Analyzing Fault Tolerant Software System Architectures with the SAE

An Incremental Life-cycle Assurance Strategy for Critical System Certification

Software Engineering Institute Carnegie Mellon University Pittsburgh, PA 15213

Peter H. Feiler Nov 4, 2014

This material is based upon work funded and supported by the Department of Defense under Contract No. FA8721-05-C-0003 with Carnegie Mellon University for the operation of the Software Engineering Institute, a federally funded research and development center.

Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the United States Department of Defense.

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Challenges in Safety-critical Software-intensive systems An Architecture-centric Virtual Integration Strategy with SAE AADL Improving the Quality of Requirements Architecture Fault Modeling and Safety Incremental Life-cycle Assurance of Systems Summary and Conclusion

Embedded software systems introduce a new class of problems not addressed by traditional system modeling & analysis

How do you upgrade washing machine software?

Incremental Life-Cycle Assurance Feiler, Nov 4, 2014 5 © 2014 Carnegie Mellon University High Fault Leakage Drives Major Increase in Rework Cost Aircraft industry has reached limits of affordability 20.5% due to exponential growth in SW size and complexity. 300-1000x Requirements Engineering Acceptance 70% Requirements & 0%, 9% 80x Test system interaction errors 80% late error discovery at high System reworkrepair cost cost Design System Test 70%, 3.5% 1x 10%, 50.5% 20x Software Architectural Integration Design Major cost savings through rework avoidance Test by early discovery and correction A $10k architecture phase correction saves $3M Component Software Design 20%, 16% Total System Cost Unit Boeing 777 $12B Where faults are introduced 5x Test Boeing 787 $24B Where faults are found

The estimated nominal cost for fault removal Software as % of total system cost Sources: 1997: 45% → 2010: 66% → 2024: 88% NIST Planning report 02-3, The Economic Impacts of Inadequate Infrastructure for Software Testing, May 2002. Post-unit test software rework cost D. Galin, Software Quality Assurance: From Theory to Code 50% of total system cost and growing Implementation, Pearson/Addison-Wesley (2004) Development B.W. Boehm, Software Engineering Economics, Prentice Hall (1981)

System Engineer Physical Plant Control Engineer Hazards Characteristics Measurement Units, value range Impact of Lag, proximity Boolean/Integer abstraction system failures Air Canada, Ariane, 7500 Boolean variable architectureApplication Developer System Control Under Data Stream System Characteristics Control Latency jitter affects

User/Environment Operator Error control behavior Automation & Potential event loss human actions Compute Runtime Application System System Platform Architecture Software Hardware Distribution & Redundancy Virtualization, load balancing, Concurrency Engineer mode confusion Communication Embedded SW System EngineerITunes crashes on dual-cores

Embedded software system Why do system level failures still occur despite fault as major source of hazards tolerance techniques being deployed in systems?

The system

Inconsistency between independently developed analytical models System models

Confidence that model reflects implementation System implementation

This aircraft industry experience has led to the System Architecture Virtual Integration (SAVI) initiative

• System engineering – Focus on system architecture and operational environment – SysML developed to capture interactions with outside world, as a standardized UML profile – 4 pillars/diagrams: requirements, parameterics (added in SysML), structure, behavior • Conceptual architecture – UML-based component model – Architecture views (DoDAF, IEEE 1471) – Platform Independent model (PIM) • Embedded software system engineering – OMG Modeling and Analysis of Real Time Embedded systems (MARTE) as UML profile • Borrowed Meta model concepts from AADL • Focus on modeling implementations – xUML insufficient for PSM (Kennedy-Carter, NATO ALWI study)

Data Provider Data Consumer

Request Sensor Data

Receive Sensor Data

Request Current State

Receive Current State

Request Target State

Receive Target State

Apply algorithm

Publish Updated State

Data Consumer Requirement • Process data in 1 second Partitions • Provide space and time boundary enforcement • Execute periodically on a static timeline at 1 second rate Data request protocols across partitions

Request1 Provide1 Request2 Provide2 Request3 Provide3 Process

Provider Consumer Provider Consumer Provider Consumer Provider Consumer

How much time does consumer actually have to process the data? Who pays for the communication overhead?

