session presentation application

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OSI model - What's the presentation and session layer for?

So I feel I pretty well understand the application layer, and everything below (and including) the transport layer.

The session and presentation layers, though, I don't fully understand. I've read the simplistic descriptions in Wikipedia, but it doesn't have an example of why separating out those layers is useful.

• What is the session layer? What does it do, and under what circumstances is it better to have a session layer than simply talk to the transport with your app?
• What is the presentation layer? (same questions as above)
• network-programming

7 Answers 7

The session layer is meant to store states between two connections, like what we use cookies for when working with web programming.

The presentation layer is meant to convert between different formats. This was simpler when the only format that was worried about was character encoding, ie ASCII and EBCDIC. When you consider all of the different formats that we have today(Quicktime, Flash, Pdf) centralizing this layer is out of the question.

TCP/IP doesn't make any allocation to these layers, since they are really out of the scope of a networking protocol. It's up to the applications that take advantage of the stack to implement these.

The reasons there aren't any examples on wikipedia is that there aren't a whole lot of examples of the OSI network model, period.

OSI has once again created a standard nobody uses, so nobody really know how one should use it.

Layers 5-6 are not commonly used in today's web applications, so you don't hear much about them. The TCP/IP stack is slightly different than a pure OSI Model.

One of the reasons TCP/IP is used today instead of OSI is it was too bloated and theoretical, the session and presentation layer aren't really needed as separate layers as it turned out.

I think that presentation layer protocols define the format of data. This means protocols like XML or ASN.1. You could argue that video/audio codecs are part of the presentation layer Although this is probably heading towards the application layer.

I can't help you with the session layer. That has always baffled me.

To be honest, there are very vague boundaries in everything above the transport layer. This is because it is usually handled by a single software application. Also, these layers are not directly associated with transporting data from A to B. Layers 4 and below each have a very specific purpose in moving the data e.g. switching, routing, ensuring data integrity etc. This makes it easier to distinguish between these layers.

Presentation Layer The Presentation Layer represents the area that is independent of data representation at the application layer - in general, it represents the preparation or translation of application format to network format, or from network formatting to application format. In other words, the layer “presents” data for the application or the network. A good example of this is encryption and decryption of data for secure transmission - this happens at Layer 6.

Session Layer When two devices, computers or servers need to “speak” with one another, a session needs to be created, and this is done at the Session Layer. Functions at this layer involve setup, coordination (how long should a system wait for a response, for example) and termination between the applications at each end of the session.

For the presentation layer :because most of communication done between heterogeneous systems (Operating Systems,programing langages,cpu architectures)we need to use a unified idepedent specification .like ANS1 ans BRE.

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What is the OSI model? How to explain and remember its 7 layers

A tutorial on the open systems interconnection (osi) networking reference model plus tips on how to memorize the seven layers..

The Open Systems Interconnect (OSI) model is a conceptual framework that describes networking or telecommunications systems as seven layers, each with its own function.

The layers help network pros visualize what is going on within their networks and can help network managers narrow down problems (is it a physical issue or something with the application?), as well as computer programmers (when developing an application, which other layers does it need to work with?). Tech vendors selling new products will often refer to the OSI model to help customers understand which layer their products work with or whether it works “across the stack”.

The 7 layers of the OSI model

The layers (from bottom to top) are: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

It wasn’t always this way. Conceived in the 1970s when computer networking was taking off, two separate models were merged in 1983 and published in 1984 to create the OSI model that most people are familiar with today. Most descriptions of the OSI model go from top to bottom, with the numbers going from Layer 7 down to Layer 1.

The layers, and what they represent, are as follows:

Layer 7: Application

The Application Layer in the OSI model is the layer that is the “closest to the end user”. It receives information directly from users and displays incoming data to the user. Oddly enough, applications themselves do not reside at the application layer. Instead the layer facilitates communication through lower layers in order to establish connections with applications at the other end. Web browsers (Google Chrome, Firefox, Safari, etc.) TelNet, and FTP, are examples of communications that rely on Layer 7.

Layer 6: Presentation

The Presentation Layer represents the area that is independent of data representation at the application layer. In general, it represents the preparation or translation of application format to network format, or from network formatting to application format. In other words, the layer “presents” data for the application or the network. A good example of this is encryption and decryption of data for secure transmission; this happens at Layer 6.

Layer 5: Session

When two computers or other networked devices need to speak with one another, a session needs to be created, and this is done at the Session Layer . Functions at this layer involve setup, coordination (how long should a system wait for a response, for example) and termination between the applications at each end of the session.

Layer 4: Transport

The Transport Layer deals with the coordination of the data transfer between end systems and hosts. How much data to send, at what rate, where it goes, etc. The best known example of the Transport Layer is the Transmission Control Protocol (TCP), which is built on top of the Internet Protocol (IP), commonly known as TCP/IP. TCP and UDP port numbers work at Layer 4, while IP addresses work at Layer 3, the Network Layer.

Layer 3: Network

Here at the Network Layer is where you’ll find most of the router functionality that most networking professionals care about and love. In its most basic sense, this layer is responsible for packet forwarding, including routing through different routers . You might know that your Boston computer wants to connect to a server in California, but there are millions of different paths to take. Routers at this layer help do this efficiently.

Layer 2: Data Link

The Data Link Layer provides node-to-node data transfer (between two directly connected nodes), and also handles error correction from the physical layer. Two sublayers exist here as well–the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. In the networking world, most switches operate at Layer 2. But it’s not that simple. Some switches also operate at Layer 3 in order to support virtual LANs that may span more than one switch subnet, which requires routing capabilities.

Layer 1: Physical

At the bottom of our OSI model we have the Physical Layer, which represents the electrical and physical representation of the system. This can include everything from the cable type, radio frequency link (as in a Wi-Fi network), as well as the layout of pins, voltages, and other physical requirements. When a networking problem occurs, many networking pros go right to the physical layer to check that all of the cables are properly connected and that the power plug hasn’t been pulled from the router, switch or computer, for example.

Why you need to know the 7 OSI layers

Most people in IT will likely need to know about the different layers when they’re going for their certifications, much like a civics student needs to learn about the three branches of the US government. After that, you hear about the OSI model when vendors are making pitches about which layers their products work with.

In a Quora post  asking about the purpose of the OSI model, Vikram Kumar answered this way: “The purpose of the OSI reference model is to guide vendors and developers so the digital communication products and software programs they create will interoperate, and to facilitate clear comparisons among communications tools.”

While some people may argue that the OSI model is obsolete (due to its conceptual nature) and less important than the four layers of the TCP/IP model, Kumar says that “it is difficult to read about networking technology today without seeing references to the OSI model and its layers, because the model’s structure helps to frame discussions of protocols and contrast various technologies.”

If you can understand the OSI model and its layers, you can also then understand which protocols and devices can interoperate with each other when new technologies are developed and explained.

