Network Working Group A. Yasuda
Request for Comments: 1043 T. Thompson
Defense Intelligence Agency
Updates: RFC 732 February 1988
TELNET Data Entry Terminal Option
DODIIS Implementation
Status of this Memo
This RFC suggests a proposed protocol on the TELNET Data Entry
Terminal (DET) Option - DODIIS Implementation for the Internet
community. It is intended that this specification be compatible with
the specification of DET Option in RFC-732. Discussion and
suggestions for improvements are encouraged. Distribution of this
memo is unlimited.
Introduction
In the early 1980s, the Defense Intelligence Agency (DIA) undertook
the tasks of developing a TELNET capability to access full screen
applications across a packet switching network. This effort was
successful by implementing Data Entry Terminal (DET) options within
the TELNET protocol based on RFC 732. These DET options have been
implemented on IAS, MVS, OS86 and UNIX operating systems. DET
options are being developed for VM and VMS operating systems.
The Department of Defense Intelligence Information System (DODIIS) is
a confederation of heterogeneous computer systems and remote
terminals utilizing the Defense Data Network (DDN) as the
communications backbone (namely the SCINET/DSNET-3).
Although the reason for implementing a DET option specification was
based upon data base application interfaces, the use of a full screen
TELNET provides a method to achieve higher efficiency on the network.
Most terminal to host applications on the ARPANET are character echo
TELNETs. This is both costly in time and network utilization, since
one character pressed on the keyboard generates a datagram composed
of TCP/IP headers plus the character sent to the host and the host
echoes back a similar datagram. In the DODIIS community, programmers
are highly encouraged to implement full screen applications; line at
a time is acceptable; and character remote echo mode is discouraged.
This RFC in its final form will be implemented on SCINET. During the
interim period, the "DODIIS TELNET Network Virtual Data Entry
Terminal (NVDET) Option Specification", DIA, April 1983, will be
implemented.
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SECTION 5 DESCRIPTION AND IMPLEMENTATION RULES 17
The DODIIS DET Model 17
Negotiating the DET Option 18
DET Facilities Negotiation 18
General DET Interaction 19
Form Construction 20
Form response 21
Function Keys 22
Field Selection 22
Out-Of-Context Data 23
Line Discipline 23
Standard TELNET Control Functions 24
Other Implementation Notes 24
APPENDIX 1 DET OPCODES AND SUBCOMMAND SYNTAX 25
APPENDIX 2 DET ERROR CODES 26
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The convention in the documentation of the TELNET NVDET Protocol is
to express numbers in decimal. Data fields are described left to
right, with the most significant octet on the left and the least
significant octet on the right.
The order of transmission of the data described in this document is
resolved to the octet level. Whenever a diagram shows a group of
octets, the order of transmission of those octets is the normal order
in which they are read in English. For example, in the following
diagram the octets are transmitted in the order they are numbered.
Whenever an octet represents a numeric quantity, the left most bit in
the diagram is the high order or most significant bit. That is, the
bit labeled 0 is the most significant bit. For example, the
following diagram represents the value 170 (decimal).
Similarly, whenever a multi-octet field represents a numeric
quantity, the left most bit of the whole field is the most
significant bit. When a multi-octet quantity is transmitted the most
significant octet is transmitted first.
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1. Command Name and Option Code
DET 20
2. Command Meanings
IAC WILL DET
The sender of this command REQUESTS permission to begin, or
AGREES that it will begin, sending and receiving Data Entry
Terminal (DET) subcommands to control session interactions.
IAC WONT DET
If the connection is already operating in DET mode, the sender
of this command DEMANDS that the connection stop operating in
DET mode and begin operating in TELNET NVT mode. If the
connection is not operating in DET mode, the sender REFUSES to
begin operating in DET mode. A connection is operating in
TELNET NVT mode when both parties are interpreting data as
described by the TELNET SPECIFICATION, MIL-STD-1782.
IAC DO DET
The sender of this command REQUESTS permission to begin, or
AGREES that it will begin, sending and receiving Data Entry
Terminal (DET) subcommands to control session interactions.
IAC DONT DET
If the connection is already operating in DET mode, the sender
of this command DEMANDS that the connection stop operating in
DET mode and begin operating in TELNET NVT mode. If the
connection is not operating in DET mode, the sender REFUSES to
begin operating in DET mode. A connection is operating in
TELNET NVT mode when both parties are interpreting data as
described by the TELNET SPECIFICATION, MIL-STD-1782.
