week 2 application layer

8/7/2019 Week 2 Application Layer

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Application Layer Application Layer



Applications Applications

A program or group of programs designed A program or group of programs designedfor end users. Software can be dividedfor end users. Software can be divided

into two general classes: systems softwareinto two general classes: systems softwareandand applications softwareapplications software. Systems. Systemssoftware consists of lowsoftware consists of low--level programslevel programsthat interact with the computer at a verythat interact with the computer at a very

basic level. This includes operatingbasic level. This includes operatingsystems, compilers, and utilities forsystems, compilers, and utilities formanaging computer resources.managing computer resources.



Applications Applications

In contrast, applications software (alsoIn contrast, applications software (alsocalledcalled endend--useruser programsprograms) includes) includesdatabase programs, word processors, anddatabase programs, word processors, andspreadsheets for example. Figurativelyspreadsheets for example. Figurativelyspeaking, applications software sits on topspeaking, applications software sits on top

of systems software because it is unableof systems software because it is unableto run without the operating system andto run without the operating system andsystem utilities.system utilities.



Application Layer Application Layer

TheThe application layerapplication layer is the seventhis the seventhlevel of the seven layer OSI model. Its thelevel of the seven layer OSI model. Its the

highest layer of the OSI model. highest layer of the OSI model.

The book and the course are organizedThe book and the course are organizedand broken down by the OSI model!and broken down by the OSI model!



What is the OSI model?What is the OSI model?

OSI (Open Systems Interconnection) is aOSI (Open Systems Interconnection) is astandard description or "reference model"standard description or "reference model"

for how messages should be transmittedfor how messages should be transmittedbetween any two points in abetween any two points in atelecommunication network. Its purpose istelecommunication network. Its purpose isto guide product implementers so that to guide product implementers so that

their products will consistently work withtheir products will consistently work withother products.other products.

Whatis.comWhatis.com



OSIOSI

The reference model defines seven layers of The reference model defines seven layers of functions that take place at each end of afunctions that take place at each end of acommunication. Although OSI is not alwayscommunication. Although OSI is not alwaysstrictly adhered to in terms of keeping relatedstrictly adhered to in terms of keeping relatedfunctions together in a wellfunctions together in a well--defined layer, manydefined layer, manyif not most products involved inif not most products involved intelecommunication make an attempt to describetelecommunication make an attempt to describethemselves in relation to the OSI model. It isthemselves in relation to the OSI model. It isalso valuable as a single reference view of also valuable as a single reference view of communication that furnishes everyone acommunication that furnishes everyone acommon ground for education and discussion .common ground for education and discussion .




OSIOSI

Developed by representatives of major computerDeveloped by representatives of major computerand telecommunication companies beginning inand telecommunication companies beginning in1983, OSI was originally intended to be a1983, OSI was originally intended to be adetailed specification of interfaces. Instead, thedetailed specification of interfaces. Instead, thecommittee decided to establish a commoncommittee decided to establish a commonreference model for which others could developreference model for which others could developdetailed interfaces, that in turn could becomedetailed interfaces, that in turn could becomestandards. OSI was officially adopted as anstandards. OSI was officially adopted as aninternational standard by the Internationalinternational standard by the InternationalOrganization of Standards (ISO).Organization of Standards (ISO).




OSIOSI

The main idea in OSI is that the process of The main idea in OSI is that the process of communication between two end points incommunication between two end points in

a telecommunication network can bea telecommunication network can bedivided into layers, with each layer addingdivided into layers, with each layer addingits own set of special, related functions.its own set of special, related functions.




OSIOSI

Each communicating user or program is at aEach communicating user or program is at acomputer equipped with these seven layers of computer equipped with these seven layers of

function. So, in a given message between users,function. So, in a given message between users,there will be a flow of data through each layer at there will be a flow of data through each layer at one end down through the layers in that one end down through the layers in that computer and, at the other end, when thecomputer and, at the other end, when themessage arrives, another flow of data upmessage arrives, another flow of data upthrough the layers in the receiving computer andthrough the layers in the receiving computer andultimately to the end user or program.ultimately to the end user or program.




