Past Papers questions

Explain why protocols are essential for communication between computer systems. [2]

Protocols establish standard rules for communication, allowing devices from different manufacturers/platforms to communicate correctly.

What is a protocol?

A protocol is an agreed set of rules that governs how devices communicate with one another. Without a shared protocol, two devices – which may be made by different manufacturers and run different software – would have no common way to format, send, receive or interpret data, so communication would not be possible.

Protocol as a stack

Rather than being a single, monolithic set of rules, network communication is implemented as a stack of layers, where each layer is responsible for a specific part of the communication process and has its own protocols. A layer only needs to know how to communicate with the layer directly above and below it, which keeps each layer's functionality independent and interchangeable.

EXAM TIP

A layered approach means a protocol at one layer can be changed or upgraded without needing to redesign the protocols used at the other layers, as long as the interface between layers stays the same.

The TCP/IP protocol suite

The TCP/IP protocol suite is the set of protocols that underpins communication across the internet. It is organised into four layers:

Application layer: Provides network services directly to the software the user interacts with, and determines the protocol needed for the type of data being sent (e.g. HTTP for web pages, SMTP for email).

Transport layer: Establishes an end-to-end connection between the sending and receiving host, splits data into segments, and ensures data is transferred reliably and in the correct order (using TCP) or with minimal overhead (using UDP).

Internet layer: Responsible for addressing, routing and packaging data into packets so that they can be sent across different networks to reach the correct destination host (handled by IP, the Internet Protocol).

Link layer: Handles the transmission of data over the physical network hardware connecting directly-connected devices, including framing and physical/MAC addressing.

Sending a massage across the internet

A user on Host A sends an email to a user on Host B using the TCP/IP suite:

•Application layer: the email client uses SMTP to format the outgoing message.

•Transport layer: TCP breaks the message into numbered segments and prepares to guarantee reliable, ordered delivery.

• Internet layer: IP adds source and destination IP addresses to each segment, forming packets, and determines how they will be routed.

• Link layer: each packet is transmitted across the physical network media to the next device (e.g. a router) on the path to Host B.

At Host B, the same four layers are used in reverse order to reassemble and pass the message up to the receiving email application.

Protocols

HTTP: Hypertext Transfer Protocol - used to request and transfer web pages and other resources between a web browser (client) and a web server.

FTP: File Transfer Protocol - used to transfer files between a client and a server on a network.

POP3: Post Office Protocol 3 - used to retrieve email from a mail server, typically downloading and then removing messages from the server.

IMAP: Internet Message Access Protocol - used to retrieve email from a mail server while keeping messages synchronised/stored on the server, allowing access from multiple devices.

SMTP: Simple Mail Transfer Protocol - used to send outgoing email from a client to a mail server, and between mail servers.

BitTorrent: A peer-to-peer protocol used to distribute files by having many hosts share pieces of the same file directly with one another, rather than downloading the whole file from a single server.

POP3 is an email communication protocol. Identify and describe two other communication protocols that are used when sending or receiving emails. [4]

SMTP - sends emails between mail servers or from a computer to a mail server.

IMAP - allows users to access/read emails from the mail server on multiple devices without removing them.

Describe two ways in which packet switching ensures a complete message is received when passing across a network.

1. Packets are checked when received; damaged or missing packets can be requested again.

2. Packets can use different routes, so a failed route can be avoided by rerouting the packet.

HTTP and IMAP are examples of protocols used in the Application Layer of

the TCP/IP protocol suite. State the purpose of the HTTP and IMAP protocols.

[2]

HTTP: Transfers web pages/hypertext documents between a web server and a client.

IMAP: Allows users to receive/access emails while keeping them stored on the mail server.

Describe how files are shared using the BitTorrent protocol. [4]

A .torrent file contains metadata and the tracker address. The tracker identifies peers in the swarm; peers download and upload pieces of the file. A peer with the complete file becomes a seed.