Modeling is used in practice • Modeling, analysis, and simulation in mechanical, control, computer hardware engineering Current practice: software modeling close to source code – Remember software through pictures – MDE and MDA with UML – Automatically generated documents We need language for architecture modeling and analysis • Strongly typed • Well-defined execution and communication timing semantics • Systematic approach to dealing with exceptional conditions • Support for large-scale development

Incremental Life-Cycle Assurance Feiler, Nov 4, 2014 13 © 2014 Carnegie Mellon University SAE Architecture Analysis & Design Language (AADL) for Software-reliant Systems SW Design & Runtime The Mechanical System Architecture Command & Control Embedded Operational Physical platform Avionics & Mission Aircraft Software The Software System Deployed on Utilizes

Physical interface Platform component Computer System Hardware & OS

The Computer System

AADL focuses on interaction between the three elements of a software-reliant mission and safety-critical systems.

Incremental Life-Cycle Assurance Feiler, Nov 4, 2014 14 © 2014 Carnegie Mellon University The SAE AADL Standard Suite (AS-5506 series) Core AADL language standard (V2.1-Sep 2012, V1-Nov 2004) • Strongly typed language with well-defined execution and communication semantics • Textual and graphical notation • Standardized XMI interchange format

Standardized AADL Extensions Error Model language for safety, reliability, security analysis ARINC653 extension for partitioned architectures Behavior Specification Language for modes and interaction behavior Data Modeling extension for interfacing with data models (UML, ASN.1, …)

AADL Annex Extensions in Progress Requirements Definition and Assurance Annex Synchronous System Specification Annex Hybrid System Specification Annex System Constraint Specification Annex Network Specification Annex

Precise execution semantics for components • Thread, process, data, subprogram, system, processor, memory, bus, device, virtual processor, virtual bus Continuous control & event response processing • Data and event flow, call/return, shared access • End-to-End flow specifications Operational modes & fault tolerant configurations • Modes & mode transition Modeling of large-scale systems • Component variants, layered system modeling, packages, abstract, prototype, parameterized templates, arrays of components, connection patterns Accommodation of diverse analysis needs • Extension mechanism, standardized extensions

Architecture-Centric Quality Attribute Analysis Single Annotated Architecture Model Addresses Impact Across Operational Quality Attributes Safety Security & Reliability •Intrusion •MTBF •Integrity •FMEA Architecture Model •Confidentiality •Hazard analysis

Auto-generated Data analytical models Quality •Data precision/ Resource accuracy Real-time Consumption Performance •Bandwidth •Temporal correctness •Execution time/ •CPU time Deadline •Confidence •Power •Deadlock/starvation consumption •Latency

System Engineer Control Engineer Operational Environment System Control Under System Control Common latency data from system engineering • Processing latency • Sampling latency • Physical signal latency

Impact of Scheduler Choice on Controller Stability A. Cervin, Lund U., CCACSD 2006

Execution time variation: algorithm, use of cache Processor speed Resource contention Preemption Legacy & shared variable communication Rate group optimization Protocol specific communication delay Partitioned architecture Migration of functionality Fault tolerance strategy

Incremental Life-Cycle Assurance Feiler, Nov 4, 2014 19 © 2014 Carnegie Mellon University Early Discovery and Incremental V&V through System Architecture Virtual Integration (SAVI) Aircraft: (Tier 0) Aircraft system: (Tier 1) Engine, Landing Gear, Cockpit, … Weight, Electrical, Fuel, Hydraulics,… LRU/IMA System: (Tier 2) Hardware platform, software partitions Power, MIPS, RAM capacity & budgets End-to-end flow latency System & SW Engineering: Subcontracted software subsystem: (Tier 3) Mechatronics: Actuator & Wings Tasks, periods, execution time Safety Analysis (FHA, FMEA) Software allocation, schedulability Reliability Analysis (MTTF) Generated executables

