How to Implement Link State Routing in NS2

To implement the Link State Routing in NS2 has includes simulating a routing protocol in which every router has comprehensive knowledge of the network topology. This router structures a map of the whole network and uses this data to establish the shortest path to each node. Generally known instance of a link state routing protocol is OSPF (Open Shortest Path First). Whereas the simulator NS2 does not have a built-in OSPF protocol, we can execute a simple link state routing behaviour by changing the routing logic or simulating the link state updates using NS2’s existing framework.

Steps to Implement Link State Routing in NS2

Set Up NS2 Environment
- Make sure that NS2 is installed and appropriately set up on the system.
Understand Link State Routing
- Link State Routing has encompasses every router maintaining a view of the whole network topology. This routers interchange link state advertisements (LSAs) to inform each other of the state of their connections. Each router uses these data to make a topology map and then runs the Dijkstra’s algorithm that helps to discover the shortest paths to all ends.
Simulate Link State Routing in NS2
- Whereas complete OSPF execution could need important coding in C++, we can mimic the needed aspects of link state routing using OTcl scripting.

Implementing a Basic Link State Routing Simulation

Given below is a basic method using OTcl scripting to replicate the behaviour of link state routing:

Define the Network Topology:

# Define the simulation environment

set ns [new Simulator]

# Create nodes

set node1 [$ns node]

set node2 [$ns node]

set node3 [$ns node]

set node4 [$ns node]

set node5 [$ns node]

# Create links between nodes with different delays and bandwidths

$ns duplex-link $node1 $node2 10Mb 10ms DropTail

$ns duplex-link $node2 $node3 10Mb 10ms DropTail

$ns duplex-link $node3 $node4 10Mb 10ms DropTail

$ns duplex-link $node4 $node5 10Mb 10ms DropTail

$ns duplex-link $node1 $node5 5Mb 50ms DropTail

$ns duplex-link $node2 $node4 5Mb 30ms DropTail

Implement Link State Updates:

To simulate a simple link state routing mechanism, we can manually configure the routes according to the precomputed shortest path.

# Manual routing to simulate link state updates

$ns rtproto LS ;# Simulate Link State Routing protocol

# Assume node1 knows the full topology and updates its routes

$node1 add-route $node2 1

$node1 add-route $node3 2

$node1 add-route $node4 2

$node1 add-route $node5 1 ;# Chooses direct link as shortest path despite higher delay

# Assume node2 updates its routes

$node2 add-route $node1 1

$node2 add-route $node3 1

$node2 add-route $node4 1

$node2 add-route $node5 2 ;# Through node4

# Similar updates for other nodes based on full topology knowledge

$node3 add-route $node1 2

$node3 add-route $node2 1

$node3 add-route $node4 1

$node3 add-route $node5 2 ;# Through node4

$node4 add-route $node1 2

$node4 add-route $node2 1

$node4 add-route $node3 1

$node4 add-route $node5 1

$node5 add-route $node1 1 ;# Direct link

$node5 add-route $node2 2 ;# Through node4

$node5 add-route $node3 2 ;# Through node4

$node5 add-route $node4 1

Simulate Traffic to Test the Routing Protocol:

# Attach TCP agents and applications to test routing

set tcp1 [new Agent/TCP]

$ns attach-agent $node1 $tcp1

set sink1 [new Agent/TCPSink]

$ns attach-agent $node5 $sink1

$ns connect $tcp1 $sink1

set ftp1 [new Application/FTP]

$ftp1 attach-agent $tcp1

# Start the FTP application

$ns at 1.0 “$ftp1 start”

# Add another traffic source to test multi-path routing

set udp1 [new Agent/UDP]

$ns attach-agent $node3 $udp1

set sink2 [new Agent/Null]

$ns attach-agent $node4 $sink2

$ns connect $udp1 $sink2

set cbr1 [new Application/Traffic/CBR]

$cbr1 attach-agent $udp1

$cbr1 set packetSize_ 512

$cbr1 set rate_ 1Mb

# Start the CBR application

$ns at 2.0 “$cbr1 start”

$ns at 8.0 “$cbr1 stop”

# Enable tracing

set tracefile [open out.tr w]

$ns trace-all $tracefile

# Define the finish procedure

proc finish {} {

global ns tracefile

$ns flush-trace

close $tracefile

exec nam out.nam &

exit 0

}

# Schedule the simulation end

$ns at 10.0 “finish”

# Run the simulation

$ns run

Explanation of the Script:

Network Topology: The script describes a network including five nodes, intersected with links of changing the bandwidth and latency, to make several potential paths.
Routing Table Updates: Manually mimic the routing table updates using add-route command that would happen in a link state routing protocol after running Dijkstra’s algorithm.
Traffic Generation: The script mimics both TCP and UDP traffic to analyse the routing tables and check that the shortest paths are being used.
Simulation and Trace Analysis: The trace file (out.tr) records the events, permitting to evaluate the packet forwarding and check correct route selections.

Running and Analysing the Simulation:

We can save the script to a file like link_state_routing.tcl.
Run the script using NS2:

ns link_state_routing.tcl

Evaluate the trace file to verify if the packets follow the correct routes based on the link state routing updates.

Enhancements:

Dynamic Topology Updates: Execute the link failures and recoveries to mimic dynamic updates and reconvergence of the network.
Multiple Traffic Flows: Append more traffic flows to examine the network under various load conditions.
Full OSPF Simulation: For a comprehensive OSPF simulation, we could want to write C++ code to manage LSAs, topology database management, and Dijkstra’s algorithm that is beyond the scope of a basic OTcl script.

Therefore, we conducted a detailed procedure on Link State Routing with implementation and analysis carried out through the simulation tool ns2. If you need more concepts and execution procedure we will be offered.

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