John Day                               Data Entry Terminal Option
June 27,1977                                            NIC 40652
                                                          RFC 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.

T__h_e_N__e_t_w_o_r_k_V__i_r_t_u_a_l_D__a_t_a_E__n_t_r_y_T__e_r_m_i_n_a_l(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. (Although most DET's will support one or more peripheral devices such as a paper tape reader or a printer, this option

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does not consider their support. Support of peripheral devices should be treated by 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.

F__a_c_i_l_i_t_y_F__u_n_c_t_i_o_n_s (for detailed semantics see Section 5.)

IAC SB DET <DET facility subcommand><facility map> IAC SE

        where  <DET  facility  subcommand>  is  one  8-bit   byte

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indicating the class of the facilities to be described, and <facility map> is a field of one or two 8-bit bytes containing flags describing the 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.)

        facility cmd:  ERASE FACILITIES     subcommand code:  2
          facility map:                                bit numbers

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             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
                                        byte 0
             Repeat                                         4
             Blinking                                       3
             Reverse Video                                  2
             Right Justification                            1
             Overstrike                                     0

byte 1

             Protection On/Off                              6
             Protection                                     5
             Alphabetic-only Protection                     4
             Numeric-only Protection                        3
             Intensity                                      0-2

where:

If the Repeat bit is set the sender requests or provides

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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 characters displayed on the screen from being altered by the user of the terminal. Setting this bit also implies

        that   ERASE   UNPROTECTED   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:

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           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 <facility map> is not set, then the sender does not request or provide that facility.

E__d_i_t_i_n_g_F__u_n_c_t_i_o_n_s

IAC SB DET MOVE CURSOR <x><y> IAC SE subcommand code: 5

where <x> is an 8-bit byte containing a positive binary

        integer   representing  the  character  position  of  the
        cursor, <y> 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.

        IAC SB DET SKIP TO LINE <y> IAC SE    subcommand code: 6

where <y> 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 <x> IAC SE   subcommand code: 7

where <x> 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.

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        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. 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

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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 <x><y> IAC SE
subcommand code: 18

where <x> and <y> are positive 8-bit binary integers.

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.

T__r_a_n_s_m_i_t_F__u_n_c_t_i_o_n_s (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.

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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.

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.

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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 DATA TRANSMIT <x><y> IAC SE
subcommand code: 27

This subcommand is used to preface data sent from the terminal in response to a user action or a TRANSMIT command. The parameters <x> and <y> indicate the initial position of the cursor. See the Transmit Subcommands subsection in Section 5 for more details.

E__r_a_s_e_F__u_n_c_t_i_o_n_s

        IAC SB DET ERASE SCREEN IAC SE     subcommand code: 28

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:  29

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:  30

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: 31

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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: 32

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: 33

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: 34

This subcommand causes all characters on the screen in unprotected fields to be removed from the screen. The cursor position after the operation is at (0,0) or, if that position is protected, at the beginning of the first unprotected field.

F__o_r_m_a_t_F__u_n_c_t_i_o_n_s

IAC SB DET FORMAT DATA <format map><count> IAC SE
subcommand code: 35

where <format map> is an 8-bit byte containing the following flags:

          Blinking                                 7
          Reverse Video                            6
          Right Justification                      5
          Protection                              3-4
          Intensity                               0-2

where:

If the Blinking bit is set, the following field of

        <count>  characters  should  have  the Blinking attribute

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applied to it by the receiver.

If the Reverse Video bit is set, the following field of

        <count>  characters  should  be displayed by the receiver
        with video reversed.

If the Right Justification bit is set, the input entered into the field of <count> 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 <count> bytes.

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 <count> 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.

        <count> 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.

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This subcommand is used to format data to be displayed on the screen of the terminal. The <format map> describes the attributes that the field <count> bytes wide should have. This field is to start at the position of the cursor when the command is acted upon. The next <count> 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.

IAC SB DET REPEAT <count><char> IAC SE
subcommand code: 36

        where  <count>  is  a  positive  8-bit  binary   integer.
        <char> 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 <negotiation> IAC SE
subcommand code: 37

where <negotiation> 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

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Negotiation of this subcommand follows the same rules as negotiations of the Telnet options.

