TCP handshake three times and wave four times. Why does TCP need handshake three times and wave four times? TCP connection establishes a failure processing mechanism

TCP Three handshakes and four cell phone swings ,TCP Why three and four handshakes ,TCP Connection establishment failure processing mechanism

1：TCP Three times mobile phone system

1. The client sends... To the server SYN package ：
   Sign a ：SYN
   Serial number ：Seq=x（ Random value ）
2. The server sends... To the client SYNACK message ：
   Sign a ：SYN ACK
   Serial number ：Seq=y
   Confirmation no. ：Ack=x+1
3. The client sends... To the server ACK message
   Sign a ：ACK
   Serial number ：Seq=x+1
   Confirmation no. ：Ack=y+1

2：TCP Four waves of mobile phone

1. The client sends a disconnection message to the server ：
   Sign a ：FIN
   Serial number ：Seq=u（ Random value ）
2. The server sends a message to the client ：
   Sign a ：ACK
   Serial number ：Seq=v
   Confirmation no. ：Ack=u+1
3. The server returns the data to be transferred to the client , Send the data to the client again after sending it TCP message ：
   Sign a ：FIN ACK
   Serial number ：Seq=w
   Confirmation no. ：Ack=u+1
4. The client sends a message to the server ：
   Sign a ：ACK
   Serial number ：Seq=u+1
   Confirmation no. ：Ack=w+1

3：TCP Handshake timeout processing mechanism

3.1： The first handshake failed

The client sends... To the server SYN Message to establish a connection . If the confirmation message from the server is not received within a period of time , It will wait for some time to continue sending SYN news . If it still does not receive the server confirmation message , Will continue to repeat the above actions , Until the number of transmissions exceeds the maximum number of retransmissions .

3.2： The second handshake failed

The server sends... To the client SYN ACK message , At this time, the server will block waiting for the client connection , Waiting for the client to send ACK message . If the client does not receive the confirmation message , Then the client will send to the server again SYN message , Execution is similar to The first handshake failed The handling action of .

3.3： The third handshake failed

Client received SYN ACK After confirming the message , Send to server ACK message . If the server does not receive the client's message for a long time ACK news , It will continue to send to the client SYN ACK news , Repeat the above , Until the maximum number of retransmissions is exceeded .

4：TCP Why three handshakes

TCP The two handshakes can only let the client know that the data it sends can be received by the server , But the server cannot judge whether the client can receive its own data .

As can be seen from the above figure , Just one client will cause the server to open multiple ports , Cause waste of service side expenses .

5：TCP Why wave four times

TCP Four waves ensure that the server is disconnected when all data has been transmitted .