Re: Review Requested: SMTP IPv6 operational experience document

[Dean Strik <dean@ipnet6.org>]

Dean Strik wrote:
> >    draft-ietf-ngtrans-ipv6-smtp-requirement-08.txt

There's one more small thing, which I didn't discuss in my earlier
comments. I quote from the draft, section 3, the dual-stack algorithm:
there's a note between steps (5) and (6), reading:

     NOTE: If one or more address records are found, some MTA
     implementation may sort addresses based on the implementation's
     preference of A or AAAA records.  To encourage the transition from
     IPv4 SMTP to IPv6 SMTP, AAAA records should take precedence.  But
     this type of sorting is optional.

I already said:

> I also would like to see a clarification that this sorting may only
> reorder addresses from MX records of the same preference! Address
> family preference can't be more important than MX preference value.

According to RFC 2821, section 5, paragraph 4:

   Multiple MX records contain a preference indication that MUST be used
   in sorting (see below).  Lower numbers are more preferred than higher
   ones.  If there are multiple destinations with the same preference
   and there is no clear reason to favor one (e.g., by recognition of an
   easily-reached address), then the sender-SMTP MUST randomize them to
   spread the load across multiple mail exchangers for a specific
   organization.

Now, a dual-stack client has two sorting parameters. Maybe it would be
nice to specify in this note that this IPv6 preference may take
precedence over the randomization specified above (so first all IPv6
addresses are tried, then IPv4).

Currently, my own implementation lets IPv6 take precedence. I'll make
this configurable soon (mostly because people may want to have it
otherwise).

-- 
Dean C. Strik             Eindhoven University of Technology
dean@stack.nl  |  dean@ipnet6.org  |  http://www.ipnet6.org/
"This isn't right. This isn't even wrong." -- Wolfgang Pauli