E-mail is networking's "killer app," but the decades-old
standards on which Internet mail is based can't meet the upsized demands of future
webs. For one thing, the locus of personal computing is no longer a desktop, thanks to notebooks,
home PCs and the Internet.
Users increasingly need to manage their e-mail, stored centrally on a remote mail server,
from multiple locations. But today's most popular mail retrieval standard, POP3, is
optimized for yesterday's one-computer-per-user model.
POP3's ability to manage large file attachments such as multimedia is also
quite limited. In this column, the third in IDM's continuing series on emerging
intranet standards, we look at IMAP, a strong candidate to replace POP3 for
sophisticated mail clients.
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. (Gray)
Background
Computer networking isn't the first technology for
sending messages electronically. Its precursors include television
and radio, the telephone, and way back at the beginning, the telegraph.
Western Union, the company that commercialized Morse Code and still a synonom
for wire transfers, recognized in telegraphy a cheaper/faster/better Pony
Express.
Today "getting the mail through" continues to be the operative metaphor for reliable messaging. Phone
companies have readily adopted the term voice mail, and e-mail, dating from the earliest LANs,
has become a pillar of enterprise communications.
Client/server architects use the term messaging to connote a superset
of e-mail that includes, in addition to the exchange of text and binary
messages between humans, store-and-forward communications between humans
and machines, and machines and machines. Message queuing, for example,
implements an asynchronous request from a client to a transaction
processing host.
In the Internet world, standards for sending and retrieving electronic
mail evolved early. While the continuing survival of
these standards is a testament to their longevity (and to their authors'
foresight), the standards grew up in a computing environment very different
from today's. New capabilities and requirements have given rise to new models
of message exchange, described below.
There are three fundamental models of client/server
e-mail: offline, online, and disconnected use. These use cases, defined
below, are where things have changed since the early days of e-mail -- and
where IMAP4 message access can add significant value over POP.
OFFLINE
The offline model is the most familiar form of client/server email
today, and is used by protocols such as POP-3
(RFC 1225) and UUCP.
In this model, a client application connects on demand to a
server and downloads all pending messages to the client machine.
It then deletes the retrieved messages from the server and disconnects (hence, "offline");
subsequent mail processing is done locally, on the client.
The offline model is store-and-forward: it moves
mail on demand from an intermediate server (or mailbox) to a single
destination machine.
ONLINE
In the online model, a client application
manipulates mailbox data directly on the server, and a connection is
maintained throughout the session. No mailbox data are
kept on the client; the client retrieves data from the server as is
needed. Online messaging is most often used with remote filesystem
protocols such as NFS (Network File System), which essentially make remote
message stores appear local.
DISCONNECTED
The disconnected use model is a hybrid of the offline and online
models, and is used by protocols such as PCMAIL
(RFC 1056). In this
model, the client downloads a user-specified set of messages from the server,
manipulates them offline, and later uploads the changes. The server remains
the authoritative repository of the messages. The problems of synchronization
(particularly when multiple clients hit against the same message store) are handled
by assigning a unique identifier to each message.
The Problem
In online and disconnected messaging, mail is left on the server,
which creates synchronization problems when people use different client computers at different
times to access their messages.
In its favor, POP3 has simplicity, stability and
near-universal platform support. It remains the most popular solution
for store-and-forward messaging, as the wild success of mail clients like
Qualcomm Eudora attests.
For online and disconnected access, though, POP3's simplicity is a drawback.
Consider two prevalent use cases for which POP lacks desirable functionality.
One is the instance of multiple user access to the same message store.
This occurs, for example, when someone has one computer at work and another
at home, and needs to retrieve mail on each at different times from the
same mailbox. POP3 provides no means for conveying the status of messages
operated on by one client to another.
The other challenging use case involves a small text message with one or
more large files attached. In this case, POP3 offers the client only
two unfriendly choices: ignore the whole message by turning off large message
retrieval, or download the "mail bomb." It would be nice if we
could identify and retrieve the various components of a multi-part
message separately.
Internet Message Access Protocol (IMAP), a more complex standard than POP, is capable of
handling the above scenarios. IMAP is designed to excel at all three modes of client/server
e-mail. The table below summarizes the functional advantages
of IMAP4, the current version, over POP3:
Server-side Mailbox Manipulation
Ability to add messages to a remote mailbox
Ability to set message flags, such as "answered" or "important"
Support for and notification of simultaneous updates in shared mailboxes
Server-based search and retrieval to minimize data transfer.
You may be wondering why, if IMAP truly brings these benefits to the table,
it isn't rapidly replacing POP3 as the basis for Internet messaging. The
reason is that until recently vendors had little incentive to "mess
with success" by adding IMAP to their POP-based software, since user
demand for advanced capabilities is only now starting to ramp up. At
this point [1996] Microsoft Exchange, Netscape SuiteSpot and SunSoft Solstice
Internet Mail Server have all announced a strategic shift to IMAP4.
The next section gives a summary of currently available IMAP products.
Simeon (formerly ECSMail)
The ESYS Corporation
Suite 835, 10040 - 104 St.
Edmonton, Alberta, CA T5J 0Z2
Voice: (403) 424-4922 Fax: (403) 424-4925 sales@esys.ca http://www.esys.ca
Solstice Internet Mail Server
Sun Microsystems Inc.
901 San Antonio Road
Palo Alto, CA 94303
email: sims-comments@sun.com
Product info: www.sun.com/sims
Z-Mail
NetManage
10725 North De Anza Blvd.
Cupertino, CA 95014
Voice: (408) 973-7171 Fax: (408) 257-6405 Evan Knuttila, Product Manager http://www.netmanage.com/
To learn more about IMAP, visit The IMAP Connection,
a complete reference site hosted at University of Washington.
[2] The current standards -- RFC-821, "Simple Mail Transfer Protocol," (SMTP) and RFC-822, "STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES" -- both date from August 1982.
[3] MIME stands for Multipurpose Internet Mail Extensions, RFC-1521 and
RFC-1522, a standard for encoding binary files as attachments to SMTP and RFC-822
mail messages.
Gordon Benett is the Founder and Editor-in-Chief
of internet.com's Intranet Design Magazine. His extensive writings
on corporate web technology include Introducing Intranets (Que, June
1996), a primer for IT decision-makers. Mr. Benett welcomes comments at
<gbenett@internet.com>.
Events
[
Home | Discussion Forum | How Do I... |
Lotus Notes Intranets |
Microsoft SharePoint |
Products |
Shopping
]
The only advantage of
POP over IMAP is that there is currently more 
(![[ Top ]](/pix/up-ball.gif){kind=small}