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3. Client Behavior

"SVCB resolution" is the process of enumerating and ordering the available endpoints for a service, as performed by the client. SVCB resolution is implemented as follows:

  1. Let $QNAME be the service name plus appropriate prefixes for the scheme (see Section 2.3).

  2. Issue a SVCB query for $QNAME.

  3. If an AliasMode SVCB record is returned for $QNAME (after following CNAMEs as normal), set $QNAME to its TargetName (without additional prefixes) and loop back to Step 2, subject to chain length limits and loop detection heuristics (see Section 3.1).

  4. If one or more "compatible" (Section 8) ServiceMode records are returned, these represent the alternative endpoints. Sort the records by ascending SvcPriority.

  5. Otherwise, SVCB resolution has failed, and the list of available endpoints is empty.

This procedure does not rely on any recursive or authoritative DNS server to comply with this specification or have any awareness of SVCB.

A client is called "SVCB-optional" if it can connect without the use of ServiceMode records; otherwise, it is called "SVCB-reliant". Clients for pre-existing protocols (e.g., HTTP) SHALL implement SVCB-optional behavior (except as noted in Section 3.1 or when modified by future specifications).

SVCB-optional clients SHOULD issue in parallel any other DNS queries that might be needed for connection establishment if the SVCB record is absent, in order to minimize delay in that case and enable the optimizations discussed in Section 5.

Once SVCB resolution has concluded, whether successful or not, if at least one AliasMode record was processed, SVCB-optional clients SHALL append to the list of endpoints an endpoint consisting of the final value of $QNAME, the authority endpoint's port number, and no SvcParams. (This endpoint will be attempted before falling back to non-SVCB connection modes. This ensures that SVCB-optional clients will make use of an AliasMode record whose TargetName has A and/or AAAA records but no SVCB records.)

The client proceeds with connection establishment using this list of endpoints. Clients SHOULD try higher-priority alternatives first, with fallback to lower-priority alternatives. Clients resolve AAAA and/or A records for the selected TargetName and MAY choose between them using an approach such as Happy Eyeballs [HappyEyeballsV2].

If the client is SVCB-optional and connecting using this list of endpoints has failed, the client now attempts to use non-SVCB connection modes.

Some important optimizations are discussed in Section 5 to avoid additional latency in comparison to ordinary AAAA/A lookups.

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