NWG/RFC# 732 DAY 13-Sep-77 18:38 41762
Data Entry Terminal Option
Network Working Group John Day
Request for Comments: 732
NIC: 41762 12 September 1977
Obsoletes: 731
Telnet Data Entry Terminal Option
1. Command Name and Code:
DET 20
2. Command Meanings
IAC WILL DET
The sender of this command REQUESTS or AGREES to send and receive
subcommands to control the Data Entry Terminal.
IAC WONT DET
The sender of this command REFUSES to send and receive subcommands
to control the Data Entry Terminal.
IAC DO DET
The sender of this command REQUESTS or AGREES to send and receive
subcommands to control the Data Entry Terminal.
IAC DONT DET
The sender of this command REFUSES to send and receive subcommands
to control the Data Entry Terminal.
The DET option uses five classes of subcommands 1) to establish the
requirements and capabilities of the application and the terminal, 2)
to format the screen, and to control the 3) edit, 4) erasure, and 5)
transmission functions. The subcommands that perform these functions
are described below.
The Network Virtual Data Entry Terminal (NVDET)
The NVDET consists of a keyboard and a rectangular display. The
keyboard is capable of generating all of the characters of the ASCII
character set. In addition, the keyboard may possess a number of
function keys which when pressed cause a FN subcommand to be sent.
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Data Entry Terminal Option
(Although most DET's will support one or more peripheral devices
such as a paper tape reader or a printer, this option does not
consider their support. Support of peripheral devices should be
treated by a is a separate option).
The screen of the data entry terminal is a rectangle M characters by
N lines. The values of M and N are set by negotiating the Output
Line Width and Output Page Size options, respectively. The next
writing position (x,y) on the screen (where x is the character
position and y is the position of the line on the screen) is
indicated by a special display character called the cursor. The
cursor may be moved to any position on the screen without disturbing
any characters already on the screen. Cursor addressing in existing
terminals utilizes several topologies and addressing methods. In
order to make the burden of implementaton as easy as possible this
protocol supports two topologies (the finite plane and the helical
torus) and three addressing methods ((x,y); x and y, and relative
increments). Since the finite plane with absolute addressing is the
least ambiguous and the easiest to translate to and from the others,
it is the default scheme used by the NVDET. The torodial form with
either relative or absolute addressing is provided for convience.
Also the NVDET provides a mechanism for defining on the screen
fields with special attributes. For example, characters entered into
these fields may be displayed with brighter intensity, highlighted
by reverse video or blinking, or protected from modification by the
user. This latter feature is one of the most heavily used for
applications where the DET displays a form to be filled out by the
user.
The definition of the NVDET uses Telnet option subnegotiations to
accomplish all of its functions. Since none of the ASCII characters
sent in the data stream have been used to define these functions,
the DET option can be used in a "raw" or even "rare" mode. In
circumstances where the application program knows what kind of
terminal is on the other end, it can send the ASCII characters
required to control functions not supported by the option or an
implementation. In general keeping all NVDET functions out of the
data stream provides better flexibility.
Facility Functions (for detailed semantics see Section 5.)
IAC SB DET IAC SE
where is one 8-bit byte indicating the
class of the facilities to be described, and is a
field of one or two 8-bit bytes containing flags describing the
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Data Entry Terminal Option
facilities required or desired by the sender. The bits of the
facility maps are numbered from the right starting at zero. Thus,
if bit 2 is set the field will have a decimal value of 4. The
values of the field are as follows:
facility cmd: EDIT FACILITIES subcommand code: 1
facility map: bit numbers
Toroidal Cursor Addressing 6
Incremental Cursor Addressing 5
Read Cursor Address 4
Line Insert/Delete 3
Char Insert/Delete 2
Back Tab 1
Positive Addressing only 0
where:
If the Toroidal Cursor Addressing bit is set, the sender requests or
provides that the SKIP TO LINE and SKIP TO CHAR subcommands be
supported.
