When is CAN the weakest link? A bound on Failures-In-Time in CAN-Based Distributed Real-Time Systems. Arpan Gujara6 Björn B.
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1 When is CAN the weakest link? A bound on Failures-In-Time in CAN-Based Distributed Real-Time Systems Arpan Gujara6 Björn B. Brandenburg
2 Failures due to Transient Faults Harsh environments Spark plugs Hard radia:on High-power machinery
3 Failures due to Transient Faults Harsh environments Spark plugs Hard radia:on High-power machinery Electromagne:c Interference (EMI) Bit-flips in the hosts and in the network
4 Failures due to Transient Faults Harsh environments Spark plugs Hard radia:on High-power machinery Electromagne:c Interference (EMI) Bit-flips in the hosts and in the network EMI-induced transient faults Manifest as program-visible failures
5 Failures due to Transient Faults Transmission failures (faults on the wire) Commission failures (bit-flips in the memory buffers) Crash failures (due to fault-induced excep:ons)
6 Failures due to Transient Faults Transmission failures (faults on the wire) Tolerated by error detec6on and retransmissions Commission failures (bit-flips in the memory buffers) Crash failures (due to fault-induced excep:ons)
7 Failures due to Transient Faults Transmission failures (faults on the wire) Commission failures (bit-flips in the memory buffers) Crash failures (due to fault-induced excep:ons) Tolerated by error detec6on and retransmissions Tolerated by ac:ve replica6on of tasks on independent hosts
8 Failures due to Transient Faults Transmission failures (faults on the wire) Commission failures (bit-flips in the memory buffers) Crash failures (due to fault-induced excep:ons) Tolerated by error detec6on and retransmissions Tolerated by ac:ve replica6on of tasks on independent hosts How to decide the best replica6on strategy? Is Triple Modular Redundancy (TMR) enough? or is Quadruple Modular Redundancy (QMR) required? Would you replicate only the high-frequency tasks? or only the high-cri:cality tasks?
9 Retransmissions vs. Replica5on Tradeoff
10 Retransmissions vs. Replica5on Tradeoff For tolera:ng retransmissions-induced delays (Ensure no deadline viola:ons) The more slack, the beeer!
11 Retransmissions vs. Replica5on Tradeoff For tolera:ng retransmissions-induced delays (Ensure no deadline viola:ons) The more slack, the beeer! versus Ac:ve replica:on of tasks Reduced slack in the schedule
12 Retransmissions vs. Replica5on Tradeoff For tolera:ng retransmissions-induced delays (Ensure no deadline viola:ons) The more slack, the beeer! versus Ac:ve replica:on of tasks Reduced slack in the schedule Correctness Replica:on factor
13 Retransmissions vs. Replica5on Tradeoff For tolera:ng retransmissions-induced delays (Ensure no deadline viola:ons) The more slack, the beeer! versus Ac:ve replica:on of tasks Reduced slack in the schedule Correctness Replica:on factor Timeliness Replica:on factor
14 Retransmissions vs. Replica5on Tradeoff For tolera:ng retransmissions-induced delays (Ensure no deadline viola:ons) The more slack, the beeer! versus Ac:ve replica:on of tasks Reduced slack in the schedule Correctness Replica:on factor Timeliness Replica:on factor Success Replica:on factor
15 Retransmissions vs. Replica5on Tradeoff For tolera:ng retransmissions-induced delays (Ensure no deadline viola:ons) The more slack, the beeer! versus Ac:ve replica:on of tasks Reduced slack in the schedule? Correctness Replica:on factor Timeliness Replica:on factor Success Replica:on factor How to sta:cally determine the op6mal replica6on factor?