DODIIS implementations of the DET option use the subcommands
described in the remainder of Section 2. A description of the
DODIIS DET model and DET subcommand usage is contained in Section
5.
FACILITIES SUBCOMMANDS. Facilities subcommands are used to negotiate
DET facilities (subcommands and attributes). The facility
subcommands indicate the DET facilities the sender supports.
Facility negotiation may be viewed as the terminal indicating the
facilities it provides and the application indicating the facilities
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it desires. 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.
IAC SB DET EDIT-FACILITIES <facility map> IAC SE
subcommand code: 1
This subcommand indicates the edit facilities the sender
supports. The <facility map> parameter is one eight bit byte
containing the following flags:
Bits 5-7 Reserved
Bit 4 Read Cursor
Bits 0-3 Reserved
where:
If the Read-Cursor bit is set, the sender supports the
READ-CURSOR and CURSOR-POSITION subcommands.
Reserved bits represent edit facilities that are not
defined for DODIIS implementations; therefore, no
descriptions are provided. Reserved bits must be zeroed
to indicate non support of the associated edit facilities.
IAC SB DET ERASE-FACILITIES <facility map> IAC SE
subcommand code: 2
This subcommand indicates the erase facilities the sender
supports. The <facility map> parameter is one eight bit
byte containing flags. Since no erase facilities are
defined for DODIIS implementations, no descriptions are
provided. The ERASE-FACILITIES subcommand is part of the
minimal DET implementation and is included for that reason.
DODISI implementors must declare non support of erase
facilities by sending this subcommand with a zeroed facility
map.
IAC SB DET TRANSMIT-FACILITIES <facility map> IAC SE
subcommand code: 3
This subcommand indicates the transmit facilities the sender
supports. The <facility map> parameter is one eight bit byte
containing the following flags:
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Bits 6-7 Reserved
Bit 5 Data Transmit
Bits 0-4 Reserved
where:
If the Data-Transmit bit is set, the sender supports the
DATA-TRANSMIT subcommand.
Reserved bits represent transmit facilities that are not
defined for DODIIS implementations; therefore, no
descriptions are provided. Reserved bits must be zeroed
to indicate non support of the associated transmit
facilities.
IAC SB DET FORMAT-FACILITIES <facility map> IAC SE
subcommand code: 4
This subcommand indicates the format facilities the sender
supports. The <facility map> parameter is two eight bit bytes
containing the following:
Byte 0
Bit 7 Function Key
Bit 6 Modified
Bit 5 Field Selection
Bit 4 Repeat
Bit 3 Blinking
Bit 2 Reverse Video
Bit 1 Right Justification
Bit 0 Reserved
Byte 1
Bit 7 Reserved for color
Bit 6 Reserved
Bit 5 Protection
Bit 4 Alphabetic-Only
Bit 3 Numeric-Only
Bits 0-2 Intensity
where:
If the Function-Key bit is set, the sender supports the
FUNCTION-KEY and ENABLE-FUNCTION-KEY subcommands.
If the Modified bit is set, the sender supports the
FORMAT-DATA subcommand's Modified attribute and the
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TRANSMIT-MODIFIED subcommand.
If the Field-Selection bit is set, the sender supports
the FORMAT-DATA subcommand's Selectable attribute and
the SELECTED-FIELD subcommand.
If the Repeat bit is set the sender supports the REPEAT
subcommand.
If the Blinking bit is set, the sender requests or
provides the ability to emphasize a string of characters
by causing them to blink when displayed. (See the
FORMAT-DATA subcommand.)
If the Reverse-Video bit is set, the sender requests or
provides the ability to emphasize a string of characters
by "reversing their video image". If characters are
normally displayed as dark characters on a light
background, they are reversed and displayed as light
characters on a dark background, or
vice versa. (See the FORMAT-DATA subcommand.)
If the Right-Justification bit is set, the sender
requests or provides the ability to cause data entered
in a field to be right justified within the field. (See
the FORMAT-DATA subcommand.)
If the Protection bit is set, the sender requests or
provides the ability to protect certain fields displayed
on the DET screen from being altered by the user and
supports the ERASE-UNPROTECTED, FIELD-SEPARATOR, and
TRANSMIT-UNPROTECTED subcommands. (See the FORMAT-DATA
subcommand.)
If the Alphabetic-Only bit is set, the sender requests
or provides the ability to constrain the user of the DET
such that only alphabetic data may be entered into
certain fields. (See the FORMAT-DATA subcommand.)
If the Numeric-Only bit is set, the sender requests or
provides the ability to constrain the user of the DET
such that only numeric data may be entered into certain
fields. (See the FORMAT-DATA subcommand.)