If the Incremental Cursor Addressing bit is set, the sender requests
or provides that the UP, DOWN, LEFT, and RIGHT subcommands be
supported.
If the Read Cursor bit is set, the sender requests or provides the
READ CURSOR subcommand.
If the Line Insert/Delete bit is set, the sender requests or
provides that the LINE INSERT and LINE DELETE subcommands be
supported.
If the Char Insert/Delete bit is set, the sender requests or
provides that the CHAR INSERT and CHAR DELETE subcommands be
supported.
If the Back Tab bit is set, the sender requests or provides that the
BACK TAB subcommand be supported.
If the Positive Addressing bit is set, then the sender is informing
the receiver that it can only move the cursor in the positive
direction. (Note: Terminals that have this property also have a Home
function to get back to the beginning.)
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Data Entry Terminal Option
facility cmd: ERASE FACILITIES subcommand code: 2
facility map: bit numbers
Erase Field 4
Erase Line 3
Erase Rest of Screen 2
Erase Rest of Line 1
Erase Rest of Field 0
where:
If a bit of the facility map for this facility command is set, the
sender requests or provides the facility indicated by the bit. For a
more complete description of each of these functions see the Erase
Functions section below.
facility cmd: TRANSMIT FACILITIES subcommand code: 3
facility map: bit numbers
Data Transmit 5
Transmit Line 4
Transmit Field 3
Transmit Rest of Screen 2
Transmit Rest of Line 1
Transmit Rest of Field 0
where:
If a bit of the facility map for this facility command is set, the
sender requests or provides the facility indicated by the bit. For a
more complete description of each of these functions see the
Transmit Functions section below.
facility cmd: FORMAT FACILITIES subcommand code: 4
facility map: bit numbers
FN byte 0 7
Modified 6
Light Pen 5
Repeat 4
Blinking 3
Reverse Video 2
Right Justification 1
Overstrike 0
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Data Entry Terminal Option
Protection On/Off byte 1 6
Protection 5
Alphabetic-only Protection 4
Numeric-only Protection 3
Intensity 0-2
where:
If the FN bit is set, the sender requests or provides the FN
subcommand.
If the Modified bit is set, the sender requests or provides the
ability to indicate fields that are modified and supports the
TRANSMIT MODIFIED subcommand.
If the Light Pen bit is set, the sender requests or provides the
support of a light pen, including the Pen Selectable attribute of
the DATA FORMAT subcommand.
If the Repeat bit is set the sender requests or provides the REPEAT
subcommand.
If the Blinking bit is set, the sender requests or provides the
ability to highlight a string of characters by causing them to
blink.
If the Reverse Video bit is set, the sender requests or provides the
ability to highlight a string of characters by "reversing the video
image," i.e., if the characters are normally displayed as black
characters on a white background, this is reversed to be white
characters on a black background, or vice versa.
If the Right Justification bit is set, the sender requests or
provides the ability to cause entries of data to be right justified
in the field.
If the Overstrike bit is set, the sender requests or provides the
ability to superimpose one character over another on the screen much
like a hard copy terminal would do if the print mechanism struck the
same position on the paper with different characters.
If the Protection On/Off bit is set, the sender requests or provides
the ability to turn on and off field protection.
If the Protection bit is set, the sender requests or provides the
ability to protect certain strings of
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Data Entry Terminal Option
characters displayed on the screen from being altered by the user of
the terminal. Setting this bit also implies that ERASE UNPROTECTED,
DATA TRANSMIT, FIELD SEPARATOR, and TRANSMIT UNPROTECTED subcommands
(see below) are supported.
If the Alphabetic-only Protection bit is set, the sender requests or
provides the ability to constrain the user of the terminal such that
he may only enter alphabetic data into certain areas of the screen.
If the Numeric-only Protection bit is set, the sender requests or
provides the ability to constrain the user of the terminal such that
he may only enter numerical data into certain areas of the screen.
