Session layer osi. The OSI model is simple! Application layer functions of the osi model

In this article, we will understand what the OSI network model is, what layers it consists of, and what functions it performs. So, the subject of conversation is a certain model of interaction between standards that determine the sequence of data exchange and programs.

The abbreviation OSI Open Systems Interconnection stands for the open systems interconnection model. To solve the problem of compatibility of various systems, the standardization organization released the OSI model standard in 1983. It describes the structure of open systems, their requirements, and their interactions.

An open system is a system compiled according to open specifications that are accessible to everyone and also comply with certain standards. For example, Windows OS is considered an open system because it is created on the basis of open specifications that describe the Internet, but the initial codes of the system are closed.

The advantage is that it is possible to build a network of devices from different manufacturers and, if necessary, replace its individual components. You can easily combine several networks into one whole.

According to the model we are considering, it is necessary that computer networks consist of seven levels. Because the model does not describe protocols defined by individual standards, it is not a network architecture.

Unfortunately, from a practical point of view, the open systems interaction model is not applied. Its peculiarity lies in mastering theoretical issues of network interaction. That is why this model is used as a simple language to describe the construction of different types of networks.

Model levelsOSI

The basic structure is a system consisting of 7 levels. The question arises: what are the seven stages responsible for and why does the model need so many levels? All of them are responsible for a certain stage of the process of sending a network message, and also contain a certain semantic load. The steps are performed separately from each other and do not require increased control on the part of the user. Isn't it convenient?

The lower stages of the system, from the first to the third, manage the physical delivery of data over the network; they are called media layers.

The remaining layers help ensure accurate delivery of data between computers on the network; they are called host machines.

Application is the closest level to the user. Its difference from others is that it does not provide services to other levels. Provides services to application processes that lie outside the scope of the model, for example, database transfer, voice, and more.

This stage is relatively simpler than others, because apart from ones and zeros there are no other measurement systems in it, this level does not analyze information and that is why it is the lowest of the levels. It mainly transmits information. The main load parameter is bit.

The main purpose of the physical layer is to represent zero and one as signals transmitted over a data transmission medium.

For example, there is a certain communication channel (CC), a message being sent, a sender and, accordingly, a recipient. The CS has its own characteristics:

• Bandwidth, measured in bits/s, that is, how much data we can transmit per unit of time.
• Latency is how long it takes for a message to travel from the sender to the recipient.
• Number of errors, if errors occur frequently, then protocols must provide error correction. And if they are rare, then they can be corrected at higher levels, for example at transport.

The information transmission channel is used:

• Cables: telephone, coaxial, twisted pair, optical.
• Wireless technologies such as radio waves, infrared radiation.
• Satellite CS
• Wireless optics or lasers are rarely used due to low speed and a lot of interference.

It is very rare for errors to occur in optical cables, since it is difficult to influence the propagation of light. In copper cables, errors occur, but quite rarely, and in a wireless environment, errors occur very often.

The next station the information will visit will resemble customs. Namely, the IP address will be compared for compatibility with the transmission medium. This is also where system shortcomings are identified and corrected. For the convenience of further operations, the bits are grouped into frames.

The purpose of the link layer is the transmission of messages via CS - frames.

Tasksdata link

• Find where in a bit stream a message begins and ends
• Detect and correct errors when sending information
• Addressing, you need to know which computer to send information to, because basically several computers are connected to a shared medium
• Provide consistent access to the shared environment so that one computer transmits information at the same time.

At the link level, errors are identified and corrected. If one is detected, the correctness of data delivery is checked; if incorrect, the frame is discarded.

Correcting errors requires the use of special codes that add redundant information to the transmitted data.

Resending data is used in conjunction with the error detection method. If an error is detected in the frame, it is discarded and the sender resends the frame.

Detect and correct errors

Practice has shown the effectiveness of the following methods; if a reliable medium for data transmission (wired) is used and errors occur rarely, then it is better to correct them at the top level. If errors occur frequently in the CS, then the errors must be corrected immediately at the link level.

The functions of this stage in the computer are performed by network adapters and drivers suitable for them. Through them, direct data exchange occurs.

Some of the protocols used at the data link layer are HDLC using bus topology and others.

(NETWORK)

The stage resembles the process of information distribution. For example, all users are divided into groups, and data packets are distributed in accordance with IP addresses, consisting of 32 bits. It is thanks to the work of routers at this instance that all differences between networks are eliminated. This is a process called logical routing.

The main task is to create composite networks built on the basis of network technologies of different channel levels: Ethernet, MPLS. The network layer is the “backbone” of the Internet.

Purpose of the network layer

We can transfer information from one computer to another via Ethernet and Wi-Fi, then why need another layer? Link layer (CL) technology has two problems, firstly, CL technologies differ from each other, and secondly, there is a scaling limitation.

What differences might there be in link layer technologies?

Different levels of service provided, some levels guarantee delivery and the required order of messages. Wi-Fi simply guarantees delivery of the message, but it doesn't.

Different addressing, by size, hierarchy. Network technologies can support broadcasting, i.e. It is possible to send information to all computers on the network.

The maximum frame size (MTU) may differ, for example, in Internet it is 1500, and in Wi-Fi it is 2300. How can such differences be reconciled at the network level?

You can provide different types of services, for example, frames from Wi-Fi are received with confirmation sent, and frames are sent to Ethernet without confirmation.

In order to reconcile the addressing difference, at the network level, global addresses are introduced that do not depend on the addresses of specific technologies (ARP for) the link layer.

To transmit data across multiple networks that have different frame sizes, fragmentation is used. Let's consider an example: the first computer transmits data to the second through 4 intermediate networks connected by 3 routers. Each network has a different MTU.

The computer generated the first frame and transmitted it to the router, the router analyzed the size of the frame and realized that it could not be transmitted completely through network 2, because its mtu2 was too small.

The router splits the data into 3 parts and transmits them separately.

The next router combines the data into one large packet, determines its size and compares it with the mtu of network 3. And it sees that one MTU3 packet cannot be transmitted entirely (MTU3 is larger than MTU2, but smaller than MTU1) and the router splits the packet into 2 parts and sends it to the next router.

The last router combines the packet and sends it to the recipient in its entirety. Fragmentation deals with combining networks and this is hidden from the sender and recipient.

How is the scalability problem solved at the network level?

Work is carried out not with individual addresses, as at the link level, but with blocks of addresses. Packets for which the path is unknown are discarded rather than forwarded back to all ports. And a significant difference from the channel one is the possibility of several connections between network level devices and all these connections will be active.

Network layer tasks:

• Combine networks built by different technologies;
• Provide quality service;
• Routing, finding a path from the sender of information to the recipient, through intermediate network nodes.

Routing

Finding the path to send a packet between networks through transit nodes - routers. Let's look at an example of performing routing. The circuit consists of 5 routers and two computers. How can data be transferred from one computer to another?

Next time the data may be sent in a different way.

If one of the routers breaks down, nothing bad will happen; you can find a way around the broken router.

