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Network Working Group                                           4691
RFC-5                                                           Jeff Rulifson
                                                                June 2, l969



                                DEL



:DEL, 02/06/69 1010:58   JFR   ;   .DSN=1; .LSP=0; ['=] AND NOT SP ; ['?];
dual transmission?

ABSTRACT

   The Decode-Encode Language (DEL) is a machine independent language
   tailored to two specific computer network tasks:

      accepting input codes from interactive consoles, giving immediate
      feedback, and packing the resulting information into message 
      packets for network transmissin.

      and accepting message packets from another computer, unpacking
      them, building trees of display information, and sending other
      information to the user at his interactive station.

   This is a working document for the evolution of the DEL language.
   Comments should be made through Jeff Rulifson at SRI.

FORWARD

   The initial ARPA network working group met at SRI on October 25-26,
   1968.

      It was generally agreed beforehand that the runmning of interactive
      programs across the network was the first problem that would be
      faced.

      This group, already in agreement about the underlaying notions of
      a DEL-like approach, set down some terminology, expectations for
      DEL programs, and lists of proposed semantic capability.

      At the meeting were Andrews, Baray, Carr, Crocker, Rulifson, and
      Stoughton.

   A second round of meetings was then held in a piecemeal way.

      Crocker meet with Rulifson at SRI on November 18, 1968.  This
      resulted in the incorporation of formal co-routines.

      and Stoughton meet with Rulifson at SRI on Decembeer 12, 1968.  It
      was decided to meet again, as a group, probably at UTAH, in late
      January 1969.

   The first public release of this paper was at the BBN NET meeting in
   Cambridge on February 13, 1969.

NET STANDARD TRANSLATORS        

   NST   The NST library is the set of programs necessary to mesh
   efficiently with the code compiled at the user sites from the DEL
   programs it receives.  The NST-DEL approach to NET interactive system
   communication is intended to operate over a broad spectrum.

   The lowest level of NST-DEL usage is direct transmission to the
   server-host, information in the same format that user programs
   would receive at the user-host.

      In this mode, the NST defaults to inaction.  The DEL program
      does not receive universal hardware representation input but 
      input in the normal fashion for the user-host.

      And the DEL 1 program becomes merely a message builder and
      sender.

   A more intermediate use of NST-DEL is to have echo tables for a
   TTY at the user-host.

      In this mode, the DEL program would run a full duplex TTY for
      the user.

      It would echo characters, translate them to the character set 
      of the server-host, pack the translated characters in messages,
      and on appropriate break characters send the messages.

      When messages come from the server-host, the DEL program would
      translate them to the user-host character set and print them on
      his TTY.

   A more ambitious task for DEL is the operation of large,
   display-oriented systems from remote consoles over the NET.

      Large interactive systems usually offer a lot of feedback to
      the user.  The unusual nature of the feedback make it
      impossible to model with echo table, and thus a user program
      must be activated in a TSS each time a button state is changed.

         This puts an unnecessarily large load on a TSS, and if the
         system is being run through the NET it could easily load two
         systems.

         To avoid this double overloading of TSS, a DEL program will
         run on the user-host.  It will handle all the immediate
         feedback, much like a complicated echo table.  At appropriate
         button pushes, message will be sent to the server-host and
         display updates received in return.

      One of the more difficult, and often neglected, problems is the
      effective simulation of one nonstandard console on another non-
      standard console.

         We attempt to offer a means of solving this problem through
         the co-routine structure of DEL programs.  For the
         complicated interactive systems, part of the DEL programs
         will be constructed by the server-host programmers.
         Interfaces between this program and the input stream may
         easily be inserted by programmers at the user-host site.


UNIVERSAL HARDWARE REPRESENTATION

   To minimize the number of translators needed to map any facility's
   user codes to any other facility, there is a universal hardware
   representation.

   This is simply a way of talking, in general terms, about all the
   hardware devices at all the interactive display stations in the initial
   network.

   For example, a display is thought of as being a square, the
   mid-point has coordinates (0.0), the range is -1 to 1 on both
   axes.  A point may now be specified to any accuracy, regardless of
   the particular number of density of rastor points on a display.

   The representation is discussed in the semantic explanations
   accompanying the formal description of DEL.

INTRODUCTION TO THE NETWORK STANDARD TRANSLATOR (NST)

   Suppose that a user at a remote site, say Utah, is entered in the
   AHI system and wants to run NLS.

   The first step is to enter NLS in the normal way.  At that time
   the Utah system will request a symbolic program from NLS.

      REP   This program is written in DEL.  It is called the NLS
      Remote Encode Program (REP).

      The program accepts input in the Universal Hardware
      Representation and translates it to a form usable by NLS.

      It may pack characters in a buffer, also do some local
      feedback.

