What is the TCP IP protocol. What is TCP-IP protocol

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In the modern world, information spreads in a matter of seconds. The news has just appeared, and a second later it is already available on some website on the Internet. The Internet is considered one of the most useful developments human mind. To enjoy all the benefits that the Internet provides, you need to connect to this network.

Few people know that the simple process of visiting web pages involves a complex system of actions, invisible to the user. Each click on a link activates hundreds of different computational operations at the heart of the computer. These include sending requests, receiving responses, and much more. The so-called TCP/IP protocols are responsible for every action on the network. What are they?

Any Internet protocol TCP/IP operates at its own level. In other words, everyone does their own thing. The entire TCP/IP protocol family does a tremendous amount of work simultaneously. And the user at this time only sees bright pictures And long lines text.

The concept of a protocol stack

The TCP/IP protocol stack is an organized set of basic network protocols, which is hierarchically divided into four levels and is a system for transport distribution of packets over a computer network.

TCP/IP is the most famous network protocol stack used in this moment. The principles of the TCP/IP stack apply to both local and wide area networks.

Principles of using addresses in the protocol stack

The TCP/IP network protocol stack describes the paths and directions in which packets are sent. This is the main task of the entire stack, performed at four levels that interact with each other using a logged algorithm. To ensure that the packet is sent correctly and delivered exactly to the point that requested it, IP addressing was introduced and standardized. This was due to the following tasks:

Addresses various types, must be agreed upon. For example, converting a website domain to a server's IP address and back, or converting a host name to an address and back. In this way, it becomes possible to access the point not only using the IP address, but also by its intuitive name.
Addresses must be unique. This is because in some special cases the packet must reach only one specific point.
The need to configure local area networks.

In small networks where several dozen nodes are used, all these tasks are performed simply, using the simplest solutions: compiling a table describing the ownership of the machine and its corresponding IP address, or you can manually distribute IP addresses to all network adapters. However for large networks for a thousand or two thousand machines, the task of manually issuing addresses does not seem so feasible.

That is why a special approach was invented for TCP/IP networks, which became distinctive feature protocol stack. The concept of scalability was introduced.

Layers of the TCP/IP protocol stack

There exists here a certain hierarchy. The TCP/IP protocol stack has four layers, each of which handles its own set of protocols:

Application layer: created to enable the user to interact with the network. At this level, everything that the user sees and does is processed. The layer allows the user to access various network services, for example: access to databases, the ability to read a list of files and open them, send an email message or open a web page. Along with user data and actions, service information is transmitted at this level.

Transport layer: This is a pure packet transmission mechanism. At this level, neither the contents of the package nor its affiliation with any action matter at all. At this level, only the address of the node from which the packet is sent and the address of the node to which the packet should be delivered matters. As a rule, the size of fragments transmitted using different protocols can change, therefore, at this level, blocks of information can be split up at the output and assembled into a single whole at the destination. This causes possible data loss if, at the time of transmission of the next fragment, a short-term connection break occurs.

The transport layer includes many protocols, which are divided into classes, from the simplest ones, which simply transmit data, to complex ones, which are equipped with the functionality of acknowledging receipt, or re-requesting a missing block of data.

This level provides the higher (application) level with two types of services:

Provides guaranteed delivery using the TCP protocol.
Delivers via UDP whenever possible .

To ensure guaranteed delivery, a connection is established according to the TCP protocol, which allows packets to be numbered at the output and confirmed at the input. The numbering of packets and confirmation of reception is the so-called service information. This protocol supports transmission in "Duplex" mode. In addition, thanks to the well-thought-out regulations of the protocol, it is considered very reliable.

The UDP protocol is intended for moments when it is impossible to configure transmission via the TCP protocol, or you have to save on the network data transmission segment. Also, the UDP protocol can interact with higher-level protocols to increase the reliability of packet transmission.

Network layer or "Internet layer": the base layer for the entire TCP/IP model. The main functionality of this layer is identical to the layer of the same name in the OSI model and describes the movement of packets in a composite network consisting of several smaller subnets. It links adjacent layers of the TCP/IP protocol.

The network layer is the connecting layer between the higher transport layer and the lower layer network interfaces. The network layer uses protocols that receive a request from the transport layer, and through regulated addressing, transmit the processed request to the network interface protocol, indicating to which address to send the data.

The following TCP/IP network protocols are used at this level: ICMP, IP, RIP, OSPF. The main and most popular at the network level is, of course, the IP (Internet Protocol). Its main task is to transmit packets from one router to another until a unit of data reaches the network interface of the destination node. The IP protocol is deployed not only on hosts, but also on network equipment: routers and managed switches. The IP protocol operates on the principle of best-effort, non-guaranteed delivery. That is, there is no need to establish a connection in advance to send a packet. This option leads to saving traffic and time on the movement of unnecessary service packets. The packet is routed towards its destination, and it is possible that the node remains unreachable. In this case, an error message is returned.