The OSI model remains relevant

In a post on GeeksforGeeks, contributor Vabhav Bilotia argues several reasons why the OSI model remains relevant, especially when it comes to security and determining where technical risks and vulnerabilities may exist.

For example, by understanding the different layers, enterprise security teams can identify and classify physical access, where the data is sitting, and provide an inventory of the applications that employees use to access data and resources.

“Knowing where the majority of your company’s data is held, whether on-premises or in cloud services, will help define your information security policy,” writes Bilotia. “You can invest in the correct solutions that provide you data visibility within the proper OSI layers once you have this knowledge.”

In addition, the OSI model can be used to understand cloud infrastructure migrations, particularly when it comes to securing data within the cloud.

And because the model has been around for so long and understood by so many, the uniform vocabulary and terms helps networking professionals understand quickly about the components of the networking system “While this paradigm is not directly implemented in today’s TCP/IP networks, it is a useful conceptual model for relating multiple technologies to one another and implementing the appropriate technology in the appropriate way,” Bilotia writes. We couldn’t agree more.

How to remember the OSI Model 7 layers: 8 mnemonic tricks

If you need to memorize the layers for a college or certification test, here are a few sentences to help remember them in order. The first letter of each word is the same as the first letter an OSI layer.

From Application to Physical (Layer 7 to Layer 1): 

• All People Seem To Need Data Processing
• All Pros Search Top Notch Donut Places
• A Penguin Said That Nobody Drinks Pepsi
• A Priest Saw Two Nuns Doing Pushups

From Physical to Application (Layer 1 to Layer 7):

• Please Do Not Throw Sausage Pizza Away
• Pew! Dead Ninja Turtles Smell Particularly Awful
• People Don’t Need To See Paula Abdul
• Pete Doesn’t Need To Sell Pickles Anymore

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The first gadget Keith Shaw ever wanted was the Merlin, a red plastic toy that beeped and played Tic-Tac-Toe and various other games. A child of the '70s and teenager of the '80s, Shaw has been a fan of computers, technology and video games right from the start. He won an award in 8th grade for programming a game on the school's only computer, and saved his allowance to buy an Atari 2600.

Shaw has a bachelor's degree in newspaper journalism from Syracuse University and has worked at a variety of newspapers in New York, Florida and Massachusetts, as well as Computerworld and Network World. He won an award from the American Society of Business Publication Editors for a 2003 article on anti-spam testing, and a Gold Award in their 2010 Digital Awards Competition for the "ABCs of IT" video series.

Shaw is also the co-creator of taquitos.net , the crunchiest site on the InterWeb, which has taste-tested and reviewed more than 4,000 varieties of snack foods.

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Presentation layer and Session layer of the OSI model

There are two popular networking models: the OSI layers model and the TCP/IP layers model. The presentation layer and session layer exist only in the OSI layers models. The TCP/IP layers model merges them into the application layer.

The Presentation Layer

The presentation layer is the sixth layer of the OSI Reference model. It defines how data and information is transmitted and presented to the user. It translates data and format code in such a way that it is correctly used by the application layer.

It identifies the syntaxes that different applications use and formats data using those syntaxes. For example, a web browser receives a web page from a web server in the HTML language. HTML language includes many tags and markup that have no meaning for the end user but they have special meaning for the web browser. the web browser uses the presentation layer's logic to read those syntaxes and format data in such a way the web server wants it to be present to the user.

On the sender device, it encapsulates and compresses data before sending it to the network to increase the speed and security of the network. On the receiver device, it de-encapsulates and decompresses data before presenting it to the user.

Examples of the presentation layer

Example standards for representing graphical information: JPEG, GIF, JPEG, and TIFF.

Example standards for representing audio information: WAV, MIDI, MP3.

Example standards for representing video information: WMV, MOV, MP4, MPEG.

Example standards for representing text information: doc, xls, txt, pdf.

Functions of the presentation layer

• It formats and presents data and information.
• It encrypts and compresses data before giving it to the session layer.
• It de-encrypts and decompresses the encrypted and compressed data it receives from the session layer.

Session layer

The session layer is the fifth layer of the OSI layers model. It is responsible for initiating, establishing, managing, and terminating sessions between the local application and the remote applications.

It defines standards for three modes of communication: full duplex, half-duplex, and simplex.

In the full duplex mode, both devices can send and receive data simultaneously. The internet connection is an example of the full duplex mode.

In the half duplex mode, only one device can send data at a time. A telephone conversation is an example of the half-duplex mode.

In the simplex mode, only one device can send data. A radio broadcast is an example of the simplex mode.

Functions of the session layer

• It is responsible for terminating sessions, creating checkpoints, and recovering data when sessions are interrupted.
• It opens and maintains logical communication channels between network applications running on the local host and network applications running on the remote host.
• If a network application uses an authentication mechanism before it opens a logical communication channel (session) with the remote host, it handles the authentication process.

Examples of the session layer

Structure Query Language (SQL), Remote Procedure Call (RPC), and Network File System (NFS) are examples of the session layer.

By ComputerNetworkingNotes Updated on 2024-09-24

ComputerNetworkingNotes CCNA Study Guide Presentation layer and Session layer of the OSI model

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This article is a part of a  series  on Packet Traveling — everything that happens in order to get a packet from here to there. Use the navigation boxes to view the rest of the articles.

Packet Traveling

• Key Players
• Host to Host Communication
• Host to Host through a Switch
• Host to Host through a Router
• Packet Traveling – Series Finale

The Open Systems Interconnect model (OSI Model) explains all the individual functions that are necessary for the Internet to work.

It is a set of seven independent functions which combine to accomplish the end-goal of Computer to Computer communication.

Much like a car is composed of independent functions which combine to accomplish the end-goal of moving the car forward: A battery powers the electronics, an alternator recharges the battery, an engine rotates a driveshaft, an axle transfers the driveshaft’s rotation to the wheels, and so on and so forth.

Each individual part can be independently replaced or worked on, and as long as each individual part is functioning properly, the car moves forward.

The OSI model is divided into seven different layers, each of which fulfills a very specific function . When combined together, each function contributes to enables full computer to computer data communication.

In the rest of this article, we will look at each of the individual layers of the OSI model and their individual responsibility.

OSI Layer 1 – Physical

The Physical layer of the OSI model is responsible for the transfer of bits — the 1’s and 0’s which make up all computer code.

This layer represents the physical medium which is carrying the traffic between two nodes. An example would be your Ethernet cable  or Serial Cable. But don’t get too caught up on the word “Physical” — this layer was named in the 1970s, long before wireless communication in networking was a concept. As such, WiFi, despite it not having a physical, tangible presence, is also considered a Layer 1 protocol.

Simply put, Layer 1 is anything that carries 1’s and 0’s between two nodes .

The actual format of the data on the “wire” can vary with each medium. In the case of Ethernet, bits are transferred in the form of electric pulses. In the case of Wifi, bits are transferred in the form of radio waves. In the case of Fiber, bits are transferred in the form of pulses of light.

Aside from the physical cable, Repeaters and Hubs also operate at this layer.