   When the program is first received at Utah it is compiled and
   loaded to be run in conjunction with a standard library.

   All input from the Utah console first goes to the NLS NEP.  It is
   processed, parsed, blocked, translated, etc.  When NEP receives a
   character appropriate to its state it may finally initiate
   transfers to the 940.  The bits transferred are in a form
   acceptable to the 940, and maybe in a standard form so that the
   NLSW need not differentiate between Utah and other NET users.


ADVANTAGES OF NST

   After each node has implemented the library part of the NST, it
   need only write one program for each subsystem, namely the
   symbolic file it sends to each user that maps the NET hardware
   representation into its own special bit formats.

      This is the minimum programming that can be expected if 
      console is used to its fullest extent.

      Since the NST which runs the encode translation is coded at the
      user site, it can take advantage of hardware at its consoles to
      the fullest extent.  It can also add or remove hardware 
      features without requiring new or different translation tables
      from the host.

      Local users are also kept up to date on any changes in the system
      offered at the host site.  As new features are added,
      the host programmers change the symbolic encode program.  When
      this new program is compiled and used at the user site, the new
      features are automatically included.

   The advantages of having the encode translation programs
   transferred symbolically should be obvious.

      Each site can translate any way it sees fit.  Thus machine code
      for each site can be produced to fit that site; faster run
      times and greater code density will be the result.

      Moreover, extra symbolic programs, coded at the user site, may
      be easily interfaced between the user's monitor system and the
      DEL program from the host machine.  This should ease the
      problem of console extension (e.g. accommodating unusual keys and
      buttons) without loss of the flexibility needed for man-machine
      interaction.


   It is expected that when there is matching hardware, the symbolic
   programs will take this into account and avoid any unnecessary
   computing.  This is immediately possible through the code
   translation constructs of DEL.  It may someday be possible through
   program composition (when Crocker tells us how??)


AHI NLS - USER CONSOLE COMMUNICATION - AN EXAMPLE

   BLOCK DIAGRAM

      The right side of the picture represents functions done at the
      user's main computer; the left side represents those done at the
      host computer.

         Each label in the picture corresponds to a statement with the
         same name.

         There are four trails associated with this picture.  The first
         links (in a forward direction) the labels which are concerned
         only with network information.  The second links the total
         information flow (again in a forward direction).  The last two
         are equivalent to the first two but in a backward direction.
         They may be set with pointers t1 through t4 respectively.

         [">tif:] OR I" >nif"]; ["<tif:] OR ["<nif"];

USER-TO-HOST TRANSMISSION

   Keyboard is the set of input devices at the user's console.
   Input bits from stations, after drifting through levels of monitor
   and interrupt handlers, eventually come to the encode translator.
   [>nif(encode)]

   Encode maps the semi-raw input bits into an input stream in a
   form suited to the serving-host subsystem which will process the
   input.  [>nif(hrt)<nif(keyboard)]

      The Encode program was supplied by the server-host subsystem
      when the subsystem was first requested.  It is sent to the user
      machine in symbolic form and is compiled at the user machine
      into code particularly suited to that machine.

      It may pack to break characters, map multiple characters to
      single characters and vice versa, do character translation, and
      give immediate feedback to the user.

   1 dm    Immediate feedback from the encode translator first goes to
   local display management, where it is mapped from the NET standard
   to the local display hardware.

      A wide range of echo output may come from the encode
      translator.  Simple character echoes would be a minimum, while
      command and machine-state feedback will be common.

      It is reasonable to expect control and feedback functions not
      even done at the server-host user stations to be done in local
      display control.  For example, people with high-speed displays
      may want to selectively clear curves on a Culler display, a
      function which is impossible on a storage tube.

   Output from the encode translator for the server-host goes to the
   invisible IMP, is broken into appropriate sizes and labeled by the
   encode translator, and then goes to the NET-to-host translator.

      Output from the user may be more than on-line input.  It may be
      larger items such as computer-generated data, or files
      generated and used exclusively at the server-host site but
      stored at the user-host site.

      Information of this kind may avoid translation, if it is already in
      server-host format, or it may undergo yet another kind of translation
      if it is a block of data.

   hrp  It finally gets to the host, and must then go through the
   host reception program.  This maps and reorders the standard
   transmission-style packets of bits sent by the encode programs
   into messages acceptable to the host.  This program may well be
   part of the monitor of the host machine. [>tif(net mode)<nif(code)]


HOST-TO-USER TRANSMISSION

   decode   Output from the server-host initially goes through decode,
   a translation map similar to, and perhaps more complicated than,
   the encode map.  [>nif(urt)>tif(imp ctrl)<tif(net mode)]

      This map at least formats display output into a simplified
      logical-entity output stream, of which meaningful pieces may be
      dealt with in various ways at the user site.

