The Dynamic Host Configuration Protocol for IPv4 (DHCPv4) has no guidance for how to secure messages exchanged between servers and relay agents. The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) states that IPsec should be used to secure messages exchanged between servers and relay agents but does not require encryption. With recent concerns about pervasive monitoring and other attacks@ it is appropriate to require securing relay-to-relay and relay-to-server communication for DHCPv6 and relay-to-server communication for DHCPv4.
RFC 8213-2017 history
2017RFC 8213-2017 Security of Messages Exchanged between Servers and Relay Agents