POP is the abbreviation of post office protocol. It is an electronic post office transmission protocol, and POP3 is its third version. It is an electronic protocol that specifies how to connect a personal computer to an Internet mail server and download e-mail. It is the first offline protocol standard for Internet e-mail. Simply put, POP3 is a simple and practical e-mail information transmission protocol.
●POP & amp； POP3: post office protocol (post office protocol 3rd Edition)
Pop Music & ampPOP3: post office protocol
POP protocol allows workstations to dynamically access the mail on the server, and has now developed to the third edition, called POP3. POP3 allows workstations to retrieve mail from mail servers. POP3 transmits data messages, which can be instructions or replies.
There are many different technical supports and methods to create a distributed e-mail system: POP (post office protocol), DMSP (Hierarchical E-mail System Protocol) and IMAP (Internet Information Access Protocol). Among them, the POP protocol is the earliest and therefore the easiest to understand. DMSP has good performance in supporting "connectionless" operation, but it is largely limited to a single application (PCmail). IMAP provides an extended set of POP and DMSP, and provides three support methods for remote mail access: offline, online and connectionless.
POP protocol supports "offline" mail processing. The specific process is: the mail is sent to the server, and the mail client calls the mail client program to connect to the server and download all unread mail. This offline access mode is a store-and-forward service, which sends mail from a mail server to a personal terminal machine, usually a PC or MAC. Once the mail is sent to a PC or MAC, the mail on the mail server will be deleted.
POP3 does not support the expansion of mail on the server, which is done by the more advanced IMAP4. POP3 uses TCP as the transport protocol.
Protocol architecture
POP3 is ASCII information sent between client and server. Summary of POP3 command: (see picture)
Optional POP3 command:
APOP domain name digest authorization status is valid;
Hot news n transaction status is valid;
UIDL[ message]
POP3 reply:
+OK.
Uh.
File Transfer Protocol (abbreviation for file transfer protocol)
FTP is a software for uploading and downloading. I don't use it much. In the past, when I made my own web pages, I used FTP to transfer things from my computer to free space on the Internet. It can also be used to download files, as long as there is an address.
FTP is defined as follows:
FTP (File Transfer Protocol) is a protocol used to control the two-way transmission of files on the Internet. At the same time, it is also an application. Users can connect their PC with all servers running FTP protocol all over the world through it, and access a large number of programs and information on the server.
The general steps for transferring files are as follows:
1 Log on to the Internet on the local computer.
2 search for a host or personal computer with files (generally posted on the website of a special FTP server, with the name, password and path to enter the host or personal computer)
After establishing a connection with the remote host or the other party's personal computer, log in to the host or the other party's personal computer with the user name and password provided by the other party.
After the remote host or the other party's personal computer logs in successfully, you can upload what you want to share with others, or download what others authorize to enjoy (here, Dongdong refers to what you can put in the computer and see on the display screen).
Close the FTP download software and cut off the connection after finishing the work.
In order to realize file transfer, users have to run special file transfer programs, such as flashget, and there are many special FTP transfer software, each with its own characteristics.
FTP (File Transfer Protocol) is the abbreviation of File Transfer Protocol.
The role of FTP
As the name implies, the main function of FTP is to let users connect to a remote computer (all of which are running FTP server programs), see what files are on the remote computer, and then copy the files on the remote computer to the local computer or send the files on the local computer to the remote computer.
The working principle of FTP
Take downloading files as an example. When you start FTP to copy files from a remote computer, you actually start two programs: the FTP client program on the local computer, which requests the FTP server to copy files. Another method is to start the FTP server program on the remote computer, which responds to your request and transfers the file you specified to your computer. FTP adopts "client/server" mode, and the client should install the FTP client program on its local computer. There are two kinds of FTP client programs: character interface and graphical interface. The FTP commands in the character interface are complex and numerous. The FTP client program with graphical interface is simpler and easier to operate.
Simply put, a server that supports FTP protocol is an FTP server. First, what is FTP protocol (file transfer protocol)?
