TCP's three handshakes and four waves (brief introduction)

TCP brief introduction ：

Transmission control protocol -- Connection oriented reliable transport protocol

After completing the basic work of the transport layer , It also needs to further ensure the reliability of transmission . and UDP The difference is ,TCP It provides reliable connection oriented transmission services , It also provides functions such as flow control .

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TCP The main identification mark bit in the message format

 

1.SYN Synchronization flag bit ,SYN = 1, This data packet represents the connection data packet between the client and the server .

2.ACK Confirm the flag bit ,ACK = 1, This data packet is a reply packet , Used to confirm received packets .

3.FIN End tag bit ,FIN = 1, It shows that this is a data packet that ends the connection unilaterally .

 

 

  Three handshakes   

The first handshake ： Establishing a connection   Among the messages sent by the client to the server SYN position Set to 1, Indicates a request to establish a connection with the server .
The second handshake ： Server received SYN Message segment The server receives the connection request message from the client , Then it sends a connection establishment request to the client and responds to the connection request sent by the client , In this case, the server sends messages to the client SYN position and ACK position Are set to 1.
The third handshake ： Client receives server's SYN+ACK Message segment After the client receives the message from the server , It is known that the server has received the connection establishment request sent by itself , And ask to establish a connection with yourself , Finally, the client responds to the message sent by the server .

  Four waves  

First wave ： The client is ready to disconnect Among the messages sent by the client to the server FIN position Set to 1, Indicates a request to disconnect from the server .
Second wave ： Server received FIN Message segment The server responds to the disconnection request sent by the client , Return the confirmation response package （ACK Message segment ）.
Third wave ： Server send FIN Message segment Request close connection    The server received a request from the client to disconnect , And there is no data to send to the client , The server will send a disconnect request to the client .
Fourth wave ： The client sends to the server ACK Message segment   The client responds to the disconnection request sent by the server , Send to the server ACK Message segment , The server will shut down directly after receiving , The client waits 2MSL After closing .

MSL refer to （ Maxium Segement Lifetime）, Maximum message lifetime . When the client receives FIN After package , Will wait for 2MSL Time , If in 2MSL Within time , I didn't receive it again FIN The package will shut down the client .

Popular speaking  

Three handshakes  

A： Do you want to have an appointment? ？

B： about , Do you have an appointment ？

A： about .（ Connection established successfully ）

Four waves  

A： I am afraid that l have to go .

B： Um. , got it , Wait for me to say two more words .

B： That's it , You can leave now. .

A： go .（ Successfully disconnected ）