This document discusses election algorithms for selecting a coordinator process in shared resource systems, particularly in mobile ad hoc networks where traditional assumptions about message reliability and network topology do not hold. It examines a proposed solution that handles node failures and network partitioning, ensuring the election of the best leader based on node values and unique identifiers. The algorithm operates by establishing a dynamic spanning tree and communicating election messages to determine and broadcast the most eligible leader.

1. A G A L Danushka
South Eastern University of Sri Lanka

2. If we are using one process as a coordinator for a shared
resource …
…how do we select that one process?
Solution – an Election
Election Algorithms
• All processes currently involved get together to
choose a coordinator
• If the coordinator crashes or becomes isolated,
elect a new coordinator
• If a previously crashed or isolated process, comes
on line, a new election may have to be held
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3.  Wired systems
• Bully algorithm
• Ring algorithm
 Wireless systems
 Very large-scale systems
Election Algorithms
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4. Traditional election algorithms are generally based on
assumptions that are not realistic in wireless
environments.
PROBLEM IN TRADITIONAL ALGORITHMS
For example, they assume that message
passing is reliable and that the topology of
the network does not change. These
assumptions are false in most wireless
environments, especially those for mobile
ad hoc networks.
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5. Vasudevan et al. (2004) propose a solution that can
handle failing nodes and partitioning networks. An
important property of their solution is that the best leader
can be elected rather than just a random.
SOLUTION
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6. • When modeling an ad hoc environment as an undirected graph
that changes over time as nodes move.
• The vertices in the graph correspond to mobile nodes and an
edge between a pair of nodes represents the fact that the two
nodes are within each other's transmission and, hence, can
directly communicate with one another.
IN WIRELESS ENVIRONMENT
• The graph can become disconnected if the
network is partitioned due to node
movement.
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7. Ensure that after a finite number of topology
changes, eventually each node i has a leader which
is the most-valued-node from among all nodes in
the connected component to which i belongs.
OBJECTIVE
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8. 1) Node Value: Each node has a value associated with it. The value of a node indicates
desirability as a leader of the network and can be any performance related attribute such
as the node's battery power, computational capabilities etc.
2) Unique and Ordered Node IDs: All nodes have unique identifiers. They are used to
identify participants during the election process. Node IDs are used to break ties among
nodes which have the same value.
3) Links: Links are bidirectional and FIFO, i.e. messages are delivered in order over a link
between two neighbors.
4) Node Behavior: Node mobility may result in arbitrary topology changes including
network partitioning and merging. Furthermore, nodes can crash arbitrarily at any
time and can come back up again at any time.
ASSUMPTION
5) Node-to-Node Communications: A message delivery is
guaranteed only when the sender and the receiver remain
connected (not partitioned) for the entire duration
of message transfer.
6) Buffer Size: Each node has a sufficiently large receive buffer
to avoid buffer overflow at any point in its lifetime.
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9. Under the assumption that nodes and links never fail. The algorithm
operates by first growing and then shrinking a spanning tree rooted
at the node that initiates the election algorithm.
We refer to this computation-initiating node as the source node. As
we will see, after the spanning tree shrinks completely, the source
node will have adequate information to determine the most-valued-
node and will then broadcast its identity to the rest of the nodes in
the network
Selecting Best Leader
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10. Messages Used in Algorithm
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11. Variables Used in Algorithm
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12. node A is a source node that starts a
diffusing computation by sending out
Election messages (denoted as “E” in the
figure) to its immediate neighbors, viz.
nodes B and C, shown in Figure
Example
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13. Example Cont..
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• As indicated in Figure ,nodes B and C set
their parent pointers to point to node A
and in turn propagate an Election
message to all their neighbors except
their parent nodes.
• Hence B and C send Election messages
to one another. These Election messages
are immediately acknowledged since
nodes B and C have already received
Election messages from their respective
parents.

14. Example
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• In Figure c , a complete spanning tree is built.
• In Figure d, the spanning tree starts shrinking as
nodes D and F send their pending Ack messages
(denoted by “A”)
• to their respective parent nodes in the spanning
tree. Each of these Ack messages contains the
identity of the most-valued node (and its actual
value) downstream to nodes D and F, in this case
the nodes themselves, since they are the leaves
of the tree.
• Eventually, the source A hears pending
acknowledgments from both B and C in Figure e
and then broadcasts the identity of the leader, D,
via the Leader message (denoted by “L” shown in
Figure f.)

15. Bootstrapping the Election Process
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• Each node starts execution by initializing the different
variables of the leader election algorithm.
• After the initialization, the algorithm in each node loops
forever, and on each iteration, checks if any of the
actions in the algorithm specification are enabled,
executing at least one enabled action on every loop
iteration.

16. Handling Multiple, Concurrent Computations
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The leader of a connected component periodically
sends heartbeat messages to other nodes. The absence
of a heartbeat message from its leader for a predefined
timeout period triggers a fresh leader election process
at a node.

17. Handling Node Partitions
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Once node i joins an election, it must receive Ack messages from
all nodes in list Si before it can report an Ack message to its
parent node.
However, because of node mobility, it may happen that node j,
which has yet to report an Ack message, gets disconnected from
nodei.
Node i must detect this event, since otherwise it will never
report an Ack message to its parent and, therefore, no leader
will be announced.

18. Handling Partition Merges
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Involves two connected components, each with a unique leader,
merging together by the formation of a new link between nodes
Nodes then exchange their leader identities over the newly
formed link. Since one node has a higher-identity-leader than
another, then broadcasts the new leader to the rest of the
nodes in its component.

19. Summary
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• At each stage the “most eligible” or “best” node will be passed
along from child to parent.
• Once the source node has received all the replies, it is in
position to choose the new coordinator.
• When the selection is made, it is broadcast to all nodes in the
network. • If more than one election is called (multiple
source nodes), a node should participate in only
one.
• Election messages are tagged with a process
id.
• If a node has chosen a parent but gets an
election message from a higher numbered node,
it drops out of the current election and adopts
the high numbered node as its parent. This
ensures only one election makes a choice.

20. References
• Design and Analysis of a Leader Election Algorithm for Mobile Ad
Hoc Networks by Sudarshan Vasudevan, Jim Kurose, Don Towsley
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21. Thank You
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