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SOME TRADE-OFFS IN SENSOR NETWORKS

SOME TRADE-OFFS IN SENSOR NETWORKS. Anthony Ephremides University of Maryland. Yale University October 8, 2003. PART I. Motivation. Layer Interaction in Wireless Networks Exploitation of Coupling for Enhancing Security Focus on Sensor Networks

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SOME TRADE-OFFS IN SENSOR NETWORKS

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  1. SOME TRADE-OFFS • IN • SENSOR NETWORKS Anthony Ephremides University of Maryland Yale University October 8, 2003

  2. PART I

  3. Motivation Layer Interaction in Wireless Networks Exploitation of Coupling for Enhancing Security Focus on Sensor Networks - Special Mission Objectives (not “just” a network) Focus on Interplay between Energy Expenditure, “Mission” Performance and Resistance to Threats

  4. Sensor Network for Detection • Ignore Routing Component control node

  5. 1 2 K Model • Simplified Model control center Each Node Collects Independently T independent Binary Measurements

  6. Model (cont.) • Binary Hypothesis: • Each node makes T observations • - Observations are independent for each node and across nodes • Each Observation: • Let be the number of “1’s” out of the T observations at node i • then ; hence form a sufficient • statistic • Final Decision at Control Center (CC) • Objective: Minimize

  7. Model (cont.) • Three Operating Options • - Centralized : • All data transmitted to CC • - Distributed : • Each node decides & transmits its decision to CC • - Quantized : • Each node sends a quantized M-bit quantity to CC • where

  8. Analysis • - Centralized Option • optimal rule is LR test

  9. Analysis (cont.) • - Distributed Option • - at each sensor node • - let represent the decision at node i • - is forwarded to CC • - at Control Center • where

  10. Analysis (cont.) • - Quantized Option • - at sensor nodes, a network-wide optimal M-bit quantization • rule would require a set of threshold • depending on • where is mapped into , which is • sent to CC • - Note if then • - at Control Center

  11. Analysis (cont.) • - Suboptimal solution for Quantized Option • - for • - if then • - if then • - else • - • - Thresholds are determined at each node based on • without exhaustive search

  12. Numerical Results • - Performance Comparison of Three Operating Options • Fix ; vary ; • and fix for quantized option，we compare • Centralized: blue • Distributed: green • Quantized: red

  13. Numerical Results (cont.) • - Explanation for the fluctuations • for distributed option

  14. Numerical Results (cont.) • - Suboptimal Solution for Quantized Option • Performance comparison between optimal solution and • suboptimal solution: • Fix , vary • optimal solution: blue • suboptimal solution: red

  15. Numerical Results (cont.) • - Suboptimal solution avoids complex computation for thresholds • - Suboptimal solution performs well enough compared to the optimal results

  16. Resistance to Attacks • - Attack 1: nodes are partially destroyed • Example: vary ,fix

  17. Resistance to Attacks (cont.) • - Attack 2: observations are partially deleted • fix , before attack: • Example 1: after attack • Example 2: after attack • Example 3: after attack

  18. Energy Consumption Analysis • - Energy for Data Processing • - based on # of comparisons • - represents the energy consumed for one comparison • - is the # of comparisons • - Energy for Transmission • - based on the distance from sensor nodes to control center and # of bits • transmitted • - represents the energy consumed for transmitting one bit data • over a unit distance, for a fixed communication system • - represents the distance from sensor nodes to control center • - is the # of bits transmitted • - Total Energy

  19. Energy ConsumptionAnalysis (cont.) • - Energy Consumption per Node for Three Options • - Centralized Option • - option 1: transmit all observations to CC • - option 2: transmit # of 1 out of T observations to CC • - Distributed Option • - Quantized Option (suboptimal solution) • where represents the expected # of comparisons needed for the suboptimal solution, which is a function of

  20. Energy ConsumptionAnalysis (cont.) • - Energy consumption comparison for fixed as a • function of • fix • and • vary , and for Quantized Option

  21. Energy ConsumptionAnalysis (cont.) • - Energy consumption comparison for fixed as a function • of • example 1: fix • vary