OEM & Subcontractor: Repeated Virtual Integration Analyses: Subsystem proposal validation Power/weight Functional integration consistency MIPS/RAM, Scheduling Data bus protocol mappings End-to-end latency Network bandwidth

Proof of Concept Demonstration and Transition by Aerospace industry initiative • Architecture-centric model-based software and system engineering • Architecture-centric model-based acquisition and development process • Multi notation, multi team model repository & standardized model interchange  Multi-tier system & software architecture (in AADL)  Incremental end-to-end validation of system properties

Embedded Software Engineering System Engineering

AADL SysML

Application Software Physical System Operational Runtime Architecture Architecture (task & communication) (interface with embedded Environment SW/HW) (People, Use scenarios) Control Application Software Engineering Components Physical Components UML (source code) (mechanical , electrical, heat) Java, UML, Simulink Simulink, Modelica

Application Computer Platform Mechanical Software Engineering Engineering Architecture (processors & networks) SAVI Approach

Hardware Model delivery with interchange standards

Electrical Components Model repository content with intra and inter- Engineering (circuits & logic) VHDL model consistency Tool chain flexibility for contractor

Incremental Life-Cycle Assurance Feiler, Nov 4, 2014 21 © 2014 Carnegie Mellon University Architecture-centric Virtual Integration Practice (ACVIP) Iterative architecture Transformation and design, safety analysis, and code generation based requirement decomposition Model-based architecture on verified architecture specifications & multi- specifications dimensional QA analysis Stakeholder and Quality Attribute (QA) driven architecture- centric requirement specification

BUSINESS AND ARCHITECTURE SYSTEM MISSION GOALS

Testing against verified specifications Architecture-centric virtual and models integration and compositional verification of requirements Assurance plan and execution

Incremental Life-Cycle Assurance Feiler, Nov 4, 2014 23 © 2014 Carnegie Mellon University Certification & Recertification Challenges Certification: assure the quality of the delivered system • Sufficient evidence that a system implementation meets system requirements • Quality of requirements and quality of evidence determines quality of system Certification related rework cost • Currently 50% of total system cost and growing Recertification Challenge • Desired cost of recertification in proportion to change

Improve quality of requirements and evidence

Perform verification compositionally throughout the life cycle

Incremental Life-Cycle Assurance Feiler, Nov 4, 2014 24 © 2014 Carnegie Mellon University Industry Practice in DO-178B Compliant Requirements Capture Tool Industry Survey in 2009 FAA Requirements Engineering Study

Notation

Need analyzable & executable specifications

There is more to requirements quality than “shall”s and stakeholder traceability Requirements % IEEE 830-1998 Recommended Practice for SW Requirements Specification error Incomplete 21%

Missing 33%

Incorrect 24%

Ambiguous 6% Browsable links/Coverage metrics Inconsistent 5%

System to SW requirements gap [Boehm 2006] How do we verify low level SW requirements against system requirements?

When StartUpComplete is TRUE in both FADECs and SlowStartupComplete is FALSE, the FADECStartupSW shall set SlowStartupInComplete to TRUE

Leveson System Theoretic Framework

System, operational environment, development and V&V process Incremental Life-Cycle Assurance Feiler, Nov 4, 2014 27 © 2014 Carnegie Mellon University Mixture of Requirements & Architecture Design Constraints

Requirements for a Patient Therapy System

Typical requirement documents span multiple levels of a system architecture We have made architecture design decisions. We have effectively specified a partial architecture

Adapted from M. Whalen presentation

Coverage Developmental Quality attribute utility tree Requirements Modifiability

Assurability … Environmental Assumptions

Requirements Guarantees Mission Dependability Assumptions Requirements Requirements Function Reliability

Behavior Safety Precondition Postcondition Performance Security Invariant … …

Exceptional condition Implementation constraints Interaction contract: match input assumption with guarantee

Error Propagation Ontology

Behavior Output Control System Input State

Leveson pattern

Behavior Actuator System Under Control Sensor State

System safety process uses many individual methods and analyses, e.g. • hazard analysis System Capture hazards • failure modes and effects analysis Subsystem • fault trees Capture risk mitigation architecture • Markov processes Component Capture FMEA model

Goal: a general facility for modeling fault/error/failure behaviors that can be used for several modeling and analysis activities. Annotated architecture model permits checking for consistency and completeness between these various declarations.

