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Request For Comments - RFC734

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NWG/RFC# 734        MRC 07-OCT-77 08:46  41953
SUPDUP Display Protocol    Page 1

Network Working Group      Mark Crispin
Request for Comments 734     SU-AI
NIC 41953    7 October 1977

  SUPDUP Protocol


This document describes  the SUPDUP protocol,  a highly efficient  display
telnet protocol.  It originally started as a private protocol between  the
ITS systems at MIT to allow a user at any one of these systems to use  one
of the others as a display.  At the current writing, SUPDUP user  programs
also exist  for  Data  Disc  and  Datamedia  displays  at  SU-AI  and  for
Datamedias at SRI-KL.  The author is not aware of any SUPDUP servers other
than at the four MIT ITS sites.

The advantage  of  the  SUPDUP  protocol  over  an  individual  terminal's
protocol is that SUPDUP defines a "virtual" or "software" display terminal
that implements relevant  cursor motion operations.   The protocol is  not
built on  any  particular  display  terminal but  rather  on  the  set  of
functions common to all display terminals; hence it is completely  device-
independent.  In addition, the protocol also provides for terminals  which
cannot handle certain operations, such as line or character insert/delete.
In fact,  it is  more than  this.   It provides  for terminals  which  are
missing any set of features, all the way down to model 33 Teletypes.

The advantage over the TELNET protocol  is that SUPDUP takes advantage  of
the full  capabilities of  display  terminals, although  it also  has  the
ability to run printing terminals.

It is to be  noted that SUPDUP operates  independently from TELNET; it  is
not an option to  the TELNET protocol.   In addition, certain  assumptions
are made about the  server and the user  programs and their  capabilities.
Specifically, it is  assumed that the  operating system on  a server  host
provides all the display-oriented features of ITS.  However, a server  may
elect not to do certain display operations available in SUPDUP; the SUPDUP
protocol is far-reaching enough so  that the protocol allows terminals  to
be handled  as well  as that  host can  handle terminals  in general.   Of
course, if a host does not  support display terminals in any special  way,
there is no point in bothering  to implement a SUPDUP server since  TELNET
will work just as well.

A more complete description  of the display facilities  of SUPDUP and  ITS
can be found by FTP'ing the  online file .INFO.;ITS TTY from ARPAnet  host
MIT-AI (host 206 octal, 134. decimal).  For more information, the  mailing
address for SUPDUP is "(BUG SUPDUP) at MIT-AI".  If your mail system won't
allow you to use parentheses, use Bug-SUPDUP@MIT-AI.
NWG/RFC# 734        MRC 07-OCT-77 08:46  41953
SUPDUP Display Protocol    Page 2


The SUPDUP protocol originated as the internal protocol used between parts
of ITS, and between ITS and "intelligent" terminals.  Over the network,  a
user host acts like an intelligent terminal programmed for ITS.

The way terminal  output works  in ITS is  as follows:   The user  program
tells the system to  do various operations,  such as printing  characters,
clearing the screen, moving the cursor, etc.  These operations are  formed
into 8-bit characters  (using the  %TD codes described  below) and  stored
into a  buffer.   At interrupt  level,  as the  terminal  demands  output,
characters are  removed  from  the buffer  and  translated  into  terminal
dependent codes.  At this time  padding and cursor motion optimization are
also done.

In some cases, the interrupt side does not run on the same machine as  the
user program.  SUPDUP terminals have their "interrupt side" running in the
user host.  When  SUPDUP is  run between two  ITS's, the  SUPDUP user  and
server programs and the network simply move characters from the buffer  in
the server machine to the buffer in the user machine.  The interrupt  side
then runs on the user machine just as if the characters had been generated

Due to the highly interactive characteristics of both the SUPDUP  protocol
and the ITS system, all transactions are strictly character at a time  and
all echoing  is  remote.  In  addition,  all padding  and  cursor  control
optimization must be done by the user.

Because this is also the internals of ITS, the right to change it any time
if necessary to provide new features  is reserved by MIT.  In  particular,
the initial  negotiation  is probably  going  to be  changed  to  transmit
additional variables, and additional %TD codes  may be added at any  time.
User programs should ignore those they don't know about.

