Network protocol details: UDP

Catalog

Network problems

Unreliable network transmission

Network insecurity

The transport layer UDP

UDP Working mechanism

Application layer observation UDP Characteristics

shortcoming

advantage

UDP Header structure

 checksum( The checksum )

UDP Receive buffer  

Buffer concept

  Receive buffer

 UDP Send success

UDP Transmission summary

1. send out

 2. receive

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Network problems

There are many variables in the network , Because there are too many propagation routes to go through , The final data may not be complete .

Unreliable network transmission

1. Sent data , The other party may not receive

2. Cannot be received by the other party strictly in the order of sending

Network insecurity

1. The data we send is likely to be viewed during route , modify , Eavesdropping, etc

2. Someone may forge the data we send directly to others

So about the above two points , It needs to be handed over to the network layer

The transport layer UDP

full name User Datagram Protocol ： User message protocol

UDP It is a very simple transport layer protocol , Only the most basic responsibilities of the transport layer can be completed .

say UDP unreliable , It's really because he didn't do anything , Not protective .

UDP Working mechanism

I think the teacher's picture is very good , So I will only make a few conceptual explanations

First map

 UDP The head of the newspaper (header), It can be understood that achievement is the address on the express list , I'll elaborate later .

payload That is, data content , Provided by the application layer and packaged by the transport layer

that UDP The responsibility of the application layer is to payload pack , label , Then send it to the network layer ,

As for whether it reaches the receiver , The sender cannot know .

Application layer observation UDP Characteristics

shortcoming

unreliable , It is reflected in the above scarlet letter .

advantage

There is no connection ： It will not occupy the server all the time

Data oriented message ： I won't open the data package to see , It will not destroy the data sequence structure and integrity

The cost is low ： Is cheap

UDP Header structure

UDP Of header Let both sides go IP And port , length 8 Bytes

The figure shows the whole message structure ,UDP Length is the message length

payload Is to remove header The remaining message length

 checksum( The checksum )

It's like verifying integrity , Like you steam After downloading and decompressing, you should verify whether the file is correct , Sending documents on the Internet should also be used .

It is mostly used for long-distance communication , Ensure data integrity and accuracy .

When I send , I will put the original data checksum Put it in header in , As soon as the receiver receives , Will calculate another checksum, Comparison of two calibrations , Dissimilarity , It proves that the data is damaged or modified .

UDP Receive buffer  

Buffer concept

From the application layer to the data link layer , The area where data is stored can be called a buffer , The concept is more abstract

  Receive buffer

UDP No send buffer , Only receive buffer .

We understand it as , Receiving party UDP After receiving data , It doesn't need to be taken away by the application layer immediately , Put it in the receiving buffer and wait silently , But if the application layer gets the data when there is nothing in the receiving buffer , If there is no timeout mechanism , Will die waiting .

 UDP Send success

In a word ,UDP As long as you put the data on the Internet, it will be sent successfully , Whether I receive it or not has nothing to do with me

UDP Transmission summary

1. send out

1. Receive from the application layer first payload data , It is equivalent to copying the memory data of the application layer to its own kernel memory area

2. Get ready header part ：

1). Source port (socket There are , You don't have to write it yourself )

2). Target port

3).UDP length

4).checksum

3.header+payload Equivalent to datagram

4. Just pack it up datagram Send it to the network layer

5. As long as the network layer sends successfully ( The data has reached the network card )

6. Notify the application layer of sending successfully

If the receiving party can receive , It must be intact data , This is the benefit of data message orientation

 2. receive

1. Put the package received from the network layer into the kernel buffer

2. adopt header Fixed length , Break up the package into header Part and payload part - Unpack

3. Read header part , Including ports ,ip Information ,checksum( The checksum ), Length etc.

4. Throw the wrong length , No one needs to tell

5.checksum( The checksum ) No, throw it directly

6. hold payload Put it in the receive buffer

7. Notify the application layer that the data has arrived

8. Application layer to get

9. The application layer has never come , Information can still be thrown