Please Whitelist This Site? I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :) If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads. If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK. Thanks for your understanding! Sincerely, Charles Kozierok Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited. If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)                      9  TCP/IP Electronic Mail Access and Retrieval Protocols and Methods                           9  TCP/IP Post Office Protocol (POP/POP3)

POP3 General Operation, Client/Server Communication and Session States 1 2 3 POP3 Transaction State: Mail and Information Exchange Process and Commands

Alternative Authentication Using APOP

Since user/password authorization is considered by many people to be insufficient for the security needs of modern internetworks, the POP3 standard also defines an alternative authentication method, using the APOP command. This is a more sophisticated technique based on the MD5 “message digest” encryption algorithm.

If the server supports this technique, in its opening greeting it provides a string indicating a timestamp that is unique for each POP3 session. The client then performs an MD5 calculation using this timestamp value and a “shared secret” known by the server and client. The result of this calculation is included in the client's APOP command. If it matches the server's calculation, authentication is successful; otherwise the session remains in the Authorization state.

The Post Office Protocol was also designed to allow it to be extended through the addition of other authentication mechanisms. This process is based on the use of the optional AUTH command, as described in RFC 1734.

POP3 General Operation, Client/Server Communication and Session States 1 2 3 POP3 Transaction State: Mail and Information Exchange Process and Commands