The following conventions are  used in this  document: function keys  (ie,
keys which represent a "function"  rather than a "graphic character")  are
in upper case in square brackets.  Prefix keys (ie, keys which generate no
character but  rather are  held  down while  typing another  character  to
modify that  character)  are  in  upper case  in  angle  brackets.   Hence
"<CONTROL><META>[LINE FEED]" refers to  the character generated when  both
the CONTROL and META keys  are held down while a LINE FEED is typed.  Case
should  be  noted;  <CONTROL>A  refers  to  a  different  character   from
<CONTROL>a.  Finally, all numbers which  do not explicitly specify a  base
(ie, octal  or decimal)  should be  read  as octal  unless the  number  is
immediately followed by a period, in which case it is decimal.
NWG/RFC# 734        MRC 07-OCT-77 08:46  41953
SUPDUP Display Protocol    Page 3


The SUPDUP server listens on socket 137 octal.  ICP proceeds in the normal
way for establishing 8-bit connections.   After the ICP is completed,  the
user side  sends several  parameters to  the server  side in  the form  of
36.-bit words.  Each  word is  sent through  the 8-bit  connection as  six
6-bit bytes, most-significant first.  Each byte is in the low-order 6 bits
of a character.  The first word is the negative of the number of variables
to follow in the high order 18. bits (the low-order 18. bits are ignored),
followed by the  values of  the TCTYP,  TTYOPT, TCMXV,  TCMXH, and  TTYROL
terminal descriptor variables (these  are the names they  are known by  at
ITS sites).  These  variables are  36.-bit binary numbers  and define  the
terminal characteristics for the virtual terminal at the REMOTE host.

The count is for future compatability.  If more variables need to be  sent
in the future, the server should assume "reasonable" default values if the
user does not specify them.  PDP-10 fans will recognize the format of  the
count (ie, -count,,0) as being an  AOBJN pointer.  At the present  writing
there are five variables hence this word should be -5,,0.

The TCTYP variable defines the terminal type.  It MUST be 7 (%TNSFW).  Any
other value is a violation of protocol.

The TTYOPT  variable specifies  what capabilities  or options  the  user's
terminal has.  A bit being true implies that the terminal has this option.
This variable also includes user options which the user may wish to  alter
at his or her own descretion; these options are included since they may be
specified along with the terminal capabilities in the initial negotiation.
See below for the relevant TTYOPT bits.

The TCMXV variable specifies the screen height in number of lines.

The TCMXH variable specifies the line width in number of characters.  This
value is one less  than the screen width  (ITS indicates line overflow  by
outputting an exclamation  point at  the end  of the  display line  before
moving to the  next line).  Note:  the terminal must  not do an  automatic
CRLF when a  character is  printed in the  rightmost column.   If this  is
unavoidable, the user SUPDUP must decrement the width it sends by one.

Note: Setting either the TCMXV or  TCMXH dimension greater than 128.  will
work, but will have some problems as coordinates are sometimes represented
in only  7 bits.   The main  problems occur  in the  SUPDUP protocol  when
sending the cursor position after an output reset and in ITS user programs
using the display position codes ^PH and ^PV.

The TTYROL variable specifies the "glitch count" when scrolling.  This  is
the number of lines to scroll up when scrolling is required.  If zero, the
terminal is not  capable of  scrolling.  1 is  the usual  value, but  some
terminals glitch up by more than one line when they scroll.

Following the transmission of the terminal options by the user, the server
should respond with an  ASCII greeting message,  terminated with a  %TDNOP
code (%TD codes are described  below).  All transmissions from the  server
after the  %TDNOP  are  either printing  characters  or  virtual  terminal
display codes.
NWG/RFC# 734        MRC 07-OCT-77 08:46  41953
SUPDUP Display Protocol    Page 4

