Models are produced that connect to the system. Theory: OSI network model. Application layer functions of the osi model

This model was developed back in 1984 by the International Standard Organization (ISO), and was originally called Open Systems Interconnection, OSI.
Interaction model open systems(in fact - a model of network interaction) is a standard for the design of network communications and assumes a layered approach to building networks.
Each level of the model serves different stages of the interaction process. By dividing into layers, the OSI network model makes it easier for hardware and software to work together. The OSI model divides network functions into seven layers: application, presentation, session, transport, network, link, and physical.

• Physical layer(Physical layer) - determines the way computers are physically connected on the network. The functions of the tools belonging to this level are the bit-by-bit conversion of digital data into signals transmitted over a physical medium (for example, over a cable), as well as the actual transmission of signals.
• Data Link Layer(Data Link layer) - is responsible for organizing data transfer between subscribers through the physical layer, therefore, at this level, addressing means are provided that make it possible to uniquely identify the sender and recipient in the entire set of subscribers connected to a common communication line. The functions of this level also include ordering transmission for the purpose of parallel use of one communication line by several pairs of subscribers. In addition, link layer tools provide error checking that may occur during data transmission by the physical layer.
• Network layer(Network layer) - ensures the delivery of data between computers in a network, which is an association of various physical networks. This level assumes the presence of logical addressing tools that allow you to uniquely identify a computer in an interconnected network. One of the main functions performed by tools at this level is the targeted transfer of data to a specific recipient.
• Transport layer(Transport layer) - implements data transfer between two programs operating on different computers, while ensuring the absence of losses and duplication of information that may arise as a result of transmission errors of lower layers. If data transmitted through the transport layer is fragmented, then the means of this layer ensure that the fragments are assembled in the correct order.
• Session (or session) level(Session layer) - allows two programs to maintain long-term communication over the network, called a session (session) or session. This layer manages session establishment, information exchange, and session termination. It is also responsible for authentication, thereby allowing only certain subscribers to participate in the session, and provides security services to regulate access to session information.
• Presentation layer(Presentation layer) - carries out intermediate conversion of outgoing message data into a general format, which is provided by means of lower levels, as well as reverse conversion of incoming data from a general format into a format understandable to the receiving program.
• Application layer(Application layer) - provides high-level network communication functions, such as transferring files, sending emails, etc.

OSI model in simple terms

Network models. Part 1. OSI.

OSI Reference Network Model

As mentioned above, the network model is a model of interaction between network protocols (standards), and at each level there are its own protocols. It’s a boring process to list them (and there’s no point), so it’s better to look at everything using an example, because the digestibility of the material is much higher with examples;)

Application layer

Executive level

Session layer

Transport layer

Network layer

Data Link Layer

Physical layer

Conclusion

And hello again, dear friends, today we will understand what the OSI network model is and what it is, in fact, intended for.

As you probably already understand, modern networks are very, very complex, many different processes take place in them, hundreds of actions are performed. In order to simplify the process of describing this variety of network functions (and, more importantly, to simplify the process of further development of these functions), attempts were made to structure them. As a result of structuring, all functions performed by a computer network are divided into several levels, each of which is responsible only for a certain, highly specialized range of tasks. Here the network model can be compared to the structure of a company. The company is divided into departments. Each department performs its own functions, but during work it is in contact with other departments.

Separation of functions using a network model

Interaction between layers of the OSI network model

Work at some levels of the OSI network model

Communication between two systems from the perspective of the OSI model

Demonstration of the principle of encapsulation

Levels of the open systems interaction model

The application layer is the point through which applications communicate with the network (the entry point into the OSI model). Using this layer of the OSI model, the following tasks are performed: network management, system busy management, file transfer management, user identification by their passwords. Examples of protocols at this level are: HTTP, SMTP, RDP, etc. Very often, application layer protocols simultaneously perform the functions of presentation and session layer protocols.

This level is responsible for the data presentation format. Roughly speaking, it converts data received from the application layer into a format suitable for transmission over the network (and, accordingly, performs the reverse operation, converting information received from the network into a format suitable for processing by applications).

At this level, the establishment, maintenance and management of a communication session between two systems occurs. It is this level that is responsible for maintaining communication between systems for the entire period of time during which their interaction occurs.

Protocols at this level of the OSI network model are responsible for transferring data from one system to another. At this level, large blocks of data are divided into smaller blocks suitable for processing by the network layer (very small blocks of data are combined into larger ones), these blocks are appropriately marked for their subsequent recovery at the receiving end. Also, when using appropriate protocols, this layer is able to provide control over the delivery of network layer packets. The block of data that this level operates on is usually called a segment. Examples of protocols at this level are: TCP, UDP, SPX, ATP, etc.

