How to Implement OSPF Routing Algorithm in NS2

To implement the OSPF (Open Shortest Path First) routing algorithm in ns2, we have to design a routing protocol that can replicate the activities of OSPF that is depends on the routing algorithm. Measure the shortest path and functions inside the autonomous system by using the Dijkstra Algorithm. Though ns2 lacks the in-build OSPF protocol, you can execute the core mechanisms of OSPF like link-state advertisement (LSAs), topology information interchange and shortest path computation by replicating the actions of OSPF with tailored scripts. Follow the below approach to accomplish this algorithm using ns2:

Steps to Implement OSPF in NS2

  1. Set Up NS2 Environment
    • Make certain that you have installed the ns2 on your computer.
  2. Understand OSPF Protocol
    • OSPF is a link-state routing protocol where each router upholds an entire map (topology) of the network. Routers send LSAs to their neighbors to share information about their associated links, and the routers execute the Dijkstra algorithm to compute the shortest path to all destinations.
  3. Simulate OSPF Behavior Using Link-State Protocol
    • NS2 contains a built-in support for link-state routing with the LS routing protocol. We can use this as a simplified version of OSPF for simulation purposes, when it models the primary functionality of a link-state routing protocol.

Example OTcl Script for OSPF Routing Simulation

In this example, we will simulate a basic OSPF-like routing activities using NS2’s link-state routing protocol (LS).

  1. Define the Network Topology and Links

# Define the simulation environment

set ns [new Simulator]

# Open the trace file for recording data

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

}

# Create network 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

$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

  1. Enable the Link-State Routing Protocol (Simulating OSPF)

OSPF is a link-state routing protocol that uses LSAs and Dijkstra’s algorithm to estimate the shortest path. In NS2, you can enforce a link-state routing mechanism with the help of the built-in LS routing protocol.

# Enable the link-state routing protocol (OSPF-like behavior)

$ns rtproto LS

This command dictates NS2 to use a link-state routing protocol where nodes will interchange topology information (related to OSPF LSAs), and the shortest path to any destination will be calculated using Dijkstra’s algorithm.

  1. Set Up Traffic to Test Routing

Authenticate the OSPF-like routing algorithm is working properly by replicating data transfers amongst various nodes using TCP and UDP traffic.

# Attach TCP agent to node1 (sender) and TCP Sink to node5 (receiver)

set tcp1 [new Agent/TCP]

$ns attach-agent $node1 $tcp1

set sink1 [new Agent/TCPSink]

$ns attach-agent $node5 $sink1

# Connect the TCP agent and the sink

$ns connect $tcp1 $sink1

# Create FTP application over TCP and start it

set ftp1 [new Application/FTP]

$ftp1 attach-agent $tcp1

$ns at 1.0 “$ftp1 start”

# Attach UDP agent to node3 (sender) and UDP Sink to node4 (receiver)

set udp1 [new Agent/UDP]

$ns attach-agent $node3 $udp1

set sink2 [new Agent/Null]

$ns attach-agent $node4 $sink2

# Connect the UDP agent and the sink

$ns connect $udp1 $sink2

# Create a CBR application over UDP and start it

set cbr1 [new Application/Traffic/CBR]

$cbr1 attach-agent $udp1

$cbr1 set packetSize_ 512

$cbr1 set rate_ 1Mb

$ns at 2.0 “$cbr1 start”

$ns at 8.0 “$cbr1 stop”

  1. Run the Simulation

# Stop the simulation at 10.0 seconds

$ns at 10.0 “finish”

# Run the simulation

$ns run

Explanation of the Script:

  1. Network Topology: The script generates a simple network with five nodes. Links amongst nodes have various bandwidths and delays, permitting the OSPF-like protocol to compute the shortest paths.
  2. Link-State Routing Protocol: The line $ns rtproto LS allows NS2’s built-in link-state routing protocol. This simulates the actions of OSPF, where each node has knowledge of the whole network topology and measures the shortest path to all other nodes using the Dijkstra algorithm.
  3. Traffic Generation: The script builds TCP traffic from node1 to node5 and UDP traffic from node3 to node4. This lets you to examine whether the OSPF-like routing protocol chooses the proper shortest path.
  4. Simulation and Trace Analysis: The simulation runs for 10 seconds, during which data is transmission amongst nodes. The trace file (out.tr) will record all packet transfers, which can be assessed to validate that the shortest path is used.

Running the Simulation

  1. Save the script to a file (e.g., ospf_routing.tcl).
  2. In ns3, run the simulation by giving the below command:

ns ospf_routing.tcl

  1. The simulation will produce a trace file (out.tr) and a Network Animator (NAM) file (out.nam). You can visualize the simulation in NAM to investigate how packets are routed and authenticate that the shortest path is being used.

Analyze the Results

  • Trace File: The trace file (out.tr) has meticulous information about each packet’s route, transmission time, and any delays. Verify that packets are routed through the shortest path as predicted by analysing the trace file.
  • NAM Visualization: Open the .nam file using NAM to visualize the network and monitor how traffic is routed amongst nodes. This will help you validate the OSPF-like behavior according to their shortest path routing.

Enhancements

  1. Dynamic Topology Changes: Simulate link failures or attachments to inspect how OSPF-like routing adapts to alter in network topology. This would imitate OSPF’s dynamic actions in real-world networks.
  2. Multiple Traffic Sources: Monitor how OSPF-like routing manages multiple concurrent routes by including extra traffic flows amongst various nodes.
  3. Advanced OSPF Features: To execute more modern OSPF features includes hierarchical areas or virtual links, you would need to extend NS2’s routing functionality using C++ to manage features like area-based routing and more advanced topology databases.

This procedure will walk you through the implementation of OSPF routing algorithm using ns2 simulation tool. It contains the essential information about the simulation set up and how to allow the Link-State Routing Protocol including sample snippets and you can also optimize the simulation by modifying it.

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