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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)

TCP/IP Post Office Protocol (POP/POP3) 1 POP3 General Operation, Client/Server Communication and Session States

Of the three mailbox access paradigms, online, offline and disconnected, the offline model is probably the least capable in terms of features. And it is also the most popular! This may seem counter-intuitive, but it is in fact a pattern that repeats itself over and over in the worlds of computing and networking: a good example would be Ethernet beating Token Ring in the LAN market. The reason is that simplicity and ease of implementation are keys to the success of any technology, and the offline mail access model beats the other two in these areas.

The history of offline e-mail access goes back farther than one might expect—to the early 1980s. Two decades ago, we didn't have nearly everyone and his brother accessing the Internet to check e-mail the way we do today. In fact, there were only a relatively small number of machines connected using TCP/IP, and most users of these machines had the ability to access their e-mail on-server, using the online access model.

However, even back then, developers recognized the advantages of being able to retrieve e-mail from a server directly to a client computer, rather than accessing the mailbox on the server using Telnet or NFS. In 1984, RFC 918 was published, defining the Post Office Protocol (POP). The idea behind POP was to provide a simple way for a client computer to retrieve e-mail from a mailbox on an SMTP server so it could used locally.

The emphasis was on simple; the RFC for this first version of POP is only 5 pages long, and the standard it defined is extremely rudimentary. It describes only a simple sequence of operations where a user gives a name and password for authentication, and then downloads the entire contents of a mailbox. Simple is good, but there are limits.

In February 1985, RFC 937 was published: Post Office Protocol - Version 2. POP2 expanded the capabilities of POP by defining a much richer set of commands and replies. This includes the important ability of being able to read only certain messages, rather than dumping a whole mailbox. Of course, this came at the cost of a slight increase in protocol complexity, but POP2 was still quite simple as protocols go.

These two early versions of POP were used in the mid-1980s, but not very widely. Again, this is simply because the need for an offline e-mail access protocol was limited at that time; most people were not on the Internet before the 1990s.

In 1988, RFC 1081 was published, describing version 3 of the Post Office Protocol (POP3). By this time, the personal computer (PC) was transitioning from a curiosity to a place of importance in the worlds of computing and networking. POP3 was based closely on POP2, but refined and enhanced with the idea of providing a simple and efficient way for PCs and other clients not normally connected to the Internet to access and retrieve e-mail.

Development on POP3 continued through the 1990s, with several new RFCs published every couple of years. RFC 1081 was obsoleted by, in turn, RFCs 1225, 1460, 1725 and 1939. Despite the large number of revisions, the protocol itself has not changed a great deal since 1988; these RFCs contain only relatively minor tweaks to the original description of the protocol. RFC 1939 was published in 1996 and POP3 has not been revised since that time, though a few subsequent RFCs define optional extensions and additions to the basic protocol, such as alternative authentication mechanisms.

While POP3 has been enhanced and refined, its developers have remained true to the basic idea of a very simple protocol for quick and efficient e-mail transfer. POP3 is a straight-forward state-based protocol, with a client and server proceeding through three stages during a session. A very small number of commands are defined to perform simple tasks, and even after all the changes and revisions described above, the protocol has a minimum of “fluff”.

For reasons that are unclear to me, almost everyone refers to the Post Office Protocol with its version number; that is, they say “POP3” instead of “POP”. This is true despite most people not using version numbers with many other protocols, and almost nobody using any other version of POP anyway. But it is the convention, and I will follow it in the rest of this section.

TCP/IP Post Office Protocol (POP/POP3) 1 POP3 General Operation, Client/Server Communication and Session States