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

Data processing starts from the application layer. After that, the data passes through all layers of the reference model, and through the physical layer is sent to the communication channel. At the reception, the reverse processing of the data takes place.

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

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

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

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

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

Layers of the OSI model:

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

After getting acquainted with the reference model, we will consider the TCP / IP protocol stack.

The TCP/IP model defines four layers. As you can see from the figure above, one TCP / IP layer can correspond to several layers of the OSI model.

Layers of the TCP/IP model:

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

Encapsulation is a method of packing a data packet, in which the service headers of the packet, independent of each other, are abstracted from the headers of lower levels by including them in higher levels.

Consider on specific example. Suppose we want to get from the computer to the site. To do this, our computer must prepare an http request to receive the resources of the web server on which the page of the site we need is stored. At the application layer, an HTTP header is added to the data (Data) of the browser. Further, at the transport level, a TCP header is added to our packet, containing the port numbers of the sender and recipient (port 80 for HTTP). At the network level, an IP header is formed containing the IP addresses of the sender and recipient. Immediately before transmission, an Ethernet header is added at the data link layer, which contains the physical (MAC addresses) of the sender and recipient. After all these procedures, the packet in the form of bits of information is transmitted over the network. On admission, the process is reversed. The web server at each level will check the corresponding header. If the check is successful, then the header is discarded and the packet goes to top level. Otherwise, the entire packet is dropped.

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The model consists of 7 levels located one above the other. Layers interact with each other (vertically) through interfaces, and can interact with a parallel layer of another system (horizontally) through protocols. Each level can interact only with its neighbors and perform functions assigned only to it. More details can be seen in the figure.

Application (Application) level (eng. application layer)

The upper (7th) level of the model provides interaction between the network and the user. The layer allows user applications to access network services such as database query handler, file access, email forwarding. It is also responsible for the transfer of service information, provides applications with information about errors and generates requests to presentation layer. Example: POP3, FTP.

Executive (Presentation layer) presentation layer)

This layer is responsible for protocol conversion and data encoding/decoding. It converts application requests received from the application layer into a format for transmission over the network, and converts data received from the network into a format understandable by applications. At this level, compression/decompression or encoding/decoding of data can be performed, as well as redirecting requests to another network resource if they cannot be processed locally.

Layer 6 (representations) of the OSI reference model is usually an intermediate protocol for converting information from neighboring layers. This allows the exchange between applications on dissimilar computer systems transparent to applications. The presentation layer provides formatting and transformation of the code. Code formatting is used to ensure that the application receives information for processing that makes sense to it. If necessary, this layer can translate from one data format to another. The presentation layer deals not only with the formats and presentation of data, it also deals with the data structures that are used by programs. Thus, layer 6 provides for the organization of data during its transfer.

To understand how this works, imagine that there are two systems. One uses extended binary ASCII information interchange code (used by most other computer manufacturers) to represent data. If these two systems need to exchange information, then a presentation layer is needed to perform the transformation and translate between the two different formats.

Another function performed at the presentation layer is data encryption, which is used in cases where it is necessary to protect transmitted information from being received by unauthorized recipients. To accomplish this task, the processes and code at the view level must perform data transformations. At this level, there are other subroutines that compress texts and convert graphic images into bitstreams so that they can be transmitted over the network.

Presentation level standards also define how to present graphic images. For this purpose, the PICT format, an image format used to transfer QuickDraw graphics between programs for Macintosh and PowerPC computers, can be used. Another representation format is the tagged JPEG image file format.

There is another group of presentation level standards that define the presentation of sound and movies. These include the MPEG electronic musical instrument interface used to compress and encode CD-ROM videos, store them digitally, and transmit at speeds up to 1.5 Mbps, and session layer)

The 5th level of the model is responsible for maintaining the communication session, allowing applications to interact with each other for a long time. The layer manages session creation/termination, information exchange, task synchronization, determination of the right to transfer data, and session maintenance during periods of application inactivity. Transmission synchronization is ensured by placing checkpoints in the data stream, starting from which the process resumes if the interaction is broken.

