1. 1 © H. Kopetz 11/09/2015 Introduction TU Wien The Time-Triggered Architecture for Real-Time Systems H. Kopetz TU Wien

2. 2 © H. Kopetz 11/09/2015 Introduction http://stf.rgai.hu

3. 3 © H. Kopetz 11/09/2015 Introduction Outline  Introduction  System Architecture  Time-Triggered Protocols  Composability--Temporal Firewalls  Fault Tolerance  Conclusion

4. 4 © H. Kopetz 11/09/2015 Introduction Our Goal Our goal is to facilitate the systematic design of large dependable control systems out of components. The interactions of the components is realized by the exchange of messages across interfaces to a real-time communication system. The driving forces for the composition of a large System of Systems (SOS) out of a set of components (component systems) are:  Cognitive complexity reduction in order to reduce the design and development effort  Reuse of components: The components may be newly designed according to a given architectural style or may be already existing systems (legacy systems).  Simplified diagnostics and repair.

5. 5 © H. Kopetz 11/09/2015 Introduction Report on US Air Traffic Control In February 1997, the United States General Accounting Office (GAO) published a report to the Secretary of Transportation, Mr.F. Pena, about the design and implementation of the new air traffic control system in the US. The author of the report was Dr. R. B. Stillman, Chief Scientist for Computers and Telecommunications.

6. 6 © H. Kopetz 11/09/2015 Introduction ATC Plagued by Problems “To illustrate, the long-time centerpiece of this modernization program--the Advanced Automation System (AAS)--was restructured in 1994 after estimated costs tripled from $2.5 billion to 7.6 billion and delays in putting significantly less-than- promised system capabilities into operation were expected to run 8 years or more.” “For example, the per-unit cost estimate for the Voice Switching and Control System increased 522 percent, and the first site implementation was delayed 6 years from the original estimate.” Source: GAO Report to the Secretary of Transportation, February 3, 1997, p.24

7. 7 © H. Kopetz 11/09/2015 Introduction Principal Findings of GAO Report  An architecture is the centerpiece of sound system development and maintenance.  FAA is developing a logical architectural component for ATC modernization and evolution.  FAA lacks a technical architectural component to guide and constrain ATC modernization and evolution.  Without a technical ATC architecture, costly system incompatibilities have resulted and will continue.  FAA lacks an effective management structure for developing and enforcing an ATC systems architecture.

8. 8 © H. Kopetz 11/09/2015 Introduction What is a Technical System Architecture? A technical system architecture is a framework for the construction of a system that constrains an implementation in such a way that the ensuing system is understandable, maintainable, extensible, and can be built cost-effectively.

9. 9 © H. Kopetz 11/09/2015 Introduction Technical System Architecture (II)  Architectural style: An architecture must provide rules and guidelines for the partitioning of a system into subsystems and for the design of the interactions among the subsystems.  Composability: An architecture must provide a framework for the systematic construction of a system out of subsystems (components).  Property Match: Components must comply with the architectural style to avoid a property mismatch at the component interfaces.  Elegance: An architecture must constrain an implementation in such a way that the ensuing system is understandable, maintainable, extensible, and can be built cost-effectively--in other words, it is elegant. Architecture Design is Interface Design

10. 10 © H. Kopetz 11/09/2015 Introduction Property Mismatches at Interfaces PropertyExample Physical, ElectricalLine interface, plugs, Communication protocol CAN versus J1850 SyntacticEndianness of data Flow controlImplicit or explicit, Information push or pull Incoherence in naming Same name for different entities Data representationDifferent styles for data representation Different formats for date TemporalDifferent time bases Inconsistent time-outs DependabilityDifferent failure mode assumptions SemanticsDifferences in the meaning of the data

11. 11 © H. Kopetz 11/09/2015 Introduction Human Mental Capability Size versus Mental Effort to Understand Mental Effort (Complexity) Size If the mental effort required to understand a particular system function grows with the system size, there is an inherent limitation to the size of the systems we can build.

12. 12 © H. Kopetz 11/09/2015 Introduction Complexity and Size  Large systems can only be built if the effort required to understand the system operation, i.e, the complexity of the system, remains under control as the system grows.  The effort to understand any particular system function should remain constant, and should be independent of the system size.  A large system contains many more different functions than a small system.  The effort needed to understand all functions of a large system grows with the system size. The design effort must be guided by technical system architecture.

