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Message Access Paradigms and Protocols

Written: 95.08.28, Revised: 95.09.28
Editor's note 02.01.24: some of the RFCs noted in this paper have been superseded. Please refer to the index of RFCs and drafts for the latest versions.

Terry Gray
Director, Networks & Distributed Computing
University of Washington
gray@cac.washington.edu

I. EXECUTIVE SUMMARY

This paper is an elaboration on a short note entitled "Comparing Two Approaches to Remote Mailbox Access: IMAP vs. POP", which was written in 1993 and recently updated. The purpose of this paper is to provide more extensive background on message access paradigms and protocols, and then to specifically compare the Internet's Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP) in the context of "online" operation.

There are three different paradigms for accessing remote message stores (or "mailboxes"). They are: offline, online, and disconnected. In "offline" operation, the mail client program, or "mail user agent" (MUA), fetches messages from a mail server to the machine where the mail program is running, and then deletes them from the server. In "online" operation, messages are left on the mail server and manipulated remotely by mail client programs. In "disconnected" operation, a mail client connects to the mail server, makes a "cache" copy of selected messages, and then disconnects from the server, later to reconnect and resynchronize with the server. In both online and disconnected access modes, mail is left on the server, which is important when people use different computers at different times to access their messages.

Any one of several client-server protocols could be used to access a remote message store, including general purpose file access protocols (e.g. NFS) and various application-specific protocols --some vendor-proprietary, others more open. Each protocol has its strengths and weaknesses, but for purposes of this discussion, the only two of interest are the Internet's Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP). Both of these protocols fully support the offline operation model. If one's only requirement is for offline access, then IMAP is overkill.

It is the thesis of this paper that (a) supporting online and disconnected access is essential for a growing fraction of mail users, and (b) POP is inadequate for proper online and disconnected support, whereas IMAP does a good job for all three message access paradigms. This is not an indictment of POP, since POP is entirely satisfactory for its intended design center of offline mail processing. Rather, it is an explanation of the importance of online and disconnected operation, and why IMAP is a much better answer for those access modes.

Here are some of the functions, important for online and disconnected access, that are available in IMAP but not in POP:

Remote Folder Manipulation:

Multiple folder support:

Online performance optimization:

Some of these capabilities are particularly important when one is connected to a server at low-speeds, e.g. via wireless phone links. Indeed, they may make the difference between usability and frustration.

Finally, IMAP has provision for negotiated extensions, and therefore its capabilities can grow incrementally to meet new requirements.

Our conclusions, then, are that a) messaging technology that only offers offline access alone is inadequate for contemporary needs; b) IMAP provides superior online and disconnected support, in addition to support for offline operation; c) an application-specific protocol such as IMAP offers important advantages over generic file-system protocols, and d) since IMAP is a functional superset of POP, the only real advantage of POP over IMAP is that there is currently more software available for POP.


II. MESSAGE ACCESS PARADIGMS

In the early days of email, messages were typically delivered to a single time-sharing machine within an organization, and everyone would login to that machine to read their email. While this model is still widely used, it has two serious limitations: first, it does not scale very well to accommodate growing user populations, and second, it does not allow use of native Mac or PC messaging applications. The first point is primarily of concern to system managers, but the second issue directly affects the usability of the system, both in terms of providing a graphical user-interface look/feel common with other personal productivity tools, and also in order to make it easy to import/export data between the mail system and the desktop or laptop machine.

Accordingly, a distributed messaging architecture that accommodates personal computer usage paradigms is of interest. However, before considering specific client-server protocols to support such an architecture, it is important to understand the different ways messaging clients and servers might be used.

The Internet document RFC-1733 defines three modes of operation, or message access paradigms, in a distributed messaging system:

In "offline" operation, messages are delivered to a (typically shared) server and a workstation or personal computer user periodically connects to the server and downloads all of the pending messages to the "client" machine. More precisely, the mail client program, or "mail user agent" (MUA), fetches messages from the server to the machine where the mail program is running, and then deletes them from the server. Thereafter, all message processing is local to the client machine.

