# Exercise 1 — Map Real Traffic to OSI Layers

## Scenario

You are debugging a report from a colleague: "The website is down." Before touching anything, you want to reason about the problem systematically by OSI layer, because different layers require completely different tools and point to completely different root causes.

## Deliverables

Answer each of the following in your own words. Write your answers directly under each question in this file.

1. **Layer identification table.** For each item below, name the single OSI layer (1–7) it primarily belongs to, and justify your answer in one sentence:
   - An Ethernet switch forwarding a frame based on destination MAC address
   - A TLS certificate expiring
   - A router with no matching entry in its routing table for a destination IP
   - A backend process not listening on the port the client is trying to connect to
   - A Wi-Fi card unable to detect any signal
   - An HTTP server returning a 404 for `/missing-page`

2. **Encapsulation trace.** A browser on your laptop sends `GET /status` to `api.example.com` over HTTPS. Write out, layer by layer from 7 down to 1, what header/wrapper gets added at each layer, and name the resulting unit of data at each step (e.g. "segment," "packet," "frame"). Use the walkthrough style from the theory file's "Under the Hood" section as a model, but write it in your own words for this specific request.

3. **Diagnosing "the website is down" by layer.** For each layer below, give one concrete, realistic failure that would cause "the website is down," and one concrete way you'd check for that specific failure (a command, tool, or observation — not just "check the layer"):
   - Layer 1 (Physical)
   - Layer 2 (Data Link)
   - Layer 3 (Network)
   - Layer 4 (Transport)
   - Layer 7 (Application)

4. **Ordering exercise.** Explain why it makes sense to debug "is the site down" roughly from layer 1 upward (or explain why you'd argue for a different order) — what does checking a lower layer first save you from wasting time on?

## Notes

- There is no single graded "correct" wording — the goal is to demonstrate you can place a real symptom at the right layer and pick a plausible, layer-appropriate diagnostic action.
- Keep each answer to 2–4 sentences unless a question asks for a list/trace.