OSIOSI

OSI divides telecommunication into seven layers.OSI divides telecommunication into seven layers.The layers are in two groups. The upper fourThe layers are in two groups. The upper fourlayers are used whenever a message passeslayers are used whenever a message passesfrom or to a user. The lower three layers (up tofrom or to a user. The lower three layers (up tothe network layer) are used when any messagethe network layer) are used when any messagepasses through the host computer. Messagespasses through the host computer. Messagesintended for this computer pass to the upperintended for this computer pass to the upperlayers. Messages destined for some other host layers. Messages destined for some other host are not passed up to the upper layers but areare not passed up to the upper layers but areforwarded to another host. The seven layers areforwarded to another host. The seven layers arein the next seven slidesin the next seven slides




This graphic is taken from The Abdus Salam International Centre for Theoretical Physics.



OSI: Layer 7OSI: Layer 7 Application Layer Application Layer

This is the layer at which communicationThis is the layer at which communicationpartners are identified, quality of service ispartners are identified, quality of service is

identified, user authentication and privacy areidentified, user authentication and privacy areconsidered, and any constraints on data syntaxconsidered, and any constraints on data syntaxare identified. (This layer isare identified. (This layer is not not the applicationthe applicationitself, although some applications may performitself, although some applications may performapplication layer functions.)application layer functions.)

Examples of Application Layer Protocols: HTTP,Examples of Application Layer Protocols: HTTP,UDP, TCP, Real Time Streaming Protocol (RTSP),UDP, TCP, Real Time Streaming Protocol (RTSP),SSLSSL



OSI: Layer 6OSI: Layer 6 Presentation LayerPresentation Layer

This is a layer, usually part of an operatingThis is a layer, usually part of an operatingsystem, that converts incoming and outgoingsystem, that converts incoming and outgoing

data from one presentation format to anotherdata from one presentation format to another(for example, from a text stream into a popup(for example, from a text stream into a popupwindow with the newly arrived text).window with the newly arrived text).

Responds to service requests from applicationResponds to service requests from application

layer and issues service requests to the sessionlayer and issues service requests to the sessionlayer.layer.



OSI: Layer 5OSI: Layer 5 Session LayerSession Layer

This layer sets up, coordinates, andThis layer sets up, coordinates, andterminates conversations, exchanges, andterminates conversations, exchanges, and

dialogs between the applications at eachdialogs between the applications at eachend. It deals with session and connectionend. It deals with session and connectioncoordination.coordination.



OSI: Layer 4OSI: Layer 4 Transport LayerTransport Layer

This layer manages the endThis layer manages the end--toto--end controlend control(for example, determining whether all(for example, determining whether all

packets have arrived) and errorpackets have arrived) and error--checking.checking.It ensures complete data transfer.It ensures complete data transfer.



OSI: Layer 3OSI: Layer 3 Network LayerNetwork Layer

This layer handles the routing of the dataThis layer handles the routing of the data(sending it in the right direction to the(sending it in the right direction to the

right destination on outgoingright destination on outgoingtransmissions and receiving incomingtransmissions and receiving incomingtransmissions at the packet level). Thetransmissions at the packet level). The

network layer does routing andnetwork layer does routing andforwarding.forwarding.



OSI: Layer 2OSI: Layer 2 Data Link LayerData Link Layer

This layer provides synchronization for theThis layer provides synchronization for thephysical level and does bit physical level and does bit--stuffing forstuffing for

strings of 1's in excess of 5. It furnishesstrings of 1's in excess of 5. It furnishestransmission protocol knowledge andtransmission protocol knowledge andmanagement.management.



OSI: Layer 1OSI: Layer 1 Physical LayerPhysical Layer

This layer conveys the bit stream throughThis layer conveys the bit stream throughthe network at the electrical andthe network at the electrical and

mechanical level. It provides the hardwaremechanical level. It provides the hardwaremeans of sending and receiving data on ameans of sending and receiving data on acarrier.carrier.