Network interface level: is responsible for ensuring that subnetworks with different technologies can interact with each other and transmit information in the same mode. This is accomplished in two simple steps:

Encoding a packet into an intermediate network data unit.
Converts the destination information into the required subnet standards and sends the data unit.

This approach allows us to constantly expand the number of supported networking technologies. As soon as it appears new technology, it immediately fits into the TCP/IP protocol stack and allows networks with older technologies to transfer data to networks built using more modern standards and methods.

Units of data transferred

During the existence of such a phenomenon as the TCP/IP protocols, standard terms in terms of units of transmitted data. Data during transmission can be fragmented in different ways, depending on the technologies used by the destination network.

To have an idea of what is happening with the data and at what point in time, it was necessary to come up with the following terminology:

Data stream- data that arrives at the transport layer from protocols of a higher application layer.
A segment is a fragment of data into which a stream is divided according to TCP protocol standards.
Datagram(especially illiterate people pronounce it as “Datagram”) - units of data that are obtained by splitting a stream using connectionless protocols (UDP).
Plastic bag- a unit of data produced via the IP protocol.
The TCP/IP protocols package IP packets into blocks of data transmitted over composite networks, called personnel or frames.

Types of TCP/IP protocol stack addresses

Any TCP/IP data transfer protocol uses one of the following address types to identify hosts:

Local (hardware) addresses.
Network addresses (IP addresses).
Domain names.

Local addresses (MAC addresses) - used in most local area network technologies to identify network interfaces. When talking about TCP/IP, the word local means an interface that operates not in a composite network, but within a separate subnet. For example, the subnet of an interface connected to the Internet will be local, and the Internet network will be composite. A local network can be built on any technology, and regardless of this, from the point of view of a composite network, a machine located in a separately dedicated subnet will be called local. Thus, when a packet enters the local network, its IP address is then associated with the local address, and the packet is sent to the MAC address of the network interface.

Network addresses (IP addresses). TCP/IP technology provides its own global addressing of nodes to solve a simple problem - combining networks with different technologies into one large data transmission structure. IP addressing is completely independent of the technology used on the local network, but an IP address allows a network interface to represent a machine on a composite network.

As a result, a system was developed in which hosts are assigned an IP address and a subnet mask. The subnet mask shows how many bits are allocated to the network number, and how many to the host number. An IP address consists of 32 bits, divided into blocks of 8 bits.

When a packet is transmitted, it is assigned information about the network number and the node number to which the packet should be sent. First, the router forwards the packet to the desired subnet, and then a host is selected that is waiting for it. This process is carried out by the Address Resolution Protocol (ARP).

Domain addresses on TCP/IP networks are managed by a specially designed Domain Name System (DNS). To do this, there are servers that match the domain name, presented as a string of text, with the IP address, and send the packet in accordance with global addressing. There is no correspondence between a computer name and an IP address, so in order to convert a domain name to an IP address, the sending device must access the routing table that is created on DNS server e. For example, we write the site address in the browser, the DNS server matches it with the IP address of the server on which the site is located, and the browser reads the information, receiving a response.

In addition to the Internet, it is possible to issue domain names to computers. Thus, the process of working on a local network is simplified. There is no need to remember all IP addresses. Instead, you can give each computer any name and use it.

IP address. Format. Components. Subnet mask

An IP address is a 32-bit number, which in traditional representation is written as numbers from 1 to 255, separated by dots.

Type of IP address in various recording formats:

Decimal IP address: 192.168.0.10.
Binary form of the same IP address: 11000000.10101000.00000000.00001010.
Recording an address in hexadecimal system notation: C0.A8.00.0A.

There is no separator between the network ID and the point number in the entry, but the computer is able to separate them. There are three ways to do this:

Fixed border. With this method, the entire address is conditionally divided into two parts of a fixed length, byte by byte. Thus, if we give one byte for the network number, then we will get 2 8 networks of 2 24 nodes each. If the border is moved another byte to the right, then there will be more networks - 2 16, and fewer nodes - 2 16. Today, the approach is considered obsolete and is not used.
Subnet mask. The mask is paired with an IP address. The mask has a sequence of values "1" in those bits that are allocated to the network number, and a certain number of zeros in those places of the IP address that are allocated to the node number. The boundary between ones and zeros in the mask is the boundary between the network ID and the host ID in the IP address.
Address classes method. Compromise method. When using it, network sizes cannot be selected by the user, but there are five classes - A, B, C, D, E. Three classes - A, B and C - are intended for various networks, and D and E are reserved for special-purpose networks . In a class system, each class has its own boundary of network number and node ID.