This level is responsible for routing (determining optimal routes from one system to another) data blocks of this level. A block of data at this level is usually called a packet. This level is also responsible for the logical addressing of systems (the same IP addresses), on the basis of which routing occurs. Protocols at this level include: IP, IPX, etc. Devices operating at this level include routers.

This layer is responsible for the physical addressing of network devices (MAC addresses), control of access to the medium, and correction of errors made by the physical layer. Data block used on link level usually called a frame. This level includes the following devices: switches (not all), bridges, etc. A typical technology using this level is Ethernet.

Transmits optical or electrical pulses over a selected transmission medium. Devices of this level include all kinds of repeaters and hubs.

OSI reference model

For clarity, the network process in the OSI reference model is divided into seven layers. This theoretical construct makes fairly complex concepts easier to learn and understand. At the top of the OSI model is the application that needs access to network resources, at the bottom is the network environment itself. As data moves from layer to layer down, the protocols operating at those layers gradually prepare it for transmission over the network. Once it reaches the target system, the data moves up through the layers, with the same protocols performing the same actions, only in reverse order. In 1983 International Organization for Standardization(International Organization for Standardization, ISO) and Standardization sectortelecommunications of the International Telecommunications Union(Telecommunication Standardization Sector of International Telecommunication Union, ITU-T) published the document “The Basic Reference Model for Open Systems Interconnection”, which described a model for distributing network functions between 7 different levels (Fig. 1.7). This seven-layer structure was supposed to form the basis for a new protocol stack, but it was never implemented in commercial form. Instead, the OSI model is used with existing protocol stacks as a training and reference tool. Most of the protocols popular today predate the development of the OSI model, so they do not exactly conform to its seven-layer structure. Often, one protocol combines the functions of two or even several levels of the model, and the boundaries of the protocols often do not correspond to the boundaries of the OSI layers. However, the OSI model remains an excellent visual aid for examining network processes, and professionals often associate functions and protocols with specific layers.

Data Encapsulation

Essentially, the interaction of protocols operating at different levels of the OSI model is manifested in the fact that each protocol adds title(header) or (in one case) trailer(footer) to the information it received from the level above. For example, an application generates a request to a network resource. This request moves down the protocol stack. When it reaches the transport layer, protocols at that layer add their own header to the request, consisting of fields with information specific to the functions of that protocol. The original request itself becomes a data field (payload) for the transport layer protocol. After adding its header, the transport layer protocol passes the request to the network layer. The network layer protocol adds its own header to the transport layer protocol header. Thus, for a network layer protocol, the payload becomes the original request and the transport layer protocol header. This entire construct becomes the payload for the link layer protocol, which adds a header and trailer to it. The result of this activity is plastic bag(packet), ready for transmission over the network. When the packet reaches its destination, the process is repeated in reverse. The protocol of each subsequent layer of the stack (now from bottom to top) processes and removes the header of the equivalent protocol of the sending system. When the process is completed, the original request reaches the application it was intended for, in the same form in which it was generated. The process of adding headers to a request (Figure 1.8) generated by an application is called data encapsulation(data encapsulation). In essence, this procedure resembles the process of preparing a letter for sending by mail. The request is the letter itself, and adding headings is the same as putting the letter in an envelope, writing the address, stamping it, and actually sending it.

Physical layer

At the lowest level of the OSI model - physical(physical) - the characteristics of network equipment elements are determined - the network environment, installation method, type of signals used to transmit binary data over the network. In addition, the physical layer determines what type of network adapter needs to be installed on each computer and what kind of hub to use (if necessary). At the physical level we are dealing with copper or fiber optic cable or any wireless connection. In a LAN, the physical layer specifications are directly related to the data link protocol used on the network. Once you select a link layer protocol, you must use one of the physical layer specifications supported by that protocol. For example, the Ethernet link layer protocol supports several various options physical layer - one of two types of coaxial cable, any twisted pair cable, fiber optic cable. The parameters of each of these options are formed from numerous information about the requirements of the physical layer, for example, the type of cable and connectors, the permissible length of cables, the number of hubs, etc. Compliance with these requirements is necessary for the normal operation of the protocols. For example, in a cable that is too long, the Ethernet system may not notice packet collisions, and if the system is unable to detect errors, it cannot correct them, resulting in data loss. Not all aspects of the physical layer are defined by the link layer protocol standard. Some of them are defined separately. One of the most commonly used physical layer specifications is described in the Commercial Building Telecommunications Cabling Standard, known as EIA/TIA 568A. It is jointly published American National Institute of Standarts(American National Standards Institute, ANSI), Associations fromelectronics industries(Electronics Industry Association, EIA) and Communications Industry Association(Telecommunications Industry Association, TIA). Included in this document detailed description cables for data transmission networks in industrial environments, including the minimum distance from sources of electromagnetic interference and other rules for laying cables. Today, cable laying in large networks is most often entrusted to specialized companies. The contractor hired should be thoroughly familiar with EIA/TIA 568A and other similar documents, as well as city building codes. Another communication element defined at the physical layer is the type of signal for transmitting data over the network medium. For cables with a copper base, this signal is an electric charge; for a fiber-optic cable, it is a light pulse. Other types of network environments may use radio waves, infrared pulses, and other signals. In addition to the nature of the signals, the scheme for their transmission is established at the physical level, i.e. the combination electric charges or light pulses, used to encode binary information that is generated by higher layers. Ethernet systems use a signaling scheme known as Manchester encoding(Manchester encoding), and in Token Ring systems it is used differentialManchester(Differential Manchester) scheme.

