Exercise 14: Testing Network Availability with Traceroute

Objective

Learn how to use traceroute to map the path packets take across a network, analyze network latency, and detect bottlenecks by simulating network congestion.

Scenario

As a network administrator, you are tasked with diagnosing network slowdowns. One essential tool for identifying where delays occur is traceroute, which shows the path packets take to reach a destination and the latency at each hop. In this exercise, you’ll use traceroute to identify network paths and simulate a network bottleneck to observe its impact on packet flow.

⚠️ Important: This exercise must be performed in a controlled lab environment. Do not intentionally disrupt production networks.

Lab Instructions

Step 1: Run Traceroute to a Public Website

On your machine, open the terminal and run:

Linux/macOS:

traceroute google.com

Windows:

tracert google.com

Step 2: Analyze Traceroute Results

• Identify each hop in the route.
• Note the IP addresses and latency for each hop.

Example Output

 1  192.168.1.1  2.456 ms  1.984 ms  1.753 ms
 2  203.0.113.1  10.231 ms  9.456 ms  9.789 ms
 3  198.51.100.1  25.678 ms  24.876 ms  26.123 ms
 4  142.250.74.142  45.567 ms  43.892 ms  44.210 ms

• Hop 1: Local router.
• Hop 2-3: ISP routers.
• Hop 4: Destination server (Google).

Step 3: Simulate a Network Bottleneck

On the test router or gateway, install the traffic control tool:

sudo apt update 
sudo apt install iproute2 -y

Add artificial network delay:

sudo tc qdisc add dev eth0 root netem delay 300ms

eth0: Network interface to throttle.

300ms: Artificial delay.

Confirm the rule:

sudo tc qdisc show dev eth0

Step 4: Re-run Traceroute and Observe Changes

Run the traceroute command again:

traceroute google.com

Observe increased latency at the hop where the delay was introduced.

Example Output (After Delay)

 1  192.168.1.1  2.456 ms  1.984 ms  1.753 ms
 2  203.0.113.1  305.231 ms  309.456 ms  308.789 ms
 3  198.51.100.1  325.678 ms  324.876 ms  326.123 ms
 4  142.250.74.142  345.567 ms  343.892 ms  344.210 ms

• Hop 2 now shows significant latency, indicating a bottleneck.

Step 5: Remove Artificial Delay

Remove the delay to restore normal traffic flow:

sudo tc qdisc del dev eth0 root netem

Verify that the rule has been removed:

sudo tc qdisc show dev eth0

Solution & Explanation

How Traceroute Works

• Traceroute sends packets with incrementally increasing TTL (Time-To-Live) values.
• Each router decrements the TTL, and when it reaches zero, the router responds with an ICMP Time Exceeded message.
• This reveals each hop along the packet’s path and the latency to each.

Identifying Bottlenecks

• High latency at a specific hop suggests congestion or issues with that router.
• Packet loss at a hop indicates possible packet filtering or routing issues.

Impact of Artificial Delay

• Introducing delay using tc simulates real-world network congestion.
• Traceroute reveals the exact point in the network where the bottleneck occurs.

Testing & Verification

• Confirm that the traceroute identifies increased latency at the simulated bottleneck.
• Verify that removing the delay restores normal latency.

Benefits of Using Traceroute

1. Path Discovery: Identifies the path packets take to a destination.
2. Latency Analysis: Detects high-latency links in the network.
3. Bottleneck Detection: Pinpoints where network slowdowns occur.
4. Routing Troubleshooting: Diagnoses routing misconfigurations.

Additional Script (Optional)

Automate delay simulation and removal:

#!/bin/bash
# Apply artificial delay
if [ "$1" == "add" ]; then
  sudo tc qdisc add dev eth0 root netem delay 300ms
  echo "300ms delay applied."

# Remove delay
elif [ "$1" == "remove" ]; then
  sudo tc qdisc del dev eth0 root netem
  echo "Delay removed."
else
  echo "Usage: $0 {add|remove}"
fi

Run the script:

chmod +x simulate_delay.sh
sudo ./simulate_delay.sh add   # To introduce delay
sudo ./simulate_delay.sh remove # To remove delay

Conclusion

In this exercise, you used traceroute to map network paths and identify bottlenecks. You simulated a network delay with tc and analyzed its impact on packet flow. This technique is essential for diagnosing network issues and optimizing network performance.