What is the difference between TCP and UDP?

Classic interview questions TCP and UDP What's the difference? ?

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One 、TCP and UDP What is it? ？

TCP：
Transmission control protocol （TCP,Transmission Control Protocol） It's a connection oriented 、 reliable 、 Transport layer communication protocol based on byte stream , from IETF Of RFC 793 Definition .

UDP：
Internet The protocol set supports a connectionless transport protocol , This protocol is called user datagram protocol （UDP,User Datagram Protocol）.UDP Provides an application with a way to send encapsulation without establishing a connection IP The packet approach .RFC 768 It describes UDP.

Two 、TCP and UDP What's the difference? ?

1.TCP and UDP Sum up the difference

TCP And UDP Sum up the difference ：

1. TCP Connection oriented , Establish a connection with three handshakes , Four waves to receive and remove the connection ;UDP It's disconnected , That is, you don't need to establish a connection before sending data , This way is UDP It brings efficient transmission efficiency , But it also makes it impossible to ensure the successful sending of data .
2. TCP Is a reliable means of communication . adopt TCP Connect the transmitted data ,TCP Retransmission via timeout 、 Data verification and other methods to ensure that the data is error free , No loss , No repetition , And arrive in order ; and UDP Because there is no need to connect , Will transmit at maximum speed , But there is no guarantee of reliable delivery , That is, there will be loss 、 Repeat and so on .
3. TCP Byte stream oriented , It's actually TCP Think of data as a stream of unstructured bytes , Due to the problem of connection , When the network fluctuates , Connection may have response problems ;UDP It's message oriented ,UDP no congestion control , Therefore, the network congestion will not reduce the transmission rate of the source host .
4. Every one of them TCP Connections can only be point-to-point ; and UDP Do not establish a connection , So it can support one-on-one , One to many , Many to one and many to many interactive communication , That is, you can accept multiple people's bags at the same time .
5. TCP Connection required , First cost 20 Bytes compared 8 Bytes of UDP It looks big .
6. TCP The logical communication channel of is a full duplex reliable channel ,UDP It's an unreliable channel .

2.TCP Three handshakes and four waves

2.TCP Three handshakes

Three handshakes are TCP The way to ensure that the connection is established reliably ：

• The first handshake ： A to B Text and say ：“B, Are you free now? ？”
• The second handshake ： B At this time, I received A Information about , Then on A say ： “ I'm free , And you? ？ Do you have a second? ？ ”
• The third handshake ： A At this time, I received B The confirmation of , And said, ：“ I'm free, too , Let me tell you something .”

After three handshakes ,A and B Can be sure of such a thing ： The communication between both parties can be carried out smoothly . such , The two sides can start a normal dialogue .

2.TCP Four waves

Four waves are TCP A way to ensure that the connection is closed reliably ：

• First wave ： A to B Text and say ：“B, I'm going to prepare for dinner ？”

• Second wave ： B At this time, I received A Information about , And then first of all A say ： “ I got it! .”

• Third wave ： B Yes A Speaking of ： “ I'm also going to prepare for dinner .” Then put down the phone ,

• Fourth wave ： A At this time, I received B The confirmation of , And then think B Send a bag and say ：“ well , I got it! .” Then I put down my cell phone and went to eat ,

After four waves ,A and B Can be sure of such a thing ： The communication between both parties can be closed normally . such , Both parties can make sure that the other party is fully aware of their confirmation to close the connection .

3.TCP Maintain reliable communication mode

1. Data fragmentation ： Segment the user data at the sending end , Regroup at the receiving end , from TCP Determine the size of the segments and control the fragmentation and recombination ;
2. Arrival confirmation ： When the receiving end receives the fragmentation data , Send a confirmation packet to the sender according to the fragment data serial number ;
3. Timeout retransmission ： The sender clocks after sending slices , If the time-out does not receive the corresponding confirmation package , The corresponding fragment will be resend ;
4. The sliding window ：TCP The receiving buffer space of both sides of the connection is fixed , The receiving end can only accept the data that the buffer can accept .
5. Out of order processing ：TCP The receiving end of needs to reorder the received data .
6. Reprocessing ： If the transmitted TCP Repeat in pieces ,TCP The receiving end of needs to discard duplicate data .
7. data verification ：TCP Detect any changes in the data during transmission by maintaining the checksum of its header and data .

4.TCP and UDP Use scenarios

1.UDP Use scenarios :

therefore UDP No complex control mechanism , utilize IP Provide connectionless communication services , Data can be sent at any time , Simple and efficient processing .
So it is mainly used in the following scenarios ：

• Communications with a small total number of packets （DNS、SNMP）
• video 、 Audio and other multimedia communication （ Instant messaging ）
  QQ It is used UDP agreement .
• Broadcast communications

Mainly in all the scenes of pursuing speed

2.TCP Use scenarios :

TCP Use scenarios : be relative to UDP,TCP Various controls in the process of data transmission are realized , Retransmission control during packet loss can be performed , It can also control the order of the disordered subcontracting . In the case of high reliability requirements , have access to TCP, That does not consider UDP When , You can choose TCP.

In particular, reliable connection is required , Like paying 、 Encrypting data and other directions need to rely on TCP

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summary

Connection oriented TCP And disconnected UDP It will be an indispensable and important knowledge point in Network Protocol ,TCP and UDP yes TCP/IP There are two representative transport layer protocols , It is also a perennial test question type .

I hope this blog will be beneficial to you . I am the light king , I represent myself. .