1. Anonymous Gossip: Improving Multicast Reliability in Mobile Ad-Hoc Networks
Ranveer Chandra , Kenneth P. Birman Department of Computer Science
Cornell University
ICDCS 2002
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2. Outline Abstract Introduction MAODV The Gossip Protocol Simulation
Conclusion
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3. Abstract: Propose a reliable multicast protocol
Improve packet delivery and decease the variation of packets received by different nodes
Two Phase Procedure
Send message using any Multicast protocol
Use Anonymous Gossip to ensure the most member receive the packets
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4. Introduction: What is an Ad-Hoc Network?
Wireless medium
Mobile nodes
No fixed infrastructure for communication
Applications: warfront
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5. Introduction: The Model
A graph with ‘n’ nodes
A node can move in any direction with any speed
Connectivity is based on transmission power and land features
Frequently changing connectivity and neighborhood of the nodes.
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6. Introduction: Salient Features
Dynamic Topologies
Bandwidth Constrained Links
Energy Constrained Operation
Limited Security
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7. Multicast Routing Protocols
Tree based: MAODV, AMRIS
Multicast performed over an underlying tree structure.
Mesh based: ODMRP, MCEDAR
Multicast over a mesh, or presence of alternate paths
None of them try to recover packets lost during reconfiguration
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8. MAODV: (Multicast Ad Hoc On-demand Distance Vector):
Group Member
Tree Member
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9. MAODV: Operation Route Reply Route Discovery Link breakages
Partitioned Trees
Leaving a group
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10. Desired Properties Improved delivery rate
Reduced variation in the number of packets received by the group members.
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11. A Modified Protocol Proceed as a combination of 2 sub-protocols
Any existing protocol is used to multicast messages (MAODV in our case)
The Gossip Protocol recovers lost messages.
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12. The Gossip Protocol A node randomly selects a group member.
Sends a message history
The receiver checks to see if it has any extra messages
The nodes then exchange messages and recover the ones that are lost.
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13. Gossip Protocol: Issues
How does a node know the group membership?
With whom does it gossip?
What is the direction of information exchange?
How is message history maintained?
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14. Group Membership Maintaining group membership:
Wired Networks: Easy because of domain sub-domain hierarchy
Ad-Hoc Networks: Very expensive to maintain information about all the group members.
Is it necessary to know the other group members?
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15. Anonymous Gossip
Randomly select one of the neighbors in the multicast tree.
Construct a gossip message and send it along the selected node.
On receiving a gossip message either forward it along the outgoing links or accept it with some probability if it is a group member.
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16. Anonymous Gossip C B D E A
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17. Ensuring Locality of Gossip
Gossiping with a near member
Ensures reduced traffic
Gossiping with a distant node
Able to recover messages that were lost in an entire locality.
Gossip locally with a very high probability and
occasionally with distant nodes
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18. Ensuring Locality of Gossip
Each node maintains a field called ‘nearest_member’
Has the information of the nearest member by taking the link along the next hop node.
The probability of taking the next hop is inversely proportional to the ‘nearest_member’ value.
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19. Example H 2 C 3 2 2 B 3 G D 1 1 2 2 E A F
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20. Probability Function k0 12 k0 k2 . 1 1 kN 5
Smaller nearst_numbrer value is chossen with higher probability
Larger nearst_numbrer value is chossen with lower probability
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21. Tree Overloading
All gossip messages are sent along the multicast tree:
Extra traffic on these links makes the tree congested
Shorter routes may exist
During tree repair no gossip messages can be sent
Cached Gossip with some probability!!!
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22. Cached Gossip Maintain a member cache:
Add a member on receiving a reply of an anonymous gossip request.
Delete a member if no route to it is known or it does not reply to a certain number of gossip requests.
Each entry is a three tuple of the address, distance and ‘last_gossip_time’ to the node.
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23. Sending Gossip Requests
In each gossip round:
Use anonymous gossip with some probability.
If cached gossip is chosen:
Select near members with a very high probability.
Among them select the one with the least ‘last_gossip_time’.
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24. Cached Gossip C B D E A
(E, 2)
(C, 2)
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25. Other Data Structures Data Structures at each group member:
History Table: A bounded FIFO buffer of received messages.
Lost Table: Fixed size buffer to store sequence numbers of lost messages.
Lost Buffer: The most recent entries of the Lost Table.
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26. Data Structures Example: History Table: Msg1, Msg5, Msg7, … Msgn
Lost Table: …….
Lost Buffer: 2 3
Expected Sequence Number: n+1
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27. Gossip Request Message
Group Address
Source Address
Lost Buffer
Size of Lost Buffer
Expected Sequence Number
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28. The Protocol
Each group member periodically sends a gossip request message
On receiving a gossip request the receiver checks to see if it has a copy of the requested messages.
It then unicasts any messages found back to the requester.
* Push would be expensive
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29. The Protocol Gossip Reply Msgs 6, 3 Gossip Request B A
History table: msg1, msg4, msg5
History table: msg1… msg6
Lost Table: 2, 3
Lost Table:
Lost Buffer: 3
Lost Buffer:
Expected Sequence Number: 6
Expected Sequence Number: 7
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30. Simulation Results Used GloMoSim
AG is implemented over MAODV and improvement is measured.
200m x 200m area
40 nodes and 13 group members
Random waypoint mobility model
One node sent 2201 packets to the multicast group over 440 seconds.
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31. Packet Delivery vs Transmission Range
max speed = 0.2m/sec
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32. Packet Delivery vs Transmission Range
max speed = 2.0 m/sec
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33. Packet Delivery vs Transmission Range
max speed = 0.2m/sec
max speed = 2.0 m/sec
Low transmission power => less connectivity and hence reduced performance.
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34. Packet Delivery vs Maximum Speed
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35. Packet Delivery vs Maximum Speed
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36. Packet Delivery vs Maximum Speed
Increased Speed => frequent link breakages and hence reduced performance
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37. Packet Delivery vs Number of Nodes
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38. Results Gossip significantly improves the number of packets delivered.
The variation in the number of packets received by the different group members is reduced
Resulting protocol is more scalable.
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39. Conclusions and Future Work
A more reliable underlying multicast protocol would yield much better results.
Anonymous Gossip can be implemented over any multicast protocol without much overhead.
Gossip for Routing
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40. AARON LEE

41. MAODV(Multicast Ad Hoc On-demand Distance Vector):
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42. MAODV: Route Discovery
Source Node sends RREQ
Sets ‘J’ Flag if joining
Retry for some time if unsuccessful
Become group leader if still unsuccessful
Other nodes set up reverse route entries
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43. MAODV: Route Reply Only tree members can respond to a join request.
RREP is generated and unicast to the source
RREP has address of group member and distance from closest tree member
Intermediate nodes also update their tables on receiving an RREP.
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44. MAODV: Route Activation
Selects route based on seq # and hopcount
Unicasts a MACT to the selected next hop
On receiving MACT, the node updates entry in multicast route table
Unicasts its own MACT if it is not a tree member.
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