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Principles of Software Safety Assurance

This image represents the contribution of programmable electronics and software to safety
Photo by Marta Branco from Pexels

This is the first in a new series of blog posts on Principles of Software Safety Assurance. In it, we look at the 4+1 principles that underlie all software safety standards.

We outline common software safety assurance principles that are evident in software safety standards and best practices. You can think of these guidelines as the unchanging foundation of any software safety argument because they hold across projects and domains.

The principles serve as a guide for cross-sector certification and aid in maintaining comprehension of the “big picture” of software safety issues while evaluating and negotiating the specifics of individual standards.

In this first of six blog posts, we introduce the subject and the First Principle.

Why Software Safety Principles?

I’ve been involved with industrial-scale software projects since 1994, and the concept of ‘principles’ is valuable for two reasons.

First, many technical people like detail and quickly bypass concepts to get to get to their comfort zone. This means that we often neglect the ‘big picture’ and are uncomfortable making high-level judgments. In turn, this makes it difficult for us to explain or justify our choices to management or other stakeholders.

The second reason is similar to the first. In the guts of a standard, we can think in terms of compliance with detailed requirements and our choices become simple – black and white. This is easy, but it does not equip us to choose between standards or help us to argue that an alternative means of compliance is valid. Thus, ‘Is this good enough?’ is not a question that we find easy to answer.

Introduction

Software assurance standards have increased in number along with the use of software in safety-critical applications. There are now several software standards, including the cross-domain ‘functional safety’ standard IEC 61508, the avionics standard DO-178B/C, the railway application CENELEC-50128, and the automotive application ISO26262. (The last two are derivatives of IEC 61508.)

Unfortunately, there are significant discrepancies in vocabulary, concepts, requirements, and suggestions among these standards. It could seem like there is no way out of this.

However, the common software safety assurance principles that can be observed from both these standards and best practices are few (and manageable). These concepts are presented here together with their justification and an explanation of how they relate to current standards.

These ideas serve as the unchanging foundation of any software safety argument since they hold across projects and domains. Of course, accepting these principles does not exempt one from adhering to domain-specific norms. However, they:

  • Provide a reference model for cross-sector certification; and
  • Aid in maintaining comprehension of the “big picture” of software safety issues;
  • While analyzing and negotiating the specifics of individual standards.

Software Safety Principles

Principle 1: Requirements Validity

The first software safety assurance principle is:

Principle 1: Software safety requirements shall be defined to address the software contribution to system hazards.

‘The Principles of Software Safety Assurance’, RD Hawkins, I Habli & TP Kelly, University of York.

The evaluation and reduction of risks are crucial to the design of safety-critical systems. When specific environmental factors come into play, system-level dangers like unintentional braking release in cars and the absence of stall warnings in aircraft can result in accidents. Although conceptual, software can implement system control or monitoring features that increase these risks (e.g. software implementing antilock braking or aircraft warning functions).

Typically, the system safety assessment process uses safety analysis methodologies like Fault Tree Analysis or Hazard and Operability (HAZOP) Studies to pinpoint how software, along with other components like sensors, actuators, or power sources, can contribute to risks.  The results of these methodologies ought to influence the formulation of safety requirements and their distribution among software components.

It is crucial for us to remember that software is now considered a black box, utilized to enable specific functions, and with limited visibility into how these functions are implemented. The risk from some system hazards can rise to unacceptable levels if hazardous software failures are not identified and suitable safety standards are not defined and applied.

Examples of software requirements not being adequately defined – and the effects thereof – were reported by the US Federal Drug Authority (FDA).

Simply put, software is a fundamental enabling technology employed in safety-critical systems. Assessing how software might increase system risks should be a crucial component of the overall system safety process. We define safety standards to minimize hazardous software contributions that are discovered in a safety process, which addresses these contributions.

These contributions must be described in a clear and testable way, namely by identifying the exact types of software failures that can result in risks. If not, we run the risk of creating generic software safety requirements—or even just correctness requirements—that don’t take into account the specific hazardous failure modes that have an impact on the system’s safety.

Principles of Software Safety Assurance: End of Part 1 (of 6)

I based this blog post on the paper ‘The Principles of Software Safety Assurance’, RD Hawkins, I Habli & TP Kelly, University of York. The original paper is available for free here. I learned safety engineering from Tim Kelly, and others, at the University of York. I am so glad that I can share their valuable work in a more accessible format.