        IAC SB DET FIELD SEPARATOR IAC SE    subcommand code:  38

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).

M__i_s_c_e_l_l_a_n_e_o_u_s_C__o_m_m_a_n_d_s

        IAC SB DET FN <code> IAC SE     subcommand code:  39

where: <code> 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 <code> field are defined by the user and server. The option merely provides the means to transfer the information.

IAC SB DET ERROR <cmd> <error code> IAC SE
subcommand code: 40

where:

               <cmd> is a byte containing the subcommand code  of
        the subcommand in error.
                <error code> 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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3. Default and Minimal Implementation Specifications

D__e_f_a_u_l_t

WON'T DET -- DON'T DET

Neither host wishes to use the Data Entry Terminal option.

M__i_n_i_m_a_l_I__m_p_l_e_m_e_n_t_a_t_i_o_n

DET EDIT FACILITIES
DET ERASE FACILITIES
DET TRANSMIT FACILITIES
DET FORMAT FACILITIES
DET MOVE CURSOR <x><y>
DET HOME
DET ERASE SCREEN
DET TRANSMIT SCREEN
DET FORMAT DATA
DET ERROR <cmd> <error code>

        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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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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5. Description

G__e_n_e_r_a_l_N__o_t_e_s

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

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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.

F__a_c_i_l_i_t_y_S__u_b_c_o_m_m_a_n_d_s

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.

W__h_e_n_S__e_n_t:_ 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.

A__c_t_i_o_n_W__h_e_n_R__e_c_i_e_v_e_d:_ 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

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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  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.

T__r_a_n_s_m_i_t_S__u_b_c_o_m_m_a_n_d_s

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 <x> and <y> give the screen coordinates of the beginning of the transmission. <x> must be less than or equal to M-1 and <y> 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.

6. Sample Interaction

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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  <cr>,  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: <IAC><DO><DET>

U: <IAC><WILL><DET>

S:<IAC><DO><OUTPUT PAGE SIZE> [First negotiate the screen
size. In this case we are
U:<IAC><WILL><NAOP> asking the user the size of the
terminal. This could have been
U:<IAC><SB><NAOP><DR><25><IAC><SE> done before the DET option was
negotiated.]
S:<IAC><SB><NAOP><DS><0><IAC><SE>

S:<IAC><DO><OUTPUT LINE WIDTH>

U:<IAC><SB><NAOL><DR><80><IAC><SE> [Defines the screen to be 25
lines by 80 characters.
S:<IAC><SB><NAOL><DS><0><IAC><SE> The server may use this
information when formatting the screen.]
S:<IAC><SB><DET><FORMAT FACILITIES> [Now set the terminal attributes.]
  <Repeat><Protection, 3 Levels
  Intensity><IAC><SE>

U:<IAC><SB><DET><FORMAT FACILITIES>
  <Repeat, Blinking><Protection, 3
  Levels Intensity><IAC><SE>

S:<IAC><SB><DET><ERASE SCREEN><IAC><SE> [Erase the screen and

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start sending the form.]

  <IAC><SB><DET><FORMAT DATA>
  <Protection=1, Intensity=1><0>
  <5><IAC><SE>Name:

  <IAC><SB><DET><MOVE CURSOR><0><1>
    <IAC><SE>

  <IAC><SB><DET><FORMAT DATA>
    Protection=1, Intensity=1><0>
    <8><IAC><SE>

Address:

  <IAC><SB><MOVE CURSOR><0><4><IAC>
    <SE>

  <IAC><SB><DET><FORMAT DATA>
    Protection=1, Intensity=1><0>
    <17><IAC><SE>
  Telephone number:

  <IAC><SB><DET><MOVE CURSOR><32><4>
  <IAC><SE>

  <IAC><SB><DET><FORMAT DATA>
    Protection=1, Intensity=1><0>
    <24><IAC><SE>
  Social Security Number:

  <IAC><SB><DET><FORMAT DATA>           [Establish a field that doesn't
    <Protection=1, Intensity=7>         display what is typed into it.]
    <0><11><IAC><SE>

  <IAC><SB><DET><MOVE CURSOR><32>
    <5><IAC><SE>

  <IAC><SB><DET><FORMAT FACILITIES>     [Get permission to use Blinking
    <Blinking><0><IAC><SE>              Attribute.]

