osi network reference model. Open Systems Interconnection (OSI) model

access to the network environment. In the same time, link layer manages the process of placing transmitted data in the physical environment. That's why link layer divided into 2 sublevels (Fig. 5.1): upper sublevel control of the logical data transmission channel( Logical Link Control - LLC), which is common to all technologies, and the lower sublevel media access control(Media Access Control - MAC). In addition, link layer tools allow you to detect errors in transmitted data.

Rice. 5.1.

The interaction of local network nodes occurs on the basis of link layer protocols. Data transmission in local networks occurs over relatively short distances (inside buildings or between closely located buildings), but at high speed (10 Mbit/s - 100 Gbit/s). Distance and transmission speed data is determined by the equipment of the corresponding standards.

International Institute of Electrical and Electronics Engineers - IEEE) the 802.x family of standards was developed, which regulates the functioning of the data link and physical layers of the seven-layer ISO/OSI model. A number of these protocols are common to all technologies, for example the 802.2 standard; other protocols (for example, 802.3, 802.3u, 802.5) define the features of local network technologies.

LLC sublayer being implemented software. At the LLC sublayer, there are several procedures that allow you to establish or not establish communication before transmitting frames containing data, to restore or not to restore frames if they are lost or errors are detected. Sublevel LLC implements communication with network layer protocols, usually with the IP protocol. Communication with the network layer and the definition of logical procedures for transmitting frames over the network implements the 802.2 protocol. The 802.1 protocol provides a general definition of local area networks, related to the ISO/OSI model. There are also modifications of this protocol.

The MAC sublayer determines the features of access to the physical medium when using various local network technologies. Each MAC layer technology (each protocol: 802.3, 802.3u, 802.3z, etc.) corresponds to several variants of physical layer specifications (protocols) (Fig. 5.1). Specification MAC layer technology - defines the physical layer environment and the basic parameters of data transfer ( transmission speed, type of medium, narrowband or broadband).

At the link level of the transmitting side, it is formed frame, in which package is encapsulated. The encapsulation process adds a frame header and trailer to a network protocol packet, such as IP. Thus, the frame of any network technology consists of three parts:

header,
data fields where the package is located,
limit switch.

On the receiving side, the reverse decapsulation process is implemented when a packet is extracted from the frame.

Heading includes frame delimiters, address and control fields. Separators frames allow you to determine the beginning of a frame and ensure synchronization between the transmitter and receiver. Addresses link layer are physical addresses. When using Ethernet-compatible technologies, data addressing in local networks is carried out by MAC addresses, which ensure delivery of the frame to the destination node.

End cap contains a checksum field ( Frame Check Sequence - FCS), which is calculated when transmitting a frame using a cyclic code CRC. On the receiving side check sum frame is calculated again and compared with the received one. If they match, then they consider that the frame was transmitted without errors. If the FCS values diverge, the frame is discarded and must be retransmitted.

When transmitted over a network, a frame sequentially passes through a number of connections characterized by different physical environments. For example, when transmitting data from Node A to Node B (Fig. 5.2), the data sequentially passes through: the Ethernet connection between Node A and Router A (copper, unshielded twisted pair), the connection between Routers A and B (fiber optic cable), a point-to-point serial copper cable between Router B and the wireless access point WAP, a wireless connection (radio link) between the WAP and end Node B. Therefore each connection has its own frame specific format.

Rice. 5.2.

The packet prepared by Node A is encapsulated into a local network frame, which is transmitted to Router A. The router decapsulates the packet from the received frame, determines which output interface to send the packet to, then forms a new frame for transmission over the optical medium. Router B decapsulates the packet from the received frame, determines which egress interface to forward the packet to, then generates a new frame for transmission over the point-to-point serial copper medium. The wireless access point WAP, in turn, forms its own frame for transmitting data over the radio channel to the end Node B.

When creating networks, various logical topologies are used that determine how nodes communicate across the medium, how access control medium. The most well-known logical topologies are point-to-point, multiaccess, broadcast and token passing.

Sharing the environment between multiple devices is implemented based on two main methods:

method competitive (non-deterministic) access(Content-based Access), when all network nodes have equal rights, the order of data transmission is not organized. To transmit, this node must listen to the medium; if it is free, then information can be transmitted. In this case, conflicts may arise ( collisions) when two (or more) nodes simultaneously begin transmitting data;
method controlled (deterministic) access(Controlled Access), which provides nodes with priority access to the medium for data transmission.

In the early stages of the creation of Ethernet networks, a “bus” topology was used, a shared data transmission medium was common to all users. In this case, the method was implemented multiple access to a common transmission medium (802.3 protocol). This required carrier control, the presence of which indicated that some node was already transmitting data over a common medium. Therefore, a node wishing to transfer data had to wait for the end of the transfer and, when the medium became free, try to transfer the data.