My name’s Simon Di Nucci. I’m a practicing system safety engineer, and I have been, for the last 25 years; I’ve worked in all kinds of domains, aircraft, ships, submarines, sensors, and command and control systems, and some work on rail air traffic management systems, and lots of software safety. So, I’ve done a lot of different things!

Principles of Software Safety Training

Learn more about this subject in my course ‘Principles of Safe Software’ here. The next post in the series is here.

My course on Udemy, ‘Principles of Software Safety Standards’ is a cut-down version of the full Principles Course. Nevertheless, it still scores 4.42 out of 5.00 and attracts comments like:

  • “It gives me an idea of standards as to how they are developed and the downward pyramid model of it.” 4* Niveditha V.
  • “This was really good course for starting the software safety standareds, comparing and reviewing strengths and weakness of them. Loved the how he try to fit each standared with4+1 principles. Highly recommend to anyone that want get into software safety.” 4.5* Amila R.
  • “The information provides a good overview. Perfect for someone like me who has worked with the standards but did not necessarily understand how the framework works.” 5* Mahesh Koonath V.
  • “Really good overview of key software standards and their strengths and weaknesses against the 4+1 Safety Principles.” 4.5* Ann H.
Categories
Blog software safety

Software Safety Principles Conclusions and References

ID & Analyze Functional Safety
Computer code - Functional Safety

Software Safety Principles Conclusions and References is the sixth and final blog post on Principles of Software Safety Assurance. In them, we look at the 4+1 principles that underlie all software safety standards. (The previous post in the series is here.)

Read on to Benefit From…

The conclusions of this paper are brief and readable, but very valuable. It’s important for us – as professionals and team players – to be able to express these things to managers and other stakeholders clearly. Talking to non-specialists is something that most technical people could do better.

The references include links to the standards covered by the paper. Unsurprisingly, these are some of the most popular and widely used processes in software engineering. The other links take us to the key case studies that support the conclusions.

Content

We outline common software safety assurance principles that are evident in software safety standards and best practices. You can think of these guidelines as the unchanging foundation of any software safety argument because they hold true across projects and domains.

The principles serve as a guide for cross-sector certification and aid in maintaining comprehension of the “big picture” of software safety issues while evaluating and negotiating the specifics of individual standards.

Conclusion

These six blog posts have presented the 4+1 model of foundational principles of software safety assurance. The principles strongly connect to elements of current software safety assurance standards and they act as a common benchmark against which standards can be measured.

Through the examples provided, it’s also clear that, although these concepts can be stated clearly, they haven’t always been put into practice. There may still be difficulties with their application by current standards. Particularly, there is still a great deal of research and discussion going on about the management of confidence with respect to software safety assurance (Principle 4+1).

[My own, informal, observations agree with this last point. Some standards apply Principle 4+1 more rigorously, but as a result, they are more expensive. As a result, they are less popular and less used.]

Standards and References

[1] RTCA/EUROCAE, Software Considerations in Airborne Systems and Equipment Certification, DO-178C/ED-12C, 2011.

[2] CENELEC, EN-50128:2011 – Railway applications – Communication, signaling and processing systems – Software for railway control and protection systems, 2011.

[3] ISO-26262 Road vehicles – Functional safety, FDIS, International Organization for Standardization (ISO), 2011

[4] IEC-61508 – Functional Safety of Electrical / Electronic / Programmable Electronic Safety-Related Systems. International Electrotechnical Commission (IEC), 1998

[5] FDA, Examples of Reported Infusion Pump Problems, Accessed on 27 September 2012,

http://www.fda.gov/MedicalDevices/ProductsandMedicalProcedures/GeneralHospitalDevicesandSupplies/InfusionPumps/ucm202496.htm

[6] FDA, FDA Issues Statement on Baxter’s Recall of Colleague Infusion Pumps, Accessed on 27 September 2012, http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm210664.htm

[7] FDA, Total Product Life Cycle: Infusion Pump – Premarket Notification 510(k) Submissions, Draft Guidance, April 23, 2010.

[8] “Report on the Accident to Airbus A320-211 Aircraft in Warsaw on 14 September 1993”, Main Commission Aircraft Accident Investigation Warsaw, March 1994, http://www.rvs.unibielefeld.de/publications/Incidents/DOCS/ComAndRep/Warsaw/warsaw-report.html  Accessed on 1st October 2012.

[9] JPL Special Review Board, “Report on the Loss of the Mars Polar Lander and Deep Space 2 Missions”, Jet Propulsion Laboratory”, March 2000.

