§1.1What Is the Internet?

One question, two answers

The book opens by asking what is the Internet? and answers it twice, on purpose. Neither answer is more correct. They describe the same thing from two distances.

1. The nuts-and-bolts answer lists the parts: the devices, the wires, the switches, the rules they follow.
2. The services answer ignores the parts and asks what the Internet does for the programs that use it.

An engineer building a router needs the first answer. A programmer writing a chat application needs the second. You will need both, so we take them in turn.

The nuts-and-bolts answer: what the Internet is made of

Hosts, at the edge

The Internet internet Simple A worldwide network that connects billions of devices so they can exchange data. Precise A computer network that interconnects billions of computing devices throughout the world, running the TCP/IP protocols. It can equally be described as an infrastructure that provides services to distributed applications. introduced in ch. 1 — open in glossary connects billions of computing devices. Not long ago these were mostly desktop computers and servers. Today most users connect with smartphones and tablets: close to half the world’s population are active mobile Internet users, and that share is expected to reach 75 % by 2025. Devices that are not computers at all — televisions, game consoles, thermostats, home security systems, watches, cars, traffic control systems — are being connected too. One estimate counted about 18 billion connected devices in 2017, rising to 28.5 billion by 2022.

In networking language, every one of these devices is a host host Simple Any device connected to the Internet that runs your programs: a laptop, a phone, a server. Precise A device connected to the Internet that runs (hosts) application programs. Also called an end system, because it sits at the edge of the network; the book uses host = end system. introduced in ch. 1 — open in glossary , also called an end system end system Simple The book's other name for a host: a device at the edge of the network that runs applications. Precise A device connected at the edge of the Internet that runs application programs. The book states explicitly that host = end system and uses the two words interchangeably. introduced in ch. 1 — open in glossary . They are called end systems because they sit at the edge of the Internet. They are called hosts because they host — that is, run — application programs.

Hosts split into two informal groups. A client client Simple The host that asks for something, usually your laptop or phone. Precise A host that requests and receives service from a server. Informally, clients tend to be desktops, laptops and smartphones. introduced in ch. 1 — open in glossary asks for things: your laptop, your phone. A server server Simple The host that stores content and answers requests, usually inside a data centre. Precise A more powerful host that stores and distributes Web pages, streams video, relays e-mail and so on. Most servers reside in large data centres. introduced in ch. 1 — open in glossary stores and sends things: web pages, video, e-mail.

Most servers today live inside a data centre data center Simple A building full of thousands of servers, joined by their own internal network. Precise A facility housing tens to hundreds of thousands of hosts, called blades and stacked 20 to 40 per rack, interconnected by a data centre network and connected to the Internet. introduced in ch. 1 — open in glossary , a building holding tens to hundreds of thousands of hosts. Google alone had 19 data centres on four continents as of 2020, holding several million servers between them.

Links, and how fast they are

Hosts are joined by communication links communication link Simple The physical connection that carries bits between two devices. Precise A physical medium — coaxial cable, copper wire, optical fiber or radio spectrum — over which bits are sent between two network devices. Its transmission rate is measured in bits/second. introduced in ch. 1 — open in glossary . A link can be coaxial cable, copper wire, optical fibre or radio. Different links carry data at very different speeds. That speed is the link rate link rate Simple How many bits a link can send each second. Precise The transmission rate R of a link, in bits/second: the rate at which bits are pushed into the link. Packets are always transmitted over a link at the full transmission rate of that link. introduced in ch. 1 — open in glossary and it is measured in bits per second.

Packets

A host almost never sends a large piece of data as one lump. Instead it cuts the data into pieces and puts a header on the front of each piece. The header carries the address of the destination and other control information. Each resulting block is a packet packet Simple A small block of data with an address on it, sent through the network on its own. Precise A package of information formed when a sending host segments its data and adds header bytes to each segment. Packets travel through communication links and packet switches to the destination host, where they are reassembled. introduced in ch. 1 — open in glossary .