The transport layer transport layer)

The 4th level of the model is designed to deliver data without errors, losses and duplication in the sequence in which they were transmitted. At the same time, it does not matter what data is transferred, from where and where, that is, it provides the transmission mechanism itself. It divides data blocks into fragments, the size of which depends on the protocol, combines short ones into one, and splits long ones. Protocols of this layer are designed for point-to-point interaction. Example: UDP.

There are many classes of transport layer protocols, ranging from protocols that provide only basic transport functions (for example, data transfer functions without acknowledgment), to protocols that ensure that multiple data packets are delivered to the destination in the correct sequence, multiplex multiple data streams, provide data flow control mechanism and guarantee the validity of the received data.

Some network layer protocols, called connectionless protocols, do not guarantee that data is delivered to its destination in the order in which it was sent by the source device. Some transport layers deal with this by collecting data in the right order before passing it to the session layer. Multiplexing (multiplexing) data means that the transport layer is able to simultaneously process multiple data streams (streams may come from different applications) between two systems. A flow control mechanism is a mechanism that allows you to regulate the amount of data transferred from one system to another. Transport layer protocols often have the function of data delivery control, forcing the system receiving data to send acknowledgments to the transmitting side that data has been received.

The network layer network layer)

The 3rd layer of the OSI network model is designed to determine the data transfer path. Responsible for translating logical addresses and names into physical ones, determining the shortest routes, switching and routing, monitoring network problems and congestion. A network device such as a router operates at this level.

Network layer protocols route data from source to destination and can be divided into two classes: connectionless and connectionless protocols.

You can describe the operation of protocols with the establishment of a connection using the example of a conventional telephone. Protocols of this class begin data transmission by invoking or setting the path of packets from source to destination. After that, the serial data transfer is started and then, at the end of the transfer, the connection is disconnected.

Connectionless protocols that send data containing complete address information in each packet work similarly to the mail system. Each letter or package contains the address of the sender and the recipient. Next, each intermediate post office or network device reads the address information and makes a decision about data routing. A letter or data packet is transmitted from one intermediate device to another until it is delivered to the recipient. Connectionless protocols do not guarantee that information will arrive to the recipient in the order in which it was sent. The transport protocols are responsible for setting up the data in the appropriate order when using connectionless network protocols.

Link layer data link layer)

This layer is designed to ensure the interaction of networks at the physical layer and control errors that may occur. It packs the data received from the physical layer into frames, checks for integrity, corrects errors if necessary (sends a repeated request for a damaged frame) and sends it to the network layer. The link layer can interact with one or more physical layers, controlling and managing this interaction. The IEEE 802 specification divides this level into 2 sublevels - MAC (Media Access Control) regulates access to the shared physical medium, LLC (Logical Link Control) provides network level service.

In programming, this level represents the driver network board, operating systems have a programming interface for the interaction of the channel and network layers with each other, this is not a new level, but simply an implementation of a model for a specific OS. Examples of such interfaces: ODI,

The physical layer physical layer)

The lowest level of the model is intended directly for the transfer of data flow. Carries out the transmission of electrical or optical signals to a cable or radio air and, accordingly, their reception and conversion into data bits in accordance with the methods of encoding digital signals. In other words, it provides an interface between a network carrier and a network device.

Sources

• Alexander Filimonov Building multiservice Ethernet networks, bhv, 2007 ISBN 978-5-9775-0007-4
• Unified Networking Technology Guide //cisco systems, 4th edition, Williams 2005 ISBN 584590787X

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See what the "OSI Model" is in other dictionaries:

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- (TCP/IP Model) (Department of Defense US Department of Defense) is a network interaction model developed by the US Department of Defense, the practical implementation of which is the TCP/IP protocol stack. Contents 1 Levels ... Wikipedia

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Books

• Computer networks. Textbook for students of institutions of secondary vocational education. Grif of the Ministry of Defense of the Russian Federation, Maksimov Nikolai Veniaminovich, 464 pages. The issues of organization of network architectures, types, topology, access methods, transmission medium, hardware components of computer networks, as well as packet transmission methods are considered ... Category: Manuals and reference books Series: Vocational Education Publisher:

Alexander Goryachev, Alexey Niskovsky

In order for the servers and clients of the network 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 referred to as 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 the list of tasks and data transfer rules. 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). It is believed that two computers can communicate with each other at a particular level of the OSI model if they software, which implements the network functions of this layer, interprets the same data in the same way. In this case, a direct interaction is established between the two computers, called "point-to-point".

