[SystemSafety] Does "reliable" mean "safe" and or "secure" or neither?
Mike Ellims
michael.ellims at tesco.net
Thu Apr 28 14:51:13 CEST 2016
Good afternoon Michael,
Well that's a bit of a damp squib then, I was expecting a fiery return
comment!
Cheers.
-----Original Message-----
From: systemsafety
[mailto:systemsafety-bounces at lists.techfak.uni-bielefeld.de] On Behalf Of
Michael J. Pont
Sent: 27 April 2016 18:41
To: 'The System Safety List'
Subject: Re: [SystemSafety] Does "reliable" mean "safe" and or "secure" or
neither?
Mike (Ellims),
Thanks for your reply.
My original email was intended to make two points, the first of which C.
Michael Holloway expressed much more clearly and succinctly than I did in a
later post:
"One would surely expect professional engineers to be less cavalier in
frequently adding new definitions to common words, which change the meanings
in confusing, unhelpful ways."
I (still) believe that this is what has happened here in the application of
the phrase reliability to software.
I'm happy to accept your argument that - when following particular standards
- we may be obliged to accept the definitions that are associated with these
standards (but my original point was intended to be more general).
Michael.
-----Original Message-----
From: Mike Ellims [mailto:michael.ellims at tesco.net]
Sent: 25 April 2016 13:25
To: M.Pont at SafeTTy.net; 'The System Safety List'
<systemsafety at lists.techfak.uni-bielefeld.de>
Subject: RE: [SystemSafety] Does "reliable" mean "safe" and or "secure" or
neither?
Good afternoon Michael,
First sorry about the length of the reply and the delay!
>>
>> We can choose to define any terms that we use in a particular domain
>> in
any way that we like.
>>
First who is we?
Second, no we can't; to maintain consistency with existing standards,
practice and research we have to maintain constancy with existing
definitions, for example IEC 61508 Part 4 states "see IEV 191-12-01 for a
definition of reliability".
IEV 191-12-01 defines reliability as, "probability that an item will perform
a required function under given conditions for a given time interval
(t1.t2)". ISO 26262 doesn't give a definition of reliability but as it
purports to be a sector specific derivative of IEC 61508 I assume it will
inherit definitions not otherwise defined (does anyone have an answer to
that?).
>>
>> Where these terms conflict with general use of the same word, we make
>> it
more likely that people reading the
>> documents - etc - will be confused (in my view).
>>
Which is why standard definitions such as that given in BS 4478 and IEV
191-12-01 exist because there is a disparity between what the man in the
street means when they say reliable and what reliability engineers have
decided that term reliable means. The same problem exists with the use of
the word theory, in science is it a reasonably precise meaning and
scientists have other words such as conjecture and hypothesis for concepts
that are not well established. However in common use the word theory has a
different less precise meaning and in general represents a plausible
scenario as may be argued in court of law.
>>
>>Confusion tends to result in Bad Things Happening in this business.
>>We
(therefore) want to avoid confusion.
>>
Which is why we have an exist precise definition... actually I'll concede
that as stated it's not actually very precise i.e. it doesn’t specify how to
perform the statistical evaluation buy that’s expanded on in textbooks such
as O'Conner and IEC 61508 etc. Note this is not an argument for the
usefulness of the term as applied for software it's only an argument against
redefining the meaning of the term. In general I would argue that if we have
a problem with the way a term is defined then we should find a similar term
that could be explicitly defined to mean what we think is necessary. So
rather than try and redefine "software reliability" or "reliability" we
should use a different term.
>>
>> Two seconds on Google gives me this definition of reliability:
>>
Sorry but I don't think that appeal to Google is a reasonable approach.
Google can be used as a mechanism for locating specific information in a
field of study e.g. reliability engineering. For example if I put the search
term "reliability" into Google the first entry is a link to the Wikipedia
disambiguation page that lists a set specialist used for the term
reliability (https://en.wikipedia.org/wiki/Reliability) the second of which
is reliability engineering. The second sentence of the entry for reliability
engineering gives the following definition "Dependability, or reliability,
describes the ability of a system or component to function under stated
conditions for a specified period of time" which is cross referenced to IEEE
Standard Computer Dictionary: A Compilation of IEEE Standard Computer
Glossaries.
