Tcp- simple understanding of three handshakes and four waves

Three handshakes （three-way handshaking）

1. background ：TCP On the transport layer , The function is to provide reliable byte stream service , In order to accurately deliver the data to the destination ,TCP The protocol adopts the strategy of three handshakes .

2. principle ：

1） The sender first sends a message with SYN（synchronize） Flag packet to receiver .

2） After receiving , Send back one with SYN/ACK The packet of the flag passes the confirmation message , Means I received .

3） Last , The sender sends back another one with ACK Packets for flags , On behalf of me , Express ’ handshake ‘ end .

In popular parlance

1）Client： well , Li Si , Is my , Do you hear me ？

2）Server： I heard , Can you hear me ?

3）Client： well , We can all hear each other , Our correspondence can begin .

Four waves （Four-Way-Wavehand）

1. significance ： When the passive party receives the FIN Message notification , It just means that the active party has no data to send to the passive party . But not all the data of the passive party is completely sent to the active party , So the passive side won't shut down immediately SOCKET, It may also need to send some data to the active party , Send again FIN Message to the active party , Tell the active party to agree to close the connection , So here ACK Message and FIN In most cases, messages are sent separately .

2. principle ：

1） First wave ：Client Send a FIN, Used to close Client To Server Data transfer of ,Client Get into FIN_WAIT_1 state .

2） Second wave ：Server received FIN after , Send a ACK to Client, Confirmation No. is receipt No +1（ And SYN identical , One FIN Occupy a sequence number ）,Server Get into CLOSE_WAIT state .

3） Third wave ：Server Send a FIN, Used to close Server To Client Data transfer of ,Server Get into LAST_ACK state .

4） Fourth wave ：Client received FIN after ,Client Get into TIME_WAIT state , And then send a ACK to Server, Confirmation No. is receipt No +1,Server Get into CLOSED state , Finish four waves

In popular parlance

1）Client： I'm done with everything

2）Server： I've heard all about , But wait for me , I haven't finished

3）Server： Okay , I have finished

4）Client： well , Then our correspondence is over l

link ：https://www.jianshu.com/p/d3725391af59