DNS serial number -- 32 bits.
A resource record.
NB. These are as stored in the DNS trie, which associates lists of payloads with each type in an attempt at a compact representation. As only one payload of each type can be marshalled into an RR in a packet, this necessitates a two-phase marshalling process. To prevent type collisions, [root:Packet] represents each RR as a variant type with the same name.