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Network Working Group                                        B. MetcalffRequest for Comments: 89                                           MITDGNIC: 5697                                                19 January 1971SOME HISTORIC MOMENTS IN NETWORKING   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.The IINCP   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   quite:   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 1971The 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 197136-Bit Words TransmittedFrom Harvard's PDP-10 toMITDG'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 TransmittedFrom MITDG's PDP-10 toHarvard'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 1971General 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]