[ih] Recently restored and a small ARPANET was run using simulated IMP hardware. (was: TTL [was Exterior Gateway Protocol])

[the keyboard of geoff goodfellow]

jack, you've raised my curiosity with respect to:

... There
*is* ARPANET IMP software which was recently restored and a small
ARPANET was run using simulated IMP hardware.

Who/What/When/Where/Why?

geoff

On Sat, Sep 5, 2020 at 8:40 PM Jack Haverty via Internet-history <
internet-history at elists.isoc.org> wrote:

> Lukasz,
>
> I think that the earliest implementations of TTL called it "Time", but
> I'm not aware that anyone actually used time per se in gateways, at
> least in the early days (1977-1982 or so).
>
> TCP implementations didn't do anything with TTL other than set it on
> outgoing datagrams, and at least in my implementation (TCP for Unix), it
> was just set to some arbitrary value.  Until we had some data from
> experimentation it was hard to evaluate ideas about what routers, hosts,
> et al should actually do.   The early TCPs did use time in handling
> retransmission timers, and there was work a bit later to incorporate
> time more powerfully into TCP behavior, e.g., Van Jacobson's work.
>
> The early gateways, IIRC, used the terminology "time", but in practice
> used just hop counts, since time measurements were difficult to
> implement.   The exception to that may be Dave Mills' Fuzzballs, since
> Dave was the implementor most interested in time and making precise
> measurements of network behavior.   I *think* Dave may have used time
> values and delay-based routing amongst his "fuzzies".
>
> The BBN doc you're seeking might have been one of many that discussed
> the ARPANET internal mechanisms, e.g., ones with titles like "Routing
> Algorithm Improvements".  The ARPANET internal mechanisms did use time.
> It was fairly simple in the IMPs, since the delay introduced by the
> synchronous communications lines could be easily predicted, and the
> other major component of delay was the time spent in queues, which could
> be measured fairly easily.
>
> I even found one BBN ARPANET Project QTR from circa 1975 that discussed
> the merits of the new-fangled TCP proposal that some professor had
> published -- and seemed to conclude it couldn't possibly work.
>
> My involvement in implementations of TCPs and gateways lasted through
> about mid-1983, so I don't know much of the detail of subsequent
> implementations.  For the various BBN gateway/router equipment, Bob
> Hinden would probably be a good source.  The other major early player
> was MIT and spinoffs (Proteon), which perhaps Noel Chiappa will
> remember.   There's also at least one paper on the Fuzzballs which may
> have some details.
>
> One thing I'd advise being careful of is the various "specifications" in
> RFCs.  Much of the wording in those was intentionally non-prescriptive
> (use of "should" or "may" instead of "must"), to provide as much
> latitude as possible for experimentation with new ideas, especially
> within an AS.   The Internet was an Experiment.
>
> Also, there was no consistent enforcement mechanism to assure that
> implementations actually even conformed to the "must" elements.   So
> Reality could be very different from Specification.
>
> I don't know of any gateway implementations that have survived.   There
> *is* ARPANET IMP software which was recently restored and a small
> ARPANET was run using simulated IMP hardware.   I still have a ~1979
> listing of the TCP I wrote for Unix, but haven't scanned it into digital
> form yet.
>
> Jack
>
> On 9/5/20 7:38 PM, Łukasz Bromirski wrote:
> > Jack,
> >
> > I was reading a lot of old BBN PDFs thanks to all good souls on
> > this list that post nice URLs from time to time.
> >
> > I remember reading in at least one of them, that apparently first
> > TCP/IP implementations were indeed using TTL as literally “time”,
> > not hop count. I believe there somewhere there between PDP docs
> > and ARPANET docs I’ve read something to the effect “and from this
> > time we changed from measuring time to simply count routing hops”.
> > Of course, right now google-fu is failing me.
> >
> > Quoting RFC 1009 that was already brought up, there’s quite
> > direct “definition” of the field:
> >
> > "4.8.  Time-To-Live
> >
> >  The Time-to-Live (TTL) field of the IP header is defined to be a
> >  timer limiting the lifetime of a datagram in the Internet.  It is
> >  an 8-bit field and the units are seconds.  This would imply that
> >  for a maximum TTL of 255 a datagram would time-out after about 4
> >  and a quarter minutes.  Another aspect of the definition requires
> >  each gateway (or other module) that handles a datagram to
> >  decrement the TTL by at least one, even if the elapsed time was
> >  much less than a second.  Since this is very often the case, the
> >  TTL effectively becomes a hop count limit on how far a datagram
> >  can propagate through the Internet."
> >
> > Were there any implementations that survived somewhere and actually
> > did exactly that - counted actual time/processing delay, not hops?
> > And if it took 2s to process packet, did they really decrement TTL
> > by two?
> >
> > Thanks for any pointers,
>
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Geoff.Goodfellow at iconia.com
living as The Truth is True