Software Fault Tolerance (SWFT) SWFT for Wireless Sensor Networks (Lec 2)

1. Software Fault Tolerance (SWFT)SWFT for Wireless Sensor Networks (Lec 2) Prof. Neeraj Suri Abdelmajid Khelil Dept. of Computer Science TU Darmstadt, Germany Dependable Embedded Systems & SW Group www.deeds.informatik.tu-darmstadt.de

2. Last lecture: .. FT environmental monitoring , e.g. target detection.... Data fusion ..But the WSN ages!The system is evolvable!“So who watches the watchmen?” Motivation

3. Self-healing • IDEALLY a self-healing network could • Identify Problem: That a bird landed on a node • Identify a Fix: Need to remove the bird • Fix the Problem: Actuation to remove the bird Goal: Enable WSN to self-monitoring system health and autonomous debug • Begin by enabling human debugging in order to learn what metrics and techniques are useful, in order to enable autonomous system debugging

4. Debugging Is Hard in WSN • Wide range of failures • node crashes, sensor fails, code bugs, transient environmental changes to the network • Bugs are multi-causal, non-repeatable, timing-sensitive and have ephemeral triggers • Transient problems are common • Not necessarily indicative of failures • Interactions between sensor hardware, protocols, and environmental characteristics are impossible to predict • Limited visibility • Hard accessibility of the system • Minimal resources, RAM, communication • WSN application design is an iterative process between debugging and deployment.

5. Some Debugging Challenges Minimal resource Cannot remotely log on to nodes Bugs are hard to track down Evolvable system/conditions Application behavior changes after deployment Operating conditions (energy ..) Extracting debugging information Existing fault-tolerance techniques are limited Ensuring system health

6. Scenario After Deploying a Sensor Network… Very little data arrives at the sink, could be…. anything! The sink is receiving fluctuating averages from a region – could be caused by Environmental fluctuations Bad sensors Channel drops the data Calculation / algorithmic errors; and Bad nodes

7. Existing Works Simulators / Emulators / Visualizers E.g. EmTOS, EmView, moteview, Tossim .. Provide real-time information Do not capture historical context or aid in root-causinga failure SNMS Interactive health monitoring focuses on infrastructure to deliver metrics, and high code size Log files contain excessive data which can obfuscateimportant events [1] focus on metric collectionand not on metric content Momento, Sympathy EmView

8. Sympathy: A Debugging System for Sensor Network Nithya Ramanathan, Kevin Chang, Rahul Kapur, Lewis Girod, Eddie Kohler, and Deborah Estrin. SenSys '05

9. Overview • System Model • Approach • Architecture • Evaluation

10. System Model Network feature some regular traffic: SN are expected to generate traffic of some kind (Monitored traffic): routing updates, time synchronization beacons, periodical data .. Sympathy suspects a failure when a node generates less monitored traffic than expected. Sympathy generates additional metrics traffic. No malicious behavior

11. Model For Correct Data Flow Sink may not receive sufficient traffic from a node for multiple reasons To determine where and why traffic is lost, Sympathy outlines high-level requirements for data to flow through the network

12. Node Not Connected If destination node is not connected, then it may not receive the packet, and it will not respond

13. Node Does Not Receive Packet If destination node does not receive certain packets (e.g. a query) from source, it may not transmit expected traffic

14. Node Does Not Transmit Traffic Destination node may receive traffic, but due to software or hardware failure it may not transmit expected traffic

15. Sink Does Not Receive Traffic • Sink may not receive traffic due to collisions or other problems along the route

16. Tracking Traffic Through The Data Flow Model Node Connected? Should Node Transmit Traffic? Did Node Transmit Traffic? Did Sink Receive Traffic? Node NOT Connected! (Asymmetric Communication) Node NOT Connected! (Node Crash)

17. Design Requirements Tool for detecting and debugging failures in pre- and post-deployment phases. Debugging information should provide Most precise and meaningful failure detection Accuracy Latency Lowest overhead Transmitted debugging information must be minimized

18. 3 Challenges in WSN Debugging Has a failure happened? Which failure happened? Is the failure important? Sympathy aids users in - finding (detecting and localizing) and - fixing failures by attempting to answer these questions

19. Sympathy Approach Idea: “There is a direct relationship between amount of data collected at the sink and the existence of failures in the system” X Sink Monitors data flow from nodes / components Sink collects stats passively & actively 2  Highlights failure dependencies and event correlations 1 3 Identifies and localizes failures 4

