Basics of Internet protocols (1)

Basics of Internet protocols (1)

We use the Internet every day. Have you ever wondered how it is implemented?

Billions of computers around the world are connected together and communicate with each other. One Network Card in Shanghai sends a signal, and the other network card in Los Angeles actually receives the signal. The two actually do not know the physical location of the other. Do you think this is amazing?

The core of the Internet is a series of protocols, called Internet Protocol Suite ). They have made detailed provisions on how computers are connected and networked. After understanding these protocols, we understand the principles of the Internet.

Below are my study notes. Because these protocols are too complex and too huge, I want to organize a simple framework to help myself grasp them in general. I have made a lot of simplification to ensure simplicity and accuracy, but I should be able to clarify the principles of the Internet.

I. Overview

1.1 five-layer model

The implementation of the Internet is divided into several layers. Each layer has its own functions. Like a building, each layer is supported by the next layer.

The user is only exposed to the top layer and does not feel the bottom layer. To understand the internet, you must start from the bottom up and understand the functions of each layer from the bottom up.

How to layer different models? Some models are divided into seven layers and some into four layers. I think it is easier to explain how to divide the Internet into five layers.

As shown in, the bottom Layer is the Physical Layer, the top Layer is the Application Layer, and the middle Layer is the bottom-up Layer) they are "Link Layer", "Network Layer", and "Transport Layer ). The lower the layer, the closer it is to the hardware. The higher the layer, the closer it is to the user.

Their names are not important. You only need to know that the Internet can be divided into several layers.

Layer 3 and Protocol

Each layer is used to complete a function. To implement these functions, everyone must abide by common rules.

The rules that everyone follows are called protocols ).

Each layer of the Internet defines many protocols. These protocols are called Internet protocols ). They are the core of the Internet. Next we will introduce the functions of each layer, mainly about the main protocols of each layer.

Ii. entity Layer

We start from the bottom layer.

What is the first thing to do when computers need networking? Of course, the computer can be connected first by means of optical cables, cables, twisted pair wires, and radio waves.

This is called the "entity layer", which is the physical means to connect the computer. It mainly defines some electrical characteristics of the network, and is responsible for transmitting 0 and 1 electrical signals.

3. Link Layer

3.1 Definition

0 and 1 alone have no meaning. You must specify the interpretation method: How many electric signals are counted in a group? What is the significance of each signal bit?

This is the function of "link layer". It is above the "entity layer" and determines the grouping method of 0 and 1.

3.2 Ethernet protocol

In the early days, each company had its own electric signal grouping method. Gradually, a protocol called "Ethernet" occupies a dominant position.

According to Ethernet rules, a group of electrical signals constitute a data packet, which is called a Frame ). Each frame is divided into two parts: header and Data ).

The "Header" contains some descriptions of the data packet, such as the sender, receiver, and data type. "data" is the specific content of the data packet.

The length of the "Header", which is fixed to 18 bytes ." The length of data. The minimum value is 46 bytes and the maximum value is 1500 bytes. Therefore, the entire "frame" can be up to 64 bytes and up to 1518 bytes. If the data is long, it must be divided into multiple frames for sending.

3.3 MAC address

As mentioned above, the "Header" of an Ethernet packet contains information about the sender and receiver. How are senders and recipients identified?

Ethernet requires that all devices connected to the network must have an "Nic" interface. Data packets must be transmitted from one network card to another. The NIC address is the packet Sending address and Receiving address, which is called the MAC address.