Protocols that operate at the osi application level. What is the seven-layer OSI model - why is it needed and how does it work. What differences might there be in link layer technologies?

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The OSI networking model is a reference model for communication open systems, in English sounds like Open Systems Interconnection Basic Reference Model. Its purpose is a generalized representation of network interaction tools.

That is, the OSI model is generalized standards for program developers, thanks to which any computer can equally decrypt data transmitted from another computer. To make it clear, I will give a real-life example. It is known that bees see everything around them in ultraviolet light. That is, our eye and the bee’s perceive the same picture in completely different ways, and what insects see may be invisible to human vision.

It’s the same with computers - if one developer writes an application in some programming language that his own computer understands, but is not available to anyone else, then on any other device you will not be able to read the document created by this application. Therefore, we came up with the idea that when writing applications, follow a single set of rules that are understandable to everyone.

OSI levels

For clarity, the network operation process is usually divided into 7 levels, each of which has its own group of protocols.

A network protocol is the rules and technical procedures that allow computers on a network to connect and exchange data.
A group of protocols united by a common end goal is called a protocol stack.

To perform different tasks, there are several protocols that serve systems, for example, the TCP/IP stack. Let's take a closer look at how information from one computer is sent over a local network to another computer.

Tasks of the SENDER's computer:

• Get data from the application
• Break them into small packages if the volume is large
• Prepare for transmission, that is, indicate the route, encrypt and transcode into a network format.

Tasks of the RECIPIENT'S computer:

• Receive data packets
• Remove service information from it
• Copy data to clipboard
• After complete reception of all packets, form an initial data block from them
• Give it to the app

In order to correctly perform all these operations, a single set of rules is needed, that is, the OSI reference model.

Let's return to the OSI levels. They are usually counted in reverse order and network applications are located at the top of the table, and the physical information transmission medium is at the bottom. As data from the computer flows down directly to the network cable, protocols operating at different layers gradually transform it, preparing it for physical transmission.

Let's look at them in more detail.

7. Application Layer

Its task is to collect data from the network application and send it to level 6.

6. Presentation Layer

Translates this data into a single universal language. The point is that everyone computer processor It has own format data processing, but they must enter the network in one universal format - this is what the presentation layer does.

5. Session Layer

He has many tasks.

1. Establish a communication session with the recipient. The software warns the receiving computer that data is about to be sent to it.
2. This is where name recognition and protection take place:
   • identification - name recognition
   • authentication - password verification
   • registration - assignment of authority
3. Implementation of which party is transferring information and how long this will take.
4. Placing checkpoints in the overall data flow so that if any part is lost, it is easy to determine which part is lost and should be resent.
5. Segmentation is breaking a large block into small packets.

4. Transport Layer

Provides applications with the required level of security when delivering messages. There are two groups of protocols:

• Protocols that are connection oriented - they monitor the delivery of data and optionally request retransmission if it fails. This is TCP - Information Transfer Control Protocol.
• Not connection-oriented (UDP) - they simply send blocks and do not further monitor their delivery.

3. Network Layer

Provides end-to-end transmission of a packet by calculating its route. At this level, in packets, the IP addresses of the sender and recipient are added to all previous information generated by other levels. It is from this moment that the data packet is called the PACKET itself, which has (IP protocol is an internetworking protocol).

2. Data Link Layer

Here the packet is transmitted within one cable, that is, one local network. It only works up to the edge router of one local network. To the received packet, the link layer adds its own header - the MAC addresses of the sender and recipient, and in this form the data block is already called a FRAME.

When transmitted beyond one local network, the packet is assigned the MAC not of the host (computer), but of the router of another network. This is where the question of gray and white IP arises, which was discussed in the article to which the link was given above. Gray is an address within one local network that is not used outside of it. White - a unique address in everything global internet.

When a packet arrives at the edge router, the IP of the packet is replaced by the IP of this router and the entire local network connects to the global network, that is, the Internet, under one single IP address. If the address is white, then the part of the data with the IP address does not change.

1. Physical layer (Transport layer)

Responsible for converting binary information into physical signal, which is sent on the physical data channel. If it is a cable, then the signal is electrical; if it is a fiber optic network, then it is an optical signal. This transformation is carried out using network adapter.

Protocol stacks

TCP/IP is a protocol stack that manages data transfer both on a local network and on the Internet. This stack contains 4 levels, that is, according to the OSI reference model, each of them combines several levels.

