Graphic network: uncover the principle behind TCP's four waves, combined with the example of boyfriend and girlfriend breaking up, which is easy to understand

Hello , This is the network technology alliance station .

In the last section, we illustrated TCP Three handshakes of , Understand the principle of the three handshake process 、 Why do you need three handshakes instead of two to establish a connection , Then shake hands three times when establishing a connection , that TCP How is disconnection broken ？

In this section, , Rego continues to show you the network , Explore TCP The principle behind four waves to disconnect .

Let's go straight to ！

TCP A brief review of the three handshakes

TCP  It's a full duplex protocol , To establish a connection , A reliable link can be established through three handshakes ：

• First, the client will send a
  SYN
    news （
  SYNchroniz
  e  Abbreviation ） Indicates that the client wants to establish a connection with the server .

• The server receives... From the client SYN After the message , reply
  SYN-ACK
  message , Including a SYN Serial number and one ACK Number .

• The client receives... From the server  SYN-ACK  After the news , Send to server
  ACK
    news .

TCP Four waves

Examples in life

Both ends of the connection can be terminated  TCP  Connect , Even unilateral termination is possible , This is also called

Half open connection

, Even if one party has disconnected , The other side can still transmit data .

We said four waves , It means that both parties disconnect .

Let's first explain with examples in life ：

Xiaomei and Xiaoming are college classmates , When I first went to college , Xiao Ming is tall and handsome , Can also play basketball , Xiaomei adores him very much , Xiao Ming also likes Xiaomei very much , That's it , On a dark night with a high wind , They became lovers .

But over time , Coupled with environmental changes , The two people gradually lost their enthusiasm , So Xiaomei broke up with Xiaoming . The process of breaking up is similar to TCP Four waves ：

• for the first time ： Xiaomei said to Xiaoming “ Xiao Ming , Have been together for so long , I don't think we're suitable , Break up ”

• The second time ： Xiao Ming was not too surprised after hearing it , But you can't promise directly , Otherwise, I'll become an iron scum man , So wave your hand for the second time , He said politely “ other , I have something else to say to you , In fact, you are quite good , Not only are people beautiful , Good personality, too , Also reasonable , I study hard , I'm very happy to meet you in College ！”

• third time ： Xiao Ming's bilibala is about the same , It's time to show your true face , Then he said to Xiaomei “ Now that you have made up your mind , It's no fun for me to pester you again , Then break up peacefully ~”

• The fourth time ： Xiaomei heard a lot of false words from Xiao Ming , Very angry , Just dumped a “ That's it , Delete each other's wechat , bye ！”

Then a big red exclamation mark will appear on Xiaoming's wechat ！

This is four waves , It's not very image ？

Next time if you forget TCP The steps of four waves , I think you can imagine yourself as a scum man

Okay , Let's get back to business , Let's have a good analysis TCP The principle of four waves .

Analysis of the principle of four waves

Pictured , The client wants to disconnect from the server ：

• The client will send a  FIN  news ,
  FIN
    yes  
  FINISH
    Abbreviation ,FIN  The purpose of the message is to make  TCP  Ability to gracefully terminate established connections , Then the client goes into what is called  
  FIN-WAIT  state
  The state of , stay  FIN-WAIT  State, , The client continues to receive messages from the server , And continue to process messages that are already in the queue , But the client will not send any additional data .

This can be associated , Xiaomei is sending the breakup message , Boyfriend Xiaoming has been sending messages to Xiaomei , Xiaomei entered the queue without reading the message , Messages in the queue , Xiaomei will still watch , But I will not take the initiative to reply to these messages .

• The server sends... To the client ACK news , Confirm that he has received the end message , At this time, the server will no longer accept data from the client .

• The server can continue to send data to the client （ Finally, tell me something ）, If the server has no more data to send , It will also be sent by  FIN  Message to terminate the connection .

• Then the client sends to the server ACK Confirm that you have received the message of active disconnection from the server .

thus ,TCP Connection is broken ！

Pictured ,M and N All serial numbers , They represent client and server respectively , It is also to distinguish between the server and the client , Like to see M Just think of the message coming from the client , notice N Just think that the message comes from the server .

So the process becomes ：

• The client sends... To the server
  FIN M
  To the server .

• The server sends... To the client  
  ACK M+1
  , Client received M+1, I know that the server has received its own Finish news .

• The server sends... To the client
  FIN N
  The representative is that the server actively sends the end message to the client .

• The client replies to the server
  ACK N+1
  news , The server sees that it is its own N, So I know that the client also received its own end message .

thus TCP You have successfully disconnected .

Maximum life cycle of disconnection

Here is another example , Xiaomei is finishing sending “ That's it , Delete each other's wechat , bye ！” After the news , Will wait at most

Four minutes

, Give Xiao Ming a chance to stay ：

If in these four minutes , Xiao Ming didn't ask to stay , I'm sorry , It's over completely ！Game Over！

stay 【 Analysis of the principle of four waves 】 In the figure , We have noticed TIME_WAIT state , The client sends the last  ACK After paragraph , The connection will not terminate immediately , Because there is no guarantee that the last sent packet has actually arrived , The client or server will initially remain in a time waiting state , until  ACK  Paragraph and any new  FIN  Maximum life cycle of segment （MSL,Maximum Segment Lifetime ） To enter

CLOSED state

.

This mechanism is based on two considerations ：

• Prevent the use of homologous addresses 、 Source port 、 Other of destination address and destination port  TCP  The connection received a delayed data segment .

• Guarantee TCP The connection was properly closed remotely , That is, wait for the party that passively closes the connection to receive ACK The corresponding message .

summary

this paper , Rego explained to you with vivid examples TCP The process of waving four times , I believe you can understand it as soon as you read it , Then it also analyzes the principle of four waves , Let's take a look at the overview of this article ：

• TCP A brief review of the three handshakes

• TCP Four waves

• Examples in life

• Analysis of the principle of four waves

• Maximum life cycle of disconnection

• summary

Thank you for reading , If you think the article will help you , Don't forget to like 、 Collection ！ Any questions , Welcome to discuss with me in the comment area below ！！！