CSEC3616Cybersecurity Engineering

    The Internet, protocols and the TCP/IP model

    What makes network security a different problem from the software security covered so far, what the Internet is actually made of, and the four-layer TCP/IP model the rest of this module is built on.

    • Explain what makes network security a different problem from the software security covered earlier in the unit.
    • Name the components the lecture uses to describe the Internet, and what each one does.
    • State the unit's definition of a protocol.
    • List the four layers of the TCP/IP model in order and what each layer is responsible for.

    15 min read

    Intuition

    Every topic in this unit so far has assumed a threat model where the attacker is already somewhere specific: on the same machine, in possession of a stolen credential, or watching a captured message. Networking removes that assumption. A networked program’s input comes from remote, and there is no prior control over who sends it or what they send. That single fact is why network security is treated as its own field rather than a variation on the software security already covered, and why the field is large enough to justify a unit of its own.

    Mechanism

    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.

    Mechanism

    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.

    Mechanism

    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.

    Aside

    The unit’s own diagram of the four-layer stack, and its side-by-side comparison of a human protocol against a network protocol, are both figures that did not survive extraction from the source material. Nothing about their content is in doubt, the layer order and the protocol definition above are both stated directly in text, but the visual comparisons themselves cannot be reproduced here.

    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

    • 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).

    Source

    Week 8 slides PDF