1. Application (OSI - application, presentation and session)
   The following protocols are responsible for this level:
   • TELNET - remote communication session in the form command line
   • FTP - File Transfer Protocol
   • SMTP - Mail Forwarding Protocol
   • POP3 and IMAP - receiving mail
   • HTTP - working with hypertext documents
2. Transport (the same for OSI) is the TCP and UDP already described above.
3. Internetwork (OSI - network) is an IP protocol
4. Network interface level (OSI - channel and physical) Network adapter drivers are responsible for the operation of this level.

Terminology when denoting a data block

• Stream - the data that is operated on application level
• A datagram is a block of data output from UPD, that is, which does not have guaranteed delivery.
• A segment is a block guaranteed for delivery at the output of the TCP protocol.
• Packet is a block of data output from the IP protocol. since at this level it is not yet guaranteed to be delivered, it can also be called a datagram.
• Frame is a block with assigned MAC addresses.

Thank you! Did not help

Alexander Goryachev, Alexey Niskovsky

In order for network servers and clients to communicate, they must work using the same information exchange protocol, that is, they must “speak” the same language. The protocol defines a set of rules for organizing the exchange of information at all levels of interaction of network objects.

There is an Open System Interconnection Reference Model, often called the OSI model. This model was developed by the International Organization for Standardization (ISO). The OSI model describes the interaction scheme of network objects, defines a list of tasks and rules for data transfer. It includes seven levels: physical (Physical - 1), channel (Data-Link - 2), network (Network - 3), transport (Transport - 4), session (Session - 5), data presentation (Presentation - 6 ) and applied (Application - 7). Two computers are considered to be able to communicate with each other at a particular layer of the OSI model if their software that implements network functions at that layer interprets the same data in the same way. In this case, direct communication is established between two computers, called “point-to-point”.

Implementations of the OSI model by protocols are called protocol stacks. It is impossible to implement all the functions of the OSI model within the framework of one specific protocol. Typically, tasks at a specific level are implemented by one or more protocols. One computer must run protocols from the same stack. In this case, the computer can simultaneously use several protocol stacks.

Let's consider the tasks solved at each level of the OSI model.

Physical layer

At this level of the OSI model, the following characteristics of network components are defined: types of connections for data transmission media, physical network topologies, methods of data transmission (with digital or analog signal coding), types of synchronization of transmitted data, separation of communication channels using frequency and time multiplexing.

Implementations of the OSI physical layer protocols coordinate the rules for transmitting bits.

The physical layer does not include a description of the transmission medium. However, implementations of physical layer protocols are specific to a particular transmission medium. The physical layer is usually associated with the connection of the following network equipment:

• concentrators, hubs and repeaters that regenerate electrical signals;
• transmission media connectors providing a mechanical interface for connecting the device to the transmission media;
• modems and various converting devices that perform digital and analog conversions.

This layer of the model defines the physical topologies in the enterprise network, which are built using a core set of standard topologies.

The first in the basic set is the bus topology. In this case, all network devices and computers are connected to a common data transmission bus, which is most often formed using a coaxial cable. The cable that forms the common bus is called the backbone. From each device connected to the bus, the signal is transmitted in both directions. To remove the signal from the cable, special interrupters (terminator) must be used at the ends of the bus. Mechanical damage to the highway affects the operation of all devices connected to it.

The ring topology provides for the connection of all network devices and computers into a physical ring. In this topology, information is always transmitted along the ring in one direction - from station to station. Each network device must have an information receiver on the input cable and a transmitter on the output cable. Mechanical damage to the information transmission medium in a single ring will affect the operation of all devices, however, networks built using a double ring, as a rule, have a margin of fault tolerance and self-healing functions. In networks built on a double ring, the same information is transmitted along the ring in both directions. If the cable is damaged, the ring will continue to operate as a single ring at double length (self-healing functions are determined by the hardware used).

The next topology is the star topology, or star. It provides for the presence central device, to which other network devices and computers are connected via beams (separate cables). Networks built on a star topology have a single point of failure. This point is the central device. If the central device fails, all other network participants will not be able to exchange information with each other, since all exchange was carried out only through the central device. Depending on the type of central device, the signal received from one input can be transmitted (with or without amplification) to all outputs or to a specific output to which the information recipient device is connected.

