4.4. EDNS Client Subnet (ECS)

The EDNS Client Subnet (ECS) option [RFC7871] allows recursive resolvers to request IP addresses that are suitable for a particular client IP range. SVCB records may contain IP addresses (in ipv*hint SvcParams) or direct users to a subnet-specific TargetName, so recursive resolvers SHOULD include the same ECS option in SVCB queries as in A/AAAA queries.

According to Section 7.3.1 of [RFC7871], "Any records from [the Additional section] MUST NOT be tied to a network." Accordingly, when processing a response whose QTYPE is SVCB-compatible, resolvers SHOULD treat any records in the Additional section as having SOURCE PREFIX-LENGTH set to zero and SCOPE PREFIX-LENGTH as specified in the ECS option. Authoritative servers MUST omit such records if they are not suitable for use by any stub resolvers that set SOURCE PREFIX-LENGTH to zero. This will cause the resolver to perform a follow-up query that can receive a properly tailored ECS. (This is similar to the usage of CNAME with the ECS option as discussed in [RFC7871], Section 7.2.1.)

Authoritative servers that omit Additional records can avoid the added latency of a follow-up query by following the advice in Section 10.2.