IP Address Classes

TO class A These include networks in which the network is identified by the first byte, and the remaining three are the node number. All IP addresses that have a first byte value from 1 to 126 in their range are class A networks. There are very few class A networks in quantity, but each of them can have up to 2 24 points.

Class B- networks in which the two highest bits are equal to 10. In them, 16 bits are allocated for the network number and point identifier. As a result, it turns out that the number of class B networks in big side differs from the number of class A networks quantitatively, but they have a smaller number of nodes - up to 65,536 (2 16) pieces.

On networks class C- there are very few nodes - 2 8 in each, but the number of networks is huge, due to the fact that the network identifier in such structures takes up three bytes.

Networks class D- already belong to special networks. It starts with the sequence 1110 and is called a multicast address. Interfaces with class A, B and C addresses can be part of a group and receive, in addition to the individual address, a group address.

Addresses class E- in reserve for the future. Such addresses begin with the sequence 11110. Most likely, these addresses will be used as group addresses when there is a shortage of IP addresses on the global network.

Setting up the TCP/IP protocol

Setting up the TCP/IP protocol is available on all operating systems. These are Linux, CentOS, Mac OS X, Free BSD, Windows 7. The TCP/IP protocol only requires a network adapter. Of course, server operating systems are capable of more. The TCP/IP protocol is very widely configured using server services. IP addresses on regular desktop computers are set in the settings network connections. There you configure the network address, the gateway - the IP address of the point that has access to the global network, and the addresses of the points where the DNS server is located.

The TCP/IP Internet protocol can be configured manually. Although this is not always necessary. You can receive TCP/IP protocol parameters from the server's dynamic distribution address automatically. This method is used in large corporate networks. On a DHCP server you can map local address to the network one, and as soon as a machine with a given IP address appears on the network, the server will immediately give it a pre-prepared IP address. This process is called reservation.

TCP/IP Address Resolution Protocol

The only way to establish a relationship between a MAC address and an IP address is by maintaining a table. If there is a routing table, each network interface is aware of its addresses (local and network), but the question arises of how to properly organize the exchange of packets between nodes using the TCP/IP 4 protocol.

Why was Address Resolution Protocol (ARP) invented? In order to link the TCP/IP family of protocols and other addressing systems. An ARP mapping table is created on each node and is populated by polling the entire network. This happens every time the computer is turned off.

ARP table

This is what an example of a compiled ARP table looks like.

The Internet, which is a network of networks and unites a huge number of different local, regional and corporate networks, operates and develops through the use of a single TCP/IP data transfer protocol. The term TCP/IP includes the name of two protocols:

Transmission Control Protocol (TCP) - transport protocol;

Internet Protocol (IP) is a routing protocol.

Routing protocol. The IP protocol ensures the transfer of information between computers on a network. Let's consider the operation of this protocol by analogy with the transfer of information using regular mail. In order for the letter to reach its intended destination, the address of the recipient (who the letter is to) and the address of the sender (from whom the letter is from) are indicated on the envelope.

Similarly, information transmitted over the network is “packed in an envelope” on which the IP addresses of the recipient’s and sender’s computers are “written”, for example “To: 198.78.213.185”, “From: 193.124.5.33”. Contents of the envelope on computer language called an IP packet and is a set of bytes.

In the process of forwarding regular letters, they are first delivered to the nearest address to the sender. Postal office, and then are transmitted along the chain of post offices to the post office closest to the recipient. At intermediate post offices, letters are sorted, that is, it is determined to which next post office a particular letter should be sent.

On the way to the recipient computer, IP packets also pass through numerous intermediate Internet servers where the routing operation is performed. As a result of routing, IP packets are sent from one Internet server to another, gradually approaching the recipient computer.

Internet Protocol (IP) provides routing of IP packets, that is, the delivery of information from the sending computer to the receiving computer.

Determining the route for information to pass through. The “geography” of the Internet differs significantly from the geography we are accustomed to. The speed of obtaining information does not depend on the distance of the Web server, but on the number of intermediate servers and the quality of communication lines (their capacity) through which information is transmitted from node to node.

You can get acquainted with the route of information on the Internet quite simply. Special program tracert.exe, which is included in Windows composition, allows you to track through which servers and with what delay information is transferred from the selected Internet server to your computer.

Let's see how access to information is implemented in the "Moscow" part of the Internet to one of the most popular search servers Russian Internet www.rambler.ru.

Determining the route of information passage

1. Connect to the Internet, enter the command [Programs-MS-DOS Session].

2. In the MS-DOS Session window, in response to the system prompt, enter the command.

3. After some time, a trace of information transfer will appear, that is, a list of nodes through which information is transmitted to your computer, and the time of transmission between nodes.

Route tracing transmission of information shows that the server www.rambler.ru is located at a “distance” of 7 transitions from us, i.e. information is transmitted through six intermediate Internet servers (through the servers of the Moscow providers MTU-Inform and Demos). The speed of information transfer between nodes is quite high; one “transition” takes from 126 to 138 ms.