A fully connected (mesh) topology has high fault tolerance. When networks with a similar topology are built, each of the network devices or computers is connected to every other component of the network. This topology has redundancy, which makes it seem impractical. Indeed, in small networks this topology is rarely used, but in large networks corporate networks a fully connected topology can be used to connect the most important nodes.

The considered topologies are most often built using cable connections.

There is another topology using wireless connections, - cellular. In it, network devices and computers are combined into zones - cells (cells), interacting only with the cell's transceiver device. Information transfer between cells is carried out by transceiver devices.

Data Link Layer

This level determines the logical topology of the network, the rules for gaining access to the data transmission medium, and resolves issues related to addressing physical devices within a logical network and control of information transfer (transmission synchronization and connection service) between network devices.

Link layer protocols are defined by:

• rules for organizing physical layer bits (binary ones and zeros) into logical groups of information called frames. A frame is a link-layer data unit consisting of a contiguous sequence of grouped bits, having a header and a tail;
• rules for detecting (and sometimes correcting) transmission errors;
• flow control rules (for devices operating at this level of the OSI model, for example, bridges);
• rules for identifying computers on a network by their physical addresses.

Like most other layers, the data link layer adds its own control information to the beginning of the data packet. This information may include source address and destination address (physical or hardware), frame length information, and an indication of active upper-layer protocols.

The following network connecting devices are typically associated with the data link layer:

• bridges;
• smart hubs;
• switches;
• network interface cards (network interface cards, adapters, etc.).

The link layer functions are divided into two sublayers ( table 1):

• media access control (MAC);
• logical link control (Logical Link Control, LLC).

The MAC sublayer defines data link layer elements such as the logical network topology, access method to the information transmission medium and the rules of physical addressing between network objects.

The abbreviation MAC is also used in determining the physical address of a network device: the physical address of a device (which is determined within the network device or network card at the manufacturing stage) is often called the MAC address of that device. For a large number of network devices, especially network cards, it is possible to programmatically change the MAC address. It must be remembered that the data link layer of the OSI model imposes restrictions on the use of MAC addresses: in one physical network (a segment of a larger network) there cannot be two or more devices using the same MAC addresses. To determine the physical address of a network object, the concept of “node address” can be used. The host address most often coincides with the MAC address or is determined logically during software address reassignment.

The LLC sublayer defines the rules for synchronizing transmission and service connections. This sublayer of the data link layer interacts closely with the network layer of the OSI model and is responsible for the reliability of physical (using MAC addresses) connections. The logical topology of a network determines the method and rules (sequence) of data transfer between computers on the network. Network objects transmit data depending on the logical topology of the network. The physical topology defines the physical path of the data; however, in some cases the physical topology does not reflect the way the network operates. The actual data path is determined by the logical topology. Network connection devices and media access schemes are used to transmit data along a logical path, which may differ from the path in the physical medium. A good example of the differences between physical and logical topologies is IBM's Token Ring network. IN local networks Token Ring often uses copper cable, which is laid in a star-shaped configuration with a central splitter (hub). Unlike a normal star topology, the hub does not forward incoming signals to all other connected devices. The hub's internal circuitry sequentially sends each incoming signal to the next device in a predefined logical ring, i.e., in a circular fashion. The physical topology of this network is star, and the logical topology is ring.

Another example of the differences between physical and logical topologies is the Ethernet network. The physical network can be built using copper cables and a central hub. A physical network is formed, made according to the star topology. However, Ethernet technology provides for the transfer of information from one computer to all others on the network. The hub must relay the signal received from one of its ports to all other ports. A logical network with a bus topology has been formed.

To determine the logical topology of a network, you need to understand how signals are received in it:

• in logical bus topologies, each signal is received by all devices;
• In logical ring topologies, each device receives only those signals that were sent specifically to it.

It is also important to know how network devices access the information transmission medium.

Media access

Logical topologies use special rules that control permission to transmit information to other network objects. The control process controls access to the communication medium. Consider a network in which all devices are allowed to operate without any rules for gaining access to the transmission medium. All devices in such a network transmit information as soon as the data is ready; these transmissions may sometimes overlap in time. As a result of overlap, signals are distorted and transmitted data is lost. This situation is called a collision. Collisions do not allow organizing reliable and efficient transfer of information between network objects.

Collisions in the network extend to the physical network segments to which network objects are connected. Such connections form a single collision space, in which the impact of collisions extends to everyone. To reduce the size of collision spaces by segmenting the physical network, you can use bridges and other network devices that have traffic filtering capabilities at the data link layer.