The information transmitted to the network can be received by any computer whose NIC network adapter address matches the destination MAC address of the transmitted frame, or by all computers on the network during broadcast transmission. However, only one node can transmit information at any time. Before transmitting, a node must ensure that the common bus is free by listening to the medium.

When two or more computers transmit data at the same time, a conflict occurs ( collision) when the data of transmitting nodes overlap each other, distortion occurs and loss of information. Therefore, collision processing and retransmission of the frames involved in the collision are required.

Similar method non-deterministic(associative) access by Wednesday received the name Multiple Media Access with Carrier Sense and Collision Detection( Carrier Sense Multiply Access

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.

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

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

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

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

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).

Data Link Layer

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 some program- the upper sublayer of the channel layer, through which it is possible to communicate with the lower levels, or rather with the microprocessor ( iron) – lower sublayer of the data 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.

Physical layer

Conclusion

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.

Ring topology involves connecting 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 of a 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 enterprise networks a fully mesh topology can be used to connect the most important nodes.

This level determines the logical topology of the network, the rules for gaining access to the data transmission medium, resolves issues related to addressing physical devices within the logical network and managing the transfer of information (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 functions of the link layer are divided into two sublevels (Table 1):

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

The MAC sublayer defines such link layer elements as the logical network topology, the method of access 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.

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.

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.
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 Apple Computer's LocalTalk protocol.

Contention-based systems are most suitable for use with bursty traffic (large file transfers) 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.
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 several network protocols (from different protocol stacks) when operating through a single network interface. In other words, if only one network card is installed in the computer, 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.

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.

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.

Every time you determine the next path for data, you must choose the best route.

The task of determining the best path is 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 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 layers, where the 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 currently available. When contacting, the logical name is matched to the physical name, and the user's workstation 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 running 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.

Simplex communication involves only unidirectional transmission of information from the source to the receiver. This method of communication does not provide any feedback (from receiver to source). 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 work differently with their file systems and 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.

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.

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 accomplishing this task, 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 thinks that the file is on the local storage device. In effect, a special piece of software intercepts the request to read the file 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
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 means of 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 an interoperability standard.
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 transmission 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

Application layer

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 transmits 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 looked at 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 with 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).

Just because a protocol is an agreement adopted by two interacting entities, in this case two computers operating on a network, does not mean that it is necessarily standard. But in practice, when implementing networks, they usually use standard protocols. These can be branded, national or international standards.

In the early 80s, a number of international standardization organizations - ISO, ITU-T and some others - developed a model that played a significant role in the development of networks. This model is called the ISO/OSI model.

Open Systems Interoperability Model (Open System Interconnection, OSI) defines different levels of interaction between systems in packet switching networks, gives them standard names and specifies what functions each layer should perform.

The OSI model was developed based on extensive experience gained from creating computer networks, mainly global ones, in the 70s. A full description of this model takes up more than 1000 pages of text.

In the OSI model (Fig. 11.6), communication means are divided into seven levels: application, representative, session, transport, network, channel and physical.

Each layer deals with a specific aspect of network device interaction.

Rice. 11.6. The OSI model describes only system communications implemented by the operating system, and hardware. The model does not include means for end-user application interaction. Applications implement their own communication protocols by accessing system tools. Therefore, it is necessary to distinguish between the level of interaction between applications and application layer.

It should also be kept in mind that the application can take over the functions of some of the upper layers of the OSI model. For example, some DBMSs have built-in tools remote access to files. In this case, the application does not use the system file service when accessing remote resources; it bypasses the upper layers of the OSI model and accesses directly the system facilities responsible for transportation messages over the network, which are located at the lower levels of the OSI model.

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 message in a standard format. A typical message consists of a header and a data field. The header contains service information that must be passed through the network to the application layer of the destination machine to tell it what work needs to be done. In our case, the header obviously must contain information about the location of the file and the type of operation that needs to be performed. The message data field can be empty or contain some data, such as data that needs to be written to a remote . But in order to deliver this information to its destination, there are still many tasks to be solved, the responsibility for which lies with lower levels.

After generating the message application layer sends it down the stack representative level. Protocol representative level based on information received from the application level header, performs the required actions and adds its own service information to the message - header representative level, which contains instructions for the protocol representative level destination machine. The resulting message is passed down session level, which in turn adds its header, etc. (Some protocols place service information not only at the beginning of the message in the form of a header, but also at the end, in the form of a so-called “trailer”.) Finally, the message reaches the bottom, physical level, which, in fact, transmits it via communication lines to the recipient machine. At this point, the message is “overgrown” with headers of all levels (