The three bits of the Intensity field will contain a positive binary
integer indicating the number of levels of intensity that the sender
requests or provides for displaying the data. The value of the 3 bit
field should be interpreted in the following way:
1 one visible intensity
2 two intensities; normal and bright
3 three intensities; off, normal, and bright
>3 >3 intensities; off, and the remaining levels
proportioned from dimmest to brightest intensity.
For the all of the above commands, if the appropriate bit in
is not set, then the sender does not request or
provide that facility.
Editing Functions
IAC SB DET MOVE CURSOR IAC SE subcommand code: 5
where is an 8-bit byte containing a positive binary integer
representing the character position of the cursor, is an 8-bit
byte containing a positive binary integer representing the line
position of the cursor.
This subcommand moves the cursor to the absolute screen address
(x,y) with the following boundary conditions:
if x>M-1, set x=M-1 and send an ERROR subcommand
if y>N-1, set y=N-1 and send an ERROR subcommand
This describes a finite plane topology on the screen.
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Data Entry Terminal Option
IAC SB DET SKIP TO LINE IAC SE subcommand code: 6
where is a positive 8-bit binary number.
This subcommand moves the cursor to the absolute screen line y. x
remains constant. For values of y>N-1
y = y mod N.
IAC SB DET SKIP TO CHAR IAC SE subcommand code: 7
where is a positive 8-bit binary number.
This subcommand moves the cursor to the absolute character position
x. y remains constant, unless x>M-1 in which case:
x' = (x mod M)
y' = (y+(x DIV N))
where x' and y' are the new values of the cursor.
These last two subcommands define a toroidal topology on the screen.
IAC SB DET UP IAC SE subcommand code: 8
IAC SB DET DOWN IAC SE subcommand code: 9
IAC SB DET LEFT IAC SE subcommand code: 10
IAC SB DET RIGHT IAC SE subcommand code: 11
These subcommands are provided as a convenience for some terminals.
The commands UP, DOWN, LEFT, and RIGHT are defined as
UP: (x,y)=(x, y-1 mod N)
DOWN: (x,y)=(x, y+1 mod N)
LEFT: (x,y)=(x-1, y); if x=0 then x-1 = 0
RIGHT: (x,y)=(x+1 mod M, y) and y = y+1 if x+1>M-1
Note: DOWN, LEFT, and RIGHT cannot always be replaced by the ASCII
codes for linefeed, backspace, and space respectively. The latter
are format effectors while the former are cursor controls.
IAC SB DET HOME IAC SE subcommand code: 12
This subcommand positions the cursor to (0,0). This is equivalent to
a MOVE CURSOR 0,0 or the sequence SKIP TO LINE 0, SKIP TO CHAR 0.
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Data Entry Terminal Option
This subcommand is provided for convenience, since most terminals
have it as a separate control.
IAC SB DET LINE INSERT IAC SE subcommand code: 13
This subcommand inserts a line of spaces between lines y (the
current line, determined by the position of the cursor) and line
y-1. Lines y through N-2 move down one line, i.e. line y becomes
line y+1; y+1 becomes y+2, ...; N-2 becomes N-1. Line N-1 is lost
off the bottom of the screen. The position of the cursor remains
unchanged.
IAC SB DET LINE DELETE IAC SE subcommand code: 14
This subcommand deletes line y where y is the current line position
of the cursor. Lines y+1 through N-1 move up one line, i.e. line y+1
becomes line y; y+2 becomes y+1; ...; N-1 becomes N-2. The N-1st
line position is set to all spaces. The cursor position remains
unchanged.
IAC SB DET CHAR INSERT IAC SE subcommand code: 15
This subcommand inserts the next character in the data stream
between the xth and x-1st characters, where x is the current
character position of the cursor. The xth through M-2nd characters
on the line are shifted one character positon to the right. The new
character is inserted at the vacated xth position. The M-1st
character is lost. The position of the cursor remains unchanged.
