Computer network application layer

application layer

1. Multi terminal system communication is actually process communication , Exchange messages over the network （ Application layer protocol generation ）
2. Processes send and receive messages to the network through sockets （ Corresponds to one or more ）
3. Socket is an interface between application layer and transport layer , In order to establish the programmable interface of network program
4. Application developers can control everything in the application layer , There is little control over the transport layer （ You can set the protocol , Maximum length ）
5. To communicate between processes, you need to know the host address of the parent （IP The network layer ） And the destination host accepts the process identifier （ Port transport layer ）
6. The services that the transport layer should provide to the application ： Reliable data transmission 、 Security 、 throughput 、 timing （ The latter two tu They don't offer ）
7. Reliable data transmission : When the sending process transfers data to the socket, it is considered to be satisfied .
8. throughput ： Guarantee the bit rate delivered to the receiver .
9. Security : encryption
10. TCP: Congestion control （ Inhibit the sender process ）
11. UDP: No congestion control . The sender can inject data into the next layer at any rate . But the throughput of the receiver is generally not up to
12. Throughput or timed transport layer protocols do not provide
13. The application layer protocol defines how programs running on different ends deliver messages
14. How to transfer ： Message type 、 grammar 、 semantics , When and how to send messages , How to respond to
15. HTTP Use TCP, Don't worry about data loss . Don't store any state about the user （ Stateless protocol ）
16. HTTP Provide continuous links （ Default ） And non persistent connections
17. HTML Basic documents .html Contains references to other objects （ Such as images ） You need to use the reference given to continue to ask for data
18. HTTP It's stateless , but web Sites often want to associate content with user identity ,cookie
19. cookie Allow sites to track users
20. cookie: Send a request ->web Generate a unique identifier and use it as an index to create a table entry in the back end
21. web Respond to the request with a setCookie The first one contains the identification code
22. If you visit this website again, you will ask for the header to add Cookie
23. web A proxy server is also called a cache , Store recently requested copies of objects
24. The user requests the cache first , The cache returns information when it is available . Didn't send a request to come back and respond to it
25. web Buffers usually have ISP Buy , For example, the school makes students point to it .
26. web Caching can be done with if-modified-since The first to prove that your object is up to date （ When to call ？）