The Internet: Hosts, Links, ISPs, and Protocols

The Internet

The Internet is a worldwide computer network connecting billions of computing devices. These devices are called hosts or end systems. Communication links and packet switches connect them. Links transmit data at different rates, measured in bits per second (bps). End systems obtain Internet access through Internet service providers (ISPs).

When one end system sends data to another, the sender divides the data into packets and the destination reassembles it. The sequence of communication links and packet switches a packet traverses is its route or path. Think of transporting a large shipment: the goods are divided among trucks, each truck travels independently, and the shipment is assembled again at the destination.

Distributed Applications

Distributed applications involve several end systems exchanging data: instant messaging, live traffic information, video conferencing, and multiplayer games are examples. The central problem is how an application on one end system sends data to an application on another.

The socket interface specifies how a program delivers data to a particular program on another end system. Alice cannot send Bob a letter simply by writing it and throwing it out of the window. She must put it in an envelope, write the recipient's name, address, and postal code in the required format, attach postage, and place it in a mailbox. Programs similarly follow the socket interface to deliver their data.

Protocols

Communication between people or devices follows protocols: agreed rules governing an exchange. Transmission Control Protocol (TCP) and Internet Protocol (IP) are two central Internet protocols; the Internet protocol suite is commonly called TCP/IP.

Internet standards specify rules used by systems communicating over the Internet. The Internet Engineering Task Force (IETF) develops Internet specifications, publishing documents called Requests for Comments (RFCs). These documents address networking problems and describe protocols including TCP, IP, HTTP, and SMTP. The RFC collection includes standards as well as other kinds of documents.

People also follow protocols constantly. Agreed ways of interacting allow a conversation to work; if the participants do not understand the same rules, communication breaks down.

Activities involving remote communicating entities follow network protocols too. In a simplified web exchange, entering a URL causes the computer to request a connection to a web server. The server responds, the computer requests the named page, and the server sends the file. The diagram presents the broad exchange rather than the detailed TCP handshake.