Data Link Layer

Protocol channel(data-link) level ensures the exchange of information between the hardware of a computer connected to the network and network software. It prepares data sent to it by the network layer protocol for sending to the network, and transmits data received by the system from the network to the network layer. When designing and building a LAN, the link layer protocol used is the most important factor in choosing equipment and how it is installed. To implement the link layer protocol, the following hardware and software: network interface adapters (if the adapter is a separate device connected to the bus, it is called a network interface card or simply a network card); network adapter drivers; network cables (or other network media) and ancillary connecting equipment; network hubs (in some cases). Both network adapters and hubs are designed for specific link-layer protocols. Some network cables are also tailored for specific protocols, but there are also cables that are suitable for different protocols. Of course, today (as always) the most popular link layer protocol is Ethernet. Token Ring is far behind, followed by other protocols such as FDDI (Fiber Distributed Data Interface). There are typically three main elements included in a link layer protocol specification: the frame format (i.e., the header and trailer added to the network layer data before transmission to the network); mechanism for controlling access to the network environment; one or more physical layer specifications used with a given protocol.

Frame format

The link layer protocol adds a header and trailer to the data received from the network layer protocol, turning it into frame(frame) (Fig. 1.9). Using the mail analogy again, the header and trailer are the envelope for sending the letter. They contain the addresses of the sending and receiving systems of the packet. For LAN protocols like Ethernet and Token Ring, these addresses are 6-byte hexadecimal strings assigned to network adapters at the factory. They, in contrast to the addresses used at other levels of the OSI model, are called appa military addresses(hardware address) or MAC addresses (see below).

Note Protocols at different layers of the OSI model have different names for the structures they create by adding a header to data coming from a higher protocol. For example, what a link layer protocol calls a frame would be a datagram to the network layer. A more general name for a structural unit of data at any level is plastic bag.

It is important to understand that link layer protocols provide communication only between computers on the same LAN. The hardware address in the header always belongs to a computer on the same LAN, even if the target system is on a different network. Other important functions of the link layer frame are identification of the network layer protocol that generated the data in the packet and information for error detection. The network layer can use different protocols, so the link layer protocol frame usually includes code that can be used to identify which network layer protocol generated the data in that packet. Guided by this code, the link layer protocol of the receiving computer forwards the data to the corresponding protocol of its network layer. To detect errors, the transmitting system calculates cyclical cue redundant code(cyclical redundancy check, CRC) of the payload and writes it to the frame trailer. After receiving the packet, the target computer performs the same calculations and compares the result with the contents of the trailer. If the results match, the information was transmitted without errors. Otherwise, the recipient assumes that the package is damaged and does not accept it.

Media access control

Computers on a LAN typically share a half-duplex network medium. In this case, it is quite possible that two computers will start transmitting data simultaneously. In such cases, a kind of packet collision occurs, collision(collision), in which data in both packets is lost. One of the main functions of the data link layer protocol is media access control (MAC), i.e., controlling the transmission of data by each computer and minimizing packet collisions. The media access control mechanism is one of the most important characteristics of a link layer protocol. Ethernet uses a mechanism with carrier sense and collision detection (Carrier Sense Multiple Access with Collision Detection, CSMA/CD) to control access to the medium. Some other protocols, such as Token Ring, use token passing.