A Repeater simply repeats a signal from one medium to the other, allowing a series of cables to be daisy chained together and increase the range a signal can travel beyond the single cable limit. These are commonly used in large WiFi deployments, where a single WiFi network is “repeated” throughout multiple access-points to cover a larger range.

A Hub is simply a multi-port Repeater. If four devices are connected to a single Hub, anything sent by one device gets repeated to the other three.

OSI Layer 2 – Data Link

The Data Link layer of the OSI model is responsible for interfacing with the Physical layer. Effectively, Layer 2 is responsible for putting 1’s and 0’s on the wire, and pulling 1’s and 0’s from the wire.

The Network Interface Card (NIC) that you plug your Ethernet wire into handles the Layer 2 functionality. It receives signals from the wire, and transmits signals on to the wire.

Your WiFi NIC works the same way, receiving and transmitting radio waves which are then interpreted as a series of 1’s and 0’s.

Layer 2 will then group together those 1’s and 0’s into chunks known as Frames.

There is an addressing system that exists at Layer 2 known as the Media Access Control address, or MAC address. The MAC address uniquely identifies each individual NIC . Each NIC is pre-configured with a MAC address by the manufacturer; in fact, it is sometimes referred to as the Burned In Address (BIA).

Aside from your NIC, a Switch also operates at this layer. A Switch’s primary responsibility is to facilitate communication  within  Networks (this idea will be expanded upon in a later article in this series).

The overarching function of the Data Link layer is to deliver packets from one NIC to another. Or to put it another way, the role of Layer 2 is to deliver packets from hop to hop .

OSI Layer 3 – Network

The Network layer of the OSI model is responsible for packet delivery from end to end .

It does this by using another addressing scheme that can logically identify every node connected to the Internet. This addressing scheme is known as the Internet Protocol address, or the IP Address.

It is considered logical because an IP address is not a permanent identification of a computer. Unlike the MAC address which is considered a physical address, the IP address is not burned into any computer hardware by the manufacturer.

OSI Model – Layer 2 vs. Layer 3

The interaction and distinction between Layer 2 and Layer 3 is crucial to understanding how data flows between two computers. For example, if we already have a unique L2 addressing scheme on every NIC (like MAC addresses), why do we need yet another addressing scheme at L3 (like IP addresses)? Or vice versa?

The answer is that both addressing schemes accomplish different functions:

• Layer 2 uses MAC addresses and is responsible for packet delivery from hop to hop .
• Layer 3 uses IP addresses  and is responsible for packet delivery from end to end .

When a computer has data to send, it encapsulates it in a IP header which will include information like the Source and Destination IP addresses of the two “ends” of the communication.

The IP Header and Data are then further encapsulated in a MAC address header, which will include information like the Source and Destination MAC address of the current “hop” in the path towards the final destination.

Here is an illustration to drive this point home:

Notice between each Router, the MAC address header is stripped and regenerated to get it to the next hop. The IP header generated by the first computer is only stripped off by the final computer, hence the IP header handled the “end to end” delivery, and each of the four  different MAC headers involved in this animation handled the “hop to hop” delivery.

OSI Layer 4 – Transport

The Transport layer of the OSI model is responsible for distinguishing network streams.

At any given time on a user’s computer there might be an Internet browser open, while music is being streamed, while a messenger or chat app is running. Each of these applications are sending and receiving data from the Internet, and all that data is arriving in the form of 1’s and 0’s on to that computer’s NIC.

Something has to exist in order to distinguish which 1’s and 0’s belong to the messenger or the browser or the streaming music. That “something” is Layer 4:

Layer 4 accomplishes this by using an addressing scheme known as Port Numbers .

Specifically, two methods of distinguishing network streams exist. They are known as the Transmission Control Protocol (TCP), or the User Datagram Protocol (UDP).

Both TCP and UDP have 65,536 port numbers (each), and a unique application stream is identified by both a Source and Destination port (in combination with their Source and Destination IP address).

TCP and UDP employ different strategies in how data streams are transferred, and their distinction and inner workings are both fascinating and significant, but unfortunately they are outside the scope of this article series. They will be the topic of a future article or series.

To summarize, if Layer 2 is responsible for  hop to hop delivery, and Layer 3 is responsible for end to end delivery, it can be said that Layer 4 is responsible for service to service delivery .

OSI Layer 5, 6, and 7

The Session, Presentation, and Application layers of the OSI model handle the final steps before the data transferred through the network (facilitated by layers 1-4) is displayed to the end user.

From a purely Network Engineering perspective, the distinction between Layers 5, 6, and 7 is not particularly significant. In fact, there is another popular Internet communication model known as the TCP/IP model , which groups these three layers into one single encompassing layer.

The distinction would become more significant if you were involved in Software Engineering. But as this is not the focus of this article series, we will not dive deep into the differences between these layers.

Many network engineers simply refer to these layers as L5-7 or L5+ or L7. For the remainder of this series, we will do the same.

Encapsulation and Decapsulation

The last item we need to discuss before we move on from the OSI Model is that of Encapsulation and Decapsulation . These terms refer to how data is moved through the layers from top to bottom when sending and from bottom to top when receiving .

As the data is handed from layer to layer, each layer adds the information it requires to accomplish its goal before the complete datagram is converted to 1s and 0s and sent across the wire. For example:

• Layer 4 will add a TCP header which would include a Source and Destination port
• Layer 3 will add an IP header which would include a Source and Destination IP address

Layer 2 would add an Ethernet header which would include a Source and Destination MAC address

On the receiving end, each layer strips the header from the data and passes it back up the stack towards the Application layers. Here is the whole process in action:

Note that this is only an example. The header that will be added will be dependent on the underlying communication protocol. For instance, a UDP header might be added at Layer 4 instead, or an IPv6 header might be added at Layer 3.

Either way, it is important to understand that as data is sent across the wire, it gets passed down the stack and each layer adds its own header to help it accomplish its goal. On the receiving end, the headers get stripped off one at a time, layer by layer, as the data is sent back up to the Application layer.

This article categorizes different network functions into different layers of the OSI model. While essential for understanding how packets move through a network, the OSI model itself is not a strict requirement as much as it is a conceptual model — not every protocol will fit perfectly within a single layer of the OSI model.

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“The overarching function of the Data Link layer is to deliver packets from one NIC to another. Or to put it another way, the role of Layer 2 is to deliver packets from hop to hop.”

Doubt: Data link layer delivers the Frame to Network layer where it gets de-encapsulated and we get packet there, am I correct? As in above statement of yours you mentioned layer 2 delivers the packets. Also, how NIC can deliver the packets as its PDU is bits.

Thanks, Vishwa

The combination of [IP][PAYLOAD] is known as a packet. And the combination of [ETH][PAYLOAD] is known as a frame.

In the above, the [ETH] header is the L2 header, and inside the frame’s payload is the packet ([IP][PAYLOAD]). So yes, L2 will deliver the packet to the next NIC.