Related analyses are also useful for other purposes, e.g. • maintainability SAE ARP 4761 Guidelines and Methods for Conducting the Safety Assessment Process on Civil Airborne Systems and Equipment • availability Demonstrated in SAVI Wheel Braking System Example • Integrity

• Security Error Model Annex can be adapted to other ADLs

Incremental Life-Cycle Assurance Feiler, Nov 4, 2014 32 © 2014 Carnegie Mellon University Legend Propagation of Error Types Error Propagation Contracts P1 Port Direction Propagated HW Binding Processor Error Type Not propagated NoData Incoming BadValue Error Flow through component Component C P1 Path P1.NoData->P2.NoData ValueError NoData Source P2.BadData P3 Outgoing Path processor.NoResource -> P2.NoData NoData LateData

P2 BadValue Processor Memory Bus NoResource Binding “Not“ on propagated indicates that this error type is intended to be contained. This allows us to determine whether propagation specification is complete.

Incoming/Assumed Outgoing/Contract Bound resources • Error Propagation • Error Propagation • Error Propagation Propagated errors • Error Containment • Error Containment • Error Containment: • Errors not propagated Propagation to resource

Anticipated: Anticipated: EGI No EGI data Flight Mgnt System NoService Actuator Cmd Oper’l NoData NoData Auto Pilot Airspeed Operational NoService Failed Data Failed Stall

FMS Processor Anticipated: No Stall Propagation Operational Failed

FMS Power

System engineering activity with focus on failing components.

Anticipated: Anticipated: EGI No EGI data Flight Mgnt System NoService EGI Logic Actuator Cmd Oper’l NoData Auto Pilot Airspeed Operational NoService Failed Data CorruptedData Failed Stall Corrupted FMS EGI HW Unexpected propagation of Processor Anticipated: No Oper’l corrupted Airspeed data results Stall Propagation in Stall due to miss-correction Operational Failed Failed Vibration causes boards to touch which causes EGI FMS Power data corruption EGI maintainer adds corrupted data hazard to model. Error Model analysis of integrated model detects unhandled propagation.

Incremental Life-Cycle Assurance Feiler, Nov 4, 2014 35 © 2014 Carnegie Mellon University Recent Automated FMEA Experience Failure Modes and Effects Analyses are rigorous and comprehensive reliability and safety design evaluations • Required by industry standards and Government policies • When performed manually are usually done once due to cost and schedule • If automated allows for – multiple iterations from conceptual to detailed design – Tradeoff studies and evaluation of alternatives – Early identification of potential problems

Largest analysis of satellite to date consists of 26,000 failure modes • Includes detailed model of satellite bus • 20 states perform failure mode • Longest failure mode sequences have 25 transitions (i.e., 25 effects)

2010 SEI Study for AMRDEC Aviation Engineering Directorate

Architecture-led Architecture-centric Static Analysis & Incremental Assurance Requirement Virtual System Compositional Plans & Cases Specification Integration Verification throughout Life Cycle

Model Mission Operational Repository Requirements & failure Function Architecture modes Behavior Model Performance Resource, Component Timing & Models Survivability Performance Analysis Requirements System Reliability Implementation Reliability, Safety Safety, Security System Security configuration Analysis

Four pillars for Improving Quality of Critical Software-reliant Systems

Generated assurance cases

Requirement coverage Assumption evidence

Safety hazards are part of the picture

Evidence records in terms of claims that requirements have been met Linkage to automated test harnesses

Key Problem TA4 – Research Integration and Formal Methods Workbench Many vulnerabilities occur at component interfaces. Rockwell Collins and How can we use formal methods to detect these University of Minnesota vulnerabilities and build provably secure systems?