In "online" operation, messages are left on the mail server and manipulated remotely by mail client programs --possibly more than one at a time, and probably more than one at different times.

In "disconnected" operation, a mail client connects to the mail server, makes a "cache" copy of selected messages, and then disconnects from the server, later to reconnect and resynchronize with the server. The user may then operate on the message cache "offline", but this model differs from the pure "offline" model in that the primary copy of messages remains on the server, and the mail client program will subsequently re-connect to the server and re-synchronize message status between the server and the client's message cache. Online and disconnected operation complement each other and one may alternate between them; however, neither is compatible with offline operation since, by definition, offline operation implies deleting the messages from the server after they've been copied to the client machine's local disk.

You can think of offline operation as providing a store-and-forward service, where the last hop transmission is initiated by the user via a mail program on the final destination host. It is intended to move mail (on demand) from an intermediate server (drop point) to a single destination machine, usually a PC or Mac. Once successfully delivered to the user's machine, the messages are deleted from the mail server.

The offline model shines when a person always accesses their mail from a single computer and is content to have the primary copy of their messages live on that single machine. If, however, one needs to access their message store from different computers at different times, or if they wish to keep the message store* on a different machine than the one they directly interact with, then the offline model is inappropriate.

A growing fraction of email users have one machine at work and a different one at home, and possibly also a laptop for travel. We feel that such users would, if given a choice, prefer online and disconnected access, and would choose offline access only when connect time and/or server resources are at a premium. And while online access does imply longer connect times than offline, disconnected use requires roughly the same connect times as offline access. A preference for offline access can not be inferred from its present market share, because much of the user community has never been offered the choice.


* Note: the terms "message store", "mailbox", and "folder" are used almost interchangeably in this paper, although a "message store" might contain more than one "mailbox" or "mail folder".
III. MESSAGE ACCESS PROTOCOLS

Most people do not have messages delivered directly to a personal computer, and instead rely upon an "always up" mail server, which is usually shared by others in the organization. Whenever mail is delivered to one machine but read on a different one, there is a need for a network protocol to access the messages on the server machine. Once a policy decision is made on where someone's message data is going to permanently reside, i.e. on a personal, workgroup, departmental, or central server, then the question becomes: which protocol should be used to access the message data when using a different machine? The question applies both to incoming message folders (e.g. one's primary INBOX) and also one's saved-message folders. When reading incoming message folders, a common operation is to save a message to an archive folder, so both data sets must be available simultaneously. It is not a requirement that the same protocol be used to access both classes of messages, but in most cases it makes sense to do so.

Protocol choices include:

Although it is tempting to think that a remote file system protocol is the best solution to the problem of accessing remote message stores, there are some problems with that strategy:

In contrast, an application-specific protocol can:

Hence, use of an application-specific protocol is preferred. Within that category, the choices are:

The proprietary approaches are herein summarily rejected for all the usual reasons having to do with getting locked into a single-vendor solution. Moreover, these approaches inevitably lead to having email gateways to the Internet, a perennial source of grief.

The X.400 approach is also rejected, principally for two reasons: first, the P7 protocol has some serious technical shortcomings, but more importantly, P7 presumes X.400 message header and transport technology rather than Internet messaging technology. X.400 technology condemns innocent users and system managers to dealing with the consequences of X.400 O/R addresses and gateways to the Internet --both of which we feel should be avoided at all costs. (Comparison of Internet and X.400 messaging technology is beyond the scope of this paper, but suffice it to say the oft-heard claims that one cannot do large-scale, mission-critical messaging with Internet protocols are simply incorrect.)

That leaves us with the Internet-oriented messaging protocols, of which there are three: POP, DMSP, and IMAP.

