Did you know,   What is a thought experiment, gedanken experiment?
This is a nonexistent practice, an otherworldly experience, an imagination of what is not really there. Thought experiments are like waking dreams. They give birth to monsters. In contrast to a physical experiment, which is an experimental test of hypotheses, a “thought experiment” focally substitutes an experimental test with desired conclusions that have not been tested in practice, manipulating logic-like constructions that actually violate the logic itself by using unproven premises as proved ones, that is, by substitution. Thus, the main task of the applicants of “thought experiments” is to deceive the listener or reader by replacing the real physical experiment with his “doll” - fictitious reasoning under parole without physical verification.
Filling physics with imaginary, “thought experiments” led to the emergence of an absurd surreal, confused, confused picture of the world. A real researcher should distinguish such "candy wrappers" from real values.

Relativists and positivists argue that a “thought experiment” is a very useful tool for testing theories (also arising in our mind) for consistency. In this they deceive people, since any verification can be carried out only by a source independent of the verification object. The applicant himself of the hypothesis cannot be a test of his own statement, since the reason for this statement itself is the absence of any contradictions visible to the applicant in the statement.

We see this on the example of SRT and GRT, which have turned into a kind of religion that governs science and public opinion. No amount of facts contradicting them can overcome Einstein’s formula: “If the fact does not correspond to the theory, change the fact” (In another version “Does the fact do not correspond to the theory? - All the worse for the fact”).

The maximum that a “thought experiment” can claim is only the internal consistency of the hypothesis within the framework of the applicant’s own, often by no means true logic. Compliance with practice does not check this. This test can only take place in a valid physical experiment.

An experiment and an experiment, that it is not a refinement of thought, but a test of thought. A consistent thought within itself cannot verify itself. This is proven by Kurt Gödel.

PresetBIOS

The meaning of customizationBIOS   consists in starting the computer from the device that contains the distribution kit of the operating system. In our case, we need to make the computer boot fromDVDdrive orUSB- drive. For this we will useBIOS

Network Theoretical Information

Basic data transfer protocols

The maindata transfer protocols

As you may have noticed, the number of protocols serving the open system interaction model is quite large. Some of these protocols, especially low-level ones, are not particularly interesting in terms of getting to know their operating principle. But the principle of operation and the capabilities of other protocols are still worth knowing, especially such as TCP / IP, UDP, POP3, etc.

Protocol stacks

As mentioned above, protocol stacks are often responsible for organizing the work of all levels of the ISO / OSI model. The advantage of using protocol stacks is that all protocols included in the stack are developed by one manufacturer, that is, they are able to work as quickly and efficiently as possible.

During the existence of networks, several different protocol stacks were developed, among which the most popular are TCP / IP, IPX / SPX, NetBIOS / SMB,Novell NetWare, DECnet, etc.

The stacks contain protocols that operate at different levels of the ISO / OSI model, but usually only three types of protocols are distinguished: transport, networkand applied.

The advantage of using protocol stacks is that the protocols operating at the lower levels use long-established and popular network protocols such as Ethernet, FDDI, etc. Thanks to the hardware implementation of these protocols, it becomes possible to use the same equipment for different types of networks and thereby achieve their compatibility at the hardware level. As for high-level protocols, each of the stacks has its own advantages and disadvantages, and very often it happens that there is no rigid binding "one protocol - one level", that is, one protocol can work at once on two or three levels.

Snap

An important point in the operation of network equipment, in particular a network adapter, is protocol binding. In practice, it allows you to use different protocol stacks when servicing a single network adapter. For example, you can use TCP / IP and IPX / SPX stacks at the same time, and if an error occurs when trying to establish a connection with the destination using the first stack, it automatically switches to using the protocol from the next stack. In this case, the priority is the sequence of binding, since it uniquely affects the use of a protocol from different stacks.

Regardless of how many network adapters are installed in the computer, binding can be either one to several or several to one, that is, one protocol stack can be bound to several adapters at once or several stacks to one adapter.

TCP / IP

Protocol Stack TCP / IP (Transmission Control Protocol / Internet Protocol ) today is the most common and functional. It works in local networks of any scale. In addition, this is the only protocol that allows the global Internet.

The protocol was created in the 70s of the last century by the US Department of Defense. It was with his submission that the development of a protocol began, the purpose of which was to connect any two computers, no matter how far they were. Of course, they pursued their goal - to ensure constant communication with the control center, even if everything around was destroyed as a result of hostilities. As a result, the global ARPAnet network was formed, which the ministry actively used for its own purposes.

The impetus for further improvement and widespread use of the TCP / IP stack was the fact that its support was implemented in computers running the UNIX operating system. As a result, the popularity of TCP / IP has grown.

The TCP / IP protocol stack includes quite a few protocols operating at different levels, but it got its name thanks to two protocols - TCP and IP.

