1. SmartGossip: A Reliable Broadcast Service for Wireless Sensor Networks
Romit Roy Choudhury
Dept. of ECE, Duke University
Joint work with
Pradeep Kyasanur (Google)
Indranil Gupta (UIUC)

2. Problem Broadcast in Sensor Networks A widely used service
Network layer functions heavily depend on it
Examples:
Directed Diffusion
Unicast or multicast routing
Instruction / code dissemination
Query propagation

3. Approaches Several approaches evolved over time Complex algo Broadcast
Storm
Single point
of failures
Load
Balance
Dominating sets
Flooding
Multi-point
relays
Gossiping

4. Recent Past Gossiping = Probabilistic flooding
Nodes forward with probability, p
Source

5. Recent Past Gossip based broadcast Nodes forward with probability, p
Tails
Source
Heads

6. Recent Past Gossip based broadcast Nodes forward with probability, p
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7. Recent Past Gossip based broadcast Nodes forward with probability, p
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8. Recent Past Gossip based broadcast Nodes forward with probability, p
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9. Recent Past Gossip based broadcast Nodes forward with probability, p
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10. Recent Past Gossip based broadcast Nodes forward with probability, p
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11. Recent Past Gossip based broadcast Nodes forward with probability, p
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12. Recent Past Gossip based broadcast Nodes forward with probability, p
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13. Recent Past Gossip based broadcast Nodes forward with probability, p
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14. Recent Past Gossip based broadcast Nodes forward with probability, p
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For carefully chosen ‘p’
the message reaches all nodes
in minimal transmissions
1. Simple,
2. Fault tolerant
3. Load-balanced
Tails

15. We Ask … Given some topology deployment
How do we choose a suitable value of “p” ?
One option is to simulate
the gossip offline,
and determine “p”
But, for this example,
simulation result will be
p = 1

16. We Ask … Given some topology deployment
How do we choose a suitable value of “p” ?
Even if topology is homogeneous
It may change over time due to failure and mobility

17. We Ask … Given some topology deployment
How do we choose a suitable value of “p” ?
Even if topology is homogeneous
It may change over time due to failure and mobility
Say computed p = 0.85

18. We Ask … Given some topology deployment
How do we choose a suitable value of “p” ?
Even if topology is homogeneous
It may change over time due to failure and mobility
Fails
Say computed p = 0.85

19. We Ask … Given some topology deployment
How do we choose a suitable value of “p” ?
Even if topology is homogeneous
It may change over time due to failure and mobility
15% of packets
will not reach
these nodes
Say computed p = 0.85

20. We Ask … Given some topology deployment
How do we choose a suitable value of “p” ?
Even if topology is homogeneous
It may change over time due to failure and mobility
Finally, what if topology is not known a priori ?
How can you choose “p” ?

21. We Argue … A broadcast service necessary
that customizes itself to the underlying topology
and
repairs itself with failures and mobility

22. Smart Gossip Intuition
Identify which of YOUR friends get to know gossips earlier than you do
Request those friends to gossip more
Friends who get to know gossips later than you will request you to gossip more
You choose your gossip probability as:
MAX value of all requests from YOUR friends

23. For Example … When H spreads a gossip F gets gossip only from G
F asks G to always gossip
Thus, pG = 1.0
B receives gossip from A,C,D,E,F
B also observes that A,C,D,E received gossip from F
Indicates that B must depend only on F (parent)
A,C,D,E and B are independent (siblings)
B asks F to always gossip, thus pF = 1.0

24. For Example … B asks F to always gossip, thus pF = 1.0
B does not require siblings
A,C,D,E to gossip at all
Thus pA = 0, pC = 0, pD = 0, pE = 0
Observe that only 2 transmissions
(from G and F) are sufficient for broadcast

25. Protocol Details For first gossip pkt, nodes transmit with p=1
Enables nodes to deduce neighbor dependences
Transmitters piggyback pkt with parent-id from which it received the pkt
Nodes record transmitter-id, and its parent-id, and deduce parent, child, sibling relationships …
… see next slide

26. Deducing RelationshipsSA SA SA
Parent = {A}
Child = {A} S A B C E

27. Deducing Relationships
Parent = {A}
Child = {A} S A B C
Sibling = {B}
Child = {B}
Parent = {B} AB AB AB E

28. Deducing Relationships
Parent = {A}
Child = {A} S A B C
Sibling = {B}
Child = {B}
Parent = {B} AE AE AE E

29. Deducing Relationships
Sibling = {E}
Parent = {A}
Child = {A} S A B C
Sibling = {B}
Child = {B,E}
Parent = {B,E} E

30. Choosing Probabilities
Each node calculates number of parents ( k )
Assume 99% assurance necessary for gossip
Node suggests each parent to gossip using ‘p’:
0.99 = ( 1 – (1 - p)k )
Each node receives multiple requests of ‘p’
Uses Max { pi } as its own gossip probability S A B C
Parent={B,E} E

