Protocols and Layers

1 State what is meant by a protocol. 1

One mark for each of the following, to a maximum of 1.

• A set of rules governing how devices communicate

Model answer. A protocol is a set of rules that governs how devices communicate, which both the sending and the receiving device must follow for the communication to be understood.

2 Explain why TCP would be used to download a file but UDP would be used for a live video call. 4

One mark for each of the following, to a maximum of 4.

• TCP checks that every packet arrives and reassembles them in order
• A downloaded file would be corrupt if any part were missing
• UDP sends packets without checking or resending, so it is faster
• A resent video packet would arrive too late to be useful, and waiting for it would add delay

Model answer. TCP numbers every packet, waits for the receiver to acknowledge each one, resends anything that fails to arrive and puts the packets back into the right order at the far end. That is essential for a file download, because a file with a missing or misordered section is corrupt and will not work at all. UDP does none of that checking, which makes it faster and removes the delay that waiting for acknowledgements introduces. For a live video call that trade is the right one: a packet that was lost and then resent would arrive after the fraction of a second it belonged to had already been displayed, so it is useless, and the delay caused by waiting for it would degrade the whole call for everyone.

3 Describe two reasons why network protocols are arranged in layers. 4

One mark for each of the following, to a maximum of 4.

• Each layer handles one part of the task, so the problem is broken into manageable parts
• Developers can work on one layer without understanding the whole system
• A layer can be changed or replaced without affecting the layers above or below
• Gives an example, such as changing from Ethernet to Wi-Fi affecting only the link layer

Model answer. The first reason is that layering divides a very large problem into parts small enough to handle. Each layer is responsible for one aspect of communication and only has to interact with the layer directly above and below it, so a developer working on an email client does not need to understand how a signal is placed onto a cable. The second is independence. Because each layer only relies on the interface the next one presents, a layer can be replaced entirely without disturbing the others. Moving a laptop from an Ethernet cable to Wi-Fi changes the link layer completely, but IP, TCP and HTTP carry on unchanged, which is why no application has to be rewritten when the connection type changes.

4 A school website changes from HTTP to HTTPS. Explain the benefit to users. 3

One mark for each of the following, to a maximum of 3.

• HTTPS encrypts the data before it is transmitted
• Intercepted traffic cannot be read without the key
• So personal data such as logins and messages is protected in transit

Model answer. HTTPS carries the same HTTP requests and responses over an encrypted connection, so the data is scrambled before it leaves the user's device and is only unscrambled at the far end. Anybody who intercepts the traffic in between, for example on a shared public network, sees only unreadable data rather than the actual content. That matters because users log in to the school site and may send personal information through it, and under plain HTTP a username and password would travel as readable text that anyone on the same network could capture.