Implementations of the OSI model by protocols are called stacks (sets) of protocols. Within one particular protocol, it is impossible to implement all the functions of the OSI model. Typically, the tasks of a particular layer are implemented by one or more protocols. Protocols from the same stack should work on one computer. In this case, a computer can simultaneously use several protocol stacks.

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

Physical layer

At this level of the OSI model, the following characteristics of network components are defined: types of connections of 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 physical layer protocols of the OSI model 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 media-specific. The connection of the following network equipment is usually associated with the physical layer:

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

This model layer defines the physical topologies in an enterprise network, which are built using a basic 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 of the devices connected to the bus, the signal is transmitted in both directions. To remove the signal from the cable, special breakers (terminators) must be used at the ends of the bus. Mechanical damage to the line affects the operation of all devices connected to it.

Ring topology provides for the connection of all network devices and computers in the physical ring (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 single ring media will affect the operation of all devices, however, networks built using a double ring usually have a margin for fault tolerance and self-healing functions. In networks built on a double ring, the same information is transmitted around the ring in both directions. In the event of a cable failure, the ring will continue to operate in single ring mode for double the 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 by beams (separate cables). Networks built on a star topology have a single point of failure. This point is the central device. In the event of a failure of the central device, 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 the 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 device is connected - the recipient of information.

Fully connected (mesh) topology has a high fault tolerance. When building networks with a similar topology, 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, this topology is rarely used in small networks, but in large corporate networks, a fully meshed 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 (cellular). In it, network devices and computers are combined into zones - cells (cell), interacting only with the transceiver of the cell. The transfer of information between cells is carried out by transceivers.

Link layer

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

Link layer protocols define:

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

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

The following network connectors are typically associated with the 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):

• control of access to the transmission medium (Media Access Control, MAC);
• logical link control (Logical Link Control, LLC).

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

The abbreviation MAC is also used when defining the physical address of a network device: the physical address of a device (which is determined internally by a network device or network card at the manufacturing stage) is often referred to as the MAC address of that device. For a large number network devices, especially network cards, it is possible to programmatically change the MAC address. At the same time, it must be remembered that the link layer of the OSI model imposes restrictions on the use of MAC addresses: in one physical network (segment of a larger network), there cannot be two or more devices using the same MAC addresses. The concept of "node address" can be used to determine the physical address of a network object. The host address most often matches the MAC address or is determined logically by software address reassignment.

The LLC sublayer defines the transmission and connection service synchronization rules. This link layer sublayer works 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 defines the way 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 functions. The actual data path is determined by the logical topology. To transfer data along a logical path, which may differ from the path in the physical medium, network connection devices and media access schemes are used. A good example of the difference between physical and logical topologies is IBM's Token Ring network. Token Ring LANs often use copper cable, which is laid in a star-shaped circuit with a central splitter (hub). Unlike a normal star topology, the hub does not forward incoming signals to all other connected devices. The internal circuitry of the hub sequentially sends each incoming signal to the next device in a predetermined logical ring, that is, in a circular pattern. The physical topology of this network is a star, and the logical topology is a ring.

Another example of the difference 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 involves 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 bus topology has been formed.

To determine the logical network topology, 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 media.

Media Access

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

Network collisions extend to the physical network segments to which network objects are connected. Such connections form a single collision space, in which the influence 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 functions at the link layer.

A network cannot function normally until all network entities can control, manage, or mitigate collisions. In networks, some method is needed to reduce the number of collisions, interference (overlay) of simultaneous signals.

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

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

• the nature of the transmissions - continuous or impulse;
• number of data transfers;
• the need to transfer data at strictly defined time intervals;
• the number of active devices on the network.

