TCP three handshakes you need to know

TCP, Full name Transmission Control Protocal. From the name, we can know that it is used for Control transmission The protocol at the transport layer .

TCP be located TCP/IP and OSI The transport layer of the model . What we use most HTTP agreement , The bottom layer usually uses TCP agreement .

If you want to create TCP Connect , We need to send three requests at the beginning to confirm that the communication ability of both sides is normal , These three connections are called TCP Three handshakes of .

TCP Three handshakes of

The following figure for TCP The structure of the head , our TCP The three handshakes mainly use the yellow part .

Now let's talk about these three handshakes .

The first handshake

The client sends a... To the server TCP message . stay TCP In the head ：

1. take SYN The flag bit is set to 1;

2. And then to seq Assign a random number （ Suppose the random number is x）

The so-called flag bit , It's actually one bit of data , Set to 1 , In fact, it is similar to programming to set a Boolean value to true.

SYN On behalf of the synchronization （sync） It means ,seq It's the serial number （Sequence number） It means , Is a place where values can be loaded , We set it as a random number , It is used as a voucher （token） Of , The server will use it later .

The second handshake

Server received SYN, Know that the client wants to establish TCP Connect , And got one seq random number .

The server then sends TCP message ,TCP In the head ：

1. take ACK The flag bit is set to 1, Means known （acknowledge） Client's connection request ;

2. ack The value of is set to that passed by the client seq value + 1（ namely x+1）;

3. take SYN The flag bit is set to 1, It means that the server should also be connected ;

4. seq Set a random number with the value of （ Suppose the random number is y）

Server through ACK and ack To inform the client that the connection request is known , And send your own SYN and seq To request a connection with the client .

The third handshake

Client received TCP request , Verify first SYC Is it 1, seq Is it x+1.

If it is , It indicates that the sending of the server is the return of the first handshake . The client will send another TCP request ：

1. ACK Set to 1;

2. ack Set as the server ack Value + 1（ namely y + 1）

After three handshakes ,TCP The connection is established .

summary

Three handshakes , Simply speaking , That's it ：

1. client -> Server side ：SYN=1, seq=x （x Is a random number ）

2. Server side -> client ：ACK=1, ack=x+1, SYN=1, seq=y （y Is a random number ）

3. client -> Server side ：ACK=1, ack=y+1

I'm brother watermelon , Welcome to follow me , Master more front-end interview questions .