lec26 networking

7/30/2019 Lec26 Networking

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Network Fundamentals. Internet.


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Network ClassificationsProtocolsCombining Networks

Methods of Process CommunicationDistributed Systems

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Computer network classified as:Local area network (LAN)

A LAN normally consists of a collection of computers in asingle building.

Metropolitan area (MAN)A MAN is a network of intermediate size such as onespanning community.

Wide area network (WAN)WAN links machines over a great distance for exampleneighboring cities.

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Another classification of networking isdone on the bases of ownership:

OpenNetworks internal operations are based on designs that

are in the public domain.ClosedNetworks internal operations are based on designs thatare owned and controlled by a particular entity orindividual.

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Still another way to classify networks isbased on Topology (configuration). Whichrefers to the pattern in which themachines are connected.

Bus (Ethernet)All machines are connected to a commoncommunication line called the bus.

Star (Wireless networks with central AccessPoint)

One machine serves as a central focal point to whichall the other machines are connected.The central focal machine is called hub.All machines communicate through an access point(AP)

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For a network to function reliably, it isimportant to establish rules by whichactivities are conducted. Such rules are

called protocols.Let us consider the problem of coordinatingthe transmission of messages among thecomputers.

Without rules all computers will insist ontransmitting messages at the same time or failto assist other machines when that assistance isrequired.

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CSMA/CD (Carrier Sense, Multiple Access withCollision Detection)

Used in Ethernet

Machine waits until the bus is silentSilent bus provides right to introduce newmessage while continuing to monitor the bus.

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CSMA/CD is not compatible with wireless starnetworks in which all machines communicatethrough a central AP.This is because a machine may be unable todetect that its transmissions are colliding withthose of another. For example:

A machine may not hear the other because its ownsignal drowns out that of the other machine.

Another reason could be that the signals areblocked by other machines or distance eventhrough they are all connected with centralAP.- this is known as hidden terminalproblem. 4-11


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CSMA/CA(Carrier Sense, Multiple Access withCollision Avoidance)

Avoids collision. If collision occurs retransmits themessages.

Used in WiFi Hidden terminal problem The basic difference is that when a machine first

needs to transmit a message and finds thecommunication channel silent , it does not starttransmitting immediately rather it waits for ashort period of time and then starts transmitting.

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Sometimes it is necessary to connectexiting networks to form an extendedcommunication system. This is done bymeans of different devices:

Repeater: Extends a network. It simply passessignals back and forth between two originalbuses.Bridge: Connects two compatible networks

Complex than repeater. It connects two buses but doesnot necessarily pass all the messages across theconnection. Instead it looks at the destination addressand than forwards them.

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Switch: Connect several compatible networksIt is essentially a bridge with multiple connections,allowing it to connect several buses rather than justtwo.

Thus a switch produces a network consisting of severalbuses extending from the switch as spokes on a wheel.

Router: Connects two incompatible networksresulting in a network of networks called aninternet

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Communication between the processes tocoordinate their actions and to performtheir designated tasks is calledInterprocess Communication.

Client-serverOne server, many clientsClient initiates communication by making requests of other processes

Server must execute continuously and satisfy therequest made by the client.

Peer-to-peer (P2P)Two processes communicating as equalsPeer processes can be short-lived

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Systems with parts that run on different computersInfrastructure can be provided by standardized toolkits

Example: Enterprise Java Beans from Sun

MicrosystemsExample: .NET framework from Microsoft

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The Internet: An internet that spans theworld

Original goal was to develop a means of connecting networks that would not be disruptedby local disasters.Today it has shifted from an academic researchproject to a commercial undertaking.

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Internet is a collection of connected networksthat are constructed and managed byorganizations called internet Service Providers.Internet Service Provider (ISP)

Tier-1: consists of high-speed, high-capacity,international WANs. Known as backbone of internet.Tier-2: tend to be more regional in scope and lesspotent in their capabilities.

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Access ISP: Provides connectivity to theInternet

Independent internet also called as intranet,operated by a single authority that is in business

of supplying internet access to individual users.Traditional telephone (dial up connection)Cable connectionsDSL

Wireless

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Domain name system (DNS)Name servers: converting mnemonic addressesinto IP addresses before transmitting a message.The conversion is performed with the aid of

numerous servers called name serves.These name servers are used as an Internet- widedirectory system called DNS.DNS lookup: the process of using the DNS toperform translation is called DNS lookup.

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Allocates IP addresses to ISPs who then assignthose addresses within their regions.Oversees the registration of domains and

domain names.

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Electronic Mail (email)Domain mail server collects incoming mail andtransmits outing mailMail server delivers collected incoming mail toclients via POP3 or IMAP

File Transfer Protocol (FTP)Telnet and SSH

Voice Over IP (VoIP)

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In this section we will focus on an Internetapplication by which multimedia informationis spread over the Internet.