Protocols used at this stage: Internet Protocol IP; IPX, necessary for routing packets in networks, etc.

(TRANSPORT)

There is the following task: a packet arrives at a computer that is connected to a composite network; there are many network applications running on the computer (web browser, Skype, mail), we need to understand which application needs to transfer this packet. The transport layer handles the interaction between network applications.

Transport layer tasks

Sending data between processes on different hosts. Ensuring addressing, you need to know for which process this or that packet is intended. Ensuring the reliability of information transfer.

Interaction modelopen system

Hosts are devices where useful user programs and network equipment, such as switches and routers, operate.

A feature of the transport layer is the direct interaction of one computer with the transport layer on another computer; at other levels, interaction occurs along the links of the chain.

This layer provides an end-to-end connection between two communicating hosts. This level is independent of the network; it allows you to hide the details of network interaction from application developers.

For addressing at the transport level, ports are used, these are numbers from 1 to 65,535. Ports are written like this: 192.168.1.3:80 (IP address and port).

Features of the transport layer

Providing higher reliability, unlike the network that is used for data transmission. Reliable communication channels are used, errors in these networks rarely occur, therefore, it is possible to build a reliable network that will be cheap, and errors can be corrected programmatically on the hosts.

The transport layer guarantees the delivery of data; it uses confirmation from the recipient; if confirmation does not arrive, the transport layer again sends confirmation of the data. Message follow-up guarantee.

Session layer (SESSION)

Session (session) is a set of network interactions aimed at solving a single task.

Now network interaction has become more complex and does not consist of simple questions and answers, as it was before. For example, you load a web page to display in the browser, you first need to download the text of the web page (.html), a style file (.css) that describes the design elements of the web page, and loading images. Thus, in order to complete the task of loading a web page, it is necessary to implement several separate network operations.

Session determines what type of information transfer will take place between 2 application processes: half-duplex (sequential transmission and reception of data); or duplex (simultaneous transmission and reception of information).

Data presentation layer(PRESENTATION)

Functions – present data transferred between application processes in the required form.

To describe this level, automatic online translation from various languages ​​is used. For example, you dial a phone number, speak Russian, the network automatically translates into French, transmits the information to Spain, where a person picks up the phone and hears your question in Spanish. This task has not yet been implemented.

To protect data sent over the network, encryption is used: secure sockets layer, as well as transport layer security, these technologies allow you to encrypt data sent over the network.

Application layer protocols use TSL/SSL and can be identified by the letter s at the end. For example, https, ftps and others. If you see in your browser that the https protocol and a lock are used, this means that data is being protected over the network using encryption.

(APPLICATION)

Necessary for network applications to interact with each other, such as web, e-mail, skype, etc.

In essence, it is a set of specifications that allow the user to enter pages to find the information he needs. Simply put, application's job is to provide access to network services. The contents of this level are very varied.

Functionsapplication:

• Solving problems, sending files; job and system management;
• Identification of users by their login, e-mail address, passwords, electronic signatures;
• Requests for connection with other application processes;

Video about all levels of the modelOSI

Conclusion

Analyzing problems using OSI network models can help you quickly find and fix problems. It is not for nothing that work on the project of a program that can identify shortcomings while having a complex step-by-step device has been going on for quite a long time. This model is truly a benchmark. After all, at the same time, work was underway to create other protocols. For example, . Today, they are used quite often.

In network science, as in any other field of knowledge, there are two fundamental approaches to learning: movement from the general to the specific and vice versa. Well, it’s not that in life people use these approaches in their pure form, but still, at the initial stages, each student chooses for himself one of the above-mentioned directions. For higher education (at least the (post)Soviet model) the first method is more typical, for self-education most often the second: a person was working on the network, solving small single-user administrative tasks from time to time, and suddenly he wanted to figure out how, Actually, how does all this crap work?

But the purpose of this article is not philosophical discussions about teaching methodology. I would like to introduce to the attention of novice networkers that general and most importantly, from which, like from a stove, you can dance to the most sophisticated private shops. By understanding the seven-layer OSI model and learning to “recognize” its layers in the technologies you already know, you can easily move forward in any direction of the networking industry you choose. The OSI model is the framework on which any new knowledge about networks will be hung.

This model is mentioned in one way or another in almost any modern literature on networks, as well as in many specifications of specific protocols and technologies. Without feeling the need to reinvent the wheel, I decided to publish excerpts from the work of N. Olifer, V. Olifer (Information Technology Center) entitled “The role of communication protocols and the functional purpose of the main types of equipment in corporate networks,” which I consider the best and most comprehensive publication on this topic .

chief editor

model

Just because a protocol is an agreement between two interacting entities, in this case two computers working on a network, does not mean that it is necessarily a standard. But in practice, when implementing networks, they tend to use standard protocols. These may be proprietary, national or international standards.

The International Standards Organization (ISO) has developed a model that clearly defines the different levels of interaction between systems, gives them standard names, and specifies what work each level should do. This model is called the Open System Interconnection (OSI) model or ISO/OSI model.

In the OSI model, communication is divided into seven layers or layers (Figure 1.1). Each level deals with one specific aspect of interaction. Thus, the interaction problem is decomposed into 7 particular problems, each of which can be solved independently of the others. Each layer maintains interfaces with the layers above and below.

Rice. 1.1. ISO/OSI Open Systems Interconnection Model

The OSI model describes only system communications, not end-user applications. Applications implement their own communication protocols by accessing system facilities. It should be borne in mind that the application can take over the functions of some of the upper layers of the OSI model, in which case, if necessary, internetworking it accesses directly the system tools that perform the functions of the remaining lower layers of the OSI model.

An end-user application can use system interaction tools not only to organize a dialogue with another application running on another machine, but also simply to receive the services of a particular network service, for example, accessing remote files, receiving mail, or printing on a shared printer.

So, let's say an application makes a request to an application layer, such as a file service. Based on this request, the application level software generates a standard format message, which contains service information (header) and, possibly, transmitted data. This message is then forwarded to the representative level. The presentation layer adds its header to the message and passes the result down to the session layer, which in turn adds its header, and so on. Some protocol implementations provide that the message contains not only a header, but also a trailer. Finally, the message reaches the lowest, physical layer, which actually transmits it along the communication lines.

When a message arrives on another machine over the network, it moves up sequentially from level to level. Each level analyzes, processes and deletes the header of its level, performs functions corresponding to this level and passes the message to the higher level.

In addition to the term "message", there are other names used by network specialists to designate a unit of data exchange. ISO standards for protocols of any level use the term “protocol data unit” - Protocol Data Unit (PDU). In addition, the names frame, packet, and datagram are often used.

ISO/OSI Model Layer Functions

Physical Layer: This layer deals with the transmission of bits over physical channels such as coaxial cable, twisted pair cable, or fiber optic cable. This level is related to the characteristics of physical data transmission media, such as bandwidth, noise immunity, characteristic impedance and others. At the same level, the characteristics of electrical signals are determined, such as requirements for pulse edges, voltage or current levels of the transmitted signal, type of coding, signal transmission speed. In addition, the types of connectors and the purpose of each contact are standardized here.

