Application Layer functions and protocols

Application Layer functions and Protocols 1. In the application layer, software programs or processes access the network in two forms: applications and services. Applications can interact with us. Is the user software. If the device is a computer, the application is usually started by the user. Although there are many levels of support below, the application software provides interfaces between people and hardware. When you click the send button or perform similar actions, the application starts the data transmission process. A service is a background program that executes specific functions in the data network. Services usually involve connections between devices, applications, and networks. For example, the network service can provide the data transmission function or data conversion in the network. Generally, services cannot be directly accessed or visible to users. They provide links between applications and networks. Some user applications are network-aware programs that implement application layer protocols and can communicate directly with lower layers of the protocol family. For example, the email client program and Web browser. Other services need to use network resources through the application layer service, such as file transmission. Different data types correspond to different network services, so that the lower layers of the OSI model can process data correctly. The Protocol defines the standards and data to be put into use. Each application and network service must use these protocols. The Service provides the function of doing things, and the Protocol provides the rules for service use. 2. The TCP/IP application layer protocol is the most widely known protocol for user information exchange. These protocols detail the formats and control information required for many common Internet communication functions. Common TCP/IP protocols include: Domain Name Service protocol (DNS), which is used to resolve Internet domain names to IP addresses; TCP/UDP port 53 Hypertext Transfer Protocol (HTTP ), it is used to transmit files that constitute Internet webpages; TCP port 80 simple mail Transmission Protocol (SMTP), used to transmit mail and its attachment information; TCP port 25 Telnet protocol (a terminal simulation Protocol ); TCP port 23 is a text transmission protocol (FTP) used for interactive file transmission between systems. TCP port 20 and port 21. 3. HTTP specifies the request/response protocol. When a client (especially a Web browser) sends a request message to the server, the HTTP protocol specifies the type of webpage message requested by the client and the type of server response information. There are three common types of messages: GET: a client data request message. POST: POST and PUT messages are used to send data upload requests to the Web server. When a user inputs data in a form on the Web page, a POST message containing data is sent to the server. PUT: used to upload resources or content to the Web server. Although HTTP is a flexible protocol, it is not secure. The POST message uploads information to the server in plain text format, which may be intercepted or read by other programs. Similarly, server responses (especially HTML pages) are not encrypted. For secure communication, you can use secure hypertext protocol (HTTPS ). HTTPS uses authentication and encryption to ensure data transmission security between the client and the server. In HTTPS, additional rules for data communication between the application layer and the transport layer are also set.