7. Eleven state sets of TCP

TCP Five states in the process of three handshakes

Three handshake process (5 States )

client and Server side The initial state : closed

The first course : The state of the server changes
closed --> listen Creating networks socket
The second course : The client state changes
closed --> syn_sent Completed the first handshake process Sent syn
The third course : The state of the server changes
listen --> syn_rcvd Receive the first handshake request , And reply send out syn ack
The fourth course : The client state changes
syn_sent --> established Receive the message of the second handshake reply , And send the final confirmation message ack
The fifth course : The state of the server changes
syn_rcvd --> established Receive the confirmation message of the third handshake reply

syn Flood attack ：
SYN The attack used TCP Three times mobile phone system , The attack side uses forged IP Address sends a request to the attacked end ,
And the response from the attacked end The message will never reach its destination , Then the attacked end consumes resources while waiting for the connection to be closed ,
If there are thousands of such connections , Host resources will be exhausted , So as to achieve the purpose of attack .
Is to keep the state of the server in syn_rcvd The state of , Consume the resources of the server

TCP Six states in the process of four waves

The normal situation is the following five states ：

A special case ：

Four waves (6 States )
client and Server side The initial state is : established
The first course : The state of the server changes
established --> fin_wait1 Complete the first wave process send out fin ack
The second course : The client state changes
established --> close_wait Receive the first wave message And send the ack Information
The third course : The state of the server changes
fin_wait1 --> fin_wait2 Receive the second wave message
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Add : A special case
The server directly received fin and ack Third wave information , Directly by the closing Status replacement fin_wait2 state , The transition to time_wait state
fin_wait1 --> closing
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The fourth course : The client state changes
close_wait --> last_ack Complete the third wave process send out fin ack
The fifth course : The state of the server changes
fin_wait2 --> time_wait Complete the fourth wave process send out ack
The sixth course : The client state changes
last_ack --> closed Receive the fourth wave message
The seventh course : The state of the server changes
time_wait --> closed Wait for a while (60s 90s 120s)

Add ： Optimization can be targeted at time_wait To optimize , prevent time_wait Too many connections in state , The new connection can't get in .