Network must know topics

application layer

1. Mail sending and receiving process ：

1. Users call user agents to compose and edit emails , User agents use smtp Send the mail to the sender's mail server ;
2. The sender's mail server puts the mail into the send cache queue ;
3. Running on the sender's mail server smtp The client process found messages waiting to be sent in the mail cache , Just to the mail server running on the recipient smtp Server process initiation tcp Connection is established ;
4. tcp After the connection is established ,smtp The client process begins to remote smtp Service process sends mail . When all emails are sent ,smtp Just close the established tcp Connect .
5. Running on the recipient's mail server smtp After the server process receives the message , Put the message into the recipient's user mailbox , Wait for the recipient's convenience to read ;
6. When the recipient wants to receive the letter , Call the user agent , Use pop3 perhaps imap Retrieve your email from the corresponding user mailbox of the recipient's email server ;