3. DATA TRANSFER FUNCTIONS
Files are transferred only via the data connection. The control
connection is used for the transfer of commands, which describe the
functions to be performed, and the replies to these commands (see the
Section on FTP Replies). Several commands are concerned with the
transfer of data between hosts. These data transfer commands include
the MODE command which specify how the bits of the data are to be
transmitted, and the STRUcture and TYPE commands, which are used to
define the way in which the data are to be represented. The
transmission and representation are basically independent but the
"Stream" transmission mode is dependent on the file structure
attribute and if "Compressed" transmission mode is used, the nature
of the filler byte depends on the representation type.
3.1. DATA REPRESENTATION AND STORAGE
Data is transferred from a storage device in the sending host to a
storage device in the receiving host. Often it is necessary to
perform certain transformations on the data because data storage
representations in the two systems are different. For example,
NVT-ASCII has different data storage representations in different
systems. DEC TOPS-20s's generally store NVT-ASCII as five 7-bit
ASCII characters, left-justified in a 36-bit word. IBM Mainframe's
store NVT-ASCII as 8-bit EBCDIC codes. Multics stores NVT-ASCII
as four 9-bit characters in a 36-bit word. It is desirable to
convert characters into the standard NVT-ASCII representation when
transmitting text between dissimilar systems. The sending and
receiving sites would have to perform the necessary
transformations between the standard representation and their
internal representations.
A different problem in representation arises when transmitting
binary data (not character codes) between host systems with
different word lengths. It is not always clear how the sender
should send data, and the receiver store it. For example, when
transmitting 32-bit bytes from a 32-bit word-length system to a
36-bit word-length system, it may be desirable (for reasons of
efficiency and usefulness) to store the 32-bit bytes
right-justified in a 36-bit word in the latter system. In any
case, the user should have the option of specifying data
representation and transformation functions. It should be noted
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."
Note that this has nothing to do with the byte size used for
transmission over the data connection, called the "transfer
byte size", and the two should not be confused. For example,
NVT-ASCII has a logical byte size of 8 bits. If the type is
Local byte, then the TYPE command has an obligatory second
parameter specifying the logical byte size. The transfer byte
size is always 8 bits.
This is the default type and must be accepted by all FTP
implementations. It is intended primarily for the transfer
of text files, except when both hosts would find the EBCDIC
type more convenient.
The sender converts the data from an internal character
representation to the standard 8-bit NVT-ASCII
representation (see the Telnet specification). The receiver
will convert the data from the standard form to his own
internal form.
In accordance with the NVT standard, the <CRLF> sequence
should be used where necessary to denote the end of a line
of text. (See the discussion of file structure at the end
of the Section on Data Representation and Storage.)
Using the standard NVT-ASCII representation means that data
must be interpreted as 8-bit bytes.
The Format parameter for ASCII and EBCDIC types is discussed
below.
3.1.1.2. EBCDIC TYPE
This type is intended for efficient transfer between hosts
which use EBCDIC for their internal character
representation.
For transmission, the data are represented as 8-bit EBCDIC
characters. The character code is the only difference
between the functional specifications of EBCDIC and ASCII
types.
End-of-line (as opposed to end-of-record--see the discussion
of structure) will probably be rarely used with EBCDIC type
for purposes of denoting structure, but where it is
necessary the <NL> character should be used.
The data are sent as contiguous bits which, for transfer,
are packed into the 8-bit transfer bytes. The receiving
site must store the data as contiguous bits. The structure
of the storage system might necessitate the padding of the
file (or of each record, for a record-structured file) to
some convenient boundary (byte, word or block). This
padding, which must be all zeros, may occur only at the end
of the file (or at the end of each record) and there must be
a way of identifying the padding bits so that they may be
stripped off if the file is retrieved. The padding
transformation should be well publicized to enable a user to
process a file at the storage site.
Image type is intended for the efficient storage and
retrieval of files and for the transfer of binary data. It
is recommended that this type be accepted by all FTP
implementations.
3.1.1.4. LOCAL TYPE
The data is transferred in logical bytes of the size
specified by the obligatory second parameter, Byte size.
The value of Byte size must be a decimal integer; there is
no default value. The logical byte size is not necessarily
the same as the transfer byte size. If there is a
difference in byte sizes, then the logical bytes should be
packed contiguously, disregarding transfer byte boundaries
and with any necessary padding at the end.