[10] Australian Transport Safety Bureau. In-Flight Upset Event 240Km North-West of Perth, WA, Boeing Company 777-2000, 9M-MRG. Aviation Occurrence Report 200503722, 2007.

[11] H. Wolpe, General Accounting Office Report on Patriot Missile Software Problem, February 4, 1992, Accessed on 1st October 2012, Available at: http://www.fas.org/spp/starwars/gao/im92026.htm

[12] Y.C. Yeh, Triple-Triple Redundant 777 Primary Flight Computer, IEEE Aerospace Applications Conference pg 293-307, 1996.

[13] D.M. Hunns and N. Wainwright, Software-based protection for Sizewell B: the regulator’s perspective. Nuclear Engineering International, September 1991.

[14] R.D. Hawkins, T.P. Kelly, A Framework for Determining the Sufficiency of Software Safety Assurance. IET System Safety Conference, 2012.

[15] SAE. ARP 4754 – Guidelines for Development of Civil Aircraft and Systems. 1996.

Software Safety Principles: End of the Series

This blog post series was derived from ‘The Principles of Software Safety Assurance’, by RD Hawkins, I Habli & TP Kelly, University of York. The original paper is available for free here. I was privileged to be taught safety engineering by Tim Kelly, and others, at the University of York. I am pleased to share their valuable work in a more accessible format.

Meet the Author

My name’s Simon Di Nucci. I’m a practicing system safety engineer, and I have been, for the last 25 years; I’ve worked in all kinds of domains, aircraft, ships, submarines, sensors, and command and control systems, and some work on rail air traffic management systems, and lots of software safety. So, I’ve done a lot of different things!

Principles of Software Safety Training

Learn more about this subject in my course ‘Principles of Safe Software’ here.

My course on Udemy, ‘Principles of Software Safety Standards’ is a cut-down version of the full Principles Course. Nevertheless, it still scores 4.42 out of 5.00 and attracts comments like:

  • “It gives me an idea of standards as to how they are developed and the downward pyramid model of it.” 4* Niveditha V.
  • “This was really good course for starting the software safety standareds, comparing and reviewing strengths and weakness of them. Loved the how he try to fit each standared with4+1 principles. Highly recommend to anyone that want get into software safety.” 4.5* Amila R.
  • “The information provides a good overview. Perfect for someone like me who has worked with the standards but did not necessarily understand how the framework works.” 5* Mahesh Koonath V.
  • “Really good overview of key software standards and their strengths and weaknesses against the 4+1 Safety Principles.” 4.5* Ann H.
Categories
Blog software safety

Software Safety Assurance

Photo by Andrea Piacquadio: man-in-black-suit-achieved-an-accomplishment
Photo by Andrea Piacquadio

Software Safety Assurance is the fourth in a new series of six blog posts on Principles of Software Safety Assurance. In them, we look at the 4+1 principles that underlie all software safety standards. (The previous post in the series is here.)

Read on for These Benefits…

This post deals with some crucial software assurance topics: what is it? what does it mean? I add [my comments] further explaining some key topics, based on my wide experience in the industry since 1994.

There are some important case studies here. They add depth and diversity to those already presented in previous posts. This post also addresses the crucial issues of diverse assurance techniques, as no one approach is likely to be adequate for safety significant software.

Content

We outline common software safety assurance principles that are evident in software safety standards and best practices. You can think of these guidelines as the unchanging foundation of any software safety argument because they hold true across projects and domains.

The principles serve as a guide for cross-sector certification and aid in maintaining comprehension of the “big picture” of software safety issues while evaluating and negotiating the specifics of individual standards.

Software Assurance = Justified Confidence

[The original authors referred to Principle 4+1 as ‘confidence’, but this term is not well recognized, so I have used ‘assurance’. The two terms are related. Both terms get us to ask: how much safety is enough? This is also the topic addressed in my blog post on Proportionality.]

Principle 4+1:

The confidence established in addressing the software safety principles shall be commensurate to the contribution of the software to system risk.

‘The Principles of Software Safety Assurance’, RD Hawkins, I Habli & TP Kelly, University of York.

All safety-related software systems must adhere to the four aforementioned principles. To prove that each of the guiding principles has been established for the software, evidence must be presented.

Depending on the characteristics of the software system itself, the dangers that are present, and the principle that is being shown, the proof may take many different forms. The strength and quantity of the supporting evidence will determine how confidently or assuredly the premise is established.