Transport protocol. Now let’s imagine that we need to send a multi-page manuscript by mail, but the post office does not accept parcels or parcels. The idea is simple: if the manuscript does not fit into a regular postal envelope, it must be disassembled into sheets and sent in several envelopes. In this case, the sheets of the manuscript must be numbered so that the recipient knows in what sequence these sheets will be combined later.

A similar situation often occurs on the Internet when computers exchange large files. If you send such a file as a whole, it can “clog” the communication channel for a long time, making it inaccessible for sending other messages.

To prevent this from happening, the sending computer must be set to large file into small parts, number them and transport them in separate IP packets to the recipient computer. On the recipient computer you need to collect original file from individual parts in the correct sequence.

Transmission Control Protocol (TCP), that is, a transport protocol, ensures that files are split into IP packets during transmission and files are assembled during reception.

Determining the time of IP packet exchange. Time of exchange of IP packets between local computer and the Internet server can be determined using the ping utility, which is included in the Windows operating system. The utility sends four IP packets via specified address and shows the total transmission and reception time for each packet

73. Searching for information on the Internet.

There is a largely fair opinion that today the Internet “has everything” and the only problem is how to find the necessary information. Herself open architecture The network is facilitated by the fact that it lacks any centralization and the most valuable data for you, which you have unsuccessfully searched all over the world, may turn out to be located on a server in the same city as you. There are two complementary approaches to collecting information about Internet resources: creating indexes And creating directories:

With the first method, powerful search servers continuously “search” the Internet, creating and expanding Database, containing information about which documents on the Network contain certain keywords. Thus, in reality the search takes place not on Internet servers, which would be technically impossible, but on the database of a search engine, and the absence of suitable information found upon request does not mean that it is not on the Internet - you can try using another search tool or resource directory. Search server databases are not only replenished automatically. On any major search engine It is possible to index your site and add it to the database. The advantage of the search server is the ease of working with it, the disadvantage is the low degree of selection of documents upon request.

In the second case, the server is organized as library catalog, containing a hierarchy of sections and subsections that store links to documents that correspond to the topic of the subsection. The catalog is usually replenished by the users themselves after checking the data they entered by the server administration. A resource catalog is always better organized and structured, but it takes time to find the right category, which, moreover, is not always easy to define. In addition, the size of the directory is usually less than the number of sites indexed by the search engine.

Working with search servers. When entering home page search server, just type your query in the input field in the form of a set of keywords and press Enter or the start search button.

Queries can contain any words, and it is not necessary to worry about cases and declensions - for example, the queries “philosophy essay” and “philosophy essay” are quite correct.

Modern search servers understand natural language quite well, however, many of them retain advanced or special search capabilities that allow you to search for words by mask, combine query words with logical operations “AND”, “OR”, etc.

After completing the database search, the server displays the first batch of 10 or more documents containing the keywords. In addition to the link, there are usually several lines of text describing the document or just its beginning. Opening links in a new or the same browser window, you can move to the selected documents, and the link line at the bottom of the page allows you to move to the next portion of documents. This line looks something like this:

Different servers sort found documents in different ways - by date of creation, by document traffic, by the presence of all or part of the query words in the document ( relevance), some servers allow you to narrow your search by selecting the category of the document you are looking for on the main page - for example, the query “banks” in the “business world” category is unlikely to find information about cans.

Among the popular Russian-language search tools we can name servers Yandex, Aport And Rambler, indexing tens of thousands of servers and tens of millions of documents. Popular from foreign servers Altavista, Excite, Hotbot, Lycos, WebCrawler, OpenText.

Finally, there are many pages on the Internet for metasearch, allowing you to access several popular search servers at once with the same query - look, for example, at the pages http://www.find.ru/ or http://www.rinet.ru/buki/.

Working with resource catalogs. When we enter the main page of the catalog, we find ourselves in an extensive menu or table for selecting categories, each of which can contain nested subcategories. There is no standard here, but still the directory structures are very similar, everywhere you can find sections “business” or “business world”, “computers”, “programming” or “Internet”, “humor” or “hobbies”, etc. . By moving through categories, you can get to links to specific documents, which, just like on a search server, are issued in portions and are accompanied by brief information.

Today there are many large directories with tens of thousands of links, from domestic directories we can name http://www.list.ru/, http://www.weblist.ru/, http://www.stars.ru/, http://www.au.ru/, http://www.ru/, http://www.ulitka.ru/, and from foreign ones - Yahoo, Magellan.

Often the catalog also has a form for searching by keywords among the documents listed in it.

