[ietf-dkim] Meta-comment re: subdomain strawpoll

[Jim Fenton]

There seems to be a little confusion in questions I have gotten about 
the "Try Parent Domain" step and the title of this straw poll.  In 
particular, whether the parent domain check is intended to apply to 
subdomains at all.

The confusion is whether a hostname, like www.example.com, is a 
subdomain or not:

(1) When used in an email address, www.example.com is called a domain, 
and since it's below example.com, it could be called a subdomain of 
example.com.

(2) In some other contexts, a hostname may not be considered to be a 
subdomain, since it has nothing below it.  If there's a 
www.eng.example.com, then eng.example.com would be a subdomain 
(regardless of whether it has its own SOA record, or a direct record for 
www.eng exists within the example.com zone).

Using definition (2), the parent domain check is beneficial only for 
hostnames, but not for subdomains.  Since any hostname can receive mail 
via its A or AAAA record, in order to achieve complete coverage for ADSP 
within a given domain, every hostname needs its own ADSP record, absent 
a reference to the ADSP record at the next higher level.  The parent 
domain check does not attempt to cover subdomains, but since they are 
much less numerous than hostnames, the overhead of publishing ADSP 
records (manually, if necessary) for subdomains is much less than for 
hostnames.

-Jim