Of the three, the oldest and best known is the Post Office Protocol (POP), which was originally defined in RFC-918 of October 1984. Like the other two, POP has gone through several revisions since its first incarnation. As of this writing, the current version is POP3 as described in RFC-1725, but there is an even later revision of POP3 pending as an Internet Draft. Recent additions include support for advanced authentication methods and unique message identifiers, both of which were derived from work on IMAP version 4. POP was designed specifically to support the offline access paradigm; however, with very significant limitations, it has also been applied to the other two paradigms.

The Distributed Mail System Protocol (DMSP) was first defined in RFC-984 of May 1986. The latest revision is in RFC-1056 of June 1988. Unlike POP, DMSP has not been widely supported, and is largely limited to a single application, PCMAIL. DMSP was designed specifically to accommodate, and is best known for, the disconnected operation support in PCMAIL.

The Internet Message Access Protocol (IMAP) dates to 1986 at Stanford University, though it was not documented until RFC-1064 of July 1988. It, too, has seen several revisions since then, with IMAP4 (described in RFC-1730) being the current version. IMAP was originally designed to support the online access model; indeed, "IMAP" once stood for "Interactive Mail Access Protocol", but the name was changed in 1993 as part of the IETF standardization effort in order to better reflect IMAP's current capabilities. Since IMAP is a functional --though not syntactic-- superset of POP capabilities, it can fully support offline access as well as POP does, and with additions made in version 4 it can also support disconnected operation. IMAP has therefore subsumed the functionality of both POP and DMSP.

The remainder of this paper will focus on comparing the two principal message access protocols, POP and IMAP, especially with respect to their use in online and disconnected operation.


IV. PROTOCOL REQUIREMENTS FOR ONLINE AND DISCONNECTED OPERATION

A proper online message access protocol must be able to manipulate remote message stores as if they were local. For the reasons cited earlier, a complete solution requires that this be accomplished without reliance on general file system protocols. So if one were to design a protocol (or protocol family) to support a high-quality client-server messaging system, what capabilities would it need? It would need to offer:

In the Internet, sending messages is accomplished via the Simple Mail Transfer Protocol (SMTP). Because SMTP is indeed simple, there is no particular advantage to duplicating this functionality in the message access protocol, and neither POP nor IMAP do so. Likewise, accessing and updating personal configuration information is relegated to a separate companion protocol. In the case of IMAP, the configuration support protocol is called IMSP (Internet Message Support Protocol) and was developed at Carnegie Mellon University. Hence, the focus of subsequent sections will be on remote folder access and manipulation capabilities, as well as performance considerations while clients and servers are connected.

Disconnected operation has all of the same requirements as online operation, plus it demands that messages in a particular folder be uniquely identifiable throughout the life of that folder. This is so that clients and servers can periodically resynchronize the status of particular messages.

V. SIMILARITIES BETWEEN POP AND IMAP

Although the protocols are not directly compatible and differ in significant ways, there are some common characteristics. Both of them:

VI. POP IN ONLINE MODE

As customers and vendors of POP-based mailers encounter the limitations of the offline access paradigm, there has been interest in trying to use POP in both online and disconnected modes. Usually this involves configuring the POP mailer to "leave mail on server" rather than deleting it after copying the messages to the client machine. However, POP has no provision for several capabilities that are essential for a high-quality online or disconnected client. These capabilities fall into three categories:

Even if POP is used in "pseudo online" mode, wherein messages are not deleted from the server, it cannot really be considered a proper protocol for online operation because a different (typically a remote file system) protocol must be used for certain online operations.

One example of POP's limitations in online mode is updating per-message state information, e.g. marking a message as Answered. Either the client mail program omits this important capability, or it must be done outside of POP, since POP has no primitives for this function. Indeed, the per-message state information would typically have to be stored separately from the messages themselves and accessed as a local file in order for a POP client to update this data.

Another example would be saving a message to a personal archive folder. Although it might seem that such saving is only done to a "local" file, in fact a key premise of client-server messaging is the need to use different clients at different times, and by definiton, the saved-message archives can't be local to more than one computer.

