Chapter 1 · Packets, cables and names, from 1969 to today
The Internet
Cut it into packets
Imagine you send a 3 MB photo to a friend. The Internet does not push it down a wire as one lump. Your device cuts it into pieces of at most about 1,500 bytes, which makes roughly 2,000 packets, and writes on each one the sender, the destination and its number in the sequence. In the animation the photo breaks into small tiles that light up in order. Why bother? Small packets let many people share one line, and if one packet is lost, only that little piece has to be sent again, not the whole photo.
Hop, hop, hop
Now every packet travels on its own. At each junction a router reads the destination address and hands the packet to the neighbour that looks closest to the goal. A trip usually takes 10 to 20 such hops, and two packets of the same photo may take different roads. At the far end the numbers put them back in order. The rules are called TCP/IP. ARPANET, the Internet's ancestor, sent its first message on 29 October 1969, and on 1 January 1983 every machine on it switched to TCP/IP, a date many people call the birthday of the Internet.
Lost, late and numbered
TCP is the careful half of the rules. The receiver sends back confirmations such as 'got packets 1 to 40', and if one does not arrive, the sender repeats the missing packet. IP is the addressing half. An IPv4 address is 32 bits long, about 4.3 billion in total, which ran short, so IPv6 uses 128 bits, a number with 39 digits. The journey itself is invisible, but you feel its cost as latency, the delay between sending and answer. In the sandbox, send a message and watch every hop add its little delay.
Cables under the sea
The Internet is not in the sky. Roughly 99 percent of the data between continents travels through cables lying on the ocean floor, and more than 500 of them are in service. Inside each cable are hair-thin glass fibres. A laser at one end blinks pulses of light, the light bounces along the glass, and a detector at the other end turns the blinks back into bits. Repeaters boost the light every few dozen kilometres. Light in glass moves at about 200,000 km/s, around a third slower than in empty space.
From Moscow to New York
Let us time one packet. The straight-line distance from Moscow to New York is about 7,500 km. At 200,000 km/s the light needs 7,500 / 200,000 = 0.0375 s, which is 37.5 milliseconds one way. Real cables wind around coasts and routers add their own pauses, so a real round trip usually takes well over 100 ms. That 37.5 ms is a floor that no engineer can lower, because nothing travels faster than light. Pick two cities in the sandbox and compare its estimate with this sum.
A phonebook for the planet
Computers want numbers, people prefer names. The Domain Name System, DNS, is a giant phonebook spread across the world, designed by Paul Mockapetris in 1983. When you type a name, your device asks a resolver, which asks the root servers, then the servers for the ending such as .com, then the servers of the site itself. The last one answers with an IP address like 203.0.113.7. Answers are remembered for minutes or hours, so repeat visits are fast. A fresh lookup normally takes only tens of milliseconds.
The Web is not the Internet
In March 1989 Tim Berners-Lee, working at CERN, proposed a system of linked pages. By the end of 1990 he had the first browser and server running, and in 1991 the Web opened to the world. It rests on three simple ideas: pages written in HTML, an address for each page called a URL, and a protocol called HTTP for fetching them. On 30 April 1993 CERN released the software into the public domain. The Web is one service riding on the Internet, like email, video calls and online games, which all share the same cables.
In numbers
- 37.5 ms
- one way, Moscow to New York, light in fibre
- 1 Jan 1983
- ARPANET switches every machine to TCP/IP
- ≈ 99 %
- of intercontinental data goes by undersea cable