Aggregation

Learning Objectives Understand why we need aggregation in WSNs Understand aggregation protocols in WSNs Understand secure aggregation protocols in WSNs

Prerequisites Basic concepts in network protocols Basic concepts of network security

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 [Aggre_1] Section 1

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 Sensor # 2 1 3 4 5 1 2 3 4 5 - Time Goal: Count the number of nodes in the network. Number of children is unknown. So many redundant message Scenario: Count adopted from slides from S. Madden

Scenario: Count 1 2 3 4 5 Sensor # 1 2 3 Time - 2 3 Time Goal: Count the number of nodes in the network. Number of children is unknown. Scenario: Count

Scenario: Count 1 2 3 4 5 Sensor # 1 2 3 Time 4 - 1 + 2 2 3 Time 4 Goal: Count the number of nodes in the network. Number of children is unknown. Scenario: Count

Scenario: Count 1 2 3 4 5 Sensor # 1 2 3 Time 4 5 - 1 + 2 1 + ½ 2 3 Time 4 5 Goal: Count the number of nodes in the network. Number of children is unknown. Scenario: Count

Scenario: Count 1 2 3 4 5 Sensor # 1 2 3 Time 4 5 - 1 + 2 1 + ½ 1+3 1+ ½ 1+1 2 3 Time 4 5 Goal: Count the number of nodes in the network. Number of children is unknown. Scenario: Count

Scenario: Count 1 2 3 4 5 Sensor # 1 2 3 Time 4 5 - 1 + 2 1 + ½ 1+3 1+ ½ 1+1 1+2/2 Time Goal: Count the number of nodes in the network. Number of children is unknown. Scenario: Count

Scenario: Count 1 2 3 4 5 Sensor # 1 2 3 Time 4 5 - 1 + 2 1 + ½ 1+3 1+ ½ 1+1 1+2/2 1+4 Time Goal: Count the number of nodes in the network. Number of children is unknown. Scenario: Count

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 Collection 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 Sensors Examples: SELECT nodeid, light FROM sensors WHERE light > 400 EPOCH DURATION 1s Epoch Nodeid Light Temp Accel Sound 1 455 x 2 389 422 405 1 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 Epoch AVG(sound) 440 1 445 2 Epoch roomNo 440 1 445 2 SELECT AVG(sound) FROM sensors EPOCH DURATION 10s Epoch roomNo AVG(sound) 1 360 2 520 370 SELECT roomNo, AVG(sound) FROM sensors GROUP BY roomNo HAVING AVG(sound) > 200 EPOCH DURATION 10s Rooms w/ sound > 200 3

Illustration: Aggregation SELECT COUNT(*) FROM sensors Slot 1 1 2 3 4 5 Sensor # 1 2 3 4 5 Slot # 1 Section 4.1 of TAG

Illustration: Aggregation SELECT COUNT(*) FROM sensors Slot 2 1 2 3 4 5 Sensor # 1 2 3 4 5 2 Slot #

Illustration: Aggregation SELECT COUNT(*) FROM sensors Slot 3 1 2 3 4 5 Sensor # 1 2 3 4 5 1 3 Slot #

Illustration: Aggregation SELECT COUNT(*) FROM sensors 5 Slot 4 1 2 3 4 5 Sensor # 1 2 3 4 5 Slot #

Illustration: Aggregation SELECT COUNT(*) FROM sensors Slot 1 1 2 3 4 5 Sensor # 1 2 3 4 5 Slot # 1

Flow Up the tree during an epoch How parents choose the duration of the interval in which they will receive values?

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 Section 7.1 of [Aggre_1]

Secure Aggregation

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 Security Goals Assumptions Confidentiality Integrity 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 P1|P2: Concatenation of message P1 and P2 MAC(K, P): Message Authentication Code (MAC) of message P using key K X0: seed of the LCG a, b, m: Parameters of the LCG

Integrity and Authenticity CBC: Cipher Block Chaining