Data is sent as small numbered packets, not one long stream. You watch a message go to pieces, invent your own fix, then put names to it: routers pass packets on by different routes, numbers put them back in order, and TCP has the receiver confirm each one so a lost packet is sent again.
The internet never sends your message in one piece. So how does it ever arrive whole?
Your mission: get the message MEET AT NOON to your friend Ali, across a network of six routers. One of them, router C, is busy.
Make your prediction first. Help opens once you've locked it in.
When you send a message, a photo or a video, it is chopped into small pieces. The pieces travel separately, through computers run by other people, and some of those computers are busy. There is no right answer yet: write what you think.
The internet never sends a message in one piece. MEET AT NOON is chopped into 4 pieces, and each piece finds its own way to Ali through routers, some of them busy. What could go wrong on the way? Write your best guess.
Your guess from the start is beside the board. Compare it with what happens.
Send the message at least three times. Follow the pieces: which way does each one go, and what does Ali get?
Use the board. This ticks when you get there.
Sort what you saw on the board.
Try your own changes: a different piece size, or click a link to break it. Send after each change. What makes things better, and what makes them worse?
Use the board. This ticks when you get there.
There is more than one way to get the message there. Find two.
What would Ali's computer need on each piece to know where it goes?
Numbers put the pieces in order, and show a gap where one is missing. What could fill the gap?
Switch on Numbers and Ask again, then send.
The trouble starts at router C.
Click links to break them so that no piece can go near C.
Break the link from C to E, then send.
Get MEET AT NOON to Ali exactly, with every link working and router C still busy. Use the switches however you like.
Use the board. This ticks when you get there.
Now find a different way, with both switches off.
Use the board. This ticks when you get there.
Before you see any new words, write how your first fix worked, your own way. Notes are fine. Then compare with a model.
What you invented has names. The pieces are packets: each carries a piece of the data, a number and the address it is going to. Routers are the computers along the way; each one reads a packet's address and passes it on. When Ali's computer confirms a packet, that is an acknowledgement. Your Ask again switch was TCP: the set of rules that has the receiver acknowledge each packet, so any packet never acknowledged is sent again. Your first fix, step by step:
In your own words: how did your first fix get the whole message to Ali, in order, with every link working? Say what each piece carried and what Ali's computer did.
Your working
A model solution
Check your own working
Match each name to what you did on the board.
Sam explains what happens to a lost packet. Click the line where it first goes wrong.
Roadworks cut the cable from D to E. Click that link to break it, then send. Now every packet has to go through busy C. Does Ali still get the message?
Use the board. This ticks when you get there.
How much can break before Ali is cut off? Break links until nothing can reach Ali, then try to send.
Use the board. This ticks when you get there.
A full 5E lesson (AC9TDI8K02 node 2). Students write what they think will go wrong, watch a message go to pieces on the board (no fixes, nothing named), invent their own fix with two plain switches (Numbers, Ask again) and find a second fix by avoiding busy router C. Only then do they explain it in their own words and meet the names: packet, router, acknowledgement, TCP. Elaborate takes the idea to broken links and live video. About 45 minutes.
Open the teacher guide: answers, solutions, slides and a worksheet.
This is inquiry first (5E): students explore and invent before anything is explained. The words packet, acknowledgement and TCP appear only in Explain, after students have written their own explanation. If you usually teach explicitly, this is the lesson to try the difference.
Try both Explore boards yourself. The first has no fix: Ali always gets a jumbled message with a gap. The second has two switches, Numbers and Ask again (TCP, not yet named); the goals can be met two ways (both switches with every link working, or no switches with C cut off).
Engage works well unplugged first: cut MEET AT NOON into four strips with no numbers, hand them out across the room, have students pass them to an "Ali" by different paths, and keep one strip "lost". Ask Ali to read out what arrived. Then have students write their guesses.
Explain: let students write first, then share two or three explanations before anyone opens Give it names. Then name each part of what they did on the projector, and reveal the worked lines. The "Explain it to me" drawer opens once Engage is answered and uses the names, so ask students to leave it until after Explain.
Simplifications: real packets carry about 1500 bytes, and real TCP uses acknowledgement numbers and timers; the board waits until everything else has arrived before asking again. Without numbers the board's Ask again gives up, since TCP needs sequence numbers to know what is missing. Router C is set to be busy (slow, and it loses the second packet).
Review: open the next lesson with a quick question on this one, and this one with one on Wired or wireless (which link loses the most packets?).
Links: Secret messages' checksum lesson is how a receiver tells a packet was damaged.
Students expect the network to be one pipe, and want to be told the fix. Let Explore stay open: the first board has no fix on it, and the second asks them to invent one. Hold back the words packet and TCP until Explain; the names stick when they label something students already did.
Stretch: Video calls often use UDP, which never resends lost packets. Why is that better for a live call than waiting for a resend, and worse for downloading a file?
Support: Give the five words with a picture each: packet, number, router, route, acknowledgement.
Next: Who can see your data? (packets pass through routers you don't control).
Code links: micro:bit radio sends short packets; Secret messages adds checksums to them.