1.3.3 Networks

Protocol

A set of rules that allow (defines how) two devices to communicate

Protocol Layering

The concept of a protocol not simply being a set of rules but those rules being built up into very specific layers and those rule layers behind built on top of each other in a deliberate order creating a layered protocol stack. This results in the rules of a protocol being executed in a specific sequence as you move through the protocol stack

HTTP/ HTTPS

Web page transfer

FTP

file transfer

TCP/IP

packet routing and delivery

POP

Retrieving/ deleting email

IMAP

Synching/ storing email

SMTP

sending email

TCP/IP Stack (Transmission Control Protocol / Internet Protocol)

The most common general-purpose standard protocol that allows any networked computers (including those on The Internet) to communicate with each other whatever their equipment

TCP/IP Stack - 1. Application Layer

Chooses protocol for the application, e.g. HTTP, FTP, POP3

TCP/IP Stack - 2. Transport Layer

Uses TCP protocol - splits data into packets, adds packet numbers and port numbers, and requests retransmission of lost packets

TCP/IP Stack - 3. Network Layer

Adds source and destination IP addresses. Routers operate here, IP + port = socket address

TCP/IP Stack - 4. Link Layer

Handles physical connection. Uses MAC addresses of NICs and uses router MAC if device is on another network

Data Packets Structure

Header: Source and destination IP, Protocol, Packet order, Time to Live

Payload: Actual data

Trailer: Checksum / CRC (error detection)

DNS (Domain Name System)

The Internet’s equivalent of a phone book. They maintain a directory of domain names and translate them to Internet Protocol (IP) addresses. This is necessary because, although domain names are easy for people to remember, computers or machines access websites based on IP addresses

Packet Switching

A method of sending data over a wide area network in which the message is broken into a number of parts which are sent independently, over whatever route is optimum for each packet, and reassembled at the destination

Packet Switching advantages

Multiple routes available, fault tolerant, suitable for large networks

Packet Switching disadvantages

Must reassemble packets, all packets must arrive first

Circuit Switching

A method of sending data over a wide area network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. All data then follows this same path for the duration of the data transfer

Circuit Switching advantages

Data arrives in order, no delay

Circuit Switching disadvantages

Wasted bandwidth, network tied up, same transmission rate required

LAN (Local Area Network)

A collection of computers / computing devices on the same network which are physically close together, for example, all located within one building or site e.g. a home or school network

WAN (Wide Area Network)

A collection of computers / computing devices on the same network which are spread out over a geographically large area, for example a university across several campuses, or a multinational corporation with offices / sites in different cities or even different countries (require extra telecommunications hardware, infrastructure usually owned by third parties)

NIC (network interface card)

Connects device to network, contains unique 48-bit MAC address and written as 12-digit hexadecimal

Switch

Directs data within a network, common in star topology

Router

Connect different networks e.g. connects LANS to internet

Wireless Access Point

Allows wireless connection, often combined with router

Gateway

Translates between different protocols, removes and replaces packet headers

Transmission Media Types

Twisted pair (copper), coaxial (shielded), fibre optic (light)

Advantages of a network

Resource sharing, centralised management, communication

Disadvantages of a network

Security risks, server dependency, traffic congestion

Client-Server

A method of network organisation in which network stations make use of resources available at one or more servers.

Client-Server advantages

Secure, central backups, easy data sharing

Client-Server disadvantages

Expensive, server failure affects all clients, requires specialist staff

Peer to Peer

A method of network organisation in which network stations can share resources on other network stations, so one station can use a printer on another station or save data on another station’s local storage

Peer to Peer advantages

Cheap, easy to set up, no specialist staff needed

Peer to Peer disadvantages

poor security, difficult to locate, separate backups requires, used in piracy

Firewall

A computer application used in a network to prevent external users gaining unauthorised access to a computer system (Prevents unauthorised access by using packet filtering and examining IPs, ports and protocols. Packets can be dropped (no response) or rejected (error message))

Proxies

A computer application that accesses data on a different computer system or network. It controls the access of authorised users to data and allows the operation of the system to be isolated from control by external users (Acts as an intermediary - it hides user identity and caches data for faster loading, reducing traffic and blocking inappropriate content)

Encryption

The process of making data in a computer system unintelligible (uses keys to encrypt/ decrypt data ensures confidentiality during transmission)

Anti-malware & Patching

Software designed to detect, prevent and remove malicious code from a system and keep the system updated

User education

Addressing several specific vulnerabilities created by human behaviour

Threats

Malware - viruses, spyware, Social engineering - phishing, farming, DoS & DDoS attacks, SQL injection, Hackers (black, white, grey hat)