[DNSOP] meaning of tag "match" for CAA RDATA

[神明達哉 <[e-mail]>]

I happen to have this question while reading RFC6844: what does the
"matching" mean in the following description of Section 5.1?

   Tag:  The property identifier, a sequence of US-ASCII characters.

      Tag values MAY contain US-ASCII characters 'a' through 'z', 'A'
      through 'Z', and the numbers 0 through 9.  Tag values SHOULD NOT
      contain any other characters.  Matching of tag values is case
      insensitive.

Although the boundary is not very clear, Section 5.1 generally seems
to talk about the DNS-level syntax (e.g. what should/should not appear
in wire as a DNS message or in a zone file), while Section 5.2 and
later mainly talk about the semantics at the application layer
(something that validates certificates).  Since the above text is in
Section 5.1, I first thought "matching of tag values" was a DNS level
concept.  But then it's not clear to me what it actually means.

Does this mean, for example, we should perform case-insensitive
comparison of this field when we compare CAA RDATAs?  (If so, at least
ISC BIND 9 isn't compliant to the spec; it doesn't care about the case
of the tag field when loading or serving or updating or signing a CAA
RR).

It may also be related to Section 5.1.1, which states:

   The canonical presentation format of the CAA record is:

   CAA <flags> <tag> <value>
[...]
   Tag:  Is a non-zero sequence of US-ASCII letters and numbers in lower
      case.

which might read, for example, as 'dig' should present the tag field
with lower-case letters.  But 'dig' currently doesn't work that way.

Could someone more familiar with the background of CAA clarify these
points?

Thanks,

--
JINMEI, Tatuya