Journey of a Message
Computing · Year 3–Year 6
Send a message and watch the packets take different routes.
About the journey of a message
Press Send and the message does not travel as a message. It is cut into four numbered packets, and every packet goes its own way through the network: some routes are shorter than others, so they arrive in the wrong order and the numbers are what put the message back together.
The board moves one hop at a time. That is deliberate: a class can be asked which packet will get there first before it does, and a teacher can stop on the hop where something went wrong. The sliding between routers is there when a computer is set to show it and the tool is complete without it.
Lose one drops a packet at whichever router it has reached, and the network sends the whole packet again from the beginning. It arrives last, and the message still gets there, which is why the internet works at all on connections that drop things.
Nothing here is random. The routes are fixed, so the same message does the same thing every time and a teacher can promise what will happen. Nothing is timed and nothing is scored: the packets are not racing, they are taking different numbers of hops.
The five routers in the middle are the whole point of the picture. A message from your computer to your friend’s goes through machines belonging to other people, and no single one of them sees the whole message.
How to use it on your whiteboard
- 1Pick a message and press Send. It splits into four numbered packets.
- 2Ask the class which packet will get there first, then press Next hop.
- 3Watch the list at the bottom: what arrives is in the wrong order.
- 4Press Lose one and see what the network does about it.
- 5Keep pressing Next hop until every packet is there and the message goes back together.
Questions
- What is a packet?
- A small piece of a message. Anything sent over the internet is cut up into packets, each one labelled with where it is going and which piece of the message it is, and they are sent separately. Your computer puts them back in order at the other end.
- Why do they take different routes?
- Because there is more than one way through, and each router passes a packet on by whichever route looks best at that moment. It is what makes the internet hard to break: if one cable is busy or broken, the next packet goes another way.
- What happens if a packet is lost?
- It gets sent again. Nothing is lost for good: your computer notices a piece never turned up and asks for it, which is what the Lose one button shows. That packet arrives last and the message is still complete.
- Is the picture what the internet really looks like?
- It is the right shape and much too small. A real message might pass through fifteen or twenty routers and cross an ocean on the way, and the numbers on the packets are more than four. Five routers is what fits on a board and shows the same idea.