Physical Layer Specifications

Link layer protocols used in LANs often support more than one network medium, and one or more physical layer specifications are included in the protocol standard. The data link and physical layers are closely related because the properties of the network medium significantly influence how the protocol controls access to the medium. Therefore we can say that in local networks Link layer protocols also perform physical layer functions. IN global networks Link layer protocols are used that do not include physical layer information, for example, SLIP (Serial Line Internet Protocol) and PPP (Point-to-Point Protocol).

Network layer

At first glance it may seem that network(network) layer duplicates some functions of the data link layer. But this is not true: network layer protocols are “responsible” for end-to-end(end-to-end) communications, while link layer protocols operate only within a LAN. In other words, network layer protocols completely ensure the transmission of a packet from the source to the target system. Depending on the type of network, the sender and recipient may be on the same LAN, on different LANs within the same building, or on LANs separated by thousands of kilometers. For example, when you communicate with a server on the Internet, packets generated by your computer pass through dozens of networks on their way to it. The link layer protocol will change several times to accommodate these networks, but the network layer protocol will remain the same all the way. The cornerstone of the TCP/IP (Transmission Control Protocol/Internet Protocol) protocol suite and the most commonly used network layer protocol is the Internet Protocol (IP). Novell NetWare has its own network protocol IPX (Internetwork Packet Exchange), and small Microsoft Windows networks typically use the NetBEUI (NetBIOS Enhanced User Interface) protocol. Most of the functions assigned to the network layer are determined by the capabilities of the IP protocol. Like a link layer protocol, a network layer protocol adds a header to the data it receives from a higher layer (Figure 1.10). A data element created by a network layer protocol consists of transport layer data and a network layer header and is called datagram(datagram).

Addressing

The network layer protocol header, like the link layer protocol header, contains fields with the addresses of the source and target systems. However, in this case, the destination system address belongs to the final destination of the packet and may differ from the destination address in the link layer protocol header. For example, when you type the address of a Web site into your browser's address bar, the packet generated by your computer specifies the address of the target network-level system as the address of the Web server, while at the link-layer the address of the router on your LAN that provides the Internet access. IP uses its own addressing system, which is completely independent of link-layer addresses. Each computer on an IP network is manually or automatically assigned a 32-bit IP address, identifying both the computer itself and the network on which it is located. In IPX, a hardware address is used to identify the computer itself, in addition, a special address is used to identify the network on which the computer is located. NetBEUI differentiates computers by the NetBIOS names assigned to each system during installation.

Fragmentation

Network layer datagrams must traverse multiple networks on their way to their destination, encountering the specific properties and limitations of various link layer protocols. One such limitation is the maximum packet size allowed by the protocol. For example, a Token Ring frame can be up to 4500 bytes in size, while Ethernet frames can be up to 1500 bytes in size. When a large datagram generated in a Token Ring network is transmitted to an Ethernet network, the network layer protocol must break it into several fragments of no more than 1500 bytes in size. This process is called fragmentation(fragmentation). During the fragmentation process, the network layer protocol breaks the datagram into fragments, the size of which corresponds to the capabilities of the data link layer protocol being used. Each fragment becomes an independent packet and continues its path to the target network layer system. The source datagram is formed only after all fragments have reached the destination. Sometimes, on the way to the target system, the fragments into which the datagram is broken must be re-fragmented.

Routing

Routing(routing) is the process of selecting the most efficient route on the Internet for transmitting datagrams from the sending system to the receiving system. In complex internetworks, such as the Internet or large corporate networks, there are often several ways to get from one computer to another. Network designers deliberately create redundant links so that traffic can find its way to its destination even if one of the routers fails. Routers are used to connect individual LANs that are part of the Internet. The purpose of a router is to accept incoming traffic from one network and forward it to a specific system on another. There are two types of systems on internet networks: terminal(end systems) and intermediate(intermediate systems). End systems are senders and receivers of packets. A router is an intermediate system. End systems use all seven layers of the OSI model, while packets arriving at intermediate systems do not rise above the network layer. There, the router processes the packet and sends it down the stack for transmission to the next target system (Figure 1.11).

To correctly route the packet to the target, routers store tables with network information in memory. This information can be entered manually by the administrator or collected automatically from other routers using specialized protocols. A typical routing table entry includes the address of another network and the address of the router through which packets must travel to that network. In addition, the routing table element contains route metric - conditional assessment of its effectiveness. If there are multiple routes to a system, the router selects the most efficient one and sends the datagram to the data link layer for transmission to the router specified in the table entry with the best metric. In large networks, routing can be an unusually complex process, but most often it is done automatically and unnoticed by the user.

Transport Layer Protocol Identification

Just as the link layer header specifies the network layer protocol that generated and transmitted the data, the network layer header contains information about the transport layer protocol from which the data was received. Based on this information, the receiving system forwards incoming datagrams to the appropriate transport layer protocol.

