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OSI model

OSI helps manufacturers to produce interoperable devices and software in the way of protocols, so that the networks of different manufacturers can work together.

The OSI model is mainly a network architecture model, which describes how data and network information propagate from the application of one host to the application of another through the network media.

OSI is a grand blueprint for how to build communications.

The main advantages of OSI:

1) dividing the network into smaller parts is beneficial to management, troubleshooting and design.

2) different devices can communicate and different networks can coordinate their work.

3) when one layer fails, the other layers will not be affected.

OSI is divided into seven layers. The upper three layers represent the communication between the terminal application and the user, and the lower four layers represent the end-to-end transmission.

Layer 7: application layer

Layer 6: presentation layer

Layer 5: session layer

Layer 4: transport layer

Layer 3: network layer

Layer 2: data link layer

Layer 1: physical layer

Specify the function of each layer

Application layer

The application layer is the highest layer of the open model. It is actually the interface of the application, which provides a guarantee for the transmission of the application. The application layer is also responsible for determining the availability of the target communicating party and determining whether there are sufficient resources to communicate.

Presentation layer

A successful transmission is to convert the data to a standard format for transmission. Therefore, the presentation layer is the converter and the translator between the network and the user program. The presentation layer has the function of decryption and encryption. For example, when you query your bank account on Internet, you use a secure connection. Your account data is encrypted before it is sent, and on the other side of the network, it means that the received data will be decrypted. For files, there are compression, decompression, coding, decoding function.

Session layer

Responsible for establishing, maintaining and terminating calls between the two nodes. There are three different modes: simplex, half duplex, and full duplex. In short, the session layer is to separate different applications. Detect errors.

Transport layer

The transport layer is responsible for dividing the data into segments and then reorganizing them into data streams. They provide end-to-end data transfer services and establish a logical connection between the sending host and the destination host.

The transport layer is divided into connection-oriented (TCP reliable transport protocol) and connectionless (UDP unreliable transport protocol).

Now let's introduce the reliable transport protocol.

In the process of reliable transmission, a connection is established between the transmitted device and the remote device, which is called three-party handshake. after the end of the transmission, the call is terminated and the virtual link is removed.

The process of three-way handshake is as follows

(1) the connection Protocol data segment, which is used to request synchronization.

(2) the main function of the next data segment can be said to determine the connection parameters.

(3) make a determination, which notifies the target host of the connection agreement to be established. Data can be transmitted.

However, when the host receives a large amount of information, it will overflow, resulting in data loss. So TCP has a buffer to hold those overflowed data. But when the data continues to come in large quantities, the memory of the device will be exhausted. So the host will send a signal "not ready" to all transmissions at this time, and a "ready" signal when the host can handle it.

confirm

Reliable transmission requires the integrity of the data, that is, so that the data will not be repeated and discarded. This is achieved through the confirmation mechanism. Each time the sender sends a piece of data to the destination, the destination needs to reply to the transmitter to confirm that the sender will not transmit the data until the acknowledgment is received. After the sender sends the data, it starts the timer and resends the message before the timer has received an acknowledgement.

Window technology

When the transmission sends a data to the host, the host sends an acknowledgement ACK message. This makes it inefficient, for example, the transmitter sends data to the host, and it takes time for the host to reply to it for confirmation. So you can use window technology to make it a little more efficient. Windowing technology even when the transmission sends multiple data to the host, the host replies to it for confirmation. When any point is not received, the transmitter needs to resend it. However, when the host receives the data from the transmitter in a disordered order, the transmission fails and needs to be retransmitted.

Reliable transport protocols have the following characteristics

(1): establish a virtual link (through a three-way handshake)

(2): receive data and send it to determine

(3): the data sent is sorted by rows.

In practice, what is sent is the number of bytes, not the segment.

Network layer

The network layer tracks the location of the device on the network and confirms the best transmission path.

There are two main groups used (more on this later)

(1) data packet

Used to transmit user data over the Internet.

(2) routing update protocol

Contains routing update information for all networks in the router, including static routes and dynamic routes.

Data link layer

Its main function is how to transmit data reliably on unreliable physical lines. To put it bluntly, it is to ensure the reliability of transmission. In order to ensure transmission, the data received from the network layer is divided 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, but also the physical addresses of the sender and receiver, as well as error detection 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. If the receiving point detects an error in the transmitted data when transmitting the data, it will notify the sender to resend the frame. Resolve the error.

Physical layer

The main function of the physical layer is to use the transmission medium to provide physical connection for the data link layer to realize the transparent transmission of the bit stream. Realize the transparent transmission of bit stream between adjacent computer nodes, shielding the differences between specific transmission media and physical devices as much as possible. So that the data link layer above it does not have to consider the specific transmission medium of the network.

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