13. 13 © H. Kopetz 11/09/2015 Introduction Summary: A Good Distributed Architecture  provides a framework and guidelines for the composition of a system out of nearly autonomous components (subsystems) without the occurrence of property mismatches.  defines an architectural style.  specifies the type of interactions among the components across well- defined and small interfaces. It thus builds structure by weak inter- component coupling and strong intra-component coupling.  provides interfaces that are flexible enough to support the intended functions, but rigid enough to act as error containment boundaries.  is based on already familiar orthogonal concepts that are used recursively.  is scalable without limits.

14. 14 © H. Kopetz 11/09/2015 Introduction Technology Trend to Distributed Systems  System on a Chip (SOC) is the components: A complete computer system, including, CPU, Memory, I/O, Communication Controller, Operating Systems, and Application Software can be implemented on a single silicon die: e.g., Motorola “Golden Oak”  Smart Sensors: Sensing Element, signal processing, calibration, diagnosis, communication control on a single die.  On-Chip Oscillators for low-cost nodes: cheap, but imprecise  COTS: Commercial off the shelf components comprising hardware and software  Integrated Fault Tolerance: to mask faults, e.g. SEU (single event upsets)--New failure modes of SOCs

15. 15 © H. Kopetz 11/09/2015 Introduction Economics of Silicon Silicon real-estate requirements (today, i.e. in the year 2002): ARMcore 32 bit CPU: 1 mm 2 Infineon 256 Mbit DRAM: < 100 mm 2 : 320 kbyte of DRAM: 1 mm 2  Marginal Production Costs of 1 mm 2 of silicon is in the order of 10 US cent (Cost at silicon foundry TSMC)  Cost of packaging, testing, pins, power-supply significant and often dominant.  Marginal production costs of 100 mm 2 silicon chip order of 10 US $. One men minute of work buys how many megabytes of RAM?

16. 16 © H. Kopetz 11/09/2015 Introduction Time-Triggered Architecture (TTA)  Safety without compromises  No single point of failure  Formal analysis of critical functions  Composability:  Building systems out of prevalidated components--Component reuse  Fully specified operational interfaces in the temporal domain and value domain  Two level design methodology  Flexibility  Flexible reuse of existing components.

17. 17 © H. Kopetz 11/09/2015 Introduction TTA Overview Controlled Object RT Communication System HHH TR H Host TR Transducer Data Sharing Interface Analog or Digital dense time-base Digital on a sparse time-base

18. 18 © H. Kopetz 11/09/2015 Introduction Design Principles of the TTA  Establishment of a Consistent Distributed Computing Base  Global Time at every Node  Temporal Accuracy of of Real-time Data  Distinction between State and Event Observations  Interfaces specified in the domains of time and value  Transparent Fault Tolerance

19. 19 © H. Kopetz 11/09/2015 Introduction Validity of Real-Time Data How long is the observation: “The traffic light is green” temporally accurate ? The validity of real-time data is time dependent.

20. 20 © H. Kopetz 11/09/2015 Introduction Definition: Temporal Accuracy The temporal accuracy of a RT image is defined by referring to the recent history of observations of the related RT entity. A recent history RH i at time t i is an ordered set of time points, where the length of the recent history d acc = t i - t i-k is called the temporal accuracy. Assume that the RT entity has been observed at every time point of the recent history. A RT image is temporally accurate at the present time t i if

21. 21 © H. Kopetz 11/09/2015 Introduction State and Event Observation An observation is a state observation, if the value of the observation contains the full or partial state of the RT-entity. The time of a state observation denotes the point in time when the RT-entity was sampled. An observation is an event observation, if the value of the observation contains the difference between the “old state” (the last observed state) and the “new state”. The time of the event information denotes the point in time of the L-event of the “new state”.