Search rules. A few simple tips regarding searching the Internet.

clearly define in advance the topic of your search, keywords and the time you are willing to spend on this search; select a search server - it is useful to store links to the best of them in Favorites;

do not be afraid natural language, but check the correct spelling of words, for example, when Microsoft help Word;

use capital letters only in names and titles. Many search engines will correctly process the query “abstract”, but not “Abstract”;

UNIX, which contributed to the growing popularity of the protocol, as manufacturers included TCP/IP in the set software every UNIX computer. TCP/IP finds its mapping in reference model OSI, as shown in Figure 3.1.

You can see that TCP/IP is located at layers three and four of the OSI model. The point of this is to leave the LAN technology to the developers. The purpose of TCP/IP is message transmission in local networks of any type and establishing communication using any network application.

The TCP/IP protocol works because it is connected to OSI model at the two lowest layers - the data transfer layer and the physical layer. This allows TCP/IP to find mutual language with virtually any network technology and, as a result, with any computer platform. TCP/IP includes four abstract layers, listed below.

Rice. 3.1.

Network interface. Allows TCP/IP to actively interact with all modern network technologies, based on the OSI model.
Internetwork. Defines how IP controls forwarding messages through routers of a network space such as the Internet.
Transport. Defines a mechanism for exchanging information between computers.
Applied. Specifies network applications to perform tasks such as forwarding, Email and others.

Due to its widespread use, TCP/IP has become the de facto Internet standard. The computer on which it is implemented network technology, based on the OSI model (Ethernet or Token Ring), has the ability to communicate with other devices. In "Networking Fundamentals" we looked at layers 1 and 2 when discussing LAN technologies. Now we'll move on to OSI stack and see how the computer establishes a connection on the Internet or private network. This section discusses the TCP/IP protocol and its configurations.

What is TCP/IP

The fact that computers can communicate with each other is itself a miracle. After all, these are computers from different manufacturers, working with various operating systems and protocols. Without some kind of common basis, such devices would not be able to exchange information. When sent over a network, data must be in a format that is understandable to both the sending device and the receiving device.

TCP/IP satisfies this condition through its internetworking layer. This layer directly matches the network layer of the OSI reference model and is based on a fixed message format called an IP datagram. A datagram is something like a basket in which all the information of a message is placed. For example, when you load a web page into a browser, what you see on the screen is delivered piecemeal by datagram.

It's easy to confuse datagrams with packets. A datagram is an information unit, while a packet is a physical message object (created at the third and higher layers) that is actually sent over the network. Although some consider these terms interchangeable, their distinction actually matters in a specific context - not here, of course. It is important to understand that the message is broken into fragments, transmitted over the network and reassembled at the receiving device.

The positive thing about this approach is that if a single packet is corrupted during transmission, then only that packet will need to be retransmitted, not the entire message. Another positive point is that no host has to wait indefinitely for another host's transmission to finish before sending its own message.

TCP and UDP

When sending an IP message over a network, one of the transport protocols is used: TCP or UDP. TCP (Transmission Control Protocol) makes up the first half of the acronym TCP/IP. The User Datagram Protocol (UDP) is used instead of TCP to transport less than important messages. Both protocols are used for the correct exchange of messages in TCP/IP networks. There is one significant difference between these protocols.

TCP is called a reliable protocol because it communicates with the recipient to verify that the message was received.

UDP is called an unreliable protocol because it does not even attempt to contact the recipient to verify delivery.

It is important to remember that only one protocol can be used to deliver a message. For example, when a web page is loaded, packet delivery is controlled by TCP without any UDP intervention. On the other hand, Trivial File Transfer Protocol File Transfer Protocol, TFTP) downloads or sends messages under the control of the UDP protocol.

The transport method used depends on the application - it could be email, HTTP, the application responsible for networking work, and so on. Developers network programs use UDP wherever possible, since this protocol reduces excess traffic. The TCP protocol is attached more effort for guaranteed delivery and transmits many more packets than UDP. Figure 3.2 provides a list of network applications and shows which applications use TCP and which use UDP. For example, FTP and TFTP do essentially the same thing. However, TFTP is mainly used for downloading and copying programs. network devices. TFTP can use UDP because if the message fails to be delivered, nothing bad happens because the message was not intended for the end user, but for the network administrator, whose priority level is much lower. Another example is a voice video session, in which ports for both TCP and UDP sessions can be used. Thus, a TCP session is initiated to exchange data during installation telephone communication, while himself phone conversation transmitted via UDP. This is due to the speed of voice and video streaming. If a packet is lost, there is no point in resending it, since it will no longer match the data flow.

Rice. 3.2.

IP Datagram Format

IP packets can be broken down into datagrams. The datagram format creates fields for the payload and for message transmission control data. Figure 3.3 shows the datagram diagram.

Note. Don't be fooled by the size of the data field in a datagram. The datagram is not overloaded with additional data. The data field is actually the largest field in the datagram.

Rice. 3.3.