That said, keep in mind the terms Frame and Packet are often used interchangeably in the industry.

Hope this helps.

Thank you for an explanation Ed, that’s helpful.

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Ok I get it. Data link layer will receive the packet from network layer that contains the IP header. Then it will add the ETH header in it and form a frame. Then what?

• Will it transfer the frame to physical layer and physical layer convert it into bits and signals(radio waves, electric signals) and transmit over the wire
• Or data link layer will convert it into bits and transmit to physical layer

I am really confused about these two. After frame we have two more steps that are bits and signals(signal depending on the medium of transfer like radio waves for wifi and electric signals for ethernet). So what actually happens. And also all the above units like frame, packet, segment, what are they actually? Aren’t they also bits since computers process data in terms of bits

Hi Nikhil, yes they are all made up of bits. You should take a look at Ethernet and IP headers (e.g. on Wikipedia) to see how they are constructed. Another way to look at this data is with WireShark, which is a free packet inspection application, or with Cisco’s Packet Tracer in simulation mode (also free, you have to sign up with Cisco to get access).

Hi Steven. Thanks for the response. Will try those tools. But I was asking that ethernet frame needs to be converted into bits, and then finally to signals. So after data link layer forms the frame, what happens exactly.

Hi Nikhil. Great questions. Be careful not trying to explicitly fit everything into one layer… the OSI model is just a model, it’s just a teaching tool. The TCP/IP stack is the model actually implemented on operating systems, and TCP/IP combines OSI L1 and L2 into a single layer. Meaning the “function of adding a L2 header and turning the packet into Frames” and “transmitting bits across the wire” is handled by the same process.

On way or another, both items are done (header + converted to bits). The distinction of exactly when either happens is ambiguous, and likely not overly helpful when trying to understand Networking.

Thanks Ed. That makes sense now

HI..Nice document

Hi Rajendra, glad you liked it!

This has to be the most extraordinary explanation work I ever came across since browsing the Internet. That someone has the talent to make things that clear and the desire to put such a content available to all of us is unbelievable.

Hi Almudena, glad you enjoyed it, and thank you for such kind words! This article has indeed become one of my favorites to share with others, so much so that I set up an easy shortcut: pracnet.net/osi.

Such an amazing discovery this site is for me. Thanks so much for sharing this with us!

Nice article dude. Thanks! Do you have an article for layers 5 6 7 as well?

Hi Ashwin, I’m afraid I don’t cover L5-7 more thoroughly than I do in this article. I did answer a question on Reddit that involved explaining L5, L6, and L7 that might help make sense of it in your mind.

This is the best of its kind.

easy to understand!!!! very Helpful!!!!!!!!!!!

Your animation drives the concept deeper and remains sealed in my mind. I can just close my eyes and visualize. Thanks for the great visualization…outstanding.

The best explanation of the OSI model I have seen so far. My computer networks professor can’t be even remotely compared. Hands down!

Really well worded article! This is a very good series and was a fun read! My boss looked at me weird when he saw me reading about the OSI model when I have my CCNA but I believe that refreshing on the basics will ALWAYS be important and will allow you to even further strengthen your foundation, even if you are already standing tall.

But back to putting credit to where it is due. I have to agree with several responses here, this is by far one of the best representations of the OSI models I have ever seen. The vocabulary and way it is explained is easy to get without rereading (unlike the Cisco books) and the diagrams are very helpful (… Again unlike the Cisco books and other sources).

I will look forward to seeing additional prints if you decide to make more. I thought I saw a comment somewhere asking to go more in depth with the 5-7 layers. And yes it is true it’s not really a need-to-know thing in the IT world, but it would be a great thing to learn, and I feel that if you explained it, it would help A LOT of people to understand it in stead of those layers being this vague, magical, thing . . . that does stuff. Because that’s what plenty of people think, especially novices or beginners in the networking field.

Once again, an amazing article, very thorough, nicely worded, and obviously very passionately wrote.

The best text lecture on topic.

Wonderful (clappings)… thanks

hi, Thank you for explaining in a simple way. i have a question– i heard that there are 3 types of sessions .resumable,non-resumable and null session can you please explain about this?

In Session layer

Hi Krishna, glad you enjoyed the articles.

The rules for the Session, Presentation, and Application layer are much less clearly defined. Essentially, these layers answer these questions:

Session – How do we keep one user’s session separate from another? How do we keep multiple transactions from the same user independent from one another? Presentation – In what format is the data transferred? Letters? If so, ASCII or EBCDIC? Binary? If so, big endian or little endian? etc Application – What sequence(s) of data make up actual data structures or transactions? For example, in FTP the sequence of letters “LIST” issue a command to view the files on a remote FTP server. In HTTP, the sequence of letters “GET” issue a command to retrieve an webpage.

Remember, once we get to these three it varies greatly between different applications. To that end, I am unable to speak specifically to what you have read as “resumable, non-resumable, and null sessions”. It could mean many different may things for many different applications.

Bro, your video on Youtube was awesome. I even can’t think that it was too easy in that way to understand Layer 2 Layer 3 Arp and function of switch, router and packet flow.

I think you should make more video on computer networking, it will be great help to everyone. mainly on the topic such as DNS DHCP VLAN and all

Regards Jony

Glad you enjoyed them! Making more videos is on my list. As is publishing more articles.

You mentioned VLANs, have you seen the VLAN articles? You can see all of them with this short link: pracnet.net/vlans . I hope you enjoy them as much as the other video / articles!

Just made that so much clearer than my Cisco courses ever did. Thank you!!!

How does the layer 2 knows the target MAC address ? I know as soon as I sent the message, I know the source IP, MAC and also destination IP, but how can I know the destination MAC , as it is specify here:

Hi Ysidro, great question!

The destination MAC address is resolved using the Address Resolution Protocol , or ARP .

Great explanation. Would like if you could cover more topics. Looking forward to your further posts.

This is VERY helpful. Thank you!

Amazing article, thanks for posting this, very useful ! Very well explained too.

Your articles are really great. It’s so simple and easy to.understand and visualise that we don’t have to browse to another sites to clear our networking concepts.

You are really boon for network engineer like us.

Keep continuing such beautiful teaching and looking for more articles.

You are truly the best tutor. God bless you.

very nice article and clear explaination

Neat stuff, it really helped me in understanding the concept in a better way. Thank you!

Wow! Informative, Thanks Man.

to understand osi this is the end…..

Crystal Clear. Makes the foundation strong. Awesome. Thanks a lot for the good work.

Hello there ! hope so you doing great mayn same I am feeling right now after reading your article .well Please tell me one thing ? does mac address changes on every node like when our packet send to router with src and destination mac addr then how new src and destination mac address makes up I am confused as far as I know router only have one macAddress with one NIC card

Loved the explanation!! Thank you so much !!!

Super Awesome Explanation… Thanks a lot for wonderful articles. All of your articles are a great source of information with clarity, granularity & preciseness.

Incredible!

