Routing packets.

Step 1 of 6

The shortest route

The internet is a huge graph: the nodes are computers and routers, and the edges are cables, optical fibres and radio links. When you send a message to a server, it does not travel there directly; it passes from router to router, and each router sends it to a neighbour nearer the destination.

Here every edge has a length, and the shortest route is the one whose lengths add up to the least, not necessarily the one with the fewest edges. It is found with Dijkstra’s algorithm: start from the source, and at each step settle the nearest node not settled yet. The route glowing on the board is the result.

Step 2 of 6

A cut cable

Cables get cut, routers fail, and a good network keeps working. The message goes out again and again, and now we will cut a cable on its route. Predict.

Step 3 of 6

Cutting the server off

Each cut in the Scene hits a cable on the current shortest route. How many cuts will it take before there is no way at all?

Step 4 of 6

A cut

The server is cut off, but most cables are whole. Look at the cut cables, and at what they separated.

Step 5 of 6

Packets

Long messages are not sent in one piece. They are split into small packets, and each packet has a header with the destination address and a sequence number. A packet damaged on the way is sent again on its own, without resending the whole message. Predict, and then Look inside will split the message.

Step 6 of 6

Putting it together

Packets that travelled different ways arrive at different times. The server collects them and puts the message back together.