US
RFC 4242: Information Refresh Time Option for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
https://www.rfc-editor.org/rfc/rfc4242
… the addition of a new NTP server or a change of address of a DNS server. See [ RFC4076 ]. 2 . Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as de…