which device runs the presentation layer

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Prerequisite : OSI Model

Introduction : Presentation Layer is the 6th layer in the Open System Interconnection (OSI) model. This layer is also known as Translation layer, as this layer serves as a data translator for the network. The data which this layer receives from the Application Layer is extracted and manipulated here as per the required format to transmit over the network. The main responsibility of this layer is to provide or define the data format and encryption. The presentation layer is also called as Syntax layer since it is responsible for maintaining the proper syntax of the data which it either receives or transmits to other layer(s).

Functions of Presentation Layer :

The presentation layer, being the 6th layer in the OSI model, performs several types of functions, which are described below-

• Presentation layer format and encrypts data to be sent across the network.
• This layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data efficiently and effectively.
• This layer manages the abstract data structures and allows high-level data structures (example- banking records), which are to be defined or exchanged.
• This layer carries out the encryption at the transmitter and decryption at the receiver.
• This layer carries out data compression to reduce the bandwidth of the data to be transmitted (the primary goal of data compression is to reduce the number of bits which is to be transmitted).
• This layer is responsible for interoperability (ability of computers to exchange and make use of information) between encoding methods as different computers use different encoding methods.
• This layer basically deals with the presentation part of the data.
• Presentation layer, carries out the data compression (number of bits reduction while transmission), which in return improves the data throughput.
• This layer also deals with the issues of string representation.
• The presentation layer is also responsible for integrating all the formats into a standardized format for efficient and effective communication.
• This layer encodes the message from the user-dependent format to the common format and vice-versa for communication between dissimilar systems.
• This layer deals with the syntax and semantics of the messages.
• This layer also ensures that the messages which are to be presented to the upper as well as the lower layer should be standardized as well as in an accurate format too.
• Presentation layer is also responsible for translation, formatting, and delivery of information for processing or display.
• This layer also performs serialization (process of translating a data structure or an object into a format that can be stored or transmitted easily).

Features of Presentation Layer in the OSI model: Presentation layer, being the 6th layer in the OSI model, plays a vital role while communication is taking place between two devices in a network.

List of features which are provided by the presentation layer are:

• Presentation layer could apply certain sophisticated compression techniques, so fewer bytes of data are required to represent the information when it is sent over the network.
• If two or more devices are communicating over an encrypted connection, then this presentation layer is responsible for adding encryption on the sender’s end as well as the decoding the encryption on the receiver’s end so that it can represent the application layer with unencrypted, readable data.
• This layer formats and encrypts data to be sent over a network, providing freedom from compatibility problems.
• This presentation layer also negotiates the Transfer Syntax.
• This presentation layer is also responsible for compressing data it receives from the application layer before delivering it to the session layer (which is the 5th layer in the OSI model) and thus improves the speed as well as the efficiency of communication by minimizing the amount of the data to be transferred.

Working of Presentation Layer in the OSI model : Presentation layer in the OSI model, as a translator, converts the data sent by the application layer of the transmitting node into an acceptable and compatible data format based on the applicable network protocol and architecture.  Upon arrival at the receiving computer, the presentation layer translates data into an acceptable format usable by the application layer. Basically, in other words, this layer takes care of any issues occurring when transmitted data must be viewed in a format different from the original format. Being the functional part of the OSI mode, the presentation layer performs a multitude (large number of) data conversion algorithms and character translation functions. Mainly, this layer is responsible for managing two network characteristics: protocol (set of rules) and architecture.

Presentation Layer Protocols : Presentation layer being the 6th layer, but the most important layer in the OSI model performs several types of functionalities, which makes sure that data which is being transferred or received should be accurate or clear to all the devices which are there in a closed network. Presentation Layer, for performing translations or other specified functions, needs to use certain protocols which are defined below –

• Apple Filing Protocol (AFP): Apple Filing Protocol is the proprietary network protocol (communications protocol) that offers services to macOS or the classic macOS. This is basically the network file control protocol specifically designed for Mac-based platforms.
• Lightweight Presentation Protocol (LPP): Lightweight Presentation Protocol is that protocol which is used to provide ISO presentation services on the top of TCP/IP based protocol stacks.
• NetWare Core Protocol (NCP): NetWare Core Protocol is the network protocol which is used to access file, print, directory, clock synchronization, messaging, remote command execution and other network service functions.
• Network Data Representation (NDR): Network Data Representation is basically the implementation of the presentation layer in the OSI model, which provides or defines various primitive data types, constructed data types and also several types of data representations.
• External Data Representation (XDR): External Data Representation (XDR) is the standard for the description and encoding of data. It is useful for transferring data between computer architectures and has been used to communicate data between very diverse machines. Converting from local representation to XDR is called encoding, whereas converting XDR into local representation is called decoding.
• Secure Socket Layer (SSL): The Secure Socket Layer protocol provides security to the data that is being transferred between the web browser and the server. SSL encrypts the link between a web server and a browser, which ensures that all data passed between them remains private and free from attacks.

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  Layer 6 Presentation Layer

De/Encryption, Encoding, String representation

The presentation layer (data presentation layer, data provision level) sets the system-dependent representation of the data (for example, ASCII, EBCDIC) into an independent form, enabling the syntactically correct data exchange between different systems. Also, functions such as data compression and encryption are guaranteed that data to be sent by the application layer of a system that can be read by the application layer of another system to the layer 6. The presentation layer. If necessary, the presentation layer acts as a translator between different data formats, by making an understandable for both systems data format, the ASN.1 (Abstract Syntax Notation One) used.

OSI Layer 6 - Presentation Layer

The presentation layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems. An example of a presentation service would be the conversion of an EBCDIC-coded text computer file to an ASCII-coded file. The presentation layer is the lowest layer at which application programmers consider data structure and presentation, instead of simply sending data in the form of datagrams or packets between hosts. This layer deals with issues of string representation - whether they use the Pascal method (an integer length field followed by the specified amount of bytes) or the C/C++ method (null-terminated strings, e.g. "thisisastring\0"). The idea is that the application layer should be able to point at the data to be moved, and the presentation layer will deal with the rest. Serialization of complex data structures into flat byte-strings (using mechanisms such as TLV or XML) can be thought of as the key functionality of the presentation layer. Encryption is typically done at this level too, although it can be done on the application, session, transport, or network layers, each having its own advantages and disadvantages. Decryption is also handled at the presentation layer. For example, when logging on to bank account sites the presentation layer will decrypt the data as it is received.[1] Another example is representing structure, which is normally standardized at this level, often by using XML. As well as simple pieces of data, like strings, more complicated things are standardized in this layer. Two common examples are 'objects' in object-oriented programming, and the exact way that streaming video is transmitted. In many widely used applications and protocols, no distinction is made between the presentation and application layers. For example, HyperText Transfer Protocol (HTTP), generally regarded as an application-layer protocol, has presentation-layer aspects such as the ability to identify character encoding for proper conversion, which is then done in the application layer. Within the service layering semantics of the OSI network architecture, the presentation layer responds to service requests from the application layer and issues service requests to the session layer. In the OSI model: the presentation layer ensures the information that the application layer of one system sends out is readable by the application layer of another system. For example, a PC program communicates with another computer, one using extended binary coded decimal interchange code (EBCDIC) and the other using ASCII to represent the same characters. If necessary, the presentation layer might be able to translate between multiple data formats by using a common format. Wikipedia

• Data conversion
• Character code translation
• Compression
• Encryption and Decryption

The Presentation OSI Layer is usually composed of 2 sublayers that are:

CASE common application service element

SASE specific application service element

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.

The OSI Model – The 7 Layers of Networking Explained in Plain English

This article explains the Open Systems Interconnection (OSI) model and the 7 layers of networking, in plain English.

The OSI model is a conceptual framework that is used to describe how a network functions. In plain English, the OSI model helped standardize the way computer systems send information to each other.

Learning networking is a bit like learning a language - there are lots of standards and then some exceptions. Therefore, it’s important to really understand that the OSI model is not a set of rules. It is a tool for understanding how networks function.

Once you learn the OSI model, you will be able to further understand and appreciate this glorious entity we call the Internet, as well as be able to troubleshoot networking issues with greater fluency and ease.

All hail the Internet!

Prerequisites

You don’t need any prior programming or networking experience to understand this article. However, you will need:

• Basic familiarity with common networking terms (explained below)
• A curiosity about how things work :)

Learning Objectives

Over the course of this article, you will learn:

• What the OSI model is
• The purpose of each of the 7 layers
• The problems that can happen at each of the 7 layers
• The difference between TCP/IP model and the OSI model

Common Networking Terms

Here are some common networking terms that you should be familiar with to get the most out of this article. I’ll use these terms when I talk about OSI layers next.

A node is a physical electronic device hooked up to a network, for example a computer, printer, router, and so on. If set up properly, a node is capable of sending and/or receiving information over a network.

Nodes may be set up adjacent to one other, wherein Node A can connect directly to Node B, or there may be an intermediate node, like a switch or a router, set up between Node A and Node B.

Typically, routers connect networks to the Internet and switches operate within a network to facilitate intra-network communication. Learn more about hub vs. switch vs. router.

Here's an example:

For the nitpicky among us (yep, I see you), host is another term that you will encounter in networking. I will define a host as a type of node that requires an IP address. All hosts are nodes, but not all nodes are hosts. Please Tweet angrily at me if you disagree.

Links connect nodes on a network. Links can be wired, like Ethernet, or cable-free, like WiFi.

Links to can either be point-to-point, where Node A is connected to Node B, or multipoint, where Node A is connected to Node B and Node C.

When we’re talking about information being transmitted, this may also be described as a one-to-one vs. a one-to-many relationship.

A protocol is a mutually agreed upon set of rules that allows two nodes on a network to exchange data.

“A protocol defines the rules governing the syntax (what can be communicated), semantics (how it can be communicated), and synchronization (when and at what speed it can be communicated) of the communications procedure. Protocols can be implemented on hardware, software, or a combination of both. Protocols can be created by anyone, but the most widely adopted protocols are based on standards.” - The Illustrated Network.

Both wired and cable-free links can have protocols.

While anyone can create a protocol, the most widely adopted protocols are often based on standards published by Internet organizations such as the Internet Engineering Task Force (IETF).

A network is a general term for a group of computers, printers, or any other device that wants to share data.

Network types include LAN, HAN, CAN, MAN, WAN, BAN, or VPN. Think I’m just randomly rhyming things with the word can ? I can ’t say I am - these are all real network types. Learn more here .

Topology describes how nodes and links fit together in a network configuration, often depicted in a diagram. Here are some common network topology types:

A network consists of nodes, links between nodes, and protocols that govern data transmission between nodes.

At whatever scale and complexity networks get to, you will understand what’s happening in all computer networks by learning the OSI model and 7 layers of networking.

What is the OSI Model?

The OSI model consists of 7 layers of networking.

First, what’s a layer?

No, a layer - not a lair . Here there are no dragons.

A layer is a way of categorizing and grouping functionality and behavior on and of a network.

In the OSI model, layers are organized from the most tangible and most physical, to less tangible and less physical but closer to the end user.

Each layer abstracts lower level functionality away until by the time you get to the highest layer. All the details and inner workings of all the other layers are hidden from the end user.

How to remember all the names of the layers? Easy.

