Catalog

Transport layer based protocols

The original problem of the network

UDP agreement  

Transport layer based protocols

The original problem of the network

• We know that the transport layer is based on the premise that the host and the host can communicate , Let the process communicate with the process , Then the implementation of the transport layer should be based on the implementation of the network layer
• Data in the network , Between routers , Jump and jump , Two problems caused by transferring to the target host （ Discussion based on the perspective of network layer ）
• To solve these problems , Let the interested people in the transport layer and application layer solve

Unreliable network transmission

1. Data sent , The other party cannot guarantee to receive
2. There is no guarantee that the other party will be received strictly in the order of sending （ Because of the router , The path and speed of data transmission may be different , For example, when we go to Beijing in the morning A Make a car ,B Chinese take the high-speed rail ,C Fly in the afternoon , So the possible order of arrival in Beijing is BAC It could be ABC BCA）

The Internet is not secure

1. All the data sent , The routers along the way can be checked or modified , hacking , Tampering
2. Others can fake the data you send

 

UDP agreement  

• User message protocol , It is a very simple transport layer protocol , It just fulfills the most basic responsibility of the transport layer （ When the host and the host can be connected , Realize process to process connection ）
• characteristic 1 There is no connection 2 Unreliable transmission 3 For datagram   4  Receive buffer , No send buffer , But the cost is cheap
• UDP It's not reliable ： Not at all UDP What did the agreement do , Become unreliable , But because UDP You've done nothing at all? , Express the unreliability of the network layer directly to the application layer , So from the perspective of application layer , We just said UDP It's not reliable

UDP How it works

1. UDP The head of the newspaper , The headers of all network protocols have responsibilities ： How to unpack （ How to put data in Chinese header( Headlines ), and payload（ The data content ） Separate ）
2. The problem of distribution , That is, how to implement application layer to application layer

Example

• The items we want to ship are payload, The package and the order on the bag are header
• We just enjoy UDP The process of service
• The delivery car is a service provided by the network layer  

UDP Header format

UDP The head has 8 Bytes , from 4 Fields make up , Each field is two bytes ,

• 1. Source port ： Source port number , Use when you need a reply , Set it all when you don't need it 0.
• 2. Destination port ： Destination port number , You need to use .
• 3. length ：UDP The length of the datagram （ Including the first and data ） Its minimum value is 8（ Only the first one ）
• 4. The checksum ： testing UDP Is there any error in the transmission of datagram , If there is a mistake, discard （ The basic principle is to use hash function , As long as the data is the same , After function processing , The hash value must be the same ）
   

  The concept of buffer

 

• What we call layering , It's all logical concepts , In terms of hardware , We only have memory （ User mode managed memory and kernel mode managed memory ）
• Let's put this memory area , Used to store the data sent or received , It's called a buffer
• Its UDP Only receive buffer （ Just like we send express , There is a rookie post station for us to temporarily store ）

In the application layer UDP Agreement , There is no send buffer , If UDP Data sent successfully , What phenomenon does it represent ？

UDP The data is just sent to the network （ In transit ）, It's like in our lives , We succeeded in sending the letter , It just means that we sent the letter , I don't know where the letter has been received

UDP Sending process  

1. Receive data from the application layer （ Remove data from the memory area of the application layer （ User mode ） Copy to the memory area of the kernel ）
2. Get ready header part 1 Source port number （socket Object with （ Port of the local process ））2 Target port （ The application layer is responsible for filling ）3 UDP length （header length + Data length ） By calculation 4 checksum, The checksum , By calculation
3. header+payload Get message segment （datagram）
4. Wait for the network layer to send successfully （ It means that the data is transferred to the network card ）
5. Notify the application layer of sending successfully

UDP Receiving process of

1. Receive a new package from the network layer （UDP Datagram）
2. ​​​​​ According to the fixed length （8 Bytes ）, Wipe split into headerz and payload—— Unpack
3. Read header Four fields of , Source port , Target port , length , The checksum
4. Judge whether the length is correct , If the length of the bag is incorrect, throw it away , No one needs to be informed （ Reflects the unreliability ）
5. Judge whether the checksum is correct （ Judge whether the data is inadvertently damaged ）, If there are questions , Directly to throw away
6. hold payload Put it in the receive buffer , Relevant information is also included
7. Inform the application layer to get information （ Find the corresponding process according to the target port ）
8. Wait for the application layer to receive information from the receive buffer
9. If you don't take it for a long time , You can also throw away the data