GCSE · Computer Science · AQA · Spec 8525

Four-layer TCP/IP model

Post a parcel and it's wrapped, labelled, then unwrapped at the other end. Data crossing a network does much the same, in a fixed order.

Computer Science · Networks

Follow one message down, then back up

Eight steps: four down the sender's stack, four back up the receiver's. All the addresses and numbers are made-up examples.

Before you press Next, guess: as the data goes down the stack, does it get bigger or smaller? Step through and check.

Message: We follow segment 2 of 3. Example values: sender IP 192.0.2.10, receiver IP 198.51.100.7, next node's MAC AA:BB:CC:00:11:22, port 6000.

Sender: application→Sender: transport→Sender: internet→Sender: link→Receiver: link→Receiver: internet→Receiver: transport→Receiver: application

Data now=DATAAdded here=Nothing to track yet. This is the data itselfLayer's job=Prepares the data for sending

Output

Step 1: Start at the top. The application layer is the one programs like web browsers use, and it prepares the data for sending. Right now it's just the data, with nothing wrapped round it.

1 / 8

Step through the trace — every value is the lesson’s, not run by the page.

Step 1 of 8: Start at the top. The application layer is the one programs like web browsers use, and it prepares the data for sending. Right now it's just the data, with nothing wrapped round it..

Computer Science · Networks

Which layer does each protocol work at?

Sort the nine protocols into the layer where each one operates.

Still to sort

Application (0)

The layer used by programs such as web browsers

Transport (0)

The layer that splits data into segments

Where the line is: Segments belong here. IP packets belong one layer down, at the internet layer.

Internet (0)

The layer that builds IP packets

Where the line is: Notice the name TCP/IP: only one of those two protocols works at this layer.

Link (0)

The layer that carries packets across each link

9 of 9 still to sort.

Different protocols operate at each layer. Commit to a layer for every protocol before you read why.

Predict, then check

Segments can arrive in a different order from the one they were sent in. Which header information gets the message back together?

The receiver has a pile of segments in the wrong order. Which header information lets it rebuild the message and know it has them all?

Two addresses, two jobs

IP addressvsMAC address

People swap these constantly. Ask yourself: is this about the final destination, or just the next node?

Focus

What it points to

IP address

The sender and the receiver, so the destination at the end of the journey

MAC address

The next node on the network

The insight

One names where the data is ultimately going. The other names only the very next stop.

Used to deliver data

IP address

Between different networks

MAC address

Between devices on the same local network

Layer and header

IP address

Internet layer, in the IP packet's header

MAC address

Link layer, in the link header

Computer Science · Networks

Which of these sounds like you?

A classmate is explaining the TCP/IP model to you and says one of the following. You're deciding which one to believe.

Which is closest to what you think right now?

The four layers are four physical boxes, and the data gets handed from one box to the next.The receiving device goes through the layers top to bottom again, application first and link last.The extra information added on the way down stays on the data, so the application layer receives it all.The layers are a stack of responsibilities. Data goes down when it's sent and back up in reverse when it's received, with each layer taking off what was added.

How sure are you?

I'm sureI think soI'm guessing

The four layers are four physical boxes, and the data gets handed from one box to the next.

This is a common misconception

That's an easy picture to have, but the TCP/IP model is conceptual. It's a way of splitting network communication into four responsibilities. Each layer is responsible for a different aspect of it, and none of them is a separate device.

The receiving device goes through the layers top to bottom again, application first and link last.

This is a common misconception

Not quite. The receiver reverses the process: it works back up the stack, from link to internet to transport to application. The data was wrapped starting at the top, so it has to be unwrapped starting at the bottom.

The extra information added on the way down stays on the data, so the application layer receives it all.

This is a common misconception

No: each layer takes off the extra information that was added, as the data goes up. By the time it gets to the application layer, the original data is what's left.

The layers are a stack of responsibilities. Data goes down when it's sent and back up in reverse when it's received, with each layer taking off what was added.

This matches the evidence

Yes. That's encapsulation going down and decapsulation coming back up. Nothing physical, no repeats, no leftovers.

Four layers, four jobs. Wrap on the way down, unwrap in reverse on the way up.

Computer Science · Networks

Now say it in your own words

Describe what happens to data as it passes down the TCP/IP stack when it is sent. [5 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Names the four layers in order from the top: application, transport, internet, link. (1 mark)
Says the transport layer splits the data into segments, each with a header. (1 mark)
Says the transport header includes information such as the segment number, the total number of segments and the port number. (1 mark)
Says the internet layer puts segments into IP packets that include the sender's and receiver's IP addresses. (1 mark)
Says the link layer attaches a header with the MAC address of the next node, and that each layer adding information on the way down is called encapsulation. (1 mark)

Model answer

The data starts at the application layer and passes down through the transport layer, the internet layer and finally the link layer. At the transport layer the data is split into segments and each segment is given a header. The header holds information such as the segment number, the total number of segments and the port number. At the internet layer the segments are packaged into IP packets, and each packet includes the sender's and receiver's IP addresses. At the link layer a header is attached that includes the MAC address of the next node. Each layer adds extra information to help with delivery, and this is called encapsulation.

