TCP three handshakes and four swings

TCP（Transmission Control Protocol, Transmission control protocol ） It's a connection oriented protocol , in other words , Before sending and receiving data , Must establish a reliable connection with the other party . One TCP The connection has to go through three times “ dialogue ” To build it up , The process is very complicated , Just briefly describe the simple process of these three conversations .

Briefly describe the process of three handshakes ：
Before we start to explain , First, let's talk about the full names of several important fields , Convenient memory ：

seq：（sequence number） Serial number
ack：（acknowledgement number） Confirmation no.
Sign a ：
SYN ：(SYNchronization） Sync
ACK ：(ACKnowlegment） confirm
FIN   :（FINish） End
Let's take a look at the general picture first ：

The first handshake ：Client The flag bit will be synchronized SYN Set as 1（SYN by 1 It means to establish a connection , After the connection is successful, the location will be set to 0）, Request number seq=x（ In all byte arrangements , Which byte does the request start from , This serial number generally indicates which serial number has been sent to , After the server agrees, it will send from the next serial number , The first handshake only has the request serial number but no confirmation number ）, And send the packet to Server,Client Get into SYN_SENT state , wait for Server confirm .

The second handshake ：Server After receiving the data packet, the synchronization flag bit SYN=1 know Client Request to establish a connection , Confirm flag bit ACK Set as 1（ Only then will there be a confirmation flag , The first handshake did not confirm the flag bit . When the confirmation flag is 0 when , Confirmation number does not work ）,ack=x+1（ Confirm that the sequence number is equal to the request sequence number +1, Express x+1 Previous Server received , from Server The request sent has been received ）.TCP It's full duplex protocol , therefore Server May also give Client send data , therefore Server Also will Client Establishing a connection ,Server The flag bit will be synchronized SYN Set as 1（Server We should also ask for help Client Send a request , therefore SYN It should also be set to 1）,seq=y It means Server to Client Start sequence number of sent data . And send the packet to Client To confirm the connection request ,Server Get into SYN_RCVD state .

The third handshake ： Because the connection should be bidirectional ,Server After confirmation, only Client To Server Connected , therefore Client Also make sure , To make Server towards Client The connection of is also connected .Client and Server Get into ESTABLISHED state , Complete three handshakes , And then Client And Server You can start transferring data between .

       
Design such a similar scenario ：

1、 Client host C say ：“ Can I send you data ？”

2、 The server S say ：“ Tolerable , But I may also send you data .”

3、C：“ good , Let me start sending data to each other .”

Four waves
Four waves （Four-Way Wavehand） Termination TCP Connect , To disconnect one TCP When the connection , The client and server need to send a total of 4 Packages to confirm disconnection . stay socket Programming , This process is performed by either the client or the server close To trigger .

because TCP Connection is full duplex , therefore , Each direction must be closed separately . The principle is that when one party completes the data transmission task , Send a FIN To terminate the connection in this direction , Receive a FIN It just means there's no data flow in this direction , No more data . But here TCP Data can still be sent on the connection , Until the other direction also sent FIN. Active shutdown will be performed by the first party to close , The other party performs a passive shutdown , This is what is described above .
 

First wave ：Client Send a FIN, And the selection number seq=u（ Express ：u All previous data has been sent , And the data is sent to u It can be cut off , There will be no more data ）, Used to close Client To Server Data transfer of .Client Get into FIN_WAIT_1 state .

Second wave ：Server received FIN after , Send a request number seq=v And confirm the serial number ack=u+1 to Client.Server Get into CLOSE_WAIT state .
Third wave ：Server Send a FIN, The request number is the latest seq=w And confirm the serial number ack=u+1, Used to close Server To Client Data transfer of .Server Get into LAST_ACK state .

Fourth wave ：Client received FIN after ,Client Get into TIME_WAIT state , And then send a ACK to Server, Confirm serial number is w+1.Server Get into CLOSED state , Finish four waves .

The scene of four waves is roughly like this ：

1、 Client host C say ：“ I have no data , Disconnect . ”

2、 The server S say ：“ good , But I still have data （ Continue to C send data , here C Can't give S Sent data , But if you have to, just accept S Data sent ）.”

3、（ When S to C After sending data ）S say ：“ I'm done , Disconnect .”

4、C say ：“ good , Disconnect .”