A network cannot function properly until all network entities are able to monitor, manage, or mitigate collisions. In networks, some method is needed to reduce the number of collisions and interference (overlay) of simultaneous signals.

There are standard media access methods that describe the rules by which permission to transmit information is controlled for network devices: contention, token passing, and polling.

Before choosing a protocol that implements one of these media access methods, you should pay particular attention to the following factors:

• nature of transmission - continuous or pulsed;
• number of data transfers;
• the need to transmit data at strictly defined time intervals;
• number of active devices on the network.

Each of these factors, combined with its advantages and disadvantages, will help determine which media access method is most appropriate.

Competition. Contention-based systems assume that access to the transmission medium is implemented on a first-come, first-served basis. In other words, every network device competes for control of the transmission medium. Contention-based systems are designed so that all devices on the network can transmit data only as needed. This practice ultimately results in partial or complete data loss because collisions actually occur. As each new device is added to the network, the number of collisions can increase exponentially. An increase in the number of collisions reduces network performance, and in the case of complete saturation of the information transmission medium, it reduces the network performance to zero.

To reduce the number of collisions, special protocols have been developed that implement the function of listening to the information transmission medium before the station starts transmitting data. If a listening station detects a signal being transmitted (from another station), it will refrain from transmitting the information and will try again later. These protocols are called Carrier Sense Multiple Access (CSMA) protocols. CSMA protocols significantly reduce the number of collisions, but do not eliminate them completely. Collisions do occur, however, when two stations poll the cable, find no signals, decide the medium is clear, and then simultaneously begin transmitting data.

Examples of such adversarial protocols are:

• Carrier Sense Multiple Access/Collision Detection (CSMA/CD);
• Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA).

CSMA/CD protocols. CSMA/CD protocols not only listen to the cable before transmission, but also detect collisions and initiate retransmissions. When a collision is detected, the stations transmitting data initialize special internal timers with random values. The timers begin counting down, and when zero is reached, stations must attempt to retransmit data. Since the timers were initialized with random values, one of the stations will try to repeat the data transmission before the other. Accordingly, the second station will determine that the data transmission medium is already busy and will wait for it to become free.

Examples of CSMA/CD protocols are Ethernet version 2 (Ethernet II, developed by DEC) and IEEE802.3.

CSMA/CA protocols. CSMA/CA uses schemes such as time slicing access or sending a request to gain access to the medium. When using time slicing, each station can transmit information only at strictly defined times for this station. In this case, a mechanism for managing time slices must be implemented in the network. Each new station connected to the network notifies about its appearance, thereby initiating the process of redistributing time slices for information transmission. In the case of using centralized access control to the transmission medium, each station generates a special transmission request, which is addressed to the control station. The central station regulates access to the transmission medium for all network objects.

An example of CSMA/CA is the LocalTalk protocol Apple Computer.

Race-based systems are most suitable for use with bursty traffic (transmitting large files) in networks with a relatively small number of users.

Systems with token transfer. In token passing systems, a small frame (token) is passed in a specific order from one device to another. A token is a special message that transfers temporary control of the transmission medium to the device holding the token. Passing the token distributes access control among devices on the network.

Each device knows which device it receives the token from and which device it should pass it to. Typically, these devices are the closest neighbors of the token owner. Each device periodically gains control of the token, performs its actions (transmits information), and then passes the token on to the next device for use. The protocols limit the time each device can control the token.

There are several token passing protocols. Two networking standards that use token passing are IEEE 802.4 Token Bus and IEEE 802.5 Token Ring. A Token Bus network uses token-passing access control and a physical or logical bus topology, while a Token Ring network uses token-passing access control and a physical or logical ring topology.

Token passing networks should be used when there is time-sensitive priority traffic, such as digital audio or video data, or when there are very large numbers of users.

Survey. Polling is an access method that allocates one device (called a controller, primary, or "master" device) to act as an arbiter of access to the medium. This device polls all other devices (secondary) in some predefined order to see if they have information to transmit. To receive data from a secondary device, the primary device sends a request to it, and then receives the data from the secondary device and forwards it to the receiving device. The primary device then polls another secondary device, receives data from it, and so on. The protocol limits the amount of data that each secondary device can transmit after polling. Polling systems are ideal for time-sensitive network devices, such as equipment automation.