Therefore, it’s crucial to confirm that the level of trust developed is always acceptable. This is frequently accomplished by making sure that the level of confidence attained corresponds to the contribution the software makes to system risk. This strategy makes sure that the areas that lower safety risk the most receive the majority of attention (when producing evidence).

This method is extensively used today. Many standards employ concepts like Safety Integrity Levels (SILs) or Development Assurance Levels (DALs) to describe the amount of confidence needed in a certain software function. [And the ‘Level of Rigor’ required for its development.]

Examples

The flight control system for the Boeing 777 airplane is a Fly-By-Wire (FBW) system … The Primary Flight Computer (PFC) is the central computation element of the FBW system. The triple modular redundancy (TMR) concept also applies to the PFC architectural design. Further, the N-version dissimilarity issue is integrated into the TMR concept.

Details are given of a ‘special case procedure’ within the principles’ framework which has been developed specifically to handle the particular problem of the assessment of software-based protection systems. The application of this ‘procedure’ to the Sizewell B Nuclear Power Station computer-based primary protection system is explained.

Suitability of Evidence

Once the essential level of confidence has been established, it is crucial to be able to judge whether it has been reached. Several factors must be taken into account when determining the degree of confidence with which each principle is put into practice.

The suitability of the evidence should be taken into consideration first. The constraints of the type of evidence being used must be considered too. These restrictions will have an impact on the degree of confidence that can be placed in each sort of evidence with regard to a certain principle.

Examples of these restrictions include the degree of test coverage that can be achieved, the precision of the models employed in formal analysis approaches, or the subjectivity of review and inspection. Most techniques have limits on what they can achieve.

Due to these limitations, it could be necessary to combine diverse types of evidence to reach the required degree of confidence in any one of the principles. The reliability of each piece of evidence must also be taken into account. This takes into account the degree of confidence in the item of evidence’s capacity to perform as expected.

This is also frequently referred to as evidence rigor or evidence integrity. The rigorousness of the technique employed to produce the evidence item determines its reliability. The primary variables that will impact trustworthiness are Tools, Personnel, Methodology, Level of Audit and Review, and Independence.

The four software safety principles will never change. However, there is a wide range of trust in how those principles are developed. We now know that a determination must be made regarding the degree of assurance required for any given system’s principles to be established. We now have our guiding principle.

Since it affects how the previous four principles are put into practice, this concept is also known as Principle 4+1.

Software Safety Assurance: End of Part 4 (of 6)

This blog post is derived from ‘The Principles of Software Safety Assurance’, RD Hawkins, I Habli & TP Kelly, University of York. The original paper is available for free here. I was privileged to be taught safety engineering by Tim Kelly, and others, at the University of York. I am pleased to share their valuable work in a more accessible format.

Meet the Author

My name’s Simon Di Nucci. I’m a practicing system safety engineer, and I have been, for the last 25 years; I’ve worked in all kinds of domains, aircraft, ships, submarines, sensors, and command and control systems, and some work on rail air traffic management systems, and lots of software safety. So, I’ve done a lot of different things!

Principles of Software Safety Training

Learn more about this subject in my course ‘Principles of Safe Software’ here. The next post in the series is here.

My course on Udemy, ‘Principles of Software Safety Standards’ is a cut-down version of the full Principles Course. Nevertheless, it still scores 4.42 out of 5.00 and attracts comments like:

  • “It gives me an idea of standards as to how they are developed and the downward pyramid model of it.” 4* Niveditha V.
  • “This was really good course for starting the software safety standareds, comparing and reviewing strengths and weakness of them. Loved the how he try to fit each standared with4+1 principles. Highly recommend to anyone that want get into software safety.” 4.5* Amila R.
  • “The information provides a good overview. Perfect for someone like me who has worked with the standards but did not necessarily understand how the framework works.” 5* Mahesh Koonath V.
  • “Really good overview of key software standards and their strengths and weaknesses against the 4+1 Safety Principles.” 4.5* Ann H.
Categories
Blog software safety

Software Safety Principles 2 and 3

System Safety Concepts and Principles
Photo by Alena Darmel

Software Safety Principles 2 and 3 is the second in a new series of blog posts on Principles of Software Safety Assurance. In it, we look at the 4+1 principles that underlie all software safety standards. (The previous blog post is here.)