Transport layer

Functions performed by protocols transport(transport) layer, complement the functions of network layer protocols. Often the protocols of these layers used for data transmission form an interconnected pair, as can be seen in the example of TCP/IP: the TCP protocol operates on transport level, IP - on the network. Most protocol suites have two or more transport layer protocols that perform different functions. An alternative to TCP is UDP (User Datagram Protocol). The IPX protocol suite also includes several transport layer protocols, including NCP (NetWare Core Protocol) and SPX (Sequenced Packet Exchange). The difference between transport layer protocols from a particular set is that some are connection oriented and others are not. Systems using the protocol connection-oriented(connection-oriented), before transmitting data, they exchange messages to establish communication with each other. This ensures that systems are turned on and ready to go. The TCP protocol, for example, is connection-oriented. When you connect to an Internet server using a browser, the browser and the server first perform a so-called three-step handshake(three-way handshake). Only after this the browser transmits the address of the desired Web page to the server. When the data transfer is complete, the systems perform the same handshake to terminate the connection. In addition, connection-oriented protocols perform additional actions, such as sending a packet acknowledgment signal, segmenting data, controlling flow, and detecting and correcting errors. Typically, protocols of this type are used to transfer large amounts of information that must not contain a single bit of error, such as data files or programs. Additional features of connection-oriented protocols ensure correct data transfer. This is why these protocols are often called reliable(reliable). Reliability in this case is a technical term and means that every packet transmitted is checked for errors, and the sending system is notified of the delivery of each packet. The disadvantage of this type of protocol is the significant amount of control data exchanged between the two systems. First, additional messages are sent when communication is established and terminated. Second, the header added to the packet by a connection-oriented protocol is substantially larger than the header of a connection-less protocol. For example, title TCP protocol/IP takes 20 bytes and the UDP header takes 8 bytes. Protocol, not connection oriented(connectionless), does not establish a connection between two systems before data is transferred. The sender simply transmits information to the target system without worrying about whether it is ready to accept the data or whether the system even exists. Typically, systems resort to connectionless protocols such as UDP for short transactions consisting of only requests and response signals. The response signal from the receiver implicitly functions as a transmission acknowledgment signal.

Note Connection-oriented and connectionless protocols are not limited to the transport layer. For example, network layer protocols are usually not connection-oriented, since they rely on the transport layer to ensure communication reliability.

Transport layer protocols (as well as network and data link layers) usually contain information from higher layers. For example, the TCP and UDP headers include port numbers that identify the application that originated the packet and the application to which it is destined. On session(session) level, a significant discrepancy begins between the actually used protocols and the OSI model. Unlike lower layers, there are no dedicated session layer protocols. The functions of this layer are integrated into protocols that also perform the functions of the representative and application layers. The transport, network, data link and physical layers are responsible for the actual transmission of data over the network. Protocols of the session and higher levels have nothing to do with the communication process. The session layer includes 22 services, many of which define how information is exchanged between systems on the network. The most important services are dialogue management and dialogue separation. The exchange of information between two systems on a network is called dialogue(dialogue). Dialogue management(dialog control) consists of choosing the mode in which the systems will exchange messages. There are two such modes: half duplex(two-way alternate, TWA) and duplex(two-way simultaneous, TWS). In half-duplex mode, the two systems also transmit tokens along with the data. Information can only be transferred to a computer that has this moment there is a marker. This avoids message collisions along the way. The duplex model is more complicated. There are no markers in it; both systems can transmit data at any time, even simultaneously. Dividing dialogue(dialog separation) consists of inclusion in the data stream control points(checkpoints) that allow synchronizing the operation of two systems. The degree of difficulty of dividing the dialogue depends on the mode in which it is carried out. In half-duplex mode, systems perform minor synchronization by exchanging checkpoint messages. In full duplex mode, systems perform full synchronization using the master/active token.

Executive level

On representative The presentation layer performs a single function: syntax translation between different systems. Sometimes computers on a network use different syntaxes. The representative layer allows them to "agree" on a common syntax for exchanging data. When establishing a connection at the presentation layer, systems exchange messages about what syntaxes they have and select the one they will use during the session. Both systems involved in the connection have abstractsyntax(abstract syntax) is their “native” form of communication. The abstract syntaxes of different computer platforms may vary. During the system coordination process, a common transfer syntaxdata(transfer syntax). The transmitting system converts its abstract syntax into data transfer syntax, and the receiving system, upon completion of the transfer, does the opposite. If necessary, the system can select the data transfer syntax with additional functions, for example, data compression or encryption.