16 This Work For CAN-based distributed real-6me systems Probabilis6c analysis Quan:fy the replica:on vs. retransmissions tradeoff
17 The Larger Picture The CAN-based system is just one component in a safety-cri:cal system
18 The Larger Picture The CAN-based system is just one component in a safety-cri:cal system = (Photo: Airbus D&S/Dassault/Finmeccanica) UAV Power CAN
19 The Larger Picture The CAN-based system is just one component in a safety-cri:cal system = (Photo: Airbus D&S/Dassault/Finmeccanica) UAV Power CAN Replicate tasks, add more ECUs to the CAN subsystem
20 The Larger Picture The CAN-based system is just one component in a safety-cri:cal system = (Photo: Airbus D&S/Dassault/Finmeccanica) UAV Add a heat-sink to the power supply unit Power CAN Replicate tasks, add more ECUs to the CAN subsystem
21 The Larger Picture The CAN-based system is just one component in a safety-cri:cal system = (Photo: Airbus D&S/Dassault/Finmeccanica) UAV Add a heat-sink to the power supply unit Power CAN Replicate tasks, add more ECUs to the CAN subsystem What if the UAV has strict weight constraints? and you can either add the heat sink or the addi:onal ECUs How do you decide the best choice?
22 Failures-In-Time (FIT) Rate Expected #failures in one billion opera:ng hours e.g., 1M UAVs flying for 1K hours each FIT rates are widely used in the industry
23 Failures-In-Time (FIT) Rate Expected #failures in one billion opera:ng hours e.g., 1M UAVs flying for 1K hours each FIT rates are widely used in the industry
24 Failures-In-Time (FIT) Rate Expected #failures in one billion opera:ng hours e.g., 1M UAVs flying for 1K hours each FIT rates are widely used in the industry When is the CAN-based distributed real-:me system the weakest link in the system?
25 This Work For CAN-based distributed real-6me systems Probabilis6c analysis Quan:fy the replica:on vs. retransmissions tradeoff FIT rate analysis Builds upon the proposed probabilis:c analysis
26 Overview Model Analysis Evalua6on
27 Fault Abstrac5on & Modeling Transmission failures (faults on the wire) Commission failures (bit-flips in the memory buffers) Crash failures (due to fault-induced excep:ons)
28 Fault Abstrac5on & Modeling Transmission failures (faults on the wire) Commission failures (bit-flips in the memory buffers) Poisson Distribu6on Crash failures (due to fault-induced excep:ons)
29 Fault Abstrac5on & Modeling Transmission failures (faults on the wire) Commission failures (bit-flips in the memory buffers) Poisson Distribu6on Crash failures (due to fault-induced excep:ons) Probability that each message is omieed / corrupted / retransmieed
30 Fault Abstrac5on & Modeling Transmission failures (faults on the wire) Commission failures (bit-flips in the memory buffers) Poisson Distribu6on Crash failures (due to fault-induced excep:ons) Probability that each message is omieed / corrupted / retransmieed We do not consider sotware defects
31 System Model Host 2 CAN bus Host N Host 1 Host 3
32 System Model Host 2 Task A CAN bus Task B Host N Host 1 Host 3
33 System Model Host 2 Task A Msg. Sender Receiver M Task A Task B Message M from Task A to Task B CAN bus M Task B OUT Host N Host 1 Host 3
34 System Model Host 2 Task A Msg. Sender Receiver M Task A Task B Message M from Task A to Task B CAN bus M Task B OUT Host N Host 1 Host 3 Is OUT generated on :me? Is OUT generated from a logically correct value of M?
35 System Model Host 2 with Task Replica6on Task A Msg. Sender Receiver Replicas M Task A Task B 3 Message M from Task A to Task B CAN bus M Task B OUT Host N Host 1 Host 3
36 System Model Host 2 with Task Replica6on Task A Msg. Sender Receiver Replicas M Task A Task B 3 Message M from Task A to Task B CAN bus M Task B OUT M M Host N Task A replica Task A replica Message M from replicas of Task A to Task B Host 1 Host 3