Generally speaking, the primary purpose of user networking is to realize information sharing, and file transfer is one of the most important contents of information sharing. It was not easy to transfer files on the Internet in the early days. We know that the Internet is a very complicated computer environment, including PC, workstation, MAC and mainframe. According to statistics, there are tens of millions of computers connected to the Internet, which may run different operating systems and Unix servers. There are also PCs running Dos and Windows, Macs running MacOS and so on. , and the file exchange between various operating systems needs to establish a unified file transfer protocol, which is called FTP. There are different FTP applications based on different operating systems, all of which follow the same protocol, so users can transfer their own files to others or get files from other users' environments.
Like most Internet services, FTP is a client/server system. Users connect to FTP server programs on remote hosts through client programs that support FTP protocols. The user sends commands to the server program through the client program, and the server program executes the commands sent by the user and returns the execution results to the client. For example, if a user sends a command asking the server to send a copy of a file to the user, the server will respond to the command and send the specified file to the user's machine. The client program receives the file on behalf of the user and stores it in the user directory.
In the use of FTP, users often encounter two concepts: "download" and "upload". "Downloading" files means copying files from a remote host to your own computer; "uploading" a file means copying it from your own computer to a remote host. In Internet language, users can upload (download) files to (from) remote hosts through client programs.
When using FTP, you must log in first, and obtain the corresponding permissions on the remote host before uploading or downloading files. In other words, if you want to use which computer to transfer files, you must have the corresponding authorization of which computer. In other words, you can't transfer files unless you have a user ID and password. This situation violates the openness of the Internet. There are more than 10 million FTP hosts on the Internet, so it is impossible to require each user to have an account on each host. Anonymous FTP is produced to solve this problem.
Anonymous FTP is a mechanism through which users can connect to and download files from remote hosts without becoming registered users. The system administrator has established a special user ID named anonymous, which can be used by anyone on the Internet.
The way to connect anonymous FTP host through FTP program is similar to that of ordinary FTP host, except that anonymous must be entered when user ID is needed, and the password of user ID can be any string. Traditionally, using your own e-mail address as a password allows system maintainers to record who is accessing these files.
It is worth noting that anonymous FTP does not apply to all Internet hosts, but only to those that provide this service.
When the remote host provides anonymous FTP service, it will designate some directories to be open to the public and allow anonymous access. The rest of the directories in the system are hidden. As a security measure, most anonymous FTP hosts allow users to download files from them, but do not allow users to upload files to them. In other words, users can copy all the files on the anonymous FTP host to their own machines, but they cannot copy any files on their own machines to the anonymous FTP host. Even if some anonymous FTP hosts do allow users to upload files, users can only upload files to the specified upload directory. Then, the system administrator will check these files and move them to another public download directory for other users to download. In this way, users of remote hosts will not upload problematic files, such as files with viruses.
As an Internet user, you can copy files between any two Internet hosts through FTP. But in fact, most people only have one online account, and FTP is mainly used to download public files, such as * * * software, technical support documents of various companies and so on. There are thousands of anonymous FTP hosts on the Internet, and countless files are stored on these hosts for users to copy for free. In fact, almost all kinds of information and computer programs can be found on the Internet. This is one of the important reasons why the Internet attracts us.
Anonymous FTP gives users the opportunity to access the world's largest information base, which is constantly accumulating and growing, and it never closes, involving almost all topics. Besides, it's all free.
Anonymous FTP is a common method to publish software on the Internet. The Internet can continue today because people use programs that provide standard services through standard protocols. Many programs like this are published through anonymous FTP, and anyone can access them.
There are a huge number of anonymous FTP hosts and more files on the Internet, so how do we know which directory of which anonymous FTP host a particular file is located? This is exactly what Archie server is trying to do. Archie will automatically search the FTP host and construct a database containing all the file directory information, so that you can directly find the location information of the required files.
American Automobile Emission Testing Association
host machine
Host refers to putting its own server in the computer room of the server hosting business unit to realize its connection with the Internet, thus avoiding the trouble of users applying for a dedicated line to connect to the Internet. The server hosting provided by digital network companies is different from that provided by ordinary companies. Because digital network companies have access centers of CHINANET, the hosted servers can be connected to the Internet through the network interface of 100M, which is difficult for ordinary companies to provide.
Among them, (new network interconnection) is a better enterprise engaged in mainframe.