If you want to pass the CCNA certification exam and obtain the CCNA certificate, you must have some knowledge about basic common protocols of Cisco’s new version of CCNA certification. We provides you with the latest CCNA exam dumps and CCNA certification training courses about basic protocols.
SMTP is a protocol that provides reliable and effective e-mail transmission. SMTP is a mail service based on FTP file transfer service. It is mainly used for the delivery of mail information between systems and providing notification of letters. SMTP is independent of a specific transmission subsystem and only needs reliable and orderly support of data flow channel. One of the important features of SMTP is that it can transmit mail across the network, namely “SMTP mail relay”. Using SMTP, mail transmission between the same networks processing process can be realized, and mail transmission between a process and other networks can be realized through the repeater or gateway.
SMTP is a relatively simple text-based protocol. One or more recipients of a message are specified on it, and then the message text is transmitted. In the early 1980s, SMTP began to be widely used. At that time, it was only a supplement to UUCP, which was more suitable for handling the transmission of mail between intermittently connected machines. On the contrary, SMTP works best when the sending and receiving machines are continuously connected through a stable network.
The working process of SMTP protocol can be divided into the following three processes:
1. Establish the connection: at this stage, the SMTP client requests to establish a TCP connection with port 25 of the server. Once the connection is established, the SMTP server and the client begin to inform each other of their domain names and confirm each other’s domain names.
2. Mail delivery: with the command, the SMTP client delivers the source address, destination address and specific content of the mail to the SMTP server, and the SMTP server responds accordingly and receives the mail.
3. Connection release: the SMTP client issues the exit command, the server responds after processing the command, and then closes the TCP connection.
SMTP usually has two modes of operation, that is, the sending SMTP and the receiving SMTP. The specific working process is as follows:
After receiving the request of user’s e-mail, the sending SMTP determines whether it is a local e-mail or not. If it is, it will be directly delivered to the user’s mailbox. Otherwise the sending SMTP queries the MX record of the remote e-mail server from DNS. And establishes a two-way transmission channel with the remote receiving SMTP.
After that, the SMTP command is sent by the sending SMTP and received by the receiving SMTP. And the response is sent in the opposite direction.
Once the delivery channel is established, the SMTP sender sends a mail command to indicate the sender. If the SMTP recipient can receive the mail, an OK response is returned. The SMTP sender sends the RCPT command again to confirm whether the email is received. If it is received by the SMTP receiver, an OK response is returned; if it cannot be received, a reject response will be sent (but the whole mail operation will not be stopped). And both parties will repeat this process for many times. When the receiver receives all the messages, it will receive a special sequence. If the receiver successfully processes the message, it will return an OK response.
POP3 has the full name “Post Office Protocol – Version 3”. It is a member of TCP / IP protocol family, defined by RFC1939. This protocol is mainly used to support e-mail managed remotely through the client server. The POP3 protocol that provides SSL encryption is called POP3S.
POP protocol supports “offline” mail processing. The specific process is as follows:
The e-mail is sent to the server. The e-mail client calls the e-mail client program to connect to the server and download all unread e-mails. This offline access mode is a store-and-forward service, which sends mail from the mail server to the personal terminal machine. Once the mail is sent to the terminal, the mail on the mail server will be deleted. However, most POP3 mail servers can provide downloading of mail without deleting it due to the improved POP3 protocol.
The differences and relations between POP3 and SMTP protocols:
Accoding to Spoto, POP3 specifies how to connect a personal computer to an Internet mail server and an electronic protocol for downloading e-mail. It is the first offline protocol standard of Internet e-mail.
For example, if three e-mails in the mailbox are received by the client and moved to other folders, the e-mails on the mailbox server are not moved at the same time.
SMTP is a set of specifications used to transfer mail from the source address to the destination address, through which the transit mode of mail is controlled. SMTP protocol belongs to TCP / IP protocol cluster, which helps each computer find the next destination when sending or transferring letters. The SMTP server is the sending mail server that follows the SMTP protocol.
SMTP certification, in short, requires that you must provide the account name and password before you can log in to the SMTP server, which makes those spammers have no opportunity. The purpose of adding SMTP certification is to prevent users from being disturbed by spam.