This layer also provides connection services. There are three types of connection service:

• unacknowledged connectionless service - sends and receives frames without flow control and without error control or packet sequencing;
• connection-oriented service - provides flow control, error control and packet sequencing by issuing receipts (confirmations);
• acknowledgment connectionless service - uses receipts to control flow and control errors during transfers between two network nodes.

The LLC sublayer of the data link layer provides the ability to simultaneously use multiple network protocols (from different protocol stacks) when operating through one network interface. In other words, if your computer has only one LAN card, but there is a need to work with various network services from different manufacturers, then the client network software at the LLC sublevel provides the possibility of such work.

Network layer

The network level determines the rules for data delivery between logical networks, the formation of logical addresses of network devices, the definition, selection and maintenance of routing information, and the operation of gateways.

The main goal of the network layer is to solve the problem of moving (delivering) data to specified points in the network. Data delivery at the network layer is generally similar to data delivery at the data link layer of the OSI model, where physical device addressing is used to transfer data. However, addressing at the data link layer applies only to one logical network and is valid only within that network. The network layer describes the methods and means of transmitting information between many independent (and often heterogeneous) logical networks that, when connected together, form one large network. Such a network is called an internetwork, and the processes of information transfer between networks are called internetworking.

Using physical addressing at the data link layer, data is delivered to all devices on the same logical network. Each network device, each computer determines the purpose of the received data. If the data is intended for the computer, then it processes it, but if not, it ignores it.

Unlike the data link layer, the network layer can select a specific route in the internetwork and avoid sending data to logical networks to which the data is not addressed. The network layer does this through switching, network layer addressing, and routing algorithms. The network layer is also responsible for ensuring the correct routes for data through the internetwork consisting of heterogeneous networks.

The network layer implementation elements and methods are defined as follows:

• all logically separate networks must have unique network addresses;
• switching defines how connections are made across the internetwork;
• the ability to implement routing so that computers and routers determine the best path for data to pass through the internetwork;
• the network will perform different levels of connection service depending on the number of errors expected within the interconnected network.

Routers and some switches operate at this layer of the OSI model.

The network layer determines the rules for the formation of logical network addresses of network objects. Within a large interconnected network, each network object must have a unique logical address. Two components are involved in the formation of a logical address: the logical network address, which is common to all network objects, and the logical address of the network object, which is unique to this object. When forming the logical address of a network object, either the physical address of the object can be used, or an arbitrary logical address can be determined. The use of logical addressing allows you to organize data transfer between different logical networks.

Each network object, each computer can perform many network functions simultaneously, ensuring the operation of various services. To access services, a special service identifier is used, called a port or socket. When accessing a service, the service identifier follows immediately after the logical address of the computer that provides the service.

Many networks reserve groups of logical addresses and service identifiers for the purpose of performing specific, predefined and well-known actions. For example, if it is necessary to send data to all network objects, the sending will be made to a special broadcast address.

The network layer defines the rules for transferring data between two network objects. This transmission can be done using switching or routing.

There are three methods of switching for data transmission: circuit switching, message switching and packet switching.

When using circuit switching, a data transmission channel is established between the sender and the recipient. This channel will be active during the entire communication session. When using this method, long delays in channel allocation are possible due to the lack of sufficient bandwidth, load on the switching equipment, or busyness of the recipient.

Message switching allows you to transmit a whole (not broken into parts) message using the “store-and-forward” principle. Each intermediate device receives a message, stores it locally, and when the communication channel through which the message should be sent is free, sends it. This method is well suited for transmitting email messages and organizing electronic document management.

Packet switching combines the advantages of the previous two methods. Each large message is broken down into small packets, each of which is sent sequentially to the recipient. As each packet passes through the internetwork, the best path at that moment in time is determined. It turns out that parts of one message can arrive at the recipient at different times, and only after all the parts are collected together will the recipient be able to work with the received data.

Every time you determine the next path for data, you must choose the best route. Definition task the best way called routing. This task is performed by routers. The task of routers is to determine possible data transmission paths, maintain routing information, and select the best routes. Routing can be done statically or dynamically. When specifying static routing, all relationships between logical networks must be specified and remain unchanged. Dynamic routing assumes that the router itself can determine new paths or modify information about old ones. Dynamic routing uses special routing algorithms, the most common of which are distance vector and link state. In the first case, the router uses second-hand information about the network structure from neighboring routers. In the second case, the router operates with information about its own communication channels and interacts with a special representative router to build a complete network map.