IAC SB DET CHAR DELETE IAC SE subcommand code: 16
This subcommand deletes the character on the screen at the x-th
position. The x-th character is removed and the characters x+1
through M-1 are shifted one character position to the left to become
the x-th through M-2nd characters. The M-1st character position is
left empty. (For most terminals it will be set to a NUL or space.)
The cursor position remains unchanged.
IAC SB DET READ CURSOR IAC SE subcommand code: 17
This subcommand requests the receiver to send the present position
of the cursor to the sender.
IAC SB DET CURSOR POSITION IAC SE subcommand code: 18
where and are positive 8-bit binary integers.
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Data Entry Terminal Option
This subcommand is sent by a Telnet implementation in response to a
READ CURSOR subcommand to convey the coordinates of the cursor to
the other side. Note: x is less than M and y is less than N.
IAC SB DET REVERSE TAB IAC SE subcommand code: 19
This subcommand causes the cursor to move to the previous tab
position. If none exists on the present line, the cursor moves to
the previous line and so on until a tab is found or the address
(0,0) is encountered. When field protection is in effect the cursor
moves to the beginning of the preceding unprotected field.
Transmit Functions (For detailed semantics see Section 5.)
IAC SB DET TRANSMIT SCREEN IAC SE subcommand code: 20
This subcommand causes the terminal to transmit all characters on
the screen from position (0,0) to (M-1,N-1). The cursor will be at
(0,0) after the operation is complete.
IAC SB DET TRANSMIT UNPROTECTED IAC SE subcommand code: 21
This subcommand causes the terminal to transmit all characters in
unprotected fields from position (0,0) to (M-1,N-1). The unprotected
fields are separated by the field separator subcommand. The cursor
will be at (0,0) or at the beginning of the first unprotected field
after the operation is complete.
IAC SB DET TRANSMIT LINE IAC SE subcommand code: 22
This subcommand causes the terminal to transmit all data on the yth
line where y is determined by the present position of the cursor.
Data is sent from character position (0,y) to the end-of-line or
position (M-1,y) whichever comes first. The cursor position after
the transmission is one character position after the end of line
condition or the beginning of the next line, (0,y+1).
IAC SB DET TRANSMIT FIELD IAC SE subcommand code: 23
This subcommand causes the terminal to transmit all data in the
field presently occupied by the cursor. The cursor position after
the operation is complete is one character position after the end of
the field or, if that
position is protected, at the beginning of the next unprotected
field.
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Data Entry Terminal Option
IAC SB DET TRANSMIT REST OF SCREEN IAC SE subcommand code: 24
This subcommand causes the terminal to transmit all characters on
the screen from position (x,y) to (M-1,N-1) or until the end of
text. (x,y) is the current cursor position. The cursor position
after the operation is one character position after the last text
character, or (0,0) if the last filled character position is
(M-1,N-1).
IAC SB DET TRANSMIT REST OF LINE IAC SE subcommand code: 25
This subcommand causes the terminal to transmit all characters on
the yth line from position (x,y) to the end of line or (M-1,y)
whichever comes first. (x,y) is the current cursor position. The
cursor position after the operation is one character position after
the last character of the line or the first character of the next
line.
IAC SB DET TRANSMIT REST OF FIELD IAC SE subcommand code: 26
This subcommand causes the receiver to transmit the rest of the
characters in the field currently occupied by the cursor. The cursor
position after the operation is at the beginning of the next field.
IAC SB DET TRANSMIT MODIFIED IAC SE subcommand code: 27
This subcommand causes the receiver to transmit only those fields
which have the modified attribute set. The cursor position after the
operation is unchanged.
