A sofa in many boxes
You send a sofa to a friend. The shop does not send it in one go. It takes it apart and packs it into numbered boxes. Each box has a label. The boxes go on different lorries and arrive in a mixed order.
Your friend uses the numbers to put the sofa back together. Computers send data in much the same way.
Key words
| Word | What it means |
|---|---|
| Packet | a small piece of a message, with its own labels |
| Header | the labels at the front of a packet |
| Payload | the piece of the message a packet carries |
| Trailer | the part at the end of a packet, with a check value |
| Packet switching | sending packets by whatever route is free, then rebuilding the message |
| Sequence number | a number that says where a packet fits in the message |
| TTL | time to live: a count that stops a packet travelling for ever |
| Router | a device that joins networks and passes packets on |
| IP address | a number that names a device on a network |
| Network | devices joined so they can share things |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can describe what a packet holds and how packet switching sends a message.
Starter
Have a go
Starter
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Small packets are easier to send
A big message is cut into many small packets. Each packet is sent on its own. If one is lost, only that small piece is sent again.
Small packets also let many people share one line. Each person's packets take turns, so no one has to wait for a huge file to finish.
What is inside a packet
| Part | What it holds | What it is for |
|---|---|---|
| Header: source address | The IP address of the sender | So the reply knows where to go |
| Header: destination address | The IP address of the receiver | So routers know where to send it |
| Header: sequence number | The place of this packet in the message | So the message can be put back in order |
| Header: TTL | A count that goes down at each router | So a lost packet does not travel for ever |
| Payload | A piece of the message | The data you actually want |
| Trailer | A check value | So the receiver can spot damage |
The cells students fill in are left blank in this preview.
Extra: Send a packet with 100 bytes of payload
sudo ip netns exec h1 ping -c 1 -s 100 -W 1 10.0.12.1A command for the lesson’s lab terminal. It is shown here and not run.
Extra: Find the TTL in the reply
sudo ip netns exec h1 ping -c 1 -W 1 10.0.12.1 | grep ttlA command for the lesson’s lab terminal. It is shown here and not run.
Check
Have a go
Check
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.