The user  and  the  server  now both  communicate  using  the  intelligent
terminal protocol (described below) from the  user and %TD codes from  the
server.  The user has two commands in addition to these; they are  escaped
by sending 300  (octal).  If following  the escape is  a 301 (octal),  the
server should attempt to  log off the remote  job (generally this is  sent
immediately before the user disconnects,  so this logout procedure  should
be done regardless of the continuing integrity of the connection).  If the
character following  the escape  is a  302 (octal),  all ASCII  characters
following up to a null (000  octal) are interpreted as "console  location"
which the server  can handle as  it pleases.  No  carriage return or  line
feed should be in the console location text.  Normally this is saved  away
to be displayed by the "who" command when other users ask where this  user
is located.
NWG/RFC# 734        MRC 07-OCT-77 08:46  41953
SUPDUP Display Protocol    Page 5


The relevant TTYOPT bits for SUPDUP usage follow.  The values are given in
octal, with the left and right 18-bit  halves separated by ",," as in  the
usual PDP-10 convention.

Bit nameValueMeaning

%TOALT200000,,0characters  175  and 176  are converted to
altmode (033) on input.

%TOERS 40000,,0this  terminal  is capable  of selectively
erasing its  screen.  That is, it supports
the %TDEOL,  the %TDDLF,  and (optionally)
the  %TDEOF  operations.   For   terminals
which   can   only   do   single-character
erasing, see %TOOVR.

%TOMVB 10000,,0this  terminal  is capable of  backspacing
(ie, moving the cursor backwards).

%TOSAI  4000,,0this   terminal   has   the   Stanford/ITS
extended ASCII graphics character set.

%TOOVR  1000,,0this terminal is  capable of overprinting;
if  two  characters  are  displayed in the
same  position, they will both be visible,
rather than one replacing the other.

Lack of this capability but the capability
to backspace (see %TOMVB) implies that the
terminal can  do single  character erasing
by overstriking with a space.  This allows
terminals without the %TOERS capability to
have display-style "rubout processing", as
this capability depends upon either %TOERS
or [%TOMVB and not %TOOVR].

%TOMVU   400,,0this terminal  is capable  of  moving  the
cursor upwards.

%TOLWR    20,,0this  terminal's  keyboard is  capable  of
generating lowercase characters;  this bit
is mostly provided for programs which want
to know this information.

%TOFCI    10,,0this  terminal's  keyboard is  capable  of
generating  CONTROL and META characters as
described below.

%TOLID     2,,0this  terminal  is  capable  of doing line
insert/delete operations,  ie, it supports

%TOCID     1,,0this   terminal   is   capable   of  doing
character insert/delete operations, ie, it
supports %TDICP and %TDDCP.
NWG/RFC# 734        MRC 07-OCT-77 08:46  41953
SUPDUP Display Protocol    Page 6


Bit nameValueMeaning

%TPCBS     0,,40this terminal is  using  the  "intelligent
terminal protocol".

%TPORS     0,,10the server should  process  output  resets
instead of ignoring them.

The following bits are user  option bits.  They may be  set or not set  at
the user's discretion.  The bits that are labelled "normally on" are those
that are normally  set on when  a terminal is  initialized (ie, by  typing
[CALL] on a local terminal).

Bit nameValueMeaning

%TOCLC100000,,0convert  lower-case  input to  upper case.
Many  terminals  have  a  "shift lock" key
which makes this option useless.

%TOSA1  2000,,0characters  001-037  should  be  displayed
using  the  Stanford/ITS  extended   ASCII
graphics character set instead of  uparrow
followed by 100+character.

%TOMOR   200,,0the   system   should  provide  "**MORE**"
processing when  the  cursor  reaches  the
bottom  line  of  the  screen.    **MORE**
processing is described in ITS TTY.

%TOROL   100,,0the terminal should scroll when attempting
output below the bottom line of the screen
instead of wrapping around to the top.
NWG/RFC# 734        MRC 07-OCT-77 08:46  41953
SUPDUP Display Protocol    Page 7


Note: only  the parts  of the  intelligent terminal  protocol relevant  to
SUPDUP are discussed here.  For more information, read ITS TTY.