Packet switches: routers and link-layer switches

A packet switch packet switch Simple A device that takes a packet arriving on one link and sends it out on another link. Precise A device that takes a packet arriving on one of its incoming communication links and forwards that packet on one of its outgoing communication links. The two most prominent types in today's Internet are routers and link-layer switches. introduced in ch. 1 — open in glossary takes a packet arriving on one of its incoming links and forwards it on one of its outgoing links. That is its entire job.

Packet switches come in many shapes, but two dominate today’s Internet:

• A router router Simple A packet switch in the network core that uses IP addresses to choose the next link. Precise A packet switch that implements layers 1 through 3 of the protocol stack and forwards datagrams using a forwarding table indexed by the destination IP address. Routers are typically used in the network core. introduced in ch. 1 — open in glossary is normally used in the middle of the network.
• A link-layer switch link-layer switch Simple A packet switch used inside a local network; it reads link addresses, not IP addresses. Precise A packet switch that implements only layers 1 and 2 and forwards frames using link-layer addresses such as Ethernet addresses. Link-layer switches do not recognise IP addresses and are typically used in access networks. introduced in ch. 1 — open in glossary is normally used in access networks — the local network in an office or a home.

Both forward packets toward the destination. How each one decides where to send a packet is different, and that difference needs a chapter each: routers in chapter 4, link-layer switches in chapter 6.

The sequence of links and packet switches a packet travels through is its route route Simple The series of links and switches a packet travels through to reach its destination. Precise The sequence of communication links and packet switches traversed by a packet from the sending host to the receiving host. Also called a path. introduced in ch. 1 — open in glossary , also called its path. Cisco predicted that global IP (Internet Protocol) Internet Protocol The network-layer protocol that defines the datagram format and addressing every Internet device must use. introduced in ch. 1 traffic would reach nearly five zettabytes — that is 10²¹ bytes — per year by 2022.

ISPs, and the shape they make

Hosts reach the Internet through an ISP (Internet Service Provider) Internet Service Provider Connects hosts, and other ISPs, to the rest of the Internet. introduced in ch. 1 internet service provider Simple A company or organisation that connects you to the rest of the Internet. Precise A network of packet switches and communication links that provides network access to hosts and to other ISPs. ISPs form a loose hierarchy of access ISPs, regional ISPs and tier-1 ISPs. introduced in ch. 1 — open in glossary . Your ISP might be a cable or telephone company, your university, your employer, the café you are sitting in, or your mobile operator. Each ISP is itself a network of packet switches and links.

The Internet exists to connect hosts to each other, so the ISPs must also be connected to each other. Lower-tier ISPs connect through national and international upper-tier ISPs, and those upper-tier ISPs connect directly among themselves. Every ISP network is managed independently, runs the IP (Internet Protocol) Internet Protocol The network-layer protocol that defines the datagram format and addressing every Internet device must use. introduced in ch. 1 protocol, and follows the same naming and address rules. Section 1.3.3 builds this structure up properly.

The picture above is the book’s Figure 1.1, redrawn. Look at what it contains: a mobile network, a home network, an enterprise network, a regional ISP, a national ISP, two data centre networks and a content provider’s own network. None of these is “the Internet”. The Internet is all of them, joined together — which is why the book calls it a network of networks.

Protocols and standards

Hosts and packet switches run protocols protocol Simple An agreed set of rules for what messages to send, in what order, and what to do with them. Precise A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event. introduced in ch. 1 — open in glossary that control how information is sent and received. The two most important are TCP (Transmission Control Protocol) Transmission Control Protocol The Internet transport protocol that delivers data reliably and in order, with flow control and congestion control. introduced in ch. 1 and IP (Internet Protocol) Internet Protocol The network-layer protocol that defines the datagram format and addressing every Internet device must use. introduced in ch. 1 . IP specifies the format of the packets that travel between routers and hosts. Together the Internet’s main protocols are called TCP/IP (Transmission Control Protocol / Internet Protocol) Transmission Control Protocol / Internet Protocol The collective name for the Internet's principal protocols. It became the ARPAnet standard on 1 January 1983. introduced in ch. 1 .