TCP (Transmission Control Protocol) is a transport protocol designed to control the transmission of data in networks using the TCP / IP protocol stack. IP (Internet Protocol) is a network layer protocol designed to deliver data on a composite network using one of the transport protocols, such as TCP or UDP. The lower level of the TCP / IP stack uses standard data transfer protocols, which makes it possible to use it in networks using any network technology and on computers with any operating system.

Initially, the TCP / IP protocol was developed for use in wide area networks, which is why it is as flexible as possible. In particular, due to the ability of packet fragmentation, data, in spite of the quality of the communication channel, in any case reaches the addressee. In addition, due to the presence of the IP protocol, it becomes possible to transfer data between heterogeneous network segments.

The disadvantage of TCP / IP protocol is the complexity of network administration. So, for the normal functioning of the network requires the presence of additional servers, such as DNS, DHCP, etc., the maintenance of which takes most of the time the system administrator.

IPX / SPX

The IPX / SPX protocol stack (Internetwork Packet Exchange / Sequenced Packet Exchange) is a development and property of Novell. It was developed for the needs of the Novell NetWare operating system, which until recently had one of the leading positions among server operating systems.

The IPX and SPX protocols work at the network and transport layers of the ISO / OSI model, respectively, and therefore complement each other perfectly. IPX can transmit data using datagrams using routing information on the network.

However, in order to transmit data along the route found, you must first establish a connection between the sender and the recipient. This is what the SPX protocol or any other transport protocol that works in conjunction with IPX does.

Unfortunately, the IPX / SPX protocol stack is initially focused on servicing small networks, therefore, its use in large networks is inefficient: excessive use of broadcasting on low-speed communication lines is unacceptable.

NetBIOS / SMB

A fairly popular protocol stack, developed by IBM and Microsoft, respectively, focused on the use in the products of these companies. Like TCP / IP, standard protocols such as Ethernet, Token Ring and others work at the physical and link layer of the NetBIOS / SMB stack, which makes it possible to use it in tandem with any active network equipment. Onat the upper levels, protocols work   NetBIOS (Network Basic Input / Output System)and SMB (Server Message Block).

The NetBIOS protocol was developed in the mid-80s of the last century, but was soon replaced by a more functional NetBEUI protocol (NetBIOS Extended User Interface), which allows organizing a very efficient exchange of information in networks of no more than 200 computers.

For exchange between computers to be possible, each of them must have a logical name. To exchange data between computers, logical names are used that are assigned to computers dynamically when they are connected to the network. In this case, the table of names applies to each computer on the network. It also supports work with group names, which allows you to transfer data to multiple destinations at once.

The main advantages of the NetBEUI protocol are its speed and very low resource requirements. If you want to organize a quick exchange of data in a small network consisting of one segment, there is no better protocol for this. In addition, for the delivery of messages, an established connection is not a requirement: in the absence of a connection, the protocol uses the datagram method, when the message is supplied with the address of the recipient and sender and “sets off”, moving from one computer to another.

However, NetBEUI also has a significant drawback: it is completely devoid of the concept of packet routing, so its use in complex composite networks does not make sense.

As for the Server Message Block (SMB) protocol, it is used to organize the network at the three highest levels - session, presentation, and application. It is with its use that it becomes possible to access files, printers, and other network resources. This protocol has been improved several times (three versions have been released), which made it possible to use it even in modern operating systems such as Microsoft Vista and Windows 7. The SMB protocol is universal and can work in tandem with almost any transport protocol, such as TCP / IP and SPX

HTTP

Perhaps the most demanded of the protocols with which tens of millions of Internet users around the world work every day.

The HTTP protocol (HyperText Transfer Protocol) was developed specifically for the Internet: for receiving and transmitting data over the Internet. It works on the client-server technology, which implies that there are clients requesting information (for example, viewing the contents of a web page), and the server part that processes these requests and sends a response.

HTTP works at the application level. This means that this protocol must use the services of the transport protocol, which is the default TCP protocol.

The first version of the HTTP protocol was developed in the early 90s of the last century and at that time completely satisfied users with its capabilities. But over time, when graphics and dynamic images came to the Internet, the protocol began to lack the capabilities and it gradually began to change.

In its work, the protocol uses the concept of URI (Uniform Resource Identifier) \u200b\u200b- a unique identifier for a resource, which is usually the address of a web page, file or any other logical object. At the same time, the URI supports working with parameters, which allows you to expand the functionality of the protocol. So, using the parameters, you can specify in which format and encoding you want to receive a response from the server.

This in turn allows using HTTP not only text documents, but also any binary data.

The main disadvantage of the HTTP protocol is the excessive amount of text information necessary for the client to correctly display the response received from the server. With a large amount of web page content, this can create unnecessarily large traffic, which affects the perception of information. In addition, the protocol is completely devoid of any state preservation mechanisms, which makes it impossible to navigate web pages using only the HTTP protocol. For this reason, third-party protocols are used together with the HTTP protocol, or the user needs to work with a browser that processes HTTP requests.

