Shulou(Shulou.com)06/01 Report--

(1) OSI reference model:

It is divided into seven layers, namely: physical layer, data link layer, network layer, transport layer, session layer, presentation layer, application layer. Each layer has its own set of functions and interacts with the adjacent upper and lower layers. At the top level, the application layer interacts with the software used by the user.

The role of each of the seven layers of OSI:

(1) physical layer

The physical layer is the lowest or first layer of the OSI reference model, which includes physical networking media, such as cable connectors. The protocol of the physical layer generates and detects the voltage in order to send and receive signals carrying data. Insert a network interface card into your desktop PC and you will build the foundation for computer networking. In other words, you provide a physical layer. Although the physical layer does not provide error correction service, it can set the data transmission rate and detect the data error rate. Network physical problems, such as disconnected wires, affect the physical layer. Similarly, if you don't plug the network interface deep enough into the computer's circuit board, the computer will have network problems at the physical layer.

IEEE has established standards for physical layer protocols, especially IEEE802, which specifies how Ethernet and token ring networks should handle data. The terms "layer 1 protocol" and "physical layer protocol" refer to standards that describe how electrical signals are amplified and transmitted over wires. [2]

(2) data link layer

The data link layer is the second layer of the OSI reference model, which controls the communication between the network layer and the physical layer. Its main function is to divide the data received from the network layer into specific frames that can be transmitted by the physical layer. A frame is a structural packet used to move data, which includes not only the original data, or "payload", but also the network addresses of the sender and receiver, as well as error correction and control information. The address determines where the frame will be sent, while error correction and control information ensures that the frame arrives error-free.

The function of the data link layer is independent of the network, its nodes and the type of physical layer used. Because they decode the frame and use the frame information to send the data to the correct receiver, they work at the data link layer.

(3) Network layer

The main function of the network layer, the third layer of the OSI reference model, is to translate the network address into the corresponding physical address and determine how to route the data from the sender to the receiver. The network layer determines the best path from node An in one network to node B in another network by comprehensively considering transmission priority, network congestion, quality of service and the cost of optional routes.

Because the network layer handles routing, and because the router connects each segment of the network and intelligently guides data transmission, it belongs to the network layer. In a network, "routing" guides the transmission of data based on addressing schemes, usage patterns, and accessibility. Network layer protocols can also compensate for the imbalance in the capabilities of devices that send, transmit, and receive data. To accomplish this task, the network layer segments and reassembles the packets. Segmentation refers to the process of reducing the size of data units in the network layer when data is transferred from a network segment that can handle smaller data units than big data.

(4) Transport layer

The transport layer is mainly responsible for ensuring that data is transmitted reliably, sequentially and without error from point A to point B. Because data cannot be sent for verification or interpretation without the transport layer, the transport layer is often considered to be the most important layer in the OSI reference model. The transmission protocol performs flow control at the same time or specifies an appropriate transmission rate based on the speed at which the data is acceptable to the receiver.

In addition, the transport layer forcibly splits longer packets according to the maximum size that the network can handle, for example, Ethernet cannot accept 1500 bytes of packets. The transmission layer of the sender node divides the data into smaller data slices, and arranges a series of data slices for each data slice, so that when the data reaches the transmission layer of the receiver node, it can be reorganized in the correct order. This process is called sorting.

(5) session layer

The session layer is responsible for establishing and maintaining communication between the two nodes in the network. The term "session" refers to a connection that establishes a data exchange between two entities; it is often used to refer to communication between a terminal and a host. The so-called terminal means that it will have almost no processing power or hard disk capacity, but will only rely on the dialogue between the application program and the data processing service provided by the host computer to determine whether the communication is interrupted and where to resend when the communication is interrupted.

(6) presentation layer

The presentation layer is like a translator between the application and the network. At the presentation layer, the data is formatted according to the scheme that the network can understand; this format also varies depending on the type of network used. The presentation layer manages the decryption and encryption of data, and the presentation layer protocol also decodes and encodes picture and file format information.

(7) Application layer

The top and seventh layer of the OSI reference model is the application layer. The application layer is responsible for providing interfaces to the software so that the program can adapt to network services. The term "application layer" does not refer to a particular application running on the network. The services provided by the application layer include file transfer, file management and e-mail information processing.