Because everyone must agree on what each protocol does, protocols are written down as standards. Internet standards are developed by the IETF (Internet Engineering Task Force) Internet Engineering Task Force The body that develops Internet standards. introduced in ch. 1 , and its standards documents are called RFC (Request For Comments) Request For Comments The name of an IETF standards document. There are currently nearly 9000 of them. introduced in ch. 1 s. There are currently nearly 9 000 of them. They define TCP (Transmission Control Protocol) Transmission Control Protocol The Internet transport protocol that delivers data reliably and in order, with flow control and congestion control. introduced in ch. 1 , IP (Internet Protocol) Internet Protocol The network-layer protocol that defines the datagram format and addressing every Internet device must use. introduced in ch. 1 , HTTP (HyperText Transfer Protocol) HyperText Transfer Protocol The application-layer protocol that requests and transfers Web documents. introduced in ch. 1 for the Web and SMTP (Simple Mail Transfer Protocol) Simple Mail Transfer Protocol The application-layer protocol that transfers e-mail messages. introduced in ch. 1 for e-mail. Other bodies standardise other parts: the IEEE (Institute of Electrical and Electronics Engineers) Institute of Electrical and Electronics Engineers The body whose 802 committee standardises Ethernet and WiFi. introduced in ch. 1 802 committee, for example, specifies Ethernet and WiFi (IEEE 802.11 wireless LAN) IEEE 802.11 wireless LAN Wireless LAN access; a user must be within a few tens of metres of the access point. WiFi is a trade name, not an abbreviation. introduced in ch. 1 .

The services answer: what the Internet does for programs

Now forget the hardware. Describe the Internet by what it offers.

The Internet is an infrastructure that provides services to applications: e-mail, web browsing, messaging, maps with live traffic, music and video streaming, social media, video conferencing, multi-player games, location-based recommendations. These are distributed applications distributed application Simple A program that runs on several hosts at once and exchanges data between them. Precise An application involving multiple end systems that exchange data with each other. Internet applications run on hosts; they do not run in the packet switches of the network core. introduced in ch. 1 — open in glossary , because they involve several hosts exchanging data.

So you have an idea for a distributed application. You write programs that run on hosts. Those programs need to send data to each other. How does one program tell the Internet to deliver data to another program on another host?

The answer is the socket interface socket interface Simple The set of rules a program follows to ask the Internet to deliver its data. Precise The interface that end systems provide, specifying how a program running on one end system asks the Internet infrastructure to deliver data to a specific destination program running on another end system. introduced in ch. 1 — open in glossary : a set of rules the sending program must follow so the Internet can deliver its data to the right destination program. Chapter 2 uses it to write real code.

What is a protocol?

First, a human protocol

You already run protocols all day. Consider asking a stranger for the time.

Good manners say you greet the person first. The usual reply to “Hi” is another “Hi”, and you read that reply as permission to ask your real question. A different reply — “Do not bother me”, or “I do not speak English” — tells you the conversation cannot happen, and you do not ask. Sometimes there is no reply at all, and you give up and ask somebody else.

Three things are doing the work here:

1. Specific messages are sent, in a specific order.
2. Specific actions are taken when a message is received.
3. An event that is not a message — no reply within some time — also causes an action.

If two people run different protocols, nothing useful happens. The same is true in networking: it takes two or more entities running the same protocol to get anything done.

Now the same shape, in a network

Here is what happens when you type a web address into a browser.

Compare the two ladders. They have the same four steps in the same order:

The difference is only that the entities exchanging messages are hardware and software components of a device, rather than people. Every activity in the Internet that involves two or more communicating remote entities is governed by a protocol. Hardware protocols control the flow of bits along a wire. Congestion control protocols in hosts decide how fast to send. Protocols in routers decide a packet’s path.

The definition

Mastering computer networking is largely a matter of understanding the what, the why and the how of networking protocols. Most of this book is about them.

Check yourself