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Re: Registration model, 2821bis-06

2008-04-02 12:39:41

Hi Arnt,
At 04:11 02-04-2008, Arnt Gulbrandsen wrote:
How many people would be happy if 5.1 were to say this?

 - if there is no MX record, but there is an A, then the sending
   host MUST synthesise an MX
 - if there is no MX record and no A, but there is an AAAA, then
   the sending host MAY synthesise an MX
 - even if a client doesn't use AAAA records for synthesising MX
   records, it MAY use them for delivering mail once an MX has
   been synthesised.

You may have noticed that draft-09 carefully sidesteps the IPv6 question by mentioning an obviousness test from an operational viewpoint. The above sounds more like a decision to be made during implementation. The MAY makes it more difficult, from the server end, to determine why a message isn't being delivered as we have to know whether the client is designed to synthesize the AAAA RR as an MX in addition to the usual failure points.

Connectivity to/from AAAA-only mail servers will suck terribly for many decades to come, but no 2821bis wordsmithing can avoid that. Only an arrangement with a dual-stack relay can improve connectivity, and that practically requires an MX RR or more.

That's a reason for having the MX RR.

This discussion has covered both sides of the (implicit MX) argument from historical, email architecture, implementation and operational viewpoints. Whichever option is accepted will be viewed as a change from RFC 2821. I would be happy if the issue could be resolved one way or another.

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