Critical Systems

Topics covered
A simple safety-critical system
System dependability
Availability and reliability
Safety
Security

Critical Systems
Safety-critical systems
• Failure results in loss of life, injury or damage to the
environment;
• Chemical plant protection system;
Mission-critical systems
• Failure results in failure of some goal-directed activity;
• Spacecraft navigation system;
Business-critical systems
• Failure results in high economic losses;
• Customer accounting system in a bank;

System dependability
For critical systems, it is usually the case that the
most important system property is the
dependability of the system.
The dependability of a system reflects the user’s
degree of trust in that system. It reflects the
extent of the user’s confidence that it will operate
as users expect and that it will not ‘fail’ in normal
use.

Importance of dependability
Systems that are not dependable and are
unreliable, unsafe or insecure may be rejected by
their users.
The costs of system failure may be very high.
Undependable systems may cause information
loss with a high consequent recovery cost.

Socio-technical critical systems
Hardware failure
• Hardware fails because of design and manufacturing
errors or because components have reached the end
of their natural life.
Software failure
• Software fails due to errors in its specification, design
or implementation.
Operational failure
• Human operators make mistakes. Now perhaps the
largest single cause of system failures.

A software-controlled insulin pump
Used by diabetics to simulate the function of the
pancreas which manufactures insulin, an
essential hormone that metabolizes blood
glucose.
Measures blood glucose (sugar) using a micro-
sensor and computes the insulin dose required to
metabolize the glucose.

Insulin pump organisation
Needle
assembly
Sensor
Display1 Display2
Alarm
Pump Clock
Controller
Power supply
Insulin reservoir

Insulin pump data-flow
Insulin
requirement
computation
Blood sugar
analysis
Blood sugar
sensor
Insulin
delivery
controller
Insulin
pump
Blood
Blood
parameters
Blood sugar
level
Insulin
Pump control
commands Insulin
requirement

Dependability requirements
The system shall be available to deliver insulin
when required to do so.
The system shall perform reliably and deliver the
correct amount of insulin to counteract the current
level of blood sugar.
The essential safety requirement is that
excessive doses of insulin should never be
delivered as this is potentially life threatening.

Dependability
The dependability of a system equates to its
trustworthiness.
A dependable system is a system that is trusted
by its users.
Principal dimensions of dependability are:
• Availability;
• Reliability;
• Safety;
• Security

Dimensions of dependability
Dependability
Availability Reliability Security
The ability of the system
to deliver services when
requested
to deliver services as
specified
to operate without
catastrophic failure
to protect itelf against
accidental or deliberate
intrusion
Safety

Other 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.

Maintainability
A system attribute that is concerned with the ease
of repairing the system after a failure has been
discovered or changing the system to include
new features
Very important for critical systems as faults are
often introduced into a system because of
maintenance problems

Survivability
The ability of a system to continue to deliver its
services to users in the face of deliberate or
accidental attack
This is an increasingly important attribute for
distributed systems whose security can be
compromised
Survivability subsumes the notion of resilience -
the ability of a system to continue in operation in
spite of component failures

Dependability vs performance
Untrustworthy systems may be rejected by their users
System failure costs may be very high
It is very difficult to tune systems to make them more
dependable
It may be possible to compensate for poor performance
Untrustworthy systems may cause loss of valuable
information

Dependability costs
Dependability costs tend to increase exponentially as
increasing levels of dependability are required
There are two reasons for this
• The use of more expensive development techniques and
hardware that are required to achieve the higher levels of
dependability
• The increased testing and system validation that is required to
convince the system client that the required levels of
dependability have been achieved

Costs of increasing dependability
Low Medium High Very
high
Ultra-high
Dependability

Reliability
• The probability of failure-free system operation over a
specified time in a given environment for a given
purpose
Availability
• The probability that a system, at a point in time, will
be operational and able to deliver the requested
services

It is sometimes possible to subsume system
availability under system reliability
• Obviously if a system is unavailable it is not delivering
the specified system services
However, it is possible to have systems with low
reliability that must be available. So long as
system failures can be repaired quickly and do
not damage data, low reliability may not be a
problem
Availability takes repair time into account

Reliability terminology
Term Description
System failure An event that occurs at some point in time when
the system does not deliver a service as expected
by its users
System error An erroneous system state that can lead to system
behaviour that is unexpected by system users.
System fault A characteristic of a software system that can
lead to a system error. For example, failure to
initialise a variable could lead to that variable
having the wrong value when it is used.
Human error or
mistake
Human behaviour that results in the introduction
of faults into a system.