There are two  character sets  available for  use with  SUPDUP; the  7-bit
character set of standard ASCII, and the 12-bit character set of  extended
ASCII.  Extended ASCII has 5 high order  or "bucky" bits on input and  has
graphics for octal 000-037 and 177 (see the section entitled "Stanford/ITS
character set" for more details).  The two character sets are identical on
output since the protocol  specifies that the host  should never send  the
standard ASCII  formatting  characters  (ie,  TAB,  LF,  VT,  FF,  CR)  as
formatting characters; the characters whose  octal values are the same  as
these formatting characters are never output unless the user job has these
characters enabled (setting %TOSAI and %TOSA1 generally does this).

Input differs dramatically  between the 7-bit  and 12-bit character  sets.
In the 7-bit character set, all characters input whose value is 037  octal
or less  are assumed  to be  (ASCII) control  characters.  In  the  12-bit
character set,  there are  5 "bucky"  bits which  may be  attached to  the
character.  The two most  important of these are  CONTROL and META,  which
form a 9-bit character set.  TOP  is used to distinguish between  printing
graphics in the extended character set and ASCII controls.  The other  two
are reserved and should be ignored.  Since both 7-bit and 12-bit terminals
are commonly in use, 0001, 0301, and 0341 are considered to be  <CONTROL>A
on input by most programs, while 4001 is considered to be downwards arrow.


Many programs and hosts do not process 12-bit input.  In this case, 12-bit
input is folded down to 7-bit as follows: TOP and META are discarded.   If
CONTROL is on, then if the 7-bit  part of the character specifies a  lower
case alphabetic it is converted to upper  case; then if the 7-bit part  is
between 077 and 137 the  100 bit is complemented or  if the 7-bit part  is
040 the 040 bit is subtracted  (that's right, <CONTROL>?  is converted  to
[RUBOUT] and <CONTROL>[SPACE] is  converted to [NULL]).   In any case  the
CONTROL bit is discarded,  and the remainder is  treated as a 7-bit  ASCII
character.  It should be noted that  in this case downwards arrow is  read
by the program as standard ASCII <CONTROL>A.

Servers which expect 12-bit input and are told to use the 7-bit  character
set should  do  appropriate unfolding  from  the 7-bit  character  set  to
12-bit.  It is up  to the individual server  to decide upon the  unfolding
scheme.  On ITS, user programs that use the 12-bit character set generally
have an alternative method for 7-bit; this often takes the form of  prefix
characters indicating that the next character should be "controllified" or
"metized", etc.
NWG/RFC# 734        MRC 07-OCT-77 08:46  41953
SUPDUP Display Protocol    Page 8



Under normal circumstances, characters input from the keyboard are sent to
the foreign host as is.  There  are two exceptions; the first occurs  when
an octal 034  character is to  be sent; it  must be quoted  by being  sent
twice, because 034 is used as  an escape character for protocol  commands.
The second  exception occurs  when  %TOFCI is  set  and a  character  with
non-zero bucky bits is to be sent.  In this case, the character, which  is
in the 12-bit form:


%TXTOP4000This character has the [TOP] key depressed.

%TXSFL2000Reserved, must be zero.

%TXSFT1000Reserved, must be zero.

%TXMTA 400This character has the [META] key depressed.

%TXCTL 200This character has the [CONTROL] key depressed.

%TXASC 177The ASCII portion of the character

is sent as three bytes.  The first  byte is always 034 octal (that is  why
034 must be  quoted).  The next  byte contains the  "bucky bits", ie,  the
%TXTOP through %TXCTL bits,  shifted over 7 bits  (ie, %TXTOP becomes  20)
with the 100  bit on.   The third  byte contains  the %TXASC  part of  the
character.  Hence the character <CONTROL><META>[LINE FEED] is sent as  034
103 012.


The  intelligent  terminal  protocol  also  is  involved  when  a  network
interrupt (INR/INS) is  received by  the user program.   The user  program
should increment a count of received network interrupts when this happens.
It should not do any output, and if possible abort any output in progress,
if this count is greater than zero  (NOTE: the program MUST allow for  the
count to go less than zero).