• Please | Physical Layer
• Do | Data Link Layer
• Not | Network Layer
• Tell (the) | Transport Layer
• Secret | Session Layer
• Password (to) | Presentation Layer
• Anyone | Application Layer

Keep in mind that while certain technologies, like protocols, may logically “belong to” one layer more than another, not all technologies fit neatly into a single layer in the OSI model. For example, Ethernet, 802.11 (Wifi) and the Address Resolution Protocol (ARP) procedure operate on >1 layer.

The OSI is a model and a tool, not a set of rules.

OSI Layer 1

Layer 1 is the physical layer . There’s a lot of technology in Layer 1 - everything from physical network devices, cabling, to how the cables hook up to the devices. Plus if we don’t need cables, what the signal type and transmission methods are (for example, wireless broadband).

Instead of listing every type of technology in Layer 1, I’ve created broader categories for these technologies. I encourage readers to learn more about each of these categories:

• Nodes (devices) and networking hardware components. Devices include hubs, repeaters, routers, computers, printers, and so on. Hardware components that live inside of these devices include antennas, amplifiers, Network Interface Cards (NICs), and more.
• Device interface mechanics. How and where does a cable connect to a device (cable connector and device socket)? What is the size and shape of the connector, and how many pins does it have? What dictates when a pin is active or inactive?
• Functional and procedural logic. What is the function of each pin in the connector - send or receive? What procedural logic dictates the sequence of events so a node can start to communicate with another node on Layer 2?
• Cabling protocols and specifications. Ethernet (CAT), USB, Digital Subscriber Line (DSL) , and more. Specifications include maximum cable length, modulation techniques, radio specifications, line coding, and bits synchronization (more on that below).
• Cable types. Options include shielded or unshielded twisted pair, untwisted pair, coaxial and so on. Learn more about cable types here .
• Signal type. Baseband is a single bit stream at a time, like a railway track - one-way only. Broadband consists of multiple bit streams at the same time, like a bi-directional highway.
• Signal transmission method (may be wired or cable-free). Options include electrical (Ethernet), light (optical networks, fiber optics), radio waves (802.11 WiFi, a/b/g/n/ac/ax variants or Bluetooth). If cable-free, then also consider frequency: 2.5 GHz vs. 5 GHz. If it’s cabled, consider voltage. If cabled and Ethernet, also consider networking standards like 100BASE-T and related standards.

The data unit on Layer 1 is the bit.

A bit the smallest unit of transmittable digital information. Bits are binary, so either a 0 or a 1. Bytes, consisting of 8 bits, are used to represent single characters, like a letter, numeral, or symbol.

Bits are sent to and from hardware devices in accordance with the supported data rate (transmission rate, in number of bits per second or millisecond) and are synchronized so the number of bits sent and received per unit of time remains consistent (this is called bit synchronization). The way bits are transmitted depends on the signal transmission method.

Nodes can send, receive, or send and receive bits. If they can only do one, then the node uses a simplex mode. If they can do both, then the node uses a duplex mode. If a node can send and receive at the same time, it’s full-duplex – if not, it’s just half-duplex.

The original Ethernet was half-duplex. Full-duplex Ethernet is an option now, given the right equipment.

How to Troubleshoot OSI Layer 1 Problems

Here are some Layer 1 problems to watch out for:

• Defunct cables, for example damaged wires or broken connectors
• Broken hardware network devices, for example damaged circuits
• Stuff being unplugged (...we’ve all been there)

If there are issues in Layer 1, anything beyond Layer 1 will not function properly.

Layer 1 contains the infrastructure that makes communication on networks possible.

It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating physical links between network devices. - Source

Fun fact: deep-sea communications cables transmit data around the world. This map will blow your mind: https://www.submarinecablemap.com/

And because you made it this far, here’s a koala:

OSI Layer 2

Layer 2 is the data link layer . Layer 2 defines how data is formatted for transmission, how much data can flow between nodes, for how long, and what to do when errors are detected in this flow.

In more official tech terms:

• Line discipline. Who should talk for how long? How long should nodes be able to transit information for?
• Flow control. How much data should be transmitted?
• Error control - detection and correction . All data transmission methods have potential for errors, from electrical spikes to dirty connectors. Once Layer 2 technologies tell network administrators about an issue on Layer 2 or Layer 1, the system administrator can correct for those errors on subsequent layers. Layer 2 is mostly concerned with error detection, not error correction. ( Source )

There are two distinct sublayers within Layer 2:

• Media Access Control (MAC): the MAC sublayer handles the assignment of a hardware identification number, called a MAC address, that uniquely identifies each device on a network. No two devices should have the same MAC address. The MAC address is assigned at the point of manufacturing. It is automatically recognized by most networks. MAC addresses live on Network Interface Cards (NICs). Switches keep track of all MAC addresses on a network. Learn more about MAC addresses on PC Mag and in this article . Learn more about network switches here .
• Logical Link Control (LLC): the LLC sublayer handles framing addressing and flow control. The speed depends on the link between nodes, for example Ethernet or Wifi.

The data unit on Layer 2 is a frame .

Each frame contains a frame header, body, and a frame trailer:

• Header: typically includes MAC addresses for the source and destination nodes.
• Body: consists of the bits being transmitted.
• Trailer: includes error detection information. When errors are detected, and depending on the implementation or configuration of a network or protocol, frames may be discarded or the error may be reported up to higher layers for further error correction. Examples of error detection mechanisms: Cyclic Redundancy Check (CRC) and Frame Check Sequence (FCS). Learn more about error detection techniques here .

Typically there is a maximum frame size limit, called an Maximum Transmission Unit, MTU. Jumbo frames exceed the standard MTU, learn more about jumbo frames here .

How to Troubleshoot OSI Layer 2 Problems

Here are some Layer 2 problems to watch out for:

• All the problems that can occur on Layer 1
• Unsuccessful connections (sessions) between two nodes
• Sessions that are successfully established but intermittently fail
• Frame collisions

The Data Link Layer allows nodes to communicate with each other within a local area network. The foundations of line discipline, flow control, and error control are established in this layer.

OSI Layer 3

Layer 3 is the network layer . This is where we send information between and across networks through the use of routers. Instead of just node-to-node communication, we can now do network-to-network communication.

Routers are the workhorse of Layer 3 - we couldn’t have Layer 3 without them. They move data packets across multiple networks.

Not only do they connect to Internet Service Providers (ISPs) to provide access to the Internet, they also keep track of what’s on its network (remember that switches keep track of all MAC addresses on a network), what other networks it’s connected to, and the different paths for routing data packets across these networks.

Routers store all of this addressing and routing information in routing tables.

Here’s a simple example of a routing table:

The data unit on Layer 3 is the data packet . Typically, each data packet contains a frame plus an IP address information wrapper. In other words, frames are encapsulated by Layer 3 addressing information.

The data being transmitted in a packet is also sometimes called the payload . While each packet has everything it needs to get to its destination, whether or not it makes it there is another story.

Layer 3 transmissions are connectionless, or best effort - they don't do anything but send the traffic where it’s supposed to go. More on data transport protocols on Layer 4.

Once a node is connected to the Internet, it is assigned an Internet Protocol (IP) address, which looks either like 172.16. 254.1 (IPv4 address convention) or like 2001:0db8:85a3:0000:0000:8a2e:0370:7334 (IPv6 address convention). Routers use IP addresses in their routing tables.

IP addresses are associated with the physical node’s MAC address via the Address Resolution Protocol (ARP), which resolves MAC addresses with the node’s corresponding IP address.

ARP is conventionally considered part of Layer 2, but since IP addresses don’t exist until Layer 3, it’s also part of Layer 3.

How to Troubleshoot OSI Layer 3 Problems

Here are some Layer 3 problems to watch out for:

• All the problems that can crop up on previous layers :)
• Faulty or non-functional router or other node
• IP address is incorrectly configured

Many answers to Layer 3 questions will require the use of command-line tools like ping , trace , show ip route , or show ip protocols . Learn more about troubleshooting on layer 1-3 here .

The Network Layer allows nodes to connect to the Internet and send information across different networks.

OSI Layer 4

Layer 4 is the transport layer . This where we dive into the nitty gritty specifics of the connection between two nodes and how information is transmitted between them. It builds on the functions of Layer 2 - line discipline, flow control, and error control.

This layer is also responsible for data packet segmentation, or how data packets are broken up and sent over the network.

Unlike the previous layer, Layer 4 also has an understanding of the whole message, not just the contents of each individual data packet. With this understanding, Layer 4 is able to manage network congestion by not sending all the packets at once.

The data units of Layer 4 go by a few names. For TCP, the data unit is a packet. For UDP, a packet is referred to as a datagram. I’ll just use the term data packet here for the sake of simplicity.

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are two of the most well-known protocols in Layer 4.

TCP, a connection-oriented protocol, prioritizes data quality over speed.

TCP explicitly establishes a connection with the destination node and requires a handshake between the source and destination nodes when data is transmitted. The handshake confirms that data was received. If the destination node does not receive all of the data, TCP will ask for a retry.

TCP also ensures that packets are delivered or reassembled in the correct order. Learn more about TCP here .

UDP, a connectionless protocol, prioritizes speed over data quality. UDP does not require a handshake, which is why it’s called connectionless.

Because UDP doesn’t have to wait for this acknowledgement, it can send data at a faster rate, but not all of the data may be successfully transmitted and we’d never know.

If information is split up into multiple datagrams, unless those datagrams contain a sequence number, UDP does not ensure that packets are reassembled in the correct order. Learn more about UDP here .

TCP and UDP both send data to specific ports on a network device, which has an IP address. The combination of the IP address and the port number is called a socket.

Learn more about sockets here .

Learn more about the differences and similarities between these two protocols here .

How to Troubleshoot OSI Layer 4 Problems

Here are some Layer 4 problems to watch out for:

• Blocked ports - check your Access Control Lists (ACL) & firewalls
• Quality of Service (QoS) settings. QoS is a feature of routers/switches that can prioritize traffic, and they can really muck things up. Learn more about QoS here .

The Transport Layer provides end-to-end transmission of a message by segmenting a message into multiple data packets; the layer supports connection-oriented and connectionless communication.

OSI Layer 5

Layer 5 is the session layer . This layer establishes, maintains, and terminates sessions.

A session is a mutually agreed upon connection that is established between two network applications. Not two nodes! Nope, we’ve moved on from nodes. They were so Layer 4.

Just kidding, we still have nodes, but Layer 5 doesn’t need to retain the concept of a node because that’s been abstracted out (taken care of) by previous layers.

So a session is a connection that is established between two specific end-user applications. There are two important concepts to consider here:

• Client and server model: the application requesting the information is called the client, and the application that has the requested information is called the server.
• Request and response model: while a session is being established and during a session, there is a constant back-and-forth of requests for information and responses containing that information or “hey, I don’t have what you’re requesting.”

Sessions may be open for a very short amount of time or a long amount of time. They may fail sometimes, too.

Depending on the protocol in question, various failure resolution processes may kick in. Depending on the applications/protocols/hardware in use, sessions may support simplex, half-duplex, or full-duplex modes.

Examples of protocols on Layer 5 include Network Basic Input Output System (NetBIOS) and Remote Procedure Call Protocol (RPC), and many others.

From here on out (layer 5 and up), networks are focused on ways of making connections to end-user applications and displaying data to the user.