Physical layer functions are implemented in all devices connected to the network. On the computer side, the physical layer functions are performed by the network adapter or serial port.

An example of a physical layer protocol is the 10Base-T Ethernet technology specification, which defines the cable used as Category 3 unshielded twisted pair with a characteristic impedance of 100 Ohms, an RJ-45 connector, a maximum physical segment length of 100 meters, Manchester code for representing data on the cable, and other characteristics of the environment and electrical signals.

Data Link Layer: The physical layer simply transfers bits. This does not take into account that in some networks in which communication lines are used (shared) alternately by several pairs of interacting computers, the physical transmission medium may be occupied. Therefore, one of the tasks of the link layer is to check the availability of the transmission medium. Another task of the link layer is to implement error detection and correction mechanisms. To do this, at the data link layer, bits are grouped into sets called frames. The link layer ensures that each frame is transmitted correctly by placing a special sequence of bits at the beginning and end of each frame to mark it, and also calculates a checksum by summing all the bytes of the frame in a certain way and adding the checksum to the frame. When the frame arrives, the receiver again calculates the checksum of the received data and compares the result with the checksum from the frame. If they match, the frame is considered correct and accepted. If the checksums do not match, an error is recorded.

The link layer protocols used in local networks contain a certain structure of connections between computers and methods for addressing them. Although the data link layer provides frame delivery between any two nodes on a local network, it does this only in a network with a very specific connection topology, precisely the topology for which it was designed. Typical topologies supported by LAN link layer protocols include shared bus, ring, and star. Examples of link layer protocols are Ethernet, Token Ring, FDDI, 100VG-AnyLAN.

In local area networks, link layer protocols are used by computers, bridges, switches, and routers. In computers, link layer functions are implemented through the joint efforts of network adapters and their drivers.

In global networks, which rarely have a regular topology, the data link layer ensures the exchange of messages between two neighboring computers connected by an individual communication line. Examples of point-to-point protocols (as such protocols are often called) are the widely used PPP and LAP-B protocols.

Network level. This level serves to form a unified transport system that unites several networks with different principles for transmitting information between end nodes. Let's look at the functions of the network layer using local networks as an example. The local network link layer protocol ensures the delivery of data between any nodes only in a network with the appropriate typical topology. This is a very strict limitation that does not allow building networks with a developed structure, for example, networks that combine several enterprise networks into a single network, or highly reliable networks in which there are redundant connections between nodes. In order, on the one hand, to maintain the simplicity of data transfer procedures for standard topologies, and on the other hand, to allow the use of arbitrary topologies, an additional network layer is used. At this level the concept of "network" is introduced. In this case, a network is understood as a collection of computers connected to each other in accordance with one of the standard typical topologies and using one of the link layer protocols defined for this topology to transmit data.

Thus, within the network, data delivery is regulated by the data link layer, but data delivery between networks is handled by the network layer.

Network layer messages are usually called packages. When organizing packet delivery at the network level, the concept is used "network number". In this case, the recipient's address consists of the network number and the computer number on this network.

Networks are connected to each other by special devices called routers. Router is a device that collects information about the topology of internetwork connections and, based on it, forwards network layer packets to the destination network. In order to transmit a message from a sender located on one network to a recipient located on another network, you need to make a number of transit transfers (hops) between networks, each time choosing the appropriate route. Thus, a route is a sequence of routers through which a packet passes.

The problem of choosing the best path is called routing and its solution is the main task of the network level. This problem is complicated by the fact that the shortest path is not always the best. Often the criterion for choosing a route is the time of data transmission along this route; it depends on the capacity of communication channels and traffic intensity, which can change over time. Some routing algorithms try to adapt to changes in load, while others make decisions based on long-term averages. The route can be selected based on other criteria, for example, transmission reliability.

At the network level, two types of protocols are defined. The first type refers to the definition of rules for transmitting end node data packets from the node to the router and between routers. These are the protocols that are usually meant when people talk about network layer protocols. The network layer also includes another type of protocol called routing information exchange protocols. Using these protocols, routers collect information about the topology of internetwork connections. Network layer protocols are implemented by operating system software modules, as well as router software and hardware.

Examples of network layer protocols are the TCP/IP stack IP Internetwork Protocol and the Novell IPX stack Internetwork Protocol.

Transport Layer: On the way from the sender to the recipient, packets may be corrupted or lost. While some applications have their own error handling, there are others that prefer to deal with a reliable connection right away. The job of the transport layer is to ensure that applications or the upper layers of the stack - application and session - transfer data with the degree of reliability that they require. The OSI model defines five classes of service provided by the transport layer. These types of services are distinguished by the quality of the services provided: urgency, the ability to restore interrupted communications, the availability of means for multiplexing multiple connections between different application protocols through a common transport protocol, and most importantly, the ability to detect and correct transmission errors, such as distortion, loss and duplication of packets.

The choice of transport layer service class is determined, on the one hand, by the extent to which the problem of ensuring reliability is solved by the applications and protocols of higher levels than the transport one, and on the other hand, this choice depends on how reliable the entire data transportation system is online. So, for example, if the quality of communication channels is very high, and the likelihood of errors not detected by lower-level protocols is small, then it is reasonable to use one of the lightweight transport layer services that are not burdened with numerous checks, handshaking, and other techniques for increasing reliability. If the vehicles are initially very unreliable, then it is advisable to turn to the most developed transport level service, which works using maximum means for detecting and eliminating errors - using the preliminary establishment of a logical connection, monitoring the delivery of messages using checksums and cyclic numbering of packets, establishing delivery timeouts, etc.

As a rule, all protocols, starting from the transport layer and above, are implemented by software of the end nodes of the network - components of their network operating systems. Examples of transport protocols include the TCP and UDP protocols of the TCP/IP stack and the SPX protocol of the Novell stack.

Session Layer: The session layer provides conversation management to record which party is currently active and also provides synchronization facilities. The latter allow you to insert checkpoints into long transfers so that in case of failure you can go back to the last checkpoint, instead of starting all over again. In practice, few applications use the session layer, and it is rarely implemented.

Presentation Layer: This layer provides assurance that information conveyed by the application layer will be understood by the application layer in another system. If necessary, the presentation layer converts data formats into some common presentation format, and at the reception, accordingly, performs the reverse conversion. In this way, application layers can overcome, for example, syntactic differences in data representation. At this level, encryption and decryption of data can be performed, thanks to which the secrecy of data exchange is ensured for all application services at once. An example of a protocol that operates at the presentation layer is the Secure Socket Layer (SSL) protocol, which provides secure messaging for the application layer protocols of the TCP/IP stack.

Application Layer The application layer is really just a set of various protocols through which network users access shared resources such as files, printers, or hypertext Web pages, and also organize their collaboration, for example, using the electronic mail protocol. . The unit of data that the application layer operates on is usually called message .