Since the server  no longer  knows where the  cursor is,  it suspends  all
output until the user informs it of the cursor position.  This also  gives
the server an idea of how much was thrown out in case it has to have  some
of the aborted output  displayed at a later  time.  The user program  does
this when it  receives a  %TDORS from the  server.  When  this happens  it
should  decrement  the  "number  of  received  network  interrupts"  count
described in the previous paragraph and then send 034 followed by 020, the
vertical position,  and  the  horizontal  position  of  where  the  cursor
currently is located on the user's screen.
NWG/RFC# 734        MRC 07-OCT-77 08:46  41953
SUPDUP Display Protocol    Page 9


Display output  is  somewhat  simpler.   Codes less  than  200  octal  are
printing characters and  are displayed  on the terminal  (see the  section
describing the "Stanford/ITS character set").  Codes greater than or equal
to 200 (octal) are known as "%TD codes", so called since their names begin
with %TD.  The %TD codes that are relevant to SUPDUP operation are  listed
here.  Any other code  received should be ignored,  although a bug  report
might be sent  to the server's  maintainers.  Note that  the normal  ASCII
formatting characters (011 - 015) do NOT have their formatting sense under
SUPDUP and should not occur at all unless the Stanford/ITS extended  ASCII
character set is in use (ie, %TOSAI is set in the TTYOPT word).

For cursor  positioning operations,  the  top left  corner is  (0,0),  ie,
vertical position 0, horizontal position 0.

%TD codeValueMeaning

%TDMOV200General cursor position code.  Followed by
four bytes;  the  first  two are the "old"
vertical  and horizontal positions and may
be  ignored.    The  next  two are the new
vertical  and  horizontal  positions.  The
cursor  should be moved  to this position.

On printing consoles (non %TOMVU), the old
vertical position may differ from the true
vertical position;  this  can  occur  when
scrolling.  In this case, the user program
should set  its  idea of the old  vertical
position to what the %TDMOV says and  then
proceed.  Hence a %TDMOV with an old  vpos
of 20. and a new vpos of 22. should always
move the "cursor" down two lines.  This is
used to prevent the vertical position from
becoming infinite.

%TDMV1201An  internal  cursor  motion  code   which
should not be seen;  but if it is,  it has
two  argument bytes after it and should be
treated the same as %TDMV0.

%TDEOF202Erase  to  end  of  screen.   This  is  an
optional function  since many terminals do
not support this.   If  the terminal  does
not  support this  function,  it should be
treated the same as %TDEOL.

%TDEOF does an erase to end of line,  then
erases  all lines lower on the screen than
the cursor.  The cursor does not move.

%TDEOL203Erase  to  end  of  line.  This erases the
character position the  cursor  is at  and
all  positions  to the right on  the  same
line.  The cursor does not move.
NWG/RFC# 734        MRC 07-OCT-77 08:46  41953
SUPDUP Display Protocol   Page 10


%TD codeValueMeaning

%TDDLF204Clear the character position the cursor is
on.  The cursor does not move.

%TDCRL207If the cursor is not on the bottom line of
the screen, move  cursor to  the beginning
of the next line and clear that line.   If
the  cursor is at the bottom line,  scroll

%TDNOP210No-op; should be ignored.

%TDORS214Output reset.   This code serves as a data
mark for  aborting  output much as  IAC DM
does in the ordinary TELNET protocol.

%TDQOT215Quotes the following  character.   This is
used  when sending 8-bit codes  which  are
not  %TD codes,  for instance when loading
programs  into  an  intelligent  terminal.
The  following  character should be passed
through intact to the terminal.

%TDFS216Non-destructive forward space.  The cursor
moves right one position;  this  code will
not be sent at the end of a line.

%TDMV0217General cursor position code.  Followed by
two bytes; the new vertical and horizontal

%TDCLR220Erase the screen.   Home the cursor to the
top left hand corner of the screen.

%TDBEL221Generate an audio tone, bell, whatever.

%TDILP223Insert blank lines at the cursor; followed
by a byte containing a count of the number
of blank lines to insert.   The  cursor is
unmoved.   The line the cursor is  on  and
all lines below it move down;  lines moved
off the bottom of the screen are lost.