How to Troubleshoot OSI Layer 5 Problems

Here are some Layer 5 problems to watch out for:

• Servers are unavailable
• Servers are incorrectly configured, for example Apache or PHP configs
• Session failure - disconnect, timeout, and so on.

The Session Layer initiates, maintains, and terminates connections between two end-user applications. It responds to requests from the presentation layer and issues requests to the transport layer.

OSI Layer 6

Layer 6 is the presentation layer . This layer is responsible for data formatting, such as character encoding and conversions, and data encryption.

The operating system that hosts the end-user application is typically involved in Layer 6 processes. This functionality is not always implemented in a network protocol.

Layer 6 makes sure that end-user applications operating on Layer 7 can successfully consume data and, of course, eventually display it.

There are three data formatting methods to be aware of:

• American Standard Code for Information Interchange (ASCII): this 7-bit encoding technique is the most widely used standard for character encoding. One superset is ISO-8859-1, which provides most of the characters necessary for languages spoken in Western Europe.
• Extended Binary-Coded Decimal Interchange Code (EBDCIC): designed by IBM for mainframe usage. This encoding is incompatible with other character encoding methods.
• Unicode: character encodings can be done with 32-, 16-, or 8-bit characters and attempts to accommodate every known, written alphabet.

Learn more about character encoding methods in this article , and also here .

Encryption: SSL or TLS encryption protocols live on Layer 6. These encryption protocols help ensure that transmitted data is less vulnerable to malicious actors by providing authentication and data encryption for nodes operating on a network. TLS is the successor to SSL.

How to Troubleshoot OSI Layer 6 Problems

Here are some Layer 6 problems to watch out for:

• Non-existent or corrupted drivers
• Incorrect OS user access level

The Presentation Layer formats and encrypts data.

OSI Layer 7

Layer 7 is the application layer .

True to its name, this is the layer that is ultimately responsible for supporting services used by end-user applications. Applications include software programs that are installed on the operating system, like Internet browsers (for example, Firefox) or word processing programs (for example, Microsoft Word).

Applications can perform specialized network functions under the hood and require specialized services that fall under the umbrella of Layer 7.

Electronic mail programs, for example, are specifically created to run over a network and utilize networking functionality, such as email protocols, which fall under Layer 7.

Applications will also control end-user interaction, such as security checks (for example, MFA), identification of two participants, initiation of an exchange of information, and so on.

Protocols that operate on this level include File Transfer Protocol (FTP), Secure Shell (SSH), Simple Mail Transfer Protocol (SMTP), Internet Message Access Protocol (IMAP), Domain Name Service (DNS), and Hypertext Transfer Protocol (HTTP).

While each of these protocols serve different functions and operate differently, on a high level they all facilitate the communication of information. ( Source )

How to Troubleshoot OSI Layer 7 Problems

Here are some Layer 7 problems to watch out for:

• All issues on previous layers
• Incorrectly configured software applications
• User error (... we’ve all been there)

The Application Layer owns the services and functions that end-user applications need to work. It does not include the applications themselves.

Our Layer 1 koala is all grown up.

Learning check - can you apply makeup to a koala?

Don’t have a koala?

Well - answer these questions instead. It’s the next best thing, I promise.

• What is the OSI model?
• What are each of the layers?
• How could I use this information to troubleshoot networking issues?

Congratulations - you’ve taken one step farther to understanding the glorious entity we call the Internet.

Learning Resources

Many, very smart people have written entire books about the OSI model or entire books about specific layers. I encourage readers to check out any O’Reilly-published books about the subject or about network engineering in general.

Here are some resources I used when writing this article:

• The Illustrated Network, 2nd Edition
• Protocol Data Unit (PDU): https://www.geeksforgeeks.org/difference-between-segments-packets-and-frames/
• Troubleshooting Along the OSI Model: https://www.pearsonitcertification.com/articles/article.aspx?p=1730891
• The OSI Model Demystified: https://www.youtube.com/watch?v=HEEnLZV2wGI
• OSI Model for Dummies: https://www.dummies.com/programming/networking/layers-in-the-osi-model-of-a-computer-network/

Chloe Tucker is an artist and computer science enthusiast based in Portland, Oregon. As a former educator, she's continuously searching for the intersection of learning and teaching, or technology and art. Reach out to her on Twitter @_chloetucker and check out her website at chloe.dev .

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Presentation Layer: Protocols, Examples, Services | Functions of Presentation Layer

Presentation Layer is the 6th layer in the Open System Interconnection (OSI) model where all application programmer consider data structure and presentation, beyond of simply sending the data into form of datagram otherwise packets in between the hosts. Now, we will explain about what is presentation layer with its protocols, example, service ; involving with major functions of presentation Layer with ease. At the end of this article, you will completely educate about What is Presentation Layer in OSI Model without any hassle.

What is Presentation Layer?

Definition : Presentation layer is 6th layer in the OSI model , and its main objective is to present all messages to upper layer as a standardized format. It is also known as the “ Translation layer “.  This layer takes care of syntax and semantics of messages exchanged in between two communication systems. Presentation layer has responsible that receiver can understand all data, and it will be to implement all data languages can be dissimilar of two communication system.

Presentation Layer Tutorial Headlines:

In this section, we will show you all headlines about this entire article; you can check them as your choice; below shown all:

Let’s Get Started!!

  functions of presentation layer.

Presentation layer performs various functions in the OSI model ; below explain each one – 

Protocols of Presentation Layer

Example of presentation layer protocols:.

Here, we will discuss all examples of presentation layer protocols; below explain each one –  

Multipurpose Internet Mail Extensions (MIME) : MIME protocol was introduced by Bell Communications in 1991, and it is an internet standard that provides scalable capable of email for attaching of images, sounds and text in a message.

Apple Filing Protocol (AFP ) : AFP protocol is designed by Apple company for sharing all files over the entire network .

Network Data Representation (NDR) : NDR is an data encoding standard, and it is implement in the Distributed Computing Environment (DCE).

Telnet (Telecommunication Network) : Telnet protocol was introduced in 1969, and it offers the command line interface for making communication along with remote device or server .

eXternal Data Representation (XDR) : This protocol provides the description and encoding of entire data, and  it’s main goal is to transfer data in between dissimilar computer architecture.

Presentation Layer Services

Design issues with presentation layer, faqs (frequently asked questions), what is meant by presentation layer in osi model.

Presentation Layer is the 6th layer in the Open System Interconnection (OSI) model that is the lowest layer, where all application programmer consider data structure and presentation, beyond of simply sending the data into form of datagram otherwise packets in between the hosts.

What protocols are used in the presentation layer?

Can you explain some presentation layer examples, what are the main functions of the presentation layer, what are services of presentation layer in osi.

Presentation layer has a responsibility for formatting, translation, and delivery of the information for getting to process otherwise display .

Also Read: Data Link Layer: Protocols, Examples | Functions of Data Link Layer

If you have any experience, tips, tricks, or query regarding this issue? You can drop a comment!

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• Andrew Froehlich, West Gate Networks

What is the presentation layer?

The presentation layer resides at Layer 6 of the Open Systems Interconnection ( OSI ) communications model and ensures that communications that pass through it are in the appropriate form for the recipient application. In other words, the presentation layer presents the data in a readable format from an application layer perspective.

For example, a presentation layer program could format a file transfer request in binary code to ensure a successful file transfer . Because binary is the most rudimentary of computing languages, it ensures that the receiving device can decipher and translate it into a format the application layer understands and expects.

How the presentation layer works

Once the application layer passes data meant for transport to another device in a certain format, the presentation layer then prepares this data in the most appropriate format the receiving application can understand.

Common data formats include the following:

• American Standard Code for Information Interchange and Extended Binary Coded Decimal Interchange Code for text;
• JPEG , GIF and TIFF for images; and
• MPEG, MIDI and QuickTime for video.

Encryption and decryption of data communications are also performed at the presentation layer. Here, encryption methods and keys exchange between the two communicating devices. Only the sender and receiver can properly encode and decode data so it returns to a readable format.

The presentation layer can serialize -- or translate -- more complex application data objects into a storable and transportable format. This helps to rebuild the object once it arrives at the other side of the communications stream. The presentation layer also deserializes the data stream and places it back into an object format that the application can understand by the application.

The tool that manages Hypertext Transfer Protocol ( HTTP ) is an example of a program that loosely adheres to the presentation layer of OSI.

Although it's technically considered an application-layer protocol per the TCP/IP model , HTTP includes presentation layer services within it. HTTP works when the requesting device forwards user requests passed to the web browser onto a web server elsewhere in the network.

HTTP receives a return message from the web server that includes a Multipurpose Internet Mail Extensions ( MIME ) header. The MIME header indicates the type of file -- text, video, or audio -- that has been received so that an appropriate player utility can present the file to the user.

Functions of the presentation layer

• ensures proper formatting and delivery to and from the application layer;
• performs data encryption; and
• manages serialization of data objects.

Editor's note: This article was republished in January 2023 to improve the reader experience.

Continue Reading About presentation layer

• What is the difference between TCP/IP model vs. OSI model?
• Data and file formatting

Related Terms

Dig deeper on network infrastructure.

What are the most important email security protocols?

file extension (file format)

network protocol

MIME (Multipurpose Internet Mail Extensions)

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Presentation Layer: What It Is, Design Issues, Functionalities

Description and Functions of Presentation Layer in the OSI model: In this tutorial, we are going to learn what the Presentation layer is and the Functions of the Presentation Layer in the OSI model in Computer Networking. We will also discuss the Design issues with the Presentation Layer and the working of the Presentation Layer with the help of its diagram. By Monika Jha Last updated : May 05, 2023

What is Presentation Layer?

The Presentation Layer is concerned with the syntax and semantics of the information exchanged between two communicating devices.

• The presentation layer takes care that the data is sent in that way the receiver of the data will understand the information (data) and will be able to use the data.
• Languages that are syntax can be different from the two communicating machines. In this condition, the presentation layer plays the role of translator between them.
• It is possible for two machines to communicate with different data representations, data structures to be exchanged can be defined in an abstract way.
• These abstract data structures will be managed by the presentation layer and this layer allows higher-level data structures (For example banking records), to be defined and exchanged.

This figure shows the relationship of the presentation layer to the session layer and application layer.

Design Issues with Presentation Layer

The following are the design issues with presentation layer:

• To manage and maintain the Syntax and Semantics of the information transmitted.
• Encoding data in a standard agreed-upon way just like a string, double, date, etc.
• It Performs Standard Encoding scheme on the wire.

Functionalities of the Presentation Layer

Specific functionalities of the presentation layer are as follows:

1. Translation

• The processes or running programs in two machines are usually exchanging the information in the form of numbers, character strings and so on before being transmitted. The information should be changed to bitstreams because different computers use different encoding schemes.
• The Presentation layer is responsible for compatibility between these encoding methods.
• The Presentation layer at the sender's side changes the information from its sender dependent format.
• The Presentation layer at the receiving machine changes the common format into its receivers dependent format.

Example: Convert ASCII code to EBCDIC code.

2. Encryption

• The system must be able to assure privacy regarding the message or information as it also carries sensitive information.
• Encryption means that the sender transforms the original information or message to another form, this data after encryption is known as the ciphertext and this ciphertext sends the resulting message out over the network.
• Decryption concerned with the transform of the message back to its original form. This decrypted data is known as plain text.

