Re: [SystemSafety] RE : Qualifying SW as "proven in use"

From: Nancy Leveson < >
Date: Fri, 28 Jun 2013 02:14:39 -0700


These IEC standards are having an impact beyond the process industries and are being pushed or influencing practice in products such as the cars we all drive. IEC 61508 is being promoted for everything.

Other standards, such as MIL-STD-882 are not perfect, but they have been used for 50 years by non-academics and have included software explicitly since 1984. We do not have to follow and promote dangerous standards simply because they are simple for unqualified people to understand. Most system safety engineers that I know are intelligent and have a college degree in engineering. Should we dumb down the standards to the level of the most unqualified person we can hypothesize might exist? This has nothing to do with academics.


On Fri, Jun 28, 2013 at 2:00 AM, RICQUE Bertrand (SAGEM DEFENSE SECURITE) < bertrand.ricque_at_xxxxxx

> Nancy,****
> ** **
> remember that the whole story is initiated by process industries
> primarily, for IEC 61508 and IEC 61511. This is the cultural background,
> and it cannot be disregarded. It is NOT raised by software experts and it
> was NOT, until a very short time, handled by sofwtarea ns safety experts.*
> ***
> ** **
> So the persons we have to convince:****
> **· **Are not sensitive to a potential difference between prrof
> and certainty****
> **· **"Never failed in past" and safe****
> ** **
> I guess that, having participated to interesting audit panels in these
> industries in the past, you have a clear view of what the stakes are.
> Declare that whole plants are OK.****
> ** **
> It is clear that if we push the standards toward the orientation and
> academic level of this forum we will gain in scientific rigour but they
> will fly miles over the readers and will be never applied.****
> ** **
> The aim is not to find something totally consensual and as perfect as
> possible from and academic point of view but to drop in 2 standards (namely
> 61508 and 61511/S84 for USA) half a dozen of requirements that:****
> **· **are intelligible to the readers****
> **· **practically applicable and economically acceptable****
> **· **as efficient as possible****
> ** **
> Not and easy thing I ackowledge it…****
> ** **
> *Bertrand RICQUE*****
> Program Manager, Optronics and Defense Division****
> ****
> *T* +33 (0)1 58 11 96 82****
> *M* +33 (0)6 87 47 84 64****
> 23 avenue Carnot ****
> 91300 MASSY - FRANCE ****
> **
> * *
> [image: cid:image002.jpg_at_xxxxxx >
> ** **
> *From:* systemsafety-bounces_at_xxxxxx > systemsafety-bounces_at_xxxxxx > Leveson
> *Sent:* Thursday, June 27, 2013 4:24 PM
> *To:* systemsafety_at_xxxxxx > *Subject:* Re: [SystemSafety] RE : Qualifying SW as "proven in use"****
> ** **
> 1. Software is *not*, by itself, unsafe. It is an abstraction without any
> physical reality. It cannot itself cause physical damage. The safety of
> safety is dependent on
> -- the software logic itself,
> -- the behavior of the hardware on which the software executes,
> -- the state of the system that is being controlled or somehow affected
> by the outputs of the software (the encompassing "system"), and
> -- the state of the environment.
> All of these things determine safety so a change in one can impact the
> so-called "software safety.:" For example, the change in the design of the
> Ariane 5 which led to a steeper trajectory than the Ariane 4 led to the
> software contributing to the explosion. The environment does matter. All
> the usage of that software on the Ariane 4 meant nothing with respect to
> its use in the Ariane 5.
> Any change in the environment, in the controlled system, in the underlying
> hardware, or in the software invalidates all previous experience unless one
> can prove that the change will not lead to an accident (and that proof
> cannot not be based on a statistical argument). Does anyone know any
> non-trivial software, for example, that is not changed in any way over
> decades of use? or even years of use? And what about changes in the
> behavior of human operators, of the system itself, and of the environment?
> Someone wrote:
> > I've been thinking about Peter's example a good deal, the developer
> seems to me to have made an implicit assumption that one can use a
> statistical argument based on successful hours run to justify the safety of
> the software.
> And Peter responded:
> >>It is not an assumption. It is a well-rehearsed statistical argument
> with a few decades of universal acceptance, as well as various successful
> applications in the assessment of emergency systems in certain English
> nuclear power plants.
> "Well-rehearsed statistical arguments with a few decades of universal
> acceptance" are not proof. They are only well-rehearsed arguments. Saying
> something multiple times is not a proof. The fact that nuclear power plants
> in Britain have not experienced any major accidents (they have had minor
> incidents by the way) rises only to the level of anecdote, and not proof.
> And that experience (and well-rehearsed arguments) cannot be carried over
> to other systems.
> I agree with the original commenter about the implicit assumption, which
> the Ariane 5 case disproves (as well as dozens of others).
> 2. It is not even clear what "failure" of software means when software is
> merely "design abstracted from its physical realization." How can a
> "design" fail? It may not satisfy its requirements (when executed on some
> hardware), but design (equivalent to a blueprint for hardware) does not
> fail and certainly does not fail "randomly."
> Perhaps the reason why software reliability modeling still has pretty poor
> performance after at least 40 years of very bright people trying to get it
> to work is that the assumptions underlying it are not true. These
> assumptions have not been proven (only stated with great certainty) and,
> in fact there is evidence showing they are not necessarily true. I tried
> raising this point a long time ago, but I was met with such a ferocious
> response (as I am sure I will be here) that I simply ignored the whole
> field and worked on things that seemed to have more promise. The most
> common assumption is that the environment is stochastic and that the
> selection of inputs (from the entire space of inputs) that will trigger a
> software fault (design error) is random. There is data from NASA (using
> real aircraft) that is evidence of "error bursts" in the software (ref.
> Dave Eckhardt). It appeared that these resulted when the aircraft flew a
> trajectory that was near a "boundary point" in the software and thus set
> off all the common problems in software related to boundary points. The
> selection of inputs triggering the problems was not random.
> As another example, Ravi Iyer looked at software failures of a widely used
> operating system in an interesting experiment where he found that a bunch
> of software errors appeared to be preceding a computer hardware failure. It
> made no sense that the software could be "causing" the hardware failure.
> Closer examination showed the problem. Hardware often degrades in its
> behavior before it actually stops. The strange hardware behavior, if I
> remember correctly, was exercising the software error handling routines
> until it got beyond the capability of the software to mitigate the
> problems. Again, in this case, the software was not "failing" due to
> randomly selected inputs from the external input space.****
> When someone wrote:
> > I don't think that's true,
> Peter Ladkin wrote:****
> >>You might like to take that up with, for example, the editorial board of
> [As a past Editor-in-Chief of IEEE TSE, I can assure you that the entire
> editorial board does not read and vet the papers, in fact, I was lucky if
> one editor actually read the paper. Are you suggesting that anything that
> is published should automatically be accepted as truth? That nothing
> incorrect is ever published?]
> Nancy
> --
> Prof. Nancy Leveson
> Aeronautics and Astronautics and Engineering Systems
> MIT, Room 33-334
> 77 Massachusetts Ave.
> Cambridge, MA 02142
> Telephone: 617-258-0505
> Email: leveson_at_xxxxxx > URL:****
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Prof. Nancy Leveson
Aeronautics and Astronautics and Engineering Systems
MIT, Room 33-334
77 Massachusetts Ave.
Cambridge, MA 02142

Telephone: 617-258-0505
Email: leveson_at_xxxxxx

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Received on Fri Jun 28 2013 - 11:14:49 CEST

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