%TDDLP224Delete lines at the cursor;  followed by a
count.  The cursor is unmoved.   The first
line  deleted is the  one the cursor is on.
Lines below those deleted move up.  Newly-
created lines  at the bottom of the screen
are blank.
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%TD codeValueMeaning

%TDICP225Insert  blank  character  positions at the
cursor;  followed by  a count.  The cursor
is unmoved.   The character the  cursor is
on and all characters to the right  on the
current line move to the right; characters
moved off the end of the line are lost.

%TDDCP226Delete characters at the cursor;  followed
by a count.   The cursor is unmoved.   The
first  character  deleted  is the  one the
cursor is on.  Newly-created characters at
the end of the line are blank.

%TDBOW227Display black characters on white screen.

%TDRST230Reset %TDBOW and such any future options.
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SUPDUP Display Protocol   Page 12


This section describes the extended  ASCII character set.   It  originated
with the character set developed at SAIL but was modified for 1968 ASCII.

This character set only  applies to terminals with  the %TOSAI and  %TOFCI
bits set in its TTYOPT word.  For non-%TOSAI terminals, the standard ASCII
printing  characters  are  the  only  available  output  characters.   For
non-%TOFCI terminals, the standard ASCII characters are the only available
input characters.


The first table describes the printing characters.  For output, the  7-bit
code is sent (terminal operations are performed by %TD codes).  For input,
the characters with values 000-037 and 177 must have the %TXTOP bit on  to
indicate the graphic is intended rather than a function or ASCII control.


 4000centered dot
 4001downward arrow
 4004logical AND
 4005logical NOT
 4017partial delta
 4020proper subset (left horseshoe)
 4021proper superset (right horseshoe)
 4022intersection (up horseshoe)
 4023union (downward horseshoe)
 4024universal quantifer
 4025existential quantifier
 4027double arrow
 4030left arrow
 4031right arrow
 4033lozenge (diamond)
 4037logical OR
 0040first standard ASCII character (space)
  ... . .
 0176last standard ASCII character (tilde)
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SUPDUP Display Protocol   Page 13



In addition, the following special characters exist for input only.  These
characters are  function keys  rather than  printing characters;  however,
some of these  characters have some  format effect or  graphic which  they
echo as; the host, not the SUPDUP program, handles any such mappings.

ValueCharacterUsual echoUsual Function

 0010[BACK SPACE]text formatting
 0011[TAB]text formatting
 0012[LINE FEED]text formatting
 0013[VT]text formatting
 0014[FORM]text formatting
 0015[RETURN]text formatting
 0032[CALL]uparrow-Zescape to system
 0033[ALTMODE]lozenge or $special activation
 0037[BACK NEXT]uparrow-underscoremonitor command prefix
 0177[RUBOUT]character delete

 4101[ESCAPE]local terminal command
 4102[BREAK]local subsystem escape
 4110[HELP]requests a help message


For all input characters, the following  "bucky bits" may be added to  the
character.  Their interpretation depends entirely upon the host.  <TOP> is
not listed here, as it  has been considered part  of the character in  the
previous  tables.

<CONTROL> is different from ASCII CTRL, however, many programs may request
the operating system to map such  characters to the ASCII forms (with  the
<TOP> bit off).  In this case <META> is ignored.


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SUPDUP Display Protocol   Page 14


Richard M. Stallman (RMS@MIT-AI)  and David A.  Moon (Moon@MIT-MC) of  the
MIT-AI and MIT-MC  systems staff  wrote the source  documentation and  the
wonderful ITS terminal support that made this protocol possible.  It  must
be emphasized  that  this is  a  functional  protocol which  has  been  in
operation for some years now.

In addition, Moon,  Stallman, and Michael  McMahon (MMcM@SRI-KL)  provided
many helpful comments and corrections to this document.

For further reference, the sources for the known currently existing SUPDUP
user programs are available online as:

[MIT-AI] SYSENG;SUPDUP >for the ITS monitor,
[SU-AI]  SUPDUP.MID[NET,MRC]for the SAIL monitor,
[SRI-KL] <MMcM>SD.FAIfor the TOPS-20 monitor.

The source for the known currently existing SUPDUP server program is:

[MIT-AI] SYSENG;TELSER >for the ITS monitor.

These programs  are written  in  the MIDAS  and  FAIL dialects  of  PDP-10
assembly language.

©2018 Martin Webb