U:<IAC><SB><DET><FORMAT FACILITIES>
    <Repeat, Blinking><Protection,
    3 Levels Intensity><IAC><SE>

S:<IAC><SB><DET><FORMAT DATA>
  <Blinking=1, Protection=1,
  Intensity=1><0><29><IAC><SE>

Your SSN will not be printed.

  <IAC><SB><DET><HOME><IAC><SE>

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  <IAC><GA>

The previous exchange has placed a form on the screen that looks like:

Name:
Address:

Telephone Number: Social Security Number:
"Your SSN will not be printed."

where the quoted string is blinking.

The terminal user is now free to fill in the form provided. He
positions the cursor at the beginning of the first field (this usually
is done by hitting the tab key) and begins typing. We do not show this
interaction since it does not generate any interaction with the User
Telnet program or the network. After the terminal user has completed
filling in the form, he strikes the transmit key to send the unprotected
part of the form, but first the User Telnet program negotiates the Byte
Macro Option to condense the Field Separator subcommand:

U:<IAC><DO><BM> [Negotiate Byte Macro Option.]

S:<IAC><WILL><BM> [Define decimal 166 to be the
Field Separator subcommand
U:<IAC><SB><BM><DEFINE> (see Appendix 3)]
  <166><6><IAC SB DET FIELD
  SEPARATOR IAC SE><IAC><SE>

S:<IAC><SB><BM><ACCEPT><166> [The server accepts the macro.]
  <IAC><SE>

U:<IAC><SB><DET><DATA TRANSMIT><0><6>
  <IAC><SE>
  John Doe <166> 1515 Elm St., Urbana, Il 61801
  <166> 217-333-9999 <166> 123-45-6789 <166>

S:<IAC><SB><DET><ERASE SCREEN><IAC><SE>
Thank you.

And so on.

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Appendix 1 - Subcommands, opcodes and syntax

               1    EDIT FACILITIES     <Facilty map>
               2    ERASE FACILITIES    <Facility map>
               3    TRANSMIT FACILITIES <Facility map>
               4    FORMAT FACILITIES    <Facility map 1> <Facility map 2>
               5    MOVE CURSOR          <x> <y>
               6    SKIP TO LINE         <y>
               7    SKIP TO CHAR         <x>
               8    UP
               9    DOWN
               10   LEFT
               11   RIGHT
               12   HOME
               13   LINE INSERT
               14   LINE DELETE
               15   CHAR INSERT
               16   CHAR DELETE
               17   READ CURSOR
               18   CURSOR POSITION      <x><y>
               19   REVERSE TAB
               20   TRANSMIT SCREEN
               21   TRANSMIT UNPROTECTED
               22   TRANSMIT LINE
               23   TRANSMIT FIELD
               24   TRANSMIT REST OF SCREEN
               25   TRANSMIT REST OF LINE
               26   TRANSMIT REST OF FIELD
               27   DATA TRANSMIT <x><y>
               28   ERASE SCREEN
               29   ERASE LINE
               30   ERASE FIELD
               31   ERASE REST OF SCREEN
               32   ERASE REST OF LINE
               33   ERASE REST OF FIELD
               34   ERASE UNPROTECTED
               35   FORMAT DATA <format map>
               36   REPEAT     <count><char>
               37   SUPPRESS PROTECTION <negotiation>
               38   FIELD SEPARATOR
               39   FN <code>
               40   ERROR  <cmd><error code>

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Appendix 2 - Error Codes

               1    Facility not previously negotiated.
               2    Illegal subcommand code.
               3    Cursor Address Out of Bounds.
               4    Undefined FN value.
               4    Can't negotiate acceptable line width.
               5    Can't negotiate acceptable page length.
               6    Illegal parameter in subcommand.
               7    Syntax error in parsing subcommand.
               8    Too many parameters in subcommand.
               9    Too few parameters in subcommand.
               10   Undefined parameter value
               11   Unsupported combination of Format Attributes

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Appendix 3 - Use of the Byte Macro Option

One of the major drawbacks of the DET option is that because the functions are encoded as Telnet option subnegotiations a fairly high overhead is incurred. A function like Character Insert which is encoded as a single byte in most terminals requires six bytes in the DET option. Originally the only other solution that would have accomplished the same transparency that the use of subcommands provides would have been to define additional Telnet control functions. However, since this would entail modification of the Telnet protocol itself, it was felt that this was not a wise

     solution.    Since  then the Telnet Byte Macro Option (RFC 729) has
     been defined.  This option allows the user and  server  Telnets  to
     map  an arbitrary character string into a single byte which is then
     transferred over the net.  Thus the Byte Macro Option provides  the
     means  for  implementations  to avoid the overhead for heavily used
     subcommands.  The rest of this appendix suggests how the Byte Macro
     Option should be applied to the DET option.