Each of these factors, combined with 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, each network device competes for control over the transmission medium. Race systems are designed so that all devices on the network can only transmit data as needed. This practice eventually results in partial or complete loss of data 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 the performance of the network, and in the event of complete saturation of the information transmission medium, it reduces the network's 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 start of data transmission by the station. If the listening station detects a signal transmission (from another station), then it refrains from transmitting the information and will try to repeat it later. These protocols are called Carrier Sense Multiple Access (CSMA) protocols. CSMA protocols significantly reduce the number of collisions, but do not completely eliminate them. Collisions do occur, however, when two stations interrogate the cable: they detect no signals, decide that the medium is free, and then start transmitting at the same time.

Examples of such contention protocols are:

• multiple access with carrier control / collision detection (Carrier Sense Multiple Access / Collision Detection, CSMA / CD);
• multiple access with carrier control / collision avoidance (Carrier Sense Multiple Access / Collision Avoidance, CSMA / CA).

CSMA/CD protocols. The CSMA/CD protocols not only listen on the cable before transmitting, but also detect collisions and initiate retransmissions. When a collision is detected, the stations that transmitted data initialize special internal timers with random values. The timers start counting down, and when zero is reached, the stations must try to retransmit the 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 medium is already busy and 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 such schemes as time slicing access or sending a request for access to the medium. When using time slicing, each station can transmit information only at times strictly defined for this station. At the same time, the mechanism for managing time slices must be implemented in the network. Each new station connected to the network announces its appearance, thereby initiating the process of redistribution of time slices for information transmission. In the case of using centralized media access control, each station generates a special request for transmission, 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.

Race-based systems are best suited for bursty traffic (large file transfers) on networks with relatively few users.

Systems with the transfer of the marker. 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 media control to the device that owns the token. Passing the token distributes access control among devices on the network.

Each device knows which device it is receiving the token from and to which device it should pass it on. Usually such devices are the nearest neighbors of the owner of the token. Each device periodically takes control of the token, performs its actions (transmits information), and then passes the token to the next device for use. Protocols limit the amount of time a token can be controlled by each device.

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 dependent 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 singles out one device (called the controller, primary, or "master" device) as media access arbiter. This device polls all other devices (secondaries) in some predefined order to see if they have information to send. To receive data from a secondary device, the primary device sends an appropriate request to it, and then receives data from the secondary device and sends it to the recipient device. Then the primary device 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 being polled. Polling systems are ideal for time-sensitive network devices such as plant automation.

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

• service without confirmation and without establishing connections (unacknowledged connectionless) - sends and receives frames without flow control and without error control or packet sequence;
• connection-oriented service - provides flow control, error control and packet sequence through the issuance of receipts (confirmations);
• Acknowledged connectionless service - uses tickets to control flow and control errors in transmissions between two network nodes.

The LLC sublayer of the link layer provides the ability to simultaneously use several network protocols (from different protocol stacks) when working 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 layer defines the rules for data delivery between logical networks, the formation of logical addresses of network devices, the definition, selection and maintenance of routing information, the functioning of gateways (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 in general similar to data delivery at the data link layer of the OSI model, where physical addressing of devices is used to transfer data. However, link-layer addressing refers only to one logical network, and is valid only within this network. The network layer describes the methods and means of transferring information between many independent (and often heterogeneous) logical networks, which, when connected together, form one large network. Such a network is called an interconnected network (internetwork), and the processes of information transfer between networks are called internetworking.

With the help of physical addressing at the data link layer, data is delivered to all devices that are part of the same logical network. Each network device, each computer determines the destination of the received data. If the data is intended for the computer, then it processes it; if not, it ignores it.

In contrast to the link layer, the network layer can choose a specific route in the internetwork and avoid sending data to those logical networks to which the data is not addressed. The network layer does this through switching, network layer addressing, and using routing algorithms. The network layer is also responsible for providing the correct paths for data across the internetwork, which is made up of heterogeneous networks.

The elements and methods for implementing the network layer are defined as follows:

• all logically separate networks must have unique network addresses;
• switching defines how connections are established 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 internetwork.

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

The network layer defines the rules for generating logical network addresses for network objects. Within a large internetwork, each network object must have a unique logical address. Two components are involved in the formation of the logical address: the logical address of the network, which is common to all network objects, and the logical address of the network object, which is unique for 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 the transfer of data between different logical networks.

