Subject: Re: ECSMail (fwd)

From: Mike Roch <[email-masked]>
Date: Thu, 24 Feb 1994 10:28:04 +0000 (GMT)

Mark,

I got this message on another list. Couldn't think of anyone better to 
answer it!

Mike

==============================================================================
Mike Roch, Computer Services Centre,                          Tel: 0734 318430
The University, Whiteknights, Reading, RG6 2AF                Fax: 0734 753094

---------- Forwarded message ----------
Date: Thu, 24 Feb 94 10:03:37 gmt
From: Andre Schappo <[email-masked]>
To: [email-masked],
    mac-supporters mailbase list <[email-masked]>
Subject: Re: ECSMail

At 11:51 21/2/94 +0000, P Browning wrote:
>Any of you out there using ECSMail? I'm just about to have
>a play with a Mac version of it. This could be the
>way forward for sites which have religous objections to
>POP - it's an IMAP client which seems to offer a similar
>level of feature-richness as Eudora.
>
>It's available for PeeCees and Unix too and apparently
>CHEST are thinking about it.
>
>I'm summarise for the list if appropriate.
>

I sometimes wonder what would be the effect of large scale usage of IMAP
clients as opposed to SMTP/POPx clients since IMAP is connection oriented
and SMTP/POPx is connectionless oriented.

Take the case of a University (say) with X thousand mail users and a single
mail server.

Now I figure if everyone used an SMTP/POPx client the system would cope
fairly well. If though everyone used an IMAP client I figure the system
would clog.

I have no proof or evidence for this. It is just something I feel in my bones.

Anyone had experience of large scale usage of IMAP clients?

Andre Schappo
Department Computer Studies,
University of Technology,
Loughborough,                     email: [email-masked]
Leics,                             talk: [email-masked]
LE11 3TU,                           tel: (0509) 222693
UK                                  fax: (0509) 610815

---

From: Mark Crispin <[email-masked]>
Date: Thu, 24 Feb 1994 02:42:09 -0800 (PST)

Andre -

     Mike Roch forwarded your questions about IMAP vs POP performance to me.
It seems that you have several misconceptions, which I hope I can clarify.

     There is no ``IMAP vs SMTP'' issue.  Like POP clients, IMAP clients use
SMTP for mail delivery.  Actually some POP clients may use an undocumented
extension feature in some POP servers to deliver mail without using SMTP, so
one could say the issue is ``SMTP/IMAP vs POP''.  However, it is generally a
bad idea to use an access protocol such as POP or IMAP to deliver mail (a long
description of why this is so is in the IMAP4 draft document).

     It is incorrect to think of POP as being ``connectionless''.  POP is as
much a connection-oriented protocol as IMAP is.  The difference is that for
the most part you use POP to download all messages from the server to the
client, delete from the server, and close the connection.  IMAP permits this
mode of operation, but also offers a mode in which you manipulate data on the
server without having to download the entire mail folder (which implies
keeping the connection open).

     IMAP servers, in general, use remarkably little CPU time.  Depending upon
folder format, my implementation may need some virtual memory (note the word
``virtual'' -- it tolerates being paged out quite nicely) but even that isn't
necessarily the case; one folder format is quite a miser on system resources.

     We have NEVER observed a system being overloaded because of IMAP servers.
In general, long before the system runs out of resources to handle IMAP
servers, you run into trouble due to other reasons related to a overlarge user
population; inadequate disk space, lock contention between sendmail processes,
access to /tmp, etc.  In fact, sendmail will drag such a system to its knees
long before IMAP servers will.

     This isn't to say that there is no cost differential between IMAP and
POP.  There is.  However, the concerns expressed tend to be overblown.  Most
IMAP users are likely to be mobile users, who would otherwise log in to the
server system as a timesharing user.  An IMAP server is ALWAYS less costly
than a user mail program run on that system.  That is, a user running PC Pine
to a UNIX based server causes MUCH LESS of a load on that server than by
running UNIX Pine on the UNIX machine.

-- Mark --