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RE: Deliver-qmails-ondemand

2003-10-28 11:44:29

        Thank you for your detailed explantaion.

On Tue, 28 Oct 2003 19:35:01 +0530, Madan Ganesh Velayudham said:

    Yes, I understand RFC 1985, talks about Server and Client 
    Interaction. And It is not clear when the client should 
    initiate ETRN.

For SMTP, "client" is always the end starting the connection, 
and "server" is the end receiving.  This means that the 
machine you call your "SMTP server" is actually operating in 
both modes all the time - when it accepts inbound mail it 
runs in server mode, and (the case you are interested in) 
when it sends mail out, it's in client mode.

To avoid this confusion, we usually talk about Mail User 
Agents (MUAs - the code that actually talks to the user, 
usually on a desktop/laptop/etc), Message Submission Agents 
(MSAs - the code that accepts mail from an MUA), Message 
Transmission Agents (MTAs - the code that moves mail from 
system to system), and Message Delivery Agents (MDAs - the 
code that actually gets mail into a user's mailbox).  It is 
quite common for one piece of software to do multiple parts 
of this process.

        Yes, I understand them clearly. I take care of MTA/MDA/MUA 
        on HP-UX.

    My comment is about when the SMTP server brings up, it should
    inform its neighbour SMTP servers.

When your MTA comes up, it may want to connect to its 
neighbor MTAs in client mode and issue an ETRN to tell them 
that it is now up, and to initiate a queue run on the 
neighboring boxes now rather than at the next scheduled time.

        My only concern was I could not get it clearer from the RFC.

As an operational note, this can be *very* dangerous in 
high-volume environments if you have multiple neighbors with 
large queues., particularly with high-speed network 
connections and SMTP Pipeline enabled at both ends.

        I may wrong here. Please correct me if I'm.
        When a server comes up, a separate thread would take care
        sending #INVITE message to its neighbours and Parallely
        it can take care other activities.

The basic problem is that most high-volume mail systems are 
Unix or Linux based, and often they include the system load 
average in the "am I too busy?" calculation. Unfortunately, 
said average is time-smoothed, and represents the average 
load over the last 1/5/15 minutes.  So the server end 
receives a client connection, which then starts sending the 
mail *very* quickly (it's possible to get well into the 
hundreds/second if you try hard), while fork()/exec()ing in 
the background to do final delivery.  By the time the load 
average starts reflecting the problem, the server has already 
done dozens/hundreds/thousands of fork/execs and the load 
average then proceeds to spike.  I've personally seen an IBM 
RS6K-250 (1 133mz 601 processor) (our Listserv box at one 
time) bring a large 8-CPU Sun (our main mail hub) to its 
knees - the fact that the
*average* number of recipients per message was in the 
hundreds didn't help...

        Thank you for your sharings. That was a useful note. 

        + MG

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