How the Internet Works

The Internet and the Web

• The Web is actually just one of many computer applications that run on the Internet

• Among others are email and file transfer programs for text, sound, and video

• The Internet provides the physical basis for sending and receiving information of any kind

• Some similarities to the telephone system, but not totally

Structure and Behavior

• The Internet has a structure or physical organization of its parts

• The Internet has a set of protocols or rules for transmitting information

Structure: Point to Point Connections

I connect my laptop to my desktop via an ethernet cable or wireless card

I connect my iPod or cell phone to my laptop or desktop via a USB line or Bluetooth card

Wide Area Network (WAN)

All connections are point to point

Not practical as the number of nodes gets large

Each computer or resource is called a node

A Central Network Server

Easy to add or remove nodes

Not robust: if the server goes down, so does the network

A Better Structure:Distributed Servers

The central unit is actually a collection of many machines through which messages are routed (one link for many machines, but many of these links are available)

The network is fault-tolerant: if a server goes down, the net stays alive

Internet Service Providers (ISPs)

A hierarchy of connections

Local ISP Local ISP

Regional ISP

National ISP International ISP

Behavior: Internet Protocols

• With so many different types of machines, operating systems, and applications, we need a set of universal rules to exchange information

• These rules are called protocols

• Established and maintained by the Internet Society

Protocol Hierarchy

5 Application HTTP, FTP

4 Transport TCP

3 Network IP

2a Logical link control PPP, ethernet

2b Medium access control ethernet

1 Physical Modem, DSL

The Physical Layer

• Sets up a “pipe” through which bits are transmitted

• Concerned with voltage levels, radio frequencies, converting between analog and digital signals, etc.

• Mechanics of phone line (simple modem) or broadband (DSL or cable modem) transmission

Application

Transport

Network

Logical link control

Medium access control

Physical

Transmission Speeds and Times

Medium Access Control

• Need to establish a connection between sender and receiver across a physical link

• Could have two or more senders competing for the same link

• Poll until the link becomes idle, then transmit

• If two senders collide, both stop sending, wait random times, and then resend

Application

Transport

Network

Logical link control

Medium access control

Physical

Logical Link Control

• Medium access sets up a channel, but logical link ensures that a message arrives correctly

• Uses ARQ (automatic repeat request) algorithm to check information for damage

Application

Transport

Network

Logical link control

Medium access control

Physical

Packets

• Before information is sent, it’s bundled up into one or more packets

• A packet contains– Beginning and ending marks of the information

– The addresses of the sender and receiver

– A number indicating the position of the packet’s information in the original source information

– A damage indicator

Receiving the Message

• When the receiver gets a packet, it– Checks for damage– Sends an acknowledgement message (ACK) to

the sender if the packet is not damaged

ARQ Algorithm

• The sender waits a brief time after sending a packet

• If an acknowledgement message is not received after that time, the sender resends a copy of the packet

• If the receiver gets the copy, it’s thrown away

Network Layer

• Maintains a universal addressing scheme for all nodes in the network

• Delivers messages between any two nodes

Application

Transport

Network

Logical link control

Medium access control

Physical

Addressing a Machine

• Use the Internet Protocol (IP) system

• An IP address is a 32-bit number that contains four 8-bit quantities, each quantity ranging from 0..255

• A domain name is a symbol associated with an IP address

Resolving a Domain Name to an IP Address

• Domain name: www.wlu.edu

• IP Address: 137.113.100.113

• A domain name service (DNS) keeps a table of domain names and their IP addresses

Routing a Message

Ideally, the network layer would find the shortest path through the network from one machine to the other

As the network gets very large, this method becomes impractically slow, so we have to approximate the shortest path

Transport Layer

• One machine might be running several applications (email server, Web server, FTP server, etc.)

• Need to locate the port that the application uses to receive information

• A port number identifies an application’s port

• Example: a Web server uses port 80

Transport Control Protocol (TCP)

• Establishes a “logical” connection between two applications running on different machines

• Example: A Web browser on my home computer connects to W&L’s Web server

• Uses the same ARQ algorithm as the network layer

Application Layer

• Web browser/server (HTTP)

• File client/server (FTP)

• Email client/server

Application

Transport

Network

Logical link control

Medium access control

Physical