The choice of the best route is most often influenced by factors such as the number of hops through routers (hop count) and the number of ticks (time units) required to reach the destination network (tick count).

The network layer connection service works when the connection service of the LLC sublayer of the data link layer of the OSI model is not used.

When building an interconnected network, you have to connect logical networks built using different technologies and providing a variety of services. In order for a network to operate, logical networks must be able to correctly interpret data and control information. This task is solved using a gateway, which is a device or application program that translates and interprets the rules of one logical network into the rules of another. In general, gateways can be implemented at any level of the OSI model, but most often they are implemented at the upper levels of the model.

Transport layer

The transport layer allows you to hide the physical and logical structure of the network from applications at the upper layers of the OSI model. Applications work only with service functions that are quite universal and do not depend on the physical and logical network topologies. Features of logical and physical networks are implemented at previous levels, where transport layer transmits data.

The transport layer often compensates for the lack of reliable or connection-oriented connection service in lower layers. The term “reliable” does not mean that all data will be delivered in all cases. However, reliable implementations of transport layer protocols can usually acknowledge or deny delivery of data. If the data is not delivered correctly to the receiving device, the transport layer may retransmit or inform upper layers that delivery was not possible. Upper levels can then take necessary corrective action or provide the user with choice.

Many protocols in computer networks provide users with the ability to work with simple names in natural language instead of complex and difficult to remember alphanumeric addresses. Address/Name Resolution is a function of identifying or mapping names and alphanumeric addresses to each other. This function can be performed by every entity on the network or by special service providers called directory servers, name servers, etc. The following definitions classify address/name resolution methods:

• consumer initiation of service;
• initiated by the service provider.

In the first case, a network user accesses a service by its logical name, without knowing the exact location of the service. The user does not know whether this service is available in this moment. When accessed, the logical name is matched to the physical name, and work station the user initiates a call directly to the service. In the second case, each service notifies all network clients about itself on a periodic basis. Each client knows at any time whether the service is available and knows how to contact the service directly.

Addressing methods

Service addresses identify specific software processes executing on network devices. In addition to these addresses, service providers monitor various conversations they have with devices requesting services. Two different conversation methods use the following addresses:

• connection ID;
• transaction ID.

A connection identifier, also called a connection ID, port, or socket, identifies each conversation. Using a connection ID, a connection provider can communicate with more than one client. The service provider refers to each switching entity by its number and relies on the transport layer to coordinate other lower-layer addresses. The connection ID is associated with a specific conversation.

Transaction IDs are similar to connection IDs, but operate in units smaller than a conversation. A transaction is made up of a request and a response. Service providers and consumers track the departure and arrival of each transaction, not the entire conversation.

Session layer

The session layer facilitates communication between devices requesting and delivering services. Communication sessions are controlled through mechanisms that establish, maintain, synchronize and manage dialogue between communicating entities. This layer also helps upper layers to identify and connect to available network services.

The session layer uses logical address information supplied by lower layers to identify names and server addresses, needed by the upper levels.

The session layer also initiates conversations between service provider devices and consumer devices. In performing this function, the session layer often represents, or identifies, each object and coordinates access rights to it.

The session layer implements dialogue management using one of three communication methods - simplex, half duplex and full duplex.

Simplex communication involves only unidirectional transmission of information from the source to the receiver. No feedback(from receiver to source) this method of communication does not provide. Half-duplex allows the use of one data transmission medium for bidirectional information transfers, however, information can only be transmitted in one direction at a time. Full duplex ensures simultaneous transmission of information in both directions over the data transmission medium.

Administration of a communication session between two network objects, consisting of connection establishment, data transfer, connection termination, is also performed at this level of the OSI model. After a session is established, software that implements the functions of this layer can check the functionality of (maintain) the connection until it is terminated.

Data presentation layer

The main task of the data presentation layer is to transform data into mutually consistent formats (interchange syntax) that are understandable to all network applications and the computers on which the applications run. At this level, the tasks of data compression and decompression and their encryption are also solved.

Conversion refers to changing the bit order of bytes, the byte order of words, character codes, and file name syntax.

The need to change the order of bits and bytes is due to the presence of a large number of different processors, computers, complexes and systems. Processors from different manufacturers may interpret the zero and seventh bits in a byte differently (either the zero bit is the most significant one, or the seventh bit). Similarly, the bytes that make up large units of information - words - are interpreted differently.