Faults and failures
Failures are a usually a result of system errors that are
derived from faults in the system
However, faults do not necessarily result in system errors
• The faulty system state may be transient and ‘corrected’ before
an error arises
Errors do not necessarily lead to system failures
• The error can be corrected by built-in error detection and
recovery
• The failure can be protected against by built-in protection
facilities. These may, for example, protect system resources
from system errors

Reliability achievement
Fault avoidance
• Development technique are used that either minimise the
possibility of mistakes or trap mistakes before they result in the
introduction of system faults
Fault detection and removal
• Verification and validation techniques that increase the
probability of detecting and correcting errors before the system
goes into service are used
Fault tolerance
• Run-time techniques are used to ensure that system faults do
not result in system errors and/or that system errors do not lead
to system failures

Reliability modelling
You can model a system as an input-output
mapping where some inputs will result in
erroneous outputs
The reliability of the system is the probability that
a particular input will lie in the set of inputs that
cause erroneous outputs
Different people will use the system in different
ways so this probability is not a static system
attribute but depends on the system’s
environment

Input/output mapping
IeInput set
OeOutput set
Program
Inputs causing
erroneous outputs
Erroneous
outputs

Reliability perception
Possible
inputs
User
1
User
3
User
2
Erroneous
inputs

Reliability improvement
Removing X% of the faults in a system will not necessarily
improve the reliability by X%. A study at IBM showed that
removing 60% of product defects resulted in a 3%
improvement in reliability
Program defects may be in rarely executed sections of
the code so may never be encountered by users.
Removing these does not affect the perceived reliability
A program with known faults may therefore still be seen
as reliable by its users

Safety
Safety is a property of a system that reflects the
system’s ability to operate, normally or
abnormally, without danger of causing human
injury or death and without damage to the
system’s environment
It is increasingly important to consider software
safety as more and more devices incorporate
software-based control systems

Primary safety-critical systems
• Embedded software systems whose failure can
cause the associated hardware to fail and directly
threaten people.
Secondary safety-critical systems
• Systems whose failure results in faults in other
systems which can threaten people
Discussion here focuses on primary safety-critical
systems.
Safety criticality

Safety and reliability are related but distinct
Reliability is concerned with conformance to a
given specification and delivery of service
Safety is concerned with ensuring system cannot
cause damage irrespective of whether
or not it conforms to its specification
Safety and reliability

Safety terminology
Term Definition
Accident (or
mishap)
An unplanned event or sequence of events which results in human death or injury,
damage to property or to the environment. A computer-controlled machine injuring its
operator is an example of an accident.
Hazard A condition with the potential for causing or contributing to an accident. A failure of
the sensor that detects an obstacle in front of a machine is an example of a hazard.
Damage A measure of the loss resulting from a mishap. Damage can range from many people
killed as a result of an accident to minor injury or property damage.
Hazard
severity
An assessment of the worst possible damage that could result from a particular
hazard. Hazard severity can range from catastrophic where many people are killed to
minor where only minor damage results.
Hazard
probability
The probability of the events occurring which create a hazard. Probability values tend
to be arbitrary but range from probable (say 1/100 chance of a hazard occurring) to
implausible (no conceivable situations are likely where the hazard could occur).
Risk This is a measure of the probability that the system will cause an accident. The risk is
assessed by considering the hazard probability, the hazard severity and the probability
that a hazard will result in an accident.

Safety achievement
Hazard avoidance
• The system is designed so that some classes of hazard simply
cannot arise.
Hazard detection and removal
• The system is designed so that hazards are detected and
removed before they result in an accident
Damage limitation
• The system includes protection features that minimise the
damage that may result from an accident

Security
The security of a system is a system property that
reflects the system’s ability to protect itself from
accidental or deliberate external attack
Security is becoming increasingly important as
systems are networked so that external access to
the system through the Internet is possible
Security is an essential pre-requisite for
availability, reliability and safety

Fundamental security
If a system is a networked system and is insecure
then statements about its reliability and its safety
are unreliable
These statements depend on the executing
system and the developed system being the
same. However, intrusion can change the
executing system and/or its data

Security terminology
Term Definition
Exposure Possible loss or harm in a computing system. This can be loss or
damage to data or can be a loss of time and effort if recovery is
necessary after a security breach.
Vulnerability A weakness in a computer-based system that may be exploited to
cause loss or harm.
Attack An exploitation of a system vulnerability. Generally, this is from
outside the system and is a deliberate attempt to cause some damage.
Threats Circumstances that have potential to cause loss or harm. You can
think of these as a system vulnerability that is subjected to an attack.
Control A protective measure that reduces a system vulnerability. Encryption
would be an example of a control that reduced a vulnerability of a
weak access control system.

Damage from insecurity
Denial of service
• The system is forced into a state where normal services are
unavailable or where service provision is significantly degraded
Corruption of programs or data
• The programs or data in the system may be modified in an
unauthorised way
Disclosure of confidential information
• Information that is managed by the system may be exposed to
people who are not authorised to read or use that information

Security assurance
Vulnerability avoidance
• The system is designed so that vulnerabilities do not occur. For
example, if there is no external network connection then
external attack is impossible
Attack detection and elimination
• The system is designed so that attacks on vulnerabilities are
detected and neutralised before they result in an exposure. For
example, virus checkers find and remove viruses before they
infect a system
Exposure limitation
• The system is designed so that the adverse consequences of a
successful attack are minimised. For example, a backup policy
allows damaged information to be restored

Critical Systems

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