Aggregation and Secure Aggregation [Aggre_1] Section 12 Why do we need Aggregation? Sensor networks Event-based Systems Example Query: What is the maximum temperature in area A between 10am and 11am? Redundancy in the event data Individual sensor readings are of limit use Forwarding raw information too expensive Scarce energy Scarce bandwidth Solution Combine the data coming from different sources Eliminate redundancy Minimize the number of transmissions Aggregation: Summary What is Aggregation? One Example of Aggregation - Count Example: consider a query that counts the number of motes in a network of indeterminate size adopted from slides from S. Madden Scenario: Count Goal: Count the number of nodes in the network. Number of children is unknown Sensor # Time 2 13 Scenario: Count Goal: Count the number of nodes in the network. Number of children is unknown Sensor # Time Scenario: Count Goal: Count the number of nodes in the network. Number of children is unknown Sensor # Time Scenario: Count Goal: Count the number of nodes in the network. Number of children is unknown Sensor # Time Scenario: Count Goal: Count the number of nodes in the network. Number of children is unknown Sensor # Time Scenario: Count Goal: Count the number of nodes in the network. Number of children is unknown / Sensor # Time Scenario: Count Goal: Count the number of nodes in the network. Number of children is unknown / / Sensor # Time Count Example A Better Scheme Each leaf node in the tree reports a count of 1 to their parents Interior nodes sum the count of their children, add 1 to it, and report that value to their parent Data Aggregation Process Sensor nodes are organized into a tree hierarchy rooted at the Base Station Non-leaf nodes act as the aggregators Example Aggregation Max, Min Count, Sum Average Median Tiny Aggregation Distribution phase Aggregate queries are pushed down into the network Collection phase Aggregate values are continuously routed up from children to parents Energy Consumption Declarative Queries for Sensor Networks Examples: SELECT nodeid, light FROM sensors WHERE light > 400 EPOCH DURATION 1s 1 EpochNodeidLightTempAccelSound 01455xxx 02389xxx 11422xxx 12405xxx Sensors Time is partitioned into epochs of duration i A single aggregate value is produced to combine the readings of all devices during the epoch Aggregation Queries SELECT roomNo, AVG(sound) FROM sensors GROUP BY roomNo HAVING AVG(sound) > 200 EPOCH DURATION 10s Rooms w/ sound > SELECT AVG(sound) FROM sensors EPOCH DURATION 10s EpochAVG(sound) EpochroomNoAVG(sound) Section 4.1 of TAG Illustration: Aggregation Sensor # Slot # Slot 1 SELECT COUNT(*) FROM sensors Illustration: Aggregation Sensor # Slot # Slot 2 SELECT COUNT(*) FROM sensors Illustration: Aggregation Sensor # Slot # Slot 3 SELECT COUNT(*) FROM sensors Illustration: Aggregation Sensor # Slot # Slot 4 SELECT COUNT(*) FROM sensors Illustration: Aggregation Sensor # Slot # Slot 1 SELECT COUNT(*) FROM sensors Flow Up the tree during an epoch How parents choose the duration of the interval in which they will receive values? Section 7.1 of [Aggre_1] Topology Maintenance and Recovery How to address the unreliable nature of WSNs in TAG? Each node maintains a fixed size of its neighbors Select a better parent node If a node does not hear from its parent for some time, it assumes that its parent has failed Secure Aggregation It is challenging to design suitable security mechanisms for Wireless Sensor Networks (WSNs) Stringent resource constraints on energy, processing power, memory, bandwidth, etc. WSNs need lightweight secure mechanisms We introduce an LCG-based secure aggregation scheme Efficiency and simplicity Security Goals Confidentiality Sensor data/readings cannot be disclosed to attackers Integrity If an adversary modifies a data message, the receiver should be able to detect this tampering Authenticity Ensures that data messages come from the intended sender Assumptions The existence of a key management scheme WSN nodes can negotiate the key and trust setup LCG-based Security Protocols Basic Hop by Hop Message Transmission Notations A, B, C: Sensor Nodes E(P, K): Encryption of plaintext message P using key K P 1 |P 2 : Concatenation of message P 1 and P 2 MAC(K, P): Message Authentication Code (MAC) of message P using key K X 0 : seed of the LCG a, b, m: Parameters of the LCG Integrity and Authenticity CBC: Cipher Block Chaining