Data transfer protocol tcp ip. What is TCP-IP protocol

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 V local networks any type and establish 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 work well 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 for performing 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 the OSI stack and look at how a computer communicates over 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 different operating systems and protocols. Without some 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. Datagram is information item, 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 is that no host has to wait indefinitely for a long time until the other host has finished transmitting to send 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, TCP controls packet delivery without any UDP intervention. On the other hand, Trivial 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 makes more effort to guarantee 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 network device programs. 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 when a telephone connection is established, while the TCP session itself telephone 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.

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.

It is important to remember that IP packets can have different lengths. In "Networking Fundamentals" it was said that information packets on an Ethernet network range in size from 64 to 1400 bytes. In the Token Ring network their length is 4000 bytes, in the ATM network - 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 CPU A person working with packets must 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 concurrent existence different versions in the internetwork 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 checksums all message packets do not match the 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 a 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 header is all of the shuttle's onboard computers 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.

In short, this is a set of rules that govern the “communication” of computers with each other over the network. There are about a dozen of them, and each of them defines the rules for transferring a specific type of data. But for ease of use, they are all combined into a so-called “stack”, calling it after the most important protocol - TCP protocol/IP (Transmission Control Protocol and Internet Protocol). The word "stack" implies that all these protocols are like a "stack of protocols" in which the upper-level protocol cannot function without the lower-level protocol.

The TCP/IP stack includes 4 layers:

1. Application - HTTP, RTP, FTP, DNS protocols. The top level; responsible for the work application applications, For example postal services, displaying data in the browser, etc.

2. Transport - TCP, UDP, SCTP, DCCP, RIP protocols. This protocol level ensures the correct interaction of computers with each other and is a data conductor between different network participants.

3. Network - IP protocol. This layer provides identification of computers on the network by giving each of them a unique digital address.

4. Channel - Ethernet, IEEE 802.11, Wireless Ethernet protocols. Lowest level; it interacts with physical equipment, describes the data transmission medium and its characteristics.

Therefore, your computer uses the HTTP - TCP - IP - Ethernet protocol stack to display this article.

How information is transmitted over the Internet

Each computer on the network is called a host and, using the protocol of the same name, receives a unique IP address. This address is written in the following form: four numbers from 0 to 255 separated by a period, for example, 195.19.20.203. To successfully communicate over a network, the IP address must also include a port number. Since it is not the computers themselves that exchange information, but the programs, each type of program must also have its own address, which is displayed in the port number. For example, port 21 is responsible for FTP, port 80 for HTTP. The total number of ports on a computer is limited and equal to 65536, numbered from 0 to 65535. Port numbers from 0 to 1023 are reserved by server applications, and the niche of ports from 1024 to 65535 is occupied by client ports, which programs are free to use as they please. "Client ports" are assigned dynamically.

Combination IP addresses and port numbers called " socket". In it, the address and port values ​​are separated by a colon, for example, 195.19.20.203:110

Thus, in order for a remote computer with IP 195.19.20.203 to receive email, you just need to deliver data to its port 110. And since this port “listens” day and night to the POP3 protocol, which is responsible for receiving emails, then further - "a matter of technology."

For convenience, all data on the network is divided into packets. A package is a file of 1-1.5 MB in size, which contains address data of the sender and recipient, transmitted information, plus service data. Splitting files into packages can significantly reduce the load on the network, because the path of each from the sender to the recipient will not necessarily be identical. If a traffic jam occurs in one place on the network, packets can bypass it using other communication paths. This technology makes it possible to use the Internet as efficiently as possible: if some transport part of it collapses, information can continue to be transmitted, but along other paths. When the packets reach the target computer, it begins to assemble them back into a single file using the service information they contain. The whole process can be compared to some kind of large puzzle, which, depending on the size of the transferred file, can reach truly enormous sizes.

As mentioned earlier, the IP protocol gives each network participant, including websites, a unique numeric address. However, no person can remember millions of IP addresses! Therefore, the Domain Name System (DNS) domain name service was created, which translates numeric IP addresses into alphanumeric names that are much easier to remember. For example, instead of dialing the dreaded number 5.9.205.233 every time, you can dial address bar browser www.site.

What happens when we type the address of the site we are looking for in the browser? From our computer, a packet with a request is sent to the DNS server on port 53. This port is reserved by the DNS service, which, after processing our request, returns the IP address corresponding to the alphanumeric name of the site. After this, our computer connects to socket 5.9.205.233:80 of computer 5.9.205.233, which hosts the HTTP protocol responsible for displaying sites in the browser, and sends a packet with a request to receive the www.site page. We need to establish a connection on port 80, since it is the one that corresponds to the Web server. If you really wish, you can specify port 80 directly in the address bar of your browser - http://www.site:80. The web server processes the request received from us and issues several packets containing HTML text, which our browser displays. As a result, we see the main page on the screen

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 different 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. it can be implemented on a 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 appeared new service Internet – WWW (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.

Interaction model open systems(OSI - Open Systems Interconnection) was developed by the International Organization for Standardization (ISO - International Organization for Standardization) for a uniform approach to building and connecting 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 of signal transmission, transmission speed, and specifications of communication channels. The layer is implemented by 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 the unification of networks operating using different channel and physical levels,into the composite network. In this case, each of the networks included in a single network is 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 reliable message transmission 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) of the upper DARPA levels with the network technologies included in the composite network (for example, Ethernet, FDDI, ATM ).

