🌐 OSI Model: Deep Dive (Layman's Terms)
OSI stands for Open Systems Interconnection. It's a 7-layer model that explains how data travels from one device to another over a network.
📦 Think of sending a parcel (data) from your home to a friend's home:
You pack it, label it, give it to a courier, they transport it, it reaches your friend, he opens it, and uses the content.
That’s exactly what the OSI model does with data, in 7 steps (layers):
🔢 OSI 7 Layers (Bottom-Up):
🧠 Tricks to Remember OSI Layers:
Mnemonic (Bottom to Top):
👉 Please Do Not Throw Sausage Pizza Away
(Physical, Data Link, Network, Transport, Session, Presentation, Application)
Or
👉 All People Seem To Need Data Processing
(Application → Physical)
📥 Practical Example: Sending a WhatsApp Message
Application (7): You type "Hi!" in WhatsApp.
Presentation (6): WhatsApp encrypts it.
Session (5): WhatsApp establishes a connection to the recipient.
Transport (4): TCP ensures the message is delivered in order, no loss.
Network (3): IP helps find the recipient’s phone (via their IP address).
Data Link (2): MAC address helps deliver data over Wi-Fi or mobile.
Physical (1): Data is turned into electrical signals sent via Wi-Fi or 4G.
🧠 Why OSI Matters in FAANG Interviews:
FAANG companies don’t expect you to just memorize OSI, but to:
Debug real network issues
Design scalable systems (e.g. load balancers, proxies)
Optimize data transfer (latency, throughput)
Build chat/video apps (protocol awareness)
🧪 20 FAANG-Level OSI Model Questions & Answers
✅ Beginner Level
Q1: What is the OSI Model? Why do we need it?
A: A conceptual framework for understanding how data flows through a network. It helps standardize communication between systems, ensuring compatibility.
Q2: Which layer is responsible for routing packets?
A: Network Layer (Layer 3) – uses IP for routing.
Q3: Which layer ensures reliable delivery?
A: Transport Layer (Layer 4) – uses TCP for reliability.
Q4: What's the difference between TCP and UDP in the OSI Model?
A: TCP (reliable, connection-oriented) vs UDP (fast, connectionless).
TCP is like sending a registered letter. UDP is like broadcasting radio.
Q5: Which layer deals with MAC addresses?
A: Data Link Layer (Layer 2).
✅ Intermediate Level
Q6: What happens at the Presentation Layer?
A: Data translation (e.g., ASCII to EBCDIC), encryption, compression.
Q7: What's the role of the Session Layer?
A: Establishes, manages, and terminates sessions. Example: FTP or video call session.
Q8: DNS works at which layer?
A: Application Layer (Layer 7) – it’s an application service.
Q9: What layers do switches, routers, and hubs operate on?
A:
Hub → Physical (1)
Switch → Data Link (2)
Router → Network (3)
Q10: What layer would SSL/TLS work on?
A: Presentation Layer (6) – for encryption/decryption.
Q11: What layer handles retransmissions and flow control?
A: Transport Layer (4) – TCP handles this.
Q12: What does the Physical Layer include?
A: Cables, switches, NICs, radio waves, fiber optics.
Q13: Difference between Layer 2 and Layer 3 switches?
A: Layer 2 switch uses MAC (LAN). Layer 3 switch does routing using IP.
✅ Advanced Level / FAANG Debugging
Q14: A packet reaches the server but the app doesn’t respond. Which OSI layer might be failing?
A: Possibly Layer 7 (Application). Lower layers worked.
Q15: If data is corrupted but reaches the destination, which layer failed?
A: Data Link (2) or Transport (4) – both handle integrity.
Q16: Which layer would you debug if DNS is failing?
A: Application Layer (7).
Q17: A website is slow to load. What layers could be bottlenecks?
A: Could be any of:
Network (3): bad routing
Transport (4): slow retransmission
Application (7): server response time
Q18: Can Layer 3 work without Layer 2?
A: No. Layer 2 (MAC addressing) is needed to deliver packets inside local networks.
Q19: If packets are being dropped but connection exists, which layer?
A: Transport Layer (4) – TCP tries to recover, but dropping continues → network congestion.
Q20: What layer is responsible for load balancer IP forwarding?
A: Network Layer (3) – routes traffic via IP.
💡 Bonus: Mind Map Style Summary
[Layer 7: Application] - Gmail, WhatsApp, HTTP
[Layer 6: Presentation] - Encryption, Encoding (TLS, JPEG)
[Layer 5: Session] - Login sessions, FTP
[Layer 4: Transport] - TCP, UDP (Port numbers)
[Layer 3: Network] - IP, Routers
[Layer 2: Data Link] - MAC, Switches
[Layer 1: Physical] - Cables, Wi-Fi, Signals
🔍 Example: Sending a Web Request
-
You type a URL → application layer sets HTTP request.
-
Presentation layer encodes it (e.g., UTF-8, TLS).
-
Session layer establishes/mantains connection.
-
Transport layer packs data into TCP segments.
-
Network layer adds IP header → routes Internet.
-
Data Link layer frames it for Ethernet or Wi‑Fi.
-
Physical layer sends actual electrical bits on the wire.
Each layer encapsulates and decapsulates headers/trailers, passing data up/down until delivery to the target application.
