§1.2The Network Edge

Starting at the edge

The book now moves from the wide picture to the parts, and it begins with the components you use every day: computers, phones and the other devices you own. In networking language these are all hosts 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 , and together they form the network edge network edge Simple The outer part of the Internet, where hosts and applications live. Precise The part of the Internet consisting of the hosts — clients and servers — and the access networks that connect them. Applications run here, never in the network core. introduced in ch. 1 — open in glossary .

They are called end systems 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 because they sit at the edge of the Internet, at the two ends of a conversation. The Internet’s end systems include desktop computers, servers 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 such as web and e-mail servers, and mobile devices such as laptops, smartphones and tablets. An increasing number of things that are not computers at all are being attached as end systems too.

They are called hosts because they host — that is, run — application programs: a web browser, a web server program, an e-mail 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 , an e-mail server.

What the edge looks like

This is the same map as Figure 1.1. Only the emphasis has changed. The book redraws this one picture several times in chapter 1. Each time it fades everything except the part under discussion: the edge here, access networks in §1.2.1, the core in §1.3.1.

Look at the thick blue arc. There is no cable running from the laptop to the enterprise server. The arc is the conversation the two hosts care about, drawn as though the network in between were not there. That gap — between what the two hosts experience and what actually happens hop by hop — is the subject of most of this book.

Clients and servers

Hosts are sometimes divided into two groups. Informally, clients tend to be desktops, laptops and smartphones, and servers tend to be more powerful machines that store and distribute web pages, stream video and relay e-mail.

The split is informal on purpose. Both are hosts. What separates them is which one speaks first and which one waits.

Today, most of the servers you receive search results, e-mail, web pages, videos and mobile app content from live in large data centres 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 .

Inside a data centre

Companies such as Google, Microsoft, Amazon and Alibaba have built very large data centres, each holding tens to hundreds of thousands of hosts. As of 2020, Google had 19 data centres on four continents, holding several million servers between them.

These buildings are not only connected to the Internet. Inside, they contain complex networks of their own that interconnect their hosts — chapter 6 covers those networks in detail.

What the machines look like

The working hosts in a data centre are called blades blade Simple One server in a data centre: a flat case holding a processor, memory and a disk. Precise A commodity host in a data centre, resembling a pizza box, containing CPU, memory and disk storage. Blades are stacked in racks, typically 20 to 40 per rack, and the racks are interconnected by the data centre network. introduced in ch. 1 — open in glossary , and the book compares them to pizza boxes: flat cases holding an ordinary processor, memory and disk storage. Blades are stacked in racks, typically 20 to 40 blades per rack. The racks are then joined by the data centre’s own network.

What they are for

The book gives three purposes, using Amazon as the example.

1. Serving pages to users — the product and purchase pages of the shop.
2. Massively parallel computing — large data-processing jobs Amazon runs on its own data.
3. Cloud computing cloud computing Simple Renting computing power and storage from another company instead of owning machines. Precise The third purpose of a data centre: providing scalable computing and storage environments to other companies, together with implicit access to the provider's high-performance private network. Companies such as Airbnb run their entire web services this way rather than owning data centres. introduced in ch. 1 — open in glossary for other companies — renting that computing power out.

The third has become a major trend. Many companies now use a cloud provider rather than owning any data centre at all. Airbnb, for example, does not own or manage data centres; it runs its entire web service in the Amazon cloud, called Amazon Web Services.

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