Geographic Hash Table

1
Geographic Hash Table

• S. Ratnasamy, B. Karp, S. Shenker, D. Estrin, R.
  Govindan, L. Yin and F. Yu

2
Overview

• Data Centric Storage
• Data is stored inside the network each name
  corresponds to a location in space
• All data with the same name will be stored at the
  same sensor network location
• E.g an elephant sighting
• Why Data centric Storage?
• Energy efficiency
• Robustness against mobility and node failures
• Scalability

3
Keywords and Terminology

• Observation
• ? low-level readings from sensors
• ? e.g. Detailed temperature readings
• Events
• ? Predefined constellations of low-level
  observations
• ? e.g. temperature greater than 75 F
• Queries
• ?Used to elicit information from sensor network

4
Performance MetricTotal Usage /Hotspot Usage

• Use communication as a cost function for energy
  consumption
• Total Usage
• Total number of packets sent in the
  Sensor network
• Hotspot Usage
• The maximal number of packets send by a
  particular sensor node
• Costs used in the evaluation
• Message flooding cost O(n)
• Point-to-point routing cost
• n is the number of nodes

5
Alternative Storage Schemes

• External Storage (ES)
• Events propagated and stored at an external
  location
• Local Storage (LS)
• Events stored locally at the detecting node
• Queries are flooded to all nodes and the events
  are sent back
• Data Centric Storage (DCS)
• Data for an event stored within the sensor
  network
• Queries are directed to the node that stores the
  data

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External Storage (ES)
External storage
event
7
Local Storage (LS)
Queries flooded at all the nodes
event
event
8
Why do we need DCS?

• Scalability
• Robustness against Node failures and Node
  mobility
• To achieve Energy-efficiency

9
Design Criterial Scalability Robustness

• Node failures
• Topology changes
• System scale to large number of nodes
• Energy Constraints
• Persistence
• (k,v) pair must remain available to queries,
  despite sensor node failures and changes in
  sensor network topology
• Consistency
• A query k must be routed correctly to a node
  where (k,v) pairs are stored if these node
  change, then they should do this consisently
• Scaling in Database Size
• Topological generality system should scale well
  on a large number of topologies

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Assumptions in DCS

• Large Scale networks whose approximate
  geographic boundaries are known
• Nodes have short range communication and are
  within the radio range of several other nodes
• Nodes know their own locations by GPS or some
  localization scheme
• Communication to the outside world takes place by
  one or more access points

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Data Centric Storage

• Relevant Data are stored by name at nodes
  within the Sensor network
• All data with the same general name will be
  stored at the same sensor-net node.
• e.g. (elephant sightings)
• Queries for data with a particular name are then
  sent directly to the node storing those named
  data

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Data centric Storage
Elephant Sighting
sourcelass.cs.umass.edu
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Geographic Hash Table

• Events are named with keys and both the storage
  and the retrieval are performed using keys
• GHT provides (key, value) based associative memory

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Geographic Hash Table Operations

• GHT supports two operations
• ? Put(k,v)-stores v (observed data) according
  to the key k
• ? Get(k)-retrieve whatever value is
  associated with key k
• Hash function
• ? Hash the key in to the geographic
  coordinates
• ? Put() and Get() operations on the same
  key k hash k to the same location

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Storing Data in GHT
Put (elephant, data)
(12,24)
Hash (elephant)(12,24)
sourcelass.cs.umass.edu
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Retrieving data in GHT
(12,24)
Hash (elephant)(12,24)
Get (elephant)
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Geographic Hash Table
Node A
Node B
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Algorithms Used By GHT

• Geographic hash Table uses GPSR for
  Routing(Greedy Perimeter Stateless Routing)
• PEER-TO-PEER look up system
• (data object is associated with key and each
  node in the system is responsible for storing a
  certain range of keys)

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Algorithm (Contd)

• GPSR- Packets are marked with position of
  destinations and each node is aware of its
  position
• Greedy forwarding algorithm
• Perimeter forwarding algorithm

B
B
A
A
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GPSR Right-Hand Rule In Perimeter Forwarding
2
x
z
3
1
y
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Home Node and Home perimeter

• Home node Node geographically nearest to the
  destination coordinates of the packet
• Serves as the rendezvous point for Get() and
  Put() operations on the same key
• In GHT packet is not addressed to specific node
  but only to a specific location
• Use GPSR to find the home node
• only perimeter mode of GPSR to find Home
  Perimeter
• Home Perimeter perimeter that encloses the
  destination
• Start from the home node, and use perimeter mode
  to make a cycle and return to the home node

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Problems

• Robustness could be affected
• Nodes could move (i.d. of Home node?)
• Node failure can Occur
• Deployment of new Nodes
• Not Scalable
• Storage capacity of the home nodes
• Bottleneck at Home nodes

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Solutions to the problems

• Perimeter refresh protocol
• mostly addresses the robustness issue
• Structured Replication
• address the scalability issue
• how to handle storage of many events

