What are the classifications of application layer protocols?

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 classification of application layer protocols are:

(1), Domain Name System (Domain Name System, DNS): A network service used to map network device names to IP addresses.

(2), File Transfer Protocol (File Transfer Protocol, FTP): used to implement interactive file transfer functions.

FTP is a file transfer protocol that supports two modes: one is called Standard (that is, Active mode), and the other is Passive (that is, PASV, passive mode). Standard mode is where the FTP client sends the PORT command to the FTP server. Passive mode is where the FTP client sends PASV commands to the FTP server. In both methods, the data and control links are transmitted separately. The only difference is that in active mode, the server initiates a link request for the data link, while in passive mode, the client initiates a link request for the data link.

During the FTP communication process, its control link and data link do not communicate on the same port, but communicate independently on two different ports. First, the client initiates a control link request to the server. When the control link is successfully established with the server, the client will send a port number to the server in active mode to tell the server the data transmission port used by the server for the current transmission. After receiving this information, a data link request is initiated to the client, and the current data transmission is performed after success. After the current transmission is completed, the data link is dismantled. If the client performs a new transmission, a new port number is sent to the server and the link is re-established. During the entire process, the control link link always exists until the entire communication process of FTP is completed, while the data link needs to establish a new link for each transmission. The passive mode process is similar to the above communication process, except that the client initiates a data link establishment request.

During the FTP interaction process, the client tells the server related information through command words. Eight commonly used access control commands are USER, PASS, CWD, QUIT, etc.; the transmission parameter commands PORT, PASV, TYPE , STRU, MODE five kinds; FTP service commands RETR, STOR, LIST, ABOR and twenty kinds. The server tells the client the current server feedback status through some status codes. Generally, 2xx indicates that the current operation is successful, 3xx indicates a permission problem, 4xx indicates a file problem, and 5xx indicates a server problem.

(3), Simple Mail Transfer Protocol(Simple Mail Transfer Protocol, SMTP): Used to realize the email transfer function

SMTP is A protocol that provides reliable and efficient email transmission. SMTP is an email service modeled on the FTP file transfer service. The SMTP server listens for client requests on the default port 25 and is mainly used to transmit email information between systems.

SMTP interaction process

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 a status code of 220 to tell the client that the service is currently ready. After receiving the status code, the client issues a HELO or EHLO command to the server to tell the server the type of service the client needs. HELO is the default SMTP service. EHLO requires support for extended services in addition to the default services. After the server tells the client the services it supports, the two parties interact with each other using command words and status codes.

(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 reliable transmission of TCP (Transfer Control Protocol, Transmission Control Protocol) and adopts a client/server working mode. In the HTTP communication process, the client first initiates a request to the server to establish a link, which is completed through the TCP three-way handshake. Then the client sends a request to the server and tells the server the information it wants. The server returns the information the client needs through a response. Finally, the link is closed through the TCP four-way handshake to complete 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 implement remote login function.

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