22. 22 © H. Kopetz 11/09/2015 Introduction Example of State and Event Observation State observation (blue): The flow is at 5 l/sec a 10:45 a.m. Event Observation (red): The flow changed by 1 l/sec at 10:45 a.m. RT Entity RT Image

23. 23 © H. Kopetz 11/09/2015 Introduction State versus Event Observations

24. 24 © H. Kopetz 11/09/2015 Introduction Message A message is an atomic data structure that is formed for the purpose of inter-component communication. The endpoints of the communication are the component interfaces. In the temporal domain, a message can be characterized by  The message send instant, i.e. the instant when the first bit of the message leaves the sender.  The message receive instant, i.e., the instant when the last bit of the message arrives at the receiver.

25. 25 © H. Kopetz 11/09/2015 Introduction Interface The interface between two subsystems (cluster, component, etc.) is characterized by  Its data properties, i.e., the structure and semantics of the data items crossing the interface  Its temporal properties, i.e., the temporal conditions that have to be satisfied by the interface: control and temporal data validity.  The functional intent, i.e., the assumptions about the functions of the interfacing partner In a non-real-time computer system, there is little concern about the temporal properties.

26. 26 © H. Kopetz 11/09/2015 Introduction Distributed System Interfaces Communication System Inter- face View Component A Component B Messages

27. 27 © H. Kopetz 11/09/2015 Introduction Elementary vs. Composite Interface Consider a unidirectional data flow between two subsystems (e.g., data flow from sensor node to processing node). We distinguish between: A B Data Control Elementary Interface: A B Data Control Composite Interface: Elementary interfaces are inherently simpler than composite interfaces Example: state message in a DPRAM Queue of event messages

28. 28 © H. Kopetz 11/09/2015 Introduction Information Push vs. Information Pull Information Push Interface: Information producer pushes information on information consumer (e.g., telephone, interrupt) Information Pull Interfaces: Information consumer requests information when required (e.g, email). What is better in real-time systems?--For whom?

29. 29 © H. Kopetz 11/09/2015 Introduction State Message versus Event Message  State Message: A periodic message that contains state observations (synchronous). Message handling: update in place and non-consuming read. Periodic state messages can be implemented as an elementary interface (no dependence of sender on receivers) with error detection at the receiver.  Event Message: A message that contains event observations (asynchronous). Message handling: exactly-once semantics, realized by message queues. Requires a composite interface (dependence of sender on receivers) for error detection at the sender. (Compare “sampled message” and “queued message” in ARINC)

30. 30 © H. Kopetz 11/09/2015 Introduction Time Triggered (TT) vs. Event Triggered (ET) A Real-Time system is Time Triggered (TT) if the control signals, such as  sending and receiving of messages  recognition of an external state change are derived from the progression of a (global) time. A Real-Time system is Event Triggered (ET) if the control signals are derived from the occurrence of events, e.g.,  termination of a task  reception of a message  an external interrupt

31. 31 © H. Kopetz 11/09/2015 Introduction Basic Elements of the TTA Assumes existence of a sparse global time and contains the following four basic elements:  Interface: a data-sharing boundary between two communicating subsystems that contains temporally accurate state observations.  Communication subsystem: transports real-time data in the from of state messages from an output interface to an input interface within a given time.  Host computer: Reads input data from an input interface (information pull), performs a data transformation and writes output data into an output interface (information push) within a given a priori known duration.  Transducer: Transforms output data from an interface into a form required by the system environment and transforms data from the environment into the form required by an input interface.

32. 32 © H. Kopetz 11/09/2015 Introduction A Time-Triggered Architecture (TTA) Node Interface to Transducerss Host computer including application software Communication Network Interface (CNI) Interface to Other Nodes Communication Network Interface (CNI) Control signals and data items to and from the controlled object Messages to and from the real-time communication system Host Computer

33. 33 © H. Kopetz 11/09/2015 Introduction TTP - Principle of Operation  TTP generates a global time-base  Media access is controlled by TDMA, based on this time  Acknowledgement implicit by membership  Error detection is at the receiver, based on the a priori known receive time of messages  State agreement between sender and receiver is enforced by extended CRC calculation  Every message header contains 3 mode change bits that allow the specification of up to seven successor modes

34. 34 © H. Kopetz 11/09/2015 Introduction How well can we synchronize clocks?

