7.3. "ipv4hint" and "ipv6hint"

The "ipv4hint" and "ipv6hint" keys convey IP addresses that clients MAY use to reach the service. If A and AAAA records for TargetName are locally available, the client SHOULD ignore these hints. Otherwise, clients SHOULD perform A and/or AAAA queries for TargetName per Section 3, and clients SHOULD use the IP address in those responses for future connections. Clients MAY opt to terminate any connections using the addresses in hints and instead switch to the addresses in response to the TargetName query. Failure to use A and/or AAAA response addresses could negatively impact load balancing or other geo-aware features and thereby degrade client performance.

The presentation value SHALL be a comma-separated list of one or more IP addresses of the appropriate family in standard textual format. To enable simpler parsing, this SvcParamValue MUST NOT contain escape sequences.

The wire format for each parameter is a sequence of IP addresses in network byte order (for the respective address family). Like an A or AAAA RRset, the list of addresses represents an unordered collection, and clients SHOULD pick addresses to use in a random order. An empty list of addresses is invalid.

When selecting between IPv4 and IPv6 addresses to use, clients may use an approach such as Happy Eyeballs. When only "ipv4hint" is present, NAT64 clients may synthesize IPv6 addresses as specified in RFC 7050 or ignore the "ipv4hint" key and wait for AAAA resolution (Section 3). For best performance, server operators SHOULD include an "ipv6hint" parameter whenever they include an "ipv4hint" parameter.

These parameters are intended to minimize additional connection latency when a recursive resolver is not compliant with the requirements in Section 4 and SHOULD NOT be included if most clients are using compliant recursive resolvers. When TargetName is the service name or the owner name (which can be written as "."), server operators SHOULD NOT include these hints, because they are unlikely to convey any performance benefit.