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 is 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 are defaults for both the user and server data ports, and under normal circumstances this command and its reply are not needed. If this command 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.
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.