Computer Networks

Why Computer Networks

• Consider computers in ancient times, say the 1970s– To communicate, two computers separated

by a distance had to be connected by a rented phone line to communicate

– If you wanted your computer to talk to two other computers, you needed two phone lines . . . and so on.

Why Computer Networks

– If you needed to communicate with 100 computers you needed 100 phone lines.

– This approach did not, scale up.

Arpanet

• DARPA Defense Advance Research Projects Administration. (The bad guys in the X files– Responsible for coordinating much Defense

Department Research– They and the Universities they dealt with

owned the lions share of the worlds computers . . . and they needed to communicate.

Arpanet

• Dedicated p• Phone lines were impossible• A better idea was needed• Researchers had been exploring the idea

of “networking” for five or six years• The better idea looked something like

this . . .

The Better Idea

• Given computers A, B and C– Where A is connected to B– Where B is connected to C– If C needs to talk to A

• Instead of renting another line• Let C send the message for A to B• Let B FORWARD the message to A• In other words, use existing phone lines

The Better Idea

• This was one of those simple ideas that would change the world– Appropriately, the contract was bid in 1968

and let in 1969– The net came up in late 1969 and included

• UCLA• UC Santa Barbara• Stanford• University of Utah

Arpanet

• The net was called Arpanet and is, in fact the internet we use today

• The protocol used was TCP/IP which we will study

• Originally, the computers that did the message forwarding were called IMPs (Interface Message Processors) We now call machines that forward, routers.

Arpanet

• Arpanet in 1977• http://en.wikipedia.org/wiki/Image:Arpnet-

map-march-1977.png

Arpanet

• Arpanet grew rapidly, by the mid eighties most colleges and Universities and private sector research facilities were on the net.

• By the time of the Morris worm in the late eighties there were 10s of thousands of machines connect to what was by then know as the internet.

Internet

• In the early 90s, AOL became the first online service to provide its customers access to the internet.

• The intenet community swelled from tens of thousands of users to millions of users quite literally overnight.

World Wide Web

• In 1990 CERN, a European physics center, began research on a hyperlinked document distribution system.

• On August 6, 1991, a prototype of the system was launched. The collection of servers was called the World Wide Web

• The first browser, Mosaic, was launched at this time.

World Wide Web

• Originally, the web included mostly researchers and research centers.

• Then, on day, when AOL made web browsing available to their clients, the world arrived.

• You know the rest of the story

Computer Networks

• In effect, the Internet makes it appear to your computer is directly connected to several million other computers.

• Your computer has, as far as its concerned, a dedicated connection to that computer in an internet cafe in Kenya where that gentleman is having trouble gain access to 10 million dollars.

Computer Network Basics

Computer Networks

• Networks make computers much more useful by connecting them to other systems worldwide.

• By connecting them to a network, we can– Look at the weather forecast

Computer Network Basics

Computer Networks

• Networks make computers much more useful by connecting them to other systems worldwide.

• By connecting them to a network, we can– Buy a textbook

Computer Network Basics

Computer Networks

• Networks make computers much more useful by connecting them to other systems worldwide.

• By connecting them to a network, we can– Check for a movie at a local theater

Computer Network Basics

Computer Networks

• Networks make computers much more useful by connecting them to other systems worldwide.

• By connecting them to a network, we can– Print out an airline boarding pass

Computer Network Basics

Computer Networks

• Networks make computers much more useful by connecting them to other systems worldwide.

• By connecting them to a network, we can– Check on your class assignments

Computer Network Basics

Computer Networks

• Networks make computers much more useful by connecting them to other systems worldwide.

• By connecting them to a network, we can– Do a literature search for your assignment

Computer Network Basics

Computer Networks

• Networks make computers much more useful by connecting them to other systems worldwide.

• By connecting them to a network, we can– Submit your assignment to your professor

Computer Network Basics

Computer Networks

• Networks make computers much more useful by connecting them to other systems worldwide.

• By connecting them to a network, we can– Contact your friends to go out to celebrate

finishing your assignment.

Computer Network Basics

Computer Networks

• Just as networks make computers much more useful by connecting them to other systems worldwide, they also make them more vulnerable.

Computer Network Basics

Computer Networks

• Just as networks make computers much more useful by connecting them to other systems worldwide, they also make them more vulnerable.

• By connecting them to a network, we can

Computer Network Basics

Computer Networks

• Just as networks make computers much more useful by connecting them to other systems worldwide, they also make them more vulnerable.

• By connecting them to a network, we can– Get attacked by a worm exploiting a buffer

overflow weakness in your operating system.

Computer Network Basics

Computer Networks

• Just as networks make computers much more useful by connecting them to other systems worldwide, they also make them more vulnerable.

• By connecting them to a network, we can– Receive an Trojan horse in an email and

inadvertently open it, turning your computer into a zombie.