There is a very wide variety of application layer protocols. Let us give as examples at least a few of the most common implementations of file services: NCP in the Novell NetWare operating system, SMB in Microsoft Windows NT, NFS, FTP and TFTP, which are part of the TCP/IP stack.

The OSI model, although very important, is only one of many communication models. These models and their associated protocol stacks may differ in the number of layers, their functions, message formats, services provided at the upper layers, and other parameters.

Characteristics of popular communication protocol stacks

So, the interaction of computers in networks occurs in accordance with certain rules for exchanging messages and their formats, that is, in accordance with certain protocols. A hierarchically organized set of protocols that solve the problem of interaction between network nodes is called a communication protocol stack.

There are many protocol stacks that are widely used in networks. These are stacks that are international and national standards, and proprietary stacks that have become widespread due to the prevalence of equipment from a particular company. Examples of popular protocol stacks include Novell's IPX/SPX stack, the TCP/IP stack used on the Internet and many networks based on the UNIX operating system, the OSI stack of the International Standards Organization, the DECnet stack of Digital Equipment Corporation, and several others.

The use of a particular communication protocol stack in a network largely determines the face of the network and its characteristics. Smaller networks may use only one stack. In large corporate networks that connect various networks, several stacks are usually used in parallel.

Communication equipment implements lower-layer protocols that are more standardized than higher-layer protocols, and this is a prerequisite for successful collaboration between equipment from different manufacturers. The list of protocols supported by a particular communication device is one of the most important characteristics of this device.

Computers implement communication protocols in the form of corresponding software elements of a network operating system, for example, link-level protocols are usually implemented in the form of network adapter drivers, and upper-level protocols are implemented in the form of server and client components of network services.

The ability to work well in a particular operating system environment is an important characteristic of communications equipment. You can often read in advertisements for a network adapter or hub that it was designed specifically to work on a NetWare or UNIX network. This means that the hardware developers have optimized its characteristics for the protocols used in that network operating system, or for a given version of their implementation if these protocols are used in different operating systems. Due to the peculiarities of the implementation of protocols in various operating systems, one of the characteristics of communication equipment is its certification for the ability to work in the environment of a given operating system.

At the lower levels - physical and data link - almost all stacks use the same protocols. These are well-standardized protocols: Ethernet, Token Ring, FDDI and some others, which allow the same equipment to be used in all networks.

The network and higher layer protocols of existing standard stacks are highly variable and generally do not conform to the layering recommended by the ISO model. In particular, in these stacks, session and presentation layer functions are most often combined with the application layer. This discrepancy is due to the fact that the ISO model appeared as a result of a generalization of already existing and actually used stacks, and not vice versa.

OSI stack

A distinction must be made between the OSI protocol stack and the OSI model. While the OSI model conceptually defines the procedure for interaction of open systems, decomposing the task into 7 layers, standardizes the purpose of each layer and introduces standard names for the layers, the OSI stack is a set of very specific protocol specifications that form a consistent protocol stack. This protocol stack is supported by the US government in its GOSIP program. All government computer networks installed after 1990 must either directly support the OSI stack or provide a means to migrate to the stack in the future. However, the OSI stack is more popular in Europe than in the US, as Europe has fewer legacy networks installed that use their own protocols. There is also a big need for a common stack in Europe, as there are so many different countries.

This is an international, manufacturer-independent standard. It can enable collaboration between corporations, partners and suppliers. This interaction is complicated by addressing, naming, and data security issues. All these problems are partially solved in the OSI stack. OSI protocols require a lot of CPU processing power, making them more suitable for powerful machines rather than networks of personal computers. Most organizations are just planning the transition to the OSI stack. Among those working in this direction are the US Navy Department and the NFSNET network. One of the largest manufacturers supporting OSI is AT&T. Its Stargroup network is entirely based on the OSI stack.

For obvious reasons, the OSI stack, unlike other standard stacks, fully complies with the OSI interconnection model; it includes specifications for all seven layers of the open systems interconnection model (Figure 1.3).

Rice. 1.3. OSI stack

On The OSI stack supports Ethernet, Token Ring, FDDI protocols, as well as LLC, X.25 and ISDN protocols. These protocols will be discussed in detail in other sections of the manual.

Services network, transport and session levels are also available in the OSI stack, but they are not very common. The network layer implements both connectionless and connection-based protocols. The OSI stack transport protocol, consistent with the functions defined for it in the OSI model, hides the differences between connection-oriented and connectionless network services so that users receive the desired quality of service regardless of the underlying network layer. To provide this, the transport layer requires the user to specify the desired quality of service. 5 classes of transport service have been defined, from the lowest class 0 to the highest class 4, which differ in the degree of error tolerance and requirements for data recovery after errors.

Services application level include file transfer, terminal emulation, directory services, and mail. Of these, the most promising are directory service (X.500 standard), electronic mail (X.400), virtual terminal protocol (VT), file transfer, access and management (FTAM) protocol, forwarding and job management protocol (JTM). Recently, ISO has concentrated its efforts on top-level services.

The X.400 Recommendations define the following minimum required set of services to be provided to users: access control, maintenance of unique system message identifiers, message delivery or non-delivery notification with reason, message content type indication, message content conversion indication, transmission and delivery timestamps, selecting a delivery category (urgent, non-urgent, normal), multicast delivery, delayed delivery (up to a specific point in time), transforming content to interface with incompatible mail systems such as telex and fax services, querying whether a specific message was delivered, mailing lists, which may have a nested structure, means of protecting messages from unauthorized access, based on an asymmetric public key cryptosystem.

The purpose of the recommendations X.500 is to develop global help desk standards. The process of delivering a message requires knowledge of the recipient's address, which is a problem in large networks, so it is necessary to have a help desk that helps obtain the addresses of senders and recipients. In general, an X.500 service is a distributed database of names and addresses. All users are potentially allowed to log into this database using a specific set of attributes.

The following operations are defined on the names and addresses database:

• reading - getting an address by a known name,
• request - obtaining a name based on known address attributes,
• modification involving deleting and adding records in a database.

The main challenges to implementing the X.500 recommendations stem from the scale of this project, which purports to be a worldwide reference service. Therefore, software implementing X.500 recommendations is very cumbersome and places high demands on hardware performance.

Protocol VT solves the problem of incompatibility between different terminal emulation protocols. Currently, a user of a personal computer compatible with an IBM PC, in order to simultaneously work with VAX, IBM 3090 and HP9000 computers, needs to purchase three different programs to emulate terminals of different types and using different protocols. If every host computer had ISO terminal emulation protocol software, then the user would only need one program that supported the VT protocol. In its standard, ISO has accumulated widely used terminal emulation functions.

File transfer is the most common computer service. Access to files, both local and remote, is needed by all applications - text editors, email, databases or remote launch programs. ISO provides such a service in the protocol FTAM. Along with the X.400 standard, it is the most popular OSI stack standard. FTAM provides facilities for localizing and accessing file content and includes a set of directives for inserting, replacing, extending, and clearing file content. FTAM also provides facilities for manipulating the file as a whole, including creating, deleting, reading, opening, closing the file, and selecting its attributes.

