Computer Networking › Section A · Computer networks and the internet › Topic A.1
What the internet is
Every message, video and web page you use crosses the same global system. This topic names its parts, explains the rules that let them cooperate, and gives the two complementary views of the internet that the rest of the course builds on.
Two ways to describe the internet
The internet can be described in two complementary ways. The first lists its parts: the devices people use, the cables and radio channels that join them, the boxes that pass data along, and the rules they all follow. The second describes what it does: it is a platform that carries data between programs on different machines, so anyone can build a new application without redesigning the network. The parts view explains how a packet gets from a phone to a server; the services view explains why one network can carry email, maps, games and video calls.
Hosts: the devices at the edge
The devices that run applications are called hosts or end systems, because they sit at the ends of every conversation. Laptops, phones and data-centre servers are hosts, and so are televisions, game consoles, thermostats, cars and traffic sensors, often grouped as the internet of things. Hosts are commonly divided into clients, which request a service, and servers, which provide it. A host needs no screen or person: a weather sensor reporting every minute is as much a host as a laptop. The topic The network edge: hosts and access networks looks at how hosts connect.
Links and transmission rates
Hosts are joined by communication links built from physical media: copper wire, coaxial cable, optical fibre and radio. Each link has a transmission rate, the speed at which it pushes bits onto the medium, in bits per second. Rates use SI prefixes, so 1 Mbps is \(10^6\) bit/s and 1 Gbps is \(10^9\) bit/s. File sizes are usually given in bytes, and 1 byte is 8 bits, so a 100 Mbps link moves at most 12.5 megabytes per second (MB/s). Lower-case b means bits and capital B means bytes; convert before you divide.
Packets and packet switches
A host does not send a large file as one block. It splits the data into smaller pieces and attaches a header to each, holding control information such as the destination address; each piece with its header is a packet. Packet switches take a packet arriving on one link and send it out on another. The two main kinds are routers, which forward packets between networks and dominate the core, and link-layer switches, which forward traffic within a local network. The sequence of links and switches a packet crosses is its route, or path. The destination host reassembles the pieces.
Worked example: splitting a photo into packets
A phone uploads a 3.2 MB photo, that is 3,200,000 bytes. Each packet carries at most 1,460 bytes of photo data plus 40 bytes of headers. 1. Packets needed: \(3{,}200{,}000 \div 1{,}460 = 2{,}191.8\), so 2,192 packets, because a part-filled packet still counts. 2. The last packet carries \(3{,}200{,}000 - 2{,}191 \times 1{,}460 = 1{,}140\) bytes. 3. Header bytes: \(2{,}192 \times 40 = 87{,}680\), about 2.7% extra. 4. In bits, the photo alone is \(3{,}200{,}000 \times 8 = 25.6\) million bits. Check: \(2{,}191 \times 1{,}460 + 1{,}140 = 3{,}200{,}000\) bytes.
A network of networks
Hosts reach the internet through internet service providers (ISPs): a cable or phone company at home, a mobile operator, a university or company network, or the Wi-Fi in a café. Each ISP is itself a network of links and packet switches, run independently by its owner. For any host to reach any other, these access ISPs connect to larger regional, national and global ISPs, which also connect to one another. No single organisation owns the whole system. What makes it one internet is that every ISP runs the Internet Protocol and follows shared addressing and naming rules.
Protocols: the rules of the conversation
Every exchange between communicating devices follows a protocol. A protocol fixes three things: the layout of the messages, the order in which the parties send them, and what each party does when it sends a message, receives one, or meets some other event, such as a reply that never comes. Both sides must run the same protocol, or nothing useful happens. Protocols operate everywhere: between two network cards on a cable, between routers choosing paths, and between programs. Two are central: IP, the Internet Protocol, which sets the packet format every router and host handles, and TCP, the Transmission Control Protocol. Together the internet's main protocols are called TCP/IP.
Worked example: reading a protocol exchange
A garden sensor reports to a server using a simple made-up protocol. The sensor sends READING 21.5; the server replies OK 17, where 17 numbers the stored reading; a malformed message gets ERR; if no reply arrives within 2 seconds, the sensor resends. 1. Format: a keyword, then any values, separated by spaces. 2. Order: the sensor speaks first and the server only answers. 3. Actions on receipt: OK means wait for the next minute; ERR means rebuild and resend. 4. Action on another event: the 2-second timeout triggers a resend. Check: all three parts of the definition appear, including an action caused by silence.
Who sets the standards
Protocols work only if everyone implements them the same way, so they are written down as standards. Internet protocols are developed openly by the Internet Engineering Task Force (IETF) and published as Requests for Comments (RFCs); the name survives from the early days, when the notes really were requests for comment. IP version 4 is RFC 791 (1981), and RFC numbers have passed 9,000: RFC 9000 (2021) defines QUIC. Other bodies standardise links: the IEEE 802 committee writes the Ethernet (802.3) and Wi-Fi (802.11) standards, and 3GPP writes the cellular standards behind 4G and 5G.
The services view: a platform for applications
Web browsing, messaging, streaming and online games are distributed applications: programs on several end systems that exchange data. They run only on hosts, never inside the routers of the core, which forward packets without caring which application sent them. That is why a new application needs no change to the network. A program hands its data to the network through a socket interface, a set of rules provided by the host's operating system for naming the receiving program. The internet offers more than one delivery service, such as reliable or best effort, and each application picks one. The topics Principles of network applications and Socket programming develop this view.
Common mistakes to avoid
First, mixing bits and bytes: a 100 Mbps link carries at most 12.5 MB/s, not 100 MB/s. Second, thinking applications run inside the network; routers forward packets, and the programs run on hosts. Third, treating a router and a link-layer switch as the same device: routers join networks, switches forward traffic within one. Fourth, defining a protocol only by its messages and forgetting the actions, including what to do when nothing arrives. Fifth, assuming one company or government runs the internet. Sixth, rounding a packet count down: a half-filled packet still has to be sent.
Key takeaways
The internet can be described by its parts or by the services it gives applications. Hosts run the applications; links carry bits at rates in bits per second; packet switches, mainly routers and link-layer switches, forward packets along a route. Data travels as packets, each with a header. Hosts reach the internet through ISPs, which interconnect into a network of networks with no single owner. A protocol defines message format, message order and the actions taken on sending, receiving or other events; IP and TCP are central. The IETF publishes RFCs, the IEEE standardises Ethernet and Wi-Fi, and 3GPP standardises cellular networks.
Flashcards
Flashcard
A device at the edge that runs applications
Test your understanding
Question 1 of 5
A home connection is rated at 50 Mbps. What is the most data it can move each second, in megabytes?
Question 2 of 5
A 1,500,000-byte file is sent in packets that each carry at most 1,460 bytes of file data. How many packets are needed?
Question 3 of 5
A device sends a request and waits; after 3 seconds with no reply it sends the request again. Which part of a protocol does the resend rule belong to?
Question 4 of 5
A start-up launches a new video-chat application. Where does its software have to run?
Question 5 of 5
Which organisation develops internet protocols such as IP and TCP and publishes them as RFCs?