Application layer

The application layer is the entry point through which programs access the OSI model and network resources. Most application layer protocols provide network access services. For example, using the SMTP (Simple Mail Transfer Protocol) protocol, most programs Email used to send messages. Other application layer protocols, such as FTP (File Transfer Protocol), are themselves programs. Application layer protocols often include session and presentation layer functions. As a result, a typical protocol stack contains four separate protocols that operate at the application, transport, network, and data link layers.

This material is dedicated to the reference seven-layer OSI network model. Here you will find the answer to the question why system administrators need to understand this network model, all 7 levels of the model will be considered, and you will also learn the basics of the TCP/IP model, which was built on the basis of the OSI reference model.

When I began to get involved in various IT technologies and began to work in this field, I, of course, did not know about any model, I didn’t even think about it, but a more experienced specialist advised me to study, or rather, simply understand this model, adding that “ if you understand all the principles of interaction, it will be much easier to manage, configure the network and solve all sorts of network and other problems" I, of course, listened to him and began to dig through books, the Internet and other sources of information, while at the same time checking on the existing network whether this was all true in reality.

IN modern world the development of network infrastructure has reached such a high level that without building even a small network, an enterprise ( incl. and small) will not be able to simply exist normally, so system administrators are becoming increasingly in demand. And for high-quality construction and configuration of any network, the system administrator must understand the principles of the OSI reference model, just so that you learn to understand the interaction of network applications, and indeed the principles of network data transmission, I will try to present this material in an accessible way even for novice administrators.

OSI network model (open systems interconnection basic reference model) is an abstract model of how computers, applications, and other devices interact on a network. In short, the essence of this model is that the ISO organization ( International Organization for Standardization) developed a standard for network operation so that everyone could rely on it, and there was compatibility of all networks and interaction between them. One of the most popular network communication protocols, which is used all over the world, is TCP/IP, which is built on the basis of a reference model.

Well, let's move directly to the levels of this model themselves, and first, get acquainted with the general picture of this model in the context of its levels.

Now let's talk in more detail about each level, it is customary to describe the levels of the reference model from top to bottom, it is along this path that interaction occurs, on one computer from top to bottom, and on the computer where data is received from bottom to top, i.e. the data passes through each level sequentially.

Description of the levels of the network model

Application layer (7) (application layer) is the starting and at the same time ending point of the data that you want to transmit over the network. This layer is responsible for the interaction of applications over the network, i.e. Applications communicate at this layer. This is the highest level and you need to remember this when solving problems that arise.

HTTP, POP3, SMTP, FTP, TELNET and others. In other words, application 1 sends a request to application 2 using these protocols, and in order to find out that application 1 sent the request to application 2, there must be a connection between them, and it is the protocol that is responsible for this connection.

Presentation layer (6)– this layer is responsible for encoding the data so that it can later be transmitted over the network and accordingly converts it back so that the application understands this data. After this level, the data for other levels becomes the same, i.e. no matter what kind of data it is, be it word document or email message.

The following protocols operate at this level: RDP, LPP, NDR and others.

Session level (5)– is responsible for maintaining the session between data transfers, i.e. The duration of the session differs depending on the data being transferred, so it must be maintained or terminated.

The following protocols operate at this level: ASP, L2TP, PPTP and others.

Transport layer (4)– is responsible for the reliability of data transmission. It also breaks the data into segments and puts them back together as the data comes in different sizes. There are two well-known protocols at this level: TCP and UDP. The TCP protocol guarantees that the data will be delivered in full, but the UDP protocol does not guarantee this, which is why they are used for different purposes.

Network layer (3)– it is designed to determine the path that data should take. Routers operate at this level. He is also responsible for: translating logical addresses and names into physical ones, determining a short route, switching and routing, monitoring network problems. It is at this level that it works IP protocol and routing protocols, e.g. RIP, OSPF.

Link layer (2)– it provides interaction at the physical level; at this level, MAC addresses network devices, errors are also monitored and corrected here, i.e. sends a re-request for the damaged frame.

Physical layer (1)– this is the direct conversion of all frames into electrical impulses and vice versa. In other words physical transmission data. They work at this level hubs.

This is what the entire data transfer process looks like from the point of view of this model. It is a reference and standardized and therefore others are based on it network technologies and models in particular the TCP/IP model.

TCP IP model

TCP/IP model is slightly different from the OSI model; to be more specific, this model combines some levels of the OSI model and there are only 4 of them:

• Applied;
• Transport;
• Network;
• Duct.

The picture shows the difference between the two models, and also once again shows at what levels the well-known protocols operate.