The Intensity parameter is three bits wide and is
interpreted as a positive binary integer indicating the
number of visible levels of intensity that the sender
requests or provides for displaying data. (See the
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FORMAT-DATA subcommand.)
Reserved bits represent format facilities that are not
defined for DODIIS implementations; therefore, no
descriptions are provided. Reserved bits must be
zeroed to indicate non support of the associated format
facilities.
EDIT SUBCOMMANDS. Edit subcommands are sent by the application to
position the cursor on the DET screen.
IAC SB DET MOVE-CURSOR <x><y> IAC SE
subcommand code: 5
This subcommand positions the DET cursor at screen location
(x,y). the <x> and <y> parameters are positive eight bit
binary integers representing the character and line positions,
respectively, of a DET screen location. Values of x range
from zero (0) through M-1, where M is the DET screen width in
characters. Values of y range from zero (0) through N-1,
where N is the DET screen length in lines.
IAC SB DET HOME-CURSOR IAC SE
subcommand code: 12
This subcommand positions the cursor at DET screen address
(0,0). It is equivalent to the MOVE-CURSOR subcommand, where
x=0 and y=0.
TRANSMIT SUBCOMMANDS. Transmit subcommands are sent by the
application to request data from the DET or by the terminal to
identify data returned from the DET.
IAC SB DET READ-CURSOR IAC SE
subcommand code: 17
This subcommand requests return of the DET cursor position.
Use of this subcommand requires facility negotiation; see the
EDITFACILITIES subcommand, Read-Cursor bit.
IAC SB DET CURSOR-POSITION <x><y> IAC SE
subcommand code: 18
This subcommand returns cursor position in response to a
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READCURSOR subcommand. The <x> and <y> parameters are
eight bit binary integers representing the cursor's position.
The <x> and <y> parameters are positive eight bit binary
integers representing the character and line positions,
respectively, of a DET screen location. Values of x range
from zero (0) through M-1, where M is the DET screen width in
characters. Values of y range from zero (0) through N-1,
where N is the DET screen length in lines. Use of this
subcommand requires facility negotiation; see the
EDIT-FACILITIES subcommand, Read-Cursor bit.
IAC SB DET TRANSMIT-SCREEN IAC SE
subcommand code: 20
This subcommand requests return of all characters on the DET
screen beginning at cursor position (0,0). M x N characters,
where M is the DET screen width in characters and where N is
the DET screen length in lines, are returned with a SPACE
character returned for each character in the unwritten areas
(the areas between defined fields). FIELD-SEPARATOR and
DATA-TRANSMIT subcommands are not required to delimit or
identify fields.
IAC SB DET TRANSMIT-UNPROTECTED IAC SE
subcommand code: 21
This subcommand requests return of all characters in
unprotected fields. Use of this subcommand requires facility
negotiation; see the FORMAT-FACILITIES subcommand, Protection
bit.
IAC SB DET TRANSMIT-MODIFIED IAC SE
subcommand code: 27
This subcommand requests return of all characters in modified
fields. Modified fields are fields that have the Modified
attribute set (see FORMAT-DATA subcommand) as well as fields
actually modified by the user. Use of this subcommand
requires facility negotiation; see the FORMAT-FACILITIES
subcommand, Modified bit.
IAC SB DET DATA-TRANSMIT <x><y> IAC SE
subcommand code: 28
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This subcommand identifies a field returned in response to
a TRANSMIT-MODIFIED subcommand. The <x> and <y> parameters
are positive eight bit binary integers indicating the cursor
position of the field that follows the DATA-TRANSMIT
subcommand. This subcommand may precede the first field of
a transmission with subsequent fields separated by the
FIELD-SEPARATOR subcommand or it may precede each field.
Use of this subcommand requires facility negotiation; see
the TRANSMIT-FACILITIES subcommand, Data-Transmit bit.
ERASE SUBCOMMANDS. Erase subcommands are used by the application to
erase the DET screen or selected DET screen areas. In performing
erase operations, the erased characters are replaced with SPACE
characters.
IAC SB DET ERASE-SCREEN IAC SE
subcommand code: 29
This subcommand erases all characters from the DET screen.
All fields regardless of their attributes are deleted. The
cursor position after the operation is at (0,0). If the
protection attribute has been negotiated, the erased screen
contains protected SPACE characters.
