F22 · Topic 4.1
Packet journey
1. Predict first
Packets 1…4 contain Meet/by/the/gate. Deliver 4,1,3. Which positions are held, what is missing, and can the whole message be reconstructed?
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 14,1,3Waiting
Ready to experiment
Why: Predict the first change, then advance one step.
Try this: Compare 1,1,1,1 with 4,2,1,3. Both have four deliveries; only one has every position.
2. Build your procedure
Describe a receiver that records unique sequence numbers, ignores duplicate deliveries and reassembles only when all four positions exist. Propose a real-protocol response to a missing packet.
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 |
|---|---|---|
| 4,1,3 | Missing 2; incomplete | An out-of-order partial delivery. |
| 4,1,3,2 | Meet by the gate | Sequence metadata supports reassembly. |
| 1,1,2,3 | Missing 4; duplicate 1 does not complete | Count unique positions, not deliveries. |
4. Explain and reflect
Why might packets from the same message arrive in a different order, and why is the Web not the name for the whole Internet?
Model limits: Four fixed packets and supplied delivery order; no actual network traffic, encryption, congestion, timing or retransmission is simulated.