A partial reply to Estell's nice piece on "Reliability, complexity and confidence in SDI software" as well as other comments about fault rates in large software: (1) The bathtub curve for reliability of engineered artifacts is usually considered to be composed of three distinct phenomena, (i) The early failures caused by manufacturing defects, (ii) The "random" failures of components for "unknown" reasons (These may be judged as defects in the design, allowed to lower the cost of the product), (iii) Wearout failures near the end of the product life. Type (i) failures give the initial declining failure rates during "burn-in", type (ii) failures during the useful product life, and type (iii) failures occur at the design-life limit. This bathtub curve is not applicable to software since the usual definition of a large software product includes many different releases. Perhaps a software product should be compared to several different models of the same car, toaster, airplane, etc. The bathtub curve describes the sum of manufacturing defects, design defects, and wear. Software ordinarily has no manufacturing defects and the usual way ordinary backups are done insures that most software does not wear out before it becomes obsolete. Perhaps the Viking 1 Lander software failure could be classified as a "wearout" due to inadequate preventative maintenance, but this seems to be streching a point. So software ordinarily fails from design defects and design defects only. These are considered so important that we classify such defects into specification, design and implementation defects. The point here is that none of these are manufacturing or wear failures. (2) The defect rate models for software all attempt to describe a process of redesigning the software after the discovery of failures, repeatedly, in a never-ending cycle of testing (either formally or via users discovering problems) and "maintenance" (which is actually redesigning a new model of the software upon discovery of problems--with so-called enhancements thrown in to confuse the issues). I shall now give a crude approximation to all of these models. Let all realize I have abstracted the essential features of these models to the point of unusability in QA practice. The essense is enough to make my point. We assume that the original release of the software has a load of N design defects and that defects are discovered and instantly and flawlessly reworked with a rate constant, a, according to the formula R(t) = N*exp(-a*t) where exp() is the exponential function, t is a measure of software use (time, person-years, cpu cycles consumed, ...) and R(t) is the remaining number of design faults in the reworked software. This formula clearly illustrates that for any t>=0, if R(t) is not zero, then more faults remain. In words, some faults mean yet more faults. The more detailed versions of this essential idea do, approximately, describe the process of removing faults from a continuing sequence of releases of a software product. Bev Littlewood has a nice survey of these, together with some practical suggestions, in a recent IEEE Trans. on Software Engineering--perhaps last Jan or Feb issue. In any case, we may see that the essential feature of "some faults imply more faults" is used in practice to estimate remaining design fault loads in software. The models have this feature because this seems theoretically sound and the actual data is not inconsistent with this class of models. (3) If faults are not repaired when discovered, there is data suggesting that software failures may be viewed as type (ii), supra: Singpurwalla and Crow have a nice paper suggesting that faults are evidenced as failures with a periodicity sufficiently good to make interesting Fourier analysis of the failure data. We may take this as suggesting that some failures imply more failures at regular times in the future. (4) Good designs have few faults and evidence few failures. In software this means few releases are necessary to correct faults. However, many software products interact primarily with that most flexible of io devices, people, People quickly adjust to the ideosyncracies and failures of the software they use. In my opinion, Unix (Reg. Trademark, AT&T) and derivatives is successful because its ideosyncracies and failures are somehow "human", but not because of low failure rates. Good software designs start with a low initial number of faults. Good design practices seem to lead to better software. But one simply requires more data than currently exists to say much definite about the advantages of Ada vs. a more traditional practice. Furthermore, new software is likely to be "more complex" than old software--leading to perhaps the same MTTF. Highly reliable software appears to be engineered in much the same manner as any other highly reliable engineered artifact: By repeatedly designing similar artifacts, obtaining experience with their use, and then redesigning anew. (5) Thus many of us are extremely dubious about the claims made for SDI (and thus its driving software). Without the ability to test in actual practice, there is no compelling reason to believe any claims made for the reliability of the