Subject: Re: Last-Call: draft-klensin-rfc2821bis

From: Mark Andrews <[email-masked]>
Date: Wed, 16 Jan 2008 11:38:05 +1100

I know this is a bit late but:

	Section 5.1's sythesis of MX records based on the presence of
	AAAA records is a bad idea.

   If no MX records are found, but an address
   RR (i.e., either an IPv4 A RR or an IPv6 AAAA RR, or their
   successors) is found, the address RR is treated as if it was
   associated with an implicit MX RR, with a preference of 0, pointing
   to that host.  

	Synthesizing a MX record on NODATA to a MX lookup and a
	subsequent successful AAAA lookup is bad engineering decision.
	It will work reasonably well for IPv4 only + dual stack
	envirionment.  It will not work well for IPv4 only + dual
	stack + IPv6 only envirionment.
  
        The reason it is a bad engineering decision is that:
  
	* the IPv4 only world needs a MX RRSet to find a dual stack
	  MTA to relay into the IPv6 network.
  
        * the IPv6 world has a raft of solutions which will allow it
          to initiate a connection to a IPv4 only MTA without having
          to find a dual stack MX for the target mail domain.
  
        * it changes the definition of what it means to exist in
          the mail domain and you will have different MTA/MSA making
          different existance decisions.  Some will say that AAAA +
          no MX exist but others will say that the site does not
          exist.

	e.g.
	a new (IPv6 aware) MSA which is configured to relay through
	a old (non-AAAA aware) MTA on its outward bound path.
  
        Do you really thing we should be trying to force a upgrade
        of all MTA's on the planet to support MX synthesis from
        AAAA when there is no engineering need to to this?
  
        MX from A was a transition strategy.  IPv6 only sites have a
        transition strategy that doesn't require synthesis.  It is
        advertise a dual stack MX.  At some point in the future sites
        will stop having a dual stack MX, the same way they stopped
        adding A records for mail only domains back in the 90's.

	Mark
-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE:	+61 2 9871 4742		         INTERNET: [email-masked]

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---

From: Mark Andrews <[email-masked]>
Date: Wed, 16 Jan 2008 12:25:17 +1100

>         adding A records for mail only domains back in the 90's.

	It should be noted that direct to address for IPv4 or IPv6
	addesses is reasonable when there isn't any appropriate
	records (MX, A or AAAA) in the DNS and the MTA is configured
	to use /etc/host, NIS etc.

	Mark