It is important to remember that IP packets can have different lengths. In "Networking Fundamentals" it was said that information packets in Ethernet networks have a size from 64 to 1400 bytes. In the Token Ring network their length is 4000 bytes, in ATM networks- 53 bytes.

Note. The use of bytes in a datagram can be confusing, since data transfer is often associated with concepts such as megabits and gigabits per second. However, because computers prefer to work with data bytes, datagrams also use bytes.

If you look again at the datagram format in Figure 3.3, you'll notice that the leftmost margins are a constant value. This happens because the CPU processing the packets needs to know where each field begins. Without standardization of these fields, the final bits will be a jumble of ones and zeros. On the right side of the datagram are packets of variable length. The purpose of the various fields in a datagram is as follows.

VER. The version of the IP protocol used by the station where the original message appeared. Current version IP is version 4. This field ensures that different versions exist simultaneously in the Internet space.
HLEN. The field informs the receiving device of the length of the header so that the CPU knows where the data field begins.
Service type. Code that tells the router the type of packet control in terms of service level (reliability, priority, deferment, etc.).
Length. The total number of bytes in the packet, including header fields and data fields.
ID, frags and frags offset. These fields tell the router how to fragment and reassemble the packet and how to compensate for differences in frame size that may occur as the packet traverses LAN segments with different network technologies (Ethernet, FDDI, etc.).
TTL. An abbreviation for Time to Live is a number that decreases by one each time a packet is sent. If the lifetime becomes zero, the packet ceases to exist. TTL prevents loops and lost packets from wandering endlessly across the Internet.
Protocol. The transport protocol to use to transmit the packet. The most common protocol specified in this field is TCP, but other protocols may be used.
Header checksum. A checksum is a number that is used to verify the integrity of a message. If the checksums of all message packets do not match correct value, this means that the message has been corrupted.
Source IP address. The 32-bit address of the host that sent the message (usually Personal Computer or server).
Destination IP address. The 32-bit address of the host to which the message was sent (usually a personal computer or server).
IP options. Used for network testing or other special purposes.
Padding. Fills all unused (empty) bit positions so that the processor can correctly determine the position of the first bit in the data field.
Data. The payload of the sent message. For example, the package data field may contain the text of an email.

As mentioned earlier, the packet consists of two main components: data about message processing, located in the header, and the information itself. The information part is located in the payload sector. You can imagine this sector as the cargo compartment of a spaceship. The title is everything on-board computers shuttle in the control cabin. It manages all the information needed by all the different routers and computers along the message path, and is used to maintain a certain order in assembling the message from individual packets.

Interaction between computers on the Internet is carried out through network protocols, which are an agreed upon set of specific rules, according to which different devices data transmissions exchange information. There are protocols for error control formats and other types of protocols. Most commonly used in global internetworking TCP-IP protocol.

What kind of technology is this? The name TCP-IP comes from two network protocols: TCP and IP. Of course, the construction of networks is not limited to these two protocols, but they are basic as far as the organization of data transmission is concerned. In fact, TCP-IP is a set of protocols that allows individual networks to come together to form

The TCP-IP protocol, which cannot be described only by the definitions of IP and TCP, also includes the protocols UDP, SMTP, ICMP, FTP, telnet, and more. These and other TCP-IP protocols provide the most complete operation of the Internet.

Below we provide a detailed description of each protocol included in general concept TCP-IP.

. Internet protocol(IP) is responsible for the direct transmission of information on the network. The information is divided into parts (in other words, packets) and transmitted to the recipient from the sender. For accurate addressing, you need to specify the exact address or coordinates of the recipient. Such addresses consist of four bytes, which are separated from each other by dots. Each computer's address is unique.

However, using the IP protocol alone may not be enough for correct data transmission, since the volume of most of the transmitted information is more than 1500 characters, which no longer fits into one packet, and some packets may be lost during transmission or sent in the wrong order, what is needed.

. Transmission Control Protocol(TCP) is used for more high level than the previous one. Based on the IP protocol's ability to carry information from one host to another, the TCP protocol allows large amounts of information to be sent. TCP is also responsible for separation transmitted information into separate parts - packets - and correct recovery of data from packets received after transmission. Wherein this protocol automatically retransmits packets that contain errors.

Management of the organization of data transfer in large volumes can be carried out using a number of protocols that have special functional purpose. In particular, there are the following types of TCP protocols.

1. FTP(File Transfer Protocol) organizes file transfer and is used to transfer information between two Internet nodes using TCP connections in the form of a binary or simple text file, as a named area in computer memory. In this case, it does not matter where these nodes are located and how they are connected to each other.

2. User Datagram Protocol, or User Datagram Protocol, is connection independent and transmits data in packets called UDP datagrams. However, this protocol is not as reliable as TCP because the sender does not know whether the packet was actually received.

