On Fri, Jan 04, 2008 at 07:57:29PM -0500, John C Klensin wrote: > > > > (3) As Keith Moore has pointed out repeatedly for the general > case and as I and others have pointed out for more specific ones > (including today's mail-and-DNS case), dual stack is a nice > thing to do if one is developing operating systems and maybe if > one is developing servers. But any form of "the application has > a choice of multiple addresses or interfaces and must choose" > puts the application into the routing business. :) welcome to the end2end model and the rise of the stupid network... :) > Even the trivial example that Jeroen and Phillip used may be > problematic, especially if there are multiple IPv4 and multiple > IPv6 addresses. There are no MX rules at all about which > address must be tried first and some handwaving in RFC 2821 > about how many addresses at a given preference level need to be > tried at all. With most SMTP senders, such things can be > configured, but the IETF has published approximately zero advice > as to how to configure them (there actually is some reasonable > advice in RFC 3974, but that isn't an IETF document and it bears > one of the stronger forms of a "you really shouldn't pay any > attention to this" disclaimer from the IESG. well the failures fo the MX record for both DNS and specific application use are well known and there was a well designed "charge" to replace that capability in the DNS with a more general purpose tool that does help w/ the "which interface to use" problem.. which is in fact, very old indeed.. I point you to the SRV RR. --bill > john -- --bill Opinions expressed may not even be mine by the time you read them, and certainly don't reflect those of any other entity (legal or otherwise). _______________________________________________ Ietf@xxxxxxxx https://www1.ietf.org/mailman/listinfo/ietf