SUPER COMPREHENSIVE. UNDERSTOOD what i was able to uderstand in the last two years. Thanks you xoooooooooooooo much sir 🙂

I see the whole world of network in a couple of minutes. thank to u and god I meet this. no more boring!!!

Hi Ed… How can I get full material to study???

these are great. thanks for these articles

I just have one word on the blog, AWESOME!!

OMG, the best ever article, thanks a lot!

8 years excluding college – this is the first time I understood OSI Model . Thank you so much.

Tremendous article!! Many thanks 😀 Networking has become a pleasure to study

Love the animations. They help me understand the concept very effectively.

This is TECHNICAL SEX!

Thanks a ton, I never read something like this in such a simple language.

This is an awesome break down of the OSI model! Thank you.

This is AWESOMEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE

Amazing Explanation , i will be so grateful if you could clear my 1 confusion , i have read that session layer’s responsibility is to start , maintain and terminate the session but we always learn that its the job of tcp to establish a connection via 3-way handshake , whats the difference between the sessions at Session and Transport Layer?

Each layer, in a way, maintains it’s own “session”. For example, L3 maintains a session between two IP addresses. L4 maintains a session between two ports (this is where TCP lives). L5+ might maintain sessions between two different users. Think of HTTP cookies, for example, they are arriving on TCP port 80, but it’s possible an application wants to differentiate multiple user streams within each “TCP port 80” stream. Hence, you need another “session” distinguisher.

Thanks a lot Ed , your explanations are always very amazing. #Respect

Dear Eddie..

Thanks for uploading this content for all of us n really appreciate for your way of explanation. I have seen your youtube videos too. It was simply amazing!! Kindly upload more videos. May God bless you!!

You’re welcome! Glad you enjoyed the content!

its 2019 and i still regard this as the clearest explanation of the User 2 User communication over the internet.

Thanks for the kind words =)

Thank You very much for this explanition

You’re welcome =)

Best article i have seen so far on L2/L3. Glad i found your article and i plan to read all your articles.. thanks very much.

You’re welcome =) Glad you liked it.

Simple and explanatory but also detailed with super animations. You are talented. I will definitely read more.

Thank you..

You’re welcome!

Wish I had your brain dude. Outstanding. I just took a picture of this whole series in my head. Very well organized. Can’t forget it now

Awesome =) Hope it helps you in your studies and Networking journey!

What is the OSI Model?

The OSI Model Defined, Explained, and Explored

The OSI Model Defined

The OSI Model (Open Systems Interconnection Model) is a conceptual framework used to describe the functions of a networking system. The OSI model characterizes computing functions into a universal set of rules and requirements in order to support interoperability between different products and software. In the OSI reference model, the communications between a computing system are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

Created at a time when network computing was in its infancy, the OSI was published in 1984 by the International Organization for Standardization (ISO). Though it does not always map directly to specific systems, the OSI Model is still used today as a means to describe Network Architecture.

The 7 Layers of the OSI Model

Physical layer.

The lowest layer of the OSI Model is concerned with electrically or optically transmitting raw unstructured data bits across the network from the physical layer of the sending device to the physical layer of the receiving device. It can include specifications such as voltages, pin layout, cabling, and radio frequencies. At the physical layer, one might find “physical” resources such as network hubs, cabling, repeaters, network adapters or modems.

Data Link Layer

At the data link layer, directly connected nodes are used to perform node-to-node data transfer where data is packaged into frames. The data link layer also corrects errors that may have occurred at the physical layer.

The data link layer encompasses two sub-layers of its own. The first, media access control (MAC), provides flow control and multiplexing for device transmissions over a network. The second, the logical link control (LLC), provides flow and error control over the physical medium as well as identifies line protocols.

Network Layer

The network layer is responsible for receiving frames from the data link layer, and delivering them to their intended destinations among based on the addresses contained inside the frame. The network layer finds the destination by using logical addresses, such as IP (internet protocol). At this layer, routers are a crucial component used to quite literally route information where it needs to go between networks.

Transport Layer

The transport layer manages the delivery and error checking of data packets. It regulates the size, sequencing, and ultimately the transfer of data between systems and hosts. One of the most common examples of the transport layer is TCP or the Transmission Control Protocol.

Session Layer

The session layer controls the conversations between different computers. A session or connection between machines is set up, managed, and termined at layer 5. Session layer services also include authentication and reconnections.

Presentation Layer

The presentation layer formats or translates data for the application layer based on the syntax or semantics that the application accepts. Because of this, it at times also called the syntax layer. This layer can also handle the encryption and decryption required by the application layer.

Application Layer

At this layer, both the end user and the application layer interact directly with the software application. This layer sees network services provided to end-user applications such as a web browser or Office 365. The application layer identifies communication partners, resource availability, and synchronizes communication.

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Session, Presentation, and Application Layers

Recall that there are two popular network models in use: the OSI Reference Model and the TCP/IP Reference Model. In this chapter you’ll learn about the top three layers of the OSI model. Those three OSI layers correspond to the Application Layer (top layer) in the TCP/IP model. FIGURE 6-1 shows how the layers in each model (OSI and TCP/IP) relate to one another.

FIGURE 6-1 OSI model compared to the TCP/IP model.

The fact that the TCP/IP model groups these layers together should tell you something. Although each layer has its own responsibilities, they are closely related. The functions you’ll ...

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  Layer 5 Session Layer

Build and control sessions

The layer 5 (control of logical connections; also session layer) provides inter-process communication between two systems. Here you can find among others the protocol RPC (Remote Procedure Call). To resolve failures of meeting and similar problems, the session layer services for an organized and synchronized data exchange. To this end, recovery points, so-called fixed points (check points) introduced, where the session can be synchronized after a failure of a transport connection again without the transfer must start from the beginning again.

OSI Layer 5 - Session Layer

In the seven-layer OSI model of computer networking, the session layer is layer 5. The session layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e., a semi-permanent dialogue. Communication sessions consist of requests and responses that occur between applications. Session-layer services are commonly used in application environments that make use of remote procedure calls (RPCs). An example of a session-layer protocol is the OSI protocol suite session-layer protocol, also known as X.225 or ISO 8327. In case of a connection loss this protocol may try to recover the connection. If a connection is not used for a long period, the session-layer protocol may close it and re-open it. It provides for either full duplex or half-duplex operation and provides synchronization points in the stream of exchanged messages. Other examples of session layer implementations include Zone Information Protocol (ZIP) – the AppleTalk protocol that coordinates the name binding process, and Session Control Protocol (SCP) – the DECnet Phase IV session-layer protocol. Within the service layering semantics of the OSI network architecture, the session layer responds to service requests from the presentation layer and issues service requests to the transport layer. Wikipedia

• Authentication
• Authorization
• Session restoration

Popular Session Layer Protocols

Layer 7   Application Layer

Layer 6   presentation layer, layer 5   session layer, layer 4   transport layer, layer 3   network layer, layer 2   data link layer, layer 1   physical layer.