  22. Energy ConsumptionAnalysis (cont.) • example 2: fix • vary

  23. Energy ConsumptionAnalysis (cont.) • - Energy Consumption vs. Accuracy • fix ; vary • example 1: • example 2:

  24. Conclusions • - Robustness to Threats • - Robustness to nodes destruction: • Loss of Performance (in terms of ratio) is least for Distributed Option • and highest for Centralized Option • - Robustness to observations deletion: • Loss of Performance (in terms of ratio) is least for Distributed Option • and highest for Centralized Option • - Trade-off in Energy / Accuracy • - Regarding Accuracy: Centralized > Quantized > Distributed • - Regarding Energy: • - low & high : Distributed Option is best • - high & low : Centralized Option is best for relatively small distances • Distributed Option is best for large distances • - for d increasing: Centralized is affected most

  25. Future Work • - Spatial Correlation • - the independence assumption does not hold for sensor nodes • or observations • - spatial correlations are stronger for neighboring sensor nodes • - Routing in Multi-hop Environment • - single-hop is not feasible • - routing is needed for multi-hop transmission • - link metrics that capture cost function are needed • - Less Restrictive Model • - non-binary data • - general parameter values • - Other Attacks • - corruption of data • - compromised nodes

  26. PART II

  27. Ad-Hoc Networks • Ad-hoc wireless networks • Infrastructureless • Multi-hop transmissions • Mobile • New vulnerabilities to various forms of attack • Focus on the covert transmission threat • Established protocols offer possibility of covert transmission • Focus on AODV • Evaluation of covert channel

  28. Destination Destination RREQ RREP Source Source AODV • Ad-hoc On-demand Distance Vector Routing • Idea: initiate a path discovery process only when a node needs to communicate with another node for which it doesn’t have an active route.

  29. AODV • Each node i maintains a sequence number Ni, which is incremented in two circumstances: • Immediately before the node originates a route discovery; • Immediately before the node originates a RREP as the destination node in response to a RREQ for itself • RREQ: <ID(Destination), NDestination , ID(Source), NSource , Hop Count, …> • RREP: < ID(Destination), NDestination, ID(Source), Hop Count, Lifetime, …> • RERR: <ID(Unreachable Destination), NUnreachable Destination , …> • RERR is disseminated when an active link breaks • Route table entry: <ID(Destination), NDestination, Next Hop, Lifetime, …> • The Lifetime field is the expiration time for an active route, determined from the route control packets. • Each time a route is used to forward a data packet, its Lifetime field is updated to the current time plus a fixed value T1.

  30. AODV • Expanding ring search technique: • The originating node initially uses a number X in the RREQ packet IP header and sets the timeout for receiving a RREP to T2,where • X specifies the number of hops that the RREQ can traverse and is usually equal to 3 or to a value based on prior hop count information. • If the RREQ times out without receiving a RREP, the RREQ is broadcast again with X incremented by Y (usually 2) • This continues until X reaches a threshold. Later attempts have X set to the network diameter so that they can traverse the entire network.

  31. Covert Operation • Denote covert transmitter and receiver as CT and CR • The on-demand property of AODV allows manipulating the routing control packets to convey information covertly: • 1) Timing of the RREQs from CT • 2) Increment CT’s sequence number within a fixed period of time • 3) Lifetime entry in the RREP from CR • 4) Destination ID entry in the RREQ from CT

  32. Covert Operation • 1) Requires synchronization between the CT and CR, which is not easily guaranteed in an ad-hoc wireless network. • 2) Either requires synchronization between the CT and CR or is easily detectible by the arbitrary size of increase in the sequence number. • 3) Has the covert information carried in the RREP, which is unicast to the originator. Probability of loss is high. • 4) Has the covert information carried in every RREQ broadcast by CT. We will focus on this covert operation.