Here are 20 FAANG-level scenario-based OSI Model troubleshooting questions with realistic interview-style context, layer identification, and answer explanations — ideal for DevOps, SRE, Backend, or Systems Engineer roles.
✅ 20 FAANG-Level OSI Troubleshooting & Scenario-Based Questions
🔧 Q1. You're pinging a server and getting “Destination Host Unreachable.” Which layer(s) could be at fault?
Answer:
Layer 3 – Network
This error implies your system can't find a route to the destination IP. Could be due to misconfigured routing tables or incorrect default gateway.
🔧 Q2. The user can open the login page of a web app but gets “500 Internal Server Error” after submitting credentials. Which layer?
Answer:
Layer 7 – Application
Transport and lower layers are fine since page loads. The backend service likely crashed or failed to handle the request.
🔧 Q3. An HTTPS site is giving certificate errors. Which layer is affected?
Answer:
Layer 6 – Presentation
TLS/SSL handshake and encryption/decryption issues happen here.
🔧 Q4. A VoIP call has choppy audio. Network looks fine. Which OSI layer and why?
Answer:
Layer 4 – Transport
If using UDP, there's no retransmission. Choppiness suggests packet loss or jitter. Inspect transport metrics.
🔧 Q5. You're able to SSH into a server using IP but not hostname. Which layer?
Answer:
Layer 7 – Application (DNS issue)
DNS resolves names → IPs. If IP works but hostname doesn't, DNS is misconfigured or unreachable.
🔧 Q6. User can access internal sites but not public websites. What OSI layers to check?
Answer:
Layer 3 (Network): Check NAT, routing to public internet.
Layer 7 (Application): Ensure DNS is resolving public names.
🔧 Q7. Data packets are arriving out of order. Which OSI layer ensures proper sequencing?
Answer:
Layer 4 – Transport (TCP)
TCP handles sequencing. UDP doesn’t. Reordering could be due to misconfigured load balancer or NIC offloading.
🔧 Q8. You're using traceroute and it times out at the third hop. What does this tell you?
Answer:
Layer 3 – Network
Traceroute uses ICMP (or UDP) and TTL. Timeout = router may drop or block packets. Check firewall or routing loop.
🔧 Q9. After a switch replacement, some devices can’t communicate on the LAN. Why?
Answer:
Layer 2 – Data Link
MAC address table may not have populated, or VLAN configuration is missing.
🔧 Q10. Developer reports REST API is not returning data, but TCP connection is successful. What layer?
Answer:
Layer 7 – Application
TCP (Layer 4) is okay. Check application logs for endpoint failures.
🔧 Q11. FTP connection is established but directory listing fails. What's wrong?
Answer:
Layer 5+ – Session/Application
FTP uses two connections: control and data. Passive mode/firewall/NAT issues may break data channel.
🔧 Q12. An EC2 instance receives ICMP ping but can’t access the internet. Possible issue?
Answer:
Layer 3 – Network
Ping proves Layer 3 connectivity internally. Check NAT Gateway or routing table for outbound internet access.
🔧 Q13. A containerized app can't connect to a database using the service name, but works with IP. What’s broken?
Answer:
Layer 7 – Application (DNS inside container)
Likely a DNS resolution issue in Docker/Kubernetes.
🔧 Q14. A VM sends packets, but the MAC address of the destination is not resolving. Where's the issue?
Answer:
Layer 2 – Data Link (ARP failure)
Check ARP cache or broadcast behavior on the network.
🔧 Q15. HTTPS site loads on Chrome but not on curl. Why?
Answer:
Layer 6/7 – TLS handshake
Curl may not trust the certificate or requires updated CA bundle.
🔧 Q16. Network speed tests show good throughput, but application is slow. Where to look?
Answer:
Layer 7 – Application bottlenecks
Check app performance, API latency, server processing time. Lower layers are fine.
🔧 Q17. Load balancer routes traffic but users experience session loss. Layer?
Answer:
Layer 5 – Session
Sticky sessions may not be preserved. Misconfigured load balancer isn’t maintaining session affinity.
🔧 Q18. Packets are dropped after firewall rules are changed. Which OSI layers are involved?
Answer:
Layer 3 (IP filtering) and Layer 4 (TCP/UDP ports)
Check firewall logs/rulesets.
🔧 Q19. Packet capture shows TCP handshake (SYN, SYN-ACK, ACK), but application request times out.
Answer:
Layer 7 – Application
Transport layer is working. App might not be bound to port or is overloaded.
🔧 Q20. Wi-Fi connects, IP assigned via DHCP, but browsing doesn't work.
Answer:
Check:
-
Layer 3 – IP: Check gateway config.
-
Layer 7 – DNS: If IP ping works but not names.
-
Layer 4 – TCP timeout: Could be ISP firewall/drop.
🔁 Cheat Sheet: OSI Troubleshooting Hints
| Symptom | Layer |
|---|---|
| No electrical signal | Layer 1 |
| ARP issues / MAC problems | Layer 2 |
| No route, IP unreachable | Layer 3 |
| Packet loss, retransmissions | Layer 4 |
| Session timeout / login fails | Layer 5 |
| SSL, encoding errors | Layer 6 |
| App doesn’t respond | Layer 7 |
 OSI stands for Open Systems Interconnection . It's a 7-layer model that explains how d...){kind=link}