Each network object, each computer can perform many network functions simultaneously, providing the operation of various services. To access services, a special service identifier is used, which is called a port (port), or a socket (socket). When accessing a service, the service identifier immediately follows the logical address of the computer that is running 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, it will be sent to a special broadcast address.

The network layer defines the rules for transferring data between two network entities. This transmission may be performed using switching or routing.

There are three methods of switching in 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, the workload of the switching equipment or the busyness of the recipient.

Message switching allows the transmission of a whole (not broken into parts) message on a store-and-forward basis. Each intermediate device receives a message, stores it locally, and, when the communication channel through which this message is to be sent, is released, sends it. This method is well suited for sending e-mail messages and organizing electronic document management.

When using packet switching, the advantages of the two previous methods are combined. Each great message is broken into small packets, each of which is sequentially sent to the recipient. When passing through the internetwork, for each of the packets, the best path at that moment in time is determined. It turns out that parts of one message can reach the recipient at different times, and only after all the parts are put together, the recipient will be able to work with the received data.

Each time a data path is determined, the best path must be chosen. 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 defining static routing, all relationships between logical networks must be defined 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 operates when the link layer LLC sublayer connection service of the OSI model is not used.

When building an internetwork, you have to connect logical networks built using different technologies and providing a variety of services. For a network to work, logical networks must be able to correctly interpret data and control information. This task is solved with the help of a gateway, which is a device or an 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 layer of the OSI model, but they are most often implemented at the upper layers of the model.

transport layer

The transport layer allows you to hide the physical and logical structure of the network from the applications of 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 the logical and physical networks are implemented at the previous levels, where the transport layer transmits data.

The transport layer often compensates for the lack of a reliable or connection-oriented connection service in the 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 of the failure to deliver. Upper levels can then take the necessary corrective action or provide the user with a choice.

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

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

In the first case, the network user accesses a service by its logical name, without knowing the exact location of the service. The user does not know if this service is available in this moment. When accessed, the logical name is mapped to the physical name, and the user's workstation initiates a call directly to the service. In the second case, each service announces itself to all network clients on a periodic basis. Each of the clients at any given time knows whether the service is available and can access the service directly.

Addressing methods

Service addresses identify specific software processes running on network devices. In addition to these addresses, service providers keep track of the various conversations they have with devices requesting services. Two different methods dialogs use the following addresses:

• connection identifier;
• transaction ID.

A connection identifier, also called a connection ID, port, or socket, identifies each conversation. With 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 particular dialog.

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

session layer

The session layer facilitates interaction between devices requesting and providing services. Communication sessions are controlled through mechanisms that establish, maintain, synchronize, and manage a conversation between communicating entities. This layer also helps upper layers identify and connect to an available network service.

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

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 conversation control using one of three communication modes - simplex, half duplex, and full duplex.

Simplex communication involves only one-way transmission from the source to the receiver of information. No feedback(from the receiver to the 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 provides simultaneous transmission of information in both directions over the data transmission medium.

The administration of a communication session between two network entities, consisting of establishing a connection, transferring data, terminating a connection, is also performed at this layer of the OSI model. After the session is established, the software that implements the functions of this level can check the health (maintain) the connection until it is terminated.

Presentation Layer

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

Transformation refers to changing the order of bits in bytes, the order of bytes in a word, character codes, and the syntax of file names.

The need to change the order of bits and bytes is due to the presence of a large number of various 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 highest bit, 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 level ensures the correct transformation of the file syntax. Different operating systems work differently with 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 the file information in its own way, but the meaning of the information should not change.

The presentation layer converts the data into a mutually agreed upon format (an exchange syntax) that is understandable by all network applications and the computers running the applications. It can also compress and decompress, as well as encrypt and decrypt data.

Computers use different rules for representing data with binary 0s and 1s. 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 some of the data protection methods. Protection is often performed by data scrambling, which uses one or more of the three methods: permutation, substitution, algebraic method.

Each of these methods is just a special way of protecting data in such a way that it can only be understood by those who know the encryption algorithm. Data encryption can be performed both in hardware and in software. However, end-to-end data encryption is usually done in software and is considered part of the functionality of the presentation layer. 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 is changed, all devices must be modified (preferably not using 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. The 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 passed to it to establish a mutually acceptable encryption value.