We can talk about the OSI network model and specifically about the interaction of computers on a network for a long time and it will not fit in one article, and it will be a little unclear, so here I tried to present the basis of this model and a description of all levels. The main thing is to understand that all this is really true and the file that you sent over the network passes simply “ huge“path before reaching the end user, but this happens so quickly that you don’t notice it, largely thanks to developed network technologies.

I hope all this will help you understand the interaction of networks.

The modern IT world is a huge, branching structure that is difficult to understand. To simplify understanding and improve debugging even at the stage of designing protocols and systems, a modular architecture was used. It is much easier for us to figure out that the problem is in the video chip when the video card is a separate device from the rest of the equipment. Or notice a problem in a separate section of the network, rather than shoveling the entire network.

A separate layer of IT - the network - is also built modularly. The network operating model is called the ISO/OSI Open Systems Interconnection Basic Reference Model network model. Briefly - the OSI model.

The OSI model consists of 7 layers. Each level is abstracted from the others and knows nothing about their existence. The OSI model can be compared to the structure of a car: the engine does its job by creating torque and transferring it to the gearbox. The engine does not care what happens next with this torque. Will he spin a wheel, caterpillar or propeller? Just like the wheel, it doesn’t matter where this torque came from - from the engine or the handle that the mechanic turns.

Here we need to add the concept of payload. Each level carries a certain amount of information. Some of this information is proprietary to this level, for example, the address. The site's IP address does not provide us with any useful information. We only care about the cats that the site shows us. So this payload is carried in that part of the layer called the protocol data unit (PDU).

Layers of the OSI Model

Let's look at each level of the OSI Model in more detail.

Level 1. Physical ( physical). Load unit ( PDU) here is the bit. The physical layer knows nothing except ones and zeros. At this level, wires, patch panels, network hubs (hubs that are now difficult to find in our usual networks), and network adapters work. It is network adapters and nothing else from the computer. Myself network adapter receives a sequence of bits and transmits it further.

Level 2. Duct ( data link). PDU - frame ( frame). Addressing appears at this level. The address is the MAC address. The link layer is responsible for the delivery of frames to the recipient and their integrity. In the networks we are familiar with, the ARP protocol operates at the link level. Second-level addressing only works within one network segment and does not know anything about routing - this is handled by a higher level. Accordingly, devices operating on L2 are switches, bridges and a network adapter driver.

Level 3. Network ( network). PDU packet ( packet). The most common protocol (I won’t talk further about “the most common” - this article is for beginners and, as a rule, they don’t encounter anything exotic) here is IP. Addressing occurs using IP addresses, which consist of 32 bits. The protocol is routed, that is, a packet can reach any part of the network through a certain number of routers. Routers operate on L3.

Level 4. Transport ( transport). PDU segment ( segment)/datagram ( datagram). At this level, the concepts of ports appear. TCP and UDP work here. Protocols at this level are responsible for direct communication between applications and for the reliability of information delivery. For example, TCP can request a retransmission of data if the data was received incorrectly or not all. TCP can also change the data transfer rate if the receiving side does not have time to receive everything (TCP Window Size).

The following levels are “correctly” implemented only in the RFC. In practice, the protocols described at the following levels operate simultaneously at several levels of the OSI model, so there is no clear division into session and presentation layers. In this regard, currently the main stack used is TCP/IP, which we will talk about below.

Level 5. Session ( session). PDU data ( data). Manages the communication session, information exchange, and rights. Protocols - L2TP, PPTP.

Level 6. Executive ( presentation). PDU data ( data). Data presentation and encryption. JPEG, ASCII, MPEG.

Level 7. Applied ( application). PDU data ( data). The most numerous and varied level. It runs all high-level protocols. Such as POP, SMTP, RDP, HTTP, etc. Protocols here do not have to think about routing or guaranteeing the delivery of information - this is done by lower layers. At level 7, it is only necessary to implement specific actions, for example, receiving an html code or an email message to a specific recipient.

Conclusion

The modularity of the OSI model allows for quick identification of problem areas. After all, if there is no ping (3-4 levels) to the site, there is no point in delving into the overlying layers (TCP-HTTP) when the site is not displayed. By abstracting from other levels, it is easier to find an error in the problematic part. By analogy with a car - we don’t check the spark plugs when we puncture the wheel.

The OSI model is a reference model - a kind of spherical horse in a vacuum. Its development took a very long time. In parallel with it, the TCP/IP protocol stack was developed, which is actively used in networks at present. Accordingly, an analogy can be drawn between TCP/IP and OSI.