IAC SB DET ERASE-UNPROTECTED IAC SE
subcommand code: 35
This subcommand erases all characters in the unprotected fields
of the DET screen. This subcommand replaces field contents
with SPACE characters; field attributes and sizes are not
changed. The cursor position after the operation is at the
beginning of the first unprotected field or, if there is no
unprotected field, at (0,0). Use of this subcommand requires
facility negotiation; see the FORMAT-FACILITIES subcommand,
Protection bit.
FORMAT SUBCOMMANDS. The format subcommands are used by the
application to define the fields of a form and by the terminal to
delimit fields sent from the DET.
IAC SB DET FORMAT-DATA <format map><count> IAC SE
subcommand code: 36
This subcommand defines the attributes and size of a DET field.
The <format map> parameter defines the field attributes and the
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<count> parameter defines the field size. The field starts at
the position of the cursor when the subcommand is acted upon.
The next <count> data characters in the data stream fill the
field.
The <format map> parameter is two eight bit bytes and contains
the following:
Byte 0
Bit 7 Blinking
Bit 6 Reverse Video
Bit 5 Right Justification
Bits 3-4 Protection
Bits 0-2 Intensity
Byte 1
Bits 5-7 Reserved
Bits 2-4 Reserved for color
Bit 1 Modified
Bit 0 Selectable
where:
If the Blinking bit is set, the following field of
<count> characters should have the Blinking attribute
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, characters
entered into the field by the user should be right
justified.
The Protection attribute is two bits wide and may take
on the following values:
0 No protection. Any valid DET data character may
be entered in the field.
1 Protected. No data may be entered in the field.
2 Alphabetic-only. Only the alphabetic characters
(A-Z and a-z) or the space character may be
entered in the field.
3 Numeric-only. Only the numeric characters (0-9),
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the plus sign (+), the minus sign (-), the decimal
point (.) or the space character may be entered in
the field.
The Intensity attribute is three bits wide and indicates
the brightness to be used when displaying the characters
in or entered into the field <count> characters wide.
The available number of visible intensity levels should
have been negotiated using the FORMAT-FACILITY
subcommand. A value of zero (0) indicates that
brightness should be OFF; that is, characters in or
entered into the field should not be displayed. The
values 1-7 indicate relative brightness; the exact
algorithm for mapping these values to the available
levels of intensity is left to the implementors.
If the Modified bit is set, the field is considered to
have been modified and will be returned, along with any
user modified fields.
If the Selectable bit is set, the field is a candidate
for field selection using the DET field selection
device.
The <count> parameter is two bytes and should be interpreted as a
positive 16-bit binary integer that defines the field size. The
high order bit is transmitted first. Data, not in the scope of
the count of a FORMAT-DATA subcommand, should be displayed with
the default field attributes (no blinking, no reverse video, no
justification, no protection, not modified, not selectable, and a
visible intensity). Minimum field size is one (1) character.
Maximum field size is determined by a field's starting location
and the end of the screen or the start of the next field.
Use of field attributes requires facility negotiation; see the
FORMAT-FACILITIES subcommand.
IAC SB DET REPEAT <count><char> IAC SE
subcommand code: 37
This subcommand permits compression of DET data by encoding
strings of identical characters as the character and a repeat
count. The <count> parameter is a positive 8-bit binary
integer. The <char> parameter is a valid DET data character.
Use of this subcommand requires facility negotiation; see
the FORMAT-FACILITIES subcommand, Repeat bit.
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IAC SB DET FIELD-SEPARATOR IAC SE
subcommand code: 39
This subcommand separates fields returned by the DET in
response to TRANSMIT-MODIFIED or TRANSMIT-UNPROTECTED
subcommands. Use of this subcommand requires facility
negotiation; see the FORMAT-FACILITIES subcommand,
Protection bit.
MISCELLANEOUS SUBCOMMANDS
IAC SB DET FUNCTION-KEY <code> IAC SE
subcommand code: 40
This subcommand transmits a user entered function key code.
The <code> parameter is one byte that identifies the virtual
function key entered. Function key <code> values range from
0 to 255. This subcommand is used in conjunction with the
ENABLE-FUNCTION-KEY subcommand. Use of this subcommand
requires facility negotiation; see the FORMAT-FACILITIES
subcommand, Function-Key bit.
IAC SB DET ERROR <cmd><error code> IAC SE
subcommand code: 41
This subcommand allows a DET option implementation to report
errors it detects to the corresponding TELNET process. The
<cmd> parameter is one byte containing the subcommand code
of the subcommand causing the error. The <error code>
parameter is one byte containing a DET error code. (See
Appendix 2 for DET error codes.)
Errors should be reported when detected. However, the
implementation should attempt to carry out the intent of
the subcommand or data in error.