Forwarding and Work Control Protocol JTM Allows users to forward work that needs to be completed on the host computer. The job control language that enables job submission tells the host computer what actions should be performed on what programs and files. The JTM protocol supports traditional batch processing, transaction processing, remote job entry, and distributed database access.

TCP/IP stack

The TCP/IP stack, also called the DoD stack and the Internet stack, is one of the most popular and promising communication protocol stacks. If at present it is distributed mainly in networks with UNIX OS, then its implementation in the latest versions of network operating systems for personal computers (Windows NT, NetWare) is a good prerequisite for the rapid growth in the number of installations of the TCP/IP stack.

The stack was developed at the initiative of the US Department of Defense (DoD) more than 20 years ago to connect the experimental ARPAnet network with other satellite networks as a set of common protocols for heterogeneous computing environments. The ARPA network supported developers and researchers in military fields. In the ARPA network, communication between two computers was carried out using the Internet Protocol (IP), which to this day is one of the main ones in the TCP / IP stack and appears in the name of the stack.

Berkeley University made a major contribution to the development of the TCP/IP stack by implementing stack protocols in its version of the UNIX OS. The widespread adoption of the UNIX operating system also led to the widespread adoption of IP and other stack protocols. This stack also powers the Internet, whose Internet Engineering Task Force (IETF) is a major contributor to the development of stack standards published in the form of RFC specifications.

Since the TCP/IP stack was developed before the advent of the ISO/OSI open systems interconnection model, although it also has a multi-level structure, the correspondence of the TCP/IP stack levels to the levels of the OSI model is rather conditional.

The structure of the TCP/IP protocols is shown in Figure 1.4. TCP/IP protocols are divided into 4 layers.

Rice. 1.4. TCP/IP stack

The lowest one ( level IV ) - level of network interfaces - corresponds to the physical and data link levels of the OSI model. This level in the TCP/IP protocols is not regulated, but supports all popular standards of the physical and data link layer: for local channels these are Ethernet, Token Ring, FDDI, for global channels - their own protocols for operating on analog dial-up and leased lines SLIP/PPP, which establish point-to-point connections via WAN serial links, and WAN protocols X.25 and ISDN. A special specification has also been developed that defines the use of ATM technology as a data link layer transport.

Next level ( level III ) is the internetworking layer that deals with the transmission of datagrams using various local networks, X.25 area networks, ad hoc lines, etc. The stack uses the protocol IP, which was originally designed as a protocol for transmitting packets in composite networks consisting of a large number of local networks connected by both local and global connections. Therefore, the IP protocol works well in networks with complex topologies, rationally using the presence of subsystems in them and economically using the bandwidth of low-speed communication lines. The IP protocol is a datagram protocol.

The level of internetworking also includes all protocols related to the compilation and modification of routing tables, such as protocols for collecting routing information R.I.P.(Routing Internet Protocol) and OSPF(Open Shortest Path First), as well as the Internet Control Message Protocol ICMP(Internet Control Message Protocol). The latter protocol is designed to exchange information about errors between the router and the gateway, the source system and the destination system, that is, to organize feedback. Using special ICMP packets, it is reported that it is impossible to deliver a packet, that the lifetime or duration of assembling a packet from fragments has been exceeded, anomalous parameter values, a change in the forwarding route and type of service, the state of the system, etc.

Next level ( level II) is called basic. The transmission control protocol operates at this level TCP(Transmission Control Protocol) and User Datagram Protocol UDP(User Datagram Protocol). The TCP protocol provides a stable virtual connection between remote application processes. The UDP protocol ensures the transmission of application packets using the datagram method, that is, without establishing a virtual connection, and therefore requires less overhead than TCP.

Top level ( level I) is called applied. Over many years of use in the networks of various countries and organizations, the TCP/IP stack has accumulated a large number of protocols and application level services. These include such widely used protocols as the FTP file copy protocol, the telnet terminal emulation protocol, the SMTP mail protocol used in Internet e-mail and its Russian branch RELCOM, hypertext services for accessing remote information, such as WWW and many others. Let's take a closer look at some of them that are most closely related to the topics of this course.

Protocol SNMP(Simple Network Management Protocol) is used to organize network management. The management problem is divided here into two problems. The first task is related to the transfer of information. Control information transfer protocols determine the procedure for interaction between the server and the client program running on the administrator's host. They define the message formats that are exchanged between clients and servers, as well as the formats for names and addresses. The second challenge is related to controlled data. The standards regulate what data should be stored and accumulated in gateways, the names of this data, and the syntax of these names. The SNMP standard defines a specification for a network management information database. This specification, known as the Management Information Base (MIB), defines the data elements that a host or gateway must store and the permissible operations on them.

File Transfer Protocol FTP(File Transfer Protocol) implements remote file access. In order to ensure reliable transfer, FTP uses the connection-oriented protocol - TCP - as its transport. In addition to file transfer protocol, FTP offers other services. This gives the user the opportunity to interact interactively with a remote machine, for example, he can print the contents of its directories; FTP allows the user to specify the type and format of the data to be stored. Finally, FTP authenticates users. Before accessing the file, protocol requires users to provide their username and password.

In the TCP/IP stack, FTP offers the most comprehensive set of file services, but is also the most complex to program. Applications that do not require all the capabilities of FTP can use another, more cost-effective protocol - the Simple File Transfer Protocol TFTP(Trivial File Transfer Protocol). This protocol only implements file transfer, and the transport used is a simpler than TCP, connectionless protocol - UDP.

Protocol telnet provides the transfer of a stream of bytes between processes, as well as between a process and a terminal. Most often, this protocol is used to emulate a remote computer terminal.

IPX/SPX stack

This stack is the original Novell protocol stack, which it developed for its NetWare network operating system back in the early 80s. The Internetwork Packet Exchange (IPX) and Sequenced Packet Exchange (SPX) protocols, which give the stack its name, are direct adaptations of Xerox's XNS protocols, which are much less common than IPX/SPX. In terms of installations, IPX/SPX protocols are the leaders, and this is due to the fact that the NetWare OS itself occupies a leading position with a share of installations worldwide of approximately 65%.

The Novell protocol family and their correspondence to the ISO/OSI model are presented in Figure 1.5.

Rice. 1.5. IPX/SPX stack

On physical and data link levels Novell networks use all popular protocols of these levels (Ethernet, Token Ring, FDDI and others).

On network level the protocol works in the Novell stack IPX, as well as routing information exchange protocols R.I.P. And NLSP(analogous to the OSPF protocol of the TCP/IP stack). IPX is a protocol that deals with addressing and routing packets on Novell networks. IPX routing decisions are based on the address fields in its packet header as well as information from routing information exchange protocols. For example, IPX uses information provided by either RIP or NLSP (NetWare Link State Protocol) to forward packets to the destination computer or the next router. The IPX protocol supports only the datagram method of message exchange, due to which it economically consumes computing resources. So, the IPX protocol provides three functions: setting an address, establishing a route, and sending datagrams.