3. Compression

• Data Compression means reduces the number of bits to be transmitted by this reduce the bandwidth of the data.
• Data Compression becomes particularly important in the transmission of multimedia such as audio, video, text, etc.

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The OSI Model - Presentation Layer

Presentation layer.

The presentation layer handles the conversion of data between a Standards-based or platform independant formats to a format understood by the local machine. This allows for data to be transported between devices and still be understood.

The presentation layer performs the folowing functions:

• Communication with the application layer above.
• Translation of data conforming to cross-platform standards into formats understood by the local machine.
• Communication with the session layer below.

Examples of Presentation Layer Functions

• Conversion of a Sun .RAS raster graphic to JPG.
• Conversion of ASCII to IBM EBCDIC
• Conversion of .PICT on a MAC to .jpg
• Conversion of .wav to .mp3

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The Open Systems Interconnection (OSI) networking model defines a conceptual framework for communications between computer systems. The model is an ISO standard which identifies seven fundamental networking layers, from the physical hardware up to high-level software applications.

Each layer in the model handles a specific networking function. The standard helps administrators to visualize networks, isolate problems, and understand the use cases for new technologies. Many network equipment vendors advertise the OSI layer that their products are designed to slot into.

OSI was adopted as an international standard in 1984. It remains relevant today despite the changes to network implementation that have occurred since first publication. Cloud, edge, and IoT can all be accommodated within the model.

In this article, we'll explain each of the seven OSI layers in turn. We'll start from the lowest level, labelled as Layer 1.

1. Physical Layer

All networking begins with physical equipment. This layer encapsulates the hardware involved in the communications, such as switches and cables. Data is transferred as a stream of binary digits - 0 or 1 - that the hardware prepares from input it's been fed. The physical layer specifies the electrical signals that are used to encode the data over the wire, such as a 5-volt pulse to indicate a binary "1."

Errors in the physical layer tend to result in data not being transferred at all. There could be a break in the connection due to a missing plug or incorrect power supply. Problems can also arise when two components disagree on the physical encoding of data values. In the case of wireless connections, a weak signal can lead to bit loss during transmission.

2. Data Link Layer

The model's second layer concerns communication between two devices that are directly connected to each other in the same network. It's responsible for establishing a link that allows data to be exchanged using an agreed protocol. Many network switches operate at Layer 2.

The data link layer will eventually pass bits to the physical layer. As it sits above the hardware, the data link layer can perform basic error detection and correction in response to physical transfer issues. There are two sub-layers that define these responsibilities: Logical Link Control (LLC) that handles frame synchronization and error detection, and Media Access Control (MAC) which uses MAC addresses to constrain how devices acquire permission to transfer data.

3. Network Layer

The network layer is the first level to support data transfer between two separately maintained networks. It's redundant in situations where all your devices exist on the same network.

Data that comes to the network layer from higher levels is first broken up into packets suitable for transmission. Packets received from the remote network in response are reassembled into usable data.

The network layer is where several important protocols are first encountered. These include IP (for determining the path to a destination), ICMP, routing, and virtual LAN. Together these mechanisms facilitate inter-network communications with a familiar degree of usability. However operations at this level aren't necessarily reliable: messages aren't required to succeed and may not necessarily be retried.

4. Transport Layer

The transport layer provides higher-level abstractions for coordinating data transfers between devices. Transport controllers determine where data will be sent and the rate it should be transferred at.

Layer 4 is where TCP and UDP are implemented, providing the port numbers that allow devices to expose multiple communication channels. Load balancing is often situated at Layer 4 as a result, allowing traffic to be routed between ports on a target device.

Transport mechanisms are expected to guarantee successful communication. Stringent error controls are applied to recover from packet loss and retry failed transfers. Flow control is enforced so the sender doesn't overwhelm the remote device by sending data more quickly than the available bandwidth permits.

5. Session Layer

Layer 5 creates ongoing communication sessions between two devices. Sessions are used to negotiate new connections, agree on their duration, and gracefully close down the connection once the data exchange is complete. This layer ensures that sessions remain open long enough to transfer all the data that's being sent.

Checkpoint control is another responsibility that's held by Layer 5. Sessions can define checkpoints to facilitate progress updates and resumable transmissions. A new checkpoint could be set every few megabytes for a file upload, allowing the sender to continue from a particular point if the transfer gets interrupted.

Many significant protocols operate at Layer 5 including authentication and logon technologies such as LDAP and NetBIOS. These establish semi-permanent communication channels for managing an end user session on a specific device.

6. Presentation Layer

The presentation layer handles preparation of data for the application layer that comes next in the model. After data has made it up from the hardware, through the data link, and across the transport, it's almost ready to be consumed by high-level components. The presentation layer completes the process by performing any formatting tasks that may be required.

Decryption, decoding, and decompression are three common operations found at this level. The presentation layer processes received data into formats that can be eventually utilized by a client application. Similarly, outward-bound data is reformatted into compressed and encrypted structures that are suitable for network transmission.

TLS is one major technology that's part of the presentation layer. Certificate verification and data decryption is handled before requests reach the network client, allowing information to be consumed with confidence that it's authentic.

7. Application Layer

The application layer is the top of the stack. It represents the functionality that's perceived by network end users. Applications in the OSI model provide a convenient end-to-end interface to facilitate complete data transfers, without making you think about hardware, data links, sessions, and compression.

Despite its name, this layer doesn't relate to client-side software such as your web browser or email client. An application in OSI terms is a protocol that caters for the complete communication of complex data through layers 1-6.

HTTP, FTP, DHCP, DNS, and SSH all exist at the application layer. These are high-level mechanisms which permit direct transfers of user data between an origin device and a remote server. You only need minimal knowledge of the workings of the other layers.

The seven OSI layers describe the transfer of data through computer networks. Understanding the functions and responsibilities of each layer can help you identify the source of problems and assess the intended use case for new components.

OSI is an abstract model that doesn't directly map to the specific networking implementations commonly used today. As an example, the TCP/IP protocol works on its own simpler system of four layers: Network Access, Internet, Transport, and Application. These abstract and absorb the equivalent OSI layers: the application layer spans OSI L5 to L7, while L1 and L2 are combined in TCP/IP's concept of Network Access.

OSI remains applicable despite its lack of direct real-world application. It's been around so long that it's widely understood among administrators from all backgrounds. Its relatively high level of abstraction has also ensured it's remained relevant in the face of new networking paradigms, many of which have targeted Layer 3 and above. An awareness of the seven layers and their responsibilities can still help you appreciate the flow of data through a network while uncovering integration opportunities for new components.

Unveiling the Interpreter: Fortifying the Presentation Layer’s Role in Cybersecurity

• March 28, 2024

Cherise Esparza

Welcome back to Tea with C. As we navigate deeper into the OSI model, today’s spotlight shines on Layer 6: the Presentation Layer. The Presentation Layer, often likened to an adept interpreter in the digital realm, plays a pivotal role in the seamless transmission and reception of data across the network.

At its essence, the Presentation Layer is tasked with data translation, encryption, and compression. It ensures that the information sent from the application layer is suitably formatted for transmission across the network and can be accurately interpreted by the receiving system. This layer is a universal translator for network communications, bridging different data formats into a universally understood language.

Encryption protocols at this layer are vital for securing data at rest and in transit. They are the bedrock of data integrity and confidentiality, shielding sensitive information from prying eyes and potential cyber threats. This is where the magic of transforming data into a secure format occurs, making it an essential battleground for cybersecurity efforts.

However, the Presentation Layer is not without its vulnerabilities. Some of the most insidious security threats stem from weaknesses in the coding practices used to develop applications interacting at this layer. Buffer overflows, SQL injections, and cross-site scripting are prime examples of exploits that can lead to significant security breaches. These vulnerabilities underscore the critical importance of secure coding practices and robust input validation procedures to fend off attackers.

The roles vital to securing the Presentation Layer span a broad spectrum of cybersecurity expertise. Network security analysts play a key role in scrutinizing the data exchange protocols and ensuring that encryption measures are robust and correctly implemented. Developers adhere to a secure development lifecycle, ensuring that code is thoroughly vetted and sanitized before deployment. Lastly, the overarching vigilance of security analysts dedicated to network monitoring and logging forms the backbone of a comprehensive security strategy at this layer.

Ensuring the security of the Presentation Layer is a multifaceted challenge that requires a concerted effort from all parties involved in the development and deployment of network applications. As we gear up for our following discussion on the final layer of the OSI model , the Application Layer, remember that each layer presents unique challenges and opportunities for enhancing our cybersecurity posture. Join us next week as we conclude our exploration of the OSI model.

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The Layers of the OSI Model Illustrated

Each layer explained

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• The Wireless Connection
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The Open Systems Interconnection (OSI) model defines a networking framework to implement protocols in layers, with control passed from one layer to the next. It is primarily used today as a teaching tool. It conceptually divides computer network architecture into 7 layers in a logical progression.

The lower layers deal with electrical signals, chunks of binary data , and routing of these data across networks. Higher levels cover network requests and responses, representation of data, and network protocols, as seen from a user's point of view. 

The OSI model was originally conceived as a standard architecture for building network systems, and many popular network technologies today reflect the layered design of OSI.

Physical Layer

At Layer 1, the Physical layer of the OSI model is responsible for the ultimate transmission of digital data bits from the Physical layer of the sending (source) device over network communications media to the Physical layer of the receiving (destination) device.

Examples of layer 1 technologies include  Ethernet cables  and  hubs . Also, hubs and other repeaters  are standard network devices that function at the Physical layer, as are cable connectors.

At the Physical layer, data is transmitted using the type of signaling supported by the physical medium: electric voltages, radio frequencies, or pulses of infrared or ordinary light.

Data Link Layer

When obtaining data from the Physical layer, the Data Link layer checks for physical transmission errors and packages bits into data frames. The Data Link layer also manages physical addressing schemes such as MAC addresses for Ethernet networks, controlling access of network devices to the physical medium.

Because the Data Link layer is the most complex layer in the OSI model, it is often divided into two parts: the Media Access Control sub-layer and the Logical Link Control sub-layer.

Network Layer

The Network layer adds the concept of routing above the Data Link layer. When data arrives at the Network layer, the source and destination addresses contained inside each frame are examined to determine if the data has reached its final destination. If the data has reached the final destination, layer 3 formats the data into packets delivered to the Transport layer. Otherwise, the Network layer updates the destination address and pushes the frame down to the lower layers.

To support routing, the Network layer maintains logical addresses such as IP addresses  for devices on the network. The Network layer also manages the mapping between these logical addresses and physical addresses. In IPv4 networking, this mapping is accomplished through the Address Resolution Protocol (ARP); IPv6 uses Neighbor Discovery Protocol (NDP).

Transport Layer

The Transport Layer delivers data across network connections. TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are the most common examples of Transport Layer 4 network protocols.  Different transport protocols may support a range of optional capabilities, including error recovery, flow control, and support for re-transmission.

Session Layer

The Session Layer manages the sequence and flow of events that initiate and tear down network connections. At layer 5, it is built to support multiple types of connections that can be created dynamically and run over individual networks.