31. Choosing Probabilities
Each node calculates number of parents ( k )
Assume 99% assurance necessary for gossip
Node suggests each parent to gossip using ‘p’:
0.99 = ( 1 – (1 - p)k )
Each node receives multiple requests of ‘p’
Uses Max { pi } as its own gossip probability
p = 1.0
p = 1.0
p = 0.9 S A B C
p = 0.9
p = 1.0 E

32. Choosing Probabilities
Each node calculates number of parents ( k )
Assume 99% assurance necessary for gossip
Node suggests each parent to gossip using ‘p’:
0.99 = ( 1 – (1 - p)k )
Each node receives multiple requests of ‘p’
Uses Max { pi } as its own gossip probability
p = 1.0
p = 1.0
p = 0.9
p = 0 S A B C E
p = 0.9

33. The Bigger Picture
Src

34. Reliability (Details in paper)
Node Failures
Node failures affect broadcast
Source node flags packet periodically (p=1)
Allows for updating dependences
Link Losses
Node requests upstream nodes to retransmit
We require each node to buffer few packets
Children overhear this request
Children do not request retransmissions themselves

35. Evaluation Qualnet Simulator, ver 3.7 Metrics used
Average Reception Percentage
Average Forwarding Percentage
Resilience to link/node failures

36. Percolation
Smart Gossip
Adaptive Overhead
Adaptive Neighbor

37. Forwarding Overhead

38. Adaption to Node Failures
Nodes gossip more to compensate for other failing nodes

39. Conclusion Broadcast is an important problem Smart Gossip Results
Gossip is good – but not practical for sensor nets
Need to adapt gossip based on topology / failures
Smart Gossip
Form dependence graphs using distributed protocol
Dependence relations suggest suitable probability
Results
Overheads are low, and yet good percolation
Robust to node and link failures

40. Questions?

41. Protocol Details For first gossip pkt, nodes forward with p=1
This is to let nodes deduce topology dependences
When forwarding, node A piggybacks the packet with parent-id from which it received the gossip
If parent-id of received packet is a neighbor of B S AS A A S A B C

42. Percolation
Smart Gossip
Adaptive Overhead
Adaptive Neighbor

43. Deducing Relationships
Assume gossip sent by node i to node j
If parent (i)  Neighbor (j)
Parent ( j )  i SA SA SA
Parent = {A}
Child = {A} S A B C E

44. Deducing Relationships
Assume gossip sent by node i to node j
If parent (i)  Neighbor (j)
Parent ( j )  i
If parent (i)  Neighbor (j)
If parent (i)  Parent (j), then Sibling ( j )  i
If parent (i)  Sibling (j), then Children ( j )  i
If parent (i)  Children (j), then Children ( j )  i
Parent = {A}
Child = {A} S A B C
Sibling = {B}
Child = {B}
Parent = {B} AB AB AB E

45. Deducing Relationships
Assume gossip sent by node i to node j
If parent (i)  Neighbor (j)
Parent ( j )  i
If parent (i)  Neighbor (j)
If parent (i)  Parent (j), then Sibling ( j )  i
If parent (i)  Sibling (j), then Children ( j )  i
If parent (i)  Children (j), then Children ( j )  i
Parent = {A}
Child = {A} S A B C
Sibling = {B}
Child = {B}
Parent = {B} AE AE AE E

46. Deducing Relationships
Assume gossip sent by node i to node j
If parent (i)  Neighbor (j)
Parent ( j )  i
If parent (i)  Neighbor (j)
If parent (i)  Parent (j), then Sibling ( j )  i
If parent (i)  Sibling (j), then Children ( j )  i
If parent (i)  Children (j), then Children ( j )  i
Sibling = {E}
Parent = {A}
Child = {A} S A B C
Sibling = {B}
Child = {B,E}
Parent = {B,E} E

47. Robustness Nodes/Links fail with time
Topology changes
Gossip probabilities need to be revised
Gossip source periodically sets flag in packet
All nodes use p=1 for flagged packets

48. Wireless Losses Resilience toward wireless losses necessary
If F does not get a packet, all its dependents will also not get it
SmartGossip:
F requests its parents for missing pkt (seq # j)
F piggybacks { j } in following gossip packets
Nodes A,B,C,D,E do not request for packet j
They know that F is trying to retrieve it