37 Aggrega5ng the replicated messages How & when to compute OUT from mul:ple copies of M?
38 Aggrega5ng the replicated messages How & when to compute OUT from mul:ple copies of M? Case 1: Synchronous Systems Common global :me base e.g. majority value at the absolute deadline
39 Aggrega5ng the replicated messages How & when to compute OUT from mul:ple copies of M? Case 1: Synchronous Systems Common global :me base e.g. majority value at the absolute deadline Case 2: Asynchronous Systems No global :me base e.g. majority value ater enough copies have been received
40 Overview Model Analysis Evalua6on
41 The Larger Picture = (Photo: Airbus D&S/Dassault/Finmeccanica)
42 The Larger Picture = (Photo: Airbus D&S/Dassault/Finmeccanica) Objec:ves: A good replica6on strategy for the CAN-based system Compare the reliability of the CAN-based system with other components in the safety-cri6cal system
43 The Larger Picture = (Photo: Airbus D&S/Dassault/Finmeccanica) Objec:ves: A good replica6on strategy for the CAN-based system Compare the reliability of the CAN-based system with other components in the safety-cri6cal system Solu:on: FIT rate analysis Using the probabilis6c analysis
44 FIT Rate Analysis of the System FIT rate of the CAN subsystem
45 FIT Rate Analysis of the System FIT rate of the CAN subsystem FIT1 FIT2 FITn
46 FIT Rate Analysis of the System FIT rate of the CAN subsystem FIT1 FIT2 FITn Lower Bound on the Probability of Successful Transmission of M1
47 FIT Rate Analysis of the System FIT rate of the CAN subsystem FIT1 FIT2 Probability Density Func:on f1(t) FITn Lower Bound on the Probability of Successful Transmission of M1
48 FIT Rate Analysis of the System FIT rate of the CAN subsystem FIT1 FIT2 Mean Time Between Failures MTBF1 Probability Density Func:on f1(t) FITn Lower Bound on the Probability of Successful Transmission of M1
49 FIT Rate Analysis of the System FIT rate of the CAN subsystem FIT1 FIT2 Mean Time Between Failures MTBF1 Probability Density Func:on f1(t) FITn Lower Bound on the Probability of Successful Transmission of M1
50 FIT Rate Analysis of the System FIT rate of the CAN subsystem FIT1 FIT2 FITn Standard procedure to compute FIT rates given the failure probabili:es, but tailored for real-6me workloads Mean Time Between Failures MTBF1 Probability Density Func:on f1(t) Lower Bound on the Probability of Successful Transmission of M1
51 FIT Rate Analysis of the System FIT rate of the CAN subsystem FIT1 FIT2 Mean Time Between Failures MTBF1 Probability Density Func:on f1(t) FITn Lower Bound on the Probability of Successful Transmission of M1
52 FIT Rate Analysis of the System FIT rate of the CAN subsystem FIT1 FIT2 Mean Time Between Failures MTBF1 Probability Density Func:on f1(t) FITn Lower Bound on the Probability of Successful Transmission of M1 Network faults Host faults
53 FIT Rate Analysis of the System Host and network faults follow a Poisson distribu6on FIT rate of the CAN subsystem FIT1 FIT2 FITn Mean Time Between Failures MTBF1 Probability Density Func:on f1(t) Lower Bound on the Probability of Successful Transmission of M1 Network faults Host faults
54 FIT Rate Analysis of the System FIT1 FIT rate of FIT2 the CAN subsystem FITn retransmiced due to transmission failures omiced due to crash failures corrupted due to commission failures Host and network faults follow a Poisson distribu6on Probabili:es that each message: Mean Time Between Failures MTBF1 Probability Density Func:on f1(t) Lower Bound on the Probability of Successful Transmission of M1 Network faults Host faults Message Retransmission Message Omission Message Corrup:on
55 FIT Rate Analysis of the System Broster et al. s probabilis6c response-6me analysis* FIT rate of the CAN subsystem FIT1 FIT2 Mean Time Between Failures MTBF1 Probability Density Func:on f1(t) FITn Lower Bound on the Probability of Successful Transmission of M1 Network faults Host faults Message Retransmission Message Omission Message Corrup:on OUT1 is computed on :me *Broster, Ian, Alan Burns, and Guillermo Rodriguez-Navas. "Timing analysis of real-6me communica6on under electromagne6c interference." Real-Time Systems (2005):