20. Network’s purpose is to communicate If nodes are communicating sufficiently, network is working Simplest solution is the best “Insufficient” Traffic => Failure Application defines “sufficient” Sympathy detects many different failure types by tracking application end-to-end data flow Has a Failure Happened? No Sensor Data Channel Contention Asymmetric Links Node Crash Sensor Failure

21. Which Failure Happened? Anybody heard from node Sink Timestamp/Neighbor Tables Node Connected? No Node Crash Yes Time Awake increases No Node Reboot Yes No Valid Neighbor Table No Neighbors Yes No Valid Route Table No Route Yes No Should Node Transmit? Bad Path to Node Sufficient #Pkts Received Yes Did Node Transmit? No Bad Node Transmit Sufficient #Pkts Transmitted Yes Did SinkReceive Traffic? No Bad Path to Sink Sufficient #Pkts Received at Sink

22. Is the Failure Important? Analyze failure dependences to highlight primary failures Based on reported node topology “Can failure X be caused by failure Y” Deemphasize secondary failures to focus user’s attention Does NOT identify all failures or debug failures to line of code Primary Failure Secondary Failures

23. Failure Localization Determining why data is missing Physically narrow down cause E.g. Where is the data lost X • Where in the transmission path was the data lost? • Was the data even sent by the component? • OR

24. Detect Failures Determine what the failure is (Root Cause) Determine if the failure is important (Primary Failures) Final Goal Bad Path To Sink (due to contention at sink) Bad Path To Node (due to contention at sink) PRIMARY Failures In Red Boxes Contention at Sink No Neighbors Node crashed Bad Node Transmit (ADC failure)

25. Sympathy System: Architecture Apps Sympathy Apps Sympathy … … Routing Routing Collect Metrics Sympathy User Processes SINK

26. Architecture Definitions Network: a sink and distributed nodes Component Node components Sink components Sympathy-sink Communicates with sink components Understands all packet formats sent to the sink Non resource constrained node Sympathy-node Statistics period Epoch Sink Sink Component Sympathy sink Sympathy node Node Component Sensor node

27. Architecture: Overview Nodes Comp 1 Sympathy 1 … Routing If No/Insufficient data If Insufficient data Collect Stats Collect Stats Run Fault Localization Algorithm Run Fault Localization Algorithm Run Tests Run Tests Perform Diagnostic Perform Diagnostic SYMPATHY SYMPATHY USER Sink Components SINK

28. Sympathy Code on Sensor Node Each component is monitored independently Return generic or app-specific statistics Retrieve Comp Statistics Sympathy - Node Stats Recorder & Event Processor Comp 1 … Ring Buffer Data Return Routing Layer MAC Layer

29. Metrics Metrics are collected in 3 ways: Sympathy code on each SN actively reports to the sink (periodically or on-demand) Sink passively snoops its own transmission area Sympathy code on sink extracts sink metrics from sink application 3 metric categories: Connectivity: ROUTING TABLE, NEIGHBOUR LIST from each node, either passively or actively. Flow: PACKETS SENT, PACKETS RECEIVED, #SINK PACKETS TRANMITTED from each SN, and #SINK PACKETS RECEIVED and SINK LAST TIMESTAMP from sink. Node: SN actively report UPTIME, BAD PACKETS RECEIVED, GOOD PACKETS RECEIVED. Sink also maintains BAD and GOOD PACKETS RECEIVED. All Metrics timeout after EPOCH

30. Node Statistics Passive (in sink’s broadcast domain) and actively transmitted by nodes ROUTING TABLE (Sink, next hop, quality) tuples. Neighbors and associated ingress/ egress NEIGHBOUR LIST Time node is awake UP TIME #Statistics packets transmitted to sink #Statistics tx #Pkts routed #Packets routed by the node

31. Component Statistics Actively transmitted by a node to the sink, for each instrumented component Number of packets component received #Reqs rx #Pkts tx Number of packets component transmitted SINK LAST TIMESTAMP Timestamp of last data stored by component

32. Sympathy System Comp 1 Sympathy Comp 1 … Routing Collect Stats Collect Stats SYMPATHY SYMPATHY 2 Sink Components Comp 1 SINK

33. Sink Interface Sympathy passes comp-specific statistics using a packet queue Components return ascii translations for Sympathy to print to the log file Comp-specific statistics Comp 1 Sympathy Comp 2 Ascii translation of statistics / Data received Comp 3

34. Sympathy System Comp 1 Sympathy … Routing If No/Insufficient data If Insufficient data Collect Stats Collect Stats Run Failure Localization Algorithm Run Fault Localization Algorithm Run Tests Run Tests Perform Diagnostic Perform Diagnostic SYMPATHY SYMPATHY 3 Sink Components SINK