Possession of only the public key is of little use to unauthorized users. The complexity of the resulting encryption key is large enough to be computed in a reasonable amount of time. Even knowing your own secret key and someone else's public key not much help to determine another 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 six lower layers combine the tasks and technologies that provide overall support for the 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 basic mechanisms for identifying offered services are provided by elements such as service addresses. In addition, servers use such methods of presenting their service as active and passive service presentation.

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

Servers implement a passive service advertisement by registering their service and address in the directory. When clients want to determine which services are available, they simply query the directory for a location. desired service and about his address.

Before a network service can be used, it must be 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:

• call pickup operating system;
• remote mode;
• collaborative 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 assumes that given file is on local storage. In effect, a special piece of software intercepts a request to read a file before it reaches the local operating system (DOS) and forwards the request to a network file service.

At the other extreme, in Remote Operation, the local operating system is aware of the network and is responsible for forwarding requests to the network service. However, the server does not know anything about the client. To the server operating system, all requests to a service look the same 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 usage is typically required for peer-to-peer collaborative data processing. Collaborative data processing involves the sharing of 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

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

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

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

chief editor

model

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

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

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

Rice. 1.1. ISO/OSI Open Systems Interoperability Model

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

An end user application can use system communication tools not only to establish a dialogue with another application running on another machine, but simply to receive the services of a particular network service, such as accessing remote files, receiving mail, or printing on a shared printer.

So, let the application make a request to the application layer, for example, to a file service. Based on this request, the application layer software generates a message in a standard format, in which it places service information (header) and, possibly, transmitted data. This message is then sent to the representative layer. The presentation layer adds its header to the message and passes the result down to the session layer, which in turn adds its header, and so on. Some implementations of the protocols provide for the presence in the message not only of the header, but also of the trailer. Finally, the message reaches the lowest, physical layer, which actually transmits it over the communication lines.

When a message arrives over the network at another machine, it moves up sequentially from layer to layer. Each level analyzes, processes and removes the header of its level, performs the corresponding given level functions and passes the message to the upper layer.

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

Layer Functions of the ISO/OSI Model

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

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

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

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

The link layer protocols used in local networks have a certain structure of connections between computers and ways of addressing them. Although the link layer provides frame delivery between any two nodes of the local network, it does this only in a network with a completely defined link topology, exactly the topology for which it was designed. Common bus, ring, and star topologies supported by LAN link layer protocols are common. Examples of link layer protocols are Ethernet, Token Ring, FDDI, 100VG-AnyLAN protocols.

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

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

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

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

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

Networks are interconnected by special devices called routers. router is a device that collects information about the topology of interconnections and, based on it, forwards network layer packets to the destination network. In order to transfer a message from a sender located in one network to a recipient located in another network, it is necessary to make a certain number of transit transmissions (hops) between networks, each time choosing the appropriate route. Thus, a route is a sequence of routers through which a packet passes.

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

The network layer defines two kinds of protocols. The first type refers to the definition of rules for the transmission of packets with data of end nodes from a node to a router and between routers. It is these protocols that are usually referred to when talking about network layer protocols. The network layer also includes another type of protocol called routing information exchange protocols. Routers use these protocols to collect information about the topology of interconnections. Network layer protocols are implemented by software modules of the operating system, as well as software and hardware of routers.

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

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

The choice of the class of service of the transport layer is determined, on the one hand, by the extent to which the task of ensuring reliability is solved by the applications themselves and protocols higher than the transport layers, and on the other hand, this choice depends on how reliable the entire data transportation system is. online. So, for example, if the quality of communication channels is very high, and the probability of occurrence of errors not detected by lower layer protocols is small, then it is reasonable to use one of the lightweight transport layer services that are not burdened with numerous checks, handshaking and other methods of improving reliability. If vehicles are initially very unreliable, it is advisable to turn to the most developed transport layer service that works using the maximum means for detecting and eliminating errors - using the preliminary establishment of a logical connection, message delivery control using checksums and round-robin numbering of packets, setting delivery timeouts, and the like.

