A detailed summary of four handshakes (or four waves) over TCP connections

The process of waving four times ：

（ client || The server can initiate a wave motion . Suppose the client starts waving first （ Close request ）. Suppose the client is considered the active party , The server is treated as a passive party .）

	First wave	Second wave	Third wave	Fourth wave
From A to B	client → The server	The server → client	The server → client	client → The server
state	client ： ESTABLISHED→ FIN_WAIT1	The server ： ESTABLISHED→ CLOSE_WAIT client ： FIN_WAIT1→ FIN_WAIT2	The server ： CLOSE_WAIT→ LAST_ACK	client ： FIN_WAIT2→ TIME_WAIT（ After waiting for the time designed by the timer ）→CLOSED The server ： LAST_ACK→ CLOSED
Message name	FIN message （ Connection release message segment ）	ACK message ( Confirm message segment )	FIN message （ Connection release message segment ）	ACK message ( Confirm message segment )
What	FIN = 1 Serial number ：seq = u	ACK = 1 Confirmation no. = Client initial serial number （ISN）+ 1： ack = u + 1, seq = v	FIN = 1, ACK = 1 Confirmation no. ack = u + 1, seq = w	ACK = 1 Confirmation no. ack = w + 1, seq = u + 1
What do you mean by that?	FIN == Finish Represents it （ client ） Don't send any more data	ACK == Acknowledge Inform the other party （ client ）： Your sending channel has been closed	FIN == Finish Represents it （ client ） Don't send any more data	ACK == Acknowledge Inform the other party （ client ）： Your sending channel has been closed
Whether the transmission channel is closed		The active party has received ACK message , State by FIN_WAIT1→ FIN_WAIT2, The transmission channel of the active party is closed .	The active party has received FIN message , The kernel automatically replies ACK message , The connection state is determined by FIN_WAIT2→ TIME_WAIT,Linux About under the system 1min after TIME_WAIT State connection will be closed completely .	The passive party receives ACK message , Connection is closed .

 

About TIME_WAIT Several problems of state ：

Why should the active party TIME_WAIT（ After waiting for the time designed by the timer ）→ CLOSED ？

Because the active side sends... To the passive side ACK message , The passive party does not receive ACK The message was still in LAST_ACK state , If the active party sends this ACK The message did not reach the passive party , The passive party will resend FIN message .

If the active party does not retain TIME_WAIT state , Directly from TIME_WAIT To CLOSED state , At this time, the port connected by the active party is free , You can create a new connection . But the passive side FIN The message may arrive again （ The router in the network repeatedly sends || The passive party has not received tcp_orphan_retries Parameter retransmitted ）, In this way, new connections for normal communication may be sent repeatedly FIN The message is closed by mistake . therefore , Retain TIME_WAIT Status can be used to cope with resending FIN message .

TIME_WAIT How to deal with too many states ？

TIME_WAIT Too many states will occupy system resources .

terms of settlement ？

•      modify TIME_WAIT Upper limit of connection status

•      Start the rapid recycling mechanism

•      Turn on the reuse mechanism

•      Modify short connection to long connection

•      The client actively disconnects

Refer to the post ：https://yuanrengu.com/2020/77eef79f.html