When the data reaches the receiving host, it will be
transformed in a manner dependent on the logical byte size
and the particular host. This transformation must be
invertible (i.e., an identical file can be retrieved if the
same parameters are used) and should be well publicized by
the FTP implementors.
For example, a user sending 36-bit floating-point numbers to
a host with a 32-bit word could send that data as Local byte
with a logical byte size of 36. The receiving host would
then be expected to store the logical bytes so that they
could be easily manipulated; in this example putting the
36-bit logical bytes into 64-bit double words should
suffice.
In another example, a pair of hosts with a 36-bit word size
may send data to one another in words by using TYPE L 36.
The data would be sent in the 8-bit transmission bytes
packed so that 9 transmission bytes carried two host words.
3.1.1.5. FORMAT CONTROL
The types ASCII and EBCDIC also take a second (optional)
parameter; this is to indicate what kind of vertical format
control, if any, is associated with a file. The following
data representation types are defined in FTP:
A character file may be transferred to a host for one of
three purposes: for printing, for storage and later
retrieval, or for processing. If a file is sent for
printing, the receiving host must know how the vertical
format control is represented. In the second case, it must
be possible to store a file at a host and then retrieve it
later in exactly the same form. Finally, it should be
possible to move a file from one host to another and process
the file at the second host without undue trouble. A single
ASCII or EBCDIC format does not satisfy all these
conditions. Therefore, these types have a second parameter
specifying one of the following three formats:
3.1.1.5.1. NON PRINT
This is the default format to be used if the second
(format) parameter is omitted. Non-print format must be
accepted by all FTP implementations.
The file need contain no vertical format information. If
it is passed to a printer process, this process may
assume standard values for spacing and margins.
Normally, this format will be used with files destined
for processing or just storage.
3.1.1.5.2. TELNET FORMAT CONTROLS
The file contains ASCII/EBCDIC vertical format controls
(i.e., <CR>, <LF>, <NL>, <VT>, <FF>) which the printer
process will interpret appropriately. <CRLF>, in exactly
this sequence, also denotes end-of-line.
3.1.1.5.2. CARRIAGE CONTROL (ASA)
The file contains ASA (FORTRAN) vertical format control
characters. (See RFC 740 Appendix C; and Communications
of the ACM, Vol. 7, No. 10, p. 606, October 1964.) In a
line or a record formatted according to the ASA Standard,
the first character is not to be printed. Instead, it
should be used to determine the vertical movement of the
paper which should take place before the rest of the
record is printed.
The ASA Standard specifies the following control
characters:
Character Vertical Spacing
blank Move paper up one line
0 Move paper up two lines
1 Move paper to top of next page
+ No movement, i.e., overprint
Clearly there must be some way for a printer process to
distinguish the end of the structural entity. If a file
has record structure (see below) this is no problem;
records will be explicitly marked during transfer and
storage. If the file has no record structure, the <CRLF>
end-of-line sequence is used to separate printing lines,
but these format effectors are overridden by the ASA
controls.
3.1.2. DATA STRUCTURES
In addition to different representation types, FTP allows the
structure of a file to be specified. Three file structures are
defined in FTP:
file-structure, where there is no internal structure and
the file is considered to be a
continuous sequence of data bytes,
record-structure, where the file is made up of sequential
records,
and page-structure, where the file is made up of independent
indexed pages.
File-structure is the default to be assumed if the STRUcture
command has not been used but both file and record structures
must be accepted for "text" files (i.e., files with TYPE ASCII
or EBCDIC) by all FTP implementations. The structure of a file
will affect both the transfer mode of a file (see the Section
on Transmission Modes) and the interpretation and storage of
the file.
The "natural" structure of a file will depend on which host
stores the file. A source-code file will usually be stored on
an IBM Mainframe in fixed length records but on a DEC TOPS-20
as a stream of characters partitioned into lines, for example
by <CRLF>. If the transfer of files between such disparate
sites is to be useful, there must be some way for one site to
recognize the other's assumptions about the file.
With some sites being naturally file-oriented and others
naturally record-oriented there may be problems if a file with
one structure is sent to a host oriented to the other. If a
text file is sent with record-structure to a host which is file
oriented, then that host should apply an internal
transformation to the file based on the record structure.
Obviously, this transformation should be useful, but it must
also be invertible so that an identical file may be retrieved
using record structure.
