Understand TCP's three handshakes and four waves with love

One 、TCP/IP The connection of

       When socket Established TCP/IP After the connection , The server will always listen to the connection request of the client , After the connection is successful, it will be a more reliable connection . Reason why TCP Reliable connection , That's because connecting and disconnecting require complex processes .
       Just like a pair of lovers . Before they became boyfriend and girlfriend , There must be one side that constantly seeks the other , Or they spent some unforgettable time together , They depend on each other . Then one of them said " Be my daughter （ male ） Friends "（ This is it. TCP The first handshake of the connection ）, Then the other party agreed （ There is a second handshake ）, Low three handshakes is naturally a formal boyfriend and girlfriend .

Two 、 Three handshakes

      TCP The third handshake is to confirm the connection , Ensure that the server and client can communicate normally .“ Three handshakes ” It is actively opened by the client , The server is turned on passively . It's not hard to understand , We are learning socket When programming , When programming the server , There is a code to listen to the connection ：listen, Then there's another API ：accpet Respond to the connection of the client ：

       Each handshake carries a message ：SYN Data packets . Every time you send data, you should confirm the integrity of the data , It's like a token of love （ The first handshake ）. In the picture ack Is the reply signal , Received SYN after , hold SYN Here you are, with your own Keepsake （ The second handshake ）, Prove that I have received your token of love SYN. The third time I received it was from myself SYN after , Confirm that you can communicate （ The third handshake ）, Complete the connection .

3、 ... and 、 Four waves

       Long period of division , A long time must be divided. , Since there is a handshake, there is a breakup , Ah No … wave .TCP The four waves are Confirm that the communication is disconnected , The process is initiated by one end disconnect request , The other party responds to the disconnection request .
       That is, between boyfriend and girlfriend, one day, someone suddenly felt that both sides were inappropriate , Break up （ Namely TCP Your first wave ）; The other party said yes , Can break up （ Produce a second wave ）; Break up can , When I get off work , Give you a love token after work （FIN）( This is a Third wave ); Received the changed Keepsake （FIN） Confirm the breakup ,（ Fourth wave ）.

       The second wave , Only the reply signal is sent , Because the receiver may still be dealing with tasks in process , Wait for the task to finish , This is the third wave .

5、 ... and 、 Why only three handshakes and four waves

       Because when that girl broke up , I'm still working overtime （ The receiving end is still processing the necessary procedures ）, I can only tell her ,“ Okay, I get it ”（ The receiving end can only pass ACK Tell the sender , A disconnect request has been received ）, Only after the overtime work is over can I return the love keepsake of that year to her （ After the receiving end processes the program , Send again FIN+(ACK+1) It means the third wave ）.

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