Software Engineering (Chapter 4)

Midterms

1. Safety-critical systems

2. Mission-critical systems

3. Business-critical systems

What are the three Critical Systems?

Safety-critical systems

Failure Results = loss of life, injury or damage

Mission-critical systems

Failure results = some goal-directed activity

Business-critical systems

Failure results = high economic losses

important system property

what is the system dependability for critical systems?

System Dependability

reflects the user’s degree of trust: it will not “fail“

1. systems that are not dependable, unreliable, unsafe or insecure = rejected by users

2. cost of system failure = very high

3. Undependable Systems = information loss = high consequent recovery cosf

What are The Three Importance of Dependability?

1. Formal Methods of software development

2. Static Analysis

3. External Quality Assurance

What are the Three Development Methods for Critical Systems?

Development Methods for Critical Systems

This may be used due to the cost of critical systems failure that are so high

1. Hardware Failure

2. Software Failure

3. Operational Failure

What are the Three Socio-Technical Critical Systems?

Hardware Failure

because of design and manufacturing errors or because components have reached the end of their natural life.

Software Failure

due to errors in its specification, design or implementation

Operational Failure

Human Operators make mistakes (the largest single cause of system failures)

Dependability

• trustworthiness

• trusted by its users

1. Availability

2. Reliability

3. Safety

4. Security

What are the 4 Principal Dimensions of Dependability?

Availability

the ability of the system to deliver services when requested

Reliability

the ability of the system to deliver services as specified

Safety

the ability of the system to operate without catastrophic failure

Security

the ability of the system to protect itself against accidental or deliberate intrusion

1. Repairability

2. Maintainability

3. Survivability

4. Error Tolerance

What are the OTHER 4 Dependability Properties?

Repairability

Reflects the extent to which the system can be repaired in the event of a failure

Maintainability

Reflects the extent to which the system can be adapted to new requirements

Survivability

Reflects the extent to which the system can deliver services whilst under hostile attack

Error Tolerance

Reflects the extent to which user input errors can be avoided and tolerated

Dependability Costs

tends to increase exponentially.

2 reasons:
1. use of more expensive techniques and hardware
2. increased testing and system validation

Dependability Economics

more cost effective to accept untrustworthy systems

System Failure (reliability)

An event that occurs when the system does not deliver a service as expected by users

System Error (reliability)

an erroneous system state that can lead to system behavior that is unexpected by system users

System Fault (Reliability)

characteristic of a software system that can lead to a system error

Human Error or Mistake (Reliability)

human behavior that results in the introduction of faults into a system

1. System Failure

2. System Error

3. System Fault

4. Human Error or Mistake

What are the 4 Reliability Terminology?

Failures

are usually a result of system errors

1. Fault Avoidance

2. Fault Detection and Removal

3. Fault Tolerance

What are the 3 Reliability Achievement?

Fault Avoidance

development technique to minimize the possibility of mistake

Fault Detection and Removal

Verification and Validation techniques before the system goes into service

Fault Tolerance

run-time techniques to ensure that system faults do not result in system errors

Reliability Modelling

input-output mapping = some input results in erroneous outputs

Safety

a property of a system that operates without danger to human injury or death

1. Primary Safety-Critical Systems

2. Secondary Safety-Critical Systems

What are the 2 Safety Criticality?

Primary safety-critical system

embedded software system whose failure can cause the associated hardware to fail

Secondary safety-critical systems

systems whose failure results in faults in other system

1. Specification Errors

2. Hardware Failures generating spurious inputs

3. Context-sensitive commands

What are the 3 Unsafe Reliable Systems

Specification Errors

system specification is incorrect

Hardware Failures generating spurious inputs

hard to anticipate in the specification

Context-sensitive commands

often the result of operator error

1. Accident

2. Hazard

3. Damage

4. Hazard Severity

5. Hazard Probability

6. Risk

What are the 6 Safety Terminology?

1. Hazard Avoidance

2. Hazard Detection and Removal

3. Damage Limitation

What are the 3 Safety Achievement?

Hazard Avoidance

system is designed so that some classes of hazard cannot arise

Hazard Detection and Removal

system is designed so that hazards are detected and removed

Damage Limitation

system includes protection features that minimizes damage

Fundamental Security

networked system is insecure then statement (reliability and safety) is unreliable

1. Exposure

2. Vulnerability

3. Attack

4. Threats

5. Control

What are the 5 Security Terminology?

1. Denial of Service

2. Corruption of Programs or Data

3. Disclosure of Confidential Information

What are the 3 Damage from insecurity

Denial of Service

normal services are unavailable

Corruption of programs or data

programs of data may be modified in an unauthorized way

Disclosure of Confidential Information

information may be exposed to people who are unauthorized to read or use

1. Vulnerability Avoidance

2. Attack Detection and Elimination

3. Exposure Limitation

What are the 3 Security Assurance?

Vulnerability Avoidance

system is designed so that vulnerabilities do not occur

Attack Detection and Elimination

system is designed so that attacks on vulnerabilities are detected and neutralized

Exposure Limitation

system is designed so that consequences of attacks are minimized