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292 Chapter 11 Dependability and security

Dependability

Availability Reliability Safety Security

The ability of the system The ability of the system The ability of the system The ability of the system
to deliver services when to deliver services as to operate without to protect itelf against
requested specified catastrophic failure accidental or deliberate
intrusion

Rather, we use relative terms such as ‘not dependable,’ ‘very dependable,’ and
Figure 11.1
Principal ‘ultra-dependable’ to reflect the degrees of trust that we might have in a system.
dependability Trustworthiness and usefulness are not, of course, the same thing. I don’t think
properties that the word processor that I used to write this book is a very dependable system.
It sometimes freezes and has to be restarted. Nevertheless, because it is very useful,
I am prepared to tolerate occasional failure. However, to reflect my lack of trust in
the system I save my work frequently and keep multiple backup copies of it. I com-
pensate for the lack of system dependability by actions that limit the damage that
could result from system failure.
There are four principal dimensions to dependability, as shown in Figure 11.1.

1. Availability Informally, the availability of a system is the probability that it will
be up and running and able to deliver useful services to users at any given time.

2. Reliability Informally, the reliability of a system is the probability, over a given
period of time, that the system will correctly deliver services as expected by
the user.
3. Safety Informally, the safety of a system is a judgment of how likely it is that the
system will cause damage to people or its environment.
4. Security Informally, the security of a system is a judgment of how likely it is that
the system can resist accidental or deliberate intrusions.

The dependability properties shown in Figure 11.1 are complex properties that
can be broken down into a number of other, simpler properties. For example, secu-
rity includes ‘integrity’ (ensuring that the systems program and data are not
damaged) and ‘confidentiality’ (ensuring that information can only be accessed by
people who are authorized). Reliability includes ‘correctness’ (ensuring the system
services are as specified), ‘precision’ (ensuring information is delivered at an appro-
priate level of detail), and ‘timeliness’ (ensuring that information is delivered when
it is required).