Presentation Layer

The Presentation layer has the simplest function of any piece of the OSI model. At layer 6, it handles syntax processing of message data such as format conversions and encryption/decryption needed to support the Application layer above it.

Application Layer

The Application layer supplies network services to end-user applications. Network services are protocols that work with the user's data. For example, in a web browser application, the Application layer protocol HTTP packages the data needed to send and receive web page content. This layer 7 provides data to (and obtains data from) the Presentation layer.

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• The OSI (Open Systems Interconnection) model

The OSI (Open Systems Interconnection) Model

1. the osi (open systems interconnection) model.

Definition: The OSI model defines internetworking in terms of a vertical stack of seven layers. The upper layers of the OSI model represent software that implements network services like encryption and connection management. The lower layers of the OSI model implement more primitive, hardware-oriented functions like routing, addressing, and flow control.

The OSI model was introduced in 1984. Although it was designed to be an abstract model, the OSI model remains a practical framework for today's key network technologies like Ethernet and protocols like IP.

The OSI model should be used as a guide for how data is transmitted over the network. It is an abstract representation of the data pathway and should be treated as such.

The OSI model was specifically made for connecting open systems. These systems are designed to be open for communication with almost any other system. The model was made to break down each functional layer so that overall design complexity could be lessened. The model was constructed with seven layers for the flow of information. These are:

• Application Layer
• Presentation layer
• Session layer
• Transport layer
• Network layer
• Data link layer
• Physical layer

1.1 Application layer

Provides a means for the user to access information on the network through an application. This layer is the main interface for the user to interact with the application and therefore the network.

The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer.

Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).

1.2 Presentation layer

Manages the presentation of the information in an ordered and meaningful manner. This layer's primary function is the syntax and semantics of the data transmission. It converts local host computer data representations into a standard network format for transmission on the network. On the receiving side, it changes the network format into the appropriate host computer's format so that data can be utilized independent of the host computer. ASCII and EBCDIC conversions, cryptography, and the like are handled here.

The presentation layer provides a variety of coding and conversion functions that are applied to application layer data. These functions ensure that information sent from the application layer of one system would be readable by the application layer of another system. Some examples of presentation layer coding and conversion schemes include common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes.

Common data representation formats, or the use of standard image, sound, and video formats, enable the interchange of application data between different types of computer systems. Using different text and data representations, such as EBCDIC and ASCII, uses conversion schemes to exchange information with systems. Standard data compression schemes enable data that is compressed. or encrypted at the source device to be properly decompressed, or deciphered at the destination.

Presentation layer implementations are not typically associated with a particular protocol stack. Some well-known standards for video include QuickTime and Motion Picture Experts Group (MPEG). QuickTime is an Apple Computer specification for video and audio, and MPEG is a standard for video compression and coding.

Among the well-known graphic image formats are Graphics Interchange Format (GIF), Joint Photographic Experts Group (JPEG), and Tagged Image File Format (TIFF). GIF is a standard for compressing and coding graphic images. JPEG is another compression and coding standard for graphic images, and TIFF is a standard coding format for graphic images.

1.3 Session layer

Coordinates dialogue/session/connection between devices over the network. This layer manages communications between connected sessions. Examples of this layer are token management (the session layer manages who has the token) and network time synchronization.

The session layer establishes, manages, and terminates communication sessions. Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer. Some 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 Decent Phase IV session layer protocol.

1.4 Transport layer

Responsible for reliable transmission of data and service specification between hosts. The major responsibility of this layer is data integrity--that data transmitted between hosts is reliable and timely. Upper layer data grams are broken down into network-sized data grams if needed and then implemented using appropriate transmission control. The transport layer creates one or more than one network connection, depending on conditions. This layer also handles what type of connection will be created. Two major transport protocols are the TCP (Transmission Control Protocol) and the UDP (User Data gram Protocol).

Important features of Transport layer:

• Transport layer ensures reliable service.
• Breaks the message (from sessions layer) into smaller packets, assigns sequence number and sends them.
• Reliable transport connections are built on top of X.25 or IP.
• In case IP, lost packets arriving out of order must be reordered.

Important features of TCP/UDP:

• TCP/IP Widely used for network/transport layer (UNIX).
• TCP (Transport Control Protocol): This is a connection oriented protocol.
• UDP (Universal Data gram Protocol): This is a connectionless transport layer protocol.
• Application programs that do not need connection-oriented protocol generally use UDP.

1.5 Network layer

Responsible for the routing of data (packets) through the network; handles the addressing and delivery of data. This layer provides for congestion control, accounting information for the network, routing, addressing, and several other functions. IP (Internet Protocol) is a good example of a network layer protocol. Network layer does not deal with lost messages.

Important features of Network layer protocols:

• Concerned with the transmission of packets.
• Choose the best path to send a packet (routing).
• The routing may be complex in a large network (e.g. Internet).
• Routing packets through a network may be accomplished by using simple static routes or by using complex dynamic routing algorithms.

1.6 Data link layer

Provides for the reliable delivery of data across a physical network. This layer deals with issues such as flow regulation, error detection and control, and frames. This layer has the important task of creating and managing what frames are sent out on the network. The network data frame, or packet, is made up of checksum, source address, destination address, and the data itself. The largest packet size that can be sent defines the maximum transmission Unit (MTU).

Important features of Data link layer:

• Handles errors in the physical layer.
• Groups bits into frames and ensures their correct delivery.
• Adds some bits at the beginning and end of each frame plus the checksum.
• Receiver verifies the checksum.
• If the checksum is not correct, it asks for retransmission. (Send a control message).
• Consists of two sub layers:

Logical Link Control (LLC) defines how data is transferred over the cable and provides data link service to the higher layers.

Medium Access Control (MAC) defines who can use the network when multiple computers are trying to access it simultaneously (i.e. Token passing, Ethernet [CSMA/CD]).

The data link layer provides reliable transit of data across a physical network link. Different data link layer specifications define different network and protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control. Physical addressing (as opposed to network addressing) defines how devices are addressed at the data link layer. Network topology consists of the data link layer specifications that often define how devices are to be physically connected, such as in a bus or a ring topology. Error notification alerts upper-layer protocols that a transmission error has occurred, and the sequencing of data frames reorders frames that are transmitted out of sequence. Finally, flow control moderates the transmission of data so that the receiving device is not overwhelmed with more traffic than it can handle at one time.

The protocols used in Data link layer are SLIP, PPP, MTU, and CSLP.

OSI network model for beginners: device, layers, protocols

What is the OSI model

Today, the OSI seven-layer model is still a very popular basis that is used to build network interaction in all computer systems.

OSI (Open Systems Interconnection) is a specially developed model, the main function of which is to build a logical diagram of interaction between computer systems, which allows them to freely interact with other similar systems.

No less important functions of this model are the determination of the logic of network operation and efficient description of the computer packet transmission using protocols of various levels. In other words, the main purpose of the OSI model is to provide a visual explanation of how various elements and technologies interact to transmit data over a network.

History of creation

The idea of creating the OSI model arose after realizing the need for a reliable tool for visualizing the various components of a network system. Computer systems needed a universal way to interact with various companies and areas of activity (including business processes). As a result, the OSI model provided a reliable way to describe and analyze network structures.

Initially, 2 large projects were in the development process:

• International Telegraph and Telephone Consultative Committee (CCITT).
• International Organization for Standardization (IOS).

In 1983, all prepared documents were combined, and in 1984 ISO published them as a single framework. It was named OSI (Basic Reference Model for Open Systems Interconnection).

Thus, OSI Model has become a recognized international standard, which is not inferior in popularity to the most popular standard among network equipment manufacturers – TCP/IP model.

Differences between OSI and TCP/IP models

TCP/IP model was developed in the 1970s. Transmission Control Protocol (TCP) and Internet Protocol (IP) are the basis for the name of the TCP/IP model.

Let's consider the first difference – the number of levels.

• OSI has seven layers.
• TCP/IP combines OSI layers 1-2 into the network interface layer and OSI layers 5-7 into one application layer.

OSI is a more general model (describes network interaction in general).

TCP/IP model simulates the operation of communication protocols with maximum accuracy (it is an excellent option for public networks).

Next difference – work profile.

• TCP/IP is better for practical operations (all levels of this model can be used by appropriate applications.
• The OSI model allows applications to use only a few layers (layers 1-3 are required for data transfer).

OSI model operation

This process can be visually represented as follows:

There are two servers whose interaction requires data exchange. To do this, data blocks travel down the network layers and eventually reach the transmission line. This process must also be performed in reverse order (until it reaches the receiving app).

Next, we will consider several basic concepts that can be used to explain the data path:

• Service Data Units (SDU) . They indicate the content of adjacent upper-layer PDUs, which is equivalent to the payload when transmitted to the lower network layer.
• Protocol Data Units (PDU) . They are transmitted between equivalent network objects. They include protocol-specific and user data (+Protocol body / payload, + Protocol header, + Protocol trailer).

With each subsequent transition from some level N to any other level N-1, the level N PDU becomes a new N-1 SDU. This payload may be located in a level N-1 PDU including appropriate trailers and headers. Data travels up the chain at the opposite end. In the transition process they unfold at each necessary stage until they become just a payload. This payload can be used by an appropriate level N device.

OSI Model layers and their functions

There are 7 separate layers of the OSI network model, which are united by a common protocol stack. General rules and guidelines are the basis of each level. They simplify the processes of creating and operating network technologies.

Each OSI model layer is essentially an associated critical data transfer process. Steps in such a path may include packet creation, flow control, encryption and presentation.

Layers in the OSI model describe the stages in which an idealized data packet passes through a communications system. More often, data is transferred from the L-7 OSI Application Layer down to the L-1 OSI Physical Layer. After that, they are transmitted in reverse order to the L-7 OSI layer. At the top level the data can be used by recipients.

L-7 OSI Application Layer

Data transfer format (PDU) : message.

Purpose : to provide the user with access to network resources.

Basic protocols : Domain Name System (DNS), Hypertext Transfer Protocol (HTTP), Simple Mail Transfer Protocol (SMTP), Secure Shell (SSH), File Transfer Protocol (FTP), Simple Network Management Protocol (SNMP), Telnet.

This level is characterized by user interaction with data. L-7 makes it possible to receive data for use by software (or another option – preparing data before sending it through the chain of layers of the OSI model). The application layer of the OSI model includes software (it allows network applications to function).

Important L-7 OSI functions:

• Directory services.
• Working with protocols and data formatting tools.
• Post services.
• FTAM (File Transfer Access & Management) – access to file transfer and management.
• Network virtual terminal.

L-6 OSI Presentation Layer

Data transmission format (PDU) : message.

Purpose : translation, encryption and compression of data.

Basic protocols : Transport Layer Security (TLS), MPEG Media Transport Protocol (MMTP), Multipurpose Internet Mail Extensions (MIME), Asymmetric Synchronous Channel Hopping (ASCH).

Raw data is located on the Presentation Layer. The main job of this layer is to process the data before it can be used by the OSI application layer. At this level, necessary data is encrypted and compressed (or vice versa, decrypted and unpacked). This process ensures secure data transfer. Transfer of large amounts of data at high speed is ensured by its compression.

Important L-6 OSI functions :

• Compression—reducing the number of bits transmitted over a network.
• Data conversion.
• Data encryption/decryption using a key value.

L-5 OSI Session Layer

Purpose : opening, managing and closing a session.