FTP

FTP (File Transfer Protocol) is the "brother" of the HTTP protocol, only, unlike the latter, it does not work with text or binary data, but with files.

This protocol is one of the oldest: it appeared back in the early 70s of the last century. Like HTTP, it works at the application level and uses the TCP protocol as a transport protocol. Its main task is to transfer files from / to the FTP server.

FTP protocol is a set of commands that describe the rules for connecting and exchanging data. In this case, commands and directly data are transmitted using various ports. Ports 21 and 20 are used as standard ports: the first is for transmitting data, the second is for transmitting commands. In addition, ports can be dynamic.

The size of files transferred using the FTP protocol is not limited. There is also a mechanism for resuming a file if a connection break occurs during the transfer.

The main disadvantage of the FTP protocol is the lack of data encryption mechanisms, which allows you to intercept the initial traffic and determine with it the user name and password for connecting to the FTP server. To avoid this situation, the SSL protocol is used in parallel, with the help of which data is encrypted.

POPZ and SMTP

Using email for messaging has long been an alternative to regular mail. Email is much more efficient and faster. Its use was made possible thanks to the POP3 (Post Office Protocol Version 3) and SMTP (Simple Mail Transfer Protocol) protocols.

The POP3 protocol works at the application level and is used to receive electronic messages from a mailbox on a mail server. At the same time, it uses one of the ports and the TCP transport protocol.

A communication session with the mail server is divided into three stages: authorization, transaction   and update. User authorization occurs when connecting to the mail server, which can be used by any mail client that supports the POP3 protocol. At the transaction stage, the client requests the server to perform the necessary action, for example, obtaining information about the number of messages, receiving the message itself, or deleting them. The update process is designed to fulfill a client request. After the update is completed, the communication session is completed before the next connection request is received.

The POP3 protocol only allows you to receive electronic messages, and to send them you have to use another protocol, which is most often used as SMTP, more precisely, its improved version - ESMTP (Extended SMTP).

Like POP3, the SMTP protocol works at the application level; therefore, it needs the services of a transport protocol, which is TCP. At the same time, sending electronic messages also occurs using one of the ports, for example, 25 ports.

IMAP

IMAP (Interactive Mail Access Protocol) is another mail protocol created using the POP3 protocol. It was developed later than the POP3 protocol. As a result, it took into account all the shortcomings and added a large number of new popular features.

The most useful among them is the ability to partially download messages, analyzing the contents of which you can effectively configure filters that sort letters or filter out spam.

Another important function is the mechanism for optimizing the use of channels through which messages are transmitted. These channels are not always fast and unloaded, so the presence of such a function significantly simplifies the life of the user. There is also the possibility of sending messages in small parts, which is very useful when the message size is large, for example 5-10 MB.

SLIP

The SLIP (Serial Line Internet Protocol) data transfer protocol was created specifically for organizing a permanent Internet connection using an existing telephone line and a regular modem. Due to the high cost, few users can afford this type of connection. Typically, such a connection is created in organizations that have a server on which the organization’s web page and other resources (database, files) are located.

This protocol works with the TCP / IP protocol and is at a lower level. Before the information from the modem goes to the processing of the TCP / IP protocol, it is preliminarily processed by the SLIP protocol. Having completed all the necessary steps, he creates another packet and transmits it to TCP / IP.

PPP

The PPP (Point-to-Point Protocol) does the same job as the SLIP described above. However, it performs these functions better, as it has additional features. In addition, unlike SLIP, PPP can interact not only with TCP / IP, but also with IPX / SPX, NetBIOS, DHCP, which are widely used in local networks. PPP is also more common due to the use of the Windows NT family of operating systems on Internet servers (SLIP is used to connect to servers running the UNIX operating system).

X.25

The X.25 protocol, which was created in 1976 and improved in 1984, operates at the physical, link, and network layers of the ISO / OSI interaction model. It was developed by a consortium consisting of representatives of many telephone companies, and created it specifically for use on existing telephone lines.

When X.25 was being developed, a digital telephone line was a rarity - it was mostly analog. For this reason, it has a system for detecting and correcting errors, which significantly increases the reliability of communication. At the same time, this system slows down the data transfer rate (maximum - 64 Kbps). However, this fact does not interfere with its use where, first of all, high reliability is required, for example, in the banking system.

Frame relay

Frame Relay is another protocol designed to transmit data over a telephone line. In addition to high reliability (like X.25), it has additional useful innovations. Since the transmitted data can be in the format of video, audio or contain electronic information, it is possible to choose the priority of the transmitted content.

Another feature of the Frame Relay protocol is its speed, which reaches 45 Mbps.