However, when POP is used in "pseudo-online" mode, its deficiencies are sometimes masked by the fact that certain online messaging operations are actually being done on ostensibly "local" archive folders via a remote file system protocol. Note that if one could depend on having a generic remote file access protocol pervasively available, one wouldn't need POP at all...

Because POP was never designed to be an online messaging protocol, it is not surprising that it lacks critical functionality in this area. The requirements for disconnected operation are a strict superset of online access functional needs, so POP also lacks the abilities to be a superior disconnected messaging protocol, even though the latest POP specification includes provision for unique and persistent message identifiers, an essential ingredient for disconnected operation.

VII. IMAP ADVANTAGES

The functional areas where POP is weak, with respect to online/disconnected operation, are strengths for IMAP, since online access was its original design center. Specific advantages of IMAP over POP (for online/disconnected use) include:

Remote Folder Manipulation:

Multiple folder support:

Online performance optimization:

In addition, IMAP has provision for negotiated extensions, and therefore its capabilities can grow incrementally.

Implications of remote folder manipulation abilities...

The ability to "append messages to a remote folder" means that you can save messages to an archive folder, or even from an archive folder back to one's inbox. Saving messages is central to message management.

The ability to "set standard and user-defined message status flags" means that the mail client program can record status information about particular messages; for example, the fact that it has been answered, or marked important by the user, or marked deleted. IMAP allows for user-defined status flags, as well as a small set of standard flags.

"Support for simultaneous update and update discovery in shared folders" is important in situations where several people are handling incoming messages to the same mailbox. For example, a help desk may have several people processing requests coming into a single mailbox. This requirement has implications for both the message access protocol and the mailbox format. In particular, the mailbox format must permit concurrent status flag updates from different sessions, and the message access protocol must provide for notification of each user session when a status flag changes. IMAP differs from many client-server protocols in that the responses from a server may not be a direct result of requests from one's own client; the server must be able to send "unsolicited" data to any client to inform it of mailbox state changes. Note that in practice, shared folder support also requires that the protocol permit access to multiple folders.

Another example of mailbox state change is "new mail notification". In IMAP, when a client program performs any operation on a mailbox, the server will automatically include in its response notification of any new messages that have arrived since the last notification.

Implications of multiple folder support...

A key objective of online and disconnected operation is to support message access from different computers at different times. This includes knowledge workers with one computer in the office and a different one at home, as well as "nomadic" students who must rely on lab machines which do not have any local per-user storage.

Thus, the ability to "manipulate remote folders other than INBOX" is fundamental to online and disconnected operation. This means being able to save messages from one folder to a different one, being able to access archived messages subsequently, and allowing for multiple incoming message folders. Accessing "multiple incoming message folders" is useful for people who have partitioned their incoming mail streams, either via delivery filters, or by having different accounts for different purposes. The same protocol issues that argue for the online or disconnected access model for one's INBOX also apply to other message folders, be they "incoming" or "archive".

"Remote folder management (list/create/delete/rename)" follows from the need to access and manipulate more than one folder. In IMAP version 4, there is also "support for folder hierarchies", which allows nesting collections of folders. This has protocol implications because the client mail program must be able to distinguish directory names from folder names.

One additional benefit of IMAP is that it is "suitable for accessing non-email data; e.g., NetNews, documents". This capability is attractive in cases where it is desirable to unify different categories of information.

Note that, depending on the IMAP client implementation and the mail architecture desired by a system manager, the user may save messages locally on the client machine, or save them on the server, or be given the choice of doing either.

Implications of online performance optimization...

Whenever the mail client is connected to the mail server, in either online mode or during cache resynchronization in disconnected mode, there is an issue of network latency --exacerbated when the link speed between client and server is low. IMAP provides several facilities that can be used to dramatically reduce the data transmitted between client and server.

First, there is "provision for determining message structure without downloading entire message". This allows the mail client to display information about MIME message attachments without transferring them.