35. Failure Localization: Decision Tree Node Rebooted Tx: transmit Rx: receive Yes No Rx a Pkt from node Node Rebooted Yes No Rx Statistics Some node has heard this node Yes No No Yes Rx all Comp’s Data No stats Some node has route to sink Node Crashed Yes No No NO FAILURE (Comp has no Data to Tx) Yes Comp Rx Reqs No Data Some node has sink as neighbor No Yes Yes No Node not Rx Reqs Comp Tx Resps No node has a Route to sink Yes No node has sink on their neighbor list No Sink Rx Resps Comp Tx Node not Tx Resps Yes DIAGNOSTIC No Insufficient Data Sink not Rx Resps Insufficient Data No Data

36. Functional “No Data” Failure Localization

37. Performance “Insufficient Data” Failure Localization

38. Source Localization Root Causes with Associated Metrics and Source: • Three localized sources for Failures: • Node self (crash, reboot, local bug, connectivity issue..) • Path between node and sink (relay failure, collisions ..) • Sink

39. Sympathy System Comp 1 Sympathy … Routing If Insufficient data If Insufficient data Collect Stats Collect Stats Run Fault Localization Algorithm Run Fault Localization Algorithm Run Tests Run Tests Perform Diagnostic Perform Diagnostic SYMPATHY SYMPATHY USER Sink Components 4 SINK

40. Informational Log File Node 25 Time: Node awake: 78 (mins) Sink awake: 78(mins) Route: 25 -> 18 -> 15 -> 12 -> 10 -> 8 -> 6 -> 2 Num neighbors heard this node: 6 Pkt-type #Rx Mins-since-last #Rx-errors Mins-since-last 1:Beacon 15(2) 0 mins 1(0) 52 mins 3:Route 3(0) 37 mins 0(0) INF Symp-stats 12(2) 1 mins Reported Stats from Components ------------------------------------ **Sympathy: #metrics tx/#stats tx/#metrics expected/#pkts routed: 13(2)/12(2)/13(1)/0(0) Node-ID Egress Ingress ----------------------------------------------- 8 128 71 13 128 121 24 249 254

41. Failure Log File Node 18 Node awake: 0 (mins) Sink awake: 3 (mins) Node Failure Category: Node Failed! TESTS Received stats from module [FAILED] Received data this period [FAILED] Node thinks it is transmitting data [FAILED] Node has been claimed by other nodes as a neighbor [FAILED] Sink has heard some packets from node [FAILED] Received data this period: Num pkts rx: 0(0) Received stats from module: Num pkts rx: 0(0) Node’s next-hop has no failures

42. Spurious Failures An artifact of another failure Sympathy highlights failure dependencies in order to distinguish spurious failures Appears to not be sending data Node Crashed Congestion Appears to be sending very little data Sympathy Sink

43. Testing Methodology Application Run in Sympathy with the ESS (Extensible Sensing System) application In simulation, emulation and deployment Traffic conditions: no traffic, application traffic, congestion Node failures Node reboot – only requires information from the node Node crash – requires spatial information from neighboring nodes to diagnose Failure injected in one node per run, for each node 18 node network, with maximum 7 hops to the sink

44. Evaluation Metrics Accuracy of Failure Detection: Number of primary failure notifications Latency of Failure Detection/notification Time from when the failure is injected to when Sympathy notifies the user about the failure  There is a tradeoff between accuracy and latency

45. Notification Latency Does Sympathy always detects an injected failure? Detection = Assign a root cause of node crash Highlight the failure as primary EPOCH

46. Notification Accuracy

47. Memory Footprint TinyOS, mica2

48. Extensibility Adding new metrics requires ~5 lines of code on the nodes and ~10 lines of code on the sink Extensible to application classes with predictable data flow within bounds of an epoch User specifies expected amount of data Extensible to different routing layers due to modular design Multihop routing plug-in was 140 lines Mintroute routing plug-in was 100 lines

49. Conclusion A deployed system that aids in debugging by detecting and localizing failures Small list of statistics that are effective in identifying and localizing failures Behavioral model for a certain application class that provides a simple diagnostic to measure system health

50. Literature [1] Zhao, J.   Govindan, R.   Estrin, D.  “Computing aggregates for monitoring wireless sensor networks” SNPA 2003. [2] Nithya Ramanathan, Kevin Chang, Rahul Kapur, Lewis Girod, Eddie Kohler, and Deborah Estrin “Sympathy for the Sensor Network Debugger”, Sensys 2005. [3] Rost, S.; Balakrishnan, H. “Memento: A Health Monitoring System for Wireless Sensor Networks“, SECON 2006.