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Anonymous Gossip:
Improving Multicast Reliability in
Mobile Ad-Hoc Networks
Ranveer Chandra
(joint work with Venugopalan Ramasubramanian and
Ken Birman)
What is an Ad-Hoc Network?
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Wireless medium
Mobile nodes
No fixed infrastructure for communication
Applications: relief operations, warfront!!
Warfront
BAS
The Model
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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.
The Model
B
C
A
Salient Features
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Dynamic Topologies
Bandwidth Constrained Links
Energy Constrained Operation
Limited Security
A Future Network
Base Station
Fixed Network
Ad Hoc Network Switch
Mobile Host
Wired Link
Wireless Link
Ad Hoc Network
Issues in Multicast Routing
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Minimize state stored in each node
Reduce the number of messages
exchanged
Active adaptability of nodes to mobility,
power
Local effect of link breakages
Multicast Routing Protocols
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Tree based: MAODV, AMRIS
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Mesh based: ODMRP, MCEDAR
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Multicast performed over an underlying
tree structure.
Multicast over a mesh, or presence of
alternate paths
None of them try to recover packets
lost during reconfiguration
MAODV: Route Discovery
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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
Multicast Tree
Group Member
Tree Member
MAODV: Route Reply
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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.
Multicast Tree
Group Member
Tree Member
MAODV: Route Activation
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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.
Multicast Tree
Group Member
Tree Member
MAODV: Other Considerations
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Link breakages
Partitioned Trees
Leaving a group
Details in draft-ietf-manet-aodv-05.txt
Charles Perkins and Elizabeth Royer,
March 2000
Desired Properties
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Improved delivery rate
Reduced variation in the number of
packets received by the group
members.
A Modified Protocol
Borrows ideas from Bimodal Multicast
Proceed as a combination of 2 subprotocols
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Any existing protocol is used to multicast
messages (MAODV in our case)
ii. The Gossip Protocol recovers lost
messages.
i.
The Gossip Protocol
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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.
Gossip Protocol: Issues
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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?
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?
Anonymous Gossip
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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.
Anonymous Gossip
C
B
A
D
E
Ensuring Locality of Gossip
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Gossiping with a near member
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Ensures reduced traffic
Gossiping with a distant node
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Able to recover messages that were lost in
an entire locality.
Gossip locally with a very high probability and
occasionally with distant nodes
Ensuring Locality of Gossip
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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.
Example
2
3
B
G
1
2
A
2
3
H
C
2
D
1
2
F
E
Probability Function
k1
k0
k2
kN
.
.
.
Probability that nbr(i) is chosen as the next hop for
gossip: 1 – ki/(k1+k2+....kN)
Tree Overloading
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All gossip messages are sent along the
multicast tree:
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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!!!
Cached Gossip
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Maintain a member cache:
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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.
Sending Gossip Requests
In each gossip round:
 Use anonymous gossip with some
probability.
 If cached gossip is chosen:
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Select near members with a very high
probability.
Among them select the one with the least
‘last_gossip_time’.
Cached Gossip
C
B
(E, 2) A
(C, 2)
D
E
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.
Data Structures
Example:
History Table: Msg1, Msg5, Msg7, … Msgn
Lost Table: 2 3 4 6 …….
Lost Buffer: 2 3
Expected Sequence Number: n+1
Gossip Request Message
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Group Address
Source Address
Lost Buffer
Size of Lost Buffer
Expected Sequence Number
The Protocol
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*
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
The Protocol
Gossip Reply
Msgs 6, 3
A
Gossip Request
B
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
Simulation Results
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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.
Packet Delivery vs Transmission Range
Low transmission power => less connectivity and hence reduced performance.
Packet Delivery vs Maximum Speed
Increased Speed => frequent link breakages and hence reduced performance
Packet Delivery vs Number of Nodes
Results
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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.
Conclusions and Future Work
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A more reliable underlying multicast
protocol would yield much better
results.
Anonymous Gossip can be implemented
over any multicast protocol without
much overhead.
Is AG well suited for the internet?