3. ICMP(Internet Control Message Protocol) exists to transmit error messages that occur during data exchange on the Internet. However, the ICMP protocol only reports errors, but does not eliminate the reasons that led to these errors.

4. Telnet- which is used for implementation text interface on the network using TCP transport.

5. SMTP(Simple Mail Transfer Protocol) is a special by email, which defines the format of messages that are sent from one computer, called an SMTP client, to another computer running an SMTP server. In this case, this transfer can be delayed for some time until the work of both the client and the server is activated.

Data transmission scheme via TCP-IP protocol

1. The TCP protocol breaks the entire amount of data into packets and numbers them, packing them into TCP envelopes, which allows you to restore the order in which parts of information are received. When data is placed in such an envelope, a calculation occurs checksum, which is then written to the TCP header.

3. TCP then checks to see if all packets have been received. If, during reception, the newly calculated one does not coincide with that indicated on the envelope, this indicates that some of the information was lost or distorted during transmission, the TCP-IP protocol again requests the forwarding of this packet. Confirmation of the receipt of data from the recipient is also required.

4. After confirming the receipt of all packets, the TCP protocol orders them accordingly and reassembles them into a single whole.

The TCP protocol uses repeated data transmissions and waiting periods (or timeouts) to ensure reliable delivery of information. Packets can be transmitted in two directions simultaneously.

Thus, TCP-IP eliminates the need for retransmissions and waits for application processes (such as Telnet and FTP).

StackTCP/ IP.

The TCP/IP stack is a set of hierarchically ordered network protocols. The stack is named after two important protocols – TCP (Transmission Control Protocol) and IP (Internet Protocol). In addition to them, the stack includes several dozen more various protocols. Currently, TCP/IP protocols are the main ones for the Internet, as well as for most corporate and local networks.

In the Microsoft Windows Server 2003 operating system, the TCP/IP stack is selected as the main one, although other protocols are also supported (for example, the IPX/SPX stack, the NetBIOS protocol).

The TCP/IP protocol stack has two important properties:

platform independence, i.e. its implementation is possible on a wide variety of operating systems and processors;

openness, i.e. the standards by which the TCP/IP stack is built are available to anyone.

History of creationTCP/ IP.

In 1967, the Advanced Research Projects Agency of the US Department of Defense (ARPA - Advanced Research Projects Agency) initiated the development of a computer network that was supposed to connect a number of universities and research centers that carried out orders from the Agency. The project was called ARPANET. By 1972, the network connected 30 nodes.

As part of the ARPANET project, the main protocols of the TCP/IP stack - IP, TCP and UDP - were developed and published in 1980–1981. An important factor in the spread of TCP/IP was the implementation of this stack in the UNIX 4.2 BSD operating system (1983).

By the end of the 80s, the significantly expanded ARPANET network became known as the Internet (Interconnected networks) and united universities and research centers in the USA, Canada and Europe.

In 1992, a new Internet service appeared - WWW (World Wide Web - The World Wide Web), based on the HTTP protocol. Largely thanks to WWW, the Internet, and with it the TCP/IP protocols, received rapid development in the 90s.

At the beginning of the 21st century, the TCP/IP stack is acquiring a leading role in the means of communication not only of global, but also local networks.

ModelOSI.

The Open Systems Interconnection (OSI) model was developed by the International Organization for Standardization (ISO) to provide a consistent approach to building and interconnecting networks. Development of the OSI model began in 1977 and ended in 1984 with the approval of the standard. Since then, the model has been the reference for the development, description and comparison of various protocol stacks.

Let's briefly look at the functions of each level.

The OSI model includes seven layers: physical, data link, network, transport, session, presentation, and application.

The physical layer describes the principles signal transmission, transmission speed, communication channel specifications. The level is implemented in hardware ( network adapter, hub port, network cable).

The data link layer solves two main tasks: it checks the availability of the transmission medium (the transmission medium is most often divided between several network nodes), and also detects and corrects errors that occur during the transmission process. The implementation of the level is hardware and software (for example, a network adapter and its driver).

The network layer provides network interconnection, operating on different channel and physical levels,into the composite network. Moreover, each of the networks included in single network, called subnet(subnet). At the network level, two main problems have to be solved: routing(routing, choosing the optimal path for transmitting a message) and addressing(addressing, each node in a composite network must have a unique name). Typically, network layer functions are implemented by a special device - router(router) and its software.

The transport layer solves the problem of reliably transmitting messages in a composite network by confirming delivery and resending packets. This level and all the following are implemented in software.

The session layer allows you to remember information about the current state of a communication session and, in the event of a connection break, resume the session from this state.

The presentation layer ensures the conversion of transmitted information from one encoding to another (for example, from ASCII to EBCDIC).

The application layer implements the interface between the other layers of the model and user applications.