IAC SB DET DATA TRANSMIT IAC SE subcommand code: 28
This subcommand is used to preface data sent from the terminal in
response to a user action or a TRANSMIT command. The parameters
and indicate the initial position of the cursor. See the
Transmit Subcommands subsection in Section 5 for more details. A
DATA TRANSMIT subcommand may precede an entire transmission with
each field being delineated by the FIELD SEPARATOR subcommand as
would be the case in a response toa
TRANSMIT UNPROTECTED. Or, it may precede each field as would be the
case in a response to a TRANSMIT MODIFIED.
Erase Functions
IAC SB DET ERASE SCREEN IAC SE subcommand code: 29
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Data Entry Terminal Option
This subcommand causes all characters to be removed from the screen.
All fields regardless of their attributes are deleted. The cursor
position after the operation will be (0,0). Most terminals set the
erased characters to either NUL or space characters.
IAC SB DET ERASE LINE IAC SE subcommand code: 30
This subcommand causes all characters on the yth line to be removed
from the screen, where y is the line of the current cursor position.
All fields regardless of their attributes are deleted. The cursor
position after this operation will be (0,y). Note: This operation
can be easily simulated by the sequence: LINE DELETE, LINE INSERT.
However, the order is important to insure that no data is lost off
the bottom of the screen.
IAC SB DET ERASE FIELD IAC SE subcommand code: 31
This subcommand causes all characters in the field occupied by the
cursor to be removed. The cursor position after the operation is at
the beginning of the field.
IAC SB DET ERASE REST OF SCREEN IAC SE subcommand code: 32
This subcommand causes all characters from position (x,y) to
(M-1,N-1) to be removed from the screen. All fields regardless of
their attributes are deleted. The cursor position after the
operation is unchanged. This is equivalent to doing an ERASE REST OF
LINE plus a LINE DELETE for lines greater than y.
IAC SB DET ERASE REST OF LINE IAC SE subcommand code: 33
This subcommand causes all characters from position (x,y) to (M-1,y)
to be removed from the screen All fields regardless of their
attributes are deleted. The cursor position after the operation is
unchanged.
IAC SB DET ERASE REST OF FIELD IAC SE subcommand code: 34
This subcommand causes all characters from position (x,y) to the end
of the current field to be removed from the screen. The cursor
position after the operation is unchanged.
IAC SB DET ERASE UNPROTECTED IAC SE subcommand code: 35
This subcommand causes all characters on the screen in unprotected
fields to be removed from the screen. The cursor position after the
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Data Entry Terminal Option
operation is at (0,0) or, if that position is protected, at the
beginning of the first unprotected field.
Format Functions
IAC SB DET FORMAT DATA IAC SE
subcommand code: 36
where is a two byte field containing the following
flags:
Byte 0
Blinking 7
Reverse Video 6
Right Justification 5
Protection 3-4
Intensity 0-2
Byte 1
Modified 1
Pen Selectable 0
where:
If the Blinking bit is set, the following field of
characters should have the Blinking attribute applied to it by the
receiver.
If the Reverse Video bit is set, the following field of
characters should be displayed by the receiver with video reversed.
If the Right Justification bit is set, the input entered into the
field of characters should be right justified.
The Protection field is two bits wide and may take on the
following values:
0 no protection
1 protected
2 alphabetic only
3 numeric only
The protection attribute specifies that the other side may modify
any character (no protection), modify no characters (protected),
enter only alphabetical characters (A-Z, and a-z) (alphabetic only),
or enter only numerical characters (0-9,+,.,and -) (numeric only) in
the following field of bytes.
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Data Entry Terminal Option
The Intensity field is 3 bits wide and should be interpreted in the
following way:
The values 0-6 should be used as an indication of the relative
brightness to be used when displaying the characters in or entered
into the following field characters wide. The number of
levels of brightness available should have been obtained
previously by the Format Facility subcommand. The exact algorithm
for mapping these values to the available levels of intensity is
left to the implementors. A value of 7 in the intensity field
indicates that the brightness should be off, and any characters in
or entered into the field should not be displayed.
If the Modified bit is set, the field is considered to have been
modified and will be transmitted in response to a TRANSMIT MODIFIED
subcommand.