Computer Network Basics

Computer Networks

• Just as networks make computers much more useful by connecting them to other systems worldwide, they also make them more vulnerable.

• By connecting them to a network, we can– Open a web page that infects your

computer with spyware.

Computer Network Basics

Computer Networks

• Just as networks make computers much more useful by connecting them to other systems worldwide, they also make them more vulnerable.

• By connecting them to a network, we can– Get an email with a phishing attack that

fools you into giving your credit card information to an online crook.

Computer Network Basics

Computer Networks

• Just as networks make computers much more useful by connecting them to other systems worldwide, they also make them more vulnerable.

• By connecting them to a network, we can– Give computer access to a friend, who sets

up file sharing, getting you sued by the RIAA.

Computer Network Basics

Computer Networks

• Just as networks make computers much more useful by connecting them to other systems worldwide, they also make them more vulnerable.

• By connecting them to a network, we can– You get your password for amazon.com

stolen by an eavesdropper, who then buys himself a new gaming system at your expense.

Computer Network Basics

Computer Networks

• Network-based vulnerabilities and attacks are one of the most important security concerns.

Computer Network Basics

Introduction

• Traditionally, the motivation in designing computer networks was to move information quickly and reliably between computers.

Computer Network Basics

Introduction

• So the goal for modern computer networks was to be able to efficiently move information worldwide.

• Security was initially not an issue. Speed and reliability were.

Computer Network Basics

Introduction

• Networks must work for many different kinds of computers.

• Computers with different– Processors,– Operating systems, and – Running different network access

programs.

Computer Network Basics

Introduction• For that reason, networks are designed using

agreed upon, standard rules, called protocols.• Any computer, operating system, and

program that uses these protocols can interact with other computers over networks using the same protocols.

• The protocol called TCP/IP, the Arpanet protocol, has become the defacto standard.

Computer Network Basics

The Client/Server Model

• Computing in computer networks is organized into a client/server model.

Computer Network Basics

The Client/Server Model• As mentioned in Unit One, server computers

provide services on the network. • For example, they host (provide)

– Web sites,– Email services,– Calendars,– Databases,– Games.

Computer Network Basics

The Client/Server Model

• Client computers and programs access servers to take advantage of their services.

Computer Network Basics

The Client/Server Model

• Client computers and programs access servers to take advantage of their services.

• For example,– Web browsers are clients that retrieve web

pages from web servers, and then display them at the client computer.

Computer Network Basics

The Client/Server Model

• Client computers and programs access servers to take advantage of their services.

• For example,– Email clients like Outlook, Mail, and others

embedded in web browsers read and send mail by communicating with mail server computers.

TCP/IP

TCP/IP Networks

• TCP/IP is the networking protocol in the widest use.

• It is the ONLY protocol used on the Internet• Networking protocols view networks as a

series of layers.• Each layer is concerned with a different level

of detail of transporting information across a network.

TCP/IP

Layers

• The TCP/IP Internet model, has the following layers:– Application Layer– Transport Layer– Network Layer– Link Layer

TCP/IP

Layers

• The different layers are how different parts of the computer view the task of transporting information across a network.

• The layers hide detail that isn’t relevant to a particular part of the computer.

TCP/IP

Packets

• As we mentioned in Section Two, information in computers is in digital format, usually organized as bytes.

TCP/IP

Packets

• Multiple bytes of information to be sent across the network are organized into packets.– For example, a web server sending an

image will send the information as one or more packets with the bytes making up the image.

TCP/IP

Application Layer

• The application layer is concerned with one program sending packets of data to another program on another computer across a network.

• They are not concerned with the details of how this happens.– That is the concern of “lower” layers.

TCP/IP

Application Layer

• The application layer is controlled by programs that are part of the operating system.

• When a program wants to send information, it calls the application layer program, and gives it the information to send.

TCP/IP

Application Layer

• The application layer is controlled by programs that are part of the operating system.

• When the information arrives at the receiving computer, the application layer program delivers it to the program that is to get the information.

TCP/IP

Transport Layer

• The next layer down is the Transport Layer.

• The transport layer handles the details of communications between a client computer and server computer.

TCP/IP

Transport Layer

• In particular, the transport layer– Establishes communications,– Oversees the communications between the

two computers, and– Terminates communications.

TCP/IP

Transport Layer

• The transport layer can also track information and verify that it has been received.

TCP/IP

Transport Layer

• The programs that handle the transport layer on a computer are contacted by the application layer programs in the operating system, not the applications themselves.

• This simplifies interactions between the application programs and the layers.

TCP/IP

Transport Layer

• There are two major protocols used for delivering data at the transport layer:– TCP, and– UDP.

TCP/IP

Transport Layer

• UDP is a simple protocol that does not promise delivery of packets.

TCP/IP

Transport Layer

• TCP is a more complicated protocol that does promise packet delivery.