Whatis.com



Application Layer Review Application Layer Review

This layer supports application and endThis layer supports application and end--useruserprocesses. Communication partners areprocesses. Communication partners areidentified, quality of service is identified, useridentified, quality of service is identified, userauthentication and privacy are considered, andauthentication and privacy are considered, andany constraints on data syntax are identified.any constraints on data syntax are identified.Everything at this layer is applicationEverything at this layer is application--specific.specific.This layer provides application services for fileThis layer provides application services for filetransfers, etransfers, e--mail, and other network softwaremail, and other network softwareservices. Telnet and FTP are applications that services. Telnet and FTP are applications that exist entirely in the application level. Tieredexist entirely in the application level. Tieredapplication architectures are part of this layer.application architectures are part of this layer.



Application Layer Protocols: In Application Layer Protocols: In--

Depth Review of HTTPDepth Review of HTTP The HyperText Transfer Protocol, orThe HyperText Transfer Protocol, or HTTPHTTP, must , must

be the most widely used Application layerbe the most widely used Application layerprotocol in the world today. It forms the basis of protocol in the world today. It forms the basis of what most people understand the Internet towhat most people understand the Internet tobebe the World Wide Web. Its purpose is tothe World Wide Web. Its purpose is toprovide a lightweight protocol for the retrieval of provide a lightweight protocol for the retrieval of HyperText Markup Language (HyperText Markup Language (HTMLHTML) and other) and otherdocuments from Web sites throughout thedocuments from Web sites throughout theInternet. Each time you open a Web browser toInternet. Each time you open a Web browser tosurf the Internet, you are usingsurf the Internet, you are using HTTPHTTP overoverTCPTCP/ /IPIP..




Depth Review of HTTPDepth Review of HTTP Example from web on basic HTTP page retrieval process:Example from web on basic HTTP page retrieval process:

The first important point to note is that a Web page isThe first important point to note is that a Web page istypically made up of many dozens of objects, rangingtypically made up of many dozens of objects, rangingfrom thefrom the HTMLHTML base through to the images that arebase through to the images that arepresent on the page.present on the page.

TheThe HTMLHTML can be thought of as the template for thecan be thought of as the template for thepage overall, instructing the browser on the layout of thepage overall, instructing the browser on the layout of thetext, font sizes and colors, background color of the page,text, font sizes and colors, background color of the page,and which other images need to be retrieved to make upand which other images need to be retrieved to make upthe page.the page.




Depth Review of HTTPDepth Review of HTTP Think of the process, taking place in theThink of the process, taking place in the

following order:following order: Client sends a request for the required page to theClient sends a request for the required page to theWeb server.Web server.

The server analyzes the request and sends back anThe server analyzes the request and sends back anacknowledgment to the client along with theacknowledgment to the client along with the HTMLHTMLcode required to make the page.code required to make the page.

The client will begin interpreting theThe client will begin interpreting the HTMLHTML andand

building the page.building the page. The client, in subsequent requests, will retrieve anyThe client, in subsequent requests, will retrieve any

embedded objects, such as images or otherembedded objects, such as images or othermultimedia sources.multimedia sources.




Depth Review of HTTPDepth Review of HTTP Once all elements of the page have beenOnce all elements of the page have been

retrieved, the client browser will displayretrieved, the client browser will display

the completed Web page. The order andthe completed Web page. The order andtiming of the process described previouslytiming of the process described previouslydepends largely on which implementationdepends largely on which implementation

of of HTTPHTTP is usedis used 1.0 or 1.11.0 or 1.1 although allalthough allbrowsers work in this way of request andbrowsers work in this way of request andresponse.response.




Depth Review of HTTPDepth Review of HTTP TheThe URLURL is the most important piece of is the most important piece of

information that the client browser includes ininformation that the client browser includes in

any GET request. Theany GET request. The URLURL is defined as being ais defined as being acombination of the host where the site iscombination of the host where the site islocated, the scheme used to retrieve the page,located, the scheme used to retrieve the page,and the full path and filename. Optionally, theand the full path and filename. Optionally, the

URLURL may include information such as themay include information such as the TCPTCPport number to be used or a unique referenceport number to be used or a unique referencepoint within a larger page.point within a larger page.



SecuritySecurity

Security of the application layer is critical.Security of the application layer is critical.

Review the Guard the Application LayerReview the Guard the Application Layerdocument.document.

week 2 application layer

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