If the Pen Selectable bit is set, the field can be selected with the
light pen. Note: Use of the light pen should be the subject of
another Telnet option.
is 2 bytes that should be interpreted as a positive 16-bit
binary integer representing the number of characters following this
command which are affected by it.
Data sent to the terminal or the Using Host for unwritten areas of
the screen not in the scope of the count should be displayed with
the default values of the format map. The default values are No
Blinking, Normal Video, No Justification, No Protection and Normal
Intensity. For example, suppose a FORMAT DATA subcommand was sent to
the terminal with attributes Blinking and Protected and a
count of 5 followed by the string "Name: John Doe". The string
"Name:" would be protected and blinking, but the string "John Doe"
would not be.
This subcommand is used to format data to be displayed on the screen
of the terminal. The describes the attributes that the
field bytes wide should have. This field is to start at the
position of the cursor when the command is acted upon. The next
displayable characters in the data stream are used to fill
the field. Subsequent REPEAT subcommands may be used to specify the
contents of this field. If the sender specifies attributes that have
not been agreed upon by the use of the Format Facility subcommand,
the Telnet process should send an Error Subcommand to the sender,
but format the screen as if the bit had not been set.
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Data Entry Terminal Option
IAC SB DET REPEAT IAC SE subcommand code: 37
where is a positive 8-bit binary integer. is an 8-bit
byte containing an ASCII character.
This subcommand is used to perform data compression on data being
transferred to the terminal by encoding strings of identical
characters as the character and a count. The repeated characters may
be part of a field specified
IAC SB DET SUPPRESS PROTECTION IAC SE
subcommand code: 38
where may have the values of the Telnet option
negotiation:
251 WILL
252 WONT
253 DO
254 DONT
This subcommand is used to suppress the field protection in a
non-destructive manner. Many data entry terminals provide the means
by which protection may be turned on and off without modifying the
contents of the screen or the terminal's memory. Thus, the
protection may be turned off and back on without retransmitting the
form.
The default setting of the option is that protection is on, in other
words
IAC SB DET SUPPRESS PROTECTION WONT IAC SE
IAC SB DET SUPPRESS PROTECTION DONT IAC SE
Negotiation of this subcommand follows the same rules as
negotiations of the Telnet options.
IAC SB DET FIELD SEPARATOR IAC SE subcommand code: 39
It is necessary when transmitting only the unprotected portion of
the screen to provide a means for delimiting the fields. Existing
DET's use a variety of ASCII characters such as Tab, Group
Separator, Unit Separator, etc. In order to maintain transparency of
the NVDET this subcommand is used to separate the fields. Clearly,
this incurs rather high overhead. This overhead can be avoided by
using the Byte Macro Option (see Appendix 3).
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Data Entry Terminal Option
Miscellaneous Commands
IAC SB DET FN IAC SE subcommand code: 40
where: is one byte.
Many data-entry terminals provide a set of "function" keys which
when pressed send a one-character command to the server. This
subcommand describes such a facility. The values of the field
are defined by the user and server. The option merely provides the
means to transfer the information.
IAC SB DET ERROR IAC SE subcommand code: 41
where:
is a byte containing the subcommand code of the subcommand
in error.
is a byte containing an error code.
(For a list of the defined error codes see Appendix 2.)
This subcommand is provided to allow DET option implementations to
report errors they detect to the corresponding Telnet process. At
this point it is worth reiterating that the philosophy of this
option is that when an error is detected it should be reported;
however, the implementation should attempt its best effort to carry
out the intent of the subcommand or data in error.
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Data Entry Terminal Option
3. Default and Minimal Implementation Specifications
Default
WON'T DET -- DON'T DET
Neither host wishes to use the Data Entry Terminal option.