IAC SB DET START-OUT-OF-CONTEXT-DATA IAC SE
subcommand code: 42
This subcommand precedes out-of-context data. The data
following this subcommand and prior to the
END-OUT-OF-CONTEXT-DATA subcommand is NOT part of the current
form. The out-out-of-context data should be interpreted as
NVT mode data (i.e., it may contain carriage return and line
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feed characters) and should be displayed in a timely and
non-destructive fashion.
IAC SB DET END-OUT-OF-CONTEXT-DATA IAC SE
subcommand code: 43
This subcommand indicates the end of the out-of-context data.
IAC SB DET ENABLE-FUNCTION-KEYS <key-map>IAC SE
subcommand code: 44
This subcommand enables (or disables) virtual function keys and
indicates the application's data requirements on function key
selection. The <key-map> parameter is a variable length byte
string. Each byte contains four bit-pairs and each bit-pair
represents a single function key. The first byte represents
function keys zero (0) through three (3); the second byte,
function keys four (4) through seven (7); and so on. Bit-pair
values and there meanings are as follows:
0 The virtual function key is disabled (i.e., locked).
1 The virtual function key is enabled. Only the FUNCTION-
KEY subcommand is returned on function key selection.
2 The virtual function key is enabled. All requested
screen data and/or cursor position, as well as, the
FUNCTION-KEY subcommand are returned on function key
selection.
3 Undefined.
Function keys not explicitly represented in the bitmap are
disabled (i.e., they are assumed to have a bit-pair value of
zero (0)).
Use of this subcommand requires facility negotiation; see the
FORMAT-FACILITIES subcommand; Function-Key bit.
IAC SB DET SELECTED-FIELD <x><y> IAC SE
subcommand code: 45
This subcommand identifies a user selected field. The <x> and
<y> parameters are the cursor position of the character
selected from within a selectable field (see the FORMAT-DATA
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subcommand, Selectable attribute.) Use of this subcommand
requires negotiation; see the FORMAT-FACILITIES subcommand,
Field-Selection bit.
3. Default and Minimal Implementation
Default.
WONT DET -- DONT DET
If the DET option cannot be negotiated, the connection is
not operated in DET mode.
Minimal DET Implementation.
The minimal DET implementation consists of all DET subcommands
that may be used without prior negotiation. These subcommands
are as follows:
The minimal DET implementation set of subcommands is not broad
enough to support forms interactions between DODIIS terminals
and DODIIS applications. Therefore, DODIIS implementations of
the DET option must support additional DET subcommands.
DODIIS terminal (User Host) implementations must implement and
support all of the DET subcommands contained in Section 2, as
well as those DET attributes supported by the terminal hardware
and any DET attributes easily emulated in software. DODIIS
application (Server Host) implementations must implement and
support those DET subcommands and attributes required by its
applications.
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DODIIS implementation recommendations are contained in the
table that follows. DODIIS implementors are cautioned that
failure to provide recommended support may limit
interoperability.
Recommended DET support levels for DODIIS implementations
USER HOST SERVER HOST
DET SUBCOMMANDS SUPPORT LEVEL SUPPORT LEVEL
--------------- ------------- -------------
EDIT-FACILITIES send & receive send & receive
ERASE-FACILITIES send & receive send & receive
TRANSMIT-FACILITIES send & receive send & receive
FORMAT-FACILITIES send & receive send & receive
REPEAT send & receive send & receive
ERROR send & receive send & receive
MOVE-CURSOR receive only send only
HOME-CURSOR receive only send only
READ-CURSOR receive only send only
TRANSMIT-SCREEN receive only send only
TRANSMIT-UNPROTECTED receive only send only
TRANSMIT-MODIFIED receive only send only
ERASE-SCREEN receive only send only
ERASE-UNPROTECTED receive only send only
FORMAT-DATA receive only send only
START-OUT-OF-CONTEXT-DATA receive only send only
END-OUT-OF-CONTEXT-DATA receive only send only
ENABLE-FUNCTION-KEYS receive only send only
CURSOR-POSITION send only receive only
DATA-TRANSMIT send only receive only
FIELD-SEPARATOR send only receive only
FUNCTION-KEY send only receive only
SELECTED-FIELD send only receive only
OTHER
-----
Page size (lines) 24-48
Line size (characters) 80
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Function keys (number) 64
(1) Implement if supported by terminal hardware.
(2) Implement if required by the application.
