R2: "OS" is used only once here in the explanation after the recommendation. Just spell it out.
R4: "in path MTU size" -> "in the path MTU size"
Section 4: The first two recommendations do not end with a period although all other recommendations do end with a period.
Section 5: I would add something right after the Section 5 header like "Some authoritative servers deviate from the DNS standard as follows:" and then make the current first two sentences in Section 5 indented and/or numbered or otherwise clearly a list.
Nits
R1: 'without "Fragment' -> 'without a "Fragment'
R2: "OS" is used only once here in the explanation after the recommendation. Just spell it out.
R4: "in path MTU size" -> "in the path MTU size"
Section 4: The first two recommendations do not end with a period although all other recommendations do end with a period.
Section 5: I would add something right after the Section 5 header like "Some authoritative servers deviate from the DNS standard as follows:" and then make the current first two sentences in Section 5 indented and/or numbered or otherwise clearly a list.