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

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

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

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

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

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

Feature of popular communication protocol stacks

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

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

The use of one or another stack of communication protocols in the network largely determines the face of the network and its characteristics. In small networks, only one stack can be used. In large corporate networks that combine different networks, as a rule, several stacks are used in parallel.

Communication equipment implements lower layer protocols that are more standardized than upper layer protocols, and this is a prerequisite for successful joint work equipment from various manufacturers. The list of protocols supported by a particular communication device is one of the most important characteristics of this device.

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

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

At the lower levels - physical and channel - almost all stacks use the same protocols. These are well-standardized Ethernet, Token Ring, FDDI and some others protocols that allow using the same equipment in all networks.

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

OSI stack

A distinction should be made between the OSI protocol stack and the OSI model. While the OSI model conceptually defines the procedure for the interaction of open systems, decomposing the task into 7 levels, standardizes the purpose of each level and introduces standard names for the levels, the OSI stack is a set of very specific protocol specifications that form an agreed protocol stack. This protocol stack is supported by the US government in its GOSIP program. All computer networks Post-1990 government installations must either directly support the OSI stack or provide the means to migrate to that stack in the future. However, the OSI stack is more popular in Europe than in the US, as there are fewer old networks installed in Europe that use their own protocols. There is also a strong need for a common stack in Europe, as there are a large number of different countries.

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

For obvious reasons, the OSI stack, unlike other standard stacks, fully complies with the OSI Interoperability Model, it includes specifications for all seven layers of the Open Systems Interconnection Model (Figure 1.3).

Rice. 1.3. OSI stack

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

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

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

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

The aim of the recommendations X.500 is the development of global standards help desk. The process of delivering a message requires knowledge of the recipient's address, which is a problem with large networks, so it is necessary to have a help desk to help you get the addresses of senders and recipients. In general, an X.500 service is a distributed database of names and addresses. All users are potentially eligible to log into this database using a certain set of attributes.

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

• reading - getting an address by a known name,
• query - getting a name from known address attributes,
• modification, including the removal and addition of records in the database.

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

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

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

Transfer and Job Control Protocol JTM allows users to submit jobs to be completed on the host computer. The job control language, which provides job transfer, tells the host computer what to do and with what programs and files. The JTM protocol supports traditional batch processing, transaction processing, remote job entry, and access to distributed databases.

TCP/IP stack

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

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

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

Since the TCP/IP stack was developed before the advent of the ISO/OSI open systems interworking model, although it also has a layered structure, the correspondence between the levels of the TCP/IP stack and the levels of the OSI model is rather arbitrary.

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

Rice. 1.4. TCP/IP stack

lowest ( level IV ) - the level of gateway interfaces - corresponds to the physical and data link layers of the OSI model. This level is not regulated in TCP/IP protocols, but it supports all popular physical and data link level standards: for local channels, these are Ethernet, Token Ring, FDDI; point-to-point connections via WAN serial links, and X.25 and ISDN area network protocols. A special specification has also been developed that defines the use of ATM technology as a link layer transport.

Next level ( level III ) is the internetworking layer that deals with the transmission of datagrams using various local area networks, X.25 territorial networks, ad hoc links, etc. As the main protocol of the network layer (in terms of the OSI model), the protocol used in the stack is IP, which was originally designed as a protocol for transmitting packets in composite networks, consisting of a large number of local networks, united by both local and global links. Therefore, the IP protocol works well in networks with a complex topology, rationally using the presence of subsystems in them and economically spending throughput low speed communication lines. The IP protocol is a datagram protocol.

The internetworking layer also includes all protocols related to the compilation and modification of routing tables, such as protocols for collecting routing information. RIP(Routing Internet Protocol) and OSPF(Open Shortest Path First), as well as the Internet Control Message Protocol ICMP(Internet Control Message Protocol). The latter protocol is designed to exchange information about errors between the router and the gateway, the source system and the receiver system, that is, to organize feedback. With the help of special ICMP packets, it is reported about the impossibility of delivering a packet, about exceeding the lifetime or duration of the packet assembly from fragments, about abnormal parameter values, about changing the forwarding route and type of service, about the state of the system, etc.

