F23 · Topic 4.2
Network failure investigation
1. Predict first
Links are S-A,S-B,A-C,B-C,C-T,A-D,D-T. Remove C-T. Predict all simple S-to-T paths.
Change it · step through · explain
Change an input, predict what will happen, then advance one step. Read why the state changes, reset and experiment. This fixed teaching model does not save progress or award XP.
Ready · Make a prediction
- Input 1C-TWaiting
- Input 2(empty)Waiting
Ready to experiment
Why: Predict the first change, then advance one step.
Try this: Fail C-T, then also D-T. Add a link and predict whether it reconnects S to T.
2. Build your procedure
Draw the graph and test every single-link failure, then remove both links into T. Add B-T and explain the new path. Give a failure that still isolates S.
Use AP CSP pseudocode, a language you know, or precise plain language. Your code is not executed here. Use the classroom assignment to save your response.
3. Check boundaries
| Input | Expected | Why this case |
|---|---|---|
| Failed C-T; no added link | Two simple paths survive | Undirected reverse hops matter. |
| Failed C-T,D-T; no added link | No path | All destination links fail. |
| Failed C-T,D-T; add B-T | S-B-T survives | A reachable replacement destination link. |
4. Explain and reflect
Does redundancy guarantee that communication never fails? State a specific limit in this graph.
Model limits: Only fixed nodes S,A,B,C,D,T and an optional single added link. It enumerates simple paths, not actual packet routes or reliability probabilities.