Device : network gateway.

Basic protocols : Session Announcement Protocol (SAP), Network Basic Input/Output System (NetBIOS).

The main purpose of L5 OSI is to define the rules for data transmission and authentication. The Session Layer also performs the function of establishing communication between devices. L5 OSI determines the accuracy of data transfer and the duration of sessions. It is also necessary to mention data checkpoints or data synchronization points. Checkpoints are used to divide data into smaller segments. Before closing the session, each segment is checked for correctness and reliability.

Important L-5 OSI functions :

• Data synchronization using checkpoints.
• Security (it is necessary to quickly end sessions and enable the authentication system).
• The most efficient data transfer with minimal use of resources.
• Dialogue controller (2 systems can communicate in half-duplex or full-duplex mode).

OSI Session Layer Tools :

Application toolkit can be applied by users. For example, let`s consider the FileZilla FTP application: it offers logs and debug menus, which helps resolve FTP connection problems at the session level.

L-4 OSI Transport Layer

Data transfer format (PDU) : fragment.

Purpose : Transfer data from a process on the source computer to a process on the target computer.

Device : network firewall.

Basic protocols : User Datagram Protocol (UDP), Transmission Control Protocol (TCP).

L-4 OSI is responsible for setting up direct communication between connected devices. Its main task is to ensure continuous data transfer (they must be sent and received in the same form).

Special tools used at this layer determine the correct data transfer rate (taking into account the connection speed of the devices used, data transfer speed may vary). The OSI transport layer controls the flow of data in end-to-end communications.

Important L-4 OSI functions :

• Error control by evaluating data packets at the receiving device. L-4 OSI service providers may request retransmission if the data did not arrive in the correct form.
• Segmentation and reassembly. The received message from a higher level is divided into smaller blocks (each of these blocks has a header associated with it). At the final destination, all fragments are reassembled into a message.
• Service Point Addressing. The L4 OSI header has an address type called service point address or port address (ensures that the message is sent to the correct process).

OSI Transport Layer Tools :

For Linux, separate solutions are used for certain protocols, since no special tools have been developed. For TCP there is a utility tcptrack , which performs the function of displaying lists of current sessions. tcptrack can be installed using the apt command:

sudo apt install tcptrack

To make active interface connections available, you must use the -i option and the interface name:

sudo tcptrack -i eth0

Monitoring of outgoing and incoming packets on a specific interface is available thanks to the tcpdump packet analyzer. The eth0 interface is used by default. The -i attribute displays the listening interface:

sudo tcpdump

L-3 OSI Network Layer

Data transmission format (PDU) : packet.

Purpose : data transfer from one host to another on different networks.

Device : router.

Basic protocols : Internet Group Management Protocol (IGMP), Internet Protocol (IP), Internet Control Message Protocol (ICMP), Multiprotocol Label Switching (MPLS), Border Gateway Protocol (BGP), Open Shortest Path First (OSPF).

The main task of L-3 OSI is to create and support the stability of network connections. The OSI Network Layer performs the function of transferring data between connected devices. The data is divided into packets ready for transmission over the network. The original data is restored by combining packets at the receiving end of the transmission

Important L-3 OSI Functions :

Logical addressing – placing the sender and recipient IP addresses in the header helps define the addressing scheme for each unique device on the network.

Routing – hardware and software determine the optimal path for transmitting data between various networks.

OSI Network Layer Tools :

For any problems at network level, the ip command is useful. The ip addr show command shows the IP address associated with each interface:

ip addr show

To view the contents of the system routing table the following command can be used:

ip route show

ping and traceroute commands help you track the path along which a packet is sent to its destination. They can be used with the IP address or router name:

ping wikipedia.org

L-2 OSI Link Layer

Data transmission format (PDU): frame.

Purpose : organize bits into frames to enable local data transfer.

Device : network bridge (switch).

Basic protocols : Asynchronous Transfer Mode (ATM), Rapid Assessment of Physical Activity (RAPA), Frame Relay (FR), Point-to-Point Protocol (PPP), fiber optic cable.

The L-2 OSI layer is closely related to the Network layer. However, it is most often referred to as communication between locally connected devices.

At this level, after receiving, all data is divided into frames. These frames interact with two sublayers of the L-2 OSI layer:

• Media Access Control (MAC) – connects appropriate local devices and controls the flow rate on the network.
• Logical Link Control (LLC) – establishes the logical basis for local data transfer.

Important L-2 OSI Functions :

• Access control (determining control priority between used devices).
• Framing (transmission of the desired set of data is possible by attaching specific combinations of bits to the beginning and end of the frame).
• Data rate control (competent distribution of information flows).
• Physical addressing (adding the MAC address of the sender and/or recipient to the headers of created frames).
• Error control mechanism (transmission of data containing any errors will be repeated).

OSI Link Layer Tool s:

To display information about network interfaces on the server, you will need the next command:

ip link show

The nast packet utility can be used to analyze local network traffic. This utility can be installed using the apt command:

sudo apt install nast

Next step – you should run the command with superuser rights and specify the interface to listen to using the -i parameter:

sudo nast -i eth0

The configuration and capabilities of each network interface can be viewed using the following command:

L-1 OSI Physical Layer

Data transmission format (PDU) : bit.

Purpose : providing electrical and mechanical resources for transmitting bits in networks.

Devices : modem, RF links, cables, hub, repeater, voltage regulators and routing devices.

Basic protocols : Integrated Services Digital Network (ISDN), Recommended Standard 232 (RS232/EIA232), 100BaseTX.

The L-1 OSI layer includes all physical infrastructure and necessary equipment used to transmit data. The digital bit stream that is converted to L2 OSI is formed from 1s and 0s at the physical layer. Before transmission, the form of this bit stream is confirmed between the two devices. This makes it possible to reconstruct the data at the receiving end.

The most common errors and network problems occur at the physical OSI layer, but they can be eliminated quite simply.

Important L-1 OSI Functions :

• Selecting a data transfer mode (simplex, half duplex and full duplex modes are included).
• Synchronization of sender and receiver bits.
• Determination of the physical topologies of network devices.
• Data transfer rate control.

OSI Physical Layer Tools :

There are no practical ways to debug problems at this level. Eliminating existing problems usually requires a series of trial and errors in the process of replacing physical ports, connectors and cables.

Cross-level functions

Cross-layer functions are different layers combined in the OSI hierarchy. These functions include the most important services for certain parts of the data transfer process. Some of these services include:

• Common security architecture recommended by ITU x.800 standard.
• Protocols for converting IP addresses to MAC addresses (they operate at the network and data link layers).
• Security management tools for configuring and controlling the process of data exchange between network devices.
• Domain Name System (DNS) lookup services.
• Multi-protocol label switching (MPLS) for transferring data frames between networks.

Cross-layer functions monitor and regulate traffic, ensuring secure and reliable data transmission. Cross-layer functions operate at different network layers and resolve problems as they arise. Thus, cross-layer services are the basis of network security planning.

The process of transferring data across the layers of the OSI model

L-7 OSI . At the Application Layer, the web browser client interacts with the application protocol. The user's request takes the form of an HTTP or HTTPS message. The DNS protocol applies to resolving a domain name to an IP address.

L-6 OSI . When using HTTPS, the Presentation Layer encrypts the outgoing request with the help of TLS socket. The data may also be encoded or translated into another character set.

L-5 OSI . At the Session Layer, a session is defined for sending and receiving HTTP/HTTPS messages. Because web browsing requires reliable data transmission, the L-5 OSI layer most often opens a TCP session. Some streaming applications may also choose a UDP session for use.

L-4 OSI . The Transport Layer TCP protocol initiates a connection with the target server. During a session, packets are transmitted in their original order, as well as being sent and received. UDP sends all packets without a direct connection and without waiting for confirmation. In some cases, data packets may be segmented into smaller parts. Further, all outgoing packets are sent to the Network layer.

L-3 OSI . Routing protocols select the output interface to use based on the destination address. The data, including address information, is encapsulated in an IP packet, which is then forwarded to the Link Layer.

L-2 OSI . The link layer converts IP packets into frames (this may lead to their fragmentation). Frames are created based on the data link protocol, which is used.

L-1 OSI . At the physical layer, frames are converted into a stream of bits and transmitted to the media.

OSI model and DDoS protection

The OSI Network model is the primary method for countering serious cyberattacks such as DDoS. Each OSI layer is distinguished by certain types of DDoS attacks and methods for eliminating them:

L-7 OSI . Common types of DDoS: "slow session" attacks, hacking the BGP protocol, HTTP(S) GET/POST flood. Protection methods: monitoring applications and tracking zero-day attacks and cyber attacks at this OSI layer.

L-6 OSI . Common types of DDoS: sending false or incorrect SSL requests. Protection methods: cleaning, filtering, and routing SSL traffic.

L-5 OSI . Common types of DDoS: attacks through vulnerabilities in network protocols for terminal interfaces. Protection methods: regularly updating software versions, restricting access to network equipment.

L-4 OSI . Common types of DDoS: SMURF attacks, SYN flood (TCP/SYN), UDP flood. Protection methods: filtering and limiting the number of connections from certain sources.

L-3 OSI . Common types of DDoS: SMURF attacks, POD (ping of death), ICMP flood (ping flood). Protection methods: using heuristic algorithms, false traffic is filtered and redirected.

L-2 OSI . Common types of DDoS: manipulation of data in the SRC/MAC and DST/MAC fields leads to disruption of the standard network data flow between devices. Protection methods: applying modern managed switches.

L-1 OSI . Common types of DDoS: any damage to the equipment or disruption of its performance. Protection methods: using the system for restricting and controlling access to equipment.

OSI Model Advantages

Cybersecurity . The OSI system makes it possible to identify security weaknesses. Information security specialists classify risks in accordance with OSI levels, which allows them to select suitable data protection tools after determining data location in the network hierarchy.

Troubleshooting . The OSI hierarchy makes it possible to detect network defects at an early stage. This model can be used by professionals to detect application problems, network-wide problems, or physical hardware failures. OSI offers a reliable way to divide problems into manageable parts.

Software development . The OSI model plays an important role in the planning and coding stages. Applications operating at different levels can be simulated by specialists. The layer model defines an algorithm for interaction of the application with other network components and tools from different vendors.

Marketing . By providing detailed product feature descriptions, marketers can accurately explain to customers where their products fit in the OSI hierarchy. Buyers can figure out how these products will fit into the network architecture.

OSI Model Disadvantages

• There is a bit of chaos due to the fact that certain functions span different OSI layers.
• Presence of redundant elements in the 7-level model is also a minus.
• Statements by many experts that the OSI system is outdated. It is believed that today the division of network structures into seven different levels is not effective anymore. The Internet is less compliant with the OSI model, which is why it never became the main industry standard among network equipment manufacturers.

Although OSI is a convenient learning model, it is rather abstract in nature. The OSI model protocols are most often used in conjunction with the TCP/IP stack.

However, a large number of popular network tools still map to different OSI layers, making the OSI model an essential part of many network techniques. Moreover, the conceptual principles of this model can be the basis for creating cybersecurity systems and protection against distributed network attacks.

Let's not forget that OSI became a successful result of an attempt to standardize a network language. This gave experts a common language to discuss IT network architecture. This hierarchy also made it easier to compare hardware profiles, protocols, applications, etc.