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

Top level ( level I) is called applied. Over the years of use in the networks of various countries and organizations, the TCP / IP stack has accumulated a large number of protocols and application-level services. These include widely used protocols such as the Copy Protocol FTP files, the telnet terminal emulation protocol, the SMTP mail protocol used in the e-mail of the Internet and its Russian branch RELCOM, hypertext services for accessing remote information, such as WWW, and many others. Let us dwell in more detail on some of them, which are most closely related to the subject of this course.

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

File Transfer Protocol FTP(File Transfer Protocol) implements remote file access. In order to ensure reliable transmission, FTP uses the connection-oriented protocol - TCP - as a transport. Besides the file transfer protocol, FTP offers other services. So the user is given the opportunity to interact with a remote machine, for example, he can print the contents of its directories, FTP allows the user to specify the type and format of the stored data. Finally, FTP performs user authentication. Users are required by protocol to provide their username and password before accessing the file.

Within the TCP/IP stack, FTP offers the most extensive file services, but it is also the most complex to program. Applications that do not need all the features of FTP can use another, more economical protocol - the simplest file transfer protocol TFTP(Trivial File Transfer Protocol). This protocol implements only file transfer, and the connectionless protocol, UDP, which is simpler than TCP, is used as a transport.

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

IPX/SPX stack

This stack is Novell's original protocol stack that it developed for its NetWare network operating system back in the early 1980s. The Internetwork Packet Exchange (IPX) and Sequenced Packet Exchange (SPX) protocols that gave the stack its name are direct adaptations of Xerox's XNS protocols, which are much less common than IPX/SPX. The IPX/SPX protocols lead in terms of installations, and this is due to the fact that the NetWare OS itself occupies a leading position with a share of installations on a global scale of about 65%.

The family of Novell protocols and their correspondence to the ISO/OSI model is shown in Figure 1.5.

Rice. 1.5. IPX/SPX stack

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

On network layer protocol running on Novell stack IPX, as well as routing information exchange protocols RIP And NLSP(similar to the OSPF protocol of the TCP/IP stack). IPX is the protocol that deals with the addressing and routing of packets on Novell networks. IPX's routing decisions are based on the address fields in its packet header, as well as information from routing information exchange protocols. For example, IPX uses information provided by either RIP or NetWare Link State Protocol (NLSP) to forward packets to the destination computer or the next router. The IPX protocol supports only datagram messaging, which saves computing resources. So, the IPX protocol performs three functions: setting the address, establishing the route, and broadcasting datagrams.

The transport layer of the OSI model in the Novell stack corresponds to the SPX protocol, which implements connection-oriented messaging.

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

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

On Novell networks, NetWare 3.x servers send SAP broadcast packets every minute. SAP packets pollute the network to a large extent, so one of the main tasks of routers that go to global links is to filter the traffic of SAP packets and RIP packets.

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

This circumstance, and the fact that the IPX/SPX stack is owned by Novell and must be licensed from Novell, for a long time limited its distribution to NetWare networks only. However, by the time NetWare 4.0 was released, Novell had made, and continues to make, major changes to its protocols to make them more suitable for corporate networks. Now the IPX/SPX stack is implemented not only in NetWare, but also in several other popular network operating systems - SCO UNIX, Sun Solaris, Microsoft Windows NT.

NetBIOS/SMB stack

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

Rice. 1.6. NetBIOS/SMB stack

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

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

• Session management. Creating and breaking a logical channel between a workstation and network resources file server.
• File access. The workstation can address the file server with requests to create and delete directories, create, open and close files, read and write to files, rename and delete files, search for files, get and set file attributes, block records.
• Print service. The workstation can queue files for printing on the server and obtain information about the print queue.
• Message service. SMB supports simple messaging with the following features: send a simple message; send a broadcast message; send the beginning of a message block; send the text of the message block; send the end of the message block; send username; cancel the transfer; get machine name.

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

Why do we need this valuable knowledge? (editorial)

Once a colleague asked me a tricky question. Well, he says, you know what the OSI model is ... And why do you need it, what is the practical use of this knowledge: is it possible to show off in front of dummies? False, the benefits of this knowledge are systems approach when solving many practical problems. For example:

• trouble shooting (