In order for users of different operating systems to receive information in the form of files with correct names and contents, this layer ensures correct conversion of file syntax. Different operating systems handle their file systems, implement different ways of forming file names. Information in files is also stored in a specific character encoding. When two network objects interact, it is important that each of them can interpret file information differently, but the meaning of the information should not change.

The data presentation layer transforms data into a mutually consistent format (interchange syntax) that is understandable by all networked applications and the computers on which the applications run. It can also compress and expand, as well as encrypt and decrypt data.

Computers use different rules for representing data using binary ones and zeros. Although all of these rules attempt to achieve the common goal of presenting human-readable data, computer manufacturers and standards organizations have created rules that contradict each other. When two computers using different sets of rules try to communicate with each other, they often need to perform some transformations.

Local and network operating systems often encrypt data to protect it from unauthorized use. Encryption is a general term that describes several methods of protecting data. Protection is often performed using data scrambling, which uses one or more of three methods: permutation, substitution, or algebraic method.

Each of these methods is simply a special way of protecting data in such a way that it can only be understood by someone who knows the encryption algorithm. Data encryption can be performed either in hardware or in software. However, end-to-end data encryption is usually done programmatically and is considered part of the presentation layer functions. To notify objects about the encryption method used, 2 methods are usually used - secret keys and public keys.

Secret key encryption methods use a single key. Network entities that own the key can encrypt and decrypt each message. Therefore, the key must be kept secret. The key can be built into the hardware chips or installed by the network administrator. Each time the key changes, all devices must be modified (it is advisable not to use the network to transmit the value of the new key).

Network objects using public key encryption methods are provided with a secret key and some known value. An object creates a public key by manipulating a known value through a private key. The entity initiating the communication sends its public key to the receiver. The other entity then mathematically combines its own private key with the public key given to it to set a mutually acceptable encryption value.

Owning only the public key is of little use to unauthorized users. The complexity of the resulting encryption key is high enough that it can be calculated in a reasonable amount of time. Even knowing your own private key and someone else's public key is not much help in determining the other secret key - due to the complexity of logarithmic calculations for large numbers.

Application layer

The application layer contains all the elements and functions specific to each type of network service. The lower six layers combine the tasks and technologies that provide general support for a network service, while the application layer provides the protocols needed to perform specific network service functions.

Servers provide network clients with information about what types of services they provide. The main mechanisms for identifying the services offered are provided by such elements as service addresses. In addition, servers use such methods of presenting their service as active and passive service presentation.

When performing an Active service advertisement, each server periodically sends messages (including service addresses) announcing its availability. Clients can also poll network devices for a specific type of service. Network clients collect representations made by servers and form tables of currently available services. Most networks that use the active representation method also define a specific validity period for service representations. For example, if network protocol specifies that service submissions must be sent every five minutes, clients will time out those service submissions that have not been submitted within the last five minutes. When the timeout expires, the client removes the service from its tables.

Servers carry out passive service advertisement by registering their service and address in the directory. When clients want to determine the types of services available, they simply query the directory for the location of the desired service and its address.

Before a network service can be used, it must be made available to the computer's local operating system. There are several methods for solving this problem, but each such method can be determined by the position or level at which the local operating system recognizes the network operating system. The service provided can be divided into three categories:

• intercepting operating system calls;
• remote mode;
• joint data processing.

When using OC Call Interception, the local operating system is completely unaware of the existence of a network service. For example, when a DOS application tries to read a file from a network file server, it believes that this file located on local storage. Actually a special piece software intercepts a file read request before it reaches the local operating system (DOS) and forwards the request to the network file service.

At the other extreme, in Remote Operation mode, the local operating system is aware of the network and is responsible for passing requests to the network service. However, the server knows nothing about the client. To the server operating system, all requests to a service look the same, regardless of whether they are internal or transmitted over the network.

Finally, there are operating systems that are aware of the existence of the network. Both the service consumer and the service provider recognize each other's existence and work together to coordinate the use of the service. This type of service use is typically required for peer-to-peer collaborative data processing. Collaborative data processing involves sharing data processing capabilities to perform a single task. This means that the operating system must be aware of the existence and capabilities of others and be able to cooperate with them to perform the desired task.

ComputerPress 6"1999

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, System Administrator must understand the principles of the OSI reference model, just so that you can 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 most top level and it is necessary to remember this when solving emerging problems.

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. TCP protocol provides a guarantee 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.