35. 35 © H. Kopetz 11/09/2015 Introduction Sparse Time Base If the occurrence of events is restricted to some active intervals with duration  with an interval of silence of duration  between any two active intervals, then we call the timebase  /  -sparse, or sparse for short.

36. 36 © H. Kopetz 11/09/2015 Introduction Uniform Time Format--OMG Standard external time format (8 bytes) Elapsed seconds since January 6, 1980 at 00:00(GPS base). 2 -24 sec 1 sec Time horizon Time granularity determined by precision of GPS 2 40 seconds Start of epoch: January 6, 1980 at 0:00:00 UTC Granularity about 60 nanosecond

37. 37 © H. Kopetz 11/09/2015 Introduction Time and State In abstract system theory (Mesarovic, p.45), the notion of state is introduced in order to separate the past from the future: “The state enables the determination of a future output solely on the basis of the future input and the state the system is in. In other word, the state enables a “decoupling” of the past from the present and future. The state embodies all past history of a system. Knowing the state “supplants” knowledge of the past. Apparently, for this role to be meaningful, the notion of past and future must be relevant for the system considered.” A precise concept of time is a prerequisite for a precise concept of state.

38. 38 © H. Kopetz 11/09/2015 Introduction Global Interactions versus Local Processing Host Computer C N I CC+MEDL Host Computer CC+MEDL Host Computer CC+MEDL Host Computer CC+MEDL Host Computer CC+MEDL C N I I/O In the TTA, the locus of temporal control is in the communic- ation system. In ET systems, the locus of temporal control is in host computers.

39. 39 © H. Kopetz 11/09/2015 Introduction TTP-Controller Protocol Engine CNI in DPRAM Host CPU TTP Control Data in MEDL TTP-Time Interrupt Replicated TTP Bus TTP Controller

40. 40 © H. Kopetz 11/09/2015 Introduction Use of Apriori Knowledge The a priori knowledge about the behavior is used to improve the Error Detection: It is known a priori when a node has to send a message (Life sign for membership).  Message Identification: The point in time of message transmission identifies a message (Reduction of message size)  Flow control: It is known a priori how many messages will arrive in a peak-load scenario (Resource planning). For event-triggered asynchronous architectures, there exists an impossibility result: ‘It is impossible to distinguish a slow node from a failed node!’ This makes the solution to the membership problem very difficult.

41. 41 © H. Kopetz 11/09/2015 Introduction Continuous State Agreement The internal state of a TTP controller (C-state) is formed by the  Time  Operational Mode, and  Membership The Protocol will only work properly, if sender and receiver contain the same state. Therefore TTP contains mechanisms to guarantee continuous state agreement (extended CRC checksum) and to avoid clique formation (counts of positive and negative CRC checks).

42. 42 © H. Kopetz 11/09/2015 Introduction TTP-A Objectives  Composability and Testability  Latency Guarantee for State Estimation  Good Error Detection for fail safe operations  Use of Standard UARTS (8 data bits with parity)  High Data Efficiency (>50 %) and small latency  Single wire (10 kbits) or twisted pair operation  Clock Synchronization better than 1 msec

43. 43 © H. Kopetz 11/09/2015 Introduction Fault-Tolerant Sensor Connection TTP/A TTP/C TTP/A A A A AA A Controlled Object Sensors TTP/A Bus Host FTU TTP/A TTP/A master controller TTP/C TTP/C controller A TTP/A slave node interfacing to sensors and actuators Fault Tolerant Unit TTP/C Bus

44. 44 © H. Kopetz 11/09/2015 Introduction TTA and the CORBA Architecture Object Request Broker (ORB)--GIOP communication ORB at A ORB at B Object A Object B Corba Facilities: Time Internationalization Domain Specific, e.g, Banking Health Care Corba Services: Naming Transaction Security Persistent State Event Notification, and more Time-Triggered Architecture TTA CNI

45. 45 © H. Kopetz 11/09/2015 Introduction Integration of TT and ET Services--the Options (i) Parallel: Time Axes is divided into two parallel windows, where one window is used for TT, the other for ET, Two media access protocols needed, one TT, the other ET TT ET TT ET Time (ii)Layered: ET service is implemented on top of a TT protocol Single time triggered access media access protocol. Time Loss of Temporal Composability Loss of Global Bandwidth Sharing What are the consequences for global time and state?