In the case of a file being sent with file-structure to a
record-oriented host, there exists the question of what
criteria the host should use to divide the file into records
which can be processed locally. If this division is necessary,
the FTP implementation should use the end-of-line sequence,
<CRLF> for ASCII, or <NL> for EBCDIC text files, as the
delimiter. If an FTP implementation adopts this technique, it
must be prepared to reverse the transformation if the file is
retrieved with file-structure.
3.1.2.1. FILE STRUCTURE
File structure is the default to be assumed if the STRUcture
command has not been used.
In file-structure there is no internal structure and the
file is considered to be a continuous sequence of data
bytes.
3.1.2.2. RECORD STRUCTURE
Record structures must be accepted for "text" files (i.e.,
files with TYPE ASCII or EBCDIC) by all FTP implementations.
In record-structure the file is made up of sequential
records.
3.1.2.3. PAGE STRUCTURE
To transmit files that are discontinuous, FTP defines a page
structure. Files of this type are sometimes known as
"random access files" or even as "holey files". In these
files there is sometimes other information associated with
the file as a whole (e.g., a file descriptor), or with a
section of the file (e.g., page access controls), or both.
In FTP, the sections of the file are called pages.
To provide for various page sizes and associated
information, each page is sent with a page header. The page
header has the following defined fields:
Header Length
The number of logical bytes in the page header
including this byte. The minimum header length is 4.
Page Index
The logical page number of this section of the file.
This is not the transmission sequence number of this
page, but the index used to identify this page of the
file.
Data Length
The number of logical bytes in the page data. The
minimum data length is 0.
Page Type
The type of page this is. The following page types
are defined:
0 = Last Page
This is used to indicate the end of a paged
structured transmission. The header length must
be 4, and the data length must be 0.
1 = Simple Page
This is the normal type for simple paged files
with no page level associated control
information. The header length must be 4.
2 = Descriptor Page
This type is used to transmit the descriptive
information for the file as a whole.
3 = Access Controlled Page
This type includes an additional header field
for paged files with page level access control
information. The header length must be 5.
Optional Fields
Further header fields may be used to supply per page
control information, for example, per page access
control.
All fields are one logical byte in length. The logical byte
size is specified by the TYPE command. See Appendix I for
further details and a specific case at the page structure.
A note of caution about parameters: a file must be stored and
retrieved with the same parameters if the retrieved version is to
be identical to the version originally transmitted. Conversely,
FTP implementations must return a file identical to the original
if the parameters used to store and retrieve a file are the same.
3.2. ESTABLISHING DATA CONNECTIONS
The mechanics of transferring data consists of setting up the data
connection to the appropriate ports and choosing the parameters
for transfer. Both the user and the server-DTPs have a default
data port. The user-process default data port is the same as the
control connection port (i.e., U). The server-process default
data port is the port adjacent to the control connection port
(i.e., L-1).
The transfer byte size is 8-bit bytes. This byte size is relevant
only for the actual transfer of the data; it has no bearing on
representation of the data within a host's file system.
The passive data transfer process (this may be a user-DTP or a
second server-DTP) shall "listen" on the data port prior to
sending a transfer request command. The FTP request command
determines the direction of the data transfer. The server, upon
receiving the transfer request, will initiate the data connection
to the port. When the connection is established, the data
transfer begins between DTP's, and the server-PI sends a
confirming reply to the user-PI.
Every FTP implementation must support the use of the default data
ports, and only the USER-PI can initiate a change to non-default
ports.
It is possible for the user to specify an alternate data port by
use of the PORT command. The user may want a file dumped on a TAC
line printer or retrieved from a third party host. In the latter
case, the user-PI sets up control connections with both
server-PI's. One server is then told (by an FTP command) to
"listen" for a connection which the other will initiate. The
user-PI sends one server-PI a PORT command indicating the data
port of the other. Finally, both are sent the appropriate
transfer commands. The exact sequence of commands and replies
sent between the user-controller and the servers is defined in the
Section on FTP Replies.
In general, it is the server's responsibility to maintain the data
connection--to initiate it and to close it. The exception to this
is when the user-DTP is sending the data in a transfer mode that
requires the connection to be closed to indicate EOF. The server
MUST close the data connection under the following conditions:
1. The server has completed sending data in a transfer mode
that requires a close to indicate EOF.
2. The server receives an ABORT command from the user.
3. The port specification is changed by a command from the
user.