4. Motivation for the option
In 1981, the TELNET DET option (RFC 732) was selected as the protocol
to support interactions between DODIIS forms applications and DODIIS
forms terminals. The intent was to foster a high degree of
interoperability between DODIIS hosts with forms applications and
terminals. Since that time, the DET option has been and is being
implemented by several independent organizations within the DODIIS
community.
Motivated by concern that the independently developed implementations
of the DET option may not interoperate with one another, DODIIS
implementors met to identify DODIIS implementation requirements and
to resolve implementation issues that affect interoperability.
This document attempts to present the agreements and recommendations
of the DODIIS implementors.
5. Description and Implementation Rules
The DODIIS DET model.
The conceptual model of the DODIIS DET is that of a half-duplex,
forms oriented device with the following:
a. A rectangular screen for displaying protected and unprotected
data (a form) and optional capability to support blinking,
reverse video, and up to seven display intensity levels.
b. A keyboard and onboard mechanisms for editing unprotected
fields of a form and returning the modified fields.
c. Function keys that may be enabled and disabled on a key-by-key
basis by the application.
d. A field selection device, similar to a light pen, that permits
user selection of characters within appropriately identified
"selectable" fields.
The DODIIS DET screen has default sizes of 80 characters and 24
lines. These defaults may be changed through negotiation using the
Output Line Width and the Output Page Size options. When the parties
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cannot agree on screen size through negotiation, the default values
will be used. By agreement, DODIIS terminal (User Host)
implementations of DET will support page sizes of 24 to 48 lines.
The next writing position (x,y) on the DET screen is indicated by a
special display character called the cursor, where x is the position
of a character on a line and y is the line position on the DET
screen. Values of x range from 0 (the left most character position
on the line) to M-1, where M is the line length. Values of y range
from 0 (the top most line on the screen) to N-1, where N is the page
length. The cursor may be moved to any position on the DET screen
without disturbing the characters already displayed.
Valid field data for DET forms are the displayable ASCII character
codes in the range 32 through 126 decimal and character 7 "BELL".
Negotiating the DET option
The DET option is negotiated when either party REQUESTS use of the
DET option and the other party AGREES to its use. The DET option
is requested by sending a DO DET and WILL DET and is accepted by
sending a WILL DET and DO DET. (In the spirit of TELNET
negotiation, the DET option must be negotiated for both directions
on the connection.)
Several TELNET options conflict with the DET option. Therefore,
when the DET option is negotiated, the following TELNET options
should be refused (or explicitly terminated): Echo, Suppress Go-
Ahead, and Binary. (The Suppress Go-Ahead is the default state of
DODIIS TELNET connections when they are first established.)
DET facilities negotiation
All implementations of the DET option are required to support the
minimal DET implementation described in Section 3. In addition,
DODIIS implementations are required to support subcommands and
attributes that are consistent with DODIIS implementation
requirements. Before any of these additional DET facilities may
be used, an implementation must negotiate with its correspondent
for permission to use them.
The four facility subcommands (EDIT-FACILITIES, ERASE-FACILITIES,
TRANSMIT-FACILITIES, and FORMAT-FACILITIES) are used to negotiate
DET subcommands and attributes. This negotiation consists of an
exchange of facility subcommands and may be viewed as the terminal
(User Host) indicating the facilities it provides and the
application program (Server Host) indicating the facilities it
desires. The facilities that are jointly supported (and may be
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used) are arrived at by forming the logical intersection of the
facility map that was sent with the facility map that was
received. (For the intensity attribute, the lesser of the number
of intensity levels sent and the number of intensity levels
received will be used.) An implementation must record the
currently agreed upon set of subcommands and attributes. Only
subcommands and attributes reflected in that set may be used
without further exchange of facility subcommands.
Either party or both parties may initiate facilities negotiation
without confusion as long as care is taken to avoid non-
terminating negotiation loops. In particular, if you initiate
negotiation by sending a facility subcommand, you must remember
that you did initiate the negotiation. On receipt of a facility
subcommand; if you initiated the negotiation, no response is
required and the negotiation is complete; if you did not initiate
the negotiation, you must respond by sending the appropriate
facility subcommand to the requester. (Note that there is no
requirement to negotiate facilities one class at a time and that
the awareness of who initiated the negotiation must be maintained
for each of the facility subcommands.)
A TELNET implementation responding to a facility subcommand is not
required to compute the logical intersection of the maps before
responding. It should respond as quickly as possible with a
facility map indicating all facilities of that class that it
supports. There is no confusion since both parties compute the
set of supported subcommands and attributes in the same fashion.
Note that while both parties must agree to the use of the optional
subcommands and attributes, either party may disable use at any
time by merely sending the appropriate facility subcommand.
