Network Working Group B. Metcalff
Request for Comments: 89 MITDG
NIC: 5697 19 January 1971
SOME 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,
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,
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.
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.
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.
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.
+-------------------------------+
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 RFCwas put into machine readable form for entry]
[into the online RFCarchives by Lorrie Shiota, 10/01]
