DNSSEC
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... To ensure the correct operation of equality comparison (section 6)
and of the DNSSEC canonical form (section 7) when an RR type is known
...
... DNSSEC Canonical Form and Ordering ...
... RDATA are converted to lower case.
The downcasing is necessary to ensure the correctness of DNSSEC
signatures when case distinctions in domain names are lost due to
compression ...
... RR types that contain embedded
domain names, and whose DNSSEC canonical form therefore involves
downcasing according to the DNS ...