Further, there are no restrictions on when facilities may be sent.
CAUTION:
All facilities maps contain reserved bits.
These reserved bits must be zeroed when
facility maps are sent to indicate non
support and/or ignorance of the associated
facility. The reserved bits may be defined
in the future.
General DET Interaction
In the general interaction, the application implementation
constructs a form, negotiates the desired options, indicates the
required responses, and sends the TELNET GO-AHEAD. The GO-AHEAD
signals that the form construction is complete and that the DET
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keyboard may be unlocked to permit a user response.
The user normally responds by editing the unprotected areas of the
form and signaling "form-complete", entering a function key,
electing a field, or performing a combination of the preceding.
In each case, the terminal implementation sends the DET
subcommands indicating the user's response and returns the GO-
AHEAD. The GO-AHEAD signals the end of the user response.
The form, as edited by the user, remains on the virtual screen so
the application may continue the interaction by altering the form.
Form construction
The application implementation constructs a form on an erased
screen by defining each of the fields in the form. The DET fields
are defined by their starting cursor position, size, attributes,
and contents (data).
A field's starting cursor position is the cursor position of the
first character in the field. The cursor may be positioned
explicitly by the MOVE-CURSOR subcommand or it may be positioned
implicitly by field data or other DET subcommands (e.g.,
ERASESCREEN and ERASE-UNPROTECTED).
Field size, attributes, and contents may be defined using the
FORMAT-DATA subcommand followed by field data. Alternatively, a
field with default attributes may be defined using only the field
data. In this case, field size is the data string length. The
data string is terminated by the GO-AHEAD or any DET subcommand,
except the REPEAT subcommand.
There are no restrictions on attribute combinations that might be
applied to a field even though some combinations may not be
supported by terminal hardware. The terminal implementation
should display the field with a "reasonable" combination of
attributes. There is an error code that might be returned when an
"unsupported combination of format attributes" is detected. It is
not clear what the application should do about the error. In any
event, this condition should not provoke session termination.
Field contents (data) are restricted to printable ASCII characters
and "BELL" (codes 32 through 126 and 7 decimal). It is the
responsibility of the application implementation to properly
translate carriage returns, line feeds, tabs, etc. to the
appropriate DET subcommands.
The maximum number of fields a screen might contain is the screen
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size in characters (the product of characters per line and lines
per screen).
Fields may not overlap. That is, a new field may not start or end
within a previously defined field. However, overwriting of a
field to change its attributes or contents is permitted.
There are no restrictions on the order in which a form is built
(e.g., left-to-right and top-to-bottom); the terminal
implementation must be prepared to handle any order. Terminal
implementations are encouraged to display data as it arrives to
accommodate applications that persist in displaying status updates
on the task(s) they are performing.
If an application elects to modify a user edited form, it must
properly position the cursor making no assumptions about where the
user might have left the cursor. Further it must exactly
overwrite the existing fields.
When form construction is complete, the application indicates its
response requirements by sending the appropriate transmit
subcommand. It may send TRANSMIT-SCREEN, TRANSMIT-UNPROTECTED, or
TRANSMIT-MODIFIED to request data and/or it may send READ-CURSOR
to request cursor position. TRANSMIT-MODIFIED should be used
whenever possible to minimize the volume of data transmitted
between user and server hosts.
Form response
A form response is generated by the terminal implementation when
the user signals "form-complete" or enters an enabled function
key. The data returned are determined by the application through
the transmit subcommands. If no transmit subcommand was sent the
Modified and Protection attributes are used to determine an
implied transmit subcommand. If the Modified attribute has been
negotiated, assume TRANSMIT-MODIFIED. If the Protection attribute
has been negotiated but the Modified has not, assume
TRANSMITUNPROTECTED. If neither has been negotiated, assume
TRANSMITSCREEN. (The intent is to achieve transmission efficiency
by returning the smallest amount of data permitted by the in-force
DET attributes.)
CAUTION:
With TRANSMIT-MODIFIED the terminal implementation
must return all fields marked with the Modified
attribute in addition to fields actually modified by
the terminal user.
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Returned fields are identified and delimited using the
DATATRANSMIT and/or FIELD-SEPARATOR subcommands. The DATA-
TRANSMIT subcommand indicates the cursor address of the field that
follows it and there are no restrictions on the order in which
fields are returned. The FIELD-SEPARATOR subcommand conveys
left-to-right and top-to-bottom field ordering. Data not preceded
by one of these subcommands is assumed to be the first unprotected
field in the form. A FIELD-SEPARATOR followed by FIELD-SEPARATOR
indicates a field was unchanged and not returned.