StructureTCP/ IP. The TCP/IP structure is not based on the OSI model, but on its own model, called DARPA (Defense ARPA - the new name of the Advanced Research Projects Agency) or DoD (Department of Defense - US Department of Defense). This model has only four levels. The correspondence of the OSI model to the DARPA model, as well as the main protocols of the TCP/IP stack, is shown in Fig. 2.2.

It should be noted that the lower level of the DARPA model - the network interface level - strictly speaking, does not perform the functions of the data link and physical layers, but only provides communication (interface) upper levels DARPA with composite network technologies (eg, Ethernet, FDDI, ATM).

All protocols included in the TCP/IP stack are standardized in RFC documents.

DocumentationRFC.

Approved official Internet and TCP/IP standards are published as RFC (Request for Comments) documents. Standards are developed by the entire ISOC community (Internet Society, an international public organization). Any ISOC member may submit a document for consideration for publication in an RFC. The document is then reviewed by technical experts, development teams and the RFC editor and goes through the following stages, called maturity levels, in accordance with RFC 2026:

draft(Internet Draft) – at this stage, experts familiarize themselves with the document, additions and changes are made;

proposed standard(Proposed Standard) - the document is assigned an RFC number, experts have confirmed the viability of the proposed solutions, the document is considered promising, it is desirable that it be tested in practice;

draft standard(Draft Standard) - a document becomes a draft standard if at least two independent developers have implemented and successfully applied the proposed specifications. At this stage, minor corrections and improvements are still allowed;

Internet standard(Internet Standard) - the highest stage of approval of the standard, the document specifications have become widespread and have proven themselves in practice. A list of Internet standards is given in RFC 3700. Of the thousands of RFCs, only a few dozen are documents with the status of “Internet standard”.

In addition to standards, RFCs can also be descriptions of new networking concepts and ideas, guidelines, results of experimental studies presented for information, etc. Such RFCs can be assigned one of the following statuses:

experimental(Experimental) – a document containing information about scientific research and developments that may be of interest to ISOC members;

informational(Informational) - a document published to provide information and does not require approval by the ISOC community;

best modern experience(Best Current Practice) - a document intended to convey experience from specific developments, such as protocol implementations.

The status is indicated in the header of the RFC document after the word Category (Category). For documents in the status of standards (Proposed Standard, Draft Standard, Internet Standard), the name is indicated Standards Track, since the level of readiness may vary.

RFC numbers are assigned sequentially and are never reissued. The original RFC is never updated. The updated version is published under a new number. An obsolete and superseded RFC becomes historical(Historic).

All existing RFC documents today can be viewed, for example, on the website www.rfc-editor.org . There were over 5,000 in August 2007. The RFCs referenced in this course are listed in Appendix I.

Overview of the main protocols.

Protocol IP (Internet Protocol) – This is the main network layer protocol responsible for addressing in composite networks and packet transmission between networks. The IP protocol is datagram protocol, i.e. it does not guarantee delivery of packets to the destination node. The transport layer protocol TCP provides guarantees.

Protocols R.I.P. (Routing Information Protocol – routing information protocol ) AndOSPF (Open Shortest Path First – « The shortest routes open first" ) – routing protocols in IP networks.

Protocol ICMP (Internet Control Message Protocol – Control Message Protocol in Composite Networks) is designed to exchange error information between network routers and the source node of the packet. Using special packets, it reports the impossibility of delivering a package, the duration of assembling a package from fragments, anomalous parameter values, changes in the forwarding route and type of service, the state of the system, etc.

Protocol ARP (Address Resolution Protocol – Address Translation Protocol) converts IP addresses into hardware addresses of local networks. The reverse conversion is carried out using the protocol RAPR (Reverse ARP).

TCP (Transmission Control Protocol – transmission control protocol) ensures reliable transmission of messages between remote network nodes through the formation of logical connections. TCP allows you to deliver a byte stream generated on one computer without errors to any other computer included in the composite network. TCP divides the byte stream into parts - segments and passes them on network level. Once these segments are delivered to their destination, TCP reassembles them into a continuous stream of bytes.

UDP (User Datagram Protocol – User Datagram Protocol) provides data transmission in a datagram manner.

HTTP (HyperText Transfer Protocol – hypertext transfer protocol) – web document delivery protocol, the main protocol of the WWW service.

FTP (File Transfer Protocol – file transfer protocol) – a protocol for transferring information stored in files.

POP 3 (Post Office Protocol version 3 – post office protocol) and SMTP (Simple Mail Transfer Protocol – Simple Mail Forwarding Protocol) – protocols for delivering incoming email (POP3) and sending outgoing email (SMTP).

Telnet – terminal emulation protocol 1, allowing the user to connect to other remote stations and work with them from their machine, as if it were their remote terminal.

SNMP (Simple Network Management Protocol – simple network management protocol) is designed to diagnose the performance of various network devices.