56 FIT Rate Analysis of the System Broster et al. s probabilis6c response-6me analysis* We extend the analysis for a set of message replicas E.g., any 1 out of 3 message replicas are transmiced on :me FIT rate of the CAN subsystem FIT1 FIT2 FITn Mean Time Between Failures MTBF1 Probability Density Func:on f1(t) Lower Bound on the Probability of Successful Transmission of M1 Network faults Host faults Message Retransmission Message Omission Message Corrup:on OUT1 is computed on :me *Broster, Ian, Alan Burns, and Guillermo Rodriguez-Navas. "Timing analysis of real-6me communica6on under electromagne6c interference." Real-Time Systems (2005):
57 FIT Rate Analysis of the System Broster et al. s probabilis6c response-6me analysis* We extend the analysis for a set of message replicas E.g., any 1 out of 3 message replicas are transmiced on :me Deadline viola:ons due to crash failures FIT rate of the CAN subsystem FIT1 FIT2 FITn Mean Time Between Failures MTBF1 Probability Density Func:on f1(t) Lower Bound on the Probability of Successful Transmission of M1 Network faults Host faults Message Retransmission Message Omission Message Corrup:on OUT1 is computed on :me *Broster, Ian, Alan Burns, and Guillermo Rodriguez-Navas. "Timing analysis of real-6me communica6on under electromagne6c interference." Real-Time Systems (2005):
58 FIT Rate Analysis of the System Broster et al. s probabilis6c response-6me analysis* We extend the analysis for a set of message replicas E.g., any 1 out of 3 message replicas are transmiced on :me Deadline viola:ons due to crash failures FIT rate of the CAN subsystem FIT1 Incorrect output due to commission failures FIT2 FITn Mean Time Between Failures MTBF1 Probability Density Func:on f1(t) Lower Bound on the Probability of Successful Transmission of M1 Network faults Host faults Message Retransmission Message Omission Message Corrup:on OUT1 is computed on :me OUT1 does not contain a corrupted value of M1 *Broster, Ian, Alan Burns, and Guillermo Rodriguez-Navas. "Timing analysis of real-6me communica6on under electromagne6c interference." Real-Time Systems (2005):
59 FIT Rate Analysis of the System Broster et al. s probabilis6c response-6me analysis* We extend the analysis for a set of message replicas E.g., any 1 out of 3 message replicas are transmiced on :me Deadline viola:ons due to crash failures FIT rate of the CAN subsystem FIT1 Incorrect output due to commission failures FIT2 FITn Mean Time Between Failures MTBF1 Probability Density Func:on f1(t) Lower Bound on the Probability of Successful Transmission of M1 Network faults Host faults Message Retransmission Message Omission Message Corrup:on OUT1 is computed on :me OUT1 does not contain a corrupted value of M1 *Broster, Ian, Alan Burns, and Guillermo Rodriguez-Navas. "Timing analysis of real-6me communica6on under electromagne6c interference." Real-Time Systems (2005):
60 FIT Rate Analysis of the System Broster et al. s probabilis6c response-6me analysis* We extend the analysis for a set of message replicas E.g., any 1 out of 3 message replicas are transmiced on :me Deadline viola:ons due to crash failures FIT rate of the CAN subsystem FIT1 Incorrect output due to commission failures FIT2 FITn Mean Time Between Failures MTBF1 Probability Density Func:on f1(t) Lower Bound on the Probability of Successful Transmission of M1 Network faults Host faults Message Retransmission Message Omission Message Corrup:on OUT1 is computed on :me OUT1 does not contain a corrupted value of M1 *Broster, Ian, Alan Burns, and Guillermo Rodriguez-Navas. "Timing analysis of real-6me communica6on under electromagne6c interference." Real-Time Systems (2005):
61 FIT Rate Analysis of the System FIT rate of the CAN subsystem FIT1 FIT2 FITn Math in the paper! Mean Time Between Failures MTBF1 Probability Density Func:on f1(t) Lower Bound on the Probability of Successful Transmission of M1 Network faults Host faults Message Retransmission Message Omission Message Corrup:on OUT1 is computed on :me OUT1 does not contain a corrupted value of M1
62 Overview Model Analysis Evalua6on
63 Mobile Robot Workload* Task Name Length (bytes) Period (ms) Deadline (ms) MotorCtrl Wheel Wheel RadioIn Proximity Logging Broster, Ian, Alan Burns, and Guillermo Rodriguez-Navas. "Comparing real-6me communica6on under electromagne6c interference." Real-Time Systems, ECRTS Proceedings. 16th Euromicro Conference on. IEEE, 2004.