The transport layer of the OSI model in the Novell stack corresponds to the SPX protocol, which carries out connection-oriented message transfer.

On the top application, presentation and session levels NCP and SAP protocols work. Protocol NCP(NetWare Core Protocol) is a protocol for interaction between the NetWare server and the workstation shell. This application layer protocol implements the client-server architecture at the upper layers of the OSI model. Using the functions of this protocol, the workstation connects to the server, maps the server directories to local drive letters, scans the server file system, copies remote files, changes their attributes, etc., and also shares a network printer between workstations.

Servers and routers use SAP to advertise their services and network addresses. The SAP protocol allows network devices to constantly update information about what services are currently available on the network. At startup, servers use SAP to notify the rest of the network about their services. When a server shuts down, it uses SAP to notify the network that its services have ceased.

On Novell networks, NetWare 3.x servers send out SAP broadcast packets every minute. SAP packets significantly clog the network, so one of the main tasks of routers that access global communications is to filter traffic from SAP packets and RIP packets.

The features of the IPX/SPX stack are due to the features of the NetWare OS, namely the orientation of its early versions (up to 4.0) for working in small local networks consisting of personal computers with modest resources. Therefore, Novell needed protocols that required a minimum amount of RAM (limited in IBM-compatible computers running MS-DOS to 640 KB) and that would run quickly on low-power processors. As a result, the IPX/SPX stack protocols until recently worked well in local networks and not so well in large corporate networks, since they overloaded slow global links with broadcast packets that are intensively used by several protocols in this stack (for example, to establish communications between clients and servers).

This circumstance, as well as the fact that the IPX/SPX stack is the property of Novell and requires a license to implement it, has for a long time limited its distribution only to NetWare networks. However, by the time NetWare 4.0 was released, Novell had made and continues to make major changes to its protocols aimed at adapting them to work in corporate networks. Now the IPX/SPX stack is implemented not only in NetWare, but also in several other popular network operating systems - SCO UNIX, Sun Solaris, Microsoft Windows NT.

NetBIOS/SMB stack

Microsoft and IBM worked together on networking tools for personal computers, so the NetBIOS/SMB protocol stack is their joint brainchild. NetBIOS tools appeared in 1984 as a network extension of the standard functions of the basic input/output system (BIOS) of the IBM PC for the PC Network network program from IBM, which at the application level (Fig. 1.6) used the SMB (Server Message Block) protocol to implement network services. .

Rice. 1.6. NetBIOS/SMB stack

Protocol NetBIOS works at three levels of the open systems interaction model: network, transport and session. NetBIOS can provide a higher level of service than the IPX and SPX protocols, but does not have routing capabilities. Thus, NetBIOS is not a network protocol in the strict sense of the word. NetBIOS contains many useful networking functions that can be attributed to the network, transport and session layers, but it cannot be used to route packets, since the NetBIOS frame exchange protocol does not introduce such a concept as a network. This limits the use of the NetBIOS protocol to local networks that are not subnetted. NetBIOS supports both datagram and connection-based communications.

Protocol SMB, corresponding to the application and representative levels of the OSI model, regulates the interaction of the workstation with the server. SMB functions include the following operations:

• Session management. Creation and breaking of a logical channel between the workstation and the network resources of the file server.
• File access. A workstation can contact the file server with requests to create and delete directories, create, open and close files, read and write to files, rename and delete files, search for files, get and set file attributes, and lock records.
• Printing service. The workstation can queue files for printing on the server and obtain information about the print queue.
• Messaging service. SMB supports simple messaging with the following functions: send a simple message; send a broadcast message; send start of message block; send message block text; send end of message block; forward username; cancel the shipment; get the machine name.

Because of the large number of applications that use the API functions provided by NetBIOS, many network operating systems implement these functions as an interface to their transport protocols. NetWare has a program that emulates NetBIOS functions based on the IPX protocol, and there are software emulators for NetBIOS for Windows NT and the TCP/IP stack.

Why do we need this valuable knowledge? (editorial)

A colleague once asked me a tricky question. Well, he says, you know what the OSI model is... And why do you need it, what is the practical benefit of this knowledge: unless you show off in front of dummies? It’s not true, the benefits of this knowledge are a systematic approach to solving many practical problems. For example:

• troubleshooting (

troubleshooting)

A user (just a friend) comes to you as an admin (an experienced networker) and says - it “doesn’t connect” to me here. There is no network, he says, and that’s it. You start to figure it out. So, based on my experience of observing my neighbors, I noticed that the actions of a person “not aware of the OSI model in his heart” are characterized by characteristic chaotic behavior: either he tugs at a wire, or suddenly he tinkers with something in the browser. And this often leads to the fact that, moving without direction, such a “specialist” will tug on anything and anywhere except in the area of ​​the problem, wasting a lot of his own and other people’s time. When realizing the existence of levels of interaction, the movement will be more consistent. And although the starting point may be different (in each book I came across the recommendations were slightly different), the general logical premise of troubleshooting is as follows - if at level X the interaction is carried out correctly, then at level X-1 most likely everything is in order too. At least for each specific moment time. When troubleshooting in IP networks, I personally start “digging” from the second level of the DOD stack, aka the third OSI layer, aka Internet Protocol. Firstly, because it is easiest to carry out a “superficial examination of the patient” (the patient is more likely to ping than not), and secondly, if, thank God, it pings, you can skip the unpleasant manipulations of testing cables, network cards and disassemblies, etc. pleasant things;) Although in especially severe cases you will still have to start from level one, and in the most serious way.

• mutual understanding with colleagues

To illustrate this point, I will give you an example from life. One day, my friends from a small company invited me to visit me to help figure out why the network was not working well, and to give some recommendations on this matter. I come to the office. And it turns out that they even have an administrator there, called according to the good old tradition “programmer” (and in fact, he mainly deals with FoxPro;) - an old pre-perestroika IT specialist. Well, I ask him, what kind of network do you have? He: “What do you mean? Well, just a network.” The network, in general, is like a network. Well, I have some guiding questions: what protocol is used at the network level? He: “WHERE is this?” I clarify: “Well, IP or IPX or whatever you have...” “Oh,” he says, “it seems yes: IPX/something else!” By the way, “there-is-something else,” as you may have noticed, is located a little higher from the network level, but that’s not the point... What’s typical is that he built this network and even poorly maintained it. It’s not surprising that it withered away... ;) If I had known about OSI, I would have scribbled out a diagram in 5 minutes - from 10Base-2 to application programs. And you wouldn’t have to crawl under the table to inspect the coaxial wires.

