TCP state transition diagram

Pictured

1  Initiate connection initiatively

1.1  theory

The status change of the active connection request is shown in the following figure .

At the very beginning , Both the active requesting end and the passive end are in CLOSED state .

The first stage ： When the active requester sends SYN After the sign , Its status changes to SYN_SEND;

The second stage ： At this time, the passive party receives the request of the active party , Will send to the active party SYN Flag bit and de confirmation information ACK, After the active party receives the information , Send to the passive party

ACK Confirmation information , At this time, the state of the active party changes to ESTABLISHED, complete TCP Three handshakes connected .

SYN_SEND Generally, it is not easy to see

State change ：
CLOSE -- send out SYN -- SEND_SYN -- receive ACK、SYN -- SEND_SYN -- send out ACK -- ESTABLISHED（ Data communication state ）

1.2  test

Use the command to view the status

View clients

netstat -apn | grep client

Check the server

netstat -apn | grep server

Here is multi process communication , One server In a listening state , the other one server be in ESTABLISHED state

View port number

netstat -apn | grep 9527

2  Active close connection

2.1  theory

The status change of the active close connection request is shown in the following figure .

Before initiating the shutdown request , Both sides are in ESTABLISHED state .

The first stage ： The active party initiates a connection closing request , send out FIN, At this time, its state changes to FIN_WAIT_1, When it receives from the passive party ACK After confirmation , Its status changes to FIN_WAIT_2, This is the semi closed state .

The second stage ： The active party receives the connection closing request from the passive party （FIN）, It sends ACK confirm , The state changes to TIME_WAIT.

The third stage ： To really enter CLOSED state , Need to wait 2MSL Duration , Different systems have different time .

State change ：
ESTABLISHED（ Data communication state ） -- send out FIN -- FIN_WAIT_1 -- receive ACK -- FIN_WAIT_2（ Half closed ）-- Receiving end to end sending FIN -- FIN_WAIT_2（ Half closed ）-- Postback ACK -- TIME_WAIT（ Only actively close the connection party , Will experience this state ） -- etc. 2MSL Duration -- CLOSE

2.2  test

Use the command to view the status

Check the status of the active shutdown party

netstat -apn | grep 9527

Check the... Of the client TIME_WAIT

  Close the client first , Shut down the server again , At this time, the client is in TIME_WAIT state , And there is no problem starting the program again .

Shut down the server first , Close the client again , At both ends TIME_WAIT state , And at this time, the server has 2MSL Duration , Port multiplexing , Can't start .

3   The passive party accepts the connection

3.1 theory

The status change of passive connection request is shown in the following figure

The first stage ： Passively accepting connections is generally a server , So from the beginning, I was in LISTEN Listening state , When it receives from the active party SYN After the sign , Then send it to the active party SYN Flags and ACK After confirming the information , Its status changes to SYN_RCVD.

The second stage ： When the passive party receives the message sent by the active party ACK After confirming the information , Its status changes to ESTABLISHED.

State change ：

CLOSE -- LISTEN -- receive SYN -- LISTEN -- send out ACK、SYN（ Send successfully ） -- SYN_RCVD -- receive ACK -- ESTABLISHED（ Data communication state ）

  If the passive party does not receive the active party ACK Confirmation information , The passive party always sends SYN and ACK.

4   Passive close connection

4.1  theory

The status change of the passive close connection request is shown in the following figure .

The first stage ： Both the active and passive parties are in the stage of data communication , When the passive party receives the message sent by the active party FIN End mark , And the passive sends ACK After confirming the information ,

The state of the passive party changes to CLOSE_WAIT, It corresponds to the semi closed state of the active party .

The second stage ： Send... In the passive direction to the active party FIN After the end flag , Its status changes to LAST_ACK.

The third stage ： When the passive party receives the ACK After the confirmation , Its status changes to CLOSE.

State change ：

STABLISHED（ Data communication state ） -- receive FIN -- ESTABLISHED（ Data communication state ） -- send out ACK 
-- CLOSE_WAIT ( Explain the opposite end 【 Actively close the connection end 】 It's half closed ) -- send out FIN -- LAST_ACK -- receive ACK -- CLOSE

2MSAL Guarantee the last one ACK Can be successfully received by the opposite end .（ Waiting period , The opposite end didn't receive my message ACK, The opposite end will send again FIN request .）

It must appear in 【 Actively close the connection requester 】. --- Corresponding TIME_WAIT state .

5  A term is used to explain

CLOSED： Represents the initial state .

LISTEN： This status indicates a status on the server side SOCKET In a listening state , Can accept connections .

SYN_SEN： This state is related to SYN_RCVD Echo each other at a distance , When the client SOCKET perform CONNECT When the connection , It first sends SYN message , Then we went to SYN_SENT state , And wait for the third handshake sent by the server 2 A message .SYN_SENT Status indicates that the client has sent SYN message .

SYN_RCVD: This status indicates that SYN message , Under normal circumstances , This state is on the server side SOCKET In establishment TCP An intermediate state in a three-way handshake session when connecting , It's short . In this state , When received from client ACK After the message , Will enter the ESTABLISHED state .

ESTABLISHED： Indicates that the connection has been established .

FIN_WAIT_1: FIN_WAIT_1 and FIN_WAIT_2 The real meaning of state is waiting for each other FIN message . The difference is that ：FIN_WAIT_1 The state is when socket stay ESTABLISHED In the state of , Want to take the initiative to close the connection , Sent... To the other party FIN message , At this point the socket Enter into FIN_WAIT_1 state .FIN_WAIT_2 The status is when the other person responds ACK after , The socket Enter into FIN_WAIT_2 state , Under normal circumstances , The other party should respond immediately ACK message , therefore FIN_WAIT_1 The state is generally difficult to see , and FIN_WAIT_2 Status available netstat notice .

FIN_WAIT_2： The party that actively closes the link , issue FIN received ACK Enter this state later . It is called a half connected or half closed state . In this state socket Only data can be received , No hair .

TIME_WAIT: It means that I have received FIN message , And sent out ACK message , etc. 2MSL And then you can go back to CLOSED Available status . If FIN_WAIT_1 State, , Receive the other party and bring FIN Logo and ACK When the message is marked , You can go directly to TIME_WAIT state , Without going through FIN_WAIT_2 state .

CLOSING: This state is special , It belongs to a rare state . Under normal circumstances , When you send FIN After the message , It should be received first （ Or received at the same time ） The other person's ACK message , I'll get it back FIN message . however CLOSING Status means you send FIN After the message , Didn't receive the other party's ACK message , But also received the other side's FIN message . When will this happen ？ If both sides are almost at the same time close One SOCKET Words , So there's a simultaneous delivery FIN Message situation , That is to say, there will be CLOSING state , That both sides are closing down SOCKET Connect .

CLOSE_WAIT: This status indicates waiting for shutdown . When the other party closes a SOCKET Post send FIN Message to yourself , The system will respond to a ACK Message to the other party , At this point, you will enter CLOSE_WAIT state . The next? , See if any data is sent to the other party , If not, you can close This SOCKET, send out FIN Message to the other party , Close connection . So in CLOSE_WAIT State, , You need to close the connection .

LAST_ACK: This state is passively closed when one party is sending FIN After the message , Finally, wait for each other's ACK message . When I received ACK After the message , That is, you can enter CLOSED Available status .