UDP protocol: simple because of good nature, it is inevitable to encounter "city can play"

TCP and UDP What are the differences ？

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I usually ask the difference between the two agreements during the interview , Most people will answer ,TCP It's connection-oriented ,UDP It's for connectionless .

What is connection oriented , What is connectionless ？ Before intercommunication , The connection oriented protocol establishes the connection first . for example ,TCP Will shake hands three times , and UDP Can't . Why establish a connection ？ you TCP Three handshakes , I UDP You can also send three bags to play , What's the difference ？

The so-called establishing a connection , To maintain the connection between the client and the server , And establish a certain data structure to maintain the state of interaction between the two sides , Use this data structure to guarantee the so-called connection oriented features .

for example ,TCP Provide reliable delivery . adopt TCP Connect the transmitted data , No mistakes 、 No loss 、 No repetition 、 And arrive in order . We all know IP There is no reliability guarantee for the package , Once sent out , It's like learning from the West , Lost it 、 Eaten by monsters , Can only let it go . however TCP It claims to be able to do what the connection maintenance program does , and UDP Inherited IP Package features , There is no guarantee that it will not be lost , There is no guarantee of arrival in order . 

Again ,TCP Is oriented to a byte stream . When sending, you send a stream , No head, no tail .IP A package is not a stream , But one by one IP package . The reason why it becomes a stream , This is also TCP Your own state maintenance . and UDP Inherited IP Characteristics of , Datagram based , One by one , One by one .

also TCP There can be congestion control . It realizes that the packet is discarded or the network environment is bad , Will adjust their behavior according to the situation , See if it's fast , Would you like to slow down .UDP Won't , The app let me send , I'll send it , Despite the flood .

thus TCP It's actually a stateful service , Generally speaking, it's a brain , It accurately remembers whether it has been sent , Received no , To which , Which should be accepted , No mistake at all . and UDP Stateless service . Generally speaking, it's mindless , Innocent , Just send it out .

 

 

UDP What is Baotou like ？

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When I send UDP When the package reaches the target machine , Find out MAC Address matching , So he took it down , Pass the rest of the packets to the processor IP Layer code . hold IP Take off your head , Finding goals IP matching , The next? ？ Who is the packet in here for ？

At the time of sending , I know I sent a UDP My bag , How did you know about the machine you received ？ So in IP There's a... In the head 8 Bit agreement , It will store , What's in the data TCP still UDP, Of course, here it is UDP. therefore , If we knew UDP The format of the head , From the data , Parse it out . After analyzing it ？ Who will process the data ？ 

After dealing with the transport layer , The kernel is basically done , The data inside should be handled by the application itself , But there are so many applications running on one machine , Who should I give it to ？

Regardless of application write usage TCP The data transfer , still UDP The data transfer , Listen to a port . It is this port , Used to distinguish applications , Let's say that ports cannot conflict . Two applications listen on one port , Who will you give it to then ？ therefore , Logically speaking , Whether it's TCP still UDP There should be a port number in the header , According to the port number , Give the data to the corresponding application .

When we see UDP When Baotou , It is found that there is indeed a port number , Active port number and target port number . Because it's communication between two ends , That makes sense . But you will also find ,UDP Except for the port number , There is nothing else . and TCP Head to head , This is simply a mess ！ 

 

 

 

UDP Three characteristics of

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UDP Just like a child , It has the following characteristics ：

• First of all , Simple communication , You don't need a belly of fancy intestines （ A lot of data structures 、 Processing logic 、 Header field ）. The premise is that it believes that the online world is beautiful , Uphold the theory of good nature , I believe that the default of network channel is easy to deliver , Not easily discarded .
• second , Credulity . It doesn't make connections , Although there is a port number , But monitoring is in this place , Anyone can send him data , He can also send data to anyone , It can even be transmitted to multiple personal data at the same time .
• Third , I'm so stupid , Don't know how to change things . I don't know when to insist , When should we give way . It will not control the congestion of the contract according to the situation of the network , No matter what happens to network packet loss , How it should be sent back .