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OCR H446 1.3.3 TCP IP stack and protocol layering

Part 2 of 10 · H446 1.3.3 · Networks

The four-layer TCP/IP model is examined twice over in H446 1.3.3: what each layer is responsible for, and separately why dividing the job into layers is useful at all. This worksheet follows a message down the sending stack and back up the receiving one, then asks students to argue the benefits of layering in their own words.

Students will:

  • state the responsibility of each of the four layers OCR requires
  • follow one message down a sending stack and up a receiving stack
  • explain benefits of layering by linking a feature of the model to the benefit that follows
  • place unfamiliar actions at the correct layer and give the direction of travel

Inside: 5 explanation cells, 1 multiple-choice question, 2 fill-in-the-blanks cells and 2 written answers. 23 marks, about 40 to 50 minutes.

Series: H446 1.3.3 · Networks, part 2 of 10.

Shared by Coding PathwayVerified teacher

  • 10 cells
  • About 45 minutes
  • CC BY-SA 4.0
  • Shared 31 Aug 2026
  • Updated 3 Sept 2026

Preview

The whole resource, exactly as a class sees it. Answers and marking are held back.

TCP/IP stack and protocol layering

OCR uses four layers: application, transport, internet, link. You must know what each does and, separately, why dividing the communication task into layers is useful. The seven-layer OSI model is not required.

A bidirectional model

Four-layer TCP IP send and receive journeySender: data moves downReceiver: data moves upApplicationservice rules and usable dataTransportend-to-end transfer and sequencingInternetIP addressing and routing between networksLinklocal-medium transfer to the next linkApplicationdelivers usable data to the serviceTransportchecks order and reconstructs the streamInternetrecognises the destination IPLinkreceives from the local mediumnetwork links carry the transmitted units

At the sender, each layer performs its job and hands the result down. At the receiver, corresponding processing happens in reverse so the application receives usable data. A named protocol can illustrate a job, but current OCR clarification does not require recall about any particular protocol.

Worked encrypted-message journey

  1. Application: the messaging service applies appropriate communication rules and encryption, then passes data down.
  2. Transport: manages end-to-end transfer, dividing/numbering data at the simplified OCR model and supporting reconstruction.
  3. Internet: adds/uses source and destination IP addressing so packets can be routed between networks.
  4. Link: handles transfer over the current local medium to the next network device/link.

On receipt the layers process upwards; the application removes non-user information and decrypts so the message can be displayed. The 2022 evidence shows why send-only explanations are not enough.

Multiple choice1 mark

Which option explains a benefit of layering rather than merely naming a layer function?

  • AThe internet layer uses IP addresses
  • BA layer can be changed or replaced while its agreed interface limits effects on other layers
  • CThe link layer transfers data over a local medium
  • DThe application layer supports a user-facing service
Fill in the blanks4 marks
A service begins at the gap 1 layer. End-to-end transfer is handled by the gap 2 layer. Routing with IP addresses belongs to the gap 3 layer. Local-medium transfer belongs to the gap 4 layer.
  • application
  • transport
  • internet
  • link
  • OSI
Written answer6 marks

Explain three benefits of protocol layering. For each, state the layer feature and the resulting benefit.

Develop modularity/independence, abstraction, manageable units, compatibility/standards, manufacture to an interface or fault isolation. Do not just describe the four layer jobs.

Students type their answer here.

Apply the model independently

The remaining tasks change the context or reduce the support. Complete them without copying the worked model, then check that each explanation connects a mechanism to its consequence.

Written answer8 marks

A sensor sends an encrypted alert over Wi-Fi to a monitoring application on another network. Place these actions at layers and explain the direction of travel: choose service rules/encrypt; manage numbered transfer; attach/use destination IP; transfer over Wi-Fi; reconstruct/decrypt/display on receipt.

Name each relevant layer and account for both down-stack sending and up-stack receiving.

Students type their answer here.

Fill in the blanks4 marks
The OCR stack has checkpoint gap 1 layers. Sender data moves checkpoint gap 2 the stack and receiver data moves checkpoint gap 3. Describing IP routing states a layer checkpoint gap 4; explaining independent replacement states a layering benefit.

Review your understanding

Before submitting, check that you can explain the main distinction in your own words, apply it in an unfamiliar context and justify each consequence rather than only naming a feature.