Layer by layer: transport splits and numbers, internet addresses, link names the next node. All of it together is encapsulation.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How a message gets wrapped on the way out and unwrapped, in reverse, on the way in.

What you need to know

The TCP/IP model is conceptual: four layers, each responsible for a different aspect of network communication.
The layers form a stack, top to bottom: application, transport, internet (sometimes called the network layer), link.
Have a goA classmate confidently writes the stack as application, link, internet, transport. Which two layers need to swap places?
Link and transport. The stack is application, transport, internet, link.
Link is the bottom layer, so it comes last, and transport sits directly under application.
Application layer: used by programs such as web browsers, it prepares data to send and decodes received data for the user.
Transport layer: splits data into segments, and each header carries a segment number, the total and the port number.
Have a goA message is split into 4 segments. What will the total in each segment's header say?
4. Every segment's header carries the same total number of segments.
The header holds the segment's own number and the total, and the total is for the whole message, not for that one segment.
Internet layer: packages segments into IP packets, each with the sender's and receiver's IP addresses so they can be routed.
Link layer: carries IP packets across each link, attaching a header that includes the next node's MAC address.
Sending is encapsulation: data passes down the stack and each layer adds extra information to help delivery.
Receiving is decapsulation: the process reverses, data goes back up the stack, and each layer takes off the information added.
Have a goThe receiving device's link layer has just removed its header. Which layer works on the data next?
The internet layer.
Receiving runs back up the stack, so the next layer above the link layer is the internet layer.
IP addresses deliver data between different networks, while MAC addresses deliver it between devices on the same local network.

The big picture

Data sent across a network is passed down a four-layer stack (application, transport, internet, link). Each layer adds the information needed to deliver it, which is called encapsulation. The receiving device reverses the process, removing that information layer by layer until the original data is left, which is called decapsulation.

Key points

1Application: prepares data for sending and decodes received data for the user. Protocols include HTTP, HTTPS, SMTP and IMAP.

2Transport: splits data into segments with a header (segment number, total, port number). Its protocols are TCP and UDP.

3Internet: builds IP packets with the sender's and receiver's IP addresses. IP operates here.

4Link: carries IP packets across each link, with a header that includes the next node's MAC address. Protocols include Ethernet and Wi-Fi.

5Sending = encapsulation, down the stack, each layer adding information. Receiving = decapsulation, back up in reverse, each layer removing it.

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Worked example

Problem

A receiver strips three labels off a message, but they've been jumbled: "segment 1 of 4", "next node's MAC address" and "sender and receiver IP addresses". Match each label to the layer that added it, then put the labels in the order the receiver removes them.

⚠ Watch out

Picturing the layers as four devices, or running them top to bottom again at the receiving end. The layers are a conceptual stack: receiving goes link, internet, transport, application, and each layer removes what its counterpart added.

🧠

Memory hook

A Tall Igloo Leans: Application, Transport, Internet, Link, from the top down. Wrap on the way down, unwrap in reverse on the way up.

✓

Check yourself

Pick one header from the journey: the segment header, the IP header or the link header. Which layer adds it, what does it hold, and which layer takes it off when the data is received?

Flashcards

(13)

What kind of model is TCP/IP, and how many layers does it have?

A conceptual model with four layers. Each layer is responsible for a different aspect of network communication.

Name the four TCP/IP layers from top to bottom.

Application, transport, internet (sometimes called the network layer), link.

What does the application layer do?

It's the layer used by programs such as web browsers. It prepares data for sending and decodes received data into a meaningful form for the user.

What does the transport layer do to the data it gets from the application layer?

It splits the data into segments and gives each segment a header with the segment number, the total number of segments and the port number.

What does the internet layer do?

It packages segments from the transport layer into IP packets, each including the sender's and receiver's IP addresses.

What does the link layer do, and what is in its header?

It carries IP packets across each individual link. Its header includes the MAC address of the next node.

What is encapsulation?

Data passing down the stack when it is sent, with each layer adding extra information to help delivery.

What is decapsulation?

The reverse process on the receiving device: data goes back up the stack and each layer takes off the extra information that was added, revealing the original data.

Which protocols operate at the application layer?

HTTP, HTTPS, SMTP and IMAP are all application-layer protocols.

Which protocols operate at the transport layer?

TCP and UDP.

At which layer does IP operate?

The internet layer. IP stands for Internet Protocol.

Which layer do Ethernet and Wi-Fi belong to?

The link layer. The protocol used depends on the transmission technology.

Which layer uses IP addresses and which uses MAC addresses?

The internet layer uses IP addresses to deliver data between networks. The link layer uses MAC addresses to deliver data between devices on the same local network.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Follow one message down, then back up

Which layer does each protocol work at?

Which of these sounds like you?

Now say it in your own words

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