Unless otherwise restricted by Numeric-only or Alphabetic-only
attributes, data entered into unprotected fields is restricted to
the printable ASCII characters and "BELL" (codes 32 through 126
and 7 decimal); no other characters are permitted.
Function keys
By general agreement, DODIIS terminal implementations will support
64 function keys (key values 0 through 63). Information on
mapping function keys to application functions is the
responsibility of the application and should be provided to the
terminal user in the form of user documentation.
The application enables and disables the function keys and
indicates its form response requirements by sending the
ENABLEFUNCTION-KEY subcommand. The terminal implementation
validates function key selections based on information received in
the ENABLE-FUNCTION-KEY bitmap. When an enabled function key is
entered, the terminal returns a form response (if indicated in the
bitmap), a FUNCTION-KEY subcommand, and the GO-AHEAD.
Virtual function keys are part of the DET's virtual keyboard and
are "locked" when the application has the GO-AHEAD. Since the
terminal sends the GO-AHEAD when a function key is entered,
entering a function key "re-locks" all function keys until the
GO-AHEAD is returned.
Field selection
Any character within a field having the Selectable attribute is a
candidate for selection. When selection is made, the terminal
returns a SELECTED-FIELD subcommand identifying the character
position selected. Multiple selections are permitted; however,
the ordering of the selections need not be preserved. Field
selection does not cause the GO-AHEAD to be sent. The GO-AHEAD
must be sent as a result of another user action such as a function
key entry or "form-complete" indication. Field selection is
disabled when the application has the GO-AHEAD.
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Out-of-context data
The out-of-context-data subcommands identify data that is clearly
not in the context of the form interaction. It is a convenient
not in the mechanism for sending ARE-YOU-THERE responses or host
advisory messages to the user without disturbing the DET's virtual
screen or altering the context of the form interaction.
The application may send out-of-context data at anytime. The data
must be preceded by the START-OUT-OF-CONTEXT-DATA subcommand and
followed immediately by the END-OUT-OF-CONTEXT-DATA subcommand.
The out-of-context data should contain carriage returns and line
feeds to facilitate formatting. The sender should limit the
amount of data sent, since most terminal implementations must
buffer the data prior to displaying it. The terminal
implementation should display the data to the user in a timely
fashion. The data is for display only, no user response is
required, and there is no mechanism for user response.
Line Discipline
The subject of DET and line discipline (controlling the connection
using the GO-AHEAD) causes a bit of confusion. The following
rules apply to GO-AHEAD and the DET option:
When DET is negotiated, the application assumes the GO-AHEAD.
GO-AHEAD is never passed implicitly; it is always passed
explicitly.
When the application has the GO-AHEAD, the terminal
implementation may send TELNET commands (INTERRUPT-PROCESS,
ABORT-OUTPUT, BREAK, and ARE-YOU-THERE). Nothing else is
valid.
When the terminal has the GO-AHEAD, the application may send
out-of-context data or MOVE-CURSOR and FORMAT-DATA subcommands
to update protected fields. Nothing else is valid. (The
terminal implementation must display the out-of-context data
and the field updates as soon as convenient.)
The terminal implementation sends the GO-AHEAD, without further
action on the part of the terminal user, when an enabled
function key or a "form-complete" is entered.
Since the terminal user must take explicit action to return the
GO-AHEAD to the application, instances will occur when the user
has the GO-AHEAD but the application needs it to display a new
form. (This is most likely to occur when the user enters an
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INTERRUPT PROCESS.) When it does occur, the application should
send an out-of-context-context message requesting the user to
enter a "form-complete". If the user cooperates, the application
can ignore any associated form response and regain control of the
connection to display its form.
The line discipline described here is more rigorous than that
described for NVT in MIL-STD-1782. These rules apply only when
operating in DET mode. At other times, the descriptions contained
in MIL-STD-1782 apply. This distinction is necessary to ensure
interoperability with non-DET implementations of TELNET.
Standard TELNET control functions
The TELNET control functions, ERASE CHARACTER and ERASE LINE, are
NOT required and should not be sent in DET mode.
Other implementation notes
a. The DODIIS DET conceptual model does not support character
editors or basic scrolling applications.
b. Implementors are cautioned that DET subcommand parameters
(e.g., facilities maps) may take on the value of the IAC
character and must be replicated if they are to be properly
interpreted.
c. Principle of Robustness: "Be conservative in what you send; be
liberal in what you accept from others."
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