Thus, the OSI model is still relevant in many areas of activity and is used all over the world.

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Application Architecture Guide - Chapter 10 - Presentation Layer Guidelines

Note - The patterns & practices Microsoft Application Architecture Guide, 2nd Edition is now live at http://msdn.microsoft.com/en-us/library/dd673617.aspx .

- J.D. Meier, Alex Homer, David Hill, Jason Taylor, Prashant Bansode, Lonnie Wall, Rob Boucher Jr, Akshay Bogawat

• 1 Objectives
• 3 Presentation Layer Components
• 5 Design Considerations
• 6 Presentation Layer Frame
• 8 Composition
• 9 Exception Management
• 12 Navigation
• 13 Presentation Entities
• 14 Request Processing
• 15 User Experience
• 16 UI Components
• 17 UI Process Components
• 18 Validation
• 19 Pattern Map
• 20 Pattern Descriptions
• 21.1 Mobile Applications
• 21.2 Rich Client Applications
• 21.3 Rich Internet Applications (RIA)
• 21.4 Web Applications
• 22 patterns & practices Solution Assets
• 23 Additional Resources
• Understand how the presentation layer fits into typical application architecture.
• Understand the components of the presentation layer.
• Learn the steps for designing the presentation layer.
• Learn the common issues faced while designing the presentation layer.
• Learn the key guidelines for designing the presentation layer.
• Learn the key patterns and technology considerations for designing the presentation layer.

The presentation layer contains the components that implement and display the user interface and manage user interaction. This layer includes controls for user input and display, in addition to components that organize user interaction. Figure 1 shows how the presentation layer fits into a common application architecture.

Figure 1 A typical application showing the presentation layer and the components it may contain

Presentation Layer Components

• User interface (UI) components . User interface components provide a way for users to interact with the application. They render and format data for users. They also acquire and validate data input by the user.
• User process components . User process components synchronize and orchestrate user interactions. Separate user process components may be useful if you have a complicated UI. Implementing common user interaction patterns as separate user process components allows you to reuse them in multiple UIs.

The following steps describe the process you should adopt when designing the presentation layer for your application. This approach will ensure that you consider all of the relevant factors as you develop your architecture:

• Identify your client type . Choose a client type that satisfies your requirements and adheres to the infrastructure and deployment constraints of your organization. For instance, if your users are on mobile devices and will be intermittently connected to the network, a mobile rich client is probably your best choice.
• Determine how you will present data . Choose the data format for your presentation layer and decide how you will present the data in your UI.
• Determine your data-validation strategy . Use data-validation techniques to protect your system from untrusted input.
• Determine your business logic strategy . Factor out your business logic to decouple it from your presentation layer code.
• Determine your strategy for communication with other layers . If your application has multiple layers, such as a data access layer and a business layer, determine a strategy for communication between your presentation layer and other layers.

Design Considerations

There are several key factors that you should consider when designing your presentation layer. Use the following principles to ensure that your design meets the requirements for your application, and follows best practices:

• Choose the appropriate UI technology. Determine if you will implement a rich (smart) client, a Web client, or a rich Internet application (RIA). Base your decision on application requirements, and on organizational and infrastructure constraints.
• Use the relevant patterns. Review the presentation layer patterns for proven solutions to common presentation problems.
• Design for separation of concerns. Use dedicated UI components that focus on rendering and display. Use dedicated presentation entities to manage the data required to present your views. Use dedicated UI process components to manage the processing of user interaction.
• Consider human interface guidelines. Review your organization’s guidelines for UI design. Review established UI guidelines based on the client type and technologies that you have chosen.
• Adhere to user-driven design principles. Before designing your presentation layer, understand your customer. Use surveys, usability studies, and interviews to determine the best presentation design to meet your customer’s requirements.

Presentation Layer Frame

There are several common issues that you must consider as your develop your design. These issues can be categorized into specific areas of the design. The following table lists the common issues for each category where mistakes are most often made.

Table 1 Presentation Layer Frame

Caching is one of the best mechanisms you can use to improve application performance and UI responsiveness. Use data caching to optimize data lookups and avoid network round trips. Cache the results of expensive or repetitive processes to avoid unnecessary duplicate processing.

Consider the following guidelines when designing your caching strategy:

• Do not cache volatile data.
• Consider using ready-to-use cache data when working with an in-memory cache. For example, use a specific object instead of caching raw database data.
• Do not cache sensitive data unless you encrypt it.
• If your application is deployed in Web farm, avoid using local caches that need to be synchronized; instead, consider using a transactional resource manager such as Microsoft SQL Server® or a product that supports distributed caching.
• Do not depend on data still being in your cache. It may have been removed.

Composition

Consider whether your application will be easier to develop and maintain if the presentation layer uses independent modules and views that are easily composed at run time. Composition patterns support the creation of views and the presentation layout at run time. These patterns also help to minimize code and library dependencies that would otherwise force recompilation and redeployment of a module when the dependencies change. Composition patterns help you to implement sharing, reuse, and replacement of presentation logic and views.

Consider the following guidelines when designing your composition strategy:

• Avoid using dynamic layouts. They can be difficult to load and maintain.
• Be careful with dependencies between components. For example, use abstraction patterns when possible to avoid issues with maintainability.
• Consider creating templates with placeholders. For example, use the Template View pattern to compose dynamic Web pages in order to ensure reuse and consistency.
• Consider composing views from reusable modular parts. For example, use the Composite View pattern to build a view from modular, atomic component parts.
• If you need to allow communication between presentation components, consider implementing the Publish/Subscribe pattern. This will lower the coupling between the components and improve testability.

Exception Management

Design a centralized exception-management mechanism for your application that catches and throws exceptions consistently. Pay particular attention to exceptions that propagate across layer or tier boundaries, as well as exceptions that cross trust boundaries. Design for unhandled exceptions so they do not impact application reliability or expose sensitive information.

Consider the following guidelines when designing your exception management strategy:

• Use user-friendly error messages to notify users of errors in the application.
• Avoid exposing sensitive data in error pages, error messages, log files, and audit files.
• Design a global exception handler that displays a global error page or an error message for all unhandled exceptions.
• Differentiate between system exceptions and business errors. In the case of business errors, display a user-friendly error message and allow the user to retry the operation. In the case of system exceptions, check to see if the exception was caused by issues such as system or database failure, display a user-friendly error message, and log the error message, which will help in troubleshooting.
• Avoid using exceptions to control application logic.

Design a user input strategy based on your application input requirements. For maximum usability, follow the established guidelines defined in your organization, and the many established industry usability guidelines based on years of user research into input design and mechanisms.

Consider the following guidelines when designing your input collection strategy:

• Use forms-based input controls for normal data-collection tasks.
• Use a document-based input mechanism for collecting input in Microsoft Office–style documents.
• Implement a wizard-based approach for more complex data collection tasks, or for input that requires a workflow.
• Design to support localization by avoiding hard-coded strings and using external resources for text and layout.
• Consider accessibility in your design. You should consider users with disabilities when designing your input strategy; for example, implement text-to-speech software for blind users, or enlarge text and images for users with poor sight. Support keyboard-only scenarios where possible for users who cannot manipulate a pointing device.

Design your UI layout so that the layout mechanism itself is separate from the individual UI components and UI process components. When choosing a layout strategy, consider whether you will have a separate team of designers building the layout, or whether the development team will create the UI. If designers will be creating the UI, choose a layout approach that does not require code or the use of development-focused tools.

Consider the following guidelines when designing your layout strategy:

• Use templates to provide a common look and feel to all of the UI screens.
• Use a common look and feel for all elements of your UI to maximize accessibility and ease of use.
• Consider device-dependent input, such as touch screens, ink, or speech, in your layout. For example, with touch-screen input you will typically use larger buttons with more spacing between them than you would with mouse or keyboard inputs.
• When building a Web application, consider using Cascading Style Sheets (CSS) for layout. This will improve rendering performance and maintainability.
• Use design patterns, such as Model-View-Presenter (MVP), to separate the layout design from interface processing.

Design your navigation strategy so that users can navigate easily through your screens or pages, and so that you can separate navigation from presentation and UI processing. Ensure that you display navigation links and controls in a consistent way throughout your application to reduce user confusion and hide application complexity.

Consider the following guidelines when designing your navigation strategy:

• Use well-known design patterns to decouple the UI from the navigation logic where this logic is complex.
• Design toolbars and menus to help users find functionality provided by the UI.
• Consider using wizards to implement navigation between forms in a predictable way.
• Determine how you will preserve navigation state if the application must preserve this state between sessions.
• Consider using the Command Pattern to handle common actions from multiple sources.

Presentation Entities

Use presentation entities to store the data you will use in your presentation layer to manage your views. Presentation entities are not always necessary; use them only if your datasets are sufficiently large and complex to require separate storage from the UI controls.

Consider the following guidelines when designing presentation entities:

• Determine if you require presentation entities. Typically, you may require presentation entities only if the data or the format to be displayed is specific to the presentation layer.
• If you are working with data-bound controls, consider using custom objects, collections, or datasets as your presentation entity format.
• If you want to map data directly to business entities, use a custom class for your presentation entities.
• Do not add business logic to presentation entities.
• If you need to perform data type validation, consider adding it in your presentation entities.

Request Processing

Design your request processing with user responsiveness in mind, as well as code maintainability and testability.

Consider the following guidelines when designing request processing:

• Use asynchronous operations or worker threads to avoid blocking the UI for long-running actions.
• Avoid mixing your UI processing and rendering logic.
• Consider using the Passive View pattern (a variant of MVP) for interfaces that do not manage a lot of data.
• Consider using the Supervising Controller pattern (a variant of MVP) for interfaces that manage large amounts of data.

User Experience

Good user experience can make the difference between a usable and unusable application. Carry out usability studies, surveys, and interviews to understand what users require and expect from your application, and design with these results in mind.

Consider the following guidelines when designing for user experience:

• When developing a rich Internet application (RIA), avoid synchronous processing where possible.
• When developing a Web application, consider using Asynchronous JavaScript and XML (AJAX) to improve responsiveness and to reduce post backs and page reloads.
• Do not design overloaded or overly complex interfaces. Provide a clear path through the application for each key user scenario.
• Design to support user personalization, localization, and accessibility.
• Design for user empowerment. Allow the user to control how he or she interacts with the application, and how it displays data to them.

UI Components

UI components are the controls and components used to display information to the user and accept user input. Be careful not to create custom controls unless it is necessary for specialized display or data collection.

Consider the following guidelines when designing UI components:

• Take advantage of the data-binding features of the controls you use in the UI.
• Create custom controls or use third-party controls only for specialized display and data-collection tasks.
• When creating custom controls, extend existing controls if possible instead of creating a new control.
• Consider implementing designer support for custom controls to make it easier to develop with them.
• Consider maintaining the state of controls as the user interacts with the application instead of reloading controls with each action.

UI Process Components

UI process components synchronize and orchestrate user interactions. UI processing components are not always necessary; create them only if you need to perform significant processing in the presentation layer that must be separated from the UI controls. Be careful not to mix business and display logic within the process components; they should be focused on organizing user interactions with your UI.