Various generic dictionaries produce the following set of definitions in
order of appearance;
dictionary.com : the ability to be relied on or depended on, as for
accuracy, honesty, or achievement.
merriam-webster.com : 1. the quality or state of being reliable 2: the
extent to which an experiment, test,
or measuring procedure yields the same results on
repeated trials
thefreedictionary.com : The ability of an item to perform a required
function under stated conditions for a
specified period of time.
en.wiktionary.org : 1. The quality of being reliable, dependable or
trustworthy. 2. In education - the ability to
measure the same thing consistently (of a
measurement indicating the degree to which the
measure is consistent); that is, repeated
measurements would give the same result (See also
validity). 3. In engineering - measurable time of
work before failure
Several definitions are a close match the IEC and IEEE definition and the
last makes a clear distinction as to where different definition apply.
However there is wide variation and taking this approach it's pot-luck
whether any particular person would derive anything useful or usable. For
example the Concise OED has no separate definition for reliability and the
Oxford Dictionary of Computing does and it matches the IEC/IEE/BS standard.
Note that if we search for the term "software reliability" we usually get
something more akin the standard meaning as used in reliability engineering
e.g. "A software quality aspect that is measured in terms of mean time to
failure or failure intensity of the software", or from ANSI " the
probability of failure-free software operation for a specified period of
time in a specified environment".
>> "Definition of reliability. 1 : the quality or state of being
>> reliable. 2
:
>> the extent to which an experiment, test, or measuring procedure
>> yields
the same results on repeated trials."
>>
>> I see "2" as the useful (and general) definition here. It may even
>> be
compatible with O'Conner.
I don't think that this is compatible with O-Conner nor the standard
definitions of engineering reliability, and to my mind the second definition
would seem to be a closer match to testing or program corretness.
As an extreme contrived counter example to the usefulness of definition 2 we
could consider the case of a software system that always halted 1/2 a second
after start producing no output. We could run this any number of times and
get the same output (whatever the input) and meet the criteria above that
repeated trails produced the same output. Thus the software system is
reliable. Useless but reliably useless... which is perhaps not useful.
>>
>> My issue is that software (whether good or bad) doesn't change over time.
>>
This statement uses the starting position that all reliability issues are a
result of wear out, i.e. X changes over time.
My original point was that reliability engineering is NOT just about wear
out. An object (mechanical, electric, software) can fail to deliver the
specified function for a number of different reasons. A simplified
mechanical example may help, consider if a part were made from aluminum as
specified, rather than stainless steel as was intended (i.e. we have a
specification error) then the part could fail more quickly than desired for
several reasons;
i) it was not strong enough,
ii) it melted,
iii) it suffered badly from corrosion,
iv) or because it wore more quickly.
Reliability isn't only about "wear-out"; as defined it's "about failure to
deliver functions", whether that function be ability of hold a load, support
a beam, maintain a current, measure a value, monitor a parameter or to
perform a calculation. In addition you can stretch the analogy of wear out
to Niklaus Wirth's concept of accumulated state. That is the longer the
program runs the more (possibly redundant) state information builds up
within in memory, the more probable the system is to go bonk (important
technical term). The patriot missile system springs to mind as a simple
example of catastrophic state accumulation and while possibly reliable when
used with the assumed timeframe (t1,t2) it proved unreliable the used in the
timeframe (t1,t3) where t3 >> t2.
>>
>> Talking about "software reliability" (therefore) doesn't make sense
>> and
is likely to lead to confusion.
>> (Hardware reliability is fine; System reliability is also fine, where
System = Hardware + Software.)
>>
At the current time, given the limits associated with research in software
reliability prediction that is not that an unreasonable position to take in
practical terms. However as stated above I don't think that gives a green
light to appropriate the term reliability.
As a topic of theoretic interest software reliability is a valid area of
research and some of the concepts have at least some utility if the
limitations of the techniques are taken into account. For example
reliability growth theory indicates that over time with a good development
process the rate of failures observed should decline; so it isn't then we
are in an effluent stream with no propulsive device (grossly simplified of
course). There is also practical application to the evaluation of systems
fielded and to comparisons between different software running on identical
platforms.
In summary:
1. Reliability is a existing defined term, even for software.
2. For software the term may have little practical predictive power, however
it can't just be redefined.
3. If necessary a new term should be defined that captures the idea of
reliability in a manner that matches common usage.
4. I suspect 3 could be really, really, really hard.
Please feel free to take pot-shots.
Cheers.
