In-Reply-To: <Pine.SOL.3.96.990305091529.17350A-100000@taurus.cus.cam.ac.uk>
Date: Fri, 5 Mar 1999 18:20:21 +0100
To: Philip Hazel <ph10@cus.cam.ac.uk>, Pete Resnick <presnick@qualcomm.com>
CC: John C Klensin <klensin@MCI.NET>, cos@leftbank.com, drums@cs.utk.edu
From: Brad Knowles <blk@skynet.be>
Subject: Re: FWD: Re: To MX or not to MX?
Message-Id: <19990305182021.011465@relay.skynet.be>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit

On Fri, Mar 5, 1999, Philip Hazel <ph10@cus.cam.ac.uk> wrote:

><pie-in-the-sky>
>The right way to fix all of this would be to start again, and start 
>registering services rather than hosts.

    You've got this today.  They're called SRV records.  Hardly anyone
uses them yet (because they're so new), but they exist and are handled
properly by current versions of BIND.

-- 
  These are my opinions -- not to be taken as official Skynet policy
 ____________________________________________________________________
|o| Brad Knowles, <blk@skynet.be>            Belgacom Skynet NV/SA |o|
|o| Systems Architect, News/mail/FTP Admin   Rue Col. Bourg, 124   |o|
|o| Phone/Fax: +32-2-706.11.11/12.49         B-1140 Brussels       |o|
|o| http://www.skynet.be                     Belgium               |o|
\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/
                The Sky is no longer the limit
or layer four switches) to hide multiple hosts behind a single name. 
You can separate the POP3/IMAP servers from the outbound mail relays, but
it is not strictly necessary (although I think it's a good idea).

    Unfortunately, MXes don't scale all that well, either.  Once you get
to hundreds of inbound mail machines for your domain, you can't
realistically list all of them in your replies, so you end up having to
use either DNS or NAT/switch tricks to get back down to a smaller subset
that any one sender might know about at any one particular point in time.


    Based on what I've seen here so far, If I have to choose between
SHOULD and MAY (or SHOULD NOT), I would vote for SHOULD NOT.

    I think we should explicitly state that how MUAs would deliver mail
to their upstream servers is a private arrangement issue (since, as far
as we're concerned, they're not truly on the Internet and we're not
expecting them to do MX resolution), and then we should describe some
typical private arrangements.


    As Pete points out, there are just too many different ways in which
things might radically vary at different sites, and I don't think that it
would be a good idea to try and encourage the adoption of any one of
those alternatives to the detriment of any other.  There's just too many
"legacy" systems out there that probably wouldn't conform to any solution
we might recommend (whatever that might be).

    How these guys go about doing their business has been unregulated and
pretty much undocumented for too long, for us to come along at this late
stage and try to impose our view of order upon them.


    Now, when it comes to the Message Submission protocol (whatever that
might be), I think this would be an ideal time to make use of SRV
records.  But I think this is a matter for a different document, if not a
different group.

-- 
  These are my opinions -- not to be taken as official Skynet policy
 ____________________________________________________________________
|o| Brad Knowles, <blk@skynet.be>            Belgacom Skynet NV/SA |o|
|o| Systems Architect, News/mail/FTP Admin   Rue Col. Bourg, 124   |o|
|o| Phone/Fax: +32-2-706.11.11/12.49         B-1140 Brussels       |o|
|o| http://www.skynet.be                     Belgium               |o|
\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/
                The Sky is no longer the limit