46. 46 © H. Kopetz 11/09/2015 Introduction Architecture Design is Interface Design A good interface within a distributed real-time system  is precisely specified in the value domain and in the temporal domain,  provides the relevant abstractions of the interfacing subsystems and hides the irrelevant details,  leads to minimal coupling between the interfacing subsystems,  limits error propagation across the interface,  Conforms to the established architectural style and thus introduces structure into a system.

47. 47 © H. Kopetz 11/09/2015 Introduction Composability Compose: “to make or form by combining things, parts, or elements” Composition: “the act of combining parts or elements to form a whole” Webster Encyclopedic Dictionary, 1989, p. 302 Composability: “The ease of forming a whole by combining parts” Parts: The component systems or the components Whole: A system of systems (SOS). A composition brings into existence new emerging services of the SOS that are more than the sum of the prior services of the components. These emerging services are the result of the integration of the component systems.

48. 48 © H. Kopetz 11/09/2015 Introduction What is a “Component”? In our context, a component is complete computer system that is time aware. It consists of  The hardware  The system and application software  The internal state The component interacts with its environment by the exchange of messages via interfaces.

49. 49 © H. Kopetz 11/09/2015 Introduction Closed Component vs. Open Component  Closed Component: Contains no local interface to the real world, but can contain local interfaces to other closed components. Semi-closed if it is time-aware.  Open Component: Contains an interface to the real world. Semi-open if no control signals are accepted from the real- world (e.g., a sampling system). The real world has an unbounded number of properties.

50. 50 © H. Kopetz 11/09/2015 Introduction Interfaces of a Component Application Software Linking Interface (LIF) Relevant for Composability Diagnostic and Management Interface (Boundary Scan in Hardware Design) Configuration Planning Interface Local Interfaces

51. 51 © H. Kopetz 11/09/2015 Introduction Interfaces of a Component (ii) Realtime Service (RS) Interface--the linking interface LIF:  In control applications periodic  Contains RT observations  Time sensitive Diagnostic and Maintenance (DM) Interface:  Sporadic access  Requires knowledge about internals of a node  Not time sensitive Configuration Planning (CP) Interface:  Sporadic access  Used to install a node into a new configuration  Not time sensitive Local Interface(s):  To other nodes or the environment  Not visible to the user of the component

52. 52 © H. Kopetz 11/09/2015 Introduction How is the “Integration” achieved?  The component systems are integrated by the exchange of messages across linking interfaces (LIF).  Our focus is on what are the contents of a message (data) and when a message is sent and received (time).  We abstract from the low-level (physical, coding) aspects of communication.  We assume that all property mismatches of the interacting systems have been resolved by a connection system.

53. 53 © H. Kopetz 11/09/2015 Introduction Only RS Interface Important for Composability An RS interface to a RT service module (e.g., a control algorithm) must specify:  At what point in time the input information is delivered to a module (temporal pre-conditions)  At what point in time the output information must be produced by the module (temporal post-conditions).  The properties of the intended information transformation provided by the module (a proper model) The RS interface contains RT images of the relevant RT entities.

54. 54 © H. Kopetz 11/09/2015 Introduction Interface Specification Operational Specification:  Operational Input Interface Specification  Syntactic Specification  Temporal Specification  Input Assertion  Operational Output Interface Specification  Syntactic Specification  Temporal Specification  Output Assertion  Interface State Meta-level Specification:  Meaning of the data elements: Means-and-ends model

55. 55 © H. Kopetz 11/09/2015 Introduction Views of a System: Four Universe Model Physical Level Analog Signals Logical Level Bits Informational Level Data Types User Level Meaning of Data Types Operational Interface Specification Value and Temporal Meta-level Specification Interpretation by the User Avizienis, FTCS 12, 1982

56. 56 © H. Kopetz 11/09/2015 Introduction Operational Input Interface Specification  Syntactic Message Specification: Forms information chunks out of the bit-stream of a message using a interface definition language (e.g., IDL of the OMG): e.g., numbers, operations, text (see: Four Universe Model)  Temporal Message Specification: Specifies when a message is expected: instant, phase, frequency  Operational Input Assertion: Specifies an executable predicate on the incoming message (and the interface state) of a component to determine whether the message is permitted at the given instant. Many specifications do not contain a precise temporal specification and the operational input assertions.