Moreover, "selective fetching of individual MIME body parts" means that if someone sends you a two line text message with a 10MB video clip attached, your mail client can choose to transfer only the two lines of text until you specifically request the attachment. If connected via dialup from a hotel room, this can be an enormous benefit. Efficient processing of MIME messages is a significant benefit of IMAP over POP. (MIME stands for Multipurpose Internet Mail Extensions. It is a technique for encoding arbitrary files as attachments to SMTP and RFC-822 compatible Internet mail messages.) In the same vein, IMAP4 also provides for selective retrieval of specific message headers.

Finally, the ability of IMAP software to use "server-based searching and selection to minimize data transfer" should not be underestimated. This is one of the key advantages of an application-specific messaging protocol in comparison to a general purpose file access protocol (or POP in pseudo-online mode, for that matter). When the client-server protocol does not include primitives to ask the server to do searching, then searching implies transferring all of the data over the net to the client. Being able to ask the server to find relevant messages by searching its local file is a huge win.

VIII. IMAP DISADVANTAGES

IMAP has two disadvantages when compared to POP:

Obviously IMAP is more complex because it does more, but the complexity concern is mitigated by two factors: first, depending on the objectives of the client, one may not need to use all of the functionality available. For example, one of the earliest IMAP clients was POPmail from the University of Minnesota. It offers only offline support, via both POP and IMAP, but it is likely that the author found that the complexity of implementing offline support via IMAP was roughly the same as for POP. A second mitigating factor is the existence of freely available support libraries which implement the IMAP protocol details. An example is the c-client library from the University of Washington.

While it is still true that POP enjoys broader vendor support than IMAP, there is already a substantial list of IMAP software (see the REFERENCES section), and many POP software vendors are working to add IMAP support as soon as possible. At the moment there are 20 different IMAP clients available, and at least seven more in development.


IX. SOME OBJECTIONS ANSWERED

There is a common misconception that IMAP implies centralized computing. This is usually said in the context of how powerful personal computers are now and therefore how irrelevant central computing has become. Regardless of one's politics or religion on the subject of central computing, this issue has nothing to do with IMAP. In fact, IMAP may be used with personal, workgroup, departmental, or enterprise-wide servers. Once the policy decision has been made on where one's message data will live, the relevant question is: which protocol will be used to access it from *other* machines? And for most folks, there will be "other" machines: a diminishing fraction of computer users will go through life using a single computer in all situations.

It has been said that POP is sufficient and IMAP is not worthwhile because it only addresses remote access to message objects, and clearly there are other classes of data to which one might want remote access. It is certainly true that IMAP is not a panacea for all remote data access, on the other hand, there is no evidence that the world is converging on a single general-purpose remote file access protocol. Neither Sun's NFS, Transarc's AFS, OSF's DFS, Apple's AFP, Microsoft's SMB, nor Novell's NCP are now, nor are likely to become, universal platform-independent remote file access protocols. Even if a single protocol did meet this need, there are still performance reasons to consider application-specific protocols. Also, certain file/object locking problems that are inevitable in a real-time messaging environment are often easier to solve in an application-specific protocol. One of the ironies of the "POP is sufficient because IMAP doesn't do everything" argument is that if one postulates the existence of a pervasive remote file-access protocol, then POP is no longer needed either.

A variation on the same theme is the hypothesis that eventually everyone will carry all of their personal files with them, so that remote access to anything other than a single mail drop is unnecessary. While carrying cache copies of relevant data with you is clearly useful, one wonders how many people would choose to have the data set on their person be their *primary* copy. The thought of losing one's email at the same time one loses keys, etc, is most unsettling, so the author certainly would not... but we have a few more years before storage technology advances even make the question relevant. Moreover, it seems just as plausible to believe that wireless technologies will keep one connected at all times and obviate the need for any on-person storage, in which case remote access protocols that will perform well for key applications over low-speed links may become even more vital.