Appletalk

AppleTalk Protocol is the property of Apple Computer. It was designed to establish communication between Macintosh computers.

Like TCP / IP, AppleTalk is a set of protocols, each of which is responsible for the operation of a certain level of the ISO / OSI model.

Unlike the TCP / IP and IPX / SPX protocols, the AppleTalk protocol stack uses its own implementation of the physical and link layers, rather than the ISO / OSI model protocols.

Consider some of the AppleTalk stack protocols.

· DDP (Datagram Delivery Protocol) - is responsible for the operation of the network layer. Its main purpose is the organization and maintenance of the data transfer process without first establishing communication between computers.

· RTMP (Routing Table Maintenance Protocol) -works with routing tables AppleTalkAny such table contains information about each segment where message delivery is possible. The table consists of the numbers of routers (ports) that can deliver a message to the selected computer, the number of ma routers, the parameters of the selected network segments (speed, congestion, etc.).

· NBP (Name Binding Protocol) - is responsible for addressing, which is reduced to binding the logical name of the computer to a physical address on the network. In addition to the process of binding a name, he is responsible for registration, confirmation, erasure and search for this name.

· ZIP (Zone Information Protocol) - works in conjunction with the NBP protocol, helping it to search for a name in work groups, or zones. To do this, he uses the information of the nearest router, which creates a request across the network where computers belonging to a given working group can be located.

· ATP (AppleTalk Transaction Protocol) is one of the transport layer protocols that is responsible for transactions. Transaction - this is a set of a request, a response to this request and an identification number that is assigned to this set.   An example of a transaction is a message about the delivery of data from one computer to another. In addition, ATP is able to break large packets into smaller ones with their subsequent assembly after confirmation of receipt or delivery.

· ADSP (AppleTalk Data Stream Protocol) -aTP-like protocol. He is responsible for delivering packages. However, in this case, not one transaction is carried out, but guaranteed delivery, which may entail several transactions. In addition, the protocol ensures that data on delivery will not be lost or duplicated.

Mobile and other communication services involves the use of various communication protocols. Which of them can be considered the most common? What might be the relevance of the relevant standards?

What are communication protocols?

A communication protocol is a list of unified instructions that establish how certain software or hardware interfaces should provide data transfer - for example, text, graphic, audio and video streams. Corresponding protocols are created mainly to facilitate the scaling of various computer networks. For example, the introduction of the TCP / IP protocol made it possible to unify data transmission virtually throughout the world, making it possible to integrate computers into

In the modern market of information technology, protocols are used at various levels of implementation of communication interfaces. They are constantly being refined, updated. New protocols are periodically developed that reflect the specifics of the development of the communication market. Various communication protocols can be used in networks of both household and industrial use, implemented on the basis of the infrastructure of research centers, etc. Among the most common standards of the corresponding type are Ethernet, CAN, HART.

The use of communication protocols is also carried out in the field of mobile communications services. Among them - 3G, 4G, GPRS.

These protocols in the networks of mobile operators differ, in particular:

By speed between the subscriber and the communication service provider;

By frequency ranges;

In terms of the maximum distance of the communication device to the base station.

As for the classification of computer communication protocols, it is characterized by a rather high level of complexity. Consider its specifics in more detail.

Classification of computer communication protocols

The classification of relevant protocols can be carried out using a sufficiently large number of approaches. Widespread is one in which communication standards can be subdivided into lower and higher levels. Among those:

Applied;

Executive;

Session

Transport;

Network

Channel;

Physical.

We will study them in more detail.

Network layer application layer

The level under consideration, within which a particular communication protocol can be classified, relates primarily to applications. That is, it provides communication between and specific user programs. It uses such protocols as HTTP, Telnet, DNS, IRC, BitTorrent and many others, through which the delivery of modern online services is carried out.

Executive Network Protocol

At an appropriate level, the communication protocol involves the presentation of certain data. Here, procedures can be implemented to convert one protocol to another, encoding, compressing files, and managing various requests.

Specific applications set certain requests to the network, after which they are converted into a language that the server understands. Next, the request is processed. Then the response from the server is converted, in turn, into a language that the application understands. Among the popular protocols of the corresponding type are ASN, FTP, SMTP. It can also be attributed to some extent to HTTP and FTP.

Session layer communication protocols

At this level, the communication protocol is used to implement a specific operation - for example, synchronizing certain tasks, creating a communication session, sending or receiving a file. Among the common protocols that are used for such purposes are ASP, DLC, SOCKS.

Transport layer of communication protocols

The corresponding type of standards are used to directly deliver certain types of data from one network object to another. In many cases, there is a division of files into separate elements - to facilitate their transfer. Protocols of the corresponding type include TCP, UDP, RMTP.

