Interview shock: why does TCP need three handshakes?

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 ：

1. I ： feed , Can you hear me ？
2. other party ： I can hear you , Can you hear me ？
3. 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 ：

TCP Three handshake process

TCP The three handshakes are as follows ：

1. The client sends SYN To the server , Indicates that you want to establish a connection ;
2. After the server receives the message , In response to a SYN and ACK（ Confirm response ） To the client ;
3. 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 ：

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 .