To coordinate the operation of network devices from different manufacturers To ensure the interaction of networks that use different signal propagation environments, a reference model for the interaction of open systems (OSI) has been created. The reference model is built on a hierarchical principle. Each level provides services to the higher level and uses the services of the lower level.

Data processing begins at the application level. After this, the data passes through all layers of the reference model, and is sent through the physical layer to the communication channel. At reception, reverse processing of the data occurs.

The OSI reference model introduces two concepts: protocol And interface.

A protocol is a set of rules on the basis of which the layers of various open systems interact.

An interface is a set of means and methods of interaction between elements of an open system.

The protocol defines the rules for interaction between modules of the same level in different nodes, and the interface - between modules of adjacent levels in the same node.

There are a total of seven layers of the OSI reference model. It's worth noting that real stacks use fewer layers. For example, the popular TCP/IP uses only four layers. Why is that? We'll explain a little later. Now let’s look at each of the seven levels separately.

OSI Model Layers:

• Physical level. Determines the type of data transmission medium, the physical and electrical characteristics of the interfaces, and the type of signal. This layer deals with bits of information. Examples of physical layer protocols: Ethernet, ISDN, Wi-Fi.
• Data link level. Responsible for access to the transmission medium, error correction, and reliable data transmission. At the reception The data received from the physical layer is packed into frames, after which their integrity is checked. If there are no errors, then the data is transferred to the network layer. If there are errors, the frame is discarded and a request for retransmission is generated. The data link layer is divided into two sublayers: MAC (Media Access Control) and LLC (Local Link Control). MAC regulates access to the shared physical medium. LLC provides network layer service. Switches operate at the data link layer. Examples of protocols: Ethernet, PPP.
• Network layer. Its main tasks are routing - determining the optimal data transmission path, logical addressing of nodes. In addition, this level may be tasked with troubleshooting network problems (ICMP protocol). The network layer works with packets. Examples of protocols: IP, ICMP, IGMP, BGP, OSPF).
• Transport layer. Designed to deliver data without errors, losses and duplication in the sequence in which they were transmitted. Performs end-to-end control of data transmission from sender to recipient. Examples of protocols: TCP, UDP.
• Session level. Manages the creation/maintenance/termination of a communication session. Examples of protocols: L2TP, RTCP.
• Executive level. Converts data into the required form, encrypts/encodes, and compresses.
• Application layer. Provides interaction between the user and the network. Interacts with client-side applications. Examples of protocols: HTTP, FTP, Telnet, SSH, SNMP.

After getting acquainted with the reference model, let's look at the TCP/IP protocol stack.

There are four layers defined in the TCP/IP model. As can be seen from the figure above, one TCP/IP layer can correspond to several layers of the OSI model.

TCP/IP model levels:

• Level network interfaces. Corresponds to the two lower layers of the OSI model: data link and physical. Based on this, it is clear that this level determines the characteristics of the transmission medium (twisted pair, optical fiber, radio), the type of signal, coding method, access to the transmission medium, error correction, physical addressing (MAC addresses). In the TCP/IP model, the Ethrnet protocol and its derivatives (Fast Ethernet, Gigabit Ethernet) operate at this level.
• Interconnection layer. Corresponds to the network layer of the OSI model. Takes over all its functions: routing, logical addressing (IP addresses). The IP protocol operates at this level.
• Transport layer. Corresponds to the transport layer of the OSI model. Responsible for delivering packets from source to destination. At this level, two protocols are used: TCP and UDP. TCP is more reliable than UDP by creating pre-connection requests for retransmission when errors occur. However, at the same time, TCP is slower than UDP.
• Application layer. Its main task is to interact with applications and processes on hosts. Examples of protocols: HTTP, FTP, POP3, SNMP, NTP, DNS, DHCP.

Encapsulation is a method of packaging a data packet in which independent packet headers are abstracted from the headers of lower levels by including them in higher levels.

Let's look at a specific example. Let's say we want to get from a computer to a website. To do this, our computer must prepare an http request to obtain the resources of the web server on which the site page we need is stored. At the application level, an HTTP header is added to the browser data. Next, at the transport layer, a TCP header is added to our packet, containing the sender and recipient port numbers (port 80 for HTTP). At the network layer, an IP header is generated containing the IP addresses of the sender and recipient. Immediately before transmission, an Ethrnet header is added at the link layer, which contains the physical (MAC addresses) of the sender and recipient. After all these procedures, the packet in the form of bits of information is transmitted over the network. At the reception, the reverse procedure occurs. The web server at each level will check the corresponding header. If the check is successful, the header is discarded and the packet moves to the upper level. Otherwise, the entire packet is discarded.