📄 rfc765.txt
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| 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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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
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