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How the Internet works

History of the Internet

  • Today’s Internet traces its beginning back to the early 1960s.
  • Back then the telephone network was the world’s dominant communication network.
  • Three research groups around the world, each unaware of the others’ work, began inventing packet switching networks: MIT (1961), Rand Institute (1964), National Physical Laboratory in England (1965).
  • The work of these research groups came together, when they went on to lead the computer science program at the Advanced Research Projects Agency (ARPA), which was the first packet-switched computer network and a direct ancestor of today’s public Internet.
  • In 1969, the ARPANET was installed at UCLA, Stanford Research Institute (SRI), UC Santa Barbara, and the University of Utah. Internet was four nodes large by the end of 1969.

Robert E. Kahn, one of the inventors of the technolgy behind Internet, explains how computers are interconnected in a packet-switching network: https://youtu.be/fVhwOaCwkb0?t=174

Basis of the Internet Communication: Packet switching

  • In a network application, end systems exchange messages with each other.
  • Messages can contain anything the application designer wants, such as an email message, a JPEG image, or an MP3 audio file.
  • The sender breaks long messages into smaller chunks of data known as packets.
  • Between sender and receiver, each packet travels through communication links and packet switches.

What is the difference between packet-switched and circuit-switched networks?

  • In circuit-switched networks, the resources needed along a path between the end systems are reserved for the duration of the communication session. Traditional telephone networks are examples of circuit-switched networks.
  • In packet-switched networks, these resources are not reserved; a session’s messages use the resources on demand.

World Wide Web

In 1980, Tim Berners-Lee, an English independent contractor at the European Organization for Nuclear Research (CERN) in Switzerland, built ENQUIRE, as a personal database of people and software models, but also as a way to play with hypertext; each new page of information in ENQUIRE had to be linked to a page.

By 1990, Berners-Lee had built all the tools necessary for a working Web: the HyperText Transfer Protocol (HTTP) 0.9, the HyperText Markup Language (HTML), the first Web browser (named WorldWideWeb, which was also a Web editor), the first HTTP server software (later known as CERN httpd), the first web server (http://info.cern.ch), and the first Web pages that described the project itself.

First webpage ever: http://info.cern.ch/hypertext/WWW/TheProject.html

Protocols

A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken using that message.

  • All activity in the Internet that involves multiple remote entities is governed by a protocol.
  • What happens when you make a request to a Web server?
    • you type the URL of a Web page into your Web browser.
    • Your computer sends a connection request message to the Web server and waits for a reply.
    • The Web server receives your connection request message and returns a connection reply message.
    • Knowing that it is now OK to request the Web document, your computer sends the name of the Web page it wants to fetch from that Web server in a GET message.
    • Finally, the Web server returns the Web page (file) to your computer.

Internet addresses

  • What are IP addresses?
  • Why have we run out of IPv4 addresses?
  • IPv6 and its sloooowww adoption
  • localhost and private IP addresses

Network communication

  • Computers run programs, which listen at ports
  • Port: a number that tells the computer which program to send the message to
  • Restricted ports: 0-1023 https://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers
  • Registered ports: everything in /etc/services
  • Once you learn enough Python, you too can write a program that listens at a port!

Network protocols

  • This is how computer programs talk to each other

  • Much like shells, programs have their own language that they expect

  • Example! Talking to a mail server

      HELO digitalbyzantinist.org
  MAIL FROM:<mickey@digitalbyzantinist.org>
  RCPT TO:<aurum@well.com>
  DATA
  From: "Mickey Mouse" <mickey@digitalbyzantinist.org>
  To: Tara <aurum@well.com>
  Date: Wed, 12 Jul 2017 20:42:01 -0500
  Subject: my message

  Hello Tara! It's your old friend Mickey. Squeak.
  .
  QUIT

  • Why might mail get caught in a spam filter?

The Hypertext Transfer Protocol (HTTP)

  • The components of a URL: protocol, host, port, path (protocol://host:port/path?query)
  • Connecting to an HTTP server
  • HTTP request verbs
    • HEAD
    • GET
    • POST
    • PUT
    • DELETE
  • HTTP headers
  • HTTP responses

List of HTTP Status Codes: https://en.wikipedia.org/wiki/List_of_HTTP_status_codes

Application Layer in Computer Networks

References

The content of this course summarizes the relevant sections from the network technolgies textbook "Computer Networking: A Top-Down Approach" by James F. Kurose, Keith W. Ross.

https://commons.wikimedia.org/wiki/File:Packet_Switching.gif by Oddbodz [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]

https://www.cisco.com/en/US/technologies/tk648/tk872/technologies_white_paper0900aecd8054d37d.html