64 Mobile Robot Workload* Only the MotorCtrl task is replicated (#replicas vary from 1 to 9) Task Name Length (bytes) Period (ms) Deadline (ms) MotorCtrl Wheel Wheel RadioIn Proximity Logging Broster, Ian, Alan Burns, and Guillermo Rodriguez-Navas. "Comparing real-6me communica6on under electromagne6c interference." Real-Time Systems, ECRTS Proceedings. 16th Euromicro Conference on. IEEE, 2004.
65 Evalua5on Assess the proposed FIT rate deriva:on Comparison with results from CAN bus simula:on
66 Evalua5on Assess the proposed FIT rate deriva:on Comparison with results from CAN bus simula:on Is the FIT rate analysis too coarse-grained? Analysis for various fault rates
67 Analysis versus Simula5on for MotorCtrl Prob. of Failure of MotorCtrl Lower means beeer MotorCtrl reliability! #Replicas of MotorCtrl Task
68 Analysis versus Simula5on for MotorCtrl 1 Prob. of Failure of MotorCtrl e-05 1e-06 1e-07 1e-08 Asynchronous Analysis Asynchronous Simula6on #Replicas of MotorCtrl Task
69 Analysis versus Simula5on for MotorCtrl 1 Prob. of Failure of MotorCtrl e-05 1e-06 1e-07 1e-08 Synchronous Analysis Asynchronous Analysis Synchronous Simula6on Asynchronous Simula6on #Replicas of MotorCtrl Task
70 Prob. of Failure of MotorCtrl Analysis versus Simula5on for MotorCtrl e-05 1e-06 1e-07 1e-08 MotorCtrl becomes more reliable with replica6on Synchronous Analysis Asynchronous Analysis Synchronous Simula6on Asynchronous Simula6on #Replicas of MotorCtrl Task
71 Analysis versus Simula5on for MotorCtrl 1 Prob. of Failure of MotorCtrl e-05 1e-06 1e-07 1e-08 Analysis tracks the simula6on results Synchronous Analysis Asynchronous Analysis Synchronous Simula6on Asynchronous Simula6on #Replicas of MotorCtrl Task
72 FIT Rate Analysis of the CAN Subsystem FIT Rate of the System Lower means beeer reliability of the CAN subsystem! #Replicas of MotorCtrl Task
73 FIT Rate Analysis of the CAN Subsystem FIT Rate FIT of Rate the System 1e e-05 1e-10 1e-15 1e-20 1e-25 1e-30 1e-35 FIT Rate vs. Replication Factor for varying EMI rates host-fault-rate=10-20 /ms, bus-fault-rate=10-6 /ms host-fault-rate=10-12 /ms, bus-fault-rate=10-8 /ms host-fault-rate=10-16 /ms, bus-fault-rate=10-10 /ms Replicaion Factor #Replicas of MotorCtrl Task
74 FIT Rate Analysis of the CAN Subsystem FIT Rate FIT of Rate the System 1e e-05 1e-10 1e-15 1e-20 1e-25 1e-30 1e-35 FIT Rate vs. Replication Factor for varying EMI rates host-fault-rate=10-20 /ms, bus-fault-rate=10-6 /ms host-fault-rate=10-12 /ms, bus-fault-rate=10-8 /ms host-fault-rate=10-16 /ms, bus-fault-rate=10-10 /ms Replicaion Factor System becomes more reliable due to beeer tolerance against crash and commission failures #Replicas of MotorCtrl Task