4. The control connection is closed legally or otherwise.
5. An irrecoverable error condition occurs.
Otherwise the close is a server option, the exercise of which the
server must indicate to the user-process by either a 250 or 226
reply only.
3.3. DATA CONNECTION MANAGEMENT
Default Data Connection Ports: All FTP implementations must
support use of the default data connection ports, and only the
User-PI may initiate the use of non-default ports.
Negotiating Non-Default Data Ports: The User-PI may specify a
non-default user side data port with the PORT command. The
User-PI may request the server side to identify a non-default
server side data port with the PASV command. Since a connection
is defined by the pair of addresses, either of these actions is
enough to get a different data connection, still it is permitted
to do both commands to use new ports on both ends of the data
connection.
Reuse of the Data Connection: When using the stream mode of data
transfer the end of the file must be indicated by closing the
connection. This causes a problem if multiple files are to be
transfered in the session, due to need for TCP to hold the
connection record for a time out period to guarantee the reliable
communication. Thus the connection can not be reopened at once.
There are two solutions to this problem. The first is to
negotiate a non-default port. The second is to use another
transfer mode.
A comment on transfer modes. The stream transfer mode is
inherently unreliable, since one can not determine if the
connection closed prematurely or not. The other transfer modes
(Block, Compressed) do not close the connection to indicate the
end of file. They have enough FTP encoding that the data
connection can be parsed to determine the end of the file.
Thus using these modes one can leave the data connection open
for multiple file transfers.
3.4. TRANSMISSION MODES
The next consideration in transferring data is choosing the
appropriate transmission mode. There are three modes: one which
formats the data and allows for restart procedures; one which also
compresses the data for efficient transfer; and one which passes
the data with little or no processing. In this last case the mode
interacts with the structure attribute to determine the type of
processing. In the compressed mode, the representation type
determines the filler byte.
All data transfers must be completed with an end-of-file (EOF)
which may be explicitly stated or implied by the closing of the
data connection. For files with record structure, all the
end-of-record markers (EOR) are explicit, including the final one.
For files transmitted in page structure a "last-page" page type is
used.
NOTE: In the rest of this section, byte means "transfer byte"
except where explicitly stated otherwise.
For the purpose of standardized transfer, the sending host will
translate its internal end of line or end of record denotation
into the representation prescribed by the transfer mode and file
structure, and the receiving host will perform the inverse
translation to its internal denotation. An IBM Mainframe record
count field may not be recognized at another host, so the
end-of-record information may be transferred as a two byte control
code in Stream mode or as a flagged bit in a Block or Compressed
mode descriptor. End-of-line in an ASCII or EBCDIC file with no
record structure should be indicated by <CRLF> or <NL>,
respectively. Since these transformations imply extra work for
some systems, identical systems transferring non-record structured
text files might wish to use a binary representation and stream
mode for the transfer.
The following transmission modes are defined in FTP:
The data is transmitted as a stream of bytes. There is no
restriction on the representation type used; record structures
are allowed.
In a record structured file EOR and EOF will each be indicated
by a two-byte control code. The first byte of the control code
will be all ones, the escape character. The second byte will
have the low order bit on and zeros elsewhere for EOR and the
second low order bit on for EOF; that is, the byte will have
value 1 for EOR and value 2 for EOF. EOR and EOF may be
indicated together on the last byte transmitted by turning both
low order bits on (i.e., the value 3). If a byte of all ones
was intended to be sent as data, it should be repeated in the
second byte of the control code.
If the structure is a file structure, the EOF is indicated by
the sending host closing the data connection and all bytes are
data bytes.
3.4.2. BLOCK MODE
The file is transmitted as a series of data blocks preceded by
one or more header bytes. The header bytes contain a count
field, and descriptor code. The count field indicates the
total length of the data block in bytes, thus marking the
beginning of the next data block (there are no filler bits).
The descriptor code defines: last block in the file (EOF) last
block in the record (EOR), restart marker (see the Section on
Error Recovery and Restart) or suspect data (i.e., the data
being transferred is suspected of errors and is not reliable).
This last code is NOT intended for error control within FTP.
It is motivated by the desire of sites exchanging certain types
of data (e.g., seismic or weather data) to send and receive all
the data despite local errors (such as "magnetic tape read
errors"), but to indicate in the transmission that certain
portions are suspect). Record structures are allowed in this
mode, and any representation type may be used.