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

DocumentsRFC.

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 an RFC editor and is passed in accordance with RFC 2026 next steps, called maturity levels:

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 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 transmits them to the network layer. 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.

To exchange information between computers, standards for transmitting and processing information were developed, which were called network protocols. The most common protocols are IP, ICMP, TCP, UDP, SMTP, POP/POP3, IMAP, HTTP/HTTPS and FTP, but there are others, less well known, such as SSH, TELNET and others.

For two people to speak, they must speak the same language. However, they do not need to strictly adhere to grammar and formal language structures in order to understand each other. To exchange information between computers, everything must be clearly defined and structured. Therefore, transmission and processing standards should be used various types information. The protocols are established by international agreement and guarantee the exchange of information between any computers anywhere. There are many different protocols for different needs and types of information.

IP, ICMP, TCP and UDP

IP (Internet Protocol) and TCP (Transmission Control Protocol) are two completely different protocols that are usually associated with each other. Combinations of several protocols are often used, since the functions of different protocols can be combined in such a way as to obtain a solution to the problem. In combination, each protocol performs operations at its own level.

When transmitting information over the Internet, it is divided into small parts - Internet packets, which are transmitted independently of each other. This significantly speeds up the transfer of information due to the fact that different parts can be transmitted along different routes, after which they are reassembled at the point of receipt into a single whole. This is also a measure to prevent information loss during transmission. The TCP protocol is responsible for creating Internet packets and reassembling them in the required order at the receiving location, and also checks the integrity of the information. If some packets are lost during transmission, they are retransmitted.

Internet Protocol (IP) is used to deliver information over to the right address. Each computer that has an Internet connection has its own unique address - . Each package sent contains a delivery address. An Internet packet may pass through many routers before reaching its destination. The Internet Protocol is responsible for routing the packet to the specified computer. IP does not create physical connections between computers. It can be used in conjunction with other protocols that create connections.

To transmit small pieces of information you can use UDP protocol(User Datagram Protocol - user datagram protocol). It is also used in conjunction with the Internet Protocol, but is much simpler than TCP. Unlike TCP, UDP does not guarantee the delivery of packets in the required sequence and does not duplicate the transmission of lost packets; accordingly, it consumes less system resources and the transmission speed is significantly higher. It is used in applications that require large throughput communication lines, or short data delivery time, for example for audio or video communications.

There is a completely different protocol low level– ICMP (Internet Control Message Protocol). It is primarily used for diagnostic or service purposes, such as reporting errors and other exceptional situations encountered during data transmission, such as the requested service is not available, or the host or router is not responding.

Mail protocols – SMTP, POP, IMAP

Sending and receiving email requires its own protocols. Mail is usually sent using SMTP (Simple Mail Transfer Protocol). It is also used to transfer mail between mail servers. When setting up email clients (for example, Outlook Express), you must specify the address SMTP servers. Mail clients typically use POP (Post Office Protocol) to receive mail from a Mailbox server. post office). Currently, its third edition (version) is in effect, which is called POP3 (Post Office Protocol Version 3 - post office protocol, version 3). To be able to receive mail, you must specify the POP3 server address when setting up your mail client. The SMTP and POP3 server addresses may or may not be the same; they should be checked with your mail provider. The SMTP and POP3 protocols work in conjunction with the TCP protocol to transmit and deliver mail over the Internet.

There is also a more functional, but less known protocol for reading email - IMAP (Internet Message Access Protocol - Internet Email Access Protocol). This protocol allows you to access messages stored in a mailbox on the server without having to upload it to local computer. This is very convenient when you need to access mailbox messages from several computers. IMAP also works in conjunction with the TCP protocol.

HTTP and HTTPS protocols

Web pages use HyperText Markup Language (HTML). HTML pages are transmitted over the Internet using a standard called HyperText Transfer Protocol (HTTP). The basis of HTTP is the client-server technology, that is, the user initiates a connection to the server to request information, and the server waits for the connection to receive the request, processes the request and returns a message with the result. HTTP works in conjunction with the TCP protocol. Addresses using the HTTP protocol begin with “http:”.

Associated with the HTTP protocol HTTPS protocol(HTTP over TLS – HTTP over TLS). It provides encryption during data transmission to protect confidential information. URLs using the HTTP protocol begin with “https:”.

File Transfer Protocol – FTP

The File Transfer Protocol (FTP) is designed to transfer files over computer networks from one computer to another. It provides the opportunity simple controls files on the remote computer. This is a fairly old protocol that was put into operation before world wide web(WWW – World Wide Web). Currently, it is used mainly for uploading files to web servers, but there are also file storages that operate using the FTP protocol. It works in conjunction with the TCP protocol. URLs using the FTP protocol begin with “ftp:”.

For simultaneous operation of servers using protocols SMTP, POP, IMAP, HTTP, HTTPS, FTP etc. separate computers or IP addresses are not required at all. All these servers can be installed on one computer with one IP address. This is achieved due to the fact that each protocol uses its own .