Detailed explanation of application layer protocol and its functions

Detailed explanation of application layer protocol and its functions

1. DNS: Domain Name Service, used to resolve names to IP addresses and vice versa
2. DNS is an inverted tree structure. At the very top is the root domain, represented by a period (.). There are 13 root domain servers around the world, one primary server in the United States, 9 secondary servers in the United States, 2 in Europe, and one in Japan.
3. Below the root domain server are first-level domains (top-level domains), second-level domains, third-level domains... up to 127 levels
4. The process of DNS domain name resolution:
   (1) Query your own cache
   ipconfig/displaydns View the local DNS cache
   ipconfig/flushdns Clear the local cache
   (2) Query the HOSTS file
   C:\Windows\System32\drivers\etc\ hosts
   (3) Local DNS server (specified when configuring network parameters)
5. FTP has two ports 21 and 20. 21 is used for control connections and 20 is used for data transfers.
6. FTP working mode:
   (1) Active connection: The client downloads files, and the server actively establishes a data connection from port 20 to the client.
   (2) Passive connection: The client downloads the file, and the client establishes a data connection to the server with a port greater than 1024. The implication is that passive connections are not using port 20.
7. DHCP (Dynamic Host Configuration Protocol) is a dynamic host allocation protocol, which uses the UDP protocol to automatically assign IP addresses to hosts, as the main management method for network administrators. Realize plug and play network function.
8. The application layer protocol of the Web is the HyperText Transfer Protocol (HTTP), which is the core of the Web.
9. SMTP (Simple Mail Transfer Protocol) Simple Mail Transfer Protocol <port number 25> is used to send mail.
10. The comparison between DHCP and SMTP
    is first: HTTP is mainly a pull protocol, that is, people can load information on a Web server at their convenience, and users use HTTP to pull information from the server. The TCP connection is initiated by the machine that wants to get the file.
    Second: SMTP is a push protocol, the sending mail server pushes the file to the receiving mail server. The TCP connection is initiated by the machine from which the file is to be sent.
    Third: the communication between the user and his remote mailbox is via HTTP. Email messages travel from the mail server to the browser, using HTTP. SMTP is still used when sending and receiving mail between mail servers and other mail servers.
11. POP3: The recipient's user agent cannot retrieve mail using SMTP, because fetching mail is a pull operation and the SMTP protocol is a push protocol. This dilemma is solved by introducing a special mail access protocol that delivers mail on the recipient's mail server to his local PC. There are several popular mail access protocols, the third version of the Post Office Protocol (Post Office Protocol-Version 3, POP3)
12. Other very common protocols:
    a) TELNET remote login protocol <port number 23>
    b) RPC (Remote Procedure Call Protocol) (RFC-1831) remote procedure call protocol
    c) RTCP (RTP Control Protocol) RTP control protocol
    d) RTSP (Real Time Streaming Protocol ) Real-time Streaming Protocol
    e) TLS (Transport Layer Security Protocol ) Secure Transport Layer Protocol
    f) SDP ( Session Description Protocol ) Session Description Protocol
    g) SOAP (Simple Object Access Protocol ) Simple Object Access Protocol
    h) GTP General Data Transmission platform
    i) STUN (Simple Traversal of UDP over NATs, UDP simple traversal of NAT) is a network protocol
    j) NTP (Network Time Protocol) network time calibration protocol.