ARCHITECTURE-CENTRIC PROOF 16 months into the project Draper Labs could not hack into the system in 6 weeks Had access to source code

Accomplishments • Created AADL model of vehicle hardware & software architecture • Identified system-level requirements to be verified Technical Approach based on input from Red Team evaluations • Develop a complete, formal architecture model for UAVs that • Developed Resolute analysis tool for capturing and provides robustness against cyber attack evaluating assurance case arguments linked to • Develop compositional verification tools driven from the AADL model architecture model for combining formal evidence from multiple • Developed example assurance cases for two sources, components, and subsystems security requirements • Develop synthesis tools to generate flight software for UAVs • Developed synthesis tool for auto-generation of directly from the architecture model, verified components, and configuration data and glue code for OS and platform verified operation system hardware

Continuous Confidence Measure throughout Life Incremental Evolution and Cycle that a System Meets its Requirements Execution of Assurance Plans

Architecture-centric Virtual Integration Incremental Architecture & Requirement Evolution Requirements Architecture Led Acceptance Verification Requirements Deployment Test Requirement Requirements SpecificationEngineering Build Flight Test RS Coverage Early Discovery through Architecture Analysis System&SW leads to Assurance Related Rework Reduction Design & Req Architecture Compositional System & SW System Verification System Integration Refinement Verification Architectural Target Test Design Build Lab Testing Virtual Architecture RS VA RS VA RS VA Integration & Analysis Design Integration Verification Component Integration Test Software Build Compositional Design Code Coverage Design & Req Testing Refinement Verification Unit Design ValidationCode by Code Test RS VA RS VA RS VA VirtualVerification Integration Development Auto-generation from verified models AADL&SCADE/Simulink Incremental Contract-based Compositional Verification Need for Multi-valued Ada SPARK/Ravenscar Argumentation Logic MISRA C

Confidence = Requirement Quality + Evidence Quality Build the System

Auto-generated Assurance Cases Build the Assurance Case FY15/16 line funded project

Challenges in Safety-critical Software-intensive Systems An Architecture-centric Virtual Integration Strategy with SAE AADL Improving the Quality of Requirements Architecture Fault Modeling and Safety Incremental Life-cycle Assurance of Systems Summary and Conclusion

Reduce risks • Analyze system early and throughout life cycle • Understand system wide impact • Validate assumptions across system Increase confidence • Validate models to complement integration testing • Validate model assumptions in operational system • Evolve system models in increasing fidelity Reduce cost • Fewer system integration problems • Incremental evidence through compositional verification • Fewer verification steps through generation from single source and verified models

Incremental Life-Cycle Assurance Feiler, Nov 4, 2014 44 © 2014 Carnegie Mellon University References AADL Website www.aadl.info and AADL Wiki www.aadl.info/wiki Blog entries and podcasts on AADL at www.sei.cmu.edu AADL Book in SEI Series of Addison-Wesley http://www.informit.com/store/product.aspx?isbn=0321888944 On AADL and Model-based Engineering http://www.sei.cmu.edu/library/assets/ResearchandTechnology_AADLandMBE.p df On an architecture-centric virtual integration practice and SAVI http://www.sei.cmu.edu/architecture/research/model-based- engineering/virtual_system_integration.cfm On an a four pillar improvement strategy for software system verification and qualification http://blog.sei.cmu.edu/post.cfm/improving-safety-critical-systems-with-a- reliability-validation-improvement-framework Webinars on system verification https://www.csiac.org/event/architecture-centric- virtual-integration-strategy-safety-critical-system-verification and on architecture trade studies with AADL https://www.webcaster4.com/Webcast/Page/139/5357

Peter H. Feiler U.S. Mail Principal Researcher Software Engineering Institute RTSS Customer Relations Telephone: +1 412-268-7790 4500 Fifth Avenue Email: [email protected] Pittsburgh, PA 15213-2612 USA

Web Customer Relations Wiki.sei.cmu.edu/aadl Email: [email protected] www.aadl.info SEI Phone: +1 412-268-5800 SEI Fax: +1 412-268-6257