     In keeping with the specification of the Byte Macro Option,   macro
     bytes  will  be  chosen  from  the  range 128 to 239.   For the DET
     option, it is suggested that macro  bytes be chosen by  adding  the
     subcommand  code to 128.  In addition, an unofficial DET subcommand
     might be defined indicating that each side was willing  to  support
     macro bytes for all subcommands (but not necessarily support all of
     the  subcommands  themselves)  according  to  this algorithm.  This
     subcommand would be:

IAC SB DET DET-MACRO <negotiation> IAC SE
subcommand code: 254

where <negotiation> may have the values of the Telnet option negotiation:

                    251       WILL
                    252       WONT
                    253       DO
                    254       DONT

This subcommand is sent by a Telnet implementation to indicate its willingness to adopt byte macros for all of the DET subcommands according to the following algorithm:

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The macro byte for subcommand i will be i+128 and will represent the following string for parameterless subcommands:

IAC SB DET <subcommand code> IAC SE

and the following string for subcommands with parameters:

IAC SB DET <subcommand code>

The default setting for this subcommand is that the macros are not in effect, in other words,

IAC SB DET DET-MACRO WONT IAC SE
IAC SB DET DET-MACRO DONT IAC SE

Negotiation of this subcommand follows the same rules as negotiations of the Telnet options.

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References

1. ADM-1 Interactive Display Terminal Operator's Handbook
Lear-Siegler, Inc. 7410-31.

2. ADM-Interactive Display Terminal Operator's Handbook
Lear-Siegler, Inc. EID, 1974.

3. Burroughs TD 700/800 Reference Manual, Burroughs Corp., 1973

4. Burroughs TD 820 Reference Manual, Burroughs Corp. 1975.

5. CC-40 Communications Station: General Information Manual.
Computer Communication, Inc. Pub. No. MI-1100. 1974.

6. Crocker, David. "Telnet Byte Macro Option," RFC 729, 1977.

7. Data Entry Virtual Terminal Protocol for Euronet, DRAFT, 1977.

8. Day, John. "A Minor Pitfall in the Telnet Protocol,"
RFC 728, 1977.

9. Hazeltine 2000 Desk Top Display Operating Instructions.
Hazeltine IB-1866A, 1870.

10. How to Use the Consul 980: A Terminal Operator's Guide
and Interface Manual. Applied Digital Data Systems, Inc. 98-3000.

11. How to Use the Consul 520: A Terminal Operator's Guide
and Interface Manual. Applied Digital Data Systems, Inc. 52-3000.

12. Honeywell 7700 Series Visual Information Projection (VIP)
Systems: Preliminary Edition. 1973.

13. An Introduction to the IBM 3270 Information Display System.
IBM GA27-2739-4. 1973.

14. Naffah, N. "Protocole Appareil Virtuel type Ecran"
Reseau Cyclades. TER 536. 1976.

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15. Postel, Jon and Crocker, David. "Remote Controlled
Transmission and Echoing Telnet Option", RFC 726 NIC 39237, Mar. 1977.

16. Schicker, Peter. "Virtual Terminal Protocol (Proposal 2).
INWG Protocol Note #32., 1976.

17. UNISCOPE Display Terminal : Programmer Reference . Sperry-
Univac UP-7807 Rev. 2, 1975.

18. Universal Terminal System 400: System Description. Sperry-
Univac UP-8357, 1976.

19. Walden, David C. "Telnet Output Line Width Option."
NIC # 20196, 1973, also in ARPANET Protocol Handbook, 1976.

20. Walden, David C. "Telnet Output Page Size" NIC # 20197,
1973, also in ARPANET Protocol Handbook, 1976.

28