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Intro to CS 9-10 / Week 11 / Mission Quest
6/6
Week 11 · Systems and the Internet

Mission Quest

Mission Quest: The family packet race
// Packets, paths and a network that keeps working
⏱ about 20 min

Mission Quest: The Family Packet Race

The relay is down for a quick upgrade, so the crew turns the galley into a network.

Comet is the greenhouse. Wren is mission control. The chairs in between are relay stations.

Comet writes a secret message on six numbered cards and sends them off, one by one, chair to chair.

Halfway through, Wren pulls one chair out of the circle. "Link broken," he announces.

Comet laughs and sends the rest of the cards the long way round.

Nova counts the cards as they arrive at Wren's seat. "All six," she says. "Out of order, but all here."

Wren reads the message in order. "Great job on the tomatoes, team."

  1. A computer network is a group of connected devices that can send or receive data. Routing finds a path.
  2. Packets carry a chunk of data plus metadata for routing and reassembly. They may arrive in order, out of order or not at all.
  3. The Internet is a network of networks using open protocols. The World Wide Web uses the Internet.
  4. Redundant paths make a network fault tolerant: it keeps working when parts fail.
  5. Parallel computing can speed up a job, but the sequential part always limits the gain.
◇ THE FAMILY PACKET RACE
You need five or six index cards or paper scraps, a pencil, and four chairs or sheets of paper to mark spots.
Choose one sender and one receiver. Everyone else is a relay station. Stand or sit so each station can reach two others.
Only two or three players? Use the chairs or sheets as extra relay stations, and carry each card from spot to spot.
The sender writes a short message, one word per card. Add To, From and a number like 3 of 5 to each card.
Pass the cards one at a time, station to station, choosing any path that reaches the receiver.
Halfway through, one station steps out. Reroute the remaining cards around the gap.
The receiver puts the cards in order using only the numbers, then reads the message aloud.
For a grown-up
Ask: what is a packet, and how does it find its way when a link breaks?
Ask: why should we be careful before joining a public wireless network? Agree on a family rule together.
The Moon relay is a story, not a description of how real space missions communicate.
QUEST CHECK
  • Read the question.
  • Tap your answer.
A station stepped out of the race. What kept the message moving?
The cards arrived out of order. How did the receiver fix it?
Which is an agreed set of rules for how a system behaves?
Which word means a system keeps working when some parts fail? Type two words, starting with fault.
WHY THIS EXERCISEFault tolerance is what redundancy gives a network.
On paper, sketch your family network: each person as a circle and each possible handoff as a line. Mark the gap.
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