About POP3 protocol working modes and principles

In daily life, correspondence is an indispensable part of our daily life. With the popularity of the Internet, the Internet version of traditional mail-email has gradually entered our life and work. When we receive regular emails on weekdays, we often use a locked box as our mailbox, the POP server is also a temporary storage location for emails we use on weekdays. In comparison with traditional mail, we use accounts and passwords to open POP servers, just like opening our daily mailbox in our daily life, let's take a look at how to use the POP server.

What is POP?
When you hear this POP, it seems like a bubble in Chinese. In fact, this is an abbreviation of an English term. The full name of POP is Post Office Protocol, which is the Post Office Protocol for receiving emails. It uses TCP port 110. Currently, the third edition is commonly used, so POP3 is short. POP3 still uses the Client/Server working mode. The Client is called the Client. Generally, we use the computer as the Client, while the Server is managed by the network administrator. For example, the Server is a collection of many small mailboxes, just like the mailbox structure of the building where we live, the client is like a person holding a key to unlock the mailbox to obtain the mail.

Layers of POP in the Network Model
As we all know, networks are layered, which is like the organizational structure of an enterprise. In daily computer use, people operate computers. People are like CEOs who direct computers to the Internet. When we open Foxmail to receive an email, Foxmail will call the application layer protocol-POP protocol in the TCP/IP Reference Model.

The application layer protocol is built on the network layer protocol and is designed to provide application services for users. It is generally visible. For example, using the FTP File Transfer Protocol) to transfer a file and request a connection to the target computer. During file transfer, you can see part of the exchange between the user and the remote computer. At this time, the POP protocol directs the lower-layer protocol to send data to the server. Finally, Foxmail successfully saves the email to the inbox of Foxmail after a series of protocol conversations. The TCP/IP Reference Model is the foundation of the Internet. Compared with OSI's layer-7 protocol, there is no Session Layer or presentation layer in the TCP/IP Reference Model. Generally, TCP/IP is a group of protocols, and TCP/IP is actually a Protocol family or Protocol package), including more than 100 associated protocols, IPInternet Protocol, internet Protocol) is the most important Protocol in the network layer; TCPTransmission Control Protocol, Transmission Control Protocol) and UDPUser datasync Protocol, User Datagram Protocol) is the most important Protocol in the transmission layer. Generally, IP, TCP, and UDP are the three most fundamental protocols and are the basis of other protocols.

I believe that readers will be interested in each layer after understanding the TCP/IP framework, but our understanding of this model is also step by step. Here, we first need to know that the corresponding software will call the corresponding protocol at the application layer. For example, Foxmail will call the POP protocol, while IE will call the DNS protocol to resolve the website address to an IP address first. In the actual process of receiving emails, the protocol at the application layer of POP directs the TCP protocol and uses the IP protocol to split a large email into several packets for transmission over the Internet.

To help readers understand this process, I will give you an example. For example, if you want to talk to a person remotely, the distance is too far, you have to divide a large part of your expression into words and shout, while the other party writes each word on the paper. After writing it down, the other party will yell and tell you that it has received it, in this way, we can overcome the weak point of remote hearing. This type of Q & A feedback mechanism is like the TCP protocol. the POP server generally uses port 110 of TCP.

How POP works
Let's take a look at the process of receiving emails from the email software. Generally, we set a POP server URL such as pop.163.com on the account attribute of the email software ), and the account and password of the mailbox. This is used in the receiving process. When we press the receiving key in the email software, the email software will first call the DNS protocol to resolve the IP address of the POP server. After the IP address is resolved, the mail program began to use the TCP protocol to connect to port 110 of the mail server. Because the POP server is busy, we have to wait a long time in this process. After the email program successfully connects to the POP server, it first uses the USER command to send the account of the mailbox to the POP server, and then uses the PASS command to send the account of the mailbox to the server, after this authentication process is completed, the mail program uses the STAT command to request the server to return the mailbox statistics, such as the total number of mails and the size of mails, and then the LIST will LIST the number of mails on the server. Then the mail program will use the RETR command to receive the mail, and then use the DELE command to delete the mail in the mail server. When QUIT is used, the mail server deletes the mail with the deletion flag. In layman's terms, the Mail Program receives emails from the server, which is actually a conversation process. The POP protocol is a language used for email.