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Network Working Group B. Metcalff Request for Comments: 89 MITDG NIC: 5697 19 January 1971

 While awaiting the completion of an interim network control program
 (INCP) for the MIT MAC Dynamic Modeling/Computer Graphics PDP-6/10
 System (MITDG), we were able to achieve a number of 'historic moments
 in networking' worthy of some comment.  First, we were able to
 connect an MITDG terminal to a Multics process making it a Multics
 terminal.  Second, we successfully attached an MITDG terminal to the
 Harvard PDP-10 System thereby enabling automatic remote use of the
 Harvard System for MIT.  Third, we developed primitive mechanisms
 through which remotely generated programs and data could be
 transmitted to our system, executed, and returned.  Using these
 mechanisms in close cooperation with Harvard, we received graphics
 programs and 3D data from Harvard's PDP-10, processed them repeatedly
 using our Evans & Sutherland Line Drawing System (the E&S), and
 transmitted 2D scope data to Harvard's PDP-1 for display.


 Our experiments were run on the MITDG PDP-6/10 using what we have
 affectionately called our 'interim interim NCP' (IINCP).  Under the
 IINCP the IMP Interface is treated as a single-user I/O device which
 deals in raw network messages.  The software supporting necessary
 system calls includes little more than the basic interrupt-handling
 and buffering schemes to be used later by the NCP.  In short, the
 user-level programs which brought us to our historic moments were
 written close to the hardware with full knowledge of IMP-HOST
 Protocol (BBN 1822).  When the INCP and NCP are completed, these
 programs can be pruned considerably (80%).  The exercise of writing
 programs which conform to IMP-HOST Protocol was not at all wasted.
 Only now can those of us who are not writing the NCP begin to grasp
 the full meaning of RFNM's and their use in flow control.  The
 penalties for ignoring an impatient IMP, for failing to send NOOPS
 (NO-OPS) when starting up, and for blasting data onto the Network
 without regard for RFNM's are now well understood.

The Multics Connection

 Our quest for historic moments began with the need to demonstrate
 that the complex hardware-software system separating MITDG and
 Multics was operative and understood.  A task force (Messrs. Bingham,

Metcalff [Page 1] RFC 89 SOME HISTORIC MOMENTS IN NETWORKING 19 January 1971

 Brodie, Knight, Metcalfe, Meyer, Padlipsky and Skinner) was
 commissioned to establish a 'polite conversation' between a Multics
 terminal and an MITDG terminal.
 It was agreed that messages would be what we call 'network ASCII
 messages': 7-bit ASCII characters right-adjusted in 8-bit fields
 having the most significant bit set, marking, and padding.  In that
 Multics is presently predisposed toward line-oriented half-duplex
 terminals, it was decided that all transmissions would end with the
 Multics EOL character (ASCII <LINE FEED>).  To avoid duplicating much
 of the INCP in our experiment, the PDP-10 side of the connection was
 freed by convention from arbitrary bit-stream concatenation
 requirements and was permitted to associate logical message
 boundaries with network message boundaries (sic).  The 'polite
 conversation' was thus established and successful.
 Multics, then, connected the conversation to its command processor
 and the PDP-10 terminal suddenly became a Multics terminal.  But, not
 First, in the resulting MITDG-Multics connection there was no
 provision for a remote QUIT, which in Multics is not an ASCII
 character.  This is a problem for Multics.  It would seem that an
 ASCII character or the network's own interrupt control message could
 be given QUIT significance.
 Second, our initial driver program did not provide for RUBOUT.
 Because the Multics network input stream bypassed the typewriter
 device interface module (TTYDIM), line canonicalization was not
 performed.  In a more elegant implementation, line canonicalization
 could be done at Multics, providing the type-in editing conventions
 familiar to Multics users.  We fixed this problem hastily by having
 our driver program do local RUBOUT editing during line assembly, thus
 providing type-in editing conventions familiar to MITDG users.  It is
 clearly possible to do both local type-in editing and distant-host
 type-in editing.
 Third, we found that because of the manner in which our type-in
 entered the Multics system under the current network interface (i.e.
 not through TTYDIM), our remotely controlled processes were
 classified 'non-interactive' and thus fell to the bottom of Multics
 queues giving us slow response.  This problem can be easily fixed.