75 FIT Rate Analysis of the CAN Subsystem 1e+10 FIT Rate vs. Replication Factor for varying EMI rates FIT Rate FIT of Rate the System System 1 reliability degrades 1e-05 1e-10 1e-15 1e-20 1e-25 1e-30 1e-35 due to less slack host-fault-rate=10-20 /ms, bus-fault-rate=10-6 /ms host-fault-rate=10-12 /ms, bus-fault-rate=10-8 /ms host-fault-rate=10-16 /ms, bus-fault-rate=10-10 /ms Replicaion Factor #Replicas of MotorCtrl Task
76 Success Replica:on factor
77 Success Failure Replica:on factor Replica:on factor
78 FIT Rate Analysis of the CAN Subsystem FIT Rate FIT of Rate the System 1e e-05 1e-10 1e-15 1e-20 1e-25 1e-30 1e-35 FIT Rate vs. Replication Factor for varying EMI rates host-fault-rate=10-20 /ms, bus-fault-rate=10-6 /ms host-fault-rate=10-12 /ms, bus-fault-rate=10-8 /ms host-fault-rate=10-16 /ms, bus-fault-rate=10-10 /ms Replicaion Factor #Replicas of MotorCtrl Task
79 FIT Rate Analysis of the CAN Subsystem FIT Rate FIT of Rate the System 1e e-05 1e-10 1e-15 1e-20 1e-25 1e-30 1e-35 FIT Rate vs. Replication Factor for varying EMI rates Analysis yields the op6mal replica6on factor host-fault-rate=10-20 /ms, bus-fault-rate=10-6 /ms host-fault-rate=10-12 /ms, bus-fault-rate=10-8 /ms host-fault-rate=10-16 /ms, bus-fault-rate=10-10 /ms Replicaion Factor #Replicas of MotorCtrl Task
80 FIT Rate Analysis of the CAN Subsystem FIT Rate FIT of Rate the System 1e e-05 1e-10 1e-15 1e-20 1e-25 1e-30 1e-35 FIT Rate vs. Replication Factor for varying EMI rates Analysis yields the op6mal replica6on factor host-fault-rate=10-20 /ms, bus-fault-rate=10-6 /ms host-fault-rate=10-12 /ms, bus-fault-rate=10-8 /ms host-fault-rate=10-16 /ms, bus-fault-rate=10-10 /ms FIT rate spans more than 20 orders of magnitude Replicaion Factor #Replicas of MotorCtrl Task
81 Summary
82 Summary? Find the best replica:on strategy for CAN-based systems Correctness Replica:on factor Timeliness Replica:on factor Success Replica:on factor
83 ? Summary Replica:on factor Success Timeliness Correctness Find the best replica:on strategy for CAN-based systems Replica:on factor Replica:on factor Compare reliability of the CAN-bus subsystem in the context of the larger safety-cri:cal system = (Photo: Airbus D&S/Dassault/Finmeccanica) UAV Power CAN
84 Future Work More complex system models CAN-based systems bridged together Sporadic DAG models
85 Future Work More complex system models CAN-based systems bridged together Sporadic DAG models Study of other technologies, e.g., Real Time Ethernet
86 Future Work More complex system models CAN-based systems bridged together Sporadic DAG models Study of other technologies, e.g., Real Time Ethernet hep://
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