Network layer protocols

The next type of standards on the basis of which a communication system can function is network layer protocols. They are responsible, first of all, for data transmission methods, address translation, switching, monitoring the quality of infrastructure work. Such protocols include, in particular, the same TCP / IP, ICMP. DHCP

Link layer protocols

These standards are used to ensure the operation of key network hardware components. Corresponding protocols allow the system, first of all, to check the data coming from the physical layer for errors. If necessary, they are also adjusted. Among these standards - the common communication protocol PPP, algorithms such as SLIP, L2F, PROFIBUS. In principle, Ethernet can also be referred to as channel protocols.

Protocol Physical Layer

The next level of validity of the standards in question is physical. Here, the communication protocol is a tool through which the digital data stream is transmitted directly - by sending a signal through a cable or via a radio channel.

In the case of wired transmission, standards such as RS-232, xDSL, 100BASE-T can be used. Common wireless protocols - in particular, implemented using Wi-Fi routers - are those that are of type IEEE 802.11.

The classification of standards we have studied can be considered very conditional. So, within the framework of it, it can be very problematic to attribute a particular protocol to a specific category: it often happens that the standard is applied at once at several levels. It will be useful to consider in more detail the specifics of the most popular protocols in the modern information technology market. Such as, for example, the PPP control protocol (communication - it is it that is the subject of the influence of the algorithms that are provided for by the corresponding standard).

What is PPP?

The protocol under consideration refers, as we noted above, to standards that are designed to ensure the functioning of network infrastructure at the data link layer. It is universal: through the appropriate protocol, you can implement device authentication, use the data encryption mechanism, and if necessary, file compression.

The protocol under consideration ensures the functioning of networks based on common communication resources - such as telephone lines, cellular communication channels. If an inscription pops up in a particular program stating thatsince the PPP connection was interrupted, this will most likely mean that the user cannot receive actual access to network resources that are provided by his provider.

There are several varieties of the corresponding standard - for example, PPPoE, PPPoA. Moreover, the protocol structure in question includes several standards: LCP, NCP, PAP, CHAP, MLPPP. Another protocol common in today's IT market is HTTP.

What is the HTTP protocol?

The corresponding standard is used to ensure the operation of the infrastructure for the exchange of hypertext data - in the general case between computers and servers on the Internet. Refers to the fundamental protocols that support the World Wide Web. By default, it is supported by most modern communication software tools in common operating systems. It is distinguished by stability - it is difficult to imagine a situation in which an HTTP program message pops up on the user’s screen like “the PPP connection protocol was interrupted”. In the extreme case, if for some reason the tools for using the HTTP standard are not available, you can use, for example, the FTP protocol for online, although in many cases its use may not be the best solution.

The standard in question involves the transfer of data from a hardware-software object in the status of a client to a server and vice versa. The first sends requests to the second, and he answers them according to the established algorithm. There are several varieties of the protocol under consideration: for example, HTTPS, HTTP-NG. The main advantages that determine the fact that the HTTP communication protocol has become one of the most popular:

Universality;

Ease of implementation;

Possibility of expansion;

Widespread support from software manufacturers.

He has a number of shortcomings highlighted by experts:

A sufficiently large value of individual messages;

Inability to distributed computing;

The inability to navigate through resources hosted on the server.

We noted above that the communication standard under consideration is supported by the main user operating systems, as well as by common software products. However, the scope of this protocol is much wider than the implementation of communication algorithms within user solutions. The HTTP standard is applicable in industry, video surveillance systems, in the SCADA infrastructure.

A large number of manufacturers, considering various communication protocols in networks as basic for building a communication infrastructure, choose HTTP - as a functional and reliable tool for organizing access to various online resources, configuring objects, and managing various devices.

Speaking specifically about the industry, the Modbus is one of the most popular protocols in the corresponding market segment.

What is the Modbus protocol?

The relevant standard is mainly used to ensure the interaction between the various elements within the framework of the automation infrastructure in the workplace. The corresponding protocol can be presented in those varieties that are adapted to transmit data on a specific type of communication channel - wired, wireless (in turn, the resources of the first type may include copper, fiber-optic cables - and for them separate modifications of the protocol about which speech).

There are versions of Mobdus adapted for transferring data over TCP / IP. Another popular solution among industrial enterprises is PROFIBUS-FDL.

What is the PROFIBUS-FDL protocol?

The protocol in question operates within the framework of the PROFIBUS network, which has become widespread among European industrial enterprises. Its prototype was developed by Siemens specialists and was to be used in production areas where controllers were involved.

Subsequently, based on the developments of the German corporation, a network infrastructure was formed in which various technological as well as functional features of serial communications related to the field level were combined. The network protocol under consideration allowed the integration of heterogeneous automation devices within a single production system. It is worth noting that the PROFIBUS-FDL protocol is not the only one that operates on the specified industrial network. However, it is uniform in terms of applicability in order to organize access to the main bus.

