The difference between TCP and UDP, the three handshakes of TCP and the four waves of TCP

One 、 TCP And UDP The difference between

1. The same thing

  All belong to the transport layer protocol 

2. Difference

	TCP	UDP
1	Based on connections , High reliability	Based on connectionless , Low reliability
2	It takes three handshakes 、 Reconfirmation and other processes , Poor real-time , At the same time, the process is complex , Vulnerable to attack	There is no connection , Therefore, the real-time performance is very strong , A little safer
3	When transmitting data of the same size , Headlines 20 byte , and tcp No packet loss , Costly	When transmitting data of the same size ,udp The first cost is only 8 Bytes , and udp Frequent packet loss , It's a small expense
4	Every one of them tcp Connections can only be point-to-point	udp Support one-to-one 、 One to many 、 For one more 、 Many to many interactive communication
5	It's called transmission control protocol , Low transmission efficiency , High reliability	Called user datagram protocol , It is suitable for low reliability transmission , A small amount of data

Two 、 TCP Three handshakes of

TCP Some nouns need to be explained , To understand 

1. A term is used to explain

SYN:  Sync serial number , Used to initiate a connection ,SYN=1 Can't carry data , But it consumes a sequence 
ACK:  Confirm logo , When ACK=1 The confirmation field is valid only when 
seq:  Serial number , That is, the serial number of the packet itself , Use... For data transmission after connection 
ack:  Acknowledgement of receipt of a packet , The value is the sequence number of the packet waiting to be accepted （ Is the serial number of the packet that the other party is expected to continue sending ）

2. Handshake process

   for the first time ：  client C To the server S Send connection request message , In the header of the message SYN = 1,ACK = 0, Pick a serial number at random seq = i  As the initial serial number ,
  		   At this time, the client enters SYN_SEDN Sync sent status .
  		  
   The second time ：  The server receives the connection request message from the client , If you agree to establish a connection , Then send a confirmation message , Confirm... In the header of the message SYN = 1 , ACK = 1 ,
   ack = i + 1 ,seq = j ( The initial serial number of the server ), here , Server entry SYN_RCVD Sync receive status .
   
   third time ：  After the client receives the confirmation message from the server , A confirmation message will be sent to the server , Tell the server that it has received its acknowledgement message and is ready to establish a connection , In the header of the confirmation message SYN = 0、
   ACK = 1、ack = j + 1、 seq = i + 1. The server receives the client confirmation message , here , Server access ESTABLISHED Connection state established .

3. TCP Why use three handshakes , Instead of shaking hands twice or four times

TCP The purpose of the third handshake is to synchronously connect the serial number and confirmation number of both parties , And exchange TCP Window size information 

 Why use three handshakes ：  To prevent invalid request message segments from being accepted by the server , So there's a mistake .

source ：TCP Why use three handshakes , Not two or four handshakes

3、 ... and 、 TCP Four waves （ Release ）

Active shutdown by one party , The other side closes passively

 for the first time ： client C Send a FIN, Used to close the client C To the server S Data transfer of , client C Get into FIN-WAIT-1 state 

 The second time ： Server side S received FIN after , Send a ACK To the client C, Confirm that the serial number is the received serial number +1, Server side S Get into CLOSE-WAIT

 third time ： Server side S Send a FIN, Used to close the server S To client C Data transfer of , Server side S Get into LAST-ACK

 The fourth time ： client C received FIN after , client C Get into TIME-WAIT state , And then send a ACK To the server S,
        Confirm that the serial number is the received serial number +1, Server side S Get into CLOSE state , Complete the fourth wave .