Identify one benefit of circuit switching and one benefit of packet switching. [2]

Circuit switching: A dedicated path gives a steady data rate.

Packet switching: Bandwidth can be shared and packets can be rerouted around failed paths.

Identify two differences between circuit switching and packet switching.

[2]

1. Circuit switching uses a dedicated path; packet switching does not.

2. In circuit switching the data arrives in order; in packet switching packets may arrive out of order and must be reassembled at the destination

Identify and describe three protocols that are used in the Application Layer of

the TCP/IP protocol suite. [6]

HTTP — transfers web pages between a web server and a client.

FTP — transfers files between computers over a network.

SMTP — sends emails from a client to a mail server or between mail server

The Application Layer and Transport Layer are two layers of the TCP/IP

protocol suite. Describe the purpose of the Application Layer and the purpose

of the Transport Layer. [5]

Application Layer: Provides services/interface for applications and users and defines protocols

for exchanging data.

Transport Layer: Provides logical communication between applications on different hosts,

ensuring reliable delivery and reassembly of data.

Describe packet switching as a method of transmitting messages across

the internet. [4]

The message is split into packets with headers such as source and destination IP addresses.

Packets are sent independently, may take different routes and may arrive out of order; the destination reassembles them and missing/damaged packets are resen

Secure Socket Layer (SSL) and Transport Layer Security (TLS) are two

protocols.

(a) State two functions of SSL/TLS. [2]

1. Provides data encryption/security.

2. Authenticates/identifies the client and server.

(b) Give two examples of situations where the use of SSL/TLS would be

appropriate. [2]

1. When transmitting passwords or other authentication data.

2. When transmitting private data that must be protected from modificatio

Identify two different layers of the TCP/IP protocol suite. [1]

Application and Transport.

Describe how the TCP/IP protocol suite is applied when a message is sent

through the internet from one host to another. Do not describe the function of

individual layers of the TCP/IP protocol suite. [4]

At the sender, the message passes down through all four layers and is encapsulated with

headers at each layer. Frames are transmitted across the network; at the receiver they pass

up through the layers, headers are removed, and the original message is delivered to the

application

Circuit switching may be used as a method of data transmission.

State two benefits and two drawbacks of circuit switching. [4]

Benefit 1: A dedicated path with the full bandwidth is reserved, giving a constant/steady data

rate.

Benefit 2: Data arrives in order (no need to reassemble it) and with little delay once the

connection is set up.

Drawback 1: The path must be set up before any data can be sent, causing an initial delay.

Drawback 2: The bandwidth stays reserved even when no data is being sent, so capacity is

wasted (and other users cannot use it)

Identify the two main protocols that form Transport Layer Security (TLS) and

state the purpose of each. [4]

Protocol 1: Handshake protocol — establishes a secure connection and allows the client and

server to authenticate and agree encryption algorithms.

Protocol 2: Record protocol — provides secure transmission of application data, encrypting

and authenticating data exchanged between client and server

The Internet layer and Link layer are two layers of the TCP/IP protocol

suite. Describe the purpose of the Internet layer and the purpose of the Link

layer. [5]

Internet layer: Identifies the intended network and host and adds source/destination IP

addresses to datagrams, which are routed across networks.

Link layer: Manages the link between directly connected devices, formats datagrams into

frames and handles delivery/error checking on the local network.

Describe the function of a router in packet switching.

The router reads the destination IP address in the packet header, uses its routing table to

determine the next/optimum hop, forwards the packet to that hop and reduces the hop

counter

Secure Socket Layer (SSL) and Transport Layer Security (TLS) are two

protocols. Explain how SSL/TLS is used when client-server communication is

initiated. [4]

A client initiates the SSL/TLS connection and a handshake begins. The server sends its digital

certificate; the client verifies it and obtains the server's public key. The encryption

algorithms/session keys are agreed, establishing a secure session between client and server