Consider the following guidelines when designing UI processing components:

• Do not create UI process components unless you need them.
• If your UI requires complex processing or needs to talk to other layers, use UI process components to decouple this processing from the UI.
• Consider dividing UI processing into three distinct roles: Model, View, and Controller/Presenter, by using the MVC or MVP pattern.
• Avoid business rules, with the exception of input and data validation, in UI processing components.
• Consider using abstraction patterns, such as dependency inversion, when UI processing behavior needs to change based on the run-time environment.
• Where the UI requires complex workflow support, create separate workflow components that use a workflow system such as Windows Workflow or a custom mechanism.

Designing an effective input and data-validation strategy is critical to the security of your application. Determine the validation rules for user input as well as for business rules that exist in the presentation layer.

Consider the following guidelines when designing your input and data validation strategy:

• Validate all input data on the client side where possible to improve interactivity and reduce errors caused by invalid data.
• Do not rely on client-side validation only. Always use server-side validation to constrain input for security purposes and to make security-related decisions.
• Design your validation strategy to constrain, reject, and sanitize malicious input.
• Use the built-in validation controls where possible, when working with .NET Framework.
• In Web applications, consider using AJAX to provide real-time validation.

Pattern Map

Key patterns are organized by key categories, as detailed in the Presentation Layer Frame in the following table. Consider using these patterns when making design decisions for each category.

Table 2 Pattern Map

• For more information on the Page Cache pattern, see “Enterprise Solution Patterns Using Microsoft .NET” at http://msdn.microsoft.com/en-us/library/ms998469.aspx
• For more information on the Model-View-Controller (MVC), Page Controller, Front Controller, Template View, Transform View, and Two-Step View patterns, see “Patterns of Enterprise Application Architecture (P of EAA)” at http://martinfowler.com/eaaCatalog/
• For more information on the Composite View, Supervising Controller, and Presentation Model patterns, see “Patterns in the Composite Application Library” at http://msdn.microsoft.com/en-us/library/cc707841.aspx
• For more information on the Chain of responsibility and Command pattern, see “data & object factory” at http://www.dofactory.com/Patterns/Patterns.aspx
• For more information on the Asynchronous Callback pattern, see “Creating a Simplified Asynchronous Call Pattern for Windows Forms Applications” at http://msdn.microsoft.com/en-us/library/ms996483.aspx
• For more information on the Exception Shielding and Entity Translator patterns, see “Useful Patterns for Services” at http://msdn.microsoft.com/en-us/library/cc304800.aspx

Pattern Descriptions

• Asynchronous Callback. Execute long-running tasks on a separate thread that executes in the background, and provide a function for the thread to call back into when the task is complete.
• Cache Dependency. Use external information to determine the state of data stored in a cache.
• Chain of Responsibility. Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request.
• Composite View . Combine individual views into a composite representation.
• Command Pattern. Encapsulate request processing in a separate command object with a common execution interface.
• Entity Translator. An object that transforms message data types into business types for requests, and reverses the transformation for responses.
• Exception Shielding. Prevent a service from exposing information about its internal implementation when an exception occurs.
• Front Controller . Consolidate request handling by channeling all requests through a single handler object, which can be modified at run time with decorators.
• Model-View-Controller . Separate the UI code into three separate units: Model (data), View (interface), and Presenter (processing logic), with a focus on the View. Two variations on this pattern include Passive View and Supervising Controller, which define how the View interacts with the Model.
• Page Cache. Improve the response time for dynamic Web pages that are accessed frequently but change less often and consume a large amount of system resources to construct.
• Page Controller . Accept input from the request and handle it for a specific page or action on a Web site.
• Passive View . Reduce the view to the absolute minimum by allowing the controller to process user input and maintain the responsibility for updating the view.
• Presentation Model . Move all view logic and state out of the view, and render the view through data-binding and templates.
• Supervising Controller . A variation of the MVC pattern in which the controller handles complex logic, in particular coordinating between views, but the view is responsible for simple view-specific logic.
• Template View . Implement a common template view, and derive or construct views using this template view.
• Transform View . Transform the data passed to the presentation tier into HTML for display in the UI.
• Two-Step View . Transform the model data into a logical presentation without any specific formatting, and then convert that logical presentation to add the actual formatting required.

Technology Considerations

The following guidelines will help you to choose an appropriate implementation technology. The guidelines also contain suggestions for common patterns that are useful for specific types of application and technology.

Mobile Applications

Consider the following guidelines when designing a mobile application:

• If you want to build full-featured connected, occasionally connected, and disconnected executable applications that run on a wide range of Microsoft Windows®–based devices, consider using the Microsoft Windows Compact Framework.
• If you want to build connected applications that require Wireless Application Protocol (WAP), compact HTML (cHTML), or similar rendering formats, consider using ASP.NET Mobile Forms and Mobile Controls.
• If you want to build applications that support rich media and interactivity, consider using Microsoft Silverlight® for Mobile.

Rich Client Applications

Consider the following guidelines when designing a rich client application:

• If you want to build applications with good performance and interactivity, and have design support in Microsoft Visual Studio®, consider using Windows Forms.
• If you want to build applications that fully support rich media and graphics, consider using Windows Presentation Foundation (WPF).
• If you want to build applications that are downloaded from a Web server and then execute on the client, consider using XAML Browser Applications (XBAP).
• If you want to build applications that are predominantly document-based, or are used for reporting, consider designing a Microsoft Office Business Application.
• If you decide to use Windows Forms and you are designing composite interfaces, consider using the Smart Client Software Factory.
• If you decide to use WPF and you are designing composite interfaces, consider using the Composite Application Guidance for WPF.
• If you decide to use WPF, consider using the Presentation Model (Model-View-ViewModel) pattern.
• If you decide to use WPF, consider using WPF Commands to communicate between your View and your Presenter or ViewModel.
• If you decide to use WPF, consider implementing the Presentation Model pattern by using DataTemplates over User Controls to give designers more control.

Rich Internet Applications (RIA)

Consider the following guidelines when designing an RIA:

• If you want to build browser-based, connected applications that have broad cross-platform reach, are highly graphical, and support rich media and presentation features, consider using Silverlight.
• If you decide to use Silverlight, consider using the Presentation Model (Model-View-ViewModel) pattern.

Web Applications

Consider the following guidelines when designing a Web application:

• If you want to build applications that are accessed through a Web browser or specialist user agent, consider using ASP.NET.
• If you want to build applications that provide increased interactivity and background processing, with fewer page reloads, consider using ASP.NET with AJAX.
• If you want to build applications that include islands of rich media content and interactivity, consider using ASP.NET with Silverlight controls.
• If you are using ASP.NET and want to implement a control-centric model with separate controllers and improved testability, consider using the ASP.NET MVC Framework.
• If you are using ASP.NET, consider using master pages to simplify development and implement a consistent UI across all pages.

patterns & practices Solution Assets

• Web Client Software Factory at http://msdn.microsoft.com/en-us/library/bb264518.aspx
• Smart Client Software Factory at http://msdn.microsoft.com/en-us/library/aa480482.aspx
• Composite Application Guidance for WPF at http://msdn.microsoft.com/en-us/library/cc707819.aspx
• Smart Client - Composite UI Application Block at http://msdn.microsoft.com/en-us/library/aa480450.aspx

Additional Resources

• For more information, see Microsoft Inductive User Interface Guidelines at http://msdn.microsoft.com/en-us/library/ms997506.aspx .
• For more information, see User Interface Control Guidelines at http://msdn.microsoft.com/en-us/library/bb158625.aspx .
• For more information, see User Interface Text Guidelines at http://msdn.microsoft.com/en-us/library/bb158574.aspx .
• For more information, see Design and Implementation Guidelines for Web Clients at http://msdn.microsoft.com/en-us/library/ms978631.aspx .
• For more information, see Web Presentation Patterns at http://msdn.microsoft.com/en-us/library/ms998516.aspx .

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Presentation layer in 3-tier architecture

My question is about various ways of implementing presentation layer in 3-tier architectures

When we talk about a 3-tier web application, it is assumed that the presentation layer is browser-oriented, and hence communicates with logic tier through HTTP protocol

I'd like to know, how presentation layer is going to communicate with logic tier, in case if the presentation layer is going to be a standalone application with its own GUI, rather than browser-based

For example, Java servlets get HTTP requests from our browser, but what about if I want to design a specific desktop application to communicate with servlets ? How my app is going to communicate with logic tier ? Which protocol is used ?

• client-server
• server-side-scripting

I guess you're misunderstanding the problems. You can say in this case the presentation layer is splitted in 2 small layers:

• Files that handle the view (JSP, Facelets, etc).
• Files that control the interaction between user and the view (Servlets, @Controller from Spring MVC, @ManagedBean from JSF, etc).

Apart from these, you can have your business logic layer (usually Service classes) and your data access layer (DAOs or whatever you feel better to call them).

If you put this in the perspective of creating GUI desktop applications, your presentation will have a similar structure:

• Classes that handle the view
• Controller classes that handle the interaction between the user and the view.

In this scenario, it usually happens that these classes are the same , but note that they are for presentation purpose and should relate with your business logic layer.

what about if I want to design a specific desktop application to communicate with servlets?

You probably mean about a client application that consumes Web Services. The Web Services (consumed by XML, JSON or plain text) could be part of a services layer that should be consumed in the business logic layer or in the presentation layer of the application, depending on what the web service returns. Still, I would find better consuming the web service layer from the business logic layer and let the presentation layer to handle its purpose: presentation logic only .

To show an example:

From comments:

Still it's unclear how business logic layer of my application is going to communicate with web service layer.

Posting a very simple skeleton sample from a Web Application Project that will consume a Web Service.

Servlet class (adapted from StackOverflow Servlet wiki ) (part of presentation)

PersonBL class (part of Business Logic Layer)

Now, posting the skeleton of the Web Service:

PersonPort class (the implementor of the Web Service)

PersonWSBL class (business logic layer in Web Service)

PersonDAO class (data access layer)

As you can notice, there's no magic in communicating the presentation with the business logic layer. Of course, this skeleton can be enhanced by using other set of technologies, but it just to illustrate the main idea.

Note: The skeleton of the Web Service was adapted from here Creating a Simple Web Service and Client with JAX-WS .

• Thanks for reply. I guess I need the ways of communication between presentation and business logic layers in your scheme above. How is that accomplished ? –  mangusta Commented Jun 10, 2013 at 23:00
• @mangusta that depends on your application design. You can consider this scenario as two applications: the first one that acts as the presentation application that will have the presentation layer and its own business logic layer (GUI, Web Application, Mobile application, it doesn't really matter as long as its purpose is presentation). Then, from your business logic layer of this app you will communicate to a web service application. Note that communication between presentation and business logic layer will depend on the technology you choose. –  Luiggi Mendoza Commented Jun 10, 2013 at 23:04
• Ok, I see. Still it's unclear how business logic layer of my application is going to communicate with web service layer. You mean that, even though if we're going to use a desktop application, we will communicate with a server part through HTTP ? –  mangusta Commented Jun 10, 2013 at 23:08
• @mangusta answer updated showing an skeleton of the proposed example. –  Luiggi Mendoza Commented Jun 10, 2013 at 23:32
• : ) so you assume that servlet is part of application's presentation layer, although it's a bit misleading because servlet is located on a server, not user machine : ) –  mangusta Commented Jun 11, 2013 at 6:44

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