It is based on the concept of hypertext :Text documents that contain links, calledhyperlinks .Hypertext have expanded to audio and video thatis why it is sometimes referred as hypermedia .When using a GUI, the reader of a hypertextdocument can follow the hyperlinks associatedwith it by pointing and clicking with the mouse.


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Hypertext


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In this manner , a reader of a hypertextdocuments can explore related documents orfollow a train of thought from document todocument.

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As portions of various documents are linkedto other documents, a tangled web of related information is found.

When implemented on a computer network, thedocuments within such a web can reside ondifferent machines, forming a network wide web.

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The web that has evolved on the Internetspans the entire globe and is known as theWorld Wide Web(WWW or W3)

A hypertext document on the World Wide Web is

often called a Web page .A collection of closely related Web pages is called aWeb site .WWW had its origins in the work of Tim Berners-

Lee who realized the potential of combining thelinked-document concept with internet technologyand produced the first software for implementingthe WWW in Dec 1990.

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Software packages that allows users to accesshypertext on the internet fall into one of twocategories:

Packages that play the role of clients

Packages that play the role of serversClient package resides on the users computer andis charged with the tasks of obtaining materialsrequested by the user and presenting these

materials to the user in an organized manner.It is the client that provides the user interface thatallows a user to browse within the WebHence the client is often referred as a browser or Webbrowser

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The server package often called the Webserver , resides on a computer containinghypertext documents to be accessed.

Its task is to provide access to the documents underits control as requested by clients.

In summary:A user gains access to hypertext documents by meansof a browser residing on the users computer. This browser, playing the role of a client, obtains the

documents by soliciting the services of the Webservers scattered throughout the Internet.Hypertext documents are normally transferredbetween browsers and Web servers using a protocolknown as Hypertext Transfer Protocol(HTTP )

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In order to locate and retrieve documents onthe WWW each document is given a uniqueaddress called a Uniform ResourceLocator(URL).

Each URL contains the information needed by abrowser to contact the proper server and requestthe desired document.Thus to view a Web page, a person first provideshis or her browser with the URL of the desireddocument and then instructs the browser toretrieve and display the document.

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Encoded as text fileContains tags to communicate with browser

Appearance to start a level one heading

to start a new paragraph

Links to other documents and content

Insert images


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XML: A language for constructing markuplanguages similar to HTML

A descendant of SGML(Standard GeneralizedMarkup Language)Opens door to a World Wide Semantic WebUsed not only for text.


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C minor 2/4 < rest> egth egth

G,egth G, egth G

hlf E


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But what if we wanted a Web pageinvolving animation or one that allows acustomer to fill out an order form? Theseneeds would required additional activityby either the browser or the Web server.

Such activities are called client-side activities if they are performed by client

Server-side activities if performed by aserver.


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Client-side activitiesExamples: java applets, javascript,Macromedia Flash

Server-side activitiesCommon Gateway Interface (CGI):

Set of standards by which clients could request theexecution of programs stored at a server

ServletsA variation to CGI is to allow clients to cause programunits called servlets to be executed at the clients end.

Persona Home Page(PHP)An open source system for implementing server-side

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We will investigate how messages aretransferred over the Internet.This transfer process requires the

cooperation of all the computers in thesystemTherefore software for controlling thisprocess resides on every computer in the

InternetWe begin by studying the overall structure of this software.

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A principal task of networking software is toprovide the infrastructure required fortransferring messages from one machine toanother.

In the internet this message-passing activity isaccomplished by means of a hierarchy of software units, which perform tasks analogous tothose that would be performed if you were to

send a gift in a package from one country toanother country.

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Application: Constructs message withaddressTransport: Chops message into packetsNetwork: Handles routing through theInternetLink: Handles actual transmission of packets


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The demand for open networks has generateda need for publishing standards by whichmanufacturers can supply equipment andsoftware that function properly with productsfrom other venders:

One standard is Open System Interconnection (OSI).This standard is based on a seven-level hierarchy.

TCP/IP protocol suite is a collection of protocol standards used by the internet toimplement the four-level communicationhierarchy implemented in the internet.

Transmission Control Protocol(TCP)Internet Protocol (IP)


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Transport LayerTCPUser Datagram Protocol(UDP)

Network LayerIP (IPv4 and IPv6)

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AttacksMalware (viruses, worms, Trojan horses, spyware,phishing software)Denial of serviceSpam

ProtectionFirewallsSpam filtersProxy Servers

Antivirus software


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FirewallsA primary prevention technique is to filter trafficpassing through a point in the network usually with aprogram called a firewall.

Spam filtersFirewall designed to block unwanted emails.

Proxy ServerA software unit that acts as an intermediary betweena client and a server with the gold of shielding the

client from adverse actions of the server.Antivirus software

Represents a broad class of software products eachdesigned to detect and remove a specific type of infection.

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FTPS, HTTPS, SSL(Secure Sockets Layer)Public-key Encryption

Public key: Used to encrypt messages

Private key: Used to decrypt messages


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lec26 networking

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