Some have observed that no mail application protocol can keep up with ever-growing needs driven by increasingly sophisticated mail client programs. They argue that defining a language, in which to write programs to be executed on the mail server, is the only way to provide sufficient generality for changing client needs. In other words, any fixed protocol --even one with negotiated extensibility features such as IMAP has-- will soon be obsolete. The counter-argument is that opening up the server to executing any program that can be expressed in the candidate language is a system manager's nightmare, since it makes it much more difficult to maintain consistent response times or plan for growth. Also, use of open-ended macro facilities can lead to architectural conflicts. Even with protocols that permit downloaded code, one is constrained by the primatives available on the server and the model that the protocol imposes. For example, if a protocol allows the text of a message to be modified, then a client cannot reasonably cache the message text. In other words, protocol generality is sometimes over-rated, and having a relatively fixed set of application-specific functions in the protocol is often considered a feature rather than an unfortunate limitation.

An important recent trend is the advent of the World Wide Web, and with it the Hypertext Transport Protocol (HTTP). Might not HTTP obsolete IMAP? It seems to us much more likely that the two will coexist to mutual advantage. The designers of the Web wisely decided to exploit existing technologies and incorporate them into URL syntax, and provided hooks for external display tools and access methods. IMAP and HTTP can both coexist and complement each other in that context. And while some very interesting work has been done recently in allowing generic WWW browsers to access email via HTTP alone, IMAP's application-specific primitives still offer significant performance and functionality advantages over a pure HTTP approach.

Finally, one might ask: why not do both POP and IMAP? Indeed, this strategy has been successfully adopted by many sites in order to provide the advantages of IMAP while maintaining compatibility with earlier mail user agents. There are no particular technical barriers to providing both protocols on a mail server, as long as both daemons use compatible mailbox formats and file locking disciplines. However, there may be non-technical reasons to support just one protocol: there are several mail user agents that support one or the other, but not both protocols; therefore, if both services are made available, client services staff must (as usual) bear the brunt of the diversity. Where support resources are stretched thin, it may be necessary to pick one family of mailers rather than provide mediocre support to all of them.


X. CONCLUSIONS

The two key points of this paper are that:

The key virtue of the offline message access paradigm is that it minimizes server connect time and server disk requirements. However, offline mode works best for people who use a single client machine all the time; it is not well-suited for the goals of accessing one's incoming or saved-message folders from different machines at different times. If one relies on a remote file system protocol to access message data on a different machine, this is really "online mode in disguise" --but without the advantages of an application-specific protocol tuned for online and disconnected operation.

IMAP is such an application-specific protocol; a client-server messaging protocol designed to permit manipulation of remote mailboxes as if they were local. In addition to fully supporting the offline access paradigm, it offers capabilities that are essential for proper online message access, and which cannot be achieved with POP mailers unless they also use general file system protocols to provide location-independent access to message archives and message status information.

A reasonable conclusion is that the only advantage of POP over IMAP is that there is currently more POP software available. However, this is changing rapidly, and IMAP's functional advantages over POP are nothing less than overwhelming.


XI. REFERENCES

The original short note upon which this paper was based, "Comparing Two Approaches to Remote Mailbox Access: IMAP vs. POP", may be found at this URL:
http://www.imap.org/papers/imap.vs.pop.brief.html

IMAP version 4 is defined in RFC-1730. A URL for this RFC is: http://www.faqs.org/rfcs/rfc1730.html.

IMAP version 4rev1 is defined in RFC-2060. A URL for this RFC is:
http://www.isi.edu/in-notes/rfc2060.txt

Additional information on IMAP may be found at the following URL:

http://www.imap.org/papers/biblio.html

Available from the directory at the URL: ftp://ftp.cac.washington.edu/mail/   are various pieces of IMAP software, including an IMAP server and also a POP server that, in addition to offering the normal POP service, can relay commands to an IMAP server, thus permitting existing POP clients to access an IMAP server.

There are currently some modifications to the POP specification being considered by the IETF, but as of this writing, the latest non-draft version of the POP specification is RFC-1725, available at: http://andrew2.andrew.cmu.edu/rfc/rfc1725.html



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