[concept of network principle]

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Network principle

Basic principle

User mode and kernel mode are just different areas of memory User mode -> Kernel mode ： The process of copying data in memory

network card ： Realize physical signal <-> Digital signal conversion

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Data transmission between two hosts

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User mode -> Kernel mode ： The process of copying data in memory

Internal replication here It's code like this ( It's just like )

byte[] User mode = new byte[1024]; // Data to be sent

byte[] Kernel mode = new byte[1024]; // Kernel buffer

os.write( User mode );

​ for(int i = 0; i < bytes.length; i++){

​ Kernel mode [i] = User mode [i];

​ }

How to deal with the data sent by the network card ？

1. Load the data into the network card ： Replication of data
2. Send data ：fire()
3. To send data, you need to change the data signal into a physical signal

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The data is on the transmission medium ( Ethernet cable 、 Optical fiber ) Spread on ( Signal decay 、 Signal error and other problems )

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When the data goes to another host , here CPU Stop the current execution cycle , Give Way CPU Execute a piece of code prepared in advance （ This is the interrupt processing flow ）. Transfer the data from the corresponding location of the network card Copied to the Kernel memory

here , Only one network medium can be used to connect the two hosts , Data exchange can be realized from the perspective of hardware

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How about the networking of multiple hosts ？

The basic structure of network topology ： Bus type 、 Stars 、 ring 、 Tree form

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agreement / layered

To complete the whole communication , The problems to be dealt with in the middle are still very complex . Therefore, it is very necessary to adopt division of labor and cooperation

When there is layering , You need to target each layer , Specify some specifications , Let everyone abide by , And then better complete the work .

These norms are authoritative , Under the network discipline , Call it agreement (protocal)

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Network standard layering ：

1. Academic school ：OSI (Open System Interconnect) Provides a 7 Layer solution

application layer

The presentation layer

The session layer

Transport layer

The network layer

Data link layer

The physical layer

2. Practical school ：TCP / IP 5 Layer protocol （ four layers , In expression The physical layer is omitted ）

   application layer (Application Layer)： Implement and apply your own business problems ( Business is different , Different agreements )

   Transport layer (Transmission Layer): Realize any process on the host To The problem of any process communication on another host

   The network layer (NetWork Layer): Based on the function of data routing , Finally, cross LAN The host To The communication problem of the host

   Data link layer (Data Link Layer) : The problem of connecting and communicating between one host and another host in the LAN ( Conflict domain / Broadcast area )

   The physical layer (Physical Layer) : Hardware ensures data conversion / How to spread in network media

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The physical layer

LAN Internal communication problems ：

1. The simplest LAN ： That is, two hosts are directly connected
2. LAN with certain topology

These hosts are using a channel together, and the channel can only travel a limited number of signals at a time , This will cause the signal to conflict on the channel

Conflict domain ：

For example, in the scene of self-study , Everyone can take the initiative to speak , As a result, they can't hear each other's statements

Hub can effectively solve the problem of data transmission in network topology

The hub works in The physical layer （1. Copy the data 2. Signal amplification ）

Use a hub for networking , Born in the same conflict domain , This is caused by the working principle of the hub

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Data link layer

Already under the same conflict domain , How to solve , Through a certain conflict avoidance algorithm , Avoid conflicts .—— Data link layer to solve

The most common conflict avoidance algorithm ： After discovering the conflict , Silent random time , Send... Again

Even if there is a conflict avoidance algorithm , The data in the same conflict domain can be effectively transmitted , But the efficiency of signal transmission will still have an impact

1. Don't have too many hosts in the conflict domain
2. Minimize the occurrence of conflict domains

Switch （switch） Used to reduce conflict domains （ Divide the conflict domain ）

A hub ： Just copy , I don't know what the data is

Switch ： You need to understand the target address put into the data link layer ( Need to be able to parse the data link layer protocol ), Destination address of format data , Decide who to give the data or not

The switch works at the data link layer , According to the target address in the data , Send data only to the corresponding target

therefore , LAN can also be built through switches .

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Address ： The address of the data link layer refers to MAC Address ( Physical address )

MAC The length of the address is 48 position (6 Bytes ), Usually expressed as 12 individual 16 Hexadecimal number , Such as 00-16-EA-AE-3C-40 It's just one. MAC Address

The network card was dead when it was produced . Network cards all over the world MAC The address will not conflict

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Between different routers Divided into different broadcast domains

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Responsibilities of data link layer protocol ： One LAN Inside , No matter what kind of equipment 、 Which topology , The host and the host can communicate normally

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The network layer

Solve through data routing Span LAN Communication problems between hosts .

The router is located in at least two LANs （ There are at least two network cards ）

Routers work on the network layer .

Data can only be in the same LAN transmission , Routers span Two LAN, So cross LAN The transmission of needs to pass through the router .

route （route）: Do pathfinding ;

Router (router): A device used to complete the pathfinding function

The network layer protocol is to complete pathfinding , Realize cross LAN Host to host

The network layer has its own address ：IP Address

IP The address is a 32 The binary number of bits , Usually divided into 4 individual ”8 Bit binary number “ （ That is to say 4 Bytes ）

Usually use ” dotted decimal “ In the way of , namely a.b.c.d In the form of （a,b,c,d All are 0~255 A decimal integer between ）

IP The address is the software address ： Within the same network IP It shouldn't be ” repeat “ Of .IP The address changes at any time .

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Transport layer

Host to host has been fully connected . A process Want to be with Processes on another host communicate .

port （port）：0~65525 Unsigned number of two bytes

process <-> port ： Write the port , Corresponding to the specific process

people <-> cell-phone number ： Express delivery ： Recipient phone

A process can have multiple ports （ No conflict ）

A port can only be assigned to one person ( Can't repeat )

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Three important network devices

A hub

Working on the physical level

Switch

Working at the data link layer

Router

Working at the network layer

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Summary ：

application layer	Business			User mode
Transport layer	process To the process	port		Inside
The network layer	Span LAN Of Host to host	IP Address / route	Router	nucleus
Data link layer	LAN Inside Host to host	MAC Address / Broadcast area / Conflict domain	Switch	state OS Completion of the code
The physical layer	Physically ensure that the line is unblocked		A hub	Hardware

Important agreement ：

HTTP/HTTPS/DNS Belongs to application layer

TCP/UDP Belong to Transport layer

IP Belong to The network layer

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send out ： encapsulation ( You must carry the distribution and unpacking information )

Accept ： Unpack / Divide up （ According to the control information of this layer ）

Packaging and distribution

Different protocol layers have different names for packets , At the transport layer, it's called segments (segment), In the network layer, it's called datagram (datagram), In the link layer, it's called frames (frame).

When the application layer data is sent to the network through the protocol stack , Each layer of protocol has to add a data header (header), It's called encapsulation (Encapsulation).

The first message contains something similar to how long the first one is , load (payload) How long is , What is the upper layer protocol .

Data is packaged into frames and sent to the transmission medium , After reaching the destination host, the corresponding header of each layer protocol is stripped , According to the first part Of “ Upper layer protocol field ” The data is handed over to the corresponding upper layer protocol for processing .

Each layer can perform necessary operations according to the control information added by each layer

Between corresponding levels , It's completely transparent , This process can be completely ignored

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