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This article mainly introduces what is the classification of application layer protocols in computers, which has a certain reference value. Interested friends can refer to it. I hope you will gain a lot after reading this article. Let's take you to know it.

The classification of application layer protocols are: 1, domain name system; 2, file transfer protocol; 3, simple mail transfer protocol; 4, hypertext transfer protocol; 5, simple network management protocol; 6, remote login protocol.

The application layer protocols are classified as follows:

(1) Domain name system (Domain Name System,DNS): a network service used to realize the mapping of network device names to IP addresses.

(2) File transfer Protocol (File Transfer Protocol,FTP): used to realize interactive file transfer function.

FTP is a file transfer protocol that supports two modes: one is called Standard (Active, active mode), and the other is Passive (PASV, passive mode). Standard mode is when the client of FTP sends PORT commands to the FTP server. Passive mode is when the client of FTP sends PASV commands to the FTP server. In the two ways, the data and the control link are transmitted separately, the only difference is that the active mode initiates the link request of the data link by the server, while the passive mode initiates the link request of the data link by the client.

In the process of FTP communication, its control link and data link do not communicate on the same port, but independently on two different ports. First, the client initiates a request for a control link to the server, and when the control link is successfully established with the server, the client will send a port number to the server in active mode, telling the current data transmission port used by the transmission server. After receiving this information, the server initiates a data link request to the client, and carries out the current data transmission after success. After the current transmission is complete, the data link is removed, and if the client makes a new transmission, a new port number is sent to the server to re-establish the link. In the whole process, the link of the control link exists until the end of the whole communication process of FTP, and a new link needs to be established for each transmission of the data link. The passive mode process is similar to the above communication process, only the client initiates the data link establishment request.

In the process of FTP interaction, the client tells the server relevant information through command words, including eight access control commands such as USER,PASS,CWD,QUIT, five transmission parameter commands PORT,PASV,TYPE and STRU,MODE, and twenty FTP service commands such as RETR,STOR,LIST,ABOR. The server tells the client the feedback status of the current server through some status codes. Generally, 2xx indicates that the current operation is successful, 3xx indicates permission problem, 4xx indicates file problem, and 5xx indicates server problem.

(3) simple Mail transfer Protocol (Simple Mail Transfer Protocol, SMTP): used to realize the function of e-mail transfer.

SMTP is a protocol that provides reliable and efficient e-mail delivery. SMTP is a mail service modeled on the FTP file transfer service. The SMTP server listens for customer requests on the default port 25 and is mainly used to transmit mail information between systems.

SMTP interaction process

It is relatively simple. First, the client initiates a request to the server's SMTP service port and establishes a link through a three-way handshake. Then the server returns the status code of 220 to tell the client that the service is currently ready. After receiving the status code, the client sends a HELO or EHLO command to the server to tell the server the type of service the client needs, where HELO is the default SMTP service, and EHLO requires that the extension service should be supported in addition to the default service. When the server tells the client which services it supports, the two parties interact with the command word and the status code.

(4) Hypertext transfer Protocol (HyperText Transfer Protocol,HTTP): used to implement WWW services.

Is an object-oriented protocol belonging to the application layer, it is based on TCP (Transfer Control Protocol, transmission control protocol) reliable transmission, using the client / server working mode. In the process of HTTP communication, the client first initiates a request to establish a link to the server, which is completed by the TCP three-way handshake, and then the client sends a request to the server to tell the server the information it wants. The server returns the information needed by the client through the response. Finally, the link is closed through the TCP four-way handshake, thus completing a basic communication process.

(5) simple Network Management Protocol (simple Network Management Protocol,SNMP): used to manage and monitor network devices.

(6) remote login protocol (Telnet): used to realize the remote login function.

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