57. 57 © H. Kopetz 11/09/2015 Introduction Operational Output Interface Specification  Syntactic Message Specification: Specifies the structure of an outgoing message: e.g., numbers, operations, text (see: Four Universe Model)  Temporal Message Specification: Specifies when a message must be sent: instant, phase, frequency  Operational Output Assertion: Specifies a predicate on the outgoing message of a component to be able to determine whether the message is well-formed.

58. 58 © H. Kopetz 11/09/2015 Introduction Interface State The state of a component as seen from the point-of-view of the interface:  Only a (small) subset of the full state of the component  Simplified if a sparse time model is supported  Methods to access the interface state should be provided at the interface

59. 59 © H. Kopetz 11/09/2015 Introduction Meta-Level Specification The meta-level specification provides an interface model in order that the meaning of the information chunks that cross the interface can be established:  Hierarchical Model according to means-and-end relationship  Understandable to the user of the interface  Limits to formalization if components are open

60. 60 © H. Kopetz 11/09/2015 Introduction Reasoning about the Emerging Services The specification of the LIF message interfaces that are involved in a composition must be sufficient to reason about the properties of the emerging services:  LIFs must be precisely specified in the time and value domain Interface model behind a LIF.  LIFs should refer only to those aspects of a component systems that are required for the composition.  Dependence of the subsystem operation on the correct functioning of a LIF partner should be minimized (Otherwise, violation of the principle of the stability of prior services). Only if the LIF specification is easier to comprehend than the full subsystem specification, a complexity reduction is achieved.

61. 61 © H. Kopetz 11/09/2015 Introduction (Cognitive) Interface Complexity An interface provides a view into a system. The cognitive complexity of this view depends on  Interface model  Number and interaction of elements visible at the interface  Representation (Documentation) of the interface  Experience of the observer ...... The time it takes for an “average” user to understand an interface documentation is a possible quantitative measure of cognitive interface complexity.

62. 62 © H. Kopetz 11/09/2015 Introduction Complexity Reduction by Partitioning Complexity Reduction: (LIF Service Interface Complexity)/(Component Complexity) A good decomposition will lead to a significant complexity reduction for the understanding of the emerging functions at the system level. The easier it is, to understand a LIF interface, the better the decomposition from the point of view of complexity management.

63. 63 © H. Kopetz 11/09/2015 Introduction Complexity Reduction by Partitioning Complexity Reduction: (LIF Service Interface Complexity)/(Component Complexity) A good decomposition will lead to a significant complexity reduction for the understanding of the emerging functions at the system level. The easier it is, to understand a LIF interface, the better the decomposition from the point of view of complexity management.

64. 64 © H. Kopetz 11/09/2015 Introduction A Composition Involving three LIFs Linking Interfaces (LIFs)

65. 65 © H. Kopetz 11/09/2015 Introduction The Five Principles of Composability (LIF) (1) Independent Development of the Components (Architecture) The message interfaces of the components must be precisely specified in the value domain and in the temporal domain in order that the component systems can be developed in isolation. (2) Stability of Prior Services (Component Implementation) The prior services of the components must be maintained after the integration and should not fail if a partner fails. (3) Performability of the Communication System (Comm. System) The communication system transporting the messages must meet the given temporal requirements under all specified operating conditions. (4)Replica Determinism (Architecture) Replica Determinism is required for the transparent implementation of fault tolerance (5)Diagnosability (Architecture) It must be possible to diagnose a faulty component

66. 66 © H. Kopetz 11/09/2015 Introduction Common Composability Violations  Missing temporal specification of interfaces concerning message rates and message receive instants (1).  Prior services are impaired by excessive load across an information push interface (e.g., interrupts) (2).  At the critical instant, the communication system does not meet the temporal requirements of the applications (3).  Missing replica determinism destroys the fault-tolerance strategy (4).  Error propagation: The prior services of a component become dependent on a fault of a LIF partner (2).  Diagnosis: Impossibility to determine the sender of an incorrect message (e.g., CAN) (5)