Minimal Implementation
DET EDIT FACILITIES
DET ERASE FACILITIES
DET TRANSMIT FACILITIES
DET FORMAT FACILITIES
DET MOVE CURSOR
DET HOME
DET ERASE SCREEN
DET TRANSMIT SCREEN
DET FORMAT DATA
DET ERROR
In the case of formatting the data, the minimal implementation
should be able to support a low and high level of intensity and
protection for all or no characters in a field. These functions,
however, are not required.
The minimal implementation also requires that the Output Line Width
and Output Page Size Telnet options be supported.
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Data Entry Terminal Option
4. Motivation
The Telnet protocol was originally designed to provide a means for
scroll-mode terminals, such as the standard teletype, to communicate
with processes through the network. This was suitable for the vast
majority of terminals and users at that time. However, as use of the
network has increased into other areas, especially areas where the
network is considered to provide a production environment for other
work, the desires and requirements of the user community have changed.
Therefore, it is necessary to consider supporting facilities that were
not initially supported. This Telnet option attempts to do that for
applications that require data entry terminals.
This option in effect defines the Network Virtual Data Entry Terminal.
Although the description of this option is quite long, this does not
imply that the Telnet protocol is a poor vehicle for this facility.
Data Entry Terminals are rather complex and varied in their abilities.
This option attempts to support both the minimal set of useful
functions that are either common to all or can be easily simulated and
the more sophisticated functions supplied in some terminals.
Unlike most real data entry terminals where the terminal functions are
encoded into one or more characters of the native character set, this
option performs all such controls within the Telnet subnegotiation
mechanism. This allows programs that are intimately familiar with the
kind of terminal they are communicating with to send commands that may
not be supported by either the option or the implementation. In other
words, it is possible to operate in a "raw" or at least "rare" mode
using as much of the option as necessary.
Although many data entry terminals support a variety of peripheral
devices such as printers, cassettes, etc. it is beyond the scope of
this option to entertain such considerations. A separate option should
be defined to handle this aspect of these devices.
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Data Entry Terminal Option
5. Description
General Notes
All implementations of this option are required to support a certain
minimal set of the subcommands for this option. Section 3 contains a
complete list of the subcommands in this minimal set. In keeping
with the Telnet protocol philosophy that an implementation should
not have to be able to parse commands it does not implement, every
subcommand of this option is either in the minimal set or is covered
by one of the facility subcommands. An implementation must
"negotiate" with its correspondent for permission to use subcommands
not in the minimal set before using them. For details of this
negotiation process see the section below on facility subcommands.
Most data entry terminals are used in a half duplex mode. (Although
most DET's on the market can be used either as data entry terminals
or as standard interactive terminals, we are only concerned here
with their use as DET's.) When this option is used, it is suggested
that the following Telnet options be refused: Echo, Remote
Controlled Transmission and Echoing, and Suppress Go-Ahead. However,
this option could be used to support a simple full duplex CRT based
application using the basic cursor control functions provided here.
For these cases, one or more of the above list of options might be
required. (Support of sophisticated interactive calligraphic
applications is beyond the scope of this option and should be done
by another option or the Network Graphics Protocol.)
In RFC 728, it was noted that a synch sequence can cause undesired
interactions between Telnet Control functions and the data stream. A
synch sequence causes data but not control functions to be flushed.
If a control function which has an effect on the data immediately
following it is present in the data stream when a synch sequence
occurs, the control function will have its effect not on the
intended data but on the data immediately following the Data Mark.
The following DET subcommands are susceptible to this pitfall:
CHAR INSERT
DATA TRANSMIT
FORMAT DATA
The undesired interactions are best avoided by the receiver
of the synch sequence deleting these subcommands and all data
associated with them before continuing to process the control
functions. This implies that the Data Mark should not occur in the
middle of the data associated with these subcommands.
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Data Entry Terminal Option
Facility Subcommands
These four subcommands are used by the User and Server
implementations to negotiate the subcommands and attributes of the
terminal that may be utilized. This negotiation can be viewed as the
terminal (User Host) indicating what facilities are provided and the
Server Host (or application program) indicating what facilities are
desired.