• Government Exam Articles
• OSI Model & Its Layers in Computer Network

OSI Model & Its Layers in Computer Network

The OSI Model or the Open Systems Interconnection Model is a conceptual framework which describes the functions of a networking system. It is used for the transfer of data over a network which moves through different layers. 

From the competitive exam perspective, the OSI Model plays a very important part and questions from this topic may be asked in the Computer Knowledge section of the question paper.

In this article, we shall discuss in detail the OSI Model and its 7 layer architecture. Also, candidates preparing for upcoming exams can get sample questions based on this concept for practice and preparation.

The seven layers of an OSI Model include Physical, Data Link, Network, Transport, Session, Presentation, and Application. Using this model, the functioning of a networking system can be easily explained. 

OSI Model and its Layers PDF:- Download PDF Here

Given below are a few other links which will help candidates understand the other important computer-related concepts in detail:

An Introduction to the OSI Model

Before learning about the detailed structure of the model and its seven layers, given below are a few points of introduction related to the Open Systems Interconnection Model which one must know. 

• In the 1970s the OSI Model was proposed and in the year 1984, it was published by the International Organisation of Standardization (ISO)
• Using this model, troubleshooting has become easier as the error can be detected at different levels
• This also helps in understanding the relationship and function of the software and hardware of a computer network
• The concept that the OSI Model should be a seven-layer structure, was proposed by Charles Bachman at Honeywell Information Systems
• The model initially did gain much popularity as it could not support the Internet protocol suite which was not acceptable to a lot of IT Companies
• The seven layers of the structure are divided into two part: the upper layer or the host layer and lower layer or the media layer

To learn in detail about the Fundamentals of Computer , candidates can visit the linked article. 

7 Layers of the OSI Model

Discussed below is each stage of the Open Systems Interconnection Model in detail. Candidates are advised to go through these carefully to understand the structure and the functioning of the model in a systematic manner:

1. Physical Layer

• It is the bottom-most or the first layer of the OSI Model
• It comprises the raw data which is further transmitted to the higher layers of the structure
• Preparing the physical devices in the network and accepting the received data for transmission
• The termination of connection between two nodes of a network also takes place at this stage
• This layer converts the digital bits into electrical, radio, or optical signals

2. Data Link Layer

• Access to get the data is achieved at this layer
• It breaks the input data into frames which makes analysing the data easier
• Ensures that the data received is free of any errors
• It controls the flow of data in the stipulated time duration and along with a set speed of transmission
• The data is sent to the next layer in the form of packets which are then reviewed for further processing

3. Network Layer

• It acts as a network controller
• Transferring of variable data from one node to another, connected in a network, takes place at this layer 
• Each node has a specific address and the network layer ensures that the data is sent to its destination address
• The data is sent in the form of fragments which are then connected to each other once the processing is done

4. Transport Layer

• The delivery of data packets is managed by the transport layer
• It manages the flow of data, segmentation and desegmentation and error control
• There are five classes of the transport protocol, starting from 0 and continuing till 4 (TP0 to TP4)
• Fragmentation and reassembly of data packets occur that this stage

5. Session Layer

• The connection between the computers connected in a network is managed at this layer
• Establishment, management and termination between the remote and local application takes place here
• Authentication and authorisation happen at this layer
• This layer can also terminate or end any session or transmission which is complete

6. Presentation Layer

• The data is converted into the syntax or semantics which an application understands
• Before passing on the data any further, the data is formatted at this stage
• Functions including compression, encryption, compatible character code set, etc. are also done at this layer of the model
• It serves as a data translator for the network

7. Application Layer

• The interaction with the user or the user application takes place at this stage
• When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit

On the whole, the entire process of transfer of raw data into processed data and finally to the user or the application can be done through this OSI model. It can detect eros, transmit the data and format it during the course of the above-mentioned seven layers.

OSI Model – Sample Questions

Questions based on the OSI Model may be asked in the Computer Awareness section for the various competitive exams, especially Bank, SSC, RRB, Insurance, etc. To be able to answer these questions efficiently, one must comprehend the information discussed above carefully and prepare themselves well enough.

Since the Computer Knowledge section is scoring and easy in comparison to the other subjects included in the exam syllabus aspirants must try scoring the maximum marks here to improve their overall scores.

Thus, to help candidates with their Government exams preparation, given below are a few sample questions which will help them apprehend the standard of the examination and analyse the type of questions which may be asked. 

Q 1. Routers work at which layer of the OSI Model?

• Data Link Layer
• Network Layer
• Physical Layer
• Application Layer
• Transport Layer

Answer: (2) Network Layer

Q 2. What is the full form of OSI?

• Operating Static Internet
• Open Systems Interconnection
• Operating System Interface
• Open Statutory Interface
• Oppressed System Interactions

Answer: (2) Open Systems Interconnection

Q 3. _________ Layer is responsible for Framing, Error Detection and Physical Addressing.

• Application 

Answer: (4) Data Link 

Q 4. _______ layer defines how data is formatted, presented, encoded, and converted for use on the network.

• Presentation 
• Application

Answer: (2) Presentation

Q 5. How many layers are there in the Open Systems Interconnection (OSI) model?

Answer: (2) Seven

Q 6. Which is the first layer of the OSI Model?

• Presentation Layer

Answer: (4) Physical Layer

Q 7. ____________ layer converts raw bits to frames and frames to raw bits in the Open Systems Interconnection Model.

Answer: (3) Data Link

Q 8. _________ is the process of detecting and correcting the error. While ______ is a mechanism to ensure the efficient delivery of Data.

• Error control, Flow control
• Process control, Network control
• Session control, Flow control
• OSI Control, Data control
• None of the above

Answer: (1) Error control, Flow control

Apart from Computer Knowledge, other subjects are also included in the competitive exam syllabus. Candidates can get free sample questions and previous year papers for the various Government exams at the links given below:

Also, Government exam aspirants can get the best Preparation Strategy for Competitive Exams at the linked article. Here they can find the best preparation tips, study material and exam information for the upcoming exams.

For any further update or notes on the various topics included in the exam syllabus, candidates can turn to BYJU’S for assistance. 

Frequently Asked Questions on OSI Model

Q 1. what is the full form of osi, q 2. what is the purpose of an osi model, q 3. what are the different layers of the osi model.

Ans. There are 7 layers of the OSI Model:

• Session Layer

Q 4. What is the difference between the OSI Model and TCP/IP Model?

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15.1.4 Check Your Understanding – Application, Session, Presentation Answers

1. This layer of the OSI model is concerned with the protocols that exchange data between programs running on hosts.

• application

Explanation: The application layer of the OSI model is the layer that is closest to the end user. It provides an interface between application protocols exchanging data between hosts.

2. MKV, GIF, and JPG standards are associated with which OSI layer?

• presentation

Explanation: The presentation layer is concerned with formatting and presenting data in a format that is compatible with the destination device. Examples of presentation layer standards are MKV, GIF, JPG, MOV, and PNG.