We outline common software safety assurance principles that are evident in software safety standards and best practices. You can think of these guidelines as the unchanging foundation of any software safety argument because they hold true across projects and domains.

The principles serve as a guide for cross-sector certification and aid in maintaining comprehension of the “big picture” of software safety issues while evaluating and negotiating the specifics of individual standards.

Principle 2: Requirement Decomposition

The second software safety principle is:

Principle 2: The intent of the software safety requirements shall be maintained throughout requirements decomposition.

‘The Principles of Software Safety Assurance’, RD Hawkins, I Habli & TP Kelly, University of York.

The requirements and design are gradually broken down as the software development lifecycle moves forwards, leading to the creation of a more intricate software design. The term “derived software requirements” refers to the criteria that were derived for the more intricate software design. The intent of those criteria must be upheld as the software safety requirements are broken down once they have been established as comprehensive and accurate at the highest (most abstract) level of design.

An example of the failure of requirements decomposition is the crash of Lufthansa Flight 2904 at Warsaw on 14 September 1993.

In essence, the issue is one of ongoing requirements validation. How do we show that the requirements expressed at one level of design abstraction are equal to those defined at a more abstract level? This difficulty arises constantly during the software development process.

It is insufficient to only consider requirements fulfillment. The software safety requirements had been met in the Flight 2904 example. However, they did not match the intent of the high-level safety requirements in the real world.

Human factors difficulties (a warning may be presented to a pilot as necessary, but that warning may not be noticed on the busy cockpit displays) are another consideration that may make the applicability of the decomposition more challenging.

Ensuring that all necessary details are included in the first high-level need is one possible theoretical solution to this issue. However, it would be difficult to accomplish this in real life. It is inevitable that design choices requiring more specific criteria will be made later in the software development lifecycle. It is not possible to accurately know this detail until that design choice has been made.

The decomposition of safety criteria must always be handled if the program is to be regarded as safe to use.

Requirements Satisfaction

The third software safety assurance principle is:

Principle 3: Software safety requirements shall be satisfied.

‘The Principles of Software Safety Assurance’, RD Hawkins, I Habli & TP Kelly, University of York.

It must be confirmed that a set of “valid” software safety requirements has been met after they have been defined. This set may be assigned software safety requirements (Principle 1), or refined or derived software safety requirements (Principle 2). The fact that these standards are precise, well-defined, and actually verifiable is a crucial need for their satisfaction.

The sorts of verification techniques used to show that the software safety requirements have been met will vary on the degree of safety criticality, the stage of development, and the technology being employed. Therefore, attempting to specify certain verification methodologies that ought to be employed for the development of verification findings is neither practical nor wise.

Mars Polar Lander was an ambitious mission to set a spacecraft down near the edge of Mars’ south polar cap and dig for water ice. The mission was lost on arrival on December 3, 1999.

Given the complexity and safety-critical nature of many software-based systems, it is obvious that using just one type of software verification is insufficient. As a result, a combination of verification techniques is frequently required to produce the verification evidence. Testing and expert review are frequently used to produce primary or secondary verification evidence. However, formal verification is increasingly emphasized because it may more reliably satisfy the software safety standards.

The main obstacle to proving that the software safety standards have been met is the evidence’s inherent limitations as a result of the methods described above. The characteristics of the problem space are the root of the difficulties.

Given the complexity of software systems, especially those used to achieve autonomous capabilities, there are challenges with completeness for both testing and analysis methodologies. The underlying logic of the software can be verified using formal methods, but there are still significant drawbacks. Namely, it is difficult to provide assurance of model validity. Also, formal methods do not deal with the crucial problem of hardware integration.

Clearly, the capacity to meet the stated software safety requirements is a prerequisite for ensuring the safety of software systems.

Software Safety Principles 2 & 3: End of Part 2 (of 6)

This blog post is derived from ‘The Principles of Software Safety Assurance’, RD Hawkins, I Habli & TP Kelly, University of York. The original paper is available for free here. I was privileged to be taught safety engineering by Tim Kelly, and others, at the University of York. I am pleased to share their valuable work in a more accessible format.

Meet the Author

My name’s Simon Di Nucci. I’m a practicing system safety engineer, and I have been, for the last 25 years; I’ve worked in all kinds of domains, aircraft, ships, submarines, sensors, and command and control systems, and some work on rail air traffic management systems, and lots of software safety. So, I’ve done a lot of different things!

Learn more about this subject in my course ‘Principles of Safe Software’ here. The next post in the series is here.