Outline

• Applications of Internet
• Behind the Scenes in Viewing a Web Page
• Another Application: Streaming Video
• Requirements of the Internet
• Expectations of Networks

Applications

Most people know the internet through its applications:

• World Wide Web
• E-mail
• Instant Messaging
• Audio/Video Streaming/Torrents
• File Transfer
• Remote Access to Machines

Behind the Scenes in Viewing a Web Page

• A Web resource is identified by a Uniform Resource Identifier (URI):

  http://www.cs.sjsu.edu/faculty/pollett/

  The first part: http://www.cs.sjsu.edu indicates that HTTP, the hypertext transfer protocol, should be used to get the resource and also the name of the machine in which the resource lives; the last part, /faculty/pollett/, actually identifies the page location on that machine.
• When a user clicks on a web page, many messages might be sent over the internet:
  • Messages to translate the server name (www.cs.sjsu.edu) to its internet address 130.65.86.56.
  • Messages to set up the Transmission Control Protocol (TCP) connection between your browser and the server.
  • Messages to send the HTTP "GET" request and get a response from the server.
  • Messages to tear down the Transmission Control Protocol (TCP) connection.

Another Application: Streaming Video

• One way to get video over the internet is just to send the whole video file first and then play it back when it has completely arrived. This is similar to what happens when downloading and displaying a web page.
• The problem is that video files are generally very large and there would be a long delay before seeing anything.
• In video-on-demand, we "stream" data, we send portions of the video file in sequence between sender and receiver and display each portion as it arrives.
• In video-on-demand, it is okay if there is a slight delay before the first image arrives. in a video-conferencing system where we expect instant two-way communication this would be unacceptable.
• The point of these examples is to show the diversity of internet applications, to see the design of the internet must be robust to a variety of usage models, and to hint at some of the complexity that might be involved in building the internet.

Requirements of the Internet

• We want to understand how to build a computer network from the ground up.
• We want to look at the kind of questions that arise naturally in building an actual network.
• We will look at various design choices which are both in current use and have been considered, and we will try to develop a framework to make it easy to understand new network protocols as they arise.

Expectations of Networks

The expectation one has on a network depends on what kind or customer of the network you are:

• An application programmer would list the services his application needs. For example, a guarantee that each message sent by the application arrives in a certain time without error.
• A network designer would list the properties of a cost-effective design. For example, the design should utilize network resources efficiently and fairly allocate resources to different users.
• A network provider would list the characteristics of a system so that it would be easy to administer and manage. For example, it would be useful if faults could be easily isolated and it would be useful to be able to easily account for how resources are being used.

Next day, we will discuss other high-level consideration are made in the design of networks and we will look at common network architectures.