fundamental concepts of dependability - computer...
TRANSCRIPT
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Fundamental Concepts ofDependability
Algirdas Avizienis, Jean-Claude Laprie, Brian Randell
Presented By
Neeraj Krishnan
CS 553, Spring 2003
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Authors
All have decades of research in fault tolerantcomputing behind them.
Avizienis, esp. on software faulttolerance
Laprie on control, m-electronics
Randell on HPCS, and dependability
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What we’ll do
Dependability
Attributes (Where we want to go)
Threats (What we are up against)
Means (How we overcome)
Conclude
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Attributes of Dependability
Integrity
Availability, Reliability, Safety
Integrity + Availability + Confidentiality = Security
Maintainability
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Attributes of Dependability
More mathematical definitions:
Reliability: Probability that the system survivesthroughout [0,t] (MTTF)
Maintainability: Probability that a system will berepaired in time less than t. (MTTR)
Likewise for others.
Availability: MTTF/(MTTF + MTTR)
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Attributes of Dependability
Any design contradictions ?
What about availability and safety ?
Any examples ?
Maintainability as a measure of the tradeoff ?
Why a time measure for everything ?
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Threats
What are we up against ?
Failure: Deviation from specified service
Error: System state that can lead to a failure
Fault: Cause of an error
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Threats
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Threats
Examples:
1. Link failure(!) - no connection - no webpage (physical)
2. Dos - low b/w - no service (interaction)
3. Math library - incorrect float - divide by 0 (design)
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Fault Classes
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Means
How do we overcome ?
Fault prevention: QC, Classical s/w engineeringprinciples. Modularity, information hiding, etc.
Fault tolerance: Deliver service in the presence offaults. Detect and recover, or maybe recover withoutdetecting. Mask.
Fault removal: test by injecting faults, system must beverifiable.