Three handshakes and four waves for the interview summary

One ： Three handshakes

Content ：

1. What is three handshakes ？

• The first handshake ： The client will SYN Set as 1, Generate an initial sequence number randomly seq=x, Send to the server , Get into SYN-SENT state ;

• The second handshake ： Server received from client server SYN After the message , take SYN Set as 1,ACK Set as 1, Generate an initial sequence number randomly seq=y, At the same time, tell the client that the serial number of the next data transmission is x+1;

• The third handshake ： Client receives server's SYN-ACK After the message , take SYN and ACK All are set for 1, At this time, the serial number sent is x+1 The data of , At the same time, tell the server the serial number of the next data transmission is y+1.

2. The function of three handshakes ？

① Confirm the receiving capacity of both parties , Whether the sending ability is normal .

② Specify your own initialization serial number , Prepare for reliable transmission later .

3. The function of the third handshake （TCP Can I make a connection by shaking hands twice ？ Why? ？）：

Prevent the invalid connection request message segment from being sent to the server .

The first connection request message segment sent by the client is not lost , But stay at a network node for a long time , As a result, it will not arrive at the server until some time after the connection is released . Originally, this is a message segment that has already failed . But after the server receives the invalid connection request message segment , Mistakenly thought that it was a new connection request sent by the client again , Then send the confirmation message segment to the client , Agree to establish a connection .

If there's no third handshake , So as long as the server confirms , A new connection will be established , But the client did not actually send a request to establish a connection , Therefore, the server will always wait for the client to transmit data , The resources of the server will be wasted .

4. If a connection has been established , But what if the client suddenly fails ？

TCP There is a live timer in , Every time the server receives a request from the client, it will reset the timer , The time is usually set to 2 Hours , If you haven't received any data from the client in two hours , The server will send a detection segment , After every 75 Seconds to send , If it happens in a row 10 There is still no response , The server thinks the client has failed , Connection is closed .

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Two ： Four waves

Content ：

1. What is four waves ？

• First wave ： The client sends FIN Message to server , At this time, the serial number of the data sent by the client is seq=u, Get into FIN-WAIT-1 state ;
• Second wave ： The server receives FIN message , Respond ACK message , Tell the client to receive his request and agree , At this time, the serial number of the data sent by the server is seq=v, And tell the server that the serial number of the next data you want to receive is ack=u+1, Get into CLOSE-WAIT state ;
• Third wave ： Wait until the server data transmission is completed , Send to the client FIN-ACK Request close connection , At this time, the data serial number sent by the server is seq=w（w Is not necessarily v+1,w It refers to the sequence number of the last byte after the end of data transmission +1）, And tell the client that the serial number of the next data you want to receive is ack=u+1, Get into TAST-ACK state ;
• Fourth wave ： The client receives the request from the server , send out ACK Message to server , Agree to the shutdown request of the server , At this time, the serial number of the data sent by the client is u+1.ack=w+1. Get into TIME-WAIT Stage .

2. Why? TIME_WAIT Status needs to pass 2MSL( Maximum segment lifetime ) To return to CLOSE state ？

• ①MLS yes “ Maximum message lifetime ”. When waving the server for the third time FIN-ACK After package , The client needs to respond , issue ACK The package agrees to the shutdown request of the server , If you wave your hand for the fourth time in this process ACK The bag is missing , The server will resend the third wave after the timeout FIN-ACK package , So at this time, the client needs to wait for the server to retransmit , And this waiting is not indefinite , But wait 2MLS after , The client did not receive the server's third wave of retransmission packets , Then the client determines the fourth wave ACK The packet was successfully received ,TCP End of connection .

• ② Prevent the invalid connection request message segment from appearing in the new connection .

  stay TIME-WAIT In the state of , Ports on both ends cannot be used , Wait until the 2ML You can continue to use it after the end of time , When the connection is in 2MLS In the waiting phase , Any late segment will be discarded .