-----Original Message-----
From: systemsafety
[mailto:systemsafety-bounces at lists.techfak.uni-bielefeld.de] On Behalf Of
Mike Ellims
Sent: 24 April 2016 13:23
To: 'Coq, Thierry' <Thierry.Coq at dnvgl.com>; 'The System Safety List'
<systemsafety at lists.techfak.uni-bielefeld.de>
Subject: Re: [SystemSafety] Does "reliable" mean "safe" and or "secure" or
neither?
> as software does not "fatigue" or randomly "break" the way hardware
> does
Reliability engineering is much more than just fatigue or randomly breaks,
it encompasses everything about the control of variation and error in a
system, most commonly using statistical methods.
BS 4478 defines reliability as "The ability of an item to perform a required
function under stated conditions for a stated period of time".
O'Conner states that "reliability is usually concerned with failures in the
time domain. This distinctions marks the difference between traditional
quality control and reliability engineering".
He goes on to list a number of reasons why a failure may occur as follows
(abridged)
- design: which may be inadequate
- overstress: i.e. analysis of condition was incomplete and/or incorrect
- variation: which includes manufacturing variation
- wear out, which is what everyone seems to think of...
- error, such as errors in specification
Henley and Kumamoto give a potted history of the development of reliability
engineering and track its roots to work done by Lusser on the V2 (V1?)
missile which was spectacularly unreliable at first. But obviously not a
wear out issue...
This is of course separate from whether in any context reliability is useful
concept e.g. software. You can obviously measure the reliability of
Windows98 is terms of mean time between crashes (failures in time) likewise
you can measure the reliability of Linux in the same manner. Whether that
has any deep meaning aside from the fact that it shows Windows98 to be pants
compared to Linux for some distribution of uses/input/outputs a different
question.
Cheers.
-----Original Message-----
From: systemsafety
[mailto:systemsafety-bounces at lists.techfak.uni-bielefeld.de] On Behalf Of
Coq, Thierry
Sent: 24 April 2016 07:24
To: The System Safety List
Subject: Re: [SystemSafety] Does "reliable" mean "safe" and or "secure" or
neither?
Hi
I find this list hugely informative. In particular, I find PBL's posts
factual, useful, interesting and intriguing. As well as the many other
debaters who agree with him or challenge him. As it should be. I wish to
express my gratitude to all debaters.
However, this last exchange seems to me a debate on authority.
On our left, we have DO-178. B now C.
On our right, we have IEC, IEEE, Musa, etc.
To go further, it is plain fact that the aeronautics industry has
demonstrated it doesn't need "software reliability" to deliver highly
reliable automated systems, or systems of systems.
It seems evident with the knowledge we have of the aeronautics success that
in order to use "software reliability" in other industries, or in
aeronautics, there needs to be a clear use case where the value of "software
reliability" is demonstrated, compared to other methods or techniques, in
order to apply "software reliability". The analogy of software and hardware
does not seem valid, as software does not "fatigue" or randomly "break" the
way hardware does, which is the basis for all probabilistic reliability
theories for hardware. The analogy that does seem valid between hardware and
software is the presence of systematic faults, in design, manufacturing,
installation, testing, misuse, etc. Which in hardware also does not have a
probability number. Formal methods can be used to identify such systematic
faults in software. If one can be found, then a test environment can be
devised in which the software will fail 100% of the time. Random hardware
faults do not behave like t hat.
Best regards,
Thierry Coq
The opinions reflected here are my own and are not necessarily those of my
employer
-----Original Message-----
From: systemsafety
[mailto:systemsafety-bounces at lists.techfak.uni-bielefeld.de] On Behalf Of
Peter Bernard Ladkin
Sent: dimanche 24 avril 2016 03:27
To: The System Safety List
Subject: Re: [SystemSafety] Does "reliable" mean "safe" and or "secure" or
neither?
On 2016-04-23 19:43 , Nick Tudor wrote:
> DO-178C
In the absence of a complete sentence, let me suggest one.
---- DO178C sees no need to assign any meaning to the term "software
reliability".
It's fine for some industry consortium to find it has no use for a specific
concept. RTCA likely has no use for the notion of a cup of tea, either
(BS6008). But that doesn't mean it makes any sense to argue that there isn't
any such thing as a cup of tea.
PBL
Prof. Peter Bernard Ladkin, Faculty of Technology, University of Bielefeld,
33594 Bielefeld, Germany Je suis Charlie
Tel+msg +49 (0)521 880 7319 www.rvs.uni-bielefeld.de
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