The header consists of the three bytes. Of the 24 bits of
header information, the 16 low order bits shall represent byte
count, and the 8 high order bits shall represent descriptor
codes as shown below.
Block Header
+----------------+----------------+----------------+
| Descriptor | Byte Count |
| 8 bits | 16 bits |
+----------------+----------------+----------------+
The descriptor codes are indicated by bit flags in the
descriptor byte. Four codes have been assigned, where each
code number is the decimal value of the corresponding bit in
the byte.
Code Meaning
128 End of data block is EOR
64 End of data block is EOF
32 Suspected errors in data block
16 Data block is a restart marker
With this encoding, more than one descriptor coded condition
may exist for a particular block. As many bits as necessary
may be flagged.
The restart marker is embedded in the data stream as an
integral number of 8-bit bytes representing printable
characters in the language being used over the control
connection (e.g., default--NVT-ASCII). <SP> (Space, in the
appropriate language) must not be used WITHIN a restart marker.
For example, to transmit a six-character marker, the following
would be sent:
+--------+--------+--------+
|Descrptr| Byte count |
|code= 16| = 6 |
+--------+--------+--------+
+--------+--------+--------+
| Marker | Marker | Marker |
| 8 bits | 8 bits | 8 bits |
+--------+--------+--------+
+--------+--------+--------+
| Marker | Marker | Marker |
| 8 bits | 8 bits | 8 bits |
+--------+--------+--------+
3.4.3. COMPRESSED MODE
There are three kinds of information to be sent: regular data,
sent in a byte string; compressed data, consisting of
replications or filler; and control information, sent in a
two-byte escape sequence. If n>0 bytes (up to 127) of regular
data are sent, these n bytes are preceded by a byte with the
left-most bit set to 0 and the right-most 7 bits containing the
number n.
Byte string:
1 7 8 8
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
|0| n | | d(1) | ... | d(n) |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
^ ^
|---n bytes---|
of data
String of n data bytes d(1),..., d(n)
Count n must be positive.
To compress a string of n replications of the data byte d, the
following 2 bytes are sent:
Replicated Byte:
2 6 8
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
|1 0| n | | d |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
A string of n filler bytes can be compressed into a single
byte, where the filler byte varies with the representation
type. If the type is ASCII or EBCDIC the filler byte is <SP>
(Space, ASCII code 32, EBCDIC code 64). If the type is Image
or Local byte the filler is a zero byte.
Filler String:
2 6
+-+-+-+-+-+-+-+-+
|1 1| n |
+-+-+-+-+-+-+-+-+
The escape sequence is a double byte, the first of which is the
escape byte (all zeros) and the second of which contains
descriptor codes as defined in Block mode. The descriptor
codes have the same meaning as in Block mode and apply to the
succeeding string of bytes.
Compressed mode is useful for obtaining increased bandwidth on
very large network transmissions at a little extra CPU cost.
It can be most effectively used to reduce the size of printer
files such as those generated by RJE hosts.
3.5. ERROR RECOVERY AND RESTART
There is no provision for detecting bits lost or scrambled in data
transfer; this level of error control is handled by the TCP.
However, a restart procedure is provided to protect users from
gross system failures (including failures of a host, an
FTP-process, or the underlying network).
The restart procedure is defined only for the block and compressed
modes of data transfer. It requires the sender of data to insert
a special marker code in the data stream with some marker
information. The marker information has meaning only to the
sender, but must consist of printable characters in the default or
negotiated language of the control connection (ASCII or EBCDIC).
The marker could represent a bit-count, a record-count, or any
other information by which a system may identify a data
checkpoint. The receiver of data, if it implements the restart
procedure, would then mark the corresponding position of this
marker in the receiving system, and return this information to the
user.
In the event of a system failure, the user can restart the data
transfer by identifying the marker point with the FTP restart
procedure. The following example illustrates the use of the
restart procedure.
The sender of the data inserts an appropriate marker block in the
data stream at a convenient point. The receiving host marks the
corresponding data point in its file system and conveys the last
known sender and receiver marker information to the user, either
directly or over the control connection in a 110 reply (depending
on who is the sender). In the event of a system failure, the user
or controller process restarts the server at the last server
marker by sending a restart command with server's marker code as
its argument. The restart command is transmitted over the control
connection and is immediately followed by the command (such as
RETR, STOR or LIST) which was being executed when the system
failure occurred.