Vinay Kumar

B090629CS

OPTIMIZATION IN ANTNET ROUTING ALGORITHM

BACKGROUND

• What is Routing?

Routing is the process of selecting paths in a network to send data packets.

It fully affects the throughput, reliability, and congestion of the network.

• What is an ideal routing algorithm?

An algorithm which can deliver a packet from source to its destination with min. amount of delay.

It must be adaptive and intelligent to make decisions according to circumstances.

BACKGROUND…• Routing algorithms

OSPF, RIP, BGP are some routing algorithms in use.

Not sufficient to tackle the complexity of networks.

Not adaptive, not intelligent and fault intolerant.

INTRODUCTION• Ant is a common insect found in your home.

• Its foraging behavior

Discovers shortest path to a food source.

Shares the info to other ants by stigmergy.

Stigmergy is a form of indirect communication used by ant for problem solving activity.

It drops pheromone through the way it goes. It follows the backtrack path with maximum pheromones. That path is actually the shortest path.

INTRODUCTION…• Ant Colony Optimization [ACO] – A family of optimization algorithms based on real ant

behavior.

• AntNet Routing Algorithm

A routing algorithm based on ACO

A group of artificial ants (agents) create the shortest path between a pair of nodes.

They update the routing tables.

Data packets are directed towards destination using the updated routing table.

DATA STRUCTURE MAINTAINED AT EACH NODE

1. Routing table [Tk] –

It contains the information specifying which is the next node should be taken in order to reach a destination.

Row represents all adjacent nodes to current node.

Column represents all possible destinations in the network.

Table entry is represented by Pnd ,

Where Pnd = goodness of choosing n as next node in order to reach destination d

OR

Pheromone concentration along the link from current node to next node n for destination d

DATA STRUCTURE MAINTAINED AT EACH NODE2. Local traffic statistics

• It follows the traffic fluctuations as seen by current node

• Stored as an array where each entry contains,

µd=estimated mean

σ2=estimated variance

Wd=moving observation window

for a destination node d.

ANTNET ROUTING ALGORITHM• The operation involves two types of agents-

Forward agents [Fs->d] who gather information about the state of network.

Backward agents [Bd->s] who use the collected information from Fs->d to update the routing tables of routers on their path.

ANTNET ROUTING ALGORITHM• Fs->d is launched towards destination d to discover a shortest and least-cost path at

regular time intervals.

• Fs->d maintain a stack which stores –

Identifier of every visited node i

Time elapsed in arriving at node i from originating node

At any node i, Fs->d selects the next adjacent node j by using the probability value,

ANTNET ROUTING ALGORITHM• If Fs->d reaches an already visited node(cycle) then the information of this node is popped

from the stack.

• If cycle is greater than half the Fs->d lifetime then the Fs->d is destroyed.

• When Fs->d reaches the destination node d then a backward agent Bd->s is created.

• Fs->d transfers its stack information to Bd->s and dies.

• Bd->s starts from d towards s following the same path as Fs->d by popping the stack at each node.

• As Bd->s reaches a node ,it updates both the data structure of the node.

UPDATING LOCAL TRAFFIC STATISTICS• The estimated mean and variance are updated as follows-

Where, O i->d is the observed ant’s trip time from node i to destination d

η weighs the number of most recent samples that will really affect average

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))(( 2222ididdiidid o

UPDATING ROUTING TABLE• If Bd->s arrives at a node i from a node f ,then the entry in the routing table is updated by

the formula-

Where r is reinforcement value depending on tables of node i, Ti and Mi

• Simultaneously, the pheromone value for nodes other than f is decreased so that total sum of probability still comes 1.

• As soon as Bd->s finishes, the data packet can be sent into the network according to updates routing tables..

)1( ifdifdifd PrPP

fnrPPP indindind ,

LIMITATIONS• Stagnation is the major problem in antnet algorithm. If a link z keeps good condition for a

long time, then its probability value will be high thus leading to more and more packets to be sent via link z. It reduces the probability of other links.

• Stagnation is very critical problem because-

Link z may lose its optimality.

If network gets failed, then link z may become unavailable.

Other long paths may become optimal which otherwise should not.

OPTIMIZATIONOptimization operation is performed on Routing table -

• The column wise values in the table are picked up and a sorting algorithm is executed on these values.

• The sorted values ranging from higher to lower are stored in a temporary array.

• The difference d, amongst the adjacent values is calculated and is compared to some

threshold value say pm.

• If the difference d is less than pm then those values are selected and comparison amongst

the adjacent values is continued until difference is greater than pm.

• Otherwise at the very first occurrence of difference greater than pm , the comparison is

stopped and the corresponding values in the array are selected.

• The interfaces corresponding to these values are stored in a new routing table which will

have the same structure as the original one, but obviously, the new table will have less

number of rows.

WORKING

WORKING…

E G F A

A .25 .39 .8 .34

C .45 .32 0 .34

D .17 .11 .1 .20

F .13 .28 .1 .12

Ist column Assumption: pm =0.1 & Node B Routing table shown

.45 .25 .17 .13

{.45} is selected

Destination nodes

Next

nod

es

WORKING

E G F A

A .25 .39 .8 .34

C .45 .32 0 .34

D .17 .11 .1 .20

F .13 .28 .1 .12

IInd column

.39 .32 .28 .11

{.39,.32,.28} is selected

Destination nodes

Next

nod

es

WORKING

E G F A

A .25 .39 .8 .34

C .45 .32 0 .34

D .17 .11 .1 .20

F .13 .28 .1 .12

IIIrd column

.8 .1 .1 0

{.8} is selected

Destination nodes

Next

nod

es

WORKING

E G F A

A .25 .39 .8 .34

C .45 .32 0 .34

D .17 .11 .1 .20

F .13 .28 .1 .12

IVth column

.34 .34 .20 0.12

{.34,.34} is selected

Destination nodes

Next

nod

es

WORKING

E G F A

A .39 .8 .34

C .45 .32 .34

F .28

Result- Thus, more than one optimal paths if exist are identified.

Next

nod

es

CONCLUSIONS• We find more than one optimal outgoing path.

• It can explore new and better paths even if network topologies change frequently.

• The problem of stagnation is resolved.

• The throughput of network is improved.

REFERENCES[1] Shuchita Upadhyaya and Richa Setiya , “Identifying multiple optimal paths in Antnet Routing Algorithm with negligible overhead,” IJCSNS International Journal of Computer Science and Network Security, VOL.9 No.2, February 2009

[2] F. Tekiner, F. Z. Ghassemlooy and S. Al-khayatt, “The Antnet Routing Algorithm – A

Modified Version”

[3] soe.northumbria.ac.uk

Thanks …. Any Questions?