To simulate Open Shortest Path First (OSPF) algorithm projects utilizing NS2, we require setting up NS2 to use Link-State Routing (LSR) since OSPF is depends on the link-state routing, and it determines the shortest path using Dijkstra’s algorithm. The network simulator NS2 does not have a built-in OSPF module, however we can replicate the behavior of OSPF by concentrating on the link-state routing mechanism taht OSPF is derived from.
Steps to Simulate OSPF-like Behavior in NS2
- Install NS2
If we haven’t installed NS2 then we can download it from the NS2 Official Website and we follow the installation steps that suitable for the operating system.
- Create the TCL Script for OSPF
The script should describe the network topology that allow the link-state routing mechanism, and mimic traffic among the nodes. OSPF is an interior gateway protocol, thus we replicate several connected routers sharing link-state information to calculate the shortest path.
Example TCL Script for Simulating OSPF-like Behavior:
# Create a simulator object
set ns [new Simulator]
# Open files for tracing and NAM animation
set tracefile [open “ospf_trace.tr” w]
set namfile [open “ospf_simulation.nam” w]
$ns trace-all $tracefile
$ns namtrace-all $namfile
# Create network nodes (routers in the OSPF network)
set r0 [$ns node]
set r1 [$ns node]
set r2 [$ns node]
set r3 [$ns node]
set r4 [$ns node]
# Create duplex links between the routers with bandwidth and delay
$ns duplex-link $r0 $r1 1Mb 10ms DropTail
$ns duplex-link $r1 $r2 1Mb 15ms DropTail
$ns duplex-link $r2 $r3 1Mb 20ms DropTail
$ns duplex-link $r3 $r4 1Mb 10ms DropTail
$ns duplex-link $r0 $r2 1Mb 30ms DropTail
$ns duplex-link $r1 $r3 1Mb 25ms DropTail
# Enable Link-State Routing Protocol (for OSPF-like behavior)
for {set i 0} {$i < 5} {incr i} {
$ns at 0.0 “$r$i start-lsr”
}
# Define a UDP agent and attach it to node r0 (simulating OSPF traffic)
set udp0 [new Agent/UDP]
$ns attach-agent $r0 $udp0
# Define a CBR (Constant Bit Rate) traffic generator
set cbr0 [new Application/Traffic/CBR]
$cbr0 set packetSize_ 512
$cbr0 set rate_ 0.5Mb
$cbr0 attach-agent $udp0
# Attach UDP sink agent to node r4 (the destination)
set udp1 [new Agent/UDP]
$ns attach-agent $r4 $udp1
# Connect the source and destination agents
$ns connect $udp0 $udp1
# Schedule the traffic
$ns at 1.0 “$cbr0 start”
$ns at 4.5 “$cbr0 stop”
# Define the end of the simulation
$ns at 5.0 “finish”
proc finish {} {
global ns tracefile namfile
$ns flush-trace
close $tracefile
close $namfile
exit 0
}
# Run the simulation
$ns run
- Run the Simulation
We can save the script as ospf_simulation.tcl and then execute it in NS2:
ns ospf_simulation.tcl
- Visualize the Network Using NAM
We can open the generated .nam file utilizing the NAM tool to envision the OSPF network and routing behavior:
nam ospf_simulation.nam
- Analyze the Trace File
The trace file (ospf_trace.tr) will include details of the packet transmissions, routing decisions, and overall performance of the network. We can examine this file to learn:
- Packet delivery ratio.
- Throughput.
- Routing table updates and the path taken by packets.
- Modifying the Topology
- We can maximize the amount of routers and links that modify the delays and bandwidths, and rerun the simulation to monitor how OSPF assesses the shortest path in numerous network conditions.
- By changing the link characteristics, we can replicate the link failures and experiment OSPF’s ability to recompute the shortest path.
Optional Enhancements:
- Simulate OSPF Area Design: Split the routers into OSPF areas to observe how the routing protocol performs in a multi-area OSPF design.
- Compare with Other Routing Protocols: Replicate the protocols such as RIP (Distance Vector Routing) and compare them with OSPF to learn performance variances such as convergence time and routing efficiency.
In this simulation, we clearly illustrated the simulation process with related examples for OSPF algorithm projects that were simulated using NS2 tool, which based on the LSR and finally it calculated the shortest path utilizing Dijkstra’s algorithm. If required, we will present more details.
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