.<CRLF>
U: Blah blah blah blah....etc etc etc
U: .
S: 250 Mail sent.
The text has now been sent to both "Foo" and "bar".
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Example 2
Example of MRSQ T (Text first)
Using the same message as the previous example:
U: MRSQ ?
S: 215 T Text first, please.
MRSQ is indeed implemented, and the server says that it prefers
"T", but that needn't stop the user from trying something else:
U: MRSQ R
S: 501 Sorry, I really can't do that.
It's possible that it could have understood "R" also, but in
general it's best to use the "preferred" scheme, since the server
knows which is most efficient for its particular site. Anyway:
U: MRSQ T
S: 200 OK, using that scheme.
Scheme "T" is now selected, and the text must be sent:
U: MAIL
S: 354 Type mail, ended by <CRLF>.<CRLF>
U: Blah blah blah blah....etc etc etc
U: .
S: 250 Mail stored.
Now recipients can be specified:
U: MRCP Foo
S: 250 Stored mail sent.
U: MRCP Raboof
S: 553 Who's that? No such user here.
U: MRCP bar
S: 250 Stored mail sent.
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Again, the text has now been sent to both "Foo" and "bar", and
still remains stored. A new message can be sent with another
MAIL/MRCP... sequence, but the fastidious or paranoid could chose
to do:
U: MRSQ ?
S: 215 T Text first, please.
Which resets things without altering the scheme in effect.
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APPENDIX ON PAGE STRUCTURE
The need for FTP to support page structure derives principally from
the need to support efficient transmission of files between TOPS20
systems, particularly the files used by NLS.
The file system of TOPS20 is based on the concept of pages. The
system level is most efficient at manipulating files as pages.
System level programs provide 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 TOPS20 file is just a bunch of words pointed to by a page table.
If those words contain CRLF's, fine -- but that doesn't mean "record"
to TOPS20.
A TOPS20 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 version
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 pages 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 TOPS20 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.
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In fact both of these special cases, "holey" files and
end-of-file pointers not at the end of the file, occur with NLS data
files.
The TOPS20 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 TOPS20 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 21 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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Data transfers are implemented like the layers of an onion: some
characters are packaged into a line. Some lines are packaged into a
file. The file is broken into other manageable units for
transmission. Those units have compression applied to them. The
units may be flagged by restart markers. On the other end, the
process is reversed.
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