How to Tackle TCP Congestion and VPN Design Assignments in Computer Networks

Let’s break down the components that typically appear in such assignments and how to best approach them.

📌 Part 1: Approaching TCP Congestion Control Questions

🔍 Break Down the Congestion Control Phases

In TCP Reno (and Tahoe), your first step is to understand the lifecycle of a TCP connection:

1. Slow Start Phase: Window size grows exponentially.
2. Congestion Avoidance (Additive Increase): Growth becomes linear.
3. Multiplicative Decrease: Reacting to congestion events (timeouts or duplicate ACKs).

A good way to tackle related questions is by drawing a time vs. cwnd graph. This helps visualize transitions and is critical in questions asking you to identify when each phase occurs.

For example, if you're given that ssthresh = 16 MSS and initial cwnd = 1 MSS, ask yourself:

• How many RTTs before slow start ends?
• At what point does linear growth begin?
• What happens when a duplicate ACK occurs?

Each congestion control event (timeout vs. triple duplicate ACKs) behaves differently:

• Timeout: cwnd resets to 1 MSS (Tahoe behavior).
• 3 Duplicate ACKs: cwnd is halved (Reno behavior).

Mark those points on the graph and note corresponding RTTs. That becomes the basis of your answers.

🧠 Identify Congestion Events with Logic

Assignments will ask you to interpret events like:

“What is the congestion event at RTT = 12?”

To answer this, correlate RTTs with the trend of cwnd. If it suddenly drops, it’s likely a multiplicative decrease triggered by congestion. You’ll need to match the behavior with Reno vs. Tahoe rules.

Don't guess. Instead, apply this mini-framework:

• Was cwnd increasing linearly or exponentially before the event?
• Did cwnd reset or halve?
• Did ssthresh change?

This logic builds accuracy into your analysis and avoids vague assumptions.

📈 Calculating ssthresh and cwnd

Many assignments ask you to compute ssthresh and cwnd at specific RTTs. Here's a pattern:

• ssthresh changes only during congestion events.
• When a timeout occurs: ssthresh = cwnd / 2, cwnd = 1 MSS.
• On 3 duplicate ACKs: ssthresh = cwnd / 2, cwnd = ssthresh + 3 MSS.

Create a table that tracks RTTs, cwnd, and ssthresh as columns. This gives you a clear way to justify your values.

Also, if asked to “redraw the graph using TCP Tahoe,” clearly highlight the difference: Tahoe resets cwnd after any congestion event, while Reno recovers faster.

🛡️ Part 2: Solving Network Security Design Questions

🔐 Understand IPsec vs. SSL/TLS VPN Use Cases

The first thing you’ll notice in such assignments is that VPN types matter based on context.

Here’s a solid comparison framework:

Assignments typically ask you to match the right VPN to the right purpose. For example, use IPsec between two branches but SSL VPN for employees accessing the system from coffee shops.

Back up your answer with reasoning:

• Security coverage
• User accessibility
• Firewall traversal

Never just define IPsec—relate it to the company’s network design.

⚠️ Identify Real-World VPN Challenges

A high-level assignment might say: “What are three issues with enabling communication between branches via HQ using IPsec?”

This is your cue to discuss:

1. Performance bottlenecks – All traffic rerouted via HQ.
2. Single point of failure – If HQ VPN fails, branch-to-branch traffic dies.
3. Complex key management – More tunnels, more chances of configuration errors.

Use short, scenario-driven paragraphs. Example:

"If the London branch wants to share files with Tokyo, but both must tunnel through Sydney, any bandwidth limits or latency at Sydney will degrade the user experience."

Assignments love that kind of applied, contextual response.

🔓 Identifying Security Risks from Tunnel Downtime

Now you’re told that the IPsec tunnel between HQ and London went down for two hours. Here's how you should think:

• Was the data exposed?
• Were users rerouted through insecure channels?
• Could someone spoof endpoints?

Three common risks to discuss:

1. Data interception via fallback paths.
2. Session hijacking if credentials were reused.
3. Man-in-the-middle (MITM) attacks during reconnections.

These aren't just buzzwords—relate them back to the use of IPsec.

“Without IPsec’s integrity checks, attackers could spoof data packets, leading to unauthorized access.”

📁 Types of Sensitive Data Exposed

This part tests your understanding of enterprise systems.

Don’t just say “files.” Instead, list:

• Authentication credentials (user logins to ERP systems).
• Customer PII (names, addresses, financial records).
• Live transactional data (orders, banking info).

Explain how these might have traveled through the VPN and why they matter if leaked.

Example:

“Real-time CRM access over the IPsec tunnel means customer notes and contact info were actively transmitted and potentially vulnerable.”

🚨 Potential Data Breaches

This is where you're expected to go from theory to practical impact.

Discuss breaches like:

1. Unauthorized data exposure – attackers sniffing traffic.
2. Credential reuse – passwords stolen and used elsewhere.
3. Compliance failure – data leak leading to GDPR/CCPA penalties.

Don't write generic sentences. Use the enterprise name and setting:

“For Future Trend Inc., a 2-hour data leak might violate regional data laws, leading to fines or customer distrust.”

Also, tie your breach type to the earlier data risk for a coherent structure.

💡 Part 4: General Tips for Scoring Well in Such Assignments

Structure Your Answers Clearly

Use headings, bullets, and paragraph breaks to avoid walls of text. Many students lose marks not because of wrong answers, but poor presentation.

Stick to Word Limits—But Don't Undershoot

If a 200-word limit is given, aim for 180–200. Undershooting shows underdevelopment of ideas. Always justify with examples and connect points logically.

Use Visuals Where Possible

TCP-based questions benefit heavily from graphs and tables. You don’t need artistic skill—just use plotting tools or even ASCII if needed.

Cite Your Sources

Even if you learned from your textbook or documentation, mention it briefly in IEEE or Harvard style. Marks are often deducted for poor citation.

Example:

[1] Kurose, J. & Ross, K. (2020). Computer Networking: A Top-Down Approach. Pearson.

Avoid Copy-Paste or AI Reliance

As clearly mentioned in many course briefs, unoriginal or AI-generated content will be flagged. Always rewrite in your own interpretation, even if you refer to tools like ChatGPT for structure.

Final Thoughts

Assignments like the one from COMP3250 are designed to prepare you for real-world networking roles—whether you become a network engineer, security analyst, or IT architect. By understanding how to interpret congestion control timelines, design VPN architectures, and think critically about downtime scenarios, you're not just submitting an assignment—you’re simulating what you’d do in a real tech role.

If you ever feel overwhelmed, remember:

• Break the problem down.
• Work from known values (like ssthresh and cwnd).
• Use visual aids.
• Structure your arguments.
• Relate concepts to the real-world business setting.

And when in doubt—don’t just memorize. Think like a network engineer. That mindset will help you ace not just the assignment, but the career ahead.