software. This point has been made several times, by several people, on RISKS and I'll not repeat the argument. It seems that the onus of compelling evidence lies with those who claim SDI "will work." So far I've found no evidence whatsoever to support the claim that ANY new military software works in its first adversarial role: i.e., in the face of enemy action or a good, determined, simulation thereof. I'd appreciate reliable evidence for such. The claim for 100,000 line programs which work reliably requires supporting evidence. I am perfectly prepared to believe that the 28th yadbm (yet another data base manager) works reliably. I'm not prepared to simply accept such claims for military software. An example: JSS is a C3I system for the defense of North America against bomber attack. JSS is currently receiving some kind of "independent operational" test in Colorado. Workers at Hughes kept careful records of defect rates during development, and reported that certain of the standard models alluded to above failed to predict defect rates at the next step of in-house testing. Will I ever be able to learn what the results of the "independent operational" test are? I doubt it. All I might be able to learn is whether the US adopts the system or not. I'm highly dubious about the reliability of JSS, despite the adoption of reasonably current SE practices. And recall, JSS is the nth yac3i. (6) Controlling complexity is a wonderful idea. But what does one do in the face of a truely complex world, in which complex decisions must be made? One designs complex software. Recall that the Enroute Air Traffic Control System has so far exercised only a minute fraction of all the paths through it, despite being installed at about 10 sites for about 10 years. At the current rate one might get to 90% path coverage by the year 2200? Yet every time you fly on a commercial aircraft, you implicitly trust this system. I suggest you trust it because it has been used operationally for 10 years and the enroute controllers view it as trustworthy. The fault rate is low enough and the controllers flexible enough and the enroute mid-air near collision rate is low enough that everyone is satisfied enough. No mathematics and little statistics here--just actual operational experience. (7) Software types need to adopt rather more of a Missouri attitude: Show me that it works. Part of the problem is defining what "works" means. Thats what makes the Viking Lander experiences so compelling. Everyone can easily agree that the software worked the only two times it was called upon to land the craft. One might think that military software experiences should be equally compelling to the senses. So consider the Navy's Aegis experiences... The result of actual data suggests that SDI software is unbuildable as a highly reliable program. I repeat my call for serious, professional papers on military software which worked the first time. So far I can only conclude than none such exist. Thereby I think I am entitled to discount any claims for the "quality" of military software in actual, operational practice. The logical, rational conclusion is that, with no data supporting claims for military software working in first use, and only data such as the Sgt. York and Aegis, SDI software will not work the first and only time it might be called upon to function.
Brian is quite right. The job of an engineer is to build systems that people can trust. By this criterion, there exist few software engineers. js
Brian Reid speaks eloquently to important issues. Virtually everything he says in this note makes perfect sense and should be taken to heart by everyone designing systems. BUT... What he says now is not exactly what he said the first time; when, I assure him, some of us were listening. His first note did in fact attribute blame, to the networking code and to the student involved (under the general rubric of 'wizards'). The designer of the gelignite-handled screwdriver has clearly got a responsibility when the screwdriver is (incorrectly) used to pound on something and explodes. The designer has little responsibility when the screwdriver is used (incorrectly and maliciously) as a sharp object to stab a co-worker during a fight. If the screwdriver is used to hit someone over the head in a fight, and explodes, the responsibility is a lot more muddled. It is not at all clear how far the designer's responsibility for protecting us from mistakes extends to protecting us from temptation. Is a car manufacturer morally liable for its cars being capable of going 120 mph, creating the potential for more serious accidents when they are used inappropriately? Is the manufacturer of autodial modems responsible because they make it possible for system crackers to try many more phone numbers per hour than manually dialled modems? Had Brian made slightly less attempt to de-jargonize his original posting and said ".rhosts" instead of "permission files", which could refer to quite a few different things ina BSD system, I would have taken a different impression of his complaint away from that original posting. I agree strongly that .rhosts files are a danger that administrators should be able to turn off, preferably