When Sent: A Server Telnet implementation using the DET option must
send a facility subcommand requesting the use of a particular
subcommand or terminal attribute not in the minimal implementation
before the first use of that subcommand or attribute. The User
Telnet implementation should respond as quickly as possible with its
reply. Neither the User nor Server are required to negotiate one
subcommand at a time. Also, a Telnet implementation responding to a
facility subcommand is not required to give permission only for that
subcommand. It may send a format map indicating all facilities of
that class which it supports. However, a Telnet implementation
requesting facilities must send a facility subcommand before its
first use of the subcommand regardless of whether earlier
negotiations have indicated the facility is provided. The facility
cannot be used until a corresponding facility subcommand has been
received. There are no other constraints on when the facility
subcommands may be sent. In particular, it is not necessary for an
application to know at the beginning of a session all facilities
that it will use.
Action When Recieved: There are two possible actions that may be
taken when a facility subcommand is received depending on whether
the receiver is a requestor or a provider (User).
Requestor: When a facility subcommand is received by a requestor and
it is in the state of Waiting for a Reply, it should go into the
state of Not Waiting. It should then take the facility map it had
sent and form the logical intersection with the facility map
received. (For the Intensity attribute, one should take the minimum
of the number received and the number requested.) The result
indicates the facilities successfully negotiated. Note: if
the receiver is not in the Waiting for Reply state, then this is the
provider case described next.
Provider: When a facility subcommand is received, it should send a
facility subcommand with a facility map of the facilities it
provides as soon as possible. It should then determine what new
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Data Entry Terminal Option
facilities it is providing for the Requestor by forming the logical
intersection of the facility map received and the one sent.
Note: Although in most cases the requestor will be the Server Host
and the provider will be the User Host supporting the terminal, this
distinction may not always be true.
Transmit Subcommands
There are two kinds of transmit subcommands: those used to request
that data be sent to the requestor, and one to preface data sent to
the requestor. The first kind allow the requestor to control when,
from where and to some degree how much data is transmitted from the
terminal. Their explanation is straightforward and may be found in
Section 2.
Data may be sent from the terminal as a result of two events: the
user of the terminal caused the transmission or in response to a
transmit subcommand. Some programs may wish to know from where on
the screen the transmission began. (This is reasonable, since the
terminal user may move the cursor around considerably before
transmitting.) Other programs may not need such information. The
DATA TRANSMIT subcommand is provided in case this function is
needed. When used this subcommand prefaces data coming from the
terminal. The parameters and give the screen coordinates of
the beginning of the transmission. must be less than or equal to
M-1 and must be less than or equal to N-1. It is assumed that
all data between this DATA TRANSMIT and the next one starts at the
coordinates given by the first subcommand and continues filling each
line thereafter according to the constraints of the screen and the
format effectors in the data. Thus an intelligent or sloppy
user-host DET implementation (depending on your point of view) need
only include a DATA TRANSMIT subcommand when the new starting point
is different from the last ending point.
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Data Entry Terminal Option
6. Sample Interaction
The nomenclature of RFC 726 will be used to describe this example. To
quote that RFC:
"S:" is sent from serving host to using host.
"U:" is sent from using host to serving host.
"T:" is entered by the terminal user.
"P:" is printed on the terminal.
Text surrounded by square brackets([]) is commentary. Text
surrounded by angle brackets (<>) is to be taken as a single unit.
E.g, carriage return is , and the decimal value 27 is
represented <27>.
We assume that the user has established the Telnet connection,
logged on, and an application program has just been started either
by the user directly or through a canned start up procedure. The
presentation on the page is meant to merely group entities together
and does not imply the position of message boundaries. One should
assume that any part of the dialogue may be sent as one or many
messages. The first action of the program or Telnet is to negotiate
the DET option:
S:
U:
S: |