The Harvard Connection

 Connecting MITDG terminals to Multics proved to be easy in that the
 character-oriented MITDG system easily assembled lines for the
 Multics line-oriented system.  We (Messrs. Barker, Metcalfe) decided,

Metcalff [Page 2] RFC 89 SOME HISTORIC MOMENTS IN NETWORKING 19 January 1971

 therefore, that it would be worthwhile to connect the MITDG system to
 another character-oriented system, namely Harvard's PDP-10.  This
 move was also motivated by MITDG's desire to learn more about
 Harvard's new language system via MITDG's own consoles.
 It was found that Harvard had already provided an ASCII network
 interface to their system which accepted IMP-Teletype style messages
 as standard.  We quickly rigged up an IMP-Teletype message handler at
 MITDG and were immediately compatible and connected.  But not quite:
 First, Harvard runs the Digital Equipment Corporation (DEC) time-
 sharing system on their PDP-10 which has <control-C> as a QUIT
 character and <control-Z> as an end-of-file (EOF).  MITDG runs the
 MAC Incompatible Time-sharing System (ITS) which has <control-Z> as a
 QUIT character and <control-C> as an EOF.  This control character
 mismatch is convenient in the sense that typing <control-C> while
 connected to Harvard system through MITDG causes the right thing to
 happen - causes the execution of programs at Harvard to QUIT, as
 opposed to causing the driver program at MITDG to QUIT.  If, however,
 a Harvard program were to require that an EOF be typed, typing
 <control-Z> would cause ITS to stop the driver program in its tracks,
 leaving the Harvard EOF wait unsatisfied and the MITDG-Harvard
 connection severed.
 Second, the Harvard system has temporarily implemented this remote
 network console interface feature using a DEC style pseudo-teletype
 (PTY).  This device vis-a-vis the DEC system behaves as a half-duplex
 terminal which wakes up on a set of 'break characters' (e.g., return,
 altmode) affording us an opportunity for an interesting experiment.
 The use of DDT (Dynamic Debugging Tool) is thereby restricted (though
 not prevented) in that break characters must be scattered throughout
 a DDT interaction to bring the PTY to life to cause DDT to do the
 right thing.  For example, to examine the contents of a core location
 one needs to type 'addr<altmode>' (address slash altmode) the altmode
 being only a call-to-action to the PTY.  To alter the contents of the
 opened location, one must then type '<rub-out>contents<return>'; the
 <rub-out> character deletes the previous action <alt-mode>, the
 contents are stashed in the open address, and the <return> signals
 the close of the address and PTY wake-up.  It would seem that DDT is
 a program that will separate the men form the boys in networking.
 Third, it was found that the response from the Harvard system at
 MITDG was seemingly as fast as could be expected from one of their
 own consoles.  This fact is particularly exciting to those who don't
 have a feel for network transit times when it is pointed out that
 such response was generated through two time-sharing systems, three
 user level processes, and three IMPs, all connected in series.

Metcalff [Page 3] RFC 89 SOME HISTORIC MOMENTS IN NETWORKING 19 January 1971

The Harvard-MIT Graphics Experiment

 At Harvard are a PDP-10 Time-sharing System and a graphics oriented
 PDP-1, both connected to Harvard's IMP.  At MITDG are a PDP-6/10
 Time-sharing System and an E&S Line Drawing System.  It was felt
 (Messre. Barker, Cohen, McQuillan, Metcalfe, and Taft) that the time
 had come to demonstrate that the network could make remote resource
 available - to give Harvard access to the E&S at MITDG via the
 network.  The protocol for such use of the network was as follows:
 (1)  MITDG starts its network monitor program listening.  (2)
 Harvard starts its PDP-10 transmitting a core image containing an
 arbitrary PDP-10 program (with an embedded E&S program in this case).
 (3)  MITDG receives the core image from Harvard and places it in its
 memory at the starting address specified, collecting messages and
 concatenating them appropriately.  (There was no word-length mismatch
 problem.)  (4) Upon collecting a complete image (word count sent
 first along with starting address), MITDG stashes its own return
 address in a specified location of the transmitted program's image
 and transfers control to another image location.  (5)  Upon getting
 control at MITDG, the transmitted program executes (in this case sets
 up and runs an E&S program) and before returning to the MITDG network
 monitor stashes in specified locations of its image the beginning and
 ending addresses of its result.  (6)  With control returned, the
 MITDG monitor program then transmits the results to a listening host
 which makes good use of them (in this case a PDP-1 which displays
 them).  (7)  Then the MITDG program either terminates, returns
 control back to the image (as in this case), or waits for more data
 and/or program.  The protocol was implemented in the hosts and used
 to run a Harvard-assembled version of the E&S Aircraft Carrier
 Program (written originally by Harvard's Prof. Cohen) at MITDG and to
 display the resulting dynamic display on Harvard's PDP-1 driven DEC
 scopes.  The Carrier Program was 'flown' from MITDG and the changing
 views thus generated appeared both at MITDG and Harvard.  The picture
 was observed to change (being transmission limited) on the order of
 twice each second (perhaps less often).  But all was not rosey:
 First, it was observed that during the experiment prompting messages
 to the IMP-Teletypes were often garbled.  Most of the garbling can be
 attributed to the ASR-33 itself, some cannot.  There were no errors
 detected during data transmissions not involving the IMP-Teletypes.
 Second, during attempts to fly the Carrier from Harvard, we stumbled
 across a yet undiagnosed intermittent malfunction of (presumably) the
 MITDG hardware and/or software which caused our network connection to
 be totally shut down by the system during bi-directional
 transmission.  This problem is currently under investigation.

