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Interview shock 68: why does TCP need three handshakes?

2022-07-26 09:37:00 【InfoQ】

TCP  Three handshakes is a classic interview question , It means  TCP  Before transferring data , Need to carry out  3  The connection can only be formally established after several interactions , And data transmission .
TCP  The reason why we need  3  This handshake is because  TCP  Both sides are full duplex
. Full duplex refers to ,TCP  Either end is the sender , It is also the receiving party , So this requires  TCP  Both sides of communication should ensure their sending ability , You have to ensure your receiving ability . It's like making a phone call , Both communication parties should ensure that they can use the microphone （ Transmit sound ） And headset （ Receiving sound ） It's all normal , Only in this way can we communicate effectively , Usually on the phone , It all starts like this ：

  • I ： feed , Can you hear me ？
  • other party ： I can hear you , Can you hear me ？
  • I ： I can hear you , Let's talk about business .

TCP  Three handshakes are the same ,3  Details of the ability to prove this handshake are as follows ：

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TCP  Three handshake process

TCP  The three handshakes are as follows ：

  • The client sends  SYN  To the server , Indicates that you want to establish a connection ;
  • After the server receives the message , In response to a  SYN  and  ACK（ Confirm response ） To the client ;
  • The client receives... From the server  SYN  After the message , In response to a  ACK  message .

The specific implementation process is shown in the figure below ：

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summary

TCP  The reason why we need  3  The second handshake , Because  TCP  Both sides of the communication are full duplex , So we have to go through  3  The sending and receiving capabilities of both parties can only be confirmed after several interactions , also  TCP  Handshake must be  3  Time , If it is  2  The second handshake , It cannot prove the sending ability of the server and the receiving ability of the client ; Nor is  4  The second handshake , because  3  Things that can be proved once , Then shake hands with each other  1  This time is completely unnecessary .
Reference resources  &  Acknowledgement
《 Code of effective ：Java Development Manual 》

《Offer  coming 》

It's up to you to judge right and wrong , Disdain is to listen to people , Gain or loss is more important than number .
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