To simulate Open Shortest Path First (OSPF) routing using NS2, we can set up NS2 utilising Link-State Routing since OSPF is according to the link-state algorithm that builds an entire view of the network topology and calculates the shortest path to each destination to use Dijkstra’s algorithm.
Even though, NS2 doesn’t have explicit OSPF modules, we can replicate OSPF-like behavior by utilizing link-state routing protocols within NS2. We provide a guide on how to simulate OSPF routing using NS2.
Steps to Simulate OSPF Routing in NS2
- Set Up NS2 Environment
Make sure NS2 is installed and running on the system. We can check this by running:
ns
It will begin the NS2 shell if NS2 is installed appropriately.
- Understanding OSPF Routing
Open Shortest Path First is a link-state routing protocol utilized within IP networks to find out the shortest path among the nodes using Dijkstra’s algorithm. Every single router maintains a map of the network topology and interchange link-state advertisements (LSAs) with neighbouring routers keeping the topology updated.
- Writing a TCL Script for OSPF-like Routing
Following is an instance TCL script to mimic OSPF-like behavior using link-state routing in NS2.
# Create a simulator object
set ns [new Simulator]
# Open trace and NAM output files
set tracefile [open out.tr w]
set namfile [open out.nam w]
$ns trace-all $tracefile
$ns namtrace-all $namfile
# Define the network topology
set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]
set n4 [$ns node]
# Create duplex links between nodes
$ns duplex-link $n0 $n1 1Mb 10ms DropTail
$ns duplex-link $n1 $n2 1Mb 10ms DropTail
$ns duplex-link $n2 $n3 512Kb 20ms DropTail
$ns duplex-link $n3 $n4 1Mb 15ms DropTail
$ns duplex-link $n4 $n0 1Mb 20ms DropTail
# Enable link-state routing (OSPF-like behavior)
$ns rtproto LS
# Create a UDP agent and attach it to node n0 (source)
set udp0 [new Agent/UDP]
$ns attach-agent $n0 $udp0
# Create a Null agent and attach it to node n4 (destination)
set null0 [new Agent/Null]
$ns attach-agent $n4 $null0
# Connect the source (n0) to the destination (n4)
$ns connect $udp0 $null0
# Create a CBR traffic generator and attach it to the UDP agent
set cbr0 [new Application/Traffic/CBR]
$cbr0 set packetSize_ 512
$cbr0 set interval_ 0.005
$cbr0 attach-agent $udp0
# Schedule the traffic to start and stop
$ns at 1.0 “$cbr0 start”
$ns at 4.5 “$cbr0 stop”
$ns at 5.0 “finish”
# Define the finish procedure
proc finish {} {
global ns tracefile namfile
$ns flush-trace
close $tracefile
close $namfile
exec nam out.nam &
exit 0
}
# Run the simulation
$ns run
- Explanation of the Script
- Link-State Routing: The command $ns rtproto LS configures Link-State Routing that works similarly to OSPF by building an entire map of the network and calculating the shortest paths using Dijkstra’s algorithm.
- Network Topology: This simulation utilizes five nodes are associated in a mesh-like topology, in which each node can attain the others via several paths.
- Traffic: A Constant Bit Rate (CBR) traffic generator is configuring at node n0 to transmit traffic to node n4.
- Running the Simulation
We can save the TCL script as ospf_simulation.tcl and execute it with the below command:
ns ospf_simulation.tcl
It will generate a trace file (out.tr) and a NAM file (out.nam) for envisioning the network behavior.
- Visualizing the Simulation
To envision the network using NAM, execute:
nam out.nam
We can monitor how packets are routed via the network and how link-state routing assesses the shortest paths among the nodes that similar to the OSPF.
- Simulating Link Failures and OSPF Adaptation
To replicate link failures and monitor how OSPF adjusts to the topology changes, which insert the following commands to mimic a link failure among two nodes:
# Simulate a link failure between n2 and n3 at 2.5 seconds
$ns rtmodel-at 2.5 down $n2 $n3
# Restore the link at 4.0 seconds
$ns rtmodel-at 4.0 up $n2 $n3
It will force the link-state protocol (OSPF-like) to recompute routes once the link among n2 and n3 goes down and comes back up.
- Customizing the Simulation
We need to change the script to replicate numerous OSPF situations:
- Increase the number of nodes: Insert more nodes and links to replicate a larger network.
- Vary the link costs: Modify link bandwidth and delay to denote diverse link weights, which impacting OSPF’s shortest-path calculation.
- Experiment with different traffic types: Attempt utilising TCP traffic rather than UDP, or differ the CBR traffic interval to mimic distinct loads.
- Analyzing the Trace File
The trace file (out.tr) encompasses detailed data regarding packet transmissions and routing updates. We can examine this data using awk or grep to extract key performance parameters like:
- Packet Delivery Ratio (PDR): Assess how many packets are attaining the destination.
- End-to-End Delay: Compute the time taken for packets to travel from origin to destination.
- Routing Overhead: Calculate the amount of control packets utilized by the link-state protocol to maintain the network topology.
For instance, to assess the number of packets received at the destination that utilize:
grep “^r” out.tr | wc -l
- Performance Evaluation
We can estimate the performance of OSPF-like routing by:
- Increasing network size: Insert more nodes and links to replicate a larger network.
- Simulating network dynamics: Mimic link failures, node failures, or topology changes to monitor how rapidly OSPF adapts.
- Traffic load testing: Change the traffic load by maximizing the amount of flows or modifying the traffic rates.
At the end of these brief explanations, you can obtain to know about the OSPF routing projects and their simulation process. Also, we can provide more specific insights regarding this subject through another manual.
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