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Perimeter refresh protocol

• Replicates stored data for key k at nodes around
  the location to which k hashes
• Stores a copy of the key value pair at each node
  on the home perimeter
• Each node on the perimeter is called a replica
  node
• How do you ensure consistency persistence
• A node becomes the home node if a packet for a
  particular key arrives at that node
• The perimeter refresh protocols periodically
  sends out refresh packets
• After a time period Th generate a refresh packet
  that contains the data for that key
• Packet forwarded on the home perimeter in the
  same way as Get() and Put()
• The refresh packet will take a tour of the home
  perimeter regardless the changes in the network
  topology since the keys insertion
• This property maintains the perimeter

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Perimeter Refresh Protocol

• How do you guard against node failures
• When a replica node receives a packet it did not
  originate, it caches the data in the refresh and
  sets up a takeover timer Tt
• Timer is reset each time a refresh from another
  node arrives
• If the timer expires the replica node initiates a
  refresh packet addressed to the keys hashed
  location
• Note That particular node does not determine a
  new home node. The GHT routing causes the refresh
  to reach a node home node

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Perimeter Refresh Protocol
E
Replica
Assume key k hashes at location L A is closest
to L so it becomes the home node
Replica
D
L
F
A
home
B
C
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Perimeter Refresh Protocol
E
Replica
D
Replica
Suppose the node A dies
L
F
home
C
Replica
B
Replica
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Time Specifications

• Refresh time (Th)
• Take over time (Tt)
• Death time (Td)
• General rule
• TdgtTh and TtgtTh
• In GHT Td3Th and Tt2Th

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Characteristics Of Refresh Packet

• Refresh packet is addressed to the hashed
  location of the key
• Every (Th) secs the home node will generate
  refresh packet
• Refresh packet contains the data stored for the
  key and routed exactly as get() and put()
  operations
• Refresh packet always travels along the home
  perimeter

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Structured Replication

• Too many events are detected then home node will
  become the hotspot of communication.
• Structured replication is used to address the
  scaling problem
• Hierarchical decomposition of the key space
• Event names have a certain hierarchy depth

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Structured Replication
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Structured Replication

• A node that detects a new event, stores that
  event to its closest mirror
• this is easily computable
• This reduces the storage cost, but increases the
  query cost
• GHT has to route the queries to all mirror nodes
• Queries are routes recursively
• First route query to the root, then to the first
  level and then to the second level mirrors
• Structured replication becomes more useful for
  frequently detected events

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Evaluation

• Simulation to test if the protocol is functioning
  correctly
• Done in the ns-2 network simulator using an IEEE
  802.11 mac
• This is a well known event driven simulator for
  ad-hoc networks
• Larger scale simulations for the comparative
  study where done with a custom simulator

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Comparative Study

• Simulation compares the following schemes
• External Storage (ES)
• Local Storage (LS)
• Normal DCS A query returns a separate message
  for each detected event
• Summarized DCS(S-DCS) A query returns a single
  message regardless of the number of detected
  events
• Structured Replication DCS (SR_DCS) Assuming an
  optimal level of SR
• Comparison based on Cost
• Comparison based on Total usage and Hot spot
  usage

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Assumptions in comparison

• Asymptotic costs of O(n) for floods and O( n) for
  point to point routing
• Event locations are distributed randomly
• Event locations are not known in advance
• No more than one query for each event type
• (Q Queries in total)
• Assume access points to be the most heavily used
  area of the sensor network

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Comparison based onHot-spot/Total Usage

• n - Number of nodes
• T - Number of Event types
• Q Number Of Event types queried for
• Dtotal Total number of detected events
• DQ- Number of detected events for queries

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DCS TYPES

• Normal DCS Query returns a separate message for
  each detected event
• Summarized DCS Query returns a single message
  regardless of the number of detected events
• (usually summary is preferred)

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Comparison Study contd..
ES LS DCS
Total
Hot spot
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Observations from the Comparison

• DCS is preferable only in cases where
• Sensor network is Large
• There are many detected events and not all event
  types queried
• Dtotalgtgtmax(Dq,Q)

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Simulations

• To check the Robustness of GHT
• To compare the Storage methods in terms of total
  and hot spot usage

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Simulation Setup

• ns-2
• Node Density 1node/256m2
• Radio Range 40 m
• Number of Nodes -50,100,150,200
• Mobility Rate -0,0.1,1m/s
• Query generation Rate -2qps
• Event types 20
• Events detected -10/type
• Refresh interval -10 s

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Performance metrics

• Availability of data stored to Queriers
• (In terms of success rate)
• Loads placed on the nodes participating in GHT
  (hotspot usage)

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Simulation Results for Robustness

• GHT offers perfect availability of stored events
  in static case
• It offers high availability when nodes are
  subjected to mobility and failures

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Simulation Results under varying Q
Number of nodes is
constant 10000
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Simulation results under varying N
Number of Queries Q 50
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Simulation Results for comparison of 3-storage
methods

• S-DCS have low hot-spot usage under varying Q
• S-DCS is has the lowest hot-spot usage under
  varying n

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Conclusion

• Data centric storage entails naming of data and
  storing data at nodes within the sensor network
• GHT- hashes the key (events) in to geographical
  co-ordinates and stores a key-value pair at the
  sensor node geographically nearest to the hash
• GHT uses Perimeter Refresh Protocol and
  structured replication to enhance robustness and
  scalability
• DCS is useful in large sensor networks and there
  are many detected events but not all event types
  are Queried