One way or another, the communication protocol in question is supplemented by the following standards:

The PROFIBUS DP protocol is used to organize data exchange between leading industrial devices of the DP type, as well as devices on which I / O is implemented according to a distributed scheme. Moreover, this protocol allows you to organize data exchange at high speed. It is also characterized by a relatively low cost of implementation, which can make it popular in small enterprises.

The PROFIBUS PA standard allows for the exchange of data between infrastructure, which consists of field-level equipment. This protocol is optimized for connecting various sensors and mechanisms to a common linear or ring bus.

The PROFIBUS FMS standard is universal. It is intended primarily for organizing the exchange of data between high-tech components of the industrial infrastructure - computers, programmers, controllers.

Among the strengths of the protocols operating in the PROFIBUS network are openness (that is, they can be used by any interested industrial enterprises), widespread use (which leads to easier scaling of the industrial infrastructure while expanding markets, opening new industries).

Summary

So, we examined the essence of communication protocols, studied the features of some popular varieties of relevant standards. Their main purpose is to ensure data transfer within the framework of unified formats. That is - those that can be scaled within the framework of the infrastructure, as a rule, significantly exceeding the scale of a single enterprise.

In fact, we are talking about international standards: modern protocols for cellular communications, wired, Wi-Fi-communications, are very widespread, publicly available, relatively easy to scale. Of course, in some cases, even in such global markets as the provision of cellular communication services, regional protocols are possible, but in the interests of the largest brands - to introduce, if this does not contradict the interests of business, and in some cases the state, as much as possible unified standards, which will enhance international communications.

There are a number of fundamental communication protocols. In the context of the Internet, these include HTTP, TCP / IP. In the segment of network access services, the PPP standard can be classified as such. If the user sees a message stating that the PPP control protocol has been interrupted, then he most likely will not be able to access online resources using either HTTP or TCP / IP. Thus, each standard is of great importance and, moreover, in many cases is inextricably linked with others. If one protocol is interrupted by communication, then it is likely that the user will not be able to access those resources for which other standards are responsible for organizing communications.

Communication protocols are an indispensable tool for solving complex problems both in the field of providing user communications and in industrial and service areas. The successful implementation of the corresponding infrastructure, as well as its effectiveness, in terms of the ratio of system performance to installation costs, depends on the competent choice of a specific standard. Thus, the advance study of the properties of network protocols, the choice of the optimal one is an important task for the enterprise managers responsible for the implementation and modernization of the company's communication infrastructure.

There are two types of protocols: basic and application. Basic protocols are responsible for physically forwarding messages between computers on the Internet. These are the IP and TCP protocols. Applied are the protocols of a higher level, they are responsible for the functioning of specialized services. For example, the HTTP protocol is used to transmit hypertext messages, the FTP protocol is used to transfer files, and the SMTP protocol is used to transfer email.

A set of protocols of different levels working simultaneously is called a protocol stack. Each underlying layer of the protocol stack has its own system of rules and provides service to the overlying ones. Similarly, each protocol in the protocol stack performs its function without worrying about the functions of the protocol of another level.

At the lower level, two main protocols are used: IP (Internet Protocol - Internet Protocol) and TCP (Transmission Control Protocol - transmission control protocol). The TCP / IP protocol architecture is designed for networking. They can be different LANs (Token Ring, Ethernet, etc.), various national, regional, and global networks. Different types of machines can connect to these networks. Each of the networks operates in accordance with its principles and type of communication. Moreover, each network can receive a packet of information and deliver it to the specified address. Thus, it is required that each network has some kind of end-to-end protocol for transmitting messages between two external networks.

Suppose there is a message sent by email. Mail is transmitted using the SMTP application protocol, which relies on TCP / IP protocols. According to
tCP protocol, the data sent is divided into small packets of a fixed structure and length, marked so that upon receipt of the data could be collected in the correct sequence.

Usually the length of one packet does not exceed 1500 bytes. Therefore, one e-mail can consist of several hundreds of such packages. The short packet length does not lead to blocking of communication lines and does not allow individual users to capture the communication channel for a long time.

To each received TCP packet, the IP protocol adds information by which it is possible to determine the addresses of the sender and receiver. This is similar to placing the address on an envelope. For each incoming packet, the router through which the packet passes determines, according to the IP address, which of the nearest neighbors it is necessary to forward the packet so that it is faster to the recipient, i.e. decides on the optimal route for the next packet. At the same time, geographically the shortest path is not always optimal (a fast channel to another continent may be better than a slow one to a neighboring city). Obviously, the speed and paths of different packets can be different. Interconnected data packets may be transmitted in various ways. It is possible that packages will travel across different continents at different speeds. At the same time, packets sent later may arrive earlier. Regardless of the path length, as a result of a finite number of forwarders, TCP packets reach the destination.

