[ih] The IMP Lights story (Was: Nit-picking an origin story)

[Steve Crocker]

Folks,

On Monday, 18 August 2025, I described how the lights on the IMPs often
burned out and caused a noticeable amount of downtime on the IMPs.  Geoff
Goodfellow asked for more details.  That exchange is copied below.

I learned of the problem with the IMP lights during a virtual roundtable
with Ben Barker and others.  We published the roundtable in [1].

I later interviewed Ben and Scott Bradner to learn more details. The
interview [2] is attached.

In the process of checking, Alex McKenzie sent me a more recent article
Dave Walden, he and Ben wrote which covers several incidents related to
reliability, and he sent a reference to the article to the list.  See [3]
below.  I also learned that Ben passed away two years ago.  I'm sad.  He
was a delightful and always positive guy.

After further discussion with Alex, we agreed [1] has the least detail.
[3] is best, but it's behind a paywall.  The interview [2] is a
close second.

I think this is all the information that's available.

Thanks to Ben for the delightful story, to Geoff for asking for the
details, to Scott for permission to use the interview, and to Alex for the
recent article and advice on how to proceed.

Steve


[1]  "The Arpanet and Its Impact on the State of Networking," Stephen D.
Crocker, Shinkuro, Inc., Computer, October 2019.  This was a virtual
roundtable with Ben Barker, Vint Cerf, Bob Kan, Len Kleinrock and Jeff
Rulifson.  Ben mentioned the problem with the IMP lights.  It's only a
small portion of the overall roundtable.  The next two references have more
detail.

[2] "Fixing the lights on the IMPs," an unpublished interview with Ben
Barker and Scott Brader, 3 July 2020.  It's attached.

[3] "Seeking High IMP Reliability in the 1970' ARPAnet" by Walden,
McKenzie, and Barker, published in Vol 44, No 2 (April - June 2022) of IEEE
Annals of the History of Computing.

---------- Forwarded message ---------
From: the keyboard of geoff goodfellow <geoff at iconia.com>
Date: Mon, Aug 18, 2025 at 8:54 PM
Subject: Re: [ih] Nit-picking an origin story
To: Steve Crocker <steve at shinkuro.com>
Cc: John Day <jeanjour at comcast.net>, Dave Crocker <dhc at dcrocker.net>,
Internet-history <internet-history at elists.isoc.org>, <dcrocker at bbiw.net>


[I] am innately curious about the ARPANET "The IMPs Lights Reliability
Issue" you mention here and wonder if some additional color could be
elucidated to the colorful story as to just HOW "the lights on the IMP
panel being a major source of outages" and specifically what
"re-engineering" was effectuated to ameliorate them from crashing the IMPs?

On Mon, Aug 18, 2025 at 7:22 AM Steve Crocker via Internet-history <
internet-history at elists.isoc.org> wrote:

> ... Ben Barker has a colorful
> story about the lights on the IMP panel being a major source of outages.
> The IMPs had a 98% percent uptime at first.  98% was astonishingly good
> compared to other machines of the day, but intolerably poor in terms of
> providing an always available service.  Ben re-engineered the lights and
> brought the reliability up to 99.98%.  How's that for a small thing having
> a big effect!
>


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