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Network Protocols and Packet Switching: Question 3

Syllabus 14.1

Structured A2 8 marks

A student's laptop runs a web browser, which sends a request over the internet to the school's web server to fetch a homework page.

(a) State the four layers of the TCP/IP protocol suite, in the order the request passes through them as it leaves the laptop. [2]

(b) Describe the purpose of the Transport layer and the purpose of the Link layer for this request. [4]

(c) State which TCP/IP layer's protocols include the specific protocol used to request and receive the homework page over the web, and name that protocol. [2]

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Worked solution

Part (a): The four TCP/IP layers, in order

As data leaves a sending host, the TCP/IP protocol suite is implemented as a stack of four layers, each responsible for a different part of preparing and moving the data:

  1. Application layer
  2. Transport layer
  3. Internet layer
  4. Link layer

The browser’s request starts at the Application layer (where it is created), then passes down through the Transport and Internet layers, and finally leaves the laptop through the Link layer onto the physical network.

[2 marks]: [1] for naming all four layers correctly, [1] for the correct order (Application, Transport, Internet, Link).

Transport layer: provides end-to-end delivery of data between the correct application on the source host and the correct application on the destination host. It breaks the browser’s request into smaller segments, and (using TCP) checks that every segment arrives at the web server complete and in the correct order, requesting that any segment which is lost or corrupted in transit be resent. It also attaches port numbers, which identify which specific application on each host the data belongs to, so the reply is delivered back to the browser and not some other application.

Link layer: handles the physical transmission of data across the one physical link connecting the laptop directly to the next device on its own local network, for example, the laptop’s Wi-Fi connection to its wireless access point. This layer deals with the hardware addressing (the MAC address) needed to move data across that single connection, and with actually putting the bits onto (or reading them off) the transmission medium. It is not responsible for the rest of the request’s journey across the internet, only this first (or, at the receiving end, last) local hop.

[4 marks]: [2] for a correct, complete description of the Transport layer’s purpose, [2] for a correct, complete description of the Link layer’s purpose.

Part (c): Which layer, and which protocol?

The protocol used to request and receive a web page is HTTP (HyperText Transfer Protocol), and HTTP operates at the Application layer of the TCP/IP suite, the same layer as other protocols such as FTP, SMTP, POP3, IMAP and BitTorrent, each serving a different application-level purpose.

[2 marks]: [1] for naming the Application layer, [1] for naming HTTP.

Final answers

  • (a) Application layer, then Transport layer, then Internet layer, then Link layer.
  • (b) Transport layer: segments the request and ensures reliable, end-to-end delivery to the correct application, using port numbers. Link layer: handles physical transmission and hardware addressing across the single local link.
  • (c) Application layer; the protocol is HTTP.