Finally, the destination TCP module collects and decompresses the IP envelopes, then decompresses the TCP envelopes and puts the data in the correct sequence. If something is missing, he needs to forward this packet again. Packets are not only lost, but can be distorted during transmission due to interference on the communication lines. TCP also solves this problem. In the end, the information is collected in the correct order and completely restored.

Thus, the IP protocol moves data over the network, and the TCP protocol provides reliable data delivery using a system of error correction codes. Moreover, two network servers can simultaneously transmit on both sides on the same line many TCP packets from different clients.

It is necessary to emphasize the main difference in the transmission of information over the telephone network and over the Internet. The telephone system, when making a phone call to another region or even to another continent, establishes a channel between your telephone and the one you are calling. A channel can consist of dozens of sections of different physical nature - copper wires, fiber optic lines, wireless sections, satellite communications, etc. These sections are unchanged throughout the communication session. This means that the line between you and the person you are calling is constant throughout the conversation, so damage to any part of the line can interrupt your conversation. At the same time, the part of the network allocated to you is no longer available to others. This is a circuit switched network. The Internet is a packet-switched network. The email forwarding process is fundamentally different.

So, Internet data in any form — an email, a Web page, or a downloadable file — travels as a group of packages. Each packet is sent to its destination along the optimal path available. Therefore, even if some part of the Internet is violated, this will not affect the delivery of the packet, which will be sent along the alternative path. Thus, during data delivery, there is no need for a fixed line between two users. The principle of packet switching provides the main advantage of the Internet - reliability. The network can distribute the load in different areas in thousandths of a second. If any part of the network equipment is damaged, the package can bypass this place and take a different path, ensuring the delivery of the entire message. Prototype Internet - Network
ARPAnet, developed by order of the US Department of Defense, was conceived precisely as a network that is resistant to damage (for example, in the event of hostilities), capable of continuing normal operation if any part of it fails.

A protocol is a set of rules in accordance with which information is transmitted through a network.

The main protocols used in the work of the Internet:

The protocol used as the main network layer protocol in the stack IP, which was originally designed as a packet transfer protocol in composite networks consisting of a large number of local networks. Protocol TCP   Provides a robust virtual connection between remote application processes.

Actually, TCP / IP is a whole set of protocols that work together. It consists of two levels. The upper level protocol, TCP, is responsible for the correct conversion of messages into packets of information from which the original message is collected on the receiving side. The lower level protocol, IP, is responsible for the correct delivery of messages to the specified address. Sometimes packets of a single message can be delivered in different ways.

HTTP (Hypertext Transfer Protocol) is a higher-level protocol with respect to TCP / IP, an application-level protocol. HTTP was designed to efficiently transfer web pages over the Internet. Thanks to HTTP, we are able to browse the web. HTTP is the foundation of the World Wide Web.

You issue HTTP commands using the browser interface, which is the HTTP client. When a mouse clicks on a link, the browser asks the Web server for the data of the resource pointed to by the link.

So that the text that makes up the content of Web pages is displayed on them in a certain way - in accordance with the intent of the page creator - it is marked using special text labels - tags of the HyperText Markup Language (HTML). Internet resource addresses look something like this: http://www.tut.by

FTP (File Transfer Protocol - File Transfer Protocol over the Internet. File Transfer Protocol FTP   (File Transfer Protocol) provides remote file access. In order to ensure reliable transmission, FTP uses TCP as the protocol for establishing connections.

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

Using this protocol, you can connect to a remote computer as a user and perform actions on his files and applications in the same way as if you were working on your computer. Work with it is carried out from the command line. The telnet client is supplied, for example, with Windows 98.

WAIS (Wide-Area Information Servers). This protocol was developed to search for information in databases. The WAIS information system is a distributed database system where individual databases are stored on different servers. Information about their content and location is stored in a special database - the server directory. Viewing information resources is carried out using the WAIS client program.

Information is searched for using keywords that the user sets. These words are entered for a specific database, and the system finds all the text fragments corresponding to them on all servers where the data of this database is located. The result is presented in the form of a list of links to documents indicating how often the searched word and all the searched words are used in this document together. The WAIS resource on the Internet looks something like this: wais: //site.edu

The Gopher Protocol is an application layer protocol developed in 1991. Before the ubiquitous distribution of the hypertext system, the World Wide Web Gopher was used to extract information (mainly textual) from a hierarchical file structure. Gopher was a forerunner of the WWW, allowing the menu to move from one page to another, gradually narrowing the range of information displayed. Gopher client programs had a text interface. However, Gopher menu items could indicate not only text files, but also, for example, telnet connections or WAIS databases. Modern web browsers support this protocol. Gopher information resource addresses look something like this: gopher: //gopher.tc.umn.edu

WAP (Wireless Application Protocol) was developed in 1997 by a group of companies Ericsson, Motorola, Nokia and Phone.com in order to provide access to Internet services to users of wireless devices using various communication standards. By typing on your mobile phone the address of the desired Web page, you can see it (in a simplified form) directly on the phone’s display.