  33. 1 Covert Transmitter 0 RREQ(1) … RREQ(0) RREQ(0) Covert Receiver (1, 0, 0, …) Covert Operation • Encode information into the destination ID entry of the route requests • CT and CR share an alphabet {ID1,ID2,…IDN-1}, ID of CT itself is not included in that alphabet • Symbol i is sent if CT sends a RREQ for destination IDi • Order of reception is assured through the NCT in the RREQs

  34. Covert Operation • Limited bandwidth: • Transmission depends on “availability” of the route to the intended node (i.e., the next covert symbol is the destination ID) • When that route does exist, transmission is “stuck” • Lossy Communication: • The RREQs may not reach the covert receiver • Intermediate nodes may reply • Expanding ring search technique will stop the spread of RREQ • The covert transmitter and receiver are not connected • Covert channel performance • Covert transmission rate • Covert channel throughput • Probability of loss • Delectability

  35. Simulation Environment • Area: 500m x 500m • Independent movement, random waypoint model • Channel Bandwidth: 2Mbits/sec • Packet length: 64 Bytes • Packets are generated independently at each node ~Poisson; • for each new packet, the destination is randomly chosen from the remaining (N-1) nodes in the network • Simplified assumption of collision free MAC layer • AODV parameters are set as in draft-ietf-manet-aodv-12.txt • 900 sec of simulation time • Varying parameters: • (on average, there is one neighbor • within a node’s transmission range)

  36. Default Values for Some Important AODV Parameters • Parameter Name Value • ---------------------- -------- • ACTIVE_ROUTE_TIMEOUT (i.e., T1): 3,000 Milliseconds • MY_ROUTE_TIMEOUT : 2 * ACTIVE_ROUTE_TIMEOUT • ALLOWED_HELLO_LOSS : 2 • HELLO_INTERVAL : 1,000 Milliseconds • RREQ_RETRIES : 2 • RREQ_RATELIMIT : 10 • RERR_RATELIMIT : 10 • NET_DIAMETER : 35 • NET_TRAVERSAL_TIME : • 2*NODE_TRAVERSAL_TIME * NET_DIAMETER • NODE_TRAVERSAL_TIME : 40 • PATH_DISCOVERY_TIME : 2 * NET_TRAVERSAL_TIME • RING_TRAVERSAL_TIME (i.e., T2) : • 2*NODE_TRAVERSAL_TIME*(TTL_VALUE+TIMEOUT_BUFFER) • TTL_START (i.e., X) : 1 • TTL_INCREMENT (i.e., Y) : 2 • TTL_THRESHOLD : 7

  37. Covert Channel Performance vs. Mobility • Smaller physical pause time: • More movement • More link breakages • & encounters • More “excuses” for covert transmission T = (1-P)R

  38. Covert Channel Performance vs. Mobility • Higher speed: • Causes link breakage • Facilitates encounter

  39. Covert Channel Performance vs. Network Size • Larger N: • Larger “vocabulary” • Greater distance between CT and CR • More congestion

  40. Covert Channel Performance vs. Transmission Range • Small transmission range: • Covert transmission may be stuck for a long time. • Large transmission range: • Less loss • Less “excuses” for covert transmission • Hence a critical maximizing point of transmission range exists

  41. Covert Channel Performance vs. Traffic Rate • Traffic generates the need for route discovery • Heavy traffic: • Tends to maintain the route • Has less “excuses” for covert transmission • Causes congestion

  42. Covert Channel Performance vs. Expanding Ring Search Technique Transmission rate is suppressed because RREQ to CT may not reach CT Larger probability of loss because RREQ from CT does not reach beyond the ring Hence, the covert channel throughput almost halved

  43. Conclusions • Clearly demonstrated that AODV can be used for covert transmission. • Quality of the covert channel is poor. • ( probability of loss : covers the interval (0,1), • but is usually high 0.4~0.9 • transmission rate : 0.5 ~ 7 bits per second • throughput : 0 ~ 3.5 bits per second) • Yet difficult to detect. • Hence, measures are needed for protection against covert transmission threat.

  44. Future Work • Comparison of network performance and covert channel performance in different environment regimes • Guidelines for secure protocol design • Eliminate the basis for covert channels • e.g., design proactive protocols • Diminish the covert channel throughput • e.g., break the order of reception, use no sequence number • Detect the covert communication • Exploration of other ad-hoc network protocols with respect to potential for covertness • Abstraction to high-level properties • (e.g., distributed, adaptive, etc)

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