computer networks getting started. who am i? 4 dr. lillian n. cassel 4 professor 4 161 mendel...

Computer Networks

Getting started

Who am I?

Dr. Lillian N. Cassel Professor 161 Mendel Science Center 610 519 7341 http://www.csc.villanova.edu/~cassel cassel@acm.org

What is a Network?

What do you think of when “network” comes to mind?– Connected machines?– Facilitated communication?– Resource sharing?– Collaboration?– Access to distributed resources?

Reasons for Installing Networks

Communication Price/Performance Ratio Performance Quality Reliability Accessible Resources Incremental Growth of Computing Power

Significance of Networks in the Computing Environment Computing platform for many applications. Cooperate with other programs running on

different systems.

Simple file access involves a file server. Printing requires contact with the right printer. Performance varies with network load. Some resources may be inaccessible. Programs may be competing for resources.

Networks, Data Communications, and Distributed Systems Closely related topics

– Distributed Systems• Applications that run on multiple computing

platforms

• Issues of concurrency, data integrity, etc.

– Networks• The systems that allow distributed applications

– Data Communications• The components that allow message flow

Network System Software

Three major components to a network:– a transmission medium, – an interface between the network station and

the medium,– and software to drive the network connection.

Support similar to what the O S provides– make life easier for the user– protect the system

Network Operating System

A layer between the application and the “real” operating system– interprets some system calls

• if the call involves network operation, capture and service it

• if the call does not involve network operation, pass the call on to the operating system

Network message exchange

Explicit message sending– attach the identification of the sender and intended

receiver– pass it on to the appropriate network support

facility for processing

Receiving a message– must be listening all the time– look for messages that identify me as the intended

recipient

Protocols

Agreements– How to interpret a message received– How to respond the the message content

• local actions

• returned messages

– What to send when initiating interaction

Protocol Example

Tiger Monet

Tiger, Monet, C, D, “Reset Printer”

Monet, Tiger, OK

Monitor

ResetPrinter

C = confirm; D = Display

Another example protocol

Neptune Jupiter

Neptune, Jupiter, S, “Reset Printer”

Jupiter, Neptune, OK

Monitor

ResetPrinter

S = show, (confirmation implicit)

Duplicate protocols

How to deal with proliferation of protocols that do not interact– Drop one protocol entirely and use the other on

all the systems– Drop both protocols and develop a new one that

all stations use– Each station runs both protocols, using the

correct one for each communication– Keep the original protocol in each set of stations

and also adapt the new global protocol to extend the range of stations each can work

Network Protocols and Standards

The concept of layering– isolate specific functions

• easier to analyze performance, revise

– make upgrades easier

The OSI Reference Model– seven layers– basis of most discussions of network operations

OSI Reference Model

Application Presentation Session Transport Network Data Link Physical

Major Types of Networks Local Area Networks

– high speed– high reliability

Wide Area Networks– speed limited by distance requirements– reliability less dependable

Metropolitan Area Networks– high speed over wider spread than LANs

Wireless Networks– Speed and reliability issues– Mobility usually

A Brief Historical Perspective ARPAnet

– four operating nodes by the end of 1969 – a worldwide network of more than 60,000 nodes in 1989– succeeded by a combination of networks using its

protocols and called the Internet. Alohanet

– began operating in June 1971– the direct ancestor of the bus protocols in very common

use today. – 1.5 years after it began operating, the ALOHA system

was connected to the ARPAnet.

More history

Aloha – lead to the Ethernet (1973)

– In many ways relevant to the wireless networking issues

Cambridge Ring– developed at Cambridge University in the mid 1970s

– similar to very modern approaches such as DQDB

Token ring network – released by IBM in 1985

Internet Addresses and Host Names

Two kinds of addresses– local area network interface address

– Internet (and/or other) address

LAN addresses– Usually, 48 bits associated with the hardware interface

unit

Internet addresses– 32 bits assigned by an authority

– dotted decimal notation

Names and addresses

LAN address is usually not of interest to the human user or to a program

Internet addresses are often required for effective communication

Names allow humans to remember machine identifiers– Servers translate the names to numbers or vice versa

as needed– We will look at that service later

IP addresses More detail will come when we look at the

Network layer 32-bit addresses are represented in dotted

decimal notation w.x.y.z– each part represents 8 bits, so the possible values

range from 0 to 255– Blocks of addresses are assigned to organizations– Network administrators then assign individual

addresses to machines

Types of IP addresses IP address structure (v4)

Class A |0xxxxxxx|yyyyyyyyyyyyyyyyyyyyyyyy| Class B |10xxxxxxxxxxxxxx|yyyyyyyyyyyyyyyy| Class C |110xxxxxxxxxxxxxxxxxxxxx|yyyyyyyy| Multicast|1110xxxxxxxxxxxxxxxxxxxx|yyyyyyyy| Reserved |1111xxxxxxxxxxxxxxxxxxxx|yyyyyyyy| Ranges: Class A = 0.0.0.0 to 127.255.255.255 . Class B = 128.0.0.0 to 191.255.255.255 . Class C = 192.0.0.0 to 223.255.255.255 . Class D = 224.0.0.0 to 239.255.255.255 . Class E = 240.0.0.0 to 247.255.255.255

Class A network addresses

Class A |0xxxxxxx|yyyyyyyyyyyyyyyyyyyyyyyy|

– 27 networks

– each with up to 224 hosts attached

Not quite. Addresses of all 0 or all 1 are special cases and not permitted for general use

Class B and Class C networks

Class B |10xxxxxxxxxxxxxx|yyyyyyyyyyyyyyyy|

– 214 networks (16,384)

– each with up to 216 hosts (65,536)– - again, not quite.

Class C |110xxxxxxxxxxxxxxxxxxxxx|yyyyyyyy|

– 221 networks (2097152)– each with up to 28 hosts (256) (approximately)

The World Wide Web

Joining network infrastructure with hypertext applications

Easily accessible networked communication Easy entry into distributed applications

Web basics Browser

– Presents a graphical display of a document, broadly defined

– Netscape, Mozilla, safari– Internet Explorer– Others

Server– Makes some part of the computer’s file system accessible

to browsers– Sends document files to the browsers

A protocol and a language

HTTP (Hyper text transport protocol)– specifies the behavior of the sending and the

receiving system

HTML (Hypertext markup language)– describes how a page should be presented– originally intended for report formats, primarily text

XML (EXtensible Markup Language)– extend the paradigm beyond documents

Basic web system

Browser

Server

File System

The server may fetch a file from a different system or from itsown file system

Web-based applications

Simplest entry into distributed computing Client and server side

– client side initiates activity– server side responds to the request.

Web Form Server

Browser

Server

Program

Program

Database

Data from the form is input to a program, which may callother programs, interact with a database, or do anything anyother program could do.

Security

A server responding to a web-based form is allowing strangers to execute code.– Obviously could be dangerous– Access is restricted to programs stored in

particular directories• If a problem arises, there is a restricted area to

search

• Programs allowed into that area should be carefully reviewed for safety

Week 1 Summary Networks include hardware and software

– hardware for physical connection– software for meaningful interaction

• protocols allow communication between cooperating processes

• resources and applications give a reason for accessing one computer from another

We will focus on some distributed computing issues and the network facilities that make those possible

We will give little attention to the data communication details

Summary continued The OSI Reference Model summarizes the layers

of software protocols needed to make distributed applications and network accessible resources

Network types – local, wide area, metropolitan area– characterized by speed and susceptibility to errors

Historically, the ARPAnet is the parent of the Internet and Alohanet is the parent of local networks such as Ethernet