3. These three OSI layers define the same functions as the TCP/IP model application layer.

Explanation: The upper three OSI layers; application, presentation, and session, define the application layer functions of the TCP/IP model.

4. Which two are protocols that belong in the OSI application layer?

Explanation: The application layer of the OSI model provides an interface between applications protocols exchanging data between hosts. Protocols at the application layer include DNS, HTTP, SMTP, FTP, and IMAP.

5. This is a function of the OSI session layer.

• compress and decompress data
• provide an interface between applications
• format data for the application layer
• exchange of information to initiate dialog between peers

Explanation: The session layer of the OSI model creates and maintains the dialogs, or sessions, between two communicating hosts.

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Session, presentation and application layer

I almost finished the ccna, in the meantime I'm following Pentesting courses, I noticed enough interesting protocols at the session level. I wish I could understand them as I did with cisco, but I found only generic materials. I wish I had something deeper, which could also include levels 6 and 7. Where can I find what I'm looking for?

The OSI model is a theoretical model, and there aren't any protocols in use today that follow it. So layers 5-7 are just "concepts." In the TCP/IP model (closer to what is actually in use today), there are no session and presentation layers. Everything is simply "application." This question and answer may help you.

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See how imperva can help, what is the osi model.

The Open Systems Interconnection (OSI) model describes seven layers that computer systems use to communicate over a network. It was the first standard model for network communications, adopted by all major computer and telecommunication companies in the early 1980s

The modern Internet is not based on OSI, but on the simpler TCP/IP model. However, the OSI 7-layer model is still widely used, as it helps visualize and communicate how networks operate, and helps isolate and troubleshoot networking problems.

OSI was introduced in 1983 by representatives of the major computer and telecom companies, and was adopted by ISO as an international standard in 1984.

OSI Model Explained: The OSI 7 Layers

We’ll describe OSI layers “top down” from the application layer that directly serves the end user, down to the physical layer.

7. Application Layer

The application layer is used by end-user software such as web browsers and email clients. It provides protocols that allow software to send and receive information and present meaningful data to users. A few examples of application layer protocols are the Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Post Office Protocol (POP), Simple Mail Transfer Protocol (SMTP), and Domain Name System (DNS).

6. Presentation Layer

The presentation layer prepares data for the application layer. It defines how two devices should encode, encrypt, and compress data so it is received correctly on the other end. The presentation layer takes any data transmitted by the application layer and prepares it for transmission over the session layer.

5. Session Layer

The session layer creates communication channels, called sessions, between devices. It is responsible for opening sessions, ensuring they remain open and functional while data is being transferred, and closing them when communication ends. The session layer can also set checkpoints during a data transfer—if the session is interrupted, devices can resume data transfer from the last checkpoint.

4. Transport Layer

The transport layer takes data transferred in the session layer and breaks it into “segments” on the transmitting end. It is responsible for reassembling the segments on the receiving end, turning it back into data that can be used by the session layer. The transport layer carries out flow control, sending data at a rate that matches the connection speed of the receiving device, and error control, checking if data was received incorrectly and if not, requesting it again.

3. Network Layer

The network layer has two main functions. One is breaking up segments into network packets, and reassembling the packets on the receiving end. The other is routing packets by discovering the best path across a physical network. The network layer uses network addresses (typically Internet Protocol addresses) to route packets to a destination node.

2. Data Link Layer

The data link layer establishes and terminates a connection between two physically-connected nodes on a network. It breaks up packets into frames and sends them from source to destination. This layer is composed of two parts—Logical Link Control (LLC), which identifies network protocols, performs error checking and synchronizes frames, and Media Access Control (MAC) which uses MAC addresses to connect devices and define permissions to transmit and receive data.

1. Physical Layer

The physical layer is responsible for the physical cable or wireless connection between network nodes. It defines the connector, the electrical cable or wireless technology connecting the devices, and is responsible for transmission of the raw data, which is simply a series of 0s and 1s, while taking care of bit rate control.

Advantages of OSI Model

The OSI model helps users and operators of computer networks:

• Determine the required hardware and software to build their network.
• Understand and communicate the process followed by components communicating across a network. 
• Perform troubleshooting, by identifying which network layer is causing an issue and focusing efforts on that layer.

The OSI model helps network device manufacturers and networking software vendors:

• Create devices and software that can communicate with products from any other vendor, allowing open interoperability
• Define which parts of the network their products should work with.
• Communicate to users at which network layers their product operates – for example, only at the application layer, or across the stack.

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OSI vs. TCP/IP Model

The Transfer Control Protocol/Internet Protocol (TCP/IP) is older than the OSI model and was created by the US Department of Defense (DoD). A key difference between the models is that TCP/IP is simpler, collapsing several OSI layers into one:

• OSI layers 5, 6, 7 are combined into one Application Layer in TCP/IP
• OSI layers 1, 2 are combined into one Network Access Layer in TCP/IP – however TCP/IP does not take responsibility for sequencing and acknowledgement functions, leaving these to the underlying transport layer.

Other important differences:

• TCP/IP is a functional model designed to solve specific communication problems, and which is based on specific, standard protocols. OSI is a generic, protocol-independent model intended to describe all forms of network communication.
• In TCP/IP, most applications use all the layers, while in OSI simple applications do not use all seven layers. Only layers 1, 2 and 3 are mandatory to enable any data communication.

See how Imperva Web Application Firewall can help you with application security.

Imperva Application Security

Imperva security solutions secure your applications across multiple layers of the OSI model, from the network layer, protected by Imperva DDoS mitigation, to Imperva’s web application firewall (WAF), bot management and API security technology that safeguards the application layer.

To secure applications and networks across the OSI stack, Imperva provides multi-layered protection to make sure websites and applications are available, easily accessible and safe. The Imperva applicati on security solution includes:

• DDoS Protection —maintain uptime in all situations. Prevent any type of DDoS attack, of any size, from preventing access to your website and network infrastructure.
• CDN —enhance website performance and reduce bandwidth costs with a CDN designed for developers. Cache static resources at the edge while accelerating APIs and dynamic websites.
• WAF —cloud-based solution permits legitimate traffic and prevents bad traffic, safeguarding applications at the edge. Gateway WAF keeps applications and APIs inside your network safe.
• Bot protection —analyzes your bot traffic to pinpoint anomalies, identifies bad bot behavior and validates it via challenge mechanisms that do not impact user traffic.
• API security —protects APIs by ensuring only desired traffic can access your API endpoint, as well as detecting and blocking exploits of vulnerabilities.
• Account takeover protection —uses an intent-based detection process to identify and defends against attempts to take over users’ accounts for malicious purposes.
• RASP —keep your applications safe from within against known and zero‑day attacks. Fast and accurate protection with no signature or learning mode.
• Attack analytics —mitigate and respond to real cyber security threats efficiently and accurately with actionable intelligence across all your layers of defense.