Metcalff [Page 4] RFC 89 SOME HISTORIC MOMENTS IN NETWORKING 19 January 1971

 Third, the response of the total system was slow compared to that
 required to do real-time dynamic graphics.  One would expect that if
 this limitation is to be overcome, higher bandwidth transmission
 lines, faster host response to network messages, and/or perhaps a
 message priority system will be required.

Metcalff [Page 5] RFC 89 SOME HISTORIC MOMENTS IN NETWORKING 19 January 1971

36-Bit Words Transmitted From Harvard's PDP-10 to MITDG's PDP-10

                   +---------------+---------------+ Image control
                   |     -count    |    origin-1   | word.
      Image:       |    start address of results   | | Filled in by
                   +-------------------------------+  -Harvard's
      Image+1:     |     end address of results    | | program during
                   +-------------------------------+-  its execution.
      Image+2:     |   ---------unused-----------  |  +--        -+
                   +-------------------------------+  |Filled in  |
      Image+3:     |      program stop address     |<-|by MITDG   |
                   +-------------------------------+  |for return |
      Image+4:     |     program start address     |  |of control.|
                   +-------------------------------+  +--       --+
      Image+5:     |                               |

Image control word | | and image arrive in | | network size buffers | | which are stripped of| | marking and padding | | and concatenated. | |


36-Bit Words Transmitted From MITDG's PDP-10 to Harvard's PDP-1

                    |               |    count      |

First word of results | | Specified in Image+0. | |

                    |      results                  |
                    |                               |
                    |                               |
                    |                               |
                    |                               |
                    |                               |
                    |                               |

Last word of results | | specified in Image+1. | |


Metcalff [Page 6] RFC 89 SOME HISTORIC MOMENTS IN NETWORKING 19 January 1971

General Comments

 In producing 'network ASCII messages' we were required to bend over
 backwards to insert marking so that our last data bit could fall on a
 word boundary.  Surely there must be a better way.  The double
 padding scheme and its variants with or without marking should be
 considered.  Given the current hardware, it would seem that double
 padding with marking would be an improvement.  A simple(?) fix to
 host IMP interfaces enabling them to send only good data from a
 partially filled last word would permit a further improvement:
 marking and host-supplied single padding.
 In these initial experiments Harvard used the IMP-Teletype message
 convention or what are call 'IMP ASCII messages' (without marking)
 because it would allow them to use IMP-Teletypes for logging in and
 testing.  Multics, on the other hand, used the standard network
 message format (with marking) to have Host-Host compatibility as per
 accepted protocols.  Both approaches have merit.  The IMP-Teletype
 message format should be changed to conform with the network standard
 - it should have marking.
 Finally, we would like to announce our readiness to participate in
 experiments which will further extend our confidence and competence
 in networking, especially experiments which, like the preceding, will
 have very large returns with relatively small investment.

Roster of those participating

 Ben Barker              Harvard, BBN
 Grenville Bingham       MITDG
 Howard Brodie           MITDG
 Dan Cohen               Harvard
 Tim Knight              MITDG, MIT/AI
 John McQuillan          Harvard
 Bob Metcalfe            MITDG, Harvard
 Ed Meyer                Multics
 Mike Padlipsky          Multics
 Tom Skinner             Multics
 Ed Taft                 Harvard
        [This RFC was put into machine readable form for entry]
        [into the online RFC archives by Lorrie Shiota, 10/01]

Metcalff [Page 7]

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