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Network Protocols and Packet Switching: Question 4

Syllabus 14.1

Structured A2 10 marks

Oakwood Books is a small independent bookshop that relies on several different application-layer protocols to run its online services.

(a) Oakwood's web developer uses an FTP client to upload updated web pages to the bookshop's web hosting server. State the purpose of FTP in this context. [2]

(b) Every month, Oakwood's mail server sends a newsletter on to each subscriber's own mail server, ready for the subscriber to read later. Describe how SMTP is used to deliver this newsletter from Oakwood's mail server to a subscriber's mail server. [2]

(c) The shop's owner checks the same order-enquiries mailbox from a desktop computer in the shop and from a phone while travelling to book fairs, and expects to see the same read/unread status and the same folders on both devices. State which is more appropriate here, IMAP or POP3, and justify your answer by describing how your chosen protocol manages the mailbox. [3]

(d) Oakwood also distributes a large digital course catalogue (several gigabytes) to several regional distributors at once, using a torrent file rather than a single direct download from Oakwood's own server. Identify the protocol being used here, and describe one way it differs from the client-server pattern used in parts (a) to (c). [3]

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Worked solution

Part (a): FTP for uploading web pages

FTP (File Transfer Protocol) is an application-layer protocol whose purpose is transferring files between a client and a server. Here, the developer’s FTP client uploads the updated web page files from their own computer to Oakwood’s web hosting server, so that the live website is updated with the new content.

[2 marks]: [1] for correctly identifying FTP’s purpose as file transfer, [1] for correctly applying it to this scenario (uploading files to the web host).

Part (b): SMTP for delivering the newsletter

SMTP (Simple Mail Transfer Protocol) is used to send and relay email. Oakwood’s mail server uses SMTP to pass each copy of the newsletter on to the mail server responsible for the corresponding subscriber’s mailbox. SMTP’s job ends once the message has been handed over to the subscriber’s mail server. It does not retrieve the message back out again for the subscriber to read; that is done separately, using a retrieval protocol such as IMAP or POP3.

[2 marks]: [1] for identifying SMTP as the protocol used, [1] for correctly describing it as relaying/passing the message from Oakwood’s mail server to the subscriber’s mail server (not retrieving it).

Part (c): IMAP or POP3 for the shared mailbox?

The owner needs the same mailbox (same read/unread status, same folders) on two different devices. IMAP (Internet Message Access Protocol) is the more appropriate choice, because IMAP keeps the mailbox, messages and folder structure, stored on the mail server rather than moving it onto a device, and synchronises what each connecting device sees, so a message read on the desktop also shows as read on the phone.

POP3 (Post Office Protocol version 3) would not suit this scenario as well: POP3 is designed to download messages to a single device, typically removing them from the server once downloaded, so the desktop and the phone could end up with two different, inconsistent copies of the mailbox rather than one shared, up-to-date view.

[3 marks]: [1] for correctly choosing IMAP, [2] for justifying it by describing how IMAP keeps mail stored and synchronised on the server (or, equivalently, explaining why POP3’s download-and-often-delete behaviour would not keep the two devices consistent).

Part (d): BitTorrent and the peer-to-peer model

The protocol being used to distribute the large catalogue to several distributors at once is BitTorrent. Parts (a) to (c) all follow a client-server pattern, where one central server (Oakwood’s web host or mail server) is the single source that every client connects to.

BitTorrent instead uses a peer-to-peer model: the catalogue file is broken into many pieces, and shared directly between the distributors’ own devices (peers). Once a peer has downloaded a piece of the file, it can start uploading that piece to other peers at the same time as it continues downloading the pieces it is still missing, so distributors end up sharing pieces with one another rather than every single copy having to be downloaded directly from Oakwood’s own server.

[3 marks]: [1] for identifying BitTorrent, [2] for describing a valid difference from the client-server model (peers share pieces directly with each other, so there is no single central source for every copy).

Final answers

  • (a) FTP, transfers/uploads the website files from the developer’s computer to the web host.
  • (b) SMTP, relays the newsletter from Oakwood’s mail server to each subscriber’s mail server.
  • (c) IMAP, keeps the mailbox stored and synchronised on the server across both devices.
  • (d) BitTorrent, a peer-to-peer protocol, where distributors share file pieces directly with each other rather than all downloading from one central server.