• learning new technologies

I have already dwelled on this important aspect in the preface and I will repeat it again: when studying a new protocol, you should first of all understand a) in which protocol stack(s) it belongs and b) in which part of the stack and with whom it interacts from below and who with him on top... :) And this will give you complete clarity in your head. And there are different message formats and APIs - well, that’s a matter of technology :)

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 the 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. it doesn't matter what kind of data it is, whether it's a word document or an 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 data transfer. 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 other network technologies and models, in particular the TCP/IP model, are based on it.

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.

This model was developed back in 1984 by the International Standard Organization (ISO), and was originally called Open Systems Interconnection, OSI.
The open systems interaction model (in fact, the network interaction model) 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

The OSI model is an abbreviation for the English Open System Interconnection, that is, a model for the interaction of open systems. Open systems can be understood as network equipment (computers with network cards, switches, routers).
The OSI networking model is a blueprint (or communication plan) for network devices. OSI also plays a role in the creation of new network protocols, as it serves as a standard for interaction.
OSI consists of 7 blocks (layers). Each block performs its unique role in the network interaction of various network devices.
7 layers of the OSI model: 1 - Physical, 2 - Channel, 3 - Network, 4 - Transport, 5 - Session, 6 - Presentation, 7 - Application.
Each level of the model has its own set of network protocols (data transfer standards) through which devices on the network exchange data.
Remember, the more complex a network device is, the more capabilities it provides, but it also occupies more layers, and as a result, the slower it works.

Network models. Part 1. OSI.

It is definitely better to start with theory, and then gradually move on to practice. Therefore, first we will consider the network model (theoretical model), and then we will lift the curtain on how the theoretical network model fits into the network infrastructure (network equipment, user computers, cables, radio waves, etc.).
So, network model is a model of interaction between network protocols. And protocols, in turn, are standards that determine how different programs will exchange data.
Let me explain with an example: when opening any page on the Internet, the server (where the page being opened is located) sends data (a hypertext document) to your browser via the HTTP protocol. Thanks to the HTTP protocol, your browser, receiving data from the server, knows how it needs to be processed, and successfully processes it, showing you the requested page.
If you don’t yet know what a page on the Internet is, then I’ll explain in a nutshell: any text on a web page is enclosed in special tags that tell the browser what text size to use, its color, location on the page (left, right, or in the center). This applies not only to text, but also to pictures, forms, active elements and generally all content, i.e. what is on the page. The browser, detecting the tags, acts according to their instructions, and shows you the processed data that is enclosed in these tags. You yourself can see the tags of this page (and this text between the tags), to do this, go to the menu of your browser and select - view source code.
Let’s not get too distracted, “Network Model” is a necessary topic for those who want to become a specialist. This article consists of 3 parts and for you, I tried to write it not boringly, clearly and briefly. For details, or for additional clarification, write in the comments at the bottom of the page, and I will certainly help you.
We, as in the Cisco Networking Academy, will consider two network models: the OSI model and the TCP/IP model (sometimes called DOD), and at the same time compare them.

OSI Reference Network Model

OSI stands for Open System Interconnection. In Russian it sounds like this: Network model of interaction of open systems (reference model). This model can be safely called a standard. This is the model that network device manufacturers follow when developing new products.
The OSI network model consists of 7 layers, and it is customary to start counting from the bottom.
Let's list them:
7. Application layer
6. Presentation layer
5. Session layer
4. Transport layer
3. Network layer
2. Data link layer
1. Physical layer

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

The application layer or application layer is the topmost level of the model. It communicates user applications with the network. We are all familiar with these applications: web browsing (HTTP), sending and receiving mail (SMTP, POP3), receiving and receiving files (FTP, TFTP), remote access (Telnet), etc.

Executive level

Presentation layer or presentation layer – it converts data into the appropriate format. It’s easier to understand with an example: those pictures (all images) that you see on the screen are transmitted when sending a file in the form of small portions of ones and zeroes (bits). So, when you send a photo to your friend by email, the SMTP Application Layer protocol sends the photo to the lower layer, i.e. to the Presentation level. Where your photo is converted into a convenient form of data for lower levels, for example into bits (ones and zeros).
In exactly the same way, when your friend starts receiving your photo, it will come to him in the form of the same ones and zeros, and it is the Presentation layer that converts the bits into a full-fledged photo, for example, a JPEG.
This is how this level works with protocols (standards) for images (JPEG, GIF, PNG, TIFF), encodings (ASCII, EBDIC), music and video (MPEG), etc.

Session layer

Session layer or session layer - as the name implies, it organizes a communication session between computers. A good example would be audio and video conferencing; at this level it is established which codec the signal will be encoded with, and this codec must be present on both machines. Another example is the SMPP (Short message peer-to-peer protocol), which is used to send well-known SMS and USSD requests. One last example: PAP (Password Authentication Protocol) is an old protocol for sending a username and password to a server without encryption.
I won’t say anything more about the session level, otherwise we’ll delve into the boring features of the protocols. And if they (features) interest you, write letters to me or leave a message in the comments asking me to expand on the topic in more detail, and a new article will not be long in coming;)

Transport layer

Transport layer - this layer ensures the reliability of data transmission from sender to recipient. In fact, everything is very simple, for example, you communicate using a webcam with your friend or teacher. Is there a need for reliable delivery of every bit of the transmitted image? Of course not, if a few bits are lost from the streaming video, you won’t even notice it, not even the picture will change (maybe the color of one pixel out of 900,000 pixels will change, which will flash at a speed of 24 frames per second).
Now let’s give this example: a friend sends you (for example, via mail) important information or a program in an archive. You download this archive to your computer. This is where 100% reliability is needed, because... If a couple of bits are lost when downloading the archive, you will not be able to unzip it, i.e. extract the necessary data. Or imagine sending a password to a server, and one bit is lost along the way - the password will already lose its appearance and the meaning will change.
So, when we watch videos on the Internet, sometimes we see some artifacts, delays, noise, etc. And when we read text from a web page, the loss (or distortion) of letters is not acceptable, and when we download programs, everything also goes without errors.
At this level I will highlight two protocols: UDP and TCP. The UDP protocol (User Datagram Protocol) transfers data without establishing a connection, does not confirm the delivery of data and does not make repetitions. TCP protocol (Transmission Control Protocol), which before transmission establishes a connection, confirms the delivery of data, repeats it if necessary, and guarantees the integrity and correct sequence of the downloaded data.
Therefore, for music, video, video conferencing and calls we use UDP (we transfer data without verification and without delays), and for text, programs, passwords, archives, etc. – TCP (data transmission with confirmation of receipt takes more time).

Network layer

Network layer - this layer determines the path along which data will be transmitted. And, by the way, this is the third level of the OSI Network Model, and there are devices that are called third-level devices - routers.
We have all heard about the IP address, this is what the IP (Internet Protocol) protocol does. An IP address is a logical address on a network.
There are quite a lot of protocols at this level, and we will examine all these protocols in more detail later, in separate articles and with examples. Now I’ll just list a few popular ones.
Just like everyone has heard about the IP address and the ping command, this is how the ICMP protocol works.
The same routers (with which we will work in the future) use protocols of this level to route packets (RIP, EIGRP, OSPF).
The entire second part of the CCNA (Exploration 2) course is about routing.