(2) TCP/IP protocol suite

TCP/IP and ISO/OSI

Comparison between TCP IP Model and OSI Model

The ISO/OSI model, the Open Communication system Interconnection reference Model (Open System Interconnection Reference Model), is a standard framework proposed by the International Organization for Standardization (ISO) to interconnect all kinds of computers into networks around the world, referred to as OSI. TCP/IP protocol model (Transmission Control Protocol/Internet Protocol), which contains a series of network protocols that constitute the foundation of the Internet, is the core protocol of Internet. Through more than 20 years of development, it has become increasingly mature, and has been widely used in local area network and wide area network, and has become a de facto international standard. TCP/IP protocol suite is a combination of multiple protocols at different levels, which is usually regarded as a four-layer protocol system, corresponding to the seven-layer model of OSI.

2. TCP/IP hierarchical model

(1)。 Link layer

Also known as the data link layer or network interface layer (the network interface layer and hardware layer in the first figure), usually includes the device driver in the operating system and the corresponding network interface card in the computer. Together, they deal with the details of the physical interface with the cable (or any other transmission medium). ARP (address Resolution Protocol) and RARP (inverse address Resolution Protocol) are special protocols used by some network interfaces, such as Ethernet and token Ring, to translate addresses used by the IP layer and the network interface layer.

(2)。 Network layer

Also known as the Internet layer (the internetwork layer in the first figure), it deals with the activities of packets in the network, such as packet routing. In the TCP/IP protocol family, the network layer protocols include IP (Internet Protocol), ICMP (Internet Internet Control message Protocol), and IGMP (Internet Group Management Protocol). IP is a network layer protocol that provides an unreliable service. It only sends packets from the source node to the destination node as soon as possible, but does not provide any reliability guarantee. Used by both TCP and UDP. Each set of data for TCP and UDP is transmitted over the Internet through the end system and the IP layer in each intermediate router. ICMP is a subsidiary of the IP protocol. The IP layer uses it to exchange error messages and other important information with other hosts or routers. IGMP is the Internet group management protocol. It is used to multicast an UDP Datagram to multiple hosts. (3)。 The transport layer mainly provides end-to-end communication for applications on two hosts. In the TCP/IP protocol family, there are two different transmission protocols: TCP (Transmission Control Protocol) and UDP (user Datagram Protocol). TCP provides highly reliable data communication between the two hosts. The work it does includes dividing the data given to it by the application into appropriate pieces to the network layer below, confirming the received packets, setting the timeout clock to send the final acknowledgement packets, and so on. Because the transport layer provides highly reliable end-to-end communication, the application layer can ignore all these details. In order to provide reliable services, TCP adopts mechanisms such as timeout retransmission, sending and receiving end-to-end acknowledgement packets. UDP provides a very simple service for the application layer. It simply sends packets called datagrams from one host to another, but there is no guarantee that the Datagram will reach the other end. A Datagram is a unit of information transmitted from the sender to the receiver (for example, a certain number of bytes of information specified by the sender). Any required reliability of the UDP protocol must be provided by the application layer.

(4)。 Application layer

The application layer is responsible for handling specific application details.

III. Encapsulation and division of data

When an application transmits data using TCP, the data is fed into the protocol stack and then traverses each layer one by one until it is sent into the network as a string of bitstreams. Each layer adds some header information (and sometimes tail information) to the received data, as shown in the figure. The data unit that TCP transmits to IP is called TCP message segment or TCP segment for short (TCP segment); UDP data is basically the same as TCP data. The only difference is that the unit of information that UDP sends to IP is called UDP Datagram (UDP datagram), and the first length of UDP is 8 bytes. The data unit that IP transmits to the network interface layer is called IP Datagram (IP datagram). The stream of bits transmitted over Ethernet is called a Frame.

When the destination host receives an Ethernet data frame, the data begins to rise from the bottom of the protocol stack, and the header added by each layer protocol is removed. Each layer protocol box has to check the protocol identification in the header of the message to determine the upper layer protocol that receives the data. This process is called Demultiplexing. The protocol is unpacked by the destination port number, the source IP address, and the source port number.

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