Second handshake?? Three waves??

When we learn 《 computer network 》 At the time of this course , I will definitely learn TCP This classic transport layer protocol ,TCP There is a feature called Connection management , This feature leads to two concepts ,TCP Three handshakes and four waves . Why three times , And why is it four times this problem has troubled me for a long time , Today I will explain this reason in detail .

On this question , Most articles will quote 《 computer network 》 A sentence in it .

Prevent invalid connection requests from being sent to the server , So there's a mistake

Actually, that's too broad , Can't really understand .

TCP What is the communication process like ？

Send messages to each other , This is a very easy thing to understand .

Such as this ：

At this time, we need to introduce a concept , be called TCP The buffer ：

TCP The buffer of can be understood as a very large array ：

When there is data transmission in our application layer , Writing data to the transport layer through the system call interface is written in TCP In the buffer of the Protocol ,TCP Meeting malloc Two segments of memory space . The role of having a buffer is obvious ：

1. Improve the efficiency of the application layer
2. Only OS,TCP The protocol can know the network , Even the status details of the other party , So only TCP agreement , Can handle how to send , When to send , How many hair , What to do if something goes wrong , Because the buffer exists , So we can achieve application layer and TCP Decoupling

my TCP There is a lot of data in the buffer , Some data has been sent , Some have not been sent , So where should we develop and send the connection this time ？ This is not necessarily from 0 Location ( Subscript ) At the beginning , therefore client End sum server The client needs to know where the buffer of the other party comes from ( The array subscript ) Develop and send data .

• For reliable data transmission , TCP The parties to the agreement , You have to maintain a serial number , To identify the packets sent , What has been received by the other party . The process of three handshakes is that the communication parties inform each other of the starting value of the serial number , And confirm that the other party has received the start value of the serial number
• If it's just two handshakes , At most, only the starting serial number of the connection initiator can be confirmed , The serial number chosen by the other party is not confirmed

So let's see TCP The whole process of communication ：

You can see the process of confirming the serial number by both parties .

Why? TCP You need to shake hands ？ Can you stop shaking hands ？

If the contrast UDP Agreement can be found ,UDP Handshake is not required in the Protocol , Here we need to draw TCP and UDP The characteristics of the Protocol , A key feature is ,TCP The agreement is a reliable agreement ,UDP An agreement is an unreliable agreement .

Reliable agreement means ： The data received by the receiver is complete , Orderly , Error free .

Unreliability means ： The data received by the receiver may be partially lost , Sequence is not guaranteed .

TCP Protocol in order to achieve reliability , Both sides of the communication need to know whether the data they send has been received by the receiver , If not received , You need to resend . In order to achieve this requirement , About to lead out Serial number and Confirm the serial number .

The sender is sending a packet （ Assume the size is 10 byte） when , At the same time, send a serial number ( Assuming that 500), After the receiver receives the packet , You can reply with a confirmation number （510 = 500 + 10） Tell sender “ I have received your packet , You can send the next packet , The serial number from 510 Start ” .

In this way, the sender can know which data has been received , What data was not received , Need to resend .

So there is the process of the above figure , This explains why 3 Time , instead of 2 Time .

Someone may be interested in the above picture TCP There are questions about the data of the communication process .

• Why three handshakes, the last handshake , Reply in the diagram above seq = x+1 .

Take a look at a passage in the big black book ：

What is said here is very complicated , It's actually ： my ACK The serial number inside is the next one I need to get .

So why four waves ？

Can't you wave your hand like three handshakes ？

The answer is no , as a result of ：TCP Agreement is Full duplex .client and server Both ends need to transmit and process data . I client End request to close the link , send out FIN, from OS The kernel rules , send out FIN It will echo immediately after ACK To show that I have received this message , But my server The client may be processing data at this time , So you can't close it immediately , You can only send it after the matter is handled FIN to client, It means I'm done ,client Echo after receiving ACK To indicate receipt of , then server close ,client stay 2MSL After that, it is also closed .

If server There is nothing to deal with , that FIN and ACK Will be merged and sent , That is the so-called three waves .

in other words , Compared to three handshakes , Four waves, one more I'll call you back when I finish my work The process of .

It's like I go out to play with walnuts ：

I ： walnut , Let's go out and play ！

walnut ： All right ！ But I have something , I'll call you after I finish the work .

walnut ： I'm done , Let's go ！

I ： well ！

Why? client etc. 2MSL Just close ？

This is also the guarantee of reliability , If I send it ACK I'll wait for the packet to be sent again , What if I quit the second ？