67. 67 © H. Kopetz 11/09/2015 Introduction Temporal Firewall Interface in the TTA A temporal firewall interface  is a unidirectional elementary data flow interface for the exchange of state information.  is located in a dual ported RAM of a communication controller--update-in-place semantics  the instants when data is fetched (delivered) from (to) the communication system are a priori common knowledge to all communicating partners (error detection!)  eliminates control error propagation since no control signal cross the temporal firewall interface Input Firewall: Assumptions Output Firewall: Guarantees

68. 68 © H. Kopetz 11/09/2015 Introduction Temporal Firewall Information Flow Information flow Control flow

69. 69 © H. Kopetz 11/09/2015 Introduction Temporal Firewall Characteristics Fully specified in the domains of time and value and of low cognitive complexity:  Information Content: State Message versus Event Message  Role: Linking Interface (LIF) versus Local Interface  Dependency: Elementary versus Composite  Control: Information Push at Sender and Information Pull at Receiver  Error Detection: Sender versus Receiver The Temporal Firewall Interface is the simplest interface we were able to come up with.

70. 70 © H. Kopetz 11/09/2015 Introduction A Temporal Firewall is a Natural Concept  A temporal firewall is a high-level abstract concept.  It is a small and stable unidirectional interface that provides understandable abstractions of the relevant properties of the interfacing subsystems.  Timeliness is an integral part of the temporal firewall concept.  Conceptually, the RT images in the temporal firewall are closely related to the image presented by a sensor of an analog RT entity in the environment.  Temporal firewalls are thus based on an accustomed view of the world.

71. 71 © H. Kopetz 11/09/2015 Introduction Localized View of Global System

72. 72 © H. Kopetz 11/09/2015 Introduction Stable Properties of Temporal Firewalls The following stable properties of temporal firewalls are known a priori to all interfacing partners:  The addresses (names) and the syntactic structure of the data items in the temporal firewall.  A (abstract) model explaining the meaning of the data items contained in the temporal firewall.  The points on the global time base when the data items in the temporal firewall are accessed by the TT communication system. This information enables the avoidance of race conditions between the producer and the consumer.  The temporal accuracy of the data items in the temporal firewall. This knowledge is important to guide the information consumer about the minimum rate of sampling of the temporal firewall.

73. 73 © H. Kopetz 11/09/2015 Introduction Temporal Firewalls and Validation Assume a host that is encapsulated between two temporal firewalls, and input firewall and an output firewall. These two firewalls form the only interfaces of this host to its environment.  The stable properties of the input firewall form important preconditions for the validation of the component under consideration. Many assumptions about the environment are contained in the specification of this input firewall.  The stable properties of the output firewall form important postconditions of the validation.  In the validation process it must be demonstrated that the postconditions, given in the output firewall specification, are always TRUE, provided the preconditions associated with the input firewall hold.

74. 74 © H. Kopetz 11/09/2015 Introduction Example: A Five Cluster System Transponder H H H H Collision Avoidance H Radar H H H H H H H H ECluster Controlled Object (State Variables are called RT-Entities) H RT Image in Temporal Accuracy Relationship to RT entity ECluster T T T T T

75. 75 © H. Kopetz 11/09/2015 Introduction Temporal Firewalls and Composability A composable architecture must support the (1) Independent development of components--relates to the architecture (2) Stability of prior services--relates to the components (3) Performability of the Communication System--relates to the communication system. (4)Replica determinism--to support transparent implementation of fault tolerance. (5) Diagnostics--It mus be possible to identify the sending FCU (Fault Containment Unit) of every message. The temporal firewall concept supports these principles of composability.

76. 76 © H. Kopetz 11/09/2015 Introduction Top-Down Design Process in the TTA Level 1: Decompose the design problem into clusters and components Allocate functions to components Investigate the data flow among the components Specify the temporal firewalls in value and time Estimate the failure rates and specify the fault-hypothesis Specify the NGU Strategy Level 2: Implement the components, taking the temporal firewall specifications as constraints.

