1.2. Overview of the SVCB RR

This subsection briefly describes the SVCB RR with forward references to the full exposition of each component. (As discussed in Section 6, this all applies equally to the HTTPS RR, which shares the same encoding, format, and high-level semantics.)

The SVCB RR has two modes: 1) AliasMode (Section 2.4.2), which aliases a name to another name and 2) ServiceMode (Section 2.4.3), which provides connection information bound to a service endpoint domain. Placing both forms in a single RR type allows clients to fetch the relevant information with a single query (Section 2.3).

The SVCB RR has two required fields and one optional field. The fields are:

SvcPriority (Section 2.4.1): The priority of this record (relative to others, with lower values preferred). A value of 0 indicates AliasMode.

TargetName: The domain name of either the alias target (for AliasMode) or the alternative endpoint (for ServiceMode).

SvcParams (optional): A list of key=value pairs describing the alternative endpoint at TargetName (only used in ServiceMode and otherwise ignored). SvcParams are described in Section 2.1.

Cooperating DNS recursive resolvers will perform subsequent record resolution (for SVCB, A, and AAAA records) and return them in the Additional section of the response (Section 4.2). Clients either use responses included in the Additional section returned by the recursive resolver or perform necessary SVCB, A, and AAAA record resolutions (Section 3). DNS authoritative servers can attach in-bailiwick SVCB, A, AAAA, and CNAME records in the Additional section to responses for a SVCB query (Section 4.1).

In ServiceMode, the SvcParams of the SVCB RR provide an extensible data model for describing alternative endpoints that are authoritative for a service, along with parameters associated with each of these alternative endpoints (Section 7).

For HTTP use cases, the HTTPS RR (Section 9) enables many of the benefits of Alt-Svc [AltSvc] without waiting for a full HTTP connection initiation (multiple round trips) before learning of the preferred alternative, and without necessarily revealing the user's intended destination to all entities along the network path.