         The Decode program was sent to the host machine at the same
         time that the Encode program was sent to the user machine.
         The program is initially in symbolic form and is compiled
         for efficient running at the host machine.
         
         Lines of charaters should be logically identified so that
         different line widths can be handled at the user site.

         Some form of logical line identification must also be made.
         For example, if a straight line is to be drawn across the
         display this fact should be transmitted, rather than a
         series of 500 short vectors.

         As things firm up, more and more complicated structural
         display information (in the manner of LEAP) should be sent
         and accommodated at user sites so that the responsibility for
         real-time display manipulation may shift closer to the user.

      imp ctrl   The server-host may also want to send control
      information to IMPs.  Formatting of this information is done by
      the host decoder.  [>tif(urt) <tif(decode)]

      The other control information supplied by the host decoder is
      message break up and identification so that proper assembly and
      sorting can be done at the user site.

   From the host decoder, information does to the invisible IMP, and
   directly to the NET-to-user translator.  The only operation done
   on the messages is that they may be shuffled.

   urt   The user reception translator accepts messages from the
   user-site IMP 1 and fixes them up for user-site display.  
   [>nif(d ctrl)>tif(prgm ctrl)<tif(imp ctrl)<nif(decode)]

      The minimal action is a reordering of the message pieces.
      
      dctrl   For display output, however, more needs to be done.  The
      NET logical display information must be put in the format of
      the user site.  Display control does this job.  Since it
      coordinates between (encode) and (decode) it is able to offer
      features of display management local to the user site.
      [>nif(display)<nif(urt)]

      prgmctrl   Another action may be the selective translation and
      routing of information to particular user-site subsystems.
      [>tif(dctrl)<tif(urt)]

         For example, blocks of floating-point information may be
         converted to user-style words and sent, in block form, to a
         subsystem for processing or storage.

         The styles and translation of this information may well be a 
         compact binary format suitable for quick translation, rather
         than a print-image-oriented format.

      (display)   is the output to the user.  [<nif(d ctrl)]

   
   USER-TO-HOST INDIRECT TRANSMISSION

      (net mode)   This is the mode where a remote user can link to a node
      indirectly through another node.   [<nif(decode)<tif(hrt)]


DEL SYNTAX

   NOTES FOR NLS USERS
  
      All statements in this branch which are not part of the compiler
      must end with a period.

      To compile the DEL compiler:

         Set this pattern for the content analyzer ( (symbol for up arrow)P1
         SE(P1) <-"-;). The pointer "del" is on the first character of pattern.

         Jump to the first statement of the compiler.  The pointer "c"
         is on this statement.

         And output the compiler to file  ( '/A-DEL' ).  The pointer "f"
         is on the name of the file for the compiler output -

   PROGRAMS

      SYNTAX

         -meta file (k=100.m=300,n=20,s=900)

         file = mesdecl $declaration $procedure "FINISH";

         procedure =

           procname (

              (

                 type "FUNCTION" /

                 "PROCEDURE" ) .id (type .id / -empty)) /

              "CO-ROUTINE") ' /

           $declaration labeledst $(labeledst ';) "endp.";

         labeledst = ((left arrow symbol).id ': / .empty) statement;

         type = "INTEGER" / "REAL" ;

         procname = .id;

      Functions are differentiated from procedures to aid compilers in
      better code production and run time checks.

         Functions return values.

         Procedures do not return values.

      Co-routines do not have names or arguments.  Their initial
      envocation points are given the pipe declaration.

      It is not clear just how global declarations are to be??

DECLARATIONS

   SYNTAX

      declaration = numbertype / structuredtype / label / lcl2uhr /
      uhr2rmt / pipetype;

      numbertype = : ("REAL" / "INTEGER") ("CONSTANT" conlist /
      varlist);

      conlist =

         .id '(left arrow symbol)constant

         $('. .id '(left arrow symbol)constant);

      varlist =

         .id ('(left arrow symbol)constant / .empty)

         $('. .id('(left arrow symbol)constant / .empty));

      idlist = .id $('. .id);

      structuredtype = (tree" / "pointer" / "buffer" ) idlist;

      label = "LABEL1" idlist;

      pipetype = PIPE" pairedids $(', pairedids);

      pairedids = .id .id;

      procname = .id;

      integerv = .id;

      pipename = .id;

      labelv = .id;

   Variables which are declared to be constant, may be put in
   read-only memory at run time.

   The label declaration is to declare cells which may contain the
   machine addresses of labels in the program as their values.  This 
   is not the B5500 label declaration.

   In the pipe declaration the first .ID of each pair is the name of
   the pipe, the second is thke initial starting point for the pipe.

ARITHMETIC

   SYNTAX

      exp = "IF" conjunct "THEN" exp "ELSE" exp;

      sum = term (

         '+ sum /