77. 77 © H. Kopetz 11/09/2015 Introduction Composability and Reuse of Components Composability and the effortless reuse of available components are highly intertwined:  The precisely defined component interfaces of a composable architecture specify clearly what a user has to supply and what a user can expect from an existing component.  The “stability of prior service” principle ensures that the functions of the existing component are not disturbed by the integration.  The “constructive integration” principle ensures that the component integration is linear and not circular.

78. 78 © H. Kopetz 11/09/2015 Introduction Bottom-up Design--Reuse of Components The bottom up design takes advantage of the existing COTS components:  The input firewall parameters determine what a user is expected to supply  The output firewall parameters determine what a user can rely upon The architecture design must proceed taking these component characteristics as constraints. The temporal firewalls of the new components can be designed according to the top-down process.

79. 79 © H. Kopetz 11/09/2015 Introduction Legacy Systems In many application legacy systems have to be integrated in a new design:  Identify the “Linking Interface” of the legacy system.  Provide a gateway component that hides the idiosyncracies of the legacy system and provides a standard interface (wrapper technology) to the new architecture.  Provide back-pressure flow control in the gateway component.

80. 80 © H. Kopetz 11/09/2015 Introduction Localized View of Global System

81. 81 © H. Kopetz 11/09/2015 Introduction States outside Fault Hypothesis States covered by Fault-Hypothesis System States of a FT System Correct States FT Mechanisms NGU Strategy Normal Failures Rare Events

82. 82 © H. Kopetz 11/09/2015 Introduction Systems on a Chip (SOC) Failure Modes In the future, new failure modes are expected to occur due to the high integration density:  Multi-bit failures caused by SEUs  Intermittent failures due to proximity effects In safety-critical applications, an SOC must be considered to form a single fault-containment region with no restricting assumptions about its possible failure modes.

83. 83 © H. Kopetz 11/09/2015 Introduction Slightly off Specification (SOS) Failure Special type of Byzantine failure: A component produces an output signal (in the value domain or in the temporal domain) that is slightly outside the specified operating interval. Some receivers interpret the result correctly, some others cannot interpret the result. Voltage Receive Window SOS Sender A B C D E Correct Sender

84. 84 © H. Kopetz 11/09/2015 Introduction Example: Brake by Wire System ABS Master A master with an SOS failure can cause inconsistencies.

85. 85 © H. Kopetz 11/09/2015 Introduction Physical Interconnection Structure GGGGGGGGGG Guardian TTP-Bus TTP-Star Arbitrary Faults Fail-silent faults

86. 86 © H. Kopetz 11/09/2015 Introduction TTA Fault Containment and Error Detection

87. 87 © H. Kopetz 11/09/2015 Introduction Order of Magnitude of Failure Rates The following table gives an order of magnitude estimate of possible failure rates in an automotive environment:

88. 88 © H. Kopetz 11/09/2015 Introduction Fault-Tolerant Unit (FTU) A fault-tolerant unit (FTU) is a set of actively redundant components that provide a fault tolerant service to its environment:  FTUs have to receive identical input messages in the same order  FTUs have to operate in replica determinism  The output messages of FTUs should be idempotent  As long as a defined subset of the components of the FTU is operational, the FTU is considered operational FTUs provide the continuous service by fault masking.

89. 89 © H. Kopetz 11/09/2015 Introduction Active Redundancy: TMR Voter

90. 90 © H. Kopetz 11/09/2015 Introduction Design (Software) Faults The application of fault-tolerance techniques to tolerate software faults by design diversity is still an open research area:  If a disciplined software development process is followed most remaining failures are due to incorrect specification  Even independent programming teams tend to make similar errors  Replica Determinism can get lost if different algorithms are used However, an independent check of safety assertions makes sense.

91. 91 © H. Kopetz 11/09/2015 Introduction Controlled Object Sensors and Actuators Field Bus High Level Cluster Lower Level Cluster with limited functionality, implemented on diverse hardware and diverse software. Real-Time Buses Multilevel Architecture

92. 92 © H. Kopetz 11/09/2015 Introduction Conclusions  The Time-Triggered Architecture provides a framework for the constructive design of dependable distributed real- time systems.  Essential system functions (clock synchronization, membership) are implemented in hardware to simplify the application development.  Major industries (aerospace, automotive, railway) are supporting the paradigm shift towards the time-triggered technology.