<shepherd hat on> During the second last call for rfc2821bis, there has been much discussion of how the "implicit MX" handling is to be handled in an IPv6-capable and IPv6-only environment. This has generated much heat, as well as numerous proposals that were both productive and counter-productive, and that were both in scope and out of scope. I came at this question with an open mind, trying to weigh each of the arguments being made both for and against different stances. My measuring stick in each case against has been: How does this measure against what is required to advance 2821bis to Draft Standard? What is the current usage? What do the implementation reports have to say on this issue? The SMTP implementations that have made the transition to support IPv6 appear to already have done it in a way that supports AAAA records for the implicit MX case. In some cases they are following RFC 3974, and other cases they are just using getaddrinfo() and letting it do the rest. Note that RFC 3974 itself was supposedly "based on experience in deploying IPv6". At least one of these MTAs is in common use around the network in the IPv4 world. In essence, these implementations are following the RFC 821 and RFC 974 (sans WKS) rules of looking for address records. They've ignored the A-only wording of RFC 2821 and are acting like we specify currently in 2821bis-09. In my queries I haven't yet found any IPv6-capable SMTP server that doesn't do it. I've seen examples of sites that are in regular use that mail would break in an IPv6-only world if implicit MX to AAAA did not work. From this viewpoint, running code wins. I'm also swayed by the principle of "least surprise". Some of the responses I've gotten have been along the lines of "Why's this a question? Of course you do AAAA lookup". One person who had a site set up with an IPv6-only connection and no MX record told me "I wanted to forward my e-mail to an account on that machine. It worked the first time, so I didn't see a need to change it." As mentioned above, at least one of the IPv6-capable MTAs is in common use around the network in the IPv4 world, and turning on IPv6 on those boxes should not cause surprises. Last of all, I'm swayed by the discussions around RFC 974 and the DRUMS archive search around the question of what led to the wording change in 2821bis saying explicitly to do A lookups. These indicate that the intent of adding the A record description was to be descriptive, not prescriptive nor proscriptive. So the bottom line is that I see sufficient support for including AAAA lookups when implicit MX comes into play. It's been suggested that 2821bis revert back to either the implicit MX description found in RFC 821 or RFC 974, although Glen Anderson had some suggested improvements to that latter's description that do make it clearer. Any of these three would satisfy this decision, and I'll let John choose the wording he prefers. </shepherd hat off> Tony Hansen tony@xxxxxxx _______________________________________________ IETF mailing list IETF@xxxxxxxx https://www.ietf.org/mailman/listinfo/ietf