DNS Domain Name System

DNS   (eng. Domain name system   - domain name system) - a distributed computer system for obtaining domain information. Most often used to obtain an IP address by host name (computer or device), to obtain information about mail routing, serving nodes for protocols in a domain (SRV record).

A distributed DNS database is maintained using a hierarchy of DNS servers that communicate over a specific protocol.

DNS is important for the operation of the Internet, because information about its IP address is required to connect to a host, and it is easier for people to remember alphabetic (usually meaningful) addresses than a series of digits of an IP address. In some cases, this allows the use of virtual servers, for example, HTTP servers, distinguishing them by request name. Initially, the conversion between domain and IP addresses was performed using a special hosts text file, which was compiled centrally and automatically sent to each of the machines on its local network. With the growth of the Web, there was a need for an efficient, automated mechanism, which DNS became.

The DNS was developed by Paul Mokapetris in 1983.

Key DNS concepts are:

Domain   (eng. domain   - region) - a node in the name tree, together with all nodes subordinate to it (if any), i.e. branch   or under the tree   in the name tree. The structure of the domain name reflects the order of nodes in the hierarchy; the domain name is read from left to right from lower domains to higher-level domains (in order of increasing importance), the dot domain (".") is the root domain of the entire system, the first-level domains (geographical or thematic) are below, then the second-level, third-level and etc. (for example, for the address ru.wikipedia.org, the first level domain is org, the second is wikipedia, and the third is ru). In practice, the point at the end of a name is often omitted, but it is important in cases of separation between relative domains and FQDN.

Subdomain   (eng. subdomain) is the subordinate domain (for example, wikipedia.org is the subdomain of the org domain, and ru.wikipedia.org is the wikipedia.org domain). Theoretically, such a division can reach a depth of 127 levels, and each label can contain up to 63 characters, until the total length with dots reaches 254 characters. But in practice, domain name registrars use more stringent restrictions. For example, if you have a domain of the form mydomain.ru, you can create different subdomains for it of the form mysite1.mydomain.ru, mysite2.mydomain.ru   etc.

Resource record   - A unit for storing and transmitting information in DNS. Each resource record has name   (i.e. attached to a specific Domain Name, a node in the name tree), type of   and data field, the format and content of which depends on of type.

Zone - part of the domain name tree (including resource records), placed as a whole on a certain domain name server, and more often on several servers simultaneously. The purpose of separating part of the tree into a separate zone is to transfer responsibility for the corresponding domain to another person or organization. This is called delegation. As a connected part of a tree, the zone inside is also a tree. If we consider the DNS namespace as a structure of zones, rather than individual nodes / names, we also get a tree; it is justifiable to talk about parent and child zones, about elders and subordinates. In practice, most zones of the 0th and 1st level (".", Ru, com, ...) consist of a single node to which the child zones are directly subordinate. In large corporate domains (2 or more levels), sometimes the formation of additional subordinate levels occurs without allocating them to child zones.

Delegation   - the operation of transferring responsibility for a part of the domain name tree to another person or organization. By delegating to DNS, administration and storage are distributed. Technically, delegation is expressed in the allocation of this part of the tree to a separate zone, and the placement of this zone on a DNS server managed by this person or organization. At the same time, “sticking” resource records containing pointers to the DNS servers of the child zone are included in the parent zone, and all other information related to the child zone is already stored on the DNS servers of the child zone.

DNS server   - This is specialized software for servicing DNS, as well as the computer on which this software runs. The DNS server may be responsible for some zones and / or may redirect requests to upstream servers.

DNS client   - This is a specialized library (or program) for working with DNS. In some cases, the DNS server acts as a DNS client.

Authority   - a sign of the zone on the DNS server. DNS server responses can be of two types: authoritative   (when the server claims to be responsible for the zone itself) and non-authoritativewhen the server processes the request and returns a response from other servers. In some cases, instead of transferring the request further, the DNS server may return the value (caching mode) already known to it (upon request).

DNS query   - a request from the client (or server) to the server.

DNS contains a hierarchy of DNS servers corresponding to a zone hierarchy. Each zone is supported by at least one authoritative DNS serverwhere the domain information is located.

The name and IP address are not identical - a single IP address can have many names, which allows you to support many websites on a single computer (this is called shared hosting). The converse is also true - multiple IP addresses can be mapped to one name: this allows you to create load balancing.

To increase the stability of the system, many servers are used that contain identical information, and the protocol has tools to maintain the synchronism of information located on different servers. There are 13 root servers, their addresses are practically unchanged. The DNS protocol uses a TCP or UDP port to respond to requests. Traditionally, requests and responses are sent as a single UDP datagram. TCP is used for AXFR requests.

Similar information.