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I was reading an English document when I noticed something:
RFC 4033 [Section 8.1.]
It is important to note the distinction between a RRset's TTL value and the signature validity period specified by the RRSIG RR covering that RRset.
I understand the use of the genitive in RRset's TTL, because they are talking about the TTL of the RRset. But why didn't they use it with the signature validity period? They are talking about the validity period of the signature, right?!
Not necessarily.
the signature's validity period
would be understood as validy period being a property of signature.
Whereas
the signature validity period
is simply a proper name composed of three words. A technical term, meaning the time during which the signature is valid.
Compare:
In English, nouns can be used as adjectives: "airplane glue", that is, the glue used when making model airplanes. Nouns are said to be used attributively when they're used as adjectives.
These nouns-as-adjectives can be chained or stacked up:
model airplane glue
In technical writing such concatenation is very typical, because it is perceived to be a terse and efficient style.
The following sentence
It is important to note the distinction between a RRset's TTL value and the signature validity period specified by the RRSIG RR covering that RRset.
could be made more verbose:
It is important to note the distinction between the RRset's time-to-live ("TTL") and the period during which the signature remains valid (the "Signature Validity Period") as specified by the RRSIG RR that covers the RRset.
With respect to your question, why didn't the author use a possessive?
The noun in question is "signature". Yes, the author could have written:
... the signature's validity period