Data Link Layer

Data link layer – we need it for the interaction of networks at the physical level. Probably everyone has heard about the MAC address; it is a physical address. Link layer devices - switches, hubs, etc.
IEEE (Institute of Electrical and Electronics Engineers) defines the data link layer as two sublayers: LLC and MAC.
LLC – Logical Link Control, created to interact with the upper level.
MAC – Media Access Control, created to interact with the lower level.
I’ll explain with an example: your computer (laptop, communicator) has a network card (or some other adapter), and so there is a driver to interact with it (with the card). A driver is a program - the upper sublayer of the link level, through which you can communicate with the lower levels, or rather with the microprocessor (hardware) - the lower sublayer of the link layer.
There are many typical representatives at this level. PPP (Point-to-Point) is a protocol for connecting two computers directly. FDDI (Fiber Distributed Data Interface) - the standard transmits data over a distance of up to 200 kilometers. CDP (Cisco Discovery Protocol) is a proprietary protocol owned by Cisco Systems, which can be used to discover neighboring devices and obtain information about these devices.
The entire third part of the CCNA (Exploration 3) course is about second-level devices.

Physical layer

Physical layer is the lowest level that directly transfers the data stream. The protocols are well known to us all: Bluetooth, IRDA (Infrared Communication), copper wires (twisted pair, telephone line), Wi-Fi, etc.
Look for details and specifications in future articles and in the CCNA course. The entire first part of the CCNA course (Exploration 1) is devoted to the OSI model.

Conclusion

So we have analyzed the OSI network model. In the next part, we will move on to the TCP/IP Network model, it is smaller and the protocols are the same. To successfully pass the CCNA tests, you need to make a comparison and identify the differences, which will be done.

After some thought, I decided to post here an article from the Network Problems website. So that everything is in one place.

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

The OSI network model is designed in such a way that higher layers of the network model use lower layers of the network model to transmit their information. The rules by which the model layers communicate are called network protocols. A network protocol at a certain level of the model can communicate either with protocols at its own level or with protocols at neighboring levels. Here again we can draw an analogy with the work of a company. The company always has a clearly established hierarchy, although not as strict as in the network model. Workers at one level of the hierarchy carry out orders received from workers at a higher level of the hierarchy.

Interaction between layers of the OSI network model

Each device operating on a network can be represented as a system operating at the appropriate levels of the OSI model. Moreover, this device can use in its work both all levels of the OSI model, and only some of its lower levels. Usually, when they say that a device operates at a certain level of the model, they mean that it operates at this level of the network model and at all levels below it.

Work at some levels of the OSI network model

When two different network devices communicate with each other, they use protocols of the same levels of the network model, while the interaction process involves both the protocols of the level at which the interaction directly occurs, and the necessary protocols of all underlying levels, since they are used for data transfer , received from the upper levels.

Communication between two systems from the perspective of the OSI model

When transmitting information from the upper level of the network model to the lower level of the network model, some service information called a header is added to this useful information (at level 2, not only the header is added, but also the trailer). This process of adding service information is called encapsulation. When receiving (transferring information from the lower level to the upper), this service information is separated and the original data is obtained. This process is called deencapsulation. At its core, this process is very similar to the process of sending a letter by mail. Imagine that you want to send a letter to your friend. You write a letter - this is useful information. When you send it by mail, you pack it in an envelope and write the recipient's address on it, that is, you add some heading to the useful information. In essence, this is encapsulation. Upon receiving your letter, your friend de-encapsulates it - that is, tears the envelope and takes out useful information from it - your letter.

Demonstration of the principle of encapsulation

The OSI model divides all functions performed during the interaction of systems into 7 levels: Physical (Physical) - 1, Channel (Data link) -2, Network (network) - 3, Transport (transport) - 4, Session (Session) -5, Presentation -6 and Application - 7.

Levels of the open systems interaction model

Let us briefly consider the purpose of each level 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. A block of data used at the data link layer is 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.

The OSI model itself is not a practical implementation; it only assumes a certain set of rules for the interaction of system components. A practical example of implementing a network protocol stack is the TCP/IP protocol stack (as well as other less common protocol stacks).

The OSI reference model is a 7-level network hierarchy created by the International Standards Organization (ISO). The presented model in Fig. 1 has 2 different models:

• a horizontal protocol-based model that implements the interaction of processes and software on different machines
• a vertical model based on services provided by adjacent layers to each other on the same machine

In the vertical one, neighboring levels exchange information using API interfaces. The horizontal model requires a common protocol for exchanging information at one level.

Picture 1

The OSI model describes only system interaction methods implemented by the OS, software, etc. The model does not include end-user interaction methods. Ideally, applications should access the upper layer of the OSI model, but in practice many protocols and programs have methods for accessing lower layers.

Physical layer

At the physical layer, data is represented in the form of electrical or optical signals corresponding to the 1s and 0s of the binary stream. Transmission medium parameters are determined at the physical level:

• type of connectors and cables
• pin assignment in connectors
• coding scheme for signals 0 and 1

The most common types of specifications at this level are:

• — unbalanced serial interface parameters
• - balanced serial interface parameters
• IEEE 802.3 -
• IEEE 802.5 -

At the physical level, it is impossible to understand the meaning of the data, since it is presented in the form of bits.

Data Link Layer

This channel implements the transportation and reception of data frames. The layer implements network layer requests and uses the physical layer for reception and transmission. The IEEE 802.x specifications divide this layer into two sublayers: logical link control (LLC) and media access control (MAC). The most common protocols at this level are:

• IEEE 802.2 LLC and MAC
• Ethernet
• Token Ring

Also at this level, error detection and correction during transmission is implemented. At the data link layer, the packet is placed in the data field of the frame - encapsulation. Error detection is possible using different methods. For example, the implementation of fixed frame boundaries, or a checksum.

Network layer

At this level, network users are divided into groups. This implements packet routing based on MAC addresses. The network layer implements transparent transmission of packets to the transport layer. At this level, the boundaries of networks of different technologies are erased. work at this level. An example of the operation of the network layer is shown in Fig. 2. The most common protocols:

Figure - 2

Transport layer

At this level, information flows are divided into packets for transmission at the network layer. The most common protocols at this level are:

• TCP - Transmission Control Protocol

Session layer

At this level, information exchange sessions between end machines are organized. At this level, the active party is determined and session synchronization is implemented. In practice, many other layer protocols include a session layer function.

Presentation layer

At this level, data exchange occurs between software on different operating systems. At this level, information transformation (compression, etc.) is implemented to transfer the information flow to the transport layer. The layer protocols used are those that use the higher layers of the OSI model.

Application layer

The application layer implements application access to the network. The layer manages file transfer and network management. Protocols used:

• FTP/TFTP - file transfer protocol
• X 400 - email
• Telnet
• CMIP - Information Management
• SNMP - network management
• NFS - Network File System
• FTAM - access method for transferring files