--On Monday, January 11, 2016 09:07 -0500 "Robert A. Rosenberg"
At 20:49 -0500 on 01/10/2016, John C Klensin wrote about Re:
[ietf-smtp] Fwd: New Version Notification for draft-fen:
(3) The EAI WG struggled for a long time with the relationship
between a requirement for certain SMTP options (i.e., if the
option is requested (or required) by the client but the server
will not accept it, the message content must not be sent) and
our MX structure. For example, suppose (to generalize, the
XYY option is at issue and required. Suppose we have
example.com. IN MX 10 A.EXAMPLE.COM.
IN MX 10 B.EXAMPLE.COM.
A supports XYZ and B does not.
There is another way to handle this situation that we already
use for connect fails. When I try B and can not connect I can
try A and if that also fails go to some MX with a higher
priority value number.
In this case of B not returning XYZ, in lieu of giving up
immediately, why not try A FIRST? Only when ALL the first
priority MXs get rejected would we give up and bail out. I
note that this may not cover the complex scenario version you
reference but it can handle some simple versions such as the
single setting support one.
Without commenting on whether I think it is a good idea or not,
it is an obvious one (and one that EAI did talk about). But it
violates the retry model in 5321 as now written (and in 974 --
nothing much has changed in that area in the 30 years (this
month) since 974 was published). Could we change SMTP or invent
an extension that would change the behavior? Probably. But
that leaves the "is it a good idea" question, especially in the
light of the observation that, if the parties configuring the MX
entries, preferences, and machines are sensitive to the issue
and really care, it is possible to have all configured servers
support the same services.
Other questions include whether, if there are a whole collection
of MX records with different preference levels and one can't get
through to any of the best-preference ones, does one try
multiple servers at the first preference level to which a
connection is possible? Higher-numbered preferences if those at
one level are not sufficient? Remember that, if one reaches a
third-preference-tier server, it has no obligation to use SMTP
and the better MX preferences to forward the message -- it can
do any or a wide range of out-of-band things. Also remember
that any of these steps converts "try" from "see if you can open
a TCP connection and whether it times out or is reset or
otherwise rejected" into "open a TCP connection and start
negotiating". The historical assumption has been that the costs
are very different.
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