on a host by host basis. It should still be noted that .rhosts files are there for a reason and that that reason is perfectly valid and the provision of .rhosts capabilities perfectly reasonable IN THE APPROPRIATE SITUATION. A campus-wide network of machines under diverse administrators may not be such a situation; I would hate to see the capabilities taken out of the system simply because there may be inappropriate situations. Ftp and telnet are still provided as well as the r-utilities. As our moderator has said, fault rarely lies on one head. I agree with Brian that the designer (of systems OR screwdrivers) has a strong responsibility to consider both unintentional and intentional misuses of her systems and to watch for aspects of her designs that could raise the consequences of such misuses. The strongest responsibility is to make the limits of appropriate use obvious to the user, by packaging, documentation, and whatever other steps may be necessary. If on mature reflection it still seems likely the user will be unaware of the problem (who reads documentation on a screwdriver), the designer has a moral obligation to seek other means to avoid misuse. Perhaps the explosive screwdriver should be sold only with with a two-foot long handle, making it unsuitable for common domestic use, or as a separately packaged replacement handle in a six-inch thick lead box bedecked with scenes of mutilation. If, however, the object is the best or only solution to a particular problem (only a gelignite screwdriver can remove red kryptonite screws from lead doorframes), it may also be morally unacceptable to suppress the product simply because it may have dangerous implications in the hands of the unwary. Hey, surprise, there's no easy answer... scott preece, gould/csd - urbana, uucp: ihnp4!uiucdcs!ccvaxa!preece [Let me commend Brian once again for having performed a truly valuable service to the community. (I notice his original message is reappearing in many places!) I don't think we should expect him to try to respond to each such comment. But -- given the ease with which system and network security can be broken -- we may see lots more of such analyses of OTHER breakins. The sad part is that most of these vulnerabilities are well known in the security community, but few other people have yet been concerned enough to do anything, including most system developers. The consensus among security folks is that it will take a Chernobyl-like event in computer security before most people wake up. PGN]
As I see it, you are all missing the point. A simulator *should* allow the plane to land with the gear up. A simulator should allow it to release a bomb in any position, *if the plane would*. The simulator should not try and stop the pilot from doing stupid things, it should react as the plane would. *If the plane will not allow something*, then the simulation should not allow it. There is a difference. the *plane* should not allow a bomb to be detached if it will damage the plane. *But if it does* the software should too. stev knowles, boston university distributed systems group CSNET: stev@bu-cs.CSNET UUCP:...harvard!bu-cs!stev BITNET:ccsk@bostonu.BITNET
From: Charles R. Fry <Chucko at GODZILLA.SCH.Symbolics.COM> No matter how many automated controls we install on cars (and airplanes) to prevent operators from exceeding their vehicles' limits, there will always be a need to allow the deliberate violation of these limits. This discussion about allowing overrides to programmed safety limits worries me. It is certainly true that there are instances in which the preservation of life requires the operator to override these devices. But these have to be weighed against the situations in which a careless operator will go beyond those limits when it is inappropriate. I haven't heard much discussion about that, and maybe it is because it is very difficult (impossible?) for the safety machinery to tell when an operator is being careless given the operative conditions at the time. There is a tradeoff here that many have resolved categorically in favor of people being able to override computers. I think only competent and sensible people people, under the right circumstances, should be able to do so. The problem is to find a mechanical system capable of making these distinctions. Thus, the comment that PGN omitted "[Chuck added an aside on the value of high performance driving schools.]" was, in my view, crucial to understanding the situation involved. Maybe a partial solution would be to allow only drivers who have passed courses at high performance driving schools to override. Herb [Shades of Chernobyl! PGN]
>Date: Wed, 24 Sep 86 18:01:18 pdt >From: Dave Benson <benson%wsu.csnet@CSNET-RELAY.ARPA> > ... Since the Viking Landers were the >first man-made objects to land on Mars, ... Actually, the first man-made object to land on Mars was a Russian craft that sent about 30 seconds of carrier signal and then died. Nobody knows exactly what happened to it. Courtenay Footman ARPA: email@example.com Lab. of Nuclear Studies Usenet: cornell!lnsvax!cpf Cornell University Bitnet: cpf%lnsvax.tn.cornell.edu@WISCVM.BITNET
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