Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 245
POP Operation

POP3 should be viewed only to retrieve your mail from the mail drop-off point. Sending mail is a different story. Your personal computer still has the ability to establish a TCP connection to a relay host (intermediate mail host) to send mail. Therefore, you should consider your host as having the ability to be an SMTP sender, and the SMTP protocol explained earlier applies. However, to retrieve your mail, POP3 comes into play.

The client (your PC with a mail application such as Eudora), once established with TCP/IP on its network, builds a connection to the POP3 server. The POP3 server configuration is built during the installation of your mail program on the PC. The connection between your PC and the POP3 server is a TCP connection on TCP port 110. Similar to SMTP, once the connection is established, the server will respond with a greeting like “POP3 server ready.”

The POP3 protocol then enters the authentication state. During this phase, you must identify yourself with a username and password. The RFC does not indicate which authentication mechanism you should use. The most common is the simple username/password combination. However, other options are available, such as Kerberos and APOP, which are beyond the scope of this book. Once you have been “authenticated,” the POP3 server puts an exclusive lock on your mailbox, ensuring that no other transactions take place on the messages while you are retrieving your mail.

The server now enters the transaction state in which each of the messages in your mailbox is assigned a number. This allows your client POP to indicate how many messages are in your mailbox. Each message can be retrieved one at a time or all can be retrieved. Furthermore, you can instruct your client POP to delete messages as they are retrieved. This can be good and bad. It would be nice to hold on to your messages as a backup on the server, but this requires disk space that can be depleted quickly.

From here, you retrieve your messages and, depending on how you configured you PC mail program, the messages are marked for deletion after your session. After you retrieve your messages, your mail program will send the QUIT command, which closes the POP3 session down. Then the UPDATE process begins on the server, which is housekeeping work on the server (deleting messages, etc.).

From here, you can read your messages locally on your PC. You are now disconnected from the POP3 server and you can manipulate the messages locally. There are many other options available for POP3 which may or may not be implemented; however, from a users’ point of view, they are not noticed.

POP Operation