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Application Layer in OSI Model

The Application Layer of OSI (Open System Interconnection) model, is the top layer in this model and takes care of network communication. The application layer provides the functionality to send and receive data from users. It acts as the interface between the user and the application. The application provides services like file transmission, mail service, and many more.

In this article we will explore the application layer in the OSI model, application layer functions, the working of the application layer, and services provided by the application layer.

Table of Content

• What is Application Layer in OSI Model?

Functions of Application Layer

Working of application layer in osi model, features provided by application layer protocols.

• Services Provided by Application Layer Protocols

Application Layer Protocols

What is an application layer in osi model.

Application Layer is the topmost layer in the Open System Interconnection (OSI) model . This layer provides several ways for manipulating the data (information) which actually enables any type of user to access network with ease. This layer also makes a request to its bottom layer, which is presentation layer for receiving various types of information from it.

The Application Layer interface directly interacts with application and provides common web application services. This layer is basically highest level of open system, which provides services directly for application process.

The Application Layer, as discussed above, being topmost layer in OSI model, performs several kinds of functions which are requirement in any kind of application or communication process. Following are list of functions which are performed by Application Layer of OSI Model:

• Application Layer provides a facility by which users can forward several emails and it also provides a storage facility.
• This layer allows users to access, retrieve and manage files in a remote computer.
• It allows users to log on as a remote host.
• This layer provides access to global information about various services.
• This layer provides services which include: e-mail, transferring files, distributing results to the user, directory services, network resources and so on.
• It provides protocols that allow software to send and receive information and present meaningful data to users.
• It handles issues such as network transparency, resource allocation and so on.
• This layer serves as a window for users and application processes to access network services.
• Application Layer is basically not a function, but it performs application layer functions.
• The application layer is actually an abstraction layer that specifies the shared protocols and interface methods used by hosts in a communication network.
• Application Layer helps us to identify communication partners, and synchronizing communication.
• This layer allows users to interact with other software applications.
• In this layer, data is in visual form, which makes users truly understand data rather than remembering or visualize the data in the binary format (0’s or 1’s).
• This application layer basically interacts with Operating System (OS) and thus further preserves the data in a suitable manner.
• This layer also receives and preserves data from it’s previous layer, which is Presentation Layer (which carries in itself the syntax and semantics of the information transmitted).
• The protocols which are used in this application layer depend upon what information users wish to send or receive.
• This application layer, in general, performs host initialization followed by remote login to hosts.

The application layer in the OSI model generally acts only like the interface which is responsible for communicating with host-based and user applications. This is in contrast with TCP/IP protocol, wherein the layers below the application layer, which is Session Layer and Presentation layer, are clubbed together and form a simple single layer which is responsible for performing the functions, which includes controlling the dialogues between computers, establishing as well as maintaining as well as ending a particular session, providing data compression and data encryption and so on.

Working of Application Layer

• At first, client sends a command to server and when server receives that command, it allocates port number to client.
• Thereafter, the client sends an initiation connection request to server and when server receives request, it gives acknowledgement (ACK) to client through client has successfully established a connection with the server.
• Therefore, now client has access to server through which it may either ask server to send any types of files or other documents or it may upload some files or documents on server itself.

To ensure smooth communication, application layer protocols are implemented the same on source host and destination host. The following are some of the features which are provided by Application layer protocols-

• The Application Layer protocol defines process for both parties which are involved in communication.
• These protocols define the type of message being sent or received from any side (either source host or destination host).
• These protocols also define basic syntax of the message being forwarded or retrieved.
• These protocols define the way to send a message and the expected response.
• These protocols also define interaction with the next level.

Services Provided by Application Layer

The services provided by application layer in OSI model are:

• It provides interface between user and application.
• It is used for remote login.
• It is used for file transfer.
• It is used for mail services and transfers.
• It is also used to transfer multimedia files.
• It is used for resource sharing.
• It is used for data synchronization.
• It is used for authentication services.

The application layer provides several protocols which allow any software to easily send and receive information and present meaningful data to its users. The following are some of the application layer protocols .

• TELNET: Telnet stands for Telecommunications Network. This protocol is used for managing files over the Internet. It allows the Telnet clients to access the resources of Telnet server. Telnet uses port number 23.
• DNS: DNS stands for Domain Name System. The DNS service translates the domain name (selected by user) into the corresponding IP address. For example- If you choose the domain name as www.abcd.com, then DNS must translate it as 192.36.20.8 (random IP address written just for understanding purposes). DNS protocol uses the port number 53.
• DHCP: DHCP stands for Dynamic Host Configuration Protocol. It provides IP addresses to hosts. Whenever a host tries to register for an IP address with the DHCP server, DHCP server provides lots of information to the corresponding host. DHCP uses port numbers 67 and 68.
• FTP: FTP stands for File Transfer Protocol. This protocol helps to transfer different files from one device to another. FTP promotes sharing of files via remote computer devices with reliable, efficient data transfer. FTP uses port number 20 for data access and port number 21 for data control.
• SMTP: SMTP stands for Simple Mail Transfer Protocol. It is used to transfer electronic mail from one user to another user. SMTP is used by end users to send emails with ease. SMTP uses port numbers 25 and 587.
• HTTP: HTTP stands for Hyper Text Transfer Protocol. It is the foundation of the World Wide Web (WWW). HTTP works on the client server model. This protocol is used for transmitting hypermedia documents like HTML. This protocol was designed particularly for the communications between the web browsers and web servers, but this protocol can also be used for several other purposes. HTTP is a stateless protocol (network protocol in which a client sends requests to server and server responses back as per the given state), which means the server is not responsible for maintaining the previous client’s requests. HTTP uses port number 80.
• NFS: NFS stands for Network File System. This protocol allows remote hosts to mount files over a network and interact with those file systems as though they are mounted locally. NFS uses the port number 2049.
• SNMP: SNMP stands for Simple Network Management Protocol. This protocol gathers data by polling the devices from the network to the management station at fixed or random intervals, requiring them to disclose certain information. SNMP uses port numbers 161 (TCP) and 162 (UDP).

From the above discussion we can conclude that application layer is the topmost layer in the OSI Model and acts as an interface between end user software like a web browser (or any other application that needs network communication) to send or receive data over network. By offering support for multiple protocols like SMTP, FTP, SNMP and DHCP the stack helps in effective communication of data. Every one who is at network management and application development should also have the idea about what are basic functions of Application Layer and Protocols.

Frequently Asked Questions on Application Layer in OSI Model – FAQ’s

What is the function of the application layer.

The function of application layer is to provide an interface to send and receive data from user.

Which is an Application Layer Service?

The application layer services are remote login, email services and file transmission.

How does the Application Layer ensure data integrity?

It uses protocols and error-checking mechanisms to detect and correct transmission errors.

Can the Application Layer handle multiple protocols simultaneously?

Yes, it supports concurrent protocols like HTTP, FTP, SMTP, and DNS for seamless network operations.

How does the Application Layer interact with other OSI model layers?

It requests data formatting from the Presentation Layer and relies on lower layers for connection and data transmission.

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