The Internet, protocols and the TCP/IP model

Remote input changes the security problem

A networked program accepts input from remote parties, with no prior control over who sends it or what they send. Earlier threat models placed the attacker on a machine, holding a stolen credential or watching a captured message. Remote, uncontrolled input is what makes network security a distinct problem.

Hosts, switches and links form a network of networks

The Internet, in the unit’s own terms, is built from four kinds of thing. Hosts, also called end systems, sit at the edge of the network and are where applications actually run: smartphones, laptops, servers, and IoT devices are all hosts. Packet switches, routers and link-layer switches, take incoming packets and forward them toward their destination. Communication links are the physical or over-the-air paths connecting devices and switches, fibre, copper, radio, satellite, and each is characterised by its bandwidth, the transmission rate of the link. Finally, a network is more than hardware: it is a collection of devices, routers and links managed by a single organisation, an ISP, a university, a corporation. The Internet is precisely a network of networks, built from these organisations connected together.

Protocols define message formats, order and actions

Communication on the Internet happens using protocols: HTTP for the Web, TCP, IP, DNS, SSH, and many more. A protocol defines the format and the order of the messages exchanged between two or more communicating entities, and the actions taken on transmission or receipt of a message or other event. The lecture’s own comparison is a human protocol: asking “what’s the time?” only works because both parties already agree on when to speak, what to say, and what a reply means. A network protocol does the same job for hardware and software components instead of people, and every activity on the Internet involving two or more communicating parties is governed by one.

TCP/IP delivers data through four ordered layers

All of this is organised into the TCP/IP model, four layers stacked in this order:

1. Application layer
2. Transport layer
3. Network layer
4. Network interface layer, itself covering the link layer and the Media Access Control (MAC) layer

Each layer’s job is covered on its own page later in this module: the application and transport layers next, then the network layer, then the link layer and MAC addresses. What matters here is the ordering itself, data moves down the stack when it is sent and up the stack when it is received, and that this four-layer split is the practical model governing how traffic on the Internet is actually packaged and delivered.

Exam detail

Keep the four layers in this exact order, and keep the network interface layer’s internal split, link layer plus MAC layer, straight: some presentations of networking split this into a separate data link layer and physical layer instead. This unit’s own four-layer diagram groups link and MAC together as one layer. If a question names five layers instead of four, it is describing a different model, not this one.

Optional aside

The older OSI (Open Systems Interconnection) model, with its seven layers, was historically significant for standardising network architecture, but the unit describes it as largely theoretical today. The four-layer TCP/IP model is the one that actually governs Internet traffic, and it is the one this module builds on.

Recall

Why is network security distinct, and how do the Internet components, protocols and four layers fit together?

• Network security is distinct because a networked program’s input can come from anyone remote, with no prior control over who sends it or what they send.
• The Internet is made of hosts (end systems), packet switches, communication links (rated by bandwidth), and networks, organisations of the first three connected together as a network of networks.
• A protocol defines the format and order of exchanged messages, and the actions taken on sending or receiving one.
• The TCP/IP model has four layers, in order: application, transport, network, and network interface (link layer plus MAC layer).