Triggering on Area: A Systems Approach

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Triggering on Area A Systems Approach

• Benjamin Greenstein
• David Culler
• Deborah Estrin
• Eddie Kohler
• 11/8/03

2
What are we talking about?

• Detection, Evaluation, Action
• A dataset cut from space/time is determined to
  contain a target phenomenon, so do something

3
Example

• A fire is localized to one square meter in a
  meadow. It is probably a campfire, so do nothing.

4
Example (continued)

• A fire has grown larger locate any sprinklers
  nearby and turn them on. If there are no
  sprinklers, alert a human.

5
Elements of the example

• Homogenous region detection
• Defined by contiguity among nodes in a
  communication graph
• Distributed area calculation
• Actuation (flooded activation, service discovery,
  etc.)

6
Goals

• Compute area of homogenous region, so triggering
  can occur if region satisfies some requirements
• Devise a flexible system, so that components can
  be swapped out to achieve an alternate
  functionality
• Provide user-level configurability via wiring and
  parameterization

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Architecture
Sink
Sampler

CombineCache
CombineStamp

MinRate

SpanningCache
SpanningStamp
InverseNeighbhood

SourceCache
SourceStamp
AM
uses BufferPool
8
Spanning Tree

• Optimal amounts to all pairs shortest paths which
  is O(N3)
• Cannot be solved greedily optimal graph does not
  necessarily contain optimal substructure
• In terms of depth, random is no worse than 2X
  optimal.
• Approximations might help
• Convex hull and center of mass for root
• Best of k random trees
• Implemented a greedy formation algorithm
• If in region, set region id to local address,
  parent to NULL, and hop count to zero, and
  advertise
• If I receive a message with a lower region id or
  same id, but lower hop count than my parent, then
  switch parent pointer to source.
• Version number incremented when parent dies or
  changes to a new region to prevent
  count-to-infinity loops

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Area Calculation

• Delaunay Triangulation
• Inverse Neighborhood
• Inverse Neighborhood w/ Location
• Voronoi Tesselation

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Delaunay Triangulation

• If the circle circumscribed about three vertices
  contains no other vertices in the graph, then it
  is part of the Delaunay Triangulation
• Requires Location Information

Sink
Sampler
CombineCache
CombineStamp

MinRate

SpanningCache
SpanningStamp

DelaunayCache
DelaunayStamp
LocationStamp
LocationCache
SourceCache
SourceStamp

AM
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Triangulation (cont.)

• Area computation
• Assign each triangle to the vertex with the
  lowest ID
• Build a count aggregation tree and weight count
  by the area of the triangles
• Consistent for any ?ABC in the triangulation when
  AB, BC, and AC are either
• Communication links
• Present in the neighborhood of each vertex.

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Accuracy vs. Hop Count
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Inverse Neighborhood

• Guess what a communication area is likely to be
• Weight count operation by that area divided by
  the communication neighbors of a node
• Advantages
• Requires no location information
• Issues
• Communication area guess has a huge impact on the
  result (unless only comparing relative sizes)

r
Q
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Comparison

• Delaunay triangulation tends to underestimate
  area because it yields the tightest polygon
  enclosing the region (even though edge nodes
  might very well represent space beyond that
  polygon)
• Inverse neighborhood can overestimate area,
  because nodes near edge of network have fewer
  neighbors

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Node Involvement Outside Region
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Accuracy vs. Degree of Connectivity
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Ceiling Data
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Systems Contributions

• Minimum Rate Component
• Tag/Value Packet Format
• Memory Layer

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Minimum Rate

• Generate NULL Packets at a minimum rate.
• Check over period to see if any packets came
  through
• If not, generate one.
• Drives soft state mechanisms of the various
  layers
• Think Elevator
• Advantages
• Fewer timers
• Fewer header bytes sent
• Reduced duplication of attributes
• Issues
• Soft state can drown a network, particularly in
  TOSSIM
• Eventual consistency isnt always timely

MinRate
Sink
Sampler
CombineCache
CombineStamp

MinRate

SpanningCache
SpanningStamp

DelaunayCache
DelaunayStamp

LocationStamp
LocationCache
SourceCache
SourceStamp

AM
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Stamping Attributes

• Tag/Value packet format
• Add as many attributes as needed or as can fit in
  data portion of TOS_Msg struct
• Limited to 8 different types of attributes in
  system
• Constant time inserts, tests for inclusion,
  linear reads
• putAttr() and getAttr() provided as inline for
  write and read
• Byte 0 of data portion contains bit field for
  test
• 1c bytes of overhead for c attributes

TOS_Msg msg-gtdata
Tag Field byte
Tag (3 bits)
Length (5 bits)
Value (length as specified)
Tag (3 bits)
Length (5 bits)
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Adding an attribute

• command SendMsgAbove.send(uint16_t address,
  uint8_t length, TOS_MsgPtr msg, uint8_t
  virtual_time)
• if (waitedLongEnough(virtual_time))
• if (putAttr(msg,
• (uint8_t )loc, / pointer
  to the attribute /
• sizeof(loc_attr_t), / size of
  the attribute /
• length, / the
  current length of msg /
• LOCATION_TYPE, / one of
  8 attribute types /
• sizeof(source_attr_t)) / only
  put if also room for this /
• )
• / There was room for this attribute /
• else
• / There was not room for this attribute /
• return (call SendMsgBelow.send(address,length,msg
  ,virtual_time))

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Reading an attribute

• event ReceiveMsgBelow.receive(TOS_MsgPtr msg)
• uint8_t sz
• if (!updatePending)
• if (pmsg-gtdata0 LOCATION_FLAG)
• sz sizeof(location_attr_t)
• if (getAttr(msg, (uint8_t )newLoc, sz,
  LOCATION_TYPE) SUCCESS)
• post updateCache()
• signal ReceiveMsgAbove.receive(msg)
• return msg

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Impact of Aggregation (degree8)
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More on Impact
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Memory Layer

• Sits directly above AM layer
• Buffer Pool
• Buffer allocation and guards handled in memory
  layer instead of in every component
• alloc(), free(), copyAlloc()
• Implicit Deallocation
• When SendMsg.send () fails and immediately after
  upcalls to SendMsg.sendDone() and
  ReceiveMsg.receive()
• Queued Send absorbs bursts (thanks, Phil B.)

Sink
Sampler
CombineCache
CombineStamp

MinRate

SpanningCache
SpanningStamp

DelaunayCache
DelaunayStamp

LocationStamp
LocationCache
SourceCache
SourceStamp

AM
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Allocation

• event result_t Timer.fired()
• virtualTime
• if (!msgReceivedFromAbove)
• if ((msg call BufferPool.alloc()) ! NULL)
• msg-gtdata0 NO_ATTRIBUTES
• return (call SendMsgBelow.send(
• TOS_BCAST_ADDR, 1, msg,
• virtualTime)

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Memory Layer (cont.)

• Advantages
• Dynamic memory allocation (potential for less
  memory usage)
• Sharing of RAM across components without the
  hassles of
• Buffer guards
• Pointer swaps
• Calling free() everywhere
• Issues
• Memory exhaustion
• Memory leaks
• Centralized thing to break (if memory layer
  breaks, then everything breaks)
• Might want greater flexibility than what our
  implicit deallocation provides (e.g., sticky
  alloc())

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Spanning Trees
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Delaunay Regions
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The End